6 Organometallic Chemistry Part (ii)Main-Group Elements By M. G.HUTCHINGS 1.C.I. Organics Division Research Department Blackle y Manchester M9 3DA. 1 Introduction To maintain continuity with last year's Report' and its two predecessors P Se and Te are excluded as are carbaboranes. Organometallic species containing Li Be and B have been the subjects of various ab initio calculations during part of a wider attempt to develop a 'model chemistry' aimed at establishing relationships between structures and energies of simple molecules rather than merely calculating isolated absolute values. Species studied include phenyl cations,*" ethyl and vinyl cations and their isoelectronic methylboranes,2b and small compounds containing Li or Be.2' Considering the number of new organic synthetic routes based on organo-metallics which have been developed in the past twenty years or so application in larger synthetic schemes had until recently been disappointing.However the full worth of such reagents is becoming recognized as evidenced further by the syntheses mentioned below all dependent on organometallic reagents and pro- cedures which have been recently developed. The enantiomer of the macrocyclic antibiotic dilactone (-)-vermiculin and the closely related fungicide (-)-pyreno-phorin have been synthesized by modifications around lithio dithiane~.~ A 'chemo- regio- and stereo-selective' route to the sex peromone (7E,9Z)-dode- cadien-1-yl acetate in 40 '/o yield from but-1-yne and hept-1-yne has been based largely on organoboron ~hemistry.~ Key reagents are NH2(CH2)3NH-K+ SiazBH (Sia = 3-methylbut-2-yl) and EtCECLi 12 and NaOAc and' finally Sia2BH followed by HOAc.Syntheses of prostaglandins5" and loganid& have included reactions of allylsilanes with electrophiles. Without wishing to labour the point it does seem worthwhile reiterating last year's Reporter's trenchant caveat concerning overliberal interpretation of product studies when assigning structures to reactive intermediates.' Instances of prema- ture inferences based on insufficient data continue to come to light. K. Smith Ann. Reports (B),1976.73 121. (a)J. D. Dill P. von R. Schleyer and J. A. Pople J. Amer. Chem. SOC.,1977,99 1; (6)Y. Apeloig P. von R. Schleyer and J. A. Pople ibid. 1977,99 5901; (c)J.D. Dill P. von R. Schleyer J. S. Binkley and J. A. Pople ibid. 1977 99 6159. D. Seebach B. Seuring H.-0. Kalinowski W. Lubosch and B. Renger Angew. Chem. Internat. Edn. 1977 16 264. E.4. Negishi and A. Abramovitch Tetrahedron Letters 1977 41 1. B.-W. Au-Yeung and I. Fleming (a)J.C.S. Chem. Comm. 1977 79; (b)ibid. 1977 81. 136 Organometallic Chemistry -Part (ii) Main -Group Elements 137 Of a general nature a review has appeared which deals with the preparation and use in synthesis of reactive metal powders by reduction of their salts with alkali metak6 A recently published volume contains articles on organometallic synthesis via metal atoms,7a T-ligands containing organosilicon structures of organometallic compounds containing electron-deficient bridge and organome talk radical anions.7d Due to an oversight' in last year's Report' the comments concerning stabilized silicenium ions failed to include reference to a note by Mislow and co-workers.' Further evidence relating to the proposed ferrocenyl silicenium ion FcSiPh2' has forced a reappraisal of the data to the effect that this species is now no longer considered to be a necessary discrete intermediate in the observed reaction sequences. Hence the unequivocal existence of a silicenium ion in solution has yet to be demonstrated. 2 Group1 Reviews have appeared concerning the generation and reactivity of di- and poly- alkali metal derivatives of heterofunctionally substituted organic molecules,1o the structure and reactivity of alkali metal enolates," and applications of a-metallated isocyanides in organic synthesis.'* A useful correlation has been drawn between 13C-lH one-bond coupling constants in benzenoid and heterocyclic molecules and the relative reactivity of the corresponding CH moiety toward 1ithiati0n.l~ Incorrect predictions caused by heterosubstituents can be related to the propensity for Li complexation and consequent ortho-directing effects.Presumably the technique could be used in reverse to assist in n.m.r. assignments. Reagents are sometimes favourably modified merely by the change of physical form they undergo when adsorbed on a passive support and methods have been described for the preparation of Na or K in reactive forms on the high surface area of charcoal graphite or alumina.14L1 These reagents and the intercalation compound CsK react with ketones giving enolates which can be subsequently monoalkylated preferably in hexane solvent where competing side reactions are minimized through the reactive species remaining adsorbed on the support.Following a similar procedure nitriles and esters may be a-metallated hetero- geneously with CsK and monoalkylated in good ~ie1ds.l~~ Lithiation of ketone dimethylhydrazones by more conventional techniques generate useful enolate equivalents. A recent study of the mechanistic stereo- chemistry of this reaction (Scheme 1)15 showed that regardless of the hydrazone 'R. D. Riecke Accounts Chem. Res. 1977,10,301. '(a) P. L. Timms and T. W. Turney Adv. Organometallic Chem.1977 15 53; (6) I. Haiduc and V. Popa ibid. p. 113; (c)J. P. Oliver ibid. p. 235; (d)P. R. Jones ibid. p. 273. 