ISSN:0069-3030    

DOI:10.1039/OC98077FX001

出版商:RSC    

年代:1980

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC98077BX003

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

3 Theoretical Chemistry By H. S. RZEPA Department of Chemistry Imperial College of Science and Technology London SW7 2AY 1 Introduction The number of reports of theoretical studies by molecular orbital and related methods now appears to be doubling annually. A notable feature is the increased application of ab initio methods compared with semi-empirical and other more approximate procedures. Clementi' has reviewed the historical and recent development of reliable approximations to the Schroedinger equation. The practical limit for accurate calculations still unfortunately a long way off is put at molecules containing between 100 and 150 atoms. Various more approximate approaches for considering interactions in biologically important molecules are also discussed and it is significant that the number of such studies has recently been increasing.Jug has briefly summarized the various theoretical procedures which have been used for studying chemical processes' and has tabulated results that have been obtained for more than 120 reactions of various types. 2 Advances in Theoretical Techniques Geometry Optimization.-The optimization of molecular geometries using ab initio methods in which energy gradients are evaluated analytically and efficient mini- mization algorithms are employed is becoming increasingly common. The appli- cation of the Hellmann-Feynman theorem to the calculation of forces on atoms has been beset by problems of inadequacy of the basis set but it has 6een successfully a~plied,~ and may be one approach to the problem of reducing the computation time required for geometry optimization.Calculation of the force-constant (or Hessian) matrix has been greatly facilitated by the formulation of the second derivatives of the total energy with respect to the nuclear co-ordinates for ab initio wave function^.^ This allows very much more effective location and characterization of stationary points in a potential-energy surface and makes calculation of the vibrational frequencies and related thermodynamic properties straightforward. It may however not always be necessary to obtain all the elements of the Hessian matrix in order to identify whether or not a given stationary point is an energy minimum (when this matrix has E. Clementi J. Phys.Chem. 1980,84,2122. K. Jug Theor. Chim. Actu 1980,54,263. H. Nakatsuji,K. Kanda. andT. Yonezawa. Chem. Phys. Lett. 1980,75 340. J. A. Pople R. Krishnan H. B. Schlegel and J. S. Binkley Znt. J. Quantum Chem. Symp. 1979,13,225. H. S. Rtepa no negative eigenvalues) or a true transition state (when this matrix has one unique negative eigenvalue). Such information can also be obtained from the rank and signature of the Hessian matrix,' the evaluation of which may require only a few elements of this matrix. Although variable metric or quasi-Newtonian minimization methods (i.e. the Davidon-Fletcher-Powell) are best suited for the optimization of the geometries of molecules with less than about 100 degrees of freedom it has been suggested6 that the geometries of macromolecules are best optimized using selective constraints (e.g.bond lengths) in conjunction with steepest descent/conjugate gradient methods. Basis Sets.-The STO basis sets introduced by the Pople group more than ten years ago have become the most widely used in ab initio calculations. The most common combination has been the use of a STO-3G basis for optimizing the molecular structure followed by evaluation of relative energies using a 4-31G basis (for which the nomenclature 4-31G//STO-3G has been proposed). It is now known that programs which calculate derivatives analytically increase greatly in efficiency if the number of primitive gaussians in the basis set is reduced. Accordingly the Pople group have proposed' new basis sets for the first-row elements termed STO 3-21G which are of comparable quality to STO 4-3 1Gand clearly superior to STO-3G basis sets but which require only as much time as the latter for the evaluation of the energy derivatives.They suggest that for calculations which are dominated by derivative evaluation the 3-21G basis offers a superior alternative to the 3G basis. Huzinaga* has used the same philosophy in developing minimum contracted gaussian basis sets of double-5 quality for Li-Ne which promise to offer similar advantages and minimum gaussian basis sets for the first three rows of the Periodic Table which offer considerable advantages to the use of STO-3G basis sets for these elements. The Pople group have also introducedg 6-31 1G"" basis sets of triple-5 quality which have been developed for use in calculations which include corrections for electron correlation using Mgller-Plesset perturbation theory and which are claimed to overcome some of the problems encountered with earlier basis sets.The charge distribution in a molecule is commonly obtained using a 'Mulliken population analysis' of the ab initio wavefunction. The results however are very dependent on the basis set and several alternative partitioning schemes have been proposed which suffer to a lesser extent from this defect." Whereas a few years ago the prospective user of theoretical methods was faced by a bewildering choice of variously parameterized semi-empirical SCFmethods to this has now been added an equally large if not greater choice of variously optimized basis sets for use in ab inifio calculations! Electron Correlation.-The graphical unitary-group approaches to direct configuration interaction (CI) offer a systematic solution to the problem of electron P.Scharfenberg Theor. Chim. Acta 1980,58 73. W.F.van Gunsteren and M. Karplus J. Comput. Chem. 1980,1,266. 'J. S. Binkley J. A. Pople and W. J. Hehre J. Am. Chem. SOC.,1980,102.939. (a) H. Tatewaki and S. Huzinaga J. Cornput. Chem. 1980 1 205; (6)A. N.Tavouktsoglou and S. Huzinaga J. Chem. Phys. 1980,72 1385. R.Krishnan J. S. Binkley R. Seeger and J. S. Pople J. Chem. Phys. 1980,72 650. lo (a)J. B. Collins and A. Streitwieser,J. Comput. Chem. 1980,1,81;(6)M.Yanez R. F. Stewart and J. A. Pople Acta Crystallogr. Sect. A 1978,34,641.Theoretical Chemistry 17 correlation" and have the advantage of requiring a relatively simple data input. Schaefer and co-workers'2 have extended the utility of such methods by formulating the analytical gradients for an arbitrary closed-shell CI illustrating the procedure with a study of the conversion of HNC into HCN that included 12 497 configura-tions. Small but significant differences were found between the CI and the single- configuration SCF structures for the transition state. Multi-configuration (MCSCF) studies in which the orbital coefficients (in the Roothaan LCAO sense) and the CI coefficients are variationally optimal are also becoming comm~n.'~ These tech- niques are expected to have the greatest advantages in the study of those electronic states which are not dominated by a single configuration or of reactions in which the dominant electronic configuration may change.Pople and co-workers14 have concentrated on the application of MBller-Plesset perturbation theory at second and higher order for the evaluation of electron correlation energy and they have now reported the formulation of analytical gradients for this level of theory. There have been many advances in pseudopotential methods for the calculation of the wavefunctions of heavy atoms. For such atoms relativistic effects can be as important as electron-correlation effects and a glimpse of what we can expect in the future is illustrated by the relativistic effective-potential MCSCF calculations on thallium h~dride.'~ For elements of the first row such as carbon relative relativistic corrections for e.g.the 'Al and 3B1 states of methylene are now considered to be negligible,16 although such corrections to the total energy can be significant." Semi-empirical and Other More Approximate Methods.-Several authors have proposed improvements to existing zero differential overlap (ZDO) methods such as CNDO or MND0,I8 and ThielIg has dissected the differences between the INDO and the NDDO approximations. These are most pronounced for molecules with adjacent heteroatoms molecules with four-membered rings classical vis a vis non-classical carbonium ions and the relative energies of u-and w-orbitals. Jug and Nanda2' have presented a statistical analysis of the SINDO/l method which is based on the INDO approximation and which represents a considerable improvement over MIND0/3 whilst requiring fewer adjustable parameters.An alternative to the Hartree-Fock (HF) approach which has been borrowed from solid-state physics and much applied in calculations involving the transition metals is the Xa multiple-scattering method. This technique can be used to calculate the one-electron proper- ties of benzene and pyridine with an accuracy comparable with those obtained from l1 (a)B. 0.Roos and P. E. M. Siegbahn Inr. J. Quantum Chem. 1980,17,485;(b)P. E.M. Siegbahn J. Chem. Phys. 1980,72 1647; (c) B. R.Brooks W. D. Laidig P. Saxe and H. F. Schaefer 111 J. Chem. Phys. 1980,72 3837. B. R.Brooks W. D. Laidig P. Saxe J. D. Goddard Y. Yamaguchi and H.F. Schaefer 111 J. Chem. Phys. 1980,72,4652. 13 (a)H.J. Werner and W. Meyer J. Chem. Phys. 1980,73,2342; (b)B. 0.Roos,P. R.Taylor and P. E. M. Siegbahn Chem. Phys. 1980,48,157. l4 R.Krishnan H. B. Schlegel and J. A. Pople J. Chem. Phys. 1980,72,4654. l5 P.A. Christiansen and K. S. Pitzer J. Chem. Phys. 1980,73,5160. l6 (a)C. P. Wood and N. C. F'yper Chem. Phys. Lert. 1980,71,368;(b)E.R.Davidson D. Feller and P. Phillips ibid. 1980 76 416. *' 0.Matsuoka N. Suzuki T. Aoyama and G. Malli J. Chem. Phys. 1980 73 1320 1329. *' (a)P.G.Nelson J. Chem. Res. 1980 (S)106 (M)1701; (b)U.Dinur and B. Honig J. Chem. Phys. 1980 72 1817. l9 W.Thiel J. Chem. SOC.,Faraday Trans. 2 1980,76,302. *' K. Jug and D. W. Nanda Theor. Chim. Acra 1980,57,95,107,131.18 H. S. Rzepa ab initio HF wavefunctions but at significantly less computational cost.21 An LCAO variation of the Xa procedure fared much better than the multiple-scattering method in the calculation of the equilibrium internuclear distance and of the dipole moment of carbon monoxide.22 The purely parametric force-field methods continue to be developed for new elements and types of Parameters have been reported which give good results for the conformational properties of crown and for the strain energies in cyclic n~cleotides.~~~ 3 Electronic Structure and Geometries of Molecules” Ab initio STO 4-21G calculations have resolved serious discrepancies between two electron-diff raction studies of t~luene,~’ and a molecular-orbital-constrained elec- tron-diffraction (MOCED) method has been used to investigate the structure and conformational properties of 1-butene.26 The force field of uracil has been analysed with the aid of interaction co-ordinates calculated using the MNDO method and the results have been used to interpret the effects of hydrogen-bonding on the observed vibrational frequencies.The m.c.d. spectrum of cyclo-propane has been investigated using an ab initio CI method.28 The calculated spectrum was in good agreement with the observed one although the interpretation afforded by the calculations differed in many respects from that obtained through a moment analysis of the experimental spectrum. Molecular orbital methods have also frequently been applied to the interpretation of photoelectron ~pectra,~’ although the dangers of correlating an ordered set of MO energy levels with observed ionization energies have been empha~ized.~’ Numerous studies have been concerned with substituent effects including the relationship between the angle of tilt of a methyl group and hyperc~njugation,~~ the possibility of rr-electron donation by CN or CHO groups attached to unstable cationic and further discussion of the separation of u/rr interactions in saturated A model for retention of configuration in SN2reactions’in terms of a perturbational approach and STO-3G calculations reproduces the experimental trends in silicon without requiring either &orbitals on silicon or pseudorotation in the transition The ab initio approach to calculations on large molecules using 21 D.A. Case M. Cook and M. Karplus J. Chem. Phys. 1980,73,3294. 22 J. W. Mintmire and J. R. Sabin Chem. Phys. 1980,50,91. 23 (a)N.L. Allinger Adu. Phys. Org. Chem. 1979 $4,1; (b)M. J. Bovill D. J. Chadwick I. 0.Sutherland and D. Watkin J.Chem. SOC.,Perkin Trans. 2,1980,1529;(c)F.J. Marsh P. Weiner J. E. Douglas P. A. Kollman G. L. Kenyon and J. A. Gerlt J. Am. Chem. SOC.,3980,102 1660. 24 P. S. Bagus B. Liu A. D. McLean and M. Yoshidne Comp. Methods Chem. (Proc. Int. Symp.) 1980 203. 25 F.Pang J. E. Boggs P. Pulay and G. Fogarasi J. Mol. Struct. 1980,66,281. 26 D. Van Hemelrijk L. Van den Enden H. G. Geise H. L. Sellers and L. Schaefer J. Am. Chem. Soc. 1980,102,2189. ’’(a)B.I. Swanson T. H. Arnold M.J. S. Dewar J. J. Rafalko H. S. Rzepa and Y. Yamaguchi J. Am. Chem. SOC.,1978,100,771;(b)W. D. Bowman and T. G. Spiro J. Chem. Phys. 1980,73,5482. 28 E.Goldstein S. Vijaya and G. A. Segal J. Am. Chem. SOC. 1980,102,6198. 29 B.KovaE M. Mohraz E. Heilbronner V. Boekelheide and H. Hopf J. Am. Chem. SOC.,1980,102 4314. 30 E. Heilbronner and A. Schmelzer Nouv. J. Chim. 1980,423. A. Pross L.Radom and N. V. Riggs J. Am. Chem. SOC.,1980,102,2253. 32 M. Paddon-Row C. Santiago and K. N. Houk J. Am. Chem. SOC.,1980,102,6561. 33 K.B. Wiberg J. Am. Chem. SOC.,1980,102 1229. 34 Nguyen Trong Anh and C. Minot J. Am. Chem. SOC.,1980,102,103. Theoretical Chemistry molecular fragments continues to be de~eloped,~' and an interesting application of the Xa valence-bond method to the study of models of 2-Fe ferredoxin has satisfactorily accounted for the observed antiferromagnetic coupling between the two iron centres in the oxidized protein.36 Conformations and Intermolecular Interactions.-The theoretical prediction that glycine should have a low-energy conformer (1) with a small dipole moment (1.1D)37led to an intensive search for the expected weak transitions in the microwave spectrum.Such transitions have indeed now been located and the derived rotational constants show excellent agreement with the theoretical values. These results have important bearing on spectroscopic searches for interstellar glycine which is likely to exist largely as conformation (1). Force-field calculations on humulene (2) have revealed an unsuspected new low-energy conformer3' which may be important in several biosynthetic trans- formations.Gorenstein et al. have continued their investigation into stereoelectronic effects in phosphate esters phosphoramidates and pho~phonates.~' The results of STO-3G calculations suggest that elimination of methoxide anion is generally favoured by about 40 kJ mol-' in cases where an adjacent lone pair on N or 0 is antiperiplanar with respect to the leaving group [i.e. (3a) us (3b)l. A number of studies of conformations and hydrogen-bonding in peptides have appeared. Ab initio calculations in particular are often in close agreement with results obtained from crystal structures of protein^.^' Haddon41 has used simple Huckel theory to study the intrinsic ability of a functional group to develop symmetrical hydrogen-bonding.The eigenvalues of the bond-bond polarizability matrix have been shown to provide a measure of the degree of distortion or asymmetry of the hydrogen bond. Maleate anion (4) for example is known to have a symmetric hydrogen bond and was found to have the lowest such eigenvalue of all the compounds studied. Ab initio methods are generally more reliable than semi- empirical procedures in their treatment of hydrogen In one interesting study ab initio calculations suggest that (5a) is 7kJ mol-' more stable than (5b) as a result of hydrogen-bonding between the hydroxyl group and the d~uble-bond.~~ It has been demon~trated~~ that the considerable variations in the calculated STO-3G 35 D.Spangler and R. E. Christoffersen Int. J. Qudntum Chem. 1980 17 1075. 36 J. G.Norman P. B. Ryan and L. Noodleman J. Am. Chem. SOC.,1980,102,4279. 37 (a)R.D. Suenram andF. J. Lovas J. Am. Chem. SOC.,1980,102.7180;(b)L.Schaefer H. L. Sellers F. J. Lovas and R. D. Suenram ibid. p. 6566. 38 H. Shirahama E. Osawa and T. Matsumoto J. Am. Chem. Soc. 1980,102,3208. 39 D.G. Gorenstein B. A. Luxon and E. M. Goldfield. J. Am. Chem. SOC.,1980 102 1757. 40 (a)D.Peters and J. Peters J. Mol. Struct. 1980,62,229; (c) E.L.Mehler J. (b)ibid. 1980,68,243,255; Am. Chem. SOC.,1980,102,4051. 41 R. C. Haddon J. Am. Chem. SOC.,1980,102,1807. 42 S.Scheiner Theor. Chim. Actu 1980 57 71. 43 K.Morokuma and G. Wipff Chem. Phys. Lett. 1980,74,400. 44 Y.-C.Tse M. D. Newton and L. C. Allen Chem. Phys. Lett. 1980,75 350. 20 H. S. Rzepa geometries for hydrogen-bonded dimers of H20 and MeOH are largely attenuated with larger basis sets such as 4-31Gor 6-31G". Numerous studies of intermolecular interactions have been de~cribed.~' These include the interactions between biomolecules and polar amino-acid~,~'~ between morphine45b or and water and model systems of choline The ultimate aim of such studies is the development of reliable structure-activity relationships. Ab initio/CI techniques have been used to investigate the So S1 and T1 hates of ethyl bacteriochlorophyllide-a and the related radical and the necessity of CI in the theoretical study of several fundamental problems of chemical carcinogenesis has been empha~ized.~~' Neutral Species.-An interesting example of the problems associated with the interpretation of wavefunctions of molecules is illustrated by three studies of methyl-lithium Collins and Streitwieser,"" on the basis of double-5 a6 initio calculations and integrated electron populations claim the C-Li bond to be wholly ionic; a study using basis sets of triple-5 quality concluded that this bond is 60% and an INDO study of the 7Li-'3C n.m.r.coupling constants suggests that the value obtained is typical of a covalent bond.466 The Schleyer group continues to predict unusual structures for lithiated specie^.^' For example of the several stable calculated structures for C2Li6 (6) contains essentially a single CGC bond and (7)a single C-C bondq4'" The species (8) appears to be an example of an aromatic compound which can be regarded as both a Huckel and a Mobius and the most stable isomer of SiHzLiF has been predicted to have carbenoid character (9) and to be a good candidate for experimental ~bservation.~~" F H \ H/c=si (9) 4s (a)E.Clementi G. Corongiu and G. Ranghino J. Chem. Phys. 1981,74,578 and references therein; (6) A. Agresti F. Buffoni J. J. Kaufman and C. Petrongolo Mol. Pharmacol. 1980,18,461; (c)Y. Orita A. Ando H. Abe S. Yamabe H. Berthod and A. Pullman Theor. Chim. Acta 1979 54 73; (d)H. Johansen S. Rettrup and B. Jensen Theor. Chim. Acta 1980,55,267; (e)J. D. Petke G. M. Maggiora L. S. Shipman and R. E. Christoffersen Phoiochem. Photobiol.1980,32 399; (f)J. J. Kaufman Int. J. Quantum Chem. Quantum Biol. Symp. 1979,6 503. 46 (a)G. D. Graham D. S. Marynick and W. N. Lipscomb J. Am. Chem. SOC.,1980,102,4572; (b)T. Clark J. Chandrasekhar and P. von R. Schleyer J. Chem. Soc. Chem. Commun. 1980,672. 47 (a)A. J. Kos D. Poppinger P. von R. Schleyer and W. Thiel TetrahedronLett. 1980,21,2151; (b)A. J. Kos and P. von R. Schleyer J. Am. Chem. SOC.,1980,102,7928; (c)T. Clark and P. von R. Schleyer J. Organomet. Chem. 1980,191,347. Theoretical Chemistry Among the many studies of silicon compounds is the prediction48 that silaethyne will rearrange with no activation barrier to the silylene (lo) and STO 3-21G calculations49 do not encourage the hope that silatetrahedrane may be more stable relative to silacyclobutadiene than the carbon analogues predicting an energy difference between the two species of more than 125 kJ mol-’.The surprising result that tetra-t-butylcyclobutadieneis essentially square and not rectangular has received theoretical upp port.'^ The energy required to distort rectangular to square cyclobutadiene has been estimated at about 42 kJ mol-’ and this would be more than offset by the relief of steric compression of the substituents. The radical cation of tetra-t-butyltetrahedrane (11) has been calculated by the MNDO method not to be stable and to rearrange (with no energy barrier) to the cyclobutadiene radical cation,’l which is fully in accord with both the photoelectron spectrum and other known properties of (11).The singlet state of cyclopropyne has been investigated using a very large basis set and extensive CLS2It has been predicted to be unstable rearranging to vinylidene (H2C=C=C:) whereas the triplet state is marginally stable and was found to have a calculated C=C vibrational wavenumber of 1840cm-l. A study of the geometry of norbornene and norbor- nadiene at the STO-3G level has revealed that the m-system is expected to be significantly n~n-planar.’~ The olefinic CH bonds are bent endo by 3-5 O and the greater exo development of the ~~~-0rbital has been used to rationalize the observed preference for electrophilic exo-substitution. The apicophilicity of electronegative substituents in five-co-ordinate phosphorus has been the subject of several papers.Two independent ab initio calculations using double-5 basis sets with polarization functions were found to predict that (12) and (13) are essentially i~oenergetic.’~ Although an earlier study using a 4-31G basis had predicted (12) to be 32 kJ mol-’ more stable this is probably due to inadequate optimization of the geometry.’’ The two forms have been predicted to interconvert via a turnstile mechanism with a barrier of about 20 kJ mol-’. A comprehensive study of carbonyl ylides using the 4-3 lG//STO-3G method with 3x3 CI has suggested that the parent compound (14) is planar with a substantial barrier to rotation about the C-0 bond and a small barrier to inversion 48 A. C. Hopkinson and M. H. Lien J. Chem. Soc. Chem. Commun. 1980 107. 49 M. S.Gordon J.Chem. SOC.,Chem. Commun. 1980,1131. ” W.T.Borden and E. R. Davidson J. Am. Chem. SOC.,1980,102,7958. 51 (a)H. Bock B. Roth and G. Maier Angew. Chem. Int. Ed. Engl. 1980,19,209;(6)E. Heilbronner,T. B. Jones A. Krebs G. Maier K. D. Malsch J. Pocklington and A. S. Schmelzer J. Am. Chem. SOC. 1980,102,564. s2 P.Saxe and H. F. Schaefer 111 J. Am. Chem. Soc. 1980,102,3239. 53 G. Wipff and K. Morokuma Tetrahedron Lett. 1980,21,4445. ” (a)R.Hoeller and H. Lischka J. Am. Chem. SOC.,1980,102,4632;(6)H.J. Bestmann J. Chandrasek- har W. G. Downey and P. von R. Schleyer J. Chem. Soc. Chem. Commun. 1980,978. ” H. J. Bestmann Pure Appl. Chem. 1980,52,771. H. S. Rzepa about the central oxygen atom. Substituent effects upon the barrier to the formation of (14) from oxiran the barrier to rotational isomerization of the C-0 bond the reactivity and regioselectivity of cycloaddition reactions and the fragmentation of (14) were A variety of methods ranging from simple Huckel to MIND0/3 and STO-3G have been used to predict the relative stabilities and the spectral properties of an almost completely unknown class of compounds the az~loquinones.~~ Of the classical isomers for which a KekulC structure is possible 1,5-azuloquinone (15) was suggested as the most promising candidate for synthesis whereas of the non-classical forms 1,3-azuloquinone (16) proved by far the most stable The first such compound recently synthesized was in fact 1,2-a~uloquinone.~~ 0 0 The evergreen problem of bond alternation in [18]annulene has been studied by an ab initio method.58 The bond-alternating form was predicted to be 150 kJ mol-' more stable than the delocalized form; a result similar to earlier MIND0/3 cal- culations but in apparent disagreement with the experimental evidence.There seems little doubt that electron correlation is unusually important in cyclically conjugated hydrocarbons and that correct geometries are only obtained when CI or other approaches are used.59 Charged Species.-The stereoelectronic properties of CH(OH),NH,' and CH(OH)(NH2)0- as models for tetrahedral intermediates in acid- and base- catalysed hydrolysis of amides have been investigated with partial optimization of the geometries at the STO-3G and 4-31G The C-N bond is calculated to be very long and weak when it is antiperiplanar to two lone pairs of electrons.Protonation on the nitrogen atom leads to a marked amplification of this effect. Experimental evidence that the allenic anion is bent i.e. (17) and not linear i.e. (18) is supported by double-l/SCEP ab initio calculations,6' which also predict a barrier to inversion of 29 kJ mol.-' H / (18) " K. N. Houk N. G. Rondan C. Santiago,C. J. Gallo R. W. Gandour and G. W. Griffin J. Am. Chem. Soc. 1980,102,1504. " L. T. Scott M. D. Rozeboom K. N. Houk T. Fukunaga H. J. Lindner and K. Hafner,J. Am. Chem. Soc. 1980,102,5169. '* R. C. Haddon Chem.Phys. Lett. 1980,70,210. "M.J. S. Dewar and M. L. McKee Pure Appl. Chem. 1980,52 1431. 6o J. M. Lehn and G. Wipff J. Am. Chem.SOC.,1980,102,1347. J. K. Wilmshurst and C. E. Dykstra. J. Am. Chem. Soc.. 1980,102,4668. Theoretical Chemistry The novel non-classical ion (19) has been suggested on the basis of MIND0/3 calculations to account for scrambling of 13C in cyclopentyl cation.62 MINDO/3 4-31G//STO-3G and 6-31G*//STO-3G calculations have indicated that the two vinyl cations (20) and (21) may be unusually stabilized in the former by hypercon- jugation of the cyclopropyl ring with the vacant vinylic p-orbital and in the latter by the non-classical structure that is formed.63 The authors of the adjacent paper in the same journal independently present experimental evidence of the unusual stability of (21).64 Radom and co-w~rkers~~ have studied 17 isomers of C,&Of.and found five to be relatively low-energy species and possible candidates for detection by ICR techniques. In a later paper such evidence is indeed presented for the detection of one of these i.e. (22).65b Substituent effects in the various intermediates in the Birch reaction have been studied with the 4-3 lG//STO-3G method including benzene radical anions,66a cyclohexadienyl radicals,66b and cyclohexadienyl anions.66c The homoaromatic species (23) was calculated by various methods to be between 143 and 180 kJ mol-’ higher in energy than cyclohexadienyl anion itself. Carbenes and Open-Shell Species.-Double-l ab initio calculations with extensive CI predict a barrier of about 21 kJ mol-’ to the interconversion of the triplet states of (24) and (25) in agreement with experiments which indicated that the barrier to geometrical isomerization in triplet carbalkoxycarbenes is larger than their barrier to intermolecular reaction.67 The singlet states were calculated to have only one non-planar low-energy form.The barrier to the [l 21 shift of hydrogen in the triplet nitrene (26) was calculated68 to be as high as 200 kJ mol-’. The species should be stable in a matrix in contrast to the singlet ‘A’ state where the [1,2] shift of hydrogen was predicted to occur with no barrier. Carbenes of the type CX2 where X is less electronegative than carbon have been predicted to be linear,69 although Pauling has put forward an explanation in terms of resonance theory and the number of available orbitals on X.70 The ‘ll’ground states and ‘2’excited states of conjugated N and 0 radicals have been investigated by ab initio methods.71A marked dependence of the calculated geometry on the basis set used was found particularly with N radicals.Ab initio 62 W. Franke H. Schwarz H. Thies J. Chandrasekhar P. von R. Schleyer W. J. Hehre M. Saunders and G. Walker Angew. Chem. Int. Ed. Engl. 1980,19,485. 63 Y.Apeloig J. B. Collins D. Cremer T. Balley E. Haselbach J. A. Pople J. Chandrasekhar and P. von R. Schleyer J. Org. Chem. 1980,45 3496. W. Franke H. Schwarz and D. Stahl J. Org. Chem. 1980,45,3493. 65 (a)W. J. Bouma J. K. MacLeod and L. Radom J.Am. Chem. Sac. 1980,102,2246; (6) B. C. Baumann J. K. MacLeod and L. Radom ibid. p. 7927. 66 A. J. Birch A. L. Hinde and L. Radom J.Am. Chem. Soc. 1980 102 (a) p. 3370; (b) p. 4074; (c) p. 6430. 67 K. S. Kim and H. F. Schaefer 111 J. Am. Chem. SOC.,1980,102,5389. J. Demuynck D. J. Fox Y.Yamaguchi and H. F. Schaefer 111 J. Am. Chem. SOC.,1980,102,6204. 69 W. W. Schoeller I. Chem. Soc. Chem. Commun. 1980,124. 70 L. Pauling J. Chem. SOC.,Chem. Commun. 1980,688. ’’ N. C. Baird and K. F. Taylor Can. J. Chem. 1980,58 733. 24 H. S. Rzepa calculations72a predict that the energy separation between the ll and X states of (27) is 115 kJ mol-’ compared with 58 kJ mol-’ obtained using the MNDO method.72b The two states in such systems eventually become degenerate if the angle at the nitrogen atom is increased to about 140 ’. The results of STO 4-31G calculations suggest that the simultaneous stabilization of a radical by a T-donor and a T-acceptor substituent is significantly greater than the additive effects of the individual groups.73 Earlier failures to find this effect theoretically were ascribed to incomplete optimization of the geometry.The magni- tude of the stabilization is about 71 kJ mol-’ for (28) and 14 kJ mol-’ for (29). (27) The occupation numbers of natural orbitals (NOONS) have been proposed as a quantitative criterion for biradical chara~ter.’~ The method has been illustrated using the pPP/.rr-CI approach on a number of conjugated hydrocarbons in their ground and excited states and on several hypothetical Mobius-type systems and species with orthogonal double-bonds. An important study of the photoisomeriza- tion of polyenes has been carried out at the STO 4-31G The transition states for this process in butadiene hexatriene and (30) and the geometries of the lowest triplet state of these species were located and full normal-co-ordinate analyses were performed.MIND0/3 calculations for several larger polyenes gave qualitatively similar results. Integrals of the Franck-Condon type were evaluated from the calculated vibrational force field and the results indicated that the energy of the triplet state is funnelled mainly into the C(l)-C(2) torsional motion of the ground state in the deactivation process. 4 Dynamic Processes aiid Reaction Hypersurfaces New approaches to the problem of locating saddle points in an energy surface continue to be One such method involves the calculation of the energy of ‘hypercubes’ of space in conjunction with ‘global’ interpolation technique^.'^" A welcome development is the increasingly common characterization of stationary points via calculation of the force-constant matrix.The information that is available from this approach is illustrated by an ab inifiostudy of the rearrangement of MeNC to MeCN.77 The calculated activation energy includes corrections for zero-point energy and is within 10 kJ mol-’ of the experimental value. The vibrational analysis but not the structure is reasonably consistent with previous purely empirical estimates used in RRKM calculations of the kinetics of this reaction. At 500 K the experimental and the calculated value (ab initio) of the pre-exponential factor for ” (a) Y.Apeloigand R.Schreiber J.Am. Chem. SOC.,1980,102,6144;(b)T.Clark ibid. 1979,101,7746. 73 D. Crans T. Clark and P. von R. Schleyer Tetrahedron Lett. 1980,21 3681. 74 D.Doehnert and J. Koutecky J. Am. Chem. SOC.,1980,102,1789. ’* I. Ohmine and K. Morokuma J. Chem. Phys. 1980,73 1907. 76 (a) D.Feller W. T. Borden and E. R. Davidson J. Comput. Chem. 1980 1 158;(b) H.Cardy D. Liotard A. Dargelos and E. Poquet Now.J. Chim. 1980,4,751. ” P.Saxe Y.Yamaguchi P. Pulay and H. F. Schaefer 111 J. Am. Chem. SOC.,1980 102,3718. Theoretical Chemistry 25 this reaction agree remarkably well. Ab initio theory has also been used to evaluate tunnelling effects and the corresponding corrections to E for proton-transfer reaction^'^' and the equilibrium constant in isotopic exchange reactions.78 A number of reactions of fundamental importance to organic chemistry have been the subject of systematic and high-quality ab initio calculations.The cycloaddition reaction of ethyne to HCNO was calculated at the 4-31G/ single determinantal level to proceed concertedly via a relatively symmetrical transition The lengths of the new C-C and C-0 bonds in the transition state were calculated to be 2.185 8 and 2.219 8,respectively although the stretching force-constant for the C-C bond was ten times larger than for the C-0 bond. Contrary to a previous suggestion by McIverg0 that symmetric structures were unlikely to be genuine transition states the force-constant matrix corresponding to the structure that was located had only one negative eigenvalue.Iriclusion of extensive CI without further optimization of geometry decreased the barrier to the reaction two-fold (from 128 to 67 kJ mol-') and the authors merely speculate that CI would not have any substantial effect on the geometry or on the properties of the force-constant matrix! The reaction of borane with ethene has been re-investigated at the ab initio 4-31G leveL8' A .rr-complex is formed in the early stages of the reaction and the transition state corresponds to a four-centre structure involving concerted formation of the CH and C-B bonds and cleavage of the B-H and C=C bonds. This scheme differs significantly in several details from earlier theoretical studies. The Wittig reaction between H3P=CH2 and formaldehyde was studied with a double-t plus polarization-function basis An oxaphosphetan ring (12) is formed initially with a very small energy barrier followed by concerted dissociation into the products accompanied by a reorganization of the co-ordination of the phosphorus atom.An orbital-correlation diagram for the reaction can be con- structed in which all occupied orbitals of the reactant can be correlated with all such orbitals in the product. This is due to the polar nature of the molecules involved and it contrasts to the situation of a symmetry-forbidden [2 + 2,] addition. Harding et al. have presented a tour de force with their study of the potential-energy surface of H4C0.82Eight minima and six transition states were located using basis sets of up to 6-3 1G** quality and fourth-order Mgller-Plesset correction for the correlation energy.Vibrational frequencies and zero-point energies were evaluated from a full normal-co-ordinate analysis using the analytically derived force-constant matrix. At this level energies of reactions were predicted to be within 21 kJmol-' of the experimental values whilst the calculated vibrational frequencies were about 10 to 15% too large. Among the results obtained was that the formation of hydroxycarbene from photolysis of formaldehyde could be competitive with the dissociation into HZ and CO. Morokuma et al. have investigated the photochemistry of HFCO in a similar manner.83 In this case elimination of CO is predicted to occur rather than rearrangement to the carbene and a normal-co-ordinate analysis suggests that the HF will be formed with considerable vibrational energy.A 78 R. F. Hout M. Wolfsberg and W. J. Hehre J. Am. Chem. SOC.,1980,102,3296. 79 A.Komornicki J. D. Goddard and H. F. Schaefer 111 J. Am. Chem. SOC.1980,102 1763. J. W. McIver Acc. Chem. Res. 1974,7,72. S. Nagase N.K.Ray and K. Morokuma J. Am. Chem. SOC.,1980,102,4536. 82 L.B.Harding H. B. Schlegel R. Krishnan. and J. A. Pople J. Phys. Chem. 1980,84 3394. 83 K.Morokuma S. Kato and K.Hirao J. Chem. Phys. 1980,72,6800. 26 H.S. Rzepa normal-co-ordinate analysis of the transition state for the elimination of HF from fluoroethaneg4 suggests that here too the HF that is formed will be vibrationally excited. A study at the double-[/CI level of the Wolff rearrangement" reaffirms that oxiren is barely stable requiring only 8 kJ mol-' to rearrange to formylmethylene which itself is not stable to rearrangement to keten.None of the postulated transition states however was characterized by inspection of the corresponding force-constant matrices. After a detailed investigation of the ene reaction of '02with olefins it was concluded that the major pathway involves biradical intermediates and the results show how many aspects of the stereospecificity and regiospecificity can be under- stood in terms of biradical-like intermediates or transition states. One important point that emerges is that normal Markownikoff directing effects are not expected in the addition of '02 to unsymmetrical olefins.86 The hydration of formaldehyde to give methanediol was calculated (using a 4-31G basis) to proceed uia a concerted proton transfer and formation of a C-0 bond [see (31)] with a calculated activation energy of 184 kJ mol-'.The addition of a single extra water molecule changed the character of the transition state dramatically [see (32)] and lowered the activation energy c~nsiderably.~~ Prior protonation of the carbonyl group or deprotonation of the water was found to remove the barrier for the process whilst the potential-energy surface for the formation of the zwitterion H&-CH,o was found to be completely repulsive. 4-H (31) An entirely different approach to nucleophilic addition reactions involved the development of intermolecular SCF perturbation theory for weak interactions." The interaction energy was expressed as the sum of electrostatic exchange and exchange-repulsion charge-transfer and polarization terms and the approach was illustrated with a discussion of the orientation of a nucleophile in carbonyl addition reactions.Whereas the frontier-orbital (ie.charge-transfer) term alone was inadequate in describing the interactions it proved a useful means of expressing Baldwin's rules for such reactions in qualitative MO terms. Another approach to the problem of solvation has been to use the valence-bond concept of ionic- covalent resonance to develop a simple model for comparing activation energies of reactions in solution with that in an enzyme. The method was used to investigate the potential-energy surface of simple general acid catalysis in solution and in ~ysozyme.~~ 84 S.Kato and K. Morokuma J. Chem. Phys. 1980,73 3900. *' K.Tanaka and M. Yoshimine J. Am. Chem. SOC.,1980,102,7655. 86 L.B.Harding and W. A. Goddard J. Am. Chem. SOC.,1980,102,439. " (a)I. H. Williams D. Spangler D. A. Femec G. M. Maggiora and R. L. Schowen J. Am. Chem. SOC. 1980,102,6619;(b)I. H.Williams G. M. Maggiora and R. L. Schowen ibid. p. 7831. 88 A. J. Stone and R. W. Erskine J. Am. Chem. SOC.,1980,102,7185. 89 A.Warshel and R. M. Weiss J. Am. Chem. SOC.,1980,102,6218.

