6 Organometallic Chemistry Part (ii) Main-group Elements By K. SMITH Department of Chemistry University College of Swansea Swansea SA2 8PP 1 Introduction As in the previous Annual Report on main-group element organometallic com- pounds,' B Si and As will be included but not P Se and Te. Perusal of the relevant 1975 literature prompts a feeling of dkjk ufi with the appearance of a number of papers whose titles recall some of the highlights from the literature of the past decade or so. The classic example is a paper by Seebach and Corey on the generation and synthetic applications of 2-lithio- 1,3-dithian (Scheme 1).2 It seems incredible that these reactions3 had not previously been reported in full detail.3 The importance of the compounds derives of course from their role as acyl carbanion equivalents (the carbonyl group is subsequently generated by mercury- catalysed hydrolysis of the dithian moiety).Scheme 1 Hardly less well-known are the hydroboration reactions of 1,1,2-trimethylpropylborane (thexylborane Scheme 2).4 New and more detailed investi-gations reveal that careful attention to experimental procedure can maximize yields of the synthetically valuable organoborane prod~cts.~ Full details of hydroborations using catecholborane have also appeared.6 1 K.Smith Ann. reports (B),1974,71 195. D. Seebach and E. J. Corey J. Org. Chem. 1975,40 231. 3 E. J. Grey and D. Seebach Angew. Chem. Internat. Edn. 1965,4,1075 1077. G.Zweifel and H. C. Brown J. Amer. Chem. Soc. 1963,85 2066. 5 H. C. Brown E.Negishi and J.-J. Katz J. Amer. Chem. Soc. 1975,97,2791; H. C. Brown J.-J. Katz C. F. Lane and E. Negishi ibid.,p. 2799. H. C. Brown and S. K. Gupta J. Amer. Chem. SOC.,1975,97,5249. 136 Part (ii) Main -group Elements Reagents i alkene A -20 "C;ii alkene B. Scheme 2 The initial communications on the synthetically important reactions of trialkyl- cyanoborates with electrophiles (e.g.Scheme 3)7 signalled a major shift in research into applications of organoboranes as reagents. Whereas in 1970 reports of reactions RB-CR2 R,B -1 Na+[R,BCN]-A 1 ,>N 0-c R,COH ' R,C=O /B-CR3 Reagents i NaCN; ii (CF,CO),O 1 mole; iii excess (CF,CO),O; iv H,O,-OH-. Scheme 3 of four-co-ordinate ('ate'-complex) organoboron compounds with electrophiles were rare chemical curiosities of little synthetic value today such reactions provide most of the new applications of organoboranes and are intensively studied (see p.143). Included with details of the original cyanoborate work are studies of solvent effects and of reactions with other electrophiles and methods for the preparation of highly hindered tertiary alcohols.' A clearly presented review of the present deliberations about the vexed problem of which factors control the stereochemistry of addition of organometallic reagents to cyclic ketones makes it clear that the 'product development' part of the 'steric approach and product development control' rationalization must finally be put to rest.' Where steric interactions are substantial hindrance to approach is clearly a major factor in determining product distributions but it is of questionable value in the light of our present understanding to discuss product distributions of ca.60:40 (i.e.transition-state energies differing by ca. 0.25 kcal mol-' at 25 "C or around one tenth of the barrier to rotation about the C-C bond in ethane); such small differences may result from factors (the phase of the moon?) quite beyond our ken. Organometallic compounds containing element-carbon pT-pT bonds for elements such as As Sb Bi Si Ge and B have been reviewed." It may be surprising to those not familiar with this area of chemistry to discover the number of such compounds which have been isolated. Compounds of Si continue to defy isolation though they are abundantly postulated as transient intermediates (see p.146). 'A. Pelter M. G. Hutchings and K. Smith Chem. Comm. 1970 1529; ibid. 1971 1048. * A. Pelter K. Smith M. G. Hutchings and K. Rowe J.C.S. Perkin I 1975 129 138. E. C. Ashby and J. T. Laemmle Chem. Rev. 1975,75,521. lo P. Jutzi Angew. Chem. Zntemat. Edn. 1975,14 232. 