12 Organometallic Chemistry Part (ii) Main-Group Elements By J. L. WARDELL Department of Chemistry University of Aberdeen Meston Walk Old Aberdeen AB9 2UE 1 Introduction The powerful combination of a prolific research area and a keen commercial sense has resulted in Volume 200 of the Journal of Organometallic Chemistry appearing just sixteen years after the Journal's inception. To mark the occasion a special issue was produced of papers by twenty-two eminent chemists giving in the main personal accounts of their research. Several of these articles too many to itemize here are concerned with Main-Group compounds but the whole volume is well worth a look. The plenary lectures delivered at the Tenth International Organometallic Chemistry Conference at Dijon have also been printed.' These include the mechan- isms of the reactions between Grignard reagents and ketones,'" the role of electron transfer and charge transfer in organometallic reactions," reactive intermediates in the synthesis and chemistry of organo-silacycles," optically active organotin compounds,ld and nickel- and palladium-catalysed cross-couplings of organometal- lic reagents with organic halides." Review articles on silicon chemistry were concer- ned with synthetic applications of Me3SiX (X = CN I N3 or SMe),2" silylated synthons,2b and organosilicon chemistry in general.*' In addition a review concerned with the synthesis structures and vibrational spectra of methylmetal compounds has been p~blished.~ 2 Group1 Two new indicators 2,5-(Me0)2C6H3CH20H and (PhCH2)2C=NNHS02C6H4-~ -Me have been used to determine concentrations of organolithium c~mpounds.~ Several theoretical calculations have been carried out on methyl-lithium oligomers.' (a) E.C. Ashby Pure Appl. Chem. 1980,52,545;(b)J. K. Kochi ibid. p. 571;(c)T.J. Barton ibid. p. 615;(d)M. Gielen ibid. p. 657;(e)M. Kumada ibid. p. 669. * (a)W. C. Groutas and D. Felker Synthesis 1980,861;(b)L. Birkofer and 0.Stuhl Top. Cum Chem. 1980,88,33;(c) R. West and T. J. Barton J. Chem. Educ. 1980,57 165. 'E. Maslowsky Jr. Chem. SOC.Rev. 1980,9,25. M. R. Winkle J. M. Lansmger and R. C. Ronald J. Chem. SOC.,Chem. Commun. 1980 87; M.F. Lipton. C. M. Sorensen A. C. Sadler and R. H. Shapiro J. Orgunornet. Chem. 1980,186 155. ' (a)G.D. Graham S. Richtsmeier and D. A. Dixon,J. Am. Chem. SOC.,1980,102 5759; (6) G. D. Graham D. S. Marynick and W. N. Lipscomb ibid. p. 4572; (c) T. Clark J. Chandrasekhar and P. von R. Schleyer J. Chem. SOC.,Chem. Commun. 1980 672; (d)J. B. Collins and A. Streitwieser Jr. J. Comput. Chem. 1980 1,81. 219 220 J. L. Wardell An approximate M.O. calculation (PRDDO) indicated stable planar arrangements of lithium and carbon atoms for (MeLi) (2 < n < 6) in addition to a condensed tetramer (Tdsymmetry) and a hexamer (D3dsymmetry). An INDO treatment of 7Li-'3C coupling constants pointed to particularly large values of these constants for the MeLi monomer consistent with a covalent carbon-lithium bond. Ab initio calculations6 indicated involvement of the metal 2p orbital in the stabilization of linear singlet carbenes M-C-M and of the anions MCH2- and MCH2CH2- (M = Li or BeH).The anions MCH2- were suggested to be planar (C2"symmetry) and more stable than CH3- in contrast to the highly destabilized and pyramidal anion NaCH2-. Minimum-energy structures that were calculated from other studies were the distorted partially bridged structure (1) for C2H2Li2 in contrast to a classical structure for vinyl-lithium,' and the three structures (2) (3) and (4) for LiCH2CN of which the last two can be considered as an ion-pair (CH2Li'CN-) and a complex (CH2:LiCN) respectively.8 Racemization occurs during the formation of the organolithium compound from [2,2,6-2H3]cyclohexy1 bromide and lithium probably either on the surface of the lithium or during the lifetime of a short-lived radical ir~termediate.