9 Organometallic Chemistry Part (ii) The Main-Group Elements By A. T. HUTON Department of Pure and Applied Chemistry The Queen's University Belfast BT9 5AG 1 Introduction In scope and coverage this year's report is similar to last year's,' though this time somewhat more emphasis has been placed on the organometallic chemistry of the elements of Groups I and IV mainly at the expense of Group 11. Next year's report will redress the balance particularly with respect to organo-zinc -cadmium and -mercury chemistry. A glance at the references cited in this report will confirm that the health of main-group organometallic chemistry is confined largely to the research groups in Germany with whom excellence in this field has been traditionally associated. 2 Groups I and I1 In principle the hydrides of alkali metals should be ideal synthetic reagents for the metallation of active hydrogen compounds hydrogen is the only by-product and the course of the reaction can be conveniently followed by gas evolution.Unfortu- nately however the alkali metal hydrides are normally not very reactive. Now the synthesis of 'superactive' alkali metal hydride metallation reagents LiH NaH and KH by hydrogenation of BUM (M = Li Na K) in hexane in the presence of TMEDA has been described.2 Precipitated n-butylpotassium (Bu"K) prepared by metal-metal exchange between Bu"Li and potassium t-amylate has been shown to dissolve in hexane (after addition of TMEDA) to give a homogeneous solution which is an effective metallation reagent,3 while simple mixing of Bu'OK Bu"Li and TMEDA in hexane or pentane at temperatures below -40 "Cgives an extremely efficient metallating agent.This has resulted in the first successful direct metallation of ethene (to form vinyl potassium); typical products are obtained with l-bromo- octane and after addition of LiBr in THF with benzaldehyde and diphenyl disul- hide.^ ' A. T. Hutton Annu. Rep. hog. Chem. Sect. B Org. Chem. 1985 82 223. ' P. A. A. Klusener L. Brandsma H. D. Verkruijsse P. von R. Schleyer T. Fried] and R. Pi Angew. Chem. Int. Ed. Engl. 1986 25 465. R. Pi W. Bauer B. Brix C. Schade and P.von R. Schleyer J. Organomet. Chem. 1986 306 C1. L. Brandsma H. D. Verkruijsse C. Schade and P. von R. Schleyer J. Chem. Soc. Chem. Commun. 1986 260.221 222 A. 7'.Hutton Reductive metallation has been used as a general preparative method for hydrocar- bon allylmetallic compounds allyl phenyl sulphides are the ideal substrates for a particularly versatile preparative method for allylic anions because of their great ease of preparation and their smooth reductive lithiation using lithium naphthalenide or l-(dimethylamin~)naphthalenide.~ The great importance of the counterion in determining the structure of a carbanion is clearly revealed both by calculations performed for allenyl anions and the results of the first X-ray structure analysis of an allenylsodium derivative viz. Na(tmeda),+ Bu'-C=C=C(Me)-CEC-Bu'. Na+ favours the allenyl structure (1) having a strongly localized charge; the free anion and the Li+ salts on the other hand are better described by the resonance structures (1) f* (2) *(3).The different behaviour of the organolithium and organosodium compounds is due to the stronger interaction of the anion with the smaller Li+ ion.6 Me Me Me Attention has been drawn to the use of the complex-induced proximity effect (CIPE) as a rationale for a number of novel reactions of organolithium compound^.^ The importance of such complexation has been recognized for some time but recent work suggests that proximity in a transition state related to the initial complex can be dominant over classical effects in determining the course of a reaction. CIPE processes are notable in the formation and reactions of a variety of carbanionic synthetic equivalents ranging from a-lithioamines allyl anions and electrophilic nitrogen to enolates and the regio- and stereo-control provided in these reactions is a matter of continuing interest.The CIPE proposal should be a useful guide for correlating observations devising new reactions and designing mechanistic probes. Recent reports which illustrate CIPE processes in novel reactions include the carbenoid anion behaviour of dilithio derivatives of thioacetal alcohols (ring-closure by oxyanion-facilitated CH bond insertion),8 the selective vulnerability towards lithium bases of a yCH bond syn to a functional group in an a,P-unsaturated amide system,' the transient introduction of a complexing group to bring the lithium base into proximity for proton abstraction e.g.in tetrahydroisoquinoline alkyla- tions," and the deprotonation of the chelating enamine derived from cyclohexanone and N,N,N'-trimethylethylenediamine which leads to complete formation of the vinyl carbanion rather than the expected allyl carbanion (see equation l)." An ' T. Cohen and B.-S. Guo Tetrahedron 1986,42 2803. C. Schade P. von R. Schleyer M. Geissler and E. Weiss Angew. Chem. Inr. Ed. Engb 1986 25 902. ' P. Beak and A. I. Meyers Acc. Chem. Rex 1986 19 356. ' R H Ritter and T Cohen. J Am Chem Ync. 1986. 108. 1718 M. Majewski J. R. Green and V. Snieckus Tetrahedron Lett. 1986 27 531. "' A. R. Katritzky and K. Akutagawa Tetrahedron 1986 42 2571. " G. Stork C. S. Shiner C.-W. Cheng and R. L. Polt J. Am. Chem. Soc. 1986 108 304. Organometallic Chemistry -Part (ii) The Main-Group Elements 223 equally surprising result is that chelating enamines derived from aldehydes unbranched at the a-position in which the competition now involves removal of either the a-or the P-vinyl hydrogen lead to p-rather than the expected a-deprotonation (see equation 2)." It seems very likely that these remarkable deproton- ations of chelating enamines arise from critical geometric constraints in the relevant Me transition states.The oxygen- and nitrogen-assisted lithiation and carbolithiation of non-aromatic compounds has been reviewed the discussion covers the properties of non-aromatic organolithium compounds capable of intramolecular coordination to oxygen and nitrogen.I2 The compounds (4) react with Bu'Li to give considerable amounts of the 'anti-Michael' adducts (5) (equation 3) and a single-electron-transfer mechanism has been proposed for these reactions; in contrast both compounds (4) behave as Michael acceptors towards Bu"Li.l3 R H But \/ 1. 2Bu'Li 2. H,O I *R CH (3) R = Ph,Me,Si (4) \CO,H H /c=c\cO,H \CH ' (5) The structural variety of organolithium compounds has been further expanded by the 1,3-diboratacyclobutadiene(6),for which n.m.r. spectroscopic results under- line the importance of the resonance structures for the description of the electron distribution. Compound (6) crystallizes as a dimer in which a planar layer of four Li atoms bridges two four-membered rings in a sandwich structure. The rings are eclipsed (a B atom lying above a C atom in each case) and strongly puckered; the separations of the Li atoms in the Li layer are in part shorter than those found in the Li tetrahedra of tetrameric lithium cornpo~nds.'