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Russian Chemical Reviews 70 (12) 991 ± 1016 (2001) Organic molecules with abnormal geometric parameters I V Komarov Contents I. Introduction II. Stability of strained molecules III. Abnormally long and short C7C bonds IV. Elongated and shortened C=C bonds V. Variations of the triple carbon ± carbon bond lengths in organic compounds VI. Distortions of bond angles at the tetrahedral carbon atom VII. Distortions of bond and torsion angles at the double bond in alkenes VIII. Distortions of the bond and torsion angles in the molecules of aromatic compounds IX. Distortions of cumulated double bonds X. Bending of the X17C:C7X2 fragment Abstract. which of parameters structural the molecules, Organic Organic molecules, the structural parameters of which (carbon angles) torsion and bond lengths, bond carbon ± (carbon ± carbon bond lengths, bond and torsion angles) differ differ appreciably from the typical most frequently encountered values, appreciably from the typical most frequently encountered values, are `record-breaking' of examples many Using discussed. are discussed.Using many examples of `record-breaking' mole- mole- cules, compounds carbon in distortions structural of limits the cules, the limits of structural distortions in carbon compounds and and their demonstrated. are properties chemical unusual their unusual chemical properties are demonstrated. Particular Particular attention is devoted to strained compounds not yet synthesised attention is devoted to strained compounds not yet synthesised whose properties have been predicted using quantum-chemical whose properties have been predicted using quantum-chemical calculations.Factors that ensure the stability of such compounds calculations. Factors that ensure the stability of such compounds are outlined. The bibliography includes 358 references. are outlined. The bibliography includes 358 references. I. Introduction The experimentally determined lengths of carbon ± carbon bonds of one type in the molecules of various organic compounds are usually very close. Thus the lengths of single and multiple carbon ± carbon bonds usually deviate only slightly (as a rule, by less than 0.01 ± 0.02A) from the values obtained by averaging over a large number of experimentally determined bond lengths.These values are often referred to as `typical' or `standard' geometric parameters. Some standard lengths of carbon ± carbon bonds determined by X-ray diffraction analysis are listed in Table 1.1 ±4 The same is true, although to a lesser extent (due to the smaller values of the corresponding force constants), for the bond angles the departure of which from the standard values is normally several degrees. The torsion angles usually vary over broader limits than bond angles; however, in some cases, in particular, for fragments with multiple bonds, these angles are also characteristic parameters. The standard molecular geometric parameters are fundamen- tal values used in stereochemistry.5 The invariance of these parameters underlies various types of molecular models and various methods for structure calculation, in particular, molecular mechanics methods.6 I V Komarov Department of Chemistry, Taras Shevchenko Kiev National University, ul.Vladimirskaya 64, 01033 Kiev, Ukraine. Fax (7-38 044) 226 12 73. E-mail: ik214@mail:yahoo.com Received 26 October 2000 Uspekhi Khimii 70 (12) 1123 ± 1151 (2001); translated by Z P Bobkova #2001 Russian Academy of Sciences and Turpion Ltd Table 1. Typical (averaged) lengths of some carbon ± carbon bonds. Type of bond Single bond CÖsp3Ü7CÖsp3Ü CÖsp3Ü7CÖsp2Ü CÖsp3Ü7CÖspÜ CÖsp2Ü7CÖsp2Ü CÖspÜ7CÖspÜ Double bond CÖsp2Ü=CÖsp2Ü CÖsp2Ü=CÖspÜ CÖspÜ=CÖspÜ Triple bond a CÖspÜ:CÖspÜ a The data of Ref. 4. Meanwhile, in the molecules of some compounds, the bond (or torsion) angles and the bond lengths sharply deviate from the typical values.The synthesis of such compounds is often stimu- lated by a chemist's desire to overstep the limits of the existing theoretical views and concepts. A vivid example is the synthesis of four- and three-membered carbocyclic compounds performed successfully by W H Perkin at the end of the XIX century,7, 8 despite the opinion of scholars of authority in those days who considered the synthesis of small rings to be impossible.9 1) NaOEt 2) Br(CH2)3Br CO2Et 1) NaOEt 2) Br(CH2)2Br CO2Et A different tendency has been observed in recent years: one can name a number of compounds with appreciably distorted molecular geometry the successful synthesis of which was pre- DOI 10.1070/RC2001v070n12ABEH000633 991 992 993 995 995 996 1001 1005 1008 1010Length /A 1.53 1.51 1.47 1.48 1.38 1.32 1.31 1.28 1.186 CO2Et CO2Et CO2Et CO2Et992 ceded by theoretical predictions of their possible existence and calculations of their spectral properties.In any case, synthesis and study of these compounds were favourable for the accumulation of knowledge on the nature of the chemical bond and mechanistic details for reactions of organic compounds.10 Research along this line has resulted in the development of new original methods of synthesis and experimental techniques which made an invaluable contribution to modern synthetic organic chemistry.11, 12 The non-trivial syntheses of highly deformed molecules, together with many total syntheses of natural compounds,13 can be regarded as classical achievements of modern chemical science.What properties are inherent in substances the molecules of which have highly distorted bond lengths and angles? What is the degree of distortion of the molecular geometry of organic com- pounds that results in a qualitative (discontinuous) change in their properties? What is the maximum attainable degree of distortion of bond lengths and bond (torsion) angles and do calculations reproduce the observed structural characteristics? In this review, we consider the recent publications concerned with these prob- lems.{ The literature on the synthesis and structural studies of compounds with distorted geometric parameters of molecules is very extensive.This review does not claim to be an exhaustive analysis of all these studies; the author's goal was to illustrate the limits of the possible geometric distortions of organic molecules using a number of interesting examples. Among the vast material, studies devoted to `record-holder' molecules with the greatest distortions of bond lengths and bond and torsion angles either observed experimentally or calculated are discussed. However, structural chemistry is so rapidly developing that any unusual molecule soon loses the record-breaker title.18 The review discusses compounds the molecules of which are stable despite substantial distortions of the molecular geometry.The structures of these compounds are studied by physical methods, most often by X-ray diffraction analysis. Therefore, first, it is pertinent to discuss the specific features of the structures of strained molecules which allow them to reach the stability needed to study them by physical methods. II. Stability of strained molecules Organic molecules with abnormal geometric parameters possess increased energy. A usual quantitative characteristic of these molecules is strain energy, which is defined as the difference between the experimental standard enthalpy of formation of the molecule and the enthalpy calculated for the `strainless' (often hypothetical) molecule containing the same number of atoms and the same sequence of bonds between them but no distortion of bond lengths or bond angles and no non-valence intramolecular interactions (for example, steric repulsion).19 The concept of molecular strain is widely used in chemistry, although it is not exact because the calculation schemes for the enthalpies of formation of strain-free molecules are arbitrary and often differ- ent.20 Therefore, when comparing strain energies, one has to be sure that they have been calculated by the same methods.Normally, calculations involve summation of the strain incre- ments known for the structural fragments present in the mole- cule.19 { This review covers studies published before the beginning of 2001. Analysis of publications on this topic up to mid-1970s was carried out by Greenberg and Liebman.14, 15 The publications for this review were collected using the STN electronic databases (Scientific Technical Net- work,16 first of all, the REGISTRY database, which contains information on more than 18 000 000 organic and inorganic compounds), Chemical Abstracts and Beilstein as well as the Cambridge Structural Database (CSDB; the version of April 2001 presents structural information on more than 233 000 compounds).17 I V Komarov Molecules for which this enthalpy difference is great are referred to as strained.It should be noted that the criteria for regarding one or another structure as strained are quite arbitrary. Even the adamantane molecule with a strain energy of 7.6 kcal mol71 can be regarded as `strained', although its molecular model seems misleadingly unstrained.14, 15 We shall discuss molecules with strain energies many times exceeding that of adamantane.Such molecules are often reactive and susceptible to intra- and intermolecular reactions which are not typical of strainless analogues. The combination of these properties is associated with the qualitative notion of `instability', while the factors due to which the chemical transformations of strained molecules can be retarded or entirely avoided are associated with `stability' of the molecules. Most highly strained `record-breaker' compounds known to date exist owing to kinetic stability. Despite the substantial strain energy of the molecules of these compounds, their structures correspond to minima on potential energy surfaces (PES),21 indicating the possibility of preventing decomposition and relaxation of the geometrically distorted fragments to the unstrained state.The stability against structural changes that bring a fragment with distorted bonds and angles to the `normal' state is provided most often by the fact that this fragment is incorporated into a ring or a rigid polycyclic cage. Alternatively, steric repulsion of bulky groups, especially tert-butyl substituents, also helps to avoid relaxation of a distorted fragment to an unstrained state.22 The molecules of compounds 1 and 2 with a twisted double bond can serve as examples of strained molecules having a distorted frag- ment within a polycycle or bulky groups.Steric repulsion 1 Rigid tricyclic cage 2 Steric repulsion To attain stability of strained molecules, it is necessary to prevent them from entering reactions either with one another (oligomerisation, polymerisation) or with other potential reac- tants (solvents, oxygen, moisture, etc.). These reactions can be prevented by creating steric restrictions around the reaction centres. For example, tetrahedrane 3 is so reactive that it has not been prepared until now, in spite of considerable effort,{ whereas tetra-tert-butyltetrahedrane (4), in which the strained C4 core is shielded, has been isolated and studied, in particular, by X-ray diffraction analysis. But H But H But H But H 5 4 3 The instability of strained molecules is also related to their tendency to undergo intramolecular rearrangements. Kinetic stability due to electronic factors is an effective method for preventing these rearrangements.A classical example is cubane (5) in which the possible rearrangement pathways are forbidden by the orbital symmetry rules. The strain energy of molecule 5 is very high (157 kcal mol71; determined using data on the heat of { From here on, the formulae of compounds not obtained experimentally are drawn in parentheses.Organic molecules with abnormal geometric parameters combustion of cubane 23);} however, cubane does not decompose even at temperatures above the melting point (130 ¡À 131 8C).26 The strain energy of compound 5, as well as that for other molecules described below, is markedly higher than that required for rupture of the C7C bond.However, the stability of cubane is secured by even distribution of the steric strain over the whole carbon skeleton. The exceptionally stable C60 fullerene (6) in the molecule of which strain is uniformly distributed over all the 90 carbon ¡À carbon bonds can serve as yet another example of stable but very strained molecules. The heats of combustion of C60 and C70 fullerenes and their standard enthalpies of formation, which can be used to estimate the strain energy, have been determined experimentally.276 In some cases, the concept of thermodynamic stability of strained molecules is discussed but this term is often used improperly.For example, it has been claimed 28 that coordination of a metal atom to a distorted fragment can ensure `thermo- dynamic stability' of strained ligands, in particular, trans-cyclo- alkenes. The overlap of molecular p orbitals of a strained alkene with the vacant orbitals of the transition metal and interaction of the occupied d orbitals of the metal with the p* orbital of the double bond (dative bond) finally stabilise the complex. However, in our opinion, this is a case of kinetic stability. Thermodynamic stability implies reversible transformations, for example, those between a complex with a ligand containing a distorted structural fragment and a complex with a strain-free analogue of the same ligand. Most transformations of strained molecules are irrever- sible; therefore, their thermodynamic stability is seldom encoun- tered.An example is provided by compound 7, stable in methanol.29 The driving force of the spontaneous formation of compound 7, the molecule of which contains a strained twisted amide fragment, in equilibrium with zwitter-ion 8 is the steric strain in the bicyclic system, which decreases upon cyclisation. 2 OH HN+ NH�¢ CO¡¦2OH Me Me Me 8 Me O N Me 7 Me It makes sense to discuss the stability of a compound when conditions for its isolation or investigation are known. For example, a large number of compounds unstable at room temper- ature have been studied in inert matrices at 4 ¡À 77 K.30 Low- temperature isolation in solid matrices of inert gases prevents } These data are somewhat outdated; however, more precise values have not yet been reported. The strain energy of the cubane molecule presented here is consistent with rough values determined from the heats of combustion of various cubane derivatives and with calculated values.24, 25 diffusion and, hence, excludes intermolecular reactions of the compounds under study.Moreover, intramolecular transforma- tions, even those having an energy barrier of only several calories per mole are also retarded.31 One more method that provides a sufficient (in particular, for spectroscopic studies at room temperature) lifetime of reactive species should also be mentioned, namely, preparation of these species inside cavities of large macromolecules D carcerands and hemicarcerands. In recent years, several studies have been pub- lished (see below) devoted to spectroscopic and chemical proper- ties of compounds the isolation and investigation of which at room temperature could not even be discussed before the advent of this method, for example, cyclobutadiene and dehydroben- zene.32 III.Abnormally long and short C7C bonds The characteristic lengths of C7C, C=C and C:C bonds (see Table 1) can be regarded as fundamental parameters in chemistry. Deviations from `normal' values are seldom encountered, and they serve as an indicator for the occurrence of steric or electronic effects. The quest for the longest C(sp3)7C(sp3) bond has been called a `real Odyssey.' 33 Considerable attention was devoted to hexa- alkyl- and hexaarylethanes such as 9 ¡À 11,34 ¡À 36 in the molecules of which the central C7C bond is longer than 1.64A.1.640A 1.639A Me Me Me 1.647A Me 9 But But But But 1.67A ButBut But But 11 The main cause for bond elongation is steric repulsion between bulky groups. The elongated bonds are weakened, which acunts for the thermal instability of these hydrocar- bons.34 In the case of compound 9, molecular mechanics (MM2) calculations} for the molecular geometry were carried out and good agreement between experimental and calculated values was obtained. In 1996, Toda et al.37 have reported exceptionally long C7C bonds in two benzocyclobutane derivatives, 12 (1.720A) and 13 (1.710 and 1.724A).Cl Ph Ph 1.720(4)A Ph Ph 12 Cl Ph 1.710(5)A Ph } Most of the calculation methods mentioned in the review are described in a book by Jensen.21 993 F 1.671A CF3 CF3 F3C F3C F 10 But But But But Me Me Cl Cl Ph Ph Ph 1.724(5)A Ph Ph Ph Cl Cl 13994 These values are almost 0.18A greater than the standard length of a single carbon ± carbon bond; therefore, it is not surprising that some doubts have been cast upon the X-ray diffraction data published. Before the study by Toda et al.,37 examples of compounds with even longer C7C bonds had been reported; however, a thorough verification showed the results of structural studies of these compounds to be incorrect.Thus the lengths of the central bonds in the molecule of compound 14, a photoisomer of [2.2]tetrabenzoparacyclophane 15, have been reported 38 to be 1.77A. Later, it was found that the measurements were carried out for a mixture of the photoisomers 14 and 15 rather than for a crystal of pure 14.39 In subsequent studies of a pure sample of 14 at 170 K, the lengths of the central bonds were found to be 1.648A.40 Dhn 15 14 Repeated studies 41 of the compound 12 have confirmed, nevertheless, the validity of the previously obtained data,37 and quantum-chemical calculations (B3LYP/6-31G*) reproduced them satisfactorily.42 Model calculations [B3LYP/DZ(2d,p)] for the 1,1,2,2-tetraphenylbenzocyclobutene molecule also provided a Ph2 C7CPh2 bond length consistent with the value found experimentally 37 in compound 12.These calculations demon- strated the importance of taking into account the electron correlation in order to obtain correct values for geometric parameters of the strained molecules. It was also shown 41 that satisfactory interpretation of the observed elongation of the C7C bond requires that only steric effects (repulsion between bulky groups) be taken into account without considering hyperconjuga- tion effects.43 This conclusion is also confirmed by recently published data on the synthesis and X-ray diffraction analysis of the even more sterically hindered compound 16.44, 45 The C7C bond length in the four-membered ring of 16 equals 1.729A, i.e., it is longer than those in the molecules 12 and 13.Cl ButBut 1.729A Ph Ph 16 Cl The distance between the carbon atoms formally not bound to each other in bicyclo[1.1.1]pentane (17) and its derivatives is close to the lengths of the `stretched' C7C bonds given above. The C(1)7C(3) distance in this simple hydrocarbon measured by electron diffraction 46 is 1.874A (or 1.845A according to another publication 47). The search through the CSDB for the shortest C(1)7C(3) distance among the bicyclo[1.1.1]pentane derivatives gave a value of 1.80A in bis(3-iodobicyclo[1.1.1]pentan-1-ylpyr- idinium)iodide triiodide 18.48 In this case, the X-ray diffraction data can best be interpreted by assuming that structures contain- ing [1.1.1]propellane contribute to the resonance hybrid.In the molecules of bicyclo[1.1.1]pentane derivatives, the non-bonded bridging carbon atoms are so close to each other that the `through- space' electronic interaction is clearly identified.49 3 1 H H 17 + N N I I I I¡3 18 I V Komarov The above-mentioned studies raise the question of what is the longest distance between carbon atoms in a molecule that enable the existence of a chemical bond between them. Proceeding from the empirical definition of the chemical bond { proposed by Pauling 50 without restricting the consider- ation to saturated systems, one can find examples of structures with formal C7C bonds longer than 1.729A, which is the value established for molecule 16. For example, a distance between carbon atoms equal to 2.83A has been found recently in the crystal of dimer 19, formed by two tetracyanoethylene radical anions (20).Nevertheless, the results of structural, magnetic, and spectroscopic studies are consistent with the existence of a chemical bond between them.51 This two- electron four-centre bond results from the p ± p binding interac- tion between the radical anions 20 and the Coulomb repulsion between them. It was shown that the dimers are metastable species, i.e., their formation is an endothermic process; in the solid phase, they can be stabilised upon interaction with metal cations. 7 N N 27 CN NC CN NC C C 52.83A C C CN NC CN NC 19 N N 20 Apparently, it is impossible to establish the limiting bond length of any type on the basis of merely geometric criteria such as `longer (or shorter) than..'; it is necessary to invoke conceptual models of chemical binding including new ones.Some new approaches to the determination of the nature of the chemical bond could be provided by Bader's theory.52 This theory is based on the mathematical analysis of topological characteristics of the total molecular electron density function either found experimen- tally or calculated. In terms of this theory, it is possible to give strict criteria for the occurrence of a bonding interaction between a pair of atoms, irrespective of the distance between them. A number of studies have been devoted to the search for strained molecules with short carbon ± carbon single bonds. For example, the length of the exocyclic bond in bicubane 21 was found to be 1.458A,53 which was explained by rehybridisation of the carbon orbitals. The exocyclic bonds in cubane have a greater s-character and, therefore, they are shorter than the standard C(sp3)7C(sp3) bonds.The steric strain that could result in elongation of the central C7C bond in compound 21 is insignif- icant, unlike that in the molecule of 1,10-biadamantane 22 (the length of the central bond in 22 is 1.578A 54). The central bond in bicubane can be compared with that in butadiene, the length of which is 1.383A, according to electron diffraction data;55 however, in the latter compound, this bond is further shortened due to conjugation.Schleyer and Bremer 56 not only compared the experimental and calculated (MNDO, MP2/3-21G, 6-31G*) lengths of the central bonds in molecules 23 ± 25, similar to the bicubane molecule, but also carried out model calculations for ethane (MP2/3-21G) with variation of the C7C7H bond angle. It was shown that an increase in this angle to 150 8 entails energy redistribution between molecular orbitals, resulting in strengthen- ing of the C7C bond. The calculated (MP2/6-31G*) length of the C7Ccentral bond in bitetrahedrane 25 is 1.438A. The compound 25 has not yet been synthesised; however, Ermer et al.57 believe that the hexa-tert-butyl derivative of this compound could be prepared. { According to Pauling, `... there is a chemical bond between two atoms or groups of atoms in the case that the forces acting between them are such as to lead to an aggregate with sufficient stability to make it convenient for the chemist to consider it as an independent molecular species.'Organic molecules with abnormal geometric parameters 1.438A 1.468A 1.480A 1.578(2)A 1.458A 25 24 23 22 21The angle strain in the cyclopropane molecule is known to result in shortening of the formally single C7C bonds (1.499A).58 In the bicyclobutane (26) molecule, which is the smallest one among the molecules of bicyclic hydrocarbons, two cyclopropane rings are fused; the central C7C bond exhibits properties of the double bond (for example, electrophilic reagents readily add at this bond to give cyclobutane derivatives) and is abnormally short (1.497A). X-Ray diffraction study of bicyclobutane derivatives showed that the length of the central bond depends on the angle between the planes of the three-membered rings.In the compound 26, this angle is *122.7 8, while in its derivatives 27, this angle is much smaller (*87 8) and the C(1)7C(3) bond is much shortened (1.408A), i.e., this is one of the shortest single bonds known to date.59 Me1.408AMe 26 O27 IV. Elongated and shortened C=C bonds Analysis of the structures deposited in the CSDB shows that abnormally long formally double C=C bonds are found in systems containing long chains of conjugation (see, for example, compound 28).60 In aromatic derivatives (see, for example, com- pound 29),61 the C C bonds can also be elongated. The shortest formally double carbon ± carbon bonds have been found in a number of strained molecules (for example, in molecule 30).62 Exceptionally long carbon ± carbon double bonds have also been found in twisted alkenes.The elongation of these bonds is due to a decrease in the degree of p-bonding. CO2Me H O O OH Cl Ph Ph Cl HO 1.413(2)A28 1.416(2)A CO2Me 29 Let us consider the compounds the molecules of which incorporate the shortest (30) and the longest (31) 63 C(sp2)=C(sp2) bonds known to date. But Ph Ph C CH 1.255A 30 Ph Ph 1.540(5)A But 31 The length of the double bond in ethynylcyclopropene 30 is approximately an average between the bond lengths found in strain-free alkene and alkyne molecules � 1.255A.Even very 995 high-level calculations provide overestimated lengths for the C=C bond in this molecule.62 Ab initio (HF/3-21G, 6-31G*) calculations for benzo[1,2 : 4,5]- dicyclobutene 32 predicted 64 two types of isomerism, one type being due to different bond lengths (32a and 32b, 32c) and the other type being due to different positions of the double bonds (32b and 32c). X-Ray diffraction analysis 63 of the compound 31 did not confirm the results of calculations reported earlier.64 The structural parameters found experimentally for 31 and those determined by quantum-chemical calculations for the hydrocar- bon 32 (B3LYP/6-31G*, MP2/6-31G*) correspond to the struc- ture 32d with elongated bonds in the benzene ring.However, it should be emphasised that in this case, we are dealing with aromatic bonds rather than purely double bonds (as in alkenes). 32c 32b 32a 32d V. Variations of the triple carbon ± carbon bond lengths in organic compounds The deviations of the lengths of triple carbon ± carbon bonds in organic compounds from typical values are less pronounced than similar deviations in the case of single or double bonds. For example, according to analysis of the CSDB data,4 the difference in the C:C bond lengths in acetylene and its alkyl derivatives is less than 0.05A. It is noteworthy that triple bond lengths determined by electron or neutron diffraction are often greater than those found by the conventional X-ray diffraction technique for the same compound (the difference can reach 0.02A or even more).This is due to the fact that X-rays are scattered by the electron density of the object and the electron density maxima (centroids) identified as `atoms' in the X-ray diffraction method do not always coincide with the positions of the atomic nuclei. The electron density maxima can be displaced towards each other in the case of high concentration of the electron density in the interatomic space, which is often the case for multiple bonds including triple bonds. However, in modern X-ray diffraction software packages, this effect can easily be taken into account. Nevertheless, statistic analysis covering only those parameters in the CSDB which have been determined by electron diffraction in the gas phase results in an even smaller scatter of the C:C bond lengths, namely, 0.012A.4 As a rule, elongation or shortening of triple bonds is mainly due to electronic factors.For example, this bond is shorter in the molecules of acetylene derivatives containing electron-withdraw- ing substituents at the triple bond.65 (Note that this is equally true for double carbon ± carbon bonds.) Elongated C:C bonds have been found in conjugated systems, for example, in butadiyne 55 [1.2176(6)A], although the lengths of these bonds in vinylacety- lene and propynal molecules exceed only slightly the average value. The steric factors in linear alkynes in which substituents at the C:C bond are far from each other are minimised.It can be suggested that the loss of p-bonding caused by bending of the X7C:C7X fragment (X is any substituent) in cycloalkynes would result in elongation of the triple bond. Never- theless, in most of the known strained molecules of cycloalkynes, the length of the triple bond is close to the typical value. For example, in compound 33, despite the substantial bending of the Si7C:C7Si fragment, the C:C bond length is equal to 1.22A, which virtually coincides with the average value (1.201A) for silyl- substituted alkynes.66996 1.22A Me Me Si Si Si Si Si Me Me Me Me MeMe Me Me 33 VI. Distortions of bond angles at the tetrahedral carbon atom The modern theory of the structure of carbon compounds is underlain by the hypothesis advanced independently by Van't Hoff 67 and LeBel 68 in 1874 stating that a tetracoordinated carbon atom has a tetrahedral or a nearly tetrahedral configuration.Later, the results of numerous studies including X-ray diffraction analysis have confirmed Van't Hoff's and LeBel's hypothesis based on empirical data, and the sp3-hybridisation concept has explained simply and conveniently the stability of the tetrahedral configuration. However, as soon as somewhat more than a decade after the publication,67, 68 Baeyer, who considered distortions of the tetrahedral configuration of the carbon atom, stated the foundations of strain theory.69 The types of distortion of bond angles at the sp3-hybridised carbon atom are illustrated in Fig.1. First of all, this is a decrease/ increase in one bond angle (the angle between the other two bonds usually correspondingly increases/decreases; this is the so-called scissors distortion) as well as distortions leading to flattening of the carbon configuration, i.e., tetrahedral compression and digo- nal twist. Special mention should be made of configurations in which all atoms attached to a given carbon atom are located on one side of the plane passing through it, namely, inverted and pyramidal configurations to which an umbrella-type distortion leads. Rigorous mathematical analysis of the disortions of the CX4 fragment has been reported.70 ± 72 It should be noted that in experimental or theoretical study of the electron density distribution in the molecules characterised by highly distorted bond angles at carbon atoms, the regions of increased electron density between the atoms { do not lie on the straight lines connecting the atomic nuclei but are displaced from these lines.These bonds have been referred to as curved (banana) bonds; a classic example of these bonds is found in the cyclo- propane molecule. The displacement of the maxima of DED (covalent electron density) outwards the ring in cyclopropane derivatives reaches 0.2A or more. An even greater shift of the DED maxima away from the internuclear lines, up to 0.5A, was found in tetrahedrane derivatives. Banana bonds are found not only in the molecules with small rings but also in highly strained cage systems.74 Subsequently, in particular in the analysis of structural data for cyclic and strained cage molecules, the notion of bond path was defined as the interatomic line along which the total electron density is a maximum relative to any `lateral' displacement and has a minimum in the so-called saddle point.52, 73 ± 75 A bond path is not necessarily a straight line; the presence of this line implies a chemical bond between the given atoms.76 These and some other definitions constitute the foundation of the modern quantum theory of interatomic interactions and chemical bonding, devel- oped by Bader.52 In Bader's theory, the bond path angle at an atom is determined by the angle between the tangents to two neighbouring bond paths.In strained molecules, the bond paths are curved, as indicated above. In particular, in a cyclopropane molecule, the { Normally, this is so-called deformation electron density (DED), which characterises the difference between the electron density of the molecule and that calculated as a sum of the electron densities of isolated spherical atoms located at the same distance fr each other.73 I V Komarov a b c dX X X X X X X X X X X X C C C C X X X X X X X X X C IV C X C X X X X X X I C C X X X X X X XX III II Figure 1. Distortions of bond angles at the tetrahedral carbon atom: scissors (a), tetrahedral compression (b), digonal twist (c), umbrella (d ). Carbon atom configuration: (I ) planar, (II ) inverted, (III ) pyramidal, (IV ) intermediate between the tetrahedral and pyramidal.bond path angle is *78.8 8 rather than 60 8, which has been confirmed by a number of X-ray diffraction studies of the electron density distribution in this type of system. Therefore, in the case of strained molecules, it would be more appropriate to discuss the bond path angles. A number of publications have been devoted to the study of electron density distribution in organic molecules. In these studies, the term `bond angle' is used to imply the angle between the bond paths and not between the straight lines connecting the atomic nuclei. However, the vast majority of structural studies discuss the angles between the internuclear straight lines.Therefore, in this review, we also consider the latter type of angle unless otherwise indicated. It should be noted that the Bader's topological approach and definition, in terms of its rigorous theory, of the bond path and some other key concepts of the molecular structure theory are drawing increasing attention of researchers. In particular, the classical term `bond length' (the distance between the nuclei) is replaced in this theory by the notion of `bond path length.' In the case of a strained molecule, the bond path length would naturally be longer than the internuclear distance. For example, in the cyclopropane molecule, the distance between the carbon nuclei (bond length) is 1.499A,77 and the bond path length between them is*1.53A, i.e., it is nearly equal to the typical C7C bond length (see Table 1).An even greater inconsistency between the inter- nuclear distances and the bond path lengths is found for cage systems, in particular, for icosahedral carboranes.78 1. Scissors type distortion: a decrease in the C7C7C bond angle A considerable decrease in the C7C7C bond angle is found in molecules of cyclic compounds with small rings. A lot of informa- tion on the methods of synthesis and properties of these com- pounds has been accumulated to date. Many fundamental theoretical concepts, for example, the concept of steric strain and banana bonds, have been developed upon investigations of molecules with three- or four-membered carbocycles.14, 26, 79 ± 81 We shall consider several prominent examples of research along this line.In the CSDB, we found eighteen structures with a bond angle at a saturated carbon atom smaller than 50 8. The molecules of these compounds contain three-membered rings, and most of them contain multiple bonds in the rings. The smallest A7C7A angles (A is any atom) are typical of compounds with C=N andOrganic molecules with abnormal geometric parameters N=N bonds in the three-membered rings. In compounds 34 82 and 35,83 the angle at the saturated carbon atom is somewhat smaller than that in cyclopropene derivatives (for example, in compound 36).84 This is due to the fact that the C=N or N=N double bonds are shorter than the C=C bond, and the C7N single bonds are longer than the C7C bonds in cyclopropane (the C7C bond length in cyclopropane is 1.499A).77 Both factors result in a decrease in the angle between the single bonds in unsaturated three-membered heterocycles.Ph 1.254A N 47.48 Cy2N MeO2S P 1.228A N 48.68 CF3 N 1.630A SiMe3 Cy2N 35 34 Pri SiMe3 But Pri Si 49.18 But 1.279A SiMe3 Pri But 36 Cy is cyclohexyl. Among the compounds containing saturated three-membered carbon rings with reduced C7C7C angles, bicyclobutane deriv- ative 37 should be mentioned.85 In the molecule of this compound, the bond between the bridging carbon atoms is shortened and, therefore, the opposing C7C7C angle in the three-membered rings is reduced. 