138 K. Smith A new general method for the synthesis of trifluoromethyl organometallic com- pounds involves the reaction of trifluoromethyl radicals generated in the glow discharge of GF6,with metal halides." Yields are good [e.g. (CF,),Hg 95% from HgI; (CF,),Sn 90%from SnI,; (CF,),Ge 64% from GeBr,). 2 Group I Lithium.-There has been much interest in 'masked' lithium-containing synthons. For example (l) formed by reaction of (2) with lithium bis(prop-2-yl)amide behaves as a masked version of LiCOCH2CH0.'2 On the other hand compounds of type (3) Li Li + CH,SC=C=CHOMe MeSCECCH,OMe -VoMe behave as masked Wittig reagents corresponding to (4)in their reactions with carbonyl compo~nds,'~ whereas (5) is a masked 3-carboxycarbanion.l4 Compound (5) is of course similar to a vinylogous protected cyanohydrin anion. Cyanohydrin anions protected by an 0-SiMe group have been shown to be comparable with those protected by an 0-methoxymethyl group." The latter compounds as well as other acyl carbanion equivalents can be used in the synthesis of small-ring ketones by reaction with a,o-dihalogenoalkanes.'6 Dianions of a -methylthio- or a-phenylthio-acetic acids also behave as acyl carbanion equi~alents,~~ whilst the dianion of a-trimethylsilylacetic acid is a masked analogue of the Wittig reagent R,P=CHCO,H in its reactions with carbonyl compounds.'8 In contrast to 2-lithio- 1,3-dithian bulky sulphur-stabilized carbanions such as (PhS),CLi (LiC0,Me equivalent) or (6; M =Si,Sn) useful for further modifications after introduction of a masked acyl group undergo conjugate addition to cyclohex-2-enone." Sulphur-stabilized carbanions of a different type have been employed in 11 R.J. Lagow L. L. Gerchman R. A. Jacob and J. A. Morrison J. Amer. Chem. Soc. 1975,97 518. 12 R. M. Carlson R. W. Jones and A. S. Hatcher Tetrahedron Letters 1975 1741. 13 E. J. Corey and P. Ulrich Tetrahedron Letters 1975 3685. l4 H.Ahlbrecht and C. Vonderheid Synthesis 1975 512. S. Hiinig and G. Wehner Synthesis 1975 180. l6 G. Stork J. C. Depezay and J. d'Angelo Tetrahedron Letters 1975,389; K. Ogura M. Yamashita S. Furukawa M. Suzuki and G. Tsuchihashi ibid. p. 2767. B. M. Trost and Y. Tamaru Tetrahedron Letters 1975 3797; P. A. Grieco and C.-L. J. Wang J.C.S. Chem.Comm. 1975,714. l8 P. A. Grieco C.-L. J. Wang and S. D. Burke J.C.S. Chew. Comm. 1975,537. l9 A.-R. B. Manas and R. A. J. Smith J.C.S. Chem. Comm. 1975,216; D. Seebach and R. Biirstinghaus Angew. Chem. Internat. Edn. 1975,14,57. Part (ii) Main -group Elements a versatile synthesis of thiirans which are of course readily converted into alkenes by sulphur extrusion (Scheme 4 X =0,S).*' -OLi 0 + LiCH2SCf] 11 -78°C I /c\ N N temp.S \ /C=CH 4 Ph,P \ 1\ C-CH + LiOC / N Scheme 4 1-Lithio-1,l-dibromoalkanesdecompose above ca. -70 "C but their formation and reactions with electrophiles occur in high yield below ca. -90 OC.*l As a general method for the introduction of ortho-substituents into a benzene ring bearing an oxygen function lithiation of methoxymethoxy-compounds is superior to lithiation of methoxy-compounds on three counts (i) faster lithiation (ii) greater positional selectivity (with BuZi) and (iii) easier removal of the protecting group (e.g. Scheme 9.'' ortho-Substituted benzoic acid derivatives may be obtained by selective ortho-lithiation of 2-aryloxazolines reaction with an electrophile and hydrolysis of the oxaz01ine.~~ OCH,OMe OCH,OMe i ii iii w DO," __* flC0,H / / / /o 95 % this isomer Reagents i Bu'Li; ii CO,; iii H,O+.Scheme 5 Amine-free monolithioacetylene may be prepared and used at low temperature in THF though it disproportionates on ~arming.'~ It is more reactive towards nu- cleophiles than the tmeda complex and adds to ketones to give essentially quantita- tive yields of alkynylmethanols. Consecutive lithiation-alkylations of allene lead to mono- di or tri-substituted allenes also in good yields.25 Reactions of organolithium compounds with water are usually thought to be extremely fast and D,O quenching is often quoted as a method of determining equilibrium proportions of interconverting organolithiums so it is particularly 20 A.I. Meyers and M. E. Ford Tetrahedron Letters 1975 2861; C. R. Johnson A. Nakanishi N. Nakanishi and K. Tanaka Tetrahedron Letters 1975 2865. 21 J. Villieras C. Bacquet and J.-F. Normant Bull. SOC. chim. France 1975 1797. 