ISSN:0069-3030    

DOI:10.1039/OC98481FX001

出版商:RSC    

年代:1984

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC98481BX003

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

3 Theoretical Chemistry By M. GODFREY Department of Chemistry University of Southampton Southampton SO9 5NH 1 Introduction There is a wide variety of theoretical models designed for the description of chemical properties in terms of the behaviour of nuclei and electrons in molecules. The models differ in the extent to which accuracy is sacrificed in favour of simplicity or feasibility. Some are designed for viewing individual trees and others are designed for viewing the wood as a whole. In compiling this year's report material concerning the nature and the application of all types of model was considered. The main criterion for selection from many hundreds of relevant references was the perceived general interest to organic chemists. Hence advances in computational methods are not usually reported unless they are exemplified by application to some important problem in organic chemistry.2 Models Concepts and Rules In this section the material considered concerns innovations in and criticisms of theoretical models the concepts used in them and rules generated from them. A special issue of Croatica Chernica Acta' was devoted to critical review articles on various models of the electronic structure of molecules. Many distinguished theoretical chemists contributed. Several interesting review papers on fundamental aspects of reaction mechanisms are among the proceedings of a colloquium on the mechanisms of elementary physicochemical processes held at Sorrento Italy in May 1983.2 Simonetta3 discussed the theoretical approach to organic reaction paths in his RSC Centenary Lecture.Calculations for isolated molecules for molecules present in a solvent and reactions in the solid state were reviewed. Ponec4 has proposed and exemplified a topological analysis of chemical reactions which he claims is more attractive for classification of reactivity in computer designed synthesis than existing techniques. Sinanoglu' has proposed a theorem and two simple rules derived from it which permit the qualitative behaviour of molecules to be predicted vis-Ci-vistheir relative stabilities deformations or reactions in a simple pictorial way. ' Croat. Chem. Acta 1984 57 765. Int. J. Quantum. Chem. 1984 26 563-964. M. Simonetta Chem. SOC.Rev. 1984 13 1. R. Ponec Collect.Czech. Chem. Commun. 1984 49 455. 0. Sinanoglu Chem. Phys. Lett. 1984 103 315. 17 18 M. Godfrey Some clarification of the properties of minimum energy paths on the potential energy surfaces (PES) of chemically reacting systems has been given6 'Structural stability' has been established as an intrinsic property of PES that holds within the adiabatic appr~ximation:~ structural stability restriction has been shown to rule out certain frontside SN2paths.* Dewar et aL9 have described a general method for the location of transition states in reaction mechanisms. The procedure is fully automatic once reactants and prod- ucts are characterized and no assumptions as to the geometry of the transition state or of the mechanism are necessary.Dewar" has also proposed the rule that syn-chronous multibond mechanisms are normally prohibited they occur only in exceptional cases where special factors operate. The best description of the reaction path may often involve co-ordinates other than the obvious one. Agmon" has shown that it is possible to generalize models for energy profiles along the reaction co-ordinate by adding an upside-down profile which describes the energy perpendicular to the reaction co-ordinate. His model involves three simple assumptions concerning the relationship between the two curves. Northrup and McCammon'* use the term 'gated' to describe reactions in which the rate of a local reaction is controlled on a larger scale by the formation of permissive structural configurations within which the local transformation can proceed relatively rapidly.Theoretical models for specific types of gated reactions have been described in previous work and a general model for analysing the kinetics of such reactions is now being developed. Houk and Rondan13 have developed a new hypothesis to show how negative activation energies and entropy control of reactivity can arise in reactions having no inherent PE barrier. The analysis of adiabatic surfaces in terms of diabatic components plays an important role in the interpretation of organic phen~mena.'~~'~ Bernardi and Robb16 have described a procedure for the computation of diabatic surfaces defined in the framework of an ab initio CI approach. The main feature of this procedure is that each diabatic surface is associated with a specific bonding situation and thus with a specific packet of configurations built from the valence orbitals of the fragments.The importance of HOMO-HOMO interactions and of T-u interactions in frontier orbital treatments of nucleophilic attack has been stressed in several studie~.'~ Parr and Yang18 have demonstrated that most of the frontier-electron theory of W. Quapp and D. Heidrich Theor. Chim. Actu 1984 66 245. A. Fernandez and 0. Sinanoglu Theor. Chim. Acta 1984 65 179. A. Fernandez and 0.Sinanoglu Theor. Chim. Actu 1984 66 147. M. J. S. Dewar E. F. Healy and J. J. P. Stewart 1. Chem Soc. Furuduy Trans. 2 1984 227. 10 M. J. S. Dewar J. Am. Chem. SOC. 1984 106 209. N. Agmon J. Am. Chem. SOC.,1984 106 6960.I2 S. H. Northrup and J. A. McCammon J. Am. Chem. SOC. 1984 106 930. l3 K. N. Houk and N. G. Rondan J. Am. Chem. Soc. 1984 106,4293. 14 R. B. Woodward and R. Hoffman 'The Conservation of Orbital Symmetry' Academic Press New York 1970. l5 N. D. Epiotis 'Theory of Organic Reactions' Springer-Verlag Heidelberg 1978. 16 F. Bernardi and M. A. Robb J. Am. Chem. Soc. 1984 106 54. 17 C. L. Liotta E. M. Burgess and W. H. Eberhardt J. Am. Chem. SOC.,1984 106 4849; S. Yamaba T. Minato and Y. Kawabata Can. J. Chem. 1984 62 235; R. D. Bach M. L. Braden and G. J. Wolber J. Org. Chem 1983 48 1439; R. D. Bach G. J. Wolber and A. Pross Isr. J. Chem. 1983 23 97; R. D. Bach and G. J. Wolber J. Am. Chem. Soc. 1984 106 1401 1410. 18 R. G. Parr and W.Yang J. Am. Chem. SOC.,1984 106 4049. Theoretical Chemistry 19 chemical reactivity can be rationalized from the density functional theory of the electronic structure of molecules. Hori et al.” have proposed a new method for the partitioning of interaction energy. It is now well established that SN2reactions of anions commonly take place with little or no activation in the gas phase indicating that the barriers in solution are due primarily to desolvation of the ion. However the rates of SN2reactions usually vary little with changes in the solvent. Carrion and Dewa8’ have proposed a resolution of this paradox. Shaik” has attempted to show that the VB state correlation diagram model2’ can provide a conceptual basis for both solvent effects and intrinsic reactivity trends in the SN2reaction.The general problem of formulating theories of nucleophilic reactivity has been discussed by Rit~hie.~~ A knowledge of the energy-transfer rates in molecular collisions is required for a basic understanding of the kinetics of gas-phase chemical reactions. A systematic method for calculating rate constants for the excitation and relaxation of the individual vibrational modes of organic molecules in collisions with atoms has been described.24 The method involves the approximate solution of the 3D quantum- dynamical equations of motion for the atom-molecule system. Siebrand and co- workersz5 have elaborated on a method26 of non-classical hydrogen atom transfer which employs a formalism analogous to that used for radiationless transitions rather than the usual tunnelling formalism.The HSAB (hard and soft acids and bases) principle27 has been proved from a simple model utilizing the concept of ‘absolute hardness’.28 An operational definition electron affinity of the species in question. Tables of values are given for a number of free atoms Lewis acids and Lewis bases. The role of c7-conjugative interactions has been considered in detail by De~ar,~~ with special reference to bond localization and aromaticity the structures of radicals and diradicals triplet carbenes stabilities of conformers including the gauche and anorneric effects and chelotropic reactions. Borden and co-workers3’ have shown using second-order Jahn-Teller theory that allylic resonance in radicals will decrease in importance with increasing differences between the electronegativities of the central and terminal heavy atoms and that electron repulsion effects will cause allylic resonance to be more important in diradicals than in the corresponding radical species containing one less .rr-electron.They have also shown how the same conclusions can be reached within the context of resonance theory provided that no-bond resonance structures which localize electron pairs on the most electronegative atoms are considered. Starting from a 19 K. Hori,Y. Asai and T. Yamabe Theor. Chim. Acta 1984 66 77. 2o F. Carrion and M. J. S. Dewar J. Am. Chem. Soc. 1984 106 3531. ” S. S. Shaik J. Am. Chem SOC.,1984 106 1227. 22 A. Pross and S. S. Shaik Acc.Chem. Res. 1983 16 363. 23 C. D. Ritchie J. Am. Chem. SOC.,1983 105 7313. 24 D. C. Clary J. Am. Chem. SOC. 1984 106 970. 25 W. Siebrand T. A. Wildman and M. 2.Zgierski J. Am. Chem. Soc. 1984 106 4083 4089. 26 J.-P. Laplante and W. Siebrand Chem. Phys. Lett. 1978 59 433. 27 R. G. Pearson J. Am. Chem. SOC.,1963 85 3535. 28 R. G. Parr and R. G. Pearson J. Am. Chem. SOC.,1983 105 7512. 29 M. J. S. Dewar J. Am. Chem. SOC.,1984 106 669. 30 D. Feller E. R. Davidson and W. T. Borden J. Am. Chem. Soc. 1984 106 2513. are the ionization potential and the AandIwhereI-A) ;( of absolute hardness is 20 M. Godfrey VB approach Howeler and Klessingefl' have developed a simple model for non-polar radical reactions which extends beyond the possibilities offered by known MO considerations.K~wajima~~ has presented an extension of the classical VB theory of aromaticity that can account for both antiaromaticity and magnetic effects. According to Wennerstrom and co-~orkers~~ the r-system of a macrocycle with 2-fold rotational symmetry and 2n conjugated nelectrons can conveniently be regarded as the sum of two cyclic n .rr-electron systems having Huckel and Mobius topology respectively. Two studies show that through-resonance is less significant than many organic chemists assume. In the influence of the mesomeric interactions of .rr-donor para-substituents on nitrobenzene was examined by means of analyses in VB terms of ab initio SCF wavefunctions. In the other,35 it was shown by ab initio MO calculations that the length of the double bond in mono- and di-substituted ethylenes is almost constant even in compound (1).This result is in keeping with previous studies36 on benzene derivatives. 02NINHz Magn~sson~~ has reconsidered sp hydridization in the light of results of an ab initio MO study of bonding by first and second row main-group elements. He finds no justification for retaining the familiar model of a strict relationship between bond angle and the participation of s and p orbitals in bonding. The Walsh-Bent hypothesis that the attachment of electronegative groups favours the use of p rather than s orbitals in bonding by a central atom is also not supported. Gready3' has re-examined the nature of the relationship between .rr-bond order and bond length in the prediction of molecular geometry and the interpretation of chemical bonding.In an extension of earlier Jug4* has suggested a generaliz- ation of the quantum-mechanical definition of valence of atoms in molecules. Valence is considered as the expectation value of diatomic parts of density operators. A novel approach to the description of chemical bonds has been reported41 which confirms indications from X-ray diffraction results that the formation of covalent bonds does not necessarily bring about an increase in electron density in the bonding region. The weakness ofthe Mulliken population formalism in determining the ionic character of the C-Li bond in organolithium compounds has been 31 U. Howeler and M. Klessinger Angew.Chem Int. Ed. Engl. 1984 23 985. 32 S. Kuwajima 1. Am. Chem. SOL,1984 106 6496. 33 U. Norrinder 0. Wennerstrom and H. Wennerstrom Tetrahedron Lett. 1984 25 1397. 34 P. C. Hiberty and G. Ohanessian J. Am. Chem. Soc. 1984 106,6963. 35 S. Marriott and R. D. Topsom THEOCHEM 1984 18 305. 36 E. von Nagy-Felsobuki R. D. Topsom S. Pollack and R. W. Taft J. Mol. Stnrct. 1982 88 255. 37 E. Magnusson J. Am. Chem. Soc. 1984 106 1177 1185. 38 J. E. Gready J. Comput. Chem. 1984 5 411. 39 M. S. Gopinathan and K. Jug Theor. Chim.Act4 1983 63 491 511. 40 K. Jug J. Comput. Chem. 1984 5 555. 41 D. Cremer and E. Kraka Angew. Chem. Int. Ed. Engl. 1984 23 627. 42 S. M. Bachrach and A. Streitwieser J. Am. Chem. Soc. 1984 106 2283. Theoretical Chemistry 21 Visual representations of electron densities can be instrumental in helping in the understanding of both the structures of molecular systems and their potential reactivity.Practical problems in representing electron densities by surfaces are being tackled by Hehre and co-worker~.~~ 3 Simple Quantitative Relationships The material considered in this section is that which contributes to the understanding of the bases of simple quantitative free energy relationships. The relationship between the free energy of activation and the free energy of reaction in electron proton atom and group-transfer reactions has attracted much attention. Murdoch’s theoretical equation,44 (l) expresses barrier heights (or well depths) in terms of kinetic and thermodynamic contributions.In equation (l),AE* describes the barrier height for a reaction AE the overall energy change AE; the intrinsic barrier and g and g are functions which govern the relative contributions of the kinetic term and the thermodynamic term. It has been shown4’ that previous empirical equations (including those of and of Agmon and Le~ine~~) obtained by a wide variety of apparently unrelated assumptions are special cases of equation (1). Murdoch and co-workers have argued4’ that applications of Marcus- like equations to double-well potential surfaces might fail and have proposed49 a new method for obtaining the height of the intrinsic barrier that can be applied to processes which have no corresponding identity reactions.AE* = AEi(1 -g2) + $AE(l + gl) (1) WolfeSo has pointed out that if Marcus’ equation is valid no stationary point should exist between the reactant state and the product state when IAEl z= 4AEi. This prediction has been confirmed by ab initio MO calculations on the deprotonation of carbon acids. Wiseman and Kestner” have made some modifications to the model of Agmon and Levine.4’ They conclude that the Bronsted coefficient is not equal to the transition state bond order in violation of the Leffler-Grunwald post~late.’~ The mechanistic significance of the Bronsted parameter has been studied by Pross within the framework of a qualitative VB Although the correlation between total N-atom charge and gas-phase basicities is poor in heteroaromatic systems there is good correlation of N lone-pair charge and protonation energy.54 An experimental result which has implications for the interpretation of Bronsted coefficients is that the slopes of log k versus pK, for 43 M.M. Franc] R. F. Hout and W. J. Hehre J. Am. Chem. SOC.,1984 106 563. 44 J. R. Murdoch and D. E. Magnoli 1. Am. Chem. SOC.,1982 104 3792. 45 J. R. Murdoch 1.Am. Chem. SOC.,1983 105 2159. 46 R. A. Marcus J. Chem. Phys. 1956 24 966. 47 N. Agmon and R. D. Levine J. Chem. Phys. 1979,71 3034. 48 J. Donnella and J. R. Murdoch J. Am. Chem. SOC 1984 106,4724. 49 M. Y. Chen and J. R. Murdoch J. Am. Chem. SOC.,1984 106 4735. 50 S. Wolfe Can. J. Chem. 1984 62 1465. 51 F. W. Wiseman and N. R. Kestner J. Phys. Chem. 1984 88,4354.52 J. E. Leffler and E. Grunwald ‘Rates and Equilibria of Organic Reactions’ Wiley New York 1963. 53 A. Pross J. Org. Chem 1984 49 1811. 54 J. Catalan J. L. G. de Paz M. Yanez and J. Elguero J. Am. Chem. Soc. 1984 106 6552. 22 M. Godfrey SN2reactions of various sets of substituted anionic nucleophiles with benzyl chloride are identical irrespective of the nature of the donor atom.” An experimental result which has implications for the mechanistic significance of the Hammett reaction constant (rho) is that the rates of the identity methyl-transfer reactions of a set of substituted benzene sulphonates fit the Hammett equation with a non-zero (+0.6) value of rho.56 The mechanistic significance of the reaction constants (rho) and the cross-interaction constant (q) in the interactive free-energy relationship for two substituents X and Y (equation 2) have been ~tudied.’~ It is concluded that the value of q reflects at least in part a change in the position of the transition state induced by the substituent.log kXYIkHH = P%X + PzHffY + PXUY (2) Swain has recently5’ reiterated an earlier59 claim that only one resonance scale is necessary in the field and resonance analysis of substituent effects. This claim has been criticized6’ on the grounds that it provides an incomplete separation of field and resonance effects and that it fails badly for systems following the cr-scale of substituent effects. Swain has also proposed61 that a dual solvent parameter equation (3) is adequate for expressing solvent effects on molecular properties (P):the solvent parameters A and B represent respectively the solvating ability of anions and cations.However Taft Abboud and Kamlet62 have reasserted that the three solvent parameters involved in their own analysis63 are necessary in the general case. Swain has replied64 to all these criticisms. Afanas’ev6’ has also argued that different scales of resonance constants are superfluous but Kevi1f6 has given evidence that A and B are considerably inferior to the N and Y parameters of the extended Grunwald-Winstein equation67 when applied to solvolytic displacement reactions. P = Po + aA + 6B (3) The correlation of octanol/water partition coefficients with the Kamlet-Abboud- Taft parameted3 gives insights into the relative importance of molecular factors that determine solution partition coefficients.68 Brady and Cad9 are pessimistic about whether physically meaningful information is extractable from correlations involving single-parameter scales following an examination of the effects of modelling dipole-dipole and dipole-induced dipole interactions with a single lumped parameter.55 F. G. Bordwell and D. L. Hughes J. Am. Chem. SOC.,1984 106 3234. 56 E. S. Lewis and D. D. Hu J. Am. Chem. SOC.,1984 106 3292. 57 J. E. Dubois M.-F. Ruasse and A. Argile J. Am. Chem. SOC.,1984 106 4840 4846. 58 C. G. Swain S. H. Ungar N. R. Rosenquist and M. S. Swain J. Am. Chem. SOC., 1983 105 492. 59 C. G. Swain and E. C. Lupton J. Am Chem. SOC.,1968,90 4328. 60 W.F. Reynolds and R. D. Topsom J. Org. Chem. 1984,49 1989; A. J. Hoefnagel W. Oosterbeek and B. M. Wepster J. Org. Chem. 1984,49 1993; M. Charton J. Org. Chem. 1984,49 1997. 61 C. G. Swain M. S. Swain A. L. Powell and S. Alunni J. Am. Chem. SOC.,1983 105 502. 62 R. W. Taft J.-L. M. Abboud and M. J. Kamlet J. Org. Chem. 1984 49 2001. 63 M. J. Kamlet J-L. M. Abboud and R. W. Taft hog. fhys. Org. Chem. 1981 13 1080. C. G. Swain J. Org. Chem. 1984 49 2005. 65 I. B. Afanas’ev J. Chem. SOC.,ferkin Trans. 2 1984 1589. 66 D. N. Kevill J. Chem. Res. (S) 1984 86. 67 S. Winstein E. Grunwald and H. W. Jones J. Am. Chem. SOC.,1951 73 2700. M. J. Kamlet M. H. Abraham R. M. Doherty and R. W. Taft J. Am. Chem. Soc. 1984 106,464. 69 J. E. Brady and P.W. Cam J. Phys. Chem. 1984.88 5796. Theoretical Chemistry The transmission of polar substituent ,eff ects in the bicycloalkane systems (2) and (3) have been studied by Adcock et aL7’ by measuring 19F substituent chemical shifts. In each case the shifts linearly correlate with a combination of field and electronegativity parameters but the sign of the coefficient of the electronegativity parameter is different in the two cases. A qualitative explanation in terms of c+-electron delocalization mechanisms is advanced. Ab initio MO calculations on suitable model systems have been used to study the polarization of welectron systems by substituent dipoles and to construct a theoretical scale of substituent field parameters,” and to provide a simple and well-defined scale of substituent electronegativity parameter^.^^ The latter scale correlates with the JC(ipso+c(ort~o) coupling constant for many monosubstituted benzenes where the directly bonded atom is a first-row element.The impossibility of obtaining a universal scale of field parameters has been discussed.72 (2) (3) The problem of through-bond versus through-space transmission of substituent effects has been discussed by Exner and co-w~rkers~~ following the failure of electrostatic theory to account for variations in the pK values of carboxylic acids in different solvents. Doan and drag^^^ have provided an answer to why free energies which are composites of two quantities (AHand AS) with independent molecular explanations are often correlated with one or two parameters.Gie~e~~ has defined the ‘isoselective relationship’ equation (4) with TiSrepresenting the isoselective temperature and discussed its significance for linear free-energy relationships. S(AH -AH:,,) = TisS(AS:,2 -AS:,,) (4) Ruoff 77 has discussed linear free-energy relationships from the viewpoint of information theory. 4 Studies of Molecular Structures and Reaction Paths The material included in this section illustrates the state of the art of theoretical studies of molecular structures and reaction paths. A critical examination of 149 landmark papers up to 1983 is presented in a book by S~haeffer.~’ A bibliography of ab initio calculations published in 1983 is available.79 70 W. Adcock A. N. Abeywickrema and G.B. Kok,J. Org. Chem. 1984 49 1387. 71 S. Mamott and R. D. Topsom J. Chem. SOC.,Perkin Trans. 2 1984 113. 72 S. Marriott and R. D. Topsom J. Am. Chem. SOC.,1984 106 7. 73 S. Marriott W. F. Reynolds R. W. Taft and R. D. Topsom J. Org. Chem. 1984 49 959. 74 K. Kalfus Z. Fried] and 0. Exner Collect. Czech. Chem. Commun. 1984 49 179. 75 P. E. Doan and R. S. Drago J. Am. Chem. SOC. 1984 106 2772. 76 B. Giese Acc. Chem. Res. 1984 17 438. 77 P. Ruoff 2. Phys. Chem (Leipzig) 1964 265 433. 78 H. F. Schaeffer ‘Quantum Chemistry The Development of ab initio Methods in Molecular Electronic Structure Theory’ Oxford University Press Oxford 1984. 79 THEOCHEM 1984 20 1. 24 M. Godfrey A fundamental weakness of Hartree-Fock MO calculations is the neglect of electron correlation.Although the energy associated with this phenomenon is a small fraction of the total energy of a molecule it is of the same order of magnitude as most energies of chemical interest. Accurate techniques for the theoretical determi- nation of electron correlation on molecular properties are therefore vital for the quantitative accuracy of predictions. A book by Wilson" describes methods for the calculation of electron correlation effects in molecules which are now widely used. Calculations on molecular systems containing up to about six first-row atoms are feasible. Kello and co-workers" have systematically calculated correlation effects and bond-correlation energies in a series of molecules which contains C1 to C4 hydrocarbons and oxygen-containing derivatives using the many body Rayleigh- Schrodinger perturbation theory up to the fourth order.Results of studies by Cizek et aZ.82on the localization of the filled and virtual orbitals in the nucleotide bases suggest that the application of localized orbitals will open new possibilities for the calculation of correlation in extended systems. Marriot and T~psom~~ have proposed standard bond lengths for a wide variety of bonds involving first-row elements to be used in fixed geometry ab initio MO calculations. They were obtained as average values from a large number of calculations at the 4-31G level with geometry optimization. The results of even the best ab initio calculations of molecular structures may not be qualitatively correct and there is still a case for performing semi-empirical calculations on small molecular systems.The inadequacies of semi-empirical methods have been discussed recently by Freed84 and by de Bruijnx5 who have suggested ways of overcoming some of the objections to these methods. The mechanisms of reactions which involve the cyclic displacement of electrons continue to attract theoretical attention. Correlation effects favour asynchronism in reaching the transition state of the Diels- Alder reaction,86 but favour C2 symmetry for the transition state structure in the Cope rearrangemente8' The transition states for 1,5-hydrogen shifts appear to have CHC angles very different from 180°.x8A MNDO study suggests that substituents should influence electrocyclic processes markedly.89 Qualitative VB theory has been usedg0 to show how the primary elec- tronic structure of the initial transition-state determines whether or not the 1,3-dipolar cycloaddition reaction is concerted.The theory also indicates that if the first transition state is primarily an extended diradical the energy minimum for an electronically excited state of the diradical must lie immediately above and be energetically close to the transition state. 80 S. Wilson 'Electron Correlation in Molecules' Oxford University Press Oxford 1984. 81 V. Kello M. Urban J. Noga and G. H. F. Diercksen J. Am. Chem. Soc. 1984 106 5864. J. Cizek W. Forner and J. Ladik Theor. Chim.Acta 1984 64 107. 83 S. Marriott and R.D. Topsom THEOCHEM 1984 19 337. 84 K. F. Freed Acc. Chem. Res. 1983 16 137. 85 S. de Bruijn Znt. J. Quantum Chem. 1984 25 367. 86 M. Ortega A. Oliva J. M. Lluch and J. Bertran Chem. Phys. Lett. 1983 102 317; F. K. Brown and K. N. Houk Tetrahedron Lett. 1984 25 4609. 87 Y. Osamura S. Kato K. Morokuma D. Feller E. R. Davidson and W. T. Borden J. Am. Chem. Soc. 1984 106 3362. 88 B. A. Hess and L. J. Schaad J. Am. Chem. Soc. 1983 105 7185; N. G. Rondan and K. N. Houk Tetrahedron Lett. 1984 25 2519; J. W. Verhoeven Red. Truv. Chim. Pays-Bas 1984 103 143. 89 A. Jensen and H. Kunz Theor. Chim. Acta 1984 65 33. 90 R. D. Harcourt and R. D. Little J. Am. Chem. Soc. 1984 106 41. Theoretical Chemistry 25 Calculations on alkenes and related compounds are inevitably numerous because of the convenient size of the molecular systems and we can mention only a few examples.There has been a systematic ab initio MO study of the structural properties of ethylene and the all-trans conjugated polyenes C4H6 C6H8 C8H10 and C10H12.~~ The cyclopropenyl cation appears to be n~n-planar,~~ and the second stable confor- mer of buta-1,3-diene appears to be gauche rather than s-c~s.~~ Stereoselectivity in hydroborati~n?~ and directive effects and selectivity in radical addition reactionsg5 have been studied. A MO of 1,3- and 1,2-sigmatropic shifts in propene has shown that the Hartree-Fock level is inadequate for determining an energy surface whose shape is dictated by the interaction and intersection of diabatic surfaces.The phenomenon of bond length alternation in large cyclic polyenes predicted at the restricted Hartree-Fock level has been confirmed at the correlated and there has been a MNDO study of bond orders in some conjugated bicyclic and tricyclic hydrocarbon^.^^ The heavy-atom chain in cumulenones CH,=(C),=O with n 3 2 has been calculated to be bent with the direction of bending alternating between in-plane and orthogonal as a function of r1.9~The influence of the lowest double- excited a-electron configuration may be responsible for this phenomenon.'" There have been a number of ab initio MO calculations on silicon-containing molecular systems that include allowance for electron correlation. If the results are valid there is no stable linear conformation of Si2H2 analogous to acetylene,"' and the thermodynamically most stable structure of H,CSiO is methylsilanone.lo2 Other studieslo3 were concerned with silylene insertion reactions into a-bonds.There has also been a study of lithium atom insertion into the C-H bond of methane.lo4 Doubleday et ~1.'~~ have concluded that singlet tetramethylene exists in an entropy- dominated free-energy minimum even though it lacks a potential-energy minimum but Bernardi et ~l.''~have concluded that the diradical exists as a stable species in two different conformations gauche and trans. Calculations support the possibility that a and T phenyl radicals are well separated energetically and may exist as discrete chemically distinguishable species.'" There have been many other MO 91 C.W. Bock P. George and M. Trachtman THEOCHEM 1984 18 1. 92 B. A. Hess L. J. Schaad and P. Carsky Tetrahedron Lett. 1984 25 4721. 93 J. Breulet T. J. Lee and H. F. Schaefer J. Am. Chem. SOC.,1984 106 6250; C. W. Bock P.George and M. Trachtman Theor. Chim. Acra 1984 64 293. 94 K. N. Houk N. G. Rondan Y.-D. Wu J. T. Metz and M.N. Paddon-Row Tetrahedron 1984,40,2257. 95 R. Ponec J. Malek W. Kuhnel and E. Gey THEOCHEM 1984 19 293; R. Amaud Y.Ellinger R. Subra and J. Douady THEOCHEM 1984 19 203. 96 F. Bernardi M. A. Robb H. B. Schlegel and G. Tonachini J. Am. Chem. SOC.,1984 106 1198. 97 J. Paldus and M. Takahashi Int. 1. Quantum Chem. 1984 25 423; M. Takahashi and J. Paldus Int. J. Quantum. Chem. 1984. 26.349. 98 C. Glidewell and D. Lloyd Tetrahedron 1984 40 4455. 99 L. Farnell and L. Radom J. Am. Chem. Soc. 1984 106 25. 100 R. D. Brown and R. G. Dittmann Chem. Phys. 1984 83 77. 101 J. S. Binkley J. Am.Chem. Soc. 1984 106,603. 102 M. S. Gordon and C. George J. Am. Chem. SOC.,1984 106 609. 103 C. Sosa and H. B. Schlegel J. Am. Chem. SOC.,1984 106 5847; K. Raghavachari J. Chandrasekhar M. S. Gordon and K. J. Dykema 1. Am. Chem. SOC.,1984 106 5853; M. S. Gordon and D. R. Gano J. Am. Chem. SOC.,1984 106 5421. 104 J. G. McCaffrey R. A. Poirier G. A. Ozin and I. G. Csizmadia J. fhys. Chem. 1984,88 2898. 105 C. Doubleday R. N. Camp H. F. King J. W. McIver D. Mullally and M. Page J. Am. Chem. SOC. 1984 106 447. 106 F. Bernardi A. Bottoni G.Tonachini M. A. Robb and H. B. Schlegel Chem. fhys. Lett. 1984,108,599. 107 R. P. Johnson J. Org. Chem. 1984 49 4857. M. Godfrey calculations of various kinds on radicals,"* radical ions,"' ions,"O carbenes and nitrenes,"' and excited states we reference just a selection. Other interesting ab initio MO studies elucidate the barrier height for double proton transfer in the formic acid dimer,'13 show that hydrogen bonding plays an important role in explaining the relative basicities of methylamines in solution,' l4 and demonstrate that the activation energy in a reaction might be lowered by a catalyst designed to compress the reactant state more than the transition ~tate."~ The proceedings of a symposium on molecular mechanics held at Indianapolis in June 1983 have been published:'16 they include a treatment of the anomeric effect.' '' An appraisal of molecular force-fields for the representation of polypeptides has been given by Hall and Pavitt.'18 A set of procedures and guidelines has been presented"' for the estimation of bond length bond angle and torsional potential constants for molecular-mechanics force-fields.Chandrasekhar Smith and Jorgensenl2O claim the first computation of an energy profile for an SN2reaction in aqueous solution using Monte Carlo simulations of the reaction in solution. Chiles and Rossky121 have reported an alternative computationally faster method of tackling the identical problem. Cournoyer and Jorgensen'22 have studied solvent effects on the relative energies of carbocations.Kollman and co-~orkers'~~ claim the first ab initio SCF correlation energy and environmental effect calculation on all the chemical steps of an enzymatic reaction. K~llman'~~ has also proposed a new force-field for molecular methanical simulation of nucleic acids and proteins. lo' C. Glidewell J. Chem. Soc. Perkin Trans. 2 1984 407. R. H. Nobes W. J. Bouma and L. Radom J. Am. Chem. SOC.,1984 106 2774; M. H. Lien and A. C. Hopkinson Can. J. Chem. 1984,62 922. M. T. Nguyen and T.-K. Ha J. Chem. SOC.,Perkin Trans. 2 1984 1401 ;J. Almlof G. Hvistendahl and E. Uggerud Chem. Phys. 1984,90 55. 'I1 G. Frenking and J. Schmidt Tetrahedron 1984 40 2123. 112 M. J. S. Dewar M. A. Fox K. A. Campbell C.-C. Chen J. E. Friedheim M.K. Holloway S. C. Kim P. B. Liescheski A. M. Pakiari T.-P. Tien and E. G. Zoebisch 1. Comput. Chem. 1984 5 480. 113 S. Hayashi J. Umemura S. Kato and K. Morokuma J. Phys. Chem. 1984,88 1330. S. Galera A. Olivia J. M. Lluch and J. Bertran THEOCHEM 1984 19 15. I. H. Williams J. Am. Chem. SOC.,1984 106 7206. 116 J. Comput. Chem. 1984 5 289-348. 117 L. Norskov-Lauritsen and N. L. Allinger J. Comput. Chem. 1984 5 326. 'lS D. Hall and N. Pavitt J. Comput. Chem. 1984 5 441. A. J. Hopfinger and R. A. Pearlstein J. Comput. Chem. 1984 5 486. 120 J. Chandrasekhar S. F. Smith and W. L. Jorgensen J. Am. Chem. SOC.,1984 106 3049. "' R. A. Chiles and P. J. Rossky J. Am. Chem. Soc. 1984 106 6867. 122 M. E. Cournoyer and W. L. Jorgensen J. Am. Chem.SOC.,1984 106 5104. lZ3 G. Alagona P. Desmeeules C. Ghio and P. A. Kollman J. Am. Chem. SOC.,1984 106 3623. lZ4 P. A.Kollman J. Am. Chem. Soc. 1984 106 765.

