ISSN:0069-3030    

DOI:10.1039/OC98279FX001

出版商:RSC    

年代:1982

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC98279BX003

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

3 TheoreticaI Chemistry By C. THOMSON Department of Chemistry University of St. Andrews St. Andrews KY169ST 1 Introduction A useful new textbook on modern quantum chemistry has appeared,' and recent reviews in this area include a review on hybrid orbitals,* on the axioms of quantum the~ry,~ orbital interactions through bond^,^ the application of molecular mechanics calculations in organic chemistry,' and intermolecular forces.6 A new bibliography of ub initio calculations has appeared covering the period 1978-1980,' and a recent issue of the THEOCHEM journal was denoted to an update of the latter through 1981.8 The use of large vector computers such as the CRAY-1 in quantum chemistry has been reviewed by Saunders and Guest.' This review will necessarily be rather subjective and is mainly restricted to the use of ub initio methods in chemistry.2 Advances in Theoretical Techniques Basis Sets.-Pople's group have developed several new useful basis sets suitable for applications to second-row atoms. 6-21G and 3-21G basis sets for Na to Ar,'" analcgous to their first-row counterparts," have been tested on numerous molecules containing second-row atoms and the 3-21G basis set in particular yields equili- brium geometries vibrational energies relative energies and dipole moments that are much superior to STO-3G values at relatively modest cost. Two basis sets supplemented with polarization functions for the second row have also been described the first a 3-21G* basis set (added d-functions on second-row elements only) and this gives a much improved description of hypervalent molecules,1z and the second are the large 6-31G* and 6-31G** basis These 'Modern Quantum Chemistry' ed.A. Szabo and N. S. Ostlund MacMillan Publishing Co. New York 1982. 'W. A. Bingel and W. Luettke Angew. Chem. 1981,93 944. ' P.-0. Lowdin Adv. Chem. Phys. 1982 21 275. M. N. Paddon-Row Acc. Chem. Res. 1982 15,245. ' E. Osawa and H. Musso Top. Stereochem. 1982,13 117. 'P. Schuster Angew. Chem. Int. Ed. Engl. 1981 20 546. 'K. Ohno and K. Morukuma 'Quantum Chemistry Literature Data Base' Elsevier Amsterdam 1982. 'J. Mol. Strucf. THEOCHEM. 1982,8 Parts 112. 'V. R. Saunders and M. F. Guest Comput. Phys. Commun. 1982 26 389. M. S. Gordon J. S. Binkley J. A. Pople W. J. Pietro and W.J. Hehre J. Am. Chem. SOC.,1982 '' J. 104,2797. S. Binkley J. A. Pople and W. J. Hehre J. Am. Chem. SOC.,1980 102 939. '* W. J. Pietro. M. M. Francl W. J. Hehre. D. J. DeFrees J. A Pople and J. S. Binkley J. Am. Chem. SOC.,1982 104 5039. l3 M. M. Francl W. J. Pietro. W. J. Hehre J. S. Binkley M. S.Gordon D. J. DeFrees and J. A. Pople 1. Chem. Phys. 1982.77 3654. 18 C. Thomson give equilibrium structures very close to experimental values and excellent values for several other properties. Nobes et al. have shown that it is possible to estimate MP3 energies for basis sets including polarization functions from results of HF and MP2 cal~ulations.'~ The use of 0s orbitals in SCF calculations on molecules has been advocated by Szalewicz based on a re-evaluation of these functions in calculations on He." Further studies on the uniform quality basis sets have appeared,16.17 and studies of heavily contracted basis sets for Fe Ni Pd and Mo.'* Standard basis sets with polarization functions have also been studied by Zhu et di9 Procedures for the determination of contraction coefficients have been studied by Graf and Mehler,20*21 and further new contracted basis sets have been reported by Sakai ef al.for the elements Ga through Cd.22 Finally a 6-311G** basis set suitable for use with correlated wave functions (5d-functions are used and the exponents optimized for correlated wave functions) has been reported by Nobes et af. and tested by computation of geometries and atomization energies of several small mo~ecules.~~ Self-consistent Field Theory.-There have been several new studies of the conver- gence problem in SCF calculations particularly the question of the intrinsic convergence of closed-shell SCF calculation^,^^-^^ and convergence in excited state ca~cu~ations.~~ A new quadratically convergent SCF method applicable to both closed2' and open-shell has been discussed by Backsay.Pulay has described a pro- cedure that seems to be somewhat superior to Backsay's method for difficult cases.30 Almlof3' has presented a view of the direct SCF approach which is very efficient for large systems. In this method all two-electron integrals are re-calculated in the SCF iterations and the method is particularly useful also on modern mini-computers.A detailed study by Almlof32 also concludes that the HF model is incapable of describing the metal-ring distance in ferrocene to better than 0.25 A and for transition-metal complexes one should really go beyond the HF approxi- mation. I' R. H. Nobes W. J. Bourna and L. Radorn Chem. Phys. Lerr.. 1982,89,497. *'K. Szalewicz and H. J. Monkhorst J. Chem. Phys. 1981,75 5785. I' R. A. Poirier R. Daudel P. G. Mezey and I. G. Csizrnadia Znt. J. Quanrum Chem. 1982 21 799. " R. Daudel R. A. Poirier and I. G. Csizrnadia In(. J. Quantum Chem. 1982 21 699. " L. Pettersson and U. Wahlgren Chem. Phys. Lerr. 1982. 89 26. I' Z. H. Zhu and J. N. Murrell. Chem. Phys. Letr. 1982.88 262. 2" P. Graf and E. L. Mehler Inr. J. Quanrum Chem. 1981 QB8.63. " P. Graf E. L. Mehler A.D. McLean and G. S. Chandler J. Chem. Phys. 1982,76 1593. 22 Y. Sakai H. Tatewaki and S. Huzinaga J. Compur. Chem. 1982 3 6. 2' R.H. Nobes,W. R. Rodwell and L. Radorn 1. Compur. Chem. 1982,3,561. 24 R. E. Stanton J. Chem. Phys. 1981 75 3426. 25 R. E. Stanton J. Chem. Phys. 1981,75 5416. 26 J. C. Facelli J. Chem. Phys. 1982 77 1076. 27 P. C. De Mello M. Hehenberger and M. C. Zerner Znr. J. Quantum Chem. 1982,21 251. 2R G. B. Backsay Chem. Phys. 1981 61 385. 29 G. B. Backsay. Chem. Phys. 1982.65 383. '' P. Pulay I. Compur. Chem. 1982 3 556. I' J. Almlof K. Faegri Jun. and K. Korsell J. Comput. Chem. 1982,3 385. 72 H. I-uthi J. H. Arnrnete J. Alrnlof and K. Faegri Jun.. J. Chem. Phys. 1982 77 2002. Theoretical Chemistry 19 Ka~fman~~ and co-workers have also emphasized the desirability of developing optimal strategies for large molecule calculations especially on molecules of biologi-cal interest The use of such strategies on vector machines’ should be very important in future work.Pople et al.34 have summarized the results of SCF calculations of vibrational frequencies and harmonic force constants for a wide variety of molecules using the 3-21G basis set. A non-conventional algorithm for SCF calculations has been described by Benzel and Dyk~tra,~’ which is essentially open ended and likely to be very cost effective on computers with relatively small core size. The doublet instability problem in radicals is well and more examples of this problem with RHF and two configuration SCF calculations on radicals (in which incorrect geometries were obtained for both singlet and triplet states of conjugated diradicals) have been reported.Multi-configuration SCF (MC-SCF) methods continue to be useful in correcting some of the deficiencies of the SCF model; there have been further studies of more efficient ways of achieving stable con~ergence.~~ Yeager and co-workers have also investigated the problems of avoided curve crossings using MC-SCF theory.38 A simplified method for determining the molecular correlation energy has been described by Lievin et al. within a MC-SCF A review of MC-SCF CI and numerical SCF calculations has been given by Hin~e.~’ Electron Correlation.-The continued search for more efficient schemes to include electron correlation continues.An important extension of the CI gradient tech- niques to MC-SCF wave functions by solving the coupled perturbed Hartree-Fock equations has been reported by Schaefer’s group.41 The (b‘B,)first excited state of CH2 was used as a test case with a DZ + P basis set. A diagrammatic formulation of the direct CI method has been reported,42 as well as renewed studies on the Unitary Group Shavitt and co-w~rkers~~ have examined various wave function optimization methods used in CI procedures. The problem of size consistency in CI calculations restricted to a limited number of excitations (such as all doubles) is well and corrections for this in CI calculations have been discussed in two paper^.^^.^^ 33 J. J. Kaufman P.C. Hariharan and H. E. Popkie fnt. J. Quantum Chem. 1981 S15 199. 34 J. A. Pople H. B. Schlegel R. Krishnan D. J. DeFrees J. S. Binkley M. S. Frisch R. A. Whiteside R. F. Hout and W. J. Hehre Znt. J. Quantum Chem. 1981 S15.269. 3s M. A. Benzel and C. E. Dykstra J. Comput. Chem. 1982,3,260. 36 W. T. Borden and E. R. Davidson Tetrahedron 1982,38,737. 37 R. C. Camp and H. F. King J. Chem. Phys. 1982 77,3056. 38 J. Olsen P. Jbrgensen and D. L. Yeager J. Chem. Phys. 1982,76 527. 39 J. Lievin J. Breulet P. Clercq and J.-Y. Metz Theor. Chim. Acta 1982 61,513. 40 J. Hinze Int. J. Quantum Chem. 1981 15 69. 41 Y. Osamura Y. Yamaguchi and H. F. Schaefer J. Chem. Phys. 1982 77,383. 42 J. M. Cullen and M. C. Zerner Chem. Phys. Lett. 1982,88 448. 43 R.D. Kent and M. Schlesinger fnt. J. Quantum Chem. 1982 22 223. 44 W. Zhen-yi Theor. Chim. Acta 1982,61 335. 45 G. Born and I. Shavitt J. Chem. Phys. 1982,76 558. “ R. Shepard I. Shavitt and J. Simons J. Chem. Phys. 1982,76,543. 47 E. R. Davidson ‘The World of Quantum Chemistry’ ed. R. Daudel and B. Pullman Reidel Dordrecht 1974. 48 S. Prime C. Rees and M. A. Robb Mol. Phys. 1981,44 173. 49 E. J. Brandas L. L. Combs and N. S. Correia fnt. J. Quantum Chem. 1982,21 259. 20 C. Thomson Integral-dependent spin couplings in CI that lead to only one spin coupling regardless of the complexity of the reference wave function yield about 98.5% of the correlation energy.5o Different ways of tackling CI calculations continue to be reported and a perturba- tive variational approach to a multireference CI which results in H and S matrices that are much reduced in size and the elements of which are obtained by diagram- matic techniques has been developed and te~ted.’~ The relationship between the CI and coupled-cluster methods (see below) has been explored by Paldus and c~-workers.~* This analysis provides a new viewpoint for the origin of Davids~n’s~’ correction for unlinked cluster contributions to CID calculations.Numerical applications to CO were reported. Nakatsuji et al.53have developed a new cluster-type approach for both closed- and open-shell systems following their earlier papers on this subject. These authors also considered the treatment of excited states. The coupled-cluster (CC) approach and its relation to many-body-perturbation (MBPT) theoretical methods has been reviewed by Bartlett.54 During 1982 there have been further developments in cluster-type theories.The formalism of the full coupled-cluster singles and doubles model has been derived by Purvis and Bartlett.” This method is a many-body theory built upon the linked-diagram theorem. Implementation of the method has been described and compared with full CI calculations on H20 and BeH,. This method should be widely applied in view of its advantages. A related approach has been taken by Banerjee and Simon~,~~ but using a multi-configuration SCF reference function. Cullen and Zerner have derived an approximation to a linked single and doubles model using diagram techniques from coupled-cluster the~ry.~’ As mentioned above MBPT methods are related to the CC methods.Redman and BartlettS8 have discussed the multidimensional many-body theory and Carsky and co-workersSg have compared CI and MBPT methods in calculations on the triplet states of CH and 0,. Carsky et af. have also compared CEPA PNO-CI and MBPT in open-shell systems.60 Further work on the formalism has appeared by Wilson,61*62 and Banerjee et al.63Several applications using Mdler-Plesset methods are dealt with below. Davidson and co-w~rkers~~ have used MBPT to ’O K. Iberle and E. R. Davidson J. Chem. Phys. 1982 76 5385. 51 G. L. Bendazolli G. Fano F. Ortolani and P. Palmieri J. Chem. Phys. 1982,76 2498. ” J. Paldus P. E. S. Wormer F.Visser and A. van der Avoird J. Chem. Phys. 1982 76 2458. 53 H. Nakatsuji K. Ohta and K. Hirao J. Chem. Phys. 1981,75 2952 and references therein. 54 R. J. Bartlett Annu. Reo. Phys. Chem. 1981 32 359. ” G. D. Purvis tert. and R. J. Bartlett J. Chem. Phys. 1982,76 1910. 56 A. Banerjee D. Mukherjee and J. Simons J. Chem. Phys. 1982,76 1972 1979 1995. ’’ J. M. Cullen and M.C. Zerner J. Chem. Phys. 1982,77,4088. ’’ L. T.Redman and R. J. Bartlett J. Chem. Phys. 1982 76 1938. 59 P. Carsky M. Svrcek I. Hubac F. Brown and I. Shavitt Chem. Phys. Lett. 1982,85 17. 6o P. Carsky I. Hubac and V. Staemmler Theor. Chim. Acta 1982 60,445. 61 S. Wilson Theor. Chim. Acta 1982,61 343. ” D. M. Silver and S. Wilson Proc. R. SOC.London Ser. A 1982 383 477. 63 A. Banerjee and J.Simons J. Chem. Phys. 1982,76,4548. “ P. Philips and E. R. Davidson J. Chem. Phys. 1982 76 516. Theoretical Chemistry 21 develop a formalism to facilitate calculation of phosphorescence lifetimes and reported applications to CH,. Carsky has given formulae for the correlation energy in triplet states using the MBPT formali~m.~’ More applications of the CEPA methods have appeared including studies on the Rydberg states of H20,66and the vibration frequencies of SF and SCl.67 One method that has received more attention recently after a relatively long gap is the theory of two electron functions or geminals. Rgeggen has developed an extended geminal and reported test calculations on H,06* and Li2.69 It is suggested that this model should recover up to 97% of the correlation energy obtainable by a full CI calculation.There have been other studies on geminal the~ry~’-~~ and applications reported on Be and LiH. A method related to electron pair theory and to CI is the theory of self-consistent electron pairs (SCEP) proposed by Me~er’~ and further extended by Dykstra et .~~ ~1 Applications were reported on the stability of glycine conformers. A recent development has been the extension of the formalism to the multi-reference a move which reduces the computational effort. Applications to CH2(3B1)+ HZ+CH3 + H have been reported. The barrier to this reaction was calculated to be 43.7 kJ mol-’ using a large basis set. An interesting recent paper by Jasien and Dyk~tra~~ has pointed out the advantages of using a common one-electron basis representation in seemingly different methods of handling electron correlation such as SCEP and CI.These methods use coefficient and associated operator matrices to represent the wave function directly. Illustrative calculations were presented on N2H2,.HCN and HNC. The energy difference between the two isomers was predicted to be 63.0 f 4 kJ mol-’ with the largest basis set in good agreement with the experimental value. Valence Bond The~ry.-Simonetta~~ and co-workers have continued their studies of ab initio VB theory and K~wajirna~~’~~ has investigated further a new self- consistent VB method and the use of an effective spin Hamiltonian in such a context. The aim was to develop a more practical method applicable to conjugated systems.Pseudopotential Methods.-Developments in this area continue since it is hoped that ab initio methods of this type will enable accurate valence electron only calculations to be made on quite complex molecules. Methods using this formalism 65 P. Carsky and J. Hubac Collect. Czech. Chem. Commun. 1981,46 1324. 66 V. Staemmler R. Jaquet and M. Jungen J. Chem. Phys. 1981 74 1285. 67 V. Staemmler Theor. Chim. Acta 1982,62 69. 68 I. Reeggen Znt. J. Quantum Chem. 1982,22 149. 69 I. Reeggen Chem. Phys. Lett. 1982,92 398. 70 A. Kurtz N. Elander 0.Goskinski and E. Sangfelt Znt. J. Quantum Chem. 1981 S15 143. 71 J. V. Ortiz B. Weiner and Y. O.hcn Znt. J. Quantum Chem. 1981 S15,113. 72 R. L. Lozes B.Weiner and Y. Ohm Znt. J. Quantum Chem. 1981 S15 129. 73 W. Meyer J. Chem. Phys. 1976,64,2901. 74 C. E. Dykstra R. A. Chiles and M. D. Garrett J. Comput. Chem. 1981 2 266. 75 H.-J. Werner and E.-A. Reinsch J. Chem. Phys. 1982,76,3144. 76 P. G. Jasien and C. E. Dykstra J. Chem. Phys. 1982 76,4564. 77 M. Simonetta M. Raimondi and G. F. Tandardini Int. J. Quantum Chem. 1981 S15,225. 78 S. Kuwajima J. Chem. Phys. 1981 74 6342. 79 S. Kuwajima J. Chem. Phys. 1982.77 1930. 22 C. Thomson have been reviewed by Topiol et u1.,80and its use in molecular structure calculations of geometries and transition states using gradient-optimization methods has been tested by Morukuma’s group” and the results found to be in excellent agreement with full ub initio calculations.The extension of such calculations to transition-metal atoms has been reported by several groups. One calculation of the barrier to internal rotation” in Ni&*- gave rather poor results and reasons for this were discussed. However some successful results have been obtained using minimum basis adapted pseudo- potentials for Cu in calculations on Cu Cu2 and CUF.~~ Gaspar’s group using pseudopotentials within the FSGO method have deter- mined improved parameters for these pseudo potentials and further applications to the structure of ethylene acetylene and H20 were rep~rted.’~ In a later paper,” the pseudopotential parameters of DZ quality were developed and used to study various properties of Liz LiH and BeH2. Generalized valence bond (GVB)-effective potentials have been used in studies of the stabilities of HSiSiH and five isomeric structures.86 The acetylene analogue was the least stable structure with the analogue of vinylidene carbene H2SiSi the most stable.In the above calculations a DZ-quality valence basis set was used with the Si pseudopotential. A scaled single zeta basis of wider applicability has been developed by the same group and used in an investigation of the structure of Si2H6.87 The structure is explicable in terms of localized u and SiH bonds analogous to C2H6 unlike the case of disilene and disilyne. 3 Potential Energy Surfaces and Reaction Mechanisms Potential Energy Surfaces.-The fundamental importance of the potential energy hypersurface in the analysis of chemical reactions is unquestioned and Mezey in particular has investigated several important fundamental questions concerning the general characteristics of these s~rfaces.~~*~~ Fukui” has reviewed the ideas of the Intrinsic Reaction Co-ordinate approach and Nauts and Chapuisat have given criteria for the definition of an Intrinsic Reaction Path.” The problem of locating transition states has been discussed by S~hlegel,~’ and a practical implementation and results described.Alternative procedures have been ‘O S. Topiol R. Osman and H. Weinstein Ann. N.Y. Acad. Sci. 1981 367 17. ” S. Obara K. Kitaura and K. Morukuma Theor. Chim. Acta 1981,60 227. 82 0.Gropen U. Wahlgren and L. Pettersson Chem. Phys. 1982 66,453 459. 83 G. H.Jeung J. C. Barthelat and M. Pelissier Chem. Phys. Lett. 1982,91 81. 84 R. Gaspar Jun. and R. Gaspar Int. J. Quantum Chem. 1982 22,415. ’’ R. Gaspar Jun. and R. Gaspar Int. J. Quantum Chem. 1982 22,421. 86 L. C. Snyder Z. R. Wasserman and J. W. Moskowitz Int. J. Quantum Chem. 1982,21,565. 87 J. W. Moskowitz S. Topiol L. C. Snyder and M. Ratner Int. J. Quantum Chem. 1981,19 131. ’’ P. G. Mezey Theor. Chim. Acta 1982,60,409. 89 P. G. Mezey Theor. Chim. Acta 1981 S15 279. 90 K. Fukui Acc. Chem. Res. 1981 14 363. 91 A. Nauts and X. Chapuisat Chem. Phys. Lett. 1982,85 212. 92 H. B. Schlegel J. Comput. Chem. 1982 3 214. Theoretical Chemistry 23 developed by Bell93 and S~harfenberg.'~ Almlof has derived a compact formalism for the calculation of MC-SCF gradients a problem also tackled by Schaefer et ~1.~' An algorithm for efficiently evaluating energy gradients using s and p functions has been described by S~hlegel.~' Turning now to particular potential energy (P.E.) surfaces computations corre- lated P.E.curves have been computed for several diatomic molecules among them a detailed study of various states of N2.96MC-SCF studies on the reaction C' + H2 -+ CH' + H have been reported,97 and a detailed MRD-CI study of the photo- dissociation of H20 -+ O('D,) + H2(1Xi).98At the SCF level a study of the NSF surface reveals NSF to be more stable than FNS by -85 kJ m01-l.~~ An interesting study has been made of various 22 electron molecules and the dissociation process leading to the isoelectronic products CO or N2 or BH3 CH2 NH and O.looCalculated values of the dissociation energies are in good agreement with experiment where known lending some confidence to predicted values for the dissociation of HNCO BH,CO CH2N2 and BH3N2.An investigation of the insertion reactions of CH and SiH2 into H has been reported by Gordon.'" A similar study of SiH + H,O has been reported using an extended basis set and correlation corrections.102 An interesting study of the oxidation mechanism of NH2 + O2 $ NH202 using calculated parameters from a 4-31G basis set calculation determined the transition- state geometry and calculated the thermodynamic parameter^."^ SN2transition states have been studied in detail by Wolfe et aZ.'04~105 following earlier ~~rk.'~~~~~~ It was shown that 4-3 1G calculations are adequate to characterize the surface and the results of the calculation of AH and for ten transition states in process (1)gives theoretical backing for Hammond's Rule.A second paper dealt with intrinsic barriers,106 rate equilibria relationships and the Marcus equation. X-+ CH3F + [X-CH3-F]-+ CH3X + F-(1) Two papers have dealt with the CH20 system important in the ozonolysis of C2H4.The minimum energy pathway diradical state and saddle point were deter- mined.108.109 Bouma el al. have explored in detail the C,H20 surface at the 4-31G level. Relative energetics were then more accurately calculated using the MP3 93 S. Bell J. S. Crighton and R. Fletcher Chem. Phys. Lett. 1.981,82 122. 94 P. Scharfenberg J.Compuf. Chem. 1982 3 277. 95 H. B. Schlegel I. Chem. Phys. 1982,77 3676. 96 W. C. Ermler A. D. McLean and R. S. Mulliken J. Phys. Chem. 1982,86 1305. 97 S. Sakai S. Kato K. Morukuma and I. Kusunoki J. Chem. Phys. 1981,75 5398. 98 G. Theodorakopoulos C. A. Nicolaides R. J. Buenker and S. D. Peyerimhoff Chem. Phys. Lett. 1982,89 164. 99 R. Seeger U. Seeger R. Bartetzko and R. Gleiter Inorg. Chem. 1982 21,3473. IonJ. Breulet and J. Lievin Theor. Chim. Actu 1982,61 59. M. S. Gordon J. Chem. SOC.,Chem. Commun. 1981,890. K. Raghavachari J. Chandrasekhar and M. J. Frisch J. Am. Chem. SOC..1982,104,3779. C. Pouchan and M. Chaillet Chern. Phys. Lett. 1982 90 310. Io4 S. Wolfe D. J. Mitchell and H. B. Schlegel Can. J. Chem. 1982,60 1291. '" S.Wolfe D. J. Mitchell H. B. Schlegel C. Minot and 0.Eisenstein Terruhedron Lett. 1982 23 615. S. Wolfe D. J. Mitchell and H. B. Schlegel J. Am. Chem. Soc. 1981 103 7692. S. Wolfe. D. J. Mitchell and H. B. Schlegel J. Am. Chem. SOC.,1981 103,7694. R. Cimiraglia T.-K. Ha and Hs. H. Gunthard Chem. Phys. Lett. 1982.85 262. R. Cimiraglia T.-K. Ha R. Meyer and Hs. H. Gunthard Chem. Phys. 1982,66 209. C. Thomson method. Ketene is the most stable isomer but hydroxyacetylene is a potentially observable isomer.' lo Several P.E. surfaces of the important combustion intermedi- ate CH30 have been studied. '" Pop1e's'l2 group have carried out a similar study of the C2H4 surface using an extended polarized basis set (6-311G**) with up to MP4 corrections. After inclusion of zero-point vibrational corrections it was found that singlet CH3CH is not a local minimum rearrangement to CzH4 should occur without activation.The structures and rearrangements of various cyclic and acyclic C4HsNO+ cations have been studied by MNDO and ab initio (4-31G) method^."^ The cyclic cations have unusual stability in this system the most stable being (1). (1) K~tzelnigg"~ has carried out a very detailed study of the reaction PH5 +PH + H2.Of the numerous other studies in this area we draw attention to the MC-SCF study of the O(3P)+ C& -+ CH,CH,O-reaction where a stable diradical is produ~ed."~ The transition state in this case has C symmetry i.e. the addition is asymmetric. The decomposition of glyoxal continues to fascinate theoreticians and the transi- tion state of C2"symmetry has been located by Schaefer's group using several different basis sets.116 Considerably different results are obtained in the vibrational analysis of the four stationary point structures and a detailed discussion of the implications of this is given.The pathway for the interconversion of the cyclobutene cation (2) to the buta-1,3- diene cation (3) has been studied by both MIND0/3 and ab initio methods with an STO-3G basis and also with a 6-31G* basis. The activation energy is in good agreement with the experimental value. A model for the Grignard reaction has been studied by Nagase et al.,"' and by Sakai and Jordan."' 1,2-Hydrogen shifts continued to be studied,12' including the barrier for reaction (2).Addition of polarization functions however does not change the predicted H H H \/ / H/sI'c\H -b H H /Si-Y-H W. J. Bouma R. H. Nobes L. Radom and C. E. Woodward J. Org. Chem. 1982,47 1869. 'I' C. F. Jackels J. Chem. Phys. 1982,76,505. 'I2 R. Krishnan M. J. Frisch J. A. Pople and P. von R. Schleyer Chem. Phys. Left. 1982.85 145. G. Frenking and H. Schwarz J. Comput. Chem. 1982 3 251. '14 W. Kutzelnigg and J. Wasilewski J. Am. Chem. SOC.,1982,104,953. I" M. Dupuis J. J. Wendoloski T. Takada and W. A. Lester Jun. J. Chem. Phys. 1982 76 481. 'I6 Y. Osamura H. F. Schaefer tert. M. Dupuis and W. A. Lester Jun. J. Chem. Phys. 1981.75 5828. '" D. J. Bellville R. Chelsky and N. L. Bauld J. Comput. Chem. 1982 3 548. '" S. Nagase and Y.Uchibori Tetrahedron Left. 1982,23,2585. 'I9 S. Sakai and K. D. Jordan J. Am. Chem. SOC.,1982,104,4019. Y. Yoshioka and H. F. Schaefer tert. J. Am. Chem. SOC.,1981,103,7366. Theoretical Chemistry 25 barrier substantially even after extensive CI. There is still a continuing discrepancy with experiment. 1,2-Shifts in open-shell systems have been studied at the POL-CI level.'*' In simple alkyl radicals and triplet carbenes the lowest energy pathway for 1,2-hydrogen migration will be CH bond cleavage in most cases followed by reattachment. More complicated systems like that of the cycobutadiene radical cation,'22 and me thylene cy~lopropane'~~ and cyclopropane cation'24 surfaces are necessarily restricted to smaller basis sets such as STO-3G or to semi-empirical studies i.e.OCS,.'~~ A reaction that is of some importance is the H20 + CO -D H2CO3 reaction. A detailed (4-31G basis) study126 of various conformers of the acid and also of the bicarbonate ion has led to an estimate of the AHo for the hydration reaction. Discrepancies between theory and experiment are ascribed to basis set effects not to correlation effects. Harding and co-w~rkers'~~~'~~ have computed the rate constants for the reactions H2 + C2H -+ H + C2H2 and also located the transition state. The method used was the GVB-CI SOGVB or POL-CI. Finally the reaction surface for the H20-Li20 system has been studied at the MP3 level of The reaction proceeds with activation to yield the cyclic dimer of LiOH. The results agree well with experiment.4 Electronic Structure and Molecular Geometries Large numbers of papers have appeared this year indeed the pace of the application of theoretical methods of ab initiu quality to organic systems continues to accelerate. The papers referred to in this review will necessarily be selected according to this reviewer's prejudices. A thorough study of 30 organic corn pound^'^^ using complete geometry optimiz- ation at the 4-21G SCF level has led the authors to suggest a series of empirical corrections to the computed geometry to give geometries in agreement with experiment. An important paper by De Frees et al.'31compares theoretical geometries calculated with the MP3 or CID levels of theory and a 6-31G* basis set for a large set of one- and two-heavy-atom molecules.The results are an improvement over the MP2 results and are in good agreement with experiment. High quality CASSCF calculations on HNOi32 have been reported but CI is necessary to obtain results in agreement with experiment. 12' L. B. Harding J. Am. Chem. SOC.,1981 103 7469. 122 W. T. Borden E. R. Davidson and D. Feller J. Am. Chem. Soc. 1981,103,5725. 123 D. Feller K. Tanaka E. R. Davidson and W. T. Borden J. Am. Chem. SOC.,1982 104. 967. 124 J. R. Collins and G. A. Gallup J. Am. Chem. SOC.,1982,104 1530. 12' L. Carlsen J. Cornput. Chem. 1982 3 123. 126 P. George C. W. Bock and M. Trachtman J. Comput. Chem. 1982,3,283. '*' L. B. Harding G. C. Schatz and R. A. Chiles J. Chem. Phys. 1982,76,5172. 12* L. B.Harding A. F. Wagner J. M. Bowman G. C. Schatz and K. Christoffel J. Phys. Chem.. 1982 86,4312. 129 K. Raghavachari J. Chem. Phys. 1982,76 5421. L. Schafer C. Van Alsenoy and J. N. Scarsdale J. Mol. Srruct. 1982,86 349. 131 D. J. De Frees K. Raghavachari H. B. Schlegel and J. A. Pople J. Am. Chem. SOC.,1982,104,5576. 13' A. Heibern and J. Almlof. Chem. Phys. Lett. 1982.85 542. C. Thomson The remarkable stabilities of Li-substituted carbenium ions CLi3- H,' (n = 0-3) have been discovered by Pople and co-worker~.~~~ CLi3+ in particular is very stable but in the triplet state. The ionization potential of CLi is predicted to be 6 eV less than that of CH,. The stabilizing influence of Li is such as to make CLi3+ one of the most stable carbenium ions known.Recently it was also shown that the CH3+ triplet state is n0np1anar.l~~ The same group have shown that in neutral AX molecules when X = Li the octet rule is violated CLi3 and OLi are predicted to be ~tab1e.l~~ The cation CLi5+ has recently been observed in the gas phase following earlier theoretical predictions and a more accurate recent study has reconsidered this species together with CLi6'+ CLi6 and CLi8 all of which are predicted to be stable.136 The structures of other carbenium ions with electropositive ~ubstituents'~~ have been studied as well as those of CH2" and CH42+.138 The latter have relatively high barriers to proton loss in contrast to the case of CH3'+ and CH'+. Various C2H6'+ structures have been studied by Olah139 and also by Pople et aI.14' The methanol cation (4) has been to be higher in energy on the CH40+ surface than the alternative structure (5).Experimental evidence for this species has been pre~ented.'~' A similar study of the pathways'43 for the decomposition of CH30H2+ has been reported. H H 1' 1' 1 ,c =c C. H H/y..H I H 0' 0 H\ ..I= H. 1; F--C=C*-F 1' I "H F' H (6) (7) (4) (5 ) Alternative structures for the ions of C2H2- have been Diffuse d-orbitals are essential to describe anions correctly and these studies utilized the 4-31G+ basis set. The conclusion was the structure is the vinylidene species (6). The entire set of anions XCH2- where X = Li BeH BH2 CH3 NH2 OH and F were studied with 4-31G+ and 6-31G+* basis sets and also at the MP2 level by Spitznagel et 133 J.Chrandresekhar J. A. Pople R. Seeger U. Seeger and P. von R. Schleyer J. Am. Chem. SOC. 1982,104,3651. 134 J. A. Pople and P. von R. Schleyer Chem. Phys. Lerr. 1982.91 9. P. von R. Schleyer E.-U. Wurthwein and J. A. Pople J. Am. Chem. SOC.,1982,104,5839. E. D. Jemmis J. Chandresekhar E.-U. Wurthwein P. von R. Schleyer J. W. Chinn Jun. F. J. Landro R. J. Lagow B. Luke and J. A. Pople. J. Am. Chem. Soc. 1982,104,4275. 13' J. A. Pople. Y. Apeloig and P. von R. Schleyer Chem. Phys. Lett. 1982 85,489. 13* J. A. Pople B. Tidor and P. von R. Schleyer Chem. Phys. Lett. 1982,88,533. G. A. Oiah and M. Simonetta J. Am. Chem. SOC.,1982 104. 330. I4O P. von R. Schleyer A. J. Kos J. A. Pople and A. T. Balaban J.Am. Chem. Soc. 1982 104 3771. 141 W. J. Bouma R. H. Nobes and L. Radom J. Am. Chem. Soc. 1982,104,2929. W. J. Bouma J. K.MacLeod and L. Radom J. Am. Chem. Soc. 1982,104,2930. 143 R. H. Nobes and L. Radom Org. Mass.Spectrom. 1982,17 340. 144 J. Chandresekhar R. A. Kahn and P. von R. Schleyer Chem. Phys. Lett. 1982,85,493. "Ii G. W. Spitznagel T. Clark J. Chandresekhar and P. von R. Schleyer J. Comput. Chem. 1982,3,363. TheoreticaI Chemistry 27 The geometry of the fluoro-substituted ethylene radical ions has been established by Merry and Th~mson,'~~ and independently by Paddon-Row et uf.14' Although the radical cations are planar substitution of only one F-atom is sufficient to give a markedly non-planar radical centre. The extreme case of C2F4- is that of CZh symmetry (7).There have been various other studies of the structure of radicals including an ab initio SCF-CI of a stable nitroxide of known geometry and in particular the calculation of the hyperfine tensors from the wave function. 14' Pacansky and co-workers have continued their studies of simple alkyl radicals. Barriers to internal rotation were found to be small in the neopentyl and isobutyl radical^.'^' The same author15' has used a very large basis set to study CH3 and shown that only if inadequate bases are used is the geometry predicted to be non-planar. Bair and Goddard have reported GVB calculations of the structures of several peroxides and peroxy radicals. There have been fewer studies of carbenes but an SCF-CI of CH3CH has suggested that this species should not exist as a stable intermediate although there is a local minimum on the P.E.surface. The singlet-triplet gap is -40 kJ mol-'. Kim So and Schaefer have looked at three low-lying states of HOCOC using SCF and CI methods (including up to 31 532 configurations). The ground state is 'A" lying 40 kJ mol-' lower than the 'A' The calculation of the acid-base behaviour of several systems has been pursued by several groups. A comprehensive study of the methyl and silyl amines and their ions using the 3-21G basis set and full geometry optimizations was reported by Thornson and Glide~el1.I~~ With the exception of (SiH3)'N- [and the isoelectronic molecules (SiH3)*0 and (SiH3)2C2-] the observed data can be rationalized without the inclusion of 3d functions in the basis set.A related study by del Bene et ~1.'~~ examined the protonation of NH3 H20 and HF and the closed-shell bases H,ABH, where A B may be C N 0,and F. The inclusion of correlation (up to MP4 level) did not change the results much from the SCF values however the barriers to proton transfer are more sensitive to correlation. Similar FSGO basis set calculations lead to similar conclusions. 156 Zielinski et af.15' have examined the structures of protonated neutral and deprotonated amidine HC(NH,)(=NH) at the 3-21G level and the structures of several transi- tion states in this system. 146 S. Merry and C. Thornson Chem. Phys. Lett. 1981,82 373. 147 M. N. Paddon-Row N. G. Rondan K. N. Hauk and K.D. Jordan J. Am. Chem.SOC.,1982,104,1143. 14* R. Briere T. A. Claxton Y. Ellinger P. Rey and J. Langier J. Am. Chem. Soc. 1982,104.34. 149 J. Pacansky and W. Schubert J. Chem. Phys. 1982.76 1459. J. Pacansky J. Phys. Chem. 1982.86,485. lS1 R. A. Bair and W. A. Goddard tert. I. Am. Chem. Soc. 1982 104 2719. 15' T.-K. Ha M. T. Nguyen and L. G. Vanquickenborne Chem. Phys. Lett. 1982,92,459. K. S. Kim S. P. So and H. F. Schaefer tert. J. Am. Chem. Soc. 1982 104 1457. C. Glidewell and C. Thornson J. Comput. Chem. 1982,3,495. 155 J. del Bene M. J. Frisch K. Raghavachari. and J. A. Pople J. Phys. Chem. 1982,86 1529. H. Huber and J. Vogt Chem. Phys. 1982,64 399. T. J. Zielinski M. R. Peterson I. G. Csizmadia and R. Rein J. Comput. Chem. 1982 3,62. C.Thomson The destabilizing effects of CN and CF3 groups on a-heteroatom stabilized cations has been rep~rted,'~~ but on the other hand CN and NC stabilize car- bani on^.'^^*'^^ Calculations on several different aldoximes RCH=NOH have been reported,161 including the low-lying states of CH2=NOH.'62 There have been two further sets of calculations on ketenimine H2C=C=NH. 163~164 A complete yo structure has been computed for pr~padienone,'~~ including a study of the effect of electron correlation. The molecule (8)is planar but with a CCC angle of 145". A long standing problem of the cis-trans energy difference between the isomers of 1,2-difluoroethylene has been studied'66 with SCF method the SCEP method and also MP2 and coupled-cluster methods.Correlation plays a crucial role in determining the structure here. Among several miscellaneous molecules studied this year are HCCNO 167 as yet not synthesized the triplet species on the C3H60 surface,'68 the mechanism of the reduction of R2C0 by LiBH4,'69 and a study of the (CO)nm- isomers.'7o HF instabilities occur in these systems and hence correlation effects are expected to be important. Several conformational studies on glycine and substituted glycines have a~peared,'~72 'J as well as studies on several alkanes:'73 n-propane n-butane iso-butane n-pentane and n-r~onane'~~ have all been studied by complete geometry optimization at the 4-21G basis set level. Turning now to the results for cyclic systems there has been a recent detailed study of C6H6 and C6F6 with 20 different basis Minimal basis contractions M.N. Paddon-Row K. N. Houk and T. T. Tidwell Tetrahedron Lett. 1982,23,383. J. B. Moffat Int. J. Quantum Chem. 1982 22 299. 160 J. B. Moffat J. Am. Chem. SOC.,1982,104 3949. 16' M. T. Nguyen and T.-K. Ha J. Mol. Struct. 1982 88 127. "* T.-K. Ha Chem. Phys. Lett. 1982,86 477. 163 J. Kaneti and M. T. Nguyen J. Mol. Struct. 1982 87 205. T.-K. Ha and M. T. Nguyen J. Mol. Struct. 1982,87,355. L. Farnell and L. Radom Chem. Phys. Lett. 1982 91 373. 166 S. R. Gandhi M. A. Benzel C. E. Dykstra and T. Fukunaga J. Phys. Chem. 1982,86 3121. S. Bell J. Mol. Strucf. 1982 87 25. M. R. Peterson and G. R. De Mare J. Mol. Struct. 1982,88 1. 169 R. Bonaccorsi P.Palla and J. Tomasi J.Mol. Srrucr. 1982,87 181. D. P. Vercanteren J. G. Fripiat and J. M. Andre Int. J. Quantum Chem. 1981 SlS,219. 17' L. Schafer C.Van Alsenoy and J. N. Scarsdale J. Chem. Phys. 1982,76 1439. 17* V. J. Klimkowski J. D. Ewbank C. Van Alsenoy J. N. Scarsdale and L. Schafer J. Am. Chem. Soc. 1982,104 1476. 173 J. N.Scarsdale C. Van Alsenoy and L. Schafer J. Mol. Strucr. 1982,86,277. 174 J. N. Scarsdale H. L. Sellers L. Schafer and N. L. Allinger J. Comput. Chem. 1982 3 269. 17' J. Almlof and K. Faegri Jun. J. Chem. Phys. 1982 xxx. Theoretical Chemistry 29 of atom-optimized basis sets are quite unreliable particularly for such basis sets on H; d-orbitals are essential for high accuracy in the case of C6F6. Boggs and co-w~rkers'~~ have also investigated various fluorobenzenes (including C6F6) using the 4-21G basis set.Several nitrogen-containing heterocycles have been studied by Boggs' group,'77 and the dihydroderivatives of benzene naphthalene and anthracene have been calculated to be ~1anar.l~~ The lowest triplet state of pyrazine has been studied at the MC-SCF particularly with regard to the e.s.r. parameters of the 3(nn") state. However the MC-SCF wave function was shown to break symmetry at D, geometries. Several cyclobutanes18' and ions derived from them have been studied,18' also bicyclobutanes,'82 a tricy~lohexane,'~~ and ~ubane.'~~*l~' Borepin (9) and cycloheptatriene'86 have also been examined and Goddard et have fully optimized structures for the classical and non-classical forms of the 2-norbornyl cation and edge-protonated nortricyclene.The calculated geometry suggests that the classical isomer is best viewed as an unsymmetrically bridged species. The tautomeric equilibria of 4-hydroxy-2-pyridinone and its other forms has been studied by Schlegel'88 using the 3-21G basis set. The relative energy differences were in reasonable agreement with the available experimental data. The same author'89 has also investigated 2- and 4-pyridone tautomers and the tautomers of uracil have been studied by Mondragon and Blake.'" Turning now to calculations on molecules containing elements of the second row there have been many interesting studies. The paperlS4 on the basicities of sub- stituted phosphines has been referred to above and Schaefer has reviewed the evidence for Si=C double bonds.'" The interest in organosilicon compounds continues C2H2 and Si2H2 have been compared,19* the disilyne being twisted in the ground-state configuration.The geometries and relative energies of singlet silylsilylene and disilene have been and it was found that the planar disilene is also not a minimum on the surface. However the energy difference between the two isomers is very small. 176 J. E. Boggs F. Pang and P. Pulay J. Comput. Chem. 1982,3 344. 177 F. Pang P. Pulay and J. E. Boggs J. Mol. Struct. 1982,88,79. 17' K. B. Lipkowitz P. W. Rabideau D. J. Raber L. E. Hardee P. von R. Schleyer A. J. Kos,and R. A. Kahn J. Org. Chem. 1982 47 1002. J. C. Ellenbogen D. Feller and E.R. Davidson J. Phys. Chem. 1982,86 1583. D. Feller E. R. Davidson and W. T. Borden J. Am. Chem. SOC., 1982,104 1216. T. Jonvik and J. E. Boggs J. Mol. Struct. 1981,85 293. P. N. Skancke J. Mol. Strucr. 1982 86 255. P. N. Skancke Acta Chem. Scand. Ser. A 1982 36 513. J. Almlof and T. Jonvik Chem. Phys. Lett. 1982 92. 267. C. Van Alsenoy J. N. Scarsdale and L. Schafer J. Comput. Chem. 1982 3 53. J. M. Schueman R. L. Disch and M. L. Sabio J. Am. Chem. SOC.,1982 104 3785. 187 J. D.Goddard Y. Osamura and H. F. Schaefer tert. J. Am. Chem. SOC.,1982,104 3258. 188 H. B. Schlegel Int. J. Quantum Chem. 1982 22 1041. H. B. Schlegel P. Gund and E. M. Fluder J. Am. Chem. SOC.,1982,104,5347. 190 A. Mondragon and I. Ortega Blake Int. J. Quantum Chem. 1982 22,89.H. F. Schaefer tert. Acc. Chem. Res. 1982 15 283. 'q2 F. Kawai T. Noro A. Murakami and K.Ohno Chem. Phys. Lett. 1982 92,479. 193 K. Krogh-Jespersen J. Phys. Chem. 1982,86 1492. C. Thornson has examined various carbon-silicon-bonded isomers with F and Li substituents and concluded that fluorine attached to Si has a strong stabilizing effect. Similar species containing Ge have also been studied i.e. H2Ge=CH2.195 Pseudopotential calculations were used in calculations on digermene.'96 Its geometry is trans-bent (10)and it is more stable than the HGe-GeH3 isomer. (10) (11) The inversion barrier in PH3 has been computed with a near-HF basis of STO's,19' and extensive CI in the valence space. The dependence of the barrier on basis set size the method of computation and geometry were studied.A moderate effect of correlation on this barrier was found. Comparisons of the results of SCF calculations and correlation calculations on the structures of HCP and FCP have been reported by Thomson and Ellam.'98 STO-3G basis sets for these molecules lead to unacceptable errors in the geometries. The structures and inversion barriers in CF3- and SiF3 have been computed.'99 The HOMO of planar SiF3- is of al' symmetry rather than the expected a,". A variety of calculations deal with the geometry of other sulphur-containing compounds such as thionylimide2" (HNSO) thioacrolein and thioglyoxal,20' and H2CS for which harmonic vibrational frequencies were calculated.20z Calculations on various sulphanes at the STO-3G" level were reported by Boyd.203 This basis is however inferior to those developed for the second row by Huzinaga's group.204 Several thiocarbonyls have been studied by Ha and co- worker~,~~~ as well as sulphido-borons206 R-B=S studied earlier by Thom~on.~~' Finally the simplest metallocyclobutane (11)has been studied at the SCF level and also by GVB-CI.208 5 Excited States and Ionization Processes There have been computations of the vertical ionization potential of OF2 HNFz and CH2F2 by a perturbation method developed by Ch~ng,~'~ but with a DZ-quality 194 M.S. Gordon J. Am. Chem. SOC.,1982 104,4352. 195 T. Kudo and S. Nagase Chem. Phys. Lett. 1981,84 375. 196 C. Trinquier J.-P. Malrieu and P. Riviere J. Am. Chem. SOC.,1982 104,4529.D. S. Marynick and D. A. Dixon J. Phys. Chem. 1982,86,914. 198 C. Thomson and P. Ellam Theor. Chim. Actu 1982 62 81. 199 D. S. Marynick J. Mol. Struct. 1982,87 161. 2oo K. Raghavachari J. Chem. Phys. 1982,76 3668. 201 T.-K. Ha M.-T. Nguyen and L. Vanquickenborne Z. Nuturforsch. Teil A 1982,37 125. '02 J. D. Goddard and D. J. Clouthier J. Chem. Phys. 1982 76 5039. 203 R. J. Boyd and J. F. Szabo Can. J. Chem. 1982,60,730. 204 Y. Sakai H. Tatewaki and S. Huzinaga J. Comput. Chem. 1981 2 108. 205 T.-K. Ha M.-T. Nguyen and L. G. Vanquickenborne J. Mol. Struct. 1982,90 107. 206 T.-K. Ha M.-T. Nguyen and L. G. Vanquickenborne J. Mol. Strwct. 1982 90 99. 207 C. Thomson Chem. Phys. Lett. 1974,25 59. '08 A. K. Rappe and W. A. Goddard tert. J. Am Chem.SOC.,1982 104,297. '09 S. R. Langhoff and D. P. Chong Chem. Phys. Lett. 1982,86,487. 19' Theoretica1 Chemistry 31 basis set. Extensive CI calculations continue to be made on the states of HC0,210 of acrolein,211 and of the ionic states of vinylidene and acetylene.212 The lowest excited state of the latter is 4A2with a cis-bent configuration. The geometry of the nII* states of p-benzoquinone have been computed with both a CI technique and a VB Calculations of the excitation energies in trimethylenemethane derivatives214 using better than DZ-quality basis sets were made to aid the understanding of substituent effects in these systems. The 'n-ll state of pyrazine was also investigated by Kleier and co-worker~.~'~ Using an STO-3G basis set and CI the triplet-triplet absorption spectrum of naphthalene has been studied,216 and the authors conclude that T-T transitions can be treated at this level of theory.6 Molecular Interactions Solvation Phenomena and the Hydrogen Bond.-The incorporation of solvent effects in quantum chemical calculations is of great importance and especially if changes in reaction mechanisms occur between the gas phase and solution reactions. More progress has been made in this area and the results of the ab initio studies are described in this Section. Some of these follow Clementi's approach utilizing intermolecular potential functions derived from high quality SCF wave functions and the properties of H20 have been investigated in detail by Reimer et al.217A further example of this approach is the study of the solvation in a dilute methanol An alternative is the use of a polarizable continuum in which the solvent is treated as a structureless polarizable continuum having a dielectric constant of the pure bulk material.Solvent effects on the internal rotation barrier of formamide were reproduced using this model and a 4-31G basis set ab initio wave function.220 Donor-acceptor complexes of the form A -OH2 (A = Li,Be,C,Na,Mg,Al) have been studied at the SCF level with a 6-31G** basis set.221 The gas-phase solvation of H' by clusters of 1-4 molecules of methanol or 1-3 molecules of (CH,),O have been studied by Hirao et a1.,222 who also studied the interaction of C1- with several RH Studies of the hydrogen bond continue.del Bene has continued her studies at both SCF and SCF-CI levels using a minimal basis set of H-bonding in H20-5-fluorouracil complexes224 and H20-thymine complexes.225 210 K. Tanaka and K. Takeshita Chem. Phys. Lett. 1982 87 373. 211 K. Valenta and F. Grein Can.J. Chem. 1982.60 601. 212 P. Rosmus P. Botschwina and J. P. Maier Chem. Phys. Lett. 1981 84 71. 213 R. L. Martin and W. R. Wadt J. Phys. Chem. 1982,86 2382. 214 S. B. Auster R. M. Petzer and M. S. Platz J. Am. Chem. SOC.,1982 104,3812. 215 D. A. Kleier R. L. Martin W. R. Wadt and W. R. Moomaw J. Am. Chem. SOC.,1982,104,60. 'I6 T.-K. Ha and U.P. Wild J. Compuf. Chem. 1982 3 1. 217 J. R. Reimer R. 0.Watts and M. L. Klein Chem. Phys. 1982,64,95. 218 K. Nakanishi S. Okazaki K. Ikar and H.Touhara Chem. Phys. 1981,84,428. 219 S. Miertus E. Scrocco and J. Tomasi Chem. Phys. 1981,55 117. 220 A. J. Duben and S. Miertus Chem. Phys. Lett. 1982 88 395. 221 J. Bentley J. Am. Chem. SOC.,1982 104 2754. 222 K. Hirao M. Sano and S. Yamabe Chem. Phys. Lett. 1982 87 181. 223 S. Yamabe N. Ihira and K. Hirao Chem. Phys. Lett. 1982.92 172. 224 J. E. del Bene J. Phys. Chem. 1982,86 1341. 225 J. E. del Bene J. Chem.Phys.. 1982 76 1058. C.Thornson The intermolecular H-bond in singlet and triplet excited states of the adenine- thymine pair has been studied,226 as well as the adenine-adenine interaction energy.227 Proton transfer in H-bonded systems has been extensively studied by Scheiner and further work has appeared on the cation dimers of NH and OHz,z28 (NzH7)' and (OzH5)',229 and a detailed studyz3' of correlation effects up to the MP3 level in (H0HOH)- with a range of basis sets up to 6-311G**.Several configurations of the complex between CzH4 and HF have been as has the barrier to proton transfer in the H-bonded dimers of benzoic and acetic Geometry optimization provides better values of the barrier heights in these cases. SCF calculations233 on the complexes of NH with I2 or HI at valence shell DZ level lead to the prediction of complete proton transfer in the case of H3N - - HI i.e. leading to a structure H4N'...I- . Studies of several other complexes have appeared but space limitations precludes any discussion of these. Metal or Metal Ion Interactions.-The interaction of Na' with ethene 2-methyl- propene and benzene has been studied and the optimum structures There have been further studiesz3* of models for cation binding to nucleic acids using HZPO4- and the ions Li' Na+ Be2' and Mg".The interactions are not totally electrostatic apart from those involving Na'. All cations affect the phosphate group substantially. The binding of Ca2' to malonate and formate has been inve~tigated,~,~ both with and without &orbitals in the basis set. d-Orbitals make little difference to the conformational minimum. The interaction between Oz NZ and COz and hydrocarbons and fluorocarbons has been investigated. The electrostatic interaction energy is greater in the former case and electrostatic potential maps were used to predict the structure of the complexes.237 These maps have continued to be of great use in the study of the behaviour of molecules.For instance the most favoured sites for hydration for the tautomeric forms of 2-and 4-hydroxypyridine have been investigated in this way,238 and the charge alternation in substituted alkanes has been investigated by Politzer et aZ.239The electrostatic potentials do not however alternate in the same way. 226 J. Maranon H. Grinberg and N. S. Nudelrnan Int. J. Quantum Chem. 1982 22 69. 227 M. Aida and C. Nagata Chem. Phys. Lett. 1982,86,44. S. Scheiner J. Phys. Chem. 1982 86. 376. 229 S. Scheiner J. Chem. Phys. 1982,75 5791. 230 M. M. Szczesniak and S. Scheiner J. Chem. Phys. 1982 77,4586. 231 D. Volkmann B. Zurawski and D. Heidrich Int.J. Quantum Chem. 1982,22,631. 232 S. Nagaoka N. Hirota T. Matsushita and K. Nishirnoto Chem. Phys. Lett. 1982 92,498. 233 P. Kollrnan A. Dearing and E. Kochanski J. Phys. Chem. 1982,86 1607. 234 J. Sauer and D. Delniger I. Phys. Chem. 1982,86 1327. 235 P. Liebman C. Loew A. D. MacLean and G. R. Pack J. Am Chem. SOC.,1982,104,691. 236 K. E. Gottschalk R. G. Hiskey L. G. Pedersen and K. A. Koehler J. Mol. Struct. 1982 87 155. 237 P. Ruelle and C. Sandorfy Theor. Chim. Acta 1982,61 11. 238 H.-J. Hofmann C. Peinel C. Krebs and C. Weiss Inr. J. Quantum Chem. 1981 20 785. 239 P. Politzer S. L. Whittenberg andT. Warnheirn J. Phys. Chem. 1982,86 2609. Theoretical Chemistry 7 Miscellaneous The effect of correlation on theoretical vibrational frequencies has been investigated in detail by Hehre and co-~orkers.~~~ The errors at the MP3/6-31G* level were found to be an average of -7% too high in some 36 molecules.Cremer has investigated the use of ab initio calculations to calculated thermochemical data such as heats of formation.241 Finally the number of topics treated and the accuracy of the calculations continues to increase and it is clear that ab initio computations will be an increas- ingly important tool in the investigation of chemical processes. Acknowledgement.-The author wishes to acknowledge the invaluable help of Mrs. Maureen Thomson in the mammoth literature search involved in preparing this Report. 240 R. F. Hout Jun. B. A. Levi and W. J. Hehre J. Comput. Chem. 1982 3 234.24 1 D. Cremer J. Cornput. Chem. 1982,3 154 165.

