3 Theoretical Organic Chemistry By J. J. W. McDOUALL Department of Chemistry University of Manchester Manchester M 13 9PL UK 1 Methods The high level of activity in the area of density functional theory continues and a number of useful assessments of various aspects have appeared. Notable amongst these is a detailed review of the methodology of the DGauss program' and its application to many organic systems. Fan and Ziegler2 also provide an assessment of their density functional formalism through its application to the calculation of activation energies for elementary organic reactions the results of which are very encouraging. Applica- tion of density functional theory to the calculation of 27 homolytic dissociation reactions using generalized valence bond (GVB) reference wavefunctions3 and a modest basis set (&3 lG(D)) show an average error of 4kcal mol- '.The use of the same basis set and Hartree-Fock reference wavefunctions with a density function treatment of exchange and correlation4 for a set of 106 experimentally well established atomization energies ionization potentials and proton affinities show a mean absolute deviation of only 4.18 kcal mol- '.Other tests of density functional using the large G1 database of Pople and co-workers' have also shown these approaches to provide accurate and computationally efficient methods for studying chemical reactions.Though not directly related to density functional theory an interesting analysis of the effect of electron correlation on charge density distributions in molecules has been carried out for the widely used MP2 CISD and QCISD wavefunctions.8 Compared to the QCI density MP2 is found to consistently exaggerate the loss in charge density in bonding regions leading to an increase in bond length whereas CISD is found to underestimate the shift.Pople and co-workers have provided a detailed overview of their approach to the molecular orbital theory ofexcited states.' The method is now applicable to quite large 1 cal = 4.1845 J. Andzelm and E. Wimmer J. Chem. Phys. 1992 96 1280. L. Fan and T. Ziegler J. Am. Chem. Soc. 1992 114 10890. E. Kraka Chem. Phys. 1992 161. 149. P. M. W. Gill B.G. Johnson J. A. Pople and M. J. Frisch Chem. Phys. Lett. 1992 197 499. ' A. Becke J. Chern. Phys. 1992 96 2155.'A. Becke J. Chem. Phys. 1992 97 9173. 'J. A. Pople M. Head-Gordon D.J. Fox K. Raghavachari and L. A. Curtiss,J. Chem. Phys. 1989,97,9173; L.A. Curtiss K. Raghavachari G. W. Trucks and J. A. Pople J. Chem. Phys. 1992 97 9173. K. B. Wiberg C. M. Hadad T.J. LePage C. M. Breneman and M. J. Frisch J. Phys. Chem. 1992,96,671. 'J. B. Foresman M. Head-Gordon J. A. Pople and M. J. Frisch J. Phys. Chem. 1992 96 135. J. J. W. McDouall systems as demonstrated by calculations on the excited states of formaldehyde ethylene pyridine and porphin. 2 Bonding and Molecular Structure It seems appropriate to begin this section by mentioning a review entitled 'The VSEPR Model Revisited'.'' An up-to-date account of the model with emphasis on an improved reformulation of some of the basic ideas together with some examples of new applications is given.As a qualitative method for predicting molecular geometries it remains hard to beat! The effects of sulfur and nitrogen substituents on the properties of 23 analogues of monoanionic methylphosphonates and methylphosphinates have been studied.' Pv compounds with oxygen bound to phosphorus are generally the most stable analogues and consecutive substitution by sulfur and nitrogen makes these compounds less stable. The sulfur analogues of PI" compounds were found to be the most stable while compounds with P=NH groups were calculated to be the most unstable analogues. A study of the structures of 12 phosphonium ylides (l),shows that they can be classified as (i) non-stabilized and (ii) stabilized ylides.'' All examples of non-stabilized ylides have non-planar ylidic carbon geometries while stabilized ylides have planar ylidic carbons.These structures have been used to support the notion that the dominant resonance structure is the ylide form. Mide