~ The first isolable crystalline compound containing the bishomoaromatic anion bicyclo[3.2.l]octa-2,6-dienide the TMEDA adduct of bicyclo[3.2.l)octa-2,6-dienyllithium,has C symmetry in solution but in the solid state the lithium is bonded unsymmetrically to all five sp2 carbon atoms.The lithium occupies an almost central position below the seven- membered ring such that not only the allylic but also the olefinic part of the carbanion l2 G. W. Klumpp Red. Trau. Chim. Pays-Bas 1986 105 1-21 containing 151 refs. l3 K. J. H. Kruithof A. Mateboer bl.Schakel and G. W. Klumpp Red. Trau. Chim. Pays-Bas 1986 105 62. 14 G. Schmidt G. Baum. W. Massa and A. Berndt Angew. Chem. Inr. Ed. Engl 1986 25 1111. 224 A. T. Hutton SiMe3 SiMe3 is engaged in metal-ligand bonding. A coordination number of seven rarely observed for solvated lithium bound to a pure hydrocarbon is thus a~hieved.'~ An X-ray structure determination has shown that the 'ketene iminate' PhCH=C=N-and not the carbanion PhCH-CEN is present in [(a-cyanobenzyl- lithium.trneda),.C,H,] as structure (7). The 'anions' are planar (measurements at -35 "C located the H atom) and it is interesting that the bond lengths in the CCN groups deviate considerably from those in ketene imines such as (8) because in .Ph ...* :C' 'I4 comparison enolates have practically the same bond lengths as enol ethers.This difference is probably due to the different stabilization of the negative charge whereas a carbonyl group is predominantly mesomeric in a nitrile group the inductive field effect also plays a prominent role.' Another organolithium com- pound without Li-C bonding is (Ph,P),CHLi-tmeda which as shown by X-ray structure analysis is a monomer containing a slightly puckered CP2Li four- membered ring (9). The transannular Li-C distance of 3.054(6) A precludes any bonding interaction." Again the crystal structure of the a-sulphinyl 'carbanion' [PhC(Me)S(O)Ph]Li shows no interaction between the benzylic carbon atom and H C 4-\ Ph2P PPh2 \/ l5 N. Hertkorn F. H. Kohler G.Muller and G. Reber Angew. Chem. Int. Ed. Engl 1986 25 468. G. Boche M. Marsch and K. Harms Angew. Chem. Int. Ed. EngL 1986 25 373. " D. J. Brauer S. Hietkamp and 0.Stelzer J. Organomet. Chem. 1986 299 137. Organometallic Chemistry -Part (ii) The Main-Group Elements the Li atom; the benzylic C atom is not planar and the salt is present as a dimer with a central Li202 unit." The stereoselectivity in reactions of a series of a-lithiosulphinyl carbanions with aldehydes (to give p-hydroxysulphoxide products) has been unambiguously estab- lished steric effects and intramolecular chelation of the associated cation are the important factors contributing to the observed ~utcome.'~ One approach to asym- metric homoaldol reactions may be provided by the surprising finding that a-deprotonation of an a-chiral 2-alkenylcarbamate proceeds with retention and the resultant lithium compound (10) racemizes only very slowly at -78 "C.Accordingly the lithium in (10) must be very strongly bonded to the chiral carbon centre despite the possibility of resonance in the anion. Furthermore lithium-titanium exchange of (10) with TiC1(NEt2) proceeds with inversion to give a species that can add to carbonyl compounds with 1,3-chirality transfer.20 For the first time q3-allyllithium entities as found in allyl-transition metal com- plexes have been shown to exist in the solid state. The X-ray structure of [1,3- diphenylallyllithium~Et20],,shows that the q3-allyllithium units aggregate to form polymeric chains and that the Li atoms lie practically symmetrically above and below the almost planar ally1 groups which are inclined at 120" to each other.The structure in solution is thought largely to correspond with this crystal structure.21 Monolithiated organic compounds can often be converted further into syntheti- cally useful dilithiated derivatives and the first lithium substituent often determines the position of the second metallation by activation of H atoms in close spatial proximity. A combination of X-ray and n.m.r. analysis of 2-lithio- 1-phenylpyrrole (11) has shown that the position of the second lithiation may be predicted. The X-ray structure revealed a dimer [(1 1)-tmeda] with the closest contact to Li shown by the H marked by an asterisk. Using 6Li-'H two-dimensional heteronuclear Li 18 M.Marsch W. Massa K. Harms G. Baum and G. Boche Angew. Chem. Int. Ed. Engl. 1986,25 1011. 19 D. R. Williams J. G. Phillips F. H. White and J. C. Huffman Tetrahedron 1986 42 3003. 20 D. Hoppe and T. Gamer Angew. Chem. Int. Ed. Engl. 1986 25 160. 21 G. Boche H. Etzrodt M. Marsch W. Massa G. Baum H. Dietrich and W. Mahdi Angew. Chem. In?. Ed. Engl. 1986 25 104. 226 A. T. Hutton Overhauser n.m.r. spectroscopy (2D HOESY) it was shown that the solution structure of (11) was similar to that in the crystal with the shortest Li-H distance being to the H marked by an asterisk (the most intense cross peak was between 6Li and this 'H). Experimentally it is indeed this H which undergoes substitution by Li to form the dimetallated product.22 0-OLi OLi (12) X = H or F (13) OH (14) X = H or F (15) Although mono-lithiated fluoro compounds capable of undergoing a-,p- and/or y-elimination have been known for a long time the reagents (12) and (13) [>go% enantiomeric excess (R)-form] are the first poly-lithiated derivatives of this type.They exhibit an unexpected stability toward fluoride elimination and offer accessibil- ity to a variety of new fluorinated organic compounds with additional functional groups. They can be prepared using Bu"Li in THF at low temperatures and converted with carbonyl compounds or alkylating reagents into (14) or (15) respectively in yields of 50'/0.~~ It has also been shown that addition of excess lithium powder to cyclooctyne in ether (-35 "C 2 h) results in the formation of a yellow solution of cis-dilithiocyclooctene while acyclic alkynes react much more slowly with lithium (20 "C,48 h) to give trans-dilithioalkenes which are insoluble in ether.Only trans-addition is observed; the long-known cis-addition of lithium to diphenylacetylene and now to cyclooctyne however suggests a sequential cis-trans isomerization as has found to be the case for unsubstituted 1,2-dilithioeth~lene.