Allylic zirconocene species

Carbozirconation. The presence of MAO is essential for the reaction of alkynes with the allylic zirconocene chlorides derived fixim allenes. The Lewis acid presumably enhances the carbozirconation of alkynes by promoting formation of cationic Zr species in the same manner as during the polymerization of a-olefins. 

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. 

The presence of a discrete organometallic species as well as the stereochemistry of the metalated diene were first checked by iodinolysis and bromolysis,but the corresponding iodo- and bromodienes were found to be unstable and rapidly isomerized to a mixture of E and Z isomers. Thus, in order to have a better picture of the stereochemical outcome of the process, the crude reaction mixture was treated with AT-chlorosuccinimide (Scheme 37), and the corresponding chlorodiene 103 was isolated in 60% yield with an isomeric ratio >98 2. As alternative solution, the stereochemistry of the reaction was also determined by addition of allyl chloride, in the presence of a catalytic amount of copper salt, to the dienyl zirconocene derivative [74]. Skipped triene 104 was... 

The second mechanism requires the formation of a zirconocene allyl complex, which is in turn formed from the Cp 2ZrH(CH2=CMeP), the product of /3-H transfer after a primary insertion (see section V.C). This mechanism is more general, since it can work with any polymerization catalysts. Here, hydrogen activates the catalyst by converting the Mt (allyl) species, which have been shown to be effectively deactivated with respect to propene insertion, into Mt—H, that is, by shifting to the left the equilibria... 

Allyl Coupling Reactions. Zirconocenes assist in the attack of allylms e-siiun species on allyl alkyl ethers (eq. 44) and other activated allylic compoimds (264). 

Organozirconocene Chlorides. Acyl zirconocene chlorides function as efficient donors of acyl anions and can react with allylic or propargylic halides (X=Cl, Br, I, OTs) under Cul catalysis. In these reactions, acyl copper species (RCOCu), generated by the transfer of acyl group fromZr to Cu, were speculated to be the reactive species. These Cul-catalyzed reactions of acyl zirconocene chloride are cort5)lementary to the palladium-catalyzed reactions and enable the formation of -y-unsaturated ketones, which suffer ready isomerization under the palladium-catalyzed conditions (eq 23). On the other hand, carbonylative cross-coupling of F-o -selanylvinylzirconiums with alkynyUodonium tosylates can be performed in THF in the presence of 3 mol% of Cul and atmospheric pressure of carbon monoxide in a palladium-free system. 

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