1.442AMe Me 48.88 Me Me O O 37 Compounds with reduced bond angles at saturated carbon atoms exhibit unusual reactivity due to the strain in their mole- cules.As an example, we shall consider derivatives of a hydro- carbon which has not yet been synthesised, tetrahedrane 3. Ab initio quantum-chemical calculations of different levels (SCF/4-31G,86 G2 87) give a strain energy of 129 ± 137 kcal mol71 for the unsubstituted tetrahedrane molecule. This value is greater than the dissociation energy of a single C7C bond (72 ± 117 kcal mol71) 88 and exceeds (in relation to one carbon atom) the molecular strain in other hydrocarbons studied to date. It is not surprising that numerous early attempts to synthesise tetrahedrane and its derivatives have failed.14, 89, 90 Only in 1978, did Maier et al.91 synthesise the first tetrahedrane derivative, tetra-tert-butyltetrahedrane 4, by photochemical transformations of tetra-tert-butylcyclopentadienone. But But But But But hn hn But But But But But But 4 But O O The seeming simplicity of the reaction scheme conceals great efforts of researchers aimed at selecting the optimal conditions for the preparation of a compound the possibility of isolation of which seemed unlikely at that time.The compound 4 is a stable crystalline substance isomerising into tetra-tert-butylcyclobuta- diene only at 130 8C. Stability is attained owing to the presence of bulky tert-butyl substituents, which ensure the steric shielding (so- called `corset effect') of the central strained fragment of the molecule by shielding it from the attack by reagents and prevent- ing intramolecular rearrangements.The tert-butyl groups in the molecule 4 are remote from each other as far as possible but in the 997 transformations of compound 4, they approach each other, which results inevitably in an increase in the strain energy. Tri-tert-butylisopropyltetrahedrane (38) proved to be much less stable than the compound 4: its thermal isomerisation to cyclobutadiene 39 proceeds over several minutes at 80 8C.92 But Pri But Pri But But But 39 38 But For a long period, researchers did not succeed in preparing other tetrahedrane derivatives. It was another 10 years after the synthesis of 4 before a new derivative was prepared; this was tri-tert-butyl(trimethylsilyl)te- trahedrane (40).93, 94 The approach to substituted cyclopropenyl- diazomethane proposed by Masamune et al.95 permitted Maier and co-workers to prepare other silyl- 96 and alkyl-substituted 92 tetrahedranes. But R But But R ButR But But But But N2 40, 41 But R=SiMe3 (40), SiMe2OPri (41). The reaction of compound 41 with the fluoride ion affords highly reactive trisubstituted tetrahedrane 42.96 But H But 42 But X-Ray diffraction data for the compound 4 showed that the C7C bond length in it (1.485A) 97 is shorter than the typical value (1.53A) and is even shorter than the C7C bond length in cyclopropane (1.499A).77 This is due to rehybridisation of the cage atoms and bond bending.Early molecular mechanics calcu- lations (MM2) showed that, due to the steric repulsion of the tert- butyl groups, the chiral structure (group of symmetry T) is energetically more favourable (by 2 ± 6 kcal mol71) than the achiral structure (Td).98 This conclusion was confirmed by ab initio quantum-chemical calculations [B3LYP/6-311+G(d)// B3LYP/6-31G(d)].99 The difference between the energies of these forms was 0.5 ± 2.0 kcal mol71. The chemical properties of the compound 4 have also been studied. Besides the above-mentioned thermal rearrangement of the substituted tetrahedrane 4 into tetra-tert-butylcyclobutadiene, protonation with gaseous HCl and oxidation into the corresponding radical cation are known.100 2. Scissors type distortion: an increase in the C7C7C bond angle Increased bond angles in the molecules with tetrahedral carbon atoms are observed, naturally, in the same cases where substan- tially reduced bond angles are found.For example, in the tetra- tert-butyltetrahedrane molecule discussed in the preceding Sec- tion, the angles between the exocyclic bonds and the bonds in the tetrahedral cage are 144.7 8, according to X-ray diffraction data.97 A search through the CSDB provided examples of structures with even larger C7C7C angles. The vast majority of these com- pounds are spiropentane (43) or bicyclobutane (26) derivatives. According to X-ray diffraction analysis, the bond angles at the spiro carbon atom in the spiropentane molecule amount to 137.6 8 and 136.7 8.101 The largest C7C7C bond angles were found in bridged derivatives of 43 � molecules 44 ± 46 (the greater of the two increased bond angles is given for each molecule).102998 O S 162.58 158.28 137.68 O 45 43 44 164.08 46 Analysis of DED maps for spiropentane and its derivatives has been carried out.101 In all cases, substantial exocyclic dispace- ment of DED maxima in the planes of the three-membered rings was found.An increase in one bond angle at the tetracoordinated carbon atom to 180 8 would result in a configuration intermediate between the tetrahedral and pyramidal ones (see Fig. 1 d ). The carbon atoms in dicubane 47 may have a similar configuration. The possibility of synthesis of dicubane has been discussed.103 The C7C7C bond angle in this molecule was calculated to be *175 8.175847 According to calculations (MP2/6-31-G*, HF/6-31G*),104 a carbon configuration with a C(2)7C(1)7C(6) angle of*180 8 is also possible in the molecule of compound 48. 61 C O 2 48 This highly reactive tricyclo[3.1.0.01,3]hexane derivative was detected by IR spectroscopy in an inert matrix at 15 K.105 3. Planar configuration of a tetracoordinated carbon atom In recent years, researchers have devoted particular attention to types of distortion such as tetrahedral compression and digonal twist, resulting in a flattened CX4 fragment.106 ± 110 The search for compounds with such an unusual structure was stimulated by a publication by Hoffmann et al.,111 in which the molecular orbitals in a hypothetical planar methane molecule were analysed and suggestions were made about possible ways of stabilisation of the planar configuration of carbon.According to qualitative analysis of the electronic structure of the planar methane molecule, the carbon atom in this molecule should be sp2-hybridised. This atom should have two electrons in a non- hybridised p orbital and should form two two-electron two-centre C7H bonds and one two-electron three-centre H_C_H bond. H HH HThe strain energy of this molecule substantially exceeds the C7Hbond dissociation energy; the energy difference (MP2/6-31- G*//6-31G*) between the tetrahedral (Td) and planar (D4h) methane configurations amounts to 159.7 kcal mol71. The pos- sibility of existence of the planar structure, even as the transition state in the methane inversion, has been called in question.112 Calculations 113 [MCSCF/6-31G(d,p), 6-31+G(d,p), TZV+ +G(d,p)] showed that the planar structure with D4h symmetry does not match any minimum on the PES, whereas the planar structure with Cs symmetry corresponds to a saddle point on the PES.A molecule with this symmetry could represent a transition state formed in the methane inversion: its energy is almost I V Komarov 40 kcal mol71 lower than the energy of the molecule with D4h symmetry (nevertheless, its strain energy is still greater than the energy of C7H bond rupture by 7 kcal mol71). The same conclusion has also been drawn by other researchers,114 who used extended basis sets [6-311+G(3df,2p), 6-311G*] to analyse the problem of methane inversion.Hoffmann et al.111 were the first to consider stabilisation factors of a planar configuration of the formally tetracoordinated carbon atom. One of these factors is electronic stabilisation caused by substituents exhibiting s-donor and/or p-acceptor properties; the former provide the deficient s-electron density, while the latter delocalise the increased p-electron density. Delocalisation of the lone electron density of the carbon atom with a planar config- uration in the (4n+2p)-system should also facilitate stabilisation. The idea of electronic stabilisation has been further extended by Schleyer and coworkers,115 who found by RHF/STO-3G calculations that the energy of 1,1-dilithiocyclopropane molecule with a planar carbon (49a) is lower by 7 kcal mol71 than the energy of the corresponding molecule with the tetrahedral carbon configuration (49b).Li Li 1 1 Li Li 49b 49a Later, Sorger and Schleyer 110 made calculations for a number of molecules that may have a planar configuration of a tetracar- oordinated carbon atom stabilised by s-donor and/or p-acceptor substituents. These include, for example, 1,1- and 1,2-dilithio- ethylenes (50 and 51, respectively) and 3,3-dilithio-1,2-diboracy- clopropane (52). Li Li H H Li HB C C H HB Li H Li Li 51 52 50 According to calculations (B3LYP/6-31G*), for the last- mentioned compound, the energy of the molecule with a planar carbon configuration is 18.7 kcal mol71 lower than that in the case of tetrahedral configuration.1,1-Dilithio-2,2,3,3-tetramethylcyclopropane (53) has been synthesised;116 however, its structure has not been determined. The structural studies for this type of compound are complicated by the high degree of aggregation. Me Me Li Li Me 53 Me Spectroscopic detection of the [CAl4]27 dianion has been reported.117 It was suggested that the planar (according to B3LYP/6-311+G* calculations) [CAl4]27 dianion can be stabi- lised in the solid phase by an appropriate counter-ion, for example, Na+. Organometallic compounds of transition metals the molecules of which contain a tetracoordinated carbon atom with a planar configuration are also known. Complex 54 was the first crystallo- graphically characterised compound of this type.It is of interest that only two years after the publication of the paper by Cotton and Millar 118 reporting the structural data for complex 54, Keese et al.119 noticed the unusual planar environ- ment of the carbon atom (marked by an asterisk) in this com- pound. Quite a few other complexes of this type are also known;107 however, the mechanisms of stabilisation of the planar config- uration of the carbon atom in them are different.108 In most of the molecules, the carbon atom under interest is a part of an unsaturated (often, aromatic) system, which is an additional stabilising factor. Molecules in which a tetracoordinated carbonOrganic molecules with abnormal geometric parameters atom with a sum of bond angles equal to*360 8 is stabilised only by s-donor and p-acceptor substituents (as has initially been suggested for planar methane analogues) are found rather rarely; among them, compounds 55, 56 were the first to be characterised and studied 108 (the carbon atom having a planar configuration is marked by an asterisk).Me MeO Me Me OMe 2 V V Me3Si Co Me * C B C Me3Si B C* OMe MeO Co Men 2 54 n=4 (55), 3 (56) 4 Stabilisation of a planar methane molecule can be attained by oxidation to give a mono- or di-cation. As shown by calculations [ST4CCD/6-311+G(2df,2p)],120 the carbon atom in CH2á has a planar environment (the symmetry group is C2u rather than D4h).This cation has been detected experimentally in mass spectra.121 The attempts to synthesise compounds in which the carbon atom with a planar configuration would be embedded into a rigid carbon cage have not yet been crowned with success. Hoffmann et al.111 suggested that this type of carbon atom could exist in [n.n.n.n]fenestranes (CH2)n (H2C)n C (CH2)n (H2C)n Numerous studies have been devoted to the synthesis of fenestranes; elegant strategies for their synthesis have been devel- oped.106 However, fenestranes with trans-fused five- and four- membered rings (the case in which the greatest distortions of the bond angles at the central carbon atom are expected) have not yet been obtained. Of the fenestranes studied by X-ray diffraction analysis, the greatest distortion has been found in the molecule of [4.4.4.5]fenestrane derivative 57,122 in which the opposing bond angles at the central carbon atom are 128 8 and 129 8. H O H Me O H HN57 Br A highly distorted tetrahedral, although still not perfectly planar, configuration of the central carbon atom is also expected in the hypothetical structure 58, which was called bowlane; initially, a pyramidal configuration of the central carbon atom was ggested for this compound.123 Ab initio calculations (HF/6-31G*) have shown 124 that the bowlane molecule should have C2u symmetry and a distorted tetrahedral configuration of the central carbon atom with max- imum sizes of the opposing angles at this atom of 170.9 8 and 148.1 8.The predicted strain energy of the molecule 58 (RMP2/6-31G*, 166 kcal mol71) is approximately the same as that found experimentally for cubane (157 ± 181 kcal mol71);23 therefore, the synthesis of bowlane appears quite possible.125 However, it is beyond doubt that the reactivity of the distorted CC4 fragment accessible to attack by reagents from outside would hamper the synthesis. Even in the case of [4.4.4.5]fenestranes (in which the distortion of this fragment is less pronounced), the 999 reactivity is still rather high. Many thermal and photochemical reactions of [4.4.4.5]fenestranes proceed with cleavage of the bonds at the central carbon atom.126 The first and not yet synthesised neutral saturated hydro- carbon with a predicted perfectly planar configuration of the carbon atom the structure of which corresponds to a minimum on the PES is dimethanospiro[2.2]octaplane (59).C C 58 59 The MP2/6-31G(d ) calculations predict for the compound 59 a structure with D2h symmetry and a perfect planar configuration of the central carbon atom.127, 128 The highest occupied molecular orbital in 59 is represented by the p orbital of the central atom with a lone electron pair. Unusual physical properties of the octaplane 59 were predicted, in particular, a low ionisation potential (*5 eV) comparable with those of alkali metals. In this com- pound, the planar carbon atom is shielded by the hydrocarbon cage, which may decrease the reactivity. 4. The inverted and pyramidal configurations of the tetracoordinated carbon atom Now we shall consider examples of molecules containing a carbon atom with bonds located on one side of the plane that passes through this atom (see Fig.1 b,c). Inverted and pyramidal con- figurations should be distinguished.123, 129 The inversion means that one bond of the carbon atom points to a direction opposite to its direction in the tetrahedral configuration, resulting in a frag- ment with C3u symmetry (see Fig. 1 b). In the pyramidal CX4 fragment, the carbon atom occupies a vertex of a tetragonal pyramid with C4u symmetry (see Fig. 1 c ). The existence of inverted configurations was first discussed in relation to propellanes, i.e., structures in which two bonded carbon atoms are additionally connected by three bridges.(CH2)n (CH2)n (CH2)n [n, n, n]propellanes The first representative of small-ring propellanes, [3.2.1]pro- pellane (60), was synthesised in 1969.130 At about the same time, discussion concerning the inverted configuration of the carbon atom was initiated. It was suggested that in small-ring propellanes such as 60 ± 63, unusual configuration of the central atoms is possible. The results of X-ray diffraction analysis of 8,8-dichlor- otricyclo[3.2.1.01,5]octane (64) provided the first evidence sup- porting this hypothesis.131 61 60 63 62 Cl Cl 67 66 65 64 [3.2.1]Propellane 60 proved to be thermally stable but still extremely reactive (it reacts with oxygen and enters into diverse radical addition reactions).130 An even more strained propellane (61) was stabilised only in an argon matrix at 29 K.132 It appeared that the simplest [1.1.1]propellane 63 could hardly be expected to1000 be more stable.However, in 1982, Wiberg and Walker 133 pre- dicted theoretically the possibility of synthesis and the highest stability, among the compounds 61 ± 63, for [1.1.1]propellane and later they synthesised it. For the compound 63, not only the possibility of synthesis and stability were predicted, but also the enthalpy of formation and photoelectron and IR spectra were calculated. The [1.1.1]propellane molecule is the first polyatomic molecule for which correct theoretical predictions preceded the synthesis.134 The relative stability of 63 is interpreted in the following way.134 The molecules 61 ± 63 have approximately equal strain, while the strain of the corresponding bicyclic hydrocarbons 65 ± 67 with no bond between the central carbon atoms sharply increases on passing from [2.2.1]bicycloheptane (65) to [1.1.1]bicy- clopentane (67).135 Since the reactions of propellanes include rupture of the bond between the bridging carbon atoms to give species having the skeleton of the corresponding bicyclic hydro- carbons, the energy barrier to the reactions of [1.1.1]propellane is higher than the corresponding values typical of its homologues.[1.1.1]Propellane was first synthesised from 1,3-dibromobicy- clo[1.1.1]pentane, which is difficult to obtain. Br CO2H MeLi HgO Br2 Br HO2C Later, a procedure for the synthesis of the propellane 63 from more easily accessible precursors was developed;136 this allows the preparation of tens of grams of this compound.CH2Cl Br Br Br CBr2 BuLi BuLi Cl Cl Cl Cl A similar approach has been used to prepare substituted [1.1.1]propellane derivatives.137 The latest achievements in the synthesis, study of the chemical properties and application of [1.1.1]propellanes are covered in a review.138 The structure of [1.1.1]propellane was determined using vibra- tional spectroscopy,139 gas-phase electron diffraction 140 and X-ray diffraction analysis.141 The results of investigations by these methods confirmed the presence of the inverted configura- tion of the carbon atoms at the bridgehead in the structure of this compound.The nature of the bond between the central carbon atoms in the compound 63 was the subject of numerous theoretical studies.134 The energy of this bond was estimated (based on MP2/6-31G* calculations) to be approximately 70% of the energy of a single C7C bond (59 kcal mol71).142 Experimental information on the nature of the central bond in propellanes are provided by studies on the electron density distribution from X-ray diffraction data. Studies of this type have been attempted;141, 143 however, the accuracy of experimen- tal data proved to be inadequate to draw ultimate conclusions. The most accurate results of X-ray structure determination (at 81 K) and electron density distribution have been reported 143 for two [1.1.1]propellane derivatives, compounds 68 and 69.68 69 As expected, the DED maxima for non-bridging bonds proved to be concentrated outside the three-membered rings, which is typical of bent (banana) bonds. However, the DED I V Komarov values between the central carbon atoms are negative for both molecules. Nevertheless, this fact alone does not prove the absence of bonds between these atoms and does not contradict the results of the electron density calculation carried out by Wiberg.144 The negative DED values on the lines connecting atoms are encountered quite often; this is due to over-subtraction of the electron density of the `pro-molecule' (combination of spherical non-interacting atoms) from the total density.This is due to the nature of the DED maps � they are differential. Therefore, in investigations of these bonds, a more promising approach includes analysis of the electron distribution topology and con- struction of non-differential maps, in particular, the Laplacian (the second derivative) of the experimental and theoretical elec- tron density.75 The inverted configuration of the bridgehead carbon atoms was also observed in bicyclobutane derivatives (the structure was determined by microwave spectroscopy 145). 11.58 H H H H 3 3 1 1 2 4 2(4) Projection along the straight line passing through C(2) and C(4) In this molecule, the C(1)7H and C(3)7H bonds form an angle of 11.5 8 with the plane passing through the C(1), C(2) and C(4) atoms.X-Ray diffraction data for other bicyclobutane derivatives with inverted configurations of bridgehead carbon atoms have also been published.146 ± 148 The Cambridge Database contains also structural data on bicyclobutane heteroanalogues which also incorporate carbon atoms with inverted configurations. Examples are provided by compounds 70 ± 72 (the carbon atoms with inverted configura- tions are marked by asterisks149 ± 151 Ph Me3Si SiMe3 * * * N Me Me Ph Si Si 70 Ph Me 71 Me Me3Si But But * But P But P 72 But But The presence of carbon with inverted configuration in 1,4- dehydrocubane has been doubted based upon the results of theoretical (ab initio 6-31G*) and experimental data.152 ± 154 It is more likely that this highly unstable molecule exists as singlet biradical 73a and contains no diagonal bond, as shown in 73b.73b 73a Theoretical calculations have been performed for the mole- cules of polycyclic hydrocarbons 74 ± 76 [MP2/6-31G(d,p), 6-311++G(d,p), B3LYP/6-31G(d,p)] 155 and [2.2.2]propella- triene 77 (CASSCF/6-31G*),156 in order to find other structure types where existence of `inverted' carbon atoms is possible.Organic molecules with abnormal geometric parameters 77 76 75 74 The pyramidal configuration of the carbon atom (see Fig. 1 d) is more exotic than the inverted one, and calculation methods still remain the most efficient tool for the investigation of compounds with this type of atom. Noteworthy are studies by Minkin et al.,157, 158 who were the first to demonstrate that among the possible non-tetrahedral configurations of the carbon atom in methane, the pyramidal rather than planar configuration is energetically most favourable.The researchers cited discuss the factors that stabilise the pyramidal configuration; they are similar to those for the planar configuration, namely, the presence of s-donor or p-acceptor substituents as well as incorporation of a pyramidal carbon atom in a rigid polycyclic cage, especially in small rings. Known and structurally characterised systems with a pyrami- dal configuration of carbon (as well as compounds with the planar configuration of carbon) are represented exclusively by organo- metallic compounds of transition metals.Some of these structures are deposited in the CSDB; most of them are carbido clusters of iron, ruthenium, or osmium in which the carbon atom occupies an open cavity formed by metal atoms. The most frequently encoun- tered types of structures are those present in compounds 78, 79 (with the so-called butterfly geometry 159, 160) and compounds 80, 81, in which the carbon atom lies below the base of the tetragonal pyramid formed by the metal atoms.161, 162 CO C Fe(CO)3 (OC)3Fe OC OC M OC M M Fe(CO)3 (OC)3Fe CO CO COCO CO OC CO A M M 78, 79 OCOC CO CO C A=H+ (78), CO (79) 80, 81 M=Fe (80), Ru (81) In recent years, interest in these clusters has substantially increased because they were shown to be the key intermediates in the catalytic reduction of CO with hydrogen to give hydrocar- bons, i.e., the Fischer ± Tropsch synthesis.163 The first complex of this type was prepared as a side product of the reaction of Fe3(CO)12 with alkynes;161 at present, preparative procedures for the synthesis of these compounds in high yields have been developed.164 The carbon atoms in carbido clusters form s- and p-bonds involving transition metal d orbitals; this stabilises the clusters.Similar complexes containing no carbido (interstitial) ligands are less stable. Numerous theoretical studies of hypothetical organic mole- cules having the carbon atoms with pyramidal configuration have been carried out. About 10 years ago, the lack of computer resources and the absence of advanced software did not allow sufficiently accurate and reliable determination of the structural parameters of strained molecules.Many structures calculated at that time have later (according to the results of more advanced methods of calculation) proved unreal, i.e., either not matched by minima on the PES or inaccurate. For example, [1.1.1.1]- and [2.2.2.2]paddlanes (82, 83), which were initially supposed to have pyramidal configurations of carbon atoms, did not correspond to minima on the PES calculated by modern ab initio methods.124 84 83 82 1001 The molecular mechanics calculations for bowlane 58 had also showed the presence of a carbon atom with pyramidal config- uration, but ab initio calculations carried out later refuted this conclusion.The C4u structure of pyramidane (84) with the pyramidal configuration of a carbon atom corresponds to a local minimum on the PES (MINDO/3,165 HF/STO-3G 166 and HF/6-31G* 124). Rasmussen and Radom 167 have reported results of ab initio calculations (B3LYP/6-31G(d)//MP2/6-31G(d)) for hemialka- plane (bowlane 58 is a representative of this class of polycyclic hydrocarbons) and hemispiroalkaplane molecules; according to their data, the central carbon atom has a pyramidal configuration. The general formulae of hemialkaplanes (on the left) and hemi- spiroalkaplanes (on the right) are shown below: C C On the basis of calculations of the molecular strain and the so- called apical strain (which characterises the strain of the separate molecular fragment containing the carbon atom with the pyrami- dal configuration), the researchers concluded that hemispirobioc- taplane 85, hemispirooctaplane 86 and hemispirobinonaplane 87 are the most promising synthetic targets.Analogues of the systems 85 ± 87 containing methyl groups (for example, compound 88) might prove to be easier to prepare because the presence of methyl groups contributes to the kinetic stability of the molecule. The compounds 85 ± 88 were predicted to be highly basic.167 Me Me Me Me 88 86 85 87 The existence of rigid polycyclic organic molecules, for which the presence of a carbon atom with a pyramidal configuration had been predicted, was later proved experimentally.For example, Wiberg et al.168, 169 have detected one of them, namely, highly reactive tricyclo[2.1.0.01,3]pentane 89. 89 VII. Distortions of bond and torsion angles at the double bond in alkenes The research into the distortions of the bond and torsion angles in the molecules of alkenes has a long-standing history. Back in 1890, Baeyer 170 arrived at the conclusion that substantial strain should be expected in the trans-cyclohexene molecule and, therefore, isomerisation of cis-cyclohexene into the trans-isomer is unlikely. Later, when analysing the strain in unsaturated bicyclic molecules, Bredt,171, 172 formulated a rule according to which the cage of camphane, pinane or other related compounds cannot have a double bond to a bridgehead position.At present, vast theoretical and experimental material on this point has been accumulated and reflected in numerous reviews (see, for example, stud- ies 28, 173 ± 176). The types of distortion of a planar alkene fragment are illustrated by Fig. 2. Mathematical analysis of the distortions of the bond and torsion angles in the ethylene molecule has been described in detail in the literature (see, for example, reviews 28, 177). Distortions of only one type are seldom encountered in alkene molecules. However, in most cases, the prevailing type of dis- tortion can be distinguished, which determines the total strain of1002 c d b a Figure 2. Types of distortion of the planar alkene fragment: (a) in-plane distortions; (b ± d) out-of-plane distortions: (b) twist, (c) syn- and (d) anti- pyramidalisation.the alkene.28 Below we consider compounds in which the strain of the molecule is mainly due to the contribution of one type of distortion. 