~~ Liz[ (C5Me4)2CH2] the dilithium salt of the novel permethylated ring-connected [ ( C5Me4)2CH2]2-dianion has been prepared as a colourless pyrophoric powder from C5Me4H2 via (C5Me4H),CH2 and subsequent reaction with Bu"Li (equation 4);it has been used as a bridging ligand between transition metal fragments.25 2Bu"Li CHZCIZ 2C,Me4H2 -2LiC,Me,H -22 W.Bauer G. Miiiler R. Pi and P. von R. Schleyer Angew. Chem. Int Ed. Engl. 1986 25 1103. 23 D. Seebach A. K. Beck and P. Renaud Angew. Chem. Inf. Ed. Engl. 1986 25 98. 24 A. Maercker T. Grade and U. Girreser Angew. Chem. Int. Ed. Engl. 1986 25 167 25 H. J. Scholz and H. Werner J. Organomef. Chem. 1986 303 C8. 232 A. T. Hutton 88%) and its transoid isomer (22; 12%). The same reaction using the germanium analogue of (20) did not give the corresponding germole but only 2,3-dimethyl- butadiene as the sole identifiable product.58 In the presence of a catalyst derived from the co-sublimation of AlCl with FeC1 (lolo) a series of permethylcyclosilanes (Me,Si), n = 5-12 rearranged to form isomeric branched cyclopentasilanes or cyclohexasilanes remarkably in each reac- tion a single isomeric product was obtained in nearly quantitative yield.Conforma- tional analyses of these branched cyclosilanes have been perf~rmed.~~ Alkylthiosilanes RSSiMe, have been found to be excellent initiators for the group- transfer polymerization of acrylic acid esters.60 The molecular and electronic structures of penta- and hexa-coordinate silicon compounds have been comprehensively reviewed.61 The reactivity of anionic pentacoordinated silicon complexes towards nucleophiles e.g. PhMgBr has been studied and this has led to new synthetic methods for various silanes while the reaction of Grignard reagents with dianionic hexacoordinated (pyrocatechol) silicon complexes has provided the way to synthesize organosilicon compounds directly from silica The chemistry of the penta-coordinated bi- and tri-cyclic organo- silicon and -tin compounds R2M(CH2CH2CH,),E and RM(CH2CH2CH2)3N (M = Si Sn; R = C1 Br I Me; E = NMe 0 S) has been reviewed the compounds which have 'diptych' and 'triptych' structures exhibit intramolecular E (or N) + M donor-acceptor interactions of varying strengths depending on the nature of the substituents R bonded to the metal centre.63 Until recently the investigation of cyclopentadienyl chemistry of main-group elements was restricted to Group IV species and some other isolated examples.This situation has changed radically during the last decade; a variety of new cyclopen- tadienyl compounds is their often- or usually-observed fluxionality and this aspect edge of the possible bonding modes in a-bonded (7')species as well as in v-bonded ( qn)complexes has increased.One of the most interesting features in 7'-cyclopen- tadienyl compounds is their often- or unusually-observed fluxionality and this aspect has been reviewed with respect to compounds of the main-group 111 IV and V elements.64 Recent developments have shown that drastic differences in fluxional behaviour exist among the cyclopentadienyl compounds of main-group elements and these differences may be ascribed to the nature of the main-group element the other ligands bonded to the main-group element and the substituents on the cyclopentadienyl ring. These factors influence the rate of prototropic shifts and the proportion of allylic and vinylic isomers present in equilibrium enormously; further- more they determine specifically the activation energy for the circumambulatory migration of the relevant main-group metal.The migrations are characterized as 58 J.-P. Beteille G. Manuel A. Laporterie H. Iloughmane and J. Dubac Organornetallics 1986 5 1742. 59 T. A. Blinka and R. West Organometallics 1986 5 128 and 133. 60 M. T. Reetz R. Ostarek K.-E. Piejko D. Arlt and B. Bomer Angew. Chern. Int. Ed. Engl. 1986,25 1108. 61 St. N. Tandura M. G. Voronkov and N. V. Alekseev Top. Curr. Chem. 1986 131,99-189 containing 1008 refs. 62 A. Boudin G. Cerveau C. Chuit R. J. P. Corriu and C. Reye Angew. Chem. Inf. Ed. Engl. 1986 25 413 and 414.'' A. Tzschach and K. Jurkschat Pure Appl. Chern. 1986 58 639. 64 P. Jutzi Chern. Rev. 1986 86 983-996 containing 94 refs. 228 A T. Hutton 1,4-addition of benzylzinc bromide or allylzinc bromide respectively to 5-benzy- lidenebarbituric acids followed by hydr~lysis.~~ It has been shown that the bromina- tion (pyridinium hydrobromide perbromide) or iodination (iodine) of allenic or propargylic organomercury halides in pyridine proceeds with rearrangement to afford the corresponding propargylic or allenic halides respectively. This procedure provides a convenient new route to 3-bromo- and 3-iodo-l,2-alkadienes useful in organic ~ynthesis.~~ The synthesis of organometallics by decarboxylation reactions both thermal and radical-initiated has been reviewed; much of this is relevant to the organic chemistry of Zn Cd and Hg.35 3 Group 111 A review on the high-resolution metal-n.m.r.spectroscopy of organometallic com- pounds has highlighted the use of 25Mg n.m.r. for the characterization of organomag- nesium compounds and the relationship between 27Al n.m.r. shift and coordination number of organoaluminium Conclusions from 27Al n.m.r. spectro- scopy invoking an equilibrium between complexes with four- and five-coordinate A1 atoms in the compound [Et2A10CH2(2-C5H4&)l2 have been shown to be incorrect. The false impressions were caused by background signals emphasizing the fact that caution is necessary when n.m.r. spectra of dilute samples of broad quadrupolar metal nuclei are recorded.37 rr-Complexes of aluminium with olefins have long been regarded as intermediates in many reactions of organoaluminium compounds but only now has a compound with aluminium-olefin T-bonds been characterized by X-ray structure analysis.Treatment of the active species AlCl (generated at elevated temperature) with MeCECMe in a low-temperature reaction led upon warming from -196 "C to room temperature to formation of dimeric 1,4-dichlor0-2,3,5,6-tetramethyl-1,4-dialumina-2,5-cyclohexadiene. The structure shows that two non-planar 1,4-dialuminacyc- lohexadiene moieties twisted through 90" with respect to each other are coupled via four aluminium-olefin .rr-bonds to give the dimer (mean rr-bonded A1-C distance 2.354 A).38 All simple organoaluminium compounds which have so far been characterized by single-crystal X-ray diffraction methods have been shown to be dimeric in the solid state with three-centred two-electron carbon bridge bonds or have chain structures as exhibited by Me2A1(C,H,) and Al(CH,Ph) .However steric interac- tions in trimesitylaluminium (16; M = Al) prepared by metal exchange between dimesitylmercury and aluminium metal give rise to a monomeric structure (16) which is planar around the A1 centre with the three mesityl groups disposed in a propeller-like fashion about the trigonal axis. The reactivity of this compound is also altered from that of other organoaluminium compounds because of the structural 33 Y. Frangin C. Guimbal F. Wissocq and H. Zamarlik Synthesis 1986 1046.34 R. C. Larock and M.