22 H. Christensen Synthetic Comm. 1975,5,65; R.C. Ronald Tetrahedron Letters 1975 3973. 23 H. W. Gschwend and A. Hamdan J. Org. am.,1975,40,2008;A. I. Meyers and E. D. Mihelich ibid. p. 3158. 24 M. M. Midland J. Org. Chem. 1975,40 2250. 25 G. Linstrumelle and D. Michelot J.C.S. Chem. Comm. 1975 561. 140 K. Smith interesting to read two reports relying upon the slowness of the reactions between organolithium compounds and water for their interpretations. Thus addition of Bu"Li to a mixture of a bromoarene and tritiated water in Et20 at -70°C is a convenient method for preparation of tritiated arenes which implies that Bu"Li reacts with the bromoarene faster than it reacts with water.26 Reaction (1)gives good yields of ketones after extended periods of reflux but if prematurely quenched with water much less of the ketone and more of the tertiary alcohol are produced implying that the excess R2Li reacts with liberated ketone faster than it reacts with Shall we await the day when reactions of organolithiums are routinely performed in aqueous solution? R'CH2C02Li +R2Li-+ R'CH2C(OLi)2R2 R'CH2COR2 (1) On the whole 1975 has been a disappointing year for mechanistic organolithium studies though a few interesting points have emerged.The reaction of (7) with MeLi to give (8) occurs with retention of configuration and without loss of deuterium so cycloalkyne or carbene intermediates are ruled out though the mechanism of the direct coupling remains unclear.28 cis-Enones of type (9; R =Bu',Ph) rapidly equilibrate with the trans-isomers via their anion radicals so they can be used to test for anion radical mechanisms in reactions of a@-unsaturated ketones with organometallic Methyl-lithium and dimethylmagnesium react with- out causing equilibration but anion radicals are clearly implicated in reactions of Me2CuLi.Sodium Potassium Rubidium and Caesium.-Benzylcaesium compounds can be prepared by direct metallation of the appropriate hydrocarbons with Cs metal in THF.30 3Group I1 Magnesium.-Transition-metal catalysis of reactions of organomagnesium com- pounds presumably involving intermediate transition-metal organometallics (though these have not been identified) can lead to some useful synthetic proce- dures.Thus the Ni-complex-catalysed reaction of 1,5-dichlorobenzenes or 2,6- dichloropyridines with XMg(CH2) MgX gives metacyclophanes in a single step albeit in yields ranging from only ca. O-lO% to ca. 30% as n is varied from 6 to ca. 12.31 Unsymmetrical ethers are obtained in high yields by reaction of alkyl 2,4-dichlorophenyl acetals with Grignard reagents under TiC1 catalysis (e.g. Scheme z6 R. Taylor Tetrahedron Letters 1975,435. 27 R. Levine M. J. Karten and W. M. Kadunce J. Org. Chem. 1975,40 1770. 28 P. G. Gassman and T. J. Atkins Tetrahedron Letters 1975 3035.29 H. 0.House and P. D. Weeks J. Amer. Chem. SOC.,1975,97 2770. 3O N. Collignon J. Organometallic Chem. 1975,% 139. 31 K. Tamao S. Kodama T. Nakatsuka Y. Kiso and M. Kumada J. Amer. Chem. SOC.,1975,97,4405. Part (ii) Main -group Elements 141 6),32whereas the regioselective insertion of isoprene into allyl-Mg bonds again Ti-catalysed is useful in the synthesis of ter~enoids.~~ TiCI, THF + PhCH,CH,MgBr . 98 7"yield CH,CH2 Ph Scheme 6 The initial product of the reaction of ethyl LY -(t-butylsulphiny1)acetate with EtMgBr reacts with carbonyl compounds to give adducts which eliminate the elements of t-butylsulphinic acid on treatment with sulphuryl ~hloride.~ The overall result is a high-yield synthesis of ap-unsaturated esters and the reagent could be considered as a masked Wittig reagent R,P=CHCO,Et.A Mg-acetylene compound of approximate composition Mg(C-CH) ,,,formed by reaction of Mg or MgSO with Na in liquid NH3 is reported to resemble LiC=CH in its reactions but to be much cheaper.35 The addition of Grignard reagents RMgX to Me,SiNCO followed by hydrolysis provides a convenient and direct method for preparation of RCONH2,which is applicable even for cases where R is an alkynyl The reaction of NN-dimethylhydrazone methiodides with Grig- nard reagents gives a more efficient synthesis of aziridines than the previously reported reaction involving oximes and is applicable to vinyl-substituted aziridines (Scheme 7) in yields around 50-75%.37 r 1 R3CCHR'R2 R3 NII \+ NMe I- H2C=CH w::N H Reagents i H,C=CHMgBr; ii H,O +.Scheme 7 Competition between 1,2- and 1,4-addition reactions of Grignard reagents with arP-unsaturated ketones is well known but a major product from the reaction of ButMgCl with ethyl cinnamate is ethyl 2-t-butyl-3-phenylpropanoate,apparently a product of €,3-additi0n.