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Russian Chemical Reviews 70 (7) 535 ± 541 (2001) Paramagnetic resonance and detection of a single electron spin A L Buchachenko, F I Dalidchik, S A Kovalevskii, B R Shub Contents VIII. Can single-spin ESR be detected by tunnelling spectroscopy? I. Introduction II. ESR sensitivity evolution III. Optical detection and magnetic resonance of single triplet molecules IV. Single-molecule tunnelling spectroscopy V. Tunnelling spectroscopy of a single electron spin VI. Tunneling spectroscopy of spin channels VII. Single electron spin relaxation IX. Excited states and single-molecule chemistry X. Single-molecule spin chemistry XI. Conclusion Abstract. spin, electron single a of detection of methods Two Two methods of detection of a single electron spin, namely, tunnelling and (1993) detection optical namely, optical detection (1993) and tunnelling spectroscopy spectroscopy (1998) the on based are methods Both described.are (1998) are described. Both methods are based on the spin spin chemistry of prospects and principles Physical laws. chemistry laws. Physical principles and prospects of application application of includes bibliography The considered. are methods these of these methods are considered. The bibliography includes 34 34 references. I. Introduction Electron spin resonance (ESR) spectroscopy has found numerous applications in physics, chemistry, molecular biology and medi- cine. This method has two key functions. First, it is used as a physical tool in studies of the structure and dynamics of spin carriers (radicals, ions, complexes, high-spin molecules, etc.). The hyperfine (electron ± nucleus) interaction constants as well as the magnitudes of dipole (electron ± electron) splitting of the Zeeman energy levels and the parameters of spin ± orbit interaction, which manifests itself as different values of g-factors, contain structural information.The width and shape of ESR spectral lines contain information on the rotational, translational and intramolecular spin carrier dynamics. This information can be extracted by means of the spin relaxation times t1 and t2 which are determined by the rate of energy relaxation of the Zeeman reservoir and the rate of phase relaxation (i.e., spin precession) respectively. Second, ESR spectroscopy is an important analytical method which is used not only in fundamental areas of science (physics, chemistry and biology), but also in ecology, archaeology, medi- cine and history (e.g., in age determination using ESR1, 2). Analytically, the key advantage of this method is its high sensi- tivity. The aim of this review is to analyse the evolution of this A L Buchachenko, F I Dalidchik, S A Kovalevskii, B R Shub N N Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul.Kosygina 4, 119991 Moscow, Russian Federation. Fax (7-095) 938 24 84. Tel. (7-095) 939 74 90. E-mail: spinchem@chph.ras.ru (A L Buchachenko) Tel. (7-095) 939 72 59 (F I Dalidchik), (7-095) 137 82 73 (B R Shub) Received 15 January 2001 Uspekhi Khimii 70 (7) 611 ± 618 (2001); translated by A M Raevsky #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n07ABEH000652 535 535 536 537 538 538 539 539 540 541 541 parameter and to outline some ways toward progress in reaching the limiting sensitivity, which allows detection of a single, indi- vidual paramagnetic species.II. ESR sensitivity evolution In his pioneering paper Zavoiskii reported a sensitivity of *1019 spins. In 1959, Semenov and Bubnov 3 improved the ESR sensi- tivity and reached a value of *1011 spins using high-frequency modulation and automatic frequency control. Since then this value appeared to be a `nearly limiting' one for all commercial ESR spectrometers including modern instruments.Further improvement of sensitivity up to *107 spins 4 appeared to be solely due to the development of high-resolution ESR spectro- scopy (an `elite' division of ESR spectroscopy) which operates with magnetic fields of*50 kê. The sensitivity of `routine' ESR spectroscopy is, as a rule,*1011. Technically, this threshold is difficult to overcome since it is determined by the low sensitivity of detectors of microwave and radiofrequency radiation (it should be remembered that classical ESR spectroscopy is based on measurements of energy losses). A great breakthrough in studies aimed at improving the ESR sensitivity is associated with the use of a combination of the microwave pumping of the spin system with optical detection of results.High photon energy allows them to be detected even in the photon-counting regime. This provides a colossal gain in sensi- tivity and starts the development of non-classical ESR spectro- scopy, viz., the so-called optically detected magnetic resonance (ODMR). The principle ofODMRis illustrated in Fig. 1. Let us assume that a triplet radical pair with the total spin S=1 is generated by photolysis or radiolysis in solution or in a solid. In the magnetic field, only two spin states are mixed. These are the triplet state T0 with the zero spin projection and the singlet state S, whereas the other two spin states (T+ and T7) with the spin projections Sz=1 are excluded from the triplet ± singlet intersystem cross- ing of the pair. Fast intersystem crossing from the non-reactive state T0 to the reactive state S followed by the formation of reaction products in the radical pair leads to depopulation of the levels T0 and S. Microwave pumping at the ESR frequency o corresponding to the difference between the Zeeman energies of536 T+ o S T0 o T7 Reaction products, luminescence Figure 1.An energy level diagram illustrating the ODMR mechanism. Here and in Fig. 2 wavy arrows denote the microwave pumping of corresponding transitions. the states T07T+ and T07T7 leads to an inversion drop and accumulation of electron spins on the level T0 , thus increasing the yield of reaction products. Clearly, the radical pair serves as a chemical microwave detector. If reaction products fluoresce, `chemical reception' of microwaves can be optically detected, which is the case with ODMR.However, this is not the only reception technique since in the case of charged reaction products magnetic resonance can be detected using changes in conductivity. Generally, this technique is well known as Reaction Yield Detected Magnetic Resonance (RYDMR). Pioneering studies on RYDMR were reported by Frankevich.5, 6 Among different versions of the RYDMR method, ODMR is the most sensitive one.The outstanding possibilities of the ODMR technique can be best illustrated by the results obtained by Molin et al.7, 8 taking detection of aromatic radical anions as an example. Generation of anion ± cation radical pairs in the experiments on radiolysis of solutions of aromatic molecules is followed by their annihilation and creation of electronically excited luminescent molecules.The yield of such molecules is strongly dependent on the microwave pumping of ESR transitions in the radical pair. In this case the ODMR sensitivity is as high as several tens of spins. It should be noted that both pulsed and CW ODMR can be observed exper- imentally. Having reached such a high sensitivity, it is logical to pose the next problem which can be formulated as detection of single- molecule ESR (or ESR of a single electron spin carrier). III. Optical detection and magnetic resonance of single triplet molecules Progress in photodetection technique and confocal optical micro- scopy has opened the possibility of visual observation and optical detection of single molecules.9 Upon being illuminated, lumines- cent molecules (e.g., rhodamine 6G, terrylenediimine, etc.) adsorbed on a surface, implanted (in low concentrations) in monomolecular layers or embedded in thin polymer films can be observed as molecular `fireflies' using a microscope.Currently, this experimental technique has been intensely developed. It is widely used in, e.g., molecular dynamics studies. Translational dynamics are studied by tracing the `firefly' trajectory while information on rotational dynamics is obtained by measuring luminescence depolarisation. Other areas of application of the `firefly' technique include measurements of the rates of energy transfer to neighbouring molecules, verification of the ergodic theorem, etc.10, 11 Many optical chromophores used in the experiments on observation of single molecules occur in the long-lived triplet states.By placing such molecules in a permanent and a microwave magnetic field one can initiate paramagnetic resonance and detect it optically by means of observation of light emission from the molecular `fireflies'. The possibility of using such a detection technique can be aptly illustrated taking the ODMR of single pentacene molecules in a p-terphenyl single crystal (the mole fraction of pentacene was *1078) as an example. A propos, it is A L Buchachenko, F I Dalidchik, S A Kovalevskii, B R Shub S1 IC T1 Tx Ty hn Tz IC S0 Figure 2.An energy level diagram illustrating transitions between differ- ent levels in the pentacene molecule. Notations of energy levels: S0 and S1 are the ground-state singlet and excited singlet levels, respectively; T1 is the lowest triplet state; Tx, Ty and Tz are the magnetic sublevels of the triplet state T1. IC denotes intersystem crossing. this system that was used in the first successful single-molecule ODMR experiments.12, 13 An energy level diagram of pentacene spins, typical of almost all molecules, is shown in Fig. 2. After absorption of a quantum hn, the molecule goes from the ground state S0 to the excited state S1 which is then depopulated via fluorescence or molecular relaxation to the lowest triplet state T1. The state T1 is para- magnetic and triply spin degenerate.The dipole interaction between two spins leads to removal of degeneracy and the appearance of three energy levels (Tx, Ty and Tz). The dipole electron ± electron interaction is described by a tensor, the principal components of which, ex, ey and ez, are different with respect to different molecular axes. In ESR spectro- scopy, they are usually expressed through the parametersDand E. D=32 ez , E=712 (ex7ey). These parameters characterise the deviation of the symmetry of the dipole interaction (and, hence, spin distribution) from spherical and axial symmetry, respectively. Dipole splitting also exists in the absence of a magnetic field. If a molecule is placed in an external magnetic field, one should take into account not only the dipole magnetic interaction, but also the Zeeman magnetic interaction, so that in the limiting case (in strong magnetic fields) the spin states Tx, Ty and Tz with the energies ex, ey and ez, respectively, are transformed into the corresponding canonical spin states T+, T0 and T7.Intersystem crossing from the state S1 to the state T1 is spin- forbidden and is thus induced by the intramolecular spin ± orbit interaction, which partially removes this forbiddenness (due to exchange between the intrinsic angular momentum of an electron and its orbital angular momentum). Since the orbital angular momentum is anisotropic (i.e., it takes different values with respect to different molecular axes), the spin ± orbit interaction is also anisotropic.Because of this, the rate of inversion drop from the state S1 also appears to be dependent on the symmetry of the molecule: the states Tx, Ty and Tz with the energies ex , ey and ez (and, correspondingly, the states T+, T0 and T7) are populated at different rates. For the same reason, the depopulation rates of these levels due to spin relaxation from the state T1 to the ground state are also different. These differences in the population and depopulation rates of the three spin sublevels of the triplet T1 are responsible for colossal differences in their steady-state populations. For instance, excitation of a pentacene molecule to the state S1 followed by red S1?S0 fluorescence also causes population of the lowest triplet level T1 with a probability of 0.5%.Among the triplet sublevels, the highest population rates (1.56105 s71) are observed for the levels Tx and Ty; the depopulation rates of these levels (transition to the ground state S0) are also high (0.256105 s71). For the level Tz, these rates are very low, so that in the steady- state regime only the levels Tx and Ty appear to be populated, whereas the level Tz remains nearly completely depopulated. If microwave pumping is tuned to resonance with the energy difference between the spin levels Tx and Tz, their populations areParamagnetic resonance and detection of a single electron spin equalised. As a result, a proportion of pentacene molecules pass to the long-lived Tz level and are temporarily excluded from the `community' of fluorescent molecules.Clearly, a decrease in the fluorescence intensity of a single pentacene molecule is an indicator of magnetic resonance. Know- ing the kinetic constants of photophysical and relaxation proc- esses, one can estimate the magnitude of this effect: it reaches *20% in the case of microwave pumping of the transition Tx±Tz.13 An ODMR spectrum of a single triplet pentacene molecule is shown in Fig. 3 a. In accord with theory, microwave pumping at the resonance frequencies of the Tx±Tz and Ty±Tz transitions (*1470 and *1410 MHz, respectively) leads to noticeable (by more than 10%) decrease in the fluorescence intensity; the latter transition is less intense since the population of the level Ty is intermediate between those of the `active' (Tx) and `passive' (Tz) levels.4000 1440 1520 1480 1360 1400Frequency /MHz Figure 3. AnODMRspectrumof a single pentacene molecule implanted in a p-terphenyl crystal (a) and hyperfine structure of theODMRspectrum of a pentacene molecule deuterated at all positions except for two positions at the carbon atoms lying on the central symmetry axis (b). If the number of magnetic nuclei is small and the hyperfine splitting exceeds the linewidth, one can resolve the hyperfine structure in the spectrum of triplet ODMR. For instance, the ODMR spectrum of a pentacene molecule deuterated at all positions except for two protons at the carbon atoms lying on the central symmetry axis exhibits a clearly seen hyperfine structure originating from two magnetically equivalent protons (a triplet with the 1 : 2 : 1 intensity ratio) and corresponding to the Tx ? Tz (Fig. 3 b) transition with an electron ± proton interaction constant of*10 MHz (*3.6 Gs).13 At first glance, observation of hyperfine structure seems to be a mysterious phenomenon.In a single-molecule experiment, nuclear configurations (a total of four for two protons of the partially deuterated pentacene) should be fixed during the time taken for one excitation ± fluorescence cycle (S0?Tz?S0), which is of the order of 1075 ± 1076 s. The unpaired electron of the triplet molecule must experience only one nuclear configuration. However, the spectral shape suggests that it somehow `sees' all the four pairs of spin configurations, namely, anan, anbn, bnan and bnbn (here, an and bn denote the proton spins; the pairs anbn and bnan are equivalent and correspond to identical nuclear config- urations).The answer to this enigma is simple. In single-molecule ODMR experiments, the molecule experiences over 106 cascade excitation ± fluorescence cycles during the typical measuring time (usually, about 10 min). Hence, the number of `arrivals' at the ground state with a residence time of 100 ms during each cascade cycle is the same. Due to the dipole interaction of pentacene protons with the nuclei of neighbouring molecules and other nuclei of the indicator pentacene molecule, this time appears to Fluorescence intensity a 6000 b 5000 x7z y7z 537 be sufficient for reorientation of the spins of the pentacene protons (in spectroscopy, this is called spin diffusion or spectral diffusion).As a result, the pentacene protons have time to `arrive' at all the four nuclear configurations during the ODMR measuring time and the final spectrum exhibits a triplet with the 1 : 2 : 1 intensity ratio (see Fig. 3 b). It should be noted that reorientation of the proton spins (spectral diffusion) occurs only at those instants when the mole- cule is in the ground state. In the triplet state, strong electron ± nuclear magnetic interaction fixes the proton spins and `excludes' them from spectral diffusion (or, as spectroscopists say, excludes these protons from the dipole ± dipole nuclear reservoir).The hyperfine structure of the ODMR spectrum of 13C nuclei of a pentacene molecule isotopically substituted at different positions is due to the same reasons.14 However, it should be noted that all the ODMR spectra of single triplet molecules observed so far were measured for only one case, viz., a pentacene molecule implanted in a terphenyl crystal. Further progress of this experimental technique and extension of the areas of its applica- tion is closely related to advances of optical detection of single molecules in crystals and on the surface of solids.15 Magnetic resonance of single spin carriers has been intensively developed. Recently, KoÈ hler 16 has reported an excellent review concerning the state-of-the-art and prospects of studies in the field, so that there is no need to dwell on this point.Mention may be made of one important circumstance: it is spectral diffusion and change in the nuclear spin configurations at a single electron spin that cause a remarkable phenomenon, viz., a single spin Hahn echo.17 It should be remembered that spin echo (Hahn echo) appears as a result of coherent behaviour of a spin ensemble; the observation of a single spin Hahn echo confirms the ergodic theorem, according to which averaging over the ensemble is equivalent to averaging over time. IV. Single-molecule tunnelling spectroscopy Recently published series of pioneering studies on tunnelling vibrational spectroscopy 18 ± 20 has promoted the transformation of scanning tunnelling microscopy from a `topographic' method for monitoring the atom-molecular relief of a surface into a new powerful method of chemical physics, which allows one not only to `see' a desired single molecule, but also to identify it from `fingerprints' (the vibronic spectrum) and even follow its chemical fate.The last-mentioned circumstance seems to be of great value owing to the possibility of observation of chemical transforma- tions of a single molecule, which has appeared for the first time. So far classical chemistry has been concerned with molecular ensem- bles even in the best versions of molecular beam techniques. Modern coherent chemistry also deals with single wave packets (ensembles of coherent molecules) rather than single molecules.21 Physically, the idea a single-molecule, resonance tunnelling vibrational spectroscopy 18 is very simple. If a scanning tunnelling microscope operates in the field emission mode and the tip voltage exceeds the work function of electrons, the tunnelling electrons leaving the tip appear to be confined in the space between the tip and the adsorbed molecule `sitting' on the surface under the tip.The electrons confined in this space can move between two potential barriers, viz., the tip and the surface. The residence time of an electron under the tip is of the order of 10713 ± 10714 s (it is much longer than the mean free time), i.e., the space between the tip and the surface represents a specific nanoscale resonator in which standing waves (field emission resonances) appear at particular energies.Since the resonance energy depends on the tip potential, the tunnelling current as function of the bias voltage exhibits some clearly seen peaks corresponding to vibronic (resonance) energy levels of the specified adsorbed molecule which interacts with an the electron trapped in the resonator. This is a specific form of manifestation of a fundamental quantum effect called resonance bleaching of double potential barriers. Such barriers can be as `wide and high' as it is wished, but if the538 energy of the species corresponds to that of a quasi-stationary level, they appear to be penetrable.22 The double potential barriers for which the energy of the tunnelling species can vary 23 (e.g., thin layers of a dielectric containing impurity atoms or the tunnelling contacts containing adsorbed molecules) also appear to be pene- trable.In such systems, energy exchange between the tunnelling electrons and vibrational degrees of freedom of the impurity atoms or adsorbed molecules can occur, thus leading to excitation of oscillations (inelastic interaction). For those electrons whose energies correspond to those of vibronic levels of the resonator containing an adsorbed molecule, the vacuum gap between the surface and the tip becomes transparent; in this case, the tunnel- ling current is a maximum. V. Tunnelling spectroscopy of a single electron spin Tunnelling spectroscopy also allows detection of a single electron spin on the surface.This requires that the tunnelling electron be able to distinguish (discriminate) two spin states of the unpaired electron with Sz=1/2 (e.g., the Zeeman energy levels of a paramagnetic species). There are two parameters suitable for performing such a discrimination, viz., energy and spin. In typical magnetic fields H&3000 Gs, the Zeeman energy of the unpaired electron is *1075 eV, which is 4 ± 5 orders of magnitude lower than the energies of the tunnelling electron and 3 orders of magnitude lower than the resolution of tunnelling spectroscopy (*0.01 eV). Clearly, it is impossible to use energy as a parameter for discriminating the Zeeman energy levels of the unpaired electron.This means that the other parameter, spin, should be used and that the detection principles of a single spin should be based on the spin chemistry laws. Let us consider from this viewpoint the spin states and spin dynamics of two electron spins, the spin of a paramagnetic (probed) centre and that of a tunnelling (probing) electron. This two-spin system represents a spin analogue of the radical pair considered in the spin chemistry. It can be in two states, viz., in a singlet state with the total spin S=0 or in the triplet state with the total spin S=1. The energy splitting between these spin states equals the exchange energy J of two electrons in the nanoscale resonator. This means that all vibronic levels of the spin carrier situated under the tip should be split into doublets corresponding to the singlet and triplet spin states of the electron pair.The nanoscale resonator is `transparent' to the tunnelling electrons of the electron pair irrespective of the state (singlet or triplet) of the pair at different energies (different tip potentials). The interaction between two electrons (the tunnelling electron and the unpaired one) is identical to the interaction between radicals in a radical pair. For instance, recombination of a singlet radical pair,CH3 and OH, involves the singlet channel only to give a singlet CH3OH molecule, while their disproportionation involves the triplet channel (the reaction CH3+OH? 3CH2+H2O) to give the triplet carbene 3CH2. Thus, formally, 2 both channels are spin-allowed (i.e., the total spins of the reagents and products in each channel conserve).However, the energy `cost' of each channel is different, viz., the former process proceeds barrierlessly (without activation energy), whereas the latter requires the overcoming of an energy barrier of*6 kcal mol71. Tunnelling spectroscopy of a single electron spin on the surface was theoretically substantiated and experimentally imple- mented by Dalidchik and Kovalevskii.24 The spectrum of a O¡ anion sitting on the TiO2 surface (Fig. 4) exhibits a doublet splitting of each line, which is in complete agreement with theory. The splittings 1 ± 2, 2 ± 3, 1 0 ± 2 0 and 2 0 ± 3 0 equal 0.15 eV, which exactly corresponds to the vibrational quantum for the O7O bond in the anion (the frequency of the stretching vibrations O7O is 1180 cm71).Singlet ± triplet splitting between the peaks, i.e., the separations between the lines 1 ± 1 0, 2 ± 2 0 and 3 ± 3 0, is equal to the exchange energy of two electrons (the tunnelling A L Buchachenko, F I Dalidchik, S A Kovalevskii, B R Shub I /nA 0 75 710 2 20 715 10 1 7.4 7.0 Figure 4. Tunnelling current I as function of the tip potential V (tunnel- ling spectrum) of a single radical anion O¡2 on the TiO2 surface. electron and the unpaired electron of the radical anion O¡2 ) in the nanoscale resonator. This experiment also allowed the exchange energy to be measured (J=0.07 eV), which provided the possibility of esti- mating the size of the nanoscale resonator, R.Let us write the known relationship J(R)=J0exp(7aR), where J0 is the exchange energy in a strongly coupled two-electron system (e.g., H2 molecule) and a is the characteristic parameter of the J decay as a function of R. Assuming that J0&4.5 eV and a&2 A71, we get R&2.1 A, which seems to be physically realistic for the nanoscale resonator. VI. Tunneling spectroscopy of spin channels As mentioned above, a single paramagnetic species can be detected using tunnelling spectroscopy due to the appearance of two resonance channels (a singlet and a triplet one) of the tunnelling current. These channels are separated by *0.1 eV on the energy scale. Such a doublet splitting of vibronic levels was confirmed experimentally and discussed above (see Fig. 4).Equi- distant doublet splitting of all lines in the tunnelling spectrum (on the plot of the dependence of the current I on the potential V) is an unambiguous indicator of the presence of a spin carrier (a radical or an ion) under the tip. However, direct attribution of each of the two spin channels to the singlet and triplet channel is impossible (we cannot determine to which channel the groups of lines 1, 2 and 3 and 1 0, 2 0 and 3 0 in Fig. 4 belong). Based on general consid- erations and following Pauli, one can only suggest that the triplet level lies above the singlet one on the energy scale, i.e., that the low-energy lines of the doublets correspond to the singlet channel, while their high-energy components correspond to the triplet channel. However, these considerations seem to be quite unreli- able since no details of the interaction between the tunnelling electron and paramagnetic centre are currently known.Microwave pumping is also unhelpful in distinguishing these channels since the singlet channel is ESR `silent' while the response of the triplet channel (transitions between the states T+, T0 and T7) does not produce any changes in the tunnelling current. Therefore, if microwave pumping causes no transitions between the states S and T, no transfer of the tunnelling electrons between the two channels occurs. On the other hand, yet another reason seems to be stronger: the lifetime of the electron pair in the nanoscale resonator is much shorter than the time taken for spin reorientation induced by microwave pumping.This means that 3 30 V /eVParamagnetic resonance and detection of a single electron spin classical ESR of a single electron spin is impossible and other approaches should be used. Imagine that the tip of a tunnelling microscope is made of a ferromagnetic material and emits spin-polarised electrons. (Such tips are widely used in topographic studies of the surface of ferromagnetic materials.25) If the voltage is chosen in such a way that the nanoscale resonator is `tuned' in resonance with the singlet level, the potential barrier on the surface is transparent to the tunnelling electrons with the spins antiparallel to the spin of the paramagnetic centre.The singlet spin state of two electrons (the tunnelling electron and the unpaired one) in the nanoscale resonator is of course a combination of the spins of these electrons. Let the emitted (polarised) electrons leaving the para- magnetic tip have the spin b and the unpaired electron of the paramagnetic centre have either the spin a of b (here, the differ- ence between the populations of the Zeeman energy levels of the electrons is neglected and the probabilities of the appearance of electrons with the spins a and b are taken to be equal). Then, the combinations ab correspond to the singlet state of the electron pair, thus providing conditions for the tunnelling current to flow, while the combinations bb correspond to the triplet state whose energy differs by a value of J.For the spin combinations bb the potential barrier is impenetrable, so this contribution to the tunnelling current is locked. Here, an important point which was first reported by Kozhushner et al.26, 27 should be taken into account. Due to exchange interaction between the electrons (more exactly, its nonsecular term (1/2)J(S+S7+S7S+), where S+ and S7 are the spin operators which increase and decrease the spin projection, respectively), the polarised electrons leaving the tip also polarise the spin of the unpaired electron in such a manner that its a spin it transformed into a b spin and the energy level a of the para- magnetic centre is depopulated. The system undergoes a transition from the singlet state to the triplet state and the resonance tunnelling current in the singlet channel no longer flows.An energy level diagram illustrating the population of the Zeeman levels of both electrons in the system `ferromagnetic tip ¡À paramagnetic centre' is shown in Fig. 5. Initially (a), both energy levels corresponding to the single spin have nearly equal populations. If the tip potential differs from zero, the tunnelling current It proportional to NbPa (Nb is the population of the Zeeman energy level of the polarised tip electrons and Pa is the population of the upper Zeeman energy level a of the para- magnetic centre) flows in the system. The singlet current will flow only provided that the single spin of the paramagnetic centre will be oriented antiparallel to the spins of the tunnelling electrons, i.e., in the combination ba.Pa Pb �� exp It is known that ¡¦gbH , kT a Na Nb b Na Nb Ferromagnetic tip Figure 5. An energy level diagram illustrating population of the Zeeman levels in the system `ferromagnetic tip ¡À paramagnetic centre'. Initial state (a) and after passage of the tunnelling current (b). Pa Pb Pa Pb Paramagnetic centre 539 where gbH is the Zeeman splitting. Since gbH 55 kT, we get Pa&Pb . Exchange interaction causes polarisation of the spin a of the paramagnetic centre (an analogue of the flip-flop transition in the nuclear Overhauser effect) and depopulation of the upper Zeeman energy level.As a result, only the spin combinations bb remain in the system (Fig. 5 b). The singlet resonance peak on the I ¡ÀV plot disappears and the tunnelling current (singlet channel) is locked. The polarisation mechanism of the singlet tunnelling current lock has not been experimentally observed so far. However, should it be found, it could be possible to distinguish between the singlet and triplet channels of the resonance tunnelling current and to determine not only the magnitude, but also sign of the electron ¡À electron exchange interaction in the nanoscale resona- tor. This opens new prospects for single-molecule tunnelling spectroscopy. VII. Single electron spin relaxation Observation of the lock effect of the singlet tunnelling channel requires that the spin of the paramagnetic species be completely polarised.This is possible only in the case of slow spin ¡À lattice relaxation where the spin orientation of the species remains fixed between the acts of electron transport between the ferromagnetic tip and the surface. For characteristic spin relaxation times (ts&1078 s), this condition is met if the tunnelling currents are rather heavy, I>0.1 nA s71. At weaker tunnelling currents, the species can have random spin orientations at the instances corresponding to the acts of interaction with the tunnelling electron; this means that both parallel and antiparallel orienta- tions of the spins of the species and the electron are possible. During the measuring time of the tunnelling current (at specified voltage), the paramagnetic species sequentially interacts with numerous electrons leaving the tip (the number of electrons is N=It, where t is the measuring time). The probabilities of the singlet and triplet states of electron pairs are determined not only be statistics of these states, but also by the kinetics of both spontaneous (relaxation) spin transitions and those induced by the polarised electron current.In the case of ferromagnetic tip the lock rate of the singlet channel is given by the relation . (1) qP qt �� ¡¦ I ¡¦ 1 ts s s s Here, P is the population of the singlet channel (i.e., the popula- tion of the spin states ab) and I is the tunnelling current of those electrons that had experienced exchange scattering by the para- magnetic centre.The first term in Eqn (1) includes the polar- isation of the paramagnetic centre due to the polarised electron current (the pumping rate of the triplet channel and the depopu- lation rate of the singlet channel) while the second term includes the depolarisation (relaxation) rate of the triplet channel and the recovery rate of the singlet channel. If I>t¡¦1 (qP=qt<0), population of the triplet channel and depopulation of the singlet channel occur; the tunnelling current in the singlet channel is locked. At weak currents, where I<t¡¦1 (qP=qt>0), the relaxation rate is higher than the population rate of the triplet channel. In this case, the weak tunnelling current cannot lock the singlet channel that remains open.In fact, the condition I=t¡¦1 determines the lock threshold of the tunnelling current. It allows measurements of the electron spin relaxation time of the single paramagnetic centre under conditions of tunnelling spectroscopy. VIII. Can single-spin ESR be detected by tunnelling spectroscopy? Novel spin spectroscopy methods, viz., optical spectroscopy and tunnelling spectroscopy are based on spin combinations of two540 electrons. In both cases, not only detection of single paramagnetic species, but also measurements of their spin relaxation parameters is possible. However, these methods use fundamentally different detection techniques. In the case of optical detection, the singlet channel manifests itself via fluorescence and, hence, some of its characteristics can be measured.The triplet channel is nondetect- able; however, both the channels are strongly coupled via the spin ¡À orbit and magnetic interactions, so that any changes in the triplet (nondetectable) channel affect some parameters of the singlet channel. In tunnelling spectroscopy, both these channels are detectable; however, no magnetic interaction between them occurs so that they are independent. There is also a number of dynamic distinctions between the two above-mentioned detection techniques. In the case of optical detection, the residence time of a molecule in the triplet state is *1075 s in each excitation ¡À deactivation cycle. This time is much longer than that required for spin reorientation under the action of microwave pumping, tmw=(gH)71 (g is the gyromagnetic ratio of an electron and H is the strength of the microwave field).Typically, tmw&3.161077 s; this means that the triplet lifetime is long enough for both spin reorientation and change in the populations of the triplet levels under the action of the microwave magnetic field. In other words, the triplet state is an effective `receiver' of external microwave radiation. In tunnelling spectro- scopy, the residence time of the electron in the nanoscale resonator is very short. As mentioned above, it does not exceed 10714 to 10713 s. This means that the triplet states in the nanoscale resonator cannot act as effective `receivers' of microwave radia- tion.In tunnelling microscopy, single-spin magnetic resonance should be detected using non-classical approaches (e.g., polarised tunnelling electrons).. tmw s In this case, the population kinetics of the spin states is similar to that considered above; however, transitions between two spin orientations of a paramagnetic species can now be stimulated not only by the tunnelling electrons, but also by microwave pumping. This can be easily understood by considering Eqn (1) again. If resonance microwave pumping occurs (i.e., the ESR conditions are met), this equation takes the form qP qt �� ¡¦ I ¡¦ 1 ¡¦ 1 (2) t As can be seen, a new term appears, which is due to the spin relaxation induced by the microwave pumping.Spin relaxation induces additional transitions from the state bb to the state ab, thus additionally populating the singlet channel with a rate of t¡¦1 mw. In other words, microwave pumping shifts the lock threshold of the singlet channel in such a manner that the old condition I=t¡¦1 mw is replaced by a new one, tmw I= 1 + 1 . ts s Since the values t¡¦1 and t¡¦1 mw are commensurable under standard ESR conditions, the additional shift of the lock thresh- old of the tunnelling current in the singlet channel due to micro- wave pumping can be noticeable and even experimentally measurable. Moreover, experiments under conditions where t¡¦1 mw55ts can be readily set up (at H 44 1 �º). In this case, the shift of the lock threshold will be nearly completely determined by the microwave pumping. This offers considerable promise for single-spin ESR detectable from the tunnelling current.A com- plete theory of this phenomenon has been developed recently.28 IX. Excited states and single-molecule chemistry In the aforesaid, we assumed conservation of the electron energy (it should be remembered that the Zeeman transition energies are negligible). Hence, the spin effects due to exchange interaction, A L Buchachenko, F I Dalidchik, S A Kovalevskii, B R Shub relaxation and microwave pumping were associated solely with changes in the spin orientations of the paramagnetic species and tunnelling electrons. However, more complex effects can also occur, where the tunnelling electrons are responsible not only for the change in the spin orientation of the adsorbed species, but also for the excitation of internal (vibrational and/or electronic) degrees of freedom of the species.In the case of tunnelling between conductors in which all electron states below the Fermi level can be considered filled, such processes can proceed only at the voltages V exceeding the energy loss de (usually, de 44 tT) and the dependence of I on the potential V exhibits the so-called threshold features (conductivity jumps), which appear at the voltage eV=de. Threshold features manifest themselves as max- ima of the second derivatives of the tunnelling current with respect to the tip potential. They are observed in those cases where the tunnelling electrons excite the molecule `sitting' under the micro- scope tip and induce its chemical transformations.The possibility of observation of inelastic processes using tunnelling microscopy was predicted in 1985; however, exper- imental studies have been reported only recently.29 ¡À 31 For instance, it was found that molecules C2H2 and C2D2 adsorbed on the (001) surface of a copper single crystal dissociate to give C2H and C2D, respectively. This occurs at potentials of 2.8 V for C2H2 and 3.9 V for C2D2.31 The C2H and C2D radicals thus formed undergo further dissociation to give C2 at potentials of 2.1 and 2.7 V, respectively. In this case, large isotope effects are due to differences between the vibrational excitation cross-sections of different isotopomers rather than to isotopism of the C7H and C7D bonds.The excitation cross-section is determined by the probability of energy transfer (an inelastic process) from the `light' tunnelling electron to the `heavy' molecule; its magnitude is proportional to a small parameter (a0/R)2Dn41072Dn (a0 is the amplitude of zero-point vibrations of the molecule or constituent atoms, R is the characteristic atomic size and Dn is the change in the vibrational quantum number upon excitation). This formula readily demonstrates large isotope effect for the excitation cross- section. Dehydrogenation was also observed for single ethylene mol- ecules adsorbed on the Ni(110) surface (finally, ethylene loses two protons to give C2H2) and for benzene, pyridine and their isotopomers n the Cu(001) surface.31 Moreover, dif- fusion displacements and rotational motions of single molecules induced by the tunnelling electrons can also be observed along with their chemical transformations. No observations of threshold features in the experiments on the excitation of adsorbed species by tunnelling electrons using a scanning tunnelling microscope (STM) have been reported so far.These features are assumed to be `large-scale' since in this case we are concerned with electron ¡À electron energy exchange. Of partic- ular interest is consideration of a physically adsorbed oxygen molecule. The ground state of the molecule (a triplet X3S¡¦g ) is separated from the nearest excited singlet state (a1Dg) by about 1 eV on the energy scale.The reactivity and relaxation of singlet dioxygen adsorbed on various surfaces were studied in detail by Ryskin 32 and Krylov and Shub.33 Using different `macroscopic', kinetic and spectroscopic methods, they first determined the relaxation times of singlet dioxygen. It was found that this parameter depends strongly on the surface characteristics and its magnitude can vary over a wide range. Optimum experimental conditions for observation of threshold features of the X3Sg?a1Dg transitions of an oxygen molecule `sitting' under the tip using a STM should be chosen so that the rates of stimulated and spontaneous (relaxation) transitions be compara- ble.This allows detection of new changes in the tunnelling current by varying the tunnelling current (i.e., by changing the popula- tions of two electronic states of the oxygen molecule). This situation is quite similar to that described above for the single spin (`self-regulation' and lock of the tunnelling current).Paramagnetic resonance and detection of a single electron spin X. Single-molecule spin chemistry In STM experiments, the operating current densities can be as high as 108 A cm72. It is for this reason that is possible to transform the equilibrium conditions into nonequilibrium ones and cause creation of nonequilibrium distributions over vibra- tional and electron energy levels of the single molecules `sitting' under the tip.34 (It should be remembered that in the case of single- molecule experiments we are dealing with ensembles of elementary events, i.e., a manifold of independent interaction acts of a species with individual tunnelling electrons.) Heating leads to accelera- tion of chemical transformations such as desorption, dissociation, transfer of the adsorbed species from the tip to the surface, etc.Experiments with ferromagnetic tips allow the observation of versatile single-molecule spin chemistry effects. The simplest case is deposition of a paramagnetic species from the ferromagnetic tip on the surface containing a paramagnetic centre. It is easy to understand that at sufficiently heavy tunnelling currents (both species are polarised) recombination is forbidden.At weak currents (the spin of the species adsorbed on the surface has no definite orientation), recombination is allowed. XI. Conclusion Thus, classical scanning tunnelling microscopy gave impetus to the emergence and development of two new, important and promising methods, viz., single-molecule tunnelling spectroscopy and single-molecule chemistry. The emergence of these methods signifies a new stage of the development of modern chemistry, which offers considerable prospects for surface sciences, catalysis, etc. In particular, the use of single-molecule spectroscopy and single-molecule chemistry in combination with photochemistry and femtochemistry presents new, inestimable possibilities for manipulating the molecules. In the framework of single-molecule spectroscopy and single-mole- cule chemistry classical spin chemistry also undergoes transfor- mation into `single-molecule' spin chemistry.This offers unique possibilities for spin channel detection, single electron spin manip- ulation and detection of ESR of a single paramagnetic centre (including triplet molecules, carbenes and ions) from the tunnel- ling current. The authors express their gratitude to the Russian Foundation for Basic Research and the Federal Programme `Integration' for financial support. References 1. D Attanasio EPR Newsletters 9 14 (1998) 2. D Attanasio, D Capitani, C Federici, A Serge Archaeometry 37 377 3. A G Semenov, N N Bubnov Pribory Tekhnika Eksperim. (1) 92 4. O Ya Grinberg, A A Dubinskii, Ya S Lebedev Usp.Khim. 52 1490 5. E L Frankevich, A I Pristupa, V I Lesin Chem. Phys. Lett. 47 304 6. A L Buchachenko, E L Frankevich Chemical Generation and 7. V O Saik, O A Anisimov, Yu N Molin Chem. Phys. Lett. 116 138 8. O A Anisimov, V M Grigoryants, Yu N Molin Pis'ma Zh. Eksp. 9. S Mais, J Tittel, Th Basche', C BraÈ uchle, W GoÈ hde, H Fuchs, (1995) (1959); Vestn. Akad. Nauk SSSR (3) 55 (1959) a (1983) [Russ. Chem. Rev. 52 850 (1983)] (1978) Reception of Radio- and Microwaves (New York: VCH, 1994) (1985) Teor. Fiz. 30 589 (1979) G MuÈ llen, K MuÈ ller J. Phys. Chem. A 101 8435 (1997) 10. W E Moerner, M Orrit Science 283 1670 (1999) 11. F Kulzer, F Koberling, Th Crist, A Mews, Th Basche' Chem. Phys. 247 23 (1999) 12. J KoÈ hler, J A J M Disselhorst, M C J M Donckers, E J J Groenen, J Schmidt,W E Moerner Nature (London) 363 242 (1993) 541 13. J Wrachtrup, C von Borczyskowski, J Bernard, M Orrit, R Brown Nature (London) 363 244 (1993) 14. J KoÈ hler, A C J Brouwer, E J J Groenen, J Schmidt Science 268 1457 (1995) 15. Chem. Phys. 247 (1) (Spec. Issue) (1999) 16. J KoÈ hler Phys. Rep. 310 261 (1999) 17. J Wrachtrup, C von Borczyskowski, J Bernard, R Brown, M Orrit Chem. Phys. Lett. 245 262 (1995) 18. F I Dalidchik, M V Grishin, N N Kolchenko, S A Kovalevskii Surf. Sci. 387 50 (1997) 19. F I Dalidchik, M V Grishin, S A Kovalevskii, N N Kolchenko Pis'ma Zh. Eksp. Teor. Fiz. 65 306 (1997) 20. S A Kovalevskii, F I Dalidchik,M V Grishin, N N Kolchenko, B R Shub Appl. Phys. A, Mater. Sci. Process. 66 (Suppl. Pt. 1) (1998) 21. A L Buchachenko Usp. Khim. 68 99 (1999) [Russ. Chem. Rev. 68 85 (1999)] 22. D Bohm Quantum Mechanics (New York: Prentise Hall Inc., 1952) 23. F I Dalidchik Zh. Eksp. Teor. Fiz. 87 1384 (1984) 24. F I Dalidchik, S A Kovalevskii Pis'ma Zh. Eksp. Teor. Fiz. 67 916 (1998) 25. R Wiesendanger, in Scanning Probe Microscopy. Analytical Methods (Berlin: Springer, 1998) 26. M A Kozhushner, B R Shub,R R Muryasov Pis'ma Zh. Eksp. Teor. Fiz. 67 484 (1998) 27. A L Buchachenko, M A Kozhushner, B R Shub Izv. Akad. Nauk, Ser. Khim. 1732 (1998) b 28. A L Buchachenko, F I Dalidchik, B R Shub Chem. Phys. Lett. 340 103 (2001) 29. B C Stipe, M A Resaei, W Ho Phys. Rev. Lett. 81 1263 (1998) 30. B C Stipe, M A Resaei, W Ho Science 279 1907 (1998) 31. J Gaudioso, H J Lee,W Ho J. Am. Chem. Soc. 121 8479 (1999) 32. M E Ryskin, Candidate Thesis in Physicomathematical Sciences, Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow, 1983 33. O V Krylov, B R Shub Neravnovesnye Protsessy v Katalize (Non-Equilibrium Processes in Catalysis) (Moscow: Khimiya, 1990) 34. M V Grishin, F I Dalidchik, S A Kovalevskii, N N Kolchenko Pis'ma Zh. Eksp. Teor. Fiz. 71 104 (2000) a�Herald Russ. Acad. Sci. (Engl. Transl.) b�Russ. Chem. Bull. (Engl. Tran

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年代:2001

