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Russian Chemical Reviews 68 (9) 709 ¡À 726 (1999) Electronic effects of substitution and reconstruction of the coordination polyhedron in adducts of Main Group IV element halides E A Kravchenko, Yu A Buslaev Contents I. Introduction II. Electronic effects of substitution in adducts of tin(IV) halides III. Electronic characteristics ofM7Hal bonds in hexa- and pentacoordinate adducts of tin(IV) tetrachlorides; similarities and differences IV. Penta- and hexacoordinate adducts of silicon and germanium tetrachlorides V. Conclusion Abstract. Nuclear quadrupole resonance and X-ray data on the electronic effects of substitution of acidoligands and reconstruc- tion of the coordination polyhedra in adducts (complexes) of tin(IV), germanium(IV) and silicon(IV) tetrahalides due to the change in the central metal coordination number are surveyed and systematically described.The s- and p-electron density redistribution over the central metal ¡À halogen bonds resulting from substitution or reconstruction of the coordination polyhedra from pseudooctahedral to trigonal-bipyramidal geometry is dis- cussed. The bibliography includes 55 references. I. Introduction Redistribution of electron density upon substitution or recon- struction of the central atom coordination polyhedron in coordi- nation compounds is a fundamental problem of the chemistry and stereochemistry of heteroligand complexes. Whereas the impor- tance of this problem in the coordination chemistry of transition elements is widely known, the electronic effects in the chemistry of main group elements have been studied less extensively.Electronic effects of substitution as well as their steric manifestations are extremely diverse in such compounds, the resulting changes in the metal ¡À halogen bonds being sensitive to details of the electronic structure of the whole adduct, viz., the central metal, ligands and substituents. In most earlier publications, the electronic mecha- nism of substituent influence was discussed based on the position of ligands in various rows, AO occupancies determined by approximate quantum-chemical calculations and X-ray data. However, all these characteristics related only indirectly to the phenomenon in question, because the contribution of the sub- stituents to the changes in the appropriate parameters is as a rule not unique and sometimes not predominant.Often, the results thus obtained do not allow determination of both the extent of the influence and even its direction. As a result, many important questions of the electronic mechanism of ligand influence in main E A Kravchenko, Yu A Buslaev N S Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky prosp., 117907 Moscow, Russian Federation. Fax (7-095) 954 12 79. E-mail: ekravch@ionchran.rinet.ru (E A Kravchenko). Tel. (7-095) 952 22 78. E-mail: buslaev@ionchran.rinet.ru (Yu A Buslaev) Received 1 December 1998 Uspekhi Khimii 68 (9) 787 ¡À 805 (1999); translated by E A Kravchenko #1999 Russian Academy of Sciences and Turpion Ltd UDC 539.143.44+541.49+541.67 709 710 717 721 725 group element coordination compounds remain still open.In particular, quantitative data on electron density redistribution over the atomic s and p orbitals as a result of either substitution of acidoligands or change in the adduct geometry, caused by the change in the central atom coordination number, were very scanty. The intention of this review is to bring to the attention of researchers a feasibility of quantitative description of electron density redistribution in Main Group IV element coordination compounds using nuclear quadrupole resonance (NQR) spectro- scopic data. Here we discuss only static influence of substituents, which reflects processes in a system in the ground state.In this case, a parameter can be used as a measure of substituent (ligand) electronic influence if it satisfies at least two requirements: first, it characterises a system in its ground state and second, it is changed predominantly under this influence. In this respect, NQR spectro- scopic parameters seem to be quite suitable, since they are the ground-state characteristics of a system directly relating to the electron density distribution in the vicinity of a quadrupolar nucleus. NQR spectroscopy investigates interactions between nuclei having nonzero quadrupole moment Q (all the nuclei with spin I>1/2 have such a moment) and their local electric environment. The latter is characterised by the electric field gradient (EFG) qij �� qEi qj �� qq2ijU, where Ei is the strength of the electrostatic field, U is the electro- static potential at the nuclear site and i, j=x, y, z.The directly measured spectroscopic parameters are the quadrupole coupling constant (QCC) QCC=e2Qqzz , h where e is the charge on the electron and h is the Planck constant, and the EFG asymmetry parameter Z �� jqxx ¡¦ qyyj . qzz Because the values of QCC and Z depend directly on the electron density distribution around the nucleus under study, any weak disturbance of the electron density upon indroduction of a substituent is clearly reflected in the NQR spectra. The physical710 principles of the NQR phenomenon, the scope and limitations of NQR spectroscopy in application to problems of inorganic and coordination chemistry have been reviewed.1 X-Ray diffraction analysis is yet another approach to the study of substituent effects.However, one should bear in mind that the changes in interatomic distances depend on numerous factors which form the optimum crystal packing and are not necessarily related to the effects of substitution. Comparison of NQR and X-ray data allows one to determine the changes in effective charge on atoms upon the appropriate bond lengthening, which, in turn, helps to outline the range of variation of interatomic distances under the influence of substituents. The use of the two experimental techniques providing complementary pieces of information, on such important properties of com- pounds as electron density distribution and spatial arrangement of atoms, permits avoiding indirect estimations in the study of the problem on a quantitative level.This review presents the results of NQR spectroscopic studies on the s- and p-electron density redistribution over metal ¡¾ halogen bonds in tin(IV), germanium(IV) and silicon(IV) halide complexes and adducts resulting from the substitution and/ or change in the central atom coordination number. The following series MHal4?MHal4L?MHal4L2, are considered: SnHal4L2?RnSnHal47nL2 and [SnHal6]27?[RSnHal5]27, where M=Sn, Ge and Si; Hal=Cl, Br and I; R=Alk and Ph; and L are oxygen- and nitrogen-containing ligands. II. Electronic effects of substitution in adducts of tin(IV) halides The problem of electron density redistribution over the atomic s and p orbitals of metal ¡¾ halogen bonds upon substitution in tin(IV) adducts of the type SnHal4L2 ([SnHal6]27)? RnSnHal47nL2 ([RSnHal5]27) seems to be most intriguing. The solution of this problem requires the knowledge of the complete set of NQR spectroscopic parameters for the complexes and adducts of parent tin tetrahalides including the asymmetry parameters Z of the halogen atoms. In this respect, iodostannates are most suitable for NQR studies, because the iodine (127I) nuclear spin is I=5/2 and the QCC and values in the iodo adducts can be found, provided both resonance transition frequencies, n1 (Dm=1/273/2) and n2 (Dm=3/275/2) are observed.1 No additional experiments are necessary in this case, whereas they are necessary for determining Z in chlorostannates, because only two degenerate quadrupole levels and one transition frequency are available for chlorine nuclei with I=3/2, which precludes simultaneous determination of the QCC and Z values from the known relationship n a e2Qqzz 2hO1 a Z2=3U1=2 .The EFG asymmetry parameter provides additional informa- tion on the electron density distribution around the nucleus in question, because Z is a measure of the bonding electron cloud axial asymmetry. Hence the problem of measuring Z on the 35, 37Cl nuclei which are most often subject to NQR studies, is considered by spectroscopists as being of current interest, and various attempts were undertaken to solve it.Most determina- tions of Z on the 35Cl atoms were made by Zeeman studies of single crystals.1, 2 These measurements provide high precision of the Z values and permit determination of the orientation of the EFG axes with respect to the crystallographic axes of the specimen. However, this approach substantially reduces the number of the compounds that can be examined, eliminating those air- or moisture-sensitive as well as presenting problems for the crystal growth. The number of reports on the determination of Z in crystalline powders is much less. The majority of the results published by foreign authors were obtained using theCWtechnique, which as a E A Kravchenko, Yu A Buslaev rule gives low accuracy of measurements 2.The advantages of pulsed methods over stationary ones for measuring Z in powders were outlined in the review.1 One approach was based on the Fourier-analysis of slow beats of the spin echo envelope in external magnetic fields of 15 ¡¾ 100 e. In powders of chlorine- containing compounds, this provided values of Z in reasonable agreement with those available for the appropriate single crys- tals.3 This approach was applied for determination of Z in organic and organoelement compounds.4¡¾7 Later, it was modified 8 to be used for measuring Z in a large number of adducts of tin, germanium and silicon tetrachlorides.9¡¾ 12 The determination of Z in compounds in question enables one to escape the s-approximation, to take into account the electron density distribution over the tin ¡¾ halogen s and p orbitals and to juxtapose trends in electron density redistribution upon substitu- tion of different groups for acidoligands.Within the Townes and Dailey theory approximations, the measured NQR spectroscopic parameters (QCC and Z) are related to the halogen atom valence p-orbital occupancies Npi (i=x, y, z) and electronic character- istics of metal ¡¾ halogen bonds as follows: U (1) z ¢§ Np p a e2Qqzz a Npx a Npy e2Qq0 2 a s ¢§ p2 , (2) Z a qxx ¢§ qyy a q p zz 2Np3zONp ¢§ OxNp ¢§xNp ayNp U yU a 3Op2xU¢§ pyU , where e2Qqzz is the QCC value at the halogen atom, e2Qq0 is the atomic QCC produced by a single electron on the valence pz orbital of the halogen atom; Npz=27s, Npx=27px, Npy=27py; px+py=p ; i+s+p=1, where i, s and p are the ionic, covalent and p-character of the bond.Whereas the mutual ligand influence in similar transition metal compounds is known to alter the metal7halogen bonds in such a way that the trans-influence is prevailng and is accompa- nied by a slight change in the cis-bonds, the resulting changes in the Sn7Hal bonds due to the introduction of substituents R into [SnHal6]27or SnHal4L2 were found to be sensitive to details of the electronic structure of the whole adduct, viz., to the AO energies for the central atom, the halogen atoms and substituents as well as to the oxidation state of the central atom.Depending on the oxidation state of the central metal and its position in the Periodic Table, cis-weakening, or trans-weakening or cis-weakening accompanied by a relatively smaller trans-influence can prevail, both strengthening and weakening being possible in principle. Various theoretical approaches have been developed to describe the effects of substituents (ligands) in main group element complexes. Based on the model of equivalent orbitals applicable to compounds having covalent two-electron two-centre bonds, the mechanism of ligand influence was described as purely induc- tive.13 As a result of substitution, the M7Hal bonds were predicted to undergo changes so as to provide prevailing of cis- over trans-influence.In the approach using the canonical s-MO within the perturbation theory approximations, the atomic orbital energies of the halogen ligands and substituents were assumed to be close to each other.14 According to this approach a trans- influence prevailing over relatively weak cis-influence was expected as the contribution of the np orbitals of the central atom became more pronounced. A consideration which took into account all the transitions between the occupied and vacant MOs suggested that the central atom valence basis set involved mostly ns and np orbitals rather than the vacant nd-AOs.15 The resultingElectronic effects of substitution and reconstruction of the coordination polyhedron in adducts of Main Group IV element halides change in the the M7Hal bond character upon substitution was determined in this case by a relationship between the energies of the levels for the valence ns- and np-AOs of the central atom and s- AOs of typical ligands.This means that the influence was expected to be essentially different for the compounds of Groups II ¡¾ IV (type I) and VII (type II) elements. Thus the substitution [MHal6]27?[RMHal5]27 in complexes of the Main Group IV element was predicted to result in contraction of the [MHal6]27 octahedron along the z axis and its expansion in equatorial directions, if substituent R is more electropositive than Hal, whereas the octahedron will be expanded along the z axis and contracted in the x, y directions, if R is more electronegative than Hal.A model of hypervalent (HV) orbital-deficient bonds 16 applied to tin(IV) compounds 17 suggests that the central atom valence basis involves only ns and np orbitals. According to this model, in the parent [SnHal6]27 complex, the 5s orbitals of the tin atom participate in the formation of three equivalent three-center sp-hybrid hypervalent (HV-I) bonds Hal7Sn7Hal, each involv- ing only one 5p orbital of the tin atom.The s-level energies of the substituents R in adducts (complexes) RnSnHal47nL2 and [SnHal6]27 are then close in value to those of the p levels of the central atom, while the s-level energies of the acidoligands are close in value to those of the s levels of the tin atom. In alkyl- and dialkyl-substituted complexes, occupation of a1 MOs occurs, which are antibonding with respect to the equatorial ligands.Because the a1 MOs contain a remarkable contribution from the s orbital of the central atom, the s-character of the equatorial bonding orbitals is reduced, so that the corresponding bonds (HV-II) are weakened compared to the bonds HV-I in the parent nonsubstituted complex [SnHal6]27. On the other hand, alkyl substituents R cannot form stable hypervalent bonds with tin, because R is less electronegative that the tin atom, so that the Alk7Sn bonds are ordinary covalent two-centre two-electron bonds. Thus, for the adducts and complexes RSnHal3L2 and [RSnHal5]27, the model of hypervalent orbital-deficient bonds predicts weakening of the Sn7Hal bonds at the cis-position to the alkyl substituent compared to the bonds in the initial SnHal4L2 or [SnHal6]27 compounds. The weakening can be accompanied by trans-influence, presumably strengthening of the Sn7Hal bond at the trans-position to the substituent R.Using NQR, the electronic effects of substitution were observed experimentally: the direction of the trans-influence was determined and the relative importance of the effects of cis- and trans-influence was quantitatively compared. Most clear manifes- tations of both effects were obtained for pseudooctahedral complexes of the type [RSnHal5]27 and molecular adducts RnSnHal47nL2 where the tin atom is in its highest oxidation state and either one or two acidoligands (Hal) are substituted for alkyl groups R, which considerably exceed the halogen ligands in donating power.The effects of substitution were especially strong in these compounds, and we discuss here how the substitution influences the s- and p-electron density distribution in the complexes and molecules of adducts. Tables 1 and 2 list the NQR spectroscopic parameters measured for the substituted tin(IV) compounds and their unsub- stituted precursors, hexahalostannates Cat2SnHal6 (Cat is cation) and pseudooctahedral adducts of tin tetrahalides SnHal4L2. The appropriate Sn7Hal interatomic distances wherever available are also given in these tables. As is seen from Tables 1 and 2, the cis- and trans-influence of the substituents is clearly reflected in the NQR spectra, which enables one to compare their contribution to the resulting electron density redistribution on a quantitative scale. In Tables 3 and 4, the halogen atom valence ps orbital occupancies (Npz) and pp orbital occupancy differences (DNpp=Npx7Npy), estimated from the NQR data, are given for the adducts and complexes of parent unsubstituted and mono- and dialkyl(diphenyl)-substituted tin halides [RSnHal5]27, RSnHal3L2, R2SnHal2L2. The main sources of inaccuracy in the 711 determination of the spectroscopic parameters as well as the reliability of interpreting the spectral data were discussed in a review.3 Based on the estimated values of the atomic orbital occupancies, the effective charges (r) on the halogen atoms and their variation (Dr) as a result of the introduction of the substituents R were determined (3) Dr a rORnSnHal4¢§nL2U ¢§ rOSnHal4L2U .rOSnHal4L2U Similarly, the variation of the Sn7Hal bond distances (Dd) was estimated in compounds whose structures were studied (4) Dd a dORnSnHal4¢§nL2U ¢§ dOSnHal4L2U . dOSnHal4L2U The values of Dd are also listed in Tables 3 and 4. The NQR spectra provide evidence for a profound electron density redistribution upon substitution. The negative effective charge on the halogen atoms in the cis-position to the substituent R increases in absolute magnitude with respect to the correspond- ing parent compounds hence yielding the positive values of Dr (Tables 3 and 4). On the halogen atoms in the trans-position to the substituent, the absolute magnitude of the charge falls as com- pared to that in the parent nonsubstituted adduct.The negative values of Dr, found for the trans-halogens in all the adducts studied, indicate that the trans-influence of substituents R always acts in these compounds as trans-strengthening of the Sn7Hal bonds. The only exception was the adduct EtSnCl3(DMSO)2 (Table 3), in which strong donor ligands compensate the effect of trans-strengthening (and accordingly trans-shortening). The introduction of an ethyl group into a [SnCl6]27complex (its singlet NQRspectrum points to the equivalence of all the chlorine atoms) results in the formation of the complex [EtSnCl5]27 characterised by an NQR spectrum with a spectroscopic splitting of 6 MHz between the frequencies, assigned to the chlorine atoms in cis- and trans-positions relative to the substituent R.The effective charges on the appropriate chlorine atoms differ on average by 0.1 e. The difference between the characteristics of the cis- and trans- bromine atoms in the complex [BuSnBr5]27 is even more pro- nounced: the splitting of the corresponding 81Br NQR frequencies exceeds 50 MHz, whereas the difference in effective charges on the corresponding Br atoms reaches 0.16 e (see Ref. 3). It is evident that such values of frequency shifts and spectro- scopic splittings are mainly due to the intramolecular electronic rather than crystal-field effects. A correlation between the 35Cl NQR frequencies and Sn7Cl bond lengths was examined for a large number of chlorine-containing tin compounds including hexacoordinate tin(IV) complexes of the types Cat2SnHal6 and Cat2RSnHal5 with various cations and anions, electroneutral adducts of types SnCl4L2 and R2SnCl2L2 and compounds RnSnCl47n, where the tin atom is tetracoordinate and both terminal and bridging chlorine atoms are available.19 Tin(II) compounds SnCl2 .xH2O (x=0, 1.5, 2) were also taken into consideration. The results of the analysis showed that the maximum contribution of the crystal field to the frequency shifts and Sn7Cl bond lengths in these compounds does not exceed 2 MHz and 0.05 A, respectively. These contributions are evi- dently less in crystallochemically similar chlorine-containing compounds listed in Tables 3 and 4.Their further decrease occurs on going to more covalent tin(IV) bromo- and iodo compounds. It should be added that the relative changes of the effective charges on atoms are considered as the measure of substituent effects rather than their absolute values. Thus the electronic effect of the ethyl substituent in pentachloroethylstannate was estimated from changes in the effective charges on the chlorine atoms compared to those in the parent hexachlorostannate with the same cation, which implies that the crystal field contributions to the 35Cl NQR frequencies of both pentachloroethyl- and hexachloro-stannate are virtually the same.20, 21712 Table 1. 35Cl and 81Br frequencies [n(35Cl) and n(81Br)/MHz], EFG asymmetry parameters (Z) in the NQR spectra of SnHal4L2, [SnHal6]27 , RnSnHal47nL2 and [RSnHal5]27 at 77 K and Sn7Hal bond distances (d).9 ± 11, 18, 19 ± 22, 23 Compound n (35Cl) (NMe4)2SnHal6 a (NMe4)2RSnHal5 a (MeNH3)2SnCl6 a (MeNH3)2EtSnCl5 a (PyH)2SnCl6 a (PyH)2EtSnCl5 a 16.683 12.329 18.070 15.815 12.072 11.537 10.770 9.969 17.032 16.588 11.322 17.926 17.91 20.02 trans-SnHal4L2 trans-RSnHal3L2 trans-PhSnCl3L2 5.6(3) 7{14.0} 7 L=HMPT 10.6 {0 ± 5.0} 2.379(1) 7 7 2.491(1) 18.525 14.045 9.450 trans-Me2SnHal2L2 trans-SnHal4L2 MeSnHal3L2 19.02 20.17 15.45 Me2SnBr2L2 trans-SnHal4L2 trans-SnHal3L2 18.24 19.35 13.47 trans-Et2SnBr2L2 cis-SnHal4L2 cis-EtSnCl3L2 18.36 17.045 13.52 16.96 10.40 cis-Me2SnCl2L2 trans-Me2SnBr2L2 cis-SnHal4L2 19.75 19.135 17.77 11.13 trans-SnCl4L2 cis-Me2SnCl2L2 cis-SnBr4L2 cis-BuSnBr3L2 b 17.70 trans-SnHal4L2 trans-Me2SnHal2L2 trans-Et2SnHal2L2 cis-SnBr4L cis-Bu2SnBr2L2 Br7Sn7Br Br7Sn7N Br7Sn7N Note.Resonance frequencies of chemically equivalent halogen atoms are averaged over crystrallographically inequivalent positions for all the compounds except for (MeNH3)2EtSnHal5. The values expected by analogy with those measured for the related compounds are given in braces. HMPT is hexamethylphosphortriamide; THF is tetrahydrofuran; TMU is tetramethyl urea; DMSO is dimethyl sulfoxide; DMF is dimethylformamide; DPSO is diphenyl sulfoxide; py is pyridine; bipy is bipyridyl.a The NQR data at 300 K. b The substituent R (Bu) is in trans-position to the ligand DPSO.24 d (Sn7Cl) /A Z (%) 7{10.0} 77 2.424(3) 2.516(10) 2.407(3) 2.417(3) 11.7(3) 2.507(1) 14.6(3) 2.531(1) 15.4(3) 2.497(1) 17.1(3) 2.549(1) 2.426(1) 2.431(1) 2.518(1) 2.401(2) 2.388(1) 72.570(1) 7 {>6.7} 7 {10.6} {10.6} L=THF13.5 {>7.0} 7 {13.5} 72.391(1) L=TMU7.5 {>1.4} 2.374(4) 2.491(4) {7.5} L=DMSO {3.0} {14.0} 14.0 3.0 {14.0} 2.382(3) 2.391(3) 2.456(4) 2.398(3) 2.51(1) 77 L=Me2CO 1.2 7.0 L=DMF 5.7 {5.7} 72.47(1) L=py15.4 Assignment n (81Br) 115.07 81.94 133.36 Cl7Sn7Cl Cl7Sn7Cl Cl7Sn7Et Cl7Sn7Cl Cl7Sn7Cl Cl7Sn7Cl Cl7Sn7Cl Cl7Sn7Cl Cl7Sn7Et Cl7Sn7Cl Cl7Sn7Cl Cl7Sn7Et 127.75 132.60 93.82 60.61 Cl7Sn7Cl Cl7Sn7Et Cl7Sn7Cl Cl7Sn7Ph Cl7Sn7Cl Cl7Sn7Cl Cl7Sn7Cl Cl7Sn7Me Cl7Sn7Cl 132.44 141.12 104.70 99.04 124.44 Cl7Sn7Cl Cl7Sn7Et Cl7Sn7Cl 75.71 118.17 127.73 Cl7Sn7Cl Cl7Sn7O Cl7Sn7O Cl7Sn7Et Cl7Sn7O 59.46 Cl7Sn7Cl Cl7Sn7O Cl7Sn7Cl Cl7Sn7O 116.63 128.30 90.72 102.40 Cl7Sn7Cl 122.66 65.76 64.61 118.87 129.92 74.69 E A Kravchenko, Yu A Buslaev d (Sn7Br) /A Z (%) 2.605(1) 2.699(5) 2.566(2) 7{10.0} 7 L=HMPT {10.6} {>6.5} {10.6} 2.55(1) 2.529(1) 2.633(1) 2.74(1) {10.6} 7777 L=THF {13.5} {>6.2} {13.5} {13.5} L=TMU {7.5} 7 {7.5} 7 L=DMSO {3.0} {14.0} 772.752(1) {14.0}L=DPSO2.556(1) 2.527(2) 2.627(2) 2.559(1) 7777 2.719(5) 7 L=py {15.4} {15.4} {15.4}L=bipy 2.565(1) 2.540(1) 2.714(1) {0.0} {15.4} {15.4} Assignment Br7Sn7Br Br7Sn7Br Br7Sn7Bu Br7Sn7Br Br7Sn7Bu Br7Sn7Br Br7Sn7Br Br7Sn7Br Br7Sn7Me Br7Sn7Br Br7Sn7Br Br7Sn7Br Br7Sn7Br Br7Sn7Br Br7Sn7O Br7Sn7Br Br7Sn7Br Br7Sn7O Br7Sn7Br Br7Sn7O Br7Sn7Br Br7Sn7Br Br7Sn7BrElectronic effects of substitution and reconstruction of the coordination polyhedron in adducts of Main Group IV element halides Table 2.127I NQR spectroscopic parameters [n1, n2, e2Qqzz/h (MHz); Z] for adducts SnI4L2 and RnSnI47nL2 at 77 K and Sn7I bond distances (d).12, 18, 25, 26 Compound n1 L=HMPT cis-SnI4L2 cis-EtSnI3L2 156.15 164.61 166.40 111.30 109.80 70.51 86.92 88.46 trans-Et2SnI2L2 trans-Ph2SnI2L2 (see b) (I) (II) L=TMU cis-SnI4L2 157.27 164.93 L=TPPO c cis-SnI4L2 158.36 163.42 L=bipy cis-SnHal4L 153.68 169.29 L=DMSO cis-SnI4L2 cis-Et2SnI2L2 Ph2SnI2L2 150.65 166.55 70.83 79.96 103.17 107.02 L=py trans-SnI4L2 Et2SnI2L2 162.66 80.92 82.01 98.11 Ph2SnI2L2 L=DPSO cis-SnI4L2 cis-EtSnI3L2 154.41 163.72 164.58 167.22 130.50 136.50 Note.Resonance frequencies of chemically equivalent iodine atoms are averaged over crystrallographically inequivalent positions. The values expected by analogy to those measured for the related compounds are given in braces.a Calculated. bThe unit cell comprises two crystalographically independent molecules of the adduct. c Triphenylphosphine oxide. As is seen from Tables 3 and 4, the prevailing effect of substitution is, in agreement with the earlier conclusions,17 weakening of the cis-Sn7Hal bonds relative to the substituent R as compared to the Sn7Cl bonds in the corresponding non- substituted complexes (adducts). The elongation of the Sn7Hal bond by 0.1 A is accompanied by an increase in the effective negative charge on the chlorine, bromine and iodine atoms by ca. 0.08, 0.1 (see Ref. 18) and 0.12 e (see Ref. 12), respectively. It is noteworthy that according to the X-ray data the elongation of the cis-Sn7Hal bond relative to the substituent R is the same for all the monoalkyl-substituted adducts of any halogen element, amounting approximately to 0.1 A. The similar bond elongation is accompanied by an increment of the negative charge on the corresponding halogen atom, which points to an increase in cis- weakening due to the alkyl group in the order Cl<Br<I.12, 18 d /A e2Qqzz/h Z (%) n2 {1.9} {6.0} 1040.59 a 1093.11 a 77 11.0 732.40 219.17 312.15 a 327.69 a 7 7 7 2.634(4) 2.886(2) 7 7 7 2.901(2) 3.016(1) 2.957(1) 2.938(1) 6.5 41.2 41.2 467.90 495.17 504.08 140.26 144.00 146.60 1.9 6.0 1048.04 1095.28 314.39 328.35 77 7 7 7 2.813(2) 7 7 7 2.782(2) 7 7 7 2.816(2) 7 7 7 2.788(2) 7 7 7 7 I7Sn7I 7 7 7 7 I7Sn7O 39.1 13.3 24.3 8.6 2.941(1) 2.982(1) 7 7 7 7 409.07 523.00 647.32 707.80 119.30 156.34 192.00 212.047 7 7 7 I7Sn7I 25.7 8.8 18.9 504.35 542.13 630.03 149.36 162.38 187.68 777 9.8 1018.70 305.00 12.4 895.10 268.00 2.806(1) 7 7 7 2.781(1) 7 7 7 2.776(1) 7 7 7 2.773(1) 7 7 7 2.869(1) 2.840(1) 2.821(1) Phenyl substituents also weaken the appropriate Sn7Hal bonds, although to a lesser extent.The effective charges born by the iodine atoms in dialkyl- and diphenyl-substituted adducts R2SnI2L2 were estimated 12 to differ, on average, by 0.08 e. In adducts of mono-substituted tin halides, both cis-weakening and trans-strengthening are also more pronounced in alkyl-substituted adducts than in phenyl analogues (Tables 1 and 3).Variations of the effective charges on the halogen atoms in cis- and trans- positions to the substituent R as well as their valence pp orbital occupancies resulting from substitution can clearly be compared on a quantitative scale. As follows from comparison of the absolute magnitudes of Dr (Table 3), a negative charge incre- ment on the halogen atoms, i.e., the effect of cis-weakening of the Sn7Hal bond as a result of substitution, is 2 ± 3 times as strong as that of trans-strengthening. Comparison of the corresponding 713 Assignment I7Sn7I I7Sn7O I7Sn7Et I7Sn7O I7Sn7O I7Sn7I I7Sn7I I7Sn7I I7Sn7I I7Sn7O I7Sn7I I7Sn7O I7Sn7I I7Sn7N I7Sn7O I7Sn7O I7Sn7I I7Sn7I I7Sn7I I7Sn7I 777I7Sn7I I7Sn7I I7Sn7O714 Table 3.Changes in the Sn7Hal bond characteristics in the adducts of tin tetrahalides as a result of substitution SnHal4L2?RSnHal3L2. Compound Nps DNpp (NMe4)2SnCl6 (NMe4)2EtSnCl5 (NMe4)2SnBr6 (MeNH3)2SnCl6 (PyH)2SnCl6 (PyH)2EtSnCl5 1.696 1.670 1.768 1.642 (NMe4)2EtSnBr5 1.585 1.733 1.712 (MeNH3)2EtSnCl51.684 1.772 1.780 1.794 1.809 1.698 1.673 1.785 0.0 a 0.0 a 0.015 0.0 a 0.0 a 0.017 0.0 a 0.012 0.017 0.020 0.020 0.021 0.0 a 0.0 a 0.019 L=TMU 0.017 SnCl4L2 EtSnCl3L2 1.659 1.645 >0.003 <0.642 <0.0 70.7 Cl7Sn7Et 1.748 0.012 L=DMSO SnCl4L2 EtSnCl3L2 0.007 0.029 0.006 0.023 1.662 1.676 1.688 1.743 L=HMPT 0.023 0.097 1.663 1.630 SnCl4L2 EtSnCl3L2 PhSnCl3L2 1.655 <0.015 <0.640 <0.0 7 0.019 0.028 1.735 1.589 SnBr4L2 BuSnBr3L2 0.640 7 7 Cl7Sn7Cl 0.620 b 73.2 70.4 Cl7Sn7Et +4.3 Cl7Sn7Cl Cl7Sn7Ph Cl7Sn7Cl 0.561 7 7 Br7Sn7Br 1.579 >0.018 <0.561 <0.0 70.8 Br7Sn7Bu 1.698 0.020 L=THF 1.639 SnCl4L2 MeSnCl3L2 1.707 1.571 SnBr4L2 MeSnBr3L2 0.031 1.624 >0.017 <0.607 <0.0 7 0.025 0.037 1.552 >0.018 <0.534 <0.0 7 0.029 1.661 abIn the absence of the experimental data, it was assumed that Z=0; from symmetry considerations, the real Z-value should not exceed 5%± 10%.At Z=4%. values for DNpp shows that the equatorial p-bonds in monoalkyl- substituted adducts remain virtually undisturbed by cis-weaken- ing, whereas trans-strengthening seems to involve both s- and p-bonds.In most of the RSnHal3L2 compounds, trans-strength- ening of the Sn7Hal bond occurs only if Z is non-zero, hence revealing disturbance of the halogen p-system. In chlorine- containing adducts, the Z-values, at which trans-strengthening takes place, show the tendency to an increase as the donating power of either substituent R (provided the ligand L remains the same) or ligand L (provided the substituent R remains the same) decreases. Thus, trans-strengthening in the adduct PhSnCl3(HMPT)2 can only be observed when the Z-value for the appropriate chlorine atom reaches 6.7%, whereas in its ethyl analogue EtSnCl3(HMPT)2, it is observed at Z=0 and reaches 3% at Z=4%.However, trans-strengthening in the adduct 7r /e Dr (%)Dd (%) Assignment (aÊ á.) Cl7Sn7Cl 0.696 7 7 Cl7Sn7Cl 0.670 73.7 70.7 Cl7Sn7Et 0.753 +8.2 +3.8 Cl7Sn7Cl 0.642 7 7 Br7Sn7Br 0.585 78.9 71.5 Br7Sn7Et 0.716 +11.5 +3.6 Br7Sn7Br 0.712 7 7 Cl7Sn7Cl 0.672 75.6 +0.4 Cl7Sn7Et 0.755 7 7 Cl7Sn7Cl 0.760 +8.0 +4.3 Cl7Sn7Cl 0.774 ??? 0.788 7 7 Cl7Sn7Cl 0.698 7 7 Cl7Sn7Cl 0.673 73.6 71.2 Cl7Sn7Et 0.766 +9.7 +2.8 Cl7Sn7Cl 0.642 7 7 Cl7Sn7Cl 0.736 +14.6 +4.2 Cl7Sn7Cl 0.655 7 7 Cl7Sn7Cl 0.647 7 7 Cl7Sn7O 0.682 +4.1 +0.7 Cl7Sn7Et 0.720 +11.2 +2.7 Cl7Sn7O 0.716 +11.9 7 0.678 +20.8 +3.2 Br7Sn7Br 0.682 +12.3 7 0.607 7 7 Cl7Sn7Cl Cl7Sn7Me Cl7Sn7Cl 0.534 7 7 Br7Sn7Br Br7Sn7Me Br7Sn7Br 0.632 +18.3 7 Table 4.Changes in the Sn7Hal bond characteristics in the adducts of tin tetrahalides as a result of substitution SnHal4L2?R2SnHal2L2. Compound L=HMPT SnCl4L2 Me2SnCl2L2 SnI4L2 Et2SnI2L2 Ph2SnI2L2(I) (II) SnBr4L2 Me2SnBr2L2 L=TMU SnI4L2 SnBr4L2 Et2SnBr2L2 L=DMF SnCl4L2 Me2SnCl2L2 L=DMSO Et2SnI2L2 Ph2SnI2L2 SnBr4L2 Me2SnBr2L2 SnCl4L2 Me2SnCl2L2 L=bipy SnBr4L Bu2SnBr2L L=THF SnBr4L2 Me2SnBr2L2 L=py Et2SnI2L2 Ph2SnI2L2 SnBr4L2 Et2SnBr2L2 Me2SnBr2L2 1.761 0.038 0.723 1.757 0.014 0.742 1.708 0.035 0.673 1.601 0.038 0.563 1.790 0.020 0.770 +36.8 1.786 0.020 0.766 +36.1 a With respect to SnI4(Ph3PO)2. b With respect to SnI4[(Me2N)2CO]2.c With respect to SnI4(bipy). d With respect to SnBr4(Ph2SO)2.23 EtSnCl3(THF)2 is only possible at Z>7%. Although the contri- bution of the p-system to chemical bonding in tin adducts is small, the above tendency is clearly seen from the NQR data. As a whole, the electronic effects of substitution in the adducts with known structure were found to correlate to their structural manifestations, although in the adducts of mono-substituted tin chlorides, a shortening of the Sn7Hal bond in the trans-position to the substituent R was small and comparable with the standard deviation values. In (MeNH3)2EtSnCl5, no shortening of the trans-bond Sn7Hal compared to the parent unsubstituted com- plex was found by X-ray analysis,19 whereas the absolute value of E A Kravchenko, Yu A Buslaev 7r /e Dr (%) Dd (%) Assignment Nps DNpp Cl7Sn7Cl I7Sn7I I7Sn7O Br7Sn7Br 1.663 0.023 0.640 1.822 0.012 0.810 +26.6 +7.6 Cl7Sn7Cl 1.543 0.006 0.537 1.514 0.019 0.495 1.791 0.009 0.782 +51.5 +7.8 a I7Sn7I 1.754 0.059 0.695 +34.7 +5.7 a I7Sn7I 1.750 0.060 0.689 +33.5 +5.0 a I7Sn7I 1.589 0.028 0.561 1.805 0.014 0.791 +41.0 +7.4 Br7Sn7Br 1.540 0.006 0.534 1.512 0.019 0.493 1.604 0.018 0.586 1.759 0.011 0.748 +27.6 I7Sn7I I7Sn7O Br7Sn7Br Br7Sn7Br 1.670 0.012 0.658 1.793 0.008 0.785 +19.3 Cl7Sn7Cl Cl7Sn7O 77Br7Sn7Br Br7Sn7O Cl7Sn7Cl Cl7Sn7O 1.798 0.046 0.752 +52.5 b +5.0 c I7Sn7O 1.762 0.020 0.742 +50.5 b +6.4 c I7Sn7O 1.695 0.046 0.649 +31.6 b 1.682 0.018 0.665 +34.9 b 1.629 0.007 0.622 1.586 0.037 0.549 1.807 0.017 0.790 +35.0 +7.7 d Br7Sn7Br 1.662 0.007 0.655 1.676 0.029 0.647 1.802 0.017 0.785 +21.3 +5.0 Cl7Sn7O Br7Sn7Br Br7Sn7N 1.630 0.004 0.630 1.577 0.041 0.536 1.757 0.024 0.733 +36.7 +6.8 Br7Sn7N 1.571 0.037 0.534 1.679 0.028 0.651 +21.9 Br7Sn7Br Br7Sn7Br I7Sn7I I7Sn7I I7Sn7I Br7Sn7Br Br7Sn7Br Br7Sn7BrElectronic effects of substitution and reconstruction of the coordination polyhedron in adducts of Main Group IV element halides the effective charge on the trans-ligand decreased by more than 5%, the appropriate frequency shift exceeding 1 MHz.The frequency shifts at the chlorine atoms in trans-positions to the substituent observed in theNQRspectra of all compounds studied notably exceeded the errors of measurement, so that the effect of trans-strengthening was readily revealed in terms of a decrease in the effective charge on the corresponding chlorine atom.The NQR parameters are more sensitive to the influence of substitu- ents than the interatomic distances (Tables 3 and 4); this is however not unexpected, considering that the substituent influ- ence is governed by the electronic effects, whereas the NQR parameters directly reflect the electron density variations on the atom under study. It is worthwhile to compare redistribution of the electron density in adducts of mono-substituted tin halides with different mutual arrangement of the ligands L. For instance, in Tables 1 and 3, the data are summarised for the adducts cis- EtSnCl3(DMSO)2 (1) and trans-PhSnCl3(HMPT)2 (2), whose strong donor ligands L are competitive in donating power with the substituents R.One can see that cis-weakening of the Sn7Hal bonds in the adduct 1, where the ligandsDMSOare in cis-position to each other, is less pronounced than that in the ethyl-substituted adducts with weaker donor ligands L. Moreover, in agreement with X-ray data, trans-strengthening of the Sn7Hal bond in 1 gives way to trans-weakening. By contrast, trans-strengthening of the Sn7Hal bond exists in the molecule 2 with trans-position of theHMPTligands relative to each other, as follows from theNQR frequency shift, and cis-weakening in this adduct is stronger than in 1, although the phenyl group is a weaker `influencing' substituent than the ethyl group. Introduction of the second substituent R to give an adduct R2SnHal2L2 increases the cis-lengthening (weakening) of the Sn7Hal bond in an approximately additive manner.As a result, in the adducts of dialkyltin dichloride, dibromide and diiodide the effects exceed 25%, 40% and 50%, respectively, the appropriate Sn7Hal bond elongation being *0.2 A. Whereas the cis-weak- ening effect of the phenyl substituents is less than that due to the ethyl groups, the effects due to different alkyl groups are approximately the same, as one can see from the data for Me2SnBr2(py)2, Et2SnBr2(py)2 and Bu2SnBr2(bipy)2 (Table 4). It thus follows that the variations from 22% to 41% in cis- weakening of the Sn7Br bonds in the adducts of dialkyldibro- mostannates are caused by different contributions of the ligands L into the combined effect of cis-weakening: THF< TMU<DMSO ^ py ^ bipy<HMPT.Table 5. 127I NQR spectroscopic parameters for adducts R2SnI2L2 and SnI4L2 at 77 K and the Sn7I bond characteristics.26 Compound e2Qqzz/h /MHz cis-Et2SnI2(DMSO)2 409.1 523.0 Ph2SnI2(DMSO)2 trans-Et2SnI2(HMPT)2 trans-Ph2SnI2(HMPT)2(I) (II) cis-SnI4(TMU)2 647.3 707.8 467.9 495.2 504.1 1048.0 1095.3 cis-SnI4 . bipy Et2SnI2(py)2 504.35 542.1 630.03 Ph2SnI2(py)2 Note. Here and in tables below: a is axial, e is equatorial halogen atoms; for the remaining notations see Table 3. a See text. a/e Npz Z (%) 1.798 1.762 39.1 13.3 1.695 1.682 1.791 1.754 1.750 1.540 1.512 24.3 8.6 6.5 41.2 41.2 1.9 6.0 aeae 1.761 1.757 1.708 25.7 8.8 18.9 In studies of adducts of dialkyl(diphenyl)diiodostannates it was found 12, 25 that the elongation of the Sn7I bond with respect to that in the parent adduct was accompanied by a dramatic increase in Z at the iodine sites.Such an increase was observed in five of six adducts studied by NQR (Table 5), which implies that the iodine valence 5pp orbitals were notably disturbed as a result of the substitution, which gives rise to an essential increase in the occupancy difference DNpp=Npx7Npy.NQRspectroscopic and X-ray diffraction data for the two adducts were correlated in order to understand the reason for this disturbance.26 As one can see from Table 5, the largest increase in Z upon replacement of two iodine atoms by the groups R is observed in the adducts Ph2SnI2(HMPT)2 (3) and Et2SnI2(DMSO)2 (4).The 127I NQR spectrum of the adduct 3 consists of two sets of lines corresponding to two crystallographically inequivalent iodine sites having close spectroscopic characteristics. The introduction of the phenyl groups resulted in the elongation of the Sn7I bonds by 0.165 A on average with respect to those in the nonsubstituted adduct SnI4(TPPO)2 whose structure is known and spectroscopic characteristics are similar to those in the yet as structurally unstudied adducts SnI4(HMPT)2 and SnI4(TMU)2 (Table 2).As the comparison of the iodine valence orbital occupancies in substituted and nonsubstituted adducts shows (Table 5), the substitution results in an increase in the iodine valence 5ps orbital occupancies (Npz), which is a natural consequence of the cis- weakening of the Sn7I bonds. The latter is however accompanied by a notable increase in the iodine valence 5pp orbital occupancy difference (DNpp=Npx7Npy) from 0.019 in nonsubstituted SnI4(TMU)2 to 0.06 in the adduct 3. The obvious suggestion accounting for such an increase in Z in compound 3 is that secondary non-valence intra- or intermole- cular contacts exist in this compound that involve the iodine atoms. The results of the X-ray study do not, however, support this suggestion: no secondary contacts were found in the structure of 3.26 In the crystals of the adduct 4, two crystallographically inequivalent iodine positions (characterised by substantially different spectroscopic parameters) were identified by NQR.According to the X-ray diffraction data, the coordination poly- hedron around tin is an octahedron with trans ethyl groups and cis DMSO molecules (Fig. 1 a). Both DMSO molecules occupy disordered positions, so that the sulfur atom of one DMSO molecule occupies the positions at the opposite sides of the O(1)C(5)C(6) plane, whereas in the other molecule, both sulfur and oxygen atoms are sited at disordered positions. Table 6 lists the bond lengths and angles for the prevailing configuration of the DNpp=Npx7Npy (Npx=2) 0.046 0.020 0.046 0.018 0.009 0.059 0.060 0.006 0.019 0.038 0.014 0.035 715 d /A 7r /e 2.941 2.982 3.016 2.957 2.938 0.752 0.742 0.690 a 0.649 0.665 0.782 0.695 0.689 0.534 0.493 2.816 2.788 0.723 0.742 0.673716 C(1) I(2) Sn C(3) I(1) c I(2) I(1) 0 a Figure 1.Structure of the adduct Et2SnI2(DMSO)2;26 (a) molecular structure of Et2SnI2(DMSO)2, illustrating disorder in the DMSO ligands; occupancies of the atomic positions are given in parentheses; (b) crystal structure of the adduct showing non-valence contacts (dotted lines); only prevailing conformation retained. adduct molecule. In the strucutre of the adduct 4, the intra- molecular [S(2)_O(1)=3.06 A, S(1)_I(1)=4.11 A], and inter- molecular [S(2)_I(20)=3.79 A] contacts (Fig.1 b) were found, hence revealing at least one of the reasons for the increase in the value of Z (39.1%) in this adduct. On the plot giving the effective charge on the iodine atom as a function of the respective Sn ± I bond lengths (Fig. 2), deviation of the point 7, corresponding to the I(1) site in the adduct 4 from the correlation line is a maximum. Note that the effective charges were evaluated assuming that the occupancy of one of the iodine 5pp orbitals (Npx) was equal to 2.26 As Fig. 2 shows, such an assumption results in a good correlation between 7r and the Sn7I bond lengths for all the points in the adducts discussed except that for the I(1) atom in compound 4.The effective charge Table 6. Bond lengths (d) and bond angles (a) in the adduct 4.26 Bond d /A 2.982(1) 2.941(1) 2.220(6) 2.253(8) 2.16(2) 2.164(10) 2.174(10) Sn7I(1) Sn7I(2) Sn7O(1) Sn7O(2) Sn7O(21) Sn7C(1) Sn7C(2) a C(2) S(21) (0.24) O(2) (0.76) C(7) C(8) S(2) (0.76) O(21) (0.24) O(1) S(11) (0.21) C(6) C(5) C(4) S(1) (0.79) b O(2) S(2) O(1) S(1) I(20) b Angle I(1)7Sn7I(2) I(1)7Sn7O(1) I(1)7Sn7O(2) I(1)7Sn7C(1) I(1)7Sn7C(3) I(2)7Sn7O(1) I(2)7Sn7O(2) I(2)7Sn7C(1) I(2)7Sn7C(3) E A Kravchenko, Yu A Buslaev 7r /e 7 2 8 0.7 6 5 1 0.6 4 3 9 0.5 10 3.00 d /A 2.80 2.90 Figure 2. Correlation between the effective charges on the iodine atom (7r) and interatomic Sn7I distances (d);12 (1) EtSnI3(HMPT)2, (2) Et2SnI2(HMPT)2, (3) SnI4(Ph2SO)2, (4) EtSnI3(Ph2SO)2, (5, 6) Ph2SnI2(HMPT)2, (7, 8) Et2SnI2(DMSO)2, (9, 10) SnI4(TMU)2.on this atom (for the same values of experimentally measured QCC and Z) for the point 7 to fit the correlation line should be equal to 70.69 e (Table 5), which gives the following iodine valence orbital occupancies: Npz=1.778, Npx=1.979 and Npy=1.933. Thus, the deviation of the point 7 from the correla- tion serves as indirect evidence that the assumption Npx=2 underestimates the Sn7I p-bonding character for the I(1) atom involved in secondary contact to sulfur. This contact is associated with the transfer of*0.042 e from the iodine valence 5pp orbitals. The probable reason for the increased Z-value (39.1%) in the adduct 3, where no shortened contacts were found, was sought in the analysis of contributions of the iodine valence 5p orbital occupancies into the measured value of Z.It was shown that despite similar values of the asymmetry parameters at the iodine sites in 3 and the I(1) site in 4 the difference DNpp=Npx7Npy in the latter adduct is less than in the former one. As has been mentioned above, the alkyl substituents weaken the respective Sn7I bonds to a greater extent than the phenyl groups, thus producing higher occupancy of the iodine 5ps orbitals. A sharp increase in the experimentally measured Z-value occurs, at the fixed DNpp=Npx7Npy difference, as the s-electron density at the iodine atom (Npz) increases (Fig.3), i.e., the enhanced cis- weakening of the Sn7I bond due to substitution results, at a given DNpp difference, in an increase in the asymmetry parameter Z caused by the increased contribution to the latter of the iodine valence 5ps orbitals. Indeed, the closely similar values of Z in 3 and at the I(1) site in 4 are associated with a relatively smaller iodine 5pp orbital occupancy difference in the latter adduct, since the alkyl substituents favour an increase in s-electron density on the iodine atoms to a larger extent than the phenyl groups do. Thus, an increase in the Z-value in the adduct 4 (Table 5) is caused by the combined contribution of the iodine valence 5ps orbitals, whose occupancy increases upon substitution, and the shortened I_S contacts responsible for an increase in DNpp.As to the increased DNpp difference in the adduct 3, this is most Angle a /deg a /deg 80.2(3) 88.9(3) 92.0(4) 85.0(4) 99.7(4) 175.3(4) (1)7Sn7O(2) O(1)7Sn7C(1) O(1)7Sn7C(3) O(2)7Sn7C(1) O(2)7Sn7C(3) C(1)7Sn7C(3) 100.40(4) 95.4(2) 173.9(2) 90.7(3) 84.6(3) 163.9(2) 83.8(2) 87.7(3) 92.7(3)Electronic effects of substitution and reconstruction of the coordination polyhedron in adducts of Main Group IV element halides Z Npz=1.80 Npz=1.90 0.8 Npz=1.75 0.6 Npz=1.65 0.4 1 2 0.2 0.06 0.04 0.00 0.02 Npx7Npy Figure 3. Measured value of the EFG asymmetry parameter (Z) at the iodine atom as a function of its valence p-orbital occupancy difference (Npx7Npy) for various s-orbital occupancies (Npz);26 (1) is the point corresponding to the point (7) and (2), to the points (5) and (6) on the plot of Fig.2. probably associated with the interaction of the iodine p-system with those of the phenyl rings. This suggestion is supported by the fact that in the Et2SnI2(HMPT)2 adduct, which is a dialkyl analogue of 3 comprising no cyclic fragments in the ligands and substituents, the alkyl groups did not give rise to an increase in DNpp and consequently in the EFG asymmetry parameter Z. However, replacement of the ethyl by phenyl substituents affected dramatically the asymmetry parameter Z, although no proper structural manifestations could be registered by X-ray diffraction.This is one of a few examples illustrating a possibility to detect, using NQR, the electronic effects, whose structural manifestations lie below the sensitivity of X-ray experiments. Yet another example was given above in the effects of trans-strength- ening. The accuracy of their determination in chlorides is, as a rule, comparable to the standard deviations, whereas the corre- sponding variations of the NQR parameters notably exceed the errors of measurement. It is however to be noted that there are many other systems in which p ± p interactions were clearly revealed by X-ray studies. For instance, the p ± p interactions between the iodine atoms and phenyl rings were earlier found by X-ray in several imidazopyridinium salts.27 The NQR and X-ray data yield complementary pieces of valuable information.A correlation between them on a quantita- tive scale contributes to the further progress in understanding the electronic effects of substitution in adducts and complexes of tin(IV) halides, augmenting the current view by information on relative importance of the s- and p-electron systems in electron density redistribution. III. Electronic characteristics ofM7Hal bonds in hexa- and pentacoordinate adducts of tin(IV) tetrachlorides; similarities and differences In this section, theM7Hal bond reorganisation in adducts of tin tetrahalides resulting from the change in the central metal coordination number is discussed. The data on electron density redistribution due to spatial rearrangement of adducts, especially those of silicon and germanium, are scanty.The literature on 35Cl NQR studies of hexa- and pentacoordinate adducts of tin tetrachloride is more extensive. However, cis ± trans isomerism of hexacoordinate tin(IV) adducts complicated comparative studies of the bond electronic characteristics in these compounds, because no reliable spectroscopic criteria of either isomerism or assign- ment of the 35Cl NQR frequencies in cis-isomers to axial ± equatorial chlorine atoms were available. 