ISSN:0069-3030    

DOI:10.1039/OC97875FX001

出版商:RSC    

年代:1978

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC97875BX003

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

3 Theoretical Chemistry Applications of Molecular Mechanics Calculations ~~~~~ ~ ~ By M.B. HURSTHOUSE G. P. MOSS AND K. D. SALES Department of Chemistry Queen Mary College Mile End Road London El 4NS 1 Introduction The current state of theory suggests that if we knew the proper Hamiltonian operators and had the necessary computational apparatus and time all the results of chemistry should follow from the solution to the relevant Shroedinger equation. In particular we would be able to predict ground state geometries heats of formation and energies of different conformations of molecules. Of course ab initio cal-culations of this nature are not possible for most molecules of chemical interest and semi-empirical theories whilst being somewhat more successful are still a long way from this ideal.However force field calculations which adopt a much more pragmatic approach have already proved themselves. The idea of force field calculations originated in vibrational spectroscopy,’ when physicists attempted to predict vibrational frequencies with a simple model. A molecule is envisaged as a system of particles held together by forces with a potential energy V given by equation (1). The molecule contains N atoms the fii are force constants and the xi are the co-ordinates of the atoms with the convention that x1,x2,x3 are the Cartesian co-ordinates of atom 1; x4 xs x6 of atom 2 etc. The form of equation (1) pre-sumes that quadratic (so-called harmonic) terms are sufficient; for other than small displacements of the atoms cubic quartic and higher anharmonic terms may be required.In general there are i(3N -6)(3N-5) independent force constants and one of the simplest approximations to reduce this number is to assume central forces forces which act only along the lines joining pairs of atoms asif the molecule was composed of ions. Another idea the valence force approximation seems more reasonable to a chemist the forces are those which resist changes in valence bond lengths and bond angles. Finally Urey and Bradley2 proposed the use of a mixed potential function a function which is basically of the valence force type but which includes central force terms between 1,3non-bonded atoms. The name Urey-Bradley is attached to a force field which specifically includes such terms.See for example E. B. Wilson J. C. Decius and P. C. Cross ‘Molecular Vibrations’ McGraw-Hill New York 1955 Chap. 8; G. Herzberg ‘Molecular Spectra and Molecular Structure’ Van Nostrand New York 1945 Vol. 2 pp. 159 and 168. H. C. Urey and C. A. Bradley jun. Phys. Rev. 1931 38 1969. 23 M. B. Hursthouse G. P. Moss and K. D. Sales Chemists approached the same sort of idea from a desire to quantify the concept of strain energy or steric effect^.^ The original attempts were directed towards highly hindered biphenyls4 and somewhat later to the stable conformations of cy~loalkanes.~~ The various names under which this topic has been known reflect its history Westheimer Method Strain Energy Minimization Technique Force Field Calculations and Molecular Mechanics which is the accepted modern term.A molecular force field describes the potential energy of a molecule relative to the energy of a reference geometry which used to be called the strain-free or natural geometry terminology not now used because of the difficulty in defining these concepts. All the force fields used in practice employ parameters derived inductively by a systematic comparison of calculated and observed molecular properties. The ultimate aim is to cover all or most of organic structures with a reasonably limited set of transferable parameters. The properties one would like to predict include heats of formation conformational energies barriers to rotation geometries of various conformations in the ground state of molecules in crystals and of transition states between conformers.The Molecular Mechanics Method with carefully chosen parameters can predict these and other quantities accurately for a wide range of molecules. The subject has been reviewed several times.g In practice the method works as follows choose the type of force field required find the best parameters for this field and for the properties to be calculated and finally minimize the energy of the molecule by varying the positions of all the atoms. These stages will now be discussed separately. 2 Types of Force Field The potential energy of the molecule may be expressed as a function of all the internal co-ordinates and interatomic distances in the general manner of equation (2). v= v,+v,+v,+v,+v,b+v,+v,b+(v,,or vet) (2) The different symbols represent the molecular potential energy due to bond length changes (vb),to valence angle changes ( Vo) to dihedral angle changes (V,) to out-of-plane bending ( V‘) to non-bonded interactions ( Vnb), to electrostatic inter- actions ( Ve),to hydrogen bonding (Vhb),and to either 1,3-interactions (V13)or cross terms (Vet) These will each be discussed below.The inclusion of non-bonded interactions means that the force field is no longer of the simple harmonic valence or Urey-Bradley type is responsible for the non-transferability of spectroscopic force T. L. Hill J. Chem. Phys. 1946,14,465;‘Steric Effects in Organic Chemistry’ ed. M. S. Newman John Wiley New York 1956,Chap. 12,by F. H. Westheimer. F.H. Westheimer and J. E. Mayer J. Chem. Phys. 1946,14 733. N. L.Allinger I. Amer. Chem. SOC.,1959 81 5727. J. B. Hendrickson J. Amer. Chem. SOC.,1961,83,4537; 1962,843355; 1964,86,4854. J. B. Hendrickson J. Org. Chem. 1964,29,991. * K. B. Wiberg J. Amer. Chem. SOC.,1965,87 1070. (a) J. E.Williams P. J. Stang and P. von R. Schleyer Ann. Rev. Phys. Chem. 1968 19 531; (b)L.S. Bartell J. Chem. Educ. 1968 45 754; H.A.Scheraga Adv. Phys. Org. Chem. 1968,6,103; H. A. Scheraga Chem. Rev. 1971,71,195; M. I. Page, D.A.Brant Ann. Rev. Biophys. Bioeng. 1972,1,369; Chem. SOC.Rev. 1973,2,295; N. L.Allinger Ado. Phys. Org. Chem. 1974,13,1;C. Altona and D. H. Faber,*Topicsin Current Chem. 1974,45,1;(c) 0.Errner Structure and Bonding 1976.27,161;(d)J. D. Dunitz and H.B. Biirgi MTP Int. Rev. Sci. Physical Chern. Series Two Butterworths London 1975 Vol. 11 Chap. 4. Theoretical Chemistry Applications of Molecular Mechanics Calculations constants to calculations of geometry enthalpy efc.,but is essential if the field is to be transferable between systems of different strain. Bond Stretching.-The form of vb will be considered rather more fully than for subsequent terms because many of the ideas are common to them all. It is usually expressed by equation (3). bonds The symbols are b the bond length bo the reference bond length and Kb the harmonic force constant. Kband boare parameters to be adjusted for each different type of bond in the molecule; note therefore that the Kbare not force constants in the normal spectroscopic sense nor are the bo equilibrium .values of the b.Anharmonic terms of the type & 1 KI,(b -b0)3were introduced by Allinger." bonds Bond Angle Bending.-Equation (4) gives the common expression for Ve. Vo=$ C He(#-#o)2 (4) bond angles The symbols are analogous to those of equation (3); He and O0 are adjustable parameters. Anharmonic terms have been introduced. 11312~13 Bond Torsion.-Despite the fact that non-bonded interactions are considered in force field calculations it has always proved necessary to include a specific term to allow for changes in dihedral angles. The torsional potential energy for rotation about a carbon-carbon bond is commonly expressed by equation (5). v,= 1 H,(l+scosnf#I) dihedral angles The angle q5 is the dihedral angle H is an adjustable parameter and n and s are fixed by the nature of the bond as follows the periodicity of the torsional motion is given by n and s is -1 if the configuration of lowest energy has eclipsed atoms and +1 for the staggered case.Thus n = 3 s = +1 for ethane and n =2 s = -1 for ethene. An alternative way to express this interaction is by equation (6),which might be expected to hold if (4-do)is reasonably small. Terms additional to that in equation (5) have been considered. I4.l5 N. L. Allinger and J. T. Sprague J. Amer. Chem. SOC., 1972,94 5734. S. Lifson and A. Warshel J. Chem. Phys. 1968,49 5116. l2 N. L. Allinger M. T. Tribble M. A. Miller and D. H. Wertz J. Amer. Chem. SOC.,1971 93 1637. l3 E. M. Engler J.D. Andose and P. von R. Schleyer J. Amer. Chem. SOC., 1973 95,8005. l4 L. S. Bartell J. Amer. Chem. SOC.,1977,99,3279;N. L. Allinger D. Hindman and H. Honig J. Amer. Chem. Soc. 1977,99,3282. '' N. L. Allinger J. Amer. Chem. SOC.,1977,99,8127. 26 M. B. Hursthouse G.P.Moss and K. D. Sales Out-of-plane Bending.-This term is required when alkenes are being considered and allows for the motion shown in the figure. H \/ "H The usual expression for V is equation (7),the reference value of x the angle describing the out-of-plane bending being taken as zero. (7) out-of-plane bends Non-bondedInteractions.-These interactions are probably the most important of all the terms in the expression for V,and yet the model from which they are calculated is the most uncertain.The usual form assumed is either equation (8)or equation (9). vnb = {Nlr," -N2ri6} non-bonded pairs separated by three or more bonds v, = C {N~ ii e-''ii-N2r+) (9) non-bonded nails separated by ihree ormore bonds The distance between the atoms is rii,x is 9 or 12; the remaining symbols are adjustable constants. The second term in each equation is the familiar London dispersion force of attraction; the first expresses the repulsion at small internuclear distances by either the Lennard-Jones 12 or 9 potential (depending upon the value of x) or the modified Buckingham exponential potential. Even when the form of V, has been decided there is still a divergence of opinion as to its use. Some authors treat the parameters as adjustable for all interactions whereas others average those for H--Hand C--Cto give values for C--H.16Some foreshorten the distance when hydrogen is involved by moving the protons.12*17~18133 interactions are usually ignored (unless explicitly included in a Urey-Bradley potential? Vl,3),and even 1,4interactions are neglected on occasion on the grounds that the bond torsion term should account for changes in dihedral angle. Finally some calculations cut offthis interaction at separations greater than about 5 A even though the longer range London forces are still contributing. Electrostatic Interactions.-A simple Coulomb expression is used for the energy of interaction between partial charges 4i and qi separated by a distance rii [equation The 4i are regarded as variable parameters.When heteroatoms are present in the l6 T. L. Hill J. Chem. Phys. 1946,16,399; T. L. Hill 'Lectures on Matter and Equilibrium' Benjamin New York 1966 p. 46. l7 D. E. Williams J. Chem. Phys. 1965 43,4424. l8 S. Fitzwater and L. S. Bartell J. Amer. Chem. Soc. 1976,98,5107. Theoretical Chemistry Applications of Molecular Mechanics Calculations 27 molecule this term is usually incl~ded,'~ although it has been omitted.20 Partial charges in saturated hydrocarbons have also been considered. '1*21*22 Hydrogen Bonding.-These contributions may be very important in the con-formational analysis of certain types of molecular system for instance biopolymers.19*23 For most applications in organic chemistry they are ignored.1,34nteractions or Cross-terms.-These attempt to take into account explicitly geminal interactions with a Urey-Bradley potential as in equation (1 1). (11) all 1.3-all 1.3-interactions interactions The adjustable parameters are F F' and ro; r is the distance between the geminal atoms. Valence Force Fields can include a term which contains two types of change simultaneously see equation (12). v=,=CF(pi-poi)(pj-~oj) (12) 1.1 The pi are internal co-ordinates and F is an adjustable parameter. This term might for instance take into consideration the interaction between a CH stretch and an adjacent HCH angle bend. It has been suggested that 1,3-interactions are equivalent to cross-terms between adjacent co-ordinate~.~~ 3 Determination of Force Field Constants One should always remember that the functions used in force field calculations are empirical being picked because they are easy to handle numerically.Their ability to give a quantitative description of molecular systems is almost entirely dependent on the choice of parameters. Obviously from the outline above there are a large number of such constants and furthermore different parameter sets may give equally good descriptions for a change in one constant can often be taken up in another. Several good force fields have emerged over the last ten years from calculations on large sets of molecules together with thorough refinement on a trial-and-error basis. A more systematic approach has been developed by Lifson and co-workers:" as many experimental values for properties such as structural quantities thermo- chemical measurements etc.are taken the corresponding values calculated and the sum of the squares of the differences minimized in an iterative process by vary- ing the potential constants. Problems can arise because of correlations between the parameters but techniques have been developed to avoid these difficulties. l9 R. A. Scott and H. A. Scheraga,J. Chem. Phys. 1966,45,2091. 2o See for instance D. N. J. White and G. A. Sim Tetrahedron,1973,29,3933. 21 A.Warshel and S. Lifson J. Chem. Phys. 1970,53,582. 22 0.Ermer and S. Lifson J. Amer. Chem. SOC.,1973,95,4121. '' R.F.McGuire F. A. Momany and H. A. Scheraga,J. Phys. Chem. 1972,76 375. 24 S.Califano PureAppl.Chem. 1969,18,353. M. B. Hursthouse G. P.Moss and K. D. Sales 4 Energy Minimization The major problem of Molecular Mechanics is how to minimize the potential energy of a molecule by varying the atomic Fositions. All the methods available are iterative and rely upon guessing a starting geometry computing the gradients of the energy with respect to changes in the atomic co-ordinates and thereby estimating the best direction in which to move the atoms to reduce the energy. This new position is used to start the procedure all over again. The Newton-Raphson pr~cedure~~(NR) gives the correction to the Cartesian co-ordinates of all the atoms Sx from a starting set xo as equations (13)-(15). sx = -F-'D(xo) a* v F..=-" ax,ax The procedure has to be iterative because of the approximations involved in deriving these equations; it is iterated until all the gradients are zero i.e.until D(xo)has zero elements. The elements of F may be evaluated analytically or more usually numerically. The technique is very good if a reasonable geometry is guessed to begin with (i.e.one which is fairly close to the minimum). Otherwise different techniques must be used and usually have been in the literature. The method of steepest descents is equivalent to setting F equal to a diagonal matrix with elements 1/L(a scaling factor) when 8x is given by equation (16). The methods of parallel tangents and of pattern search have been discussed by Schleyer ~?tal.'~ AllingerI2 has adopted a different modification of the NR method in which each atom is considered in turn.Other changes include taking F to be a diagonal matrix" with elements Ei=d2 V/dx or a block diagonal matrixZ0*26 with elements Ei= d2V/dxi axj for i j =s3 (i.e. each atom has a block to itself). As mentioned above the NR method is sensitive to poor starting positions and is therefore often used together with one of the other techniques which converge more slowly but are not so sensitive. For example the steepest descents method may be used until the energy change at a given iteration is about 0.1 kcal mol-' A-' when the NR method will quickly converge to changes of kcal mol-' Having found a minimum one has to determine if it is unique and if not to find other local minima to determine the absolute minimum.One technique is to start a new search from each of a number of points chosen randomly or in some predeter- mined manner and check that each search reaches the same minimum. The big drawback is the amount of work required. False minima can occur because of stationary points such as saddle points or can be an artefact of an inadequate minimization technique. The NR procedure is the only one which guarantees a 25 See for example reference 9(c). 26 N. L. Allinger and G. A. Lane J. Amer. Chem. SOC.,1974,96,2937. Theoretical Chemistry Applications of Molecular Mechanics Calculations Table 1 Some force fields for saturated hydrocarbons" Field 1 ? 3 4 5 6 7 8 9 10 Reference 28 29 30b 12' 22' 13' 31' 32 18' 15' -590.4 655.2 662.4 654.0 662.4 600 662.4 554.4 662.4 -1.056 1.090 1.094 1.105 1.100 1.056 1.100 1.0203 1.113 -316.8 633.6 633.6 645.3 633.6 300 633.6 337.0 633.6 -1.240 1.530 1.512 1.501 1.520 1.250 1.520 1.166 1.523 0.0230 0.0228 0.0223 0.0088 0.0241 0.0145 0.0158 0.0144 0.0154 0.0140 ---0.0007 -0.0004 -112.0 109.47 -111.2 106.4 109.2 108.2 109.47 109.0 112.0 109.47 107.9 108.5 109.6 109.1 109.41 109.1 109.47 109.4 0.0230 0.0140 0.0267 0.0105 0.0268 0.0175 0.0140 0.0176 0.0141 0.0158 ----0.0005 -0.0005 -112.0 109.47 -107.8 112.39 109.5 -109.0 109.47 110.0 112.0 109.47 109.5 112.8 109.18 109.0 109.47 109.2 109.47 109.4 108.2 109.47 109.5 108.4 -109.2 -109.2 109.47 109.4 0.0230 0.0301 0.0351 0.0175 0.0284 0.0250 0.0274 0.0240 0.0276 0.0197 lccc HBn ---0.0004 -0.0007 - 112.0 109.47 -110.2 110.5 110.4 112 110.4 109.47 109.5 Itlo 110.7 109.47 -110.6 -110.1 -110.1 109.47 109.5 -109.47 111.0 109.47 109.47 109.5 -109.5 109.47 109.5 2.65 2.73 2.10 0.50 2.85 0.68 2.60 0.22 0.34 0.237 2300 6591 2650 49680 903.8 6515 5524 2604 2120 13630 HH 3.60 4.08 3.74 9.06 9.00 3.75 4.00 3.90 3.4 4.17 49.2 49.2 27.4 1.54 28.3 86.0 49.2 28.4 48.0 77.1 4012 44710 4320 69077 4979 4875 67925 4866 19498 13340 VL CH 3.40 2.04 3.42 9.06 9.00 3.58 4.40 3.60 3.75 3.59 125 125 138 2.14 163 84.7 126 84.5 540 144 7000 2199300 14976 96048 27372 15466 99787 19531 72317 12760 I cc 3.2 12 3.1 9.06 9.00 3.12 4.00 3.10 3.75 3.16 325 325 641 2.97 943 619 322 782 400 298 The units throughout this table are kcal mol-' for energies 8 for length and degrees for angles.The fields have been derived for different purposes which explains why certain entries are blank and usually have extra detail which is given in the following notes. The symbols are defined in equations (3)-(6) (a) and (9). 'Kb for CH is for >CH2 and % CH groups; that for -CH3 is 676.8 A Urey-Bradley potential [equation (1l)]was included. Cross terms [equation (12)] were included. The bond lengths were assumed invariant in field 1 at CC = 1.533A CH = 1.109 A. Fields 9 and 10 included anharmonic terms in V,. 'The numbers to the right of B0 refer to the substitution pattern of the carbon atom 1 is primary 2 secondary etc. A different anharmonic term was used for field 10. Field 2 used equation (6) for V, and field 10 the additional terms fV,(l+ cos 4) +fV2(-cos 24).V, is given by equation (9)for all fields except for number 5 which used equation (a) and number 2 which used equation (8) for the C-€ interaction. Fields 4 and 5 used a potential derived from four independent parameters only [see discussion after equation (9)}. M. B. Hursthouse G. P.Moss and K.D. Sales proper minimum for although the gradients (8V,8xi)are zero at a maximum as well as a minimum the F matrix has negative eigenvalues at improper minima. For example a one dimensional partial maximum is a saddle point which can be recognized because F will have one negative eigenvalue a fact which can be used to find transition states. Furthermore although the energy may not change appreci- ably for a given change of the atomic positions the partial derivatives may change considerably and lead to sizeable adjustments to for instance torsion angles.27 Thus in conclusion it is worth noting that if the NR method has not been used to refine the minimum the final geometries obtained with a given force field may not be correct in that they may not correspond to the partial derivatives being zero.5 Stereochemistry Molecular mechanics calculations were originally devised to analyse saturated hydrocarbons. These compounds still excite interest especially when strained or sterically crowded. For example cis-1,4- and trans- 1,2-di-t-butylcyclohexanewere shown to prefer a twist conformation of the ring;33 or 1,1,2,2-tetra-t-butylethane does not adopt an alternating arrangement between the groups at each end when viewed in a Newman pr~jection.~~ Steric hindrance from the alkyl groups of a series of methyl ketones showed a good correlation with Taft’s E-scale of steric parameter^.^^ In a study of a peri-substituted naphthalene with two t-butyl groups it is predicted that the ring system will be n~n-planar.~~ Large rings may exist in many different conformations.Molecular mechanics calculations offer one of the few methods of analysing the various alternatives and allow an estimate of the energy barrier for their interconversion. For example calculations by Anet3’ on 11- 12- 13- and 15-membered ring cycloalkanes cyclo-octa-1,3- and -1,4-diene and cyclododeca-1,5,9-trienehave been used to analyse their low temperature ‘H and 13Cn.m.r.spectra. After a similar examination of humulene (1)d~awa~~ showed that the more stable conformers were also those required for cyclization in the biosynthesis of the illudane group of sesquiterpenoids. 27 D. N. J. White and 0.Ermer Chem. Phys. Letters 1975,31,111;J. M. A.Baas B. van de Graaf A. van Veen and B. M. Wepster Tetrahedron Letters 1978 819. 28 J. B. Hendrickson J. Amer. Chem. SOC.,1967,89,7036,7043,7047. 29 E.J. Jacob H. B. Thompson and L. S. Bartell J. Chem. Phys. 1967,47,3736. 30 S. Chang D. McNally S. Shary-Tehrany M. J. Hickey and R. H. Boyd 1.Amer. Chem. Soc.,1970,92 3109. 31 R.L.Hilderbrandt J. D. Wieser and L. K. Montgomery J. Amer. Chem. SOC.,1973.95,8598. 32 D.N.J. White and M.J. Bovill J.C.S. Perkin 11 1977 1610. 33 B. van de Graaf J. M. A. Baas,and B. M. Wepster Rev. Trav. Chim. 1978,97,268. 34 W. D.Hounshell D. A. Dougherty and K. Mislow J. Amer. Chem. Soc. 1978,100,3149. 35 J.-E.Dubois J. A. MacPhee and A. Panaye Tetrahedron Letters 1978,4099. 36 J. Handal J. G. White R. W. Franck Y. H. Yah and N. L. Allinger J.Amer. Chem. Soc. 1977,99,3345. 37 F.A. L.Anet and I. Yavari J.Amer. Chem. SOC.,1977,99,6986; 1978,100,7814; F.A. L.Anet and T. N. Rawdah 1978,_100,5003,7166,7810. 38 H. Shirahama E. Osawa and T. Matsumoto Tetrahedron Letters 1978 1987. Theoretical Chemistry Applications of Molecular Mechanics Calculations 31 In a study3' of some substituted trans-2-decalones molecular mechanics cal- culations of the conformation were combined with ab initio methods (STO-3G) to calculate the energy.It was found that a significant proportion of flexible conformers should be present with 3a-methyl and l,l,lO-trimethyl cases but not with the la-methyl compound. This result should be contrasted with the study of 4,4-dimethylandrostran-3-one,where a chair conformer was predicted although a twist ring A was found for the A5 ana10gue.~' Hexaphenylethane notwithstanding claims to the contrary has not yet been synthesized. However it is predicted41 that it should be stable enough to isolate although it should have an elongated central C-C bond. Examination of related molecules with other aryl systems suggested that they too should have a similar long bond; however with 9,9'-bitriptycyl this bond was found4* to be only 1.558& compared with a calculated value of 1.589 A.With a conjugated polyene it is necessary to distinguish between the various bonds along the chain. This was acc~mplished~~ by assuming the bond stretching force constant was proportional to the bond order calculated by quantum mechanics (VESCF). In a study of some substituted di- and tri-enes S-cis conformers were shown to be present to a significant extent. The predicted conformation of a diene can then be used to calculate the U.V. absorption maxima.44 A similar study of enones4' was used to modify the Woodward-Fieser rules. It distinguished between S-cis- and S-trans-enones and suggested the parent molecules absorb at 215 and 209 nm respectively.Values are also calculated for all intermediate conformations of an a-,@- a@-,@p-,and a@@-substituted enone. Molecular mechanics calculations in general refer to isolated molecules at 0 K. Hence caution must be exercised in assuming these conclusions also apply at room temperature in the liquid phase. In a study of solvent interactions with molecules containing two polar groups it was necessary to consider not only dipole terms but also quadrupole contribution^.^^ 6 Elements other than Carbon and Hydrogen Most force fields were initially developed for hydrocarbons and have since been extended to include other elements. Replacement of a carbon atom by Si Ge Sn or Pb was examined by Mislo~,~' who showed that (M(Bu') or M(SiMe3)4 adopt a ground state with T symmetry previously unknown with organic molecules.The related (Bu')~S~H is interesting as it exhibits4' correlated rotation of the t-butyl groups; a prediction confirmed by low temperature I3C n.m.r. Tetra-alkyldisilane 39 M. Askari N. S. Ostlund and L. Schafer J. Amer. Chem. Soc. 1977,99 5246. 40 U.Burkert and N. L. Allinger Tetrahedrp 1978 34 807. 41 W. D. Hounshell D. A. Dougherty J. P. Hummel and K. Mislow J. Amer. Chem. Soc. 1977,99.1916. 42 M. H. P. Ardebili D. A. Dougherty K. Mislow L. H. Schwartz and J. G. White J. Amer. Chem. SOC. 1978,100,7994. 43 J. C.Tai and N. L. Allinger J. Amer. Chem. Soc. 1976,98,7928. 44 N.L.Allinger and J. C. Tai J. Amer. Chem. SOC.,1977,99,4256. 45 T. Liljefors and N. L. Allinger J. Amer. Chem. Soc.1978,100,1068. 46 L.DoJen-MikoviE and N. L. Allinger Tetrahedron 1978,34,3385; N. L.Allinger L. DoSen-Mibvik J. F.Viskocil jun. and M. T. Tribble ibid. p. 3395. 47 L. D. Iroff and K. Mislow J. Amer. Chem. Soc. 1978,100,2121. 48 W. D.Hounshell L. D. Iroff,R.J. Wroczynski and K. Mislow I.Amer. Chem. SOC.,1978,100,5212;R. J. Wroczynski L. D. Iroff and K. Mislow J. Org. Chem.. 1978,43,4236. M. B. Hursthouse G. P.Moss and K. D. Sales was found to prefer a gauche-conformation although anti-forms may be present. In the case of (BU')~S~HS~H(BU')~ the gauche-conformer was calculated49 to be 49.7 KJ mol-' more stable than the dnti-form. An additional complication occurs when alcohols or ethers are considered due to the oxygen lone pair electrons.However inclusion of them as a pair of additional pseudoatoms results in a better prediction of stereochemistry and conformational energy differences5' An alternative treatment" of lone pairs with 1,3-dioxans and alcohols is to consider them as point charges taken from quantum mechanical calculations (CND0/2) preferably with additional weak van der Waals interactions between the lone pairs. Other elements such as halogen,53 or phosphoruss4 are treated by adjustment of the normal parameters. With quinquevalent phosphorus the unusual rectangular pyramidal structure of (C61-&02)2PCH3 was predi~ted.~~" Carbonyl compounds such as carboxylic acids or amide~~~ have also been examined. In a study of germacranolide (2) it was correctly predicted5' that hydrolysis and re-lactonization would favour the formation of a lactone to C-8 rather than to C-6.0 R=OH R=H 7 Reaction Mechanisms Molecular mechanics calculations have been used in two different ways for the analysis of reaction mechanisms. One approach is to predict the most probable pathway for a rearrangement when there are many possible alternative reactions. In the other type of study steric factors or transition states are analysed to predict the kinetics and stereospecificity of the reaction. Examir~ation~~ of the 69 isomers of tricycloundecane which do not have three- or four-membered rings or alkyl groups suggests a preferred pathway for the known conversion of tricyclo[6.2.1 .02*']undecane into 1-methyladamantane (Scheme 1)by 49 S.G. Baxter D. A. Dougherty J. P. Hummel J. F. Blount and K. Mislow J. Amer. Chem. SOC.,1978 100,7795. N.L. Allinger and D. Y. Chang J. Amer. Chem. SOC.,1976,98,6798. 51 U. Burkert Tetrahedron 1977,33,2237; 1979 35 209. 52 N. L. Allinger J. Kao H.-M. Chang and D. B. Boyd Tetrahedron 1976 32 2867. 53 A. J. Meyer J. Mol. Struct. 1977,40 127. 54 (a)J. A. Dieters J. C. Galluicci,T. E. Clark and R. R. Holmes J.Amer. Chem. SOC.1977,99,5461; (6)N. L. Allinger and H. von Voithenberg Tetruhedron 1978,34,807. 55 N.L. Allinger and S.H. M. Chang Tetrahedron 1977 33 1561. 56 D. N. J. White and M. H. P. Guy J.C.S. Perkin II 1975,43. 57 M. H. P.Guy G. A. Sim and D. N. J. White J.C.S. Perkin 11 1976 1917. E. Osawa K. Aigami N.Takaishi Y. Inamoto Y. Fujikura Z.Majerski P. von R. Schleyer E. M. Engler and M. FgrcaSiu J. Amer. Chem. SOC. 1977,99,5361. Theoretical Chemistry Applications of Molecular Mechanics Calculations Scheme 1 a series of 1,2-shifts. With pentacycloundecane the most stable isomer was predic- ted’’ to be D3 trishomocubane (4); a result confirmed by the AlBr,-catalysed isomerization of a suitable precursor. Steric effects in the SN2displacement of bromide from bromoalkanes by bromide were examined by De Tar et a1.,60 who found a close correlation between the transition state energy and the observed kinetics. Rates of solvolysis of polycyclic alcohol derivatives were similarly correlated with the strain of the system.61 The rate of oxidation of secondary alcohols to ketones was related to the structure of both alcohol and ketone.62 Reduction of cyclic ketones by hydride in general gives a mixture of epimeric alcohols.The yields of each isomer and the kinetics were predicted63 with reason- able precision by molecular mechanics calculations on the transition state. With nucleophiles larger than hydride the limiting step is controlled by steric factors in the approach of the reagent. Calculations based on the cone of access to the carbonyl group correlate closely with the observed stereo~electivity.~~ 8 X-Ray Structures One of the basic problems in the application of molecular mechanics calculations to the study of molecular structure and conformation is the choice of a suitable starting model especially if the molecule has a considerable degree of fiexibility.One simple solution has been to use the results of crystallographic investigations. Not only is this very convenient from a practical point of view (a set of atomic co-ordinates are easily deduced from the unit cell fractional co-ordinates) but it immediately leads to a means of studying the problem of how similar are the crystal and gas phase molecular structures. 59 G. J. Kent S. A. Godleski E. Osawa and P. von R. Schleyer J. Org. Chern. 1977,42,3852. 6o D. F. de Tar,D. F. McMullen and N. W. Luthra J. Amer. Chem. Soc.,1978,100,2484. 61 M. R. Smith and J. M. Harris,J. Org. Chem. 1978,43 3588; D. Fkcqiu ibid. p. 3878. 62 P. Miiller and J.-C. Perlberger 1Amer. Chem. Soc. 1976,98 8407. 63 J.-C. Perlberger and P.Muller J. Amer. Chem. SOC.,1977,99 6316. W. T. Wipke and P. Gund J. Amer. Chem. Soc. 1976,98,8107. M. B. Hursthouse G. P.Moss and K.D. Sales Molecular mechanics calculations of varying degrees of sophistication have been applied in this area. Huler and War~hel~~ have described a method for the simul- taneous calculation of the effect of inter- and intra-molecular forces on crystal packing which allows for the study of effects of intermolecular forces on molecular conformation using the potential surfaces of the quantum mechanical extension of the consistent force field to conjugated molecules. The applicability of the method to both rigid and flexible molecules is demonstrated by calculations on benzene biphenyl and p-ionylidenecrotonic acid.A variety of conformations are found for acetylcholine (AcOCH2CH2NMe3X) as its halide salt and lattice energy calculations have been made in order to examine the preferred conformation found for each halide.66 Similar studies have been made on crystals containing adrenaline (5),67and one significant result to come from this work is that the requirement of minimizing the lattice energy overrides the require- ment of minimizing the conformational energy. The presence of different con- formers in different crystal structures especially in systems containing ions is therefore not surprising. Studies on molecular crystals also show that the balance between crystal and intermolecular forces is very fine. Vos and co-workers have made an extensive study of molecules with folded conformations using only non-bonded interactions.The most recent work has shown that p-dimethylaminobenzyl-p-nitrophenylsulphone68 and p-dimethylaminophenyl-N-methyl-N-( p-nitrophenylsulphonylmethy1)car- bamate6’ have a folded conformation whereas p-chlorophenyl-p-methoxybenzyl-sulphone68 has a stretched conformation. Potential energy calculations however predict a folded conformation for all three although the stretched conformation is only -4 kJ mol-’ higher energy. Presumably crystal packing forces can provide the necessary difference. The importance of the attractive component of van der Waal’s forces and the interplay between the preferred free-molecule and crystal con- formations is well demonstrated by 1,3,5-trineopentylbenzenederivatives.Previous n.m.r. studies and molecular mechanics calculations on TNB (the parent compound) Me,TNB and Me,TNB showed a preference for a rotamer with all three neopentyl groups on the same side of the ring and it was suggested that attractive steric forces between the bulky hydrocarbon residues were significant in this choice.70 The crystal structure of 2,4,6-tribromo-TNB71 interestingly contains two independent rotamers-the preferred one and also one with a neopentyl group on the opposite 65 E. Huler and A. Warshel Acru Cryst. 1974 B30 1822. J. Caillet P. Claveni and B. Pullmann Actu Cryst. 1978 B34 3266. 67 J. Caillet P. Claveni and B. Pullmann Actu Cryst.,1976 B32 2740. 68 I. Tickle J. Hess A. Vos,and J. B.F. N. Engberts J.C.S. Perkin II.1978 460. 69 R.J. J. Visser A. Vos and J. B. F. N. Engberts J.C.S. Perkin II 1978,634. ’O R.E. Carter and P. Stilbs J. Amer. Chem. Soc. 1976,98,7515. 71 B. Aurivillius and R. E. Carter J.C.S. Perkin [I 1978 1033. Theore tica 1 Chemistry Applications of Molecular Mechanics Calculations side of the ring to the other two. Again crystal packing forces are assumed to help stabilize this latter conformation which lies about 4 kJ mol-’ above the preferred one. The effect on the molecular geometry of peri-substitution in naphthalenes has been studied crystallographically and by molecular mechanics calculations with an ad hoc force field.72 The results indicate that the C(8)-C(9)-C(l) angle is the most constant indicator of the steric crowding due to the peri-substitution and the difference in steric energies of 19 kJ mol-1 for the 1-methyl and 1,g-methyl deriva- tives of a 2-naphthyl acetate give a good idea of the energies involved.Conformational studies of cyclic molecules provide a very useful application of molecular mechanics calculations. For such systems differences between experi- mentally determined crystal and calculated gas phase conformations are generally small but for some ring systems several conformers may lie close in energy. The structure analysis of and molecular mechanics calculation on bicyclo- [4.4.l]undecane-l,6-di01’~have provided interesting information not only on the conformation of the cycloheptane rings (which are locked into C2 symmetric twist/chair forms) but also on the ten-membered ring the conformation of which was used as a starting point for strain energy minimization studies and which was found to resemble a low-energy cyclodeca- 1,6-diene conformer.A of a cyclodeca-1,5-diene ring system in the germacranolide costunolide (3)shows that the calculated free molecule conformation is only slightly perturbed by crystal packing forces-the main distortion being a small deviation from the prefer- red C2symmetry. Quite large differences in strain energies of various final con- formers were found for the ten-membered ring systems in the sesquiterpenes agerol diepoxide (6) and ageratriol (7)but again the crystal conformations correspond quite closely to the calculated minimum energy conforrner~.~~ 0’ 72 D.N. J. White J. Carnduff M. H. P. Guy and M. J. Bovill Acta Crysf.,1977 B33 2986. 73 D. N. J. White and M. J. Bovill Actu Crysr. 1977 B33. 3029. 74 M. J. Bovill P. J. Cox P. D. Cradwick M. H. P. Guy G. A. Sim and D. N. J. White Actu Cryst. 1976 B32,3203. ’’ W. Messerotti V. M. Pagnoni R. Trave R. Zanasi G. D. Andreetti G. Bocelli and P. Sgarabotto J.C.S. Perkin 11 1978 217.