~ The cleavage reaction of alkoxy-substituted organotin compounds by RLi has been further studied." Organolithiums prepared by this route include LiCH=CHCH20Li Li(CH2)30Li and enantiomerically pure EtCHLiOCH20CH2Ph.While cleavage of Bu3SnCH20H by RLi does lead to a compound that is capable of hydroxymethyl- ating electrophiles," it was considered to be more complex e.g. (9,than LiCH20Li. Bu3SnCH20H + 2BuLi --+ ii ('CH,)" HCd -1 (6) IZ = 2-5 lC\ \J I The products of reaction of (cycloalkeny1)benzenes (6) with Bu'Li in the presence of THF or TMED were found to be polymers for n = 2 both those of allylic deprotonation and of addition to the double-bond [see reaction (l)]for n = 3 products of allylic deprotonation for n = 4 and finally for n = 5 of metallation of the ring." The rotational barriers of the aryl-C bonds in the compounds (7) (a) T.Clark H. Korner and P. von R. Schleyer Tetrahedron Lett. 1980 21 743; (6) A. Pross and L. Radom Aust. J. Chem. 1980,33,241;(c)W. W. Schoeller I. Chem. SOC.,Chem. Commun. 1980 124. Y. Apeloig T.Clark A. J. Kos E. D. Jemmis and P. von R. Schleyer Zsr. J. Chem. 1980 20,43. J. B. Moffat J. Chem. SOC.,Chem. Commun. 1980,1108. J. B. Lambert M. W. Majchrzak B. I. Rosen K. P. Steele and S. A. Oliver Isr. J. Chem. 1980 20 177. (a) W.C.Still and C. Sreekumar J. Am. Chem. SOC.,1980,102,1201;(6)S.D.Burke S. A. Shearouse D.J. Burch and R. W. Sutton Tefrahedron Lett. 1980,21,1285;(c)N.Meyer and D. Seebach Chem. Ber. 1980 113 1290. G.Fraenkel D.W. Estes and M. J. Geckle J. Organomet. Chem. 1980,185 147. Organometallic Chemistry -Part (ii) Main-Group Elements 22 1 Bu' Li + Pha + Bu'Li + . , .-' Ph Li+ Li + YoCzMe + Bu'Li +Y -CH2 CH But (7) which were prepared by the addition of Bu'Li to a-methylstyrenes [reaction (2)] and in p-MeC6H4CHC6H4SiMe~-p Li+(8) depend on the substitutents; e.g. at 255 K,the p-MeC6H4 ring in (8) rotates ca 200 times faster than the p-Me3SiC6& group i.e. there is more delocalization of the charge into the silylated ring.I2 An interesting intramolecular 1,4-migration of a Me3Si group occurs in 9,9-(Me3Si)2- 10-lithio-9,10-dihydroanthraceneto give 9,10-(Me3Si)2-9-lithio-9,10-dihydroan-thracene protonation of which provides the cis-product." Other rearrangements occur14 in the (Me3Si-substituted alky1)-lithiums Me3Si(CH2),Li in THF solution but not in Et20; e.g.reactions (3) and (4). Me3Si(CH&Li Me2PrSiCH2Li (3) Me,Si(CH2)5Li -Me,Si + MeLi + Me2(n-C5H,,)SiCH2Li (4) Metallations of 1,3-dithiolans (see Scheme 1)provide a complex range of prod- ucts in contrast to the simple 2-lithiation of 1,3-dithians.15 The lithiated 2-lithio-2- methyl-l,3-dithian-TMED complex crystallizes as a centrosymmetric dimer (9). Lithium bridging leads to a central ring in which the lithium is bonded equally strongly to carbon and to sulphur.16 "S) -* [RCH=S] + H,C=CHSLi HS BuLi yy/ lBUhRCH2SLi R(BuS)CHBu ,BuLiR(BuS)CHLi RCHSLi I Bu Scheme 1 Me n Me (9) l2 G.Fraenkel and M. J. Geckle J. Am. Chem. SOC.,1980,102,2869. l3 M.Daney R. Lapouyade B. Labrande and H. Bouas-Laurent Tetrahedron Lett. 1980,21,153. l4 A. Maercker M. Eckers and M. Passlack J. Organomet. Chem. 1980,186,193. Is S.R. Wilson G. M. Georgiadis H. N. Khatri and J. E. Bartmess J. Am. Chem. SOC.,1980,102 3577. R. Amstutz D. Seebach P. Seiler B. Schweizer and J. D. Dunitz Angew. Chem. Int. Ed. Engl. 1980,19,53. 222 J. L. Wardell High-field 13C n.m.r. spectroscopy of isotopically labelled CH3CHZl3CHz6Li in cyclopentane revealed the existence of hexamer octamer and nonamer units all undergoing rapid intra-aggregate C-Li bond exchanges.Above 250 K the hexamer is the predominant aggregate. Inter-aggregate exchanges also occur at higher temperat~res.~' For allyl-lithium a n-bridged structure rather than rapidly equilibrating a-bonded unsymmetrical structures was indicated from the 13C n.m.r. spectrum of [1-2H]allyl-lithium in THF using the Saunders isotope-perturbation met hod. l8 Solid PhCHLiS(O)Me obtained from lithiating PhCH2S(0)Me in THF reacts with methyl iodide vapour at 250°C to give one diastereoisomer of PhCHMeS(0)Me [(RS) and (SR)]; no selectivity is obtained from the reaction with H20 nor when liquid Me1 is used.lg 3 Group2 Reactions of atomic magnesium with MeX (X = C1 Br or I) in argon matrices and with a number of organic chlorides2' at 77K have been reported.From the i.r. and e.s.r. spectra the following steps were proposed for the organic chloride reaction Mg + RCl + RCl'Mg+ + RMgCl Mg + RCI + + MgCl' MgCl' + RCI (RXt + R' R' + Mg + RMg' RMgCl + R' (st = matrix-stabilized) The mechanisms of the more traditional Grignard formation in solution have been also considered.2' In general the rates of reaction of alkyl halides RX with magnesium are proportional to [RX] and to the surface area of the magnesium. For RI and for most RBr transport to the magnesium is overall rate-limiting while reactions with RCl are slower. Either (i) an initial transfer of an electron from Mg to RX or (ii) a collision of RX and Mg followed by extractions of halogen were possible critical steps. Specifically for the reaction of cyclopentyl bromide (10) in Et,O it was established that -d[(lO)]/dt is proportional not only to [(lo)] and the surface area of magnesium but also to the rate of stirring of the solution and it is inversely proportional to the viscosity.Only for the less reactive RX compounds such as Me3CCH2Br [but not (lo)] are the rates also dependent on the dielectric constant. " G. Fraenkel M. Heinrichs J. M. Hewitt B. M. Su and M. J. Geckle J. Am. Chem. SOC., 1980 102 3345. W. Neugebauer and P. von R. Schleyer J. Organomet. Chem. 1980 198 C1;M.Schlosser and M. Stahle Angew. Chem. Int. Ed. Engl. 1980 19,487. I9 J. F.Biellmann J. F. Blanzat and J. J. Wicens J. Am. Chem. SOC.,1980,102 2460. 2o B. S. Auk J. Am. Chem. SOC.,1980 102 3480; G.B.Sergeev V. V. Smirnov and V. V. Zagorskii J. Organomet. Chem. 1980 201 9. tI. R. Rogers G. L. Hill Y. Fujiwara R. J. Rogers H. L. Mitchell and G. M. Whitesides J Am. Chem. SOC.,1980,102 217; H. R. Rogers J. Deutch and G. M. Whitesides ibid. p. 226;H. R. Rogers R. J. Rogers H. L. Mitchell and G. M. Whitesides ibid. p. 231;J. J. Barber and G. M. Whitesides ibid. p. 239. -/ Organometallic Chemistry -Part (ii) Main-Group Elements 223 Hammett p values were found to be similar for the reactions between ArBr and Mg and between ArBr and Bu3SnH in EtzO (but not in THF). The rate-determining step of the reaction between ArBr and Mg in EtzO was one of the following (i) transfer of an electron to ArBr (ii) formation of Ar’ radical or less probably (iii) insertion of Mg into the Ar-Br bond.Reactions in THF and more polar solvents like the reactions between ArI and Mg even in Et20 are mass-transport-limited. At least 80% of (10) reacts with magnesium via free radicals.22 The reaction of (10) with magnesium produces a radical precursor to cyclopentylmagnesium bromide (1l),which is trapped by 2,2,6,6-tetramethylpiperidinenitroxyl(R2N0) as cyclo-C5H90NR2 even in the presence of Bu‘OH. The Crignard reagent (ll) on the other hand reacts preferentially with Bu‘OH. Evidence was also gained from the simultaneous reactions of RBr (e.g. R = Et) and of p-XC6H4CH2Br with Mg in THF-C6D6 that radicals are generated in the Grignard formation itself rather than in side-reactions such as Wurtz couplings or metal-halide exchange^.'^ The exchange reaction shown in reaction (3,which is catalysed by [(Cp)2TiC12] provides good yields of allyl-Grignards; styrene similarly reacts to give a-phenyl- ethylmagnesium [(Cp)2TiCI21 +PrMgBr -MeCH=CH + MeCH=CHCH,MgBr (5) CH,-PMe, / \ Li BH2 \ CH,-PdIe (12) A set of tetrahedral complexes (12; M = Be Mg Zn Cd or Hg) were prepared as shownz5 in reaction (6).The preparation and properties including structures in solution and in the solid state of bis-(2,4-pentadienyl)M (M = Be Mg or Zn) and related compounds have been investigated.26 The acyclic compounds are terminally a-bonded species under- going rapid 1,3-exchange in solution. The zinc compounds are the least thermally stable. The first example of electron-transfer in reactions of primary Grignard reagents with ArzCO has been rep~rted.’~ The cyclized product (13) in reaction (7) can only be obtained from a radical source.Cross-coupling reactions of alkenyl OH Ph CCH PhZCO ‘ QMe (7) (i) Et20 at r.t. Me (13) H2C=CH(CH,),CMe2CH,MgC1 (ii) H20 + H,C = CH (CH2),CMe,CH2CPh20H 22 L. M. Lawrence and G. M. Whitesides J. Am. Chem. SOC.,1980,102,2493. 23 B. J. Schaart C. Blomberg 0.S. Akkerman and F. Bickelhaupt Can. J. Chem. 1980,58,932. 24 F.Sato H. Ishikawa and M. Sato Tetrahedron Lert. 1980 21 365. ” H.Schmidbaur and G. Miiller Monarsh. Chem. 1980,111,1233. 