'K. Smith personal communication. P. Bickart F. M. Llort and K. Mislow J. Organometallic Chem. 1976,116 C1. lo E. M. Kaiser J. D. Petty and P. L. A. Knutson Synthesis 1977 509. L. M. Jackman and B. C. Lange Tetrahedron 1977 33 2737. '* U. Schollkopf Angew. Chem. Internat. Edn. 1977 16 339. l3 E. B. Pedersen J.C.S. Perkin II 1977,473. 14 (a)H. Hart B. Chen and C. Peng Tefruhedron Letters 1977,3 121; (b)D. Savoia C. Trombini and A. Umani-Ronchi ibid. 1977 653. '' M. E. Jung and T. J. Shaw Tetrahedron Letters 1977 3305. M. G. Hutchings 16' cyclopentanones. Me i,ii + CD,CH2 lNMe2 kN + CD,CH2 +N ,NMe2 CH,CH2 CH,CH- Lit or It CD,CH CD,CH 111t Et Pr' NMe Reagents i.LiNEt2; ii CD31;iii CH31 Li+ Scheme 1 orientation the primary carbanion is initially generated. The second anti depro- tonation is kinetically favoured presumably because of Me2N hindrance to the alternative but the syn carbanion is thermodynamically favoured as evidenced by subsequent isomerization. Alkylation of enolates resulting from the addition of lithium organocuprates to cyclopentenones usually gives the 2,3-dialkylated cyqlopentanone with trans-orientation of the alkyl substituents. However the presence of a 3-phenyl group causes the incoming alkylating agent to attack cis,in a highly stereoselective fashion. 16a The reasons for this reversal of stereochemistry are presently unknown but a possibly related phenomenon concerns the directing effect of a 3-aryl or -benzyl group during the formation of lithium enolates of The thermodynamically favoured 2-enolate is formed pref- erentially the 3-substituent possibly acting to co-ordinate and thereby direct and stabilize the adjacent Li' by n-complexation.This model finds support from an independent theoretical study (CND0/2) which predicts Li+-C6& to exist as a pyramidal face-metallated species (c& symmetry) in contrast to H+-C6H6 (corner protonated; C2,symmetry).l6' The preference for this structure can be rationalized by the .rr-bonding ability of the unoccupied p-orbitals on Li' in conjunction with filled n-orbitals of C6H6 of suitable symmetry. An anionic equivalent of the Friedel-Crafts cycloacylation forms the basis of a general ring-forming synthesis (Scheme 2 e.g.Z = CH2CH2CH2).17a For Z = CH2 LiO OLi 0 Reagents i RLi; ii H20 Scheme 2 or CMe2 ring formation is not observed but rather reactions of an alternative type occur that for Z=CMe2 E=C02H being depicted in its most general form in Scheme 3. In principle Z and E can assume a variety of chemical identities (e.g. l6 G. H. Posner and C. M. Lentz (a) Tetrahedron Letters 1977,3215; (b)ibid. 1977,321 1 (c)D. Heidrich and D. Deininger ibid. 1977 3751. " (a) R. J. Boatman B. J. Whitlock and H. W. Whitlock jun. J. Amer. Chem. Suc. 1977 99,4822; (b) D. Hellwinkel G. Hofmann and F Lammerzahl Tetrahedron Letters 1977 3241. Organometallic Chemistry -Part (ii) Main -Group Elements 139 Oz\Eef nz-Li+ -P -b \ E-Li' ' Li \E Scheme 3 2=NR,0;E = COPh) and other examples have been gi~en."~ Furthermore if Z and E themselves are initially part of a ring the product itself will be bicyclic.The inclusion of a negative!y charged atom a to the carbonyl carbon of an a',6'-unsaturated ketone system suppresses addition of RLi to the carbonyl group with consequent preference for P'-attack as exemplified by reactions of the ylid in Scheme 4." An alternative approach to the inhibition of conventional addition to a carbonyl group relies on the effect of extreme steric hindrance to reagent attack. 0 0 .. ... +cO,tl fl.111 ,9c02Me +CozEt +PPh +PPh Bun Reagents i MeLi; ii Bu"I; iii HC-MeOH Scheme 4 Metallations at surprising sites are thereby promoted as in the generation of (la) from (lb),19" and the metallation of the Y-Me group of (3),where Y = NR S,I9' or 0.19' Reaction of any of these species with electrophiles and suitable work-up leads to products derived from the uncommon synthons HYCH2-.By formally adding two electrons to the pentadienone system electrophilic attack at the terminal 5-position ('E' activity') is enhanced at the expense of the conventional nucleophilic attack ('Ns activity'). In practice a direct E5synthon (3) has been generated by double deprotonation of allylacetophenone and shown to react regio- and stereospecifically with electruphiles.20 Studies of polymer-bound organolithium species have given interesting synthetic and mechanistic results as for instance in the reaction between @-C6H4SCH2-Lit and (4a) followed by MeI/NaI.*l" The product ratio [(4c)]/[(4b)] gives a measure of the extent of site interaction on the polymer; high PhCH 0 a N - R P r i ~ ~ o 0 ~ ~ e \/ PhCH 0 Pr' KH2)n I (1) a; R=CH2Li b; R=CH3 (3) (4) a; n =4 b;n=5 c; n=6 M.P. Cooke jun. and R. Goswarni J. Amer. Chem. SOC.,1977 99,642. l9 (a)R. Schlecker and D. Seebach Helv. Chim. Acta 1977,60 1459; (b) P. Beak B. G. McKinnie and D. B. Reitz Tetrahedron Letters 1977 1839; (c)P. Beak and B. G. McKinnie J. Amer. Chem. SOC. 1977,99,5213. 2o M. Pohrnakotr and D. Seebach Angew. Chem. Infernat. Edn. 1977,16 320. 21 (a)G. A. Crosby and M. Kato J. Amer. Chem. SOC.,1977,99,278; (b)B.J. Cohen M. A. Kraus and A. Patchornik ibid. 1977 99 4165. 