ISSN:0069-3030    

DOI:10.1039/OC9848100017

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (i)Pericyclic Reactions By R. S. ATKINSON Department of Chemistry University of Leicester Leicester LE1 7RH 1 General A monograph on natural product synthesis using pericyclic reactions has appeared.' A computer-assisted procedure has been devised to treat competitive thermal pericyclic reactiom2 2 Electrocyclic Reactions A remarkable stereospecificity has been demonstrated in the thermal conrotatory ring-opening of the trans-substituted cyclobutene (1) to the 2,Z-substituted butadiene (2).3This conclusion was arrived at by a study of the equilibria in Scheme 1. It has previously been assumed that such substituted cyclobutenes always preferen- tially ring-open to E,E-butadienes (3) to minimize repulsive steric effects. A survey of a number of substituted cyclobutenes has shown that this aberrant behaviour is not restricted to trifluoromethyl groups as substituents but is also exhibited to a lesser degree by other substituents which are not electron donating.A rationalization of the results in molecular orbital terms has been offered? F a F F 3 c'F E E Scheme 1 (3) ' 'Natural Products Synthesis through Pericyclic Reactions' ed. G. Desimoni G. Tacconi A. Barco and G. P. Pollini ACS Monograph No. 180 1983. * J. S. Burnier and W. L. Jorgensen J. Org. Chem. 1984,49 3001. W. R. Dolbier H. Koroniak D. J. Burton A. R. Bailey G. S. Shaw and S. W. Hansen J. Am. Chem. Soc. 1984 106 1871. W. Kirmse N. G. Rondan and K. N. Houk J. Am. Chem. Soc. 1984 106 7989. 27 R.S. Atkinson Dissymmetric cis-1,4-disubstituted cyclobutenes in which the substituents are very similar can nevertheless be coaxed into selecting one of the two possible conrotatory ring-openings preferentially. In model studies towards the synthesis of verrucarin A conrotatory ring-opening of the cyclobutene (4) gave the dienes (5) and (6) in a 2 :1 ratio with the major isomer having the required stereochemistry of verrucarin.' Electrochemically induced electrocyclization of (7) via a radical anion has been shown to proceed in a conrotatory fashion and the product (8) is identical with the photolysis product of (7). The nodal character of the highest occupied molecular orbital is presumably the same in both ground-state radical-anion and neutral photoexcited state.6 A cascade of four consecutive pericyclic reactions is involved in the reaction of alkynes and cyclobutenones to give (polysubstituted) aromatics (Scheme 2).This annulation method has been used to synthesize the antifungal antibiotic DB-2073 (9h7 R' R' R' R4 IX R4 R3 2. 2si 2s X R3 R2 R3 1;a:;;e2m Scheme 2 B. M. Trost and P. G. McDougal J. Org. Chem. 1984 49 458. M. A. Fox and J. R. Hurst J. Am. Chem. Soc. 1984 106 7626. ' R. Danheiser and S. K. Gee J. Org. Chem. 1984 49 1672. Reaction Mechanisms -Part (i) Pericyclic Reactions > -Ll;ic<. i;r. 61r + 47~ @ -~ / 41r + 677 \ ~ 1 H [1,5] sig. rearr. 3 Cycloaddition Reactions A number of reviews on aspects of the Diels-Alder reaction have appeared this year including recent advances and synthetic applications of the intramolecular Diels- Alder ;lo reactivity of exocyclic dienes tetraenes and hexaenes ;ll asymmetric Diels- Alder (and ene) reactions ;12-14 and theoretical considerations of regio- and stereo-~electivity.’~ Acetylene equivalents in cycloaddition reactions have been reviewedI6 and many anthracyclinone syntheses include different Diels- Alder reac- tion~.‘~ Aspects of the mechanism of the Diels-Alder have aroused some controversy.Dewar et al. adopt a fundamentalist position with regard to the concertedness of pericyclic reactions and in particular to whether both new C-C bonds in this reaction are formed synchronously.’8 Their conclusion is that all the experimental evidence can be interpreted in terms of formation of one of these bonds being almost complete while the other is very incomplete.A probe for the symmetry of the transition state of this reaction has used chiral prosthetic groups at the termini of the dienophile and an examination of co-operativity in asymmetric induction on addition to a two-fold symmetric diene.” The enantiomeric ratio produced when a dienophile containing two independent chiral moieties (e.g. dibornyl fumarate) reacts with anthracene is found to be the square of the ratio obtained when the dienophile contains only one of the chiral moieties (e.g. bornyl methyl fumarate) and therefore the C-C bond formations are adjudged to be synchronous. (Dewar has an interpretation of these results which ’A.Beck L. Knothe D. Hunkler and H. Prinzbach Tetrahedron Lett. 1984 25 1785. J. Breulet and H. F. Shaefer J. Am. Chem. Soc. 1984 106 1221. 10 A. G. Fallis Can. J. Chem. 1984 62 183. P. Vogel Methods Stereochem. Anal. 1983 3 147. W. Oppolzer Angew. Chem. Int. Ed. Engl. 1984 23 876. l3 L. A. Paquette in ‘Asymmetric Synthesis,’ vol. 3 ed. J. D. Morrison Academic Press Orlando Florida 1984 pp. 455-501. l4 P. Welzel Nachr. Chem. Tech. Lab. 1983 31 979. 15 R. Gleiter and M. C. Boehm Methods Stereochem. Anal. 1983 3 105. 16 0. DeLucci and G. Modena Tetrahedron 1984,40,2585; see also 0. DeLucci V. Lucchini L. Pasquato and G. Modena J. Org. Chem. 1984,49 596. 17 T. R. Kelly Tetrahedron 1984 40,4537 et seq. 18 M. J. S. Dewar and A.B. Pierini J. Am. Chem. Soc. 1984 106 203 209. 19 L. M. Tolbert and M. B. Ali J. Am. Chem. Soc. 1984 106 3806; see also R. GrCe M. Laabassi P. Mosset and R. Carrie Tetrahedron Lett. 1984 25 3693. 30 R. S. Atkinson supports his position.) The co-operativity in this reaction vanishes when Lewis acid catalysis is applied and an asymmetric transitiomstate is presumed to obtain in this case. Ab initio (STO-3G basis set) calculations for the Diels-Alder reaction suggest that a symmetrical transition-state structure exists.20 Reaction of unsymmetrically substituted tropones and dienes (Scheme 3) are ,6 + ,4 cycloadditions which are highly regioselective and exo-stereoselective. Clearly therefore the recent suggestion of Alston et al. -that in some Diels-Alder reactions secondary frontier molecular orbital (FMO) interactions control regioselectivity rather than primary FMO interactions -cannot apply to these ,6 + ,4 cycloadditions.To go on to conclude however as the authors do that secondary FMO interactions are unlikely to control regioselectivity in Diels- Alder reactions either seems unwarranted in the view of the present author.21 4-\ \ &Q gd B Scheme 3 Considerable effort continues to be expended on the synthesis and study of dienophiles bearing chiral auxiliaries with a view to maximizing the diastereoisomeric purity of the Diels-Alder adduct. Dienophiles which have been employed successfully for this purpose include (12),22(13),23 and (14)24,25 and cyclopentadiene invariably seems to be the 'test' diene used.An X-ray structure determination on (14) shows that the environment of the acrylate double bond makes its facial selectivity explicable. 0-\\ Co2Me 7% [aID-15" 93% [a]D-0.6" Et02C oq0 Me+ -I TiCI4 -63°C H 0 HO (13) 93:7 d.e. 32 1 endolexo 2o F. K. Brown and K. N. Houk Tetrahedron Lett. 1984 25 4609. 21 M. E. Garst V. A. Roberts K. N. Houk and N. G. Rondan J. Am.Chem. SOC.,1984 106 3882. 22 C. Maignan A. Guessous and F. Rouessac Tetrahedron Lett. 1984 25 1727. 23 T. Poll G. Helmchen and B. Bauer Tetrahedron Lett. 1984 25 2191. 24 W. Oppolzer C. Chapuis and G. Bernardinelli Tetrahedron Lett. 1984 25 5885. 25 W. Oppolzer M. J. Kelly and G. Bernardinelli Tetrahedron Lett. 1984 25 5889.Reaction Mechanisms -Part (i) Pericyclic Reactions 31 (14) R = cyclohexyl Addition of the p-tolylsulphinylacrylate(15)to cyclopentadiene gave (16) as the major diastereoisomer whose absolute configuration was determined by correlation with (+)-2-endo-methyl-2-exa-norbornanecarboxylic acid.26 It is argued that the absolute configuration of the major product is inconsistent with an s-cis-conformation of the S-0 and C=C bonds for the acrylate in the Diels- Alder transition-state. The preferential endo-overlap of the sulphinyl (and Me) group over the ethoxycarbonyl group is however noteworthy. + I '* (15) (16) 63% Ho2cb Me Aldehydes are increasingly seeing use as 27r-components in Diels- Alder additions usually catalysed by Eu(fod) but also accelerated by high pressure e.g.(17)+ ( 18).27The absolute configuration of (18)was proved by chemical correlation. Et ?Me FHO H+OH \ -20 kbar 53 "C,20 h High threo-selectivity (60 1) is observed in the formation of (19).The sense and efficiency of diastereoselection in these ,4 + ,2 reactions is sensitive both to the Lewis acid catalyst and to the substitution pattern of the diene chelation-controlled addition (20)may be important in some cases; in others the bulky solvated metal-ion may prefer the less hindered exo-site (21).28 26 T. Koizumi I. Hakamada and E. Yoshii Tetrahedron Lett. 1984 25 87. 27 J. Jurczak T. Bauer and S. Jarosz Tetrahedron Lett. 1984 25 4809. 28 M. M. Midland and R. S. Graham J.Am. Chem. SOC.,1984 106,4294; see also S. Danishefsky W. H. Pearson and D. F. Harvey ibid. 1984 106 2455 2456; S. Danishefsky and M. Bednarski Tetrahedron Lett. 1984 25 721; S. Danishefsky and R. R. Webb J. Org. Chem. 1984 49 1955. R. S. Atkinson OSiMe 0 A number of examples of Diels-Alder reactions in which cation radicals are proposed intermediates have been reported re~ently,*~,~~ but attention has been drawn to the possibility of these reactions in some cases being acid-catalysed as a result of the aminium cation radical-initiator generating a proton source.31 An Fe"'-doped montmorillonite or bentonite clay has been found to accelerate the rate of Diels- Alder reaction of furan with ap-unsaturated carbonyl compounds32 and the dimerization of cycl~hexadiene.~~ Diels- Alder reaction of cyclohexadiene with the enantiomerically pure a-chloronitroso compounds (22)33and (23)34 gave the corresponding enantiomeric bicyclic oxazines in high enantiomeric purity as shown by their specific rotations and by the n.m.r.spectra of the corresponding 10-camphorsulphonyl derivatives. -20 "C 69% 3 95% e.e. 69% 3 95% e.e. Dialkyldiimides do not undergo Diels- Alder addition to dienes the retro-Diels- Alder is almost invariably thermodynamically preferred. Protonation of the bicyclic azo compound (24) however renders it dienophilic as exemplified by cycloaddition 29 R. A. Pabon D. J. Bellville and N. L. Bauld J. Am. Chem. SOC. 1984 106 2730 and refs. therein. 30 P. Laszlo and J. Lucchetti Tetrahedron Lett.1984 25 1567. 31 P. G. Gassman and D. A. Singleton J. Am. Chem. SOC.,1984 106 6085 7993. 32 P. Laszlo and J. Lucchetti Tetrahedron Lett. 1984 4387 2147. 33 M. Sabuni G. Kresze and H. Braun Tetrahedron Lett. 1984 25 5377. 34 H. Felber G. Kresze H. Braun and A. Vasella Tetrahedron Lett. 1984 5381. Reaction Mechanisms -Part (i) Pericyclic Reactions to cyclohexadiene. Reversal of the thermodynamic equilibrium in this cycloaddition in the presence of acid is attributed to the conversion of the weakly basic (but protonated) azo function into the more basic hydra~ine.~~ Deprotonation of (25) gives the free base which undergoes a retro-Diels-Alder reaction at room temperature (?+CQ. 7h). Another unusual dienophile is singlet sulphur IS2 which is apparently generated when the trisulphides (26) are treated with Ph3PBr2 a reaction in which the sulphur analogue of phosphine ozonide Ph3PS3 is a presumed intermediate.In the presence of conjugated dienes the singlet sulphur is trapped but unlike singlet oxygen it appears to show no 2.rr + 27r or enophilic activity.36 CH-Cl. R3MSSSMR3+ Ph,PBr2-L' 2R,MBr + [Ph3PS3] -S2 + Ph,P=S 25 "C (26) A number of interesting and potentially useful dienes have been synthesized including (27),37(28),38(29),39 and (30).40The trimethylstannyl group in (27) can be further substituted by electrophiles if the X group is base stable. Me3Sn HsnMe3 1. MeLi SnMe3 E = SiMe, SPh C1 SnMe, CH2SiMe3 C(OH)R2 (27) R4yCHo R3 R'O R3 &R4 R3 = H alkyl (28) 35 S.F. Nelson S. C. Blackstock and T. B. Frigo 1 Am. Chem. SOC.,1984 106 3366. 36 K. Steliou Y. Gareau and D. N. Harpp J. Am. Chem. SOC.,1984 106 799. 37 H. J. Reich K. E. Yelm and I. L. Reich 1. Org. Chem 1984 49 3438. 38 J. I. Luengo and M. Koreeda Tetrahedron Lett. 1984 25 4881. 39 W. A. Nugent and J. Calabrese J. Am. Chem. SOC.,1984 106,6422. 40 Y. Eta H. Yasuda 0.Tamura and Y. Tamura Tetrahedron Lett. 1984 25 1813. R S. Atkinson R' Ti catalyst R' R2=alkyl Ph,OEt R'C C(CH2) C CR2 -Me Me \/ RZ (29) Unlike the corresponding 5-methoxycarbonylcyclopentadiene,the 5-ferrocenyl- substituted cyclopentadiene (3 1) can be obtained isomerically pure. Reaction with activated dienophiles followed by ferrocenyl +methoxycarbonyl interconversion gave (32) and thus (31) is in practice a substitute for 5-methoxycarbonylcyclopen-tadie~~e.~* Me0,C H v Fp =T~-C,H,F~(CO)~ A useful addition to the Danishefsky suite of dienes is the l-t-butoxy-3-[(trimethyl-sily1)oxylbutadiene (33) which has been synthesized in 50 g quantities.Diels- Alder adducts of this diene with electron-poor alkenes as dienophiles can be converted into 3-t-butoxycyclohexanones (34) without enone formation.42 OBu' OBu' OBu' (33) (34) The Bradsher-Falck version of the inverse Diels-Alder reaction -the use of isoquinolinium salts as dienes with electron-rich dienophiles -has been cleverly extended to the synthesis of C-naphthylglycosides by using cyclic enol ethers (glycals) as dienophiles (Scheme 4).43 The formal ,4 +,4 and ,4 +,2 adducts of benzene and anthracene (35) and (36) respectively have been synthesized.Their thermodynamic parameters for thermal decomposition and quantum yields in photochemical decomposition have been interpreted as evidence for orbital-symmetry control in both ground-state and excited-state reactions.@ 41 M. E. Wright J. F. Hoover G. 0. Nelson C. P. Scott and R. S. Glass J. Org. Chem. 1984 49 3059. 42 R. P. Potman N. J. M. L. Janssen J. W. Scheeren and R. J. F. Nivard J. Org. Chem. 1984,49 3628. 43 R. W. Franck and R. B. Gupta J. Chem. Soc. Chem. Commun. 1984 761. N. C. Yang M.-J. Chen and P. Chen J. Am. Chem. Soc. 1984 106 7310. Reaction Mechanisms -Part (i) Pericyclic Reactions CHO H+/H,O / OH Scheme 4 Thermal stabilities of (37) and (38) were examined to test for the facility of retro-Diels-Alder fragmentation.At 160 "C(37) was stable over 90 h whereas (38) fragmented smoothly at 95 "C with first-order kinetics (k= 1.67 x lop4s-'). It would be anticipated that (38) be less stable than (37) if the fragmentation is concerted and naphthalene resonance energy contributes to a lowering of the transition-state energy. That (38) does not spontaneously disintegrate at room temperature can be attributed to the significant barrier required for deformation from its preferred conformation (39) (or its mirror image) to the conformation required for concerted fragmentati~n.~' L. A. Paquette H. Jendralla and G. Lelucca J.Am. Chem. SOC.,1984 106 1518. R. S. Atkinson 4 1,3-Dipolar Cycloadditions A treatise on 1,3-dipolar cycloaddition reactions (in two volumes) has been pub- lis hed.46 Aromatic diazonium salts as well as reacting concertedly with diene~,~' show dipolarophilic activity with e.g. azomethine ylides and thiocarbonyl ylides leading to triazolium salts (40) and thiadiazolines (41) re~pectively.~~ Me N PhfiPh $iy ArN ph + BF Arf;12BF B F4 (40) 47% Ar = p-N02C& BiphC CH S Biphc ) +Biph( S149% ArN'N ArN-N Biph = BF4 (41) 49% Hunig et al. have observed some unusual intramolecular 1,3-dipolar cycloaddition reactions in systems49 having C=C and 1,3-dipole in enforced proximity. Thus the azoxy compounds (42) readily undergo thermal or acid-catalysed cycloaddition to form unusually stable 1,2,3-oxadiazolidines ;" previous azoxy 1,3-dipole and alkene cycloadditions have led to ring-opened products from these oxadiazolidines.Methyl- ation of (43) does not give the expected quaternary salts (44) but pyrazolidinium salts (45).51 The authors suggest that a 1,3-dipolar addition is involved here also 0-Me \ (43) (44) 46 '1,3-Dipolar Cycloaddition Chemistry,' ed. A. Padwa Wiley New York N.Y. 1984. 47 F. Bronberger and R. Huisgen Tetrahedron Lett. 1984 25 57. 48 F. Bronberger and R. Huisgen Tetrahedron Lett. 1984 25 65. 49 K. Beck A. Hohn S. Hunig F. Prokschy Chem. Ber. 1984 117 517; S. Hunig and F. Prokschy ibid. 1984 117 534. so S. Hunig and M.Schmitt Tetrahedron Lett.1984 25 1725. 51 S. Hunig and F. Prokschy Chern. Ber. 1984 117 2099. 37 Reaction Mechanisms -Part (i) Pericyclic Reactions and presume that deprotonation of the methyl group is dramatically enhanced by the presence of the neighbouring double bond [in its absence no H-D exchange in the (normal) quaternary salt occurs in the presence of D+-MeOD]. How the double bond brings about this dramatic enhancement of acidity of the methyl group is not clear. Decarboxylative transamination of a-amino-acids has been assumed to proceed via the concerted process (46) += (47). Evidence for an alternative mechanism which involves decarboxylation via the zwitterionic form (48) is the trapping of the latter by 1,3-dipolarophiles (Scheme 5).52 This trapping procedure has also been accomplished intramolecularly e.g.Scheme 6. Ph R2 R’ R2 Scheme 5 H CHO DMF A do ____* 2L \/ 58% Scheme 6 A study of stereoselectivity in nitrile oxide addition to a variety of chiral allylic ethers (49) has led to a proposal for the preferred transition-state geometry shown in (50).s3 In this geometry the alkoxy function occupies an ‘inside’ site and the alkyl substituent the sterically less-crowded anti-position. It is believed that the wbond becomes electron-deficient in the transition state and consequently is stabil- ized by the electron-donating effect of CT~-~-T overlap and least destabilized by UT.-~-V interaction when the latter is in the inside position close to the plane of the wbond.52 R. Grigg and S. Thianpatanagul J. Chem. SOC.,Chem. Commun. 1984 180; see also R. Grigg F. M. Aly V. Sridharan and S. Thianpatanagul ibid. 1984 182. 53 K. N. Houk S. R. Moses Y.-D. Wu N. G. Rondan V. Jager R. Schohe and F. R. Fronczek J. Am. Chem. SOC.,1984 106 3880. R. S. Atkinson OR V (49) erythro OR' threo anti (50) The proposal above is in contrast to that of Kozikowski and Gho~h~~ who envisage that the allylic oxygen is aligned anti to the developing C-0 bond in the transition state. A variety of 1,3-dipoles have been reacted with phosphaethyne (51) regiospecifi- cally to give phosphorus-containing heterocycles e.g. (52).5' Bu'C=P + PhN -PhN I 52% 'p5CBu' (51) (52) 5 Cheletropic Addition and Elimination The ,4 + "2 addition of free germylenes Me,Ge to 1,3-dienes (Scheme 7) has been shown to proceed faster with more electron-deficient dienes suggesting that the reaction is LUMOdiene-HOMOgermylene ~ontrolled.'~ Me Me , 70"C d M; he Scheme 7 Similarly 1 -stannacyclopent-3-enes are now readily available by addition of stannylenes R,Sn (including SnCl, SnBr, and SnI,) to 1,3-dienes.Judging from the stereospecificity of the reaction a ,4 + ,2 addition is operating here also and the yields with various substituted dienes suggest that a LUMOdie,e-HOMO,t,n,ylene is likewise of major irnp~rtance.~' Acyl- and sulphonyl-thionitroso compounds have been obtained by extrusion from the intermediate ylides themselves obtained by Diels- Alder addition of thiophene S-N ylides to dienophiles (Scheme 8).These reactive thionitroso 54 A. P.Kozikowski and A. K. Ghosh J. Org. Chem. 1984,49,2762. 55 Y.Y.C. Yeung Lam KO R. Came A. Muench and G. Becker J. Chem SOC.,Chem. Commun. 1984 1634; W. Rosch and M. Regitz Angew. Chem lnt. Ed. Engl 1984 23 900; see also A. Schmidpeter and A. Willhalm ibid 1984 23 901. 56 J. Kocher and W. P. Neumann J. Am. Chem SOC 1984 106 3861. 57 R. Marx W. P. Neumann and K. Hillner Tetrahedron Lett. 1984 25,625. Reaction Mechanisms -Part ( i) Pericyclic Reactions cyl CI’ ‘Cl + (’I -CI&R:\ +S-NR -CI R’ CI R’ c$: CI -NR CI 1 +R-N=S Scheme 8 compounds are trapped non-periselectively giving both 47r + 27r cycloaddition and ene reactions.’* Di-iminosuccinonitrile (DISN) (53) is prepared by base-catalysed addition of HCN to cyanogen and is a versatile polyfunctional reagent whose heterodiene unit HN=C-C=NH undergoes apparent 47r + 27r cycloaddition with electron-rich alkenes e.g.dimethoxyethene (Scheme 9). H -NcaI Nc@(NH +MeOCH=CHOMe OMe H+ NC[) ~ OMe orA NC HN CN NC (53) 76% it NcgH R = R’ = OMe I N [ 1,4] sig. rearr. ** C NH CH,CN + -H wcN HY CN R R’ A R R’ \ (55) R = Ph NcgNH2 R‘= H 58% P Ph (54) Scheme 9 Intriguingly however styrene yields aziridine (54) as the only isolable product with DISN. It is suggested that DISN reacting via its latent nitrenium resonance hybrid in a ,2 + ,2 cycloaddition to the alkene gives the aziridinium ion (55) which is the intermediate in formation of both aziridines and dihydropyridazines.This mechanism avoids the necessity for the cisoid-form of DISN to be invoked 0. Meth-Cohn and G. van Vuuren J. Chem. SOC.,Chem. Commun. 1984 1144. R. S. Atkinson but it does require C(CN)-=C(CN) bond rotation within the anionic portion of (55) before sigmatropic rearrangement is p~ssible.’~ Thermal cheletropic elimination from the iminonaphthalene (56) generates the arylsulphenylnitrene which has been trapped by alkenes to give aziridines in good yields.60 SAr 6 Sigmatropic Rearrangements Recent reviews in this area include those on mercury and palladium catalysed [3,3] sigmatropic rearrangements,61 chirality transfer uia sigmatropic rearrangements,62 and walk rearrangements in [n.1.O]bicyclic corn pound^.^^ Striking contrasts have been found in the stereoselectivities of Claisen rearrange- ments of pyranoside and carbocyclic ally1 vinyl ethers.Thus in contrast to the Claisen rearrangement of (57) which gives a 48% and 52% yield of (58) and (59) respectively rearrangement of e.g. (60) and (61) gave the corresponding aldehydes /CO,Et PhCN phTc%‘ phq;% ___) OHC 0 OMe I+‘/OMe -Si Si 59 T. Fukunaga and R. W. Begland J. Org. Chem. 1984,49 813. 60 R. S. Atkinson M.Lee and J. R. Malpass J Chem. SOC.,Chem. Commun. 1984 919. 61 L. E. Overman Angew. Chem. Int. Ed. Engl. 1984 23 579. 62 R. K. Hill in ‘Asymmetric Synthesis’ vol. 3 ed. J. D. Morrison Academic Press Orlando Florida 1984 pp.503-572. 63 F. G. Klaerner Top. Stereochem. 1984. 15. 1. Reaction Mechanisms -Part ( i) Pericyclic Reactions 41 stereospecifically. The anomeric oxygen does not appear to play an important role in determining its stereoselectivity and neither does the trans-ring fusion.@ Claisen rearrangement of the chiral alcohol (62) gave (63) (E:Z = 83 17). Both these stereoisomers were converted into the same enantiomer of the alcohol (64) by cyclization with Tic&. This is taken to mean that chiral transfer proceeds 100% efficiently with the (R)-configuration in (62) yielding (RE)-and (S,Z)-(63) followed by anti-attack in the SE2’-type cyclization of both these stereoisomers to (64).65 +he ,J OHC Y TiCI “‘‘0 100% HO’ (64) Diastereoselection has also been studied in the chelation-controlled ,reland- Claisen rearrangement of (65).Formation of the major diastereoisomer is rational- ized by assuming an anti-relationship of the newly formed C-C bond and the allylic alkoxyl function in a chair transition-state.66 A (major product) This transfer of chirality in the Claisen rearrangement is only possible using secondary (as in the cases above) or tertiary alcohols. The a-silylcrotyl alcohol (66) has been synthesized and resolved and the propionate esters of the enantiomers subjected to Claisen rearrangement uia the ester enolates (Scheme 10).The configur- ation of the enolate double bond generated can be controlled by the choice of base used and the overall transformation here after removal of the silicon is chirality- transfer in the Claisen rearrangement using a chiral primary alcohol eq~ivalent.~’ 64 D.B. Tulshian R. Tseng and B. Fraser-Reid J. Org. Chem. 1984 49 2347; B. Fraser-Reid D. B. Tulshian R. Tseng D. Lowe and V. G. S. Box Tetrahedron Lett. 1984 25 4579. 65 K. Mikami T. Maeda N. Kishi and T. Nakai Tetrahedron Lett. 1984 25 5151. 66 J. K. Cha and S. C. Lewis Tetrahedron Lett. 1984 25 5263. 67 R. E. Ireland and M. D. Varney J. Am. Chem. Soc. 1984 106 3668. R. S. Atkinson (66) \iii iv vii viii ix. x HO,C BzO-/-> vii viii ix x Ho2Cyii+ BzO7 I Me Me 93:7 Reagents i 1.5 eq. Bu'Li-TMEDA; ii MeC02H-THF -78°C; iii resolve; iv EtCOCI-pyr; v (Me,Si),NLi-THF -78 "C; vi TBSiCI -78 "C;vii CH,N,; viii LAH; ix PhCH,Br KH THF; x 50% HBF, MeCN; xi LDA Scheme 10 Claisen rearrangement of 2-morpholino-substituted allyl vinyl ethers (67) proceeds at a rate estimated to be several thousand times faster than for allyl vinyl ether itself (the rate acceleration is significantly affected by the nature of the amine).68 In contrast P-furfuryloxyenamines (68) are transformed into products (69) apparently by [1,3]sigmatropy.It would be of interest to know what part if any radicals play in both these rearrangements since the 2-aminovinyloxy radical (70) is capto-dative stabilized. The facility with which anionic Claisen rearrangement of the vinyl ethers (71) occurs to produce two vicinal chiral centres in (72) is attributed to the drop of ca.10 pK units between (71) and the product.69 68 J. Barluenga F. Aznar R. Liz and M. Bayod 1. Chem. SOC.,Chem. Commun. 1984 1427. 69 S. E. Denmark and M. A. Harmata Tetrahedron Lett. 1984 25 1543. 43 Reaction Mechanisms -Part (i) Pericyclic Reactions p-TolSO 1 ~ , KH-THF so' R'~ \ o~RLiCH,SOCH R2 'n P' n R' M&,* 4 +G-CO,Bu' -M%5 -i/ Bu'0,C C02Bu' R' R2 (75) (73) (74) ha-Claisen rearrangement of (73) + (74) has been shown to depend critically on the purity of the t-butyl propiolate used for formation of (73) from the amine (75)?* The Carroll rearrangement (Scheme 11) like many other [3,3] sigmatropic rear- rangements has been found to be dramatically accelerated by base.-M Me& er -Meno ,O -00 OH H' Scheme 11 Thus whereas this thermal rearrangement is usually carried out at 13O-22O0C the dianions of allylic acetoacetates rearrange at room temperature or in refluxing THF (Scheme 12) the mono-anion is unreactive to boiling in THFfor several hours. The acetoacetates were obtained in good yield and purity by treatment of the ally1 "+SiMe3 2:%. Me iMe3*oc Me-o 0 0 0 iMe, R' = Me R' = H 84% R' = H,R' = Me 40% Me 0 Scheme 12 S. Chao F. A. Kunng J.-M. Gu H. L. Ammon and P. S. Mariano J. 01%Chem. 1984,49 2708. R S. Atkinson alcohol in ether at -20°C with diketene and a catalytic amount of 4-dimethyl- amin~pyridine.~' Cope rearrangement of (76) has been shown to proceed in the gas phase using ion cyclotron resonance spectrometry for the case where R = Me.The secondary alcohol (R = H) is slower to react. Since a similar rate difference is observed in THF or DMSO the effect is unlikely to be the result of differential ion-pairing or solvation An ab initio calculation for the transition state of the Cope rearrangement (using a 3-21G multiconfiguration SCF wave function) suggests that bond making and bond breaking occur in unison,73 contradicting Dewar's general assertion18 that multibond reactions are not synchronous. Other theoretical studies (of substituent effects on the rate) of the Cope and Claisen rearrangements have appeared.74 Diastereoselectivity at an increasingly high level is being accomplished in [2,3]- sigmatropic rearrangements also.Thus Wittig rearrangement of (77)bearing the 'Meyers' chiral oxazoline auxiliary gives an erythro threo (2R,3S:2S,3S) ratio of 90 10 with 78% e.e. for the erythro-i~omer.'~ \ \ OMe OMe (77) Reagents i BuLi or LDA -85 "C; ii H+/H20; iii CH2Nz Even more impressive is the similar [2,3]sigmatropic rearrangement of the car- banion derived from (78) which after protodesilylation gave (79) in high (>99%) diastereoisomeric purity and with an enantiomeric excess of 98% as judged by the conversion into the naturally occurring pheromone (80). The nearly 100% efficiency of chirality transfer in this example has been explained using the transition-state geometry (81).~~ " S. R. Wilson and M.F. Price J. Org. Chem.1984. 49 722. 72 M. D. Rozeboom J. P. Kiplinger and J. E. Bartmess J. Am. Chem. SOC.,1984 106 1025. 73 Y.Osamura S. Kato K. Morokuma D. Feller G. R. Davidson and W. T. Borden J. Am. Chem. soc. 1984 106 3362. 74 F. Delbecq and N. T. Anh Noun J. Chem. 1983,7 505; M. J. S. Dewar and E. F. Healy J. Am. Chem. SOC.,1984 106 7127. 75 K. Mikami. K. Fujimoto T. Kasuga and T. Nakai Tetrahedron Lett. 1984 25 6011. 76 N. Sayo K. Azuma K. Mikami and T. Nakai Tetrahedron Lett. 1984,25 565; K. Mikami K. Azuma and T. Nakai Tetrahedron 1984. 40,2303. Reaction Mechanisms -Part (i) Pericyclic Reactions \\\ 1 \ \ SiMe SiMe (78) (79) (81) Reagents i Bu"Li -85 "C; ii CsF-MeOH; iii H,-Raney Ni The tendency for groups to undergo [1,5] sigmatropy in substituted cyclopen- tadienes and cyclohexadienes is in the order CHO > MeCO > H > C02Me > CN -C=C > alkyl.The sluggish rate of rearrangement of CN seems anomalous particularly by comparison with its ready migration in substituted cycloheptatrienes where the corresponding order is CHO > CN CGC > MeCO > C02Me > alkyl. The promotion of the triply bonded groups in this latter order is attributable to reduced strain in the bridged rearrangement transition-state (82) compared to (83) (cJ methylenecyclopropane versus methylenecyclopentane) .77 N The ease of [1,5]sigmatropic rearrangement of the alkoxide (84) (in HMPA) is another example of the acceleration of a pericyclic reaction that can be brought about by placement of an alkoxide substituent at the terminus of a breaking a-bond.K'O-Me HMPA HOAc ____ -,,GPh 0-Kt Ph Ph 20 "C Ph Ph Ph Ph + (84) 0 Since the uncatalysed rearrangement of pentaphenylcyclopentadienol requires heating at 173 "C and the migratory aptitude of methyl is much less than that of phenyl the acceleration here is considerable. These rearrangements have been shown to be stereospecific with retention of configuration in the migrating group.78 77 P. J. Battye and D. W. Jones J. Chem. Soc. Chem. Commun. 1984 1458. 78 . P. J. Battye and D. W. Jones J. Chem. Soc. Chem. Commun. 1984 990. R. S. Atkinson At 126 "C the ratio kendo/k,, for [1,5]sigmatropic rearrangement of hydrogen in (85) has been determined as 6.8 by indirect means and has been interpreted in terms of an interaction of gnorbo,.nyl and n-molecular orbitals inducing a twisting of the C-4 Ir-orbital and facilitating migration of the endo-H.79 Carpenter's model for [1,5]sigmatropic rearrangement in a cyclopentadiene is a bicyclo[3.1 .O]hexatriene (86) polarized as shown with the negative dipole distributed over C-2 (C-3) and C-4.A study of solvent effects kinetics thermodynamic parameters and regiospecificity for [1,5]sigmatropic rearrangement of a number of substituted spiro[4,4]nona-l,3-dienessupports this model with (87) rearranging most rapidly and regiospecifically to (88) and (89) rearranging least rapidly to (90).*' 2 4 / (86) (87) R' = CN,R2 = H (88) (89) R' = H R2 = OMe 79 W. N. Washburne and R.A. Hillson 1. Am. Chem. Soc. 1984 106,4575. 80 K. S. Replogle and B. K. Carpenter J. Am. Chem. SOC.,1984 106 5751.