ISSN:0069-3030    

DOI:10.1039/OC9787500023

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (i) Pericyclic Reactions By R. J. BUSHBY Department of Organic Chemistry The University Leeds LS2 9JT A simple HMO model for estimating the effect of substituents on the rates of pericyclic reactions has been described and predictions for most cases of monosub-stitution presented in clear tabular form.’ 1 Cycloadditions and Cycloreversions Dewar2 has published details of his calculations on the interconversion of cyclo-hexene and butadiene +ethylene. Despite indications from ab initio calculations3 that this reaction is concerted he remains unconvinced and is still prepared to defend the conclusion of his MIND0/3 method which suggests that it is of the unsymmetrical biradical type. Taken the results illustrate the problems involved in this sort of calculation.At present time-saving approximations have to be made somewhere but whether these are made mainly in the MO method itself or in the method used to search the energy surface the conclusions are never beyond doubt or criticism. Most treatments of reactivity and selectivity in cycloaddition reactions seem to have been in terms of approximate frontier molecular orbital (FMO)theory. Hence although rates of addition of diphenylketen to exocyclic double bonds have been shown not to correlate with alkene ionization potential^,^ a linear relationship has been noted between the ionization potentials of N-acylamino-dienes and their reactivity towards methyl acrylate.’ It has also been shown that the rate of the Diels-Alder reaction between the tetracyclic alkene (1)and tetrazine (2) Py = 2-B.K. Carpenter Terrahedron 1978,34 1877. ’ M. J. S.Dewar S.Olivella and H. S. Rzepa J. Amer. Chem.Soc. 1978,100 5650. L. A. Burke G. Leroy,and M. Sana Theor. Chim.Acfa,1975,40,313; R. E.Townshend G. Ramunni G. Segai W. J. Hehre and L. Salem,J. Amer. Chem. SOC.,1976,98,2190. R.W.Hofhnann and J. Becherer Tetrahedron 1978,34 1193;cf. K.N.Houk Accounts Chem. Res. 1975 11,361. L. E.Ovennan G. F. Taylor K.N. Houk,and L. N. Domelsmith J. Amer. Chem. SOC. 1978,100,3182. 37 R. J. Bushby pyridyl is enhanced by substituents (X =OH or OMe) which interact with the alkene T orbital and raise its energy.6 Secondary orbital interactions (marked *) have been invoked to explain the preferred syn transition state in the addition of phthalimidonitrene to dienes (3)' and LUMO HOMO similar secondary orbital interactions to explain the preference of this nitrene for addition to the 4,5-bond rather than the 2,3-bond of tropone.' The importance of both steric and secondary orbital interactions in determining the stereochemistry of the Diels -Alder reaction has attracted further attention' and is quite nicely illustrated by the work of Martin et a1." Hence the decrease in endo selectivity for the addition of dimethylenecyclobutene to cyclopentadiene (endo :ex0 = 1)relative to the corresponding addition of cyclobutadiene (endo :ex0 >25) can be explained in terms of the smaller orbital coefficients for the centres involved in the secondary orbital interactions as may be seen from the HOMO'Sshown in formulae (4) and (5).(4) For the reaction of cyclobutene derivatives however the decrease in endo selectivity engendered by replacing a methylene by a CMe2 or SO2 grouping is the result of steric effects. Contrary to previous reports the kinetic product of the reaction between furan and maleic anhydride has been shown to be the expected endo adduct" and whereas thiophen is usually unreactive towards this dienophile an adduct is obtained if the reaction is conducted at a pressure of 15 kbar.12 Although the orientation of most photochemical 2 +2 reactions between benzene and alkenes seems to be determined by the natural association between the two reactants it is interesting to note that benzene and vinyl ethers associate in solution in an endo sense but give exo 2 +2 adducts on irradiation.13 M.N. Paddon-Row H. K. Patney and R. N. Warrener J.C.S. Chem. Comm. 1978,296. R. S. Atkinson and J. R. Malpass J.C.S. Perkin I 1977,2242. * D. W. Jones J.C.S. Chem. Comm. 1978,404. J. Kalo D. Ginsburg and J. J. Bloomfield Tetrahedron 1978 34 2153 and following pap&. lo H.-D. Martin R. Iden and H.-J. Schiwek Tetrahedron Letters 1978 3337. M. W. Lee and W. C. Herndon J. Org. Chem. 1978,43,518;cf F. A. L. Anet Tetrahedron Letters 1962 1219. l2 H. Kotsuki S. Kitagawa H. Nishizawa and T. Tokoroyama J. Org. Chem. 1978,43 1471. l3 R. J. Atkins G. I. Fray M. G. B. Drew A. Gilbert and G. N. Taylor Tetrahedron Letters 78,2945. Reaction Mechanisms There seems to have been even more interest than usual in the problem of regioselectivity in the Diels-Alder rea~tion'~*~~ and in other 2 +416*17 and 4 +618 cycloaddition reactions.In many cases these have also been interpreted in terms of FMO theory and in the case of addition of benzonitrile oxide to 3-substituted cyclopentenes the theory has been extended to explain the moderately good linear relationship obtained between selectivity and a function of alkene ionization poten- tial." In view of the number of approximations involved (for example the neglect of product stability coulombic and exchange terms) the degree of success of FMO theory in this area is remarkable and the occurrence of some exceptions not at all surprising. Such anomalies where either the orientation is incorrectly predicted or where large selectivities are only reflected by small differences in the frontier molecular orbitals have however attracted attention and explanations have been sought in terms of the theory itself.Hence Houk has suggested that polarization of the frontier orbitals brought about by the approach of the reagent may be important" and Alston and others have suggested that secondary orbital inter- actions are to blame.".17 Hence the observation that N-t-butyl-nitrone adds to acrylonitrile to give only the adduct (6) but adds to cyanoacetylene to give a 1:1 mixture of adducts (7)and (8)has been attributed to the secondary orbital interaction [marked * in formula (9)] which favours the orientation found in adduct (8) and c/o O\N TCN 6" .-jN + + (6) (7) (8) (9) l4 P.-A.Carrupt M. Avenati D. Quarroz and P. Vogel Tetrahedron Letters 1978,4413;I. J. Westerrnann and C. K. Bradsher J. Org. Chem. 1978 43 3002; T. Imagawa A. Haneda T. Nakagawa and M. Kawanishi. Tetrahedron,1978,34.1893;A. Oliva J. I. Fernandez-Alonso and J. Bertran ibid. p. 2029; P.S.Mariano P. L. Huesmann R.L. Beamer and D. Dunaway-Mariano ibid. p. 2617. lS P.V.Alston R. M. Ottenbrite and T. Cohen J. Org. Chem. 1978,43,1864; T.Cohen R.J. Ruffner D. W. Shull W.M. Daniewski R.M. Ottenbrite and P. V. Alston ibid. p. 4052. l6 J. J. Tufariello and R. C. Gatrone Tetrahedron Letters 1978,2753;D.Mukherjee C. R.Watts and K. N. Houk J. Org. Chem. 1978 43 817; P. Caramella G.Cellerino K.N. Houk F.M.Albini and C. Santiago,ibid. p. 300;G.Cauquis and B. Chabaud Tetrahedron,1978,34,903;H. Benhaoua F.Texier P. Guenot J. Martelli and R.Carrie ibid. p. 1153;E.Stephan Bull. SOC. Chim. France 1978,364;P. Buttioni L.Vo-Quang and Y.Vo-Quang ibid. 1978,401; 415. M. D.Gordon P. V. Alston and A. R.Rossi J. Amer. Chem. SOC. 1978,100,5701. A. Padwa and F. Nobs Tetrahedron Letters 1978.93;L.C. Dunn and K. N. Houk ibid. p. 3411. l9 E.J. McAlduff P. Caramella. and K. N. Houk. 1.Amer. Chem. Soc. 1978,100 105. 2o K.N.Houk L. N. Domelsmith R.W. Strozier and R. T. Patterson J. Amer. Chem. SOC.,1978,100 6531. R. J. Bushby which should be more important in the acetylene than in the alkene.17 The importance of secondary orbital interactions in determining regioselectivity for the Diels-Alder reaction has however been questioned.21 Isotope effects for the addition of diphenylketen to styrene suggest a concerted mechanism22 and X-ray crystallographic studies have confirmed that the stereo- chemistry of the t-butylcyanoketen + 2-methylbut-2-ene adduct (10) is as expected for a ~2a (keten)+w2s (alkene) process.23 Good evidence has however been obtained for a zwitterionic intermediate in the addition of this keten to activated ~lefins,~~ and other ketens.26 Hence its addition to dimethylketen or the imidate~,~~ decomposition of the 4-azido-5-t-butylcyclopentene-1,3-dione (1 1; R = Me) gives compound (12) but its addition to methylketen or the decomposition of dione (11; R = H) gives the lactone (13).These results are best explained in terms of a common zwitterionic intermediate (14;R = Hor Me) in which the CO’may cyclize either to the carbon or the oxygen and it has been shown that this intermediate can be trapped by methanol. (12) (13) (14) Allenyl cations have been shown to add to diene~~~ in a formal 3 + 4 sense in much the same way as oxyallyl cations although on the basis of the regioselectivity of this latter reaction it has been suggested that the mechanism is stepwise rather than concerted.28 Further examples have appeared relating the geometry of 1,3-dipoles to the extent of carbene character.*’ Hence nitrile imines are thought to be less bent than the corresponding nitrile ylides and show more reactivity in the 1,3 sense and less in the 1,1 sense.3o Additions to the planar singlet trimethylenemethane (15;X = H)have been reinterpreted on the assumption that the HOMO has the symmetry shown in formula ” I.Fleming J. P. Michael L. E. Overman and G. F. Taylor Tetrahedron Letters 1978 1313. ” C. J. Collins B. M. Benjamin and G. W. Kabalka J. Amer. Chem. SOC.,1978 100,2570. 23 P. R.Brook A. M. Eldeeb K. Hunt and W. S. McDonald J.C.S. Chem. Comm. 1978 10. 24 D. Becker and N. C. Bodsky J.C.S. Chem. Comm. 1978,237. 25 H.W. Moore L. Hernandez and R. Chambers J. Amer. Chem. SOC.,1978,100,2245. ’‘ H.W. Moore and D. S. Wilbur J. Amer. Chem. Soc. 1978 100 6523. 27 H.Mayr and B. Grubmiiller Angew. Chem. Internat. Edn. 1978,17,130. 28 H.M.R. Hoffmann and R. Chidgey Tetrahedron Letters 1978,85.29 A.Padwa and A. Ku J. Amer. Chem. SOC.,1978,100,2181;A. Padwa P.H. J. Carlsen and A. Ku ibid. p. 3494;A Padwa and P. H. J. Carlsen. J. Org. Chem. 1978,43,3757. 30 A. Padwa S. Nahm and E. Sato J. Org. Chem. 1978,43,1664. Reaction Mechanisms (16) and the LUMO that in formula (17). Hence reaction with alkenes (YCH= CHY) gives fused adducts (18)bur cyclopentadiene a bridged adduct (19). For the methoxy-substituted TMM (15; X =MeO) however the order of the HOMO and LUMO is reversed and hence reaction with alkenes gives bridged adducts (20).31 (18) (19) (20) Whereas it had originally been claimed that the adducts (21) formed between tropone and fulvenes were the result of a direct 6 (fulvene)+4 (tropone) cyclo- addition as a result of stereochemical studies it is now conceded that they arise through a 3,3-sigmatropic rearrangement of an initial 6 (tropcne)+4 (fulvene) adduct (22).32 RK The barrier to thermal loss of nitrogen from the diazetine (23; R=Me) is unexpectedly high33" and since the reaction is highly exothermic it has been suggested that a triplet electronically excited product could result.This however cannot be the case since nitrogen elimination from (23; R=Et) is stereospecific (cis).33bThermochemical data for the elimination of N2and of N20from compound (24; X = N) and (24; NO) suggest that the relative slowness of elimination of N20 may be related to the lower exothermicity of this reaction rather than to orbital symmetry 31 J. A. Berson R. Siemionko A.Shaw G. O'Connell R. D. Little B. K. Carpenter and L. Shen Tetrahedron Letters 1978 3529. 32 I.-M. Tegmo-Larsson and K. N. Houk Tetrahedron Letters 1978 941. 33 (a)P.S. Engel R. A. Hayes L. Keifer S. Szilagyi and J. W. Timberlake J. Amer. Chem. SOC.,1978,100 1876; (b)D. K. White and F. D. Greene J. Amer. Chem. SOC.,1978,100,6760. J. F.M.0th. H. Olsen and J. P. Snyder J. Amer. Chem. SOC.,1977,99,8505. R.J. Bushby 2 Sigmatropic Reactions The skeletal rearrangements and particularly the sigmatropic rearrangements of carbanions have been reviewed.35 It is interesting to note that the 1,4 shift (25)+(26) R = benzyl which is thought to be concerted has a negative volume of activation but the corresponding shift of a benzhydryl group which is thought to involve a radical pair shows a positive volume of a~tivation.~~ OR (25) (26) An example of a thermal 1,3 acyl shift has been described3’ and further evidence presented of the high migratory aptitude of acyl and vinyl groups in 1,5 shifts.38 In one case however it is claimed that 1,5 migration of allyl is faster than that of vinyl.” The kinetics of the rearrangement (27) -B (28)+ (29) have been studied both for (27) (28) (29) the parent hydrocarbon and for its radical anion.4o Quite remarkably the radical anion rearrangement is 10’’ times faster.This could be explained if both reactions involved the intermediate (30;* = -or -) since in the radical anion translation of an electron from an antibonding to a nonbonding orbital would provide the extra driving force.Detailed studies of the interconversions of the bicycloC6.1 .O]nona- trienes (31)-+(32)+etc. show that the 1,7shift is stereospecific with inversion at C-9 which is consistent with a concerted a2a + 7r6s me~hanism.~~ The stereochemistry of 2,3 sigmatropic rearrangements has been the subject of several investigations although no real pattern seems to have emerged. Unlike their open-chain analogues cyclic allyl sulphoxides rearrange through an ex0-transition 35 E. Grovenstein,Angew. Chem. Internat. Edn. 1978,17,313. ” W. J. le Noble and M. R. Daka J. Amer. Chem. SOC.,1978,100 5961. 37 J. M. Janusz L. J. Gardiner and J. A. Berson J. Amer. Chem. Soc. 1978,99,8509. D. J.Field D. W. Jones andG. Kneen J.C.S. Perkin I 1978,1050;M.F. Semme1hack.H.N. Weller and J. Clardy J Org. Chem. 1978,43 3791. ’’ R. D. Miller D. Kaufmann and J. Mayerle J. Amer. Chem. Soc. 1977,H. 8511. *O F. Gerson W. Hukr and K. Mulen Angew. Chem. Internat. Edn. 1978,17,208. 41 F.-G. Klarner and M. Wette Chem. Ber. 1978,111 282. Reaction Mechanisms 43 state (33)."* Also whereas 2,3 rearrangements through the conformation shown in formula (34) leading to trans products are usually preferred exceptions involving conformation (35) and leading to cis products have been reported especially in cyclic sy~tems."~ (33) (34) (35) The mechanism of 3,3-sigmatropic rearrangements and the possible involvement of 1,4-biradicals (36) has remained a major area of interest and controversy. The stereochemistry of the conversion of allylcyclopropene (37) into compounds (38) Ph Ph cis (38) and (39) R=H and Me has been interpreted in terms of a common 1,4-biradical intermediate (40) which gives rise to (38) via a chair-like conformation and (39) (39) (40) through a boat-like onf formation.^" The effects of and ~heny1"~ substituents on the rates of 3,3-sigmatropic rearrangements whilst substantial seem less than would be expected for full 1,4-biradical character and studies of the phenyl-substi- tuted systems (41) do not show the expected linear Hammett plot."' Furthermore 42 R.W. Hoffmann R. Gerlach and S. Goldmann Tetrahedron Letters 1978,2599. '' E. Vedejs. M. J. Arw and J. M. Renga Tetrahedron Letters 1978,523; E. Vedejs J. P. Hagen B. L.Roach and K. L. Spear J. Org. Chem. 1978,43 1185; W. C. Still and A. Mitra J. Amer. Chem. Soc. 1978,100,1927. A. Padwa and T. J. Blacklock J. Amer. Chem. SOC.,1978,100 1321. 45 K. J. Shea and S. Wise J. Org Chem. 1978 43 2710. P. Metzner T. N. Pham and J. Vialle J. Chem. Res. (S) 1978,478. 47 E. N. Marvel1 and T. H.-C. Li J. Amer. Chem. SOC.,1978,100,883. R. J. Bushby Gajewski4* has advanced a thermochemical argument that the 1,4-biradical involved in the isomerization and ring opening of bicyclo[2.2.0]hexane cannot also be involved in the Cope rearrangement of hexa-1,5-diene. However the situation is somewhat complicated by the possibility of several geometrically distinct 1’4- biradical~.~’He has also provided evidence from secondary isotope effects which seems to suggest that the Cope reaction is concerted but readily diverted towards radical character by suitable substituents.Hence for compound (42) secondary isotope effects suggest that bond making and breaking are almost equally advanced in the transition state (concerted mechanism?). For compound (43) bond making seems to be in advance of bond breaking (1,4-biradical-like?). For compound (44) bond breaking seems to be in advance of bond making (tending towards two ally1 radicals?). Q It is interesting to note that the 1,2 rearrangement of ylide (45) to give the compound (46) involves 7 1-8 1YO stereoselectivity (for the intramolecular component) whereas the 1,3 rearrangement which gives rise to (47) involves only H H Ph )-Me PhTMePhYMe PhCO-C-NMe2 Ph-C-COMe ph+Me2Me MeI IOH (46) (45) 56-57% stereoselectivity.This observation seems consistent with the idea that the degree of racemization is related to the degree of translation involved within the radical pair.50“ In related ylide rearrangements there seems to be some evidence to support the notion that ‘allowed’ and ‘forbidden’ processes involves distinct types of radical pairs or transition ~tates.”~ It is also interesting to note that whilst ylide (48) apparently prefers the ‘allowed’ 3,2 rearrangement to the ‘forbidden’ 1,2 re-arrangement for ylide (49) the situation is reversed.’’ A number of interesting studies have appeared of the photochemical di-v- methane rearrangement which may be regarded as involving a formal 1,2 sig- matropic shift.Hence Houk has shown that the propensity of bicyclic systems such as compound (50),to undergo this rearrangement is related to the v/vinteraction as 48 J. J. Gajewski and N. D. Conrad J. Amer. Chem. SOC.,1978,100.6268 6269 49 M. J. S. Dewar S. Kirschner H. W. Kollmar and L. E. Wade J. Amer. Chem. Soc. 1974,% 5242. (a)K. Chantrapromma W. D. Ollis and I. 0.Sutherland J.C.S. Chem. Comm. 1978,672; (b) ibid. pages 670 and 673. ’’ W. D. Ollis M. Rey and I. 0.Sutherland J.C.S. Chem. Comm. 1978,675. Reaction Mechanisms (48) (49) (50) estimated by photoelectron spectros~opy.~~ Similarly the regioselectivity for systems bearing a ring substituent can be rationalized in terms of PMO theory although other treatments give equivalent results.Hence inspection of SOM02for the nitrile (51) and of SOMO1 for the amine (52) shows that bridging to the ortho position (marked +-+)should be favoured whether the substituent is of the acceptor or the donor type.53 Zimmerman has shown that for aryl-substituted 1,4-dienes regioselectivity is moderately well correlated by the substituent constant cand that in the biradical(53) an electron-donating substituent X leads to preferred homolysis of bond b but an electron-accepting substituent cleavage of bond a.54 3 Ene and Related Reactions X-Ray crystallography has confirmed that the product of the ene reaction of a-pinene and chloral as catalysed by ferric chloride has the correct stereochemistry for the least crowded concerted transition state (54),55 but the presence of halogen- H (54) L.N. Dornelsmith P. D. Mollere K. N. Houk R. C. Hahn and R. P. Johnson J. Amer. Chem. Soc. 1978 100,2959. C. Santiago E. J. McAlduff K. N. Houk R. A. Snow and L. A. Paquette J. Amer. Chem. soc. 1978 100,6149. H. E. Zimmerman and T. R. Welter J. Amer. Chem. Suc. 1978 100,4131. M. J. Begley G. B. Gill and B. Wallace J.C.S. Perkin I 1978 93. R. J. Bushby transfer products in the equivalent reaction of 1-substituted and 1,2-disubstituted alkenes has been interpreted in terms of a dipolar intermediate which may react in either of the two modes (55) or (56).56 (55) (56) A P-methoxy substituent has a strong accelerating effect on the rate of decar- boxylation of Py-unsaturated acids,” an observation which is consistent with the changes in charge distribution in the transition state proposed by Dewar (57).58 d+ ;;(::<- H no n-(57) 4 Electrocyclic Reactions STO-3G and 3-41G MO calculation^^^ on the opening of cyclopropylidene to allene have provided a substantially different picture to that based on INDO calculations.60 Kirmse and co-workers have produced an interesting series of papers giving details of their work on the opening of ring-fused cyclopropyldiazonium ions.61 Normally but perhaps not invariably,62 these proceed in the expected disrotatory sense and it seems likely that reaction with solvent occurs before opening to the planar truns,truns-ally1 cation e.g.(58) is complete. Often the highly strained products undergo secondary transformations.Solvolysis of compound (59)61gives among other things cis-3-methoxycycloheptene. This however is formed by iso-merization of the intermediate trans-isomer which can be trapped as its furan adduct t3 pN2+ H@oMe -(58) (59) (60) (60). Thermal ring opening of the aziridines (61)63 proceeds in the expected conrotatory sense to give the azomethine ylide (62),which can be stereospecifically G. B. Gill S. J. Parrott and B. Wallace J.C.S. Chem Comm. 1978 655. ” D. B. Bigley and A. Al-Borno J.C.S. Chem. Comm. 1978,1025. ” M.J. S. Dewar and G. P. Ford J Amer. Chem. SOC.,1977,99 8343. s9 D. J. Pasto M. Haley and D. M. Chipman J. Amer. Chem. SOC.,1978,100 5272. 6o P.W.Dillon and G. R. Underwood J. Amer.Chem. SOC.,1977,99,2435. 6’ W. Kirmse and H. Jendralla Chem. Ber. 1978 111 1857 and following papers. 62 W. Kirmse and U. Richarz Chem. Ber. 1978,111 1895. 63 A. C. Oehlschlager A. S. Yim and M. H. Akhtar Cunad. J. Chem. 1978,56 273. Reaction Mechanisms Ar Ar I I PhAPh Ph Pheyl-trapped by addition to double bonds. As expected for a process dominated by the ArN (HOMO)/(r bond (LUMO) interaction variation of the N-aryl substituent yields a negative Hammett constant (p= -0.80). The equivalent opening of the aziridines (63)is however forced to be disrotatory and seems to be controlled by an ArN (LUMO)/(r bond (HOMO) interaction since the sign of p is reversed (+0.74). It is interesting to note that whilst the potassium salt of the ketone (64)is a stable 107.r-electron aromatic system the lithium salt which is thought to be more covalent undergoes a rapid disrotatory ring closure to give compound (65).64 64 G.Boche and F. Heidenhain Angew. Chem. Internat. Edn. 1978,17,283.