140 M. G. Hutckings ratios signify ionic clustering as is in fact observed in this particular system. Polymer-bound trityl-lithium ((&CC,H4C-Ph2Li') has been used to generate the enolate from acetophenone which then reacts with an independent polymer-bound acylating agent (@-C6H3N020COPh) to give high yields of the diketone (PhCOCH2COPh).21b In contrast the corresponding reaction with wholly soluble reagents gives a much inferior yield. The reaction sequence is capable of extension by use of a third reagent giving rise to what is termed a 'chemical cascade'. Among other useful new organolithium reagents are RCC12Li (from RCC12H including CH2C12) which reacts with a variety of electrophiles,22 and cis-X-CH=CH-Li (X = OEt..NR,) which act as CHOCH2-equivalents.23 In a study of the gas phase acidities of cis- and trans-but-2-enes by pulsed ion-cyclotron resonance spectroscopy the cis-butenyl anion has been found to be 0.2 kcal mol-' less stable than its trans-is~rner.~~ This result has relevance to solution-phase studies of butenyl anions where the cis-isomer has been found to predominate to a greater extent as the electropositivity of its alkali metal coun- terion M+ increased. Since ion-separation is expected to increase with more electropositive M+,it had been assumed that the cis-preference was to be ascribed to the relative stabilities of the free anions. However it now appears to arise because of differences in interaction energies among the allylic skeleton M' and the solvent.Ab initio calculations at minimal basis and split basis levels on the species C3Li4 including geometry optimization have predicted the strange ground-state geometry depicted in (5) (C2"symmetry) where the angle between the two central Li-C bonds is only 93°.25The central Li' ions bond with the highest occupied nonbonding .rr-orbitals of the C3 chain (formally and conveniently considered a C34- ion isoelectronic to C02)(6). Li\ /c\ Li c \c / Li Li Attention is drawn to a review of the umpolung of the reactivity of carbonyl compounds with particular reference to lithiated sulphur heterocyclic reagents.26 Besides this review and work cited elsewhere in this Chapter Seebach and his prodigious group have published in 1977 a series of papers in Annalen and Chem.22 J. VilliCras P. Parriot and J. F. Normant Bull. SOC.chim. France 1977 765. 23 J. Ficini S. Falou A.-M. Touzin and J. d'Angelo Tetrahedron Letters 1977 3589; R. H. Wollenberg K. F. Albizati and R. Peries J. Amer. Chem. Soc. 1977,99,7365;L. Duhamel and J.-M. Poirier ibid. 1977,99,8356. 24 J. E. Bartmess W. J. Hehre R. T. McIver jun. and L. E. Overman J. Amer. Chem. SOC. 1977 99 1976. 25 E. D. Jemmis D. Poppinger P. von R. Schleyer and J. A. Pople J. Amer. Chem. SOC. 1977,99,5796. 26 B. T. Grobel and D. Seebach Synthesis 1977,357. Organometallic Chemistry -Part (ii) Main -Group Elements 141 Ber. which give details of previously communicated material (much of which has been highlighted in Annual Reports in previous years) but which also include new illuminating insights.3 Group I1 Magnesium.-A useful review has appeared on the one-step reaction of organic halide Mg and substrate (Barbier rea~tion).~' Species of general formula HMgR are surprisingly ill-characterized. However the preparation of a particularly active form of MgH from the reaction of LiAlH4 and Et2Mg has opened up routes to these species based on 1:1 redistribution reactions with appropriate R2Mg. Thus HMgAlkyl may be prepared in THF in which it is stable at room temperature contrary to earlier reports.28"*b The species is dimeric in dilute solution (bridging hydrogens) and is reported to give the products of both reduction and alkylation on reaction with ketones.Perhaps the most significant result from this series is the observation that HMgCl and HMgBr add to alkenes and alkynes.28c No details are yet available but hopefully we can look forward to a hydrometallation agent analogous to diborane but which leads to Grignard reagents. Other related species are HMgOAr which are highly stereoselective reducing agents,28d and HMgPh prepared directly by reaction between LiAIH4 and Ph2Mg.28e An interesting new addition reaction of organomagnesiums is the transition metal catalysed addition of Me3SiCH2MgC1 to diketene yielding Me3SiCH2C(:CH2)CH2C02H.zg The reaction provides the first example of vinyl-oxygen bond cleavage in diketene but conceivably the catalysts rather than the Grignard itself could be responsible for the mode of ring opening.Copper- promoted additions of Grignard reagents continue to offer useful synthetic sequences including the addition to 3-alken- 1-ynes predominantly at the 2-posi- tion as exemplified in a synthesis of myrcene 30a the direct stereospecific replace- ment of iodide from vinyl iodides,30b and regiospecific addition of unsymmetrical ally1 Grignards to alkyl halides used in a simple synthesis of gerani01.~'~ A purely electrophilic mechanism is proposed for the addition of 2-norbornyl- magnesium bromide to formaldehyde based on similar exo :endo ratios for product and starting material.31 In contrast an electron-transfer mechanism is believed to apply in the rare 'wrong end' addition of RMgX to the carbonyl oxygen in o-quinol acetates.32 Zinc Cadmium and Mercury.-Alkynylzinc reagents couple with alkenyl halides in the presence of Pd catalysts to give high yields of conjugated en~nes.