717 NQR studies of pseudooctahedral adducts SnCl4L2 have received major attention. The subjects of investigations were the structures of the compounds,28�34 charge distribution over acceptor molecules,33 ± 39 relation of the ligand donating power to the strength of the produced adducts judging from spectro- scopic data 40 and identification of isomers based on their NQR spectra.40 ± 42 A bibliography on these problems covering the period since 1980 to the beginning of the 90's may be found in a review.43 Studies on isomerism of pseudooctahedral adducts of tin tetrachloride show 40 that the frequency splitting in the NQR spectra of cis-isomers (i.e., the difference between the frequencies of equatorial and axial chlorine atoms) cannot serve as a reliable criterion of isomerism, because its magnitude decreases as the donating power of ligands L increases.For strong adducts, this prevents the spectroscopic identification of cis- and trans-isomers.An unambiguous identification of isomers is further complicated where the NQR spectra of trans-isomers are affected by crystal- field effects, which enhance the frequency splitting, and mask their difference from the spectra of cis-isomers. Temperature depend- ences of the NQR frequencies of equatorial and axial chlorine atoms do not always clarify the situation.41, 42 For the isomers with established cis-configuration, the assignment of resonance frequencies to equatorial ± axial atoms is also ambiguous. Accord- ing to the X-ray data available on a relatively few cis-SnCl4L2 adducts, the number of adducts whose axial Sn7Cl bond distances are shorter than the equatorial ones is approximately the same as that whose axial Sn7Cl bond distances exceed equatorial.The identification of a large number of SnCl4L2 isomers was made 40 assuming that in NQR spectra of both s- and trans-isomers the 35Cl frequencies of the atoms making a linear Cl7Sn7Cl fragment are close to each other if the donating powers of ligands L are similar. As a result, the higher 35Cl frequencies in the NQR spectra of cis-isomers were erroneously assigned to linear Cl7Sn7L fragments. Thus, a considerable body of experimental data showed that neither frequency splittings and line multiplicity nor temperature dependence of the NQR frequencies can be recognised as reliable criteria for identification of isomers. However, the symmetry considerations suggest that the EFG asymmetry parameters at axial ± equatorial chlorine sites may possibly serve as such criteria, although the values of Z and QCC cannot be simultaneously found, without additional experiments, from only one transition frequency for nuclei with I=3/2 having two degenerate quadru- pole levels.Conventional Zeeman NQR experiments with single crystals are however complicated and tedious, serious limiting factors being duration of experiments and problems of growing large single crystals of tin tetrachloride adducts, most of which are hygroscopic. Attempts at experimental determination of Z at the chlorine atoms in powder samples were undertaken 8 using a modified approach 3 based on Fourier analysis of slow beats of the spin- echo envelope in an external magnetic field. The experiment was run according to the scheme:NQRspectrometer programmer and digitiser box-car integrator computer.The program- mer and digitiser were adapted to the tube levels of NQR signals. Instead of the conventional Fourier transform used in NMR spectroscopy, the data processing involved expansion of a signal to harmonics followed by numerical integration for determining the expansion coefficients. As a result, the accuracy of the analysis increased, whereas its duration remained shorter than the time of signal accumulation. The Fourier transform programme was used which yielded a graphic representation of both the accumulated spectrum of slow beats and its expansion into harmonics. This easy-to-grasp form of representation of the results enabled the authors to monitor the data processing checking the operation for errors.The accuracy of Z measurements performed by this procedure even on weak 35Cl signals was fairly high. In subsequent experiments, the results of determination of Z in a number of cis- and trans-isomers of SnCl4L2 adducts con-718 firmed 11 that the EFG asymmetry parameters can actually be used for identification of isomers (Tables 7 and 8). Thus the magnitudes of Z on axial chlorine atoms in cis-isomers appeared to be zero unless lattice contributions raised its value by several percent. On equatorial Cl atoms of both isomers, the Z-values were usually noticeably higher, varying around 10% in the majority of the compounds. For comparison, the results of Zeeman analysis of single crystals at 297 K, 293 K and 235 K available for several hexa- coordinate adducts, are also listed in Tables 7 and 8.44 ± 46 They are in reasonable agreement with those obtained with powder samples: the magnitudes of orbital occupancies and effective charges on the chlorine atoms derived from both experiments are consistent with each other.The approach for measuring Z in powder samples simplified the experiment and markedly cut its time. The important benefit to the user consists in avoiding a tedious long-term procedure of growing large single crystals required for the NQR Zeeman experiments. The determination of the total set of NQR spectroscopic parameters (the transition resonance frequencies n, quadrupole coupling constant QCC and EFG asymmetry parameter Z) for a number of adducts, which became possible due to the use of powder samples, revealed the regularities of electron density redistribution between axial7equatorial bonds (Fig.4) which thereafter allowed identification of cis- and trans-isomers of some adducts that had not been studied structurally based only on the NQR frequency measurements.10 The 35Cl NQR frequencies assigned to the axial chlorine atoms in the spectra of all the cis-isomers examined 10 appeared to be higher and the s-electron density on the respective Cl atoms lower than those on the equatorial chlorine atoms. The frequency ranges assigned to axial and equatorial Cl atoms did not virtually overlap, so that the corresponding frequencies in the spectra of cis- isomers were found in the intervals (MHz) 17.5< <ne<19.8<na<22.0, whereas the region covered by the 35Cl frequencies in the spectra of trans-isomers was slightly shifted downwards: 17.2<ne<19.4. The effective charges on the Cl atoms were practically insensitive to the kind of isomerism of the adduct, their magnitudes being approximately the same on both Table 7.35Cl NQR spectroscopic parameters for trans-isomers of hexacoordinate adducts of tin tetrachloride at 77 K and the M7Cl bond characteristics.9 L n /MHz Diox DMF TMU THF DEE HMPT py DBS acac 19.41 17.77 18.36 18.16 19.39 19.25 18.91 18.54 19.44 19.47 19.341 a 19.440 a 17.77 18.04 17.76 17.64 18.48 18.18 19.94(2) 19.48 18.28 adduct.a The results of Zeeman analysis of a single crystal at 235 K.44 Note. Diox is dioxane, DEE is diethyl ether, DBS is dibenzyl sulfide, acac is acetylacetonate; |Pr| is the total effective charge on the chlorine atoms in the 10.52.1 Z (%) 6.11.5 5.71.0 8.40.5 6.71.9 15.83.0 13.41.1 12.21.1 12.81.2 13.63.1 13.63.1 11.4 11.6 10.82.1 10.41.4 15.42.6 15.42.6 15.24.1 15.53.4 5.50.9 5.21.0 e2Qqzz/h /MHz 38.80 35.52 36.68 36.29 38.62 38.38 37.73 36.98 38.76 38.82 38.600 38.793 35.47 36.02 35.38 35.14 36.81 36.22 39.86 38.94 36.49 18 17 2 1 Figure 4. The diagrams of 35Cl NQR spectra of pseudooctahedral adducts: trans-SnCl4L2 (a), cis-SnCl4L2 (b), trigonal-bipyramidal adducts SnCl4L (c) and parent tin tetrachloride (d).The 35Cl frequencies with measured (1) and unknown (2) magnitudes of Z; the dotted bars are axial, the solid bars are equatorial chlorine positions. axial and equatorial chlorine sites and in both types of isomers (Tables 7 and 8). The results of the study cited (Ref. 10) show that the cis-isomer of adduct SnCl4L2 can be identified by the presence in its NQR spectrum of the 35Cl signals at frequencies above 19.8 MHz, which should be assigned to the axial Cl7Sn7Cl fragments. More than 80% of trans-isomers examined so far gave NQR spectra with the 35Cl average frequencies below 18.3 MHz, while 80% of cis-isomers had them above 19.3 MHz.It is to be emphasised however that the above regularities in electron density distribution are only valid for the molecular adducts, whose chlorine atoms are bonded only to the central atom and are not involved in secondary inter- or intramolecular interactions. Nps 1.639 1.670 1.656 1.662 1.628 1.634 1.641 1.648 1.630 1.629 1.665 1.660 1.660 1.662 1.647 1.653 1.630 1.639 1.655 E A Kravchenko, Yu A Buslaev abc 21 22 23 20 19 23 7r /e DNpp 0.625 0.658 0.637 0.647 0.591 0.603 0.613 0.619 0.598 0.597 0.014 0.012 0.019 0.015 0.037 0.031 0.028 0.029 0.032 0.032 0.642 0.637 0.627 0.629 0.613 0.619 0.617 0.627 0.632 0.023 0.023 0.033 0.033 0.034 0.034 0.013 0.012 0.023 24 d 24 n /MHz |Pr| /e 2.500 2.632 2.568 2.426 2.390 2.558 2.512 2.464 2.493Electronic effects of substitution and reconstruction of the coordination polyhedron in adducts of Main Group IV element halides Table 8.35ClNQRspectroscopic parameters for cis-isomers of hexacoordinate adducts of tin tetrachloride at 77 Kand theM7Cl bond characteristics.9 a/e LPOCl3 P2O3Cl4 Me2Ob MeCN ButCNc Me2C MeOH EtOH C4H8(CN)2 SeOCl2 a BrCH2COOMe ClCH2COOMe aeea a e a e a aaeeaeaaeea c a c e c e c aaeeaaeeaaeeaaeeaeaeaaeaae Note. The results of Zeeman analysis of single crystals: a at 297 K;45 b at 293 K;44 c at 235 K.46 For the latter compounds, the results of X-ray study are consistent with those of NQR measurements: their axial Sn7Cl bond distances were found to be shorter than the equatorial ones.However, it is clear that the EFG asymmetry parameter on the chlorine atom will increase, if this atom is involved in additional (inter- or intramolecular) interactions, as it is observed in SnCl4(SeOCl2)2.45 The respective 35Cl frequency in the NQR spectrum of the adduct decreased, and the ratios between the axial ± equatorial resonance frequencies and Sn7Cl bond distan- ces were inverted (Table 8). The spectroscopic regularities found for the isomeric chloro adducts are not universal. Tin tetrabromide and tetraiodide adducts show different positional order of the NQR frequencies assigned to axial and equatorial halogen atoms due to different relationships between the electron-donating (withdrawing) prop- erties of ligands L and halogens and relatively lesser contribution Z (%) n /MHz 2.30.5 11.12.0 11.72.4 1.20.1 8.80.4 9.20.1 4.81.6 5.61.6 12.83.0 19.62.1 2.7 4.9 5.61.5 3.41.5 7.32.2 7.82.2 3.1 2.5 5.3 4.9 2.8 1.3 7.8 8.1 1.20.8 1.20.8 7.31.3 6.61.2 2.10.4 3.10.5 13.11.5 12.20.1 3.81.5 3.91.2 19.43.5 23.23.2 1.80.5 7.11.8 39.50.2 6.90.3 1.70.5 1.70.5 21.13 19.79 19.03 20.924 19.093 18.852 22.05 21.87 19.77 18.49 20.287 18.807 20.67 19.87 19.59 19.20 20.466 19.752 19.513 19.207 20.039 19.426 18.981 18.859 20.09 19.48 19.43 18.84 20.32 19.87 17.87 17.50 20.57 20.12 17.75 17.50 20.30 19.06 17.030 19.551 21.91 18.57 19.59(2) 21.82 18.53 19.51(2) 9.01.6 1.10.5 1.70.5 9.00.8 e2Qqzz/h /MHz Nps 1.612 1.627 1.640 1.617 1.642 1.646 1.592 1.594 1.625 1.643 1.627 1.651 1.616 1.643 1.634 1.641 1.623 1.637 1.638 1.644 1.632 1.644 1.645 1.648 1.632 1.644 1.637 1.649 1.627 1.634 1.661 1.669 1.620 1.629 1.657 1.659 1.623 1.644 1.657 1.636 1.598 1.660 1.632 1.601 1.660 1.634 42.26 39.50 37.97 41.847 38.134 37.651 44.08 43.72 39.43 36.74 40.569 37.600 41.32 39.73 39.14 38.36 40.93 39.45 39.01 38.40 40.07 38.85 37.92 37.68 40.17 38.96 38.82 37.65 40.64 39.73 35.63 34.91 41.13 40.23 35.27 34.69 40.60 38.08 33.219 39.069 43.82 37.14 39.13 43.64 37.06 38.97 to the EFG of crystal-field effects as compared to the contribution of their own valence orbitals.This is illustrated in the results of NQR studies of hexacoordinate adducts SnBr4L2.23 Although the 81Br frequency ranges assigned to axial and equatorial Br atoms in the NQR spectra of cis-isomers also did not overlap (Table 9), their mutual positions were inverted with respect to those in the 35Cl NQR spectra of the chloro analogues, i.e., the 81Br NQR frequencies of axial bromine atoms were lower than those of equatorial in all the cis-isomers studied: 112<na<126 MHz; 128<ne<142 MHz.In agreement with the NQR data, the results of X-ray studies available for the cis-SnBr4L2 adducts gave axial Sn7Br bond distances exceeding the equatorial ones.23 In the spectra of trans-isomers, the 81BrNQRfrequency range was relatively larger than the corresponding 35Cl frequency range in the spectra of chloro adducts, the former interval covering almost the entire (ne+na) frequency range of cis-isomers: 7r /e DNpp 0.606 0.600 0.613 0.614 0.622 0.625 0.579 0.579 0.594 0.599 0.620 0.640 0.602 0.635 0.617 0.623 0.615 0.631 0.625 0.633 0.625 0.641 0.627 0.630 0.629 0.641 0.620 0.634 0.622 0.627 0.633 0.643 0.611 0.619 0.615 0.610 0.623 0.628 0.577 0.620 0.598 0.660 0.611 0.601 0.660 0.613 0.006 0.027 0.027 0.003 0.020 0.021 0.013 0.015 0.031 0.044 0.007 0.011 0.014 0.008 0.017 0.018 0.008 0.006 0.013 0.011 0.007 0.003 0.018 0.018 0.003 0.003 0.017 0.015 0.005 0.0075 0.028 0.026 0.009 0.010 0.042 0.049 0.004 0.016 0.080 0.016 0.005 0.004 0.021 0.003 0.004 0.021 719 |Pr| /e 2.425 2.475 2.351 2.520 2.477 2.504 2.523 2.524 2.525 2.455 2.502 2.394 2.480 2.487720 Table 9.81Br frequencies in theNQRspectra of isomers of hexacoordinate adducts of tin tetrabromide at 77 K.23 L n /MHz trans-Isomers TMU DMF DPF DBS Diox py 2,3-Mepy TPPO HMPT Et2O Me2O THF 125.30(2) 123.68 123.47 124.57 122.18 125.22 124.72 127.63 127.18 136.20 122.66 119.54(2) 117.26 115.74 127.91 121.14 127.73 135.31 135.58 135.82 136.47 136.72 137.27 128.28 131.26 134.30 135.93 Note.DPF is diphenylformamide; DESO is diethyl sulfoxide; relative intensities of resonance lines are given in parentheses. Table 10. 35Cl NQR spectroscopic parameters for pentacoordinate adducts of tin tetrachloride at 77 K and theM7Cl bond characteristics.9 a/e LMeNO2 4-MeC6H4COCl PhMe 2,5-Me2C6H3OMe C5H5COCl eeeaeeaeeeaeeeaeea SnCl4 a a Data from Ref.47. a/e L n /MHz cis-Isomers bipy 119.07 118.67 128.78 aae NC(CH2)4CN 130.95(2) 140.96 141.29 aae DESO 118.24 127.07 ae DPSO 116.63 128.34 ae DMSO 118.17 127.73 ae 115<ne<138 MHz. Thus, the 81Br NQR frequency ranges in the spectra of cis- and trans-isomers of the SnBr4L2 adducts were found to overlap. This implies that, unlike in the related chloro adducts, the EFG asymmetry parameters at the Br nuclei in the bromo adducts should be measured for the spectroscopic identi- fication of isomerism. The mutual position of the 81Br frequency ranges assigned to axial ± equatorial bromine atoms indicates the prevailing electronic influence of ligands on the electron density redistribution in cis- and trans-isomers of the adducts SnBr4L2.As a whole, the NQR results on isomers of hexacoordinate adducts SnHal4L2 show 9, 11, 18, 23 that the measurement of the EFG asymmetry parameters along with examination of the spectroscopic shifts, splittings and temperature dependence of the resonance frequencies made it possible to identify the type of isomerism of the adducts from their NQR spectra, even though the latter exhibit complicated patterns of the shifts and splittings. The average NQR frequency shifts at halogen atoms in adducts with respect to their frequencies in parent SnHal4 can serve as a measure of the strength of the adducts formed provided the resulting increase in the halogen s-electron density in the acceptor is mainly due to intramolecular electronic effects, the contributions of the crystal-field effects being much less signifi- cant.CH3OH 120.90 125.75 135.55 140.46 aaee C2H5OH 113.83(2) 140.51 141.44 aee Table 10 lists the 35Cl NQR spectroscopic parameters for trigonal-bipyramidal adducts of tin(IV) having the coordination number 5. The corresponding spectra differ notably from those for pseudooctahedral adducts (Tables 7 and 8). They comprise the triplets from equatorial Cl atoms at frequencies slightly different from those of the parent tetrachloride, and the singlests from axial chlorine atoms, shifted considerably to lower frequencies, which indicates that the electron density in such adducts is mainly transferred from the ligands L to the axial chlorine atoms.(C2H2Ph)2CO 112.57 133.62 ae In both trigonal-bipyramidal and pseudooctahedral adducts, the asymmetry parameters on equatorial chlorine atoms exceed those on the axial atoms and vary within similar limits. Accord- ingly, the corresponding valence 3pp orbital occupancy differences are also close to each other. The main difference between these two types of adducts consists of the magnitudes of the 3ps orbital occupancies (Npz) at equatorial chlorine sites. In trigonal-bipyr- Z (%) n /MHz Nps 10.60.7 14.01.2 8.40.4 4.50.6 14.21.0 9.61.0 1.50.5 12.01.0 9.21.0 13.31.0 0.90.5 13.03.0 1.540 1.537 1.550 1.626 1.555 1.551 1.635 1.548 1.556 1.555 1.631 1.546 1.553 1.563 1.634 1.551 1.548 1.604 12.53.0 10.53.0 0.0 5.52.0 4.61.5 <1.557 24.39 24.26 24.01 20.21 23.34 23.89(2) 19.91 23.85 23.66 23.39 20.18 23.90 23.56 23.19 20.11 24.22 21.38(2) 21.04 24.30 24.23 24.14 23.72 0.90.5 2.90.5 2.90.5 2.90.5 2.90.5 E A Kravchenko, Yu A Buslaev 7r /e DNpp 0.509 0.496 0.525 0.615 0.515 0.523 0.631 0.513 0.530 0.517 0.629 0.508 0.517 0.533 0.634 0.535 0.534 0.602 0.031 0.041 0.025 0.011 0.040 0.028 0.004 0.035 0.026 0.038 0.002 0.038 0.036 0.030 0.0 0.016 0.014 0.002 |<0.54| >0.02 |Pr| /e 2.145 2.192 2.189 2.192 2.205 <2.15Electronic effects of substitution and reconstruction of the coordination polyhedron in adducts of Main Group IV element halides amidal adducts, the absolute values of Npz are by 0.1 lower than 3pp orbitals in bond formation results in their reduced occupancies those in the pseudooctahedral adducts, whereas the spectroscopic Npx and Npy , whose sign in equation (1) is opposite to that of the and electronic characteristics of their Sn7Cl axial bonds differ 3ps orbital occupancy Npz.This causes a decrease in both Up and insignificantly. As is seen from Tables 7, 8 and 10, the overall frequency with respect to those for the purely s-bonded chlorine effective charge |Pr| on all the chlorine atoms in the octahedral atom.In the latter case, the value of e2Qqzz/h becomes a measure adducts is markedly larger than that in the parent tetrachloride SnCl4, while in trigonal-bipyramidal adducts, the corresponding difference is small. This supports a conventional idea of relatively larger extent of charge transfer in octahedral adducts. The NQR data on pseudooctahedral adducts show that the overall s-electron density on chlorine atoms in trans-isomers is on average slightly higher than that in cis-isomers. IV. Penta- and hexacoordinate adducts of silicon and germanium tetrachlorides The 35Cl NQR spectroscopic parameters and M7Cl bond characteristics in adducts MCl4L (M=Si, Ge) are summarised in Table 11, which clearly demonstrates the similarities of the spectral patterns to those of the related tin compounds: the NQR spectra of the adducts of all three elements consist of triplets from equatorial atoms, slightly shifted relative to the frequencies of parent tetrachlorides MCL4, and lower-frequency singlets from axial chlorine atoms.However, the electronic characteristics of equatorial M7Cl bonds in adducts of different central metals M are markedly different. Thus Z on the equatorial chlorine atoms was found to increase considerably in the order: Sn<Ge<Si. In silicon adducts, this increase was accompanied by a fall of the 35Cl frequencies with respect to both the germanium and tin adducts and CCl4, which can be accounted for by an increase in the p- character of the Si7Cl bonds. The participation of the chlorine Table 11.35Cl NQR spectroscopic parameters for pentacoordinate adducts of silicon and germanium tetrachlorides at 77 K and the M7Cl bond characteristics.47 a/e Compound SiCl4 .Me3N SiCl4 .TMUa eeaeeaa SiCl4 GeCl4 .Me3N GeCl4 .TMU GeCl4 .HMPA eeaeeeaeeea GeCl4 Note. a The ligand TMU is located in the equatorial plane of the adduct. b The results of Zeeman analysis of a single crystal at 77 K.48 n /MHz 21.50 21.30(2) 18.86 21.65 21.16 18.45 17.34 20.464 20.415 20.408 20.273 24.83(2) 24.66 20.59 25.09 24.86 24.34 20.67 24.92 24.66 24.57 20.53 25.745 25.739 25.715 25.450 25.745 b 25.735 b 25.713 b 25.449 b of the chlorine valence 3ps orbital occupancy.One cannot make an independent estimation of the M7Cl bond s- and p-charac- ters, i.e., determine three unknowns, s, px and py from two equations (1) and (2). A spatial arrangement of ligands is however possible where the central atom M can interact with only one chlorine pp orbital (3px), the occupancy of the other (3py) remaining undisturbed (Npy=2). Denoting px=p, we obtain equation (2) in the form Z à 3p , 2Up so that Z becomes a measure of the M7Cl p-bonding. One can now determine both parameters, s and p, from equations (1) and (5). Such an arrangement is often the case in trigonal-bipyramidal adducts MCl4L. The estimation of their M7Cl bond s- and p-characters (Table 11) shows that in the silicon adducts the decrease in the equatorial 35Cl frequency and increase in Z result from substantial increase in the Si7Cl p-bonding.In the germa- nium adducts, the p-bonding is relatively weaker, whereas in SnCl4L the Z values exhibit a further considerable fall. They comprise a significant contribution from external charges, which is more pronounced than in analogous germanium and silicon adducts because of the larger Sn7Cl bond ionicity. This is supported by relatively higher values of Z on axial Cl atoms in comparison with equatorial in the SnCl4L adducts than in related silicon and germanium adducts. e2Qqzz/h /MHz Z (%) Nps 42.17.2 36.64.0 1.70.5 43.14.1 1.566 1.574 1.654 1.562 1.575 1.656 1.682 41.78 41.68 37.77 42.02 41.30 36.86 34.68 38.82.6 7.52.0 1.70.5 2.90.5 <1.59 40.77 2.90.5 2.90.5 2.20.5 25.33.9 24.1 3.3 2.00.9 26.45.0 25.34.2 27.03.8 5.61.0 31.85.0 28.04.3 1.516 1.519 1.623 1.510 1.514 1.522 1.616 1.506 1.515 1.516 1.622 48.92 48.85 41.17 49.06 49.19 48.10 41.33 49.02 48.69 48.51 41.05 <1.51 51.32 28.04.3 3.40.5 2.20.5 2.20.5 7 7 7 7 7 2.20.5 2.20.5 77.8 3.7 3.4 7r /e DNpp 0.459 0.481 0.650 0.452 0.478 0.639 0.679 0.107 0.093 0.004 0.110 0.097 0.017 0.003 |<0.52| >0.07 0.441 0.448 0.618 0.431 0.438 0.443 0.602 0.411 0.432 0.433 0.614 0.075 0.071 0.005 0.079 0.076 0.079 0.014 0.095 0.083 0.083 0.008 |<0.47| >0.04 721 (5) |Pr| /e 2.081 2.248 <2.08 1.948 1.914 1.890 <1.89722 Table 12.35Cl and 81Br Frequencies (n) in the NQR spectra of pseudooc- tahedral adducts of silicon and germanium tetrachlorides and EFG asymmetry parameters (Z) at 77 K. e/a LSiCl4L2 trans-py e trans-TPPO e cis-4,40-Me2-a,a-bipy ea GeCl4L2 (see Ref. 49) trans-py trans-HMPT trans-DMF cis-DMSO cis-DESO eeeeeaea GeBr4L2 (see Ref. 49) trans-py HMPT e 7 DMF 7 Only a few results of NQR studies are currently available on pseudooctahedral adducts of silicon and germanium tetrahalides (Table 12), most important being the value of the asymmetry parameter Z on the chlorine atoms in the only cis-isomer of the adduct SiCl4L2.This enables one to assign the 35Cl NQR frequen- cies to equatorial and axial chlorine atoms and find that their positional order on a frequency scale is inverted with respect to that in analogous tin adducts: unlike in the latter, the lower- frequency components in the spectra of silicon adducts are assigned to axial Cl atoms.Unfortunately, lack of data on cis- isomers does not allow one to conclude whether this order is typical of silicon adducts. Table 13 lists the M7Hal bond characteristics and effective charges on halogen sites in all the spectroscopically examined pseudooctahedral (MCl4L2) and trigonal-bipyramidal (MCl4L) Table 13.Average effective charges (7r) on chlorine atoms and chlorine valence 3p orbital occupancies (Nps=Npz , DNpp=Npx7Npy) in adducts MCl4L and MCl4L2. a/e Atom M Nps MCl4L2 Sn Ge Si 1.645 1.624 1.614 1.641 1.641 1.662 eaeaea Cl Cl Cl Cl Cl Cl MCl4L Sn Ge Si 1.550 1.627 1.510 1.619 1.572 1.654 eaeaea Cl Cl Cl Cl Cl Cl Z (%) n /MHz 6.60.6 19.15 18.29 19.03(2) 18.4 13.01.0 2.00.7 6.71.0 7.81.2 8.20.8 {5.7} {13.0} 20.45 21.10 20.67 20.32 20.82 19.69 20.56 19.59 139.78 135.09 146.46 138.43 145.46 7r /e DNpp 0.620 0.617 0.593 0.641 0.618 0.657 0.025 0.007 0.021 0.0 0.023 0.005 0.521 0.622 0.430 0.610 0.470 0.650 0.029 0.005 0.080 0.009 0.102 0.004 E A Kravchenko, Yu A Buslaev adducts of silicon and germanium tetrachlorides.The correspond- ing characteristics for SnCl4L2 and SnCl4L averaged over all the known examples are also included in the table. One can see that the chlorine 3pp orbital occupancy differences at equatorial sites (DNpp=Npx7Npy) in all the hexacoordinate adducts MCl4L2 (M=Si, Ge, Sn), are close to each other, the main difference between the adducts consisting in the chlorine 3ps orbital occupancies. The absolute magnitudes of the effective charge r on the equatorial chlorine atoms vary in the order: Sn*Si>Ge. When the adduct of pseudooctahedral configuration is compared to that of trigonal-bipyramidal geometry, one can find that the axial M7Cl bond characteristics differ insignificantly in both types of the adducts, whereas the equatorial bonds are markedly different, the difference being determined by the nature of the central atom.The effective negative charge on the equatorial chlorine atom increases in absolute value as the coordination number of the central atom increases (Table 14). In the tin adducts, this increase in the charge is nearly fully induced by the increase in chlorine 3ps orbital occupancy, whereas in the germanium and especially in the silicon adducts, it is provided by both the increased chlorine 3ps orbital occupancy and reduced DNpp. This may be accounted for by the increased local symmetry of the Cl site in a crystal as well as the M7Cl bond elongation accompanying an increase in the coordination number.In germanium tetrachloride adducts, the contributions of the 3ps- and 3pp-systems to the effective charge increment on the Cl atom amount in absolute magnitude to 0.104 and 0.059, respectively; in the silicon adducts, the contribution of the chlorine 3pp orbitals to the respective increment exceeds that of the 3ps orbitals (0.079 and 0.069, respectively). Table 14. Average values of effective charges on equatorial chlorine atoms and chlorine valence 3p orbital occupancies in adducts MCl4L and MCl4L2 and variations of their absolute magnitudes (dp, dNps, dDNpp) due to the change MCl4L?MCl4L2. Adduct dr /e dDNpp DNpp dNps 7r(Cl) /e Nps SiCl4L?SiCl4L2 0.470 SiCl4L 1.572 1.641 0.102 +0.148 +0.069 70.079 0.023 SiCl4L2 0.618 GeCl4L?GeCl4L2 GeCl4L 0.430 1.510 1.614 0.080 +0.163 +0.104 70.059 0.021 GeCl4L2 0.593 SnCl4L?SnCl4L2 SnCl4L 0.521 +0.099 +0.095 70.004 0.029 0.025 1.550 1.645 0.620 SnCl4L2 Thus, the NQR data available on the trigonal-bipyramidal SiCl4L and GeCl4L adducts provide evidence that the 3pp-systems of the equatorial Si7Cl and Ge7Cl bonds contribute consider- ably to the bond formation, the estimated p-character amounting on average to 0.1 and 0.08, respectively.It should be noted however that the NQR data enable one to compare an extent of participation of the equatorial chlorine 3pp electrons in theM7Cl bonding, whereas the question of the mechanism of the p-interaction remains open.In addition, the magnitude of DNpp may not be a measure of the M7Cl bond p-character in pseudooctahedral adducts SiCl4L2 and GeCl4L2, if the electron density is withdrawn from both chlorine 3pp orbitals; in this case, the true extent of the p-character will be higher than that estimated from DNpp. This also holds for the parent tetrachloride molecules having slightly distorted tetrahedral structures.48 The variation of the chlorine valence p orbital occupancies upon adduct formation was discussed based on the results of PM3 calculations of model acceptor molecules and adducts MCl4L and MCl4L2 (M=Si, Ge, Sn).37, 38 The M7Cl bond p-character wasElectronic effects of substitution and reconstruction of the coordination polyhedron in adducts of Main Group IV element halides identified by the authors with the magnitude of DNpp determined in the parent tetrachlorides by the NQR experiments.47 For SiCl4 and GeCl4, such an estimation gave p=0.007, which resulted in the considerably overestimated absolute value of the negative effective charge on the chlorine atom: r(Cl)=0.618 The problem of electron density distribution in Main Group and 0.525 e in SiCl4 and GeCl4, respectively.Hence, the entire charge on the chlorine atoms |Pr(Cl)| was 2.472 e in SiCl4 and Refs 34 ± 39. The results of semiempirical quantum-chemical IV element chloro complexes has received much attention in 2.1 e in GeCl4, which notably exceeded in absolute magnitude the appropriate charges in adducts MCl4L, namely, 1.983 e in SiCl4 .NMe3 and 1.912 e in GeCl4 .NMe3.The contradictions arisen were accounted for 37, 38 by the difference between the symmetry of free MCl4 molecules (Td) and the symmetry of the same molecules in the adducts (D3h in MCl4L and D4h in MCl4L2). The results of PM3 calculations showed that the total effective charge on the chlorine atoms |Pr(Cl)| in hypothetical MCl4 molecules, whose symmetry and chlorine atoms in the adducts MCl4L2 either remains unchanged M7Cl bond distances are the same as those in the corresponding or decreases as compared to freeMCL4. The reader is referred to a adducts, was reduced as a result of the molecular spatial review 43 where the criticism of this conclusion was given.rearrangement only (the charge transfer being ignored) up to The quantum-chemical calculations of the trigonal-bipyrami- 2.184 e in SiCl4 and 1.988 e in GeCl4. However, it is to be noted dal adducts SiCl4 .NMe3 (5), SiCl4 .CO(NMe2)2 (6) and pseu- that similar characteristics (|Pr(Cl)|) of the MCl4 molecules in dooctahedral SiCl4 . (py)2 adduct (7) were made within the the adducts SiCl4 .NMe3 and GeCl4 .NMe3 estimated from the orbital occupancies using the PM3 approach 37, 38 decrease accordingly: Atom a/e Npx+Npy Npz Npx7Npy 7r /e Pr(Cl) /e SiCl4 .Me3N 1.504 7 0.490 0.338 0.016 0.044 3.911 3.898 1.579 1.440 Cl Cl ae GeCl4 .Me3N 1.176 7 0.384 0.264 0.019 0.055 3.903 3.883 1.481 1.381 Cl Cl ae One can see that the calculated Z values at the equatorial Cl atoms in the adducts in question (16.4% in SiCl4 .NMe3 and 20.7% in GeCl4 .NMe3)37,38 do not reflect even qualitatively the trends observed in experiments (see the measured magnitudes of Z in Table 11).47 Hypothetical molecules MCl4 having the D4h symmetry and M7Cl bond distances characteristic of those in pseudooctahedral adducts MCl4L2 were calculated to give the largest values of Z at the chlorine atoms, namely, Z&31% in SiCl4, 37% in GeCl4 and 33% in SnCl4.37, 38 Meanwhile, the experimental Z values at chlorine atoms did not exceed 15% in any of the measured MCl4L2 adducts (Tables 7 and 8), unless this atom is involved in secondary inter- or intramolecular interac- tions, as in the adducts SnCl4(SeOCl2)2 and SnCl4(EtOH)2 (Table 8).Moreover, the average values of Z at the equatorial chlorine atoms in the adducts SiCl4L2 and GeCl4L2 are notably lower than in the related trigonal-bipyramidal adducts SiCl4L and GeCl4L. There is no question that spatial reconstruction of an acceptor molecule upon complexation is accompanied by electron density redistribution. However, the orbital occupancies calculated within the PM3 approximations do not account for the inconsistency between the effective charges on the chlorine atoms in free acceptor molecules and their adducts,37, 38 nor the reasons for the increase in the p-character of the M7Cl equatorial bonds in the adducts MCl4L (Sn<Ge<Si) found in NQR experiments.47 But no inconsistency arises if one takes into account that the Td symmetry of the parent tetrachloride molecules allows for the participation of both chlorine 3pp orbitals in bond formation, whereas one can measure by NQR only the difference between their occupancies.Let us take the values of Npx and Npy in SiCl4 (fitting the measured NQR frequencies and Z), which give rise to the Si7Cl bond p-character typical of the equatorial Si7Cl bonds in the adducts SiCl4L (for example, p=0.093, Npx=1.957, Npy=1.950). One can find herefrom that the corresponding effective charges on the Cl atoms in the SiCl4 723 molecules (r=70.49 e, ~r(Cl)=71.96 e) are consistent with those in the SiCl4L adducts (72.1>~r(Cl)>72.3 e, see Table 11), hence allowing for the charge transfer from the ligand L to the acceptor SiCl4.calculations were reported, and their conformity to the data obtained using the NQR, MoÈ ssbauer and X-ray fluorescence spectroscopies were discussed. It was found that upon complex- ation, the positive charge on the central atom of the acceptor molecule increases with respect to that in the free MCl4 molecule, which is in agreement with the results of other publications.43 It was however noted that according to the X-ray fluorescence spectroscopy data (ClKa-shifts), the electron density on the MNDO approximations.49 ± 52 The changes in silicon, nitrogen, oxygen and chlorine valence p orbital occupancies as well as the effective charges on the appropriate atoms were considered as functions of the reaction coordinate of complexation, the consid- ered separation between the components of the adducts varying from infinity (d=200 A) to 1.5 A.As the calculated heats of formation showed, the most energetically favourable structures of 5 and 6 were those with the ligand L in axial and equatorial positions of the bipyramid, respectively, which is in agreement with their 35Cl NQR spectral patterns (Table 11). When the shape of the SiCl4 molecule changes from tetrahe- dral to that in the adduct 5 (at d=200 A), the effective charge on the silicon atom decreases in absolute magnitude by 0.116 e, and that on the equatorial chlorine atom, by 0.059 e, whereas the absolute magnitude of the negative charge on the axial chlorine atom increases by 0.061 e.When the components of 5 approach each other to reach d=2.25 A, at which distance the heat of adduct formation is a minimum, the effective charge on silicon begins to grow. At the same time, the negative charge on nitrogen decreases and that on chlorine, increases in absolute value, the increment being larger on the equatorial Cl atoms. It was estimated 50, 52 that the formation of 5 results in the transfer of electron density of 0.2 e from the donor (mainly, from the nitrogen lone pair px orbital) to the acceptor (mainly, to the equatorial chlorine ps orbitals) molecule, which is in good agree- ment with experiment.47 However, low sensitivity of this semi- empirical method to the chlorine 3pp orbital occupancies was noted. The calculated differences between the equatorial chlorine 3px and 3py orbital occupancies in the adduct 5 were negligibly small, whereas the experimental asymmetry parameters Z on these atoms were large, the corresponding DNpp values contributing considerably to the effective charge values (Table 11).Thus, the estimation of the effective charge on chlorine atoms from the calculated orbital occupancies 50, 52 is not quite reliable. A similar calculation for the adduct 6 51, 52 with a tetramethyl- urea ligand occupying an equatorial position shows that the formation of 6 also results in the transfer of the charge from the donor to chlorine atoms of the acceptor molecule, but in this case, the electron density is mainly transferred to the axial chlorines.The donor atoms are carbon of the carbonyl group, one of the nitrogen atoms and hydrogens from the dimethylamino groups, both ps and pp orbitals of the carbonyl group contributing to the charge transfer. An increase in positive charge on silicon and in negative charge on oxygen upon adduct formation is caused presumably by polarisation of the Si7Cl and Si7O bonds.51, 52 The ps orbital occupancies of the axial (1.591 e) and equatorial (1.523 e) chlorine atoms in 6, found by the MNDO calculations, are in agreement with the observed trends (Table 11), while the724 values of Npx and Npy at the equatorial chlorine atoms showed, as in 5, poor agreement with experiment.47 The results of MNDO calculations of 5, 6 and pseudooctahe- dral 7 adducts were summarised 52 to conclude that the formation of all the three adducts results in an increase in positive charge on the silicon atom, the conclusion being supported by the MoÈ ssba- uer and X-ray fluorescence spectroscopy data 34 ± 36 available on the adducts of Main Group IV element tetrachlorides. The increasing charges, positive on silicon and negative on nitrogen, gave the authors 52 grounds to believe that the electrostatic interactions resulting in polarisation of the bonds contribute considerably into the electron density distribution upon complex- ation.The electrostatic interactions initiate the charge transfer: the estimated total charge located on the acceptor fragment of the adduct was negative (70.523 e in 7) and that on the donor fragments, positive (+0.206 e per pyridine ring in 7).In order to compare the chlorine orbital occupancies calcu- lated within the MNDO approximations with those derived from the NQR experiment the assumption was made 52 (in the absence of the published Z values) that the values of Z at the equatorial chlorine atoms in pseudooctahedral adducts SiCl4L2 are similar to those for trigonal-bipyramidal adducts SiCl4L. Thus, the value Z=0.4, which is the average for the SiCl4L adducts, was used to estimate Npx (at Npy=2) at the equatorial Cl atoms in 7. This gave Npx=1.909 and DNpp=0.09, the latter being considerably overestimated compared to the correct value DNpp=0.015 derived from the recently measured Z in SiCl4 .2py. As was noted above, the values of DNpp at the equatorial chlorine atoms, derived from the NQR spectra of SiCl4L2 and GeCl4L2, are markedly lower than those in analogous trigonal-bipyramidal adducts. But even this correct magnitude of DNpp exceeded considerably the value DNpp=0.004 (and hence Z=0.016) found by the MNDO calculations.52 Therefore, for the pseudooc- tahedral adduct 7, the MNDO approach also gave an Z value in poor agreement with experiment (Tables 11 and 12). It was however reported 53 that the results of calculations of Z can be in much better agreement with the experiment if one takes into account that the EFG (qzz, qxx, qyy) at the quadrupole atom is produced not by its entire valence p shell, but its less diffuse part which can be determined by ab initio calculations of a large Table 15.Variation of chlorine valence 3pz orbital occupancies (dNps) and equatorialM7Cl bond p-character (dp) in adducts MCl4L in relation to the difference D between the total effective charges on the chlorine atoms in the adduct and parent tetrachloride MCl4. a/e Compound 7r(Cl) /e Pr(Cl) /e 2.07 SiCl4L SiCl4 2.07 1.96 1.924 1.89 1.86 1.836 1.77 1.89 GeCl4L ea7777777ea GeCl4 1.89 1.84 1.607 1.53 2.15 SnCl4L 7777ea SnCl4 0.473 0.650 0.517 0.490 0.481 0.472 0.465 0.459 0.442 0.425 0.614 0.472 0.460 0.402 0.382 0.511 0.615 0.537 0.513 0.490 0.470 2.15 2.052 1.96 1.88 7777 Nps 1.571 1.654 1.591 1.582 1.579 1.576 1.573 1.571 1.566 1.512 1.622 1.512 1.508 1.489 1.482 1.543 1.626 1.557 1.545 1.537 1.531 E A Kravchenko, Yu A Buslaev number of molecules in the split valence basis set 6-31G* using full optimisation of the molecular geometry.It is reasonable to discuss how the differences in the M7Cl bond characterisitics influence the formation by silicon, germa- nium and tin tetrachlorides of pentacoordinate adducts with ligands of various donating power. Before discussing this prob- lem, the bonding in the parent tetrachlorides should be considered in more detail. Note once more that the values of Z measured in tetrahedral MCl4 molecules and octahedral adducts MCl4L2 using spin-echo technique and powder samples 9, 47 were in agreement with those measured with single crystals of similar com- pounds,44 ± 46, 48 whereas earlier experiments 54, 55 using continu- ous-wave techniques for measuring Z with powder samples of MCl4 (M=Si, Ge, Sn), yielded the asymmetry parameters Z, whose values were an order of magnitude higher than those given in Tables 10 and 11, which is unlikely on symmetry grounds.An independent estimation of the M7Cl bond s- and p-characters from equations (1) and (2) is impossible in these molecules, because Z at chlorine sites is equal to zero (Npx=Npy), and two unknowns, s and p, remain in equation (1). The probable intervals for the values of M7Cl bond s- and p-characters in MCl4 were found based on the data obtained for the adducts MCl4L.47 The magnitude of the entire effective charge |~r(Cl)|MCl4 , born by all the chlorine atoms in the parent molecule MCl4, was varied with respect to the related value for the adducts |~r(Cl)|MCl4 , assuming that the latter is at least no less than the former: in fact, an increase in the entire effective charge on the chlorine atoms |~r(Cl)|MCl4 , depending on the amount of charge transferred from the donor L, is normally observed upon adduct formation. For the silicon compounds, the results of varying the differ- ence D=|~r(Cl)|SiCl4L7|~r(Cl)|SiCl4 showed that the strength- ening of the equatorial Si7Cl bonds by virtue of both s and p orbitals is expected in the adduct with respect to SiCl4 as D varies in the interval 0<D<0.18 e (Table 15).This implies that if the extent of charge transfer from the ligand L is small (due, for instance, to the weak donating power of ligands L), the adduct formation will be accompanied by an additional charge transfer from the equatorial chlorine atom s and p orbitals to the axial ones. When the extent of charge transfer is increased, so that D varies in the interval 0.18<D<0.21 e, weakening of the equa- p D/e dp dNps +0.024 +0.006 0.0 70.06 70.11 70.015 70.026 0.0 0.11 0.146 0.18 0.21 0.234 0.30 70.020 70.011 70.008 70.005 70.002 +0.00 +0.005 +0.047 +0.039 0.0 70.013 0.00 +0.004 +0.023 +0.030 0.0 0.05 0.283 0.36 0.098 0.004 0.074 0.092 0.098 0.104 0.109 0.113 0.124 0.087 0.008 0.040 0.048 0.087 0.100 0.032 0.011 0.020 0.032 0.047 0.061 +0.012 0.0 70.015 70.029 0.0 0.098 0.19 0.27 70.014 70.002 +0.006 +0.012Electronic effects of substitution and reconstruction of the coordination polyhedron in adducts of Main Group IV element halides torial Si7Cl bonds through the p-mechanism is expected, i.e., increase in the electron density will partly localise on the equatorial chlorine atom 3pp orbitals of the acceptor molecule being withdrawn from their 3ps orbitals until D reaches 0.21 e, after which the weakening will occur through the s-system also.The variation of D should evidently proceed within reasonable limits. The situation where the p-bonding in SiCl4 exceeds the equatorial Si7Cl bond p-character in the adducts seems improb- able. From the other side, the assumption of pure s-bonds in SiCl4 leads to the conclusion that the total s-electron density in the acceptor molecule decreases considerably upon adduct formation, which is also unlikely. It thus follows that the most probable limits for variation of D, associated with the formation of the adducts SiCl4L, are 0<D<0.30 e. Similarly, strengthening of the equatorial Ge7Cl bonds in the adducts GeCl4L with respect to GeCl4 is first expected as the extent of charge transfer increases within 0<D<0.22 e, which is then followed by their weakening at D>0.22 e.But unlike the silicon adducts, the Ge7Cl bond strengthening involves the p-system only, whereas the chlorine 3ps orbital occupancy can only increase, whatever small extent of charge transfer may be in the germanium adducts (Table 15). The range for variation of D associated with the formation of the adducts GeCl4L (0<D<*0.40 e) is here larger than for the silicon adducts. As to the adducts SnCl4L, their equatorial Sn7Cl bonds weaken at D>0.13 e. Thus, the NQR data, if not providing the undisputable absolute magnitudes of the electronic characteristics, reveal considerable difference between the equatorial M7Cl bonds in trigonal-bipyramidal adducts of silicon, germanium and tin tetrachlorides.This difference determines specific changes in their s- and p-components when going to the adducts of pseudooctahedral configuration. Several characteristic distinc- tions, depending on the nature of the central atom M and donating power of ligand L, were revealed when the electron density redistribution due to formation of pentacoordinate adducts by the parent MCl4 molecules was compared using the NQR data. V. Conclusion The NQR spectroscopic data refine our understanding of the electronic effects due to substitution and spatial rearrangement of the coordination polyhedra in adducts and complexes of tin, germanium and silicon tetrachlorides. They provide information on the relative contribution of the s- and p-systems to the electron density redistribution.The results of NQR studies of adducts and complexes of monoalkyl(phenyl)-substituted chloro, bromo and iodostannates enabled one to estimate and compare the contribu- tions of their s-electron systems to weakening of the Sn7Cl bonds in cis-positions relative to the substituents. An estimation of the effective charges on the halogen atoms in cis- and trans- positions to the substituents made it possible to compare quanti- tatively the contributions of cis-weakening and trans-strengthen- ing to the resulting electron density redistribution in the adduct due to replacement of the acidoligands; participation of the halogen p-electron system in the M7Cl bond trans-strengthen- ing was revealed. The NQR spectra provide evidence that the length of the chain in the alkyl groups of the substituent influences insignificantly the overall cis-weakening of the Sn7Cl bonds.This permitted one to estimate the contribution of the ligands L to the Sn7Cl bond weakening in the presence of the alkyl groups in dialkyldibromostannates. An examination of dependence of the EFG asymmetry parameters at the iodine atoms in the adducts of type R2SnI2L2 on the iodine valence orbital occupancies showed that an increase of the halogen s-electron density due to cis- weakening leads to an increase in Z at a fixed DNpp=Npx7Npy difference. Comparison of the NQR and X-ray diffraction data enabled one to correlate on a quantitative basis the change in the Sn7Hal 725 bond lengths with the effective charge increments on the appro- priate halogen atoms.The NQR spectroscopic parameters directly determined by the electron density distribution at the halogen atoms were found to be more sensitive to the electronic effects than the interatomic distances. However, X-ray data on the spatial arrangement of atoms and atomic groups, especially in isomers, appeared to be very important for the correct assignment of resonance lines and hence correct interpretation of the NQR spectra. However, examples are known where the electronic effects registered by NQR gave structural evidences below the accuracy of X-ray experiments. Thus the structural effects of trans-strengthening in adducts and complexes of mono- alkyl(phenyl)tin trichlorides were small, so that the correspond- ing trans-shortening was comparable with the standard deviation values, whereas the electronic effects involved were registered by NQR with an accuracy notably exceeding the errors of measure- ment. Similarly, no structural evidence for any intermolecular (interligand) interactions were found by X-ray in the adduct Ph2SnI2(HMPT)2, whose 127I NQR spectral patterns were con- sistent with a considerable disturbance of the iodine valence p-electron system caused seemingly by the p ± p interactions between the iodine atom and phenyl rings of the substituents.The measurement of the EFG asymmetry parameters at chlorine atoms in pseudooctahedral adducts SnCl4L2 enabled one to find a spectroscopic criterion of cis ± trans isomerism of the adducts.The type of isomer could be identified, using this criterion, by measuring the 35Cl NQR frequencies only. The identification of isomers led to better understanding of the electronic reorganisation of the adducts as a result of their rearrangement from pseudooctahedral to trigonal-bipyramidal geometry. It was found that the transformation SnCl4L2?SnCl4L is accompanied by an insignificant change in the axial Sn7Cl bond character. The equatorial Sn7Cl bond p-system also remains undisturbed by this transformation. The main influence was exerted on the equatorial halogen atom valence 3ps orbitals, whose occupancies decreased on average by 0.1 e. The measurement of Z at chlorine atoms in analogous adducts of germanium and silicon tetrachlorides demonstrated both similarities and differences between the M7Cl bond character- istics.According to the NQR data, the equatorial M7Cl (M = Sn, Ge, Si) bond p-character considerably increases in the trigonal-bipyramidal adducts in the order Sn<Ge<Si (0.03<0.08<0.1, respectively). On going from these adducts to the adducts of pseudooctahedral configuration, the axial M7Cl bond exhibits only minimal changes. However, the electron density redistribution over the s- and p orbitals of the equatorial M7Cl bonds strongly depends on the nature of the central atom: an increase in the coordination number of the latter results in an increase in absolute magnitude of the effective charges on the equatorial chlorine atoms.In tin adducts, this increase is com- pletely provided by the increased halogen 3ps orbital occupancy, whereas in the silicon adducts, the charge increment, caused by the reduced chlorine 3pp orbital occupancy difference (0.079), exceeds that contributed by its 3ps orbital (0.069). The difference between the bonds of tin, germanium and silicon tetrachlorides causes the different character of s- and p-electron density redistribution upon formation of trigonal- bipyramidal adducts with ligands L of various donating powers. The work was supported by the Russian Foundation for Basic Research (Project No. 96-03-34280). References 1. Yu A Buslaev, E A Kravchenko, L Kolditz Coord. Chem. Rev. 82 1 (1987) 2.K V Raman J. Mol. Struct. 345 31 (1995) 3. Yu E Sapozhnikov, Ya B Yasman Izv. Akad. Nauk SSSR, Ser. Fiz. 42 2148 (1978)726 4. V P Feshin, Yu E Sapozhnikov, G V Dolgushin, Ya B Yasman, M G Voronkov Dokl. Akad. Nauk SSSR 247 158 (1979) a 5. V P Feshin, G V Doldushin, M G Voronkov, Yu E Sapozhnikov, Ya B Yasman, V I Shiryaev J. Organomet. Chem. 295 15 (1985) 6. V P Feshin, M G Voronkov, G V Dolgushin, P A Nikitin, I M Lazarev, Yu E Sapozhnikov, Ya B Yasman Dokl. Akad. Nauk SSSR 268 1163 (1983) a 7. V P Feshin, G V Dolgushin, I M Lazarev, Yu E Sapozhnikov, Ya B Yasman,M G Voronkov Dokl. Akad. Nauk SSSR 300 1181 (1988) a 8. V G Morgunov, E A Kravchenko Pribory Tekhn. Eksper. (6) 199 (1988) 9. E A Kravchenko, V G Morgunov,M Yu Burtsev, Yu A Buslaev Zh. Obshch. Khim. 60 1945 (1990) b 10. E A Kravchenko, M Yu Burtsev Z. Naturforshch., A Phys. Sci. 47 134 (1992) 11. V P Feshin, G V Dolgushin, I M Lazarev, M G Voronkov, E A Kravchenko, V G Morgunov,M Yu Burtsev Dokl. Akad. 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ISSN:0036-021X    