ISSN:0069-3030    

DOI:10.1039/OC9807700015

出版商:RSC    

年代:1980

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 The ene-type reactions of singlet oxygen with olefins have continued to attract attention with the main interest focussed upon attempts to delineate mechanisms. Recent advances in the area have been reviewed.' Results of a theoretical treatment based on ab initio calculations for the reaction between ethylene and '02combined with thermochemical methods for estimating the effects of substituents to distinguish the pathways involving peroxy diradical open 1,4-zwitterion and perepoxide intermediates have been reported.* It was concluded that the major reaction pathway involves a diradical and that solvent and substituent effects can be accounted for by assuming variable diradical-zwitterionic character.The reaction of 2-methyoxybut-2-ene is predicted to proceed via the diradical (1)in non-polar solvents whereas in polar solvents the opposite regiochemistry should be favoured leading to the formation of the zwitterion (2); this prediction should be verifiable. Stephenson3 has suggested that as activation barriers in the olefin-'02 reactions are very low future developments may necessarily lean towards theoretical treatments. He proposes that the interactions of frontier orbitals between highly electrophilic singlet oxygen (LUMO) and both the olefinic 7r-orbitals and C-H bonds (HOMO) lead to complex formation. In the case of cis-2-butene the complex can be pictured as in (3) in which the pseudo-.rr-orbitals of CH3 contribute to the .rr-HOMO.The interaction is expected to be stronger than for trans-2-butene where only one carbon Meope Meo8Me Me H (2) ' L. M. Stephenson M. J. Grdina and M. Orfanopoulos,Acc. Chem. Res. 1980.13,419. 'L. B. Harding and W. A. Goddard J. Am. Chem. SOC.,1980,102,439. 'L.M.Stephenson Tetrahedron Lett. 1980,21,1005. 28 G.B. Gill pseudo-rr-orbital can be involved but weaker than in trisubstituted olefins or with CH30-substitution (CH30 is a strong cis-director). The attraction of this model is that it does not conflict with the present (isotope-effect) studies or with previously established data and allows for the possibility of relaxation to perepoxide diradical or zwitterionic species depending upon the nature of the alkene undergoing reaction.The reaction of lo2with the optically active olefin (4) followed by reduction afforded the alcohols (R)-(5)and (S)-(6)in 1:1 ratio; while the process is highly stereospecific the lack of stereoselectivity (isotope effect) is not what would be anticipated for a normal ene reaction but is consistent with for example the perepoxide me~hanism.~ The finding5 of an isotope effect in the ene reaction of N-phenyltriazolinedione with (E)-2,3-bis(trideuteriomethyl)-2-butene (the Stephenson model3) but not with the (2)-olefin has led to the suggestion that an aziridinium imide (cf.perepoxide intermediate) is intermediate in the change. The formation of a diazetidine from adamantylideneadamantane and the triazolinedione implies from steric considerations that orthogonal approach does occur.A number of papers have been concerned with the stereoelectronic effects in the cycloaadition reactions of norbornene and related molecules. A field effect rather than through-space orbital mixing has been suggested as the origin of the polariz- ation of the rr-orbitals of 7-substituted norbornadienes and provides a satisfactory rationalization of the deactivation toward exo-anti-attack by electronegative sub- stituents. However the more subtle rate effects of syn-endo- and anti-endo-attack are less well understood.6 Comparison of the dipolar and Diels-Alder cycloaddition rates of norbornene bicyclo[2.2.2]octene (7) and (8)reveal that the rate constants for (7) and (8) are similar to or smaller than those for n~rbornene.~ Molecular mechanics MM2 calculations of strain energies and heats of formation (which provide quite good agreement between calculated AH,“values and experimental values where available) reveal that loss of strain in (7) and (8) in becoming dihydro-(7) or -(8) should provide a relative rate acceleration upon cycloaddition of 103-104.The high rate constants for norbornene therefore originate from another source -possibly from non-equivalent orbital extension (Fukui’s concept).Electronic control of stereoselectivity in cycloadditions to molecules of the type (9) and (10) has been examined in detail.’ Whereas (9) undergoes [4+2] rr-M. Orfanopoulos and L. M. Stephenson J. Am.Chem. SOC.,1980,102,1417. C. A.Seymour and F. D. Greene J. Am. Chem. SOC.,1980,102,6384. P. H. Mazzocchi B. Stahly J. Dodd N. G. Rondan L. N. Domelsmith M. D. Rozeboom P. Caramella and K. N. Houk J. Am. Chem. SOC.,1980,102,6482. ’R. Huisgen P.H. J. Ooms M. Mingin and N. L. Allinger J. Am. Chem. SOC.,1980 102 3951. (a) L. A. Paquette V. C. Carr M. C. Bohm and R. Gleiter J. Am. Chem. SOC.,1980,102,1186; (6)hi. C. Bohm R. V. C. Cam R. Gleiter and L. A. Paquette ibid.,p. 7218;(c) L.A.Paquette R. V. C. Carr E. Arnold and J. Clardy J. Org. Chem. 1980 45,4907; (d)L.A. Paquette F. Bellamy M. C. Bohm and R. Gleiter ibid.,p. 4913;(e) L.A.Paquette R. V. C. Carr P. Charumilind and J. F. Blount ibid. p. 4922. Reaction Mechanisms -Part (i) Pericyclic Reactions OH (10) X = (CHA (11) cycloadditions with typical dienophiles entirely by endo-attack both endo and em products are obtained from (10)in most cases.sa*b*e Since steric factors are ruled out in determining the endo stereoselectivity in (9) the results have been rationalized in terms of cT-orbital mixing with the T diene level leading to a disrotatory tilt of the diene orbitals thereby minimizing antibonding interactions on the endo face in the interaction with the enophile T-level.Only moderate endo stereoselectivity was observed in the case of the cycloadditions of 102,8cTd and this is presumably the result of the very low energy of the 7rl(S) level for lo2,and hence a much smaller interaction with the diene nl(S)level so that differences between endo and ex0 approach are not large.Diels-Alder reactions of the fused cyclopentadiene (11) occur exclusively to the face bearing the OH group in accord with the bias of the T-HOMO in the syn-direction (Fukui et ~21.)~~ 2 Cycloadditions and Cycloreversions An algorithm of the form y(rc)= (E$ +E; -E:)/C*,based on the Michl model has been derived for prediction of reactivity in allowed [2 + 21 and [4 + 41 singlet-state photo-cycloadditions.'O Favourable features are high singlet-state energy (E is that for excited reactant A) low triplet energy (E$ and E:! are triplet energies for reactants A and B) and a high frontier-orbital density at the reacting positions (c2 is the sum of products of HOMO and LUMO orbital coefficients over reacting positions); y(rJ is the resonance integral for end-on interaction of carbon 2p orbitals at distance r,.There is reasonable correlation of the algorithm with known photo- reactivities which allows segregation into three categories which are highly reactive [y(rc)f 20 kcalmol-')I moderately reactive [20 s y(rc)s 251 and unreactive [y(rc)b 25 kcal mol-'1. The electronic structure of the first excited states of hypostrophene (12) deduced from analysis of the first bands in the He(1) photoelectron spectra of (12) and of its di- and tetra-hydro- and its homo- and bis-homo-derivatives in combination with semi-empirical calculations has been discussed in the light of its inability to undergo [,2 + ,2,] photo-cylcoaddition." The results confirm that for (12) T+(the in-phase combination of the two ethylenic orbitals) is the HOMO arising from the mixing in (12) D.W. Jones J. Chem. Soc. Chem. Commun. 1980,739. lo R. A. Caldwell J. Am. Chem. SOC., 1980 102 4004. J. Spanget-Larsen R. Gleiter G. Klein C. W. Doecke and L. A. Paqiiette Chern. Ber. 1980,113,2120. 30 G. B. Gill of several totally symmetric bicyclohexane orbitals thus placing it above *-(the out-of-phase combination). However the LUMO previously assigned to d,is not so readily deduced because of the closeness in energy of rrT and T?.Possibly therefore derivatives of (12) may be ‘allowed’ to follow the proscribed [,2 + ,2,] pathway. The regioselectivities of the [,4 + ,2,] cycloadditions of alkyl-allenes to tetraphenylcyclopentadienone differ markedly from those for the formal [2 + 21 additions of maleic anhydride.Coupled with PMO predictions the results have been interpreted in favour of the [,2 + (,2 + ,2,)] ,or diradical pathway rather than the [,2 + ,2,] mechanism.12 Experimental and theoretical criteria are thoroughly examined in a review which attempts to draw a line of distinction between concerted and multi-step mechanistic alternatives in Diels-Alder reaction^.'^ The [4 + 21 cyclo-dimerization of 2,3-dimethyl-1,3-butadiene has been examined at high pre~sure.’~ Study of the pressure dependence of the rate constant reveals that A V*represents only about 70-75% of A V (the reaction volume) indicating that interatomic distances between atoms that are participating in bond formation are not as close as those distances in the adduct.Both concerted and diradical mechanisms are considered to compete; however the A V* criterion (if applicable) assumes rate-determining formation of the inter- mediate. The considerable recent interest in intramolecular Diels-Alder reactions con- tinues unabated; reviews” covering various features of such processes and methods for the generation of the necessary intermediates are particularly welcome. Such has been the number and diversity of papers in this area (estrone has been a favoured target) that only a few representatives can be considered here. An innovative one-step synthesis of polycycles including (f)-estrone has been reported which utilizes a [CpC~(CO)~]-catalysed co-oligomerization of substituted 1,5-hexadiynes with bis(trimethylsily1)acetylene solvent to produce the requisite benzocyclo- butenes.16 An example of the overall procedure is shown by the transformation (13) -+ (14) -+ (15) in 60% yield.A heteroatom variant with inverse electron demand of the Diels-Alder reaction has been used in the preparation of (16) (33’/0) which is formed stereo~electively.~~ The ene-diene precursor of (16) was formed simply by condensation of (R)-citronella1 with cyclohexane-1,3-dione the cyclo- addition occurring at room temperature. ’’ D. J. Pasto Tetrahedron Lett. 1980,21 4787. l3 J. Sauer and R. Sustmann,Angew. Chem. Int. Ed. Engl. 1980,19 779. l4 G.Jenner and J. Rimmelin Tetrahedron Lett. 1980 21 3039. Is G.Brieger and J.N. Bennett Chem. Rev. 1980,80,63;R.L.Funk and K. P. C. Vollhardt Chem. SOC. Rev. 1980 9 41;J. J. McCullough,Acc. Chem. Res. 1980,13,270. l6 R. L. Funk and K. P. C. Vollhardt J. Am. Chem. SOC.,1980,102 5245,5253. l7 L.-F. Tietze G. von Kiedrowski K. Harms W. Clegg and G. Sheldrick Angew. Chem. Znt. Ed. Engl. 1980 19 134. Reaction Mechanisms -Part (i) Pericyclic Reactions (16) (17) Interesting use has been made of bridgehead alkenes (formed by way of intramolecular Diels-Alder reactions) of the type (17) to control the relative configurations of a number of asymmetric centres.” Additions to the bridgehead double-bond occur with almost complete syn -stereoselectivity as reagents are denied access to the back side of the molecule.Pyrolysis of the diene (18)afforded the single cyclo-adduct (19); if the (a-acylimine is intermediate then there is a preference for endo placement of the non-terminal unsaturated group (N-acyl) in contrast to the analogous all-carbon systems.” The severe limitation in the use of (a-dienes in that two relatively easily accessible transition states are available for intramolecular cycloaddition has been discussed. Differences due to geometric factors are highlighted in the synthesis of optically active (20) by refluxing the (2)-amide in toluene (95O/O yield).20 The (E)-amide remained largely unaffected under similar conditions. Me Me .CO,Me heat 0 CHiPh Two ap-unsaturated aryl sulphones are reported to be fairly reactive dienophiles and the ready replacement of the ArS02 grouping is likely to recommend their further use in synthesis.Thus phenyl vinyl sulphone is an equivalent of ethylene and of terminal olefin,’l and ethynyl p-tolyl sulphone is an acetylene and trimethyl- silylacetylene equivalent.22 It appears however at least for phenyl vinyl sulphone that Lewis-acid catalysis is not effective in promoting reaction. Following upon domino Diels-Alder reactions which found favour in recent years (notably in the construction of sub-units of dodecahedrane) further development may be anticipated for ‘timed Diels-Alder reaction~’.~~ Thus the reaction of a bis-diene e.g. (21) with a bis-dienophile e.g. (22) results in intermolecular [4+2] reaction e.g. to give (23) followed by slower intramolecular [4 +21 cycloaddition e.g.to give (24). Regiochemical control of course is all-important in the first addition and requires careful consideration of the structures of the bis-dienes and bis-dienophiles. K. J. Shea P. S. Beauchamp and R. S. Lind J. Am. Chem. SOC.,1980,102,4544. l9 B.Nader R. W. Franck and S. M. Weinreb J. Am. Chem. SOC.,1980,102,1153. 2o S.G.Pyne M. J. Hensel S. R. Bryn A. T. McKenzie and P. L. Fuchs J. Am. Chem. SOC.,1980,102 5960. 21 R. V. C. Carr and L. A. Paquette J. Am. Chem. SOC.,1980,102 854. 22 A. P.Davis and G. H. Whitham J. Chem. SOC.,Chem. Commun. 1980,639. 23 G. A. Kraus and M. J. Taschner J. Am. Chem. SOC.,1980,102 1974. 32 G.B. Gill R Me J-Ld [77 "CI __* (23) (R = OSiMe2Bu') Studies of thermal [4 +21 cycloreversions of (25)-(27) have revealed that AGS decreases in the series respectively 29 26 and ~20 kcal mol-'.The rate acceler- ation of at least lo6by the 1-alkoxide substituent in (27) has been ascribed to the loss in basicity of ca 8 pKb units in the conversion of alcoholate into phenolate ion adding -11 kcal mol-' [i.e. -2.303RTA(pKb)] to the negative free enthalpy of reaction of the parent system. Since the transition state of a [4 +21 cycloreversion is product-like it should participate appreciably in the gain in free energy.24 (25) X=H (26) X =OSiMe (27) X=O-NMe4 There are few recorded instances of photochemical [2 +2 +2 +21 processes involving bis-terminal addition to an acetylene component. Nevertheless this procedure has been employed in a new route to the elassovalene system (28).25 Further developments have been reported this year in [4 +3'1 cycloadditions of ally1 carbonium ions to dienes.An alternative method for the construction of seven-membered rings involves cycloaddition of enophiles to the pyrylium zwitterion (29) affording adducts (30). Reactions were generally regiospecific but variable stereoselectivity was observed; however em-addition was normally favoured.26 A concerted mechanism appears to be involved in the [6+3-1 reactions of cyclohep- tatriene with 1,l-diphenyl- and 1,3-diphenyl-2-a~a-allyl-lithium.~~ 3 Ene Reactions Full details have now appeared of Lewis-acid-catalysed ene reactions of alkenes with acetylenic esters28a and with methyl a-chloroacrylate,286 and recent develop- ments in this area have been reviewed.28c Initial studies on the ene additions to alkenes of formaldehyde and higher aldehydes2'" and of terminal alkynes to formal- 24 0.Papies and W. Grimme Tetrahedron Lett. 1980,21,2799. R. Askani and B. Pelech Tetrahedron Lett. 1980,21,1841. 25 26 J. B. Hendrickson and J. S. Farina J. Org. Chem. 1980 45 3359 3361. 2'7 D. J. Bower and M. E. H. Howden J. Chem. SOC.,Perkin Trans. 1 1980 672. 28 (a)B. B. Snider D. M. Roush D. J. Rodini D. Gonzalez and D. Spindell J. Org. Chem. 1980 45 2773; (b)B. B. Snider and J. V. DunEia J. Am. Chem. Soc. 1980 102 5926; (c) B. B. Snider Acc. Chem. Res. 1980 13,426. 29 (a)B. B. Snider and D. J. Rodini Tetrahedron Lett.1980,21 1815; (b)D. J. Rodini and B. B. Snider ibid. p. 3857. Reaction Mechanisms -Part (i)Pericyclic Reaction dehyde2" have been reported. In both cases the proton-scavenging properties of Me2AlCl are put to good effect. The formation of a-allenic alcohols and (2)-3-chloroallylic alcohols in the terminal alkyne-formaldehyde reactions indicates the probable intermediacy of the dipolar species (31). Large rate accelerations coupled with greatly improved diastereoselectivities have been reported for the Et2AlC1-promoted intramolecular ene reactions of the (E)-and (2)-dienes (32) and (33) re~pectively.~'In the related reaction of the (-)-8-phenylmenthyl ester analogue of (33; Y = chiral carboxylate) cyclization afforded the trans-pyrrolidine (34) enantioselectively; a total synthesis of (+)-a -allokainic acid the natural epimer has therefore been achieved./COCF3 /COCF3 C02Et "Rf"_) H c1. -0 \=-C0,Et /\ (34) 7 Me Me XY (3 1) (32) X = C02Et Y =H (33) X = H Y = COZEt Quantitative studies of the reaction of dimethyl mesoxalate with alkenes which include the measurement of ahvation parameters and primary and secondary kinetic isotope effects the illustration that there is little variation of rate with change in solvent polarity and the establishment of the preponderant formation of (E)-adducts have been interpreted in favour of a late (product-like) transition state utilizing an exo arrangement in which the transfer of H occurs n~n-linearly.~~ Hammett correlations for thermal and Lewis-acid-catalysed ene additions of diethyl mesoxalate to 1-aryl-cyclopentenes have revealed a strong electronic bias in the catalysed process (p = -3.9) relative to the thermal reaction (p = -1.2) which is manifested in striking differences in regioselectivity in additions to other 01efins.~~ Hydrolysis and oxidative decarboxylation of the ene-adducts gives allylcarboxylic acids; hence oxomalonate is a C02synthon in ene reactions.Inability to achieve a planar arrangement of reaction centres in the transition states of acetylenic retro-ene processes prevents reaction. Hence vapour-phase thermolysis of (39 (36) and (38) afforded the retro-ene products whereas the alkyne (37) was recovered ~nchanged.~~ Thermolysis of (R)-(+)-laurolenal (39) at 297 "Cyields CO and (R)-(+)-(42)in high optical purity by a formal retro-nor-ene process; the acid (R)-(+)-(40)likewise was cleanly converted into (R)-(+)-(42)by retro-ene cleavage.34 In contrast the alcohol (R)-(+)-(41) initially underwent ene-retrogression to give (42) and CH20 slowly but oxidation of (41) by formal- dehyde yielded (39) and CH30H in the later phases of the thermolysis leading to a more rapid formation of (R)-(+)-(42).Relative rates were approximately 9 :2 :1 for (40) (39) and (41).30 W. Oppolzer and C. Robbiani Helv. Chim. Acta 1980,63 2010;W.Oppolzer C. Robbiani and K. Battig ibid. p. 2015;for related thermal studies see P. D. Kennewell S. S. Matharu J. B. Taylor and P. G. Sammes J. Chem. SOC.,Perkin Trans. 1 1980 2542.31 0.Achmatowicz and J. Szymoniak J. Org. Chem. 1980,45 1228,4774. 32 M.F.Salomon S. N. Pardo and R. G. Salomon J. Am. Chem. SOC.,1980,102,2473. 33 A.Viola and J. J. Collins J. Chem. SOC.,Chem. Commun. 1980 1247. 34 R. J. Crawford and €4. Tokunaga Can. J. Chem. 1980,58,463. 34 G.B. Gill H H&n U X Me Me Me% Me Me ri (42) (35) n = 2 X = CH=CH2 (39) x=co (36)n = 3,X = CH=CH2 (40)X=COO (37) n = 2 X = CGCH (41) X=CH20 (38) n = 3 X = CrCH N-Sulphinylnonafluorobutanesulphonamide(n-C4F9SO2N=S=O) is a 'super- enophile' being ca. 103-1O4 times more reactive than N-sulphinyltoluene-p- sulphonamide; reactions with typical olefins have tl, = 1-5 min at 20 "C and hence even electron-deficient alkenes can be converted into the ene adducts.Enolizable dialkyl ketones react by way of the enol in an autocatalytic process.35 The high ene reactivity of SO is revealed by the production of adducts (43; X = H or Me) at -78 "C from 1,l-dimethylallene and tetramethylallene; unusually the products have stability with respect to the starting materials although decomposition occurs at 25 0C.36The limited stereoselectivity in allylic oxidation of the isotopomeric olefins (E)-and (Z)-RCH=C(CH3)14CH3 (R = 2-cyclopentylethyl) by Se02 can be traced to differences in steric interactions in the chair-like transition states there being a 7 :3 preference for pseudo-equatorial R.37 Me =$q HO,S X (43) (44) The intramolecular ene reactions of 1,2-diallylcylohexanes have been investi- gated with a view to ring expansion by six carbon atoms.However the ene adducts (44) have yet to be persuaded to undergo the indicated further retro-ene cleavage that is required for the ring-expan~ion.~~ Transannular ene reactions have been used in an elegant approach to the synthesis of eudesmane-type corn pound^.^^ Retro-ene reactions may be involved in the ammonolysis of a-methoxyvinyl carbonates40a and in hydroxyammonolysis of enol ester~,~" and an ene-type reaction has been proposed involving acyl-group transfer in the addition of N-phenyltriazolinedione to a-angelica la~tone.~~ However a dipolar mechanism seems to be more likely for the acyl-transfer. 4 Sigmatropic Rearrangements Applications of sigmatropic processes in stereo-controlled syntheses of acyclic systems have been reviewed,42 as have energy-surface considerations in sigmatropic 3s R.Bussas and G. Kresze Angew. Chem. Znt. Ed. Engl. 1980,19,732; G.Kresze and R. Bussas ibid. p. 737. G. Capozzi V. Lucchini F. Marcuzzi and G. Melloni Tetrahedron Lett. 1980,21,3289. 37 W.-D. Woggon F. Ruther and H. Egli J. Chem. SOC.,Chem. Commun. 1980,706. 38 E. N. Marvel1 and J. C.-P. Cheng J. Org. Chem. 1980 45,4511. 39 P. A. Wender and J. C. Hubbs J. Org. Chem. 1980,45 365; P. A. Wender and L. J. Letendre ibid. p. 367. 40 (a) Y. Kita J.4. Haruta H. Tagawa and Y. Tamura J. Org. Chem. 1980 45 4519; (b) F. W. Lichtenthaler and P. Jarglis Tetrahedron Lett. 1980,21 1425 1429. 41 W. E. Bottomley G. V. Boyd and R.L. Monteil J. Chem. SOC.,Perkins Trans. 1 1980 843. 42 P. A. Bartlett Tetrahedron 1980 36 3. Reaction Mechanisms -Putt (i) Pericyclic Reactions shifts.43 The claim of base-assisted carbon-Claisen rearrangement of 4-phenyl-l- butene could not be ~ubstantiated.~~ Hence the all-carbon [3,3] rearrangement must be a high-energy process. Competitive formation of tricyclohexanes and Cope products in the thermal isomerization of allyl-substituted cyclopropenes indicates a non-concerted mechanism passing through intermediates analogous to the 1,4- cyclohexylene diradi~al.~' Relative rates and energy differences between the diastereoisomeric transition states in the Cope rearrangements of (*)-(45) and (&)-(46) involving a chair-like geometry (of local C2hsymmetry) and meso -(47) and meso-(48) involving a boat-like geometry (local Cz,symmetry) have been dete~mined.~~ The sizeable AASS terms (11-13 cal mol-' deg-') for (45) (47) (i.e.-11.4 -0.4 cal mol-' deg-') and (46) (48) (i.e. -8.3 +5.0 cal mol-' deg-') are puzzling. Since it appears that a dissociation-recombination mechanism can be discounted for the rneso-compound (47) (48) reactions the AS*values do not provide a valid criterion of concertedness as these Cope reactions satisfy other mechanistic criteria for concerted rearrangements. The [1,3] rearrangements of 5 -X-bicyclo[2.1 .O]pent-2-enes have been examined by ab initio calculations; no evidence for participation of lone pairs as required by the pseudopericyclic concept was Thermal rearrangement of (+)-(49) by the norcaradiene walk mechanism occurs predominantly (295%) with inversion at the migrating carbon C-7.48" Analogous but non-degenerate rearrangement of (50) proceeds with ca 80% inversion of configuration at the migrating carbon under thermal control; predominant inversion of configuration also occurs in the photo-induced [1,5] sigmatropic reaction.The relevance of orbital symmetry considerations of the Woodward-Hoffmann type to either process is therefore open to Details have been reported of the stereochemistry of and migratory aptitudes in [1,5] sigmatropic shifts of acyl and vinyl groups in 1,3-dimethylindene~.~' Preference for the exo alignment of the unsaturated terminus of the migrating group has been established and rationalized as an avoidance of antibonding secondary interactions in the endo transition-state geometry.Details of the stereochemical course in the intramolecular [1,7] antarafacial H shift leading to vitamin D3 have been unravelled by labelling studies. Thus the triene precursor adopts a left-handed twist of the reaction centres placing the A ring above the C/D rings and resulting in 9p transfer of the pro-R hydrogen atom.50 43 J. J. Gajewski Acc. Chem. Res. 1980 13 142. 44 M. Newcomb and R. S. Vieta J. Org. Chem. 1980,45,4793. 45 A. Padwa and T. J. Blacklock J. Am. Chem. SOC., 1980,102,2797. " K. J. Shea and R. B. Phillips J. Am. Chem. SOC.,1980,102 3156. 47 J. P. Snyder and T. A. Halgren J. Am. Chem. SOC.,1980,102,2861. 48 (a)F.-G.Klarner and B. Brassel J. Am. Chem. SOC., 1980 102 2469; (6) W. T. Borden J. G. Lee and S. D. Young,ibid. p. 4841; for related [1,6] shifts see R. F. Childs and C. V. Rogerson ibid.,p. 4159. 49 D. J. Field and D. W. Jones J. Chem. SOC.,Perkin Trans. 1 1980,714 1909. R. M. Moriarty and H. E. Paaren Tetrahedron Lett. 1980 21 2389. G.B. Gill Cope rearrangements of acyclic 1,Sdienes can be accelerated in appropriate cases by Pd” catalysts. The presence of an alkyl group at either C-2 or C-5 (not both) appears to be necessary presumably because of the need to provide stability in an intermediate species such as (5l)? Preference for chair-like transition states in anion-accelerated oxy-Cope rearrangements follows from the results of the rearrangements of the potassium salts of the four individual dienols of gross structure (52).52Application to the stereoselective synthesis of (+)-erythru-juvabione is also reported.Although formation of the oxy-anion from ~is~-2,4,7-~yclononatrienol results in enhancement of both [1,5] hydrogen shift and oxy-Cope rearrangement there is a bias of ca 104-105in favour of the [3,3] process resulting in quantitative conversion (following work-up) into (53).53 The rearrangement of the lithium salts of 2-vinylcyclopropanols to cyclopentenols apparently occurs by concerted [1,3] rearrangement. 54 Concerted [5,4] rearrangements proceeding uiu a nine-membered ten-electron transition state have been demonstrated in the base-catalysed transformations of allyl(pentadieny1)ammoniumcations.Analogous rearrangements of the propynyl- (pentadieny1)ammonium ylides however afforded only products resulting from [1,2] [3,2] and [5,2] sigmatropic shifts.55 5 Electrocyclic Reactions The chemistry of allene oxides has been reviewed.56 Simple Woodward-Hoff mann predictions are at variance with the conclusions based on CNDO/B potential-energy surfaces for ring-closure of 1-and 2-heterosubstituted butadienes and can be rationalized in terms of nodal shifts within the T moleculai orbitals as ring-closure sets in.57 Whereas photochemical (conrotatory) ring-opening of the trans-bicyclic dienes (55) and (56) affords trans,cis,truns-cyclo-deca-and -undeca-1,3,5-trienes by the least-motion pathway ‘discordant’ ring-opening of the lower homologue (54) occurs since only cis,cis,cis-cyclonona-l,3,5-triene is formed.Relief of strain is responsible. Thermal (disrotatory) cyclization of the trienes afforded the cis-fused isomers of (54)-(56)? ” L. E. Overman and F. M. Knoll J. Am. Chem. SOC.,1980,102,865. ” D. A.Evans and J. V. Nelson J. Am. Chem. SOC., 1980,102,774. 53 L.A.Paquette G. D. Crouse and.A. K. Sharma J. Am. Chem. SOC.,1980,102,3972. 54 R. L. Danheiser C. Martinez-Davila and J. M. Morin J. Org. Chem. 1980 45 1340. ” T.Laird W. D. Ollis and I. 0.Sutherland J. Chem. SOC.,Perkins Trans. 1,1980,2033;see also previous papers in this series. T. H. Chan and B. S. Ong Tetrahedron 1980,36,2269. ” J. P. Snyder J. Org. Chem. 1980,45 1341. ’* W.G.Dauben and E. G. Olsen J. Org. Chem. 1980,45,3377.