~~~ Besides 27 C.Blomberg and F. A. Hartog Synthesis 1977 18. E. C. Ashby and A. B. Goel (a)J. Org. Chem. 1977,42,3480;(6) J.C.S. Chem. Comm. 1977,169; (c) Inorg. Chem. 1977 16 2941; (d) E. C. Ashby A. B. Goel and J. J. Lin Tetrahedron Letters 1977 3133; (e)E. C. Ashby and A. B. Goel Inorg. Chem. 1977,16,1441. 29 K. Itoh T. Yogo and Y. Ishii Chem. Letters 1977 103. 30 (a) H. Westmijze H. Kleijn J. Meijer and P. Vermeer Tetrahedron Letters 1977 869; (b) A. CommerGon J. F. Normant and J. VilliCras J. Organornetallic Chem. 1977 128 1; (c) F. Derguini-Boumechal R. Lorne and G. Linstrumelle Tetrahedron Letters 1977 1181.31 D. E. Bergbreiter and 0.M. Reichert J. Organometallic Chem. 1977 125 119. 32 B. Miller J. Org. Chem. 1977 42 1402 1408. 33 (a)A. 0.King N. Okukado and E.-I. Negishi J.C.S. Chem. Comm. 1977,683; (b)E.-I.Negishi A. 0. King and N. Okukado J. Org. Chem. 1977,42 1821. M. G. Hutchings being >97 ‘/o stereospecific the procedure is compatible with the presence of various electrophilic functionalities in the alkene precursor and is unique in allow- ing the direct preparation of terminal enynes CH,=CH-C=C-R without pro- tection-deprotection sequences. A related reaction involving aryl- or benzylzinc compounds and aryl halides leads to high yields of unsymmetrically coupled pro- ducts and is again tolerant of further substitution in the aryl halide.33b The silylated cyclohexenol (7) undergoes synthetically useful spirocyclo-propanation to (8) in the presence of excess Et2Zn-CHJ2 in concentrated benzene solution (Scheme 5).34” In contrast trimethylsiloxyrnethylenecyc’fopentanegives (9) under Simmons-Smith conditions (CH212-Zn-C~).34b In both these reactions Zn12 apparently causes ring opening of the cyclopropane and either H-or ring C- migration.Me,SiO Reagent i Et2Zn-CH212; ii ZnIz I Scheme 5 The conventional view that substituents preferentially adopt the equatorial orientation in monosubstituted cyclohexanes is no longer tenable in the light of variable temperature I3C and Ig9Hgn.m.r. studies of various cyclohexylmercury It is shown unambiguously that the mercury substituents preferentially occupy an axial position.Photolysis of ty -amino-acids with various aliphatic side chains in the presence of HgC12 leads to methylmercury chloride from an apparent fragmentation of the side chain of the amino-a~id.~~ It is tempting to infer a connection with the formation of highly neurotoxic methylmercury found in the natural environment. 4 Group I11 Boron.-A review has appeared dealing with boraheterocycles prepared via hydroboration . their use as reagents and intermediates being particularly emphasized.37 By redistribution of Me2S,BH3 with BC13 and BBr3 in the appropriate stoi- cheiometry Me2S,BHX2 and Me2S,BH2X (X = C1 Br) are made readily avail- able.38 Reagents such as these are particularly important in preparations of unsymmetrical trialkylboranes or variously substituted mono- and di-alkylboranes 34 (a) I.Ryu S. Murai and N. Sonoda Tetrahedron Letters 1977,4611; (6) I. Ryu S. Murai S. Otani and N. Sonoda ibid. 1977 1995. ’’ P. F. Barron D. Doddrell and W. Kitching J. Organometallic Chem. 1977 139 361. 36 K. Hayashi S. Kawai T. Ohno and Y. Maki J.C.S. Chem. Comm. 1977. 158. 37 H. C. Brown and E.-I. Negishi Tetrahedron 1977 33 2331. H. C. Brown and N. Ravindran (a) J. Amer. Chem. Soc. 19?7,99,7097; (6) Synthesis 1977,695; (c)J. Org. Chem.. 1977,42 2533. Organorne ta llic Chemistry -Part (ii) Main -Group E1ements all of which have their place in the repertoire of synthetic organic chemistry. Significant observations are that the bromo compounds give appreciable amounts of tertiary alkylboranes with trisubstituted alkenes and the ease with which Me2S,BHBr2 effects hydroboration in comparison with Me2S,BHC12.38" It is rele- vant that there is evidence for .rr-interaction between benzene and Me2S,BHBr2 which is absent from the chloro analogue.Reaction between BBr3 and (Me3Si)3N gives MeBBr2 essentially quantitatively and free from di- or tri-methyl derivative^.^^ Easy routes to methylboranes are always interesting since hydroboration is obviously inapplicable for their preparation. Tertiary alkylboranes are also rela- tively inaccessible so the sequence bromination (Br2-hv) and alkylation (RM) when applied inter alia to secondary alkyl boronate esters giving tertiary alkyl boron derivatives is of potential synthetic value.4o The reagent Li'Et,BH- may be used in a route to other novel boranes and borates involving nucleophilic hydro- boration of styrenes under relatively mild condition^.^^ Finally w -alkynylalkyl-boranes inaccessible by hydroboration may be prepared by prototropic iso-merization [K+NH2(CH2)3NH- catalyst] of the available internal alkynylborane itself prepared by hydroboration of a terminal nonconjugated enyne.'* The increasing problems associated with the use of racemic materials as phar- maceuticals (etc.) has heightened the need for asymmetric reagents.Therefore the renewed interest in chiral boranes as reducing agents is particularly timely and is reflected in several publications dealing with modified isopinocampheylborane systems.The reagent (10) derived from 9-borabicyclononane reduces aldehydes (10) with concommittant elimination of alkene and has been used to reduce deutero- benzaldehyde to (S)-benzyl-a-&alcohol (important for mechanistic biochemical studies) with essentially quantitative asymmetric indu~tion.