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Russian Chemical Reviews 68 (9) 727 ± 736 (1999) Radical pairs in redox reactions involving sterically hindered quinones and phenols { A I Prokof'ev Contents I. Introduction II. Radical pairs of phenoxyl radicals in frozen solutions III. Characteristic features of photoreduction of quinones in single crystals of organic hydrogen donors IV. Other quinoid systems in hydrogen atom transfer reactions in single-crystalline phenols V. Anomalous relaxation in the ESR spectra of radical pairs VI. Conclusion Abstract. The review summarises the results of studies on the redox reactions involving sterically hindered quinones and pyro- catechols (phenols) occurring in glassy and single-crystal matrices. Exposure of these systems to light results in the transfer of a hydrogen atom or an electron from the phenol to the quinone accompanied by the formation of radical pairs, viz., two para- magnetic species existing in the triplet state.Studies of the transformations in single-crystal matrices enabled separation of the mechanisms of electron and hydrogen atom transfer. Syn- chronous transfer of two hydrogen atoms from two sterically hindered phenol molecules to o-benzoquinone is also possible. The bibliography includes 39 references. I. Introduction The simplest quinones can be considered as model systems that mimic biologically important quinoid compounds (plastoqui- nones, ubiquinones, vitamin K, flavin adenine dinucleotide, etc.). Therefore, studies of the photochemistry of quinones are of undeniable interest due to elucidation of their role as electron acceptors in photosynthesis. For example, photoreduction of quinones results in semiquinone radicals, which are active species in electron transfer reactions in biological systems.Studies of photoreduction reactions in the liquid phase make use of methods of flash photolysis with optical detection of intermediate products and chemically induced dynamic polarisation of electrons and nuclei, while studies of these reactions in the solid state are carried out with the use of ESR spectroscopy. Although the ESR data obtained in the solid state are essentially depleted due to unaveraged rotational dipole ± dipole electron ± nucleus interac- tions, studies of solid-state processes and the identification of the radical species formed in these processes are still of special interest.The present review generalises the results of studies dealing with the solid-state photoreduction of quinones and quinone-like systems with sterically hindered phenols. A characteristic feature of sterically hindered phenols and their derivatives is that they are readily involved in redox reactions to give relatively stable radicals. The application of radio- A I Prokof'ev A N Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, 117813 Moscow, Russian Federation. Fax (7-095) 135 50 85. Tel. (7-095) 135 93 63 Received 15 January 1999 Uspekhi Khimii 68 (9) 806 ± 816 (1999); translated by S S Veselyi #1999 Russian Academy of Sciences and Turpion Ltd UDC 541.515 727 728 730 732 735 736 frequency spectroscopic methods gave strong impetus to the studies on the structure and reactivity of phenoxyl-type radicals and promoted progress in the chemistry of this class of com- pounds; this resulted in the synthesis of metal complexes with paramagnetic ligands.1, 2 The rather high stability of phenoxyl radical species and the relative simplicity of their ESR spectra made it possible to solve diverse structural and kinetic problems based on the analysis of the hyperfine structure (HFS) of the spectra.2 ±4 Taken together, all this has made the sterically hindered phenols ± quinones system a convenient model for studying the photooxidation by the ESR method.UV irradiation of aromatic ketones in the presence of hydro- gen or electron donors results in the reduction of carbonyl groups.5 ±7 In a discussion of the elementary act of transfer of an H atom from a hydrogen donor (AH) to an excited quinone (Q*), Porter suggested in 1958 two possible variants of this process: (1) transfer of a hydrogen atom as a whole (one-step process) and (2) transfer of an electron followed by transfer of a proton (two- step process).8 Q Q hn *, QH.+A., Q*+AH Q*+AH QH.+A.. Q¡.+AHá. In certain cases, it was possible to detect the formation of both neutral and radical-ion intermediates in these systems and to demonstrate the independence of their formation and dark decay.This fact implies that the solid-state reactions of this type are quite unusual. Irradiation of a solution of 3,6-di-tert-butyl-1,2-benzoqui- none (1, 3,6-Q) and 2,6-di-tert-butyl-4-methylphenol (2) in tolu- ene or CCl4 results in the phototransfer of an H atom from the phenol 2 to the quinone 1 accompanied by the formation of 2,6-di- tert-butyl-4-methylphenoxyl (3) and 3,6-di-tert-butyl-2-hydroxy- phenoxyl (4, 3,6-SQH.) radicals, as indicated unambiguously by the HFS of the spectra of these species (Fig. 1 a). But OH But O. But O But But O. But hn + + OH O Me But Me But 4 3 1 2 { In memory of Professor Ya S Lebedev.728 ab c Figure 1. ESR spectra of a solution of 3,6-di-tert-butyl-1,2-benzoqui- none (1,3,6-Q) and 2,6-di-tert-butyl-4-methylphenol (2) in CCl4 (UV irradiation, 30 8C) (a) and of radical pairs (3,6-Q +NEt3) (b) and (3,6-Q +phenol 2) (c) in solid phase (toluene, 77 K).On the other hand, irradiation of quinone 1 in the presence of amines results in semiquinone (SQ) radical anions.9 HFS of the ESR spectra of biradicals formed upon photolysis of the quinone 1 ± phenol 2 system at 77 K points to their triplet state (Fig. 1 b,c).Asimilar pattern is also observed for the efficient dipole ± dipole interaction of unpaired electrons of two mono- radical species located in a solid phase at a distance of up to 7 ± 10A from each other (radical pairs). Atheoretical consideration of systems in a triplet state enables an experimental determination of the zero splitting parameters (D and E) from the lines of the fine structure corresponding to the allowed transitions (DmS=1).10 The parameter D (in mT) in the point dipole approximation depends on the distance between the radical centres: D=3gb/r3av, where g is the spectroscopic splitting factor, b is the Bohr magneton, and rav is the average distance between the unpaired electrons.In this approximation, r is defined as rav(A)=14.06 D71/3. As regards the parameter E, E 6à 0 in the absence of axial symmetry, while E=0 in the case of axial symmetry where the system should have, at least, a three-fold axis. The recorded absorption (DmS=1, g=2) due to reorienta- tion of one electron spin in the external magnetic field (H) and in the field of the other electron of the radical pair (RP) is anisotropic in principle.Therefore, two components are distinguished for chaotically oriented RP, which correspond to RP oriented along the direction of H, i.e., parallel (Dk) and perpendicular (D?); in this case, Dk &2D?. In addition, the ESR spectra of triplet systems often display forbidden transitions (DmS=2) in the range g=4 (half-field signal); the intensity of these increases with a decrease in the distance between the electron spins. The signal with DmS=2 is intrinsically isotropic, as it originates from simultaneous reorien- tation of two electrons of the RP in the field H. Detailed analysis of the ESR spectra of single crystals of phenols doped with quinones and similar compounds and subjected to photolysis made it possible to reveal fundamentally new types of solid-state reactions, viz., synchronous transfer of two hydrogen atoms from a single-crystal phenol matrix to a quinone, relay transfer of a hydrogen atom in the system 2,6-di- tert-butylphenol ± 2,6-di-tert-butyl-1,4-quinonediazide, and tun- nel transfer of a hydrogen atom in the dark decay of RP in a single 0.5 mT 5.0 mT A I Prokof'ev crystal of 2,6-di-tert-butylphenol doped with 2,6-di-tert- butylquinoneimine. The effects of abnormal relaxation of RP detected due to the different saturation of certain lines of the fine structure proved to be useful in the study on the chemical polarisation of nuclei in liquid solutions of the above systems.In certain cases, X-ray diffraction analysis of single crystals of phenols made it possible to determine specific ways of reverse transfer of a hydrogen atom in the dark decay of the RP.Analysis of kinetic curves of the RP decay showed the possibility of intramolecular migration of a hydrogen atom in semiquinone radicals. Thus, detailed investigations into the spectral and relaxation characteristics of RP can provide additional information regard- ing the nature of the RP and the mechanism of processes resulting in their formation. II. Radical pairs of phenoxyl radicals in frozen solutions Specific features of solid-state phototransfer of a hydrogen atom in frozen solutions have been studied for the system 2,4,6-tri-tert- butylphenol (5) ± 3,6-di-tert-butyl-1,2-benzoquinone (1) as an example.11 The ESR spectra of the radical pairs formed upon photolysis of the phenol 5 and the quinone 1 in Nujol at 77 K (Fig.2) attest to the formation of two types of RP, viz., RP1 with rav=5.4A and RP2 with rav=6.3A. The lines corresponding to the D? component of RP1 coincide with the lines of the Dk component of RP2. A study of the static saturation of these types of RP showed that the RP1 lines are saturated with much greater difficulty than the RP2 lines (Fig. 3) because of efficient relaxation in RP1 due to stronger spin ± spin interaction of unpaired electrons. It was shown that the accumulation of RP1 and RP2 during photolysis by filtered light (l=365 ± 460 nm) occurs in parallel, but the ratio of their maximum concentrations at 77 K is 1 : 4.The presence of two types of RP in this system is due to the formation of `sandwich' donor ± acceptor complexes (Fig. 4) between the quinone 1 and the phenol 5 (NMR data) in which b a A0 A0 B0 B 0 C0 B C A B A A0 d c A0B 0 B 0 C0 C B B A 20 mT A Figure 2. ESR spectra of radical pairs formed after 10-min irradiation of frozen solutions of 3,6-Q and 2,4,6-tri-tert-butylphenol (5) in Nujol at 77 K with light at a wavelength of 365 ± 450 nm (a, b) and after 6 min of annealing the samples at 133 K (c, d). The spectra were recorded at a microwave irradiation power of 150 (a, c) and 0.2 mW (b, d).Radical pairs in redox reactions involving sterically hindered quinones and phenols qI=qH 3.0 2 2.0 1 RP1 2 0 1 0 1.0 RP2 10 20 30 40 50 r /dB Figure 3.Dependence of saturation of RP1 (1, 2) and RP2 (1 0, 2 0) formed upon irradiation of solutions [3,6-Q + 2,4,6-tri-tert-butylphenol (5)] (1, 1 0) and (3,6-Q + OH-deuterated 2,4,6-tri-tert-butylphenol) (2, 2 0) in Nujol on the microwave irradiation power (r). O(3) HO(1) O(2) Figure 4. Tentative structure of the complex of the quinone 1 with the phenol 5. the distances between the oxygen atoms of the OH group of the phenol 5 and two carbonyl groups of the quinone 1 [O(1)...O(2) and O(1)...O(3)] differ noticeably. In addition, account of the electric dipole interaction between the oxygen atoms of the OH group of the phenol 5 and the carbonyl groups of the quinone 1 in the sandwich complex (Fig.4) shows that the relative depth of the proton trap at the O(2) atom is greater than that at the O(3) atom by 0.2 kcal mol71. As a result, the ratio between the equilibrium concentrations of RP1 and RP2 is approximately equal to 1 : 4, which is consistent with the experimental data. The hydroxyphe- noxyl radical 4 formed is characterised by intramolecular migra- tion of a hydrogen atom between the oxygen atoms at a frequency of n '109 s71 (at 300 K) or n '103 s71 (at 77 K), which is much slower than the ESR time scale (1076 ± 10710 s).12 But But OH O. OH O. But But 4 As a result, the spectra of RP1 and RP2 are recorded separately. Studies of the dark decay of RP1 and RP2 showed that both types of pairs decay in the same way and that the kinetics of this process is stepwise, which is probably caused by the heterogeneity of the matrix. The similarity of the character of accumulation and decay of RP1 and RP2 is apparently due to intramolecular migration of the H atom in the hydroxyphenoxyl radical, which causes the kinetic interrelation between the states determining the existence of both pairs.It should be noted that multiple cycles of generation and subsequent dark decay of the pairs do not result in the consumption of the starting reagents, which suggests that only two processes occur in these and similar systems, viz., photo- 729 Table 1. The values of fine structure constants D (mT) for neutral and radical-ion (Di) pairs formed upon irradiation of the system quinone 1 ± pyrocatechol 6 with light at l=480 ± 650 nm in various solvents.Solvent D2 (B,B0) a Di D1 (C,C0) a DDi (see b) 22.5 0.5 21.1 c 0.9 14.6 14.2 25.7 24.2 ± 0.5 22.9 ± 14.3 12.5 24.9 23.1 22.0 0.9 21.5 1.6 20.9 0.9 21.4 0.7 20.3 0.7 21.3 1.2 20.5 1.4 21.0 0.6 21.7 1.1 21.0 0.5 20.5 0.8 22.4 1.2 20.9 1.3 0.9 14.2 13.4 13.9 12.4 13.1 12.3 13.1 13.2 13.4 13.9 13.8 12.1 13.3 12.6 25.5 24.5 24.7 23.5 23.5 23.2 23.8 23.3 23.0 24.5 24.1 21.5 23.8 23.3 ± 3-Methylhexane 1,1,2,2-Tetrachloro- ethane Toluene Carbon tetrachloride Pentane Hexane Heptane Octane Nonane Decane Tridecane Benzene Chlorobenzene Bromobenzene Dichloroethane m-Ethyltoluene Tetramethylsilane Decalin a D1 and D2 are dipole ± dipole coupling constants for neutral RP with the perpendicular components C,C0 and B,B0.b DDi is the change in Di after 30 min of dark annealing at 77 K. c In this case it was possible to measure E, which was found to be 0.47 mT. transfer of the hydrogen atom from the phenol to the quinone and its reverse transition in the dark process. A detailed study of the dependence of the structure of RP formed in the system quinone 1 ± 3,6-di-tert-butyl-1,2-dihydroxy- benzene (6, 3,6-QH2) in glassy frozen solutions at 77 K on the solvent nature (Table 1) and the wavelength of the incident light was carried out.13, 14 Irradiation of this system with light at l=365 ± 500 nm results in the phototransfer of a hydrogen atom with formation of three types of neutral RP,{ which differ in their fine structure parameters.The kinetic characteristics of accumulation and decay of all of these types of RP are identical due to the intramolecular migration of theHatom in the phenoxyl radical 3,6-SQH., and hence, the difference between the accumu- lation and decay curves of the RP disappears. However, irradiation of this system with light at l=480 ± 650 nm gives, in addition to the types of pairs specified above, one more type which has an intermediate D value and corresponds to a radical-ion pair formed due to the phototransfer of an electron: 3,6-Q¡.+3,6-QHá..3,6-Q+3,6-QH2 2 The kinetic stability of this type of RP is much smaller (Fig. 5) than that of neutral ones. An important feature of radical-ion pairs is the absence of an induction period in their accumulation curves; this made it possible to draw the conclusion that the formation of the neutral and radical-ion pairs occurs independently, i.e. the two processes occur in parallel.14, 15 The main argument for the suggested mechanism of the formation of neutral and radical-ion pairs is the increase in the relative photoyield of ionic RP in comparison { In solution, the quinone 1 and the pyrocatechol 6 give donor ± acceptor complexes;13 in this case, the phototransfer of an H atom formally results in four types of RP (generally, four localisation variants of twoHatoms on four O atoms are possible in each hydroxyphenoxyl radical; however, the parameters D of two of these variants are equal).730 c/c0 1.0 2 0.5 1 60 20 t /min 40 Figure 5.Kinetic curves of decay of ionic (1) and neutral (2) radical pairs in 3-methylhexane. with that of the neutral ones (Fig. 6) with an increase in the wavelength of the incident light (l). Similar results were obtained for the photoreduction of p-benzoquinone in an ethanolic solution at room temperature.16 The long-wave threshold for the gener- ation of neutral semiquinone radicals is 45010 nm, while that of radical ions is 58010 nm.16 Hence, the transfer of a hydrogen atom requires irradiation in a shorter wavelength range than that required for the electron transfer.This feature of ionic RP is apparently due to the formation of donor ± acceptor complexes in which the single-electron phototransfer is facilitated.17 [Q¡. . . . AH+. ]/[QH. . . . A. ] 3 8.0 6.0 2 4.0 1 2.0 400 300 500 600 l /nm . . . Figure 6. Dependence of the relative photoyield of radical-ion pairs [Q¡. AH+. ]/[QH. . . . A. ] in different solvents on the wavelength of the exciting light. The abscissa axis represents the value of l corresponding to the maximum of the transmission band of a set of light filters (the half- width of the transmission band was 50 nm); (1) toluene; (2) 1,1,2,2-tetrachloroethane; (3) 3-methylhexane. III. Characteristic features of photoreduction of quinones in single crystals of organic hydrogen donors Doping of single-crystalline phenolic compounds with sterically hindered quinones enabled the development of new interesting systems characterised by specific behaviour when exposed to photoirradiation.Single crystals of 3,6-QH2 doped with 3,6-Q (1072 mol litre71) were obtained 18 and the ESR spectra of radical pairs formed upon irradiation of this system (l=425 ± 475 nm) at 77 K were studied. Figure 7 shows the ESR spectra of single-crystalline 3,6-QH2 doped with 3,6-Q (Fig. 7 a) and a polycrystalline sample obtained from a melt of a b A0 C A B c A0 B 0 D1 C D01 B D A Figure 7. ESR spectra of a single crystal of 3,6-QH2 doped with 3,6-Q (1072 mol litre71) (a) and a polycrystalline sample obtained from a melt of the corresponding single crystals (b, c); (a) is the single-crystal sample 10 min after irradiation with light at a wavelength of 425 ± 475 nm; (b) is the polycrystalline sample 10 min after the irradiation; (c) is the polycrystalline sample during irradiation in the ESR spectrometer resonator.The components A and A0 can be assigned to RP with D=7.50.2 mT and rav=7.2A; B and B 0, to the perpendicular componentsD?;CandC0, to the parallel components Dk. The additional short-lived components D, D0 and D1, D01 belong to RP with E=0. The parallel components of the short-lived RP coincide with those for the stable RP. the corresponding single crystals (Fig. 7 b,c).The calculated value of parameterD=28.60.4 mTfor this system corresponds to the mean distance between the unpaired electrons, rav=4.6A. The equality between the Dk components of short-lived and stable RP suggests that their rav are approximately equal to 4.6A. The essential difference in the stability of the RP in question enables one to assign the short-lived RP to radical-ionic ones (AH+.+Q7. ) formed upon phototransfer of an electron. Stable RP (A.+SQH. ) result from the phototransfer of an H atom. As in glassy frozen solutions (see Section II), the long-wave threshold of the formation of stable RP is 46015 nm, while that of radical- ionic RP is 56015 nm. It should be noted that irradiation of a sample with light at l=475 nm for 2 h virtually does not give stable RP.Analysis of the ESR spectra of radical pairs shows that the fraction of the radical-ion pairs is 99.9%, and their subsequent dark decay occurs without conversion into stable neutral RP.15 These data confirm the conclusions on the independence of two photoreduction mechanisms of excited 3,6-Q made for glassy frozen solutions. Single crystals of 3,6-QH2 were studied by the X-ray diffrac- tion method.19 They were found to have a tetragonal structure (space group I41/a). Analysis of packing of the 3,6-QH2 molecules in the crystal indicates that they form a helix relative to a four-fold axis (41) (Fig. 8) with a rotation angle of 90 8 relative to each other. The X-ray diffraction data were compared with those of ESR spectroscopy by calculating the zero splitting parameters (D and E) for the case where one 3,6-QH2 molecule is replaced by 3,6-Q A I Prokof'ev B 0 C0 D0 C0 20 mTRadical pairs in redox reactions involving sterically hindered quinones and phenols O(2) O(1) O(10) O(20) y x Figure 8.Fragment of a projection of the unit cell of 3,6-QH2 on the xy plane. and two identical 3,6-SQH. radicals are formed upon irradiation. The values of D were calculated for various pairs of adjacent molecules using the method described in Ref. 20. The spin densities on various atoms in the 3,6-SQH radical were taken from Ref. 21 where they are given with an accuracy of 15%. The transfer of the H atom (Fig. 8) can occur through the channels O(1) ± O(10) and O(1) ± O(20).The calculated values of D are 33.55 mT for the channel O(1)7O(10) and 33.4 mT for the channel O(1)7O(20). These values are virtually identical and differ from the experimental one by 14% (Dexp=28.6 mT). Since the spin density distribution in 3,6-SQH. was estimated to the same accuracy, the agreement between the theoretical and experimental D values may be considered as satisfactory. By comparing the theoretical and experimental angular dependences D(y), where y is the angle between the axis x (Fig. 8) and the direction of the external magnetic field H (the crystal rotates around a four-fold axis), it was shown that the O(1)7O(20) channel of the transfer of the H atom is more efficient.19 The interest in the photoprocesses in single-crystal matrices is caused by the fact that single crystals of sterically hindered phenols doped with quinones have diverse crystal structures and unit cell parameters, which depend on the structure of the phenol. In turn, this determines the specific features of the photo- and dark processes of the H atom transfer in each particular case.It is known that in some redox processes, the transfer of two electrons can occur within one elementary act.22 Studies 13, 23, 24 of the transfer of H atoms in single crystals of sterically hindered phenols doped with quinones showed the possibility of synchro- nous transfer of two H atoms from the phenol single-crystal matrix to quinones. The ESR spectra that appear upon photo- irradiation (l=285 ± 470 nm) of single-crystalline 2,6-di-tert- butyl-4-methylphenol doped with 3,6-Q are given in Fig.9. Visually, the single crystals of the phenol 2 have a two-fold axis (axis z) and two faces parallel to this axis; the axis y is chosen along these faces. The spectral pattern shown in Fig. 9 a corresponds to the direction of the external magnetic field H along the axis z. Six groups of lines of the fine structure of the RP formed comprise HFS lines, which indicates unambiguously the interaction of the unpaired electrons with the protons of two equivalent methyl groups in the phenoxyl radicals 3 present in the pair. Each component of the fine structure contains seven lines with a binomial intensity distribution, aMe H =0.53 mT.Such a pattern can only be caused by the transfer of twoHatoms from two matrix molecules of the phenol 2 to the quinone 1: OH But But But 2 +3,6-Q hn 2 1 3 2 Me O. But+3,6-QH2 6 Me 731 a B B 0 A0 C A C0 bc 10 mT Figure 9. ESR spectra of a single crystal of the phenol 2 doped with 3,6-Q (1073 mol litre71) (a) (b is the forbidden signal of single-crystalline phenol 2 (DmS=2) in half-field) and polycrystalline sample (c). The isotropic forbidden signal (DmS=2) also contains seven lines (Fig. 9 b) from two equivalent methyl groups. The HFS constant in this system is half that in the original radical existing in solution (aMe H =10.7 mT), which also proves the existence of two phenoxyl radicals 3 in the triplet state.In other words, along with the dipole ± dipole interaction of two unpaired electrons giving fine structure lines, their strong isotropic exchange interaction due to overlapping of the wave functions of the unpaired electrons is observed.25 The presence of six groups of fine structure lines (A,A0, B,B0 and C,C0) in the single-crystalline phenol 2 suggests the possibility of three orientations of the three RP with respect to the external magnetic field H; on the other hand, only one type of RP with D=15.80.3 mT (rav=5.4A) is observed in the polycrystalline matrix of this sample (Fig. 9 c). The spectral pattern is completely the same in the system phenol 2 ± 3,5-di-tert-butyl-1,2-benzoquinone (3,5-Q).Thus, one can state that the transfer of two H atoms from two molecules of the phenol 2 to the quinone 3,5-Q accompanied by the formation of two phenoxyl radicals 3 existing as a pair also occurs in this case. The photoirradiation of the same single-crystalline phenol matrix doped with 2,6-di-tert-butyl-1,4-benzoquinone (7, 2,6-Q) results in a spectral pattern where the fine structure components of the three types of RP contain HFS from only one methyl group (a quadruplet with a 1 : 3 : 3 : 1 intensity ratio, aMe H =0.53 mT) (Fig. 10). This indicates unambiguously that the transfer of only one H atom occurs. 10 mT Figure 10. ESR spectrum of irradiated single crystal of the phenol 2 doped with 2,6-di-tert-butyl-1,4-benzoquinone.732 O OH But But But But hn + O Me 7 2 O.O. But But But But + OH 8 Me 3 It should be noted that the D parameter (13.4 mT) in this polycrystalline sample is noticeably smaller than that in the sample doped with 3,6-Q; this suggests that the relative arrange- ment of molecules of the corresponding quinones and the phenol 2 in these systems is different in principle. The question arises as to the mechanism of phototransfer of two H atoms in the system phenol 2 ± 3,6-Q. It was shown that the RP accumulation in this system is proportional to the radiation intensity, i.e., the transfer of two H atoms is not a two-quantum process. In addition, the long-wave threshold frequency of the RP formation in the single-crystalline phenol 2 doped with 3,6-Q is in the range from 290 to 330 nm, while in glassy frozen solutions where the transfer of one H atom occurs the long-wave threshold frequency of the RP formation is 560 ± 630 nm; this indicates that a much smaller energy (by a factor of 1.7 ± 2.2) is required for the phototransfer of one H atom.Let us note that the long-wave threshold frequency of the RP formation in the system single crystal of the phenol 2 ± 2,6-Q where phototransfer of one Hatom occurs is noticeably higher: l=390 ± 445 nm. These data alto- gether support the one-quantum mechanism of phototransfer of two H atoms in the system single crystal of phenol 2 ± 3,6-Q (or 3,5-Q). Nevertheless, the principal proof of a one-quantum process is that the RP yields, or, in other words, the efficiencies of phototransfer of one and two H atoms in all three systems (phenol 2 ± 3,6-Q, phenol 2 ± 3,5-Q and phenol 2 ± 2,6-Q) are all almost the same.In the case of the two-quantum process, the efficiency of phototransfer of two H atoms would be much lower. It was found in a study of the kinetics of RP decay in all the three systems that it is exponential and is the same in the systems phenol 2 ± 3,6-Q and phenol 2 ± 3,5-Q (k=104 s71, Ea= 23.8 kJ mol71) but differs strongly from the kinetics of RP decay in the system phenol 2 ± 2,6-Q (k=108 s71, Ea= 22.8 kJ mol71). Apparently, the decay of RP occurs due to the reversible dark transfer of H atoms. Thus, is has been shown that the phototransfer of two H atoms requires much higher excitation energy and that this act occurs due to the absorption of one quantum of light.IV. Other quinoid systems in hydrogen atom transfer reactions in single-crystalline phenols The interest in the use of 2,6-di-tert-butyl-1,4-quinonediazide (2,6- QN) as a potential acceptor of anHatom from phenolic hydrogen donors is due to the fact that the photoirradiation of 2,6-QN results in its dissociation into a carbene 9 (2,6-SQN) and molecular nitrogen.26 ± 28 O O. O But But But But But But hn +N2 . 9 Figure 11 a shows a part of the ESR spectrum of the carbene 9 in single-crystalline phenol 2 doped with 2,6-QN upon irradiation : N2 a 460 450 b 5 mT c 2 1 5 mT 2 0 Figure 11.ESR spectra of the carbene 9 (a), the radical pair appearing upon irradiation of a single crystal of the phenol 2 doped with 2,6-QN (77 K) (b) and a polycrystalline sample obtained from a melt of single crystals of the phenol 2 and 2,6-QN (c); (1, 1 0) are perpendicular components of RP1; (2, 2 0) are perpendicular components of RP2. with light at a wavelength of l=53020 nm at 77 K. In this case, two triplets (1 : 2 : 1) are observed which correspond to the interaction of unpaired electrons with benzene ring protons.} This spectrum corresponds to the high-field transition DmS=1. Depending on the sample rotation angle, four sets of triplets, which reflect four orientations of the 2,6-QN molecules in a single crystal, are observed.27 Depending on the conditions, it is possible to observe two types of radical pairs in the system phenol 2 ± 2,6-QN, viz., RP1 (D=13.2 mT, rav=6.0A) and RP2 (D=3.9 mT, rav=8.9A) (Fig.11 b,c). The fine structure param- eters were determined from the spectra of polycrystalline samples. The radical pairs RP1 and RP2 appear upon irradiation with light atl4420 nm andl4350 nm, respectively. In the short-term (30 s) irradiation at 77 K, only the spectrum of RP2 and the carbene is observed. After a period of three days, the signal of the carbene disappears, but the intensity of the signal of RP2 (D=3.9 mT) increases. The rate of RP1 formation at the beginning of photolysis with light at l 5310 nm at 77 K is practically zero and its increase is observed after some time; this is apparently due to the transfer of anHatom from the phenol 2 to the carbene 9 with the formation of two phenoxyl radicals, viz., 3 and 10.O: OH But But But + 2 9 Me } The magnitude of D for the system phenol 2 ± 2,6-Q is comparable with the magnetic field strength in the ESR spectrometer. A I Prokof'ev 470 H /mT 1 0 But l4420 nmRadical pairs in redox reactions involving sterically hindered quinones and phenols O. O. But But But But + 10 3 Me RP1 The characteristic time of RP1 accumulation (D=13.2 mT) is 30 min at 110 K. As the temperature is increased to 300 K, the RP2 species disappear and only the RP1 species remain. These data suggest that there are two different processes of the H atom phototransfer.Specifically, these include the phototrans- fer itself (the formation of RP2) resulting in the formation of a `primary' pair, OH O But But But But l4350 nm + 2 N2 Me O. OH But But But But + , . 3 Me N2 RP2 and the reaction of the carbene with the matrix (the formation of RP1). The eight groups of lines containing a multiplet (1 : 4 : 6 : 4 : 1) correspond to different orientations of RP1 relative to the external magnetic field H in the single crystal. The relative intensity of the lines corresponding to each type of pairs depends on the photo- reaction efficiency, i.e. on the orientation of the 2,6-QN molecules relative to the direction of the incident light. The photoselection effect in the photolysis of 2,6-QN has been studied in detail.28 The line positions vary as the crystal is rotated relative to the external magnetic field, but the hyperfine coupling (HFC) remains unchanged and corresponds to the interaction of the unpaired electrons of the carbene 9 with four equivalent protons (aH=0.6 mT).The hyperfine structure (multiplet) orig- inates from the interaction of the unpaired electrons of RP1 with three protons of the methyl group in the radical 3 and the H atom at the para position of 2,6-di-tert-butylphenoxyl (radical 10). This conclusion was confirmed by a study of photoreactions that occur in the matrix of the phenol 2 with a deuterated OH group. In this case the multiplet is transformed into a quadruplet due to the HFC only with the hydrogen atoms of the methyl group.28 The particular channels of the H atom phototransfer were determined from X-ray diffraction data 24 on the phenol 2 crystal structure and with consideration of the spin density distribution in the phenoxyl radicals formed. The experimental results obtained allow one to assume a different mechanism of the RP1 and RP2 formation as well.The radical pair RP2 (the `primary' RP) eliminates N2 to give the `secondary' RP1; the difference between the rav of the unpaired electrons is due to the change in the spin density distribution in the radical partners of the phenoxyl radical 3 and the relative change in the distances between the radicals in the corresponding pairs. The formation of symmetric RP was observed in the photo- lysis of single-crystalline 2,6-di-tert-butylphenol (11) doped with 2,6-QN.29 Two types of RP were detected in the ESR spectra of this system due to different orientations of the RP in the crystal.Owing to the specific features of the 2,6-di-tert-butylphenol crystal lattice, the phototransfer of an H atom can involve two possible different RP orientations relative to the external magnetic field. The small magnitude of the fine structure parameter D (4.3 mT) in this case suggests a large distance between the radical centres. X-Ray diffraction data and calculations of the D and E hyperfine structure parameters and data on the spin density distribution in phenoxyl radicals suggest that the RP are formed in this case through a two-step (relay) process.30, 31 OH O But But But But + 11 9 OH O.But But But + . 12 OH 10 O. But But But+ 10 11 The carbene 9 formed in the photolysis abstracts the para- hydrogen atom from the nearest 2,6-di-tert-butylphenol molecule to give a phenoxyl radical (10) and a phenyl radical (12). In turn, the phenyl radical abstracts an H atom from the nearest 2,6-di- tert-butylphenol molecule. In this manner, an RP is formed from rather remote phenoxyl radicals. Experiments with OH-deuter- ated single crystals confirmed the formation of such RP with identical hyperfine structures, which proves the validity of the suggested relay scheme of the H atom transfer. The ESR spectra of the radical pairs formed upon photolysis of single-crystalline phenol 2 doped with 2,6-di-tert-butylimino- quinone (l=350 nm) and those of the corresponding polycrys- talline sample are given in Fig.12. The spectrum of the polycrystal (Fig. 12 b) implies the formation of only one type of RP (D=12.20.2 mT). The observed combination of the fine struc- ture lines suggests different orientation of radical pairs relative to the external magnetic field due to the crystal structure of the phenol 2.28 The nature of the radicals that form RP in a single crystal of the phenol 2 doped with 2,6-di-tert-butylquiononeimine was established using computer modelling based on the set of HFS a A 345 340 335 330 : 325 b C B B 0 10 mT C0 Figure 12. ESR spectra of the radical pairs formed upon photolysis of a single crystal of the phenol 2 doped with 2,6-di-tert-butyliminoquinone (a) and the corresponding polycrystalline sample (b).733 OH But But But O. But But But + 10 A0 H /mT 350734 lines (Fig. 12 a).30, 31 It was found that the HFS is due to the interaction between the unpaired electrons and four equivalent protons, which implies the phototransfer of the H atom and formation of the corresponding phenoxyl and aminyl radicals. NH OH But But But But hn + 2 O Me O. N .H But But But But + 3 OH Me The multiplet is formed by three protons of the methyl group and the proton of the aminyl radical. The long-wave threshold frequency of the RP formation in this system is 43010 nm, which corresponds to the quinoneimine absorption band. The interpre- tation of the RP spectra was confirmed completely by experiments with single crystals of deuterated phenol 2.30 The exponential character and an unexpectedly large kinetic isotope effect (kH/kD=103) of the dark RP decay allow one to assume the tunnel character of the H atom reverse transfer.32 The redox properties of 2,4,6,8-tetrakis-tert-butylphenoxazin- 1-one (13) are rather similar to those of sterically hindered o-quinones.The phenoxazinyl radical 14 formed in the reduction of phenoxazinone 13 manifests high stability and very character- istic ESR spectrum with a hyperfine coupling constant aN=0.9 mT.33 O O. But N But But But HN [H] O O But But But But 13 14 The high spin density on the nitrogen nucleus makes it possible to obtain additional information on the structure of paramagnetic complexes incorporating this ligand and on the structural changes in RP formed upon phototransfer of the hydrogen atom in single crystals of 3,5-QH2 doped with the phenoxazinone 13.34 Figure 13 presents the optical absorption spectra of 3,5-QH2 and the phenoxazinone 13.The ESR spectra of single-crystalline 3,5-QH2 doped with the phenoxazinone 13 contain only a combination of lines of the original pyrocatechol and phenoxazi- none, i.e., no additional bands are present. On irradiation of single-crystalline phenol 2, RP of various types appear in the absorption band of the phenoxazinone (Fig. 14 a, b).Analysis of the spectral pattern obtained suggests I 2 1 0.50 20 30 40 103 n /cm71 Figure 13. Optical absorption spectra of 3,5-QH2 (1) and phenoxazinone 13 (2). A I Prokof'ev a 1,210,20 3 4 30 40 5 M 50 6 60 b 50 5 4 1,2M 3 6 60 30 40 10,20 5 mT c 4 3 5a 5b 50b 30 50a 40 Figure 14. ESR spectra of the radical pairs formed upon photolysis of a single-crystalline 3,5-QH2 doped with the phenoxazinone 13 (a, b) and polycrystalline sample (c). Microwave power /mW: (a) 1, (b) 50. that there are six types of RP with different spectral characteristics (D) and hence with different distances between the radical centres. The radical pairsRP1±RP4 appear upon irradiation with light at l 460020 nm, RP5 at l 452020 nm and RP6 at 4504l4550 nm.The observed RP have the following dipole ± dipole coupling constants: D1=D2=1.6, D3=8.0, D4=13.0, D5=25.0 and D6=0.7 mT. The values of D3, D4 and D5 were determined from the spectra of polycrystalline samples (Fig. 14 c), and those of D1 and D2, from studies of the dependence of these parameters on the angle relative to the external field. Studies of the dark decay of RP1±RP6 at 77 K showed that the kinetic curves are satisfactorily linearised in the I/I0 ± log(t/t0) coordinates,34 which indicates that there is a set of different rates of reverse transfer of the H atom from the radical 14 to the 3,5- SQH. radical. This situation is typical of processes occurring in solids and is determined by the structural features of the RP formed in this system.In the case under consideration, the distribution of the RP decay rates can be determined by the small changes (*0.1A) in the position of the phenoxazinone 13 in the crystal lattice of 3,5-QH2. Moreover, the presence of six types of RP and, hence, the difference in their kinetic behaviour can be due to different arrangement of hydrogen atoms at the oxygen atoms of the 3,5-SQH. radical (non-equivalent shielding of the oxygen atoms by the tert-butyl groups) and in the radical 14. O But N But OH But hn + O OH But But But 13 3,5-QH2 O. But But But NH O. + O OH But But But 14 3,5-SQH.Radical pairs in redox reactions involving sterically hindered quinones and phenols Photolysis of the system 3,5-QH2 ± phenoxazinone 13 in the region from 450 to 550 nm at 77 K gives rise to RP6 with additional HFS caused by the nitrogen nucleus of the radical 14 (Fig.14); the intensity of the signal is small under these con- ditions. The existence of reversible temperature evolution of RP3 and RP6 in the temperature range from 20 to 77 K has also been shown.34 Only the ESR spectrum of RP3 (Fig. 15 a) was observed during irradiation of a single crystal at 55 K. The spectrum of RP6 was observed when the irradiation was terminated and the temperature was decreased to 20 K (Fig. 15 b); the components of this spectrum contain HFS from the 14N nucleus (aN=0.9 mT). This temperature-reversible transformation is caused by spin density redistribution in the radical 14 as a result of H atom intramolecular transfer between the nitrogen and oxygen atoms (Fig.16). O. But But NH O. + O OH But But RP3 But But O. + OH But RP6 a 3 30 b 6 60 5 mT Figure 15. ESR spectra of the radical pairs RP3 and RP6 at 55 K (a) and 20 K (b).H 55 K 20 K N O Figure 16. Potential energy curve for the migration of anH atom between the oxygen and nitrogen atoms in the radical 14. V. Anomalous relaxation in the ESR spectra of radical pairs The ESR spectra of radical pairs sometimes show anomalies which can provide additional information on the interaction of the spin centres. For example, anomalies have been reported 35 such as changes in the widths of the fine structure lines for the RP formed upon the hydrogen atom phototransfer in single-crystalline 3,5-QH2 doped with 3,5-Q (system A) and in single-crystalline 3,30,5,50-tetrakis- tert-butyl-6,60-dihydroxydiphenylmethane doped with 2,6-Q (system B).Figure 17 shows the ESR spectra of the system A obtained upon irradiation with light at l>720 nm. It can be seen that at low power of the microwave field (0.02 mW), the fine structure components have identical intensities, while at high power (2 mW), the high-field component has a considerably higher intensity implying shorter relaxation time. But a 0.02 mW But B 0 B OH But N . 2 mW O 5 mT But But E b T+ T0 J ST7 H Figure 17.ESR spectra (a) and diagram of magnetic levels of the radical pair energy (b) for the system 3,5-QH2 ± 3,5-Q. The times of spin ± lattice (T1) and spin ± spin (T2) relaxations for the low- and high-field components were determined from the saturation curves using the technique described in Ref. 36 and based on a model of non-interacting Bloch packets: T1=5.761074, T2=6.661077 s (low-field component); T1=1.261074, T2=2.861077 s (high-field component). The system B was found to behave similarly. In the systems studied, the ESR spectra at low power of the microwave field are centrosymmetric at any orientations of single crystals relative to the external magnetic field. Therefore, a mechanism of interaction of triplet levels was suggested 39 which dictated nonsymmetrical relaxation of the fine structure compo- nents but did not result in their asymmetry relative to the centre of the spectrum.Additional relaxation channels according to the scheme shown in Fig. 17 b were considered: T0?S?T7 (high-field component)7channel 1; T+?S?T0 (low-field component)7channel 2. For these channels, the distances between the levels S ± T7 and S ± T+ are different. In the cases J gbHor J gbH(J is the distance between the T0 and S levels), this difference is insignif- icant, and the rates of relaxation through the channels 1 and 2 are 735736 the same. However, if the magnitudes of J and gbH are compa- rable (Fig. 17), differences in the relaxation S?T7 and T+?S can be expected and, hence, the rates of relaxation through the channels 1 and 2 should differ.The rates of the spin ± lattice relaxation can be represented as: T 01ÖBÜ á t11, 1ÖBÜ à T 1 T T0Ö1B0Ü á t12 , 1ÖB 0Ü à 1 where T0(B) and T0(B0) are the times of spin ± lattice relaxations through the channels 1 and 2, respectively; t1 and t2 are the times of relaxation through the channels 1 and 2, respectively. The difference between 1/t1 and 1/t2 can explain the effect of abnormal asymmetrical relaxation provided that 1/T0(B)41/t1 and 1/T0(B0)41/t2, i.e., where the relaxation channels 1 and 2 under consideration contribute significantly to the relaxation rate. Thus, the asymmetrical relaxation effect can take place if the singlet ± triplet coupling is comparable with the Zeeman coupling, but is still different from the latter.These results can be useful in studies of the effect of a magnetic field on the rates of chemical reactions involving radical pairs as transition states.37 VI. Conclusion The processes of solid-state hydrogen and electron transfer under photoirradiation considered above provide unique information on the nature of the elementary act in organic matrices which represent potential hydrogen donors. The advantage of studying solid-state systems, particularly single crystals, is that it provided the possibility of ESR in-depth studies of the structural features and kinetic behaviour of radicals which exist as radical pairs as a function of the features of the matrix and its crystal structure.It was found that solid-state transfer of a hydrogen atom and an electron is possible not only under photoirradiation but also under elastic-wave impact on mechanical mixtures of the corre- sponding pyrocatechols and quinones or on the same mixtures in the presence of polymeric materials.38, 39 The results summarised in this work were obtained with direct participation of Professor Ya S Lebedev, whose top professional- ism, authority, and sharp scientific intuition much favoured the development of chemical physics of solid-state processes in general and of the hydrogen atom and electron transfer in particular. References 1. E R Milaeva, Doctoral Thesis in Chemical Sciences, Moscow State University, Moscow, 1997 2.G A Abakumov, in Metalloorganicheskie Soedineniya i Radikaly (Organometallic Compounds and Radicals) (Ed.M I Kabachnik) (Moscow: Nauka, 1985) p. 8 3. N N Bubnov, S P Solodovnikov, A I Prokof'ev, M I Kabachnik Usp. Khim. 47 1048 (1978) [Russ. Chem. Rev. 47 549 (1978)] 4. R R Rakhimov, A I Prokof'ev, Ya S Lebedev Usp. Khim. 62 547 (1993) [Russ. Chem. Rev. 62 509 (1993)] 5. A I Terenin Fotonika Molekul Krasitelei i Rodstvennykh Organiches- kikh Soedinenii (Photonics of Molecules of Dyes and Related Organic Compounds) (Leningrad: Nauka, 1967) p. 345 6. J A Howard Adv. Free Radical Chem. 4 49 (1971) 7. J C Scaiano J. Photochem. 2 81 (1973/74) 8. N K Bridge, D Porter Proc. R. Soc. Chem. A244 259; 276 (1958) 9.N N Bubnov, A I Prokof'ev, A A Volod'kin, I S Belostotskaya, V V Ershov Dokl. Akad. Nauk SSSR 210 100 (1973) a 10. A Carrington, A D McLachlan Introduction to Magnetic Resonance (New York, London: Evanston, 1967) 11. A I Aleksandrov, N N Bubnov, G G Lazarev, Ya S Lebedev, A I Prokof'ev,M V Serdobov Izv. Akad. Nauk SSSR, Ser. Khim. 515 (1976) b A I Prokof'ev 12. A I Prokof'ev, N N Bubnov, S P Solodovnikov, M I Kabachnik Tetrahedron Lett. 2479 (1973) 13. G G Lazarev, Ya S Lebedev,M V Serdobov Izv. Akad. Nauk SSSR, Ser. Khim. 2520 (1978) b 14. G G Lazarev, Ya S Lebedev, A I Prokof'ev, R R Rakhimov Khim. Fiz. 3 867 (1984) c 15. G G Lazarev, Ya S Lebedev, A I Prokof'ev, R R Rakhimov Chem. Phys. Lett. 95 262 (1983) 16. H Yoshida, K Hayashi, T Warashina Bull.Chem. Soc. Jpn. 45 3515 (1972) 17. B L Tumanskii, A I Prokof'ev, N N Bubnov, S P Solodovnikov, A A Khodak Izv. Akad. Nauk SSSR, Ser. Khim. 268 (1983) b 18. G G Lazarev, Ya S Lebedev, A I Prokof'ev, R R Rakhimov Khim. Fiz. 809 (1982) c 19. G G Aleksandrov, V G Andrianov, G G Lazarev, Ya S Lebedev, A I Prokof'ev, Yu T Struchkov Khim. Fiz. 3 960 (1984) c 20. S N Dobryakov, G G Lazarev,M V Serdobov, Ya S Lebedev Mol. Phys. 36 877 (1978) 21. A I Prokof'ev, S P Solodovnikov, N N Bubnov, A S Masalimov Izv. Akad. Nauk SSSR, Ser. Khim. 2488 (1976) b 22. Yu N Kozlov, A P Purmal', A M Uskov Zh. Fiz. Khim. 54 1745 (1980) d 23. G G Lazarev, Ya S Lebedev, A I Prokof'ev, R R Rakhimov Khim. Fiz. 525 (1983) c 24. G G Lazarev Z. Phys. Chem. 173 141 (1991) 25. V I Parmon, A I Kokorin, G M Zhidomirov Stabil'nye Biradikaly (Stable Biradicals) (Moscow: Nauka, 1980) p. 73 26. G G Lazarev, V L Kuskov, Ya S Lebedev Chem. Phys. Lett. 170 94 (1990) 27. V V Ershov, G A Nikiforov, J De Jonge Quinonediazides (Amsterdam: Elsevier, 1981) p. 350 28. G G Lazarev, V L Kuskov, Ya S Lebedev, A Rieker Chem. Phys. Lett. 181 512 (1991) 29. G G Lazarev, V L Kuskov, Ya S Lebedev,W Hiller, M Kretschmar, A Rieker Z. Phys. Chem. 176 41 (1992) 30. G G Lazarev, F Lara, F Garcia, A Rieker Chem. Phys. Lett. 199 29 (1992) 31. GMZhidomirov, Ya S Lebedev, S N Dobryakov, N Ya Shteinshneider, A K Chirkov, V A Gubanov Interpretatsiya Slozhnykh Spektrov EPR (Interpretation of Complex EPR Spectra) (Moscow: Nauka, 1975) 32. E D Spraque, K Takeda, J T Wang, F Williams Can. J. Chem. 52 2840 (1974) 33. I V Karsanov, Ye P Ivakhnenko, V S Khandkarova, A I Prokof'ev,A Z Rubezhov,M I Kabachnik J. Organomet. Chem. 34. G G Lazarev, V L Kuskov, Ya S Lebedev, A I Prokof'ev, 35. R R Rakhimov, G G Lazarev, A I Prokof'ev, Ya S Lebedev Khim. 36. Ya S Lebedev, V I Muromtsev EPR i Relaksatsiya Stabilizirovan- 379 1 (1989) A Rieker Chem. Phys. Lett. 185 375 (1991) Fiz. 5 1085 (1986) c nykh Radikalov (EPR and Relaxation of Stabilised Radicals) (Moscow: Khimiya, 1972) 37. A L Buchachenko, in Fizicheskaya Khimiya. Sovremennye Problemy (Physical Chemistry. Modern Problems) (Ed. YaMKolotyrkin) (Moscow: Khimiya, 1980) p. 7 38. A I Aleksandrov, A I Prokof'ev, I Yu Metlenkova, N N Bubnov, D S Tipikin,G D Perekhodtsev, Ya S Lebedev Zh. Fiz. Khim. 69 739 (1995) d 39. A I Aleksandrov, V P Zhukov, A I Prokof'ev, N N Bubnov, G D Perekhodtsev, Ya S Lebedev Izv. Akad. Nauk, Ser. Khim. 1192 (1996) b a�Dokl. Chem. Technol., Dokl. Chem. (Engl. Transl.) b�Russ. Chem. Bull. (Engl. Transl.) c�Russ. J. Chem. Phys. (Engl. Transl.) d�Russ. J. Phys. Chem. (Engl.