ISSN:0069-3030    

DOI:10.1039/OC9848100027

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (ii) Polar Reactions By D. J. McLENNAN Department of Chemistry University of Auckland Auckland New Zealand 1 Introduction Some journals take some time to arrive in the Southern Hemisphere thus the 1984 literature has not been completely covered. The deficiency will be made up in next year's Report. A new section in this Report deals with free energy relationships isotope effects etc. as they pertain to the study of polar mechanisms. Indeed a monograph on methods of determining reaction mechanism has appeared.' Two deservedly popular texts have been produced in second edition guise.293 A mono-graph on the neutral reactive intermediates in organic chemistry discusses ionic routes to some of the intermediate^.^ Russian authors have reviewed non-classical carbocations' and rearrangements of carbocations by 1,2-shifts6 in exhaustive detail.Simonetta has reviewed his work on the theoretical calculation of reaction paths in SN2 and other proce~ses.~ An iconoclastic paper by Shaik and Bar questions the importance of rr-resonance stabilization in organic chemistry and suggests that the delocalized rr-systems of the ally1 anion (and radical) may well be unstable or metastable electronic species which have geometric symmetry conferred on them by the a-skeleton.' In a dmilar vein Dewar argues that multibond reactions (except for E2 and S,2') cannot normally be synchr~nous,~ and reviews the evidence. A review on the use of pyridines as leaving groups in SNreactions emphasizes the mechanistic diversity of these processes." Kirby has reviewed stereoelectronic effects on acetal hydrolysis with particular reference to lysozyme binding and catalysis." Among other things he draws attention to the hitherto unacknowledged insight possessed by Samuel Taylor Coleridge (1772-1834) on the subject of enzymatic catalysis.The reactivity of pure B. K. Carpenter 'Determination of Organic Reaction Mechanisms' Wiley New York 1984. * F. A. Carey and R. J. Sundberg 'Advanced Organic Chemistry-Part A Structure and Mechanisms' 2nd edn. Plenum New York 1984. R. A. Y. Jones 'Physical and Mechanistic Organic Chemistry' 2nd edn. Cambridge University Press 1984. C. Wentrup 'Reactive Molecules' Wiley New York 1984. V. A. Barkhash Topics Cum Chem.. 1984.116/117. 1. V. G. Shubin Topics Cum.Chem. 1984 116/117 267. ' M. Simonetta Chem. SOC.Rev. 1984 14 1. S. S. Shaik and R. Bar Nouv. J. Chim. 1984 8 411. M. J. S. Dewar J. Am. Chem. Soc. 1984 106 209. A. R. Katritzky and G. Musumarra Chem. SOC.Rev. 1984 14 47; A. R. Katritzky and C. M. Marson Angew. Chem. Int. Ed. Engl. 1984 23 420. I' A. J. Kirby Acc. Chem. Res. 1984 17 305. 47 D. J. McLennan conformational isomers has been reviewed,12 and the application of linear free- energy relationships (LFERs) to the related Curtin-Hammett principle has been disc~ssed.'~ The evidence afforded by primary kinetic isotope effects (KIEs) to the investigation of internal return processes dominates a review on carbanion formation and proton transfer^,'^" whilst acidity and structural factors are featured in an account of work on carbanion ion pairs and tri~1ets.l~' A review of intramolecularly H-bonded acids has a~peared.'~ Page has reviewed work on the mechanisms of reactions of p-lactam antibiotics.16 Co-operative effects between substrate binding and rate enhancements dominate a discussion on models for enzymatic catalysis." Polar and electron-transfer (ET) mechanisms are important in copper-assisted nucleophilic substitution of halide ion in aryl halides.18 The conventional wisdom is that if one wishes to study the properties of reactive ionic intermediates one stabilizes them; Tidwell demonstrates that as much if not more insight into carboca- tions can be afforded by de~tabilization.'~" A review on the homotropylium cation concludes that it is homoaromatically ~tabilized.'~' The evidence for Hg" and Pd" catalysis of hetero-Cope rearrangements proceeding via electrophilic cyclization (1) +(2) +(3) has been reviewed.Cope rearrange- ments of acyclic 1,Sdienes catalysed by PdC12 are believed to proceed by the same E route.20 Enantioselection in reductive addition of nucleophiles to carbonyl groups has been reviewed for cases where chelation control can be adjustedz1 and where microbial enzymes are involved.22 2 Solvolysis and Carbocations It is not generally appreciated that carbocationic intermediates can be involved in the photolytic decomposition of alkyl halides. Kroppz3 has reviewed the evidence for products arising from both homolytic and heterolytic pathways and the 12 M.Oki Acc. Chem. Res. 1984 17 154. l3 C. L. Perrin and J. I. Seeman J. Org. Chem. 1984,49 2887. l4 (a) H.F. Koch Acc Chem. Res. 1984 17 137; (b) A. Streitwieser ibid. 353. F. Hibbert Acc. Chem. Res. 1984 17 115. 16 M. I. Page Acc. Chem. Res. 1984 17 144. 17 J. Rebek Acc. Chem. Res. 1984 17 258. 18 J. Lindley Tetrahedron 1984 40,1433. 19 (a) T. T. Tidwell Angew. Chem. Int. Ed. Engl. 1984 23,20; (b)R. F. Childs Acc. Chem. Res. 1984 17 347. 20 L. E. Overman Angew. Chem. Int. Ed. Engl. 1984 23 579. 21 M.T.Reetz Angew. Chem. Znt. Ed. Engl. 1984 23 556. 22 C. J. Sih and C.-S. Chen Angew. Chem. Int. Ed. Engl. 1984 23 570. 23 P. J. Kropp Acc. Chem. Res. 1984 17 131. Reaction Mechanisms -Part (ii) Polar Reactions 49 dichotomy has been exemplified in the photosolvolyses of ArCH2X compounds (Ar = naphthyl; X = C1 Br I).24 The room-temperature photolytic decomposition of triarylvinyl halides in solution25 yields a spread of products similar to that obtained from thermal solvolysis at higher temperatures,26 thus implicating vinyl cations in the photolytic process.N.m.r. techniques have been used to demonstrate the angular dependence of hyperconjugative stabilization of bicyclo[2.2.2]0cty1~~~**and bicycl0[2.2.l]heptyl~~ cations. Hyperconjugation vanishes when the C -H bond axis is orthogonal to the axis of the empty p-orbital confirming the point elegantly made by Shiner some 25 years ago. Olah has succeeded in generating the first a-nitrodiaryl methyl cations from Ar2C(N02)2 in FS03H/S02C1F.29 No such cation is however obtained from 9,9-dinitrofluorene which is not surprising since the parent 9-fluorenyl cation has yet to be observed under stable ion conditions.The behaviour of 1-phenethyl derivatives has been a cornerstone of solvolysis theories for some 50 years yet Jencks and co-workers have found good reasons for furthering these studies on the SN1-&2 borderline. The lifetimes of intermediates resulting from solvolyses of activated 1 -phenethyl derivatives have been estimated?' Capture of 1-phenethyl cations by alcohols shows a surprisingly large Pnuclof 0.5 the selectivity of the cations is inversely proportional to their rea~tivity,~~" and general base catalysis of nucleophilic attack is required for alcohol addition to the more stable cation^.^' The nature of the bimolecular substitutions with less activated derivatives has been probed and it is concluded that these are concerted sN2 processes proceeding through 'exploded' transition-states that have carbocationic character.32 Virtually the whole spectrum of solvolytic behaviour is exhibited by the solvolyses of secondary (4) and tertiary (5) a-keto mesylates and triflate~.~~ a-Keto carbocations and ion pairs are strongly implicated and it is suggested that destabilizing inductive electron-withdrawal by the a-C=O is at least balanced by resonance stabilization OY OY I I MeCHCOR MeCCOR Y = Ms or Tf I Me (4) (5) 24 G.H. Slocum and G.B. Schuster J. Org. Chem. 1984 49 2177. 25 T. Kitamura S. Kobayashi H. Taniguchi C. Y. Fiakpui C. C. Lee and Z. Rappoport J. Org. Chem. 1984,49 3167. 26 Z. Rappoport C. Y. Fiakpui X.-D. Lu and C. C. Lee J. Org. Chem. 1984,49,470; D. Wanigasekera C. C. Lee Y. Houminer M. Aviv and Z. Rappoport ibid.,4367. H.-U. 27 Siehl and H. Walter J. Am. Chem. Soc. 1984 106 5355. 28 D. A. Forsyth J. H. Botkin and V. M. Osterman J. Am. Chem. SOC,1984 106 7663. 29 G. A. Olah G. K. Surya Prakash M. Arvanaghi V. V. Krishnamurthy and S. C. Narang J. Am. Chem. SOC.,1984 106 2378. 30 J. P. Richard M. E. Rothenberg and W. P. Jencks J. Am. Chem. SOC.,1984 106 1361. 31 (a) J. P. Richard and W. P. Jencks J. Am. Chem. SOC.,1984 106 1373; (b) 1396. 32 J.P. Richard and W. P. Jencks J. Am. Chem Soc. 1984 106 1383. 33 X. Creary J. Am. Chem. SOC., 1984 106 5568. D. J. McLennan (equation 1). Accurate MO calculations confirm this view.34 Other destabilized (via CF3 substitution) carbocations have been investigated by Ga~sman.~' 0' Me-C-C + ;* f* Me-C=C / I I Me \R Me \R Solvolyses of primary alkyl acridinium ions (6) in alcoholic solvents are proposed to proceed by SN2 reactions in nucleophilic solvents and via ion-molecule pairs in less nucleophilic solvents.36" In cases where rearrangement occurs (e.g. R = neopentyl) it is concluded that anchimeric assistance is absent.36a A study of secondary alkyl derivatives36b is also reported. The question of anchimeric assistance in solvolyses of secondary alkyl substrates rests heavily on the value of the a-deuterium isotope effect for limiting S,l solvolysis of a 2-propyl arenesulphonate as a model compound.A revised value of this isotope effect for trifluoroacetolysis (from 1.22 to 1.17) reveals that the reaction is not limiting.37 Earlier results for trifluoroacetolysis of secondary alkyl tosylates can now be interpreted in terms of either solvent or anchimeric assistance. Ab initio MO calculations confirm that the rearrangement attending solvolyses of simple neopentyl derivatives provides anchimeric a~sistance.~~ .. Ph The norbornyl cation controversy refuses to die. Formation of an ion pair is the rate-controlling step in the ethanolysis of endo-2-norbornyl mesylate since "0 n.m.r.studies reveal no sulphonate oxygen scrambling and hence no internal return.39 The em endo rate ratio of ca. lo3is thus confirmed and the extreme reluctance of 2-norbornyl cations to capture nucleophiles on the endo face is further exemplified. The bridging in 2-norb0rnyl~~" and bicyclo[2.2.2]octy140b cations is considered on the basis of substituent effects on solvolysis rates to involve all neighbouring C-C a-bonding electron pairs graded according to steric strain consequences. Deamina- tion studies of isomeric amines potentially capable of producing common delocalized 34 D. A. Dixon R. A. Eades R. Frey P. G. Gassman M. L. Hendewerk M. N. Paddon-Row and K. N. Houk J. Am. Chem. SOC. 1984 106 3885. 35 P. G. Gassman and J. B. Hall J.Am. Chem. SOC,1984,106,4267; P. G. Gassman and C. K. Harrington J. Org. Chem. 1984 49 2258. 36 (a) A. R. Katritzky Z. Dega-Szafran M. L. Lopez-Rodriguez and R. W. King J. Am. Chem. Soc. 1984 106 5577; (b) A. R. Katritzky M. L. Lopez-Rodriguez and J. Marquet J. Chem. SOC.,Perkin Trans. 2 1984 349. 37 H. Yamataka S. Tamura T. Hanafusa and T. Ando J. Chem. SOC.,Chem. Commun. 1984,362. 38 H. Yamataka T. Ando S. Nagase M. Hanamura and K. Morokuma J. Org. Chem. 1984 49 631. 39 C. Chang and W. J. le Noble J. Am. Chem. SOC.,1984 106 810. 40 (a) C. A. Grob and P. Sawlewicz Tetrahedron Lett. 1984,25,2973; (b) Helv. Chim. Acta 1984,67,1906. Reaction Mechanisms -Part (ii) Polar Reactions bicyclo[2.2.2]octyl cations reveal product differences and it is proposed that the non-classical ion has classical precursors which may also give rise to Rate studies and rate-product correlations for the solvolysis and aminolysis of benzoyl chloride point to SN2 character in these reactions.42 Establishment of a rate-product correlation for aminolysis over a 300-fold concentration range was achieved by using quantitative h.p.1.c.for both kinetic and product monitoring. The solvolyses of ring-substituted 4-dibromomethylphenoIs (7) in dioxane-water exhibit mechanistic complexities but at all acidities investigated quinone methides (8) are found as transient intermediates. Both carbocationic routes and a formal 1,6-elimination (EIcB) pathway (Scheme 1) are impli~ated.~~ Scheme 1 3 Other Nucleophilic Substitutions When a trimethyloxonium salt (Me30+) is treated with a strong hingered base C-C bond formation occurs.44 An oxonium ylide intermediate Me20-CHi is proposed which can then act as a nucleophile towards another molecule Me30+ to achieve C-C bond formation equation 2.Similarly the reaction of Me30+ with NaH yields C2 products whose isotopic composition is consistent with the inter- mediacy of an oxonium ~lide.~~" The action of singlet methylene on dialkyl ethers likewise produces oxonium ylides (directly) as well as products derived from normal C -H insertion^.^^ Most significantly the conversion of heterosubstituted methanes such as MeOH Me20 etc. into ethene and hydrocarbons derived therefrom can be achieved using bifunctional acidic-basic catalysts such as W03 on alumina.46 Oxonium ions uia SN2 reactions and oxonium ylides are the important intefmediates and ethene from MeOH is envisaged as arising from @elimination of Me20CH,CH3 (uia an equation 2 process) equation 3.Me26-EH2 + MebMe -+ Me26-CH2-CH + Me20 (2) Me26CH2CH CH2=CH2 + Me20 (3) 41 H. Maskill and A. A. Wilson J. Chem. Soc.. Perkin Trans. 2. 1984 119 1369. T. W. Bentley G. E. Carter and H. C. Harris J. Chem. Soc. Chem. Commun. 1984,387; T. W. Bentley and A. E. Freeman J. Chem. SOC.,Perkin Trans. 2 1984 1115. 43 P. B. D. de la Mare and P.A. Newman 1. Chem. SOC.,Perkin Tram. 2 1984 231 1797. 44 P. Rimmelin H. Taghavi and J. Sommer J. Chem. SOC.,Chem. Commun. 1984 1210. 45 (a) G. A. Olah H. Doggweiler and J. D. Felberg J.Org. Chem. 1984,49 2112; (b) 2116. 46 G. A. Olah H.Doggweiler J. D. Felberg S. Frohlich M. J. Grdina R. Karpeles T. Keumi S. Iuaba W. M. Ip K. Lammertsa G. Salem and D. C. Tabor J. Am. Chem. SOC.,1984 106 2143. 52 D. J. Mctennan The emphasis on oxonium ylides provides an answer to the question of C2 hydrocarbon formation in the Mobil methanol-to-gasoline process using the shape- and size-selective bifunctional ZSM-5zeolite catalyst whereby methane from natural gas can be converted via methanol into synthetic liquid gasoline. The first commer- cial plant is presently being constructed in New Zealand and it is satisfying to have some idea now of what will be going on in the catalytic reactors. A nucleophilic substitution reaction likewise appears in a proposed mechanism for carbonyl-assisted thermal dissolution of coal at high temperature^.^^ Even KIEs have a place in the study of coal conversion into more useful fuels,48“ and ionic pathways in superacid coal liquefaction (HF-BF,-H,) have been de~cribed.~” Industrial matters now make way for biologically-related studies.The role of metaphosphate ion PO, in the solvolysis of phosphate monoesters has been probed by studying the solvolysis of chiral phenyl [l60, 170 ‘80]phosphate?9 Complete inversion of configuration at P is observed so if PO is an intermediate equation 4 it mast not leave nor rotate within the solvent cage in which it is generated. It is 0 0 II Ph-ayG + PhOH + P=O ++ products (4) II I H 0-0-more plausible to suppose that backside pre-association of a nucleophilic solvent molecule occurs but whether this happens in a concerted SN2-like or in a stepwise fashion is ~ncertain.~’ In contrast migration of the phospho group from the 2-to the 1-position of phosphopropane-l,2-diol,equation 5 occurs with the retention of OH OH lo,7<& = 1;ky (5) OH configuration at P.” This results from an associative process in which a penta-coordinate intermediate must undergo a pseudorotation to yield the product.Two other groups concur in the rejection of a discrete metaphosphate intermediate. If a stepwise mechanism involving intermediates were operating one would expect a change in rate-limiting step and hence a break in the Bronsted plot as the basicities of nucleophiles were systematically changed yet the Bronsted plot for reaction of pyridines with pyridinio-N-phosphonates is linear.5’ Thus the process can be symbolized by equation 6 as involving a single transition-state.Consideration of the response of rate and equilibrium constants to changes in basicity of entering r 0 013-(6) 47 C. Choi and L. M. Stock J. Org. Chem. 1984,49 2871. 48 (a) K. R. Brower and J. Pajak J. Org. Chem 1984 49 3970; (b) G. A. Olah and A. Husain Fuel 1984 63 1427; G. A. Olah M. R. Bruce E. H. Edelson and A. Husain ibid. 1432. 49 S. L. Buchwald J. M. Friedman and J. R. Knowles J. Am. Chem. Soc. 1984 106 4911. S. L. Buchwald D. H. Pliura and J. R. Knowles J. Am. Chern. SOC.,1984 106 4916. 51 N. Bourne and A.Williams J. Am. Chem. SOC.,1984 106 7591. Reaction Mechanisms -Part (ii) Polar Reactions 53 and leaving groups suggests however that these are but weakly bonded to P in the transition state which will therefore possess a degree of metaphosphate character. A parallel study has produced similar results and a like conclusion; the transition state is seen as an open or ‘exploded’ Sulphonyl transfer is similarly concerted.52 The bicyclic phosphite (9) is a remarkably unreactive n~cleophile,~~ and steric effects cannot be invoked in view of the enhanced reactivity of species such as (9) (10) quinuclidine (10). In the systems studied (EtO),P is quite reactive.53 A stereoelec-tronic effect is advanced to account for these results. The results of ab initio MO calculations add credence to this hypothe~is.’~” Other stereoselective effects in phosphite reactions have been reported.54b Such stereoelectronic effects are of course related to the a-eff ect on nucleophilic reactivity which continues to attract attention.Both a caution on interpretation” and the view that the a-effect is a transition-state stabilization eff ect56 have been published. Both the kinetics of and the position of bond cleavage in the enzymatic and acid-catalysed hydrolysis of benzyl phosphate can be obtained from the l8Oisotope-induced shifts in the I3C and 31P n.m.r. spectra of the esters.57 Schowen’s hypothesis that compression of sN2 transition states is a necessary condition for enzymatic catalysis of methyl transfers has teen examined.MO calculations on the degenerate methyl transfer between MeNH and NH in the presence of a simple compression-attraction enzyme model indeed show that a compressed transition-state is selectively bound by the ‘en~yme’.’~ Calculated KIEs (CH Versus CD,) are in accord with the experimental values that originally gave rise to Schowen’s proposal. Other theoretical studies have explored areas of con- troversy and uncertainty. Pross and a co-worker have examined the acceleration of some SN2 reactions by a-carbonyl groups and a reactivity-selectivity anomaly thereof using the Pross-Shaik VBCM In these terms the carbanion (enolate) configuration (11) is seen to be the principal transition-state contributor amongst those commonly used to depict an SN2process Scheme 2 but only if the nucleophile Nu C..X e Nu 6 :X-e Nu? C :X-f) Nu C .X I I I II c=o c=o c=o C-0-Scheme 2 (11) 52 (a) M.T. Skoog and W. P. Jencks J. Am. Chem. Soc. 1984,106,7597; (b) P. D’Rozario R. L. Smyth and A. Williams ibid. 5027. 53 K. Taira W. L. Mock and D. G. Gorenstein J. Am. Chem. Soc. 1984 106 7831. 54 (a) K. Taira and D. G. Gorenstein J. Am. Chem. Soc. 1984 106 7825. (b) K. Taira and D. G. Gorenstein Tetrahedron 1984,40 3215; K. Taira T. Fanni and D. G. Gorenstein J. Org. Chem. 1984 49 4531. 55 S. Hoz and E. Buncel Tetrahedron Lett. 1984 25 3411. 56 M. Laloi-Diard P. Gesselin and F. Terrier Tetrahedron Lett. 1984 25 1267. 57 J. E. Parente J. M. Risley and R. L. Van Etten J. Am. Chem. Soc. 1984 106 8156.50 I. H. Williams J. Am. Chem. SOC.,1984 106 7206. 59 D. J. McLennan and A. Pross J. Chem. SOC. Perkin Trans. 2 1984 981. D. J. McLennan is a good electron donor. Computations and i.c.r. studies show that tetrahedral carbon and silicon differ in their behaviour towards nucleophiles ;5-co-ordinate Si anions appear to be stable but short-lived species!’ An MNDO study which leads to the conclusion that SN2 barriers at carbon are a consequence of the,small size of carbon inhibiting binding to five ligands6’ is seemingly at odds with the finding that SiI occupies a local minimum on the I- + Si14 potential surface.60 The crucial role of solvation in SN2‘61 and SN261’62 reactions has been theoretically probed and the importance of u*-T* orbital mixing in SNreactions at sp2 carbon (carbonyl and aromatic) has been empha~ized.~~ Attack of nucleophiles on acetyl chloride is rate-limiting rather than the break- down of a tetrahedral intermediate.64 It is concluded that the Ritchie N scale is not suitable for correlating acylation rates.Activation-volume measurements in particular provide evidence for rate-limiting attack of the piperidine nucleophile on an ion-molecule pair formed from (12) by pre-equilibrium heterolysi~.~’ This is the Ph most compelling item of evidence in favour of the operation of a Sneen-type S,2 mechanism under non-solvolytic conditions. It is significant that reaction through an apparently classical SN2 pathways competes. Bimolecular nucleophilic displace- ments at carbon-nitrogen double bonds are like those at C=C addition-elimination processes.66 In one case66a stereoelectronic control leads to a stereospecific reaction wherein the 2-chloride (13) produces the 2-azide by a retentive process whilst the E-chloride (14) reacts with inversion Scheme 3.Ar \ /OAc 7N3’ \OAc /C=N c1 Scheme 3 60 J. C. Sheldon R. N. Hayes and J. H. Bowie J. Am. Chem. Soc. 1984 106 7711. 61 F. Camon and M. J. S. Dewar J. Am. Chem. Soc. 1984 106 3531. 62 J. Chandrasekar S. F. Smith and W. L. Jorgensen J. Am. Chem. Soc. 1984 106 3049. 63 S. Yamabe T. Minato and Y. Kawabata Can. J. Chem. 1984 62 235. 64 D. J. Palling and W. P. Jencks J. Am. Chem. Soc. 1984 106 4869. 65 A. R. Katritzky K. Sakizadeh B. Gabrielsen and W.J. le Noble J. Am. Chem. SOC.,1984 106 1879. 66 (a) A. F. Hegarty and M. Mullane J. Chem. SOC.,Chem. Commun. 1984,913; (b)J. E. Rowe and A. F. Hegarty J. Org. Chem. 1984 49 3083. Reaction Mechanisms -Part (ii) Polar Reactions The archetypal SN2reaction is the Williamson ether synthesis using an alkoxide and an alkyl iodide. One example involving a primary (though neopentylic) iodide has been shown to go at least partly through an ET This is one of an ever-increasing number of cases of reactions hitherto thought to be polar two- electron processes that are falling to the axes of radical ion and radical pathway but survivors remain.68 4 Elimination Reactions It is difficult enough to measure a p-tritium secondary KIE in an elimination reaction equation 7 L = H or T since the isotope is in a labile position; interpretation is even more difficult when the kinetic effect is larger than the equilibrium isotope effect.Saunders has dealt with both the e~perimental~~" and theoretical problems69b and it transpires that such a &isotope effect can no longer be reliably accepted as an index of C-..H rupture at the transition state. The E2 exo-syn elimination of exo-norbornyl tosylate promoted by bulky alkoxides is reported to exhibit a temperature-independent primary kH/kD.70A bent C. -.H. -0arrangement is proposed to account for this but such interpretations have been ~hallenged.'~ In view of the demonstrations of E 1cB elimination from p-N0,C6H,CH2CH2NR3 substrates in recent years the reactions of the 2-(2,4-dinitrophenyl)ethylhalides are of interest.They are general base-catalysed and the Bronsted p-value increases in the order F = C1 > Br > I which is the inverse of the reactivity order. It is proposed that the mechanism is E2 with a major proton-transfer cornp~nent.~~ ArCHLCH,X + OR-+ ArCL=CH + X-+ ROH (7) Intramolecular base-promoted elimination of lactone (13; X = 0) to give (14; Y = COY) is surprisingly rapid and is general base-catalysed. Even though a proton alpha to carbonyl is removed an E2 mechanism is favoured on the grounds that strain in the 5-membered ring forces C-0 cleavage to accompany proton transfer.73" On the other hand curved buffer plots in the intramolecular elimination from the alkoxy-derivative (13; X = H2) to yield (14; Y = 0-)indicate a change in rate-limiting step and the intermediacy of an enolate anion73b The changeover (13) (14) 67 (a) E.C. Ashby D.-H. Bae W.-S. Park R. N. Depriest and W.-Y. Su Tetrahedron Lett. 1984 25 5107; (b) E. C. Ashby and J. N. Argyropoulos ibid. 7. 68 M. Newcomb and M. T. Burchill J. Am. Chem. Soc. 1984 106 8276. 69 (a) R. Subramanian and W. H. Saunders J. Am. Chern. Soc. 1984 106 7887; (b) W. H. Saunders ibid. 2223. 70 H. Kwart A. H. Gaffney and K. A. Wilk J. Chem. SOC.,Perkin Trans. 2 1984 565. 71 B. Anhede and N.-A. Bergman J. Am. Chem. Soc. 1984 106 7634. See also N. G. Rondan and K. N. Houk Tetrahedron Lett. 1984 25 2519. 72 J. R. Gandler and T. Yokoyama J. Am. Chem. SOC.,1984 106 130.73 (a) B. J. Mayer T. A. Spencer and K. D. Onan J. Am. Chem. SOC.,1984 106 6343; (b) B. J. Mayer and T. A. Spencer ibid. 6349. D. J. McLennan from E2 to ElcB is attributed to a diminution of nucleofugality as one goes from carboxylate to alkoxide. In another study of nucleofugality Issari and Stirling find that for once leaving-group rank and pK correlate in activated 1,3-eliminations from carbanions to form cycl~propanes.~~ Triose phosphates undergo non-enzymatic elimination and isomerization through a common enediolate intermediate Scheme 4 and catalysis studies show that the ElcB elimination proceeds via rate-limiting depr~tonation.~~ CH,OH -o\ C/H o\\ /H I -H+ -PO C L C=O II I I +H+ C CH ,O PO -HO CH,OPO:-HO/ %-I CHOH I CH,OPO:-Scheme 4 Further examples of E 1cB ester hydrolyses have been presented.Volume of activation studies confirm the earlier mechanistic assignment for p-HOC6H4CO2Ar (Ar = 2,4-dinitro~henyl).~~ The alkaline hydrolysis of thiocarbamate esters ArNHCO-SR involves the intermediacy of the isocyanate ArNCO which is formed from the thiocarbamate anion.77 The 1’3-dipole benzonitrile oxide can be formed by base-induced 1,3-dehydro- chlorination of the hydroxyimidoyl chlorides Scheme 5. The rate ratio kZ/kE is 6 x lo7 which represents a very large stereoelectronic effect indeed.78 Classical studies on alkyl halides and ‘onium’ salts are diminishing nevertheless the reason why the effect of successive P-methyl sutstitutions on the rates of Hofmann elimination from (R’CH,CH,)( R2CH2CH2)NMe20H- is non-additive has not yet been answered.After rejecting tunnelling and syn-elimination alterna- tives Saunders and co-workers account for this effect in terms of a sterically-induced shift in the transition-state character from ElcB-like towards a more central E2 character with increasing bulk at C,. Consequently substitution of a second p-methyl which is expected to favour a developing double bond leads to rate changes through a shift in transition-state character.79 74 B. Issari and C. J. M. Stirling J. Chem. SOC.,Perkin Trans. 2 1984 1043. 75 J. P. Richard J. Am. Chem. SOC.,1984 106 4926. 76 N. S. Isaacs and T. Najem J. Chem. Soc. Chem. Commun. 1984 1361; G. Cevasco G. Guanti S. Thea and A.Williams ibid. 783. 17 J. Mindle V. StErba V. Kadeifibek and J. Klicnar Coll. Czech. Chem. Commun. 1984 49 1577; N. Bourne A. Williams K. T. Douglas and T. R. Penkava J. Chem. SOC.,Perkin Trans. 2 1984 1827. 78 A. F. Hegarty and M. F. Mullane J. Chem. Soc. Chem. Commun. 1984 229. 19 S.-L. Wu Y.-T. Tao and W. H. Saunders J. Am. Chem. SOC.,1984 106 7583. Reaction Mechanisms -Part (ii) Polar Reactions Ph Ph OH \/ /c=N c1 It *H+ll Ph Ph 0-\/ ,C=N c1' + h PhCrN-0-Scheme 5 Phase transfer" and micellar" catalysis of elimination reactions has attracted attention and in the complete absence of solvent and catalyst gas-phase pyrolyses of alkyl acetates exhibit ion-pair characteristics as evidenced by the finding of strong anchimeric assistance being provided by the Me,N group.82 5 Addition Reactions A reliable method for obtaining values of k2 and k3 for mixed-order bromination of alkenes in non-polar solvents has been described.83 The question of bromonium ion formation in alkene bromination being reversible has been examined and examples provided.84 This has serious implications as far as structure-reactivity analysis is concerned since one must be sure that all alkenes in a series have the same rate-limiting step before making comparisons based on rates of bromination.Full details of intramolecular phenolate addition to unactivated double bonds have appeared. A primary carbanion is not formed since concerted general acid- catalysis is observed.85" Such reactions are considered to be models for enzyme- catalysed nucleophilic hydration of unactivated double bonds.On the other hand the analogous intramolecular addition of phenolate to unactivated triple bonds proceeds by way of a discrete vinyl carbanion and is some 104-times faster than cyclization of corresponding alkene~.~~ Evidence supporting the existence of weak nucleophilic solvent-assistance in the bromination of alkenes in protic solvents has been obtained. Data beautifully exemplifying the reactivity-selectivity principle are presented.86 80 J. Barry G. Bram G. Decodts A. Loupy P.Pigeon and J. Sansoulet J. Org. Chem. 1984,49 1138; M. Halpern Y. Sasson and M. Rabinovitz ibid. 2011. 81 E. Stadler D. Zanette M. C. Rezende and F. Nome J. Phys.Chem. 1984 88 1892. 82 G. Chuchani A. Rotinov R. M. Dominguez and N. Gonzalez J. Org. Chem. 1984,49 4157. 83 G. H. Schmid and B. Toyonaga J. Org. Chem. 1984,49 761. 84 R. S. Brown R. Gedye H. Slebocka-Tilk J. M. Buschek and K. R. Kopecky J. Am. Chem. Soc. 1984 106 4515. 85 (a) C. M. Evans and A. J. Kirby J. Chem. SOC.,Perkin Trans. 2 1984 1259; (b) 1269, 86 M.-F. Ruasse and B.-L. Zhang J. Org. Chem 1984,49 3207; M.-F. Ruasse and E. Lefebvre ibid. 3210. 58 D. J. McLennan An analysis of the preferred trajectory of approach of a nucleophile to a 7r-system considers both stabilizing HOMO-LUMO interactions and destabilizing HOMO- HOMO interaction^.'^ It is predicted that hard nucleophiles (low-lying HOMOs) will approach a 7r-centre at a smaller angle than soft nucleophiles (higher energy HOMOs).6 Aromatic Substitution and Rearrangements The Mills-Nixon effect concerns 7r-bond localization in polycyclic aromatic systems which was once thought to be of importance in determining rates and orientation in electrophilic substitution reactions. Clear evidence against ?r-localization has now been presented. An n.m.r. study of cyclobutane-annelated analogues of the dimethyl- dihydropyrene (15) shows that bond fixation is insignificant.” Ab initio MO calculations on protonated nitrous acid and on NO+ reveal that the former is best regarded as hydrated NO+.89” A similar conclusion is reached regarding protonated nitric acid and In both cases the production of electrophile by dehydration is calculated to be endoenergetic; it could indeed be the rate-limiting step in electrophilic attack upon reactive msystems.Nitrous acid is often a nuisance-impurity in nitration studies; the relative reac- tivities of several HN02 scavengers have been determined.” In 0.05 M acid 4-nitroaniline is the most efficient of those studied whilst in 1.3 M acid the scavenger of choice is NH,NH,+; urea is the least effective. The rate-limiting step is usually N-nitrosation and a study is reported wherein proton transfer between electronega- tive atoms is thought to be the rate-limiting step in the acid-catalysed decomposition of some N-nitroso comp~nds.~~ A sequence such as that in Scheme 6 (k- << k,) is advanced. k, RR’N-NO + H30+ -RR~~H-NO+ H,O k-I RR~~H-NO+ Y-2RR’NH + NOY NOY + scavenger -decomposition products Scheme 6 87 C.L. Liotta E. M. Burgess and W. H. Eberhardt J. Am. Chem. Soc. 1984 106 4849. 88 R. H. Mitchell P. D. Slowey T. Kamada R. V. Williams and P. J. Garrett J. Am. Chem. Soc. 1984 106 2431. 89 (a) M.-T. Nguyen and A. F. Hegarty J. Chem. Soc. Perkin Trans. 2 1984 2037; (b) 2043. 90 J. Fitzpatrick T. A. Meyer M. E. O’Neill and D. L. H. Williams J. Chem. Soc. Perkin Trans. 2 1984 927; L. R. Dix and D. L. H. Williams ibid. 109. 9’ S.S. Al-Kaabi G. Hallett T. A. Meyer and D. L. H. Williams J. Cbem. Soc. Perkin Trans.2,1984,1803. Reaction Mechanisms -Part (ii) Polar Reactions The Brown selectivity relationship concerning the correspondence of substrate (intermolecular) and positional (intramolecular) selectivity has often been observed in the breach for Friedel-Crafts alkylations.Careful studies are required to avoid isomerization and mixing complications and several are reported this year. Olah has investigated the alkylation of anisole and shows that under kinetic control the proportion of meta-product from this reactive substrate is small.92 De Haan and his (numerous) co-workers report both concordant and discordant cases using direct rather than competitive kinetic studies.93 Deviations are rationalized in terms of diffusion control. The cation radical ArCHz is the prime transient intermediate in the oxidative substitution of methylarenes by FelI1 complexes to yield benzylic The ET step is analysed in terms of Marcus theory and the non-equivalence of the measured a and the barrier position is emphasized.A detailed study of aromatic thallation reveals the presence of ArCH which is produced in an ET step with (CF3C02),T1+ as acceptor which in turn arises from the (CF3C02)3Tl reagent.95 The radical cation is important in side-chain substitution biaryl coupling and oxidative substitution. The (CF3C02)2Tl+ ion is also the active reagent in elec- trophilic (2-electron) nuclear thallation and while the question of whether the 1-electron and 2-electron processes proceed in parallel or by sequential pathways involving a common intermediate is not yet answered it is clear that the latter process possesses many of the characteristics of the former. No such difficulties exist with the Sandmeyer reaction induced by reducing cations single electron transfer to the diazonium cation is the rule and appears to be rate-limiting.96 A plausible mechanism involving nitrenium ions PhNH+ can be written for the acid-catalysed rearrangement of N-phenylhydroxylamines PhN( R)OH to the corre- sponding 4-aminophenols.Substituent effects however suggest that the intermediate is better represented by another resonance structure the imine-like structure (16).97 KIE studies have shown that two-proton catalysed benzidine rearrangements are concerted [5,5]-sigmatropic rearrangements. The same method now reveals that a one-proton rearrangement proceeds in a like fashion:* thus the rules of orbital symmetry govern the reaction irrespective of the proton count.Probing the nitramine rearrangement equation 8 in the same way yields a different result.99 A substantial 92 G. A. Olah J. A. Olah and T. Ohyama J. Am. Chem. Soc. 1984 106 5284. 93 F. P. De Haan G. L. Delker W. D. Covey J. Ahn M. S. Anisman E. C. Brehm J. Chang R. M. Chicz R. L. Cowan D. M. Ferrara C. H. Fong J. D. Harper C. D. Irani J. Y. Kim R. W. Meinhold K. D. Miller M. P. Roberts E. M.Stoler Y. J. Suh M. Tang and E. L. Williams J. Am. Chem. Soc. 1984 106 7038; see also earlier papers in the series. 94 C.J. Schlesener C. Amatore and J. K. Kochi J. Am. Chem. Soc. 1984 106 3567. W. Lau and J. K. Kochi J. Am. Chem Soc. 1984 106 7100. 95 96 C. Galli J. Chem. Soc. Perkin Trans. 2 1984 897. 97 G.Kohnstam W. A. Petch and D. L. H. Williams J. Chem. Soc. Perkin Trans. 2 1984 423. 98 H. J. Shine K. H. Park M. L. Brownawell and J. San Filippo J. Am. Chem. Soc. 1984 106 7077. 99 H. J. Shine J. Zygmunt M. L. Brownawell and J. San Filippo 1.Am. Chem. Soc. 1984 106 3610. D. J. McLennan MewO MeNH MeNH nitrogen (NO,) KIE is not accompanied by carbon KIEs of any magnitude. Thus N-N bond breaking is rate+limiting and earlier arguments in favour of the radical-cation-radical pair PhNHMe NO as an intermediate are strengthened. 7 Carbanions and Proton Transfer A6 initio calculations on the structures and stabilities of monomeric organo-lithium and -sodium compounds MCH2X provide a starting point for insight into syntheti- cally useful aggregated organometallics.Carbanions are more stabilized by a second- row element X and negative hyperconjugative and polarizability factors are cited.'" &Orbital effects are energetically unimportant in contradiction of earlier calcula- tions but do improve geometries. Stereoelectronic effects on the kinetic acidities of diastereotopic hydrogens have been theoretically examined and the results are in accordance with experimental findings on for example benzyl sulphoxides."' The trapping of fluorocarbanions by C02 has been achieved equation 9 and can be shown to be successful only when the most stable conformer of the carbanion has its lone pair anti to the position of the recently-entered nucleophile.102 A full account of cleavage of R-Si bonds in silanols RSiMe20H by OMe- in slightly aqueous methanol has appeared.lo3 For R = m-C1C6H4 and starting with the methyl ether the novel mechanism proposed (Scheme 7) avoids direct base attack at Si and involves a silanone intermediate.RSiMe,OMe + H20 eRSiMe,OH + MeOH RSiMe,OH + OMe-eRSiMe20-+ MeOH RSiMe20-__* R-+ Me2Si=0 R-+ MeOH -RH + OMe-Me,Si=O + MeOH -Me,Si(OMe)OH Thus the presence of OH or OMe on silicon can facilitate the removal of an R group from Si by dilute base which could be valuable in removal of protecting groups. Slow proton-transfers (N to 0)are reported for the deprotonation of monoproton- ated (and intramolecularly hydrogen-bonded) proton sponge.'04 It is contended that 100 P. von R. Schleyer T. Clark A. J. Kos G. W. Spitznagel C.Rohde D. had K. N. Houk and N. G. Rondan J. Am. Chem. SOC.,1984 106 6467. 101 S. Wolfe A. Stolow and L. A. La John Can. J. Chem. 1984 62 1470. 102 C. G. Krespan F. A. Van-Catledge and B. E. Smart J. Am. Chem. SOC.,1984 106 5544. 103 C. Eaborn and W. A. Stanczyk J. Chem. SOC.,Perkin Trans. 2 1984 2099. 104 G. B. Barnett and F. Hibbert J. Am. Chem. SOC.,1984 106 2080. Reaction Mechanisms -Part (ii) Polar Reactions the open form of the acid must be formed before proton loss can occur and that even then steric hindrance to proton transfer must be involved. Carbanions resulting from addition of MeO- to ArCH=CF2 are neutralized by solvent MeOH (MeOD) in a step having a primary KIE that increases with increasing temperature in some cases.lo5A two-step mechanism for protonation involving preassociation of MeOH with the carbanion allows the results to be rationalized.Carbanion production by the decarboxylation of (nitropheny1)acetate anions'06 and trichloroacetate ions'" is reported. In the latter case CCl; reacts as rapidly with 1,3,5-trinitrobenzene to form a Meisenheimer adduct as it does with H+in dimethyl sulphoxide. One of the clearest examples of stereoelectronic control of enolization is provided by the acid- and base-catalysed enolizations of trans-hexahydrofluorenone (17).'08 In a comparison with cyclohexyl phenyl ketone the substantially faster enolization of (17) (a rate factor of between 25 and 60) is in part attributed to the fact that the cleaving C-H bond is held rigidly parallel to the carbonyl .rr-orbital.Rates of proton transfer from transient methylarene cation radicals equation (9) have been successfully measured and cover a 105-fold range. Substantial primary KIEs are reported and the Bronsted a is ca. 0.28. ArCHj' + B + ArCH; + BH+ (10) Relative acidities of the cation radicals are also reported"' and Marcus theory parameters are derived which establish the reaction as being in the exergonic region of the driving force. 8 Carbonyl Derivatives and Tetrahedral Intermediates Stereoelectronic effects dominate the research in this area as is the case for some other areas. The variability of the C-0 single bond Fngth in ethers and esters R'-OR2 has been analysed; the resonance hybrid R'OR' is considered to be important and the C-0 bond lengthens as it acquires greater ionic character.' lo The exocyclic C-0 bond in axial compounds represented by (18) is systematically longer than in their equatorial analogues (19) and a bond length-reactivity correla- tion is obtained for hydrolyses in the (18) series where the stereoelectronic effect 105 H.F. Koch and A. S. Koch J. Am. Chem. SOC.,1984 106 4536. 106 E. Buncel T. K. Venkatachalam and B. C. Menon 1. Org. Chem 1984,49,413. 107 P. J. Atkins V. Gold and R. Marsh J. Chem. SOC., Perkin Trans. 2 1984 1239; P. J. Atkins V. Gold and W. N. Wassef ibid. 1984 1247. 108 R. M. Pollack R. H. Kayser and M. J. Cashen J. Org. Chem. 1984 49 3983. 109 C. J. Schlesener C. Amatore and J. K.Kochi J. Am. Chem.SOC.,1984 106 7472. 'lo F. H. Allen and A. J. Kirby J. Am. Chem. Soc. 1984 106 6197. D. J. McLennan OR ensures that molecules with a longer and more ionic C-0 bond gain a 'head start'."' These structure-reactivity correlations then allow a preliminary quantitative mapping of the reaction co-ordinate for spontaneous hydrolysis of axial cyclic acetals in terms of variable electron-demand at the departing oxygen.112 Sinnott and co-workers are not however as convinced that the steric relationship of an oxygen lone-pair with the axis of the cleaving bond is im~0rtant.l'~ In the case of spontaneous hydrolyses of pyridinium glycosides the stereoelectronic effect is considered to have at best a small accelerating effect after conformational factors are taken into account.Instead least-motion effects are favoured and stereoelec- tronic effects are seen as special cases rather than as dogma. However the principle of least motion may be inapplicable when extremely reactant- or product-like transition states are involved. It is noteworthy that least-motion effects were not ignored in Deslongchamps' 1983 monograph on stereoelectronic effects. Other interesting work on acetals114 and orthoe~ters'~~ has been reported. The f~rmation"'~and breakd~wn"'~ of a cyclic tetrahedral intermediate in a dead-end pathway accompanying the hydrolysis of the p-nitrobenzoate of pinacol equation 11 Ar = P-No2C6H4) have been studied. ArCOOCMe,CMe,O-Ar)(I$ -0 Rate and equilibrium data for the addition of nucleophiles to PhCOCF are reported,"6 which should be useful in allowing estimation of the behaviour of unstable tetrahedral intermediates.Extensive kinetic studies of the hydrolysis of ketene dithioacetals via initial C-protonation are reported."' The Thorpe-Ingold gem-dialkyl effect is manifested in the intramolecular car- boxyl-catalysed hydrolysis of sulphonamides ;it is rationalized in terms of (i) relief of unfavourable van der Waals interactions on proceeding from the initial to the transition state and (ii) the decrease in ring strain during activation."' X-ray crystallography establishes a splendid linear correlation between reactivity and the initial state S--0distance. 111 A. J. Briggs R. Glenn P. G. Jones A. J. Kirby and R. Ramaswany J.Am. Chem SOC. 1984,106,6200. 112 P. Jones and A. J. Kirby J. Am. Chem. SOC.,1984. 106 6207. 113 L. Hosie P. J. Marshall and M. L. Sinnott J. Chem. SOC.,Perkin Trans. 2 1984 1121. 114 R. A. Burt C. A. Chambers Y. Chiang C. S. Hillock A. J. Kresge and J. W. Larsen J. Org. Chem. 1984,49 2622. lL5 (a) R. A. McClelland N. E. Seaman and D. Cramm J. Am. Chem. SOC.,1984 106 4511; (b) R. A. McClelland ibid. 7579. 116 C. D. Ritchie J. Am. Chem. SOC. 1984 106 7187. 117 T. Okyama 1. Am. Chem. SOC.,1984 106 7134; T. Okuyama S. Kawao W. Fujiwara and T. Fueno J. Org. Chem. 1984 49 89; T. Okuyama S. Kawao and T. Fueno ibid. 85. 118 J. Jager. T. Graffland H. Schenk A. J. Kirby and J. B. F. N. Engberts,1.Am. Chem. SOC.,1984,106,139. Reaction Mechanisms -Part (ii) Polar Reactions A novel n.m.r.technique involving the l80isotope effect on 13C resonance^"^ has been used to monitor the oxygen exchange of a pyrylium cation in H2180. Acyclic dicarbonyl intermediates are proposed. The mechanism of p-lactam ring- opening in cephalosporins has been investigated in detail.12' Enzymatic transition- state analogues can be investigated by 13Cn.m.r.; the fact that a peptide aldehyde complexes with the serine active site of a-chymotrypsin in a hemiacetal form suggests that transition states bound by the enzyme may resemble hemiacetals.121 Weakly basic nucleophiles are normally ineffective in hydrolysing esters with more strongly basic leaving groups. The complexed zinc hydroxide (20) is an 20H exception; it is claimed that a metal-oxygen bond in the resulting tetrahedral intermediate cleaves more rapidly than the intermediate reverts to reactants.This has implications as far as metal ions co-ordinated close to enzyme active-sites are concerned.'22 In view of the large amount of work on carbon isotope effects in biological processes it is surprising that nothing is known on the hydration of C02 and the dehydration of HCO,. This gap has now been filled;'23 the respective k1Jk13 ratios at 24 "C are 1.0069and 1.0147,whilst the equilibrium isotope effect is 1.0077. The effective concentration of an intramolecular hydride-transfer to carbonyl carbon is several orders of magnitude higher than is normally found for proton transfer.'24 Tighter transition-states for the former process are suggested.The concept of virtual transition-states allows quantitation of &deuterium KIEs in all steps of hydrolysis of an a~etani1ide.l~~" Acyl substituent effects are similarly ana1~sed.l~~' This is a worthwhile advance since in both cases the interpretation of observed rate constants and KIEs is anything but straightforward. At moderately high concentrations of Br2 the acid-catalysed bromination of 2,4,6-trimethylacetophenoneis first-order in Br2 in contrast to the textbook case of the Lapworth zero-order result.'26 A systematic investigation of the thermodynamics of ionization of P-di- and tri-carbonyl compounds has been provided together with 119 J. M. Risley R. L. Van Etten C. Uncuta and A. T. Balaban J.Am. Cbem. SOC.,1984 106 7836. 120 M. I. Page and P. Proctor 1. Am. Cbem. SOC.,1984 106 3820. 121 D. 0. Shah K. Lai and D. G. Gorenstein J. Am. Cbem. SOC., 1984 106 4272. 122 J. Chin and X. Zou J. Am. Cbem. SOC.,1984 106 3687. 123 J. F. Marlier and M. H. O'Leary J. Am. Cbem. SOC., 1984 106 5055. 124 A. M. Davis M. I. Page S. C. Mason and I. Watt J. Cbem SOC., Cbem. Commun. 1984 1671. 125 (a) R. L. Stein H. Fujihara D. M. Quinn G. Fischer G. Kullertz A. Earth and R. L. Schowen J. Am. Cbem. SOC.,1984 106 1457; (b)J. K. Young S. Pazhanisamy and R. L. Schowen J. Org. Cbem. 1984,49 4148. A. G. Pinkus and R. Gopalan J. Am. Cbem. Soc. 1984 106. 2630. 12' 64 D. J. McLennan association constants for pairing of the enolate ions to alkali-metal cation^.'^' The patterns fit what the authors ciaim is 'common knowledge' but a caution against over-interpretation of small (<2 kcal mol-') rate or product differences is issued.9 Some Probes of Polar Mechanisms Included in this section are discussions of some of the tools used to probe the mechanisms of polar reactions -substituent effects and free energy relationships (linear and otherwise) solvent effects and isotope effects. The validity of Swain's 1983 fF + rR dual substituent parameter equation has been questioned. The separation of field and resonance effects and the fixed R scale give the greatest cause for concern.12* Similarly the aA + bB solvent-effect equation has been adversely critici~td.'~~ Swain has replied to criticisms and has emphasized his intention of simplifying analyses over those used in for instance the plat + pRi?R DSP equation where the best of four oRscales is picked Another DSP equation having but one resonance scale has been propo~ed.'~' For what it is worth the DSP equation is superior to Swain in correlating 13C 'H and vco spectral parameters for substituted acetanilides and 4-nitrophenylben~oates.'~~ What is more significant is that there are excellent intercorrelations amongst these and between them and hydrolysis rates and solution basicities even for cases where DSP and Swain correlations are bad.Ab initio calculations are used to derive a scale of pure field-effe~ts'~~" and to analyse the polarization of 7r-electron systems by substituent dipoles and poles.'33b Alkyl inductive effects have been analysed in terms of base constants to which are added numbers characteristic of the group connectivity of carbon atoms in the various groups.'34 The influence of solvent on the inductive order of substituents has been probed by i.r.methods; oIcorrelates results except for H-bond acceptor substituents in H-bond donor solvents and H-bond donor substituents in H-bond acceptor solvents. In the amphoteric solvent MeOH the above effects cancel and a true inductive order is ~bserved.'~' Enhanced substituent solvation-assisted reson- ance effects are observed for phenol ionization in solvents such as DMS0.'36 The Hammett p value for phenol ionization in DMSO (5.3) lies between the H20and the gas-phase value thus less stabilized anions with localized charge (gas phase) are more stabilized in polar solvents and acidity differences are le~elled.'~' 'Through 127 E.M. Amett S. G. Maroldo S. L. Schilling and J. A. Harrelson J. Am. Chem. Soc. 1984 106 6759. 12* W. F. Reynolds and R. D. Topsom J. Org. Chem. 1984,49 1989; A. J. Hoefnagel W. Oosterbeek and B. M. Wepster ibid. 1993; M. Charton ibid. 1997. 129 R. W. Taft J.-L. M. Abboud and M. J. Kamlet J. Org. Chem. 1984 49 2001; D. N. Kevill J. Chem. Research (S) 1984 86. 130 C. G. Swain J. Org. Chem. 1984 49 2005. 131 1. B. Afanas'ev J. Chem. SOC.,Perkin Trans. 2 1984 1589. 132 C. J. O'Connor D. J. McLennan D. J. Calvert T. D. Lomax A. J. Porter and D. A. Rogers Aust. J. Chem..1984 37 497. 133 (a) S. Mamott and R. D. Topsom J. Am. Chem. SOC.,1984 106 7; (b)1. Chem SOC.,Perkin Trans. 2 1984 113. 134 P.Hanson J. Chem. Soc. Perkin Trans. 2 1984 101. 135 C. Laurence M. Berthelot M. Lucon M. Helbert D. G. Moms and J.-F. Gal J. Chem. SOC.,Perkin Trans. 2 1984 705. 136 M. Mashirna R.T. McIver R. W. Taft F. G. Bordwell and W. N. Olmsted 1.Am. Chem. Soc. 1984 106 2717. 137 F. G. Bordwell R. J. McCallum and W. N. Olmsted J. Org. Chem. 1984 49 1424. Reaction Mechanisms -Part (ii) Polar Reactions 65 resonance' is usually invoked to explain the influence of a para-nitro group on the properties of aniline and phenol; VB analyses based on ab initio wavefunctions suggest that through resonance is negligible and that ring polarization has a greater effect than charge transfer.'38 Some 30 years ago H.C. Brown correctly surmized that a steric effect caused 2,6-di-t-butylpyridine to exhibit an anomalously low basicity in solution but the exact nature of the effect has since been the subject of debate. A combination of gas-phase and aqueous-solution measurements now shows that both the neutral and protonated forms are substantially solvated in water but for the latter the barriers to internal rotation for the water molecule and the substituents are am~1ified.l~' This accords with the entropic origin of the lowered basicity. The ubiquitous Marcus equation for proton (and electron) transfer predicts the Bronsted Q = 0.5 for a particle half-transferred at the transition state.Analysis of the dependence of AG* on AGO for a reaction series using different barrier models reveals that this may not necessarily be the case.'4o The qualitative VB treatment of Pross and Shaik lends credence to this view. Pross shows that two-configuration (i.e. reactant and product) systems are likely to exhibit normal behaviour e.g. SN2 methyl transfers but that in multi-configuration systems e.g. SN2 benzyl transfer or nitroalkane deprotonation anomalous rate-equilibrium relationships are likely and depend as much on the position of the probe relative to the reaction site as on the reaction type. A provocative suggestion is that even in a simple SN2 reaction charge may be 50% transferred at the transition state no matter where it lies on the reaction co-ordinate in the geometric sense.14' Simple Marcus theory is of course a quantitative expression of the Bell-Evans-Polyani-Leffler-Hammond principle which deals with parallel effects in the reactant-product dimension.Kreevoy and Lee now introduce factors related to perpendicular effects operating in the loose-tight dimension and show that for hydride transfers between NAD+ analogues the measured Bronsted a is not an index of the degree of hydride transfer.'42 Marcus theory can also be used to predict the existence or otherwise of transition states using the relative magnitudes of AGO and the intrinsic barrier AG;. The predictions for gas-phase deprotonations of C-acids (by ab initio MO calculations) are verified.'43 The Marcus equation correctly predicts the Bronsted slope for the gas-phase reaction of CH3Br with substituted benzyl anions.'44 Two papers by Murdoch consider further Marcus business.In the the proposition that AG; should be the arithmetic mean of the AG* values of the two identity reactions is examined using models from SCF-MO calculations and is found to hold in most cases. The second'45b considers cases such as internal rotation and conformational rearrangements where no identity reactions exist and a novel method for obtaining AG is developed. 138 P. C. Hiberty and G. Ohanessian J. Am. Chem. SOC.,1984 106 6963. 139 H. P. Hopkins D. V. Jahagirdar P. S. Moulik D. H. Aue H. M. Webb W. R. Davidson and M. D. Pedley J. Am. Chem. SOC.,1984 106,4341. 140 F. Wiseman and N.R. Kestner J. Phys. Chem. 1984 88 4354. 141 A. Pross J. Org. Chem. 1984 49 1811. 142 M. M. Kreevoy and I.& H. Lee J. Am. Chem. SOC.,1984 106 2550. 143 S. Wolfe Can. J. Chem. 1984 62 1465. 144 J. A. Dodd and J. I. Brauman J. Am. Chem. SOC.,1984 106 5356. 145 (a) J. Donnella and J. R. Murdoch J. Am. Chem. SOC.,1984 106 4724; (6) M. Y. Chen and J. R. Murdoch ibid. 4735. 66 D. J. McLennan The arithmetic mean aspect of the Marcus equation has been experimentally tested for sN2 methyl transfers of arenesulphonates and is found to hold (quadratic term omitted hence linear Hammett and Bronsted plots).l4 Surprisingly the Ham- mett p for ArS0; + CH303SAr identity reactions is 0.6 and the Kreevoy-Ross idea of a looser transition-state is provided as a rationalization.Linear Bronsted plots are also generated by the SN2 reactions of nitranions derived from carbazoles phenothiazines and diphenylamides with benzyl chlorides in DMS0147 and it is shown that anion basicity is the primary factor in controlling nucleophilicities of nitranions carbanions and oxyanions. Surprisingly neutral amines are 10-100times more nucleophilic than comparably basic nitranions. The Marcus equation is also a quantitative expression of the much-debated reactivity-selectivity principle (RSP),which in turn requires multiple substitution for exemplification. Dubois and co-workers have examined multiple substituent effects in the context of the Hammett equation and find a cross-term is usually significant thus guaranteeing n~n-additivity.'~~" The cross-coefficient is then a measure of substituent-induced transition-state structural changes which may none- theless be compatible with linear individual Hammett plots.The contribution of transition-state variation to p variation has been assessed for carbocation-forming alkene br~minations'~~~ and conditions for the applicability or otherwise of the RSP have been established. Both p and Winstein-Grunwald rn values obtained from rates of bromination of styrenes ArC(Y)=CH2 vary with Y in a beautiful RSP fa~hi0n.l~~ The reactivity order of bases in nitroethane deprotonation is tertiary amines > secondary and primary amines > oxyanions. Electrostatic effects are in~0ked.l~' The Bronsted p parameters are in accord with the RSP and in such a case are expected to be a valid measure of the extent of charge (and perhaps proton) transfer at the transition state.141 If KIEs are regarded as a measure of transition-state structure the variation of primary carbon and secondary hydrogen isotope effects in the SN2 reactions of substituted dimethylanilines with benzyl arenesulphonates variously substituted in both ringslsl constitutes the finest piece of evidence in favour of transition-state variation in accordance with the Thornton-More O'Ferrall-Jencks rules yet pro- vided.Both parallel and perpendicular shifts are beautifully illustrated. Modification of the Marcus equation allows quantitative rationalization of trends in primary kH/kD values with changing ApK in proton transfer reactions.152 The question of whether hydride transfers in NADH/NAD+ systems proceed via linear or bent transition-state structures has been investigated using MNDO ;the former alternative is fa~0ured.l~~ Secondary deuterium isotope effects in SE2reactions are reported for the first time,154 for electrophilic attack on dialkylmercurials.Alpha 146 E. S. Lewis and D. D. Hu J. Am. Chem. Soc. 1984 106 3292. 147 F. G. Bordwell and D. L. Hughes J. Am. Chem. Soc 1984 106 3234. 14' (a)J.-E. Dubois M.-F. Ruasse and A. Argile J. Am. Chem. SOC.,1984 106 4840; (b)4846. 149 M.-F. Ruasse and J.-E. Dubois J. Am. Chem. Soc. 1984 106 3230. 150 P. Y. Bruice J. Am. Chem. SOC.,1984 106 5959. 15' T. Ando H. Tanabe and Y. Yamataka J. Am.Chem Soc. 1984 106 2084. 152 D. J. Hupe and E. R. Pohl J. Am. Chem. Soc. 1984 106 5634. 153 S. M. van der Kerk W. van Gerresheim and J. W. Verhoeven Rec. Truv. Chim. Pays-Bus.,1984,103,143. 154 D. J. Nencivengo M. L. Brownawell M.-Y. Li and J. San Filippo J. Am. Chem. Soc. 1984 106 3703. Reaction Mechanisms -Part (ii) Polar Reactions effects are large which is surprising in terms of SN1-SN2theory if a pentaco-ordinate transition state is involved but bond stretching factors may outweigh angle bending factors in SE2. Isotope effects b/k14 for enzyme-catalysed transmethylation from S-adenosyl- methionine to the rneta- and para-positions of dopamine are markedly and surpris- ingly different.155 para-Methylation seems to be normal SN2,but the isotope effect for rneta may be ‘diluted’ by a binding step or a conformational change.The primary (and perhaps the secondary) deuterium isotope effects for enzymatic oxidation of glucose 6-phosphate by TPN are different in H20and D20? Thus hydride transfer to TPN may be coupled with transfer of the OH proton to an enzyme site with tunnelling playing an important role. A major disincentive to the evaluation of deuterium isotope effects is the effort needed to synthesize the labelled substrate. When intramolecular competition between hydrogen isotopes is involved measurement by natural abundance ’H n.m.r. may obviate this diffic~lty.’’~ 155 S.-E. Wu W. P. Huskey R. T. Borchardt and R. L. Schowen J. Am. Chem. SOC.,1984 106 5762. 156 J. D.Hermes and W. W. Cleland J. Am. Chem. SOC.,1984 106 7263. 157 R. A. Pascal M. W. Baum C. K. Wagner and L. R. Rodgers J. Am. Chem. Soc. 1984 106 5377.