1. In-plane distortions of an sp2-hybridised carbon atom The bond angles at the sp2-hybridised carbon atom can differ appreciably from the ideal value, equal to 120 8. Indeed, the internal C7C=C angle in cyclopropene derivatives can be twice as small. The results of a search through the CSDB demonstrated that this angle is about 60 8 in the molecules of cyclopropene derivatives that also contain an exocyclic double bond (there are 10 structures of this type), for example, 90, 91.178, 179 + Me Me NMe2 61.9(1)8 OMe NMe2 Me2N Cl 92 91 90 In these molecules, a major contribution to the resonance hybrid is made by canonical forms with the cyclopropenylium cation.In other cyclopropene derivatives, the internal C7C=C angle is close to 60 8; for example, in molecule 92 this angle is 61.9 8.180 The external R7C=C angles in cyclopropene derivatives can be greater than 150 8 in the case of bulky R substituents, for example, the angle is 155.6 8 in molecule 93.181 Bond angles exceeding 150 8 have also been found at exocyclic double bonds in compounds with saturated three-membered rings. The devia- tions of these angles from 150 8 are caused by steric repulsion of bulky groups (as in compound 94) 182 or by the fact that double bonds are incorporated in rings (as, for example, in compound 95).183 Me Me CO2H Me Me 158.18 Ph 155.68 But Me Me Me Me 93 S 94 O 155.68 Cl OMe C C 95 A decrease in the R7C=C angle results in pyramidalisation of the carbon atoms of the double bond.This fact was predicted theoretically:184 model calculations (RHF/STO-3G) showed that if the H7C=C angles in the planar ethane molecule are less than 100 8, pyramidalisation becomes energetically favourable. X-Ray diffraction analysis of compound 96 confirmed this fact experimentally.185 In molecule 96, the C7C=C angles in the cyclopropene fragment amount to 64.8 8 and 65.3 8, and the c angle characterising pyramidalisation equals 17.6 8. Ph Ph Me c=17.68 Me Me N O N Me N O Me 96 O O c=7.48 H H O 98 For the norbornene molecule (97), pyramidalisation of the carbon atoms of the double bond was predicted theoretically (for example, according to the STO-3G ab initio calculations,186 the hydrogen atoms at the double bond are displaced in the endo- direction and the c amounts to 4.9 8).X-Ray diffraction data for norbornene have not been obtained. In a norbornene derivative, anhydride of exo,exo-2,3-nor- born-5-enedicarboxylic acid (98), the structure of which has been determined by neutron diffraction, the c angle proved to be 7.4 8.187 In compound 99, as shown by X-ray diffraction analysis, the fusion of two norbornyl cages (and, probably, steric repulsion between the cyclopentane rings) accounts for quite a noticeable pyramidalisation: c=22.7 8.188, 189 An exceptionally high pyramidalisation of the carbon atoms at the double bond can be expected, according to calculations (HF, TCSCF, MP2, B3LYP, B3PW91/6-31G*),190 in compounds 100 ± 103, the synthesis of which has been reported.191 ± 195 101 100 It follows from calculations that each of the tricyclic structures 100 ± 103 is matched by two minima on the PES, which corre- spond to the endo- and exo-isomers.In the case of the compounds 100 and 101, a lower energy is found for endo-isomers (their energy is 2.2 ± 7.5 kcal mol71 lower than that of the corresponding exo- isomers), whereas for the compound 103, the exo-isomer is lower in energy (by 2.2 ± 3.3 kcal mol71). The carbon atoms of the double bonds in the compounds 100 ± 103 should be strongly pyramidalised (c>408).Experimental verification of these the- oretical conclusions would be a complicated task because the strained molecules 100 ± 103 are highly reactive (for example, the existence of the compound 103 can only be proved by analysing the products of reactions involving this molecule).193 Br Cl Bu4N+F7 Cl Cl SiMe3 MeLi Cl Br PhO Ph I V Komarov c=4.98 H H 97 c=22.78 99103 102 Br Ph O PhOrganic molecules with abnormal geometric parameters 2. Twisting of the double bond The loss of p-bonding upon distortion of the torsion angles at the double bond (twist) is partially counterbalanced by rehybridisa- tion of carbon atoms, resulting in their pyramidalisation.196 ± 198 Therefore, when comparing the degree of twisting of double bonds in the molecules of different alkenes, it is not always legitimate to use the dihedral angle between the aC(1)b and cC(2)d planes (Fig.3). Therefore, the so-called twist angle t was introduced, which is found as the arithmetic mean of the aC(1)C(2)d and bC(1)C(2)c dihedral angles. t=aCÖ1ÜCÖ2Üd á bCÖ1ÜCÖ2Üc . 2 This angle is equal to the angle between the aC(1)b and cC(2)d planes in the case where no pyramidalisation takes place. The degree of pyramidalisation of the C(1) and C(2) atoms is evaluated from the departure of the aC(1)C(2)c and bC(1)C(2)d dihedral angles from 180 8 and from the pyramidalisation angles wC(1) and wC(2): wC(1)=bC(1)C(2)c7bC(1)C(2)d+p, wC(2)=aC(1)C(2)d7bC(1)C(2)d+p.The results of theoretical investigations demonstrate that twisting of the molecule of an alkene with all electron-donating or all electron-withdrawing substituents by 90 8 (t=908 or 790 8) should give rise to a singlet biradical,199 ± 201 and the maximum twisting in the molecules of non-symmetrical alkenes (so called push ± pull alkenes) yields bipolar species.202 Twisted systems correspond to the transition state of the cis ± trans- isomerisation of alkenes. For simple alkenes, the activation free energy of the cis ± trans-isomerisation found experimentally lies in the range from 51 to 67 kcal mol71 (see Refs 203 ± 205). For push ± pull alkenes, this barrier is substantially lower.202 This is why the `record-holder' molecules in which the t angle for the C=Cbond is close to 90 8 (or790 8) are represented in the CSDB by push ± pull alkenes. For example, according to X-ray diffrac- tion data, in the molecules 104 and 105, the t angles are 85.1 8 and 84.3 8, respectively.206, 207 Me O O2N HN NO2 O Me HN NPri PriN HN NO2 O2N HN 104 105 When the double bond is conjugated with other p-bonds or aromatic systems, the barrier to the cis ± trans-isomerisation also decreases.Among the structures of compounds with conjugated double bonds, numerous examples can be found in which the jtj values are close to 90 8. Thus in the octaisopropylcyclooctate- traene molecule (106), the t angles for all multiple bonds exceed 64 8.208 IV II I III d b b c c c Stereo- formulae b d c d a a d a d a t b d c c c b c d da b a t b t a Newman projections along the C=C bond a d Figure 3.Twist distortion of the C=C bond and definition of the twist angle t: (I ) unstrained double bond; (II ) twisting of double bond without pyramidalisation of the carbon atoms; (III, IV ) twisting+pyramidalisa- tion; a ± d are substituents. 1003 Pri Pri Pri Pri Pri Pri Pri Pri106 If push ± pull alkenes and compounds with conjugated dou- ble bonds are excluded from consideration, the number of structures with t angles lying in the range of 20 8<jtj<908 included in the CSDB sharply decreases. Considerable twist-type distortions of the C=C bond were found both in acyclic and in cyclic or polycyclic compounds.a. Acyclic twisted alkenes In acyclic alkenes, twist distortions take place if bulky substituents are present at the double bond. The largest twist of the double bond not incorporated in a ring or a polycyclic system and bearing only hydrocarbon substituents is expected in the tetra-tert-butyl- ethylene molecule (107). Numerous attempts at synthesising this compound were undertaken; however, they have not yet met with success. Ab initio (BLYP/DZd) calculations for the molecule 107 209 predict a singlet ground state (D2 symmetry) with t= 45 8, slight pyramidalisation and a strain energy of 93 kcal mol71. The energy of the triplet state of 107 with t &87 8 would be 12 kcal mol71 higher than the energy of its singlet state.But But But But107 The alkenes 2, 108, and 109 have been prepared by methods used in the attempts to synthesise the compound 107. In the molecules of these compounds, as in the molecule of 107, the quaternary carbon atoms are linked directly to the double bonds. However, the steric repulsion in the molecules 2, 108 and 119 is weaker than that in tetra-tert-butylethylene due to the presence of additional bonds between substituents.210 ± 213 Krebs et al. 214, 215 approached very closely the synthesis of the compound 107; they succeeded in synthesising tetrakis(2-formyl- propan-2-yl)ethylene (110) (t=28.6 8, according to X-ray dif- fraction data). However, the formyl groups proved to be so non- reactive that the researchers were unable to reduce them to the methyl groups.CHO OHC CHO OHC 109 108 110 The relatively low reactivity of the double bond in the compounds 2 and 108 ± 110 should also be mentioned. In all probability, this is due to the shielding of the p-system by bulky substituents. The cis ± trans-isomerisation of strained alkenes proceeds much more readily than isomerisation of unstrained compounds (the energy barrier can be almost twice as low in the former case). This was used in a study 205 to estimate the energy barrier to the isomerisation of alkenes. Since isomerisation of strained alkenes takes place at relatively low temperatures, side reactions do not complicate the kinetic measurements. The stand- ard enthalpies of formation (with an applied correction for steric effects found by MM2 calculations) of these alkenes were employed to calculate the activatiom barrier to the cis ± trans- isomerisation (69.50.9 kcal mol71), which does not depend on the nature of substituents at the double bond.1004According to the results of a search through the CSDB, the greatest twist distortion of the double bond among acyclic compounds is found in compound 111.216 SiMe3 ButMe2Si ButMe2Si SiMe3 111 In the tetra(trialkylsilyl)ethylene 111, the t angle equals 49.6 8 (note that this value is greater than that predicted for tetra-tert- butylethylene 107), no pyramidalisation of the carbon atoms of the C=C bond takes place, and the double bond length (1.370A) exceeds the typical value (1.32A). b.trans-Cycloalkenes Among monocyclic compounds, a substantial distortion of the double bond (mainly twist-type distortion) can be attained in trans-cycloalkenes in which the number of carbon atoms in the ring does not exceed nine. trans-Cyclooctene (112) is the unsub- stituted compound with the smallest ring that can be isolated in a pure state at room temperature.217, 218 The structure of trans- cyclooctene was determined by electron diffraction.219 According to these studies, the C7C=C7C dihedral angle in the molecule 112 amounts to 136 8. For a derivative of trans-cyclooctene, compound 113, X-ray diffraction data have been published.220 The C(8)7C(9)7C(10)7C(11) dihedral angle in the molecule 113 is 139.3 8.9 11 Me Me 8 10 Me Bu O O 112 113 O Further contraction of the ring containing a trans-double bond results in a substantial increase in the reactivity and the tendency for isomerisation to the cis-form. trans-Cycloheptene (114) can be synthesised at low temperatures.221 Complexes of 114 with transition metals stable at room temperature have also been obtained.222 Unsubstituted trans-cyclohexene (like trans-cyclo- pentene) has not been experimentally detected yet. Monocyclic compound 115 with the smallest-possible ring containing a trans-double bond and stable at room temperature contains a silicon atom in the seven-membered ring and methyl groups in the a-positions relative to the double bond.223 This compound proved to be sufficiently stable to be studied by X-ray diffraction analysis.The kinetic stability is ensured by the methyl group, while the presence of two C7Si bonds, which are longer than C7C bonds, makes the strain in the molecule 115 lower than that found in trans-cycloheptene. Si Me Me Me Me 114 115 A substantial distortion of the double bond in 115 is reflected in the size of the C7C=C7C dihedral angle, which is 130.97 8. It is of interest that the H7C=C7H dihedral angle is 173 8, and pyramidalisation of the carbon atoms in the molecule 115 is insignificant. The C=C bond length (1.330A) deviates only slightly from the typical value, despite the substantial distortion of the dihedral angles. It should be noted that trans-cycloalkenes are chiral; the racemic 115 was partially resolved by column chromatography using a chiral adsorbent.c. Polycyclic compounds with twisted double bonds: anti-Bredt alkenes Among numerous anti-Bredt alkenes deposited in the CSBD, one can find structures in whose molecules a trans-double bond is I V Komarov incorporated into an eight-membered ring. As examples, one can mention compounds 116 and 117 (the molecule of 116 contains also a twisted amide bond 224). Br EtO N O MeO2C 116 117 Cl The following parameters have been found for the molecule 116:225 the angle t=10.8 8, the average degree of pyramidalisa- tion of the carbon atoms at the double bond w=28.45 8. The size of the t angle attests to an even greater twist distortion of the double bond (t=29.3 8) in the molecule 117, although the degree of pyramidalisation in this molecule is lower, w=17.1 8.226 The alkene 117 is very sensitive to moisture and atmospheric oxygen; the compound 116 is less reactive.Anti-Bredt alkenes with a trans-double bond incorporated in rings containing not more than seven carbon atoms are known. The substantial strain in the molecules of these compounds accounts for their enhanced reactivity. The exceptionally reactive alkene 1 contains a trans-cyclohexene ring.227, 228 This compound was stabilised in a solid argon matrix, and the IR absorption frequency for its double bond was determined. The unusually low frequency (1481 cm71) indicates that the p-bonding has been considerably reduced as a result of angle distortion in the molecule of 1.An even more strained molecule of 9-phenyl-1(9)homocubene (118) contains a trans-`double' bond in the five-membered ring.No spectroscopic or structural characteristics have been obtained so far for this compound because of its high reactivity.229 The compound 118 rearranges into carbene 119 even at low temper- atures. Ph Ph 119 118 This reaction is uncommon for alkenes (the reverse trans- formation takes place much more often); the fact that this reaction does proceed implies that the properties of the double bond change substantially upon geometric distortion. Although the compounds 1 and 118 can be regarded as `record-breaking twisted alkenes', the p orbitals of their `double' bonds are not orthogonal: the calculated t values are much smaller than 90 8 (for example, t=648 in the molecule of 1).227, 228 Due to the pyramidalisation of non-bridging carbon atoms of the C=C bonds, the orbital overlap is partly restored and the t value decreases.Perhaps, an ideally orthogonal arrangement of the p orbitals could be realised in `fixed betweenenes' (or orthogonenes) 120, 121 (MM2 and semiempirical calculations; no ab initio calculations for these structures have been carried out).230 Tenta- tive results on the synthesis of orthogonenes have been pub- lished.231 n n n n 120, 121 n = 0 (120), 1 (121). 3. Out-of-plane distortions of angles at the C=C bond: pyramidalisation It is well-known that alkenes in which the double bond occurs in a non-symmetric environment are often characterised by a pyrami- dal configuration of the double-bond carbon atoms.This pyr- amidalisation is slight; the reasons for it are discussed in theOrganic molecules with abnormal geometric parameters literature (see, for example, the reviews 232, 233). Pyramidalisation accompanies also in-plane and twist distortion of a double bond (see above). Calculations 234 have shown that anti-pyramidalisa- tion of the carbon atoms of the double bond is energetically more favourable than syn-pyramidalisation. However, this conclusion requires verification by means of more advanced methods of calculations than those used in the study cited.234 In this Section, we consider compounds the structural features of which impose pronounced syn-pyramidalisation. As a rule, these compounds contain a rigid polycyclic cage having two mutually perpendicular planes of symmetry one of which accom- modates the double bond.175 Examples of non-symmetric mole- cules have also been reported.235 When discussing pyramidalisation in these molecules, in addition to w, it is convenient to use the angle f a R R f b R R cosf=7 cosb cos(0.5a) The first synthesised compound with a considerable pyramid- alisation of the carbon atoms at the double bond (122) was reported more than 30 years ago;236 at present, dozens of compounds of this type are known, for example, 123,237, 238 124,239 ± 241 125 242 and 126.126 125 124 123 122 In the cubene molecule (126), the angle f, according to the SCF/3-21G calculations, is equal to 84.1 8.243 The possibility of synthesis of the compound 126 was predicted theoretically and then realised in practice.244 Rough thermodynamic data for cubene 245 and the first results of the study of chemical properties of this highly reactive compound 246 have been published. The calculations showed that, despite the pronounced pyramidalisa- tion of the double-bond carbon atoms, the overlap of the p orbitals in the molecule 126 is sufficient for regarding this compound as an alkene rather than a biradical (TCSCF/6-31G* for the singlet state, ROHF/6-31G* for the biradical).153 This is also indicated by an observed reaction typical of alkenes with highly pyramidalised carbon atoms of the double bond,247 namely, cubene dimerisation to give the corresponding propellane 127 with subsequent transformation into tetraenes 128 and 129.Br F7 SiMe3 127 126 + 129 128 The attempts to synthesise derivatives with even more pyr- amidalised carbon atoms of the C=Cbond, prismene, have so far been unsuccessful.248 Both prismene isomers, 130 and 131, have 1005 been studied theoretically [HF/6-31G(d), MP2/6-31G(d)].249 According to calculations, the energy of the molecule 131 is lower than the energy of 130. 131 130 VIII. Distortions of the bond and torsion angles in the molecules of aromatic compounds Cyclic unstrained aromatic systems are normally planar.The benzene structure,250 which serves as a sort of standard in discussion of the nature of aromaticity,251 has been studied in the gas phase (spectroscopic methods and electron diffraction) with high accuracy: the molecule has D6h symmetry, the carbon atoms form a regular hexagon, and the C7C bond length is 1.3990.001A. The types of distortion investigated for benzene derivatives include changes in the bond angles in the aromatic ring plane and out-of-plane distortions; compounds with `conformations' similar to those of cyclohexane, i.e., a chair, a boat, and a twist `conformation,' have been obtained (Fig. 4). Below we consider only benzene derivatives (strained molecules of aromatic hetero- cyclic compounds have received much less attention in the literature).a c d b X Asymmetrical Symmetrical a a a g f e Xb Figure 4. Bond angle distortions in benzene derivatives. (a) in the ring plane; (b ± f ) out-of-plane distortions: (b) chair, (c) symmetrical boat, (d ) asymmetrical boat, (e) twist, (f ) deviations of the C7X bond from the ring plane. The angles characterising the degree of distortion are marked.} 1. Changes in the bond angles in the plane of the benzene ring The C7C7X bond angles at the carbon atoms of the benzene ring can markedly depart from the ideal value, equal to 120 8. These distortions may be caused, in particular, by annelation of small rings to benzene. Mills and Nixon 252 postulated that annelation of cyclopentane to benzene shifts the equilibrium between the two Kekule structures.Later, the hypothesis of the existence of an equilibrium between the Kekule structures in benzene derivatives was admit- ted to be faulty;50 however, localisation of the double bonds induced by annelation of strained rings was actually discov- ered.253 The first experimental proof of this phenomenon was provided by the synthesis of cyclohexatrienes 132 and 133.254 } These designations of angles are used only in this Section.1006 R R 1.502A 1.333A R R 132 R R 133 (R=SiMe3) The compound 133 contains, according to X-ray diffraction analysis, noticeably alternating bonds in the central ring. The unusually easy hydrogenation of the central ring in 132 also points indirectly to a substantial localisation of the double bonds.255 Thermochemical data for this reaction have been published.256 However, in this type of compound, localisation of the double bonds can be interpreted by assuming a smaller contribution of the structure 134, with antiaromatic cyclobutadiene rings, to the resonance hybrid.Calculations (B3LYP/6-31G*) demonstrated that it is the bond localisation caused by annelation rather than other p-effects (aromaticity ± antiaromaticty) that is responsible for the structure of these compounds.257 Ph Ph 888 1.363A 1.429A 1.381A 1.371A 1.418A 134 135 The clear-cut bond alternation has been found by X-ray diffraction analysis 258 in the molecules of naphthalene derivative 135. The bond angles at the carbon atom at the peri-positions of the naphthalene ring differ appreciably from 120 8.Alternation of single and double bonds was to be expected in dehydrocyclobuta- and cyclopropabenzenes. It proved to be very difficult to perform the synthesis and X-ray diffraction analysis of strained compounds such as 136 ± 143. However, unexpectedly, no noticeable bond alternation was found in the molecules of these compounds.259 139 138 137 136 143 142 141 140 On the one hand, these results are at variance with the calculations (MP2/3-21G, HF/6-31G*) for model systems (ben- zene molecules with distorted C7C7H bond angles), according to which the decrease in the C7C7H angle to 90 8 is accompa- nied by pronounced localisation of the double bonds.260 However, on the other hand, clear-cut alternation of bonds was found in the benzene ring annelated to bicyclic fragments.In the most strained molecule of this type of compound, tris(tetrahydrobicyclo[2.1.1]- hexa)benzene (144), noticeable bond alternation was found, first, by ab initio calculations [6-31G(D)(LDF)],261 [MP2/6-31G(D)],262 and then by X-ray diffraction analysis.263 The C7C7C angle in the structure 144 is equal to 102.3 8 (which is much smaller than 120 8). The reason for the observed bond alternation in the structure 144 is currently under discussion. I V Komarov 1.390A 1.438A 1.414A 1 1.398A 1.390A 1.349A 2 1.401A 102.38 109.78 1.387A 145 144 The alternation might be caused by the strain in the bicyclic fragments of 144, which results in elongation of those bonds in the benzene ring at which the bicyclic fragments are annelated.A similar effect was observed for the compound 145.101 2. Out-of-plane distortions of the benzene ring a. Compounds with benzene rings in the boat `conformation' Among the molecules in which the benzene rings are distorted to form a boat `conformation' typical of the cyclohexane ring, cyclophanes should be mentioned first of all.264 In recent years, strained molecules of meta-, para- ([n]- and [n.n]-) cyclophanes have been vigorously studied. The synthesis and properties of these compounds have been considered in detail in reviews.265 ± 268 (CH2)n (CH2)n [n]metacyclophanes [n]paracyclophanes (H2C)n (H2C)n (CH2)n (CH2)n [n,n]metacyclophanes [n,n]paracyclophanes The interest in these compounds, like that in the benzene derivatives 133 ± 143 described above (cyclobuta- and cyclo- propabenzenes can be treated as [n]orthocyclophanes), is related to some extent to the discussion of the nature of aromaticity. In the case of [n]paracyclophanes, the benzene ring is not strained when n=9; however, with a decrease in the bridge length, the ring becomes non-planar; in addition, the exocyclic C7C bonds are also distorted, in particular, they decline from the plane of the two adjacent bonds of the benzene ring (see Fig.4 for the definition of the a and b angles characterising these distortions). Analogous distortions of the benzene rings in [n]metacyclophanes take place when n47.In [n.n]para- and -metacyclophanes, the benzene rings deviate substantially from the planar configuration at n43. [2.2]Metacyclophane was synthesised in 1899,269 and [2.2]paracyclophane was prepared in 1949 270 by closure of the bridge. In the case of homologues with smaller n values, [n]para- cyclophanes with n47, or [n]metacyclophanes with n46, this classical approach is inapplicable. The syntheses of cyclophanes with small n values include isomerisation of appropriately sub- stituted valence isomers of benzene as the key stage. Six isomers of this type can be conceived (if only the CH fragments are combined, while charged species are excluded).267 Different research groups have used four isomers,271 most often, 3,30- bicyclopropenyl derivatives.265 A decrease in the length of the bridge in cyclophane molecules results in a dramatic increase in their reactivity.The unsubstituted cyclophanes with the smallest n (with a saturated bridge) known to date, [4]paracyclophane (146) 272, 273 and [1.1]paracyclophane (147),274 can be stabilised only in inert matrices at low tem- peratures, while [4]metacyclophane (148) 275 and an extremely unstable [1.1]metacyclophane derivative (compound 149) 276 were detected as short-lived intermediates.Organic molecules with abnormal geometric parameters Cl Cl 149 148 147 146 A highly unstable and, apparently, the most strained among the known [n]paracyclophanes, bicyclo[4.2.2]decapentaene, has been synthesised.277 The results of experimental and theoretical studies (MP2, taking account of electron correlation) 278 suggest that the compound has the structure 150a, which has a lower energy than the other isomer (150b).150b 150a To study the chemical and spectral properties and the struc- ture of these compounds, it is necessary for them to be stable under experimental conditions. Sufficient stability was attained for strained cyclophanes having electron-withdrawing substituents or bulky groups shielding the most reactive centres, i.e., bridging carbon atoms. Among the known stabilised cyclophane deriva- tives, the molecules of compounds 151 ± 154 contain the most distorted benzene rings.279 ± 282 R X NTos R R Cl Cl R 152 151 X R=CH2SiMe3 , X=CONMe2 , Tos is p-toluenesulfonyl.NC CN NC CN Cl Cl 154 153 X-Ray diffraction data are available for the compounds 151 and 152. Appreciable distortion of the benzene ring is indicated by the a and b angles: in the molecule 151, a=25.6 8 and 24.3 8, b=26.8 8 and 22.9 8, while in the molecule 152, these parameters are even greater: a=27.4 8, g=12.3 8, bmax=48.7 8. The distance between the opposing bridging carbon atoms in the cyclophane derivative 151 is equal to 2.376(5)A. Despite this heavy distortion of the aromatic rings, no bond alternation was detected in the compounds 151 and 152, and the bond lengths fall in the ranges typical of unstrained benzene derivatives. The presence of p-electron delocalisation is also indicated by NMR studies of strained cyclophanes including compounds 153 and 154.The 1H and 13C NMR signals are exhibited in the chemical shift regions characteristic of aromatic compounds. Thus, despite the substantial distortions, the strained molecules of cyclophanes can still be regarded as aromatic. The system of p-bonds in these molecules is an `observer' rather than a `manager' and adapts itself to changes in the s-core. This was confirmed by ab initio calculations for strained [n]metacyclophanes.283 The benzene ring distortion of up to a=308 is not accompanied by noticeable bond alternation. It may seem that these conclusions contradict the data on the chemical properties of strained cyclo- phanes, most of all, the data on the ability of these compounds to enter readily into Diels ± Alder and nucleophilic substitution 1007 reactions.266 However, one should take into account the fact that the molecules of compounds with non-planar benzene rings possess substantial strain.The reactions of the compounds result in strain relief, which is the driving force of their unusual chemical transformations.265 Distorted benzene rings serve as structural blocks of full- erenes; therefore, studies of strained cyclophanes are related to fullerene chemistry. Particular interest was aroused by the syn- thesis of compounds with fused non-planar aromatic rings, full- erene fragments,284 for example, of 1,7-dioxa[7](2,7)pyrenophane (155).285 In accordance with X-ray diffraction data, the pyrene residue in this compound is distorted to a greater extent than the similar fragment in C70 fullerene; the planes of non-annelated benzene rings make an angle of 109.2 8.O O 155 In conclusion, it should be noted that distortions of the benzene rings to a boat `conformation' can be found not only in cyclophanes. In the molecules of 1,2,3-trisubstituted benzene derivatives, this conformation results in a decrease in the steric strain if the substituents are sufficiently bulky. According to analysis of CSDB structures, among benzene derivatives with bulky substituents in the 1,2,3-positions, the molecule 156 has the most distorted benzene ring.286 The angle values a=30.1 8, g=11.6 8 , and bmax=16.4 8 are comparable with the sizes of these angles in the cyclophane derivative 152.Nevertheless, in compound 156, no double bond localisation is found either. The strain in 156 accounts for the unusual reactivity of this compound, first of all, the tendency for isomerisation into prismane 157 [with the intermediate formation of the corresponding derivative of the Dewar benzene (158)], in which the steric repulsion between the tert-butyl groups is weaker than that in 156. ButBut But CO2Me MeO2C 157 CO2But But MeO2C But hn But But ButO2C CO2Me MeO2C CO2But CO2Me 156 hn But But 158 b. Twist and chair `conformations' of benzene rings Due to steric repulsion of bulky substitiuents, the benzene ring can assume twist and chair `conformations'. According to calcula- tions, the chair `conformation' can be found in the benzene ring of hexa-tert-butylbenzene (159).287 This compound has not been synthesised so far but its less strained silyl- 288 and germyl- containing 287 analogues 160 and 161 are known.But SiMe3 1 But But Me3Si SiMe3 6 2 5 3 But But 4 Me3Si SiMe3 But 159 SiMe3 1601008 GeMe3GeMe3 Me3Ge GeMe3 Me3Ge GeMe3 161 X-Ray diffraction analysis of the compound 160 showed that four carbon atoms of the benzene ring [C(1), C(3), C(4), C(6)] are located approximately in one plane, whereas the C(2) atom lies above this plane by *0.1A and the C(5) atom is *0.1A below this plane. The mean value of the C7C7C7C dihedral angle amounts to 9.8 8 and the C7Si bond deflects from the plane of the benzene-ring bonds adjacent to it by 22.2 8.Despite the consid- erable distortions of the bond angles comparable to the distortions in the molecules 151 ± 154, the bond lengths in the benzene ring are normal for aromatic compounds. Analysis of the spectroscopic properties of the compound 160 shows that the compound can be considered to be aromatic but having an unusually high reactivity. The molecule 160, similarly to the molecule 156, is readily rearranged on exposure to radiation giving rise to a Dewar benzene derivative, hexakis(trimethylsilyl)bicyclo[2.2.0]hexa-2,5- diene (further [2+2]-cycloaddition to a substituted prismane was not observed). The rearrangement of the compound 160 occurring on heating resulted in benzene ring cleavage; in this case, 1,1,3,4,6,6-hexakis(trimethylsilyl)-1,2,4,5-hexatetraene was iso- lated.Interestingly, the benzene derivative 162 containing four tert-butyl groups in the adjacent positions is thermodynamically less stable than the isomer 163, which is a Dewar benzene derivative.289 But But But But But CO2Me CO2Me hn hn D D But CO2Me But CO2Me 162 But But But CO2MeCO2Me But 163 But The twist `conformation' of the benzene ring has been observed in molecules of fused aromatic compounds. Typically, these molecules contain an aromatic polycyclic system twisted to form a helix the axis of which is either perpendicular or parallel to the benzene-ring planes. In the molecule 164, the angle between the terminal bonds of the linear polybenzene system is 105 8.Despite the steric strain, the 164 I V Komarov compound 164 is exceptionally stable: it does not decompose on heating to 400 8C.290 IX. Distortions of cumulated double bonds Cumulenes are hydrocarbons containing two (or more) double bonds with shared carbon atoms. Strain-free cumulene molecules, for example, allene (1,2-propadiene) (165), 1,2,3-butatriene (166) and 1,2,3,4-pentatetraene (167) have a linear carbon skeleton of cumulated double bonds. The four substituents are located either in mutually perpendicular planes (for an even number of cumu- lated double bonds) or in one plane (for an odd number of these bonds). H H H H C C C C C C C 166 165 H H H H H H C C C C C 167 H H Distortions of cumulenes can include bending and twisting of the linear system of bonds.For allene, these distortions can be represented as follows: z=908H H H H C C C C C C j=1808 H H H H twist bending Below we consider only allene and 1,2,3-butatriene derivatives in which cumulated double bonds are incorporated in a ring. In the molecules of these compounds, the cumulene fragment is not only bent; twist type distortion is also always observed. The properties of strained cyclic allenes have been analysed in several reviews (see, for example, reviews 291, 292). Only few publications have been devoted to the study of molecules with predominating twist distortion such as [n.n]betweenallenes.In the known betwee- nallenes, the z angle between the planes formed by substituents at the terminal C(sp2) atoms deviates insignificantly from 90 8.293, 294 CC Xn Xn C [n,n]betweenallenes Even strain-free molecules of cumulenes are highly reactive (reactions involving them readily give stabilised intermediates).295 Molecular distortions result in a further increase in reactivity and even in a qualitative change in the chemical properties of cumu- lenes. 