~~" Bu'Li donates H- to the B atom of (lo) the new chiral trialkylborohydride reducing even relatively hindered ketones rapidly and q~antitatively.~~' Optical purities of products range only from 3-37 % but of more importance is the finding that the predominant enantiomers of all alcohols produced to date have the same absolute configuration. In a reaction analogous to the well known radical addition of R3B to enones and analogues B-alkynyl-9-BBN undergoes conjugate addition to those a$ -unsaturated ketones which are capable of assuming a cisoid conformation but apparently by a nonhomolytic pathway.Further elimination in the reaction with (11)leads to conjugated enynone~.~~ Dialkyldialkynylstannanes react with Et3B or 39 I<. Barlos and H. Noth Chem. Ber. 1977,110 3460. 40 H. C. Brown N. R. DeLue Y. Yamamoto K. Murayama. T. Kasahara S.-I. Murahashi. and A. Sonoda J. Org. Chem. 1977,42 4088. 41 H. C. Brown and S. C. Kim J. Org. Chem. 1977.42,1482. 42 C. A. Brown and E.-I. Negishi. J.C.S. Chem. Comm. 1977 318. *' (a) M. M. Midland A. Tramontano and S. A. Zderic J. Amer. Chem. SOC.,1977,99,5211; (b) S. Krishnamurthy F. Vogel and H. C. Brown J. Org.Chem. 1977,42,2534. 44 J. A. Sinclair G. A. Moiander and H. C. Brown J. Amer. Chem. Soc. 1977 99,954; G.A. Molander and H.C. Brown J. Org. Chem. 1977,42,3106. M. G. Hutchings Me0 (11; Bu;B to give the novel heterocyclic dienylboranes (12) which can be used for further synthetic m~dification.~~ Of relevance to the mechanism of the photo- chemically induced rearrangement of dialkyl-trans -1,3-dienylboranes is the finding that the independently generated cis-isomers rapidly cyclize with concur- rent alkyl migration to (13).46 The rearrangement is formally isoelectronic to the second migration step in the reactions between acyIating agents and alkynyl- or cyano-borates. R (12) (13) Reports of new reactions of borate salts have been few in 1977.However a reaction of great potential value in view of the number of structural features capable of modification is the Michael addition of alkynylborates to a,p-unsaturated systems depicted in general form by equation (l).47As yet only nitroethenes have given synthetically useful yields of organic products although reaction with even these is inhibited by excessive substitution [R3 = R4= Me equa- tion (l)]. R’ Li+R:BCECR2 +R3R4C=C(X)R5 + R:BL=CR2-CR3R4-CR5XLi+ (1) Following on from last year’s claim for the generation of ‘methylborylene’ (MeB:) the analogue ‘naphthylboryne’ (a-NpB:) has been proposed as a primary product in the photolysis of ~-NP,B.~~ Evidence for this species rests solely on indirect product studies. Firstly oxidation products observed were cyclohexanol when cyclohexane was solvent and cyclohexane-cis- 1,2-diol for reaction in cyclo- hexene the latter uia the assumed ‘stable’ species (14); and secondly Cl- ions and COCl were observed after hydrolysis and oxidation with CC14 as solvent If (14)is (14) indeed stable further data on its characterization are awaited but in the meantime readers are invited to make up their own minds on the rightness of the claim for the generation of a boryne.45 L. Killian and B. Wrackmeyer J. Organometallic Chem. 1977,132 213. 46 G. Zwiefel S. J. Backlund and T. Leung J. Amer. Chem. SOC.,1977.99 5192. 4’ A. Pelter and L. Hughes J.C.S. Chem. Comm. 1977,913. 48 B. G. Rarnsey and D. M. Anjo J. Amer. Chem. SOC.,1977,99 3182.Organometallic Chemistry-Part (ii) Main -Group Elements The methylated cyclopentadienylborane (15; X =BC12) prepared from (15; X = Me3Ge) and BC13 is fluxional on the 'H n.m.r. time scale down to -80°C due to rapid BCl sigmatropic shifts.49" In contrast (15; X = B12) undergoes further reac- tion with B13 yielding the novel salt (16) characterized by a full range of spec-troscopic and conductivity techniq~es.~~' The bonding of the Csvpyramidal cation in (16) is assumed to be similar to that in the dication C,(CH3b2'. An interesting (15) (16) series of 1,3-shifts has been observed for (17; Scheme 6).50Addition of MeLi to (17; R2 = Me) leads to fragmentation products resulting from migration of R' from C to B (Scheme 6). When R' # R3 R3BMe2 is also observed consistent with the rapid sigmatropic shift of the BMe unit prior to ate-complex formation.R'QR3 d- R'@R3BR~( R'@R3BRZ( A.-e*c. R' , R3 R' R3 R' R' R " BMe2 R' R' 1 ArLi L 7+ BR~+borepin R' R'BMez Ri R3 R'BMe2-Li' R3 Reagent i LiMe (R2=Me) Scheme 6 The recently published MNDO method has been employed to calculate various data for boranes (AHf geometries IP dipole moments and proton affinities) which agree much more satisfactorily with known experimental data than did the previous MIND0/3 values5* Aluminium Gallium and Thallium.-The lack of ready synthetic approaches to alkylalanes has prevented exploitation of their full potential in organic synthesis so approaches reported in 1977 which go some way in removing this deficiency open up important new areas for future research.The alkene- or alkyne-derived metal P. Jutzi and A. Seufert (a)Angew. Chem. Internal. Edn. 1977 16,41; (b) ibid. 1977 16,330. J. J. Eisch and J. E. Galle J. Organometallic Chem. 1977 127,C 9. " M. J. S. Dewar and M. L. McKee J. Amer. Chem. SOC. 1977,99 5231. 