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Russian Chemical Reviews 70 (7) 543 ± 576 (2001) Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis Yu N Ogibin, G I Nikishin Contents I. Introduction II. Reactions induced by direct anodic oxidation III. Reactions induced by indirect anodic oxidation IV. Conclusion Abstract. anodic of use the in trends and achievements main The The main achievements and trends in the use of anodic oxidation in alkenes of reactions electrochemical and oxidation and electrochemical reactions of alkenes in organic organic synthesis The generalised. and systematised analysed, are synthesis are analysed, systematised and generalised. The large large potential of methods electrochemical of advantages and potential and advantages of electrochemical methods of synthesis synthesis of methods These demonstrated.are compounds organic of organic compounds are demonstrated. These methods allow allow transformation the derivatives functional into alkenes of transformation of alkenes into functional derivatives the prepa- prepa- ration of which by conventional chemical methods is either ration of which by conventional chemical methods is either difficult the to paid is attention Primary impossible. or difficult or impossible. Primary attention is paid to the influence influence of on conditions electrolysis and substrates of structures the of the structures of substrates and electrolysis conditions on the the regio- and stereoselectivities of the reactions. The bibliography regio- and stereoselectivities of the reactions.The bibliography includes 305 includes 305 references. references. I. Introduction Anodic oxidation and electrochemical reactions of unsaturated organic compounds represent a vigorously developing line of research in organic electrochemistry. These reactions allow trans- formation of alkenes into functionally substituted compounds the synthesis of which by chemical methods is either impossible or difficult from the experimental standpoint. In many cases, electro- chemical reactions are more advantageous from the environ- mental standpoint than chemical reactions. This review describes systematically the published data on the reactions of alkenes taking place at the anode and in electrolyte solutions and on the use of these reactions in organic synthesis.More than 200 studies on electrooxidation of alkenes have now been published (most of them, during the last 15 years). There are several reviews 1± 18 and monographs 19 ± 23 containing data on the anodic processes involving unsaturated compounds. However, the information on electrooxidation of alkenes presented in these sources is not systematised in any way and does not reflect adequately the knowledge accumulated to date. The present review is meant to fill these gaps, to shed light upon the new trends in the development of alkene reactions induced by anodic oxidation, and to demonstrate the use of these reactions in organic synthesis taking electrooxidation of unsubstituted and function- ally substituted alkenes and cycloalkenes, substituted dienes, and some other types of alkenes as examples.The review covers publications of the last two decades and also some earlier studies Yu N Ogibin, G I Nikishin N D Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp. 47, 119991 Moscow, Russian Federation. Fax (7-095) 135 53 28. Tel. (7-095) 938 35 95 (Yu N Ogibin), (7-095) 135 53 05 (G I Nikishin) Received 9 January 2001 Uspekhi Khimii 70 (7) 619 ± 655 (2001); translated by Z P Bobkova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n07ABEH000650 543 543 558 573 that have not been reflected adequately in the above-mentioned reviews and monographs.An alkene can be directly involved in an electrochemical process upon transfer of an electron from its higher occupied molecular orbital at the anode (direct route). Alkenes can also participate in the electrochemical process via the reaction with electrogenerated organic or inorganic radicals (X.), ions (X+), or oxidants (Ox), which can be formed from the corresponding precursors at potentials lower than the potential of alkene oxidation (indirect route).24 ± 26 The role of precursors can be played by the solvent, organic or inorganic anions and mediators (Scheme 1). In accordance with the possibility of electrochemical trans- formations of alkenes into final products by both direct and indirect routes, the reactions considered in the review are classified into two groups.Scheme 1 + 7e R C1 C2 R C1 C2 products +e A X R C1 C2 + R=CH C C1 C2 C C1 C2 products 7e 7XH B X X R C1 C2 + R C1 C2 7e X X + + products R C1 C2 + R C1 C2 D C X+ products C+D R C1 C2 Ox products II. Reactions induced by direct anodic oxidation The efficiency of direct anodic oxidation depends, most of all, on the rate of electron transfer from the alkenes adsorbed on the anode and is determined by their oxidation potentials. The oxidation potentials of the alkenes considered in this Section are listed in Table 1.27 ± 39544 Table 1. Potentials of oxidation half-waves (E1/2) for alkenes of various types. Electrolyte Solvent Alkene E1/2 /V Alkenes and cycloalkenes 2.7 ± 2.8 2.2 ± 2.3 1.65 Et4NBF4 Et4NBF4 Bu4NClO4 Bu4NClO4 2.14 LiClO4 LiClO4 Alk-1-enes Alk-2-enes 2-Methylbut- 2-ene 2,3-Dimethyl- 1.29 but-2-ene Cyclohexene 1-Methylcyclo- 1.70 hexene a-Pinene Norbornene 1.3 ± 1.4 2.02 LiClO4 LiClO4 Arylalkenes and arylcycloalkenes Styrene NaClO4 NaClO4 1.45 1.95 ± 2.05 Et4NBF4 1.37 1.80 ± 1.89 Et4NBF4 1.50 LiClO4 NaClO4 1.38 Et4NClO4 1.15 Et4NClO4 Allylbenzene Et4NClO4 LiClO4 LiClO4 LiClO4 1.38 1.19 NaClO4 NaClO4 NaClO4 1.25 2-Phenyl- propene 1-Phenyl- propene 1,1-Diphenyl- 1.48 ± 1.55 Bu4NClO4 ethylene 1-Methoxyphe- 0.90 nylethylene 4-Methyl- styrene 4-Methoxy- styrene 4-Chlorostyrene 1.76 1.89 3-Methoxy-3- 1.95 phenylprop- 1-ene 1-Methoxy-3- 1.42 phenylprop- 1-ene cis-Stilbene trans-Stilbene 1-Phenylcyclo- 1.36 hexene Indene Acenaphthene 0.95 Acenaphthylene 1.32 NaClO4 NaClO4 Et4NClO4 Enol ethers and esters 0.95 ± 1.00 NaClO4 1.04 NaClO4 1.18 LiClO4 1-Ethoxy- cycloalkenes 1-Methoxy- hex-1-ene 1-Acetoxy-2- methylcyclo- pentene Note.Hereinafter, the oxidation potentials are given in the unified scale versus a silver halide electrode at 25 8C; the potentials determined versus saturated calomel and standard hydrogen reference electrodes were decreased by 0.30 and 0.54 V, respectively. The high oxidation potentials of alkenes with a terminal double bond hamper their direct anodic oxidation.The replace- ment of hydrogen atoms at double bonds by alkyl or other electron-donating groups decreases the oxidation potential and Electrode Ref. 27 ± 31 27 ± 31 32 MeCN MeCN MeCN Pt Pt Pt 32 MeCN Pt MeCN 28, 32 32 Pt Pt 30 30 MeCN MeCN Pt Pt 33 33 33 27 27 MeOH MeCN MeOH MeCN MeCN CPt CPt Pt 32, 35 MeCN Pt 34 MeOH C 35 MeCN Pt 35 MeCN Pt 35 36 36 MeCN MeCN MeCN Pt Pt Pt 36 MeCN Pt 27 27 32 MeOH MeOH MeOH CCC 28, 29 27 37 MeOH MeCN MeCN CPt Pt 38 MeOH 38 MeOH glass carbon glass carbon Pt 39 MeCN Yu N Ogibin, G I Nikishin thus facilitates this process. To reach the anodic potentials at which alkenes undergo effective direct oxidation, tetrafluoro- borates are used as supporting electrolytes and acetonitrile, 2,2,2-trifluoroethanol, trifluoroacetic acid or fluorosulfonic acid are used as solvents.Oxidation of activated alkenes, for example, styrenes and enol ethers (or esters), is normally carried out in methanol and ethanol, which are the best solvents for most of the electrochemical processes considered in this review. In undivided cells, these protic solvents suppress side cathodic reactions of alkenes due to easy discharge of protons. Owing to their versatile solvent capacity, they permit the use of high concentrations of the electrolyte and the organic substrate. The transformation route of radical cations A (see Scheme 1) depends on their reactivity with respect to other compounds present in the reaction mixture.In most cases, radical cations behave as proton donors or electrophiles and react with nucleo- philes or bases. They can also be oxidised or be involved in radical dimerisation, recombination, disproportionation and addition processes. The most typical reactions of the radical cations electrogenerated from alkenes are considered below (Sections II.1 ± II.8). 1. Allylic substitution in alkenes The allylic substitution in activated alkenes occurs via direct anodic oxidation, and allylic substitution in non-activated or weakly activated alkenes can occur via either direct or indirect anodic oxidation, or involve both types of reaction. For represent- ing the full picture of allylic substitution, the direct and indirect routes of anodic oxidation are considered together in Section II.In both cases, cationic intermediates B, C and D are generated at initial stages (see Scheme 1); they enter into solvolysis (Scheme 2, pathway a) and deprotonation reactions (Scheme 2, pathways b, c).30 Scheme 2 Pathway a Solv Solv + C C1 C2 C C1 C2 C C1 C2 + SolvH 7H+ B SolvH is protic solvent. X X R= CH + Pathway bX R C1 C2 C C1 C2 C C1 C2 + 7H+ C Pathway c X X + CH R C1 C2 R C1 C2 CH2 7H+ D Allylic substitution in alkenes has been studied in relation to diisobutene,40 ± 42 cyclohexene,30, 43 ± 48 1-, 3- and 4-methylcyclo- hexenes,30 a- and b-pinenes,30 allylbenzene,36, 49 4-allylanisole and some other allylarenes.50 The efficiency and the direction of the direct anodic oxidation in undivided cells at a controlled anode potential or in the direct-current mode depends substantially on the nature of the anode material, supporting electrolyte and the solvent.This effect is attributed to different susceptibilities of electrolytes, alkenes and solvents for chemo- and electrosorption on the anode surface. Allylic substitution in non-activated alkenes � diisobutylene and cyclohexene � is most efficient in methanol with tetraalkylammonium toluene-p-sulfonate as the supporting electrolyte and with graphite or lead dioxide supported on graphite as the anode material. Diisobutylene is converted into a complex mixture of products mainly consisting of isomeric methoxyoctenes 1 ± 6.41545 Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis MeO + + MeOH 7e 2 OMe 1 OMe It is worth noting that acetoxylation of 1-, 3- and 4-methyl- cyclohexenes, oct-1-ene and oct-2-ene, gives the same products both in the electrochemical 30 and standard versions.50 ± 52 This is evidence in favour of participation of carbenium ions in electro- chemical acetoxylation.However, the cis- to trans-isomer ratios obtained in the electrochemical and chemical acetoxylations are different; the electrochemical process yields mainly cis-isomers. + + + MeO4 3 OMe Electrochemical allylic acetoxylation and methoxylation of a-pinene, unlike those of methylcyclohexenes, give predominantly trans-isomers of substituted cyclohexenes 11a,b and 12a,b.30 + + OMe 6 5 + RO ROH, Et4NOTs 7e OR OR 12a,b 11a,b Yield (%) (trans : cis ratio) R 12 11 12 (2.9 : 1) 41 (2.6 : 1) 24 (2.2 : 1) 22 (6.2 : 1) Ac (a) Me (b) The electrolysis of cyclohexene in acetic acid gave 3-acetoxy- cyclohexene (7a) as the major product; that in methanol yielded 3-methoxycyclohexene (7b).In both cases, 1,2-disubstituted cyclohexanes 8a,b are also formed. The composition of reaction products depends on the solvent nucleophilicity: when the elec- trolysis is carried out in acetic acid, radical cationsA are converted predominantly into cations B and then into the product 7a, whereas in more nucleophilic methanol, they mainly react with the solvent to give intermediates C and then the products 8b and 9b.30 In wet acetonitrile, 3-acetamidocyclohexene (10) is Under similar conditions, allylic methoxylation of b-pinene gives formed.30, 43, 45 rise to a mixture of mono- and dimethoxy derivatives 13 ± 16.30 + OMe OMe 7e + + MeOH, Et4NOTs 7e A OR + OMe ROH 7H+ 14 (7.4%) 13 (2.5%) 7e, 7H+ B 7a,b OMe OMe OR ROH 7H+ OR + + + 8a,b ROH 7e OR OR OR C + ROH 7H+ OMe OR 9b 16 (23.4%) 15 (14.7%) NHCOMe MeCN±H2O 7e Electrooxidation of allylbenzene in methanol results initially in 3-methoxy-1-phenylprop-1-ene (17) and 3-methoxy-3-phenyl- prop-1-ene (18).36 10 (13 ± 17%) OMe Yield (%) R Ph MeOH, 60 8C 7e OMe + Ph Ph 18 17 7 8 9 OMe 17 OMe Ph 2.6 23 1.1 2.5 55 24 Ac (a) Me (b) MeOH 7e OMe 19 Under conditions of electrolysis, compound 17 is converted almost entirely into 1,2,3-trimethoxypropylbenzene (19) because of its lower oxidation potential compared to those of 18 or allylbenzene (1.42, 1.95 and 1.89 V, respectively, see Table 1).The degree of conversion and the yields of electrolysis products depend on the supporting electrolyte used (Table 2). To increase the selectivity of electrochemical acetamidation of hydrocarbons, in particular, cyclohexene, the use of an ion exchange resin with sulfonate groups has been proposed.48 In the presence of an ion exchange resin, the reactive nitrilium ions D formed in acetonitrile solutions are transformed into stable imidoyl sufonates E attached to the polymeric support.With this approach, the yield of the reaction products markedly increases and the isolation of products is substantially facilitated. + +NC Me N C Me SO3H MeCN 7e D + NHAc NH C OSO2 NaOH±H2O Me E 10 (64%) In the presence of acidic electrolytes (TsOH, H2SO4), 3-methoxy-3-phenylprop-1-ene (18) undergoes fast isomerisation into 3-methoxy-1-phenylprop-1-ene (17); as a consequence, the product of allylbenzene trimethoxylation, namely, the compound 19, is produced predominantly. The major products formed in the presence of basic electrolytes are the allylic methoxylation prod- ucts 17 and 18. When neutral salts are used as electrolytes (Bu4NBF, TsONa), the products of allylic methoxylation and trimethoxylation of allylbenzene are formed in comparable amounts.546 Table 2.Effect of the supporting electrolyte on the degree of conversion and the yield of products of allylbenzene electrolysis.36 Yield (%) Conversion (%) Electrolyte 19 18 17 MeOLi MeONa Bu4NBF4 59 54 36 32 77 58 24 26 63 62 82 49 43 25 55 44 TsONa TsOH H2SO4 845442 Note. Electrolysis under amperostatic conditions in an undivided cell (ACU), Pt anode, Cu cathode, 220 mA cm72, 6.8 F mol71, MeOH, 60 8C. Here and below, F is the quantity of electricity equal to one Faraday. In an initial stage of electrolysis in methanol with a graphite anode and with sodium methoxide as the supporting electrolyte, 4-allylanisole is methoxylated similarly to allylbenzene at the benzylic carbon atom; however, during the subsequent, more extensive electrooxidation, it is converted into 1,1-dimethoxyallyl- anisole.50 OMe MeOH 7e MeOH 7e (44%) MeO MeO OMe MeO(34%) MeO In the anodic oxidation of methyl oleate, allylic substitution also takes place.This yields predominantly unsaturated mono- acetates 20 and 21 (yield of up to 60%); further electrooxidation affords unsaturated diacetates 22 and 23 (upon subsequent allylic substitution). The highest yield of the diacetates (85%) was attained when the anode current density was 8 mA cm72 and the consumption of electricity was*4 F mol71 (see Ref.53). a Me(CH2)7CH CH(CH2)7CO2Me Me(CH2)7CHCH CH(CH2)6CO2Me+ 20 OAc +Me(CH2)6CH CHCH(CH2)7CO2Me 21 OAc OAc b 20+21 Me(CH2)7CHCHCH CH(CH2)5CO2Me+ 22 OAcOAc +Me(CH2)5CH CHCHCH(CH2)7CO2Me 23 OAc (a) ACU, Pt anode, 1.3 F mol71, AcOH, LiClO4; (b) ACU, >1.3 Fmol71. Apart from unsaturated mono- and diacetates 20 ± 23, small amounts of the corresponding saturated mono- and diacetates are also formed. Allylic substitution can follow an intramolecular pathway provided that the electrogenerated cation of type B (see Schemes 1 and 2) contains an internal nucleophile. An example of this reaction is anodic oxidation of citronellol in acetonitrile or propylene carbonate giving rise to 2-(2-methylprop-1-enyl)-4- methyltetrahydropyran (24), a valuable fragrance compound Yu N Ogibin, G I Nikishin known as rosenoxid.54, 55 The use of methanol instead of acetoni- trile changes the electrochemical behaviour of citronellol: the yield of the product 24 decreases to 7%, while 3,7-dimethyl-6-methox- yoct-7-en-1-ol (25) becomes the r product of electrolysis. O 24 (19% ± 26%) a HO MeO MeOH HO 24+ 25 (31.5%) (a) ACU, C anode, 40 mA cm72, 2 F mol71, MeCN or propylene carbonate, Et4NOTs.2. Transformation of enol acetates into enols and a-acetoxy ketones During the electrolysis of enol acetate in acetic acid, radical cations A, generated in the first stage, undergo electrooxidation, deprotonation, and solvolysis to give a-acetoxy-substituted allylic cations B and a,b-diacetoxy-substituted carbenium ions C.Elim- ination of acetyl cations from these intermediates results in enones and a-acetoxy ketones.39, 56 R R + C OAc CH C C OAc CH C AcOH 7e A R R + C C C O C C C OAc 7Ac+ 7e,7H+ B R R + AcOH CH C C O CH C C OAc 7Ac+ 7e,7H+ AcO AcO C The nature of the supporting electrolyte exerts a crucial influence on the composition of the electrolysis products. Indeed, when a solution of Et4NOTs in AcOH or in MeOH is used as the electrolyte, amperostatic electrolysis of enol acetates in a divided cell (ACD) with a carbon anode, as a rule, results predominantly (in some cases, solely) in enones;39 when the anodic oxidation is carried out in potassium acetate or triethylamine, a-acetoxy ketones are produced (Table 3). It is noteworthy that in an undivided cell, enol acetates are converted almost entirely into 1,6-diketones as a result of cathodic dimerisation of the enones formed.For example, enol acetates derived from cyclohexanones are transformed into bis(3,30-di- oxo)cyclohexyls.57 O R R Et4NOTs ±AcOH OAc R (56% ± 65%) O R=H, Me. In the presence of catalytic amounts (0.4 mol %± 1.0 mol %) of metal salts (iron and copper sulfates, cobalt acetate), the cathodic dimerisation is completely suppressed and cycloalk-2- enones become the major product of electrolysis, as in a divided cell (yield 80%± 86%).57 Electrochemical transformation of enol acetates into enones served as the basis for the development of a general method for the synthesis of 2-mono- and 2,3-disubstituted cyclopent-2- enones.56, 57Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis Table 3.Composition of products of anodic oxidation of enol acetates in AcOH.39, 56, 58 Substrate R2 R3 R1CH2C COAc R1 R2 OAc R2 OAc R1 Electrolyte R3 R2 R1 H Me Et4 NOTs n-C5H11 Me H Alk(C1±C5) Et H H Ph H,Me H i AcOK or Et3N AcOK or Et3N AcOK or Et3N H Me Pr AcOK or Et3N H H Et4 NOTs H Alk(C1±C9) Et3 NOTs Me H AcOK or Et3N H Et4NOTs (CH2)6CO2Me H Bn Et4 NOTs Et4NOTs CH2CO2Me n-C5H11 H CH2 CH=CH2 AcOK or Et3N H CH2 CH=CHMe AcOK or Et3N H H Et4 NOTs Products n-C6H13CH(OAc)COMe n-C5H11CH=CHCOMe AlkCH2CH(OAc)COMe AcO(Me)CHCOEt AcO(Me)C(R2)COPh AcO(Me)C(Me)COPri O AcO O Alk O Me O AcO Me O MeO2C(CH2)6 O Bn O n-C5H11 MeCO2CH2 O CHCH2 CH2 O AcO CHCH2 CH2 O MeCH CHCH2 O AcO CHCH2 MeCH O OOAc O 547 Yield (%) 246 54 ± 78 66 47 ± 73 203 37 66 ± 90 24 59 84 45 69 12 593 43 259548 Table 3 (continued). Substrate R1 R2 OAc R1 C(Me)OAc Note.Electrolysis under amperostatic conditions in a divided cell (ACD), carbon anode; a electrolysis was carried out in methanol. R2R1 AcOH or MeOH, Et4NOTs 7e OAc Methoxylation of enol acetates, which occurs similarly to acetoxylation, is used to accomplish 1,2-shift of the carbonyl group in ketones.An example of such transformation for sub- stituted cyclohexanones is given below.58 R1 R1 Ac2O TsOH OAc O R1 OMe OH R2 3. Reactions of enamines Electrooxidation of enamines affords radical cations A, which then add a nucleophile to give radical intermediates B. Electro- oxidation of these species followed by deprotonation of cationic intermediates C gives rise to enamines containing a nucleophilic substituent in the b-position. Hydrolysis of the enamines furnishes the corresponding a-substituted ketones.59, 60 This electrochemical process was implemented for cyclopen- tanone (n=1) and cyclohexanone (n=2) enamines. Methanol and the anions derived from acetylacetone, ethyl acetoacetate and Products Electrolyte R3 R2 O H H Et4 NOH OAc O Me O H Me Et4 NOTs a OMe Me Me Pri O AcOK or Et3N Pri OAc Me O Pri OAc Et4NOTs COMe COMe + R2 N N NuH Nu R1 7e 7H+ n n O (45% ± 90%) A N N + Nu Nu 7H+ n n R1 C n=1, 2.R2MgX MeOH, Et4NOTs 7e OMe O diethyl malonate were used as nucleophiles, and a methanolic solution of sodium methoxide was used as the electrolyte.59, 60 N-Methoxycarbonyl-substituted enamines 26a ± d are con- R1 H2SO4 verted into halomethoxylation products 27a ± d upon electrolysis in methanol in ammonium bromide or iodide as the supporting electrolyte. O R R R2 n n a XOMe N NCO2Me 26a ± d Compound 26 CO2Me 27a ± d R n The yield of compounds 27 (%) abcd 38 42 66 93 1212 Me Me Et Et Yu N Ogibin, G I Nikishin Yield (%) 391 55 60 253 55 N 7e n B O H2O Nu nElectrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis Compound 27d has been used to synthesise a bicyclic derivative of a-carboxybutyrolactone 28, which is a key intermediate in the synthesis of the alkaloid eburnamonine (29).61, 62 CO2H Et Et b, c CO2Me d 27d O CO2Me O N N 28 CO2Me CO2Me N H N O Et 29 (a) ACU, Pt anode, 75 mA cm71, 3.5 F mol71, MeOH, NH4X (X=Br, I); (b) CH2(CO2Me)2, Et3N, TiCl4; (c) MeOH, 20 equiv.KOH, D; (d ) DMF, D. 4.Dimethoxylation and diacetoxylation of alkenes The scheme of electrochemical dimethoxylation and diacetoxyla- tion of non-activated alkenes includes the formation of radical intermediate A, its electrooxidation and the reaction of the carbenium ion B with MeOH or AcOH. RO ROH 7e,7H+ + ROH RO C C OR C C C C OR 7e 7H+ B A C C OR RO R=Me, Ac. Electrochemical dimethoxylation of alkenes, for example, of conjugated arylalkenes and enol ethers (or esters) occurs with high selectivity and results in the formation of products in high yields. The most interesting examples are listed in Table 4. Electrochemical dimethoxylation of weakly activated double bonds in arylalkenes (styrene, 2-phenylpropene and so on) takes place as an irreversible two-electron process (one two-electron oxidation wave is observed).26, 75 Highly activated double bonds in vinylanisole, 4-propenylanisole, a,a0-dimethoxystilbene and so on are dimethoxylated in two steps (two single-electron waves are observed), the first being reversible and the second being irrever- sible.24, 25, 75, 76 + X X ROH C C C C 7e 7H+ C X OR ROH C C OR C C RO X 7e,7H+ X=Ar, Bn, AlkO; R=Me, Ac. The transfer of the first electron from the substrate molecule adsorbed on the anode, which occurs on the anode, is the rate- limiting step of this reaction.In the radical cations C thus formed, the highest spin density and the positive charge density are located on the b-carbon atom.75 The process selectivity is markedly affected by the arylalkene concentration both in the solution and on the anode surface, by the current density at the anode, the anode material and the nature of the electrolyte anion.26, 40, 75, 77 The rate of oxidation of trans-alkenes is about an order of magnitude higher than that for the cis-isomers.78 The yield of 549 dimethoxylation products increases with a decrease in the sub- strate concentration in the solution and on the anode surface and with an increase in the current density up to values close to limiting ones; the yield sharply increases when a platinum, lead dioxide, or magnetite anode is used instead of the graphite anode 75 and sodium toluene-p-sulfonate is used as the electrolyte.77 Unlike NaClO4, sodium toluene-p-sulfonate exhibits desorption-replac- ing properties with respect to arylalkenes and other organic substrates; as a consequence, the substrate concentration on the anode surface during the electrolysis with TsONa is 50 times lower than that in the presence of NaClO4 .The same effect, although less pronounced, can be traced for tetrabutylammonium tetrafluoroborate. The highest yields (substance yield 60% ± 80% and current yield 66%± 71%) of the products of dimethoxylation of arylalkenes are attained with a Pt anode, Bu4NOTs or Bu4NBF4 as the supporting electrolyte and when the substrate concentration is close to 1 mol litre71 (see Ref. 64). Under similar conditions of electrolysis in the presence of potassium hydroxide as the electrolyte, enol ethers are converted predominantly into acetals and ketals of a-methoxy-substituted aldehydes and ketones.For example, alkyl vinyl ethers give rise to a-methoxy- aldehyde acetals 30, 1-alkoxycycloalkenes are converted into a-methoxycycloalkanone ketals 31,39, 59, 72, 79 while bicyclic enol ethers yield dimethoxy-substituted oxabicycloalkanes 32.73, 74 OMe OR2 CHOR2 R1CH R1 MeOH 7e 30 OMe Yield of 30 (%) R2 R1 60 68 75 60 Me Et Bun Me HHHn-C6H13 OMe OMe MeOH 7e OR OR n n 31 (45% ± 60%) n=0, 1; R=Me, Et. OMe MeOH 7e n n O O OMe 32 (64% ± 68%) n=2, 8. The selectivity of dimethoxylation of arylalkenes and enol ethers is substantially affected by the competing electrooxidative dimerisation and oligomerisation reactions, which will be consid- ered in Section II.5.These side reactions are effectively suppressed by tris(4-bromophenyl)amine. In the presence of this compound, the yield of the reaction product in the dimethoxylation of propenylbenzene increases to 93%.80 Electrochemical diacetoxylation of alkenes at the double bond is usually accompanied by allyllic substitution; this diminishes the synthetic value of this process. This reaction carried out in acetic acid with potassium or sodium acetate as the supporting electrolyte occurs with moderate selectivity but only for alkenes the structure of which rules out the possibility of allylic substitution, for example, stilbenes, 1,1- diphenylethylene,81 indene and 1-alkylindenes.82 ± 85 In these cases, the yields of diacetoxylation products do not exceed 40%.An example is provided by 1-alkylindenes, which are converted into 1-alkyl-2,3-diacetoxyindans in 55% ±75% yields; small amounts of 2-acetoxy-1-alkyl-3-hydroxyindans (yields 6%± 20%) are formed simultaneously.84 A mechanism for the anodic acetoxylation of indenes involving the intermediate for-550 Table 4. Electrochemical dimethoxylation of arylalkenes and enol ethers under conditions of diaphragmless anodic oxidation in methanol. Substrate Ph C CH2 R1 R1 R2 Ph Me Ph Me R1 Me R2 R1 Ph Ph n n=1, 2 Ph n n=1±3 R1 O OMe n n=1, 2 n O n=1, 2OO Note. Method A: ACU, Pt anode, 100 ± 220 mA cm72, 1.9 ± 2.5 F mol71, 60 8C; method B: ACU, Pt anode, 8 ± 80 mA cm72, 2.0 ± 6.2 F mol71, 75 ±20 8C; method C: ACU, C anode, 100 mA cm72, 2.0 F mol71, 60 8C; method D: ACU, C anode, 8 mA cm72, 2.0 ± 3.5 F mol71,*20 8C.a The substance yield (or the current yield, in parentheses) is given relative to the converted substrate; b n = 1; c n = 2; d n = 3. mation of radical cation 33, radical 34 and carbenium ion 35 has been proposed.85 The side products, namely, acetoxy-3-hydroxy- derivatives 36, are produced from cation 37, which results from an intramolecular attack of the OAc group on the electron-deficient centre of the carbenium ion 35. R1H Bu4 NBF4 A KF KF Et4NClO4 KF HMe Ph Ph HMe Me Me Me Cl MeO MeO MeO OCH2O Me Et Bu Electrolyte R2 KF KF Me Et Bu4NBF4 KF MeONa Bu4NBF4 MeONa HHMeO Bu4NBF4 KF NaClO4 , NaOH KF KF MeONa Bu4NBF4 Bu4NBF4 Bu4NBF4 Bu4NBF4 Bu4NBF4 Bu4NBF4 Bu4NBF4 Bu4NBF4 MeONa KOH KOH KOH MeONa KOH KOH KOH Bu4NBF4 Yu N Ogibin, G I Nikishin Ref.Product Method Yield (%) a 63, 64 R1 OMeOMe Ph 65 86 63 ± 80 A 80 64 A 70 65 B 69 33 A R1 OMeOMe Ph 65 65 76 73 AA R2 OMe OMe 63 75 (63) A Ph Me Me R1 OMe OMe 65 33 63 33 88 36 65 (52) 31 ABAB OMe R2 Me OMe 65 76 70 (70) 58 A 76 65 A 65 B 66 A R1 Ph(OMe)CHCH(OMe)Ph 41 64 67 68 AB OMe 63 A b OMe c 87 (72) b C 88 69 C 90 69 n bc n Ph MeO MeO d C 81 69 A 71 (71) c 63 C 76 69 C 90 69 OMe MeO A 80 70 B, D 71 7 OMe R1 OMe O 68 60 75 72 72 72 BBB OMe OMe OMe 59 72 45 b 60 c DB n OMe n 73, 74 73, 74 68 b 80 c BB O OMe MeO O 73, 74 70 C O MeO + AcO7 7e R R 33Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis OAc 7e 34 R OAcOAc R Me O + AcO7 O 35 37 RH Me O+ OAc O ROH O + OAc Me R R=H, Me, Et, Prn, Pri, Bun, But.5. Dimerisation of alkenes The electrochemical transformation of alkenes into dimerised products of type 38 ± 40 takes place via several consecutive steps. Radical cations A generated in the first step undergo recombina- tion and add to the initial alkene molecules.The radical-cation and dication intermediates B and C undergo solvolysis, electro- oxidation and deprotonation giving rise to unsubstituted and substituted (saturated and unsaturated) dimeric compounds. H H + C+ C + C C C C C C A H H C C C C 7e,72H+ 38H H 2 SolvH Solv C C C C 7e,72H+ 39 H SolvH C C C C Solv 72H+ 40 H H +C C C C C+ C + A H C SolvH is a protic solvent. The optimal conditions for these reactions are as follows: the initial concentration of alkene >1 mol litre71, methanol as the solvent, NaClO4 as the supporting electrolyte, and a graphite anode.33, 35, 38, 72 Under these conditions, the aliphatic and alicy- clic vinyl ethers 41, 42 are converted into ketals of 1,4-dicarbonyl + AcO7 OAc 35 R Me O OAc H+ O R OH OAc 7Ac+ 36 R B Solv 38 72H+ + 2 SolvH 39 72H+ SolvH 40 72H+ 551 compounds, which are hydrolysed to give the corresponding diketones; 5,6-dihydro-4-pyran (43) is converted into dimethoxy derivative 44.38, 72 R1O R3 R2 OMe H3O+ R1O C CH R3 MeOH 7e OR1 41 R2 MeO R2 R3 O R3 R2 R2 O R3 (42% ± 72%) R1=Me, Et; R2=H, Et, Ph; R3=H, Me, Bun. H3O+ MeOH 7e n n OR n RO OMe RO OMe 42 n n O O (48% ± 61%) R=Me, Et; n=2, 4. MeO O MeOH 7e O OMe O43 44 (28%) Dimerisation of vinyl ethers 41 ± 43 is accompanied by side addition of methanol to the double bond, which is catalysed by the protons resulting from anodic oxidation of the solvent and solvolysis of electrogenerated radical cations.To suppress this reaction, electrolysis of vinyl ethers is usually carried out in the presence of bases, for example, pyridine or 2,6-lutidine.38 Unlike vinyl ethers, arylalkenes are unable to add methanol under conditions of electrooxidative dimerisation; therefore, the reaction can be carried out without a base. The composition, the structure and the yield of dimerisation products of arylalkenes depend on the type of substitution at the double bond, the nature of the supporting electrolyte and the anode potential.33, 34 Thus electrolysis of arylalkenes 45a,c in the presence of sodium per- chlorate yields 1,4-dimethoxy-1,4-diarylbutanes 46a,c as the only or major products, while in the presence of sodium camphorsul- fonate, 1,4-diaryl-1-methoxybut-1-ene 47a and 1,4-diarylbuta- 1,3-diene 48a are formed (Table 5).Electrolysis of 2-phenylpro- pene (45b) with the NaClO4 electrolyte at an anode potential of 1.6 ± 1.7 V yields dimer 48b; at lower potentials (1.1 ± 1.2 V), a mixture of dimers 46b, 47b and 48b in 4.2 : 2.5 : 1 ratio is formed. In the case of 4-methoxystyrene (45c), an increase in the potential from 0.8 to 1.2 ± 1.3 V increases the yield of the dimer 46c from R2 R1 MeO R1 Ar R2 + MeOH 7e Ar Ar 45a ± d R1 OMe R2 46a ± d R2 R1 R2 MeO R1 Ar Ar + Ar + Ar R1 R1 R2 48a ± d R2 47a ± d Ar=Ph, 4-MeOC6H4; R1=H, Me; R2=H; R17R2=(CH2)3 .552 Table 5.Electrooxidative dimerisation of arylalkenes 45 in methanol.33, 34 Electrolyte Ar R2 Compound 45 R1 HHMe Me HH HHHHHH Ph Ph Ph Ph 4-MeOC6H4 4-MeOC6H4 Ph NaClO4 ±MeONa (10 : 1) Sodium camphorsulfonate NaClO4 ±MeONa (10 : 1) NaClO4 ±MeONa (10 : 1) NaClO4 , MeOH, CH2Cl2 NaClO4 , MeOH, CH2Cl2 NaClO4 , MeOH, CH2Cl2 abcd (CH2)4 Note. Electrolysis was carried out under potentiostatic conditions in an undivided cell (PCU) with a graphite anode; the current yield was determined relative to the isolated products; the initial concentration of the substrate was 5.8 ± 6.0 mol litre71. 66% to 79%. Alkenes with a greater extent of substitution at the double bonds, for example, 1-phenylcyclohexene 45d give the corresponding dimer 46d in a yield of only 3.4%.Apart from dimer 49, the electrooxidative dimerisation of indene gives dimethoxylation product 50.33, 34 MeOH 7e OMe OMe H + H OMe 50 (45%) MeO 49 (26%) 6. Intramolecular electrooxidative cyclisation Intramolecular electrooxidative cyclisation is possible for alkenes the molecules of which contain, in addition to the C=C bond, a group capable of electrooxidative transformation into a radical- cation or a radical centre and the chain separating them consists of three to five atoms. The radical cations and radical intermediates electrogenerated from alkenes of this type, for example, from unsaturated carboxylic acids, unsaturated ethers and carbamates, activated a-stannyl or a-silyl groups, aryl(hetaryl)alkenes and non-conjugated a-alkoxy- and a-acetoxydienes, are converted into cyclic products upon intramolecular addition at the double bond followed by solvolysis or other secondary reactions.These processes are considered in more detail below, in Sections II.6.a ± e. a. Cyclisation of unsaturated carboxylic acids In the cyclisation of unsaturated carboxylic acids, the reaction products result from intramolecular cyclisation of alkenyl radicals 51 generated by the anodic oxidation of the anions of unsaturated acids under conditions of the Kolbe reaction. R1 R1CO¡27e,7CO2 R2 R2 R2 R1 R1 7e,7CO2 CO¡2R3 51 R3 R3 The recent comprehensive reviews devoted to this known reaction 1±4, 86 describe, along with other aspects, the results of application of anodic oxidation to the generation of radicals from carboxylic acids and investigation of the electrochemical cyclisa- tion of unsaturated acids.Further progress along this line 87 ± 90 resulted in the elaboration of new methods for the synthesis of natural and related cyclic products, for example, pyrrolidines Yu N Ogibin, G I Nikishin Reaction product (yield) (%) Potential /V 46a (64) 47a (28)+48a (36) 48b (67) 46b (42)+47b (17)+48b (10) 46c (79) 46c (66) 46d (3.4) 1.1 1.6 ± 1.7 1.6 ± 1.7 1.1 ± 1.2 1.2 ± 1.3 0.8 0.9 52,88, 89 precursors of prostaglandins 53 and 54 90 and angular triquinanes 55.91 CO2H R2 N N R2CO2H 72 e,72H+, 72CO2 O R1 R1 O 52 (46% ± 58%) R1=H, Me; R2=Me, n-C5H11, (CH2)4CO2Me.AcO AcO AcO R R RCO2H + 72 e,72H+, 72CO2 O O CO2H 54 53 OEt OEt EtO R Yield (%) 53 54 Me 50 18 n-C8H17 40 14 (CH2)2CO2Me 42 14 CH2SiMe2CH2 30 8 Me3SiCH2 29 10 n-C5H11COCH2CH(SiMe3) 4±5 7 a R RCO¡27CO2 O O n n a 7H+,7CO2 m m 57 56 CO2H O O H H R n n R m m 58 55 (26% ± 42%, endo : exo=2.7 : 1) R=Me, n-C5H11 , (CH2)2CO2Me; n=1±3, m=1, 2; (a) ACU, Pt anode, steel cathode, 25 mAcm72, 1.2 Fmol71, MeOH, 4 equiv. of RCO2H neutralised by KOH to 5%, 40 ± 45 8C. The formation of the compounds 55 from acids 56 takes place via regio- and stereoselective radical tandem cyclisation initiated by anodic oxidation and giving rise to three newC7Cbonds.TwoElectrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis bonds are formed at the stage of cyclisation of radical 57, and the third one, upon recombination of tricyclic radical 58 with the R. radical generated from the saturated acid, which is used as the second reactant. This distinguishes electrochemical cyclisation from tandem cyclisations initiated by chemical methods: in this case, C7H or C7I bonds are formed at the final stage.92 ± 94 An advantage of the electrochemical tandem cyclisation is that it does not use organotin reagents, normally employed in the chemical version, and the combination of the initial unsaturated and saturated acids makes it possible to change the side chain of the product 55.b. Cyclisation of unsaturated ethers and carbamates activated by a-stannyl and a-silyl groups Silyl, stannyl or thioaryl groups in the a-position markedly decrease the oxidative potentials of ethers or carbamates 59; this facilitates the electrochemical transformation in oxonium (X=O) or N-acyliminium (X=NCO2Me) ions.95 ± 101 Intramo- lecular cyclisation of substrates 59 yields products containing an endocyclic fluorine atom, indicating a cationic mechanism of the reaction (in the case of a radical mechanism, an exocyclic product would be formed at n=1). The supporting electrolyte, namely, tetrabutylammonium tetrafluoroborate or hexafluorophosphate, serves as the source of the fluorine anion at the final stage of the process.98 MR2 MR2 3 3 a X +X +X 7MR3 R1 R1 R1 n n n 59 X X F7 + F R1 R1 n n (50% ± 98%, cis : trans=1.2 : 3) M=Sn, Si; R1=H, Me; R2=Alk; X=O, NCO2Me; n=1, 2; (a) ACU, graphite anode, CH2Cl2, Bu4NBF4 .Trialkylstannyl groups exert a more pronounced activating effect than trialkylsilyl groups; as a consequence, cyclic products are formed in higher yields from stannyl derivatives. This method has been used to convert a-stannyl-substituted arylaliphatic ethers 60a ± c into isochroman (61a), 6-fluoroiso- chroman (61b) and N-methoxycarbonyl-1,2,3,4-tetrahydroiso- quinoline (61c). The transformation of trans-1-(tributyl- stannylmethoxy)-2-(hex-1-enyl)cyclohexane (62) gave rise to trans-3-butyl-4-bromo-1-oxadecalin (63).99, 101 SnBu3 X X CH2Cl2, Bu4NClO4 7e R R 61a ± c 60a ± c X=O:R = H (a, 55%), F (b, 80%); X=NCO2Me, R = H (c, 54%).H H O O SnBu3 CH2Br2, Bu4NClO4 7e Bu Bu 62 H H Br 63 (72%) c. Cyclisation of aryl and hetarylalkenes Intramolecular cyclisation of aryl- and hetarylalkenes is a result of transformations of radical cations 64 electrogenerated from these substrates. This cyclisation is one of the most useful reactions induced by direct anodic oxidation.102 ± 118 R1 n 7e R2 XR4 R3 (+) R1 n R2 XR4 R3 +( ) X=O, S. The structure of the bi- and polycyclic compounds thus obtained depends on the nature and the stereochemistry of substituents at the double bond. For example, cyclisation of substrates 65 ± 67 having related structures but differing in the nature and the positions of substituents at the double bond affords absolutely different tricyclic compounds 68 ± 70, precursors of natural products.102 ± 111 OH MeO Me MeO 65 H Me OH MeO Me MeO 66 Ar Me OH MeO MeO 67 Me MeMe O O Me MeO70 (80%) OAc Mild experimental conditions and high product yields are important advantages of this method for the preparation of this type of polycyclic polyfunctionalised compound.4-Hydroxy-20-alkenylbiphenyls 71a ± e have been used as examples to study the influence of the structure of the initial substrates, electrolysis conditions and some other factors on the cyclisation. Presumably,112, 113 the initial step represents gener- ation of cationic intermediates A and B induced by anodic oxidation; this is followed by their cyclisation and the reaction of cations C with methanol giving rise to compounds 72a ± e.112, 113 OH R1 a 7e R2 R5 R3 71a ± e R4 553 R1 n + R2 XR4 R3 64R1 n MeOH 7e R2 XR4 R3MeO O O Ar 72e,7H+ Ar HMe MeO Me68 (69%) Me O 72e, 7H+ H MeO Me HAr O 69 (66%) 72e, 7H+ CH2OAc O+ R1 R2 R3 R5 R4 A554 O+ R2 R1 R3 R5 R4 B R1 R2OMe R3 O R4 72a ± e R5 (a) ACD, platinum wire or glass carbon anode, 1.6 ± 2.0 F mol71, MeOH or MeOH±AcOH (CH2Cl2 , MeCN), LiClO4 , 0±20 8C.R3 R2 R1 Com- pound 71 HHHMe HMe Ph H (CH2)3HHMe Me HMe Me HHHHHHH abcdefghijkl HHHHHMe HHHHHH The cyclisation of 4-(20-alkenylphenyl)phenols 71b,c,e, con- taining a-substituents in the alkenyl group, occurs with high yields (65% ± 85%). In the case of the compounds 71 in which R2=H the yields are much lower.The compound 71m, containing a phenylthio group at the double bond, is converted into diketone 73. OH MeOH±AcOH, LiClO4 7e SPh 71m The yields of reaction products also depend on the solvent, current density, anode material and addition of mild acids, for example, acetic acid. The highest yields are attained when the reaction is carried out in a mixture of acetonitrile with methanol (4 : 1) containing 5 ± 10 equiv.of AcOH, using lithium perchlorate as the supporting electrolyte and a current density of 0.84 mA cm72 on a platinum electrode.113 The increase in the yields of the compounds 72 following the replacement of hydrogen at the a-carbon atom in the olefinic group by a methyl group is presumably 115 due to the fact that in this case, the cationic intermediate exists preferably in conforma- tion A, in which the double bond is close to the reaction centre. For the substrates 71 in which R2 = H, conformation B is R1 R2 + MeOH R3 O R4 C R5 R5 R4 The yield of Ref. compounds 72 (%) HHHHHH HHHHHHHHOMe H 112, 113 112, 113 112, 113 112, 113 112, 113 114 115 115 115 115 115 115 16 85 65 35 69 70 OMe 47 OMe 84 80 OMe OMe 70 OMe OMe 92 70 OCH2O O O 73 (72%) preferred.A similar effect is exerted by the methyl group in the ortho-position to the alkenyl substituent. For example, the trans- formation of 4-hydroxy-20-vinyl-30-methylbiphenyl (71f) gives the cyclisation product�spirodienone 72f�in 70% yield.114 Cyclisation of 4-hydroxyarylalkenes with R2 = H containing methoxy groups in the para-position or both in the para- and meta-positions relative to the vinyl group (the compounds 71g,j) is more efficient. Similar meta-methoxy-derivatives do not cyclise. Cyclisation of para- and meta-methoxy-substituted 71h,i,k,e in which R2 = Me gives products in high yields.115 The trimethylsilyl ethers of 4-hydroxyarylalkenes, for exam- ple, compound 74, unlike the corresponding phenols, are con- verted into cyclic products in high yields even in the absence of AcOH.114 Me3SiO 74 Silyl ethers can cyclise in the presence of bases.Thus, depend- ing on the nature of the functional groups present in the substrate, this process can be carried out both in a neutral medium and in the presence of acids or bases. Within the framework of investigation of cyclisation, anodic oxidation of aryl- and hetarylalkenes with electron-enriched double bonds has been vigorously developing in recent years.116 ± 118 The interest in these reactions is determined by the possibility of performing the cyclisation of arylalkenes with a regioselectivity (pathway b) differing from that observed in the case of the Friedel ± Crafts cyclisation (pathway a).The cyclisa- tion process during the anodic oxidation is based on the inversion of polarity of enol ethers. This reaction makes it possible to prepare cyclic products of various structures from the same initial substrate, depending on the method used to initiate cyclisation. R3 XR4 R2 n R1 R3 R2 Pathway a H+ R1 R3 R2 Pathway b 7e R1 R2 R1 X=O, S. Thus by anodic oxidation of enol ethers and enol thioethers 75a ± c, cyclisation products, namely, substituted tetralins 76a ± c, 77, and 78 have been prepared in high yields. Under the same conditions, the enol ether 75d is converted into spiro derivative 79 (Scheme 3).116, 118 It should be noted that cyclisation of the enol ether 75a to give the tetralins 76a and 77 is accompanied by the formation of 1,6- and 1,8-dimethoxytetralins, resulting from further electrooxida- tion.The tetralin 76b, formed from the enol ether 75b, is converted into 1,6,8-trimethoxytetralin 78.118 Yu N Ogibin, G I Nikishin Me OMe Me MeOH, LiClO4 7e O (73%) XR4 n + R3 XR4 XR4 + R2 MeOH n n R1 + MeO R3 XR4 nElectrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis Scheme 3 XMe R3 R2 a R1 75a ± d OMe CH(OMe)2 CH(OMe)2+ MeO 77 (20%) 76a (33%) OMe OMe OMe CH(OMe)2+MeO MeO 78 (34%) 76b (31%) SMe MeO OMe MeO 76c (72%) O MeO OMe 79 (51%) 75: R1=OMe, R2=R3=H,X=O(a); R1=R3=OMe, R2=H, X = O (b), S (c); R1=R3=H, R2=OMe, X=O (d); (a) ACD, glass carbon anode, 1.6 ± 2.0 Fmol71, MeOH±CH2Cl2 (1 : 4), LiClO4 , 5 equiv.of 2,6-lutidine (2,6-Lut),*20 8C. These side reactions do not take place if electrolysis is carried out with a controlled potential (1.1 V). Unlike cyclisation of the enol ethers 75a,b, cyclisation of the enol thioether 75c under similar conditions occurs without further oxidation of the reaction product 76c. OMe CH(OMe)2 + 76b 7e 7CH(OMe)á2MeOOMe 1)7e 2) MeOH, 7H+ 78 MeO The cyclisation of furyl- (80) and pyrrolyl-substituted alkenes 81 containing electron-enriched double bonds induced by anodic oxidation has opened a new way for assembling bicyclic structures incorporating five-membered heterocycles.116, 117 A compound of type 82 can be used in the synthesis of furan-containing bi- and polycyclic natural products.119 ± 126 R2 R1 O MeOH, LiClO4 , 2,6-Lut 7e n 80 555 OMe R2 OMe R2 R1 R1 TsOH O O MeO n n 82 R1=SMe, OMe, Ph, Me, CH2SiMe3; R2=H, Me, n=1, 2.O SMe SMe OMeO But But N N MeOH, LiClO4 7e 81 (66%) d. Cyclisation of non-conjugated 1-alkoxy-dienes Cyclisation of non-conjugated 1-alkoxy-dienes has not yet been adequately studied. This electrochemical process, which has much in common with cyclisation of aryl- and hetarylalkenes, has found application for the synthesis of carbocyclic compounds. The value of this method is dictated by the possibility of reverting the polarity of the b-carbon atom in the initial 1-alkoxy-dienes 83a ±m from nucleophilic to electrophilic due to the fact that an initial stage of the process is transformation of the substrates into radical cations 84a ±m.127 ± 134 The electrolysis conditions and the yields of the reaction products 85a ±m are listed in Table 6.+ R2 R2 R3 R3 R1 R1 7e R1 R1 n n 83a ±m OMe OMe R2 R3 OMe R1 R1 OMe n OMe 85a ± g R2 R3 + Ac R1 MeOH R1 R1 7e,7H+ R1 n + n CH(OMe)2 85h 84a ±m OMe R3 R1 R1 n CH(OMe)2 85j ±m + OMe SiMe3 a MeOH R1 R1 MeO R1 7e,7H+, 7Me3Si+ R1 85i OMe (a) R1=Me, R2=SiMe3, R3=H. The 1-methoxy-dienes 83 (n = 1 ± 3), which contain substitu- ents R2 and R3 activating the double bond, ensure the most selective formation of cyclic compounds (see Table 6).127 ± 134 Cyclisation of 1-methoxy-dienes 86a,b does not proceed under similar conditions: these compounds are converted into unsatu- rated dialdehyde diacetals 87a,b.556 Table 6.Anodic oxidation of 1-methoxy dienes 83a ±m. R2 n R1 Com pound 83 1 H 2 H 3 H 1 H 2 H 1 H 1 Me OMe OMe OMe OMe Ph Ph Me 1 Me OMe 1 Me SiMe3 1 Me CH2 SiMe3 1 Me CH2 SiMe3 2 H CH2 SiMe3 abcdefghijkmNote. Conditions of electrolysis: ACU, Pt or wire glass carbon anode, 15 mA cm72, 2 F mol71, methanol ±THF (1 : 1) or MeOH±CH2Cl2 (1 : 4), LiClO4 , 2,6-Lut. a A mixture of trans- and cis-isomers.OMe MeOH 7e OMe n 86a,b n = 4 (a), 12 (b). The principle of electrochemical cyclisation of linear 1-methoxy-dienes has been extended successfully to compounds 88, containing cyclopentane (m=1) and cyclohexane (m=2) fragments.130, 132, 134 MeO R1 R3 n R2 m 88 R1, R2=H, Me; R3=OMe, SiMe3; m, n=1, 2. e. Cyclisation of non-conjugated 1-acetoxy-dienes Cyclisation of non-conjugated 1-acetoxy-dienes 89 has many common features with the transformation of enol acetates into enones and a-acetoxy ketones (see Section II.2). A fundamental distinction between these electrochemical processes is that radical cations A electrogenerated from the 1-acetoxy-dienes 89 can react with a second C=C bond acting as an internal nucleophile.The intramolecular electrophilic attack on the double bond by the cationic centre of the radical cation gives rise to cyclic radical cation B. The subsequent deprotonation and electrooxidation of intermediate B are accompanied by elimination of the acetyl cation, resulting in the formation of cyclisation product 90 and competitive formation of conjugated enones 91 or 92.135 Ref. R3 Reaction Yield (%) product 68 70 65 50 HHHHHHHH 85a 85b 85c 85d 85e a 85f Me 85g Me 85h 85i 85j a Me 85k 85m 128 128 128 128 65 ± 73 127, 128 128 128 132 129 132 128 128 65 60 62 29 84 89 59 H OMe OMe n OMe OMe 87a,b m n CH(OMe)2 CH(OMe)2 R1 (44% ± 70%) MeOH 7e mCH(OMe)2 (62% ± 75%)R3 Yu N Ogibin, G I Nikishin OAc OAc R1 R1 a + R2 R2 A 89 OAc O + R17R2=(CH2)n , n=3±6 (CH2)n R1 R2 B 90 (22% ± 50%) O (CH2)n R17R2=(CH2)n n=3, 4 91 n=3 (77%), 4 (14%) O R1 R1, R2=H,Me R2 92 (47%) (a) ACU, graphite electrodes, 4 ± 10 Fmol71, AcOH, Et4NOTs (1 mol litre71),*20 8C.Cyclisation of radical cations A by a radical mechanism does not occur under these conditions. 7. Transformations of alkenes with skeletal rearrangement The transformations of alkenes with skeletal reconstruction take place as a result of rearrangement of carbenium ions, which are intermediates in the anodic oxidation of alkenes. For example, cyclohexene is converted into cyclopentanecarbaldehyde dimethyl acetal (93),136 while translbene yields diphenylacetaldehyde dimethyl acetal (94).67 + + MeOH 7H+ MeOH 7e,7H+ OMe OMe OMe OMe 93 OMe Ph + Ph MeOH 7e,7H+ Ph Ph OMe OMe MeOH + Ph Ph 7H+ OMe 94 Ph Ph Since carbenium ions react competitively with MeOH, the dimethyl acetals 93 and 94 are formed as mixtures with 1,2- dimethoxycyclohexane and 1,2-dimethoxy-1,2-diphenylethane.When anodic oxidation of cyclohexene, trans-stilbene and related alkenes is carried out in trimethyl orthoformate in the presence of iodine, this side process is suppressed and acetals are produced as the only electrolysis products.136 The promotory effect of iodine is related presumably 136 to the formation of iodonium salts, which efficiently assist the rearrangement.Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis I+ I OMe HC(OMe)3 a (CH2)n (CH2)n (CH2)n 7I2 OMe OMe (51% ± 70%) n=4, 5.R2 a Ar + HC(OMe)3 I I ArC(R1) CHR2 7I2 MeO R1 R2 MeO Ar MeO R1 (35% ± 97%) Ar=Ph, 4-MeC6H4, 4-MeO2CC6H4, 4-ClC6H4; R1=H, Me; R2=H, Me, Ph, CH2OMe; (a) ACU, graphite anode, 0.1 mAcm72, 3 ± 17 Fmol71, HC(OMe)3, LiClO4 .3H2O, 0.25 ± 1.2 equiv. I2 , 20 ± 100 8C. 8. Cleavage of the double bond in alkenes Electrochemical cleavage of the double bond in alkenes takes place in an aqueous or alcoholic solution. This is a two-stage process consisting of vicinal dioxygenation of the alkene double bond at the initial stage and cleavage of the C7C s-bond in the resulting species at the subsequent stage.products C C RO C C OR ROH 7e ROH 7e ROH is water or alcohol. For conjugated arylalkenes, aryl(areno)cycloalkenes and enol acetates, the two stages can proceed successively as a one-pot reaction because the same electrolytes are effective for both processes. The attempts to cleave the double bond in enol ethers in the same way failed (see Section II.8.b). a. Direct cleavage of the C=C bond in arylalkenes, aryl(areno)cycloalkenes and enol acetates Stepwise investigation of styrene electrooxidation in methanol in an undivided cell showed that the initial and the subsequent stages of this process are affected in different ways by the nature of the electrolyte, anode material and temperature.The highest selectiv- ity (76% ± 80%) of the first stage (degree of substrate conversion 90%± 93%) is attained in potassium fluoride or tetrabutylammo- nium tetrafluoroborate as the supporting electrolyte with a platinum anode, while for the second stage a selectivity of 70%± 83% (degree of conversion 85%± 98%) is observed in the presence of sodium toluene-p-sulfonate and tetrabutylammonium tetrafluoroborate with a graphite anode.64, 65 Operation of the overall process under conditions favourable for the second stage permits preparation of the target products in higher yields. Under these conditions, conjugated arylalkenes were converted into benzaldehyde acetals and arylketone ketals (the consumption of electricity is 4 ± 8 F per mole of alkene).64, 137 ± 139 R1 R1 OMe R2 MeOH 7e OMe Ar Ar (54% ± 84%) Ar=Ph, 4-MeC6H4, 4-MeOC6H4; R1=H, Me, Ph; R2=H, Me, Et.1-Phenylcycloalkenes give acetal and ketal derivatives of o- benzoylalkanals.64, 140 557 OMe Ph n Ph MeOH 7e MeO OMe n OMe (58% ± 76%) n=1±3. Indene (95a) and 1,2-dihydonaphthalene (95b) form the bis(acetals) 96a,b.64, 140 MeO OMe OMe MeOH 7e n OMe n 95a,b 96a,b (61% ± 75%) n = 1 (a), 2 (b). Acyclic (97) and cyclic (98a,b) acetals of naphthalene-1,8- dicarboxaldehyde were prepared from acenaphthene.70 The 97 to 98a,b ratio depends substantially on the electrolyte used: in the presence of potassium fluoride, these products are formed in 30% and 34% yields, respectively; in the presence of tetrabutylammo- nium tetrafluoroborate, the ratio is*1 : 25 (overall yield 57%).O OMe MeO OMe MeO R OMe MeO + MeOH 7e 97 98a,b R = H (a), OMe (b). The process selectivity sharply decreases when the initial alkene concentration is higher than 1.0 mol litre71 because of the increase in the relative amount of the alkene involved in the side dimerisation, oligomerisation and polymerisation reactions (see Section II.5). The decrease in the alkene concentration in the reaction mixture by using its gradual addition to the electrolyte during electrolysis makes it possible to increase the product yield to 15%± 20%.65 The predominant formation of the cyclic acetals 98 from acenaphthene is, apparently, caused by the fact that, during the electrolysis in the presence of Bu4NBF4, the space near the anode surface gets acidified by the electrogenerated HBF4, which catal- yses the transformation of the acetals 97 into 98a,b.The rupture of the C7C bond in glycols formed upon the electrooxidation of the double bond in conjugated arylalkenes in aqueous and water-organic media proceeds at a substantially higher rate than that in their alkyl ethers. In this case, the process cannot be stopped at the stage of formation of glycols due to the fast rupture of the C7C bond in the HO7C7C7OH fragment. Direct anodic oxidation of propenylbenzene, anethole and isosa- frole has been studied in most detail.138 ± 142 Anisaldehyde was prepared from anethole in high substance yield (81% ± 85%) but a low current yield (*20%).141, 142 The cleavage of double bond in isosafrole has not resulted in the synthesis of 3,4-methylenedioxy- benzaldehyde (piperonal) in a yield higher than 34%.Electrochemical rupture of the C=C bond in enol acetates proceeds via consecutive transformation into radical-cation and radical intermediates A and B, a-methoxy ketones, their semi- ketals, and radical cationsC and gives rise to esters of oxoalkanoic acids.143 + R1 R1 a n n OAc OAc MeOH 7H+ R2 R2 A MeOH n n R1OMe O R1OMe OAc 7Ac+ R2 R2 B558 n n + R1OMe OH OMe R2 R2 O O R2 OMe R1 n (82% ± 97%) n=1, 2; R1, R2=H, Me, Pri; (a) ACU, Pt anode, 4 F mol71, MeOH±AcOH (10 : 1), LiClO4, 2±8 8C.Starting from (+)-menthone enol acetate, methyl (+)-3,7- dimethyl-6-oxooctanoate was prepared by this method in 74% yield.143 b. Direct cleavage of the double bond in cyclic enol ethers It has been noted above that cleavage of the double bond in enol ethers cannot be carried out as a one-pot reaction with the same selectivity as for conjugated arylalkenes, aryl- and benzocycloal- kenes. This is due to the fact that the electrochemical vicinal dialkoxylation of enol ethers proceeds with a satisfactory selec- tivity only with basic electrolytes, which are inefficient at the stage of electrooxidative cleavage of dialkoxylation products. There- fore, the process is carried out as two separate electrochemical stages with isolation of the products of dialkoxylation of enol ethers 99 ± 101 after the first stage.59, 144 ± 146 OMe OMe a OMe OMe n n 99 (45% ± 66%) OMe + OMe 2 MeOH, H2O 7e,72H+ OMe n n=1, 2.MeO O O a O O MeO 100 (75%) MeO O + 2MeOH 7e,72H+ O MeO OMe a n n O O OMe 101 (65% ± 68%) OMe CO2Me n O 102 (45% ± 75%) n=2, 8; (a) ACU, Pt anode, 3.5 ± 6.2 F mol71, MeOH, MeONa or KOH,75 ±20 8C; (b) ACU, graphite anode, 6 ± 8 F mol71, MeOH, Et4NOTs or Bu4NBF4 , 20 8C. The electrochemical processes at the second stage result in ring opening in the dialkoxylation products 99 ± 101. In the case of the compounds 101, the reaction is accompanied by their electro- oxidative rearrangement into methyl o-(2-methoxytetrahydrofur- yl)alkanoates 102.The rearrangement takes place due to isomerisation of intermediates A into carbocations B via oxonium MeOH R1OMe OH OMe Cb OMe CO2Me MeO n (75%) b CO2Me MeO2C 4 b Yu N Ogibin, G I Nikishin ions C. The subsequent alcoholysis of cations B results in the reaction product 102.145, 146 OMe MeO+ 2MeOH + 101 n O 72 e,72H+ n O MeO OMe MeO OMe C A OMe OMe MeOH, H2O + 102 7H+ OMe n O B III. Reactions induced by indirect anodic oxidation In the reactions induced by indirect anodic oxidation, unlike reactions induced by direct anodic oxidation, no direct electron transfer from the alkene molecule takes place at the anode.In this case, alkenes are involved in the electrochemical process through interaction with electrogenerated inorganic or organic radicals, ions or reagents (see Scheme 1). These intermediates arise from their precursors at potentials lower than the potentials of alkene oxidation. The process of alkene transformation into products takes place partly or entirely in the bulk solution rather than on the anode surface and is markedly facilitated and accelerated. This strategy allows one to perform electrochemical transformations of alkenes impossible in the case of direct anodic oxidation and thus extends the scope of applicability of anodic processes in organic synthesis. The most typical precursors for the generation of oxidants and reagents in the alkene transformations induced by indirect anodic oxidation are anions of organic and inorganic acids, variable- valence metal compounds and triarylamines.1. The reactions of alkenes with radicals electrogenerated from carboxylate anions and 1,3-dicarbonyl and related compounds In the anodic oxidation of carboxylate, acetoacetate, malonate and cyanoacetate anions, carbon-centred radicals are gener- ated;1 ± 4, 86 in the presence of alkenes, they add to the double bond. The resulting radical adducts 103 are converted along two routes: they either undergo pairwise or cross recombination (Scheme 4, pathways a, b) or are oxidised to give carbocations (pathway c). Subsequently the carbocations react with internal or external nucleophiles.1 ±4 Scheme 4 RCO¡2Y 7e,7CO2 R R Y 103 R7 7e Pathway a Y R R 2 R Y 103 Y Pathway b R R 103+R Y Pathway c Nu + Nu7 R R 103 Y Y 7e a.Reactions of alkenes with alkanoic acids The reactions of alkenes with radicals generated from saturated acids and 1,3-dicarbonyl and related compounds have been generalised in a review,5, 6 which covers the data published up to 1981. In this review, we consider later publications.559 Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis and/or cyclic products (110, 111). The adducts formed with styrene also give dimers 112.163 OEt X X=CO2Me, CN MeO2C a XCH2CO2Me + OEt OEt 108 (19% ¡À 37%) Ac X=Ac Me EtO O 110 (36%) Ph MeO2C a + CH2(CO2Me)2+ Ph MeO2C The main difference between alkene reactions with radicals generated by the anodic oxidation of alkanoic acids and their reactions with similar radicals generated by chemical methods is that the electrochemical method makes it possible to vary the radical concentration on the electrode surface over a broad range by varying the current density and thus to control the process.An increase in the current density favours recombination of radicals, while a decrease facilitates the oxidation of radicals. The electro- chemical and chemical processes also differ in the way in which electron-withdrawing substituents Y in the alkene molecule influence the reaction route. The formyl, acetyl and alkoxycar- bonyl groups direct the electrochemical reaction exceptionally to pathway a 147 and the chemical reaction, to pathway a or b.The alkenes CH2=CHY with electron-donating substituents Y, such as alkyl, aryl, vinyl and alkoxyl groups are converted along each of the three pathways: a, b and c (see Scheme 4).5, 6 OMe 109 (17%) MeO2C Ph MeO2C + CO2Me MeO2C + MeO MeO Ph Ph CO2Me O 111 (40%) 112 (10%) (a) ACU, 14 mA cm72, 1.5 Fmol71, EtOH or MeOH, EtONa or MeONa. Joint electrolysis of cycloalkane-1,3-diones 113, 114 with alkenes affords the products of formal [3+2]-cycloaddition, dihydrofuran (115, 116) and tetrahydrofuran (117) deriva- tives.164 ¡À 167 O R1 R2 O O a R1 R3 + R3 Y O Y R2 115 113 (a) ACU, 14 mAcm72, 1.5 F mol71, MeCN.Yield of 115 (%) Y R3 R2 R1 The electrochemical reactions under consideration have been studied in relation to the methylation 147 and trifluoromethyla- tion 148 ¡À 162 of acrylates,147 ¡À 159 methacrolein, vinyl ketones,147 acrylo- and fumaronitrile,149, 160 esters of fumaric, maleic 151, 152 and but-3-enoic acids,150 alkenes,148, 149, 153, 154 vinyl ace- tate,151, 152 isopropenyl acetate, allyl chloride,153, 154 but-3-en-1- ol 161 and undec-10-enoic acid.162 These reactions are normally carried out in an aqueous solution of a mixture of acetonitrile with partially (to 10%) neutralised acetic or trifluoroacetic acid in a cell with platinum electrodes at an anodic current density of 50 ¡À 100 mA cm72 and with passage of 1.1 ¡À 1.5 F of electricity per mole of the acid.147, 159 Methylation and trifluoromethylation of methacrolein, vinyl ketones and acrylates results under these conditions in pairwise and cross coupling of the arising radicals to give 2 : 2 (104, 105) and 2 : 1 (106) adducts in 40% ¡À80% yields.147 ¡À 149, 156, 157 The decrease in the anodic current density to 1 mA cm72 and a decrease in the temperature from 20 to 0 ¡À 5 8C directs the reaction of trifluoroacetic acid with methyl methacrylate almost entirely along pathway c giving rise to methyl 2-acetamido-2-methyl-4,4,4-trifluorobutanoate (107).The com- pound 107 is formed upon oxidation of the initial radical adducts and the reaction of the resulting carbocations with the solvent (MeCN).156 This change in the direction of the process is due to the fact that electrooxidation of radical adducts occurs much faster than their dimerisation.159 R1 R1CO¡¦27e,7CO2 Ph CH=CH2 OEt Ph CH2SiMe3 85 62 57 97 45 HMe HHH HHHMe Me HHHMe Me O Y a Me Me Ph R1=R2=H MeCN 104 Y R1 O a R2 Y CF3 R2 + Ph R1 b O 116 (54%)O CF3 Me O + CF3 Y CF3 Y 114 R1=H, R2=Me 106 105 Y Ph MeCN, R3OH O Me NHAc OR3 c CF3 117 (46% ¡À 70%) CO2Me 107 R3=H, Me; (a) ACU, 14 mAcm72, 1.5 F mol71.Y=CHO, Ac, CO2H, CO2Me, CN; (a) R1=Me, R2=H, 50 mA cm72; (b) R1=CF3, R2=H, 50 ¡À 100 u�� A. cu�� 72; (c) R1=CF3, R2=Me, Y=CO2Me, 1 mA cm72, 0¡À5 8C. 2. Reactions with electrochemically generated o-quinonemethides o-Quinonemethides 118 are generated upon the anodic oxidation of o-[1-(phenylthio)alkyl]phenols 119 in nitroethane followed by fragmentation of the radical cations 120 thus formed with elimination of the phenylthiol radicals. In the presence of alkenes, which are active acceptors of o-quinonemethides, cycloaddition to give chromans 121 takes place.168, 169 b.Reactions of alkenes with 1,3-dicarbonyl and related compounds Radical adducts formed by ethyl acetoacetate, diethyl malonate or malononitrile with vinyl ethyl ether or styrene are oxidised at the anode to cationic intermediates, which react with external or internal nucleophiles being thus converted into linear (108, 109)560 OH OH R2 SPh 7e MeO MeO R1 119 R1 R1 R2 O MeO R1 MeO 118 R1 R1=H, OMe; R2=H, Alk.3. Arylthiocarbonylation Electrochemically initiated arylthiocarbonylation is typical of alkenyl sulfides and alkenylsilanes. In this reaction, they are converted into a-phenylthio ketones 122.170 ± 172 X R1 R2 PhS PhSH O2 7e PhSHR1 O X O2H R1 R1 R2 R2 7XOH SPh 122 SPh X=SPh, SiMe3; R1, R2=H, Alk. The reaction is carried out in an undivided cell (at*20 8C) in acetic acid with Et4NOTs as the supporting electrolyte in the presence of 2 ± 4 equiv. of benzenethiol with passage of 20 mA current for 1-min periods at 30-min intervals (0.16 ± 1.19 F mol71) and with bubbling oxygen. 