1. Strained cyclic allenes If the allene fragment is incorporated in a carbocycle that consists of not more than ten atoms, it inevitably becomes bent and, in addition, substituents tend to be arranged in one plane, which implies twist distortion. Model calculations showed that bending is always accompanied by twist � optimisation of an artificially bent system of two cumulated double bonds results in a twisted structure.296 The strain energy of the molecules of cyclic allenes gradually increases up to j=208 (by*4 kcal mol71, according to the SCF/4-31+G calulations);297 however, subsequently it rises very steeply.Even slight angle distortions in allene molecules entail a pronounced increase in the compound reactivity, which is anyway rather high, first of all in the tendency for polymerisation. 1,2- Cyclooctadiene the strain energy of which is calculated to be onlyOrganic molecules with abnormal geometric parameters 14 kcal mol71, has not yet been isolated in a pure state.291 Bulky substituents at double bonds ensure the kinetic stability of allenes; the tert-butyl derivative of 1,2-cyclooctadiene 168 represents the smallest-ring carbocyclic allene that has been isolated at room temperature.298 A cyclic allene with a smaller ring, 1,2-cyclo- heptadiene, has been stabilised in metal complexes.For com- pound 169, X-ray diffraction data have been published,299 pointing to a substantial distortion (bending and twist) of the allene fragment. Among metal complexes of this type deposited in the CSDB, the most pronounced bending of the allene fragment is found in the molecule of compound 170, in which the j angle is 135 8.300 CO PF¡¦6 + Ph3P Fe Cp Pt 1358 PPh3 138.18 Ph3P But 170 169 168 1,2-Cyclohexadiene (171) has been known since 1968 when this compound was detected as a short-lived intermediate.301 Later, it was detected by IR spectroscopy in an argon matrix at 11 ¡À 170 K.302 The compound 171 has now been thoroughly studied; according to calculations, the j angle in this molecule is 138 8 (MINDO) or 134.8 8 (HF/6-31G**).291 The attempts at synthesising 1,2-cyclopentadiene (172) have not yet met with success.303 H H 172 171 Cyclohexa-1,2,4-triene (`isobenzene') (173) has also been studied.304 There is evidence that deprotonation of the compound 173 gives rise to the phenyl anion (174).305 H KOBut 7 K+ 174 H 173 Stable six-membered heterocyclic compounds with a 1,2-diene fragment containing two or three heteroatoms, silicon and phos- phorus, have been synthesised.The allene fragment in the cyclic allene 175 is less distorted than that in the six-membered carbo- cycle 171; however, the former is stable, which allowed X-ray diffraction analysis of this compound.306 According to X-ray diffraction data, the allene fragment is bent (j=166.4 8) and the z angle between the Si7C(1)7C(2) and Si7C(3)7C(2) planes is 64.6 8. Among stabilised cyclic compounds, the most distorted 1,2- diene group is found in diphosphaisobenzene 176. A derivative of 176, compound 177, has been investigated by X-ray diffraction analysis.307 But P But ButO But Ph Ph N P Si But P But SiMe2 Me2Si Me 155.88 P SiMe3 Me3Si But Me 166.48 But 176 175 177 Me The values for the j (155.8 8) and z (40.7 8) angles in the molecule 177 significantly depart from the ideal values (180 8 and 90 8, respectively); steric shielding of the central fragment caused by the tert-butyl groups is responsible for the rather low reactivity of compound 177.1009 The accumulated knowledge makes it possible to form a picture of molecular and electronic structures of highly strained allenes. Several variants of the spatial and electronic structures of these compounds can be suggested. For example, for 1,2-cyclo- hexadiene, apart from the chiral allene 171, structures with the planar HC(1)C(2)H fragment and a non-bonding sp2-hybridised orbital located in the plane of this fragment are also possible, i.e., zwitter-ions 178, 179, and singlet (180) and triplet (181) biradical.7 + H H H H H H 7 + 179 178 S- 180, T- 181 The MCSCF/3-21G 308 and HF/6-31G* 309 calculations and experimental data 302, 309, 310 favour a non-planar chiral structure, at least, in 1,2-cycloheptadiene and 1,2-cyclohexadiene. The calculated t angle at each double bond amounts to *20 8; this is sufficient for effective overlap of p-orbitals.309 In the case of the 1,2-cyclopentadiene 172, ab initio (STO, MP2/3-21G) calculations predict a relatively low energy barrier to racemisation (which might involve the intermediate formation of singlet biradical 182) D 2 ¡À 5 kcal mol71 (see Ref. 311). This low value is within the error of the calculation techniques; therefore, experimental data are required to verify the results of theoretical methods.H H H H H H 182 (R)-172 (S)-172 As regards the chemical properties of cyclic allenes, one should mention their enhanced tendency to enter into cyclo- addition reactions, which were used in experiments on trapping short-lived molecules such as 1,2-cyclohexadiene. Strained allenes undergo the Diels ¡À Alder reactions even with dienes having very low reactivity. These reactions are regioselective and endo-stereo- selective; however, in the case of chiral allenes, no diastereoselec- tivity was observed. This fact was interpreted using the results of ab initio calculations (B3LYP/6-31G*),312 which showed that in this case, the mechanism involving biradicals is preferred over the synchronous mechanism. 2. Cyclic butatrienes The unstrained 1,2,3-butatriene fragment can be incorporated into a carbocycle if the number of carbon atoms in it is more than ten.Ring contraction causes bending of the linear C=C=C=C fragment, resulting in rehybridisation of carbon atoms and a substantial increase in the reactivity. Tentative experimental results showed 313 that, among unsubstituted cyclic 1,2,3-buta- trienes, the minimum size of the ring which allows isolation of this compound at room temperature, is found in 1,2,3-cyclooctatriene (183). Homologues with smaller rings, 1,2,3-cycloheptatriene 314 (184) and a benzene isomer, 1,2,3-cyclohexatriene 315 (185), are highly reactive short-lived intermediates; only the products of cycloaddition of these species to dienes have been isolated.185 184 183 1,2,3-Cyclopentatriene (186) has not been prepared to date; however, its heterocyclic analogues, 3,4-didehydrothiophene (187) 316 and 1-tert-butoxycarbonyl-3,4-didehydro-1H-pyrrole (188),317 have been synthesised, and their chemical properties have been studied using trapping reactions.1010 N But S 187 186 O O188 The compound 187 was prepared and identified without substantial difficulties, although it is highly reactive. Meanwhile, the high reactivity of the heterocyclic triene 188 became a serious obstacle in the attempts to prove its existence. Among the strained butatrienes known currently, the geometry of the C=C=C=C system of bonds deviates most appreciably from linearity in the molecule 188.317 Cyclic 1,2,3-butatrienes, like highly strained allenes, readily react with dienes.The high reactivity of the compounds 187 and 188 can be illustrated by their reaction with benzene, which affords cycloadducts 189 and 190. X X 189, 190 X = S (189), NCO2But (190). It was suggested that rehybridisation of the carbon atoms of the bent C=C=C=C system of bonds (like pyramidalisation of double bonds, see above) changes the electronic properti of the compounds 187 and 188 so dramatically that they can be regarded as biradicals. To support this, the researchers cited 316 reported the reaction of the heterocyclic triene 187 with tetrahydrofuran. O(CH2)4OH O S S However, an ionic mechanism of this reaction is also possible. It has been shown above in relation to cubene that, even in the case of a very strong pyramidalisation of the carbon atoms of the double bond, alkenes do not exhibit properties of biradicals.Therefore, the conclusion that the compounds 187 and 188 are biradicals appears unlikely and requires further analysis. Among isolated and structurally characterised cyclic buta- trienes, we shall consider metallacyclocumulenes (191) with a five- membered ring consisting of a 1,2,3-butatriene fragment and a metal (titanium or zirconium).318 R Cp2MR 191 M=Ti, R=But, Ph; M=Zr, R=But. The stability of compounds of the type 191 is attributable to the additional stabilisation upon the coordination of the central double bond to the metal. This coordination changes the elec- tronic structure of cumulenes; nevertheless, for some of the complexes 191, dimerisaition to give radialene (192) has been observed.319 Ph Ph Ph Cp2Ti Cp2Ti TiCp2 192 Ph Ph Ph This reaction is also characteristic of purely organic strained cumulenes.320 Metallacyclocumulenes have been investigated by I V Komarov X-ray diffraction analysis.The data obtained indicate that the central double bond is elongated and that the cumulene fragment deviates substantially from the linear geometry: the j angle varies from 147 8 to 150 8. X. Bending of the X17C:C7X2 fragment The unstrained X17C:C7X2 fragment is linear. The deviations of the bond angles at the carbon atoms from 180 8 are similar to the analogous deviations found in allene and butatriene mole- cules.The geometric distortions of the X17C:C7X2 fragment can be characterised by variation of the a1 , a2 and y angles. a1 C X1 a2 C X2 y Below, we shall discuss the cis- (y=08) and trans- (y=180 8) bendings. Note that it is the cis-bending that is found in each organic molecule with a strained triple bond. The calculations showed, however, that trans-bending requires less energy than cis- bending (in a similar way as anti-pyramidalisation of the double bond is energetically more favourable than syn-pyramidalisa- tion).234 Probably, the lower energy of the transition state which contains a trans-bent fragment with a triple bond can account for the preferential trans-attack of alkyne molecules by nucleophilic reagents.This hypothesis has not been verified experimentally because no organic molecules with a trans-bent X17C:C7X2 fragment have been synthesised to date. 1. cis-Bending The only known way of realisation of cis-bending is incorporation of the X17C:C7X2 fragment into a ring or a polycyclic cage.292, 321 This fragment is bent in carbocyclic alkynes with the number of carbon atoms not exceeding ten. The first strained cyclic alkyne, cyclooctyne (193), was synthesised in 1938;322 later, it was isolated in a pure state.323 Cyclooctyne is smallest-ring carbocyclic alkyne stable at room temperature. Cycloheptyne (194) polymerises in less than 1 min even at 725 8C in dilute solutions (at 776 8C, it is stable over several hours).324 Homologues with smaller ring sizes, cyclohex- yne (195) 325, 326 and cyclopentyne (196),327 ± 333 are highly reac- tive.They have been detected as intermediates and studied in inert matrices. 196 195 194 193 The highly strained molecules 194 ± 196 can be prepared using elimination and cycloelimination reactions and by carbene rear- rangements.321 Elimination from appropriate precursors, hyper- valent iodine compounds, has proved effective for this purpose.334 Using this reaction, it was possible to prepare, perhaps, the most strained known (according to MP2/6-31G* calculations) carbo- cyclic molecule with a bent C7C:C7C fragment, namely, bicyclo[2.2.1]hept-2-en-5-yne (197) molecule.335 SiMe3 Bu4N+F7 7OSO2CF3 + 197 IPh Cyclobutyne (198) and cyclopropyne (199) have not yet been synthesised. The results of calculations (MCSCF(4,4)/6-31G*, MP4/6-31G*//MP2/6-31G*) indicate that the compound 198 isOrganic molecules with abnormal geometric parameters highly susceptible to a rearrangement to cyclopropylidenemethy- lene;336 apparently, this makes this molecule `elusive'.The exis- tence of perfluorocyclobutyne (200) as an intermediate, possibly, more stable than 198, has now been proven experimentally.337 Metal complexes with cyclobutyne acting as a two-electron ligand have been prepared; however, cyclobutyne itself was not formed as an intermediate during the synthesis of these complexes.338, 339 F F F F 199 198 200 The structure of cyclopropyne (unlike cyclobutyne) does not correspond to a minimum on the PES, even when the PES is calculated using advanced quantum-chemical techniques [such as SCF, TCSCF, CISD, TC-CISD/DZP; CCSD(T)/TZ(2df,2dp)];340 Cyclopropyne is the only C3H2 isomer that has not yet been prepared.The other three isomers � cyclopropenylidene (201), propargylene (202) and propadienylidene (203), have been syn- thesised and studied in inert matrices.341 It should be noted that investigation of the potential energy surface of C3H2 arouse considerable interest, among other reasons, due to the fact that the molecules 201 342, 343 and 203 344 were found in the interstellar space. C H H H C C C C C C C C H H 203 202 H 201 Maier et al.345, 346 synthesised a heteroanalogue of cyclo- propyne, silacyclopropyne (204).H H Si hn (254 nm) Si C C hn (>395 nm) C H H C204 The identification of the compound 204 was at variance with the results of SCF/DZ calculations published previously, which indicated that the silacyclopropyne structure has properties of a saddle point rather than a minimum on the PES.347 Maier et al., who used higher-level calculations (MP2/6-31G**) demonstrated that the structure of 204 still corresponds to a local minimum on the PES; however, the IR frequencies they obtained differed somewhat from those observed experimentally. Only recent calculations 340 have given vibration frequencies on the basis of which the compound 204 was identified. The necessity of using advanced quantum-chemical methods for the prediction of prop- erties of strained molecules such as 204 was demonstrated once again.The difference between the stabilities of cyclopropyne and silacyclopropyne can be explained by the difference between the energies of the carbenes and silapropadienylidenes ..C=C=XH2 (X=C, Si), which are formed presumably upon decomposition of the compound 199 and 204. The energy of this species with X=Si is higher than the energy of the initial molecule 204, whereas in the case where X=C, calculations predict the opposite situation.346 Cycloalkynes with a bent X17C:C7X2 fragment are sus- ceptible to addition reactions with electrophiles, nucleophiles, and radicals.321 Cycloaddition and oligomerisation also proceed with these molecules much more easily than with strain-free alkynes.Indeed, whereas cyclotrimerisation of strain-free alkynes requires catalysts (transition metal compounds), a similar reaction in the case of cyclic strained alkynes proceeds easily without catalysts. Moreover, the detection of benzene derivatives in the products of elimination reactions of cyclic alkenes can serve as evidence for the formation of highly strained alkynes as intermediates.348 Trimer- isation gave the above-discussed benzene derivative with anne- lated bicyclic fragments (144). Cl ButOK, ButLi Ni(Cp)2 Cyclic alkynes stable due to either bulky substituents at the a-position relative to the C:C bond (for example, as in molecule 205 292) or annelation of benzene rings (for example, as in molecule 206 349) have been studied by X-ray diffraction analysis.The search through the CSDB showed that in the molecules characterised by the most bent X17C:C7X2 fragment, the a1 and a2 angles approach*30 8. Me Si Pri2 146.88 Me Pri2Si Pri2Si 150.58 Si Pri2 146.78 205 The structure of compound 207 has been studied by electron diffraction in the gas phase.350 ± 352 The a1 and a2 angles in the molecule 207 are 34.2 8 and 30.7 8, which isomewhat greater than those in 205 or 206. Despite the pronounced bond angle distortions, the C:C bond lengths in the compounds 205 and 207 remain within the limits typical of acyclic linear alkynes. Only in some cases (for example, in the molecule 206), was the triple bond elongated (1.20 ± 1.23A).Finally, we should mention a new, original method of stabi- lisation of strained molecules and reactive intermediates, which allows one to gain spectroscopic data for these species in solutions at room temperature. The idea of this approach is to generate unstable molecules inside `molecular vessels' � carcerands and hemicarcerands. Cyclobutadiene 353 and 1,2-dehydrobenzene (208) 354, 355 have been fixed in this way. Warmuth 354 has tackled the question whether the molecule 208 should be assigned to strained alkynes or to bent cumulenes. 1,2-Dehydrobenzene was prepared by exposure to UV light of benzocyclobutenedione inside hemicarcerand (209). For the compound 208 stabilised in 208b 208a Ph Ph H HO O O O O (CH2)4 O (H2C)4O O O O O O H H Ph Ph 209 1011 144 MeMe S 146.18 207 206 Ph Ph H HO O O O O O (CH2)4 (CH2)4 O O O O O OH H Ph Ph1012 this way, it was possible to record the 1H and 13C NMR spectra before the compound has reacted with the host molecule. Analysis of the NMR spectra led to quite an unexpected conclusion: the spectroscopic data are best described by a hybrid of two contributing structures, 208a and 208b, with a predominant contribution of 208b.This interpretation, however, is inconsistent with the results of other studies (see, for example, Ref. 356). 2. trans-Bending Synthesis of a molecule with a trans-bent X17C:C7X2 frag- ment is a difficult task: the shortage of substituents at a triple bond precludes creation of such a bending.However, trans-bending of an alkyne ligand can be attained in metal complexes. The first complex of this type 210 was prepared in 1995.357 But C Cp2Ti C H Si Me Me 210 A whole series of titanium and zirconium complexes of this type are currently known.358 In the compound 210, the agostic Si_H_Ti interaction ensures the trans-arrangement of substitu- ents at the C:C bond coordinated to titanium. The angles a1=30.5 8 (for Si7C:C) and a2=44.8 8 (forC7C:C) indicate a substantial distortion of bond angles at a triple bond. However, it should be noted that metal coordination also changes substan- tially the properties of a triple bond, for example, in the complex 210, the bond length is increased to 1.275(9)A.The author is grateful to professors M Yu Kornilov and R R Kostikov, colleagues S P Verevkin, V V Burlakov and V I Tararov for critical remarks, corrections and assistance in preparing the manuscript, and to a colleague, mathematician O B Pikhurko, for consultation and help in the solution of mathematical problems. 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ISSN:0036-021X    

出版商:RSC    

年代:2001

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Russian Chemical Reviews 70 (12) 1017 ± 1036 (2001) Fluorescent and photochromic chemosensors V A Bren Contents I. Introduction II. Fluorescent cation sensors III. Fluorescent chemosensors for anions IV. Luminescent sensors for molecules V. Photochromic chemosensors VI. Conclusion Abstract. recent over published chemosensors organic on Data Data on organic chemosensors published over recent years The systematically. described and surveyed are years are surveyed and described systematically. The bibliography bibliography includes references 165 includes 165 references. I. Introduction The creation and development of highly sensitive and reliable methods for the quantitation of various elements and their compounds in the environment (atmosphere, hydrosphere and lithosphere) is of current interest in chemistry.Success in solving this task requires that scientific studies be carried out to find the most selective reactions of atoms, ions or molecules which make it possible to determine qualitative and quantitative parameters of an object under study. The search for a selective reaction implies that one needs to find the transformation of the object being studied from a series of other transformations, which converts the substrate into the most convenient form with specific physical characteristics. Of the many methods used to analyse substrates based on their physical properties (atomic absorption spectroscopy, ion-selective pM-metry (where pM is the negative logarithm of the metal ion concentration), electron microprobe analysis, neutron activation analysis, etc.), a special place belongs to electron spectroscopic methods,1 fluorescence analysis being the most sensitive of these methods.2 An organic analytical reagent employed in fluorescence anal- ysis should have a receptor which reacts selectively with the substrate, a signal-generating fragment and a conductor (bridge) between these parts of the molecule.3 Such a system is called a chemosensor.If a fluorophore is the signal-generating part of a chemosensor, then this is a fluorescent chemosensor. Interaction of the receptor with the substrate followed by transfer of the energy of this interaction through the bridge to the fluorophore changes the spectral signal of the fluorophore, which allows one to make conclusions about the nature of the substrate. It became obvious while developing chemosensors that the set of fluoro- phores is rather limited, whereas the types of receptor are rather V A Bren Institute of Physical and Organic Chemistry, Rostov State University, Prosp.Stachki 194/2, 344090 Rostov-on-Don, Russian Federation. Fax (7-863) 243 46 67. Tel. (7-863) 243 47 77. E-mail: bren@ipoc.rsu.ru Received 12 March 2001 Uspekhi Khimii 70 (12) 1152 ± 1174 (2001); translated by S S Veselyi #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n12ABEH000676 1017 1018 1026 1027 1029 1034 diverse. The most common chemosensors are represented by ionofluorophores; of these, reagents for various cations are used most widely.Figure 1 shows a schematic diagram of the operating principle of chemosensor for cations.4 The receptor of such a chemosensor is an electron donor (D), whereas the fluorophore is an electron acceptor.5, 6 Excitation of the fluorophore results in electron transfer from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). This enables electron transfer from the HOMO of the donor (receptor) to the HOMO of the acceptor (fluorophore), thus quenching the fluorescence of the latter. This phenomenon is known as the PET (Photoinduced Electron Transfer).7 Once a cation (shown as a ball in Fig. 1) has been attached to the receptor, theHOMOenergy level of the latter becomes lower than that of the fluorophore, the PET is no longer possible, and the sensor starts to fluoresce.The majority of fluorescent sensors fit this scheme. However, a different PET mechanism is possible in the case of transition metal cations. In this case, the electron transfer is directed from the fluorophore to the bound metal ion.8 If the fluorophore of the sensor incorporates an electron- donor fragment in addition to the electron-withdrawing one, then the so-called PCT mechanism (Photoinduced Charge Transfer) is also observed due to the interaction between the similarly or oppositely charged poles of the fluorophore and the receptor. This effect is observed as a bathochromic or hypsochromic shift of E E LUMO LUMO HOMO HOMO HOMO HOMO Receptor Fluorophore Receptor Fluorophore hn 0 e7 hn hn D D Strong fluorescence Figure 1.Schematic diagram of cation recognition fluorescent PET sensor.41018 absorption and emission bands in the spectra, and also as changes in the extinction parameters.4 Fluorescence of sensors may get enhanced or quenched if exciplexes { or excimers { appear due to the formation of com- plexes between the receptor and the substrate.4 Fluorescent sensors have certain advantages over other opti- cal chemosensors. The fluorescence method is very sensitive (in certain cases, a few molecules of an object can be detected). These experiments are inexpensive and easy to perform; the method has high resolving power in the wavelength and time measurement scales. Furthermore, luminescence spectra are quite sensitive to both structural changes in the sensor and substrate molecules and the external conditions.6, 9 ± 12 Thus, the fluorescence method makes it possible to both solve purely analytical problems and control chemosensor processes.When Fabbrizzi and Poggi 10 introduced the concept of `switchable sensors', they distinguished such systems that can be triggered or blocked by changing the pH or the concentration of metal ions in the environment, by applying an external redox potential, etc. The original photo-switching method is based on the photo- chromism phenomenon. If the sensor is associated with one state of a bistable photochromic device, the sensor function can be reversibly switched on or off by means of a photoreaction.In this review, recent achievements in the development of organic chemosensor systems are classified and generalised. Fluorescent sensors are considered in the most detail. II. Fluorescent cation sensors According to a classification by de Silva,11, 13 fluorescent cation sensors can be subdivided into four types. 1. Organic molecules that emit light due to the presence of a metal ion. 2. Organometallic complexes containing a metal which emits light. 3. Organometallic complexes which emit light due to inter- action with a different metal ion. 4. A complex of two organic compounds which emits light in the presence of an organic `guest'. Although this classification of sensors is quite versatile, the need arose recently to expand it and make it more specific, because many new publications have appeared in the field of chemo- sensors.Detailed reviews have been published in which chemo- sensors were classified according to the type of interaction between the coordinating, luminescent and conducting parts of the system,14, 15 as well as reviews dealing with the fundamentals of designing ionofluorophores for general analysis of cations,4 transition metal ions,16 anionic substrates,17, 18 etc. In this review, we have tried to emphasise the role of the receptor as the most functional part of a chemosensor molecule and to show the sensor capacities of various reactive series of compounds containing the same molecular receptors.1. Azacrown ethers Chemosensors in which crown ethers serve as receptors belong to the most popular type of cation sensors of various metals. In order to enhance the efficiency of the crown ether receptor, the macro- cycle is usually modified by incorporating nitrogen atoms, which are potent complexation centres. Compound 1a is the simplest azacrown-containing PET sensor.19 { An exciplex is an excited molecular complex. It is formed upon association of two molecules (which may be similar or different), one of which is in the ground state while the other is in an excited state. { An excimer (a specific case of an exciplex) is an excited dimer formed from two similar molecules, one of which is in an excited state while the other one is in the ground state.O O R Mn+ O N O O 1a ± c (a), R= O O N N O O 2 For example, if compound 1a coordinates the K+ ion, its fluorescence quantum yield in methanolic solutions increases from 0.003 to 0.14. Incorporation of several (more than three) methylene units between the nitrogen atom of the azacrown ether and the aromatic substituent, i.e., between the receptor and the fluorophore, results in an abrupt decrease in the PET.20 Aza(diaza)-18-crown-6 ethers bound to fluorophores (poly- nuclear aromatic hydrocarbons) through an N-methylene bridge (1b,c and 2) were used in order to discriminate between mono- and divalent metal ions.15, 21 The nature of the solvent and the type of anion in the salt being analysed may affect considerably the character of coordination, and hence the strength of the PET.22, 23 The interaction of the sensor 1c with thiocyanates of mono- and divalent metals can serve as an example; the spectral pattern of this interaction is shown in Fig.2.15 Divalent and ammonium cations display the standard PET with luminescence enhance- ment. Conversely, coordination of monovalent cations results in luminescence quenching. This anomaly can be rationalised by the existence of specific coordination of monovalent cations with the I (rel.u.) 1234567 8 10 9 300 Figure 2. Fluorescence spectra of compound 1c and its complexes with various metal ions in methanol;15 (1) Ba2+ complex; (2) Ca2+ complex; (3) Zn2+ complex; (4) NH4 + complex; (5) Mg2+ complex; (6) compound 1c; (7) Li+ complex; (8) Na+ complex; (9) Cs+ or Rb+ complex; (10) K+ complex. V A Bren O O R O Mn+ N O O (c).(b), 400 l /nmFluorescent and photochromic chemosensors azacrown ether and the thiocyanate anion, providing additional transfer of electron density from the anion to the naphthalene chromophore.15 Using electronic spectra, 1H and 13C NMRspectra and X-ray diffraction analysis, it was shown that the PET affects the physical characteristics of complexation for a number of sensor series. For example, enhancement of fluorescence in the presence of various `guest' cations occurs in derivatives of diazacrown ethers contain- ing two, three, or four oxygen atoms in the ring and two fluorescent substituents.There are two reasons that can explain this phenomenon. 1. Quenching of fluorescence in the chemosensor molecules due to intramolecular electron transfer between the fluorescent `pendants' and two nitrogen atoms of the ring. As a result, coordination of a cation by the crown ether enhances light emission. 2. Coincidence between the steric parameters of the receptor and the substrate and a suitable rate of cation exchange between the free ligand and the complex in the ground state. It was found 15 that Zn2+ ions are coordinated to both nitrogen atoms of the ring, which results in enhancement of luminescence of diaza-12-crown-4 ether 3 by a factor of 182. O N N O 3 It was shown 15 that diaza-18-crown-6, which has two 1-pyr- enyl substituents, can be used as a fluorescent sensor for alkali metal ions.Ionofluorophores 4a,b are suitable for the quantitative deter- mination of Ca2+ ions in solutions based on changes in lumines- cence spectra upon complexation,24 but they behave differently. For example, the Ca2+ . 4a complex displays an intense fluores- cence band, unlike the Ca2+ . 4b complex which is characterised by weak long-wave emission. O O N R Me N O O 4a,b R = H (a), NMe2 (b). The N-aryl receptor of the azacrown sensor 5 with decreased basicity of the nitrogen atom of the crown macrocycle is charac- terised by strong luminescence enhancement (by a factor of 28) upon interaction with the Ca2+ ion in acetonitrile,25 the fluores- cence life time being increased from 0.28 to 3.6 ns.S O O N N N O N O 5 Ph The coordination sphere of crown ethers can be expanded by introducing substituents containing additional donors. For exam- ple, the sensor 6 is a good fluorescent reagent for `softer' sodium ions 13 due to involvement of the electron pairs of the o-methoxy groups in coordination. 1019 OMe C6H4CN-4 O N N N N O O OMe C6H4CN-4 6 Additional coordination sites are also present in compound 7a, which is a highly selective luminescent sensor for magnesium ions effective in solutions with pH>6.26 R N O O OH N N HO O O N 7a,b R R=Cl (a), NO2 (b). An analogous ion-selective chemosensor ligand 7b has high sensitivity and affinity for Hg2+ ions over a wide range of pH.27 Yet another way to enhance the efficiency of sensors is to incorporate zwitter-ionic fragments. For example, excess electron density on the oxygen atoms of the central zwitter-ionic fragment in the luminescent sensor 8 makes it highly efficient with respect to lithium ions in acetonitrile ± dichloromethane system.28 Me Me O N O O O N O 2+ 7O O7 N O O O O O N Me Me 8 Similarly, the efficiency of chemosensors 9a,b for divalent metal ions (Ca2+, Mg2+, Ni2+, Zn2+, Cu2+) can be explained by the presence of a quinolizine and coumarin nuclei in the fluo- rophore.29, 30 It was shown 30 that charge transfer from the nitro- gen atom of the quinolizine part of the molecule to the carbonyl group of the coumarin part occurs in compounds 9a,b.A strong PCT effect is realised in these molecules, and the electronic spectra manifest bathochromic shifts.30 O R N O O O O N Mn+ O O 9a,b R = H (a), CF3 (b). Yet another approach to specific chemosensors is the intro- duction of extra functional groups into the known sensors result- ing in alteration of their properties. For example, introduction of carboxyl-containing thiophene derivatives as fluorophores into dipyridino-crown ether makes the resulting caesium tetracarbox- ylate 10 water-soluble, which is very important for practical use. The fluorescent sensor itself has high selectivity for potassium1020 ions, because the bulky caesium cations do not compete with potassium ions in solution for the coordination by the cavity of this dipyridinocrown ether.31 Cs+ 7O2C S N Cs+ 7O2CModification of compound 1a by addition of one more azacrown fragment results in the sensor 11 which can recognise dications.For example, ammonium groups located at a- and o-carbon atoms of the substrate alkane chains enhance consid- erably the fluorescence of the sensor 11; they are probably incorporated into both fragments of the crown ether.32 O O O N O O Azacrown ethers are characterised by rather a high basicity of the nitrogen atoms. On the one hand, the high basicity of the nitrogen atom increases the chemosensor efficiency and provides a stable PET; on the other hand, undesirable protonation occurs in acidic solutions.Crown-ether ligands are devoid of this drawback. 