26 H.Yasuda Y. Ohnuma A. Nakamura Y. Kai N. Yasuda and K. Kasai Bull. Chem. SOC.Jpn. 1980 53,1101;H.Yasuda M. Yamauchi A. Nakawara T.Sei Y. Kai N. Yasuoka and N. Kasai ibid. p. 1089. ” E.C.Ashby J. Bowers and R. Depriest Tefrcihedron Lett. 1980 21 3541. 224 J. L. Wardell aryl and allylic28 selenides with RMgX are catalysed by [NiC12{Ph2P(CH2)3PPh2}]; some rearrangements occur with ally1 selenides. A reactivity sequence of PhSeMe >> PhCl > PhSMe was established. The compounds {(Me3Si)3C}2M (14) (M = Zn or Cd) have appreciable thermal and chemical stability; e.g. (14; M = Zn) can be steam-distilled and it does not react with concentrated HCl or with Br2 in CCl,.29 Good yields of alkenes R'R2C=CH2 are obtained from the reaction of CH2Br2 R'R2C=0 and zinc in THF in the presence of TiC14 [R' = R2 = C8H17 or R'R2 = -(CH2)ll-etc.]; in a related manner (2)-and (E)-R'W2C=C(C1)CO2Me are produced from CC13C02Me R1R2C0 and Zn plus Et2AlCl in THF [for R' = Me R2 = Ph (2) (E) = 91 9].30 The mechanism of mercuriation of ArH by Hg(OCOCF3)2 in CF3C02H involves a rapidly formed r-complex which slowly converts to products [reaction (S)]."" ArH + Hg"(OCOCF3)2& ArH+ Hgx1(OCOCF3)2 ArHgOCOCF3 (8) Some values (at 25 "C) of K1/l mol-' and of k2/103 s-' for ArH in reaction (8) are 0.8 1 for PhCl 8.2 3.53 for PhH and 10 52 for PhMe.On standing changes in product composition of the PhMe reaction were monitored by 'H n.m.r. spec-trometry the equilibrium distribution at 18"C was established as 31 38 31 for 0- m- p-MeC6H4HgOCOCF3. Addition of CF3S03H to CF3C02H provides a much more effective medium.31b Acetoxymercuriation of PhCzCR (17) in HOAc occurs in a trans manner but is not regioselective [see reaction (9)] as by PhCZCR + Hg(OAc)z + Ph(AcO)C=C(HgOAc)R + Ph(AcOHg)C=C(OAc)R (9) (17) (15) (16) the ratios of products i.e.[(15)]:[(16)] for R of 3 for Me 5 for Et 11for Pr and 16.5 for Bun. Only (15; R = Pi) is obtained from (17; R = Pri) and no reaction occurs if R is But. Monocyclic peroxides have been produced from the peroxymer- curiation of dienes [reaction (lo)];"" a preparation of 2,6-dimethyl-18-crown-6 the reaction of (H2C=CHCH2)20 and tetraethylene glycol with Hg(OAc)2 followed by reductive demercuriation using NaBH4 and OH-. Replace- ment of mercury in vinyl-mercurials R'CH=CHMgX has been achieved34 by various functional groups including S02R2 OP(OR2)2 and (stereospecifically) by OAc and YR2 (Y = S Se or Te using R2XYR2; see Scheme 2 for the proposed mechanism).CH,H~OAC 28 H. Okamura M. Miura K. Kosugi and H. Takei Tetrahedron Lett. 1980,21 87. 29 C. Eaborn M. Retta and J. D. Smith J. Organomet. Chem. 1980 190 101. 30 K. Takai Y. Hotta K. Oshima and H. Nozaki Bull. Chem. SOC.Jpn. 1980 53 1698. 31 (a) C. W. Fung M. Khorramdel-Vahed R. J. Ransom and R. M. G. Roberts J. Chem. SOC.,Perkin Trans. 2,1980,267; (b)G. B. Deacon and M. F. O'Donoghue J. Organomet. Chem. 1980,194 C60. 32 S. Uemura H. Miyoshi and M. Okano J. Chem. SOC.,Perkin Trans. 1 1980 1098. 33 (a)A. J. Bloodworth and J. A. Khan J. Chem. SOC.,Perkin Trans. I 1980,2450 (6)A. J. Bloodworth D. J. Lapham and R. A. Savva J. Chem. SOC.,Chem. Commun.1980,925. 34 R. Clarock K. Oertle and K. M. Beatty J. Am. Chem. SOC.,1980,102 1966; J. Hershberger and G. A. Russell Synthesis 1980,475;J. Am. Chem. SOC.,1980,102 7603. Organometallic Chemistry -Part (ii) Main-Group Elements R2YYR2 2R2Y-H R'CH=CHHgX + R2Y.+ [R'CH-A-YR2 +R'CH=CHYR2 + HgX Agx 1 .HgX + (R2Y)2 + R2YHgX + R2Y* Scheme 2 4 Group3 Reactions of R3Al (R = Me or Et) or of R2AlCl with aza-aromatics (such as 2,2'-bipyridyl) and alkali metals lead to persistent free radicals e.g. (18);large 27Al coupling constants were measured.35 Treatment of PhCGC-CECPh with RzAICl (R = Me or Et) and lithium in ether at -25"C a tetrasubstituted product (19). Reactions of (19) that were studied were its hydrolysis to + RJAl % + NaAlR L L J .c PhC R2A1 AIR2 Li I I +PhC-CEC-CPh + I I R2Al AlR2 t t L L (19) (L PhCH=CHCH=CHPh and its dimerization to (20).Other organo-aluminium heterocycles were obtained as shown3' in Scheme 3. The complex formed from [Ni(aca~)~] and DiBAH catalyses the conjugate addition of R2AlCGCR (obtained from RCECLi and Me2AlCl) to ap-en~nes;~~ 3-alkynyl ketones are obtained on hydrolysis. Scheme 3 Trimethylsilylmethyl-indiumand -gallium compounds M{CH(SiMe3)2}3 (M = In or Ga) and Ga(CHzSiMe3) have been obtained from the organo-lithium or -magnesium compound and MC13. In contrast LiCHPh2 has been shown to reduce 3s W. Kairn J. Organomet. Chem. 1980 201 C5. 