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.