~~ Rearrangement of a cyclopropanone hemiketal inter- mediate is suggested as the likely route to this unusual product. Reactions of organomagnesium reagents with thiocarbonyl compounds have been reviewed.39 In recent years the mechanism of formation of Grignard reagents from organic halides and Mg has been shown to involve initial electron transfer from Mg to RX followed by loss of halide to give a radical R. However it was uncertain whether the 32 H.Ishikawa and T. Mukaiyama Chem. Letters 1975 305. 33 S. Akutagawa and S. Otsuka J. Amer. Gem. Soc.,1975,97 6870. 34 J. Nokami N. Kunieda and M. Kinoshita Tetruhedron Letters 1975 2179. M J. N. Gardner Canad. J. Chem. 1975,53 2157. 36 K. A. Parker and E. G. Gibbons Tetrahedron Letters 1975,981. 37 R. Chaabouni and A. Laurent Synthesis 1975,464. 38 I. Crossland Acra Chem. Scund. (B).,1975 29 468. 39 D. Paquer Bull. SOC. chim. France 1975 1439. 142 K. Smith Grignard reagent was formed directly from R and 'MgX (path a Scheme 8) or from R2Mg and MgX via the Schlenk equilibrium (path b). In the cases R =But or C,F the Schlenk equilibria are established relatively slowly and n.m.r. methdds can be used to observe the initial products; it is clear that in these cases at least RMgX is the primary product not R2Mg4* RX +Mg ARX' +Mg' Ml3s 1 RMg; t-R' +'MgX combine path b\ Ipath a 1+J 1 R2Mg +MgX2 +RMgX subscript s =surface bound Scheme 8 Zinc,Cadmium and Mercury.-Activated Zn prepared by refluxing ZnC1 with K metal in THF can be used for Reformatsky reactions in Et,O at low temperatures with excellent The reaction of Et,Zn with CHBr in the presence of air (radical initiator) provides a bromocarbenoid of Zn which in the presence of alkenes produces monobromocyclopropanes in good yields.42 Previously such compounds have usually been obtained by partial reduction of gem-dibromocyclopropanes.Homogeneous catalysts overcome the problems previously encountered in cataly- tic hydrogenolysis of organomercury compounds when heterogeneous catalysts were employed.Furthermore carbonylation of the intermediate in the presence of MeOH gives good yields of carboxyl derivatives [(Ph,P),RhCl ~atalyst].~ Carbonylation of vinylmercurials is easier occurring at -20 "C with a PdC1,-LiCI catalyst and is stereospecific(e.g.Scheme 9).44 R' H R' H \/ i\/ c=c -+ c=c /\ /\ H HgX H CO,RZ Reagent i CO(1 atm) R20H,PdC1,-LiCl -20 "C. Scheme 9 Trihalogenomethylmercury compounds are well known as dihalogenocarbene precursors but extended periods at elevated temperatures are generally necessary for their complete decomposition. Iododihalogenomethylmercurycompounds (10; X =Cl Br; Y =C1 Br) however generate the corresponding dihalogenocarbenes 40 H.W. H. J. Bodewitz C. Blomberg and F. Bickelhaupt Tetrahedron Letters 1975 2003. 41 R. D. Rieke and S. J. Uhm Synthesis 1975,452. 42 S. Miyano Y. Matsumoto and H. Hashimoto J.C.S. Chem. Comm. 1975,364. 43 W. C. Baird and J. H. Surridge J. Org. Chem. 1975,40 1364. 44 R. C. Larock J. Org. Chem. 1975,40,3237. Part (ii) Main -group Elements 143 slowly at 20°C or within minutes at 80°C.45By analogy with ketone analogues HgC1,-catalysed hydrolysis of compounds of type (11) to aldehydes (12) had generally been considered as a formality with synthesis of (11)being thought of as synonymous with synthesis of (12) but it has been shown that the usual conditions are not successful in these cases.46 Successfu! hydrolysis is achieved by prior addition of either HCl or PhSH.SPh \/ PhHgCXYI c=c ‘CHCHO / ’ ‘H (10) (11) (12) Use of the system Hg(OAc),-Bu‘O,H for peroxymercuration of alkenes suffers from competitive acetoxymercuration resulting in mixtures which are difficult to separate but use of Hg(OCOCF,) (CF,CO; is less nucleophilic than CH,CO,) overcomes this difficulty and high yields of 2-peroxyalkylmercurials or their bromodemercuration products may be thus ~btained.