146 M. G. Hutchings complexes Cp2Zr(C1)R readily transfer an alkyl or alkenyl group respectively to AICl3. The resulting species react rapidly with acyl chlorides at -30°C to give quantitative yields of ketones whereas the starting Zr complex reacts at best only slowly.52" An even greater synthetic potential results from modifications which use the transition metal catalytically. Both AIH3 and more significantly LiAIH4 in the presence of TiC14 or ZrC14,52b as well as readily prepared (PrlNkAlH in the presence of Cp2TiC12,52c give alkylalanes or alanates on reaction with alkenes.The reactions show analogies to hydroboration with BH3 in their sensitivity to steric effects (allowing preferential reduction of the terminal alkene bond in noncon- jugated dienes) and preference for strongly basic ethers as solvents. By suitable choice of reagents further reaction generates alkane alkyl halide or alcohol (02 oxidation) in high yields. The reagents are cheap and the conditions mild but a critical limitation to the generality of the reaction is the conversion of internal alkenes to terminal alkyl groups. Of course this may be turned to advantage when mixtures of isomeric alkenes are used as starting material and the terminally substituted product is required.A further type of novel organoalane formulated as (18) is also derived by transfer of a group from the more readily formed Zr derivative Cp2Zr(CI)COR.53 0 I1 (C12Al-C -R)x (18) While masked carbonyl functionalities have found favour as one source of acyl anion equivalents the more direct progenitors of this synthon acylmetallics have proved singularly disappointing as synthetic intermediates. However the acyl moiety present in (18) is highly susceptible to electrophilic attack. Thus (18) is protonated (or deuterated) at the carbonyl carbon by water (or D20),and is both C-and 0-acylated with CH3COCl. The possible existence of a stable Group I11 acylmetallic species such as (18) is noteworthy in that no stable acylborane analogue has yet been prepared.Attempts at further structural characterization of (18)would be desirable. BukAlH reacts with Cp2ZrC12 to give (19) which acts as one of the few homo- geneous catalysts for the industrially significant Fischer-Tropsch process (reductive polymerization of CO).54 Straight chain C1-Cs alcohols are obtained remarkably from a room temperature reaction at atmospheric pressure all carbon atoms of the products being incorporated from the CO feedstock. H-AlBu; /\ cp2zrH /c' H-AIBu; (19) " (a)D. B. Carr and J. Schwartz J. Amer. Chem. SOC. 1977 99 638; (b) F. Sato S. Sato H. Kodama and M. Sato J. Organometallic Chem. 1977 142 71; (c)E. C. Ashby and S.A. Noding Tetrahedron Letters 1977 4579. 53 D. B. Carr and J. Schwartz J. Organometallic Chem. 1977 139 C 21. 54 L. I. Shoer and J. Schwartz J. Amer. Chem. SOC. 1977,99,5831. Organom eta llic Chemistry -Part (ii) Main-Group Elem en ts An unusual type of isomerism exists in the oxamide complexes (20) and (21) of Me I Thallium (111) trifluoracetate brings about nonphenolic oxidative coupling of aromatic systems. The reaction is tolerant of nitrogen- and oxygen-containing functionalities and is applicable to intramolecular couplings as in a synthesis of the aporphine alkaloid (f)-~coteine.~~ The corresponding thallium (111) acetate effects an unprecedented aromatic acetoxylation during the coupling. 5 GroupIV Silicon.-One of the archetypal components of organic chemistry the benzene ring has still to find a Si analogue.However a report in 1977 claims evidence for the generation and trapping of the first silabenzene (22; Scheme 7).57 Unfortunately an alternative route to the observed (23) involving carbanionic attack by the lithiated intermediate (24) and subsequent ring closure cannot be excluded on the evidence available. Nor are other anticipated products observed which might be indicative of the intermediacy of (22) such as dimers. In the (24) (22) Reagents i (Me3Si)zNLi -77 "C; ii CF3C=CCF3 Scheme 7 absence of more compelling data the case for a silabenzene must at best remain unproven. Two related synthetic approaches have been used to prepare species which undergo reactions now generally ascribed to ~ilaethenes.~' By varying R (Scheme 8) the silaethene can be generated in solution and trapped at tempera- tures as low as -50 "C (R =Ts) to give products resulting from initial 2 +2 2 +3 and 2 +4 cycl~additions.~~" In the absence of trapping agents dimers are formed.58 Furthermore other more stable lithiated intermediates (e.g.R = Ph2PO) can be isolated and subsequently thermolysed in vucuo the silaethene being detected 5s P. Fischer R. Graf J. J. Stezowski and J. Weidlein J. Amer. Chem. Soc. 1977 99,6131. E. C. Taylor J. G. Andrade and A. McKiIlop J.C.S. Chem. Comm. 1977,538. ''T. J. Barton and D. S. Banasiak J. Amer. Chem. SOC.,1977,99 5199. '* (a)N. Wiberg and G. Preiner Angew. Chem. Internat. Edn.1977,16,328; (b)P. R. Jones and T. F. 0. Lim J. Amer. Chem. SOC.,1977,99,2013. 148 M. G. Hutchings Br Br OR Br OR Li II II II MezSi-C(SiMe3)~ Me2Si-C(SiMe& b Me2Si-C(SiMe3)2 -% Me2Si=C(SiMe3)2 Reagents i AgOR; ii Bu"Li; iii A Scheme 8 directly by mass spe~trometry.~~" After last year's report of preparations of stable silacyclopropenes there have now been several discussions of their reactions with methanol and with ketones alkenes alkynes dienes and dimethylsilylene all leading to insertion and ring Pd catalyses dimerization to dis- ilacycl~hexadienes.