ISSN:0069-3030    

DOI:10.1039/OC9827900017

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (i) Pericyclic Reactions By G. B. GILL Department of Chemistry University of Nottingham Nottingham NG7 2RD 1 Introduction All chemists and particularly those with special interests in pericyclic reactions will take delight in the award of the Nobel Prize for chemistry 1981 to Roald Hoffmann and Kenichi Fukui; the texts of their Stockholm lectures have now been published.' The full papers of several pieces of work previously discussed in Annual Reports have now appeared. Limitations on space however prevent any further discussion this year. Review articles include photochemical cycloadditions uia exciplexes excited complexes and radical ions,2 preparative organic photo~hemistry,~ (2 + 2) cyclorever~ions,~ and some aspects of orbital interactions through bonds.' 2 Cycloadditions and Cycloreversions The Diels-Alder reaction has been modelled by new quantum chemical approxima- tions in terms of a charge flux from space regions in which bonds are broken into those formed rather than in terms of changes in nuclear co-ordinates.The method is 1-2 orders of magnitude faster than the geometrical procedure at a comparable level of sophistication and is suggested to be convenient for the study of medium- sized molecules of ca. 30-40 atoms.6 Volume effects of regioselectivity VAc -VLc and V& -Vk,(where oc mc ot and mt refer to ortho-cis meta-cis ortho-trans and meta-trans adducts respectively) and the volume effects of stereoselectivity V& -VAt and VLc -Vk, have been determined for the Diels-Alder reactions of trans-piperylene with alkyl acrylates CH2=CHC02R (R = Me Pr CMe2Et).The volume effects of regioselectivity became more negative with increasing size of R whereas the volume effects of stereoselectivity varied irregularly; the results were attributed to conformational effects in the ester.' MIND0/3 MO calculations reveal that 1,4-dyotropic" H migrations in H2NC( =NH)C02H H2NCOCONH2 ' R. Hoffmann Angew. Chem. Int. Ed. Engl. 1982,21,711; K. Fukui ibid. p. 801. S. L. Mattes and S. Farid Acc. Chem. Res. 1982 15 80. 'P. Margaretha Top. Curr. Chem. 1982,103 1. E. Schaumann and R. Ketcham Angew. Chem. Int. Ed Engl. 1982 21,225. M. N. Paddon-Row Acc. Chem. Res. 1982,15,245. J. Pantif J. Am. Chem. SOC.,1982 104,7424.'S. K. Shakhova and B. S. El'yanov lzv. Akad. Nauk SSSR Ser. Khim. 1982 1194. * Dyotropic rearrangements are defined by M. T. Reetz Angew. Chem. Znt. Ed. Eng.. 1972 11 129 130. 36 G. B. Gill and CH2=CMeCMe=CH2 proceed by a stepwise mechanism whereas 1,5-and 1,6-migrations in HOCH=C(OH)COCHO and compound (l) respectively pro- ceed by concerted mechanisms.8 0 OH (2) X=H,Y=OH (4) X = H Y = OH (1) (3) X=OH,Y=H (5) X = OH,Y = H x- x@o Y 0 (7) X = D,Y = H (8) X = H,Y = D The simplest enediols (2) and (3) have been generated successfully by flash vacuum pyrolysis at 600 0C/10-5 Torr of the corresponding (4 + 2) adducts with anthracene (4) and (5),respectively. The enediols were characterized specti oscopi- cally.' Results obtained for the (4 + 2) addition of the deuterium-labelled butadiene (6) with maleic anhydride which gave (7) and (8)in 85 :15 ratio leads to a value for AAG* of 1.2 kcal mol-' at 80 "C.Since the endu-transition state is sterically disfavoured in this case the 1.2 kcal mol-' represents the minimum energy advan- tage associated with electronic explanations of the Alder endu rule." Orbital correlation diagrams reveal that the (4 + 2)' reaction is allowed for the case (diene) + (dienophile)' but not for the case (diene)' + (dienophile); thus in the radical cation Diels-Alder reaction diene radical cations function as highly electron- deficient dienophiles. 'la Radical cation polymers with radical cation functions of up to 5% of the cross-linked polystyrene monomer sites have been prepared and are effective in catalysing for example the (4 + 2) dirnerization of cyclohexa-1,3- diene.' Id Danishefsky et al.have published several papers this year on the Lewis acid- catalysed cyclocondensations of (E)-l-methoxy-3-[(trialkylsilyl)oxy]buta-l,3-dienes with aldehyde^'^"^" and with imines,13' affording rapid entry to 5,6-dihydro- B. Ya. Simkin B. V. Golyanskii and V. Minkin Zh. Org. Khim. 1981 17 1793. M.-C. Lasne and J.-L. Ripoll Tetrahedron Lett. 1982 23 1587. L. M. Stephenson D. E. Smith and S. P. Current J. Org. Chem. 1982,47,4170. (a) D. J. Bellville and N. L. Bauld J. Am. Chern. Soc. 1982 104 2665; (b)N. L. Bauld D. J. Bellville S. A. Gardner Y. Migron and G. Cogswell Teruhedron Lett. 1982 23 825.l2 (a) S. Danishefsky J. F. Kerwin and S. Kobayashi J. Am. Chem. SOC. 1982 104 358; (b) S. Danishefsky N. Kato D. Askin and J. F. Kerwin ibid. p. 360; (c) S. Danishefsky and J. F. Kerwin J. Org. Chem. 1982 47 1597; (d) S. Danishefsky S. Kobayashi and J. F. Kerwin ibid. p. 1981; (e) S. Danishefsky E. R. Larson and D. Askin J. Am. Chem. Soc. 1982 104 6457; (f) E. R. Larson and S. Danishefsky ibid. p. 6458. l3 (a) E. R. Larson and S. Danishefsky Tetrahedron Lett. 1982 23 1975; (6) J. F. Kerwin and S. Danishefsky ibid. p. 3739. Reaction Mechanisms -Part (i) Pericyclic Reactions y-pyrones and 5,6-dihydro-y-pyridones, respectively. The ZnC12-catalysed addition of Danishefsky's diene with (R)-glyceraldehyde acetonide in benzene at room temperature afforded the (5S,6R)-heptulose (9) in 72% yield with no detectable contamination by the (5R,6R)-dia~tereoisomer.'~~ The product (9) arises from the reaction of the chiral aldehyde with the diene by &attack in accordance with the classical Cram's rule model.The stereochemical outcome of these cyclocondensa- tions is considerably influenced by the choice of the Lewis acid catalyst. For example predominant trans-addition has been observed using BF,-OEt2 at -78 "C whereas use of ZnC12 at room temperature afforded mainly cis-adducts.12e The ZnC12- catalysed process exhibits all of the characteristics expected from a pericyclic addition mechanism whereas with BF3.0Et2 the evidence indicates that part of the products arise from intermediate siloxonium species.12f*130 Virtually quantitative asymmetric induction (99.3%d.e.) has been achieved in the TiC12(0R)2 catalysed Diels-Alder addition of the acylate ester (10) and its enantiomer [prepared from (R)- (+)-and (S)-(-)-camphor] to ~yclopentadiene.'~ The re-face of the acrylate moiety C carbon atom is sterically blocked by the pendant t-butyl group forcing addition at the si-face of (10) thereby affording adduct (11) of (R)- configuration at the new chiral centre (*). Diels-Alder additions of carbohydrate-derived dienophiles bearing an allylic oxygen function as shown by the generalized structure (12) with the o-quinone methide (13) have been studied to elucidate the stereochemical control exerted by the allylic oxygen func- tion." The preferential formation of adducts of the general type (14) indicates a pronounced selectivity in favour of orbital control over steric control in accordance with previous predictions by Houk ef af.,in that the incoming group should bond to the face opposite to the allylic oxygen function (si-face) so as to minimize secondary orbital antibonding effects.The new chiral centre (*) in (14) was shown to possess the expected (S)configuration. l4 W. Oppolzer C. Chapuis G. M. Dao D. Reichlin and T. Godel Tetrahedron Leu. 1982,23,4781. Is R. W. Franck T. V. John and K. Olejniczak J. Am. Chem. Soc. 1982 104 1106. G. B. Gill Isodicyclopentadiene undergoes a [1,5] H shift at temperatures a170 "Cto form the isomeric highly reactive diene (15) which readily underwent (4 + 2) cycloaddi-tions by attack of the dienophile at the exo-face but giving adducts in accord with Alder's endo rule.16 This reaction has been used as a strategy for the preparation of fused norbornene/norbornadiene adducts.16' The intramolecular Diels-Alder reaction naturally continues to be the focus of considerable interest because of its unique value for the construction of complex molecules of biological importance.Attention has been drawn to the problems associated with such reactions of an (E)-diene;17 these are the ready accessibility of the alternative endo-and exo-transition state topologies and in the case of the introduction of an asymmetric centre at the pentadienylic position of the diene the further complication arising from the four possible modes of addition namely endo-a endo-p exo-a and exo-p.Chiral (2)-dienes on the other hand undergo intramolecular Diels-Alder reaction with enones to afford cis-fused products with complete stereo-and enantio-specificity since the (2)-diene because of its geometry can only attain a single transition-state ge~metry.'~ The utilization of a (2)-diene unit in (16) a relatively unactivated system for the stereoselective construction of the cis-lactone (17) which has previously been transformed into marasmic acid has been reported.'' Comparison of the relative rates of cyclization C0,Me of the (Z,E,Z)-triene with the (Z,Z,Z)-triene (16) at 205 "C indicates the former to be more reactive as expected by a factor of CQ. 42 (AAG*at 205 "C ca.3.5 kcal-mol-'). Comparison with previous work on the stereoselectivity of the (Z,E,Z)-(a) R. Subramanyam P. D. Bartlett G. Y. M. Iglesias W. H. Watson and J. Galloy J. Org. Chem. 1982 47 4491; (6)L. A. Paquette R. V. Williams R. V. C. Carr P. Charumilind and J. F. Blount ibid. p. 4566. " S. G. Pyne M. J. Hensel and P. L. Fuchs J. Am. Chem. SOC.,1982,104 5719; see also S. G. Pyne, D. C. Spellmeyer. S. Chen and P. L. Fuchs ibid.,p. 5728. IX R. K. Boeckman and T. R. Alessi J. Am. Chem. SOC.,1982 104 3216; see also R. K. Boeckman and D. M. Demko J. Org. Chem. 1982 47 1789. Reaction Mechanisms -Part (i)Pericyclic Reactions 39 triene reaction indicates that selection of the transition state is primarily governed by diene geometry that dienophile orientation is independent of dienophile geometry and that the dienophile orientation has no great effect on the activation energy.Of further interest is the finding that [1,5] sigmatropic H shifts apparently do not present a limitation even in highly substituted systems.'* Heating the pentenyl derivative (18) at 150 "C for 16 h in acetonitrile afforded the perhydro- azulene derivative (20) by way of the oxidopyrylium ylide intermediate (19).19 The yield (61%) can be improved by use of a base promoter such as DBN (75%). A claim has been made for the first example in which a monosubstituted benzene ring assumes the role of the diene function in an intramolecular (4 + 2) sycloaddi-tion; thus heading the allenecarboxanilides (21) in xylene at reflux for 4-9 h effects conversion into the spiro-fused lactams (22).20 In one case (R'= R2 = Me R3= SiMe,Ph) an equilibrium process is suggested since the lactam on heating in xylene was partially reconverted into the allene.R2 Much work also continues to be done on 1,3-dipolar cycloadditions and only one or two examples will be considered heie. Thermal or copper-catalysed decomposition of dimethyl diazomalonate [NEN-C(CO~M~)~] in the presence of benzaldehyde afforded mainly the 1,3-dioxolanes (23) and (24) but the oxirane (25) was also formed.21 Although (25) could be converted into (23) and (24) under the reaction conditions this is only a minor pathway and the evidence including that from trapping experiments [e.g. the formation of (26) and (27) in the presence of dimethyl fumarate iqdicates that reaction involves the formation of the carbonyl ylide PhCH= -CC(CO2Me),.Interesting use has been r; made of X=Y-ZH systems as potential 1,3-dip+oles by means of B;onstedy and Fewis acid-catalysis22n (X=Y-ZH $ X=Y(L)-ZH X=Y(L) -Z t* X-Y(L)-Z). Thus for example treatment of the benzylidene imine of methyl- 19 P. G. Sammes and L. J. Street J. Chem. SOC.,Chem. Commun. 1982 1056. *' G. Himbert and L. Henn Angew. Chem. Int. Ed. Engl. 1982,21,620. 21 R. Huisgen and P. de March J. Am. Chem. SOC.,1982,104,4952,4953. 22 (a) R. Grigg and H. Q. N. Gunaratne. J. Chem. Sou Chem. Commun. 1982 384; (b)R. Grigg L. D. Basangoudar D. A. Kennedy J. F. Malone and S. Thianpatangul Tetrahedron Lett. 1982,23,2803. 40 G.B.Gill x (23) X = Ph,Y = H (24) X = H,Y = Ph YX (26) X = H Y = C02Me (27) X = C02Me,Y = H phenylglycinate with a Lewis acid [Z~(OAC)~ > AgOAc > LiOAc > Mg(OAc),] in the presence of methyl propiolate afforded the adduct (28); in the analogous addition catalysed by a Bronsted acid the rate was found to be related to the pKa of the acid.22" The pressure dependence of the rate of the (2 + 2 + 2) cycloaddition of norbor- nadiene to dimethyl acetylenedicarboxylate or tetracyanoethylene has been st~died.'~ The reactions were accelerated by pressure to the same extent as for Diels-Alder reactions and on the basis of the activation volume the establishment of the two new bonds is concluded to occur simultaneously.The AV*:AV ratio (20.85) indicates a product-like transition state. The mechanism of the thermal isomerization of cyclododeca- 1,5,9-triyne to hexaradialene has been investigated by means of the doubly labelled (I3C2)starting material. From chemical degradation of the product and analysis as biacetyl to determine doubly singly or unlabelled material the results indicate that the (2 + 2 + 2) pathway by way of 1,2 :3,4 :5,6-tricyclobutabenzene as intermediate is not followed.24" This conclusion accords with theoretical prediction that thermal alkyne (2 + 2 + 2) cycloadditions are highly unfavourable and instead hexaradialene appears to be formed by a sequence of three Cope rearrangements. Related results have been obtained in the flash and flow pyrolysis of de~a-1,5,9-triyne.~~' In contrast cobalt-mediated intramolecular (2 + 2 + 2) cyclization of a,S,w-diynenes to annulated cyclohexadienes [e.g.(29) + (30)] occurs readily,25" .and Wilkinson's catalyst RhCl(PPh,) is effective at the 0.5-2 mol% level for the intermolecular trimerization of hepta-lY6-diynes with monoacetylenes under mild conditions [e.g. (31) + (32)].25b The mechanism of the photochemical rneta-cycloaddition of olefins to the benzene ring has attracted further study.26 The effects of bulky substituents on the course of the reaction have been rationalized in terms of a mechanism involving an exciplex '' G. Jenner and M. Papadopoulos Tetrahedron Lett. 1982 23,4333. 24 (a)W. V. Dower and K. P. C. Vollhardt J. Am. Chem. Soc.1982,104 6878; (6) W. V. Dower and K. P. C. Vollhardt Angew. Chem. Znt. Ed. Engl. 1982 21 685. '' (a) E. D. Sternberg and K. P. C. Vollhardt J. Org. Chem. 1982 47 3447; (b) R. Grigg R. Scott and P. Stevenson Tetrahedron Lett. 1982 23 2691. 26 (a)D. Bryce-Smith G. A. Fenton and A. Gilbert Tetrahedron Lett. 1982 23 2697; (6) J. Mattay J. Runsink H. Leismann and H.-D. Scharf ibid. p. 4919. Reaction Mechanisms -Part (i ) Pericyclic Reactions intermediate and formation of bonds b and c concertedly but prior to a formula (33) through interaction betweeen S1benzene and Soalkene with meta-cycloaddi-tion being favoured by low vibrational levels of the S1arene.26a The photochemical Y (33) cycloadditions of 1,3-dioxoles to benzene affords predominantly products of exo- configuration in both ortho- and rneta-addition modes.266 The results are rationalized in terms of a mechanism involving an exciplex intermediate with a product-like structure.Wender's group continue to make elegant use of arene-olefin cycloaddi- tions for natural products synthesis; the latest advances are concerned with syntheses of (+)-hir~utene~~" and of (f)-m~dhephene.*~~ Direct excitation at 254 nm of the N-oxides (34)/(35) led to N2/N20metathesis giving (36)/(37) and (38) respectively.28 The analogous azo-compound to (34)/(35) underwent N2 elimination. The reaction between OsO and olefins leading to cis-hydroxylation has been generally assumed to proceed by way of a 67r electron cyclization. Evidence has now been presented in favour of an alternative mechanism involving the formation of an asymmetric (2 + 2) addition intermediate which rearranges and dimerizes in a rate-determining step to give the osmium(v1) ester complex that is subsequently hydrolysed reductively or oxidatively to furnish the cis-diol products.The slow reaction of 1,l-diphenylethylene with Os04 enabled the detection of the asymmetric intermediates (39) and (40) by 'H n.rn.~.'~ Asym-metric (27r + 27r) photopolymerization in chiral crystals has been examined as a " (a)P. A. Wender and J. J. Howbert Tetrahedron Lett. 1982 23,3983; (b)P. A. Wender and G. B. Dreyer J. Am. Chem. Soc. 1982 104 5805. *' H.Prinzbach G. Fischer G. Rihs G. Sedelmeier E. Heilbronner and Yang Z.-z. Tetrahedron Lerr. 1982 23 1251; see also J.M. Mellor R. N. Pathirana and J. H. A. Stibbard ibid. p. 4489; J. M. Mellor and R. N. Pathirana ibid. p. 4493. 29 M. Schroder and E. C. Constable J. Chem. Soc.. Chem. Commun. 1982,734. G. B. Gill X N'N (39) X = Ph,Y = H (40) X = H,Y = Ph means for 'absolute' asymmetric ~ynthesis.~' Thus crystalline chiral phases com- posed of unsymmetrically substituted phenylene-1,4-diacrylateswith the two different double bonds correctly aligned for asymmetric (2 + 2) photodimerization along a translational axis have been engineered. Dimers and oligomers of either chirality with quantitative enantiomeric yield were obtained in several experiments. A new (4 + 4) annulation approach applicable to the synthesis of unsymmetrically substituted and functionalized cyclo-octane derivatives has been devised.31 The strategy involves the (2 + 2) cycloaddition of a vinylketene to a 1,3-diene the 2,3-divinylcyclobutanoneintermediate reacting further by Cope rearrangement to give the eight-membered ring system.Hence reaction of cyclohexa-1,3-diene for example at 120 "C for 4 days in the presence of 2,3-dimethylcyclobut-2-en-l-one (the vinylketene precursor) gave an 81% yield of (41). A high-yield dehydrogena- tion-free synthesis of azulenes is possible through the use of the (6 + 4) cycloaddi- tions of dienamines to fulvenes that possess a potential leaving group at the C-6 position. The 6-p-nitro-benzoyloxy group is particularly well-suited to this pur- The intramolecuIar variant of the (6 + 4) cycloaddition process has now been examined.32b Thermolysis of (42) in o-dichlorobenzene at 180 "C for 7 h gave (43) as the major product of a mixture of olefin isomers arising from [1,5] H shifts following upon the inital (6 + 4) cycloaddition.The analogous intermolecular reaction afforded only endo-(4 + 2) cycloadducts. The alteration in periselectivity may be rationalized in terms of conformational requirements in ring-opened (42) which make the (6 + 4) process favourable in comparison with the spiro-(4 + 2) or ring-(4 + 2) addition modes. It has been suggested that a classification system based on the mode of cycloaddition [i.e. (6 + 2) (4 + 2) or (2 + 2)] of electron- deficient ,2-cycloaddends to tricyclo[5.3.1 .O]undeca-2,4,9-triene (44) could be 30 L.Addadi J. van Mil and M. Lahov J. Am. Chem. SOC. 1982 104 3422; see also J. van Mil L. Addadi E. Gati and M. Lahov ibid. p. 3429; J. van Mil L. Addadi M. Lahov and L. Leiserwitz J. Chem. SOC.,Chem. Commun. 1982,584. 31 R. L. Danheiser S. K. Gee and H. Sard J. Am. Chem. Soc. 1982,104,7670. 32 (a) Y. N. Gupta S. R. Mani and K. N. Houk,Tetrahedron Lett. 1982 23 495; (b)Y. N. Gupta M. J. Doa and K. N. Houk J. Am. Chem. SOC.,1982,104,7336. Reaction Mechanisms -Part (i) Pericyclic Reactions (* +2) (4 +2) valuable in predicting the outcome of new cycloadditions in other These results suggest that 4-phenyl-l,2,4-triazoline-3,5-dione is a better model for singlet oxygen additions than is tetracyanoethylene. Stereospecific endo-(10 + 8) cyclo-addition has been observed between isobenzofuran (8~ component) and 8,8-dimethylisobenz~fulvene.~~ 3 Ene Reactions Intramolecular 'magnesium-ene' reactions of type-13' and t~pe-11~~ have been defined and then employed in a most interesting way for the construction of carbocyclic frameworks.Two type-I processes in sequence were employed in the key steps for the preparation of (f)-A9"2'-~apnellene,35a and the type-I1 reaction for the key step (45)+ (46)in the construction of (f)-khusim~ne.~~' The central carbonyl group of indane-1,2,3-trione exhibits very marked enophilic character; the adduct (47),for example was obtained in high yield from hept-1-yne at moderate temperatures (80-1 10 0C).37 These ene adducts are cleaved essentially quantitatively by periodic acid thus providing a very simple route to allyl- and to 2,3-dienyl-carboxylic acids (i.e.indanetrione is a CO,ene-equivalent).Many potential enophiles are insufficiently reactive to overcome the relatively high energy barriers to the ene additions and hence catalysis of the reactions by Lewis acids continues to be a profitable area for study. Diethylaluminium chloride is an effective promoter for the addition of a-substituted acrylate esters to tri- and 33 L. T. Scott I. Erden W. R. Brunsvold T. H. Schultz K. N. Houk and M. N. Paddon-Row J. Am. Chem. SOC.,1982,104 3659. 34 R. N. Warrener D. A. C. Evans M. N. Paddon-Row and R. A. Russell Ausr. J. Chem. 1982,35,757. " (a) W. Oppolzer and K. Battig Tetrahedron Lett. 1982 23 4669; (6) W.Oppolzer H. F. Strauss and D. P. Simmons ibid. p. 4673. 36 (a)W. Oppolzer R. Pitteloud and H. F. Strauss J. Am. Chem. SOC.,1982,104,6476;(6)W. Oppolzer and R. Pitteloud ibid. p. 6478. 37 G. B. Gill and K. S. Kirollos Tetrahedron Lett. 1982 23 1399. 44 G. B. Gill truns-l,2-di-substitutedalkene~.~~~ The stereochemical courses of the additions occur by way of the least hindered transition state approach geometry and are regio- and stereo-selective in that the ester group adds endo and a hydrogen atom is transferred selectively from the alkyl group syn to the vinylic hydrogen atom. In a synthetic approach to the (*)-pseudomonic acids A and C sequential ene additions of formaldehyde under alkylaluminium chloride 'catalysis' have been used to great effect in constructing the diene (48) from hexa-1,5-diene (Scheme l).38b Elimination of ethane from (48) led to the formation of the Lewis acid (49) Me,AICI (49) Et (i) HCHO (ii) HZO Scheme 1 which in the presence of formaldehyde underwent a quasi intramolecular Diels- Alder reaction giving (after hydrolytic work-up) the dihydropyran (50).Two sequential ene reactions the first intermolecular and the second intramolecular have been used to construct bicyclic alcohols through the Me2A1C1-promoted addition of a,P-unsaturated carbonyl compounds to alkylidene cy~loalkanes.~~" The regio- and stereo-chemical outcome of the BF,-OEt catalysed ene addition of formaldehyde to (E)-and (Z)-l-ethylidene-2-methylcyclopentaneshas been examined and applied to a synthesis of (*)-Prelog-Djerassi la~tone.~~' The stereocontrolled synthesis of (20s)- steroidal side chains from the Lewis acid- catalysed addition of methyl propiolate to the (20E)-steroid has been de~cribed.~~' 38 (a)J.V. Duncia P. T. Lansbury T. Miller and B. B. Snider J. Am. Chem. Soc. 1982 104 1930; (b) B. B. Snider and G. B. Phillips ibid. p. 11 13. 39 (a) B. B. Snider and E. A. Deutsch J. Org. Chem. 1982 47 745; (b) P. M. Wovkulich and M. R. UskokoviC ibid. p. 1600; (c) W. G. Dauben and T. Brookhart ibid. p. 3921. Reaction Mechanisms -Part (i)Pericyclic Reactions 45 The rate of reaction is at least an order of magnitude less rapid than for the (2)-isomer which gives the (20R)-steroid but the reasons for the retardation are not clear at present.Several important papers have appeared this year concerned with the transition- state geometry and mechanisms of ene reactions Lewis acid-catalysed ene reactions and superene reaction^.^^.^' Kinetic isotope-eff ect data for the addition of mesoxalic esters to allylben~ene~~" suggest that a symmetrically structured (2 + 2) charge- transfer complex is formed in a preliminary step followed by a pseudopericyclic transition state similar to that proposed previously41b for superene reactions. In particular the data imply a bent rather than a linear transition state for the H transfer. Further support comes from a study of the activation volumes and pressure dependence of rate constants for the thermal ene reactions of dimethyl mesoxalate with substituted alkene~.~'~ A concerted mechanism in which the transfer of the H atom occurs non-linearly [i.e.(AVf/Ae), ratios are close to unity] in a highly product like transition state is proposed. The SnC14-catalysed addition of diethyl mesoxalate to allylbenzene affords the oxetane (51)rather than the tetrahydrofuran (52).4uc This and previous results can be accomodated by assuming the preliminary formation of a three-membered charge-transfer complex for which there are two possible olefin/electrophile orientations one suitable for the formation of ene adduct (i.e.Markovnikov-type addition with olefins possessing bulky substituents) and the other for formation of oxetane (i.e.anti-Markovnikov-type addition where there is little steric hindrance between the ene and enophile substituents).The Stephenson isotope test using 2-(53) E-(53) and gem-(53) and [2H0]-(53)and 5:. 2 Et12 (51) (52) 2-(53) W = X = CD3 Y = CH3 E-(53) W = Y = CD3 X = CH3 gem-(53) W = CH3,X = Y = CD3 [2H12]-(53)has been applied to the addition of nitrosopentafluorobenzene to obtain the intra- and inter-molecular isotope effect^.^'" The lack of an intermolecular isotope effect compared with the large isotope effects with E-(53)and gern-(53) and the substantial difference in isotope effects for E-(53) compared with 2-(53) are regarded as strong evidence against cleavage of the allylic C-H(D) bond in the rate-determining step. Instead the results are consistent with the rate-deter- mining formation of an intermediate followed by cleavage of the allylic C-H(D) bond in a subsequent faster step with isotopic discrimination when the allylic C-H is cis to C-D [i.e.in E-(53) and gem-(53)].The finding that the reaction rate for the ene addition of 'Ago2 to alkenes is almost entirely determined by changes in 40 (a)H. Kwart and M. W. Brechbiel J. Org. Chem. 1982,47,3353; (b)G.Jenner and M. Papadopoulos ibid. p. 4201; (c)H. Kwart and M. Brechbiel ibid. p. 5409. 41 (a) C. A. Seymour and F. D. Greene J. Org. Chem. 1982 47 5226; (6)H. Miinsterer G. Kresze M. Brechbiel and H. Kwart ibid. p. 2677; (c) J. R. Hurst and G. B. Schuster J. Am. Chem. Soc. 1982,104,6854. 46 G. B. Gill AS*betweeen cis-olefins and non-cis-olefins lends support to a previous proposal that the inital encounter complex of '0 and olefin is not in the required reaction geometry.41" Movement to this geometry is the irreversible entropy-controlled process that brings the reactants to the transition state.The favoured ene transition states for the addition of glyoxylate N-sulphonylimine to prochiral olefins (e.g. cyclohexene and trans-but-2-ene) are those with the ester carbonyl group Highly regio- and stereo-selective SnC1,-catalysed intramolecular ene reaction is observed in the transformation (54) + (55).42bAsym-metric induction levels consistently above 93% were obtained in the additions of olefins to 8-phenylmenthol glyoxylate promoted by SnC14.42c 0 (54) (55) Photobisdecarbonylation of benzonorbornene-2,3-dioneat low temperatures afforded a diastereoisomeric mixture of 1-(indanyl)- lH-indenes which are con- sidered to arise by an [BT + 8~ + 2u] cyclodimerization (56)of intermediate isoindene by an ene-type me~hanism.,~ 4 Sigmatropic Rearrangements Optimized but semi-empirical CND0/2 and MIND0/3 calculations have been performed on the isolated molecule intramolecular [1,3] H shift that converts vinyl alcohol into a~etaldehyde.,~ Surprisingly the barrier to the forbidden suprafacial shift in the plane perpendicular to the molecule is lower than that of the allowed antarafacial shift in the plane of the molecule which is explained by the interaction of the migrating H atom with bonds.Existence of the relatively high barriers indicates that a spontaneous H shift in vinyl alcohol is essentially impossible.The difference between the allowed [1,5]H shift and the forbidden [1,3] and [1,7] H shifts can be traced to the orbital interaction between the migrating proton and the .rr-system of the remaining conjugated anion (the model system employed in the calc~lations).~~ The temperature dependence of the kinetic hydrogen isotope effect for the [1,5] sigmatropic rearrangement (57) (58) has been determined 42 (a) D. M. Tschaen and S. M. Weinreb Tetrahedron Lett. 1982 23 3015; (6) D. L. Lindner J. B. Doherty G. Shoham and R. B. Woodward ibid. p. 5111; (c) J. K. Whitesell A. Bhattacharya D. A. Aguilar and K. Henke J. Chem. SOC.,Chem. Commun.. 1982,989. 43 R. N. Warrener I. G.Pitt and R. A. Russell J. Chem. SOC.,Chem. Cornmun. 1982 1136; see also R. N. Warrener P. A. Harrison and R. A. Russell ibid. p. 1134. 44 M.Zakova and J. Leska Collect. Czech. Chem. Commun. 1982,47 1897. 4s G. Klopman 0.Kikuchi H. Moriishi and K.Suzuki Tetrahedron Lett. 1982 23 3447. Reaction Mechanisms -Part (i) Pericyclic Reactions (57) (E = C0,Me).46 Over a 65 "C temperature range kH/kDwas independent of tem- perature with AH/AD = 5.11 (average value) which can only be equated with an angular geometry for the H transfer; More O'Ferrall calculations provide an estimate of the angle at ca. 145 "C. Accordingly the [1,5] H shift in pentadiene itself [(AE) = 1.4 kcal mol-' and A,/AD = 1.15 from temperature dependence studies] is considered to occur by a concerted linear H-transfer mechanism.Thermal isomerization of (+)-2-deuterio-3,7-dimethyl-7-methoxycycloheptatriene (59) occurs by degenerate [1,5] carbon shifts with inversion by way of the norcaradiene form^.^' This reaction stereochemistry is favoured by least motion but forbidden by orbital symmetry theory and may result from subjacent orbital effects or may involve a diradical intermediate in which rotation at the tertiary radical site occurs much more slowly than closure to form the inversion product. Thermolysis of (0-tolylcarbony1)trimethylsilane at 450 "C-0.05 mm Hg afforded o-trimethylsilyl- rnethylben~aldehyde.~' It appears that the reaction path followed involves acylsilane to siloxycarbene rearrangement insertion of the carbene into the adjacent benzylic C-H bond to give the trimethylsilyloxybenzocyclobutane ring opening to the o-quinone methide and [1,5] sigmatropic shift of the silicon atom from oxygen to carbon.The preparation of 2,6-dicyano- 1,5-dimethylsemibullvalene(60) has been repor- Low-temperature I3C n.m.r. data indicates that (60) possesses an even lower activation barrier (4 kcal mol-') towards degenerate Cope rearrangement than the parent unsubstituted system. The presence of two discrete C=N and two C=C bands in the room-temperature i.r. spectrum however militates against C2-symmetry indicating that (60) hits not achieved homo-aromaticity on the i.r. time scale. The asymmetric induction observed in the thermal (240°C) and PdC12- catalysed Cope rearrangement of (3R,5E)-2,3-dimethyl-3-phenylhepta-1,5-diene 46 H.Kwart M. W. Brechbiel R. M. Acheson and D. C. Ward J. Am. Chem. Soc. 1982,104,4671. 47 J. E. Baldwin and B. M. Broline J. Am. Chem. SOC.,1982 104 2857. 48 C. Shih and J. S. Swenton J. Org. Chem. 1982,47 2668. *9 R. Askani and M. Littmann Tetrahedron Lett. 1982 23 3651; H. Quast J. Christ Y. Gorlach and W. van der Saal ibid. p. 3653; H. Quast Y. Gorlach G. Meichsner K. Peters E.-M. Peters and H. G. von Schnering ibid. p. 4677. 48 G. B. Gill at room temperature occur in the same sense and demonstrates that the PdC1,- catalysed process takes place preferentially (AAG' > 1.9 kcal mol-' at 25 "C) by way of a chair transition Kinetic data and activation parameters are in agreement with a concerted symmetry-allowed unturu-unturu Cope rearrangement for the transformation of (61) into (62) in boiling x~lene.~' Inductively stabilized a-sulphonyl carbanions greatly accelerate the Claisen rear- rangement.Thus treatment of (63) with potassium hydride (1.5 equiv.) in HMPA at 50°C for 3.5 h afforded on work-up the ketone (64).52 The regioselectivity is completely unaffected by the position of the double bond in the vinyl ether portion of the substrate and methyl substitution of the vinyl unit in (63) accelerated the rearrangement. Me Me 5 Electrocyclic Reactions Experimental evidence has been provided that the thermal ring-opening of cis-fused 3-aminocyclobutenes proceeds in a conrotatory fashion to give (initially) the cis,truns-monocyclic system e.g.(65) + (66).53 Irradiation of the diene (-)-(67) at 193 K in pentane with 254 nm light produced the thermally labile chiral triene (+)-(68); irradiation of the triene with 300 nm light at 193 K regenerated (-)-(67) demonstrating that least motion ('accordant') conrotatory processes occurred in the reverse photo reaction^.^^ At 218 K disrotatory cyclization of (68) occurred to give the achiral cis-hexahydronaphthalene (fIl2 = 29 min). g2Me $?2Me S C02Me C0,Me L. E. Overman and E. J. Jacobsen J. Am. Chem. Soc. 1982,104 7225. " R. K. Hall H. J. den Hertog and D. N. Reinhoudt J. Org. Chem. 1982 47,972. " S. E. Denmark and M. A. Harmata J. Am. Chem. SOC.,1982,104,4972. 53 G. W. Visser W. Verboom D. N. Reinhoudt S. Harkema and G. J.Van Hummel J. Am. Chem. SOC. 1982,104,6842. 54 B. Matuszewski A. W. Burgstahler and R. S. Givens 1.Am. Chem. SOC., 1982 104,6874. Reaction Mechanisms -Part (i) Pericyclic Reactions a3 In a series of four short papers Nicolaou et al. have outlined stepwise stereocon- trolled syntheses of endiandric acids A-G including biomimetic studies." The key step in the construction of the common bicyclo[4.2.0]octane moiety present in these systems was the conrotatory cyclization of a 1,8-disubstituted octa- 1,3,5,7-tetraene to a 7,8-disubstituted cyclo-octa-1,3,5-triene which then underwent disrotatory six-electron electrocyclization to give the bicyclic system. Further synthetic elabor- ation included the utilization of intramolecular Diels-Alder reactions to construct the additional carbocyclic rings.The vinylogous fidecene (69) possessed a half-life of ca. 5 min at 150 "Cbeing converted in 55% yield into the pentacyclic hydrocarbon (70).5" The reaction pathway visualized was (in sequence) an 18-electron conrotatory electrocyclization (symmetry-forbidden) sigmatropic [1,9] and [1,5] H shifts and a 14-electron disrotatory electrocyclization. " K. C. Nicolaou N. A. Petasis R. E. Zipkin and J. Uenishi J. Am. Chem. SOC.,1982 104 5555 5557; K. C. Nicolaou R. E. Zipkin and N. A. Petasis ibid. p. 5558 5560. A. Beck L. Knothe D. Hunkler and H. Prinzbach Tetrahedron Lett. 1982,23,2431.