~’ The displacement of alkali metals from their organcmetallic compounds by Hg metal4’ and the role of Hg‘ intermediates in reactions of related types4’ have been reviewed. 4 Group I11 Boron.-An excellent book on synthetic reactions of organoboranes which has appeared is especially valuable for the non-specialist ; it contains many detailed experimental procedures and a whole chapter is devoted to techniques useful for handling organ~boranes.~~ Reactions of allylboranes with unsaturated com-pound~,~~ arid applications of methane-tetra- and -tri-boronic esters5 and of organoborates and organoboranesS3 as synthetic reagents have been reviewed.Of the wealth of new synthetically useful reactions of organoboranes the majority involve the reactions of four-co-ordinate (‘ate’-complex) boron compounds with electrophiles. Trialkylalkynylborates (1 3) continue to be involved in the lion’s share and further studies of their behaviour [reaction (2)] have been numerous. Protona- tion of (13; R2=H) can be controlled to give either a single migration (leading to ‘Markovnikov’ vinylboranes) or two rnigration~.’~ Functionalized electrophiles react with (13)to give functionalized products.Thus methyleneimmonium salts give cis-trans mixtures of (14; E =CH2NMe2),55 whereas iodoacetonitrile and propargyl bromide undergo a stereospecific reaction to give (14; E =CH,CN or CH,CrCH) 45 D. Seyferth and C. K. Haas J. Org. Chem. 1975,40 1620. 46 A. J. Mura G. Majetich P. A. Grieco and T. Cohen Tetrahedron Lerters 1975 4437. 47 A. J. Bloodworth and I. M. Griffin J.C.S. Perkin I 1975 195 695. 48 A. A. Morton Chem. Reo. 1975,75767. 49 0.A. Reutov and K. P. Butin J. Orgunometallic Chem. 1975,99 171. H. C. Brown ‘Organic Syntheses via Boranes’ Wiley-Interscience New York 1975. s1 B. M.Mikhailov Pure Appl. Chem. 1974,39 505. 52 D. S. Matteson Synthesis 1975 147. 53 E. Negishi J. Chem. Educ. 1975,52 159. 54 H. C. Brown A. B. Levy and M. M. Midland J. Amer. Chem.Soc. 1975,97,5017;M. M. Midland and H. C. Brown J. Org. Chem. 1975,40,2845. 55 P. Binger and R. Koster Chem. Ber. 1975,108 395. 144 K. Smith with E and R1 cis (not trans as wrongly reported for the reaction with a-bromocarbonyl compounds) .56 Li+[R:BC-CR*]-+ EX +RiBCR'=C R2E +further? (2) (13) (14) A reaction of type (2) between (13; R1=Pr" R2=SiMe3) and TsO(CH,),CZX(CH,),OTHP (Ts = tosyl THP = tetrahydropyranyl) is a key step in a new synthesis of propylure the sex attractant of the pink bollworm A modification of reaction (2) involves the reaction of di-sec-alkyldialkynylborates with iodine; this provides a convenient synthesis of conjugated diynes (Scheme R',BX + 2LiCGCR2 -+ Li+[R1 2B(CrCR2),]- 1 R~C~C-C~CR~ (ca.90%) R' = 1,2-dimethylpropyl cyclohexyl ; X = C1 Br Scheme 10 Reactions of trialkylvinylborates with aldehydes provide a direct route to 1,3-diols though yields are whilst trialkyl-isopropenylboratesreact with iodine to give 2-alkylpropenes in good yields.60 Lithium tetra-alkylborates and trialkylarylborates react with acyl halides to give ketones by intermolecular transfer of an alkyl or aryl group.61 Since ketones rather than tertiary alcohols are obtained the trialkylborane may be said to 'moderate' the reactions of the organolithium compounds used to form the organoborates.The major products in the reactions of magnesium trialkylarylborates with acyl halides are ortho-alkylaryl ketones formed by attack of the acyl group on the aromatic ring and intrumoleculur transfer of an alkyl group.62 The reasons for the differences are not clear but may have something to do with the relative solubilities (Li > Mg). Borate complexes may be key intermediates in the synthesis of (E,E)-dienes by MeCu-induced coupling of dialkenylchloroboranes,63the production of 'mixed' trialkylmethanols from lithioaldimines and dialkylchloroboranes,64 the formation of alkyl phenyl ketones from trialkylboranes and Q -azido~tyrene,~~ and the homologa- tion of organoboranes with sulphur-stabilized anions (Scheme 1 1).66 Whereas Scheme 11 leads to a particular type of 'mixed' trialkylborane a two-step process [reaction (3)] involving bromination of dialkyl(methy1thio)boranesand 56 A.Pelter K. J. Gould and C. R. Harrison Tetrahedron Letters 1975 3327. 57 K. Utimoto M. Kitai M. Naruse and H. Nozaki Tetrahedron Letters 1975,4233. 