-S. Chow Organometallics 1986 5 603. 35 G. B. Deacon S. J. Faulks and G. N. Pain Adu. Organomet. Chem. 1986 25,237-276 containing 169 refs. 36 R. Benn and A. Rufinska Angew. Chem. Int. Ed. Engl. 1986 25 861-881. containing 181 refs. 37 R. Benn A. Rufinska E. Janssen and H. Lehmkuhl Organometallics 1986 5 825. 38 H. Schnockel M. Leimkuhler R. Lotz and R. Mattes Angew. Chem. Int. Ed. Engl. 1986 25 921. Organometallic Chemistry -Part (ii) The Main-Group Elements effects of the mesityl group reduced reactivity towards hydrolysis oxidation and complex formation is found.39 Trimesitylgallium has been found to have virtually exactly the same structure (16; M = Ga).,’ An attempt has been made to introduce Ga+ cations in between the parallel rings of [2.2]paracyclophane.However an X-ray structure of the 1 :1 complex with Ga[GaBr,] showed that the Ga+ cations are not accommodated within the cyclo- phane cages but are approximately centrically ( v6)-bonded to the arene rings from their outer sides (Ga- -.ring plane 2.72 A) giving rise to zig-zag chain-like polydecker columns (17). These are interconnected by [GaBr,]- anions each of which provides one bromine atom to bridge two Ga’ centres thereby forming a highly symmetrical three-dimensional network.41 Dimethylgallane best synthesized by the reaction between GaMe3 and NaGaH, has now been characterized by its spectrmcopic and chemical properties and the predominant vapour species at low pressure has been established by electron diffraction as the dimer Me,Ga( p-H),GaMe Although the structure of cyclopentadienylindium(1) in the solid state has long been known to comprise zig-zag polymeric chains of In( q5-C5H5) units with very long In-‘centroid’ distances (3.19 A) the presence of an octahedral cluster has now been established for the permethylated derivative In( v’-C5Me5).The v’-C,Me units are arranged on the exterior of an octahedral In core with an In-‘centroid’ distance of 2.302 A. The volatility of h6(C5Me5)6 however suggests that the octahedral cluster has only marginal stability and monomeric species are probably formed in the gas phase.43 The synthesis and reactions of fulvalenedithallium (18) have been reported; it is obtained as an air-sensitive chocolate-brown solid from the reaction of an ethyl ether/ hexane solution of dihydrofulvalene with thallium ethoxide.44 It has been shown that room temperature reaction of simple aliphatic ketones with an aqueous solution of TlC13 leads to mono-oxoalkylthallium( III) derivatives 39 J.J. Jerius J. M. Hahn A. F. M. M. Rahman 0.Mols W. H. Ilsley and J. P. Oliver Organomeraflics 1986 5 1812. 40 0. T. Beachley jun. M. R. Churchill J. C. Pazik and J. W. Ziller Organomeraflics 1986 5 1814. H. Schmidbaur W. Bublak B. Huber and G. Muller Organometallics 1986 5 1647. P. L. Baxter A. J. Downs M. J. Goode D. W. H. Rankin and H. E. Robertson J. Chem. Soc. Chem. 41 42 Commun. 1986 805. 43 0.T. Beachley jun. M. R.Churchill J. C. Fettinger J. C. Pazik and L. Victoriano J. Am. Chem. SOC. 1986 108 4666. 44 W. C. Spink and M. D. Rausch J. Organomet. Chem. 1986 308 C1. 230 A. T Hutton T1 I T1 (18) of type C12T1CH2C(0)CH2R followed by formation of the selectively a-monochlori- nated ketones MeC(0)CHClR it is not clear as to why the chlorination site of the final product is not the same as the thallation site of the intermediate though it is known that de-thallation can involve rearrangement of the product.45 4 Group IV The organic chemistry of silicon has been succinctly described in a monograph which includes a discussion of the applications of organosilicon compounds in industry and medicine,46 while a review of the chemistry of silenes (=Si=C=) covers their synthesis physical properties calculations concerning their properties and their addition reactions dimerization and molecular rearrangement^.^^ There has been a review on the synthesis of ylides by the desilylation of a-trimethylsilyl onium salts,48 while silyl-substituted cyclopropanes which are emerging as versatile synthetic reagents have been reviewed as Another review focuses on vinylsilane- and alkynylsilane-terminated cyclization reactions.’’ As a result of the direct attachment of the silicon atom to the participating T bond the chemistry of vinyl- and alkynyl-silane terminators is similar.Vinylsilane-terminated cyclizations are of value for the synthesis or carbocycles as well as oxygen and nitrogen heterocycles and their utility for the synthesis of complex target structures alkaloids in particular has been amply demonstrated.Continued evaluation of the useful cyclization chemistry of vinyl- and alkynyl-silanes seems likely the combination of these 7r nucleophiles with other initiating electrophiles as well as the use of these organosilanes to terminate polyene cyclizations that form two or more rings are obvious areas for future development. The chemistry of organometallic compounds with highly sterically hindered organosilicon ligands such as (Me3S&C or (Me2PhSi)3C attached to a variety of metal centres has been reviewed and these include a range of new boron compounds such as (Me,PhSi),CBF(OH) and (Me,PhSi),CB(p-H),Li(thf) ,which have inter- esting structures.It is worth noting that each of the three metal-carbon bonds in the Me-SiMe2-C(SiMe3)2-M systems may participate in reactions at the metal ~entre.~’ The synthesis properties and reactivities of stable compounds featuring double bonding between the main-group IV elements e.g.,the disilenes (R2Si=SiR2) 45 J. Glaser and I. Toth J. Chem. Soc. Chem. Commun. 1986 1336. 46 S. Pawlenko ‘Organosilicon Chemistry’ Walter de Gruyter Berlin and New York 1986. 41 A. G. Brook and K. M. Baines Adu. Organomer. Chem. 1986 25 144 containing 209 refs. 4R E. Vedejs and F. G. West Chem. Rev. 1986 86 941-955. 