Ylene (1) Molecular electrostatic potentials have been used to determine the effect of the substituent groups -F -NH, -OH -CH, CH,=C- HCEC-on the carbon- phosphorus triple bond in phosphaalkynes.' In spite of the varying electronegativity of the groups studied a characteristic feature found is the weak sensitivity of the C-P bond length to the substituent at the carbon atom. The electrostatic potential reflects the relative electronegativity of these groups and their ability to attract the polarizable charge from the carbon-phosphorus triple bond.This also suggests that these derivatives of phosphaalkynes are most likely to attract incoming electrophiles to the n-electron region of the carbon-phosphorus triple bond. Transition metals are coordinated 'sideways' with phosphaalkynes and in this configuration the lone-pair electrons of the phosphorus atom are unlikely to interact with the metal. A number of studies of tautomeric equilibria in the gas phase and solution have appeared. These include the keto-enol tautomerism of 3-and 5-hydroxyisoxazole,'4 lo R. J. Gillespie Chem. SOC.Rev. 1992 21 59. l1 M. Perakyla T. A. Pakkanen J.-P. Bjorkroth E. Pohjala and H. 0.Leiras J. Chem. SOC.,Perkin Trans. 2 1992 1167.l2 S.M. Bachrach J. Org. Chem. 1992 57 4367. '' K. Jayasuriya Int. J. Quantum Chem. 1992 44,321. l4 S. Woodcock D. V. S. Green M. A. Vincent I. H. Hillier and M. F. Guest J. Chem. SOC., Perkin Trans. 2 1992 2151. Theoretical Organic Chemistry three tautomers of 3-amino- 1,2,4-triazole,' and tautomerism in cytosine.' Note also a high-level gas-phase study of the four most important tautomers of uracil' and the infrared spectrum of the dioxo form.'* The structures and energies of MeCH,NO (2) and MeNHNO (3) and their tautomeric (E,Z)-and (Z,Z)-nitronic acids (4) and (5) respectively have been investigated." Also the nitroanions MeCH=NO; (6)and MeN=NO; (7) which are intermediates in the transformations (2) -P (4) (3) -,(5) have been determined.(Z,Z)- (4)and (Z,Z)-(5) were calculated to be 18.4 kcal mol -' and 14.0 kcal mol- ' less stable than (2) and (3) respectively (the (Z,Z)-isomers being the most stable); (3)was found to be essentially nonplanar. Ab initio calculations of tautomerism between formhydroxamic acid (8) and formhydroximic acid (9)20 H(C=O)NHOH eHC(OH)=NOH (81 (91 show that formhydroxamic acid is more stable by 40.7 kJ mol- '. The activation energy for the tautomerism via a 1,3-intramolecular hydrogen shift is 15 1.4 kJ mol- '. The formhydroximic acid is predicted to have a possibility of existence in conditions of low temperature and pressure. The structures of the fluoromethylplumbanes CH,PbF (lo) (CH,),PbF (1 l) and CH,PbF (12) show that the bond angles in all of the unsymmetrically substituted species deviate significantly from the idealized tetrahedral values.' F-Pb-F angles are less than 109.5 O C-Pb-C angles are considerably larger. The most extreme case being (11) with a C-Pb-C of 135". In (10) the C,Pb unit approaches planarity. Pb" compounds are destabilized by electronegative substituents in contrast to alkanes and silanes which are stabilized by geminal fluorine substitution. Transition metal activation of C-C bonds in ethane and cyclobutane have been studied for the entire second row of transition metals. For cyclobutane the first transition metal series has also been studied,22 and cyclopropane has also been studied for rhodium and palladium. Palladium is found to have the smallest barriers for the IS O.G.Parchment I. H. Hillier D. V. S. Green N.A. Burton. J. 0.Morley. and H. F. Schaefer 111 J. Chem. SOC.,Perkin Trans. 2 1992. 1681. 16 I. R. Gould. D. V. S. Green P. Young and I. H. Hillier J. Org. Chem. 1992 57. 4434. 17 J. Leszczynski J. Phys. Chern. 1992 96 1649. I8 I. R. Gould. M. A. Vincent and I. H. Hillier J. Chem. Soc.. Perkin Trans. 2 1992 69. 19 V.G. Arakyan and O.V. Fateyev THEOCHEM 1992 262. 39. 20 D. Wengui. Int. J. Quunturn Chem. 1992 44 319. 21 M. Kaupp and P. von R. Schleyer. Angeu.. Chern. Inr. Ed. Engl. 1992 31 1224. 22 P. E. M. Siegbahn and M. R. A. Blornberg J. Am. Chem. Sac.. 1992 114 10548. 