ISSN:0069-3030    

DOI:10.1039/OC9787500037

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (ii) Polar Reactions By H. R. HUDSON Department of Chemistry The Polytechnic of North London Holloway Road London N78DB 1 Introduction This year’s report covers nucleophilic substitution (at saturated allylic and vinylic centres) carbocations and their rearrangements carbanions &elimination elec- trophilic addition and nucleophilic addition to carbonyl compounds. The aim has been to select material of a novel character where possible and not to give a comprehensive coverage of the literature which is dealt with elsewhere.’ Polar reactions in the gas phase have not been included as these were highlighted in last year’s report which referred to some of the more significant lines of research in this area. 2 Nucleophilic Aliphatic Substitution Substitution at Saturated Carbon.-The role of ion-pair intermediates in nucleo- philic aliphatic substitution has been referred to previously [Ann Reports (B),1974 71 114; 1976 73 551.As a further means of exploring this topic the reactivity- selectivity principle (r.s.p.) has been applied to studies of the solvolysis of n-octyl 1-methylheptyl and benzyl substrates (halides and/or arenesulphonates) in aqueous ethanol.* According to this prin~iple,~ highly reactive species such as ion-pair intermediates would be expected to show less discrimination in their reactions with competing reagents than would the less reactive neutral substrates. Selectivity values were determined and were expressed in terms of the ratio (kE/kw) of the rate constants for the reactions of each of the substrates with ethanol and with water respectively.Low selectivity values (0.63-3.61) coupled with the total absence of a leaving group effect were interpreted as possible evidence for product-determining attack on a highly reactive ion-pair intermediate not only for benzyl and 1-methylheptyl but more remarkably for n-octyl also. The nature of any ion-pair that might be involved was not however clear and the results were not claimed to be more than speculative. A similar use of the r.s.p. approach in the solvolysis in aqueous ethanol of a series of para-substituted benzyl derivatives has led to a more definitive identification of ’ ‘Organic Reaction Mechanisms 1978’ ed. A. R. Butler and M. J. Perkins Wiley London 1979.* A. Pross H. Aronovitch and R. Koren J.C.S. Perkin 11 1978 197. ’ A. Pross Adv. Phys. Org. Chem. 1917,1469. 48 Reaction Mechanisms the species involved and to the proposal of a new diagnostic tool for the identification of solvent-separated ion pairs in such reactions viz. that a leaving group effect on selectivity will be ob~erved.~ For p-chlorobenzyl and benzyl halides which solvolyse to yield products from intimate ion pairs a plot of log(k,/kw) against solvent ionizing power Y was linear and was essentially the same for both chlorides and bromides. In contrast the plots for p-methyl and p-methoxybenzyl halides deviated considerably from this pattern and indicated product formation from at least two solvolytic intermediates one being the solvent separated species (Scheme 1).R-X $ RtX-$ R+IIX-1 1 products products Scheme 1 In studies of the use of chlorine leaving group kinetic isotope effects (k3’/k3’)for the investigation of ion-pairing a series of benzyl chlorides p-XC6H4Cl (X = H Me or PhO) has been subjected to solvolysis in aqueous acetone with or without the presence of azide ion or thiophenoxide For processes thought to involve classical sN2 displacements values of ca. 1.0092 (X= H) 1.0094 (X = Me) and 1.0097 (X=PhO) were obtained. Smaller kinetic isotope effects were correlated with competing SN2 and ion-pair pathways and a lower limiting value of 1.0059 (X = Me) or 1.0058(X = PhO) was observed for solvolysis in 97 % trifluoroethanol. These figures are very close to the calculated equilibrium isotope effect (1.0057) obtained from i.r.vibrational frequency data and indicate rate-determining ion-pair dissociation in these cases. Complications in interpretation could however arise from compensating solvation of the incipient chloride ion in the transition state.6 To investigate the effect of solvent on transition state structure for a conventional SN2 process the displacement of chloride ion from n-butyl chloride by thiophenoxide [Equation (l)]was ~tudied.~ Although rate constants changed by three orders of PhS-+RCI + PhS--R--CI + PhSR +C1-(1) [ 6-6-1 [R = n-C4H9] magnitude over the range of solvents used (alcohols diglyme tetraglyme DMF and DMSO) the chlorine kinetic isotope effect (k3’/k3’) was essentially constant (1.0094f0.00012).This result is in contrast to that reported earlier’ for the effect of solvent variation on the displacement of dimethyl sulphide from tri-methylsulphonium by ethoxide ion [Equation (2)] for which k32/k34decreased from EtO-+MeSMe2 * EtO--Me--SMe2 + EtOMe+SMe2 (2) + [” 6+ 1 1.0095to 1.0035as the solvent was changed progressively from ethanol to a mixture of ethanol (35%) and DMSO (65 %). It has however been pointed out that the H. Aronovitch and A. Pross J.C.S. Perkin 11 1978 540. D. G. Graczyk J. W. Taylor and C. R. Turnquist J. Amer. Chem. SOC.,1978,7333. ‘G. W. Burton L. B. Sims J. C. Wilson and A. Fry,J. Amer. Chem. SOC.1977,99 3371. ’R. T. Hargreaves A. M. Katz and W. H. Saunders jun. J. Amer. Chem. SOC.,1976,98,2614.H. R.Hudson charge distributions at the transition states for these two systems are different.8 From a consideration of these observations and of other literature data on secondary a-deuterium kinetic isotope effects it has been concluded that there will be little or no solvent effect on SN2 transition state structure if the two nucleophiles involved are of like charge [Equation (l)]but that the transition state structure will be sensitive to solvent effects if one nucleophile is negatively charged and the other is neutral [e.g. Equation (2)].8 A novel approach to mechanisms of aqueous solvolysis has been based on the concept that water contains an equilibrium concentration of ‘free’ hydroxyl and ‘free’ lone-pair groups as indicated by the overtone regions of the 0-H stretching band for water [Equation (3)].9It was suggested that these free groups are the reactive (H2O)bul~e (0H)free+ (LPhree (3) ingredients which may initiate hydrolyses involving for example C-H ionization [Equation (4)Jor C-Cl heterolysis (Scheme 2).The hypothesis gives a qualitative R-CI + (OH)fre $ Rf+ (Cl--H0)-HO (R-Cl--HO)+(OH)free $ R++Cl-:j ‘HO Scheme 2 explanation of the effect on reaction rates of the addition of an alcohol or basic aprotic co-solvent which will reduce the concentration of (OH), [Equation (5)] and RH + (LP)free + R-+ (H--LP)’ (4) Base + (0H)free + Base--(HO) (5) consequently increase that of (LP)free. SN2displacements which are relatively insensitive to the addition of co-solvents were explained on the basis of a two-stage process (Scheme 3) in which relative changes in the concentrations of (OH),, and (LP)freenearly compensate one another.R-hal+(OH)free T! Rhal--(HO) (LP)free+ Rhal--(HO) -+ Products Scheme 3 Investigations of the controversial role of a-participation in the solvolysis of exo-2-norbornyl systems” have continued unabated and further evidence has been produced in support of the view that high exolendo rate ratios are steric in origin. Comparisonsof solvolytic data for em- (1)and endo-2-norbornyl arenesulphonates (2) with those for adamantyl derivatives were made in aqueous ethanol or trifluoro- K. C. Westaway Cunud. J. Chem. 1978,56,2691. M. C. R.Symons J.C.S. Chem. Comm. 1978,418. lo H.C. Brown and P.von R.Schleyer ‘The Nonclassical Ion Problem’ Ch. 4 Plenum Press New York,N.Y. 1977. Reaction Mechanisms x (1) (2) ethanol,” and in a wide range of other media of differing nucleophilicities and ionizing powers ranging from methanol and ethanol through various aqueous solvents to hexafluoroisopropanol and trifluoroacetic acid. l2 The excellent cor- relations which were obtained indicated that there was no significantly larger solvent contribution to the rates of solvolysis for the 2-norbornyl systems than for 2-adamantyl a standard k substrate. The similarity in solvent effects on both the endo-and ex0 -2-norbornyl systems was also taken as strong evidence against participation in the latter by either solvent or neighbouring carbon.Apart from the semantic problem of whether such processes should be designated k unless there is absolutely no participation involved,12 it is clear that the results are in conflict with the usual interpretation of high exolendo rate ratios which is based on distinctly different kA and k processes. Increasing electron demand has also been used as a tool to show lack of significant electron release from exo-2-norbornyl to the a-carbon of aryl (em-2-norbornyl) methylcarbinyl p-nitrobenzoates (3) during solvolysis in aqueous acetone. l3 The endo-isomer (4) in this case solvolyses four times faster a result attributed to greater Ar (4) relief of steric strain on ionization. The importance of steric factors in other examples of this type has been demonstrated by comparison of reaction rates with those for various sterically hindered acyclic analogues.l4 Allylic Substitution.-A reinvestigation of certain aspects of the solvolysis of 1-and 3-substituted ally1 chlorides” has produced new evidence in support of the contro- versial sN2’ mechanism [Ann. Reports (B) 1971 68 2521. Product composition studies and in particular the results of work with optically active substrates (+)-RCHClCH=CH2 (R =Me or Bui) indicated that in anhydrous alcohols both the rearranged and unrearranged products were formed in concerted reactions. The stereochemical consequences of sN2’ reactions appear to cover the whole range of possibilities from syn-to anti-displacement according to the nature of the displacing and displaced groups counterions and solvent.l6 Preferential syn-attack (by a J. M. Harris D. L. Mount and D. J. Raber J. Amer. Chem. Soc. 1978,100,3139. I2 H. C. Brown M. Ravindranathan F. J. Chloupek and 1. Rothberg. J. Amer. Chem. Soc. 1978,100,3143. l3 H. C. Brown and M. Ravindranathan J. Org. Chem. 1978,43 1709. l4 H.C.Brown and M. Ravindranathan J. Amer. Chern. Soc.,1978,100,1865. l5 C.Georgoulis and G. Ville J. Chem. Research (S),1978 248. l6 T.Oritani and K. H. Overton J.C.S. Chem. Comm. 1978,454,and cited references. H. R.Hudson factor of 1.4-1.8) was demonstrated in the aminolysis of (R)-or (S)-[1-2H]oct-l- en-3-y1(5; X = n-C5H,,) or [1-2H]but-l-en-3-y12,6-dichIorobenzoate(5;X = Me) by (R)-or (S)-a-methylbenzylamine (Scheme 4). The results indicated a difference (5) Scheme 4 in activation energies for the two processes of not more than 0.5 kcalmol-'.The syn-geometry which has recently been demonstrated for the reaction of a-methyl- ally1 chloride with diethylamine" was therefore considered16 to reflect more effective hydrogen-bonding between the amine and allylic chlorine. In contrast intramolecular SN2'displacement of 2,4,6-trimethylbenzoate by an anionic nucleo- phile (RS-)proceeded entirely with anti-geometry (Scheme 5).Is Me Scheme 5 Vinylic Substitution.-The role of vinyl cations in solvolytic displacements has been reviewed" and the first examples of vinyl cations as intermediates in the solvolysis of vinyl fluorides have been reported.20 In buffered aqueous ethanol or trifluoro- ethanol the fluorides [(6)-(9)] gave the corresponding ketones together with (6) (7) (8) (9) Ar = p-MeOC6H4 Ar = p-MeC6H4 various elimination and/or rearrangement products.High negative entropies of activation dependence of reaction rates on solvent ionizing power and Winstein- R. M. Magid and 0.S. Fruchey J. Amer. Chem. SOC. 1977,99,8368. '' G. Stork and A. F. Knight J. Amer. Chem. SOC. 1977,99,3851. M. Hanack Angew. Chem. Internat. Edn. 1978 17 333; P.J. Stang Accounts Chem. Res. 1978 11 107. *' L. Eckes and M. Hanack Chem. Ber. 1978,111 1253. Reaction Mechanisms 53 Grunwald values of 0.53 [for (S)] and 0.86 [for (9)] were indicative of a mechanism which involved rate-determining formation of a vinyl cation as first intermediate.Additional information an the effect of bulky substituents on the properties of vinyl cations has been obtained by studying the solvolysis of 9-(a-chloroviny1)-anthracene In 80 YO ethanol 90 YO acetone and acetic acid the mechanism of reaction was entirely sN1 unlike that for the solvolysis of (R)-(a-chloroviny1)- anisole in which the AdE-E route contributes appreciably. The reaction showed a common ion rate depression not previously observed for a-arylvinyl systems devoid of P-substituents and it was thought that the relative stability of the sN1 intermediate (11) was a consequence of the fact that maximum conjugative interaction between the aromatic ring and the vinyl cation will occur when the ortho hydrogens and the &hydrogens are in perpendicular planes.In contrast stabilization of the AdE intermediate (12)by conjugation will be associated with steric interference between the ortho hydrogen atoms and the coplanar methyl and chlorine groups. HVC *=H CI Me II \/ CCI=CH2 H C+ H H C+ H m(yJ$pJyJ // // (10) (1 1) (12) The alkylation of aromatic compounds by vinyl derivatives is well known although the mechanisms of these reactions have received little attention. In a recent study of the reactions of a series of monosubstituted benzene derivatives CsHsX (X= MeO Me F H or C1) with vinyl triflates [R:C=CR20S02CF3 (R' = R2= Ph; R1 = Me R2= Ph; R' = R2= Me); cyclohexenyl cycloheptenyl and cyclo-octenyl triflate] an unusually low p value (-2.57) has been obtained.22 The result indicated the involvement of a highly reactive and intermolecularly non-selective electrophilic intermediate uiz.the vinyl cation and it was suggested that this intermediate may also be involved in alkylations with vinyl halides vinyl esters and alkynes. Vinyl substitutions which involve neighbouring group participation by sulphur iodine or phenyl etc. proceed with net retention of configuration as the result of a double inversion process (Scheme 6). It has been pointed out that each of these Y R' Y*+ Y+ 'I \/ / /\ -+ R~-C=C\-R~ + R~-C=C-R~ R 'X*-/\ Y= S I Ph etc. Nu*-R2 Nu Scheme 6 21 Z. Ramport P. Shulman and M. Thuval (Shoolman) J. Amer. Chem. SOC..1978,100,7041. 22 P. J. Stang and A. G. Anderson J. Amer. Chem. Soc.,1978,100 1520.H. R.Hudson displacement steps provides an interesting example of a transition state involving planar four-co-ordinate carbon.23 The stability of this transition state compared to that of the more common tetrahedral arrangement was attributed to enforced reduction in bond angle by the small ring and to delocalization of the lone pair by rr-conjugation over the molecular rr-orbital of the double bond. 3 Carbocations Preparations and Properties in Super-acid Media.-The combined use of 13C and 'H n.m.r. spectroscopy has continued to provide detailed information on the behaviour of carbocations in super-acid media. In the case of simple tertiary cations which undergo rapid degenerate rearrangement by 1,2-hydride or 1,2-methide shift sharp n.m.r.spectra have previously been observed down to -160 "C. The use of high field (67.9 MHz) 13C n.m.r. has now made possible the observation of line broadening at temperatures of -110 to -139 "C for the ions (13)-(19).24sRate~ of degenerate rearrangement have thus been measured and the free energies of activation have been calculated (AG*= 3.0-3.7 kcal mol-'). Ions (13)and (14) showed an inverse @-deuterium isotope effect which suggests that the methyl C-H force constants are greater (and that hyperconjugation is therefore less) in the transition state than in the ground state. The energy difference between a protonated cyclopropane and a proton bridge is reflected in the lower rate of methide shift in (18) than of hydride shift in (13). The still lower rate of rearrangement by hydride shift in (17) is correlated with the need for an accompanying change in conformation of the six-membered riag and in (15) with a steric barrier to rotation about the C3-C4 bond.The low temperature "C n.m.r. spectrum of the l-methylcyclobutyl cation has also been re-e~amined.~~ At -158 "C the ion (which at -80 "C undergoes rapid equilibration to give an averaged methylene signal) was previously claimed to be 'frozen out' and was assigned the unusual sp3 hybridized carbocation structure (20) on the basis of high 13C shielding.26 Comparison with other strained non-planar carbocations which do not show similar high shielding now suggests that the above 23 Z. Rapport Tetrahedron Letters 1978 1073. 24 M.Saunders and M.R. Kates J. Amer. Chem. Soc. 1978,100,7082. '' G.A.Olah G. K. S. Prakash D. J. Donovan and 1. Yavari J. Amer. Chem. Soc.. 1978,100,7085. 26 R.P.Kirchen and T. S. Sorensen,J. Amer. Chem. SOC.,1977,99,6687. Reaction Mechanisms interpretation is incorrect and that the ion has a a-delocalized structure containing two different methylene groups [S72.7 and -2.83].*' The high-field signal is indicative of five-co-ordinate carbon. It was not however possible to distinguish between a single symmetrical puckered species (21) and a degenerate set of rapidly Me &A .:+,* Me %; H-'H (20) (21) equilibrating bicyclobutonium-like ions (e.g. Scheme 7)with the symmetrical ion (21) as intermediate. It was emphasized that other a-delocalized ions (e.g.2-norbornyl) may not necessarily be static symmetrically bridged ions as has been claimed," but could be still equilibrating non-classical delocalized types. Scheme 7 A study of the conformations of t-cycloalkyl cations [(22) (23) (24)] in super-acid media has suggested that the chair conformations are relatively less stable than those (22) R = Meor n-C4H9 of the neutral analogues (i.e.the cycloalkanones and methylenecycloalkanes).27The twist-boat conformation (26)for each of the t-cyclohexyl cations (22;R = Me or Bun) is more stable than the corresponding chair form (25) by ca. 0.5 kcal mol-' (Scheme 8). In the 13Cn.m.r. spectrum of the methylcycloheptyl cation (23),a single peak was observed for each of the a-,p- and y-carbon atoms with no evidence of line broadening down to -130 "C.This result is similar to that for cycloheptanone and is ''R.P.Kirchen and T. S. Sorensen J. Amer. Chem. SOC.,1978,100 1487. 56 H.R. Hudson indicative of a fluxionally mobile system with low pseudorotational energy. In contrast line broadening was observed for the 1-methylcyclo-octyl cation (24) below -80 "C and at lower temperatures the system could be frozen out and assigned the chair-twist boat conformation (27),analogous to that of cyclo-octanone. In the bicyclic series the 9-methyl-9-bicyclo[3.3. llnonyl cation was unstable above -1 15 "C but below this temperature it appeared to consist of an equilibrating mixture of the two boat-chair conformers [(28) and (29)] in contrast to the 9-keto derivative which has a di-chair conformation.Me Me (27) (28) (29) The preferred ground state rotamer conformations of a series of bisected cyclo- propylcarbinyl cations in which the carbocation centre is conjugated to a second cyclopropyl group a vinyl group or a phenyl ring have been shown to be those which involve the most extended conjugation [(30) (33) (35)] unless significant steric factors are In suitable cases it was possible to investigate the equilibrium between each of these structures and the corresponding less stable conformer [(3 l) (34) or (36)] rotational barriers being mainly ca. 8-11 kcal mol-'. Steric hindrance played a significant role in determining the preferred conformation of the phenyldicyclopropylcarbinyl cation (32; R =Ph) in which interaction between the ortho-hydrogen of the phenyl group and one of the cyclopropane rings is minimized.In the less stable conformer (30; R=Ph) the phenyl group was shown to be non-coplanar with the carbocation centre. R R * -.+.-A R R (35) (36) Enthalpies of formation of simple alkyl cations from alkyl halides and antimony pentafluoride have been determined for the first time.29 The calorimetric measure- ments were made in SO2C1F usually at -55 "C. Values for the chlorides showed a very good linear correlation (of slope ca. unity) with the differential limiting free energiesof solvolysis for the same halides in ethanol. Support is thus given to the use of enthalpies of ionization and rate constants as guides to carbonium ion stabilities.28 N. Okazawa and T.S. Sorensen Canad. J. Chem. 1978,56,2355. 29 E. M. Arnett and C. Petro J. Amer. Chem. Soc. 1978,100,2563. Reaction Mechanisms Attempts to correlgte I3C chemical+shifts with c+constants for a ra9ge of substituted cumyl (ArCMe,) styryl (ArCHMe) and benzhydryl cations (ArCHPh) [Ar=p-XC6H4 (X=H MeO Me F C1 Br or CF,); m-XC6H4 (X=Me or F); 3,5-(CF3)&H3] have shown that for carbon at or adjacent to the ionic centre considerable deviations from linearity may A better correlation was obtained with an enhanced substituent constant (a")which described the effects of resonance-stabilizing substituents in super-acid media. Caution was therefore urged in the use of 13C n.m.r. chemical shift correlations with single substituent parameters to reach conclusions on the mode of stabilization of carbocations.Good correlations were however obtained31 in a range of 120 p.p.m. between I3Cchemicql shifts for C in substituted diphenylmethylcarbenium [(p-Xc$%)(p-YC6&)CMe] and diphenylhydroxycarbenium ions [( p-XC6&)( p-YC&C.OH](X and/or Y =MeO Me F C1 H CF, or NOz)(the latter in 98 % sulphuric acid) and INDO .rr-electron densities for C,. Rearrangements in Deamination and Solvo1ysis.-Em-and endo-bicyclo-[4.1.O]heptane-7-diazonium ions have been shown to favour synchronous disrotatory ring ~pening.~' Generated by diazotization of the corresponding amine and in the presence of sodium bromide the exo-isomer (37) gave mainly 7-exo-bromobicyclo[4.l.0]heptane (39; X =Br) whilst the endo-isomer (38)gave cyclohept-2-en-1-01 (40; X= OH).The latter was formed by hydrolysis of the corresponding bromide. When the same two ions were prepared by the action of weakly alkaline methanol on the corresponding nitrosoureas the same proportions of the products (39; X =OMe) and (40; X =OMe) were obtained in each case the cis-3-methoxycycloheptene being shown to arise by isomerization of the first formed trans-isomer (41). The results are consistent with a reaction scheme which involves a (37) (38) (39) (40) (41) partially opened cyclopropyl cation (42) as first intermediate in the formation of both products (Scheme 9). An analogous 'bent' cyclopropyl cation was reported previously on the basis of I3C n.m.r. studies in super-acid [Ann.Reports (B) 1977,74,78].Similar intermediates were thought to occur in the decompositions of exo- bicyclo[4.1 .O]hept-2-ene-7 -diaz~nium~~ and exo- bicyclo[S .1.O]oct-2-ene-8-diazonium ions.34 In contrast the em-bicyclo[S. 1.O]octa-2,4-diene-8-diazonium ion (43)appears to undergo a novel conrotatory ring-opening process to give the cyclo-octatrienyl cation in a configuration (44)which facilitates subsequent ring closure to yield bicyclo[3.3.0]octadiene derivatives (Scheme 10) instead of the energetically more favourable homotropylium ion.,' 30 D. P. Kelly and R. J. Spear Austral. I. Chem. 1977,30 1993; 1978,31 1209. 31 B.Ancian F.Membrey and J. P. Doucet J. Ore. Chem. 1978,43 1509. 32 W.Kirmse and H. Jendralla Chem. Ber. 1978 111 1857 and cited refs.33 W. Kirmse and H. Jendralla Chem. Ber. 1978,111 1873. 34 W.Kirmse and U. Richarz Chem. Ber. 1978,111.1883. 35 W.Kinnse and U. Richarz,Chem. Ber. 1978,111,1895. H. R.Hudson Scheme 9 (44) Scheme 10 Neighbouring group participation by the cyclopropane ring has been shown to constitute a significant reaction pathway in the solvolysis of 2-cyclopropylethyl tosylate in the weakly nucleophilic triflu~roethanol.~~ Apart from the major products which were formed either by direct nucleophilic displacement by solvent or via 1,2-hydride shift cyclopentyl derivatives were obtained in which deuterium scrambling had occurred and it was concluded that an intramolecularly corner- alkylated cyclopropane intermediate (45) was involved (Scheme 11).A similar R =CF3CHz Scheme 11 route with cyclopropane participation enhanced by over 20 YO,was thought to be followed by the analogous 2-( 1-methylcyclopropy1)ethylt~sylate.~' An intermediate of this type would be analogous to the corner-protonated cyclopropanes which have been reported elsewhere [Ann.Reports (B) 1973,70 1361. '' I. M. Takakis and Y. E. Rhodes Tetrahedron Letters 1978 2475. 37 Y. E. Rhodes I. M. Takakis P. E. Schueler and R. A. Weiss Tetrahedron Letters 1978,2479 Reaction Mechanisms An unequivocal demonstration of sequential rearrangement involving a bridged phenonium ion has been provided by the solvolysis in aqueous dioxan of the tosylate of (-)-(R)-3-methyl-2-phenylbutan-l-o1(46; X =OTs) and by the deamination of the corresponding amine (46;X=NH2) with nitrous acid.38 In addition to the expected major product (47),arising from 1,2-phenyl shift products [(48)and (49)J were formed; these were explicable on the basis of further rearrangement in which either hydrogen or methyl acted as an internal nucleophile for displacement of phenyl (Scheme 12).In the course of deamination a 1,2-shift of isopropyl also occurred. HO ,H H' Ph (46) (47) Me H Ph + 1 1 4-m FPh OH OH C (49) Scheme 12 Side reactions in the preparation of cyclo-octa-2,7-dienone oia elimination from the trans-2,8-dibromocyclo-octanone ketal(50) provide what appears to be a novel example of homallyl-cyclopropylcarbinyl cation rearrangement under strongly alk- aline condition^.^' Heated under reflux with methanolic sodium hydroxide thc ketal (50)led to formation of minor products identified as the orthoesters (55) and (57).It was assumed that the elimination of one molecule of hydrogen bromide gave an intermediate ketal(5 1)which could ionize under the polar conditions used to give the homoallyl cation (52) and its cyclopropylcarbinyl counterpart (53). Rearrangement to the oxygen-stabilized cations (54) and (56),followed by capture by methoxide accounts for the products formed. 4 Carbanions The stabilities of carbanions in solution are generally expected to decrease with increasing alkyl substitution in accord with inductive electron release. The tri-t- butyl carbanion (59) would thus be highly unstable.Nevertheless evidence for its '* W. Kirmse and B.-R.Gunther J. Arner. Chem. SOC.,1978,100,3620. 39 H. 0.Krabbenhaft J. Org. Chem. 1978,43,4556. H. R. Hudson Br OMe (54) (55) MeOT WO] (53)- OMe (57) Scheme 13 formation in solution has been obtained in the interaction of trj-t-butylmethanol with dimsylate anion in DMSO (Scheme 14).40 The drivingforce for the formation of the carbanion from the first-formed alkoxide (58) was thought to be the release of steric strain. It was noted that the heat of reaction (-23.2 kcal mol-') was very close to the difference in calculated strain energies for tri-t-butylmethane (40.4 kcal mol-') and 1,l-di-t-butylethane (15.0 kcal mol-') which are sterically similar to the alkoxide (58) and its other decomposition product di-t-butyl ketone (60),respectively.BuiCOH + MeSOCH2-K' + C +Me2S0 Bu" Bu)iJ (58) 1 Me SO Me3CH 2Me& -K' + Bu$CO (59) (60) Scheme 14 Quantitative information on the effects of a-cyclopropyl substituents on carbanion stability has been obtained by studying base-catalysed hydrogen-deuterium exchange in a range of cyclopropylcarbinyl ketones [(61) and (62)].41Relative rates of exchange at 35.5 "C,compared to that for isovalerophenone fell within the range 0.76-50.6. The results showed that cyclopropyl groups exert little stabilizing effect 40 E. M. Amett L. E. Small R. T. McIver jun. and J. S. Miller 1. Org. Chern. 1978 43 815. 41 M. J. Perkins N. B. Peynircioglu and B. V. Smith J.C.S. Perkin II 1978 1025.Reaction Mechanisms $CHR2COPh (61) R'= R2= H; R' = H R2= Me; (62) R = Ph or p-02NC6H4 R' = Ph R2= H; R' = p-OzNC6H4 R2= H. on an adjacent carbanionic centre and that the transmission of substituent effects by cyclopropyl is poor. Vinyl groups on the other hand exhibited a considerable stabilizing influence in keeping with earlier observations on the effects of unsaturated groups. Stabilization by dipole interaction is believed to account for carbanion formation adjacent to oxygen nitrogen or sulphur when the heteroatom is conjugated to carbonyl (Scheme 15).42The importance of the carbonyl group in providing this I1 Base e.g. s-BuLi 1 -AAr-C,r-C, Ar-C \x/ Me ,CH in TMF or TMEDA X \v/ A X = 0,NR or S TMEDA = tetramethylethylenediamine Scheme 15 stabilization has been demonstrated in the case of thioesters by showing that the equilibrium obtained by adding methylthiomethyl-lithium to methyl 2,4,6-tri-iso-propylbenzoate (Scheme 16) is displaced to the right by at least one order of magnitude.43 4-P \/ \ SMe~2H8~HF,at-780c,~c40 \SCH Li,TMEDA + + MeSCHzLi,TMEDA MeSMe Scheme 16 pp-Diphenylacrylonitrile (63) has been shown to be a good model substrate for studyingthe formation of vinyl carbanions by reaction with strongbases.44 The steric effect of the two phenyl groups prevents the alternative possibility of nucleophilic addition at C, as occurs in the Michael reaction and the equilibrium with n-butyl-lithium (Scheme 17) is reached rapidly at -78 "C.Competing nucleophilic addition at C occurred however to give the carbanion intermediate (64) which 42 P. Beak and B. G. McKinnie J. Amer. Chem. Suc. 1977,99,5213; P. Beak B. G. McKinnie and D. B. Reitz Tetrahedron Letters 1977 1839; P. Beak and D. B. Reitz Chem. Rev. 1978,78 275. 43 D. B. Reitz P. Beak R. F. Farney and L. S. Helmick J. Amer. Chem. Soc. 1978 100 5428. 44 U. Melamed and B. A. Feit J.C.S. Perkin I 1978 1228. H. R.Hudson Ph,C=CHCN + BuLi $ Ph2C=CCN Li' + BuH (63) Scheme 17 formed products by loss of either cyanide or hydride ion (Scheme 18). Elimination of hydride under these circumstances was unexpected in view of the better leaving ability of cyanide and it was assumed that a suitable acceptor e.g.a molecule of alkene (63) must be involved. Ph2C=CHCN + BuLi S Ph2CCH(Bu)CNLi' (63) (64) 7I-.- Ph2C=CHBu Ph2C=C(Bu)CN Scheme 18 Kinetic isotope effects (kH/kD)for the interactions of carbanions with MeOH or MeOD have been obtained in terms of product isotope effects (pie.) in the reactions of substituted benzyltrimethylsilanes with methanolic sodium methoxide (Scheme 19)."' Although an overall increase in kH/kDwas observed with increasing reactivity MeO-+ Me3SiR -* MeOSiMe3+ R-R-+ MeOH (orMeOD) + RH (orRD) + MeO-R = XC6H4or XYC6H3 Scheme 19 of RSiMe3 (and thus with pK for the corresponding carbon acid RH) the values remained effectively constant at 1.2-1.3 for X or XY = H C1 rn-CN m-NO, or 3,5-C12. Intermediate values were observed for X = p-CN (2.0) and p-SO,Ph (2.9) followed by a steep increase to 7.0 for X = p-COPh and to 10 for X or XY = #-NO2 p-N02 2-Me-4-N02 or 2-Me-6-N02.These results together with redetermined p.i.e. values for Ph2CHSiMe3 and Ph3CSiMe3 of 1.3 * 0.2 are inconsistent with an earlier literature k,/k value of 4.2 for proton abstraction from triphenylmethane (and hence by calculation of ca. 8.1 for the reverse reaction). The reason for this discrepancy is however not clear. Other aspects of carbanion chemistry are referred to in the following section on Elimination Reactions. 5 &Elimination For E2 eliminations from @-aryl-activated alkyl halides ArCH2CH2X(X = C1or Br) values of p (kH/kD)B,and k,,/k,- are very similar whether the attacking base is phenoxide or the much bulkier 2,6-di-t-butylphenoxide The steric require-45 D.Macciantelli. G. Seconi and C. Eaborn J.C.S. Perkin 11,1978 834. 46 S. A. E. Baciocchi and P. Perucci J. Org. Chem. 1978 43 2414. Reaction Mechanisms ments of the base appear therefore to have little effect on transition state structure. Some of the dangers inherent in relating kinetic isotope effects to transition state geometries have been reported as a result of studies of methoxide-induced elimina- tions from a number of 1,2-diaryl-l-chloroethanes,p-RC6&CH2CHPhC1 (R = NO2,C1,H or Me) substituted with deuterium at the a!-and/or p-po~ition.~~ Certain features of the reactions were in agreement with expectation from previous work. Thus Hammett plots confirmed the carbanionic character of CB in the transition state primary kinetic isotope effects (kH/kD)B increased from 2.1 to 8.8 with change of R in the order Me<H<C1<N02 (i.e.as the transition state became more reactant-like) and secondary isotope effects (kH/kD) changed in the opposite direction from 1.375 (R = Me) to 1.08 (R = NO2). However high zero-point energy differences (E -ED)B == 13 kJ mol-' and abnormally low pre-exponential factors AH/AD= 0.001 to 0.08 indicated that proton tunnelling or an internal-return mechanism may be complicating this E2 elimination. Furthermore the energies of activation showed different reactant-like relationships for the transition states from those based on the kinetic isotope effects. Further information has been obtained on the factors controlling elimination from carbanion intermediates (Scheme 20; R' = R2= H).The effects of the activating group G (PhS02 CN or Bz) on leaving group ability are seen to be c~mplex.~' G.CHR'.CHR~.Z+B- kl k2G.CR'-CHR~.Z+GR'C=CHR* + :Z k-1 Scheme 20 Although similar ranking orders for the leaving ability of Z were obtained in some cases e.g. PhAMez>OPh >SPh >OMe > CMe2N02 for G = CN or PhS02 and OMe >NRAc >CN CMe2N02 for G = Bz or PhS02 there were variations in others and it was not possible to make simple correlations of orders with any molecular parameters. The results suggest that there is little cleavage of the bond to the leaving group in the transition state in which Z is expelled; this conclusion is supported by a lack of sensitivity to changes in solvent and to ion association.Substituent effects in these reactions (Scheme 20; Z = OPh OMe S02Ph; R' R2= Me and/or Ph) have also been determined and are in accord with the same general For nitriles and sulphones (G= CN or PhS02) the (ElcB)R mechanism operates (k2 is rate-determining) and both methyl and phenyl substituents at C or Cpgive rise to mild acceleration of the expulsion of the leaving group. Ketones (G= Bz) react by the irreversible (ElcB)I route and the effect of substitution is on the rate-determin- ing deprotonation only. The reactions in these cases are sensitive to steric inter- ference by substituents at CBbut not at C,. Markedly different effects of P-phenyl substitution on the rates of formation of carbanions from nitriles ketones and sulphones have been discussed in terms of steric interference with the formation of a planar carbanion.47 F. M. Fouad and P. G. Farrell Tetrahedron Letters 1978,4735. 48 D. R. Marshall P. J. Thomas,and C. J. M. Stirling,J.C.S. Perkin 11,1977,1898; P. J. Thomas and C. J. M. Stirling ibid. 1978 1130. 49 R. P. Redman P. J. Thomas and C. J. M. Stirling.J.C.S. Perkin IZ 1978 1135. H. R.Hudson The first quantitative information on the contribution of ring strain to an elimina- tive ring opening has been reported for the reaction of the sulphone (65) with ethanolic sodium ethoxide (Scheme 21).50Ring strain accelerated the elimination to Scheme 21 such an extent that 6-deprotonation became rate-determining i.e.the mechanism was (ElcB), as shown by a primary isotope effect (kH/kD)of 2.5. Compared to the acyclic analogue P-methoxypropyl ethyl sulphone which undergoes elimination by the (ElcB) mechanism the rate was 2.46X lo6times faster. Elimination reactions of all types in which expulsion of a leaving group is accompanied by ring fission have been reviewed.51 6 Electrophilic Addition The most common topic for investigation with respect to both alkenes and alkynes has been that of open versus bridged transition state structures and methods for their differentiation. As one approach to this problem in the alkene series it has been proposed that acid-catalysed hydrations [Equation (6)]and the addition reactions of arenesulphenyl chlorides [Equation (7)] should be taken as models for these two \/ HO I1 c=c S \C+-C-H AHO-C-C-H+H+ (6) / \ slow / I II Ar I.Ar S extreme reaction types.52 From these it should then be possible to obtain informa- tion on the structures of rate-determining transition states for other electrophilic additions e.g. bromination by comparison of structure reactivity profiles. The method was illustrated by showing that the logarithms of rate constants for bromina- tion in methanol (kgBr2)gave a poor correlation with those for acid-catalysed hydration (kZH+). On the other hand a good correlation was obtained with log kZArSC' for the addition reactions of 4-chlorobenzenesulphenyl chloride with a wide range of alkenes (excepting a few in which steric or other special features were present).The result is consistent with the usually accepted bromonium-type struc- ture for the rate-determining transition rate for brominations and the method was advocated for the testing of other controversial mechanisms such as those of oxymercuration and thallation. R. J. Palmer and C. J. M. Stirling J.C.S. Chem. Comm. 1978,338. C. J. M. Stirling Chem. Rev. 1978,78 517. 52 G. H. Schmid and T. T. Tidwell J. Org. Chem. 1978,43,460. Reaction Mechanisms Further confirmation of the symmetrical nature of the transition state in alkene brominations has been obtained by the use of a new procedure of internal comparison which avoids resorting to external structural scales and substituent constant^.^^ This involved the measurement of rate constants for the brominations in methanol of four isosubstituted series of alkenes RCH=CH2 trans-RCH=CHMe RMeC=CH2 and RCH=CMe2 (R=Me Et Prh PhCH2 CH3C02CH2 or C1CH2).Plots of log k for each series against log k for the first series (RCH=CH2) gave sets of parallel lines. The result is most significant for those alkenes (RMeC=CH2 and RCH=CMe2) which are the ones most likely to involve carbo- nium-ion-like pathways. It was argued that since the carbocatio? intermediates which eacp of these two series would involve would be RMeC-CH2Br and RCHBr-Me2 respectively they would be expected to respond differently to the polar influence of substituent R. The parallel plots showed however that this was not so and it was concluded that the transition state was a symmetric entity.A slight secondary effect that was detected was attributed to hyperconjugation. The electrophilic bromination of alkynes is generally considered to follow a course (Scheme 22) which is similar to that for alkenes. Recent of solvent and AaH products Scheme 22 structural effects on rates of bromination in acetic acid formic acid trifluoroacetic acid and methanol-water mixtures show that nucleophilic solvent assistance cas be important in the reactions of alkylacetylenes e.g. hex-l-yne and hex-3-yne whereas arylacetylenes are virtually unaffected by solvent changes. The alkylacetylenes were considered to react through a bridged transition state with assistance from hy- droxylic media e.g. as shown [(66) or (67)].A bridged intermediate (68)was also SH ‘0’ s R-C=,C-R ,‘a+.\ I R-C=C-R ‘d?-:Bk Brh-X-__. I, . / Br --Br*-H (66) (67) indicated in the addition of bromine monochloride to hex-l-yne which yielded only the trans-isomers of both the Markownikoff and anti-Markownikoff addition products (Scheme 23).55 Structural effects on the rates of bromination in methanol of a series of substituted diarylacetylenes pointed to the involvement of a highly 53 E. Bienvenue-Gotz and J. E. Dubois Tetrahedron 1978 34 2021. 54 G. Modena F. Rivetti and U. Tonellato J. Org. Chem. 1978,43 1521. 55 V. L. Heasley D. F. Shellhamer J. A. Iskikian and D. L. Street J. Org. Chern.,1978 43 3139. 66 H. R. Hudson R-C~C-H BrCl -+ +Br /’ R-cLC-H (68) R Br Br H a-\ / \/+ c=c H R’ \Cl/c=c\-+ c1 R = n-CdH9 (90 Yo) (10 Yo) Scheme 23 asymmetrical vinyl-cation-like transition state in which the positive charge developed essentially on the carbon which was attached to the better substituted benzene ring (69)[(-a&) > (-at)].’“ (69) Evidence for the intervention of a bridged phenonium ion has been obtained in the bromination in acetic acid of 3-(p-methoxyphenyl)propyne which yielded mainly the product of syn addition (Scheme 24) unless lithium bromide was present when the AdE3 mechanism operated and anti addition occ~rred.’~ The intermediate (70) H Br2 Br -/ _I* p-MeOC6H4CH2CrCH CH,-C=CHBr -p-MeOC6H4CH2C=C I (70) Br ‘Br Scheme 24 was described as an aryl-stabilized vinyl cation.With less electron-attracting groups in the ring (H p-Me rn-CF,) the products were almost entirely trans-isomers and were presumably formed via cyclic bromonium intermediates. 7 Nucleophilic Addition to Carbonyl Compounds The first systematic study has been made of the effect of structure upon equilibrium constant for the aldol condensation [Equation (8)].” The number of data that can be CH3COR*+ R2COR3 $ R2R3C(OH)CH2COR’ (8) obtained directly for this reaction are limited by several factors such as easy dehydration in certain cases of the aldol product to the corresponding enone and the fact that cross-product formation from two reactants might not occur in the desired direction. Experimentally accessible equilibrium constants have therefore been ’‘ J.A. Pinock and C. Somawardhana Canad.J. Chem. 1978,56 1164. ’’ J. P. Guthrie Canad.J. Chem. 1978 56 962. Reaction Mechanisms augmented by values calculated from thermodynamic data for the enones and their precursors equilibrium constants for the dehydration reaction and estimated free energies of formation based on hypothetical disproportionation reactions. In this way a full set of equilibrium constants was obtained for the condensations of acetaldehyde acetone acetophenone and acetic acid (acting as nucleophiles) with formaldehyde acetaldehyde benzaldehyde acetone and acetophenone (acting as carbonyl acceptors). The results showed that a previously determined free-energy relationship for the additions of alcohols thiols amines hydrogen cyanide and bisulphite to carbonyl compounds (E.G. Sander and W. P. Jencks J. Arner. Chern. SOC., 1968,90,6154) can be extended to include the aldol reaction. Prediction of the equilibrium constant for any aldol condensation therefore becomes possible from a knowledge of that for the addition of another nucleophile such as water or HCN to the acceptor and the y-value of the carbonyl nucleophile. The equilibrium and kinetics for cyanohydrin cleavage and formation in aqueous solution at 25 "C and at ionic strength 1.0 M have been determined for a range of substituted benzaldehydes (4-N02; 3-C1,4-C1; 4-C1; 4-H; 4-Me; 4-Me0; 4-MezN).'* The rates of formation and breakdown showed no significant general-base-catalysed contribution in the pH range 2.5-7.4 and it was concluded that neutral reactants and neutral products were involved.In the breakdown of the cyanohydrin complete removal of the proton to form the oxyanion was necessary for the CN-group to depart. The major contribution of the substituent effect to the equilibrium constant was its effect on the stability of the reactant aldehyde. However the anionic cyanohydrins were less sensitive to electronic effects than were the neutral cyano- hydrins the a-carbon atom of which is relatively more electron-deficient. There have been a number of recent investigations into the mechanism of carbonyl reduction by borohydride. The overall process is first order in each of the reactants and is generally considered to proceed by a stepwise series of hydride transfers to yield R2CHOBH3- (R2CH0)2BH2- (R2CHO),BH- and (R2CHO),B- followed by ,&ydrolysis of the tetra-alkoxyborate.Previous evidence for this mechanism and for other possibilities has been s~mmarized.~~ Strong support for the stepwise process has been obtained by the isolation of sodium tetrakisbenzyloxy- borate from the reduction of benzaldehyde in DMS0.59 Reductions of acetone pivaldehyde and benzaldehyde in DMSO and DMSO-water mixtures all showed a marked fall in reaction rate as the water content of the medium was decreased indicating the importance of hydrogen-bonding from a hydroxylic solvent in the transition state (71). Further detailed information on the mechanism was obtained 6"- ,' ; .> /H H\ '* Wei-Mei Ching and R.G. Kallen J. Amer. Chem. Soc. 1978 100 6119. s9 C. Adams V. Gold and D. M. E. Reuben J.C.S. Perkin ZZ 1977 1466. 6o C. Adams V. Gold and D. M. E. Reuben J.C.S. Perkin ZZ 1977 1472. H. R. Hudson by showing that the reduction of benzaldehyde by tritiated sodium borohydride in DMSO and DMSO-water mixtures resulted in the incorporation of tritium into unchanged aldehyde.60 This implies reversibility of the hydride-transfer process which must therefore precede that in which B-0 bond formation occurs. As there was no evidence for the formation of free borane in the reaction medium it was believed that the initially formed benzyl oxide ion and borane must react competi- tively within a solvent cage (74) without diffusing apart to give either starting materials with hydride exchange or the alkoxyhydridoborate ion (Scheme 25).Alternative formulations for the reaction involving four-centre (72) or six-centre .. (72) (73) (73)transition states are not compatible with this result although the six-centre process had been suggested in order to account for the formation of the alkoxyborate corresponding to solvent alcohol rather than that corresponding to the alcohol derived from the carbonyl compound when reductions were carried out in alcoholic PhCTO +B&-f PhCHO +BH3T-S {PhCHTO- BH3} L (74) PhCHTOBHj-Scheme 25 media.61 Solvolysis of the first-formed alkoxyborate should however be considered as a possibility in this context.” The possible disproportionation of reaction intermediates (Scheme 26) has been excluded by carrying out the reduction of cyclohexanone or 3,3,5,5-tetramethylcyclohexanonewith mixtures of NaBa and NaBD in propan-2-01 when no isotope exchange between the reagents occurred.62 2ROBH3-+ BH4-+(R0)2BH2-3(R0)2BH2-+ BH4-+2(R0)3BH-4(R0)3BH-+ BH4-+3(R0)4B-Scheme 26 This also confirms the stepwise nature of the reduction by successive alkoxyborate intermediates and shows that no more than 25% of the carbonyl compound is reduced by BH,-itself a factor relevant to selectivity.The influence of alkali cations on the mechanisms of nucleophilic additions to carbonyl compounds including metal hydride reductions has been reported by a D. C. Wigfield and F. W. Gowland Tetrahedron Letters 1976,3373;J. Org. Chem. 1977,42 1108.62 D. C. Wigfield and F. W. Gowland Canad. J. Chem. 1978,56,786. Reaction Mechanisms number of workers in recent years.63 Such reactions are considered to occur either under ‘complexation control’ which involves loose ion-pair association with carbo- nyl oxygen [Equation (9)],or ‘association control’ in which tight ion-pairing with the attacking nucleophile is involved [Equation (lo)]. As an experimental criterion to \ I NU-+ C=O--M++ NU-C-0-M’ (9) / I \ I M’NU-+ c=o + NU-C-0-M’ / I determine the dominant factor controlling the reactivity of a carbonyl compound under given conditions the kinetic effect of adding cryptand or crown-ether is These reagents will trap the cation with a consequent decrease in reaction rate as the effect of carbonyl complexation is removed or an increase in rate as the effect of association with the nucleophile is removed.The principle has also been applied to regioselectivity of attack in a-enones but with the added compli- cations of hard or soft character at the two possible sites of attack and in the attacking nucleophile. J.-M. Lefour and A. Loupy Tetrahedron,1978 34 2597 and cited references.