49 L. A. Paquette Chem. Rev. 1986 86 733-750 containing 146 refs. so T. A. Blumenkopf and L. E. Overman Chem. Reo. 1986 86 857-873 containing 105 refs.” J. D. Smith Pure Appl. Chem. 1986 58 623. Organometallic Chemistry -Part (ii) The Main-Group Elements 231 digermenes ( R2Ge=GeR2) and distannenes (R,Sn=SnR,) have been reviewed,52 as have derivatives of divalent Si Ge Sn and Pb (silylenes germylenes etc.) as ligands in transition-metal complexes.53 Silabenzene silaethene ( H2Si=CH2) 1,4- disilabenzene and related compounds such as borabenzene and boraethene have been commented on briefly.54 'Thermal elimination of lithium chloride' reactions continue to be popular result- ing e.g. in the synthesis of the very stable free silanimine (19) which is an orange crystalline solid melting at 97-99 "C without decomposition to give a deep-red liquid. This stability is due to the good steric shielding of the Si=N bond.Neither in solution nor as a solid does (19) exhibit the often common tendency to undergo dimerization though it is very sensitive to water and air.55 A Y"'" (19) The application of difunctional organosilicon compounds to organic synthesis can be seen in the simultaneous use of a silicon atom as an activating influence and a link between reagent and substrate. This forms the basis of a method which reduces P-hydroxy-ketones to anti-1,3 -diols with diastereoisomeric excesses exceeding 95% the reducing agent is an organosilane e.g. Pr',SiHCl which is initially attached to the hydroxy group of the P-hydroxy-ketone and then induced to react with the carbonyl group by a Lewis acidic catalyst e.g. SnC14. The reaction is presumed to involve intramolecular transfer of hydrogen from the silicon atom to the carbonyl carbon.56 The utility of trimethylsilylation in organic synthesis is well recognized and a large number of methods have been described for the introduction of the trimethylsilyl group.Now trimethylsilylazide ( Me3SiN3) has been shown to react very rapidly with primary or secondary alcohols at room temperature to give high yields of trimethylsilyl ethers.57 The double C-methylation of main-group IV metalloles causes a stabilization toward the [4 + 21 dimerization reaction and an attempt to produce stable monomeric 3,4-dimethylmetalloles with a Si-H or Ge-H bond has resulted in the first.stable C-methylated silole having a Si-H bond. Flash vacuum pyrolysis of the starting silacyclopentene (20) was used in reaction (5) to obtain the silole (21; FVP -C,H 0+ 0 R = Me Ph Si R/si-R/\ Pi\ H RH (20) (21) (22) 52 A.H. Cowley and N. C. Norman Prog. Inorg. Chem. 1986 34 1-63 containing 156 refs. 53 W. Petz Chem. Reu. 1986 86 1019-1047. 54 G. Maier Pure Appl. Chem. 1986 58 95. 55 M. Hesse and U. Klingebiel Angew. Chem. In?. Ed. Engl. 1986 25 649. 56 S. Anwar and A. P. Davis J. Chem. Soc.,Chem. Commun. 1986 831. 57 D. Sinou and M. Emziane Synthesis 1986 1045. 232 A. T. Hutton 88%) and its transoid isomer (22; 12%). The same reaction using the germanium analogue of (20) did not give the corresponding germole but only 2,3-dimethyl- butadiene as the sole identifiable product.58 In the presence of a catalyst derived from the co-sublimation of AlCl with FeC1 (lolo) a series of permethylcyclosilanes (Me,Si), n = 5-12 rearranged to form isomeric branched cyclopentasilanes or cyclohexasilanes remarkably in each reac- tion a single isomeric product was obtained in nearly quantitative yield.Conforma- tional analyses of these branched cyclosilanes have been perf~rmed.~~ Alkylthiosilanes RSSiMe, have been found to be excellent initiators for the group- transfer polymerization of acrylic acid esters.60 The molecular and electronic structures of penta- and hexa-coordinate silicon compounds have been comprehensively reviewed.61 The reactivity of anionic pentacoordinated silicon complexes towards nucleophiles e.g. PhMgBr has been studied and this has led to new synthetic methods for various silanes while the reaction of Grignard reagents with dianionic hexacoordinated (pyrocatechol) silicon complexes has provided the way to synthesize organosilicon compounds directly from silica The chemistry of the penta-coordinated bi- and tri-cyclic organo- silicon and -tin compounds R2M(CH2CH2CH,),E and RM(CH2CH2CH2)3N (M = Si Sn; R = C1 Br I Me; E = NMe 0 S) has been reviewed the compounds which have 'diptych' and 'triptych' structures exhibit intramolecular E (or N) + M donor-acceptor interactions of varying strengths depending on the nature of the substituents R bonded to the metal centre.63 Until recently the investigation of cyclopentadienyl chemistry of main-group elements was restricted to Group IV species and some other isolated examples.This situation has changed radically during the last decade; a variety of new cyclopen- tadienyl compounds is their often- or usually-observed fluxionality and this aspect edge of the possible bonding modes in a-bonded (7')species as well as in v-bonded ( qn)complexes has increased. One of the most interesting features in 7'-cyclopen- tadienyl compounds is their often- or unusually-observed fluxionality and this aspect has been reviewed with respect to compounds of the main-group 111 IV and V elements.64 Recent developments have shown that drastic differences in fluxional behaviour exist among the cyclopentadienyl compounds of main-group elements and these differences may be ascribed to the nature of the main-group element the other ligands bonded to the main-group element and the substituents on the cyclopentadienyl ring.These factors influence the rate of prototropic shifts and the proportion of allylic and vinylic isomers present in equilibrium enormously; further- more they determine specifically the activation energy for the circumambulatory migration of the relevant main-group metal. The migrations are characterized as 58 J.-P. Beteille G. Manuel A. Laporterie H. Iloughmane and J. Dubac Organornetallics 1986 5 1742. 59 T. A. Blinka and R. West Organometallics 1986 5 128 and 133. 60 M. T. Reetz R. Ostarek K.-E. Piejko D. Arlt and B. Bomer Angew. Chern. Int. Ed. Engl. 1986,25 1108. 61 St. N. Tandura M. G. Voronkov and N. V. Alekseev Top.Curr. Chem. 1986 131,99-189 containing 1008 refs. 62 A. Boudin G. Cerveau C. Chuit R. J. P. Corriu and C. Reye Angew. Chem. Inf. Ed. Engl. 1986 25 413 and 414. '' A. Tzschach and K. Jurkschat Pure Appl. Chern. 1986 58 639. 