Olefin carbometalation with (alkoxy)allylic lithium and zinc reagents. Four-centered vs six-centered mechanism of allylmetalation reaction

Addition of substituted (alkoxy)allyllithium and zinc reagents to cycloprop enone acetal takes place smoothly to give carbometalation products of well- defined regio- and stereochemistry. The pathways of product formation depen ds on the metal. The (alkoxy)allylzinc reagents add to the cyclopropene in such a manner that the alpha-carbon attached to the alkoxy group becomes bo und to the olefin. The regioselectivity of the (alkoxy)-allylzincation is i ndependent of the allyl substituents, the diastereoselectivity for the newl y formed carbon-carbon bond is excellent (> 97%), and the geometry of the o lefinic bond in the product was always exclusively cis (if applicable). On the other hand, the regioselectivity of the (alkoxy)-allyllithiation is dep endent on the substituent, while the diastereoselectivity remains constantl y high (>97%). Theoretical studies supported this conjecture by revealing t hat a(hydroxy)allyllithium species of pi-allylmetal nature can react with c yclopropene via two [2 + 2]-type four-centered transition states of similar energies leading alpha- and gamma-adducts, while the zinc species of sigma -allylmetal nature reacts via a single [2 + 4]-type six-centered transition state leading to an alpha-adduct.