2. Crown ethers Benzocrown ether 12a was one of the first to be suggested as PET sodium ion sensor. When this compound is coordinated by the sodium cation in suitable solvents, fluorescence is enhanced.33, 34 O O O O O R 12a,b O (b). R=CN (a), CH2N Compound 12b is a so-called `switchable' fluorescent sensor which operates in acidic media, where the electron-donating morpholine substituent is protonated in the first stage.35 Other fluorescent indicators for the Na+ and K+ ions based on benzocrown ethers have been synthesised and studied (see Ref. 36). 1,8-Anthraceno-18-crown-5 is a luminescent sensor for ions of alkali and alkaline-earth metals.37 Fluorometric titration was used to determine the stability constants of its complexes and their dependences on the cation parameters.Bis(anthracene-9,10-diyl)coronand 13 is a sensor for Na+ and K+ ions. It was shown 38, 39 that gradual addition of sodium perchlorate to solutions of compound 13 in methanol or acetoni- trile gives a stable excimer complex 14 with the composition 1 : 2, which is the reason for enhancement of the corresponding luminescence. K+ ions form a 1 : 1 complex. Cs+ CO¡ O O 2 S N Cs+ CO¡ O O 2 10 O O O N O O 11 O O O O OO O O O O 13 O O O Na+ O O O O 14 Combination of the coumarin fluorophore with benzocrown receptors results in efficient chemosensors.For example, strong substituent polarisation in sensor 15 in an excited state results in additional coordination of alkali and alkaline-earth metal ions with the carbonyl group of the coumarin fragment. CF3 O O O O N O 15 This effect is observed in luminescence spectra as a bath- ochromic shift of the bands (Fig. 3);4 the strongest quenching is observed in the presence of the Ca2+ ion. The selectivity of Ca2+ I (rel.u.) 1 1.0 2 0.8 34 0.6 5 0.4 0.20 500 600 550 Figure 3. Fluorescence spectra of compound 15 and its complexes with metal ions in acetonitrile;4 (1) the ligand; (2) K+ complex; (3) Na+ complex; (4) Li+ complex; (5) Ca2+ complex. V A Bren O O Na+OOO O 650 l /nmFluorescent and photochromic chemosensors sensor 15 is very high: the ratio of stability constants of the complexes is k(Ca2+) : k(Mg2+)=12 500; k(Na+) : k(K+)=16. A few other sensors containing the coumarin fragment are known.For example, 4-substituted benzo-a-pyronebenzo-12- crown-4, -15-crown-5, and -18-crown-6 in acetonitrile are lumi- nescent sensors for Li+, Na+, K+, and Mg2+ ions (see Refs 40 and 41). The effect of the macrocycle size on the extent of fluorescence quenching during the formation of 1 : 1 complexes was studied.42 The crown ethers 16 and 17 containing the dihydroxyxan- thone fragment have different properties: compound 16 has high affinity for barium ions, whereas the cavity in compound 17 binds selectively potassium ions. In the former case, luminescence is enhanced as the ion concentration increases; in the latter case, it is quenched.43 O O 4 4 O O O O O O O16 O 17 Unusual fluorescent sensors, namely, optically active sila- crown ethers 18a,b were synthesised.44 Compound 18a is a selective reagent for lithium ions and viologen bromide, while compound 18b reacts with the latter to give rotaxane-like struc- tures.O O O SiPh2 O O O 18a O O SiPh2 O O 18b The enantiomerically pure chiral ligand, viz., acridinodi- methyl-18-crown-6 (19), displays high enantioselectivity and gives luminescent complexes with chiral ammonium ions in acetonitrile.45 The authors of this study give well-reasoned evi- dence that it is possible to design a series of efficient enantiose- lective chemosensors for chiral substrates.N Me Me O OO O O 19 Xia et al.46 described the synthesis of the luminescent sensor 20 containing 1,4-di(phenylethynyl)tetramethylbenzene as the chro- mophore and 18-crown-6 as receptors and discussed its photo- chemical properties. The chromophore luminescence is considerably quenched only by those lanthanide ions which have large ionic radii and in which f ± f transitions occur (Ce3+, Pr3+, Nd3+). Ions of alkali and alkaline-earth metals, as well as lanthanide ions with smaller radii, do not greatly affect the emission spectra of this sensor. 1021 Me MeC C C C O O O O O O Me Me O O O O O O 20 S S O S S O 21 Thiacrown ether 21 is selective for Cu2+ ions.It is interesting that a `reverse' PET operates in this complex, and fluorescence quenching occurs due to electron density shift from the anthracene substituent to the copper cation possessing strong oxidising properties.47 Thus, depending on the ring size and configuration, crown ethers may be selective for ions of alkali, alkaline-earth, or rare- earth metals. 3. Sensors with polyamine receptors Even compound 22, which is a rather simple diamino derivative, can form complexes with Cr3+, Fe3+, Co2+, Ni2+, and Cu2+ ions; it manifests the PET and acts as a chemosensor with 30-fold fluorescence enhancement.48 The sensor 23 acts in a similar way but with higher selectivity for copper ions.49 O O NMe2 NH N C6H13N NH2 H2N O O 22 23 The anthracene derivative 24 was the first example of a chemosensor with a short polyalkylamine chain, which forms chelate complexes with transition metal ions.50 The ligand fluo- rescence, which is very weak due to the PET process, becomes 1000 times more intense upon addition of zinc chloride in acetonitrile.NH NMe2 HN Me2N 24 The action of chemosensors of the type 25 depends on the nature of the substrate cations and on the acidity of solutions. Coordination of Cu2+ and Ni2+ ions with compound 25 does not result in intense light emission in weakly acidic and neutral solutions (pH>4) where the amino groups are not protonated and the PET operates. This results from deactivation of the excited state via the energy levels of the metal centre.HN HN NH2 NH 25 On the contrary, a complex of compound 25 with Zn2+ luminesces strongly due to the d 10 configuration of the Zn2+ ion and an increase in the oxidation potential of amines.511022A large series of open-chain and cyclic N-mono- and N,N- diarylpolyalkylamines has been synthesised and analysed.52 Com- pounds 26 are chemosensors for proton and transition metal ions: Co2+, Ni2+, Cd2+, Pb2+, Cu2+, Zn2+. Protonation or coordi- nation of cations change the fluorescence accordingly. Sensors 26 where Ar are terminal anthracene substituents can form excimers. NH HN NH n Ar Ar 26 n=0±4. Ligands containing aminoethylamide fragments (e.g., com- pound 27) are effective in recognition of Ni2+ and Cu2+ cations.It is believed that photoexcitation involves electron transfer from the metal ions to the ligand.53 NH2 O NH CH2 NH O NH2 27 Fluorescent complexes with the composition ML2á [where L 2 is 6-(9-fluorenyl)-1,4,8,11-tetraazaundecane-5,7-dione andM is a divalent ion of copper, nickel or cobalt] were studied in non- aqueous and aqueous media.54 It was found that open-chain polyamines are more efficient sensors than cyclic polyazacyclophanes 28.52 HN HN HN NH 28 In a molecule of compound 28, the polyamine chain is arranged as a strained arch above the benzene ring plane. As a result, simultaneous coordination of both benzyl nitrogen atoms by a single metal ion becomes impossible. However, the PET can still operate in the complex.The situation changes if the ring size increases or if additional nucleophilic centers are introduced. For example, macrocyclic ligands 29a,b, in which the receptor ring contains a 1,10-phenan- throline fragment, are chemosensors for hydrogen and zinc cations.52 N NHN NHHN HN n 29a,b n = 1 (a), 2 (b). Compound 29a forms only a mononuclear complex with zinc, whereas compound 29b forms both mono- and binuclear com- plexes. Thus, fluorescence spectra depend on the degree of protonation and complexation. V A Bren Rather efficient chemosensors for Zn2+, Cd2+, and Hg2+ were obtained based on anthracene-substituted polyazacrown ethers 30 (see Ref. 55). NH n N 30a ± e n = 1 (a), 2 (b), 3 (c), 4 (d), 5 (e).The luminescence of compounds 30c,e is pH-dependent and increases abruptly upon complexation with Zn2+ or Cd2+ in water. On the contrary, Hg2+ ions quench the residual fluores- cence of compounds 30a ± e. The fluorescent properties of macrocyclic ligands 31a ± d, containing N-(b-naphthylmethyl)amide groups as fluorophores, were studied in water and acetonitrile. All ligands 31 display the typical fluorescence of the naphthalene chromophore (lmax= 337 nm); however, it is weakened due to the PET. The emission spectra of compounds 31b ± d contain additional broad bands typical of the naphthalene excimer (lmax=397 ± 401 nm). The ligands show different luminescent properties in the presence of quenching ions (Pb2+, Ni2+, Cu2+, Eu3+ ) and ions that cause luminescence enhancement (Zn2+, Cd2+).56 C(O)NR5R6 R1R2N(O)C N NN N C(O)NR1R2 R3R4N(O)C 31a ± d R1=R3=R5=H,R2=R4=Me, R6=2-CH2C10H7 (a); R1=R3=R5=H, R2=R4=R6=2-CH2C10H7 (b); R1=R3=H,R2=R4=Me, R5=R6=2-CH2C10H7 (c); R1=H, R2=Me, R3=R4=R5=R6=2-CH2C10H7 (d).When the ligand 31a is coordinated with zinc or cadmium ions, the emission intensity increases by a factor of two; addition of Pb2+ salts quenches fluorescence by 25% (the heavy atom effect), while addition of Cu2+ quenches it by 95%. In the Tb3+ . 31a complex, fast deactivation processes predominate, which complicate spectroscopic studies. If Cd2+ ions are added, the intensity of the short-wave radiation of protonated sensor 31b in acetonitrile increases, whereas the excimer band disappears quickly. Similarly, emission is quenched by Pb2+ ions.The reason for this is that naphthyl excimers cannot be formed in these complexes because of spatial hindrance. Complexation of the ligand 31c with metal ions (Zn2+, Cd2+, Pb2+, Ni2+, Cu2+, Eu3+) occurs quickly even in aqueous solutions. Its behaviour in the presence of various ions resembles that of compound 31a. Coordination of compound 31d (which is more strained spatially) with metal ions is hindered. The Zn2+ and Cd2+ ions affect very weakly both the total intensity of fluorescence of the ligand 31d and the ratio of emission from the monomer and excimer. On the other hand, the Pb2+, Ni2+ and Cu2+ ions decrease the total emission (by 36%, 37% and 86%, respectively) by affecting considerably the excimer band.Various physical methods were used to study whether it is possible to determine copper, nickel, zinc, cadmium, mercury, and lead cations using a series of polyaza- and oxazacycloalkanes (14 to 18 units) containing peripheral anthrylmethyl fluorophores. It was found that all metals studied form stable complexes with these ligands in dioxane ± water and THF± water media.57 Copper(II) ions selectively quench the fluorescence of sensors in neutral and acid media; the intensity of luminescence of the 1,4,10-trioxa-7,13-Fluorescent and photochromic chemosensors diazacyclopentadecane ligand increases selectively in the presence of mercury(II) ions in basic medium. Novel fluorescent chemosensors 32 and 33 for heavy metal ions were suggested.58 Since the reduced form 33 is readily trans- formed into the oxidised form 32, these compounds may be considered as two components of a switchable redox system.NHPh Me Me NHPh HN N S Red N N S NH N Ox 33 32 The oxidised form 32 binds mercury ions with a 44-fold increase in fluorescence; the carbamimidoylthiourea fragment of molecule 33 interacts with cadmium ions with a six-fold increase in the fluorescence intensity. In the majority of fluorescent chemosensors considered above, which incorporate monocyclic or open-chain receptors containing nucleophilic atoms such as O, N, and S, the structures and electronic properties are affected by substrates (in most cases, cations).Nitrogen-containing podands and rings have sensor properties only with respect to hydrogen and transition metal ions. Incorporation of oxygen receptor atoms makes it possible to expand the range of cations that can be analysed (alkali and alkaline-earth metals; rare-earth elements; ammonium ions) and to improve selectivity. The selectivity and sensitivity of chemo- sensors increase upon transition from two-dimensional molecules to three-dimensional systems that coordinate substrate species in a `host ± guest' fashion in molecular cavities of specific sizes. 4. Tetraaminoalkyl three-dimensional sensors Some planar chemosensors contain functional substituents which fix a cation or some other substrate outside the plane.As an example, the molecules 6 ± 9a,b, 15, and 31a ± d have additional complexation sites located outside the main coordination plane.A tertiary nitrogen atom in a pyramidal configuration often serves as a vertex of a complex molecular frame forming a cavity. For example, compound 34 containing four nitrogen atoms is a selective sensor for Cu2+ and Hg2+. In neutral media, its fluorescence is quenched due to formation of chelates with copper (18-fold) or mercury cations (fourfold).59 NH2 N HN NH 34 The sensor 35a containing p-N,N-dimethylaminobenzyl fluo- rophores has an emission band at l=360 nm.60 The fluorescence of sensor 35a in solution in the presence of Cu2+ ions reaches a maximum at pH 3.8 (protonated form); as the pH is increased and protons are replaced by copper ions, fluorescence is progressively quenched and disappears at pH 5.5.Thus, the system 35a±Cu2+ is an efficient switchable pH-dependent sensor. The fluorescence of compound 35b, which contains 5-dime- thylaminonaphthalenesulfonyl (dansyl) substituents as chromo- phores, does not depend on the pH of the medium in the range from 3 to 11. However, complexation with such ions as Cu2+, Co2+ , Zn2+, and Cd2+ at pH 9.5 affects strongly the lumines- cence: Cu2+ and Co2+ quench luminescence, whereas Zn2+ and Cd2+ enhance it. At pH 6, only Cu2+ ions form a complex, which makes compound 35b a highly selective sensor for copper ions under physiological conditions.61 NH N HN R R NH 35a,b R SO2 (a), NMe2 R=CH2 In certain sensors, some of the nitrogen atoms may be involved in heterocycles; for example, this is the case for the sensor 36a which has high selectivity for zinc ions and may be used to analyse biological objects.Its isomer 36b, in which the poly- amine substituent is at position 6 of the benzene ring, is also highly selective for Zn2+ ions. The fluorescence of original starting compounds is almost independent of the medium acidity; it is virtually absent at pH 7.5. However, the fluorescence of com- pound 36a becomes 17 times more intense as zinc ions are added, whereas that of the 6-substituted isomer 36b becomes 51 times more intense.62N N NH N CO2HO O 36a The fluorescence of the sensor 37 is quenched owing to the PET.At appropriate pH values and concentrations of the coordi- nating Zn2+ ions, reversible formation (decomposition) of the exciplex occur because the phenanthroline and anthracene parts of the molecule become closer (or farther, respectively). This allowed Bencini et al.63 to classify this system as a `molecular machine' controlled by pH and light. N N HN NH N NH 37 Balzani et al.64, 65 used polymeric compositions (dendrimers) based on polypropyleneamine with dansyl end groups (e.g., compound 38) as luminescent Co2+ sensors. The amine fragments create a polycellular internal cavity which coordinates the cations. The formation of the complex with the Co2+ ions does not affect the absorption spectra, but it strongly quenches the radiation from the peripheral fluorophores; one cell of the dendrimer reversibly coordinates one cation. Zinc ions do not affect the properties of dendrimers; in the presence of Cu2+ cations, the macromolecule undergoes irreversible changes.1023 (b). NMe2 N N N NH CO2HO O 36b1024 R HN NH R HN H R N N R HN N N N R H R N N NHHN N R HN R HNR NH RNMe2 R= SO2 . 5. Cryptands Attachment of an out-of-plane bridge to azacrown ethers results in cryptands, which represent sensors with enhanced receptor capacity.66 For example, the PET is observed in the molecules of cryptands 39 ± 41. The cavity size in these compounds governs the selectivity: the parameters of the receptors in compounds 39 and 40 match the diameter of the K+ ion (see Ref.67), whereas the cavity in compound 41 is suitable for coordinating the sodium cation.68 O O O O N N O O 39 OO CF3 O O N N O O O 41 R R R R R R NH HN HN NH HN N N N N N N N N N N N N N N N N N N NH NH NH NH HN R R NHR R R R O O O O N N O O Me O Me40 O N O O ON N N N 42 R HN RNH HN R R N NH N N HN RR N N NH HN N R R NH NH R NH R 38 In order to determine transition metal ions, it was suggested to use cryptand sensors of the type 42, which form 1 : 1 adducts with Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+ and Pb2+ cations in THF with an accompanying strong increase in the sensor fluores- cence.69 Of these cations, the strongest effect is observed in the case of Zn2+. Silver and mercury ions, which have larger ionic radii, cannot form complexes. The properties of compounds 40 ± 42 are strongly pH-affected.4, 70 6. Calixarenes Various calixarene and -hetarene derivatives can also be used as efficient sensor systems.Spatially hindered calixarenes are not very good ligands to form stable complexes; however, they can serve as a supramolecular base for adding diverse receptors and fluorophores. This results in sensors containing cavities with different sizes and properties suitable for several types of `guest' species. One of the first simple calix[4]arene systems 43 containing a benzothiazole fluorophore showed selective properties as a sensor for lithium ions.71 The Li+ cations replace the phenol proton and coordinate the methoxy groups, thus causing a bathochromic PCT-shift of the fluorescence band.This system shows no response to Na+ and K+ ions. OMe OMe OHOMe N S 43 V A Bren NO2 EtO O O O OEt O O O O O 44Fluorescent and photochromic chemosensors The cavity of yet another calix[4]arene sensor 44, which incorporates ester groups, matches the size of Na+ (see Ref. 72). Coordination with Na+ breaks the spatial contact between the pyrene and nitrophenyl rings, which results in disappearance of the PET and enhances fluorescence by a factor of six. Calixarene 45 is also suitable for the identification of Na+ (see Ref.73). Incorporation of the cation into the inner sphere of the sensor prevents the formation of an excimer from the pyrene units; as a result, only the monomer fluorescence remains in the spectrum of the complex. The selectivity factor [the ratio k(Na+) : k(K+)] is 154; the presence of lithium cations does not disturb the system. EtO O O O OEt O O O O O But But ButBut 45 Ionophore 45 has recently been modified 74 by introducing a 9-anthroyloxyphenyl fluorophore into the molecule. The electronic interactions in a series of calix[4]arene crown ethers and their complexes with alkali metal ions were studied. Absorption and emission electron spectroscopies have been used to reveal the effects of p-binding between the cations and the ligand; it was assumed that they have a major effect on the luminescent properties of the fluorophore.75 Many calixarene sensors contain crown ether fragments as receptors.For example, calix[4]crown ether 46 is a strong ionochromo- phore.76 If Ca2+ ions are added, they enter the cavities and interact with the electrons of the imino, keto, and crown-ether groups; a bathochromic shift occurs, which changes the colour of the compound from blue to green (Dl=112 nm). This phenom- enon is not observed in the presence of potassium ions; if the latter are coordinated with compound 46, the spectrum of the latter is restored due to displacement of calcium ions from the inner sphere. Me X O O N N Et2N NEt2 Me 46 X=OCH2(CH2OCH2)3CH2O.Alkali metal cations affect strongly the excitation spectra of other calix[4]crown ethers containing two peripheral acetami- doanthraquinone residues.77 1,3-Alternating calix[4](9-cyano-10-anthrylmethyl)benzo- crown-6 and its dideoxygenated analogue showed high selectivity towards the caesium ion when it was determined in the presence of a tenfold excess of potassium and rubidium cations.78 A more complicated calix[4]arene system 47 containing two crown receptors was suggested as an integrated sensor for the determination of potassium cations in alkaline medium and caesium cations in acidic medium.79 The PET in alkaline solutions results in a very weak residual fluorescence of the anthracene fluorophore.Addition of caesium ions, which are coordinated by the cavity A, does not change the spectral characteristics. On the contrary, K+ cations are incorporated into the cavity B of the crown ether, quench the PET and cause a sevenfold increase in fluorescence. In acidic media, where the nitrogen atom of the ring B is protonated, the PET is supported by the crown ether fragment A, and the fluorescence intensity is low. In this case, the ring B cannot coordinate potassium cations. Addition of Cs+ ions to the solution results in their coordination by the ring A and a fourfold increase in fluorophore emission. Li+ and Na+ ions do not compete with each other in these processes, but rubidium cations display an effect resembling that of potassium cations in alkaline medium and that of caesium cations in acidic media.Pri O Pri O O O A O O O O e7 N O O B O O O 47a The range of cations that can be determined by sensors based on calixarenes is rather wide. Such sensors are suitable for analysis of not only alkali but also transition metal cations. For example, the bispyrenyl calix[4]arene 48 with two hydroxamic acid linkers is a good Cu2+ and Ni2+ sensors (see Ref. 80). N OH O O OMe But But 48 In solutions, compound 48 generates a double fluorescence spectrum, which corresponds to the monomeric and excimeric states of the pyrene rings. Addition of copper or nickel salts to the solution quenches abruptly a part of the fluorescent irradiation 1025 Pri O Pri O O O A O O O O H+ 7H+ e7 + NH O O B O O O 47b 21026 depending on the solution acidity.In the opinion of Bodenant et al.,80 the excimer photosensitivity of the ligand makes it possible to consider this system as a new type of chemosensor for the selective determination of transition metal ions in aqueous media. The selectivity of sensors based on calixarenes may be enhanced by replacing the oxygen atoms in the calixarene skeleton by sulfur atoms. For example, water-soluble thiacalix[4]arenes 49 modified by dansyl fragments were suggested for the first time as fluorescent chemosensors for Co2+, Ni2+, and Cd2+.81 (But)m S 4 OSO2 NMe2 n 49 m=0±4; n=2±4. tert-Butyl groups decrease the sensitivity of sensor molecules. The thiacalixarene 49 containing one tert-butyl group (m=1) and three dansyl residues (n=3) was found to be the most efficient.The complexation constants of this compound with Co2+ and Cd2+ are 2380 and 1510, respectively. For comparison, the corresponding constants for the chemosensor 49 containing four tert-butyl groups (m=4) and three dansyl residues (n=3) are 130 and 250. Calix[4]arene systems can be luminescent chemosensors for medium-radius lanthanides. The recently obtained macrocyclic water-soluble tetra-p-sul- fonylmethylcalixresorcin[4]arenes coordinated with Nb3+ or Eu3+ ions show a triplet transition with an analytical frequency of 24 400 cm71 (see Ref. 82). In acetonitrile, the same ions form highly luminescent complexes with calix[4]arene crown ethers containing a 2,20-bipyridine fragment.83 III. Fluorescent chemosensors for anions The main types of sensor used for the determination of anions have been considered in detail in reviews 17, 18 and in a mono- graph,84 hence here it is appropriate to dwell on the general principles for constructing such chemosensors and to discuss recent publications. Design of luminescent sensors for anions is a more complex task than that for cations, for the following reasons.1. Anions are usually larger than cations, hence the receptor should have a larger coordination space; in addition, because of a low charge density on the anion, the receptor should form rather strong bonds with the anion (at the hydrogen bond level).2. Receptors for anions are commonly constructed on the basis of cation receptors by incorporating strong electrophiles (protons, metal cations) enabling reversible coordination of the substrate anion, into the molecule. + H NH2 O NH2 H2N P 7 O O N 7O + H NH3H+, HPO2¡ 4 NH + HH2N NH 50 V A Bren 4 The system 50 was the first example of a fluorescent molecular sensor for anions.85 In aqueous solutions at pH 6, only three nitrogen atoms are protonated (the nitrogen atom closest to the anthracene fragment remains non-protonated), therefore the PET is effective and emission of the anthracene ring is small. If an HPO2¡ anion is coordinated, the fourth H7N bond appears and fluorescence is enhanced.4 and HSO¡4 can be detected using the Such anions as H2PO¡ fluorescent system 51, which incorporates three naphthylurea residues. The sensor 51 forms a 1 : 1 complex with H2PO¡4 in which the ligand fluorescence is enhanced owing to the PET.86 N O HNHN O NHHN O NH NH 51 Compound 52 is an example of molecular sensor for amino acids.87 In this compound, the bifunctional receptor is suitable for coordinating a zwitter-ionic amino acid molecule. The fluores- cence of the anthracene fragment increases severalfold due to complexation. O O O H NH2 H O + 7 + NH2 NO O O H N NH 52 Polyazacycles are commonly used as selective sensors for anions. At pH 6, six of the eight nitrogen atoms in the cage of compound 53 are protonated (except the two apical ones). In this case, the fluorescence of the acridine fragment is quenched almost completely.When a dicarboxylate anion of suitable size is incorporated into such a protonated cell, intramolecular isolation NN NH HN 6H+ HN NH N N HO2C(CH2)4CO2H HN NH N 53 N + N +NH2 H2N+ +NH2 H2N N N O 7 + OO 7 N +H2 O NH2 NFluorescent and photochromic chemosensors of the acridine rings occurs; the latter can no longer form an excimer and hence show strong one-ring fluorescence.88 An anion may also be fixed by a cation incorporated into the receptor and capable of coordination. For example, the tetramine 54, which is a well-known chemosensor for zinc ions, displays secondary sensor capabilities in the presence of aromatic carbox- ylate anions.89 e7 RC Zn2+ O O7 NH RCO¡2 H2N NH Zn2+ N NH2 H2N 54 H2N N R= , .Fe NO2 NMe2, The fluorescence of the excited anthracene fragment is quenched when the carboxylate anion is bound with Zn2+, which is explained 89 by electron transfer between the aromatic moiety of the carboxylic acid and the anthracene fluorophore. The SO2¡ 4 ions can be determined selectively in acidic aqueous media using a complex of 1,4,8,11-tetrakis(naphthylmethyl)- 1,4,8,11-tetraazacyclotetradecane with Cu2+. The fluorescence intensity of this system increases in the presence of sulfate ions.90 A number of calix[4]pyrroles bound with anthracene moieties were synthesised and used as fluorescent chemosensors for anions (F7, Cl7, H2PO¡4 ).91, 92 A bulky dizinc octamine complex 55 suitable for the selective determination of a number of anions was synthesised.93 At pH 8 this complex reacts with ambident anions to give stable 1 : 1 adducts.The cavity of the molecule containing two zinc cations can accommodate linear anions such as N¡3 (2.34 A), NCO7 (2.42 A), and (partially) NCS7 (2.75 A). If any of these anions is added to the sensor, fluorescence of the anthracene moiety is quenched due to electron density transfer. HN NHNH N HN Zn2+ N N Zn2+ N N7 HN NH 55 Certain other anions, such as NO¡3 , HCO¡3 , SO24 ¡, Cl7 and Br7, do not affect the fluorescence intensity of the anthracene moiety even if they are present in tenfold excess.Other, less bulky molecular systems can also be effective for the determination of anions. In particular, Sharma and Street 94 reported a fluorescent determination method for nitrate and nitrite ions in water using simple photoreactions resulting in a fluorescent compound with unknown structure. This method is used for monitoring the content of NO¡3 anions in tap water. SO3HNH2 hn, H+, H2X NO¡ NO¡ 2 3 fluorescent compound X=S27, SO2¡ 3 . 