ISSN:0069-3030    

DOI:10.1039/OC9848100047

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (iii) Free-radical Reactions By D. GRILLER Division of Chemistry National Research Council of Canada Ottawa Ontario Canada KIA OR6 1 Synthesis Giese and his colleagues have made good use of known chemistry to effect carbon- carbon bond formation.’ For example a yield of 87% (gas chromatography) of (1) was obtained when RX was t-butyl iodide and the olefin was acrylonitrile (Scheme 1). Tributyltin hydride was only required in sub-stoicheiometric amounts since it was regenerated in situ by the simple expedient of having sodium borohydride present in the reaction mixture. R.+ \/ I IC=C + R-C-C. /\ I I I I I 1 R-C-C. + Bu3SnH -+ I 1 I 1 R-C-C-H + Bu,Sn. (1) Bu,Sn. + RX -+ Bu,SnX + R-Scheme 1 An elegant use of stannyl radicals was made by Baldwin et al.,’ who devised an addition-elimination sequence for carbon-carbon bond formation (Scheme 2).This method avoids the risk of telomerization inherent in approaches that rely upon simple radical addition to an olefin as a propagation step of 8 chain reaction. R R*+ -Bu,Sn C0,Et Bu,Sn C0,Et I R Rx etc. -Bu,Sn-+ C0,Et Scheme 2 ’ B. Giese J. A. GonzLlez-G6mez and T. Witzel Angew. Chem. Int. Ed. Engl. 1984 23 69. ’ J. E. Baldwin D. R. Kelley and C. B. Ziegler 1. Chem. Soc. Chem. Commun. 1984 133. 69 70 D. Griller Three further communication^^-^ have appeared on the use of thiohydroxamic- 0-esters as synthetic sources of alkyl radicals. One of them' describes a method for the radical decarboxylation of N-protected a-amino-acids after first converting them into their N-hydroxypyridine-2-thioneesters (Scheme 3).This method is a slight variation on previous work6 where tributyltinhydride was used as a hydrogen donor instead of t-butyl thiol. +*SBu' + +RCO,* NS N S-SBu' I 0 I co I R Scheme 3 Silylmethyl radical cyclizations have been used to convert allylic alcohols into 1,3-diols ~tereoselectively,~ (Scheme 4). In general the 5-exo mode of cyclization was dominant and in some instances exclusive. Isolated yields were normally greater than 75YO. I AH :vH R ,BuiSnH Si Si HO OH Me/ \Me Me/\Me Scheme 4 2 Mechanism 1984 saw few striking developments in the area of free-radical reaction mechanisms although many workers continued to develop existing themes.Some showed con- siderable entrepreneurial skill in convincing editors to continue publishing these extensions as urgent communications. D. H. R. Barton and D. Crich J. Chem. SOC,Chem. Commun. 1984 774. D. H. R. Barton D. Crich and W. B. Motherwell J. Chem. SOC.,Chem. Commun. 1984 242. 'D. H. R. Barton Y.HervC P.Potier and J. Thierry J. Chem. SOC.,Chem. Commun. 1984 1298. D. H. R. Barton D. Crich and W. B. Motherwell J. Chem. SOC.,Chem Commun 1983 939. 'H. Nishiyama T. Kitajima M. Matsumoto and K.Itoh J. Org. Chem. 1984,49 2298. Reaction Mechanisms -Part (iii) Free-radical Reactions 71 Two have appeared in defense of the hypothesis that the sigma and pi states of the succinimidyl radical can be differentiated on the basis of their chemical reactions.The arguments for and against are now so intricate and conflicting that it is almost impossible to make an objective judgement on the subject. However several simple facts emerge. First results from one laboratory are not reproduced in another. This is not entirely surprising since much of the debate rests upon the presence or absence of small quantities of P-bromopropionyl isocyanate an extremely moisture-sensitive product which is formed as the result of ring-opening of the succinimidyl radical (Scheme 5). Secondly the reaction mixtures are frequently heterogeneous making a precise description of reaction conditions and concentra- tions essentially impossible. Thirdly chain lengths are seldom determined so that the extent to which initiation and termination reactions contribute to the product distribution is unknown.Fourthly complete product balances are rarely undertaken. As noted in last year's report the principal players in this drama continue to ignore a key paper in the field." (2)-Br ** + OCN(CO)CH,CH -OCN(CO)CH,CH,Br (2) Scheme 5 The ephemeral capto-dative effect again eluded all attempts to prove its existence. The simple requirement for proof is that an electron-donating and an electron- withdrawing substituent at a radical centre produce a stabilizing effect which is greater than the sum of the effects produced by the individual groups. t-Butoxyl radicals were added to a series of 'capto-dative' olefins and for each case the yield of radical dimers formed was taken as a guide to the efficiency of the initial addition reaction (Scheme 6)." It was concluded that electron-rich olefins were the most efficient scavengers of t-butoxyl radicals although a complete interpre- tation of the data was reserved for a future publication.The experiments were generally carried out using photolysis at 20°C to decompose the peroxide. Yields were low ca. 30% and the basis for their calculation was not given. Moreover there were few experimental details so that it was impossible to ascertain if the starting olefins and/or products were stable under the photolysis conditions. Without this information interpretation in terms of the capto-dative and frontier molecular- orbital approaches hinted at in this work must be treated with caution.H X X \/ / Bu'O.+ C=C + Bu'O-CH,-C. H' \Y 'Y (3) (3) + (3) -(3)-(3) Scheme 6 P. S. Skell and S. Seshadri J. Org. Chem. 1984,49 1650. P. S. Skell J. Am. Chem. SOC.,1984 106 1838. 10 A. G. Davies B. P. Roberts and J. M. Smith J. Chem. SOC.,Perkin Trans. 2 1973 2221. " S. Mignani Z. Janousek R. Merenyi H. G. Viehe J. Riga and J. Verbist Tetrahedron Lett. 1984 25 1571. 72 D. Griller In similar vein relative rate-constants for radical addition to diphenylethylenes were interpreted in terms of a capto-dative interaction.” The results were analyzed in terms of a Hammett plot. Taken as a whole the 11 data points for a range of substitution patterns showed a fair correlation with the sum of substituent constants.However two capto-dative substituted olefins were cited as being unusually reactive. By removing these from the graph it became possible to fit the remaining nine points to two separate correlation lines and claims were made that the quality of this revised fit was proof that the mechanism changed with substituent. However the total spread in relative rate-constants was only a factor of six. There was no description of the experimental work nor were any estimates of error given making all of the conclusions somewhat suspect. A serious attemptt3 was made to measure the combined effect of capto-dative substituents by taking the rotational barriers in ally1 radicals to be a measure of the radical stabilization energies.Some values were CH,=CHCHD (15.7 f l.O) CH,=CHCH(OCH,) (14.3 f 1.4) CH,=CH-eHCN (9.8 f l.l) and CH,=CH-C(OCH3)CN (6.0 f 0.4) (kcal mol-I). Despite the care with which the experiments were carried out it is clear that the combined stabilizing effect of a ‘capto’ and a ‘dative’ group is within experimental error equal to the sum of the effects provided by the groups individually. Hence there was no evidence for a substantial synergistic interaction. Photolysis of the peroxydisulphate dianiont4 was shown to be an effective source of SOT particularly when acetone was present in the solvent mixture to act as a photosensitizer. The radical was an effective oxidizing agent. For example when sulphides were present as substrates dimer radical-cations were observed as products (Scheme 7).S04T+ R2S -SO,,-+ R2S’ R,S? + R,S -R2StSR2 Scheme 7 Zimmt Doubleday and Turro” investigated the magnetic field dependence of CIDNP signals obtained during the photolysis of dibenzyl ketones in micellar media. The photolysis produced pairs of radicals which were effectively trapped within the micelles. The singlet-to-triplet To,energy gap for these pairs was determined from the magnetic field dependence of the polarization. This exhibited a maximum when there was efficient S-T- mixing. Analysis of the data suggested that the energy gaps were of the order of .03 cm-’ and as expected decreased with increasing micellar size. The experimental technique was both simple and effective. Samples were flowed through the field of a small electromagnet where the photolysis was carried out and then onto an n.m.r.spectrometer where the polarization was detected. 12 F. Lahousse R. Mertnyi J. R. Demurs H. Allaime A. Borghese and H. G. Viehe Tetrahedron Lett. 1984 25 3823. 13 H. Korth P. Lommes and R. Sustmann J. Am. Chem. Soc.. 1984 106. 663. 14 M. J. Davies B. C. Gilbert and R. 0.C. Norman J. Chem. SOC.,Perkin Trans. 2 1984 503. 15 M. B. Zimmt C. Doubleday jun. and N. J. Turro J. Am. Chem. Soc. 1984 106 3363. Reaction Mechanisms -Part ( iii) Free-radical Reactions 73 3 Structure Wendt and Hunziker16 obtained the gas-phase U.V. spectra of a variety of alkyl radicals including ethyl i-propyl s-butyl and t-butyl. The radicals were generated by mercury photosensitization and were detected by modulation spectroscopy.The pattern of bands agreed with ab initio predictions for the 3s 3p and 3d Rydberg transitions. The spectra became increasingly red-shifted with successive methyl substitution at the radical centre. This observation was consistent with the trend in ionization potentials for the radicals. The experimentally difficult technique of time-resolved microwave dielectric absorption was used to measure the dipole moment for the benzylperoxy radical.” The dipole moment was found to be 2.4 f0.2 D. Since the benzyl radical is thought to have a dipole moment of 0.2 D in the opposite sense it was concluded that simple alkylperoxyl radicals should have dipole moments of ca. 2.6 f 0.2 D. This value falls between those for alkyl chlorides and cyanides and indicates that peroxyls should be fairly hydrophilic; a property that may be of importance for oxidations carried out in vesicles or micelles.The e.s.r. spectrum of the bromoalkyl radical (Scheme 8) was interpreted in terms of a rapid migration of the bromine atom between the two central carbons at 100 K.’* At 77 K this motion was thought to be effectively frozen. The rapid migration of the bromine atom was consistent with conclusions drawn from earlier mechanistic studies which were carried out in solution at room temperature. However a complete analysis of the spectrum depended upon the interpretation of hyperfine splittings which were only discernible after resolution-enhancement techniques were applied and they were therefore interpreted with some caution.Br Br Me,C-&Me Me,L-CMe Scheme 8 The need for care in applying the radiolysis method for the generation of radicals was emphasized in a corrigendum” withdrawing the assignment of a spectrum to the p-benzoquinone radical cation. The signal detected was actually due to an impurity in the sample. An analysis of p-hyperfine uH@splittings in the 3-methyl-3-phenylbut-1 -yl radical has been reported,20 which is more important from the point of view of the underlying principle involved than for the specific example in question. It was shown that the simple equation (1) is frequently used in error. This equation relates uH@ to two aH8 = A + B cos‘ 8 (1) constants (A and B) and the dihedral angle 0 between the C-H bond and the orbital containing the unpaired electron.Errors in applying the equation have arisen because the observed value of aH@ is an average over quantum states and corresponds 16 H. R. Wendt and H. E. Hunziker J. Chem. Phys. 1984 81 717. 17 R. W. Fessenden A. Hitachi and V. Nagarajan J. Phys. Chem. 1984,88 107. 18 S. P. Maj M. C. R. Symons and P. M. R. Trousson J. Chem. SOC Chem. Commun. 1984 561. 19 H. Chandra and M. C. R. Symons J. Chern. Soc. Chem. Commun. 1983 1044. 20 K. U. Ingold D. C. Nonhebel and T. A. Wildman J. Phys. Chern. 1984 88 1975. 74 D. Griller to an average over cos’ 8 i.e. (cos28) which is not equal to cos’ (8). It is therefore not possible directly to apply equation (1) to obtain physically meaningful values of the average value of 8 from the hyperfine splittings.In fact by assuming the correctness of equation (l) data for the P-hyperfine splittings were used to derive the potential function for the rotation of the radical in question.” Studies of radical cations derived from conjugated systems have continued,21-26 and perhaps the most interesting of these26 deals with that derived from pentamethyl- cyclopentadiene. This radical cation was generated by photolysis of trifluoroacetic acid solutions of its parent hydrocarbon. The hyperfine coupling constants were consistent with values calculated by substituting Huckel coefficients into the McCon- nell equation. The hydrogen on C-1 which lies in the nodal plane of the orbital containing the unpaired electron was correctly predicted to have a very small hyperfine splitting (aH = 0.16 mT).The e.s.r. spectrum of the gallane radical anion has been obtained by photolysis of tetrahydrofuran solutions containing tetra-n-butylammoniumtetrahydrogallate and di-t-butyl peroxide (Scheme 9).27The isotropic gallium hyperfine splittings showed that the radical was pyramidal and that the orbital containing the unpaired electron had ca. 16% s-character. The temperature-dependence of the splittings was extremely small indicating that there was a substantial barrier to the umbrella inversion of the radical. Bu‘OOBu‘ -% 2Bu‘O-Bu‘O. + H,Ga-__* Bu‘OH + H,GaS Scheme 9 The opticai absorption spectra of diarylphosphonyl radicals were detected on flash photolysis of acylphosphine oxides28 and had absorption maxima at 330 nm.Since dialkylphosphonyl radicals absorb below 310 nm the use of aryl groups clearly served to red-shift the spectra to an experimentally accessible range of wavelengths. However a more careful reading of the published literature on these radicals would have prompted the authors to check their assignments by olefin benzene or alkyl bromide quenching of the transient absorption spectra. 4 Kinetics Work has continued on persistent peduoroalkyl radical^.'^ These were generated by radiolysis at 77 K of perfluoroalkane fractions and had general formula C,F2,+1. On warming to 300 K the concentration of radicals dropped to 10-70% of the value initially detected in the frozen solution.However the residual radicals showed no ” J. L. Courtneidge A. G. Davies T. Clark and D. Wilhelm J. Chem SOC.,Perkin Trans. 2 1984 1197. 22 Q. 3. Broxterman H. Hogeveen and R. F. Kingma Tetrahedron Letf. 1984. 25 2043. 23 J. L. Courtneidge A. G. Davies E. Lusztyk and J. Lusztyk J. Chem. Soc. Perkin Trans. 2 1984 155. 24 J. L. Courtneidge and A. G. Davies J. Chem. Soc. Chem. Commun. 1984 136. 2s J. L. Courtneidge and A. G. Davies BulL SOC.Chim. Belg. 1984 93 329. 26 J. L. Courtneidge A. G. Davies and S. N. Yazdi J. Chem. Soc. Chem. Commun. 1984 570. 27 J. C. Brand and B. P. Roberts J. Chem. SOC Chem. Commun.,1984 109. T. Sumiyoshi A. Henne P. Lechtken and W. Schnabel 2.Naru$orsch. Teil A 1984,39 434. 29 S. R. Allayarov S. V.Demidov D.P. Kiryukhin A. I. Mikhailov and I. M. Barkalov DOH.Akad. Nauk SSSR 1984 214,91. Reaction Mechanisms -Part (iii) Free-radical Reactions 75 signs of decay in the absence of oxygen but were immediately destroyed when oxygen was admitted. The persistence of these radicals was ascribed to steric protection of the radical centres. The diphenylketyl radical was formed in its excited state by photolysis of phenyl- benzoin using an intense laser pulse.30 The fluorescence of the excited state was quenched by a variety of electron acceptors at rates close to the diff usion-controlled limit. The quenching reactions for the excited state were several orders of magnitude faster than the corresponding processes for the ground-state radical. The excited-state reactions were therefore thought to be electron-transfer processes which were ther- modynamically viable by virtue of the additional energy of the excited transient.The reaction products were not investigated. Absolute rate constants for the reactions of the cyclopropyl radical have been measured by a flash photolysis technique which makes use of photolysis of bis(cyc1o- propylformyl) peroxide as the radical prec~rsor.~' The rate constant for the reaction with P-methylstyrene was 2 x lo6 M-'s-' at 298 K. This substrate was used as a probe of the lifetime of the cyclopropyl radical when other substrates were added as quenchers. Rate constants for a variety of C-H abstractions by cyclopropyl were ca. lo6 M-' s-l and were fairly insensitive to the strength of the bond being broken.They varied by about an order of magnitude when the latter changed by 20 kcal mol-'. Laser flash photolysis was also used to measure rate constants for the reactions of tributyl-stannyl and -germyl radicals with carbonyl compounds and organic halides.32 The y-scission of a series of P-peroxyalkyl radicals has been investigated (Scheme Rate constants for scission were ca. lo6s-' at 298 K. Variations reflected the conformational requirement for the carbon-oxygen bond to be aligned with the axis of the p-orbital containing the unpaired electron. It was also that this scission should contribute to the thermal decomposition of di-t-butylperoxide at room temperature since the t-butoxyl radical formed should abstract hydrogen from the parent peroxide to propagate a chain reaction.The normal rate of peroxide thermoly- sis at room temperature should be sufficient to initiate this chain decomposition and lead to degradation of this peroxide within a few weeks. In fact di-t-butyl peroxide has a long shelf-life. It was concluded that this stability was due to-impurities of hydroperoxide dissolved oxygen and the oxirane itself all of which serve as chain-breaking inhibitors. Quantum mechanical tunnelling in hydrogen-transfer reactions has been treated by a combination of theory and e~periment.~~-~* The reaction of methyl radicals *C-C-0-OR 4 C+ + *OR '0' Scheme 10 30 H. Baumann C. Merckel H.-J. Timpe A. Graness J. Kleinschmidt I. R. Gould and N. J. Turro Chem.Phys. Lett. 1984 103 497. 31 L. J. Johnston J. C. Scaiano and K. U. Ingold J. Am. Chem. SOC.,1984 106,4877. 32 K. U. Ingold J. Lusztyk and J. C. Scaiano J. Am. Chem. SOC.,1984 106 343. 33 A. J. Bloodworth J. L. Courtneidge and A. G. Davies J. Chem. SOC.,Perkin Trans. 2 1984 523. 34 S. A. Davis B. C. Gilbert D. Griller and A. S. Nazran J. Org. Chem. 1984 49 3415. 35 W. Siebrand T. A. Wildman and M. Z. Zgierski 1.Am. Chem. SOC.,1984 106 4083. 36 W. Siebrand T. A. Wildman and M. Z. Zgierski J. Am. Chem. SOC.,1984 106 4089. 37 T. Doba K. U. Ingold and W. Siebrand Chem. Phys. Lett. 1984 103 339. 38 T. Doba K. U. Ingold W. Siebrand and T. A. Wildman J. Phys. Chern. 1984 88 3165. 76 D. Griller in methanol glass was taken as a model system.38 To a first approximation the radical decay was exponentially related to It was shown that this rate law could be reproduced by considering that the methyl radicals occupied a distribution of sites each of which had a characteristic rate constant for decay which depended exponentially upon r the distance between the methyl radical and methanol.The 'most probable* site was found to be that in which the methyl radical and methyl group of the alcohol were separated by a distance ro which was approximately equal to the sum of their Van der Waals radii. The experimental data were reproduced with a very narrow distribution of sites varying by only a few percent from that of the most probable. This approach is certainly the most promising and intuitively reasonable treatment of tunnelling in chemical reactions which has yet emerged.5 Thermochemistry 1984 was a bumper year for papers on thermochemistry. Lossing and Homes reported heats of formation of 18 radicals as measured from appearance energies.39 These included vinyl (AH = 64 f2 kcal mol-') and phenyl (AH = 75.8 * 2 kcal mol-') which lead respectively to carbon-hydrogen bond dissociation energies of 104 f 2 and 108 * 2 kcal mol-I. Two determination^^^*^' of the heats of formation of ethyl (28 kcal mol-I) and one of t-b~tyl~~ (9.2 kcal mol-') were in excellent agreement with the results of an earlier but much criticised solution and led to bond strengths of 100 and 94 kcal mol-' respectively. Stratt and Desjardins& have addressed the frequent criticism that thermodynamic data obtained in solution are often contaminated by solvent effects.They calculated the magnitude of the interaction between the dipole induced on vibration of the methyl radical and solvent molecules. The solvation free-energies due to the 'vibra- tional polarizability' were only 4.0 and 33.0 cal mol-' for methane and dichloromethane respectively. However the difference in the I3C hyperfine splitting as measured in the two solvents was predicted to be 0.028 mT which is about 1% of the total hyperfine splitting. This prediction and the dependence of I3C on the dipole moment of the solvent can clearly be tested by experiment. The relationship between bond dissociation energy and radical stability was reviewed in some detail.45 It was argued that the definition of radical stabilization energy E = BDE(S-X)-BDE(R-X) (where S refers to a standard moiety and R to the radical in question) depended on the nature of X and therefore was not a measure of the intrinsic stability of the radical.However the data cited for a variety of X gave fairly consistent values of E, which were within the normal range of experimental errors. In a few instances the fit for X = halogen was poor but showed no well defined trend suggesting that the experimental determinations were suspect in those cases rather than the E definition itself. The data do not support 39 J. Holmes and F. P. Lossing Int. J. Muss Spectrom. Ion Processes 1984 58 113. 40 J.-R. Coa and M.H. Back Znt. J. Chem. Kinet.1984 16 961. 41 P. D. Pacey and J. H. Wimalasena J. phys. Chem. 1984,88 5657. 42 T. S. A. Islam and S. W. Benson Znt. J. Chem. Kinet. 1984 16 995. 43 A. L. Castelhano and D. Griller J. Am. Chem. SOC.,1982 104 3655. 44 R.M. Stratt and S. G. Desjardins J. Am. Chem. SOC.,1984 106 256. 45 A. M.de P. Nicholas and D. R. Arnold Can. J. Chem. 1984 62 1850. Reaction Mechanisms -Part ( iii) Free-radical Reactions the argument attractive as it may be. Variations of the kind expected by the authors ought only to be expected when perturbation caused by an electronegative substituent shows a strong dependence on the structure of R. In an imaginative approach Nonhebel and Walton showed that a linear correlation existed between the C-H bond dissociation energy of a compound and the barrier to internal rotation (as measured by e.s.r.) of the corresponding radical.& The results suggested that literature value of the C-H bond strength in acetone (98 kcal mol-') was too high by ca.8 kcal mol-' and that BDE(RSCH,-H) = 93 kcal mol-'. This concept was extended in a study of heptatrienyl and other polyenyl radicals47 and an empirical equation was developed which related radical stabilization energies as estimate from e.s.r. rotational barriers to aH-the average of the anti-and syn-hydrogen hyperfine splittings of the terminal methyl groups. While relationships of this kind are unlikely to replace thermodynamic measurements of stabilization energies they are certainly very effective methods for screening the accuracy of the available literature data.Finally Chen and Mendenhal148 measured the heat of combustion of di-t-butyl hyponitrite by bomb calorimetry and determined its heat of vaporization by an effusion technique. These results led to the gas-phase heat of formation of -41.3 2.9 kcal mol-' and to estimates of group equivalents for oxygen-nitrogen combina- tions. This work emphasises the need for fundamental thermochemical data and the importance of simple classical techniques for their determination. 46 D. G. Nonhebel and J. C. Walton J. Chem. Soc. Chem. Commun. 1984 731. 47 I. G. Green and J. C. Walton 1. Chem. Soc. Perkin Trans. 2 1984 1253. 48 H.-T. E. Chen and E. D. Mendenhall 1. Am. Chem. Soc. 1984 106 6375.