64 P. Jutzi Chern. Rev. 1986 86 983-996 containing 94 refs. Organometallic Chemistry -Part (ii) The Main-Group Elements 1,5-sigmatropic rearrangements which can proceed either with retention or with inversion of configuration depending on the circumstances. Last year's report' suggested that a new approach to the main-group IV metal-locenes which had resulted in the synthesis of decamethyl-germanocene and -stannocene pointed a possible path to the (then unknown) analogous silicon compound.65 Indeed the reduction of (Me5C5)2SiC12 with naphthalene-lithium -sodium or -potassium in THF has now been reported to afford decamethyl- silicocene the first molecular compound of divalent silicon that is stable under normal conditions and the first .ir-complex with silicon as the central atom.The colourless crystalline silicocene Si( q5-C5Me,)2 readily undergoes sublimation and is thermally stable (m.p. 171 "C) but is extremely air-sensitive (presumably under- going oxidation). Two conformers are present in the crystal structure in the ratio 1:2. Whereas in the first conformer the two C5Me5 rings are staggered and their planes strictly parallel in the second conformer the rings are staggered and form an interplanar angle of 25.3" probably due to intermolecular interactions and packing effects in the crystal.Interestingly the colours of the decamethylmetallocenes of this group proceed from colourless (Si) through light-yellow (Ge) and dark-yellow (Sn) to orange-red (Pb).66 The molecule with the longest Si-Si bond is now (hexa-t-buty1)disilane (23). The X-ray structure determination of (23) shows an unusually long Si-Si bond of 2.697 A which is about 0.35 8 greater than that usually found in normal disilanes (ca. 2.34 A). This distance apparently results (despite a staggered conformation) from mutual repulsion of the But groups. The air- and moisture-stable compound (23) is formed via a radical intermediate upon reaction of nitrosyl cations with (tri-t-buty1)silyl-sodium or -potassium (equation 6).67 Two neighbouring very long +2NO' 2But,SiNa (or K) 2{But,Si}' + Bu',Si-SiBu' (6) -2N0 -2Na(or K)' (23) Si-Si bonds are found in the diiodotrisilane (24) [2.581(1) and 2.644(1) A Si-Si-Si = 115.8(1)0].68 The great steric crowding in the molecule significantly (24) lengthens the Si-C and Si-I bonds as well and also increases the Si-Si-Si and Si-C-C bond angles.A linear correlation between the average number of bulky substituents per Si atom (n) and the length of the Si-Si bond (d) has made it possible to classify substituents according to their steric effect [d(Si-Si) = (0.186n + 2.14) A].680n the other hand the weak and highly reactive central Si-Si 62 Full paper P. Jutzi and €3. Hielscher Orgnnornetallics 1986 5 1201. 66 P. Jutzi D. Kanne and C. Kriiger Angew.Chem. Int. Ed. Engl. 1986 25 164. 67 N. Wiberg H. Schuster A. Simon and K. Peters Angew. Chem. In?. Ed. Engl. 1986 25 78. 68 M. Weidenbruch B. Flintjer K. Peters and H. G. von Schnering Angew. Chem. Int. Ed. Engl. 1986 25. 1129. 234 A. T. Hutton bond in the tetrasilabicyclo[ 1.1.O]butane (25) which undergoes extremely facile ring-inversion is surprisingly short at 2.373(3) A. This distance is typical for a cyclic Si-Si bond but was expected to be particularly long in (25) owing to the high p-character of the bond and in analogy to the bonding in bicycl~butanes.~~ / \ But But (25) R = 2,6-Et,C,H3 A highly efficient method of uniting inorganic and organic matter requires ‘bireac- tive’ molecules containing a silicon functionality for bonding to inorganic material on the one hand and a carbon functionality for anchoring to an organic counterpart on the other.The 3-chloropropyltrialkoxysilanes,(R0)3SiCH2CH2CH2C1, and their derivatives epitomize such molecules owing to their bifunctionality they are capable of binding to inorganic (especially siliceous) systems as well as to organic polymers. They are key intermediates in the commercial production of organofunctionalized silanes and polysiloxanes and were originally used as coupling agents mainly in glass-fibre reinforced thermosetting or thermoplastic resins (e.g.,for boat building) in organic sealants containing inorganic fillers and in casting moulds prepared from foundry sand and organic binders. More recently the benefit of silanes to the rubber industry has been established since sulphur derivatives of the 3-chloropropyltrialkoxysilanes have been found to be useful as coupling agents in silica-reinforced rubber articles (e.g.special tyres) or as essential ingredients of equilibrium cure systems for sulphur-curable elastomers. Other applications are to be found in the fields of dental materials coatings and adhesives and catalysis and even in the immobilization of enzymes on glass spheres for use in commercial enzyme reactors. In all cases organofunctionalized silanes guarantee a reliable and permanent union between two otherwise ‘incompatible’ material systems. Along with silicones organosilicates and silylating agents they have become the fourth important class of commercially-used organosilicon compounds and an interesting review of their synthesis and applications was most ~elcorne.~’ Several papers at the ‘Symposium on the Biogeochemistry of Silicon and Related Group IV Organometallics in Fresh Water Ecosystems’ at the 192nd ACS National Meeting at Anaheim in September 1986 noted that organosilicon compounds are not so inert in the environment as thought.Polydimethylsiloxane or silicone as it is widely called in commerce is by far the most widespread organosilicon compound in the environment and has been found to settle out of water into sediment layers and stay there. It has been found that inorganic mercury( 11) salts can be methylated in the presence of polydimethylsiloxane by thermal energy in aqueous solution and that there is the potential for producing methylmercury which could bioaccumulate in fish and other aquatic organisms and which is highly toxic to humans.Organosi- latranes and several other organosilicon compounds also transfer a variety of organo 69 R. Jones D. J. Williams Y. Kabe and S. Masamune Angew. Chem. Int. Ed. Engl. 1986 25 173. 70 U. Deschler P. Kleinschmit and P. Panster Angew. Chem. Inr. Ed. Engl 1986 25 236. Organometallic Chemistry -Part (ii) The Main-Group Elements 235 species to mercury. Fortunately the most reactive methylsiloxane with mercury salts hexamethyldisiloxane has extremely low water solubility and is highly volatile and consequently is not a component of the aquatic environment. Also temperatures of 60-80°C were required to yield significant methyl transfer to mercury which would not appear to be environmentally relevant.Even at these higher temperatures the polydimethylsiloxanes which comprise most of the material that can be found in the environment are markedly unreactive relative to hexamethyldisiloxane. The symposium concluded that the silicone trace-components of aquatic sediments are extremely unlikely to contribute significantly to environmental methylmercury levels but pointed out that these compounds which were previously considered to be inert in the environment do have the potential to react.71 Organotin chemistry continues to be lively both in the organic synthesis and structural areas. Recent results on the use of tin(rr) compounds in highly diastereo- and enantio-selective synthesis have been summari~ed,~~ and the use of transition- metal catalysts in organotin chemistry has been re~iewed.'