38 J.J. W. McDouall C-C breaking reaction with the bond in cyclopropane being the easiest to break.The binding energy curve between ethene and all second row transition metals from yttrium to palladium has been ~alculated.~~ The strongest bonds are formed by the atoms to the right for which covalent and donation-back-donation bonding is optimally mixed. The atoms to the left form metallocyclopropanes in which the C-C n-bond is fully broken. Theoretical binding energies have also been obtained for all of the first row and selected members of the second row transition metal ion-benzene complexes.24 Although the bonding is found to be predominantly electrostatic a significant enhancement to the bonding does arise from metal donation into the n* orbitals of benzene. The structure of azole-X+ (X = Na K Al) complexes show the A1 complex closely resembles the corresponding protonated species while those of Na' and K+ are similar to that found for Li+.25 The Al+-azole interaction has a non-negligible covalent character due to the low lying p orbitals of Al'.It is suggested that this interaction is responsible for the enhanced stability of some A1 complexes. + The acid-strengthening and base-weakening effect of the acetylenic linkage as illustrated by a comparison of ethyneamine (HC-CNH,) and etheneamine (H2C=CHNH,)26 shows ethyneamine to have a very high relative acidity due to the relative stabilization of the ethynylamide anion. The corresponding low basicity is due to the relative destabilization of the ethynylammonium cation. For both ethyneamine and etheneamine the preferred site of protonation is on carbon rather than nitrogen due to strong stabilization of the carbon-protonated species.In the geometries of diethers diketones and diamines and their protonated species a good correlation is found between proton affinity and the hydrogen bond angles.27 As the alkyl size increases the angles and the ring strain at the sp3 carbons become larger which leads to a levelling-off of proton affinity values. The splitting between the two lowest energy cation states and the two lowest energy anion states have been investigated for a series of rigid non-conjugated dienes with the two double bonds separated by 4-12 carbon-carbon a-bonds. The dependence of the n+,n-splittings on the number of carbonxarbon a-bonds separating the ethylenic groups is found to have an exponential dependence for dienes with bridge^^^^^^ containing 8,10 and 12 C-C bonds.When four- and six-bond bridges are included the dependence is no longer a simple exponential; the origin of this effect is suggested to be due to multiple through-bond interactions. A high level ab initio study of the structure of acetamide finds that the lowest energy conformer in the gas phase and condensed media has a nonplanar geometry. One of the methyl C-H bonds is almost perpendicular to the plane of the non-hydrogen atoms and the amino group is slightly ~yramidal.~' The preference for this non-planar structure is attributed to the favourable interaction of the nitrogen lone-pair and one of the methyl hydrogens. A recent theoretical study of the diketene (13) produced an equilibrium 23 M.R. A. Blomberg P. E.M. Siegbahn and M. Svensson J. Phys. Chem. 1992 96 9794. 24 C. W. Bauschlicher Jr. H. Partridge and S. R. Langhoff J. Phys. Chem. 1992 % 3273. 25 M. Alcami 0.Mo and M. Yanez J. Phys. Chem. 1992 % 3022. 26 B. J. Smith and L. Radom J. Am. Chem. SOC. 1992 114 36. 27 S. Yamabe K. Hirao and H. Wasada J. Phys. Chem. 1992 % 10261. K. D. Jordan and M. N. Padden-Row J. Phys. Chem. 1992,% 1188. 29 K. D. Jordan and M.N. Padden-Row Chem. Rev. 1992 92 395. 30 M.W. Wong and K.B. Wiberg J. Phys. Chem. 1992,96 668. 