58 A. Pelter K. Smith and M. Tabata J.C.S. Chem. Comm.,1975 857. 5q K. Utimoto K. Uchida and H. Nozaki Chem. Letters 1974 1493. 6o N. Miyaura H. Tagami M. Itoh and A. Suzuki Chem. Letters 1974 1411. 61 E. Negishi K.-W. Chiu andT. Yoshida J. Org. Chem. 1975,40,1676;E. Negishi A. Abramovitch and R. E. Merrill J.C.S. Chem. Comm. 1975 138. 62 K. Utimoto K. Okada and H. Nozaki Tetrahedron Letters 1975,4239. 63 Y.Yamamoto H. Yatagai and I. Moritani J. Amer. Chem. SOC.,1975,97 5606.64 Y. Yamamoto K. Kondo and I. Moritani J. Org. Chem. 1975,40 3644. 65 A. Suzuki M. Tabata and M. Ueda Tetrahedron Letters 1975 2195. 66 E. Negishi T. Yoshida A. Silveira and B. L. Chiou J. Org. Chem. 1975,40,814. Part (ii) Main-group Elements 145 R',B +LiCH2SR2 Li+[R',BCH,SR']-'3 R',BCH2R' +MeSR2 +LiI Scheme 11 reduction-hydroboration of the product provides a more general approach to these A general synthesis of cis-alkenyldialkylboraneshas also beenyeported.68 NaH RiPiSMe 3MeSSMe +R;BBr -R:BR2 (3) alkeneZ A different kind of borate complex (15) a complex of borabenzene is obtained by treatment of the Co derivative (16) with KCN.69 Another type sodium diphenyl- borate(1) (17) has been postulated as an intermediate in the formation of yet a further type (18) formed on photolysis of NaBPh in the presence of PhCZCPh.70 Aliphatic analogues of (17) have previously been postulated in the reactions of alkali metals with dibutylchloroborane but it has been shown that subsequent reaction with benzoyl chloride does not give the previously claimed boryl ketones.'l The actual product is a mixture of Bu2BOCH2Ph and PhC02CH2Ph [reaction (4)] and it seems very unlikely that aliphatic analogues of (17) are the stable intermediates in these reactions.Ph Ph co K+ [Q-R] -<F>B Ph2B-Na+ B-Na' -R /\ Ph Ph PhCOCl Bu,"BCI %K [Bu,"B-M'] -BuqBOCH,Ph +PhC02CH2Ph (4) alloy Reactions of alkeneboronic acids with Br in the presence of MeO-MeOH provide simple one-stage syntheses of a-brom~acetals,~~ whilst reactions of aldehydes with bis(ethylenedioxybory1)methyl-lithium followed by oxidation give the homologated aldehydes in high yields.73 An earlier report that mercurideboro- nation at C-2 of the norbornyl system involved retention of configuration is not general inversion occurring to 3 95% in an open-chain example.74 Aluminium Gallium Indium and Thallium.-A convenient method of attaching a t-alkyl group to an alkyne is provided by the reaction of a t-alkyl halide with a trialk~nylalane.~~ Only one of the three alkynyl residues is utilized but the other two 67 A.Pelter K. Rowe D. N. Sharrocks and K. Smith J.C.S. Chem. Comm. 1975,53 1 ;A. Pelter K. Rowe and K. Smith ibid. p. 532. 68 E. Negishi R. M. Williams G. Lew and T.Yoshida J. Organometallic Chem. 1975,92 C4. 69 G. E. Herberich and H. J. Becker Angew. Chem. Internat. Edn. 1975,14 184. 'O 3. J. Eisch K. Tamao and R. J. Wilcsek J. Amer. Chem. Sac. 1975,97 895. 71 K. Smith and K. Swaminathan J.C.S. Chem. Comm. 1975 719. 72 T. Hamaoka and H. C. Brown J. Org. Chem. 1975,40 1189. 73 D. S. Matteson R. J. Moody and P. K. Jesthi J. Amer. Chem. SOC.,1975 97 5608. 74 M. Gielen and R. Fosty Bull. SOC.chim. belges 1974,83 333. 75 E. Negishi and S. Baba J. Amer. Chem. SOC. 1975,97,7385. 146 K. Smith cm be recovered as alkyne. The lower basicity of the aluminium reagents compared with e.g. lithium compounds may be responsible for the success of this method and for the successful synthesis of ketones by Ni(acac),-catalysed reactions of Me,Al with nitriles even when the latter contain highly acidic hydrogens (e.g.PhCH,CN).76 Nickel compounds also catalyse conjugate addition reactions of Me& to aP-unsaturated or cyclopropyl ketone^.'^ Whereas previous studies showed little reaction between tetra-alkylaluminates and oxiransjn ether solvents the reactions occur readily in hydrocarbons especially under Ni ~atalysis.~’ High yields of alcohols are obtained and the regiochemistry is opposite to that obtained in the corresponding reactions of trialkylalanes. Scrambling of stereochemistry in the addition product of an optically active organoaluminium compound and an aromatic ketone indicates a non-synchronous transfer whereas the same aluminium compound abstracts a deuterium atom from [2&]acetone with complete retention of ~tereochemistry.