36 H. Hoberg and F. Aznar J. Organomet. Chem. 1980,193,155 161. 37 H.Hoberg and W.Richter J. Organomet. Chem. 1980,195,347. 38 J. Schwartz D. B. Cam R. T. Hansen and F. M. Dayrit J. Org. Chem. 1980.45 3053. 226 J. L. Wardell InC13 to indium. Sterically crowded In{CH(SiMe,),} (21) is the first reported monomeric trialkyl-indium.39 In addition to these M"' compounds some associated M' species have been prepared; see reaction (11).Examples of these hydrocarbon- soluble compounds are [KGa(CH2SiMe3)2] (22) and [NaGa(CH2SiMe3)2]3; ether-ates (via complexation with the alkali metal) were also produ~ed.~' PhH Ga(CH2SiMe3)3+ MH +MGa'(CH2SiMe3)2+ SiMe4 (11) DME (22) R = Me3SiCH2 The reactivity of PhR towards Tl"' (OCOCF3)3 in CF3C02H was in the sequence R = But > Me > H. The reversible thalliations (whose progress was followed by n.m.r.) are ca 10-100 times slower than mercuriations.As in mercuriations a large primary isotope effect was found (ca 5.0).41Thalliation using TI"'203 in C13CC02H was also st~died,~' at 70°C. In addition to all the usual replacement reactions ArC6H4Tl"'(OCOCC13)2 was shown to react with H2C=CMeCN in the presence of PdC12 and NaOAc to give H2C=C(CH2Ar)CN and (2)-and (E)-ArCH=C(CN)Me. A synthesis of esters from ArH utilizes the PdCl,-catalysed carbonylation of ArTl(OCOCF3)2 (Scheme 4).No additional oxidant is required to oxidize Pd" to Pd" since the T1"' that is present does this.43 A new hydrodethalliation agent is N-benzyl-1,4-dihydronicotinamide(Scheme 5);its use unlike that of NaBH4 does ArH A ArTl(02CCF3)2-% ArCOzMe Reagents i Tl(O,CCF,), CF,CO,H; ii CO (1atm) LiCl MgO MeOH PdCl Scheme 4 RCH=CH2 A RCH(OMe)CH2Tl(OAc)2 RCH(OMe)CH3 & ii iv RCH(0Me)CHzOH Reagents i Tl(OAc), MeOH; ii N-benzyl-1,4-dihydronicotinamide; iii MeOH N,;iv MeOH 0 Scheme 5 not lead to any regenerated alkene amongst the In the presence of oxygen PhCH(OMe)CH20H is formed instead.The radical PhCH(0Me)CH; can be trapped. The TI-C bond in RCH(OM~)CHDTI(OAC)~ (23) can be cleaved by Cu'X in MeCN in both ionic and radical routes; the stereochemistry of the products RCH(0Me)CHDX from threo-(23) depends on R and the temperat~re.~' 39 A. J. Carty M. J. S. Gynane M. F. Lappert S. J. Miles A. Singh and N. J. Taylor Inorg. Chem. 1980,19,3637; 0.T. Bleachley and R. G. Simmons,ibid.p. 1021. 40 0.T. Bleachley and R. G. Simmons Inorg. Chem. 1980,19,3042. 41 R. M. G.Roberts Tetrahedron 1980 36 3281. 42 S. Uemura H.Miyoshi M. Wakasugi M. Okano 0.Itoh T. Izumi and K. Ichikawa Bull. Chem. SOC.Jpn. 1980,53 553. 43 R. C. Larock and C. A. Fellows J. Org. Chem. 1980,45,363. 44 H.Kurosawa H. Okada and M. Yasuda Tetrahedron Lett. 1980 21 959. 4s J. E.Backvall M. U. Ahmad S. Uemura A. Toshimitsu and T. Kawamura Tetrahedron Lett. 1980 21 2283. Organometallic Chemistry -Part (ii) Main-Group Elements 5 Group4 Many reactions of dimethylsilylene (Me2Si; usually generated photolytically from Mel2%,) with oxygen-containing substrates e.g. aUp -unsaturated epoxides and oxetans proceed via 1,2-mitterionic complex intermediate^^^ (e.g.Scheme 6). SiMe, H + Me,Si % MeCH=CHCH,OSiMe,H Me -&vJ Me H SiMe, I Me Me Scheme 6 Reactions af Me2Si are more selective in ethereal solution than in hydrocarbons as a result of its complexation with the ether to produce a less reactive species; e.g. the relative reactivities towards EtOH and Bu‘OH (giving Me2SiHOR) are 1.8,4.7 and 9.6 in cyclohexane Et,O and THF re~pectively.