~ Hexaorganoditins are finding increasing applications in reduction reactions as organic radical sources or as precursors of tin-metal derivatives and they also show interesting bactericidal and fungicidal activity.An easy high yield route to these &Sn2 compounds now exists in the reduction of bis(triorganotin)oxides (R3Sn)20 by Ti Mg K or Na 1,l-Distannyl-1-alkenes of the type RR'C=C(SnMe3)2 can now be readily prepared from Me,SnLi and geminal dibromoalkenes; hydro- stannation of the geminal distannylalkenes gives tristannylalkanes and bromodes- tannylation using N-bromosuccinimide occurs readily as does halogenodemethyla- tion at tin using dimethyltin dihalides.Palladium-catalysed coupling reactions between geminal distannylalkenes and organic halides are also possible.'' The cross-coupling of organotin reagents with organic electrophiles catalysed by pal- ladium provides a novel method for generating a carbon-carbon bond and this area has been reviewed.76 The organotin compounds may be prepared by several routes can bear a variety of functional groups and moreover are not very sensitive to air or moisture. This versatile reaction takes place under mild conditions and is tolerant of a wide variety of functional groups on either coupling partner and so is ideal for use in the synthesis of elaborate organic molecules. A simple example is reaction (7) in which a wide variety of R and R groups may be employed.When [L,PdOl R-COCl + Bu",Sn-CEC-R -R-CO-CEC-R' + Bu",SnCl (7) the coupling reaction is carried out in the presence of carbon monoxide instead of a direct coupling CO insertion takes place stitching the two coupling partners together and generating a ketone (equation 8). 71 192nd American Chemical Society National Meeting Anaheim California 7-12 September 1986 Abstracts of Papers GEOC 115-119 and 126138; see also Chem. Eng. News Vol. 64 No. 39 (29 Sept. 1986) p. 73. 72 T. Mukaiyama Pure Appl. Chem. 1986 58 505. 73 T. N. Mitchell J. Organomet. Chem. 1986 304,1-16 containing 68 refs. 74 B. Jousseaume E. Chanson and M. Pereyre Organometallics 1986 5 1271. 7s T. N. Mitchell and W.Reimann Organometallics 1986 5 1991. 76 J. K. Stille Angew. Chem. Int. Ed. Engl. 1986 25 508-524 containing 125 refs. 236 A. T. Hutton IPdol RX + R'SnR + CO -R-CO-R' + R",SnX (8) The highly stereoselective acid-catalysed cyclization of an epoxystannane is a key step in a simple synthesis of an aromatic 6,11,17-triketo~teroid.~' A competing 1,3-rearrangement of allyl stannanes has been demonstrated to occur under the normal thermal homolytic allyl transfer reaction conditions which limits the substitu- tion patterns in these processes and two methacrylyl stannanes have been described which allow the direct transfer of the methacrylyl moiety to alkyl halides under mild condition^.^^ For the nucleophilic alkyllithiums RLi the stannocene molecule provides two sites of attack the protons of the cyclopentadienyl ligands and the central tin atom.While unsubstituted stannocene undergoes metallation of cyclopentadienyl protons decamethylstannocene is attacked only at the central tin atom and the reaction products strongly suggest the formation of a [ (ql-Me5C5),Sn(R)Li] intermediate. In the presence of Me1 the intermediate is trapped yielding tin(1v) compounds; otherwise loss of Me5C5Li takes place to give substituted diorganotin species [ R2Sn] .79 The cyclopentadiene rings in decaphenylstannocene ( Slosymmetry) are exactly parallel thus violating the VSEPR model and the question has been raised as to where the lone pair resides seeing as it is not stereochemically active. Fenske- Hall MO calculations now predict that the tin lone pair in (q5-Ph5C5),Sn resides in the HOMO a Sn 5s-like orbital and is not (as was earlier suggested) delocalized onto the ring system of the ligands.80 The X-ray structure and solution behaviour of the organotin(1v) compound (26) derived from the reaction of 2-Me2NC6H4CH(SiMe3)Li with PhMeSnBr has been studied.The compound (26) contains two chiral centres which are formed stereo- specifically during the reaction and which have either the Rc,Rs or the SC,SSn @n='ph 'Me \ I Br '' SiMe combination of configurations at the benzylic C and five-coordinate Sn centres.81 Li[CH( PPh,),] reacts with SnCl or PbCl in THF to yield the homoleptic complexes [M{CH(PPh,),},] (M = Sn or Pb) X-ray crystallographic studies reveal similar structures with a monomeric pyramidal MCP core i.e.each complex contains two distinctly diflerent bis(dipheny1phosphino)methanide ligands one binding as a 77 D. N. Jones and M. R. Peel J. Chem. SOC.,Chem. Commun. 1986 216. 78 J. E. Baldwin R. M. Adlington D. J. Birch J. A. Crawford and J. B. Sweeney J. Chem. Soc. Chem. Commun. 1986 1339. 79 P. Jutzi and B. Hielscher Organometallics 1986 5 2511. 80 R. L. Williamson and M. B. Hall Organometallics 1986 5 2142. " J. T. B. H. Jastrzebski G. van Koten C. T. Knaap A. M. M. Schreurs J. coon and A. L. Spek Organometallics 1986 5 155 1. Organometallic Chemistry -Part (ii) The Main-Group Elements monodentate ligand through carbon (its P atoms remaining uncoordinated) the other binding as a bidentate chelating ligand through two P atoms.' The crystal structures of (Bu',Sn) and (Am',Sn) show that the four-membered tin ring of (Bu',Sn) is planar and the molecules are highly ordered in the crystal; the ring of (Amt2Sn) is puckered.In both compounds the tin-tin bonds are longer than in other cyclostannanes which is probably caused by the size requirements of the organyl group^.'^ Hexakis(trimethylgermyl)benzene c6(GeMe3) has been synthesized from the reaction of C6Br6 with Me3GeC1 and magnesium; the X-ray structure shows that the benzene ring is very slightly puckered and the six germanium atoms are located alternately above and below the average ring-plane. The mean bond-distance between aromatic carbons 1.418 A is significantly longer than that in benzene (1.39 A).84 Interactions of basic organic compounds especially unsaturated hydrocarbons with transition-metal centres form the starting point for various catalytic processes.Thus any comparable catalytic activity of a main-group element requires the ability to form .rr-complexes. Surprisingly crystallographic evidence has not previously been presented for an interaction between a main-group metal and the wsystem of an ordinary carbon-carbon double-bond and the compound ( 77,-Me4C5H)MezSi( q5-Me4C5)Ge+GeC13- (27) provides the first example of this type of bonding since its diene is dihapto-coordinated to the germanium atom.85 The parent permethylated silyl-bridged dicyclopentadienyl ligand Me,Si( Me4C5H)2 was obtained from reaction of Me4C5HLi with MezSiC1,; with C2H4(Me2SiC1)2 the 1,2-disilylethane-bridged dicyclopentadienyl ligand C2H4{ Me,Si( Me,C,H)} is formed.86 1' GeCl; 5 Groups V and VI Despite their potential importance as precursors to semiconductors such as gallium arsenide and indium phosphide relatively little is known about organometallic compounds featuring bonding between the heavier main-group I11 and V elements.A flurry of such compounds has now appeared. For example a product of the 82 A. L. Balch and D. E. Oram Organornefaflics 1986 5 2159. 