31 (a)E. T. Seidl and H. F. Schaefer 111 J. Phys. Chem. 1992,% 657; (b)J. Am. Chem.SOC.,1990,112,1493. Theoretical Organic Chemistry geometry quite different from that determined experimentally by either electron diffraction or X-ray crystallography.An improved level of theory has been applied 31b and the predicted rotational constants are in much better agreement with microwave experiments than are those derived from electron diffraction and crystal structures. A case is put forward that there are serious deficiencies in the experimental molecular structures for diketene. The geometries electron distribution and spin distribution for three oxidation forms of lumiflavin (14) (15) and (16) and the 2+ and 2-charged species have been obtained.32 The reduced form (1,5-dihydrolumiflavin) is found to have a non-planar structure. H3C’ (14) Lumiflavin (oxidized form) (15) 5-hydrolumiflavin (radical form) (1 6) 1,5-dihydrolumiflavin (reduced form) Calculations of various protonation states of DNA base and DNA radical-ions including each of the protonation states important to proton-transfer reactions in base pair radical-ions found in irradiated DNA have been perf~rmed.~~,~~ Evidence is provided for proton transfer in guanine-cytosine base pair radical-anions.Conforma- tional features of amrinone (1 7) and milrinone (18) and their molecular electrostatic 32 S.A. Vasquez J.S. Andrews C. M. Murray R. D. Amos and N.C. Handy J. Chem. SOC.,Perkin Trans. 2 1992 889. 33 A.-0. Colson B. Besler D. M. Close and M. D. Sevilla J. Phys. Chem. 1992 96,661. 34 A.-0. Colson B. Besler D. M. Close and M. D. Sevilla J. Phys. Chem. 1992 % 9787. J. J. W. McDouall potentials have been studied.35 It is suggested that the twisted conformers of these molecules are responsible for their observed cardiotonic properties.H HH HH Nmo:wo H HH NH2 H HH CN Finally there have been a number of studies of the geometries and singlet-triplet energy gaps in ~henylnitrene,~~ (in which the ground state is a triplet) rn-q~inone~~ and tetramethyleneethane38~3gand its cyclic analogue^.^' 3 Reactivity Pericyclic Reactions.-A new secondary orbital interaction has been suggested4l for the Diels-Alder reactions of butadiene with vinyl boranes. In these systems the archetypal [4 + 21 cycloaddition appears to take place via a [4 + 3) transition state in which the distance of the boron atom in the vinyl borane is closer to the terminal carbon atom of the butadiene moiety than is the carbon atom in the vinyl borane.The reaction path for the Diels-Alder reaction of s-cis-1,3-butadiene cation-radical with ethene yielding the cyclohexene cation-radical has been studied42 and found to follow a concerted but non-synchronous path which proceeds without a barrier in the gas phase. In Diels-Alder and nitrile oxide intramolecular cycloadditions it has been found that when the addends are linked by three CH groups (leading to the formation of a five-membered ring) the strain in the transition state causes the addends to twist about the forming bonds resulting in a skewed transition state compared to the intermolecular transition state.43 However when the addends are linked by four CH groups (forming a six-membered ring) there is little strain and the addends do not twist.The different reactivity of oxazole and isoxazole in Diels-Alder cycloadditions is ascribed to the higher energy of activation (13 kcal mol- ') and endothermicity of the isoxazole reactions.44 The activation energies and asynchronicities of the Diels-Alder reactions of 2-azabutadiene with alkenes and alkynes follow the same trends as that of b~tadiene.~' The effect of the aza group in the diene is very small. In the dimerization of silaethylene to form 1,3-disilacyclobutane and 1,2-disilacyclobutane the 1,3 product is formed via a concerted [2s + 2s] mechanism with a barrier of 5.2 kcal mol- '. The 35 A. K. Bhattacharjee D. Majumdar and S. Guha J. Chem. Soc. Perkin Trans. 2 1992 805. 36 D.A.Hrovat E. E. Waali and W.T. Borden J. Am. Chem. SOC. 1992 114 8698. R. C. Fort Jr. S.J. Getty D. A. Hrovat P.M. Lahti and W. T. Borden J. Am. Chem. Soc. 1992,114,7549. 38 P. Nachtingall and K. D. Jordan J. Am. Chem. Soc. 1992 114 4743. 39 P. Nachtingall P. Dowd and K.D. Jordan J. Am. Chem. Soc. 1992 114,4747. 40 J.J. Nash P. Dowd and K.D.Jordan J. Am. Chem. Soc. 1992 114 10071. " D.A. Singleton J. Am. Chem. Soc. 1992 114 6563. 42 N.L. Bauld J. Am. Chem. Soc. 1992 114 5800. 