4. Halogenation, halooxygenation, haloacetamidation, epoxidation and other related reactions Anodic oxidation initiates halogenation, halooxygenation and related reactions in aqueous MeCN orDMFor in water-alcoholic media, in the presence of chloride, bromide or iodide anions.The halide anions are readily oxidised due to the low oxidation potentials (0.8, 0.4 and 0.2 V vs. Ag/Ag+, respectively), being converted into molecular chlorine, bromine and iodine; these reagents are responsible for alkene halogenation, halooxygena- tion and haloacetamidation 173 ± 184 and, under certain conditions of electrolysis, epoxidation 184 ± 193 and other related reactions (for example, azidoalkoxylation,194 oxyselenenation, acetamidosele- nenation,195 ± 198 acetamidosufenation,199 intramolecular arylthio- and arylseleno-esterification and lactonisation,200 ±d nitroacetamidation 203, 204).2 X7 X2 72e X2 C C C C + 7X7 X R2 +SPh 7PhS ,7H+ R1 120 O R1 R2 R1121 X R1 R2 SPhO2 X O O R2 SPh Yu N Ogibin, G I Nikishin DMF±H2O C C OCHO X 7NHMe2 C C NHAc X MeCN±H2O 7H+ X7 C C X X C C C C OR X R=H 7HX ROH 7H+ O X=Cl, Br, I. In the case of alkenes activated by electron-donating substitu- ents, competitive formation of these products via radical cations is also possible. X7 C C C C X C C + 7e 7e products. C C +X Due to the high discharge potential of fluoride anions (2.5 V), fluorination, fluorooxygenation and fluoroacetamidation of alkenes occurs exclusively with participation of radical cati- ons.205 ± 207 The composition of oxidation products in the presence of hydrogen halide salts depends considerably on the substrate type, the nature of the solvent and electrolysis conditions (Table 7, 8).A highly selective transformation of alkenes into halohydrins and epoxides takes place in water and aqueous acetonitrile or dime- thylformamide with participation of bromides; sodium and tet- raalkylammonium bromides as well as readily regenerable poly- (4-vinylpyridine) hydrobromide are the most effective among them.192 The selectivity of epoxidation is greatly affected by the medium pHand the anode current density.In a solution of sodium bromide in aqueous acetonitrile, the reaction medium becomes alkaline at the final stage of electrolysis in an undivided cell; this induces the transformation of the bromohydrin formed into the epoxide.Oxidation of an alkene under similar conditions with a small amount of an acid added (normally, sulfuric and hydro- chloric acid) permits bromohydrin to be produced in a nearly quantitative yield. In the case of a-arylalkenes (for example, isosafrole), glycols are formed under these conditions due to hydrolysis of a-bromo-b-hydroxyalkylarenes.184 The selectivity of transformation of alkenes into bromohydrins and epoxides sharply decreases for an anode current density of >20 mA cm72 and an anode potential of >1.5 V. For alkenes reactive towards halogen molecules, the optimal values are 1 ± 2 mA cm72 and 0.8 V.179, 180 Alkene bromoalkoxylation is highly selective in anhydrous alcohols at an anode current density of 100 ± 200 mA cm72 under conditions that ensure complete cathodic reduction of the diha- lides formed in a competing reaction to give the initial alkene (graphite anode, nickel cathode, 30 8C, 5.6 F mol71 of electricity) (see Table 8).182 Br R1 R1CH CHR2+R3OH R2 NH4Br 7e OR3 (66% ± 94%) R1=Alk, Ar; R2=H, Alk; R3=Me, Et, Pri, But.Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis Table 7.Influence of electrolysis conditions on the composition of products of anodic oxidation of acyclic alkenes in the presence of halogen-containing mediators (20 ± 45 8C). Alkene R1CH=CHR2 R1 CH2 R2 R1 Note.Method A: ACU, PbO2 anode, 50 ± 200 mA cm72; method B: PCU, Pt anode, 0.4 ± 1.5 V; method C: ACU, Pt anode, 20 mA cm72, 2.0 F mol71; method D: ACU, graphite anode, 110 mA cm72, 5.6 ± 9.5 F mol71; method E: ACU, Pt anode, 6 ± 13 mA cm72, 4 ± 10 F mol71; method F: ACU, Pt anode, 0.3 ± 6.0 mA cm72, 2.1 ± 3.5 F mol71; method G: ACD, graphite anode, 25 mA cm72, 3.0 F mol71. a Substance yield (in parentheses, current yield); b poly(4-vinylpyridine) hydrobromide; c 1-[3,4-(methylenedioxy)phenyl]propane-1,2-diol; d a-azido ketone ketal. When the quantity of electricity increases to 10 ± 14 F mol71 and the nickel cathode is replaced by a brass one, or by a nickel cathode coated with powdered Raney nickel, not only dibromide but also the bromo ether formed is reduced.The reaction yields ethers and thus it can formally be regarded as a very facile and effective method for the addition of alcohols to alkenes.63, 208 R2 R1 HHHAlk Me Me Me Me Ph Ph Ph HHHH Alk CH2Cl Alk Alk Bn Bn Ph Ph (E)-Ph (E)-Ph (E)-Ph Ph Ph Ph Ph O Me OO Me OO Me O Me HHPh HHMe OMe OMe OBun OMe Me Alk Cl(CH2)10 Ph Bn Bn Bn HHHMe CH2OAc CO2H R1 CO2Me, CH2OAc, CH2SO2Ph Mediator HCl HCl NaBr NaX (X=Cl, Br) Et4NBr, NaClO4 Et4NI, LiClO4 Et4NCl, LiClO4 Et4NCl, LiClO4 Et4NCl, LiClO4 Et4NCl, LiClO4 Et4NF. 3HF Et4NF. 3HF Et4NCl, LiClO4 NH4Cl NH4Br NaBr NaBr, H2SO4 NaX (X=Cl, I) HCl see b see b Et4NF.3HF NH4Br NH4Br NH4Br NH4X (X=Cl, Br) MeO(CH2)2OH MeO(CH2)2OH MeOH MeOH NH4I NaN3±NH4OTs NaN3±NH4OTs Et4NBr, LiClO4 NaCl, HCl NaCl, HCl MBr (Na, K, Li) Et4NBr 561 Method Yield of the reaction product Ref. Solvent (%) a dihalide epoxide mono- halide 77 (85 ± 90) (10) (85 ± 90) (10) 60 (90) AAB H2O H2O H2O (35) 7 (50) (51) BB MeOH MeCN±H2O 65 23 77 MeCN±H2O C 747 34 40 CBB DMF±H2O MeCN MeCN traces 7 173 173 7 7 174 H2O B 7 7 65 ± 88 186 ± 189 177 7 7 177 MeCN±H2O C 65 7 7 178 DMF±H2O C 85 7 7 178 178 178 7 7 205 ± 207 206 DMF±H2O C 75 7 7 178 95 (35) 65 ± 78 DD MeOH±MeCN ROH 7 7 182 7 7 182 c 184 71 MeCN±H2O E 23 77 7 184 MeCN±H2O E *100 39 ± 43 c E MeCN±H2O 7 7 184 H2O 7 (85 ± 90) 10 7 7 80 7 7 62 48 ± 63 7 MeCN MeOH EtOH MeOH 173 192 192 205 ± 207 7 7 182 7 7 182 7 7 182 7 7 180 7 7 180 7 7 194 7 7 194 A DMF± PhH ±H2O E DMF± PhH ±H2O EBDDDEEGG 0 ± 21 93 63 94 87 ± 92 60 66 ± 81 d 82 d 50 MeOH C 7 7 176 85 7 7 179 Cl(CH2)2Cl ±H2O E (2 : 1) CH2Cl2±H2O E 55 7 7 179 MeCN±H2O E 7 7 75 ± 91 190, 191 (5 : 1) MeCN±H2O E 7 7 82 190, 191 (5 : 1) OR +ROH NH4Br 7e n n (85% ± 92%) n=1, 2, 7; R=Me, Et, CH2CH2OMe.Yu N Ogibin, G I Nikishin 562 Table 8.Influence of electrolysis conditions on the composition of products of anodic oxidation of cyclic alkenes in the presence of halogen-containing mediators (20 ± 45 8C).Ref. Method Solvent Mediator R2 Alkene R1 Yield of the reaction product (%) (see a) dihalide mono- halide 1 H N4 Br H R 1 H E4 NBr t n MeOH A 75 MeOH B 54 MeCN±H2O B 40 1 H Et4 NCl 1 H Et4 NBr 1 H I2 , LiClO4 1 H E4 NCl t 1 H Et4 NBr 1 H Et4 NBr ± (PhSe)2 1 H Et4 NBF4 ± (PhSe)2 OMe OAc OAc OSiMe3 1, 2, 7 1, 2, 7 1, 2, 3, 7 7 O 63, 182 175 175 175 177 175 175 196 197 180 180 194 180 180 180 180 180 194 182 714 768 722 64 777777777777 MeCN±H2O B 7 MeCN±H2O DMF±H2O DMF±H2O MeOH±H2SO4 MeCN MeOH MeCN±H2O MeOH MeCN±H2O (3 : 1) MeOH MeOH MeOH MeOH MeOH MeOH NH4X (X=Cl, Br, I) NH4X (X=Cl, Br) NaN3±NH4OTs NH4I MBr (M=Li, K, Na) NH4X (X=Br, I) NH4Cl Et4NBr NaN3±NH4OTs NaBr 66 C (85) B 58 B 16 D 95±98 C 77 E 85 ± 94 b E 55 ± 95 c F 67 ± 83 d E 97 c E 69 ± 74 E 93 ± 96 E 60 E 97 F 60 d A Note.Method A: ACU, graphite anode, 110 mA cm72, 5.6 ± 9.5 F mol71; method B: potentiostatic conditions, a divided cell (PCD), Pt anode, 0.8 ± 1.3 mA cm72; method C: PCU, Pt anode, 0.4 ± 1.5 V; method D: ACU, Pt anode, 6 ± 13 mA cm72, 4 ± 10 F mol71; method E: ACU, Pt anode, 0.3 ± 6.0 mA cm72, 2.1 ± 3.5 F mol71; method F: ACD, graphite anode, 25 mA cm72, 3.0 F mol71. a Substance yield (in parentheses, current yield); b a-halocycloalkanone dimethyl ketals.c a-halocycloalkanones. d a-azido ketone ketal. R=Me XOMe OMe n 126 X7, MeOH 7e In more profound electrooxidation of arylalkenes in the sodium bromide electrolyte, the bromo ethers formed are dehy- drobrominated and the resulting enol ethers 123 are converted into a-bromoalkyl aryl ketone dimethyl ketals 124 under the action of methanol and bromine.63, 209 X OR n Br Br R =Ac, SiMe3 a Ar Ar a Ar ArCH CHR R R O n 127 OMe OMe n=1, 2, 7; X=Cl, Br, I. 123 R MeO OMe 124 (62% ± 91%) R=H, Alk; (a) ACU, Pt anode, brass cathode, 220 mA cm72, 9 F mol71, MeOH, NaBr, 60 8C. Other reactions induced by anodic oxidation proceed similarly to electrochemical halooxygenation and haloacetamidation of alkenes.For example, azidomethoxylation of alkenes is docu- mented.194 Electrolysis of styrene in ethylene-, 1,3-propylene- or 1,3- butylene glycol gives rise to cyclic ketals of bromomethyl phenyl ketones 125.63 MeO C C N3 C C +NaN3 MeOH, Et4NOTs 7e Br R O Ph+ HO NaBr 7e OH n Ph R O n 125 (66% ± 80%) n=1, 2; R = H,Me. Two mechanisms have been proposed for this reaction, namely, pathway a for non-activated alkenes and pathway b for activated alkenes.194 Pathway a N3 N¡37e MeOH N3 C C OMe C C C C N3 N3 7e,7H+ Anodic oxidation of enol ethers in the presence of ammonium bromide yields ketals of a-halo ketones 126. When lithium, sodium, or potassium bromide is used, the yield of the ketals decreases by 20%± 30%.a-Halo ketones 127 are formed in 55%± 97% yields from enol acetates and trimethylsilyl ethers under similar conditions (see Table 8).180Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis Pathway b OR OR + + MeOH 3 C C C C C C N3 N¡¦ 7e 7e 7H+ OR C C OMe N3 OR Alkene oxyselenenation ¡À deselenenation results in allyl ethers such as 128.195, 196 OAc C CH a 7e OAc C CH ROH 7e,7PhSeOR RO Y OAc C CH RO 128 (70% ¡À 90%) R=H, Me; Y=SePh, Se(O)Ph; (a) ACU, Pt anode, 10 ¡À 20 mA cm72, ROH or MeCN¡ÀH2O, 0.1 equiv. (PhSe)2 , 1.5 equiv. MgSO4, 60¡À70 8C. Electrolysis of alkenes in the presence of diselenides and disulfides is accompanied by hydroxy-, alkoxy- and acetamidose- lenenation 196 ¡À 198 and acetamidosulfenation.199 Hydroxy-, alkoxy- and acetamidoselenenation reactions are initiated by electrogenerated halogenonium ions.Under the action of these species, diphenyl diselenide and diphenyl sulfide are converted into phenylselenenyl and phenylsulfenyl halides. These electrophilic intermediates add to alkenes, and the resulting adducts react with external or internal nucleophiles to give reaction products. ROH C C SePh RO R=H, Me, Ac a +(PhSe)2 C C MeCN C C SePh AcNH a 2 R1S+ R1SSR1 NHAcSR1 R2=Alk, Ar; R3=H R2 R1S+ R2CH CHR3 MeCN SR1 R27R3=(CH2)4 NHAc (a) ACU, Pt anode, 6.7 ¡À 13 mA cm72, MeOH, AcOH, MeCN¡ÀH2O, Et4NX¡ÀH2SO4 (X=Cl, Br, I). Unsaturated alcohols and acids undergo intramolecular arylthio and arylseleno esterification and lactonisation during electrolysis in the presence of disulfides and diselenides.200 ¡À 203 The reaction mechanism is similar to the mechanism of electro- chemical alkoxyselenenation and acetamidosulfenation.R5 R4 R2 R4NBr7CH2Cl2 +(PhX)2 7e R1 Y n R3 563 R4 R4 n R3 Y=OH n + PhX R5 O R5 O R3 PhX R2 R1 R2 R1 PhX R3 R2 R4 R1 R4 Y=CO2H PhX n + R3 R2 R1 O n O O O R5 R5 R1, R2, R3, R4, R5=H, Me; R17R2 u�¢ o�� u�¢ R17R3=(CH2)m (m=2, 3); n=0, 1, 2; Y = OH, CO2H; X=S, Se. Electrochemical intramolecular phenylthio and phenylseleno esterification and lactonisation of unsaturated alcohols and carboxylic acids proceed with high selectivity (50% ¡À 86%) only in the case where five- and six-membered rings are formed.If the substrate double bond is endocyclic as, for example, in cyclo- hexen-3-ylacetic acid, stereospecific trans-addition of phenylsul- fenyl and phenylselenenyl halides takes place to give cis-fused bicyclic system 129.201, 202 CO2H a O O SePh 129 (73%) (a) ACU, graphite anode, Cu cathode, 400 mA cm72, 6 F mol71, MeOH, 0.5 equiv. (PhSe)2, NH4Br, 15 ¡À 20 8C. Nitronium tetrafluoroborate resulting from anodic oxidation of N2O4 in MeCN adds to alkenes to give nitroacetamidation products.203, 204 NO2 X=H, n=1 NHAc (20%) a NO2 X n X =OSiMe3, n=1, 2 O n (80% ¡À 95%) R1 R1 NHAc R2 a X C CH R2 H NO2 X (50% ¡À 82%) X=H, Br; R1=H, Me; R2=H, Me, Ph; (a) NO�¢2 BF¡¦4 (0.25 mol litre71 in MeCN),770 8C. 5.o-Nitration and diacylation Electrochemical o-nitration and diacylation involve the nitrate radical NO3 generated by the anodic oxidation of the nitrate anions.210 In the presence of terminal alkenes and cycloalkenes, the nitrate radicals add to them, and the resulting radical adducts are converted into alkyl nitrates upon detachment of hydrogen atoms from the solvent. The resulting alkyl nitrates are trans- formed into the corresponding alcohols without isolation.211564 a NO¡ NO3 3 SolvH R2 R1CH CHR2 R1 NO3+ 7Solv ONO2 R2 R2 b R1 R1 OH ONO2 R1=Alk, R2=H; R1±R2=(CH2)m (m=4, 6); (a) ACU, Pt anode, 50 mA cm72, 8 F mol71, MeCN±H2O±Et2O (10 : 2 : 1), LiNO3 , 10 8C; (b) NaSH.Under similar conditions of electrolysis, 1,1-di- and 1,1,2-tri- substituted alkenes furnish oxazoline derivatives.211 R1 ONO2 ONO2 MeCN NO3 R1 C R1 CHR3 + 7e R3 R3 R2 R2 R2 R2 R2 R3 R3 R1 R1 O N N ONO2 + R1=Me, Et, n-C9H19; R2=Me, Et; R3=H, Me. In the case where the reaction mixture contains, apart from nitrate (usually, lithium, ammonium, sodium, or potassium nitrate) and alkene, an aldehyde in a concentration sufficient for trapping nitrate radicals, diacylation of the alkene takes place.212 NO3 R1C O R1CHO O R3 R3 R1C O R1C O R2 R1 CO2Me CO2Me R2 R3 O CO2Me R1 R1 O R2 6. Hydroboration Electrochemical hydroboration of alkenes includes two stages, namely, electrogeneration of diborane from borohydride anions and addition of the diborane formed in situ to alkenes to give trialkylboranes; treatment of these products with hydrogen hydroperoxide in an alkaline medium results in alkanols.213, 214 2 BH¡ B2H6 4 7H2 ,72e H2O2, NaOH B2H6 OH R R (RCH2CH2)3B Hydroboration is carried out with the sodium iodide electro- lyte under galvanostatic conditions in an undivided cell with a platinum anode and a steel cathode by passing 2 F of electricity per mole of alkene.The reaction is regio- and stereoselective and gives products in high yields. This procedure has been used to prepare primary alkanols from unsaturated and hydroxy- and acetoxy-substituted terminal alkenes (yields 64%± 81%), cyclo- alkanols from cycloalkenes (70% ± 72%), (7)-cis-myrtanol from (7)-b-pinene (82%), exo-borneol from norbornene (72%), etc.The method is promising for organic synthesis. The electrogen- erated diborane can also reduce carboxylic acids to the corre- sponding alcohols.214 Yu N Ogibin, G I Nikishin 7. Electrochemical cyclisation assisted by tin hydrides The electrochemical cyclisation assisted by tin hydrides is illus- trated by transformation of 2-halocyclohexyl allyl ethers into octahydro-3-methylbenzofuran 130 (yield 65%). Tin hydrides act in this reaction as precursors of tin-centred radicals and as hydrogen atom donors, while the electric current initiates this radical-chain process.215 Me H X O O H 130 (endo : exo=2:1) Ph3Sn 7e,7H+ Ph3SnX Ph3SnH O O X=Br, I; (a) ACD, Pt electrodes, 10 mA cm72, 0.2 ± 0.5 F mol71, THF, Bu4NBF4 , 1.2 equiv. Ph3SnH,*20 8C.Similarly, octahydro-3-methylidenebenzofuran was prepared from 2-halocyclohexyl propargyl ethers (65% ± 69%), b-lactam derivative 131 was converted into dihydrocephalosporin 132. PhOCH2CO PhOCH2CO SSR S12 65 78 3 N 4 Ph3SnH 7e N Me Me O O SO2Me 131 CO2Me 132 (80%, 3a : 3b=1:2) R is 2-benzothiazolyl. These reactions can also be carried out in undivided cells. However, complete conversion of the substrate in an undivided cell required a four times greater quantity of electricity. An advantage of the electrochemical method over the chem- ical methods in which azoisobutyronitrile, benzoyl peroxide, trialkylboranes, etc., serve as initiators 216, 217 is higher stereo- selectivity.8. Phosphorylation Indirect electrochemical phosphorylation of alkenes and cyclo- alkenes occurs upon the electrophilic addition of triphenylphos- phinium radical cations generated by anodic oxidation of triphenylphosphine followed by electrooxidative deprotonation of the adduct thus formed. The reaction yields 1-alkenyl- and 1-cycloalkenyl-substituted triphenylphosphonium salts.18, 218 ± 220 a Ph3P+ X7 Ph3P a + Ph3P+ X7 CH CH CH CH PPh3X7 7H+ + CH C PPh3X7 X=ClO4, BF4; (a) X7, ACU, graphite anode, Fe cathode, 1.07 mA cm72, 2 F mol71, CH2Cl2 , 0.2 equiv. Ph3P, 0.5 equiv. 2,6-LutHClO¡4 or 1.33 equiv. K2CO3, N2 atmosphere, *20 8C. This reaction has been used to prepare phosphonium salts from ethylene, C5±C8 cycloalkenes, norbornene,ed styrenes, vinyl and allylsilanes, a,b-unsaturated compounds, etc.(Table 9).218 ± 225 When vinyl- and allylsilanes are used, desilyla- tion of the adducts formed initially takes place, and phosphory- lated alkenes are formed as the reaction products.220 ± 222Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis Table 9. Electrochemical triphenylphosphorylation of alkenes. Alkene n n=1±4 RR=H, Me SiMe3 R R=H, Me SiMe3 SiMe3 SnR3 Note. Method A: ACU, graphite anode, stainless-steel cathode, 1.07 mA cm72, 2 F mol71, CH2Cl2 , 0.2 ± 1 equiv. of Ph3P, 0.5 equiv. of 2,6-LutHClO4 or 2,6-LutHBF4 ,*20 8C; method B: ACU, glass carbon anode, Pb cathode, 1.1 mA cm72, 1.8 F mol71, CH2Cl2 , 2 equiv.of Ph3P, 0.5 equiv. of 2,6- LutHClO4 or 2,6-LutHBF4, K2CO3 ,*20 8C; method C: ACU, glass carbon anode, Pb cathode, 1 mA cm72, 2 F mol71, MeCN, 1 ± 3 equiv. of Ph3P, 0.5 equiv. of LiBF4, N2 atmosphere,*20 8C. R2 C CH R1CH R17R2=(CH2)n (n=3±6) R3=H Electrochemical reactions of triphenylphosphine with vinyl ketones, acrolein, cycloalk-2-enones, ethyl acrylate, acryla- mide 220, 223 and cycloalkenol ethers 220, 224 give rise to phosphory- lated ketones. R C CH O R=H, Me, Et, EtO. X O X=CH2O, n=1, 2. Method Reaction product + A PPh3 n +PPh3 A + A PPh3+ R PPh3 A +PPh3 B R + B PPh3 + B PPh3 + B PPh3 SiMe3+PPh3 7e R3 R2 R1, R3=H C CH2 CH2 R2=H, Me PPh3 (68% ± 71%) R1, R2=H Ph CH R3=Ph CH (51%) (CH2)n +PPh3 (24% ± 53%) O PPh3 CH2 R 7e PPh3 (72% ± 84%) X n n PPh3 + 7e O PPh3 (61% ± 63%) Ref.Yield (%) 219 ± 221 50 ± 60 219 ± 221 65 220, 221 40 220, 221 80 ± 100 220, 221 68 ± 71 221 ± 223 21 221 ± 223 24 221 ± 223 61 + +PPh3 CH2 + Alkene R C CH CH2 O R=H, Me, Et, NH2, OEt X n OX=CH2 , n=1, 2; X=O, n=2 OAc R n R =H, Me; n=1± 3 OP(O)(OEt)2 OSiMe3 n n=1, 2 OY R +PPh3 n R=H, Me; n=1±3; (a) Y=Ac (47% ± 96%); (b) Y =P(O)(OEt)2 (23% ± 51%); (c) Y=SiMe3 (11%). Electrochemical phosphorylation of carboxy-substituted alkenes 133 by phosphines with Bu4NBr as the supporting electro- lyte results in acylphosphonium salts 134, which are readily reduced at the cathode under the conditions of diaphragmless electrolysis to give intermediates 135.These undergo radical cyclisation, electroreduction and protonation and are thus con- verted first into cyclic intermediates 136 with separated anionic, cationic and radical centres and then into cyclopentanones 137. The reaction is carried out in an undivided cell (graphite electro- des, 2.4 mA cm72, 4 F mol71, CH2 Cl2 , 3 equiv. of X3P (X = Ph, Bun), 1 equiv. of Bun4 NBr, 2 equiv. of MeSO3H). Together with cyclopentanones 137, aryl-substituted alkenals 138 are produced (Scheme 5).225 This electrochemical reaction is an example of successful use in organic synthesis of paired electrolysis in which the processes occurring at both the cathode and the anode are involved in the assembling of the final product.By phosphorylation of alk-1-enes with the phosphines X3P in the presence of thiols RSH (R = Prn, Bun) under conditions of electrolysis differing substantially from those used in the studies cited above,218 ± 225 alk-2-enylphosphonium salts 139 have been Method Reaction product Yield (%) + R PPh3 B OX B n+ O PPh3 O + R PPh3 C n O + C PPh3 O C +PPh3 n O + R PPh3 a ± c 7e n 565 Ref. 55 ± 84 221, 224 61 ± 67 221, 224 47 ± 96 221, 225 23 ± 51 221, 224 11 ± 30 221, 224566 Scheme 5 X3PBr2 X3P 2 Br7 7e X3PBr2 ArCH CH(CH2)3C(O)Br X3P ArCH CH(CH2)3CO2H 133 O7+ + e PX3 CHAr CH ArCH CH(CH2)3C(O)PX3 Br7 134 135 O7 + O PX3 e, 2 H+ CHAr CH2Ar 136 137 (27% ± 49%) e, 2 H+ 135 ArCH CH(CH2)3CHO 138 (38% ± 68%) X=Ph, Bun; Ar=4-YC6H4, Y=H, Me, MeO, Cl, Br.prepared (yields 46%± 85%). The products of alkene thioalkyla- tion are formed as side products.226 + a R +X3P X3PCH2CH CHR 139 R=n-C5H11, n-C6H13; X=Mes, Bun; (a) ACD, Pt anode, Ni cathode, 4 mAcm72, 2.2 ± 4.8 F mol71, MeCN, 0.05 equiv. X3P, 0.75 equiv. RSH, 0.5 equiv. NaClO4 , 0.6 equiv. Na3PO4. The processes of regiospecific phosphorylation of alkenes presented in Table 9 take place only with triphenylphosphine. If it is replaced by trialkylphosphines, trialkyl phosphites, dialkyl trimethylsilyl phosphites or phosphorous amides, the reactions yield mixtures of phosphorylation products with different posi- tions of the C=C bond in the alkenyl or cycloalkenyl fragment.For example, electrolysis of Et3P, Prn3 P or Bun3 P in the presence of cycloalkenes gives rise to regioisomeric cycloalkenyl phospho- nium salts with the double bond in positions 1, 2 or 3.227 This difference between the behaviours of triphenylphosphine and other phosphines in this reaction is related, in the researchers' opinion, to higher thermodynamic stability of alk-1- and cycloalk- 1-enyltriphenylphosphonium salts compared to the correspond- ing trialkylphosphonium salts caused by the effective stabilisation of the positive charge with participation of the p-systems of the phenyl groups.Phosphorylation of hex-1-ene and cyclohexene with trialkyl and dialkyl trimethylsilyl phosphites gives rise to three types of isomeric products, while the reactions involving phosphorous mono- and diamides, yield two types of iso- mers.228, 229 Phosphorous amides are more basic than alkyl phosphites; thus, they can bind more efficiently the protons generated during the electrolysis, and the prototropic isomer- isation of the intermediates is suppressed (Scheme 6).229 X a OR3 R1CH CHCH2CH2R2+ P Y OR3 X=R3O, Y=R3O, Me3SiO R3O P C CHCH2CH2R2+R3O O R1 NAlk2 X =Alk2N; Y=R3O, Alk2N Y P CHCH CHCH2R2+ Y 141 O R1 R1, R2 = H, Bun; R17R2 = (CH2)2; R3=Alk; (a) ACD, Pt anode, 0.1 ± 8 mA cm72, 1.1 ± 1.7 F mol71, MeCN, 5 ± 6 equiv.of alkene, 0.25 equiv. NaClO4 , 8 equiv. Na3PO4 . Yu N Ogibin, G I Nikishin The yields of alk(cycloalk)-2-enylphosphonates of type 140, 141 are 2 ± 4 times as high as the yields of other regioisomers. In some cases, they are formed as the only products.228, 229 In the presence of diethyl phosphite or O,O-dibutyl phosphorothioite as a mediator which changes the nature of the phosphorus-contain- ing active intermediates, the regiodirectivity of phosphorylation of alkenes by phosphorous mono- and diamides changes towards the predominant formation of alk(cycloalk)-1-enyl-substituted phosphonamidates of type 142. For example, cyclohexenyl phos- phonamidates 141 and 142 (1 : 2 ratio) were prepared in the presence of diethyl phosphite, together with the side product diethyl cyclohexylphosphonate (143).229 H RO + (EtO)2PHO +P(RO)2PNR2 (RO)2PNR2 7e 7ROH 7(EtO)2PO RO NR2 + ROP(O)NR2 RO RO + + ROP(O)NR2 P P R2N R2N 7H+ (49%) O O (EtO)2P O (EtO)2PHO (EtO)2P 7(EtO)2P O O (EtO)2PO 143 (17%) 9.Hydrophosphonylation and hydrothiophosphonylation Indirect electrochemical hydrophosphonylation of alkenes with dialkyl phosphites takes place in the presence of trialkylphos- phines, trialkyl and dialkyl trimethylsilyl phosphites [Bun3 P, (PriO)3P, (EtO)2POSiMe3].230 HP(OR)2 H C C + C C P(OR)2 O O The promotory effect of these compounds is based on electric generation of the radical cations X3P+.at potentials at which dialkyl phosphites and alkenes are electrochemically inactive (0.8 ± 1.6 V). The products of hydrophosphonylation are formed upon the detachment of hydrogen atoms from dialkyl phosphites by the X3P+. radical cations and subsequent addition of the generated (RO)2P(O). radicals to alkenes. A chain transfer takes place in the reaction between the radical adducts and dialkyl phosphites.230 The products of hydrophosphonylation of alkenes, for example, cyclohexene,230 are formed selectively only with the assistance by trialkylphosphines. Scheme 6 R3O OR3 CHR2 PCH(R1)CH CHCH2R2+ R3O P CHCH2CH 140 O R1 O NAlk2 P C CHCH2CH2R2 142 O R1Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis Trialkyl and dialkyl trimethylsilyl phosphite-initiated reac- tions give rise to saturated and unsaturated phosphonates derived from the mediator and the dialkyl phosphite. The triisopropyl phosphite-assisted reaction of cyclohexene with diethyl phosphite yields eight products including diethyl and diisopropyl cyclo- hexyl-, cyclohex-1-enyl-, cyclohex-2-enyl- and cyclohex-3-enyl- phosphonates. Presumably,230 this is due to active oxidation of the intermediate radical adducts A by the radical cations gener- ated from triisopropyl phosphite and to addition of the latter species to cyclohexene.(EtO)2P A O + (PriO)3P+ (EtO)2P (EtO)2P 7P(PriO)3 7H+ 144 O O SH 143 7S + (PriO)3P+ 1) SH 2)7e (PriO)3P (PriO)2PO Similarly, the radical cations electrogenerated from trialkyl phosphites initiate hydrothiophosphonylation of alkenes with O,O-dialkyl phosphorothioites.231, 232 C C H (R1O)2P (R1O)2P(S)H+ C C (R2O)3P 7e S R1=Et, Pri, Bu; R2=Alk.Joint electrolysis of alkenes with dialkyl phosphites or O,O- dialkyl phosphorothioites or with their sodium or lithium salts affords mixtures of alkene hydrophosphonylation (hydrothio- phosphonylation) products and the products of substitution of the diethoxyphosphoryl or -thiophosphoryl group for the allylic hydrogen atom.231 ± 235 Thus the reaction of cyclohexene with sodium diethyl phosphite yields diethyl cyclohexyl- and cyclo- hexenylphosphonates 143 and 144 (overall yield 52%, 2 : 1 ratio).232, 233 A similar reaction with sodium O,O-diethyl phos- phorothioite gives compounds 145 and 146 in a total yield of 87% (11 : 1 ratio).(EtO)2PXNa + MeCN 7e X=O 143+144 X=S + (EtO)2P (EtO)2P 146 S 145 S X=O, S. In the reactions of alkenes with dialkyl phosphorothioites, the formation of phosphorylation products is completely suppressed when a 1 : 1 acetonitrile ±THF mixture is used as the solvent.232 10. Radical-cation [4+2]-cycloaddition of styrenes and enamines to vinylindoles and -pyrroles induced by anodic oxidation The radical-cation [4+2]-cycloaddition reactions have been vigo- rously studied during the last two decades. Owing to the very low activation barriers, they proceed tens or hundreds times as fast as the ordinary Diels ± Alder reactions; hence, they can be performed under milder conditions.The radical-cation [2+4]-cycloaddition is highly regio- and chemoselective. Moreover, they can be carried out for dienophiles with electron-donating substituents at the double bond, which do not enter into conventional Diels ± Alder reaction. There are strong grounds for believing that they will find extensive application in organic synthesis.236 The [4+2]-cycloaddition of enamines to 2-vinylindoles or -pyrroles acting as dienes induced by anodic oxidation 237 ± 240 proceeds efficiently only in the case where the difference between the oxidation potentials of dienophiles and dienes does not exceed 500 mV.241 To avoid polymerisation, there should not be hydro- gen atoms at the 3-position of the pyrrole or indole ring.The radical-cation cycloaddition of enamines to 2-vinylindoles 147 includes generation of radical cations 148 at the anode, their addition to enamines 149a ± d and subsequent cyclisation, electro- oxidation and deprotonation of the radical-cation cycloadducts 150a ± d to give cycloaddition products 151a ± d.239, 240 The struc- ture and the yield of the reaction product depend most of all on the structure of the initial substrates. Electrolysis of 2-vinylindoles 147 together with methyl b-(N,N-dimethylamino)acrylate (149a) or substituted b-(N,N-dimethylamino)acrylonitriles 149b ± d affords compounds of the pyrido[1,2-a]indole series (151a ± d), which are of interest as building blocks in the synthesis of indole alkaloids.237, 239 R1 Me NH 147 CN R3 148 X NMe2 149a ± d R2 R1 CN +NH Me R3 Me2N R2 X 150a ± d R2=H 150a ± c 7Me2NH X 151a ± c (32% ± 50%) (a) PCU, graphite anode, 0.49 ± 0.67 V, CH2Cl2 ±MeCN (1 : 1) or MeCN, 0.1 equiv. LiClO4 .R1 Compounds 149 ± 151 Me CH2CH2OEt Me Me abcd Cycloaddition of the indoles 147 to 1,4,5,6-tetrahydropyri- dines 152a ± d or to a-(N,N-dimethylaminomethylene)butyrolac- tone (153) results in tetracyclic compounds 154a ± d, 155 of the indolo[1,2-a]pyridine series incorporating the skeleton of pyrroco- line alkaloids such as goniomitine.238, 239 567 R1 a + Me NH 148 CN R1 CNMe NH R3 + Me2N R2 X R1 CN R2=Me N Me 7e,72 H+ R3 Me2N Me X 151d (50%) R1 CN N Me R3 R3 R2 X Yield of 151 (%) 90 ± 91 62 68 53 CO2Me CO2Me CN CN HHMe H HHHMe568 X a 147+ RN2 152 ± d R2 Compound 154 R1 H Me Me Me H Me H CH2 CH2OEt CN abcd O a, 147 O Me2N 153 (a) PCU, graphite anode, 0.49 ± 0.67 V, CH2Cl2 ±MeCN (1 : 1) or MeCN, 0.1 equiv.LiClO4 . Polysubstituted indolizines, difficult to synthesise by other methods, have been prepared by joint electrooxidation of 2-vinyl- pyrroles 156 with enamines 149.240 R1 X + Me2N Me 149 156 X=CO2Me, CN, NO2; Y =CO2Me, CN; R1, R2=Me, CO2Me. Me Me NMe2+Me CN NH Me CO2Me+Me NHHN Me O + Me2N O Me 153 Me N Me Me2N (33%) X=CO2Me.Yu N Ogibin, G I Nikishin R1 CN N Me RN X 154a ± d The tentative asynchronous mechanism of the radical-cation cycloaddition 241, 242 allows the formation of the final product as a mixture of cis- and trans-isomers because rotation around the C7C bond in the initial radical-cation adduct is possible. The formation of only trans-isomers indicates that cyclisation occurs much faster than the rotation around the C7C bond. Stabilisa- tion of the radical intermediates by radical-stabilising substituents such as nitrile or alkoxycarboxyl group is favourable for the reaction.238, 239 Yield (%) X 11. Reactions of alkenes induced by electrooxidation of variable-valence metal compounds CO2 Me 91 CO2Me 62 CN 6853 R1 CN N Me2N Me O R RH RH++Med products Compounds of variable-valence metals (manganese, copper, pal- ladium, ruthenium, nickel, cobalt, cerium, and thallium) act in electrochemical processes as mediators (Med) and participate in the electron transfer at the anode and in the reaction of the oxidised form of the mediator (Medox) with the substrate (RH).The reaction between RH and Medox can follow two pathways, one giving rise to a radical cation (pathway a) and one giving rise to a radical (pathway b). Pathway aMedox 7Med 7H+ O 155 (52% ± 53%) Pathway b B Medox Med+BH+ RH R+MedH+ products B is proton acceptor. R2 R1 R2 Y In many cases, compounds and complexes of variable-valence metals are used in these processes as binary mediator systems in combination with other mediators including organic compounds (CH acids, benzoquinone or triarylamines).N Me 7e Y N Y Y (43% ± 66%) XMe Me Me CN N Me 7e CN Me2N CN CN Me CN (54%) Me Me Me X Me N 7e X HN X X X (25%) Me 7e X NH X Me a. Reactions induced by electrooxidation of manganese compounds Despite the fact that the addition of carboxylic acids, aldehydes, ketones and amines to alkenes initiated by manganese triacetate or oxidative systems based on it have been comprehensively studied,243, 244 their extensive use is held up, most of all, by the need to introduce relatively large amounts of initiators and by problems associated with their regeneration.In the last decade, a strategy has been developed according to which this type of electrochemical reaction is carried out in the presence of a catalytic amount (0.03 ± 0.2 equiv.) of Mn(OAc)2 , acting as a mediator and the source of electrogenerated manganese triacetate. Examples of the addition of CH acids to alkenes initiated by Mn(OAc)2-based oxidative systems are listed in Table 10. Using the Mn(OAc)2 ±Cu(OAc)2 mediator system, g-lactones have been obtained in acceptable yields from acetic acid and styrenes,246, 250 and alk-2-enyl- and cycloalk-2-enyl-substituted cyanoacetic esters were synthesised from ethyl cyanoacetate, alkenes and cycloalkenes.246, 247 In these reactions the carboxy- methyl and (ethoxycarbonyl)cyanomethyl radicals together with their adducts with alkenes are formed as intermediates; the former species result from single-electron oxidation of acetic acid and ethyl cyanoacetate by electrogenerated Mn(OAc)3 and the latter arise upon the addition of the primary radicals to alkenes.These secondary intermediates (A and B) are further oxidised by regenerable Cu(II) ions, which react with radical adducts A and B by different mechanisms, namely, single-electron transfer for A and oxidative b-deprotonation for B.246, 247 X R2 R1 R1 a CH C C CHR2 +CH2CO2H 7e,7H+ XO Ar Ar CH2CO2H A OElectrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis Table 10. Electrochemical addition of CH acids to alkenes initiated by manganese diacetate-based oxidative systems.Alkene CH acid R1 XCH2CO2Et (X=CN, CO2Et) (CH2)3C CHR3 XCH2CO2Et (X=CN, CO2Et) R1CH=CHR2 NCCH2CO2Et R1CH2C CH2 AcCH2CO2Et R2 R1CH=CHR2 PhCH2CH(CO2Et)2 PhCOCH2NO2 R1CH=CH2 BunCH=CH2 NCOCH2NO2 BunCH=CH2 Me PhCOCHNO2 Note. Method A: ACD, graphite anode, 20 ± 30 mA cm72, 2 F mol71, AcOH ± AcOEt (13 : 3), 0.03 ± 0.25 equiv. of AcONa, Mn(OAc)2 .4H2O, 60 ± 70 8C; method B: ACD, graphite anode, 0.2 ± 0.5 mA cm72, 2 F mol71, 0.1 equiv. of AcOH, Bu4NBF4 , Mn(OAc)2 .4 H2O, 60 8C. a Succinimido. R2 R1 O Ar O (58% ± 84%) R1=H, Me; R2=H, Ph, CO2Et, CH2OH; Ar=Ph, 4-ClC6H4 , 4-MeC6H4 . CN a R1CH CHR2+ HC CO2Et CN R1 CO2Et 7e,7H+ B R2 R1=Me(CH2)n (n=2, 4, 6), R2=H; R17R2=(CH2)n (n=2 ± 5, 9); (a) ACD, graphite anode, 10 mA cm72, 2 F mol71, AcOH7Ac2O (2 : 1), AcONa, 0.065 equiv.Mn(OAc)2 .4H2O, 0.025 equiv. Cu(OAc)2 .H2O, 95 ± 97 8C. The binary mediator system, Mn(OAc)2±NCCH2CO2Et, proved to be a good initiator of free-radical chain addition of polyhalomethanes 251 (bromomalonic esters and methyl bromoa- cetate) to alkenes.251, 252 Method R3 R2 R1 R2 AAAAA HHHHMe HHHMe H HMe FHH7 7 7 A Alk A H 7 Alk A H, Me 7 Alk CO2Et Ph AcO see a 77777 AAAAA HHPh HH Bun AcOCH2 7 7 B 7 7 B 7 7 7 B 7 7 7 B Br CY2 X CN R1 CO2Et R2 (44% ± 58%) X=Y=Br, Cl; X=Br, Y =F; A=O, NAc; (a) ACU, graphite anode, 0.06 ± 0.6 F mol71, AcOH, AcOK, 0.05 equiv.Mn(OAc)2, 0.1 equiv. NCCH2CO2Me, 40 8C. Certain progress has taken shape in the field of practical implementation of indirect electrooxidation with manganese diacetate as a mediator. For instance, electrosynthesis of a precursor of sorbic acid (157), namely, g-vinyl-g-butyrolactone (158), from butadiene and acetic acid, which allows preparation of kilograms of this compound, has been developed.253 Reaction productCO2Et R3 R2 R1 X X CO2Et HH CN R1(CH2)nCHCHCO2Et R2 n=2±5, 10 CO2Et Me R2 R1CH2 OCO2Et CO2Et R2 R1 R1 PhCO N O + 7OO Bun NC N+O 7O Me PhCOCCH2CH2Bun NO2 R R(R=Alk) Br A a X=Y=Cl, Br 569 Ref.Yield (%) 245 245 245 245 245 70 ± 79 58 ± 61 53 ± 60 60 ± 73 37 ± 39 246 76 ± 78 246, 247 50 ± 64 246 80 ± 86 249 249 249 249 249 86 40 75 24 55 248 248 59 37 248 48 248 54 CY2 X Br CXY2 A (60% ± 72%) CCl3 (83%)570 a OAcO 158 (94% ± 96%) (a) ACU, AcOH ±Ac2O (1 : 1), AcONa, 0.1 equiv. Mn(OAc)2 .4H2O, 0.037 equiv. Cu(OAc)2 .H2O, 95 ± 125 8C. b. Reactions induced by electrooxidation of palladium, ruthenium, osmium and silver compounds The chemical prototypes of these reactions such as the Wacker process of oxidation of a-alkenes to methyl ketones,254 ± 256 ethyl- ene to vinyl acetate,257 hydroalkoxycarbonylation 258 ± 260 and epoxidation of alkenes,261 suffer from a number of drawbacks including the low rate of oxidation of alkenes with internal double bonds, the explosive risk from the gaseous mixtures of alkenes with oxygen, fast corrosion of the equipment induced by corrosive reaction media, etc.Numerous publications have been devoted to the search for new procedures for performing the above-listed reactions.262 ± 277 Methods of indirect anodic oxidation of alkenes using redox systems consisting of Pd(OAc)2 and an organic mediator (hydroquinone, benzoquinone, their analogues or tri- arylamines) have been developed to date.263 ± 269 The mediator acts as an electrogenerated oxidant with respect to Pd(0), which results from the reaction of alkene with Pd(II). Table 11. Indirect anodic oxidation of alkenes with mediator systems containing catalytic amounts (0.01 ± 0.1 equiv.) of Pd(OAc)2 or PdCl2 .Alkene Dec-1-ene (E)-Oct-2-ene (Z)-Hept-2-ene PhCH=CH2 Cyclohexene Cycloheptene Cyclohexylethylene OCO2R R=Me, Et OMe Ph OCO2Me Dec-1-en-3-ol Note. PCD with alternation of electrode polarity (graphite, Pt, Ti ±MnO2), MeCN±H2O, DMF±H2O, MeOH or AcOH; BQ is benzoquinone, CDA is copper diacetate, TBA is tri(4-bromophenyl)amine,HQis hydroquinone. a Bubbling ofCOduring the electrolysis; b ratio of esters of linear and branched acids. a CO2K AcO CO2K + H+ CO2H Me O 157 Co-mediator (equiv.) Electrolyte BQ (0.2) CDA (0.1) see a BQ (0.2) BQ (0.2)a BQ (0.2) CDA (0.1) see a BQ (0.2) CDA (0.1) see a CDA (0.1) CDA (0.1) Bu4NBF4 ± HClO4 NaClO4 ±AcONa LiCl Bu4NBF4 ± HClO4 LiCl Bu4NBF4 ± HClO4 NaClO4 ±AcONa LiCl Bu4NBF4 ± HClO4 NaClO4 ±AcONa LiCl NaClO4 ±AcONa NaClO4 ±AcONa BQ (1.1) NaClO4 ±AcONa TBA (0.05) Et4NOTs TBA (0.05) Et4NOTs TBA (0.05) Et4NOTs HQ LiClO4 ± AcOLi H2O Pd(II) Med 7e Pd(0) Medox H+ Med is hydroquinone or (4-BrC6H4)3N; MedOx is benzoquinone or (4-BrC6H4)3Ná .This procedure has been used to perform electrooxidative transformation of alkenes into ketones 263 ± 266 or into vinyl or allyl acetates.267, 269 Electrooxidation of alkenes in alcohols in the presence of CO yields esters of alkanoic acids (Table 11).268 CH C CH2+AcOH 7e CH C CHOAc +AcO C C +CO+MeOH 7e Ruthenium and silver polypyridyl complexes, the ruthenium heteropolyanion [SiRu(H2O)W11O39]57 and potassium osmate have also shown themselves as effective mediators in indirect anodic oxidation of alkenes.Thus the complex [Ag(bipy)2]+ (bipy is 2,20-bipyridine) catalyses epoxidation of alkenes.270 Reaction product (isomer ratio) decan-2-one 2- and 3-acetoxydec-1-enes (2 : 3) methyl undecanoates (1 : 3)b octan-2,3,4-ones (6 : 3 : 1) methyl octanoates (1 : 3)b acetophenone a-acetoxystyrene methyl phenylpropionates (1 : 3)b cyclohexanone 2- and 3-acetoxycyclohexenes (1 : 1) methyl cyclohexanecarboxylate 2- and 3-acetoxycycloheptenes (1 : 13) 1-acetoxy-1-cyclohexylethylene OCO2R COMe O MeO Ph Me OCO2Me COMe 3-hydroxydecan-2-one OAc AcO Yu N Ogibin, G I Nikishin R O Me R C C CH2 H C C CO2Me Ref.Yield (%) 264 269 268 264 268 264 269 268 264 269 268 269 269 92 91 73 96 50 79 35 75 100 29 50 72 33 265 87 266 89 266 60 266 41 267 7Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis R2 LAgO+ L72 H+ R1 R3 a R2 O L2Ag+ H2O R3 R1 R2 R3 R1 Oxirane yield (Current yield) (%) H H Bun 7(30) H Me CO2Bui 87 (21) n-C5H11 80 (20) H CO2Me MeO2CCH2 H CO2Me 90 (20) L=bipy; (a) ACU, Pt or graphite anode, MeCN±H2O (19 : 1), LiClO4 , 0.0125 ± 0.05 equiv. AgOAc, 0.025 ± 0.1 equiv. L. Oxidation of allylic and benzylic C7H bonds and rupture of double bonds in alkenes are catalysed by the complex [Ru(tripy)- (bipy)(H2O)]2+ (tripy is 2,20,200-terpyridyl).271 ± 273 O O R=CH2CH CH2 R O (65%) O a CHO O O R=CH CHMe (49%) O (a) PCD, Pt anode, 0.8 V, 3.9 F mol71, ButOH±H2O (1 : 1), 0.065 equiv. [Ru(tripy)(bipy)(H2O)]2+, pH 6.8 (phosphate buffer), 30 8C.Rupture of double bonds in alkenes also takes place in the presence of SiRu(H2O) .W11O5¡ 39 ± NaIO3 274 andK2OsO2(OH)4 ± HIO4 275 mediator systems. CHO R3 a R1 R1 R2 R2 The yield of aldehyde (%) R3 R1 R2 Ph H H 79 Me MeO H 59 H MeO OCH2O 4-MeOC6H4 Me 38 70 (a) ACD, PbO2 anode, 8 mA cm72, 3.5 ± 5.5 F mol71, CH2Cl2±H2O, 0.0034 ± 0.006 equiv. SiRu(H2O)W11O5¡ 39 , 2 equiv. NaIO3, 50 8C. The K2OsO2(OH)4 ± [K3Fe(CN)6] system catalyses alkene dihydroxylation and the subsequent cleavage of diols giving rise to ketones (total yields 62%± 91%).276, 277R3 R1 R3 R1 a O+ O C C C C R4 R2 R4 R2 R1, R3=H, Me, Ph; R2=H, Me; R4=Ph, (CH2)2OCOPh, 3,4-(OCH2O)C6H3; (a) ACU, PbO2 anode, Pt cathode, 5.6 mA cm72, 5 F mol71, MeCN±H2O (10 : 3), Et4NOTs, 0.04 equiv.K2OsO2(OH)4 , 0.5 equiv. HIO4. The electrochemical asymmetric dihydroxylation of alkenes in the presence of the K2OsO2(OH)4± [K3 Fe(CN)6] system and the Sharpless ligand is markedly superior to the corresponding chemical process in enantioselectivity and in yields of target products.278 The main advantage of the electrochemical variant 571 is that it implies the use of catalytic amounts of the osmate (0.002 equiv.) and the electrically regenerable oxidant, [K3Fe(CN)6] (0.1 equiv.).In the classical version, at least a stoichiometric amount of the oxidant is required. The electro- chemical dihydroxylation of styrene, 2-methyl-1-phenylpropene, trans-stilbene and 1,2-dihydronaphthalene results in the corre- sponding chiral diols; the degree of conversion is 90%± 97% and the enantiomeric yield is 90%± 100%.276, 277 H R H OH H O a O R R HOCH2 Os H O O L* L* is the Sharpless ligand. R Configuration (ee) The yield of diol (%) 95 Ph R(7) (97.3) 71.2 Bun R(+) (84.1) n-C8H17 69 2-Naphthyl 87 R(+) (91.8) R(7) (86.3) (a) ACU, Pt anode, ButOH±H2O (1 : 1), 0.1 equiv. [K3Fe(CN)6], 0.002 equiv. K2OsO2(OH)4, 0.01 equiv.L*, 0 ± 20 8C. 12. Indirect electrochemical rupture of double bonds in alkenes Indirect electrochemical rupture of double bonds in alkenes takes place with assistance of mediators and mediator systems such as manganese sulfate,279 methanesulfonate and cerium sulfate,280, 281 triarylamines,80 RuSi(H2O)W11O5¡ 39 ± NaIO3 ,274 K2OsO2(OH)4 ± HIO4 ,275 PhSSPh ± NaBr 80 and CuCl2±O2 .282 Two procedures have been developed: a single-stage procedure in which the electric generation of the oxidised form of the mediator (Medox) and the substrate oxidation by this species take place simultaneously in an electrolytic cell and a two-stage procedure in which the electric generation of Medox takes place in the cell, while the alkene oxidation by Medox occurs in a separate reaction vessel.These methods have been used to cleave the double bonds in 3,7-di- methyloct-6-enyl benzoate (159) in order to prepare 4-methyl-6- benzoyloxyhexanal (160);275 and in styrenes (161a ± d),276, 280, 282 trans-stilbene (161e),274, 275 anethole (162a) 275 and isosafrole (162b) 80, 271, 275, 276, 279 in order to prepare aromatic carbonyl compounds (163a ± d and 164a,b). The conditions of synthesis and the product yields are summarised in Table 12. O Med OCOPh 7e OCOPh 159 160 (72%) Ph Ph O C C +R2CHO CH R2 Med 7e R1 R1 163a ± e 161a ± e R1=R2=H(a); R1=Me, R2= H (b); R1=Ph, R2=H(c); R1=H,R2=Me (d); R1=H, R2=Ph (e). R1 Me R1 CHO+MeCHO Med 7e R2 R2 164a,b 162a,b R1=H,R2=MeO (a); R17R2=OCH2O (b).Electrooxidative cleavage of the double bonds in alkenes solves the same problems as chemical methods such as ozono- lysis,283 ± 289 oxidation with chromic 290, 291 or cerium reagents,292572 Table 12. Indirect electrooxidative cleavage of the C=C bond in alkenes. Alk- Mediator or mediator system (equiv.) ene 159 K2OsO2(OH)4 ±HIO4 (0.04 : 0.5) 161b K2OsO2(OH)4 ±HIO4 (0.04 : 0.5) 161c K2OsO2(OH)4 ±HIO4 (0.04 : 0.5) 161e K2OsO2(OH)4 ±HIO4 (0.04 : 0.5) 162b K2OsO2(OH)4 ±HIO4 (0.04 : 0.5) 161a (MeSO3)2Ce(OH)2(H2O) 161e SiRu(H2O)W11O5¡ 39 ± NaIO3 [(0.003 ± 0.006) : 0.5] 161a SiRu(H2O)W11O5¡ 39 ± NaIO3 [(0.003 ± 0.006) : 0.5] 162b SiRu(H2O)W11O5¡ 39 ± NaIO3 [(0.003 ± 0.006) : 0.5)] 162b MnO2 Ce(SO4)2 [Ru(tripy)(bipy)(H2O)]2+ (0.065) PhSSPh ± NaBr (0.37 : 0.2) 161d (4-BrC6H4)3N (0.041) 161a CuCl2 (0.05) ±O2 Note.Method A: ACD, PbO2 anode, 6.7 mA cm72, 2.5 ± 10 F mol71, MeCN±H2O (10 : 3), Et4NOTs, 20 8C; method B: ACD, PbO2 anode, 8 mA cm72, 3.5 ± 5.5 F mol71, dichloroethane ±H2O (1 : 1), 50 8C; met- hod C: PCD, Pt anode, 0.8 V, ButOH±H2O (4 : 1), phosphate buffer, 20 8C; method D: ACU, Pt anode, 8 mA cm72, 6.4 F mol71, MeCN±H2O (2 : 1), *20 8C; method E: ACU, graphite anode, 5 mA cm72, 3.5 F mol71, MeOH, LiClO4, 18 8C; method F: PCD, Pt anode, 1.0 V, MeCN, NaClO4 , 80 8C. a Substance yield (in parentheses, current yield). b Two-step cleavage; conditions of electric generation of Medox for (MeSO3)2Ce(OH)2(H2O) and Ce2(CO3)3: ACD, Pt anode, 200 mA cm72, 1.0 F mol71 , MeSO3H±H2O, 50 8C; for MnO2 and MnSO4 .4H2O: ACD, PbO2 anode, 200 mA cm72, 2.2 F mol71, 2 equiv.of H2SO4, 20 8C; for Ce(SO4)2 and Ce2(SO4)3: ACD, Pt anode, 200 mA cm72, 1.2 F mol71, H2SO4 , 20 8C. c PhCH(OMe)2 . d PhCHO. the RuO4 ±MeCHO±O2 system,293 etc.294, 295 The drawbacks peculiar to chemical methods have been largely overcome in the electrochemical method for the cleavage of double bonds in alkenes. 13. Reactions involving sacrificial metallic anodes Electrochemical alkylation or acylation of alkenes with organic halides and acyl halides giving rise to C7C bonds is carried out using sacrificial metallic (usually, aluminium, zinc and magne- sium) anodes. Their role is to suppress the formation of halogen molecules and thus to prevent halogenation of alkenes.The electrochemical formation of a C7C bond is an alternative to the reaction including deprotonation of organic CH acids induced by metals, organometallic compounds or strong bases and sub- sequent alkylation with the organic halide. In this case, the C7C bond formation is usually accompanied by side reactions, namely, O-alkylation, dialkylation or condensation of the substrate. In the processes with sacrificial anodes, the C7C bond is formed more selectively. For example, the reactions of activated alkenes with organic 1,1-, 1,3- and 1,4-dihalides,296 non-activated alkenes with acylating reagents,297, 298 and cyclisation of unsaturated bromo- acetals 299 afford compounds 165 ± 169 in high yields.Ref. Yield (%) a Meth- Pro- duct od 160 275 72 (29) A 163b 275 71 (40) A 163c 275 94 (42) A 163e 275 66 (33) A 164b 275 62 (99) A 163a 163e 280 274 89 79 (57) see b B 164a 274 59 (23) B 164b 274 70 (44) B 164b 164b 164b 279 281 271 54 83 49 see b see b C 164b 80 53 (70) D 80 282 35 (70) EF see c see d Yu N Ogibin, G I Nikishin X Al XCH Br CHY+ Br n 7e,7AlBr3 Y n 165 X=H, Ph; Y =Ph, CO2Me, Ac; n=1, 2. Al +AcCl or Ac2O 7e,7AlCl3 R n Ac R=H n 166 Ac Me Me R=Me + Ac n n 168 167 R=H, Me; n=1, 2. CAlk Br Alk Zn C 7e,7ZnBr2 O O O O 169 Besides preventing the generation of molecular bromine, a sacrificial aluminium anode participates (through Al3+ ions) in stabilisation of the type A dianions generated by cathodic reduc- tion of the activated alkene.Anode Cathode Br Br n 72e 7 7 XCH CHY A Al Al3+ 2e XCH CHY AlBr3 X n Y 165 The substituted cycloalkanes 165 are formed in reasonable yields (30% ± 60%) only from activated alkenes, namely, esters of maleic, fumaric, cinnamic and acrylic acid and 4-phenylbut-3-en- 2-one, upon reaction with 1,3- and 1,4-dihaloalkanes. The reac- tion proceeds with high stereoselectivity (isomer ratio trans : cis> 10 : 1). Cyclobutane derivatives are obtained by this method in very low yields, and only from 1,2-dichlorides. When 1,2-dibro- mides are used, electrochemical debromination takes place.In the reaction of dimethyl maleate with 1,3-dibromopropane, dimethyl 2-allylbutanedioate (yield 10%) and 1,2-cyclopentadicarboxylate (50%) are formed.296 In acylation of cycloalkenes with acetyl chloride, an alumi- nium anode not merely suppresses the generation of molecular chlorine but also catalyses the reaction, due to the influence of AlCl3 formed during the electrochemical process. The electro- chemical acylation occurs more selectively than the classical Friedel ± Crafts reaction.300 ± 304 Joint electrolysis of equimolar amounts of acetyl chloride and cyclohexene or cycloheptene in an undivided cell at 710 to 75 8C (3 F mol71) results in complete conversion of cycloalkenes into 1-acetylcycloalkenes 166 (yields 60%± 80%).297, 298 The transformation of 1-methyl-substituted cycloalkenes gives rise to two isomers, 1-acetyl-2-methylcycloal- kenes 167 and 3-acetyl-2-methylcycloalkenes 168, in a total yield of 65%± 85%; the compounds 168 markedly predominate.Elec-Electrochemical reactions of alkenes induced by anodic oxidation and their applications in organic synthesis trochemical acetylation of acyclic alk-1-enes involves only the terminal carbon atom but the yields are relatively low (22% ± 25%). The side products formed are a,b-dichlorides (1% ± 10%), which become the only reaction products when a carbon or platinum anode is used. CR Br C O O Cathode Anode ZnBr2 [Co(I)] 2e Zn 72e [Co(II)] Zn2+ Br7 [Co(III)] R CR SolvH C 7Solv O O O O 169 A The intramolecular electrochemical cyclisation of unsaturated bromoacetals to dioxabicycloalkanes 169 is an example in which the use of sacrificial metallic anodes is combined successfully with the catalysis of the cathodic single-electron reduction of organic halides with cobaloxime.The cathodic generation of Co(I) com- plexes, single-electron reduction of bromoacetal to radical A by these complexes, cyclisation of the radical and subsequent trans- formation into the final product (yields 70%± 77%) are the key steps of this process. Electrochemical reactions of alkenes with sacrificial metallic anodes provide good results in many cases where the chemical analogues either do not occur at all or are hardly suitable due to the low yields of the target products.This also eliminates the necessity of performing a labour-consuming and, in some cases, unsafe step of the synthesis of organometallic compounds. By using high current densities (up to 1 A dm72), one can produce substantial amounts of reaction products. IV. Conclusion Although electrochemical reactions are a powerful tool for trans- forming and assembling organic molecules, they are still used in most cases only by specialists and have not become a routine method of chemical synthesis. We hope that this review would be useful not only as a source of specially selected information but also as a manual for the practical use of electrochemical reactions for solving problems of organic synthesis. From this standpoint, reactions carried out in an electrolytic cell not separated into electrode compartments, which are simple in experimental imple- mentation and require much less electricity, are more advanta- geous.The review demonstrates the substantial progress achieved in recent years in the investigation of alkene reactions induced by anodic oxidation and in development, on this basis, of theoretical and procedural aspects of modern organic chemistry. First of all, this refers to indirect anodic oxidation of alkenes with assistance by inorganic and organic mediators and mediator systems. Study of electrochemical processes involving these systems made it possible not only to establish the regularities and propose mech- anisms of transformation of alkenes into functionally substituted products but also to elaborate simple methods which ensure high degrees of alkene conversion with minimum electricity consump- tion to give compounds the synthesis of which by chemical methods is either impossible or difficult from the experimental standpoint. Substantial progress has also been achieved in the study of electrochemical cyclisation, [4+2]-cycloaddition, phos- phorylation, rupture of double bonds, and reactions involving sacrificial anodes.Methods of electrochemical transformation of alkenes and other organic compounds under conditions of dia- 573 phragmless electrolysis (in which both anodically and cathodically induced processes are involved in the assembling of the final product) have been developed.Vivid examples of this type of transformation are cyclisation of carboxy-substituted arylalkenes to give 2-benzylcyclopentanones in the presence of a tertiary phosphine (Scheme 5) 225 and indirect electrochemical reduction of carboxylic acids to the corresponding alcohols with assistance by the same reagent.305 In our opinion, the scope of application of organic electrosynthesis should be extended. 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出版商:RSC    