~’ Addition of Me2Si to 2-butenes occurs stereospecifically; as the resulting silirans are cleaved by MeOD stereospecifically one (MeOSiMe2)CHMeCHDMe. diastereoisomer is produced from each 2-butene isomer.48 The addition of Me2Si to alkenes has been to be reversible on heafing; e.g. hexamethylsiliran (24) decomposes at 75°C to Me2C=CMe2 and Me2Si which can combine with (24) to provide octamethyl-1,2- disilacyclobutane.The reaction of MePhSi (generated photolytically from Me3SiSiPhMeSiMe3) with the conjugated diene H2C=CMeCMe=CH2 initially provides products from 1,2-addition e.g. vinylsiliran (25) from 1,4-addition e.g. 1-silacyclopent-3-ene (26) and from insertionSo into the C-H bond of the methyl group e.g. (27). Me H2C \ Si + /si -&eMe+ Me0Me Ph /\ MeSiH Ph Me I Ph H2C=CMeCMe=CH2 (27) The radicals Ar3M’ (Ar = 2,6-Me2C6H3 or 2,4,6-Me3C6H2; M = si Ge Of Sn) have been directly detected when Ar3MCl are irradiated in the presence of (RNCH2CH2NRC+j=2 Whereas Ar3Ge‘ have (R = Me or Et) in toluene ~olution.~* 46 D. Tzeng and W. P. Weber J. Am. Chem. SOC.,1980,102 1451; T.Y. Yang and W. P. Weber ibid. p. 1641. 47 K. P. Steele and W. P. Weber J. Am. Chem. Soc. 1980,102 6095. 48 V. J. Tortorelli and M. Jones J. Am. Chem. SOC.,1980,102 1425. 49 D. Seyferth D. C. Annarelli S. C. Vick and D. P. Duncan J. Organomet. Chem. 1980,201,179. 50 M. Ishikawa K. I. Nakagawa R. Enoxida and H. Kumada J. Organomet. Chem. 1980,201,151. ’’ M. J. S. Gynane M. F. Lappert P. I. Riley P. RiviBre and M. RiviBre-Baudet J. Organomet. Chem. 1980 202 5. 228 J. L. Wardell appreciable thermal stability (tl is several hours at 20 "C) both Ar3Si' and Ar3Sn' are only detected under constant irradiation at -20 "C. The two competing reac- tions of Me3Si' [see reactions (12) and (13)] were investigated in three kdi Me3SiH + [H2C=SiMe2] (12) 2Me3Si' 57Me3SiSiMe3 (13) krccomb These studies following different routes to Me3Si' [namely from Me3SiH on mercury-sensitized photolysis (gas-phase reaction) and on reaction with Bu'O' (in the liquid phase) and also from photolysis of (Me3Si)2Hg] all proved the occurrence of the disproportionation step but did not agree on kdisp:krecomb values.An electron- diffraction study of Me2Si=CH2 (dimethylsilaethylene; generated via pyrolysis of 1,l-dimethyl-1 -silacyclobutane) has been made;53 bond lengths (re values) were calculated to be 1.83 f 0.04 (Si=C) and 1.91 f 0.02A (Si-C). Two separate Russian groups have reported54" the i.r. spectra of Me2Si=CH2 and (CD3),Si=CH2 in argon matrices at low temperatures; the v(C=Si) stretch was given as 1001 and 1003.5 cm-'.The i.r. and mass spectra of MeHSi=CH2 H2Si=CH2 [v(Si-C) 1155 cm-'I and D2Si=CH2 were also studied; it was quoted that H2Si=CH2 can be stored at -196 "C for several months.54b Prochiral silaethylenes PhR'Si=CH2 (R' = Et or Bu') produced by irradiatiod5 of 1-R-1-phenyl-1-silacyclobutaneat 2537& react with chiral alcohols (R20H) to give unequal amounts of diastereoisomeric pairs of R'PhSiHOR2; this was reported to be the first example of asymmetric induction involving a trigonal silicon species to be observed at a silicon centre. As well as routes to silaethylenes from silacyclobutanes the following have been employed (i) the reaction56 of H2C=CHSiMe2C1 and Bu'Li in hexane (forms Bu'CH2CH=SiMez in contrast to the stable organolithium adduct Bu'CH,CHLiSiMe2C1 which forms in THF) and (ii) (Me3Si)2C=SiPh2 (28) from thermolysis of (Me3Si)3CSiPh2F at 450 "C;the silaethylene (28) takes part in rapid equilibriation prior to internal cyclizations," involving addition of C6H4-H to Si=C bonds as shown in Scheme 7.Flash therrnolysi~~~ of (29; Y = CH2CH=CH2 R = H or Me) or of (29; Y = OAc R = H) and co-deposition of the products (Me3Si)(Ph2MeSi)C=SiMe2$(Me3Si)(PhMe2Si)C=SiMePh Jt .lt (Me3Si)2C=SiPh2 (Me2PhSi)2C=SiMe2 (28) Scheme 7 52 S.K. Tokach and R. D. Koob J. Am. Chem. SOC.,1980,102 376; B. J. Cornett K. Y. Choo and P. P. Gaspar ibid. p. 377; L. Gammie I. Safarik 0.P. Strausz R. Roberge and C. Sandor ibid. p. 378. " P. G. Mahafly R. Gutowsky and L. K. Montgomery J.Am. Chem. Soc. 1980,102,2854. 54 (a) 0.M. Nefedov A. K. Maltsev V. N. Khamashesku and V. A. Korolev J. Organomet. Chem. 1980 201 123; L. E.Gusel'nikov V. V. Volkova V. G. Avakyan and N. S. Nametkin ibid. p. 137; (6) N. Auner and J. Grobe Z. Anorg. Allg. Chem. 1979,459 15. " G. Bertrand J. Dubac P. Mazerolles and J. Amchelle J. Chem. SOC.,Chem. Commun. 1980,382. 56 P.R.Jones T. F. D. Lim and R. A. Pierce J. Am. Chem. SOC.,1980,102,4970. 