ISSN:0036-021X    

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Russian Chemical Reviews 68 (9) 737 ± 763 (1999) Cyanoacetamides and their thio- and selenocarbonyl analogues as promising reagents for fine organic synthesis V P Litvinov Contents I. Introduction II. Synthesis of cyanoacetamides, their thio- and selenocarbonyl analogues and aryl- and hetarylmethylidene derivatives III. The use of cyanoacetamides and their thio- and selenocarbonyl analogues in the synthesis of heterocyclic compounds IV. Conclusion Abstract. Published data on the use of cyanoacetamides, cyano- thio- and -selenoacetamides in fine organic synthesis and the prospects of application of these compounds in combinatorial synthesis are analysed and generalised. The bibliography includes 653 references. I. Introduction Cyanoacetic acid derivatives are important intermediates for preparation of various organic and, especially, heterocyclic compounds possessing diverse biological activities and many other practically useful properties.In this group of compounds, cyanoacetamides and their thio- and selenocarbonyl analogues are of particular interest as very promising reagents for cascade heterocyclisation, which will undoubtedly become one of the main approaches to the targeted synthesis of heterocycles in the near future, and for combinatorial chemistry experiencing nowadays a sharp rise (see, for example, Refs 1 ± 26). This new methodology based on automatic, high-tech synthetic methods enables syn- thesis of a large number (up to several thousand) of novel organic compounds as subjects for biological screening.The present review systematises for the first time published data on the reactivity of cyanoacetamides and their thio- and selenocarbonyl analogues (for selected aspects of the chemistry of these compounds, see reviews 27 ± 34). The presence of several reactive centres in the molecules of these reagents provides ample opportunities to synthesise a great variety of novel compounds under relatively mild conditions and using rather simple labora- tory equipment. Syntheses of many polyfunctional heterocyclic compounds, including those prepared by multicomponent con- densations, are based on cyanoacetamides and their S- and Se- containing analogues. The reactions involving these reagents occur, as a rule, with high regioselectivity (often, stereoselec- tively) and their course can easily be controlled by changing reaction conditions and varying substituents in the molecules of initial compounds.All this makes cyanoacetamides and their S- and Se-analogues very useful reagents for combinatorial synthesis aimed at the design of novel biologically active compounds with a targeted mode of action. V P Litvinov N D Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp. 47, 117913 Moscow, Russian Federation. Fax (7-095) 135 53 28. Tel. (7-095) 135 88 37. E-mail: vpl@carc.ioc.ac.ru Received 13 May 1999 Uspekhi Khimii 68 (9) 817 ± 844 (1999); translated by S V Chapyshev #1999 Russian Academy of Sciences and Turpion Ltd UDC 547.7/8 737 737 738 755 The present review aims to demonstrate the synthetic poten- tial of cyanoacetamides in the synthesis of novel heterocyclic compounds. Primary attention is paid to the methods for preparation of the final products and much less to mechanistic aspects of the reactions surveyed. The structures of the feasible intermediates are considered only in the most synthetically important reactions.II. Synthesis of cyanoacetamides, their thio- and selenocarbonyl analogues and aryl- and hetarylmethylidene derivatives Cyanoacetamide (1a) can readily be obtained by several methods, viz., using the reactions of cyanoacetates (2) with ammonia,35 of cyanoacetic acid (3) with NH4OH36 and of chloroacetamide (4) with NaCN.37 EtOH BuOH TsOH NCCH2COOR+NH3 2 NCCH2COOH+NH4OH 3 NCCH2C(O)NH2 1a ClCH2C(O)NH2+NaCN 4 There are several methods for the preparation of cyanothio- acetamide (1b), viz., treatment of an ethanolic solution of malononitrile with H2S in the presence of triethylamine 38 ± 41 or triethanolamine,42, 43 the reaction of cyanoacetamide (1a) with phosphorus pentasulfide in pyridine or ethyl acetate 44 ± 46 (the yield in the latter reaction does not exceed 36%), and the reaction of 1-ethoxyethylidenemalononitrile (6) with H2S in ethanol.39 B CH2(CN)2+H2S 5 NCCH2C(S)NH2 1b NCCH2C(O)NH2+P2S5 1a EtO(Me)C C(CN)2+H2S 6 B=Et3N, (HOCH2CH2)3N.Cyanoselenoacetamide (1c) was synthesised for the first time in 1985 by the reaction of malononitrile with H2Se in diethyl ether in the presence of triethylamine at 5 8C under argon in 75% yield.47738 Et3N NCCH2C(Se)NH2 1c CH2(CN)2+H2Se 5 Aryl- and hetarylmethylidene cyanoacetamides 7 are also promising reagents for the synthesis of functionally substituted heterocycles.These compounds are obtained by the reactions of aromatic or heteroaromatic aldehydes 8 with amides 1a ± c in the presence of organic bases, under argon in the case of cyanosele- noacetamide (1c).39, 44, 45, 48 ± 56 Thioamides 7b can also be pre- pared by the reactions of arylmethylidenemalononitriles 9 with H2S.48 In some cases, amides 7b (R=Ph, 4-FC6H4, 4-ClC6H4, 4-BrC6H4, 4-NO2C6H4) readily undergo spontaneous [2+4]- cycloaddition to give 3,4-dihydro-2H-thiopyrans 10.48 RCHO +NCCH2C(X)NH2 8 C(X)NH2 1a ± c B CN RCH 7a ± c CN RCH + H2S CN 9 R=Ar, Het; X=O (a), S (b), Se (c).C6H4Y-4 NC C(S)NH2 CN 4-YC6H4CH H2N(S)C CN R NH2 S 10 Y=H, F, Cl, Br, NO2. 1-[(4-Alkoxycarbonylphenyl)pyrrolidin-3-ylidene]cyanothio- acetamide (11) has been obtained by the reaction of pyrrolidone 12 with cyanothioacetamide (1b). The reaction of the same reactants in the presence of b-alanine with azeotropic distillation of water from the reaction mixture results in tetrahydrothieno- [2,3-b]pyrrole 13.57 NC C(S)NH2 1b B N O C6H4COOAlk-4 11 CN N NH2 C6H4COOAlk-4 1b S 12 H2N(CH2)2COOH NC6H4COOAlk-4 13 Cycloalkylidenecyanothioacetamides 14 were synthesised in a similar way.58 NC O C(S)NH2 1b (H2C)n (H2C)n B 14 n=1, 2.Some other reactions involving the methylene group of cyanothioacetamide (1b) are known. Thus the condensation of this amide with nitroso-arenes affords the corresponding arylimi- nocyanothioacetamides 1533 and the reaction with aryldiazonium salts affords compounds 16.59 The latter were used in the synthesis of thiazolidin-3-one and pyrimidine-2-thione derivatives.59 C(S)NH2 ArNO ArN CN 15 NCCH2C(S)NH2 + C(S)NH2 1b ArN2Cl7 ArNHN CN 16 V P Litvinov Cyanoacetamide 1b adds unsaturated compounds. Thus its reaction with 3b-acetoxy pregna-5,16-dien-20-one (17) in the presence of sodium ethoxide involves the double bond of the ring D yielding (3b-hydroxy-20-oxopregn-5-en-16a-yl)cyanothio- acetamide (18).60 ± 62 Ac 1b EtONa, D 17 RO Ac CH(CN)C(S)NH2 RO 18 R=H, Ac.Arylmethylidenecyanothioacetamides 7b are alkylated with methyl iodide to give the corresponding methyl thioimidates 19, which can be used in the synthesis of heterocyclic systems.63, 64 CN CN MeI ArCH ArCH EtONa NH C(SMe) C(S)NH2 19 7b III. The use of cyanoacetamides and their thio- and selenocarbonyl analogues in the synthesis of heterocyclic compounds Owing to the presence of several highly reactive centres, first of all, the methylene and carbonyl or thio(seleno)carbonyl groups, cyanoacetamides have found wide use in the synthesis of diverse five- and six-membered heterocycles and their annelated ana- logues.All reactions in this Section are systematised in accord with the size of the heterocycle, while each Subsection deals with a certain type of the reacting substrates. 1. Synthesis of five-membered heterocycles a. Five-membered heterocycles with one heteroatom in the ring Reactions of the a-mercapto carbonyl compounds 20 with nitriles that contain an active methylene group, including cyanoacet- amide (1a), in the presence of bases at room temperature afford 2-aminothiophenes 21 in good yields (the Gewald reaction).65 ± 74 The first stage of these reactions presumably involves the condensation of a nitrile with the carbonyl group of compounds 20 (the Knoevenagel-like reaction) and formation of 4-mercapto nitriles (path a) the structures of which are suitable for intra- molecular cyclisation. However, two other possible mechanisms involving the initial nucleophilic attack on the cyano group by thiolate (path b) and the concerted cyclocondensation (path c) cannot also be excluded from consideration.Z R2 H Nu O R2 a CHZ C R1 SH R1 SH CN N 20 Nu H O R2 O R2 CH2Z b Z C S R1 S R1 N H Nu O R2 c NHHCHZ C SH R1 NCyanoacetamides and their thio- and selenocarbonyl analogues as promising reagents for fine organic synthesis Z R2 Z R2 R1 NH R1 NH2 S S 21 R1, R2=H, Alk, Ar; Z=CN, CO2R3, CONH2, COAr, 4-NO2C6H4SO2R4; R3, R4=H, Alk, Ar. The use of dimeric 2,5-dihydroxy-1,4-dithianes 73 instead of the a-mercapto carbonyl compounds 20 in the reactions with amides 1a,b and other CH acids essentially simplifies the synthesis of substituted 2-aminothiophenes 21.A yet more efficient version of the Gewald reaction consists in the use of a mixture of a carbonyl compound 22 (aldehyde, ketone or b-dicarbonyl compound) and sulfur in the presence of an organic base in ethanol or DMF instead of the a-mercapto carbonyl compounds 20. This method results in higher yields of 2-aminothiophenes 21.66, 68, 69, 73 ± 77 B 21 R1CH2COR2+NCCH2Z+S8 22 Treatment of cyanothioacetamide (1b) or its N-aryl deriva- tives with sulfur in the presence of bases affords 2,5-diaminothio- phenes 23 78 or 24.79 NC C(S)NH2 R=H NH2 H2N S8, B S 23 NCCH2C(S)NHR NC C(S)NHAr R=Ar ArHN NH2 S 24 Heating of solutions of equimolar amounts of 2-cyano- methylbenzothiazole (25a) or 2-cyanomethyl-1H-benzoimida- zole (25b) and sulfur in dry DMF in the presence of a base affords 3,5-diamino-4-[benzothiazol(or imidazol)-2-yl]-2-cyano- thiophenes 26a,b in 61%± 64% yields.80 H2N CN N N 1a, S8 CH2CN Et3N, DMF S X X 25a,b H2N 26a,b X = S (a), NH (b).Treatment of a mixture of cyanothioacetamide (1b) and cyclic ketones 27 with sulfur in the presence of a base gives rise to thieno[2,3-d]pyrimidine-4-thiones 28. Compounds 29 are assumed to be the intermediates of this reaction.78 C(S)NH2 O 27 1b, S8 (H2C)n (H2C)n NH2 B 27 S29 S NH (H2C)n S (CH2)n NH 28 n=2, 3. A series of thiophene derivatives 30 ± 32 has been obtained by the reactions of cyanothioacetamide (1b) with ethyl 3-chloro-2,4- dioxopentanoate (33)81 or ethyl bromopyruvate (34).82 In ethanol in the presence of a base, the reaction of the amide 1b with compound 33 involves its acetyl group, whereas the reaction of the same compounds in a mixture of pyridine and acetic acid involves 739 the a-carbonyl group of compound 33 and results in ethyl 5-acetyl-2-amino-3-cyanothiophene-4-carboxylate (31).81 Me CN EtOH, D EtO2C(O)C NH2 1b S 30 MeCOCH(Cl)COCO2Et 33 CN EtO2C Py, AcOH Ac NH2 S 31 The course of the reaction of the amide 1b with ethyl bromopyruvate (34) also depends on the solvent.Thus the reaction of these compounds in a mixture of pyridine and acetic acid results in ethyl 2-amino-3-cyanothiophene-4-carboxylate (32), whereas ethyl 2-cyanomethylthiazole-4-carboxylate (35) is formed in DMF.81 CN EtO2C 1b, B, EtOH NH2 S 32 BrCH2COCO2Et 34 EtO2C N 1b, DMF D CH2CN S 35 The use of pyridinium ylides in reactions with nitriles containing the activated methylene group has led to the creation, in recent years, of methods of regio- and stereoselective synthesis of partially hydrogenated five-membered heterocycles.83 ± 91 The course of the reactions of pyridinium ylides 36 (prepared in situ by treatment of the corresponding pyridinium salts with an equimolar amount of triethylamine) with aryl- or hetarylmethyli- dene cyanothioacetamides 7b depends on the temperature.Thus the condensations of these compounds at 20 8C follow the AdN-E1,6 mechanism to yield stereoselectively substituted 3,4- trans-1,2,3,4-tetrahydropyridines 37, whereas the reactions of these compounds under reflux follow the AdN-E1,5 mechanism and give rise to 2,3-dihydrothiophenes 38.83 ± 89 Ar R1=Ph + CN 20 8C R2 S7 R2 C(S)NH2 NH +ArCH NH HOPh37 CN N + CN Ar 7b 7CHCOR1 D 36 R1CO NH2 S 38 Ar=4-FC6H4, 4-ClC6H4, 4-BrC6H4, 4-MeOC6H4, ; N ; R2=H, Br.R1=Ph, Compounds 38 were also obtained by a three-component condensation of pyridinium ylide 36 (R1=1-adamantyl) with aromatic or heteroaromatic aldehydes and cyanothioacetamide (1b).83, 84, 87, 88 A novel stereoselective method for the preparation of trans- dihydrothiophenes 38 and trans-dihydrofurans 39 has been developed based on the three-component condensation of ylides 40 with aldehydes and amides 1a,b in ethanol in the presence of triethylamine.89740 Et3N + NCCH2C(X)NH2 + ArCHO + EtOH 1a,b N N 7CHCOR 40 CN Ar RCO NH2 X 38, 39 R=MeO, Ph; Ar=2-MeOC6H4, 4-FC6H4, , N N ; X = S (38), O (39).Functionally substituted trans-dihydrothiophenes 38 have also been obtained by the reactions of arylmethylidenecyano- thioacetamides 7b with stabilised sulfonium ylides generated in situ from bromides 41. Cyclopropanethiocarboxamides 42 were found to be the minor products of these reactions.92, 93 + Et3N MeOH Me2SCH2COR Br7+7b 41 NC C(S)NH2 CN Ar + COR Ar RCO NH2 S 42 (11% ± 24%) 38 (19% ± 70%) ; R=Ph, S Ar=2-MeC6H4, 2-NO2C6H4, 4-MeOC6H4, , .N S First publications on the use of the Gewald reaction in the synthesis of sulfur-, selenium- and tellurium-containing five- membered heterocycles have appeared recently.65 ± 74 Com- pounds 43 ± 45 were prepared in 60% ±85% yields by treating mixtures of ethyl cyanoacetate (cyanoacetamide, malononitrile) and isothiocyanates, cyclohexanone or camphor with sulfur, selenium or tellurium. It has been found that ultrasound or microwave irradiation considerably accelerate these reac- tions.94, 95 Y X NR RN C S+YCH2CN Et3N, DMF S H2N X 43 Y O X +YCH2CN NH2 Et3N, DMF X 44 Me Me Y O X +YCH2CN NH2 Et3N, DMF X 45 R=Ph, PhCO, CH2=CHCH2, ButCO; X=S, Se, Te; Y=CO2Et, CONH2, CN.The addition of cyanoacetamide (1a) to 1,1-diphenylethylene in acetic acid in the presence of Mn(OAc)3 results in a complex mixture of amides 46 and 47, lactones 48 ± 50, lactam 51 and benzophenone.96 1a, Mn(OAc)3 Ph2C CH2 Ph2CHCH(OAc) C(CN)C(O)NH2+ AcOH 46 (22%) O O NC H2N(O)C Ph2C(OAc)CH2CHCN + + + O O + 47 (3%) C(O)NH2 Ph Ph Ph 48 (15%) Ph 49 (2%) V P Litvinov O O CH CH Ph2C Ph2C NC NH + Ph2CO NC O + + (2%) Ph Ph Ph Ph 50 (3%) 51 (5%) A new approach to the synthesis of substituted pyrrolinone derivatives has been developed using the Knoevenagel condensa- tion of a-hydroxy ketones 52 with substituted cyanoacetamides aimed at comparing biological activities of oxygen- and nitrogen- containing five-membered heterocycles.These reactions occur via iminolactams 53, which are readily converted into the correspond- ing pyrrolinones 54 under the reaction conditions.97 R1MeC(OH)Ac+NCCH2C(O)NHR2 52 C(O)NHR2 C(O)NHR2 Me Me Me Me O NH O R1 R1 NH 54 53 R1=Alk, CH2=CHCH2, Bn; R2=H, Alk, Bn, Het. The condensation of cyanothioacetamide (1b) with diethyl oxalate results in 4-cyano-5-thioxopyrrolidine-2,3-dione (55). The latter reacts with arylmethylidene malonates yielding pyrroli- dino[1,2-b]-1,3-thiazine-5,6-diones 56.42, 43, 98 (CO2Et)2+NCCH2C(S)NH2 1b O CN Z CN O ArCH O CN S N S O NH NH2 Ar Z 55 56 Z=CN, C(S)NH2, CO2Et. The reactions of amide 1b with maleic anhydride or N-(2- chlorophenyl)maleimide afford pyrrolidine (57) or pyrrolo- [3,2-b]pyrrole (58) derivatives.99 NC CH2CO2H 1b OH S O O N O 57 NH 1b S O O O N N CN 2-ClC6H4 C6H4Cl-2 58 2,3,6-Trimethylbenzoquinone (59) reacts with cyanoaceta- mide (1a) in the presence of sodium methoxide yielding 5-hydroxy-4,6,7-trimethyl-3-cyanoindolin-2-one (60).100, 101 Me Me CN HO O 1a, MeONa O NH Me Me O Me Me 59 60 (78%) Polysubstituted phthalimide 61 has been obtained by conden- sation of methyl acetylenedicarboxylate with the amide 1a in the presence of Cu2(OAc)4 in dioxane.102Cyanoacetamides and their thio- and selenocarbonyl analogues as promising reagents for fine organic synthesis NH2 O MeO2C 1a, Cu2(OAc)4 NH MeO2CC CCO2Me MeO2C O CO2Me 61 (16%) Cyanoacetamide (1a) has been used as the starting material in the synthesis of dye 62.The cyclocondensation of phthalonitrile with the amide 1a was carried out in methanol in the presence of NaOMe and then the reaction mixture was treated with barbituric acid at 65 8C.103, 104 C(CN)C(O)NH2 CN a, b NH CN O O NH HN 62 (96.5%) O OC NH ( H a) 1a, MeONa, MeOH; (b) , 2C CO658C. OC NH The reaction of enamino ketone of the indoline series 63 with the amide 1a (refluxing in benzene in the presence of triethyl- amine) gives rise to 3-amino-2-carbamoyl-9-oxopyrrolo[1,2- a]indole 64.105 O O 1a, Et3N CHNMe2 PhH, D, 6 h N CONH2 NH 63 64 (14%) NH2 A series of pyrrolothiolates has been synthesised by reaction of the amide 1a with sulfur in the presence of catalytic amount of diethylamine.106 b. Five-membered heterocycles with two heteroatoms in the ring Cyanoacetamides and their chalcogen analogues have been successfully used in syntheses of five-membered heterocycles with two heteroatoms in the ring: thiazoles, isothiazoles, thiazo- linones, oxazoles, pyrazoles, imidazoles, dithioles, etc. Thus the condensation of cyanothioacetamide (1b) with a-halogeno carbonyl compounds involving their methylene group has been employed in the synthesis of 2-thiazolylacetoni- triles 65 as well as thiazolylthiones and other five-membered heterocycles.34, 59, 80, 81, 107 R N 1b HalCH2COR CH2CN S 65 R=Alk, Ar, CO2Et.Thiazole derivatives 66 were synthesised by the reaction of 2-acetyl-2-cyanothioacetamide 67 (prepared by the acetylation of the amide 1b) with ethyl bromoacetate or 1-chloroacetonitrile in good yields.108 R BrCH2CO2Et N 67 CH(CN)Ac S ClCH2CN 66 R=OH, NH2.3-(Bromoacetyl)coumarins 68 react with activated thioamides 69 on heating in ethanol to give substituted thiazoles 70 in yields of up to 93%.109 741 S X R1 R1 COCH2Br N +H2NC(S)X 69 O O O O R2 R2 70 68 R1=R2=H;R1±R2= ;R1=7-NEt2, R2=H; X=CH2CN, NH2, NHNH2, C(S)NMe2. The reaction of the pregnenolone derivative 18 containing the cyanothioacetamide fragment with phenacyl bromide was employed for the synthesis of thiazolyl-substituted steroids 71 (yields 85%± 97%).60, 62, 110 Ac 4-XC6H4COCH2Br CH(CN)C(S)NH2 KOH, EtOH 18 RO Ac C6H4X-4 N CH(CN) S 71 HO R =Alk; X=H, Cl, Br. Cyanothioacetamide (1b) reacts with sulfur and isothiocya- nates in the presence of bases yielding thiazoline-2-thione deriva- tives 72.The latter are cyclised on treatment with formic acid to give thiazolo[4,5-d]pyrimidine-2,7-dithiones 73.78 Similar reaction of the amide 1b with sulfur and hydrogen sulfide affords 1,2- dithiol-3-thione derivative 74, which is converted into 5-amino-4- cyano-1,2-dithiol-3-thione (75) on treatment with formic acid rather than undergoes cyclisation.78 H2N N NR RNCS, S8 HCOOH NR S EtONa HN S H2N(S)C S S 72 S 73 1b S S NC H2N(S)C CS2, S8 HCOOH EtONa S S H2N H2N S 74 S 75 R=Me, Ph, CH2=CHCH2. Reactions of the amide 1a and its derivatives with isothiocya- nates have also been employed in the synthesis of other substituted thiazoles.111, 112 Thiazolyl-substituted acrylonitriles 76 can be prepared in two ways.The first one is the reaction of phenacyl bromide with salts 77. The latter are obtained by the reaction of Meldrum's acid (78) with aromatic aldehydes and cyanothioacetamide (1b) or with arylmethylidenecyanothioacetamides 7b (ethanol, 20 ± 25 8C, equimolar amount of the secondary amine). The second method of preparation of acrylonitriles 76 is based on the reaction of phenacyl bromide with amides 7b.113 ± 117 Ar O Ph CHCHC(S)NH2 Ar N O 7 +PhCOCH2Br CN Me + CN S O O H2NR1R2 Me 76 77V P Litvinov 742 O O 1b The condensation of the amide 1a with CS2 in the presence of KOH results in 3,5-dimercaptoisothiazole-4-carboxamide 85.125 KS CN +ArCHO Me MeI KOH O O Me NHR1R2 KS NCCH2C(O)NH2+CS2 1a C(O)NH2 78 77 C(S)NH2 SH H2N(O)C CN MeS S8 78 +ArCH CN 7b N MeS C(O)NH2 HS S 85 C(S)NH2 76 ArCH +PhCOCH2Br CN 7b 5-Amino-3-methyl-4-cyanoisothiazole (87) has been synthes- ised by the reaction of the amide 1b with methyl dithioacetate (86).126 EtOK MeI KSC CC(S)NH2 HCl EtOH Thiazolinone 79 was obtained by the reaction of cyanoacet- amide (1a) with mercaptoacetic acid or its ester.118, 119 This compound can also be synthesised from cyanothioacetamide (1b) and ethyl chloroacetate.120 MeC(S)SMe +NCCH2C(S)NH2 86 1b Me CN 1a, Et3N Me NC HSCH2CO2R O AcOH 1) NH3 2) H2O MeSC CC(S)NH2 N N Me CN H2N CH2C(O)NH2 1b S ClCH2CO2Et S 87 79 R=H, Alk.Thiazole derivatives 80 and 81 also have been synthesised using the amide (1b).81, 82, 109 Ar N-Substituted cyanothioacetamides 88 were prepared from ethyl cyanoacetates and aryl isothiocyanates,127 their reactions with hydrazines 89 yielded 5-amino-3-arylaminopyrazoles 90.128 The analogous reaction of the amide 1a with hydrazonoyl chloride 91 afforded 5-amino-1,3-diphenylpyrazole-4-carboxamide.129 N 1b NHR1 ArCOCH2Hal CH2CN POCl3, DMF EtOH S N 80 H2N 7H2S R1NHC(S)CH2CN+H2NNHR2 89 88 NR2 90 EtO2C 1b N R1=Ar; R2=H, Ar. DMF BrCH2COCO2Et 34 H2NCO Ph H2N CH2CN S 1a Ph(Cl)C NNHPh 81 N H2N 91 NPh Cyanoacetamide derivatives 92 on heating with hydrazine give pyrazoles 93, while their reactions with mercaptoacetic acid result in thiazolinones 94.130 N ArN NH2 NH2NH2 4-Thiazoline-2-thione derivatives 82 were obtained by the reaction of cyanoacetamide (1a) with isothiocyanates in the presence of elemental sulfur.121 The reaction of the same amide with 2-mercaptoaniline gives 2-carbamoylmethylbenzothiazole (83a).122 The latter is employed in light- and waterproof azo- dyes used in polycaprolactam manufacture.Benzooxazole 83b is obtained in a similar way from the amide 1a and 2-aminophe- nol.123 N HO H2N(O)C 1a, S8 NH 93 NR RN C S O S H2N ArNHN C(CN)C(O)NH2 92 H2NCO N S 82 HSCH2CO2H ArNHN C R=Ar, Alk. S 94 NH2 N 1a Ar=Ph, 4-MeC6H4, 4-ClC6H4.CH2C(O)NH2 X XH 83a,b X = S (a), O (b). The reaction of N-[bis(methylthio)methylidene]toluene-4-sul- fonamide (95) with the amide 1a affords compound 96 in 94% yield. The latter gives pyrazoles 97 in yields of up to 98% on treatment with hydrazines.131 SMe 1a, K2CO3 4-MeC6H4SO2N DMSO SMe 95 SMe 4-MeC6H4SO2N Reactions of the amide 1a with phenacyl benzoates in boiling xylene in the presence of BF3 afford 2-aryl-4-phenyloxazoles in 75%± 90% yields.124 Isothiazole 84 has been obtained in good yield by bromination of acetyl(cyano)thioacetamide 67.108 Ac Br CH(CN)C(O)NH2 Br2 AcCH(CN)C(S)NH2 7HBr N 67 H2N S 84Cyanoacetamides and their thio- and selenocarbonyl analogues as promising reagents for fine organic synthesis H2NCO NHSO2C6H4Me-4 CN MeS H2NNHR N C(O)NH2 H2N NR97 4-MeC6H4SO2NH96 R=H, Ph, 4-ClC6H4, 4-NO2C6H4. Phenylhydrazone 98 reacts with the amide 1a in methanol in the presence of sodium methoxide yielding pyrazole 99.132 An analogous reaction of hydrazone 100 with a bulky substituent affords pyrazole 101 in only 6% yield.133 Isooxazole 103 is obtained from hydroximoyl bromide etherate under similar conditions.132 H2NCO CF3 Br 1a, MeONa N H2N NNHPh MeOH F3C 98 NPh 99 (53%) NC CF3 Cl H2N N N Br 1a, EtONa NNH CF3 Cl Cl EtOH F3C Cl 100 101 CF3 H2NCO CF3 Br 1a, MeONa NOH.OEt2 MeOH N F3C H2N 102 O 103 (30%) High yields of isoxazoloquinazolines 104 were obtained upon cyclocondensation of 4-methylthioquinazoline 3-oxides 105 with the amide 1a in the presence of ButOK.134 R N N R 1a, ButOK N O N O SMe H2N(O)C NH 105 104 R=Ph, 4-MeC6H4.Reactions of 1-oxa-2-azaspiro[2.5]octane (106) with cyano- amides 107 give spiroimidazoline derivatives 108 in high yields.135 O NH B O R +NCCH(R)C(O)NH2 HN H2N(O)C NH 107 108 106 R=cyclo-C5H9, cyclo-C6H11, Me(CH2)7, PhCH2. A four-step synthesis of a xanthine analogue, viz., 5,6,7,8- tetrahydro-4H-imidazo[4,5-c]-1,4-diazepine-5,8-dione (109), has been developed starting from cyanoacetamide (1a).136 O C(O)NH2 NH N a, b, c d e N NCCH2C(O)NH2 NH2 O 1a NH NBn NBn O NH HN O NH N109 (a) HNO2; (b) [H]; (c) CH2(OEt)2, BnNH2; (d) KOH; (e) H2/Pd.743 2-Cyanomethylbenzoimidazole (110) has been obtained by condensation of cyanoacetamide (1a) with o-phenylenediamine in good yield.137 This reaction, carried out in the presence of concentrated sulfuric acid, was accompanied by hydrolysis of the cyano group resulting in 2-carbamoylmethylbenzoimidazole 111.138, 139 NH2 NH2 NH2 NH 1a CH2CN TsOH, PhNO2 N NHC(O)CH2CN 110 NH 1a, H2SO4 CH2C(O)NH2 N 111 The reaction of o-phenylenediamine with substituted cyanoacet- amide 112 under similar conditions gives rise to benzimidazole derivative 113.140 NH2 CN + NHN C(O)NH2 NH2 112 NO2 O2N NNH N C(O)NH2 NH113 c. Five-membered heterocycles with three heteroatoms in the ring The amide (1a) and thioamide (1b) are widely used in the synthesis of five-membered heterocycles with three heteroatoms in the ring, in particular, of 1,2,3-triazoles possessing a wide range of bio- logical activities.Synthesis of substituted 1,2,3-triazoles by the reactions of cyanoacetamide (1a) with alkyl-, aryl- and hetaryla- zides can be mentioned as an example.141 ± 145 It is noteworthy that the preparation of 1,2,3-triazole derivatives from compounds with an activated methylene group and phenyl azide was first reported by Dimroth in 1902.146, 147 1-Azidoglycosides 114 react with the amide 1a in aqueous DMF in the presence of KOH to give triazolonucleosides 115 in high yields. The reactions proceed with inversion of configuration yielding 1,2-trans adducts from 1,2-cis azides.144 H2NCO KOH DMF, H2O NN RN3+NCCH2C(O)NH2 114 1a H2N NR 115 (70% ± 80%) ButPh2SiO Ph3CO O O O , , O O O .R= O O O O O The synthesis of 2-substituted 9-b-D-ribofuranosyl-8-azahy- poxanthines 116 from b-D-ribofuranosyl azide, sodium salt of the amide 1a and carboxylic acid esters has also been described.145744 O N HN N N R N HOH2C O O O 116 (R=H, Alk, Ar) Synthesis of nucleosides with 1,2,3-triazoles and 8-azapurines as aglycones by reactions of azides 117 with the amide 1a has been undertaken aimed at access to novel potential pharmaceutical preparations. In the presence of KOH in aqueous DMF at 0 8C, these reactions gave rise to 1,2,3-triazole derivatives 118. The latter were readily converted into 8-azapurine derivatives 119 on treatment with ethyl formate.148, 149 KOH DMF, H2O, 0 8C N3CH2XR+NCCH2C(O)NH2 1a 117 O H2NCO N HN HCO2Et N NNH HN EtONa, EtOH N N N 119 118 CH2XR CH2XR R=PhCH2O(CH2)2, (PhCH2OCH2)2CH; X =O, S.The reaction of benzyl azide with the amide 1a in EtOH in the presence of NaOEt followed by treatment of the reaction mixture with carboxylic acid esters affords 3,5-disubstituted 7-hydroxy- 3H-1,2.3-triazolo[4,5-d]pyrimidines 120. 5-Amino-1-benzyl-1,2,3- triazole-5-carboxamide (121) has been postulated as an intermedi- ate.150 O H2NCO N NH 1a, EtONa R1CO2R2 N BnN3 NN EtOH H2N R1 N Bn N120 (43% ± 95%) NBn 121 R1=H, Me, Et, Pr, CH2OCH2Me, CH(OCH2Me)2, (CH2)2CO2H, CO2Et, Ph, PhCO, CH2NHCOPh; R2=Me, Et.Many other 1,2,3-triazole derivatives possessing coccidio- static, anticonvulsive, antiproliferative and other types of bio- logical activities have been synthesised using the reactions of aromatic and aliphatic azides with the amide 1a.151 ± 157 The reaction of 6-nitroazolo[1,5-a]pyrimidines 122 with the amide 1a in refluxing ethanol involves transformation of the pyrimidine ring into the pyridine ring and affords substituted 2-azolyl-2-pyridylamines 123. The latter are converted into 7- nitro-9-oxo-4,9-dihydroazolo[1,5-a]pyrido[2,3-d]pyrimidines 124 on subsequent treatment with sodium carbonate in ethanol and acidification with HCl.158, 159 NO2 N 1a, EtOH, B N R X N 122 C(O)NH2 X HN R 1) Na2CO3, EtOH 2) HCl N NH N NO2 123 O N N R X N NH 124 R=H, Me, SMe, CF3, Cl, NH2, NMe2, Ph; X=N, CH, CNO2.Synthesis of 5-amino-4-cyano-1,2,3-thiadiazole (126) by the reaction of tosyl azide 125 with cyanothioacetamide (1b) has been described.160 NC 1b 4-MeC6H4SO2N3 H2N 125 Later,161 1,2,3-thiadiazolium bromides 127 were obtained by bromination of arylhydrazonocyanothioacetamides 16. NC NC C(S)NH2 Br2 ArNHN H2N CN 16 A group of Japanese scientists 162 have developed a nine-step method of synthesis of an useful chemical modificator of cepha- losporin antibiotics, viz., (Z)-2-(5-amino-1,2,4-thiadiazol-3-yl)-2- [(fluoromethoxy)imino]acetic acid (128), from cyanoacetamide (1a) according to the scheme shown below. The yield of the target product with respect to the initial cyanoacetamide was 17.6%.CN a HON NCCH2C(O)NH2 1a C(O)NH2 CN d FCH2ON FCH2ON CN CN FCH2ON g N N NH2 S CO2Me FCH2ON i N N NH2 S CO2H FCH2ON N N NH2 128 S (a) HNO2 (81%); (b) BrCH2F, Et3N (75%); (c) POCl3 (72%); (d) NH4OH, NH4Cl (84%); (e) Br2; ( f ) KSCN, Et3N (88%); (g) MeONa, MeOH (92%); (h) H2SO4, H2O (84%); (i) HCO2H, Ac2O (84%); ( j) NaOH, H2O (84%). 2. Synthesis of six-membered carbo- and heterocycles Of six-membered heterocycles synthesised on the basis of cyano- acetamides, pyridin-2-one (-thione and -selenone) derivatives are of major interest (see, for example, reviews 27, 28, 30 ± 34, 74 and papers 163 ± 175).Many of these compounds have been found to be highly effective cardiotonics 176 ± 251 (e.g., `amrinone' and `milrinone' 242 ± 251), non-nucleoside inhibitors of HIV reverse transcriptase (potential pharmaceuticals for treatment of V P Litvinov NO2NN S 126 Br7 Br7 + + NNAr NNAr H2N S S 127CN b c FCH2ON C(O)NH2 CN e, f C(NH2) NH MeO NH FCH2ON h N N NH2 S CO2Me FCH2ON j N N NHCHO SCyanoacetamides and their thio- and selenocarbonyl analogues as promising reagents for fine organic synthesis AIDS 252 ± 265), tranquilisers,266 ± 278 compounds with antiinflama- tory, hypoglycemic, anticonvulsant, antiulcerous, fungicidal, bactericidal, pesticidal and many other types of biological activity.272 ± 308 Pyridin-2-ones are also very useful starting mate- rials in the design of novel calcium antagonists,309, 310 CH antioxidants,311 ± 315 folic acid analogues,316 vitamin B6,317 and other biologically active compounds,311, 316 ± 324 as well as models of redox coenzymes NAD and NADP.285 As an example, the use of substituted spiro[cyclopentane-piperidone] in the synthesis of `buspirone', which is an antidepressant and an analgesic, and also prevents an affection to narcotics,269 can be mentioned.The only known natural alkaloid with the cyano group in a molecule is a representative of the class of pyridones (`ricyane', 3-cyano-4- methoxy-1-methylpyridin-2-one).325 Pyridin-2-one derivatives are employed as stabilisers of polymers and lacquers,326 as pigments and diazo-components in the synthesis of dyes,327 ± 334 as acid ± base indicators in titrimetric analysis;335 they possess liquid-crystalline 336 and some other practically useful properties.The simplest and most convenient methods for the prepara- tion of functionally substituted pyridin-2(1H)-ones, -thiones and -selenones are based on the use of cyclisation reactions of carbonyl compounds and their enamines with compounds containing the activated methylene group, first of all, malononitrile and cyano- acetamides. This opens an indirect route to regioselective synthesis of functionally substituted pyridines. Direct functional- isation of the rather inert pyridine ring by electrophilic substitu- tion reactions is quite difficult.Condensations of symmetrical 1,3-dicarbonyl compounds with cyano(chalcogeno)acetamides 1a ± c occur smoothly in the presence of basic catalysts to give 3-cyanopyridin-2(1H)-ones, -thiones and selenones 129 in high yields.28, 85, 337 ± 352 Similar reaction of monothiodibenzoylmethane (130) with cyanothioacet- amide (1b) in ethanol in the presence of piperidine gives rise to 3-cyano-4,6-diphenylpyridine-2(1H)-thione.353 R R CN CN RCOCH2COR 1a7c, B 7H2O R X X R NH O NH2 129a ± c R=Alk, Ar, Het; X =O (a), S (b), Se (c). Ph CN 1b, B PhC(S)CH2COPh S Ph 130 NH (87%) It is noteworthy that pyridin-2(1H)-ones 129a were obtained by the above method as far back as the end of the past century, while the first thione analogue, viz., 3-cyano-4,6-dimethylpyri- dine-2(1H)-thione, was synthesised by condensation of acetylace- tone with the amide 1b at the end of the 1950's,338, 341 and the first selenone analogue was prepared in a similar way from the amide 1c only in 1985.47 Reactions of cyanothioacetamide (1b) occur under milder conditions and give higher yields of pyridines than with cyanoacetamide (1a).The reactions of cyanoselenoacetamide (1c) also occur under mild conditions, though an inert atmosphere is required in this case. The reaction of 1,3-diketones with dimethylformamide dimethyl acetal affords b-enamino carbonyl compounds 131, which are condensed with amides 1a,b yielding functionally substituted pyridones and pyridinethiones 132.232, 235, 354 ± 356 R1CO DMF 1a,b R1COCH2COR2+Me2NCH(OMe)2 R2CO NMe2 131 745 CN R2CO R1CO R2CO R1 X CH(CN)C(X)NH2 NH 132 R1=Me, Et, CH2CO2Et; R2=Me, MeO, EtO; X=O, S.Synthesis of pyridinethiones 133 by the reaction of ethoxyme- thylidene diketones 134 with the amide 1b in ethanol in the presence of piperidine has also been described.357 R1 R1CO CN R2CO 1b, B OEt R2CO S 134 133 NH Condensations of unsymmetrical 1,3-dicarbonyl compounds with cyano(thio, seleno)acetamides 1a-c leads to two isomeric pyridines with different substituents in positions 4 and 6, which allows one to elucidate the effect of the reactivities of the carbonyl groups on the regioselectivity of reactions. Thus it was shown 85, 337, 347, 358 that the condensation of benzoylacetone with the amide 1b gives both isomeric pyridinethiones 135b and 136b (Ar=Ph) in the ratio 2 : 1.Earlier, only the isomer 135b was thought to be the reaction product.341 However, the analogous reaction of benzoylacetone with cyanoselenoacetamide (1c) occurs with high regioselectivity to give only one isomer, viz., 3-cyano-4-methyl-6-phenylpyridine-2(1H)-selenone (135c, Ar= Ph).358, 359 Reactions of cyanothioacetamide (1b) with other unsymmet- rical 1,3-diketones 137 (Ar=4-MeC6H4, 4-MeOC6H4, 4-BrC6H4, 2-thienyl) also give rise to mixtures of the correspond- ing isomers 135b and 136b.360, 361 Only the isomer 135a (Ar=Ph) was earlier assumed 362, 363 to be the reaction product of benzoylacetone with cyanoacetamide (1a) or ethyl cyanoacetate in the presence of ammonia or diethyl- amine. In the condensation of nicotinoylacetone with cyanoace- tamide (1a), a mixture of isomers 135a and 136a (Ar=3-pyridyl) in the ratio 3 : 2 is formed, while in the reaction of 4-pyridylcarbo- nylacetone, the isomer 136a (Ar=4-pyridyl) is the major prod- uct.167 1a ± c ArCOCH2COMe Ar Me CN CN + MeC(O)CH2 C(X)NH2 C(X)NH2 ArC(O)CH2 Ar Me CN CN + X Me X Ar NH NH 136a ± c 135a ± c Ar=Ph, 4-MeC6H4, 4-MeOC6H4, 4-BrC6H4, , , S N ; X = O (a), S (b), Se (c).N The regioselectivity of the reaction is the higher the stronger are the electron-withdrawing properties of a substituent at one of the carbonyl groups. Thus 1,3-diketones with the trifluoromethyl group 137 react with cyanoacetamides 1b,c to give only 3-cyano-6- R-4-trifluoromethylpyridine-2(1H)-thiones and -selenones 138 in fairly high yields.345, 364 ± 366 The change in the order of operations, i.e., the preliminary generation of a carbanion from the thioamide 1b by treating it with sodium ethoxide followed by the addition of trifluoroacetylacetone has led to the other isomer, viz., 3-cyano-4- methyl-6-trifluoromethylpyridine-2(1H)-thione, in 94% yield.365, 367746 CF3 CN 1b, c, B CF3COCH2COR EtOH X R 137 NH 138 ; X =S, Se.R=Alk, Ph, 4-MeC6H4, 4-MeOC6H4, 4-ClC6H4, S Reactions of cyanoacetamide (1a) with 1,3-dicarbonyl com- pounds and their derivatives have successfully been employed in the synthesis of pyrimidines, pyrazines and other hetero- cycles.368 ± 380 Thus the condensation of the amide (1a) with methyl 2-acetylpent-4-ynoate (139) in concentrated aqueous ammonia solution gives rise to pyridone 140.An analogous reaction of ethyl 2-acetylpent-4-enoate (141) with the amide 1a in piperidine yields pyridone 142.371 Me Ac NC CH2C CH 1a, NH3, H2O CH CHCH2C MeO2C OH O 139 NH 140 (74%) Me Ac NC CH2CH CH2 NH 1a, CH2 CHCH2CH EtO2C OH O 141 NH 142 (51%) Substituted dispiro(dipyrano[2,40 : 6,40]bisdithiolo[4,5-b : 40,50- e])-4,8-benzoquinone 144 was obtained by the reaction of 2,6-bis(diacetylmethylidene)bisdithiolo[4,5-b : 40,50-e]benzoquinone (143) with a twofold excess of the amide 1a in ethanol in the presence of piperidine.380 O NH Ac 1a, Ac S S D, 3±5 h S S Ac Ac 143 O C(O)NH2 O Me H2N S S O O S S Me NH2 O H2N(O)C 144 (68%) The kinetics of condensation of symmetrical and unsymmet- rical 1,3-diketones (pentane-2,4-dione, 5-methylhexane-2,4- dione, 5,5-dimethylhexane-2,4-dione) with cyanoacetamide (1a) in the presence of piperidine as a catalyst under various reaction conditions has been studied. Judging from the rate constants and activation parameters as well as the structure of unsymmetrical 4,6-disubstituted 3-cyanopyridin-2(1H)-ones obtained, a plausi- ble scheme for the reaction mechanism was suggested, which gives a rationale for the regioselectivity of the condensation and the positions of the substituents in the reaction products.381 Cyclic 1,3-diketones have been successfully employed as substrates in the reactions with cyanoacetamides 1a-c, and regioselective syntheses of bicyclic 3-cyanopyridin-2(1H)-ones, -thiones and -selenones 146 were proposed.85, 346, 382 ± 394 (H2C)n (H2C)n O CN CN 1a ± c, B (H2C)n R 7H2O R X COR X 145 O NH2 NH 146 n=1, 2; R=Alk, Ar; X=O, S, Se.V P Litvinov This approach has also been applied to the preparation of pyridone 147 having the skeleton of the alkaloid papaverine.386 CN CN 1b Bn 7H2O S COBn Bn S O NH2 NH 147 7-Acetyl-8-aryl-4-cyano-1,6-dimethyl-6-hydroxy-5,6,7,8-tet- rahydroisoquinolin-2(1H)-ones 149a, -thiones 149b and their sodium salts 150a,b were obtained in high yields in reactions of the amides 1a,b with 2,4-diacetyl-3-aryl-5-hydroxy-5-methyl- cyclohexanones 148.394 Ar CN Ac HO Ac 1a C(O)NH2 Me Me O HO Ac Ac Ar 148 CN HO O Me NH Et3N Ac Me Ar 149aOH7 H+ CN HO O MeONa 7 Me Na+ N Ac Me Ar 150aCN HO HO S S 7 Me Me H+ 1b 148 NH N MeONa Ac Ac OH7 Na+ Me Me Ar 149b Ar150b Of reactions of cyclic 1,3-diketones with cyanoacetamide (1a),277, 280, 395 ± 399 the condensations of pyrans or thiopyrans 151 in ethanol in the presence of triethylamine can be noted as the method for preparation of 3-oxopyrano(thiopyrano)[3,4-c]- pyridines 152.277, 395, 396 CN O Me COR 1a, Et3N C(O)NH2 Me Me EtOH X COR X Me 151 CN Me O Me NH X 152 R=Alk, Ar; X=O, S.It is known that substituted coumarins and their annelated analogues exhibit bactericidal, spasmolytic and hypothermic activities. In order to develop methods of synthesis of substituted coumarins, which could be used in design of novel pharma- ceuticals, it was necessary to study reactions of 3-acetylcoumarin747 Cyanoacetamides and their thio- and selenocarbonyl analogues as promising reagents for fine organic synthesis ONa O O Me Me C(S)NH2 NH 1b, AcOH (153) with the amides 1a,b in the presence of bases.This resulted in oxo- and thioxo-derivatives of benzopyrano[3,4-c]pyridines 154.280 X CN EtOH EtOH NC Me Me N NMe Me N 163 162 Ac Me 1a,b, Et3N Me EtOH O O O O C(S)NH2 EtO2C 154 153 Me N CN Me X = O (a), S (b). Me Me MeN MeN CN CN 7H2 CHC(S)NH2 Me EtO2C Me S O Compared with cyclic 1,3-diketones, cyclic b-oxo aldehydes react with cyanoacetamides more selectively.Thus the condensa- tions of sodium salts of 2-hydroxymethylidenecycloalkanones 155 with amides 1b,c occur regioselectively to give only one of two possible isomers, viz., 5,6-annelated 3-cyanopyridine-2(1H)-thio- nes 156a and -selenones 156b.85, 387, 400 ± 406 HN Me CN MeN CHONa CN 1b,c X (H2C)n (H2C)n Me O O O NH2 155 HN S 164 CN (H2C)n X NH 156a,b n=1, 2; X = S (a), Se (b). Similarly to cyclic b-oxo aldehydes, their acyclic analogues also react with high regioselectivity.38, 85, 174, 395, 410 ± 422 Thus 6-substituted 3-cyanopyridine-2(1H)-thiones and -selenones (166) are formed in the reaction of amides 1b,c with sodium enolates of b-oxo aldehydes (165).The regioselectivity of the reaction is independent of the nature of substituents (electron- donating or electron-withdrawing) in the benzene ring of the substrates 165.85, 397 ± 399, 402 CN CN Yet another example of high regioselectivity is the reaction of sodium salt of 2-formylquinuclidone (157) with cyanothioacet- amide (1b), which affords only bridged 3-cyano-1,5-naphthyri- dine-2(1H)-thione (158).407 RCO ONa 1b,c, AcOH CN N O X R X R 1b 165 O NH2 NH 166 S CHONa N R=Pr, Ph, 4-ClC6H4, 4-BrC6H4, 3,4-Cl2C6H3, 4-MeOC6H4, NH 157 158 2,4-(MeO)2C6H3, N ; X=S, Se. Cyanothioacetamide has been used in the synthesis of steroids annelated with the pyridine ring.Thus pyridinethione 161 was obtained in 80% yield by condensation of the amide 1b with the sodium enolate of 16-formyl-5a-androstan-17-one (159).60, 408 The latter has been prepared by formylation of 5a-androstan-17- one (160) with methyl formate. The key step in the synthesis of 2-pyridinone derivatives 167 possessing high anti-HIV activity 253, 257 is the condensation of sodium enolate of 2-ethyl-3-oxobutane (168) with cyanoacet- amide (1a) in the presence of piperidinium acetate. Et CN O CHO Et ONa Ac 1a O Me OMe Me N Et 1) MeONa 2) HCO2Me 168 NH R Et (CH2)2R 160 Et S HN O CN O Me O Me NH NH 1) NaH 2) 1b, AcOH CHO 167 161 159 O N N , , , , R= O NH O N N , .O S An example of recyclisation of (1,2,5-trimethyl-4-oxopiperid- 3-ylmethylidene)cyanothioacetamide (162), prepared from the sodium salt of 1,2,5-trimethyl-3-formylpiperid-4-one (163) and the amide 1b, into 4-cyano-6,7,8a-trimethyl-3-thioxo- 1,2,3,5,6,7,8,8a-octahydro-2,7-naphthyridin-1-one (164) has been described.173, 409 The regioselective reaction of sodium enolates of 2- and 3-thenoylacetaldehydes 169 with cyanothioacetamide (1b) in ethanol in the presence of acetic acid, giving rise to the corre- 170,421 3-cyano-6-thienylpyridine-2(1H)-thiones sponding748 presents yet another example of successful synthesis of substituted pyridine-2-chalcogenones using condensations of oxo aldehydes with cyanoacetamides. COCH CHONa S 169 The condensations of cyanoacetamides with aliphatic, aro- matic and heterocyclic aldehydes also lead to functionalised heterocycles.423 ± 436 Among this type of the reactions, cyclo- condensations of 5-chloro-3-methyl-1-phenylpyrazole-4-carbox- aldehyde (171) with amides 1a,b, giving rise to pyrazolopyridines 172a,b,426 and of 5-aminopyrazole-4-carboxaldehydes 173 with the amide 1a, which makes possible the preparation of amino- substituted pyrazolo[3,4-b]pyridines 174,429 are worth mention- ing.Me CHO 1a,b N Cl NPh 171 X = O (a), S (b).R CHO 1a N NH2 NPh 173 R=Me, Pr, Ph. 6-amino-3,5-dicyanopyridine-2(1H)-thiones 4-Substituted 175 are convenient building blocks in the synthesis of biologically active compounds. It is remarkable that they can be prepared from aliphatic or aromatic aldehydes, cyanothioacetamide (1b) and malononitrile in a one-pot preparative step including three consecutive reactions.432, 434 ± 442 RCHO + NCCH2C(S)NH2 1b R=Alk, Ar.Analogous reactions of aromatic and heterocyclic aldehydes with cyanoacetamide (1a) in the presence of ammonium acetate had earlier been employed for the synthesis of 4-substituted 3,5- dicyano-6-hydroxypyridin-2(1H)-ones 176.437 RCHO + NCCH2C(O)NH2 1a R=Ar, Het. The cyclisation of glutaraldehyde with cyanoacetamide (1a) occurs in aqueous dioxane in the presence of triethylamine to give 1-cyano-2,6-dihydroxycyclohexane-1-carboxamide (177) in 75% yield.425 1a, Et3N OCH(CH2)3CHO CN 1b, AcOH S NH EtOH S 170 (60% ± 72%) Me CN N X NH NPh172a,b R C(O)NH2 N N NH2 NPh 174 R CN NC CH2(CN)2, Et3N S H2N NH 175 R CN NC NH4OAc O HO NH 176 CN H2N(O)C HO OH 177 V P Litvinov Reactions of the amide 1a with 2-nitro- and 2-aminobenz- aldehyde have been successfully used in the synthesis of 2-amino- quinoline-3-carboxamide 178.438 CN CH CHO 1a Fe C(O)NH2 AcOH NO2 NO2 C(O)NH2 N NH2 178 CHO 1a 178 NH2 Analogous reactions of amides 1a,b with hydroxybenz- aldehyde and hydroxybenzo[b]thiophenecarbaldehyde have led to coumarin derivatives 179a,b and 180a,b.439 ± 442 C(X)NH2 CHO 1a,b NH OH O 179a,b CHO HO 1a,b S C(X)NH2 C(X)NH2 O HN HCl, H2O O O S S 180a,b X = O (a), S (b).b-Enamino carbonyl compounds are widely used in the synthesis of various heterocyclic compounds. They are succes- sfully employed for regioselective synthesis of 3-cyanopyridin-2- (1H)-ones, -thiones and -selenones 129a ± c (see, for example, reviews 28, 30, 85 and papers 443 ± 454). R1 CN NR2R3 R1CO 1a ± c X R1 R1 NH 129a ± c R1=Alk, Ar, Het; R2, R3=H, Ph; R2±R3=(CH2)5, (CH2)2O(CH2)2; X = O (a), S (b), Se (c). It was found 47, 85, 341 ± 343, 349, 353, 387 that unlike the condensa- tions of 1,3-dicarbonyl compounds, the reactions of the corre- sponding symmetrical b-enamines with cyanoacetamides occur under mild conditions (20 ± 40 8C) and in the absence of basic catalysts to give selectively and in higher yields 4,6-disubstituted 3-cyanopyridin-2(1H)-ones, -thiones and -selenones 129a ± c.Synthesis of 5-substituted 3-cyanopyridin-2(1H)-ones 181 is based on the use of the reactions of b-enamino aldehydes 182 with cyanoacetamide (1a) in the presence of a base.272, 455 ± 462 R CN OHC NMe2 1a, B R O NH 181 182 R=Ar, Het.Cyanoacetamides and their thio- and selenocarbonyl analogues as promising reagents for fine organic synthesis Condensations of b-enamines prepared from unsymmetrical 1,3-diketones with amides 1a ± c can afford isomeric 4,6-disubsti- tuted 3-cyanopyridin-2(1H)-ones, -thiones and -selenones depending on whether the keto or the amino group of enamines is subjected to the initial nucleophilic attack by amides.85, 339, 345, 347, 358, 387, 446, 463 ± 467 Varying the substituents in initial enamines and the reaction conditions, one may control the direction of these reactions leading to one particular isomer or to a mixture of both isomers.Thus 2-amino-1-benzoylpropene (183) reacts with cyanoacet- amide (1a) in the presence of sodium ethoxide to give 3-cyano-6- methyl-4-phenylpyridin-2(1H)-one (184),339, 464, 466, 467 while the condensation of these compounds at 150 8C for 1 h results in the isomeric 3-cyano-4-methyl-6-phenylpyridin-2(1H)-one (185a).464, 465 Ph CN EtONa EtOH, D, 6 h O Me NH Me 184 1a Me COPh H2N 183 CN 150 8C, 1 h O Ph NH 185a Only 3-cyanopyridine-2(1H)-thione 185b and -selenone 185c are obtained on brief refluxing of a mixture of b-enamino ketones 186 or 187 and amide 1b or 1c in ethanol in the presence of acetic acid.85, 347, 358, 446, 463 Ph Me N O CN 1b,c MeC(O) 186 Ph X Me O N NH 185b,c 187 PhC(O) X = S (b), Se (c).The stronger the electron-withdrawing properties of the substituent at the sp2 hybridised carbon atom linked with the amino group, the higher the regioselectivity of the reaction. This is true even for those enamines which have the phenyl and 2-thienyl substituents at the carbonyl group. Thus condensations of trifluoromethyl-substituted enamino ketones 188 with amides 1b,c in ethanol at 25 ± 30 8C in the absence of a catalyst afford 3-cyano-6-phenyl- or -6-(2-thienyl)-4-trifluoromethylpyridine- 2(1H)-thiones and -selenones 138.85, 345 CF3 CN CF3 1b,c R O N X R NH O 138 188 ; X=S, Se.R=Ph, S b-Enamino ketones can also be used for the preparation of 5,6-disubstituted 3-cyanopyridin-2(1H)-ones. Thus 6-alkyl-5- aryl-3-cyanopyridin-2(1H)-ones 189 possessing cardiotonic activ- ity have been obtained by condensation of enamino ketones 190 (R1=R2=Me) with cyanoacetamide (1a) in the presence of sodium methoxide.86, 87, 468 ± 472 3-Cyano-5-aryl-6-methylpy- 749 ridine-2(1H)-thiones 189 (X=S) were synthesised in a similar way from the enamino ketone 190 [R17R2=(CH2)2O(CH2)2].473 Ar CN Ar 1a,b R1R2NCH X R3 COR3 NH 190 189 Ar=Ph, 4-MeSC6H4, 4-SO2C6H4, 3,4-(MeO)2C6H3; R1, R2=Alk; R1±R2=(CH2)2O(CH2)2; R3=Me, CH(OMe)2, cyclo-Alk; X=O, S.The well-known cardiotonic preparation `milrinone' (192) is obtained by the condensation of the enamine 191 with cyanoace- tamide (1a) or malononitrile (5).474 Me Ac NH 1a O N N NHMe 191 192 CN The reaction of enamine 193 with the amide 1a in isopropyl alcohol in the presence of sodium alkoxides is nonselective and results in a mixture of pyridones 194 and 195.475Me Me N N 1a Ac CN + Pri(Ac)N N(Ac)Pri S S Me2NCH Me O 193 Me 194 NH N + C(O)NH2 Pri(Ac)N SMe O NH 195 5,6-Disubstituted 3-cyanopyridin-2(1H)-ones 132 have been obtained by the reactions of 2-aminomethylidene derivatives of 1,3-dicarbonyl compounds 196 with the amide 1a in the presence of sodium hydride in THF369 or sodium methoxide in DMF.476 Both symmetrical 369, 476 and unsymmetrical 477 ± 479 b-diketones were used in these reactions.Compounds 196 with substituents R1=Bun, OEt and R2=Me react regioselectively yielding only one isomer. CN R2CO COR1 1a Me2NCH COR2 O R1 HN 196 132 R1, R2=Me, Pri, But, Bun, OEt, Ph. 3-Cyano-4,4,6-trimethylpyridin-2(1H)-one (197) has been obtained by the reaction of 4-amino-3-methylpent-3-en-2-one (198) with the amide 1a at 150 8C for 30 min.465 This compound was also synthesised by the condensation of the enamine 198 with malononitrile (5) in THF in the presence of triethylamine at 20 8C.454 Me Me Me CN Me 1a Ac H2N Me O 198 HN 197 The condensations of b-enamino carbonyl compounds 199 with substituted cyanoacetamides 200a,b occur with the elimina- tion of methanethiol and result in 4-dialkylaminopyridones 201a,b bearing the b-enamino amide fragment.Compound 201b reacts with N-methylcyanoacetamide 200b to give naphthyridine- dione 202.480750 NR3R4CN R2 SMe R1CO PriONa PriOH +NCCH2C(O)NHR5 200a,b NR3R4 R2 O R1 RN5 199 201a,b O NC NMe 200b 201b NH2 PriONa, PriOH O 202 Me N R1=Me, Ph, 4-ClC6H4, 4-MeOC6H4; R2=H; R3=Et, Ph, CH2CH(OEt)2, Bn; R4=H; R3±R4=(CH2)2O(CH2)2; R5 = H (a), Me (b). Synthesis of fused pyridinones, -thiones and selenones to with takes advantage of the high reactivity of cyclic b-enamino ketones respect 1a ± c.28, 30, 85, 346, 349, 382, 384 ± 387, 446, 481, 482 cyanoacetamides Condensations of enamino ketones 203 with amides 1a ± c occur regioselectively to give in each case only one of two possible isomers in the absence of basic catalysts and afford the products in higher yields (on average, by 10%) than those obtained from 1,3-dicarbonyl compounds.Condensations of b-enamino ketones 203 bearing an aryl substituent instead of an alkyl one, demand more drastic reaction conditions. Thus 3-cyano-6-phenyltetrahydroiso- quinolin-3-one 146 (n=4, X=O, R=Ph) can be obtained on refluxing of the equimolar amounts of the starting compounds in isopropyl alcohol for 1.5 h.384 CN COR COR Z NCCH2Y (H2C)n (H2C)n (H2C)n NH CN N Y R 146 X 203 n=1, 2; R=Alk, Ar; X=O, CH2; Y=CN, C(Z)NH2; Z=O, S, Se. It has been noted 369, 481 that condensations of enamines prepared from cyclic oxo aldehydes and secondary amines are more regioselective.Hydrogenated 3-cyanoquinoline-2(1H)-thiones and -selen- ones 156a,b were obtained by the condensation of 2-piperidino- methylidenecycloalkanones 204 with amides 1b,c in ethanol in the presence of acetic acid in high yields.85, 352, 387, 402 ± 404, 446 CN CHN 1b,c (H2C)n (H2C)n X NH 156a,b O 204 n=2, 3; X = S (a), Se (b). The reaction of 2-dimethylaminomethylidenenecyclohexane- diones 205 with cyanoacetamide (1a) in THF in the presence of NaOH gives rise to a mixture of cyanopyridones 206 and carbamoylpyrones 207. When enamines 205 are made to react with the amide 1a in ethanol under reflux, only carbamoyl- pyridones 208 are obtained in high yields.369 O CHNMe2 1a R1 O R2 205 V P Litvinov CN C(O)NH2 O O O O NaOH, THF + O NH R1 R1 207 206 R2 R2 C(O)NH2 O O EtOH, D NH R1 R2 208 R1=H, Me; R2=H, Me, Ph.The reaction of the enamine 209 prepared from dimedone with N,N-dimethylacetamide diethyl acetal (210) affords diaminodiene 211 in high yield. The latter reacts with amides 1a,b with the elimination of dimedone to give 3-cyano-6-dimethylamino- pyridin-2(1H)-one (212a) or -thione 212b, respectively.449 CHCH C(NMe2)2 CHNMe2 O O O O 1a,b + Me2 NC(Me)(OEt)2 210 Me Me Me Me 209 211 O O CN + X Me2N NH Me Me 212a,b X = O (a), S (b). 