~~~~' Sila-allene (25a) has been proposed as a product of the photolysis of (26) on the basis of isolation of (25b) following initial in situ cyclo-addition with solvent a~etone.~'" A convenient and eacient synthesis of Me2Si =O results from abstraction of oxygen from DMSO by dimethylsilylene.60 Intermediacy of the first oxasilacyclopropane (27) has been claimed but as usual depends solely on product analyses.61o Novel intramolecular reactions of the carbene (28) have been reported.6l6 Me,Si / Me 0 Ph \ /\ Ph Me PhCGC -SiMe2SiMe3 MezSi -CPhz Me2ii -&Ph (25) a; X=Si b;X=C In the near absence of experimental data on free sila-alkenes the latest compu- tational study of HzSi=CHz offers several useful insights.62 Ab initio calculations which take into account electron correlation effects have predicted HzSi=CH2 to be a singlet planar species in its ground state 28 kcal mol-' below the first triplet.The difference in energy between the planar and orthogonal geometries taken as a reflection of the .n-bond strength has the surprisingly high value of 46 kcal mol-' whereas the barrier to the exothermic dimerization is less than 14 kcal mol-'.The polarity (6 -C=Si S +) and low lying T*level are responsible for the experiment- ally observed reactivity of such species. Moreover the polarization causes the C 2p.n and Si 3p.n orbitals to be enlarged and contracted respectively thus vitiat- ing the conventional view that 'mismatching' of these orbitals is responsible for the weakness of the .n-bond. A cheerless prospect is that bulky substituents will be insufficient to prevent dimerization and attempts to alter the polarity will only result in counterproductive weakening of the .n-bond.A provocative note concerning ab initio calculations on divalent Si has predicted that CHz=Si is considerably more stable than HCESiH although further results predict 59 (a)M. Ishikawa T. Fuchikami and M. Kumada J. Amer. Chem. Soc. 1977,99,245;(b)H. Sakurai Y. Kamiyama and Y. Nakadaira ibid. 1977,99,3879;(c)D. Seyferth S. C. Vick M. L. Shannon T. F. 0. Lim and D. P. Duncan J. Organometallic Chem. 1977,135 C37; (d)D. Seyferth and S. C. Vick ibid. 1977 125 C 11; (e) M. Ishikawa K.-I. Nakagawa and M. Kumada ibid. 1977 131 C 15; (f)M. Ishikawa T. Fuchikami and M. Kumada J.C.S. Chem. Comm. 1977,352. 6o H. S. D. Soysa H. Okinoshima and W. P. Weber J. Orgunometallic Chem. 1977,133 C 17. '' (a)W. Ando M. Ikeno and A. Sekiguchi J. Amer. Chem. Soc.1977,99,6447; (b)W. Ando and A. Sekiguchi J. Organometallic Chem. 1977 133 2 19. 62 R. Ahlrichs and R. Heinzmann J. Amer. Chem. SOC.,1977,99,7452. Organometa11ic Chemistry -Part (ii) Main -Group Elem en ts 149 CH2=SiH2 to be somewhat more stable than H2+ CH2=Si.63 Nevertheless it was suggested that it would be yorth searching experimentally for unsaturated divalent organosilicon species. The burgeoning synthetic chemistry based on vinylsilanes has prompted several new reports of syntheses of these species. Alkynylsilanes react with hydroborating agents leading to a,P -substituted vinylsilanes useful for conversion to a-silaketone~,~~~ carboxylic and stereospecifically a-alkylated vinyl-silane~,~~' and with Cu-promoted Grignard reagent^.^' An alternative approach relies on the addition of vinyl-lithiums to Me3SiC1.66 An important reaction of vinylsilanes is their oxidation to epoxysilanes themselves precursors to carbonyl and stereochemically defined' hetero-substituted alkenes.68 a-Chloroalkylsilanes may be lithiated and react with ketones to give epoxysilanes directly.Since these may be cleaved to aldehyde the overall procedure is one of reductive nucleophilic a~ylation.~~ Because of decreased C-C1 polarity a-chloroalkylsilanes undergo anomalous nucleophilic substitution reactions both in solution (Scheme 9)69aand vapour-solid D + -n R3 i-CH2 + D3SiCH2CI -R3P-CH2SiD2-CH2 pCI -+ R3kH2SiD2CH2DCI-Scheme 9 heterogeneous phases.69b Another paper on substitution reactions at Si has poin- ted out that reagents which favour 1,2-addition to enones (hard bases charge control) substitute at Si with retention of configuration whereas others which add 1,4 to enones (soft bases frontier orbital control) react with inversion at Si.This correlation supports the hypothesis that the electronic character of the nucleophile dictates the stereochemistry of nucleophilic substitution at Si7* A range of tech-niques has shown that both a concerted dyotropic process as well as a 2-step radical process occurs in the thermal rearrangement of (29) to (30),71" whereas silylmethyl acetates are believed to rearrange (alkyl migration) via an unprecedented inverted Si ylid (31),71breminiscent of species frequently invoked in borate chemistry. 63 J.N. Murrell H. W. Kroto and M. F. Guest J.C.S. Chem. Comm. 1977 619. 64 (a)A. Hassner and J. A. Soderquist J. Organometallic Chem. 1977 131,C 1; (6)G. Zwiefel and S. J. Backlund J. Amer. Chem. SOC. 1977 99 3184; (c) K. Uchida K. Utirnoto and H. Nozaki Tetra-hedron 1977 33,2987. 65 (a) H.Westmijze J. Meijer and P. Vermeer Tetrahedron Letters 1977 1823; (b)M. Obayashi K. Utimoto and H. Nozaki ibid. 1977 1805. 