ISSN:0069-3030    

DOI:10.1039/OC9787500048

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (iii) Free Radical Reactions By A. G. DAVIES Department of Chemistry University College 1ondon 20 Gordon Street London WClH OAJ 1 General For organic chemists who are concerned with e.s.r. spectroscopy the latest volumes of Landolt-Bornstein are invaluable. In 1965 Volume II/1 listed the magnetic properties of all the radicals which were then known. Data on the radicals which have been studied in the succeeding decade are now being compiled in five supplementary volumes of which the first three have already appeared. Volume 9/a lists the inorganic radicals volume 9/b the organic carbon-centered radicals and volume 9/cl the organic N-centred and NO radicals up to the end of 1975.’ Volumes 9/c2 (organic radicals with 0 P Si Ge Sn Sb As or Se as the central atom) and 9/d (radical ions polyradicals and spin-labelled biomolecules) should be published during the coming year.Other monographs include Symons’ ‘Chemical and Biological Aspects of E.s.r. Spectroscopy’,* Box’s ‘Radiation Effects E.s.r. and Endor Analysi~’,~ and Davies and Parrott’s ‘Free Radicals in Organic Synthe~is’.~ The papers delivered at the symposia on organic free radicals in Aix-en-Provence’ and in Chicago6 in 1977 have been published in bookform and an issue of Journal of Physical Chemistry contains a state-of-the-art collection of papers on radical ions7 2 Kinetics Most values for the rate constants of elementary homolytic processess have been measured by comparison against a small number of absolute standards which must be determined as accurately as possible.One such standard is the rate constant for abstraction of hydrogen by t-butoxyl radicals and this has now been measured more accurately by the use of laser flash Landolt-Bornstein’s ‘Numerical Data and Functional Relationships in Science and Technology’ New Series II/1 (1965) II/9a (1977) I1/9b (1977) I1/9c 1 (1978) ‘Magnetic Properties of Free Radicals’ Springer Berlin. M. C. R. Symons ‘Chemical and Biological Aspects of E.s.r. Spectroscopy’. Van Nostrand Reinhold New York 1978. H. C. Box ‘Radiation Effects E.s.r. and Endor Analysis’. Academic Press New York,1978. D. I. Davies and M. J. Parrott ‘Free Radicals in Organic Synthesis’. Springer-Verlag Berlin 1978. ’ ‘Radicaux Libres Organiques’ ed.J.-M. Suzur C.N.R.S. Paris 1978. ‘‘Organic Free Radicals’ ed. W. A. Pryor A.C.S. Symposium Series Washington 1958 Vol. 69. ’J. Phys. Chem. 1978,82 [lo]. 70 Reaction Mechanisms 71 photolysis.* Di-t-butyl peroxide is photolysed by a laser pulse of a few nanoseconds to generate t-butoxyl radicals in the presence of diphenylmethanol or cumene (RH). The build-up of the concentration of the diphenylhydroxymethyl or cumyl radicals over a few microseconds is monitored by U.V. spectroscopy giving the rate constant for the reaction B~O.+RHk B~OH+R. For other substrates R'H where the radical R'* does not absorb in the u.v. t-butoxyl radicals are generated in the presence of a mixture of R'H and cumene and the effect of R'H on the rate of formation of the cumyl radical is again followed by U.V.absorption. Typical values which have been obtained for k at room temperature with 1:2 benzene-di-t-butyl peroxide as solvent are toluene 2.3 X lo5,cyclopentane 8.8 X lo5,methanol 2.9 x lo' t-butyl hydroperoxide 2.5 x lo8,di-t-butyl peroxide ~1.6 x lo51mol-1 s-l. The relative values of these rate constants are similar to those which have been determined by other methods but the absolute values are substantially higher than those which have been used as reference standards in the past with the result that many rate constants quoted in the literature are too low. The absolute rate constants for the reactions of aryl radicals have been determined by time-resolved U.V. or e.s.r. ~pectroscopy.~ Pulse radiolysis of the p-bromoben- zoate ion ArC02- gave the Ar*radical and the rate of the formation of the adduct [ArArC02-].was monitored by optical absorption or of the decay of the radical Ar. by e.s.r. spectroscopy. It is concluded that the rate constants for the reaction of phenyl radicals with most aromatic systems are in the range 5-10 X lo6 1mol-' s-l and that the rate of hydrogen abstraction from isopropyl alcohol is 5.2X lo61 mol-' s-'; with such a high reactivity the life-time of the phenyl radical in most organic media will be very short. In an alternative approach,1° phenyl radicals formed by thermolysis of phenyl- azotriphenylmethane (1)are caused to abstract hydrogen from mineral oil (RH) in competition with a scavenging reaction with Iz or CBr4 the rate of which in the viscous medium is diffusion controlled and can be calculated from diffusion theory.R/phH Ph3CN=NPh -* Ph3C. + N2 +Ph. (1) CRr4 \tt.. PhX The absolute rate constants for other reactions of phenyl radicals can then be determined from appropriate competition reactions based on the rate constant for the reaction of mineral oil as the primary standard. For example the rate constant for the reaction Ph. + 0,+PhOZ* has been found to be 5 x lo91mol-' s-l and not abnormally low as had been suggested. R. D. Small and J. C. Scaiano J. Amer. Chem. Suc. 1978 100 296; H.Paul R.D. Small and J. C. Scaiano J. Amer. Chem. Suc. 1978,100,4520. V.Madhavan R.H. Schuler and R. W. Fessenden J. Amer. Chem. SOC.,1978,100,888.lo R. G. Kryger J. P. Lorand N. R. Stevens and N. R. Herron J. Amer. Chem. Suc. 1977,99,7589. A. G. Davies Full details of Schmid and Ingold's measurements of the rate of spin-trapping of primary alkyl radicals by 2-methyl-2-nitrosopropane,nitrosodurene 2,3,5 -tri- t- butylnitrosobenzene 1,l-di-t-butylethylene and di-t-butyl thioketone," and of Schuh and Fischer's thorough study of the kinetics of the self-reaction of t-butyl radicals in so1ution,12 which were briefly reported last year have now been pub- lished. The isomerization of the 2,4,6-tri-t-butylphenyl (2) radical had been shown to occur in solution by quantum-mechanical tunneling of a hydrogen atom from an ortho t-butyl group. These studies have now been extended down to 28 K in solid matrices.Below 40 K the rates become virtually independent of temperature and show enormous deuterium kinetic isotope effects of greater than 104.13 (2) Work on nanosecond time-resolved e.s.r. spectroscopy is going on in a number of laboratories and many applications of this technique in the study of homolytic kinetics and mechanisms can be expected in the near future. 3 Carbon-centred Radicals A linear relationship has been established between the free activation energy of the thermolysis of a large number of aliphatic hydrocarbons (3) and their strain energies ESl4as estimated by Engler-Schleyer force field" calculations R' IR2-C-C-R2 IR3 R' I k3 -P R' I2R2-C. IR3 (3) (4) AGS(300"C)= -2.51 (*0.13)Es+274.5 (k4.2) kJ mol-' The ground state strain energy is thus crucial in determining the thermal stability about 40% of E prevailing in the transition state; conversely the activation energy for the combination of the radicals (4) corresponds to 40% of the ground state energy of the dimer of the radical.1,1,2,2-Tetra-t-butylethane has a strain energy of 264 kJ mol-' and an activation enthalpy for decomposition of 152 kJ mol-' and as such is the most thermally labile alkane which is known.16 P. Schmid and K. U. Ingold J. Amer. Chem. Soc. 1978,100,2493; S. Y. Maeda P. Schmid D. Griller and K. U. Ingold J.C.S. Chem. Comm. 1978,525. H. H. Schuh and H. Fischer Helu. Chim. Acfu 1978 61 2130. l3 G. Brunton D. Griller L. R. C. Barclay and K. U. Ingold J. Amer. Chem. SOC.,1976 98 6803; G.Brunton J. A. Gray D. Griller L. R. C. Barclay and K. U. Ingold ibid. 1978 100 4197. l4 C. Ruchardt H.-D. Beckhaus G. Hollmann S. Weiner and R. Winiker Angew. Chem. Internat. Edn. 1978 17 593. Is E. M. Engler J. D. Andose and P. von R. Schleyer J. Amer. Chem. Soc. 1973 95,8005. l6 H.-D. Beckhaus G. Hellmann and C. Riichardt Chem. Ber. 1978 111,72. Reaction Mechanisms The heats of formation of the hydrocarbons R-But and R-Me as calculated using the Engler-Schleyer force field have been used to define a frontal strain parameter .Yf for the group R by the expression Yf(R)= AH,"(R-But)-AH (R-Me) + 8.87 ( lo4J mol-') where the constant normalizes .Yf(CH3)to zero. Values for Yf have been listed for many acyclic and cyclic alkyl groups.17 The Yf parameter is useful for analysing the effect of frontal strain on reactivity.For example if the experimental activation enthalpies AH&-AH; for a variety of radicals (5) are plotted against the Yfvalues two straight lines are obtained one for u-radicals and a second one of steeper slope for w-radicals which exert a greater repulsion toward XCC13.'8 t-Alkyl radicals lie on the line correlating n-radicals indicating that they must already have or can readily achieve a near-planar structure Although it is generally accepted that the methyl radical is planar the possibility that t-alkyl radicals may be non-planar continues to attract attention. A sensitive test is the value of a [l3Ca) which should increase monotonically with temperature as the amplitude of vibration increases if the configuration at the minimum of the potential energy curve is planar.A careful study of the e.s.r. spectrum of the Me313C. radical in propane or iso-octane over the temperature range 120-380 K shows that a('3Ca)has a well- defined minimum at 220 K suggesting that there is a small energy barrier for the inversion of the radical. This barrier was calculated to be 1.88 kJ mol-' with the most stable structure distorted by 11.5" from planar." The self-reaction of triphenylmethyl radicals is well established to occur by addition of the central carbon atom of one radical to a carbon atom of a phenyl ring in the second radical. In an elegant application of CIDNP n.m.r. spectroscopy it has now been shown that benzyl radicals give bibenzyl by direct aa coupling and indirectly uia unstable semibenzenes (6) by CYO and arp coupling which can be trapped by acids as u-and p-benzyl-toluenes.*' At 30 "C the total yield of semibenzenes is 19%.The product distribution and temperature dependence point to energetically different bimolar complexes of benzyl radicals acting as product-controlling intermediates.H.-D. Beckhaus Angew. Chem. Internat. Edn. 1978,17,593. ** B. Giese and H.-D. Beckhaus Angew. Chem. Internat Edn. 1978,17,594. D. Griller K. U. Ingold P. J. Krusic and H. Fischer 1.Arner. Chem. SOC.,1978,100,6750. *' H. Langhals and H. Fischer Chem. Ber. 1978 111,543. A. G. Davies 4 Nitrogen-centred Radicals An interesting development in nitrogen radical chemistry is the proposal that succinimidyl (NS*) and related radicals can exist in distinct T (7) and u (8) states which are not readily interconvertible.2' The basis of the suggestion is that suc- cinimidyl radicals derived from different reactions show different selectivities in their reactions with compounds such as neopentane and dichloromethane.If the NS-radical is generated in the presence of bromine by the reaction NBS+Br*-+Br2+NS- it reacts unselectively with neopentane and dichloromethane whereas if an alkene is present to scavenge any bromine so that the NS. radical is generated by the reaction NBS + R*+RBr +NS- the selectivity is about 20 1.22 The results of earlier INDO calculations that the NS. radical from the former route is a ground state wradical(7) but that from the latter is an excited u (probably uN)radical (8) which in its reactions shows a selectivity similar to C1-.ao 043-0 o N N (7) (8) The reactions with hydrogen donors are therefore as follows Br.+NBS -* Br2+.rr-NS* v-NS*+RH -+ NHS+R* R*+Br2 + RBr+Br. and R.+NBS + RBr+c-NS. u-NS*+RH + NHS+R* The u-NS-radical will also add to alkenes and arenesZ4 and is also responsible for the formation of 3-bromopropionyl isocyanate (9) by reversible ring-opening. The 21 P. S. Skell and J. C. Day ref. 6 p. 290;Accounts Chem. Res. 1978 11 381. 22 P.S.Skell and J. C. Day J. Amer. Chem. Soc. 1978,100 1951. 23 T.Koenig and R. A. Wielesek Tetrahedron Letters 1975 2007. 24 J. C.Day M. G. Katsaros W. D. Kocher A. E. Scott and P.S. Skell J. Amer. Chem. SOC.,1978,100 1950. Reaction Mechanisms 75 w-NS- radical does not correlate with the ground state of the ring-opened radical but the u-NS-radical does and the P-scission is rapid and reversible. The reaction of ci~-2,3-[~H~lNBS with neopentane in the presence of bromine (conditions for the formation of the w-NS- radical) gave pure cis -[2H2]succinimide but C~S-[~H~]NCS in the presence of an alkene (now to give the a-NS- radical) gave r2H2]NHS which was 70% cis and 30% trans.25 Attempts to observe the e.s.r. spectra of succinimidyl radicals have as yet been unsuccessful but the hyperfine coupling constants of some other amidyl radicals appear best interpreted in terms of a wconfiguration.26 Other u-amidyl radicals have also been suggested to be implicated in some intramolecular hydrogen abstrac- tions reaction with aromatic rings and additions to alkene~.~~ An interesting comparison can be drawn between the properties of the radicals '+ R2N* R2NH and (Me3Si)2N*.Simple dialkylaminyl radicals R2N-,are rather inert towards alkanes alkenes and arenes whereas the dialkylaminium analogues *+ R2NH are much more reactive; thus the photolysis of tetramethyltetrazene Me2NN=NNMe2 in acetonitrile containing an alkene in the presence of .+ trifluoroacetic acid yields a mixture of products resulting from attack of the Me2NH radical on the double bonds.28 The bis(trimethylsily1)aminylradical (Me3Si)2N* (10) has now been generated by the photolysis of tetrakis(trimethylsilyl)hydrazine (Me3Si)2N-N(SiMe3)2 tris(trimethylsilyl)hydroxylamine (Me3Si),NOSiMe3 or bis(bistrimethylsily1-aminyl)mercury [(Me3Si)2N]2Hg.29 E.s.r.spectroscopy showed that its reactivity is similar to that of an alkoxyl radical. Reaction with isobutane at 170 K gave the isobutyl(l1) and t-butyl(l2) radicals in the ratio of 4.2 :1 whereas t-butoxyl radicals under the same conditions give the ratio 0.02 :1;this probably reflects the greater steric demands of the disilylaminyl radicals. (Me3Si)2NN(SiMe3)2 2(Me3Si)2N* Me2CHCH2+Me3C. (12) " P. S. Skell J. C. Day and J. P. Slanga Angew. Chem. Internat. Edn. 1978 17 515. W. C. Danen and R. W. Gellert J. Amer. Chem. SOC.,1972,94,6853;W.C. Danen and F. A. Neugebauer Angew. Chem. Intemat. Edn. 1975,14 783.27 T. C. Joseph J. N. S. Tan M. Kitadani and Y. L. Chow Canad.J. Chem. 1976,54,3517; S. A.Glover and A. Goosen. J.C.S. Perkin I 1977 1348; P.Mackiewicz R.Furstoss B. Waegell R. Cote and J. Lessard I. Org. Chem. 1978.43 3746 3750. '* L. J. Madgzinski K. S. Pillay H. Richard and Y. L. Chow Canad. J. Chem. 1978,56 1657. 29 B.P.Roberts and J. N. Winter J.C.S. Chem. Comm. 1978 545. 76 A. G. Davies As this would imply bis(trimethylsily1)bromamine will brominate hydrocarbons by a radical chain mechanism AIBN (Me3Si)2N.+RH d(Me3Si)2NH+R. R. +(Me3Si)2NBr __* RBr .t(Me3Si)zN. In the presence of norbornene or t-butylethene to scavenge bromine toluene gives benzyl bromide in 90% yield and ring-substituted toluenes show a Hammett p value of -0.62 (against cr+;cf.Bu'O* -0.35 Me2N-1.08 Br-1.36).30 The interaction of the occupied N 2p orbital with the vacant Si 3d orbital increases the electrophilic power of the radical and accomplishes intramolecularly what protonation does intermolecularly the effect is similar to that which is observed in the series Me3CO* Me3COH and Me3SiO*.31 5 Oxygen-centred Radicals The e.s.r. spectra of alkoxyl radicals cannot be observed in solution because the px and p, orbitals are degenerate allowing the unpaired electron to have orbital angular momentation about the z-axis with consequent broadening of the In the solid state strong hydrogen bonding can remove this degeneracy and quench the orbital angular momentum and under these conditions the e.s.r.spectra of a number of alkoxyl radicals have been observed by the irradiation of biologically important molecules. The first example to be reported was the radical (13),derived from the radiolysis of serine at 4.2K,33to be followed by a number of primary or secondary alkoxyl radicals at temperatures up to 165 K derived from the ribose or deoxyribose moiety of nucleosides [(14) and (15); R =H or OH] or from inositol (16)34and the spectrum of the methoxyl radical itself a(3H) 52 G has recently been observed at 4.2 K in X-irradiated polycrystalline methan01.~' OH OH R 00 OH ' OH (13) (14) (15) (16) Part of the evidence for the identification of the alkoxyl radicals is that the direction of g, in the radical is parallel to that of the C-0 bond in the parent which 30 B.P. Roberts and C. Wilson J.C.S. Chem. Comm. 1978,752. 31 P. G. Cookson A. G. Davies B. P. Roberts and M. W. Tse J.C.S. Chem. Comm. 1976 937; P. G. Cookson A. G. Davies N. A. Fazal and B. P. Roberts,J. Amer. Chem. SOC.,1976,98,616;A.G.Davies ref. 5 p. 399. 32 M. C. R. Symons J. Amer. Chem. SOC.,1969,91,5924;J.C.S. Perkin 11 1974 1618. 33 J. Y.Lee and H. C. Box J. Chem. Phys.. 1973,59,2509. 34 H.C.Box and E. E. Budzinski J. Chem. Phys. 1975,62,197; J.C.S.PerkinII 1976,553;J. Chem. Phys. 1977,67,4726; W.A. Bernhard D. M. Close J. Hiittermann and H. Zehner J. Chern. Phys. 1977,67 1211; E.Sagstuen and C. Alexander J. Chem. Phys. 1978,68,762; H. C.Box E. E. Budzinski and G. Potienko J. Chem. Phys. 1978,69 1966. 35 M. Iwasaki and K.Toriyama J. Amer. Chem. Soc. 1978,100,1964. Reaction Mechanisms forms the strongest hydrogen bond which is the site from which a proton will be most easily transferred. When they decay the alkoxyl radicals R2CHO* are converted principally into the hydroxyalkyl radical R2C0H. The g-values are highly anisotropic with g, ca. 2.028 and the magnitude of the isotropic coupling to the P-hydrogen atoms is given by the expression a(H@)= Bo+B2cos28 where Bo=ca. 0 and B2= ca. 100 G and 8 is the dihedral angle between the PC-H bond and the principal axis of the orbital containing the unpaired electron. This value of p2 is about double that which is observed for carbon-centred ?r-radicals. Against this picture of hydrogen bonding it has recently been claimed that irradiation of deoxycytidine 5'-phosphate showed the spectrum of the 5'-alkoxyl radical by cleavage of the 0-P bond.36 A radical formed at this site could hardly be hydrogen bonded; the g-value was similar to that of the alkoxyl radicals previously reported but the value of a(2HP) at 8.0 G was much lower.6 Metal-centred Radicals The trialkyltin radicals are usually obtained by the photolysis of a hexa-alkylditin or in chain reactions from a trialkyltin hydride. Some new sources have been developed which render these useful radicals more readily available. The simple hexa-alkylditins are thermally stable up to about 200 "C when they decompose irreversibly into hydrocarbons and metallic tin. Very bulky groups however can weaken the tin-tin bond and the first examples of hexa-arylditins have been prepared which undergo reversible thermal dissociation.Hexakis( 2,3,5 -trimethylphenyl)ditin and hexakis( 2,3,5 -triethylphen y1)ditin were prepared by the reaction between the corresponding triaryltin hydrides and azoiso- butyronitrile at 100 "C. The former compound at 180 "C and the latter compound at 100"C show the e.s.r. spectra of the appropriate triaryltin radicals (17; Ar =2,4,6-Me&& or 2,4,6-Et3C6H2) and measurements of the intensities gave the dis- sociation energy of the Sn-Sn bond as 190* 8 and 125 f5 kJ mol-' respectively to be compared with the value of 210-240 kJ mol-' for he~amethylditin.~' Ar3Sn-SnAr3 S 2Ar3Sn. (17) A second new source of R3Sn- radicals depends on the high reactivity of a CH bond @ to tin coupled with the ease of fragmentation of P-stannylalkyl radicals.Bu'O. +HCMe2CH2SnMe3+ Bu'OH +Me2CCH2SnMe3+ Me2C=CH2+.SnMe3 (18) Photolysisof di-t-butyl peroxide in the presence of trimethylisobutyltin (18)shows the e.s.r. spectrum of the trimethyltin radical which demonstrates its usual reactivity towards alkenes and alkyl halides. The reagent is more readily prepared and stored than is hexamethylditin and it is more stable towards oxidizing agents. Trimethyltin isopropoxide HCMe20SnMe3 can be used in the same way.38 36 D. Krilov A. Velenik and J. N. Herak J. Chem. Phys. 1978,69,2420. 37 H.U.Buschhaus and W. P. Neumann Angew. Chem. Internat. Edn. 1978,17,59. 38 A.G. Davies B. P. Roberts and M.-W. Tse J.C.S.Perkin 11 1978 145. A. G. Davies The cyclopentadienyltin compounds CpSnX3 promise to be a useful source of a variety of tin-centred radicals. Among the organotin compounds they are unique in readily undergoing photolysis and for example the Bu3Sn* radical from CpSnBu3 shows its usual reactivity toward reagents such as alkenes and alkyl halides. Tetra- cyclopentadienyltin readily reacts with a variety of protic reagents HX making available a series of new *SnX3 radical^.^' 3HX hw CpNa+SnC14 -B Cp& -CpSnX3 +Cp.+4nX3 39 A. G.Davies and M.-W. Tse J.C.S. Chem. Comm. 1978 353.