ISSN:0069-3030    

DOI:10.1039/OC9827900035

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (ii) Polar Reactions By D. G. MORRIS Department of Chemistry University of Glasgow Glasgow G12 8QQ 1 Introduction In the present Report there are sections on substitution on which much relevant work has appeared eliminations ester hydrolyses micellar reactions and miscel- laneous topics. A review entitled ‘How does a reaction choose its mechanism?’ has appeared’ and other relevant reviews include ‘Enolisation of simple carbonyl compounds’,* ‘Degenerate Carbocation rearrangements’,’ and ‘Mechanisms of nitro~ation’.~ 2 Substitution Reactions At very low concentrations (5 x 10-3moldm-3) in benzene (1) is transformed into (2) by an intramolecular endocyclic SNmechanism to the extent of 16%;the balance of the reaction is made up of an intermolecular component.’ The proportion of intramolecular reaction decreased regularly with increase in the initial concentra- tion of (1).In the case of the lower homologue (3) however the reaction was intermolecular at all concentrations.The authors note that the intramolecular substitution undergone by (1)defines a minimum ring-size for the transition state in this type of process as nine Nucleophilic attack on sp2 and sp3 hybridized carbon by pyridines and imidazoles between 0-2 a-methyl groups has been examined6 in the light of a theory that ‘conesof trajectory’ are permitted for most reactions with each trajectory associated with a particular degree of formation and of cleavage of bonds. A single methyl produced a relatively minor retardation of the reaction whereas a second methyl W.P. Jencks Chem. SOC.Rev. 1981,10 345. ’J. Toullec Adv. Phys. Org. Chem. 1982 18 1. P.Ahlberg G. Jonsell and C. Engdahl Adv. Phys. Org. Chem. 1983,19 223. ‘D.L.H. Williams Ado. Phys. Org. Chem. 1983 19 381. ’J. F.King and M.J. McGarrity J. Chem. SOC.,Chem. Commun. 1982,175. F. M. Menger and D. Y. Williams Tetrahedron Lett. 1982,23,3879. D. G. Morris was much more effective in accord with expectations based on a fairly wide angular window for reaction. The observed changes had their origins principally in AHf effects. In the synthesis of dl-coriolin A the transformation of (4) into (5)with potassium superoxide under the conditions indicated proceeded in over 82% yield at the neopentylic position.7 (i) 5 equiv.dibenzo-18-crown-6 ether _____) DMSO DME at r.t. for 48 h H (ii) H,O NaBH H The first unambiguous SN2reaction at a cyclopropane carbon was observed in the conversion of (6)into (7) by Bu'OK in dimethyl sulphoxide.8 CI Rate constants for displacements of exo-and endo-2-norbornyl brosylate by azide ion in toluene are ~imilar.~ Substantial though not excessive steric congestion is indicated by the fact that these substrates react ca 500 times faster than the seriously encumbered 2-adamantyl brosylate. By means of a rapid-injection n.m.r. technique in 95% aqueous CF,COOD and in the temperature range 5-65 "C it was found that CF3S03- is a better leaving group from MeOS02CF3 than is dimethyl ether from Me,O' BF4- when allowance is made for the statistical factor of three in the latter case." Pyramidal inversion at three-co-ordinate nitrogen and inversion of configuration at saturated carbon respond inversely to substituent and angular-constraint effects." In particular the enhancement of rate that is brought about by introduction of an a-carbony1 substituent is related to the presence of a stabilizing orbital interaction 'of a new type' in the transition state.From application of a configuration-mixing model it was found that the extent of development of charge in an SN2transition state is not a function of the position of the transition state along the reaction co-ordinate. l2 'F. Ito N.Tomiyoshi K. Nakamura S. Azuma M. Izawa F. Marnyama M. Yanagiya H. Shirahama and T.Matsumoto Tetrahedron Lett. 1982,23 1721. * L. A. M. Turkenburg W. H. de Wolf F. Bickelhaupt C. H. Stam and M. Koniju J. Am. Chem. Soc. 1982,104,3471. K. Bauert and W. Kirmse J. Am. Chem. SOC.,1982,104,3766. lo J. F. McGarrity and J. W. Prodolliet Tetrahedron Lett. 1982 23 417. S. Wolfe D. J. Mitchell and H. B. Schlegel Can. J. Chem. 1982 60 1291. l2 A. Pross and S. S. Shaik Tetrahedron Lett. 1982,23. 5467. Reaction Mechanisms -Part (ii) Polar Reactions The activation barrier for sN2 reactions in the gaseous phase has been attributed to an avoided crossing of two curves which contain the reactant- and product-like Heitler-London valence-bond forms R=R=X and N=R=X-.l3 The reaction barrier was shown to be a fraction 3f the energy gap (IN:-A& where IN:equals the ionization energy of the nucleophile and ARXis the electron affinity of the substrate.The size of the fraction depends on the slopes of the two curves which in turn are influenced by inter alia the extent of delocalization of three-electron- bonds and differences in strength between C-X and N-C. In the reaction between para-substituted benzyl chlorides and 3Ar electron- withdrawing substituents in the chloromethyl-bonded aromatic ring lead to a transition state with a shorter S--C bond and less conjugation between the aromatic ring and the a-carbon.14 When conjugation is important in the transition state the secondary a-deuterium kinetic isotope effects are appreciably lower. U-shaped Hammett plots in these reactions have been attributed to a variation in the importance of resonance and polar effects rather than to a change in mechanism.McLennan and Martin15 have noted that multiple equilibria involving the solvent accompany substitution processes at saturated carbon in solution; thus the observa- tion of curved Arrhenius plots is in order. Accordingly attempts to rationalize this behaviour in terms of ion-pair return in all cases are ill-advised. The s01id-state'~C n.m.r. spectrum of the 2-norbornyl cation has been determined in the range 77-200 K using cross-polarization magic-angle spinning.'6 Although the results 'do not remove contentious issues' no evidence was adduced for a classical norbornyl cation; from analysis of the lineshapes of a chemical exchange process that was observed in the solid state the value E = 5.9 f0.2 kcal mol-' was found for the 6,1,2 hydride shift.If the norbornyl cation is classical then the barrier for degenerate Wagner-Meerwein rearrangement is <ca 3 kcal mol-'. At O'C in methylene chloride in the presence of TiC14-HCI for 8 hours the [2.2]paracyclophanes (8) and (9) were converted into (lo) which was stable to the (8) R' = R2 = Me,R3 = H (10) R = Me (9) R' = R3 = Me,R2 = H l3 S. S. Shaik and A. Pross,J. Am. Chem. SOC.,1982 104 2708. l4 K. C. Westaway and Z. Waszczylo Can. J. Chem. 1982,60,2500. D. J. McLennan and P. L. Martin J. Chem. SOC.,Perkin Trans. 2 1982 1099. C. S. Yannoni V. Macho and P. C. Myhre. J. Am. Chem. SOC.,1982,104,907. D.G. Morris reaction conditions. ' ' The reaction proceeds by way of intramolecular migration of a methyl group from one deck of the paracyclophane to the other. The key steps the methyl migrations are exemplified when (8)is the starting material by the conversion of (11)into (12). In this instance (12) was not directly observable although evidence for its intermediacy was obtained when the initial substrate was (9). The first example of a 1,2 intramolecular migration of a -COOH group towards an electron-deficient centre has been reported,'' occurring as a consequence of the subjection of PhCH(OH)CMe,COOH that is doubly labelled with 13C at C-1 and C-3 to HS03F and SO2C1F (1:3) at -100 "C followed by warming to 0 "C. The preference for migration of COOH to give PhC(COOH)=CMe, is accounted for on the basis of the stability of the carbo-cation precursor.At -2O"C in the presence of AgSbF6 in benzene-methylene chloride the a-bromo-ketone (13) gave mainly (i.e. >95%) (14) whose formation was rationalized in terms of the highly stereoselective cyclopropylmethyl-cyclopropyl-methyl rearrangement that is depicted in Scheme l.19 H +,-COPh /O PhH D-yH-C,, -Ir -PhCH2 Br H'A'Coph H (13) (14) Scheme 1 The pentamethoxyallyl cation (16) which has been generated from (15) under a variety of conditions at -30 "C shows variable-temperature n.m.r. spectra. These have been attributed to equilibration of inner and outer terminal methoxy-groups brought about by rapid rotation around the C-1-C-2 (or C-2-C-3) bonds; this process is associated with AGS = 11kcal mo1-'.20 Me0 OMe OMe MeOyOMe CI OMe (15) (16) The major (>8O%) product that is formed from hydrolysis of either the cyclopro- pylcarbinyl (17) or the homoallyl (18) derivatives was the alcohol (19).2' The enhancement of rate that is associated with the formation of the purported carbo- cation intermediate (20) is 2 x lo4with respect to the 4-homoadamantyl analogue (21); that for (22) is very high i.e.3 x lo' for an asymmetric double-bond with respect to the epimer (23). The ratio of rate constants of (23) in which participation of a double bond is precluded to (21) uiz 0.1 provides the best estimate yet of the inductive destabilization of a developing carbo-cation centre by a homoallylic double-bond.J. Kleinschroth S. El-tamany and H. Hopf TetrahedronLert. 1982 23 3345. D. Berner D. P. Cox and H. Dahn J. Am. Chem. SOC.,1982,104 2631. l9 C. Pardo and M. Charpentier-Morize J. Chem. SOC.,Chem. Commun. 1982 1037. 'O R. A. Moss W. Guo A. Hagedorn and R. Beveridge J. Chem. SOC.,Chem. Commun. 1982 1102. " J. M. Harris J. R. Moffatt M. G. Case F. W. Clarke J. S. Polley T. K. Morgan T. M. Ford and R. K. Murray J. Org. Chem. 1982,47,2740. Reaction Mechanisms -Part (ii) Polar Reactions (17) X = DNB (18) X = C1,Y = H (20) (19) X = H (22) X = OTs,Y = H (23) X = H,Y = OTs The similarity of the 13C n.m.r. spectra of the 2-hydroxytropylium ion (24) in the solid state and in solution in methylene chloride indicates that there are no substantial differences in structure or in distribution of charge.*' X-Ray crystal structure analysis has revealed a shallow boat for the seven-membered component of this ring with the 'bridging' C-8 carbon held over the ring.The C-1-C-7 separation [1.626(8) A] is greater than that for a free cyclopropane ring (1.510 A) whereas the C-1-C-8 bond [1.488(7) A] is shorter; the respective lengthening and shortening of these bonds is in accord with delocalization which affects only the internal 'cyclopropane' bond. Direct observation of the special salt effect originally proposed on the basis of kinetic analysis of acetolyses of sulphonate esters has been provided by analysis of the photoreduction of benzophenone by NN-diethylaniline in acetonitrile in the presence of NaC104.Irradiation in the absence of salt gave after rapid electron transfer a solvent-separated ion-pair within 300 ps; after diffusion together a contact ion-pair whose components were the amine radical cation and a benzo- phenone radical anion was formed. In the time interval between 500 ps and 10ns A,, shifts to 645 nm in the presence of sodium perchlorate to give a sodium contact ion-pair with a rate which depends on [NaC104].23 The limiting rate of formation of this ion-pair was determined by photolysis of (25) which is so constructed that the methylene bridge precludes formation of an amine contact ion-pair although a charge-transfer intermediate analogous to an amine-solvent-separated ion-pair is formed initially.Rate constants for formation of the sodium contact ion-pair are 7.2 x 108mol-'s-' for both (25) and NN-diethylaniline; this accords with the conclusion that NaC104 intercepts both the inter- and intra-molecular systems at the same stage namely the solvent-separated ion-pair. A somewhat smaller rate constant that was observed for (25) and NaI has been attributed to the need to effect solvent separation of the component inorganic ions. 22 R. F. Childs A. Varadajan C. J. L. Lock,R. Faggiani C. A. Fyfe and R. E. Wasylishen J. Am. Chem. SOC.,1982,104,2452. 23 J. D. Simon and K. S. Peters 1.Am. Chem. SOC.,1982 104,6142. D. G.Morris The specific rates for solvolysis of (26) respond to changes in the solvent in the same way inter afia,as exo-norbornyl tosylate; accordingly (26)is taken to solvolyse via a kAprocess with assistance provided by the strained carbon-carbon bond in common with the lower cyclopropylcarbinyl hom01ogue.~~ However cyclopentyl- methyl brosylate reacts by way of competitive kA and k processes whose ratio varies with nucleophilicity of the solvent In the weakly nucleophilic solvents formic acid trifluoroethanol and hexafluoroisopropyl alcohol there is a significant increase in 1,3-shift of hydride during the solvolysis of (27) as the nucleophilicity of the solvent decrease^.^' No detectable (<O.S0/o) 1,3-shift of hydride was observed in the case of (28) in which there is no tertiary carbo-cation to provide a driving force; this work clarifies earlier cited reports.TsO@R2 R' (27) R' = H R2.= Me (28) R' = D,R2 = H Attribution of lower a-deuterium kinetic isotope effects than the limiting value of ca 1.22 to steric effects in the initial state during the solvolyses of 1-(1-adamanty1)ethyl sulphonates prompted a more detailed investigation; this revealed in particular that the lowest a-D kinetic isotope effect i.e.1.111 in 97% CF3CH,0H-3 O/O H20 corresponds to the greatest proportion of substitution with rearrangement.26 A rearranged or partially rearranged transition-state structure is proposed. It is calculated that replacement of the oxygen of the leaving group by a transition-state bond to carbon would lead to an a-D kinetic isotope effect of ca 1.07. Conversion of [1-'*0,8,9-14C]geranyl pyrophosphate (29) into d-bornyl pyrophosphate (30)by the enzyme system from Salvia officinalis was effected with essentially no positional exchange of oxygen isotope; 32Plabelling indicated that two ends of the pyrophosphate do not become equivalent during this interconver- ion.'^ The very tight restriction on the motion of the transiently generated inorganic pyrophosphate is unexpected.The mechanism is indicated in Scheme 2. The same precursor (29) was converted into the enantiomeric 1-bornyl pyrophosphate by the supernatant of whole leaf homogenates in Tanacetum vulgare. 24 D. D. Roberts J. Org. Chem. 1982,47 561. 25 H.-J. Scheider and R. Busch J. Org. Chem. 1982.47 1766. 26 V. J. Shiner T. E. Neumann and R. D. Fisher J. Am. Chem. SOC.,1982,104,354.*' D. E. Cane A. Saito R. Croteau J. Shaskus and M. Felton J. Am. Chem. SOC.,1982 104 5831. Reaction Mechanisms -Part (ii) Polar Reactions (30) Scheme 2 The question of whether the nortricyclylcarbinyl cation (3 1) has an enhanced vinyl-bridged 2-norbornyl character (32) has been addressed by both MIND0 and STO-3G calculations; these methods indicate an enhanced contribution of (32) to the structure with respect to the simple unbridged case. Experimental methods were indecisive in assessing the relative weighting of (31) and (32) to the stru~ture.~~*~~ 3 Elimination Reactions Solvolyses of 1-[*Hl]cyclo-octyl brosylate in a variety of solvents have shown that elimination is favoured from the C-1 site rather than from C-5 by a factor of 1.5 to 3.2.30 This indicates that a tightly paired counter-ion is the principal base in the E 1 reaction leading to cyclo-octene; there is appreciably less preference between the two sites in substitution reactions which take place via more dissociated intermediates.*' L. R. Schrnitz and T. S. Sorensen J; Am. Chem. SOC.,1982,104 2605. 29 L. R.Schmitz and T. S. Sorensen J. Am. Chem. SOC.,1982,104,2601. 30 J. E. Nordlander P. Ownor D. J. Cabral and J. E. Haky J. Am. Chem. SOC.,1982,104 201. D. G.Morris In the conversion of (33) into (36) the Elcb mechanism shown in Scheme 3 was propo~ed,~' in which deprotonation is not rate-determining; (34) and (35) gave the diene at least lo3 times faster than did (33).The increased rate of elimination brought about by relief of ring strain is estimated to be >lo" by comparison with acyclic analogues.S02Ph ,S02Ph ,S02Ph S02Ph PhS02-PhSOl Scheme 3 The same group3' noted that carbon leaving groups (i.e.those in which the leaving group is bonded to C through carbon) exhibit very low nucleofugality in elimination reactions; thus the formation of (38)from (37) as shown in Scheme 4 is associated with kobs= 1.5 x 10-9dm3mol-1s-'. EtOH A PhS02 N 0 2 +'OEt PhSOz (37) 1 )-NO2 + PhS02- (38) Scheme 4 With (39),an enhancement of kobsby 4 X 10" is observed and for this substrate in which incorporation of the leaving group into a cyclopropane ring has generated an excellent nucleofuge the mechanism is now E2 rather than Elcb.@& \ / PhSO,aNO' H27 NMe3 (39) MeOMe (40) 3' G. Griffiths S. Hughes and C. J. M. Stirling J. Chem. SOC..Chem. Commun. 1982 236. 32 P. P. Piras P. J. Thomas. and C. J. M. Stirling J. Chem. Soc. Chem. Commun. 1982 658. Reaction Mechanisms -Part (ii) Polar Reactions At 50.6"C in chloroform alkene is formed exclusively from (40) with a rate constant kl = 128 x 10-3s-'. The driving force for this El mechanism support for which is provided by Hammett p values and p deuterium kinetic isotope effects is relief of a steric interaction between the relatively poor leaving group and the ortho substituents of the benzyl group.33 By means of a qualitative valence-bond method the E2C-E2H mechanistic spectrum of elimination reactions has been j~stified.~~ The authors claim that the E2C transition state is 'looser' with respect to B-C approach (B = base) and hence less sensitive to steric influences than the tighter sN2 transition state.Accor- dingly a cited test case which provided evidence against the E2C mechanism is not considered appropriate. Nevertheless rate constants for bimolecular elimination of cyclohexyl bromide with thiophenoxide naphthoxide carbazole and fluorene nitranions were com- pared at the same basicity by means of Bronsted correlation^^^ and found to give a markedly different sequence to that exhibited for sN2 reactions. In particular oxanions and nitranions promote elimination much more readily whereas S,2 reactions are favoured much more by carbanions.Apparent correlations previously observed between SN2and E2 rate constants are considered to be fortuitous and the authors3' consider that 'there is no reason to believe that in some E2 transition states the anion is bonded to carbon as well as to hydrogen'. This study does not concern itself with bromide the archetypal E2C reagent however. Pross and Shaik34 have also proposed a mechanism for elimination depicted in Scheme 5 in which an electron is transferred from B to the C-X moiety leading to (41)and thence to olefin. I \ -C-C/ \> -+ H-BB+ + ,C=C + X-I X- Scheme 5 4 Ester Hydrolyses and Related Reactions The stereochemistries of methanolyses of phenyl phosphate monoanion (pK of the leaving group is 9.9) and of 2,4-dinitrophenylphosphatedianion (pK of the leaving group 4.1) have been investigated with both phosphates labelled as R-[l60 I7O '*O],in the light of the mechanism shown in Scheme 6.36Both reactions R-0-PO3H-$ R-6-P032-+ ROH + PO3-I H 1 H2PO4-Scheme 6 33 J.Pradham and P. J. Smith Tetrahedron Lett. 1982,23 611. 34 A. Pross and S. S. Shaik J. Am. Chem.SOC.,1982 104 187. 35 F. G. Bordwell and S. R. Mrozack J. Urg. Chem. 1982,47,4813. 36 S. L. Buchwald and J. R. Knowles J. Am. Chem. Soc. 1982. 104. 1438. D. G.Morris proceed with complete inversion of configuration at phosphorus. The results require that if metaphosphate forms it is not symmetrically solvated but is captured exclusively by nucleophilic attack from the side opposite to the leaving group.A pre-associative mechanism involving (42),where A is a nucleophilic acceptor is also feasible. Staphylococcal nuclease catalyses the hydrolysis of one of the diastereoisomers of thymidine 5'-(4-r1itrophenyl)['~O,~*O]phosphate in isotopically normal water to yield 4-nitr0phenyl['~O,'~O,~~O]phosphate with inversion of configuration at phosphorus (Scheme 7).37The most probable mechanism involves direct attack of water on phosphorus general-base-catalysed by glutamate with displacement of thymidine (which is the poorer leaving group) taking place. 170 170 180. I 02N~o~P-ovThy H2'hO) I OH Scheme 7 Hydrolysis of (43)takes place via allylic substitution under specific conditions to give (44),as shown in Scheme 8.38 H C02Ar2 H C02Ar2 \/ \ -/ -HO-C-C Ar' ,c=c \CN Ar '/ 'CN C02H 1 / HO-CH-CH HO-CH-C=C=O I I Ar' 'CN Ar' &bJ Ar'= O O M e H c02-\/ c=c Ar I/ 'CN (44) Scheme 8 37 S.Mehdi and J. A. Gerit J. Am. Chem. Soc. 1982,104 3223. '' M. Inoue and T. C. Bruice J. Am. Chem. SOC., 1982 104 1644. Reaction Mechanisms -Part (ii) Polar Reactions In the pH range 8-12 the rate constant for the hydrolysis of ester (45) shows a plateau and the reaction is characteri~ed~~ by a rate-determining unimolecular decomposition of the conjugate base (46)in a process with AS' = +40 kJ mol-' K-'. This dissociative mechanism thus operates in a carboxylic ester that is devoid of a-protons and is mediated by the para-oxoketen (47). OH 0-8CO Ar product CII 0 (47) Scheme 9 In aqueous acetonitrile acid-catalysed hydrolysis of inter alia (48) is a minimum at 20-25 mol dm-3 water; the rate increases at lower concentrations in contrast to that of the corresponding carboxamide.Nucleophilic catalysis was shown not to be imp~rtant.~' The mechanism shown in Scheme 10 (in which S is H20or MeCN) 0 OH--S I1 +I S--H-b-H--S + R'2P-NR22 $ R12 P-NR22 I Jr (49) 0 H--S s It + I products + R12P-NR22 Scheme 10 is considered to account for the observed results with the rate increase attributed to the increased concentration of the reactive intermediate (49); in addition desolvation of the ground state at higher concentrations of water may also contribute to the increase in the rate.This result is possibly relevant to enzymic catalysis where media with low aqueous content might lead to catalytic effects; it is proposed that any acidic functional group on the enzyme could therefore have stronger acidity and that desolvation of the substrate could render it more basic. 39 S. Thea G. Guanti G. Petrillo A. Hopkins and A. Williams J. Chem. SOC.,Chem. Commun. 1982 577. 40 J. M. Bonicamp and P. Haake Tetrahedron Lett. 1982 23 3127. D. G.Morris 5 Carbonyl Compounds The zero-order component in -OH which was present in the rate equation for the chlorination of ketones by hypochlorous acid and which probably represents the reaction of enolate with hypochlorous acid has provided a method for determination of pKa values of ketones in aqueous media.41 For acetone a value of ca 19 was determined.In the case of isobutyraldehyde Kresge's group4* has found a pK of 15.53 in aqueous solution; the derived enol of this aldehyde had pKa 11.63.The formation of the carbonyl compound from this enol was subject to general acid catalysis with kH+/kD+ = 2.83. Perrin and Arrheniu~~~ showed that the hemiorthoamide (51)that is formed during the hydrolysis of (50)gave only the amino-amide (52),with no (53) although this was formed subsequently under thermodynamic control. This is claimed to be the first definitive support for Deslongchamps' proposal that preferential cleavage of a tetrahedral intermediate occurs when two lone pairs are anti-periplanar to the leaving group;43a however it is pointed that the original work featured lactones which are less stable than esters and whose absence from the product(s) lessens the conviction of any mechanistic argument.(50) (51) (52) (53) The original data of De~longcharnps~~" have been revised; thus hydrolysis of 2,2-diethoxytetrahydropyrangave 2040% of 6-valerolactone shown to be a primary product and 60-80% of ethyl S-hydr~xyvalerate.~~ Conformational inver- sion of (54) is sufficiently rapid that reaction via another conformation must be allowed for. (54) Hydrolysis of the bicyclic ketal (55) has been attempted but this compound is inert in 50% aqueous dioxan at 390C.45The reason for this behaviour lies in the fact that the leaving group is fixed in an equatorial configuration by the geometry of the ring system; accordingly the lone pairs of the ring oxygen are syn-clinal to the C-OAr bond thereby minimizing n + u*overlap.The model tetrahydropyran ketal (56) is too reactive and decomposes with an estimated rate constant of 600 s-'; under the same conditions (55) is inert thereby making (56)lo'* times more reactive than (55). J. P. Guthrie J. Cossar and A. Klym J. Am. Chem. Soc. 1982 104,895. 42 Y. Chiang A. J. Kresge and P. A. Walsh J. Am. Chem. Soc. 1982,104 6122. 43 C. L. Perrin and G. M. L. Arrhenius J. Am. Chem. Soc. 1982 104,2839. P.Deslongchamps Tetrahedron 1975 31 2463. '' B. Capon and D. McL. A. Grieve Tetrahedron Lett. 1982,23,4823. 45 C. M. Evans R. Glean and A. J. Kirby J. Am. Chem. SOC.,1982,104,4706.Reaction Mechanisms -Part (ii) Polar Reactions cQo OAr OAr (55) (56) Ar = 2,4-dinitrophenyl 6 MiceUar Reactions A number of interesting papers concerned with micelles have recently appeared; and these are conveniently collected here rather than under reaction type. Nucleophilic aromatic substitution of (57) by azide ion has been found to be much faster in the micellar pseudophase than in water and is the only known reaction to behave The cationic micelles are considered to be affecting the free energy of the transition state only; in the absence of micelles the azide ion is very unreactive in this reaction. Hydrolyses of benzyl halides in particular are fas-x in sodium lauryl sulphate micelles than in their cationic counterparts where Coulombic repulsions between the micelle and the incipient carbo-cation centre are rate-retarding.47 Surfactants are known to aggregate in apolar aprotic solvents; the aggregates reversed or inverted micelles are stabilized by dipole-dipole and ion-pair interac- tions and can solubilize considerable quantities of water within their hydrophilic cavity.@ These water pools constitute a unique environment for substrates.In cyclohexane (58) exists with an aggregation number of ca 16 over a wide concentration range. Solutions of (58) and picric acid separately in chloroform or cyclohexane are colourless; however a yellow colour indicative of dissociation of the picric acid was construed as ion-pair formation and was found to occur even at very low concentrations of s~rfactant.~~ In the presence of anilino surfactants e.g.(59) the dediazoniation of simple aromatic diazonium ions proceeds via rate-determining formation of AT+ even in the positively charged Stern layer of a cationic micelle in the presence of bromide 46 C. A. Bunton J. R. Moffatt and E. Radenas J. Am. Chem. SOC.,1982,104,2653. 47 H. Al-Lohedan C. A. Bunton andM. M. Mhala J. Am. Chem. SOC.,1982,104,6654. 48 N. W. Fadnavis and J. B. F.N. Engberts J. Org. Chem.. 1982,47,2923. D. G.Morris ions.49 The product from (59),however consisted of >95% of the aromatic bromide with very little phenol; these values are typically inverted for (60). The selectivity enhancement of >380 that is experienced by (59) in favour of the formation of bromide by (59) arises since the micellar aryl cation selectively reacts with the bromide ions that are concentrated in the micellar Stern layer.A much greater rate of consumption (ca 1 x 10’) of (61) was observed during hydroxymercuration in sodium lauryl sulphate (SLS) solution as compared with aqueous tetrahydrofuran.” Moreover the ratio of rate constants k,/kb was 1.5 in aqueous tetrahydrofuran and 2.5 X 10’ in SLS [k,and k refer to the rate constants for the reaction of (61) and the monofunctionalized substrate i.e. the mono-011. a. This micelle-ind,uced chemoselectivity is considered to arise from anisotropic solubilization of the monofunctionalized substrate in the micellar site with varying degrees of penetration by water and/or the water-soluble reagent.This potentially very useful selectivity is achieved without any intrinsic difference in reactivity of the two sites. Aromatic molecules are solubilized in a relatively more polar micellar region than are simple hydrocarbons. Thus oxymercuration of p -diallylbenzene with one equivalent of mercuric acetate in the presence of SLS was essentially non-selecti~e.~’ Catalyses by reversed micelles are relevant to enzymic processes since active sites of proteolytic and lipolytic enzymes contain regions in which the polarity resembles that of micellar pools of water. Substrates usually concentrate in micellar cones of hydrophilic groups where enhanced reactivities for proton transfer and favourable entropies of activation contribute to ~atalysis.~~ With dodecylammonium propionate (62) aminolysis by these reversed micelles indicated a charge-relay mechanism similar to that for enzymic catalysis (Scheme 11).G 0-Me -C -0Ar I L .-Me-C-OAr + EtC0,H R-N-AP0,CEt I H RNH (62) J MeCONHR +OAr Scheme 11 Lower diastereoselectivities were observed when functional vesicular systems exemplified by (n-C16H33)2N+(Me)CH2CH2SH C1- were employed in the cleavage of dipeptide and tripeptide p -nitrophenyl esters such as N-carbobenzyloxy-(D or O9 R. A. Moss F. M.Dix and L. Rornsted J. Am. Chem. SOC.,1982 104 5048. J. K. Sutter and C. N. Sukenik J. Org. Chem. 1982,47,4175. ’* M.I. El Seoud R. C. Vieira and 0.A. El Seoud J. Org. Chem. 1982,47,5137. Reaction Mechanisms -Part (ii) Polar Reactions L)-Trp-(L)-Pro-PNP than when comparable micelles were used.52 The enhanced molecular restriction that is obtainable in vesicular systems does not necessarily induce greater stereoselectivity which is a function of the particular reaction mechanism.7 Miscellaneous By the use of zinc tosylate diethyl azodicarboxylate and triphenylphosphine in tetrahydrofuran alcohols are each converted into the corresponding tosylate with inversion of c~nfiguration.~~ This is exemplified by the formation of (64)from (63); one exception is however reported. The dilatometric method of determining the rate constant of hydrolysis of methylal H,C(OMe), was attended by serious kinetic complications when the initial concentration of substrate was 0.2 mol dm-3.54 Significant changes were noted in both the product composition and the acidity of the medium during a kinetic run.Reversible formation of an oxocarbo-cation was also noted from exchange experiments between H,C(OMe)2 and CD30H during hydrolysis. Proton transfers between acidic and basic centres are generally diffusion- controlled although they can be rate-determining when ApK = 0. In an investiga- tion of proton transfer between p-nitrophenol (pK is 7.16) and imidazole (pK of the conjugate acid is 6.99) it is possible to work at pH = 7 where contributions to relaxation times 7 from proton transfer uia protolysis or hydrolysis reactions are at a minimum.5s In aqueous solution at 25"C the kinetic isotope effect is kH/kD= 2.8 in accord with the maximum values previously measured between normal acid-base centres in complex systems.Phenolate oxygen adds to a neighbouring mono-alkylalkene when the two groups are juxtaposed in a system exhibiting high effective molarity. Thus at pH = 10 t1I2 for (65) is 82 s at 39 0C.56 Proton transfer is thought to be only weakly coupled with the formation of C-0 bonds and substantial carbanion character is indicated in the transition state of the reaction indicated in Scheme 12. Electrophilic aromatic bromination of activated compounds has been carried out with a new reagent hexabromocyclopentadiene in e.g. acetonitrile in the presence of Et,N (1 1).The reagent is considered to be a source of Br+ although in its 52 R. A. Moss T. Taguchi and G.0.Bizzigotti Tetrahedron Lett. 1982 23 1985. 53 I. Galynker and W. C. Still Tetrahedron Lett. 1982 23 4461. '' W. M. Schubert and D. W. Brownawell I. Am. Chem. SOC.,1982,104,3487. 55 Y. Chiang A. J. Kresge and J. F. Holzwarth,J. Chem. Soc.. Chem. Commun. SOC.,1982 1203. 5b C. M. Evans and A. J. Kirby J. Am. Chem. SOC., 1982,104,4705. D. G.Morris (65) Scheme 12 reaction with NN-dimethylaniline to give the p -bromoderivative an intense blue- green colour is attributed to a complex (66)between the amine and the undissociated hexabromo-compound.57 -BrSBr Br Br Acid-catalysed proton exchange in N-methylated amides may occur via proton-ation on oxygen (Scheme 13) which is the more basic site. A plot of the rate constants for exchange kH+,for ZCHzCONHMe versus the pK values for the corresponding carboxylic acids gives a slope of 0.43 for electron-withdrawing substituents rising to +1.84 when Z is less electron-~ithdrawing;’~ this latter value RCONHMe f H+ $ RC(OH)=hHMe + RC(OH)=NMe + H+ Scheme 13 signifies an N-protonation mechanism.In the case of N-acetylglycine methylamide which most closely resembles peptides and proteins the N-protonation exchange route is followed only to the extent of 6%; accordingly these classes of compound undergo proton exchange predominantly via the imidic acid mechanism. The diastereoisomeric complex (67)that is formed by the reaction of the ketone (68) with the chiral compound lithium (S,S)-a,a’-dimethylbenzylamide leads to (67) (68) ’’ B.Fuchs Y. Belsky E. Tartakovsky J. Zizuashvili and S. Weinman J. Chem. Soc. Chem. Commun. 1982,778. C. L. Perrin and G. M. L. Arrhenius J. Am. Chem. Soc. 1982,104,6693. Reaction Mechanisms -Part (ii)Polar Reactions the regeneration of optically active ketone (68) with an enantiomeric excess of 48% after reaction with water.59 Potassium in liquid ammonia (inter alia ) reduces racemic camphor to isoborneol (69) and borneol (70) in a ratio of 18:82 whereas when enantiomerically pure camphor is used the corresponding ratio is 62 38. The solution to this tailor-made seminar problem is given in reference 60. ’’ H. Hogeveen and L. Zwart Tetrahedron Lett. 1982 23 105. 6o V. Rautenstrauch Helv. Chim. Acta 1982,65,403.