年代:2001

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Russian Chemical Reviews 70 (7) 577 ± 606 (2001) Spectral properties of porphyrins and their precursors and derivatives N Zh Mamardashvili, O A Golubchikov Contents I. Introduction II. IR spectra of porphyrins and their precursors and derivatives III. 1H NMR spectra of porphyrins and their precursors IV. Electronic absorption spectra of porphyrins and metalloporphyrins Abstract. absorption electronic and NMR IR, of data The The data of IR, 1H NMR and electronic absorption spectroscopy and precursors their and porphyrins of spectroscopy of porphyrins and their precursors and derivatives derivatives are The generalised. and systematised analysed, are analysed, systematised and generalised. The bibliography bibliography includes references 105 includes 105 references.I. Introduction Problems of spectroscopy of porphyrins have been considered in detail in a number of papers and monographs (see, for example, Refs 1 ± 8). However, the last 10 ± 15 years have been marked by impressive progress in the synthetic chemistry of porphyrins resulting in the determination of the spectral characteristics of new heterocyclic compounds (porphyrins and their intermediates) with very complex structures. It should be noted that most studies dealt with the spectra of particular porphyrins and their precur- sors with the aim of establishing the structures of the newly synthesised compounds. However, the results of investigations by different spectral methods, which were performed under differ- ent conditions and using a limited number of compounds, cannot necessarily be compared.In order for the relationship between the structure and the spectral properties of complex multicontour chromophores (among which are, undoubtedly, porphyrins) to be revealed, comprehensive investigation of a diversity of compounds whose structures vary from simple to complex must be carried out. Pyrrole intermediates are of obvious interest because they can be used to follow changes in the spectral characteristics as the macrocycle undergoes chemical modifications. In the present review, we analyse, systematise and generalise the recent data of IR, 1H NMRand electronic absorption spectroscopy of porphyr- ins and their derivatives and precursors, viz., of mono-, di- and tetrapyrrole compounds.NZh Mamardashvili Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, 153045 Ivanovo, Russian Federation. Fax (7-093) 237 85 09. Tel. (7-093) 237 85 12. E-mail: ngm@ihnr.polytech.ivanovo.su O A Golubchikov Ivanovo State University of Chemistry and Technology, prosp. F Engelsa 7, 153460 Ivanovo, Russian Federation. Fax (7-093) 241 79 95. Tel. (7-093) 232 73 78 Received 29 March 2001 Uspekhi Khimii 70 (7) 656 ± 686 (2001); translated by T N Safonova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n07ABEH000661 577 577 585 595 II. IR spectra of porphyrins and their precursors and derivatives IR spectroscopy enables one not only to identify compounds, but also to follow changes in various characteristics of individual chemical bonds and the molecule as a whole that occur upon chemical modifications.For example, information on the relative stabilities of corresponding chemical bonds in the molecules can be obtained by comparing the force constants of these bonds, which can be calculated from the spectral data. Since all force constants cannot necessarily be determined from the spectral data, qualitative conclusions about the relative stabilities of the bonds are often made based on the experimental vibration frequencies observed in IR spectra. These frequencies are functions of the atomic weights and the force constants of the bonds. A compar- ison of the frequencies of analogous vibrations for different compounds can be performed only in the case of the characteristic vibrations. The shifts of characteristic vibration frequencies are directly related to the changes in the electronic states of the bonds and the fragments within which the vibrations under consider- ation are localised.Investigation of a large number of structurally similar mole- cules can be a help in refining the assignment of vibrations to particular fragments of the porphyrin ring. 1. Pyrroles The assignment of virtually all bands in the IR spectrum of pyrrole (1a) was made 9, 10 and this spectrum can be used in studies of pyrrole derivatives (Table 1). The NH stretching vibrations in the spectrum of solid pyrrole are observed in the region of *3500 cm71.In chloroform, this band is split into two compo- nents (at 3400 and 3497 cm71). The intensity of the band at 3497 cm71 increases upon dilution. Therefore, the band at 3400 cm71 corresponds to associated NH bonds, whereas the band at 3497 cm71 is assigned to the free bonds. According to the results of calculations carried out for the isolated pyrrole mole- cule, theNHstretching vibration band in its spectrum is located at 3491 cm71 (Table 2). Two pronounced bands at 3100 and 3130 cm71 are assigned to CH stretching vibrations of pyrrole. Calculations by the semi- empirical PM3 method demonstrated 11 (see Table 2) that the more intense band (at 3100 cm71) corresponds to vibrations of the C(a)7H bonds, whereas the less intense band corresponds to vibrations of the C(b)7H bonds.The frequencies in the region of 1350 ± 1600 cm71 belong to skeletal vibrations of pyrrole. In the spectrum, these vibrations are observed as an intense band at 1350 cm71 and a medium intensity band at 1530 cm71.N Zh Mamardashvili, O A Golubchikov 578 Table 1. Positions of the absorption maxima (cm71) in the IR spectra of the pyrroles 1a ± l (in KBr pellets).9, 10 Alkyl substituent Pyrrole fragment Compound 1 n(C7O) n(C=O) g(CH) d(CH) n(C7C) n(CH) n(NH) 1350, 1530 1350, 1508 1348, 1510 1346, 1515 1360, 1530 1338, 1514 1328, 1517 1316, 1514 1320, 1517 1384, 1540 1357, 1578 1352, 1564 3100, 3130 77777777773108 7 7 7 7 1667 1664 1680 1670 1690 1685 1702 1687 1686 1680 1637 1284 1290 1290 1280 1287 1273 1255 1268 1265 1295 1290 1004 1022 1018 1010 1015 1090 1065 1116 1088 1000 994 1380 1379 1386 1380 1380 1377 1378 1384 1370 1382 1378 a a bcdefghijkl 3495 3458 3456 3458 3450 3451 3450 3453 3452 3466 3450 3454 a According to the published data.2 Examinations 9±12 of the spectra of substituted pyrroles 1b ± l demonstrated that chemical modifications of the molecule lead to essential spectral changes.Thus, the spectrum of 2-ethoxycar- bonyl-3,4,5-trimethylpyrrole (1b) shows CH deformation vibra- tions of the alkyl groups. R2 R3R4 R1 NH The skeletal vibration bands are shifted to the lower-fre- quency region and are more intense than the corresponding bands in the spectrum of unsubstituted pyrrole.Apparently, a rise in the intensities of these bands can be accounted for by the effect of the electron-withdrawing ester group. 1a ± l R4 R3 R2 R1 Compound 1 Me Me Me Me Me CO2Et CO2Et Me Et Bun Bun Et Et Et CO2H EtEt CO2Et CO2Et CO2Et CO2Bn CO2Bn CO2Bn CO2H CO2 Et I CO2Et n CO2Et Me H HMe Me Me This assumption is supported by the following fact. In the spectrum of 2-benzyloxycarbonyl-5-ethoxycarbonyl-4-ethyl-3- methylpyrrole (1g), the vibration bands of the pyrrole ring are shifted further to the low-frequency region with a simultaneous increase in their intensities.Of the molecules under consideration, the largest low-frequency shifts of skeletal vibrations are observed for the a-carboxypyrroles 1h,i. Noteworthy are also the low- frequency shifts of NH vibrations of these pyrroles. The spectrum of 4-butyl-2-ethoxycarbonyl-3-methylpyrrole (1l) has an intense band at 3108 cm71 corresponding to CH stretching vibrations of the pyrrole ring. Investigations demonstrated that this band corresponds to vibrations of the C(a)7H bond of pyrrole (the b positions are occupied by substituents). The spectral character- istics of the positional isomers 1h,i and 1j,k differ only slightly from each other. An increase in the size of the alkyl substituent (R) in 4-R-2-ethoxycarbonyl-3,5-dimethylpyrroles 1b ± d (R = Me, Et or Bun) is accompanied primarily by an increase in the intensity of the CH vibration bands of the alkyl groups. It should be noted H H H H Me Me Me Me Me Me Me Me Me I Et Me CO2 Et H Bu Me H Me Me H HMe H Me Et Me H Me Et CO2 Et that the C=O, C7O, C7OH and C7I vibrations are character- istic.The bands belonging to these vibrations are present in the spectra of all compounds of the series 1b ± l and allow one to CHO CHO Me Et Et Me Me Me abcdefghijklmnopqrsTable 2. Vibration frequencies (cm71) in the IR spectra of the pyrroles 1a ± l calculated by the PM3 method.11 Alkyl substituent Pyrrole fragment Compound 1 n(C7O) g(CH) d(CH) n(C7C) n(CH) n(NH) 1355, 1530 1350, 1512 1356, 1516 1348, 1510 1364, 1528 1340, 1516 1325, 1515 1320, 1510 1316, 1519 1386, 1544 1350, 1576 1354, 1564 3100, 3133 77777777773112 3491 3453 3459 3462 3447 3453 3450 3450 3448 3466 3454 3456 71289 1293 1296 1284 1280 1278 1252 1270 1266 1290 1284 7 71007 1027 1021 1012 1011 1094 1069 1119 1086 1002 994 1384, 1390 1386, 1399 1382, 1394 1380, 1392 1380, 1391 1379, 1395 1376, 1397 1380, 1391 1373, 1389 1376, 1382 1371, 1384 abcdefghijklSpectral properties of porphyrins and their precursors and derivatives identify the pyrrole synthesised and to establish its structure and purity.2. Dipyrrolylmethanes Compound 2 R1 The IR spectra of dipyrrolylmethanes 2a ± x were measured and examined. 12 ± 14 The structures of these compounds have been previously investigated by calculation methods (theMM+molec- ular mechanics method and the semiempirical PM3 method). The calculated energy parameters and geometric characteristics corre- sponding to the energy minima are summarised in the studies.15, 16 The energy parameters of the dipyrrolylmethane molecule are substantially affected by the substituents at positions 5 and 50, while the effects of the alkyl substituents at positions 3, 30, 4 and 40 are insignificant. As mentioned above, the NH stretching vibrations of pyrrole are observed at 3495 cm71.On going to dipyrrolylmethane (2x), the NH vibration band is shifted to the low-frequency region by *40 cm71. Introduction of the phenyl substituent at the meso- position of dipyrrolylmethane (the compound 2w) leads to a further short-wavelength shift (by 15 cm71) of this band (Table 3). Below are given the charge distributions on the atoms of the dipyrrolylmethane molecules, which show the influence of the electronic factors upon chemical modifications of the mole- cule. abcdefghijklmnopqrstuvwx Thus, the presence of electron-withdrawing substituents (for example, the carboxy group) at positions 5 and 50 of dipyrrolyl- methane leads to a decrease in the electron density on the nitrogen atoms (the compound 2b), which is accompanied by lowering of the n(NH) frequency by*40 cm71 (Tables 3 and 4).The presence of the electron-donating methoxy groups in the meso substituents of the carboxy derivatives 2n,p,r causes a high-frequency shift (by *15 cm71) of the NH vibration band compared to that in the spectrum of the carboxy derivative 2b. The larger the electron density on the nitrogen atom, the higher the vibration frequency. Charges on atoms (arb.u.) Compound C(5) C(4) C(3) C(2) N(1) The intense band at 3100 cm71 was assigned 12 to CH stretching vibrations of pyrrole [C(a)7H stretching vibrations]. The formation of the dipyrrole structure is accompanied by the high-frequency shift of this band by*20 cm71 (see Table 3). The introduction of the aryl substituent at the meso position of dipyrrolylmethane has virtually no effect on CH vibrations of the pyrrole rings.The spectral characteristics of the positional isomers 2h,i are very similar (see Table 4). 1a 2a 2b 2f 2w 0.337 0.368 0.425 0.339 0.387 70.309 70.331 70.358 70.312 70.311 70.164 70.101 70.068 70.133 70.164 70.162 70.159 70.184 70.133 70.164 70.315 70.296 70.279 70.312 70.305 The intense band at 1350 cm71 and the medium intensity band at 1530 cm71 belong to skeletal vibrations of pyrrole. In the dipyrrolylmethane 2w, the C7C vibration frequency of the pyrrole fragment is shifted to the low-frequency region by *5 cm71. Alkylation of the pyrrole rings and the replacement Table 3.Positions of the absorption maxima (cm71) in the IR spectra of the pyrrole 1a and the dipyrrolylmethanes 2c ± g,m,o,q,s,u,w,x.12 ± 14 Compound n(CH) n(NH) B A B A 1a 2c 2d 2e 2f 2g 2m 2o 2q 2s 2u 2w 2x 3100, 3130 3120 3125 3120 3125 3120 3120 3120 3122 3124 3128 3129, 3170 3122, 3160 3100, 3133 3124 3122 3123 3122 3123 3121 3121 3120 3122 3124 3124, 3166 3122, 3160 3495 3452 3432 3444 3430 3440 3430 3430 3421 3442 3440 3440 3455 3491 3448 3436 3446 3433 3434 3428 3432 3426 3448 3442 3438 3453 Note. Here and in Tables 4 and 5, the calculated and experimental (in KBr pellets) spectra are denoted by A and B, respectively.a Stretching vibrations of the pyrrole fragment; b stretching vibrations of the methine bridge. 579 R3 H R4 R3 R2 R2 NH HN R1 R1 2a ± x R4 R3 R2 n n n nnnnnnnnnn CO2Et Me Me H CO2H Bu Me H CO2Et Et Me H CO2Et Bun Me H H Me Me H CO2Et H Me H H Me Bu H CO2H Me BunPh CO2H Bu Me Ph COH Bun Me Ph CO2Et Bun Me Ph CO2Et Bun Me Ph H Me Bu 4-MeOC6H4 CO2H Me Bu 4-MeOC6H4 H Me Bu 3-MeOC6H4 CO2H Me Bu 3-MeOC6H4 H Me Bu 2-MeOC6H4 CO2H Me Bu 2-MeOC6H4 H Me Bu 4-NO2C6H4 CO2H Me Bu 4-NO2C6H4 H Me Bu 3-NO2C6H4 CO2H Me Bu 3-NO2C6H4 H H H Ph H H H H n(C7C) b n(C7C) a A B B A 1350, 1530 1335, 1520 1330, 1525 1335, 1520 1330, 1520 1330, 1530 1330, 1530 1325, 1540 1330, 1525 1330, 1527 1330, 1530 1350, 1540 1330, 1530 1355, 1530 1335, 1517 1333, 1514 1330, 1507 1325, 1510 1325, 1512 1328, 1518 1324, 1520 1325, 1515 1326, 1521 1328, 1518 1346, 1525 1345, 1524 7 71370 1370 1375 1380 1380 1360 1360 1365 1368 1360 1370 1360 1364 1362 1364 1360 1360 1358 1358 1360 1362 1360 1362 1359580 Table 4. Positions of the absorption maxima (cm71) in the IR spectra of the dipyrrolylmethanes 2b,h ± l,n,p,r,t,v.15, 16 Compound 2 n(C7C) a n(NH) B A A B 1310, 1520 1318, 1512 1320, 1522 1323, 1517 1322, 1520 1326, 1525 1330, 1515 1324, 1522 1320, 1524 1324, 1618 1319, 1520 3412 3410 3427 3435 3440 3445 3427 3429 3427 3434 3438 3416 3418 3421 3438 3444 3452 3430 3432 3433 3429 3433 bhijklnprtv 1315, 1520 1321, 1510 1326, 1510 1332, 1510 1330, 1510 1330, 1514 1335, 1520 1330, 1530 1324, 1520 1329, 1522 1324, 1530 a Stretching vibrations of the pyrrole fragment; b stretching vibrations of the methine bridge.positions 5 and 15 are equally probable. The corresponding conformers shown in Fig. 1 a,b have equal energies of steric strain (365 kJ mol71). The meso-phenyl or meso-aryl substituents in the porphyrins 3h ± s are located perpendicular to the plane of the porphyrin macrocycle (Fig. 1 c,d).22 R2 of one of the meso-hydrogen atoms by the aryl group are accompanied by an increase in this shift (see Tables 3 and 4).The presence of electron-withdrawing substituents at positions 5 and 50 of dipyrrolylmethane leads to lowering of the n(C7C) frequency of the pyrrole fragment compared to that in the spectrum of unsubstituted dipyrrolylmethane. Calculations of the spectra of dipyrrolylmethanes demon- strated that the intense band in the region of 1340 ± 1365 cm71 corresponds to C7C vibrations of the methane bridge. The presence of the electron-withdrawing nitro groups in the aryl meso-substituents leads to a substantial increase in the intensity of this band. Compound 3 On going from pyrroles to dipyrrolylmethanes, the C=O, C7O and C7OH vibrations remain characteristic and allow the identification of the dipyrrolylmethane synthesised.It can be seen from the data presented in Tables 1 ± 4 that the IR spectra of pyrroles differ substantially from those of dipyrrolylmethanes. The involvement of pyrrole in the open conjugated system of hexamethyldipyrrolylmethene as well as in the cyclic octamethyl- porphyrinogen system 7 is accompanied by further complications of the spectra. However, the most pronounced effect is observed when pyrrole is involved in the conjugated porphyrin macrocycle. 3. Porphyrins The effects caused by an increase of the molecular symmetry (for example, upon formation of metal complexes) or its lowering (for example, in the case of introduction of substituents in the macro- cycle) are manifested in the IR spectra of porphyrins along with the changes in stretching vibration frequencies.When studying these effects, it is useful to consider two series of compounds: (1) porphyrins containing the hydrogen atoms at the meso positions of the macrocycle and substituents at the b positions of the pyrrole fragments and (2) porphyrins containing substituents at the meso positions and the hydrogen atoms at the b positions of the pyrrole fragments. The IR spectra of b-alkylated porphyrins 3a ±w in which two and four meso-hydrogen atoms are successively replaced by the alkyl (CH37C6H13 or C11H23), phenyl or aryl (C6H4X, X=H, 4-OMe, 3-OMe, 2-OMe, 4-NO2 or 3-NO2) substituents were investigated.14, 17 ± 21 abcdefghijklmnopqrstuvw The out-of-plane CH deformation vibrations of porphyrins are observed in the region of 830 ± 850 cm71.Among these are the CH vibrations of the pyrrole rings and methine bridges of the macrocycle. The g(CH) vibration frequency of the pyrrole frag- ments of porphyrins increases compared to the corresponsing frequency in the spectrum of pyrrole as the interactions between The geometric and energy parameters of the porphyrin molecules corresponding to the energy minima are summarised in the studies.22, 23 The experimental vibration frequencies of the porphyrins 3a ± w and those calculated by the PM3 method are given in Table 5. The meso-alkyl substituents in the porphyrins 3a ± g can adopt various conformations. However, calculations by the semiempirical PM3 method demonstrated 24 that the extended zigzag chain typical of n-alkanes in the crystalline state is a preferential conformation of the alkyl substituents.The cisoid and transoid relative arrangements of the alkyl substituents at N Zh Mamardashvili, O A Golubchikov n(C=O) n(C7C) b A B B1675 1670 1685 1670 1675 1680 1685 1680 1690 1675 1678 1360 1358 1357 1355 1360 1360 1366 1365 1350 1342 1340 1354 1350 1354 1350 1350 1356 1360 1362 1356 1340 1342 R4 R3 Me Me NH N R5 R1 HN N Me Me R6 R8 R7 3a ±w R3, R7 R2, R4, R6, R8 R1, R5 nnnnnnn Ph Ph 4-OMeC6H4 Me Me H Et Me H Prn Me H Bun Me H C5H11 Me H C6H13 Me H C11H23 Me H Bun Ph H 4-OMeC6H4 Bun H 3-OMeC6H4 Bun H 2-OMeC6H4 Bun H 4-NO2C6H4 Bun H 3-NO2C6H4 Bun H H Bu Ph H Bu 4-OMeC6H4 H Bu 3-OMeC6H4 H Bu 2-OMeC6H4 H Bu 4-NO2C6H4 H Bu 3-NO2C6H4 H Bu H Et Me C6H13 Me C6H13 MeSpectral properties of porphyrins and their precursors and derivatives Table 5.The NH and CH vibration frequencies (cm71) of the porphyrin ring in the b-octaalkylporphyrins 3a ±w.14, 17, 24 d(CH) b n(CH) a g(NH) d(NH) n(NH) Com- pound 3 A B A B A B A B A B A B abcdefghijklmnopqrstuvwa Stretching vibrations of the alkyl groups; b deformation vibrations of the alkyl groups; c out-of-plane deformation vibrations of the methine bridges. 1502 1497 1491 1484 1486 1489 1486 1488 1490 1487 1486 1500 1485 1489 1500 1485 1500 1500 1486 1484 1500 1496 1492 1507 1487 1483 1479 1480 1478 1477 1485 1484 1478 1481 1494 1479 1490 1490 1479 1492 1493 1479 1476 1492 1484 1480 3027 3016 3008 2940 2920 2900 2880 2960 3000 2980 2962 3030 2990 2990 3010 2940 2920 3020 2995 2980 3060 3048 3035 3022 3011 3002 2942 2926 2907 2906 2960 3006 2984 2970 3025 2994 2884 2986 3012 2942 2931 3014 2994 3007 3028 3020 695 695 700 700 705 705 710 700 709 702 700 710 705 690 706 690 690 710 704 702 710 705 700 708 707 710 712 714 715 721 720 714 717 710 715 719 700 718 704 705 721 731 714 728 730 728 960 963 964 970 978 984 980 980 990 965 960 985 980 960 986 978 970 990 980 984 974 982 990 980 974 986 992 1001 991 1003 992 1007 974 970 994 996 997 1005 994 981 1002 994 990 1010 1013 1017 3346 3342 3340 3345 3346 3348 3350 3320 3315 3318 3320 3310 3320 3327 3315 3320 3324 3310 3324 3335 3325 3328 3330 3348 3343 3342 3346 3348 3347 3352 3323 3321 3323 3332 3325 3332 3327 3318 3328 3330 3319 3336 3338 3342 3340 3347 the C(b)7C(b) bonds and the p-system of the macrocycle are weakened due to the introduction of alkyl or aryl substituents.In the spectra of b-unsubstituted porphyrins (meso-tetraalkyl- and -tetraphenylporphyrins), an intense band in the region of 772 ± 805 cm71 corresponds to g(CH) vibrations. This band is absent in the spectra of octamethyl-, octaethyl-, dodecaalkyl- and dodecaphenylporphyrins.25 ± 27 The g(CH) vibration frequency depends substantially on the presence of substituents at the meso positions of the macrocycle. Thus, the bands at 772 and 780 cm71 correspond to these vibrations in the spectra of porphyrin and meso-tetra(iso-butyl)porphyrin, respectively. This effect is more pronounced in the spectrum of tetraphenylporphyrin in which the g(CH) vibration band is observed at 797 cm71.25 In the IR spectrum of 2,8,12,18-tetrabutyl-3,7,13,17-tetrame- thylporphyrin (3t), out-of-plane C(b)7H deformation vibration bands are absent, whereas an intense band at 835 cm71 belongs to g(CH) vibrations of the meso-hydrogen atoms (see Table 5).This band is absent in the spectra of the 5,15-dialkyl-10,20-diphenyl- porphyrins 3u ± w, whereas it is observed in the region of *830 ± 850 cm71 in the spectra of the 5,15-dialkyl- and 5,15- diaryl-b-octaalkylporphyrins 3a ±m. b a NH NH N N HN HN N N d c OMe OMe OMe NH NH N N HN N N HN Figure 1. Cisoid (a) and transoid (b) conformers of 5,15-di-n-heptyl-2,3,7,8,12,13,17,18-octamethylporphyrin and the atropoisomers [a,a (c) and a,b (d)] of 2,8,12,18-tetrabutyl-5,15-di(2-methoxyphenyl)-3,7,13,17-tetramethylporphyrin.22 581 g(CH) c 830 825 829 835 831 832 833 840 845 840 839 850 845 830 845 830 849 842 839 835 832 832 834 836 836 837 838 842 849 845 844 853 848 840 850 840 852 846 844 844 7 7 7 7 7 7 MeO582 Table 6.The CH vibration frequencies (cm71) of the phenyl groups in the diphenyl(aryl)octaalkylporphyrins 3h ±m,o ± w (in KBr pellets).4, 18 Compound 3 g(C(3)7H) g(C(2)7H) g(C(1)7H) 1170, 1050 7 7 7 1174, 1065 780 840 7 7 7 1164, 1060 1179, 1050 7 7 7 1185, 1069 7857 7 7 1180, 1068 782 846 7844 7 7 7 1170, 1067 1180, 1055 7 7 7 1184, 1073 787 848 7 7 7 7 7 745 78707 7 7 750 7890 78747 7 7 754 78967 7 7 7 747 7 7 7 7 750 7 7 7 1170, 1068 1172, 1060 1178, 1050 7 7 7 1174, 1075 hijklmopqrsuvw 840 Since the out-of-plane deformation vibrations of the adjacent CH bonds of the phenyl fragments, which occur in phase, strongly interact with each other, the positions of the corresponding bands in the spectra depend on the number of the above-mentioned bonds.These bands are intense and can be used for the determi- nation of the position of the substituent in the phenyl fragment (Table 6). The spectra of the 5,15-diphenylporphyrins 3h,n have an intense band at 745 cm71 due to interactions between five adjacent CH bonds (for example, in monosubstituted benzene).25 The spectra of the di(meta-R-phenyl)porphyrins 3j,m,p,s have bands in the regions of 780 ± 787 cm71 (three adjacent CH bonds) and 870 ± 896 cm71 (one CH bond).The spectra of the di(ortho- R-phenyl)porphyrins 3k,q have a band at *750 cm71 (four adjacent CH bonds), whereas the spectra of the di(para-R- phenyl)porphyrins 3i,l,o,r have a band in the region of 840 ± 848 cm71 (two adjacent CH bonds) (see Table 6). The identification of in-plane CH deformation vibrations presents a more complicated problem. The bands in the region of 964 ± 1060 cm71 are generally assigned 28 to d(CH) vibrations of the pyrrole fragments of porphyrins. These bands are observed as two narrow closely spaced intense bands. The spectra of b-octaal- kylporphyrins have no intense bands in this region.The medium intensity bands in the region of 1484 ± 1500 cm71 belong to d(CH) vibrations of alkyl groups. Their positions remain virtually unchanged as the length of the alkyl substituent increases (see Table 5). The in-plane CH deformation vibrations of the phenyl fragments are observed in the region of 1000 ± 1300 cm71.25 In the spectra of meso-diphenyloctaalkylporphyrins, these vibrations are observed as medium intensity bands at 1170 and 1050 cm71, which are insignificantly shifted upon introduction of electron- withdrawing or electron-donating substituents into the benzene rings (see Table 6). The spectra of the positional isomers 3h,n are similar. In the spectra of tetraphenylporphyrins, the n(CH) vibrations of the phenyl substituents are observed at 3100 cm71.Therefore, it is conceivable that the band at 3000 cm71 has a complex nature because the CH vibrations of both the porphyrin and benzene rings are observed in this region. The spectra of the b-alkylated porphyrins 3a ±m,u ± w have bands in the region of 2940 cm71, which, apparently, belong to CH stretching vibra- tions of the alkyl groups. The skeletal vibrations of the porphyrin ring are observed in the region of 1100 ± 1250 cm71.24, 26 The IR spectra of the b-octaalkylporphyrin 3t and the 5,15-dialkyl-b-octamethylpor- phyrins 3a ± g have several medium intensity bands in this region. The assignment of the skeletal vibration bands in the spectra of meso-phenyl-substituted porphyrins is hampered by the fact that vibrations of the porphyrin ring overlap with those of the benzene fragments. The bands in the region of 1350 ± 1600 cm71 were assigned to n(C7C) vibrations of the benzene rings.26 In the N Zh Mamardashvili, O A Golubchikov d(CH) g(C(5)7H) g(C(4)7H) 7 7 7 1162, 1071 7 spectrum of meso-tetraphenylporphyrin, these vibrations are manifested as three rather weak bands (at 1489, 1570 and 1590 cm71) whose intensities become higher as the frequency increases. It is known 29 that the ratio of the intensities of these bands in the spectra of functional benzene derivatives is deter- mined by the presence or absence of conjugation between the phenyl fragments and functional groups. In the spectrum of meso- tetraphenylporphyrin, alternation of the intensities of the bands is analogous to that observed in the spectrum of biphenyl. Chemical modifications of the phenyl substituents in meso- phenyl-substituted porphyrins exert a substantial effect on the skeletal vibrations of the macrocycle.25, 26 Tetra(ortho- and meta- R-phenyl)porphyrins exist as two atropoisomers due to which the band at 1200 cm71 is split.The spectra of tetra(para-R-phenyl)- porphyrins have one intense band at about 1100 cm71, which is shifted to the low-frequency region in parallel with the increase in the mass of the substituent. The spectra of the di(ortho- and meta- R-phenyl)porphyrins 3j,k,m,p,q,s have a pronounced doublet at 1200 cm71, whereas the spectra of the para-derivatives 3i,l,o,r show an intense band at 1100 cm71 (for the nitro derivatives) or at 1180 cm71 (for the methoxy derivatives). Since the nitrogen atoms in porphyrins are more electro- negative than those in pyrroles, the N7H bond length (90 pm) in porphyrins is 10% smaller than that in pyrrole.Because of this, the bands at 719 and 725 cm71 (which are 160 cm71 higher than those in the case of pyrrole) are assigned to out-of-plane g(NH) deformation vibrations of the porphyrin molecule, while the bands at 970 and 980 cm71 (which are 160 cm71 lower than those in the case of pyrrole) are assigned to in-plane d(NH) deformation vibrations.26, 30 The validity of this assignment is additionally confirmed by the absence of the corresponding bands in the spectra of metalloporphyrins.In the spectra of the b-octa- alkylporphyrins 3a ±w, the g(NH) and d(NH) vibrations are observed in the regions of 690 ± 710 and 960 ± 990 cm71, respec- tively (see Table 5).The positions of the bands change only slightly upon meso-substitution. No relationship between the effects of the substituents (OMe or NO2) in the benzene rings of the porphyrins 3i ±m,o ± s and the NH deformation vibration frequencies was revealed. The NH stretching vibrations of porphyrins are characteristic and their identification presents no problems. The n(NH) fre- quency of porphyrins is *190 cm71 lower than that of pyrrole because the electron density is transferred from the pyrrole frag- ments to the macrocyclic aromatic p system, the lowering of the n(NH) vibration frequency being accompanied by stabilisation of the porphyrin anion.Further lowering of the stretching vibration frequency (by *15 cm71) and stabilisation of the porphyrin anion (i.e., an increase of the acid ionisation constant) occur as the aromaticity increases in the porphyrin ± tetraazapor- phyrin ± phthalocyanine series (Table 7).Spectral properties of porphyrins and their precursors and derivatives Table 7. Frequencies of NH symmetrical stretching vibrations and the first-stage acid ionisation constants (K1) for porphyrins in DMSO solutions.7 Porphyrin pK1 n(NH) /cm71 Porphin Octamethylporphyrin meso-Tetraphenylporphyrin Tetrabenzoporphyrin Tetraazaporphyrin Phthalocyanine Mesoporphyrin Deuteroporphyrin 3305 3311 3315 3305 3300 3290 3315 7 22.350.02 721.150.03 18.530.05 12.360.18 10.730.03 725.300.04 The n(NH) frequency becomes higher on both deep alkylation and hydrogenation of porphyrins due to an increase of the electron density on the nitrogen atoms.The n(NH) vibration band in the spectrum of the octaalkylporphyrin 3t is shifted to the high-frequency region by *30 cm71 compared to that in the spectrum of porphyrin. This effect is further enhanced (by *10 cm71) in the presence of the electron-donating alkyl sub- stituents (CH37C6H13 or C11H23) at the meso positions of the macrocycle. At the same time, the introduction of aryl substitu- ents at the meso positions leads to the low-frequency shift (by *15 cm71) of the n(NH) vibration bands compared to the analogous band observed in the spectrum of the porphyrin 3t (see Table 5).4. Metalloporphyrins The molecular symmetry of metalloporphyrins is higher than that of porphyrins due to the replacement of two hydrogen atoms in the coordination centre of the porphyrin ligand by the metal atom and equalisation of the geometric parameters of the pyrrole fragments in metal complexes. Changes in the geometric param- eters of the porphyrin molecule upon complex formation are clearly manifested in the IR spectra. Thus, the number of absorption bands decreases and their symmetry and resolution increase. These facts are associated with degeneration of the energies of particular types of vibrations as the molecular symme- try becomes higher.2, 31 ± 33 In the spectra of metalloporphyrins, the following NH absorption bands disappear first: the NH stretching vibration bands in the region of 3345 ± 3360 cm71 and the NH deformation vibration bands in the regions of 720 ± 740 and 960 ± 980 cm71.2, 7 In the spectra of mesoporphyrin 4a, protoporphyrin 4b, deutero- porphyrin 4c and hematoporphyrin 4d, the fingerprint region where the skeletal vibrations of the porphyrin molecule are located is most sensitive to the introduction of the metal atom.34 R Me R Me NH N HN N Me Me (CH2)2CO2H HO2C(H2C)2 4a ± d R=Et (a), CH=CH2 (b), H (c), CH(Me)OH (d).The formation of metalloporphyrin from porphyrin leads to sharp changes in the intensities of the bands in the regions of 675 ± 683 (I) and 705 ± 711 cm71 (II).Asubstantial decrease in the intensity of the band I is accompanied by an increase in the intensity of the band II.The positions of these bands depend only slightly on the nature of the metal atom. Other skeletal vibrations of the pyrrole rings of porphyrin are less sensitive to the intro- duction of the metal atom into the coordination centre of the molecule. All absorption bands caused by vibrations of the peripheral substituents of porphyrin are weakly sensitive to the introduction of the metal atom. Thus, the intense band at 900 cm71 observed in the spectrum of the protoporphyrin 4b is retained on complex formation with divalent metals. On going to the porphyrin complex with Mn(III), the relative intensity of this band decreases, but its position remains unchanged.Analogous situations are observed for the complexes of the mesoporphyrin 4a and the hematoporphyrin 4d.34 The presence of at least two free b positions in the pyrrole rings of porphyrin leads to substantial changes in the spectral pattern. Thus, the number of the bands in the region of 900 ± 1000 cm71 in the spectra of complexes of the deuteroporphyrin 4c is smaller and their intensities are higher compared to those observed in the spectrum of the ligand. The changes in the IR spectra of porphyrins on complex formation with divalent metals depend on the nature of the metal atom (Tables 8 and 9). However, the most substantial changes in the spectral characteristics of the porphyrin complexes are observed in the case of polyvalent metal ions.The formation of these complexes results in essential changes in the structure and symmetry of the metalloporphyrin molecule due to the displace- ment of the cation from the plane of the macrocycle.7, 35 In the case of meso-tetraphenylporphyrin complexes, the most essential changes associated with complex formation are observed in the region of the spectrum where the out-of-plane g(CH) deformation vibrations of the pyrrole ring are manifested (*800 cm71).25 The IR spectrum of meso-tetraphenylporphyrin has three bands in this region (at 783, 797 and 810 cm71), the intensities of the first and third bands being equal and low. The band at 797 cm71 is the most intense one.In the spectrum of the corresponding metalloporphyrin, the band at 783 cm71 disap- pears, the intensity of the band at 810 cm71 decreases sharply and the latter is shifted to the high-frequency region due, apparently, Table 8. Positions of absorption maxima (cm71) in the IR spectra of the porphyrin complexes 4a ± d.7, 34 Complexes with Com- pound 4 Cu Zn abcd 832 897 951 980 1194 1433 833 898 948 987 1196 1433 833 989 952 984 1200 1437 832 920 952 982 1002 1433 a Porphyrin ± Mn(Cl) complexes. 831 896 950 982 1193 1435 835 900 947 984 1193 1434 833 898 950 981 1200 1434 834 928 952 982 1193 1433 583 Pd Co Mna 839 898 958 989 1195 1434 838 917 958 993 1200 1435 850 920 956 989 1205 1437 837 930 955 990 1210 1433 834 898 952 987 1194 1433 835 902 950 992 1200 1434 834 900 954 984 1200 1438 834 832 956 985 1208 1432 832 897 951 985 1194 1432 834 902 948 990 1200 1435 833 898 952 983 1200 1437 832 928 954 984 1203 1433584 Table 9.Positions of absorption maxima (cm71) in the IR spectra of meso- tetraaryl-substituted porphyrin complexes.20, 25 Substituent in the benzene ring H2-OMe 3-OMe 4-OMe 2-Me 3-Me 4-Me to the fact that complex formation leads to lowering of the electron density in the macrocycle.The high-frequency shift of the band at 797 cm71 is observed in the spectra of porphyrin complexes with polyvalent metal ions. In the case of Mo(V) and W(V), this band is observed at 805 cm71.36 Apparently, the shift of the band at 797 cm71 in the spectra of complexes with polyvalent ions is associated with the geometric characteristics of these complexes in which the metal ion deviates substantially from the plane of the molecule. The noncoplanar arrangement of the metal atom reduces the possibil- ity of transfer of the s- and p-electron density from the nitrogen atom to the metal atom as a result of which the complexes should be destabilised. Complex formation causes substantial changes in the frequen- cies and intensities of the bands in the region of 964 ± 1030 cm71 where the in-plane d(CH) deformation vibrations of the pyrrole fragments of the porphyrin molecule are manifested.It is worthy of note that the intensity of the band at 1003 cm71 in the spectrum of meso-tetraphenylporphyrin rises sharply upon complex forma- tion. Koifman 34 believed that this band is associated with two types of vibrations of the pyrrole fragments, viz., with in-plane d(CH) deformation vibrations and n[C(b)7C(b)] stretching vibra- tions. As mentioned above, the introduction of electron-with- drawing substituents into the pyrrole ring results in a more Complexes with Ni Zn 752 809 980 1079 1166 758 808 1007 1050 1184 785 810 864 1009 1092 1155 811 854 1001 1107 1186 752 816 999 1067 1176 784 899 792 1005 1107 1186 804 859 1007 998 1090 1175 737 802 971 1070 1171 742 805 994 1050 1182 771 806 847 1005 1076 1172 794 850 990 1110 1174 740 811 993 1055 1182 770 892 790 1001 1107 1155 787 854 996 992 1110 1188 Mg Cu Co 735 800 982 1075 1164 740 801 1004 1055 1183 774 800 855 1002 1079 1166 792 853 995 1109 1174 738 810 990 1056 1188 768 890 789 1000 1083 1164 789 852 1001 990 1109 1174 745 797 977 1070 1173 751 798 1000 1049 1179 781 803 860 1003 1080 1162 804 840 998 1107 1174 747 804 998 1051 1181 776 885 799 1003 1092 1151 796 847 1003 999 1108 1180 740 802 974 1073 1176 747 803 996 1051 1181 776 808 852 1005 1084 1165 800 844 993 1109 1178 743 809 995 1054 1183 773 890 792 1001 1100 1153 792 852 999 996 1112 1183 N Zh Mamardashvili, O A Golubchikov substantial change in the dipole moment of the molecule in the course of vibration.This leads to significant changes (generally, to a rise) in the intensities of the out-of-plane and in-plane CH deformation vibration bands. By analogy, the increase in the intensity of the band at 1003 cm71 in the spectrum of meso- tetraphenylporphyrin can be related to the electron density redis- tribution in the molecule upon complex formation.Apparently, the d(CH) vibrations make the major contribution to this absorp- tion band. Complex formation with meso-tetraaryl-substituted porphyr- ins as well as the change of the metal atom lead to substantial changes in the region of 690 ± 758 cm71 (see Table 9) in which two out-of-plane deformation vibration bands of the aryl groups are observed. The position of one band depends on the nature of the metal atom and changes from 690 to 708 cm71.20, 25 The second band belongs to the most intense bands manifested in the IR spectra of metal complexes and is observed in the region of 732 ± 758 cm71. The frequencies and intensities of the n(C7C) stretching vibration bands of the benzene rings are also changed substantially upon complex formation.Thus, two intense bands appear instead of three medium intensity bands in the region of 1430 ± 1590 cm71. It was noted 37 that the changes in the elec- tronic structure of the p and d shells of the metal atom as well as the changes in the geometric parameters of the coordination centre caused by complex formation have no effect on the position and intensity of the n(C7C) band (1590 cm71) in the spectrum of meso-tetraphenylporphyrin. Hence, there is no clear correlation between the electronic structure of the coordination centre and the n(C7C) vibration frequency. However, it can be stated that the C7H and C7C vibrations of the benzene rings depend on the change in the symmetry of the p system of the macrocycle due to complex formation.The spectral characteristics of the complexes of 2,8,12,18- tetrabutyl-3,7,13,17-tetramethylporphyrin (3t) and its meso-di- (3a ± c,g ±m) and -tetrasubstituted derivatives (3u,w) with Zn, Cu and Ni 38 are given in Table 10. Complex formation leads to the disappearance of the NH vibration bands in the IR spectra and to a decrease in the number of the bands in the regions where C7C vibrations of the pyrrole and benzene rings are manifested. In the case of both protoporphyrin and deuteroporphyrin, the spectral characteristics are less affected by the introduction of the metal ion into the coordination centre of the porphyrin molecule. However, the changes are comparable with those observed on coordination of meso-tetraphenylporphyrin.The region about *1000 cm71 in which the metal-sensitive bands are most pronounced is of particular interest. The spectra of all complexes have three bands at *980, 955 and 910 cm71, the positions of the first and third bands being dependent on the nature of metal.27, 38 The high-frequency shifts of these bands increase in the series Zn<Cu<Ni. It should be noted that the above-mentioned sequence corresponds to the hypsochromic shifts of the absorption bands in the electronic absorption spectra of the complexes of 3a ± c,g ±m,t,u,w (see Table 10).39 The bands at 955 and 910 cm71 in the IR spectrum of the copper complex of octaethylporphyrin belong to deformation vibrations of the alkyl groups, which is confirmed by the absence of these bands in the spectrum of the copper complex of porphyrin.34 The band at 980 cm71 may be assigned to deformation vibrations of the porphyrin ring.An analogous assignment was made for the copper complex of octaethylporphyrin. 25 The CH vibration frequencies of the methine bridges in the meso-unsubstituted (3t) and the meso-disubstituted b-octaalkyl- porphyrins (3a ± s) depend substantially on the nature of the metal ion.27 These frequencies at 3110, 1220 and 835 cm71 belong to stretching and deformation (in-plane and out-of-plane) CHvibra- tions, respectively. The positions of the absorption maxima of the CH deformation vibrations of the methine bridges in porphyrins are given in Table 10.The stronger the coordination of the metal atom and the higher its electronegativity, the larger the high- frequency shift. The in-plane deformation vibrations depend onSpectral properties of porphyrins and their precursors and derivatives Table 10. Positions of absorption maxima in the IR (cm71) and electronic absorption (nm) spectra of the b-octaalkylporphyrin complexes 3a ± c,g ±m,t,u,w.38, 39 n a Com- pound 3 for complexes with Zn abcghij 835 925 982 1220 837 924 987 1236 836 925 988 1238 836 926 987 1239 837 919 985 1223 842 923 986 1225 839 916 994 1215 a In KBr pellets; b a solution in chloroform. 