ISSN:0069-3030    

DOI:10.1039/OC9848100069

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

5 Arynes Carbenes Nitrenes and Related Species By M. S. BAlRD School of Chemistry University of Newcastle upon Tyne Newcastle upon Tyne NEl 7RU 1 General The transition states for insertion of methylene and silylene into the C-H and Si-H bonds of methane and silane have been probed by ab initio methods at the 3-21 G level. Only the insertion of silylene into methane has a non-zero barrier.’ A6 initio studies of the insertion of methylene silylene and various fluorinated deriva- tives into hydrogen reveal a dramatic increase in the barrier height with increasing fluorine substitution.2 MNDO methods have been used to study the 1,2-rearrangement of substituted singlet methylcarbenes carbonylcarbenes methylnitrenes and carbonylnitrenes. The C-R bond of the migrating R-group prefers to be eclipsed to the vacant p-orbital of the carbene or nitrene and the migratory aptitudes of various groups are explained in terms of nucleophilicity and leaving group character of R.3 2 Arynes Generation of a 1,2-doubly labelled benzyne from phthalic anhydride at 830°C leads to biphenylene with a rearrangement of the label consistent with a 1,2- to 1,3-rearrangement either in the benzyne itself or in an intermediate C7H40.Forma-tion of the carbene (1) is one possible source of the ~crambling.~ The isolation of acridines from the reaction of benzyne with imines such as N-benzylideneaniline and with amidines such as N,N-dimethyl-N’-phenylformamidinehas been taken as unambiguous evidence for formation of an intermediate benzazetidine formed by [2 + 21 cycloaddition (Scheme l).5 Trapping of benzyne by 1,2-diketone monoketal enolates leads to benzocyc- lobutenes (2) in reasonable yield while thi-iranes react with benzyne in a stereos- pecific reaction which leads to phenyl vinyl sulphides ;thus cis-2,3-diphenylthi-irane is converted into cis-(pheny1thio)stilbene (79%).The reaction is thought to proceed by way of betaines (3).’ ’ M. S. Gordon and D. R. Gano J. Am. Chem. Soc. 1984 106 5421. ’ C. Sosa and H. B. Schlegel J. Am Chem. SOC.,1984 106 5847. G. Frenking and J. Schmidt Tetrahedron 1984,40 2123. M. Berry R. F. C. Brown F. W. Eastwood D. A. Gunawardana and C. Vogel Aust. J. Chem. 1984 37 1643. C. W. G. Fishwick R. C. Gupta and R. C. Storr J. Chem. SOC.,Perkin Trans.1 1984 2827. M.-C. Carre B. Gregoire and P. Caubere J. Org. Chem. 1984 49 2050. ’ J. Nakayama S. Takeue and M. Hoshino Tetrahedron Lett. 1984 25 2679. 79 0,H M. S. Baird + MezN>NPh -Q)-J,NMe2 Ph 1 H NMe, =+m+ Scheme 1 Substituted 3-amidobenzynes have been generated under neutral mild conditions by reaction of 2-trimethylsilyl-3-triflyl benzamides (4) with fluoride ion; the benzynes form [4 + 21 cycloadducts with dienes and undergo attack of nucleophiles to produce rn-disubstituted benzamides.* Benzynes derived from 3-halogenoaryloxazolines react with a-lithio-alkyl nitriles to give (5)in a reaction thought to involve addition directed by the oxazoline to produce (6) followed by cyclization to (7) and ring- opening.' The reaction of (8) with lead tetra-acetate provides the formal equivalent of 1,4-dibenzyne which is trapped by double addition to dienes in relatively good yield.This method is somewhat more tolerant of functionality in the diene than dehalogena- tion of tetrahalogenoarenes with an organolithium and with unsymmetrical dienes leads predominantly to one regioisomer e.g. (9) from 3-bromofuran." Thermal decomposition of ( 10) provides a convenient source of 3,4-didehy- dropyridine. This has been trapped by (11) to provide after decarboxylation a ' K. Shankaran and V. Snieckus Tetrahedron Left. 1984,25 2827. A. 1. Meyers and P. D. Pansegrau Tetrahedron Lett. 1984 25 2941 H. Hart and D. Ok Tetrahedron Lett. 1984 25 2073. 81 Arynes Carbenes Nitrenes and Related Species I N."*\\ N Brm Rr simple route to elliptycene (12) ;unfortunately the cycloaddition shows no regioselec- tivity and isoelliptycene is also obtained." Trapping of (13) with the pyridyne also provides a route to elliptycene though in this case isoelliptycene is the major product and overall yields are 10w.l~ Me Me I Me S02Ph Me MNDO calculations on all didehydro-derivatives of pyridazine pyrimidine and pyrazine suggest that these species generally resemble benzyne but there are some interesting differences. For example the two o-pyridazynes are predicted to be of very different energies and several of the species are expected to ring-open or lose nitrogen relatively easi~y.'~ 3Carbenes A re-examination of the reaction of singlet methylene with dialkyl ethers has been carried out using CD2N2.It is found that in addition to insertion of the carbene into various C-H bonds about 10% of the product consists of methyl alkyl ethers and dimethyl ether.This indicates that carbene attack takes place on the ether oxygen to produce ylides which are protonated by traces of methanol or water and then dea1k~lated.l~ The addition of difluorocarbene to propene has been examined by ab initio methods using the 3-21 G basis set. Two equal-energy transition structures have I' C. May and C. J. Moody J. Chem. SOC. Chem. Commun. 1984 926. 12 G. W. Gribble M. Saulnier M. P. Sibi and J. A. Obaza-Nutaitis J. Org. Chem. 1984 49 4518. l3 M. J. S. Dewar and D. R.Kuhn J. Am. Chem. SOC.,1984 106 5256. 14 G. A. Olah H. Doggweiler and J. D. Felberg J. Org. Chem. 1984 49 2116. 82 M. S. Baird been located with the fluorines approaching syn-or anti-to the methyl group; an activation energy 1.3 kcal mol-' lower than that for addition to ethene is f~und.'~ While calculations show that difluorocarbene does form a weakly bound complex with ethene as an intermediate in cyclopropanation no stable complex is predicted in the addition of dichlorocarbene.'6 Where complexes are found they are not likely to energy minima at room temperature and ether or hydrocarbon solvents should be better Lewis base co-ordinators than the alkenes. The conclusion that stable ncomplexes are not formed between reactive carbenes and alkenes requires a new explanation of negative activation energies and of entropy control of carbene selectivity.A model relating the position of the transition state in additions of :CX2 (X = F C1 Br) to alkenes to AH and -TAS has been presented and this shows that the AG maximum which corresponds to the transition state for reaction does not correspond to the AH maximum. In contrast to the .rr-complex model this one predicts that reactions of :CC12 :CClBr and :CBr2 will be unselective and diffusion- controlled at low temperature." The use of diazirines as sources of carbenes has continued to provide a wealth of interesting results. Fluorophenylcarbene generated from photolysis of the corre- sponding diazirine shows a relative reactivity very similar to that observed when the carbene is generated by an a-elimination from PhCHBrF in the presence of a crown ether.'* Chlorophenylcarbene generated by thermolysis of the diazirine is trapped with about equal efficiency by both vinyl ethers and a,p-unsaturated esters.The rate of nitrogen evolution is not markedly dependent on the alkene and cyclopropane formation is remarkably free of competing reactions such as formal C-H insertion. Reaction with dimethyl fumarate leads to a single cyclopropane but with dimethyl maleate the same (E) product is observed together with the two possible 2-diesters. The authors propose a mechanism involving formation of a nucleophilic ylide between the maleate (B) and the carbene followed by Michael addition of this to maleate to produce (14) and cyclization of this with elimination of (B).19 Phenylbromodiazirine reacts with a variety of primary and secondary amines at 25 "C to produce aminophenylcarbenes (19 probably through the corresponding aminophenyldiazirine.When R = H the product is an imine presumably formed by a 1,2-hydrogen shift but when R = R' = alkyl the product is (16) derived by a formal insertion of the carbene into the N-H bond of a second molecule of amine. The carbenes can also be trapped by intramolecular insertion into N-H or 0-H bonds when these are 5,6-related to the carbene centre; thus reaction of the diazirine with N-methylethanolamine leads to (17). Although the philicities of these species remain to be determined their behaviour in 1,2-shifts and insertions is typical of electrophilic carbenes.20 Irradiation of 3-chloro-3-methoxydiazirinein an argon matrix at 10 K leads to a photolabile product which is characterized as methoxychlorocarbene and which decomposes to acetyl chloride ketene and HCl ;this is in marked contrast to normal N.G. Rondan and K. N. Houk Tetrahedron Lett. 1984,25 5965. 16 K. N. Houk N. G. Rondan and J. Mareda J. Am. Chem. SOC., 1984 106 4291. I' K. N. Houk and N. G. Rondan J. Am. Chem. Soc. 1984 106,4293. 18 R. A. Moss and W. Lawrynowin 1. Org. Chem. 1984 49 3828. 19 M. P. Doyle J. W. Terpstra and C. H. Winter Tetrahedron Lett. 1984 25 901. 20 R. A. Moss D. P.Cox and H.Tomioka Tetrahedron Lett. 1984 25 1023. Arynes Carbenes Nitrenes and Related Species Ph 'C /R-N I R' PhCH(NRR') H N I Me (16) (17) thermal decomposition of the carbene.In a 3-methylpentane glass at 80 K (18) and (19) are formed.2' The photochemical or thermal decomposition of 3-chloro-3-benzyldiazirine leads to benzylchlorocarbene which either rearranges to E -and Z-chlorostyrenes or reacts with the environment. In the presence of acetic acid the main product is l-chloro-2- phenylethyl acetate and experiments with ['H,]acid show that some styrene is formed from the carbocation ;however cyclopropanation of 2,3-dimethylbut-2-ene shows that the carbene is formed even in the presence of the acid.22 Photolysis of the diazirine in the presence of alkenes leads to cyclopropanes as well as to the isomeric chlorostyrenes.An analysis of the E/Z ratio in the presence of alkene and the fact that the ratio falls in the presence of electron-withdrawing groups on the aromatic ring have been used to analyse the stereochemistry of the migration while the non-linear dependence of product distribution upon the alkene concentra- tion has been interpreted in terms of an intermediate in the addition of the carbene to the alkene.23 However a re-analysis of the kinetic equations presented in this paper has shown that in fact the results provide no evidence that there is a carbene-alkene complex!24 Singlet arylchlorocarbenes rearrange to E -and 2-p-chlorostyrenes but do not undergo phenyl migration -unlike 1,2-diphenylethyl- idene. In ethanol an additional product is the acetal(20; R = Et) which is derived from the product of insertion of the carbene into the 0-H bond.Ratios of alkenes to acetal increase as the p-aryl substituent changes from C1 to H to Me in support of a mechanism for hydrogen migration which involves considerable hydride charac- ter.25 The presence of ethanol also changes the E/Z ratio of Llkenes produced probably due to differences in the reactivity towards alcohols between the carbene conformations leading to each alkene. Relative yields of alkenes decrease at low temperature but increase sharply as the solid state is rea~hed.~' Trapping of ben- zylchlorocarbene by methanol which leads to (20; R = Me) has been shown to be termolecular with a frequency factor of 2 x lo5l2 mo1-2 s-' and an activation energy 21 R.S. Shendan and M. A. Kasselmayer,J. Am. Chem SOC.,1984 106 436. 22 M. T. H. Liu N. H. Chishti M. Tencer H. Tomioka and Y. Izawa Tetrahedron 1984,40 887. 23 H. Tomioka N. Hayashi Y. Izawa and M. T. H. Liu J. Chem. Soc. Chem. Commun. 1984 476; J. Amer. Chem SOC.,1984 106,454. 24 P. M. Warner Tetrahedron Lett. 1984 25 4211. 25 H. Tomioka N. Hayashi Y. Izawa and M. T. H. Liu Tetrahedron Lett. 1984. 25. 4413. 84 M. S. Baird of -4.5 kcal mol-'. The ratio of inter- to intra-molecular products does not vary linearly with methanol concentration and once again this has been interpreted in terms of a model in which a complex is formed reversibly between the carbene and two methanol molecules in the oligomer chain.26 Photolysis of (21) leads to a highly reactive carbene capable of C-H or 0-H bond insertion; the use of this diazirine in photo-affinity labelling is being examined." Ultra-violet excitation of diphenyldiazomethane leads to singlet diphenylcarbene from the excited state of the diazo-compound.Energy relaxation occurs through inter-system crossing to the triplet. The rate of this process has been measured using laser-induced fluorescence and is found to be strongly dependent on solvent polarity. The rate is higher in less polar solvents and there is a linear correlation between log(rate) and the empirical solvent polarity parameter ET(30).The singlet is more polar and is stabilized by interaction with the solvent; this infers that the rate of inter-system crossing increases as the S-T energy gap increases.28 The triplet carbene adds to ring-substituted styrenes as an ambiphile -reacting more rapidly with both electron-poor and electron-rich styrenes than with styrene itself presumably due to stabilization of a diradical intermediate by the aryl Diphenylcarbene also reacts with oxygen at a rate of 5 x lo9 M-'s-' at 300 K leading to an intermediate with an absorption maximum at ca.410nm which is believed to be the carbonyl oxide (22) the precise structure of which is unknown.30 CO,H I Ph,COO (22) The singlet carbene (23) inserts into 0-H bonds of alcohols to produce ethers whereas the triplet forms cyclopropanes in a non-stereospecific manner with alkenes and abstracts hydrogen from hydrocarbons and alcohols. Kinetic and product analysis shows that singlet-triplet crossing is slow compared with most bimolecular reactions of the triplet unlike the situation with fluorenylidene.In this case this is believed to be a consequence of a larger S-T gap.3' The chemistries and kinetics of six other diarylcarbenes in polycrystalline methanol at 77 K have been studied by means of e.s.r. and isotope effects for reaction with deuterated solvent. The singlet-triplet separation decreases in the order (24) diphenylcarbene (25) 1-naphthylcarbene fluorenylidene and (26).32 26 M. T. H. Liu and R. Subramanian J. Chem. SOC.,Chem. Commun. 1984 1062. 21 M. Nassal J. Am. Chem. SOC 1984 106 7540. 28 E. V. Sitzmann J. Langan and K. B. Eisenthal J. Am. Chem. SOC.,1984 106 1868. 29 H.Tomioka K. Ohno Y. Izawa R. A. Moss and R. C. Munjal Tetrahedron Lett. 1984 25 5415. 30 N. H. Werstiuk H. L. Casal and J. C. Scaiano Can. J. Chem. 1984 62 2391. 31 S. C. Lapin B.-E. Brauer and G. B. Schuster J. Am. Chem. SOC.,1984 106 2092. 32 B. B. Wright and M. S. Platz J. Am. Chem. SOC.,1984 106 4175. Arynes Carbenes Nitrenes and Related Species mw / / ? Carbene (27) generated by flash thermolysis of the tosyl hydrazone sodium salt in the gas phase is largely converted into 1- and 2-methylanthracenes through trapping of the corresponding anthrylcarbenes. In the condensed phase (27) can be trapped by addition to alkenes while (28) -an intermediate in the formation of the rearranged carbenes -may be trapped by [4 +21 cycloaddition.It is suggested that the singlet and triplet species (27) and cyclopropene (28) are in eq~ilibrium.~~ The use of excimer laser flash photolysis in studying the kinetics of carbene reactions has been reviewed.34 Laser flash photolysis of diphenyldiazomethane leads to triplet diphenylcarbene which may be characterized by an absorption at 314 nm. The carbene abstracts a hydrogen atom from cyclohexane toluene or cyclopentane and the observed activation energies for these processes are much lower than those extrapolated from matrix experiments. The results clearly show the radical nature of the carbene which is found to be much more reactive than the diphenylmethyl radical.35 The activation energy for reaction of triplet diphenylcarbene with methanol has been measured in various solvents; e.g.the figure in acetonitrile is 1.66 * 0.20 kcal mol-'. The results are inconsistent with earlier interpretations and may suggest a mechanism involving thermal triplet-singlet equilibrium followed by reaction of the latter with methanol.36 The singlet and triplet states of dimesitylcarbene show quite different chemistries the former reacting with cg. methanol and cyclohexa-l,3-diene while the latter 33 A. Hackenberger and H. Dun,Chem. Ber. 1984 117 2644. 34 D. Griller A. S. Nazran and J. C. Scaiano Acc. Chem. Res. 1984 17 283. 35 L. M. Hadel M. S. Platz and J. C. Scaiano J. Am. Chem. SOC.,1984 106 283. 36 D. Griller A. S. Nazran. and J. C. Scaiano. 1.Am. Chem. Soc. 1984 106 198. M. S.Baird dimerizes to alkenes or reacts with oxygen. The steric effect of the o-methyl groups has a considerable effect on this chemistry the triplet state being orders of magnitude less reactive to standard substrates than those of diphenylcarbene or fluorenylidene while the singlet shows all the normal reactivity expected of a diarylcarbene. Although singlet to triplet intersystem crossing is very efficient the reverse process is not and the sharp differences in the chemistries of the two species suggest a substantial free-energy separation between them.37 A series of carbenes (29) has been generated by photolysis of the corresponding diazo-compound in paraffin matrices at 93 K. When n = 9,11 or 12 persistent e.s.r. signals are observed but with n = 8 or 10 no signals are seen.The e.s.r. characteristics have been correlated with the shapes of the carbene~.~~ Laser flash photolysis of 9-diazofluorene leads to a transient species characterized as the triplet carbene which is believed to be in thermal equilibrium with the singlet -their energies being within a few kcals of each other. The singlet is frequently more reactive and tends to dominate the chemistry; the lifetime of this is s5 ns. In nitrile solvents a nitrile ylide is formed which reacts rapidly with electron-deficient alkene~.~~ Photolysis of 9-diazofluorene in trans-1,2-dichloroethene leads to >92% stereospecific addition; the stereoselectivity of addition to the cis-isomer is <86'/0. In the presence of styrene or butadiene the selectivity increases and absolute and relative yields of 9-( 2,2-dichloroethylidene)fluorene,a rearrangement product from a diradical intermediate fall.It is suggested that both singlet and triplet species are present and the former undergoes stereospecific cis-addition and the latter leads to mixtures of stereoisomeric products as well as to diradical rearrangement. The S-T interconversion is rapid compared to reactions with dichloroethene and methanol but not as rapid as triplet scavenging by styrene or butadiene. The effect of added hexafluorobenzene may be to form a carbenoid which mimics triplet fl~orenylidene.~' Irradiation of 2-halogeno- 1,3-diphenylindenyl anions causes loss of halide ion to produce an intermediate best described as the carbene (30) or the related allene form; this undergoes ready C-H insertion addition to electron-rich alkenes and halide exchange!l The heterocyclic carbene (31) has been generated thermally or photochemically from the corresponding diazo-compound ; it inserts into C-H bonds e.g.in cyclohexane in competition with hydrogen ab~traction.~~ Ph 37 A. S. Nazran and D. Griller J. Am. Chem. SOC.,1984 106 543. 38 R. Alt H. A. Staab H. P. Reisenauer and G. Maier Tetrahedron Lerr. 1984 25 633. 39 D. Griller L. Hadel A. S. Nazran M. S. Platz P. C. Wong T. G. Savino and J. C. Scaiano J. Am. Chem. SOC.,1984 106 2227. 40 P. P. Gaspar C.-T. Lin B. L. W. Dunbar D. P. Mack and P. Balasubramanian J. Am. Chem. SOC. 1984 106 2128. 41 L. M. Tolbert and S. Siddiqui J. Am.Chem. SOC.,1984 106 5538. 42 M. Nagarajan and H. Shechter J. Org. Chem. 1984 49 62. Arynes Carbenes Nitrenes and Related Species Pyrolysis of (32) leads to cyclopropenylidene which is stable in an argon matrix at 10 K and has a singlet ground-state with the aromatic ylide structure (33) making a considerable contribution. Irradiation of the carbene in a matrix leads to propyny- lidene (34).43 (32) (33) (34) (35) There has been continued interest in the chemistry of vinylmethylenes and in their relationship to cyclopropenes. Irradiation of the four diazo-compounds (35 ; R' or R2or R3or R" = Me others = H) in matrices at 10-15 K leads to the four corresponding carbenes which are shown by e.s.r. to have triplet ground-states. Photolysis of p-and rn-tolylcarbenes leads to methylcyclohepta- 1,2,4,6-tetraenes with no evidence for bicyclo[4.1 .O]hepta-2,4,6-trienes.The o-tolylcarbene leads largely to o-xylylene though a small amount of l-methylhepta-l,2,4,6-tetraene is formed; the latter is in turn photolysed to phenylmethylcarbene and thence to styrene.44 Carbon and deuterium labelling studies of the rearrangement of benzocyc- lobutene to styrene at high temperatures have also suggested the formation of intermediate o-tolylcarbene and methyl~ycloheptatetraene.~~ Extensive ab initio studies of the C3H4 energy-surface suggest that the allene to methylacetylene rear- rangement should proceed through vinylmethylene cyclopropene and prop-1 -eny- lidene. The 3A" states of trans and cis vinylmethylenes are shown to have the allylic structure (36) and to be isoenergetic and 46 kcal mol-' above the ground state of methyla~etylene."~ Thermolytic or photolytic ring-opening of a range of aryl- and heteroaryl-substituted cyclopropenes has been rep~rted"~ (for example see Scheme 2).Moreover 1,2-dichloro-3,3-dimethylcyclopropene undergoes ring-opening to car- bene (37; X = Y = C1) even at temperatures below ambient and can be trapped Ph Scheme 2 43 H. P. Reisenauer G. Maier A. Riemann and R. W. Hoffmann Angew. Chem. Znf. Edn. Engl. 1984 23 641. 44 0. L. Chapman R. J. McMahon and P. R. West J. Am. Chem. Soc. 1984 106 7973. 45 0. L. Chapman and U.-P. E. Tsou J. Am. Chem. SOC.,1984 106 7974; W. S. Trahanovsky and M. E. Scribner ibid.7976. 46 N. Honjou J. Pacansky and Y. Hoshimine J. Am. Chem. Soc. 1984 106 5361. 47 A. Padwa U. Chiacchio A. Compagnini A. Corsaro and G. Purrello J. Chem. SOC.,Perkin Trans. 1 1984 2671; A. Padwa M. J. Pulwer and R. J. Rosenthal J. Org. Chem. 1984,49 856; A. Padwa and G. D. Kennedy ibid. 4344; 3113; A. Padwa L. A. Cohen and H. L. Gingrich J. Am. Chem. !984, 106,1065. ' 1 8' Q M. S. Baird readily with alkenes,48 while 1 -bromo-2-chloro-3,3-dimethylcyclopropene (38) opens to both carbenes (37; X = Br Y = C1) and (37; X = C1 Y = Br) at 10 "Cand reacts with methyl lithium at -70 "C in the presence of alkenes to give products apparently derived by trapping of the carbene (39) or a related carbenoid. Labelling studies indicate that C-2 of the cyclopropene becomes the central allenic carbon of (39).49 Reaction of oligohalogenoprop- 1-enes with base provides a convenient source of the corresponding oligohalogenovinylcarbene,50 while reaction of 1,3,3-trichloroprop-1-yne with potassium t-butoxide leads to 3,3-dichloropropa-l,2-dien-1-ylidene;the precursor acetylene may be obtained in situ by dehydrohalogenation of 1,1,3,3- or 1,2,3,3-tetrachloroprop-l-enes or of 1,1,2,3,3-penta~hloropropane.~~ The carbene selectivity index for perchlorovinylcarbene has been found to be ca.0.38 whereas a figure of 0.34 f 0.1 is calculated from the Moss equation; however it is certainly an electrophilic carbene.'* Photolysis of the 5-nitro-3H-pyrazole (40) in furan leads to (41) (42),and (43). The furan (42) appears to be derived by cycloaddition of the nitrovinylcarbene (44) to furan while (41) is the result of ring closure of the carbene to the corresponding cyclopropene followed by cycloaddition.It is felt that the most likely origin of the aldehyde is a rearrangement of an intermediate cyclopropane derived from the carbene and furan. Photolysis of the pyrazole (45) leads to open-chain oximes which are explained in terms of oxygen transfer from the nitro-group to an intermediate carbene (46) followed by rea~~angement.'~ Ngo2 (45) 48 M. S. Baird S. R. Buxton and J. S. Whitley Tetrahedron Lett. 1984 25 1509. 49 M. S. Baird Tetrahedron Lett. 1984 25 4829. 50 W. Gothling S. Keyaniyan and A. deMeijere Tetrahedron Lett. 1984 25 4101. 51 S.Keyaniyan W. Gothling and A. deMeijere Tetrahedron Lett. 1984 25 4105. 52 R. Kostikov and A. de Meijere J. Chem. SOC.,Chem. Commun. 1984 1528. 53 M. Franck-Neumann and M. Miesch Tetrahedron Lett. 1984 25 2909. Arynes Carbenes Nitrenes and Related Species Photolysis of (47) in wet benzene in the presence of 2,3-dimethylbut-2-ene leads to the cyclopropene (48). This is thought to be the result of formation of (49) which cyclizes to the corresponding cyclopropene form and is then hydroly~ed.’~ There are also several reports of differently substituted vinylmethylenes obtained from photolysis of unsaturated epoxides undergoing ring-closure to cyclopropenes e.g. (50)leads to (51).” (50) There has also been continued interest in rearrangement of cycloalkyl carbenes.The effect of alkyl and phenyl substituents on the rearrangement of cyclopropylidenes (52) generated from the corresponding dibromides has been examined. When R’ and R6 are alkyl groups the formation of cyclopentadienes from the rearranged carbene (53) is suppressed and acyclic allenes pred~minate.~~ An analysis of the kinetics of decomposition of the diazo-compound (54) shows that there is a very low barrier to rearrangement of the carbene (55) to (56) in support of earlier work proposing this rearrangement,” while carbene (57) also undergoes a 1,2-alkyl shift leading eventually to dihydr~pentalenes.’~ Temperature and solvent effects in the trapping of the carbenes (55) and (56) by methanol are interpreted in terms of reversible formation of an ylide for the former but of a different mechanism for the latter.59 (54) (55) (56) (57) 54 S.Wolff and W. C. Agosta 1. Am. Chem. SOC. 1984 106 2363 55 A. O’Sullivan B. Frei and 0.Jeger Helu. Chim. Acta 1984 67 815; A. Siewinski B. Henggeler H. R. Wolf B. Frei and 0.Jeger ibid. 120; A. Pascual T. Nishio B. Frei and 0.Jeger hid 129. 56 K. H. Holm and L. Skattebdl Acta Chem Scand. 1984 38 783. 57 P. Warner and I.-S. Chu J. Org. Chem 1984 49 3666. 58 U. H. Brinker and K. Lothar Chem. Lett. 1984,45. 59 P. M. Warner and I.-S. Chu J. Am. Chem SOC. 1984 106 5366. 90 M.S. Baird A full account has also appeared of the use of intramolecular insertion of cyclopropylidenes into C-H bonds in the synthesis of ishwarane aAd ishwarone together with a discussion of insertions in several model systems.60 Intermolecular additions of cyclobutylidene generated by reaction of 1,l-dibromocyclobutane with methyl lithium or from diazocyclobutane have also been reported.61 Reaction of N,N-dialkylated pyruvamides (58) with diethyl (diazomethy1)phos- phonate (59) leads to either (60) or (61) by insertion of an intermediate methylenecar- bene (62) into a 5,6-related C-H bond or a 1,2-shift.Although the reactions were carried out in methanol no intramolecular insertion into the 0-H bond was observed. Moreover in the intermolecular insertion primary C-H bonds appeared to be the most reactive probably reflecting conformational factors leading to pre- ferred orientation of the amide with primary bonds syn to the diazoalkenyl-group of the intermediate.62 R 0 II TR Me-C-C-N A LRI (EtO),P(O)CHN (59) /p Me-CGC-C \ Me%l!l-R' R' 0 The phosphonate route has also been used to generate (63) which rearranges to cyclopentyne and is trapped in a 99% stereospecific [2 + 21 cycloaddition with cis-1-meth~xyprop-l-ene.~~ It is interesting to note that a cyclobutyne (64) is reported to undergo the reverse rearrangement to produce the methylene carbene (65).6" Mide formation is presumably involved in the formation of (66; X = Se Te) from Me,C=C and diphenyldiselenides and ditell~rides,6~ while a N-ylide (67) may well be involved in the transformation of azobenzenes to 2H-indazoles e.g.(68) in reasonable yield on reaction with the same carbene."6 [2H]-Labelling implicates S-ylides in the intramolecular reactions of o-alkylthiophenylcarbenesin the gas phase.67 (63) (64) (65) 60 R.M. Cory L. P. J. Burton D. M. T. Chan F. R. McLaren M. H. Rastall and R. M. Renneboog Can. J. Chem. 1984 62 1908. 61 U. H. Brinker and M. Boxberger Angew. Chem. Int. Edn. Engl. 1984 23,974. 62 J. C. Gilbert and B. K. Blackburn Tetrahedron Lett. 1984,25 4067. 63 J. C. Gilbert and M. E. Baze J. Am Chem. Soc. 1984 106 1885. 64 K.-D. Baumgart and G. Szeimies Tetrahedron Lett. 1984 25 737. 65 P. J. Stang K. A. Roberts and L. E. Lynch J. Org. Chem 1984 49 1653. 66 K. Krageloh G. H. Anderson and P. J. Stang J. Am. Chem. Soc. 1984 106 6015. 67 W. D. Crow and Y.T. Pang Aust. J. Chem. 1984 37 1903. Ayes Carbenes Nitrenes and Related Species Reaction of P-hydroxyselenides (69; R = R’ = alkyl) with thallium ethoxide and chloroform leads to rearranged ketones in a reaction thought to involve dichlorocarbene.68 When R = H the product is the epoxide (70),formed in a stereospecific reaction with inversion at the carbon-bearing selenium.69 Presumably both reactions involve initial ylide-formation between selenium and dichlorocarbene. In the same way optically pure P-ethanolamines are converted into epoxides by reaction with dichlororocarbene in a reaction which proceeds in high yield and with high enantiomeric excess.7o The effect of added ethanol on the efficiency and rate of trapping of dibromocarbene generated under phase-transfer conditions has been disc~ssed.~’ Reaction of the carbene under these conditions with trans-cyclo- octene leads to partial isomerization to the cis-isomer.This has been explained in terms of the formation of an intermediate between the carbene and the trans-alkene which can revert to ~is-alkene.~~ Other unusual reactions to be reported are the apparent 1,4-addition of dibromocarbene to 1,2-dimethylene~ycloheptane,~~ and the formation of (7 1) from quadricyclane and bis(ethoxycarbony1)carbene which has been formulated as an addition of a carbene to two u-bond~.~~ (69) X = Se (71) 4 Nitrenes Ab initio calculations of the reaction of hydrogen with the lowest singlet-state of :NH are rep~rted.~’ Similar calculations on the vinyl azide to 2H-azirine conversion indicate a very weak bond between vinyl nitrene and nitrogen parts of the molecule; a singlet planar nitrene is predicted to cyclize with no activation barrier to give the a~irine.~~ The photochemistry of phenylazide has been examined using laser flash photolysis in inert and nucleophilic solvents.Direct and triplet-sensitized irradiation produces J. L. Laboureur and A. Krief Tetrahedron Lett. 1984 25 2713. 69 J. L. Laboureur W. Durnont and A. Krief Tetrahedron Lett. 1984 25 4569. 70 L. Castedo J. L. Castro and R. Riguera Tetrahedron Lett. 1984 25 1205. 71 E. V. Dehrnlow and J. Wilkenloh J. Chem. Res. 1984 396. 72 E. V. Dehrnlow and R. Kramer Angew. Chern. Int. Edn. Engl. 1984 23 706. 73 L.W. Jenneskens F. J. J. DeKanter L. A. M. Turkenburg H.J. R. DeBoer W. H. DeWolf and F. Bickelhaupt Tetrahedron 1984 40,4401. 74 M. L. Tetef and M. Jones Tetrahedron Lett. 1984 25 161. 75 T. Fueno 0.Kajirnoto and V. Bonacic-Koutecky J. Am. Chem. SOC.,1984 106 4061. 76 T. Yamabe M. Kaminoyarna T. Minato K. Hori K. Isomura and H. Taniguchi Tetrahedron 1984 40.2095. M. S. Baird triplet phenyl nitrene but the reaction products depend dramatically on the con- centration of the azide and the intensity of the irradiation. In the direct irradiation a relatively long-lived singlet precursor of triplet nitrene is also observed; this species is probably the dehydroazepine (72). Formation of triplet nitrene from the transient species is relatively slow compared to other reactions it can undergo.77 Quantum yields for the disappearance of phenyl azide on photolysis in acetonitrile increase exponentially with log(concentration) and can reach three thousand.This provides evidence for a branching chain-reaction which is thought to result from reaction of a phenyl nitrene intermediate with ground-state phenylazide to form two nitrene~.~* An analysis of the photochemistry of 1-and 2-naphthylazides and 1-and 2-pyrenylazides in benzene and diethylamine has also been carried out using product analysis and laser flash spectroscopy. In each case two intermediates were character- ized and identified as the triplet nitrenes and ground-state singlet azirines. The key to the photochemistry seems to be the relative energies of the singlet nitrene and these two species.When the singlet-to-azirine separation is small the lifetime of the azirine is short and its reactions with nucleophiles do not compete efficiently with triplet formation. When the singlet-triplet separation is small inter-system crossing is reversible and singlet species have longer lifetimes than triplets leading to efficient trapping by nu~leophiles.~~ The carbamates (73; X = H Y = C1) and (73; X = C1 Y = H) react with sodium hydride in dimethylformamide at 70°C to give the same azo-compound (74). This has been explained in terms of the formation of the common anion (75) which ring-opens selectively to the more stable nitrene (76).80 c1a N-cozEt N c1n:):Et The bis-azide (77) undergoes thermal decomposition to (78) and (79).The latter apparently arises through loss of nitrogen from an intermediate ylide (80) as indicated followed by ring cleavage with loss of ethyne.81 77 A. K. Schrock and G. B. Schuster J. Am. Chem. Soc. 1984 106 5228. 78 C. H. L. Go and W. H. Waddell J. Am. Chem. Soc. 1984 106 715. 79 A. K. Schrock and G. B. Schuster J. Am. Chem. SOC.,1984 106 5234. R. K. Smalley and A. W. Stocker Tetrahedron Lett. 1984 25 1389. 81 C. J. Moody C. W. Rees and S. C. Tsoi J. Chem. Soc. Perkin Trans. I 1984 915. Arynes Carbenes Nitrenes and Related Species dN \/ When o-azidobiphenyl is photolysed in acetonitrile in the presence of tetracyanoethylene the major product is carbazole derived from the phenylnitrene but a considerable amount of (81) apparently derived by trapping of a 2-azacyc- loheptatrienylidene is also obtained.The formation of the carbene may be explained in terms of a rearrangement of phenylnitrene. Thermolysis of the product leads to carbazole apparently by cheletropic elimination of tetracyanoethylene followed by a rearrangement in the opposite sense -from carbene to nitrene.82 Laser flash photolysis has been used to examine the absolute rate of formation of (82) by intramolecular addition of the corresponding nitrene to the fluorene ring-sy~tem,~~ while spray pyrolysis of aryl 2-azidobenzoates leads to carbazoles in a reaction apparently involving spiro-intermediates (Scheme 3). Scheme 3 H The corresponding benzyl 2-azidobenzoates are converted into (83) in a reaction thought to involve an insertion of a nitrene into the C-0 bond.84 Photolysis of the acylazide (84) which is readily derived from 19,20-dihydromutilin leads to nitrene insertion into the C-H bond indicated despite the proximity of the methyl group at ~-9.~' 82 S.Murata T. Sugawara and H. Iwamura J. Chem. SOC.,Chem. Commun. 1984 1198. 83 S. Murata T. Sugawara N. Nakashima K. Yoshihara and H. Iwamura Tetrahedron Lett. 1984,25,1933. 84 M. G. Clancy M. M. Hesabi and 0.Meth-Cohn J. Chem. SOC.,Perkin Trans. 1 1984 429. 85 H. Berner H. Vyplel G. Schulz and P. Stuchlik Tetrahedron 1984 40,919. 94 M. S. Baird Photolysis of p-toluenesulphonyl azide in p-xylene leads primarily to products derived from insertion reactions of the corresponding nitrene.An intermediate is formed which decomposes both to the insertion products and to p-toluene sul- phonamide. Photolysis of the ground-state charge-transfer complex between the azide and aniline leads to the product of nitrene insertion into the solvent (among others) giving evidence for nitrene production from an excited charge-transfer complex.86 First-order rate constants for the thermolysis of a series of sulphonyl azides bearing nucleophilic neighbouring groups reveal no anchimeric assistance and are best interpreted in terms of rate-limiting formation of s~lphonylnitrenes.~’ Solution pyrolysis of 3-arylpropanesulphonyl azides in Freon 113 leads to 7-membered ring sultams e.g. (85) while in hydrocarbon solvents hydrogen-abstraction and solvent- insertion products are observed.An interesting rearrangement is seen with 3-(2,6- dichloropheny1)propane sulphonyl azide in which the product (85; X = C1) has the chlorines para-related.88 0 Y x N P On heating to 80-120 “C the sulphenamides (86) decompose to naphthalene and arenesulphenylnitrenes which can be trapped by alkenes in quantitative yield. This method is more efficient than the oxidation of arenesulphenamides although both routes apparently proceed through the same intermediate r~itrene.~~ The kinetics of thermal decomposition of benzenesulphinyl azide is first order and consistent with the formation of a dipolar sulphinylnitrene intermediate.” The highly non-selective nitrenes (87) are unusual in that they show no tendency to undergo intramolecular reactions and instead are presumably captured on almost 86 C.E. Hoyle R. S. Lenox P. A. Christie and R. A. Shoemaker 1. Org. Chem. 1983 48 2056. ” S. P. McMinus M. R. Smith R. A. Abramovitch and M. N. Offor 1. Org. Chem. 1984,49 683. 88 R. A. Abramovitch A 0. Kress S. P. McManus and M. R. Smith J. Org. Chem. 1984,49 3114. 89 R. S. Atkinson M. Lee and J. R. Malpass J. Chem Soe. Chem. Commun. 1984,919. 90 T. J. Maricich C. N. Angeletakis and R. Mjanger 1. Org. Chem. 1984 49 1928. Arynes Carbenes Nitrenes and Related Species every collision in intermolecular processes. The collision frequencies apparently overcome the normal entropic effect favouring intramolecular proces~es.~' N-Nitrenes (88) derived -by oxidation of the corresponding amines undergo intramolecular addition to the double bonds and the effect of various R and R' groups on the competiGve addition suggests a non-concerted reaction through a 7-membered transition-state with the nitrene behaving as an ele~trophile.~~ When the double bonds are replaced by a 2-arylethyl group the product is for example (89) apparently derived by addition of the nitrene to the benzene ring followed by ring-opening and hydrogen transfer.93 Me0 Although singlet ethoxycarbonylnitrene adds to cyclohexene reasonably efficiently it does not attack the annelated sulphone (go) or several related molecules under the same conditions.The effect is apparently not due to interception of the nitrene by the sulphone group itself but may reflect the unusually high ionization potentials of these corn pound^.^^ The nitrene derived by (Y -elimination adds to allylic ethers to form aziridines whereas the corresponding nitrenium ion (EtOCONH+) forms P-amino-alcohol derivative^.'^ 91 R.Breslow F. Herman and A. W. Schwabacher J. Am. Chem SOC.,1984 106 5359. 92 R. S. Atkinson J. R. Malpass K. L. Skinner and K. L. Woodthorpe J. Chem. SOC.,Perkin Trans. I 1984 1905. 93 R. S. Atkinson J. Fawcett N. A. Nawad D. R. Russell and L. J. S. Sherry J. Chem. Soc. Chem. Commun. 1984 1072. 94 R. A. Aitken J. I. G. Cadogan H. Farries I. Gosney M. H. Palmer I. Simpson and E. J. Tinley J. Chem SOC.,Chem Commun. 1984 791. 95 M. A.Loreto L. Pellacani P. A. Tardella and E. Toniato Tetrahedron Lett. 1984 25 4271. M. S. Baird 5 Silylenes Calculations using the 3-21G basis set show that abstraction of a hydrogen from methane by triplet silylene should be an endothermic reaction with a barrier of 32.6 kcal mol-' while abstraction of a hydrogen from silane should be nearly thermoneutral with a barrier of 15.9kcal mol-'. In contrast the abstraction of a hydrogen from silane by triplet methylene should be an exothermic process with a barrier of 9.1 kcal m01-l.~~ Laser flash photolysis of dodecamethylcyclohexasilane in a hydrocarbon solvent at 293 K leads to dimethylsilylene which has an absorption band at 350 nm that is quenched by the silylene scavenger triethylsilane and by methanol.No band is seen for the silylene at 450 nm as previously reported in matrices. The rate of quenching by methanol in dilute solution was 3.1 x lo7M-'s-'.~~ An analysis of the products from generation of the silylene at high temperature in the presence and absence of trapping agents has led to the conclusion that an equimolar equilibrium mixture of dimethylsilylene and HMeSi=CH2 is produced irrespective of the direction from which the equilibrium is appr~ached.~~ Potential energy surfaces for silylene insertion into a range of single bonds have been reported. The reaction with water is found to involve a complex with a fairly high rearrangement barrier while that with ammonia involves a very deep minimum with a rearrangement barrier of 38 kcal mol-' -suggesting that the addition complex may be a suitable candidate for spectroscopic dete~tion.~~ 96 M.S. Gordon J. Am. Chem. SOC.,1984 106 4054. 97 A. S. Nazran,J. A. Hawari D. Griller I. S. Alnaimi. and W. P. Weber J. Am. Chem. SOC.,1984,106,7267. 98 I. M. T. Davidson S. Ijadi-Maghsoodi T. J. Barton and N. Tillman J. Chem. SOC.,Chem. Commun. 1984 478. 99 K. Raghavachari J. Chandrasekhar M. S. Gordon and K. J. Dykema J. Am. Chem SOC,1984,106,5853.

ISSN:0069-3030    

DOI:10.1039/OC9848100079

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

6 Photochemistry By J. D. COYLE Chemistry Department The Open University Milton Keynes MK7 6AA 1 General A collection has appeared’ of short ‘state-of-the-art’ reports covering a wide range of physical and organic photochemistry each written by an active leader in the field. There is a continuing strong interest in the study of photochemical reactions carried out in a heterogeneous medium and it is suggested2 that examination of radical processes on silica or other porous surfaces using cage and magnetic effects can be used not only to study radical reactions but also to obtain useful information about the surfaces. In a synthetic application of heterogeneous photochemistry irradiation of ethyl cinnamate or benzalacetone (4-phenylbut-3-en-2-one) in a stirred suspension of aluminosilicate or a solid acidic material such as Nafion leads to a much higher proportion of the cis isomer than can be achieved in homogeneous solution;3 the effect is attributed to the different relative degree of adsorption by the two isomers.Titanium dioxide is widely used in different contexts as a heterogeneous promoter of photochemical reactions and a careful study4 of amorphous and colloidal forms suggests that the properties are not adequately explained by a semi-conductor model but better by postulating the existence of Ti”’ species. 2 Alkenes Irradiation of peduoronorbornadiene gives the corresponding bicyclo-[3.2.0]heptadiene but it is now revealed’ that at low temperature the expected quadricyclane is formed (Scheme l) which undergoes a sequential thermal iso- merization to perfluorocycloheptatriene;this is converted photochemically into the bicyclo[3.2.0]heptadiene or on treatment with boron trifluoride it yields the hepta- fluorotropylium cation.The copper( 1)-promoted internal photocycloaddition of myrcene leads to a bicyclo[3.2.0]heptane (1) as a significant product;6 this compound is not formed on direct photolysis nor on triplet-sensitization demonstrating again the usefulness of copper(1) in enhancing this mode of cycloaddition. A different use of a copper ‘Photochemistry Past Present and Future,’ ed. R. P. Wayne and J. D. Coyle J. Photochem. 1984 25 issue I. ’ N. J. Turro and Chen-Chih Cheng J. Am. Chem. Soc. 1984 106 5022. R. F. Childs B. Duffey and A.Mika-Gibala J. Org. Chem. 1984 49 4352. R. S. Davidson C. L. Morrison and J. Abraham J. Photochem. 1984 24 27. ’ W. P. Dailey and D. M. Lemal J. Am. Gem. Soc. 1984 106 1169. K. Avasthi S. R. Raychaudhuri and R. G. Salomon J. Org. Chem. 1984,49,4322. 97 J. D. Coyle hv I25°C 25 'C/ Scheme 1 reagent this time copper( 11) in conjunction with an electron-transfer photosensitizer is seen in the oxidative addition of methanol to certain cy~lopropylstyrenes,~ which gives ketones (2) derived by a mechanism that includes a 1,2-shift of the cyclopropyl group. p h ! hv,MeOH CU~',C,,H&CN) qB R (R= Me c-C,H,) O -55% A study of the photo-oxidation of 2,5-dimethylhexa-2,4-dieneby singlet oxygen highlights the considerable effect of solvent on the products observed.8 In methanol the major product (53%) is a 1,2-dioxetane; in acetonitrile the predominant pathway leads to the 'normal' allylic hydroperoxide (89O/0 ); however in benzene the major product (45%) is a different hydroperoxide (3) formed in a previously unreported vinylogous ene-reaction possibly by way of a non-polar singlet biradical.The use of electron-transfer photosensitizers for the ring-opening oxidation of cyclobutanes was described recently; now the same 1,2-dioxan products (4) are isolated in very high yields starting from the aryl-alkene rather than the cyclobutane.' ' K. Mizuno K. Yoshioka and Y. Otsuji J. Chem. SOC.,Chem. Commun. 1984 1665. K. Gollnick and A. Griesbeck Tetrahedron 1984,40 3235. K. Gollnick and A.Schnatterer Tetrahedron Lett. 1984 25 185 and 2735. Photochemistry 3 Aromatics The valence isomerizations of benzene continue to interest theoretical chemists and a MIND0/3 and ab initio study of the triplet benzene + benzvalene surface suggests that the prefulvene biradical may be a relatively stable species.” The aromatic ring photoisomerizations that find synthetic applications are more usually of systems such as 2-pyridones and such a photoreaction of a 4-acetoxypyridone (Scheme 2) gives a product that after catalytic hydrogenation and alkaline reductive hydrolysis provides in good yield an intermediate for carbapenam synthesis.” The photoreduc- tion of benz[ alanthracene in the presence of N N-dimethylaniline and sodium borohydride yields 6 1‘/o of the 7,12-dihydro derivative.I2 This method for reducing polycyclic aromatic hydrocarbons overcomes the problem of low solubility in ammonia that hinders the use of Birch reduction; the reducing agent is the tertiary amine and the sodium borohydride is said to serve as a scavenger to reduce the formation of by-products.Scheme 2 Little work has been reported with regard to photochemical electrophilic substitu- tion of aromatic compounds but an important study13 shows that tryptophan irradiated in D20 exchanges deuterium for hydrogen at qng carbon number 4 in high yield (96% 4 = 0.14). Attack occurs from the -NH3 substituent on the side-chain and the result is significant in understanding the mechanisms for non- radiative decay of tryptophan one of the amino-acids most commonly implicated in photochemical damage to proteins.An interesting rep~rt,’~ that deals with the much more widely investigated nucleophilic photosubstitution reactions of aromatic compounds shows that methylamine replaces the methoxy group meta to nitro in 1,2-dimethoxy-4-nitrobenzene as expected but dimethylamine replaces the methoxy group para to nitro (Scheme 3). The authors suggest that charge-density calculations provide a satisfactory explanation of reactivity for small nucleophiles such as ammonia methylamine or hydroxide ion but that with larger (‘softer’) nucleophiles frontier orbital considerations become important. lo S. Oikawa M. Tsuda Y. Okamura and T. Urabe J. Am. Chem Soc. 1984 106 6751.I’ C. Kaneko T. Naito and A. Saito Tetrahedron Left. 1984 25 1591. I’ N. C. Yang W.-L. Chiang and J. R. Langan Tetrahedron Lett. 1984 25 2855. l3 I. Saito H. Sugiyama A. Yamamoto S. Muramatsu and T. Matsuura J. Am. Chem. Soc. 1984,106,4286. J. Cervell6 M. Figueredo J. Marquet M. Moreno-Mahas J. Bertran and J. M. Lluch Tetrahedron Lett. 1984,25 4 147. 100 J. D. Coyle OMe ON""' No2 81% NO 68% Scheme 3 Chlorobenzene is hydrolysed to phenol (100%,$I = 0.1) when irradiated in dilute aqueous solution.' The reductive dehalogenation of halogenoaromatics (as well as vinyl or cyclopropyl halides) can be carried out with improved yield and efficiency when lithium aluminium hydride is present. l6 For example p-bromochlorobenzene gives chlorobenzene in 77% yield or more usefully 7,7-dichloronorcarane gives the 7-chloro compound in a similar yield.The related reductive decyanation of cyanoaromatics occurs by way of the aromatic radical-cation and this process is enhanced by the additon of a dialkyl ~u1phide.l~ One cyano group in 1,2- or 1,4-(but not 1,3-) dicyanobenzene can be replaced by a 2-methoxyalkyl group (Scheme 4) by irradiating with an alkene and methanol in acetonitrile so1ution.l8 The presence of phenanthrene as sensitizer improves the yield and efficiency which is consistent with a mechanism involving radical ions. Electron transfer is also invoked to account for the side-chain substitution in which toluenes are converted into benzyl nitrates (96% for the p-methyl system) on irradiation with cerium( IV) ammonium nitrate." A very extensive compilation of the photocyclization reactions of stilbenes and related compounds has been published.20 An unusual photochemical cyclization is I CN CN 70% Scheme 4 l5 A.Tissot P. Bode and J. Lemaire Chemosphere 1984 13 381. 16 N. Shimizu K. Watanabe and Y. Tsuno Bull. Chem. SOC.Jpn. 1984 57 885. l7 R. A. Beecroft R. S. Davidson D. Goodwin and J. E. Pratt Tetrahedron 1984 40,4487. I* R. M. Borg D. R. Arnold and T. S. Cameron Can J. Chem 1984 62 1785. 19 E. Baciocchi C. Rol G. V. Sebastiani and B. Serena Tetrahedron Lett. 1984 25 1945. 20 F. B. Mallory and C. W. Mallory Org React. 1984 30 I. Photochemistry 0 0 72‘10 Scheme 5 involved in the reaction (Scheme 5) that leads from phenacyl chloride to a 1-tetralone in the presence of an alkene and silver triflate.2’ 4 Carbonyl Compounds There have been several reports of interesting reactions that involve photochemical cleavage in a carbonyl compound.A mechanistic study22 of the replacement of aromatic aldehyde C-H by C-D on irradiation in D20 shows that the process is efficient ( = 0.95 and benzoin derivatives are formed in only small amounts) and occurs through the (n,T*)triplet state. The photolysis of glycylglycine or alanylgly- cine causes loss of both carbon dioxide and ammonia and the major organic product after thermal treatment is acetamide or propionamide (-50% isolated yield).23 These results together with the structures of minor products point to the involvement of intermediates obtained by electron transfer and not a-carbon radicals such as those detected in e.s.r.studies of irradiated peptides. Photochemical a-cleavage in cyclic ketones generally occurs on the more heavily substituted side of the carbonyl group but for a pair of epimeric tetracyclic cyclopentanones (Scheme 6) there is very strong control by the stereochemical c~nfiguration,~~ leading in one case to almost complete cleavage on the less substituted side. a-Cleavage is also involved in the reactions that lead from 6-(w-hydroxyalkyl)cyclohexa-2,4-dienonesto macrocyclic lactones (Scheme 7).25In some systems (e.g. R = H n = 4)dilactones rCOOMc Scheme 6 2’ T. Sato and K. Tamura Tetrahedron Lett.. 1984 25 1821. 22 A.Defoin R. Defoin-Straatmann and H. J. Kuhn Tetrahedron 1985 40,2651. 23 D. Birch J. D. Coyle R. R. Hill G. E. Jeffs and D. Randall 1.Chem. Soc,Chem. Commun.,1984,796. 24 J. M.Trendel and P.Albrecht Tetrahedron Lett. 1984 25 1175. 25 G. Quinkert G. Fischer U.-M. Billhardt J. Glenneberg U. Hertz G. Diirner E. F. Paulus and J. W. Bats Angew. Chem. Int. Ed. Engl. 1984 23,440. 102 J. D. CoyZe 0 hv(R = Me n = 9) DABCO Go 73 % / Scheme 7 (22-membered 30%) and trilactones (33-membered 6%) are formed by inter- molecular reaction of the intermediate ketene. Photochemical &cleavage can occur in carbonyl compounds where the a-/3 bond is relatively weak as with N-halogenoimides to give halogen atoms and imido radicals.N-Bromo- and N-chloro-succinimides give reasonable yields of photo- adducts such as (5) with cyclohexene by such a route.26 Extensive use has been made of the photochemical cleavage of N-acyloxypyridine-2-thiones for the decar- boxylation of N-protected amino-acids (Scheme 8);27 a modification of the pro- cedure allows halogen derivatives RBr to be formed. A 0 NHBoc e.g. R = CH / 78% 'CH2CH2SMe Scheme 8 A re-investigation is reported28 of the photoreduction of ketones by tributylstan- nane; the major products are alkoxytributylstannanes (Bu3SnOCHR2 from O=CR2) the process involves a chain mechanism and some ring reduction occurs for aromatic ketones. The Norrish type 2 biradicals from aromatic ketones PhCO(CH,),Me can be oxidized by chromium(v1) or maganese(vI1) species to give good yields (around 26 J.Lessard Y. Couture M. Mondon and D. Touchard Can. J. Chem 1984 62 105. 27 D. H. R.Barton Y. HervC P. Potier and J. Thierry 1. Chem. Soc. Chem. Commun. 1984 1298. 28 M. H. Fisch J. J. Dannenberg M. Pereyre W. G.Anderson J. Rens,and W. E. L. Grossman Tetrahedron 1984,40 293. Photochemistry 70% ) of 1,4-diketones PhCO( CH2)2CO(CH2) -3 Me.29 Related biradicals from o-alkylbenzyl phenyl ketones ArCH2COPh can be excited photochemically in solution if a laser source is used for their generation;30 the process leads to biradical cleavage products. Such photochemistry of a transient intermediate in fluid solution is uncommon. The Norrish type 2 elimination process continues to find applications in generating multiply bonded species and a patented example involves the oxidation of a 4-mercapto p-lactam to the corresponding thione by way of the S-phenacyl derivative (Scheme 9).31 Scheme 9 Intramolecular hydrogen abstraction by the oxygen of a 2-pyridone is responsible (Scheme 10) for converting rhombifoline a common lupin alkaloid to tsukush- inamines A and B which co-exist in small quantities in the plant.32 In many a,p-enones however the abstracted hydrogen is taken by the p-carbon atom; this is responsible for the production of tricyclic ketones from certain unsaturated bicyclic ketones (Scheme 11),33 and for the formation of spiro-P-lactams in reasonable yield (28-65% ) from 2-( acylamino)cyclohex-2-enones.34 0 Scheme 10 Scheme 11 29 M.Mitani M. Tamada S. Uehara and K. Koyama Tetrahedron Lett. 1984 25 2805. 30 J. C. Sciano and P.J. Wagner J. Am. Chem. SOC.,1984 106 4626. 31 L. Re A. Brandt and L. Bassigani Pat. Spec$ (Aust.) AU 532 930 (Chem. Abstr. 1984 101 23212). 32 S. Ohmiya H.Otomasu and I. Murakoshi Chem. Pharm Bull. 1984 32 815. 33 Y. Tobe T. Iseki K. Kakiuchi and Y. Odaira Tetrahedron Lett. 1984 25 3895. 34 M. Ikeda T. Uchino H.Ishibashi Y.Tamura and M.Kido J. Chem. SOC.,Chem. Commun. 1984,758. 104 J. D. Coyfe Intramolecular photocycloadditions of carbonyl compounds like their more widely studied intermolecular counterparts have been employed in synthetic approaches to natural products. A y-vinyloxy ketone gives as minor product a homo-analogue to thromboxane A2 (Scheme 12) accompanying the major dioxabicyclo[4.2.0] prod~ct.~’ In an attempt to make perhydrohistrionicotoxin a route has been developed that involves as a key stage the internal photocycloaddition of an alkyne to an enone in the cyclohexenone (6).36 Scheme 12 0 5 Sulphur Nitrogen and Other Compounds The irradiation of N-vinylthiobenzamides gives 2-arylthiazolidines in good yield (Scheme 13) possibly by way of the thiol ta~tomer.~’ N-Alkylthioimides (which are red) are converted photochemically into isomeric a-amidothioacetophenones (which are purple); if the reaction is carried out at low temperature the reaction mixtures are colourless reflecting the existence of aziridinethiols as intermediates (Scheme I 4).38Thioimides derived from methacrylic acid have been used in another Scheme 13 Scheme 14 35 H.A. J. Carless and G. K. Fekarurhobo J. Chem. SOC.,Chem. Commun. 1984. 667. 36 E. R.Koft and A. B. Smith J. Org. Chem. 1984,49 832. 37 A. Couture R. Dubiez and A. Lablache-Combier J. Org. Chem. 1984. 49 714. 38 M. Sakamoto H.Aoyama and Y.Omote J. Org. Chem. 1984.49 1837. Photochemistry 0 5 5-95 % Scheme 15 photochemical approach to p-lactams (Scheme 15),39 but one that employs the photocycloaddition reaction with alkenes rather than a hydrogen abstraction process. The photochemistry of C=N compounds has developed into an area of consider- able interest. Hydrogen transfer reactions involving this group have occasionally been reported and an intramolecular example is seen in the conversion of 1-(0-toluoyl)-3,4-dihydroquinolinesinto spiro-indanones (Scheme 1 6).40 The electron- transfer photoaddition of allylsilanes or benzylsilanes to iminium salts has proved to be especially useful.In the basic intermolecular reaction with allylsilanes 2-allylpyrrolidines are formed from pyrrolinium perchlorates (Scheme 17).41Several intramolecular examples are reported including the use of o-substituted N-benzyl-pyrrolinium salts (Scheme 18) where the absence or presence of a trimethylsilyl group controls the direction of reaction and hence the size of ring formed.42 Me0 Me0 / hv -Meo?ilH R Scheme 16 Scheme 17 As part of a strategy to prepare larger quantities of 20-methylisobacteriochlorins a mild synthetic route has been developed43 that involves in the final stage a photochemical ring-closure between two C=N functions one of which carries a methylthio group (Scheme 19).The photochemistry of N-oxides has been reviewed,44 39 M.Sakamoto Y. Omote and H. Aoyama J. Org. Chem. 1984 49 396. 40 Y. Hirai H. Egawa and T. Yamazaki Heterocycles 1984 22 1359. 41 K. Ohga U. C. Yoon and P. S. Mariano J. Org. Chem. 1984,49 213. 42 A. J. Y. Lucan S. L. Quillen R.0.Heuckeroth and P. S. Mariano J. Am. Chem. Soc. 1984,106 6439. 43 D. M. Amott A. R. Battersby P. J. Harrison G. B. Henderson and Zhi-Chu Sheng J. Chem. Soc. Chem. Commun. 1984 525. 44 A. Albini and M.Alpegiani Chem. Rev. 1984 84 43. 106 J.D. Coyle 90% b R (R = CH,SiMe,) F Scheme 18 Scheme 19 and an interesting example is reported4’ of an azoxy compound that undergoes intramolecular metathesis with a neighbouring alkene [-N=N(O)-+ C=C +-N=C-+ -C=N(O)-1 whereas the corresponding azo-compound gives a 1,2-diazetidine in quantitative yield. Alkyl halides in solution undergo reactions that involve radicals carbocations or carbenes as intermediates and these processes have been reviewed.46 The use of deuterium-labelled 1-iodo-octane helps to elucidate the various pathways for this primary halide.47 Nucleophiles such as alcohols or even acetonitrile or benzene can replace the phosphate group in benzyl diethyl phosphates irradiated in and these reactions most probably occur through a benzyl cation intermediate.45 G. Fischer D. Hunkler and H. Prinzbach Tetrahedron Lett 1984 25 2459. 46 P.J. Kropp Acc. Chem. Res. 1984 17 131. 47 P. J. Kropp J. A. Sawyer and J. J. Snyder J. Org. Chem. 1984 49 1583. 48 R. S. Givens and B. Matuszewski J. Am. Chem. Soc. 1984 106 6860.

