Allylzincation alkenylmetals

Preliminary studies demonstrated that, under the above conditions, the more substituted the alkenylmetal, the less reactive it was towards addition of allylzinc bromide. Disubstitution at the /J position significantly retarded the addition127. The reaction still occurred efficiently in THF at 35 °C if silyl or aryl groups were present at the a-position... 

For alkenylmetals bearing two stereocenters at both the allylic and homoallylic positions, two situations have to be distinguished depending on their relative influence (matched or mismatched) with respect to the stereochemical outcome of the allylzincation. Not surprisingly, in the matched case, as illustrated for substrate 232, the diastereoselectivity was excellent and 233 was obtained as a single diastereomer. The allyl moiety was delivered anti to both the allylic and homoallylic substituents in the chelated allyl alkenylzinc species (equation 113). 

Substrate-induced diastereoselective allylzincations essentially rely on coordination of the alkenylmetal by an appropriately located heteroatom, but a zinc-alkene -interaction23 27 can also exert a remarkable stereodirecting effect. Indeed, the alkenyllithium derived from 248, bearing an appropriately located carbon—carbon double bond, underwent highly diastereoselective allyl- and crotylzincations which led after hydrolysis to the corresponding 1,6-dienes 249 and 250. The stereochemical outcome... 

Thus, the weak interaction between the zinc and a double bond was remarkably efficient at promoting the differentiation of the two pro-stereogenic faces of the alkenylmetal during the allylzincation process. Nevertheless, recent results have demonstrated that n-interactions with arenes or alkenes are less powerful in magnitude than the coordination by an alkoxy group such as a MOM ether, when both can exert a role on the stereochemical outcome158. 

Allylzincation of alkenylmetals provides a useful entry to the diastereoselective synthesis of sp3 yem-dirnclallic species that can react with two different electrophiles in a one-pot protocol, leading to elaborated acyclic structures with control of the configuration of up to three adjacent stereocenters, as well as to cyclopropanes bearing various substitution patterns126,163. 

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