43 F. K. Brown U. C. Singh P. A. Kollman L. Raimondi K. N. Houk and C. W. Bock J.Org. Chem. 1992 57 4862. 44 J. Gonzalez E.C. Taylor and K. N. Houk J. Org. Chem. 1992 57 3753. 45 J. Gonzalez and K. N. Houk J. Org. Chem. 1992 57 3031. Theoretical Organic Chemistry 41 1,2-product is higher in energy by 19.8 kcalmol-' and is formed via a two-step mechanism involving a diradical intermediate.46 The [2s + 2s] cycloaddition of ethylene is a classical forbidden Woodward-Hoffman process.The reaction lends itself to catalysis via complexation with metal radical- cation^.^' In the case of Be-+ the reaction proceeds via a metallocyclopentane (see Scheme 1 ) radical cation intermediate and two successive 1,2-hydrogen shifts to form the isomeric acyclic butenes. No points on the reaction path have an energy higher than the starting materials. Be" + C2H4 -4be.+ + C2H4 -[[::Bet511 -Scheme 1 The dimerization of carbodiimides (HN=C=NH) to 1,3-diazetidinediimines (19) appears to favour an asynchronous pathway.48 For the case of N-amino-"methyl carbodiimide 12 asynchronous transition structures have been located which lead to isomeric products.The mechanism of the cycloaddition of ketene and imine is found to H I I H be a two-step process with a zwitterionic intermediate.49 The electrocyclic conrotatory closure of this intermediate is predicted to be the rate determining step. The conrotatory ring opening in phospha- and azacyclobutenes is found to be thermoneut- ral or very slightly endothermic for dihydrophosphates but exothermic for dihyd- r~azetes.~'The energies of activation for the opening of 2,3-dihydrophosphete and 3,4-dihydroazete are 41 and 37 kcal mol -',respectively. The opening of 1,2-dihyd- rophosphete and 1,2-dihydroazete can occur via two diastereomeric pathways.Inward rotation of the heteroatom lone pair is favoured for both systems. The barriers are 25 and 30 kcal mol- ' for 1,2-dihydrophosphete and 2,3-dihydrophosphete respectively. The conrotatory ring opening of dihydrodiphosphetes is endothermic while the opening of the dihydrodiazetes is ex other mi^.^' Inward rotation of the heteroatom lone pair is favoured in all cases. A detailed study of the transition structures of pericyclic hydrocarbon reactions finds common features in the geometries energies and electronic characteristics.s2 General- " E.T. Seidl R. s. Grev and H. F. Schaefer 111 J. Am. Chem. Soc. 1992 114. 3643. '' A. Alex and T. Clark 1. Am. Chem. SOC.. 1992 114 506. '' J. Bertran A. Oliva J. Jose M. Duran P. Molina M. Alajarin C.L. Leonardo and J. Elguero J. Chem. SOC.,Perkin Trans 2 1992 299. 49 J.A. Sordo J. Gonzalez. and T. L. Sordo. J. Am. Chem. Soc. 1992. 114 6249. " S.M. Bachrach and M. Liu J. Org. Chem. 1992 57 209. " S.M. Bachrach and M. Liu J. Org. Chem. 1992 57 2040. " K.N. Houk Y. Li and J. D. Evanseck Angew. Chem.. Int. Ed. Enyl.. 1992 31 682. 42 J.J. W. McDouall ization permits the prediction of other transition state geometries and energies. At the same time a great diversity of electronic structure is observed from rigid closed shells to floppy diradicals. Other interesting results on pericyclic processes include evidence for the photo- chemical addition of halo(trifluoromethy1)carbenes to dinitr0ge1-1~~ and a study of the thermal and photochemical cycloaddition of Dewar benzene.54 Oxygen Transfer.-A study of the oxygen transfer from an oxaziridine to the lithium enolate of acetaldehyde shows it to proceed by S,2 attack of the p-carbon on the enolate along the 0-N bond of the parent ~xaziridine.~~ In the transition structure the Li' cation is coordinated to both the enolate and the oxaziridine oxygen atoms.A review of the structure and mechanism of formation of ozonides studies a number of questions regarding the three step Criegee mechanism including evidence for intermediates and whether cycloaddition and cycloreversion are ~oncerted.