ISSN:0069-3030    

DOI:10.1039/OC9848100097

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

7 Aliphatic Compounds Part (i) Hydrocarbons By D. F. EWING Department of Chemistry University of Hull Hull HU6 7RX 1 Alkanes Ever since the pioneering work of Bouveault and Blanc at the beginning of this century the action of sodium on esters has been of great interest to chemists principally as a means of converting the acid moiety into the corresponding alcohol. However recently attention has turned to investigation of the fate of the original alcohol moiety and the conditions under which it is converted into an alkane. Sodium in hexamethylphosphortriamide will convert' the acetyl derivatives of primary and secondary alcohols into the corresponding alkanes in 40-50% yield and in the case of highly hindered alcohol esters the alkane yield can exceed 95%.The relative proportion of alkane and alcohol shows some dependence on the structure of the esterifying acid.The halogenation of alkanes is usually restricted to reaction with chlorine or bromine by a radical process. In contrast elemental fluorine has been found2 to react as an electrophile with tertiary C-H bonds in cyclic and acyclic hydrocarbons to give the fluoride in reasonable yield e.g. 3-methylnonane is converted into 3-fluoro-3-methylnonane in 60% yield by a 2% stream of F2 in nitrogen at -75 "C. 2 Alkenes Synthesis.-From Alkynes. The reduction of acetylenic derivatives with zinc powder has been thoroughly ~haracterized.~ Two methods of activating the zinc were com- pared. The reagent Zn-C2H4Br2 is highly selective for conjugated diynes isolated triple bonds being unreactive.Only one triple bond is reduced to give the 2-enzyne in at least 70% yield. The presence of a heteroatom in the substrate usually leads to enhanced yields. A more powerful system is Zn-Cu-C2H4Br2 which will reduce both triple bonds in conjugated diynes and will react at isolated triple bonds in some cases. A more interesting catalyst system can be prepared from NaH t-amyl alcohol and palladium( 11) acetate in a 3 :2 :1 molar ratio in tetrahydrof~ran.~ With this catalyst the hydrogenation of acetylenes is cu. 99% selective for conversion into alkenes with a 99% stereoselectivity for the 2-product. Quinoline is required as a H. Deshayes and J.-P. Pete Can. J. Chem. 1984 62 2063. 'C. Go1 and S. Rozen Tetrahedron Lett.1984 25 449. M.H.P. J. Aerssens antl L. Brandsma J. Chem. SOC.,Chem Commun. 1984 735. J.-J. Brunet and P. Caubete J. Org. Chem. 1984 49 4058. 107 108 D. E Ewing modifier but exceptional reproducibility of results can be maintained from batch to batch making this catalyst the most attractive to date for monohydrogenation of alkynes. Calcium in a 1 1 mixture of methylamine and ethylenediamine has been investigated' as an alkyne reducing system. Internal alkynes give the corresponding 2-alkene in 80% yield with small amounts of other isomeric alkenes. By Elimination. The reduction of vicinal dibromides with sodium sulphide facilitated by the use of a phase-transfer reagent was reported on last year. The usefulness of this approach has been confirmed independently by other workers.6 Another useful reducing agent for this type of elimination reaction is the species Cp2TiC1 made from titanocene ( Cp2TiC12) by reduction with zinc.' The reaction is stereoselective for anti-elimination and the yield of olefin ranges from 66 to 95% over twelve examples.a-Silyl esters (1) are known to be vinyl di-cation equivalents at least for the unsubstituted compound (R' = R2 = H) and as such can be alkylated twice at carbon-1 to give an alcohol which in turn undergoes facile elimination to afford an alkene (Scheme 1). This reaction has now been extended to substituted compounds R' IPh,MeSiCCOOEt R2I R'R'IA Ph2MeSiCCOR3 * Ph2MeSiLCR3R40H R2I R2I -+ R' R3 \/C=C R2' \R4 (1) (2) Reagents i R3MgBr; ii R4MgBr or R4Li Scheme 1 to give ultimately tri- and tetra-substituted alkenes (2).8The yields are very variable and as might be expected very sensitive to the cumulative steric effect of R' R2 R3,and R4 in (2).a-Lithiated benzyl phenyl sulphones ArCH2S02Ph undergo elimination to afford E-1,2-diarylethenes only very slowly at ambient temperature. In the presence of elemental tellurium the rate of this reaction is significantly enhanced' but some of the cis form is also obtained. However isomerization to give a pure trans product is simply achieved by refluxing with TeC1,. Alkyl or ally1 sulphones are unreactive even in the presence of tellurium. Other Syntheses. Direct arylation of alkenes can be affected by one equivalent of t-butyl perbenzoate in presence of a catalytic amount of a palladium salt (Scheme 2)." The perester acts as a hydrogen acceptor and this oxidative coupling reaction can be extended to furan with yields in the 50-70% range.The reaction of alkenes with trialkylsilanes usually gives a silylated alkane. However in the presence of high concentrations of rhodium( I) catalysts the reaction is more complex and gives R. A. Benkeser and F. G. Belmonte J. Org. Chorn. 1984,49 1662. D. Landini L. Milesi M. L. Quadri and F. Rolla J. Org. Chern. 1984 49 152. ' S. G. Davies and S. E. Thomas Synthesis 1984 1027. D. Hernandez and G. L. Larson J. Org. Chern. 1984,49 4285. L. Engman J. Org. Chem. 1984 49 3559. J. Tsuji and H.Nagashima Tetrahedron 1984 40,2699. Aliphatic Compounds -Part ( i) Hydrocarbons Y C6H + r-/ 2'Fy R Ph Reagent i PhC03CMe3(100 mol YO),Pd(OCOPh)2 (5 mol Oh) CH,COOH 110 "C Scheme 2 a much increased proportion of silylated alkenes." There is clearly some potential in this reaction but further work is required.Reactions.-There is much interest at present in the oxygenation of alkenes by methods which mimic the action of natural mono-oxygenases such as the cytochrome group. Tetraphenylporphyrin (TPP) complexes of several metals have been investi- gatedt2 using NaOCl as oxygen source. This reagent has the advantage that it can be effective in dilute solution. Epoxidation of styrene with Mn(TPP)OAc can be achieved with 80% conversion but analogous complexes of Co Fe and Cr are decreasingly effective.Selectivity for epoxidation can vary between 60 and 100%. In another studyt3 with iodosylbenzene as oxidant Fe(TPP)Cl shows 100% selec-tivity for epoxidation but other species such as FeC13 and Fe(acac)j are much less selective. The overall yield of oxidized products is also lower. In contrast copper(I1) species with iodosylbenzene catalyse alkene epoxidation efficiently without the mediation of a porphyrin moiety.I4 It is postulated that the active species is [Cu2OIPhI4+ or a derived (p-0xo)dicopper complex. Some interesting mechanistic revelationst5 have come from work with a TPP complex of cobalt with O2as oxidant and BH as reductant. This system leads to regioselective oxidation of styrenes to benzylalcohols in high yield (75-95%). Other arylalkenes are less reactive and the overall results suggest that activation of the alkene is the primary role of the catalyst.This is in clear contrast to the ideas of most other workers who claim that TPP complexes activate 02. A very high selectivity for terminal double bonds is observed for epoxidation with hydrogen peroxide using a hydroxy platinum complex as catalyst.16 This catalyst probably functions as a base providing hydroxide ion by dissociation and generates no oxidation products other than the epoxide. The reagent KHS05 in the presence of a ketone results in epoxidation of unfunctionalized alkenes. The active intermedi- ate is a dioxirane species derived from the ketone and thus if the ketone is chiral the oxidation of prochiral alkenes shows" significant enantioselectivity (enan- tiomeric excess in the range 5 to 12.5%).This is an interesting example of asymmetric induction particularly since the chiral agent is not only catalytic (rather than stoicheiometric) but can be easily recovered. The stereorelationship between the plane of a C=C moiety and the per-acid group -C03H in the transition state of an epoxidization reaction is not easily examined. However some work with the A. Onopchenko E. T. Sabourin and D. L. Beach J. Org. Chem. 1983,48 5101; 1984,49,3389. 12 B. Meunier E. Guilmet M.-E. De Carvalho and R. Poilblanc J. Am. Chem Soc. 1984 106 6668. l3 M. Fontecave and D. Mansuy J. Chem. SOC Chem. Commun. 1984 879. 14 C. C. Franklin R. B. Van Atta A. Fan Tai and J. S. Valentine J. Am. Chem.SOC.,1984 106 814. T. Okamoto and S. Oka J. Org. Chem. 1984 49 1589. 16 G. Strukul and R. A. Michelin J. Chem. SOC.,Chem. Commun. 1984 1538. " R. Curci M. Fiorentino and M. R. Serio J. Chem. Soc. Chem. Commun. 1984 155. 110 D. F. Ewing per-acid (3) (generated in situ from the acid chloride) suggests18 that the C=C is probably co-planar with the -C03H moiety (rather than orthogonal). This inference is based on a selectivity for 1,2-over 1,I-disubstituted alkenes and a cis-selectivity of between 4 and E relative to the trans compound. Catalysis of the oxidation of olefins to ketones by palladium species (variants on the Wacker process) has been reviewed." Emphasis is placed on the synthetic usefulness of this reaction. Alper and co-workers2' have found that PdC1,-CuC1,- HC1-0 is an exceedingly mild catalytic system for the hydrocarboxylation of alkenes with CO and H20 to give the branched acid only i.e.RCH=CH2 to RCH( CH3)COOH. At room temperature and atmospheric pressure terminal alkenes give the corresponding acid in 60-100% yield. A high regioselectivity is also found for alk-2-enes but not for alk-3-enes. Dienes give a monocarboxylic acid. The influence of the structure of the olefin and the nature of the mercury salt and the solvent have been studied in detail for the solvomercuriation-demercuriation reaction.21 Hg(OAc) is effective only in MeOH whereas Hg(02CF3)2 works well in primary secondary or tertiary alcohols. Much poorer selectivity for ether forma- tion is observed with the corresponding nitrate and methanesulphonate salts.A very complex dependence on olefin structure is evident. Two novel mercuriation reactions are shown in Scheme 3. The reaction with a sulphinate salt22 offers a convenient upR1CH=CR2S02Ph R'C H =cHR* jiLA R1CH2CH2NHS02C,H,Me Reagents:i HgCL2-PhS02Na H20 2 days at RT ii 50% NaOH-dioxane; iii TsNH2-Hg(N03), CH2C12; iv NaBH Scheme 3 route to vinylsulphones in 70-80% yield via an arenesulphonomercuric intermedi- ate. Reductive demercuriation of the analogous sulphonamidomercuric derivative provides an entry to N-alkylsulphonamides in 70-100% yield. Starting with a 1,4-or 1,S-diene bis-alkylation at nitrogen can be achieved thus indicating an intriguing route to N-hetero~ycles.~~ An improvement in the stereospecificity of the trans- 18 J.Rebeck L. Marshall R. Wolak and J. McManis J. Am. Chem. SOC.,1984 106 1170. 19 J. Tsuji Synfhesis 1984 369. 20 H. Alper J. B. Woell B. Despeyroux and D. J. H. Smith J. Chem SOC.,Chem. Commun. 1983 1270; B. Despeyroux and H. Alper Ann. N.Y.Acad. Sci 1984,415 148. 21 H. C. Brown J. T. Kurek M.-H. Rei and K. L. Thompson J. Org. Chem. 1984,49 2551. 22 W. Sas J. Chem. SOC.,Chem. Commun. 1984 862. 23 J. Barluenga C. Jimtnez C. Nijera and M. Yus,J. Chem. Soc. Perkin Truns. 1 1984 721. Aliphatic Compounds -Part (i) Hydrocarbons 111 metallation of vinylboranes to vinylmercury compounds has been obtained,24 par- ticularly for longer-chain alkenes such as dec-2-ene (Scheme 4). Catecholborane is the best borane for the first step and Hg(OAc),-NaOAc or HgC12 the best mercuric salt for the second step.With optimization of reaction conditions a trans-selectivity of 99% was possible. H H i \/ ii \/ R'C=CH -R' -R' c=c H/c=c\BR H/\HgOAc Reagents i BRiH; ii Hg(oA~)~-NaoAc Scheme 4 A number of minor studies of alkane hydroboration reactions includes the selection of a primary alkene to react with 9-borobicyclo[3.3.1]nonane followed by reaction with a lithium acetylide. Specific rearrangement of the primary alkyl group of the borate is induced by (Bu)~S~C~.~~ The synthesis of boranes containing the isopinocampheyl (Ipc) group has been improved to give even higher levels of optical purity enantiomeric excess (e.e.) 99.9O/0.~~ Not only does this lead to higher levels of optical induction in alkene adducts but careful recrystallization of the initial adduct boranes allows the Ipc group to exercise a normal diastereoisomeric role resulting in boranes of ca.100% e.e.27 Many optically pure compounds are thus readily accessible from alkenes by this route. Lithium triethylborohydride has been examined as a reducing agent for styrenes and similar conjugated alkenes.28 The reaction is highly regiospecific (Markovnikov) and electron deficient olefins are most reactive as would be expected. A full report has now appeared29 on the activation of alkenes by the presence of esters to hydroboration with LiBH4. The product obtained after suitable hydroly- sis is usually a dialkylborinate thus providing access to compounds such as alcohols ketones or iodides.Either vinyl- or divinyl-borinates are formed from alkynes. The regioselectivity is not very high in some cases styrene being a good example but this is generally a useful reaction. Asbestos fibres (chrysolite) have been developed3' as a novel hydrogenation catalyst for linear and cyclic alkenes. Mild conditions (1 atmosphere at 27 "C) are sufficient. Treatment of the fibres with titanocene increases the rate of H2 addition for n-alkenes but cyclohexene is unreactive suggesting that the treated and untreated fibres have different catalytic sites. Another catalyst [(hexa-1,5-diene)RhI2 is effec- tive for the hydrogenation of alkenes and polyenes in an aqueous-organic two-phase system under ambient condition^.^' In many cases the yields are 100% and carbonyl groups are not reduced.24 R. C. Larock and K. Narayanan 1. Org. Chem 1984,49 3411. 2s K. K. Wang and K.-H. Chu J. Org. Chem. 1984,49 5175. 26 H. C. Brown and B. Singaram J. Org. Chem. 1984 49 945. 27 H. C. Brown and B. Singarem J. Am. Chem Soc. 1984 106 1797. 2s H. C. Brown and S.-C. Kim,J. Org. Chem. 1984,49 1064. 29 H. C. Brown V. Somayaji and S. Narasimhan 1 0%.Chem 1984,49 4822. 30 D. Conk and C. De Blois Can. J. Chem 1984 62 392. 31 K. R. Januszkiewin and H. Alper Can. J. Chem. 1984,62 1031. 112 D. F. Ewing A novel approach to the functionalization of isoprenoids is shown in Scheme 5.32 The adduct (4) from arylsulphenylation can be converted into either the trans-allylic or internal allylic alcohols with full retention of the original stereochemistry.This method is suitable for large-scale production of oxygenated terpenes with excellent site regio- and stereo-control. The sulphenamides PhSNH(CXC,H,) (X = NOz Ii \v (4) lv R>oAc PhS '7PACvi PhS \vii Reagents i PhSCI-CH2C12,0 "C; ii Excess Et3N-DMF 20 h at 60 "C;iii 30% H202-AcOH 20 h at 20 "C; iv (MeO),P-MeOH 48 h at 20 "C; v Heat; vi TsOH 1 h at 0 "C; vii 10% KOH(aq)-EtOH 3 h at 20 "C Scheme 5 C1 Me or OMe) react readily with alkenes at ambient temperature in the presence of 1.5 equivalents of BF3-Et,0.33 High regioselectivity (Markovnikov) and stereoselectivity (trans) suggest the involvement of an episulphonium ion. The substituent effect on yield (lowest for donor substituents) is also in keeping with this electrophilic mechanism.32 Y. Masaki K. Hashimoto K. Sakuma and K. Kaji J. Chern. SOC.,Perkin Trans. 1 1984 1289. 33 L. Benati. P. C. Montevecchj and P. Spagnoko Tetrahedron Left. 1984 25 2039. Aliphatic Compounds -Part ( i) Hydrocarbons 113 Arylselenenylation is known to proceed stereospecifically to give a trans adduct. However reaction of the adduct with further ArSeCl results in formation of RSG(Ar)SeAr. This species will react with chloride sources such as Bu,N+Cl- to give the cis-dichloride in 55-100% yield for terminal and cis- and tr~ns-alkenes.~~ Arylselenenylation followed by treatment with silver nitrate in presence of HgC12 has been investigated3' as a route to nitroselenides R',SiCH(SePh)CHR2N02.This addition-substitution appears to operate with full regio- and stereo-control but yields are variable (3746%). Oxidation in the normal way gives alkenes such as Me3SiCH=CRN02. Some useful extensions to the selenosulphonation of alkenes have been Alkaneselenosulphonates RS02SePh are readily obtained from the reaction between the corresponding sulphinic and seleninic acids. The photochemically promoted reaction of these compounds with alkenes depends upon the nature of R. The addition reaction does not occur if extrusion of SO2 (to give RSePh) is favoured (e.g. for R = benzyl). Bis-sulphonylselenides (ArSO,),Se (Scheme 6) undergo radical addition to cyclohexene to yield the expected product (5) and the corresponding selenide probably formed by photodissociation of (5).Styrene affords the analogous selenide (6) as the sole product.ArS0,H -!+ ArS0,SeS02Ar -!!+ " rrso2Ar 1 iii -SeSO,Ar (5) PhCHCH2S02Ar I Se I PhCHCH2S02Ar (6) Reagents i Se0,-THF 24 h at 20 "C; ii Cyclohexene hv; iii Styrene hv Scheme 6 A comprehensive investigation3' of the addition of hydrazoic acid to 23 alkenes has demonstrated that activated systems (enol ethers) do not require a catalyst but for styrene 1,l-dialkyl- and trialkyl-alkenes TiCl promotes the formation of the azide in 5540% yield. Curiously monosubstituted and 1,2-disubstituted alkenes are unreactive. The mechanism is unclear and further study of this reaction is required.Another synthetically useful reaction recently developed38. is the reaction of styrenes at -70 "C with NOT BF in MeCN-CH2C12 to give the nitroacetamidation products (50-80% yield) with high regioselectivity but variable stereocontrol. Simple alkenes are liable to polymerize under these conditions. The new electrophilic reagent chlorine chlorosulphate (7) is obtained from C12 and SO3 at -78 "C. This reagent leads to more control over the addition reaction 34 A. M. Morella and A. D. Ward Tetrahdron Lett. 1984 25 1197. 35 T. Hayama S. Tomoda Y. Takeuchi and Y. Nomura J. Org. Chem. 1984 49 3235; Tetrahedron Lerr. 1983 24 2795. 36 Y.-H. Kang and J. L. Kice J. Org. Chem. 1984 49 1507. 37 A. Hassner R. Fibiger and D. Andisik J. Org. Chem. 1984 49 4237.38 A. J. Bloom M. Fleischmann and J. M. Mellor 1. Chem. Soc. Perkin Trans. I 1984. 2357. 114 D. F. Ewing 0 rl c1-0-s-c1 II 0 than is the case with the more reactive species C10S02F and C1OSO2CF3 and generally behaves as a good electrophile although the regioselectivity is Activation of such weak electrophiles (in this case C12) by SO3 may have wider application and further interesting results can be expected from this work. Bromina- tion4' of 1-phenylpropenes with tetrabutylammonium dichlorobromate ( i.e. BrC12) shows anti-stereospecificity but little regioselectivity in contrast to addition of bromine chloride. Reactions of alkenes with nucleophiles catalysed by palladium( 11) species have been re~iewed.~' This article also covers certain insertion reactions.Fleming has previously demonstrated that the reagent (PhMe2Si)2CuLi is a versatile source of a nucleophilic silyl group which is reactive towards various alkenic and alkynic substrates. One of the shortcomings of this reagent is the inevitable loss of the second silyl group and this is overcome42 with the mixed cuprate Me(PhMe,Si)CuLi which preferentially transfers the silyl group to an alkene. Another variant is (Me3Si),CuLi-HMPA which has the advantage that the ultimate fate of the silyl group will be a volatile species thus obviating problems of product purification. 3 Polyenes Synthesis.-Recent developments in allene chemistry have been reviewed,43 includ- ing methods of synthesis. A new route4 to terminal allenes involves the transfer of the alkynyl moiety in Ph3SnCH2C-CH to suitable alkyl bromides or iodides (chlorides react poorly).This radical reaction appears to tolerate protected amino or carboxyl functions but yields are only fair (40-60%). Several notable routes to conjugated dienes have been reported recently. The di-anion (8) reacts readily45 with monofunctional electrophiles (alkyl halides) to give the corresponding disubstituted butadienes (9; R = alkyl allyl or benzyl) in variable yield (20-70% ). Dihalides afford cyclic compounds incorporating one or more diene moiety. Extrusion of the heteroatom from simple heterocycles (10) (Scheme 7) by treatment with an aliphatic Grignard reagent produces a diene with a 2,Zgeometry exclusively,& but PhMgBr gives a mixture of Z,Z and E,E dienes.Tellurophene is the most reactive furan the least reactive heterocycle and yields are high (80%) for non-alkylated heterocycles. An E-selective route to 1,3-dienes has been developed4' using the Wittig reagent Ph2( RCH,)P=CH(alkenyl). With 39 N. S. Zefirov A. S. Koz'min and V. D. Sorokin J. Org. Chem. 1984 49 4087. 40 T. Negoro and Y. Ikeda Bull. Chem. SOC.Jpn. 1984 57 2111 2116. 41 L. S. Hegedus Tetrahedron 1984 40 2415. 42 I. Fleming and T. W. Newton J. Chem. SOC.,Perkin Trans. I 1984 1805. 43 D. J. Pasto Tetrahedron 1984 40 2805. 44 J. E. Baldwin R. M. Adlington and A. Basak J. Chem. SOC.,Chem. Commun. 1984 1284. 45 R. B. Bates B. Gordon T. K. Highsmith and J. J. White J. Org. Chem.1981 49 2981. 46 E. Wenkert M. H. Leftin and E. L. Michelotti J. Chem. SOC.,Chem. Commun. 1984 617. 47 E. Vedejs and H. W. Fang J. Org. Chem. 1984.49 210. Aliphatic Compounds -Part (i) Hydrocarbons R3 Reagents i RBr or RCI; ii R3MgBr Ni(PPh3)2C12 Scheme 7 aliphatic aldehydes an E 2 ratio of 15 :1 can be achieved in good cases which compares favourably with that for the analogous triphenyl phosphorus ylide. A similar improvement4* in established synthetic methodology is the dehydrobromination of 1,2-dibromocyclohexane with Li2C03-LiCl at 160 "C in HMPA. The diene distils from the reaction vessel directly in up to 90% yield. This procedure may well be of value in other syntheses. Dimerization of butadiene to octa-1,3,6-triene in 95% selectivity can be achieved49 with a new aminophosphinite nickel(0) complex at ambient temperature.The proton donor need be present only in equimolar amounts and the catalytic activity is very high (TN > 5000). Aprolonged reaction time results in isomerization to an octa-2,4,6- triene in 85% yield. l-substituted 1,6-dienes can be formed" by treatment of an a-chloroester with the bis-Grignard reagent BrMg(CH2),MgBr followed by lithium powder. With shorter-chain bis-Grignards a cyclic alkylidene derivative is formed. A range of polyunsaturated compounds are accessible5' by elimination from p-acetoxy sulphones (Scheme 8). The starting aldehyde can contain a double or triple OAc SO Ph iv ii RIM ,o R,+ iii R' SO Ph Reagents i PhS02CH2R2-BuLi-THF -78 "C; ii Ac,O-pyridine TsOH; iii BU'OK-THF 20 "C; iv PhS02C2Hs-BuLi-THF -78 "C 48 A.Weisz and A. Mandelbaum J. Org. Chem. 1984 49 2648. 49 P. Denis A. Montreux F. Petit G. Buono and G. Peiffer J. Org. Chem 1984 49 5274. so J. Barluenga M. Yus J. M. Concellon P. Bernad and F. Alvarez J. Chem. Res.(S) 1984 122. 51 T. Mandai T. Yanagi K. Araki Y. Morisaki M. Kawada and J. Otera J. Am. Chem. SOC.,1984,106,3670. 116 D. F. Ewing bond thus leading to enyne or diyne structures. With alkylaldehydes the double elimination gives a diene rather than an alkyne. This reaction is clearly a useful entry to complex polyunsaturated compounds of many types. A timely survey of synthetic methods has appeared52 for the class of polyenes known as dendralenes e.g.(1 1). This is a somewhat neglected area which contains many interesting synthetic problems. Reactions.-Two useful papers53 have described the reaction of allenylzinc com- pounds (RC=C=CH)ZnCl with aldehydes to give homopropargylic alcohols with high regioselectivity (98-99% ). Similar results are found with the analogous aluminium compounds but the stereocontrol is much worse in this case. The activity of the system PdCl2-CuCl2-HC1-O2-CO for hydrocarboxylation of alkenes is dis- cussed above.20 With allene this reagent will induce oxidative carbonylation to afford CH2=C(CH20Me)COOMe in 85% yield.s4 There must be some commercial poten- tial in this reaction. Arylpolyenes are of potential interest as organic semiconductors and Pd(0Ac)- (0-tolyl) has been investigateds5 as a catalyst for the arylation of 1,3-dienes and conjugated trienes.Both mono-and di-arylated species are obtained with aryl bromides and iodides but the yield is very variable depending on the structure of both aryl halide and diene. The site-selectivity of 1,3-dienes in the reaction with HgO-HBF has been studieds6 for a range of compounds. Formation of 1,4-diethers is favoured in most cases with high regio- and stereo-selectivity. 1 ,CAddition to dienes mediated by acylcobaltcar- bony1 complexes is a useful route to nitroenones.” Both 1,2- and 1,Caddition is found with PhSeC1 but subsequent reaction with a range of nucleophiles results in exclusive formation of 1,4-isomers.~~ 4 Alkynes Synthesis.-Although most types of unsaturated group have been successfully cross- linked by Pt or Ni catalysis formation of conjugated diynes by this method has proved difficult.This problem has now been resolveds9 using E-2-chloro-iodoethene with a Pd complex as coupling agent (Scheme 9). Possibly this reaction could be used to prepare conjugated poly-ynes. An analogous procedure has been described for the synthesis of Me,SiC_CCH=CHC=CSiMe3 which is likely to be a valuable precursor of poly(diacetylenes) hexatriene or other polyenes. In this case an alkynyl 52 H. Hopf Angew. Chem. Int. Ed. Engl. 1984 23 948. 53 G. Zweifel and G. Hahn J. Org. Chem. 1984 49 4565. 54 H. Alper F. W. Hartstock and B. Despeyroux J. Chem. SOC.,Chem. Commun. 1984,905. 55 T.-A. Mitsudo W.Fischetti and R. F. Heck J. Org. Chem. 1984 49 1640. 56 J. Barluenga J. Pirez-Prieto and G. Asensio J. Chem. SOC.,Perkin Trans. I 1984 629. 57 L. S. Hegedus and R. J. Perry J. Org. Chem 1984,49 2570. 58 R. S. Brown S. C. Eyley and P. J. Parsons J. Chem SOC.,Chem Commun. 1984,438. 59 E.-I. Negishi N. Okukado S. F. Lovich and F.-T. Luo J. Org. Chem. 1984 49 2629. 117 Aliphatic Compounds -Part ( i) Hydrocarbons / H iii Reagents i BuLi; ii Zn CI, ICH=CHCI Pd(PPh,),; iii NaNH,-NH,(liq) 1 h; iv H+; v Me,SiCl; vi R2Cl Scheme 9 Grignard reagent is coupled with dichloroethene.60 Direct coupling of an arylbromide or iodide with propargyl alcohol is best achieved with PhPdI( PPh& Et,N-CUI in an organic solvent.6' The resulting arylynols ArCrCCH20H are easily converted in an overall one-pot process into arylacetylenes with Mn0,-KOH in benzene.In a recent report62 the CuBr-Me2S-mediated coupling of acetylenic Grignard reagents (RCECMgBr) with propargylic tosylate (CH-CCH20Ts) is applied to the synthesis of the diyne moiety RC_CCH2CrCH for subsequent elaboration into an insect pheromone. The isomerization of long-chain aliphatic alkynes is readily promoted by LiNH(CH2)3NH2. In the presence of sodium or potassium alkoxide this reaction is even more efficient,63 and dec-2-yn-1-01 for example is converted into dec-9-yn- 1-01 with 99% selectivity (93% isolated yield) in 15 minutes at room temperature. Another intriguing aspect of this isomerization reagent is the exchange of the NH hydrogen atoms with the alkyne CH2 atoms.64 If C6H,C~C-cH20H is treated with sodium or potassium in D2NCH,CH2CH,ND2 the isomerized alkyne incorporates 86-90% deuterium.The initial position of the triple bond is relatively unimportant. Reactions.-Nitrosochlorination of alkyne carboxylic esters has been achieved6' with NOCl gas at 0°C. Terminal alkynes also give some oxime by rearrangement. The reagent TiCl,-A12Et3Cl gives rise to carbometallation of but-3-yn-1-01.~ Only one ethyl group is incorporated with high regio- and stereo-selectivity to give the resulting enol in 70% yield at -23 "C. At higher temperatures this yield is much reduced and a complex mixture of products is formed. A convenient method of a-deuteriation of terminal alkynes in the presence of other functional groups (such as OR COOR NR,) involves exchange with D20 catalysed by SO,.At 90-95 "C one pass with this reagent gives 60-8O% incorpor-ation of deuterium. A mechanism is postulated for this exchange rea~tion.~' M) J. A. Walker S. P. Bitler and F. Wude J. 0%.Chem 1984 49 4733. 61 N. A. Bumagin A. B. Ponomaryov and 1. P. Beletskaya Synthesis 1984 728. 62 R. Baker M. J. O'Mahony and C. J. Swain J. Chem. Res.(S) 1984 190. 63 S. R. Abrams Can. J. Chem. 1984 62 1333. 64 S. R. Abrams J. Org. Chem. 1984 49 3587. 65 M. M. Siddiqui F. Ahmad and S. M. Osman J. Chem. Res. (S) 1984 186; (M), 1984 1801. 66 F. W. Schultz G. S. Ferguson and D. W. Thompson J. Org. Chem. 1984,49 1736. 67 C.-A. Chang K. G. Cronin D.D. Crotts E. Dunach T. R. Gadek and K. P. C. Vollhardt J. Chem. SOC.,Chem. Commun. 1984. 1545. 118 D. F. Ewing Organocuprates ( i. e. RMgBr-CuBr-Me,S) can react with propargylic ethers and acetates in two competing ways (Scheme The usual alkylation reaction is predominant with acetates using Et,O as solvent but the reverse is true for methyl ethers using THF as solvent. The mechanism which accounts for all the subtleties of these results is likely to be complex and may involve a Cu"' species. The influence of reactant structure on the ratio of allenic to alkynic reduction products suggests that these may arise by different mechanisms. CH,CH,C=CR + CH,CH=C=CHR 3 '..C R =C H3C HC I ox CH3CHCECR + CH,CH=C=CREt I Et X = Me,COMe Reagents i EtMgBr-CuBr-Me,S-THF -30 "C; ii H20 Scheme 10 C.Sahlberg and A. Claesson J. Org. Chem. 1984 49 4120.