ISSN:0069-3030    

DOI:10.1039/OC9787500070

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

5 Arynes Carbenes Nitrenes and Related Species By S. A. MATLIN Chemistry Department The City University St John Street London EC1 V 4PB 1 Arynes The use of complex bases for the generation of arynes has been reviewed by Caubkre.' Diels-Alder reactions of benzyne with 1-vinylcyclobutene or with 1,2-dimethyl- enecyclobutane lead to tricyclic adducts (Scheme 1)which can be dehydrogenated with DDQ to give naphtho[a]cyclobutene or naphtho[b]cyclobutene respectively. Similar reactions of 2,3-didehydronaphthalenewith the dienes provide synthetic routes to anthrocyclobutenes.' Scheme 1 Arynic condensations have been used in several new syntheses of heterocyclic systems. Condensation of benzyne with diazocyclopentadienes gives 3H-indazole- (3-~piro)-cyclopentadienes.~ Treatment of N-phenyl 2-chlorobenzylamines or of N-phenyl 3-bromobenzylamines with potassium amide in liquid ammonia affords mixtures of dihydrophenanthridines and dihydrobenzazetes (Scheme 2).4 1 Ra/ph R \ Scheme 2 ' P.Caubere Topics Current Chem. 1978,73,49. * R. P. Thummel W. E. Cravey and W. Nutakul J. Org. Chem. 1978 43,2473. H. Dunand A. Hackenberger Synthesis 1978 594. K. Krohn D. Carboo and U. Puttfarcken Annulen 1978,608. 79 S. A. Madin 2-Isopentoxy-l,3-benzothioleshave been prepared by aprotic diazotization of substituted anthranilic acids in the presence of carbon disulphide and isopentyl alcohol.' The dithioles (1) are useful intermediates in the synthesis of diben- zotetrathiafulvalenes.(1) The initial adduct formed by reaction of the 1,4-dithiin- 1,l -dioxide (2) with benzyne extrudes sulphur dioxide and phenylacetylene to furnish the benzo[bJthio- phen (3). This appears to be the first example of the thioethylene moiety reacting as a 1,3-dip0le.~ Following an earlier report of the generation of 4,5 -didehydrotropone its reac- tions with cyclic dienes have now been described.' The first example of the trapping of 4,5-didehydropyrimidine with a cyclic diene has also been reported (Scheme 3).' Scheme 3 2 Nitrenes A theoretical study of CHNO isomers has been carried out by Poppinger and Radom.' CH3C(0)N ClC(O)N and perpendicular H,NC(O)N were found not to correspond to true energy minima on their respective potential energy surfaces and to collapse without activation to the corresponding isocyanates.However planar H,NC(O)N emerged as a stable structure 4 kcal mol-' below the perpendicular geometry with HOC(0)N and FC(0)N also having energy minima. Direct observation of the N-nitrene (4a) has been made by i.r. and electron spectroscopy,lo which provide evidence for considerable double bond character (4b) in the ground state. A new route to N-dibenzylaminonitrene (5) involves deoxy- genation of N-nitrosodibenzylamine with phenacyl bromides in the presence of silver hexafluoroantimonate.' ' J. Nakayama E. Seki and M. Hoshino J.C.S. Perkin I 1978 468. K. Kobayashi and K. Mutai Tetrahedron Letters 1978 905. T.Nakazawa Y.Niimoto and I. Murata Tetrahedron Letters 1978 569.* D. Christophe R. Promel and M. Maeck Tetrahedron Letters 1978,4435, D.Poppinger and L. Radom J. Amer. Chem. SOC.,1978,100,3674. lo W. D. Hinsberg and P. B. Dervan J. Amer. Chem. SOC.,1978,100,1608. I' K. Nishiyama and J.-P. Anselme J. Org. Chem. 1978,43,2045. Arynes Carbenes Nitrenes and Related Species \T ~ N=N N-N Spin delocalization in 4-substituted triplet phenylnitrenes has been examined by e.s.r. and by INDO calculations. The latter indicated an essentially constant spin density (ca.1.83)on the nitrene nitrogen the e.s.r. data being interpreted in terms of the influence of substituents on the delocalization of the remaining spin. l2 A new procedure for the generation of acylnitrenes reminiscent of the Lossen rearrangement involves the pyrolysis of the readily prepared N,O-bistrimethylsilyl hydroxamic acids (6)with elimination of hexamethyldisiloxane.l3 Another method of nitrene generation which involves N-0 bond cleavage is the reaction of oxinses with Grignard reagents. A cyclic elimination mechanism (Scheme 4) is implied by the observation that the intermediate vinylnitrene cyclizes regio- specifically onto the C atom syn to the 3bxime OH group. 31 Direct deamination of primary amines has been acc~mplished'~ by treatment with hydroxylamine 0-sulphonic acid and base. This Combination of reagents results in the formation of hydrazine intermediates and also in the generation of nitrene. The latter oxidises the hydrazines to diazenes which readily lose nitrogen (Scheme 5).NH,OSO,H :NH -N RNH2 ARNHNH2 -RN=NH 2RH OH-Scheme 5 Carbethoxynitrene is formed initially in the singlet state by a-elimination from ethyl p-nitrobenzenesulphonyloxycarbamate under phase transfer conditions and has been trapped by addition and insertion reactions with olefins.I6 Competition J. H. Hall J. M. Fargher and M. R. Gisler J. Amer. Chem. Soc. 1978 100 2029. l3 F. D. King S. Pike and D. R. M. Walton J.C.S. Chem. Comm. 1978 351. l4 G. Alvernhe and A. Laurent J. Chem. Res. (S),1978 28. lS G. A. Doldouras and J. Kollonitsch J. Amer. Chem. SOC.,1978 100,341. '' M. Seno T. Namba andH. Kise J. Org. Chem. 1978 43 3345. 82 S. A. Math experiments on the selectivity of singlet nitrene insertions into the tertiary secon- dary and primary C-cH bonds of hydrocarbons provide further evidence for the stabilization of carbethoxynitrene by solvents such as dichloromethane and 1,4- dioxan." The observed changes in product ratios with increasing concentration of the stabilizing solvent can be accounted for by the formation of bulky solvent- nitrene complexes whkh discriminate in favour of less hindered positions.Unlike dichloromethane in which singlet stabilization is counterbalanced by inter-system crossing 1,4-dioxan also stabilizes the singlet nitrene in addition reactions with olefins the solvent-nitrene complex favouring addition over insertion reactions and giving improved selectivity in the additions.I8 Azabicyclohexanes and/or pyridines are formed in high yields by the thermolysis of 2-allyl-3-phenyl-2H-azirines(7; R1= Ph) the suggested mechanism involving initial rearrangement to vinylnitrenes and recycli~ation.'~ The isomeric 2-allyl-2- phenyl derivatives (7; R2= Ph) undergo competitive cyclization of the nitrene onto the aromatic ring affording indoles.I I R3 R3 R3 R3 p'= (7) Ph R3 0LJR4 I H The photolysis of aroyl azides in the presence of diketene leads to l-aroyl-4- hydroxy-3-pyrrolin-2-ones(8) via intermediate aziridines.*' Bis-aziridines have been obtained by cycloaddition reactions of aziridinonitrenes.2' (8) 2-Substituted azepines are formed by a singlet pathway when phenyl azide is irradiated in solutions containing nucleophiles such as amines. Attempts to use alcohols as the nucleophiles have hitherto met with little success but it has now been '' P.Cassagrande L. Pellacani and P. A. Tardella J. Org. Chem. 1978 43 2725; H.Takeuchi Y. Kasamatsu M. Mitani T. Tsuchida and K. Koyama J.C.S. Perkin 11 1978 780. H. Takeuchi T. Igura M. Mitani T. Tsuchida and K. Koyama J.C.S. Perkin II 1978,783. l9 A. Padwa and P. H. J. Carlsen J. Org. Chem. 1978,43,2029;Tetrahedron Letters 1978,433. *' T. Kato Y. Suzuki and M. Sato Chem. Letters 1978 697. 21 L.Hoesch N. Egger and A. S. Dreiding Helu. Chim. Actu 1978,61 795. 83 Arynes Carbenes Nitrenes and Related Species shown that the carbonyl function in 2-azidobenzoyl esters and amides promotes 2-alkoxy-3H-azepine formation on irradiation in alcoiio1s.22 The previous assump- tion that the reaction involves cyclization of an intermediate phenylnitrene to an azirine followed by addition of nucleophile and ring expansion (Scheme 6) has now Scheme 6 been challenged.Chapman and Le ROUX*~ have observed an intermediate in the photolysis of phenyl azide in argon at 8 K which has an intense absorption at 1895cm-'. The same intermediate could be generated by irradiation of the triazene (9) and was suggested to be l-aza-l,2,4,6-~ycloheptatetraene (10). It is this cyclic ketenimine formed either via singlet phenylnitrene or directly from singlet excited phenyl azide which is claimed to add nucleophiles to form 2-substituted azepinesZ3 and to lie on the pathway between isomeric phenylnitrenes and pyridylmethylene~.~~ Irradiation of the azidopyrimidine derivatives (1 1 ;R =Me) in alkylamines gives the 1,3,5-triazepines (12;R =Me X = NR'R') and similarly the azide (11;R = CN) affords (12; R = CN X =OR') on irradiation in alcohols.25 However the azide (1 1; R =H) behaves differently,26 photolysis in alkylamines leading to the 6-alkylamino- 5-aminouracils (13).The differences were attributed to substituent-dependent behaviour of the intermediate (14)formed by addition of the nucleophile to the azirine (15). '* R.Purvis R. K. Srnalley W. A. Strachan and H. Suschitzky,J.C.S. Perkin I 1978 191. 23 0.L.Chapman and J.-P. Le Roux,J. Amer. Chem. Soc. 1978,100,282. 24 0.L.Chapman R. S. Sheridan and J.-P. LeRoux,J. Amer. Chem. Soc. 1978,100,6245. 25 S.Senda K. Hirota and T. Asao Tetrahedron Letters 1978 1531.26 S.Sendo K. Hirota T. Asao and K. Murahashi J. Amer. Chem. SOC.,1978,100,7661. S. A. Math 0 0 Me Me (14) (15) Studies continue on intramolecular insertion pathways for substituted aromatic nitrene~.~’The azidophenyl thienyl sulphides (16) yield pyrrolo[2,1 -b]benzo- thiazoles (17) with elimination of sulphur on thermolysis cyclization of the inter- mediate nitrene being followed by ring opening and reclosure (Scheme 7).28 -1 Scheme 7 Further studies of the manganese dioxide oxidation of aryl 1,2-diaminoirnidazoles (18; R=H or Ar) now provide evidence for formation of the N-nitrene (19)as a minor intermediate the major product-forming pathway being via the C-nitrene (201.~~ Ar Ar Ar ,&H2 R&: RN I R&HI!? NH, I I NH2 (18) (19) (20) A nitrene complex MO~O,(NH)[S~P(OE~)~]~ has been isolated from the reaction of MoO~[S~P(OE~)~]~ with HN3and was shown by X-ray crystallography to contain a nitrene group bridging the two molybdenum Whereas the 2-ketovinyl azirine (21) affords a high yield of the oxazepine (22) on thermolysis decomposition of (21) in the presence of MO(CO)~ gives a mixture of products (22)-(25) which result from an intermediate molybdenum-nitrene ’’ R.N. Carde,G. Jones W. H. McKinley,andC.Pn0e.J.C.S.PerkinI 1978,1211; R.A. Abramovitch C. I. Axogu I. T. McMaster and D. P. Vanderpool J. Org. Chem. 1978,43 1218. J. M. Lindley. 0.Meth-Cohn and H. Suschitzky,J.C.S. Perkin I 1978 1198. 2q M. Nakajima R. Hisada and J.-P. Anselme J.Org. Chem. 1978,43 2693. 30 A.W. Edelblut. B. L. Haymore and R. A. D. Wentworth J. Amer. Chem. Soc. 1978,100,2250. Arynes Carbenes Nitrenes and Related Species complex. Interestingly reaction of (21) with Fe2(C0)9 gives as well as the products (22)-(25) an additional product (26) arising from attack of the metallonitrene on the carbonyl gr~up.~' Ph Ph Ph Ph I 'Ph Ph COPh Ph Ph 1 1 3 Carbenes A review has been published of the synthesis of carbocyclic spiro compounds via cycloaddition routes including carbene additions to exocyclic double bonds.32 The results of theoretical and experimental assessments of the triplet-singlet energy separation in methylene are gradually converging. The latest configuration interaction (CI) calculation^^^ indicate an upper limit of 10.6 kcal mol-' for the separation compared with a new experimentally determined value34 of around 8.1 kcal mol-' .Like methylene diphenylmethylene has a triplet ground state. New e.s.r.studies of the triplet-singlet energy separation in diarylmethylenes isolated at low tempera- 31 F. Bellamy J.C.S. Chem. Comm. 1978,998; Tetrahedron Letters 1978,4577. 32 A. P. Krapcho Synthesis 1978.77. 33 C. W. Bauschlicher jun. and I. Shavitt J. Amer. Chem. Soc.,1978,100,739. 34 R.K.Lengel and R. N. Zare. J. Amer. Chem. Soc. 1978,100,7495. 86 S. A. Matlin tures in a rigid glass show an unexpectedly large stabilization when the p and p' positions are substituted by strong electron-withdrawing and electron-releasing groups re~pectively.~~ This is attributed to merostabilization in which charge- separated resonance structures [e.g.(27)Jpermit increased delocalization of the unpaired electron in the v orbital onto the positions ortho to the substituents. \ \ 0-1 0-+m+,o-Me,N N\ CI calculations indicate an allenic structure HC=C=N for cyanomethylene in agreement with earlier i.r. and U.V. studies. This result contrasts with restricted open-shell SCF theory which predicts a bent triplet carbene ground state. The discrepancy is explained by an inability of the restricted open-shell SCF theory to describe reliably the energy changes involved in simultaneous breaking of one bond and formation of an adjacent bond owing to an overemphasis of ionic character in bond breaking.36 MIND0/3 calculations on silylated carbenes have also been made.Trimethylsilylmethylene is predicted to be a bent triplet the result again conflicting with earlier Theoretical studies of singlet methylene 1,2-addition to ethylene and 1,4-addition to butadiene have also been reported.38 Generation.-Carbene transfer reactions of organomercury compounds have been reviewed.39 Optimization studies4' of dichlorocarbene addition to olefins by phase transfer catalysis (FTC) indicate the following conditions to be the best 4-molar excess each of chloroform and 50% aqueous sodium hydroxide over alkene 1mol% catalyst mixing at 0-5 "C stirring at >800 rev min-' for 1-2 h at room temperature then heating for 2-4 h at 50 "C. An improved electrochemical method41 for the generation of dichlorocarbene utilizes galvanostatic reduction of CCl and CHClj at lead cathodes in CHC13/Bu4NBr or CH2C12/Bu4NBr at -5 "C.The formation of carbenes by thermal decomposition of tosylhydrazone monoanions generally requires temperatures in excess of 130 "C. It has now been 35 D. R. Arnold and R. W. R. Humphreys J.C.S. Chem. Comm. 1978,181. 36 J. I. Harrison A. Dendramis and G. E. Lepoi J. Amer. Chem. Suc. 1978,100,4352. 37 R.Noyori M. Yamakawa and W. Ando Bull. Chem. SOC.Japan 1978,51,811. 38 B.Zurawski and W. Kutzelnigg J. Amer. Chem. SOC.,1978,100,2654: W.W.Schoeller and C. W. Kern ibid,p. 7548. 39 R. C. Larock Anpew. Chem. Internal. Edn. 1978,17,27. 40 E.V.Dehmlow and M. Lissel J. Chem. Res.(S),1978,310. O1 H. P.Fritz and W. Kornrumpf Annalen 1978 1416. Arynes Carbenes Nitrenes and Related Species shown that the anions of trisylhydrazones (2,4,6-tri-isopropylphenylsulphonyl-hydrazones) decompose to carbenes under significantly milder conditions.42 Carbenes are often formed during photochemical fragmentations and rear- rangements of 01efins.~~ Irradiation of cyclic olefins at 185nm leads to carbene intermediates by both C-C and C-H bond migration^.^^ Trimethylsilylcarbene can be generated from chloromethyl trimethylsilane by the use of the hindered base lithium 2,2,6,6-teramethylpiperidide in a hydrocarbon solvent and will cyclopropanate olefins. An analogous reaction was also obtained with chloromethyl trimethyl~tannane.~’ Evidence has been reported for the existence of free bi~(pheny1thio)carbene~~ and for the generation of dimethyl-aminocyanocarbene in the thermolysis of dimethyl- amin~malononitrile.~’ Reactions.-The reactivity of dichlorocarbene and related species continues to receive intensive scrutiny.Ausloos and Lia~*~ have carried out further measure- ments of the proton affinity of :CCl and reaffirm that its value is slightly below that of NH,. Giese and Mei~ter~~ have compared the selectivities of addition of dihalogeno- carbenes to 2-methylbut-2-ene and 2-methylpropene at different temperatures. A strong temperature dependence of the selectivities was observed with an isoselective temperature at about 330K. Below this temperature the selectivity order was :CF2> :CCI2> :CBr2 whereas above it the order was reversed.These results point to the caution which must be exercised in drawing conclusions about the stability or reactivity of individual carbenes on the basis of selectivity comparisons at one temperature. The partitioning between different reaction pathways for a given carbene is also temperature de~endent.~’ Diphenylcarbene reacts with 2-methyl- propene only by addition at 25”C but allylic C-H insertion becomes the near- exclusive process at -196 “C. The observation of rearranged alkene products (e.g. Scheme 8)during halogeno- carbene additions to olefins has now been re-interpreted in terms of ready intercon- version between the singlet and triplet states of the carbene the former being responsible for stereospecific cyclopropanation and the latter for rearrangement via diradi~als.’~ c1 Br CC1,H % + :CBr -C1 Br Br Scheme 8 *’ A.R. Chamberlin and F. T. Bond J. Org. Chem. 1978,43,154. 43 G. Kaup Angew. Chem. Internat. Edn. 1978,17 150. R. Srinivasan and K. H. Brown J. Amer. Chem. SOC.,1978,100,4602; Tetrahedron Letters,1978,3645. ‘’ R. A. Olofson D. H. Hoskin and K. D. Lotts Tetrahedron Letters 1978 1677. 46 M. Nitsche I).Seebach. and A. K. Beck Chem. Ber. 1978,111,3644. 47 L. De Vries J. Amer. Chem. SOC.,1978,100,926. ‘13 P. Ausloos and S. G. Lias J. Amer. Chem. Soc. 1978,100,4594. 49 B. Giese and J. Meister Angew. Chem. Internat. Edn. 1978,17 595. R. A. Moss and J. K. Huselton J. Amer. Chem. Soc. 1978,100,1314; R. A.Moss and M. A. Joyce ibid,p. 4475. 51 J. B. Larnbert K.Kobayashi and P. H. Mueller Tetrahedron Letters 1978,4253.; M. Jones jun.. P. P. Gaspar and J. B. Lambert ibid,p. 4257. 88 S.A. Math Insertion of dichlorocarbene into the C(2)-H bond of 2-substituted 1,3-diox- olans (28) affords the 2,2-disubstituted dioxolans (29). Fur 2-aryl derivatives (28; no:cc1* n 0 YH +Ox0 R R CCI,H (28) (29) R =m-or p-substituted phenyl) a good Hammett correlation with u+was obtained yielding p =-0.63. For 2-alkyl derivatives (28; R =alkyl) use of the modified Taft equation [Equation (l)]gave a good correlation with p =-0.73 and 6 =0. The p values indicate a largely concerted process with some positive charge development at C-2 in the transition state and it is apparent that the steric effect of a 2-alkyl substituent is of little imp~rtance.~~ gem-Dichloroaziridines have been synthesized by PTC addition of dichlorocar-bene to imine~.’~ This contrasts with the behaviour of diarylcarbenes with the imine (30) which give the product (31) of N-H insertion rather than a~irine.~~ :CF2 inserts directly into the N-H bond of bis(pentafluoropheny1) amine leading to an isolable difluoromethylamino compound (32).55 Ph Cu(acac) \ ArzC=N2 + C=N-CHAr2 Ph2C=NH -/ Ph (30) (31) :CF, (CzF5hNH -(C~FS)~NCF~H (32) Dichlorocarbene will efficiently abstract oxygen from many sulph~xides’~ and causes cleavage of a-hydroxyketoximes to ketone and nitrile products (Scheme 9).57 R \ / :cc1 \ C-C d C=O+RCrN /AH MOH / Scheme 9 Contrary to some earlier expectations it has been that alkylchlorocar- benes (methyl ethyl t-butyl) and cyclopropylchlorocarbene can be efficiently 52 K.Steinbeck and J. Klein J. Chem. Res. (S),1978 396; K. Steinbeck Tetrahedron Letters 1978 1103. 53 M. K. Meilahn D. K. Olsen W. J. Brittain and R. T. Anders J. Org. Chem. 1978,43 1346. 54 K. N. Mehrotra and G. Prasad Tetrahedron Letters 1978,4179. ’’R. Koppang J. Fluorine Chem. 1978,11,19. 56 H.S.D. Soya and W. P. Weber Tetrahedron Letters 1978 1969. 57 J. N. Shah J. P. Mehta and G. M. Shah J. Org. Chem. 1978,43,2078. R. A. Moss and R. C. Munjal J.C.S. Chem. Comm. 1978,775; R. A. Moss and M. E. Fantina J. Amer. Chem. SOC.,O1978,100,6788. Arynes Carbenes Nitrenes and Related Species trapped by addition to olefins.Only in the case of isopropylchlorocarbene did the alternative intramolecular 1,2-H shift leading to 1-chloro-2-methylpropene compete effectively. Phenylsulphinylcarbene is unusually stable and! shows a selectivity similar to that of methylchlorocarbene. The stability has been attributed to a powerful electron- donating ability of the PhS(0) substituent under conditions of high electron demand.59 Small amounts of asymmetric induction are observed in the cuprous chloride- catalysed additions of chiral carboalkoxycarbenes to olefins6' Substituted methyl- enecyclopropanes are formed by the addition of carboethoxycarbene to 1,l-dimethylallene.61 Rhodium(I1) carboxylates are particularly effective catalysts for the formation of cyclopropanecarboxylatesfrom ethyl diazoacetate and acetylenes.62 In the presence of rhodium(I1) acetate dimethyl diazomalonate reacts with thiophen to give the ylide (33; R=H) whose structure was confirmed by X-ray crystallography.The dichlorothiophenium ylide (33;R =C1) is remarkably stable C0,Me 00 and can be stored as a crystalline solid without special precautions. In the presence of Rh" acetate or copper acetylacetonate it serves as a useful precursor to bis- methoxycarbonylcarbene for cyclopropanation of ~lefins.~~ Carbonylcarbene :C=C=O formed by gas-phase photolysis of carbon suboxide reacts with the cyclic ethers tetrahydrofuran and oxetan to give products arising from both C-0 insertion and deoxygenation of the ether.64 Both types of reaction can be explained in terms of a common intermediate the ylide (34; n =0 or 1).The (34) reaction of ethyl diazoacetate with unsymmetrical ketones leads to highly selective formation of the least substituted enol ether consistent with the expected steric and 59 C.G. Venier and M. A. Ward Tetrahedron Letters 1978 3215. M P. E. Krieger and J. A. Landgrebe J. Org. Chem. 1978,43,4447. 61 X. Creary J. Org. Chem. 1978,43 1777. 62 N. Petiniot A. J. Anciaux A. F. Noels A. J. Hubert and P. TeyssiC Tetrahedron htters 1978 1239. 63 R. J. Gillespie J. Murray-Rust P. Murray-Rust and A. E. A. Porter J.C.S. Chem. Comm. 1978,83;J. Cuffe R. J. Gillespie and A. E. A. Porter ibid p. 641. T. R. Forbus P. A. Birdsong and P.B. Shevlin J. Amer. Chem. Soc. 1978,100,6425. 90 S. A. Matfin electronic effects for proton abstraction in the presumed carbonyl ylide intermediate (35)? R'R'CH \ &\ ,CO,Et R'R~CH \c/o\ CH ,CO ,Et C + I\ R3Ha LC\H R3HC H (35) Cyclopropanation of cycloheptatriene has been studied with diazomalonate and diazoacetate esters. In the case of the latter carbene precursor the products of addition to all three double bonds could be isolated.66 Bis-carbomethoxycarbene adds in the singlet state to cyclobutene to give the bicyclopentane (36),whereas under triplet conditions the vinylcyclopropanes (37) and (38) are also obtained. The latter are evidently formed via the diradical(39) and it has been argued that a similar mechanism is involved in vinylcyclopropane formation from methylene and cyclobutene .67 Me0,C C0,Me hulPh2CO + n -Meo2cm b 2 Me0,C C0,Me + MeO& C0,Me (36) (37) .-There is a growing interest in carbenes which exhibit nucleophilic character as a result of electron donation by a substituent into the vacant p-orbital of the carbene.As a model for the insertion reactions of such carbenes a theoretical study of the addition of hydrogen to cyclopropylidene has been carried out.68 As in the case of :CH2+ Hz, the reaction follows a pathway in which the H2a-orbital interacts with the carbene p-orbital (electrophilic phase) followed by population of the H20"-orbital by the carbene lone pair and rotation of the HCH plane to give the final tetrahedral arrangement (nucleophilic phase).A high activation energy (41 kcal mol-') is predicted with the electrophilic phase being rate-determining. Thioxanthenylidene (40a) shows evidence of nucleophilic character (40b) in its reactions with fumaric and maleic esters the former undergoing stereospecific cyclopropanation whereas the latter affords the olefin (41).69 By contrast a re- examination of xanthenylidene (42) for which nucleophilic or weak electrophilic " J. A. Landgrebe and H. Iranmanesh J. Org. Chem. 1978,43,1244. 66 B. Decock-le-Reverend M. Durand and R. Merenyi Bull. SOC. Chim. France 1978,369. '' M. E. Hendrick and M. Jones jun. Tetrahedron Letters 1978,4249. '* H. Kollmar J. Amer. Chem. Soc. 1978,100,2660. 69 T. B. Patrick M. A. Dorton and J. G.Dolan J. Org Chem. 1978,43 3303. Arynes Carbenes Nitreries and Related Species (42) (41) properties had earlier been claimed is now more indicative of typical electrophilic rea~tivity.~’ As expected methylchlorocarbene shows evidence of both electrophilic and nucleophilic character in its reactions with 01efins.~’ 1,3-Bis(dimethylamino)vinylcarbene(44a) generated by base attack on the perchlorate (43; R =Ph or OEt) has nucleophilic properties as a consequence of the allylic resonance (44b) and has been trapped as a dipole in a variety of Me,N*NMe H C=O H H H 8-. Me2N-YNMe2 I R (43) (444 (44b) Star~g~~ has reviewed the properties of vinylidenecarbenes and their formation from vinyl triflates and tosylazoalkenes. The unsaturated carbenes derived from these precursors exhibit mildly electrophilic character giving stereospecific olefin cyclopropanation consistent with the theoretical prediction74 of a ground state singlet.The free carbene (47) is a common intermediate generated by elimination from either of the isomeric vinyl triflates (45) or (46) and rearranges to l-phenyl- propyne by 1,2-phenyl migration.75 For vinylidene itself a low barrier of 8.6 kcal mol-’ has been calculated 74n for rearrangement to acetylene. Reversal of this rearrangement accounts for the scrambling of labels in HC_”CD observed on pyr~lysis.’~ 70 G. W. Jones K. T. Chang R. Munjal and H. Shechter J. Amer. Chem. Soc. 1978,100,2922. 71 N. P. Smith and I. D. R. Stevens Tetrahedron Letters 1978 1931 ;R.A. Moss and W.-C. Shieh ibid p. 1935. 72 R. Gompper and R. Sobotta Angew. Chem. Internat. Edn. 1978,17,762. 73 P. J. Stang Accounts Chem. Res. 1978,11 107; Chem. Rev. 1978,78,383. 74 (a)C. E. Dykestra and H. F. Schaefer,J. Amer. Chem. SOC., 1978,100,1378;(b)J. W. Kenney J. Simons G. D. Purvis and R. J. Bartlett ibid p. 6930. ” P. J. Stang D. P. Fox,C. J. Collins and C. R. Watson jun. J. Org. Chem. 1978,43,364. 76 R. F. C. Brown F. W. Eastwood and G. P. Jackman Austral. J. Chem. 1978,31,579. S. A. Matlin Me ‘&c Me*CECPh Ph/ (47) Cycloalkylides generated by flash vacuum pyrolysis of the leldrum A acid derivatives (48; n = 1-3) give products derived from rearrangement to bicy- clopropenes as well as cy~loalkynes.~~ Although ring closure to cyclopropenes is an important reaction for vinyl- carbene~,’~ intermolecular trapping by olefins to give vinylcyclopropanes is also readily Several studies have been reported of the reversal of the intramolecular process in which photolysis of phenyl-substituted cyclopropenes leads to vinylcarbenes.The latter may recyclize by an alternative mode affording indenes” or in the case of vinylcyclopropenes cyclopentadienes.81 The thiovinylcarbene (49) undergoes migration and dimerization reactions apparently without cyclopropene formation. Trapping with ethyl dimethylallyl sulphide leads uia rearrangement of the ylide (50) to the thioketal(51) of artemisi- aketone.82 Cycloheptatrienylidene has been generated by the fluoride ion induced desilyl- ation of the cation (52; R=H)83 as well as by decomposition of the anion of tropolone to~ylhydrazone.~~ In the absence of trapping reagents it undergoes dimerization but this process can be inhibited by the presence of 2,7-substituents which favour rearrangement to the arylcarbene (53).The reactions of the cyclopropanated cycloheptatrienylidenes (54)-(56) provide an interesting contrast each showing a unique rearrangement path~ay.~’ ’’ G. J. Baxter and R. F. C. Brown Austral. J. Chem. 1978,31 327. W. Welter A. Hartmann and M. Regitz Chem. Ber. 1978,111,3068. M. Franck-Neumann and C. Dietrich-Buchecker Tetrahedron 1978,34,2797. J. 0.Stoffer and J. T.Bohanon J.C.S. PerkinfZ 1978,692;M. I. Komendantov,R. R. Bekmukhamentov and I. N. Domnin Tetrahedron 1978,34,2743.A. Padwa T. J. Blacklock D. Getman N. Hatanaka and R. Loza J. Org. Chem. 1978,43,1481. M. Franck-Neumann and J. J. Lohmann Tetrahedron Lerters 1978,3729. 83 M. Reiffen and R. W. Hoffmann Tetrahedron Letters 1978 1107. IM C. Mayor and W. M. Jones J. Org. Chem. 1978,43,4498. M. Oda Y. Ito and Y. Kitahara Tetrahedron Letters 1978 977. Arynes Carbenes Nitrenes and Related Species 3S/Et -I-SEt (49) SiMe S. A. Matlin A new SCF of the rearrangement of cyclopropylidene to allene contradicts earlier calculations predicting barrier heights of 18 and 19 kcal. mol-' respectively for the singlet and triplet processes with the triplet carbene being more stable than the singlet by 8.4 kcal. mol-'. The related facile vinylcyclopropylidene-cyclo-pentenylidene rearrangement has also been examined using MIND0/3.In the singlet state the rearrangement is pictured*' as involving initial w-complex forma- tion between the double bond and the p-orbital of the carbene with synchronous opening of the three-membered ring leading to a non-classical carbene inter- mediate. For cyclopropylidenes the alternative reaction pathway to allene formation is intramolecular C-H insertion leading to bicyclobutanes.88 This process has hitherto not been observed for cycloprop-2-ylidene carbinols but evidence for the formation of intermediate bicyclo[ 1.1.O]butan-2-olates (58) has now been presen- ted.89 Reaction of the dibromocyclopropanes (57) with MeLi gave after work-up the aldehydes (59) and/or alcohols (60).R2 R* K R2 O * MeLi* R' R2&- "- Br Br D D D (57) R' R' R2 ,MeLi DH Intramolecular 1,2-migrations to carbenes are a useful method of olefin forma- tion. Adamantene (62) has now been generated by this type of reaction following pyrolysis of the anion (61). Biadamantyl isomers were obtained amongst the products and their formation is presumed to occur via radical intermediates generated from the strained olefin (62). Their isolation from this reaction raises questions about the nature of the processes involved in previous attempts at adamantene formation when no such products were ob~erved.~' In the deuterium-labelled 1-aryl-2-diazopropanes (63),kinetic isotope effects for H(D) migration in the thermally generated carbene provide evidence for a pull-push N2 II ArCHD-C-CH 86 D.J. Pasto M. Haley and D. M. Chipman J. Amer. Chem. SOC.,1978 100 5272. W. W. Schoeller and U. H. Brinker J. Amer. Chem. SOC.,1978,100,6012. " S. A. Matlin Ann. Reports (B),1977,74 105. 89 N. 0.Nilsen L. K. Sydnes and L. Skattebol J.C.S. Chem. Comm. 1978,128. 90 D. J. Martella M. Jones jun. and P. von R. Schleyer J. Amer. Chem. SOC.,1978 100 2896. Arynes Carbenes Nitrenes and Related Species mechanism.” Electrophilic attack on the C-H bond by the carbene p-orbital is accompanied by backside nucleophilic attack of the carbene electron pair assisting the H-transfer. According to SCFcalculations the preferred pathway for rearrangement of triplet formaldehyde to hydroxycarbene is a concerted process (1,2-H shift from C to 0) whereas for alkyl migration in ketones a diradical process involving dissociation- recombination is energetically favoured (Scheme lo).’’ Scheme 10 Some evidence consistent with this picture emerges from a study of the photorear- rangement of 1-alkoxytriptycenes (64).In alcoholic solvents the ketals (66) are formed by trapping of the oxacarbenes (65).However in inert solvents the aldehyde (67) and/or ketones (68) become major products and their formation can be rationalized by a homolysis of the 0-R bond in (65) to give acyl and alkyl radicals which may either recombine or abstract hydrogen atoms from the hv OR -I + I 91 D. T. T. Su and E. R.Thornton J. Amer. Chem.SOC.,1978,100 1872. 92 J. A. Altmann I. G. Csizmadia M. A. Robb,K. Yates and P. Yates J. Amer. Chem. SOC.,1978,100 1653. 93 H.Iwamura and H. Tukada Tetrahedron Letters 1978,3451.

