Allylmetal-directed addition of 1O2: Scope, mechanism and synthetic utility

Abstract

Allylic silyl and stannyl groups strongly influence the regio- and stereochemistry of alkene oxygenations by ¹O₂, even within functionalized systems. Allylstannanes undergo anti-S_E2′ oxygenation to form both Z-stannylalkenyl hydroperoxides and 4-stannyl-1,2-dioxolanes; the ene-like reaction is generally preferred unless limited by allylic strain. The alkenylstannane products, as well as the derived iodides, are effective substrates for palladium-mediated cross-couplings, additions, carbonylations, and acylations to form peroxydienes, peroxyenones, and peroxyenoates. Allylsilanes are less effective directing groups, possessing reactivity surprisingly similar to simple alkenes, and undergoing oxidation to form regioisomeric mixtures of hydroperoxides. The different reactivities and product distributions observed for allylstannanes and allylsilanes reflect different nucleophilicities of the ground state alkenes as well as variable polarization of the developing perepoxides by the neighboring C–Sn or C–Si bond. The observed selectivity for production of Z-alkenylmetals appears to result from the preferential formation of a single perepoxide pyramidal isomer and the tendency for this perepoxide to abstract the inside hydrogen on the metal-bearing carbon at a rate which is faster than either perepoxide inversion or single bond rotation to deliver the outside hydrogen for abstraction.