~~ A study of the reaction of organic sulfides with singlet oxygen57 finds only a peroxy sulfoxide as an intermediate at the Hartree-Fock level of theory.However another intermediate thiadioxirane is found at the MP2 level. In the gas phase these are essentially isoenergetic but peroxy sulfoxide may be preferred in solution. The energy of activation for their interconversion is 20 kcal mol- *. A study on the mechanism of oxidation of sulfides and sulfoxides by dioxirane suggests that the oxidation of sulfoxide to sulfone by parent dioxirane in the gas phase is greatly favoured over the oxidation of sulfide to sulf~xide.~~ Using a simple electrostatic model of solvent the sulfide to sulfoxide oxidation is shown to be stabilized by the presence of solvent while the sulfoxide to sulfone oxidation is destabilized. Hence the difference in the predicted reactivity of dioxiranes with sulfides and sulfoxides in solution may be quite small and solvent dependent.The relative reactivity of dioxirane and its isomeric form carbonyl oxide have been investigated at a number of levels of theory regarding their ability to transfer oxygen to ethylene and ammonia.59 The question of the intermediacy of diradical structures in the transition states of these reactions is analysed and rejected. Unsaturated Systems.-An elaborate calculation of the energy barrier for the isomerization of vinylidene6' finds this system to violate the Hammond postulate. The barrier found is 2.2 kcal mol-' and the reaction exothermicity is 43.9kcal mol-'. In spite of the small barrier and highly exothermic nature of the reaction the transition structure is located halfway between reactant and product.The result is rationalized by considering the two components of the process the location of the transition state is determined by hydrogen migration which has a significant barrier whereas the exothermicity results from the conversion of the lone pair of vinylidene to a 7t bond in acetylene. Each process individually satisfies the Hammond postulate. A study of the mechanism and selectivity of electrophilic aromatic nitration finds '' J.E. O'Gara and W. P. Dailey J. Am. Chem. SOC.,1992 114 3581. 54 I. J. Palmer M. Olivucci F. Bernardi and M.A. Robb J. Org. Chem. 1992 57 5081. 55 R.D. Bach J.L. Andres and F.A. Davis J. Org. Chem. 1992 57 613. 56 R.L. Kuczkowski Chem. SOC.Rev. 1992 21 79. '' F. Jensen J. Org. Chem. 1992 57 6478. 58 J. J. W. McDouall J.Ory. Chem. 1992 57 2861. 59 R. D. Bach J. L. Andres A. L. Owensby H. B. Schlegel and J. J. W. McDouall J. Am. Chem. SOC.,1992 114 7207. 6o G. A. Peterson T.G. Tensfeldt and J.A. Montgomery Jr. J. Am. Chem. SOC. 1992 114 6133. Theoretical Organic Chemistry that unsolvated nitronium ion reacts with benzene to form the Wheland intermediate without an activation barrier.61 However solvated nitronium ion (protonated methyl nitrate) reacts with an activation barrier that is substituent dependent and also dependent on the solvating species. The conversion of diazafulvenone to cyanovinyl isocyanate according to Scheme 2 is predicted to have a barrier to reaction of 44kcal mol- '. The oxocarbene (20)is not an intermediate but the transition state corresponding to the rate determining step.This process constitutes a retro-Wolff rearrangement.62 Scheme 2 In the elimination of molecular hydrogen from 1,4-cyclohexadiene to form benzene it is found that the reactant carbon framework is planar and the transition state has a C, symmetry boat-like conformation. The transition structure has the H moiety located centrally over the ring.63 The predicted barrier is 60.1 kcalmol-' and the exothermicity is 9.3 kcal mol- '. Analysis of relative reactivity in radical addition reactions of the type A-D A-A and D-D (A = acceptor and D = donor) in 1,1-disubstituted ethenes and related monosubstituted ethene and 2-substituted propenes have been perf~rmed.