ISSN:0069-3030    

DOI:10.1039/OC9827900051

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (iii) Free-radical reactions By R. A. JACKSON School of Chemistry and Molecular Sciences University of Sussex Brighton BN 1 9QJ 1 General Using the radial buffer technique to obtain values for the equilibrium constants for the reaction of an alkyl iodide with a different alkyl radical Castelhano and Griller have derived values for for alkyl radicals in solution for Me* Eta Pr'. and But-values are 144 (standard) 117 80 and 39 kJ mol-' corresponding to bond dissociation energies of -418 402 393 kJ mol-' for primary secondary and tertiary C-H bonds respectively.' Studies on the relative importance of polar and resonance effects in abstraction of hydrogen from substituted toluenes are often complicated by competing ring attack.Pryor's group have shown that for (nucleophilic) t-butyl radicals no addition to the benzene ring takes place,' whereas more electrophilic radicals (e.g. p-NO&H,*) add to the ring rather than abstract benzylic hydrogen. For reaction (l) a dependence of log k on Hammett's CT was found with p = +0.49. An e.s.r. study of the same reaction gave similar res~lts,~ with p = +0.59. But*+H-CH2C6H4X -b Bu'-H + 'CH2C6H4X (1) Reviews on laser flash photolysis of 1,4-biradi~als,~ on radical anion fragmenta- tion during SRNlreaction^,^ and on phosphoranyl radicals6 have appeared. Tedder7 has discussed the factors which determine the reactivity and regioselectivity of free-radical substitution and addition reactions and proposes some 'rules' which govern these reactions.2 Structural Studies Low-temperature e.s.r. studies of n-propyl isobutyl and neopentyl radicals indicate that couplings due to y-hydrogens vary markedly with the C-C-C-H dihedral angle and the results are interpreted in terms of positive values of uH for a dihedral angle of 180"and negative values of uHfor the 60" conformation.' A. L. Castelhano and D. Griller J. Am. Chem. SOC.,1982,104,3655. W. A. Pryor F. Y. Tang R.H. Tang and D. F. Church J. Am. Chem. SOC. 1982,104,2885. ' H. R.Dutsch and H. Fischer Inf.J. Chem. Kinef. 1982,14 195. J. C. Scaiano Acc. Chem. Res. 1982 15 252. R. A. Rossi Acc. Chem. Res. 1982,15 164. W. G. Bentrude Acc. Chem. Res. 1982,15,117. J. M. Tedder Angew. Chem. Znt. Ed. Engl. 1982,21,401. K. U. Ingold and J.C. Walton J. Am. Chem. Soc. 1982,104,616. R. A. Jackson There has been considerable interest in small-ring radicals. Theory predicts that the trimethylcyclopropenyl radical (1) will deviate from D3,,symmetry because of the Jahn-Teller effect. E.s.r. ENDOR and ELDOR experiments on a yirradiated matrix of trimethylcyclopropenium fluoroborate in aqueous LiCl supports structure (l),with a(3H) = 12.5 G and a(6H) = 3.5 G; a barrier of 15-29 kJ mo1-l for interconversion between valence isomers was estimated.' Compound (1)has also Me H Me A M Me A M e Dq (1) (2) (3) been prepared in fluid solution by reaction of the cyclopropene with t-butoxyl radicals:" at 113 K splittings of 11.9 G (3H) and 3.0 G (6H) were observed in good agreement with the matrix results whereas at 240 K equilibrium between the three equivalent isomers gives aH = 6.0 G (9H).The corresponding dimethylcyc- lopropenyl radical (2) has aH= 37.09 G (lH) 3.25 G (6H) and does not readily interconvert to its isomers. The large value of aHfor the single hydrogen in (2) implies a positive sign and a structure with the C-H bond bent much further out of the plane of the cyclopropene ring than is the case for the corresponding bond in the cyclopropyl radical. The spiropentyl radical (3) has been obtained by hydrogen abstraction from spiropentane:" no rearrangement or ring opening takes place up to 270 K. Cycloalkylmethyl radicals with C4 and C rings adopt the bisected conformation (4) preferentially but for larger rings (C6 C8 Cll) the eclipsed conformation (5) is favoured.Steric effects are thought to dominate except for cyclobutylmethyl where hyperconjugation involving C and the two C,-C bonds is thought to be important." Cyclopropylacyl radicals along with oxa- and aza-analogues favour the cis-and trans-conformations such as (6) and (7).13 H I (4) n = 3,4 or 5 (5) n = 6,8,or 11 Substituents in cyclopentadienyl radicals break the degeneracy of the qSand qA orbitals with one node electron-releasing substituents raise the qslevel relative to qA,whereas electron-withdrawing substituents have the opposite effect. From the e.s.r. spectra of the l-substituted radicals it was concluded that electron release G. L. Closs W. T. Evanochko and J. R. Norris J. Am. Chem. SOC.,1982,104,350.lo R. Sutcliffe D. A. Lindsay D. Griller J. C. Walton and K. U. Ingold J. Am. Chem. SOC.,1982 104 4674. l1 A. J. Kennedy J. C. Walton and K. U. Ingold I. Chem. SOC.,Perkin Truns. 2 1982 751. M.L.Kemball J. C. Walton and K. U. Ingold J. Chem. SOC.,Perkin Truns. 2 1982 1017. l3 A.G. Davies and R. Sutcliffe J. Chem. SOC.,Perkin Trans. 2 1982 1483. Reaction Mechanisms -Part (iii)Free-radical Reactions 71 to the 7~ system falls in the sequences Me > Et -Pr > H and Me3C >> Me3Ge > H > Me,Sn > Me3Si > C13Si.14 Studies of the 2-adamantyl radical in fluid solution indicate a nearly planar structure but 2-trimethylsiloxy-Zadamantyl appears to be pyramidal at the radical site with strong correlation between the pyramidal inversion and rotation of the trimethylsilyl group.:5 The radicals Me2C-CN and Me$-C02Me form complexes with trimethyl- aluminium and are formulated as involving a donor bond from the nitrile nitrogen and the carbonyl oxygen atom respectively to the aluminium atom and with aAI= 1.7 and 0.5 G respectively.16 3 Formation Destruction and Radical Stability 1,2-Diphenylethane has been thermolysed in the gas phase and in solution in tetralin and dodecahydrotriphenylene (DHTP). The gas phase results give D(PhCH2-CH2Ph) = 265.7 kJ mol-' in reasonable agreement with recent values for AH of the benzyl radical. Decomposition in solution is slower by a factor of 2-3 since k(gas)> kctetrali,,> /qDHTP) corresponding to a decrease in rate with increasing viscosity it is suggested that the cage effect is' responsible for the difference between the gas phase and solution rates.17 Very low-pressure pyrolysis of pent-2-yne gives AH for the 3-methylpropargyl radical CH3C~CCH2* of 294.1 kJ mol-' corresponding to a stabilization energy of 44 f 10 kJ mol-' for this radical." The stabilization energy of the aminopropynyl radical H2N-CH-C=CH has been estimated from the temperature dependence of the hyperfine coupling by the two NH2 hydrogen atoms in the e.s.r.~pectrum'~ to be 107 kJ mol-'. The thermochemistry of other Group IV elements maintains its interest. A study of the reaction of iodine with phenylsilane" is consistent with a chain reaction; the I* + C6H5SiH3 + C6HSSiH2*+ HI (2) activation energy for reaction (2) gives D(C6H5SiH2-H) = 374 kJ mol-'.Com-parison with other Si-H dissociation energies suggests that the 'silabenzyl' stabiliz- ation energy is only about 7 kJmol-' in accord with much qualitative data. D(H,Ge-H) has been derived from analogous studies on Ge& as 346 kJ mol-'. This is very similar to the value of 340 kJ mol-' established earlier for Me3GeH and parallels the results for the analogous silicon compounds. Although methyl groups appear to have little influence on the X3Si-H or X3Ge-H bond dissociation energies for both X3Si-SiX3 and X3Ge-GeX3 the central bond is stronger by about 30 kJ mol-' for X = Me compared with X = H indicating that the methyl l4 A. G. Davies E. Lusztyk and J. Lusztyk J. Chem. Soc. Perkin Trans.2 1982 729; A. G. Davies J. P. Goddard E. Lusztyk and J. Lusztyk ibid. p. 737; P. J. Barker A. G. Davies R. Henriquez and J.-Y. Nedelec ibid. p. 745. Is M. Kira M. Watanabe. M. Ichinose and H. Sakurai J. Am. Chem. SOC.,1982,104,3762. 16 S. Brumby J Chem. SOC.,Chem. Commun. 1982,677. l7 S. E. Stein D. A. Robaugh A. D. Alfieri and R. E. Miller J. Am. Chem. Soc. 1982,104,6567. T. T. Nguyen and K. D. King Int. J. Chem. Kinet. 1982,14,613. D. Griller D. C. Nonhebel and J. C. Walton J. Chem. Soc. Chem. Commun. 1982 1059. *' M. Barber A. M. Doncaster and R. Walsh Int. J. Chem. Kinet. 1982 14. 669. R. A. Jackson groups are acting as bond strengtheners in these compounds.21 Shock-tube studies on the decomposition of tetramethylgermane in the presence of excess toluene give D(Me,Ge-Me) = 332 f 8 kJ mol-’ falling between the values for Me4Si (higher) and Me4Sn (lower).22 ?-Irradiation of polycrystalline alkanes with 11or more carbon atoms gives e.s.r.spectra at 77 K which differ markedly depending on whether the alkane has an even or an odd number of carbon atoms. For the even alkaqes the spectra can be simulatFd by assuming a mixture of penultimate (RCH2CHCH3) and internal (RCH2CHCH2R) radicals but for odd alkanes chain end radicals (RCH2CH2-) appear to be important too indicating that molecular packing and alignment in the crystal affects the radio~hemistry.~~ The kinetics of decomposition of PhCH(CH,)Co(drngH),OH in aqueous methanol indicates parallel reactions of the parent molecule and its conjugate acid (faster).It is suggested that both homolysis of the C-Co bond (-72%) and &elimination (-28%) to PhCH=CH2 and the Co-H compound take place.24 The rates of cleavage of the t-butyl radical from t-BuRR’COH (R and R’ are bridged-ring and non-cyclic tertiary groups) have been measured and compared with molecular mechanics calculations indicating that the radical strain energies in the bridgehead systems are slightly greater than in the corresponding alkanes but considerably less than those of the related carbo~ations.~’ Aryl radicals are produced efficiently from the tetrazene (8) and from the hexazadine (9)both thermally and photolytically. Aromatic compounds are attacked to give compounds substituted in the nucleus and propan-2-01 is oxidized to ace tone.26 COMe Ip-CIC6H4-N=N-N-NHCOMe COMe Ip-CIC6H4-N=N-N-N-N=N-c6H4cl-~ICOMe (8) (9) A new technique Laser-Powered Homogeneous Pyrolysis has been described. An infra-red laser transfers energy to an inert gas which transfers its energy to the substrate. Wall reactions tend to be unimportant and the need to define a reaction temperature is avoided by the use of a standard (e.g. t-butyl acetate) whose Arrhenius parameters for decomposition are known. It is suggested that 2,2’-azoisopropane decomposes via a concerted [reaction (3)] rather than a stepwise process in contrast to some previous findings although the alternative that decomposition involves a rate-determining trans to cis isomerization of the azo compound cannot entirely be ruled Me2CH-N=N-CHMe2 + 2Me2CH.+ N2 (3) 21 Y.Fujimoto Ching-Shih Chen Z. Szeleczky D. DiTuJlio and C. J. Sih J. Am. Chem. SOC.,1982 104,4718. 22 J. Dzarnoski M. A. Ring and H. E. O’Neal Znt. J. Chem. Kinet. 1982,14,279. 23 K. Toriyama M. Iwasaki and M. Fukaya J. Chem. SOC.,Chem. Commun. 1982 1293. 2* H. B. Gjerde and J. H. Espenson Organomefulfics,1982,1,435. ” J. S. Lomas and J.-E. Dubois J. Org. Chem. 1982 47,4505. ’‘ D. Mackay and D. D. McIntyre Can. J. Chem. 1982,60,990. ” D. F. McMillen. K. E. Lewis G. P. Smith and D. M. Golden J. Phys. Chem. 1982 86 709. Reaction Mechanisms -Part (iii) Free-radical Reactions Secondary deuterium kinetic isotope effects indicate that in 4-ethylidene-l- pyrazoline (lo) the C-N bond anti to the methyl groups breaks first to give an intermediate radical (1 1).Deuterium-substitution studies show that the product- determining step involves intramolecular displacement of nitrogen [reaction (4)] rather than the intermediacy of a trimethylenemethane diradical(12).*’ Trimethyl- enemethane-type radicals are however in reactions of the 5-alky- lidenebicyclo[2.l.0]pentanes (13) [e.g. reaction (5)] in which thermolysis breaks the bridging C-C bond to give the diradical (14). (13) (14) It is possible that the spiro-conjugated tetraradical (16) is formed on sensitized photolysis of the azo-compound (15). The products indicate involvement of one of two unprecedented processes either the formation of the tetraradical (16) or the frontside attack of a radical on a carbon-carbon bond [reaction (6b)I.” DD I1 ii hv ___) Sens.?? U ’* R. J. Crawford and Moon Ho Chang Tetrahedron 1982,38,837. 29 M. Rule J. A. Mondo and J. A. Berson J. Am. Chem. Soc. 1982 104.2209; M. R. Mazur and J. A. Berson ibid. p. 2217. ’O L. McEIwee-White and D. A. Dougherty J. Am. Chem. SOC.,1982,104,4722. 74 R. A.Jackson CIDNP studies at pressures in the range 0-200 MPa have been carried out on the thermolysis of acetyl benzoyl per~xide.~' High pressure retards the peroxide cleavage and increases the ratio of cage to escape product. AVS for the homolysis is 4 cm3 mol-' and that for the difference in formation of escape to cage products is 8 cm3 mol-'. At moderate viscosities the disproportionation :combination ratio for the cage reactions of t-butyl radicals generated by photolysis of 2,2'-azo- isobutane is similar to that for encounter pairs from di-t-butyl ketone photolysis but at viscosities of 12-808 cP the orientation of the cage t-butyl radicals favours a higher proportion of omb bin at ion.^^ 4 Radical Transfer Absolute rate constants have been measured by laser flash photolysis for the reactions of t-butoxyl radicals and some ketone triplets with several organ~silanes.~~ Triethylsilane n-C5HllSiH3 and C6H5SiH3 react with t-butoxyl at similar rates (within a factor of 2 at 300 K) the main reaction being the abstraction of hydrogen attached to silicon.Trichlorosilane reacts seven times more rapidly than triethyl- silane. Triethoxysilane reacts virtually exclusively by abstraction from the CH2 groups.A similar study on the reactions of t-butoxyl radicals with acyclic and cyclic ethers shows that abstraction of hydrogen from a C-H bond a to the oxygen is enhanced when there is a small (-30") dihedral angle with a v-type orbital on the oxygen. Highest rates were observed for 2-methyl-1,3-dioxolane and THF.34 Arrhenius parameters for the abstraction of Si-H from trimethylsilane by t-butoxyl radicals in the gas phase have been by a competition method to be lo8.' M-' s-' and 15.5 kJ mol-'; by consideration of reactions of t-butoxyl radicals with nine hydrogen donors it is suggested that logA (per hydrogen) = 8.4 f 0.5 M-'s-' and E/kJ mol-' = 0.42 AH + 36.4 (k2.9). Nitrogen dioxide reacts at high concentrations with alkenes by (reversible) addi- tion but at low concentrations (below 10 000 p.p.m.) hydrogen abstraction predomi- nates; the nitrous acid formed is likely to be harmful in biological This reaction parallels the addition/substitution reactions found for bromine and alkenes.Alkanes react with chlorosulphonyl isocyanate to give alkyl chlorides as the main organic product~.~' Selectivity studies indicate that the abstracting radical is prob- ably *NCO rather than *S02NC0 =SO,Cl or C1.. An ab initio approach to frontier orbital theory has been used to illuminate the different positions of attack of methyl radicals and chlorine atoms on propionic acid. 'Hydrogen atomic sphere charges' are calculated by integrating q2within the limit of a sphere (radius 0.5 A) round the H atom concerned both for the HOMO and the LUMO.For the electrophilic chlorine atom interaction of the 31 E. M. Schulman A. E. Merbach and W. J. le Noble J. Org. Chem. 1982,47,431. '* D. D. Tanner and P. M. Rahimi J. Am. Chem. Soc. 1982,104,225. 33 C. Chatgilialoglu J. C. Scaiano and K. U.Ingold Organometaflics 1982 1,466. 34 V. Malatesta and J. C. Scaiano J. Org. Chem. 1982,47 1455. " Chan Ryan Park Se Ahn Song Yong Em Lee and Kwang Yul Choo J. Am. Chem. Soc. 1982,104 6445. W. A. Pryor J. W. Lightsey and D. F. Church J. Am. Chem. SOC.,1982,104,6685. 37 M. W. Mosher J. Org Chem. 1982,47,1875. Reaction Mechanisms -Part (iii) Free-radical Reactions SOMO is mainly with the HOMO of propionic acid where the charge is greater at C-3 whereas for methyl the SOMO-LOMO interaction is more important with attack taking place at To explain the temperature-independent k,/k = 2.9 for abstraction of hydro- gen from allylbenzene by t-butoxyl radicals along with an inverse secondary deuterium isotope effect at both ends of the double bond it is postulated that the bent transition state (17) is involved with an unusual bridging effect of the t-butoxyl oxygen atom.39 (17) From studies of chlorination of 1-substituted butanes in various solvents it appears that solvents are of three types.Fluorocarbons act as inert solvents selectivity is approximately the same as in the gas phase. Selectivity in benzene and carbon disulphide is greater than in the gas phase probably due to a rr-complex and the formation of the adduct radical CS2Cl*.For the neat compound or solutions in polarizable solvents such as CC14 selectivity is less than in the gas phase this is attributed to stabilization of the polar transition state causing a levelling of sele~tivities.~' Absolute rate constants for abstraction of halogen from 21 organic halides by triethylsilyl radicals have been obtained by the laser flash photolysis method.41 The high log (A/M-'s-') factors (>lo) for CC14 C6H5CH2Br and C,H,I are attributed at least partially to extensive polar contributions to the transition state. For complete electron transfer the gain of two rotational degrees of freedom in the transition state would enhance the pre-exponential factor by ca.lo2.Iodine transfer between two aryl groups may be direct or may involve an Ar-I-Ar' intermediate evidence for the latter route has been obtained by studies of the reduction of diaryliodinium salts to give this intermediate.42 It is suggested that the abstraction of bromine from N-bromosuccinimide can follow two routes. Less reactive radicals such as bromine atoms may react via an out-of-plane transition state (18)to give a rr-succinimidyl radical (no ring opening) whereas more reactive radicals (e.g. primary alkyl radicals) react by an in-plane transition state (19) to give the cr-succinimidyl radical which can undergo ring opening.43 SH2 reactions at saturated carbon involving the stable cobalt (11) cobaloxime as the leaving group continue to a.ttract interest.Kinetic rtsults have been obtained for the reactions with CH3CHOC2H5 and (CH3)2COH radicals,44 and in R. J. Elliott and W. G. Richards J. Chem. SOC.,Perkin Trans. 2 1982 943. 39 H. Kwart M. Brechbiel W. Miles and L. D. Kwart I. Org. Chem. 1982,47,4524. 40 A. Potter J. M. Tedder. and J. C. Walton I. Chem. SOC.,Perkin Trans. 2 1982 143; A. Potter and J. M. Tedder ibid.,p. 1689. 41 C. Chatgilialoglu K. U. Ingold. and J. C. Scaiano J. Am. Chem. SOC.,1982,104.5123. 42 D.D.Tanner D. W. Reed and B. P. Setiloane J. Am. Chem. SOC.,1982,104,3917. 43 R.L.Tlumak and P. S. Skell I. Am. Chem. Soc. 1982,104,7267. 44 R. C.McHatton J. H. Espenson and A. Bakac J. Am. Chem. Soc. 1982,104,3531. R. A. Jackson *X 0 1 *S +X-Br 4,+ X-Br intramolecular sH2 reactions cyclopentane and sulpholane rings have been closed in this way.45 "F atoms react with tetramethyltin to give CH3"F.The &2 reaction (7)is postulated to occur with an efficiency of -& of the hydrogen-abstraction F-+ (CH3)4Sn -+ F-CH3 + (CH3)3Sn* (7) 5 Addition to Multiple Bonds and Homolytic Aromatic Substitution When positive muons are passed into alkenes adduct radicals are observed. Muonium (p+e-) behaves as a light isotope of hydrogen (mass = iH) and adds to the C=C bond in alkenes with a regioselectivity similar to that of hydrogen atom^.^' Addition and allylic hydrogen abstraction are competitive processes for t-butoxyl radicals with acyclic and monocyclic alkenes but for norbornene and norbor- nadiene addition is the only mode of A factors for attack on alkenes are 'normal' but activation energies are low compared with hydrogen abstraction from comparable saturated compounds.Isopropylperoxyl radicals react with 2,3- dimethylbut-2-ene according to equation (8),yielding the epoxide (20) as the only alkene-derived 0 /\ Me2CHO0,+ Me2C=CMe2 +Me2C-Me2 + Me2CH0. (8) (20) Stoicheiometric hydrogenation of alkenes by HCo(CO) can be achieved by a process formulated by Scheme 1.The deuterium isotope effect varies widely from 0.43 to 2.0 this is explained as due to the differing extent of hydrogen transfer to the alkene in the transition state.so ArRC=CR'R2 Arc-CR'R2H M dArC-CR'R2H+M* I I R R ArC-CR'R2H +M.MH ArCH-CR'R2H + M2 I I R R Scheme 1 45 P.Bougeard A. Bury C. J. Cooksey M. D. Johnson,J. M. Hungerford and G. M. Lampman J Am. Chem. SOC.,1982,104,5230. M. Kikuchi J. A. Cramer R. S. Iyer J. P. Frank and F. S.Rowland. J. Phys. Chem. 1982 86,2677. 47 E. Roduner W. Strub P. Burkhard J. Hochmann P. W. Percival H. Fischer M. Ramos and B. C. Webster Chem. Phys. 1982,67,275. 48 P. C. Wong,D. Griller and J. C. Scaiano,J. Am. Chem. Soc. 1982 104,5106. 49 M. I. Sway and D. J. Waddington J. Chem. SOC.,Perkin Trans. 2 1982,999. T. E. Nalesnik J. H Freudenberger and M. Orchin I. Mol. Caral. 1982,16.43. Reaction Mechanisms -Part (iii) Free-radical Reactions The ring-closure reaction (9) gave only moderate yields when carried out with tributyltin hydride but with a polymer-supported tin hydride the yields were excellent and the polymeric reagent could be regenerated for further use.51 Vinyl radicals generated by photolysis of the iodide in the presence of tributyltin hydride undergo ring closure reactions [e.g.reaction (lo)].The 5/6and 6/7ring-size preference is similar to that found for alkyl radicals. These cyclizations may be useful in synthesis of compounds with double bonds in known Above 150"C,the addition of trichlorosilyl radicals to alkenes becomes revers- ible,53 leading for example to isomerization of cis-but-2-ene to the cisltrans mixture and a value of D(CI,Si-C,,) = 328 f 8 kJ mol-'. For addition of trialkylsilanes across a C=C bond low yields are partially caused by telomerization competing with the transfer step. Good yields of adduct can be obtained by use of reaction conditions that assure a large excess of the silane throughout the reaction.54 In contrast to the cyclization of o-alkenyl radicals o-alkenyldimethylgermyl radicals derived from (21,n = 2-4) react predominantly by attack at the terminal CH2 group probably because of the geometric constraints caused by the longer Ge-C bond." n/Me Me2HGe(CH2),CH=CH2 -% (CH2),,+2 Ge (11) 'Me (21) Alkyl radicals react less rapidly with alkynes than with alkenes.It is suggested that an early transition state is involved and the LUMO in an alkyne is higher in energy than the LUMO of an alkene.56 s1 Y. Ueno K. Chino M. Watanabe 0.Moriya and M. Okawara I. Am. Chem. Soc. 1982 104 5564. '* G. Stork and N. H. Baine J Am.Chem. Soc. 1982,104,2321. 53 T. Dohmaru and Y. Nagata J. Phys. Chem. 1982 86 4522; J. Chem. Soc.. Faraday Trans. 1 1982 78 1141. 54 N. M. K. El-Durini and R. A. Jackson J. Organomet. Chem. 1982,232 117. 55 K. Mochida and K. Asami J. Organomet. Chem. 1982,232 13. " B. Giese and S. Lachhein Angew. Chem. Int. Ed. Engl.. 1982 21,768. R. A. Jackson Although hydroxyl radicals react with benzene by addition at room temperature at 290°C hydrogen abstraction takes place. In the absence of air biphenyl i!j produced but if air is present phenol is produced uia the PhOO. radical." Absolute rate constants have been determined for the addition of triethylsilyl radicals to twenty carbonyl compounds. Rates varied by more than 6 orders of magnitude from duroquinone (most reactive) to methyl acetate.'* Radical rear- R R R R I I rangements of type (12)have been studied by e.s.r.spectroscopy. It is suggested that the rearrangements do not go uia ring closure (to 23)followed by ring opening on account of the high A factors observed (-1013 s-l) and the fact that for (23 R = cyclopropyl) ring opening of the cyclopropyl group is observed rather than carbon-oxygen fission to give (22,R = cyclopropyl). It is suggested that a loose transition state approximating to (24)is inv01ved.'~ Triethylaluminium reacts with 3,6-di-t-butyl-1,2-benzoquinone in toluene to give an e.s.r. signal attributed to (25).In ether species (26)is observed indicating that polar solvents can break the chelation.60 Trimethysilyl radicals add to alkyl isocyan- Y (25) (26) ate to give imidoyl radicals (27).Ease of addition falls in the series R = Me > Et > Pr' > But suggesting a steric effect with the Me3% group closer to the N-alkyl group in the transition state than in the product.61 Me3Si.+ R-N=C=O -B R-N=C-OSiMe3 (13) (27) 57 P. Mulder and R. Louw,Tetrahedron Lett. 1982.23 2605. '13 C. Chatgilialoglu K. U. Ingold. and J. C. Scaiano J. Am. Chem. SOC., 1982 104 5 119. 59 L. R. C. Barclay D. Griller and K. U. Ingold J. Am. Chem. Soc. 1982 104,4399. 60 A. G. Davies Z. Florjanczyk,E. Lusztyk and J. Lusztyk J. Organomet. Chem. 1982,229 215. " J. A. Baban M. D. Cook,and B. P. Roberts J. Chem. Soc. Perkin Trans. 2 1982 1247. 79 Reaction Mechanisms -Part (iii)Free-radical Reactions Aromatic nitroso compounds can be used as spin traps in non-polar media but are relatively insoluble in polar solvents such as water.Compounds (28)-(30) have been suggested as water-soluble spin traps.62 0 N/p N+ N+O I 6 Fragmentation and Rearrangements Reduction of 0-thiocarbonyl derivatives of secondary alcohols in the presence of tributyltin hydride takes place in good yield at 100 "C but primary alcohol deriva- tives require much higher temperatures. However primary alcohol derivatives with P-oxygen substituents are reduced at lower temperatures the reaction involves the fragmentation (14) and it is suggested that these reactions are promoted by stabilization of the product radical Re by P-oxygen sub~tituents.~~ S-SnBu3 -+ R'-C + R* FnBu3 R1-C BS -B R'-C ./' \ 0-R \O-R \O Studies of pivaloxy radicals (ButCO2-) suggest that they decarboxylate more slowly than do acetoxy radicals in spite of the greater stability of the t-butyl radical produced. A possible interpretation is an early transition state with little C-C bond stretching and the increase in 0-C-0 bond angle causing greater steric inhibition of reaction for the pivaloxy radicals.64 Decomposition of t-butyl peroxide in the presence of NO leads to values of log (Als-') = 14.6 and E = 66.5 kJ mol-' for the fragmentation of the t-butoxyl radical [equation (15)].65 Bu'O. -B MezCO + Me-(15) In alcohols arenediazonium ions undergo competing processes an ionic reaction with the solvent to give the aryl alkyl ether and free-radical decomposition to give the arene.In the presence of P-cyclodextrin the arene only is formed even in the presence of oxygen and it is suggested that the radical species are surrounded by the cyclodextrin preventing approach by oxygen.66 " J. K. Brown P. J. Coldrick and E. J. Forbes J. Chem. SOC.,Chem. Commun. 1982 770; H. Kaur and M. J. Perkins Can. J. Chem. 1982 60 1587; R. Konaka and S. Sakata Chem. Lett. 1982,411. 63 D. H. R. Barton W. Hartwig and W. B. Motherwell J. Chem. SOC.,Chem. Commun. 1982,447. 64 D. D. May and P.S.Skell J. Am. Chem. SOC.,1982,104,4500. 65 L. Batt and G. N. Robinson Znt. J. Chem. Kinet. 1982 14 1053. 66 S. P. Breukelman G. D. Meakins and M. D. Tirel J. Chem. SOC.,Chem. Commun.1982,800. R. A. Jackson Studies of the mercury-photosensitized decomposition of hexamethyldisilane give Arrhenius parameters for reactions (16) and (17).67 Log (A16/S-l) = 12.3 the low value being appropriate for a unimolecular reaction with a tight transition state. For reaction (17) the A factor is normal [log (AI7/s-') = 15.01 and from E17 = 171 kJmol-' an estimate for the n bond energy of MezSi=CHz = 188 f 20 kJ mol-' is obtained slightly higher than a previous estimate of 163 f 20 kJ mol-'. Me3SiSiMezCHz*-+ Me3SiCHzSiMez* (16) Me3SiCHzSiMez*-+ Me3Si*+ MezSi=CHz (17) Although the decarboxylation of diacyl peroxides with a secondary chiral centre occurs with retention of configuration for peroxide (31) with a primary chiral centre racemization takes place during decarboxylation suggesting that a radical cage process is involved.68 CF3CONHCH(CO~Et)CHD-CO-O-O-CO-C~I&-CI-~ (31) Chloro-substituted radicals such as (33) and (34) formed radiolytically in aqueous solution undergo a loss of chloride ion which is rapid compared with (32) and with the SN1/E1 reaction of t-butyl chloride itself.The increase in rate caused either by a or p methylation of (32) suggests that a polar SN1-like transition state is &Cl C1 (32) (33) (34) 7 Electron Transfer The involvement at least to a partial extent of free radicals is increasingly being postulated in processes formerly thought to be ionic or molecular. For Diels-Alder reactions of anthracene with tetracyanoethylene transient charge-transfer absorp- tion was observed suggesting an intermediate donor-acceptor complex.7o In reac-tions of R3SnNa compounds with alkyl halides to give tetra-alkyltin compounds competing sN2 and electron transfer can take place.71 Radical involvement in the reaction is shown by a cyclization of the intermediate radicals (when suitable alkenyl halides are used) and by trapping with dicyclohexylphosphine.Experiments with (+)-2-bromo-octane show predominant inversion of stereochemistry which taken 67 I. M. T. Davison P. Potzinger and B. Reimann Ber. Bunsenges. Phys. Chem. 1982,86,13. 68 S.J. Field and D. W. Young J. Chem. SOC.,Perkin Trans. l. 1982 591. 69 G.Koltzenburg G. Behrens and D. Schulte-Frohlinde J. Am. Chem. SOC.,1982,104,7311. 'O S.Fukuzumi and J. K. Kochi Tetrahedron 1982,38 1035. E. C. Ashby and R. DePriest J. Am. Chem. SOC.,1982,104,6144. Reaction Mechanisms -Part (iii)Free-radical Reactions with the evidence for predominant radical character of the reaction indicates a mechanism for the reaction corresponding to (18). In reactions of alkyl halides with ketones in the presence of lithium an organolithium intermediate is often formed but for halides such as 1-bromoadamantane there is competing radical reaction in which the anion radical (R-X)-resulting from reaction of the alkyl halide with lithium reacts directly with the ketone without forming an intermediate organometallic Lithium isopropoxide acts as a good reducing agent for aromatic ketones. Radical intermedi- ates which are not the free ketyls are detected;73 it is suggested that reaction takes place according to equation (19).Ph2C=O + LiOPr' $ Ph2C=0 3 (Ph2C=O)-(LiOPri)+ + Ph2COLi + MeCOMe I LiOPr' H (19) The reaction of Ph2As- and Ph2Sb- with haloaromatic compounds gives products involving scrambling of the aryl groups which indicates that the addition of Ph2As- or Ph2Sb- to Are is rever~ible.~~ Ph2P- and Ph2As- ions also react with 1-bromoadamantane to give good yields of substitution products; the SRNl mechan-ism is thought to be Primary and secondary alkylmercury halides react with the secondary nitro-alkane salts by a light-initiated process formulated as the SRNl sequence (20)-(23).76 initiation RHgX + Rl2C=N0 2 RHg-+ X-+ R12CN02 (20) RHg.+ R-+ Hgi[RR12CN0,]'+ RHgX + RR'2CN02 + RHg-+ X-propagation Re + R12C=N02-B [RR12CN02] ' (21) (23) (22) Finally it is suggested that the 'a-effect' -enhancement of nucleophilicity by a non-bonding pair of electrons on an atom a to the nucleophilic centre -is partially due to development of some diradical character in the transition state.-To the 74-7 " G. Molle and P. Bauer J. Am. Chem. SOC.,1982 104,3481. 73 E. C. Ashby A. B. Goel and J. N. Argyropoulos Tetrahedron Lett. 1982,23,2273. 74 R.A. Alonso and R. A. Rossi J. Org. Chem.. 1982 47.77. 7s R. A. Rossi S. M. Palacios and A. N. Santiago J. Org. Chem. 1982,41,4654. 76 G. A. Russell J. Hershberger and K. Owens J. Organomet. Chem. 1982,225,43. a2 R. A. Jackson extent that electron transfer from the nucleophile to the electrophile takes place in the transition state stabilization by the lone pair will take place (35),as in hydrazyl nitroxide and other radical^.^' Note.Readers interested in a computer-readable version of the references in this Section for their own data systems are invited to contact the Reporter. 77 S. Hoz J. Org. Chem. 1982,47,3545.