ISSN:0069-3030    

DOI:10.1039/OC9787500079

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

6 Organometal tic Chemistry Part (i)The Transition Elements By H. M. COLQUHOUN and J. HOLTON ICI Corporate Laboratory The Heath Runcorn Cheshire WA7 4QE and M.V. TWlGG ICI Agricultural Division Billingham Cleveland TS23 1LD 1 Introduction This Report deals with applications of transition-metal species in organic synthesis. During 1978 a high level of interest has been maintained and the discovery of useful new procedures continues. There are a large number of publications in this area and in general references have been selected for their practical significance. A number of relevant reviews have been published and some of the more important are given below. A general review on asymmetric synthesis illustrates the growing r81e of organo-transition-metal species in the preparation of optically active compounds.’ Use of metal clusters in catalysis another growth area has been reviewed.2 Reviews dealing with cobalt-catalysed syntheses of pyridines from alkynes and nit rile^,^^ transition-metal 7r-complexes of heterocyclic and reactions of 0-bonded organometallic complexes with ele~trophiles~‘ have also been published.A particularly readable article on mechanistic aspects of olefin metathesis a~peared,~ and several conference reports contain much relevant information.’ 2 Metal-catalysed Hydrogenation and Hydrogen-transfer Reactions Selective reduction of ap-unsaturated carbonyl compounds has been achieved using iridium and lanthanide catalysts. a$-Unsaturated aldehydes are reduced to unsaturated alcohols by the hydridoiridium sulphoxide catalyst [Ir(H)C12(Me2S0)J in propan-2-01 the solvent being the source of hydrogen.6 Under mild conditions ’ D.Valentine and J. W. Scott Synthesis 1978,329; see also A. Nakamura Pure Appl. Chem. 1978,50 37. * C. U. Pittman and R. C. Ryan Chemtech. 1978,170. (a)H. Bonnemann Angew. Chem. Internat. Edn. 1978,17,505;(b)K. H. Pannell B. L. Kalsotra and C. Parkanyi,J. Heterocyclic Chem. 1978,15,1057; (c) M. D. Johnson Accounts Chem. Res. 1978,11,57. T. J. Katz Ado. Organometallic Chem. 1977 16 283. ’‘The Place of Transition Metals in Organic Synthesis’ ed. D. W. Slocum Ann. New York Acad. Sci. 1977 295; ‘Fundamental Research in Homogeneous Catalysis’ ed. M. Tsutsui and R. Ugo Plenum Press London 1977;‘Eighth International Conference on Organometallic Chemistry’ Pure Appl.Chem. 1978 50 677. B. R. James and R.H. Morris J.C.S. Chem. Comm. 1978.929. 97 H. M. Colquhoun J. Holton and M. V. Twigg 90% conversion with >80% selectivity was achieved with cinnamaldehyde a-methylcinnamaldehyde and crotonaldehyde. This may be compared with the rhodium system [RhC1(C0)J2-tertiary amine which gave only 50% selectivity with crotonaldehyde. The reasons for preferential reduction of the carbonyl function are thought to be electronic rather than steric even though reduction of the olefinic bond is observed with unsaturated ketones. Iridium is generally considered to be a less effective hydrogenation catalyst than rhodium and the other platinum metals but this and other reports7 suggest this may not always be the case.ap-Unsaturated ketones are selectively reduced to allylic alcohols using lanthanide chlorides (e.g. samarium chloride hexahydrate) with sodium boro- hydride.* The equimolar reactions take place at room temperature without exclusion of air or moisture giving excellent yields of product (>90%) within minutes. Nearly exclusive selective carbonyl reduction is obtained under conditions which do not affect functional groups such as carboxy- ester and nitro-groups. Linear and branched aliphatic as well as aromatic aldehydes are effectively hydrogenated by molecular hydrogen in the presence of [RuC12(CO),(PPh3),] to the corresponding alcohol.' High catalyst activities and turnover numbers up to 95 000 are observed with yields up to 99%.The facile reduction of substituted benzyl alcohols and styrenes is reported using hydrogen transfer from cyclohexene catalysed by Pd/C and AlCl3.l0* Other hydro- gen donors such as limonene or tetralin may be used. This method offers an alternative to catalytic hydrogenation replacing hydrogen gas by cyclohexene as the hydrogen source. The same catalyst system also reductively cleaves benzylic ethers to give the corresponding alcohols and aryl-alkanes.'Ob Due to the mild conditions used the reduction can be selective; for instance in cholesteryl benzyl ether the isolated double bond is not reduced. The first example of hydrogenation of nitro and nitrile groups using a Pd" catalyst has been reported." PdC12 supported on derivatized polystyrene (1)hydrogenates 0 (1) nitrobenzene to aniline (97%),and the more difficult to hydrogenate benzonitrile to N-benzylbenzamidine (40%)and a-(benzylideneamino)toluene (20%).In general Pd" has only a limited range of hydrogenation activity and this system represents a significant departure from previous Pd chemistry. 'R. H. Crabtree Plutinum Met. Rev. 1978 22 126; W. Strohmeier H. Steigerwalds and M. Lukacs J. Organometullic Chem. 1978 144 135. ' J -L. Luche J. Amer. Chem. SOC. 1978,100,2226. W.Strohmeier and L. Weigelt J. Organometallic Chem. 1978 145 189. lo (a)G. A. Olah and G. K. S. Prakash Synthesis 1978 397; (6)G. A. Olah G. K. S. Prakash and S. C. Narang ibid. p. 825. N. L. Holy J.C.S. Chem. Comm.. 1978 1074. Organometallic Chemistry-Part (i) The Transition Elements 99 A new homogeneous catalyst for the reduction of both benzene and olefins has been reported.[Ru(H)C~(~~~-C~M~~)PP~~] is a stable long-lived catalyst for the hydrogenation of benzene to cyclohexane under mild conditions. l2 No cyclo-hexadienes or cyclohexene were detected suggesting that the hydrocarbon remains strongly co-ordinated to the metal centre throughout all the hydrogenation steps. Olefins are also catalytically reduced and using transfer hydrogenation from secondary alcohols both olefins and diolefins may be reduced with this ruthenium catalyst. Other transfer hydrogenation catalysts [RhH(PPh3L] and [RuH2(PPh3)J will not reduce diolefins. The scope of arene hydrogenation using [Co( T-C~H~){P(OM~)~}~] has been studied using a large selection of substrates.l3 Catalytic hydrogenation has been demonstrated for benzenes with substituent groups that include R OR C02R and NR2 but electron-withdrawing groups e.g. halogen NO2,and CN inhibit the reduction. The matching of ligands to substrate is now an established part of asymmetric hydrogenation and therefore only those ligands of special interest are included in this year's review. One such ligand is (R)-prophos [(R)-l,2-bis(diphenyl- phosphino)propane] (2).14 Whereas the tendancy has been for more complexity in ligand designs this simple ligand has only a single methyl group at a chiral centre to constrain the chirality of the chelate ring. Even so the (R)-prophos-rhodium(1) system [Rh{(R)-prophos)(norbornadiene)] Clod is an excellent catalyst for producing optically active (S)-amino-acids from (2)-a-acylaminoacrylic acids (3).H3C H C02R2 \/ H&-CH2 c=c Ph2P/\PPh2 R3''NHCOR' (2) (3) Optical yields of ca. 90% are obtained and appear to be insensitive to the nature of the substituent on the substrate. Hence neither changes at the amine function (R' =Me or Ph) a change of the acid to an ester (R2 =H or Et) nor the nature of the @-vinyl substituent (R3=e.g. H Ph Pr' or p-hydroxy phenyl) affects the optical yield. Of further interest is that the catalyst is capable of breeding its own chirality. Readily available ethyl pyruvate may be converted into its enol acetate which is rapidly hydrogenated by the (R)-prophos catalyst to ethyl (S)-0-acetyl-lactate in 81% optical purity.By standard procedures the lactate is easily converted into (R)-prophos and hence a small amount of (R)-prophos can produce much larger amounts of itself. (S)-Prophos was prepared in an analogous manner and as may be predicted produces (R)-amino-acids. Further studies have appeared on the use of pyrrolidine-phosphine-rhodium systems in asymmetric hydrogenation. Substrates investigated include @-acetyl-aminoacrylic acid derivatives"" (optical yield C 55%) itaconic (optical yield 12 M. A. Bennett T. Huang A. K. Smith and T. W. Turney J.C.S. Chem. Comm. 1978 582. l3 L. S. Stuhl M. Rakowski Du Bois F. J. Hirsekorn J. R. Bleeke A. E. Stevens and E. L. Muetterties J. Amer. Chem. SOC.,1978,100 2405.l4 M. D. Fryzuk and B. Bosnich J. Amer. Chem. SOC., 1978,100,5491. l5 (u)K. Achiwa and T. Soga Tetrahedron Letters 1978 1119; (b)K. Achiwa ibid. p. 1475;I. Ojima T. Kogure and K. Achiwa Chem. Letters 1978 567; K. Achiwa ibid. p. 561; (c) I. Ojima T. Kogure T. Terasaki and K. Achiwa J. Org. Chem. 1978,43,3444; (d)K. Achiwa Tetrahedron Letters 1978,2583; (e)K. Achiwa Chem. Letters 1978 905. H. M. Colquhoun,J. Holton and M. V. Twigg >go%) ketopantoyl lactone 15' (optical yield 85%),and 2-acetoamido-3-methyl- fumaric acid ester 15d (optical yield -55%). To help overcome problems with catalyst separation from the reaction mixture a supported pyrrolidine-phosphine- rhodium system has been prepared.15= The first examples of chelating chiral ligands derived from a sugar have been reported.Diphosphinites synthesized from (a) D-glucose,'6"*b(6) D-galactose,16b and (c) 1,6-anhydro-~-glucose," in rhodium systems gave optical yields of up to 80% in the asymmetric hydrogenation of a-acetamidoacrylic acid derivatives. The latter (c) required addition of a base triethylamine to achieve high optical yields. A detailed investigation of the mechanism of asymmetric hydrogenation has been performed by a 31P n.m.r. spectroscopic study of rhodium(1) cationic complexes under experimental conditions." These systems with monophosphine ligands were thought to operate by dihydride formation followed by olefin complexation. Results suggest that the actual course of reaction is critically dependent on the structure of the phosphine and that dihydride formation may not always precede olefin complexation.With chelating phosphines it is thought that olefin complexation occurs first. A study of the asymmetric hydrogenation of a-benzamidocinnamic acid using a rhodium(1)-( +)-diop complex leads to the proposal that the stereochemistry is defined in an olefin-binding step via a square-planar chelate intermediate (4). The H H (4) subsequent addition of hydrogen is thought to have only a minor irfluence on the optical yield. 3 Dimerization Oligomerization and Polymerization Although the field of olefin polymerization is characterized by a large volume of experimental results little is known about the nature of active centres or the detailed mechanism.Three papers attempt to correct this situation. Fully characterized Group 3A and lanthanoid metal complexes [M(q-C5H4R)*MeI2 (M =Y Er or Yb) and [M(q-C5H4R)2Me2AlMe2] (M =Y Er Ho or Yb) are homogeneous ethylene polymerization catalysts. l8 A deactivation process involving abstraction of a cyclopentadienyl hydrogen was identified and could be suppressed by using peralkylated derivatives. Comparisons between these catalysts support the suggestion that the cocatalyst species (AlMe,) in Ziegler-Natta catalysis l6 (a)W. R.Cullen and Y. Sugi Tetrahedron Letters 1978 1635; (b)R.Jackson and D. J. Thompson J. Orgunometallic Chem. 1978,159 C29; (c)G. Descotes D. Lafont and D. Sinou ibid. 1978,150,C14. " (a)J. M. Brown and P. A. Chaloner,J.C.S. Chem. Cumm. 1978,321; (b)J.M. Brown P. A. Chaloner and P. N. Nicholson ibid. p. 646; (c)J. M.Brown and P. A. Chaloner Tetrahedron Letters 1978 1877. '*D. G. H. Ballard A. Courtis J. Holton J. McMeeking and R.Pearce J.C.S. Chem. Cumm. 1978,994. Organometallic Chemistry-Part (i) The Transition Elements serves both to alkylate M-Cl and to stabilize co-ordinately unsaturated active centres via alkyl bridges. The reaction mixture of a Nio complex plus the ylide Ph3P=CHCOPh has been described in patent literature as an ethylene oligomeriza- tion catalyst. Complex (5) has been isolated from this mixture and fully charac- terized (X-ray)." This species exhibits an interesting solvent effect. In toluene oligomeric <C30 linear n-olefins are formed whereas a suspension of (5) in hexane produces high-molecular-weight linear polyethylene.(5) A new mechanism for stereospecific olefin polymerization by Ziegler-Natta catalysts has been proposed.20 The key steps involve elimination of an a-hydrogen to form a metal-hydrido-carbene species olefin co-ordination and rearrangement with formation of a metallocyclobutane (Scheme 1). Although the mechanism appears Ill 'CHM~ \ .eMe \ C' CH e-/\ H+W\ /CH2 FC< H Me C. "Me H Scheme 1 very complex (compared with the Cosse mechanism) for what is a very rapid process of polymerization this scheme does present a common mechanism for both olefin polymerization and metathesis. The catalysis of both processes by a number of systems is now simply explained. If the proposed hydrogen-transfer step is slow or if there is a means of removing hydrogen from the metal centre then the catalyst becomes a metathesis catalyst.The Cosse mechanism with slight adaptations has stood the test of time and it will be interesting to see how this new mechanism fares and whether evidence can be found to substantiate what is certainly a novel theory. The metallocyclopentane [Ta(q -C,H,)Cl2C4H8] is a selective catalyst for dimerization of ethylene to but-l-ene.21 This represents a rare example of a w.Keim F. H. Kowaldt R. Goddard and C. Kriiger Angew. Chem. Internat. Edn. 1978,17,466. (a)K. J. Ivin J. J. Rooney C. D. Stewart M. L. H. Green and R. Mahtab J.C.S. Chem. Comm. 1978 604; (b)K. J. Ivin J. J. Rooney and C. D. Stewart ibid. p.603. S. J. McLain and R. R. Schrock J. Amer. Chem. Soc. 1978,100,1315. H. M. Colquhoun J. Holton and M. V.Twigg high-oxidation-state metal complex being involved in olefin oligomerization. Curiously niobium is inactive as a dimerization catalyst; normally second-row metals are better catalysts than their third-row counterparts. The nickel alkoxide system [N~(OR)(V~-C~H~)PP~(NE~~)~] (R= n-CltHZ3 or n-C15H31) oligomerizes isoprene to give up to 60% of the linear trimer trans-p-farnesene (6).22The crude product may then be used to synthesize sesqui- and di-terpenoids in good overall yield based on isoprene. (6) The recently prepared complexes [M(cod),] (M = Pd or Pt) catalyse the dimeriza- tion and telomerization of butadiene at lower temperatury and more selectively than previous cataly~ts.’~ The [Pd(~od)~]-catalysed reactions of butadiene with secondary amines preferably cyclic amines (e.g.morpholine piperidine) afford octa-2,7- dienylamines as pure isomers. With added phosphine this system catalyses the addition of acetaldehyde to butadiene giving 2-methyl-3,6-divinyltetrahydropyran (7) (33% yield) and the addition of phenyl isocyanate gives an isomeric mixture of piperidones (8)and (9). (7) (8) (9) A number of groups have described the telomerization of butadiene in the presence of C02 leading to interesting carboxylated products. Japanese workers using [Pd(Ph2PCH2CH2PPh2)J as catalyst obtained 5.4% of 2-ethylidenehept-5- en-4-olide (lo),but the bulk of the product was butadiene linear dimer.24 However Italian workers using Pdo catalysts derived from non-chelating phosphines report much higher levels of COz incorporation giving three products [(1l),(12) and (13)].The ratio in which (1 1) and (12) are formed depends on the steric requirement of the phosphine ligand bulky phosphines apparently favouring compound (1l),and the overall reaction is markedly accelerated by traces of 22 S. Akutagawa T. Taketomi H. Kumobayashi K. Takayama T. Someya and S. Otsuka Bull. Chem. SOC. Japan 1978 51 1158. 23 M. Green G. Scholes and F. G. A. Stone J.C.S. Dalton 1978 309. 24 Y.Inoue Y. Sasaki and H. Hashimoto Bull. Chem. SOC.Japan 1978,51,2375. ‘’ A. Musco C. Perego and V. Tartiari Inorg. Chim. Acta 1978,28 L147. Organometallic Chemistry-Part (i) The Transition Elements (13) A series of stoicheiometric insertion reactions of CO with ally1 nickel complexes has been described.26 A number of products were characterised by X-ray crystall- ography revealing a strong tendency for such compounds to form polynuclear aggregates [Equation (l)].Me,P-Ni 0 I I (1) 2c0 0 0 The oligomerization of hex-3-yne by [Ni(cod),] plus PPh2CH2CH2PPh2 in the presence of CO, affords a 57% yield of tetraethyl-2-pyrone (14) but when the Et 0 (14) chelating phosphine is replaced by PPh3 incorporation of CO is not observed only the cyclic trimer pentaethylprop-5-enylcyclopentadienebeing obtained in 66% yield.,’ 4 Carbon Monoxide Chemistry Carbony1ation.-A very mild regiospecific synthesis of hydroxybut-2-enolides (15) involves the cobalt-catalysed carbonylation of alkynes by carbon monoxide and methyl iodide under phase-transfer conditions [(Equation (2)].Yields vary from C02(Co),,CTAB RCrCH + Me1 + CO -& 5MdNaOH PhH M~ OH (15) CTAB = Cetyl trimethylammonium bromide 26 P.W. Jolly S. Stobbe G. Wilke R. Goddard C. Kruger J. C. Sekutowski,and Y. H. Tsay Angew. Chem. Internat. Edn. 1978,17,124. 27 Y.Inoue Y.Itoh and H. Hashimoto Chern. Letters 1978,633. H.M. Colquhoun J. Holton and M. V.Twigg 18% (cyclohexylacetylene) to 68% (17-ethynyltestosterone) and carbonylation of dienes under these conditions affords acetyl dienes in modest yield.28 These reac- tions probably proceed by successive insertions of CO and alkyne or diene into the Me-Co bond of [COM~(CO)~ 3 followed by hydrolysis and subsequent ring closure (alkyne) or dehydration (diene).Aromatic nitroso-compounds are carbonylated to isocyanates in the presence of rhodium and iridium carbonyls. The reaction is facilitated both by bulky ortho-substitutents in the substrate and by the presence of n-acceptor ligands in the The palladium-catalysed amidation of o-halogenophenyl alkylamines (16) provides a facile synthesis of otherwise relatively inaccessible benzolactams (17). Five- six- or even seven-membered lactams are obtained in this way in moderate yield.30 Catalytic decarbonylation of aldehydes under mild conditions has been achieved using cationic rhodium and iridium complexes with chelating phosphine ligand~.~~ Turnover numbers >100 000 are reported for the decarbonylation of benzaldehyde at 178 "C by [Rh{PPh2(CH,)2PPh,}2]+Cl-.Acyl-metal hydrides have previously been proposed as intermediates in such decarbonylation reactions and a stable acyl-rhodium(rI1) hydride complex (18) has been isolated from the reaction between 8-quinoline carboxaldehyde and [RhCl(PPh3)3] (Scheme 2).32 Decarbonylation to +[(PPh3)2Rh(CO)Cl] \/ Scheme 2 quinoline occurs only at elevated temperatures perhaps because the intermediate alkyl contains a strained four-membered chelate ring.H. Alper J. K. Currie and H. Des Abbayes J.C.S. Chem. Comm. 1978,311. 29 K.Unverferth C. Rueger and K. Schwetlick J. Prakt. Chem. 1977,319,841. 30 M. Mori K. Chiba and Y.Ban J. Org. Chem. 1978,43 1684. 31 D.H.Doughty and L. H. Pignolet J. Amer. Chem. SOC.,1978 100,7083. 32 J. W.Suggs J. Amer. Chem. SOC.,1978,100,640. 105 Organometallic Chemistry-Part (i) The Transition Elements The reactions of several relatively stable alkyl-metal carbonyls with hydrogen and carbon monoxide at elevated temperature and pressure have been used as models for intermediates in hydr~formylation.~~ However only [CH,Mn(CO),] could be carbonylated and subsequently hydrogenated to the aldehyde. Interestingly the manganese product was not [HMn(CO)5] but [Mn2(CO)lo]. The role of monophosphine ligands in rhodium-catalysed hydroformylation has been extensively studied but little has been done on the effect of cis-chelating pho~phines.~~ This has been corrected by an extensive study using bidentate phosphines including a polymer-bound rhodium 1,2-bis(diphenylphosphino)ethane complex.Catalysts containing bidentate phosphines are more active than those with PPh3. However selectivity for linear aldehyde is considerably lower and isg- merization activity is much higher. A supported catalyst of high activity is obtained by coating phosphinated polystyrene containing [RhCl(CO),P] onto a high-surface- area silica. Activity is approximately proportional to the surface area of the silica.35 The Reppe modification of hydroformylation uses water in place of molecul r hydrogen. In this process iron pentacarbonyl is a good catalyst although in the normal procedure using molecular hydrogen it is a poor catalyst.This is now rationalized in terms of Scheme 3 where the key intermediate [H2Fe(C0)4] is more easily formed from [Fe(CO)5] and water than from molecular hydrogen.36 Scheme 3 Ruthenium and rhodium carbonyl clusters also catalyse this reaction and it is that cluster rather than monomeric species are the catalytically active intermediates. Ruthenium systems have exceptionally high selectivity for linear aldehydes which subsequently undergo aldol condensation in the basic solution used. This does not happen with the less selective rhodium systems because reduction of the first-formed aldehyde to alcohol takes place rapidly. Carbon monoxide similarly reduces aldehyde to alcohol in the presence of rhodium trichloride. However when triethylamine is added aldol condensation takes place more rapidly than reduction with the result that the major product is the branched alcohol from reduction of the aldol reaction Carbon monoxide and water also reduce aromatic nitro-compounds to amines rhodium iridium osmium and iron carbonyls are effective catalysts in the presence of trieth~larnine.~~' The carbon monoxide-water system catalysed by rhodium trichloride will reduce Schiff bases to form N-alkyl-amine~.~~~ 33 R.B. King A. D. King M. Z. Iqbal and C. C. Frazier J. Amer. Chem. Soc. 1978 100 1687. 34 C. U. Pittman and A. Hiras J. Org. Chem. 1978 43 460. 3s H. Arai J. Catalysis 1978 51 135. 36 H.-C. Kang C. H. Mauldin T. Cole W. Slegeir K. Cann and R. Pettit J Amer. Chem. Soc. 1977,W.8323. 3'7 (a)R. M. Laine J. Amer. Chem. Soc. 1978,100,6451; (b)Y. Watanabe K. Takatsuki. and Y. Takegami Tetrahedron Letters 1978 3369; (c)K. Cann T. Cole W. Slegeir and R. Pettit J. Amer. Chem. Soc. 1978,100,3969; (d)Y. Watanabe M. Yamamoto T.-A. Mitsudo and Y. Takegami TetrakedronLetters 1978,1289. 106 H. M. Colquhoun J. Holton and M. V. Twigg Synthesis Gas Chemistry.-Selective conversion of carbon monoxide and hydrogen into alkanes alcohols etc. is an area of growing interest because it may provide a basis for future large-scale processes based on gasified coals and heavy oils. Surface- bound formyl is thought to be an intermediate in many of these heterogeneously catalysed reactions and it is therefore relevant that a number of metal formyl complexes have now been characteri~ed.~~ Several groups are examining the homogeneously catalysed formation of hydrogen from water and carbon monoxide It is apparent that under appropriate conditions most metal carbonyls will catalyse this reaction and not just those of the precious metals.However in spite of much work this year there remain many mechanistic uncertain tie^.^' In dioxan solutions of [Co2(CO),] at 182"C,carbon monoxide is hydrogenated by molecular hydrogen to methanol some higher alcohols and their formate Under the experimental conditions the major cobalt species is [HCo(CO),]. Ano- ther instance of insertion of carbon monoxide into a metal hydride bond provides the first example of a stable metal-carbon bond from this type of reaction.The reaction of [(C,H,),Zr(H)Cl] with carbon monoxide gives [{(C5H5)2ZrC1}2CH20] in which the CH,O group is C- and O-bonded to different metal atoms. Carbon dioxide reacts with [(C5H5),Zr(H)Cl] to form bound methoxide and formaldehyde as in equations (3) and (4); by keeping CO :Zr <1:3 only methoxide is formed.,' 2(C5H&Zr(H)Cl +COz +[(C5H5)2ZrC1]20+CH20 (3) 3(C5H5)2Zr(H)Cl+C02 +[(C5H5)ZrC1I20 +(C5H5)Zr(OMe)Cl (4) The related compounds [(C,Me,),MMe,] (M =Zr Th or U) react with carbon monoxide to form unusual dicarbonylation products. In the case of M =Th reaction at -80 "C (!)quantitatively affords (19) which has been characterized by X-ray Me Me \/ /c=c\ 0 0 '0 0-\/ c=c Me/\ Me (19) Homologation of dimethyl ether to ethyl acetate by reaction with carbon monox- ide and hydrogen is catalysed by [RuI,(CO),] and iodide.Contrary to what is D. A. Slack D. L. Egglestone and M. C. Baird J. Organometallic Chem. 1978 146,71; J. A. Gladysz and J. C. Selover Tetrahedron Letters 1978,319; C. P. Casey and S. M. Neumann J. Amer. Chem. SOC. 1978 100,2544; J. A. Gladysz and W. Tam ibid. p. 2545; J. A. Gladysz and J. H. Merrifield Inorg. Chim. Ada 1978 30 L317. 39 R. B. King C. C. Frazier R. M. Hanes and A. D. King J. Amer. Chem. SOC.,1978 100,2925; T. Yoshida Y. Ueda and S. Otsuka ibid.,p. 3941; P. C. Ford R. G. Rinker C. Ungermann R. M. Laine V. Landis and S. A. Moya ibid. p. 4595 C.-H. Cheng and R. Eisenberg ibid. p. 5968. 40 J. W. Rathke and H. M. Feder J. Amer. Chem.SOC. 1978,100,3623. 41 G. Fachinetti C. Floriani A. Roselli and S. Pucci J.C.S. Chem. Comm. 1978,269. 42 J. M. Manriquez D. R. McAlister R. D. Sanner and J. E. Bercaw I. Amer. Chem. SOC. 1978,100,2716; J. M. Manriquez P. J. Fagan T. J. Marks C. S. Day and V. W. Day ibid.,p. 71 12. Organometallic Chemistry-Part (i) The Transition Elements observed in related systems methyl iodide does not seem to be involved in this homologation and it appears that protonated dimethyl ether reacts with an anionic ruthenium complex to form a methyl-ruthenium intermediate.43 5 Reactions of Co-ordinated Ligands Fischer-type carbene complexes such as [Cr(CO),(MeCOMe) J react with iso- cyanides to give ketenimine complexes which liberate free ketenimines on treat- ment with excess isocyanide.Reactions of the latter complexes with acids methanol and water produce a variety of aminocarbene-chromium derivative~.~~ Methoxycarbene-chromium complexes are known to react with nucleophilic alkynes such as ynamines or ynediamines and it has now been shown4' that the reaction product of [Cr(CO),(PhCOMe)] with Et,NCrCNEt undergoes decar- bonylation and rearrangement on heating to 125 "C to give (20) the chromium tricarbonyl complex of a functionalized indene (Scheme 4). H OMe OMe Scheme 4 Arene ligands may be displaced from chromium tricarbonyl by refluxing in pyridine the complex [Cr(CO)3(py)s J being recovered in high yield.46 This pyridine derivative may itself be used for the synthesis of arene-Cr(CO) complexes and so the potential usefulness of such compounds in organic synthesis is significantly increased.The propargylation of carbanions by cationic propargyl-Co,(CO) complexes gives high yields by avoiding the formation of the allenic elimination and other unwanted by-products produced when propargyl halides or tosylates are used in such reactions. Oxidative demetallation of the products with iron(II1) affords almost quantitative yields of free alk~ne.~' Dimethyl acetylenedicarboxylate reacts with cum-dodecatrienediylnickel (21) to give after demetallation with CO a mixture of 12- and 14-membered-ring products 43 G. Braca G. Sbrana G. Valentini G. Andrich and G. Gregorio J. Amer. Chem. Soc. 1978,100,6238. 44 C.G.Kreiter and R. Aumann Chem. Ber. 1978,111 1223.45 K. H.Dotz and D. Neugebauer Angew. Chem. Internat. Edn. 1978,17,851. 46 G. Carganico P. D. Buttero S.Maiorana and G. Riccardi J.C.S. Chem. Comm. 1978,989. 47 H. D.Hodes and K. M. Nicholas Tetrahedron Letters 1978,4349. H. M. Colquhoun J. Holton and M. V.Twigg in proportions which vary according to the reaction tem~erature.~~ The correspond- ing reaction with methyl propiolate however gives only the 12-membered product (22) regardless of temperature. Phosphine-induced ring-opening of penta-arylcyclobutenylpalladium(rr) complexes proceeds stereospecifically in the expected conrotatory manner to give cr-butadienyl complexes. Cyclobutenylpalladium thiocarbamates however also undergo spontaneous ring-opening to give a mixture of conrotatory and disrotatory ring-opened cr,r-butadienyl derivatives.A mechanism involving the equilibration of these two isomers via a metallocyclopentenyl intermediate has been proposed to account for the appearance of the thermally forbidden disrotatory ring-opened product .49 The synthetic possibilities of homogeneous C-H bond activation continue to be of interest. The reaction of bis(ethylene)(q5-indenyl)rhodium(~) with but-2-yne affords two crystalline dinuclear complexes [(23) and (24)] in one of which an ethylenic C-H bond has been cleaved to generate a vinyl-bridged dirhodium species (24). This complex [but not (23)] is an effective catalyst for alkyne trimerization. It is also readily carbonylated to give dicarbonyl (q’-indeny1)rhodium and the a@-unsaturated ketone (25).” 6 Asymmetric Synthesis of Carbon-Carbon Bonds Catalytic methods are now well established for the creation of chirality by the synthesis of C-H bonds with optical yields of not uncommon.Correspond- ing reactions to form C-C bonds have been much less successful in the past but a number of promising reports have appeared this year. 48 R. Baker P. C. Bevan R. C. Cookson A. H. Copeland and A. D. Gribble J.C.S. Perkin I 1978,480. 49 S.H.Taylor and P.M. Maitlis J.Amer. Chem.SOC.,1978,100,4700; P.M. Bailey S. H. Taylor and P. M. Maitlis ibid. p. 4711. 50 P.Caddy M. Green L. E. Smart and N. White J.C.S. Chem. Comm. 1978 839. Organometallic Chemistry-Part (i) The Transition Elements 109 Nakamura and co-workers have described extensive synthetic and mechanistic studies of olefin cyclopropanation by alkyl diazoacetate esters.’l Chiral oic-dioxi- matocobalt(I1) complexes when used as catalysts produce in high optical yields (up to 88%)approximately equal amounts of cis-and trans-cyclopropane carboxylates [Equation (5)].Kinetic and other mechanistic data indicate that the reaction Ph CO,R 81% Opt. yield Co(a-CQD) PhCH=CH2 + N2CHC02R 4+ (5) HZO &ozR88Y~ Opt. yield R = neopentyl Ph CQD = ( -)camphorquinone-a -dioximate proceeds by C-co-ordination of diazoacetate to cobalt(II) loss of N2 to give a Co” complex and attack by olefin on the co-ordinated carbene with formation of a cobaltacyclobutene followed by decomposition to release the cyclopropane product.The catalytic asymmetric allylic alkylation of carbanions in the presence of Pdo (diop) was first described in 1977 and gave optical yields in the range 35-45% starting from a racemic mixture.52 A related series of reactions has been de~cribed,’~ involving the catalytic transfer of an ally1 group from allylic ethers or esters to ‘active hydrogen’ compounds (see Miscellaneous Section). Using a palladium(0)-diop catalyst optical yields of ca. 10% were obtained. Oxidative ring closure of (26)by palladium(I1) acetate in the presence of a catalytic amount of P-pinene as the source of chirality gives optically active (27) (12% optical yield) together with a small amount of (28). Cyclization is not observed however when P-pinene is present in large excess.54 (26) (27) (28) Palladium-catalysed hydroesterification of a-methylstyrene using chiral diben- zophosphazole ligands in propan-2-01 gives optical yields of ca.40% at up to 83% conversion of olefin [Equation (6); R = Pri]. In Bu‘OH however -69% optical yield is obtained but at only 8% conversion.55 Ph Ph 0 II \C=CH2 + CO + ROH -+ \C*-CH2COR (6) Me/ Me’ ‘H ” A. Nakamura A. Konishi R. Tsujitani M. Kudo and S. Otsuka J. Amer. Chem. SOC. 1978,100,3449; A.Nakamura A. Konishi Y. Tatsumo and S. Otsuka ibid. p. 3443;A. Nakamura Pure Appl. Chem. 1978 50 37. 52 B. M. Trost and P. E. Strege J. Amer. Chem. SOC.,1977,99 1649. 53 J. C.Fiaud A. H. de Gournay M. Larcheveque and H. B. Kagan J. Organometallic Chem. 1978,154 175. ’‘ T.Hosokawa S.Miyagi S. I. Murahashi and A. Sonoda J.C.S. Chem. Comm. 1978,687. 55 T. Hayashi M. Tanaka and I. Ogata Tetrahedron Letters 1978,3925. 110 H. M. Colquhoun J. Holton and M. V.Twigg 7 Metal Clusters In Catalysis Application of metal carbonyl clusters as catalysts for organic reactions continues to be an area of considerable interest. This year most effort has been directed towards methods of preparing heterogeneous systems and reactions of organic nitrogen compounds. Heterogeneous Systems.-Gates and co-workersS6 used poly(styrene4ivinylben- zene) functionalized with PPh2 groups to form polymer-bound catalysts containing characterized iridium and rhodium clusters. Monosubstituted clusters of the type [IT~(CO)~~(P~~P polymer)] were generated by reduction of [Ir(C0)2( p-toluidine)Cl] in the presence of CO and phosphinated polystyrene containing a low concentration of donor groups.The disubstituted derivative was obtained when polymer contain- ing a high concentration of phosphine groups was used. The fomer is an active hydrogenation catalyst but it readily forms aggregated Interaction of phosphinated polymer with a solution of [Rh6(CO)16] in benzene leads to incorpora- tion of the cluster into the which then has an i.r. spectrum similar to that of [Rh6(C0)13(PPh3)3] in the carbonyl region. The polymer-bound cluster is an active hydrogenation catalyst but it is sensitive to oxygen. Oxidation of the phosphine groups to oxide takes place and the unco-ordinated metal agglomerates to form interesting small uniform-sized metal crystallites.Clusters bound to a polymer with a very high ratio of phosphorus to metal have low catalytic activity due to inhibition by excess phosphine groups but this activity remains even after heating in air for several days at 125 "C. Appealingly simple procedures have been devised for anchoring tri-osmium clusters to silica by organic ligands and results of reactivity studies are awaited.s7 Zeolite has been used as a novel support for an incompletely characterized rhodium cluster which was generated in sit~.'~This is a good catalyst for liquid-phase hydroformylation of olefins. Rhodium loss from the catalyst appears to be low. Ichikawas9 obtained active catalysts for methanol synthesis from carbon monoxide and hydrogen by decomposing rhodium carbonyl clusters dispersed on zinc oxide and other supports.Activity depends on the nature of the cluster and significantly the selectivity of these catalysts is higher than that prepared from rhodium tri- chloride which produces mainly methane. Activity and selectivity also depend on the support used. Catalysts prepared from rhodium carbonyl clusters on ZnO MgO BeO and CaO selectively produce methanol. Ethanol and other oxygenated C2 species were obtained with La203 Ce02 Ti02 Zr02 and Tho2. Methane is selectively formed in low conversions over Sn02- V2OS-,and P20s-supported catalysts. Reactions involving Organic Nitrogen Compounds.-Small quantities of [Os,(CO),,] or [H40~4(C0)12] catalyse the conversion of refluxing NMe2Ph into (4-NMe2C6H4)2CH2.Suppression of the reaction by carbon monoxide indicates that co-ordination of the amine to an osmium atom is an important step. A similar " (a)J. J. Rafalko J. Lieto B. C. Gates and G. L. Schrader J.C.S. Chem. Comm. 1978 540; (b) M. S. Jarrell B. C. Gates and E. D. Nicholson J. Amer. Chem. Soc. 1978,100,5727. s7 S. C. Brown and J. Evans J.C.S. Chem. Comm. 1978 1063. E.Mantovani N. Palladino and A. Zanobi J. Mol. Catalysis 1977178,3 285. 59 M.Ichikawa Bull. Chem. SOC.Japan 1978 51,2268 2273. Organometallic Chemistry-Part (i) The Transition Elements reaction takes place with NHMePh but this is complicated by disproportionation to aniline and its dimethyl derivative.60 Nitrobenzene and meta- or para- substituted analogues react with [CO,(CO)~] in refluxing benzene to form azobenzene in moderate yield.61 The cluster [C04(CO)gC6H6] is formed during the reaction.A possible intermediate [Co,(CO),,] also reacts but no [C04(CO)gC6H6] is formed. In contrast to [Co,(CO),] which catalyses the deoxygenation of diphenylketen [Rh4(C0)12] in the presence of carbon monoxide at about 200 "C catalyses the insertion of keten into an arene hydrogen-carbon bond giving Ph,CHCOAr.62 Benzene also reacts under these conditions with para-substituted isocyanates to give benzanilides in moderate yield. Brief details of the first homogeneous catalytic hydrogenation of an isocyanide have been The reaction of [Ni(CNCMe,),] (a non-reactive source of CNCMe3) at 90 "C with hydrogen in the presence of [Ni,(CNCMe,),] gave 99% of Me,CNHMe.Other isocyanides behave similarly and the NiL4-Ni4L7 system can be conveniently generated in situ from [Nqcod),] and isocyanide. The reaction of [H20~3(C0)10] with phenyl isocyanide gives [HOs3(CO),(CHNPh)] whose X-ray structure (29) unfortunately does not locate the bridging hydride ligand. This (29) complex contains an N-phenyl formidoyl ligand bridging three metal atoms. It is possible that this type of bonding pattern represents a step in the cluster-catalysed hydrogenation of is~cyanides.~~~ An interaction involving two carbon atoms and a nitrogen atom with three rhodium metal centres on a face of an octahedral Rh6 unit (30)is proposed to explain Et H I H (30) catalysed specific deuterium exchange in triethylamine.Exchange of methyl hydro- gens and one methylene hydrogen in a single ethyl group takes place when 'O C. Choo Yin and A. J. Deeming J. Organometallie Chem. 1978,144,351. 61 H. Alper and H.-N. Paik J. Organometallic Chem. 1978 144 C18. " P. Hong H. Yamazaki K. Sonogashira and N. Hagihara Chem. Letters 1978 535. " (a)E. L. Muetterties Pure Appl. Chem. 1978,50,941; (6)R. D. Adams and N. M. Golembeski J. Amer. Chem. Soe. 1978,100,4622. 112 H,M. Colquhoun J. Holton and M. V.Twigg triethylamine is heated with D20in the presence of [Rh,(cO),,] under an atmos- phere of carbon monoxide.64 8 Olefin Metathesis The high level of interest in olefin metathesis of previous years has not been maintained. This is largely due to its limited application resulting from failure to devise catalyst systems that are not inhibited by most common functional groups.Most publications this year are concerned with mechanistic aspects and there are indications that the now familiar carbene chain mechanism may be challenged. Few soluble catalyst systems have been successfully supported. However poly(styry1bipyridine) has been used to prepare catalysts for the metathesis of internal ~lefins.,~ In conjunction with AlEtCl, polymer-bound [W(CO),(bipy)] is a better catalyst than the molybdenum compound and both are an order of magnitude more active than the unsupported systems. Moreover they have the advantage of being easily recovered by filtration from reaction mixtures and in some circum- stances they can be re-used.A study of the reaction between [W(CO),L] (L=PPh or PBun3) and A1Br3 in chlorobenzene provides some insight into the role of cocatalyst in methathesis. The first formed 1 1adduct in which aluminium interacts with carbon monoxide trans to the phosphorus ligand subsequently forms a relatively stable zerovalent co-ordina- tively unsaturated species.66 The first example of a stable complex in which the metal is bonded to an olefin and carbene ligands has been rep~rted.~’ It would be of interest to know if this complex (31) is active in olefin metathesis since such a bonding pattern may be a transitory species in the carbene chain mechanism. An intriguing report concerns ring-opening polymerization of cycloalkenes (a special case of metathesis68).Under particular conditions a frequently used cocata- lyst AlEtCl, can induce the formation of ring-opened polymers and oligomers of norbornene similar to those previously observed in the [WCl,]-initiated reaction. In order to account for their observations the authors*’’ suggest the generation of a carbene intermediate (32) and its involvement in a chain reaction. OMe AlCl It has been proposed20a that the carbene/metallocyclobutane chain mechanism might be common to oligomerization Ziegler-Natta polymerization and ole fin 64 R. M. Laine D. W. Thomas L. W. Cary and S. E. Buttrill J. Amer. Chem. SOC.,1978 100 6527. 65 S. Tamagaki R. J. Card and D. C. Neckers .I. Amer. Chem. SOC.,1978,100,6635. 66 Y. B. Taarit J.L. Bilhou M. Lecomte and J. M. Bassett J.C.S. Chem. Comm. 1978 38. 67 W. Priester and M. Rosenblum J.C.S. Chem. Comm. 1978,26. 68 J. J. Rooney and A. Stewart in ‘Catalysis’ ed. C. Kemball (Specialist Periodical Reports) The Chemical Society London 1977 Vol. 1 p. 277. Organometallic Chemistry-Part (i) The Transiiion Elemen, metathesis (see Section 3) and it will be of interest to see whether support for this novel hypothesis is forthcoming during next year. Equilibria between bis-olefin complexes and metallocyclopentanes a key step in a now superseded olefin metathesis mechanism may be more general than previously thought. Labelling experiments have shown that evolution of ethylene from nickel and titanium metallocyclopentanes proceeds via a bis-olefin intermediate.69 The reaction involves initial loss of PPh3 (addition of excess PPh decreases the reaction rate) so that the process does not involve 20-electron intermediates.An appealing application of olefin metathesis from the patent literature” provides a route to styrene from toluene via oxidative coupling of toluene giving stilbene which undergoes metathesis with ethylene to give the desired product [Equation (7)]. lo1 2PhCH3 -+ PhCH=CHPh C2H4 2PhCHzCHZ (7) 9 Isomerization Rhodium-trichloride-catalysed migration of double bonds to form conjugated systems is well established. None-the-less isomerization of the cyclohexenone (33) HOIo^c 0m-. (33) and related molecules to phenols uia remote double-bond migration catalysed by rhodium trichloride in ethanol is surprising.Migration of remote olefinic bonds in the corresponding conjugated imines to form substituted anilines is also catalysed by this reagenf.’l The first stereoselective isomerization of allyl ethers has been Air-stable [Ir(~od)(PMePh~)~]+ after ‘activation’ with hydrogen rapidly catalyses the isomerization of primary allyl ethers to the corresponding trans-propenyl ether at room temperature (Scheme 5). Yields and stereoselectivity are exceptionally high. R2 R2 R2 RI+OR~ - R~+oR’ -+R’,-A,,oR~ ’I‘ H [IrHI H R’,R2 = Hor Me Scheme 5 At high temperatures (180-210 “C),tris(acetylacetonato)ruthenium(m)has been shown to be an effective catalyst for cis-trans isomerization of trisubstituted iso-prenoid olefins.Selectivity is high and no hydrogenation or double-bond migration takes place.” 69 R. H. Grubbs and A. Miyashita,J. Amer. Chem. SOC.,1978,100 1300. 70 P. D. Montgomery R. N. Moore and W. R. Knox U.S. P. 3 965 206 (1976) (Chem. Abs. 1976 85 123 540). 71 P. A. Grieco and N. Marinovic Tetrahedron Letters 1978 2545. 72 D. Baudry M. Ephritikhine and H. Felkin. J.C.S. Chem. Comm. 1978,694. ‘3 Y. Fujita Chem. Letters 1978 533. H. M. Colquhoun J. Holton and M. V.Twigg Incorporation of palladium acetate into poly(styry1)bipyridine followed by reduc- tion with LiAlH4 in THF produces an effective catalyst for the isomerization of quadricyclene to norbornadiene. The reduced species is considerably more active than palladium on charcoal but the unreduced catalyst is ~nreactive.~~ 10 Syntheses involving Iron Carbonyls Useful developments have been made in the use of iron carbonyls in organic synthesis.Italian have demonstrated that the [FeH(C0)4]- anion rapidly and quantitatively exchanges with ion-exchange resin. The exchanged resin in refluxing THF smoothly converts alkyl halides into homologous aldehydes while a-bromo carbonyl compounds aromatic and certain other bromides are dehalo- genated. Work-up of products from these high-yield reactions is simplified by the iron compounds remaining bound to the polymer which is recovered by filtration. A solution of Na[HFe,(CO),] and acetic acid in THF is a mild selective reagent for reduction of olefinic bonds in a range of ap-unsaturated carbonyl compounds.The dimeric carbonyl hydride is prepared in two steps from Na2[Fe(C0)4].76 An almost quantitative one-flask synthesis at room temperature of analytically pure K,[Fe(CO)J from [Fe(CO)J and K[Bu,BH] has been described.77 The potassium salt unlike the more common sodium salt is not spontaneously flammable in air. An important range of carbocyclic compounds with an odd number of carbon atoms rather than the even-numbered units produced by most conventional pro- cedures are obtained from [Fe2(CO)g]-induced reactions of aa’-dibromo-ketones with olefins. Full details of these procedures have been p~blished,~ which are based on trapping the three-carbon oxyallyl-iron intermediate formed from a bromo- ketone and [Fe2(CO)g]. Thus cycloheptenones (34) are obtained from coupling a variety of 1,3-dienes with polybromo-ketones so providing a flexible route to R Ar RA RE (35) Scheme 6 74 R.J. Card and D. C. Neckers J. Amer. Chem. SOC.,1977,99,7733; J. Org. Chem. 1978,43 2985. 75 G.Cainelli F. Maneschalchi A. Umani-Ronchi and M. Panunzio J. Org. Chem. 1973 43 1598. 76 J. P.Collman R. G. Finke P. L. Mallock R. Wahren R. G. Komoto and J. I. Brauman J. Amer. Chem. SOC.,1978,100,1119. 77 J. A. Gladysz and W. Tan J. Org. Chem. 1978 43 2279. 78 Y. Hayakawa F. Shinizu and R. Noyori Tetrahedron Letters 1978,993;R.Noyori Y.Hayakawa H. Takaya S. Murai R. Kobayashi and N. Sonoda J. Amer. Chem. SOC.,1978,100 1759; and following papers. Organometallic Chemistry-Part (i) The Transition Elements 115 tropone alkaloids from readily available materials [Scheme (6)J In a similar fashion aryl-substituted olefins give arylcyclopentanones (35).Variations of these reactions provide a wide range of products many via single-flask procedures and it is to be expected that these methods will be much used in the future. 11 Miscellaneous Catalytic Reductions with Formate Ion.-Reports from several groups are concer- ned with the use of formate ion in selective catalytic hydrodehalogenation reductive coupling of aryl halides to biaryls and reduction of aromatic nitro-compounds to amines. These simple reactions are carried out in an open vessel and if necessary exact amounts of reducing agent (formic acid) can be measured allowing in some instances selective reduction.When a heterogeneous catalyst is employed it is easily isolated from the product mixture and can be re-used. With excess formic acid and a relatively large amount of palladium on charcoal aromatic nitro-compounds are quickly reduced to amine in high yield." No reaction takes place if the nitro-compound contains a halogen other than fluorine. Indeed addition of halide ion prevents reaction and it is suggested that halides poison the catalyst. Presumably for the same reason compounds containing divalent sulphur fail to react. However nitro-groups in chlorine-containing compounds are reduced without removal of halogen by phosphinic or phosphorous acid in the presence of palladium on charc~al.'~ In DMF aryl bromides are dehalogenated by methoxide ion in the presence of [Pd(PPh3)J but this procedure is improved by using formate ion.The proposed" mechanism of the latter reaction involves oxidative addition of aryl bromide to a co-ordinatively unsaturated palladium(0) species exchange of bromide for formate ion subsequent loss of carbon dioxide from co-ordinated formate to give a hydride followed by reductive elimination of ArH (Scheme 7). ArBr -L 'J co2 Scheme 7 When small amounts of palladium on charcoal are used the rate of reaction is markedly enhanced by a large excess of triethylamine.81," With this reagent substituents clearly influence the reaction. For instance the double bond in methyl 4-chlorocinnamate is reduced at a rate comparable with halogen reduction whereas methyl cinnamate does not react.Reduction and cyclization of o-nitrocinnamate gives the saturated lactam (36)in good yield. 79 I. D. Entwistle A. E. Jackson R. A. W. Johnston and R. P. TeIford J.C.S. Perkin I 1977,443. 8o A. Zask and P. Helquist,J.Org. Chem. 1978 43 1619; P. Helquist Tetrahedron Letters 1978 1913. (a)N. A. Cartese and R. F. Heck J. Org. Chem. 1977,42 3491; (6)ibid. 1978,43,3985. H.M. Colquhoun J. Holton and M. V. Twigg The nature of the catalyst also influences the selectivity of the reaction. o-Bromonitrobenzene is dehalogenated by formic acid and triethylamine in the presence of palladium on charcoal whereas reduction of the nitro-group to amine with retention of halogen takes place with platinum on charcoal.81Q The formic acid-triethylamine-palladium system is also effective for reducing ap-unsaturated carbonyl compounds to the saturated carbonyl compound.For example the enone (38)was obtained in 69% yield from the conjugated dienone /3 -ionone (37). Insome instances conjugated dienes and acetylenes can be selectively reduced to mono- enes.'lb Reduction of an aryl halide with aqueous alkaline sodium formate in the presence of palladium on charcoal and a surfactant produces biaryls in moderate yield. Nitro-groups present in the aryl halide are reduced to amine in the biaryl. The role of the surfactant is important since its nature can influence the amount of product.82 It may be expected that further work on these interesting and synthetically important reactions will be reported during the coming year.Palladium-catalysed Reactions.-Palladium is the most commonly used transition metal in organic synthesis particularly in C-C bond-forming reactions. A new general synthesis of ketones from acid chlorides by palladium-catalysed coupling with organotin compounds has been rep~rted.'~ This method [Equation (Sj] is [PdCI(CHzPh)(PPh3)zl R'COCI + RiSn b R1COR2+ R:SnCl (8) synthetically attractive because yields are high sensitive functional groups (e.g. aldehyde) are tolerated no inert atmosphere is required and reaction times are short. A short review on the use of mono-organopalladium(I1)derivatives [Pd(R)L,X] in organic synthesis has a~peared.'~ The most useful reaction [Equation (9)] is vinylic substitution using organic halides (e.g.aryl heterocyclic benzylic and vinylic bromides or iodide^).'^ P. Banfield and P. M. Quan Synthesis 1978,537;see also U.K. P. 1 457 608 (1976) (Chem.Abs. 1977 87,22 732); 1 458 633 (1976) (Chem.Abs. 1976,84 164 376). 83 D. Milstein and J. K. Stille J. Amer. Chem. SOC.,1978 100 3636. 84 R. F. Heck Pure Appl. Chem. 1978 50,691. 85 J. E. Plevyak and R. F. Heck J. Org. Chem. 1978 43 2454; C. B. Ziegler and R. F. Heck ibid. pp. 2941,2949; W. C. Frank Y. C. Kim and R. F. Heck ibid. p. 2947; N. A. Cortese C. B. Ziegler B. J. Hrnjez and R. F. Heck ibid. p. 2952. Organometallic Chemistry-Part (i) The Transition Elements H R3 R1 R3 Common functional groups other than ap-unsaturated carbonyls are unaffected by the reaction.Carbonyls may be protected by ketal or acetal formation. An extensionof this reaction provides convenient syntheses of 3-alkyl-pyridine~,'~"and 2-(3'-0xoalkyl)-furans'~~ from allylic alcohols. Palladium-catalysed allylic alkyl- ation of olefins has been further studied. .rr-Allylpalladium complexes from a variety of olefins have been prepared and characteri~ed,'~" and their reactions with nucleo- phile The effect of ligands (phosphines and phosphites) on product selectivity is also This reaction has been applied in steroid synthesis to obtain stereocontrol in the partial synthesis of 5a -ch~lestanone.'~~ A supported system Pdo on either phosphinated silica gel or phosphinated polystyrene has shown an increase in regioselectivity over homogeneous systems especially for nitrogen nucleophiles (e.g.Et,NH) and may enhance the scope of the Direct catalytic allylic alkylation of olefins uses [PdC12(CH3CN)2] or [Pd(.rr-allyl)C1]2,88 but unfortunately this new reaction is slow catalyst turnover is poor and considerable decomposition of the catalyst occurs during the reaction. Full experimental details have appeared on the palladium-assisted intramolecular amination of olefins to produce indoles quinolines and is~quinolines.'~ The cyclization reaction was achieved catalytically using benzoquinone as oxidant {Pdo-+ Pd"}. Hydrozirconation.-The hydrozirconation/protonationof vitamin D is reported to be superior to previously described hydroboronation procedures for regio-specific reduction of the 10(19)-double bond."" The usefulness of hydrozirconation in organic synthesis is extended by the discovery that alkenyl-zirconium complexes derived from alkynes and [Zr(C,H,),(H)Cl] may be coupled with unsaturated organic halides in the presence of catalytic quantities of Nio or Pdo complexes to give high yields of conjugated dienesgob [Equation (lo)].This coupling reaction is R2 H \/ 86 (a)Y. Tamaru Y. Yamada and 2.Yoshida J. Org. Chem. 1978,43,3396;(6) Y. Tamaru Y. Yamada and Z. Yoshida Chem. Letters 1978 529. *' (a)B. M. Trost P. E. Strege L. Weber T. J. Fullerton andT. J. Dietsche J. Amer. Chem.Soc. 1978,100 3407; (b)ibid.,p. 3416; (c) ibid.,p. 3426; (d)B. M. Trost and T. R. Verhoeven ibid.,p. 3435; (e)B. M. Trost and E. Keinan ibid. p. 7779.88 L. S. Hegedus T. Hayashi and W. H. Darlington J. Amer. Chem. Soc. 1978,100,7747. 89 L. S. Hegedus G. F. Allen J. J. Bozell and E. L. Waterman J. Amer. Chem. Soc. 1978 100 5800. 90 (a)A. W. Messing F. P. Ross A. W. Norman and W. H. Okamura Tetrahedron Letters 1978,3635;(b) N. Okukado D. E. Van Horn W. E. Klima and E. Negishi ibid.,p. 1027; (c)E. Negishi N. Okukado A. 0.King D. E. Van Horn and B. I. Spiegel J. Amer. Chem. Soc. 1978,100 2254. H.M. Colquhoun J. Holton and M. V.Twigg extremely sluggish when a disubstituted alkenyl group is present in the zirconium complex (probably for steric reasons) but addition of catalytic amounts of ZnC12 or CdC12 facilitates the reaction and high yields of coupled products are Alkenyl-zirconium complexes do not react with ap-unsaturated ketones but on addition of a catalytic amount of [Ni(acetylacetonate)J rapid conjugate addition of the terminal vinylic units occurs giving after hydrolysis good yields of a& unsaturated ketones.91a The initial product of conjugate addition is a zirconium 0-enolate and such compounds react with formaldehyde to give a-(hydroxy-methyl)-cycloalkanones [Equation (1l)],a reaction recently used in the synthesis of prostaglandin derivatives." ,Zr(C5H5)2c1 0 0 Aldehydes and ketones are rapidly reduced to alcohols by [Zr(C5H5),(CI)BH4] in benzene whereas carboxylic acids esters nitriles and nitro-compounds react much more slowly so that selective reactions are possible.92 Since the reactivity of this reagent is similar to that of NaBH, its major advantage appears to be the ability to carry out reductions of non-polar substrates which are insoluble in alcoholic media.91 (a)M.J. Loots and J. Schwartz J. Amer. Chem. SOC.,1977,99,8045;(b)M.J. Loots and J. Schwartz Tetrahedron Letters 1978,4381. 92 T. N. Sorrell Tetrahedron Letters 1978 4985.