ISSN:0069-3030    

DOI:10.1039/OC9848100107

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

7 Aliphatic Compounds Part (ii) Other Aliphatic Compounds By B. V. SMITH Department of Chemistry King's College London Kensington Campus Campden Hill Road London W8 7AH 1 Alcohols and Ethers Several papers have concentrated on protection-deprotection methods for the OH-group. Trimethylsilyl bromide cleaves MOM (methoxymethyl) ethers cleanly and in high yields; bis-i-propylthioboron bromide was effective for MOM and MEM (methoxyethoxymethyl) derivatives. The reaction of Et,AlCN with MOM ethers has been exploited in the synthesis of cyanomethyl ethers a useful synthetic equivalent for RO-C=O.' Selectivity toward fluoride ion has been observed for the t-butyl- dimethylsilyl-group (but not for the tetrahydropyranyl group) ; diethylaluminium chloride will remove the latter but not the former.Ethers derived from t-butyl- methoxyphenylsilyl bromide But(MeO)Si( Ph) Br are even more labile towards fluoride ion which leaves other protecting groups unaff ected. Asymmetric induction from a chiral centre has been exploited in the addition of protected allyloxyboronate (1) to ethyl pyruvate; 14:1 diastereoselection was observed in this novel application (Scheme l).3 Conversion of the halide (or enone) as shown in Scheme 2 takes advantage of the dimethylphenylsilyl group acting as a masked OH-group." C-Trapped azo- compounds are a source of alcohols (and alkanes or alkenes) by reaction of (3) [from (2) and a ketone R3R4CO] with a thi01.~ Free radical-initiated decomposition of esters derived from N-hydroxypyridine-2-thione(4) gave nor-hydroperoxides which could be transformed into alcohols (or carbonyl compounds).6 Stereoselec- tivity in transition-metal-catalysed reduction of allylic (and cyclic) alcohols showed a striking dependence on the catalyst used and the catalyst substrate ratio but was independent of hydrogen pressure (with one e~ception).~ The ring-opening (by AlC1,-NaI- MeCN) of unsymmetrical 5-membered ethers occurred preferentially at the less hindered carbon affording the S-iodoalcohol.8 ' S.Hanessian D. Delorme and Y. Dufresne Tetrahedron Lett. 1984 25 2515; E. J. Corey D. H. Hua and S. P. Seitz ibid. p. 3. Y. Ogawa and M. Shibasaki Tetrahedron Lett. 1984 25 663; Y. Guindon R. Fortin C. Yoakim and J. W. Gillard ibid. p. 4717. R. Metternich and R.W. Hoffman Tetrahedron Lett. 1984 25 4095. I. Fleming R. Henning and H. Plant J. Chem. SOC.,Chem. Commun. 1984 29. J. E. Baldwin J. C. Bottaro J. N. Kolhe and R. M. Adlington J. Chem. SOC.,Chem. Commun. 1984.22. D. H. R. Barton D. Crick and W. B. Motherwell J. Chem. Soc. Chem. Commun.,1984 242. D. A. Evans and M. M.Momssey Tetrahedron Lett. 1984 25 4637. 'M. Noda T. Kajimoto K. Nishide E. Fujita and K. Fuji Tetrahedron Lett. 1984 25 219. 119 120 B-V. Smith f /. a C02Me /=C02Me /.. r-r Ox0 Ox0 90 10 Reagents i .to,$ -6 kbar ii H,/Pt; iii TsOH-MeOH; iv TsOH-Me,CO; v KOH-MeOH; vi 0 CH,N,-Et,O Scheme 1 -+ Ph-MgBr Ph -Ph R2 4 PhdRe:l I1 Ph R' R2 R2 Reagents i HBF,; ii MCPBA-Et,N-Et,O RT Scheme 2 An improved route to dialkylboranes of very high optical purity has been described.The method is conceptually simple ;optically pure isopinocampheylalkyl boranes were prepared from prochiral olefins and IpcBH of 100% e.e. and purified further by crystallization. Reaction with MeCHO gave a boronic ester oxidized in the usual way to alcohol of 100% optical purity (Scheme 3). Chiral transfer from boron to Aliphatic Compounds -Part (ii) Other Aliphatic Compounds almost all other elements of interest will allow a simple synthesis of a large number of chiral compounds essentially optically pure; some examples are shown in the Scheme. This method opens up one of the most far-reaching vistas ever seen in asymmetric synthesis of a considerable range of chiral molecule^.^ Chiral acetals undergo TiC1,-mediated coupling with organometallics to form (5) and (6) which are precursors of chiral alcohols [e.g.(5) 4 (7) formed with high optical purity (Scheme 4)]. Silyl derivative (8a) has been used as a chiral primary alcohol equivalent; ~B(OR)Iiv / 1ii * R RB(OMe) I ,-OH H (S) ‘B+< -RBH2- iBX2 H’ (R* = chiral group) Reagents i MeI THF 0°C; ii H,O, NaOH iii RCHO; iv OH- then re-esterification Scheme 3 RY 0.- i 1ii iii Reagents i ZM TiCI,; ii PCC; iii HO-Scheme 4 H. C. Brown and B. Singaram J. Am. Chem. SOC.,1984 106 1799. 122 B. V. Smith (8a) R = H (8b) R = COEt Me Claisen-Ireland rearrangement of (8b) gave 9a :9b in the ratio 94 6 or 7 :93 according to the method used to generate the enolate." Carbanions a to alkoxy- or amino-silyl groups act as a-hydroxyalkylating agents ie.the synthetic equivalent of an a-hydroxyalkyl anion; chiral silyl compounds e.g. (10) react with an alkyl-lithium to form an adduct converted into a chiral alcohol with modest e.e. (Scheme 5)." This reaction represents a new approach through a chiral silane. Racemic esters can be converted by an esterase-catalysed transesterification in biphasic systems into active esters (and alcohols)." The heterocycle (1 1 a) (2-halogeno-l,3,2-oxaphospholidine 2-sulphide) has been recommended for the rapid measurement of enantiomeric purities of alcohols (and amines) by looking at 31Pn.m.r. shifts of (llc).'The oxide (1 lb) was less ~atisfactory.'~ (-Si( Me)O) i ii \ -Li +(lo)? 111.IV lv OH Br Reagents i Et,O 0 "C; ii MgBr,; iii. ,CuI THF; iv 6M HCI; v 90% H202 KHF, DMF Scheme 5 Me (Ila) 2 = S,Y = C1 (llb) Z= 0,Y= C1 (11~) Z = S,Y = OR 10 S. D. Lindell J. D. Elliott and W. S. Johnson Tetrahedron Lett. 1984 25 3947; R. E. Ireland and M. D. Varney 1. Am. Chem. SOC.,1984 106 3668. K. Tamao R. Kanatani and M. Kumada Tetrahedron Lerr. 1984 25 1905 1909 1913. 12 B. Carnbou and A. M. Klibanov 1. Am. Chem. Soc, 1984 106 2687. l3 C. R. Johnson R. C. Elliott and T. D. Penning J. Am. Chem. SOC.,1984 106 5019. Aliphatic Compounds -Part (ii) Other Aliphatic Compounds Secondary alcohols with bromine and a silver salt formed tetrahydrofurans via 6-hydrogen ab~traction.'~ Tertiary alcohols underwent deoxygenation in a radical- initiated decomposition of a mixed oxalate-N- hydroxypyridine-2-thione ester." Hindered alcohols especially secondary and tertiary types are benzoylated cleanly by PhCOOTf (Tf = trifluoromethane sulphonyl) in CH2C12 or CH2C12-CSH5N at -78 OC.I6 Selectivity in oxidation has been reported for secondary alcohols by suitable protection of any primary alcohol and PCC oxidation/deprotection or by use of MeV'/ H202; K2Fe0,/ PTC is selective for allylic and benzylic alcohols (but not other primary gro~ps).'~ Stereodifferentiation in 1,2-diols has been effected by reaction between a chiral acid chloride (12) and the cyclic derivative (13).Variable d.e. was observed in (14). Extension of this approach to prochiral (2-0-protected) glycerol by using the analogue of (13) and a chiral auxiliary gave only modest optical purity.Kinetic resolution of substituted propan- 1,2-diols with D-camphorquinone gave a mixture of diastereoisomeric acetals (which showed some lability when heated). By this method (*)-3-chloropropan- 1,2-diol afforded four separable products one of which was transformed by base into ( R) -chloromethyloxiran.18 Alkenes via an addition-elimination sequence were elaborated to substituted allylic alcohols; chiral epoxyhalides with CH2=CHMgBr/Cu'I followed by iodide gave chiral allylic alcohol^.'^ Branched or linear homoallylic alcohols can be pre- pared from a trialkylborane and the anion of PhSeCH2CH=CH2. The key to this remarkable observation is that rearrangement of (15) is slow; short reaction-time generates linear product and a longer time gives branched product.20 The [2,3] Wittig sigmatropic rearrangement of chiral allylic ethers proceeds with very high chirality transfer (94-97Oh ) and erythro-selectivity (90-96% ) (see Scheme 6).Since (16) has been transformed into (-)-ephedrine this pathway constitutes a formal (chiral) total synthesis. Similar selectivity was noted for the crotyl propargyl ether (17) ;the 2-ether gave 88 :12 and the E-ether 7 :93 respectively erythro threo-product ratio. The silyl ether (S)-2-( 18) after rearrangement and reduction afforded I4 N. M. Roacher and D. K. Shaffer Tetrahedron 1984,40 2643. Is D. H. R. Barton and D. Crich J.Chem. SOC.,Chem. Commun. 1984 774. 16 L. Brown and M. Koneeda J. Org. Chem. 1984,49 3875. 17 T. Nonako S. Kanemoto K. Oshima and H. Nozaki Bull. Chem. Soc. Jpn. 1984 57 2019 8. M. Trost and Y. Masuyama Tetrahedron Lett. 1984,2S 173; K. S. Kim Y. K. Chang S. K. Bae and C. S. Hahn Synthesis 1984 866. 18 T. Mukaiyama 1. Tomioka and N. Shimizu Chem. Lett. 1984 49; M. K. Ellis B. T. Golding and W. P. Watson J. Chem. SOC.,Chem. Commun. 1984 1600. 19 J. Rodriguez J.-P. Dulcere and M. Bertrand Tetrahedron Lett. 1984 25 527; K. C. Nicolaou M. E. Duggan and T. Ladduwahetty ibid. p. 2069. 20 Y. Yamarnoto Y. Saito and K. Maruyama 1.Org. Chem. 1983.48 5408. 124 B. V. Smith .. ... HO Ph (R = Me) (2S 3s)-( 16) Reagents i BuLi (5eq)-THF -85 "C; ii NaI04 KMn04 Bu'OH; iii CH2N2-Et20 Scheme 6 SePh o\ * R' H (S)-(+) -(19) and further reduction gave the alcohol (3S,4S) -(20) the aggregation pheromone of the bark beetle which was formed in 98% e.e.by this path. Trans- mission of chirality via the transition state (21) is therefore highly efficient.21 Allylic butenols undergo Pd-catalysed phenylation with a concurrent 1,3-hydrogen shift. Acyclic allylic alcohols by addition formed iodo-diols or acetoxy-iodoalcohols with high regio- and stereo-selectivity (Scheme 7) ;the iodohydrin gave selectively epoxide (22).22 Syntheses of allenic alcohols and ethers have been reported.23 A2 + OH OR ii 'Ho 0-.. OH I (22) R = H or Ac Reagents i 12/H20 or MeC0,I; ii NaOH Scheme 7 21 N.Sayo E. Kitahara and T. Nakai Chem. Lett. 1984 259; K. Mikami K.-I. Azuma and T. Nakai Tetrahedron 1984,40 2303; N. Sayo K.-I. Azuma K. Mikami and T. Nakai Tetrahdronn Lerr. 1984 25 565. 22 W.Smadja G. Ville and G. Cahiez Tetrahedron Lett. 1984 25 1793; A. R. Chamberlin and R. L. Mulholland jun. Tetrahedron Len 1984 25 1793; A. R. Chamberlin and R. L. Mulholland jun. Tetrahedron 1984 40,2297. 23 J. Pornet B. Randrianoelina and L. Miginiac Tetrahedron Lett. 1984 25 651. Aliphatic Compounds -Part (ii) Other Aliphatic Compounds Other methods used for epoxidation include use of (i) NaI04-RuC13( H20),- bipyridyl (ii) a microsomal method (iii) NaOCl- meso-tetra(halogenopheny1)por-phyrinato-manganese complexes (iv) a-azohydroperoxides and (v) a peracid de- signed to exploit cis.- trans- alkene reactivity diff erence~.’~ Asymmetric epoxidation has attracted further interest; a chiral ketone and peroxomonosulphate gave oxidation of simple prochiral olefins with small e.e.(9-13%) by a probable path set out in Scheme 8.” Sharpless-type epoxidation of seven homoallylic alcohols using Ti(OPr’),-(+) or (-)-DET-Bu‘OOH gave 23-55% e.e. and enantioface selection opposite to that observed for allylic alcohols. An unprecendented and unexpected 2 + HSO; - I R’ ,Me 2O+ RZ Scheme 8 Ph Ph Ph Ph Ph Ph R’ = RZ= NHCHzPh Reagents “O+ (R3) HO’ 0 i Ti(OPri)4:Bu‘OOH:(R3) = 2:2 1; ii Ti(OPr’),:Bu‘OOH:(R3) = 2:2:2.4. Scheme 9 reversal of selectivity was observed with PhCH=C(Ph)CH20H when the ratio of titanium isopropoxide tartramide was changed (Scheme 9).Use of TiClz(OPri)z afforded chlorodiols from regiospecific opening of intermediate epoxides ; in this case the products are formed with opposite enantioselectivity to those produced by normal asymmetric ep~xidation.~’ Such reversal adds a useful bonus to this valuable method (Scheme 10). G. Balavoine C. Eskenazi F. Meunier and H. Rivikre ibid. p. 3187; V. Schurig and D. Wistuba Angew. Chem. Znt. Ed. Engl. 1984 23,796; B. de Poorter and B. Meunier Tetrahedron Lett. 1984 25 1895; T. Tezuka and M. Iwaki J. Chem SOC.,Perkin Trans. 1 1984 2507; J. Rebek jun. L. Marshall R. Wolak and J. McManus J. Am Chem. SOC.,1984 106 1170. ’’ R. Curci M. Fiorentino and M.R. Serio J. Chem. SOC.,Chem. Commun. 1984 155; B. E. Rossiter and K. B. Sharpless J. Org. Chem. 1984,49 3707; L. D.-L. Lu R. A. Johnson M. G. Finn and K. B. Sharpless ibid. p. 728. 126 B. V. Smith / \ R R OR' I I ii iv Reagents i Ti(OBu'), DET Bu'OOH; ii TiCl,(Oh'),; iii TiC12(0Pri)2DET Bu'OOH; iv NaOH (R' = Ac DET = (+)-diethy1 tartrate) Scheme 10 Stereoselective ring-closure to form trans-epoxides has been noted for P-hydroxy- sulphonium salts and base and from optically pure P-ethanolamines and dichlorocar- bene.26 By this latter route a-amino-acids can be transformed into synthetically useful epoxides. Deoxygenation of epoxides occurs efficiently with diazomalonic ester-Rh" acetate (with stereospecificity) with metals in aprotic solvents (probably via SET) and uia the use of alkyl manganese( 11) Di-lithium tetrabromonickelate( 11) in THF has been recommended as a source of 'soft' bromide ions; regioselective ring-opening gave bromohydrins in good yield and selectivity.28 Other examples of ring-opening reported include the reaction of epoxides with Me,SiNEt2 cuprates and R,CuC N Synthetically useful reactions of epoxides and transformations of 2,3-epoxyal- cohols have been re~iewed.~' 26 M.Shimagaki Y.Matsuzaki I. Hori T. Nakata and T. Oishi Tetrahedron Lett. 1984 25 4775 4779; L. Castedo J. L. Castro and R. Roguera ibid. p. 1205. 27 M. G. Martin and B. Ganem Tetrahedron Lett. 1984 25 251; K. N. Gurudutt M. A. Pasha B. Ravindranath and P.Srinivas Tetrahedron 1984,40 1629; T.Kauffmann and M. Biding Tetrahedron Lett. 1984 25 293. 28 R. D. Dawe T. F. Molinski and J. V. Turner Tetrahedron Lett. 1984 25 2061. 29 A. Papini A. Ricci M.Taddei G. Seconi and P. Dembech J. Chem. SOC.,Perkin Trans. 1 1984,2261; A. Ghibri A. Alexakis and J. F. Normant Tetrahedron Lett. 1984,25,3075,3079,3083; B. H.Lipshutz R. S. Wilhelm J. A. Kozlowski and D. Parker J. 0%.Chem. 1984,49,3928,3938. 3943. 30 J. G. Smith Synthesis 1984 629; C. H. Behrens and K.B. Sharpless Aldrichirnica Acta 1983 16 67. Aliphatic Compounds -Part (ii) Other Aliphatic Compounds 127 2 Alkyl Halides Chloroalkanes were obtained in excellent yields from an alcohol and N,N-diphenylchlorophenylmethyleneiminium chloride-Et,N ; (-)octan-2-01 gave 2-chloro-octane with inversion.Allylic propargylic (and glycosidic) hydroxy-groups were converted into chlorides via TsCl-DMAP-Et3N ; dienyl and other sensitive halides were obtained by use of ZnC1,-EtO2CN=NCO2Et-Ph3P in THF at ambient temperat~re.~~ Polymer-supported Ph3PBr2 (or Ph3P-polymer-CBr,) gave good yields of RBr.32 Direct a-iodination of an acid RCH2C02H was achieved in good yield by 12-AcOH-Cu" (OAC)~.~~ cis-172-Dichlorides were formed indirectly from 1-chloro-2-phenyl selenides by alkylation and subsequent displacement of selenium by chloride ion.34 Several applications of sonication have appeared. Asymmetric induction 30-60% has been noted for addition of a perfluoroalkyl iodide RFI to a chiral arene- chromium complex bearing an aldehyde function (23) ;subsequent decomplexation of the product gave (24).A remarkable change of pathway was claimed for PhCH2Br- KCN-A1203 in an aromatic solvent when sonication promoted nucleophilic displace- ment instead of Friedel-Crafts substitution observed with stirring alone. Alkyl halides in the presence of zinc under sonication add cleanly to a,p-enones in a conjugate manner. Di-iodomethane-zinc is effective for methylenation under such condition^.^' (OC)&r(Ar)CHO AZH(OH)R (23) (24) The species XCH2Li from Br-Li exchange is a precursor for halogenohydrins epoxides and a-halogenomethyl ketones by reaction at low temperature (-1 15 "C) with a carbonyl compound and appropriate recovery pr~cedure.,~a,o-Dihalogenoalkanes with Bu'Li undergo metal-halogen exchange almost certainly involving an SET pathway.Such a path is also suggested for reduction of an alkyl halide by LiAlH4 or AH,; traces of transition metals are not responsible for the effects observed. Ashby and co-workers have also adduced evidence for an SET mechanism in a Williamson ether synthesis and in the reaction between the lithium enolate of PhCOEt and a primary alkyl iodide.37 Reduction of alkyl chlorides was achieved in good yield and with a simple work up by use of Amberlyst A-26 anion exchange resin (BH form).38 31 T. Fujisawa S. Iida and T. Sato Chem. Lett. 1984 1173; C. K.'Hwang W. S. Li and K. C. Nicolaou Tetrahedron Lett. 1984 25 2295; P.-T. Ho and N. Davies J. 0%.Chem. 1984 49 3027. 32 P. Hodge and E. Khoshdel J.Chem. SOC. Perkin Trans. I 1984 195. 33 C. A. Horinchi and J. Y. Satoh Chem. Lett. 1984 1509. 34 A. M. Morella and A. D. Ward Tetrahedron Lett. 1984 25 1197. 35 A. Solladie-Cavallo D. Farkhani S. Fritz T. Lazrak and J. Suffert Tetrahedron Lett. 1984 25 4117; T. Ando S. Sumi T. Kawate J. Ichihara and T. Hanafusa J. Chem. SOC Chem Commun. 1984 439; C. Petrier J.-L. Luche and C. Dupuy Tetrahedron Lett. 1984 25 3463; J. Yamashita Y. Inoue T. Kondo and H. Hashimoto BulL Chem. SOCJpn. 1984 57 2335. 36 R. Tarkhouni B. Kirschleger M. Rambaud and J. Villibas Tetrahedron Lett. 1984.. 25 835. 37 W. F. Bailey R. P. Gagnier and J. J. Patricia J. Org. Chem 1984,49,2098; E. C. Ashby R. N. DePriest A. B. Goel B. Wenderoth and T. N. Pham ibid. p. 3545; E.C. Ashby D.-H. Bae W.-S. Park R. N. DePriest and W.-Y. Su Tetrahedron Lett. 1984 25 5107; E. C. Ashby and J. N. Argyropoulos ibid. p. 7. 38 J.-V. Weber P. Faller and M. Schneider C. R Hebd. Seances Acad. Sci. Il 1984 299 1259. 128 B. V. Smith A general review of photochemical behaviour of alkyl halides in solution has appeared and the competition between carbene and cationic pathways stressed.39 A route to E-1-bromo and 2-1-iodo-alkenes formed in very high purity relies on the treatment of (25) with Br2-MeOH or 12-NaOH.40 Direct preparation of o-bromoalk-1-enes has been effected by dehydrobromination of an a,o-dihalide with HMFT at 195-220 0C.41Functionalized allylic bromides in the presence of Zn and with sonication add regioselectively to terminal alkynes ;the dienes produced were cyclized to 6-or 7-membered carbo- or hetero-cycles.Regioselective head-to-tail coupling of allylic halides and allylic trialkyl stannanes (under high pressure) gave 1,Sdienes; in the presence of a sterically demanding proton equivalent at the y-carbon regioselective alkylation was possible at one site (see Scheme 11). This R -I Ryx -111 Me$ Me3Si Me,Si Reagents i LiCuBu, Et20 -78-100 "C (quant); ii NaCH(C02Et)2-THF; iii LiCuPh2 Et20 -78-+0"C (R = Bu X = Br) Scheme 11 is a valuable method since it allows functionalization at stereodefined alkenyl silanes to form avariety of other compounds!2 Preferential y-attack was noted for alkylation of the trimethylsilylallyl anion with Schlosser's base (K0Bu'-Bu'Li in he~ane).~~ The synthesis and reactivity of organoaluminiums (derived from unsaturated halides) has been reviewed.Copper( 1)-catalysed reactions of RLi/ RMgX have been summarized.44 39 P. J. Kropp Acc. Chem. Res. 1984 17 131; P.J. Kropp J. A. Sawyer and J. J. Snyder J. Org. Chem. 1984,49 1583. 40 H. C. Brown and V. Somayaji Synthesis 1984,919. 41 G. A. Kraus and K. Landgrebe Synthesis 1984 885. 42 Y. Yamarnoto K. Maruyama and K. Matsumota J. Chem. SOC.,Chem. Commun. 1984,548; P. Knochel and J. F. Normant Tetrahedron Left. 1984 25 1475; J. Kang W. Cho and W. K. Lee J. Org. Chem. 1984,49 1838. 43 K. Koumaglo and T. H. Chan Tetrahedron Lett. 1984 25 717. 44 F. Barbot Bull. SOC.Chim. Fr. 11 1984 83; E. Erdik Tetrahedron 1984.40 641.Aliphatic Compounds -Part (ii) Other Aliphatic Compounds 129 3 Aldehydes and Ketones This section again the largest in this Report attempts to highlight initiation or development of processes of interest and therefore cannot be considered a complete summary of all published work. Grignard reagents react efficiently with lithium or sodium formate in boiling THF forming aldehydes in good yield.45 Oxidation of vinylsilanes to carbonyl compounds can be directed according to conditions (and the structure of the substrate) to form aldehyde or acid ur ketone.46 2-and E-epoxysilanes were transformed into corresponding 2-and E-silylenol ethers of an aldehyde; by this approach an alkyne was converted into a 2,2,2-trialkylated aldehyde in which each alkyl group could be different (see Scheme 12).47Chiral acetals e.g.(26) with h S i Me Ph Reagent i BF3:OEt2-CH2C12,-78 "C Scheme 12 CONMe2 R' XYoNMe2 R CONMe2 trialkylaluminium gave an adduct (27) formed in eve. which gave in turn a chiral &substituted aldehyde.48 Chiral oxathione (28) prepared from (+)-pulegone gave (29) in high e.e. (100% when R = Ph) and (30) gave with MeMgI (31) in which the product ratio was 96:4 in favour of (31a); methylation and cleavage of the thiane gave (S)-(-)-atrolactic acid methyl ether (100% e.e.).49 HO rzeH &0 R I (28) (29) krcoR k&R1 kdoH LNTR (30) (314 (31b) (32) 45 M. Bogavac L. ArsenijeviC S. Pavlov and V. ArsenijeviC. Tetrahedron Lett. 1984 25. 1843.46 K. Tamao M. Kumada and K. Maeda Tetrahedron Lett. 1984,25 321. 47 I. Fleming and T. W. Newton J. Chem SOC.,Perkin Trans. I 1984 119. 48 J. Fujiwara Y. Fukutani M. Hasegawa K. Maruoka and H. Yamamoto J. Am Chem SOC.,1984 106 5004. 49 E. L. Eliel and S. Moms-Natschke J. Am. Chem Soc.. 1984 106 2937; J. E. Lynch and E. L. Eliel ibid. p. 2943. 130 B. V. Smith Acyl chlorides reacted smoothly with Grignard reagents in the presence of Fe"'(a~ac)~,in a convenient ketone synthesis; organomanganese reagents RMnI (from RLi or RMgX + MnI,) reacted analogo~sly.~~ Organolithiums (or Grignard reagents) coupled efficiently with N-acylaziridines ;the intermediate (32) on hydroly-sis gave ketones in yields greater than 70% .'l 1,2-Migration of alkyl groups in chiral P-mesyloxyalcohols effected by Et2AlC1 gave optically pure a-alkylketones ;this method looks promising and capable of elaboration.The stereospecificity was presumed to arise from chelation control in (33).52 As shown in Scheme 13 rearrange- ment of (34) gave (35) cleanly at low temperature. Dilithioacetoacetate gave after alkylation and decarboxylation methyl ketones and is a synthetic equivalent to acetone en01ate.~~ The substituted imidazole (36) (from N-methylimidazole-BuLi- R'RZC=O) acts as a masked carbonyl compound since on quaternization and hydrolysis it regenerated the ketone. This may be a useful way of protecting the carbonyl gro~p.'~ Me BU OH . Bu )-COBu MsO H Bu Me H (90%)(35) Reagent Et2AICI -78 "C 1.5h Scheme 13 I I -Me Me Some other published methods for ketone synthesis include carbonylation of ArCHzX which gave ArCH2COCH2Ar and phase transfer catalysis (with Fe(C0)') or with Ni-CO.Palladium-catalysed oxidation of alkenes to ketones has been re~iewed.'~ Several methods for reduction of carbonyl groups have been reported. Photoreduc- tion of aldehydes and ketones with Bu3SnH led to tributylstannyl ethers of derived alcohols and as minor products the analogous derivatives of pinacol~.~~ Methyl ketones MeCOAr were reduced by monoisopinocampheyl borane in modest e.e. V. Fiandenese G. Marchese V. Martina and L. Ronzini Tetrahedron Lett. 1984,25 4805; G. Friour G. Cahiez and J. F. Normant Synthesis 1984 37. " S. Wattanasin and F.G. Kothawala Tetrahedron Lett. 1984 25 84. 52 G. Tsuchihashi K. Tomooka and K. Suzuki Tetrahedron Lett. 1984,25 1817,4253. 53 R. A. Kjonaas and D. D. Patel Tetrahedron Lett. 1984 25 5467. 54 S. Ohta S. Hayakawa K. Nishimura and M. Okamoto Tetrahedron Lett. 1984,25 3251. 55 G. Tanguy B. Weinberger and H. des Abbayes Tetrahedron Lett. 1984,25 5529; S. Inaba and R. D. Rieke Chem. Lett. 1984 25; J. Tsuji Synthesis 1984 369. 56 M. H. Fisch J. J. Dannenburg M. Pereyre W. G. Anderson J. Rens and W. E. L. Grossmann Tetrahedron 1984 40,293. Aliphatic Compounds -Part (ii) Other Aliphatic Compounds which was dependent on the ketone reagent ratio; a 1 :1 mixture gave best results.’’ High pressure (6000atm) gave enhanced e.e. and faster reaction in reduction of ketones by ‘Alpine borane’ and suppressed an undesirable side-reaction ; thus PhCOMe gave optically pure alcoh01.’~ An interesting switch in enantioselectivity on changing from boron to aluminium was reported for 23-( cis-lO-pinany1)-9-borabicyclo(3,3,l)nonane (37) and its aluminium counterpart.” Such a switch in selectivity is useful and promises further application.LAH which has been modified by reaction with R-(or S)-(38) gave high e.e. in ketone reductions and with (37a) (37b) (38) PhCOCH2Br yielded styrene oxide with 95% e.e.60 An improvement in enantioselec- tivity was observed when ketones with iodo- or phenylsulphonyl-groups were reduced by yeasts; subsequent removal of these groups gave alcohols with very high e.e. and much higher than that observed with unsubstituted starting materials.Such microbial asymmetric catalysis has been reviewed.61 N-Benzoylcysteine was an efficient chiral ligand in the reduction of RCOAr by LiBHq?’ At low temperature (-100 -+ -78 “C) Pr”C0Phgave R-PrCH(0H)Ph (92% e.e.). Two applications of Raney Ni in reduction involve firstly modification by (KR)-tartaric acid-NaBr for reduction of ketones (interestingly traces of carboxylic acids e.g. Bu‘C02H improved the optical yield) and secondly ‘deuteration’ which gave exchanged products.63 Stereoselective reduction of acyclic ketones with introduction of chiral centres has been reviewed.64 The reaction shown in Scheme 14 has been achieved (R = Ph) and was selective (e.g. PhCH=CHCHO gave 83% yield of unsaturated al~ohol).~’ Scheme 14 57 H.C.Brown and A. K. Mandal J. Org. Chem. 1984 49 2558. 58 M. M. Midland and J. I. McLoughlin J. Org. Chem. 1984 49 1316. 59 M. M. Midland and J. I. McLoughlin J. Org. Chem. 1984 49 4101; G. Giacomelli L. Lardicci and F. Palla ibid. p. 310. 60 R. Noyori I. Tomino M. Yamada and M. Nishizawa J. Am. Chem. Soc. 1984 106 6717. 61 K. Nakamura K. Ushio S. Oka and A. Ohno Tetrahedron Lett. 1984 25 3979; C. J. Sih and C.-S. Chen. Angew. Chem. Int. Ed. Engl. 1984 23 570. 62 K. Soai T. Yamanoi and H. Oyamada Chern Lett. 1984 251. 63 T. Osawa and T. Harada Bull Chem. Soc. Jpn. 1984 57 1518; P. M. Pojer Tetrahedron Lett. 1984 25 2507. 64 T. Oishi and T. Nakata Acc. Chem. Res. 1984 17 338. 65 J.D. Wuest and B. Zacharie J. Org. Chem. 1984 49 163 166. 132 B. V. Smith Organocerium compounds (from RLi-CeCl, -78 "C THF) reacted cleanly with enolizable ketones giving tertiary alcohols. The advantage of the method is that it avoids enolate formation (from ketone and RLi or RMgX) and undesired side- reactions. Thus (PhCH2)2C0 gave (PhCH2)2C(OH)Bu (96%) by this route cj 20% obtained uia the Grignard reagent.66 The method may also be applied to synthesis of alkenyl- and alkynyl-derivatives. A striking improvement in stereoselectivity of addition of p-C7H7SOCH2X to RCHO was noted when X = ZnCl rather than with X = Li; desulphurization of the adduct (Raney Ni) gave chiral alcohols with very high e.e.67 A chiral amino- alcohol improved the e.e.during addition of Et,Zn to RCH0.68 The nucleophilic acylation (umpolung) of an aldehyde has been achieved by chiral(39) as the synthetic equivalent of (40);69 high e.e. was achieved. The effect (or absence) of chelation control in nucleophilic addition to chiral a-and P-alkoxyaldehydes has been thoroughly explored and s~rveyed.~' A discrepancy between the work of Heathcock and Reetz's group has been cleared up.7' 0 Several papers have exploited boron reagents in synthesis. H. C. Brown has reported very high (86-99%) e.e. in homoallyl alcohols formed from B-allyldi- isocaranyl borane and RCHO; a corresponding selectivity was found with B-methylallyldi-isopinocampheylborane (e.e. >go% ).72 Chiral a-chloroallylboronate esters or 2-y-alkylthioallylboronates,with RCHO gave a high degree of control in formation of homoallylic alcohols and these methods would allow introduction of other functional groups uia the chlorine or sulphur atoms in the products.73 Enol boronates (from aldehydes or ketones) react readily with aliphatic or aromatic aldehydes or ketones; crossed-aldolization of ketones was achieved in moderate yields.High erythro-threo-diasteroselectionwas achieved by this route. A convenient one-pot method exploited the addition of an allylboronate to an aldehyde; complete regio- and stereo-control was noted in this simple and improved process (Scheme 15). The use of (41) implied a synthetic equivalent for (42) or (43).74 Addition of 2-enolborate (44) to RCHO gave the same product ratio as from the E-isomer a 66 T.Imamoto Y. Sugiura and N. Takiyama Tetrahedron Lett. 1984 25 4233. 67 M. Braun and W. Hild Chem. Ber. 1984 117 413. N. Ogumi and T. Omi Tetrahedron Left. 1984 25 2823. 69 M. Braun and W. Hild Angew. Chem. Inf. Ed. Engl. 1984 23 723. 70 M. T. Reek and K. Kesseler J. Chem. SOC Chem. Commun. 1984 1079; M. T. Reetz K. Kesseler and A. Jung Tetrahedron Lett. 1984 25 729; M. T. Reetz Angew. Chem. Int. Ed. En& 1984 23 556. 71 H. C. Brown and P. K. Jadhav. J. Org. Chem. 1984,49,4089. 72 H. C. Brown P. K. Jadhav and P. T. Perumal Tetrahedron Lett. 1984 25 5111. 73 R. W. Hoffmann and B. Landmann Angew. Chem. Int. Ed. Engl. 1984 23,437; R. W. Hoffmann and B. Kemper Tetrahedron 1984 40,2219. 74 C. Gennari L. Colombo and G.Poli Tetrahedron Letf. 1984 25 22;3 2283; P. G. M. Wuts P. A. Thompson and G. R. Callen. J. Org. Chem. 1983,48 5398. Aliphatic Compounds -Part (ii) Other Aliphatic Compounds THPO- Li R O V\ RO CO,R (41) (42) (43) Reagents i BCH,Cl ; ii RCHO 0 Scheme 15 surprising result. Kinetic control of product formation was ass~med.~’ Another promising development is the synthesis of chiral boronic and borinic esters uia asymmetric hydroboration-displacement. In the first case 2-butyldi-isopinocam- pheyl borane (from Ipc,BH + but-2-ene) gave with 2 eq MeCHO diethyl 2-butyl- boronate hydrolysed and re-esterified to the (+)-dimethyl analogue; oxidation of this in the usual way gave butan-2-01 of 97% e.e. A chiral borinate (49 with LiCC1,OMe and oxidation gave RCOCH(Me)Et in 70% e.e.76 Chiral or achiral oxazolines e.g.(46) as boronazaenolates reacted with aldehydes to give adducts which on hydrolysis and methylation gave e.g. p-hydroxyester (47) with high threo-preferen~e.~~ The chiral analogue (48) showed a high selectivity but modest e.e. in the formed hydroxye~ter.~~ Enol stannanes of ketones reacted with RCHO at low temperature (with kinetic control) and formed threo-aldols preferentially. Highly diastereoselective cross-ado1 reactions were carried out using tin enolates derived from reaction of stannous 75 R. W. Hoffmann and K. Ditrich Tetrahedron Lett. 1984 25 1781. 76 H. C. Brown P. K. Jadhav and M. C. Desai Tetrahedron 1984,40 1325. 77 A. I. Meyers and Y. Yamamoto Tetrahedron.1984 40,2309. 134 B. V. Smith OMe (47) (48) syn-(49) triflate with carbonyl compounds (93:7 erythro :threo aldol ratio for Et2CO- Pr'CHO). The value of chiral auxiliary in these reactions has been stressed.78 Extension of this approach to the reaction of R'COCH2Br and R2CH0 gave syn- and anti- bromoaldols (49) smoothly converted into oxiranes by KF-18-crown-6. Crotylmetal compounds e.g. (50) with RCHO-BF3 showed high erythro-selectivity which was independent of geometry in the starting material. Under pressure the erythro-threo ratio was lower. The ratio of Cram anti-Cram product was examined and it was concluded that a crown-like transition-state was implicated.79 Keck and co-workers" have studied related processes with chiral a-alkyloxyaldehydes and emphasized the crucial role of the Lewis acid required ; for P-hydroxyaldehydes MgBrz gave a high erythro- threo ratio.Almost three-fold increase in erythro-selec- tivity was achieved by using 2 eq of stannane; interestingly the TiC1,-mediated reaction of crotyltributylstannane and RCHO gave high erythro- or threo-sensitivity according to the order of mixing. The stereochemistry of allylmetal-aldehyde addi- tion was probed via the ring-closure of (51) to (52) and (53); the syn-anti-ratio was 82 18 (TiCl,) but 99 1 with CF3C02H.8' Another variation on this theme was achieved by using stannous azaenoalates derived from chiral 1,3-0xazolidines and aldehydes in the presence of a chiral norephedrine which formed aldols in high e.e.82 CCH0 Q/SnBu3 The significance of chelation control in Lewis acid-mediated aldolization via enol silanes and chiral a-and P-alkoxyaldehydes has been stressed since it represents the only way known at present to achieve such control.With a prochiral enolsilane the selectivity was surprisingly good. This unusual effect has been attributed to syn-complexation of the aldehyde e.g. (54); such an effect in an achiral aldehyde should be expected to lead to selectivity and this was demon~trated.'~ This interesting 78 S. S. Labadie and J. K. Stille Tetrahedron 1984,40 2329; T. Mukaiyama N. Iwasawa R. W. Stevens and T. Haga ibid. p. 1381. 19 Y. Yamamoto H. Yatagai Y. Ishihara N. Maeda and K. Maruyama Tetrahedron 1984,40 2239. G. E. Keck and E.P. Boden Tetrahedron Lett. 1984 25 265 1879; G. E. Keck D. E. Abbott E. P. Boden and E. J. Enholm ibid. p. 3927. 81 S. E. Denmark and E. J. Weber J. Am. Chem Soc,1984 106,7970. 82 K. Narasako T. Miwa H. Hayashi and M. Ohta Chem. Lett. 1984 1399. 83 M. T. Reetz K. Kesseler and H. Jung Tetrahedron 1984.40 4327. 135 Aliphatic Compounds -Part (ii) Other Aliphatic Compounds paper also pointed out that the sense of diastereoselection was independent of geometry in the silane and that for (54) the TiCI,-mediated addition of (55) took place with very high selectivity whereas SnCl or BF3gave stereorandomness. Dubois has discussed the conclusions to be drawn from addition of (56) to R’CHO-TiC1,; the erythro-threo-ratio was determined for R = R’ = But (89:11) and R = Me R’ = But (1 1 :89) and was held to be inconsistent with a cyclic tran~ition-state.~~ Hwos RZ RO H R = PhCH Ph (R = alk Z = OSiMe,) (54) (55) (56) Crotyl metal compounds as enolate equivalents e.g.(57) with RCHO usually give threo-alcohols preferentially; another remarkable reversal occurred with BF3 however to form erythro-alcohols as major products. As an example of the latter process E-(57)(X = Br) and Et,CHCHO gave an erythro threo-ratio of 6:94. Reetz has suggested that (58) satisfactorily accounted for this process; but it should be noted that BF3 did not reverse the threo-diasteroselectivity of MeCH=CHCH2Ti(0P3),.” A new process for chain elongation (by 3 units) has been developed; an aldehyde and a Ti-ate complex (59) [from 1-lithio-(1- a1koxy)allyltrimethyl silane and Ti(OPe),] reacted with high regioselectivity to form dienylethers in high yield.86 The net effect is thus to convert RCHO into RCH2COCH=CH2 and this approach was applied to some natural product syn- theses.Chiral metal complexes e.g. (60) were used as a source of enolate which gave aldolization with high stereoselectivity. As shown in Scheme 16 some choice in selection was achieved by changing conditions although kinetic control was assumed on the basis of time/temperature studies. The aluminium enolate of (60) was used 84 J.-E. Dubois G. Axiotis and E. Bertonnesque Tetrahedron Lett. 1984 25 4655. 85 M. T. Reetz and M. Sauerwald J. Org. Chem 1984,49 2292. 86 A. Murai A. Abiko N.Shimada and T. Masumume Telrahedron Lett. 1984 25 4951. 136 B. V. Smith (60) (XFe = chiral Fe complex) Reagents i LDA; ii MX = BuiAlCI then EtCHO; A:B = 5 1; iii MX = SnCI, then EtCHO; A:B = 1 11.6 Scheme 16 as the key intermediate in preparation of P-hydroxyacids; it is therefore acting as a chiral acetate enolate eq~ivalent.’~ Similar reactions were employed with (61) and C,F,Li; the adduct was transformed into S-(+)-(62) (88% e.e.) and in another path into chiral derivatives of phenylacetic acid.88 A new method for hydroxymethylation which uses PhCH20CH2Cl as a practical equivalent for CH,OH relied on samarium(11) iodide to couple a chloro-ether and a ketone.” Stereoisomerically pure sulphinylmethyldihydroisoxazoleshave allowed a highly stereocontrolled entry into a route leading to chiral p,/?’-dihydroxyketone~.~~ A route to a-ketoesters from aldehydes has been explored.” The keto-enol ratio for aqueous solutions of aliphatic ketones has been remea~ured;’~ for Me,CO the best value of pK was 19.16 * 0.04 and for pK was 8.22 f 0.08.The direct method for resolution of ketones via reaction with the a-lithio- derivative of N,S-dimethyl-S-phenylsulphoximine(63) has been de~eloped.’~ The diastereoisomeric adducts were separated by flash chromatography and separately pyrolysed at 130 “C to regenerate ketone enantiomers. The success of the method depended on lack of racemization during work-up; so in some cases complete resolution was achieved but in others racemization was a severe drawback.The superiority of lithium t-octyl-t-butylamide (over e.g. LDA) in regioselective and stereoselective generation of enolates has been stressed. With Et2C0 the E :2 ratio in the enolate was 98 :2 the highest yet; this ratio was reversed (18 :82) in the presence of HMPA. The hindered base could also be used for near-regiospecific preparations of trimethylsilyl enol esters. Silyl enol ether anions with base rearranged to form a-silylated ketones ;the tri-isopropylsilyl enol ether of e.g. Et2C0 gave with BuLi-Bu‘OK the rearrangement product in 60% yield in 24h. This intramolecular rearrangement occurred preferentially towards a less-hindered ter- minus since Pr’CH,COMe gave Pr‘CH2COCH2Si(Pr’)3 as sole The use of ultrasound-colloidal potassium in toluene has been applied to several processes 87 L.S. Liebeskind and M. E. Walker Tetrahedron Lett. 1984 25 4341 :S. G. Davies 1. M. Dordor and P. Warner J. Chem. Soc. Chem. Commun. 1984 956. 88 A. Solladie-Cavallo and J. Suffert Tetrahedron Lett. 1984,-25 1897. 89 T. Imamoto T. Takeyama and M. Yokoyama Tetrahedron Lett. 1984 25 3225. w R. Annunziata M. Cinquini F. Cozzi and A. Restelli J. Chem. SOC.,Chem. Commun. 1984 1253. 91 D. Home J. Gandino and W. J. Thompson Tetrahedron Lett. 1984 25 3529. 92 J. Toullec Tetrahedron Lett. 1984 25 4401; Y. Chiang A. J. Kresge Y. S. Tang and J. Win J. Am. Chem. SOC.,1984 106 460. 93 C. R. Johnson and J. R. Zeller Tetrahedron 1984,4Q 1225. 94 E. J. Corey and A. W. Gross Tetrahedron Lett. 1984,25,495; E.J. Corey and C. Rucher Tetrahedron Lett. 1984 25 4345. Aliphatic Compounds -Part (ii) Other Aliphatic Compounds e.g. preparation of silyl ethers (and Dieckmann and Wittig reactions). This approach looks useful in a range of carbanion-generating application^.