ISSN:0069-3030    

DOI:10.1039/OC9827900069

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

5 Arynes Carbenes Nitrenes and Related Species By M. S. BAIRD School of Chemistry University of Newcastle upon Tyne Newcastle upon Tyne NEl7RU 1 Arynes An extensive review of heterarynes has appeared.' Benzyne is efficiently trapped by phenoxide or thiophenoxide leading to diaryl ethers and diaryl sulphides.* Trapping of substituted benzynes by [2 + 21-cycloaddition to 1,l-dimethoxyethene is a highly regioselective process; a non- synchronous mechanism is consistent with the steric and inductive effects of the substituents. The reaction has been applied as a key step in the synthesis of (A)-t axodione 3,4-dimet hoxy-5 -(1-me thyle t hyl)benzyne adding to the a1 kene to give initially (11.~ [4 + 21-Cycloaddition of methyleneisoquinoline derivatives and benzyne leads to compounds containing the phenanthrene skeleton; e.g.(2) +(3). This provides a potentially valuable route to alkaloids of the dehydroaporphine and aporphine type; for example compound (3) is norcepharadione B.4 Addition of benzyne to furans by a similar process is remarkably efficient; thus (4) is obtained from the Me0 Me0 (2) (1) (4) ' M. G. Reinecke Tetrahedron 1982,38,427. R. B. Bates and K. D. Janda J. Org. Chem. 1982 47,4374. R. V. Stevens and G. S. Bisacchi J. Org. Chem. 1982,47 2393,2396. L. Castedo E. Guitian J. M.Saa. and R. Suau Tetrahedron Lett. 1982 23 457. M. S.Baird corresponding tetrafuran in 84% yield.5 Arynes are also trapped in a 1,3-dipolar cycloaddition with 9-diazofluorenes and 10-diazo-anthrones leading to indazoles (5),which are further transformed into (6) on heating.6 Li (7) Treatment of N-pivaloyl-rn -fluoroaniline with excess n-butyl-lithium at -20 "C leads cleanly to (7) by intramolecular trapping of the intermediate benzyne.The lithium derivative (7) may then be further functionalized by reaction with a range of electrophiles.' 2 Carbenes The interaction of carbenes and carbenoids with neighbouring heteroatoms has been reviewed,8 while a review of lithium halocarbenoids contains a number of examples of typical reactions of carbenes.' Ab initio molecular orbital theory has been used to assess the effect of dimerization on the electronic and structural characteristics of a prototype carbenoid (CH,LiF). The structure found for the monomer is similar to that obtained earlier; the CH2LiF units in the dimer have essentially the same geometries as those in the monomer with relatively strong carbon-lithium bonds but the lithium-fluorine distance is increased.It is concluded that the CH,Li' F- ion-pair nature that is found for the monomer remains unaffected by dimerization and that calculations on the monomer are a valid method for investigating carbenoids and related species. lo However dissociation to singlet methylene is thermodynamically much more favourable from the dimer than from the monomer. Transient triplet carbenes have been converted into relatively long-lived imi- noxyls by reaction with nitric oxide and have then been detected indirectly by e.s.r. in a process that is formally analogous to spin-trapping of radicals by nitrones and nitroso-compounds.Triplet nitrenes could not be trapped in this way.'' Cycloadditionsof singlet carbenes to alkenes depend on the electrophilic proper- ties (p/n interaction) and nucleophilic properties (u/n*interaction) of the carbene. Because of the energy level involved the p/n interaction is dominant for dichlorocarbene and dibromocarbene and substituents which raise the energy of H. Hart and Y. Takehira J. Org. Chem. 1982,47,4370. W. Burgert M. Grosse and D. Rewicki Chem. Ber. 1982 115 309; K. Hirakawa Y. Minami and S. Hayashi J. Chem. SOC.,Perkin Trans. 1 1982 577. 'R. D. Clark and J. M. Caroon J. Org. Chem. 1982,47,2804. K. G. Taylor Tetrahedron 1982 38 2751. H. Siegel Top.Curr. Chem. 1982,106 55. in C. Rohde T. Clark E. Kaufmann and P. von R. Schleyer J. Chem. Soc. Chem. Commun. 1982,882. " A R. Forrester and J. S. Sadd J. Chem. SOC.,Perkin Trans. 2 1982 1273. Arynes Carbenes Nitrenes and Related Species the .rr-orbital should lower the activation energy for cycloaddition. Competition experiments show however that Me MeO and Ph substituents increase the enthalpy of activation. This can be shown not to be a steric effect and indicates that the nucleophilic U/T* interaction plays a dominant role in the addition; it is explained in terms of the existence of an intermediate in the carbene addition as in Scheme 1. For rapid equilibrium between (8)and (9)and (10) the experimental k2 ,A Scheme 1 Mixp= AHo+ AH:,where AHois the reaction enthalpy for complex formation; the substituents raise the U/T* gap and therefore A two-step model for addition of phenylchlorocarbene has also been proposed (see ref.58).The rate of addition of dichlorocarbene to styrenes (11) is increased by electron-donating substituents (X) but relative rates are independent of temperature. By contrast the relative rates of addition to (12) are temperature-dependent. The electronic effect of the substituents in (11) is an entropy effect and does not depend on temperature. A steric effect is operating in the case of (12); this controls activation enthalpies and therefore does depend on temperat~re.'~ The isoselective tem- perature for addition of dichlorocarbene to alkenes has been calculated from isokinetic temperatures and activation enthalpies as 368 K; this is in good agree- ment with the experimental value of 360 f 10 K.14 Me Me Differential frontier-orbital theory allows the prediction of the relative reactivity of a carbene with an alkene based only on the knowledge of the ionization potential of the alkene and the orbital coefficients of its HOMO.A scale of relative selectivity for carbenes compared to dichlorocarbene is in good agreement with that derived from other methods." The selectivity of phase-transfer-generated dichlorocarbene towards mono- bis- or tris-addition to non-conjugated polyenes has been examined in the presence of a number of catalysts. Earlier claims that the presence of a hydroxyl group in l2 B.Giese W.-B. Lee and C. Neumann Angew. Chem. Inf. Ed. Engl. 1982,21 310. l3 B. Giese and C. Neumann Tetrahedron Lett. 1982 23 3557. l4 B. Giese and W.-B. Lee Tetrahedron Len. 1982,23 3561. '' W. W. Schoeller N. Aktekin and H. Friege Angew. Chem. Int. Ed. Engl. 1982 21,932. M. S. Baird the catalyst leads to greater selectivity towards mono-attack are not borne out but the selectivity is found to depend on the nature of the alkene and on the nature and concentration of the catalyst. Generally the more hydrophilic catalysts lead to higher selectivity to the mono-adduct with tetramethylammonium chloride being most selective.16 The generation of the carbene from chloroform and solid sodium hydroxide does not actually require a catalyst provided that efficient stirring and ultrasonic irradiation are used; in this way very good yields of adducts with alkenes can be isolated although only in small-scale reaction^.^' Addition of dichlorocarbene to 1,2-bismethylenecycloheptanegives the 1,4- adduct (13) as well as the 1,2-adduct (ratio 1:99) -the first example of inter- molecular 1,4-addition by a singlet carbene.'' Quantum-mechanical calculations (13) at the MNDO level have been carried out for 1,4-addition of singlet carbenes to butadiene and for 1,2-addition to ethene.For a range of carbenes there is a linear relation between E for the two processes but in all cases the barrier to 1,4-addition is greater than that for 1,2-addition. The carbene :C(NH2) is noticeably off the line having a lower barrier to 1,2-addition than :C(OH)* but a higher barrier for 1,4-addition.Geometries of the transition states for typical electrophilic and nucleophilic carbenes are similar for the 1,4-process but not the 1,2-process when :C(NH& approaches for maximum nucleophilic interaction of the carbene (c-approach). This indicates that nucleophilicity in carbenes is of greater advantage to 1,2- than to 1,4-addition and suggests that neither electrophilic nor nucleophilic singlets promote synchronous 1,4-addition.19 Homo-1,4-addition of difluorocarbene to the 2-methoxy- and 2-carbomethoxy- 10-norbornadienes (14; X = Me0 or Me02C) proceeds to the exclusion of 1,2- addition whereas (14 X = H) gives 9% of 1,2-adduct; the relative rates of reaction indicate an electrophilic addition of the carbene.Difluoro- dichloro- and dibromo-carbenes all add to (14; X = H) exclusively from the endo-face to give 1,2- or 1,4-adducts. The homo-cheletropic reaction is increasingly sensitive to the bulk of the carbene and is rationalized in terms of linear approach of the carbene in the mirror plane bisecting the ring (15) so that both lobes of the p-orbital overlap the HOMO. The ligands must invert during the addition; difluorocarbene has the shortest carbon-halogen distance and van der Waals radii and hence leads to most homo-1,4-addition.20" A minor product from the reaction of unsubstituted trinor- bornadiene with dichlorocarbene has been identified as (16). This has been explained in terms of a stepwise addition from the exo-face to produce (17) which may either ring-close to a cyclopropane or rearrange to (16).20b l6 E.V. Dehmlow and M. Prashad J. Chern. Res. (S) 1982 354. S. L. Regen and A. Singh J. Org. Chem. 1982,41 1587. l8 L. A. M. Turkenburg W. H. de Wolf and F. Bickelhaupt Tetrahedron Lett. 1982 23,769. l9 W. W. Schoeller and N. Aktekin J. Chem. Soc. Chem. Commun. 1982 20. (a)C. W. Jefford and P. T. Huy Tetrahedron Lett. 1982,23,391; (b) C. W. Jefford G. Bernardinelli J.-C. Rosier and J. A. Zuber Helv. Chim. Acta 1982 55 1467. Arynes Carbenes Nitrenes and Related Species Dichlorocarbene that has been generated thermally from phenyl(bromodi- chloromethy1)mercury reacts selectively with aldehydes in the presence of dimethyl acetylenedicarboxylate to give a carbonyl ylide (18) which is then trapped by the acetylene to give after dehydrochlorination (19).2' Photolysis of di-t-butyldiazomethane that has been deposited on a caesium iodide window at 14 K leads to (20) (21) and (22).The first two are readily explained in terms of expected carbene reactions whereas the latter may originate from migration of a hydrogen atom (via a tunnelling mechanism) in a triplet di-t- butylcarbene. In addition to these products a moderately intense band is observed at 1290 em-' which disappears above ca. 100 K. This was found to be due to a primary photoproduct and when the irradiation was carried out in a 2-methyl- tetrahydrofuran matrix at 20 K a stable e.s.r. spectrum due to triplet di-t-butylcar- bene was seen. The triplet is either the ground state or is close to it in energy; the zero-field splitting parameters indicated a bond angle about the central carbon of 143" close to that in triplet methylene and in the carbene (F3C),:.22a It is inter- esting to note the prediction of a singlet ground state for di-t-butylcarbene.22b A detailed examination is reported of the products derived from the carbene (23; R = Ph) generated under a range of conditions.Pyrolysis of (24; M = Na) at 160-200°C in triglyme leads to (25) (26) and (27) but not to (28) or (29). At 80-120 "C less (25) is obtained but (28) (29) and (30) are observed together with an increased amount of (26). Pyrolysis of (24; M = Li) gives similar products but in differing ratios; results for (24; M = Na) at 80-120 "C and for (24; M = Li) at 80-200 "C correspond to competitive carbenic and cationic decomposition of p-tosylhydrazonates.Thermolysis of (2-phenyl)-prop-2-yldiazirineor (2-pheny1)prop-2-ylidiazomethane at 180 "C or their photosensitized decomposition lead to identical mixtures of (25)-(27); this implies that the carbene (23; R = Ph) 21 H. S. Gill and J. A. Landgrebe Tetrahedron Lett. 1982 23 5099; for carbonyl ylides derived from a-diazo-ketones see A. Giilon D. Ovadia M. Kapon and S. Bien Tetrahedron. 1982 38 1477. 22 (a)J. E. Gano R. H. Wettach M. S. Platz and V. P. Senthilnathan J. Am. Chem. Soc. 1982,104 2326; (6) P. H. Mueller N. G. Rondan K.N. Houk J. F. Harrison D. Hooper B. H. Willen and J. F. Liebman ibid. 1981,103 5049. 88 M.S. Baird ~ Me Ph-CMe2 -CH =N-N-S02C7 H7 IMe R-hH M+ Me Me)_QhMe H (23) (24) (25) (26) generated either as singlet or triplet rearranges and inserts via the singlet species. The product ratios indicate a 9 1 preference for phenyl- rather than methyl- migration under these conditions. Photolysis of a diazirine or a diazo-compound in basic environments leads to different product ratios and to the formation of an aromatic substitution product (31); this appears to involve an electronically excited carbene for which the phenyl/methyl migratory order is about 2 :l.23 Product analysis from reactions of (23; R = cyclopropyl) that has been generated by thermolysis indicates that the principal process is insertion into a C-H bond of a methyl group but that some insertion into the tertiary hydrogen-carbon bond of the cyclopropyl group does occur.Minor products are derived from migration of methyl or cyclopropyl groups in the ratio 1:2.8; this ratio is explained by delocaliz- ation of the a-bonds of the cyclopropane on interacting with the electrophilic carbene but it is much smaller than the migratory preference of phenyl over methyl (9:1).Carbene (32) also undergoes insertion into the C-H bonds of the 2-methyl groups but the major product is derived by a 1,2-shift of the alkenyl group; only a trace of methyl migration is seen (migratory ratio ca. 85 :1).It is not clear how the alkene migrates e.g. via a dipolar species (33) or a bicyclobutane or even that the product is certainly derived from a carbene intermediate.Migratory aptitudes of the alkyl groups in (23; R = Et or Pri) which are found to be Me > Et > Pr are opposite to those predicted on the basis of inductive electron release and the ease of insertion into C-H bonds is primary > secondary > tertiary -again the reverse of the expected order. When two alkenes can be produced by a rearrangement in which there are steric preferences the less strained product predominates. The transition states leading to rearrangement insertion and E/Z isomerism are 23 A. R. Kraska K.-T.Chang S.-J. Chang C. G. Moseley and H. Shechter Tetrahedron Lett. 1982 23,1627. Arynes Carbenes Nitrenes and Related Species apparently controlled by steric factors in which there is preferred attack on the smaller alkyl group in the order of Scheme 2.24 Scheme 2 When 1,l-di-iodoneopentane is passed through a hot tube that contains methyl- lithium-coated Pyrex chips 1,l -dimethylcyclopropane and 2-methylbut-2-ene are formed in nearly quantitative yield.The product ratio indicates that the intermediate t-butylcarbene is the same as that produced from thermal or photosensitized decomposition of t-butyldiazomethane. The ratios are however different from those in the direct irradiation of the diazo-compound and the authors suggest that this is the unusual reaction. This is clearly of importance in relation to the'two papers discussed immediately It is known that a perpendicular alignment of a migrating group Z as in (34) is preferred in a 1,2-shift to a singlet carbene relative to the vacant p-orbital.n However when the group Z does not have axial symmetry it may adopt a variety of orientations. For a migrating phenyl-group two extremes [(35) and (36)] may be visualized; these have been termed M-face and M-edge arrangements. Pyrolysis of the sodium salts of tosylhydrazones of two epimeric ketones was used to produce the epimeric carbenes (37; R' = H R2= Ph) and (37; R' = Ph R2= H); both led to (38; R' = H R2= Ph) with no detectable formation of (38; R' = Ph R2= H) i.e. a H/Ph migratory aptitude ratio of >lo00 for each carbene. This selectivity is exceptionally high compared to the corresponding value in non-rigid carbenes. For (37; R' = H R2= Ph) this may be due to the favourable alignment of the hydrogen but for (37; R' = Ph R2= H) the phenyl group is better aligned.It is suggested that the hydrogen at C-7 interferes with the ortho-hydrogen on the phenyl group if it adopts the M-face orientation. Since the M-edge orientation appears relatively feasible it seems that migration of a phenyl group may occur by a preferential M-face geometry.26 24 A. R. Kraska L. I. Cherney and H. Shechter TetrahedronLett. 1982 23 2163. 25 M. Fukushima M. Jones and U. H. Brinker Tetrahedron Len. 1982,23 3211. 26 A. Nickon and J. K. Bronfenbrenner J. Am. Chem. Soc. 1982,104 2022. M. S. Baird The carbene (39; R = Me) has been reported to lead to the corresponding cy~lobutadiene.~' However a more recent paper reports that both (39; R = Me) and (39; R = Bu') also fragment to di-t-butylacetylene and the corresponding alkyl t-butylacetylenecarboxylate.In both cases the ratio of cyclobutadiene to alkynes is ca.70 30 but is markedly dependent in temperature and solvent. By analogy with the Wolff rearrangement the singlet carbene should rearrange and the triplet should fragment; in confirmation of this the ratio of products was changed to about 20 70 by adding benzophenone sensitizer.28 2,3-Di-(tri-methyl- silyl)cycloprop-2-en-l-yl carbene also fragments giving ethyne and di-(trimethyl- ~i1yl)ethyne.~' The cycloalkylcarbene (40) has been generated photolytically at -78 "Cand found to lead to indene as the major product (15%) together with 7-ethynyl- bicyclo[2.2.l]hepta-2,4-diene,(4l),and ethynylcycloheptatriene.Labelling studies rule out intramolecular addition of the carbene (40) to the alkene bond and the most likely mechanism leading to the first three products involves a 1,2-alkyl shift to produce (42) followed by a retro-Diels-Alder reaction a rearrangement or a hydrogen abstraction. The cycloheptatriene may be derived through an alternative 1,2-alkyl shift. in (40) although a direct fragmentation to 7-ethynyl-bicyclo[4.1.0]hepta-2,4-dienealso seems likely.30 Two labelling studies in rearrangements of cyclopropylidenes are reported. Car- bene (43) generated by the reaction of the corresponding dibromide with methyl- supports a sequence involving a carbene-carbene rearrangement leading to (44) which then (.)labelling12C1-t-butyl-3-methylpyrrole;lithium rearranges to *' S.Masamume N. Nakamura M. Suda and H. Ona J. Am. Chem. SOC.,1973,95,8481. P. Eisenbarth and M. Regitz Angew. Chem. Int. Ed. Engl. 1982 21,913. 29 G. Maier M. Hoppe. H. P. Reisenauer and C. Kruger Angew. Chem. fnt. Ed. Engl. 1982,21,437. 3" J. Stapersma I. D. C. Rood. and G. W. Klumpp Tetrahedron 1982 38 3051; see also ibid. p. 191. Arynes Carbenes Nitrenes and Related Species undergoes a 1,2-shift of hydrogen3’ Treatment of the dibromocarbene adduct of homobenzvalene with methyl-lithium leads to 5-ethynylcyclohexa-l,3-diene.In this case ’H and 12C labelling indicate a rearrangement as in Scheme 3.32In related Scheme 3 reactions the cyclopropylidenes (45;XiY = CC12 or-O-CH2CH2-O-) are reported to undergo efficient insertion into a 5,6-related carbon-hydrogen bond to produce the compounds (46) [the corresponding reaction of the parent system (X = Y = H) proceeds only in low yield]33 and (47) undergoes a similar reaction when n = 5 or 6 but can lead to high yields of allenes when n is larger; the latter reaction leads to chiral allenes in the presence of (-)-~parteine.~~ Decomposition of cyclopropyl- diazonium salts in the presence of sodium methoxide leads to products that are apparently derived from cyclopropylidenes; thus one product from em -9-bicyclo[6.1 .O]nonane-9-diazonium ion is cyclonona- 1,2-diene.The carbene bicyclo[6.1 .O]non-2-en-9-ylidene rearranges to the allene cyclonona-l,2,4-triene with no evidence of insertion into C-H bonds (which is the reported reaction when the carbene is derived from 9,9-dibromobicyclo[6.l.0]non-2-ene) or rearrangement to bridged carbenes (as reported in lower hom~logues).~~ Decomposition of (48) under strongly alkaline conditions is also reported to lead to cyclopropylidene- derived products such as (49).36 This product is also obtained together with (50; X = H Y = Br) and (51) when (50; X = Y = Br) is heated with methyl-lithi~m.~’ 31 J.Arct and L. Skattebd Tetrahedron Lett. 1982,23 113. 32 M. Christ1 and M. Lechner Chem. Ber. 1982 115 1. 33 A. R. Allan and M. S. Baird J. Chem. Res. (S) 1982 290. 34 M. Nakazaki K. Yarnamoto M. Maeda 0.Sato and T. Tsutsui J. Org. Chem. 1982.47 1435. ” W. Kirmse and G. Hellwig Chem. Ber. 1982 115 2744. 36 H. Jendralla Chem.Ber. 1982,115,220; see also H. Jendralla and W. Pflaurnbaum ibid. p. 229. 37 H. Jendralla and W. Pflaumbaurn Chem. Ber. 1982 115 210. 92 M. S. Baird Valence-bond and molecular-orbital theories have been used to show that triplet cyclopropylidene can rearrange to singlet (orthogonal) a11ene.38 Although cyclopro- pylidenes have been studied extensively cyclobutylidenes are less well known. However the reaction of various 1,1-dihalogenocyclobutaneswith methyl-lithium at 0 to -78 "C leads to methylenecyclopropane and cyclobutene in a ratio similar to that obtained from cyclobutylidene that is generated by the Bamford-Stevens reaction. 39 The regioselectivity of insertion into C-H bonds at positions A and B in the cycloalkylidenes (52) is found to be highly dependent on small changes in geometry and electron distrib~tion.~' The related cycloalkylidene (53) undergoes addition to the alkene rather than insertion and produces an exceptionally strained [3.l.l]pr0pellane.~*a! -AlkyI-a! -trimethylsilylcarbenes (54) have been generated by the reaction of the dibromides (R'R2C=CR3CH2CBr2SiMe3) with methyl-lithium.These carbenes also undergo intramolecular addition to the alkene bond to produce 1-trimethylsilylbicyclo[l. l.O]butanes (55) rather than undergoing a 1,2-shift of hydrogen.42 "ZC% SiMe, Bb R2 (53) (54) (55) Photolysis of phenylcyclopropyldiazomethane in a matrix of 2-methyltetra-hydrofuran at 10-25 K does not produce the ex. spectrum of phenylcyclo- propylcarbene; only a weak spectrum is observed on similar treatment of phenylcyclobutyldiazomethane.However intense spectra for triplet carbene are observed for phenylcyclopentyl- and phenylcyclohexyl-carbenes even at 77 K; phenyl-( 1-benzylcyclopropy1)- and phenyl-( 1-benzylcyclobuty1)-carbenesalso give e.s.r. spectra. By extrapolation it is concluded that the triplet state of phenylcyclo- propylcarbene is the ground state or is within a few kcal of the ground state. Zero-field splitting parameters indicate that the cyclopropyl ring is much less effective than a phenyl group in delocalizing the spin density of the ~arbene.~~ Irradiation of phenyldiazomethane at >478 nm in an argon matrix at 10 K leads to phenylmethylene. Further irradiation at shorter wavelength leads to a second product which is also obtained by irradiation of phenyldiazirine in an argon matrix or by the thermolysis of phenyldiazomethane followed by trapping in a matrix.3R Y.-N. Chiu J. Am. Chem. SOC.,1982,104,6937. 39 U. H. Brinker and G. Schenker J. Chem. SOC.,Chem. Commun. 1982,679. " S. Hirsl-Starcevic and Z. Majerski J. Org. Chem. 1982,47 2520. 4' V. Vinkovic and Z. Majerski J. Am. Chem. SOC.,1982 104,4027. 42 M. S. Baird. S. R. Buxton. and M. Mitra Tetrahedron Lett. 1982 23 2701. " R. L. Barcus E. C. Palik and M. S. Platz Terrahedron Lett. 1982 23 1323. Arynes Carbenes Nitrenes and Related Species 93 Deuterium labelling has been used to show that this product is cyclohepta-1,2,4,6- tet ~aene.~~ Photolysis of PhC(N2)C02Me in styrene at 0 "C gives the corresponding 1,2- diarylcyclopropanes in 10 :1 &/trans ratio; in contrast irradiation in a frozen styrene matrix at -78 or -190 "C reduces the ratio to 2.8 :1and 1.5 :1 respectively.A similar change is observed for PhCHN or 4-BrC6H4CHN2 but for photolysis of phenyldiazomethane in para-substituted styrenes the reverse result is obtained. These findings are not mirrored in solution at -78 "C and are also dependent on the nature of the matrix. The reaction may involve stepwise addition of a triplet carbene to produce a diradical; when such species are generated independently by photolysis of 1,3-diarylpyrazolines there is also a marked change in degree of retention of configuration in the solid phase. Orientation of the diradicals in the matrix may be dependent on the relative size of host and guest molecules leading to differences in stereochemical Photolysis of PhCHN in ether leads to PhCH,OEt PhCH(Et)OEt PhCH,CH(Me)OEt and Ph(CH,),OEt.Product distri- butions are not altered by a triplet quencher but all products are quenched with differing efficiencies by a singlet quencher. The effect of concentration of singlet quencher suggests that the first two products which arise from an 0-ylide come from singlet carbene but that the last two arise from both singlet and triplet reactions. As the temperature is decreased the changes in product ratios indicate either a gain of triplet reaction over singlet or solvation effects. The selectivity between the final two products increases as the temperature is reduced but dramatically decreases at the phase change to a solid -indicating both temperature and matrix effects.In the matrix the characteristic reactions of singlet carbene are suppressed and C-H insertion by triplet carbene results.46 Photolysis of ArCHN in simple alcohols at ambient temperature leads to products of arylcarbene insertion into 0-H bonds and small amounts of C-H insertion. At lower temperatures more C-H insertion is observed; attempted sensitized decomposition of the diazo- compound does not lead to an increase in this process presumably due to rapid triplet-singlet equilibration. The OH/CH insertion ratio is also increased at ambient temperature by electron-donating substituents on the aromatic ring -the transition state for OH-insertion involving electron deficiency at the benzylic carbon.At low temperature the ratio is increased by both electron-donating and electron-with- drawing sub~tituents.~' Hammett plots for the selectivity of insertion or addition (ki/k,) for reaction of 'aryl carbenes' with ethyl vinyl ether and for the stereochemical selectivity in the addition process (kciS/ktrans) show that the p values are highly dependent on the mode of generation. The free carbene appears to be involved when photolytic generation from diazo-compounds is used but in the thermal process a ground-state diazo-compound seems to be masquerading as a ~arbene.~~ 44 P. R. West 0. L. Chapman and J.-P. Le Roux,J. Am. Chem. SOC.,1982 104 1779; for trapping of cyclohepta-1,2,4,6-tetraenesee J.W. Harris and W. M. Jones ibid. p. 7329,and for reactions of a naphthalene-fused analogue see M. Balci W. R. Winchester and W. M. Jones J. Org. Chem. 1982 47,5180. 45 H. Tomioka Y. Ozaki Y. Koyabu and Y. Izawa Tetrahedron Lett. 1982,23 1917. 46 H. Tomioka S. Suzuki and Y. Izawa Bull. Chem. SOC.Jpn. 1982,55,492. 47 H.Tomioka S. Suzuki and Y. Izawa J. Am. Chem. Soc. 1982 104,3156. 48 H.Tomioka S. Suzuki and Y. Izawa J. Am. Chem. Soc. 1982,104 1047. 94 M. S. Baird A number of naphthylcarbenes (56) have been generated by pyrolysis of the correspondingmethoxytrimethylsilanes. When Y = Z = H the carbene inserts into the C(8)-H bond to produce a cyclobutarene (57; X = Y = H); however for (56; Z = H Y = Me) two cyclobutarenes (57; Y = Me X = H) and (57; Y = H X = Me) are obtained (ratio 11:l) suggesting an equilibration between (6-methyl- nap ht h-1-y1)carbene and (7-methylnaph th- 1-yl)carbene.Moreover (nap hth-2-y1)carbene appears to rearrange to (56; Y = 2 = H) and isomeric (phenan- threny1)carbenes also eq~ilibrate.~~" The bridged naphthylcarbenes (58; X Y = H Me or -OCH2CH20-) undergo a 1,2-shift of the X group to produce an alkene rather than a 1,2-shift of the labelled carbon-carbon bond which would produce the skeleton present in (57). The keto-carbene (58; XY = 0)does not undergo a Wolff rearrangement but can be trapped by alcohols alkanes and electron-deficient alkene~.~~' Excited triplet diphenylcarbene has been studied using picosecond lasers and unlike the low-lying triplet it reacts with methan01.~' The pseudo-first-order rates for abstraction of hydrogen from toluene by triplet diphenylcarbene in a matrix at 77-106 K a reaction which proceeds by quantum- mechanical tunnelling of hydrogen atoms have been determined using e.s.r.Barrier heights for the process are in good agreement with a theoretical model and the calculated activation barrier for classical abstraction of hydrogen of 8.7 kcal mol-' is consistent with a relatively low rate of triplet abstraction of hydrogen atoms in s~lution.