57 C.Eaborn D. A. R. Harper P. B. Hitchcock S. P. Hopper K. D. Safa S. S. Washburne and D. R. M. Walton J. Organomet. Chem. 1980 186,309. G. Maier G. Mihm and H. P. Reisenauer Angew. Chem. Int. Ed. Engl. 1980 19 52; B. Solouki D. Rosmus H. Bock and G. Maier ibid. p. 51; H. Bock R. A. Bowling B. Solouki T. J. Barton and G.T. Burns J. Am. Chem. Soc. 1980 102 429; C.L.Kreil 0.L. Chapman G. T. Burns and T. J. Barton ibid. p. 841. Organometallic Chemistry -Part (ii) Main-Group Elements with argon on a nearby spectral window at low temperatures was carried out. The spectral data (u.v. i.r. and mass) indicated5' the presence of (30). In addition the photoelectron spectra were also obtained after generating (30) close to the ioniz- ation region of the spectrometer. The ionization potentials that were obtained were 8.0 and 9.3 eV for (30; R = H) and 7.7 and 9.16 eV for (30; R = Me). Trapping of (30; R = Me) by MeOH its dimerization and the photoconversion of (30; R = H) into Dewar-silabenzene were also reported. On heating (Me3Si)2C=C(SiMe3)2(31) in decalin solution at 150"C the e.s.r.spectrum of RH (Me3Si)2C=C(SiMe3)2 $(Me3Si),C-C(SiMe3)~ -(Me3Si)2CH-C(SiMe3) (31) (32) (32) was ~bserved.'~ An unusually rapid (2)-(E)isomerization of the tetrasilyl- ated ethylene (PhMe2Si)(Me3Si)C=C(SiMe2Ph)(SiMe3) occurs in THF with an activation energy of ca 30 kcalmol-l; at 68"C = 3.7 x lOP4s-' and K[(E)/(Z)] The = 0.6. The isomerization could also be induced phot~chemically.~~ isomerization of the readily available (2)-RCH=CHSiMe3 compounds to the (E)-isomers also occurs on U.V. irradiation in the presence of NBS and pyridine.60" Another new route to (E)-Me,Si-substituted alkenes is illustrated in reaction (14)?Ob (Me3Si)3Al+ HCECR -+ (E)-Me3SiCH=CHR (14) (R=aryl or CH20H) The cuprate (MezPhSi)&uLi*LiCN prepared from Ph'MeSiLi and BuCN has been used6'" in the synthesis of various (alkeny1)silanes (see Scheme 8); addition of the cuprate (Me3Sn)(PhS)CuLi to cup-acetylenic esters has also been reported; some control of the stereochemistry of the products R'(Me3Sn)C=CHC02R2 can be achieved.61b cu ii X \ /SiMe2Ph ' \ PMezPh (Me2PhSi)2CuLi.LiCN -P c=c /c=c\ /\ R H R H Reagents i RCECH; ii X' = H' or X-Y = Me-I I, etc.Scheme 8 Direct routes from toluene and m-xylene to 2,5-(Me3Si)2-toluenes and -m-xylenes (33)are possible6' by utilizing the sequence (i) treatment with Me3SiC1 and lithium in THF [which gives 2,5-R2-2,5-dihydrotolueneand 2,5-R2-m-xylene (34) 59 H. Sakurai H. Tobita M. Kira and Y. Nakadaira Angew. Chem. Int. Ed. Engl. 1980,19 620.6o (a) G. Zweifel and H. P. On Synthesis 1980 803; (b) G. Altnau L. Rosch F. Bohlmann and M. Lomitz Tetrahedron Lett. 1980 21 4069. 61 (a) I. Fleming and F. Roessler J. Chem. SOC.,Chem. Commun. 1980 276; (6) E. Piers and H. E. Morton J. Org. Chem. 1980 45,4263. 62 M. Laguerre J. Dunogues and R. Calas Tetrahedron Lett. 1980 21 831; G. Felix M. Laguerre J. Dunogues. and R. Calas J. Chem. Res. (S) 1980 236. 230 J. L. Wardell (R = Me3Si)] followed by (ii) oxidation e.g. by air. Either (33) or (34) can be used as regiospecific precursors of other functional groups; e.g. (34) as a source of 2,s -(MeC0)2 derivatives. Germaethylenes (or germene~)~~ e.g. Ph2Ge=CHR1 (prepared from Ph2Ge and diazomethanes R1CHN2) have been trapped by MeOH [as Ph2Ge(OMe)CH2R1)] by R2CH0 and by PhNO (see Scheme 9).Diphenylger-Ph2Ge-CHR [Ph2Ge=CHR] + PhNO __* I I -[Ph2GeO] + PhN=CHR 0-NPh Scheme 9 mylene (Ph,Ge:) was obtained from Ph2GeClH and excess NEt3. Dimethylgermyl- ene has been prepared by two thermoly~es,~~ i.e. (i) of Me3GeGeMe2H at 250°C and (ii) of the 7,7-dimethyl-7-germanorbornadienes(35) (see Scheme 10)-a route used previously for stannylenes; r4 = 0.67 and 120 h at 70 "C for (35; R = H) and (35 ;R = Me) respectively. Dehydrobromination of 1,4-di-t-butyl-1-bromo-1-germacyclohexa-2,4-diene,using Bu'Li or LiNPri2 produces transient 1,4-di-t- butyl-1 -germabenzene which will dimerize unless intercepted (e.g. by dienes in Diels-Alder reaction^).