~~ Ground state effects in the 1,1-disubstituted ethene were evaluated using the isodesmic scheme H,C=CXY + H,C=CH -+ H,C=CHX + H,C=CHY (Y X = F OH NH, C1 SH CN CHO BH,).The mechanism of ferrocene formation from atomic iron and cyclopentadiene is found to proceed by insertion of iron into a C-H bond of cyclopentadiene. The resulting high-spin complex then binds another molecule of cyclopentadiene and undergoes conversion to a low-spin complex. Elimination of hydrogen from the final complex is predicted to be facile.65 The bifunctional catalysis by HF of the hydrogenation of ethylene has been studied.66 The presence of the catalyst makes possible a HOMO-LUMO interaction leading to the formation of the two new C-H bonds. The bifunctional catalyst provides an alternative path to the direct charge transfer from ethylene to hydrogen by acting as an electronic bridge (2 1).(21) 61 K. J. Szabo A.-B. Hornfeldt and S. Gronowitz J. Am. Chem. SOC. 1992 114 6827. M.T. Nguyen M. R. Hajnal T.-K. Ha L. G. Vanquickenborne and C.Wentrup J. Am. Chem. SOC.,1992 114 4387. 63 R.J. Rico M. Page and C. Doubleday Jr. J. Am. Chem. SOC. 1992 114 1131. 64 D.J. Pasto J. Org. Chem. 1992 57 1139. 65 M.L. McKee J. Phys. Chem. 1992 % 1683. 66 M. I. Menendez J. A. Sordo and T. L. Sordo J. Phys. Chem. 1992 % 1185. J. J. W. McDouall Transition structures for the syn and anti attack of diazomethane on cis-3,4-dichlorocyclobutene have been The syn attack is predicted to be dominant. General.-There have been a great many excellent studies on the subtleties of reactivity and mechanism.The following is an incomplete list rather than a general discussion of some of them hydration of carbon dioxide by carbonic anhydrase a comparison of the Lipscomb and Lindskog mechanisms including a treatment of solvent effects;68 the reaction path for the alkaline hydrolysis of ester via a hydrogen bonded intermediate;69 a detailed analysis of the reaction of L-ascorbic acid with OH radical including a discussion of the site of attack and mechanism of dehydrati~n;~' the decarboxylation and dehydration reactions of monomeric formic acid;71 dissociation pathways of formic acid;72 molecular orbital calculations on the mechanisms of repair of alkylated n~cleosides;~ modelling of receptor-ligand interactions in proteins specifically for asparagine glutamine serine threonine and tyro~ine;~~ a theoretical study and comparison with experiment of the structure and n-facial regioselectivity of 9-chloro-l,4,5,8-tetrahydro-4a,8a-methanonaphthalene (22);75an investigation of the effects of P-substituents on C-OX bond length in Y-C-C-OX systems (Y = F H SIR,; X = CH, CHO NO,);76pyramidal inversion energies of hypervalent selenox- ides R,SeO (R = H F CH3);77 the mechanism of photolysis and solvolysis of arylvinyl halides;78 primary and secondary hydrogen abstraction from propane by CN radical;79 homolytic substitution by hydrogen atom and methyl radical in alkyl sulfides and sulfoxides;80 quantitative valence-bond curve-crossing calculations for S,2 reactions;' secondary kinetic isotope effects and transition structure geometries for the Cope rearrangement.82 67 M.Bagatti A. Ori A. Rastelli M. Burdisso and R. Gandolfi J. Chem. SOC.,Perkin Trans. 2 1992 1657. 68 M. Sola A. Lledos M. Duran and J. Bertran J. Am. Chem. SOC. 1992 114 869. 69 K. Hori J. Chem. SOC.,Perkin Trans. 2 1992 1629. 70 Y. Abe S.Okada R. Nakao,T. Horii H. Inoue S.Taniguchi,and S. Yamabe,J. Chem. SOC.. Perkin Truns. 2 1992 2221. 71 J. D. Goddard Y. Yamaguchi and H. F. Schaefer 111 J. Chem. Phys. 1992,96 1158. 72 J. S. Francisco J. Chem. Phys. 1992 96,1167. 73 R. H. D. Lyngdon J. Chem. SOC..Perkin Trans. 2 1992 1173. 74 J. Lindroos M. Perakyla J.-P. Bjorkroth and T. A. Pakkanen J. Chem. SOC.,Perkin Trans. 2 1992,2271. 75 B. Halton R. Boese and H.S. Rzepa J. Chem. SOC.,Perkin Trans. 2 1992 447. 16 R. D. Amos N. C. Handy P. G. Jones A. J. Kirby J. K. Parker J. M. Percy and M. D. Su J. Chem.SOC.. Perkin Trans. 2 1992 549. 77 H. Fueno S. Ikuta H. Matsuyama and N. Kamigata J. Chem. SOC.,Perkin Trans. 2 1992 1925. 78 K. Hori H. Kamada T. Kitamura S. Kobayashi and H. Taniguchi J. Chem. SOC.,Perkin Trans. 2 1992 871. 79 J. J. W. McDouall THEOCHEM 1992 255 35. no J. E. Lyons and C. H. Schiesser J. Chem. SOC. Perkin Trans. 2 1992 1655. 81 G. Sini S. Shaik and P.C. Hiberty J. Chem. SOC.,Perkin Trans. 2 1992 1019. 82 K. N. Houk S. M. Gustafson and K.A. Black J. Am. Chem. SOC. 1992 114 8565.