ISSN:0069-3030    

DOI:10.1039/OC9787500097

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

6 OrganometalIic Chemistry Part (ii) Main-group Elements By M.G. HUTCHINGS I.C.I. Organics Division Research Department Blackle y Manchester M9 3DA 1 Introduction Although no one single contribution has been particularly noteworthy this year a few emerging trends are worthy of mention. New synthetic approaches to hydro- metallation of alkenes using Mg derivatives and a development based on hy-drosilation have opened new areas for research. The modification of the synthetic behaviour of one organometallic by a second has long been known but the increasing application of this technique or even combinations involving three organometallics emphasizes the potential value of the method. Moreover resulting polymetallated species containing sites of differing reactivity are becoming more common.Further synthetic exploitation of such molecules can be anticipated in the near future. The increasing attention devoted in recent years to the traditionally less popular elements (especially A1 and Sn) continues in 1978. The whole of a recent volume is devoted to the rearrangements of organometallic compounds. Contributions concerned with main-group elements include 1,2-anionic rearrangements of Si and Ge compounds,1n dyotropic rearrangements and related 0-0 exchange processes," rearrangements of B and A1 compounds and their Group I11 analogues,'csd organomagnesium rearrangements,le and aryl migra- tions in organometallic compounds of alkali metals." The last topic is also reviewed elsewhere.' Two other reviews on Group I derivatives deal with the reactivity of carbanion~~~ and dipole-stabilized car bani on^.^' Reviews on Group I1 elements include the less familiar reactions of organocadmium reagent^,^ and several concerning organomercury chemistry a new book containing in particular a chapter on oxymerc~ration,~~ the application of organomercurials in organic ~ynthesis,~' an account of the aqueous solution chemistry of MeHg" of particular environmental and ecological ~ignificance,~' and a Tetrahedron Report which also discusses some aspects of organotin hemi is try.'^ The following reviews are of particular interest from the point of view of organic synthesis organoborates as (a)R.West Ado. Organometullic Chem. 1977,16 1; (b)M. T. Reetz ibid. p. 33; (c)J. J. Eisch ibid.p. 67; (d)J. P. Oliver ibid. p. 111; (e)E. A. Hill ibid. p. 131; (f) E. Grovenstein jun. ibid. p. 167. E. Grovenstein jun. Angew. Chem. Internat. Edn. 1978,17,313. (a)A. A. Solov'yanov and I. P. Beletskaya Russ. Chem. Rev. 1978,47,425;(b)P. Beak and D. B. Reitz Chem. Rev. 1978,78,275. P. R. Jones and P. J. Desio Chem. Rev. 1978,78,491. (a) A. J. Bloodworth in 'The Chemistry of Mercury' ed. C. A. McAuliffe McMillan London,1977;(b)R. C. Larock Angew. Chem. Internat. Edn. 1978,17,27;(c)D. L. Rabenstein Accounts Chem. Res. 1978 11 100; (d) 0.A. Reutov Tetrahedron,1978 34 2827. 119 M. G. Hutchings synthetic intermediates,6 selective reactions with organoaluminium compounds,’ and Si in organic synthesis.’ The main contributor to the area has summarized the chemistry of the Group V heterabenzenes.’ The format and contents of the remainder of this year’s Report follow those of the recent past .O 2 Group1 On photolysis in liquid NH, Li2C2 gives an unknown compound which is believed to have the formula Li4C4 on the basis of mass spectrometry but which only yields acetylene on hydrolysis.” Ab initio calculations (STO-3G and 4-3 1G basis sets) have led to the suggestion that Li4C4 may be the first isolated derivative of tetrahedrane the bridged structure (1) being favoured by 272 kJ mol-’ over (2). Li Li The propensity for Li to stabilize bent bonds is established and there are analogies for bridging structures in lithiocarbons. Mechanisms for the inversion of RLi involving the monomer or dissociation-recombination can be ruled out but cal- culations at a similar level to those above have shown that the ground state of the dimer (MeLi) (3; C,,symmetry) is only about 80 kJ mol-’ lower in energy than a feasible model for inversion (4).12 It is suggested that movement of CH groups from H H I H-1 ,H C /H ;\ Li -Li -H H (3) (4) face to face via the edge is responsible for inversion in hexamers and tetramers (in solution) and that this mechanism is inapplicable to secondary and tertiary RLi because of the increased steric constraints.Calculations at the highest level of sophistication so far reported for CH2Li2 -a candidate for carbon planarity -have shown that the singlet and triplet tetrahedral and square-planar structures are nearly G.M. L. Cragg and K. R. Koch Chem. SOC. Rev. 1977,6,393. ’ H. Yamamoto and H. Nozaki Angew. Chem. Internat. Edn. 1978,17 169. E. W. Colvin Chem. Soc. Rev. 1978 7 15. A. J. Ashe Accounts Chem. Res. 1978 11 153. M. G. Hutchings Ann. Reports (B),1977 74 136. G.Rauscher T. Clark D. Poppinger and P. von R. Schleyer Angew. Chem. Internat. Edn.,1978,17,276. T.Clark P. von R. Schleyer and J. A. Pople J.C.S. Chem. Comm. 1978 137. Organometallic Chemistry-Part (ii) Main-group Elements degenerate.13 It is concluded that there is an urgent need for experimental studies. The propene (5)is surprisingly lithiated exclusively at a vinylic carbon by Bu'Li in THF-HMPA Calculations(4-31G basis) on model systems indicate that BuO VOBu (5) the BuO substituent stabilizes the vinylic anion inductively and destabilizes the comparable allylic anion by v-intera~tions.'~~ Additionally the trend in kinetic acidities suggests that the vinylic proton would be more readily removed as observed.A new innovation in metallation which has promise of wider applicability centres on the intentional inclusion of a chelating substituent in (6)to stabilize the ion pair." The corresponding simple Me0 derivative is inactive when treated with BuLi and RBr whereas (6) gives (7) on work-up. The directing effect of CONR in the (6) (7) lithiation of benzenoid systems16 has been applied to the synthesis of contiguously tri- and tetra-substituted alkoxybenzenes such as those found in naturally occurring anthraq~inones"~ and alkaloid^."^ Further studies have focused on the products resulting from lithiation and alkylation of sterically protected carbonyl derivatives particularly ready methods for cleavage to synthetically useful products derived from the synthons Me2NCH and MeNHCH;.Thus (8)and (9) are reduced by LiA1H4 after alkylation to Me2NCH2R,18apb and the products of reaction between (10; X2= H2or OC2H40) and carbonyl compounds are hydrolysed to amino-alcohols.'8" The search for improved more flexible acyl anion equivalents has uncovered two new heterocyclic carbonyl synthons. Both (11) and (12) may be converted into Ph,CCON(Me)CH ,Li Y"CH,Li N-CH2Li Me +qo .;CQ 0 ' I Me (8) (9) (10) la W. D. Laidig and H. F.Schaefer J. Amer. Chem. SOC.,1978,100 5972. l4 (a)S. J. Gould and B. D. Remillard Tetrahedron Letters 1978,4353; (b)A. R. Rossi B. D. Remillard and S. J. Gould ibid. p. 4357. '' J. Amupitan and J. K. Sutherland,J.C.S. Chem. Comm. 1978 852. l6 P. Beak and R. A. Brown I. Org. Chem. 1977,42,1823. (a)S. 0.de Silva and V. Snieckus TetrahedronLettem 1978,5103;J. E. Baldwin and K. W. Bair ibid. p. 2559; (b) S. 0.de Silva I. Ahmad and V. Snieckus ibid. p. 5107. (a) R. Schlecker,D. Seebach and W. Lubosch Helv. Chim. Acta 1978,61,512; (b)D. Seebach and T. Hassel Angew. Chem. Internat. Edn. 1978 17 274; (c)T. Hassel and D. Seebach Helv. Chim. Acta 1978,61,2237. M. G. Hutchings (11) X=Li (12) X=CH2Li 2-vinylbenzthiazoles which act in turn as Michael acceptors (unlike ~p-enals).’~ A range of procedures leads to aldehydes and ketones including new annulation processes for fused and spiro rings.Compound (13)is readily prepared and gives ketones by alkylation and HgO-BF3,Et20 hydrolysis.” Carbanions (14) derived (13) (14) from an amino analogue of formaldehyde cyanohydrin also act as good formyl or acyl anion equivalents.’la After alkylation the carbonyl functionality is liberated under sufficiently mild conditions (CuS04,SH20-EtOH) to tolerate such protecting groups as acetals. Alternatively conversion of (14) into the Michael acceptor CH2=C(CN)NR2 gives the ketone RCH,COR’ by the reaction sequence RLi R’X H30+.21 A method for overcoming the tendency of dienolates to alkylate Q to the carbonyl and induce y-alkylation instead uses the LUMO-filled species (15),as for instance wN0,Li2 (15) in Michael additions to unsaturated ketones and subsequent liberation of the ap-enal functionality with TiCl (Nef conditions).*’ Deprotonation of primary nitroalkanes and silylation gives silyl nitronates which can be used in improved nitro-aldol reactions; the products are reduced to give a new general synthesis of 2-amino-al~ohols.~~ Compound (16) (Scheme 1)has been introduced as a synthetic equivalent to the hypothetical dipolar enone synthon (17).24aThe stereochemistry with respect to the alkoxide of the initially introduced alkyl group is changed from mainly cis when the counterion is Li’ to mainly trans if the alkoxide is silylated prior to alkylati~n.~~’ Regiospecifically generated metalloenamines serve as inter- mediates in a homologation-alkylation procedure for carbonyl compounds in non- optimized yields of 40-80’/0 (Scheme 2).2s Several simple fluorocarbon derivatives have found application as new ‘reagents’ as for instance CF3CH20Ts (18) in an intriguing new high-yield synthesis of l9 E.J. Corey and D. L. Boger Tetrahedron Letters 1978,S 9 13. 2o S.Ncube A. Pelter K. Smith P. Blatcher and S. Warren Tetrahedron L-etters 1978 2345. 21 (a)G.Stork,A. A. Ozorio and A. Y. W. Long Tetrahedron Letters 1978,5175;(b)H.Ahlbrecht and K. Pfaff Synthesis 1978 897. ** D. Seebach R. Henning and F. Lehr Angew. Chem. Internal. Edn. 1978,17,4S8. 23 E.W.Colvin and D. Seebach J.C.S. Chem.Comm. 1978,689. 24 (a)P. C. Conrad and P. L. Fuchs J. Amer. Chem. SOC.,1978,100,346;(b)J. C. Saddler. P. C. Conrad and P. L. Fuchs Tetrahedron Letters 1978,5079. 2s S. F. Martin and G. W. Phillips J. Org. Chem. 1978,43 3792. Organometallic Chemistry-Part (ii) Main-group Elements 0-0 Reagents i 2PhLi-THF; ii MeI; iii H,Cr,O,; iv diazabicycloundecene Scheme 1 Me Reagents i (EtO),P(=O)cHN=CHPh; ii Bu"Li; iii MeI; iv H,O' Scheme 2 a -keto-acids (Scheme 3),26a and CF3CH2XR' (X = 0or S)in its reaction with excess R2Li to give R2CrCXR' in good yields.26b Treatment of CC12=CF2 with BuLi gives LiCC1=CF2." Ketones (R1R2C=O) are converted into CYP-unsaturated (CF~CH~OTS) (18) 1' OH OTs OTs 0 iv. iii II [CF2=CLiOTs] -% R1R2&-&F2 -% R1R2C=C/ R ' R*CHCCO~H \ COzH Reagents i 2LiNPri,; ii R1R2C=O; iii H,O'; iv OH- Scheme 3 a-chloro-aldehydes or -ketones (19) by reaction with this reagent followed by LiAlH4 reduction or further alkylation (R3Li) respectively and hydrolysis.A further ketone synthesis derives from the reaction of the carbenoid LiCC1,CH=CH2 with primary alkyl ketones to give (20)which undergoes acid-catalysed elimination and ionization to a chloropentadienyl cation and subsequent cyclization to (2l)." R1y!yR3(H) R1$ c1 H°F R' R2 0 R2 R2 26 (a)K. Tanaka T. Nakai and N. Ishikawa Tetrahedron Letters 1978,4809;(b) K. Tanaka S.Shiraishi T. Nakai and N. Ishikawa ibid. 1978 3103. 27 D. Masure C. Chuit R. Sauvgtre and J. F. Normant Synthesis 1978,458.'* T. Hiyama M. Shinoda and H. Nozaki Tetruhedron Letters 1978 771. M. G. Hutchings 3 Group I1 Several studies have appeared which are concerned with the mechanism of formation of Grignard reagents. A particularly revealing observation is the lack of a 12C/13C kinetic isotope effect in the formation of MeMgI from Me1 when one would be expected if cleavage of the C-I bond were involved in the rate-determining Instead radical-ion pair formation by outer-sphere electron transfer is suggested to be rate-determining. The result of a product study of the reaction between 1-adamantyl halides Bu',CO and Mg indicates that nu discrete organometallic intermediate is formed but that a radical process is f~llowed.~' As anticipated last year," MgH2 can be induced to add to alkenes and alkynes.Terminal alkenes give (terminal) organomagnesium compounds {[(C5H5),TiC1,] catalyst} which are readily hydrolysed to the alkane.31 Unfortunately the inter- mediate seems to be unstable under the reaction conditions as indicated by the extent of D-incorporation from D20work-up. In contrast Cu' catalyses addition of MgH to alkynes only hydrolysis leading in this case to cis-alkenes in high yield and stereoselectivity. 32 In the carbonylation of Grignard reagents Fe(CO)5 may act as a one-carbon Further reaction with RI leads to ketone formation in high yield (the intermediate is acting as an acyl anion equivalent) whereas ROH-I furnishes carboxylic esters directly. Amides (22) are efficient reagents for formylating or acylating Grignard reagent^.'^ Modification of Grignard addition with Cu is now well established but it has proved difficult to introduce a Me group into tri-substituted alkenes by this method.However use of CuBr,Me2S promotes addition of MeMgBr to terminal acetylenes the resulting vinylmetallic species reacting with a variety of electrophiles in high yield.35 An alternative synthesis of tri-substituted alkenes involves the low-temperature addition of RMgBr to an a-chloro-ketone followed by lithiation and room-temperature elimination of 'LiMgOX' to give the The fact that spiroactivated cyclopropanes are susceptible to nucleophilic attack has been extended to organometallic compounds and exploited in the conversion of (23) into (24) by acetylenic Grignard addition during a synthesis of (k)-brefeldin A.37 (22) (23) R = SiBu'Me2 (24) One of the year's most remarkable new C-C bond-forming reactions involves a high-yield trimolecular condensation between alkyl halides activated alkenes and 29 E.A. Vogler R. L. Stein and J. M. Hayes J. Amer. Chem. SOC.,1978,100 3163. 30 P. Bauer and G. Molle Tetrahedron Letters 1978,4853. 31 E. C. Ashby and T. Smith J.C.S. Chem. Comm. 1978 30. 32 E. C. Ashby J. J. Lin and A. B. Goel J. Org. Chem. 1978,43 757. 33 M. Yamashita and R. Suemitsu Tetrahedron Letters 1978 761 1477. 34 D. Comins and A. I. Meyers Synthesis 1978,403; Tetrahedron Letters 1978 5179. " A. Marfat P. R. McGuirk and P. Helquist Tetrahedron Letters 1978 1363 2465. 36 J. Barluenga M. Yus and P.Bernad J.C.S. Chem. Comm. 1978 847. ''T. Livinghouse and R. V. Stevens J.C.S. Chem. Comm. 1978,754. Organom eta 1lic Chem istry-Pa rt ( ii ) Ma in -group Elem en ts carbonyl compounds promoted by Zn in MeCN [equation (l)].'*Cyclic bifunctional products are obtained if halogeno-ketones react under these conditions with acry- lonitrile or methyl acrylate. Cyclopentene derivatives are produced in modest yield by reaction between substituted propargyl bromides and activated alkenes again under the influence of metallic Zn.39 )-I -I-&,-N + Ac,O -+ 81% New applications of peroxymercuration continue to appear particularly for the synthesis of cyclic peroxides. The first example of four-membered ring formation provides a novel synthetic entry into 1,2-dioxetans (Scheme 4).40 Allylic mercura- Reagent i Hg(OCOCF,),-CFCI, -40 "C Scheme 4 tion occurs besides addition to the unsaturated bond.The endoperoxide (25) is of interest as a near strain-free homologue of the nucleus of prostaglandin G or H; derivatives are formed by reaction of cyclo-octa-1,4-diene with Hg(OCOCF3)2-H,02.41 Alkynes can be dimerized in a head-to-tail fashion to unsymmetrical 1,3-dienes (26) in excellent yield via vinylmercury 0 ?-(25) (26) 1,4-Dienes are obtained from the same intermediates and excess allylic both reactions being catalysed by transition-metal systems. 4 Group111 Boron.-This year has seen two theoretical studies of the fundamental hydro- boration reaction in each case dealing strictly with the gas phase but still shedding light on the synthetically more important solution phase.The semi-empirical 38 T. Shono I. Nishiguchi and M. Susaki J. Amer. Chem. SOC. 1978,100,4314. 39 M. Bellasoued Y. Frangin and M. Gaudernar Synthesis 1978 150. O0 W. Adam and K. Sakanishi,J. Amer. Chem. SOC. 1978,100,3935. " A. J. Bloodworth and J. A. Khan Tetrahedron Letters 1978 3075. O2 (a) R. C. Larock and B. Riefling,J. Org Chem. 1978,43,1468; (b)R. C. Larock J. C. Bernhardt and R. J. Driggs J. Organometallic Chem. 1978,156 45. 126 M. G. Hutchings MNDO method has been applied to the addition reactions of BH3 and its alkylated derivatives to simple substituted alkenes and alkyne~,~~ while ab initio methods (STO-3G with geometry optimization and 4-31G basis sets) were used in a study restricted to the system BH3+CH2=CH2.44 The latter work showed that the reaction proceeds exothermically without an overall activation barrier via an inter- mediate .rr-complex which rearranges to EtBH (AH= -136 kJ mol-' in good 'pstepwise reaction of Bu'Li with B(XMe)3 (X = 0or S).48 Although Bu' derivatives simple addition is found to follow a similar pathway but in this case a barrier to formation of the .rr-complex is predicted.43 MeBH also adds via a .rr-complex but Me2BH addition is calculated to proceed via a four-centre transition state which is orbital-symmetry thermally 'allowed' through use of the unoccupied B pz-orbital.The calculated effect of introduction of methyl groups into alkenes reflects the experimentally observed decreased rate while alkylated acetylenes are predicted to undergo hydroboration faster than HC=CH itself.The stereoselectivity in the gas phase is calculated to be less than that observed in ethereal solution and if this result is real it lends support to the contention that THF,BH3 adds to alkenes directly with the solvent inducing a greater activation barrier and greater distinction between possible orientations rather than by prior fission to free BH3. Fewer new hydroboration reagents have been introduced but there has been a greater concentration on stabilizing those alkylboranes which already have established synthetic use but limited stability. (CH2NMe2)2 (TMED) complexes of RBH are air-stable and can be stored for long periods and subsequently regenerated with BF3.45 The PhNEt adduct of thexylborane is stable for 2 months at 0 "C unlike the free reagent and demonstrates graded hydroboration and reduction activity compared with the latter.46 Monoisopinocampheylborane has been prepared quan- titatively by displacement of Me2C=CMe from the Et3N adduct of the~ylborane~'" and in 100% optical purity from the TMED adduct of 94% optically pure di- isopinocampheylborane .47b The previously unknown But3B has now been claimed to have been prepared by stepwise reaction of Bu'Li with B(XMe)3 (X = 0 or S).48 Although Bu' derivatives are formed at 130 "C the molecule is not sufficiently overcrowded to prevent adduct formation with NH3 MeLi or H- (from Bu'Li).Further new organometallic routes to highly substituted organoboranes proceed via reaction of the a-carbanion derived from (27)with electrophiles or by replacement of the PhS substituent of (27)by I and subsequent reaction with a Grig~~ard.~~" When (27) reacts with N-chloro- succinimide in MeOH it gives the protected aldehydes (28) directly in good yield.49b Alkenylboranes have usually been hydrolysed under acidic conditions but it has now been found that Pd(OAc) efficiently catalyses neutral protonolysis the solvent (THF or Me2CO) acting as proton When P~(OAC)~ and PhI(OAc);! oxidize 43 M.J. S. Dewar and M. L. McKee Inorg. Chem. 1978,17 1075. 44 T. Clark and P. von R. Schleyer J. Organometallic Chem. 1978,156 191. " B. Singaram and J. R. Schwier J. Organometallic Chem.1978,156 C1. '' A. Pelter D. J. Ryder and J. H. Sheppard Tetrahedron Letters 1978 4715. 47 (a)H. C.Brown and A. K. Mandal Synthesis 1978,146;(b)H. C. Brown J. R. Schwier and B. Singaram J. Org. Chem. 1978 43 4395. 48 H. Noth and T. Taeger J. Organometallic Chem. 1977,142 281. 49 (a) D. S. Matteson and K. Arne J. Amer. Chem. SOC.,1978 100 1325; (b) A. Mendoza and D. S. Matteson J. Organometallic Chem. 1978 156 149; J.C.S. Chem. Comm. 1978 357. H. Yatagai Y. Yamamoto and K. Maruyama J.C.S. Chem. Comm. 1978 702. Organometallic Chemistry-Part (ii) Main-group Elements RCH-OMe RCH-B 1 SPh SPh I T-(27) (28) R,B to ROAc the former reagent reacts preferentially with secondary R but the latter oxidizes only primary R and each only oxidizes two of the three available R groups.51* These same reagents bring about R migration in vinyl-BR2 yielding R-vinyl including brominated derivati~es.~'~ The stereochemistry of the product alkene is dependent on the conditions used but it is notable that these reagents give the opposite preferred stereochemistry.Propenoate esters are p -alkylated by R3B electrochemically in fair to excellent yields but only when the anion of the support- ing electrolyte is Br- or I-.52 After last year's lull there has been considerable renewed activity in the chemistry of organoborate salts. Besides re-emphasizing the considerable difference in degree of mildness between the three migrations of the cyanoborate and borane carbo- nylation procedures for C-C bond formation Scheme 5 illustrates a difference in .... t H-'H 'H 'H Reagents i KCN; ii (CF,CO),O 40 "C; iii OH-H,O,; iv CO-(CH,0H)2 150"C 70atrn Scheme 5 stereochemistry between the two reaction~.~~ The thermodynamically more stable cyanide adduct is formed prior to electrophile-induced migrations in contrast to the kinetically determined product resulting from carbonylation. An analogous migra- tion is assumed to occur in the reaction of Me3SnCECR1 with Et3B which gives (29).54A second molecule of an alkynylstannane reacts in a similar fashion under more forcing conditions to give the novel polymetallated species (30) (after Me& Et R)=4 Et2BMSnMe3 Et R' C-BEt, /\ Me,% R' (29) (30) subsequent allylic rearrangement) which is potentially synthetically useful in view of the variety of sites available for further reaction.Allylic borate complexes undergo regiocontrolled head-to-tail coupling with allylic halides resulting in the formation in " (a)Y. Masuda and A. Arase Bull. Chem. SOC.Japan 1978,51,901;(6)Y.Masuda A. Arase and A. Suzuki Chem. Letters 1978,665. 52 Y. Takahashi K. Yuasa M. Tokuda M. Itoh and A. Suzuki Bull. Chem. Sue. Japan 1978,51 339. 53 A. Pelter P. J. Maddocks and K. Smith J.C.S. Chem. Comm. 1978,805. 54 B.Wrackrneyer and R. Zentgraf J.C.S. Chem. Comm. 1978,402. M. G. Hutchings high yields of 1,5-dienes." The boron-bound allylic moiety undergoes double-bond migration as in the formation of (31) from [Bun3BCH2CH=CHMe]- and (31) ClCH2CH=CHPh.An R group in the ate complex from R3B and lithium chloro- propargylide migrates spontaneously at -90 "C further reaction leading to homo- propargylic or a-allenic alcohols depending on conditions (Scheme 6).56Despite R R,BCrCCH,CI A )=C=CH RCGCCH,BR R2B liii. iv iii i.1 RCGCCH,CHR ' R i )=C=CH OH R'CHOH 77-89 '/o 7 3-86 O/o Reagents i -90 "C;ii 25 "C;iii R'CHO -78 "C;iv LO] Scheme 6 the range of C-C bond-forming procedures now available based on organoboron intermediates an instance of a direct synthesis of carboxylic acids has only just appeared [Equation (2)].57 A similar type of ate complex intermediate which R l R3B +PhOCHC022-ate 4 R2BCHC02-+ RCHzCOzH (2) rearranges essentially spontaneously with elimination of an electronegative a-substituent occurs in a synthesis of tosylated alkylamines from R3B and chloramine T (TSNCI-N~').~~ Unfortunately only one R per R,B is used in the reaction and preferential migration of the ring carbon atoms is observed with 9-alkyl-9-borabi- cyclo[3.3.llnonane. Aluminium Gallium and Thallium.-Acetylenes are carbometallated by the reagent system R3Al-[(C5H5)2ZrC12] giving rise to vinylalanes [e.g. (32)] which may be hydrolysed halogenysed or ethoxycarbonylated (ClC0,Et) in high yield and stereo- and regio-selectivity to trisubstituted alkene~.~~~'~ The aluminate inter- mediates derived from the vinylalane and Bu"Li offer a similarly highly stereoselec- tive entry to terminally functionalized alkenes including terpenes (Scheme 7).596 " Y.Yamamoto and K. Maruyama J. Amer. Chem. Soc. 1978,100,6282. G. Zweifel S. J. Backlund and T. Leung J. Amer. Chem. Soc. 1978 100,5561. 57 S. Hara K. Kishimura and A. Suzuki Tetrahedron Letters 1978 2891. 58 V. B. Jigajinni A. Pelter and K. Smith Tetrahedron Letters. 1978 181. " (a)D. E. Van Horn and E.-i. Negishi J. Amer. Chem. Soc. 1978,100,2252; (b)N. Okukado and E.-i. Negishi Tetrahedron Letters 1978 2357. Urganometallic Chemistry-Part (ii)Main-group Elements 129 Reagents i Me,Al-(C,H,),ZrCI,; ii Bu"Li; iii (CH,O) Scheme 7 Unlike related systems vinylalanes cannot be induced to cross-couple with Pd or Ni complex catalysis. However a new approach of possibly fundamental importance makes use of double metal catalysis where the adjunct has an electronegativity between that of A1 and the Pd or Ni cocatalyst.Thus ZnC12 or CdClz and [Pd(PPh3)4] effect the reaction depicted in Equation (3) in high yield and stereoselectivity.60 /-\ I II Et Et Et In contrast to other reagents alkynylalanes undergo conjugate addition to s-trans-ap -enones in good yields.61 Again Ni catalysis is necessary. Hydroalumination of alkenes gives R4Al-which with LiAlH4 and TiC14 undergo several synthetically useful functionalizations including halogenation (CUX~),~~~ homologation by three carbon atoms (ally1 halide +CuC1),62b and conversion into terminal allenes (propargyl bromide +CuC1).62' Other new synthetic routes involving R-A1 deriva- tives are discussed in the Group IV section following.Unlike the above mentioned reactions of organoalanes no transition metal is necessary to induce EtAlCl to effect ring-opening polymerization of n~rbornene.~~ The reaction is claimed to be the first example of olefin metathesis in the absence of a transition metal; a carbene complex (33) is suggested as an intermediate. The reaction of K and Bui3AI at 20°C leads to a complex formulated as (34) containing an Al- A1 bond.64 Although the available physical and chemical evi- dence does support the assigned molecular composition the possibility of rapidly exchanging AI-Bu'-AI bridges and consequent lack of necessity for direct u-bonding Al- A1 interaction is not excluded. An X-ray stucture determination seems to be called for. K2[Bui3Al-AIBui3] AlCl 60 E.-i.Negishi N. Okukado A. 0.King D. E. Van Horn and B. I. Spiegel J.Amer. Chem. Soc. 1978,100 2254. R. T. Hansen D. B. Carr and J. Schwarz,J.Amer. Chem. SOC., 1978,100,2244. 62 (a)F. Sato Y. Mori and M. Sato Chem. Letters 1978 833; (b)F. Sato H. Kodama and M. Sato J. Organometallic Chem. 1978,157 C30; (c)F. Sato K. Oguro and M. Sato Chem. Letters 1978 805. ''K. J. Ivin J. J. Rooney and C. D. Stewart J.C.S. Chem. Comm.. 1978,603. 64 H. Hoberg and S. Krause Angew. Chem Internat. Edn. 1978,17,949. M. G. Hutchings A novel functionalization of the allylic methylene groups at C-7 of the pros- taglandin skeleton is brought about by reaction of TllI1 triacetate with PGF2 methyl uia a highly unstable intermediate previously reported as an intermediate in the enzymic conversion of arachidonic acid.The newly available reagent Tl’CN converts acyl chlorides into acyl cyanides in good yield.66 5 GroupIV Silicon.-The claimed synthesis of a silabenzene was one of the more noteworthy highlights of last year’s Report.” This has been followed by several more studies in this field including an ab initio MO calculation (STO-3G with geometry optimiza- tion) of the structure of silabenzene itself (35) (Scheme 8).67 It is calculated to be (35) R=H (37) X=H (36) R=Me (38) X=CF3 Reagents i 428 OC quartz in a carrier of XC-CX ii XC-CX (X = Hor CF,) Scheme 8 planar with possible ylide character. The elusive nature of (35)has been assigned to high reactivity as with other unsaturated Si-containing species rather than to any lack of aromaticity in fact the resonance energy of (35) is about two-thirds that of benzene and it is concluded that (35) ‘appears to have all the attributes expected of an analogue of benzene’.A second ‘unambiguous’ route to a silabenzene (36) follows the pathway of Scheme 8.68Propene is identified as by-product by g.c.-m.s. The identity of the isolated adduct (38) to that isolated from the first reported route lends considerable credence to the possible transient intermediacy of a true silaben- zene. Interest in other doubly bonded Si species is scarcely diminishing and the breadth of studies is now extending to include analogues of many other well known multiply bonded carbon species. Examples (39)-(43) were ‘identified’ by more-or-less R36-SiMe3 CH2=Si=X Me2Si=SiMe2 Me2Si=NSiMe3 e,SiMe (39) R = Ph or Me (40) (41) (42) (43) indirect routes in 1978.The ylide (39)is proposed as an intermediate in the reaction of hexamethylsilacyclopropane with ketones in the presence of a tertiary phos- ~hine.~’ Subsequent reaction gives a siloxiran which dimerizes to give the isolated 65 V. Simonidesz,Z. Gombos-Visky G. Kovics E. Baitz-Gics and L. Radics J. Amer. Chem. SOC.,1978 100,6756. 66 E. C. Taylor J. F. Andrade K. C. John and A.McKillop J. Org. Chem. 1978,43,2280. 67 H. B. Schlegel B. Coleman and M. Jones jun. J. Amer. Chem. SOC.,1978 100,6499. T. J. Barton and G. T. Burns J. Amer. Chem. SOC.,1978,100,5246. 69 D. Seyferth and T. F. 0.Lim J. Amer. Chem. Soc. 1978,100,7074.Organometa11ic Chemistry -Part ( ii Main-group Elements 131 product. The first digonal Si intermediate (40; X = CH or 0)has heen postulated on the basis of the characterization of co-pyrolysis Inthe gas phase (41) is suggested as the precursor of a series of reactive Si-containing intermediates culminating in the production of a 1,3-disilacyclobutane (Scheme 9)." The evidence Me ,Me H I (41) -+ Me,Si-$iMe + Me-Si /H + Me,Si(H)CH,'S'iMe Me-Si LSi-H I Me 1-* Scheme 9 for the silaimine (42) is substantial including adduct formation with multiple bond systems insertion into Si-X and dimeri~ation.~~ Thermal elimination in dimethyldiallylsilane analogous to that of Scheme 8 gives (43) which undergoes electrocyclic ring closure to an unsubstituted silacyclobutene the overall procedure comprising a straightforward synthesis of such species.73 Hydrosilation has long been known as an efficient method for formation of the Si-C bond but there has been little previous methodology for breakingsuch bonds.A simple new development now opens up a new area of considerable synthetic potential.74 Thus H2PtC16-catalysed addition of HSiCl to alkenes or alkynes followed by reaction with KF gives K',[RSiF5l2- (44) quantitatively. Reaction of (44) with halogens NBS or CuX2 yields the anti-Markovnikoff hydrohalogenated alkenes and alkynes (the latter stereoselectively oxidative cleavage with rn -chloroperbenzoic acid gives high yields of alcohols (from terminal alkene~),~~* and the vinyl derivatives can be induced to couple with ally1 chloride to give 1,4-diene~.~~~ The considerable advantages inherent in this procedure include the mildness and tolerance of other functional groups the well-established nature of hydrosilation and its catalysts the ready availability and air-stability of HSiCl (contrast comparable B Al and Zr reagents) the stability of (44) (beaker reaction) and the ready removal of insoluble fluorosilicate by-products.Scheme 10 depicts a reaction pathway which can be accomplished by the use of a variety of Al-based reagent combinations. Significantly the vinylsilane product is further functionalized by a second metal with the consequence that further synthetic R SiMe R SiMe RCrCSiMe + &( + -+ etc. XM XE (45) (46) Scheme 10 70 G.Bertrand G. Manuel and P. Mazerolles Tetrahedron 1978 34 1951. 71 W. D. Wulff W. F. Goure andT. J. Barton J. Amer. Chem. Soc. 1978,100,6236. 72 N. Wiberg and G. Preiner Angew. Chem. Internat. Edn. 1978 17 362. 73 E. Block and L. K. Revelle J. Amer. Chem. Soc. 1978,100 1631. 74 (a)K. Tamao J.-i. Yoshida M. Takahashi H. Yamamoto T. Kakui H. Matsumoto A. Kurita and M. Kumada J. Amer. Chem. Soc. 1978,100,290; (6)K. Tamao T. Kakiu and M. Kumada ibid. p. 2268; (c) J.4. Yoshida K. Tamao A. Kurita and M. Kumada Tetrahedron Letters 1978 1809; (d) J.4. Yoshida K. Tamao M. Takahashi and M. Kumada ibid. p. 2161. 132 M. G. Hutchings modification by reaction with electrophiles is possible. Simple hydroalumination gives (45; X = H M = Bu12AI) as an intermediate in a high-yield stereoselective route to (46;E = C1 Br or I).75 MeMgBr and EtMgBr add to give (45; X =Me and H respectively) in the presence of [Ni(a~ac)~] and Me3A1 or HA1Bui2 but with less ~tereoselectivity.~~ The titanium analogue (45; X = R1,M =Ti) results from non- stereoselective addition of Cl2A1R' -[(C5H&TiC12] hydrolysis leading to good yields of cis/ trans mixture of ~inylsilanes.~~ Peroxidation liberates a,a-dialkylated ace t alde hydes.Related work concerns the use of a-lithiated silanes in alkene preparation. Silylated ally1 carbanions add to ketones regioselectively at the carbon atom (Y to Si under the influence of MgBr2.78" An elimination gives rise to 1,l-dialkylated 1,3-dienes. A mixture of isomers of a-chloro-a@-unsaturated esters is derived from chloroacetate ester and ketones via the intermediacy of silylated carbanionic derivative~,~~' and tetra-substituted alkenes may be prepared as in Scheme 11.78c Me,Si OAc Me& R' X R' Reagents i R1R2C=O; ii Ac'; iii R3,CuM (M =Li or MgI); iv X' (X= Br or I) Scheme 11 The reaction between LiCH2SiMe3 and esters R1C02R2 (R2preferentially secon- dary or tertiary) leads to a -silyl-ketones in high yield.79 Known methodology allows conversion into methyl ketones.@ -Silyl-ketones can be considered synthetically equivalent to masked a@-enones as exemplified in Scheme 12.80" Silyl anions also undergo conjugate addition to ap-enones in the presence of CuTI to give enolates which may be alkylated.80b Desilylation of @ -silyl-ketone with Cur*Br2 gives the (Y -alkylated original enone.The reagents Me3SiONXSiMe3 (X=H or Me,%) react with acyl chlorides thermolysis giving quantitative yields of isocyanates.8' This method has been applied to effect the first authentic synthesis of HC=CNCO. Me,Si Me,Si Reagents i NaH-DMF; ii Bu'I; iii NaCN-(Me,N),PO 90 "C; iv Br,-CCI,; v NaF Scheme 12 " G. Zweifel and W. Lewis J. Org. Chem. 1978,43 2739. '6 B. B. Snider M. Karras and R. S. E. Conn J. Amer. Chem. SOC. 1978 100,4624. ''J. J. Eisch R. J. Manfre and D. A. Komar J. Organometallic Chem. 1978,159 C13. " (a) P. W. K. Lau and T. H. Chan TetrahedronLetters 1978,2383; (b)T.H. Chan and M. Moreland ibid. p. 515; (c) R. Amouroux and T. H. Chan ibid.p. 4453. 79 M. Dermuth Helv. Chim. Acta 1978 62 3136. 'O (a)1. Fleming and J Goldhill J.C.S. Chem. Comm. 1978,176; (b)D. J. Ager and I. Fleming ibid.,p. 177. 81 F. D. King S. Pike and D. R. M. Walton J.C.S. Chem. Comm. 1978 351. Organometallic Chemistry-Part (ii) Main-group Elements 133 Recent detailed studies of hindered usually symmetrical main-group organo- metallic derivatives have shed light on both intricate stereochemical behaviour and general stereochemical properties and principles. New mathematical techniques such as group theoretical analysis using dynamic symmetry groups as well as computer modelling via the empirical force field method have been applied to these problems as in the latest objects of study viz. the ground-state properties 82a of the highly symmetrical M1 (M2Me3)4 and the dynamic behaviours2' of But3SiH.The latter is particularly interesting from the point of view of correlated rotation (or gearing) where the lowest-energy (threshold) process involves a net conrotation of all three But groups through staggered conformations. Germanium Tin and Lead.-Silirans have become available in the past few years and now the first germiran (47)has been postulated on the basis of the identification of styrene as an unstable rearrangement product of gas-phase-generated phenyl- trimethylgermyl~arbene.~~ Other new Ge-containing reactive intermediates include (48) and (49) which have been trapped with various reagentsg4 While stannyl radicals (R3Sn*) are intrinsically interesting they are also particularly useful for the generation of alkyl radicals by reaction with RBr.A convenient source of Me3Sn* is (50),which eliminates Bu'H and depends on the propensity for P-H abstraction from suitable alkylstannanes by Bu'O- for its e~istence."~ Alternatively (5 1) can be PhkGeMe2 R2Ge=X Me3SnCH2CMe2 (7'-CgH5)SnR3 (47) (48) X=O (50) (49) x=s photolysed to R3Sn* and Cp*.85' Sterically encumbered distannanes R,Sn-SnR3 undergo ready reversible thermal dissociation to R,Sn-(R = 2,4,6-trialkyl~henyl).~~ Tin derivatives seem to be playing an increasingly important role in synthetic organic chemistry either directly or as precursors to other reactive organometallic species. A new highly convenient ketone synthesis from &Sn and acid chlorides is one such example.87a By using a Pd" catalyst near quantitative yields are obtained in a rapid reaction which is tolerant of a wide variety of functionalities needs no special experimental precautions and is easily worked up.A disadvantage is the use of only one R per R4Sn. Unsaturated ketones are available albeit in inferior yield from an analogous reaction using vinylstannanes and AICl as ~atalyst.~'' The new reagent PhS(Me3Sn)CuLi efficiently replaces the halogen of P -iodo-cup -enones by Me,Sn in contrast to Me3SnLi.'$ However the relative ease of addition of the two reagents to simple CUP -enones is reversed. Organostannanes are often easily pre- (a)L. D. Iroff and K. Mislow J. Amer. Chem. SOC.,1978,100,2121;(b)W.D. Hounshell L. D. Iroff.R. J. Wroczynski and K. Mislow ibid. p. 5212. 83 E. B. Norsoph B. Coleman and M. Jones jun. J. Amer. Chem. SOC.,1978,100,994. H. Lavayssiere J. Barrau G. Dousse J. SatgB and M. Bouchant J. Organometallic Chem. 1978,154 C9;ibid. 1978,161,C59. (a)A.G. Davies B. P. Roberts and M.-W. Tse J.C.S.Perkin II 1978 145; (b)A.G.Davies and M.-W. Tse J.C.S. Chem. Comm. 1978 353. 86 H. U. Buschhaus and W. P. Neumann Angew. Chem. Internat. Edn. 1978,17 59. *' (a)D. Milstein and J. K. Stille J. Amer. Chem. SOC.,1978,100,3634; (b)M. L.Sai'hi and M. Pereyre Bull. SOC.chim. France 1977 1251. E.Piers and H. E. Morton J. C. S. Chem. Comm. 1978 1033. M. G. Hutchings pared precursors of less accessible organolithiums by metal-exchange processes. Two notable synthetic applications are summarized in Scheme 13 where the Reagents i KH; ii Bu,SnCH,I; iii Bu"Li; iv Ac,O-py Scheme 13 formation of the trisubstituted alkene via a [2,3] sigmatropic shift is highly stereoselective,89" and in Scheme 14 which depicts a route to an a-alkoxy-organolithium reagent.89b Reagents i BupSnLi; ii EtOCH(Me)Cl; iii Bu"Li; iv RCl (R =geranyl) Scheme 14 6 GroupV Despite the amount of work over the past few years on Group V derivatives of benzene there is little chemicd evidence for the aromaticity of arsabenzene.However it has now been shown that arsabenzene undergoes electrophilic acetyl- ation under classical Friedel-Crafts conditions (AcC1-AlCl3) to give the 2-and 4-substituted isomers (20 :80; 80% overall no 3-substituted product)." (Ph3BiCl)20 is a new mild reagent for the oxidation of (allylic) The reagent is easily prepared and used (rigorously anhydrous conditions are not necessary) and unlike Mn02 or Ag2C03-celite no excess is necessary.89 (a)W. C. Still and A. Mitra J. Amer. Chem. SOC.,1978 100,1927; (6)W. C. Still ibid. p. 1481. 90 A. J. Ashe W.-T. Chan and T. W. Smith Tetrahedron Letters 1978 2537. 91 D. H. R. Barton J. P. Kitchin and W. B. Motherwell J.C.S. Chem. Comm. 1978 1099.

