Zirconocene hydride species

Recently, Oshima et al. developed the conversion of acid chlorides into the corresponding homoallylic alcohols catalyzed by in r(/ -prepared hydridozirconium allyl reagents (Scheme 41),147 147a The proposed mechanism suggests an initial hydride transfer from the zirconocene crotyl hydride species, in equlibrium with its Cp2Zr(l-alkene),147a to the acid chloride with subsequent allylation to afford the corresponding homoallylic alcohols. 

Heterobimetallic zirconocene hydride complexes of the type [M][(CsH4R)2ZrH2] were first characterized by ESR in 1991 (M = Li, Na, K R = Me, Bu, SiMe3).2,43,44 Aluminum-zirconium hydride complexes are involved in a number of catalytic and stoichiometric processes.4 The presence of Zr(m) species in reaction mixtures has been inferred from the color45 and, in the case of CpZr(//-I I)2AI(Mc)(2,4,6-tllu ((, I I2), detected by ESR.46... 

In 1978, Schwartz and Gell found that CO would induce reductive elimination of alkane in various zirconocene alkyl hydride complexes with concurrent formation of Cp2Zr(CO)2 (2) (52,53). It was postulated that CO initially coordinates to the 6-e complex 23 forming the coordina-tively saturated species 24 which can then reductively eliminate alkane and/or rearrange to a zirconocene acyl hydride intermediate. When R = cyclohexylmethyl, methylcyclohexane reductively eliminated and Cp2Zr(CO)2 was isolated in 25% yield. 

Analogically to the titanocene(III) hydride catalyzed reactions, the zir-conocene 1 can be used as well. However, in this case the active species is assumed to be a cationic zirconium(IV) hydride generated by the reaction of the zirconocene 1 with MAO. The reaction mechanism copies the one described in Scheme 40. However, the course of the reaction favors antz-Markovnikov addition of the zirconium hydride to the double bond to give solely the methyl-enecycloalkanes 93 [50,51]. 

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