Olefin Carbometalation with (Alkoxy)allylic Lithium and Zinc Reagents. Four-Centered vs Six-Centered Mechanism of Allylmetalation Reaction

Abstract

Addition of substituted (alkoxy)allyllithium and zinc reagents to cyclopropenone acetal takes place smoothly to give carbometalation products of well-defined regio- and stereochemistry. The pathways of product formation depends on the metal. The (alkoxy)allylzinc reagents add to the cyclopropene in such a manner that the α-carbon attached to the alkoxy group becomes bound to the olefin. The regioselectivity of the (alkoxy)allylzincation is independent of the allyl substituents, the diastereoselectivity for the newly formed carbon−carbon bond is excellent (>97%), and the geometry of the olefinic bond in the product was always exclusively cis (if applicable). On the other hand, the regioselectivity of the (alkoxy)allyllithiation is dependent on the substituent, while the diastereoselectivity remains constantly high (>97%). Theoretical studies supported this conjecture by revealing that a (hydroxy)allyllithium species of π-allylmetal nature can react with cyclopropene via two [2 + 2]-type four-centered trans...