ISSN:0069-3030    

DOI:10.1039/OC9787500119

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

7 Electro-organic Chemistry By R. LlNESt Laboratory of Organic Chemistry The Norwegian Institute of Technology The University of Trondheim N-7034 Trondheim -NTH Norway 1 Introduction In this Report the emphasis has been placed on those papers which display the utility of electrochemical methods in organic synthesis or in the investigation of organic systems. As in previous years the review is organised according to the nature of the species which is believed to participate in the initial electrode reaction. 2 Anodic Processes The Anodic Oxidation of Carboxy1ates.-The synthesis of configurationally pure disparlure (1)(the sex attractant of the gypsy moth) has been reported by Kliinenberg and Schafer,’ using the two cross-coupled Kolbe reactions outlined in Scheme 1.i. co-electrolysis +pelargonic acid ii. co-electrolysis +4-methylvaleric acid then epoxidation Scheme 1 The ‘two electron’ Kolbe reaction also offers a means of carrying out oxidative substitutions a case in point being the synthesis of 2-methoxy-l,4-dioxans from p-0x0 carboxylate acetals2 (Scheme 2). Oxidative decarboxylation of carbamoylaspartic acids or ethoxycarbonylaspara- gine has been reported by Iwasaki and co-workers’ to be a mild method for the H.Kliinenberg and H.J. Schafer Angew. Chem. Internat. Edn. 1978,17,47. D.Lelandais C.Bacquet and J. Einhorn J.C.S. Chem. Comm. 1978 194. T.Iwasaki H.Horikawa K. Matsumoto and M. Miyoshi Tetrahedron Letters 1978,4799. t Present address Chemistry Department The University Southampton SO9SNH 135 136 R.Lines FOH OMe Scheme 2 formation of uracii and its derivatives. These compounds are of current interest owing to their biological activity. The anodic decarboxylation of carboxylates normally requires high potentials (32V us. SCE) because electron transfer at the electrode is slow. A paper4 has now appeared in which electron transfer is achieved homogeneously by the use of electron-transfer reagents. Under these conditions reasonably fast reaction rates can be achieved at potentials of ca. lV for example by use of tris-(4-bromophenyl)-amine Eo= 1.3V. The reaction is also catalytic in transfer reagent (Scheme 3). In Scheme 3 contrast to the normal Kolbe reaction the alkyl radicals generated in this procedure4 undergo H-abstraction rather than radical coupling and disproportionation -a reflection of the different radical concentrations of the two methods.The Anodic Oxidation of Organic Anions.-A synthesisof azoalkanes' which avoids the use of chemical oxidants e.g. chlorine is shown in Scheme 4. Oxidation of the anions of NN'-dialkyl-sulphamides leads to a cyclic thiadiaziridine 1,l -dioxide intermediate' which cleaves to give azoalkanes in high yields (ca. 90Oh). Scheme 4 Some interesting electrochemistry stemming from the oxidation of the p-diethyl- amino-dithiobenzoate anion (2) in acetonitrile has been reported by Cauquis and Deronzier.6 The initial product is the disulphide (3) which undergoes further W. Schmidt and E. Steckhan J. Electroanalyt. Chem.Interfacial Electrochem. 1978,89 215. R.Bauer and H. Wendt Angew. Chem. Internat. Edn. 1978,17,370. G. Cauquis and A. Deronzier J.C.S. Chem. Comm. 1978,809. Elec tro-orga n ic Chemistry 137 oxidation in two le reversible steps (Scheme 5). The final product is a dicationic species containing a tetrathian ring (4). Depending on the potential (4) can be formed by either an ece or an eec process. 2e e L 7 L (312+ E2 The Anodic Oxidation of Neutral Organic Compounds.-The oxidation of cyclo- pentane and cyclohexane at Pt in fluorosulphonic acid has been re-examined by Coleman and Pletcher7 with the aim of distinguishing between the preprotonation mechanism [Ann. Reports (B) 1973 70 2991 and the elce2 (e2 >el) type process proposed by a French group.* The authors' concluded that the alkanes in the presence of KFS03 underwent a single 2e oxidation but that the role of the protonated alkanes remained unclear and must await more detailed knowledge of the fluorosulphonic acid system.Another reinvestigation concerned the effects of the presence of trace amounts of polynuclear aromatics on the o/p product ratio in the acetoxylation of anis01e.~ Contrary to an earlier report that the presence of coronene or hexahelicene increased the o/p ratio it was foundg that the ratio in fact decreased. This decrease conforms with the generally observed isomer ratios in the presence of additives an effect attributed to co-adsorption at the anode.9 A new synthesis of indigos has been reported by a Japanese group" and is outlined in Scheme 6.The starting materials can be either N-acetyl-indolines (5)or N-acetyl- indoles (6). Acetoxylation of either (5) or (6) followed by thermolysis yields the intermediate (7) from which the indigos can be obtained by hydrolysis. The use of emulsion electrolyses with phase-transfer reagents [Ann. Reports (B) 1977,74,155] has been extended to the acyloxylation of aromatic compounds'1 and to the cyanation of stilbenes.12 For the acyloxylation reaction," good yields of aryl carboxylates were obtained from anisole with various aliphatic carboxylic acids whereas methylbenzenes gave mainly alcohols and aldehydes. This latter result was attributed" to the preferential adsorption of water at the anode thereby reversing J.P. Coleman and D. Pletcher J. Electroanalyt. Chem. Interfacial Electrochem. 1978,87 111. a S. Pitti. M. Herlem and J. Jordan Tetrahedron Letters 1976 3221. W.J. M.van Tilborg J. J. Scheele and L. Eberson Acta Chem. Scand. 1978 B32,36. lo S. Torii T. Yamanaka and H. Tanaka J. Org. Chem. 1978,43,2882. l1 L.Eberson and B. Helgte Acta Chem. Scand. 1978 B32 157. l2 L.Eberson and B. HelgCe Acta Chem. Scand. 1978 B32,313. 138 R. Lines I Ac Ac OAc (5) (6) J. X=HorBr OAc (7) Scheme 6 the usual order of nucleophilicity of water us. carboxylate. The anodic cyanation of stilbene and its derivatives’* resulted in the first examples of 1,2 addition of cyanide across the double bond. It was pointed out by the authors that this was probably due to the enhanced reactivity ofthe cyanide ion in the dichloromethane phase compared to the more commonly used solvents for cyanation such as methanol and acetonitrile.Electrolysis of aromatic compounds in trifluoroacetic acid (TFA) appears to be an excellent method for the synthesis of the corresponding hydroxy derivatives. l3 Thus chlorobenzene could be selectively mono- di- or tri-fluoroacetoxylated depend- ing on the charge passed.13 The easy hydrolysis of the trifluoroacetates together with the high anodic limit of TFA makes this reagent clearly superior to acetic acid for hydroxylation reactions. Schfnnidt and Steckhan14 have described the mild oxidative removal of the 4-methoxybenzyl ether protecting group by a homogeneous electron-transfer reaction.The electron-transfer reagent was the tris-(4-bromophenyl)amine cation radical either generated in situ electrochemically in moist acetonitrile or as the hexachloroantimonate salt. The electrochemical method has the advantage that only a catalytic quantity of the reagent is required. Chemical methods for the removal of the dithian protecting group are scarce so the efficient removal of this group by anodic cleavage in neutral conditions (MeCN-H20) demonstrated by Porter and Utley,” may well prove to be the method of choice. Two comprehensive papers have appeared which describe the anodic methoxyl- ation of phenols16 and the methoxylation of methylbenzenes and anisole deriva- tive~.~~ Phenols were found to undergo ortho and para methoxylation and dimeriza- tion16 although conditions were found which allowed the selective formation of each product.Products from the oxidation of methylbenzenes and anisole derivatives” depended on the electrolyte composition. Electrolysis at Pt in the presence of NaOMe gave predominantly nuclear methoxylation whereas at Pt or carbon anodes l3 G. Bockmair H. P. Fritz and H. Gebauer Electrochim. Actu 1978,23,21. l4 W. Schmidt and E. Steckhan Angew. Chem. Intemat. Edn. 1978,17,673. *’Q. N. Porter and J. H. P. Utley J.C.S. Chem. Comm. 1978 255. l6 A. Nilsson U. Palmquist T. Pettersson and A. Ronlh J.C.S. Perkin I 1978 696. ” A. Nilsson U. Palmquist T. Pettersson and A. Ronlkn J.C.S. Perkin I 1978 708. Electro-organic Chemistry with LiBF as supporting electrolyte side-chain oxidation became important (Scheme 7).By careful selection of the starting materials and electrolysis conditions the method may have some application to the synthesis of aromatic aldehydes and acids.6-4+$ \ \ ++\ R2 CH,OMe CH(OMe) C(OMe) R' = Me or Me0 R' = OMe R' = Me or Me0 R' = OMe R2= Me Scheme 7 The electrochemical properties of the sandwich compound uranocene (8)have been probed" in an attempt to test a model of its bonding arrangement. Uranocene is inert to water but removal of two electrons (predicted to be from non-bonding orbitals) would lead to a dication isoelectronic with thoracene -a highly water- sensitive compound. Contrary to expectations it was found" that oxidation of uranocene in benzonitrile gave only an unstable cation which reacted irreversibly with the parent compound to give a product of uncertain structure; possibly a dimer or cluster product cation (Scheme 8).COT = cyclo-octatetraene Scheme 8 3 Cathodic Processes The Cathodic Reduction of Organic Cations.-An interesting product has been reported" to arise from the reduction of the dihydrodiazepinium salt (9) (Scheme 9). It is thought" that the pyrrolodiazepine (10) is formed by dimerization of the initially formed radicals followed by an intramolecular displacement of ethyl- enediamine. The base is also reversibly converted by acid into the conjugated salt (11). A Japanese group" have developed a simple electrosynthesis of isoquinoline and indole alkaloids involving the electroreductive addition of alkyl halides to immonium salts.The reaction is suggested to proceed via electron transfer from the salt followed by the addition of the alkyl halide to the resulting anion. The authors2' specify the need for a lead plate in the cell during electrolysis presumably as a J. A. Butcher jun. J. Q. Chambers and R. M. Pagni J. Amer. Chem. Soc.,1978; 100,1012. D. Lloyd C. A. Vincent D. J. Walton J. P. Declerq G. Germain and M. van Meerssche J.C.S. Chem. Comm. 1978,499. 2o T. Shono K. Yoshida K. Ando Y. Usui and H. Hamaguchi TetrahedronLetters 1978,4819. 140 R. Lines H Ph (.>Ph _- (Lg H+ OH-A7 N H ClO N H .Ph Scheme 9 halogen scavenger. An example of the method applied to the synthesis of hetero-cycles containing two heteroatoms is shown in Scheme 10.OMe OMe Scheme 10 The Cathodic Reduction of Neutral Organic Compounds.-[24]Paracyclo-phanetetraene (12) is formally a derivative of [24]annulene and by analogy with the smaller 4nn electron systems would be expected to undergo stepwise reduction to the dianion. Lamm and co-workers21 have reported however that at Hg in DMF (12) is reduced in a single reversible two-electron step to the dianion. Although cyclopropanes can be synthesized by the reduction of 1,3 -dihalides extension of the method for the preparation of the higher cycloalkanes has until now been unexplored. Japanese investigators” have found that moderate yields of 3-7-ring cycloalkanes can be formed by the reductive cyclization of dimethyl K.Ankner B. Lamrn B. Thulin and 0.Wennerstrom Acra Chem. Scand. 1978,B32,155. 22 S. Satoh M. ltoh and M. Tokuda,J.C.S. Chem. Comm. 1978,481. Electro-organic Chemistry 141 2,(0 -1)-dibromo-alkanedioates (Scheme 11). The procedure represents a con- venient route to cycloalkane- 1,2-dicarboxylic acid esters. [-CHCO,Me Pt-2e MeO2CCH(CHZ),-2CHCOzMe (CHd-2 I Br Br -CHCO,Me I I (n= 3-7) Scheme 11 The cathodic addition of carbon tetrachloride to various carbonyl compounds has been reportedz3 to give the corresponding trichloromethyl carbinols in yields comparable to the phase-transfer-catalysed addition of chloroform. The same paper23 also reports the cathodic addition of ethyl trichloroacetate to cyclic ketones (Scheme 12). Scheme 12 A simple but potentially very useful electrocatalytic hydrogenation procedure for aromatic compounds has been described by Miller and Chri~tensen.‘~ The reduc- tions were carried out in 0.2M-sulphuric acid at Pt- or Rh-coated carbon cathodes in a divided cell.Phenol for example gave 92% yield of cyclohexanol. An apparent limitation of the method is that addition of organic co-solvents markedly decreases the product yield presumably by competitive adsorption at the cathode. A similar procedure for the reduction of 3-cyanopyridine to the amine has been reported by an Indian Conditions necessary for electrocatalytic aromatic nucleophilic substitution reac- tions are discussed by Pinson and SaveantZ6 and summarized in Scheme 13. The ArX+ e $ ArX (EY = electrolysis potential) ArY’-e $ ArY (Ey) ArX+Y-+ ArY +X- if EY > Ei Scheme 13 basic argumentz6 is as follows if the reduction potential E’ of the substrate (ArX) is greater than the oxidation potential E of the product radical anion (ArYT) then the oxidation of ArY; into the product occurs spontaneously and the process is catalytic.This scheme was successfully testedz6 using thiolates as nucleophiles and halogen 23 F. Karrenbrock and H. J. Schafer Tetrahedron Letters 1978 1521. 24 L. L. Miller and L. Christensen J. Org. Chem. 1978,43,2059. ’’ V. Krishnan K. Raghupathy and H. V. K. Udupa J.Electroanalyt. Chem. Interfacial Electrochem.. 1978 88,433. 26 J. Pinson and J.-M. Saveant J. Amer. Chem. SOC.,1978,100 1506. 142 R.Lines derivatives of benzophenone benzonitrile and naphthalene as substrates.For the cyanide ion as nucleophile Ey <Eg and a further separate oxidation was necessary to obtain the products. Reductive acylation previously restricted to activated olefins has now been extended to aryl~lefins~~ by carrying out the reductions in DMF or alkyl cyanides at low temperatures (-10 "C). The reaction is thought to proceed through reaction of the solvent with cathodically generated substrate anions. Formylation by this procedure is reported2' to be particularly successful. Reductive acylation has also been used in the carotenoid field this time for the synthesis of astaxanthin (14)28 (Scheme 14). Controlled-potential electrolysis of astacene (13) in the presence of Scheme 14 acetic anhydride gave either a 2e or 4e reduction product depending upon the potential and solvent system.The 2e product was identified as a new retro tetra-acetate while astaxanthin was obtained (10% yield) after hydrolysis of the tetra- acetate from the 4e reduction pathway. The reductive cyclization of some non-conjugated olefinic ketones to cyclic t- alcohols has been reported by Shono and co-w~rkers~~ to proceed with remarkable regio- and stereo-selectivities (Scheme 15). The reaction apparently takes place between the inner carbon atom of the double bond and the carbonyl carbon to give exclusively cis alcohols. CHR 0-Ketone C cathode -[.'d ' R'] -MeOH-OM F OH Scheme 15 Another paper by Shono and co-~orkers~~ describes the preparation of cyclo- propanes starting from a@-unsaturated carbonyl compounds (Scheme 16).The method is reported to be more selective and efficient than the corresponding Li-NH3 chemical method. *' R. Engels and H. J. Schafer Angew. Chem. Internat. Edn. 1978 17,460. ** E. A. H. Hall G. P. Moss,J. H. P. Utley and B. C. L. Weedon J.C.S. Chem. Comm. 1978,387. 29 T. Shono I. Nishiguchi H. Ohmizu and M. Mitani J. Amer. Chem. SOC.,1978,100 545. 30 T. Shono,Y. Matsumura S.Kashimura and H. Kyutoku Tetrahedron Letters 1978 1205. Electro -orga nic Che rnistr y R' -::q' Rz-R3 Pbcathode-4; R' Dry DMF AR3 MsO R4 R2 R4 R' -R4 =H or alkyl Ms =MeS02-Scheme 16 The cathodic cleavage of alkyl a-benzenesulphonyl carboxylates has been pro- posed" as an alternative to the malonic ester synthesis of carboxylic acids especially where strong acids or bases must be avoided.Preparation of the starting materials is achieved by alkylation of the sulphone by ion-pair extraction. The electrolyses are performed at Hg in DMF to give the benzenesulphinate ion and a good yield of the alkyl ester. Carbon-sulphur cleavage is also the feature of a new synthesis of olefins from &hydroxy-~ulphides,~~ The method is claimed to have wide applicability (Scheme 17). R' ko i. PhSCHILi R' OH Ptcathode4e ii.H+ R2fis,, R2 R2 Scheme 17 A different interpretation of the H-D exchange in recovered starting material from the reduction of phenacyl chloride in DMF-D20 [Ann. Reports (B),1977,74 1613 has been proposed by Merz and Th~mm.~~ The investigators found that reduction of (15; X = C1) in DMF-D20-tetraethylammonium bromide (TEABr) gave deuterium incorporation into the product (15;X =D) and also significantly into the aryl methyl groups.Recovered starting material from interrupted elec- trolysis was similarly deuteriated. Since regeneration of (15) through any inter- mediate on the electrochemical reaction path was rejected by the authors the H-D exchange phenomenon was suggested to be an example of the reactions of electro- generated bases i.e. of carbanion (16). In this system the TEA cation is thought3' to play the important role of excluding water from the electrode double layer thus preventing the immediate protonation of the carbanion by water.c1 c1 (4-MeCsH4)2C=C/ (4-MeC,jH4)2C=C-/ 'x (16) X = C1 or D(H) (15) 31 B. Lamm and K. Ankner Actu. Chern. Scund. 1978 B32 193. 32 T. Shono Y. Matsumura S. Kashimura and H. Kyutoku Tetrahedron Letfers 1978 2807. 33 A. Merz and G. Thumm Tetrahedron Letters 1978 679. 144 I?. Lines An X-ray crystallographic e~amination~~ of the product from the electro- hydrocyclodimerization reaction of dimethyl benzene-l,2-diacrylate has revealed that the originally proposed structure is incorrect [Ann.Reports (B),1976,73,148] and is in fact the tetracyclic compound (17). I CH,R (17) R=COzMe 4 Miscellaneous Chemically Modified Electrodes.-An important aspect of chemically modified electrodes is their potential catalytic applications and in this connection it would be useful to compare their characteristics with the corresponding homogeneous systems.With this aim Andrieux and Sa~eant~~ have now presented an analysis of the cyclic voltammetric response of catalytic electrodes thus enabling the catalysis kinetics to be extracted from experimental data. Another important aspect of modified electrodes is the ability to predict the redox potentials of the immobilized species. To this end twenty three redox couples have been both in solution and bound on an electrode surface. The results showed that the bound redox species were thermodynamically predictable i.e. the immobilization had only a slight effect on the formal potentials. In this connection it has been pointed that second harmonic a.c.voltammetry is a much more sensitive technique than cyclic voltammetry for the measurement of redox potentials and surface characteristics of electrode-bonded species. Following last year's report on the favourable properties of sulphur polynitride (SN) for chemical modification [Ann. Reports (B) 1977 74 1631 a paper has appeared3' in which pretreatment of the perpendicular fibre ends of (SN),by a Ru"' complex resulted in an electrode catalytic for the &/I-couple. Interestingly pretreatment of the parallel fibre ends resulted in no catalytic The attachment of redox groups to electrode surfaces generally requires a linking agent. Such agents must be both chemically and electrochemically inert and be reasonably small so that electron transfer from the base electrode material is not impeded.Until recently the most common linking agents were silane derivatives but cyanuric chloride (C1CN)3appears to be a good alternati~e.~~ The compound 34 J. Anderson L. Eberson and C. Svensson Actu Chem. Scand. 1978 B32,234. 35 C. P. Andrieux and J.-M. Saveant J. Electroanalyt. Chem. Interfacial Electrochem. 1978 93 163. 36 J. R. Lenhard R. Rocklin H. Abruna K. Willman K. Kuo R. Nowak and R. W. Murray J. Amer. Chem. SOC.,1978,100,5213. 37 A. F. Diaz and K. K. Kanazawa J. Electroanalyt. Chem. Interfacial Electrochem. 1978,86 441. 38 A. N. Voulgaropoulos R. J. Nowak W. Kutner and H. B. Mark jun.,J.C.S. Chem. Comm. 1978,244. 39 A. M. Yacynych and T. Kuwama Analyt. Chem. 1978,50,640.Electro-organic Chemistry 145 binds to the surface hydroxy-groups (of graphite or Sn02 electrodes) to give stable ether linkages while the chloride groups allow the attachment of a wide variety of redox reagents. Electrode surfaces can also be modified by coating with an organic polymer. These 'polymer electrodes' are simple to prepare are tough and have some interesting properties; e.g. Miller and van de Mark4' have shown that the reduction of FeC12 although reversible at Pt is irreversible at a polyester-coated electrode. The same have also demonstrated that a poly(4-nitrostyrene)-coated electrode is catalytic for the reduction of oxygen. A point that has been made by several investigators is that charge transfer to a solution species via an electroactive polymer electrode only takes place at potentials where the polymer is charged.Thus Merz and Bard4' have suggested that for polyvinylferrocene electrodes electron transfer at potentials where the polymer is not reduced takes place through holes or channels in the coating. Miscellaneous Processes.-The electrochemical oxidation of titanium in a 1 :1 :1 mixture of methanol acetonitrile and acetic acid has been found4j to be a con- venient source of Ti"'. The electrogenerated reagent can be used to reduce organic compounds in the same way as aqueous Tic& and gives similar yields and reaction rates. An interesting synthesis of carboxylic acid has resulted from the elec- trolysis of trialkylboranes in the presence of ap-unsaturated esters (Scheme 18).Pt. Undivided cell R3B +CH,=CCO,Et MeCN-TEABr 'RCH2CHCO2Et I I R' R' Scheme 18 The mechanism is thought to involve the products of both electrodes in a similar fashion to that reported for the synthesis of nitroalkanes from organoboranes [Ann. Reports (B),1976 73 1381. A novel application of strong acid ion-exchange resins has been in trapping electrogenerated intermediate^.^' Nitrilium ions generated by the oxidation of hydrocarbons in acetonitrile were trapped on a sulphonic acid resin. Filtration of the resin followed by treatment with aqueous NaOH liberated the amide products and regenerated the resin. Higher yields of amides are reported45 for this method compared to oxidation in the absence of resin. The first example of a synthetic compound displaying the sequential transfer of two electrons at the same potential has been claimed46 for the binuclear Cu" complex (18).In DMF (18) underwent reversible electron transfer at -0.47V (SCE) with a peak separation of 42mV in agreement with the Polcyn and Shain theory for reactions involving sequential transfer of electrons. 40 L. L. Miller and M. R. van de Mark J. Electroanalyt. Chem. Interfacial Electrochem. 1978,88 437. 41 M. R. van de Mark and L. L. Miller J. Amer. Chem. Soc. 1978,100,3223. 42 A. Merz and A. J. Bard J. Amer. Chem. Soc. 1978,100,3222. 43 0. Christofis,J. J. Habeeb R. S. Steevensz and D. G. Tuck Canad. J. Chem. 1978,56,2269. 44 Y. Takahashi K. Yuasa M. Tokuda M. Itoh and A. Suzuki Bull.Chem. Soc. Japan 1978,51,339 45 A. Bewick J. M. Mellor and B. S. Pons J.C.S. Chem. Comm. 1978,738. 46 D. E. Fenton R. R. Schroeder and R. L. Lintvedt J. Amer. Chem. SOC.,1978,100 1931. 146 R. Lines (18) Recent interest in the area of radical ion chemistry has centred on the anomalous reactivities of aromatic cation radicals with various nucleophiles. A theory which prompted some discussion4’ was put forward by Eberson and co-w~rkers,~~ based on the Dewar-Zimmerman (D-Z) rules. A paper has now appeared by Eberson and Nyberg4’ in which cation radical reactivity vs. nucleophiles is re-examined in the light of thermochemical calculations. The conclusion49 is that the previous arguments based on the D-Z rules are open for discussion in that the reactivity is mainly determined by the relative oxidation potentials of the parent substrate and nucleo- phile respectively.The question ‘do ece mechanisms occur under conditions where they could be characterized by electrochemical kinetic techniques?’ has been raised by Amatore and Saveant.” The authors considered the ece route (i) and the disproportionation mechanism (ii) (Scheme 19) where C is more easily oxidized (or reduced) than the A*e $ B A*e =k B 1. B+C C*e * D 11. B*C B+C $ A+D Scheme 19 starting material. It was concludedSo that measurement times <0.1 ps would be required to characterize the ece process and therefore ece mechanisms of this type do not occur in conditions where they could be directly characterized by electrochemical techniques.A potentially valuable source of data on electrode reactions is to carry out the reactions in a calorimeter. The feasibility of this approach has been explored51 using the reduction of the trichloroacetate ion in aqueous solution. To give meaningful data the chosen reaction must have unit current efficiency and no side reactions. The results of the AH calculation on the trichloroacetate ion reduction indicated5’ that although further refinement of the apparatus was required the method showed promise where alternative forms of calorimetry would be difficult. 47 M. J. Shine B. K. Bandlish and M. T. Stephenson Tetrahedron Letters 1978 733. 48 L Eberson Z. Blum B. HelgCe and K. Nyberg Tetrahedron Letters 1978,731. 49 L. Eberson and K.Nyberg Acta Chem. Scand. 1978 B32,235. c.Amatore and J.-M. Saveant J. Electroanalyt. Chem. Interfacial Electrochem. 1978,86 227. 51 p. J. Turner and H. 0.Pritchard Canad. J. Chem. 1978,56,1415.

ISSN:0069-3030    

DOI:10.1039/OC9787500135

出版商:RSC    

年代:1978

数据来源: RSC