3-Cyanopyridin-2(1H)-ones, -thiones and -selenones can also be synthesised from b-enamino esters.Thus the condensation of the b-enamino ester 214 with the amide 1a affords 3-cyano-6- hydroxy-4-methylpyridin-2(1H)-one (213), which is employed in the synthesis of dyes.465 Me CN 1a H2N(Me)C CHCO2Et 214 HO O 213 NH It was found 447, 483, 484 that enamines 215a and 215b prepared from ethyl acetoacetate or acetoacetanilide are more reactive toward amides 1a ± c than ethyl acetoacetate itself. Thus the reactions of enamines 215a,b with amides 1a ± c in ethanol at room temperature afford salts 216 in high yields. The latter are readily converted into the corresponding pyridinones, -thiones or -selenones 217 on treatment with HCl. The pyridone 217 (X=O) was also prepared by the reaction of the enamine 215a (X=CH2) with the amide 1a in water followed by acidification of the reaction mixture.Me COR CN HCl 1a ± c + X H2N N Me Y7 O X NH 216 215a,b Me CN Y HO NH 217 R=OEt (215a), NHPh (215b); X=O, CH2; Y=O, S, Se.Cyanoacetamides and their thio- and selenocarbonyl analogues as promising reagents for fine organic synthesis The b-enamino ester 215a (X=CH2) reacts with benzylide- necyanoacetamide 218 to give substituted 3,4-dihydropyridone 219. The latter is transformed into ethyl 3-cyano-6-methyl-4- phenyl-1,2-dihydropyridine-5-carboxylate (220) on treatment with dilute nitric acid.485 CO2Et N Me +PhCH C(CN)CONH2 218 215a Ph Ph CN CN EtO2C EtO2C HNO3 O Me O Me NH NH 220 219 Tetrahydroisoquinolinone 222 has been obtained by the condensation of b-enamino ester 221 with cyanoacetamide (1a).465 CO2Et CN 1a NH2 O HO HN 221 222 a,b-Unsaturated carbonyl compounds, owing to the substan- tial difference in reactivities of their nucleophilic centres, react with cyanothioacetamides yet more regioselectively than the corresponding enamino carbonyl compounds.In this case, the synthetic strategy consists in the condensation of unsaturated carbonyl compounds with cyanothioacetamide (1b) leading to 5-oxo thioamides 223. The reason for regioselectivity is that Michael addition occurs preferentially, rather than the Knoeve- nagel condensation involving the carbonyl group. Further intra- molecular cyclisation of the adducts 223 and dehydrogenation of pyridinethiones 224 result in 3-cyanopyridine-2(1H)-thiones 225 (see reviews 28, 41 and papers 486 ± 500).R3 CN R2 R1 R2 1b R1 S O R3 O NH2 223 R3 R3 CN R2 CN R2 S S R1 R1 NH NH 224 225 R1=H, Alk, Ph, Bn, ; R2=H;R3=Ph, 4-ClC6H4, N 4-MeOC6H4, 4-NO2C6H4, 4-MeNC6H4; R2±R3=(CH2)4. These reactions were initially used in the synthesis of 4,5- diaryl-3-cyanopyridine-2(1H)-thiones.343, 486, 488 Later, a series of 4,6-dihetaryl-pyridinethiones was obtained.352, 491, 492 a,b-Unsa- turated carbonyl compounds were also employed in the synthesis of 4-aryl-6-methyl- 347, 358 and 4,5,6-trisubstituted 3-cyanopyr- idine-2(1H)-thiones.41, 346, 385, 386, 487, 490 The direction of any par- ticular reaction depends on the structure of the initial compounds, reaction conditions and catalyst.Thus reactions with the amide 1b in boiling ethanol or methanol in the presence of sodium alkoxides result in the formation of substituted 3-cyanopyridine-2(1H)- thiones 225.41, 343, 346, 347, 352, 358, 385, 386, 486, 487, 491, 492 However, if the sodium alkoxide catalyst is replaced by an equimolar amount of an organic base and the reaction is carried out at 20 8C, only tetrahydropyridines 224 are formed.489, 493 ± 498 In this case, an 751 organic base acts not only as a catalyst but also as a stabiliser of the partially hydrogenated pyridinethiones, which can be isolated as stable salts. The latter are converted into pyridinethiones 225 on treatment with HCl and oxidation with atmospheric oxygen. Although the condensations of a,b-unsaturated ketones with cyanothioacetamide (1b) afford the final products in somewhat lower yields compared to those obtained from 1,3-dicarbonyl compounds, this is counterbalanced by the selectivity of the reaction.Thus only the corresponding 3,4- or 4,4-bipyridines are formed from 1-(3- or 4-pyridyl)but-1-en-3-ones, respectively.167 The reactions of 2-arylidenecyclopentan(or -hexan)ones 226 and of 2,5-dibenzylidenecyclopentanone (228) with the amide 1b give rise to 5,6-annelated 4-aryl±3-cyanopyridine-2(1H)-thiones 227 499 and 229,501 respectively. Ar CHAr CN 1b (H2C)n (H2C)n S 226 NH 227 n=1, 2; Ar=Ph, 4-ClC6H4. Ph CN 1b CHPh PhCH S NH PhCH O 229 228 Pyridinethiones annelated with steroids were obtained using the reactions of the amide 1b with a,b-unsaturated ketones 17 and 230.The condensation of the amide 1b with the ketone 17 is accompanied by hydrolysis of the acetoxy group and results in pyridinethione 231. The reaction of the amide 1b with arylidene- ketone 230 affords 4-arylpyridinethione 232.60, 62 Me NH Ac S 1b CN HO AcO 17 231 (76%) S O HN CN CHAr 1b Ar 230 232 (81%) Ar=Ph, 3-FC6H4, . N The condensations of 2-arylmethylidene-3-oxoquinuclidines 233 with the amide 1b lead to the formation of stable salts 235, which are converted into partially hydrogenated 4-aryl-3-cyano- 1,5-naphthyridine-2(1H)-thiones 234 or 236 on treatment with HCl.502 Ar N O 1b, B BH+ S7 CHAr 235 NH N233 HClAr Ar N N CN CN S NH NH S 236 234752 Reactions of cyanoacetamides with aroyl- and hetaroylimino dithioethers have been described.273, 503 ± 506 Thus reactions of compounds 237 with cyanoacetamide (1a) in methanol in the presence of sodium methoxide afford 2-aryl(hetaryl)-5-cyano-6- methylthio-3,4-dihydropyrimidin-4-ones 238 in 40%± 60% yields. However, a vinylogue of compounds 237, viz., iminodi- thioether 239, gives under similar conditions 5-cyano-6-methyl- thio-4-phenyl-3,4-dihydropyridin-2-one (240) in 70% yield.504SMe R N SMe R HN1a RCON C(SMe)2 HN N CN CN 237 O O 238 R=Ar, Het.O Ph 1a NH PhCH CH2CON C(SMe)2 239 NC SMe 240 2-(Diacetylmethylidene)benzooxazole (241) prepared from dimethyl dithio acetal 242 and 2-aminophenol reacts with the amide 1a in boiling methanol in the presence of piperidine to give spiro compound 243 in 58% yield.505 NH2 NH 1a EtOH CAc2 Ac2C C(SMe)2+ 242 O OH 241 Me NH ONH2 O H2N(O)C 243 O,S-Acetals 244 react with the amide 1a on heating in ethanol in the presence of sodium alkoxides yielding 4-alkoxy-3-cyano- pyridin-2(1H)-ones 245.Similar reactions of cyclic O,S-acetals 246 give rise to annelated 4-alkoxy-3-cyanopyridin-2(1H)-ones 247.507 OR1 CN 1a, NaOR1 ArCOCH C(OR1)SR2 R1OH, 10 ± 12 h Ar O 244 NH 245 (74% ± 83%) Ar=Ph, 4-MeOC6H4, 4-ClC6H4, ;R1=Me, Et, Prn; R2=Me, Et. O CN SMe O HN 1a, NaOR OR OR ROH (CH2)n (CH2)n X X 247 (61% ± 74%) 246 n=1, 2; R=Me, Et; X=S, CH2.Salt 248 prepared by the reaction of diacetyl dimethyl ketal 249 with methyl formate in the presence of sodium methoxide reacts with the amide 1a to give trimethyl orthopicolinate 250.508 1a H2O HCO2Et 249 MeC(OMe)2Ac NaOMe MeC(OMe)2COCH CHONa 248 CN O Me(MeO)2C NH 250 V P Litvinov The use of a,b-ynones as substrates in the reactions with cyanoacetamides has been described.85, 347, 463, 509 3-Cyano-6- methyl-4-phenylpyridine-2(1H)-thione (136b) was obtained on stirring a solution of acetylphenylacetylene (251) with cyanothio- acetamide (1b) in ethanol in the presence of morpholine at 25 8C.85, 347, 463 However, the reaction of equimolar amounts of the ketone 251 with morpholine followed by the addition of the amide 1b to the reaction mixture gave isomeric 3-cyano-4-methyl- 6-phenylpyridine-2(1H)-thione 135b.It was concluded 85 that the Michael addition of the amide 1b to the ketone 251 is realised in the first reaction (path a) in which morpholine plays the role of a basic catalyst increasing the nucleophilicity of the methylene group in the amide 1b. In the latter case (path b), b-enamino ketone 252 is initially formed, which then reacts with the amide 1b in a selective manner to give the isomer 135b. This mechanism was proved by isolation of intermediate b-enamino ketone 252.85 AcC CPh 251 b a b, c Ph Me Ph AcCH CN CN N c 135b S Ph S Me O 252 NH 135b NH 136b O, EtOH; (b)HN (a) 1b, HN O, EtOH, 25 8C, 3 ± 4 h; (c) 1b. The use of a,b-unsaturated nitriles in reactions with amides 1b,c for the preparation of 3-cyanopyridine-2(1H)-thiones and selenones is based on the formation of d-oxothio- and d-oxo- selenoamide intermediates, which easily cyclise into a pyridine ring under the reaction conditions.49, 115, 353, 447, 498, 510 ± 521 This is exemplified in the preparation of 6-amino-4-aryl(hetaryl)-3,5- dicyanopyridine-2(1H)-thiones and -selenones 253 by the reac- tions of arylmethylidenecyanothioacetamides 7b with malononi- trile (5) or of arylmethylidenemalononitriles 254 with thio- and selenoamides 1b,c.Pyridinethiones and -selenones 253 can also be prepared by recyclisation of intermediate 2,6-diamino-4-aryl-3,5- dicyanothio(or -seleno)pyranones 255.The latter were isolated in some cases.49, 510, 512, 515 ± 518 4-Aryl-3,5-dicyano-6-hydroxypyri- dine-2(1H)-thiones and -selenones 257 are synthesised by analo- gous reactions of arylmethylidenecyanoacetates 256 with amides 1b,c.519 ± 521 5 ArCH C(CN)C(S)NH2 7b 1b,c ArCH C(CN)2 254 Ar Ar CN NC CN NC C C X X NH2 N N H2 N 258 259 Ar Ar CN NC NC CN D X X NH2 H2N H2N NH 255 253 ; X=S, Se. , Ar=Ph, 4-BrC6H4, N NCyanoacetamides and their thio- and selenocarbonyl analogues as promising reagents for fine organic synthesis Ar CN NC CN 1b,c ArCH CO2Et X HO 256 NH 257 X=S, Se. The ambident nature of the thioamide and selenoamide groups in intermediates 258 and 259 is believed to be the reason for the formation of pyridinechalcogenones 253 or thio(seleno)pyrans 255, the outcome of the reactions depends on the reaction conditions.Thio(seleno)pyrans 255 are formed under kinetically controlled conditions, while pyridinechalcogenones 253 are formed under thermodynamic control. Such a dual chemical behaviour of intermediates 258 and 258 often prevents the unambiguous elucidation of the structure of the reaction products and often leads to misinterpretations. Thus the struc- ture of 5-arylmethylidenpyridinethione has been erroneously attributed to the products of thermodynamically controlled reactions of arylmethylidenemalononitriles 254 with cyanothio- acetamide 1b.520, 521 This incorrect structure was then reproduced in reviews.34, 166 Another similar mistake has been made with respect to the product obtained in the reaction of (1-amino-2,2,2- trichloroethylidene)malononitrile with the amide 1b;522 this is not pyridinethione derivative 253 (X=S), but rather the thiopyran derivative 255.523 Once again, no evidence in favour of earlier 524 postulated formation of 3,4-dihydropyridinethiones has been found later.One-step condensation of cycloalkylidenemalononitriles 260 with cyanothioacetamide (1b) or the corresponding cyanothio- amides 261 with malononitrlile proceeds via intermediates 263 and results in spirodihydropyridines 262.58 CN 1b (H2C)n CN CH(CN)2 260 (H2C)n CN CH2(CN)2 CH(CN)C(S)NH2 263 (H2C)n C(S)NH2 261 SH NC S NC NH NH (H2C)n (H2C)nNC NH2 NH2 NC262 n=1, 2.The reactions of arylmethylidenecyanothioacetamides 7b with aldehydes, ketones, esters, amides and the corresponding enam- ines are also widely used in the synthesis of 4-aryl-3-cyanopyr- idine-2(1H)-thiones. Depending on the structure of the initial carbonyl compound and the reaction conditions, these reactions can stop in the stage of either hydrogenated pyridinethiones or their oxidised forms.49, 510 ± 512, 525 ± 530 Unlike monocarbonyl compounds, acyclic 1,3-dicarbonyl compounds and b-enamino carbonyl derivatives react with the amides 7b in ethanol in the presence of an organic base (piperidine, morpholine, diethylamine, hexamethyleneimine) at 25 8C to give 1,4-dihydropyridine-2-thiolates 264.The latter are converted into 3-cyano-3,4-dihydropyridine-2(1H)-thiones 265 upon treatment with 10% HCl.49, 113, 349, 352, 446, 488, 495, 510 ± 512, 529, 531 ± 538 753 B R1COCH2COR2+ArCH C(CN)C(S)NH2 7b Ar Ar CN R1CO CN R1CO H+ HCl [O] S R2 R2 NH NH S7BH+ 264 Ar CN R1CO R2 NH S 265 R1=Me, OEt, Ph; R2=Me, CF3; Ar=4-FC6H4, 4-BrC6H4, 4-ClC N 6H4, 2-NO2C6H4, , . N The reaction of the amide 7b with a cyclic dicarbonyl compound such as dimedone allowed one to isolate the inter- mediate Michael adducts 266 as salts.113, 512 Acidification of the salts 266 affords compounds 267. Heating of the latter in ethanol in the presence of a base followed by acidification results in 3,4- dihydropyridinethiones 268.O O CH(Ar)CH(CN)C(S)NH2 HCl 7b, B Me Me EtOH O7BH+ O Me Me 266 Ar O Ar O CN CN 1) D, B 2) HCl Me Me S OH C(S)NH2 Me Me NH 267 268 Ar O CN Me SH Me NH Ar=4-FC6H4, ,N ,B=HN O,HN . N 4-Aryl-5-arylcarbamoyl-3-cyano-6-methylpyridine-2(1H)- thiones,539 stable 3-cyano-4-[2-furyl(thienyl)]-3,4-dihydro- pyridine-2(1H)-thiones,540 3-cyano-4-(2-furyl)-6-methyl-5-phe- nylcarbamoyl-1,4-dihydropyridine-2-thiolates and -selenolates 54 have been synthesised using the reactions of acetoacetanilides with the amides 7b. This approach was also used for the preparation of 60-methyl-50-phenylcarbamoyl-30,40-dihydro-spiro[cyclohexane- 1,40-pyridine]-20(10H)-thione (269) from acetoacetanilide and cyclohexylidenecyanothioacetamide.541 PhNH(O)C CN AcCH2CONHPh+ Me S NC C(S)NH2 O NH2754 PhNH(O)C CN S Me NH 269 Synthesis of 30,50-dicyano-60-oxo-spiro[cyclopentane-1,40- 10,40,50,60-tetrahydropyridine]-20-thiolate 270 by the reaction of ethyl cyclopentylidenecyanoacetate (271) with cyanothioacet- amide (1b) in the presence of N-methylmorpholine or by the reaction of cyclopentylidenecyanothioacetamide (272) with ethyl cyanoacetate under similar conditions is yet another example of the design of spiro-fused hydrogenated pyridine derivatives.542 CN 1b, B CO2Et NC CN 271 CN 7EtOH NCCH2CO2Et, B EtO S O NH2 C(S)NH2 272 S7BH+ NC NH NC O 270 All experimental results suggest that reactions of arylmethyl- idenecyanothioacetamides 7b and their carbocyclic derivatives with 1,3-dicarbonyl compounds and their enamines always involve the stage of intermediate formation of Michael adducts such as 1,4-dihydropyridine-2-thiolates and 3,4-dihydropyridine- 2(1H)-thiones. The only exception is the condensation of benzo- thiazolone 273 with the amides 7b, which results in the formation of annelated system 274.543 SH S OH S N 7b N N CHAr O NH O 273 274 As to the stereochemical aspects of reactions of cyanoacet- amides and their S- and Se-analogues, they remained unexplored until recently.Pyridinium ylides are very convenient model compounds for the study of regio- and stereoselectivity of formation of 3-cyanopyridine-2(1H)-thio- nes.58, 71, 83, 85 ± 88, 288, 492, 544 ± 559 It has been shown that condensa- tions of arylmethylidenecyanothioacetamides 7b with pyridinium ylides 275 generated in situ from the corresponding salts result in 3,4-trans-1,2,3,4-tetrahydropyridine-6-thiolates 276.The high selectivity of these reactions is due to the stereoselective addition of the ylides 275 (X=S) to the amides 7b. Subsequent cyclisation of adducts 277 into betaines 276 occurs with retention of the trans- arrangement of the hydrogen atoms.549, 554 Betaines 276 were obtained in high yields from pyridinium salts 278 and cyanothioa- cetamide (1b).551 When malononitrile (5) was used in the reaction instead of amide 1b, trans-adducts 279 were obtained from compounds (E)-278; these were isolated and examined.Treat- ment of the adducts 279 with H2S afforded trans-tetra- hydropyridines 276. The latter gave 3-cyanopyridine-2(1H)- thiones 280 upon boiling in AcOH in the presence of ammonium acetate. The condensations of amides 7b with pyridinium ylides 275 (X=O, S) give 2-oxo(thioxo)tetrahydropyridine-6-thiolates 281 in high yields. Ar H N + 7b + H N R 7 O NH2 CHC(O)R 275 277Ar H + CN D N H S7 HOR NH 276 1b 276 Y7 + Ar N ROC H Ar H ROC H 5 278 +N Y=Cl, Br. +N H Ar CN H 7b 275 S7 X NH 281 X=O, S. A great number of other examples of the use of cyanoacet- amides and their S- and Se-derivatives in the synthesis of functionalised 3-cyanopyridin-2(1H)-ones, -thiones and -sele- nones,560 ± 604 3-cyanopyridin-2-olates and -thiolates,605 ± 609 pyr- imidin-2-ones,610 ± 616 tetrahydroquinolin-2-ones,617, 618 5-cyano- 3,4-diphenylpyridazin-6-one,619 2-aryl-5-cyano-4-methylthio-1,3- oxazin-6-ones 620 and other heterocyclic compounds are also documented. Cyanoacetamides 1a,b have found application in the synthesis of substituted pyridines 282,621 quinolines 178,438 283,622 284,623, 624 isoquinolines 285,388, 389, 625 286,626 pyrazine 287,627 dihydropyrrolopyrimidines 288 628 and triazines 289.629, 630 Among other heterocyclic systems synthesised with the use of amides 1a ± c and their derivatives, thio- and selenopyr- ans,49, 510, 512, 515 ± 518, 631, 632 coumarins and thiocoumarins,439 442, 633, 634 and thiazines 635 are worth mentioning.Apart from that, the preparation of carbocyclic systems of the cyclohexane series from cyanoacetamide (1a) has been described.425, 636 NH2 CN NC N NH2 RHNC(O)CH2 282 (R=H, Ph) 283 (X=CN, CO2Et, CONH2; Ar=Ph, 4-ClC6H4, 4-BrC6H4, 3-NO2C6H4) C(O)NH2 OO N NH2 284 V P Litvinov CN S7 Ar CN S R NH 280 CN H2S 7 276 CN 279 Ar O X Me NH2 Me NPhCN OH N Me 285755 Cyanoacetamides and their thio- and selenocarbonyl analogues as promising reagents for fine organic synthesis R Me Me R N C(O)NH2 CN CN NH N CHC(O)NH2 NH2 SH O S O 286 NH NH287 292 EtO2C NH2 R1 Me N R=4-MeC MeN 6H4, , , ;B= O. S S O NH N R2 N CH2R4 NR3 N The use of pyridinium ylides 275 (X=O) in the N 288 (R1=H, Ph; R2=H, Me; R3=CO2Et; R4=OMe, OEt, NH2) three-component condensations allows one to perform a one- pot synthesis of 3-cyanopyridin-2(1H)-ones without preliminary synthesis of arylmethylidenecyanothioacetamides 7b.58, 83, 85, 288, 492, 549, 550 IV.Conclusion X 289 (X=CN, C(O)NH2, C(S)NH2) In the last decade, many derivatives of pyridin-2(1H)-ones, -thiones, -selenones and their salts have successfully been synthes- ised using a three-component condensation of cyanoacetamides or their derivatives with aliphatic, aromatic or heterocyclic aldehydes and 1,3-dicarbonyl compounds or their derivatives in the presence of an excess of organic base.495, 531, 532, 534, 535, 637 ± 653 B R1CHO+NCCH2C(X)NH2+R2COCH2COR3 1a ± c R1 R1 CN R2CO CN R2CO R3 X R3 X7BH+ NH NH R1=Alk, Ar, Het; R2=Alk, 2-MeC6H4NH; R3=Alk, NHPh; The data considered here clearly demonstrate the high synthetic potential of cyanoacetamides and their thio- and selenocarbonyl analogues.Many biologically active heterocyclic compounds have been obtained based on these reagents. This suggests that cyanoacetamides and their arylmethylidene and hetarylmethyl- idene derivatives can be particularly promising starting materials in combinatorial synthesis of functionalised carbo- and hetero- cyclic compounds used in the design of novel highly effective pharmaceuticals with a broad range of action. The great interest of chemists in these reagents is confirmed by the fact that more than 490 papers of 653 cited in this review are dated to the last 10 ± 15 years and more than 20% of them are patents.The review was written with financial support of the Russian Foundation for Basic Research (Project No. 99-03-32965). X=O, S, Se; B=HN , MeN O O, Et2NH. References Synthesis of 4-(aryl or hetaryl)-3-cyano-6-methyl-5-phenyl- carbamoyl-1,4-dihydropyridine-2-thiolates 290 from aromatic or heterocyclic aldehydes, amide 1b and acetoacetanilide in the presence of an excess of N-methylmorpholine may be regarded as an illustration of such an approach.642 ± 644 1. 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ISSN:0036-021X    

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Russian Chemical Reviews 68 (9) 765 ± 779 (1999) Synthesis and reactions of mixed halogenobuta-1,3-dienes R V Kaberdin, V I Potkin, V A Zapol'skii Contents I. Introduction II. Synthesis of mixed halogenobuta-1,3-dienes III. Chemical properties of mixed halogenobuta-1,3-dienes IV. Conclusion Abstract. The published data on the methods for the preparation of mixed halogenobuta-1,3-dienes are generalised and syste- matised with a special emphasis on compounds of high practical value. Chemical transformations of mixed halogenodienes are considered and broad possibilities of the use of these compounds for the synthesis of various valuable, but difficultly accessible polyfunctional products are demonstrated. The bibliography includes 115 references. I.Introduction The first representative of halogenobutadienes, viz., hexachloro- buta-1,3-diene, was obtained as early as 1876.1 However, in the subsequent 50 years studies on the synthesis of halogenobuta- dienes were episodic and only in the early 1930s did they become purposeful and systematic. The development of preparation methods and the search for possible uses of this class of compounds in organic synthesis were stimulated by the discovery that mono- and dichlorobuta-1,3-dienes easily polymerise. The first data on the synthesis of halogen derivatives of butadiene were generalised by Petrov in 1944.2 Later studies devoted mainly to the synthesis and transformations of chloro and fluoro derivatives of butadiene were systematised in a fundamental Houben ± Weyl monograph.3 Particularly intensive studies were carried out in the field of chloro derivatives of butadiene and their results were surveyed in reviews 4± 6 and monographs.7±9 At present, there are numerous publications devoted to the synthesis of mixed halogenobutadienes (MHB) containing atoms of different halogens, viz., fluorine, chlorine, bromine and iodine.Mixed halogenobutadienes possess a broad spectrum of useful properties: they are good dielectrics, refrigerants and heat-transfer agents; they are used as aerosols, lubricants and floatation agents and possess algicidal, bactericidal and fungicidal activities.10 ± 15A number of MHB manifest high antitumour activity.16 Chlor- ine ± fluorine-containing butadienes are employed as monomers for the preparation of valuable polymers and copolymers resistant to heat, light, chemical corrosion, etc.13, 17, 18 And, finally, MHB R V Kaberdin, V I Potkin, V A Zapol'skii Institute of Physical Organic Chemistry, National Academy of Sciences of Byelarus, ul.Surganova 13, 220072 Minsk, Byelarus. Fax (7-017) 284 16 79. Tel. (7-017) 284 16 00. E-mail: ifoch@ifoch.bas-net.by (R V Kaberdin, V A Zapol'skii). Tel. (7-017) 284 09 72. E-mail: potkin@ifoch.bas-net.by (V I Potkin) Received 2 March 1999 Uspekhi Khimii 68 (9) 845 ± 860 (1999); translated by V D Gorokhov #1999 Russian Academy of Sciences and Turpion Ltd UDC 547.413 765 765 774 778 are promising semi-products in organic synthesis. For example, the preparation of polyfluoromethine compounds is based on MHB, which opens a way to the synthesis of novel polymethine dyes.19 It should be noted that the introduction of atoms of different halogens into the butadiene molecule leads to a considerable increase in the number of theoretically possible isomers.Thus consideration of structural and geometric isomers shows that dichlorobuta-1,3-diene has nine isomers,7 while for butadiene derivatives with two different halogen atoms the number of possible isomers increases tenfold. From a theoretical aspect, mixed halogenobutadienes, like other compounds with the buta- diene system, are convenient subjects for studying the s-cis ± trans- isomerism associated with the appearance of conformationally non-equivalent structures resulting from the hindered rotation of the vinyl fragments around the central C7C bond.20 ± 22 They are also promising as model substrates for studying regularities of the processes of nucleophilic vinylic substitution (SNVin).This review surveys the literature data on the chemistry of mixed halogen- obuta-1,3-dienes published up to 1999. We presume that such a review will fill the gap in the relevant literature and will contribute to the further development of research into the chemistry of halogen derivatives of alkenes and alkadienes. II. Synthesis of mixed halogenobuta-1,3-dienes The basic methods for the preparation of MHB are: (1) Elimination reactions (dehydrohalogenation and dehalo- genation) of polyhalogenobutadienes and -butenes; (2) halogenation and hydrohalogenation of acetylenic hydro- carbons of the C4 series; (1) thermal decomposition of accessible polyhalogenated compounds.Other methods of the synthesis of individual representatives of MHB are also known. 1. Elimination reactions Numerous chlorofluoro- and bromochlorobutadienes were obtained by dehydrohalogenation and dehalogenation of poly- halogenobutanes and -butenes). Dehydrohalogenation of polyhalogenobutenes under the action of various dehydrohalogenation agents proceeds as the 1,4- or 3,4-elimination of a molecule of hydrogen halide depend- ing on the position of the C=C bond. Another conventional way for the synthesis of MHB is dehalogenation of polyhalogenobu- tanes and -butenes, which is carried out under the action of metals, zinc in most cases.766 a.Dihalogenobutadienes The simplestMHBcontaining two different halogen atoms can be obtained from trihalogenobut-1- and -2-enes. Thus the dehydro- bromination of 1,4-dibromo-2-chlorobut-2-ene (1), which is the main product of chloroprene bromination (2), resulted in 1-bromo-2-chlorobuta-1,3-diene (3) in preparative yield.23, 24 The diene 3 formed in this reaction is a mixture of geometric isomers with the predominance of the Z-isomer 3a.25 Br2 CH2 7HBr CClCH CH2 2 CH2BrCCl CHCH2Br 1 Cl Cl H Br + H H C C C C Br C C C C H H H H H 3b (E-isomer) 3a (Z-isomer) Papazyan et al. 26 have proposed performing the dehydrobro- mination of butene 1 in the presence of phase-transfer catalysts (PTC) that considerably simplified the process and made it more economical (the conditions for the synthesis of the diene 3, its yield and the data related to all subsequent syntheses of MHB by the elimination reaction are presented in Table 1).Chlorination of bromoprene (4) and subsequent dehydro- chlorination of 2-bromo-1,4-dichlorobut-2-ene (5) leads to a mixture of E- and Z-isomers of 2- (6) and 3-bromo-1-chloro- buta-1,3-diene (7).27, 28 Cl2 CH2 7HCl CH2ClCBr CHCH2Cl 5 CBrCH CH2 4CHCl CBrCH CH2+ CHCl CHCBr CH2 7 6 A more convenient synthesis of E- and Z-isomers of the diene 7 is based on the dehydrobromination of 3,4-dibromo-1-chloro- but-1-ene (8), which is the product of bromination of 1-chloro- buta-1,3-diene (9).29 Br2 7 7HBr CHCl CHCHBrCH2Br 8 CHCl CHCH CH2 9 Dehydrohalogenation of the by-products of the chlorine addition to the diene 4, viz., a mixture of 3- (10) and 2-bromo- 3,4-dichlorobut-1-enes (11), yielded a mixture of 2-bromo-3- chlorobuta-1,3-diene (12) and (E)-1,2-dichlorobuta-1,3-diene (13) (GLC).27 CH2ClCBrClCH CH2+ CH2 10 CBrCHClCH2Cl 11 CClCH CH2 7HBr (E)-CHCl 13 CH2 7HCl CClCBr CH2 12 The diene 12 was initially obtained by dehydrobromination of 3,4-dibromo-2-chlorobut-1-ene (14), which is a by-product of chloroprene 2 bromination.24 Br2 2 CH2 CH2 7HBr CClCBr CH2 12 CClCHBrCH2Br 14 The preparative synthesis of the diene 12 is based on the 1,4- elimination of hydrogen halide from 1,2-dibromo-3-chlorobut-2- ene (15) or 2-bromo-1,3-dichlorobut-2-ene (16) or a mixture of these butenes with an alkali (KOH or NaOH) in the presence of PTC.30 R V Kaberdin, V I Potkin, V A Zapol'skii 7HBr CH3CCl CBrCH2Br 15 12 7HCl CH3CCl CBrCH2Cl 16 Of the disubstituted chlorofluorobutadienes, the following three compounds are known: 1-chloro-1-fluoro- (17), 1-chloro-2- fluoro- (18) and 3-chloro-2-fluoro-buta-1,3-dienes (19) (see Table 1).The diene 17 was obtained as the result of dehydrochlorina- tion of 4,4-dichloro-4-fluorobut-1-ene (20),31, 32 whereas the diene (18) resulted from the dechlorination of 3,4,4-trichloro-3-fluoro- but-1-ene (21).33 The diene 19 is formed upon dehydrohalogena- tion of a liquid mixture of mono-, di- and trifluoro compounds which are the products of the reaction of 1,2,3,4-tetrachlorobu- tane with HF.34 Yet another route to the diene 19 consists in the dehydrohalogenation of 2,4-dichloro-1,3-difluorobutane (22), which is the main product of ClF addition to butadiene (see Table 1).35, 36 b.Trihalogenobutadienes The first representative of mixed trihalogenobutadienes, viz., 3-bromo-1,1-dichlorobuta-1,3-diene (23), was initially prepared by the thermal dehydrochlorination of 3-bromo-1,1,1,4-tetra- chlorobutane (24) 37 and later, of 2-bromo-4,4,4-trichlorobut-1- ene (25), which is the product of bromotrichloromethane addition to allene 38 (see Table 1). 3-Bromo-1,2-dichlorobuta-1,3-diene (26) was obtained by a multistep synthesis the last stage of which was dehydro- bromination of 3,4-dibromo-1,2-dichlorobut-1-ene (27), which is the main product of 1,2-dichlorobuta-1,3-diene (13) bromination.The latter is obtained by the dehydrochlorination of 1,2,4- trichlorobut-2-ene (28).39 Papazyan et al.26 simplified the syn- thesis of the diene 26 from the butene 28. The target product 26 was obtained by direct bromination of the butene 28 and subsequent dehydrohalogenation of the adduct formed, viz., 2,3- dibromo-1,2,4-trichlorobutane (29). The by-product of this reac- tion is 2-bromo-1,3-dichlorobuta-1,3-diene (30). Br2 CH2ClCBrClCHBrCH2Cl 29 CH2ClCCl CHCH2Cl 287HCl 7HCl, 7HBr CHCl CClCH CH2 13Br2 7HBr CHCl CClCHBrCH2Br 27 CHCl CClCBr CH2 26 Two methods for the synthesis of 1-bromo-2,3-dichlorobuta- 1,3-diene (31) are known.It is obtained by dehydrobromination of either 1,4-dibromo-2,3-dichlorobut-2-ene (32) 39 [the product of 2,3-dichlorobuta-1,3-diene (33) bromination] 39 or 1,2-dibromo- 2,3,4-trichlorobutane (34) [the product of bromine addition to the accessible 2,3,4-trichlorobut-1-ene (35)].26 CH2 CClCCl CH2 33 CH2ClCHClCCl CH2 35Br2 Br2 CH2ClCHClCBrClCH2Br 34 CH2BrCCl CClCH2Br 32 7HCl,7HBr 7HBrCHBr CClCCl CH2 31767 Synthesis and reactions of mixed halogenobuta-1,3-dienes Table 1. Synthesis of mixed halogenobuta-1,3-dienes by the elimination from halogenobutanes and -butenes. Ref. Yield (%) Halogenobutadiene Reaction conditions Elimination agent Starting compound 23, 24 25 KOH KOH CH2BrCCl=CHCH2Br (1) CH2BrCCl=CHCH2Br (1) 38 31.5, 1.5 10% NaOH NaOH CH2BrCCl=CHCH2Br (1) CH2ClCBr=CHCH2Cl (5) 27, 28 7 EtOH 96% EtOH, 0 8C, 14 h water, 1% katamin AB, EtOH, 40 8C, nitrosodiphenylamine 29 10% NaOH water, katamin AB, 40 8C, 3 h water, katamin AB the same EtOH, nitrosodiphenyl- 30 30 27 60.2 51.6 7 NaOH NaOH NaOH CHBr=CClCH=CH2 (3) (Z)-CHBr=CClCH=CH2 (3a), (E)-CHBr=CClCH=CH2 (3b) CHBr=CClCH=CH2 (3) 72 26 (E)-CHCl=CBrCH=CH2 (6a)+ +(Z)-CHCl=CBrCH=CH2 (6b)+ +CHCl=CHCBr=CH2 (7) (6a : 6b : 7=72 : 2 : 26) (E)-+(Z)-CHCl=CHCBr=CH2 (7) 66.8 (E:Z=82 : 18) CH2=CClCBr=CH2 (12) CH2=CClCBr=CH2 (12) CH2=CClCBr=CH2 (12)+ amine, 46 ± 48 8C +(E)-CHCl=CClCH=CH2 (13) (12 : 13=9:1) CFCl=CHCH=CH2 (17) CHCl=CFCH=CH2 (18) CH2=CClCF=CH2 (19) CCl2=CHCBr=CH2 (23) CHCl=CHCHBrCH2Br (8) (E:Z=3:1) CH3CCl=CBrCH2Br (15) CH3CCl=CBrCH2Cl (16) CH2=CHCBrClCH2Cl (10)+ +CH2=CBrCHClCH2Cl (11) (10 : 11=1:4) CFCl2CH2CH=CH2 (20) CHCl2CFClCH=CH2 (21) CH2FCHClCHFCH2Cl (22) CCl3CH2CHBrCH2Cl (24) 31 33 35, 36 37 53.5 7745.0 KOH Zn KOH 7 38 39 26 CCl3CH2CBr=CH2 (25) Et3 N KOH NaOH *90 65.0 80.0 CHCl=CClCHBrCH2Br (27) CH2ClCBrClCHBrCH2Cl (29) EtOH, boiling the same EtOH thermal dehydro- chlorination, 450 8C boiling, 2 h MeOH, 20 8C 80% EtOH, 40 ± 50 8C 39 26 26 54.0 47.0 90.0 KOH NaOH NaOH CH2BrCCl=CClCH2Br (32) CH2ClCHClCBrClCH2Br (34) CH2BrCBrClCHBrCH2Br (37) MeOH, 20 8C EtOH 80% EtOH CH2ClCBr=CBrCH2Cl (41) 58.0 777 42 43 43 16 47 Zn KOH KOH CCl3CH2CBrFCH3 (46a) Bu3 N CBr3CH2CBrFCH3 (46b) Bu3 N CBrF2CCl2CH=CH2 (48) CFCl=CFCH2CH2Br (50) (E:Z=44 : 56) CCl2=CClCHBrCH2Br (52) 26 93.0 NaOH EtOH, 25 8C 140 8C, 100 Torr the same EtOH EtOH (KOH : EtOH= =1:1) 70% EtOH, 20 ± 25 8C 49 water, TEBAa CHBrClCHBrCHClCHCl2 (57) KOH 91.0 43.5 717.0 the same EtOH, boiling the same EtOH 50 51 51 353 KOH Zn Zn Zn KOH CHBrClCHBrCH=CCl2 (58) CBrF2CFClCCl=CH2 (59) CBrF2CFClCHClCHCl2 (60) CH2=CBrCClFCBrF2 (64) CCl2=CHCBrClCHBrCl (65) 54 20% NaOH CCl2=CHCBrClCHBrCl (65) 55 10% NaOH CBr2ClCHBrCH=CCl2 (67) water, TEBA,a 40 ± 45 8C, 4 h water, TEBA,a 25 8C, 15 h the same 55 10% NaOH CBrCl2CHBrCH=CBrCl (69) 51 19.0 EtOH, boiling CBrF2CFClCCl=CHCl (71) PrnOH or PriOH, boiling CCl2=CHCBr=CH2 (23) CHCl=CClCBr=CH2 (26) CHCl=CClCBr=CH2 (26)+ +CHCl=CBrCCl=CH2 (30) (26 : 30=94.5 : 5.5) CHBr=CClCCl=CH2 (31) CHBr=CClCCl=CH2 (31) CHBr=CClCBr=CH2 (38)+ +CHBr=CBrCCl=CH2 (39) (38 : 39=93 : 7) CHCl=CBrCBr=CH2 (40) CCl2=CHCF=CH2 (42) CBr2=CHCF=CH2 (43) CF2=CClCH=CH2 (44) (E)-+(Z)-CFCl=CFCH=CH2 (45) 80.0 (E:Z=44 : 56) CCl2=CClCBr=CH2 (54)+ +CCl2=CClCH=CHBr (55) (54 : 55=10 : 1) (Z)-+(E)-CBrCl=CHCH=CCl2 (56) 60 (Z: E=4:1) CBrCl=CHCH=CCl2 (56) CF2=CFCCl=CH2 (61) CF2=CFCH=CHCl (62) CF2=CFCBr=CH2 (63) EtOH, 25 8C (Z)-+(E)-CCl2=CHCCl=CBrCl (66) 68.0 (Z: E=1:1) (Z)-+(E)-CCl2=CHCCl=CBrCl (66) 85 (Z: E=1:3) (E)-+(Z)-CBrCl=CBrCH=CCl2 (68) 55.0 (E:Z=7:3) (E)-+(Z)-CBrCl=CHCBr=CCl2 (70) 49 (E:Z=3:2) CF2=CFCCl=CHCl (73) CFCl=CHCF=CF2 (74) 64 56 47.7 73.0 Zn CFCl=CHCFClCF2Cl (72) ZnZn Zn EtOH, boiling EtOH CF2=CFCF=CFCl (75) (E)-+(Z)-CF2=CFCF=CFCl (75) CFCl2CFClCFClCF2Cl (76) CFCl2CFClCFClCF2Cl (76)+ 10, 12 57 +CFCl2CFClCFClCF2I (79) (E:Z=1:1) 10 60, 61 62 EtOH BuOH, 85 ± 95 8C dioxane, boiling Zn Zn Zn CF2ClCFClCCl2CF2Cl CFCl2CFClCFClCFCl2 (77) CFCl2CFClCFClCFCl2 (77) 748.0 82.5 CF2=CFCCl=CF2 (81) CFCl=CFCF=CFCl (82) CFCl=CFCF=CFCl (82) (E,E : E,Z:Z,Z=1 : 2.5 : 1.5)768 Table 1 (continued).Reaction conditions Elimination agent Starting compound EtOH Zn CFCl2CFClCFClCF2I (79)+ CF2ClCCl2CCl2CF2Cl (93) 58 +CFCl2CFClCF2CFClI (89) CF2 =CFCF=CFCl (75) 41 10, 69 70 EtOH water, 25 8C Zn CBrCl2CHBrCCl2CHCl2 (98) Et2 NH KOH 20% KOH, TEBAa (E)-CBrCl=CClCHBrCBrCl2 (101) a Triethylbenzylammonium chloride. 1,1-Dibromo-3-chlorobuta-1,3-diene (36) is obtained as an insignificant admixture in the synthesis of 1,1,3-tribromobuta-1,3- diene.40 Upon dehydrobromination of 1,2,3,4-tetrabromo-2-chloro- butane (37) [the product of exhaustive bromination of chloro- prene (2)], 1,3-dibromo-2-chlorobuta-1,3-diene (38) is mainly formed. In addition, a small amount of 1,2-dibromo-3-chloro- buta-1,3-diene (39) was detected by GLC.26 Br2 CH2 7HBr CH2BrCBrClCHBrCH2Br 37 CClCH CH2 2 CH2BrCBrClCBr CH2+CH2BrCCl CBrCH2Br 7HBr CClCBr CHBr 39 CHBr CClCBr CH2+ CH2 38 2,3-Dibromo-1-chlorobuta-1,3-diene (40) was obtained by of 2,3-dibromo-1,4-dichlorobut-2-ene dehydrochlorination (41) 41 (see Table 1).As regards the fluorine-containing trihalogeno derivatives of MHB, 1,1-dichloro-3-fluoro- (42), 1,1-dibromo-3-fluoro-(43), 2-chloro-1,1-difluoro- (44) and 1-chloro-1,2-difluorobuta-1,3- dienes (45) were obtained. The synthesis of the dienes 42 and 43 is based on dehydrohalo- of 3-bromo-3-fluoro-1,1,1,-trihalogenobutanes genation (46a,b).42 CX2 7HBr,7HX CHCF CH2 42, 43 CX3CH2CBrFCH3 46a,b X=Cl (42, 46a), Br (43, 46b).The butadiene 44 was first synthesised from 2-chloro-1,2- difluoro-4-iodobut-1-ene (47) 43 and later from 1-bromo-1,1- difluoro-2,2-dichloro-2-chlorobut-3-ene (48) 16 and 2,4-dichloro- 1,1,1,-trifluorobut-2-ene (49).44 CF2 7HI CClCH2CH2I 47 Zn CF2 7BrCl CClCH CH2 44 CBrF2CCl2CH CH2 48 Zn 7ClF CF3CCl CHCH2Cl 49 The butadiene 45 is formed as a mixture of E- and Z-isomers upon dehydrobromination of the isomeric forms of 4-bromo-1- chloro-1,2-difluorobut-1-ene (50) obtained by dehalogenation of 1,4-dibromo-1,2-dichloro-1,2-difluorobutane (51), which is the product of telomerisation of CBrFClCBrFCl with ethylene under the action of benzoyl peroxide. It was shown that the starting butene 50 and the target diene 45 have the same ratio of isomers, i.e., the elimination reaction occurs stereospecifically.45 R V Kaberdin, V I Potkin, V A Zapol'skii Ref.Yield (%) Halogenobutadiene 45 CFCl=CFCF=CFCl (82), 788.0 10.0 16 CF2=CClCCl=CF2 (92) CCl2=CBrCCl=CCl2 (99), CBrCl=CBrCCl=CCl2 (100) (E)-CBrCl=CClCBr=CCl2 (102) 60 71 (E,E)-+(E,Z)-CBrCl=CClCBr=CBrCl (103) (E,E : E,Z=2:3) CH2 CH2 CBrFClCBrFCl 7BrCl CBrFClCFClCH2CH2Br 51 7HBr CFCl CFCH2CH2Br 50 (E,Z)F F F Cl + H H C C C C C C C C F Cl H H H H 45b (E-isomer) 45a (Z-isomer) 1,1,3-Trihalogenobutadienes are convenient subjects for the elucidation of characteristic features of conformational behav- iour. The use of 1H NMR and UV spectroscopy made it possible to establish that the bulky substituents (Cl, Br) in the 1,3 (or 2,4) positions of the diene chain destabilise the s-trans-form, which is characteristic of haloprenes and disubstituted butadienes, hence the 1,1,3-trihalogenobuta-1,3-dienes 23 and 36 exist in a skew conformation in the form of a single conformer.40, 46 According to the 1H NMR and IR spectroscopic data, the dienes 42 and 43 having a fluorine atom in the position 3 exist as a mixture of two conformers, viz., the skew and s-trans-forms.42, 47 c.Tetrahalogenobutadienes Several examples of the synthesis of tetrasubstituted MHB based on the elimination reaction are given below. Dehydrobromination of 3,4-dibromo-1,1,2-trichlorobut-1-ene (52), which is the product of bromine addition to the accessible 1,1,2-trichlorobuta-1,3- diene (53),48 affords a mixture of 3- (54, the main product) and 4-bromo-1,1,2-trichlorobuta-1,3-dienes (55, the by-product).26 Br2 CCl2 CCl2 7HBr CClCH CH2 53 CCl2 CClCHBrCH2Br 52CClCH CHBr 55 CClCBr CH2+ CCl2 54 Two convenient methods have been developed for the syn- thesis of 1-bromo-1,4,4-trichlorobuta-1,3-diene (56).The first method consists in dehydrohalogenation of 1,2-dibromo-1,3,4,4- tetrachlorobutanes (57) in the presence of PTC. It was shown that the reaction product is a mixture of geometric isomers with the predominance of the Z-form.49 The second method of the syn- thesis of the diene 56 is based on dehydrobromination of 3,4- dibromo-1,1,4-trichlorobut-1-ene (58) under the conditions of phase-transfer catalysis in the presence of triethylbenzyl- ammonium chloride (TEBA).50Synthesis and reactions of mixed halogenobuta-1,3-dienes CHBrClCHBrCHClCHCl2 577HBr,7HCl H Br H Cl Cl C C Cl C C C C Cl C C Br 56b 56a Cl H H Cl 7HBr CHBrClCHBrCH CCl2 58 Information about tetra(chloro, fluoro) MHB can be found only in the patent literature.Thus dehalogenation of 4-bromo-2,3- dichloro-3,4,4-trifluorobut-1-ene (59) and 1-bromo-2,3,4,4,-tetra- chloro-1,1,2,-trifluoro-butane (60) result in 3-chloro-1,1,2-tri- fluoro- (61) and 4-chloro-1,1,2-trifluorobuta-1,3-dienes (62), respectively.51 CF2 7BrCl CFCCl CH2, 61 CBrF2CFClCCl CH2 59 CHCl. CF2 7BrCl, 7Cl2 CFCH 62 CBrF2CFClCHClCHCl2 60 There is only one known representative of the bromine- containing tetrahalogenobutadienes, viz., 3-bromo-1,1,2-tri- fluorobuta-1,3-diene (63), which was obtained in high yield by dehalogenation of 2,4-dibromo-3-chloro-3,4,4-trifluorobut-1-ene (64) (see Table 1).3 CH2 CH2 7ClBr CBrCClFCBrF2 64 CBrCF CF2 63 d.Pentahalogenobutadienes Some pentasubstituted chlorobromo and chlorofluoro derivatives of butadiene can be obtained by an elimination reaction. Thus dehydrobromination of 3,4-dibromo-1,1,3,4-tetrachlorobut-1- ene (65) (the product of bromine addition to the Z- or E-isomers of 1,1,3,4-tetrachloro-1,3-butadiene) affords a mixture of geo- metric isomers of 1-bromo-1,2,4,4,-tetrachlorobuta-1,3-diene (66), the yield of which depends to a considerable extent on the reaction conditions (see Table 1).52 ± 54CBrCl CCl2 7HBr CHCBrClCHBrCl 65 (E,Z) CClCH CCl2 66 (E,Z) Dehydrobromination of 3,4,4-tribromo-1,1,4-trichlorobut-1- ene (67) in the presence of PTC leads to a mixture of E- and Z-isomers of 1,2-dibromo-1,4,4-trichlorobuta-1,3-diene (68a,b) Similarly, a mixture of E- and Z-isomers of 2,4-dibromo-1,1,4- trichlorobuta-1,3-diene (70a,b) 55 was obtained from 1,3,4-tri- bromo-1,4,4-trichlorobut-1-ene (69).7HBr CBr2ClCHBrCH CCl2 67 Cl Br Br Br Cl C C Cl C C + C C Cl C C Br 68b 68a Cl H H Cl 769 7HBr CBrCl CHCHBrCBrCl2 69 Br H Cl H Cl Cl C C C C + C C Br C C Cl Cl 70b Br Cl 70a Br Dehalogenation of 4-bromo -1,2,3-trichloro-3,4,4-trifluoro- (71) and 1,3,4-trichloro-1,3,4,4-tetrafluorobut-1-ene (72) (see Table 1) yielded 3,4-dichloro-1,1,2-trifluorobuta-1,3-diene (73) and 4-chloro-1,1,2,4-tetrafluorobuta-1,3-diene (74),56 respecti- vely.e. Hexahalogenobutadienes The synthesis of hexasubstituted MHB is documented; in partic- ular, the methods for the preparation of some chlorine, fluorine- containing representatives are well developed. Thus 4-chloro- 1,1,2,3,4-pentafluorobuta-1,3-diene (75) was first obtained by dehalogenation of 1,2,3,4,4,-pentachloro-1,1,2,3,4-pentafluoro- butane (76), which is the reaction product of 1,1,2,3,4,4-hexa- chloro-1,2,3,4-tetrafluorobutane (77) with a slight excess of SbF3Cl2.10, 12 SbF3Cl2 CFCF CFCl Cl(CFCl)4Cl CF2 F(CFCl)4Cl 72Cl2 75 77 76 Later,57, 58 two new methods for the