834 917 977 1217 836 918 982 1231 835 918 982 1233 836 922 984 1234 836 912 980 1220 840 916 980 1222 838 910 988 1209 the nature of the metal atom more strongly than the out-of-plane vibrations.The regularity revealed for the d(CH) and g(CH) vibrations of the methine bridges agrees with the changes that occur at 1000 cm71 in the IR spectra upon complex formation and is confirmed by the results of studies by electronic absorption spectroscopy. Therefore, examination of the IR spectra demonstrated that the introduction of the metal ion into the porphyrin molecule affects substantially the macrocycle (particularly, its p-electron system) and causes changes in the frequencies and intensities of vibrations of most of the bonds and molecular fragments.III. 1H NMR spectra of porphyrins and their precursors Studies performed in the last decades demonstrated that 1H NMR spectra are very informative and adequately reflect the structural features of porphyrins.40 ± 59 The presence of the extended delo- calised p-electron system of the porphyrin macrocycle gives rise to a strong ring current in the molecules placed in the magnetic field. The ring current causes anisotropic shielding of the protons located in the field of its action and (together with the diamagnetic component of paired s-electrons) leads to a substantial shift of their signals in the 1H NMR spectra. It can be stated that the ring current and aromaticity of porphyrins change in a similar way in response to the analogous changes in the molecular structure of the porphyrin and the medium, which is most clearly seen on comparison of the spectra of porphyrins and their precursors.lmax(log e) b for complexes with Ni Cu Zn Ni Cu 556(3.42) k 526(3.18) 398(4.17) 568(3.58) 534(3.74) 408(4.56) 510(3.45) l 526(3.15) 398(4.20) 568(3.60) 535(3.42) 408(4.48) 558(3.38) m 526(3.19) 398(4.18) 568(3.62) 536(3.42) 409(4.50) 558(3.40) t 525(3.19) 397(4.19) 569(3.64) 535(3.50) 409(4.50) 570(3.79) u 536(4.28) 408(5.13) 573(3.87) 539(4.31) 410(5.08) 569(3.90) w 536(4.40) 406(5.21) 571(3.91) 538(4.31) 409(5.11) 5.68(3.92) 535(4.36) 409(5.27) 571(3.80) 537(4.38) 411(5.13) 836 570(3.66) 929 537(3.78) 989 409(4.63) 1226 838 570(3.65) 927 538(3.78) 992 409(4.62) 1239 837 572(3.64) 929 538(3.74) 994 410(4.61) 1242 838 572(3.60) 932 539(3.75) 995 411(4.58) 1246 838 575(3.99) 923 541(4.26) 992 412(5.01) 1229 843 573(3.96) 925 540(4.26) 992 411(5.08) 1231 840 574(3.87) 920 540(4.30) 998 413(5.20) 1220 n a Com- pound 3 for complexes withNi Cu Zn 841 924 999 1232 843 921 911 1219 847 928 996 1244 836 922 997 1237 839 920 997 1234 839 924 995 1233 840 920 994 1228 842 912 988 1214 846 924 990 1240 835 918 991 1231 838 917 992 1227 837 920 990 1228 839 912 986 1226 840 907 982 1207 846 918 985 1234 834 911 986 1228 838 912 988 1223 837 915 986 1225 1.Pyrroles The chemical shifts of protons depend on the electron density on the hydrogen atom, the ring current of the molecule, the p-electron charge on the atom bound to the proton under consideration and the steric effects resulting from interactions with the substituents or the heteroatoms involved in the ring. Generally, separation of these effects presents a considerable problem. The introduction of substituents into the pyrrole molecule leads to essential spectral changes.9, 10, 13 A comparison of the characteristics of pyrrole (1a) and 3,4-dimethylpyrrole (1m) (Table 11) demonstrated that the positions of the signals for the protons of the CH groups of the pyrrole rings remain unchanged.Apparently, the b-substitution in the pyrrole ring exerts only a slight effect on the p-electron system of the molecule. In the spectrum of 2,3,4-trimethylpyrrole (1o), the signals for a-H are shifted upfield by 1.06 ppm compared to those in the spectrum of unsubstituted pyrrole due, apparently, to the positive inductive effect of the methyl group. Correspondingly, the downfield shift of the signal for a-H (0.43 ppm) observed on going from 3,4- dimethylpyrrole (1m) to 2-ethoxycarbonyl-3-ethyl-4-methylpyr- role (1q) occurs due to the effect of the electron-withdrawing ester group. The most essential changes associated with chemical modifi- cations of the molecule are observed in the region of the signals for the protons of the b-methyl and NH groups (see Table 11).An increase in the number of methyl groups in the pyrrole ring on going from the compound 1m to the compound 1o causes upfield shifts of the signals for the protons of the alkyl substituents by 0.13 ppm. The introduction of the electron-withdrawing ester or formyl group into the molecule results in downfield shifts of these signals by *0.2 ppm. Of all the structures under consideration, 585 lmax(log e) b for complexes with Ni Cu Zn 569(3.82) 535(4.26) 408(5.20) 570(3.90) 538(4.19) 410(5.17) 574(3.86) 540(4.29) 413(5.31) 568(3.81) 536(4.20) 406(5.31) 572(3.70) 539(4.25) 410(5.20) 574(3.78) 541(4.30) 413(5.27) 567(3.72) 536(4.01) 410(5.39) 570(3.59) 538(4.09) 412(5.41) 574(3.59) 540(4.17) 414(5.34) 557(3.91) 526(3.60) 396(5.14) 569(3.86) 534(3.54) 399(5.17) 569(3.95) 538(3.62) 403(5.21) 572(3.60) 539(3.84) 414(4.89) 576(3.60) 542(3.80) 418(4.90) 578(3.67) 544(3.82) 420(4.91) 578(3.60) 542(3.81) 416(4.89) 580(3.69) 544(3.78) 420(4.94) 582(3.62) 547(3.80) 422(4.92)586 Table 11.Positions and multiplicities of the signals in the 1H NMR spectra of pyrrole (1a) and its derivatives 1b ± s, 5a,b.9, 10, 12 d(NH) Com- pound 1a 1b 1c 1d 1e 1f 1g 1h 1i 1j 1k 1l 1m 1n 1o 1p 1q 1r 1s 5a 5b 7.62 s (1H) 9.00 s (1H) 9.68 s (1H) 8.62 s 8.84 s 8.80 s 8.75 s 9.88 s 10.05 s 8.88 s 9.01 s 9.82 s (1H) 7.17 s (1H) 6.97 s (1H) 6.72 s (1H) 6.99 s (1H) 9.88 s (1H) 10.87 s 10.09 s 9.73 s 9.84 s Note.Hereinafter, the 1H NMRspectra were measured in CDCl3; the chemical shifts (d) are given in ppm. a Signals for the protons of the phenyl groups; b signals for b-H. this chemical shift is maximum (*0.5 ppm) in the case of the carboxy derivatives. The chemical shifts of the signals for the protons of the NH groups are changed more substantially. Methylation of the pyrrole ring (the compounds 1a and 1o) is accompanied by the upfield shifts of these signals by *0.9 ppm (see Table 11). The signals for the protons of theNHgroups are shifted downfield (up to 3.3 ppm) upon introduction of electron-withdrawing groups at the a positions of pyrroles.The spectral characteristics (the intensities and multiplicities of the signals) agree well with the structures of the pyrroles synthesised. The signals for the protons of the ester (at d 1.2 ± 4.5) and carboxy (at d 9.45 ± 12.65) groups remain very characteristic and do not overlap with the signals of other proton- containing groups. No substantial changes in the chemical shifts of the signals for the protons of the b-alkyl substituents and functional groups are observed on going from the compounds 1c,q to b-bound pyrroles 5a,b. Me R (CH2)11 HN Me 5a,b R=CO2Et (a), CHO (b). In the 1H NMR spectra of the compounds 5a,b, the arrange- ment of the signals of the (CH2)11 methylene bridge linking the b positions of the pyrrole fragments is of interest.60 The signals for the protons of two methylene groups directly bound to the pyrrole ring (see Table 11) are resolved from the signals for the protons of the remaining nine units of the bridge.The intensity distribution of the signals provides information on the relative number of equivalent protons (4 and 18, respectively). The signals for the protons of theNHgroups in the spectrum of the compound 5b are shifted upfield and are broadened compared to those observed in the spectrum of the pyrrole 1r. Since the bispyrroles 5a,b consist of d(b-Alk) d(b-CH3) 7 71.75 s (3H) 1.17 t (3H), 2.45 q (2H) 0.95 t, 1.28 m, 1.82 m, 2.28 t 0.84 t, 1.30 m, 1.82 m, 2.27 t 0.89 t, 2.41 q 0.84 t, 2.69 q 0.87 t, 2.69 q 0.88 t, 2.71 q 0.85 t, 2.71 q 0.84 t, 2.74 q 0.87 t, 1.87 m, 1.29 m, 2.65 t 7 6.18 s (2H) 777777 712.60 s 12.64 s 7 7 7 7 6.20 s (1H) 6.18 s (2H) 7 7 5.11 s (6H) 6.02 s (1H) 5.09 s (1H) 6.61 s (1H) 7777 2.08 s (3H) 2.24 s (3H) 2.20 s 2.21 s 2.23 s 2.24 s 2.54 s 2.53 s 2.20 s 2.18 s 2.22 s (3H) 2.01 s (6H) 1.50 s (2H) b 2.02 s (6H) 1.86 s (3H) 2.24 s (3H) 2.27 s 2.24 s (3H) 2.22 s 2.27 s 71.05 t (3H), 2.37 q (2H) 1.14 t (3H), 2,40 q (2H) 1.05 t, 2.36 q 1.21 t (3H), 2.74 q (2H) 1.08 ± 1.58 m, 2.18 ± 2.44 m 1.26 m, 2.27 ± 2.48 m Me R NH Me d(a substituent)another substituent H 7 7 7 2.02 s 2.59 s 2.13 s 2.09 s 2.01 s 77 5.12 s (3H) 77 7 7 7 7 7 7 7 7 7 79.45 s 6.89 s (3H), 9.58 s (1H) 2.14 s 9.54 s two pyrrole rings separated from each other, the observed changes are, apparently, associated with the fact that solutions contain several conformers with different relative arrangements of the pyrrole nuclei. As a result, the positions of the NH groups in the bispyrrole molecule are nonequivalent (different shielding of the nuclei in different molecules or in different positions of a single molecule).On the contrary, the signals for the protons of the NH groups in the spectra of analogous dipyrrolylmethenes and biladienes are shifted downfield due to extension of the conjugated system and enhancement of the aromaticity of the pyrrole rings.61 2.Dipyrrolylmethanes A comparison of the 1H NMR spectra of the compounds 1b and 2a revealed no substantial changes in the region of the signals for the protons of the NH groups 12, 13 (Tables 11 and 12) due, apparently, to the absence of conjugation between the nearby p-electron systems of the pyrrole rings. On going from unsubsti- tuted dipyrrolylmethane to the carboxy derivatives, the signals for the protons of the NH groups are shifted downfield (by *0.5 ppm) due, presumably, to the electron-withdrawing effect of the carboxy groups. Methylation at the b positions of dipyr- rolylmethane is accompanied by slight upfield shifts of the signals for the protons of the NH groups. The shifts successively increase on going from b-dialkyl-substituted dipyrrolylmethane to the b-tetraalkyl-substituted analogue and as the length of the alkyl substituent increases (the compounds 2a,c,d), which is consistent with the positive inductive effect of the alkyl substituents.The introduction of the phenyl substituent at the meso position of dipyrrolylmethane (the compound 2k) leads to a slight downfield shift of the signal for the protons of the NH groups by 0.2 ppm compared to that in the spectrum of unsubstituted dipyrrolylmethane. A more substantial shift is observed in the spectrum of the structural isomer 2l. Apparently, the above- considered changes can be attributed to the small negative inductive effect of the phenyl groups. Chemical modifications at positions 5 and 50 in the dipyrrolyl- methane molecule (on going from the ethoxycarbonyl derivatives to the carboxy analogues) have virtually no effect on the positions of the signals for the b-alkyl protons. However, it should be noted N Zh Mamardashvili, O A Golubchikov d(OCH2CH3) CH2 CH3 4.16 q 4.35 q 4.23 q 5.22 m 5.19 m 4.32 m 4.31 q 4.30 q 4.24 q 4.23 q 4.31 q 1.30 t 1.38 t 1.28 t 7.27 a m 7.25 a m 7.30 a m, 1.33 t 1.32 t 1.33 t 1.26 t 1.27 t 1.32 t 1.32 t 1.31 t 4.27 q 7 7 7 74.25 q 7 7Spectral properties of porphyrins and their precursors and derivatives Table 12.Positions and multiplicities of the signals in the 1H NMR spectra of dipyrrolylmethanes and their derivatives.12 ± 14 d(NH) Com- pound 2a 2b 9.24 s (2H) 9.55 s (2H) 2c 2d 9.14 s (2H) 9.10 s (2H) 2f 2h 9.20 s (2H) 9.75 s (2H) 2i 9.69 s (2H) 2j 9.81 s (2H) 2k 9.32 s (2H) 2l 9.52 s (2H) 2n 9.69 s (2H) 2p 9.48 s (2H) 2r 9.52 s (2H) 2t 9.54 s (2H) 2v 9.56 s (2H) 2x a 6a 7.81 s (2H) 9.07 s (4H) 6b 9.62 s (4H) 6c 9.20 s (4H) a According to the published data.30 that the introduction of the phenyl substituent at the meso position of the molecule is accompanied by the upfield shifts (by *0.7 ppm) of the signals for the protons of the methyl substitu- ents at positions 3 and 30 (see Table 12) due to the shielding effect of the ring current of the phenyl substituent.In the 1H NMR spectra of the 5,50-diesters 2a,c,d,f,k,l, the signals for the meso-protons are observed as a pronounced singlet in the region d 3.73 ± 4.12.As expected, the signals are shifted downfield (by*2 ppm) on going to the 5,50-dicarboxy derivatives 2b,h,i,n,p,r, (see Table 12). The electron-withdrawing effect of the carboxy group is identical in both the meso-unsubstituted and meso-aryl-substituted dipyrrolylmethanes. The introduction of the phenyl substituent at the meso position of porphyrin leads to an increase in the downfield shift (by *0.5 ppm) in the case of both the ethoxycarbonyl and carboxy derivatives. Apparently, this fact can be explained by a small negative inductive effect of the substituent. The signals for the meso-protons and the protons of the phenyl fragments in the structural isomers 2k,l are observed at d *4.0 and 6.6 ± 6.9, respectively (see Table 12).In the 1H NMR spectra, the signals for the protons of the unsubstituted meso- phenyl groups in the compounds 2h ± l are observed as a doublet of the ortho-protons and a multiplet of the meta- and para-protons. The para-substitution (the compounds 2n,t) results in the appear- ance of a doublet for the meta-protons along with a doublet for the ortho-protons. In the spectra of the meta-substituted dipyrrolyl- methanes 2p,v, the signals for the meta- and para-protons again coalesce into a multiplet. d(b-CH3) 2.92 s (6H) 2.94 s (6H) 2.97 s (6H) 2.91 s (6H) 2.94 s (6H) 2.21 s (6H) 2.22 s (6H) 2.16 s (6H) 2.22 s (6H) 2.23 s (6H) 2.20 s (6H) 2.22 s (6H) 2.20 s (6H) 2.24 s (6H) 2.27 s (6H) 72.17 s (12H) 2.19 s (12H) 2.21 s (12H) d(Alk) d(meso-H) 3.87 s (2H) 5.30 s (2H) 3.73 s (2H) 3.80 s (2H) 3.78 s (2H) 5.81 s (1H) 5.89 s (1H) 4.84 s (1H) 4.01 s (1H) 4.12 s (1H) 5.88 s (1H) 5.31 s (1H) 5.89 s (1H) 5.83 s (1H) 5.81 s (1H) 1.87 s (6H) 0.82 t (6H), 1.25 m (4H), 1.85 m (4H), 2.30 t (4H) 0.98 t (6H), 2.30 q (4H) 0.84 t (6H), 1.25 m (4H), 1.86 m (4H), 2.31 t (4H) 6.49 s (2H) 0.80 t (6H), 1.21 m (4H), 1.87 m (4H), 2.23 t (4H) 0.79 t (6H), 1.22 m (4H), 1.90 m (4H), 2.25 t (4H) 0.94 t (6H), 1.20 m (4H), 1.92 m (4H), 2.34 t (4H) 0.81 t (6H), 1.19 m (4H), 1.87 t (4H), 2.28 t (4H) 0.83 t (6H), 1.20 m (4H), 1.85 m (4H), 2.32 t (4H) 0.81 t (6H), 1.19 m (4H), 1.89 m (4H), 2.28 t (4H) 0.82 t (6H), 1.21 m (4H), 1.84 m (4H), 2.34 t (4H) 0.84 t (6H), 1.20 m (4H), 1.79 m (4H), 2.34 t (4H) 0.80 t (6H), 1.24 m (4H), 1.80 m (4H), 2.36 t (4H) 0.83 t (6H), 1.19 m (4H), 1.82 m (4H), 2.36 t (4H) 1.82 s (6H) 0.86 t (12H), 2.29 q (8H) 3.80 s (2H) 5.44 s (2H) 0.84 t (12H), 2.31 q (8H) 5.36 s (2H) 0.82 t (12H), 2.30 q (8H) 5.28 s (2H) In the spectra of the linear tetrapyrrole compounds 6a ± c, the signals for the meso-protons are shifted downfield by *1.5 ppm, whereas the positions of the signals for the protons of the b-alkyl and NH group change only slightly compared to those in the spectra of the monomeric analogue 2c.HO2C HO2C R1=Me, R2=Et, n = 3 (a); R1=Et, R2=Me, n = 4 (b); R1=Me, R2=Et, n = 4 (c).More substantial changes are observed in closed tetrapyrrole structures. It was found 61 that the signals for the protons of the b-alkyl and NH groups are shifted upfield by 0.65 and 0.28 ppm, respectively, on going from pyrrole to octamethylporphyrinogen due, apparently, to the positive inductive effect of the methyl groups at the b positions. The introduction of the methyl group at the a position of pyrrole also leads to the upfield shift of the signal for the b-alkyl protons. 587 d(Ar) 777776.68 m (3H; 3,4-H), 6.90 d (2H; 2-H) 6.61 m (3H; 3,4-H), 6.90 d (2H; 2-H) 6.65 m (3H; 3,4-H), 6.90 d (2H; 2-H) 6.60 m (3H; 3,4-H), 6.90 d (2H; 2-H) 6.69 m (3H; 3,4-H), 6.90 d (2H; 2-H) 6.70 d (2H; 3-H), 6.92 d (2H; 2-H), 3.66 s (3H; CH3) 6.72 m (2H; 3,4-H), 6.92 d (2H; 2-H), 3.68 s (3H; CH3) 6.74 m (3H; 3,4-H), 6.96 d (1H; 2-H), 3.70 s (3H; CH3) 6.68 d (2H; 3-H), 6.90 d (2H; 2-H) 6.74 m (2H; 3,4-H), 6.98 d (2H; 2-H) 76.95 t (4H; 3,4-H), 7.13 m (4H; 2-H), 4.12 m (2H; CH2), 3.54 t (4H; CCH2) 7.03 m (4H; 3,4-H), 7.28 m (4H; 2-H), 4.10 m (4H; CH2), 3.78 t (4H; CCH2) 7.14 m (4H; 3,4-H), 7.31 m (4H; 2-H), 4.12 m (4H; 2-CH2), 3.84 t (4H; OCH2)R2 R2 CO2H R1 R1 O(CH2)nO HN NH NH HN R1 R1 CO2H 6a ± c R2 R2588 3. Porphyrins Conjugated macrocyclic structures are of most interest.The imposition of the external magnetic field on these molecules gives rise to ring currents, which make the major contribution to the observed chemical shifts. Theoretical analysis of the 1H NMR spectra shows that the positions of the signals for the protons of porphyrins are determined primarily by the strength of ring p-electron currents (the macrocyclic current enclosing the mole- cule as a whole and local currents localised in the pyrrole nuclei).It should be noted that the macrocyclic current (Fig. 2) possesses the major `bed' corresponding to the main 18-membered contour of conjugation and `arms' formed by the semi-isolated C(b)7C(b) bonds in the pyrrolenine nuclei. a b NH N NH N HN N N HN Figure 2. Schematic representation of the circuits of the ring p-electron currents in the porphyrin molecule; (a) the macrocyclic current, (b) local currents.7, 25 The protons at the b and meso positions are exocyclic with respect to the macrocyclic and local pyrrole currents and experi- ence the deshielding influence of the latter.The signals for these protons are observed at low field (at d from 9.7 to 11.2 for the meso-protons and at d from 8.5 to 9.9 for the b-protons; see Table 13). The protons of the NH groups are exocyclic with respect to the local ring currents and endocyclic with respect to the macrocyclic current. Undoubtedly, the shielding effect of the latter prevails and the signals for the protons of theNHgroups are observed at very high field (at d from71.4 to74.4; see Table 13). The b-hydrogen atoms of the pyrrole and pyrrolenine frag- ments are in principle nonequivalent. However, rapid (within the NMRtime scale) transformations occur at room temperature and Table 13.Chemical shifts (ppm) of the b- and meso-protons and the protons of the NH groups in the 1H NMR spectra of porphyrins.7 Porphyrin d(meso-H) d(NH) d(b-H) 10.58 73.76 11.22 74.40 10.18 73.70 10.10 73.67 7 72.73 7 72.75 7 72.81 7 72.38 7 72.66 7 72.68 7 72.63 7 72.72 7 72.85 7 72.96 7 72.91 7 72.45 9.96 9.65 11.21 74.08 71.40 7 9.74 9.92 779.41 9.28 8.70 8.45 8.60 8.65 8.45 8.82 8.85 8.77 8.82 8.52 779.66 Porphin Porphin a Octaethylporphin Ethioporphyrin-I Tetra-iso-butylporphyrin Tetra-n-pentylporphyrin Tetraphenylporphyrin Tetraphenylporphyrin a Tetra(2-methylphenyl)porphyrin Tetra(2-methoxyphenyl)porphyrin Tetra(2-bromophenyl)porphyrin Tetra(2-aminophenyl)porphyrin Tetra(3-methoxyphenyl)porphyrin Tetra(3-nitrophenyl)porphyrin Tetra(3-bromophenyl)porphyrin Tetra(3-hydroxyphenyl)porphyrin a Protoporphyrin Chlorin Deuteroporphyrin a Note. In all tetrasubstituted porphyrins, the substituents are located in the meso positions.a The spectrum of a solution in CF3CO2H was measured. N Zh Mamardashvili, O A Golubchikov the signals for the b-protons are averaged. Two different peaks corresponding to resonance absorption of the energy by the protons of the b-CH groups of the pyrrole and pyrrolenine fragments can be observed at low temperature.7, 62 On going from porphin to chlorin, the signals for the protons of the NH groups in the 1H NMR spectra are shifted downfield, whereas the signals for the meso-protons are shifted upfield, i.e., these signals are shifted toward each other, which is indicative of a decrease in the strength of the induced magnetic field produced by the macrocyclic ring current.7, 62 In our opinion, two consequen- ces of hydrogenation of one of the C(b)=C(b) bonds are respon- sible for these changes.First, due to hydrogenation of the C(b)=C(b) bond, the number of charge carriers providing the macrocyclic ring current is decreased by two electrons and, correspondingly, the strength of the macrocyclic ring current is diminished. Second, one of the `arms' through which this current flows disappears, i.e., the effective diameter of the ring conductor is reduced.As a result, the strength of the magnetic field induced by the macrocyclic ring current decreases. On going from a solution of porphin (P) in deuteriochloro- form to a solution in trifluoroacetic acid, the neutral form H2P is converted into the doubly protonated form H4P2+ in which the excess positive charge is located primarily on four nitrogen atoms. In this case, the electron density on the central hydrogen atoms is sharply reduced, which was confirmed by calculations for the neutral and protonated forms of porphin by the PM3method.63 In this connection, one would expect that the signals for the protons of the NH groups in the 1H NMR spectrum will be shifted downfield upon protonation of porphin. However, the signals are shifted upfield by *0.7 ppm (see Table 13). Simultaneously, the signals for the b- and meso-protons are shifted downfield. This character of the spectral changes indicates that protonation of porphin leads to a substantial increase in the strength of the magnetic field induced by the macrocyclic ring current.Calculations 23 carried out by the PM3 method demonstrated that protonation of porphin is accompanied by a change in the geometric structure of the molecule. The C(a)7NH7C(a) frag- ments in the initial porphin are planar and the nitrogen atoms are sp2-hybridised. In the protonated porphin, these fragments adopt a pyramidal structure (the nitrogen atoms have nearly sp3 hybrid- isation) and the degree of involvement of the nitrogen atoms in conjugation with the a-carbon atoms is substantially reduced.This fact is confirmed by an increase in the C(a)7N bond length observed upon protonation of porphin. The above-mentioned rearrangements lead to a change in the contour of macrocyclic conjugation. In the case of protonated porphin, this conjugation is realised primarily along the outer contour of the molecule. There- fore, protonation causes an increase in the diameter of the con- jugation ring resulting in upfield shifts of the signals for the internal protons and downfield shifts of the signals for the external protons. In the case of b-alkyl substitution in porphin (on going to octaethylporphin and ethioporphyrin), the signals for the protons of the NH groups and the signals for the meso-protons are shifted downfield and upfield, respectively.Analogous, but more pro- nounced, changes are observed in the presence of a substituent in the meso position. It should be noted that the introduction of both electron-donating (meso-tetra-iso-butyl- and meso-tetra-n-pentyl- porphyrin) and electron-withdrawing (tetraphenylporphyrin and its derivatives) substituents gives the same results, viz., the signals for b-protons and for the protons of the NH groups are shifted upfield and downfield, respectively. It seems likely that the introduction of substituents both at the b- and meso positions leads to reduction in the strength of the aromatic macrocyclic current regardless of the electronic nature of the substituent. The reasons for this phenomenon remain unclear.The currents in the porphyrin molecule are substantially affected by complex formation.7 The higher the degree of cova- lence of theN7Hbond, the larger the decrease in the ring current due to the presence of the coordinated metal atom. Coordination to the medium-sized M2+ cations (M=Mg, Zn, Cd, Ni or Pd) isSpectral properties of porphyrins and their precursors and derivatives accompanied by upfield shifts of the signals for the meso-protons (the ring current is reduced). In the spectra of the complexes in which the M2+ metal ion deviates from the plane due to its large radius and weak coordination interactions (M=Sn or Pb) or in the spectra of the complexes with the M3+ or M4+ ions in which the metal atom deviates from the plane of the macrocycle under the action of the extra ligands, the signals for the meso-protons are shifted downfield relative to those in the spectra of the porphyrin ligand.These shifts may be due to an increase in the ring current. Table 14. Positions and multiplicities of the signals in the 1H NMR spectra of the b-octaalkylporphyrins 3a ± f, h ± w.14, 19, 20, 24, 27 d(NH) Com- pound 3 a 72.11 s (2H) 72.06 s (2H) 72.04 s (2H) bcd 72.03 s (2H) e 72.07 s (2H) f 72.14 s (2H) 72.42 s (2H) 72.38 s (2H) 72.49 s (2H) hijk a 72.38 s (2H) 72.37 s (2H) 72.49 s (2H) 72.47 s (2H) 72.32 s (2H) 72.27 s (2H) 72.23 s (2H) 72.27 s (2H) 72.20 s (2H) 72.23 s (2H) k b lmnopqrst 73.80 s (2H) uv 77 w 7 a a,a-Atropoisomer; b a,b-atropoisomer; c signals for the protons of the meso-alkyl groups.d(b-CH3) 3.01 s (24H) 3.09 s (24H) 3.12 s (24H) 3.11 s (24H) 3.14 s (24H) 3.18 s (24H) 2.41 s (12H) 2.42 s (12H) 2.50 s (12H) 2.53 s (12H) 2.54 s (12H) 2.27 s (12H) 2.33 s (12H) 3.41 s (12H) 3.99 s (12H) 3.92 s (12H) 3.98 s (12H) 3.94 s (12H) 3.92 s (12H) 3.47 s (12H) 2.42 s (24H) 2.44 s (24H) 2.33 s (24H) However, this effect is insignificant and depends on the nature of the ions serving as the extra ligands.7 The following characteristic features should be noted in the 1H NMR spectra of b-octaalkylporphyrins and their 5,15- diaryl-, 5,15-dialkyl- and 5,15-dialkyl-10,20-diaryl deriva- tives.14, 19, 20, 24, 27 On going from 2,8,12,18-tetrabutyl-3,7,13,17- tetramethylporphyrin (3t) to 2,8,12,18-tetrabutyl-3,7,13,17-tetra- methyl-5,15-diphenylporphyrin (3h), the most substantial changes are observed in the region of the signals for the protons of the b-methyl and NH groups (Table 14).The upfield shifts of the d(b-Alk) d(meso-H) 7 7 7 7 7 7 7 7 7 7 7 7 9.82 s (2H), 4.01 s (6H) 9.79 s (2H), 4.08 q (4H), 2.24 t (6H) 9.77 s (2H), 4.10 t (4H), 2.20 m (4H), 1.80 t (6H) 9.78 s (2H), 4.14 t (4H), 2.22 m (4H), 1.92 m (4H), 1.54 t (6H) 9.80 s (2H), 4.20 t (4H), 2.25 m (4H), 1.90 m (4H), 1.50 m (4H), 1.01 t (6H) 9.82 s (2H), 4.32 t (4H), 2.50 m (4H), 2.16 m (4H), 1.64 m (4H), 1.09 m (4H), 0.78 t (6H) 10.17 s (2H) 10.12 s (2H) 10.18 s (2H) 10.19 s (2H) 3.91 t (8H), 2.11 m (8H), 1.63 m (8H), 1.03 t (12H) 3.85 t (8H), 2.10 m (8H), 1.67 m (8H), 1.03 t (12H) 3.93 t (8H), 2.14 m (8H), 1.67 m (8H), 1.03 t (12H) 3.99 t (8H), 2.18 m (8H), 1.67 m (8H), 1.09 t (12H) 10.17 s (2H) 10.20 s (2H) 10.17 s (2H) 10.16 s (2H) 10.07 s (2H) 10.15 s (2H) 10.14 s (2H) 10.16 s (2H) 10.18 s (2H) 9.91 s (4H) 3.97 t (8H), 2.17 m (8H), 1.64 m (8H), 1.02 t (12H) 3.91 t (8H), 2.07 m (8H), 1.63 m (8H), 1.10 t (12H) 3.87 t (8H), 2.03 m (8H), 1.66 m (8H), 1.02 t (12H) 2.91 t (8H), 1.40 m (8H), 1.36 m (8H), 1.01 t (12H) 2.79 t (8H), 2.38 m (8H), 1.50 m (8H), 1.02 t (12H) 2.92 t (8H), 2.42 m (8H), 1.56 m (8H), 1.00 t (12H) 2.90 t (8H), 2.44 m (8H), 1.52 m (8H), 1.01 t (12H) 2.94 t (8H), 2.46 m (8H), 1.50 m (8H), 1.01 t (12H) 2.87 t (8H), 2.42 m (8H), 1.48 m (8H), 1.04 t (12H) 3.92 t (8H), 2.19 m (8H), 1.64 m (8H), 1.11 t (12H) 777 4.0 q (4H), 0.65 t (6H) c 4.30 t (4H), 2.54 m (4H), 2.20 m (4H), 1.64 m (4H), 1.09 m (4H), 0.62 t (6H) c 4.50 t (4H), 2.84 m (4H), 2.20 m (4H), 1.67 m (4H) 1.01 m (4H), 0.60 t (6H) c 589 d(meso-Ar) 7.98 d (4H; o-H), 7.68 m (6H; m,p-H) 7.82 d (4H; o-H), 7.14 d (4H, m-H), 3.98 s (6H; CH3) 7.61 m (4H; o-H), 7.55 m (4H; m,p-H), 3.88 s (6H; CH3) 7.67 d (2H; o-H), 7.60 t (2H; m0-H), 7.19 t (2H; p-H), 7.10 d (2H; m-H), 3.84 s (6H; OCH3) 7.57 ± 7.65 m (4H; o,m0-H), 7.17 t (H; p-H), 7.10 d (2H; m-H), 3.78 s (6H; OCH3) 8.39 d (4H; o-H), 7.96 d (4H; m-H) 8.92 s (2H; o-H), 8.60 d (2H; o0-H), 8.24 d (2H; p-H), 7.77 t (2H; m-H) 7.99 d (4H; o-H), 7.72 m (6H; m,p-H) 7.87 d (4H; o-H), 7.70 d (4H; m-H), 3.99 s (6H; OCH3) 7.84 m (4H; o-H), 7.64 m (4H; m,p-H), 3.90 s (6H; OCH3) 7.91 m (2H; o-H), 7.66 m (6H; m,p-H), 3.81 s (6H; OCH3) 8.42 d (4H; o-H), 7.92 d (4H; m-H) 8.76 s (2H; o-H), 8.54 d (2H; o0-H), 8.20 d (2H; p-H), 7.72 t (2H; m-H) 77.70 m (6H; m,p-H), 7.96 d (4H; o-H) 7.78 m (6H; m,p-H), 7.99 d (4H; o-H) 8.42 d (4H; o-H), 7.94 d (4H; m-H), 3.92 s (6H; CH3)590 signals for the protons of the methyl groups (by 1.06 ppm) can be attributed to the shielding effect of the ring currents of the phenyl fragments.Apparently, the signals for the protons of the NH groups are substantially shifted downfield (*1.4 ppm) due to both the negative inductive effect of the phenyl fragments and a reduction in the strength of the macrocyclic ring current.The planar porphyrin macrocycle is deformed and its aromaticity is decreased due to steric strains caused by the phenyl groups. On going from the compound 3h to the compound 3n, the porphyrin macrocycle undergoes further deformation due to steric interac- tions between the butyl and phenyl substituents resulting in the downfield shifts (by 0.1 ppm) of the signals for the protons of the NH groups. The shielding effect of the ring currents of the phenyl fragments is manifested in the upfield shifts of the signals for the protons of the butyl substituents.These shifts are maximum (1 ppm) for the CH2 groups directly bound to the macrocycle, whereas the positions of the signals of the terminal CH3 groups remain virtually unchanged (0.02 ppm). The introduction of the substituents into the phenyl rings of 5,15-diphenylporphyrin (o,m,p-OMe or m,p-NO2) exerts only a slight effect on the positions of the signals for the meso protons and for the protons of the NH groups (see Table 14). In the 1H NMR spectra of the 5,15-dialkyloctamethylpor- phyrins 3a ± f, the signals for the protons of the NH groups are shifted downfield compared to those in the spectrum of the porphyrin 3t. Apparently, this is associated with the deformation of the molecule caused by the meso-substitution.The magnetic field of the p-electron ring current of the porphyrin, in turn, affects the positions of the signals for the protons of the alkyl meso- substituents. In this case, the spectra contain individual non- overlapping signals for the protons of the methylene and methyl fragments of the alkyl substituents. Thus, the spectrum of 5,15- dihexyl-2,3,7,8,12,13,17,18-octamethylporphyrin (3f) has the sig- nal for the protons of the methylene group, which is directly bound to the macrocycle, as a triplet at d 4.32, the signal of the adjacent methylene group as a quintet at d 2.5 and the signals for the protons of the next two CH2 groups as quintets at d 2.16 and 1.64.12 The signals of the last methylene group and the terminal methyl group are observed as a sextet at d 1.09 and a triplet at d 0.78.The signals of the b-methyl groups in the spectra of the compounds 3a ± f are shifted downfield by *0.4 ± 0.5 ppm com- pared to the corresponding signals in the spectra of 5,15-diaryl- porphyrins, which is attributed to elimination of the shielding effect exerted by the ring currents of the meso-aryl substituents. Not only the signals for the meso-protons, but also the signals for the protons of the NH groups disappear in the 1H NMR spectra on going from the meso-disubstituted porphyrins to the meso-tetrasubstituted compounds (3u ± w). The reasons for this phenomenon remain unclear. Analogous changes are observed in the spectrum of tetraphenyl-substituted b-octamethylpor- phyrin.27 The signals for the b-methyl protons of meso-tetrasubstituted porphyrins are observed in the region (at d*2.4) identical to that of 5,15-diarylporphyrins (see Table 14).It should be noted that the above-mentioned region is intermediate between the regions in which these signals are observed in the spectra of the octaalkyl- porphyrin 3t (at d 3.47) and its meso-tetraphenyl derivative (at d 1.76),25 which confirms the assumption that the magnetic anisotropy of the aryl meso-fragments exerts an effect on the protons of the b-substituents. The chemical shifts of the protons of the phenyl fragments are observed in the region d *7.7 ± 8. The signals of the unsubstituted phenyl groups of the compounds 3h,n,u,v are observed as a doublet for the ortho-protons and a multiplet for the meta- and para-protons. The chemical shifts of the protons of the substituted phenyl fragments vary depending on the nature of the substituent.Among the spectra of the porphyrins containing the electron- donating methoxy group in the meso-phenyl fragment, only the spectra of the para-substituted compounds 3i,o,w have pro- nounced individual signals, viz., a doublet for the ortho-protons and a doublet for the meta-protons. In the spectra of the meta- methoxy-substituted compounds 3j,p, the corresponding signals are observed as multiplets. In the spectra of porphyrins containing the electron-with- drawing nitro group in the meso-phenyl fragment (both in the meta- and para-substituted compounds), the signals are observed as pronounced multiplets.The spectra of the para-nitro deriva- tives 3l,r have a doublet for the ortho-protons and a doublet for the meta-protons. The signals of the meta-nitro derivatives 3m,s are observed as a doublet for the para-protons, a triplet for the meta- protons and a singlet and a doublet for the nonequivalent ortho- protons (see Table 14). The ortho-methoxy-substituted porphyrin 3k can exist as two atropoisomers (a,a and a,b), which differ by the orientation of the methoxy groups with respect to the porphyrin plane. The 1H NMR spectra of these compounds are characterised by the presence of the signals for the phenyl protons as multiplets. The spectrum of the a,a-atropoisomer has two doublets for the ortho- and meta-protons and two triplets for the meta- and para-protons.The spectrum of the a,b-atropoisomer has a doublet for the meta- protons, a triplet for the para-protons and a multiplet for the ortho- and meta-protons (see Table 14). 4. Bisporphyrins As mentioned above, the porphyrin fragments in cyclophane dimers are brought into proximity due to the bridging bonds located at the meso or b positions of the macrocycles. The lengths and rigidity of the bridges are responsible for the conformational flexibility of the dimer. Interactions between the p-electron systems of the macro- cycles are clearly manifested in the 1H NMR spectra. The chem- ical shifts of the protons depend on the length of the bridge and the mutual orientation of the macrocycles.64 ± 86 Dimeric porphyrins 7a ± f in which the porphyrin rings are linked through the b positions of the pyrrole rings by two amide bridges can exist as two isomers (syn and anti).The possibility of (CH2)mNHCO(CH2)n Et MeN N M1 N N Me Et Me Compound 7 M1 H2 H2 H2 H2 H2 H2 Co Co Ag Ag Mn Mn Fe Ag abcdefghijklmn N Zh Mamardashvili, O A Golubchikov Et Me Me Me N N M2 N N Me Et Me (CH2)mNHCO(CH2)n 7a ± n n m M2 H2 H2 H2 H2 H2 H2 H2 Co H2 Ag Mn Co Co Co 11111 0 1 1 1 0 2 2 1 1 2 3 1 1 1 1 1 1 1 11111Spectral properties of porphyrins and their precursors and derivatives formation of a particular isomer depends on the orientation of the porphyrin fragments at the instant the second amide bond is formed.72, 73 Figure 3 presents the 1H NMRspectrum of the anti-isomer of the bisporphyrin 7c, which has signals for four meso-protons (at d 9.29, 8.84 and 8.78) and signals for the protons of the alkyl substituents (at d 3.60, 3.64, 3.68 and 3.74).The signals for the protons of the NH groups are shifted upfield (d 77.78 and 77.81) compared to those observed in the spectra of their monomeric analogues (d 73.75). The amide bridges in the compound 7c contain two methylene groups each. Two of these groups bound to the NH groups give a quadruplet (d 5.8) and a doublet (d 4.7), whereas two other groups bound to the porphyrin fragment of the dimer give signals at d 4.1.The signals for the methylene protons of the ethyl groups are observed in the same region. The signals for the protons of the amide groups are observed as a doublet at d 6.97. The chemical shifts of the protons of the alkyl substituents in the compound 7c differ only slightly from those of the analogous protons in the monomeric analogue 8 (Table 15). 3 5 4 78 d /ppm 9 8 7 6 Figure 3. 1H NMR spectrum of the bisporphyrin 7c in CDCl3.73 Three homologous dimers 9a ± c were investigated.74 It was found that the porphyrin fragments approach each other by 100 ± 120 pm as the number of the methylene groups in the bridges is decreased by one. The spectral properties are indicative of the `face-to-face' orientation of the macrocycles. Me H13C6 Me HN N N NH Me Me R C6H13 Me H13C6 R Me HN N N NH Me Me C6H13 9a ± c R=(CH2)2CO(Bun)(CH2)3 (a), CH2CO(Bun)(CH2)3 (b), CH2CO(Bun)(CH2)2 (c).Compound 9 R/pm (see a) d(NH) (see b) l /nm (see c) a 76.2 640 383 b 76.6 540 540 c 78.5 420 420 a The distance between the porphyrin fragments; b in CDCl3; c the absorp- tion band in the electronic absorption spectrum measured in CH2Cl2. Table 15. Chemical shifts (ppm) of the protons of 4,8-di(2-ethoxycarbo- nylethyl)-2,6-diethyl-1,3,5,7-tetramethylporphyrin (8) and the dimeric porphyrins 7a ± f.72 Compound d(meso-H) d(NHpyrrole) 7a 7b 7c 7d 7e 7f 8 8.90 ± 8.98 8.70 ± 8.91 8.78 ± 9.29 8.70 ± 8.90 8.70 ± 8.90 9.00 ± 9.40 10.10, 10.16 77.81,77.93 78.14 77.78,77.81 78.00,77.90 78.30,78.00 76.2 73.75 1H NMR spectral studies revealed the existence of two con- formers of dimer 10a.In one of them, the porphyrin rings are located parallel to each other (anti). In another conformer, the porphyrin rings are bent with respect to each other (syn, `half- open shell').80 The conformers occur due to rigidity of the amide bonds and their appearance depends on the relative orientation of the carbonyl groups. The spectrum of a mixture of the conformers has four signals for the protons of the NH groups (at d 78) and twelve signals for the meso-protons (at d 7 ± 10). Four signals (at d 9) correspond to the meso-protons of the anti conformer and eight signals belong to the meso-protons of the syn conformer.The characteristic features of the crystal structure of the compound 10a are responsible for the rather steady existence of both con- formers in equal amounts. A decrease in the intensities of the signals for the protons in the spectrum of the anti conformer of the zinc complex of the dimer 10b compared to the signals for the protons of the syn conformer indicates that the anti : syn ratio is 1 : 9. On protonation of the porphyrin 10a, the amount of the anti conformer increases and the anti : syn ratio becomes equal to 1 : 6. No hypsochromic shift of the Soret band (see Section IV) is observed in the electronic absorption spectrum of the dimer 10a compared to the spectrum of the monomeric analogue.Et N Et N N M N N Et Et N O Et Et C Et N N M N N Et Et Et 10a,b M=2H (a), Zn (b). 1H NMR spectral studies established the characteristic struc- tural features of cyclophane dimers 11a ± d in which the tetraphe- nylporphyrin fragments are linked through the meta or ortho positions of the phenyl substituents by four ether bridges.77, 78 The spectrum of the dimer 11a differs from that of monomeric meso-tetra(3-butoxyphenyl)porphyrin in that the signals for the b-protons of the pyrrole rings are substantially split and the protons of the central CH2 groups of the bridges give two signals. The signals for the protons of the OCH2 and CH2 groups are broadened. These characteristic features are retained on going from the neutral to protonated form of the dimer.591 d(CH3) 3.65 ± 3.90 3.58 ± 3.60 3.60 ± 3.74 7773.67 O C592 N R N M N N R R N N M N R N11a ± g R=m-O(CH2)nO-m, n=4:M=H2 (a), Zn (b); R=o-O(CH2)nO-o, n=3:M=H2 (c), Zn (d); R=m-CH2NHCH2-m,M=H2 (e); R=m-CH2NTsCH2-m,M=H2 (f); R=m-CH2NTs-m,M=H2 (g). The cyclophane structure of the dimer imposes rigid limita- tions on the conformational flexibility of the O(CH2)nO chains. The fact that the protons of the bridging CH2 groups give two signals is indicative of a skewed conformation of the ether bridges due to which the benzene rings of the adjacent macrocycles are inclined relative to each other. As a result, both fragments of tetraphenylporphyrin in the dimer adopt a nonplanar dome- shaped conformation.The resulting strain is partially relieved due to corrugation of the tetrapyrrole rings, viz., two opposite pyrrole fragments are bent upward, while two other fragments are bent downward. The inclination of the phenyl groups in combi- nation with corrugation of the porphyrin macrocycle are respon- sible for the magnetic nonequivalence of the ortho-protons of the phenyl groups (external and internal) and the b-protons of the pyrrole rings. The external ortho-protons are located in proximity to the b-protons of the pyrrole rings directed upward, whereas the internal protons are located in proximity to the b-protons of the pyrrole rings bent downward. The distances between the pairs of the protons of the first type are smaller than those between the pairs of the protons of the second type due to which the deshield- ing effect of the ring currents of the macrocycle and the benzene ring in the first case is more pronounced (Table 16).The corrugated dome-shaped conformation of the macrocycle is retained in the protonated form of the dimers. In the latter case, Table 16. Positions and multiplicities of the signals in the 1HNMRspectra of the cyclophane dimers of tetraphenylporphyrin 11a,c and meso- tetra(3-butoxyphenyl)porphyrin.77 Solvent Compound d(b-H) d(Hbridge) a d(NH) 11a CDCl3 71.94 s 8.37 s 9.02 s 1.85 m (HCHCH2O) 2.27 m (HCHCH2O) 4.35 m (CH2O) 70.27 s CDCl3 ± CF3COOH 11c 71.32 s 8.22 s 8.43 s CDCl3 ± CF3COOH 71.03 s 8.89 s CDCl3 72.81 s 8.35 d 2.09 m (HCHCH2O) 8.51 d 2.34 m (HCHCH2O) 4.51 m (CH2O) 0.95 m (HCHCH2O) 1.93 m (HCHCH2O) 4.11 t (CH2O) 4.25 t (CH2O) 1.86 m (CH2CH2O) 4.15 t (CH2O) meso-Tetra- (3-butoxy- phenyl)por- phyrin a The d values are given for the underlined protons.N Zh Mamardashvili, O A Golubchikov one pair of the protons of theNHgroups is located in the centre of the macrocycle and is substantially shielded by the ring current, whereas the second pair of the protons is bent outward and the shielding effect of the ring current of the macrocycle exerted on the latter pair is weakened. Corrugation of the porphyrin macrocycles in the protonated form of dimeric porphyrin leads to weakening of the macrocyclic ring current and strengthening of the local currents in the pyrrole fragments.For the external b-hydrogen atoms, the above-men- tioned two effects compensate each other, whereas these effects with respect to the internal protons of the NH groups add up together resulting in substantial downfield shifts of the signals for these protons in the case of the dimeric porphyrin 11a compared to its monomeric analogue (see Table 16). The mutual effect of the macrocyclic ring currents of the adjacent porphyrin fragments leads to the nonequivalence of the protons of the NH groups in the dimer 11c. Due to corrugation of the porphyrin rings, the distance from one pair of the protons of the NH groups to the adjacent tetrapyrrole fragment is 0.8A larger compared to that for the second pair.78 This is a sufficient reason for the appearance of two signals for these protons in the 1H NMR spectrum (see Table 16).The bridging groups in the dimer 11c, unlike those in the dimer 11a, are conformationally flexible. Judging from the fact that the positions of the signals of two OCH2 groups are somewhat different (d 4.25 and 4.11), two conformations of the bridge in solutions are rapidly (within the NMR time scale) transformed into one another. Apparently, the conformation with the unsymmetrical arrangement of the OCH2 groups with respect to the tetraphenylporphyrin rings prevails. In the 1H NMR spectra of the cyclophane dimers 11e ± g in which the tetraphenylporphyrin fragments are linked through the meta positions of the phenyl groups by two- or three-membered bridges, the ring current exerts an even more pronounced shield- ing effect on the chemical shifts of the protons located below the plane of the adjacent porphyrin ring.The largest shifts are observed for the signals for the protons of the NH groups and for the ortho-protons of the phenyl groups because they are located between the porphyrin fragments.79 The shifts of the signals for the b-protons of the pyrrole rings are much smaller because these protons are located at large distances from the shielding region. Compound d /ppm phenyl o-H NH b-H 8.43, 8.46 8.17, 8.15 8.25, 8.43 8.86 11e 73.78 7.81 11f 74.42 7.14 11g 74.11 7.78 Monomer a 72.81 8.25 a 5,10,15,20-Tetra(3-bromomethylphenyl)porphyrin. It can be seen that the mutual effect of the porphyrin frag- ments is strengthened as the lengths of the bridges in the compounds 11e and 11g decrease.Steric restrictions resulting from the presence of the bulky substituents in the bridges of the dimer 11f lead to rotation of the macrocycles with respect to each other about the axis passing through the centres of both porphyrin rings (in the dimers 11f and 11e, the angle of rotation of the macrocycles with respect to each other is 26 8 and 13 8, respec- tively). The molecule adopts an energetically more favourable conformation in which the porphyrin fragments, while remaining parallel to each other, are located at shorter distances and exert a more substantial shielding effect.This fact is confirmed by the 1H NMR spectral data. As in the case of the dimers 11a,c, the characteristic features of the cyclophane structures of the dimers 11f,g are responsible for the magnetic nonequivalence of the ortho- protons of the benzene rings (internal and external) and the b-protons of the pyrrole rings. In dimeric porphyrins 12a ± d, the aromatic bridge does not provide the `face-to-face' orientation of the macrocycles.81 ThisSpectral properties of porphyrins and their precursors and derivatives Table 17. Positions and multiplicities of the signals in the 1H NMR spectra of the dimeric porphyrins 12a ± d and 2,8,12,18-tetraethyl- 3,7,13,17-tetramethyl-5-phenylporphyrin.81 Compound d(meso-H) d(Ar) d(NH) 12a 10.15 s (4H) 72.27 s (4H) 12b 10.34 s (4H) 71.90 s (4H) 12c 73.0 s (2H), 72.1 s (2H) 12d 7.94 s (1H), 8.32 t (1H), 8.89 d (2H) 8.00 d (2H), 8.70 d (2H) 7.95 s (1H), 8.29 t (1H), 8.95 d (2H) 8.01 d (2H), 8.74 d (2H) 73.0 s (2H), 71.95 s (2H) 72.39 s (2H) 9.83 s (1H), 10.10 s (2H), 10.15 s (2H) 9.95 s (1H), 10.26 s (2H), 10.34 s (2H) 9.79 s (1H), 10.21 s (2H) 8.00 d (2H), 8.24 t (2H), 8.75 t (1H) 2,8,12,18-Tetra- ethyl-3,7,13,17- tetramethyl-5- phenylporphyrin fact is clearly manifested in the 1H NMR spectra of the above- mentioned compounds (Table 17).The spectra of the unsym- metrical dimers 12c,d are of most interest. The signals for the protons of the NH groups in these spectra are observed as singlets atd 72 and73, which is indicative of the nonequivalence of the p-electron systems of the porphyrin fragments of these dimers.Neither the spectra of the unsubstituted dimers 12e,f nor the spectra of the symmetrical compounds 12a,b show analogous splitting of the signals for the protons of the NH groups. The spectra of the dimers 12a,b have one singlet each at d 72.27 and 71.90, respectively. The high-field signal (at d 73.0) was assigned to the meso-unsubstituted porphyrin fragment in the dimers 12c,d, while the low-field signal (at d 72.1 or71.95) was assigned to the meso-substituted porphyrin fragment.81 The chemical shifts of the protons in these compounds correspond to those in the linear meso-bound dimers of porphyrins whose spectra have signals for the protons of the aliphatic alkyl groups at d 1.0 ± 4.0, signals for the meso-H atoms at d 9.8 ± 11.0 and signals for the protons of theNHgroups at d 72.0 and73.0.The signals for the protons of the phenyl fragments are observed at d 7.95 ± 8.95. These chemical shifts have different values in the spectra of the dimers 12a,c and 12b,d (see Table 17). The multi- plicities and intensities of the signals correspond to the assumed structures of the compounds. Et Et Et Et Me Me Me Me NH NH N N R1 Ar R2 N N HN HN Me Me Me Me Et Et Et Et 12a ± f O O R1=H, R2= : Ar= (b); (a); R1=R2= : O O (f). (e); (c), (d); R1=R2=H: Ar = Ar=Dimeric meso-diphenyl-b-octaalkylporphyrins 13a ± i con- taining the bridging O(CH2)nO groups at different positions of the phenyl substituent were investigated.82 ± 86 593 R2 R2 R1 R1 NH N HN N R1 O O R1 R2 R2 (CH2)n (CH2)n R2 R2 R1 R1 O O NH N HN N R1 R1 R2 R2 13a ± i R1=Me, R2=Bu: n=2, para (a); n=3, para (b); n=4, para (c); R1=Me, R2=Et: n=3, meta (d); n=4, meta (e); R1=Et, R2=Me: n=4, meta (f); R1=Me, R2=Bu: n=2, ortho (g); n=3, ortho (h); n=4, ortho (i).The cyclophane structures of these compounds were con- firmed by the 1H NMR spectral data (Table 18). The meso- protons and the protons of the b-CH3 and NH groups give singlet signals. This indicates that the dimers adopt a symmetrical conformation in which the porphyrin macrocycles are located one above the other.However, it is conceivable that the singlet signals are observed due to the time-averaging of different (including unsymmetrical) conformations of the porphyrins. In the spectra of the dimers under consideration, the signals for the protons of the NH groups are shifted upfield compared to those of the monomeric analogues 3i ± k, which is a manifestation of the mutual magnetic shielding of the ring currents of the aromatic macrocycles. Judging from the positions of the signals, the distance between the porphyrin fragments increases as the length of the bridging group increases (see Table 18). This distance is responsible also for the upfield shifts of the signals for the protons of the methylene groups in the spectra of the compounds 13a,b,d,g,h (in which the number of the methylene units in the bridges n=2 and 3) compared to the signals for the protons of the methoxy groups in the spectra of meso-di(methoxy- phenyl)-substituted monomeric porphyrins.Starting from n=4, these signals in the spectra of both the dimeric and monomeric compounds are observed virtually in the same region regardless of the positions of the bridging units in the benzene rings of the compounds 13c,e,i. The chemical shifts of the b-alkyl protons change only slightly on going from the monomers to the corresponding dimers. However, these shifts are substantially different for different structural isomers. Thus, the dimers 13e,f differ by the arrange- ment of the methyl and ethyl groups at the b positions of the porphyrin fragments.In the porphyrin 13e, the protons of the methyl groups experience the shielding effect of the ring currents of the phenyl substituents. In the case of the dimer 13f, the shielding effect is exerted on the protons of the ethyl groups. The maximum shifts (1.08 ppm) are observed for the protons of the methylene groups directly bound to the macrocycle, while virtu- ally no shifts (0.02 ppm) are observed for the signals for the protons of the CH3 groups. The signals for the protons of the bridging OCH2 groups and the signals for the protons of the OCH3 groups of the monomeric analogues are located nearly in the same region. The protons of the bridging CH2 groups give signals at d *1.90, which is characteristic of the protons of the corresponding substituents in the monomeric porphyrins.12 Apparently, this is attributable to the fact that the peripheral substituents are not virtually enclosed by the ring current of the conformationally flexible dimers.The signals for the protons of the phenyl groups in the spectra of the dimers 13a ± c are observed as a doublet for the ortho-594 Table 18. Positions and multiplicities of the signals in the 1H NMR spectra of the dimeric porphyrins 13a ± i and their monomeric analogues 3i ± k.82 ± 86 d(NH) Com- pound 13a 75.27 s (4H) 13b 74.92 s (4H) 13c 74.64 s (4H) 3i 72.38 s (2H) 13d 75.82 s (4H) 13e 74.32 s (4H) 13f 74.28 s (4H) 3j 72.49 s (2H) 13g b 7 13h b, c 7 13h b, d 7 13h b, e 7 13i b 7 3k 72.38 s (2H) a Signals for the protons of the methoxy group; b zinc complexes.Atropoisomers of the zinc complex of the dimer 13h: c a,a,a,a; d a,b,a,a; e a,b,a,b. protons (at d *7.8) and a doublet for the meta-protons (at d *7.15). The dimers linked through the meta (13d ± f) or ortho positions (13h,i) of the phenyl substituents exist as three atropoisomers, which differ by the orientations of the oxygen atoms of the ether bridges (Fig. 4). The compound 13g containing short ether bridges (n=2) does not form atropoisomers. The assignment of the signals to particular atropoisomers was made based on comparison of the chromatographic mobilities and polarities of the porphyrin molecules. From Table 19 it can be seen that the experimental data (Rf) agree with the calculated characteristics d(Alk) 1.03 t (24H), 1.63 m (16H), 2.10 m (16H), 3.90 t (16H) 0.98 t (24H), 1.36 m (16H), 2.11 m (16H), 3.84 t (16H) 1.00 t (24H), 1.48 m (16H), 2.12 m (16H), 3.87 t (16H) 1.03 t (12H), 1.67 m (8H), 2.10 m (8H), 3.85 t (8H) 0.97 t (24H), 2.97 q (16H) 1.03 t (24H), 3.91 q (16H) 1.00 t (24H), 2.99 q (16H) 1.03 t (12H), 1.67 m (8H), 2.14 m (8H), 3.93 t (8H) 0.98 t (24H), 1.36 m (16H), 2.01 m (16H), 3.85 t (16H) 0.96 t (24H), 1.48 m (16H), 2.12 m (16H), 3.84 t (16H) 0.99 t (24H), 1.52 m (16H), 2.12 m (16H), 3.87 t (16H) 1.01 t (24H), 1.49 m (16H), 2.13 m (16H), 3.86 t (16H) 1.07 t (24H), 1.57 m (16H), 2.07 m (16H), 3.89 t (16H) 1.09 t (12H), 1.67 m (8H), 2.18 m (8H), 3.99 t (8H) d(Hphenyl) d(b-CH3) 2-H, 6-H 7.80 d (8H) 2.39 s (24H) 7.84 d (8H) 2.41 s (24H) 7.81 d (8H) 2.42 s (24H) 7.82 d (4H) 2.42 s (12H) 7.69 m (8H) 3.42 s (24H) 7.71 m (8H) 2.40 s (24H) 7.80 m (8H) 3.38 s (24H) 7.61 m (4H) 2.50 s (12H) 7.74 m (4H) 2.50 s (24H) 7.70 d (4H) 2.46 s (24H) 7.60 m (4H) 2.48 s (24H) 7.57 m (4H) 2.49 s (24H) 7.82 m (4H) 2.44 s (24H) 7.67 d (2H) 2.53 s (12H) 3-H, 5-H 4-H 7.10 d (8H) 7 7.14 d (8H) 7 7.24 d (8H) 7 7.13 d (4H) 77.22 m (4H) 7.22 m (4H) 7.38 m (4H) 7.38 m (4H) 7.42 m (4H) 7.42 m (4H) 7.55 m (2H) 7.55 m (2H) 7.36 m (4H) 7.36 m (8H) 7.21 t (4H) 7.64 t (4H) 7.12 d (4H) 7.19 t (4H) 7.65 m (4H) 7.11 d (4H) 7.17 t (4H) 7.65 m (4H) 7.11 d (4H) 7.42 m (4H) 7.42 m (8H) 7.19 t (2H) 7.60 t (2H) 7.10 d (2H) (the dipole moments of the molecules).The smaller the dipole moment of the molecule, the higher the chromatographic mobility of the atropoisomer. The atropoisomerism of porphyrins is well studied.89, 90 For the phenyl fragments to be rotated, the macrocycle must be deformed to reduce steric interactions between the ortho-substitu- ents and the protons of theNHorCH2 groups. The addition of the side methyl groups would be expected to hinder rotation. How- ever, the activation energy of thermal atropoisomerism is increased only slightly on going from meso-tetraphenylporphyrin to (meso-diphenyl)octamethylporphyrin.This phenomenon was N Zh Mamardashvili, O A Golubchikov d(Hbridge) d(meso-H) OCH2 CH2 9.10 s (4H) 4.01 t (8H) 7 9.43 s (4H) 2.01 m (4H) 3.92 t (8H) 10.12 s (4H) 1.94 m (8H) 3.90 t (8H) 3.98 s (6H) a 10.12 s (2H) 7 9.12 s (4H) 1.92 m (4H) 4.25 t (8H) 10.08 s (4H) 1.90 m (8H) 4.02 t (8H) 10.08 s (4H) 1.88 m (8H) 4.02 t (8H) 3.88 s (6H) a 10.18 s (2H) 7 9.08 s (4H) 4.06 t (8H) 7 9.52 s (4H) 1.98 m (4H) 4.01 t (8H) 9.50 s (4H) 2.01 m (4H) 4.03 t (8H) 9.54 s (4H) 2.04 m (4H) 4.11 t (8H) 10.09 s (4H) 1.96 m (8H) 3.94 s (8H) 3.66 s (6H) a 10.19 s (2H) 7Spectral properties of porphyrins and their precursors and derivatives O (CH2)n OO (CH2)nO O (CH2)nO Figure 4. The a,a,a,a- (a), a,b,a,a- (b) and a,b,a,b-atropoisomers (c) of the dimers 12d ± f.86 accounted for by the flexibility of diphenylporphyrins.89, 90 The molecules of these compounds possess two free meso positions Table 19.Dipole moments (m) and chromatographic mobilities (Rf) on Silufol of the monomeric (3h ±m) and dimeric (13h) porphyrins.87, 88 Substituent in the benzene ring Com- pound 3h 3i 3j 3k H4-OMe 3-OMe 2-OMe 3l 3m 13h 4-NO2 3-NO2 2-O(CH2)4O-2 0.60 0.43 0.45 0.30 0.65 0.50 0.75 0.61 0.49 0.31 a The dipole moments were calculated by the semiempirical CNDO and INDO methods. The conformations with the minimum energies were optimised by the block-diagonal Newton ± Raphson method; b in the case of monomeric porphyrins, the spots were visualised with toluene; in the case of the atropoisomers of the dimeric porphyrin, the spots were visualised with a 1 : 4 pyridine ± hexane mixture.a O(CH2)n O b O(CH2)n O c O (CH2)n O Atropo- isomer m /D (see a) a,a a,b 0.107 0.681 0.674 0.970 0.100 0.424 0.026 0.075 0.755 1.398 a,a,a,a a,b,a,a a,b,a,b along which the ring is, apparently, folded. Therefore, steric restrictions arising from the introduction of additional b-methyl groups are compensated by flexibility of the diphenylporphyrin molecule. It was demonstrated 12 that, unlike the monomeric porphyrin 3k, which readily underwent atropoisomerisation in boiling o-xylene (starting from one isomer, a mixture of the a,a and a,b isomers was formed in a ratio