83 H. Puff C. Bach W. Schuh and R. Zimmer J. Organomet. Chem. 1986 312 313. 84 W. Weissensteiner I. I. Schuster J. F. Blount and K. Mislow J. Am. Chem. Soc. 1986 108 6664. 85 F. X. Kohl R.Dickbreder P. Jutzi G. Muller and B. Huber J. Organomet. Chem. 1986 309,C43. 86 P. Jutzi and R. Dickbreder Chem. Ber. 1986 119 1750. 23 8 A. T. Hutton reaction of PhAsH with (Me,SiCH,),Ga is the novel organoarsenic-organogallium cluster [(PhAsH)( R,Ga)( P~As)~( RGa),] (R = Me3SiCH2) which contains a As7Ga core; the known compound (PhAs) is also isolated.87 Silylarsines R2AsSiMe3 (R = Me3SiCH2 or mesityl) have been used to synthesize (arsino)gallanes and e.g. the dimeric bis( arsino)gallane shown in equation 9 displays fluxional properties R=Me3SiCH 4R,AsSiMe3 + 2GaC1 [(R,As)~G~CI],+ 4Me3SiC1 (9) above room temperature due to rapid exchange of the endocyclic (bridging) and exocyclic (terminal) bis[ (trimethylsilyl)methyl]arsino groups.88 The first crystal structure of a dimeric arsenic-gallium compound that of [(Me3SiCH2),AsGaPh2], shows that the four-membered As2Ga2 ring is strained though dissociation does not occur in The reaction of GaC1 with three equivalents of Bu',AsLi affords the mononuclear perarsenido gallium compound Ga(AsBu',) as a red air-sensitive crystalline solid while the reaction of GaCl with one equivalent of Bu',AsLi and two equivalents of MeLi results in another four-membered As2Gaz ring-compound viz.[Bu',AsGaMe,] .90 The synthesis properties and reactivities of stable compounds featuring double bonding between main-group V elements e.g. diphosphenes (RP=PR) phos-phaarsenes (RP=AsR) phosphastibenes (RP=SbR) and diarsenes (RAs=AsR) have been reviewed.52 Diels-Alder adducts have been prepared in a one-pot pro- cedure by [2 + 41 cycloaddition of the perfluoro-2-arsapropene F3CAs=CF2 to 1,3-dienes; the arsapropene was produced in situ by thermal (70-80 "C) elimination of Me,SnF from Me3SnAs(CF3) .91 The exchange reactions of tetramethyl-diphos- phine -diarsine -distibine and -dibismuthine have been studied (equation 10); the Me,E + Me4Ef2 2Me,EE'Me (10) (E,E' = P As Sb Bi) tetramethyldipnictogens Me,E2 (E = P As Sb Bi) also undergo exchange reactions with dimethyldichalcogenides Me2A2 (A = S Se Te) to produce the corresponding (dimethy1pnicto)methylchalcogenides Me,EAMe.These latter compounds have been fully characterized by n.m.r. (including 12'Te) Raman and mass spec-tro~copy.~~ The first stable compound containing an arsenic-carbon triple bond is 2-(2,4,6-tri-t-butylpheny1)-1-arsaethyne (30) which is pale yellow and crystalline melting at 114-116 "C.It was prepared from the acid chloride (28) and the arsenide (29) R-COC1 + LiAs(SiMe,) --* [R-CO-As(SiMe,),] + R-CEAs (11) (28) (29) (30) (R = 2,4,6-But3C6H2) 87 R. L. Wells A. P. Purdy A. T. McPhail and C. G. Pitt J. Chem. SOC.,Chem. Commun. 1986 487. 88 C. G. Pitt A. P. Purdy K. T. Higa and R. L. Wells Organornetallics 1986 5 1266. 89 R. L. Wells A. P. Purdy A. T. McPhail and C. G. Pitt J. Organomef. Chem. 1986 308 281. 90 A. M. Arif B. L. Benac A. H. Cowley R. Geerts R. A. Jones K. B. Kidd J. M. Power and S. T. Schwab J. Chem. Soc. Chem. Commun. 1986 1543. 91 J. Grobe and D. Le Van J.Organomef. Chem. 1986 311 37. 92 A. J. Ashe 111 and E. G. Ludwig jun. J. Organomef. Chem. 1986 303 197; 1986 308 289. Organometallic Chemistry -Part (ii) The Main-Group Elements which is readily accessible from tris(trimethylsily1)arsane (equation 1 l).93One of the first lithium organoarsenides to be structurally characterized is [Li(thf)-{As( But)As( produced from the reaction of LiAs( with MgBr2 (2 :1) in THF and involving As-As bond formation and As-C bond cleavage. It is a dimer with two Li atoms bridging two -AS(BU')AS(BU')~ groups and with a planar Li2As2 The crystal structure of 2SbC13.C6H6 a prototype of the Menshutkin complexes shows that two independent SbC13 molecules with a benzene molecule between them form a molecular complex.The metal atoms are acentrically q2-bound on either side of the benzene ring plane with Sb...ring plane distances of 3.30 and 3.22 A similar to the Sb....rr interactions in other Menshutkin complexe~.~~ BiC13 BiC13 BiC13 (31) (32) The arene complexes of bismuth (3 1) and (32) display fascinating structure^.^^ They were prepared by simply dissolving BiC13 in mesitylene or hexamethylben- zene/ toluene respectively followed by crystallization. The layer structure of the 1:1 half-sandwich complex (31) is characterized by q6-coordination of the mesityl- ene to the metal atom; this is the first structurally characterized arene complex of bismuth. The novel 'inverted' sandwich structure of (32) also displays q6-coordina- tion but this time ring-shaped Bi4ClI2 structural units in the crystal are linked through the Bi(arene)Bi bridges to form a three-dimensional network.It is significant that no stereochemical activity of the lone pair in the sense of ligand-free polyhedral corners is recognizable either in (31) or (32). Here -and in the T6-bonding -the stabilization of the 6s' level (inert-pair effect) manifests itself. Chemical transformations involving tellurium were until recently very rare. However the explosive development of selenium chemistry in the last decade has called attention to the potential of tellurium reagents and a number of interesting transformations based on tellurium-containing species are now known; several of these were mentioned in last year's report.' It is therefore appropriate that the applications of tellurium reagents in organic synthesis and the transformations of organotellurium compounds exhibiting potential synthetic utility have been reviewed;97 this article also contains a useful section on the experimental procedures for preparation of the more important organotellurium reagents.93 G. Mark1 and H. Sejpka Angew. Chem. Inf. Ed. Engl. 1986 25 264. 94 A. M. Arif R. A. Jones and K. B. Kidd J. Chem. SOC.,Chem. Commun. 1986 1440. 95 D. Mootz and V. Handler Z. Anorg. Allg. Chem. 1986 533 23. 96 A. Schier J. M. Wallis G. Miiller and H. Schmidbaur Angew. Chem. Znf. Ed. Engl. 1986 25 757. 97 N. Petragnani and J. V. Comasseto Synthesis 1986 1-30 containing 119 refs.