synthesis of the diene 75 have been proposed.The first method is based on the trans- formation of a 1 : 1 mixture of erythro- and threo-forms of 3,4,4- trichloro-1,1,2,3,4-pentafluoro-1-iodobutane (78), which is the product of free-radical addition of 1,2,2-trichloro-1,2-difluoro-1- iodoethane to trifluoroethylene (GLC). Dehydrohalogenation of this product and subsequent chlorination of intermediate poly- halogenobutenes yield a mixture of polyhalogenobutanes (76 and 79). Dehalogenation of these compounds gives high yield of the diene 75 as a mixture of E- and Z-isomers.57 CFCl2CFClI+CHF CF2 CFCl2CFClCHFCF2I 78 7HCl, 7HI Cl2 [CFCl2CFClCF CF2+CFCl2CF CFCF2I] [Cl(CFCl)4F+Cl(CFCl)3CF2I] 7Cl2,7ClI 79 76 F F F F + Cl F C C C C C C C C F F Cl F F F 75b (Z-isomer) 75a (E-isomer) Another way to the synthesis of the diene 75 is based on the transformation of 1,3,4-trichloro-1,2,3,4,4-pentafluoro-1-iodo- butane (80), which is the reaction product of 1,2-dichloro-1,2,2- trifluoro-1-iodoethane with 1-chloro-1,2-difluoroethylene.58 Suc- cessive dehydroiodination, chlorination and dechlorination of polyhalogenobutane (80) yields the diene 75. CFCl CHF CF2ClCFClCHFCFClI CF2ClCFClI + 7HI 80 Cl2 F(CFCl)4Cl CF2ClCFClCF CFCl 72Cl2 76 CF2 CFCF CFCl 75 The synthesis of 3-chloro-1,1,2,4,4-pentafluorobuta-1,3-diene (81) is also based on elimination reactions.10, 43770 CHF CClCHFCF2I CF2 CClI+CF2 CF2 7HI CF2ClCFClCCl2CF2Cl CF2 72Cl2 CClCF CF2 81 The synthesis of 1,4-dichloro-1,2,3,4-tetrafluorobuta-1,3- diene (82) has attracted much attention.The diene 82 was obtained for the first time by dechlorination of perhalogeno- butane 77,59 which in turn was synthesised by fluorination of hexachlorobuta-1,3-diene (83),60 the reaction of 1,2,3,4-tetra- chlorobuta-1,3-diene (84) with ClF3 (see Ref. 61) or chloroiodi- nation of 1,2-dichloro-1,2-difluoroethylene (85) with subsequent heating of the resulting 1,2,2-trichloro-1,2-difluoro-1-iodoethane (86) with acetic anhydride, dichloromethane and granulated zinc at 160 8C (Scheme 1). Scheme 1 CCl2 CFCl CFCl 85 CHCl CClCCl CHCl 84 CClCCl CCl2 83 ClF3 F2 ICl Zn CFCl2CFClI 86 Cl(CFCl)4Cl 7772Cl2 CFCl CFCF CFCl 82 7Cl2 7Br2,7Cl2 CFCl CFCFClCFCl2 87 CBrFClCBrFCFClCFCl2 88 Yet another method for the synthesis of the diene 82 is based on dechlorination of 1,3,4,4-tetrachloro-1,2,3,4-tetrafluorobut-1- ene (87), which is the product of thermal dimerisation of the ethylene 85.63, 64 The diene 82 is formed in preparative yield upon treatment of 1,2-dibromo-1,3,4,4-tetrachloro-1,2,3,4-tetrafluoro- butane (88), which is the product of bromine addition to the butene 87,64 with zinc in dioxane (Scheme 1).And, finally, there are data on the synthesis of diene 82 based on transformations of a mixture of perhalogenobutanes.58 Thus dehalogenation of a mixture of telomers Cl(CFCl)3CF2I (79) and Cl(CFCl)2CF2CFClI (89) or F(CFCl)4F (90), Cl(CFCl)4F (76) and Cl(CFCl)4Cl (77) affords a 1 : 1 mixture of the dienes 75 and 82 (in the ratio 1 : 1) in the former case or a mixture of dienes 75, 82 and hexafluorobuta-1,3-diene in the latter case.CFCl CFCl2CFClI+CF2 89 [Cl(CFCl)3CF2I+Cl(CFCl)2CF2CFClI] 79 CFCl CFCF CF2+ CFCl CFCF CFCl 75 82 Zn F(CFCl)4F+Cl(CFCl)4F+Cl(CFCl)4Cl 77 76 90 75+82+ CF2 CFCF CF2 91 Theoretically, the diene 82 can exist in the form of three geometric isomers. F F Cl F F F F C C F C C Cl C C C C F C C Cl C C Cl Cl F Cl F F F Z,Z-form E,Z-form E,E-form R V Kaberdin, V I Potkin, V A Zapol'skii Indeed, as was shown by Kremlev et al.,62 the diene 82 is an equilibrium mixture of E,E-, E,Z- and Z,Z-isomers in the ratio of 1 : 2.5 : 1.5.The 19F NMR (Ref. 65) and UV (Ref. 21) spectral data and the values of dipole moments 66 make it possible to conclude that the isomers are in the non-planar s-cis-conforma- tion similarly to hexachloro- (83) 67 and hexafluorobutadienes (91).68 Two methods for the synthesis of 2,3-dichloro-1,1,4,4-tetra- fluorobuta-1,3-diene (92) are known, viz., dehalogenation of 1,2,2,3,3,4-hexachloro-1,1,4,4-tetrafluorobutane (93) 10 and 1,4- dibromo-2,2,3,3-tetrachloro-1,1,4,4-tetrafluorobutane (94).60, 69 72Cl2 CF2ClCCl2CCl2CF2Cl 93 CF2 CClCCl CF2 92 72BrCl CF2BrCCl2CCl2CF2Br 94 Some other difficultly accessible MHB, viz., 3,4-dichloro- 1,1,2,4-tetrafluoro- (95), 1,2,3,4-tetrachloro-1,4-difluoro- (96) and 2,3,4-trichloro-1,1,4-trifluorobuta-1,3-dienes (97), were obtained similarly.10, 60 CF2 CF2ClCFClCCl2CFCl2 72Cl2 CFCl2CCl2CCl2CFCl2 72Cl2 CF2 CF2ClCCl2CCl2CFCl2 72Cl2 CFCCl CFCl 95 CFCl CClCCl CFCl 96 CClCCl CFCl 97 Bromine ± chlorine-containing hexahalogenobutadienes were synthesised by an elimination reaction by the authors of this review.Thus dehydrohalogenation of 1,2-dibromo-1,1,3,3,4,4- hexachlorobutane (98) with aqueous-ethanolic solutions of alka- lis yields two MHB, viz., 2-bromo-1,1,3,4,4-pentachloro- (99) and 1,2-dibromo-1,3,4,4-tetrachlorobuta-1-3-diene (100), in the ratio of 2 : 1. CCl2 CBrCl2CHBrCCl2CHCl2 98 7HCl, 7HBr CBrCCl CCl2 99 CBrCl CBrCCl CCl2 100 The use of an aqueous solution of diethylamine shifted the reaction towards the predominant formation of the diene 99.70 Dehydrohalogenation of (E)-1,3,4-tribromo-1,2,4,4-tetra- chlorobut-1-ene (101) under the conditions of phase-transfer catalysis yielded (E)-1,3-dibromo-1,2,4,4-tetrachloro- (102) and 1,3.4-tribromo-1,2,4-trichlorobuta-1,3-dienes (103) as a mixture of two geometric isomers.Since the b-bromo-a,b-dichlorovinyl group of the butene (101) having the E-configuration is not directly involved in the reaction, the formation of the two isomeric forms of the diene 103 occurs due to the E,Z-isomer- isation of the a,b-dibromo-b-chlorovinyl group in the molecule. Consequently, the diene 103 is a mixture of E,E- (103a) and E,Z- isomers (103b).71 Cl Cl Cl C C C C Br Cl Cl KOH Cl 102 Br C C Br CHBrCBrCl2 Cl Cl Cl Cl 101 + Cl C C Br C C C C Br C C Br Br Br Cl Br 103b (E,Z-isomer) 103a (E,E-isomer)Synthesis and reactions of mixed halogenobuta-1,3-dienes 2.Halogenation and hydrohalogenation of the acetylenic hydrocarbons of the C4 series Acetylenic hydrocarbons, mainly conjugated halogenobutenynes and dihalogenobutadiynes, have served as the starting com- pounds for the synthesis of the first representatives of MHB.2 The syntheses based on acetylene derivatives have not found wide uses because of their poor accessibility and explosiveness. None- theless, nowadays procedures for the preparative synthesis of a number of di-, tri- and hexasubstituted MHB have been devel- oped. It is also noteworthy that a new catalytic reaction of acetylene dimerisation has been recently discovered. This is carried out in methanolic solution of NaI in the presence of Pt(IV); it is accompanied by the addition of an iodine molecule and results in (E,E)-1,4-diodobuta-1,3-diene.72 PtIV (ICH=CH)2 HC:CH I7, I2/MeOH E,E-isomer This reaction seems to be promising for the synthesis of certain MHB.The currently used procedures for the synthesis of MHB by the addition of halogens and hydrogen halides to the car- bon ± carbon triple bond imply the use of the existing C4 carbon chain. a. Dihalogenobutadienes The first representative of the dihalogenobutadienes, viz., 2-chloro-1-iodobuta-1,3-diene (104), was prepared by hydro- chlorination of 4-iodobut-1-en-3-yne (105a) in the presence of catalysts CuCl+NH4Cl.73 In the reaction of this butenyne (105a) with HBr, the predom- inant direction is the reductive elimination of the iodine atom at the triple bond with the formation of vinylacetylene (106).Among the reaction products, a considerable amount of (E)-2-bromo-1- iodobuta-1,3-diene (107a) was also found as the addition product of BrI to the enyne (106). The yield of (Z)-2-bromo-1-iodobuta- 1,3-diene (107b), which is the addition product of HBr to the initial butenyne 105a, is insignificant.74 Table 2. Synthesis of mixed halogenobutadienes based on acetylene derivatives. Starting compound Reaction conditions Electrophilic agent (Hal2 or HHal) HCl HBr HI HI HBr CuCl+NH4Cl CuBr CuI CuI CuBr 740 8C, CHCl3 the same CH2=CHCCl=CHI (104) (Z)-CHCl=CBrCH=CH2 (6b) (Z)-CHCl=CICH=CH2 (108) (Z)-CHBr=CICH=CH2 (109) (E)-+(Z)-CHI=CBrCH=CH2 (107) CH2=CHCCl=CClI (112) CH2=CHCBr=CBrI (113) " CH2 =CHCBr=CBrCl (114) " CH2 =CHCCl=CBrCl (115) CCl4 CH2ClCH2Cl CH2=CHC:CI (105a) CH2=CHC:CCl (105b) CH2=CHC:CCl (105b) CH2=CHC:CBr (105c) CH2=CHC:CI (105a) CH2=CHC:CI (105a) Cl2 CH2=CHC:CI (105a) Br2 CH2=CHC:CCl (105b) Br2 CH2=CHC:CBr (105c) Cl2 CH2ClC:CCH2Cl (117) Br2 , 1,8-diazabicyclo- [5.4.0]undec-7-ene CH2ClC:CCH2Cl (117) I2, 1,8-diazabicyclo- [5.4.0]undec-7-ene ClI EtCl, 750 8C CH:CC:CH CuBr2 MeOH, 1 h CCl4, 25±60 8C CHCl3 CHCl3 CHCl3 CCl4 CCl4 dry ether CHCl=CClC:CH CCl:CC:CCl (121a) Br2 CCl:CC:CCl (121a) I2 CBr:CC:CBr (121b) I2 CI:CC:CI (121c) Br2 CCl2=CClC:CCl (128) Br2 CCl2=CClC:CBr CCl2=CClC:CI Br2 I2 771 CH2 CHC CI 105a HCl CH2 CHCCl CHI 104 H H BrI HBr I C C CH2 7BrI CHC CH 106 C C H H 107a Br H H HBr H C C C C H I 107b Br The addition of hydrogen halides to the conjugated enynes with a halogen atom at the triple bond in the presence of copper(I) halides is widely used for the synthesis of MHB.74 It was established that 4-halogenobut-1-en-3-ynes (105b,c) add HBr and HI at the triple C:C bond giving the corresponding dihalogenobutadienes. Thus hydrobromination of 4-chlorobut- 1-ene-3-yne 105b proceeds stereospecifically with the formation of the Z-form of 2-bromo-1-chlorobuta-1,3-diene (6b) in high yield.The reaction of hydroiodination of chloro- (105b) and bromo- substituted (105c) enynes proceeds similarly and results in the formation of (Z)-1-chloro- and 1-bromo-2-iodobuta-1,3-dienes (108 and 109), respectively (Table 2). HHal CH2 CHC CX 105b,c (Z)-H2C CHCHal CHX 6, 108, 109 6: X=Cl, Hal=Br; 108: X=Cl, Hal=I; 109: X=Br, Hal =I. The addition of mixed halogens (ClI and BrI) to the con- jugated butenyne 106 was studied.74, 75 It turned out that the 1,4- addition of halogens occurred predominantly. Only with the use of a large excess of enyne and in the presence of CuCl were the 1,2- addition products, viz., the dienes 104 and 107 in the form of E-isomers, obtained.74, 75 Ref.Yield (%) Halogenosubstituted butadiene 73 74, 75 74, 75 74, 75 74, 75 74 74 74 74 795 45.2 70.8 41.5 9.7 14.2 7.3 8.5 (Z)-CHCl=CBrCBr=CH2 (40) 91 76 (Z)-CHCl=CICI=CH2 (118) 71 7678 84.0 CHI=CClCCl=CHI (119) (Z,Z+Z,E+E,E ) CHCl=CClCBr=CBr2 (120) CBrCl=CBrCBr=CBrCl (122) CICl=CICI=CICl (123) CBrI=CICI=CBrI (125) CBrI=CBrCBr=CBrI (126) CCl2=CClCBr=CBrCl (100) CCl2=CClCBr=CBr2 (130) CCl2=CClCI=CI2 (131) 79 11 80, 81 80, 81 80, 81 42 42 42 50.0 *98 77784.0 77772 b. Trihalogenobutadienes An attempt to obtain mixed trihalogenobutadienes by the addi- tion of halogens to the triple bond of enynes 105a ± c did not give satisfactory results.74, 75 The reaction of chlorine and bromine with these enynes proceeded mainly as the addition of halogens to the double bond and the 1,4-addition of enynes with the formation of butynes 110 and allenes 111, respectively. The addition of one mole of a halogen to the triple bond of enynes, which would lead to the corresponding trihalogenobuta-1,3-dienes, is less prefera- ble, the yields of products 53, 112 ± 116 are low.74, 75 Y2 CH2+ CHC CX 105a ± c CH2YCH C CXY+CH2 111 CH2YCHYC CX+ 110CHCY CXY 53, 112 ± 116 X=I: Y=Cl (112), Br (113); X=Y=Cl (53); X=Cl, Y=Br (114); X=Br, Y=Cl (115); X =Y=Br (116).The addition of bromine or iodine to 1,4-dichlorobut-2-yne (117) yielded the corresponding halogeno-substituted butenes, which gave (Z)-2,3-dibromo-1-chloro- (40) and (Z)-1-chloro-2,3- diiodobuta-1,3-dienes (118) upon catalytic 1,4-dehydrochlori- nation.76, 77 CH2ClC CCH2Cl 117 Br2 I2 CH2ClCI CICH2Cl CH2ClCBr CBrCH2Cl 7HCl 7HCl CICI CHCl CBrCBr CHCl (Z)-CH2 (Z)-CH2 118 40 c.Tetra- and pentahalogenobutadienes The synthesis of tetra- and pentasubstituted buta-1,3-dienes is demonstrated in separate examples. 2,3-Dichloro-1,4-diiodobuta- 1,3-diene (119) was obtained as a mixture of Z,Z-, Z,E- and E,E- isomers by the reaction of diacetylene with chloroiodine.78 ClI CH CC CH Cl Cl Cl H I I I C C C C C C I + H + C C I C C H C C H H Cl H Cl I Cl 119c (E,E-isomer) 119b (Z,E-isomer) 119a (Z,Z-isomer) The reaction of copper(II) bromide with 1,2-dichlorobut-1-en- 3-yne gives 1,1,2-tribromo-3,4-dichlorobuta-1,3-diene (120) (see Table 2).79 d.Mixed perhalogenobutadienes In 1930, difficultly accessible perhalogenobutadienes (122 ± 127), including first representatives of MHB, were obtained by halo- genation of dihalogenobuta-1,3-dienes (121).80, 81 Y2 CXY CYCY CXY 122 ± 127 CX CC CX 121a ± c X=Cl (a), Br (b), I (c); Y=Br, I. 127 126 124 123 122 Compound 125 X Cl Cl Br Br I I Y Br I Br I Br I R V Kaberdin, V I Potkin, V A Zapol'skii According to the patent data,11 1,4-dichloro-1,2,3,4-tetrabro- mobuta-1,3-diene (122) is formed in quantitative yield upon bromination of dichlorobuta-1,3-diene (121a). Synthesis of some mixed perhalogenobutadienes was carried out based on the accessible perchlorobutenyne (128).42 Thus direct bromination of the enyne 128 gave 1,3,4,4-tetrachloro-1,2- dibromobuta-1,3-diene (100) in high yield, whereas the synthesis of 1,1,2-tribromo- (130) and 3,4,4-trichloro-1,1,2-triiodobuta-1,3- dienes (131) was carried out using copper trichlorovinyl acetyle- nide 129 (Scheme 2).42 Scheme 2 Br2 CCl2 CClCBr CBrCl 100 CCl2 CuCl, NH3 CClC CCl 128 CCl2 CClC CCu 129 Br2 I2 CClC CI CClC CBr CCl2 CCl2 I2 Br2 CCl2 CCl2 CClCBr CBr2 130 CClCI CI2 131 3.Synthesis of mixed halogenobutadienes by pyrolysis of polyhalogenated compounds A number of preparative methods for obtaining MHB containing from four to six halogen atoms in the molecule are based on the pyrolysis of cyclic halogen-containing compounds.Thus the passage of 4-chloro-3,3,4-trifluoro- (132a) or 4,4-dichloro-3,3- difluoro-tricyclo[4.2.1,02,5]non-7-enes (132b) through a red-hot tube filled with rings of quartz glass yielded 4-chloro-1,1,4- trifluoro- (133) and 4,4-dichloro-1,1-difluorobuta-1,3-dienes (134), respectively.82 F FX CXCl CHCH CF2 133, 134 Cl 132a,b X = F (132a, 133); X=Cl (132b, 134). A more convenient method for the preparation of the diene 133 is based on the pyrolysis of 1-acetoxy-2-chloro-2,3,3- trifluorocyclobutane (135) or 1-acetoxy-3-chloro-2,2,3-trifluoro- cyclobutane (136).83 F X AcO F CFCl CHCH CF2 133 Y 135, 136 135: X =Cl, Y =F; 136:X=F,Y=Cl. ManyMHBwere obtained by the pyrolysis of the correspond- ing halogeno-substituted cyclobutenes.Thus the pyrolysis of 1-chloro-3,3,4,4-tetrafluorocyclobut-1-ene (137) gives 2-chloro- 1,1,4,4-tetrafluorobuta-1,3-diene (138) in preparative yield (Table 3).13, 83 F Cl F CF2 F CClCH CF2 138 137 F The main pyrolysis products of 1,2,3,4-tetrafluoro-3,4-diiodo- and 4-chloro-1,2,3,4-tetrafluoro-3-iodocyclobutenes (139a,b) are 1,2,3,4-tetrafluoro-1,4-diiodobuta-1,3-diene (140), formed as aSynthesis and reactions of mixed halogenobuta-1,3-dienes Table 3. Miscellaneous methods for the preparation of mixed halogenobutadienes. Halogeno-substituted butadiene Starting compound Reaction conditions Method of preparation a 132a (see b) 132b (see b) 136 (see b) 137 (see b) 139a 700 8C (1 Torr) 700 8C (1 Torr) 700 8C (10 Torr) 700 8C (1 Torr) 580 8C (0.1 Torr) AAAAA CFCl=CHCH=CF2 (133) CCl2=CHCH=CF2 (134) CFCl=CHCH=CF2 (133) CF2=CClCH=CF2 (138) CFI=CFCF=CFI (140) (E,E : E,Z:Z,Z=21 : 16: 8) CFI=CFCF=CFCl (141) CFCl=CHCH=CH2 (17) Cu, 180 ± 190 8C heating AA 139b CFCl2SOnCH2CH=CH2 (n=1, 2) CHCl=CFCF2CF2COONa (142) A (Z)-CF2=CFCF=CHCl (143) CCl2=CHCH=CBr2 (149) heating with a burner (1 Torr) CCl2=CHCHO (145) B Ph3 P=CBr2, CH2Cl2, 0 8C the same " BB CCl2=CClCH=CBr2 (150) CCl2=CBrCH=CBr2 (151) CI2 =CClCH=CBr2 (152) (Z)-CHBr=CHCCl=CCl2 (55) CCl2=CClCHO (146) CCl2=CBrCHO (147) CI2=CClCHO (148) B " CCl2=CClCHO (146) B Ph3 P=CHCOOEt, B CCl2=CBrCH=CCl2 (154) CCl2=CBrCHO (147) CBr2=CClCH=CCl2 (155) CHCl=CClCCl=CClI (157) CBr2=CClCHO (153) CHCl=CClCCl=CClLi BC Br2 CCl2=C=O, dry ether the same I2, FeCl3, ether,7110 8C Cl Cl C SF4 , 140 8C CF2 =CHCH=CCl2 (134) COOH 158 C CFCl=CFI Cu, 145 8C CFCl=CFCF=CFCl (82) (E,E : E,Z:Z,Z=19 : 51 : 30) aA is pyrolysis; B is synthesis of MHB from polyhalogenoacroleins; C is other methods.b The starting compounds were obtained by the [2 + 2]- cycloaddition reaction. mixture of E,E-, E,Z- and Z,Z-isomers, and 4-chloro-1,2,3,4- tetrafluoro-1-iodobuta-1,3-diene (141), respectively.84 CFCl+ CFCl CF2 F2C X = I F F CF2 X CFI CFCF CFI 140 CClCFClCF2Cl 144 I X=Cl F Cu F CFI CFCF CFCl 141 139a,b 4. Synthesis of mixed halogenobutadienes based on polyhalogenoacroleins X = I (139a), Cl (139b).An original way of the preparation of some bromine, chlorine- containing MHB is based on the reaction of polyhalogenoacro- leins 145 ± 148 with dibromomethylenetriphenylphosphor- ane.88, 89 CHal CHal2 CXCHO CXCHO+ Ph3P CBr2 145 ± 148 Mixed halogenobutadienes are also formed by the pyrolysis of some aliphatic compounds. Thus heating of sulfoxide CFCl2SOCH2CH=CH2 or sulfone CFCl2SO2CH2CH=CH2 yielded 1-chloro-1-fluorobuta-1,3-diene as one of the reaction products (17).85 Hal=Cl: X=H (145, 149), Cl (146, 150), Br (147, 151); Hal = I: X = Cl (148, 152). Decarboxylation of sodium 5-chloro-2,2,3,3,4-pentafluoro- pent-4-enoate (142) in vacuo gives high yield of (Z)-4-chloro- 1,1,2,3-tetrafluorobuta-1,3-diene (143).86 The first representative of MHB containing three different F F H C C CHCl CFCF2CF2COONa 142 C C F halogen atoms, viz., 4,4-dibromo-2-chloro-1,1-diiodobuta-1,3- diene (152) was obtained in 1970 by the reaction of a-chloro-b,b- diiodoacrolein (148) with Ph3P=CBr2.88, 89 Cl 143 F A method of preparation of (Z)-4-bromo-1,1,2-trichlorobuta- 1,3-diene (55) from the acrolein 146 and phosphorylide Ph3P=CHCOOEt has been developed.89 The butadiene 81 is present among the pyrolysis products of chlorofluoroethylene.Presumably, this resluts from thrmal de- chlorination of butene 144, which appears in the reaction due to the addition of chlorofluorocarbene to the initial chlorotrifluoro- ethylene. 773 Ref. Yield (%) 82 82 83 83 84 7756.0 38.0 37.0 84 85 77 86 82.0 88, 89 76.0 88, 89 88, 89 88, 89 89 62.0 57.0 44.0 82.0 42 63.0 42 90 55.0 60.0 91 7 92, 93 49.0 CFCl CFCl CFCl CF2 7Cl2 CClCF CF2 81 CHal2 CXCH CBr2 149 ± 152774 CCl2 7P(O)Ph3 CClCHO+ Ph3P CHCOOEt 146 H COOH COOEt H Br2 H2O C C C CH CCl H CCl2 CCl2 CClCl Cl CClCHBrCHBrCOOH Br C C CCl2 C C Cl 7CO2, 7HBr H H (Z)-55 Yet another method of the synthesis of MHB is based on the reaction of halogenoacroleins with dichloro ketene.Starting from the acrolein 147 and b,b-dibromo-a-chloroacrolein (153), 2-bromo-1,1,4,4-tetrachloro- (154) and 1,1-dibromo-2,4,4-tri- chloro-buta-1,3-dienes (155) were obtained, respectively.42 C O CHal2 CCl2 CHal2 CXCHO+ 147, 153 CXCH CCl2 154, 155 147, 154: Hal=Cl, X=Br; 153, 155: Hal=Br, X=Cl.This reaction is supposed to proceed through the initial formation of a lactone 156, which is easily decarboxylated with the formation of the target products 154 and 155. CCl2C O CXCH CHal2 156 O The yields of the dienes 55, 149 ± 152, 154, 155 and conditions of their synthesis are listed in Table 3. 5. Miscellaneous reactions for the preparation of mixed halogenobutadienes It was shown 28 that chlorination of bromoprene (4) yields, along with different addition products, also 2-bromo-1-chlorobuta-1,3- diene (6) as a mixture of Z- and E-isomers. Cl2 CH2 CHCl CBrCH CH2 6 CBrCH CH2 4 The reaction of lithium 1,2,3,4-tetrachlorobuta-1,3-dien-1-ide with iodine yielded 1,2,3,4-tetrachlorobuta-1-iodo-1,3-diene (157).90 I2 CHCl CClCCl CClLi CHCl CClCCl CClI 157 The synthesis of 1,1-dichloro-4,4-difluorobuta-1,3-diene (134) in low yield by the reaction of sulfur tetrafluoride with 2,2- dichlorocyclopropanecarboxylic acid (158) has been reported.91 Cl Cl CF2 +SF4 CHCH CCl2 134 COOH 158 A convenient method for the preparation of 1,4-dichloro- 1,2,3,4-tetrafluorobuta-1,3-diene (82) is the reducive dimerisation of a mixture of Z- and E-isomers of 2-chloro-1,2-difluoro-1- iodoethylene in the presence of a copper powder.92, 93 The diene 82 is formed as a mixture of E,E-, E,Z- and Z,Z-isomers in approximately the same ratio as reported elsewhere 62 (see Table 3).Cu CFCl CFI CFCl CFCF CFCl 82 Unfortunately, the attempt to extend this reaction for the synthesis of other MHB has been unsuccessful because of inaccessibility of iodine-containing halogenoethylenes. R V Kaberdin, V I Potkin, V A Zapol'skii III. Chemical properties of mixed halogenobuta- 1,3-dienes 1. Addition reactions. Halogenation of mixed halogenobutadienes Despite the fact that MHB possess electrophilic properties, they add, like other diene compounds with conjugated double bonds, halogens to both the terminal positions of the diene system (1,4- addition) and to one of the double bonds (1,2- or 3,4-addition). The direction of this reaction is determined to a considerable extent by the nature of MHB and the halogenation agent; experimental conditions are also highly important.The 1,4- addition is characteristic of the dienes having a small number of bulky substituents (Cl, Br or I) and low asymmetry. In the MHB with a high extent of substitution with bulky substituents, the p,p- conjugation is disturbed so that these dienes react as alkenes. Sometimes, an exhaustive halogenation takes place at both C=C bonds and results in the formation of polyhalogenobutanes. It was shown that the bromination of a mixture of E- and Z-isomers of 1-bromo-1,4,4-trichlorobutadienes (56) (E:Z ratio=1 : 4) occurs at the 1,2- and 3,4-positions and results in a mixture of two products, viz., 3,4,4-tribromo-1,1,4-trichloro- and 1,3,4-tribromo-1,4,4-trichlorobut-1-enes in the ratio 70 : 30 with a total yield of 76 %.55, 94 CBr2ClCHBrCH CCl2 CBrCl2CHBrCH CBrCl CBrCl CHCH CCl2 56 Bromination of the E-isomer of the diene 66 (45 ± 50 8C, illumination with an incandescent 60 W lamp, 8 h) occurs exclusively at the b,b-dichlorovinyl group and results in (E)-1,3,4-tribromo-1,2,4,4-tetrachlorobut-1-ene in a yield of 93%.71 Cl Cl Cl Cl Br2 C C Cl C C Br C C Br CHBrCBrCl2 66 Cl H Monochloropentafluoro- and dichlorotetrafluorobuta-1,3- dienes (their structure has not been unequivocally established) add one molecule of bromine with the formation of the corre- sponding butenes of the general formulae C4ClBr2F5 and C4Cl2Br2F4 at 20 ± 150 8C.61 The diene 81 adds two molecules of bromine at 50 8C and gives 1,2,3,4-tetrabromo-3-chloro-1,1,2,4,4-pentafluorobutane, the structure of which was confirmed by the results of chromato- mass spectrometry.87 According to Petrov et al.,95 the chlorina- tion and bromination of the diene 81 occur exclusively as the 1,4- addition and lead to the corresponding but-2-enes.Thus treat- ment of the diene 81 with a mixture of N-bromosuccinimide (NBS) andHFinTHFyielded a mixture of E- and Z-isomers of 3-chloro- 1-bromo-1,1,2,4,4,4-hexafluorobut-2-ene (E:Z=5 : 1) in a total yield of 82 %. Fluorination of the diene 81 with VF5 from720 to 730 8C occurs predominantly as the 1,4-addition and results in 2-chloroheptafluorobut-2-ene as a mixture of Z- and E-isomers (yields, 20% and 36%, respectively).A small amount (8%) of 2-chlorononafluorobutane is formed as the product of exhaustive fluorination.96 VF5 CF3CCl CFCF3+CF3CFClCF2CF3 CF3CCl CFCBrF2 CClCF CF2 CF2 81 HF NBS 2Br2 CBrF2CBrClCBrFCBrF2 According to the patent literature,97 the diene 92 reacts with bromine (illumination with a 100 W lamp) to give 1,4-dibromo- 2,3-dichloro-1,1,4,4- tetrafluorobut-2-ene (1,4-addition),Synthesis and reactions of mixed halogenobuta-1,3-dienes Br2 CBrF2CCl CClCBrF2, CF2 CClCCl CF2 92 whereas with the diene 82 1,2-addition occurs, which results in 3,4- dibromo-1,4-dichloro-1,2,3,4-tetrafluorobut-1-ene. Its chlorina- tion gives 1,2-dibromo-1,3,4,4-tetrachloro-1,2,3,4-tetrafluoro- butane.64 Chlorination of the diene 82 affords 1,1,2,3,4,4- hexachloro-1,2,3,4-tetrafluorobutane.64 2Cl2 CFCl2CFClCFClCFCl2 Br2 CBrFClCBrFCF CFCl CFCl CFCF CFCl 82 Cl2 CBrFClCBrFCFClCFCl2 2.Cycloaddition reactions a. Reaction of mixed halogenobutadienes with difluorocarbene The addition of carbenes to MHB was studied only for the reaction of difluorocarbene with the diene 81. Petrov et al.98 used hexafluoropropylene oxide as the source of difluorocarbene. Heating of an excess of the epoxide with the diene 81 in an autoclave gave the halogeno-substituted cyclopentene 159 in high yield. The cyclopentene 159 is one of the pyrolysis products of chlorotrifluoroethylene.87 According to Petrov et al.,98 the difluorocarbene formed in the course of pyrolysis adds to the diene 81 to give polyhalogenovinylcyclopropanes, whose intra- molecular rearrangement yields the cyclopentene 159.87 CF2 CF2 CClCF CF2 81F2C CClCF CF2 CFCCl CF2 + F2C CF2 Cl F CF2 F F FF 159 b.Diels ± Alder syntheses involving mixed halogenobutadienes Chloro-substituted buta-1,3-dienes (mostly, mono- and dichloro- derivatives) are convenient subjects for studying the Diels ± Alder reaction and preparation of practically valuable compounds.7 Some MHB were also studied in the [4+2]-cycloaddition. Thus upon prolonged boiling of an excess the dienes 40 and 118 with methyl vinyl ketone in the presence of Na2CO3, the corresponding 1-acetyl-3,4-dihalogenocyclohexa-1,3-dienes 160 (X=Br) and 161 (X=I) were obtained. Upon treatment of the cyclohexa- dienes 160 and 161 with 1,8-diazabicyclo[5.4.0]undec-7-ene, they are readily aromatised into the corresponding halogenoacetophe- nones 162 and 163.77, 99 Cl X C H C CHCOMe + CH2 7HCl C CH2 X 40, 118 COMe X COMe X 7HX X 162, 163 160, 161 The [4+2]-cycloaddition of (ClSN)3 and FSN to the buta- diene 81 yielded the difficultly accessible and highly reactive 6- membered heterocycles 164 and 165, respectively, which are the derivatives of 1,2-thiazines (Scheme 3). Treatment of the 1,2- thiazine 164 with water yields the 1,2-thiazin-3-one-1-oxide 166.The reaction of methanol with the 1,2-thiazine 165 results in the 775 Scheme 3 O F F Cl Cl H2O NH N (ClSN)3 SCl SO F F F F F166 F164 CF2 KF CClCF CF2 81 F F F F Cl Cl N MeOH N FSN SF SOMe F F F F F167 F165 substitution of the methoxy group for the sulfur-bound fluorine atom by the methoxy group with the formation of the product 167.Thiazine 164 is easily fluorinated with KF and gives a high yield of the thiazine 165.100 3. Elimination reactions As has been shown above (see Section II.2), butenynes are widely used in the synthesis of MHB. In turn, the accessible MHB can be used in the synthesis of some highly reactive halogen-containing butenynes. Thus the preparation of 4-bromo-1,1,2-trichlorobut- 1-en-3-yne (168) has been developed based on dehydrobromina- tion of the dienes 55 and 150. The synthesis of the butenyne 168 (yield 60 %) from the diene 55 (Z-isomer) includes two stages: the elimination of HBr under the action of the methanolic solution of potassium carbonate and the substitution of bromine for a hydrogen atom in the 1,1,2-trichlorobut-1-en-3-yne obtained under the action of sodium hypobromite.The second method of the synthesis of the butenyne 168 consists in the dehydrobromi- nation of the diene 150. It was established that the yield of the target product depends to a considerable extent on the nature of the dehydrohalogenation agent. When the methanolic solution of potassium carbonate is used, the yield of butenyne 168 is 57.5 %; this amounts to 70% with the use of benzyltrimethylammonium hydroxide (Triton B).89, 101, 102 CCl2 CCl2 7HBr CClC CBr 168 CClCH CBr2 150 NaOBr CClC CH CCl2 CCl2 7HBr CClCH CBrH 55 It was shown that the dehydrobromination of the diene 40 under the action of finely ground KOH at 100 8C results in 2-bromo-4-chlorobut-1-en-3-yne in 51% yield.42 CBrC CCl CH2 CH2 7HBr CBrCBr CHCl 40 Dehydrochlorination of some pentahalogenosubstituted MHByielded the previously inaccessible tetrahalogenobutenynes. CXC CCl CHal2 CXCH CCl2 CHal2 7HCl Hal=Cl, X=Br; Hal=Br, X=Cl.4. Substitution reactions a. Interaction of mixed halogenobutadienes with organometallic compounds Organolithium and -magnesium reagents found rather limited uses in the chemistry of MHB. The pronounced `affinity' of these compounds to halogens, on the one hand, and the presence of a large number of halogen atoms in the MHB molecules, on the776 other hand, largely prevent selective nucleophilic substitution in these substrates, though such reactions are known (see below).The metal ± halogen exchange is more characteristic of MHB. Thus starting from 1,2,3,4-tetrachlorobuta-1,3-diene 84 and butyllithium, mono- and dilithium derivatives were obtained, based on which a number of fine syntheses were carried out.103, 104 Syntheses involving MHB were studied in the example of 1,4- dichloro-1,2,3,4-tetrafluoro-1,3-butadiene (82).62 Treatment of an isomeric mixture of the diene 82 (E,E :E,Z:Z,Z=1 : 2.5 : 1.5) with butyllithium and then with CO2 gave (E,Z)-5-chloro-2,3,4,5- tetrafluoropenta-2,4-dienoic acid (169) in 25% yield. Comparison of the ratio of isomeric forms of this diene and the yield of the acid shows that it is the Z,Z-isomer of the diene 82 that reacts.This reaction proceeds through the stage of exchange of an exo- chlorine atom for lithium (according to Kremlev et al.,62 the starting diene exists in a non-planar s-cis-conformation). F F F F C C C C 1) BuLi 2) CO2 COOH Cl C Cl Cl C C C F F F F (E,Z)-169 (Z,Z)-82 The reaction of organomagnesium compounds with MHB occurs in a different way. The reaction of 1-chloroperfluorobuta- 1,3-diene (75) (E :Z=1 : 1) with phenylmagnesium bromide (170) includes the nucleophilic substitution of the phenyl radical for the terminal fluorine atom of the trifluorovinyl fragment, which results in the formation of 4-chloro-1,2,3,4-tetrafluoro-1-phenyl- buta-1,3-diene (171) as a mixture of E,E-, E,Z- andZ,Z-isomers in the ratio 2 : 1 : 2 and in a total yield of 77%.57 Various para-substituted arylmagnesium bromides (172 ± 175) were used in this reaction, and this extended appreciably the synthetic possibilities of this method.105 It was shown that the resulting 1-aryl-4-chloro-1,2,3,4-tetrafluorobuta-1,3-dienes (176 ± 179) (yields of 40%± 49 %) represent mixtures of E,E-, E,Z- and Z,Z-isomers in the ratio 1 : 1 : 1 (NMR data).The crystalline E,E-isomer of compound 176 was isolated in individ- ual form. Metallation of products 171, 176 ± 179 with butyllithium and subsequent carbonisation yielded a series of (E,E)-5-aryl- 2,3,4,5-tetrafluoropenta-2,4-dienoic acids (180 ± 184).CFCF CFCl CF2 p-RC6H4MgBr 170, 172 ± 175 BuLi, CO2 p-RC6H4CF CFCF CFCl 171, 176 ± 179 p-RC6H4CF CFCF CFCOOH 180 ± 184 R = H (170, 171, 180), Me2N (172, 176, 181), MeO (173, 177, 182), Me (174, 178, 183), CF3 (175, 179, 184). 19F NMR studies of the electronic effect of substituents and determination of the acidity have shown that the 1,2,3,4-tetra- fluorobuta-1,3-diene system is involved in the transfer of the electronic influence from a substituent in the aromatic ring to the corresponding reaction centre, viz., the carboxyl group or a fluorine atom.105 Compounds 171, 176 ± 179 served as the basis for the synthesis of symmetrical a,o-diarylperfluoropolyenes (185).19 BuLi CFCF CFCl 2 p-RC6H4CF 171, 176 ± 179 91 2 p-RC6H4CF CFCF CFLi p-RC6H4(CF CF)6C6H4R-p 185 R V Kaberdin, V I Potkin, V A Zapol'skii Compounds of the type 185 with the (CF=CF)n groups are good conductors of the conjugation effect because the fluorine atoms do not create any steric hindrance to its transfer.This opens up a broad perspective for the synthesis of a novel type of polymethine dyes with a totally or partially fluorinated chain.19, 105 b. Nitration reactions Of all the principal methods for the preparation of nitrobuta- dienes, the leading part belongs to the nitration of dienes with nitric acid or nitration mixtures. Nitro- and especially polychloronitrobutadienes are widely employed in organic synthesis.106 For example, pentachloro-2- nitrobuta-1,3-diene, which was obtained by the reaction of pentachloro-2H-buta-1,3-diene with nitric acid, proved to be a versatile starting material for numerous syntheses of compounds belonging to different classes.107 ± 109 The first representative of nitroderivatives ofMHB[4-bromo- 1,1,3,4-tetrachloro-2-nitrobuta-1,3-diene (186)] was obtained by the reaction of 60% nitric acid with a 1 : 1 mixture of Z- and E-isomers of 1-bromo-1,2,4,4-tetrachlorobuta-1,3-diene 66 at 80 ± 85 8C.The nitrodiene 186 is a mixture of two isomers (GLC). Along with the nitration of the initial diene 66 at the b,b- dichlorovinyl group, competitive oxidative destruction takes place, which leads to the formation of 3-bromo-2,3-dichloro- acrylic acid (187) as a by-product.110 HNO3 CBrCl CClCH CCl2 66 CBrCl CClC CCl2+CBrCl CClCOOH NO2 187 186 Recently,54 the nitration of the diene 66 with nitrating mixtures HNO3±H3PO4 and HNO3±H2SO4 has been studied over a broad range of concentrations and temperatures.It was found that the optimum conditions for the nitration process are as follows: the temperature 95 ± 100 8C, the ratio of the components of the nitration mixture HNO3:H3PO4 (or HNO3:H2SO4) equal to 10 : 1 and the ratio of diene : nitration mixture equal to 1 : 3 or 1 : 4. Under these conditions, the reaction is completed in 23 h; the yield of the nitrodiene 186 reaches 56% upon nitration with the HNO3±H2SO4 mixture and 58% upon nitration with the HNO3±H3PO4 mixture. The nitration of the diene 66 (as a mixture of Z- and E-forms in the ratio 1 : 3) yields an isomeric mixture of the nitrodiene 186 in virtually the same ratio.Among by-products of this reaction, b-bromo-a,b-dichloroacrolein (yield 8%) and 3-bromo-2,3- dichloroacrylic acid (187) were detected. HNO3±H3PO4 or HNO3±H2SO4 CBrCl CClCH CCl2 66 CBrCl CClC CCl2+CBrCl CClCHO+CBrCl CClCOOH 186 187 NO2 The nitration of other pentasubstituted MHB, viz., 1,2- dibromo-1,4,4-trichloro- (68) and 1,3-dibromo-1,4,4-trichloro- buta-1,3-dienes (70) with nitric acid or nitration mixtures occurs similarly. Thus nitration of the E-isomer of the diene 68 with the 55%± 60% nitric acid, fuming nitric acid or a nitration mixture HNO37H3PO4 yielded (E)-3,4-dibromo-1,1,4-trichloro-2-nitro- buta-1,3-diene (188).Synthesis and reactions of mixed halogenobuta-1,3-dienes Br Br CH CCl2 C(NO2) CCl2 Y C C C C Br Br Cl Cl 68 Nitration agent Y 188 Yield of 188 (%) T /8C 40 45 50 80 ± 85 20 90 ± 95 55%± 60% HNO3 Fuming HNO3 HNO3±H3PO4 The nitration of the diene 70 with the above-cited reagents under similar conditions gives 1,3-dibromo-1,4,4-trichloro-2- nitrobuta-1,3-diene in yields of 35%, 43% and 48%, respectively.1-Bromo-1,4,4-trichlorobuta-1,3-diene (56) reacts differently with nitric acid and with nitration mixtures. In this case, the nitration proceeds in the two directions: (1) substitution of a hydrogen atom in the b,b-dichlorovinyl group and (2) substitution of a bromine atom in the b,b-bromochlorovinyl group resulting in the formation of 1,1,4-trichloro-1,3-dinitrobuta-1,3-diene (189).Another reaction product is 4-bromo-1,1,4-trichloro-3-nitrobut- 1-ene (190). Under the optimum conditions [the nitration mixture HNO37H3PO4 (10 : 1), 90795 8C, 2 h], the yields of 1,4-dinitro- buta-1,3-diene 189 and nitrobutene 190 are 18% and 40%, respectively.49 H + CHClBrCH(NO2)CH CCl2 CBrCl CHCH CCl2 56NO2 C C Cl C C Cl 190 189 Cl NO2 According to the IR spectroscopy and the X-ray diffraction analysis, the diene 189 is the Z-isomer and has a virtually planar s-trans-configuration.111, 112 5. Isomerisation reactions It was shown above (see Section II.3) that the accessible poly- halogenocyclobutenes are convenient starting compounds for the synthesis of some MHB.As early as 1949, the thermal cyclisation of hexafluorobuta-1,3-diene (91) at 150 ± 180 8C was carried out and resulted in hexafluorocyclobutene (191) and a mixture of dimers C8F12 and trimers C12F18.113 Later, these transformations were also performed for hexachlorobuta-1,3-diene (83), and hexachlorocyclobutene (192) was isolated by distillation.7 The isomerisation of halogenobutadienes into the corresponding cyclobutenes is a reversible reaction.X X t X CX2 X CXCX CX2 83, 91 X X 191, 192 X = F (91, 191), Cl (83, 192). The isomerisation of MHB was studied in the example of 1,4- dichloro-1,2,3,4-tetrafluorobuta-1,3-diene (82) and 2,3-dichloro- 1,1,4,4,-tetrafluorobuta-1,3-diene (92). It was shown that heating of the diene 82 at 250 8C in a sealed glass tube results in 3,4- dichlorotetrafluorocyclobutene (193) in 56% yield.Upon heating of this diene in a platinum crucible over activated carbon to 320 8C, the yield of the butene 193 reaches 74%. The reverse process (transformation of cyclobutene into diene) occurs at 380 8C at a lower rate, therefore, the content of butadiene in the thermolysis products of the cyclobutene 193 is only 14%.64 F F Cl 3208C Cl CFCl CFCF CFCl 82 3808C F F 193 777 Heating of a mixture of E,E-, E,Z- and Z,Z-isomers of the diene 82 in a nickel tube at 330 8C yielded three products, viz, 3,4- dichloro- (193), 1,4-dichloro- (194) and 1,2-dichlorotetrafluoro- cyclobutenes (195). Presumably, the cyclobutenes 194 and 195 are formed due to the double bond migration catalysed by the fluoride ion.114 The diene 92 undergoes only partial transformation when heated in a sealed steel tube at 170 8C for 44 h.The unreacted diene 92 was recovered, 1,2-dichlorotetrafluorocyclobutene (195) and a small amount of unsaturated dimers and a dimer 196 97 were isolated from the reaction mixture. The total conversion of the diene 92 into the cyclobutene 195 occurs after 89 h.97 F F Cl Cl F Cl F F F + 170 8C 44 h F F F Cl F Cl F CF2 F 195 Cl196 CClCCl CF2 92 195 170 8C 89 h It was supposed 64 that the reaction diene.cyclobutene is a general characteristic of the polyhalogenobutadienes. 6. Miscellaneous reactions The reduction of MHB was studied only in the example of (Z)-1- bromo-2-chlorobuta-1,3-diene (3).It was shown that the reduc- tion of Z-diene (3a) with LiAlD4 in diethyl ether occurs with high stereoselectivity and leads to the formation of a mixture of (E)- (197a), (Z)-2-chloro-1-deuteriobuta-1,3-diene (197b) and 2-chlo- robutadiene (2) in the ratio 94.2 : 2.2 : 3.6, respectively.25 Cl H H C C C C D H H 197a (E-isomer) Cl Br Cl D H C C LiAlD4 H C C C C H C C H H H H H 3a 197b (Z-isomer) CH2 CClCH CH2 2 In example of the reaction of pentachloro-2H-buta-1,3-diene (198) with fuming nitric acid it was shown that the reaction product is 3,4,4-trichloro-2-nitrobut-3-enoic acid (199), i.e., the b,b-dichlorovinyl group of the diene 198 is involved in the reaction and an earlier transformation into the CH(NO2)COOH group occurs.115 An analogous reaction giving 4-bromo-3,4-dichloro-2- nitrobut-3-enoic acid (200) in high yield also occurs upon interaction of fuming nitric acid with 1-bromo-1,2,4,4-tetrachlor- obuta-1,3-diene (66).110 HNO3 CClX CClCH(NO2)COOH 199, 200 CClX CClCH CCl2 66, 198 X=Cl (198, 199), Br (66, 200).The reaction of the E-isomer of the diene 66 with chlorosul- fonic acid proceeds in an unusual manner. Under the optimum conditions (70 ± 80 8C, the diene : acid ratio equal to 1 : 2; 5 h), the reaction gives (E)-1,3-dibromo-1,2,4,4-tetrachlorobuta-1,3-diene 102 in 31% yield as the main product. In addition, bromochloromaleic acid 201 (yield 6%) and hexachlorobuta-1,3-diene 83 (yield *2%) were also found among the reaction products.Dienes 102 and 83 are apparently778 formed upon halogenation of the starting compound 66 with chlorine and bromine, which appear in the reaction mixture upon destruction and resinification of the substances involved. Acid 201 is the hydrolysis product of the diene 102. Cl Cl HSO3Cl Cl C C C C Br 66 Cl H Br Cl CCl2+ + CCl2 C C CClBr CClCBr 102 CClCCl CCl2 83 HOOC COOH 201 When the diene 66 is used in the reaction in the form of E- and Z-isomers (1 : 1), the diene 102 is also formed as a mixture of two isomers in the ratio 1 : 1.71 IV. Conclusion Analysis of the published data shows that constant attention is paid to the synthesis of mixed halogenobuta-1,3-dienes. Nowa- days, there are rather well developed methods for the synthesis of hexasubstituted chloro-, fluoro- and chlorobromobutadienes, while the methods for the synthesis of MHB containing from 2 to 5 atoms of halogens are somewhat less developed.Most of the methods for the preparation ofMHBare based on the elimination reactions of accessible polyhalogenobutenes and -butanes, i.e. they rely on the use and transformations of the existing C4 carbon chain. At present, novel methods have been proposed for the synthesis ofMHBunder the conditions of phase- transfer catalysis, which makes their preparation simpler and more efficient. Virtually all of the synthesised MHB contain two kinds of the halogen atoms. At present, there is a convenient procedure for the synthesis of only one MHB representative containing three kinds of the halogen atoms.Unfortunately, until now no MHB have been prepared with four different halogen atoms (F, Cl, Br, I) which could be used as convenient models for studying regularities of competitive processes of nucleophilic vinylic substitution (SNVin) of different halogen atoms. Transformations of MHB are represented in the relevant literature to a lesser extent; nonetheless, the available data show that MHB possess broad possibilities for the development of new methods for the synthesis of polyfunctional compounds which are difficultly accessible or totally inaccessible by other ways. We hope that a stimulus for the further development of studies in this field will be the fact that many MHB possess a broad spectrum of useful properties. References 1.F Krafft Chem. Ber. 10 801 (1876) 2. A A Petrov Usp. Khim. 13 203 (1944) 3. 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ISSN:0036-021X    

出版商:RSC    

年代:1999

数据来源: RSC