of 1 : 1), the phenyl fragments in the dimeric porphyrins cannot rotate and the atropoisomers of these compounds are stable.The 1H NMRspectral data for the a,a,a,a, a,b,a,a and a,b,a,b isomers of the zinc complex of the dimer 13h are given in Table 18. As in the case of the monomer 3k, the spectra of the atropoisomers of the dimeric porphyrin 13h differ only by splitting of the signals for the protons of the phenyl groups. The spectrum of the a,a,a,a atropoisomer, which is, apparently, characterised by the higher degree of equivalence of the protons, has a doublet for the ortho-protons and a triplet for the para-protons. The nonequivalent meta-protons give individual signals as a doublet and a triplet. The spectra of the a,b,a,a and a,b,a,b atropoisomers have a pronounced triplet for the para- protons and a doublet from a portion of the meta-protons. The signals for the other portion of the meta-protons and the signals for the ortho-protons coalesce into a multiplet.IV. Electronic absorption spectra of porphyrins and metalloporphyrins Electronic absorption spectra of porphyrins are very character- istic and contain four low-intensity absorption bands in the visible region and one very intense band in the violet region of the spectrum (the Soret band). The commonly accepted classification of these bands is as follows. The bands I and III in the visible region of the spectrum (Fig. 5 a) belong to quasi-forbidden electron transitions, whereas the bands II and IV are of elec- tronic-vibrational origin, i.e., are vibrational satellites of the bands I and III, respectively.Although possessing a number of common features, the spectra of porphyrins show substantial variations, which reflect the changes in the molecular structure and the effect of the solvent. According to the universally accepted concepts, light absorption is accompanied by excitation of the porphyrin molecule and the characteristic features of the absorp- tion spectrum are determined by transitions of the p-electrons between two higher occupied and two lower unoccupied orbitals (the four-orbital Platt ± Gouterman model 1). The intensity ratio of the absorption bands in the spectra of porphyrins depends on their structures in a peculiar fashion.The ethio type of the spectrum (see Fig. 5 a) with the intensity ratio IV>III>II>I is typical of meso-tetraphenylporphyrin, ethio- a D Rf (see b) IV 1.0 III II I 0.5 0 700 500 600 l /nm D c 1.0 0.5 0 600 500 700 l /nm 595 b D 1.0 0.5 0 700 600 l /nm 500 Figure 5. Basic types of elec- tronic absorption spectra of ethio- (a), rhodo- (b) and phylloporphyrins (c).1, 7596 porphyrin and other symmetrically substituted porphyrins. For porphyrins characterised by the rhodo type of the spectra (Fig. 5 b), another sequence, viz., III>IV>II>I, is realised. Among the latter compounds are rhodoporphyrin and other porphyrins containing an electron-withdrawing substituent (COOH, NO2, Cl, etc.) in the pyrrole fragment.Phylloporphyrin and other porphyrins containing one or two meso-substituents are characterised by the phyllo type of the spectrum (IV>II>III>I ) (Fig. 5 c). Chlorin, tetrabenzoporphyrin and phthalocyanine give characteristic spectra of their own. In their spectra, the long-wavelength band I has the maximum intensity. On going from porphyrin to metalloporphyrin, the symmetry of the planar macrocyclic fragment (of the p-electron cloud of the macrocycle) increases due to which the spectrum is simplified. The Soret band changes only slightly upon complex formation. The visible region of the spectra of metal complexes has two absorp- tion bands, viz., the band I corresponding to the electron tran- sition and the band II corresponding to the electronic-vibrational transition. The energy of the electron transition for the band I and its position in the spectrum (l1) can serve as reliable criteria for determining the strength and the type of the metal (M)7nitrogen chemical bond in metalloporphyrins. The band I is shifted hypsochromically as the strength of the M7N coordination bond in metalloporphyrins containing the planar MN4 coordina- tion unit increases.91, 92 One of the most important features of metalloporphyrins is the deviation of the metal atom from the plane of the MN4 coordination unit under the influence of the strongly coordinated molecular ligands or acido ligands.In most cases, the deviation of the metal atom from the plane is accompanied by a long-wave- length shift of the band I, whereas the movement of the metal atom toward the plane is accompanied by a short-wavelength shift.7 Therefore, electronic absorption spectroscopy is a powerful tool in studying the type and strength of the coordination bonds in metalloporphyrins, the degree of aromaticity of the macrocycle and its change upon chemical modification of the porphyrin.1. Porphyrins Electronic absorption spectra of porphyrins depend substantially on both the electronic and steric factors. Not only the positions and intensities of the absorption bands, but also their number can be changed. From this viewpoint, of special note is the introduc- tion of four substituents at the meso positions of porphyrin.A rise in the aromaticity and an increase in the rigidity of the macrocycle lead to the complete disappearance of vibrational satellites and to a sharp increase in the intensities of the electron transition bands (Fig. 6). The band I in the electronic spectrum of tetraazopor- phyrin is shifted to the short-wavelength region.6 On the whole, the introduction of substituents at the meso positions of the porphyrin macrocycle has a more substantial D 1.0 0.5 1 2 0 700 500 600 l /nm Figure 6. Electronic absorption spectra of porphyrin (1) and tetraaza- porphyrin (2) in chloroform.6, 7 N Zh Mamardashvili, O A Golubchikov D 1.0 1 0.5 2 0 600 500 700 l /nm Figure 7. Electronic absorption spectra of meso-tetramethylporphyrin (1 ) and meso-tetraphenylporphyrin (2 ) in chloroform.25 effect on the absorption spectra than the introduction of sub- stituents at the b positions.The meso-tetrasubstituted porphyrins are characterised by a distorted ethio type of spectra with rather large (25 ± 40 nm) bathochromic shifts of the bands (Fig. 7, Table 20). It should be noted that small hypsochromic shifts of all absorption bands are observed on going from meso-tetrame- thylporphyrin to its homologues containing longer alkyl substitu- ents.38 Similar bathochromic shifts are observed in the electronic absorption bands on going from porphyrin to meso-tetraphenyl- or meso-tetramethylporphyrin (see Table 20). In these cases, the steric factor, apparently, prevails over the electronic effect. The introduction of various substituents at the para positions of the phenyl rings of meso-tetraphenylporphyrin causes batho- chromic shifts of the absorption bands in the visible region of the spectra.A comparison of the spectra of tetraphenylporphyrin and its para-substituted derivatives demonstrated that the intensities of the electron transition bands (I and III) rise as the electron- donating properties of the substituents increase. Simultaneously, the intensities of the vibrational bands (II and IV) are dimin- sihed.93 The ethio type of the spectrum of tetraphenylporphyrin is distorted because the band I becomes more intense than the band II. The presence of electron-donating substituents at the meta positions also leads to bathochromic shifts of the bands, the intensities of the vibrational bands being increased, while the intensities of the electron transition bands remaining virtually unchanged.25 The spectra of ortho-substituted tetraphenylpor- phyrins are characterised by lower intensities of the electron transition bands compared to those observed in the electronic absorption spectrum of tetraphenylporphyrin. In some cases, this gives rise to the spectra of the phyllo type, for example, in the case of tetra(2-halogenophenyl)porphyrins and, particularly, in the case of tetra(2,6-dihalogenophenyl)porphyrins.The absorption spectra are also dependent on the solvent. Thus, the bands in the electronic absorption spectra of substituted meso-tetraphenylporphyrins are shifted bathochromically and the intensities of the electron transition bands grow on going from nonpolar hexane to polar pyridine.25 It is known 39 that the introduction of methyl groups at the b positions of the porphyrin ring causes only slight bathochromic shifts of the absorption bands (5 ± 10 nm).The effect of the meso- substitution on the spectral characteristics of b-alkylporphyrins and their complexes with some transition metals (Zn, Cu or Ni) was investigated.14, 17, 38, 39 2,8,12,18-Tetrabutyl-3,7,13,17-tetra- methylporphyrin (3t), which is similar in the electronic effect of the substituents to natural porphyrins, was used as the starting compound. Derivatives of the porphyrin 3t containing aryl (3h ±m) or alkyl (3a ± g) substituents or aza bridges (14m) at positions 5 and 15 are structurally related to the most importantSpectral properties of porphyrins and their precursors and derivatives Table 20.Electronic absorption spectra of meso-tetrasubstituted porphyrins in chloroform.25, 38 meso-Substituents HHa Me Ph Ph b 4-NH2C6H4 4-HOC6H4 4-MeOC6H4 4-MeOC6H4 (see b) 4-MeC6H4 4-BrC6H4 4-NO2C6H4 4-NO2C6H4 (see b) 3-NH2C6H4 3-HOC6H4 3-MeOC6H4 3-MeC6H4 3-BrC6H4 3-NO2C6H4 2-NH2C6H4 2-HOC6H4 2-MeOC6H4 2-MeC6H4 2-BrC6H4 2-NO2C6H4 C6F5 The spectral characteristics are given: a for b-octamethylporphyrin; b for meso-tetraaryl-b-octamethylporphyrins. synthetic porphyrins, viz., to meso-tetraphenylporphyrin and tetraazaporphyrin.R2 R1 N X N R1 R2 14a ±m R1=R2=Me, X=CMe: M=Zn (a), Cu (b), Ni (c); R1=Me, R2=Bun: X=CPh, M=Zn (d), Cu (e), Ni (f); X=CH,M=Zn (g), Cu (h), Ni (i); X=N,M=Zn (j), Cu (k), Ni (l), H2 (m). As can be seen from Fig. 8, the absorption spectra of these compounds, as well as of their zinc complexes, are similar. The spectral changes are quantitative rather than qualitative. This fact indicates that the corresponding electron transitions are of the same type and suggests that the introduction of two phenyl or alkyl substituents or the replacement of the carbon atoms at the meso positions by the nitrogen atoms exerts a rather small perturbing effect on the p-system of the porphyrins and their zinc complexes under study, and the p-system retains the approx- imate symmetry D2h and D4h, respectively.It was demonstrated 17 that electron transitions in metal- loporphyrins occur from the a1u and a2u orbitals to the egx and egy orbitals. The a1u orbital has four nodal planes passing through the meso positions and the central nitrogen atoms. The a2u orbital possesses zero electron density on the b-carbon atoms. Hence, perturbation of the p-system of metalloporphyrins at the b positions, for example, upon alkylation, destabilises the a1u lmax /nm (e61073 /litre mol71 cm71) II I 562 (3.5) 568 (6.3) 610 (2.7) 590 (6.1) 602 (8.0) 599 (5.5) 595 (5.07) 595 (6.1) 608 (4.0) 593 (6.2) 592 (7.4) 592 (5.4) 600 (3.9) 592 (5.2) 588 (7.5) 590 (6.7) 592 (6.0) 592 (6.6) 592 (7.1) 595 (5.2) 590 (5.3) 590 (7.6) 590 (6.4) 590 (7.7) 596 (6.7) 583 (8.0) 615 (1.0) 620 (4.4) 664 (3.7) 648 (6.4) 694 (9.1) 654 (7.6) 650 (5.2) 652 (7.4) 703 (3.8) 648 (6.8) 650 (6.5) 650 (3.5) 697 (3.8) 649 (3.9) 645 (5.1) 648 (5.2) 648 (6.5) 652 (6.7) 655 (6.6) 653 (3.1) 652 (5.6) 646 (4.1) 648 (4.6) 647 (2.6) 654 (3.0) 657 (4.4) R2 R1 N X M N R1 R2 IV III 490 (9.3) 500 (12.1) 524 (6.8) 516 (19.4) 553 (9.4) 524 (12.0) 519 (11.4) 520 (17.7) 556 (5.27) 518 (18.8) 516 (21.9) 518 (15.3) 552 (4.09) 518 (15.1) 514 (20.2) 516 (20.6) 517 (18.7) 516 (20.2) 516 (20.6) 519 (16.2) 515 (16.6) 514 (21.9) 515 (20.5) 515 (22.5) 518 (19.4) 507 (22.3) 515 (2.7) 535 (9.1) 560 (5.2) 550 (8.6) shoulder 562 (12.0) 556 (7.9) 557 (11.9) shoulder 554 (10.3) 551 (10.6) 552 (7.6) shoulder 557 (7.1) 550 (8.4) 550 (7.8) 551 (8.7) 549 (8.5) 550 (7.3) 556 (5.8) 548 (5.6) 547 (7.3) 546 (6.3) 551 (4.3) 552 (6.8) 538 (4.1) orbital, while the energy level of the a2u orbital remains unchanged.On the contrary, perturbation at the meso positions and the nitrogen atoms has no effect on the a1u orbital and affects the a2u orbital. Two degenerate antibonding eg orbitals differ in that one of the nodal planes of the egx orbital and one of the nodal planes of the egy orbital pass through different pairs of the nitrogen atoms (in porphyrins, NH7NH).Figure 9 shows the changes in the energy levels of the a1u , a2u and eg orbitals (taking into account their nodal properties) that occur on going from the zinc complexes of the octaalkylporphyrin 14g to the zinc complex of the meso-diazaoctaalkylporphyrin 14j. The energy levels of the higher occupied (b1 and b2) and lower log e 63 54 213 3 500 400 Figure 8. Electronic absorption spectra of 2,8,12,18-tetrabutyl- 3,7,13,17-tetramethylporphyrin (1) and its 5,15-diaza- (2) and -diphenyl derivatives (3) in chloroform.17 597 Soret 394 (174) 400 (141) 420 (112) 419 (499) 454 (164) 428 (313) 423 (272) 423 (491) 454 (189) 412 (504) 412 (567) 426 (228) 454 (246) 427 (155) 419 (413) 420 (499) 420 (485) 420 (456) 424 (352) 422 (186) 419 (354) 419 (432) 418 (477) 420 (475) 424 (283) 414 (317) 600 l /nm598 eg eg x y a1u a1u a2u x x y y x b2 b1 a2u 1 0 2 0 Figure 9.Scheme of the energy levels of the higher occupied and lower unoccupied orbitals of octaalkylporphyrin (1), its 5,15-diaza derivative (2) and their zinc complexes (1 0 and 2 0, respectively).17 unoccupied (c1 and c2) orbitals of free porphyrins are also presented in Fig. 9. The replacement of the carbon atoms at the meso positions by the nitrogen atoms results primarily in lowering of the energy levels of the a2u and eg orbitals compared to the levels of these orbitals in the compound 14g and, as a consequence, in weakening of the configuration interactions between the excited one-electron states a1uegx and a2uegx and between a1uegy and a2uegy.Corre- Table 21. Electronic absorption spectra of b-octaalkylporphyrin derivatives.38, 39 Compound lmax /nm (log e) I In chloroform 625.0 (2.98) 777626.3 (2.98) 625.1 (2.93) 626.3 (2.97) 626.1 (2.94) 626.3 (2.96) 626.7 (2.91) 622.0 (3.42) 777623.6 (3.60) 621.9 (3.49) 622.4 (3.60) 624.6 (3.41) 622.5 (3.51) 77629.1 (3.48) 634.5 (3.28) 622.1 (4.01) 777 3a 14a 14b 14c 3b 3c 3d 3e 3f 3g 3h 14d 14e 14f 3i 3j 3k 3l 3m 14h 14i 3u 3v 14m 14j 14k 14l In acetic acid 3a 14a 3h 14d 3t 14g 14m 14j 77621.9 (3.78) 621.9 (3.90) 77626.1 (3.81) 624.6 (3.93) c1 c21 II 574.1 (3.17) 570.3 (3.66) 568.3 (3.58 556.0 (3.42) 572.4 (3.19) 573.7 (3.15) 572.7 (3.14) 572.7 (3.13) 573.0 (3.15) 576.2 (3.14) 574.7 (3.92) 575.1 (3.99) 573.2 (3.89) 570.1 (3.79) 572.1 (3.99) 574.2 (3.71) 573.8 (3.87) 576.0 (3.70) 576.4 (3.90) 569.2 (3.86) 557.1 (3.91) 574.5 (3.62) 576.1 (3.58) 580.4 (3.64) 589.3 (4.24) 587.6 (4.14) 576.4 (4.08) 564.1 (3.89) 570.3 (3.64) 574.7 (4.03) 574.7 (4.15) 592.2 (3.49) 592.4 (3.55) 7 7 7 393.5 (5.10) 7 7 7 379.9 (5.08) N Zh Mamardashvili, O A Golubchikov c1 c2 y b2 b1 2 spondingly, the intensity of the absorption band I in the electronic absorption spectrum of the complex 14j is higher than that present in the spectrumof the complex 14g.An analogous effect is observed in the case of free porphyrins.Thus, the bands I and II in the spectrum of the meso-diazaoctaalkylporphyrin 14m are more intense than those in the spectrum of the octaalkylporphyrin 3t. Weakening of the configuration interaction would be expected to cause hypsochromic shifts of the absorption bands in the visible region of the spectra of the aza derivatives 14j,m and the bath- ochromic shift of the Soret band. However, experimental studies demonstrated that the replacement of the carbon atoms by the nitrogen atoms has the opposite effect. Apparently, the direction of the shift of the absorption bands on going from the porphyrins 3t and 14g to their aza analogues 14m and 14j is determined by a change in the energy of the corresponding one-electron transi- tions, i.e., by stabilisation of the a2u and eg levels in the compound 14j and the b1 and c2 levels in the ligand 14m.On going from the above-mentioned porphyrins to their aza derivatives, the inten- sities of the electronic-vibrational bands II and IV of the por- phyrin and the band II of the complex are sharply diminished (see Fig. 8, Table 21). These bands borrow the intensity from the adjacent allowed Soret band.17 The smaller the difference between the energies of the interacting transitions, the higher the efficiency of this rearrangement of the intensities. The replacement of the carbon atoms by the nitrogen atoms is accompanied by a IV III 505.4 (3.77) 777509.4 (3.80) 509.1 (3.79) 509.2 (3.77) 509.7 (3.76) 508.4 (3.74) 507.9 (3.76) 508.1 (4.16) 777508.4 (4.12) 508.0 (4.01) 508.2 (4.09) 506.9 (4.16) 507.1 (4.13) 77507.1 (4.01) 508.4 (4.03) 506.0 (3.69) 777 538.2 (3.21) 537.1 (3.78) 534.1 (3.74) 526.4 (3.19) 539.0 (3.22) 538.5 (3.20) 539.7 (3.19) 539.1 (3.19) 539.4 (3.18) 540.1 (3.17) 541.1 (3.71) 541.4 (4.26) 539.6 (4.31) 536.1 (4.28) 542.4 (3.80) 542.4 (3.60) 541.8 (3.74) 544.0 (3.52) 543.0 (3.71) 534.1 (3.54) 526.1 (3.60) 540.4 (3.68) 542.1 (3.72) 544.4 (4.17) 548.5 (3.52) 544.1 (3.41) 536.4 (3.38) 537.0 (3.70) 7 7 7 419.0 (5.24) 405.1 (4.60) 7 7 434.8 (5.21) 7 7 434.0 (5.20) 407.8 (5.08) 407.2 (5.18) 548.2 (3.81) 548.7 (3.88) 77 Soret 405.1 (4.61) 408.6 (4.63) 405.7 (4.56) 398.4 (4.17) 405.3 (4.58) 408.7 (4.60) 405.1 (4.59) 405.3 (4.56) 407.1 (4.56) 409.0 (4.62) 409.8 (5.19) 412.3 (5.08) 410.4 (5.08) 408.6 (5.14) 409.4 (5.10) 410.1 (5.14) 409.0 (5.02) 409.0 (5.17) 410.4 (5.09) 399.7 (5.17) 396.7 (5.14) 421.4 (4.89) 422.1 (4.92) 375.1 (5.20) 378.6 (5.24) 376.2 (5.14) 370.1 (5.07)Spectral properties of porphyrins and their precursors and derivatives hypsochromic shift of the Soret band resulting in the low intensities of the electronic-vibrational bands in the spectra of the compounds 14m and 14j.Acomparison of the electronic absorption spectra of the metal complexes of octaalkylporphyrin derivatives (M=Zn, Cu or Ni) demonstrated that the bands are shifted to the low-frequency region.These shifts increase in the series Zn<Cu<Ni, i.e., as the strength of the metal7nitrogen coordination bond increases. An enhancement of the electronegativity of the metal atom leads to lowering of the electron density on the occupied p-orbitals of the porphyrin molecule, which is accompanied by hypsochromic shifts of the bands (particularly, of the band I ). An analogous regularity is also observed for the positions of the `metal-sensitive' bands in the IR spectra of porphyrins.11 The deformation vibration bands of the alkyl groups (at 955 and 910 cm71) and the stretching and deformation vibration bands of the meso-protons (at 3110, 1220 and 835 cm71) are shifted to the high-frequency region as the degree of coordination of the metal atom increases.On going from the octaalkylporphyrin 3t to the meso-diphe- nyloctaalkylporphyrin 3h, the Soret band is shifted bathochromi- cally (see Table 21). It was demonstrated 38 that the introduction of strong donor or acceptor substituents into the phenyl rings of the diphenyl-substituents porphyrins 3i ±m has virtually no effect on the absorption spectra. Apparently, all absorption bands are shifted bathochromically due to distortion of the planar structure of the porphyrin macrocycles due to the presence of the phenyl substituents. This suggestion is evidenced by the 1H NMR spec- tral data.12, 14 It is known that steric restrictions are responsible for analogous changes in the electronic absorption spectra of ortho-derivatives of tetraphenylporphyrin 81 and cyclophane dimeric porphyrin.76 A comparison of the electronic absorption spectra of the structural isomers 3h and 3n showed that the replacement of the methyl groups at positions 3, 7, 13 and 17 of the porphyrin macrocycle by the butyl groups leads to a sub- stantial increase in the bathochromic shift of the absorption bands resulting from additional deformation of the porphyrin molecule due to the mutual effect of the adjacent butyl and phenyl frag- ments. The electronic absorption spectra of the meso-dialkyl-substi- tuted octaalkylporphyrins 3a ± g belong to the ethio type.For the first four members of the series, the odd-even alternation of the positions of the bands I and III is observed (see Table 21). The porphyrin rings in the meso-tetrasubstituted octaalkyl- porphyrins 3u,v have distorted `corrugated' structures giving rise to substantial bathochromic shifts and broadening of the bands in their electronic absorption spectra compared to those observed in the spectra of tetraphenylporphyrin and meso-disubstituted octaalkylporphyrins (see Table 21). 2. Bisporphyrins The electronic absorption spectra of cyclophane dimeric porphyr- ins have the following distinguishing features as compared to the monomeric analogues:76 ± 86, 94 ± 102 (1) a relatively large hypsochromic shift and substantial broadening of the Soret band; (2) one diffuse absorption band at the long-wavelength side of the Soret band; (3) a small bathochromic shift in the visible region.The shifts of the bands in the absorption spectrum are determined by two factors, viz., by essential interactions between two identical porphyrin fragments whose nature depends on the symmetry of the dimer (the factor of exciton interactions) and by the effect of the solvent, which reflects the different degree of solvation of the ground and excited states (the factor of solvation). In the short-wavelength region, the effect of the first factor leads to the hypsochromic shift of the Soret band in the electronic absorption spectra of symmetrical porphyrins, while the second factor causes broadening of this band.The manifestation of the low-energy excited states is possible in the case of displacement or 599 D 1.0 63 0.8 0.6 0.4 2 0.2 1 3 0 400 600 500 l /nm Figure 10. Electronic absorption spectra of the monomeric porphyrin 8 (1) and the dimeric porphyrins 7a (2) and 7b (3) in dichloromethane.72 inclination of the porphyrin macrocycles due to which the Soret band has a shoulder at 450 nm. Small bathochromic shifts of the bands in the visible region are attributable to weakening of the effect of exciton interactions in the long-wavelength region of the spectrum. The electronic absorption spectra of the monomeric por- phyrin 8 and the b-bound dimers 7a,b72 are shown in Fig.10. The hypsochromic shift of the Soret band increases and the absorption bands in the visible region are shifted bathochromi- cally and become more diffuse as the number of units in the bridge increases. The changes in the positions of the amide groups in the bridges also exert an effect on the electronic absorption spectra (Table 22). In the electronic absorption spectra of the dimers 15a ± d and 16a,b linked through the flexible hydrocarbon bridges and rigid aromatic fragments, respectively, the Soret band is shifted hyp- sochromically and the bands in the visible region are shifted bathochromically compared to those in the spectra of their monomeric analogues.57, 70, 100 These shifts are determined by the strength of interactions between the porphyrin fragments, which depends not only on the distance between the macrocycles, but also on their mutual arrangement.The hypsochromic shift of the Soret band in the electronic absorption spectrum of the compound 15c containing two eight- atom bridges is 10 nm (Table 23). A decrease in the length of one bridge by two methylene units (the compound 15b) leads to a Table 22. Electronic absorption spectra of monomeric porphyrin (8) and the dimeric porphyrins 7a ± n in dichloromethane.72, 73 lmax /nm Com- pound III IV I II Soret 566 577 578 536 544 548 502 502 507 538 505 569 7 7 554 572 566 7644 626 640 7 7 7 627 7 7 7 626 7 7 7 624 622 7640 7 7 7 5607 7 558 566 370 376 372 381 380 384 380 382 386 392 352 382 380 384 400 548 526 556 534 552 520 536 510 7468 473 77498 7620 7a 7b 7c 7d 7e 7f 7g 7h 7i 7j 7k 7l 7m 7n 8600 Table 23.Electronic absorption spectra of monomeric and dimeric porphyrins in dichloromethane.57, 70, 100 Compound lmax /nm Soret 2,7,12,17-Tetraethyl- 385 390 386 392 394 378 402 396 3,8,13,18-tetramethylporphyrin 15a 15b 15c 15d 16a 16b 2,8,13,17-Tetraethyl- 3,7,12,18-tetramethyl- 5-phenylporphyrin decrease in this shift by 4 nm. Apparently, the interactions between the p-electron systems of the macrocycles are weakened and the effect of exciton interactions on the spectral properties is reduced due to inclination of the porphyrin macrocycles arising from the different length of the bridges. An analogous spectral behaviour was observed 94, 103, 104 in the case of dimers containing two flexible ester bridges.Me R Me HN N N NH Me RMe R (CH2)n Me HN N N NH Me R 15a ± d m=n=5 (a), m=8, n=6 (b), m=n=8 (c), m=n=10 (d). Et Et Me NH N HN N Me Et Et X Et Et Me NH N HN N Me Et Et 16a,b (a), X= The Soret band in the electronic absorption spectra of the dimer 17a and its zinc complex 17b is broadened and is hypsochromically shifted relative to the Soret band in the spectra of the monomer 8 and its zinc complex. The distance between the macrocycles in the N Zh Mamardashvili, O A Golubchikov Table 24.Characteristics of absorption and emission of the porphyrins 8 and 17.94, 103 Compound l /nm a I II III IV 619 566 531 496 397 402 401 385 388 397 620 620 623 622 625 630 626 567 567 569 570 572 578 559 532 531 534 535 537 542 537 498 497 501 502 503 508 501 88 d 8 e 17a 17b 17c a The absorption maximum of the Soret band; the electronic absorption spectrum was measured in dichloromethane; b the emission spectrum was recorded in dichloromethane on irradiation in the Soret region; c the quantum yield of fluorescence of the dimeric complexes with respect to the corresponding monomeric complexes; d the Zn complex of the compound 8; e the protonated form of the compound 8.dimers increases and interactions between the porphyrin frag- ments are weakened upon protonation (Table 24). Et Me Me N N M N N Et Me Et Me Me Me O N N O M N N (CH2)m Me Et 17a ± c M=H2 (a), Zn (b), H4 (c). Me The dimers adopting a `face-to-face' conformation are char- acterised by fluorescence quenching due to interactions between the porphyrin fragments. In the case of the dimer 17a, this quenching was *50% of the corresponding value for the mono- meric analogue. Me Me In the absorption spectra of the dimers containing four bridges, no hypsochromic shift is observed in the short-wave- length region (Table 25). The spectrum of the dimer 18a contain- ing the ester bridges is characterised (Fig.11) by noticeable broadening and the bathochromic shift of the band in the visible region of the spectrum, while the position of the Soret band D 1.0 Me 2 0.8 Me 0.6 1 0.4 (b). 0 500 400 Figure 11. Electronic absorption spectra of the dimer 18a (1) and tetra- meso-[p-2-(hydroxyethoxy)phenyl]porphyrin (2) in pyridine.105 frel c lem /nm b 1.0 1.0 1.0 0.54 0.20 0.71 621 572 596 624 581 596 MeO O Me 62 600 700 l /nmSpectral properties of porphyrins and their precursors and derivatives Table 25. Electronic absorption spectra of monomeric and dimeric porphyrins in pyridine.77, 104 Compound lmax /nm III IV Soret 542 7 444 446 435 414 414 415 581 7 7 620 675 590 591 593 594 572 550 550 552 550 533 515 516 517 515 558 433 7 562 562 523 7 426 422 11a 11b 11c 11e a 11f a 11g a Tetra(m-butoxyphenyl)- 420 porphyrin Zn complex of tetra- (m-butoxyphenyl)- porphyrin 18a 18b a The spectra were measured in chloroform.remains unchanged.70, 105 The absorption bands in the visible region of the spectrum of the dimer 11a containing ether bonds are shifted bathochromically 77 by 25 ± 40 nmand the Soret bands are shifted by 20 ± 25 nm compared to those observed in the spectrum of the monomeric analogue (Fig. 12 a). O N N M O N N O O O N O N M N N O O 18a,b O M=H2 (a), Zn (b). This spectral behaviour of the dimer 11a is attributed to extension of the conjugated p-electron system due to partial conjugation between the p-systems of the phenyl rings and the porphyrin macrocycles.77 This effect is most clearly pronounced in the electronic absorption spectrum of a solution of the compound 11a in CH3CO2H in which the dimer 11a exists in the protonated form (Fig.12 b). Protonation leads to an essential increase of deformation of the macrocycle, while the dihedral angle between the mean porphyrin plane and the plane of the benzene p-systems decreases significantly. A decrease in the length of the bridge by one methylene group on going from the dimer 11a to the dimer 11c is accompanied by hypsochromic shifts of all bands by 7 ± 21 nm (see Table 25).However, the bathochromic shift with respect to I II 694 666 694 646 649 650 651 597 7677 7 584 606 O OO a D 1.2 0.8 1 0.4 2 0 600 500 D b 1.2 0.8 1 0.4 2 0 600 500 Figure 12. Electronic absorption spectra of the dimer 11a (1) and tetra- meso-(m-butoxyphenyl)porphyrin (2) in pyridine (a) and acetic acid (b).77 the absorption bands in the spectrum of the monomeric analogue is retained. In the electronic absorption spectra of the cyclophane dimers 11e ± g, the Soret band is shifted to the short-wavelength region by 4 ± 5 nm and is broadened compared to the Soret band in the spectrum of the monomer (see Table 25).79 Broadening is more pronounced in the case of the dimer 11f in which the porphyrin fragments, while remaining parallel, are rotated with respect to each other (amax=268).The spectral characteristics are changed due to interactions between the dipoles of the electronic transi- tions in two porphyrin fragments located at angles other than 0 or 180 8 with respect to each other. Bookser and Bruice 79 believed that the observed broadening and shifts of the bands indicate that solutions contain the dimers 11e ± g predominantly in a twist conformation. In this case, steric restrictions are less significant and the adjacent porphyrin rings are brought into proximity, which is also confirmed by the 1H NMR spectral data. The electronic absorption spectra (Fig. 13 a,b) of the dimeric meso-diphenyl-b-octaalkylporphyrins 13a ± i containing the O(CH2)nO bridging groups were investigated.82 ± 85 The absorp- tion bands of all dimeric porphyrins 13 are broadened and shifted hypsochromically by 5 ± 20 nm compared to those in the spectrum of the monomeric porphyrin 3h (Table 26).The short-wavelength shifts of the bands increase regularly as the length of the O(CH2)nO bridge decreases and the adjacent porphyrin fragments approach each other. Considering this shift as a characteristic of the average distance between the porphyrin fragments in the dimer, it can be concluded that this distance decreases on going from the a,a,a,a to a,b,a,b atropoisomer (see Fig. 4). Calculations of the structures of the octamethyl analogues of the dimers by molecular mechanics demonstrated that the distance between the porphyrin fragments is no less than 3.6A.87 Consequently, exciton interactions between the porphyrin rings are responsible for the shifts of the absorption bands.Since the bands are shifted hypsochromically, the dimers have predom- inantly the `face-to-face' orientation with the parallel arrangement of the interacting dipole moments of the electronic transitions. Each atropoisomer of the dimer exists as an equilibrium mixture of a large number of conformers. The structures of some of these 601 700 l /nm 700 l /nm602 a D 1.0 62 0.5 2 1 0 400 500 Figure 13. Electronic absorption spectra of the porphyrin 3h (1) and the meta-bound dimer 13d (2) in chloroform (a) and acetic acid (b).82, 83 Table 26.Electronic absorption spectra of the dimeric porphyrins 3h and 13a ± i.83, 86 Compound lmax /nm (log e) Soret In chloroform 411 (5.35) 405 (4.33) 407 (4.91) 410 (4.96) 393 (5.01) 395 (4.98) 394 (5.04) 395 (5.00) 396 (5.07) 395 (5.01) 397 (5.11) 399 (5.01) 397 (5.07) 386 (5.10) 390 (4.99) 391 (4.92) 390 (4.99) 391 (5.08) 393 (5.00) 393 (5.01) 3h 13a 13b 13c 13d a 13d b 13d c 13e a 13e b 13e c 13f a 13f b 13f c 13g 13h a 13h b 13h c 13i a 13i b 13i c In acetic acid 3h 13a 13b 13c 13d a 13d b 13d c 13e a 13e b 13e c 13f a 13f b 13f c 13g 13h a 13h b 13h c 13i a 13i b 13i c Atropoisomers: a a,a,a,a; b a,b,a,a; c a,b,a,b.436 (5.38) 432 (4.90) 435 (4.96) 438 (4.93) 435 (4.92) 436 (4.87) 435 (4.96) 438 (4.91) 437 (4.90) 438 (4.94) 440 (4.92) 439 (4.81) 439 (4.86) 427 (4.90) 430 (4.87) 431 (4.73) 430 (4.81) 433 (4.73) 434 (4.60) 434 (4.86) 600 l /nm IV 506 (4.21) 503 (3.81) 506 (3.99) 507 (4.06) 497 (4.15) 497 (4.09) 496 (4.11) 498 (3.94) 500 (3.96) 497 (3.98) 501 (4.06) 503 (4.11) 500 (4.03) 494 (3.96) 495 (3.92) 496 (3.89) 495 (3.92) 495 (3.90) 498 (3.87) 496 (3,94) 77777777777777777777 N Zh Mamardashvili, O A Golubchikov b D 1.0 0.5 2 1 0 500 600 700 l /nm III II 576 (3.89) 573 (3.59) 575 (3.63) 576 (3.57) 565 (3.54) 568 (3.48) 566 (3.51) 569 (3.19) 571 (3.27) 569 (3.49) 573 (3.50) 576 (3.49) 576 (3.52) 566 (3.27) 563 (3.26) 566 (3.36) 563 (3.26) 561 (3.28) 566 (3.21) 562 (3.20) 540 (3.81) 538 (3.38) 537 (2.45) 540 (3.31) 531 (3.80) 534 (3.72) 530 (3.79) 534 (3.56) 537 (3.58) 533 (3.69) 541 (3.89) 542 (3.82) 539 (3.82) 530 (3.60) 531 (3.56) 533 (3.42) 531 (3.56) 530 (3.57) 535 (3.52) 533 (3.60) 572 (4.16) 556 (4.51) 558 (4.37) 559 (4.09) 552 (4.42) 552 (4.40) 552 (4.42) 552 (4.42) 552 (4.48) 552 (4.45) 555 (4.43) 555 (4.43) 556 (4.45) 549 (4.48) 550 (4.27) 551 (4.14) 550 (4.39) 551 (4.37) 552 (4.19) 552 (4.41) 77777777777777777777 I630 (3.45) 626 (3.01) 627 (3.19) 629 (3.07) 621 (3.28) 622 (3.39) 621 (3.40) 623 (3.01) 623 (3.08) 622 (3.11) 624 (3.28) 625 (3.37) 623 (3.42) 618 (3.08) 619 (3.08) 620 (3.13) 619 (3.08) 619 (3.07) 620 (3.04) 620 (3.00) 619 (3.88) 589 (3.17) 593 (3.29) 597 (3.11) 591 (3.59) 590 (3.50) 590 (3.54) 595 (3.51) 595 (3.49) 595 (3.54) 592 (3.35) 592 (3.29) 592 (3.54) 583 (3.47) 587 (3.61) 588 (3.40) 587 (3.52) 591 (3.62) 591 (3.36) 592 (3.50)Spectral properties of porphyrins and their precursors and derivatives conformers were studied by molecular mechanics (the MM+ force field).The atomic coordinates and the components of the potential energy of the optimised structures are summarised in the study.87 From Table 27 it follows that a change from one con- former to another is generally accompanied by insignificant changes in the energy of the molecule (at most 10 kJ mol71).In spite of the fact that it is presently impossible to carry out complete analysis of the potential energy surface of such complex mole- cules, the existence of various conformational isomers in the gaseous phase and solutions is beyond question. The results of geometry optimisation of dimeric porphyrins by theMM+method (Fig. 14) demonstrated 87, 88 that the aromatic tetrapyrrole macrocycle is deformed very readily. None of 15 conformers contains the planar porphyrin fragment. Conforma- tional rearrangements of the tetrapyrrole nucleus occur through changes of the dihedral angles C7C7C7C, C7C7C7N, etc., while the bond lengths and bond angles remain constant.Each conformer of the dimeric porphyrins 13a ± i has its own individual absorption spectrum. It is needless to say that these spectra are qualitatively similar to the spectra of the monomeric analogues. However, in a quantitative sense these differences may be significant. One would expect that the absorption bands of the conformers with the parallel and perpendicular arrangement of the porphyrin rings will be shifted hypsochromically and bath- ochromically, respectively, under the influence of the exciton factor. The experimental spectra represent superpositions of the spectra of all conformers present in solution due to which the absorption bands of dimeric compounds are broadened and their intensities are lower compared to those of monomeric porphyrin.At the same time, the corresponding absorption bands in the spectra of monomeric and dimeric porphyrins do not differ in oscillator strength ( f ), which was calculated according to the approximate equation f=(4.661079) emaxDn1/2 , where emax is the absorption coefficient at the absorption max- imum and Dn1/2 is the half-width of the band. The data in Table 26 show that the absorption bands in the spectra of the para-bound dimers 13a ± c are shifted bathochromi- cally compared to those of their meta- (13d ± f) and ortho- analogues (13g ± i). This phenomenon may be attributed both to an increase in the proportion of conformers of the `half-open shell' 1 4 7 2 5 8 3 6 9 Figure 14.Conformers of the dimeric porphyrins 13h (1 ± 3), 13d (4 ± 9) and 13a (10 ± 12).87, 88 603 Table 27. Strain energies of the conformers of the ortho-, meta- and para- bound dimers 13a,d,h (E /kJ mol71) and their components (kJ mol71): strain energies of bonds (E1), bond angles (E2), torsion angles (E3), van der Waals interactions (E4), stretching ± bending (E5) and Coulomb interac- tions (E6).87, 88 E6 E5 E4 E3 E E1 E2 Compo- Con- und 13 former a hda 56 56 56 55 54 54 52 54 58 53 54 53 74 71 63 35 20 65 40 79 14 90 84 71 78 86 91 78 83 80 85 81 81 72 92 99 19 20 20 18 18 20 20 18 18 21 20 21 650 653 684 618 599 646 623 654 591 669 674 683 12345678910 11 12 311111111212 412 421 453 430 423 425 424 421 419 431 423 437 a For notations of the conformers, see Fig.14. type and to a more substantial deformation of the porphyrin macrocycle in the para-bound dimers. It should be noted that not only the positions of the absorption bands, but also the ratio of their intensities in the electronic absorption spectra of dimeric porphyrins are changed. In the case of para-bound dimers, the phyllo type of the spectrum, which is characteristic of monomeric b-octaalkyl-meso-diphenylpor- phyrins, is retained, while the spectra of the ortho- and meta- analogues belong to the ethio type (Fig.15). The differences in the electronic absorption spectra are reduced due to protonation of the porphyrin rings of the dimers 13a ± i in acetic acid and, correspondingly, to an increase of the distance between the likely charged fragments. * * * The data surveyed in the present review give convincing evidence that the combined use of modern spectroscopic methods enables one to obtain comprehensive information on the molecular 1011 12604D 0.8 62 0.4 21 0 500 600 400 l /nm Figure 15. Electronic absorption spectra of the dimers 13a (1) and 13d (2) in chloroform.83, 86 structures of the newly synthesised compounds. The development of the spectroscopy of porphyrins provides the theoretical basis for the wider use of this unique class of tetrapyrrole macrocyclic compounds in technology, medicine and biology because the determination of the correlations between the structure and the spectral properties of the molecules can help in solving the problem of the construction of new materials with the desired properties. This review has been written with the financial support of INTAS (Grant YSF-00-37).References 1. G P Gurinovich, A N Sevchenko, K N Solov'ev Spektroskopiya Khlorofilla i Rodstvennykh Soedinenii (Spectroscopy of Chlorophyll and Related Compounds) (Minsk: Nauka i Tekhnika, 1968) 2. K N Solov'ev, L L Gladkov, A S Starukhin Spektroskopiya Porfirinov: Kolebatel'nye Sostoyaniya (Spectroscopy of Porphyrins: Vibrational States) (Minsk: Nauka i Tekhnika, 1985) 3.M Gouterman J. Chem. Phys. 30 1139 (1959) 4. M Gouterman J. Mol. Spectrosc. 6 138 (1961) 5. 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年代:2001

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Russian Chemical Reviews 70 (7) 607 ± 626 (2001) Detonation synthesis ultradispersed diamonds: properties and applications V Yu Dolmatov Contents I. Introduction II. Synthesis and morphology III. Elemental composition and admixtures IV. Surface properties V. Phase composition, microstructure and surface texture VI. Sorption properties and exchange capacity VII. Chemical properties of ultradispersed diamonds VIII. Spectroscopy IX. Thermal analysis X. Defects of the bulk structure XI. Electrophysical properties of ultradispersed diamonds XII. Fractionation process and sedimentation stability XIII. The diamond ± graphite structural phase transition XIV. A cluster model and the fractal nature of UDD aggregates XV. Comparison of properties of diamonds of different origins.XVI. Application fields of ultradispersed diamonds Abstract. nanomaterial novel a of properties the on data Published Published data on the properties of a novel nanomaterial viz. are (UDD), diamonds ultradispersed synthesis detonation , detonation synthesis ultradispersed diamonds (UDD), are summarised and systematised for the first time. Certain properties summarised and systematised for the first time. Certain properties of UDD synthesised under different conditions are compared. of UDD synthesised under different conditions are compared. Particular attention is drawn to substantiation of the cluster Particular attention is drawn to substantiation of the cluster model of UDD and the fractal nature of their aggregates. The model of UDD and the fractal nature of their aggregates.The potential application fields of UDD are considered. Operating potential application fields of UDD are considered. Operating parameters of new materials are presented. The bibliography parameters of new materials are presented. The bibliography includes 110 references includes 110 references. I. Introduction Detonation synthesis ultradispersed diamonds (UDD or nano- diamonds) pertain a class of nanomaterials the fabrication, modification, and application of which attract the attention of scientists and engineers from different countries. This area of science and technology is at the boundary of solid-state physics and chemistry and thus becomes the world's most rapidly growing field as regards fundings provided for its development.At present, a wide spectrum of nanoparticles is studied which differ in both their nature (metals, non-metals, chemical com- pounds) and the technologies for their production (gas-phase and plasmochemical syntheses, precipitation from colloidal solutions, thermal decomposition and reduction, mechanical synthesis, electrical-detonation and detonation syntheses) and the unique properties of particles and materials and articles manufactured on V Yu Dolmatov Special Design Bureau (SDB) `Tekhnolog' at the St. Petersburg State Technological Institute (Technical University), Sovetskii prosp. 33A, 193076 St. Petersburg, Russian Federation. Fax/Tel. (7-812) 100 38 98. E-mail: alsen@comset.net Received 28 February 2001 Uspekhi Khimii 70 (7) 687 ± 708 (2001); translated by T Ya Safonova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n07ABEH000665 607 608 608 610 611 611 613 615 616 616 617 617 618 618 621 621 their basis.The latter aspect is of prime importance in view of the development of novel industrial technologies which include miniaturisation of facilities. We believe that, among all types of nanomaterials, UDD represent the most interesting subject. Indeed, first of all, even a bulk diamond (e.g., natural diamond) exhibits a unique combina- tion of chemical, physicochemical (particularly, thermophysical) and mechanical properties. Second, diamonds resulting from miniaturisation or, more precisely, prepared in a form of nano- particles (mean size 4 ± 6 nm) from a plasma upon detonation of powerful explosive mixtures (EM), acquire more pronounced anomalous properties.Whereas ordinary (natural) diamonds are applied in practice, as a rule, as abrasives and cutting tools and substrates for micro- electronics, the application sphere of UDD is much wider. By chemical, barochemical and thermal treatment of UDD one can impart to them different surface properties. In this connection, the attention of scientists is focused on detailed studies of character- istics and properties of diamonds and a search for new application fields. In Russia, UDD are used as a composite material in electro- chemical and chemical metal ± diamond coatings, polymeric films and membranes; in different kinds of rubbers; as anti-friction and anti-wear additives to lubricants of different purposes; in finishing and super-finishing polishing compositions; in abrasives.Nano- diamonds serve in fabrication of magnetically soft and hard input media, anti-friction tribotechnological diamond-containing com- positions based on thermoplastic materials (used in friction joints), abrasion-resistant structure materials based on ethylene thermoplastic co-polymers and in manufacture of radiation- and ozone-resistant rubbers. Moreover, they are most important components in the reprocessing of worn tire rubber. Studies are launched on sintering of UDD under high dynamic and static pressures (for manufacture of cutting tools, abrasives, substrates for microelectronics), on using them in medicine (as anti-cancer drugs and a component of such drugs, an effective gastroenteritis608 remedy, an immobiliser of biologically active substances, a super- effective sorbent for purification of blood and lymph, etc.).In Russia, particularly, at SDB `Tekhnolog' industrial tech- nologies are developed for the detonation synthesis of UDD, which allow production of diamonds (as powders and suspen- sions) in amounts that run into tons. In this review, for the first time, an attempt is undertaken to describe the properties of detonation synthesis diamonds in sufficient detail. All the available literature on UDD is analysed for the last 15 years including publications by the author.Inasmuch as UDD are extremely complex materials and their characteristics are largely determined by the methods of prepara- tion, chemical purification and surface modification, the proper- ties of nanodiamonds reported in different studies can differ. The literature data reviewed do not exhaust the subject, because the approaches of different authors were different, the UDD samples used differed from one another and the studies were carried out under different conditions and often were sporadic. II. Synthesis and morphology Detonation-induced transformations of powerful explosives and their mixtures with the composition CaHbNcOd with a negative oxygen balance (i.e., with the oxygen content lower than the stoichiometric value) in a non-oxidising medium yields a con- densed carbon phase which involves ultradispersed diamonds.1, 2 The pressure and temperature in the detonation wave (P± T parameters) correspond to the region of thermodynamic stability of diamonds (P 510 GPa, T53000 K).3, 4 Such condensed carbon, which is commonly named a diamond blend (DB), can contain up to 75% UDD.Calculation and experimental methods have shown 5, 6 that the primary UDD particles with diameter of *4 nm form fractal cluster structures (of 30 ± 40-nm size) from which larger aggregates (of an order of magnitude of hundreds of nanometers) are formed. The clusters are built of a limited number of atoms (from several tens to several thousands) and, with regard to their properties, occupy an intermediate position between individual atoms and bulk solids.7, 8 In the liquid-phase oxidation of condensed carbon, non- diamond structures gradually decompose, the most sophisticated method of chemical purification of DB being their treatment by dilute nitric acid at elevated temperature and pressure.9, 10 According to the literature data,11 purified UDD contain no intermediate amorphous phase.This is indicated by the shape of the (111) band in X-ray diffraction spectra and the results of electron microscopy. Depending on the conditions of the deto- nation synthesis, the mean size of UDD grains varies from 2 to 20 nm. Diamond grains with sizes up to 100 nm are shown to exhibit highly defective block structures (1 ± 3-nm blocks were observed in dark-field images).12 With an increase in the grain size, the crystalline structure becomes more perfect.These results are confirmed in studies by high-resolution electron microscopy.13 It was found 14 that UDD are single-phase materials and contain no Lonsdalite, graphite and other admixtures. The size of subgrains (mosaic blocks) was found to be 4 nm by the method of diffractometry of co-ordination spheres,15 no lattice micro- distortions were revealed and the dislocation density determined from the value of subgrain size by the Williamson ± Smallman relationship N=3/z2 was found to be equal to 1.861017 m72. An X-ray diffraction pattern for UDD powders 16 also char- acterises UDD as a diamond phase containing no admixtures of Lonsdalite and graphite. The coherent-scattering regions (CSR) were found to be of a mean size of 4.20.2 nm and represent the properUDDgrains of diamond-lattice structure and an insignificant amount of aggre- gates of carbon atoms dispersed throughout the bulk which form no lattice and are mainly spaced at*0.15 nm from one another.16 Along intergrain boundaries, carbon-atom aggregates are located for which the distribution in interatomic distances is close to V Yu Dolmatov Gaussian.The latter fact indicates that the structure of intergrain boundaries is amorphous. Sizes of UDD crystals were estimated based on the so-called phonon confinement model.17 ± 19 It was shown that the nano- crystal size is 4.3 nm, which coincides with the results X-ray diffraction analysis of the same samples.20 In their usual state, UDD represent a powder of a specific surface area of 250 ± 350 m2 g71 and a pore volume of 0.3 ± 1.0 cm3 g71 (see Ref.21). Heating of samples to 1273 K does not result in any decrease in the specific surface area. In a suspension, the size of UDD particles reaches 0.0561076 m; in a dried form, nanodiamonds represent a polydispersed powder. On heating above 873 Kin an inert atmosphere,UDDparticles begin to grow and acquire spherulitic shapes which measure up to (150 ± 200)61076 m and disintegrate at a load of 10 ± 15 kg mm72. No polycrystals are formed in this case. Due to the non-equilibrium conditions of their synthesis, UDD particles display a high density of defects, an active and well-developed surface (reaching 450 m2 g71) and an excess enthalpy of formation.III. Elemental composition and admixtures Ultradispersed diamonds are characterised by the presence of a substantial amount of difficultly removable volatile and solid admixtures. The former include adsorbed gases (CO, CO2, N2), water and acids remaining after chemical purification (HNO3, H2SO4). The latter group of admixtures involves non-diamond carbon, insoluble metal compounds (oxides, carbides) and salt admixtures. Powders of UDD can be considered as a certain diamond-containing composite material that consists of different carbon forms (80% ± 89%), nitrogen (2% ± 3%), hydrogen (0.5% ± 1.5%), oxygen (up to 10%), and an incombustible residue (0.5% ± 8.0%).Carbon represents a mixture of proper ultradis- persed diamond (90% ± 97%) and non-diamond carbon (3% ± 10%). In detonation synthesis nanodiamonds, the impurity percentage is higher compared with other artificial diamonds (diamonds prepared by steady-state synthesis contain no less than 96% carbon); and, therefore, the effect of impurities on physical and chemical properties should be more pronounced for UDD compared with other diamonds. Impurities present in UDD can be arbitrarily divided into the following groups: (1) water-soluble ionised (free electrolytes); (2) chemically bonded to the diamond surface and prone to hydrolysis and ionisation (salt forms of functional surface groups); (3) water insoluble (mechanical impurities, non-dissociating salt and oxide forms of surface impurities); (4) incorporated into the diamond lattice and encapsulated.The impurities of the first and second groups are formed in the stage of chemical purification of UDD by acids.9 Major water- soluble admixtures (first group) are removed by washing UDD with water. To make the removal more efficient, the suspensions are additionally treated by ion-exchange resins. Surface functional groups (the second group of impurities) of the COOH, OH, SO3H, NO3 and NO2 types can be considered as ionogenic with the diamond itself as the ion-exchange material.22 In this case, treatment of UDD