1027 A method for the quantitative determination of nitrite ion in water and foodstuffs was elaborated 95 based on quenching of the indole fluorescence upon its interaction with nitrite ions in acidic medium. The detection limit is 2.5 mg per litre of the solution.IV. Luminescent sensors for molecules In Section III, we have already considered luminescent sensors for the determination of amino acids in acidic media 87 and dicarbox- ylic acids in alkaline solutions.88 Recently, intense development of methods for the analysis of neutral organic molecules belonging to various classes has started. A number of sensors based on cationic and anionic receptors have been created for their determination. For example, the luminescent sensor 56 has been patented; its non-cyclic analogue 57 has also been synthesised.96 Complexes of these compounds with lanthanide cations are luminescent; the active cavities and diverse radicals R1±R19 at the periphery of the macromolecules make it possible to detect a number of organic compounds, including peptides, nucleic acids and enzymes, and also to use these systems as luminescent markers for various purposes.N O O HN HN NH O HO OH HO HN O NH HN O O N 56 R1 R5 O O R19 ( ) R3 ( ) R7 R2 R6 N N N R4 N R8 0.1 0.1 R18 OR14 R13O R9 R17 NR15R16 R11R12N R10 O O 57 The fluorescent monomer 58 has been developed for the selective determination of phenols and anilines in non-polar solvents, even in the presence of aliphatic alcohols and amines, oxygen, and water.97 The monomer can be involved in copoly- merisation. N OMe MeO CH2 OMe MeO CO2CH CHO2C CH2 58 A new sensitive fluorimetric method for the determination of formaldehyde is based on the quenching of fluorescence of the aluminium complex of a tetra-substituted aminophthalocyanine in the presence of sulfite and formaldehyde.The latter can be determined at a concentration of 0.040 ± 1.19 mg ml71, the detec- tion limit being 7.5 ng ml71 (see Ref. 98). In order to determine traces of aldehydes and ketones in water, a fluorescent sensor was suggested, viz., 2-aminooxy-N-(3- dansylaminopropyl)acetamide.99 It can be used for identification of formaldehyde, acetaldehyde, acetone, and other aldehydes and ketones at concentrations <1 mmol litre71. The very low detec- tion threshold makes this method suitable for the analysis of snow, ice and water in clouds for the content of carbonyl compounds.1028The content of hydrogen peroxide in snow and ice can be determined with high accuracy using a new chemiluminescent reagent, viz., 7-(4,6-dichloro-1,3,5-triazinylamino)-4-methylcou- marin.100 The chemiluminescence intensity is directly propor- tional to the concentration of H2O2 in the range 161071 ± 461074 mol litre71.The majority of non-transition metals do not interfere with this analysis. The first fluorescent sensor for the determination of boronic acids based on an anthracene fluorophore was created; a dieth- anolamine fragment was used as the receptor in this sensor. Complexation increases fluorescence by a factor of 19 due to the PET.101 Synthesis of a new water-soluble reagent 59 for carbohydrates based on 3-aminophenylboronic acid and naphthalene-1,8-dicar- boxylic anhydride was reported.102 Naphthalimide is the fluoro- phore in compound 59.Significant change in the irradiation intensity (I/I0=0.25) makes compound 59 a useful sensor in neutral media.102 O (HO)2B NO59 The symmetrical molecule 60 with two phenylboronic acid residues allows one to detect carbohydrates in aqueous solutions owing to a considerable enhancement of fluorescence.103 Its characteristic feature is a low-frequency range; lmax=645 nm, with a shoulder at l=695 nm. Me + Me N O O7 (HO)2B MeMe N (HO)2B 60 A fluorescent sensor for certain choline phospholipids was obtained based on squaramide.104 The selective fluorescent sensor 61 for D-glucosamine contains two receptors per molecule: monoaza-18-crown-6 (or monoaza- 15-crown-5) and a boronic acid residue.105 O NMe N O B(OH)2 O O O 61 n n=0, 1.Fluorescent sensors were found even for such chemically inert organic compounds as hydrocarbons. For example, alkanes increase the fluorescence of berberine sulfate 62 under monochro- matic UV irradiation. This effect was explained 106 by ion ± dipole interaction of the hydrocarbon with berberine sulfate. V A Bren O OHSO¡4 N+ MeO 62 OMe A method for selective determination of certain polycyclic aromatic hydrocarbons was suggested, involving the recording of the phosphorescence of sodium dodecyl sulfate micelles in the presence of acridine dyes (as donors of triplet energy) and polycyclic compounds (pyrene, anthracene, etc.) as acceptors.107 Cyclodextrins (CD) are also of considerable interest as chemo- sensors.A cavity in a cyclodextrin has low polarity, and its size is suitable for many `guest' molecules. The properties of the sub- strate in the complex change considerably; this is reflected in the absorption and emission electronic spectra. Cyclodextrins are valuable sensors, as they often form water-soluble complexes owing to the presence of hydroxy groups; this expands consid- erably their scope. More than 30 years ago, an inclusion complex was obtained, consisting of b-cyclodextrin 63 and 8-amino-1-naphthalenesul- fonic acid. The molecules of the latter in the bound state showed intense fluorescence.108 Later, a series of fluorescent sensors were obtained on the basis of this complex.H H O H O O H O O O H O O HO O HO O O HO H O O H O O OH O HO H O O H O OH OH O H O O O H O O O O O H H H 63 Two types of fluorescence, namely, monomer and excimer fluorescence, were studied for inclusion complexes of various naphthalene derivatives with b-CD.109 Excimer fluorescence pre- dominates for equimolar concentrations of naphthalene and b-CD. Complexes of 2,7-dimethylnaphthalene and 2-benzylnaph- thalene with b-CD do not show excimer fluorescence. Structures of various complexes of b-CD with naphthalene, phenanthrene, fluorene and cyclohexane were studied both by experimental and theoretical methods.110 ± 112 The complexation energy ranges within 9 ± 12 kcal mol71. Long-lived fluorescence at room temperature was found and studied for 2 : 2 complexes of b-CD with naphthalene.113 De la Pena et al.114 studied the effect of various factors on the phosphorescence of the complex of a-CD with 6-bromo-2-naph- thol in deaerated aqueous solutions at room temperature. Emis- sion maxima were detected at l=500 and 535 nm.The determination threshold of bromonaphthol is 0.26 mg litre71 with a standard deviation of 4%. It was found subsequently that the addition of small amounts of various non-polar liquids (particularly cyclohexane) results in a very strong enhancement of phosphorescence. However, the presence of reducing agents (sodium sulfite) quenches the sig- nal.115 Hydroxypropyl-b-CD in aqueous media forms a supramolec- ular inclusion compound with 8-anilino-1-naphthalenesulfonicFluorescent and photochromic chemosensors acid, which results in significant luminescence enhancement of the latter.116 Triethylenetetraminebis(b-CD) was synthesised from hydr- oxypropyl-b-CD and triethylenetetramine; after modification by Cu2+ ions, it showed specific luminescence upon interaction with `guest' molecules, such as 6-(p-toluidino)-2-naphthalene- and 8-anilino-1-naphthalenesulfonic acids.117 A naphthol-modified b-CD was successfully used as a fluo- rescent sensor in the determination of trace amounts of CCl4 and 2-methylisoborneol in water.118 b- and g-Cyclodextrins modified by pyrrolinone residues are fluorescent chemosensors for a number of carboxylic acids, terpenoids and bile acids.119 The cyclodextrin `host' has a fluo- rescence band at l=472 nm, the intensity of which decreases if adamantane-1-carboxylic, p-aminobenzoic, benzoic, phthalic or cyclohexenecarboxylic acid is added as the substrate in aqueous medium at pH 5.90.However, if these acids are added in a buffer solution with pH 9.09, fluorescence is enhanced. The same effect is achieved if terpenoids, amino compounds and bile acid salts are added as the `guests'. The sensitivity of the cyclodextrin `host' to the nature of the `guest' molecules can be varied by changing the cavity size in the `host'. For example, the terphenyl-substituted b-cyclodextrin 64a, which has a smaller cycle than the g-cyclodextrin derivative 64b, `recognises' well small terpenoid molecules, especially in the dimethyl sulfoxide ± water medium (1 : 9).Bulkier bile acids are detected with lower sensitivity. It is interesting to note that the radiation intensity of cyclodextrin 64a increases due to interac- tions with `hosts', whereas that of compound 64b decreases.120 O O Ph OH NHCCH2CHN O O O O OH HO OH HO n 64a,b n=6 (a), 7 (b). Dansyl- and tosyl-modified b- and g-CD are used as chemo- sensors for bile acids and terpenoids.121 The luminescence of b-CD is quenched in the presence of `guest' molecules, while that of g-CD is enhanced as a result of interactions with monocyclic and non-cyclic molecules. Chenodeoxycholic acid, ursodeoxy- cholic acid, and (7)-borneol can be recognised with high sensi- tivity using b-CD, while g-CD can be used to recognise lithocholic acid and (7)-borneol.A system comprising a peptide, g-CD and naphthalene frag- ments can also respond to the presence of various cholic acids due to the formation of a luminescent excimer.122 Mono[6-deoxy-6-(1-methyl-2-pyrrolidinylidenesulfamido)]- b-CD synthesised recently is also a promising sensor.123 A highly sensitive water-soluble fluorescent sensor system 65 was obtained; this is a b-CD (receptor) substituted with 2-naph- thoic acid (fluorophore).124 CO2H O O HO OH O 7 65 The sensor 65 was used to detect surfactants (mostly cations) of the types 66 ± 69 with long hydrocarbon chains. 1029 + C16H33NMe3 Cl7 66 +NC16H33 Cl7 67 + Na+ 3 HO(CH2)11NMe2(CH2)2OH Br7 C12H25OSO¡ 69 68 The fluorescence spectra of the sensor 65 undergo specific changes upon micellation or complexation.7 7 7 7 7 7 7 + 7 7 7 7 7 Complexation 7 7 +7 7+ ++ + 7+ 7 + + 65 Micellation The nature of many sensor molecules involves a mechanism to control them, which makes it possible to turn on and off or switch the sensor process. When such systems are exposed to external factors (change in temperature, polarity, pH of the medium, concentration of metal ions, electromagnetic potential, chemical reactions), they undergo intramolecular rearrangements affecting the sensor properties. The following luminescent sensors can serve as examples: pH-controlled ones (azacrown ethers, polyaza com- pounds), those controlled by redox reactions (compounds 32, 33), pK+- (compound 46) and pNa+-dependent (compound 44).V. Photochromic chemosensors New highly sensitive photodynamic fluorescent sensor systems (i.e., those that can be turned on and/or off upon irradiation of a solution) can be created based on photochromic spirocyclic compounds, such as spiropyrans 70a and spirooxazines 70b. The general scheme of these transformations is shown below. Me Me R2 X hn, D N R3 O hn, D R1 R4 R5 70a,b R3 Me Me R2 R4 X Mn+ + hn N R5 7O A R1 R3 Me Me R2 R4 X +N R5 7O R1 Mn+ B X=CH (a), N (b); R1±R4 are various substituents, R5 is complex-forming group. Exposure of photochromic spirans 70a,b to short-wave irra- diation results in the merocyanine form A.The latter can undergo1030 coordination with metal ions Mn+ to give the complex B having specific absorption and fluorescence spectral characteristics dif- ferent from those of the form A. Studies of these systems as chemosensors started in the 1960s.125, 126 It was shown that complexes of such ions as Zn2+, Co2+, and Cu2+ with the spiropyrans 70a have characteristic spectral bands in solution and can be used in analytical chemistry. In 1982 ± 1988, a number of papers were published (see, e.g., Refs 127 and 128) reporting detailed studies on the complexation of various metal ions (Pb2+, Co2+, Ni2+, Zn2+, Cd2+, Cu2+, Ce3+, La3+, etc.) with molecules of spiropyrans 70a (R1=Me, R2=H, R3=H, NO2, R4=H, R5=OMe) in organic solvents.The complexes B have characteristic bands in the absorption and emission electronic spectra. Certain cations coordinate the oxygen atom of the methoxy group and thus stabilise the open form. The open form B is also stabilised for R3=NO2. In solutions of spiropyrans 70a with R1=(CH2)2CO¡2 or (CH2)2SO¡3 , R2=NO2, R3=R4=R5=H, the merocyanine form A is strongly stabilised by divalent metal ions (Zn2+, Ca2+). Specific fluorescence of solutions in acetone was noted.129 Spiropyrans 70a with R1=Me, R2=H, R3=H, NO2, R4±R5=N=CHCH=CH, which form B-type strongly fluorescent complexes with Mg2+, Ca2+, Ba2+, Zn2+, Cd2+, Hg2+ and other cations, were found to be very sensitive sensors for these cations.Incorporation of crown ether fragments as receptors into the molecules of spiro compounds changes the properties of these photochromic compounds. For example, nitrobenzospiropyran 71 containing monoazabenzo-15-crown-5 is a good Li+ and Na+ sensor with pronounced negative photochromism.130 Me Me M+, D N O NO2 hn Alk N O OO O 71 M=Li, Na, K. Me Me N Mm+ Me N O R(CH2)nOCO 72b,c 72a ± c O O R=H, n = 1 (a); R = N O O Me Me NO2 +NAlk 7O O N M+ O O O Me Me N 72a + 7O OCOMe NMe Mm+ A Me Me N + 7 O C O O NMe B , n = 1 (b), 2 (c); Mm+=Mg2+, Ca2+, Ba2+, Eu3+, Tb3+. V A Bren Spiropyrans 70a and spirooxazines 70b with polyfunctional receptors undergo more complex transformations.131 ± 134 For example, for compounds 72a ± c there are two competing com- plexation pathways (Scheme 1) involving free alkaline-earth or rare-earth cations (structure A) or the same cations bound with the crown ether fragment (structures B, C). Each type of coordi- nation is characterised by appropriate changes in the electronic spectra.Complexation can be monitored coulometrically, by femto- second spectrochromatography,135 or this monitoring can be combined with mass spectrometric determinations.136 Light-sensitive membranes based on polyvinyl chloride with built-in fragments of lipophilic crown-containing spirobenzopyr- ans were obtained and studied. The photochromic parameters in such systems depend on the pH of the medium and on the concentration of alkali metal ions.137 The spirobenzothiapyran 73 containing the aza-12-crown-4 residue can form a complex with Li+.The coloured photoinduced form of this compound has enhanced thermal stability, partially owing to stabilisation upon complexation.138, 139 Me Me NO2 Me N S N O O O 73 Stabilisation of an open merocyanine fragment also accom- panies the complexation of Li+ and especially Ag+ with ana- logues of compound 73 containing monoaza-15-crown-5 and -18-crown-6.139 The two spirobenzopyran fragments linked with diaza-18- crown-6 (compound 74) bind the Ca2+ ions more strongly than alkali metal cations due to an apical coordination of divalent cations by the anionic oxygen atoms of the ligand.This type of calcium complexation can be controlled by irradiation.140Scheme 1 Me Me N + O 7 O O O (CH2)n N NMe C Mm+ (CH2)n O O O O N O Mm+O O CFluorescent and photochromic chemosensors NO2 O O R N Me Me Mn+ O N N O Me Me N R O O 74 O2N O2N O7 O N Me Me + R +N Mn+ O O N Me R Me O7 N O NO2 Upon complexation with Na+, K+ and alkaline-earth metal cations, the shift of the short-wave absorption band of phenyl- fulgide 75 containing a benzo-15-crown-5 fragment fused to the benzene ring was 28, 12, and >40 nm, respectively, both in the open and cyclic forms.141, 142 Me O O hn O O O Me OO O Me 75 Me O O O D O O O O Me Me O Me O O O O O O O Me Me O Indolylfulgides 76 containing 14-crown-4, 17-crown-5 and 20-crown-6 fragments are sensors for Li+, Na+ and K+, respec- tively; in this case, both the E- and Z-isomers of the open forms 76a are more sensitive for alkali metal ions than the totally cyclic system 76b.143 Me O O O hn hn, D O O Me NMe n Me O Me 76a Me O O O O O Me N Me n Me O Me 76b n=1±3.Dihetarylcyclopentenes 77 containing crown-ether fragments were synthesised and studied as photochromic sensors. These compounds can decrease considerably the degree of coordination of K+, Rb+ and Cs+ when exposed to UV irradiation at a wavelength of l=33070 nm (picrates in aqueous-organic media were analysed).144 ± 146 This phenomenon was explained by the ability of the original compound 77 to coordinate these cations by both crown-ether fragments simultaneously, whereas the more rigid photoreaction product, viz., isomer A, is devoid of this ability. The possibility of selective coordination of cations with certain radii is provided by the crown ether ring sizes.O O O O O n O O O O O n n=0±2. The novel photochromic compound 78a based on a Malachite Green analogue containing two crown-ether fragments was syn- thesised.147 Colourless complexes 78b with metal cyanides are leuconitriles; their solutions are coloured reversibly under irradi- ation due to photodissociation.147 The system 78 is a chemosensor for metal cyanides. O N O O O O OO O +M++CN7 Alfimov et al.148 ± 152 described crown-containing styryl dyes 79, 80, which are fluorescent sensors for alkaline-earth metal cations (Mg2+, Ca2+, Ba2+).F F F F F F Me Me Me Me S S 77 hnUV hnVIS F F F F F F Me Me S S Me Me A NC Ph hn O N D M+ O O O 78b Ph N C+ 78a 1031 O O O O O n O O O O O n O O + N O O1032Compounds 79b ¡À e with trans-configuration of the double bond S +NR trans-79a ¡À e R=Et ClO¡¦4 (a), (CH2)2SO¡¦3 (b), (CH2)3SO¡¦3 (c), (CH2)4SO¡¦3 (d), CH2C6H4SO¡¦3 (e). form (depending on the type of the substituent R, which serves as a spacer, and the nature of the counter-ion) either monomolecular complexes, in which the metal ion located within the crown is additionally coordinated by the sulfonic group oxygen, or type A dimers resulting from reversible [2+2]-cycloaddition.148 hn Dbenzo-15-crown-5 fragment with a metal cation; Dbenzothiazole residue; Illumination with light at a wavelength of 436 nm results in isomerisation of trans-complexes 79 .M2+ to give cis-complexes.The toluenesulfonic acid residue in the complex (cis-79e) .M2+ is arranged above the crown-ether plane to give a molecule `capped' with the anion.150 For dimeric molecules 79f with a trimethylene bridge connect- ing the benzothiazole residues, the most stable configuration of the complexes is the structure 79f .M2+ (see Ref. 149). Quantum-chemical calculations of the energies of ground and excited electronic states of the molecules 79a ¡À f, their dimers and complexes confirmed the conclusions based on the experimental studies.151 D 0.7 0.6 0.5 0.4 0.3 0.2 0.10 400 350 Figure 4.Dependence of absorption spectra (a) and fluorescence spectra (b) of ligand trans-80a (c80a=1.061075 mol litre71) in MeCN on the Ca2+ concentration at a constant concentration of ClO¡¦4 (0.01 mol litre71);148 cCa2+ : c80a=50 (1), 100 (2), 200 (3), 300 (4), 400 (5), 500 (6). O O O O O A D(CH2)nSO¡¦3 (n=3, 4). a trans-80a 123456 550 450 500 O S O N+(CH2)3 N+ O S O 79f .M2+ The trans-80a dye containing the N-phenylaza-15-crown-5 fragment manifests a considerable ionochromic effect registered in electronic spectra (Fig.4),148 which, according to the calculated complexation constants, implies the usual complex structure where the Ca2+ cation is coordinated in the azacrown fragment. S +NRtrans-80a,b R=Et ClO¡¦4 (a), (CH2)3SO¡¦3 (b). The data obtained for the complexation of compound trans- 80b with Ca2+ and Zn2+ can be visualised schematically by the following sequence of equilibria S C+M S CM, MS C, S C+M M(S M(S C)n , C)n¡À 1+ S C MS MS CM, C+M I (rel.u.) b 6 54 321 trans-80a 650 600 600 V A Bren O O O M2+O O OO O N O O l /nmFluorescent and photochromic chemosensors where S7C is a trans-80b molecule (S is a sulfonic group, C is an azacrown ether fragment);Mis a metal ion, n=2±4.This means that, in addition to 1 : 1 complexes, the betaine 80b forms aggregates comprising four dye molecules and one M2+ ion. Probably, the interaction between the metal cations with the sulfonic groups of the molecules makes the major contribution to the stability of these aggregates.148 Studies of ion exchange processes involving amphiphilic photochromic compounds 79 in the `monolayer ± aqueous phase' system showed that the Na+, Ca2+ and Mg2+ ions increase the strain in the monolayer, whereas the K+ ions decrease it consid- erably.153, 154 Compound 81, which comprises thioindigo and dithiocrown- ether fragments, is a novel photochromic sensor for alkali metal cations and Ca2+.155 It exists as cis- and trans-isomers, each isomer having characteristic bands in the absorption spectra: lmax=485 and 545 nm for cis- and trans-81, respectively.The trans-isomer of compound 81, which has weak coordinating properties, is transformed into the cis-isomer upon exposure to light with a wavelength of 550 nm; O O S n O S hn hn, D S O S O O n trans-81 O O O O S S n n S S O O cis-81 n=2, 3. the cis-isomer can coordinate a `guest' cation. The complexation equilibrium constants of metal ions, determined by the extraction method, suggest a high sensitivity of the cis-isomer for Na+, Cs+ and Ca2+; however, such ions as Hg2+, Cd2+, Cu2+, Co2+ and Ag+ do not form these complexes. The lifetime of the cis-isomer is hundreds times longer than that of its analogue (cis-thioindigo), even in the absence of metal ions.We obtained and studied 156, 157 2-(2-acyloxyphenylamino- methylene)-2,3-dihydrobenzo[b]thiophen-3-ones 82a,b. These are solvato- and thermochromic compounds undergoing the follow- ing transformations. O H O DMSO N hn S NC(O)R D PhH R(O)CO S OH A 82a,b OC(O)R OC(O)R Mn+ 7Mn+ S S N N Mn+ B HO C O Me R=Me (a), (b). O O CH2O 1033 Under irradiation, the form A is transformed into the form B, thus creating conditions for the formation of the complex C with a metal ion; the spectral characteristics of the latter differ from those of compounds A and B, i.e., the system 82 manifests metal-sensor properties. The acyl moiety can contain a fluorophore group, e.g., the residue of 4-methylcoumarin-7-yloxyacetic acid (compound 82b).It was shown by 1H NMR and electronic spectroscopy 158 that an equilibrium is established in solutions between the original compound 82b and the form A; the content of the latter increases with the solvent polarity. Illumination of the equilibrium mixture with light at 436 nm results in transfer of the acyl group from the nitrogen atom to the thiophene fragment. This produces an (E)-C=N bond. The absorption spectra of the system 82b change if the solution contains Ca2+, Mg2+, Zn2+ or Ni2+ cations. However, the fluorophore residue does not behave as a lumines- cent indicator, probably because of preferential deactivation of excited molecules. The system 83 comprising not only a benzothiophene frag- ment and an N-acyl residue but also an annelated crown ether can be used as a photochromic sensor for alkali metal ions.159 Illumination of compound 83 and its complexes with alkali metals with light at 436 nm results in migration of the acyl group from the nitrogen atom to the oxygen atom to give O-isomers 84 and 85, respectively.Complexation with lithium ions results in the biggest changes in the absorption spectra. OC(O)Me O S S N NC(O)Me hn O O D O O O O O O O O 84 83 M+ M+ 7M+ 7M+ N O O hn O O M+ O M+ O D O O O O 85 M=Li, Na, K. Figure 5 shows a plot of the optical density of a solution of the complex 85 versus the concentration ratio of LiI and the ligand 84 DD 0.15 0.10 0.050 4 6 8 100 200 2 cM+ : cA Figure 5. Dependence of the optical density variation (DD) of a solution of the complex 85 on the LiI (cM+) and the ligand 84 (cA) concentration ratio.159 * ** * * ** * NC(O)Me1034 at the absorption wavelength of the complex (l=357 nm) upon addition of lithium iodide to an illuminated solution of the O-acetyl form of 85 (at a concentration of 6.5561075 mol litre71).The maximum extinction is reached at a tenfold excess of LiI. This specific behaviour of this system and its high sensitivity for Li+ make it possible to place it among photo- chromic sensors.160 VI. Conclusion Thus, recent studies demonstrate a considerable interest in the creation of novel chemosensor systems applicable to the analysis of inorganic and organic substrates.Table 1 shows the character- istics of certain organic sensors. Table 1. Characteristics of certain organic sensors. Substrate Compound Ca2+ Ba2+ Ca2+ Ca2+ Mg2+ Hg2+ Ce3+ Nd3+ Zn2+ Hg2+ Hg2+ Cd2+ Zn2+ Zn2+ Co2+ Ni2+ 1c 1c 4a 4b 7a 7b 20 20 30c 30c 32 33 36a 36b 38 40 45 47a 47b 48 49 (m=4, n=3) 4 4 2 51 51 59 84 Indole a-CD Na+ K+ Cs+ Cu2+, Ni2+ Cd2+ Co2+ H2PO¡¦ HSO¡¦ D-fructose Li+ NO¡¦ 6-bromo- 2-naphthol a Absorption band; b mg litre71; c mg litre71. Currently, diverse types and classes of both organic and inorganic compounds are used as abiotic sensors or substrates. The use of crown-ether compounds as receptors stimulated the creation of chemosensors based on complex three-dimensional systems, including polyamines, dendrimers, calixarenes, cyclo- dextrins and their combinations.Fluorescent chemosensors with high sensitivities and selectiv- ities have been synthesised; the action of these sensors is based on specific but reliable intramolecular interaction mechanisms (the PET and PCT effects; excimers and exciplexes). Studies aimed at the external controlling of sensor processes are of current interest. The search for new types of molecular `switches' is under way. Some chemosensor systems are created using elements of supra- molecular chemistry and nanotechnologies; this makes it possible Detection threshold /mol litre71 161076 161076 361076 361076 161078 161078 261076 261076 161076 561077 161075 161075 361078 161078 561077 561078 661076 161076 161078 561078 261077 161077 261076 361076 161073 261075 2.5 b 0.04 c Ref.Analytical fluorescence wavelength /nm 15 15 24 24 27 27 46 46 55 55 58 58 62 62 65 80 74 79 79 80 81 81 86 86 102 159 95 115 333 333 417 404 530 476 406 406 416 416 413 413 514 514 514 400 480 427 429 400 520 520 377 377 400 357 a 350 500 V A Bren to create sensors with unique capabilities, e.g., properties of molecular machines 52, 161 or artificial memory elements.14, 52, 79 The scope of application of fluorescent sensors expands: from routine laboratory reagents for cations and anions, they grow into systems for identifying complex organic molecules, analysing various biological substrates,35, 55, 61 pesticides,162 and food- stuffs,163 monitoring the state of the atmosphere and oceans,29, 164 etc.Although the theory of chemical sensors has certain achieve- ments,165 it is developing somewhat more slowly than experimen- tal and applied studies. 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Russian Chemical Reviews 70 (12) 1037 ± 1065 (2001) Structural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic hydrogenation, hydrosilylation and cross-coupling reactions V A Pavlov Contents I. Introduction II. Catalyst ± substrate structural relationships III. Catalyst ± product configurational relationships in reactions on C2-symmetrical chiral metal complexes IV. Catalyst ± product configurational relationships in reactions on asymmetric metal complexes V. The structure of the intermediate complex VI. Conclusion Abstract. relationships the on data published present-day The The present-day published data on the relationships between nickel ruthenium, iridium, rhodium, of structures the between the structures of rhodium, iridium, ruthenium, nickel and and cobalt homogeneous in substrates and complexes catalytic cobalt catalytic complexes and substrates in homogeneous asym- asym- metric cross-coupling and hydrosilylation hydrogenation, metric hydrogenation, hydrosilylation and cross-coupling reac- reac- tions number the between ratio optimum The systematised.are tions are systematised. The optimum ratio between the number of of coordination of number the and complex the in vacancies coordination vacancies in the complex and the number of func- func- tional of terms in characterised been has substrate the of groups tional groups of the substrate has been characterised in terms of a criterion catalytic the of enantioselectivity maximum of criterion of maximum enantioselectivity of the catalytic complex.complex. The the of configurations the between relationships The relationships between the configurations of the complexes complexes and the products formed in the reactions are analysed. In the case and the products formed in the reactions are analysed. In the case of catalysts as complexes diphosphine chiral C2-symmetrical -symmetrical chiral diphosphine complexes as catalysts (some an represent ligands their of elements structural (some structural elements of their ligands represent an incomplete incomplete turn substrates atoms), metal central the surrounding helix a of turn of a helix surrounding the central metal atoms), substrates of of the definite having products the form kind same the same kind form the products having definite configurations configurations characteristic helix.the of orientation particular the of characteristic of the particular orientation of the helix. For For example, an form that ligands with complexes rhodium example, rhodium complexes with ligands that form an incom- incom- plete of formation the catalyse atoms metal around turn P-helical -helical turn around metal atoms catalyse the formation of ( ligands the If precursors. corresponding the from acids R)-amino )-amino acids from the corresponding precursors. If the ligands contain fragments of ( is it atoms, rhodium around contain fragments of M-helices -helices around rhodium atoms, it is (S )- amino includes bibliography The formed. are that acids amino acids that are formed. The bibliography includes 234 234 references.I. Introduction Considerable recent interest in asymmetric synthesis is explained not only by the objective logic of organic chemistry, but also by the fact that in the past decade the pharmaceutical industry has largely concentrated on the preparation of drugs in an enantio- merically pure form. Therefore, a search for relatively simple and inexpensive techniques for separation of enantiomers or the syn- thesis of compounds in the form of individual enantiomers has become a very urgent problem. In this respect, priority is given to asymmetric catalysis, which allows one-step synthesis of products (or intermediates) from prochiral compounds with high enantio- V A Pavlov N D Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp.47, 119991 Moscow, Russian Federation. Fax (7-095) 135 53 28. Tel. (7-095) 938 35 02. E-mail: pvlv@cacr.ioc.ac.ru Received 25 April 2001 Uspekhi Khimii 70 (12) 1175 ± 1205 (2001); translated by R L Birnova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n12ABEH000676 1037 1038 1042 1056 1059 1062 meric excess (ee). The latter is expressed in per cent as the ratio of the difference in the yields of two enantiomers to their total yield (the terms `optical yield' and `enantiomeric purity' are also used to describe stereoselectivity). The costs of enantiomeric products are sometimes close to those of racemates, since the concentrations of expensive chiral metal complex catalysts in the reaction mixture are usually low and the substrate : catalyst ratios are rather high (up to 70 000).The progress in asymmetric metal complex catalysis has been documented in numerous publications (see the monographs 1 ±4 and the reviews 5± 19), and their number is steadily increasing. However, the `catalyst ± substrate ± product' structural and con- figurational relationships remain relatively little-studied. Analysis of these relationships is the first step in the elucidation of the mechanism of a transfer of asymmetry from the catalyst to the reactive substrate, i.e., asymmetric induction. Most probably, the crucial role in asymmetric induction belongs to chiral ligands of metal complexes.It is these ligands that determine the chiral environment of metal atoms 20 in the catalyst. Therefore, special attention in the analysis of general regularities of `catalyst ± substrate ± product' structural and con- figurational relationships should be given to reactions where the greatest number of different ligands have been used. These reactions include hydrogenation, hydrosilylation and cross-cou- pling. These were chosen for two additional reasons. First, in these reactions very high enantiomeric excesses of the products have been achieved. Second, this minimum set of reactions allows one to obtain chiral products with the largest number of functional groups (e.g., alcohols, amines, organic acids, amino alcohols, amino acids, hydroxy acids and hydrocarbons), which is of considerable value in the synthesis of drugs and synthons. By now, a vast variety of ligands, which represent components of C1- and C2-symmetrical chiral complexes (see, e.g., reviews 20 ± 32), have been synthesised and analysed in these and some other reactions.Configurational relationships of chiral complexes having C2 symmetry axes can be systematised most easily. Complexes with five- and seven-membered chelate cycles formed by bis(diphenylphosphine) ligands (from the simplest 33 to exotic 34, 35) shown below manifest the highest enantioselectivities.1038 tion arouses considerable interest,59 ± 63 since it gives rise to chiral hydrocarbons. II. Catalyst ± substrate structural relationships Me Me Ph2P Ph2P Ph2P PPh2 (S,S)-( ± )-chiraphos PPh2 Ph2P Substrates, such as a-acetamidocinnamic acid, which allow coordination by metals in the catalytic complexes both at the double bonds and at two additional functional groups, are especially suitable for the synthesis of chiral products by hydro- genation.Thus the ee of the hydrogenation product reaches 99% (see Ref. 36). The enantioselectivity of this reaction decreases dramatically upon removal of at least one functional group. Ruthenium complexes with atropoisomeric diphosphine ligands of the binap type are most suitable reagents for hydrogenation of such substrates.37 ± 41 Me PPh2 PAr2 PAr2 Me PPh2 PAr2 PAr2 In a search for the most suitable substrates for performing their conversions on metal complexes containing structurally different ligands, enantiomeric purity of the product serves usually as the main criterion.Asymmetric hydrogenation on rhodium ± phos- phine complexes is certainly one of the most extensively studied reactions of this kind. The ee values of hydrogenation products of various substrates depending on the structures of the ligands of the catalytic com- plexes are listed in Table 1. As can be seen from these examples, any substitution of the alkyl radical for the coordination-active substituent at the double bond results in a drastic decrease in the reaction enantioselectivity. Hence, the double bond should con- tain at least two groups of the typeNHCORorCO2R. However, if a catalytic complex contains the bppm ligand, which can be coordinated by three groups (also in the presence of Et3N), replacement of the acetamido group in the substrate by a group with weaker coordination ability has a smaller effect on the enantioselectivity in comparison with reactions catalysed by complexes with bidentate ligands.Similarly, the number of coordination-active groups in the substrate can be reduced to one (with the exception of C=C) on going from the planar Rh(I) complexes to the octahedral Ru(II) complexes containing ligands of the binap type (Scheme 1).