~' Decay of the e.s.r. signal of Ph2C in protonic and perdeuteriated organic matrices corresponds to sites of very different reactivity. Although the formation of cyclopropane in alkene matrices could occur by tunnelling of heavy atoms it is more likely that the carbene is generated directly over the double bond in suitable matrix sites and reacts with minimal motion in the Irradiation of diazofluorene has been carried out in acetonitrile on a picosecond time-scale at very low temperature and has led to a re-assignment of the structures of transient species.Two main products are observed; one gives rise to an absorption at 470 nm; as this decays the second appears absorbing at 400 nm. The latter is probably due to the formation of a nitrile ~lide;~* the former is now assigned to 49 (a) T. A. Engler and H. Shechter Tetrahedron Lett. 1982 23 2715; (b) S.-J. Chang B. K. Ravi Shankar and H. Shechter J. Org. Chem. 1982,47,4226. '"Y. Wang E. V. Sitzmann F. Novak C. Dupuy and K.B. kisenthal J. Am. Chem. SOC.,1982 104 3238. " B. B. Wright V.P. Senthilnathan M. S. Platz and C. W. McCurdy Tetrahedron Lett. 1982.23 833; M. S. Platz V. P. Senthilnathan B. B. Wright and C. W. McCurdy J. Am. Chem.SOC.,1982,104,6494. 52 D. Griller C. R.Montgomery J. C. Scaiano M. S. Platz and L. Hadel J. Am. Chem. SOC.,1982 104.6813. Arynes Carbenes Nitrenes and Related Species triplet fluorenylidene. Results suggest that there is a second species preceding the one that produces the 400nm peak and that this is singlet fl~orenylidene.~~ Fluorenylidene adds to aliphatic ketones to give carbonyl ylides which may be detected spectroscopically in solution. In the absence of quenchers these can undergo ring-closure to oxiranes but the ylides are quenched with rate constants of about lo7dm3 mol-’ s-’ by electron-deficient alkenes or by oxygen.54 The car- bene (59; X = Si) generated by photolysis of the corresponding diazo-compound has been observed by e.s.r.spectroscopy in a matrix at 40K and the triplet has been shown to be the ground state; reaction with cis-2-butene gives no cyclopro- pane but only products derived by abstraction of hydrogen. The carbene (59; X = C) reacts with cis-butene apparently by hydrogen abstraction followed by dimerization.” Photoinduced loss of halide from the anion (60; X = Br or C1) leads to 1,3-diphenylisoindenylidene (61),which again undergoes abstraction of hydrogen atoms rather than addition to alkenes; e.g. with dihydropyran (62) is formed. In a most unusual reaction the carbene is trapped by ethyl vinyl ether to produce (63) apparently through an initial cyclo-addition followed by loss of ethanol to produce the alkene at AB.56 X @x Me’ ‘Me Ph (59) (60) Ph Ph Ph WB\ Ph (62) (63) 4H-1,2,3-Triazolylidenes(64) isomerize to a-diazonitriles which in turn lead to carbenes Z-C-CN (64a).Benzene is trapped by (64) in a substitution reaction leading to (65) but with (64a) addition and ring-expansion or substitution are ob~erved.~’ 7 53 B.-E. Brauer. P. E. Grasse K. J. Kaufmann and G. B. Schuster J. Am. Chem. Soc. 1982. 104,6814. ” P. C. Wong D. Griller and J. C. Scaiano J. Am. Chem. Soc. 1982 104 6631. ” A. Sekiguchi W. Ando T. Sugawara H. Iwamura and M. T. H. Liu Tetrahedron Lett.1982,23,4095. 56 L. M. Tolbert and S. Siddiqui J. Am. Chem.SOC.,1982,104,4273. ” H. K.-W. Hui and H. Shechter Tetrahedron Len. 1982 23 5115. M. S.Baird Absolute rate-constants and activation parameters are reported for the addition of phenylchlorocarbene to alkenes. The values are dependent on the experimental system but a precision of >5% and reproducibility of >lo% are obtained. The Arrhenius plot for addition to tetramethylethylene shows a pronounced curvature and as the temperature decreases from 301 to 213 K the value of koW increases; Eib" is found to be -1.7 f 0.5 kcalmol-'. At temperatures below 213 K koW decreases and Eib" approaches the value for diffusion control. The results are interpreted in terms of a kinetic model for addition represented by two steps A + PhCCl kl PhCCl/A k-I k2 PhCCl/A -+ cyclopropane A structure for the intermediate that is consistent with the data would involve interaction of the vacant p-orbital of the carbene with the 7r-orbital of the alkene in the form of a loose 7r-complex.More reactive alkenes would lead to looser complexes. The kinetics are also consistent however with an intermediate 'proxim- ity pair' of reactants in a solvent cage.58 These conclusions should be compared to the two-step model arrived at for addition of dichlorocarbene (see ref. 12). A Hammett plot for the reaction of phenylchlorocarbene with styrenes indicates that it reacts as an electrophile; the p values obtained are markedly dependent on temperat~re.'~ Laser flash photolysis of arylchlorodiazirines leads to singlet arylchlorocarbenes which are trapped by alkenes in a first-order process.However when trapped by methanol the reactions are not first-order with respect to alcohol. Solvent and concentration effects indicate that OH bonds that are involved in hydrogen-bonding in methanol oligomers are more reactive towards carbenes than those in unassoci-ated molecules possibly due to a decrease in the bond dissociation energy. For t-butyl alcohol the reverse result was obtained apparently due to a steric effect.60 Flash vacuum pyrolysis of N-allyl-substituted 2-phenyl- 1,3,4-oxadiazolin-5- ones e.g. (66),can be explained by initial decarboxylation to (67),which undergoes a 3,3-sigmatropic shift to a diazoalkene PhC(N2)CH2CH=CH2.Loss of nitrogen leads to an allylphenylcarbene which undergoes migration of hydrogen or a vinyl group or insertion into C-H bonds. 2-Phenyl-4-(prop-2-yn-l-~!)-1,3,4-oxadiazolin-5-one leads to l-phenylbut-3-en-l-yne apparently via Ph-C-CH=C=CH2 and l-phenyl-3-methylene~yclopropene.~' This route is also useful in generating benzylp hen ylcarbene. 58 N. J. Turro G. F. Lehr J. A. Butcher R. A. Moss,and W. Guo J. Am. Chem. SOC.,1982,104 1754. 59 W. Bruck and H. Durr Tetrahedron Len. 1982 23 2175. 6o D. Griller M.T. H. Liu and J. C. Scaiano J. Am. Chem. SOC.,1982,104 5549. '' A. Padwa T. Caruso S. Nahm and A. Rodriguez J. Am. Chem. SOC.,1982 104,2865. Arynes Carbenes Nitrenes and Related Species Pyrolysis of vinyldiazomethanes (68; R' R2 = alkyl and aryl) gives mixtures of 3H-pyrazoles and cyclopropenes.By analysing the kinetics and the distribution of products in each case the rate of formation of vinylcarbene and therefore the relative stabilities of these intermediates has been assessed. It is found that the carbene from (68; R' = Me R2= Ph) is more stable than that from (68; R' = Ph RZ= Me). The results can be related to those from singlet carbenes derived by photolysis of unsymmetrical cycloprbpenes.62 Direct irradiation of 1,l -diphenylallenes Ph2C=C=CHR produces the corre- sponding indenes (69) 3,3-diphenylcyclopropenes,and 1,l -diphenylprop-2-ynes as primary products. The reaction occurs via a singlet excited state and is thought to involve a 1,2-shift of hydrogen to produce a.vinylmethylene diradical which can equilibrate with a vinylcarbene PhzC=CH-C-R.63 Pyrolysis of (70) in a flow system at 800°C leads to the carbene (71) after a shift of deuterium and loss of nitrogen.The carbene rearranges to penta- 1,3-diene and the deuterium label appears at C-2 indicating that this process occurs by a 1,4-shift of hydrogen.64 D The cyclic vinylcarbene (72) has been generated by pyrolytic or photolytic decomposition of alkali-metal salts of the corresponding tosylhydrazone and leads to a complex mixture of C,H and C8Hl products. The former which include (73) 5-ethynylcyclohexa-l,3-diene,and semibullvalene are ascribed to insertion reac- tions and rearrangement of a singlet carbene; the latter which include bicyclo[3.2.lloctadiene (74) and (73 arise through abstraction of hydrogen by a triplet carbene or by a closely related species.65 Substituent effects in the addition of photolytically generated (72) and (76) to styrenes correlate well with Hammett a constants and produce p values of +0.25 and +0.68 indicating nucleophilic character in the carbenes. Calculations indicate that the HOMO'S of (72) and (76) are close to that of vinylmethylene and higher than that of nucleophilic dimethoxy- carbene; they further show that steric effects play a part in the nucleophilic approach of (76) to styrenes.66 (72) (73) (74) (75) (76) 62 J. A. Pincock and N. C. Mathur J. Org. Chem. 1982 47 3699; for cyclopropene photolysis see A. Padwa M. Akiba C. S. Chou and L. Cohen ibid.,p.183. " M. G. Steinmetz R. T. Mayes and J.-C. Yang J. Am. Chem. SOC.,1982,104,3518. 64 J. D. Perez and G. I. Yranzo J. Org. Chem. 1982 47 2221. P. K. Freeman and K. E. Swenson J. Org. Chem. 1982,47 2033. 66 S.-I. Murahashi K. Okumura T. Naota and S. Nagese J. Am. Chem. Soc. 1982 104 2466. M. S.Baird Generation of (77) by pyrolysis of the lithium salt of the corresponding tosyl- hydrazone leads to indene and 7-ethynylcycloheptatriene; the related carbene (78) leads only to indene. The reactions of (77) are discussed in terms of a number of carbene-carbene rearrangements but it is interesting that unlike (72) nohydrogen-abstraction products are formed; this may reflect the involvement of allenic inter- mediates e.g. (79). Further calculations suggest that cyclohexa-1,2-diene has a triplet ground state but that cyclohepta-1,2-diene is a singlet.67 The carbene (80) has also been examined.68 Irradiation of diphenylcyclopropenethion in methanol leads to products which are apparently derived by trapping of Ph-C-C(Ph)=C=S either as the diradical PhC=C(Ph)-C=S or after ring-closure to (81).69 Calculations of the energies of conjugated cyclic carbenes (82) using various theoretical models are rep~rted.~' Ultraviolet photolysis of microcrystalline 5-diazoimidazole-4-carboxamidegives two S = 1species which have been detected by e.s.r.One of these has been characterized as the carbene (83).71 0 Generation of the carbene (84)in methacrylonitrile leads to some addition to the alkene bond.However the major product is (85) and control studies suggest that this is derived by reversible formation of the nitrile ylide (86) in competition with irreversible addition to the alkene. In support of this the carbene is trapped by acetonitrile in the presence of N-phenylmaleimide to produce (87).72 The base-promoted reaction of dimethyl diazomethylphosphonate (Me0)2POCHN2 with aldehydes or aryl ketones leads predominantly to alkynes. The results are consistent with the intermediacy of diazoethenes e.g. ArC(R)=CN2 from ArCOR which undergo unimolecular decomposition at -78 "C to form nitrogen and alkylidenecarbenes ArC(R)=C:.73 Treatment of (88) under similar 67 P. K. Freeman and K. E. Swenson J. Org. Chem. 1982,47,2040. D. 0.Farnum M. Ghandi S. Raghu and T.Reitz J. Org. Chem. 1982,47,2598. 69 S. Singh M. M. Bhadbhade K. Venkatesan and V. Ramamurthy J. Org. Chem. 1982,47 3550. 70 M. Kausch and H. Durr J. Chem. Res. (S),1982 2. 71 H. B. Ambroz B. T. Golding T. J. Kemp and V. S. Shukla J. Chem. Soc. Chem. Commun. 1982 414. 72 A. S. Kende P. Hebeisen P. J. Sanfilippo and B. H. Toder J. Am. Chem. Soc. 1982,104,4244. 73 J. C. Gilbert and U. Weerasooriya J. Org. Chem. 1982. 47 1837. Arynes Carbenes Nitrenes and Related Species N' %N Me 0 Ill C Me A MeA conditions leads to a methylenecarbene which undergoes intramolecular addition to the alkene to produce (89); this in turn undergoes ready ring-opening to a di-t-butylmethylcyclopentadipe.Derivatives involving the ring system (89) have also been obtained from intramolecular addition of methylenecarbenes that were derived by the reaction of 1,l-dibromides with methyl-lithi~rn.~~ Irradiation of the diradical (90) in a glass at 77K leads to 6-methylhept-5-en-1-yne as the major monomeric product.This process may formally be explained by ring-closure to produce 6,6-dimethylbicyclo[3.1 .O]hex-1 -ene and cheletropic elimination of a methylenecarbene -the reverse of the proces described above. The carbene could then undergo a 1,2-shift of hydrogen; however the typical barrier for the latter process is ca. 8.6 kcal mol-' and such a process would be immeasurably slow at 77 K.75 The methylenecarbene-alkyne rearrangement is reversed in the pyrolysis of ynones; this leads to carbenes such as (91) which undergo insertion into a 5,6-related C-H bond to produce cycl~pentenones.~~ 0 Neither oxirenes nor acylcarbenes can be detected directly on irradiation of diazo-ketones in an argon matrix at 10 K.This applies even to those diazo-ketones which do not undergo a Wolff rearrangement under standard condition~.~' Copper-catalysed decomposition of a diazo-ketone in the presence of an alkene has provided the cyclopropyl ketone (92) an intermediate in a synthesis of (*)-~pirolaurenone.~* The copper-sulphate-assisted decomposition of diaz~methyl-ketones~~ and the decomposition of a-diazo-& keto-esters (93) in the presence of rhodium acetate 74 R. F.Salinaro and J. A. Berson Tetrahedron Lett. 1982 23 1447 1451; M. Rule R. F.Salinaro D.R. Pratt and J. A. Berson J. Am. Chem. SOC.,1982,104,2223. 75 S. P. Schmidt A. R. Pinhas J. H. Hammons and J. A. Berson J. Am. Chem. SOC.,1982 104 6822. 76 M. Karpf J. Huguet and A. S. Dreiding Helv. Chim. Acra 1982 65 13. 77 G. Maier H. P. Reisenauer andT. Sayrac Chem. Ber. 1982,115 2192. '13 A. Murai K.Kato and T. Masamune Tetrahedron Lett. 1982 23 2887; for a more simple but related reaction see F.Bohlmann and W. Rotard Liebigs Ann. Chem. 1982 1220. 79 E. Wenkert L. L. Davis B. L. Mylari M.'F. Solomon R. R. da Silva S. Shulman R. J. Warnet P. Ceccherelli M. Curini and R. Pellicciari J. Org. Chem. 1982,47 3242. 100 M. S. Baird lead to good yields of cyclopentanones by insertion into the C(5)-H bond." In a similar reaction intramolecular addition of a carbene from another a-diazo$ -keto- ester has been used to obtain (94) a key step in a synthesis of cycloeudesmol.81 All of these reactions underline the preference for formation of five-membered rings in both addition and insertion reactions.0 Br-The regioselectivity of cyclopropanation of 1,3-dienes by ethyl diazoacetate has been examined in the presence of a range of catalysts and has been used to define a 'metal carbene regioselectivity index'.82 The reaction of l,l-dichloro-4-methy1- penta- 1,3-diene with 1-menthyldiazoacetate in the presence of a chiral catalyst gave (95)as a cis trans-mixture but the enantiomeric excesses for the two isomers were only 3 1and 51YO respectively. However addition to 2-methyl-5,5,5-trichloropent-2-ene followed by de hydrochlorination of the intermediate (trichloroethy1)cyclopro-pane gave largely cis-(95) with an enantiomeric excess of over 90% of the opposite enantiomer to that obtained from the diene.83 The rhodium-catalysed reaction of diazo-esters leads to a carbene which undergoes selective insertion into the HO bond of unsaturated and acetylenic alcohols.Depending on the catalyst counter- anion and the nature of the ester some addition to the double or triple bond can be Copper-catalysed decomposition of MeCOC(N2)C02R and (MeC0)2CN2 in the presence of vinyl ethers leads to 4-(alkoxycarbony1)- and 4-acyl-2,3-dihydrofurans respectively. Substituent effects indicate a non-synchronous stereospecific addition of a metal-carbene intermediate to the alkene viz.(96). This leads to cyclopropanes 8o D. F. Taber and E. H. Petty J. Org. Chem. 1982,47,4808. E. Y. Chen Tetrahedron Lett. 1982,23,4769. 132 M.P. Doyle R. L. Dorow W. H. Tamblyn and W. E. Buhro Tetrahedron Lett. 1982 23 2261. 83 T. Aratani Y. Yotleyoshi and T. Nagase Tetrahedron Lett. 1982 23 685. A. F. Noels A. Demonceau N. Petiniot A. J. Hubert and P. Teyssie Tetrahedron 1982 38 2733. Arynes Carbenes Nitrenes and Related Species dihydrofurans and products of apparent insertion of a carbene into allylic Thermolysis of dimethyl diazomalonate in the presence of benzaldehyde leads to diastereomeric dioxolanes (97) together with (98). The reaction is presumed to occur by initial formation of di(methoxycarbonyl)carbene and kinetic measure- ments provide no evidence for induced decomposition of the diazo-compound by benzaldehyde.The yield of oxirane was dramatically increased by decomposition of the diazo-compound in the presence of copper. The oxirane is+no_t a precursor of the dioxolanes and an intermediate carbonyl ylide PhCH=O-CH(C02Me)2 which can lead to either product is consistent with the kinetics and with the trapping with dimethyl fumarate that leads to (99).86 n qO,Me Oxiranylidene (100) is predicted to be a stable observable species; its lowest energy unimolecular decomposition is to methylene and carbon monoxide. By contrast formylmethylene and hydroxyvinylidene are predicted to rearrange to ketene and hydroxyacetylene respectively without activation energy.87 Oxirene intermediates that are formed by oxidation of acetylenes may be further oxidized to diketones or may rearrange without the involvement of keto-carbenes; in consequence keto-carbene-keto-carbeneinterconversions should proceed via an intermediate which is not an oxirene.88 The ambiphilic behaviour of methoxychlorocarbene has been demonstrated in its reactions with substituted styrenes; thus the reactivities of 4-methoxy- and 4-nitro-styrenes are 1.50 f 0.03 and 1.27 f 0.02 respectively relative to styrene itself.89 Phenoxychlorocarbene generated by thermolysis of 3-chloro-3-phenoxydiazirine also shows an ambiphilic reactivity pattern with a series of alkene~.~' This carbene may also be generated by reaction of a,a-dichloroanisole with sodium hydroxide under phase-transfer conditions; it reacts with styrenes by addition and the selectivity is found to increase at lower temperatures.In this study the carbene behaves as a nucleophilic species.'' The reaction of bromo(pheny1)diazirine with methoxide ion gives methoxy(pheny1)diazirine as an unstable species which generates methoxyphenyl- carbene on photolysis. The latter can be trapped by ethanol as PhCH(0Me)OEt and undergoes addition to alkyl-substituted alkenes albeit in low yield.92 " M. E. Alonso A. Morales and A. W. Chitty J. Org. Chem. 1982,47 3747. 86 P. de March and R. Huisgen J. Am. Chem. Soc. 1982 104 4952; R. Huisgen and P. de March ibid. p. 4953. '' W. J. Bourna R. H. Nobes L. Radorn and C. E. Woodward J. Org. Chem. 1982,47,1869. " Y.Ogata Y. Sawaki and T. Ohno J. Am. Chem. SOC.,1982,104 216. 89 R. A. Moss W. Guo and K. Krogh-Jespersen Tetrahedron Lett. 1982,23 15. 90 R. A. Moss L. A. Perez J. Wlostowska W. Guo and K. Krogh-Jespersen J. Org. Chem. 1982,47 4177. '' W. Bruck and H. Durr Angew. Chem. Int. Ed. Engl. 1982,21,916. 92 J. Wlostowska R. A. Moss W. Guo and M. J. Chang J. Cltem. SOC.,Chem. Commun. 1982,432. 102 M. S. Baird Vacuum pyrolysis of the aryl trimethylsilyl ketones (101; R' = H R2= SiMe3) leads to (101; R' = SiMe, R2= H) and trimethylsilyloxybenzocyclobutene.Both products apparently arise uia an intermediate siloxycarbene (102).93 Oxadiazoline (103) decomposes thermally in carbon tetrachloride with first-order kinetics. The reaction apparently proceeds by initial loss of nitrogen to form a carbonyl ylide; this fragments to dicyclopropyl ketone and (acetoxymethy1)carbene.The carbene either undergoes a 1,2-acyl transfer to produce biacetyl or abstraction of a chlorine atom from solvent to give the 1-acetoxy-1-chloroethyl radical.94 3 Nitrenes Photolysis of several polycyclic azides has been used to generate strained cyclic imines as intermediates; for example bicyclo[3.2.l]oct-l-yl azide leads to (104) which may be trapped by methan01.~~ Under particular conditions quantum yields as high as three hundred can be observed in the photodecomposition of phenyl azide; this is consistent with a chain reaction in which phenylnitrene reacts with phenyl azide to produce 1,4-diphenyltetra-azadiene Ph-N=N-N=N-Ph which then decomposes into two phenylnitrenes.Direct reaction of the nitrene and azide to produce two nitrenes and nitrcgen is also possible.96 Photolysis or thermolysis of the azide in acetic acid leads to complex product mixtures including (105) (106) and various products of nucleophilic aromatic substitution. Evidence from reactions in the presence of various addends indicates a reaction leading to 1-azacyclohepta-1,2,4,6-tetraene formed either via a singlet nitrene or a singlet excited phenyl azide; the reaction between nitrene and acetic acid leads to (107) and thence to the aromatic substitution Thermal decomposition of PhN3 passing the products into thioanisole leads to 2-[(phenylthio)methyl]aniline; initial attack of & HN&)\/ aNYMe '0 .-OAc b (104) (105) (106) (107) 93 C.Shih and J. S. Swenton J. Org. Chem. 1982,47 2668. 94 M. Bekhazi and J. Warkentin J. Org. Chem. 1982 41 4870. 95 K. B. Becker and C. A. Gabutti Tetrahedron Lett. 1982 23 1883; T. Sasaki S. Eguchi and T. Okano ibid. p. 4969; H. Quast and B. Seiferling Liebigs Ann.Chem. 1982 1553. 96 W. H. Waddell and C. L. Go J. Am. Chem. Soc. 1982,104,5804. ''H.Takeuchi and K. Koyama J. Chem. Soc. Perkin Trans. I 1982 1269; for a related decomposition of ethyl azidoformate in trifluoroacetic acid see J. Chem. Soc. Chem. Commun. 1982,226. Arynes Carbenes Nitrenes and Related Species singlet phenylnitrene on sulphur followed by a Sommelet-Hauser-type rearrange- ment is thought to explain the The reaction of ethoxycarbonylnitrene with enamines also leads to initial formation of an ~lide.~'~.Pyrolysis of the azide (108; X = N2)leads to a carbazole produced by apparent insertion of nitrene into the C-H bond at A; in contrast photolysis leads to an apparent insertion at the C-H bond at B. Deoxygenation of (108; X = 0)gives the carbazole confirming that formation of the latter is a low-energy process that is favoured by singlet nitrenes. Formation of phenothiazine is also feasible with the triplet species.99 fi ' NX s A (108) Photolysis of NN'- diarylbenzoquinonedi-imine"'-dioxides (109) and N-aryl- benzoquinoneimine N -oxides leads mainly to azo-arenes formed by the dimeriz- ation of triplet arylnitrenes;lOO similar photolysis of N-t-alkyl-benzoquinoneimine N-oxides leads to alkylnitrenes which generally abstract hydrogen to produce amines.lo0 C02Et m:OIFt Me Thermal decomposition of vinyl azides (110) leads to isoquinolines benzazepines or aziridines.When R is vinyl preferential reaction occurs at the R group and (111) is formed in good yield. When R is allyl the result is a much more complex mixture of products. lo' An examination of the activation parameters for thermal isomeriz- ation of 5-methylisoxazole leads to the conclusion that the rate-determining step is R1QR3 R2 Scheme 4 98 (a) L. Benati M. Grossi P. C. Montevecchi and P. Spagnolo J. Chem. SOC.,Chem. Commun. 1982 763; (b) L. Pellacani P. Pulcini and P. A. Tardella J. Org. Chem. 1982,47 5023. 99 D. G. Hawkins and 0.Meth-Cohn J.Chem. Res. (S),1982 105. loo A. R. Forrester M. M. Ogilvy and R. H. Thornson J. Chem. SOC.,Perkin Trans. I 1982 2023; P. J. Baldry. A. R. Forrester M. M. Ogilvy and R. H. Thornson ibid.. p. 2027; see also ibid. p. 2035. I"' D. M. B. Hickey C. J. Moody and C:W. Rees J. Chem. Soc. Chem. Commun. 1982 1419. 104 M. S. Baird not a simple ring-opening but rather a rearrangement to a 3-acyl-l-azirine which can then equilibrate with the ring-opened vinylnitrene (Scheme 4). lo* Oxidation of 3-amino-2-(arylalkyl)quinazolin-4(3H)-ones leads to N-nitrenes (112) which react intramolecularly with methoxy-substituted benzene rings. The substitution pattern required for this to occur suggests a reaction occurring through a seven-membered transition state involving electron donation from the aromatic ring to the vacant p-orbital of the nitrene.lo3 The synthesis direct spectroscopic observation and kinetics of decomposition of the persistent 1,l-diazenes N- (2,2,6,6-tetramethylpiperidyl)nitrene and N-(2,2,5,5-tetramethylpyrrolidyl)-nitrene have been rep~rted."~ Oxidation of 1-nitroso-1-alkylhydrazineswith a variety of reagents leads to products apparently derived by coupling of radical fragments that ar? produced by the extrusion of nitrogen from N-nitrene intermedi- ates viz.PhCH2 and NO from the PhCH2N(NO)-N radical;lo5 however oxida- tion of hydrazines with benzeneseleninic acid to produce tetrazenes has been found by trapping experiments to be uniikely to involve N-aminonitrenes. lo6 Oxidation of 2,4-dinitrobenzenesulphenamidewith lead tetra-acetate in the presence of (2)-1-phenylpropene leads to a ca.3 :1 mixture of cis-and trans-aziridines apparently by addition of the ArSN radical. The ratio of products is unaffected by traps for triplet nitrenes such as 1,l-diphenylethene 2-phenylpropene or oxygen although the alkenes do react rapidly. The ratio is changed by the addition of a trap for singlet nitrenes i.e. ally1 aryl sulphide. It seems that two intermediates are involved in the reaction; one is the singlet nitrene but the other is not the triplet."' 4 Silylenes Flash vacuum pyrolysis of (113; R = Me) at 680°C leads to MeOSiMe3 and the silacyclobutane (114). This can be explained by loss of the former from (113; R = Me) to produce a-cyclopropyl-a -methylsilylene (115) which ring-expands to J.D. Perkz G. I. Yranzo and D. A. Wunderlin J. Org. Chem. 1982,47,982. lo3 R. S. Atkinson J. R. Malpass and K. L. Woodthorpe J. Chem. SOC.,Perkin Trans. 1 1982 2407. W. D. Hmsberg P. G. Schultz and P. B. Dervan J. Am. Chem. SOC.,1982,104 766; P. G. Schultz and P. B. Dervan. ibid. p. 6660. K. Kano C. A. Kelly and J.-P. Anselme Tetrahedron Lett. 1982,23 1427. T. G. Back and R. G. Kerr Cuk. J. Chem. 1982,60,2711. lo' R. S. Atkinson B. D. Judkins and N. Khan J. Chem. SOC.,Perkin Trans. I 1982,2491. Arynes Carbenes Nitrenes and Related Species 1-methyl-1-silacyclobuteneby a silylene-to-silene rearrangement; this then ring- opens to 2-methyl-2-silabuta-1,3-dieneand undergoes head-to-tail dimerization.Co-pyrolysis of (113; R = Me) with 2,3-dimethylbutadiene produces (116) by trapping of the silylene. It is interesting to note that cyclopropylmethylsilylene seems to undergo an alkyl shift to the silacyclobutene rather than a hydrogen shift. Flash vacuum pyrolysis of (117) leads to MeSiH,C=CH among other products; this may arise uia a -vinyl-a- methylsilylene ring-closure to 1-methyl-1-silacyc-lopropene rearrangement to 3-methyl-3-silacycloprop-l-ene,and then further rearrangement to the alkyne."' Pyrolysis of (118) at 540°C in the presence of methanol leads to (119); this may be explained once again by formation of a silyl- ene a-cyclopropyl-a-phenylsilylene,which ring-expands to 1-phenyl-1-silacyclo-butene ring-opens to 2-phenyl-2-silabuta- 1,3-diene and is trapped by Me Me&- C1 ISi Ph IH&=CH-Si-OMe SiMe I Me 1 Me Pyrolysis of (113; R = SiMe,) leads to complex products including vinyltrimethyl- silane and 1-trimethylsilylprop-1-yne.An intermediate a-trimethylsilyl-a-cyclo-propylsilylene can again be trapped by 2,3-dimethylbutadiene; the authors speculate that in the absence of a trap this silylene may again ring-expand to produce 1-trimethylsilyl-1-silacyclobutene,and that a shift of a trimethylsilil group may then occur to give (120). This second silylene (120) might then fragment to H,C=Si :and vinyltrimethylsilane. It is interesting that the silylenes Me3Si-SiCH2CH=CH2 (12 1) and Me3Si-Si-CH=CH-CH3 (122) also lead to vinyltrimethylsilane and trimethyl- silylprop-1-yne in similar ratios.The similarity of the products from the three silylenes suggests that in the absence of traps they merge onto the same energy surface. When (121) was generated in the presence of 2,3-dimethylbuta-1,3-diene one of the products was (1231 apparently produced by trapping (124)."'Generation of the silylene Me3SiSiMe2SiMe in the presence of the butadiene trapping agent leads to (125); this is the first clear evidence of a silylene4silene rearrangement the species actually trapped apparently being Me2Si=Si(Me)SiMe3.' l1 log T. J. Barton G.T:Burns W. F. Goure and W. D. Wulff J. Am. Chem. SOC.,1982,104 1149. W. Ando Y.. Hamada and A. Sekiguchi J. Chem. SOC.,Chem. Commun. 1982,787. 'lo S. A. Burns G. T. Burns and T. J. Barton J. Am. Chem. SOC.,1982,104,6140.''I H. Sakurai H. Sakaba and Y. Nakadaira J. Am. Chem. SOC.,1982,104,6156. 106 M. S. Baird SiMe (125) Contradictions in the literature concerning the ease of rearrangement of MeSi(H)=CH to dimethylsilylene have been reconciled in terms of the kinetics of competing processes involved in the different conditions for silene generation." Ab initio calculations on the insertion of silylenes into 0-Hbonds indicate that H20-SiH2 is a stable complex formed from singlet silylene and water and has a significant barrier to rearrangement to more stable silanol; the carbon analogue is highly unstable and has been predicted to rearrange without a barrier.'13 R. Walsh J. Chem. SOC.,Chem. Commun. 1982 1415. n3 K. Raghavachari J. Chandrasekhar and M.J. Frisch J. Am. Chem. SOC.,1982,104,3779.