ISSN:0069-3030    

DOI:10.1039/OC9807700027

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (ii) Polar Reactions By D. G. MORRIS Department of Chemistry The University Glasgow G12 8QQ 1 Introduction The present Report covers nucleophilic substitution carbo-cations carbanions nucleophilic addition to carbonyl compounds and to olefins reactivity of esters and of carbodi-imides and elimination reactions. Two significant reviews one on intramolecular catalysis’ and one entitled ‘When is an intermediate not an intermediate’,2 have appeared. A very readable review of methods of esterification and carbonyl protection has been given.3 Other reviews of note concern the use of reactive and selective leaving groups (e.g. PhCOOS02CF3) which obviate the necessity for Friedel-Crafts catalysis displacement reactions of allylic corn pound^,^ and a -acyl carbo-cations.6 2 Nucleophilic Substitution A perturbational study using Salem’s frontier-orbital method of bimolecular nucleophilic substitution at saturated carbon has been carried out to determine conditions under which the reaction can proceed with retention of configuration.’ Such a stereochemical outcome though of course never observed at saturated carbon is favoured by low electronegativity at the reaction centre a hard nucleophile and a leaving group of high electronegativity.When silicon is the central atom neither invocation of orbitals nor pseudorotation in the transition state is required to account for the significant retention of configuration that is observed with cyclic silicon compounds if they are not too strained.In the gas phase where SN2reaction rate constants span ca three powers of ten in contrast to proton transfers it has been shown that alkoxides and fluoride are still poor leaving groups.’ Although alkoxides will displace halides from saturated carbon as will halide displace halide alkoxide will not displace alkoxide in the thermoneutral reactions. Indeed alkoxides are relatively poor nucleophiles in the gas phase. The poor leaving-group ability of alkoxides has traditionally been ascribed to their high basicity as most displacements would be endothermic. Low nucleofugicity of fluoride persists in the gas phase and is considered an intrinsic property not a question of solvation or thermodynamics. ’ A. J. Kirby Chem. Br. 1980 16 36. W. P. Jenks Acc.Chem. Res. 1980 13 161. E. Haslam Tetrahedron 1980,36 2409. A F.Effenberger Angew. Chem. Int. Ed. Engl. 1980,19,151. ’ R. M. Magid Tetrahedron 1980 36 1901. J.-P. Begue and M. Charpentier-Morize Acc. Chem. Res. 1980 13 207. Nguyen Trong Anh and C. Minot J. Am. Chem. SOC. 1980,102,103. ’M.J. Pellerite and J. 1. Brauman J. Am. Chem. SOC. 1980 102 5993. 37 D. G. Morris The a effect i.e. the enhanced nucleophilicity of a reagent with at least one pair of unshared electrons adjacent to the nucleophilic centre generally decreases according as to whether the carbon being attacked is sp- sp2- or sp'-hybridized. For methyl aryl sulphates in MeOH at 25"C the ratio of second-order rate constants for hydrazine and glycine ethyl ester which have comparable basicities is 5.21.9This value increases as the leaving group becomes poorer in accord with an interpretation indicating a larger interaction between nucleophile and sp3- hybridized carbon in the transition state and as dictated by the reactivity-selectivity principle.Rate enhancement via a major elevation of the ground state of the a-enhanced nucleophiles is not considered to be of major importance; rather the enhanced reactivity resides in the antibonding HOMO of inhomogeneous polariza- bility and donation of these electrons to the reaction centre lowers the energy as reaction proceeds. The same effect termed supernucleophilicity has been considered by MO theory and related to the number of valence electrons and a atoms associated with the reactive centre.lo There is ambivalence in the case of three-co-ordinate entities since NC13 is only weakly nucleophilic whereas the sulphite ion S032- is a known supernucleophile. For the substrates MeTh' (1-methylthiophenium ion) MeOC103 and MeOTf (Tf = triflate) the rate-constant ratio kH20/kD;0 approximates to unity for nucleophilic attack by water at the methyl carbon when water is in dilute solution in a dipolar aprotic solvent e.g. acetonitrile or sulpholane." Water is monomeric in MeCN at concentrations of less than 0.1mol l-l in which concentration range MeTh' is mainly unhydrated. In H20 or D20 the ratio of pseudo-first-order rate constants k:/k is 1.128at 25 "C. The solvent isotope effect arisesfrom differences in the rotational relaxation times for liquid H20 and D20.Rate constants kI and koTs have been measured for the reaction of Me1 and MeOTs with a number of transition-metal nucleophiles.12 Values of kI embrace more than eleven powers of ten from 2.8 X lo6for [CpFe(C0)I2- in THF at 25 "C to 3.6 x lo-' 1mo1-ls-l for [RhCl(CO)(PPh3)2] in MeI at 25 "C; the rate-constant ratio kI/kOTsvaries from lo8for [Co(CN),I3- to for [LiCuMe2I2. The first evidence for an intermediate S,2 mechanism has been claimed by the Casadevalls' group,13 in an examination of the solvolysis of (1) in hexafluoroisopropyl alcohol and trifluoroacetic acid. Solvolysis of secondary tosy- lates take place in CF,C02H and 9'7% aqueous (CF3)2CHOH without solvent assistance; such is the case for 2-adamantyl tosylate (2-AdOTs) in all solvents.For (l),the ratio ks/ kc [as defined in equation (l)]with X = Bu'OH is 64; values greater E. Buncel C. Chuaqui and H. Wilson J. Org. Chem. 1980,45 3621. lo W. B. England P. Kovacic S. M. Hanrahan and M. B. Jones J. Org. Chem. 1980,45 2057 J. L. Kurzand J. Lee J. Am. Chem. SOC.,1980,102,5428. l2 R.G.Pearson and P. E. Figdore J. Am. Chem. SOC.,1980 102 1541. l3 J. Laureillard A. Casadevall and E. Casadevall Tetrahedron Lett. 1980 21 1731. Reaction Mechanisms -Part (ii) Polar Reactions 39 than 10correspond to solvent assistance. A distinction between an SN2-intermediate mechanism rather than the classical sN2 counterpart is made on the basis of product analysis. Thus 63% of olefin (2) is formed which together with at least a part of the 33% of olefin (3) arises from cation-mediated cis -elimination.The intermediate ion-pair if 'nucleophilically solvated' would carry a significant positive charge on the reactive carbon. The /3 axial group serves to hinder approach of the solvent. It is also noted that weak nucleophilic assistance can bring about large rate effects. ks [kt(ROTs)/k,(2-AdOTs)]{in solvent X} -= CF3C02H [or (CF3)2CHOH]} (1) kc [kt(ROTs)/k,(2-AdOTs)]{in The reaction of 2,4,6-triphenylpyridinium ions e.g. (4) with for example piperidine results in transfer of the benzyl substituent as indicated in Scheme 1.14 From a kinetic analysis of the reaction of (5) the authors concluded that reaction takes place via unimolecular and bimolecular mechanisms which are independent and provide no evidence for an intermediate mechanism.Ph Ph R R (4) R=CH2Ph (5) R=Pr' Scheme 1 The special salt effect that is usually manifested by dramatically increased polarimetric rate constants at low concentrations of added LiClO during acetolyses of diverse tosylates has been shown for the first time to affect the nature of the pr~ducts.'~ Thus during acetolysis of optically active (6),the phenyl-assisted (kA) route accounts for 30% of the reaction pathway in the absence of LiClO,. Addition of 0.02M-LiC10 causes an increase in retained acetate to ca 48% on account of stabilization of the cation as depicted in (7). PhCHzCHMe 1 -0Ts OTs H-OS (6) (7) Analysis of crystal structure parameters of S-methylmethionine chloride hydro- chloride (8) shows that the 0 --C distance is 2.97 % and the S-C...O angle is 143"; in basic solution homoserine lactone (9) is readily formed from S-methyl- methionine.16 The relatively short C..-0contact is regarded as an incipient stage of an exocyclic nucleophilic displacement. 14 A. R. Katritzky G. Musumarra K. Sakizadeh S. M. M. El-Shafie and B. Jovanovic Tetrahedron Lett. 1980,21,2697. L. S. Miller D. Zazzaron J. J. Dannenberg R. Metras and M. Gillard J. Org. Chem. 1980 45 641. l6 D. Britton and J. D. Dunitz Helv. Chim. Actu 1980 63 1068. D. G. Morris The stereochemistry of replacement at the y-carbon of 0-succinylhomoserine (10)during conversion into cystathionine (12) by cystathionine y-synthetase occurs with retention of configuration at the y-carbon (C-4).l7 The reaction is mediated by syn-elimination of P-HR and y-0-succinyl to give (11).c0,-(10) A diminution in rate constant of 3.5 x lo3has been found for an a-cyano-group from consideration of solvolyses of 2-propyl sulphonates." This value arises since the a-cyano-group though inductively strongly destabilizing toward formation of a carbo-cation can provide a countervailing mesomeric stabilization. A P-cyano substituent is capable of causing a rate retardation of lo4-10'; here the possibility of mesomeric stabilization of positive charge is denied to the cyano-group. The mesomeric effect of the cyano-group in a position a to a positive charge is significantly diminished when charge delocalization is extensive in the transition stage for ionization while the inductive effect remains strong.Thus the value obtained for the relative rate constants kH/kCNfor (13) and (14) is ca lo6 and represents a minimal value for the inductive effect of an a-cyano substituent in an ionization reaction. NC OTs CF3IAr-C-OTs I CH3 (13) (14) (15) The solvolyses of substrates such as (15) have been executed in 80% aqueous ethanol in which there is insignificant solvent participation." The compound (15; Ar=Ph) is even less reactive than benzyl tosylate. When Ar in (15) has activating substituents a plot of the logarithm of the relative rate constants versus (T+ gives a slope p+ = -8.82 that is the most negative solvolytic value hitherto recorded and which is indicative of intense electron demand at the reaction centre.l7 M. N. T. Chang and C. Walsh J. Am. Chem. SOC.,1980,102,7368. P. G. Gassmann and J. J. Talley J. Am. Chem. Soc. 1980,102 1214. l9 K-T. Liu and C-F.Sheu Tetrahedron Lett. 1980 21 4091. Reaction Mechanisms -Part (ii)Polar Reactions A highly stereoselective though not apparently stereospecific route to secondary alkyl bromides has been described,20 and it is outlined in Scheme 2. Stereoselec- tivities in excess of 90% are cited. R2. i or ii RZ iv R2 ,qOH -PhSe RH Reagents i PhSeCN Bu,P; ii MeS0,Cl; iii PhSeNa; iv Br, NEt Scheme 2 A mechanistic bifurcation has been reported from the reaction of the selenonium ion (16).With potassium t-butoxide a mixture of selenides (17) and (18) together with methyl and benzyl t-butyl ethers was formed; these products arise from attack on the selonium salts rather than iodides.’l However with sodium hydride in liquid ammonia appreciable quantities of the product of [2,3] sigmatropic rearrangement were observed consequent upon the formation of the ylide (19) from (16) and (16) (19) The enzyme-catalysed cyclization of specifically labelled copalyl pyrophosphate (20) a frequently encountered step in the biogenesis of (inter alia) ent- sandarocopimaradiene has been investigated during incubation of (9-[1-2H1]-geranylgeranyl pyrophosphate precursor.22 The incubation produced (E)-[16-2Hr]-ent-sandarocopimaradiene(21)as a consequence of SN’cyclization occurring with anti stereochemistry.Examples of both syn and anti SN’cyclizations have been reported. ‘H 2H ‘H A stereoelectronic effect which finds analogy in the anomeric effect of acetals has been proposedz3 from the reaction of iodide ion with cyclic dioxenium salts e.g. (22) which are planar and which exist in the (2)form shown. From (22) 29% of lactone (23) together with an unexpected product the iodo-ester (24) are formed. In the pathway indicated in (22) the C-5-0-2 bond that is being broken is anti-periplanar to the c-1-0-1bond; thus the electron pair of the c-5-0-2 bond can be delocalized via interaction with a T* antibonding orbital of the C-1-0-1 polar bond. Such an energy-lowering interaction is denied to the lactone pathway where by contrast the bond being broken is anti-periplanar to the C-1-C-2 bond.2o M. Sevrin and A. Krief J. Chem. SOC.,Chem. Commun. 1980 656. 21 P. G. Gassrnann T. Miura and A. Mossrnan J. Chem. SOC.,Chem. Commun. 1980 558. 22 K.A. Drengler and R. M. Coates J. Chem. SOC.,Chem. Commun. 1980,856. 23 N.Beaulieu and P. Deslongchamps Can. J. Chem. 1980,58 164. D. G.Morris 3 Carbo-cations The norbornyl cation and its derivatives continue to provide good copy and two novel experiments concerned with this ion have been described recently. A calorimetric determination of the heat of isomerization of the 4-methyl to the 2-methyl 2-norbornyl cation [(25) -+(26)] in S02C1F-SbF5 has been madez4 and compared with the heat of isomerization of the s-butyl to the t-butyl cation.No neutral molecules are involved in the comparison(s); in general contributions of the initial state to the free-energy diagram can bedevil attempts to interpret rates of solvolysis or heats of ionization. At -100 "C both of the carbo-cations (25) and (26) can be generated from the respective chlorides whereas at -55 "C the chlorides are immediately converted into the 2-methylnorbornyl cation (26). For the secondary chloride the heat of isomerization of (25) to (26) is the difference between the heat of ionization at -100 "C and that at -55 "C. A value of 6.57*0.41 kcal mol-' was found as compared with a 'normal' value of 14.20 f0.6 kcal mol-1 for the isomeriz- ation of s-butyl to t-butyl cation. The difference between these values i.e. 7.5 kcal mol-' has been ascribed to the special stabilizing feature enjoyed by secondary norbornyl cations in the 'most compelling piece of evidence yet pre~ented'.~~ Substitution by deuterium perturbs the functional symmetry that is brought about by rapid degenerate rearrangement in a carbo-cation.Thus perturbation occurs in the averaging of the I3C n.m.r. absorptions of those carbons which are interchanged by rearrangement such that large splittings are observed in the averaged resonances. At -150 "C an upper limit of 2.3 p.p.m. is put on isotope splitting for (27) (shown as a classical ion for convenience) since for C-1 and C-2 WIl2=2.3 p.p.m.; peak areas show that deuterium is not scrambled. A static symmetrical structure is consistent with the n.m.r. evidence which is not in accord with a rapidly equilibrating species.25 As a control to allow the validity of the method to be assessed the absorptions of C-1 and of C-2 in (28) which exists as a rapidly equilibrating pair of cations show a splitting of 105 p.p.m.at -130 "C whereas C-1 and C-3 are split by only 0.33 p.p.m. in (29). Interestingly in the 1,2-dimethylnorbornyl cation (30) which is taken to be partially delocalized a corresponding splitting of intermediate value (23.9 p.p.m.) is obtained. 24 E. M. Arnett N. Pienta and C. Petro J. Am. Chem. SOC.,1980 102 398. *' M. Saunders and M. R. Kates J. Am. Chem. Soc. 1980 102,6867. Reaction Mechanisms -Part (ii) Polar Reactions The rate of solvolysis of (31) is 3.5~10~ times faster than that of t-butyl p-nitrobenzoate in 80% aqueous dioxan thus making it the most reactive saturated tertiary derivative known to date.26 The products are unrearranged alcohol and olefin.The high solvolysis rate has been attributed to relief of severe non-bonded interactions between the inner protons of the ethano bridges. This endo-side congestion of (3 1)is diminished in the benzo-fused analogue (32) which accordingly solvolyses less frenetically. Thermodynamic and spectroscopic properties of 2-bicyclo[2.1. llhexyl cations are intermediate between those of 2-norbornyl and cyclopentyl cations; thus the difference in free energy between ions (33) and (34) is 7.0-9.8 kcal mol-' as compared with values of 5.5 and 12 kcal mol-' respectively for norbornyl and cyclopentyl analog~es.~' The I3C chemical shifts for the positively charged carbons indicate an intermediate degree of charge delocalization for (33) [S(C') = 322.01 2-methylnorbornyl cation [270.2] and methylcyclopentyl cation [336.7 p.p.m.1 with the proviso that the correlations hold between chemical shift and charge density.An unsymmetrically bridged ion (35) has been proposed in which the distance C-1-C-6 >C-2-C-6. Further bond shifts are considered to be a requisite of the symmetrical n.m.r. spectrum; thus the ion must undergo a very rapid shift of C-2-C-6 -+ C-2-C-5 and a degenerate Wagner-Meerwein shift that interchanges the bonds C-1-C-6 and C-2-C-6. (33) (34) (35) In their investigations on deuterium isotope effects on carbo-cations Saunders and Kate~~~ concluded that the bicyclo[2.1.llhexyl cation had a static bridged structure on account of a deuterium-induced splitting of 1.18 p.p.m. between C-1 and C-2 at -150 "C and that the cation should be represented as (36). Formolysis of specifically labelled cyclopent-3-enyl tosylate proceeds with >99'/0 retention of configuration as the result of participation of the double-bond to the exclusion of any competitive participation by the solvent.*' This r-participation in the ionization process has not been picked up in previous studies of products or 26 L. A. Paquette K. Ohkata and R. V. C. Carr J. Am. Chem. Soc. 1980,102,3303. 27 L. R. SchmitzandT. S. Sorensen J. Am. Chem. SOC.,1980,102,1645. 28 J. B.Lambert R. B. Finzel and C. A. Belec J.Am.Chem. Suc. 1980,102 3821. D. G. Morris rates; the stereochemical analysis is thus more sensitive rate enhancements not being obligatory for participation. The tricyclopropylcyclopropenium ion (37) has been prepared and is one of the most stable hydrocarbon cations known presumably on account of a bisected conf~rmation.~~ Potentiometric titration in 50% aqueous MeCN indicated a pKR+ value of 1O.Ok 0.3; a-conjugation with cyclopropyl groups is thus more effective than wconjugation with phenyl groups or the inductive effect of alkyl groups in stabilizing the cyclopropenium ior,. A series of 2,5-diaryl-2,5-norbornyldications (38) has been prepared; their remarkable stability is achieved through charge dispersal into the aryl rings thereby decreasing charge-charge The positively charged carbons C-2 and C-5 are shielded by ca 15p.p.m.from the corresponding carbon in the monocation. A plot of S(C-1) vs. S(C-3) is linear in the dications (38) indicating a similar response at these carbons to change in the substituent on the aromatic ring. Such is not the case in 2-aryl-2-norbornyl cations where departure from linearity is observed for electron-withdrawing substituents; this is in accord with the incursion of non-classical delocalization since the aromatic ring is now less able to stabilize positive charge. Diprotonation of the 1,6-methano-[ lO]annulene system (39) with magic acid leads to formation of the stable dication (40) in which two allylic cations are bound to the same carbon of a cyclopropane ring.31 A preference exists for (40) over (41) [from which (40) (with its C,symmetry) can be readily distinguished] on account of the cyclopropane-enhanced homoconjugation between the two allylic cationic centres in (40) the conjugation between the allylic cation centres in (41)being very poor.(39) (40) (41) 2-Adamantyl ONN-azoxytoluene-p-sulphonate(42) has been prepared and the activation parameters have been determined from solvolysis in a number of solvents e.g. aqueous triflu~roethanol.~’ Parameters in accord with S,l reactions were observed and the authors claim that a deaminative fragmentation is for the first time amenable to kinetic analysis. The products are alcohols and ethers of un-rearranged structure. 29 K. Komatzu I.Tomioke and K. Okamoto Tetrahedron Lett. 1980 21 947. 30 G. A. Olah G. K. S. Prakash andT. N. Rawdah J. Am. Chem. SOC.,1980,102,6127. 31 K. Lammertsma and H. Cerfontain J. Am. Chem. Soc. 1980 102 3257. 32 H. Maskill P. Murray-Rust J. T. Thompson and A. A. Wilson J. Chem. Soc. Chem. Commun. 1980 788. Reaction Mechanisms -Part (ii) Polar Reactions 4 Carbanions Ab initio MO calculations of XCH2- in which X was Li BeH BH2 CH3 NH2 OH and F have been carried out. The ion H2C=BH2- is calculated to have a planar structure in its most stable form with a very short (1.44A)carbon-boron bond. Also CH2Li- is indicated to have a planar C2"structure (43),which is stabilized with respect to CH3- the stabilization being provided (rather unexpectedly) by an alkali metal of low electronegativity and this has been attributed to the p-orbitals on The p-orbitals of sodium are of much higher energy than those of lithium and accordingly the ion CH2Na- is massively destabilized (by cu 100kcal mol-') with respect to CH3-.Both CH2Na-.and CH2F- have been calculated to be pyramidal and in the latter case the withdrawal of a-electrons by the electronegative fluorine and four-electron 7-destabilization are in opposition; the importance of the withdrawal of cr-electrons is manifest in the stabilization energy which is the largest for electronegative substituents. Protons can be abstracted from Bu'NO and Bu'CHO in the gas phase with unexpected ease.34 The (M-H)- ions of Bu'NO and Bu'CHO are stabilized by 25-37 kcal mol-' with respect to that derived from methane (AHacid = 416kcal mol-').The gas-phase acidities of Bu'NO and Bu'CHO are intermediate between those of water and methanol. The authors attribute a large part of the stabilization of the anion derived from 2,2-dimethylpropionaldehydeto interaction between the charge in an sp3-hybridized orbital and the dipole moment of the carbonyl group (44).Polarizability and hyperconjugation effects may also contribute to the stabilization energy; homoconjugation may occur but this could lead to cyclopropoxide anions. However this isomerization (if occurring at all) must on the basis of 'H-'H exchange experiments be very slow. H. ,C\ 'c /c=o / Q H/ H (44) 7-Phenylnorbornyl carbanions with Li K (and Cs) as counter-ions have been prepared in [2H8]THF.35Carbon-13 chemical shifts for C-7 and the pura carbon differ by 30 p.p.m.according as to whether Li'or K' is the counter-ion. The authors consider that the 7-phenylnorbornyl anion is intrinsically planar but that when the anion is co-ordinated to the strongly polarizing lithium cation the Coulombic stabilization increases electron localization at C-7. Thus a tetraco-ordinate carbon 33 T. Clark H. Korner and P. von R. Schleyer Tetrahedron Lett. 1980,21,743. 34 A. J. Noest and N. M. M.Nibbering J. Am. Chem. SOC.,1980,102,6427. " P. R. Peoples and J. B. Grutzner J. Am. Chem. SOC.,1980,102,4709. 46 D. G. Morris is formed yielding a pyramidal organolithium compound that is capable of inversion the rate-determining step of which is ionic dissociation with a magnitude AG&s = 9.4*0.2 kcal mol-'.The 13Cn.m.r. spectra of the K and Cs salts show little temperature variation and are consistent with a symmetrical planar benzylic carbanion. Consideration of other literature together with these results indicates that a simple carbanion e.g. CH3- has an inversion barrier of <5 kcal mol-'. By means of cyclic and second-harmonic a.c. voltammetry the electrochemical oxidation potentials have been determined for a series of organolithium compounds in HMPA;36 from these data the bond-dissociation energies and the pKa values of Ph3CH (the ion-pair acidity) are estimable. The authors claim that the electro- chemical thermodynamic method where applicable is probably the most re-liable way for estimation of pKa values of very weak acids.The ally1 anion [pKa(R-H) =47.1-48.01 is reasonably more basic than the benzyl anion [pKa(R-H) =44.2-45.41. Corresponding values of 44.2-45.4 were found for t-butylpropargyl anion (though this value may need modification on account of overpotential) and 22.2-23.4 for cyclopentadienyl anion. The latter value is not unreasonable for the solvent THF with'30'h v/v HMPA though rather higher than the figure of 16 that was obtained in water. A rather stronger acid 5H-perfluoropentamethylcyclopentadiene (45),has been prepared by Lemal's group.37 In water this compound is readily soluble to give the anion (46);this is reprotonated in concentratedH2S04. The PKa of (45)is 4 -2; thus (45) is the strongest acid without conjugating substituents and the introduction of five CF groups is responsible for an acidity increase of at least 18 orders of magnitude.However this consistent acidifying effect of CF is not mirrored by fluorine which stabilizes CH3- (pKa of CF3H is 30.5) while destabilizing by means of electron repulsion the anions of lower energy that are derived from fluorene or nitro-alkanes. Me Me F3C I I o=s s=o I I .-_* F3Cp$ H CF3 CF3 CF3 Me-C-H H-C-Me F3:y:F3 Ph Ph I I (45) A white precipitate was obtained by cooling a solution of the lithiated carbanion from racemic PhCH2S(0)Me in THF to -100°C. After work-up this solid was dispersed at 25 "C and a mixture of nitrogen and methyl iodide was passed over it to give a single diastereoisomer [(R,S)and (S,R),38 (47a) and (47b)l.In the presence of chelating agents such as D20 and ethanol only slight stereoselectivity was retained. Members of a new class of reactive intermediates a-keto-dianions have been prepared. Their formation follows the reaction of lithium enolates of primary or secondary a-bromo-ketones e.g. (48) with t-butyl-lithium to give (49) in which the lithium may well be covalently Consistent with the assigned structure 36 B. Jam J. Schwarz and R. Breslow J. Am. Chem. SOC.,1980 102,5741. 37 E.D. Laganis and D. M. Lemal J. Am. Chem. SOC.,1980,102,6633. 38 J. F.Biellmann J. F. Blanzat and J. J. Vicens J. Am. Chem. SOC.,1980 102,2460. 39 C.J. Kowhlski M. L. O'Dowd M. C. Burke and K. W. Fields J.Am. Chem. SOC.,1980 102 5411. Reaction Mechanisms -Part (ii) Polar Reactions (48) (49) (50) quenching with DOAc-D20 gives a dideuterio-ketone. Unique to a-keto-dianions is the silylation on the carbon atom that was originally adjacent to the keto-group e.g. to give (50). Rearrangement of (51)with potassium t-butoxide in HMPA gave (53) via a [l,21 shift of a methyl group in (52).40This rearrangement represents the first example of base-catalysed migration of an alkyl group from a non-heterosubstituted carbon to an adjacent carbonyl carbon atom. The anion of (51)is a novel member of a class of blocked aromatic molecules which can become aromatic by migration of a single substituent. 0 0- 0 (51) (52) (53) 5 Reactivity of Carbonyl Groups Ab initio MO calculations (4-31G) indicate that the reaction €320 + H2C0-P CH2(OH) has EA= 44.1kcal mol-' and is exothermic by 16.8kcal mol-'.A planar four-membered transition state is impli~ated.~~ A second water molecule was introduced (as a token solvent) and the acidic and basic features of the catalyst were combined to give significant energy lowering at C-0 separations of 1.7A with proton transfer and nucleophilic bond formation occurring simultaneously as shown in (54). (54) The reactions of simple nucleophiles e.g. H- OH- or MeO- with formaldehyde have been allowed to take place at ca 0.5 Torr enabling the first observation of stabilized adduct~.~~ Under the reaction conditions energy which appears as internal hbrations of MeO- after addition is removed by collision with the atoms.At 297 K the two-body rate constant for addition of NH2-to H2C0 is (1.9kO.5)~ cm3 molecule-' s-'. Transfer of hydride ion between two carbonyl groups can be affected in opposite ways by the cation and the base depending on whether or not the transition state permits complexation of both oxygen atoms by a single cation.43 The rate of 40 B. Miller and A. K. Bhattacharya J. Am. Chem. SOC.,1980,102 2450. 41 I. H. Williams D. Spangler D. A. Femec G. M. Maggiora and R. L. Schowen J. A.m. Chem. SOC. 1980,102,6619. 42 D. K. Bohme G. I. Mackay and S. D. Tanner J. Am. Chem. SOC.,1980,102,407. 43 E. W. Warnhoff P. Reynolds-Warnhoff and M. Y.H. Wong J. Am. Chem. SOC.,1980,102 5956. 48 D.G.Morris intermolecular hydride transfer from Pr'OM to (53 leading to (56),increases as the Lewis acidity of the cation increases i.e. AI3+> Li' > Ba2' > Na' > K' and thus decreases with increasing basicity of the medium; this is in accord with the presence of a cyclic transition state. Concurrently the rate of intramolecular transfer of hydride increases in essentially the reverse order as is also the case for acyclic ketones which are mediated by an ionic transition state (58). The reversal of order is a function of how the oxygens are incorporated into the transition state for hydride transfer. A cyclic transition state (57) either intra- or inter-molecular will be favoured by the better Lewis acid. However hydride transfer is preferred by greater negative charge on oxygen i.e.by the poorer Lewis acid when as in the intramolecular reaction of (53 a cation-bridged transition state is not possible. .M.. ,li O'' .?/\ Px OH 239. ,,@-\/ c...HJ\ 0P (55) OH HO(56) P (57) (58) The sodium alkoxides derived from the ketols (59)-(61) undergo 1,4 hydride transfer with (61) rearranging lo0.'times faster than (60),which in turn rearranges a102 times faster than (59).44These rate sequences correspond to the separation between carbonyl carbon and migrating hydrogen. (59) n = 1; (60) n = 2 (61) n = 3 Monomeric methyl metaphosphate MeOP02 prepared and then allowed to react in situ with acetophenone gives an enol phosphate via (62),which is formed by direct attack on carbonyl oxygen; in (62) the carbonyl group is now activated for nucleophilic The hydrolysis of phosphate esters including ATP and other high-energy phosphates occurs uia a monomeric metaphosphate or by a kindred mechanism in solution and the idea has been floated that enzyme-catalysed phosphoryl transfers could be mediated by a monomeric metaphosphate or by a related mechanism.This would ascribe a kinetic role to ATP in addition to its better-known thermodynamic properties. One experiment is cited where carbonyl oxygen is transformed into inorganic phosphate during the conversion of C=O into C=N by ATP-dependent amidotransferases. Whereas in competitive experiments the value of AAG* for the reactions of PhCHO and PhCOMe with MeLi is <1 kcal mol-' the reagent (Me2CH0)3TiMe is appreciably more selective with AAG* 'approximately one order of magnitude larger' and the aldehyde being the more reactive.46 A variety of other functionalities e.g.ester epoxide and nitrile are tolerated. 44 G.-A. Craze and I. Watt J. Chem. SOC.,Chem. Commun. 1980 147. 45 A. C. Satterthwait and F. H. Westheimer J. Am. Chem. SOC.,1980 102,4464. 46 B.Weidmann and D. Seebach Helv. Chim. Acta 1980,63 2451. Reaction Mechanisms -Part (ii) Polar Reactions 6 Nucleophilic Addition to Olefins In (63) nucleophilic addition readily takes place to the unactivated do~ble-bond.~' The ready formation of (64) has been attributed to steric compression. Nucleophiles add readily to an olefin that is co-ordinated to a transition-metal centre and electrostatic forces provide only slight activation; the role of the metal acting as electron-attractor and back-donator is ambigu~us.~' Calculations indicate that as the ligand ML is displaced by an amount A [as in (65)] an energy lowering of the LUMO occurs and the LUMO becomes concentrated on the remote carbon C-a.A carbanionic intermediate has been proposed for the first time in the substitution of an activated halogeno-olefin (66) by a non-amine nucleophile; this stereoconver- gence is not the result of prior equilibration of the ~lefin.~' 7 Esters and Carbodi-imides On the basis of orbital-steering postulates the rates of lactonization of (67) and (68) should differ by ca lo4 on account of a difference of 10" in the angle 0-1-C-2-C-3 as calculated by a force-field method.However the relative rate constants for acid-catalysed lactonization were 1:1.2 thereby refuting the above p~stulate.~' The derived lactones showed Av(C0) = 7 cm-' which indicates that there are only slight differences in strain energy and thus renders the substrates valid for comparison. (67) (68) Exceptional micellar stereoselectivity is shown in the cleavage of diastereoisomeric dipeptide esters (69). Solubilization of (69) in (70) gives very large rate enhancements relative to cetyltrimethylammonium chloride together with marked selectivity for cleavage of the ~,~-substrates." Models suggest that 47 R. A. Pfund W. B. Schweizer and C. Granter Helv. Chim. Acta 1980,63,674. 48 0.Eisenstein and R. Hoffrnann J. Am. Chem.SOC.,1980,102,6148. 49 Z. Rappoport and A. Topol J. Am. Chem. SOC.,1980,102,406. F. M. Menger and L. E. Glass J. Am. Chern. SOC.,1980,102 5404. '' R. A. Moss,Y-S. Lee and K. W. Alwis J. Am Chem. Soc. 1980,102,6646. 50 D. G. Morris PhCH2O-C-NH-CH-C-N It I * 0 H C-0002 c1-! 0 n-C16H33Nf(Me)2CH2CH2SH [(Z)-Ala-Pro-PNP] (69) R = Me (70) uniquely for L,L-substrates the methylene chain of (70) fits into the clefts defined by the Pro and PNP moieties and the R group of the variable amino-acids such that the -CH2CH2S-functionality is ideally located behind and above the apposite carbonyl group. This arrangement is also beneficial in that the hydrophobic groups are located in the micellar interior whereas the carbonyl oxygens and the aromatic rings of PNP interact in favourable electrostatic manner with the quaternary nitrogen atom of the surfactant.The rate constant for alkaline hydrolysis of (71) is 5 x lo5 times greater than that for (72) via formation of a lactone which at alkaline pH is further hydr~lysed.~~ From comparison of the intramolecular first-order rate constant of (71) with the intermolecular second-order rate constant for (72) an effective concentration of OH-of ca 5 X lo7moll-' is given as that concentration of OH- which is required for the intermolecular reaction to proceed at the same rate as its intramolecular counterpart. This translates to a corresponding value of ca lo8moll-' for alkoxide and has been ascribed to the large loss of translational and rotational entropy that is associated with those concerted reactions for which there is a negligible contribu- tion from solvation effects.Protonated intermediates HC(OH)2&H3have been considered as models for acid-catalysed decomposition of the tetrahedral intermediate in the hydrolysis of amide~.~~ Protonation leads to lengthening and weakening of the C-N bonds particularly [e.g. see (73)] where the C-N bond is anti-periplanar to two lone pairs. A more muted and opposite effect is indicated for the C-0 bonds. The largest stereoelectronic effect comes from an interaction between a lone pair on oxygen and a cr* orbital of CN'. Base-catalysed hydrolyses have also been con- sidered with less emphatic conclusions. A H Acylation of the water-soluble carbodi-imide (74) shows a Brgnsted slope of -0.67 and embracing a large pK range which is consistent with concerted attack by -0Ac and general acid catalysis (see Scheme 3).54 52 J.J. Morris and M. I. Page J. Chem. SOC.,Perkin Trans. 2 1980 679. '' J. M. Lehn and G. Wipff J. Am. Chem. SOC.,1980,102,1347. 54 I. T. Ibrahim and A. Williams J. Chem. Soc. Chem. Commun. 1980.25. Reaction Mechanisms -Part (ii) Polar Reactions Et\ AcO-C Et AH N N H~A112 It N --* AcO‘+NR EtNH I -* +NHR \ R + (74) R = CH2CH2CH2NMe3 Scheme 3 8 Elimination Reactions The reaction of (75) with -0Bu‘ in Bu‘OH gave exclusively 1-chloroacenaphthylene via an irreversible Elcb mechanism.” The loss of fluoride which is formally the poorer leaving group occurs since the carbanion (76) is much the more stable of the two possibilities.However the difluoro-analogue of (75) probably also reacts via an (Elcb) mechanism. Hno + R2 R’ H\_/Co2-R2 R’ (77) (78) (75) The 3-deprotonated oxetanones (77) are unexpectedly stable to elimination on account of an orthogonal disposition of the relevant orbitals so much so that (77) can be intercepted by ele~trophiles.’~ However elimination does proceed (albeit lethargically) to give acrylic acid anions (78). The primary process in elimination reactions of N-[P-(p-nitrophenylethy1)Iquin-uclidinium ions e.g. of the chloride (79) with aqueous hydroxide ions is proton transfer with little driving force from explusion of the leaving group.57 Although an E2 mechanism cannot be excluded values of p and isotope effects indicate a transition state in which the proton is ca two-thirds removed with the bond to quinuclidine unchanged except for expectations based on the formation of a carb- anion on a P-carbon atom.CI -(79) On treatment with NH2-in liquid ammonia 3-phenylpropyl iodides and bromides underwent p -elimination the chloride a mixture of p-and y-eliminations whereas the fluoride and tosylate underwent y-eliminati~n.~~ The position of tosylate is anomalous in that &elimination was expected. Moreover the strong base was also removing the methyl protons of the group; thus only 20% of phenylcyclopropane 55 E. Baciocchi R. Ruzziconi and G. V. Sebastiani J. Chem. SOC.,Chem. Commun. 1980 807. 56 J. Mulzer and T.Kerkmann J. Am. Chem. SOC., 1980,102,3620. ” S. Alunni and W. P. Jencks J. Am. Chem. SOC.,1980,102,2052. C. L. Bumgardner J. R. Lever and S. T. Purrington J. Org Chem. 1980,45,749. D. G.Morris was formed. Similar treatment of 3-phenylpropyl benzenesulphonate however gave only substitution products. Buffered solvolyses (in 80% aqueous ethanol) of a number of deuteriated norbor- nyl derivatives exemplified by (80) and (81) under El-like conditions gave a stereoselectivity for loss of the endo-proton that is attached to C-6 over loss of its exo-counterpart in the range of 10 1 to 15 :1 in the formation of 7-halogeno- nortricyclenes by 1,3-eliminati0n.~~ A new method has been proposed for distinction between (Elcb) and E2 mechanisms based on a linear free-energy relationship between rates of elimination (k,)and Taft's polar substituent parameter (u"), together with leaving-group ability (L),which is derived from 9-(X-methyl)fluorene as a standard system.60 In equation (2) 1 (the sensitivity of the system to change of leaving group) monitors a change in mechanism insofar as 1 measures the slope of data that are treated graphically when an Elcb mechanism holds and the incursion of an E2 pathway is manifest in curvature in the graph that may be drawn.log(k,/k,0) =p*a" +1L (2) 59 N. H. Werstiuk F. P. Cappelli and G. Timmins Can. J. Chem. 1980 58 2093. 6o A. Thibblin Chern. Scr. 1980,15 121.