^' Further work has shown that a-alkylation of ketones using activated alkyl halides (or ROAc-Lewis acid) and a ketone was regiospecific for unsymmetrical ketones and avoided polysubstitution. The ZnC1,-promoted addition of (64) to ketones (or enones) allowed entry into a-ketoesters. Tertiary halides also reacted furnishing R3CCOC02Et.96A different approach to alkylation used chiral hydrazones (65) which by metallation reaction with an electrophile and cleavage gave a chiral ketone in good yield and e.e.> 94Y0.~'The reagent Cp2Ti=CH2 methylenates enolizable ketones in a process analogous to the Wittig reaction.98 Other processes repoped and reagents used include a-bromination of ketones (Bu'Br-Me,SO ; via BrSMe2?) generation of enolates from a-halocarbonyls (by Bu3SnAlEt2 or Bu,PbAlEt) cleavage of hydrazones [Fe"'(NO,) on clay] and construction of synthetic equivalents of synthons with three consecutive chiral centres.99 Routes to the thiols (66) and (67) have been developed since they are precursors to a,P-unsaturated aldehydes."' Addition of RLi to a,P-unsaturated acetals and hydrolysis of the formed enol ethers was reported as another entry to enals.'O1 Synthetic methods for enones include the addition of enolates from trimethylsilyl ketones to aldehydes (an efficient process with E-selectivity) alkylation (by Rx) of the carbanion from (68) followed by desulphurization-hydrolysis [it is notable that (69) the precursor of (68) thus acts as a novel homoenolate dianion equivalent] S0,Tol R OMe 95 J.L. Luche C. Petrier and C. Dupuy Tetrahedron Lett. 1984 25 753. 96 M. T. Reetz P. Walz F. Hubner S. H. Huttenhain H. Heimbach and K. Schwellnus Chem Ber. 1984 117 322; M. T. Reetz H. Heimbach and K.Schwellnus Tetrahedron Lett. 1984 25 511. 91 D. Enders H. Eichenauer U. Baus H. Schubert and K. A. M. Kremer. Tetrahedron 1984,40 1345. 98 L. Clawson S. L. Buchwald and R. H. Grubbs Tetrahedron Lett. 1984 25 5733. 99 E. Armani A. Dossema R. Marchelli and G. Casnati Tetrahedron 1984 40,2035; N.Tsuboniwa S. Matsubara Y. Morizawa K. Oshima and H. Nozaki Tetrahedron Lett. 1984,25,2569; R. M. Moriarty and K.-C. Hou ibid. p. 691; P. Laszlo and E. Polla ibid. p. 3309 3313; T. Nakata M. Fukui H. Ohtsuka and T. Oishi Tetrahedron 1984 40,2225. 100 K. Oguro T. Iihama K. Takahashi and H. Iida Tetrahedron Lett. 1984 25 2671; M. Julia and C. Lefebvre ibid. p. 189. 101 C. Mioskowski S. Manna and J. R. Falck Tetrahedron Lett. 1984 25 519. 138 B. V. Smith and palladium-catalysed addition of ArI to a terminal alkyne in the presence of Zn-Cu/CO (other products were also formed).'02 A one-pot process for chiral P-alkoxy-a#-enones used the sodium derivative of an unsymmetrical 1,3-dicarbonyl and chiral alk~xides."~ Identification (and differentiation) of E-and 2-a$-unsaturated ketones (and esters) has been suggested on the basis of differences in 3JC0,Hcoupling constants measured in n.m.r.spectra.'@' Reduction (by L-selectride) of a-methyl-P y-unsaturated ketones showed high threo-selectivity in the formed homoallylic alcohol. The selective reduction of the carbon-carbon double bond in an enone (or a,@-unsaturated nitro-compound) was achieved in excellent yield by using Hantzsch's ester (70) with silica gel in benzene.'" Hindered allylic alcohols and crowded saturated ketones have been obtained from enones in sequences of reduction and addition of organometallics ; addition of (PhMe2Si),CuLi to an a$-unsaturated ketone gave a P-silylated ketone whose enolate showed significant diastereoselection on alkylation.lM In this latter process an open-chain transition state (71) was favoured since the E-enolate from certain substrates cannot form a chelate.Michael addition of P-dicarbonyls to a,p-enones occurred in the presence of CU"(OAC)~. Although yields were variable the use of neutral conditions does have some advantages.'" HH High e.e. (>99%) was noted for the Et,Al-mediated pinacol-type rearrangement of mesyloxylalcohols to substituted p,y-unsaturated ketones. Thus (72) by mesyla- tion and rearrangement gave (73) in a stereospecific process.'o8 Regiospecific formation of y,S-unsaturated ketones was achieved by intramolecular decarboxyla- 102 I. Matsuda H. Okada S. Sat0 and Y. Izumi Tetrahedron Lett. 1984,25,3879; T.Mandai T. Moriyama Y. Nakayama K. Sugino M. Kawada and J. Otera ibid. 1984 25 5913; Y. Tamaru H. Ochiai and Z.-I. Yoshida ibid p. 3861. 103 A. Lubineau and A. Malleron Tetrahedron Lett. 1984 25 1053. 104 B. Gregory W. Hinz R. A. Jones and J. S. Arques J. Chem. Res. (S) 1984 31 1. '05 K. Suzuki E. Katayama and G. Tsuchihashi Tetrahedron Lett. 1984,25,2479; K. Nakamuro M. Fujii A. Ohno and S. Oka ibid. p. 3983. 106 C. Lion J.-E. Dubois and I. Saumtally C. R. Hebd. Seances Acad. Sci. II 1984,298,783; W. Bernhard I. Fleming and D. Waterson J. Chem Soc. Chem. Commun. 1984 28. 107 A. C. Coda G. Desimoni P. Righetti and G. Tacconi Gazz. Chim. Ztaf. 1984 114 417. 108 K. Suzuki E. Katayama and G. Tsuchihashi Tetrahedron Lett. 1984 25 1817.Aliphatic Compounds -Part (ii) Other Aliphatic Compounds tive allylation of ally1 p-ketocarboxylates (or alkenylallyl carbonates) in the presence of Mo- Ni- and Rh-cornple~es.'~~ Stereospecific preparation of E-and 2-enones from y-hydroxyalkylstannanes-Pb(OAc)4reflected the stereochemistry of the starting material e.g. (74) gave only (75)."O Acetylenic ketones have been obtained in an improved method based on acylation of Li( RC_C.BF,) or by phase-transfer-catalysed alkynylation of R'COCH2R2 by BrCH2C_CR3."' The coupling of RCOCl (with Sm"12) formed a-diketones probably via a sequence involving acyl radicals and anions.'12 Acylation of a chiral imide enolate has been developed as a route using chiral P-dicarbonyl synthon equivalents.' l3 A general synthesis of lY4- lY5- and 1,6-diketones is an ingenious exploitation of borane chemistry (see Scheme 17).Il4 Ozonolysis of 1,2-dialkylcyclopentenesaffords a practi-cable specific route to 1,5-diketone~."~ OAc Reagents i CH,=CHR'; ii A(CH,).J\RJ :iii co; iv wc Scheme 17 109 J.Tsuji I. Minami and I. Shimizu Chem. Left. 1984 1721. 110 K. Nakatami and S. Isoe Tetrahedron Lett. 1984 25 5335. Ill H. C. Brown U.S. Racherla and S. M. Singh Tetrahedron Leu. 1984 25 2411; E. V. Vasil'eva E. M. Auvinen and I. A. Favorskaya J. Org. Chem. U.S.S.R 1984 19 1266. 112 J. Souppe J.-L. Namy and H. B. Kagan Tetrahedron Lett. 1984 Z,2869. I I3 D. A. Evans M. D. Ennis T.Le,N. Mandel and G. Mandel 1. Am Chem SOC,1984 106 1154. I14 H.C. Brown U.S. Racherla and S. M.Singh Synthesis 1984,922. I15 K. Abe H.Okumura T. Tsugoshi and N. Nakamura Synthesis 1984,603. 140 B. V. Smith 4 Carboxylic Acids and Esters Anhydrous salts of carboxylic acids were obtained from acyl halides or esters by action of M+O-SSiMe3 in ether or THF.l16 Cu' salts have been employed for decarboxylation or oxidative-decarboxylation of acids yielding hydrocarbons and ketones respe~tively."~ 1,l '-Oxalyl-di-imidazole a new reagent for activating acids reacts to form an acylimidazole which is converted into esters by an alcoho1.118 Zinc salts of acids (or phenols) react with t-alkyl halides in non-polar solvents in the presence of a base forming esters (or ether^)."^ Alkyl halides react with (R0)3B-C0 in the presence of Rh' diene complex and Pdo forming esters in good yield.12' Alkenes when treated with a catalytic amount of Pd(OCOCF,) in the presence of benzoquinone and o-methoxyacetophenone (ligand) gave rise to allylic acetates; the composition of the mixture was variable.The Fe"-S20i-AcOH system gave truns-vicinal diacetates probably uia a free-radical route.121 A range of acids was reduced cleanly to aldehyde by thexylchloroborane-SMe2 reagent usually in yields of 90% (by 2,4-DNP). Other functionalities were unaff ected.'22 Acids with 2eq of amine in PPSE gave imidates presumably uiu the amide. Chiral imidoesters RCH2C(OEt)NR* (* = chiral group) were deprotonated and alkylated in excellent yield and with good/very good asymmetric induction to form a,a-disubstituted carboxylic acids e.g.BuCH( Me)C02Et (optical purity 94% ).123 An asymmetric synthesis of quaternary centres (a,a-disubstituted acids) depended on the chiral bicyclic lactam (76) [from L-valinol and PhCO(CH2)2C02H] ;deproton-ation and sequential alkylation gave after hydrolysis (77) with >95% enantiomeric purity. Control in the alkylation was indicated by obtaining each enantiomer of (77) depending 0.n the order of introduction of the alkyl groups.'24 Alkanoate esters with Et3SiOC103-R3N gave the useful O-silylketen acetals in high yields. Allylic acetates/trifluoroacetatesunderwent CF3C02H-catalysed pheny- lation ;a cationic pathway was s~ggested.'~' 116 E. D. Langanis and B. L. Chenard Tetrahedron Lett. 1984 25 5831. 117 0.Toussaint P.Capdeville and M. Maumy Tetrahedron 1984 40 3229; Tetrahedron Lett. 1984 25 3819. S. Murata Chem. Lett. 1983 1819. B. Ravindranath and P. Srinivas Tetrahedron 1984 40 1623. J. B. Woell and H. Alper Tetrahedron Lett. 1984 25 3791; K. E. Hashem J. B. Woell and H. Alper ibid. p. 4879. 121 J. E. McMurry and P. KoEovskL Tetrahedron Lett. 1984 25 4187; W. E. Fristad and J. R. Peterson Tetrahedron 1984 40 1469. 122 H.C. Brown J. S. Cha B. Nazer and N. M. Yoon J. Am. Chem. Soc. 1984 106 8001. 123 M. Kakimoto S. Ogata A. Mochizuki and Y. Imai Chem Lett. 1984 821; C. Gluchowski T. Tiner-Harding J. K.Smith D. E. Bergbreiter and H.Newcomb J. Org. Chem. 1984 49 2650. 124 A. I. Meyers M. Hanne and R. Garland J. Am. Chem. Soc. 1984 106 1146.125 C. S. Wilcox and R. E. Babston Tetrahedron Lett. 1984 25 699; Y.Fujiwara H. Kuromaru and H. Taniguchi J. Org. Chem. 1984 49 4309. Aliphatic Compounds -Part (ii) Other Aliphatic Compounds 141 (R)-and (S)-2-acetoxy-1,2,2-triphenylethanol,via deprotonation/aldolization has been converted into chiral P-hydroxyacids; the reagent acts as the synthetic equivalent of a chiral acetate enolate. The lithiated camphor-based oxazoline (78) served as a precursor to chiral a-hydroxyacids and is thus equivalent to a chiral glycolate synthon. a-Hydroxy (and a-mercapto-) acids can be alkylated without racemisation via the method shown in Scheme 18. The 'self-reproduction of chirality' affords a useful entry to this type of system and a chiral auxiliary is not necessary.P-Hydroxyesters (from P-ketoesters and yeast-mediated reduction) were alkylated (2 eq LDA then RX) with high selectivity.'26 R~-CHCO,H I __* XH H x __ H H cis trans + 1 R'R~CCO,H iii "~o~oR *-I XH HX R2 Reagents i Me,CCHO; ii LDA-THF then R2X (or RCHO); iii H+-H~O Scheme 18 Enhancement of selectivity in ketoester reduction has attracted attention. a-Ketoesters gave excellent e.e. (lOOo/~ for PhCOC0,Bu') with Alpine borane or with complexes from Et,Al and Darvon alcohol (2S,3R)-(+)-4-dimethylamino-1,2-diphenyl-3-methylbutan-2-01.In the latter case reduction of menthyl esters was accompanied by a-alkylation. The stereochemistry of the product was dependent on the use of (+)-or (-)-menthol in e~terification.'~' Reduction of P-ketoesters by yeast was improved by a-sulphenylation prior to reduction separation of the erythro-and threo-products and desulphurization; by this route MeCH(OH)CH2C02Et was prepared optically pure.Enhanced e.e. was claimed for reduction of y,y,y-trihalogeno-derivatives. High e.e. (80% +) was noted in the use of a thermophilic bacterium Thermounaerobium brockii to reduce the P-carbonyl function of ketoesters.12* a-Alkylated-P-ketoesters are alkylated cleanly and with good-excellent e.e. by prior conversion into chiral enamines based on S-valine t-butylester followed by the usual deprotonation-alkylation sequence. A solvent dependence in selectivity was noted for the alkylation step.'29 A Wittig sequence was used to prepare y,8-unsaturated-f3-ket0esters.'~~ I26 M.-Braun and R.Devant Tetrahedron Lett. 1984 25 5031; T. R. Kelly and A. Arvanitis ibid. p. 39; D. Seebach R. Naef and G. Caldebari Tetrahedron 1984 40,1313; G. Frater U. Muller and W. Giinther ibid. p. 1269. 127 H. C. Brown G. G. Pai and P. K. Jadhav J. Am Chern. Soc. 1984,106,1531; A. Deberley G. Boireau and D. Abenhaim Tetrahedron Lett. 1984 25 655. 128 T. Fujisawa T. Itoh and T. Sato Tetrahedron Lett. 1984 25 5083; D. Seebach P. Renaud W. B. Schweizer and M. F. Ziiger Helv. Chim Act4 1984.67 1843; D. Seebach M. F. Ziiger F. Giovannini B. Sonnleitner and A. Fiechter Angew. Chem. Int. Ed. Engl. 1984 23 151. 129 K. Tomioka K. Ando Y. Takemasa and K. Koga J. Am. Chem Soc. 1984 106 2718. 130 J. A. M. van den Goorbergh and A.van der Gen 1. R Nerh. Chern. Soc. 1984 103 90. 142 B. V. Smith Nitro-olefins react with dilithiated carboxylic acids (or lithium enolates of esters) at low temperature forming adducts which afford y-keto-acids or esters in a con- venient one-pot process. Silyl nitronates have been used in synthesis of y-ketoesters (and aldehyde^).'^' RgozLi R:B R~ H RZ (79) (80) A novel stereosp cific synthesis of a,P-unsaturated acids has b en developed; a trialkylborane by sequential reaction with a lithiated alkyne and carbon dioxide afforded (79) and .hence the acid (80) as a single isomer.'32 Saturated esters via trimethylsilyl enolates underwent smooth palladium-mediated elimination to their a,@-unsaturated analogues. Stereoselective 2-step preparation of a-alkyl-a,P-unsat-urated esters was achieved using the addition of a-diphenylmethylsilyl enolates to a ketone with high Z-~electivity.'~~ Phase-transfer catalysis of reaction between aryl iodides and unsaturated esters (Heck reaction) has been re~0rted.l~~ Deconjugation of an E-alk-2-enoate (with a bulky alkoxy-group) by potassium disilazide gave 2-alk-3-enoate preferentially.13' A synthesis of allylcarboxylic acid relied on enecar- boxylation using diethyl ketomalonate as the enophilic equivalent of C02. Car- bethoxy-allylation was achieved through a radical pathway.t36 Allylic carbonates with CO and a palladium catalyst gave P,y-unsaturated esters in good ~ie1d.l'~ Addition of BuCu.BF3 to enoates gave predominantly syn-isomers in agreement with prediction of the model adopted by Fleming.'38 Simple ester enolates add to a,P-unsaturated esters with high erythro-selectivity;'39 tellurium tetrachloride addi- tion to allylic esters (and other derivatives) occurs via 1,3-addition and 1,2-migration of an acyloxy group; thus (81) (92% e.e.) gave erythro-(82) which in sequential reaction with Raney nickel and base gave trans- 1,2-dimethyloxiran (goo/ e.e.).131 M. Miyashita R. Yamaguchi and A. Yoshikoshi J. Org. Chem. 1984 49 2857; K. K. Sharma and K. B. G. Torsell Tetrahedron 1984 40,1085. D. Min-zhi T. Yong-ti and K. Wei-hua Tetrahedron Lett. 1984 25 1797. 133 J. Tsuji K. Takahashi I. Minami and I. Shimizu Tetrahedron Lett. 1984,25,4783; G. L. Larson,C. F. de Kaifer R.Seda L. E. Torres and J. R. Ramirez J. Org. Chem. 1984 49 3385. 134 T. Jeffery 1. Chem. SOC.,Chem. Commun. 1984 1287. I35 Y. Ikeda and H. Yamamoto Tetrahedron Lett 25 5181. 136 M. F. Salomon S. N. Pardo and R. G. Salomon J. Am. Chem. Soc. 1984 106 3797; D. H. R. Barton and D. Crick Tetrahedron Lett. 1984 25 2787. 137 J. Tsuji K. Sato and H. Okumoto J. Org. Chem. 1984 49 1341. 138 Y. Yamamoto and K. Maruyama J. Chem. Soc.. Chem. Commun. 1984,904. 139 M. Yamaguchi M. Tsukamoto S. Tankaka and I. Hirao Tetrahedron Lett. 1984,25,5661; L. Engman J. Am. Chem SOC.,1984 106 3977. Aliphatic Compounds -Part (ii) Other Aliphatic Compounds Chiral a,P-unsaturated carboximides have been used in asymmetric Diels-Alder reactions as practical chiral acrylate and crotonate dienophile equivalents.These reactions mediated by Et2AlC1 or Me2AlCl proceeded with high ~electivity.'~' The Ireland-Claisen rearrangement has been studied in detail. Very high chirality transfer was observed for the transformations shown in Scheme 19 and this was i ii 'POMe "OM e og 0 OH \ / A (83) Reagent i LDA-THF or THF-HMPA then R,SiCI; ii Hydrolysis Scheme 19 dictated by two factors. These were the control over enolate geometry and the relative ease of attainment of chair- or boat-like transition states e.g. (83). Chelation control in enolate geometry was important in rearrangement of 0-protonated allylic glycollate esters. The rearrangement of (R)-1 -methyl-E-but-2-enyl hydroxyacetate (84) with ( Me3Si)2NLi occurred with 98% erythro-selectivity and 100% asymmetric transfer giving (85).14* 2-Hex-Cenolide with alkyl cuprates gave 2-alk-4-enoic acids in excellent yield.'42 A facile Pd"-mediated Claisen rearrangement leading to y,S-unsaturated esters has been p~b1ished.l~~ The use of tungsten-[ ( MeCN)3W(C0)3]-catalysed allylic alkyla- tion has been recommended on grounds of selectivity and cheapness (compared to Mo- or Pd-compounds) ;striking results were achieved in the transformation of e.g.(86) into (87) with NaCH(C02Et)2.144 The transformation of RX into 140 D. A. Evans K. T. Chapman and J. Bisaha J. Am. Chem. SOC.,1984 106 4261. 141 M. Nagatsuma F. Shirai N. Sayo and T. Nakai Chem. Left. 1984 1393; S. D. Burke W. Fobare and G.J. Pacofsky 1. Org. Chem. 1983 48 5221; T. Fujisawa K. Tajima and T. Sato Chem. Letf. 1984 1669. 142 T. Fukisawa K. Umezu and M. Kawashima Chem Lett. 1984 1795. 143 M. Ohshima M. Murakami and T. Mukaiyama Chem. Lett. 1984 1535. 144 B. M. Trost and M.-H. Hung. J. Am. Chem. SOC.,1983 105 7757. 144 B. V; Smith X \ S02Me (89) RCH=CH(CH2)n+2C02Hhas used (88) as an ethene di-ide equivalent; deproton- ation and alkylation of (88) followed by base-promoted halogenation gave (89) which with sodium ethoxide gave the product in good ~ie1ds.l~~ Synthesis of terminal allenic acids relied on addition of free radicals to an alkynylstannane.'46 Phenylation of malonic acid derivatives was reported for reaction of aryl-lead triacetate and sodiomalonate esters.14' Microbial hydrolysis (Acinetobac-ter fowji) of diethyl 3-hydroxyglutarate was enantioselective furnishing (S)-ethyl hydrogen 3-hydroxyglutarate and hence in two steps ~-carnitine.'~' 5 Lactones Asymmetric reduction of propargyl ketones (by Alpine borane) furnished the corre- sponding alcohols in high e.e.; from these alcohols routes to a-and &substituted y-lactones were deve10ped.l~~ The ene-reaction of alkenes and diethyl ketomalonate was catalysed by clays; with K10 montmorillonite y-lactones were formed.No reaction was observed in the absence of clay."0 The syn-diastereoselective reactions of the lithiated carbamate (90) gave mainly E-syn-(91) transformed into (92)which offered a selective route to such systems and could be further exploited.'" A stereoselective synthesis of 2,2-dialkylbutyrolactones oia repeated deprotonation- alkylation of (93) is another example of a process leading to an asymmetric quater- nary centre.15* Some other methods for y-lactones include reaction of iodostannyl esters and alkenes in the presence of AIBN the horse liver alcohol dehydrogenase- mediated oxidation of diols (which gave remarkably high e.e.) and selective enzyme- catalysed hydrolysis of cyclic diester~.''~ E-P-t-butylcinnamic acid lactonized to (94) and a bridged carbonium ion was thought the most likely i~~termediate.'~~ D.Scholz Liebig's Ann. Chem. 1984 264. 146 J. E. Baldwin R. M. Adlington and A. Basak J. Chem. SOC.,Chem. Commun. 1984 1284. 147 R. P. Kopinski J.T. Pinhey and B. A. Rowe Aust. J. Chem 1984 37 1245. 148 A. S. Gopalan and C. J. Sih Tetrahedron Lett. 1984 25 5235. 149 M. Midland A. Tramontano A. Kazubski R. S. Graham D. J. S. Tsai and D. B. Cardin Tetrahedron 1984,40 1371. Is' J.-F. Roudier and A. Foucaud Tetrahedron Lett. 1984 25 4375. 151 D. Hoppe and F. Lichtenburg Angew. Chem Znt. Ed. EngL 1984 23. 239. P. J. Curtis and S. G. Davies X Chem SOC.,Chem. Commun. 1984 747. lS3 G.A. Kraus and K. Landgrebe Tetrahedron Lett. 1984,25,3939; G. S. Y. Ng,L.-C. Yuan I. J. Jakovac and J. B. Jones Tetrahedron 1984,40 1235; M. Schneider N. Engel R. Honicke G. Heinemann and H. Gorisch Angew. Chem. Znt. Ed. Engl. 1984 23 67. 154 L. Jalander Tetrahedron Lett. 1984 25 457. 14' Aliphatic Compounds -Part (ii) Other Aliphatic Compounds 145 Me2P Ph Me [(77' -C,HS)Fe(PPh& CO) (=COCH,CH,CH,)]+ 3- 4- and 5-alkynoic acids have been cyclised to enolides by PdC12.(PhCN)2 in THF-Et3N.155 Homoallylic alcohols have been converted into &lactones uia a hydroformylation-oxidation sequence.Chiral &lactones have been obtained by pig-liver esterase-catalysed hydrolysis of diesters of 3-substituted glutaric acids; the chiral monoesters could be transformed into either enantiomer of the 1act0ne.l~~ 6 Amines and Amides Preparative methods reported for primary amines include reduction of nitro- compounds by anhydrous ammonium formate-Pd/ C-MeOH a versatile process in which hydrolysis of an alkyl bis(trimethylsily1)amine is the final step use of (Me3Si),NCH20Me as a synthetic equivalent of +CH2NH2 (hence RX -+ RCH2NH2) and oxidative rearrangement of aliphatic amides uia [I,I-bis(trifluoroacetoxy)iodo]benzene in acid or aqueous-organic solvent^.'^' The Cur- tius decomposition of azides in the presence of Me3SiCH2CH20H gave trimethylsilyl ethyl carbamates which were smoothly cleaved with Bu~N+F- to primary amines ; the reaction was cleaner than usually observed.'58 Unsymmetrical s-amines were obtained from primary amines uia RNHCH2CN as intermediate followed by hydr01ysis.l~~ Activation of a primary amine via PtC12(PPh3)2-SnC12 gave for e.g.Bu"NH2+ Bu;NH a 64% yield; reductive amination of an aldehyde was effected in the presence of borane-pyridine.'60 A route to (bulky) di-t-alkylamines shown in Scheme 20 was very effective; such crowded molecules could only be methylated with Magic Methyl (MeOS0,F) but were inert towards Me1 or K-MeOCH2CH20Me.'61 155 C.Lambert K. Ultimoto and H. Nozaki Tetrahedron Lett. 1984 25 5323. 156 P.G. M. Wuts M. L. Obrzut and P. A. Thompson Tetrahedron Lett. 1984 25. 4051; C. J. Francis and J. B. Jones J. Chem. SOC.,Chem. Commun 1984 579 157 S. Ram and R. E. Ehrenkaufer Tetrahedron Lett. 1984,25,3415; H. J. Bestmann and G. Wolfel Angew. Chem. Int. Ed. Engl. 1984 23 53; T. Morimoto T. Takahashi and M. Sekiya J. Chem SOC,Chem Commun. 1984 794; G. M. Loudon et aL J. Org.Chem. 1984,49 4272 4277. 158 T. L. Capson and C. D. Poulter Tetrahedron Letf. 1984 25 3515. 159 L. E. Overman and R.M. Burk Tetrahedron Lett. 1984 25 1635. 160 Y. Tsuji J. Shida R. Takeuchi and Y. Watanabe Chem. Lett. 1984 889; A. Pelter R. M. Rosser and S. Mills J. Chem. SOC.,Perkin Trans 1 1984 717. 161 E. J. Corey and A. W. Gross Tetrahedron Left 1984 25 491. 146 B. V. Smith ii '\ &NH2 -!+ &NO + N-OBu' -% &NHBu' But/ Reagents i MeC03H-EtOAc; ii 2eq But' ( ButNHNH2-Pb02);iii Scheme 20 Such hindered bases have value in altering direction of enolate alkylation. Stereocontrolled synthesis of 1,2-diaminesY via Diels- Alder adducts derived from a diene and a bis-imide derived from sulphur dioxide makes possible the transforma- tion of dienes of known geometry into diastereoisomeric carbamates. Vicinal diamines have also been obtained from a sequence in which addition ( NBS-NH2CN) to an alkene was followed by ring-closure to an imidazoline hydrolysed to form the product.In this way trans-but-Zene gave (*)-2,3-diaminobutane (61YO,as hydrochloride).'62 A cyclic a-complex has been suggested for such aminations on the basis of low-temperature n.m.r. studies.163 Reduction of oximes to imines was effected by Bu3P-PhSSPh.lW Use of LiAlH4- sugar complexes led to low e.e. in reduction of imines to secondary amine~.'~~ Cram selectivity was high in the reaction between imines and allyl-9-BBN and was rationalized in terms of attainment of the less-hindered transition-state (95). Allenic organometallics and imines showed high threo-selectivity and this affords a method which is complementary to the erythro-selective reaction of allylic systems.166 Oxidative carbonylation of amines (catalysed by Pd) gave carbamates in good or very good ~ie1ds.l~~ Amines are converted into thiols via intermediate (96) ; methyla-tion and hydrazinolysis completed the process.16* Oxidative deamination of primary and secondary amines occurred with (ArS02)202; yields of imines were variable.Phase-transfer catalysis was used to 4 162 H. Natsugari R. R. Whittle and S. M. Weinreb J. Am Chem Soc 1984 106 7867; S.-H. Jung and H. Kohn Tetrahedron Lett. 1984 25 399. 163 B. hermark and K. Zetterberg J. Am Chem Soc 1984 106 5560. 164 D. H. R Barton W. B. Motherwell E. S. Simon and S. Z. Zard J. Chem Soc. Chem Commun. 1984 337. S. R. Landor 0.Sonola and A.-R.Tatchell Bull. Chem. Soc. Jpn. 1984 57 1658. 166 Y. Yamamoto T. Komatsu and K. Maruyama J. Am Chem. Soc 1984 106,5031; Y. Yamamoto W. Ito and K. Maruyama J. Chem. Soc. Chem. Commun,1984 1004. 167 S. Fukuoka M. Chono and M. Kohno J. Chem SOC Chem Commun 1984,399. 168 R. Ueno C. Tanaka and M. Okawara Tetrahedron Lett. 1984 25 2675. 147 Aliphatic Compounds -Part (ii) Other Aliphatic Compounds generate N-nitrosamines from secondary amine~.'~~ Dealkylation of tertiary amines was noted for reaction with a-chloroethyl chlor~formate.'~~ Two reviews of enamine chemistry have appeared.171 Enamine formation is said to be improved by adding the carbonyl component to preformed TiCl,-amine complex.'72 Thermal isomerization of N-carbethoxyaziridines gave primary allylic amines in good yield; allylic phenyselenides were rearranged smoothly and in good yield to N-allylic p-toluenesulphonamides presumably viu a [2,3] sigmatropic shift.'73 High diastereoselectivity was observed in addition to an N-sulphinyl dienophile (97) to E,E-hexa-2,4-diene [15 1 in favour of (98)]; subsequent transformation of (98) by sequential reaction with PhMgBr then (MeO),P-MeOH gave E-threo- hydroxycarbamate (99) as a single isomer.Use of E,Z-hexa-2,4-diene gave a single adduct which in turn gave only E-erythro-hydroxycarbamate. This interesting route thus gives an entry into unsaturated vicinal aminoalcohols with stereo~ontrol.'~~ N-Alkylation of MeCONH in the presence of KF-MeCN or KF-DMF gave modest yields of secondary amide~.'~~ Isocyanates and isothiocyanates with RLi-CO gave after a hydrolytic work up a-0x0-amides or -thi~amides.'~~ Me S/p It N \CO,Bu I Me NHC0,Bu 7 Other Nitrogen Compounds Sonication has been used to speed up hydrolysis of nitriles even under reflux and was shown to be more efficient than mechanical ~tirring.'~~ It has been shown that MeCN and CN-do not form an add~ct.'~~ Palladium-mediated coupling of ArX and CH2(CN) (as the anion) gave excellent ~ie1ds.l'~ Aldehydes reacted with trimethylsilyl cyanide to form a-trimethylsilyloxynitriles useful precursors for a-aminoacids heterocyclic systems etc.18' Chemistry of 3-oxoalkanenitriles has been reviewed.'" a-Tosylisocyanides can be cleaved (Li-liq.NH3) to hydrocarbons.'82 169 R.V. Hoffman and A. Kumar J. Org. Chem. 1984 49 4011 4014; M. Nakajima J. C. Warner and J.-P. Anselme Tetrahedron Lett. 1984 25 2619. I70 R. A. Olofson J. T. Martz J.-P. Senet M. Piteau and T. Malfroot J. Org. Chem 1984 49 2081. V. G. Granik Russ. Chem. Rev. 1984 53 383; P. W. Hickmott Tetrahedron 1984 40 2989. 172 R. Carlson and A. Nilsson Actu Chem. Scand. Ser. B 1984 38 49. 173 A. Laurent P. Mison A. Nafti R. Cheikh and R. Chaabouni J. Chem. Res. (S) 1984 354; J. E. Frankhanser R. M. Peevey and P. B. Hopkins Tetrahedron Lett 1984 25 15. I74 R. S. Garigipati A. J. Freyer R. R. Whittle and S. M. Weinreb J. Am. Chem. Soc. 1984 106 7861. 175 G. 0.Torosyan N. K. Tagmazyan and A. T. Bayaban J. Org. Chem. USSR 1984,20 456.176 D. Seyferth and R. C. Hui Tetrahedron Lett. 1984,25 5251. 177 J. Elguero P. Goya J. Lissavetzky and A. M. Valdeomillos C. R Hebd. Seances Acad. Sci 11 1984 298 877. 178 D. Fgrcasiu G. Marino K. D. Rose G. A. Digenis and M. Jay Tetrahedron 1984,40 1487. 179 M. Uno K. Seto and S. Takahashi J. Chem. Soc. Chem. Commun. 1984 932. 180 K. Mai and G. Patil Tetrahedron Lett. 1984 25 4583. 181 M. H. Elnagdi M.R. H. Elmoghayar and G. E. N. Elgemei Synthesis 1984 1. 182 J. S. Yadav and P. S. Reddy Tetrahedron Lett. 1984 25 4025. 148 B. V. Smith A new route for azides utilised reaction between Pb(OAc),,(N,) (from Pb(OAc),-Me,SiN,) and a borane R,B; unfortunately a mixture of products was formed in this route.ls3 Aldoximes are dehydrated to nitriles by dimethylsuccinimidylsulphonium chloride-Et,N in high yield.The dianions of aldoximes can be alkylated efficiently e.g. MeCH=NOH gave with PhCH,Br Ph(CH,),CH=N-OH (2-isomer 1OOoh).ls4 Spontaneous rearrangement was reported in the attempted preparation of N-methyl pentanehydroxamic acid (from BuC02Me and MeNHOH) in which only BuC0,NHMe was is01ated.l~~ A new route to nitro-compounds relies on ozonolysis of RN=PRi (from RN3 and R4P) since the ozonide is presumed unstable and gives RN02 and RiP0.1s6 The Bu,P-PhSSPh reagent reduced nitro-groups to imines trapped by pyrrole f~rmation.'~~ a-nitro-olefins react with secondary amines and a Pd(0) catalyst to form allylic amines. Allylic nitro compounds undergo regioselective y-attack with LiCuR in an SN2'process.'88 8 Sulphur Compounds Synthetically useful OL -trimethylsilyl vinyl sulphides were formed in high yield by deoxygenation of sulphoxides by LDA-Me3SiC1.1s9 Addition of an allylic sulphide to e.g.methyl propiolate showed dependence on the Lewis acid used; AlCI favoured the E-and ZnCl the Z-add~ct.''~ A molybdenum persulphide complex (NH4)2[(S2)2Mo(S2)2Mo(S2)2]brings about alkylation and coupling; e.g. RBr fur- nished RSR and RSSR. The acid chlorides RSOCl and RSO2C1 also formed disul- phides.'" A range of organosulphur compounds showed copper-induced solvolysis with CU"(OAC)~-ACOH.'~~ 2-1-Phenylpropene added [(ArS),SAr]+SbC& to form thiiranium ion which added chloride ion to form P-chlorosulphidesof Markovnikov orientation.An open ion could not however be e~c1uded.l~~ Asymmetric oxidation of a dialkyl sulphide by Corynebacterium equi IF0 3730 gave sulphoxides in surprisingly high e.e. (80-1000/,). Sulphone formation was observed with long-chain systems but in other cases was absent e.g. MeSC,H,Me gave sulphoxide and recovered starting materials only. The Sharpless reagent cleanly brought about the same oxidation and the optimum mixture was Ti(OPr'), DET :H20:Bu'OOH = 1:2 :1:2. Again no significant sulphone formation was noted.'94 Ring closure of a-sulphinyl carbanions is a convenient route to methylene 183 Y. Masuda M. Hoshi and A. Arase Bull. Chem SOC.Jpn. 1984 57 1026. 184 N. K. A. Dalghrd K. E. Larsen and K. B. G. Torsell Acta Chem. Scand.Ser. B 1984,38 423; R. E. Gawley and T. Nagy Tetrahedron Lett. 1984 25 263. G. I. Nikishin E. I. Troyansky I. V.Svitanko and 0. S. Chishov Tetrahedron Lett. 1984 25 97. E. J. Corey B. Samuelsson and F. P.Luzzio J. Am. Chem. SOC.,1984 106 3682. 187 D. H. R.Barton W.B. Motherwell and S. Z. Zard Tetrahedron Lett. 1984 25 3707. R. Tamura K. Hayashi Y. Kai and D. Oda Tetrahedron Lett. 1984,25,4437; N. Ono I. Hamamoto and A. Koji J. Chem. SOC.,Chem. Commun. 1984 274. 189 R.D. Miller and R.Hossig Tetrahedron Lett. 1984 25 5351. 190 K. Hayakawo Y.Kamikawaji A. Nakita and K. Kanematsu J. Org. Chem. 1984 49 1985. 191 D. N. Harpp and J. G. McDonald Tetrahedron Lett. 1984 25 703. 192 D. Uguen Chem. Lett. 1984,25 541. 193 G. H. Schmid and D. I. Macdonald Tetrahedron Lett.1984 25 157. 194 H. Ohta Y. Okamoto and G. Tsuchihashi Chem. Lett. 1984 205; P. Pitchen and H. B. Kagan Tetrahedron Lett. 1984 25. 1049. Aliphatic Compounds -Part (ii) Other Aliphatic Compounds cyclopentenones.19' The Diels- Alder addition of chiral sulphur-containing dienophiles to cyclopentadiene has been explored as a method for selective prepar- ation of nor-bornenyl dia~tereoisomers.'~~ Reduction of a-arylthio-P-oxosul-phoxides by complex hydride was an efficient route to protected a -hydroxyaldehydes of high optical purity; thus from (loo) (101) was obtained in >99 1 isomer yield.Ig7 Cholesterol with MeSOC1-Et3N gave separable epimeric esters MeS02C27H45 ;reac-tion of each ester with RMgX gave MeSOR in 95-100% optical 0 STol To1 (To1 = C7H7) STol Regiocontrolled polyalkylation of a -trifyl-dimethylsulphoneenabled four alkyl groups to be introduced at the a-sites; elimination of SO2 gave the alkene RR4C=CR2R3.In this ingenious reaction the overall change is equivalent to 2-2-successive regiospecific alkylation of a polyanion C =C ! Unfortunately the final elimination was not stereospecific since the fourth alkylation gave rise to diastereois~mers.'~~ The chiral auxiliary (102) with the magnesium derivative of C7H,S02CH2CH=CH2 formed a complex which with acetone gave high selectivity in formation of (103).200 Allylic sulphonates were rearranged to sulphones by palladium(0)-mediated catalysis.20' The preparation and uses of chiral [l60,170,180] sulphate esters has been described.Organic reactions of carbon disulphide have been reviewed.*02 Me HO TolSO 9 Phosphorus Compounds Asymmetric reduction of (*)-phosphine oxides gave phosphines with low e.e. Efficient chiral synthesis of a range of phosphines and other compounds was 195 M. Pohmakotr and S. Chancharunee Tetrahedron Lett. 1984 25 4141. 196 T. Koizumi I. Hakamada and E. Yoshii Tetrahedron Lett. 1984,25 87; C. Maignan A. Guessos and F. Rouessac ibid. p. 1727. 197 G. Guanti E. Narisamo F. Pero L. Banfi and C. Scolastico J. Chem SOC.,Perkin Trans. 1 1984 189. 198 K. K. Andersen B. Bujnicki J. Drabowin M. Mikolaynyk and J. B. O'Brien J. Org. Chem. 1984 49 4070. 199 J. B. Hendrickson G. J. Boudreaux and P. S. Palumbo Tetrahedron Lett.1984 25 4617. 2oo T. Akiyama M. Shimizu and T. Mukaiyama Chem Lett. 1984 611. 201 K. Hiroi R. Kitayama and S. Sato J. Chem. SOC Chem Commun. 1984 303. 202 G. Lowe and S. J. Salamone J. Chem. SOC.,Chem Commun. 1984,466; M. Yokoyama and T. Imamoto Synrhesis 1984 797. 150 B. V. Smith achieved by reaction of alkylthiophosphonium salts and (Me2N)3P-CH2C12; thus (I?)-(-)-Et(Ph)P(=S)OPr’ by sequential treatment with CF3S03Me and tris-DMAP gave (S)-(-)-Et( Ph)POPr’ converted back into starting material (by reaction with S) with no loss of optical activity.203 The preparation and properties of functionalized tertiary phosphines have been reviewed.204 The merits of carrying out phosphonium salt preparations under pressure have been ill~strated.~~’ The diphosphine BINAP {2,2‘-bis(dipheny1phosphino)-1,l’-binaphthyl}has been prepared and resolved.As a hydrogen-transfer catalyst it was highly efficient and selective; thus 2-a-(benzamid0)cinnamic acid gave with (R) -BINAP N-benzoylphenylalanine (96% 96%e.e.) and with (S)-BINAP (97% 100% e.e. of the isomer). This report represents the first account of synthesis and resolution of an atropoisomeric bis-triaryl diphos- phine.206 Stereospecificity in elimination from the adduct of EtPh,PO-BuLi-PhCHO has been confirmed by X-ray structural analysis of 2( 1RS,2SR)-diphenylphosphinoyl- 1 -phenylpropan-1-01 and exclusive formation of 2-1 -phenylpropene by base. A vari- ation on this theme led to single isomers of homoallylic Deproton-ation/alkylation of Et( Ph)P( =O)CH2C02Men gave alkylation 01 to the carboxyl group; this product with LiC1-H20-Me2S0 gave Et(Ph)P( =O)CH2R in almost complete optical purity.208 Some evidence for reversibility in the Wittig reaction of Ph3PCH(CH2)0Li and hexanal was secured by addition of octanal and observation of crossover.A possible bonus here was predominance of the E-isomer of either product.2w The use of heterogeneous media for the Wittig-Horner reaction has been extended and a mild method recommended for base-sensitive aldehydes or phosphonates in which LiCl and an amine with a pK comparable to that of (EtO),P(=O)CH,CO,Et may be used. With DBU in MeCN-LiCl reaction was swift and for e.g. Bu’CHO gave high yield and E :Z-ratio.210 Exploratory work using a-alkoxyphosphonium bromides (as precursors to enol ethers) a chiral phosphonate as a precursor for chiral enediols and a route to homologation of aldehydes has been published.2” The Petersen reaction (the silicon equivalent of the Wittig reaction) has been reviewed as has a broad scope of phosphorus chemistry (in honour of Arbuzov’s eightieth birthday).212 10 Miscellaneous Two new powerful and versatile oxidants have been introduced.Magnesium and zinc permanganates are safe only when supported on silica gel and used in CH2C12. 203 A. J. Macpherson and D. J. H. Smith J. Chem. Res. (S) 1984,32; J. Omelannuk and M. Mikolajayk Tetrahedron Lett. 1984 25 2493. 2w0. A. Erastov and G. N. Nikonov Russ. Chem. Rev. 1984 53 369. 205 W. G. Dauben J. M. Gerdes and R.A. Bunce J. Org. Chem 1984,49,4293. 206 A Miyashita H. Tayako T. Souchi and R. Noyori Tetrahedron 1984,40 1245. 207 A. D. Buss W. B. Cruse 0.Kennard and S. Warren J. Chem. SOC,Perkin Trans. 1 1984 243; A. D. Buss N. Greeves. D. Levin and S. Warren Tetrahedron Lett. 1984 25 357. 208 K. M. Pietrusiewin M. Zablocka and J. Monkiewin J. Org. Chem 1984,49 1522. 209 A. B. Reitz and B. E. Maryanoff J. Chem Soc. Chem Commun. 1984 1548. 210 J. VilliCras and M. Rambaud Synthesis 1984 406;M. A. Blanchette W. Choy,.J. T. Davis A. P. Essenfeld S. Masumune. W. R. Roush and T. Sakai Tetrahedron Lett. 1984 25 2183. 211 J. B. Ousset C. Mioskowski Y.-L. Yang and J. R. Falck Tetrahedron Lett. 1984 25 5903; T. Hiyama K. Kobayashi and M. Fujita ibid. p. 4959; N.L. J. M. Braekhof and A. van der Gen 1. R Nefh. Chem. SOC..1984. 103 305 312. 212 D. J. Ager Synthesis 1984 384; Arbuzov dedication Russ. Chem. Rev. 1983 52 1012. Aliphatic Compounds -Part (ii) Other Aliphatic Compounds A wide range of compounds is susceptible; clearly the compound needs careful handling since a number of solvents inflamed including AcOH. Bis(2,2'-bipyridyl)copper( 11) permanganate seems safer and versatile in the number of compounds Trost has reviewed selectivity -its pursuit and the role of strain in achieving it.214 Reviews of three carbon homologating agents and synthesis of chiral (non- racemic) compounds have appeared.215 213 S. Wolfe and C. F. Ingold J. Am Chem. SOC.,1983 105 7755; H. Firouzabadi A. R.Sardarian M. Naden and B. Vessal Tetrahedron 1984 40 5001. 214 B. M. Trost Chem. Bn'f. 1984 315; Gazz. Chim. IfaL 1984 114 139. 215 J. C. Stowell Chem. Reu. 1984 84 409; A. I. Meyers (ed.) Tetrahedron 1984,40 1213.

ISSN:0069-3030    

DOI:10.1039/OC9848100119

出版商:RSC    

年代:1984

数据来源: RSC