^^ Me Me \/ Ph Ph R4 + Me,Ge Ph R (35) R = MeorH Reagents i R,benzyne; ii heat Scheme 10 Partial resolution66 has been achieved of tri(organo)germanium hydrides and chlorides as well as of tetra(organo)tins e.g.Me(neophyl)Ph(Ph3C)Sn by inclusion chromatography on microcrystalline cellulose triacetate. The "J(Sn-D) coupling constants easily deduced from the triplet observed by l19Sn Fourier-transform n.m.r. spectroscopy with proton decoupling have been used for the direct identification of isomeric and diastereoisomeric organotin corn pound^.^^ Reactions of allyltin compounds have continued to find use in organic syntheses; e.g. the Pd"-catalysed cross-coupling6'" of allyltin with ally1 bromides or acetates and the erythro-selective addition of crotyltin,68b either (E)or (Z),to aldehydes [reaction (1 5)].This reaction contrasts with the rhreo-selective addition of (E)-MeCH=CHCH2BR13 Li' to R2CH0.68' The tran~formation~~ of -CH=C(SnMe3)-into -CH=C(N02)- in cyclic derivatives has been made using C(NOZ)4 in DMSO at 25 "C. 63 P. RiviCre A. Castel and J. SatgC J. Am. Chem. SOC., 1980 102,5413. E. C. L. Ma D. P. Paquin and P. P. Gaspar J. Chem. SOC.,Chem. Commun. 1980 381; W. P. Neumann and M. Schriever Tetrahedron Lett. 1980,21,3273. G. Mark1 and D. Rudnick Tetrahedron Lett. 1980 21 1405. 66 I. van den Eynde and M. Gielen J. Organornet. Chem. 1980,198,(255. 67 J. P. Quintard M. Dequeil-Castaing G. Dumartin A. Rahm and M. Pereyre J. Chem. SOC., Chem. Commun. 1980,1004. 68 (ti) J. Godschalx and J. K. Stille Tetrahedron Lett.1980 21 2599; B.M.Trost and E. Keinan ibid. p. 2595;(6) Y.Yamamoto H. Yatagai Y. Naruta and K. Maruyama J. Am. Chem. SOC., 1980 102 7107;(c) Y.Yamamoto H. Yatagai and K. Maruyama J. Chem. SOC., Chem. Commun. 1980,1072. 69 E. J. Corey and H. Estreicher Tetrahedron Lett. 1980 21 1113. Organometallic Chemistry -Part (ii) Main-Group Elements 231 OH + R2CH0 (i) BF,. CHzClz R~pp *SnR13 il (ii) H,O+ (R2 = Me or Ph) Me [ >90% erythro] The photochemically induced iodinolysis of tetra(alky1)tins (36) with iodine in CCl solution is a radical chain process with a long chain length.70 The homolytic substitution (SH2)is a two-step process (Scheme 11)in which the activation process (fast) R4Sn + I' [R4Sn+ I-] -R3SnI + R' Scheme 11 is an electron transfer having a similar selectivity to that shown in the reaction of (36) with IrCls2-.Reactions with molecular halogens and with HgC12 involve charge-transfer mechanisms the rate-determining step being the unimolecular decay of the charge-transfer complex by transfer of an electron from &Sn to the electrophile. Rate constants for other electron-transfer reactions of R4Sn (as well as of %Pb and R2Hg) with (NC),C=C(CN) and Fell1 compounds have been discussed in terms of inner-sphere and outer-sphere mechanism^.^^ Alk~ltin-mediated~l carbo-cyclizations[e.g. reaction (16)] have been carried out. SnMe 6 Group5 The persistent radical [(Me3Si),CHI2As' (ti = 10 d at 300 K) was obtained by U.V. irradiation of [(Me3Si)2CH]2AsC1 in the presence of the electron-rich olefin (Et16CH2CH2NEt-d=)=2.72 Further compounds containing isolated As=C bonds have been obtained (Scheme 12); on irradiation (37)forms a dimer.73 Organobis- muth compounds have been used in organic synthesis; e.g.Ph5Br will o-phenylate phenols and 6-phenyl-2,6-dimethylcyclohexa-2,4-dieneis formed with 2,6-Me2C6H30H. In addition Ar3BiV carboxylates oxidize glycols and allylic RAs(SiMe,) RAs(SiMe3)COBu' -B RAs=C(OSiMe3)Bu' R = Me But Ph or o-Me3SiOC6H4 (37) Scheme 12 70 S.Fukuzumi and J. K. Kochi J. Org. Chem. 1980,45,2654;J. Am. Chem. SOC.,1980 102 2141; S.Fukuzumi C. L. Wong and J. K. Kochi ibid. p. 2928;S.Fukuzumi and J. K. Kochi J. Phys. Chem. 1980,842246,2254;Inorg. Chem. 1980,19,3022;J. Am. Chem.SOC.,1980,102,7290. 71 T. L. MacDonald and S. Mahalingam J. Am. Chem. SOC.,1980,102,2113. 72 M. J. S.Gynane A. Hudson M. F. Lappert P. R. Power and H. Goldwhite J. Chem. SOC.,Dalton Trans. 1980 2428. 73 G. Becker and G. Gutekunst Z. Anorg. Allg. Chem. 1980,470,131 144;J. Heinicke B. Raap and A. Tzschach J. Organomet. Chem. 1980,186 39. '' D. H. R. Barton J. C. Blazejewski B. Charpiot D. J. Lester W. B. Motherwell and M. T. B. Papoula J. Chem. SOC.,Chem. Commun. 1980 827; D.H. R.Barton D. J. Lester W. B. Motherwell and M. T. B. Papoula ibid. p. 246.