ISSN:0069-3030    

DOI:10.1039/OC9807700037

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (iii) Free-Radical Reactions By R. A. JACKSON School of Molecular Sciences University of Sussex Brighton BN 1 9QJ 1 General After a gap of five years Volume 6 of ‘Advances in Free Radical Chemistry’ has appeared,’ with a new publisher and articles on radical rearrangements polar effects halogenation of cycloalkanes acyl-aryl-nitrosamines and phosphoranyl radicals. Volume 4 of ‘Free Radicals in Biology’ provides further reviews2 in this important interface area. Useful accounts of ipw-s~bstitution,~ electron spin resonance and physical-organic ~hemistry,~” ‘free radical locks',^' and the decomposition of azoalkanes have appeared,’ and further reviews are found in the report OR a Free Radical Symposium held at Baton Rouge6 in 1979.A compilation of e.s.r. para- meters for non-carbon-centred radicals has appeared in the Landolt-Bornstein ~eries.~ Onthe whole 1980has seen steady advances rather than spectacular discoveries but of more than usual interest are some guidelines for chemical reactivity which emphasize stereo-electronic effects suggested by Beckwith Easton and Serelis.8 A number of reactions of potential synthetic value are considered in the section on fragmentation. 2 Structural Studies Electron spin resonance studies dominate this area. Hydrogen abstraction by photolytically produced t-butoxyl radicals is commonly used to produce radicals for structural studies. In some systems stronger signals are obtained if p-methyl- acetophenone is used as a sensitizer for the photolysis of the t-butyl peroxide.’ There has been a continuing interest in persistent radicals.The sterically shielded alkoxyaminyl radical Bu‘ONBU‘ can be prepared by photolysis of the parent hydrox- ‘Advances in Free Radical Chemistry’ Vol. 6 ed. G. H. Williams Heyden London 1980. ‘Free Radicals in Biology Vol. 4’ ed. W. A. Pryor Academic Press New York 1980. M. Tiecco Acc. Chem. Res. 1980,13 51. D. Griller and K. U. Ingold Acc. Chem. Res. 1980,13 (a)p. 193; (b)p. 317. P. S. Engel Chem. Rev. 1980,80,99. ‘Frontiers in Free Radical Chemistry’ ed. W. A. Pryor Academic Press New York 1980. ‘Landolt-Bornstein. New Series Group 11 Vol. 9 Part c2. Organic 0-,P- S- Se- Si- Ge- Sn-,Pb- As- Sb-centered Radicals’ by A. G. Davies J. A. Howard M. Lehnig B.P. Roberts H. B. Stegman and W. Huber Springer Verlag Berlin 1979. A. L. J. Beckwith C. J. Easton and A. K. Serelis J. Chem. SOC.,Chem. Commun. 1980,482. D. Griller K.U. Ingold and J. C. Scaiano J. Mugn. Reson. 1980 38 169. 53 54 R. A. Jackson ylamine in sufficient concentration for the hyperfine splittings due to the "0 that is present in natural abundance to be determined." By use of the very bulky groups R=CH(SiMe3)2 or N(SiMe3I2 radicals of the type *PR2 and -AsR2 have been obtained." The radical *P[CH(SiMe3)2]2 has a half-life >1 yr at room temperature in toluene solution. At the other extreme spin trapping continues to be used for radicals which cannot be observed directly. The azidyl (Ng a) cyanatyl (OCN-) and cyanyl CN) radicals and chlorine atoms have been trapped by a-pheny1-N-t- (a butylnitrone.12 The most recent contribution to the controversial question of the shape of the t-butyl radical is an ab initio calc~lation'~ which suggests a pyramidal geometry with an out-of-plane bending angle of 10".In another controversial area that of radical cations derived from alkanes strong evidence for the formation of [Me3CCMe3]t has been obtained by y-irradiation of the alkane in a glassy Freon solution. A septet of a =29.0 G was interpreted in terms of a structure with staggered conformations about all the C-C bonds and a strong interaction with one (axial) proton from each methyl group.14 The conformations of substituted cycloalkyl radicals (1; n = 3-6) have been studied by adding triethylsilyl radicals to the appropriate methylenecycloalkane.l5 For the substituted cyclohexyl radical (1; n = 4) at -120"C couplings to four P-protons were observed i.e. two axial (38.5G) and two equatorial (6.25 G). At +120 "C these protons became equivalent and a value of AG' of 22 kJ mol-' was obtained from a computer simulation of the lineshape. For the smaller ring sizes the P-protons were all equivalent. It was suggested that the substituted cyclopentyl radical is probably non-planar whilst the substituted cyclobutyl radical is essentially planar. (CH2)" (CH,) SiEt +*SiEt3 -* (1) Triarylsilyl radicals pose the problem of the extent of conjugation of the free electron on the silicon atom with the aromatic n-system but such radicals have not hitherto been observed probably because they appear to be very reactive in aromatic substitution reactions.This year two groups have solved the problem. Tris-(3,5-di-t-butylphenyl)-silyland -germyl radicals have been prepared16 by abstraction of hydrogen from the corresponding silane or germane and trimesityl- silyl -germyl and -stannyl radicals (2,4,6-Me3C6H2)3M have been made17 by U.V. irradiation of the triarylmetal chloride with an electron-rich olefin [RNCH2CH2NRC=I2. The coupling constants of the hydrogen atoms of the ring are smaller than in the analogous triphenylmethyl radical indicating reduced but lo H. Woynar and K. U. Ingold J. Am. Chem. SOC., 1980,102,3813. I' M. J. S. Gynane A. Hudson M. F. Lappert P. P. Power and H.Goldwhite J. Chem. Sac. Dalton Trans. 1980 2428. '' W. Kremers and A. Singh Can. J. Chem. 1980 58 1592; E. G. Janzen H. J. Stronks D. E. Nutter Jr. E. R. Davis H. N. Blount J. L. Power and P. B. McCay ibid. p. 1596. l3 R. E. Overill and M. F. Guest Mol. Phys. 1980.41 119. l4 J. T. Wang and F. Williams J. Phys. Chem. 1980,84 3156. Is L. Lunazzi G. Placucci and L. Grossi J. Chern. Soc. Perkin Trans. 2 1980 1761. l6 H. Sakurai H. Umino and H. Sugiyama J. Am. Chem. SOC.,1980,102,6837. 17 M. J. S. Gynane M. F. Lappert P. I. Riley P. Rivikre and M. Riviere-Baudet J. Organornet. Chem. 1980 202 5. Reaction Mechanisms -Part (iii) Free-Radical Reactions 55 significant delocalization. The value of ~(~'si) in (2,4,6-Me3C6H2),Si* is 135 G; the reduction from the value of -182 G that is found in Me&* indicates some delocalization of the unpaired electron or flattening of the pyramidal structure or both.Photolysis of the phenylazo aryl sulphide PhN=NSC6H4Bu'-p gave an e.s.r. spectrum attributed to the phenyldiazenyl radical PhN=N* with couplings a(N)' = 23 G a(N)* = 9.4 G and aH = 1.1G (two meta protons) and g = 2.0006. The data" support a a-radical structure (as in the benzoyl radical) with free rotation round the Ar-N bond. A study of the e.s.r. spectra of amidyl radicals at different temperatures indicates a wstructure for these radicals;'9a in a contribution to the question of the possibility of two different structures for succinimidyl radicals,lgb the ground state is predicted to be the v-form by a UHF calculation.Sulphur-centred radicals show increasing variety." Sulphinyl (RSO *) and sul- phony1 (RS02 *) radicals are implicated in the anti-oxidant activity of organic sulphides and have been prepared for e.s.r. structural studies by abstraction of chlorine from sulphonyl chlorides by triethylsilyl radicals or by thermolysis or photolysis of sulphoxides and thiolsulphonates ArS02SAr. Addition of CF3S radicals to dialkyl sulphides R,S gives sulphuranyl radicals R2SASCF3 in which the electron is thought to be in the a*-orbital:*l analogous selenuranyl radicals,22 e.g. R2Se'OBu' can be prepared by photolytic production of Bu'O. in the presence of R2Se. The cyclic sulphur radical (3) can be made by photolysis of the peroxide (2) a new system of nomenclature for radicals of this type has been by which the radical (3) would be 9-S-3 (nine electrons centred on sulphur with three bonds).Me 0 + Bu'O* 0-OBu' 0 0 3 Formation Destruction and Radical Stability Thermolysis of suitable organic compounds provides information about stabilization in the radicals formed on the assumption that E = AH for the homolysis. Ther- molysis of hexa- 1,3-diene gives methyl and 2,4-pentadienyl radicals a resonance energy of 77 kJ mol-' for the latter radical was derived.24 In a very-low-pressure pyrolysis (VLPP) study the effects of ring-methyl substituents on the rate of ArCH2-CH3 bond scission were determined.2s The activation energy was lowered T. Suehiro T. Tashiro and R. Nakausa Chem.Lett. 1980 1339. l9 (a)J. Lessard and K. U. Ingold J. Am. Chem. SOC.,1980,102 3262; (b)Y. Apeloig and R. Schreiber ibid. p. 6144. 20 C. Chatgilialoglu B. C. Gilbert B. Gill and M. D. Sexton J. Chem. SOC.,Perkin Trans. 2 1980 1141; C. Chatgilialoglu B. C. Gilbert and R. 0.C. Norman ibid. p. 1429. 21 J. R. M.Giles and B. P. Roberts J. Chem. SOC., Perkin Trans. 2 1980 1497. J. R. M. Giles B. P.Roberts M. J. Perkins and E. S. Turner J. Chem. SOC.,Chem. Commun. 1980,504. 23 C. W. Perkins J. C. Martin A. J. Arduengo W. Lau A. Alegria and J. K. Kochi J. Am. Chem. SOC. 1980,102,7753. 24 A. B. Trenwith. J. Chem. SOC.,Faraday Trans. 1 1980,76,266. 25 B. D. Barton and S. E. Stein J. Phys. Chem. 1980,84 2141. 56 R. A. Jackson by 1.3-1.7 kJmol-' for m-and p-methyl groups and by 5.0-6.3 kJ mol-' for o-methyl groups the larger value being ascribed to a partial relaxation of steric strain in the transition state.The same technique when applied to l-ethylnaph- thalene and 9-ethylanthracene gives values for the extra delocalization energy of the 1-naphthylmethyl and 9-anthrylmethyl radicals over that of the benzyl radical as 19 and 35 kJ mol-' respectively.26 The P-keto-diazene (PhCOCMe2N=)2 has been synthesized and found to decompose 1.1~10~ times faster at 100 "C than the corresponding compound in which the benzoyl groups have been replaced by methyl gro~ps.~' The effect was attributed to the stabilizing effect of the benzoyl group on the incipient radical the order of effectiveness of substituents in speeding up decompositions in compounds (RCMe2N=)2 is Me < OMe < C1 < CN < COPh < Ph < CH=CHZ.In studies of the thermolysis of highly substituted alkanes (R'R2R3C-)2 [R # HI it was concluded that homolysis of the central bond was rate-determining and from a comparison of AG'(300 "C) with strain enthalpies for the compounds obtained from force-field calculations it was concluded that about 40% of residual strain is still present in the transition state of these C-C cleavage reactions.28 The thermal decomposition of azoalkanes shows certain anomalies when com- pared for example with decomposition of peresters. In particular the relative rate of decomposition of RN=NR for R = 1-adamantyl compared with R = t-butyl is only 0.0004 even though other evidence points to the 'normal' nature of the 1-adamantyl radical.To take account of this and other evidence it has previously been suggested that azo-compounds do not decompose in a 'least motion' process. Firestone2' suggests that this process involves a Linnett-type transition state in which the electrons are removed from the breaking bond one at a time with the temporary development of charge during the reaction the transition state being of the type 66+ a+ 6-R -N=N--R'. This fits in with the speeding-up effect of electron-withdrawing substituents on such decompositions and thus one should be cautious about attribut- ing differences in rates of decomposition of azo-compounds mainly to radical stabilization. Pressure effects have been used as a probe for thermal decompositions.Azocumene decomposes with A V+of +5 ml mol-' with pressure favouring dispro- portionation over radical co~pling.~' The decomposition of liquid bibenzyl to toluene and stilbene shows A V+= +31mi mol-' indicating that there is a transi- tion state involving a very wide separation of the two benzyl radicals and the value of the activation energy is in line with other estimates of the stability of the benzyl radi~al.~' A variety of reaction types may be involved in induced decompositions of organic peroxides. For ally1 t-butyl peroxide induced decomposition is responsible for about half the observed rate and studies of the products have shown that both radical 26 D. F. McMillen P. L. Trevor and D.M. Golden J. Am. Chem. Soc. 1980,102 7400. '' R. C. Zawalski M. Lisiak P. Kovacic A. Luedtke and J. W. Timberlake TetrahedronLeft.,1980 21 425. 28 H.-D. Beckhaus G. Kratt K. Lay J. Geiselmann C. Riichardt B. Kitschke and H. J. Lindner Chem. Ber. 1980,113 3441; R. Winiker H.-D. Beckhaus and C. Riichardt ibid. p. 3456. 29 R. A. Firestone J. Org. Chem. 1980 45 3604. 30 R. C. Neumann Jr. and M. J. Amrich Jr. J. Org. Chem. 1980,45,4629. 31 K. R. Brower J. Org. Chem. 1980 45 1004. Reaction Mechanisms -Part (iii) Free-Radical Reactions 57 addition to C=C and abstraction of H from the allylic CHz group take Dibenzenesulphenimide (PhS)2NH induces the decomposition of benzoyl peroxide at room temperature. An intermediate adduct PhSNHS(0Bz)zPh was postu-lated but the precise mechanism of its formation and breakdown (in particular the question of whether electron-transfer reactions are involved) is still not clear.32b Homosolvolysis (radical transfer to a solvent which is itself a stable free radical usually di-t-butyl nitroxide) has been used in an elegant demonstration of capto- dative stabilization of radicals.33 For example BrCH2COZEt (with an acceptor group) is quite inert towards abstraction of the bromine atom by t-butyl nitroxide and BrCH20Me (which provides a donor group only) reacts in about seven hours at room temperature whereas BrCH(OMe)C02Me which contains both a donor and an acceptor group reacts immediately.Although alkyl radicals with @-hydrogen atoms react together in pairs by dispro- portionation as well as by combination it had become accepted that silicon-centred radicals do not undergo disproportionation.That trimethylsilyl radicals do indeed disproportionate has now been established by three independent groups.34 Trimethylsilyl radicals were generated in the gas phase and in solution by photolysis (at 147 nm) of Me6Si2 by photolysis of bis(trimethylsilyl)mercury by mercury- photosensitized decomposition of trimethylsilane or by abstraction of hydrogen from trimethylsilane by t-butoxyl radicals. The MezSi=CHz that was produced in the disproportionation was trapped as Me3SiOBu' or Me2Si(CH2D)OCD3 in the presence of Bu'OH or CD30D respectively (D=2H). Values of kdisp/kcomb of 0.31 (in the gas phase) and 0.19 (in solution) were obtained.A novel technique to determine self- and cross-termination rate constants invol- ves the photolytic generation of two different radicals by a harmonically modulated light source and then following the phase shifts of the two radicals compared with the incident light.3S This general approach should prove very useful in unravelling complex radical kinetics. 4 Radical Transfer SH2Reactions at multivalent centres continue to attract attention. Trifluoromethyl radicals react with neopentane at 300 "C to give isobutane as a primary product and more CF3CH3 than would be expected by cross-combination of methyl and trifluoromethyl radicals.36 The results were interpreted in terms of reaction (1). CF3. + H3C-C(CH3)3 -* CF3CH3 + .C(CH3)3 (1) Even in this relatively favourable case (unhindered position of attack; stable radical formed; high temperature) the SH2reaction only plays a minor role in the overall reaction which suggests that other examples will be difficult to find in the absence of special features.One such feature involves attack by or displacement of 32 (a)R. Hiatt and V. G. K. Nair Can.J. Chem. 1980,58,450; (b)D. F. Church and W. A. Pryor J. Org. Chem. 1980,45,2866. 33 H. Singh and J. M. Tedder J. Chem. SOC.,Chem. Commun. 1980,1095. 34 S. K. Tokach and R. D. Koob J. Phys. Chem. 1980 84 1; J. Am. Chem. SOC. 1980,102 376; L. Gammie I. Safarik 0.P. Strausz R. Roberge and C. Sandorfy ibid. p. 378; B. J. Cornett. K. Y. Choo and P. P. Gaspar ibid. p. 377. 35 H. Paul and C. Segaud Int.J. Chem. Kinet. 1980,12,637. 36 R. A. Jackson and M. Townson J Chem. SOC.,Perkin Trans. 2,1980,1452; Tetrahedron Lett. 1973,193. 58 R. A. Jackson organometallic radicals (see e.g. ref. 37a). For some organometallic radicals the ratio (bond strength to C)/(bond strength to H) is higher than for organic radicals (see e.g. ref. 37b) which is a factor that will favour SH2 reactions by these organometallic radicals compared with -competing hydrogen-transfer reactions. Intramolecular SH2 reactions at carbon are better established. In a recent example,38 bromotrichloromethane reacts with (but-3-enyl)cobaloximes e.g. [(H,C=CHCH,CHM~)CO(~~~H)~~~], as shown in Scheme 1. [( H2C=CHCH2CHMe)Co(dmgH),py] + cc13 + [(C13CCH2CHCH2CHMe)Co(drngH)2py] [BrCo(dmgH)2PYl k[Co"(dmgH)zpy] +14 CI,C Scheme 1 SH2 Reactions at the other Group IVB elements are well established and a smooth SN2-like process is often assumed.Ko~hi~~ argues for a charge-transfer mechanism (Scheme 2) in the reaction of tetra-alkyl-tins with iodine atoms based on selectivity studies and the effect of solvent on the rates of reaction. &Sn + I*-+ [&Sn+.I-]' -+ R3SnI + R* Scheme 2 An excess of tributyltin hydride has been used to reduce ethylene thioketals or thioacetals to the corresponding hydrocarbons in synthetically significant yields (Scheme 3). A 1 1molar ratio gives the ,!?-(alky1thio)ethyltributyltinsulphide SH2 attack at sulphur is thought to be inv01ved.~' "x;] "'x), Bu,SnH+ AIBN R2 R2 H SSnBu3 Scheme 3 The reaction of methylperoxyl radicals with alkenes gives the epoxide and a methoxyl radical.41 It has been suggested that the /3 -methylperoxyalkyl radical (4) undergoes an internal SH2 attack at the peroxide oxygen atom to close the ethylene oxide ring and displace methoxyl radical (Scheme 4).Turning to radical transfer of univalent atoms the contribution of polar influences to such reactions continues to receive attention. Reduction of substituted benzyl CH302*+ H2C=CH2 + CH3-O-O-CH2CH2-+ CH30. + H,C-CH, \/ (4) 0 Scheme 4 37 (a)T. Funabiki B. D. Gupta and M.D. Johnson J. Chem. SOC.,Chem. Commun. 1977,653; (b)R. A. Jackson J. Organomet. Chem. 1979,166 17. M.R. Ashcroft A. Bury,C. J. Cooksey A. G. Davies B. D. Gupta M. D. Johnson and H.Morris J. Organomet. Chem. 1980 195 89. 39 S.Fukuzumi and J. K. Kochi J. Org. Chem. 1980,45,2654. 40 C. G. Gutierrez R. A. Stringham T. Nitasaka and K. G. Glasscock J. Org. Chem. 1980 45 3393. 41 K. Selby and D. J. Waddington J. Chem. SOC. Perkin Trans. 2 1980 65; D. A. Osborne and D. J. Waddington ibid.,p. 925. Reaction Mechanisms -Part (iii) Free-Radical Reactions 59 halides by tributyltin gave a Hammett plot that was best fitted by u-values and values of p of +0.34 (Cl) +0.17 (Br) but +0.81 (I). The high positive value for benzyl iodides suggests that electron transfer may be important for this series. Kinetic isotope effect studies using 13C also support a significant change in the transition state between Br and I in the reactions of triphenyltin radicals with the methyl halides.42b For CH3Cl and CH3Br the kinetic isotope effect is in excess of the equilibrium value signifying a late transition state but the value for CH31is significantly less than the equilibrium value.When irradiated in the presence of t-butyl hydroperoxide the 8-phenylthio-5'- deoxyadenosine derivative (5)was converted into the cyclized product (6).43Pre-sumably the radical that is originally formed having a free electron at the 8-position abstracts a hydrogen atom from the 5'-methyl group and this intermediate methyl- ene radical then adds back at the 8-position followed by loss of a hydrogen atom to another radical in a normal homolytic aromatic substitution sequence to give the observed cyclized product (6).This provides a parallel in vitro to the enzymic functionalization of the methyl group in 5'-deoxyhdenosine in vivo. N 4Nfj ++ ".;Y$ H& H Ox0 Me Me Me Me CHzZ (5) (6) (7) Finally chlorination of the triptycene derivative (7; X = Y = 2 = H) by sul-phuryl chloride initiated by benzoyl peroxide gives a mixture of the three rotamers (7; X or Y or Z = C1; other two atoms = H).44Chromatography on alumina destroys the two synclinal forms allowing isolation of the anti-periplanar rotamer (7; X = C1 Y = 2 = H). Kinetic measurements on the equilibration of the rotamers at 185 and 208 "C indicate a rotational barrier of 126 kJ mol-'. 5 Addition and Homolytic Aromatic Substitution Tedder and Walton have reviewed fifteen years of work on the addition of radicals to C=C double They concluded that for monosubstituted ethylenes steric hindrance to approach is the most important feature favouring attack at the methylene group.Polar influences are also important; methyl radicals are nucleophilic and their addition is facilitated by electron-withdrawing substituents in the alkene while trifluoromethyl radicals are electrophilic with reaction facili- tated by electron-releasing substituents. In view of the exothermic nature of most 42 (a)E. V. Blackburn and D. D. Tanner J. Am. Chem. Soc. 1980,102 692; (b) W. H. Tamblyn E. A. Vogler and J. K. Kochi J. Org. Chem. 1980 45 3912. 43 D. Gani A. W. Johnson and M. F. Lappert J. Chem. SOC.,Chem. Commun. 1980 1244. 44 T. Morinaga S. Seki H. Kikuchi G.Yamamoto and M. bki J.Am. Chem. Soc. 1980 102,1173. 45 J. M. Tedder and J. C. Walton Tetrahedron 1980 36 701. 60 R. A. Jackson additions there is an early transition state; thus stabilization of the incipient radical adduct which is much favoured by textbooks has only minor significance. In cyclizations of hex-5-enyl radicals to cyclopentylmethyl radicals (again not giving the thermodynamically favoured radical) 1-and 3-substituents favour cis -disub-stituted products whereas for 2-and 4-substituents the trans product is The chair-like transition state (8) was suggested with 2- 3- and 4-substituents taking up preferentially the 'equatorial' positions shown and thus giving the observed cis/ trans selectivity. These rules apply to ring-closure of peroxy- radical^:^' the reaction shown in Scheme 5 was reported to occur with complete stereospecificity.H2C H R' R2 R' R2 R;T$ 3R PhS -+ PhS H 0-0' 0-0 H Scheme 5 (8) Asymmetric induction has been observed in the reaction of thiols with (-)-menthyl cr~tonate.~~ For example an 18% enantiomeric excess of the (+)-enan- tiomer of the appropriate alcohol was obtained as shown in Scheme 6. i,ii I7 CH3COSH + CH3CH=CHC02Menth -CH3CO-*C-CH2CH20H Reagents i,azoisobutyronitrile (AIBN); ii LiAIH Scheme 6 E.s.r. evidence for the intermediacy of a cyclohexadienyl radical in the reaction of triphenylsilyl radicals with cyanobenzene has been The -CN group stabilizes the adduct radical (9)sufficiently to make the radical detectable and also accounts for the decrease in coupling constants compared with the unsubstituted cyclohexadienyl radical.Homolytic aromatic arylation of 4-methyipyridine and the 4-methylpyridinium ion by substituted aryl radicals p-XC6H4*shows that the isomer distribution is insensitive to X though the ratios of 2-:3-isomers change from 0.82 :1 to 9 :1 as the nitrogen atom is protonated.'' The total rate factors for arylation of the 4-methylpyridinium ion vary from 5.0 (X=Me) to 0.19 (X= NOz) but all values are in the region of 1 for 4-methylpyridine. Frontier-orbital calculations indicate that p -XC6H4 radicals behave as nucleophiles towards 4-methylpyridinium ion and as electrophiles towards 4-methylpyridine whatever the nature of X. (9) (10) 46 A. L.J. Beckwith T. Lawrence and A. K. Serelis J. Chem. SOC.,Chem. Commun. 1980,484. 47 A. L. J. Beckwith and R. D. Wagner J. Chem. SOC., Chem. Commun. 1980,485. 48 M. Yoshihara H. Fujihara and T. Maeshima Chem. Lett. 1980 195. 49 A. Alberti and G. F. Pedulli Gazz. Chim. Ital. 1979 109 395. R. Arnaud J. Court J. M. Bonnier and J. Fossey Noun J. Chim. 1980 4,299. Reaction Mechanisms -Part (iii) Free-Radical Reactions 61 Nitro-substituted aromatic compounds undergo homolytic aromatic substitution at unsubstituted positions but ipso-attack also occurs facilitated by the stability of the *NO2leaving radical. The thiophen derivative (10)reacts with methyl radicals by homolytic substitution at the 4-position whilst adamantyl radicals displace the nitro-group by ipso-attack.’l It has been suggested that polar influences stabilize the transition state for the addition of the relatively nucleophilic adamantyl radical.6 Fragmentation There is a growing interest in utilizing radical chain reactions that involve fragmenta- tion steps for synthetic purposes. Primary and secondary alcohols can be converted into the corresponding hydrocarbon in good yield by the radical-initiated reaction of the corresponding chloroformate with tripropylsilane (Scheme 7).52 Pr3Si-Pr3SiH ROH -B ROCOCl -Pr3SiC1+R-0-C=O -+ COz + R. -RH + Pr3Si. Scheme 7 A number of bridgehead iodides have been made from the corresponding carboxylic acids in reasonable yields by photolysis with t-butyl hypoiodite (Scheme 8y3 hv RC02H + Bu‘OI + Bu‘OH + RC02-I -+ R-COZ.-* COZ + R* -+ RI Scheme 8 A novel process involving a double fragmentation has been used in the decar- boxylation of carboxylic acids RC02H -+ RH (Scheme 9) via the dihydrophenan- threne derivative (11;X = C1 or PhS).54 RC0,H RH 1 t OCOR C02 + R’ 1 T + RC02‘ q \ Scheme 9 Deamination of aromatic amines can be carried out by their conversion into the diazonium salt with n-pentyl nitrite followed by photolysis to give the aromatic radical which gives the arene if THF is used as the solvent or the corresponding chloride or iodide if CCl or CH212 is ernpl~yed.~’ 51 P. Cogolli,F. Maiolo L. Testaferri M. Tiecco and M. Tingoli,J. Chem. SOC.,Perkin Trans.2,1980,1331. 52 R. A. Jackson and F. Malek J.Chem. Soc. Perkin Trans. 1 1980 1207. ’’ R. S. Abeywickrema and E. W. Della J. Org. Chem. 1980,45,4226. 54 D. H. R. Barton H. A. Dowlatsahi W. B. Motherwell and D. Villemin J. Chem. Soc. Chem. Commun. 1980,732. 5s V. Nair and S.G. Richardson J. Org. Chem. 1980,45 3969. 62 R. A. Jackson Amino-acids and amino-glycosides have been deaminated'6 by their conversion into the corresponding isocyanide and treatment of this with tributyltin hydride in the presence of AIBN at 80 "C (Scheme 10). I I1 iii RNHz +RNHCHO +RNC --+ RH Reagents i MeC0,CHO; ii POCl, Et,N; iii Bu,SnH AIBN at 80 "C Scheme 10 In the mechanistic area a number of stereochemical studies have been made. The reaction of trichloromethyl radicals with 2-(trimethylstanny1)butanegives cis-and trans-2-butene among the products with 75% of anti-periplanar eliminati~n:~~ the elimination may be concerted with anti and syn pathways or an initial anti abstraction of hydrogen may give an intermediate radical which can either immedi- ately collapse to the alkene (see Scheme 11)or undergo rotation before it produces the butene.CI,C* + /ySnMe -"Xiel + +.ke Scheme 11 Phosphoranyl radicals (12) can undergo p-scission5* to give the alkyl radical R* and the phosphate esters (RO),P=O. Fragmentation was thought previously to be preferentially from apical sites but kinetic e.s.r. studies now indicate that fragmenta- tion takes place predominantly from the equatorial positions. A number of e.s.r. kinetic studies of ring-opening reactions of compounds with a radical centre that is a to a cyclopropane or a cyclobutane ring have been made.Cyclopropylmethyl radicals undergo ring-opening much more readily than do cyclobutylme thy1 radicals. 59 7 Rearrangements Rearrangements are not as common in radical as in cationic systems but are still involved in a number of interesting processes. Amongst the products of reaction of recoil tritium atoms with CHF=CHF CHT=CF2 was unexpectedly found. The sequence shown in Scheme 12 was suggested.60 56 D. H. R. Barton G. Bringmann and W. B. Motherwell Synthesis 1980,68;J. Chem.SOC.,Perkin Trans. 1 1980 2665. 57 T. J. Stark N. T. Nelson and F. R. Jensen J. Org. Chem. 1980,45,420. B.P.Roberts and K.Singh J. Chem. Soc. Perkin Trans. 2 1980 1549.59 Y.Maeda and K. U. A. L. J. Beckwith and G. Moad J. Chem. SOC.,Perkin Trans. 2,1980,1083,1473; Ingold J. Am. Chem. SOC. 1980,102,328;K.U.Ingold and J. C. Walton J. Chem. SOC.,Chem. Commun. 1980,604. 6o E. E. Siefert and Y.-N.Tang J. Chem. SOC.,Chem. Commun. 1980 814. Reaction Mechanisms -Part (iii) Free-Radical Reactions H F \/ c=c /\F H-H T 1 T -H* Scheme 12 that a [1,2]hydrogen shift occurs first (to give the less stable n-propyl radical) followed by fragmentation as shown in Scheme 13. A similar 'uphill' rearrangement has been invoked to explain the formation of some 1,l-diphenylethane when bibenzyl is thermolysed62 at 366-400 "C (Scheme 14). (CH3)2CD*+ CH3CHDCH2. + CH3. + CHD=CH2 Scheme 13 PhCHzCH2Ph .(PhCH2)2 -B PhCH2. APhCHCH2Ph -+ Ph2CHCH2. -* Ph2CHCH3 Scheme 14 2-Tetralyl radicals (13) and 1-indanylmethyl radicals (14) prepared by thermoly- sis of their t-butyl peresters were found to interconvert readily equilibration via the intermediate neophyl-type radical (15) was In an attempt to observe a spiro-radical intermediate in reactions of this type hydrogen was abstrac- ted from the spiro-diene (16) but fragmentation of the intermediate radical (17) was fast and only the P-phenylethyl radical was observed by e.~.r.~~~ The acetylenic radical (18)was shown to rearrange to radical (19) at 45-88 "C,presumably via the cyclopropyl radical (20).64 61 I. Szirovicza and I. Sziligyi Znt. J. Chem. Kinet. 1980 12 113. 62 M. L. Poutsma Fuel 1980 59 335.63 (a)J. A. Franz and D. M. Camaioni J. Org. Chem. 1980 45 5247; (b) A. Effio D. Griller K.U. Ingold J. C. Scaiano and S.J. Sheng J. Am. Chem. SOC.,1980 102 6063. 64 K. U. Ingold and J. Warkentin Con. J. Chem.. 1980 58 348. R. A. Jackson ++-+%-+3-I (18) (20) (19) In experiments related to the enzymic conversion of diols into aldehydes and ketones photolysis of 4,5-dihydroxycyclo-octyl(pyridine)cobaloxime gave cyclo-octanone in 40% yield.65 A favourable transannular 13-H shift converts the 4,5-dihydroxycyclo-octylradical into the 1,2-dihydroxycyclo-octyl radical (Scheme 15). Scheme 15 8 Electron Transfer In the past decade there have been various suggestions for involvement of electron transfer in the addition of Grignard reagents to ketones.An intermediate charge- transfer complex (21) might collapse directly to adduct or break up to give Ph2COMgX and R*in a solvent cage. The free radical R.could now combine with Ph,d-O* *. .+ Mg-X R/ (21) Ph,COMgX to give the adduct or escape from the cage. Evidence for the inter- mediacy of a free radical in at least some cases is provided by the formation of the cyclic product (22) in 12% yield in the reaction shown; this cyclic product is likely I Me (22) [12%] I + Ph2C0 + + H2C=CHCH2CH2yCH2MgC1 I Me Me OH I I H2C=CHCHzCHzC-CH2-CPh2 I Me " B. T. Golding C. S. Sell and P. J. Seliars J. Chem. Soc. Perkin Trans. 2 1980,961. E.C. Ashby J. Bowers and R. Depriest Tetrahedron Lett. 1980 21 3541.Reaction Mechanisms -Part (iii) Free-Radical Reactions to have arisen by cyclization of the hex-5-enyl radical H2C=CHCH2CH2CMe2CH2 to give a substituted cyclopentylmethyl radical intermediate.66 When dimesityl ketone is allowed to react with metal hydrides such as AlH3 deeply coloured solutions are formed6' which show intense and complex e.s.r. spectra and which decay slowly as the reduction product is formed in 100% yield. It has been suggested that a radical anion/radical cation pair is formed as an intermediate as shown in Scheme 16. Mes2C0 + MH fast .+ __* Mes2C-0 M-H slow __* Mes2CH-OM Scheme 16 On mixing iodine with organometallic compounds R,M (M = Sn Pb or Hg) the absorption spectrum of a charge-transfer complex is observed immediately.The subsequent disappearance of this charge-transfer absorption is accompanied by iodinolysis of R,M at a rate which strongly depends on solvent polarity and this suggests that the rate-determining step is transfer of an electron from the alkylmetal to the iodine to form the ion pair [R,Mt I:] which subsequently gives the iodinolysis products.68 Polarographic reduction of substituted benzyl chlorides or bromides show Ham- mett plots which correlate best with K,suggesting that the potential-determining process involves C-Hal bond breaking or the formation of a radical ion intermedi- ate. Benzyl iodides react differently and form benzylmercuric iodide as an inter- mediate in the redu~tion.~~ Finally Walling has considered the general question of electron transfer in slow organic reactions exemplified by the reaction of an electron donor with an organic peroxide.It is commonly observed that when the donor is varied a plot of RT In k versus the one-electron oxidation potential of the donor is linear but with a slope of less than unity. This has been interpreted as an electron-transfer process which is neither rate-determining nor complete in the transition state and it has been pointed out that in the reaction of ROOR with a donor D the transition states to the different possible products {D' + RO' + RO-} {ROD * + RO a} and {ROD' + RO-} may be very similar.7o 67 E. C. Ashby A. B. Goel and R. N. DePriest J. Am. Chem. SOC., 1980,102,7779. 68 S. Fukuzumi and J. K. Kochi J. Am. Chem. SOC.,1980,102,2141.69 D. D. Tanner J. A. Plambeck D. W. Reed and T. W. Mojelsky J. Org. Chem. 1980,45 5177. '"C. Walling J. Am. Chem. SOC.,1980 102,6854.