ISSN:0069-3030    

DOI:10.1039/OC9827900083

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

6 Electro-organic Chemistry By M. SAINSBURY School of Chemistry University of Bath Claverton Down Bath BA2 7AY 1 Introduction The subject of electro-organic chemistry continues to grow and more papers than ever have appeared during the last year.* Despite this increase however there is still a general reluctance on the part of organic chemists to incorporate the technique in multistage syntheses. Fortunately a very useful and detailed review of anodic and cathodic carbon-carbon bond-forming reactions has appeared,' which may help to redress this situation Of similar interest are surveys of the utility of electrochemical methods in the steroid field' and of the electrochemistry of the sulphonium group the latter being published as a chapter in a book in the Patai ~eries.~ From a commercial standpoint a review on electrosynthetic reactions used in the fine chemicals industry is also very timely.4 2 General and Mechanistic Aspects In a series of papers Savkant and his associatesSa-' have considered the complex relationships that exist between the distribution of products and the characteristic rates of reactions or rate ratios and the magnitudes of the various operational factors which are commonly encountered in preparative scale electrolyses.For example electrochemically induced aromatic nucleophilic substitution reactions compete with hydrogen abstraction from the solvent and also with electron transfer at the electrode and in the bulk solution. This four-way system has been studied by voltammetric analysis and by product isolation and it appears that in most cases the experimental conditions can be adjusted so that the competition only involves nucleophilic attack heterogeneous electron transfer and hydrogen-atom abstrac- tion.One consequence of the existence of these competing pathways is that there is a requirement for high reactivity of the nucleophile towards the aryl radicals in * Based on Chemical Absrracrs during the period November 1981 to November 1982. ' H. J. Schafer Angew. Chem. Int. Ed. Engf. 1981 20 911. K. Ponsold and H.Kasch 2.Chem. 1982,22 157. J. Grimshaw in 'The Chemistry of the Sulphonium Group,' ed. C. J. M. Stirling and S. Patai J. Wiley and Sons Ltd. 1981,Chapter 7,pp. 145-154. 'D. Pletcher Chem. Ind. 1982 358. (a)C.Amatore and J. M. Saveant J. Electroanal. Chem. Interfacial Electrochem. 1981 123 189. (6) C. Amatore and J. M. Saveant ibid. p. 203. (c) C. Amatore. F. M. Halla and J. M. Saveant ibid. p. 219.(d) C.Amatore J. Pinson and J. M. Saveant ibid. p. 231.(e)C. Amatore and J. M. Saveant ibid. 1981 125 1. (f) C. Amatore and J. M. SavCant ibid. p. 23.(g) C. Amatore and J. M. Saveant ibid.,1981 126,1. Ih 1 C. Amatore. J. Pinson and J. M.Saveant ibid. 1982.137.143.(i)J. Pinson J. M. Savtant and A. Thitbault J. Am. Chem. SOC.,1982,104,817 108 M. Sainsbury order that good yields of substituted products result. It is established that SRN1 type mechanisms operate in such reactions rather than the alternative SRN2 pro-cesses. Similarly Parker's have analysed the effects which the steric and electronic properties of the substrates have upon the entropies of formation of ion radicals These workers6e have also returned to the thorny question of the mechan- isms of aryl-aryl coupling reactions.In the case of 1,2-bis(3,4-dimethoxypheny1)ethane the intramolecular cyclization reaction follows the rate law (i) Rate = k,pp[ArCH2CH2Art]2 (9 This is consistent with either disproportionation (ii) followed by cyclization (iii); or with cyclization (iv) followed by rate-determining electron transfer (v). It is concluded that the second alternative is the more likely with electron transfer (v) being the rate-determining step (Scheme 1). 2Ar Art- Art Art + Ar- Ar (ii) Art Art + Ar+-Ar+ (iii) Ar Art Ar+-Ar'- - (iv) Ar+-Ar' + Ar Art + Ar+-Ar+ + Ar- Ar (v) Ar+-Ar' -+products (vi> Scheme 1 Aromatic chloro- and fluoro-compounds when oxidized undergo apparent nucleophilic substitution reactions with surprising ease.It seems that the initial radical cation undergoes ips0 attack by the nucleophile and that this is followed by loss of the substituent as a species at the same oxidation level as the nucleophile giving the radical cation of the product to be formed. A chain-transfer step involving this ion and a second substrate molecule then completes the reaction sequence which is summarized below (i-iv Scheme 2).' ArX 5[ArX]" (9 [ArX]? + Nu-B Ar'(Nu)X (ii) Ar'(Nu)X + [ArNu]+ + X-(iii) [ArNaIt + ArX -P ArNu + [ArX]? (iv) Scheme 2 Little is known about the nature of 1,3-radical cations derived from alkanes so that the anodic oxidation of the cyclopropane (1)is of some interest.However the first observable product is the cation (2),and from cyclic voltammetric studies there is no evidence for two one-electron transfer steps even at sweep rates as high as 2OVs-'. In the absence of base the cation cyclizes to 1,1,3-triphenylindenes (3) although if base is added further deprotonation to tetraphenylallene (4) occurs '(a)M. Svaan and V. D. Parker Acra Chem. Scand. Ser. B,1981 35 559; (b) ibid.. 1982 36 351. (c) ibid. p. 357. (d) ibid. p. 365. (e) B. Aalstad A. Ronlan and V. D. Parker ibid. p. 171. (f) B. Aalstad and A. RonlBn ibid. p. 317. (g) 0.Hammerich and V. D. Parker ibid. p. 519. ' L. Eberson L. Jonsson and L.G.-Wistrand Tetrahedron 1982,38,1087. Electro-organic Chemistry (Scheme 3). When substituted phenyl rings are attached to the cyclopropane unit the initially formed radical cation may be stabilized since now the voltammograms show two closely spaced equally intense waves.8 -"+I Ph,C =C =CPh2 (4) Scheme 3 The fate of the intermediate radical cations formed during the anodic oxidation of epoxides is changed dramatically when the solvent is free of efficient n~cleophiles.~ Thus if acetonitrile containing water is the solvent ketones are formed e.g. (5)+(6) + (7),but in super-dry acetonitrile or in methylene chloride the amount of current consumed is low (ca. 0.1 Fmol-') and the main product is the rearranged ketone (8)-a result which is comparable to ring opening of the epoxide in strongly acidic media.This electrocatalytic process is based upon the assumption that the standard oxidation potential of the ketone is larger than that of the corresponding epoxide thus ensuring that chain propagation is allowed (see Scheme 4). f 1 (8) Scheme 4 The factors that favour cyanomethylation rather than reductive saturation or hydrodimerization of a,@-unsaturated nitriles in acetonitrile have been ascer- tained.lo High dilution tends to effect reduction whereas an increase in temperature D. D. M. Wayner and D. R. Arnold I. Chem. SOC.,Chem. Commun. 1982,1087. J. Delauney A. Lebouc A. Tallec and J. Simonet J. Chern. SOC.,Chem. Commun. 1982 387. lo A. J. Bellamy J. B. Kerr C. J.McGregor and I. S. MacKirdy I. Chem. SOC.,Perkin Truns. 2,1982 161. 110 M. Sainsbury promotes cyanomethylation over hydrodimerization. The addition of water to the electrolyte medium suppresses cyanomethylation and also reduces the overall product yield but leaves the proportion of the hydrodimer in the mixture largely unchanged. An interesting comparison has been drawn between the electroreductive and the hydride-initiated cyclizations of w-bromoalkylidenemalonates." On cathodic reduction these substrates yield ring structures (9) with one atom less in the carbocycle than the products obtained by treatment with L-selectride (Scheme 5). During the electrochemical reactions the electrode potential is more positive than that needed for the reduction of an alkyl bromide so that initial carbon-bromine bond cleavage is unlikely.Overall however two electrons are transferred and so it is probable that one-electron addition and ring-closure occur in concert. Scheme 5 The electroreduction of carbon dioxide in the presence of buta-l,3-diene leads to a mixture of the anions of pent-3-enoic acid (ll),hex-3-enedioic acid (12) deca-3,7-dienedioic acid (13) and other isomeric decadienedioic acids.'* The first two products arise through the interaction of the radical anion of carbon dioxide with one or both termini of the diene whereas the Clo-acids result from the dimerization of the intermediate radical anion" (see Scheme 6). +e. +H' MeCH=CHCH2CO; (11) O;CCH2CH=CHCH2CO; (12) (-CH2CH=CHCH2CO; )2 (13) Scheme 6 All the chlorinated nitrobenzenes except the pentachloro-compound exhibit reversible one-electron waves at the potential of the first reduction peak during cyclic voltammetry in dimethylsulphoxide s~lution.'~ For these structures a pXo correlation of the peak potentials shows that the u constants are non-additive.Additivity is obtained however by applying an empirically determined correction factor for adjacent chlorine pairs. In a continuation of earlier work designed to examine the effect of the addition of electrochemically generated cyanomethyl anion to various electrophiles the '' S.T. Nugent M. M. Baker and R. D. Little Tetrahedron Lett. 1982 23 1339. j2 W. J. M. van Tilborg and C. J. Smit Rec. Trau. Chirn. Pays-Bas.1981 100 437. l3 R.D. Geer and H. J. Byker J. Org. Chern. 1982,47 1662. Electro-organic Chemistry relative acidities of a number of substituted acetonitriles [XCH2CN where X = Ph3P+ Ph3As+ (me~ityl)~P+ or PhS02] versus acetic acid have been determined. l4 Ph3PCH2CN is a stronger acid than acetic acid the others are weaker but (me~ityl)~P+CH~CN I- and PhS02CH2CN prove to be useful precursors of the cyanomethyl anion. Interestingly PhS02CH2CN undergoes one-electron rather than two-electron reduction at the potential of the first polarographic wave. It appears that half the molecules of PhS02CH2CN reaching the electrode surface accept two electrons while the other half are rendered inactive by deprotonation of the liberated cyanomethyl anions.In acetic acid however the latter process is inhibited and a two-electron wave is now observed since the anion formed is rapidly protonated by the acid present. A similar process arises during the electrochemical reduction of N -haloamides (RiCONClR2) in acetonitrile solution. This involves two consecutive one-electron transfer reactions ultimately generating the amide anion. In the case of N-haloamides where R2= H the anion formed may deprotonate incoming substrate so that the true nature of the electrode process is masked. From a synthetic point of view the reduction of N-haloamides can be used to prepare heterocyclic com- pounds for example the anion of the amide (14) undergoes an internal displacement reaction to yield the isoindolinone (15) (Scheme 7).15 +2e --c1-N CI’ ‘Me (14) Scheme 7 When 1,2-dibromobenzene is reduced in the presence of furan 1-naphthol is produced and it has been proposed that the reaction proceeds through the gener- ation of benzyne either via an ionic stepwise or a concerted mechanism.New work16 suggests that the first alternative is the more likely for when 1,2-dichloroben- zene is reduced in the presence of carbon dioxide the products are benzene and 2-chlorobenzoic acid. Added furan does not now afford 1-naphthol. The electrochemical reduction of N-bromosuccinimide in acetonitrile at a platinum cathode affords the succinimidyl radical in an overall one-electron transfer process. An intermediate in this reaction is the succinimide anion and its interaction l4 A.J. Bellamy and I. S. MacKirdy J. Chem. SOC.,Perkin Trans. 2 1981 1093. D. Berube and J. Lessard Can. J. Chem. 1982,60 1127. l6 F. Barba A. Guirado and A. Zapata Electrochim. Actu 1982 27 1335. 112 M. Sainsbury with a molecule of the substrate forms two equivalents of the radical. Thus three steps are involved the second of which is a relatively fast reduction of initially generated succinimidyl radical" (Scheme 8). Scheme 8 Cyclovoltammetric studies show that aromatic amines in aqueous or non-acidic media give rise to mono-cation radicals at the anode which in the absence of water then dimerize by tail-to-tail head-to-tail and even head-to-head coupling. '* Since the products are more readily oxidized than the parent amines further reactions occur at the same potential.Aniline for example yields 4-aminodiphenylamine benzidine hydrazobenzene and other related structures. Tetraphenylcyclobutane and pyridyl(pheny1)cyclobutanesundergo rapid [u2s + ,2,] cycloreversion on electrochemical reduction after one-electron transfer; on the other hand the tetra-cation (17) accepts two electrons and undergoes ring-opening to give the butane (18).19 3 Anodic Processes Various allenic hydrocarbons have been electrolysed in acetonitrile or methanol solution at a platinum anode maintained at ca. 2V.'' All undergo the loss of two electrons which is followed by nucleophilic attack of the solvent and/or water to afford products containing at least two of the functional groups C=C C=O NHCOMe and OH.2-Methylpenta-2,3-diene (19) is typical and gives the three amides (20) (21) and (22) in 14.4 5.3 and 3.9% yields respectively. The product composition is changed by variation in the concentration of the substrate and J. E. Barry M. Finkelstein W. M. Moore S. D. Ross L. Eberson and L. Jonsson J. Org. Chem. 1982,47,1292. '* L. R.Sharma A. K. Manchandra G. Singh and R.S. Verma. Electrochim. Acta 1982,27 223. l9 M. Horner and S. Hunig Justus Liebigs Ann. Chem. 1982 1409. *' J. Y.Becker and B. Zinger J. Chem. SOC. Perkin Trans. 2 1982,395;Tetrahedron 1982,38 1677. Electro-organic Chemistry electrolyte (usually lithium perchlorate) the electrode potential the temperature and the anode material. In some cases cyclized structures which include arenes pyrimidines and s-triazines are isolated.Heterocumulenes have also been studied.*l NHCOMe Me NHCOMe -Pr-LiCIO,/MeCN \/ Me>C=CHMe (MehC,/ + CCOCH Me 1.85 V // COCOMe ‘Me (19) NHCOMe + (Me)2C\/ CO CH (0H )Me (22) Stable triarylamine radical cations (23) have found use as electron-transfer ‘catalysts’ in the oxidative cleavage of benzyl esters. The parent amines are added either in stoicheiometric quantities to the esters and then electrolysed or the radical cations are generated and regenerated indirectly using an external divided cell (see Scheme 9).22 -e Scheme 9 Esters can be synthesized through the electrochemical oxidation of hydroquinone carboxylates in the presence of alcohols. High yields are reported for this transacyla- tion process which occurs either in acidic or basic media.23 Amines may replace alcohols in this procedure in which case the products are amide~~~ (see Scheme 10).(X = 0 or NH) Scheme 10 *’ J. Y. Becker and B. Zinger J. Am. Chem. SOC.,1982,104,2327. 22 S.Dapperheld and E. Steckhan Angew. Chem. Int. Ed. Engl. 1982,21 780. 23 R.W.Johnson M. D. Bednardki B. F. O’Leary and E. R. Grover Tetrahedron Lett. 1981,22,3715. 24 R. W.Johnson E. R. Grover and L. J. MacPherson Tetrahedron Lett. 1981,22 3719. 114 M. Sainsbury Further examples of indirect electro-oxidations (ex-cell methods) have been described. In one such report 4-methoxytoluene has been converted into anisal- dehyde by chemical oxidation with ammonium hexanitratocerate(Iv) followed by electrochemical re-oxidation of the spent reagent at carbon or platinum electrodes in methanol as Some aryl-aryl coupled product (25) is formed in aqueous methanol together with the aryl methyl ether (24) which is considered to be an intermediate in the oxidation (see Scheme 11).OMe0 OMe Me0 QMe Me I Me IMeI OMe u’ (24) Scheme 11 CHO 5-Cyclopentyl-5-hydroxypentanoicand 4-(2-hydroxycyclohexyl)butanoic acid lactones are obtained by the anodic oxidation of 1-decalone. The two products are assumed to arise through ring-closures of the cations (26) and (27) respectively and in this process the cation (26) is formed first and interconverted into the isomeric species (27) by a 1,2-sigmatropic rearrangement (see Scheme 12).26 A new means of protecting amino-acids uia their coupling with 2,6-di-t-butyl-4- phenylphenol (28 R = Ph) has been used in the synthesis of the hydrophobic segment of human lymphoblastoid interferon.Anodic oxidation of amino-acid esters in dichloromethane in the presence of the phenol leads to N-(3,5-di-t-butyl-4-oxo-l-phenyl-2,5-cyclohexadienyl)amino-acid esters (29) that are stable towards bases but which may be cleaved by hydrogenolysis or treatment with acids. ’’ S. Torii H. Tanaka T. Inokuchi S. Nakane M. Akada N. Saitoh and T. Sirakawa J. Org. Chem. 1982,41 1647. ’‘ F. Barba A. Guirado I. Barba and M. Lopez Tetrahedron Lett. 1982,23,463. Electro-organic Chemistry 115 aO+dH Scheme 12 Furthermore the amino-acids couple without racemization and the products crys- tallize readil~.~' The same phenol and a series of its analogues when oxidized in acetonitrile solution trap a molecule of solvent to yield benzo-1,3-oxazoles (30).28 Both processes proceed through the intermediacy of cations (see Scheme 13).R' R3 -B II BU1fjBUi -2e BUi$BUi HN-:40,R4 U i ~ B U t + -H+ .-(R= Ph) I I R3 \ I R R Ph N-C-C02R4 RI' I (28) R2 (29) lMeCN Me I R (R= alkyl aryl acyl halogen or nitro) Scheme 13 '' M. H. Khalifa. G. Jung,and A. Rieker Justus Liebigs Ann. Chem.. 1982. 1068. '* E.-L. Dreher J. Bracht M. El-Mobayed P. Hutter W. Winter and A. Rieker Chem. Ber. 1982 115 288. 116 M. Sainsbury In a continuation of studies reported last year Shono and his have shown that the a-methoxylated derivative of N,N-dimethylaniline prepared by the anodic oxidation of the parent amine in methanol solution may be treated with Lewis acids in the presence of electron-rich alkenes to afford tetrahydroquinolines (31).It is suggested that iminium species are intermediates in the final reactions (Scheme 14). Me Scheme 14 Although a number of methods are available for the selenation of double bonds few are known for triple bonds however an efficient route to a-arylseleno-a,& unsaturated aldehydes is provided by the electrochemical oxidation of 3-hydroxyalkynes in the presence of diaryldiselenides at a platinum foil anode.30 The electrolyte is aqueous acetonitrile containing tetraethylammonium perchlorate and the reaction is considered to involve the initial formation of a cationic species (32) which is attacked by water prior to final dehydration (see Scheme 15).L 1 SeAr Scheme 15 29 T. Shono Y.Matsumurz K. Inoue H.Ohmizu and S.Kashimura J. Am. Chem. Soc. 1982,104,5753. 30 K.Uneyama K. Takano and S.Torii Tetrahedron Lett. 1982.23 1161. Electro-organ ic Chemistry 117 Selective chlorination of the methyl group of the 3-methylbut-3-enoate unit of certain penicillin derivatives (e.g. 33 -+ 34) can be achieved through electrolysis in an undivided cell containing a mixture of dichloromethane sodium chloride and sulphuric acid using platinum foil electr~de.~~ Variations in sodium chloride con- centration can lead to both allylic and benzylic chlorination.Related studies leading from 4-arenesulphonylt hioazetidin-2 -ones (3 5)to 3 -chloromethyl-A3 -cephems (3 6) have also been reported by the same Me C0,Me C0,Me (33) (34) R~CONH R *CON PAC1 C0,R2 (35) Whereas the anodic oxidation of arylhydrazones simply regenerates the parent ketone and thus provides a mild method for the 'hydrolysis' of hydrazones the cyclic hydrazone (37) in the presence of one molar equivalent of pyridine gives 3,3-dimethylbutan-2-one (38).33A Favorskii-like rearrangement is presumed to be involved (Scheme 16). H' (37) 1 Scheme 16 Morphinandienones (41) of the flavinantine type are obtained in high yield via the anodic coupling of N-trifluoroacetyl-l-benzyl-l,2,3,4-tetrahydroisoquinolines (39) in acetonitrile-methanol solution but when the methanol is omitted neos- 31 S.Torii H. Tanaku N. Saitoh T. Siroi M. Sasaoka and J. Nokami TerruhedronLert, 1981,22 3193. 32 S.Torii H. Tanaku N. Saitoh T. Siroi M. Sasaoka and J. Nokami Tetrahedron Lett. 1982,23 2187; see also Chem. Lett. 1982 1829. 33 E.-C. Lin and M. R. Van de Mark J. Chem. SOC.,Chem. Commun. 1982. 1176. 118 M.Sainsbury pirodienones(42) are formed It seems likely that the effect of the methanol is to facilitate 0-demethylation of the initial reaction product (40) before rearrangement may occur (see Scheme 17). OMe ::$ocF3+MeoT& NCOCF,+ ' Me0 Me0 OMe (39) ?Me (40)IMeOH OMe I QMe OMe OMe (42) Scheme 17 Should 0-methylflavinantine (43) itself be oxidized in acidic solution the 10a- hydroxy derivative (44) is obtained together with the pentacycle (45).The formation of these two products is considered to be interrelated as indicated in Scheme 4 Cathodic Processes Although the electroreductive conversion of 1,3-dihalides into cyclopropanes is a published procedure the reduction of 1,3-dimethanesulphonatesappears to be a superior method.36 The starting materials can be readily made and in great variety from 1,3-dicarbonyl compounds by C-alkylation followed by reduction and treat- ment with methanesulphonyl chloride and base (see Scheme 19). Details of the mechanism of the ring-closure reaction have not been established. '' H. Klunenberg C. Schaffer and H.J. Schiifer Terrahedron Left. 1982,23,4581. 35 L. Christensen and L. L. Miller J. Org. Chem. 1981,46,4877. 36 T. Shono Y. Matsumura K. Tsubata and Y. Sugihara J. Org. Chem. 1982,41,3090. Electro-organic Chemistry OMe MeO$ --H' -e -e d Me NMe NMe Me 0 MeO MeO Me0% 0 0 (43) Meof$ -e H e--e CHO +---2H' NMe NMe \iMe H 1 MeO' Me0 ).I Meo II 0 0 0 (44) Me0 Me0 goEdMeo$:H Me0 NMe + Meo$, NMe Me0 NMe 0 0 0 (45) Scheme 18 Scheme 19 120 M. Sainsbury Hydroxy-ketones (47) have a number of practical uses but as yet a general synthetic route to them has been lacking. Polish workers have recently shown that such compounds are available through the electrochemical reduction of hydroper- oxides formed by ozonolysis of a-alkylcycloalkenes (46) at a lead ~athode.~' In the examples quoted product yields ranged from 45-70%.Me Pbcathode Me(=) 0,AcOt-OOH -MeCO(CH,),CH,OH (CH,) HOAc AcOH-NaOAc (CH )n CHO (47) (46 n 3 3) Scheme 20 Ammonium graphite lamellar compounds formed by the reduction of tetra- alkylammonium salts in the presence of graphite can act as efficient reductants for organic halides and ~ulphones.~~ The reactions are usually carried out in dipolar solvents such as N,N-dimethylformamide and the results compare favourably with similar reductions which use tetra-alkylammonium amalgams as reductants. Cathodic reduction of the bromo-ester (48) at a vitreous carbon electrode gives the corresponding DL-and rneso-succinates (49) and a triester tentatively con- sidered to have structure (50).The two types of products are thought to arise by a single mechanistic sequence rather than through the operation of alternative pathways.39 C0,Me PhC(Me)(Br)COZMe+ PhC(Me)COZMe + Me0,C -C(Me) e+-~(Me)CO,Me I I PhC(Me)COzMe Ph Me Ph (48) (49) (50) An optical yield of 45% is reported for the asymmetric reduction of the dibromocyclopropane (51 R = Br) to its monobromo-analogue (51 R = H) at a mercury pool cathode made chiral by adsorbed emetine cations. This result is assumed to reflect preferential presentation of one stereotopic face of the substrate to the electrode surface. Overall two electrons are transferred and the anion of the cyclopropane is protonated through interaction with the adsorbed alkal~id.~' Further work has shown that the reductive cyclization of i-(2-halophenyl)-j-phenyl compounds is a general process and offers an effective synthetic approach to a wide range of structures providing that the two aryl units are held close together in the substrate.The chlorophenylpyridine (52) for example affords the dibenzo[f h]isoquinoline (53)in 95% yield.41 37 J. Gora K. Smigielski and J. Kula Synthesis 1982 310. J. Berthelot M. Jubault and J. Simonet J. Chem. SOC.,Chem. Commun. 1982 759. 39 C. de Luca A Inesi and L. Rampazzo J. Chem. SOC.,Perkin Trans. 2 1982 1403. 40 R. Hazard S. Jaouannet and A. Tallec Tetrahedron 1982,38 93. 41 J. Grimshaw R. Hamilton and J. Trocha-Grimshaw J. Chem. SOC.,Perkin Trans.1 1982,229. Elec tro-organ ic Chemistry Reduction of aroylchlorides at a mercury pool cathode yields first a,& diketones and then 1,2-diaroylethene-l,2-diolates.The latter products are acylated by two molecules of the aroyl chlorides to give the corresponding E-and Z-ester~.~' The whole sequence is outlined in Scheme 21. 2ArCOCI 5 2ArCO' + 2C1-2ArCO' + ArCOCOAr 0-0-0-ArCO Ar ArCozIAr I 4ArCOCI 2ArCOCOAr 2Ar-(!=C-Ar + ArC=CAr x-I -4CI'-0-ArCO Ar Ar C0,Ar Scheme 21 When diary1 ketones or their N-phenylimines are reduced electrochemically in the presence of 4-bromo (or chloro) butanoyl chlorides cyclopropylcarbonyl deriva- tives are formed. The reduction of the anil (54) is illustrative (see Scheme 22),43 where B is a strong base which may be the radical anion of the amine or hydroxide ion.The solvent has a considerable effect upon the product distribution and in some cases dehydrodimers are formed as well as heterocyclic structures. +H,O Ph2C=NPh + CI(CH2)3COCI-PhzC(OH)N(Ph)CO(CH2)3Cl -HCI (54) (55) B PhZCO + PhNCO(CH2)3CI -BH' Ph2CHOCOq PhNHCOq Scheme 22 A model for the in vim electrochemical release of neurotransmitters such as dopamine (56)from presynaptic termini has been described.44 The dopamine unit in the form of its amide with isonicotinic acid is bound to a polystyrene polymer 42 A. Guirado F. Barba C. Manzanera and M. D. Velasco J. Org.Chem.,1982 47 142. 43 G. Belot C. Degrand and P.-L. Compagnon J. Org. Chem. 1982 47 325. 44 L.L. Miller A. N. K. Lau and E. K. Miller J. Am. Chem. SOC.,1982,104 5242. 122 M. Sainsbury as the unit (57) and this is then coated onto the surface of a glassy carbon electrode. Cleavage of the amide function is achieved and dopamine is released when the electrode is maintained at -1.2 V in a divided -CCH2CH-);;-I -H2NCH2CH2 O -O H O\CH2 -k>CONHCH2CH2 G O H \ OH 'OH (56) (57) 'Doubly dimeric' structures result from the cathodic reduction of 4-pyrones an acetonitrile-water solution. In these reactions a certain degree of selectivity is observed thus the pyrone (58 R' = R2 = Me) affords the dimer (59 R' = R2 = Me) in 45% yield and only minor amounts of the isomers (60) and (61).However when the pyrone (58; R' = Me R2 = Ph) is reduced structure (61; R' = Me R2 = Ph) is the only 0 R' 0 Rlfj-: R' R' R2 R'QlR2 0 0 R2 Hoe R2 OH R' (61) A dimeric compound (62) is also formed when phthalic anhydride is reduced in the presence of trimethylsilyl chloride.N-Phenylphthalimide similarly affords an isoindole (63) which may be trapped by the addition of dienophiles such as maleimide.46 45 G. Mason G. Le Guillanton and J. Simonet J. Chem. SOC.,Chem. Commun. 1982 571. 46 T. Troll and G. W. Ollmann Tetrahedron Lett. 1981 22 3497. Electro-organic Chemistry When 3-(2-furyl)propenenitriles (64) are reduced at a mercury pool cathode dehydrodimers (65) are the initial products. However these products may cyclize either spontaneously or in step-wise fashion depending upon the relative stereochemistries of the intermediates to enamines (66) (Scheme 23).47 R2 / R'0 'CH=C \ CN Scheme 23 J.Delaunay A. Lebouc G. Le Guiltanton L. M. Gomes and J. Simonet Electrochim. Actu 1982 27 287.

ISSN:0069-3030    

DOI:10.1039/OC9827900107

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

7 Photochemistry By J. D. COYLE Chemistry Department The Open University Milton Keynes MK7 6AA 1 General The involvement of exciplexes excited complexes and radical ions in photochemical cycloadditions has been reviewed,’ and this reflects the increasing frequency with which electron-transfer steps are postulated in photoreaction mechanisms. Typical experimental criteria for justifying such hypotheses are given in a report2 of the quenching of aromatic hydrocarbon fluorescence by 2,3-bis(methoxycarbonyl)nor-bornadiene to give a radical-ion pair (and then diene triplet) and in a paper3 demonstrating that the photoisomerization of p -nitrobenzaldehyde to p -nitrosoben-zoic acid proceeds by way of electron transfer from water. In effect water acts as a catalyst carrying an electron ‘hole’; this is related to the use of semiconductors in photoreactions (e.g.for water-splitting) and one of the few examples of this in non-aqueous solvent is the cadmium sulphide catalysed photodimerization of phenyl vinyl ethere4 Photocycloreversion of a caged compound sensitized by a pyrylium salt goes by way of electron transfer and a radical-cation chain process,’ with a limiting quantum yield of 80. A review6 of photochemical reactions in organized assemblies (micelles vesicles films multilayers interfaces) is timely in view of a number of reports of the effects of a micellar medium on known photoreactions. One group’ has begun to put on a more quantitative basis a description of the photochemistry of molecules on silica-gel surfaces.Another growth area in organic photochemistry is the use of lasers to provide narrow-band radiation for synthetic purposes. An example is the successive use of 254nm (mercury arc) and 350 or 355 nm (Rayonet or Nd-YAG laser) light to improve the yields of pre-vitamin D3from 7-dehydrocholesterol (83% at 95% conversion analytically; 66% isolated on a preparative scale).’ ’ S. L. Mattes and S. Farid Acc. Chem. Res. 1982 15 80. * G. Jones 11 W. Schwarz and V. Malba. J. Phys. Chem. 1982,86 2286. G. G. Wubbels T. F. Kalhorn D. E. Johnson and D. Campbell J. Org. Chem. 1982,47,4664. R. A. Barber P. DeMayo and K. Okada J. Chem. SOC.,Chem. Commun. 1982 1073. K. Okada K. Hisamitsu T. Miyashi andT. Mukai J. Chem. SOC.,Chem. Commun.1982,974. ‘D. G. Whitten J. C. Russell and R. H. Schmehl Tetrahedron 1982 38 2455. ’R. K. Bauer R. Borenstein P. DeMayo K. Okada M. Rafalska W. R. Ware and K. C. Wu J. Am. Chem. SOC.,1982 104,4635. * W. G. Dauben and R. B. Phillips J. Am. Chem. SOC.,1982,104,355. 126 J. D. Coyle Finally in this introductory survey a collection of short articles has been pub- lished,' aiming to provide a general account of the very widespread applications of photochemistry. The continuing contribution of the subject to the well-being of mankind is highlighted" in a patent describing the formulation of a photochromic nail-polish (colourless indoors maroon outdoors) ! 2 Alkenes Direct ultraviolet irradiation of simple trans acyclic alkenes is not a good method for preparing the corresponding cis compounds because of limited conversion and competing side-reactions.However using pulsed infrared radiation high conversion can be achieved (90% cis for pent-2-ene)" because small differences in absorption cross-section are amplified in multiphoton absorption. The photochemical conver- sion of the A8"4'-16-ketosteroid (1)into the corresponding A'-compound is thought to go by way of a transoid intermediate,'* and this is the first example of a Py to yS enone photoisomerization. The excited states of styrenes and stilbenes have been studied intensively. Two significant new findings are that styrene undergoes quite efficient intersystem crossing (4 = 0.4 k = 3 x lo' s -') contrary to the most widely held assumptions about the excited states of alkene~;'~ and that the directly measured rate constants for photoisomerization of stilbene are very high (k,,, = 1.2 x 10'0s-'),'4 which strongly supports the view that the reaction occurs entirely on the singlet potential surface.The cis-trans photoisomerization of azobenzenes is different from that of stilbenes and a st~dy'~ of two azobenzenophanes for which nitrogen inversion is thought to be the only likely isomerization mechanism suggests that in normal azobenzenes different excited states isomerize by different routes. The use of metal trifluoromethylsulphonates to promote alkene photoreactions is well established. This has been used to produce high yields of the norbornene- acetonitrile adduct (2);16 the mechanism involves electron transfer from Ag' to co-ordinated alkene and subsequent formation of the CH2CN radical which adds to the alkene.Copper(1) triflate promotes the internal photocycloaddition of 1,6- 'Light Chemical Change and Life' ed. J. D. Coyle R. R. Hill and D. R. Roberts Open University Press 1982. I" JP 81 100 709 (Chem.Abstr. 1982.96 11 514). P. P. Teng. E.Weitz and F. D. Lewis J. Am. Chem. SOC., 1982 104 5518. l2 J. R. Williams A.Abdel-Magid W. Ricker and H. Salama J. Org. Chem. 1982 47 2536. I3 R. Bonneau J. Am. Chem. SOC.,1982,104,2921. '' M. Sumitani and K. Yoshihara Bull. Chem. SOC.Jpn. 1982 55 85. Is H. Rau and E. Luddecke J. Am. Chem. SOC.,1982,104 1616. I6 J. W. Bruno T. J. Marks and F. D. Lewis J. Am. Chem. SOC.,1982,104 5580.Photochemistry 127 dienes and the scope of this reaction has been extended" to provide routes to a variety of substituted bicyclo[3.2.0]heptanes [e.g. (3)]. Singlet oxygen reactions with alkenes and dienes continue to attract the interest of synthetic and mechanistic chemists. Myrcene can be converted into perillenal" by a double attack of singlet oxygen carried out under reducing conditions followed by oxidation of the allylic alcohol and then iron(I1)-promoted dehydration of the 1,2-dioxene (Scheme 1).The lack of stereospecificity in the formation of hydroper- oxides from the isomers of the constrained alkene (4) is taken as evidence against a concerted mechanism in this system." hv 02 sensitizer ' Bu~NBH~ X" :" CHO Scheme 1 3 Aromatics The full complexity of aromatic photonucleophilic substitution reactions is probably not yet revealed.In the reactions of the chloroanisoles with alcohols three mechan- isms are proposed;20 via aryl radical cations to give substitution products via aryl R. G. Salarnon D. J. Coughlin S. Ghosh and M. G. Zagorski J. Am. Chem. SOC.,1982 104 998. '* P. Baeckstrom S. Okecha N. DeSilva D. Wijekoon and T. Norin. Acra Chem. Scand. Ser. B,1982 36 31. '9 E. W. H. Asveld and R. M. Kellogg J. Org. Chem. 1982,47 1250. '"J. E. Siegman and J. J. Houser J. Org. Chem. 1982 47 2773. 128 J. D. Coyle radical anions to give reduction (dechlorination) products and uia aryl radicals also to give reduction products. The excited states involved in such reactions have not always been identified and although exciplex intermediates have often been invoked only recently have they been detected directly.With 3,5-dinitroanisole and nucleophiles exciplexes can be seen by flash spectroscopy,” as well as u-complexes that revert to starting materials. Arguments are presented for the involvement of a second type of exciplex and u-complex as well as substrate radical-anions which do not give rise to substitution products. The photochemistry of phenols and phenolates can lead to ring-contracted products. In the presence of trifluoromethanesulphonic acid phenol itself gives bicyclo[3.l.0]hex-3-en-2-one(5) in a method that overcomes the problems (low temperature low conversion extremely strong acid) associated with the previous use of fluorosulphuric acid.” 0-Chlorophenolates give a dimer derived from a cyclopentadiene-5-carboxylate(Scheme 2);’3 the mechanism is probably related to that of the reaction of 0-hydroxybenzenediazonium salts.0‘’ [0c02R] +dimer Scheme 2 The photochemical ring-contraction in 2-pyridones has been to make functionalized p -1actams (Scheme 3) by subsequent silylation ozonization and reduction of the initial bicyclic compound. An unrelated but fascinating method based on the irradiation of (pentacarbony1)chromium carbene complexes and imines gives good yields of p lac tarn^,^^ including those derived from thiazolines (Scheme 4). OMe Me02C. -do -+++ OHC SiMe,Bu’ H Scheme 3 ” C. A. G. 0.Varrna J. T.Tarnminga and J. Cornelisse J. Chem. SOC., Faraday Trans. 2 1982,265. 22 R. F. Childs G. S. Shaw and A. Varadarajan Synthesis 1982 198. 23 C. Guyon P. Boule and J. Lemaire Tetrahedron Lett. 1982 23 1581. 24 T. Kametani T. Mochizuki and T. Honda Heterocycles 1982 19 89. 25 M. A. McGuire and L. S. Hegedus J. Am. Chem. SOC.,1982,104,5538. Photochemistry 129 Scheme 4 Photocycloadditions of alkenes dienes or alkynes to aromatic compounds have provided a rich ground for mechanistic speculation but synthetically useful reactions are not common. 1,2-Cycloaddition (via excited-state alkyne) is the major reaction for alkynes with benzene and the bicyclo[4.2.0]octatriene products normally isomerize to cyclo-octatetraenes. With bulky groups (Scheme 5)the relative stability of the bicyclo[4.2.O]octatriene allows a caged isomer to be formed in reasonable yield,26 in a process that resembles the reactions of naphthalenes with alkynes.The 1,3-cycloaddition of vinyl acetate to indane gives a product (6) that has been elaborated into other [3.3.3]propellane~.*~ 0 Bu' + Bu'CECC0,Me -% 40% Scheme 5 &0Ac v 21% 1,3-Cycloadducts of alkenes with naphthalenes are uncommon but it is reported2* that trans (but not cis) cyclo-octene gives a mixture of 1,3-and 1,4-photoaddition products with naphthalene. The mixtures of products formed from anthracenes and dienes have provided apparent inconsistencies between results from different laboratories but this is now ascribed29 to the fact that the [4 + 41 adducts can be converted into the [4 + 21 adducts in a triplet-sensitized process.It is possible to prepare each type of adduct separately and it appears that they are not formed by way of a common biradical intermediate. Benzenoid compounds do not undergo photodimerization involving the ring as anthracenes do but the first such (intramolecular) reaction is reported3' based on a constrained multilayered cyclo- phane (7). 26 Y. Hanzawa and L. Paquette Synthesis 1982 661. " P. A. Wender and G. B. Dreyer J. Am. Chem. SOC.,1982 104,5805. *' Y. Inoue K. Nishida K. Ishibe T. Hakushi and N. J. Turro Chem. Lett. 1982,471. 29 T.-Y. Wang J.-D. Ni J. Masnovi and N. C. Yang Tetrahedron Lett. 1982 23 1231. 30 H. Higuchi K. Takatsu T. Otsubo Y. Sakata and S.Misumi Tetrahedron Lett. 1982,23,671. 130 J. D. Coyle Photocyclization reactions of aromatic compounds provide many useful routes to polycyclic products. The scope of the reaction of N-allylpyridinium (and quino- Mum) salts has been investigated (limitations arise because internal electron- transfer is involved) and the reaction can be used to prepare indolizidine derivatives (Scheme 6).31Aromatic enethioamides (8) give isoquinoline- 1-thiones and their 3,4-dihydro-derivatives on irradiati~n,~~ and these can be converted into isoquinolones and 1,2,3,4-tetrahydroisoquinolines respectively. The corresponding enamides undergo a 1,3-acyl shift and do not cyclize on irradiation. ROH 33 (& OR OR 90% (R= Me) Scheme 6 4 Carbonyl Compounds The photoreduction of benzophenone by alcohols has previously provided photo- chemists with surprises.Now it is clearly dem~nstrated~~ that with methanol ethanol or isopropanol three products are formed -benzpinacol a mixed pinacol and the oxidized para-adduct (9) -in ratios and with quantum yields that vary U. C. Yoon S. L. Quillen P. S. Mariano R. Swanson J. L. Stavinoha and E. Bay Tetrahedron Lett. 1982,23,919. 32 A. Couture R. Dubiez and A. Lablache-Combier J. Chem. SOC.,Chem. Commun. 1982 842. 33 M. B. Rubin. Tetrahedron Lett. 1982,23 4615. Photochemistry 131 0 significantly with light intensity. The intramolecular photochemical hydrogen abstraction and cyclization of carbonyl compounds has been used many times in synthesis but a particularly appealing application is in its repeated use to build up the dodecahedrane skeleton in twenty-three stages from ~yclopentadiene.~~ The mechanism for cis-trans isomerization of thioindigoid dyes (with an enedione grouping) has been shown3' to be unusual in that the trans +cis reaction occurs by way of the trans-triplet whereas the cis +trans process foIlows a singlet route.Substituted enediones (lo) obtained from furans give high yields of 2,5-dialkoxy- (or diacetoxy-) dihydrofurans on irradiation in hydroxylic and these are of interest for the synthesis of other heterocyclic systems. The triplet-sensitized oxa-di-7r-methane rearrangement of bicyclo[2.2.2]oct-5-en-2-ones(1l),usually prepared by way of the Diels-Alder cycloaddition of cyclohexa- 1,3-dienes and acrylonitriles gives products that can be elaborated to make bi- and tri-cyclic cyclopentanoid natural products and this process has been re~iewed.~' Mechanistic on the monocyclic P,y-unsaturated ketone (12) show that the oxa-di-r- methane reaction occurs through the TI state (which is not available by internal conversion from T2),and the 1,3-acyl shift through S or T2; unusually for a ketone xenon enhances S1*T2 intersystem crossing.hu 0 R'O 34 R. J. Ternansky D. W. Balogh and L. A. Paquette J. Am. Chem. SOC.,1982,104,4503. 35 C. P. Klages K. Kobs and R. Memming Chem. Phys. Len. 1982,90,46. 36 R. Antonioletti M. D'Auria G. Piancatelli S. Santucci and A. Scattri Tetrahedron Lett. 1982,23,2981. '7 M.Demuth and K. Schaffner Angew. Chem. Int. Ed. Engl. 1982,21,820. 38 D. I. Schuster and L. T. Calcaterra J. Am. Chem. SOC.,1982,104 6397. 132 J. D. Coyle Irradiation of N-methylphthalimide with allyltrimethylsilane gives a substituted hydroxyisoindolinone (Scheme 7) together with a smaller amount (1 1 Oh) of the corresponding -OSiMe derivati~e.~~ This is an unusual type of reaction in that phthalimides and alkenes normally give benzazepinediones but the difference can be rationalized on the basis of a similar process observed with a cyclic iminium salt (Scheme 8) where a mechanism is proposed4' that involves electron transfer from the allylsilane and subsequent generation of ally1 radicals. Scheme 7 + &SiMe3 A @Ph I I Scheme 8 42% Dioxetanes are normally obtained by cycloaddition of an alkene and singlet oxygen although they have occasionally been postulated as intermediates in the photoreactions of carbonyl compounds (e.g.1,3-dioxetanes in oxygen isotope exchange). Now a species (13) has been obtained by irradiation of N-methylacridone and acetone at -78"C that has the characteristics of a 1,2-dioxetane and gives rise to cherniluminescence on warming.41 The exchange of methyl groups that occurs on irradiation of a mixture of hexadeuterioacetone and the N-cyclohexyl- imine of acetone is thought to occur by way of a 1,3-oxazetidine (14).42 The formation of oxetanes from benzophenone or benzoquinone and alkenes is quenched by oxygen and 1,2,4-trioxanes [e.g. (15)] can be isolated.43 It is suggested that the species quenched is a charge-transfer exciplex rather than a biradical and that the quinone process is a model relevant to the photo-oxidation of vitamin K.y 3 ,C,Hl1 H3C-C-N I II Oao>Ph & 0-C-CD3 0-0 I CD3 (14) 39 JP 91 125 365 (Chem. Abstr. 1982,96 35 085). 40 K. Ohga and P. S. Mariano J. Am. Chem. SOC.,1982,104,617. 4' N. Suzuki Y. Kazui and Y. Izawa Tetrahedron Lett. 1982,23,95. 42 P. Margaretha Helv. Chim.Acta 1982 65 290. 43 R. M. Wilson S. W. Wunderly T. S. Walsh A. K. Musser R. Outcalt F. Geiser S. K. Gee W. Brabender L. Yerino J. T. Conrad and G. A. Tharp J. Am. Chem. Soc. 1982,104,4429. Photochemistry Of the many photochemical reactions that have been employed in synthesis the largest group is the photocycloaddition of alkenes to a,@-unsaturated carbonyl compounds.A review44 this year describes such reactions that can be followed by ring-opening in an alternative mode to the cycloaddition with special emphasis on intramolecular examples involving substituted cyclopentenones or cyclohexenones. A typically valuable example of such a reaction is the key stage (Scheme 9) in a seven-stage synthesis of sarracenin from methyl lactate,45 which is to be compared with the earlier fifteen-stage synthesis. 3-Cyanocyclohexenone and related systems give rise to a major alternative product (Scheme 10) that is formed by way of the normal intermediate biradical and an (isolable) irni~~e.~~ hu Me0 CHO C0,Me major Scheme 9 0 0 ah- CN 0 NH 8 1'/o 12% Scheme 10 44 W. Oppolzer Acc. Chem. Res. 1982 15 135. 4s S. W. Baldwin and M. T. Crimmins J. Am. Chem. SOC.,1982,104 1132. 46 I. Saito K. Shimozono and T. Matsuura Tetrahedron Lett. 1982 23 5439.