~’ In favourable cases nitrodethalliation of arylthalliumbis(trifluoroacetate)swith NO in CF,CO,H gives high yields of nitroarenes.80 On the other hand acetyl nitrate causes ring nitration without dethalliation and subsequent iododethalliation provides a useful synthesis of nitroaryl iodides.81 5 Group IV Silicon.-Interest in transient silicon analogues of unsaturated compounds continues to gather momentum and a fine review of the subject has Unfortunately the topic is getting out of hand in the sense that many papers whose specific purpose is to ‘show’ the presence of such intermediates contain little or no evidence to support the claims the inferences arising from ‘reasonable’ mechanistic rationaliza- tions for the formation of identified products.A timely reminder of the dangers of such overassumption is provided by a reinvestigation of the reactions of (19) generated in the presence of alcohols. The production of (20) in MeOH had previously been attributed to the intermediacy of a sila-alkene (path a Scheme 12) but in other alcohols the product is not (21) but (22) now rationalized according to pathway b.’ Despite the above reservations it is interesting to record the first claims for Si=S speciess4 and for the parent sila-ethene and silanone ~ystems.’~ Some effort including the use of labelling techniques was used to try to eliminate non-sila-alkene 76 L. Bagnell E. A. Jeffery A. Meisters and T. Mole Austral.J. Chem. 1974 27 2577. 77 L. Bagnell E. A. Jeffery A. Meisters and T. Mole Austral. J. Chem. 1975 28 801; L. Bagnell A. Meisters and T. Mole ibid. pp. 817 821. 78 G. Boireau D. Abenhaim C. Bernadon E. Henry-Basch and B. Sabourault Tetrahedron Letters 1975 2521. 79 J. J. Eisch and K. C. Fichter J. Amer. Chem. SOC.,1975,97 4772. B. Davis and C. B. Thomas J.C,S. Perkin I 1975,65. 81 E. C. Taylor H. W. Altland and A. McKillop J. Org. Chem. 1975,40,3441. 82 L. E. Gusel’nikov N. S. Nametkin and V. M. Vdovin Accounts Chem. Res. 1975,8 18. W. Ando A. Sekiguchi T. Migita S. Kammula M. Green and M. Jones J. Amer. Chem.SOC.,1975,97 3818. 84 L. H. Sommer and J. McLick J. OrganometullicChem.,1975,101 171. 85 C. M. Golino R. D. Bush and L. H. Sommer J. Amer.Chem. SOC. 1975,97,7371. 147 Part (ii) Main -group Elements Me,SiC(N,)CO,Me Me,Si=CMeCO,Me J$ Me,SiCHMeCO,Me I OMe pa/ (20) ;I \ Me SiCCO Me Me,SiCHMeCO,Me I OR ... path b \ (21) Me I Me,SiC=C=O 3Me,SiCHMeCO,R I I OR Reagents i A; ii MeOH; iii ROH. Scheme 12 pathways in the proGaction of disilacyclobutanes on thermal decomposition of Me3SiCHN2.86 Labelling also shows that production of (23)by rearrangement of the carbene (24) does not involve the previously suggested siliran intermediate but rather it proceeds by a well-documented set of phenylcarbene rearrangement^.^' The isolable siliran (25) provides a convenient method for low-temperature (60-80 "C) thermal generation of :SiMe, but is itself very reactive with many types of compounds.88 After 25 years of unsuccessful attempts to prepare tri-t-butylsilane derivatives such compounds have been independently reported by three laboratorie~.~~ The most successful approach involves the reaction of Bu'Li with fluorosilane~.~~~*~ Organic 1,2-disilane derivatives undergo Pd-complex-catalysed reactions with aryl halides giving the corresponding arylsilanes and halogeno~ilanes,~~ whilst cyclic examples add across CrC bonds to give cyclic cis-disilylethenes." Acyclic cis-disilylethenes are obtained by a novel dehydrogenative double silylation of alkynes by hydrosilanes catalysed by a Ni complex.92 Reductive silylation of anisole using R.L. Kreeger and H. Shechter Tetrahedron Lefrers 1975,2061. 87 T.J. Barton J. A. Kilgour R. R. Gallucci A. J. Rothschild J. Slutsky A. D. Wolf and M. Jones J. Amer. Chem. SOC. 1975,97,657. 88 D. Seyferth and D. C. Annarelli J. Amer. Chem. SOC.,1975,97,7162. 89 (a)M. P. Doyle and C. T. West J. Amer. Chem. Soc. 1975,97,3777;(b)M. Weidenbruch and W. Peter Angew. Chem. Internat. Edn. 1975,14642;(c)E. M. Dexheimer and L. Spialter Tetrahedron Letters 1975 1771; (d) E. M. Dexheimer L. Spialter and L. D. Smithson J. Organometallic Chem. 1975 102 2 1. 