ISSN:0069-3030    

DOI:10.1039/OC9807700053

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

5 Arynes Carbenes Nitrenes and Related Species By D. W. KNIGHT Department of Chemistry University of Nottingham Nottingham NG7 2RD 1 Arynes There has been relatively little activity in this area during the past year. The mechanism by which aryl halides undergo dehydrohalogenation by hydroxide ions has been shown to involve a benzyne intermediate.' An alternative pathway via @so-hydroxydehalogenation,appears to operate only in the presence of copper salts. Thermolysis of titanocene (1) at 80-100°C gives rise to the titanium-benzyne complex (2) which subsequently can react with molecular nitrogen to give anilines.' The intermediacy of the benzyne (2) is supported by the nature of the products arising from titanocenes having substituted phenyl groups. Full details have been given3 for the preparation of perchlorobenzyne (3)(from C&16 and Bu"Li) and for its subsequent trapping with benzene leading to benzobarrelene after dechlorina- tion.Perchlorobenzyne gives much higher yields of addition products than benzyne itself in this case and perhaps could find use in place of the latter elsewhere provided that the intermediate from (3)could be successfully dechlorinated as in this instance. 3,6-Disubstituted 1,2,4,5-tetrabromobenzenesserve as biaryne equivalents giving adducts such as (4) in good yield on reaction with butyl-lithium and f~ran.~ This procedure which is an improvement of a similar reaction reported some time ago by Wittig is apparently a stepwise process. Various pyrolysis experiments with 3,4-dialkyl-hexa-1,5-diyn-3-eneshave pro- vided further evidence that 1,4-dehydrobenzene exists as a diradical species.' The formation of 9,lO-phenanthrynes (5)by pyrolysis (at 700-900°C) of the corresponding diacid anhydrides at very low pressure has been reported toget her with some comparison of their reactivities relative to other arynes6 The transient M.Zoratti and J. F. Buqnett J. Org. Chem.. 1980,45,1769. 1776. E.G. Berkovich V. B. Shur M. E. Vol'pin B. Lorenz S. Rummel and M. Wahren Chem. Ber. 1980 113,70. N.J. Hales H. Heaney J. H. Hollinshead and P. Singh Org. Synth. 1979 59 71. H.Hart C.Lai G. Nwokogu S. Sharmouilian,A. Teuerstein and C. Zlotogorski J. Am. Chem. SOC. 1980,102,6649. T. P.Lockhart C. B. Mallon and R. G. Bergman J. Am. Chem.Soc. 1980,102 5976. H.-F.Grutzmacher and U. Straetmans Tetrahedron 1980,36 807. D. W.Knight @/J(-J \ R' / R 2 2 &J\ - (4) R \'I (6) B (5) aryne (6) has been obtained by dehydrobromination of a monobromo-derivative of the corresponding 127r antiaromatic system; (6) can be trapped with for example 1,3-diphenyli~obenzofuran.~ 2 Nitrenes There now seems to be little doubt about the intermediacy of azacycloheptatetraenes and cyclic carbodi-imides (7) in the reactions of phenylnitrenes and pyridylnitrenes. Further evidence for the participation of (7) rather than the corresponding carbene (8)in the 2-pyridylnitrene + 2-pyridylnitrene equilibration is the direct observation of (7) by matrix-isolation techniq~es.~*~ Similarly direct observation as well as detailed product analysis and labelling studies support the formation of carbodi-imide (9) during the thermolysis of both tetrazolo[ 1,5-~]quinazolines and tetrazolo[l,5-a]quinoxalines.10 However it also is clear that these are not the exclusive pathways by which such nitrenes react.Flow pyrolysis of azidopentafluorobenzene at 330°C gives rise to a dimer the structure of which suggests the involvement of a monoaza-carbene analogous to (8).11 Moreover photolysis of 6-azidobenzothiazoles at ambient temperatures in the presence of secondary amines gives products which are consistent with the intermediacy of azirines (10) rather than the corresponding cycloheptatetraene;12 possibly the former is trapped by the amine before it can ring-expand to the latter.During infrared studies of the photolysis of naphthyl azides in argon matrices at 12 K both azirine and aza-cycloheptatetraene species have been ob~erved.'~ A full report of studies on the reactions of pyrazolenitrenes (11) has been p~blished.'~ Briefly the singlet state of nitrene (11)attacks the 2-nitrogen to give ' H. N. C. Wong and F. Sondheimer Tetrahedron Lett. 1980,21 983. * C.Wentrup and H.-W. Winter J. Am. Chem. SOC.,1980 102 6159. For a review of some aspects of this work see 0.L. Chapman Pure Appl. Chem. 1979,51 331. For a useful review of matrix-isolation techniques see I. R. Dunkin Chem. SOC.Rev. 1980,9,1. lo C. Wentrup C. Thitaz E. Tagliaferri H. J. Linder B. Kitschke H.-W. Winter and H. P. Reisenauer Angew.Chem. Znt. Ed. Engl. 1980 19,566. " R. E. Banks N. D. Venayak and T. A. Hamor J. Chem. SOC.,Chem. Commun. 1980,900. '* P.T. Gallagher B. Iddon and H. Suschitzky J. Chem. SOC.,Perkin Trans. I 1980,2362. l3 I. R.Dunkin and P. C. P. Thomson J. Chem. SOC.,Chem. Commun. 1980 499. l4 J. M. Lindley I. M. McRobbie 0.Meth-Cohn and H. Suschitzky J. Chem.SOC.,Perkin Trans. 1 1980 982.For some related work see P. C. Hayes G. Jones C. Keates I. Kladko and P. Radley J. Chem. Res. (S) 1980,288;P.C.Hayes and G. Jones J. Chem. SOC.,Chem. Commun. 1980,844;A.Yabe Bull. Chem. SOC.Jpn. 1980,53 2933. Arynes Carbenes Nitrenes and Related Species Me h4e)r-I N ON>.b S N N R\ ylide (12) whereas the corresponding triplet of (11)attacks the 5-methyl group or becomes protonated.By varying the substituent in (1l) the latter's multiplicity can be altered. Thus when R the para-substituent of (1l) is electron-withdrawing more singlet-derived products are formed whereas when it is electron-donating only triplet-type products are obtained. A first example of intramolecular ring-expansion of a benzene to an azepine involves pyrolysis of benzyl azidoformates (Scheme 1).l5 The initially formed azepine dimerizes under the reaction conditions. Arylnitrenes derived from for example 2-azidophenyl phenyl sulphides are known to react via intramolecular ipso-attack leading to five-membered spiro-intermediates. Much the same process but with novel six-membered spiro-intermediates (13) has been evoked to explain why pyrolysis of aryl 2-azidobenzoates leads to carbazoles when R'= M [see (13)j and to acridines or acridones when R' # H.16 Scheme 1 NR // MeO-c 3 R' R2 (14) R = CN or S02Me (13) A useful review has appeared on the rearrangements of N-phenylcarb-imidoylnitrenes." The preparation of two new preparatively useful imidoyl- nitrenes (14) has been reported." Both react stereospecifically with olefins to give aziridines in good yield.It has been provenlg that photolysis of 1-azatriptycene gives rise to nitrene (15); an alternative cleavage leading to a phenylcarbene does not occur to any significant extent. Some evidence for the intermediacy of vinylnitrenes (17) in the thermal rearrange- ment of substituted phenylazirines to indoles comes from the observation that Is 0.Meth-Cohn and S. Rhouati J. Chem. SOC.,Chem. Commun. 1980,1161. l6 M. G. Clancy M. M. Hesabi and 0.Meth-Cohn J. Chem. SOC.,Chem. Commun. 1980 1112. " C. W. Rees PureAppl. Chem. 1979,51 1243. *' W. Lwowski and 0.S. Rao Tetrahedron Lett. 1980,21,727. l9 T. Sugawara and H. Iwamura J. Am. Chem. Soc. 1980,102,7134. \ \ thermal racemization of the phenylazirine (16) is much faster than its rearrangement to an indole.20 There still exists some doubt about the existence of simple alkyl-nitrenes. Calcula- tions suggest*l that alkyl-nitrenes are stable in the absence of collisions and that the methylnitrene (triplet ground state) -+ methylenimine rearrangement is endother- mic by 18 kcal mol-’ with an activation energy of 48 kcal mol-l.Ethoxycarbonylnitrene is usually rather unselective in its reactions; however it has been found to add to vinyl chlorides to give the expected aziridines in 1748% yield.22 There is some evidence to suggest that this is due to co-ordination between the nitrene and the chlorine atom. Some sources of fl~oronitrene~~ have been recorded. and of amino-nitrene~~~ 3 Carbenes Singlet carbenes are normally considered to possess a ‘bent’ structure. It has now been s~ggested’~.~~ that when the substituents are less electronegative than carbon (e.g.Li B) then the carbene has a linear structure as such substituents have a vacant p-orbital which can stabilize the carbene by two-electron three-centre overlap. The electronegativity of its substituents not only determines the stability of a carbene,26 but is also probably the crucial factor influencing the multiplicity of the ground state of the species,27 electronegative substituents favouring singlet ground states whereas electropositive atoms or groups favour triplet ground states.A review of the selectivities of carbenes in cyclopropanation reactions has appeared,” containing a useful summary of the electrophilicities of many carbenes. The first practical example of the use of the classical Simmons-Smith procedure for methylcyclopropanation has been claimed,29 in which 1,2-bis(trimethylsiloxy)cyclo-hexene is allowed to react with 1,l-di-iodoethane to give the expected product (18) in 76% yield. Carbenoids can also be generated from some aldehydes and ketones by deoxygenation using zinc-copper couple and either dichlorodimethylsilane or 20 K.Isomura G.-I. Ayabe S. Hatano and H. Taniguchi J. Chem. SOC.,Chem. Commun. 1980 1252; see also K.Isomura S. Noguchi M. Saruwatari S. Hatano and H. Taniguchi Tetruhedron Lett. 1980 ’’ J. 21 3879. Demuynck D. J. Fox Y. Yamaguchi and H. F. Schaefer 111 J. Am. Chem. SOC.,1980,102,6204. See also A. Mavridis and J. F. Harrison ibid. p. 7651. ’’ L. Pellacani F. Persia and P. A. Tardella Tetrahedron Lett. 1980 21,4967. 23 D.L. Klopotek B. G. Holrock P. Kovacic and M. B. Jones J. Org. Chem. 1980,45,1665. 24 E.Fahr and K. H. Koch Liebigs Ann. Chem. 1980,219. ” W. W. Schoeller J. Chem. SOC.,Chem. Commun. 1980,124. 26 L.Pauling J. Chem. SOC.,Chem.Commun. 1980 688. ’’ J. F. Harrison R. C. Liedtke and J. F. Liebman J. Am. Chem. SOC.,1979,101,7162. R.A. Moss Acc. Chem. Res. 1980 13 58. 29 S. Lewicka and W. H. Okamura Synth. Commun. 1980 10,415. Arynes Carbenes Nitrenes and Related Species OSiMe cl\ PMe *OSiMe c1 chlorotrimethylsilane.30Thermolysis of the cyclopropene (19)leads to the vinylcar- bene (20) which adds to olefins to give vinylcyclopropanes in reasonable yield.3' Further evidence has been found to suggest that direct irradiation of some diazo-compounds can give rise to excited singlet carbene~.~~ Thus pyrolysis or sensitized irradiation of (21) in the presence of cis-2-butene gives the diene (22) and a mixture of the cis- and trans-isomers of the cyclopropane (23); these are products which could reasonably be expected to come from the singlet and triplet carbenes derived from (2l),respectively.However direct irradiation results only in the formation of (22) and the cis-isomer of the dimethylcyclopropane (23) suggestive of the generation of another presumably excited singlet-state carbene in addition to that which leads to (22). The possible involvement of an excited diazo species cannot be completely discounted. (21) (22) (23) Calculations indicate that singlet cyclopropylcarbene rearranges to cyclobutene via initial electrophilic attack of the empty p-orbital of the carbene on an electron-rich bond of the cy~lopropane.~~ Other theoretical work on the nature of the transition states in the additions of various electrophilic ambiphilic and nucleophilic carbenes to ethylene has been reported;34 values for the HOMO and LUMO levels of the carbenes studied are quoted.Competition experiments between a range of dihalogeno-carbenes and mixtures of olefins at various temperatures suggest that variation of the substituents in the order F 3C1 +Br results in the activation enthalpies and activation entropies being changed in the same dire~tion.~~ It has been reported36 that the energy level of the singlet state of dibromocarbene is some 8 kcal mol-' below that of its triplet state. Difluorocarbene can be obtained at relatively low temperatures (-25 "Cor lower) by reactions between the metal complex [(CF3)2Cd.glyme] and acyl halides.37 Contrary to previous reports the addition of dichlorocarbene to cycloalkenols does appear to be significantly influenced by the hydroxyl group as a greater proportion of syn-addition by the carbene OCCU~S.~' 30 C.L. Smith J. Arnett and J. Ezike J. Chem. SOC.,Chem. Commun. 1980 653. 3' W. Weber and A. de Meijere Angew. Chem. Int. Ed. Engl. 1980,19 138. 32 G. R. Chambers and M. Jones jun. J. Am. Chem. SOC., 1980,102,4516. 33 W. W. Schoeller J. Org. Chem. 1980 45 2161. 34 N. G. Rondan K. N. Houk and R. A. Moss J. Am. Chem. SOC.,1980,102 1770. B. Giese,W.-B. Lee and J. Meister Leibigs Ann. Chem. 1980 725. 36 C. W. Bauschlicher jun. J. Am. Chem. SOC.,1980 102 5492. 37 L. J. Krause and J. A. Morrison J. Chem. SOC.,Chem. Commun. 1980 671. R. H. Ellison J. Org. Chem. 1980,45 2509.72 D. W.Knight Carbenoids (24) generated from [m.n.llpropellanes show a distinct se!ectivity towards insertion into neighbouring axial C -H bonds in the order cyclopentyl > norcaranyl 4.1-tetralyl > cyclohexyl > cyclohexenyl; presumably both the relative proximity and the nucleophilicity of the C-H bonds are the major controlling A novel example of a ‘tandem’ vinylcyclopropylidene-cyclopentadiene rearrangement has been used to prepare the triene (27) from the bis-dibromo- cyclopropane (25) presumably by way of the cyclopentadiene (26).40 p<”’ .. Br4 Br (25) (26) (27) A review of the chemistry of arylcarbenes and arylnitrenes in the gas phase has been published as part of a larger work on reactive intermediate^.^^ The triplet states of diphenylcarbene fluorenylidene and phenylcarbene have been monitored in matrices by e.s.ra4* This work serves to emphasize that especially in the case of the first two species such intermediates have long lifetimes in matrices and therefore that any reactivity studies at these low temperatures must be allowed to proceed for a sufficient length of time to ensure that all of the products that are isolated have arisen from carbenes that have reacted at the temperature of the matrix and not above that temperature.Phenylcarbenes generated by photolysis of aryl-diazomethanes in matrices of t-butyl alcohol (at -196 “C) give olefinic products presumably by dimerization of triplet species whereas carbenes that have been derived from a-diazocarbonyl compounds under the same conditions do not give olefinic products suggesting that only the singlet-state carbenes are Photolytically generated phenylcarbene reacts with liquid 2-chloropropane at low temperatures predominantly by insertion into the C-Cl bond whereas in a solid matrix the carbene inserts into the primary C-H bonds of the propane almost excl~sively.~~ This change in reactivity could be due to the occurrence in the C-Cl insertion reaction of an intermediate halonium ylide species which can be stabilized by solvation in the liquid phase but not in the solid phase.The stable salt [Cp(CO)2FeCHPh]+PF6- serves as a good source of phenylcarbene for the efficient and largely stereoselective phenylcyclopropanation of a range of 01efins.~~ Diphenylcarbene generated by U.V.laser irradiation of diphenyldiazomethane shows a markedly different reactivity and gives rise to different products to those formed when the carbene is generated by conventional photolytic means. The 39 L. A. Paquette E. Chamot and A. R. Browne J. Am. Chem. SOC.,1980 102 637; L.A. Paquette A. R. Browne E. Chamot and J. F. Blount ibid. p. 643. 40 U. H. Brinker and I. Fleischhauer Angew. Chem. Int. Ed. Engl. 1980,19,304. 41 C. Wentrup ‘The Behaviour of Arylcarbenes & Nitrenes in the Gas-phase’ in ‘Reactive Intermediates’ ed. R. A. Abramovitch Plenum New York Vol. 1 1980. 42 C.-T. Lin and P. P. Gaspar Tetrahedron Lett. 1980 21 3553. 43 H.Tomioka T. Miwa S. Suzuki and Y. Izawa Bull. Chem. SOC.Jpn. 1980,53,753. 44 H.Tomioka S.Suzuki. and Y. Izawa Chem. Lett. 1980,293. 4s M. Brookhart M. B. Humphrey H. J. Kratzer and G. 0.Nelson J. Am. Chem. Soc. 1980,102,7802. Arynes Carbenes Nitrenes and Related Species reasons for this are not as yet clear.46 Laser flash photolysis of diazofluorene at room temperature has been shown to give fluorenylidene in the singlet state; this is short-lived relative to the triplet state into which it converts ~nidirectionally.~~ This type of laser experiment looks to be a useful way to measure the rates of reactions of both singlet and triplet carbenes. In contrast to this are some reactions between aryl-carbenes and 1,l-dimethylallene where it is assumed that singlet carbenes add to the more substituted double-bond of the allene while triplet species add to the less substituted double-bond to give the thermodynamically preferred isopropyl- idenecyclopropanes.The results48 indicate that many monoaryl-carbenes react by way of the singlet state even though initially generated in the triplet state. In addition electron-donating substituents seem to stabilize the singlet species (cf.refs. 14 and 27). An investigation into the chemistry of benzocyclobutenylidene (28) has produced some odd results.49 In the gas phase (28)mainly undergoes dimerization while intermolecular additions together with some insertion reactions occur in solution. Additions to olefins are stereoselective implying the presence of a singlet species but oddly such additions display the largest positive p value yet observed for this type of reaction; i.e.(28)appears to be nucleophilic in contrast to the related phenylcarbenes. Absolute rate constants have been measured directly for the first time for the addition of a singlet carbene (PhCCl) to alkyl-olefins in ~olution.~~ The results show a regular rate decrease with decreasing substrate alkylation which is an understandable finding in view of the electrophilicity of the carbene. Cycloheptatrienylidene (29)can be obtained by pyrolysis (at 450OC) of 7-acetoxynorbornadiene or 7-aceto~ycycloheptatriene;~~ the carbene dimerizes under these conditions but at higher temperatures (>600 "C) it rearranges to fulveneallene (30). The carbene (29)has also been obtained from the reaction of benzene with arc-generated carbon atoms at -195 Cycloheptatrienylidene(29)undergoes a OC5* formal [4 +21reaction with anthracene; the precise mechanism by which this occurs is unclear.53 Thermally generated 1-naphthylcarbene gives cyclobuta[de]naphthalene (31)by intramolecular C-H insertion apparently before any rearrangements involving for example benzocycloheptatrienylidene species occur.54 Remarkably however 2-46 N.J. Turro,M. Aikawa J. A. Butcher jun. and G. W. Griffin J. Am. Chem. SOC., 1980 102 5127. See also P. P. Gaspar B. L. Whitsel M. Jones jun. and J. B. Lambert ibid. p. 6108. 4' J. J. Zupancic and G. B. Schuster J. Am. Chem. SOC.,1980,102,5958. 48 X. Creary J. Am. Chem. Soc. 1980,102 1611. 49 H. Durr H. Nickels L. A. Pacala and M. Jones jun. J. Org. Chem.1980 45 973. so N. J. Turro J. A. Butcher jun. R. A. Moss W. Guo R. C. Munjal and M. Fedorynski J. Am. Chem. SOC.,1980,102,7576. s1 R. W. Hoffmann I. H. Loof,and C. Wentrup Liebigs Ann. CFem. 1980 1198. s2 K. A. Biesiada C. T. Koch and P. B. Shevlin J. Am. Chem. SOC., 1980,102,2098. 53 K. Saito Y. Omura andT. Mukai Chem. Lett. 1980 349. 54 J. Becker and C. Wentrup J. Chem. SOC.,Chem. Commun. 1980 190. D. W.Knight naphthylcarbene gives the same product (31) presumably by equilibration to 1-naphthylcarbene via the cyclo heptatrienylidene (32). Some experiments involving the pyrolysis of various 4-phenylbut-3-en-1 -ynes have further delineated the mechanism of the azulene-naphthalene rearrange-ment.55aa@-O& 1 .-. \/ \ (31) (32) (33) (34) pc e p-c Me2C=C=C=C=C=C (35) (34) (37) Ab initio calculations suggest that cyclopentadienylidenecarbene (33) should be highly electrophilic because there is a significant contribution from the resonance structure (34) which contains an aromatic cyclopentadienyl Similar arguments lead to the conclusion that cyclopropenylidenecarbene [(35) C) (36)] should behave as a nucleophilic (or ambiphilic) alkylidenecarbene.Stang and Ladika have gone one better than last year by succeeding in preparing the 1,2,3,4-hexatetraenyl- idenecarbene (37),57 using the same approach (i.e. y-elimination from l-ethynyl- vinyl triflates) as for the lower homologues. Calculations suggest that the sequence of events in the photochemical reaction between carbon suboxide and ethylene involves the intermediacy of carbonylcar- ber~e,~* as outlined in Scheme 2.The initial reaction of carbon suboxide proceeds via its second excited state. The next higher homologue of carbonylcarbene can be stabilized by formation of the complex (38) using pentacarbonylchromium.59 Scheme 2 The first (spectroscopic) characterization of an electrophilic transition-metal- methylene complex has been reported.60 The complex (39) shows non-equivalent carbene protons in its ‘H n.m.r. spectrum and the freelenergy of activation for Fe-carbene bond rotation was found to be 10.4*0.1 kcal mol-1 by variable- temperature studies. The complex (39) is capable of effecting cyclopropanation of olefins (cf. ref. 45). The role of carbenes in olefin metathesis continues to attract interest.The bridging methylcarbene complexes (40; M =Fe or Ru) react with alkynes under ultraviolet irradiation to give intermediates that are reminiscent of vinylcarbene-metal com-plexes and which can be further allowed to react with carbon monoxide to give actual ” J. Becker C. Wentrup E. Katz and K.-P.Zeller J. Am. Chem. SOC.,1980 102 5110. ”Y.Apeloig R. Schrieber and P. J. Stang Tetrahedron Lett. 1980 21,411. ’’P.J. Stang and M. Ladika J. Am. Chem. SOC.,1980,102 5406; see also L. T. Scott and G. J. DeCicco J. Org. Chern. 1980,45,4055. ”T. Minato Y. Osamura S. Yamabe and K. Fukui J. Am. Chem. SOC.,1980,102,581. 59 H.Berke and P. Harter Angew. Chem. Znt. Ed. Engl. 1980,19,225. ‘O M. Brookhart J. R. Tucker T. C.Flood and J. Jensen J. Am. Chem. SOC.,1980 102,1203; for some calculations relevant to the structure of such complexes see R. J. Goddard R. Hoffmann and E. D. Jemmis ibid. p. 7667. Arynes Carbenes Nitrenes and Related Species 0 + II C II C H II / II :.co \ OC-Cr-CO /\ H /I Ph Ph oc co vinylcarbene complexes (41; M = Fe).61 This overall acetylene ‘insertion’ reaction provides a mechanism for polymerization of alkynes using transition metals. A careful analysis of the products that formed in some crossed olefin-metathesis reactions shows that carbene complexes RCH=M (M = metal) are preferred to H2C=M as chain carriers and that the latter species are very reactive towards C-1 of the terminal olefin.62 A very useful review of intramolecular insertion reactions of a-diazocarbonyl compounds has recently appeared.63 Epoxy-diazo-ketones (42) rearrange to diones (43) (isolated as their dimethyl acetals) on treatment with copper in methanol; a cyclic oxonium ylide may be an ix~termediate.~~ (42) (43) (44) The Wolff rearrangement continues to attract a considerable amount of interest.It has been briefly reported that vinylene thioxocarbonates (44) can serve as precursors of a-keto-carbenes as The initial cis-conformation of the carbene could favour the formation of oxirens i.e. the elusive species which are probably intermediates in the Wolff rearrangement. Various experiments in which a-keto-carbenes have been trapped by olefins during the Wolff rearrangement of some a-diazo-ketones strongly support the idea that singlet keto-carbenes are in equilibrium during the reaction and that the position of equilibrium depends upon the nature of the substituents.66 Similar experiments with dialkyl a-diazo-ketones 61 A.F. Dyke S. A. R. Knox,P. J. Naish and G. E. Taylor J. Chem. SOC. Chem. Commun. 1980 803; A. F.Dyke S. A. R. Knox P. J. Naish and A. G. Orpen ibid. p. 441.See also J. Levisalles H. Rudler F. Dahan and Y. Jeannin J. Organomet. Chem. 1980,188 193. 62 L. Benne K. J. Ivin and J. J. Rooney J. Chem. SOC. Chem. Commun. 1980 834. 63 S.D. Burke and P. A. Grieco Org. React. 1979,26 361. 64 L.Thijs and B. Zwanenburg Tetrahedron 1980,36 2145. ” M.Torres A. Clement and 0.P. Strausz J. Org. Chem. 1980 45 2271. H.Tomioka H. Okuno S. Kondo and Y. Izawa J. Am. Chem. SOC. 1980 102 7123. See also H.Tomioka H. Okuno and Y. Izawa J. Org. Chem. 1980,45,5278. D. W.Knight lend further weight to the notion of equilibrating keto-carbenes and also suggest the intermediacy of an oxiren as the epoxidation of an unsymmetrical dialkyl-acetylene leads to the same ratio of enone products as that produced by Wolff rearrangement of the corresponding dia~o-ketone.~~ Calculations indicate that oxirens would be rather short-lived species as expected.68 The photochemical Wolff rearrangement of monothiolo-esters (45)of malonic acid results in a remarkably selective migration of the sulphur group to give the keten (46) which can be trapped by a variety of reagent^.^' The reason behind this could be participation of a cyclic sulphonium ylide (cfiref.64). 0 N2<c02MeC-SR 44c-c II 0 (45) (46) Me0/\H (47) Carbenes with electron-withdrawing substituents usually have triplet ground states and hence it was surprising that methoxycarbonylcarbene showed what is clearly a singlet ground state. MIND0/3 calc~lations,~~ however have led to a formulation of a 'closed' structure (47)for such carbenes where the singlet state is favoured by donation of a non-bonding pair of electrons from the carbonyl oxygen into the vacant p-orbital of the carbene. In solution an 'open' structure may predominate with solvent molecules acting as electron donors. Work on the selectivity of insertion of (47)into C-H bonds further shows it to possess a singlet ground state whereas the corresponding diester :C(CO,Et), displays both singlet and triplet reactivity when generated by photolysis of diethyl diaz~malonate.~' What is still not clear is the role if any played by excited diazo-ester species in the overall insertion reaction (cf.ref.32). Cyclopropanation of olefins with diazo-esters catalysed by rhodium(I1) carboxyl- ates appears to proceed via a carbenoid mechanism [as with catalysis by copper(^)] and to be a good synthetic procedure especially with isolated disubstituted olefins whereas the use of palladium(I1) carboxylates is much less effe~tive.~ The cyclopro- panation of electron-poor olefins (e.g. ap-unsaturated nitriles and esters) by ethyl diazoacetate and a-diazoacetophenone is viable when [Mo(CO),] is used as the catalyst.73 An extensive study of the reactions between pyrroles and ethyl diazoace- tate catalysed by various copper salts leads to the conclusion that they are best described as electrophilic substitutions rather than insertions involving homopyrrole intermediate^.^^ 67 R.A. Cormier Tetrahedron Lett. 1980 21 2021. 68 K.Tanaka and M. Yoshimine J. Am. Chem. SOC.,1980,102,7655; for a review see M. Torres E. M. Lown H. E. Gunning and 0.P. Straw Pure Appl. Chem. 1980,52 1623. 69 V. Georgian S. K. Boyer andB. Edwards J. Org. Chem. 1980 45 1686. 'O R.Noyori and M. Yamakawa TetrahedronLett. 1980,21,2851.See also K.S. Kim and H. F. Schaefer 111 J. Am. Chem. SOC.,1980 102 5389. 71 H. Tomioka M. Itoh S. Yamakawa and Y.Izawa J. Chem. SOC.,Perkin Trans. 2 1980 603; H. Tomioka H.Okuno and Y. Izawa ibid. p. 1636. 72 A. J. Anciaux A. J. Hubert A. F. Noels N. Petiniot and P. Teyssit J. Org. Chem. 1980,45,695; A.J. Anciaux A. Demonceau A. J. Hubert A. F. Noels N. Petiniot and P. Teyssib J. Chem. Soc. Chem. Commun. 1980,765. 73 M. P. Doyle and J. G. Davidson J. Org. Chem. 1980 45 1538. 74 B. E. Maryanofi J. Org. Chem. 1979,444410. Arynes Carbenes Nitrenes and Related Species 4 Silylenes The volume of work reported in this area during the past year merits a separate section. Calculations show that methylsilylene MeSiH has a singlet ground state the lowet! triplet state being 19 kcal mol-' higher.75 By contrast silylmethylene H3SiCH has a triplet .Fround state with a lowest singlet state that is 25 kcal mol-' higher.Although MeSiH and silaethylene H2C=SiH2 are of approximately the same energy their interconversion via a [1,2]-hydrogen shift is difficult owing to a large energy barrier of ca 40 kcal mol-'. Silylenes (R2Si:) are often prepared by pyrolysis of 7-silanorbornadienes; by studying the thermal stability of a series of such precursors the formation of the silylenes has been-found to be a two-step process via the diradical (48).76 Germylenes (R2Ge:) can be similarly prepared again by a two-step me~hanism.~' Calculations predict that such germylenes (with R=H F or Me) have singlet ground states with 10 64 and 14kcalmol-' respectively between the singlet and lowest triplet Insertion reactions of silylenes sometimes require higher than expected activation energies.This has been-explained by assuming that such insertions by ground-state singlet silylenes are forbidden and thus the activation energy partly consists of the energy needed to promote the silylene to a triplet Dimethylsilylene requires approximately zero activation energy to insert into Si-H bonds whereas insertion into the H-Cl bond requires ca 28 kcal mol-1.80 (48) Dimethylsilylene generated at 0 "C by photolysis of dodecamethylcyclo-hexasilane adds to oxetan to give two products (50) and (51) both of which probably arise from the ylide (49)." Much the same type of ylide intermediates have been postulated to account for the products obtained by reactions between dimethyl- silylene and @-unsaturated epoxidesS2 or ally1 Competition experiments in which Me2Si has been allowed to react with pairs of alcohols show that the silylene has a much greater selectivity of insertion into the 0-H bonds when ether is used as solvent.This suggests the involvement of a less reactive silylene probably due to some interaction with the ether oxygens4 [cf. (49)]. Dimethylsilylene reacts with olefins to give silirans which can be trapped by methanolysis. The initial reaction has now been shown to be stereospecific as is the 75 J. D. Goddard Y. Yoshioka and H. F. Schaefer 111 J. Am. Chem. SOC.,1980 102 7644. 76 B. Mayer and W. P. Neumann Tetrahedron Lett. 1980,21,4887. 77 W. P. Neumann and M. Schriewer Tetrahedron Lett. 1980,21 3273. 78 J.-C.Barthelat B. S. Roch G. Trinquier and J. Satgt J. Am. Chem. Soc. 1980 102 4080. 79 T. N. Bell K. A. Perkins and P. G. Perkins J. Chem. SOC.,Chem. Commun. 1980 1046. 8o I. M. T. Davidson F. T. Lawrence and N. A. Ostah J. Chem. SOC.,Chem. Commun. 1980,859. T.-Y. Yang Gu and W. P. Weber J. Am. Chem. SOC.,1980,102 1641. '* D. Tzeng and W. P. Weber J. Am. Chem. Soc. 1980,102 1451. 83 V. J. Tortorelli and M. Jones jun. J. Chem. SOC.,Chem. Commun. 1980 785. 84 K. P. Steele and W. P. Weber J. Am. Chem. Soc. 1980 102 6095. D. W.Knight addition of methanol but it is still not proven to be cis or trans although the former seems highly likely.85 In the gas phase difluorosilylene appears to behave similarly to a carbene adding to butadiene for example to give a siliran.However its reactions in the condensed phase follow a rather different path which could involve diradicals such as (52; n = 2,3 . . .).86 Trapping experiments in which difluorosilylene is co-condensed at -196"C with for example propene suggest that the initial stages of the overall sequence involve formation of a siliran perhaps in an excited state which can either ring-open to give the diradical(53) or relax to the ground The main product of the reaction is a regular polymer which seems to support the idea of radical intermediates. 85 V. J. Tortorelli and M. Jones jun. J. Am. Chem. SOC.,1980,102 1425. 86 T. Hwang Y. Pai and C. Liu J. Am. Chem. SOC.,1980 102,7519; T. Hwang and C. Liu ibid. p. 385. W. F. Reynolds J. C. Thompson and A.P. G. Wright Can. J. Chem. 1980,58,419,425,436.