ISSN:0069-3030    

DOI:10.1039/OC9827900125

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

8 Aliphatic Compounds Part (i)Hydrocarbons By D. F. EWING Deparfment of Chemistry The University of Hull Hull HU6 7RX 1 Alkanes Although of limited interest it can often be quite difficult to make very long chain alkanes or derivatives when a specific length is required. A convenient procedure has been described' recently which starts from the bromoaldehyde (I) readily prepared by standard methods from cyclododecanone. Reaction of (1)with a Wittig reagent made from the acetal of (1)gave a 24-carbon bromoacetal(2) and repetition of this coupling step was carried out to give finally a bromoacetal containing 192 carbon atoms. The residual aldehyde function could be removed at any stage by treatment with a simple Wittig reagent and hydrogenolysis followed by hydrogena- tion was carried out to give a 104-carbon alkane.Extrusion of sulphur from sulphides or sulphones may be employed as a route to hydrocarbons. The usefulness of this approach can be enlarged by analogous reactions of selenides.2 Although the products of flash pyrolysis of selenides are dependent on the character of the bonding involving the selenium atom particularly useful starting materials are phenyl benzyl selenides which pyrolyse to give bibenzyls ArCH2CH2Ar (and diphenyldiselenide) in 65 to 90% yield. In some cases this reaction is cleaner than the corresponding reaction of sulphides or sulphones. Radical substitution reactions of alkanes are little studied now but one interesting report3 considers the reactive compound C1SO2NCO. Several possible products could be envisaged in this reaction but substitition by chlorine radical is the predominant step alkylisocyanates and alkanesulphonylchlorides being formed only to a very small extent.This reaction proceeds by either thermal or photo- chemical initiation and the hydrogen abstracting radical is probably NCO. 2 Alkenes Synthesis.-From Alkynes. The usefulness of the intermediates obtained by the addition of organometallic reagents to alkynes is legion and a very timely review * 0. I. Paynter D.J. Simmonds and M. C. Whiting J. Chem. Soc. Chem. Commun. 1982 1145. H. Higuchi T. Otsubo F. Ogura H. Yamaguchi Y.Sakata and S. Misumi Bull. Chem. SOC.Jpn. 1982,55 182. M. W. Mosher J. Org. Chem.. 1982,47 1875. 136 D.F.Ewing of this area has a~peared.~ This valuable account surveys carbometallation reactions of alkynes involving Li Zn Mg B Al and Cu as the 'metal' and highlights the activity in this field directed to the formation of specifically substituted alkenes particularly in natural product chemistry. Brown and co-workers5-' have developed several new variations of the hydro- boration of alkynes which significantly extend the usefulness of this reaction. The bromoborane complex R'BrBH-SMe is much more effective than a dialkylborane in the reaction with alkynes. Not only is there no competition from dihydroboration but the vinylborane (3) produced contains only one boroalkyl group which is subsequently transferred stereospecifically to the vinyl group [Scheme 1;reaction (a)].Thus no redundant boroalkylation is required in the formation of the reagent. R'BBr R'BOMe ii +L R' (a) R' R (3) R'BBr R' +p-b (b) )-7 Br R' (OMe)*B R' (4) R' % R' TxBR ' 7 Reagents i NaOMe -25 "C;ii NaOMe-I, 0 "C;iii H' heat Scheme 1 The only complication is formation of up to 15% of the corresponding vinyl iodide but this is less of a problem with bulky R' groups. This one-pot stereospecific synthesis of cis-alkenes has yields of 60-70%. If the hydroboration with R'BrBH-SMe is carried out on a bromoalkyne a bromovinylborane (4) is formed which can be stereospecifically converted to a trans-alkene [Scheme 1;reaction (b)]. Another similar route to trans-alkenes involves' formation of a thexyl vinyl- borane [5,Tx =Me2HCC(Me2)-] which shows the same specificity in the deboron- ation step.The starting point for this route is the versatile thexylchloroborane dime t h ylsulp hide complex TxClB H.Me2S obtained from 2,3-dime th ylbut -2-ene and BH2Cl.Me2S. This species affords an unusually stable alkyl derivative TxClBR' J. F. Normant and A. Alexakis Synthesis 1981,841. H. C. Brown and D. Basavaiah J. Org. Chem. 1982,47,3806. H. C. Brown D. Basavaiah and S. U. Kulkarni J. Org. Chem. 1982 47,3808. 'H. C. Brown H. D. Lee and S. U. Kulkarni Synthesis 1982,173. 137 Aliphatic Compounds -Part (i) Hydrocarbons when reacted with an alkene. Low-temperature hydridation regenerates the borane TxBHR' which is reacted with an alkyne to give (5).It is the high selectivity for the transfer of a primary alkyl group in (5) which provides this route for the formation of trans-alkenes under very mild conditions. The reaction appears to be general tolerating many common functional groups. The stereospecificity of the alkyl-group transfer in the deboronation of vinylboranes is responsible for a con- venient route to trisubstituted alkenes.8 Previous reports have described routes to 2-and E-1-halo-1-alkenylsilanes. It has now been established' that a generally reliable desilylation procedure is treat- ment with sodium methoxide in methanol. The 1-haloalkenes are obtained in 70 to 90% yield with no loss in stereochemical purity. Substitution of the Me& group by another halogen atom can be carried out by first performing an addition step trans -halogenation followed by base-induced desilico-halogenation.lo This pro- cedure gives E-1-bromo- 1-chloro- 1-alkenes. The isomeric 2-compounds were prepared via a hydroboration adduct of a 1-haloalkyne. Treatment of this borane directly with Br followed by NaOMe was not very effective in this case unless the borane was first converted to a borinic ester by oxidation with trimethylamine oxide. Synthesis of Alkenes by Elimination. A useful alternative" to the conversion of some primary alcohols to a selenate ester followed by oxidative elimination is to convert the alcohol to the iodide (via the tosylate) then induce elimination by in situ reaction with 1,5-diazabicyclo[4.3.0]non-5-ene or the analogous 1,8- diazabicyclo[5.4.0]undec-7-ene.These widely used reagents do not normally induce elimination in primary halides or sulphonates and only work with iodides if the @-carbon is disubstituted.p-Trimethylsilylethyl sulphoxides undergo elimination of the silylsulphonate rather faster than sulphenic acid is eliminated in the absence of a trimethylsilyl group.12 However this promoting effect is restricted to @-substituted systems since hydrogen is preferentially eliminated when both H and SiMe are present on the @-carbon. It is often the case that working on the synthesis of complex natural products provides a very severe test of currently available procedures. One example of a milder modification of an existing reaction which has been developed to meet the demands of natural product work is the conversion of 1,2-diols to 01efins.l~ The diol is first converted to a thionocarbonate by treatment with a slight excess of thiophosgene in presence of excess 4-dimethylamino-pyridine for one hour at 0 "C.Tertiary alcohols do not react. Elimination of the thionocarbonato group is promoted by reaction with 1,3-dimethyl-2-phenyl-l,3,3-diazaphospholidine. Yields are good (70 to go0/,) but perhaps not good enough to justify the use of these unusual reagents especially when another novel approach14 to this transforma- tion is considered. Treatment of a cis-or trans-diol with sodium iodide and CISiMe at room temperature is reported to effect elimination to the olefin in 80-90% yield for several steroidal compounds. H. C.Brown D. Basavaiah and S. U. Kulkarni J. Org. Chem. 1982 47 171. H. P. On W. Lewis and G. Zweifel Synthesis. 1981,999. lo R. P. Fisher H. P. On J. T. Snow and G. Zweifel Synthesis 1982. 127. S. Wolff M. E. Huecas and W. C. Agosta J. Org. Chem. 1982,47,4358. I. Fleming J. Goldhill and D. A. Perry J. Chem. SOC.,Perkin Trans. 1 1982 1563. l3 E. J. Corey and P. B. Hopkins TefruhedronLett. 1982.23 1979. l4 N. C. Barua and R. P. Sharma. Tetrahedron Lett. 1982,23,1365. 138 D. F.Ewing The direct transformation of a carbonyl compound into a vinyl derivative (Scheme 2)15 may prove valuable in the synthesis of natural products. An effective 6-trimethylsilylanion is provided by the new reagent Me3SiCH2CH2Li (or the analogous Grignard reagent). Attack on a carbonyl group gives the expected alcohol in 65 to 90% yield.In most cases this alcohol can be quantitatively converted to the required olefin. R' R' OH R' \CEO A \c/ -% \CHCH=CH2 R2/ R2/ \CH2CH2SiMe3 R2/ Reagents i Me3SiCH2CH,Li -78 "C; ii BF,.MeCOOH 25 "C Scheme 2 Nitroalkenes can be prepared by introducing a double bond into a nitroalkane via the phenyl selenide (cf. Annu. Rep. Prog. Chem. Sect. B 1981,78 149). An alternative approach is to introduce the nitro group directly.16 Formation of an adduct between an alkene and phenylselenyl bromide gives a 2-bromoalkyl phenyl selenide which reacts with silver nitrite in presence of mercuric chloride to give a nitroalkyl derivative. The nitroalkene is obtained in high yield by oxidative elimina- tion.A by-product produced at the addition stage is the analogous alcohol presum- ably arising from reaction of the nitrite nucleophile through an oxygen atom. Synthesis by other Methods. A preliminary rep~rt'~ gives details of a method for a direct alkylidenation reaction (Scheme 3) using a 1,l-dimetalloalkane. Little stereochemical control appears to operate. R2 R'CECH & R'CH=CHAI(Bu')2 A R*CH*CH=C/ 'R3 Reagents i (Bu'),AlH; ii R2R3C0 Scheme 3 Substitution of an alkylthio group on an olefin by alkyl or aryl functions under catalysis by low-valent nickel species has been studied by several workers recently. This reaction has now been extended" to replacement of an alkylthio group by hydrogen using a secondary Grignard reagent to achieve reduction.The competition between reduction and alkylation is controlled by the nature of the ligands on the nickel catalyst. Sequential replacement of bromo and alkylthio groups can be used to give E-dialkyl- and E-diarylolefins with total stereoselectivity whereas the corresponding 2-olefins undergo isomerization to a small extent (3% to lo%)." Is S. R. Wilson and A. S. Hedrinsky J. Org. Chem. 1982.47 1983. l6 T. Hayama S. Tomoda Y. Takeuchi and Y. Nomura Tetrahedron Lett. 1982.23.4733; Chem. Lett. 1982,1109. T. Yoshida Chern. Lett. 1982 429. E. Wenkert and T. W. Ferreira I. Chem. SOC.,Chem. Commun. 1982,840. l9 V. Fiandanese G. Marchese F. Naso and L. Ronzini J. Chem. SOC.,Chem. Commun. 1982,647. Aliphatic Compounds -Part (i) Hydrocarbons 139 Reactions of Alkenes.-There has been much interest in the addition reactions of organoselenium compounds in the last few years and the results of some very detailed studies are now being published.Investigation of the addition of ben- zeneselenenylchloride to 1,l -disubstituted olefins reveals that careful control of the reaction conditions can give rise to very high regiospecificity.20 The most crucial parameter is temperature. At -70°C chloride ion attack is controlled by steric factors and the resulting product always has anti-Markovnikov regiochemistry. Isomerization occurs at higher temperature. Peracid oxidative elimination shows poor specificity. Another elegant study21 of addition to allylic alcohols shows that the hydroxy group directs the selenenium ion to the syn face of the double bond and that under kinetic control the chloride ion favours axial attack.A series of rules are proposed which allow prediction of the course of this reaction. Two novel selenylation reactions have been reported. Phenylselenocyanate adds to olefins with complete stereospecificity and in high yield (70-95%)22 in presence of a Lewis acid catalyst (e.g. AlC& or BF3).With unsymmetrical alkenes a mixture of regioisomers was obtained but perhaps better control of the regiochemistry can be achieved. The reaction shown in Scheme 4 is brought about by benzeneselenenyl- iodide generated in sit^.^^ The chloride and bromide produce no products of this type which are thought to arise from formation of an episelenonium ion at one double bond which is then attacked by the second double bond.Reagent i PhSeSePh-I,-MeCN reflux Scheme 4 Diethylphosphoramidate is sufficiently nucleophilic to react with an alkene in presence of mercury(@ nitrate.24 The amidomercuration adduct is formed with high regioselectivity (Markovnikov) but in variable yield. Demercuration is achieved in situ with NaBH4 and subsequent hydrolysis gives an amine in 60 to 85% yield overall an economical amination reaction at least for reactive alkenes. In 1959 Reppe discovered that amines could be formed in low yield by reaction of an alkene with water carbon monoxide and a primary or secondary amine. This reaction has now been explored in more detail2' and it is found that with a suitable rhodium catalyst cyclohexene can be converted to amines of the type C6HI1CH2NR2 in up to 90% yield.A wide variety of nitrogen sources can be used and other types of alkene also react readily. This procedure clearly has considerable potential for one-step synthesis of amines. Oxidation of alkenes by thallium(II1) salts is a useful synthetic procedure and the sensitivity of the reaction to the nature of the anion has been exploited26 to 2o P.-T. Ho and R. J. Kolt Can. J. Chem. 1982,60,663. D. Liotta G. Zima and M. Saindane J. Org. Chem. 1982 47 1258. 22 S. Tomora Y. Takeuchi and Y. Nomura J. Chem. SOC.,Chem. Commun. 1982,871. 23 A. Toshimitsu S. Uemura and M. Okano J. Chem. SOC..Chem. Commun. 1982,87. 24 A. Koziara B. Olejniczak K. Osowska and A.Zwierak Synthesis. 1982,918. 25 F. Jachimowicz and J. W. Raksis J. Org. Chem. 1982,47,445. 26 A. J. Bloodworth and D. J. Lapham. J. Chem. SOC.,Perkin Trans. 1,1981,3265. 140 D. F. Ewing control the type of product (Scheme 5). Thallium acetate forms a stable adduct (6) whereas with the trifluoroacetate salt spontaneous oxidative dethalliation occurs readily to give (7) and (8).Regioselectivity in the oxidation of internal olefins is BuCH=CH2 i,BuCHCH2Tl(OCOMe)2 BuCHCHzBr I I iii OMe OMe (6) c BuCHCH2Tl(0COCF3) 4 BuCHCH20Me BuCHCHzOH +I [ &Me 1 OMe OMe (7) (8) Reagents i TI(OCOMe),-MeOH; ii KBr-crown ether-MeOH; iii TI(OCOCF,),-MeOH Scheme 5 difficult to achieve unless a carbonyl group is present when 1,3-diketones are formed preferentially in presence of PdC12/CuC1.The same catalyst also promotes oxidation of ally1 ethers or esters to the corresponding P-alkoxy or @-acetoxy The yields are not very high (40% to 75%) but the specificity is complete probably indicating co-ordination of the allylic oxygen to palladium in the active complex. Similar influence is observed in the reduction of unsaturated amides R2C=CHCONR1 by magnesium in methanol since unconjugated amides do not react.” Such a,@-unsaturated amides also co-ordinate with lithium directing lithiation with sec-butyl lithium to the p’-p~sition.~’Isomerization usually occurs with acyclic alkenes resulting in formation of two substitution products -CHX -C( CONR,) =CH -and -CH=C( CONR,) -CHX- . Alkenes are normally inert to LiBh but in presence of an ester rapid hydrobor- ation occurs at 25 “C to give a dialkylb~rinate.~’ It is also observed that the presence of the alkene has a rate-enhancing effect on the reduction of the carbonyl group indicating that a.coupled reaction is involved (Scheme 6).A second hydroboration step follows or even a third at high concentrations of alkene. This interesting observation may point to other areas where coupled reactions can occur. OR’ OR OR’ I / RC/ R-CO=O--Li RCH 4 $ -* ‘OLi LiBH4 k R2CH2CH2BH2 R,CH=CH~ H’A‘H RZCH=CH2 Scheme 6 27 J. Tsuji H. Nagashima and K. Hori Tetrahedron Lett. 1982,23,2679. 28 R. Brettle and S. M. Shibib J. Chem. SOC.,Perkin Truns. 1 1981 2914. 29 D. J. Kempf K. D. Wilson and P.Beak J. Org. Chem. 1982,47 1612. 30 H. C. Brown and S. Narasimhan Orgunometuffics,1982 1 763. Aliphatic Compounds -Part (i) Hydrocarbons 141 A clever method for cis-hydroxylation of olefins has been presented by Corey and Das31 The bromohydrin is formed first and then esterified with excess cyanoacetic acid (Scheme 7).The key step is enolization of the resulting P-ketonitrile which facilitates an internal nucleophilic displacement. This procedure will be of value for the mildness of the conditions used and the resulting stereospecificity. liii 0 1\=CHCN 0 li" VI YOH YOH *OCOCH,CN *OH Reagents i NBS 0 "C; ii CNCH,COOH-TosC1; iii;NaH 0 "C; iv H'; v K2C03 Scheme 7 Oxidation without chemical oxidant (and hence without contaminating reduced species) is an attractive proposition and just such a possibility has been described3' for the epoxidation of olefins.The oxygen is provided by water and the reaction is carried out in an electrochemical cell in presence of a anion-exchange resin containing quaternary ammonium groups. The best resin anion is bromide and the amount of water in the solvent (dimethylformamide-benzene) is critical. Several facets of this reaction would repay further investigation since its scope is fairly wide (groups such as C1 COOR CONEt are tolerated) and the resin beads can be reused several times at least. Another reaction involving a heterogeneous phase is that shown in Scheme 8.This simple but efficient transformation uses t-butyl hypo- chlorite in presence of silica gel.33 The relative proportions of monochloro (9) and CI (9) Reagent i Me,COCl/SiO Scheme 8 31 E.J. Corey and J. Das Tetrahedron Lett. 1982,23,4217. " J. Yoshida J. Hashimoto and N. Kawabata J. Org. Chem. 1982 47 3575. 33 W. Sato N. Ikeda and H. Yamamoto Chern. Lett. 1982 141. 142 D. F. Ewing dichloro (10) products can be varied by choice of solvent e.g. 5% (9) and 69% (10) in ether compared with 73% (9) and 5% (10) in hexane. 3 Polyenes Synthesis.-The bistrimethylsilyl species (11)has been shown to react with carbonyl compounds to give dienes directly but in low yield and with poor stereoselectivity. However in the presence of MgBrz the intermediate alcohols (Scheme 9) are isolable Lit .-* -.Me,%-SiMe (11) li RhSiMe 5R-SiMe SiMe Yh R *SiMe Reagents i 2MgBr -78 "C RCHO; ii H,SO,-THF; iii THF heat Scheme 9 in over 80% yield with excellent stereo~electivity.~~ With aliphatic aldehydes in particular B(OMe) is even better than MgBrz and the lE,3E-butadienes are obtained in ca. 90% yield. Ally1 sulphides R'R2C=CR3CH2X (X= Spy) or sul-phoxides (X = S(O)Ph) react via the corresponding carbanion with Bu3SnCH31 at -78 "Cto form (12) which spontaneously eliminates to give dienes in 50 to 90% yield.3' R' R3 M ~2 CHCH,SnBu, I X (12) Direct coupling of lithium vinyl cuprates (Vi)2CuLi with vinyl iodides in presence of ZnBr (cf. Annu. Rep. Prog. Chem. Sect. B 1981 78 156) is an inefficient procedure since only one of the vinyl groups (Vij is transferred.However vinylmag- nesium compounds (Vi)Cu MgX2 react with vinyliodide in presence of Pd(PPh& to produce the diene with complete stereoselectivity albeit in rather variable yield (55 to 70O/0),~~The addition of MgBrz to a solution of a lithium cuprate generates 34 T. H. Chan and J. S. Li J. Chem. Soc. Chem. Commun. 1982,909. 35 M. Ochiai S. Tada K. Sumi and E. Fujita Tetrahedron Lerr. 1982,23,2205. 36 N. Jabri A. Alexakis and J. F. Normant Tetrahedron Lert. 1982 23 1589. Aliphatic Compounds -Part (i) Hydrocarbons the magnesium cuprate which reacts smoothly with two equivalents of vinyl iodide in presence of ZnBr,. Dienoic esters with a conjugated E,Z geometry are important as aroma agents and as insect pheremones and an interesting application of heterogeneous catalysis with alumina is reported37 to give such compounds in high yields.The key step is the conversion of (13)to (14) by A1203. Complexes of the type Ti(Cp),R (R = alkyl alkenyl allyl) cause convergent double bond shifts in 1,5-dienes such that conju- gated 2,4-dienes are formed.38 Unfortunately the stereochemical control is poor since a mixture of (E,E)-,(E,Z)-,and (2,Z)-isomers are obtained. R'CH=C=CHCH,COOR~ (13) (14) One structure which occurs commonly in terpenoid compounds is the 1,5-enyne moiety and two groups have reported on routes to such compounds (Scheme 10). R2 R'C=CCH2CH2CH=C / LiCrCCH2CI . R~CECCH~BR \L ii (16) 'R3 (15) R' ICHZ=C=CCH2CH=C R2 / 'R3 (17) Reagents i BR, -78 "C; ii LiOMe-CuI -78 "C Scheme 10 Alkyne boranes (15) couple with allylic bromides in presence of cuprous iodide almost entirely at the a-position to give the required enynes (16) in 70% yie1d.j' Only small amounts of the allenic isomers are formed.Other workers4' report exactly the opposite when the coupling reaction is carried out in the absence of CuI. Attack occurs cleanly at the y-position to give the allenic isomers (17). These borates (or corresponding alanates) also react at the y-position with COz to give an allenic acid and with a ketone to give an allenic alcohol. Nickel(I1) acetylacetonate has been found4' to be an efficient catalyst for the dimerization of the anions of aryl allyl sulphones R2C=CHCHLiS02Ph to form trienes of the type R2C=CHCH=CHCH=CR2 in good yield (ca.80%) but with no stereochemical control. The mechanism may involve oxidative dimeriz- ation of the sulphone with subsequent desulphonylation. Another interesting appli- 37 S. Tsuboi T. Masuda and A. Takeda J. Org. Chem. 1982.47,4478. 38 K. Mach F. Turecek H. Antropiusova L. Petrusova and V. Hanus Synthesis 1982. 53. 39 S. Hara Y. Satoh and A. Suzuki Chem. Lett.. 1982 1289. 40 N. R. Pearson G. Hahn and G. Zweifel J. Org. Chem. 1982,47,3364. 41 M. Julia and J. N. Verpreux Tetrahedron Lett. 1982 23 2457. 144 D. F. Ewing cation42 of sulphone chemistry is shown in Scheme 11. The addition to (19) occurs exclusively across the 1,4 positions but not with total stereocontrol (two isomers in ratio 7 to 3).Alkenylcuprates are also effective in addition leading to a tetraene after sulphur extrusion. These polyenes [and the analogous isomers formed from (l8)l are valuable perfume additives. A to E,Z-1,Sdienes involving flash pyrolysis of a bicyclic sulphone is not likely to be of wide application. Reagents i Bu'OK-DMSO; ii Ally1 Br-DMSO; iii I,; iv R,CuLi-Et,O; v KOH-Bu'OH Scheme 11 Arylallenes are conveniently obtained by the reaction of an aryl halide with an allenyl-lithium derivative in presence of a palladium(0) complex.44 Only two examples are described but the yields are high (78% and 9OOh). Coupling with vinyl halides is also described. t -Butyl silver reacts with 2,4-diynylsulphinates such as HCsCCrCCR'R20S(0)Me to give alkylation at position 5 with resulting transmutation to 'a tetraene such as Me3CCH=C=C=C=CR'R2.The yields of these cumulene molecules are good (60 to 80%) but other alkyl silver derivatives are inactive as are also the analogous sulphonate substrate 4 Alkynes Synthesis.-A has appeared recently entitled 'Synthesis of Acetylenes Allenes and Cumulenes A Laboratory Manual'. This book is a superb collection of information concerning the practical preparation of the title compounds pre- sented in the style of Organic Syntheses. Over 200 procedures are described in detail and this is an indispensable (if expensive) volume for workers in this area. 42 F. Naf R. Decorzant and S. D. Escher Tetrahedron Left. 1982,23 5043. 43 J. 1. Cadogan C. M. Buchan I. Gosney B.J. Harnill and L. M. McLaughlin I. Chem. Soc. Chem. Commun. 1982,325. 44 T. Jeffrey-Luong and G. Linstrumelle Synthesis 1982 738. " E. A. Oostveen C. J. Elsevier J. Meijer and P. Vermeer J. Org. Chem. 1982,47,371. 46 L Synthesis of Acetylenes Allenes and Cumulenes A Laboratory Manual,' ed. L. Brandsrna and H. D. Verkruijsse Elsevier Scientific Publishing Company Amsterdam 1981. Aliphatic Compounds -Part (i) Hydrocarbons 145 The preparation of acetylenes with tertiary alkyl substituents cannot be achieved by the standard reaction of metal alkynide with the appropriate alkyl halides. However tertiary halides do react with silylalkynes in presence of AlCl to give addition followed by loss of silyl halide. Compounds such as di-( 1-adamanty1)- acetylene can be obtained4’ by this method.One drawback to the formation of alkynes by elimination of hydrogen halide is the need for a very strong base such as alkoxide or metal amide. Hence the discovery that elimination from the allylic alcohol ArOCH2CH=C(Br)CH20H only requires treatment with potassium car- bonate in refluxing methyl ethyl ketone is most intere~ting.~’ The essential presence of the allylic oxygen atom suggests that anchimeric assistance may be involved. It is not clear whether the presence of the aryloxy group is important. A thorough study has appeared49 of the reaction between (Me0)2P(0)CHN2 and aldehydes or ketones. The carbonyl group in R’R2C0 is thought to be converted to a diazoethene R’R2C=CN2 which eliminates nitrogen to give a carbene.Rearrangement affords an alkyne R1C=CR2. The metathesis of functionalized aryl acetylenes has been studied” using Mo(CO),-4-C1C6H4OH as a homogeneous catalyst with variable results. A rather better catalyst” is M00~(acac)~-AlEt,-PhOH which efficiently gives the statistical distribution of the product alkynes with negligible isomerization of the triple bond. Functional groups which are tolerated include double bond halogen and ester. Reactions.-Catalytic hydrogenation of the alkyne bond in RC6H4CECC6H4R over Raney Ni occurs readily when the pura-substituent is H Me or Et. However with R = Pr’ or But the triple bond is not reduced.” Calculation on model com- pounds indicate that the CEC moiety would lie at 0.1125 nm from the metal surface and hence this must represent a distance greater than the maximum for efficient hydrogenation.The fact that di-t-butylacetylene is readily hydrogenated suggests that the extended conjugated system in the diarylacetylenes may prevent bending of the molecule to place the triple bond closer to the surface. A novel addition reaction of alkynes is shown in Scheme 12. Arylselenosulphon- ates add to aryl and alkyl substituted acetylenes to afford P-(phenylse1eno)vinylsul-phones in 52 to 86% yield.’ The mechanism is thought to involve free radicals. Alkynylsulphones are readily formed by oxidative elimination. Further detailed studys4 of the stereochemical course of the acetoxymercuration of alkynes (Scheme 12) reveals that an antarafacial mode of addition operates for hex-3-yne to give the E-adduct in high yield.This was unequivocably established by X-ray structure analysis. In contrast stilbene reacts in a suprafacial mode to give the 2-adduct. The conventional procedures for hydration of a triple bond are relatively unselective between terminal and internal positions. The reagent PhHgOH has been found’’ to show a high reactivity with terminal triple bonds forming a phenylmercury 47 G. Capozzi G. Romeo and F. Marcuzzi J. Chem. SOC.,Chem. Commun. 1982,959. 48 P. F. Schuda and M. R. Heimann J. Org. Chem. 1982,47,2484. 49 J. C. Gilbert and U. Weerasooriya I. Org.Chem. 1982 47 1837. 50 D. Villemin and P. Cadiot Tetrahedron Lett. 1982,23 5139. ’’ M. Petit A. Mortreux and F. Petit J. Chem. SOC.,Chem. Commun.1982 1385. 52 G. Y. Han P. F. Han J. Perkins and H. C. McBay J. Org. Chem. 1981,46,4695. 53 T. Miura and M. Kobayashi J. Chem. SOC.,Chem. Commun. 1982,438. 54 R. D. Bach R. A Woodward T. J. Anderson and M. D. Glick J. Org. Chem. 1982,41 3707. 55 V. Janout and S. L. Regen J. Org. Chem. 1982 47,3331. 146 D. F. Ewing Reagents i ArS0,SePh; ii. excess 30% H202, 70 "C; iii Hg(OAc),-HOAc 25 "C Scheme 12 acetylide which is easily hydrolysed to give a methyl ketone. Internal triple bonds are totally unreactive to this reagent as are a wide range of other groups such as acetal thioacetal lactone alkene epoxide and secondary bromide. Apart from derivatives of titanocene very few titanium complexes are used in organic chemistry. The reagent Ti(OCHMe2)4 has been to generate propar- gylic titanium complexes (20)that show remarkable variations in the regiochemistry of the reaction with aldehydes (Scheme 13).For methyl acetylene complexes (20 R2= H) the electrophile attacks y-carbon exclusively to give an allenic alcohol but if R2 = Me in (20) the selectivity is completely reversed leading to formation of the acetylenic al~ohol.~' This (presumably) sterically controlled regioselectivity is better than that observed for propargylic salts with Li Mg or Zn species and may be of importance in the synthesis of useful alkynes such as those discussed above. R'C=CCHR2CHR30H + R'C=C=CHR~ I CHR30H Reagents i. Me,CLi 0 "C;ii Ti(OCHMe,), -78 "C; iii R'CHO Scheme 13 Two new reagents have been disc~vered~'.~~ which generate 1-alkynyl phenyl- selenides in high yield under mild conditions.Phenylselenocyanate reacts with terminal alkynes at room temperature in presence of CuCN and triethylamine but 56 M. Ishiguro N. Ikeda and H. Yamamoto J. Org. Chem. 1982,47,2225. '' T. Hayama S. Tomoda Y. Takeuchi and Y. Nomura Chem. Lett. 1982,1249. 58 S. Tomoda Y. Takeuchi and Y. Nomura Chem. Lett. 1982,252. Aliphatic Compounds -Part (i) Hydrocarbons even more convenient is the analogous reaction with benzeneselenenyl nitrite formed in situ from PhSeBr and AgN02. Brief mention is made of a hydroformyla- tion reaction of alkynes using Rh4(C0),* as catalyst.59 Although the conversion rate is high (ca. 100%)the yield of a,&unsaturated ketones is very variable.This reaction requires further development. *’ T.Mise P.Hong and H. Yamazaki Chem. Lett. 1982,401.

ISSN:0069-3030    

DOI:10.1039/OC9827900135

出版商:RSC    

年代:1982

数据来源: RSC