90 H. Matsumoto S. Nagashima K. Yoshihiro and Y. Nagai J. Organometallic Chem. 1975,85 C1. 91 H. Sakurai,Y.Kamiyama and Y. Nakadaira J. Amer. Chem. Suc. 1975,97,931. 92 K. Tamao N. Miyake Y. Kiso and M. Kumada J. Amer. Chem. SOC.,1975,97,5603. 148 K. Smith the system Me,SiCl-Li-THF under mild conditions gives (26) in high yield.Hydrolysis then gives (27).93 SiMe3 OoMeoo SiMe Si SiMe3 8 NMe2 (28) (29) Silicenium ions have proved elusive but now a particularly favourable example (28) has been demonstrated in the reaction of (29; R=H) with Ph3C+ at low temperature. Addition of NaBD to the green solution without prior warming gives (29; R =D).94 Since (28) presumably owes its stability (? !) albeit at relatively low temperature to extensive conjugation with the aromatic rings could it justifiably be claimed as a Si=C species stable at such temperatures? Treatment of vinylsilanes with halogenocarbenes followed by elimination of halogenotrimethylsilane under the action of fluoride ion produces cyclopropenes which can be identified as benzofuran addition Friedel-Crafts reactions of alkenes with acyl halides and AlCl usually give low yields of a@-unsaturated ketones but use of vinylsilanes instead of simple alkenes gives both good yields and site-specificity the carbonyl group becoming attached to the carbon which initially held the silyl group.96 A number of 6-silasteroids have been synthesized and investigated for ant if ertili ty activity .97 Germanium Tin and Lead.-Bis(triethylgermy1)keten has been isolated from the reaction of Et,GeC(N,) Et with (Et,Ge),Hg.98 A review of industrial applications of organotin compounds has appeared.99 Compound (30),available by a hydrostannylation reaction is a useful purveyor of a sterochemically defined -CH=CHCH20H group via the corresponding Cu ate complexes,1oo whereas (31) reacts with ketones to give y-hydroxyalkyltin com-pounds which can be converted into cyclopropane carbonitriles.lo' Although tetra-alkyltin compounds generally show no Lewis acidity the reaction of l,l-dibromo-2,3,4,5-tetraphenylstannolewith cyclopentadienyl-lithium seems to give (32),a five-co-ordinate tin compound. lo2 A convenient method for generation of dialkylstannylenes :SnR, involves photolysis of cyclopolystannenes (R,Sn), in 93 M. Laguerre J. Dunogues R. Calas and N. Duffaut J. Organometallic Chem. 1975,93 C17. 94 J. Y. Corey J. Amer. Chem. SOC.,1975,97 3237. 95 T. C. Chan and D. Massuda Tetrahedron Letters 1975,3383. 96 I. Fleming and A. Pearce J.C.S. Chem. Comm. 1975 633.97 S. Barcza and C. W. Hoffman Tetrahedron 1975,31,2363; C. G.Pitt A. E. Friedman D. Rector and M. C. Wani. ibid. p. 2369. 98 0.A. Kruglaya I. B. Fedot'eva B. V. Fedet'ev and N. S. Vyazankin Zzvest. Akad. Nauk S.S.S.R. Ser. khim.,1975 199 (Chem. Abs. 1975.82 171 157d). 99 P. Smith and L. Smith Chem. in Britain 1975 11 208. loo E. J. Corey and R. H. Wollenburg J. Org. Chem. 1975,40,2265. Io1 S. Teratake Chem. Letters 1974 1123. 102 W. Z. M. Rhee and J. J. Zuckerman J. Amer. Chem. SOC.,1975,97,2291. Part (ii) Main -group Elements H \ /CH20THP /c=c\ HBu,Sn Bu,SnCH,CH,(CN)Li Li' THP = tetrahydropyranyl (30) (31) (32) benzene and in the absence of any suitable reactant the starting material is simply re-formed.lo3 Peroxides and disulphides react to give RiSn(OR2) and R;Sn(SR2), whereas two molecules of a ketone react wi?h one of a stannylene to give a 1,3,2-dioxastannolan.6 Group V Arsenic @timony and Bismuth.-Compounds such- as (33; X =CO,Et OH) cannot be obtained by electrophilic substitution reactions of arsabenzene but may be obtained by sequences involving As-Sn exchange of appropriately substituted organotin intermediate^."^ A compound previously reported (in 19 12!) to be (34) has now been shown to be the reductive dimer (35).lo5 (33) (34) H (35) +-Crystallinemethylenetriphenylarsorane Ph3As-CH, has been isolated from the reaction of methyltriphenylarsonium bromide with sodamide. lo' *03 W. P. Neumann and A. Schwarz Angew. Chem. Internat. Edn.1975,812. lo4 G. Markl H. Kellerer and F. Kneidl Tetrahedron Letters 1975 2411; G. Markl H. Baier and S. Heinrich Angew. Chem. Internat. Edn. 1975 14 710. lo5 H. Vermeer R. Lourens and F. Bickelhaupt Tetrahedron 1975,31 2529. lo6 Y. Yamamoto and H.Schmidbaur,J.C.S. Chem. Comm. 1975,668.