ISSN:0069-3030    

DOI:10.1039/OC9807700067

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

6 Electro-organic Chemistry By J. H. P. UTLEY* Department of Chemistry Queen Mary College Mile End Road London El 4NS 1 General and Mechanistic It is the custom in this section to concentrate upon preparatively useful electro- organic reactions and purely mechanistic studies have previously received scant attention. Recent advances in technique and interpretation have been such however that it is timely to summarize some of the more important findings. Expert reviews have appeared concerning two burgeoning fields of study i.e. electro-catalytic reactions' and chemically modified electrodes.' The latter review is helpful in that it concentrates upon the more rigorous and reproducible work in this area. The details of electron transfer at an electrode or between species in homogeneous solution are much discussed and an important contribution has been made3 concern- ing the distance over which electrons may rapidly be transferred.The macrocycle (1)is reduced cathodically in a two-electron step i.e. its radical anion disproportion- ates readily with Kd= 0.14* 0.1. However partial reduction (with potassium) allows e.s.r. spectroscopic observation of the radical anion and allowance can be made for contamination with the dianion. The spectra show that the benzene rings of the radical anion of (1)are non-equivalent but that electron transfer between the benzene rings is detectable on the e.s.r. time-scale when the macrocycle is com- plexed with sodium or potassium ions. It was consequently determined in the range 309 to 195 K that the energy of activation for electron transfer over the inter- nuclear distance of 7 A was 5.85 kJ mol-' with a pre-exponential factor of 4.6* 0.1 x lo7s-'; at 291 K tb rate constant for electron transfer is 3.9 x lo6s-'.Theoretical treatments of the rates and energetics of electro-organic reactions are often hampered by lack of fundamental data such as reversible electrode potentials. (1) * The author gratefully acknowledges a current-awareness service provided by Helen Thomas (of Queen Mary College Library). ' J. M. Saveant Acc. Chem. Res. 1980,18,323. R. W.Murray Acc. Chem. Res. 1980,18,135. S. Mazur V.M. Dixit and F. Gerson J. Am. Chem. SOC.,1980 102,5343. 79 J. H. P.Utley In recent years voltammetric studies at fast scan rates have provided much informa- tion but the technique of choice for such measurements is second-harmonic a.c.voltammetry. A detailed account has been published4 of the scope and limitations of the technique. Parker and his group are also engaged in the useful process of simplifying (for more general use) the sophisticated voltammetric techniques and the interpretation of the relevant data. In this context it has been shown that potentials at half peak height (EPl2) may be measuredS to f1mV from a pen-and-ink trace. A large amount of work has concerned the use of plots of dE/d[log (scan rate)] and of dE/d[log (concentration)] in diagnosing the sequence of electron-transfer and chemical-reaction steps; the general conclusions of this approach have been summarized in a qualitative set of relatively easily applied rules.6 It is claimed7 that double potential-step chronoamperometry is particularly useful for discriminating between the ECE and the DISP 1mechanisms; the two mechan- isms are exemplified for hydrogenation of aromatic hydrocarbons in Scheme 1.The point at issue is whether the second electron-transfer process occurs at the electrode or in solution by homogeneous electron transfer.For hydrogenation of anthracene and naphthalene the clear conclusion is that the DISP 1mechanism is followed. The A & A7 ArX & ArX' [ArX = 2-chloroquinoline] k AT + PhOH + AH' + PhO-ArX' -b Ar' + X-AH' + A; + AH-+ A (DISP 1) Ar' + Nu-+ ArNu' [Nu-= PhS-] [or AH' + e-+ AH-(ECE)] ArNu' + ArX + ArNu + ArX' (DISP 1) AH-+ PhOH + AH;?+ PhO-[or ArNu' -e-+ ArNu (ECE)] Scheme 1 Scheme 2 general comment is made' that the ECE mechanism has not yet been observed under the conditions of voltammetric or amperometric techniques i.e.at low concentration and with non-steady-state measurements. In an example of an electrochemically induced chemical reaction (SRN1), namely that involving 2-chloroquinoline and thiophenolate anion in liquid ammonia solution (Scheme 2) a thorough analysis* of the kinetic characteristics of the reaction suggests that the DISP 1route is followed for experiments involving high rates of potential scan whereas at low scan rates the results are best accommodated by the ECE route. For this particular reaction the rate constant kl for cleavage of the initially formed radical anion is 1.7 x lo4s-l.Similar reactions take place in solutions in dimethyl sulphoxide (DMSO) and for these cases a question has been raised' concerning the likely importance of hydrogen abstraction from the solvent by intermediate aryl radicals; it has previously been reported" that a-naphthyl radicals react exclusively with thiophenolate anion with no concurrent hydrogen abstraction in DMSO solution. By generating a-naphthyl E. Ahlberg and V. D. Parker Acta Chem. Scand. Ser. B 1980,34,91. B. Aalstad and V. D. Parker J. Electroanal. Chem. Interfacial Electrochem. 1980,112 163. V. D.Parker Acta Chem. Scand. Ser. B 1980,34,359. C. Amatore and J. M. Saveant J. Electroanal. Chem.Interfacial Electrochem. 1980,107 353. C. Amatore J. M. Saveant and A. Thiebault J. Electroanal. Chem. Interfacial Electrochem. 1979,103 303. B.Helgee and V. D. Parker Acta Chem. Scand. Ser. B 1980,34,129. lo J. Pinson and J. M. Saveant J. Am. Chem. SOC. 1978,100 1506. Electro-organic Chemistry radicals in homogeneous solution the absolute rates of reaction with various solvents and with thiophenolate anion have been measured.' For the reaction between a-naphthyl radical and DMSO DMF acetonitrile and thiophenolate in DMSO the rate constants are respectively 3 x lo5 8 x lo6 2.5 x lo5 and 1.7 x lo81mol-' s-l. Rates of abstraction of hydrogen are therefore significant. The anodic pyridination of 9,lO-diphenylanthracene (DPA) has for some years been an example of at least one electrochemical reaction with a well-understood ECE mechanism.Linear-sweep voltammetry reveals,ll however that at low con- centrations the rate of disappearance of 9,lO-diphenylanthracene radical cation has a second-order dependence on its concentration. The voltammetric data are best explained by Scheme 3. \ rn Scheme 3 There is now considerable evidence to show that at least under conditions amenable to study by electroanalytical techniques alkyl-aromatics are oxidized anodically according to the DISP 1rather than to the ECE mechanism (Scheme 4). The combined results12 of electrochemical and spectroelectrochemical methods are particularly persuasive. For the oxidation of pentamethylbenzene in acetonitrile the decay with time of the corresponding benzyl cation (ArCH,') initially formed anodically was followed by reflectance spectroscopy.The observed curve nicely fits that predicted for the DISP 1mechanism and is clearly distinct from the very rapid decay predicted for the ECE route. The loss of proton from the radical cation is judged to be the slow step the subsequent oxidation in homogeneous solution being diff usion-controlled. Similar conclusions have been reached following the applica- tion13 of cyclic voltammetry and double-step chronoamperometry to the anodic RH RH? RHt-* R'+ H' (slow?) RH' + R' S Rf + RH (DISP 1) [or R' -e RC (ECE)] + R+ + MeCN + RN=CMe Scheme 4 E. Ahlberg and V. D. Parker Acta Chem. Scand. Ser. B,1980 34 97.l2 A. Bewick J. M. Mellor and B. S. Pons Electrochim. Acta 1980,25 931. l3 (a) J. Barek E. Ahlberg and V. D. Parker Acta Chem. Scand. Ser. B 1980 34 85; (b) R.S. Baumberger and V. D. Parker ibid. p. 537. J. H. P. Utley oxidation of hexamethylbenzene in methylene chloride-trifluoroacetic acid (TFA) solution or in acetonitrile with and without added TFA. In the latter study however it is suggested that the rate of deprotonation of radical cation will depend on the nature and concentration of an unspecified base and that in acetonitrile solution the rate of reaction between benzyl cation and solvent is sufficiently slow possibly to qualify as a rate-limiting step. The SRNl reaction (see above) is now well established and it is not surprising therefore that formal consideration has been given14 to electrophilic substitutions induced at electrodes.It has been suggested that the reactions outlined in Scheme 5 should be observable; in their de~ignation'~" as SoEl,SRE2 and SON29 0and R (a) SoEl (for H-D exchange) (6) SRE2 (forH-D exchange) RH+ + B + R' + BH+ RH; + BD -+ [RHD]' + B- R' + BD+ -+ RDt + B [RHD]' + B- -+ RD7 + BH RDf + RH -P RD + RHf RD' + RH -+ RD + RHT (c) SON2 (nucleophilic substitution) RXt + Nu-B [RXNu]' [RXNu]' -+ RNut + X-RNut + RX + RNu + RX? Scheme 5 stand for oxidative and reductive respectively and the other symbols have the usual mechanistic meaning. (In SRNl the R stands for radical!) Attention has been drawn14b to several possible cases of the SON2 mechanism and one of the most convincing is the previously puzzling acetoxylation of 4-fluoroanisole which involves dis- placement of fluorine [cf.Annu.Rep. Prog. Chem. Sect. B 1976,73 1421. Further work on this reaction has now shown that at low conversions and at low substrate concentration the current efficiency approaches 300% i.e. it is catalytic and these observations may be explained by the SON2 mechanism (Scheme 6). -e ArF F=? ArFt (ArF = p-MeOC6H4F) ArFt + AcO-+ [Ar/F -+ ArOAc' + F-]* 'OAc ArOAc+ + ArF -+ ArOAc + ArFt Scheme 6 l4 (a) R. W. Alder J. Chem. Soc. Chem. Commun. 1980,1184; (b)L. Eberson and L. Jonsson ibid. p. 1187. Electru-organic Chemistry A thorough re-examination has been made15 of the voltammetry of 9-diazofluorene (FIN2).The cyclic voltammetry is complex and several redox couples are observed; profound changes with temperature are seen with various couples dominating at various temperatures. The observations are well rationalized by a scheme in which the key features are the formation of a dimeric dianion which is relatively long-lived at 223 K but which at higher temperatures may lose nitrogen to give a cis-azine dianion which can then undergo isomerization and oxidation to the fluorenone azine product (Scheme 7).The loss of nitrogen from the initially formed radical anion [cf. Annu. Rep. Prug. Chern. Sect. B 1977 74 1621 is also considered unlikely as in linear-sweep voltammetry the gradients of plots of dE/d[log (scan rate)] and of dE/d[log (concentration)] would be respectively 29.6 and OmV per decade.The values now determinedI5' are 20.7 and 19.2mV per decade i.e. near to those predicted for the mechanism in Scheme 7. Fl-F1-F1-\ \/ N=N N-N -N FIN;?' I II -A dr Fl-/ N-N F1-\N=N (F1 = 9-fluorenyl) \F1-Scheme 7 That radical anions may be photo-excited to even more highly reducing species has been proved in an ingenious fashion.I6 The basic idea is that if photoexcitation induces rapid transfer of electrons from the radical anion the initial substrate is regenerated and it may accept another electron from the cathode with a concom- itant increase in current i.e.photochemical reduction is catalysed. The experimental difficulty is that cyclic voltammetric peak currents are diff usion-controlled and any incursion of convection caused by the local heating of photoexcitation will also increase peak currents.The temperature rise close to the electrode can easily be of the order of 10-15 K. Several pieces of evidence16 now support the contention that the major factor causing increase of current in these systems is electron transfer from the photoexcited state. For pyrene in DMF with chlorobenzene present as an irreversible electron acceptor concurrent U.V. irradiation and measurement of the peak current shows that around 500nm the peak-current heights vary with the absorbance. Even more compelling is the observation that for concurrent irradiation and cyclic voltammetry of dimethyl terephthalate (DTP)in the presence of chlorobenzene considerable enhancement of the second wave is found (formation of DTP2-) whereas in the same experiment only slight increases in current are seen for the first wave (formation of DTP').The explanation is that only the photoexcited dianion is capable of reducing chlorobenzene (Scheme 8). Discussion of the mechanism of the anodic cleavage of dibenzyl ether continues" [see Annu. Rep. Prug. Chem. Sect. B 1972,69,310; 1975,72,157]. As a result of l5 (a)D. Bethell P. J. Galsworthy K. L. Handoo and V. D. Parker J. Chem. Soc. Chem. Commun. 1980 534;(6)V.D.Parker and D. Bethell Acru Chem. Scand. Ser. B 1980 34,617. l6 H.S.Carlsson and H. Lund Acra Chem. Scand. Ser. B 1980,34,409. l7 (a)J. W. Boyd P. W. Schmalzl and L. L. Miller J. Am. Chem. Soc. 1980,102,3856; (b)E.A.Mayeda L.L. Miller and J. F. Wolf ibid. 1972,94,6812;(c)R.Lines and J. H. P. Utley J. Chem. Soc. Perkin Trans. 2 1977,803. J. H. P. Utley DTP & DTPT % [DTP']*(unreactive) 11.-PhCl DTP2--%[DTP2-]* -DTP' + Ph' + C1-T e Scheme 8 the most recent st~dy'~" there is better agreement on experimental facts; it is now acknowledged that in acetonitrile benzoic acid is produced during work-up and not directly as was originally rep~rted,"~ and also that benzylacetamide is a major product which was initially. The main point is whether in neutral or acidic solution the reactive intermediates are the cation (2) which is produced by deprotonation of the radical cation of the ether with further oxidation or PhCH20' and PhCH2+ which are formed by fission of the radical cation of the ether.17c From the relative amounts of labelled products following oxidation of PhCH20CD2Ph values of kH/kD depending on electrolyte conditions of 1.9k0.2 and 1.56 (error limits not given!) are and claimed as primary isotope effects connected with loss of a proton from the radical cation of the ether i.e.in support of intermediate (2). The possibility remains however that these are secondary isotope effects or the small primary effects that are characteristic of radical abstraction. PhcHOCH2Ph (2) These ambiguities combined with the imprecision of the experiments and the fact that 180-labelling show that oxygen in each of the products derives from the ether leave the question open. Hydrogen-deuterium kinetic isotope effects in the range kH/kD= 1.4 to 1.6f0.2 have also been used to support the suggestion of intramolecular abstraction of hydrogen in the anodic oxidation" of hexan-2-one (Scheme 9); similar values of kH/kD are obtained for rearrangement following photochemical excitation and electron-impact ionization.li Reagents i -e- MeCN H20 Scheme 9 l8 M. M. Green G. J. Mayotte L. Meites and D. Forsyth J. Am. Chem. SOC.,1980,102,1464. Elec tro -orga nic Chemistry 85 2 Anodic Processes The Kolbe reaction remains the most versatile method for the unambiguous syn- thesis of hydrocarbons and fatty acids. Schafer's studies in this direction continue" with the synthesis by successive Kolbe couplings of a pheromone (3)of the German cockroach.ClgH37CH(Me)[CH2I7CH(Me)COMe (3) The desilylation of cations is rapid compared with competing deprotonation and this has been put to good use in a synthesis2' of terminal alkenes. The method is highly selective seems to have considerable scope and there is good entry to the required carboxylate (Scheme 10).Similarly the oxidation21 of hydroquinone silyl ethers to quinones is very efficient. RCH=CH2 i ii (Et02C)2CH2 +Me3SiCH2CH(R)C02H-% [Me3SiCH26HR] -B + Me3Si+ Reagents i Base Me,SiCH2Cl; ii base RX; iii carbon anode MeCN-MeOH Scheme 10 A thorough study has been reported22 of the anodic alkoxylation of cyclic lactams; lowest yields (ca 40%) of the a-alkoxylated products (4) are found for n = 3 R=n-C4H9 whereas highest yields (ca 90Y0)are when n =4 R=Me.A related reaction has been used23 for the synthesis (Scheme 11)of several lactones one of which i.e. (9,is used as an intermediate in a route to (*)-eburnamonine (6). Lactonization does not take place in the absence of an angular substituent. @co2H ilJ$J0 ' 0 H [58%] R=Et (5) .. (6) H % [42'/0] R = CH~OAC [21%] R = C02Me Reagents i Pt anode MeCN-H,O constant current 4 F mol-'; ii several steps Scheme 11 l9 W. Seidel and H. J. Schiifer Chem. Ber. 1980 113,451. *' T. Shono H.Ohmizu and N. Kise Chem. Left.,1980,1517. 21 R. F. Stewart and L. L. Miller J. Am. Chem. SOC.,1980 102,4999. 22 M. Mitzlaff K. Warning and H. Rehling Synthesis 1980 315. 23 K. Irie M. Okita T. Wakamatu and Y.Ban Nouo.J. Chim.1980. 4 275. J. H. P. Utley The anodic of disulphides and diphenyl diselenide in acetonitrile solutions gives an electrophilic species that is capable of trans addition to alkenes. For disulphides at it has been supposed that the intermediate is a nitrilium + ion such as RSN=CMe. In the presence of acetate sulphides undergo" anodic a-acetoxylation which provides an efficient route to unsaturated sulphides. Examples of the application of each of these reactions are given in Scheme 12. Me(CH2)&H=CH2 Me(CH2)5CH(NHCOMe)CH2SPh[84%] " cyclohexene & mNHCOMe C0,Me AcO C0,Me C0,Me A S P h --*iii d s p h d S p h [7So/o] [94%] Conditions i Divided cell at +1.4 V (us AglAg') 2F mol-' MeCN (PhS),; ii as i except that at +1.3 V (PhSe), iii HOAc NaOAc constant current 2-3 F mol-'; iv 163-6 "C at 0.45 Torr Scheme 12 Zinc octaethylporphyrin (7) may be cyanated at the meso-positions anodically,26 via the cation radical; by variation of the oxidation potential and the extent of electrolysis it is possible to obtain optimum yields of mono- di- tri- and tetra- cyano-octaethylporphyrins.The monocyano-product may be obtained quantita- tively. Both dicyano-isomers are obtained in a 1 :1 mixture with an overall yield of 78%. At the other limit the tetracyanoporphyrin is obtained in 40% yield. (7) The anodic decarb~xylation~~ of tetrahydro-@-carbolinecarboxylic acids (Scheme 13) is achieved at very low potentials [0.13-0.77 V (us S.C.E.)]. This is almost certainly an example of the pseudo-Kolbe reaction [see Annu.Rep. Prog. Chem. Sect. B 1971,68 313; 1976,73 1391 whereby rapid decarboxylation of a radical cation is followed by further oxidation. 24 (a)A. Bewick D. E. Coe J. M. Mellor and D. J. Walton J. Chem. SOC.,Chem. Commun. 1980,51; (b)A. Bewick D. E. Coe G. B. Fuller and J. M. Mellor TetrahedronLett. 1980,21 3827. 25 J. Nokami M. Hatate S. Wakabayashi and R. Okawara TetrahedronLett. 1980,21,2557. 26 H.J. Callot A. Louati and M. Gross TetrahedronLett. 1980 21 3281. 27 J. M. Bobbitt and J. P. Willis J. Org. Chem. 1980,45,1978. Electro-organic Chemistry N Me [6O%] Conditions i Carbon felt anode MeOH-H,O divided cell phosphate buffer Scheme 13 3 Cathodic Processes 1,4-Diazepinium cations provide an e~ample~**~~ where one-electron reduction to a radical intermediate is clearly defined; formation of an anion by transfer of a second electron is possible but it occurs at ca 0.8 V cathodic of the first reduction wave.28 Controlled-potential reduction of the 5,7-diphenyl derivative in DMF at the first reduction potential gives a 1Fmol-' reaction and the most likely fate of the radical is disproportionation.In contrast the reduction of the 6-phenyl deriva- tive gives one-electron reduction with rearrangement to a single product which is isolated in 92% yield. The contrasting results are summarized in Scheme 14. [47%] (hydrolyses on work-up) H Conditions i DMF at -1.23 V (us AglAgCl) 1Fmol-' [92%] Scheme 14 28 D. Lloyd C. A. Vincent and D. J. Walton J.Chem. SOC., Perkin Trans. 2 1980,668. 29 D. Lloyd C. Nymo,C. A. Vincent and D. J. Walton J. Chem. Soc. Perkin Trans. 2 1980 1441. CH,Br / \ + Conditions i DMF A1 cathode undivided cell 0.7 A cm-* 2.5F mol-' Scheme 16 30 T. Shono Y. Usui T. Mizutani and H. Hamaguchi Tetrahedron Lett. 1980 21,3073. 31 J. H.P. Utley and A. Webber J. Chem. SOC.,Perkin Trans. 1 1980 1154. Elec tro -orga nic Chemistry A surprisingly simple and potentially large-scale method for the cathodic Birch reduction of benzene has been described.32 In an undivided cell with an electrolyte of tetra-n-butylammonium hydroxide in aqueous ethylene glycol benzene is reduced to cyclohexa-l,4-diene in 81'/o yield accompanied by about 10% of cyclohexene.Large amounts of the quaternary hydroxide are required and it must be that reduction by solvated electrons is possible at an essentially hydrophobic cathode environment with sufficient benzene dissolved in the electrolyte. Shono and his group have cleverly exploited33 cathodic elimination reactions in a method for extending the carbon chain of carbonyl compounds. Several examples are given and the yields of the enol ethers or sulphides are high (60-96%). The reaction is run at constant current and presumably cathode potentials are relatively high which will limit the method to compounds without other easily reducible functions. Some examples are contained in Scheme 17. PhCHZCH2CHO PhCH2CH,CH(OH)CH(SPh)2 & PhCH2CH2CH=CHSPh PhCH2CH2CH2CHO [92O/o ] Reagents i (PhS),CHLi; ii Pb cathode divided cell DMF-Et,NOTs; iii PhS(0Me)CHLi Scheme 17 The ketones (8) and (9) are known to react with respectively lithium in liquid ammonia and a dialkylcopper lithium reagent to give the tricyclic compounds (10) and (11).For both ketones reduction potentials are less cathodic with the sub- stituents at the 5-position than without and this suggests that cathodic cyclization might be effected.In the case of (9) cathodic to (11)is smooth provided that a good hydrogen-atom donor (e.g. isopropyl alcohol) is present to trap the intermediate radical. In the absence of such a donor the dimer (12) is obtained in high yield. The cathodic of (8) was not tried in the presence of a hydrogen-atom donor and in aqueous methanol the major product (71%) was the dimer (13).The results and their rationalization are given in Scheme 18.32 J. P. Coleman and J. H. Wagenknecht U.S. P. 4 187 156 (1980). 33 T. Shono Y. Matsumura and S. Kashimura Tetrahedron Lett. 1980 21 1545. R. A. J. Smith and D. J. Hannah Tetrahedron Lett. 1980 21 1081. 35 L. Mandell H. Hamilton and R. A. Day J. Org. Chern. 1980,45 1710. J. H. P.Utley OMS 0f"r, (9) 0&<*oyJ%% (10) (8) HO (12) X=H2 (13) X=O X Conditions i Hg cathode at -2.2 V (US S.C.E.) DMF-Pr'OH 1.05F mol-'; ii Hg cathode at -1.7 V (us S.C.E.) 50% aq. MeOH 1 Fmol-' Scheme 18 One-step reductive cyclization is achieved when suitable radical anions are allowed to react with 'dielectrophiles'. A full by Degrand and her co- workers contains several impressive examples and some of these are given in Scheme 19.The case involving reduction of an imine [at -1.8 V (us S.C.E.)]is reminiscent of the annelation described in Scheme 15; in Degrand's examples the electro-active species is more clearly defined. Ph N=N N-N + 6 c7 PhN=CHPh -b [59%] [78%] Conditions i Hg cathode DMF Br(CH,),Br cu 3 F mol-' Scheme 19 Selective cleavage at cathodes competes well with alternative methods and for potentially large-scale reactions it may be preferable to reduction with metals for environmental reasons. In this context the conversion of thiophen into 3- bromothiophen has been st~died;~' the key step is selective cleavage of 2,3,5- tribromothiophen. The results of this pragmatic investigation are that in aqueous dioxan high yields are obtained (80-93%) at Hg Pb Zn or graphite cathodes.A route to cyclopropanones has been explored which is based on earlier methods for ring formation in the reduction of 1,3-dihalides. In the latest however protection of the carbonyl function is built in from the start. In the absence of such protection but in nucleophilic solvents which permit hemiacetal formation cycliz- ation is achieved only for highly alkylated compounds (Scheme 20). 36 C. Degrand P. L. Campagnon G. Balot and D. Jacquin J. Org. Chew. 1980.45 1189. " D. Pletcher and M. Razaq J. Appl. Electrochem.,1980 10.575. '' W. J. M. van Tilborg R. Plomp R. dc Ruiter and C. J. Smit Recl. Trav. Chim. Pays-Bas 1980,99,206. Electro-organic Chemistry [75Yo ] [85'/o ] Conditions i at ca -2.5 V (us S.C.E.) MeCN 0 to -15 OC; ii MeOH-MeCN reduction as before.Scheme 20 The cathodic reactions of several classes of organosulphur compounds are proving to be of considerable Strictly the case of reductive rearrangement found for SS-diary1 benzene-1,2-dicarbothioates(14) and shown in Scheme 21 is an SRNlreaction without an external nucleophile. The product of rearrangement is isolated in 85% yield with the consumption of only 0.1F mol-'. Aromatic dithio- and thiol-esters undergo cathodic co~pling,~' with subsequent elimination to form a diarylacetylene; an example is shown in Scheme 22. The formation4* of tetrathioethylenes from dithiocarbonates in nearly quantitative yields (outlined in Scheme 23)is more straightforward involving ring-opening of the radical anion decarbonylation further reduction and finally alkylation by added alkyl halide.0-SAr 0 7orosAr ~cosAr -* Go -+ GO + ArS ' COSAr CSAr I SAr SAr (14) 0-ArS SAr I 0- [85'/o ] Scheme 21 s- s- i I 1 -2RS 2PhCSSR 2PhCSSR7 + PhC-CPh +PhCSCSPh I1 RS SR Y PhMSCSPh PhCECPh [90O/o 3 PhCSS Ph Conditions i Hg cathode; ii 2e- 2 PhCSSR; iii 2e- -2 PhCSS- Scheme 22 39 K. Praefcke C. Weichsel M. Falsig and H. Lund Acta Chem. Scand. Ser. B 1980,34,403. 40 M. Falsig and H. Lund Acta Chem. Scand. Ser. B 1980,34,585. M. Falsig and H. Lund Acra Chem. Scand. Ser. B 1980,34,591. 92 J. H. P. Utley i ii Conditions i Hg cathode at ca -1.5 V (us Ag IAgI) 2 F mol-' DMF; ii RZX Scheme 23 4 Indirect Processes An increasing number of preparatively significant electro-organic reactions may be classed as catalytic and/or indirect reactions.Particularly in those cases where electrode filming is a problem there may be advantage in using the electrode to generate and regenerate an organic or inorganic redox reagent which performs the desired reduction or oxidation in homogeneous solution. For several reactions including the important examples of anodic substitution into the side-chains of alkyl-aromatics and a-acetoxylation and alkoxylation of amides controversy has simmered for years over whether these are direct or indirect oxidations. In these cases a key step would be rapid abstraction of hydrogen by an anodically generated radical (e.g.MeO' NO3',or AcO') with subsequent anodic oxidation of ArCH2 or R'CHNHCOR'. Apart from the nitrate case there has generally been scepticism concerning the methoxyl radical (should it not be 'CH,OH?) and the acetoxyl radical (does it not have a half-life of a mere 60 ns?). In this context Eberson has recently used. bond energy-bond order (BEBO) and equibonding methods to estimate42 activation energies for relevant hydrogen-abstraction reactions and found them to be relatively small .e.g. 12.5-16.5 kJ mol-'. Where comparison with experiment is possible agreement between observed and calculated values is good; the calcula- tions are however wildly out for abstraction by cyano radicals. The indications are that the abstraction processes are likely to be so rapid that they may need more serious consideration.Several interesting examples of organic redox reactions that are catalysed by inorganic species have recently been An especially versatile system is that using43 the [(trpy)(bpy)R~(OH)~]'+-[(trpy)(bpy)RuO]'+ couple. These com- plexes were chosen because of their relative stability and indeed catalyst recovery is ca 75% after 100 catalytic cycles. Using this system dehydrogenation reactions may be effected e.g. of alcohols to carbonyl compounds as well as oxygenation reactions such as ArCH3 to ArC02H and cyclohexene successively to cyclo- hexenone and quinone. Ethylene is not oxidized in this system. The example given in Scheme 24 is relatively trivial but it does exemplify a likely mechanism.(bpy)= 2,2'-bipyridyl (trpy) =2,2',2"-terpyridyl Conditions i Pt or vitreous carbon anode at 0.6-0.8V (us S.C.E.) Scheme 24 42 L. Eberson Acfa Chem. Scand. Ser. B 1980,34,481. 43 B. A. Moyer M. S. Thompson,and T. J. Meyer J. Am. Chem. Soc. 1980,102,2310. Elec tro -organic Chemistry 93 The cathodic of alkyl bromides may be catalysed by nickel complexes. The products are dimers alkenes and alkanes which is a mixture that is consistent with the intermediacy of alkyl radicals. For dimers of the type (XCH2CH2)2 formed from reduction of XCH2CH2Br in the presence of NN'-ethylenebis(salicy1-ideneiminato)nickel(II) the yields are 55-77% with X = Ph C02Et or II-C~H~~. The reaction takes place at the reduction potential of the complex i.e.-1.75 V (vs S.C.E.) which is several hundreds of millivolts less cathodic than that of the bromide. A plausible reaction scheme is given in Scheme 25. [Ni"L2] h[Ni'LzI-(Ni'LJ + RBr + Ni"'L2 + R' + Br-+ CNi"L21 r 1- LBr J L = NN'-ethylenebis(salicy1ideneiminato) Scheme 25 The selective debromination of P-hydroxy-bromides by Barton's reagent [chromium(II) acetate DMSO and n-C4H9SH] is often accompanied by substantial elimination of both bromide and hydroxyl groups. This may be explained by electron transfer to the initially formed radical R'CHCH(OH)R* being faster than abstraction of hydrogen from the thiol. The P-hydroxy-carbanion would rapidly lose hydroxide ion. It has now been shown4' that cathodic regeneration of chromium(I1) allows the choice of conditions in which elimination is effectively suppressed.The hydroxyl groups are protected as tetrahydropyranyl ethers and the electrolyte consists of DMF containing chromium(I1) perchlorate a ligand (ethylenediamine) and butane- 1-thiol. The molar ratio of substrate :chromium(I1) species is about 4.5 and reaction is usually complete after 2 F mol-' based on hydroxy-bromide. As an example of the efficiency of this method 1-hydroxyindane is formed in 80% yield from 1-hydroxy-2-bromoindane; in this case the driving force for elimination to indene must be considerable. The anodic oxidation of iodine in acetonitrile is known to produce an electrophilic species that is capable of substituting into reactive aromatic systems [see Annu.Rep. Prog. Chern. Sect. B 1976 73 1411. The scope of this reaction has now been extended46 by the use of 1,2-dichloroethane containing 10% trifluoroacetic acid as the solvent. Under these conditions iodine undergoes two-electron oxidation at 2.0 V (vs Ag/Agf) to a highly reactive but unidentified species. One candidate iodinium trifluoroacetate is known to be less reactive than the species in question; another possibility is that iodine cation radical (I2?) is the electrophile with the second electron-transfer being a follow-up step of an ECE process. Whatever the exact nature of the electrophile it is capable of iodinating such compounds as benzaldehyde nitrobenzene iodobenzene benzene and benzotrifluoride in high yields (74-97%).Benzonitrile and 4-chloronitrobenzene are iodinated less efficiently (40 and 56% yields respectively) and puzzlingly acetophenone does not react. 44 C. Gosden and D. Pletcher J. Organomet. Chem. 1980,186,401. 45 J. Wellmann and E. Steckhan Angew. Chem. Int. Ed. Engl. 1980,19,46. 46 R.Lines and V. D. Parker Acta Chem. Scand. Ser. B 1980.34,47. J. H.P. Utley Several recent examples4749 point to the convenience and advantage for organic reactions of generating bromine in situ by anodic oxidation of bromide ion. These examples refer to the indirect generation of PhSeBr by anodic generation and regeneration of bromine in the presence of diphenyl diselenide. The reactions are typically performed at constant current in hydroxylic solvents and with a tetra- alkylammonium bromide as electrolyte.In this fashion the a-phenyl~elenylation~' of ketones and the oxyselenation of alkene~~**~~ have been achieved and examples are given in Scheme 26. Me0,C SePh mo [90%] (PhSe) -3 !PhSeBr] \& Br2 Br - i"i OH -* OMe \ [~OYO] [94%] Reagents i Et,NBr MgBr,.6Hz0 MeOH Pt anode constant current; ii OH ; iv BF * Et,O Scheme 26 In these days of looming shortages of petrochemicals the importance of the conversion of carbon monoxide and methanol into useful chemicals is self -evident. An interesting example of this which is essentially an indirect electrochemical reaction is a synthesis" of N-alkyl-formamides. The reactants are carbon monoxide (at pressures up to 100atm) methanol and (n-alky1)amine.A specially constructed pressure vessel-cum-undivided cell is used which includes a carbon anode and a stainless-steel cathode. The choice of electrolyte is important; using tetra-n-butyl- ammonium tetrafluoroborate (or bromide) conversions of 75-90% may be achieved. The mechanism is not clear but possibilities are given in Scheme 27. 2MeOH 52Me0-+ H2 (at the cathode) either M$O-+CO + MeOCO-5MeO-+HC02Me HCOzMe+ R2NH + R2NCH0+ MeOH or MeO-+ R2NH + MeOH + R2N-R,N-+CO + R2NCO-%R2NCHO+MeO-Scheme 27 S. Torii K. Uneyama and K. Handa Tetrahedron Lett. 1980 21 1863. 48 S. Torii K. Uneyama and M. Ono Tetrahedron Lett. 1980,21,2741. 49 S. Torii K. Uneyama and M.Ono Tetrahedron Lett. 1980 21,2653. D. Cipris J.Electrochem. SOC. 1980,127 1045.