{ { Hereinafter we give the best or close to best parameters cited in the relevant publications. (S)-biphemp (S)-H8-binap Ar=Ph: (S)-binap Ar=p-MeC6H4: (S)-Tolbinap MeO In the presence of these complexes, the maximum value of ee exceeds 99.5% (see Ref.40). The enantioselectivity of hydro- genation of substituted ketones in the presence of these complexes is at the same level.42 ± 50 Asymmetric hydrosilylation of ketones 3, 4 H3O+ MLnR1R2C O+HSiR3 HR1R2COSiR3 R1R2CHOH+HOSiR3 * * * ; the chiral fragment , M=Rh, Ir, Ru; Ln =P*, N N N , P P P or the asymmetric atom is marked with an asterisk Me H2C H2C can be regarded as a variant of their asymmetric hydrogenation, since the silyl ether formed can be hydrolysed into an alcohol with retention of the configuration of the asymmetric carbon atom.51 ± 53 Very often, the same catalysts manifest similar effi- ciencies in both reactions, which also suggests their similarity.54 However, specific hydrosilylation catalysts are known 55 ± 57 which result in a maximum ee of 97.6% (see Ref. 58).Asymmetric cross-coupling of organometallic compounds (R1M0X) with alkenyl or aryl halides and related compounds (R2Y) MLnO (R1 R2)* R1M0X+R2Y O M0 =Mg, Zn etc.; X=Hal; M=Ni, Pd; Y=Hal, OR, OH, etc.; * * Ln =P*, P P N , P Me results in the formation of a C*7C bond in the product; its enantiomeric purity reaches 95%. In terms of enantioselectivity, this reaction is somewhat inferior to hydrogenation and hydro- silylation, however, cross-coupling combined with allylic alkyla- (CO2) is supercritical CO2, Tc=31.0 8C, pc=72.9 atm, V A Pavlov Scheme 1 CH2 Ru(OAc)2[(S )-binap] COOH H2, MeOH, 20 8C, 135 atm, 12 h MeCOOH MeO ee=97% (S) 37 COOH Ru(OAc)2[(R)-binap] COOH H2, MeOH, 20 8C, 4 atm, 12 h Me Me MeCH2 ee=91% (R) 37 Me O O {RuCl[(S )-binap](C6H6)}Cl/NEt3 H2, THF, 50 8C, 100 atm, 44 h O O ee=92% (R) 38 Me Me Me Ru(OCOCF3)[(R)-binap] Me Me O O O H2, CH2Cl2, 20 8C, 100 atm, 18 h O ee=95% (S) 39 CO2H Ru(OAc)2[(S )-H8-binap] COOH Me Me MeCH2 H2, (CO2)/CF3(CF2)6CH2OH, 50 8C, 5 atm ee=89% (S ) 40Structural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions Table 1.The dependence of enantiomeric excesses of hydrogenation products of various substrates on the ligand structure of the catalytic complex. Ligand R2 Ph R1 An Ph P P Ph An dipamp R3 R1 R2 O OPPh2 Ph2P (R,R)-diop R3 R1 R2 Ph2P NCO2But bppm Note.Hereinafter, hydrogen pressure (pH2) is included, among other factors, into the description of reaction conditions. Abbreviations: cod is cycloocta- 1,5-diene; An is o-MeOC6H4. MeO MeO MeO MeO These and some other data 70 ± 73 suggest that high enantiose- lectivity of hydrogenation of substrates, such as tiglic acid in the presence of complexes of the Ru(OAc)2[binap] type is provided by only one coordination group, which is located in the a-position relative to the double bond. Interestingly, hydrogenation of a-acetamidocinnamic acid on such catalysts is less enantioselec- tive than that in the presence of rhodium complexes with binap 1039 Ref.ee (%) R3 R2 R1 R2 [Rh(cod)(dipamp)]BF3 * H2, MeOH, 50 8C, 3 ± 27 atm Ph R1 CO2H 94(S) 64 NHCOMe 47 (S) 64 CO2Me 96 (S) 64 NHCOMe 23 (S) 64 CO2H 93(S) 64 NHCOPh 39 (S) 64 CN 89 (S) 64 Me 51 (R) 64 NHCOMe 9 (R) 64 CO2H 1(R) 64 <1(R) 64 NHCOMe CO2H NHCOMe CO2Me NHCOPh CO2H NHCOPh NHCOMe Me Me CO2H Me R3 [RhCl(cyclooctene)2]2 / (R,R)-diop * H2, EtOH ±C6H6 (2 : 1), 20 8C, 1.1 atm R1 R2 OAc H CO2H 73(R) 65 NHCOMe Ph CO2H 82(R) 66 NHCOMe Pri NHCOMe CO2H 22(R) 65 H Ph CO2H 63(S) 67 H Ph Et 24.5 (S) 67 CO2Et H 27 (R) 68 OAc H CF3 0 68 R3 [RhCl(1,5-hexadiene)]2 / bppm / Et3N * H2, MeOH, 20 8C, 50 atm R1 R2 Ph H PPh2 NHCOMe OAc H CF3 CO2H 83(R) 69 67 (R) 68 10 (S) 68 CO2Et CO2Et NCHO LiAlH4 Ru(OAc)2[(R)-binap] ligands.74 Thus for ruthenium complexes, the presence of two coordination-active groups at the double bond of the substrate is excessive and does not favour maximum enantioselectivity. OMe H2, EtOH ±CH2Cl2 (5:1), 23 8C, 4 atm, 48 h OMe NMe OMe OMe ee>99.5% (R) 41 Simple ketonesR1COR2 (R1=Ar, Alk,R2=Alk) are hydro- genated on chiral rhodium diphosphine complexes with low enantioselectivities. This can be improved by addition of an acid or a base to complexes which have proved to be efficient in asymmetric hydrogenation of enamide substrates. Thus in the presence of Rh/camp/PriCO2H, where camp is the ligand synthes- ised from camphor, ee = 12% (R) (see Ref.75) and in the presence of Rh/(S,S)-diop/Et3N, ee=68% (R) (see Ref. 76). An alternative approach to increase enantioselectivity consists in the introduction of groups into a- or b-positions of ketones which can coordinate the metal. For example, hydrogenation of the ketone ArCOCH2NEt2 proceeds with high enantioselectivity without any base [in the presence of Rh/(S,S)-diop, ee=93% (see Ref. 77)]. Presumably, complexes with bidentate diphosphine ligands (or diamino ligands in hydrogen-transfer hydrogenation reac- tions) do not provide a sufficiently high level of `chiral recogni- tion' of simple ketones due to small molecular sizes of the latter, nor do they favour their enantioselective hydrogenation. The results of hydrogenation of various ketones on such complexesV A Pavlov 1040 Table 2.Asymmetric hydrogenation of simple and b-substituted ketones on chiral diphosphine and diimine metal complexes. Catalyst O O O O O NR2 NBut Ph Ph Me Ph OEt R Ref. ee (%) Ref. ee (%) Ref. ee (%) Ref. ee (%) 77 83 35 (R) 78 ± 82 3 ± 54 (R) 78(R) a 7 7 7 85 (S ) (R=Ph) 427 4 (S ) (R=Me) 7 99 (R) (R=Me) 93(+) (R=Et) 7 7 6 (R) 83 7 7 7 30 ± 74 (S) 42 7 7 7 7 95 (S ) (R=Me) 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 48 (S) 85 7 7 7 7 7745 7 745 45 7 99 (R) (R=CH2Cl2) 84 7 99 (S ) (R=CH2Cl2) 847 7 7 7 7 7 55 (R) 85 7 7 7 Rh-(S,S )-diop Rh-(R)-binap Ru-(R)-binap Ru-(S )-binap Ru(OAc)2[(R)-binap] RuCl2[(R)-binap] RuCl2(C6H6)/[(S )-binap] RuCl2(C6H6)/[(R)-binap] Rh-(1R,2R)-N,N0-dibenz- ylidene-1,2-diamino- cyclohexane Ir-(1S,2S)-N,N0-dibenzyl- idene-1,2-diaminocyclo- hexane a Our data obtained with {Ru2Cl4[(R)-binap]2}NEt3.Me Me Me Me [Rh(cod)Cl]2/(S )-Cy,Cy-5-oxoProNOP/CF3CO2H O O O H2, C6H5Me, 20 8C, 1 atm, 1 h HO O Cy2P O ee=97.7% (R),88 (S)-Cy,Cy-5-oxoProNOP= ; O N O PCy2 PCy2 O NH2 . HCl [Rh(cod)Cl]2/(S )-Cy,Cy-5-oxoProNOP/CF3CO2H H2, MeOH, 20 8C, 50 atm, 4.5 h OH NH2 . HCl are presented in Table 2. It can be seen that the enantioselectivity increases considerably with an increase in the size of the fragment of the substrate involved in coordination. If an additional coordi- nation-active group of a ketone is blocked by a bulky substituent, e.g., NHBut, or cannot compete with an achiral ligand of the catalytic complex for vacancies (e.g., in the presence of the b-ester group in the substrate and the carboxyl ligand in the Ru(AcO)2..[binap] complex), the enantioselectivity of this reaction at room temperature approximates a level characteristic of unsubstituted ketones [ee=4% (S)]; however, at high temperatures the situa- tion is different.48 The introduction of an additional coordination- active group into a ketone can change both the value and the sign of enantioselectivity (as in reactions of ruthenium catalysts with the binap ligands (see Table 2), which points to the change in the mechanism of asymmetric induction. Enantioselectivity can be increased by using complexes with ligands coordinated to metals by three groups.86 Me Me ee=93% (S);88 Me Me [Rh(cod)Cl]2 / bcpm O O O H2, THF, 50 8C, 50 atm, 45 h HO O NHMe.HCl [Rh(nbd)/(R)-(S )-bppfOH]ClO4/Et3N HO H2, MeOH, 20 8C, 50 atm O HO O ee=92% (R),86 OH Cy2P NHMe. HCl HO bcpm= N HO , Cy is cyclohexyl. PPh2 CO2But ee=95% (R),89 Me H nbd is norbornadiene, (R)-(S )-bppfOH= Fe And, finally, the maximum enantioselectivity is achieved through a combined use of all the methods described above. OH O OH ; PPh2 PPh2 Me Me NEt2 NEt2 [Rh(cod)Cl]2/mccpm/Et3N Me Me [Rh(cod)Cl]2/(S )-Cp,Cp-5-oxoProNOP/CF3CO2H H2, MeOH, 50 8C, 20 atm, 20 h O O HO O H2, C6H5Me, 20 8C, 1 atm, 0.1 h ee=97% (S),87 Cy2P O mccpm= ; O ee=98.7% (R),88 N PPh2 CONHMe O N , Cp is cyclopentyl; (S)-Cp,Cp-5-oxoProNOP= O PCp2 PCp2Structural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions Me O O Me RuCl2(PPh3)[(S )-biphemp] Me Me [Rh(cod)Cl]2/(S )-Cp,PhProNN0P/CF3CO2H H2, MeOH, 50 8C, 100 atm, 15 h O O HO O H2, C6H5Me, 20 8C, 1 atm, 0.4 h RuBr2[(R,R)-Pri-bpe] O O O O ee=83% (S),90 H2, MeOH:H2O (9 : 1), 35 8C, 4 atm OMe Ph N .(S )-Cp,PhProNN0P= N PCp2 PPh2 bpe is bis(phospholano)ethane. As in the hydrogenation of substrates containingC=Cbonds, the use of octahedral ruthenium complexes with such ligands as binap, biphemp, etc., is advantageous for an increase in the enantioselectivity of hydrogenation of ketones possessing coordi- nation-active groups in the b-positions relative to carbonyl group. O OH RuCl2[(R)-binap] OH OH H2, EtOH, 20 ± 32 8C, 93 atm, 32 h Special mention should be made of a group of highly enantio- selective catalytic complexes, which are active at atmospheric pressure of H2.These complexes have the in situ composition {RuCl2[binap(or biphemp)]}2 .MeCOMe and are used for hydro- genation of b-oxo esters,91 b-oxo phosphonates, b-oxo thio- phosphonates 92 and phenylthio ketones.93 Other catalytic complexes of this kind are also known.34, 94 ± 100 The hydrogena- tion of a- and g-substituted ketones occurs less enantioselec- tively.101 ± 103 ee=92% (R),42 95% (R),43 Ruthenium complexes with tri- and tetradentate ligands, such O O OH OH {Ru2[(R)-binap]2Cl4}NEt3 H2, MeOH, 50 8C, 50 atm, 20 h as the ligands 1 ± 8 and the complex 9, are used as catalysts for increasing the enantioselectivity of hydrogen-transfer hydrogena- tion of simple ketones.ee=99% (R,R),44 The effects of these complexes on the enantioselectivities of different hydrogenation reactions are demonstrated in Table 3 in the example of acetophenone. OH O Ru(OAc)2[(S )-binap] NMe2 NMe2 H2, EtOH, 20 ± 32 8C, 50 atm, 12 h ee=96% (S),42 Ph Ph RuCl2[(R)-binap] OH O O O H2N NHSO2-p-Tol H2, MeOH, 23 ± 30 8C, 100 atm, 36 h 1 OMe OMe ee=99% (R),45 RuCl2[(R)-binap](DMF)n OH O O O Ph Ph P(OMe)2 P(OMe)2 H2, MeOH, 25 8C, 4 atm, 72 h S S N N ee=98% (R),46 NHPh PhHN Me Me Ru(OAc)2[(S )-binap] O O 3 Cl H2, EtOH, 100 8C, 100 atm, 5 min OEt O OH O N Cl Pri Ph2P OEt ee=97% (R),48 6 Table 3.Hydrogen-transfer hydrogenation of acetophenone on chiral ruthenium complexes. Conversion (%) Base Ligand T /8C Precursor of the complex or the complex [RuCl2(1,3,5-Me3C6H3)]2 [RuCl2(1-Me-4-PriC6H4)]2 (see a) [RuCl2(1-Me-4-PriC6H4)]2 [RuCl2(Z6-C6Me6)]2 [RuCl2(1-Me-4-PriC6H4)]2 RuCl2(PPh3)3 RuCl2(PPh3)3 20 730 728 20 82 82 28 80 80 1234566778 95 91 98 94 70 24 83 80 87 72 93 KOH KOH ButOK KOH KOH NaOH NaOH PriOK Me2CHOK NaOH Me2CHOK [RuCl2(C6H6)]2 9 104 105 106 107 108 109 109 110 110 111 112 a 1-Acetylnaphthalene was used as a substrate.b The reaction time was 5 min. c The decrease in ee at higher levels of conversion is attributed to the reversibility of the hydrogen transfer reaction.113 1041 OH OH ee=99% (S,S),49 OH O OMe ee=99.3% (S ),50 NHCH2Ph Ph Fe Ph NHCH2Ph 2 OH Ph Ph NH2 NHMe HO 4 5 Ph NO PPh2 Fe 7 Ref. ee (%) 97 (S ) 90 (R) 87 (S ) 92 (S ) 91 (S ) 94 (R) b 73 (R) c 97 (R) 88 (R) c 79 (R) 97 (R)1042 Table 4. Hydrosilylation of acetophenone on chiral rhodium complexes. Complex precursor [Rh(cod)Cl]2 RhCl3 ON P NO 8 In order to achieve maximum enantioselectivity of hydro- silylation, bi-, tri- or tetradentate ligands should be taken in 2 ± 13- fold excess with respect to the metal. This is sufficient to provide the substitution of at least four coordination vacancies in a catalytic complex. But even in this case achiral ligands should sometimes be added.The necessary ratios of the chiral ligands 10 ± 16 to rhodium in catalysts for acetophenone hydrosilylation are shown in Table 4. O N N 10 S N HN 13 O N N N Pri 15 Thus, the maximum enantioselectivity will be provided by catalysts which ensure the minimum degree of freedom in the coordination of substrates in the intermediate complexes. The dependence of enantioselectivity on the ligand structures of catalytic complexes is well studied and is not considered in this review. Until very recently, most of the studies devoted to asymmetric metal complex catalysis were aimed at obtaining maximum enantiomeric excesses of reaction products by simple search for all possible variants of ligand structures.A great body of evidence has been accumulated in this field, the effect of such structural parameter of a complex as symmetry deserves special mentioning. Apparently, it is the difference in the symmetries of structur- ally related complexes, viz., {Rh[(S,S)-chiraphos](nbd)}ClO4 (I) Chiral ligand (L) L: Rh 10 11 12 13 14 14 15 16 5889 13244H Cl Pri N Ru Pri Cl PPh2 9 Me Me O O O O N N Et R R 11: R = Pri 12: R = But S N CO2Me HN 14 N N O O N Pri 16 Pri Achiral ligand 7 7 0 CCl4 CCl4 77 83 90 84 7 7 715 98 7 7 720 99 794 98 7 7 005 2AgBF4 AgBF4 H NPPh2 CO2EtO NPri V A Pavlov Ref.ee (%) Yield (%) Solvent T /8C 75 75 48.1(R) 64.0 (R) 65.0 (R) 86.7 (R) 114 115 115 116 97.6 (R) 58 75.6 (R) 58117, 118 119 95.0 (S) 90.0 (S) 7THF and {Rh[(R)-prophos](nbd)}ClO4 (II), that can explain different enantioselectivities of hydrogenation in the presence of these catalysts (Table 5). Table 5. The enantiomeric excess of the product in asymmetric hydro- genation of enamides on complexes (I) and (II).33, 120, 121 Substrate (S) II (S :Rh=250) I (S :Rh=100) EtOH THF EtOH THF 74 (R) 89(R) 91(S) 90(S) 80 (R) 88(R) 92(S) 89(S) 80 (R) 83(R) 89(S) 87(S) 100 (R) 93(R) 87(S) 87(S) 99 (R) 95(R) 93(S) 91(S) 87 (R) 72(R) 87(S) 87(S) AcDPhe AcDTyr AcDDOPA AcDLeu BzDPhe BzDLeu Note.Hereinafter, the following designations were used: AcDPhe is (Z)-a- acetylaminocinnamic acid; BzDPhe is (Z)-a-benzoylaminocinnamic acid; AcDTyr is N-acetyldehydrotyrosine; AcDDOPA is 2-N-acetyl-b-(4- hydroxy-3-methoxyphenyl)-a-dehydroalanine; AcDLeu is N-acetyldehy- droleucine; BzDLeu is N-benzoyldehydroleucine. As can be seen from the table, the enantioselectivities achieved with the asymmetric complex II are less prone to variation than with the C2-symmetrical chiral complex I. The ee value in the reactions catalysed by the former is virtually independent of the solvent or the nature of the substituent in the substrate. III. Catalyst ± product configurational relationships in reactions on C2-symmetrical chiral metal complexes A search for relationships between configurations of catalytic complexes and products of asymmetric reactions is best carried out for complexes with ligands of the same type.Chiral bis(diphe- nylphosphine) ligands which form C2-symmetrical complexes or C2-symmetrical chelate rings (without substituents) meet these requirements. Me Me Me Me Ph2P Ph2P Ph2P Ph2P PPh2 (R,R)-norphos PPh2 (S,S )-norphos PPh2 (S,S )-chiraphos PPh2 (R,R)-chiraphos Ph Ph Me H11C6 Ph2P PPh2 Ph2P PPh2 Ph2P (S )-phenphos PPh2 (R)-cycphos PPh2 (R)-phenphos Ph2P (R)-prophosStructural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions Ph Ph P P An Ph2P PPh2 An(R,R)-dipamp PPh2 (S,S )-phellanphos Ph2P (R,R)-dpcb Me Me O O Ph2P Ph2P Ph2P Ph2P PPh2 (R,R)-dipmcb PPh2 (R,R)-diop PPh2 (S,S)-dipmcb PPh2 (R,R)-dipmcp O Ph Me Me O Me O O O Ph2P Ph2P Ph2P PPh2 (S,S )-dipmch PPh2 (S,S )-diop PPh2 (S,S )-glup Ph2P Ph2P PPh2 (R)-binap PPh2 (S )-binap Regular relationships between the configurations of products and catalytic complexes in hydrogenation reactions on rhodium ± phosphine catalysts have been established only for complexes with C2-symmetrical chelate rings.Formal configurational relation- ships between catalysts and products are not always related to concrete reaction mechanism.A mnemonic dependence was proposed by Knowles in 1983.122 Me H H H H O O O Rh C C P P HO OH O O O Me Favourable arrangement Unfavourable arrangement This dependence follows the `quadrant rule' { which relates the structure of the rhodium complex containing the dipamp ligand to the orientation (or coordination) of the substrate in the inter- mediate complex. It is assumed that the substituents occupying the hatched quadrants (i.e., those blocked by the phenyl groups edge facing the viewer; the metal atom is located in the centre of the coordinates), experience larger steric hindrances. Therefore, the bulky carboxy group occupies preferably the unhatched quad- rant. It is in this orientation that the formation of a particular enantiomer is experimentally observed.These graphical represen- tations were used to describe the enantioselective behaviour of binap 14 and duphos [bis(phospholano)benzene] 124 complexes in asymmetric hydrogenation reactions. The use of such graphical representations implies that the asymmetric induction step coincides with the coordination of the substrate in the intermediate complex. However, this conclusion { For the first time, this structural approach in the form of the `octant rule' has been used to describe the chiral environment of carbonyl groups in steroids.123 demands further verification. The scheme proposed for the description of catalyst ± product configurational relationships in the hydrogenation of a-, b- and g-substituted ketones on ruthe- nium complexes containing binap ligands is even more formal.The configuration of the product depends exclusively on the configuration of the complex, rather than on the position of the functional group in the ketone.14 O X R Y O R O Z R where X, Y, Z are N, O or Hal. This has led to the development of a formal scheme for the formation of an intermediate complex using hydrogenation of a b-oxo ester as an example (Scheme 2). A similar scheme of configurational relationships has been proposed for hydrogenation of substituted b-oxo esters.125 The configuration of the product does not depend on the nature of the substituent at the carbonyl group, whereas the change in the order of priority changes the configuration symbol.X OP Ru R1 H Favoured OH O R1 X=Hal. 1043 OH H2, (R)-binap-Ru(II) X R OH H2, (S )-binap-Ru(II) X R Y OH H2, (R)-binap-Ru(II) R Y OH H2, (S )-binap-Ru(II) R OH Z H2, (R)-binap-Ru(II) R OH Z H2, (S )-binap-Ru(II) R Scheme 2 (R)-binap-Ru(II) H2 R1 O CO2R2 H2 (S)-binap-Ru(II) X O P PO P O Ru R1 R2O OR2 H Unfavoured OH O R1 R2O OR21044 O OH O O H2 OEt OEt Ph P 2 (S) Ru O O OH O H2 PPh2 Cl Cl OEt OEt (R) (S )-binap O OH O O H2 OEt OEt Ph P 2 (R) Ru O OH O O PPh2 H2 Cl Cl OEt OEt (S ) (R)-binap A large body of evidence has been gained concerning the application of rhodium complexes with these ligands as catalysts in hydrogenation of the same kind of substrates, viz., a-acetami- docinnamic acid derivatives. CO2R1 CH2Ph RhL* * H2 R2CONH Ph NHCOR2 CO2R1 * L*= .Ph2P PPh2 Earlier, it was suggested that enantioselective effects of such complexes are related to chiral arrangements of the phosphine phenyl groups.120 We suppose that the arrangement of phenyl groups forming the chiral environment of the metal atoms can be related to the sign of helicity or the conformation of the chelate complex (Scheme 3) (view from the side of the metal atom; the chelate complex without substituents is shown by a dotted line). The schematic representations of the helix turns are shown in the right part of the Scheme, where the P-helix moves off clockwise, while theM-helix moves off anti-clockwise.The analysis of molecular models has demonstrated that the conformations of l- or d-five-membered chelate complexes can be distorted to some extent, but their interconversions are either impossible (e.g., for chelates based on cyclic 1,2-diphosphines, such as norphos, phellanphos, dpcb, etc.) or result in the for- mation of chelate complexes with unfavourable axial positions of substituents 126 ± 128 (e.g., prophos, chiraphos, cycphos, phenphos, etc.). In the case of seven-membered chelate complexes, a change in the sign of chirality in the orientation of the phenyl groups is also accompanied by the transition from the favourable chair-l (or -d) conformation to the unfavourable boat-d (or -l) confor- mation.However, in this case the advantage is less apparent. Ph Ph Ph Ph P M P P M P Ph Ph Ph Ph chair-l boat-d A correlation between the conformations of five-membered chelate rings of rhodium catalytic complexes with bis(diphenyl- phosphine) ligands and the configurations of the hydrogenation products of acetamidocinnamic acid and its derivatives was found in 1982 simultaneously and independently by two groups of authors (see Refs 129 ± 131 and 132).} The l-conformation of the } There are many examples of simultaneous publications by different research groups of experimental results concerning asymmetric hydro- genation on rhodium phosphine catalysts in which the same catalyst (ligand) was used or, as in our case, the same regularity of the reaction was established (see, e.g., Refs 133 and 134; 135,136 and 137; 138 and 139, etc.).Five-membered chelate complexes Phax Pheq : P P M Pheq Phax l Phax Pheq P P M Pheq Phax d Seven-membered chelate complexes Ph Ph P : M P Ph Ph chair-l Ph Ph : M P P Ph Ph chair-d Ph Ph : M P P Ph Ph boat-l Ph Ph : M P P Ph Ph boat-d chelate complex determines the formation of (S)-, whereas the d-conformation determines the formation of (R)-products. Quite naturally, this effect is directly related to the chiral positions of the phosphine phenyl groups corresponding to these conformations rather than to the conformations themselves. The conformations and the conformational rigidities of chelate rings in catalytic complexes play a crucial role in stabilisation of chiral positions or helical chiralities of the phosphine phenyl groups.In a simplified form, this regularity may be described by the formula: l-(S), d-(R).140, 141 Another representation of this rule appears to be more complete: (l) : PPh2 Ph2P M-helix (S) (d) : PPh2 Ph2P P-helix (R) Some known ligands of rhodium complexes and the results of hydrogenation of a-acetylamidocinnamic acid derivatives in the presence of these complexes are shown in Table 6. As can be seen, the results of hydrogenation in the presence of five-membered chelate rhodium complexes obey the l-(S), d-(R) rule 140, 141 irrespective of whether theC2 symmetry axis is inherent V A Pavlov Scheme 3 Phax M Phax M-helix Phax M : Phax P-helix M MM M MMStructural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions Table 6.Arylphosphine ligands forming five-membered rhodium chelate complexes and hydrogenation of a-acetylaminocinnamic acid and its derivatives in the presence of these complexes. Ligand Ligands forming chiral C2-symmetrical complexes OMe Ph P P Ph MeO (R,R)-( ± )-dipamp Et Ph P P Ph Et Me Me Ph2P PPh2 (S,S)-( ± )-chiraphos Me Me Ph2P PPh2 (R,R)-(+)-chiraphos Ph2P PPh2 (R,R)-(+)-dpcp Ph2P PPh2 (S,S)-( ± )-dpcp Bz N Ph2P PPh2 Ligands forming chiral C2-symmetrical complexes (without substituents in the chelate rings) Me Ph2P PPh2 (R)-(+)-prophos Pri Ph2P PPh2 (R)-(+)-valphos Ph3COCH2 Ph2P PPh2 ButOCH2 Ph2P PPh2 PhCH2OCH2 Ph2P PPh2 Ph3COCH2 Ph2P PPh2 Conforma- Substrate tion (helix) l (M) AcDAla AcDPhe d (P) AcDAla AcDPhe d (P) AcDAla AcDPhe l (M) AcDAla AcDPhe BzDPhe l (M) AcDPhe d (P) AcDPhe BzDPhe BzDPheOMe l (M) AcDPhe l (M) AcDPhe BzDPhe AcDPhe BzDPhe l (M) BzDPhe AcDAla l (M) AcDPhe AcDAla l (M) AcDPhe AcDAla l (M) AcDPhe AcDAla AcDPhe AcDAla d (P) AcDAla Solvent ee (%) Type of the catalytic complex MMor E CC 90 (S ) 94 (S ) 93 (R) i EPr C 90 (R) C EE 91 (R) C 89 (R) C EEE CCC 91 (S ) 88 (S ) 94 (S ) M±B 91 (S ) N M±B M±B M±B NNN 90 (R) 98 ± 100 98 ± 100 M 99 (S) C EEMM CCNN 90 (S ) 91 (S ) 83 (S ) 88 (S ) EE NN 95 (S ) 88.6 (S ) EE CC 85 (S ) 78 (S ) EE CC 86 (S ) 92 (S ) 87 (S ) 91 (S ) 86 (S ) 75 (S ) 78 (R) EEEEE CCCCC 1045 /atm Ref.pH T /8C 2 142 64,a 143 50 50 33 50 50 144 a 144 a 33 20 1 33 20 1 33 145 145 145 1 ± 4 1 ± 4 1 ± 4 20 20 20 125 30 1 125 146 146 30 30 30 111 20 40 36 120 120 147 147 25 25 20 20 1111 148 148 20 20 11 149 149 20 20 11 150 150 20 20 11 150 150 149 149 149 20 20 20 20 20 111111046 Table 6 (continued).Ligand Ligands forming chiral C2-symmetrical complexes (without substituents in the chelate rings) cyclo-Hex Ph2P PPh2 (R)-(+)-cycphos PhCH2 Ph2P PPh2 (R)-(+)-phephos Ph Ph2P PPh2 (R)-( ± )-phephosOO O Hex-cyclo OCH2Ph PPh2 glucophos Ph2P OO O Hex-cyclo OCH2Ph PPh2 idophos Ph2P Ph Ph2P PPh2 (S)-(+)-phenphos PPh2 Ph2P (R,R)-( ± )-norphos PPh2 Ph2P (S,S)-(+)-norphos PPh2 Ph2P (+)-renophos PPh2 Ph2P (S,S)-phellanphos HPPh2 Ph2P (R,R)-nopaphos Note. Hereinafter, the following designations are used. For the substrate: AcDPheOMe is (Z)-a-acetylaminocinnamic acid methyl ester; AcDAla is a-acetylaminoacrylic acid; for the type of the catalytic complex:Nis neutral including that formed in situ; Cis cationic; for the solvent:Mis methanol; E is ethanol; B is benzene; T is tetrahydrofuran.a Data from X-ray diffraction analysis.Conforma- Substrate tion (helix) l (M) AcDPhe AcDAla BzDPhe l (M) AcDPhe AcDAla l (M) AcDPhe AcDPheOMe AcDAla BzDPhe l (M) AcDPhe AcDPheOMe d (P) AcDPhe AcDPheOMe d (P) AcDPhe AcDPhe AcDPheOMe BzDPhe BzDPhe AcDPheOMe l (M) AcDPhe AcDAla d (P) AcDPhe d (P) AcDPhe AcDAla d (P) AcDPhe l (M) AcDPhe AcDAla Solvent ee (%) Type of the catalytic complex MMM NNN 84 (S ) 93 (S ) 90 (S ) E± B (2 : 1) E± B (2 : 1) CC 99 (S ) 84.5 (S ) MMMM NNNN 79 (S ) 88 (S ) 82 (S ) 84 (S ) TT CC 54 (S ) 41 (S ) TT CC 56 (R) 27 (R) TTTTTT CCCCCC 78 (R) 82 (R) 8 (R) 84 (R) 76 (R) 76 (R) 95 (S ) 90 (S ) CC EEM 97 (R) C MM CC 95 (R) 95 (R) E C 95 (R) EE CC 80 (S ) 81 (S ) V A Pavlov /atm Ref.pH T /8C 2 147 147 147 20 20 20 111 148 148 20 20 20 20 147 147 147 147 20 20 20 20 1111 151 151 20 20 11 151 151 20 20 11 152 152 152 152 152 152 30 30 30 30 30 30 111111 20 20 153 a 153 a 11 153 20 1 154 154 20 20 11 155, 156 25 1 155 155 15 15 25 25Structural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions in the whole complex or in its chelate ring without substituents.The implementation of this rule is not influenced by the nature of substituents and experimental conditions. The situation is more complicated in the case of seven-membered } chelate rhodium complexes in which the conformational transitions chair-l ± boat- d and chair-d ± boat-l are possible (other intermediate conforma- tions have also been described 158). Such transitions change the helicity sign of the phosphine phenyl groups, as a result of which the configuration of the product also changes. Unfortunately, X-ray diffraction analysis does not provide any unambiguous information about the conformations of the corresponding mol- ecules in solution and especially in the intermediate catalytic complex, since the factors involved in crystal formation are stronger than those inducing conformational changes.Indeed, the conformations acquired by the diop ligand in Ir[(S,S)-(+)-diop](cod)Cl 159 and [(R,R)(7)-diop]NiCl2 160 crys- tals differ from the more favourable ideal conformations chair-l and chair-d. In the case of [(R,R)(7)-diop]NiCl2, the distortions are so great that the position of the phenyl groups relative to the } Six-membered chelate complexes have been studied relatively little, apparently due to their low efficiencies in asymmetric reactions resulting from conformational labilities of six-membered compounds. However, one such complex, viz., Rh(S,S)-skewphos [skewphos is 2,4-bis(diphenyl- phosphino)pentane], manifests high enantioselectivity [AcDPhe, 93% (R)].157 Table 7.Arylphosphine ligands forming seven-membered rhodium complexes for which the chair-l-(S), chair-d-(R) rule holds. Ligand Ligands forming chiral C2-symmetrical complexes Me Me O O PPh2 Ph2P (R,R)-( ± )-diop Me Me O O PPh2 Ph2P (S,S)-(+)-diop Me Me O O P(C6H4Me-o)2 (o-MeC6H4)2P Me Me O O P(C6H4Me-m)2 (m-MeC6H4)2PMe Me O O P(C6H3Me2-2,5)2 (2,5-Me2C6H3)2P metal becomes close to symmetrical. If such conformations were maintained in solution, this would contradict the Curie ± Pasteur principle (`there is no asymmetry without asymmetry') and would be in conflict with the very logic of asymmetric catalysis.The same complex, viz., [(R,R)-(7)-diop]NiCl2, manifests high enantiose- lectivity in various cross-coupling reactions;161 hence, its phenyl groups occupy chiral positions relative to the metal. Therefore, the opposite statement, viz., if the l-(S), d-(R) rule is observed, the true conformations of the ligands in the complexes are close to the predictable chair conformation, seems to be more applicable to seven-membered chelate rhodium complexes. Some examples of fulfilment of the chair-d-(R), chair-l-(S) rule are given in Table 7,130, 140 viz.: Ph2P PPh2 chair-l It is easy to ascertain that in the case of both seven-membered chelate complexes with second-order symmetry axes and com- plexes where only chelate rings without substituents possess C2 symmetry the chair-l-(S), chair-d-(R) rule [or the boat-l-(S), ee (%) Substrate Conforma- tion (helix) 82 (R) 73 (R) AcDPhe AcDAla chair-d (P) 81 (S ) AcDPhe chair-l (M) 27 (R) AcDPhe chair-d (P) 87 (R) AcDPhe chair-d (P) 44 (R) AcDPhe chair-d (P) M : ; M-helix (S) Solvent Type of the catalytic complex E± B (2 : 1) E±B (2 : 1) NN E±B (2 : 1) N E±B (2 : 1) N E±B (2 : 1) N E±B (2 : 1) N 1047 M : Ph2P PPh2 P-helix chair-d (R) Ref./atm pH T /8C 2 1.1 1.1 20 20 65, 66 65, 66, 159 a 162, 163 1.0 20 164 1.0 20 164 1.1 20 164 1.1 201048 Table 7 (continued). Ligand Ligands forming chiral C2-symmetrical complexes Me Me O O P(Np-2)2 (2-Np)2P CH2O(CH2CH2O)5Me O O PPh2 Ph2P cyclo-Hex O OPPh2 Ph2P Me Me O OP P Ph Ph PPh2 Ph2P (R,R)-( ± )-dipmcb PPh2 Ph2P (S,S)-(+)-dipmcb PPh2 Ph2P (R,R)-( ± )-dipmcp PPh2 Ph2P (S,S)-(+)-dipmcp PPh2 Ph2P PPh2 Ph2P Substrate Conforma- tion (helix) AcDPhe chair-d (P) AcDPhe chair-d (P) AcDPhe chair-d (P) AcDPhe chair-d (P) AcDPhe chair-d 86 (R) (P) AcDAla 72 (R) 91 (R) AcDPhe AcDPhe chair-l (M) AcDPhe chair-d 63 (R) (P) AcDAla 72 (R) AcDPhe chair-l 73 (S ) (M) AcDAla 68 (S ) AcDPhe chair-l 81 (S ) (M) AcDAla 52 (S ) AcDPhe chair-l (M) Solvent ee (%) Type of the catalytic complex E±B (2 : 1) N 83 (R) E C 64 (R) E±B (1 : 2) 77.5 (R) N E±B (1 : 2) N 71 (R) E± B (2 : 3) E± B (2 : 3) E± B (1 : 2) NNN E± B (1 : 1) C 87 (S ) E± B (1 : 2.3) E± B (1 : 2.3) NN E± B (2 : 1) E± B (2 : 1) NN E± B (2 : 1) E± B (2 : 1) NN E± B (1 : 2.3) N 35 (S ) V A Pavlov Ref./atm pH T /8C 2 164 1.1 20 165 1.1 25 164 1.1 20 166 1.1 20 167 167 168 1.0 1.0 1.1 25 25 25 169 1.0 20 167 167 1.0 1.0 25 25 168 168 1.0 1.0 25 25 168 168 1.0 1.0 25 25 170 1.0 25Structural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions Table 7 (continued). Ligand Ligands forming chiral C2-symmetrical complexes PPh2 Ph2P PPh2 Ph2P PPh2 Ph2P PPh2 Ph2P Ph2P PPh2 Me Me HN NH Ph2P PPh2 Ph Ph HN NH Ph2P PPh2 HN NH Ph2P PPh2 HN NHN PPh2 Ph2P O OPPh2 Ph2P Ph N O OO O Ph2P PPh2 Conforma- tion (helix) chair-d N (P) AcDAla 39 (R) N chair-l (M) chair-d (P) chair-d (P) chair-l (M) chair-l (M) chair-l (M) chair-d C (P) AcDPhe 41 (R) CC chair-l (M) chair-l N (M) AcDAla 79 (S ) N chair-l (M) Solvent ee (%) Substrate Type of the catalytic complex 36 (R) AcDPhe E± B (2 : 1) 25 E± B (2 : 1) 25 E± B (2 : 1) 25 N 81 (S ) AcDPhe 74 (R) N E±B(2:1) 7 AcDAla E± B (2 : 1) 20 79.5 (R) N AcDPhe E N 35 (S ) AcDPhe E C 45 (S ) AcDPhe E C 93.8 (S ) AcDPhe 49 (R) AcDPhe EEE 47 (R) AcDPhe E C 41 (S ) AcDPhe 68 (S ) AcDPhe EEM N 36 (S ) AcDPhe 1049 /atm Ref.pH T /8C 2 168 168 1.0 1.0 168 1.0 169 1.0 171 1.1 171 1.0 25 137, 172 1.0 20 173 1.0 25 136 137 135 1.0 1.0 1.0 20 20 20 137 8.0 20 174 174 50 50 20 20 175 50 201050 Table 7 (continued).Ligand Ligands forming chiral C2-symmetrical complexes Bn N O O O O Ph2P PPh2 Ph2P PPh2 (R)-(+)-binap Ph2P PPh2 (S)-( ± )-binap Ph2P PPh2 RhMeOH MeOHHb Ha Ha Hb Ph2P PPh2 (R,R)-bicp Ligands forming chiral C2-symmetrical complexes (without substituents in the chelate rings) Me HN NH PPh2 Ph2P Ph H O O Ph2P PPh2 Conforma- tion (helix) chair-l (M) Ph P Ph boat-l (M) boat-d C (P) AcDAla 67 (R) CCC boat-d (P) Ha Ph P Ph boat-l, favourable conformation (M) Ph Ph P Ha boat-d, unfavourable conformation (see b) chair-d (P) chair-d (P) Ph Rh P Ph Hb Ph P Rh Ph Hb Ha Hb Ha P Rh Ph Ph Hb Solvent ee (%) Substrate Type of the catalytic complex M N 16 (S ) AcDPhe E C 100 (S ) BzDPhe 84 (R) AcDPhe 84 (R) 96 (R) AcDPhe BzDPhe EEEEE C 98 (R) BzDPhe TT CC 89 (S ) 99 (S ) AcDPhe BzDPhe E C 28 (R) AcDPhe B± E N 79.5 (R) AcDPhe V A Pavlov /atm Ref.pH T /8C 2 176 1.0 20 175 3 ± 4 20 178 177 177 177 3 ± 4 3 ± 4 3 ± 4 3 ± 4 20 20 20 20 179 1.0 20 180 180 1.0 1.0 20 20 136 1.0 20 163 1.0 20Structural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions Table 7 (continued).Ligand Conforma- tion (helix) Ligands forming chiral C2-symmetrical complexes (without substituents in the chelate rings) chair-l Ph O C (M) AcDPhe 60.5 (S ) N O H O OPh O O PPh2 chair-l Ph2P Ph-a-glup Ph O C (M) AcDPhe 87 (S ) N O H O OPh O O PPh2 Ph2P Ph-b-glup chair-l Ph O O C (M) AcDPhe 73 (S ) CC H O OMe O O PPh2 Ph2P Ph O chair-d (P) O H O OMe O O PPh2 Ph2P chair-l Ph N (M) AcDAla 91 (S ) N O O H O OMe O O PPh2 Ph2P Et O chair-l (M) O H O OMe O O PPh2 Ph2P O O chair-l (M) O Hex-cyclo O OCH2Ph O Ph2P PPh2 idophinite O O chair-d (P) O Hex-cyclo O OCH2Ph O Ph2P PPh2 glucophinite a Data from X-ray diffraction analysis.b The positions of the phosphine phenyl groups characteristic of the boat-d conformation (in this case, boat-l and boat-d cannot be mutually interconverted without the bond cleavage) is associated with considerable steric hindrances. boat-d-(R) for the atropoisomeric binap ligands] is obeyed.140, 141 Thus, the intermediate catalytic complexes have predominantly the chair-l and chair-d conformations in solution. In the chelate Solvent ee (%) Substrate Type of the catalytic complex 54 (S ) AcDPhe MM 96 (S ) AcDPhe MM 61 (S ) AcDPhe EME 75 (S ) AcDPhe E C 70 (R) AcDPhe 91 (S ) AcDPhe EEM 80 (S) N AcDPhe T C 54 (S ) AcDPhe T C 40 (R) AcDPhe complexes, the atropoisomeric ligands binap exist in the only possible boat-l, boat-d conformation.1051 /atm Ref. pH T /8C 2 181 181 1.0 1.0 25 25 181 181 1.0 1.0 25 25 182 181 183 1.0 1.0 1.0 25 25 0 182 1.0 0 184 184 1.0 1.0 30 30 185 50 725 151 1.0 20 151 1.0 251052The ligands which form complexes with seven-membered chelate rings, but do not formally follow the chair-l-(S), chair-d- (R) rule are shown in Table 8. Let us consider molecular models. In the ligands based on trans-disubstituted cyclopropane, the a-angles between the planes of the rings and the substituents are close to 123 8. Ph Ph a a Ph a Ph P P P M P a Ph Ph a Ph Ph Ma chair-l boat-d The boat-d conformation of the chelate ring based on this ligand is preferable to the chair-l conformation, since the a-angles are closer to 123 8, and the l-(S), d-(R) rule is not violated in this case.The structures of the following four ligands, viz.: Me Me N Ph Ph N Ph Ph Rh P P Rh P P Ph Ph N N Ph Me Ph Me chair-l(1) boat-d(1) Me Me Ph Ph N N Ph Ph M P P P P Rh Ph Ph N N Ph Ph Me Me chair-l(2) boat-d(2) are of the same kind. In our opinion,140 these ligands can theoretically have four conformers, viz., chair-l(1), boat-d(1), chair-l(2) and boat-d(2), owing to the well-known ability of pyramidal nitrogen atoms to invert. Of the four possible conformers of the ligand (S,S)-Medi- op(N), the boat-d(2) conformer is the most sterically favourable. The chair-l(1) conformation is characterised by spatial proximity of the Ph and Me groups, whereas in the chair-l(2) and boat-d(1) conformations non-bonding interactions of the methyl groups with the apical orbitals of rhodium atoms in planar-square complexes are observed.Another disadvantage of the chair-l(2) and boat-d(1) con- formations is the interaction of Me and Ph groups (they are marked by bidirectional arrows). We have carried out a series of non-empirical quantum-chemical calculations for the interactions of isolated fragments of these structures.140 It was found that the boat-d(2) conformation (or its mirror-opposite boat-l(2) confor- mation) is more favourable for NMe-ligands taking into account the interaction between the Me and Ph groups. In the presence of complexes with these conformations of ligands, the reaction should proceed according to the d-(R) [or l-(S)] rule as is actually the case in experiment.Formally, the o-MeOC6H4-phosphine diop analogue does not conform to this rule, which can also be explained by stabilisation of the `unfavourable' conformation. The conformational equili- brium in complexes containing this ligand seems to be shifted towards the boat conformation due to the interactions of the Me Rh P P O Rh P P O Me boat-l chair-d V A Pavlov electron pairs of the methoxy group oxygens with the apical orbitals of the rhodium atoms. The existence of similar coordination bonds between the o-MeOC6H4 groups and rhodium atoms in complexes with dipamp ligands has been confirmed by X-ray diffraction analy- sis.64 The boat-l conformation is geometrically more fit for the formation of such bonds than the chair-d conformation.It can therefore be assumed that the l-(S), d-(R) rule is also applicable to the rhodium complex with this ligand. The catalytic system containing the phosphinite ligand [Rh(nbd)2]ClO4/CuCl (see Table 8) cannot be considered as an exception to this rule, since it is possible that it is a bimetallic cluster or a complex where the necessary stereochemistry that manifests the catalytic activity under the reaction conditions is violated. Examination of molecular models of the other two ligands leads to a conclusion that only the boat-l conformation in the strained bridged structure of 9,10-dihydro-9,10-ethanoanthra- cene enables the formation of seven-membered chelate complexes with a metal, viz., in this case, the l-(S) rule is fulfilled.In the case of 1,4-bisdiphenylphosphino-(2S,3S)-diphenylbutane, the boat-d conformation of the chelate complexes, in which both phenyl rings occupy equatorial positions and thus give an additional gain in energy, is more favourable.127 Therefore, the l-(S), d-(R) rule holds true for complexes containing this ligand. Five-membered chelate rhodium complexes based on alkyl- phosphine ligands with second-order symmetry axes (Table 9), conform also to this rule,{ however, in its basic `helical' form. It is impossible to isolate the helical component of the rhodium complex with the (S,S)-Pri-cnrphos ligand without recourse to X-ray diffraction analysis.However, the P-helical structure can be predicted from the configuration of the hydro- genation product. The data presented in Table 9 suggest that the product configuration is determined by the helical component present in all complexes with alkylphosphine ligands. Sometimes, the enantioselectivity of reactions in the presence of bisdiphenylphosphine complexes is associated with chirality, which is determined by a definite inclination of the planes of the phenyl groups relative to one another.194 However, the values and the signs of enantioselectivity in complexes with alkylphosphine ligands,124, 191 ± 193 cyclodiop 166 and Cydiop,195 which are devoid of such chirality, do not differ from the corresponding parameters of complexes with bisdiphenylphosphine ligands.Therefore, the helical component makes the main contribution. An experimental proof of the effect of the helical component on enantioselectivity follows from the enantioselective properties of the complex the achiral ligands in which form helices around the metal ions.196 The ligands forming eight- and nine-membered chelate com- plexes with C2 symmetry axes (with the exception of atropoiso- meric non-substituted aminophosphines 197 ± 200) are not very effective in the majority of asymmetric hydrogenation reactions. Enantioselectivity can be increased either through the use of bimetallic complexes or through a decrease in reaction temper- ature.There is no reason to suppose that the above-considered rule concerning the conformation of five- or seven-membered bis(di- phenylphosphine) chelate complexes with the C2 symmetry axes is only applicable to hydrogenation reactions. Presumably, similar rules exist for other reactions, too. Indeed, the hydrosilylation of acetophenone by diphenylsilane in the presence of rhodium complexes proceeds in accordance with the l-(S), d-(R) rule (Table 10). { In our opinion, the critical attitude 190 to the conformational expression of this rule, viz., l-(S), d-(R), is too formal.Structural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions Table 8. Arylphosphine rhodium complexes with seven-membered chelate rings which do not formally obey the chair-d(S), chair-d(R) rule. Ligand PPh2 Ph2P Me Me N Me N Me PPh2 Ph2P (S,S)-Mediop(N) N Me N Me PPh2 Ph2P Ph Ph N Me N Me PPh2 Ph2P Me N Me Me N PPh2 Ph2P Me Me O O (o-MeOC6H4)2P P(C6H4OMe-o)2 O O Ph2P OPPh2 (see b) Ph2P PPh2 Ph Ph Ph2P PPh2 a The favourable conformation is indicated (see text).b The catalytic system [Rh(nbd)2]ClO4/CuCl was studied using this ligand. Solvent ee (%) Conformation a Substrate (helix) Type of the catalytic complex AcDPhe boat-d E± B (2 : 1) E± B (2 : 1) 15 (R) (P) AcDAla 23 (R) NN E± B C 30 (R) AcDPhe boat-d(2) (P) AcDPhe boat-l(2) 92 (S ) (M) AcDAla 89 (S ) EE CC AcDPhe boat-d(2) 68.4 (R) (P) AcDAla 86.3 (R) EE CC E C 70 (S ) AcDPhe boat-l(2) (M) AcDPhe boat-l MMM CCC 31 (S ) (M) AcDPheOMe 71 (S ) 8 (S ) AcDAla E±B 90 (S) C AcDPhe 7 E N 50 (S ) AcDPhe boat-l (M) AcDPhe boat-d 54 (R) (P) AcDAla 61 (R) EE CC 1053 pH /atm Ref.T /8C 2 167 167 1.0 1 25 25 186 20 1 187 136 25 20 11 135, 173 135, 173 25 25 55 136 1 20 188 188 188 20 20 20 111 189 1.48 715 171 25 1 173 173 25 25 111054 Table 9. Alkylphosphine ligands forming five-membered chelate rhodium complexes and hydrogenation of a-acetylaminocinnamic acid and its derivatives in the presence of these complexes. Ligand Conforma- tion (helix) P-helix Me RP P CH2 Me R (R,R)-miniphos But But cyclo-C6H11 Pri Ph Me R P P R Me BisP* But Et3C AcDPheOMe 1-Adamantyl cyclo-C5H9 cyclo-C6H11 Pri Pri 77 P P Pri Pri (R,R)-Pri-bpeR R P P R Me Me Et Et Prn Prn R (S,S)-R-duphos Pri Pri 7 P P Pri Pri (S,S)-Pri-cnrphos M-helix R R P P R R Me Me Et Et Prn Prn Pri Pri 77 P P Pri Pri (R,R)-Pri-duphos a Data from X-ray diffraction analysis. The cross-coupling reaction has been most studied for secon- dary alkyl or arylalkyl Grignard reagents which react in the following way:59 R1 Fast C MgX R2 H racemate R1 R2 *CR3 H ee (%) Substrate 97 (R) >99.9 (R) 99.1 (R) 98 (R) 26 (R) AcDPhe AcDAla AcDAla AcDAla AcDAla AcDPhe 98.4 (R) 94.7 (R) 99.9 (R) 43.0 (R) 47.1 (R) AcDPheOMe AcDPheOMe AcDPheOMe 93 (R) 96.4 (R) AcDPheOMe AcDAlaOMe 98 (R) 99 (R) 99 (R) 99.4 (R) >99 (R) 99.8 (R) AcDPheOMe AcDAlaOMe AcDPheOMe AcDAlaOMe AcDPheOMe AcDAlaOMe 74 (R) AcDPheOMe 85 (S ) 91.4 (S ) 93 (S ) 98.1 (S ) 92 (S ) 97.7 (S ) AcDPheOMe AcDAlaOMe AcDPheOMe AcDAlaOMe AcDPheOMe AcDAlaOMe 87 (S ) 95.4 (S ) AcDPheOMe AcDPheOMe The cross-coupling of BusMgX with PhY in the presence of bis(diphenylphosphine) nickel complexes: R1 R3X1 C XMg R2 ML* H obeys the rule l-(R), d-(S) (Table 11).(1) Solvent Type of the catalytic complex MMMMM CCCCC MMMMM CCCCC MM CC MMMMMM CCCCCC7 7 7 7 193 MMMMMM CCCCCC MM CC MgX+PhY THF or ether, from 0 to 20 8C V A Pavlov Ref./atm pH T /8C 2 191 191 191 191 191 a 20 20 20 20 20 11111 192 192 192 192 192 a 20 20 20 20 20 22222 124 124 a 20 20 22 124 124 124 124 124 124 20 20 20 20 20 20 222222 124 124 124 124 124 124 20 20 20 20 20 20 222222 124 124 20 20 22 * NiCl2 P P * PhStructural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions Table 10. Hydrosilylation of acetophenone by diphenylsilane on chiral rhodium complexes.Catalytic system [Rh(cod)Cl]2/(S,S)-norphos [Rh(cod)Cl]2/(S,S)-chiraphos O (S,S)-norphos Rh(cod) O [Rh(cod)Cl]2/(S,S)-diop [Rh(cod)Cl]2/(R,R)-diop [Rh(cod)Cl]2/(S,S)-glup [Rh(cod)Cl]2/(S,S)-cyclo-Hex . diop O (R,R)-diop Rh(cod) O a Hydrosilylation of propiophenone. b Hydrosilylation with a-NpPhSiH2. Table 11. The optical yields of asymmetric cross-coupling products of BusMgX with PhY in the presence of nickel complexes. * (NiCl2/ ). P P * P P Conforma- Ref. tion of the complex (R,R)-dipamp (R)-prophos llll 216 34 210 210 213 214 213, 215 14.5 (S ) 26.7 (R) 214, 217 10.9 (R) 214, 217 7 (R)-cycphos (R)-phenphos l (S,S )-chiraphos d (R,R)-norphos l (R,R)-(7)-diop chair-d chair-d 209 209 212 209 209 209 209 161 218 a Presumably, the (S) conformation is indicated erroneously, since more recent studies by the same authors 209, 211 suggest the formation of (R)- products in analogous experiments.b In this case, X=Br, Y=Cl. The following catalytic reactions also represent cross-cou- pling: k1 [(S)-R1] (S)-R1MgX +R2X k2 (R)-R1MgX [(R)-R1] RMgX+ Y RMgX+ Y * RMgX+ Y * RMgX+ R Y Conformation of the complex Ref. 153 202, 203 dd 153 d 159, 176 chair-l 159, 176 chair-d 159, 176, 200 159, 176 chair-l chair-d 159, 176 chair-d R1 MgX + R3 Y R2 R4 R1 Optical yield (%) MgX + Y R2 X=Y= X=Y= Ref. Br Cl 4.5 (R) 14.4 (R) 14.4 (R) 18.5 (R) The reaction of a vinyl halide with a non-racemisable Grignard reagent also belong to the type of reaction (2).218 ± 220 In this case, the resolution of the racemic Grignard reagent occurs due to different reactivities of the enantiomers of the racemate in the presence of chiral catalysts (kinetic resolution). This enantio- selective reaction results in the enrichment of the non-consumed Grignard reagent with one of the enantiomers.4.2 (R) 39.9 (R) 21.0 (S) a 44.0 (R) 45.8 (R) 50.0 (R) 43.1 (S ) 50.7 (R) 717.0 (R) b The results obtained in the study of reaction (3c) are given in Table 12. The data concerning reactions (3a), (3b) and (3d) (Table 13) illustrate the applicability of this rule: , d H R , l R H R2 + (R)-R1MgX Table 12.The optical yields of 2-ethylcyclohexane in asymmetric cross- coupling of EtMgBr with cyclohexene derivatives in the presence of , (2) bisdiphenylphosphine nickel complexes. R2 + (S)-R1MgX * (NiCl2/ ).211 P P * (3a) +MgXY, * R Conformation of the complex P P * (3b) +MgXY, R (3c) R +MgXY, (3d) +MgXY, (R)-phephos (S,S)-chiraphos (R)-phephos (S,S)-chiraphos (R)-prophos (R)-cycphos (R,R)-norphos (R,R)-dipamp ldldllll 1055 Enantiomeric excess Ref. ee (%) 15 (R) 201 3 (R) (see a) 53204 5.4 (R) 31 (R) 22.5 (R) 28 (S) 52 23 (S) 53205 206 55 (R) (see b) 51207 208 47 (S ) 24 (R) 204 32.7 (R) R1 (4) R3 +MgXY, * R4 R2 R1 * (5) R2+MgXY.H R R H Optical yield (%) Substituent in cyclohexene 28.4 (S ) 49.5 (R) 26.5 (S ) 51.2 (R) 29.7 (S ) 16.2 (S ) 48.1 (S ) 11.2 (R) OH OH OPh OPh OPh OPh OPh OPh1056 Table 13. Asymmetric cross-coupling of Grignard reagents (R1MgX) with allylic alcohols, ethers and esters (R2Y) on chiral nickel complexes at room temperature. R2Y R1MgX EtMgBr OPh PhMgBr OPh EtMgBr OPh PhMgBr OPh MeMgBr OH MeMgBr OH OMe OCOMe OMe OCOBut MeO OCOBut PhMgBr Br PhMgBr OH PhMgBr OPh PhMgBr O O OEt PhMgBr OSiMe3 Tables 11 and 12 each contain one entry where the l-(S){(R)}, d-(R){(S)} regularities are not fulfilled. These examples may be regarded as the exceptions that corroborate the rule.More generally, if we consider the helical structure of a ligand around the central metal atom rather than the conformation of the chelate complex, this rule can be formulated as follows.125 In homogeneous asymmetric catalytic hydrogenation, hydrosilyla- tion and cross-coupling reactions C2-symmetrical chiral com- plexes with ligands in the form of an incomplete turn of a helix around the central metal atom catalyse, from the same kind of substrates, the formation of products having strictly specific configurations characteristic of this particular helicity sign, the metal and the reaction. For example, (R)-amino acids are the products of hydrogenation of amino acid precursors on rhodium complexes in which the ligands form incomplete turns of P-helices around the rhodium atoms.If the ligands in such complexes form a part of M-helices around the rhodium atoms, (S)-amino acids are formed. Complex NiCl2/(S,S)-chiraphos NiCl2/(S,S)-chiraphos NiCl2/(S,S)-chiraphos NiCl2/(S,S)-chiraphos NiCl2/(R,R)-diop NiCl2/(R,R)-diop NiCl2/(S,S)-chiraphos MgBr NiCl2/(S,S)-chiraphos MgBr NiCl2/(S,S)-chiraphos MgBr NiCl2/(R,R)-diop NiBr2[(R,R)-dpcp] NiBr2[(R,R)-dpcp] NiBr2[(R,R)-dpcp] NiBr2[(R,R)-dpcp]IV. Catalyst ± product configurational relationships in reactions on asymmetric metal complexes With respect to asymmetric catalytic reactions catalysed by chiral complexes devoid of any symmetry elements (with the exception of C1), the regular catalyst ± product configurational relationships have been gained exclusively in the cross-coupling reactions catalysed by nickel (NiCl2/L*) and palladium (PdCl2/L*) com- plexes with ferrocene-based (amino)diphenylphosphine ligands (L).The structural formulae of such complexes are given below. Conformation Product d R H d R H d R H d R H chair-d H R chair-d H R d H R d H R d H R chair-d R H l H R l H R l H R l H RPPh2 PPh2 NMe2 Fe Fe C C Me Me H (R)-(R)-ppfa (18) (S )-(R)-ppfa (17) V A Pavlov Optical yield (%) Ref. 221 22 (S ) 221 58 (R) 221 17 (S ) 221 58 (R) 222 15 (R) 222 8.5 (R) 223 68 (R) 223 89 (R) 223 68 (R) 224 6 (R) 225 41 (S ) 225 53 (S ) 225 52 (S) 225 67 (S) CH2NMe2 NMe2 H Fe PPh2 (S )-fcpn (19)Structural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions PPh2 Fe Fe CH2Me H (S )-(R)-bppfa (21) (R)-ppef (20) PPh2 Fe NR2 C Me H (S )-(R) (23a ±g) Me (a), 22: Ar = 23: NR2=NEt2 (a), NPri2 (b), NBui2 (c), N NMe (g).N O (f), NExamination of the cross-coupling reaction (1) (Table 14) demonstrates that the configurations of the products obtained on nickel and palladium complexes containing the same ligand are identical. Moreover, even the value of enantioselectivity virtually does not change. The introduction of ortho-substituted aryl radicals at the phosphorus atoms into the ligands decreases the enantioselectivity in comparison with complexes with non-sub- stituted (phenyl) or meta-substituted radicals.The absence of amino groups in the ligands, the introduction of bulkier (in Table 15. Cross-coupling reactions catalysed by nickel and palladium complexes with ferrocene-based ligands (0<T<25 8C). Entry Ligand Nickel complexes 1 (S )-(R)- ppfa 2 (R)-(S )-ppfa 3 (S )-(R)-ppfa 4 (S )-(R)-ppfa Palladium complexes 5 (S )-(R)-ppfa 6 (S )-(R)-bppfa 7 (R)-(S )-ppfa 8 (R)-(S )-bppfa PPh2 PPh2NMe2 C Me H (S )-(R) (22a ± d) Me CH2 H NMe2 Fe PPh2 (R)-(S) (24) Me (b), Grignard reagent Ph MgCl Me Ph MgCl Me p-Tol MgCl Me Ph MgCl Me Ph MgCl Me Ph MgCl Me Me3Si MgCl Ph Me3Si MgCl Ph PAr2 Fe NMe2 C Me PPh2 Fe NMe2 C Me H (S )-(R) (25) OMe Me (d).(c), Me (e), (d),N Alkenyl (alkynyl) halide CH2=CHBr CH2=CHBr CH2=CHBr CH2=CHBr CH2=CHBr CH2=CHBr CH2=CHBr CH2=CHBr Table 14. Asymmetric cross-coupling of 1-PhEtMgCl with vinyl bromide on chiral nickel and palladium complexes.226 ± 228 Ligand Yield (%) >95 >95 62 82 >95 >95 86 73 93 79 >95 (S)-(R)-ppfa (R)-(S) ± ppfa 17 17 (Pd) 18 19 20 21 21 (Pd) 22a 22b comparison with methyl) substituents at the nitrogen atoms and removal of the amino functional groups at a distance of one CH2 group from the ferrocenyl ring decrease drastically the optical yields of the target products.Therefore, any disturbances in the optimum structures of ligands in catalytic complexes have neg- ative effects on the enantioselectivity of the reaction. Reaction (4), which proceeds in the presence of (amino)- diphenylphosphine nickel and palladium complexes, is a some- what more complicated version of reaction (1). The corresponding experimental data { are presented in Table 15. The coincidence of configurations and optical yields of reaction { The catalyst ± product configurational relationships have not been analysed in Refs 219, 220, 227 ± 231. Configuration of the product Me Ph H Ph Me H Me p-Tol H Me Ph H Me Ph H Me Ph H SiMe3 Ph H SiMe3 Ph H 1057 Ligand ee (%) ee (%) Yield (%) 22c 22d 23a 23b 23c 23d 23e 23f 23g 24 25 65 (R) 57 (R) 35 (R) 7 (S ) 15 (S ) 62 (R) 42 (S ) 17 (R) 65 (R) 15 (S ) 57 (R) 90 >95 65 49 50 >95 43 68 >95 88 95 61 (R) 68 (S ) 52 (R) 61 (R) 54 (R) 65 (S ) 5 (S ) 65 (R) 61 (R) 33 (R) 65 (R) Ref.Optical yield (%) 228 66 (R) (see a) 228 68 (S ) (see a) 219, 220 66 (R) 227 61 (R) 227 61 (R) 227 61 (R) 56 (R) (see b) 219, 220, 229 21 (R) (see b) 219, 220, 2291058 Table 15 (continued). Entry Ligand 9 (R)-(S )-ppfa (R)-(S )-ppfa 10 (R)-(S )-ppfa 11 (R)-(S )-ppfa 12 (R)-(S )-ppfa 13 (R)-(S )-ppfa 14 (R)-(S)-ppfa 15 (R)-(S )-ppfa 16 (R)-(S )-ppfa 17 a At720 8C.b At 50 8C. products in the presence of catalytic complexes NiCl2/(S)-(R)- ppfa and PdCl2/(S)-(R)-ppfa (entries 4 and 5), PdCl2/(S)-(R)-ppfa and PdCl2/(S)-(R)-bppfa (entries 5 and 6) as well as NiCl2/(S)-(R)- ppfa (entries 1 and 3), prompt a conclusion about the same type of their structures. Thus the bppfa ligand is coordinated through a metal atom (Ni, Pd) to the PPh2 and NMe2 groups belonging to the same five-membered ring of ferrocene (the second PPh2 group in bppfa is hardly involved in coordination). The palladium catalyst PdCl2/(R)-(S)-ppfa can be used for the synthesis of optically active allylsilanes (entries 7 ± 15), being much more enantioselective than the PdCl2/(R)-(S)-bppfa complex (entry 8). In the presence of PdCl2/(R)-(S)-ppfa, the (E)-isomers of alkenyl bromides generate products of higher optical purity than the (Z)-isomers (entries 10, 11, 13 and 14).Substitution of the phenyl group for the methyl group in the Grignard reagent Me3Si(PhCH)MgBr (entries 7 ± 11 and 12 ± 15) has no effect on the configuration of the product, since substitution of the (S) symbol for the (R) symbol reflects the changes in the order of priority of the substituents only. The actual configurations of products do not change in reactions involving vinyl bromide and phenylethynyl bromide (entries 7 and 16). The substitution of zinc for magnesium in the Grignard reagent does not affect the product configuration either (entries 2 and 17). The analysis of configurational relationships between the catalytic complex and the product (Table 16) shows that the Ni- Grignard reagent Me3Si MgCl Ph Me3Si MgBr Ph Me3Si MgBr Ph Me3Si MgBr Me Et3Si MgBr Me Et3Si MgBr Me Ph3Si MgBr Me Me3Si MgBr Ph Ph ZnCl Me Alkenyl (alkynyl) halide CH2=CHBr Ph Br H H Br Ph Ph Br H Ph Br H H Br Ph Ph Br H Ph C C Br CH2=CHBr Configuration of the product SiMe3 Ph H SiMe3 Ph Ph H HSiMe3 H Ph Ph H SiMe3 Ph Me H H SiEt3 Ph Me H HSiEt3 H Me Ph H SiMe3 Ph Me H H SiMe3 Ph C C C Ph H Ph Me H and Pd-complexes with ferrocenyl(amino)diphenylphosphine ligands belonging to the (S)-(R) and (R)-(S) configurational series catalyse the formation of enantiomeric products or products with different actual configurations in reactions (3) and (4).Table 16. Catalyst ± product configurational relationships in cross-cou- pling reactions catalysed by ferrocenyl nickel and palladium com- plexes.219, 220, 224, 227 ± 231 Configuration Configuration of the ligand of the product a Reaction (3b) (S )-(R) (S) (S )-(R) RCH CH (R)-(S) (R) (R)-(S ) RCH CH a One of the experiments yielded V A Pavlov Ref. Optical yield (%) 95 (R) 219, 220, 229 95 (R) 219, 220, 229 13 (R) 219, 220, 229 71 (S ) 219, 220, 229 93 (S ) 219, 220, 229 14 (S ) 219, 220, 229 36 (S ) 219, 220, 229 230 18 (S ) 231 85 (S ) Configuration of the product Configuration of the ligand Reaction (4) R2 R1 H R1 R2 H (see a) R1R2.RC C HStructural and configurational relationships `metal complex ± substrate ± product' in asymmetric catalytic reactions Table 17. Enantioselectivity of the cross-coupling reaction catalysed by nickel complexes with aminophosphine ligands prepared from amino acids. ee (%) R Ligand Me Bui Bn Ph Pri Bui cyclo-Hex But (S )-alaphos (S )-leuphos (S )-phephos (R)-phglyphos (S )-valphos (S )-ilephos (R)-chglyphos (R)-t-leuphos 38 (S ) 57 (S ) 71 (S ) 70 (R) 81 (S ) 81 (S ) 77 (R) 83 (R) 94 (R) a a The optical yield is adjusted for the optical purity of the ligand. Reaction (5) has been studied relatively little. In this reaction, asymmetric catalytic effects are produced by complexes (R)-(S)- C5H5FeC5H3(CHMeNMe2).(SR)PdCl2, where R=Me, Pri, Ph, p-Tol, 4-ClC6H4. The optical purity of the product does not depend on the nature of the radical R (S-configuration, ee=26%, 22%, 18%, 25% and 16%, respectively).232 Interesting relationships were found between the configura- tions of the metal complexes and the products as well as between the enantioselectivities and the structures of complexes for cross- coupling under the action of chiral nickel complexes with amino- phosphine ligands based on amino acids (Table 17).233, 234 R H NiCl2 Ph Ph PPh2 Me2N * MgCl + ether, 0 8C, 48 h Me Me Br In this reaction, the configurations of the products depend on the configuration of the ligand.Special mention should be made of the stable tendency of enantioselectivity to increase with an increase in the molecular size of the radical R in the ligand. Reaction (2) of a vinyl halide with a non-racemisable Grignard reagent proceeds with low enantioselectivity 218 ± 221, 234 and does not present special interest from the standpoint of configurational relationships. V. The structure of the intermediate complex Knowing the configurations of the catalytic complexes and the products in cross-coupling reactions of Grignard reagents with allylic derivatives, one can conjecture upon the structure of the intermediate complex.125 Cross-coupling of Grignard reagents with allyl phenyl ethers gives rise to an intermediate compound.221 Ph g b A Ph P P Ni Ph a d Ph B Figure 1.The intermediate compound in the cross-coupling reaction (see Scheme 4), a view from the side of the coordinated allylic group. NiCl2/(S,S)-chiraphos OPh MgBr+ Et Et P P P Ni Ni Et Et P P P Ni Ni Et Et P P P Ni Ni Et Et P P P Ni Ni R P * . NiP Thus the formation of eight intermediates may be expected resulting in two enantiomers of the product (Scheme 4). Figure 1 shows a structural scheme for the intermediate compound with a fixed position of the allylic fragment (a view from the side of the coordinated Z3-allylic group). The four positions of the methyl substituent in the allyl group are marked by the letters a, b, g and d, whereas the two positions of the apical radical at the nickel atom are designated by the letters A and B (in total, eight possible intermediates).All these variants as applied to different reactions of the same kind and analysed in terms of their stereochemical properties have been compared with the known experimental data. The a-position of the Me group is unfavourable at any position of the apical substituent at the nickel atom (A and B), because of the unfav- ourable (`eclipsed') position of this radical relative to the phos- phine equatorial phenyl group. Position b is also unfavourable due to the spatial proximity of the Me group to the phosphine axial phenyl group; if a radical of the Grignard reagent occupies position A, this is proximal to the Me group, too.Position g is unfavourable only in the case where the apical substituent at the nickel atom occupies position A. And, finally, position d is favourable at any position (A or B) of the apical radical. Thus only three out of eight intermediates are theoretically more or less preferred. An attack of the radical R(Et) from A or B at the nickel atom on the prochiral allylic carbon atom results in the formation of products with (S)-, or (R)- and (S)-configurations, respectively. In general, a small preference (2/3) goes to the (S)-configuration, which is consistent with the experimental data.221 This simple analysis demonstrates a good coincidence with experimental results, which, in turn, corroborates the correctness of the state- ment about the structure of the intermediate complex (cf.Table 18). 1059 Scheme 4 Et P Me H (S) + P Et P H Me (R) + PTable 18. Relationships between the experimentally determined product configurations of cross-coupling reaction of alkyl (aryl) Grignard reagents with allylic compounds and those predicted from stereochemical models. Experimental results Grignard reagent Alkene EtMgBr OPh PhMgBr OPh EtMgBr OPh PhMgBr OPh MeMgBr OH MeMgBr OH MeMgBr OH MeO OCOMe MeO OCOBut MeO OCO2Me Catalytic system NiCl2/(S,S )-chiraphos NiCl2/(S,S )-chiraphos NiCl2/(S,S )-chiraphos NiCl2/(S,S )-chiraphos NiCl2/(R,R)-diop NiCl2/(R,R)-diop NiCl2/(R,R)-diop NiCl2/(S,S )-chiraphos MgBr NiCl2/(S,S )-chiraphos MgBr NiCl2/(S,S )-chiraphos MgBr Product Conforma- tion of the complex d Et H d Ph H d Et H d Ph H chair-d Me H chair-d Me H chair-d H Me Ar d H Ar H d Ar d H Ref.ee (%) 221 22 (S ) 221 58 (R) 221 17 (S ) 221 58 (R) 222 15 (R) 223 8.5(R) 223 1 (S ) 223 68 (R) 223 67 (R) 223 41 (R) Preferential intermediates and configurations of reaction products predicted using the model shown in Fig. 1 a b g d C PC B A 7 H H H Me S S Et 77 Et H H Me R H H Et 7 H H H Me R R Ph 77 Ph H H Me S H H Ph 7 H H H Me S S Et 77 Et H H Me R H H Et 7 H H H Me R R Ph 77 Ph H H Me S H H Ph 7 H H H Et R R Me 77 Me H H Et S H H Me 7 H H H Et R R Me 77 Me H H Et S H H Me 7 H H H Et R R Me 77 Me H H Et S H H Me 7 Me Ar 7 Me Ar 7 Me Ar 7 Me Ar 7 Me Ar 7 Me Ar S Me H H R Me H H S Me H H R Me H H R Et H H R Et H H R Et H H Ra R Me H H Ra Me H H Ra R Me H H Ra Me H H Ra R Me H H Ra Me H HTable 18 (continued).Experimental results AlkeneOMe OCOBut OCO2Me OMe OMe OMe OCOBut Br Note. Designations: C is configuration of the product formed from the intermediate (Fig. 1); PC is predicted preferential configuration of the product. a The introduction of Ar into the allylic positions (a,b) is less favourable, being sterically more hindered than that into positions (g,d) [eclipsed conformation of the C(a,b)7Ar and the P7Ph bonds of the ligand]. Grignard reagent MeO MgBr MeO MgBr MeO MgBr MeO MgBr PhMgBr MeO MgBr MeO MgBr PhMgBr Catalytic system NiCl2/(S,S )-chiraphos NiCl2/(S,S )-chiraphos NiCl2/(S,S )-chiraphos NiCl2/(S,S )-chiraphos NiCl2/(S,S )-chiraphos NiCl2/(S,S )-chiraphos NiCl2/(S,S )-chiraphos NiCl2/(R,R)-diop Product Conforma- tion of the complex d Ar H d Ar H d Ar H d Ar H d Ph H Ar d H Ar d H chair-d Ph H Preferential intermediates and configurations of reaction products predicted using the model shown in Fig.1 Ref. ee (%) AAr 223 58 (R) 7Ar 223 89 (R) 77Ar 223 64 (R) 77Ar 223 67 (R) 77Ph 223 47 (R) 7Ar 223 30 (R) 7Ar 223 68 (R) 7Ph 224 6 (R) 77 a b g d C PC B7 Me Me Ar 7 H H H Me R R Ar H H Me S H H Ar 7 H H H Me R R Ar H H Me S H H Ar 7 H H H Me R R Ar H H Me S H H Ar 7 Me Me Ph 7 Me Me Ar 7 Me Me Ar 7 H H H Me R R Ph H H Me S H H Ph Ra R Me H H Ra Me H H R Me H H R Me H H R Me H H Ra R Me H H Ra Me H H Ra R Me H H Ra Me H H Ra R Me H H Ra Me H H R Me H H1062 VI.Conclusion The analysis of experimental data presented in this review suggests that the highest enantiomeric excess of the target product can be reached if an optimum correlation is established between the number of coordination vacancies on the metal complex catalyst accessible to the substrate and the number of coordination-active functional groups of the substrate. Presumably, the nature of asymmetric catalysis is such that high enantioselectivity is ensured with the minimum degree of freedom of the substrate where the latter is coordinated to the catalyst through all of its functional groups.For example, high enantioselectivity in asymmetric hydrogenation of acetamidocin- namic acid, which contains coordination-active groups in addi- tion to the double bond, is attained with rhodium catalytic complexes with bidentate ligands. In hydrogenation of simple ketones, which are coordinated exclusively through one carbonyl group, this is insufficient and catalytic systems with tridentate ligands should be used which produce a chiral catalytic complex devoid of any symmetry elements other than C1, where one of the vacancies is additionally blocked by an achiral monodentate ligand (e.g., Et3N, CF3CO2H, PriCO2H, etc.). But even in such complicated catalytic systems, the most enantioselective hydro- genation is observed for ketones possessing additional coordina- tion-active groups in the a- or, which is better, the b-position relative to the carbonyl.The ratios of coordinating and coordination-active vacancies of the catalyst and the substrate should be an optimum. For example, substrates of the tiglic acid type, which contain only one coordination-active group (in addition to C=C), are hydrogen- ated with much higher enantioselectivity on Ru(OAc)2[binap] complexes than on Rh complexes containing the binap ligand. Contrariwise, the hydrogenation of a-acetamidocinnamic acid occurs with higher enantioselectively in the presence of rhodium complexes.If the hydrogenation is carried out on octahedral ruthenium complexes with the binap ligand, substrates with two coordination-active groups seem to be less optimal as regards enantioselectivity than those with only one coordination-active group. A similar situation is observed in hydrosilylation reactions where rhodium complexes with tri- and tetradentate ligands manifest the maximum enantioselectivity in hydrosilylation of simple ketones. Simple graphical dependences between the structures of catalytic complexes and the configurations of the reaction prod- ucts can be useful for predicting the configurations of reaction products at the design stage of asymmetric catalytic synthesis. The relationships between the configurations of reaction products and the signs of the helical components of the complexes seem to suggest the similarity of asymmetric induction mechanisms of all reactions which obey these regularities.The analysis of putative structures of intermediate complexes based on stereochemical concepts further corroborates the sim- ilarity of asymmetric induction mechanisms of certain reactions. 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