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Russian Chemical Reviews 68 (9) 781 ± 799 (1999) Aryl (meth)acrylates and polymers based on them V G Syromyatnikov, L P Paskal', I A Savchenko Contents I. Introduction II. Methods for the synthesis of aryl (meth)acrylates III. Polymerisation of aryl (meth)acrylates IV. Aryl (meth)acrylates in copolymerisation reactions V. Structure and properties of poly[aryl (meth)acrylates] VI. Chemical and photochemical properties of poly[aryl (meth)acrylates] and their copolymers VII. Practical application of aryl (meth)acrylates Abstract. Data on the synthesis, polymerisation and copolymer- isation of aryl (meth)acrylates are generalised and systematised. Chemical and photochemical properties of the polymers and copolymers are considered. Basic directions of practical applica- tion of poly[aryl (meth)acrylates] and copolymers are demon- strated.The bibliography includes 449 references. I. Introduction Aryl (meth)acrylates H2C=C(R)CO2Ar (hereinafter R=H, Me) occupy a special place in the series of (meth)acrylates due, first of all, to the diversity of functional groups which may be introduced into the aromatic ring of these monomers and the derived polymers. Specificity of physical properties of poly[aryl (meth)acrylates] is determined by the enhanced rigidity of the polymeric chains (because of the presence of aromatic rings). Aryl (meth)acrylates are easily copolymerised with most of the known monomers, which opens broad prospects for their application. Until recently, the results of numerous studies on the synthesis and properties of aryl (meth)acrylates have not been generalised.The present review is intended to fill this gap. II. Methods for the synthesis of aryl (meth)acrylates Earlier methods for the synthesis of aryl (meth)acrylates were rather complicated and were not of a general character. Thus phenyl acrylate (PA) was prepared by direct esterification of phenol with acrylic acid in the presence of trifluoroacetic anhy- dride 1 and also by the Reppe reaction, i.e., by the reaction of acetylene and carbon monoxide with phenol.2 Phenyl 3 and naphthyl acrylates 4 were obtained by pyrolysis of 2-acetoxypro- pionates MeCH2(OAc)CO2Ar. The systematic studies of the methods for the preparation of aryl acrylates were initiated by Magagnini and Pizzirani 5 who investigated the reactions of V G Syromyatnikov, L P Paskal', I A Savchenko Taras Shevchenko Kiev University, ul.Vladimirskaya 60, 252033 Kiev, Ukraine. Fax (38-044) 225 12 73. Tel. (38-044) 221 02 82 (V G Syromyatnikov), (38-044) 221 02 29 (L P Paskal') Received 21 August 1998 Uspekhi Khimii 68 (9) 861 ± 880 (1999); translated by V D Gorokhov #1999 Russian Academy of Sciences and Turpion Ltd UDC 541.64 : 547.564.4 781 781 783 785 788 790 792 acryloyl chloride with phenols. In addition to PA, these authors described acrylates of isomeric cresols, chlorophenols, thymol, hydroxybiphenyl and 4-methoxyphenol. It should be noted that the reactions were carried out under conditions precluding the addition of hydrogen chloride to the double bond.In earlier attempts of the synthesis of acrylates by this method, this addition took place; therefore, b-chloropropionates were the major prod- ucts. Conducting of these reactions in the presence of triethyl- amine allowed a considerable increase in the yields of methacrylates of substituted phenols. This method was success- fully used to prepare isomeric phenol di(meth)acrylates,6 2-biphenylyl acrylate 7 and 4-biphenylyl (meth)acrylate 8 and mono- and di(meth)acrylates of substituted 2,2 0-methylenebis- phenols.9 Aryl (meth)acrylates are usually synthesised by the reaction of phenols with (meth)acryloyl chloride in the presence of hydrogen chloride acceptors. This process may be carried out in inert solvents in the presence of Na2CO3,10 in pyridine,11 in ethanolic solution of KOH12 or triethylamine.7 It is also convenient to use the Schotten ± Baumann reaction, which involves the acylation of phenolates with (meth)acryloyl chloride in aqueous solutions at reduced temperature in the presence of an excess of alkali.13 This reaction occurs efficiently at the interface of the aqueous and organic phases.14 Monomers based on halogenated 2,2-di(4- hydroxyphenyl)propanes 15 and hydroxyphenyl derivatives of 2H-benzotriazole 16 were prepared in this manner.The methods described above have allowed the preparation of numerous alkyl- (see Refs 4, 6, 13, 17, 18), halogeno- (see Refs 4, 6, 11, 14) and nitro-substituted 4, 10, 19 phenyl (meth)acrylates (PMA). At the same duration and temperature of the reaction, the yields of esters decreased in the following order: meta- isomer>para-isomer>ortho-isomer>disubstituted phenol.10 Optimum conditions were found for the preparation of 2,6-dimethylphenyl acrylate, 2,4,6-trichloro-, 2,4,6-tribromo-, 2,4,6-trimethyl-, 2,6-diisopropyl- and 2,6-di(tert-butyl)-phenyl methacrylates, 2,4,5-trichlorophenyl acrylate 21 and meth- acrylate.22 PMA with substituents in the aromatic ring containing the aldehyde and ketone,23 carboxy,24, 25 benzyl,26 tetracyanocyclo- butyl 27 and dioxolane 28 groups have been described. Special mention should be made of (meth)acrylates of hydroxyphenyl- acetic 29 and hydroxybenzoic 24, 25 acids as well as of surfactant salts 30 of the general formula CH2=CMeC(O)OC6H4(CH2)nX [n=3, 7;X=CO2Na,N+(CnH2n+1)3Z7;Z=Cl, Br, I,MeSO4].782 Bun,32 ± 34, 36 4-(Meth)acryloyloxybenzoic acids form liquid-crystalline pol- ymers upon polymerisation and give birth to a large number of mesomorphic monomers and polymers.31 Of all monomers of this type, the best studied are the esters of the general formula CH2=CRC(O)OYC(O)OYR 0 (R 0=H,32 Me,33 OR00; R00=Me,32 ± 35, Pr,34 n-C6H13 37 and Alk(C87C16) 36 (hereinafter Y=1,4-C6H4).Monomers with more complex structures have been also described, e.g., meth- acrylates with the mesogenic side groups YC(O)OYOC(O)YOR0 (R0=H, Alk);38 or thermotropic bisacrylates R0XR0 (X=(CH2)n; R0=CH2=CHC(O)O. n=6 ± 10, 12; .YC(O)OYC(O)O.39 It should be noted that mesomorphic mono- mers are also found among aryl (meth)acrylates with a different character of substitution.These are acrylates and methacrylates of hydroxybiphenyl with alkyl 40 and alkoxy 41, 42 substituents or the CN group 43 at the position 40. It is noteworthy that poly(4,40- biphenylene diacrylate) does not possess liquid-crystalline proper- ties.44 The group of mesomorphic monomers also includes easily polymerisable acrylates of azomethines derived from 4-hydroxy- benzaldehyde. Polymers with Schiff's bases as the pendent groups can be obtained by polymer-analogous transformations from polyacryloyl chloride and the corresponding hydroxybenzylide- neaniline.45 A large group of this type of monomers of the general formulaCH2=CRC(O)OYCH=NX [X=Alk(C17C4), C6H4Z; Z=OH, OMe, OEt, NMe2, NEt2, NHPh, naphthyl, pyridyl] has been proposed 46 for the preparation of non-aqueous dispersions of polymers used in polygraphy.4-(4-Acryloyloxybenzylideneamino)phenyl acrylate 44 and N-(4-cyanobenzylidene)-4-acryloyloxyaniline are azomethines based on 4-aminophenol.47 Mesomorphic monomers (as well as the derived polymers) are rarely found among methacryloylazo- methines.48 (Meth)acrylates of hydroxyazobenzenes 49 ± 51 and hydroxy- azoxybenzenes 52 were used to prepare liquid-crystalline poly- mers. Azo-monomers possess, as a rule, alkyl, alkoxy or aryl substituents in the para-position relative to the azo group. The respective polymers are lyotropic or thermotropic liquid crystals, which undergo reversible changes under the action of UV radiation.53 Liquid-crystalline polymers based on 4,4 0-(bisac- ryloyloxy)azoxybenzene 44 are also obtained by the reaction of 4-hydroxyazobenzene with poly(acryloyl chloride).54 Comb-like polymers with liquid-crystalline properties were obtained by polymerisation of hydroquinone mono(meth)acrylates esterified at the second OH group with 4-n-alkoxybenzoic acids.55 ± 58 The results of studies on liquid-crystalline polymers derived from aryl (meth)acrylates have been described rather comprehen- sively in monographs.59, 60 Syntheses of (meth)acrylates of derivatives of condensed aromatic systems (naphthalene, anthracene, phenanthrene) have been described.61 Syntheses of a- and b-naphthyl esters of some a,b-unsaturated acids,62 2-(b-substituted acryloyl)-1-methacry- loyloxynaphthalenes, which are cross-linking agents,63 and 4-benzoylamino-a-naphthyl methacrylate,64 which is an efficient thermostabiliser, are documented.Monomeric dyes of the anthra- quinone series were obtained by the reaction of unsaturated acid chlorides with 4-amino-1,5-dihydroxy-2,6-diisobutyl-, 5-(3-ami- nophenylthio)-1,5-dihydroxy-2,6-diisobutyl- and 4,5-diamino- 1,8-dihydroxy-2,7-diisobutylanthraquinones.65 Copolymerisa- tion of these dyes with styrene and butyl methacrylate yields polymers self-coloured in yellow, red and blue. Kimura et al.66 have described 2- and 9-fluorenyl methacrylates. Upon their polymerisation in the presence of a telogen, the degree of telomerisation of the second monomer is lower.A number of monomers based on fluoranthene, including 3-fluoranthenyl methacrylate, produce polymers of the donor type, which form charge-transfer complexes possessing electrical properties with tetracyanoquinodimethane and tetracyanoethylene.67 Methacrylates of the 8-hydroxyquinoline series 68 of the general formula V G Syromyatnikov, L P Paskal', I A Savchenko R00 X=H, Br, Cl, I; R0=C(O)CMe CH2; R00=H, Br, Cl, CH2OMe, CH2OEt X N OR0 were synthesised in order to obtain subsequently medicinal preparations with prolonged action.69, 70 Halogens were shown to favour polymerisation. Methacrylates of hydroxyphenyl deriv- atives of other nitrogen-containing heterocycles are also known, e.g., 1,3-diphenyl-5-(4-methacryloyloxyphenyl)-2-pyrazoline.71 Of these, the most complex are acrylate and methacrylate of 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin 72 (the respec- tive polymers possess semiconducting properties and adsorb oxygen at temperatures below 200 K) and of 5-(4-acryloyloxy- phenyl)-15-phenyl-2,8,12,18-tetraethyl-3,7,13,17-tetramethylpor- phyrin.73 Some monomers contain the methacryloyloxy group bound directly to the heterocyclic ring, e.g., 2-pyridyl methacrylate.74 Aryl (meth)acrylates with the nitrogen-containing heterocyclic substituents in the para- and meta-positions of the benzene ring have been described.75 The initial N-(hydroxyphenyl)imides were obtained by condensation of aminophenols with the correspond- ing anhydrides. Monomers were synthesised by acylation with unsaturated acid chlorides.As regards aryl (meth)acrylates with nitrogen-containing substituents in the phenyl group, monomers with the nitro (see Refs 4, 10, 19) and cyano 76 groups have been described; a number of monomers with the azo (see Refs 77, 78), azoxy and azomethine groups 79 ± 82 are also known. 4-Azidophenyl methacrylate,83 salicinide,84 N-benzyl- and N-phenyl-4-hydroxybenzamide acryl- ates and methacrylates 85 were synthesised. 4-Aminophenyl methacrylate proved to be sensitive to oxida- tion, which reduces the possibility of its practical application.86 However, 3-(N,N-dimethylamino)phenyl methacrylate is fairly stable,76 it forms charge-transfer complexes with electron accept- ors and is promising for the synthesis of polymeric photosemi- conductors.Many reactive monomers, e.g., acylaminophenyl methacry- lates of the general formula R0C(O)NHC6H4OC(O)CR=CH2 (R0=H, Me, Et, Pr, Bu, C5H11, PhCH=CH, O2NC6H4 and BrC6H4) were obtained by N-acylation of isomeric aminophenols with equimolecular amounts of carboxylic acid chlorides (in anhydrous acetone, methanol or DMF) with subsequent esterifi- cation by the Schotten ± Baumann reaction with acryloyl or methacryloyl chlorides.87 ± 90 If 4-, 3- or 2-aminophenols are esterified with methacrylic acid, both functional groups react to give 4-, 3- or 2-(meth)acryloylaminophenyl (meth)acrylates.These reactive monomers undergo cross-linking in the polymerisa- tion.14, 91, 92 Di- and tri(meth)acryloyl derivatives of 2,4-diamino- phenol (amidol) were synthesised by successive acylation.92 Diacryloyl and di(meth)acryloyl derivatives of nitroamino- phenols O2N OC(O)CR CH2 NHCOCR CH2 were prepared 93 by acylation of 2-amino-4-nitro- and -5-nitro- phenols with unsaturated acid chlorides in tetrahydrofuran in the presence of triethylamine. Based on 4-amino-a-naphthol, azonaphthyl methacry- lates,94, 95 CMeC(O)O N N R0 CH2 R0=H, OMe, Br, NO2,Aryl (meth)acrylates and polymers based on them imidonaphthyl methacrylates 94 and 4-benzoylamino-a-naphthyl methacrylate 95 were synthesised. Thus, aryl (meth)acrylates with diverse functional groups in the aryl residue have been prepared to date.III. Polymerisation of aryl (meth)acrylates Aryl (meth)acrylates are relatively easily polymerised by a radical mechanism, the polymerisation rate being mainly determined by steric effects of substituents.96 Thus ortho-substituted PMA are appreciably less reactive than the para-isomers, the chemical nature of the substituent does not play any substantial role, which has been confirmed by the results of polymerisation of nitro- and methoxyphenyl methacrylates.13, 97, 98 Polymerisation was carried out in block and in different solvents in the presence of benzoyl peroxide. Since the electronic effect of the nitro and methoxy groups is opposite, one could expect that the rates of polymerisation of o- and p-nitrophenyl methacrylates and o- and p-methoxyphenyl methacrylates would be different; however, polymerisation of para-derivatives occurred faster in both cases. The EPR spectra of radicals devoid of ortho-substituents, e.g., poly(phenyl methacrylate) (PPMA) and poly(4-methylphenyl methacrylate), are identical, whereas the spectra of radicals of 2,6-diisopropylphenyl methacrylate and 2,6-di-tert-butylphenyl methacrylate revealed differences related to the shielding of the growing end of the polymeric chain by the ortho-substituents and the a-methyl group.99 The overall rate constant of the bulk polymerisation of alkylphenyl methacrylates (alkyl=Et, Pri, But) is higher for the para-substituted monomers than for the ortho-substituted ones and is virtually independent of the substituent size, whereas for the ortho-isomers this constant decreases with the increase in the substituent size.100 The reaction order with regard to the initiator (dilauryl peroxide) is 0.5 for non-substituted PMA, while it is 0.64 ± 0.94 for the sterically hindered monomers. The presence of methyl groups in the ortho-position (e.g., in 2,6-dimethylphenyl methacrylate) decreases appreciably the ceiling temperature of polymerisation (down to 73 8C) compared to that of non- substituted PMA (140 8C).101 The overall activation energy (kJ mol71) increases in the following order: 2,6-diisopropyl- phenyl methacrylate (45.7)<4-tert-butylphenyl methacrylate (49.9)<4-(1,1,3,3-tetramethylbutyl)phenyl methacrylate (59.6)< 4-methyl-, 4-ethyl- and 4-isopropylphenyl methacrylates (70 ± 76).102,103 Substitution of the cyclohexyl radical for the phenyl radical in the molecule of aryl methacrylate affects the polymerisation rate.For example, methacrylate of thymol, which contains a bulky isopropyl residue, is polymerised with difficulty, while methacry- late of menthol (hydrogenated thymol) analogous to the preced- ing monomer is polymerised easily under the same conditions.13 This is rationalised by the non-planarity of the cyclohexane ring and, consequently, by a lower rigidity of this monomer molecule compared to the methacrylate of thymol. Block polymerisation of substituted PMA is accompanied by gel-effect, which is observed in 2,4,6-tribromophenyl, 4-tolyl, 4-cyclohexylphenyl and 4-methoxyphenyl methacrylates at 35%, 35%, 15% and 2% conversions of the monomer, respectively [the concentration of the polymerisation initiator, viz., azobisisobu- tyronitrile (AIBN) is 0.4%].104 The kinetic parameters of PMA polymerisation in block and in solvent (60 8C, AIBN) were compared with those for alkyl methacrylates.105 The overall polymerisation rate has an order of 0.5 with respect to the initiator and 1 with respect to the monomer, i.e., it is described by the usual equation of radical polymerisation. The ratio of the chain termination constant (kt) to the squared chain propagation constant (kp) increases in the following order: (phenyl<isobutyl &n-butyl<ethyl<methyl) methacrylate.According to the radiometric analysis of the initiator fragments, the share of disproportionation in PMA is 89% [72% in methyl methacrylate (MMA)]. The initiation rate of radical polymerisation of PMA by 783 t AIBN depends slightly on the nature of the solvent, while kp (litre mol71 s71) increases in the following series of solvents: benzene (176)<fluorobenzene (180)<chlorobenzene (223)< anisole (230)<bromobenzene (235)<benzonitrile (243).106 The magnitude of kp was found to correlate with the difference in chemical shifts of b-alkene protons of monomers depending on the solvent. TheMOLCAO method was used to calculate the stabilisation energy in the formation of complexes of a radical with a solvent; kp was found to decrease with the increase in the complexation energy.It was shown later that this trend holds for MMA107 and PMA.108 The rate of radiation-induced polymerisation of PMA is proportional to the radiation dose to the power 0.65;109 in the temperature range 20 ± 34 8C, the activation energy of the process is equal to 14.15 kJ mol71, kp/k0:5=0.0198. At high degrees of conversion, a three-dimensional network is formed owing to the involvement of the terminal double bonds. Investigation of the influence of substituents on the radical polymerisation of PA has shown that the rate of this process increases in the order: methyl acrylate<2-chlorophenyl acryl- ate<PA<4-chlorophenyl acrylate.110 This is rationalised by changes in the flexibility of the propagating polymer chains. The increase in the overall polymerisation rate (toluene, 60 8C, lauryl peroxide) was found to increase in the following series:111 (2-tert- butyl- < 2-isopropyl- & 2-propyl- < 2-ethyl- < 2-methyl- < PMA < 4-methyl- < 4-isopropyl- < 4-tert-butyl-< 4-propyl- phenyl) methacrylate. The reaction order with regard to lauryl peroxide is 0.51 ± 0.87 depending on the monomer concentration.In the radical polymerisation of 2,4,5-trichlorophenyl meth- acrylate 112 in dioxane (kp=2.736103, kt = 7.226107 litre mol71 s71), the polymerisation rate is proportional to the monomer concentration to the power 1.5, which is explained by the possible formation of complexes of the monomer with the solvent molecules.In the case of radical polymerisation of nitro- phenyl methacrylates, at a low monomer concentration (0.01 mol litre71) one observes an inhibitory effect of the intra- and intermolecular chain transfer, the former being preva- lent.113 ± 115 For this reason, 2,6-dinitrophenyl, 2-methyl-4,6- dinitrophenyl and picryl methacrylates are not polymerised under these conditions, and the conversion of 2,4-dinitrophenyl methacrylate hardly reaches 2.6% in 2 h. The reaction orders of 4-biphenyl methacrylate polymerisation (inDMFwith AIBN) are 0.55 and 1.1 with respect to the initiator and monomer respec- tively; the total activation energy is 58 kJ mol71 (see Ref. 8). Replacement of an alkyl substituent by the electron-with- drawing acyl substituent in the benzene ring leads to an increase in the polymerisation rate of substituted phenyl (meth)acrylates because of the lower activation energy of the C=C bond cleavage.17 DTA studies of thermal bulk polymerisation of 4-acetylphenyl acrylate and methacrylate have shown116 that this process is the first-order reaction with respect to the mono- mer, the activation energies are equal to 228.1 and 120.5 kJ mol71, respectively.High polymerisation rates were observed for acylaminophenyl (meth)acrylates;87, 88 however, the rate of polymerisation decreased with increase in the length of the acyl radical. On the other hand, 4-diethylaminophenyl acrylate and methacrylate polymerised at a lower rate than non-substi- tuted PMA.89 Radical polymerisation of mixed diesters of 2,20-methylenedi- phenols with methacrylic and crotonic acids 117 OR00 OR0 R000 CH2 , Me NO2 whereR0 and/orR00 are methacrylic and crotonic acid residues and R000=H, Me, But, resulted in soluble polymers where R0 = R00 (polymerisation involved the methacryloyl group alone),784 insoluble dimethacrylates where R0 and R00 were the methacrylic acid residues and no polymerisation occurred where R0 and R00 were the crotonic acid residues.(Meth)acryloyl derivatives of N-hydroxyphenyl-substituted phthalimide 118 R0C6H4OC(O)CR=CH2 (R0=phthalimido) II IV III Monomer I R H H MeMe para meta meta para Position of R0 and alicyclic diacylamines 118, 119 polymerise at high rates and are more reactive than PMA, which seems to be due to the electron- withdrawing character of the imide fragment.The reactivity of monomers decreases in the following order: I >III>II>IV> p-acetylaminophenyl methacrylate (p-AAPMA), acrylates being more reactive than the corresponding methacrylates and the para- isomers more reactive than the meta-isomers.118 The reactivity of m-phthalimidophenyl (meth)acrylates 120 R00 O R000 N R000 O OC(O)CR0 CH2 R000 Monomer R000 R00 R0 Me I H H H II Me H H III H Cl Cl IV Me Cl Cl V Me Br Br VI NO2 H decreases with in the series: IV>V>II>m-AAPMA>VI, i.e., with the weakening of the electron-withdrawing character of the phthalimido group.The reduced reactivity of the monomer with the nitro group in the phthalimide cycle may be explained by its inhibitory action. The polymerisation rate of imidonaphthyl (meth)acrylates is lower than that of analogous imidophenyl methacrylates, and, according to Vretik et al., 94 this is due to the reduced reactivity of radicals because of a stronger electronic conjugation in the naphthyl-containing system. The most reactive of all the naph- thyl (meth)acrylates studied proved to be bismethacrylate with an additional cycloalkene double bond. This compound was obtained by the reaction of endic dianhydride with 1-amino-4- methacryloyloxynaphthalene.94 Irrespective of the initiation mode (thermal or photo-initia- tion),m-(succinimido)phenyl methacrylate (m-SIPMA) and acryl- ate (m-SIPA) occupy an intermediate position with regard to their reactivity between m-phthalimidophenyl methacrylate (m-PTIPMA) and acrylate (m-PTIPA), and acetylaminophenyl methacrylate (m-AAPMA) and acrylate (m-AAPA): m-PTIPMA>m-SIPMA>m-AAPMA>PMA;m-PTIPA> m-SIPA>m-AAPA>PA, i.e., there is correlation between the electron-withdrawing properties of the substituent and polymer- isability of substituted phenyl (meth)acrylates.121 High reactivity of these aryl (meth)acrylates allows the introduction of the above- mentioned heterocyclic fragments into the side chains of macro- molecules in order to impart definite photochemical, biological and other properties to the polymeric materials.Based on imidoaryl (meth)acrylates,122, 123 R0 O R00 X N n O X=OC(O)CR=CH2; R0, R00=NO2, NH2; n=1, 2; V G Syromyatnikov, L P Paskal', I A Savchenko O R0 X N n O X=NH2, OC(O)CR=CH2; R0=NO2; n=1, 2 homopolymers were obtained with the controllable intramolecu- lar energy transfer. In such polymers, movement of singlet or triplet excitons from one part of the macromolecule to another or from one unit chain to another is observed. The prerequisite for such a transfer is the presence of isolated aromatic systems with different excitation energies in each of the neighbouring units; the initial excitation should be generated in a definite element of the system. Such compounds are promising for the design of micro- electronic devices in which advantage is taken of photophysical processes at the molecular level.124 Polymerisability of bifunctional monomers deserves special mention.Cross-linked polymers are formed upon polymerisation of p- and m-phenylene di(meth)acrylates; o-phenylene diacrylate can undergo cyclopolymerisation 7 with the formation of ladder polymers; the extent of cyclisation increases with the decrease in the monomer concentration and the increase in the polymer- isation temperature.125 At a monomer concentration above 0.5 mol litre71, an insoluble polymer is formed. The intramolec- ular chain growth in the cyclopolymerisation follows mostly the `head-to-tail' mechanism, and a small number of units are formed by the `head-to-head' mechanism.Radical polymerisation of 2-allylphenyl (meth)acrylates is a two-step process:126 at an 18%± 30% monomer conversion, a soluble low-molecular-mass polymer is formed, which is then cross-linked. The low molecular mass of the product and the absence of the gel-effect are associated probably with the ability of the allyl group to undergo chain transfer. A detailed study of the polymerisation of 2-allylphenyl acrylate in toluene at 60 8C in the presence of AIBN revealed high content of the residual double bonds in the pendent allyl groups of the polymer.127 Subsequent cyclopolymerisation occurs due to the reaction of theC=Cbonds of the acryloyl and allyl groups. Cyclopolymerisation of 2-allylphenyl, 2-(o-allylphenoxy)ethyl and 4-(o-allylphe- noxy)butyl acrylates catalysed by alkylaluminium chlor- ides 127 ± 129 yields 8-, 11- and 13-membered rings; however, the catalyst's activity is insignificant for the formation of the 15-membered ring from 6-(o-allylphenoxy)hexyl acrylate.Polymerisation of ortho-, meta- and para-(meth)acryloyl- aminophenyl (meth)acrylates initiated by AIBN occurs with the formation of cross-linked polymers.92 A decrease in the reactivity in the order meta-> ortho->para-aminophenol di(meth)acryl- ates is associated with the character of conjugation in these monomers. Tri(meth)acryloyl derivatives of 2,4-diaminophenol (amidol) are more reactive than the corresponding (meth)acryloylaminophenyl (meth)acrylates, whereas di(meth)- acryl derivatives of amidol (with a free OH group) do not polymerise.Upon polymerisation of monomers based on 2-amino-4- and -5-nitrophenols, no formation of cross-linked polymer is observed even at a 60% conversion,93 as is the case of polymerisation of di(meth)acryloyl derivatives of 2-aminophenol. The presence of the nitro group decreases the number of active centres in the system resulting in the higher probability of chain transmission and formation of cyclolinear structures in macro- molecules. Cinnamoylaminophenyl (meth)acrylates 90 H2C=CRC(O)OYNHCOCH=CHPh polymerise at a lower rate than di(meth)acryloyl derivatives of isomeric aminophenols; the early chain termination at low conversions appears to be due to the inability of the cinnamoyl C=C bond, which acts as a trap for free radicals, to polymerise.This conclusion is prompted by comparison of the IR spectra ofAryl (meth)acrylates and polymers based on them the monomer and the polymer. Photosensitivity of such mono- mers (photoinitiator, benzoin), like that of 4-cinnamoylamino- phenol, confirms the non-reactivity of the cinnamoyl double bond in radical polymerisation and its capability of photoaggrega- tion.130 The nitro group in 4-nitrophenyl cinnamate or in 4-nitro-1-naphthyl cinnamate exerts a photosensitising effect. These monomers are light-sensitive and belong to the group of self-sensitised ones.131 Poly(4-cinnamoylphenyl methacrylate) obtained in the presence of benzoyl peroxide as an initiator with a degree of conversion of 78% is insoluble in common organic solvents.132 In contrast to MMA, radical telomerisation of aryl (meth)acrylates with the use of bromotrichloromethane as the telogen is not accompanied by the formation of lactones;133 however, the distribution with regard to the degree of polymer- isation and tacticity are analogous.Pyrolysis of telomers at 150 ± 200 8C leads to the formation of unsaturated compounds and in part lactones. The behaviour of 4-methacryloyl- and 4-acryloyloxybenzoic acids is highly specific under thermal treatment.134 At 250 8C (above the upper limit of polymerisation temperature), the former undergoes polycondensation with the elimination of methacrylic acid and formation of a rigid-chain polymer, viz., poly(4-hydroxy- benzoate). The latter undergoes simultaneous polymerisation and polycondensation.In the polymerised state, both acids undergo polycondensation upon thermal treatment. Methacrylic monomers with the azo group based on amino- phenols 77, 135, and 4-amino-a-naphthol 136 N N R0 CMeC(O)O CH2 NHCOMe R0=H, Me, NO2; N N R0 OC(O)CMe CH2 R0=H, Br, OMe, NO2 attract attention because of the possibility of using them for the preparation of `self-coloured' polymers containing dye residues incorporated into the polymeric chain. Of the monomers based on azo compounds of 3-aminophenol, only the azo-coupling prod- ucts at the para-position relative to the methacryloyl group can polymerise. The monomers based on 4-amino-a-naphthol are incapable of radical homopolymerisation because of a decreased reactivity of their radicals due to high electronic conjugation in the azo system.However, they copolymerise with styrene forming coloured polymers. Copolymers of trans-4-(phenylazo)-1-naph- thyl acrylate and trans-4-(phenylazo)phenyl acrylate with (7)-menthyl acrylate are optically active.137 The data on ionic polymerisation of aryl (meth)acrylates are scarce. They are known to polymerise by an anionic mechanism. Polymerisation of PMA on organolithium catalysts proceeds more slowly than that of alkyl methacrylates;138 this process is characterised by the existence of a `limit' of monomer conversion and the fact that the degree of polymerisation does not increase as the process proceeds further.Anionic polymerisation of PMA, cresyl acrylate 139, 140 and 4-(a,a-dimethylbenzyl)phenyl metha- crylate 141 in the presence of n-butyllithium, as well as anionic polymerisation of p-cresyl methacrylate 142 with naphthyllithium, butyllithium and LiAlH4 in the hydrogen atmosphere are docu- mented. Reduction of the polymers thus obtained with lithium aluminium hydride yields poly(a-methylallyl alcohol) (in the case of p-cresyl methacrylate) and polyallylic alcohol (in the case of p-cresyl acrylate). Phenyl (meth)acrylate also polymerises readily with alkylaluminium chlorides by an anionic mechanism.143 Some PA, e.g., 4-nitrophenyl acrylate, do not polymerise by an anionic mechanism.140 785 Comparative studies of radical and anionic polymerisation of 9-fluorenyl and 9-phenylfluorenyl methacrylates 144 in THF or toluene at temperatures from 778 to 760 8C in the presence of AIBN or butyllithium revealed substantial differences in the reaction course.Upon initiation with AIBN, the yield of poly- mers amounted to 60% ± 95%, the degree of polymerisation ranged from 9 to 37 with Mw/Mn=1.8 ± 3.6, while upon initia- tion with butyllithium, the yield of polymers ranged from 3% to 40%, andMw/Mn=1.9 ± 2.0. IV. Aryl (meth)acrylates in copolymerisation reactions Aryl (meth)acrylates copolymerise easily with most monomers. For example, copolymers of PMA with ethylene 145 and buta- diene 146 have been described. But of greatest interest are numer- ous data 147, 148 on copolymerisation of substituted phenyl (meth)acrylates with styrene, acrylonitrile and MMA.Parame- ters of copolymerisation (AIBN, in bulk or in benzene at 60 8C) of PMAand its derivatives with substitutions in the ring with styrene are presented in Table 1. It was noted that the reactivity of monomers towards styrene radical (1/r1) is determined by the polar (s) and resonance (Er) effects of substituents and is satisfactorily described by the modified Hammett equation 149 log 1 à rs á gEr r1 for r=0.21 and g=1. Low values of the copolymerisation constants r1 and r2 point to the tendency of aryl methacrylates for the alternation with styrene in the copolymerisation. The electron-donor substituents in PMA decrease polarity (e2), while the electron-withdrawing substituents, especially those in the para-position, increase polarity.This is accompanied by a decrease in the frequencies of stretching vibrations of the 7C=C7 and C=O groups.150 Linear correlation is observed between the Hammett constants and the e2 criteria as well as between logQ2 (Q2 is the criterion of the monomer reactivity) and Er.147 In the case of p- andm-nitrophenyl methacrylates, the linear dependences are distorted because of the inhibitory effect of the nitro group. Introduction of several functional groups enhances the effect, as is observed for the criteria Q2 and e2 in 2,4,6- tribromophenyl and pentabromophenyl methacrylates.151, 152 The azo group manifests rather strong acceptor properties (e2=1.0).153 ortho-Substituents, e.g., the carboxy group, strongly increase the values of Q2 but virtually do not affect the e2 values, which is well explained by considering the increased steric hindrances.154, 155 In copolymerisation with styrene, 2-naphthyl methacrylate (2-NMA) manifests a weaker (compared to PMA) trend for alternation, which is, however, enhanced in polar solvents (e.g., in acetonitrile).156 ± 158 Irrespective of the solvent nature, the polydispersity and molecular mass increase with the increase in the content of 2-NMA.In complex aromatic systems, the methacryloyloxyphenyl group behaves in the usual manner, as it does in PMA.159 Parameters of copolymerisation of substituted PA with styrene are presented in Table 2. Comparison of the data presented in Tables 1 and 2 shows that the values of the para- meters Q and e are higher for PA than for the corresponding MA, which is associated with the absence of the effect of the a-methyl group.160 For example, e2 is equal to 1.32 for 2,4,6-tribromophenyl acrylate 161 ± 163 and to 0.91 for the corresponding methacrylate.Copolymers of PA with MMA164 are characterised by the predominance of MMA units (r>1) in the macromolecule. In this case, the reactivity of substituted PA165 is determined by their ability to form p-complexes with the double bond ofMMA, which decreases in the series: PA>2-chlorophenyl acrylate>4-chloro- phenyl acrylate. The values of parameter e increase in the same order (up to 1.46 for the latter member).786 Table 1.Parameters of copolymerisation of substituted PMA with styrene. Substituent in PMA 7 7 7 0.30 0.60 0.45 0.40 0.38 0.22 0.36 0.36 0.34 0.35 0.27 0.26 0.70 0.60 0.66 0.50 0.49 0.16 0.13 1.18 0.41 0.47 0.39 0.14 0.10 0.39 2.53 0.34 0.54 0.31 1.23 4.45 0.75 0.18 0.03 3.66 0.21 0.28 4-Chloro 3-Chloro 2-Chloro 4-Nitro 3-Nitro 2-Nitro 2,4,6-Trichloro Pentachloro 2,4,6-Tribromo Pentabromo 4-Methyl 4-Methoxy 2-Methoxy 4-Formyl 4-Acetyl 2-Isopropyl 2,6-Diisopropyl 2-Carboxy 2-Carboxy (DMSO) 2-Methoxycarbonyl 2-Phenyl 4-Phenylazo 2,4,6-Tricyano 4-(1,3-Diphenyl)-2-pyrazolyl-5 4,4-Bishydroxyphenyl-o-carboranyl 2-Naphthyl (dioxane) 2-Naphthyl (acetone) 4-Cinnamoylamino 4-Acetylamino 3-Acetylamino 3-Butyrylamino 4-Methacryloylamino 3-Methacryloylamino 4-Nitro-2-methacryloylamino 3-Methyltetrahydrophthalimido 3,5-Bis(N,N-dimethylamino) Note.The subscripts 1 and 2 in the parameters refer to the first (styrene) and the second (PMA) monomers, respectively; here and in other tables, the solvent for copolymerisation is indicated in parentheses. Table 2. Parameters of copolymerisation of substituted PA with styrene. Substituent in PA r2 r1 0.37 0.50 0.36 0.15 0.25 0.29 0.14 0.08 0.90 2.11 0.09 0.26 0.20 0 0.49 0.61 0.20 0.53 0.06 0.64 1.08 0.35 0.31 0.55 0.83 2.34 0.05 0.88 72,6-Dimethyl 2,4,6-Trichloro 2,4,6-Tribromo Pentachloro Pentabromo 2,4,6-Tricyano 2-Carboxy (DMF) 2-Carboxy (DMSO) 2-Carboxy (benzene) 4-Cinnamoylamino 4-Propylamino 4-Bityrylamino 4-Nitro-2-acryloyl- amino Note.The subscripts 1 and 2 in the parameters refer to the first (styrene) and the second (PA) monomer, respectively. Er r1 s 0.10 0.08 70.41 0.35 7 0.226 0.373 0.560 0.778 0.710 0.650 1.07 70.03 0.11 0.170 0.268 0.21 0.516 0.22 0.21 0.40 0.19 0.18 0.13 7 7 0.19 2.00 7 7 0.20 7 7 0.09 0.30 0.28 7 7 0.33 7 7 0.27 0.15 7 7 0.34 7 7 0.39 7 7 0.16 7 7 0.80 7 7 0.27 7 7 0.57 7 7 0.29 7 7 0.18 7 7 0.26 7 7 0.56 7 7 0.49 7 7 0.63 7 7 0.62 7 7 0.29 7 7 0.63 7 7 0.38 7 7 0.14 7 7 0.26 7 7 0.023 7 7 0.55 7 7 0.32 Ref.Q2 1/r1 e2 0.50 0.91 0.82 2.70 0.96 2.77 0.71 42.00 1.09 7.14 1.31 1.18 0.19 11.11 0.81 5.00 1.03 2.04 0.85 5.00 1.50 16.60 3.90 0.43 0.42 3.21 1.11 20 20 91 1.32 161 1.11 7 7 163 1.14 161 76 0.30 160 0.70 155 1.02 154 90 0.53 148 1.20 7 7 148 0.97 148 20.00 4.86 V G Syromyatnikov, L P Paskal', I A Savchenko 1/r1 r2 3.33 4.55 4.76 2.50 5.26 5.56 7.69 5.26 0,50 5.00 11.10 3.33 3.57 3.03 3.70 6.67 2.94 2.56 6.25 1.25 3.70 1.75 3.45 5.55 3.85 1.78 2.04 1.58 1.61 2.81 1.59 2.67 7.14 3.85 43.48 1.82 3.13 It was noted 165 that the presence of aromatic substituents in copolymers of MMA with phenyl (meth)acrylates increases the number of hetero- and isotactic sequences in macromolecules. As in the case of 4-phenylazophenyl methacrylate,153 copolymers of MMA with 2-methacryloyloxybenzoic acid are enriched in the units of the latter (r1=0.74, r2=1.22).The rate of copolymerisa- tion decreases with the increase in the content of the second component.166 For 4-acetylphenyl methacrylate, the influence of the penultimate unit 167 was observed, which is due to strong steric hindrances and high polarisability of the aromatic oxo group. Constants of copolymerisation of a number of substituted phenyl (meth)acrylates with acrylonitrile are presented in Table 3; their reactivity towards the acrylonitrile radical varies depending on the nature of the substituent group and its position in the ring.168 The trend for alternation is not strong, the values of r1 fit well the dependence log 1 a ¢§0:98Os a 1:66DsU, r1 where s is the Hammett polar constant, Ds is the difference between the Brown and Hammett constants for substituents.Accumulation of substituents in the ring of phenyl acrylates decreases even more strongly the trend for alternation, as is seen in the example of 2,4,6-tribromophenyl acrylate.169 However, the Ref. e2 Q2 1.17 1.35 1.36 0.83 1.27 1.47 1.90 1.40 0.31 1.27 2.35 1.23 1.23 1.13 1.20 1.82 0.75 0.79 2.22 0.54 1.18 0.66 1.28 1.56 1.15 0.14 0.70 0.69 0.58 1.75 147 147 147 20 147 147 20 20 152 151 151 147 147 20 23 147 20 20 154 155 154 20 153 76 71 159 158 158 90 148 148 148 148 148 148 119 0.51 0.72 0.77 0.83 0.98 0.86 0.95 0.86 70.20 0.91 1.14 0.45 0.53 0.43 0.61 0.82 0.90 0.66 0.50 0.25 0.63 0.43 1.00 0.82 0.71 0.57 0.54 0.24 0.48 0.31 7 70.33 1.13 1.39 0.77 0.67 0.75 1.08 1.53 0.66 12.37 0.56 1.03Aryl (meth)acrylates and polymers based on them Table 3.Parameters of copolymerisation of substituted phenyl (meth)- acrylates with acrylonitrile.a r2 r1 Substituent in phenyl meth- acrylates Acrylates 0.36 0.94 0.86 1.16 1.02 0.80 2.05 0.46 0.33 1.26 1.12 0.82 0.73 0.96 74-Chloro 3-Chloro 2-Chloro 4-Bromo 4-Methyl 2,4,6-Tribromo Methacrylates 0.98 0.12 0.96 0.20 0.28 0.02 0.84 0.04 0.52 0.02 0.51 0.03 2,4,6-Tribromo Pentabromo 2-Naphthyl (chloroform) 2-Naphthyl (benzene) 2-Naphthyl (acetone) 2-Naphthyl (acetonitrile) Note.The subscripts 1 and 2 in the parameters refer to the first (acrylonitrile) and the second (PMA) monomer, respectively. a For acrylonitrile e1=1.2, Q1=0.6. trend for alternation was observed for 2,4,6-tribromophenyl and pentabromophenyl methacrylates.170 Copolymerisation of 2-NMA with acrylonitrile in chloroform or benzene proceeds with the formation of donor ± acceptor complexes between the monomers; therefore, the values of r1 and r2 differ in different solvents.171, 172 Complexation was also found to occur in the copolymerisation of 2-NMA with monomethyli- taconate;173, 174 thus the alternation of monomers of this pair is different in different solvents: in chloroform, r1=2.440.19, r2=1.200.28; in dioxane, r1=0.490.04, r2=0.970.29; in acetone, r1=0.150.01, r2=0.280.03 and in acetonitrile, r1=0.100.01, r2=0.350.14.Such changes in the copolymer- isation constants reflect interactions between the monomer molecules and the corresponding solvent. The constants of copolymerisation of PMA with methacrylic acid (MAA) are as follows: r1=0.40 and r2=0.52 (benzene), r1=0.21 and r2=1.52 (DMF/benzene), which points to a lower reactivity ofMAAand higher reactivity ofPMAin the presence of DMF, which appears to act as a complexation agent.175 Copoly- mers of pentachlorophenyl acrylate with acrylic acid (AA) are enriched in the AA units.163 In 2-butanone, PMA and glycidyl methacrylate give a copolymer enriched in PMA (r1=0.840.51, r2=1.570.56).176 Optically active copolymers were obtained upon copolymerisation of phenyl, benzyl and 1-naphthyl meth- acrylates with maleic anhydride in the presence of b-cyclodex- trin.177 Copolymerisation of N-vinylpyrrolidone with 2-carboxyphenyl acrylate 160 and 2,4,5-trichlorophenyl acryl- ate 178 was also studied; in the latter case, copolymers with a high degree of alternation were obtained.In copolymerisation of p- and m-phenylene diacrylates with styrene, a copolymer with acrylate pendent groups is formed in the initial stages; these groups then react with styrene to form a cross- linked copolymer, which precipitates from the reaction mixture in the form of a gel.o-Phenylene diacrylate forms a non-cross-linked ladder copolymer with styrene due to cyclopolymerisation. The smallest values of the polarity criterion e were obtained for the ortho-isomer.6 The values of the copolymerisation constants for isomeric phenylene diacrylates (PDA) are presented in Table 4. Ref. e2 Q2 1/r1 1.4 0.36 0.37 70.77 168 70.47 168 168 70.12 168 7 7 168 7 7 168 0.07 0.11 7 7 169 2.17 3.03 0.79 0.89 1.22 1.37 1.04 0.08 170 0.64 171 0.79 172 0.78 1.09 18.30 5.0 3.57 50.0 0.98 172 46.40 25.0 0.94 172 21.80 50.0 1.09 172 53.00 33.3 787 Table 4.The values of copolymerisation constants for phenylene diacryl- ates (FDA) and styrene. Solvent r1 e2 Q2 r2 FDA isomer ortho- meta- para- 0.37 0.49 0.90 0.72 0.82 0.70 0.74 0.83 0.78 0.46 0.79 1.00 THF benzene THF benzene THF benzene 0.48+0.19 0.45+0.19 0.17+0.02 0.15+0.04 0.21+0.07 0.35+0.09 0.55+0.15 0.42+0.28 0.33+0.06 0.67+0.11 0.35+0.06 0.30+0.08 Note. The subscripts 1 and 2 in the parameters refer to the first (styrene) and the second (FDA) monomer, respectively. In radical copolymerisation of p-chlorophenyl methacrylate with ethylene glycol, 2,2-di(4-hydroxydiphenyl)propane and hydroquinone dimethacrylates,179, 180 the most reactive is ethyl- ene glycol dimethacrylate as the second monomer, which is devoid of the mesomeric effect.The copolymer produced both upon catalytic and photochemical initiation is enriched in this com- pound (r1=0.290.03 and r2=1.310.06). Methacrylamide 181 and peroxide monomers can be used for subsequent cross-linking in copolymerisation with PMA. The constants of copolymerisation of PMA with tert-butyl p-vinyl- perbenzoate are as follows: r1=1.35, r2=0.32.182 Copolymerisation of aryl methacrylates in donor ± acceptor complexes bears a special character. For example, complexes of monomers and polymers of carbazolyl methacrylates with dini- trophenyl-containing monomers and polymers were obtained.183 Charge-transfer complexes (CTC) are formed in copolymerisation of the donor monomers, viz., N-(2-hydroxyethyl)carbazolyl acryl- ate (1) and -methacrylate (2) with the acceptor monomers, viz., 2,4-dinitrophenyl acrylate (3) and methacrylate (4).114, 183, 184 The extent of intramolecular charge transfer in copolymers (according to the chemical shift values of aromatic protons in the 1H NMR spectra of 3 and 4) increases as the content of 4 in the alternating diads with 1 is increased and is larger in the 2 ± 3 copolymer than in the 1 ± 3 copolymer. In copolymers containing only methacrylate units, no intramolecular CTC are revealed, the most stable CTC are formed in copolymers having only acrylate units.In copoly- merisation of 1 with 4, the intermonomeric CTC are not added as a whole,184 and the relative reactivity is determined solely by the sequence of distribution of units rather than by the configuration factors. With the increase in the temperature, the CTC present in the copolymer disintegrate due to the higher segmental mobility.Intramolecular CTC are also formed in the copolymerisation of picryl (meth)acrylate (incapable of homopolymerisation) with N- (2-hydroxyethyl)carbazolyl (meth)acrylate. The copolymerisation constants indicate that the most stable CTC are formed in dioxane.185 ± 187 Photoconductivity of poly(N-vinylcarbazole) increases when electron-acceptor comonomers forming CTC are used, e.g., isomeric 2,2-dicyanovinylphenyl (meth)acrylates (NC)2C=CHC6H4OC(O)CR=CH2 (see Ref.188). Mixing of poly(N-vinylcarbazole) with the same amount of the low-molec- ular-weight acceptor leads to a less pronounced effect. Directed intramolecular transfer of electronic triplet excita- tions was studied in a copolymer of 1-naphthyl acrylate (1-NA) and 4-acetylphenyl methacrylate.189 The copolymer contains two p-electronic systems in which unidirectional transfer of electronic excitation is brought about. Upon selective excitation of the singlet level of 4-acetylphenyl methacrylate units, phosphores- cence of only 1-NA units is observed. Such a molecular structure can function as a specific exciton gate.190 ± 192 In a ternary styrene ± acrylonitrile ± 2,4,6-tribromophenyl (meth)acrylate system, the third component decreases the molec- ular mass and thermal stability of the terpolymers formed.193 ± 195 In copolymerisation of phenyl, 4-butylphenyl, 4-nitrophenyl and788 1-naphthyl acrylates with SO2 and also in three-component copolymerisation of these monomers with hept-1-ene (see Ref.196), the maximum content of sulfonyl monomeric units in copolymers ranges from 13% to 24% depending on the inductive steric effects. The composition of terpolymers varies within a wide range depending on the molar ratio of reagents and conversion. It is also possible to copolymerise PMA one with another.197 Changes in the enthalpy and parameters of PMA copolymerisa- tion with 2-methylphenyl and 2-ethylphenyl methacrylates were studied in the temperature range 80 ± 100 8C.The problems of the relationship between the structure of polymeric carriers based on cross-linked PA polymers and their copolymers with styrene and the reactivity of such carriers were reviewed.198 V. Structure and properties of poly[aryl (meth)acrylates] Esterification of atactic, syndiotactic and isotactic polyacrylic acids with phenols yields only atactic polymers. This points to the spatial isomerisation of the tertiary carbon atom in the course of esterification and the impossibility of obtaining stereoregular poly(aryl methacrylates) by polymer-analogous transformations. Radical polymerisation of PMA yields an atactic polymer which contains nonetheless more syndiotactic structural blocks than the structurally similar poly(benzyl methacrylate) and poly(2-phenylethyl methacrylate).199 This is related to the higher polarity (s=0.6) of the phenyl groups than that of the alkyl groups (s varies from 70.3 to 0.215) rather than to their higher rigidity or small volume.Jimmy et al.200 studied the influence of the side groups and the size of the polymer coil of poly(benzyl methacrylate), poly(triphenylmethyl methacrylate) and poly(diphenylmethyl methacrylate). The overall activation energy of radical polymerisation of PMA is equal to 69.0 kJ mol71, which is somewhat lower than that for poly(methyl methacrylate) (PMMA).201 In polymerisation with AIBN in the temperature range 30 ± 100 8C, a Bernoulli distribu- tion of stereochemical structures in the PPMA chains is observed.The content of iso- and syndiotactic triads in them decreases as the temperature increases, which is characteristic of a,a-disubstituted monomers. The triads are disposed in the chain in an isolated manner. The phenyl radical is oriented with its maximum polar- isability axis along the direction of the main trans-chain.202 In poly(aryl methacrylates), the content of isotactic, atactic and syndiotactic triads is 15%, 40% and 45%, respectively.203 The use of high-frequency NMR spectroscopy (500 MHz for 1H and 125 MHz for 13C) allowed calculation of isotactic parameters for poly(phenyl acrylate) (PPA) (Pm=0.44) and poly(4-fluorophenyl acrylate) (Pm=0.48) 204 obtained by radical polymerisation. Isotactic PPA can be obtained by anionic polymerisation in the presence of butyllithium.140 Kempf and Harwood 205 have shown that PPMA of a predetermined configuration can nonetheless be obtained from polymethacrylic acid if certain conditions are met. Studies of the influence of solvents on the microstructure of poly(1-naphthyl methacrylate) and poly(2-naphthyl methacry- late) (1-PNMA and 2-PNMA, respectively) in comparison with poly[cumylphenyl meth)acrylate] and PMMA have shown 206 ± 208 that the polymerisation of 2-NMA in methanol, acetonitrile and nitromethane occurs heterogeneously.Even in the presence of non-polar solvents, the percentage of isotactic triads in 1-PNMA and 2-PNMA is higher and the percentage of syndiotactic triads is lower than in PMMA. Syndiotacticity of 2-PNMA is higher than that of 1-PNMA.In solvents with higher dielectric constants, PNMA with a still higher content of isotactic triads is obtained. Poly(1-naphthyl methacrylate) possesses a higher heterotacticity than 2-PNMA. In copolymerisation of 2-NMA with MMA (r1=1.930.16, r2=0.330.08), syndiotactic addition proved to be preferential.209 The behaviour of PA is analogous.210 However, PPMA, poly(p-cresyl methacrylate), 2- and 4-methoxy- phenyl methacrylates, obtained by polymerisation in the presence V G Syromyatnikov, L P Paskal', I A Savchenko of butyllithium at low temperatures in toluene, have an isotactic structure with 90% of isotriads,211 while syndiotactic polymers are formed in THF.212 The temperature characteristics of poly[aryl (meth)acrylates] are determined by their structural peculiarities. As was indicated above,4 the thermostability is determined by the character of the substitution: in the para-substituted poly[aryl (meth)acrylates] the glass transition temperature (Tg) is always lower than in the ortho- substituted ones.This is explained by the shielding effect of ortho- substituents on the ester bond. Tg increases with the increase in the size of substituents and their polarity. Thus PPA, poly(o-tolyl acrylate) and its para-isomer have Tg values of 56.5, 51.5 and 24.5 8C, respectively. For 1-PNMA and 2-PNMA, Tg values are 135 and 119 8C, respectively.214 The relationship between Tg and molecular mass (M) of poly(o-alkylphenyl methacrylates) is non- linear and may be described by the following equation:215 Tg=Tg(0)+klnP, where Tg(0) is the temperature characteristic of the