ISSN:0069-3030    

DOI:10.1039/OC9807700079

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

7 Photochemistry By A. COX Department of Chemistry and Molecular Sciences University of Warwick Coventry CV4 7AL 1 Introduction Reviews have appeared of a number of important fields including the excited-state chemistry of cyclopropene derivatives,' photochemical rearrangements of benzene derivatives and annelated arenes,2 the photochemistry of protonated unsaturated carbonyl c~mpounds,~ the photo- the photochemical decomposition of a~oalkanes,~ chemistry of organic bichromophoric molecule^,^ and the control of unimolecular photorearrangements that is exerted by the crystal lattice.6 2 Alkenes A hydrocarbon analogue of the Type I1 photoeliminations of ketones has been reported.' Direct irradiation of 1,4-diphenylpent-4-en-1-01leads to 2-methyl-2,S- diphenyltetrahydrofuran probably via the radical anion of the alkene.However on sensitization using benzophenone the products are a-methylstyrene aceto-phenone and 1,4-diphenylpentan-1 -one. Following excitation these arise by abstraction of hydrogen by the methylene carbon of the excited alkene which then collapses to products. Similar results have been obtained for a number of other substituted phenylpentenes. Photoaddition of methanol to 1-phenylcyclohexene to give a Markownikov-type ether has been shown' to involve an intermediate that is common to both the direct and the sensitized pathways. Low-temperature studies have enabled this intermediate to be identified as trans-1 -phenylcyclohexene. Exciplex isomerizations of dewarbenzene have been discussed,' together with the features which control exciplex formation and the efficiency of diabatic and adiabatic isomerizations.Evidence is cited which suggests that isomerization of the exciplex of hexamethyldewarbenzene occurs adiabatically and this seems to be favoured when a reaction leading to an excited-state product is least endothermic. Irradiation of certain quadricyclenes in the presence of electron acceptors such as 9,lO-dicyanoanthracene brings about valence isomerization to the corresponding ' A. Padwa Acc. Chem. Res. 1979 12 310. G. Kaupp Angew. Chem. Int. Ed. Engl. 1980,19,243. R. F.Childs Rev. Chem. Zntermed. 1980 3,285. P. S.Engel Chem. Rev. 1980,80,99. H. Morrison Acc. Chem. Res. 1979 12 383. J. R. Scheffer Acc. Chem. Res.1980,13,283. J. M.Hornback and G. S. Proehl J. Am. Chem. SOC. 1979,101,7367. W.G. Dauben H. C. H. A. van Riel J. D. Robbins and G. J. Wagner J. Am. Chem. Soc. 1979,101 6383. G.Jones and S.-H. Chiang J. Am. Chem. SOC.,1979,101,7421. 96 A. Cox norbornadiene." Various other strained hydrocarbons are however found to be rather less effective. Sensitized valence isomerization also occurs in the presence of fumaronitriles but geometrical isomerization is not apparent. The photochemistry of bicyclo[4.l.O]hept-3-ene has been investigated," using light of wavelength 185 nm (6.7 eV photons) and it was found that five isomeric compounds account for over 92% of the product. Two important conclusions have been drawn from the study. It has been shown that although the photon is absorbed by the .n-bond reaction largely occurs at the cyclopropyl group.Secondly there are two examples of secondary products being formed by carry-over of excitation energy to the initial product. One is hepta-1,3,6-triene whose precursor is the cis-isomer and the other is bicyclo[4.1 .O]hept-2-ene which comes from an electronically 'hot' cyclohepta-1,4-diene. A study has been madeI2 of cis-fused bicyclo[4.n.0]-2,4- dienes (n =3 or 4) and bicyclo[5.4.0]undeca-8,lO-diene.Depending on wavelength and temperature bicyclo[4.3.0]nona-2,4-dienegives rise either to cyclic trienes or to cyclobutenes. The initial products of photolysis of bicyclo[4.4.0]deca-2,4-diene could not be detected but bicyclo[5.4.0]undeca-8,lO-diene also gives a triene and a cyclobutene as primary photoproducts.These results suggest that the conformation of the ground state exercises a degree of control over the outcome of these photoreactions. A new practical synthesis of semibullvalene has been rep~rted.'~ Irradiation of cyclo-octatetraene in the vapour phase leads to the desired product in essentially quantitative yield. For its success however the reaction requires a light source that has the majority of its emission at 300 nm as well as an experimental arrangement that is designed such that liquid cyclo-octatetraene is not irradiated. The dynamics of cis-trans photoisomerization of alkenes are usually explained in terms of a twisted 'common intermediate' whose vibrational levels are in thermal equilibrium.This description fails however if the lifetime of the excited state is too short for vibrational equilibrium to be attained. The semi-classical trajectory approach has now been found to be more satisfactory for these cases and it has been appliedI4 to the study of the cis-trans isomerization of alkenes and polyenes. The mechanism of the redox-photosensitized cycloreversion of the trans,syn-dimer of indene (1)has been the subject of a kinetic study.15 The results show that following photochemical transfer of an electron to dicyanobenzene or to the radical cation of phenanthrene or another selected aromatic hydrocarbon a ?r-complex is formed with (l),and this complex plays a key role in the cycloreversion (see Scheme I). This sequence may provide a model for enzymatic photo-re-activation of damaged DNA.3 Aromatic Hydrocarbons A report has appearedI6 in which it is suggested that in the photocyclization of cata-condensed polynuclear aromatic hydrocarbons with cyclohexa-193-diene the lo G. Jones S.-H. Chiang W. G. Becker and D. P.Greenberg J. Chem. SOC.,Chem. Commun. 1980,681. R. Srinivasan and J. A. Ors J. Am. Chem. SOC.,1979,101 3411. '* W. G. Dauben and M. S. Kellogg J. Am. Chem. SOC.,1980,102.4456. l3 N.J. Turro J.-M. Liu H. E. Zimmerman and R. E. Factor J. Org. Chem. 1980 45,3511. l4 R.M. Weiss and A. Warshel J. Am. Chem. SOC.,1979,101,6131. '' T. Majima C. Pac and H. Sakurai J. Am. Chem. SOC.,1980,102 5265. l6 N.C. Yang J. Masnovi and W. Chiang J. Am. Chem. SOC.,1979,101,6465.Photochemistry 'S* (1) DCNB =dicyanobenzene Scheme 1 favourable pathway is determined by the local symmetry of the frontier orbitals about the reactive positions. Thus in the presence of cyclohexadiene irradiation of dibenz[a,c]anthracene which is a compound that is known to react readily with maleic anhydride gives the [T +T:] adduct (2) as the major product. A new photochemical method for preparation of a [3.l]metacyclophane and of a [3.2]metaparacyclophane has been described." Irradiation of the benzyl alcohol (3 ; n =3) gives (4) together with other products and irradiation of (3; n =2) gives N-methyl-l-aza[3.2]metaparacyclophane(5). The reaction probably occurs by an internal electron-transfer mechanism to give an ion pair which can collapse by two different routes leading to (4) and (5).hv @-\\ \ CH20H Q (3; n=3) (CH2)" (3) n =2 or 3 (4) " C.-I. Lin. P. Singh M. Maddox and E. F. Ullman. J. Am. Chern. Soc. 1980 102 3261. 98 A. Cox In view of the known intermolecular photosubstitution reactions of amines on the 3-and 4-nitroanisoles a study has been made1* of the photoreactions of the ortho- meta- and para-isomers of P-(nitrophen0xy)ethylamine.The results show that although the ortho-and para-isomers are photosensitive no product appears to arise as a result of a Smiles rearrangement. However it is observed that the meta-isomer photorearranges to P-(3-nitroanilino)ethanol with a quantum yield of 0.23 at less than 10% conversion. Compounds that are known as [1]-and [2]-rods consisting of bicyclo[2.2.2]octane moieties and having suitable donor and acceptor groups at terminal bridgehead sites have been prepared." As these molecules have little or no molecular flexibility they have been used to provide a test of mechanisms of energy transfer.It is shown that energy transfer over short distances occurs partly by dipole-dipole coupling between the chromophoric groups and also by trans- mission of excitation energy through the molecule. The first piece of direct evidence2' is now available for the existence of a biradical intermediate in the di-wmethane rearrangement of the bicyclo[3.2.2]nonanaph- thalene (6) using low-temperature spectroscopy. Irradiation of (6) in an MTHF matrix at 77 K with light of wavelength 313 and >340 nm leads to a species that has an e.s.r.spectrum consistent with its being the triplet state of (6). A second species also identified by e.s.r. spectroscopy is probably the extensively delocalized biradical (7). D ? A report has appeared2* on the ability of cadmium sulphide and titanium dioxide to function as photocatalysts for the oxygenation of aromatic olefins. Typically a suspension of the semiconductor powder is irradiated in an olefin solution and under an atmosphere of oxygen. In the particular case of 1,l-diphenylethylene the prod- ucts are benzophenone 2,2-diphenyloxiran and 2-methoxy-2,2-diphenylethanol this last product being derived by methanolysis of the oxiran. The oxygenation appears to involve free-radical species rather than singlet oxygen and the reaction pathway shown in Scheme 2 has been suggested.CdS+hv + CdS" CdS*+02 + CdS++02' CdS++ PhZC=CHz -* CdS + [Ph2C=CH$ [Ph2C=CH2]++ 0z7 + PhZC-CH202. Scheme 2 l8 G. G. Wubbels A. M. Halverson and J. D. Oxman J. Am. Chem. SOC.,1980,102,4848. l9 H. E. Zimmerman T. D. Goldman T. K. Hirzel and S. P. Schmidt,J. Org. Chem. 1980,45 3933. 2o M. Demuth D. Lemmer and K. Schaffner J. Am. Chem. SOC.,1980,102,5407. 21 T. Kanno T. Oguchi H. Sakuragi and K. Tokumaru TetrahedronLett. 1980,21,467. Photochemistry 4 Carbonyl Compounds In an attempt to characterize some of the species involved in the photo-enolization of ortho-alkyl-substituted carbonyl compounds electron-transfer properties of the intermediate biradicals have been examined.22 Paraquat (Pa) has been used as the electron trap and the formation of PQ? has been monitored in time-resolved experiments.In this way o-methylbenzaldehyde has been shown to have two triplet states that are responsible for biradical production and these have lifetimes of 1.1 and 9.5 ns accounting for 58% and 42% of the reaction respectively. The biradical itself has a lifetime of 1500ns and undergoes electron transfer to paraquat dications with a rate constant of 6.2 x lo91mol-' s-l. The photochemistry of 1,5-diaryl-1,5-diketones of the form (8)-(10) has been in~estigated,~~ stimulated by an interest in energy-migration processes. In (8) and (9) the triplet energy is found to migrate between the two chromophores with a frequency that is in excess of lo9s-' so that there is complete excitation equilibra- tion.However equilibrium is not reached between the two triplet states of (lo) (8) R1=R2=H (9) R' = H R2 = Me (10) R' =Me R2 = H which in this case correspond to its syn and anti conformers. Energy migration of the type that has been described causes a decrease in yield of both modes of Type I1 photocleavage. The primary intermediates generated in the photoreduction of benzophenone by EtOH have been to be EtO' and the p-benzosemiquinone anion by using phenyl N-(t-buty1)nitrone as a spin trap. This observation provides support for the 'anionic' mechanism for the conversion of quinone into hydroquin- one that is shown in Scheme 3.3BQ* + EtOH + BQ-+ Et6H 2BQ-B BQ+BQ2-BQ2-+2H+ + BQH2 Scheme 3 Flash photolysis studies have revealed25 that the primary reaction of the benzo- phenone triplet with a variety of aliphatic amines in benzene is conversion into the benzophenone ketyl radical (a= 0.9-1.0). The low overall yields for reduction of ketones may not be assigned to partial quenching within the primary excited reaction complex and they must result from disproportionations which regenerate the starting materials. 22 P. K. Das M. V. Encinas R. D. Small and J. C. Scaiano J. Am. Chem. SOC.,1979,101,6965. 23 J. P. Bays M. V. Encinas R. D. Small and J. C. Scaiano J. Am. Chem. SOC.,1980,102,727. 24 S.Noda T. Doba T. Mizuta M. Miura and H. Yoshida J. Chem. SOC.,Perkin Trans.2,1980,61. *' S.Inbar H. Linschitz and S. G. Cohen J. Am. Chem. Soc. 1980,102 1419. 100 A. Cox Benzocyclobutanols have been preparedz6 by irradiation of benzocycloalkenones in t-butyl alcohol (Scheme 4). If R = MezCH a second tricyclic benzocyclobutenol is formed and the results suggest that this is a good method of synthesizing both of these structural types. A biradical intermediate seems to be involved which can collapse along the two competing pathways of cyclization to benzocyclobutenol and formation of a dienol. hv -& H n=5-7 Scheme 4 Although the photochemistry of ketones has been extensively investigated the same is not true of esters and this has stimulated a study of the photochemical behaviour of (R,S:S,R)-and (R,R:S,S)-1,2-dimethylbutyl trifluoroa~etate.~~ Broadly the photochemistry of alkyl trifluoroacetates parallels that of similar ketones.Differences are however found to exist. Ester singlets are much more reactive than ketone singlets and this leads to a greater proportion of reactions through S1 than through TI states. The ester biradical is probably longer-lived than T*). the ketone counterpart and the ester triplet of lowest energy is the '(7 Moreover simple esters do not undergo a Type I1 reaction exclusively through the triplet state. The photochemical kinetics of salicylideneaniline have been examined,z8 in both protic and aprotic solvents in order to determine the time-scale for transfer of a proton within the excited state and to identify the intermediate species before the photochromic species.At room temperature the quinoid fluorescence produced by excitation of the enol is found to have a rise time of 4ps and as such is in close agreement with the rate of tautomeric proton transfer. The fluorescence at low temperature shows a short-lived component (which is probably a vibrationally excited fluorescence) and a component of much longer lifetime which has the short-lived component as precursor. A new procedure has been announcedz9 for the one-step synthesis of y-keto- carboxylic acids and esters by irradiation of an aldehyde and an ap-unsaturated ester in the presence of benzophenone as sensitizer. The transformation probably occurs by abstraction of the aldehydic hydrogen by the triplet state of benzophenone to give an acyl radical which then adds to the carbon-carbon double-bond of the enone.Yields are good for those substrates having a P-substituent that is not in conjugation with the enone moiety and the reaction should find application in the synthesis of butenolides. It has been established,'' by studies of lifetimes and of quenching using laser flash photolysis that transient species having lifetimes of the order of tens of nanoseconds 26 M.-C. CarrC M.-L. Viriot-Villaume and P. Caubkre J. Chem. SOC.,Perkin Trans. 1,1979,2542. '' J. E.Gano and D. H.-T. Chien J. Am. Chem. SOC.,1980,102,3182. 28 P.F.Barbara P. M. Rentzepis and L. E. Brus J. Am. Chem. Soc. 1980,102,2786. 29 H. Cerfontain and P. C. M. van Noort Synthesis 1980,490.'O R.Bonneau J. Am. Chem. Soc. 1980,102,3816. Photochemistry 101 and produced on excitation of enones such as methyl vinyl ketone acetyl- cyclohexene cycloheptenone cyclohexenone cyclopentenone and testosterone are orthogonal triplet states. The angle of twist is found to vary with the rigidity of the molecule and the results indicate that the relaxed triplet is not the reactive intermediate that leads to the cyclodimerization of cyclohexenone or cyclopen- tenone. Substituent effects on the photochemistry of some hexa-l,5-dien-3-ones have been examined31 and have been shown to parallel the substituent effects observed in the cyclization of hex-5-enyl radicals. A model is used which treats the &carbon of the enone system as a radical centre and this has enabled variations in the regiospecificity of [2 +21 cycloadditions of those dienones to be rationalized.The key step in a new method3* providing access to a variety of substituted cyclohexenones is a four-carbon annelation sequence in which alkenes are photo- cyclo-added to 2,2,6-trimethyl- 1,3-dioxolenone. Following mild reduction (using di-isobutylaluminium hydride) and aldol cyclization cyclohexenones are obtained in high yield. A broad spectrum of regioselectivities is obtained from unsymmetrical alkenes and although a rationalization for this behaviour is not clear at the present time the outcome does seem to be especially sensitive to the nature of the substituent at the &position. Irradiation of (11)is to bring about cycloaddition to form the cage structure (12) which is readily converted into the acetoxy-cyclobutane (13).In essence this transformation is the [2 +21 cycloaddition of the enol acetate of formyl acetic ester to the double-bond of 4-hydroxycyclohexene. Furthermore since (13) will undergo retro-aldolization this sequence constitutes a method of placement of vicinal carboxaldehyde and acetic ester groupings on to a double-bond and may have implications for the synthesis of prostaglandins. An example has been of crystal-lattice restraints controlling the outcome of unimolecular photorearrangements. Thus direct and benzophenone- sensitized irradiation (A >330 nm) of the hydroxycyclohexenone (14) gives high yields of the [2 +21 intramolecular cycloaddition product (15).In the solid state however the outcome is the tricyclic ketone (16). It has been established that solid-phase reactions are controlled by the crystal lattice and are least-motion 31 W. C. Agosta and S. Wolff J. Org. Chem. 1980,45,3139. 32 S. W. Baldwin and J. M. Wilkinson J. Am. Chem. SOC.,1980 102. 3634. 33 B. A. Pearlman J. Am. Chem. SOC.,1979,101,6398. 34 W. K.Appel T. J. Greenhough J. R. Scheffer J. Trotter and L. Walsh J. Am. Chem. SOC.,1980,102 1158. 35 W. K. Appel T. J. Greenhough J. R. Scheffer J. Trotter and L. Walsh J. Am. Chem. SOC.,1980,102. 1160. 102 A. Cox Me processes. Reaction is thus limited to one stable conformational isomer of a given substrate in contrast to the reaction in the liquid phase where a minor high-energy conformation is reacting.The behaviour of duroquinone triplets has been to vary widely in the range pH 12 to Ho= -2. This is reflected in their reactivity towards inorganic ions such as halide and hydroxide and suggests the formation of a charge-transfer complex followed predominantly by a back-reaction rather than complete transfer of an electron. The reactivity towards some amines was also examined. Evidence has been made available37 to suggest that the intermediate in the photocycloaddition of 3(n T*)p-benzoquinone to olefins to give oxetans is not always the pre-oxetan 1,4-biradical as previously thought (see Scheme 5). The experimental results are best interpreted in terms of a quinone-olefin charge- transfer complex and suggest the possibility of new ionic photochemical reactions.Insupport of this irradiation of p-benzoquinone in the presence of pent-4-en01 leads to the cyclic ether (17). In addition to the well-known [2+6] and [2+4] carbocyclic adducts that are formed on irradiation of p-benzoquinone with either cycloheptatriene or cyclo- octatetraene (COT) duroquinone has now been reported3* to yield new cage oxetans 0 1,4-biradicals CT complexes peroxides sulphones acetates HO (17) Scheme 5 36 J. C. Scaiano and P. Neta J. Am. Chem. SOC.,1980,102 1608. 37 R.M. Wilson and A. K. Musser J. Am. Chem. SOC.,1980,102,1720. 38 K.Ogino T. Minami and S. Kozuka J. Chem. SOC.,Chem. Commun. 1980,480. Photochemistry 103 compounds on photoreaction with cyclic polyenes.Good yields of [2 +21 photo- adducts are obtained from cycloheptatriene but conversion is low with COT. Investigations indicate that in the case of COT the reaction that occurs is with the thermal dimer of the cycloalkene. It has been argued that planar enone triplets relax by twisting around the C=C bond following which rapid crossing to the potential surface of the ground state is possible. From this it follows that enones which by reasons of structural rigidity are constrained from twisting around the C=C bond should be inhibited from displaying typical enone molecular rearrangements. Thus irradiation of (1 8) in degassed Bu'OH with light of wavelength 300nm does not lead to products of a lumiketone rearrangement but rather gives3' a mixture of (19) and (20).0 hu + BU'OH or Pr'OH Earlier work has shown that an enhanced cage effect of radical pairs in a micelle and a magnetic isotope effect ("C) are responsible for the 13C enrichment that is discovered in recovered dibenzyl ketone after partial photolysis in micellar solution. A number of striking predictions have been made about the quantum yields €or the cage reaction and for product formation for the photolysis of dibenzyl ketone in HDTCl micellar solutions and a correlation has been established4' between 13C- enrichment parameters and measurements of quantum yields. Studies of structural and viscosity effects on 13C isotope enrichment have also been made.4' An interest- ing paper has appea~ed,~' describing an investigation of the effect of microemulsions on aspects of photochemical reactions and comparing the effects with those apparent in micelles.The effect of microemulsions on the photophysical properties of probe molecules was also examined. Nucleophilic substitution reactions between 2-methoxy- 1,4-naphthoquinone and methylamine have been induced thermally and photochemically and have been found to follow different pathways with high regio~pecificity.~~ In the thermal process it is the 2-methoxy-group which is replaced by the amine whereas the photoreaction leads to replacement of the 3-hydrogen atom. The transformation probably occurs as shown in Scheme 6. As 1,4-naphthoquinone shows a well- resolved n + v* band and is unreiictive towards methylamine and as the reaction is inhibited by benzophenone it has been concluded that the reaction occurs from the first (v,T*)singlet state of (21).The photochemical nucleophilic substitution reactions of 2,6-dimethoxy-l,4-benzoquinone have also been examined. The photochemistry of a number of selenoketones is reported44 to be similar to that of thioketones in that they abstract hydrogen from an upper excited state. The lifetime of the abstracting state of di-t-butyl selenoketone is lo-" s and irradiation into the Sz band gives a diselenide. 39 D. I. Schuster and S. Hussain J. Am. Chem. SOC.,1980,102,409. 40 N. J. Turro B. Kraeutler and D. R. Anderson J. Am. Chem. SOC., 1979,101,7435. 41 N.J. Turro D. R. Anderson and B. Kraeutler Tetrahedron Lett.1980 21 3. 42 M.Almgren F. Grieser and J. K. Thomas J. Am. Chem. SOC.,1980,102 3188. 43 S.M.Drew J. Griffiths and A. J. King J. Chem. SOC.,Chem. Commun. 1979 1037. 44 N. Y. M. Fung P. de Mayo B. Ruge A. C. Weedon and S. K. Wong Can. J. Chem. 1980,58,6. 104 A. Cox @Me +{+;Me -{$OMe .$'". KNMe NHMe NHMe NHMe 0 0 OH 0 ?7T 7T*> (21) Scheme 6 5 Singlet Oxygen The results of ab initio calculations on the mechanism of the ene reaction of singlet oxygen with olefins have been rep~rted.~' These suggest that the reaction involves a biradical intermediate of the peroxy type whose zwitterionic character can be greatly enhanced by both solvent and substituent. Solvent variation has a predictable effect on the outcome of the reaction as shown in Scheme 7 and it is also claimed that the known directing effect of Me0 substituents should also be exhibited by F or C1 substitution.A new view of the mechanism of the ene reaction of singlet oxygen has been and it incorporates the existence of a complex whose structure is dominated by a HOMO-olefin/LUMO-oxygen interaction. Such interactions might be especially important in a system such as but-2-ene in which there are contributions from the olefin .rr-orbitals and the CH pseudo-+orbitals to give an orbital that is similar to 9b3 in butadiene. This proposal satisfactorily explains a number of recent experimental observations. lo2 Non-polar Polar -00 M~OR~ ~ ~ M~OHM~ solvent solvent M~O&M~ Me H Me H Me H Scheme 7 Results have been to show that the singlet oxygen-ene reaction is a highly stereospecific suprafacial process.Oxidation of (22) followed by its reduc- tion may if the process is concerted give two products namely (23)-(R H) and (23)-(S D). These arise from approach of the oxygen from above to give (following removal of D) (23)-(R H) and following removal of H the (S)akohol'is obtained. Crossover products (R D) and (S H) are possible only to the extent that the other H (22) bottom attack Me* %..Ph HO D (23)-(S,D) " L. B. Harding and W. A. Goddard J. Am. Chem. SOC.,1980 102,439. 46 L. M. Stephenson Tetrahedron Lett. 1980,21 1005. '' M. Orfanopoulos and L. M. Stephenson J. Am. Chem. SOC.,1980,102,1417 Photochemistry 105 isomer of the olefin or the opposite enantiomer is present in the starting material.Precisely the expected quantity of crossover products is obtained and no isotope discrimination is apparent. The dye-sensitized photo-oxygenation of imidazole has attracted much interest stimulated by the knowledge that photo-oxidative damage to histidine residues causes loss of activity in many enzymes. 2,5 -Endoperoxides have now been reported48 as the initial oxidation products of the dye-photosensit- ized oxygenation of imidazoles and of histidines themselves. These intermediates may suffer a number of different fates depending upon the precise nature of the substituents. Loss of oxygen from the initial adduct and regeneration of the starting material appear to feature importantly.1,2-Benzodithiole-3-thione has been used to study4’ the singlet oxygenation of thiones. Irradiation of a solution of the substrate containing 1YO of cross-linked polystyrene-anchored Rose Bengal leads to 4.5% of a sulphine together with about 3% of 1,2-benzodithio1-3-one. Although this mechanism may not be general it is the first evidence supporting a sulphine mechanism for the singlet oxygenation of unhindered thiones. 6 Heterocycles Interest is still clearly evident in permutation-pattern analysis. A study of the phototranspositions of the first excited singlet states of cyano-thiophens suggestsso that 2,5-bonding occurs followed by a ‘walk’ of a sulphur atom; the rate of the walk is similar to that of re-aromatization. These conclusions are supported by the isolation of furan-thiabicyclopentene adducts and suggest that there are similarities to the photochemical behaviour of cyano-pyrroles.Evidence has been presented51 for the intermediacy of oxa-azabicyclo[2.2.0]hexenones in the photoisomerization of a six-membered heterocycle. Thus irradiation of oxazinone (24) establishes an equilibrium with the isomeric oxazinone (25). It is suggested that (26; R1 =Me R2=Ph) is thermodynamically more stable than (26; R’=Ph R2 =Me) and that their mutual interconversion takes place through the zwitterion (27). n2 3 Yk R1 Me ~ (24) (25) (26) (27) The photolysis and thermolysis of the tricyclic azoalkane (28) has been studied5* with a view to generating (29) and hence providing a direct entry into the biradical manifold of the di-vmethane rearrangement of benzonorbornadiene.Direct photo- lysis gives the biradical(29) which collapses to 2,3-benzotricyclo[2.2. 1.05*’]hept-2- ene rather than rearranging to (31).This suggests that (29) probably lies at an energy minimum on the energy surface of the singlet excited state of the alkane. Sensitized photolysis leads to (30) which is a transformation analogous to the photochemical conversion of norbornene into norcar-2-ene. 48 Ryang and C. S. Foote J. Am. Chem. SOC.,1979,101,6683. H.-S. 49 S. Tamagaki and K. Hotta J. Chem. SOC.,Chem. Commun. 1980,598. 50 J. A. Barltrop A. C. Day and E. Irving J. Chem. SOC.,Chem. Commun. 1979 881. ” P. de Mayo A. C. Weedon and R. W. Zabel J. Chem. SOC.,Chem.Commun. 1980,881. 52 W. Adam and 0.De Lucchi J. Am. Chem. SOC.,1980,102.2109. 106 A. Cox F A general oxindole synthesis has been de~cribed.’~ Treatmentof N-alkyl-N-acyl-o-chloroanilines or N-acyl-o-chloroanilines with an excess of lithium di-isopropyl- amide in THF-hexane gives the corresponding anion which on irradiation cyclizes to the oxindole. Preliminary investigations have shown that the reaction may proceed by ‘an intramolecular SRNlmechanism. ’’ J. F. Wolfe M. C. Sleevi and R. R. Goehring J. Am. Chem. Soc. 1980,102,3646.