66 (a)R. T. Taylor C. R. Degenhardt M. P. Melega and L. A. Paquette Tetrahedron Letters 1977 159; (6) D. Seyferth J. L. Lefferts and R. L. Lambert jun. J.Organometallic Chem. 1977 142 39. (a) C. Burford F. Cooke E. Ehlinger and P. Magnus J. Amer. Chem. SOC. 1977 99 4536; (b) F. 67 Cooke and P. Magnus J.C.S. Chem. Comm. 1977 5 13.68 P. F. Hudrlik A. M. Hudrlik R. J. Rona R. N. Misra and G. P. Withers J. Amer. Chem. SOC.,1977 99 1993. 69 (a) H. Schrnidbaur and B. Zirnmer-Gasser Angew. Chem. Internat. Edn. 1977 16 639; (6) S. P. Hopper M. J. Tremelling R. J. Ginsberg and P. C. Mendelowitz J.Organometallic Chem. 1977 134 173. 70 R. J. P. Corriu and C. Guerin J.C.S. Chem. Comm. 1977 74. 71 (a)M. T. Reetz Chem. Ber. 1977,110,954,965;(b)M. T. Reetz and N. Greif Angew. Chem. Internat. Edn. 1977 16 712. M. G. Hutchings (29) (30) (31) Further parametrization has permitted extension of empirical force field cal- culations to p~lysilanes,~~ and using this technique Ph3SiSiPh3 is calculated to adopt a D3 conformation and to be strain free unlike the unknown Ph3CCPh3.72 Ph3SiOSiPh3 has a linear Si-0-Si unit in the crystalline phase and in agreement with predictions of the above calculations has Sg symmetry (two staggered antichiral C3 propeller^).^^ Finally attention is drawn to the remarkable observation that (32) shows Me group inequivalence in its ‘H n.m.r.spectrum up to at least 150°C whereas its C and Sn analogues have much lower barriers to substituent rotation.74 What happens in the Ge and mixed derivatives? Me,Si SiMe Germanium Tin and Lead.-The rearrangements of benzylideneaminotin (IV) halides (33) to (44) are the first examples of ortho-metallation involving Sn.75 The full scope of the reaction has yet to be demonstrated but it fails in analogous Si and Ge systems. The cycloperoxidation of bistriflates by means of R3Sn02SnR3 has given a route to the bicyclic endoperoxide (35) (of relevance in the prostaglandin field) from (36).76 Me3SnLi is readily prepared and adds 1,4 to conjugated enones Vn, fifi&-JTfOyJ-OTf R \ ’R R \ ’R (33) (34) (35) (36) the resulting stannylated enolate being set up for alkylation and possible further modification (Scheme lo).” The transposition of the carbonyl group by means of oxidative cleavage of the C-Sn bond is particularly noteworthy and demonstrates the superiority of this reageqt-over Me3SiLi which otherwise reacts analogously.The Sn-substituted ylid Ph3PCHCH2SnMe3 undergoes the Wittig reaction with carbonyl compounds to give allyltrimethylstannanes which are valuable precursors 72 J.P. Hummel J. Stackhouse and K. Mislow Tetrahedron 1977 33 1925; W. D. Hounshell D. A. Dougherty J. P. Hummel and K. Mislow J. Amer. Chem. Suc. 1977,99 1916. ’’ C. Glidewell and D. C. Liles J.C.S. Chem. Comm. 1977,632. 74 D. Seyferth and S. C. Vick J. Orgunometallic Chem. 1977 141 173. ’’ B. Fitzsimmons D. G. Othen H. M. M. Shearer K. Wade and G. Whitehead J.C.S. Chem. Comm. 1977,215. 76 M. F. Salomon and R. G. Salomon (u)J.Amer. Chem. Suc. 1977,99 3500; (b) ibid. 1977,99 3501. 77 W. C. Still J. Amer. Chem. SUC.,1977 99,4836. Organometallic Chemistry -Part (ii) Main -Group Elements 151 i,ii &c5H11 &c5H11 (j,c5H11 Me SnMe SnMe 0 Reagents i Me3SnLi; ii C5Hl1I; iii MeLi; iv CrO-jZpy Scheme 10 to difficultly accessible allyllithi~ms.~~ Moreover the Sn compounds themselves are intrinsically useful in that they add to acyl chlorides with Rh catalysis giving good yields of allylket~nes.~' Hyperconjugation by CH2MMe3 (M = Si Ge Sn or Pb) has been investigated using various n.m.r.probes in naphthyf'" and styry1806 systems. The overriding conclusion is that hyperconjugative electron-release in the neutral ground-state follows the order Pb -Sn >Ge -Si in contrast to the commonly-accepted order Pb >Sn >Ge >Si based on studies of electron deficient compounds.8oa Me3M groups which are bound directly to an aromatic nucleus donate electron density by an inductive/field effect but act as negative hyperconjugative substituents.80b 6 GroupV A synthetically useful series of reactions based on lithiated alkylarsine oxides and dependent on the thermal reaction of the halogenated arsorane RPh2AsX2(X = C1 Br or I) for the critical cleavage of the R-As bond is summarized in Scheme 11.81a.6 Two-carbon homologation of alkyl halides may be effected indirectly in R4R5C= CHBr -CH~R' J R'=H ii,iii.v.vi R'R2CHR3 II I 'ii,viii R4R5C=CBr2 Ph2As-CHR' R1= Reagents i LiNPri; ii R2X; iii LAH; iv ha12 or S02C12-A; v Br2-A; vi R3(=OH-or PhS-); vii R4R5D=0; viii excess BrZ-A.Scheme 11 good yield by addition of their alkyllithium derivatives to the vinyl group of diphenylvinylarsane followed as above by bromine or sulphuryl chloride.81' The '* D. Seyferth K. R. Wursthorn and R. E. Mammarella J. Org. Chem. 1977,42 3104. 79 M.Kosugi Y.Shimizu and T.Migita J. Organometallic Chem. 1977,129,C 36. *' (a)W. Adcock D. P. Cox and W. Kitching J. Organometaiiic Chem. 1977 133 393; (b) W. F. Reynolds G. K. Hamer and A. R. Bassindale J.C.S. Perkin II 1977 971. 81 (a)T. Kauffmann H. Fischer and A. Woltermann Angew. Chem. Internat. Edn. 1977 16 53; (b)T. Kauffmann R. Joussen and A. Woltermann ibid. 1977,16 709;(c)T. Kauffmann H. Ahlers H.-J. Tilhard and A. Woltermann ibid. 1977 16 710. M. G. Hutchings + As-containing a-amino-acid analogue Me3AsCH2C0g H20 has been isolated from the western rock lobster and fully characterized. Despite its emotive charac- ter the authors say that its existence does not necessarily reflect environmental pollution.82