monomeric unit and P is the degree of polymerisation.For the infinitely large PPMA molecules, Tg=118 8C, while for poly(2,6-dimethyl- phenyl methacrylate) (PMPMA) Tg=177 8C. In copolymers of phenyl, 2-chlorophenyl and 4-chlorophenyl acrylates with MMA, Tg decreases with the increase in the concentration of MMA.216 In this case, the concentration dependence of Tg obeys Johnson's equation, which considers the contribution of diads in the polymer chain to the glass transition effect. A comparative calorimetric study of substituted PPA 217 and poly(vinyl benzoates) made it possible to establish that the dependence of specific heat capacity (Cp) on temperature in the range of glassy state is described by the equation Cp=A+BT+CT2+DT2.The increase in Cp in the glassy state was observed for all substituted polymers with increase in the volume of substituents. For a number of alkyl- and alkoxy-substituted PPA the values of Tg were found and the heat capacity jumps in the glass transition region and coefficients A, B, C and D in the proposed equation were determined. Thermal stability of poly(aryl methacrylates) was studied by the TGA method;218 the curves for poly(tolyl methacrylates) (PTMA) and poly(benzyl methacrylate) are S-shaped and similar to the curves for PMMA. For PPMA, the process was charac- terised by several ranges of decomposition. ortho-PTMA proved to be the most stable isomer.Effective activation energies of thermal destruction for the polymers studied vary from 81 to 159 kJ mol71 (210 kJ mol71 for PMMA). Thermal destruction of poly(2-methacryloyloxybenzoic acid) 219 occurs in three steps in temperature ranges of 140 ± 220, 250 ± 330 and 350 ± 450 8C with mass losses (according to TGA data) of 33%± 35%, 30% and 30%, respectively, which corre- sponds to the stoichiometric content of salicylic acid in the polymer in the first two stages. The effective activation energy in the first stage is 79 kJ mol71. Upon destruction, fragments of the glutaric anhydride type accumulate first in the polymer (up to 330 8C); after this, the macrochains undergo disintegration. Copolymers of PMA with MMA are more thermostable than the homopolymers;220 monomers are the main degradation products.At a low content of PMA, oligomers containing anhydride groups are also formed; with the increase in the PMA content the ability to form anhydrides drops. In contrast to homopolymers, the destruction of copolymers is characterised by relatively short kinetic chains of depolymerisation and the cleavage of the principal bonds in the macrochain. Among the copolymers of acrylonitrile with 2,3-dibromopropyl, pentabro- mophenyl and 2,4,6-tribromophenyl acrylates, the highest ther- mostability was observed for copolymers with the latter.221 In studies of poly[aryl (meth)acrylates], a prominent place is occupied by the investigations of properties of their dilute solutions. By measuring the swelling degree, solubility para-Aryl (meth)acrylates and polymers based on them meters (d) were determined and their dependence on the character of forces of interaction of polymer with solvent was confirmed.222 Chloroform, tetrachloroethane, chlorobenzene and ethyl acetate are the best solvents for poly(aryl methacrylates).The values of d for poly(alkylphenyl methacrylates) were determined in 20 sol- vents of different nature, considerably differing in their polarities (the contribution of dp) and ability to form hydrogen bonds (the contribution of dH), and in the widest possible range of the chain flexibility parameter s. The magnitude of dH decreases slightly with the enhancement of the `paraffinic character' of the sol- vent.223 The influence of alkyl residues is weakly manifested and the values of dH found differ somewhat from the calculated ones.Investigation of properties of dilute solutions by the osmometry and viscosimetry methods showed that the ortho-substituents in the benzene ring of the side chain increase appreciably the rigidity of the polymer. For PMPMA, the solubility parameter in toluene is equal to 9.6 cal71/2 cm3/2 (see Ref. 224). The power index a in the dependence [Z]& Maw , where [Z] is the specific viscosity, increases as the thermodynamic quality of the solvent is enhanced (a is equal to 0.5 in toluene, 0.59 in THF and 0.69 in chlorobenzene). It was established that <r20 >1/2 (characteristic chain ratio) and Tg increase upon transi- tion from PPMA to PMPMA.For poly(2,6-diisopropylphenyl methacrylate), the solubility parameter in toluene is also equal to 9.6 cal71/2 cm3/2 (see Ref. 225), while the Mark ± Hauwink equa- tion ([Z]=KMan ) (25 8C) has the following form for THF [Z]=1.0261074M0:71, n for toluene [Z]=1.2261074M0:69, n and the THF: water mixture (90.9 : 9.1) [Z]=8.5161074M0:50, n for the ratio of the mean squared distance between the ends of the polymer coil and the molecular mass of the polymer (see Ref. 225) <r20 >=0.677M1/2. For p-AAPMA226 in DMF at 25 8C [Z] =1.4061073M0:85. n Investigation of the influence of temperature on the thermo- dynamic properties and undisturbed dimensions ofPPMAmacro- molecules in isobutyl methyl ketone showed 227 that for all fractions an anomalous dependence of [Z] on temperature takes place at 20 ± 35 8C, which becomes more pronounced with the increase in the molecular mass.The value of the coefficient K in the Mark ± Hauwink equation is maximum at 25 8C, which implies changes in the character of hydrodynamic interactions inside the macromolecular coil, whereas the value of a is a minimum at the same temperature, which means drastic deterio- ration of the solvent quality. These phenomena point to a conformational transition associated with the increased chain flexibility resulting from the changes in specific interactions between the phenyl groups. The side chains in PMMA, poly(isobutyl acrylate) and poly(cyclohexyl methacrylate) have a strong influence on the temperature of the conformational transition and changes in the undisturbed dimensions of the chains, which is accompanied by the appearance of a jump in the temperature dependence of viscosity, refraction index and partial specific volume of polymers.228 Gargallo et al.229 analysed the influence of the size and the nature of the side groups in poly(alkylphenyl methacrylates) on the magnitude of partial volumes.The intramolecular character of the conformational transition is confirmed in the example of poly(4-tert-octylphenyl methacrylate).230 It was established for PPMA solutions that the conformational transition occurs owing to the changes in specific near-order interactions between the phenyl rings in the pendent 789 groups of the polymer studied.231 Comparison of magnitudes and orientation of dipole moments related to the pendent groups of poly(chlorophenyl acrylates) revealed their strong dependence on the position of the substituent in the benzene ring.232 The example ofPPMAwith the phenyl benzoate side groups showed 233 that the influence of thermodynamic quality of the solvent on the dipole moment is characteristic of polymers with highly rigid polymeric chains and strong intramolecular interactions of the side groups.For poly(2-biphenylyl methacrylate), rather insignificant (5% ± 18%) variation of [Z] in different organic solvents was observed,234 whereas in other poly(aryl methacrylates) the varia- tion of [Z] is 21% ± 61%, which is due to the presence of the bulky o-biphenylyl group localised close to the basic chain.In poly(4- biphenylyl methacrylate),235 the undisturbed dimensions of the chain calculated by the Stockmayer ± Fixman method with con- sideration of the dependence of [Z] onMproved to be three times as large as those in the case of free rotation in the main valent chain, which is indicative of an appreciable inhibition of rotation in poly(aryl methacrylates). For this polymer, the temperature coefficient of undisturbed dimensions of the chain was determined (2.361073 at 13 ± 40 8C and 1.261073 at 40 ± 60 8C);236 similar calculations were performed for the ortho-isomer and for poly(2,4,5-trichlorophenyl methacrylate).237 In the case of poly(2-biphenylyl methacrylate), the value of the chain flexibility parameter s (2.8) was larger than that for poly(4-biphenylyl methacrylate) (s=2.7), which points to a larger flexibility of the former due to a close position of the o-diphenylyl groups to the basic chain.238 Studies of the mobility in poly(2-biphenylyl methacrylate) by the DSC method pointed to the occurrence of four relaxation processes with the temperature increase, viz., g, b, a and r.The processes of a- and b-relaxation are related to vitrification and subvitrification, respectively.239 For PPA, it was found that the values of temperature coefficients of the undisturbed dimensions range from 73.461073 to 74.761073, the upper critical temperature of dissolutionYin ethyl lactate is 14 8C and the values of [Z] increase with the temperature increase within the range 7 ± 77 8C.240 As for PMMA, the constants of the Mark ± Hauwink equation for PPMA fit the van Krevelen equation.241 The use of the light scattering method in ethyl lactate for PPA allowed determination of values ofMw, second virial coefficients A2, mean squared radii of inertia <S2> and coefficients of swelling at different temper- atures.242 This method was also applied to study PNMA solu- tions 243, 244 in comparison with polystyrene.The ratios of the threshold concentration c0 in light scattering measurement to the second virial coefficient A2 for these polymers were the same and equal (in non-polar solvents) to 0.93. For dilute solutions of homopolymers with the main methacrylate chain and methoxy- biphenyl mesogenic groups and their copolymers,245 the constants in the Mark ± Hauwink equation, parameters of Flory ± Huggins interaction and the mean squared distances between the chain ends<r2>were determined. The latter parameter is greater than that in PMMA, which indicates an appreciable increase in the rigidity of the polymethacrylate chain upon introduction of the mesogenic side groups.As regards PMA copolymers 246 and substituted PMA,247 for them, too, the values of K, a and Y were found in certain solvents, and the undisturbed dimensions of the polymer coils and parameters of the chain rigidity were calculated. The data on electrical properties of poly(aryl (meth)acrylates) are scanty.The spectra of dielectric losses at different temper- atures were studied for specimens ofPPMAand polymers of o-,m- and p-PTMA 248 and for polymethoxy-substituted PMA.249 Three relaxation processes were found to occur. The high-temperature process (a-relaxation) occurs in the glass transition range and is related to the micro-Brownian mobility in the basic chain of the polymer. The maximum temperature of this process increases in the following order:m-PTMA<o-PTMA<PPMA<p-PTMA; the crystallinity of polymers increases in the same order. In the middle-temperature range, b-relaxation is observed due to the micro-Brownian mobility of the side groups, which is particularly790 manifested in o-PTMA.Increased activation energies of this process were observed for specimens of higher crystallinity (PPMA and p-PTMA). The low-temperature g-relaxation is associated with the motion of the tolyl and methoxyphenyl pendent groups. Investigation of the dielectric relaxation of poly(phenyl and chlorophenyl acrylates) revealed that the inten- sity of the b-process in PPA and poly(4-chlorophenyl acrylate) is lower than in poly(2-chlorophenyl acrylate) and poly(3-chloro- phenyl acrylate); in the last two polymers the intensity of b-process is higher than that of a-process. a-Relaxation is determined by the free volume and is well described by the binding model;250 in polymers the time of dielectric relaxation is always shorter than the time of mechanical relaxation.251 Noticeable changes in the electrical characteristics of poly- ethylene in strong fields (electrical strength and I ±V character- istics in the pre-breakdown voltage range) are observed when small amounts (1% ± 2%) of comonomers, such as pentabromo- phenyl and 2,4,6-tribromophenyl methacrylates, were intro- duced.252 The maximum value of breakdown voltage is observed at the content of the former additive of 0.1% ± 0.2%, the doping action is produced by units of the above-mentioned PMA.Such an effect is not exerted by benzyl methacrylate. In a magnetic field, a noticeable effect of the increase in spontaneous orientation of PPMA may be observed only in mesomorphic polymers, in particular in those with the phenyl benzoate pendent groups.253 Poly[aryl (meth)acrylates] are very transparent (transmission coefficient >90%) and possess good mechanical properties.254 Therefore, copolymers containing up to 99% of these monomers are used as materials for manufacturing lenses.255 Some of them, in particular 1- and 2-NMA, determine the high refraction index of copolymers.256 Some aryl (meth)acrylates and their polymers may exist in the liquid-crystalline state with characteristic anisotropy of optical properties.31, 257 Their polymeric nature is manifested in consid- erable enhancement of thermal stability of the mesophase, its temperature range is restricted by the glass-transition and clar- ification temperatures and is determined to a considerable extent by the nature of the basic chain and its flexibility.258 In such polymers, structures of various mesophases and different variants of the packing of mesogenic groups can be realised. Most polymers can form lamellar structures of the smectic type.Nematic structures arise in polymers with shorter substituents and flexible hinges. In particular, all polymers with the lateral mesogenic groups possess nematic mesomorphism.259 Clarifica- tion temperatures of polymers with the same mesogenic groups but different chemical composition of the basic chain coincide, whereas in these cases the Tg values decrease with the increase in flexibility of the basic chain or the length of the terminal fragments of the mesogenic pendent groups.For all the polymers having one mesogenic side unit per elementary unit of the chain, it was established that the global anisotropy is independent of steric inhibition of the a-methyl groups. A decrease in temperature results in a basic polymer chain extension normally to its mesogenic units, which thus acquires an elongated shape.260 According to the results of X-ray diffraction analysis, the macromolecules of polymers of phenyl and 4-alkoxyphenyl esters of acryloyloxybenzoates,261 which form anisotropic systems on cooling of melts or concentrated solu- tions, represent cylinders 8 ± 10 nm long and 2.5 ± 3.9 nm in diameter formed by aperiodic helices. Acrylic polymers that contain mesogenic phenyl benzoate groups with short alkyl substituents form only nematic meso- phases.262, 263 In studies of the comb-like liquid-crystalline poly- mers, which differ in the nature of the basic chain (acrylate or methacrylate) but contain identical phenyl benzoate groups,264, 265 the low- and high-temperature smectic phases of polyacrylate are referred to the smectic types B and A, respec- tively.For polymethacrylates, the following scheme of polymorphic transformation was proposed: smectic F ?smec- V G Syromyatnikov, L P Paskal', I A Savchenko tic C?smectic A?isotropic liquid. Investigation of conforma- tional changes in poly(methacryloyloxyphenyl 4-n- alkoxybenzoates) 266 in the temperature range including the glassy, liquid-crystalline and isotropic states revealed that the conformational changes are observed in the glassy state 40 ± 80 8C before the transition to the liquid-crystalline state depending on the alkyl chain length and that they are accompanied by the homeotropic orientation in the polymeric films.Poly(biphenylyl acrylate) prepared by the reaction of polyacryloyl chloride with 4-hydroxybiphenyl forms a smectic mesophaseAat 144 8C, which is totally destroyed at 250 8C. The mesophase ± isotropic transi- tion is nearly fully reproduced in repeated cycles of heating.267 Poly(4-acryloyloxyazobenzene) is in turn in the mesomorphic state at 100 ± 240 8C. Polyacrylates and polymethacrylates that contain the cyano-substituted aromatic azomethine and cyanobi- phenyl mesogenic groups form liquid-crystalline phases of the nematic type and also of the smectic type A and C.268, 269 In polymethacrylate with the biphenyl fragments as mesogens in the side chains, a poorly ordered smectic phase exists at 136 ± 157 8C and above these temperature, the isotropic phase.270 Thus, among poly[aryl (meth)acrylates] it is possible to find polymers that have mesophases in different temperature ranges.Such polymers are capable of reflecting selectively light in the IR, UV and visible parts of the spectrum, manifesting transitions induced by electric and magnetic fields, and in some cases they even possess ferroelectrical properties.258 Owing to such electrical and other effects, the field of application of these polymers in modern technology is extended. VI. Chemical and photochemical properties of poly[aryl (meth)acrylates] and their copolymers Chemical properties of poly[aryl (meth)acrylates] and their monomers are determined primarily by the presence of the ester group, the aromatic ring and its substituents.The rate of hydrolysis of the pendent groups in PPMA at different temper- atures in aqueous DMSO is described by three different constants corresponding to the hydrolysis of the ester groups of the polymer with the adjacent 0, 1 and 2 hydrolysed ester groups.271 The activation energies that correspond to these constants are equal to 40.6, 58.2 and 74.0 kJ mol71, respectively. Alkaline hydrolysis of PPMA with p-Cl, p-OMe, p-Me and m-NO2 substituents in the aromatic ring is of the pseudo-first order;272 the first two constants increase with the higher KOH concentration, whereas the third constant remains virtually unchanged.The values of these constants correlate well with the values of the Hammett constants for substituents. Enzymatic hydrolysis of the side ester groups in copolymers of 4-nitrophenyl methacrylate with acrylamide 273 in the presence of lipase did not take place, whereas esterase and a-chymotrypsin acted very slowly. The rate of alkaline hydrolysis increased with the higher content of acrylamide in the copolymer, which was due to the influence of acrylamide units on the reactivity of ester groups in the 4-nitrophenyl fragments. The action of a-chymo- trypsin on 2-biphenylyl methacrylate (BPMA) and its copolymers induced hydrolytic liberation of hydroxybiphenyl; the Michaelis constant for the copolymers was lower than for the homopolymer and decreased in the following series of comonomers: methacry- loylglycine>vinyl acetate>methacryloylalanine>styrene.274 The catalytic effect of a-chymotrypsin was weaker in the case of poly(2-biphenylyl acrylate).275 The possibility of modifying poly- mers and copolymers of 2,4,5-trichlorophenyl acrylate 276 and 4-nitrophenyl methacrylate 277 ± 279 through their aminolysis has been demonstrated. Optimisation of the reaction conditions makes it possible to introduce various substituents, for example, perfluoroalkyl groups,280 into the phenyl groups of PPMA.Reactive functional groups can be subjected to further transformations 281 and be used for the chelate formation.282, 283Aryl (meth)acrylates and polymers based on them Chemical transformations of poly[aryl (meth)acrylates] under the action of different types of radiation are rather diverse. Investigation of the photobehaviour of phenyl-containing meth- acrylate polymers showed 284 that the lifetime of the singlet state in polymers is shorter than in model small molecules, which is related to the higher sensitivity of this parameter to steric hindrances.A high yield of long-living triplet states was observed in the presence of naphthalene rings.285 The acetyl groups favour quenching of the triplet. 2,4-Diacetyl-1-naphthyl methacrylate units induce quenching even at their content *1 mol.% with a quantum yield of 0.4.286 In poly(2-naphthyl methacrylate), insignificant migration of the singlet energy occurs.287 Because of a large difference in the energies of the excited singlet and triplet states, the singlet energy transfer to the ketone traps in a copolymer of 2-NMA with arylvinyl aromatic ketones and the triplet energy is transferred from these traps again to the naphthalene chromo- phores.The fluorescence (FL) spectrum of 2-PNMA 288 is characterised by the presence of two bands which are attributed to monomeric (340 nm) and excimeric (400 nm) luminescence. The luminescence spectrum of copolymers strongly depends on the amount of a comonomer. In copolymers with MMA, the correlation between the excimeric and monomeric fluorescences is determined basically by the molar fraction of naphthyl methacry- late units.289 The ratio of intensities of excimeric and monomeric fluorescences is taken as the compatibility parameter of copoly- mers of naphthyl and anthryl methacrylates with MMA and n-butyl methacrylate (BMA).290, 291 In copolymers of PMA,292 the band of excimeric FL is at 355 nm.The FL spectra of copolymers of vinyl 3-(1- and 2-naphthyl)acrylate with vinyl hexanoate and vinyl acetate were analysed in order to reveal the monomeric and excimeric FL of the naphthalene groups at 340 and 390 nm, respectively. It was shown 293 that the excimer is formed by two naphthalene groups bound to cyclobutane in the 1,2 or 1,3 positions cis-arranged relative to the cyclobutane ring.The curves of FL decay are well described by three exponents attributed to three fluorescence centres: excimers, monomeric chromophores capable of transition to the excimer form and isolated monomeric chromophores incapable of excimer forma- tion.Copolymers of 2-[(2,4,6-tricyanophenyl)thio]ethyl metha- crylate with 3,5-bis(N,N-dimethylamino)phenyl methacrylate which are obtained in bulk phase and form rigid films possess photoconductivity. The photocurrent in such films was observed upon illumination with light with wavelengths of 200 ± 700 nm.294 Photodestruction of PPMA in vacuum and in air occurs with the random scission of macrochains and is intensified under short- wave UV irradition.295 The presence of chlorine atoms in the phenyl radical accelerates photodestruction. For 1-PNMA and its copolymers with BMA it was established that in contrast to thermodestruction leading to depolymerisation, the photo- and radiation destructions involve processes of the basic chain scission, the photodestruction rate depending on the intramolec- ular excimer formation.296 For this reason, the quantum yield of the chain scission decreases monotonically (down to 0.015) with the increase in the content of 1-NMA units. Excimers are not formed for the BMA±NMA±BMA sequences.297, 298 Upon irradiation with electrons (20 keV) and UV light, the perfluoro- alkylated PPA is cross-linked in contrast to the non-perfluoro- alkylated one which is destroyed under these conditions.299 In the presence of oxygen, photooxidation of poly[aryl (meth)acrylates] occurs with the formation of carbonyl groups.Investigation of the photooxidation of 2-NMA copolymers with styrene, MMA and acrylonitrile revealed 300 that where the content of 2-NMA was 7 mass %, the concentration of carbonyl groups was considerably higher than in polystyrene (0% ofNMA) or in copolymers with 13% or 26% of 2-NMA. The number- average molecular mass is also most strongly decreased in copolymers with 7% of 2-NMA. On the whole, poly[phenyl (meth)acrylates] are regarded as radiation-resistant materials;301 however, their resistance is 791 sharply decreased upon introduction of halogens into the aro- matic rings. Upon irradiation of poly(4-chlorophenyl methacry- late) with ionic beams,302, 303 the primary process is the elimination of the chlorine atom with the formation of reactive centres in the basic chain under the action of atomic chlorine.Recombination of these two types of polymeric radicals leads to the cross-linking of the polymer. The radiation yield of the basic chain scission in the radiolysis of MMA copolymers with trichloro- and tetrabromophenyl methacrylates 304 is strongly enhanced by small amounts of the latter, which is related to the important role of the processes of dissociative electron capture. The yield drops dramatically with the increase in the concentra- tion of chlorine-containing monomer to 17% when the processes of cross-linking become noticeable. An analogous behaviour is displayed by halogen-substituted PPMA upon irradiation with a beam of electrons.305 Homopolymers of 4-acryloyloxyazobenzoate and 4-meth- acryloyloxyazobenzene copolymers with methyl acrylate 306 and MMA,307 respectively, possess photochromic properties.The azo group in their units undergoes trans ± cis-isomerisation under UV irradiation (l=370 nm). The increase in absorption is observed at l<280 nm and l>400 nm. Photoisomerisation occurs as the first-order reaction and is reversible, the rate constant for this reaction is independent of temperature. The reverse dark reaction (thermal cis ± trans-isomerisation) is also of the first order. In homopolymers, the kinetics of photoisomerisation is of a more complex character.For them, this is an energetically more favourable process than for copolymers, which may be explained by the interaction of pendent groups. The absorption band of the azobenzene group corresponds to the p ± p*-transition and is shifted from 321.5 to 324 nm with the decrease in the content of the trans-isomer.307 Like other phenyl esters, PA and PMA as well as their polymers and copolymers undergo the photochemical Fries rearrangement under the action of UV light 308 ± 313 COR0 COR0 OCOR0 hn OH + OH R1=CR=CH2. The rearrangement is inhibited after transformation of certain amount of phenyl esters into o- and p-hydroxyphenyl ketones (a 2 : 1 mixture). The ketones were identified by the appearance of absorption maxima at 265 ± 270 and 294 nm in the UV spectra of irradiated monomers or new absorption bands at 1620 and 1660 cm71 in the IR spectra of UV-irradiated poly[phenyl (meth)acrylates].The stability of the polymer with respect to further UV irradiation is associated with the formation of hydroxy ketones on its surface and their photoshielding effect due to higher extinction coefficients. It is known 314 that 2-hydroxybenzophenones are good light stabilisers. It is pre- sumed that their light-stabilising effect is related to the fast tautomerism of excited states. Spectral measurements showed 315 that under prolonged action of the UV light the molecules of m- and p-(N-acylamino)phenyl (meth)acrylates undergo the Fries rearrangement involving mainly the ester groups rather than the acylamino groups and resulting in the appearance of the hydroxy and amino groups, respectively.OC(O)CR CH2 H O CCR CH2 hn O NHC(O)R0 NHC(O)R0792 OC(O)CR CH2 H O CCR CH2 O hn NHC(O)R0 NHC(O)R0 R0=Me, Et, Pr. Poly(1- and 2-naphthyl methacrylates) also undergo photo- chemical transformations by the Fries reaction upon UV irradi- ation.316, 317 This reaction may be of practical value for the modification of polymeric materials. Yet another type of light- sensitive polymers, viz., poly(methacryloyloxybenzylidene- acetophenones) 318, 319 and copolymers of 4-methacryloyloxyben- zylideneacetophenone with BMA,320 have been described. Upon UVirradiation, they become insoluble in organic solvents because of photodimerisation resulting in the formation of cyclobutane structures.Spectral sensitisation of such copolymers is possible with triplet sensitisers, viz., 5-nitroacenaphthene, picramide, N- acetyl-4-nitronaphthylamine and some quinones. Evidently, this process is similar to the photoaggregation of cinnamates. The polymers containing 4-acryloyloxybenzophenone units are highly efficient photoinitiators of polymerisation.321 ± 323 After light absorption, the units of this monomer pass into the singlet- excited state with subsequent intercombinational conversion into p,p-triplet state and transition to n,p-triplet state, which may be quenched by oxygen molecules or be deactivated in the presence of a hydrogen donor with the formation of ketyl radicals.323 Homopolymer is the most efficient as the polymeric photoinitia- tor and its copolymers with 4-dimethylaminostyrene are also highly active.321 Some PMA copolymers possess unusual photoluminescent properties, whereas the corresponding low-molecular-mass com- pounds and homopolymers do not luminesce under experimental conditions.324 The large diversity of properties opens wide possibilities for the application of poly[phenyl (meth)acrylates].VII. Practical application of aryl (meth)acrylates The data on liquid-crystalline aryl (meth)acrylates were generalised in a number of monographs (see, e.g., Refs 59, 60), and this allows us not to go into details of this information. At present, several types of phenyl (meth)acrylates are produced in small quantities and their variety increases continually.An analysis of most recent scientific and patent publications shows that these monomers and polymers, novel for modern industries, will find wide uses basically as modifiers of properties of polymers produced on a large scale. In order to manufacture lenses with high refractive indices (up to 1.67), thermal stability, resistance to shock and solvents and surface hardness, use is made of copolymers of the known monomers (MMA, styrene, etc.) with 3%±40% of PMA,325 ± 328 halogenoaryl methacrylates,329 ± 332 tribromophenyl methacryl- ate,333 ± 337 brominated phenyl acrylates,338 as well as 1- and 2-naththyl (meth)acrylates and benzylphenyl methacrylate.339, 340 Pentachlorophenyl (meth)acrylates may be used as comonomers in the manufacture of light-conducting fibres.341 PMA and PA may be the components of oxygen-permeable plastics intended for manufacturing contact lenses.342 Poly(fluorophenyl acrylates) are used for manufacturing glass for aircraft port-holes.343 Halogen-containing phenyl (meth)acrylates [e.g., 2,4,6-tribro- mophenyl (meth)acrylate] were recommended as components of rubbers 344 and plastics 345 in order to increase their fire resistance.For the same purpose, pentabromophenyl (meth)acrylates are incorporated as the third component into a styrene ± acrylonitrile copolymer.346, 347 V G Syromyatnikov, L P Paskal', I A Savchenko Aryl (meth)acrylates are introduced into plastics to improve their service properties, e.g., shock resistance.348 Poly[3-pentade- cylphenyl (meth)acrylates] are used as pressure-sensitive adhesives for binding various porous materials.349 2,2-Di(4-hydroxy- phenyl)propane di(meth)acrylates exert a particularly noticeable effect on general properties of plastics.350 ± 352 It is proposed to use di(meth)acryloyl derivatives of dihydroxybiphenyl oxides as cross-linking agents for polymeric materials.Fluoro-substituted diacrylates, for example, 1,3-bis(hydroxy-hexafluoropropan-2-yl)benzene diacrylate, are used for making coatings.353 It is proposed to use alkoxylated dimethacrylates of bisphenol-A,354 ± 356 xylylene dimethacry- lates 357 and aryl or naphthyl (meth)acrylates 358, 359 in composi- tions for photo-solidified coatings and glues. High-strength solidifying compositions may be prepared using hydroxyaryl (meth)acrylates that can form urethanes with diisocyanates or isocyanatoethyl methacrylate.360 PMA and its derivatives and 2-NMA (510%) are incorpo- rated into copolymers used in the production of precision optical devices and video discs for recording information.361 ± 364 Low- hygroscopic methacrylic polymers with the water absorption 40.58% at 50 8C have been patented.365 Polymeric surfactants based on copolymers of hydrophilic and hydrophobic monomers are highly effective.366 These include di(meth)acrylates of hydro- quinone containing in its ring such substituents as halogens, alkyls (C1±C20) and biphenyl.Copolymers which are components of hydrosol compositions with improved adhesive capacity and cohesive strength may contain 2-acryloyloxynaphthalene-2-sul- fonic acid along with other monomeric acids.367 Emulsifying monomers of the general formula SO3Na CHC(O)O H2C C12H25 accelerate polymerisation and increase the yield of the target polymer.368 The adhesively strong coatings resistant to the action of water and ethanol were obtained based on film-forming aryl (meth)acrylate copolymers.369, 370 Aqueous dispersions of such copolymers applied onto the polymer surface prevent sweating of plasticising agents from them.Phenyl (meth)acrylates are used for the modification of well- known polymeric products. In order to prepare self-coloured polypropylene, polycaprolactam or polyethylene terephthalate fibres 371 and films,372 these are grafted (with the aid of UV light) to azo monomers, for example, Me N N OC(O)CH N N CH2.Azo monomers are also used in the synthesis of polymeric luminophores.373 Units of tribromophenyl methacrylate are incorporated into polystyrene in order to improve its colour- ability.374 Coloured polymeric films were prepared by dispersing dyes based on 3,3 0-diethoxydicarbocyanine iodide in PPMA.375 Phenyl (meth)acrylates containing groups with antioxidant or UV-stabilising activities may be introduced into macromolecules as intrachain stabilisers.376, 377 This method of stabilisation is more efficient than the conventional introduction of antioxidants because it prevents their losses from polymers due to volatilisation during processing or diffusion in the course of their service.p-Anilinophenyl methacrylate 378 may be regarded as one of the efficient thermostabilisers of this type for polyethylene.376 Copolymers of dienes and vinylarenes (styrene) are stabilised by introducing up to 1.5% of a complex acrylate 379Aryl (meth)acrylates and polymers based on them CR00R000 OC(O)CR0 OH But But CH2 Me Me R0, R00, R000=H, Alk(C1±C16), Ar, cyclo-Alk(C5±C9), AlkAr(C7±C12). Monomeric amines of the type of 4-hydroxydiphenylamine (meth)acrylate belong to the group of non-sweating and non- washed off antioxidants for rubbers.380 2-tert-Butyl-6-(3-tert- butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate is a high-temperature thermostabiliser belonging to the group of polymer-bound antioxidants of the phenol type.381, 382 Aminophenyl (meth)acrylate derivatives have been used as antioxidants 383 NH XC(O)CR CHR00 R0 R0=H, Me, OMe; R00=H, Me, Ph; X=NH, O.Such antioxidants are not washed off from rubbers by solvents, are not evaporated and do not cause their bleaching, which is highly important in the technology of rubber processing. (meth)acrylates, Acylaminophenyl imidophenyl (meth)acrylates and imidonaphthyl (meth)acrylates 384, 385 are efficient light stabilisers for styrene. The mechanism of stabilisa- tion is associated with the intramolecular energy transfer in molecules under the action of UV light. Aryl methacrylates introduced into the polystyrene macromolecule in the amount of 5 mass% undergo the Fries photorearrangement, which is accel- erated owing to the intramolecular energy transfer of triplet excitations from the phenyl to the naphthalene moieties of the molecule.The photorearrangement rate is an order of magnitude higher in naphthyl (meth)acrylates than in phenyl (meth)acrylates. The use of monomers providing for self-stabilisation upon exposure to natural conditions is rather promising for the preparation of copolymers.316 Under the action of light, the Fries rearrangement occurs in films, for example, of copolymers of 1-NMA or 2-NMA with styrene or MMA, with the formation of structures of the type of 2-acetyl-1-naphthol with absorption at 320 ± 400 nm.Even at a degree of conversion of *5%, one observes a substantial decrease in the photorearrangement rate, which leads to the increased light-resistance of NMA units situated at a distance of *2.6 nm from the photoproduct.386 The application of such light stabilisers is advisable for coatings and fibres. Polymeric absorbers can also be obtained by copoly- merisation of monomers which contain light-stabilising fragments or groups, e.g., of phenyl 5-(meth)acryloyloxysalicylate with vinyl acetate, vinylidene chloride and vinyl chloride and of 2-hydroxy- 4-(meth)acryloyloxybenzophenone with MMA.387, 388 The effect of light stabilisation is also achieved by grafting these monomers to polystyrene, polypropylene and polyethylene; the activity of methacrylate is higher and this is explained by additional stabilisation of the radical owing to hyperconjugation with the methyl group.389 An analogous effect is also observed for di(meth)acryloyl derivatives of 2,4-dihydroxybenzophenone.390 To prepare transparent articles with improved UV absorption and weather resistance, it is proposed to introduce phenyl (meth)acrylates with benzotriazole substituents into copoly- mers.391, 392 HO N OC(O)CR CH2 N X0 N X00 X0, X00=H, Hal, Alk, Ar.The photochemical behaviour of numerous monomers and polymers with benzotriazole fragments has been described in 793 several publications.393 ± 395 It was shown that the light-protective effect of polymeric UV absorbers depends on the nature of the groups which determine the nearest environment of the fragment and their influence on the strength of the intramolecular hydrogen bond in the benzotriazole fragment. Polyethylene used for manufacturing a packing material containing 4-acryloyloxy- benzophenone units (from 0.1% to 40%), which is destroyed under the action of light in 120 h, has been patented.396 Monomers with the nitro groups, for example, 4-nitrophenyl methacrylate 397 or 4-nitro-1-naphthyl methacrylate can be used as photosensitisers. Monomers with 4-dialkylaminobenzyl- ideneacetophenone residues, e.g., 4-(4-dimethylaminobenzyl- idenecarbonyl)phenyl methacrylate can be used as 398 photoinitiators, including the intrachain ones,399, 400 sensitive to light with a wavelength of 488 nm. Aryl (meth)acrylates undergo photopolymerisation, which makes them promising components of photopolymerisable com- positions (PPC) for printed circuit boards and photoresists.Di(meth)acrylates of the general formula H2C=CRC(O)NH± C6H4OC(O)CR=CH2 are introduced into PPC for improving the technology of manufacturing offset type forms,401 upgrading printing-technical properties of compositions for obtaining thin copying layers.402 It was proposed to introduce 3-acetylamino- phenyl acrylate into PPC for the same purpose.403 Dimethacry- lates of isomeric aminophenols are also used as light-sensitive compounds in compositions for ideally transparent planographic printed circuit boards.404 Two preparative methods for obtaining light-sensitive polymers containing p-phenylene diacrylate groups (PDAG) have been described.Under the action of radiation from an argon laser, PDAG is photodimerised with a quantum yield of 0.44, which allows one to use these polymers as resists.405 Comparison of the reactivities of PA in UV-compositions with alicyclic and aliphatic analogues (including diacrylates) showed its highest efficiency.406 A copolymer of 4-nitrophenyl methacrylate with 2-cinnamoyloxyethyl methacrylate is an efficient negative photoresist with a sensitivity of 17 ± 34 mJ cm72 and a resolution of 1.5 ± 3 mm for a layer 0.5 ± 1 mm thick.407 Homo- and copoly- mers of 4- or 2-tert-butoxycarbonyloxyphenyl (meth)acrylates are used as binding agents in irradiation-sensitive polymer mixtures and depending on the development technique of the irradiated photoresists, negatives or positives are obtained.408 The use of phenyl acrylates in resists for X-ray microlithography with the plasma-based development makes it possible to attain submicron resolution (*0.5 mm) at a sensitivity of 2.5 mJ cm72 (see Ref.409). The content of aryl (meth)acrylate in X-ray resists reaches 30%.410 Positive photoresists with high sensitivity and resolution are obtained by polymerisation of esters of 2-cyanoa- crylic acid in the presence of sulfur-containing compounds.411 Irradiation of poly(halogenophenyl methacrylates) with elec- trons induces their cross-linking;412 therefore, such polymers may be used as negative electron resists.Poly(4-chlorophenyl meth- acrylate) is an example. However, the increase in the number of chlorine atoms in the ring makes destructive processes more pronounced and the resist may become negative. Poly(phenyl methacrylates) with the perfluoalkyl groups are also regarded as negative electron resists. The sensitivities of C3F7- and n-C6F13- containing polymers are 20 and 7 mC cm72, respectively.413 In the case of copolymers, the fluorine atoms may be present in comonomer units. For example, a copolymer of PMA containing up to 50% of a-trifluoromethylacrylic acid is an electron resist with a sensitivity of 10 mC cm72 at a zero residual thick- ness.414, 415 The halogen-free poly[aryl (meth)acrylates] are pos- itive electron resists with an insufficiently high electron sensitivity.It is 50 mC cm72 for PPMA, 200 mC cm72 for PNMA, 65 mC cm72 for poly(fluorenyl methacrylate), 550 mC cm72 for poly(anthryl methacrylate) and differs considerably from that for PMMA (10 mC cm72).416 Introduction of halogen, e.g., by chloromethylation, changes drastically the situation.417 Elec- tron-ray resists become negative with a sensitivity of 3.7 ± 11.0 mC cm72; their sensitivity increases up to a 20%794 content of chloromethyl groups. The crucial role in the cross- linking process is played by recombination of free radicals formed as the result of cleavage of the carbon7halogen bond. In contrast to vinyl and epoxide polymers, virtually no postradiational chain polymerisation, which decreases the resolution obtained, occurs in 2-PNMA.In this particular case it reaches 0.5 mm. This polymer may be sensitised to the action of light with l=365 nm (3% of Michler's ketone), the resultant sensitivity is*100 mJ cm72. The above-considered polymers may be used in the so-called hybrid lithography, where relatively large details of microrelief are obtained under the action of UV light, while fine details are obtained with the aid of a well-focused electron beam. Copolymers of ethylene with halogenoalkyl-substituted PMA are excellent dielectrics.418, 419 Polymers of 4-hydroxystyrene or its halogen derivatives esterified with methacryloyl chloride,420 exhibit good dielectric properties, heat and fire resistance. These are also suitable for manufacturing bases for semiconductor devices.As in the case of copolymers of styrene with 2,4,5- trichlorophenyl acrylate,421 modification of functional groups makes it possible to prepare electron-conducting metallocom- plexes. Polymethacrylates, containing aromatic amino groups in the side chain, were found to possess the hole-type photoconduc- tivity with rather high mobility of charges comparable to that in polyvinylcarbazole.422 There are data on piezoelectrical 423 and magnetic 424 proper- ties of poly[aryl (meth)acrylates]. Specific behaviour of such polymers with liquid-crystalline properties in electric and light fields allows prediction of their wide use in electronics and optics, in particular, in birefringent and colour displays using the `guest ± host' effect, phase lenses and high-resolution laser devices.425, 426 Like phenols, phenyl (meth)acrylates are antiseptics. Intro- duction of chlorine atoms into the aromatic rings increases biocidal activity; however, the low rate of the ester bond hydrolysis 427 of poly[aryl (meth)acrylates], e.g., 2,4,5-trichloro- phenyl methacrylate, may lead to the total loss of this activity.Nonetheless, materials based on copolymers of styrene with pentachlorophenyl acrylate,428, 429 p-chloro-, p-cresyl or biphe- nylyl acrylate and analogous copolymers with MMA and acryl- ates 429 possess noticeable antimicrobial activities. Homo- and copolymers of pentachlorophenyl methacrylate 430 and 2-car- boxyphenyl methacrylate 431 are analogous in this respect.It is proposed to use polymers of 2- and 4-(meth)acryloyloxy- phenylacetic acids 432, 433 and their alkyl esters as bactericidal agents for the purification of sewage. High fungicidal activity is displayed by copolymers of penta- chlorophenyl (meth)acrylate 434 with vinyl acetate, ethyl and butyl acrylates, MMA and MAA. Their emulsions possess good film- forming properties and may be used as components of fungus- resistant dyes 434 for indoor and outdoor work, while copolymers with acrylamide 435 are applied for impregnation of paper. It is proposed to use 2,2-bis(4-methacryloyloxyphenyl)- propane in compositions for fillings in dentistry.436 ± 438 Dentists also make use of solidified compositions which possess enhanced adhesion to metals and the dental tissue and contain from 0.5% to 10% of hydroxybenzoic (meth)acrylate 439 or aromatic di(meth)- acrylates.440 A hemocompatible polymeric material for coating blood-contacting surfaces was obtained from dimethyl (2-hydr- oxyethyl)ammonium salt of poly(2-acryloyloxybenzoic acid).441 Some phenyl methacrylates, e.g., 4-azidophenyl methacrylate, are components of copolymers possessing hemoglobin functions.442 Others, e.g., 4-nitrophenyl acrylate, are introduced into polymeric carriers for the isolation of biologically specific molecules such as proteins, which may be used for radiodiagnostics of thrombi, antigens and tumours 443 or for immobilisation of enzymes, amino acids, proteins 444, 445, e.g., serum albumin.446, 447 Polymers based on 2,4,5-trichlorophenyl, 4-nitrophenyl or 2,4-dinitrophenyl (meth)acrylates are X-ray contrasting preparations.448 Introduction of residues of unsaturated acids into medicinal substances and subsequent polymerisation yield drugs with prolonged action.69 ± 70 Resorption of polymers of acrylic acid V G Syromyatnikov, L P Paskal', I A Savchenko with b-naphthyl acrylate as potential coatings of such medicinal preparations was studied by liquid chromatography by monitor- ing the liberation of the b-naphthol upon hydrolysis of the copolymer.449 Thus, aryl (meth)acrylates possess a very broad spectrum of useful properties and their further studies are highly advisable.References 1. 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年代:1999

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