ISSN:0069-3030    

DOI:10.1039/OC9807700095

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

8 Aliphatic Compounds Part (i)Hydrocarbons By K.J. TOYNE Department of Chemistry The University of Hull Hull HU6 7RX 1 Alkanes The reactions of alkyl halides with metal hydrides and complex metal hydrides have been examined in order to distinguish those reagents which are effective for hydrodehalogenation from those which can be utilized for reduction of functional groups without significant attack on the halogen substituent.' Lithium triethyl- borohydride and lithium aluminium hydride were found to be useful for hydro- dehalogenation reactions whereas borane and dialkylboranes were essentially inert toward alkyl halides. The reactions of lithium hydride with several functional groups in the presence of transition-metal halides have shown that terminal alkenes can be reduced to alkanes with LiH + VC13 whereas internal alkenes and alkynes are completely unreactive.* This reagent could therefore be used for selective reduction of a terminal double- bond in the presence of a triple-bond (in enynes) or an internal double-bond (in dienes).Complex aluminohydrides in the presence of catalysts have also been used for the reduction of alkynes and alkenes. Sodium and lithium aluminium hydrides and some of their derivatives were allowed to react with alkenes and alkynes using titanocene dichloride [(c~)~TiCl~] With terminal alkenes the reactions were as a cataly~t.~ complete in 10 minutes at room temperature (Scheme l),and internal alkynes also led readily to cis-alkenes but the reaction was unsatisfactory with internal alkenes and terminal alkynes.The reactions also provide a route to alkyl- and vinyl- aluminium compounds. Hydroalumination of alkenes has also been achieved with [(c~)~TiCl~] [(c~)~Ti(H)Cll .-+ LiAIH3CI 11 1 RCH=CH2 -+ [RCH2CH2Ti(CI)(Cp)2]+RCH2CH2AIH3Li+ [(c~)~Ti(H)Cll I \ iii I \" c L RCH2CH3 RCHzCHzD Reagents i LiAIH, THF; ii [(Cp),Ti(H)CI] THF; iii H,O; iv D,O Scheme 1 ' S. Krishnamurthy and H. C. Brown,J. Org Chem. 1980.45 849. ' E. C. Ashby and S. A. Noding J. Org. Chem. 1980,45 1041. ' E. C. Ashby and S. A. Noding J. Org Chem. 1980,45 1035. 107 108 K. J. Toyne tri-isobutylalane and [(Cp),ZrC12] (as the catalyst) and this reaction is successful in the presence of groups such as OH SPh and Br which may interfere with alternative hydroalumination procedures (Scheme 2).4 In this procedure both terminal and internal alkenes react to give the alkane.I [(Cp)2ZrC12]-[(Cp)2Zr(C1)Bui]+[(Cp)zZr(H)Cl]+ Me2C=CH2 [(Cp)2Zr(H)Cl]A [RZr(Cl)(Cp)z] RAIBu’2 + [(Cp),ZrCl2] 1 iv RH Reagents i Bu’,AI 1,2-dichloroethane; ii alkene; iii Bu’,AICI; iv H20 Scheme 2 Decarboxylation of acids to alkanes by using an ester from which the efficient generation of carboxyl radicals was achieved in an ‘a1kene’-forming radical-frag- mentation reaction has been described (Scheme 3).’ Esters of truns-9-hydroxy-10- phenylthio-9,lO-dihydrophenanthrene(1)or its 10-chloro-analogue (2),which lead to a fully aromatic system on fragmentation have been used and the hydrocarbon is formed in mild neutral conditions from primary secondary and tertiary acids with tri-n-butylstannane.R’ R‘ 1 RH tR-Reagents i Bu”,SnH benzene or toluene azobisisobutyronitrile reflux (i.e. Bu”,Sn -); ii Bu”,SnH Scheme 3 (1) X = SPh (2) x = CI Two methods for deoxygenation of alcohols to alkenes were mentioned in the Annual Report for 197fL6‘ One of these methods for tertiary alcohols and sterically hindered secondary alcohols involved the reduction of the derived carboxylic ester by lithium in ethylamine and now non-hindered secondary and primary alcohols can be reduced in high yield by using their (dialky1amino)thiocarbonylderivatives E. Negishi and T. Yoshida Tetrahedron Left. 1980 21 1501. D. H. R. Barton H. A.Dowlatshahi W. B. Motherwell and D. Villernin J. Chern. Soc. Chem. Cornrnun. 1980.732. K. J. Toyne Annu. Rep. Prog. Chem. Sect. B 1978,75 (a)p. 159 (b)p. 163. Aliphatic Compounds -Part (i) Hydrocarbons S II I ROH __* ROCNEt2 +RH Reagents i K Bu'NH, 18-crown-6 THF Scheme 4 to give the alkane and some of the original alcohol (Scheme 4).' The radical anion which is formed by electron transfer from potassium collapses to form pre- dominantly a stabilized thiocarboxylate anion and a carbon radical; the latter leads to the alkane. It has also been shown that the similar reduction of carboxylic esters in non-nucleophilic media occurs by cleavage of the alkyl-oxygen bond of the derived radical anion to give alkane and carboxylate anion by the major pathway.8 An alternative procedure for the deoxygenation of primary and secondary alcohols is achieved by reduction of chloroformates with tri-n-propylsilane (Scheme 5)9 (see also ref.lOa) and an exceedingly mild desulphurization of thiols using sodium triethylborohydride and ferrous chloride has been used for aromatic benzylic and aliphatic systems." I I1 111 ROH -+ ROCOCI -+ RO-C=O -R. -+ RH + Pr3Si* Reagents i COCI,; ii Pr3SiH (Bu'O),; iii Pr,SiH Scheme 5 The cyano-group in primary secondary or tertiary nitriles is reductively cleaved by highly dispersed potassium on neutral alumina in hexane at room temperature to give the alkane in excellent yield.12 The reagent is prepared by melting potassium over neutral alumina at 150 "C with vigorous stirring in an inert atmosphere.2 Alkenes Synthesis.-Several alternative methods for conventional elimination procedures have been reported. Carefully dried copper(I1) sulphate is a useful catalyst for the dehydration of secondary tertiary benzylic and allylic alcohol^.'^ The procedure simply involves heating the alcohol and the solid catalyst until the alkene distils from the mixture. The predominant Saytzeff and trans-alkene products suggest that formation of a carbonium ion is involved. On the other hand Grignard or organolithium compounds react rapidly with ethylene in the presence of nickel chloride at -20 to 0 "C in ether to give the alkene by abstraction of P-hydride which preferentially gives the less stable Hofmann Terminal (R'CH=CH2) and disubstituted (R'CH=CHR*) alkenes are formed in high yield by the reaction of vicinal dihalides [R'CHXCHY(R* or H)] with sodium methyl- or phenyl-selen01ate.l~ The elimination occurs rapidly at room 'A.G. M. Barrett P. A. Prokopiou and D. H. R. Barton J. Chem. SOC.,Chem. Commun. 1979,1175. A. G.M. Barrett P. A. Prokopiou D. H. R. Barton R.B. Boar and J. F. McGhie J. Chem.SOC.,Chem. Commun. 1979,1173. R. A. Jackson and F. Malek J. Chem. SOC.,Perkin Trans. 1 1980 1207. lo K. J. Toyne Annu. Rep. Prog. Chem. Sect. B 1979 76 (a)p. 127 (b)p. 135 (c) p. 145 (d)p. 139 (e) p. 145. l1 H. Alper and T. L. Prince Angew. Chem. Znt. Ed. Engl. 1980,19 315. l2 D. Savoia E. Tagliavini C. Trombini and A. Umani-Ronchi J. Org. Chem. 1980 45 3227. R. V. Hoffman R.D. Bishop P. M. Fitch and R. Hardenstein J. Org. Chem. 1980,45,917. l4 M. T. Reetz and W. Stephan Liebigs Ann. Chem. 1980 171. M. Sevrin J. N. Denis and A. Krief Tetrahedron Lett. 1980 21,1877. 110 K. J. Toyne temperature except for dichlorides (which need to be heated at SOOC) and the diselenide product can be separated and re-used for preparing the selenolate. syn-Elimination occurs for vicinal dichlorides but in other cases the process involves anti-elimination. The full report has now appeared16 of the method for synthesis of an alkene which uses the triphenylmethyl carbonium ion to abstract hydride ion from an alkyl-iron compound [(C~)Fe(co)~ (CHR'CH2R2)] to give an alkene complex from which the alkene can then be liberated (see also ref.6b).The sequence is useful because 1-and 2-alkyl-iron compounds usually give the terminal alkene and 3-alkyl-iron compounds give only the less stable (Z)-alk-2-ene isomer. The regioselective transformation of a ketone into an alkyl-substituted alkene is now possible by three procedures based on the reactions of enol derivatives. Enol trifluoromethanesulphonatesreact with lithium dialkylcuprates to give the coupling products in high yields (Scheme 6)." Alkyl- aryl- vinyl- and cyclopropyl-cuprates R2 \c=o iii,iv / OR'-CH2 vii H R' CH2 CH2 (4) Reagents i 2,6-di-t-butylpyridine (CF,S02),0 CH,Cl,; ii R3,CuLi THF; iii Li NH, ether; iv (PhO),POCI ether; v R4,Al [Pd(PPh,),] 1,2-dichloroethane; vi Me,SiCI Et,N DMF; vii R5MgX [Ni(acac),] or [NiC12(PPh3)2]or [NiCl,(dppf)] ether; viii 2,4,6-tri-isopropylben- zenesulphonyl hydrazide {'trisyl hydrazide) HCI; ix NNN'N'-tetramethylethylenediamine (TMEDA) hexane BuLi; x R6,B THF; xi I, ether; xii NaOH H,O Scheme 6 have been used and the vinyl reagent gives a route to 1,3-dienes.In the second method enol phosphates (3) react with trialkylaluminiums in a palladium-catalysed reaction to provide a specific transformation of ketones into aIkyl-substituted alkenes (Scheme 6)." Alkenylation and alkynylation are also possible by this method to give dienes and enynes. In the third method a new carbon-carbon bond is formed when the silyloxy-group of silyl enol ethers is substituted by an alkyl or aryl group from a Grignard reagent in a nickel-complex-catalysed reaction (Scheme ,).I9 " D.E. Laycock J. Hartgerink and M. C. Baird J. Org. Chem. 1980 45,291. " J. E.McMurry and W. J. Scott Tetrahedron Lett. 1980 21,4313. K.Takai K. Oshima and H. Nozaki Tetrahedron Lett. 1980 21 2531. l9 T. Hayashi Y. Katsuro and M. Kumada Tetrahedron Lett. 1980 21 3915. Aliphatic Compounds -Part (i) Hydrocarbons 111 The stereo- and regio-chemistry of the reaction depends on the nature of the catalyst; with dichlorobis(triphenylphosphine)nickel(II) [NiC12(PPh3)2] the coup- ling process is regio- and stereo-specific. Phosphine complexes of nickel(@ chloride have also been used to catalyse the coupling of Grignard reagents with alkenyl aryl or allylic selenides.20 The reaction of alkenyl selenides is highly stereospecific and it proceeds with retention of configuration.With alkenyl aryl selenides bond- cleavage occurs on both sides of the selenium atom and two equivalents of the Grignard reagent are required. A further procedure achieves an equivalent transformation of methyl ketones into 1,l-dialkyl-ethenes (Scheme 6).21The trisylhydrazone of the methyl ketone is converted into a trialkyl(viny1)borate (4) and then an alkyl migration from boron to carbon is induced by iodine. Three new procedures permit alkenes to be formed from epoxides in mild stereospecific reactions and one of these methods allows the stereochemistry of a disubstituted ethene to be inverted. The method for inversion can be applied to the alkene directly or to the epoxide and it is based on the formation of a vic-bromo- or vic-chloro-hydrin trifluoroacetate (Scheme 7).22 The halogeno-ester is then HH M R' R' 17"R2+ -MH [X = a1 H H R2 CFJOO CF,COO 0 7 H-A-H R' R2 Reagents i N-bromosuccinimide CF,CO,H or N-chlorosuccinimide CF,CO,H; ii NaI DMF heat; iii LiBr (CF,CO),O or LiCI (CF,CO),O; iv Zn HOAc DMF; v NaI (CF3C0),0 Scheme 7 converted into the iodo-ester by an SN2reaction; the iodo-ester then decomposes to the alkene with inverted geometry.The original alkene can however be regener- ated by reduction of the chloro-ester. A review has appeared which discusses numerous other methods for achieving inversion of alkene~.~~ The second method for generating alkenes from epoxides is based on the reaction of iodohydrins with triphenylphosphine di-iodide to produce an alkene by stereospecific trans-elimina- tion.Since iodohydrins can be formed by the anti-opening of epoxides with hydrogen iodide it is possible to use a mixture of triphenylphosphine hydriodide and triphenylphosphine di-iodide to reduce an epoxide to an alkene of the same stereo~hemistry.~~ The third procedure uses sodium diethyl phosphite and a catalytic amount of tellurium powder to obtain the alkene from an epoxide stereo-~pecifically.~~ Terminal epoxides are deoxygenated most readily and (2)-isomers react more readily than the (,!?)-isomers. Selective deoxygenations can be carried out in some cases especially when a more reactive terminal group is involved. 2o H. Okamura M. Miura K.Kosugi and H. Takei Tetrahedron Lett. 1980 21 87 21 K. Avasthi T. Baba and A. Suzuki Tetrahedron Lett. 1980 21 945. 22 P. E. Sonnet J. Org. Chem. 1980 45 154. 23 P. E. Sonnet Tetrahedron 1980,36,557. 24 P. E. Sonnet Synthesis 1980 828. 25 D. L. J. Clive and S. M. Menchen J. Org. Chem. 1980,45 2347. 112 K.J. Toyne Complex reducing agents such as NaH-RONa-Ni(OAc) are new heterogeneous hydrogenation catalysts which can be used for the partial hydrogenation of alkynes to alkenes at atmospheric pressure and room The selective hy- drogenation of carbon-carbon double-bonds in dienes mixtures of alkenes or unsaturated carbonyl systems is sometimes possible and hydrogenation of aldehydes and ketones can be achieved. The cyano-cobalt complex K3[Co(CN),H] has been known for many years and its further use as a hydrogenation catalyst has now been reported.28 The water-soluble complex is readily prepared from cobalt(@ chloride and potassium cyanide under an atmosphere of hydrogen and it is reactive at room temperature in phase-transfer conditions.It is capable of reducing conju- gated dienes to mono-enes (generally by 1,4-addition) and of converting a@-unsaturated ketones into saturated ketones but isolated carbon-carbon double- bonds and carbonyl groups are not hydrogenated. Regiospecific deoxygenation of allylic alcohols to terminal alkenes can be carried out by the three-step sequence shown in Scheme 8.29The xanthate derivative of the R2 II 0 Reagents i NaH cs,;MeI C6H6 reflux; ii Bu3SnH C6H6 azobisisobutyronitrile at 80 "c;iii HCl or MeC0,H Scheme 8 ally1 alcohol undergoes a [3,3] sigmatropic rearrangement to give a dithiocarbon- ate.The reaction of the dithiocarbonate with tributyltin hydride gives an allylic stannane which is then cleaved to give the terminal alkene. Some other methods for preparing alkenes are given in the section dealing with the reactions of alkynes (p. 120). Reactions of A1kenes.-Alternative methods continue to appear for fundamental alkene reactions such as hydration halogenation hydrohalogenation hydroxyla- tion and epoxidation. Alkenes are converted into alcohols with the hydroxy-group introduced in an anti-Markovnikov fashion by their reaction with TiCI,-NaBH, followed by decomposition of the intermediate with water.30 Vicinal dibromides can be converted into alkenes by reduction at a mercury cathode in DMF; in the case of tetrabromides from dienes the higher alkylated double-bond can be regenerated selectively and so the less alkylated double-bond can be selectively protected as the dibr~mide.~' Complementary to this procedure is the use of pyridinium hydrobromide perbromide in THF or chloroform under mild conditions to achieve the selective dibromination of a non-conjugated diene at the more alkylated (and hence more reactive) d~uble-bond.~ 26 J.Brunet P. Gallois and P. Caubere J. Org. Chem. 1980 45 1937. '' P. Gallois J. Brunet and P. Caubere J. Org. Chem. 1980 45 1946. *' D. L. Reger M. M. Habib and D. J. Fauth J. Org. Chem. 1980,45 3860.29 Y. Ueno H. Sano and M. Okawara Tetrahedron Lett. 1980 21 1767. 30 S. Kano Y. Tanaka and S. Hibino J. Chem. SOC.,Chem. Commun. 1980,414. 31 U. Husstedt and H. J. Schafer Synthesis 1979 964. " U. Husstedt and H. J. Schafer Synthesis 1979 966. Aliphatic Compounds -Part (i) Hydrocarbons Markovnikov hydrohalogenation of alkenes is readily achieved in nearly quanti- tative yields under phase-transfer conditions at 80-1 15 "C using aqueous hydro- chloric hydrobromic and hydriodic The cis-hydroxylation of unsaturated substrates (including alkynes) by osmium tetroxide has been reviewed,34 and a procedure for the hydroxylation of sterically hindered alkenes uses trimethylamine N-oxide with pyridine as the oxidizing system in an osmium-tetroxide-catalysed reaction.35 A catalytic epoxidation process using hexafluoroacetone and hydrogen peroxide is an attractive alternative to peroxy-acid methods.2-Hydroperoxyhexafluoropro-pan-2-01 (5) is the effective reagent and disodium hydrogen phosphate in refluxing 1,2-dichloroethane acts both as a buffer and as a dehydrating agent to regenerate (5)from hydrogen peroxide and the hydrate of hexaflu~roacetone.~~ Novel applications of boron compounds in alkene chemistry continue to appear. Borane-1,4-oxathian (6),obtained by passing diborane into 1,4-oxathian at 250 "C hydroborates alkenes rapidly to trialkylboranes in excellent yield.37 An excess of the reagent can be oxidized with aqueous sodium hypochlorite to the sulphoxide which is readily extracted into an aqueous phase and the organoborane which is unaffected by this treatment can be used in the organic phase.Carbonylation of B-alkyl-9-borabicyclo[3.3. llnonanes and reduction of the intermediate provides a high yield stereospecific synthesis of the homologous borane as shown in Scheme 9.38Straight-chain alkyl groups can be lengthened by one carbon atom and some cyclic alkenes for example norbornene give cycloalkylmethyl derivatives which are unobtainable by normal hydroboration procedures. i-iii R-B Reagents i,,CO Li(MeO),AlH; ii LiAIH,; iii CH,SO,H Scheme 9 Thexylborne (thexyl = t-hexyl) is not a suitable reagent for producing dialkyl(thexy1)boranes that contain different unbranched primary alkyl groups but now it is possible to produce mixed dialkyl(thexy1)boranes by using thexylchloroborane in two ways (Scheme 10).In one procedure the alkyl(thexy1)chloroborane (7) reacts with a primary organolithium or Grignard reagent39and in the other (7) is reduced to alkyl(thexy1)borane in the presence of another alkene.40 The dialkyl(thexy1)boranes can then be transformed into the 33 D. Landini and F. Rolla J. Org. Chem. 1980 45 3527. 34 M. Schroder Chem. Rev. 1980,80 187. 3s R. Ray and D. S. Matteson Tetrahedron Lett. 1980 21 449. 36 A. J. Biloski R. P. Heggs and B. Ganem Synthesis 1980,810. '' H. C. Brown and A. K. Mandal Synthesis 1980 153. H. C. Brown T. M. Ford and J. L. Hubbard J. Org. Chem. 1980,454067. 39 G. Zweifel and N. R. Pearson J. Am. Chem. Sac. 1980,102 5919. 40 S. U.Kulkarni H. D. Lee and H. C. Brown J. Org. Chem. 1980 45 4542. 114 K. J. Toyne ii R' R' R' BH2 -yB:" -\ CI c1 \ R2 R2 (7) Reagents i HCI ether; ii alkene; iii RZM (M = Li or MgX)39or potassium tri-isopropoxyborohydride alkene;40 iv NaCN; v (CF,CO),O or PhCOCl vi NaOH H,O Scheme 10 corresponding ketones so that a general conversion of two different alkenes into the corresponding ketone is available for the first time.40 The conversion of a terminal alkene (RCH=CH2) into an aldehyde (RCH,CHO) has been achieved previously by hydroboration with the borane-dimethyl sulphide complex followed by oxidation with pyridinium chlorochromate (Py.HC1CrO3). There were certain limitations to this procedure which have now been largely overcome by using disiamylborane [bis-(3 -methyl-2-butyl)borane] as the hydrobor- ation reagent and a selective reaction that affects only the terminal double-bond in dienes is now possible.41 The less common reactions of alkenes include three different kinds of addition to produce amino-compounds.One of these provides the first general method for the direct conversion of alkenes into primary vicinal diamines and it uses the reaction of cyclopentadienylnitrosylcobalt dimer to give an adduct which can be reduced directly to the amino-containing product (Scheme 1lh4*Another procedure 0 II R' / R3 \ / N-CR'R~ ii R2-c-c-cp-co-\ I I + I I R4 H2N NH2 R3/'/T N-CR3R4 R' II \/ 0 c=c /\ R2 K' K- R' R3 PhCN I/ R;N -c-c I-pd -CI 5 R3N -c I-CI -0AC II II ~2 ~4 \ PhCN R2 R4 (8) Reagents i [(Cp)Co(NO)], THF NO; ii LiAlH, THF; iii [(PhCN),PdCI,] THF; iv R',NH THF; v N-bromosuccinimide or Pb(OAc) or Br, MeC0,H Scheme 11 achieves direct stereospecific oxyamination of an alkene to a vicinal amino-alcohol derivative e.g.an acetate (8),as also shown in Scheme 11.43 A P-amino-palladium complex is readily obtained from an alkene and oxidative cleavage of the palladium- carbon bond in the presence of an oxygen-containing nucleophile gives the amino- alcohol derivative in a 'one-pot' reaction. The reaction proceeds by overall cis-addition as a result of trans-aminopalladiation followed by an inversion of configur- ation in the oxidative substitution reaction. Primary amines did not give the 41 H.C. Brown S. U. Kulkarni and C. G. Rao Synthesis 1980 151. 42 P. N. Becker M. A. White and R. G. Bergman J. Am. Chem. SOC.,1980 102,5676. 43 J. E. Backvall and E. E. Bjorkman J. Org. Chern. 1980,45 2893. Aliphatic Compounds -Part (i) Hydrocarbons 115 N-(alky1)amino-alcohols,but these compounds could be prepared by a modified route [using N-(alkyl)benzylamine followed by debenzylation]. Finally terminal alkenes R2CH=CH2 have been used to alkylate secondary amines (R'CH2),NH in the presence of [Nb(NMe,),] or [Ta(NMe2)5] at 160-200 "C although in some cases the yields are rather low and the reaction fails with primary or tertiary amine~.~~ The 2-position of the alkene is linked to the a-carbon of the secondary amine to give R'CH2NHCHR'CHR2Me.A simple general method for preparing P-hydroxy-selenides in excellent yields uses the reaction of phenylselenenyl chloride (PhSeC1) with alkenes in aqueous acetonitrile at room temperature; the phenylseleno-group adds to the less- substituted carbon atom of the do~ble-bond.~ Terminal alkenes react with chloroiodomethane in a free-radical addition to give 1-chloro-3-iodo-alkanes and the subsequent reaction of these products with malon- ate esters gives a mixture of esters arising either from elimination of hydrogen iodide and substitution of the chloro-group or from the usual cyclization of malonate esters with 1,3-dihalogeno-a1kanes (Scheme 12).46With different experimental conditions either of these products can be obtained preferentially and the olefinic product can be used for the synthesis of y6-unsaturated acids or y-lactones.Several general guidelines have been presented to enable predictions to be made of the rate and orientation of free-radical additions to alkene~.~~ R 'CH=CHCH2CH2CO2H R'CH=CH2 iii-v 9 / \.L R 'CH=CHCH2CH(C02R2)2 R'CHICH2CH2CI + wi 0 Reagents i CH2CII azobisisobutyronitrile at 80 "C;ii CH2(C02R2), R20Na R'OH; iii KOH; iv H'; v heat; vi KOH; vii aq. H,SO, reflux Scheme 12 Further Lewis-acid-catalysed additions to alkenes have been reported this year. Last year the reactions of methyl propiolate with alkenes were discussed,'ob and in a continuation of that work the reactions of alkenes with methyl chloropropiolate dimethyl acetylenedicarboxylate and methyl a-halogeno-acrylates have been studied.Ene reactions and/or stereospecific [2 + 21 cycloadditions occur with the first two (using ethylaluminium dichloride as catalyst) and the ene adducts and the cyclobutenecarboxylates (9) that are derived from methyl chloropropiolate undergo substitution reactions with organocuprates and can also be hydrolysed to P-keto-esters and substituted dimethyl glutarates (lo) respectively. When methyl 44 M. G. Clerici and F. Maspero Synthesis 1980,305. 45 A. Toshimitsu T. Aoai H. Owada S. Uemura and M. Okano J. Chem. SOC.,Chem. Commun. 1980 412. 46 S. Miyano H. Hokari Y. Umeda and H. Hashimoto Bull Chem. SOC.Jpn. 1980,53,770. 47 J. M. Tedder and J. C. Walton Tetrahedron 1980,36 701. B. B. Snider D. M. Roush D.J. Rodini D. Gonzalez and D. Spindell J. Org. Chem. 1980,45,2773. 116 K. J. Toyne R4 c1 R2 9.02Me R3.JT( Rz CHzC02Me ~1 C0,Me R' (9) (10) a-chloroacrylate is used a stereo- and regio-selective ene reaction occurs in which the new C-C bond is formed exclusively at the less-substituted carbon atom of the alkene and the hydrogen is transferred from the alkyl group that is syn to the alkenyl hydrogen (Scheme 13).49Good yields of ene adducts are obtained with 1,l-disubstituted and trisubstituted alkenes and the a-chloro-esters that are produced are useful synthetic intermediates. :cR3 CH2R2 Rt. .c=c.-.CH2R2 H' 'CH2R3 c1 CO2Me Reagents i H,C=CCICO,Me EtAlCI, C,H Scheme 13 Ene reactions of alkenes and aldehydes have been achieved for the first time with dimethylaluminium chloride which acts as a catalyst and prevents any proton- initiated rearrangements by producing methane and a non-acidic aluminium alkoxide from the initial product (1 1) (Scheme 14).50 The reaction therefore gives a route to homoallylic alcohols from alkenes; by using formaldehyde alk-3-en-1-01s can be prepared.Diethyl oxomalonate has been used as an enophilic reagent with mono- di- and tri-substituted alkenes to give an overall reaction which is equivalent to an ene reaction of carbon dioxide (Scheme 14).51 The a-hydroxymalonic ester intermediate C02H R R Reagents i RCHO ArCHO or CHzO Me,AICI CH,Cl,; ii pH4 phosphate buffer; iii diethyl oxomalonate at 145-180°C; iv aq. KOH;v HCI; vi NaIO, pyridine or cerium@/) ammonium nitrate Scheme 14 49 B.B. Snider and J. V. DunEia J. Am. Chem. SOC.,1980 102,5926. 50 B. B. Snider and D. J. Rodini TetrahedronLett. 1980 21 1815. 51 M. F.Salomon S. N. Pardo and R. G. Salornon J. Am. Chem. SOC.,1980,102,2473. Aliphatic Compounds -Part (i)Hydrocarbons was hydrolysed to the diacid and then oxidized to the @?-unsaturated acid. Lower reaction temperatures can be used in a catalysed reaction and some reactions occur at or below room temperature when using tin(1v) chloride zinc(I1) chloride or mercury(I1) trifluoroacetate as catalyst. A new four-carbon annelation of alkenes offers easy access to a variety of cyclohexenones and the sequence is complementary to reactions such as Diels- Alder and Robinson annelations (Scheme 15).s2 2,2,6-Trimethyl-1,3-dioxolenone (12) readily prepared from diketen and acetone undergoes a photochemical [2 + 21 cycloaddition to an alkene and mild reduction of the photo-product (13) gives a hemiacetal which spontaneously loses acetone and then fragments to form a keto- aldehyde.Treatment of the keto-aldehyde with toluene-p-sulphonic acid in benzene and azeotropic removal of water gives the cyclohexenone. R’ i n (13) / R4 R4 Reagents i hv hexane; ii di-isobutylaluminium hydride; iii p-MeC6H4S03H C6H6,heat Scheme 15 Alkane- and arene-sulphonyl iodides add to alkenes using copper(I1) chloride as catalyst to give @-iodo-sulphones e.g. BuCHICH2S02R (or Ar) which can be dehydroiodinated to produce ap-unsaturated sulphones (E)-BuCH=CHS02R (or AT).^^ Some of these vinylsulphones may be useful dienophiles in Diels-Alder reactions.3 Dienes Symmetrical (E,E)-1,3-dienes have been prepared from terminal alkynes by the hydroboration sequence shown in Scheme 16 which does not require the isolation of any borane precursor and which gives only small amounts of mono-ene impurities arising from alkyl-group tran~fer.~~ The dienes are formed with retention of the configuration of the alkenylborane and (Z,Z)-1,3-dienes have also been prepared from (1Z)-1-alkenyldicyclohexylboranes. A palladium-catalysed decarboxylative elimination of the adducts from enals and carboxylate enolates presents a highly stereocontrolled synthesis of 1,3-dienes that has high or exclusive preference for the formation of an E-double-bond without affecting the stereochemistry of the double- bond that is present in the precursor (Scheme 17).55 52 S.W. Baldwin and J. M. Wilkinson J. Am. Chem. SOC., 1980 102 3634. s3 L. K. Liu Y. Chi and K. Jen J. Org. Chem. 1980 45 406. 54 J. B. Campbell and H. C. Brown J. Org. Chem. 1980,45 549. ss B. M. Trost and J. M. Fortunak J. Am. Chem. SOC., 1980,102,2841. 118 K. J. Toyne B(Sia) H CH,CH=CH H R' H \/ vii \ / \/ c=c -* c=c H ,c=c\ /\ R'/\H R' H ,c=c\\ H (15) R' H Sia = CHMeCHMe2 Reagents i 9-borabicyclo[3.3.1]nonane or dicyclohexylborane; ii NaOMe; iii CuBr.SMe, at 0 "C; iv CuBr-SMe at -15 "C; v H,C=CHCH,Hal (Hal = Br or I) at -15 to +25 "C; vi HB(Sia),; vii H,C=CHCH,Br [Pd(PPh,),] C,H, aq.NaOH Scheme 16 H OAc . .. R2 iii R -R1-R2 c0,-Reagents i R'CHCO,-;'ii MeCOCl; iii [Pd(PPh,),] Et,N toluene at 85 "C Scheme 17 Three procedures have been reported for the preparation of 1,4-dienes from alkynes. One of these is linked to the synthesis of 1,3-dienes mentioned above (Scheme 16) in which an alkenylcopper derivative is a proposed intermediate. At -15 "C the formation of the symmetrical conjugated diene is slow and addition of ally1 bromide leads to a (4E)- 1,4-diene (Scheme 16).56 The reaction sequence has also been used for internal alkynes and for the synthesis of (42)-1,4-dienes. A similar report describes the use of the reaction of alkenyl-9-borabicyclo[3.3. llnonanes with methylcopper and allylic halides to give the (4E)- 1,4-dienes but the reaction using alkenyldisiamylboranes leads to the dime^-.^' Dialkenylchloroboranes (14) were also used to form 1,4-dienes by their reaction with methylcopper at -30 to -40 "C followed by cross-coupling with allylic halides; the cross-coupling reaction of (14) with simple alkyl halides in the presence of PhSLi or triethyl phosphite gave an alkene.57 1-Alkenyldisiamylboranes (15) are also readily obtainable from alk- 1-ynes and they undergo a palladium-catalysed cross-coupling with allylic bromides to produce c1 (14) 56 H.C. Brown and J. B. Campbell J. Org. Chem. 1980,45,550. 57 H. Yatagai J. Org. Chem. 1980 45 1640. Aliphatic Compounds -Part (i) Hydrocarbons 119 1,4-dienes stereoselectively (see Scheme 16).58 Alkenylboranes and palladium acetate can react to give products by intramolecular migration protonolysis or cross-~oupling.~~ With (E)-alkenyldialkylboranes from terminal alkynes intramolecular migration gives (E)-alkenes whereas the alkenylboranes from inter- nal alkynes undergo protonolysis to produce (2)-alkenes; the alkenylpalladium intermediates can be trapped by allylic chlorides to give 1,4-dienes.The coupling of lithium acetylides and allylic halides to give 1,4-enynes is improved by the addition of lithium iodide and the ratio of a-and y-coupling products is approximately 95 :5;60the enynes can then be used to give 1,4-dienes. A two-step sequence has been developed to convert an alk-1-yne into a 1,5-enyne (Scheme 18).61 The method allows 1,5-dienes to be obtained directly by using homoallylzinc chloride [H2C=CH(CH2)2ZnC1] or indirectly by reduction of the enyne (16).Repetition of the sequence leads to long-chain 1,5-diene units and this facility should prove useful in terpenoid synthesis. Me I Me CH,CH,C=CH ... \/ RCGCH 'J'b \c=c/ iii,iv - R/ 'H Reagents i Me,AI [(Cp),ZrCI,] (CH,Cl),; ii 12 THF; iii [Me,SiC~C(CH,),ZnCI] [Pd(PPh,),] THF; iv KF.2H20 DMF Scheme 18 4 Alkynes Synthesis.-A simple and mild preparation of alkynes uses solid potassium t- butoxide and catalytic amounts of 18-crown-6 in petroleum ether.62 1,2-Dibromides (from terminal alkenes) and 1,l-dichlorides (from aldehydes) give terminal alkynes and internal geminal dihalides (from symmetrical ketones) give internal alkynes (Scheme 19) whereas 2,2-dichlorides (from methyl ketones) only give alk-1 -ynes if I I RCHBrCH2Br __* RCeCH c-RCH2CHC12 I RCH2CC12CH2R __* RCrCCH2R Reagents i KOBu' 18-crown-6 light petroleum at 60-100 "C Scheme 19 the 3-position is blocked.The second stage of these reactions involves dehy- drohalogenation of a halogeno-alkene and an alternative preparation of l-bromo- alkenes which could then be used to give terminal alkynes employs a Wittig reaction of an aldehyde with bromomethylenetriphenylphosphorane(Ph3P=CHBr) which is formed from triphenylphosphine and dibr~momethane.~~ N. Miyaura T. Yano and A. Suzuki Tetrahedron Lett. 1980 21 2865. ''H. Yatagai Bull. Chem. SOC.Jpn. 1980,53 1670. 60 H.Yatagai Y. Yamamoto and K. Maruyama Chem. Lett.. 1980,669. 61 E. Negishi L. F. Valente and M. Kobayashi J. Am. Chem. SOC.,1980 102 3298. 62 E. V. Dehmlow and M. Lissel Liebigs Ann. Chem. 1980. 1. 63 M. Matsumoto and K. Kuroda Tetrahedron Lett. 1980 21,4021. 120 K. J. Toyne Enol phosphates (17) are readily formed from methyl ketones; on @-elimination with a strong base the phosphates (17)give the terminal alkyne (Scheme 20).64This procedure represents the first general method for converting methyl ketones into terminal alkynes. I I1 RCOCH3 RC=CH2 RC=CH I OPO(0Et)z (17) Reagents i lithium tetramethylpiperidide CIPO(OEt),; ii lithium tetramethylpiperidide aq. HCI Scheme 20 Aldehydes (R'CHO) can be readily converted into 1,l -difluoro-alkenes (R1CH=CF2) by using dibromodifluoromethane and triphenylphosphine in the presence of zinc dust.The difluoro-alkenes will then react with alkyl-lithiums (R2Li) either in THF to give internal alkynes R'C=CR2 or in ether to give a monofluoro- alkene R'CH=CFR2 from which the alkyne can be generated by reaction with lithium di-i~opropylamide.~' In several cases however the alkyne is contaminated with a small amounbof allenic product. Reactions of A1kynes.-A review on the formation of copper and silver organo- acetylides and their uses in organic synthesis has appeared.66 Three papers report some reactions of 1,3-dilithio-alk- 1-ynes with various elec- trophiles (see also ref. 1Oc). In almost all cases epoxides benzyl chloride and trimethylchlorosilane have been found to react exclusively on the propargylic site to give (18) (Scheme 21).67 It appears that the terminal alkyne function is RCGCH R1CH(OH)CH2CH20H R'CHXCGCH (18) x = CH~CHR'OH CH2Ph,or SiMe3 Reagents i THF BuLi-hexane at -30°C and then at 20-30"C6' or BuLi-hexane at -2O"C for 16-24 h;68 ii BuLi ether at -20 "C and then at room temperature for 30 h;69iii BH, THF; iv HzO NaOH; v H,O,; vi BD3 THF; vii COz slurry in hexane; viii \RZ or PhCH,CI or Me,SiCI as appropriate 0 Scheme 21 conveniently protected by lithium and that the relative nucleophilicities of positions 1 and 3 are determined by their basicities.The reaction of the dilithio-compound with formaldehyde and cyclic ketones occurs at both C-1 and C-3 to give alk-2-yne- 1,5-diols but with carbon dioxide the allene- 1,3-dicarboxylic acid is produced.68 The dilithio-derivatives have also been prepared from the alk-2-ynes and sub- 64 E.Negishi A. 0.King W. L. Klima W. Patterson and A. Silveira J. Org. Chem. 1980 45 2526. 65 S. Hayashi T. Nakai and N. Ishikawa Chem. Lett. 1980 935. 66 A. M. Sladkov and I. R. Gol'ding Russ. Chem. Rev. (Engl. Trans.),1979 48 868 (Usp. Khim. 1979 48 1625). 67 H. Hommes H. D. Verkruijsse and L. Brandsma Red. Trav. Chim. Pays-Bas 1980 99 113. K. A. Pover and F. Scheinmann J. Chem. SOC.,Perkin Trans. 1 1980 2338. Aliphatic Compounds -Part (i) Hydrocarbons 121 sequent hydroboration-oxidation gives a convenient method for the exclusive formation of 1,3-diols (Scheme 21).69 Terminal alkynes react with carbon monoxide and an alcohol under mild condi- tions to give acetylenecarboxylates R'CGCCO~R~.~' The reaction is catalysed by palladium dichloride and a stoicheiometric amount of copper(r1) chloride is used to regenerate the catalyst.A new synthesis of 2-bromo-alk-1-enes in high yield is accomplished by direct 'Markovnikov' hydrobromination of alk-1-ynes with tetraethylammonium hydro- gen dibr~mide.~~ A solution of tetraethylammonium bromide in dichloromethane absorbs the hydrogen bromide that is required for the addition and the bromide can subsequently be recovered. Zinc-copper couple in boiling methanol reduces alkynes to alkenes in nearly quantitative yield. Terminal alkynes give alk- 1-enes and internal alkynes give (Z)-alkenes without any trace of the (E)-i~omer.~~ A new and potentially powerful method for the synthesis of specifically substituted alkenes from internal alkynes has been demonstrated using but-2-yne (Scheme z).'~ The alkyne complex (19) is attacked by nucleophiles to give (20) and the alkenyl group can be cleaved by halogen with retention of stereochemistry.When the nucleophile is hydride ion (20) can be isomerized to (21) and then cleaved. Ae NU Me F MeJ Fe*. Me Br Me (19) (20) iiil [Nu= HI H Me \/ C ii 'C/ -H II Me II C C /\ /\ Fe* = (Cp)Fe(CO)(PPh3); Fe* Me Br Me Nu = H SPh SBu OEt CN CzCH or CH(C02Et)2 (21) Reagents i Nucleophile; ii Br, ether at -78 "C; iii toluene at 40 "C Scheme 22 (E)-Alkenyl-pentafluorosilicates(22) (see ref.10d) react with alcohols R30H in the presence of oxygen and catalytic amounts of copper(I1) acetate to give (E)-alkenyl ethers (23) of high isomeric purity; when R2 = H the reaction with water gives aldehydes R'CH2CH0.74 69 A. Medlik-Balan and J. Klein Tetrahedron 1980 36 299. 70 J. Tsuji M. Takahashi and T. Takahashi Tetrahedron Lett. 1980 21,849. J. Cousseau Synthesis 1980 805. 72 B. L. Sondengam G. Charles and T. M. Akam Tetrahedron Lett. 1980 21,1069. 73 D.L. Reger and P. J. McElligott J. Am. Chem. SOC.,1980 102,5923. 74 K.Tarnao T. Kakui and M. Kumada Tetrahedron Lett. 1980 21,4105. 122 K. J. Toyne R' R2 \/ c=c H OR^ (22) (23) 5 Allenes A general regioselective synthesis of allenes based on the alkylation of primary secondary or tertiary propargyl alcohols has been reported (Scheme 23).75The frequently used SN2'reaction between propargyl derivatives and an alkyl group in diorganocuprates is one of the most valuable methods for the synthesis of allenes (24) R' 1 \ C=C=CR2R3 R4' Reagents i MeLi ether; ii Cur THF at -70 "C; iii R4Li; iv [Bu",hN(Me)Ph] I- DMF at -70 "C Scheme 23 but it requires an excess of alkyl groups in the cuprate reagent and also propargyl derivatives which are not always readily accessible.The new procedure uses the readily accessible propargyl alcohol to give an organocopper intermediate and an excess of the alkyl-lithium is not required. Nucleophilic attack by the alkyl(methy1- pheny1amino)cuprate counter-ion on the y-carbon of the ion (24)gives the allene.With enyne alcohols however the coupling occurs on the olefinic carbon and conjugated (2)-enynes are formed predominantly [e.g. (25) gives mainly (26),using MeLi]. Et \ //C-H R'C=CCH(OH)CH= CH R'CrC-C (25) \ H (26) A further has appeared on the homologation of terminal alkynes to allenes using formaldehyde di-isopropylamine and copper(1) bromide in dioxan (see also ref. 10e). '' Y. Tanigawa and S. Murahashi J. Org. Chem. 1980,45,4536. 76 H. Fillion D. AndrC and J. Luche Tetrahedron Lett. 1980 21 929.

ISSN:0069-3030    

DOI:10.1039/OC9807700107

出版商:RSC    

年代:1980

数据来源: RSC