aqueous suspensions by ion- exchange resins, which involves demineralisation of surface groups, is also the most efficient.Water-insoluble impurities (the third group) represent both individual microparticles of metals, oxides, carbides, salts (sulf- ates, silicates, carbonates) and surface compounds of salts and metal oxides incapable of dissociation. For their removal, i.e., transformation into soluble forms, UDD are treated by acids. By using different methods of UDD purification, one can remove 40%± 95% of the impurities of these three groups. All attempts to obtain individual UDD have failed. This is explained by the fact that the impurities of the fourth group are virtuallyDetonation synthesis ultradispersed diamonds: properties and applications impossible to remove by chemical methods; moreover, the tran- sition of third-group impurities into a soluble state is incomplete. The main admixed elements are silicon, calcium, iron and sulfur; titanium, copper, chromium, and potassium are present virtually always in small amounts. Having an active and well- developed surface, nanodiamonds can sorb solution impurities.Hence, the presence of certain impurities, namely, silicon, potas- sium and, partially, iron can be attributed to the hardness of water employed in UDD purification. Being among the major technological impurities in UDD, iron in concentrations of no less than 0.1 mass% ¡¾ 0.5 mass% (which corresponds to the presence of insoluble iron compounds, prefer- entially on the surface) is difficult to remove.Powders of UDD also contain substantial amounts of volatile admixtures (*10 mass%);23 thermal treatment of UDD in vac- uum (0.01 Pa) allows one to either remove the volatile impurities or substantially decrease their content. The optimum temperature of annealing is 673 K. Ultradispersed diamonds studied in Ref. 21 were of the following composition: carbon, up to 88%; hydrogen, up to 1%; nitrogen, up to 2.5%, oxygen, up to 10%. The same ratio of elements remains after a thermal treatment up to 1273 K in either reductive (hydrogen) or neutral (argon) atmosphere. The carbon content in UDDsamples passes through a maximum in a range of 973 ¡¾ 1073 K, whereas hydrogen and nitrogen contents remain virtually unchanged. The maximum density of UDD was reached after heating in an argon atmosphere at 1073 K and amounted to 3.216103 kg m73.Scientists from the Russian Federation Nuclear Centre�¢All- Russian Research Institute of Technical Physics (RFNC-ARITP, Snezhinsk) have determined the contents of admixed elements in UDD samples manufactured at SDB `Tekhnolog' and RFNC- ARITP (Table 1). Table 1. Content (mass %) of admixed elements in UDD samples (AES data). Admixture UDD powder of UDD-STP brand a Fe 0.150 Cr 0.070 Si 0.300 Al 0.005 Na 0.030 K 0.002 Cu 0.005 Ca 0.002 Mg 0.005 Mn 0.001 Ti 0.010 Pb 0.001 Total 0.580 Incombustible 0.950 residue a Produced at SDB `Technolog' in accordance to specification TU 05121441-275-95. bProduced at RFNC-ARRITP.c Undetected at a sensitivity of 561074 mass %. Due to the fact that the time of synthesis ofUDDis very short, surface carbon atoms have no time to stabilise by closure of all the bonds in the newly formed diamond lattice. They form a bond or two bonds with other elements of explosive materials, viz., hydro- gen, nitrogen and oxygen. Hence, it is of great importance to assess the changes in the relative contents of heteroatoms that stabilise surface carbon atoms on the interface as a function of chemical treatment conditions and the gree of oxidative decom- position of DB.24 For this purpose, an original diamond blend is Typical UDDb Aqueous UDD suspension of UDD-TAH brand a 0.100 0.500 0.150 0.010 0.050 0.002 0.003 0.010 0.005 0.001 0.002 0.001 0.840 1.400 0.100 0.050 0.100 0.001 0.003 0.002 0.002 0.002 0.003 see c 0.005 0.001 0.270 0.400 Table 2.Elemental composition of DB.24 Treatment conditions DB sample Original, a=0 Treated by hydrocarbons CnH2n+2 Treated by alcohols CnH2n+1OH Oxidative decomposition, a=26.3% Oxidative decomposition, a=31.8% UDD sample Original, a=55.0% Oxidative decomposition, Oxidative decomposition, Oxidative decomposition, a=64.9% a=74.4% a=75.6% Note: a is the degree of oxidative decomposition determined from the expression a=aCoxa ¢§ aCoxa0 , aCoxa where [Cox] and [Cox] 0 are the mass fractions of the oxidisable carbon in original DB and UDD and in treated (oxidised) samples, respectively.treated by oxidising systems based on nitric acid and also by organic solvents, hydrocarbons and alcohols in the temperature interval of 358 ¡¾ 578 K. Table 2 shows the results of the analysis as arbitrary molecular formulas calculated per 100 carbon atoms. Generally, the relative content of heteroatoms varies in a wide interval: from 5 to 35 hydrogen atoms and from 4 to 32 oxygen atoms per 100 carbon atoms. The nitrogen content varies from 2 to 4 atoms per 100 carbon atoms and is independent of the treatment conditions. Non-oxidative treatment of DB by organic solvents (hydro- carbons CnH2n+2 and alcohols CnH2n+1OH) does not affect the carbon skeleton of particles, but causes changes in the composi- tion of surface functional groups.In this process, the elemental composition of DB changes regularly: relative concentrations of hydrogen and oxygen increase owing to the addition of hydro- carbon- and hydroxyl-containing fragments. The total number of heteroatoms is doubled. In the oxidative decomposition of DB, the spectrum of carbon-to-heteroatom ratio in the products may be wider. As the action of oxidants on DBis stronger (increase in temperature, acid concentration, etc.), carbon gradually transforms into carbon dioxide. First of all, the most disordered amorphous part of carbon is oxidised, next comes the turn of micrographite structures. After practically complete removal of non-diamond carbon forms from DB, the resulting UDD represent a chemically inactive material difficultly susceptible to further oxidation.Nonetheless, the author has succeeded in decomposing partially the diamond substance by long-term etching. The etching degree was 45% of UDD mass or 75.6% of the mass of original DB. The dependence of DB composition on the degree of oxidative decomposition (Fig. 1) has a complicated shape. The minimum contents of heteroatoms correspond to both the DB original DB and purified UDD, the maximum values (53.5 and 44.9 heteroatoms per 100 carbon atoms) are typical of partially oxidised and etched UDD (a=26% ¡¾ 31%) 609 Arbitrary molecular formula Number of heteroatoms per 100 carbon atoms C100H5.3N2.84.1 12.2 21.3 C100H13.8N2.94.6 C100H15.5N2.68.0 C100H25.4N2.922.5 26.1 50.8 60.9 C100H34.9N2.923.1 22.5 C100H11.2N2.29.1 C100H19.3N2.123.5 C100H18.7N2.022.8 44.9 43.5 48.8 C100H23.7N2.422.9610 n /atoms per 100 C atoms 60 40 200 0.4 0.2 Figure 1.Elemental composition of DB as a function of the degree of oxidative decomposition (a).24 Relative content (n) of (1) oxygen, (2) hydrogen, (3) all heteroatoms. (a=65%± 75%), respectively. The concentrations of hydrogen and oxygen bonded to the surface vary in parallel. This makes it possible to assume that the chemical composition of surface functional groups is constantly reproduced. For metastable structures, like partially oxidised DB and products of UDD etching, the recovery of surface-group compo- sition results from active interactions with the reaction medium and the formation of a maximum number of heterobonds.The more stable forms, viz., the original DB and UDD, contain the smallest numbers of heteroatoms, which however is higher com- pared with that in the well-known diamond micropowders of steady-state synthesis (C100H1.1O2.2) and soot (C100H5.1O4.1). From the chemical standpoint, the curves obtained allow the process of oxidative decomposition to be interpreted as two consecutive stages: (1) preferential etching of the carbon matrix at structural defects and along links between species, in the course of which the reaction surface increases and is enriched with oxidation intermediates; (2) etching of the loosened surface, gas formation and removal of surface oxidation products.A cyclic nature of these stages suggests that decomposition affects only structurally heterogeneous materials, whereas the oxidation selec- tively involves different structural forms. Inasmuch as the concentrations of heteroatoms in DB and UDD are very high, a question arises of a possible location of these heteroatoms and the nature of their bonds with carbon. Calculations show that a 4-nm diamond particle consists of *126103 carbon atoms, nearly 36103 of which are located on the surface. For example, for UDD, we can write the following arbitrary molecular formulas: C75 for bulk and C25H11.2N2.8O9.1 for the surface atoms. Thus almost every carbon atom on the surface should be bonded to a heteroatom.IV. Surface properties The concentration of active hydrogen ([Ha]) on a UDD surface has been determined.25 It is commonly agreed that a hydrogen atom bonded to an atom of any other element except carbon is active. For a carbon surface, the quantity [Ha] may serve as a measure of its enrichment by functional groups (hydroxy, car- boxy, amine, sulfonic, etc.). In the studies on the reaction of methylmagnesium iodide in anisole with functional surface groups of DB and UDD, three stages can arbitrarily be distinguished, viz., (1) the reaction with molecular impurities and the external surface (with functional groups most easily accessible), (2) the reaction with the surfaces of 31 2 0.6 a (%) V Yu Dolmatov open pores and (3) the reaction with the surface becoming free upon mechanical destruction of UDD aggregates.Depending on treatment conditions of DB and UDD, the density of protogenic groups in them varies from 0.34 to 2.52 mg-equiv m72, and the concentration of active hydrogen varies from 0.49 to 7.52 mg-equiv m72. The proportion of the mobile hydrogen onDBandUDDsurfaces amounts to4%± 22% of the total hydrogen content in the powders. Surface and electrophysical properties of nanodiamonds of two different specimens (UDD-1 and UDD-2) differing in the content of incombustible residue ([I.r.]) have been studied (Table 3).26 Table 3. Characteristics of the nanodiamond powders studied.26UDD-2 UDD-1 Parameter [I.r.] (%) w /m3 kg71 Ssp /m2 g71 CBET 0.75 70.35 .1078 167 303 4002.4 1.16 0 162 814 5503.4 A /J g71 A0 /J m72 Note. The parameters are specified in the text. The amounts of incombustible residues in powders and their specific magnetic susceptibilities (w) were measured following the procedures developed at the Bakul' Institute of Superhard Mate- rials (Kiev, Ukraine). Adsorption and structural characteristics, viz., the CBET constant, the adsorption potential (A), the specific adsorption potential of UDD (A0 ) and the specific surface area (Ssp) were determined by the BET method from isotherms of nitrogen low-temperature adsorption measured on an `Akusorb- 2100' instrument. For UDD samples from both specimens studied, the isotherms of the second kind are typical.As can be seen from Table 3 the UDD-1 and UDD-2 samples have equal specific surfaces but differ somewhat in the degree of purity and especially in the adsorption potential (for UDD-2 this is 37% larger). According to the values of the CBET constant found, the UDD-2 powder exhibits higher adsorption activity, which suggests that the use of the degree of purity for character- ising UDD is ambiguous. It is found 26 that the absorption band in the IR spectrum of UDD at 1640 cm71 is associated with the deformation vibrations of water, both bound and free. A band at 1750 cm71 corresponds to vibrations of the OH groups. Drying of UDD powders at 393 K did not result in any noticeable qualitative changes in the spectra. This suggests that the UDD surface bears active functional groups which favour adsorption of water.On the basis of absorption spectra of UDD powders, it was concluded that the amount of moisture in the form of bound water molecules on the surfaces of nanodiamonds is higher compared with common diamond micropowders obtained in steady-state synthesis. Carboxy and carbonyl groups were also identified. The contents of methyl and methylene groups in UDD powders were insignificant.26 An analysis of IR spectra revealed the presence of potassium and aluminium sulfates, chromates and their complex compounds involving silicon in incombustible residues of UDD. Most prob- ably, all these compounds have appeared as a result of a contamination during chemical purification of UDD by chromic anhydride in sulfuric acid.26 The changes in the nature of a `coat' of functional groups on the surface of nanodiamonds upon their treatment in different media, affects the susceptibility of UDD to cold pressing.27 Table 4 shows the values of limiting pressures during the compac- tion and the densities of compacted UDD both non-modified andDetonation synthesis ultradispersed diamonds: properties and applications Table 4.The effect of UDD pretreatment on the limiting compacting pressure (Plim) and the density (r) of compacts.27 UDD modified by treatment with Parameter Initial UDD CO2 H2 concentrated HNO3 1200 300 ± 400 1200 1.00 ± 1.15 1.23 ± 1.30 1.3 ± 1.4 800 ± 900 1.2 Plim /kg cm72 r /g cm73 modified by treatment with concentrated nitric acid, hydrogen and carbon dioxide.V. Phase composition, microstructure and surface texture Depending on the synthesis conditions, the explosion condensed products contain either only a cubic phase of diamond or also Lonsdalite as an admixture (up to 30%).21 The X-ray diffraction pattern of a UDD sample contains five reflections characterised by the following intensity distribution: (331) (400) (113) (220) (111) Reflection Fraction (%): UDD sample reference sample 0.2 12 0.3 4 0.5 18 14.0 22 85.0 44 It may be assumed that the predominance of the reflection from the (111) plane in the spectrum is associated with the spherical shapes of UDD particles. Table 5 shows microstructure parameters ofUDDspecimens.Nanodiamonds formed during detonation of a trotyl ± hexogen melt (60 : 40) 21 exhibit the highest microstresses of the second kind and, as a consequence, have most deformed lattices. It was found that the microstresses ofUDDlattice do not change until the onset of graphitisation at 1473 K. Thus, a highly strained structure of UDD is thermally stable. Nonetheless, UDD synthesised by the aforementioned method should be expected to possess an enhanced reactivity. Table 5. Microstructure of detonation synthesis nanodiamonds. Ref. a /nm Microstresses /GPa Coherent a/a0 (%) a scattering region /nm 21 28 29 10 4.2 ± 4.7 1.0 1.0 0.42 ± 0.47 0.1 4 ± 6 5 ± 10 5 ± 15 0.35620.0003 70.35720.0004 Note.a Relative lattice distortion (a is the parameter of a real lattice, a0 is the parameter of the diamond cubic lattice). Peculiarities of the surface structure were studied for DB, partially oxidised DB and UDD,30, 31 and the degree of oxidative decomposition was estimated (see footnote to Table 2). The BET method was used for the determination of the specific surface area (SBET) and the size distribution of pores from the adsorption isotherms of nitrogen. By gradually increas- ing the degree of oxidative decomposition (a) ofDB by acids it was possible to obtain a series of specimens of partially oxidised DB containing different amounts of oxidised carbon, viz., from the initial DB (sample 1) to UDD (sample 12).Table 6 shows the texture parameters of the specimens. The dependences of the specific surface area, the volume, and geometrical parameters of pores on the quantity a reach an extremum at a=0.63; at this point, SBET and the volume of pores are a minimum, and CBET is a maximum. At this stage of 611 Table 6. Parameters of DB and UDD textures.30 CBET SBET da a dd /nm /nm Vp /cm3 g71 Sample [Cox] (%) /m2 g71 8.8 8.0 7.9 7.6 6.8 8.8 9.1 9.3 108 112 107 109 133 136 155 182 1.2451 1.0746 0.9931 0.7488 0.6621 0.5406 0.5236 0.5089 9.1 8.1 7.7 7.5 8.6 8.7 9.1 9.5 7 7 7 7 7 7 7 7 9.8 9.2 404 409 399 314 244 209 198 195 240 252 276 290 00.17 0.28 0.32 0.48 0.49 0.56 0.63 0.81 0.85 0.94 0.98 53.4 44.4 38.4 36.3 27.8 27.6 23.7 19.5 9.9 7.9 3.4 1.2 123456789 10 11 12 9.6 9.2 135 121 0.8241 0.8396 Note.Here, [Cox] denotes the mass fraction of oxidised carbon; Vp is the limiting volume of the sorption space at P/Ps=0.995 (P is the partial pressure of nitrogen, Ps is the pressure at which the whole internal surface of pores is covered by a nitrogen monolayer); da is the size of an open pore; dd is the critical neck size, i.e., the maximum pore size at which the atoms of an adsorbed gas can enter the adsorbent. decomposition, a partially oxidised DB retains *20% of non- diamond carbon. In the range of a=0.63 ± 0.98, carbon particles are etched at the stressed and defective areas.Here, a new kind of surface with predominant relaxed diamond-type structures is formed. Thus, when passing from the original DB to UDD, the texture characteristics of the substances change non-monotoni- cally. This distinguishes DB from other carbon phases (techno- logical carbon, coals, carbon fibres) being associated, first of all, with the presence of two carbon phases, viz., diamond-type and non-diamond in DB, which differ in their chemical activities and oxidation mechanisms. Comparing the values of SBET, CBET and the volume of pores found forDBand UDD, on the one hand, and those obtained for intermediate oxidation products, on the other hand, we can conclude that the powders of detonation carbon represent organised space structures formed during multistep aggregation of original particles rather than a mechanical mixture of UDD and non-diamond carbon.In the liquid-phase oxidation, chemical decomposition (etch- ing) of secondary aggregates occurs, which entails regular changes in the surface area and in texture characteristics of the powders. Primary aggregates remain practically unchanged. These data taken altogether point to a structural self-organisation of deto- nation synthesis ultradispersed carbon. As was discussed above, the properties of UDD surface strongly depend on the conditions of their chemical pretreatment, which in a number of cases plays a key role in the production of new composite diamond-containing materials with the required characteristics. VI.Sorption properties and exchange capacity The sorption properties of DB and UDD, which are extremely important characteristics for different application fields of UDD have been studied.32 Oxygen-containing groups present on nanodiamond surfa- ces 33 impart a charge to UDD aggregates in aqueous solutions; the value and sign of this charge depend on the concentration and the dissociation constant of these groups, the solution pH and the background-electrolyte concentration. Ions H+ and OH7 are potential-determining. The dependence of the specific adsorption on the solution pH, which is observed for all powders under study, can be attributed to an increase in the degree of dissociation of surface groups with acidic properties. In the gradual oxidation of the detonation-derived carbon, the specific adsorption changes non-monotonically and passes612 103 G /mg-equiv.kg71 1.00 0.75 0.50 0.250 40 20 [Cox] (mass%) Figure 2. Specific adsorption (G) as a function of oxidisable carbon content in DB (pH 11).32 through a pronounced extremum (Fig. 2). A low specific adsorp- tion value for the original DB is explained by the fact that this DB has been synthesised in a non-oxidative medium and thus involves a small amount of surface oxygen-containing groups. When a DB is exposed to oxidants, two processes proceed simultaneously, viz., surface enrichement with oxygen-containing groups and etching of carbon.With an increase in the degree of oxidative decomposition, the surface is enriched with oxygen- containing groups, the specific adsorption would have been expected to reach a maximum and then remain constant. How- ever, starting from 18%± 20% of residual oxidisable carbon, the specific adsorption decreases. The presence of an extremum in the curve (see Fig. 2) reflects qualitative changes in the surface structure of DB upon treatment, namely, a transition from a graphite-like form to a diamond-like form.31 In this transitional state, the material exhibits the best adsorption properties. The DB subjected to drastic oxidation retains only a stable diamond form. Under the conditions of mild oxidative etching, the boundary between diamond-like and non-diamond carbon shifts.The residual oxidisable carbon (the remaining 18% ± 20%) actually represents a diamond-like phase, i.e., corresponds to the periphery of a diamond cluster. ForUDDsamples listed in Table 1, the sorption capacity with respect to benzene and chloroform vapours was estimated at 292 ± 294 K by a weighing method under steady-state conditions close to the isothermal ones. The results obtained are as follows. 1. The sorption capacity of a dry powder of UDD-STP (produced at SDB `Tekhnolog') amounts to 9 mol kg71 with respect to benzene vapour and 10.5 mol kg71 with respect to chloroform vapour. The observation time was 15 days; 90%± 95% of adsorbate was absorbed in 2 days. 2. The sorption capacities of four fractions isolated from the suspension of UDD-TAH decrease monotonically from 8 mol kg71 for the first fraction to 7 mol kg71 for the fourth fraction with respect to benzene vapour and, correspondingly, from 8.5 to 7.5 mol kg71 with respect to chloroform vapour.The observation time was 13.3 days; 95% ± 100% of the adsorbate was adsorbed in 2 days (more quickly for chloroform). 3. For comparison, sorption capacities of certain UDD samples made at RFNC-ARITP were determined, which varied from 12 ± 16 to 10 ± 14 mol kg71 for benzene vapour and from 14 ± 19 to 12 ± 17 mol kg71 for chloroform vapour.A distinguish- ing feature of this type of UDD is that its sorption capacity gradually increases: 55%± 65% of adsorbate is adsorbed in 2 days. The adsorption capacities and rates virtually coincide for DB and diamonds isolated from it (i.e., the sorption capacity in fact is independent of the content of non-diamond carbon in the diamonds).Inasmuch as the adsorption phenomenon forms the basis of many technological processes, the degrees of adsorption of heavy- V Yu Dolmatov Table 7. Results on the adsorption activity studies of UDD-1 and UDD-2 powders with respect to heavy metal ions.26 Mn+ y (%) [Mn+] in electrolyte /mol kg71 UDD-2 UDD-1 26.5 82.5 33.0 80.0 40.0 53.3 18.0 80.6 16.0 75.0 12.0 48.0 0.1 100.1 100.1 10 Fe2+ Fe2+ Ni2+ Ni2+ Cr6+ Cr6+ metal ions on the UDD surface were estimated. For this purpose, an electrochemical technique of cathodic potentiodynamic pulses was applied.34 It was shown that the adsorption activities of UDD powders with respect to metal ions (iron, nickel, chromium) differ sub- stantially (Table 7).In all experiments, the surface coverages (y) of UDD-2 were higher. On the whole, the adsorption activity of nanodiamonds with respect to the ions under study decreases in the following order: Fe2+>Ni2+>Cr6+ (Ref. 26). However, at low concentrations of metal ions (0.1 mol kg71), this sequence for UDD-2 is reversed: Cr6+>Ni2+>Fe2+. With an increase in the ion concentrations from 0.1 to 10 mol kg71, the degree of surface coverage of diamond particles increases 3 ± 4-fold and reaches 70%± 80%. It was thus confirmed 26 that the adsorption activity of UDD surface depends substantially on the method of chemical purifi- cation of nanodiamonds of detonation synthesis.Studies of surface properties of UDD isolated from DB under the action of nitric acid and atmospheric oxygen 35 and modified by different reagents 36, 37 showed UDD to possess pronounced cation-exchange properties, which manifest themselves in the exposure of nanodiamonds to aqueous solutions. It was also found that UDD represent sorbents of high activity with respect to gases.23 As was mentioned above, chemical purification of UDD enriches the diamond surface with oxygen-containing groups that determine its cation-exchange properties as a sorbent. The surface of UDD oxidised by atmospheric oxygen bears a greater amount of oxygen-containing groups of the acid type (i.e., protogenic) compared with the diamonds of liquid-phase oxida- tion.As a result, the former nanodiamonds possess better ion- exchange properties. Hence, UDD can serve as promising sorb- ents for ions of a basic nature. On the other hand, the method for cooling of the products of detonation synthesis plays a significant role in the formation of the nanodiamond surface. Comparison 38 of nanodiamonds obtained from the same explosive compositions and with charges of the same geometry (i.e., fused and pressed in the same moulds) but with different cooling procedures and isolated from the synthesis products by the same methods has shown that their surfaces differ in the composition and the number of admixed functional groups including those groups that affect the ion- exchange properties. Particularly, the use of ice as a cooling agent (i.e., as an armour for the charge) is beneficial for the maximum protection of UDD from the subsequent graphitisa- tion.This refers primarily to its finest crystals, which have the highest specific surface area and, hence, high sorption activity. The sorption capacity of UDD obtained in different condi- tions or purified by different methods was studied with respect to cesium ions.38 It was found that the sorption capacity of nano- diamonds synthesised under identical conditions but purified using different oxidants and different treatment conditions varied from 0.01 to 0.50 mol kg71. This suggests that this depends on the nature of the oxidant and the conditions (duration, temperature) of UDD isolation from synthesis products.Samples obtained from DB by ozone purification exhibited the highest sorptionDetonation synthesis ultradispersed diamonds: properties and applications abilities (0.4 ± 0.5 mol kg71). The value found exceeds substan- tially those of known natural sorbents 39 (0.01 ± 0.15 mol kg71 with respect to cesium) and is commensurable with the sorption capacity of KU-8-2 resin (0.8 mol kg71), which is a widely used ion-exchange resin. An analysis of IR spectra showed that the UDD samples contain different polar functional groups: OH, NH, C7O, C=O, C7H, C7N, C=N, C7O7O in different concentrations. Nanodiamonds of higher sorption capacities are characterised by higher intensities of absorption bands of oxygen-containing (C=O, in the region of 1700 cm71) and peroxide (C7O7O, 1800 ± 1900 cm71) groups compared with other samples.Appa- rently, it is the presence of these groups that is responsible for the enhanced cation-exchange ability of the samples under discussion. An IR spectrum of UDD synthesised in a gaseous cooling medium shows a wide absorption band in the region of 1600 ± 1900 cm71, which points to a higher concentration of oxygen-containing groups which determine the cation exchange. It was proposed to use UDD as a potential sorbent for cesium radionuclides.38 Compared with conventional ion-exchange res- ins, ion-exchange materials based on nanodiamonds possess basically better new characteristics: substantially higher thermal stability (400 8C in air compared with 120 ± 150 8C for KU-8-2 resin), resistance against aggressive media, mechanical strength, the absence of swelling and high sorbent density (3.1 ± 3.3 g cm73 against 0.7 ± 0.8 g cm73 for KU-8-2).In the context of the commercial application of UDD for deposition of composite electroplates, it is of interest to study the behaviour of UDD hydrosols in the presence of different ions under the action of an external electrical field and without it. Electrochemical studies of an aqueous suspension of UDD were carried out in the pH range of 3 ± 10, at an ionic strength varying from 1074 to 1071 in a temperature interval from 20 to 70 8C.40 A combination of macroelectrophoresis and potentio- metric and conductometric titrations were applied.The exper- imental data were analysed within the framework of a model that takes into account the complex nature of UDD aggregation with the formation of primary and secondary structures.6, 41 The dependence of the exchange capacity (G) for UDD on the pH of KCl solutions with concentrations (c) of 0.1 and 0.01 mol litre71 was examined by potentiometric titration. The titration curves for UDD powders and sedimentation-unstable hydrosols virtually coincide, which points to the similarity of the structures (or specific surface areas) of coagulated precipitates and dried powders. A threshold of fast coagulation of the hydro- sols and loss of sedimentation stability corresponds to pH 5.5 in KCl solution with c=0.01 mol litre71.With an increase in pH, the exchange capacity of UDD increases from 0.056103 mg-equiv. kg71 (pH 3) to (0.4 ± 0.6)6103 mg-equiv. kg71 (pH 10). In the pH range of 9 ± 10, the exchange capacity of the hydrosol substantially increases, in contrast to that of the UDD powder. This reflects different dispersion of nanodiamond par- ticles. On the basis of the specific surface value of 322 m2 g71 found by the BET method for powder particles,40 the surface charge density was calculated, which made it possible to roughly assess the potential of theUDDsurface for both powders and hydrosols. Figure 3 shows the dependences of the surface, diffusion and electrokinetic potentials on the solution pH.The pH-dependences of the surface (curve 1) and diffusion (curve 2) potentials have similar shapes. The electrokinetic poten- tial (curve 3) was estimated from the results of macroelectropho- resis by using a formula valid for particles with unipolar conduction.42 From comparison of the potentials (see Fig. 3), it follows that a substantial volume ascribed to the immobile part of the electrical double layer can be attributed not only to the presence of water layers with different structures near the nanodiamond surface, but also to the immobilisation of the dispersion medium inside the UDD aggregates. Such a conclusion is confirmed by the different 613 ji /mV 180 12 140 100 3 60 20 pH 9 7 5 3 Figure 3.Dependences of (1) surface, (2) diffusion and (3) electrokinetic potentials (ji) on pH for UDD particles in a hydrosol in KCl solution (c=0.01 mol litre71).40 shapes of the temperature dependences for electrokinetic and diffusion potentials. At a low ionic strength (in a KCl solution with c=1074 mol litre71), no apparent flocculation of primary aggregates occurs, and the destruction of the interfacial water layers, which proceeds at elevated temperatures, is accompanied by an increase in the electrophoretic mobility of UDD. In solutions of higher ionic strengths, the destruction of interfacial layers at elevated temperatures facilitates flocculation of primary aggregates. Water immobilised inside newly formed secondary aggregates does not necessarily take part in the motion; however, the exchanged ions retain their mobility.In this case, the electro- phoretic mobility decreases and the diffusion potential remains unchanged. Indeed, the diffusion potential calculated for the isopolarisation mode from the data on the conductivity of UDD hydrosols, under an assumption of a constant size of hydrosol particles, does not decrease with an increase in the temperature (in contrast, a slight increase is observed). Electrochemical studies of the surface allowed refinement of the structure of solid phase inUDDhysrosols. Using the values of the diffusion potentials found in potentiometric titration and the results of conductivity measurements at the isoconduction point (pH 6, KCl solution, c=0.01 mol litre71), the sizes of non- conducting spherical UDD formations were determined and proved to coincide with the radii of primary aggregates (10 ± 30 nm) estimated by electron microscopy.6 This implies that primary aggregates are non-conducting, i.e., have dense ion- impermeable structures.Thus, UDD represent presumably a cation-exchange system with a variety of functional groups of different acidic strengths.40 Hydrosols of UDD consist of dense primary aggregates which form ion-permeable secondary structures. Different shapes of the pH dependences for diffusion and electrokinetic potentials, as well as many other properties of UDD hydrosols as a function of pH, ionic strength and temperature are determined by the changes in their secondary structures.VII. Chemical properties of ultradispersed diamonds Studies of the chemical composition ofDB andUDDwere carried out by the methods of gas chromatography (CG), polarography, IR spectroscopy and X-ray photoelectron spectroscopy (XPS).43 Table 8 shows the results of polarographic and GC studies of DB and UDD samples. It follows from the data in Table 8 that the carbon of theUDD surface groups is oxidised to a higher degree and the hydrogen content is substantially lower compared with DB.614 Table 8. Composition of desorbed gases and their activation energies.43 Sample Composition of desorbed gases a Activation energy of desorption b /kJ mol71 gas content /cm3 g71 gas DBc 23.4 22.5 47.6 UDDd 7 103.6 27.2 48.5 23.0 28.4 3.9 0.7 4.03 7.8 0.09 1.43 11.4 0.83 1.7 traces CO2 N2 CH4 H2 CO NH3 CO2 N2 CH4 HCN 7 Note.a Determination conditions: 673 K; desorption time, 2 h; pressure, 0.1 Pa. b At 573 ± 773 K. c Elemental composition (%): C, 86.3; H, 0.4; N, 3.7; O, 9.6. d Elemental composition (%): C, 86.0; H, 0.1; N, 2.5; O, 11.4. The IR spectroscopy data suggest the presence of the car- bonyl, carboxy, hydroxy, methyl and nitrile groups on the surfaces ofUDDand DB. Thermodesorpsion of large amounts of methane and hydrogen from a DB sample (Table 8) gives us grounds to assume that it has a different composition of surface groups. Apparently, this is associated with the following reactions involv- ing carbon surface atoms with uncompensated bonds R17CH27CR2R3R4+0.5H2 , (1) 7R17Me+.CR2R3R4 (2) R1=CHR2+CH4 .7R17Me+.CH2R2 If reactions (1) and (2) do occur, then the fraction of carbon atoms with uncompensated bonds (surface atoms) amounts to no less than 1.6 %. An estimation based on these data showed that the mean particle diameter is 45 nm (no more than 300 carbon atoms). According to XPS results 21, 43, the C1s band in the spectrum of DB represents a wide asymmetric peak (half-width, DE1/2= 4.1 eV), which narrows after bombardment with argon (DE1/2=2.5 eV) and takes a shape typical of graphite and fine- grain coals. In this case, the surface charging is zero, which is typical of conducting substances.These results suggest that the bulk of the sample represents the graphite phase, whereas, in the surface layer (several tens of angstrom deep), the diamond carbon phase is located. For UDD samples (see Table 8), a symmetrical C1s peak (DE1/2=3.3 eV) is typical, and the surface charging is +3.3 eV. After bombardment with argon, the shape of the carbon peak remains unchanged, and the surface charging reaches + 6.2 eV, DE1/2=2.9 eV. It was concluded that carbon in the surface layer (no less than 10-nm deep) of a sample is represented exclusively by the diamond phase.43 The initial sample includes nitrogen and oxygen (O :C=0.027, N:C=0.020) as the admixtures. After bombardment with argon, the admixture content becomes lower than the threshold sensitivity of a spectrometer.Thus, in DB particles the diamond-like carbon phase sur- rounds the graphite phase or, more precisely, a non-diamond carbon form. UDD particles consist entirely of a diamond-like carbon phase. Admixtures in the form of carbon compounds with oxygen and nitrogen are accumulated preferentially in the surface layer of a particle. In accordance with a classification of functional derivatives of diamonds, UDD pertain to a group of `colloidal diamonds' as regards the crystal sizes and the ratio of surface carbon atoms (Cs) to the total number of carbon atoms (Ctot) in a diamond crystal.44 V Yu Dolmatov For nanoparticles measuring 2 ± 10 nm, the number of surface C atoms with respect to their total number in the core varies from 63% to 15%, i.e., from 4 to 18 internal carbon layers are present.Hence, UDD properties should largely be determined by the chemical states of their surfaces, in contrast to coarse diamond crystals with Cs/Ctot?0. However, to date the chemical proper- ties of UDD remain less understood as compared with diamonds of other types. The functional `coat' of UDD particles, the possibilities of their chemical modification in different gaseous media, and their behaviour at elevated temperatures have been studied.27 The UDD samples were produced at the Research Industrial Association (RIA) `Altai' (Biisk) and purified by using oleum ± nitric acid mixtures at 523 K. The isolated UDD were dark-grey powders with microcrystals of 3 ± 8 nm, which, being placed in water, formed aggregates measuring up to 261074 m.The elemental composition of these UDD (%) is given below. N O H C Ash content 1.34 ± 2.47 1.78 ± 2.16 16.5 ± 17.0 0.65 ± 0.70 78.6 ± 78.8 The high content of oxygen in UDD agrees with the Cs/Ctot ratio and the amount of possible functional oxygen-containing groups on the surfaces of diamond particles of such sizes.44 The first step of UDD modification included drying them in air and heating the fine fraction in a flow of nitrogen at 348 K in a flow-through system. In the second stage, UDD were heated at 673 ± 973 K in a flow of a gas (N2, CO2, CH4, H2, air) passed at a rate of 5.661077 m3 s71. The results obtained 27 allowed the authors to assess the effect of pretreatment of UDD on the temperature of oxidation initiation.After treatment with water oxidation starts at 673 K, whereas upon exposure to a CH4+CO2 mixture, it begins at 773 K, i.e., hydroxylated surfaces are oxidised more easily. Studies on the behaviour of UDD in oxidising gas mixtures under isothermal heating showed that the oxidants may be arranged in the following series with respect to their activities: air CO2>H2O. It was shown that all the temperature dependences of UDD oxidation rate clearly manifest two regions. In the low-temper- ature region (below 673 K), the change in theUDD mass depends little on the temperature, whereas in the high-temperature region oxidation sharply accelerates with an increase in temperature.On the basis of the results obtained, the apparent activation energies of UDD oxidation were calculated (Table 9). This table also shows the temperatures of transition to high-temperature regions for different oxidation conditions. As is seen from the data of Table 9, the activation energies in two regions differ by a factor of 3 ± 6 for the same oxidant. Probably, this is associated with the fact that, in the low-temper- ature region, oxidation actively involves the `coat' of functional groups present on the surface of UDD particles and their decom- Table 9. Apparent activation energies (Ea) of UDD oxidation in low- temperature (I) and high-temperature (II) regions. Oxidant Ea /kJ mol71 Temperature of I to II transi- tion /K region II region I 96.14.2 96.14.2 91.113.0 120.415.0 24.71.7 24.71.7 25.55.9 36.47.1 673 773 773 898 838 933 703 O2+N2 (20% O2) O2+N2 (10% O2) O2+N2 (4% O2) CO2 CO2, K2CO3 (see a) H2O H2O, K2CO3 (see a) 88.65.0 152.27.1 126.710.0 18.85.0 22.213.4 20.512.1 Note.a Successive treatment.Detonation synthesis ultradispersed diamonds: properties and applications position products, whereas in the high-temperature region only hydroxy groups take part in the reaction. In the presence of a catalyst (K2CO3), the activation energies of UDD oxidation are somewhat lower than that of the noncatalysed process. On the whole, the activation energies of UDD oxidation in the high temperature region are slightly lower compared with detonation diamonds synthesised from graphite and soot 45 and twice as low as those of kimberlite diamonds.46 The aforementioned results confirm the presence of a `coat' of different functional groups on the surfaces of UDD particles and suggest that this `coat' changes as a result of chemical modifica- tion and has a pronounced effect on physical and chemical properties of UDD, particularly, the resistance of the latter against oxidising gas mixtures, their compactibility (see Table 4), etc.Furthermore, the results obtained allow us to conclude that UDD represent not simply diamond-like materials (e.g., see Ref. 4), but typical diamonds, the surface of which, as for the other diamond types, contains different functional groups that saturate free valences of carbon surface atoms.A unique specific feature of UDD is that, with respect to the Cs/Ctot ratio, they occupy an intermediate position in a series of functional deriva- tives of diamond cores, between low-molecular forms similar to adamantane functional derivatives and the polymeric forms representing diamond macro-crystals with specific functional `coats'. It has been shown 44 that such a mesoregion corresponds to colloidal sizes of the particles. Therefore, bearing in mind that, due to the presence of a functional `coat', an electrical double layer can surround UDD particles in solution, it is quite reasonable to call the latter particles colloidal diamonds.VIII. Spectroscopy Absorption spectra of UDD powders do not reveal any specific features in the optical range.Amicroscopic study have shown that UDD microparticles by themselves are colourless, so that the typical colour of the powder is, most probably, caused by diffuse scattering and reflection. The microparticles demonstrated no optical anisotropy. The refractive index of UDD estimated by the immersion technique amounted to*2.55 at the wavelength of 580 nm, which is substantially higher than that of natural diamonds and artificial diamonds of steady-state synthesis. 1. Infrared spectroscopy The diamond lattice is known to possess over 50 defect centres active in the optical spectra. Most synthetic diamonds contain nitrogen as atomic dopant of substitution.With respect to shapes of the IR spectra, diamonds are divided into two types, viz., (I) with one-phonon absorption and (II) without absorption in the single-phonon spectrum range. The first group includes diamonds of the Ia type the one-phonon absorption of which is caused by three independent defective centres: A (1282, 1210, 1110, 480 cm71), B1 (1332, 1175, 1000, 775, 328 cm71) and B2 (1350 ± 1380 cm71), and the diamonds of the Ib type the one- phonon absorption of which is caused by C centres. A well developed surface of UDD (up to 4.56105 m2 kg71) and high Cs/Ctot ratios that determine the high concentration of surface functional groups allow the method of IR spectroscopy to be used 27 with the aim of reliable identification of the nature of these functional groups.For insufficiently purified UDD (produced by RIA `Altai'), the spectra measured both before and after modification revealed absorption bands of carbonyl (1730 ± 1790 cm71) and hydroxy groups (1640, 3400 cm71). A wide band with a maximum in the range of 1100 ± 1140 cm71 is probably related to the overall absorption of a number of admixed nitrogen centres of types A (two nitrogen atoms in neighbouring substitution cites), B1 (an accumulation of vacancies `decorated' by nitrogen), B2 (laminar formations in the cube's plane) and C (isolated nitrogen atoms 615 substituting carbon atoms in the lattice) and the vibrations of C7O7C fragments (NO2 groups at primary and secondary carbon atoms, SO2OH groups, OH groups in CO2H and at ternary carbon atoms absorb in the same range, which compli- cates unambiguous interpretation).Comparison of the spectra shows that the positions and intensities of the bands of the carbonyl-group absorption vary widely depending on the UDD modification conditions. Treat- ment in a nitrogen atmosphere at 937 K results in elimination of carbonyl and carboxy groups, which is indicated by reduced intensities of the corresponding bands. Independently of the modifying gas used, treatment at 673 K shifts the maximum at 1730 cm71 to the region of 1780 ± 1790 cm71, which indicates the formation of 7C(O)7O7C(O)7 structures. Insignificant changes in the spectral patterns observed after treating UDD by different kinds of gases suggest that these gases affect insignif- icantly the composition of surface functional groups of these UDD samples under the conditions specified.For UDD purified by the nitric-acid procedure (SDB `Tekh- nolog'), the IR spectrum slightly differs from previous spectra and contains characteristic bands at 3500 cm71 (strong, wide), 1740 and 1640 cm71 (both medium), 1260 and 1170 cm71 (both strong and wide) and 610 cm71 (wide, of medium intensity). These bands are well reproducible irrespective of the purification procedure used and, hence, can be assumed to be caused by UDD intrinsic absorption. A band at 1260 cm71 corresponds to one-phonon absorption of the diamond lattice. Presumably the width and intensity of this band are determined by distortion of the diamond lattice,18 diamonds of the IIa type, which contain no admixtures, do not absorb in this region.The nature of the other bands in UDD IR spectrum is unclear. Evidently, they do not coincide with any known absorption centres in diamonds. It is possible that some of these bands are caused by adsorbed nitrogen and water. Our results allowed also us to reveal additional bands depend- ent on the procedure used for the isolation of UDD. For instance, the spectrum of UDD treated by nitric acid shows bands at 2940, 1390, 1505, 650 cm71, which were absent in the spectrum ofUDD treated by sulfuric acid (produced at RIA `Altai'); in turn, the latter spectrum contained bands at 2800, 2205 and 670 cm71.Apparently, the latter bands are caused by the presence on UDD surfaces of conglomerates of functional groups either chemically bound or adsorbed in the course of treatment. In contrast, due to a substantial amount of background impurities, IR spectra of non-purified DB appear to have a substantially uniform shape with prominent regions of enhanced absorption at 3300 and 1170 cm71. The presence and width of the latter correlate with the concentration of the diamond component in DB. The spectrum of diamond-free DB reveals none of these peculiarities. It cannot be ruled out that processing of a sufficient body of experimental data will allow development of a quantita- tive method for the estimation of the diamond concentration in DB.On the whole, IR spectra ofUDDandDBare a good source of information on their structures.2. NMR and ESR spectra 13C NMR spectra 21 of nanodiamonds involve four signals. A signal with d 34.5 corresponds to the diamond carbon atoms; a broad signal in the region of d 30 is due to the fact that carbon atoms are not structurally equivalent, first of all, due to the presence of defects and the Lonsdalite phase. Weakly pronounced peaks with d 68 and 53 can be attributed either to the effect of partially oxidised structures or the presence of nitrogen atoms in the sample. In the 1H NMR spectrum,21 three overlapping peaks were observed. A signal with d 2.2 corresponds to the isolated C7OH groups, whereas a signal with d 3.8 corresponds to bound C7OH groups.A signal with d 6.7 most probably belongs to S7OH groups formed on the surface of a sample during its chemical616 purification (in Ref. 21, oleum ± nitric acid treatment of DB was applied). In the ESR spectra, theUDDsignal at 293 Kis a singlet with a g-factor of 2.003 and a half-width DH1/2=6.3 ± 8.3 ê.21 The concentration of paramagnetic particles ranges from 2.761021 to 2.061022 kg71. The parameters of this signal are most close to the ESR signal of broken bonds on the diamond surface (g=2.0027, DH1/2=5.5 ê).47 A similar ESR spectrum was also observed for AV detonation diamonds.48 It is of note that, despite a high concentration of nitrogen atoms in UDD (*1.661024 kg71) no triplet signal associated with doped nitrogen atoms that substitute carbon in the diamond lattice was observed.3. Raman spectra Deconvoluted profiles reveal maxima at 1580 and 1620 cm71, which coincide well with the maxima of the density-of-states function of the sp2-bound carbon atoms. This suggests that UDD involve an amorphous graphite phase.20 A band with a maximum at 1530 cm71 can be attributed to the sp3-bound carbon atoms of that part of the structure which is susceptible to a severe distortion. Apparently, the appearance in the spectrum of a number of specific features of the phonon density of states for the sp3-bound carbon atoms suggests that, in addition to an amorphous graphite phase,UDDinvolve an amorphous diamond carbon phase. 