Zirconocene hydride alkenes

Usefid reviews address alkyl zirconocene catalysts for the pol)mierisation of silanes to poly silanes by a a-bond metathesis mechanism, chiral titanates as promoters in aldol reactions, and MeTiCl3 as a reagent for chelate-controlled carbonyl addition reactions. 5 xhe reactions of terminally functionalized alkenes with zirconocene hydrides are reviewed. Thermochemical studies show that while the bond dissociation enthalpies of Zr—C6H13 and Zr— CgHjj in zirconocene systems are comparable, the insertion of cyclohexene into the Zr—bond is more exothermic than the insertion of hexene. 

As amply demonstrated by the success of the metathesis reaction [ 127], alkenes and alkynes are synthetically extraordinarily versatile functional groups for transition metal-mediated transformations. Hydrozirconation with readily available zirconocene hydrides allows the direct reductive conversion of... 

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. 

Acylzirconocene chloride derivatives are easily accessible in a one-pot procedure through the hydrozirconation of alkene or alkyne derivatives with zirconocene chloride hydride (Schwartz reagent) [Cp2Zr(H)Cl, Cp = cyclopentadienyl] and subsequent insertion of carbon monoxide (CO) into the alkyl— or alkenyl—zirconium bond under atmospheric pressure (Scheme 5.1) [2],... 

The addition of Cp2Zr(H)Cl, known as the Schwartz reagent [30], to different alkenes and alkynes is known to be a facile process [31]. Therefore, the hydrozirconation of a variety of readily available enynes 12 is among the first methods developed for the stereoselective preparation of dienyl zirconium reagents 13. This process is both completely chemo- and regioselective with a syn addition of the zirconium hydride across the alkyne [32] (Scheme 5). From the same intermediate, the Zr atom can be isomerized in its internal position such as in 15 via a zirconacyclopropene intermediate 14. Moreover, the addition of trimethylstannyl chloride to 14 led to the stannylated dienyl zirconocene 16 [33] (Scheme 5). 

Zirconocene alkene complexes 29 are readily prepared by the p-hydride activation route from dialkyl zirconocenes 28 or methyl alkyl zirconocenes 27 (Scheme 5.10). It is also possible to prepare the zirconocene complexes of ethene, styrene, and, to a lesser extent, vinyltrimethylsilane by displacement of the weakly bound 1-butene ligand from 4. ... 

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