4. X-Ray diffraction spectra X-Ray diffraction spectra 20 of UDD samples contain wide, sym- metrical diffraction maxima at 2y=43.9, 75.3 and 91.5 8, which can be adequately described by Lorentz profiles and correspond to the (111), (220) and (311) reflections of the diamond-like lattice with a parameter a0=0.35650.0005 nm.The average size of UDD particles determined by using the Selyakov ± Scherrer relationship from the half-widths of these lines for all diffraction maxima is equal to 4.50.5 nm. In the region 2y=178, an intense localised halo is detected; further, as y decreases, an increase in the scattered radiation is observed. An intense scattering near the incident beam is typical of diffraction on disordered amorphous structures. Apparently, the observed halo cannot be caused by diffraction on macrostructures with a long-range order and should be associated with its scatter- ing on structural units of a middle-range order.For UDD, not only spherical particles with sizes <4 nm, but also ordered carbon chains and packages of planes can serve as such structural units. This makes it possible to explain the high intensity of the halo compared with the intensity of the basal (111) maximum. The size of these particles roughly estimated from the halo half-width (in accordance with the Selyakov ± Scherrer formula, with assumption of a spherical shape) amounts to *1.5 nm. Apparently, it is these small (<4 nm) ordered particles that represent the `amorphous' diamond and graphite phases detected in the Raman spectra.IX. Thermal analysis Thermal analyses of UDD were carried out either in air or in an atmosphere of an inert gas.21 On heating in air at a rate of 10 K min71, oxidation of UDD starts at 703 K,49 whereas oxidation of diamonds of DSD (detonation synthetic diamond), DDG (detonation diamond with graphite nuclei) and SDB 1/0 (synthetic diamond micropowder) brands begins at 863, 843 and 923 K, respectively.50 After heating in a neutral atmosphere below 1273 K, the mass loss is3%± 4%. Heating in aCO2 atmosphere in the temperature range from 443 to 753 K increases the sample's mass by 5%, which can be attributed to CO2 adsorption and displacement of lighter molecules. Heating in a hydrogen atmos- phere is accompanied by HCN evolution. A complex thermal analysis ofUDDcarried out in our studies showed that thermograms in air manifest three characteristic regions.At 373 ± 383 K, the mass loss is 5%± 7%. This process V Yu Dolmatov is endothermic and reversible. An analysis of gaseous products showed that 97%± 98% of the gas evolved at this temperature represents nitrogen. Hence, it can be assumed that nitrogen captured from the atmosphere desorbs. At 523 K, the decrease in the mass of a sample is accompanied by a positive thermal effect. At 753 ± 773 K, an extesive mass loss occurs (up to 95%), which is accompanied by a substantial thermal effect up to 1023 ± 1053 K. Thereafter, no changes take place, and the residual mass corresponds with a 10% accuracy to the ash content of the initial sample estimated according to a standard procedure.It can be assumed that in the interval from 773 to 1023 K carbon is intensively oxidised yielding an incom- bustible residue. It is noteworthy that oxidation is accompanied by an intense glow. WetUDDlose the main mass of water in the interval from 403 to 433 K. Thereafter, the parameters vary practically in the same way as for the predried samples. Heating in an inert gas atmosphere (helium) at 383 ± 393 K is also accompanied by desorption of nitrogen captured from the atmosphere. In the temperature range of 673 ± 1173 K, a sample loses up to 10% of its mass and heat is liberated. Carbon dioxide and nitrogen are evolved (the molar ratio is 4 : 1) and the morphology of the material changes.At 1153 ± 1163 K, a slight positive thermal effect is observed; however, the mass of the sample remains unchanged. The process is accompanied by changes in the morphology and colour of the material. Note that according to the literature data 21 heating in an inert atmosphere entails a practically complete removal of surface groups (adsorbed and chemically bound) from diamond crystals. X. Defects of the bulk structure Bulk structural defects of UDD were studied 2 by the method of electron ± positron annihilation for all the modes of diamond crystallisation discussed below. Nanodiamonds were synthesised by detonation of trotyl ± hexogen welded charges with 5 mass%± 70 mass% of highly dispersed hexogen in water. To estimate the effect of the hexogen concentration and the type of bulk-structure defects and the degree of dispersion ofUDDon the sintering process, not only the carbon/hydrogen ratio in the explosive mixture, but also the geometry of charges affecting the time of manifestation of high pressures and temperatures and quenching conditions were var- ied.After chemical purification, the crystalline structures ofUDD were studied by the method of electron ± positron annihilation of bulk structural defects. Moreover, a technique of low-temperature adsorption of nitrogen was used for estimating the specific surface area. For studying the sintering processes, UDD of the maximum density of bulk defects (the pycnometric density of 3.05 ± 3.10 g cm73), the mean CSR size of 1.5 ± 2.0 nm and the maximum degree of dispersion (the specific surface area of 420 m2 g71) were chosen.UDD powders were sintered under a pressure of 4 ± 12 GPa. Then, the microhardnesses of sinters and the crushing strengths of the polycrystalline grinding powders obtained were determined. It was found that with an increase in the carbon temperature and its concentration in the detonation of carbon-containing explosives the distribution curve of vacancy clusters and pores passes through a maximum, after which their concentration begins to decrease. In the region of the maximum corresponding to 3900 K, both an increase in the concentration of vacancy clusters (i.e., agglomerates of vacancies) and the highest concen- tration of micropores of radii of 1 ± 2 nm in UDD are reached.The aggregates, which contain several vacancies and serve as nuclei for submicropores, represent the centres that capture positrons. The presence in UDD of positronium which has never been observed in diamonds suggests that it appears on the internal surfaces of submicropores formed from vacancy agglomerates. The absence of submicropores and a low volume density of structural defects (pycnometric density of 3.36103 kg m73) inDetonation synthesis ultradispersed diamonds: properties and applications UDD crystallised from trotyl brings these diamonds closer to SDB 1/0 diamonds. This is indicated, for instance, by identical IR spectra of their powders. XI. Electrophysical properties of ultradispersed diamonds Diamonds are diamagnetics with a constant magnetic suscepti- bility (w=70.6261078 m3 kg71).51 The magnetic susceptibility of UDD substantially differs from this value (see Table 3).The specific magnetic susceptibility of any powder material character- ises quantitatively the magnetic properties of the powder as a whole, being the sum of specific magnetic susceptibilities (wi) of all components present in the powder with account of their compo- sitions. Table 10 shows the values of magnetic susceptibilities for UDD admixtures.52 Table 10. Magnetic susceptibility of admixtures in UDD powder.52 UDD Component Micropowder of steady-state synthesis 108wi /m3 kg71 (see a) 7(0.1 ± 0.5) 103 ±104 7(0.1 ± 8.2) +++7 7(0.1 ± 2.0) +traces ++ Diamond Metal Graphite Carbon materials Gelatine + 7(0.5 ± 0.9) 7 Note.a Tentative value. According to the literature data,21 the conductivities of UDD are minimal for samples heated at 573 K (resistivity of *1012 O m). Subsequent heating increases the conductivity (resistivity decreases to 6.061010 ± 2.061011 O m). If the samples are heated in a carbon-dioxide atmosphere, then, at temperatures above 1173 K, the resistivity sharply decreases to 2.36104 O m, which apparently manifests the beginning of graphitisation. The permittivity (e) of different UDD samples varies in a range from 1.7 to 2.7 (Ref. 21). The dissipation factor is 0.561073 ± 1.061072. Thus, UDD exhibit a number of properties that distinguish them from the known types of synthetic diamonds.Despite enhanced reactivity, the diamond-like carbon phase remains stable in neutral and reductive media up to 1273 K, as regards their physicochemical characteristics. According to our results, at *293 K the resistivity of a pressed UDD pellet is 106 ± 107 O m. After moistening the pellet, the resistivity sharply increases (<103 O m for a sample with 5% of water). With further moistening, the resistivity remains unchanged and appears to be determined by the amount of absorbed water. It cannot be ruled out that these data can serve as a basis for the development of a method for determining the residual moisture in UDD. Yet another important characteristic of the diamond surface is the electrokinetic potential (z potential).Taking into account the fact that its value depends substantially on the state of the nanodiamond surface, z potential values should be expected not to differ for different fractions of the sameUDDand, more so, for UDD samples obtained by different methods of purification and surface modification. Determination of z potential was carried out by electropho- resis based on an electrical current-induced directed transfer of particles of the dispersed medium with respect to the liquid phase.53 The values of z potential were found for three fractions, viz., precipitated (I), intermediate (II) and suspended (III), which were obtained from UDD manufactured at RFNC-ARITP (Table 11).Table 11. Basic physicochemical parameters of isolated UDD fractions.53 Fraction I Parameter Appearance light-grey readily crumbling powder 3.3 1.6 1.0 Pycnometric density (1073 r) /kg m73 [I.r] (mass %) [Cox] (mass %) Viscosity a of UDD aqueous suspension /MPa s at [UDD]=10 kg m73 1.04 at [UDD]=60 kg m73 1.32 +16 Electrokinetic potential /mV Note. Fractions are specified in the text. a At 293 K. The value of z potential was also determined for UDD of UDD-TAH (produced at SDB `Tekhnolog'). The samples were preliminarily purified by ion-exchange resins. For non-fractio- nated UDD (UDD-TAH), the value of the z potential of 32 ± 34 mV was found (from three experiments) at 297 ± 298 K (for a comparison, z potential of non-fractionated and non-dried UDD produced at RFNC-ARITP and similarly pretreated was 25 ± 26 mV). For four isolated fractions of UDD (UDD-TAH), values of +55, +76, +25 and +33 mV, respectively, were obtained.The resistances (R) of the suspensions (not exceeding 261075 O) differed only insignificantly after the measurements. XII. Fractionation process and sedimentation stability The fractionation process has been discussed in detail.53 It was claimed that the traditional methods of fractionation of particles with respect to composition and size based on stirring of nano- diamonds in a chosen liquid and subsequent sedimentation under the action of gravity, cannot be applied to highly dispersed colloidal systems including UDD.In the best case, fine fractions are decanted and later form aggregates during drying. The use of organic liquids cannot be accepted due to the high sorption capacity of nanodiamonds (the author of the present work thinks differently). UDD of different purity grades are shown to include a typical set of functional groups, which remain unchanged up to the destruction of the diamond structure. These are polar OH, NH, C=O, CH and C=N functional groups. It is these groups, especially, the carbonyl and hydroxy groups that determine the trend of UDD particles to form aggregates in aqueous solutions. Centrifugation of a sedimentation-stable suspension for dif- ferent periods allowed isolation of UDD fractions (see Table 11).53 Analysis of IR spectra of the fractions isolated showed that practically the same functional groups were present in all the fractions; however, their concentration differed for different fractions.UDD was considered as a complex composite material, which consists of individual fractions.53 The latter differ from one another in the nature and composition of surface groups, as well as in the sizes of aggregates. Ultrasonication of a dry UDD powder (UDD-STP, produced by Joint Stock Company `Diamond Centre', St. Petersburg) in water allowed preparation of a stable suspension the dispersed composition of which remained unchanged for a month and 617 Fraction II Fraction III grey powder black thread- like quasi- crystal formations 3.1 3.2 0.9 1.9 1.3 1.5 1.12 5.15 +39 1.07 1.63 +32618 longer.54 The average size of formed aggregates was 300 nm.An attempt to stabilise the suspension by using different surfactants failed. The UDD particles appear to be surrounded by surfactant molecules with hydrophobic tails directed towards the aqueous medium. Thus, additional hydrophobisation of UDD particles takes place, and the stability of the latter in water decreases. It was found 55 that, with an increase in the ability of molecules of the medium to association (acetone<benzene<propan-2- ol<water), the degree of dispersion of UDD particles in suspen- sions also increases. Hence, the solvation of the accessible surface of diamond clusters is favoured not only by the polarity, but also by the tendency of substances to form p-complexes.From the practical viewpoint, the most important goal is the preparation of stable UDD suspensions in non-polar organic solvents. Nanodiamond suspensions of such a kind will allow cluster compositions based on elastomers to be developed. A real way of solving this problem is to change the nature of the surface of UDD particles from hydrophilic to hydrophobic. For this purpose, a dry nanodiamond powder was added to a benzene solution of commercial co-polydimethyl siloxane and polyiso- prene.55 The adsorption of hydrophobic polymer chains on the surface of UDD clusters was assumed to be beneficial for stabilisation of the suspension. Indeed, the authors managed to enhance the degree of dispersion ofUDD in an organic solvent.A method for modifying surfaces and optimising suspensions was developed;54, 55 for this purpose, polyisoprene proved to be most suitable. Its use for the modification of the surfaces of UDD clusters gave a suspension in which particles measuring 300 nm predominated. This suspension retained its sedimentation stabil- ity for no less than 10 days. XIII. The diamond ± graphite structural phase transition Systematic studies of the diamond ± graphite structural phase transition were carried out for UDD clusters.20 The phase transition occurs in the course of heating in an inert atmosphere in the temperature range of 720 ± 1400 K. To detect the transition, the Raman spectroscopy and X-ray diffractometry were used.The analysis of Raman spectra and X-ray diffraction patterns makes it possible to conclude that UDD represent a cluster material of a diamond crystalline structure with nanocrystals with a characteristic size of*4.3 nm. A narrow range of sizes of UDD nanoclusters, viz., 4 ± 5 nm, was often reported.17, 56, 57 Apparently, this is explained by the fact that for nanoclusters of small sizes diamond rather than graphite constitutes the thermodynamically stable form. This assumption is confirmed by the results of calculations.58 The fact that the lines in the Raman spectra of nanodiamonds coincide with the maxima of the function of phonon density of states in diamond and graphite suggests that UDD involve only small amounts of amorphous diamond and graphite.Inasmuch as nanodiamond clusters, like other ultra-dispersed materials, exist only in an aggregate form,6 the amorphous phase apparently is located inside such aggregates on the surfaces of diamond cores. The presence of the amorphous phase was also confirmed by the results of X-ray diffractometry, which made it possible to assess the characteristic sizes of particles as*1.5 nm. The invariant position of a maximum at 1322 cm71, which is observed in the Raman spectra with an increase in the support temperature (Tapp) up to 1000 K, points to the absence of any substantial changes in the structure of the diamond core at such annealing temperatures. This is confirmed by the data from X-ray diffractometry, which state that the graphite phase appears only at Tapp>1200 K.The diamond ± graphite structural phase transition in the annealing in an inert atmosphere starts from the cluster surfaces. As follows from the analysis of X-ray diffraction data, the graph- ite phase originates as a set of equidistant graphite nanoplates V Yu Dolmatov with a characteristic size <4 nm and then at Tapp>1200 K the formation of this phase predominantly involves the diamond core. The temperature of phase transition (Tpt) found correlates well with the results of electron microscopy,59 according to which the core of a nanocrystalline diamond begins to decrease only as a temperature of 1300 Kis reached. This process is accompanied by the transition of carbon into an onion-like structure.A peak at 1575 cm71, which appears in the Raman spectrum and is clearly pronounced at 1400 K, was interpreted 59 as the manifestation of the carbon onion-like structure. This fact confirms the correctness of the Tpt value determined. It was noted 59 that the diamond ± graphite phase transition starts in UDD at substantially lower temperatures compared with diamond single crystals for which Tpt>1900 K (see Ref. 60). Such a decrease in a phase-transition temperature, e.g., a melting point, was observed earlier for metal clusters.61 In parallel with the formation of the graphite phase on the diamond core of a cluster, ordering of sp2-bound carbon frag- ments present on original UDD occurs at Tapp>720 K.Such an sp2-coordinated crystallisation, in the course of which a disor- dered (amorphous) phase of carbon located between diamond cores forms continuous graphite nets (patterns), takes place beyond the crystalline diamond core and reflects the changes in the amorphous sp2-bound carbon. This crystallisation is indicated by an increase in the band intensity at 1350 and 1600 cm71 in the Raman spectra and the appearance of a fine structure in the diffraction pattern in the region of low and medium angles at Tapp>1300 K. XIV. A cluster model and the fractal nature of UDD aggregates Being a structure of the classical type, aUDDparticle consists of a relatively dense and ordered crystal-like core and a loose chemi- cally labile shell.The diamond core determines the basic diamond- type properties of UDD, namely, their thermal and chemical stability, heat conduction and thermal diffusivity, low electrical conductivity, the X-ray diffraction pattern quasi-abrasiveness and quasi-hardness. The cluster shell is responsible for such properties of the substance as the sign and value of the UDD surface charge, the chemical composition of surface functional groups, absorp- tion, adsorption, chemisorption and colloidal stability of UDD particles in liquid and other media. In contrast to `classical' metal clusters the cores and ligand shells of which are built of chemically different elements, viz., metal atoms and complexing ions, in UDD clusters both the cores and the stabilising shells are largely the carbon atoms.In this case, the diamond lattice of the core can gradate to a non-diamond periphery through a system of dia- mond-like polyhedral polycyclic structures and net patterns. The cluster boundary is stabilised by the products of interaction of periphery carbon atoms with the environment, viz., gaseous detonation products, oxidising mixtures and modifying reagents. Peripheral structures play a decisive role in aggregation of diamond clusters and their interaction with matrix substances of composite materials and coatings. The presence of two carbon components in UDD particles is confirmed by a number of experimental data (e.g., see Refs 62 and 63). The structure of condensed detonation products is formed during both the main process, viz., in the chemical-reaction zone of the detonation wave, and in the discharge stage where the explosion products are scattered and undergo the action of reflected shock waves.1, 4, 64 In the latter case, such secondary processes as graphitisation of diamonds and amorphisation of the crystal phase are highly probable.In addition to structural and phase transformations that affect the carbon core of the particles, chemical reactions of the condensed matter with gases take place in the detonation chamber. These interactions are highly diversi- fied due to the wide temperature range and the impact loads that operate throughout the `life-time' of the carbon condensate in the reactor.Detonation synthesis ultradispersed diamonds: properties and applications Taking into account the fact that condensation of the dia- mond substance from detonation products proceeds in fractions of several tenths of a millisecond,1, 2, 4 we assumed that, first of all, in the chemical-reaction zone, the primary products of explosive decomposition of theEMare not completely homogenised 64 and, secondly, segregation of condensed and molecular components of detonation products (DP) does not manage to proceed com- pletely. Hence, condensed DP can retain certain `chemical markers' of the diamond formation process, namely, molecular compounds and fragments of condensed structures, the composi- tion and structure of which can indicate the mechanism of theEM free carbon condensation and reveal the intrinsic structure of a carbon particle.The expected chemical markers can be divided into the following four groups: �cage, bridged, alicyclic carbon compounds, which represent the fragments of diamond and diamond-like structures charac- terised by sp3-hybridisation of the carbon atoms; � derivatives of mono- and polycyclic aromatic compounds, which represent the fragments of graphite-like structures (sp2- hybridisation of the carbon atoms); � aliphatic (linear and branched) compounds, which repre- sent the fragments of the maximally amorphous carbon-cluster periphery, as well as indications of the presence of carbyne structures; �compounds that involveC7NandC7Obonds, represent- ing the fragments of the surface layer of particles.To detect and analyse these `chemical markers', we exper- imentally studied the products of mild thermal and organolytic decomposition of UDD and DB. Low-temperature (cold) extraction was carried out in a Soxhlet apparatus at a solid : liquid ratio of 1 : 10 and an extrac- tion time of 10 ± 12 h. Organolytic decomposition was accom- plished under the conditions of the supercritical state of the solvent where the extractive power of the liquid is a minimum. This treatment was carried out in a steel autoclave of a volume of 0.4 litre at 573 ± 673 K under a pressure>5 MPa. The extracts obtained were studied by the methods of low- temperature luminescence and IR spectroscopies, gas-liquid chro- matography, chromato-mass spectrometry and 1H NMR spec- troscopy.Different types of detonation charges including a sample which contained no diamonds (the so-called `under-developed' charge) were subjected to cold extraction. Depending on the synthesis conditions, the DB contained variable amounts of extractable substances. These are largely bi- and polycyclic aromatic hydrocarbons with a single or several substituents in the rings. Thus compounds containing sp2- and sp3-hybridised carbon atoms were obtained among molecular products of low- temperature extraction of DB. However, it is evident that such compounds are formed most probably from ultradispersed graph- ite and turbostratic carbon rather than from diamond. Under cold extraction, the solid carbon matrix is not split, but its extractant- soluble molecular components are desorbed and washed out.Hence, it can be concluded that the compounds identified are the intermediates of a transition from primary products of EM detonation to a condensed carbon phase. The following correlation is observed between the amount of extracted polyaromatic compounds, on the one hand, and the composition and perfectness of the diamond phase in the mixture (i.e., the amount of various defects on the surface and in the bulk of diamond grains, the presence of lattice distortions dependent on both the detonation conditions and the configuration of the blasting chamber), on the other hand: the maximum yield of soluble substances (*5%) is reached for detonation carbon that contains no diamond.A deeper extraction in combination with partial decomposi- tion of the solid matter was carried out at 473 ± 637 Kand elevated pressure, i.e., under conditions of supercritical states of organic solvents used. The supercritical liquefaction of carbon proceeds most completely in pyridine, which ranks among the most active solvents. At present, we know the composition of simplest high-temper- ature (hot) extracts obtained by using the following relatively mild hydrocarbon solvents (Table 12): of normal structure (heptane and decane), aromatic (benzene and toluene) and alicyclic hydro- carbons (cyclohexane) and hydronaphthalenes (tetralin and dec- alin). These solvents are relatively stable under experimental conditions; their extraction ability increases in the following sequence: normal<aromatic<alicyclic hydrocarbons<hydro- naphthalenes.Table 12. Aromatic heterocyclic compounds in high-temperature extracts from UDD and DB isolated using different solvents. Compound NNNN N N N , N N N N N NH N N Note. Sign `+' denotes the presence of a compound in the extract. The extracts obtained differed in colour: from light yellow (normal hydrocarbons) to dark brown (hydronaphthalenes). Upon the treatment, the ratio of diamond to non-diamond phases changed, as well as the surface properties. Supercritical liquefac- tion allows solubilisation of up to 10 mass% of diamond-con- taining phase in 30 min. In this case, relatively weak and reactive chemical bonds are cleaved including elimination of the surface groups and most disordered near-surface carbon layers of clusters.Stable macroscopic structural units are retained as a solid carbon residue, microunits survive as individual molecules which pass into solution. Among the latter, polycyclic nitrogen-containing heretocycles (Table 12) with one and two nitrogen atoms in the ring and a number of rings up to four were detected. Taking into account the laws of organic chemistry, the formation conditions of these compounds allow us to conclude that nitrogen is incorporated into the condensed carbon (includ- ing the diamond phase) as a result of primary reactions, most probably, due to polycondensation of nitrogen-containing mono- mers with carbon ± nitrogen bonds.At first glance, this contra- dicts the ESR spectra for nanodiamonds 21 which demonstrated 619 UDD, extraction by DB, extraction by toluene benzene cyclo- hydro- hexane naphthalenes + 7 7 7 + 7 7 + 7 7 + + + + + ++ + + 7 7 7 + +7 7 7 + + 7 7 7620 no characteristic triplet signal related to admixed nitrogen atoms replacing carbon in the diamond lattice. However, this a compar- ison rather emphasises the difference between `common' diffu- sional growth of a diamond crystal and the polycondensation mechanism of the formation of UDD during the detonation process. The former involves capture of admixed nitrogen atoms into the newly formed diamond lattice and distribution of such admixed atoms throughout the crystal bulk.In contrast, the formation of UDD first involves incorporation of nitrogen (more precisely, a nitrogen ± carbon pair) into an aromatic ring (with a high binding energy in the ring, which is typical of aromatic structures), and therings are built into a compacting prototype structure of condensed carbon. In the latter case, the paramagnetic properties of nitrogen should differ from those of admixed nitrogen atoms. The structure of the outer shell of a diamond cluster was also assessed on the basis of the results on thermodesorption. The studies were carried out on an LKB-2091 chromato-mass spec- trometer (Sweden). Thermodesorption was carried out at 573 Kin a flow of helium with collection of the products in a capillary cooled by liquid nitrogen.Then, thermodesorption products were vaporised and separated by chromatography on a capillary column with a weakly polar phase (SPB-5; column length, 60 m; diameter 3.261074 m), at a helium flow (rate of 2.561076 m3 s71) in a mode of programmed temperature increase from 293 to 543 K (heating rate, 4 K min71). The mass spectra were processed and the products were identified using a built-in computer RDR-2 using a mass spectra data base. Table 13 shows the composition of substances des- orbed from the surface of the diamond-containing mixture and UDD. Table 13. Products of thermodesorption (T=253 K) from UDD and DB surfaces. Compound Acetonitrile Nitromethane Butanal Butan-2-one Tetrahydrofuran Ethanol Acetone Ethyl acetate Benzene and its homologues Alkylbenzenes: C9 C10 Alkanes: C7 C8 C9 C10 C11 Alkenes: C7 C8 C9 C10 Terpene C10 Alkylcyclopentanes Naphthalene Note.Three samples of each type were analysed. The signs `+', `++' and `+++' show that the compound was detected in thermodesorption products for one, two and three samples, correspondingly. a After hydro- gen treatment. UDD UDD-Ha DB 777 ++ +++ + 77777 + 7 7 7 + 7 7 ++++ ++ ++ +++ 7++ +++ + ++++ 77 + +++ + + 777 + 7 +++ 7 +++ + + 7 7 + 7 + + 7 7 + 7 ++ 7 7+ ++ 7 ++ 7 7 + 7 V Yu Dolmatov Only hydrocarbons are desorbed from the DB surface: satu- rated C8±C11, unsaturated C8±C9, alicyclic and aromatic.The predominance of C10 alkanes and, probably, n-decane (C10H22) agrees with the result that detonation products contain carbyne, the structure of which is most thermodynamically favourable in the condensation of C10±C12 carbon chains with the formation of cumulene bonds. The proportion of aromatic hydrocarbons including C10 alkylbenzenes among the substances desorbed from DB is comparatively small, although the presence of poly- cyclic aromatic nets in DB itself is unambiguously proved.63 Hence, the degree of sp2-type condensation of carbon is high enough; however, the polyaromatic nets have a very disordered, predominantly aliphatic coating, viz., a peculiar type of hydro- carbon `fringe'.In the desoption products from the DB surface, hydrogen is present in an inactive C7H form, which agrees with the results of determination of active hydrogen on the surface of ultradispersed carbon.31 The composition of the desorption products from the UDD surface is more complex and diverse. In addition to hydrocarbons, oxygen- and nitrogen-containing compounds were identified, which are formed as a result of oxidation of the carbon surface. Benzene and C7±C10 homologues are abundant and are second only to C10-alkanes, n-decane prevailing among the latter. Of bridged alicyclic structures, camphene and terpene C10H16 are present. Such a composition of the desorption products suggests that the appearance of the diamond structures themselves at the interface is highly improbable.Stabilisation of the diamond structure in a UDD cluster occurs via various transition carbon structures. An exposure of UDD to a hydrogen flow at 673 K (UDD-H in Table 13) results in irreversible desorption of most of the C8±C11 hydrocarbons; however, the decomposition of surface carbon structures with the formation of C2±C7 hydrocarbons goes on, i.e., the surface metastability is reproduced. Below, certain characteristics of UDD cluster particles are summarised together with the methods of their estimation. Method Characteristic X-ray diffractometry Crystalline core with 4 ± 6-nm diameter The presence of peripheral condensed structures of the following types: �polyaromatic nets selective oxidation, organolysis organolysis organolysis, thermode- sorption, hydrogenolysis �aromatic heterocycles Stabilising system of surface compounds: �aliphatic units �bridged structures �aromatic rings Surface functional groups: �carboxyl �carbonyl �ester �quinone �nitrile IR spectroscopy, chemical analysis, potentiometry, polaro- graphy, thermode- sorption, thermovacuum desorption These so very diverse and different fragments are combined into a single particle and then into an ensemble of particles in accordance with the fractal rules by a non-continuous filling of a random volume by the mass of a substance.65 Two- and three- dimensional classical models fail to describe these particles due to their very high Cs/Ctot ratio.The packing and arrangement of UDD crystals also cannot be described within the frameworks of the classical theory of filling. It is assumed6 that the aggregates formed can have a fractal structure. The practical application of UDD requires these properties to be taken into account. An original procedure has been developed for the direct determination of the fractal dimension of UDD aggregates in hydrosols.41 The fractal dimension (D), which varied insignif- icantly in the range of 2.1 ± 2.3, was studied as a function of theDetonation synthesis ultradispersed diamonds: properties and applications pH, ionic strength and temperature. It was found that an increase in the temperature and ionic strength and a decrease in the pH favour flocculation accompanied by a decrease in the density and fractal dimension of UDD aggregates.A two-stage mechanism for UDD aggregation was assumed. In the first stage, relatively compact primary aggregates are formed by a particle ± cluster mechanism, which takes place in the oxidation of a non-diamond component of DB during its chemical purification. The second stage of cluster ± cluster aggre- gation with the formation of a looser secondary structure is reversible and preferentially associated with the flocculation of primary aggregates. It is by the structural heterogeneity of the interactions of clusters both with one another and with individual carbon- containing molecules (this is associated with the fact that different particles in powder systems are surrounded by different numbers of active `neighbours'), as well as by highly irreversible conditions of UDD formation that the fact that the experimental values of UDD fractal dimension do not correspond to any definite type of aggregation (cluster ± cluster, cluster ± particles) can be explained.XV. Comparison of properties of diamonds of different origins. Table 14 (see Ref. 66) presents comparison of the properties of finely dispersed diamond powders of different origins synthesised by three different methods. The chemical compositions of UDD synthesised by detona- tion ofEMwith a negative oxygen balance differ in the presence of substantial amounts of oxygen. Studies of electrophoretic proper- Table 14.Characteristics of diamond powders of different nature. SDB 1/0 a Characteristic Phase composition diamond of a cubic symmetry (a=0.357 nm) Substructure: CSR size /nm microstresses of II kind (Da/a) dislocation density /m72 r /g cm73 Particle size /nm undetected d 77 7 1.861017 (see d) 3.49 0 ± 2000 13.5 (see 39) Ssp /m2 g71 Elemental composition (mass %) [I.r.] (mass%) Tox /K (see f) Tg /K (see g) C�99.0; (Ni, Mn, Cr, Fe) e�0.5; Si�0.2; B�0.2; (O, H) e�0.1 <0.1 723 1373 R /Om 161011 (see 70) 0.0100 0.5 (see 70) 71480 (see 70) 76.53 Dissipation factor at a frequency of 103 Hz 108 w /m2 kg71 Degree of hydrophilicity /J mol71 kg71 Electrophoretic charge of the surface /mV Adsorption potentials: A /J g71 A0 /J m72 14.2 (see 70) 1.005 Note.Based largely on the results given in Ref. 66; the data of other studies are specified. a Synthesised by a steady-state method. b Synthesised by detonation-impact of graphite or soot in EM. c Synthesised by detonation of an EMwith a negative oxygen bSozin (Bakulev Institute of Superhard Metals, Kiev). e Total content. f Initial temperature of oxidation in air. g Initial temperature of graphitisation in vacuum. 621 ties of aqueous dispersions of UDD showed that the surface of particles acquires a considerable negative charge (778.44 mV) caused apparently by ionogenic oxygen-containing groups. Such a powder possesses high adsorption capacity.Being a characteristic of UDD, the adsorption potential exceeds that of SDB 1/0 powder by an order of magnitude, and the specific adsorption potential exceeds the corresponding value more than twice. On the other hand, with respect to magnetic and electro- physical characteristics UDD powders differ insignificantly from commercial diamond powders SDB 1/0 synthesised under steady- state conditions. Substantial differences are also observed in the values of the degree of surface hydrophilicity and electrophoretic charge: for UDD, they are two- and ten-fold higher, respectively. Despite their fine degrees of dispersion, UDD have the same graphitisation threshold, although the initial temperature of oxidation in air is 50 K lower for UDD as compared with SDB 1/0 diamonds synthesised under steady-state conditions.Diamonds obtained by a detonation impact on graphite and soot in an explosive mixture consist of two phases, cubic and hexagonal (Lonsdalite). UDD powders involve only a cubic modification. XVI. Application fields of ultradispersed diamonds 1. Electrochemical and chemical co-deposition of ultradispersed diamonds with metals Co-deposition of UDD with metals is applied in machine-build- ing, ship-building, aircraft, electronic, radio-engineering, medical, and jewelry industries.71 ± 83 UDDc Finely dispersed diamond b diamond of a cubic symmetry (a=0.357 nm) diamond of a cubic (a=0.357 nm) and hexagonal (a=0.252 nm) symmetry or a cubic symmetry 67 4 d absent d 10 ± 12 161073 ± 1.961073 3.20 ± 3.40 (see 67) 41 ± 82 (see 67), 10 ± 50 (from graphite) 20 ± 800 (from soot) 68 20 ± 42 (see 67) 20 ± 25 (from graphite) 35 (from soot) 68 7 3.30 2 ± 50 2 ± 20 (see 69) 4 (see 1) 217 150 ± 450 (results of the author) C�93.2 ± 100; O �0 ± 6.8; Si7traces <2.0 673 1373 ± 1473 7.76109 (see 70) 0.0145 0.1 (see 67) 7>1073 (see 69) 777 <1.0 (see 70) 7 73100 (see 70) 7 778.44 384 (see 70) 2.16 77622 Metal ± diamond composition coatings are characterised by the following common features: superior wear-resistance and microhardness, substantial corrosion stability, low porosity, a sharply reduced friction coefficient, substantial adhesion and cohesion, high throwing power of electrolytes. Microhardness, (103 h) /kg mm73 The advantages that may be achieved by using UDD in electroplating processes are listed below.1. The quality and competitive characteristics of manufac- tured articles are improved. The articles are characterised by �better resistance to wear and high microhardness; Resistance to wear increases by a factor of n, where n Coating UDD+M, where M 1.4 0.68 0.154 0.25 0.18 71.5 2 ± 12 5 ± 8 9 ± 10 1.8 ± 5.5 4 ± 12 310 ± 13 Cr Ni Cu Au Ag Sn Al �enhanced adhesion and a reduced friction coefficient; � reduced porosity (the porosity decreases six- to eightfold for UDD coatings with Zn, sevenfold with Sn, sixfold with Au, eightfold with Ag, and a pore-free coating is formed with Cu); � high insulating ability of anodic alumina films (a relative weight gain of such a film is higher by a factor of 2.0 ± 3.5); �corrosion stability (for UDD co-deposits with Cu, no mass loss in standard tests was observed; for co-deposits with Zn, the corrosion resistance increased two- to fourfold); � a sharp increase in the throwing power of electrolytes (threefold for UDD with Cu, 1.5 ± 2.0-fold for co-deposits with Zn);� enhanced elasticity (for UDD±Cu deposits, by a factor of 2 ± 4); 2.The service life of articles increases two- to tenfold. 3. The economy in materials, energy and labour are achieved (the coating thickness is recommended to decrease two- to three- fold); 4.The effectiveness of an electroplating unit increases by 20%± 50% due to a decrease in the coating thickness and (in a number of cases) an acceleration of film formation. 5. The environmental parameters of the electroplating process improve. Addition ofUDDto the electrolyte for nickel plating results in a four- to ninefold increase in the wear-resistance of nickel plates. The UDD content in metal films varies from 0.1 mass% to 1.5 mass% for all metals and from 0.5 mass% to 40 mass% for anodic oxide films. For a 161076-m thick metal layer, UDD consumption is *1 carat (261074 kg) per 1 m2. 2. Ultradispersed diamonds in polymer compositions Modified polymers are applied in the manufacture of aircraft, cars, tractors and ships, in medicine, chemical and petrochemical industries, production of seals, cut-off devices of various kinds, in protective and anti-friction film coatings.79, 83 ± 91 a.Film coatings based on fluoroelastomers and polysiloxanes A technology of cold vulcanisation and filling of fluoroelastomers with UDD particles allows one to obtain coatings with a number of advantages. 1. The permeability with respect to hydrocarbons and polar solvents decreases by a factor of 50, viz., from 1.38961077 to 0.027861077 kg m72 s71. Co-polymers with ethylene and with perfluoroalkyl vinyl ethers (protective coatings) exhibit the high- est chemical resistance to acidic and alkaline salt media. 2. The coefficient of dry friction with metal decreases to 0.01.V Yu Dolmatov 3. The elasticity and strength parameters improve: the stretch modulus at 100% elongation and the conditional rupture strength increase more than tenfold (from 8.5 to 92 MPa and from 15.7 to 173 MPa, respectively). In this case, the elongation increases by a factor of 1.6 (from 280% to 480%), and the residual elongation decreases by a factor of 1.2 (from 108% to 81%). 4. The strength of adhesion contacts increases (a) for active surfaces (St.3 steel, zinc, aluminium), by 200% ± 500% (from 1.7 to 5.1 kN m71 for St.3 and from 0.5 to 3.3 kN m71 for aluminium); (b) for inactive surfaces (lead, copper), the strength of adhe- sion contact is 2.8 ± 3.1 kN m71; (c) the coating developed has the following dielectric charac- teristics: � at 4000 MHz, the dissipation factor is 2.58 ± 2.71 depend- ing on the film thickness; � at 5000 MHz, the transmission and reflection factors may reach 15.0 and 12.4, respectively, depending on the film thickness; �at 11 000 MHz, the transmission and reflection factors may reach 14.3 and 12.4, respectively, depending on the film thickness.The coating retains its physical and mechanical properties and operation efficiency under a load up to 26106 kg m72. Film coatings based on UDD-modified polysiloxanes possess enhanced elasticity and strength characteristics: the modulus at 100% elongation increases by a factor of 3 (from 19 to 53 MPa); the rupture strength also triples both at 100% (from 52 to 154 MPa) and absolute (from 730% to 1970%) elongation; the tear strength increases by a factor of 1.4.Fluorine-containing elastomers modified with UDD possess improved elasticity and strength characteristics and are resistant to thermal ageing; the modulus at 100% elongation increases by a factor of 1.6 (from 7.9 to 12.5 MPa), the resistance to rupture is 1.1-times higher (from 15.4 to 16.5 MPa), the rupture elongation increases by a factor of 1.35 (from 210% to 285%). The resistance of UDD-modified fluoroelastomers to abrasive wear increases by a factor of 1.5 ± 2.0, i.e., with respect to this parameter, they are similar to polyisoprene rubbers. b. Rubbers Under thermal ageing at 573 K, UDD-containing rubbers either virtually retain their physical and mechanical characteristics at a level of analogous characteristics of standard compositions (non- subjected to heating) or exceed this level.The use of UDD is beneficial for cross-linking in non-destructive changes in the course of thermrubbers modified by UDD demonstrate enhanced elasticity and strength characteristics. Thus, for the SKI-3 isoprene rubber, the modulus at 300% elongation increases by a factor of 1.6 (from 7.7 to 12.3 MPa), the conditional tensile strength increases by a factor of 1.4 (from 20.5 to 28.2 MPa), and the tear strength increases from 139 to 148 kN m71. The max- imum swelling degree of rubbers in toluene decreases by 45%. These rubbers have a higher degree of vulcanisation and increased strength characteristics (approximately 30% higher than that of standard rubbers based on SKI-3) and higher wear endurance.The increase in elongation with an increase in the modulus observed for these rubbers contradicts the commonly accepted notions. This points to the changes in the supramolecular struc- ture of the rubber, evidenced by a substantial cohesion increase (by a factor of 1.6: from 1.7 to 2.7 MPa). Modification of the SKI-5 isoprene rubber by UDD particles results in a 1.6 ± 1.8-fold increase in the controlled strength parameters (modulus at 300% elongation, rupture and tear strengths) with retention of elastic properties. Oil-containing rubbers modified by UDD possess higher degrees of vulcanisation compared with those without UDD (the modulus at 300% elongation increases from 5.8 to 7.4 MPa, relative elongation decreases from 700% to 610%). Moreover, rubbers that contain UDD together with technological carbon have a tear strength 25%± 35% higher then the reference samples.Detonation synthesis ultradispersed diamonds: properties and applications With introduction of UDD into a standard rubber mixture SKMS-30 ARK based on a butadiene ± styrene co-polymer with 30% styrene units, the cohesion strength increases by a factor of 1.5 ± 2.0 (from 1.6 to 3.1 MPa) compared with reference samples. Rubbers modified by UDD are distinguished by high degrees of vulcanisation, their strength characteristics being practically equal to those of reference rubber, and twice surpass them in tear strength (which increases from 71 to 135 kN m71).The modulus at 300% elongation increases by a factor of 1.44 (from 7.9 to 11.4 MPa). Butadiene ± nitrile V-14 rubbers modified by UDD exhibit improved operating characteristics: the friction coefficient decreases by a factor of 1.5, wear resistance is enhanced by a factor of 1.3, the elasticity increases by a factor of 1.7, frost resistance also improves (the glass transition temperature decreases by 8%± 10%). For RSS natural rubber (Malaysia) modified by UDD, the modulus at 300% elongation increases threefold (from 1.8 to 5.4 MPa), which is accompanied by a certain improvement in operational characteristics. Epoxy glues based on UDD exhibit good adhesion and cohesion properties.Nanodiamonds are applied in technologies for polymerisation from solutions and melts, chemical solidification, and in the techniques for electron-beam, flame, and electrostatic sputtering. Standard equipment is used for fabrication of UDD-containing compositions. Polymeric compositions containing UDD display substantial advantages. 1. The quality and commercial characteristics of the produc- tion improve: the articles exhibit superior strengths, wear resist- ance, resistance to ageing and action of abrasives. The friction coefficient is low for polyfluoroelastomers (such as co-polymers of polyvinylidene fluoride with perfluorinated co-monomers, e.g., perfluoropropene) and high for polyisoprenes (SKI-3 and SKI-5). The materials and coatings can be adapted to particular technol- ogies and articles.2. Cheap polymeric starting materials can be substituted for expensive ones. 3. Expensive and deficient components and materials may be saved. Nanodiamond consumption amounts to 1 ± 5 kg per 1000 kg rubber (polymer) and 1 ± 5 kg per 1000 m2 of a polymeric coating or a film. 3. Ultradispersed diamonds in lubricants, greases, and lubricating ± cooling liquids. Modified lubricant compositions are used in machinery, metal working, engine-building, ship-building, the aircraft industry and transport.79, 83, 92 ± 100 Addition of nanodiamonds to oils allows one to obtain high- quality sedimentation-resistant and environmentally safe systems with superhard particles that measure<561077 m.The merits of UDD additives to oil compositions are as follows: 1. The quality and competitive characteristics of articles improve: the service life of vehicles and equipment increases; the economy of fuel and lubricants is achieved, these additives can serve as an alternative to expensive oils and special lubricants. 2. The friction torture is reduced by 20%± 40%. 3. The wear of friction surfaces decreases by 30%± 40%. 4. Fast run in of friction gears is observed. The consumption of nanodiamonds is 0.01 ± 0.2 kg per 1000 kg of oil. 4. Ultradispersed diamonds in abrasive tools Tools fabricated from nanodiamonds are used in abrasive, mining and metal working industries, in machinery, electronics, medicine and jewelry production.83, 101 ± 103 623 Below, the quality indices [microhardness (h), electrical resis- tivity (R), heat-conductivity (g) and permittivity (e)] are shown for clinkers obtained from diamond powders subjected to high pressures and temperatures, both for UDD themselves and diamond-oxide compositions with vitreous oxides (diamond ceramics).71 K71 1073h R g e /kg mm72 /Om /Wm 7 ± 9 8 ± 10 100 ± 150 120 ± 160 109 ±1012 1010 ±1012 Individual UDD 3.0 ± 3.5 3 ± 4 Diamond ceramics (UDD+MgO) For individual UDD, the density of defect-free fragments is (3.35 ± 3.50)6103 kg m73. These clinkers (the so-called diamond glasses) can easily be cut by ultrasound and laser, ground and polished. They can be used as abrasives in the working of materials of average hardness and in electronics.Ultradispersed diamonds metallised by transition-metal clus- ters possess high microhardness [h=(6 ± 7)6103 kg mm72] and represent excellent tool materials. The use of UDD (0.5% ± 1.5%) for enhancing the strength properties of polymeric matrices of grinding and polishing tools results in a simultaneous increase in both strength (by a factor of 1.3 ± 1.5) and elastic (by a factor of 1.8 ± 2.0) properties of an abrasive tool. 5. Ultradispersed diamonds in microabrasive and polishing compositions. Polishing systems (PS) that contain UDD are used in precision finishing of materials in radio engineering, electronics, optics, medicine and machinery.79, 104 ± 109 The compositions allow one to obtain perfect mirror solid surfaces of required planeness, plane-parallelism, of any geo- metrical shape, containing no defects and dislocations, with the contour interval Rz=2 ± 8 nm (with an average deviation from a mean hypothetical surface s=0.5 ± 2.0 nm).The advantages of UDD-containing polishing systems are as follows. 1. The minute sizes of UDD ensure the minimum surface roughness and the colloidal stability of PS. 2. The chemical stability ofUDDallows one to use chemically active additives to PS and to recover PS. 3. The amount of material removed from the surface being polished is small (the material losses decrease). 4. Owing to ion-exchange and sorption properties of UDD, the removed ionic and molecular products are immobilised on the UDD surface, which provides a clean surface.5. The fractal structure of UDD aggregates ensures a con- trolled structurisation in PS suspensions and the absence of stressed and disrupted surface layers on a material being polished. 6. These systems are non-toxic. 7. Polishing systems with UDD allows one to enhance the quality and competitive characteristics of polished articles and to ensure the polishing of materials technologically difficult to work. Nanodiamond consumption is 0.001 ± 0.01 kg per 1 m2 of the surface worked. 6. Ultradispersed diamonds in magnetic recording systems. Nanodiamonds are used, first of all, as anti-friction additives and physical modifiers in ferromagnetic-varnish coatings of magnetic films and disks and, secondly, as the additives to electrochemical composition films destined to improve the magnetic recording device characteristics.77, 79, 83 1.The addition of UDD to a ferromagnetic layer sharply reduces its domain sizes (ferromagnetic grains), i.e., the density of recording increases substantially.624 When introduced into a varnish to form a special film destined for cleaning of recording heads, UDD provides polishing of the latter and enhances their resistance to wear. Magnetically soft information carriers which contain UDD display the following advantages: abrasive wear of a magnetic- carrier layer decreases, optimum conditions are provided for operation of magnetic heads and reading devices, friction of the magnetic carrier decreases and stability of its motion is provided.The consumption of nanodiamonds is 1%± 2%of the varnish mass. 2. As compared to individual CoP coatings, magnetically soft amorphous CoP ±UDD films are shown to display the following advantages: a 1.3-fold increase in microhardness, a 4.5-fold increase in the resistance to wear, a 1.4-fold decrease in friction coefficient, a 2-fold increase in the service life of magnetic-head cores without any changes in their effectiveness. For magnetically hard polycrystalline CoP ±UDD coatings compared to individual CoP coatings both electrochemically and chemically deposited, the microhardness increases by a factor of 1.2, corrosion currents decrease by a factor of 1.6, the service life of the carriers of magnetic information increases.Magnetic characteristics of both magnetically soft amorphous films and magnetically hard coatings remain unchanged with an addition of UDD. Nanodiamond consumption is *1 carat for a layer with 161076 m thick. 7. Biological activity of ultra-dispersed diamonds When UDD are used as potential medical drugs, their anoma- lously high adsorption capacities, high specific surface areas, abundance of free electrons on the surface (a multiple radical- donor), nanoscale sizes, a great amount of oxygen-containing functional groups on crystal surfaces, chemically inert grains and surface hydrophilicity are of great importance.79, 110 Nanodiamonds can be applied in oncology, gastroenterology, cardiology, dermatology, for curing vessel diseases, etc.They display no carcinogenic and mutagenic properties and are non- toxic. Owing to their high adsorption capacities and other specific features, nanodiamonds exhibit extremely high activities with respect to pathogenic viruses, microbes and bacteria by absorbing them intensively. Nanodiamonds are selective sorbents and sup- ports for biologically active substances, they can significantly enhance the action of drugs. The use of UDD normalises blood pressure. Moreover, UDD are effective drugs for different gastro- enteric diseases, an excellent remedy for after-effects of burns, different skin diseases and intoxication. The use of UDD in the form of aqueous and oil suspensions is beneficial for curing cancer patients; these tend to increase the effect of drugs, relieve pains, normalise peristalsis of the bowels, improve blood characteristics and activate the immune system, remove toxins from the organism and prolonge the patient's life.The use of UDD in chemical and radio-therapy appears to be promising in the cure of malignant tumours for prevention of the mutagenic effect of drugs. At the same time, UDD do not reduce the therapeutic effect and are capable of preventing the appear- ance of mutations in normal cells or generation of secondary tumours under the action of anti-tumour drugs. 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年代:2001

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