Palladium-catalyzed isomerization of aryl-substituted epoxides: A selective synthesis of substituted benzylic aldehydes and ketones

Aryl-substituted epoxides bearing multiple methyl substituents on the epoxide ring isomerize in the presence of 5 mol % Pd(OAc)(2)PR3 (R = n-Bu, Ph) to form the corresponding benzylic aldehyde or ketone, with complete regioselectivity for the carbonyl compound formed via cleavage of the benzylic C-O bond. No allylic alcohols or products arising from alkyl migration are observed. Rapid reaction rates and nearly quantitative yields are obtained, even with highly sterically hindered epoxides, using tri-n-butylphosphine as ligand and tert-butyl alcohol as solvent. 2-Aryl-substituted epoxides with two methyl substituents on C3 are completely unreactive, consistent with an oxidative addition/beta-hydride elimination mechanism. Catalyst variation studies show that both Pd(OAc)(2) and PR3 are essential for optimal activity and that palladium catalysts formed in this manner are superior to other Pd(O) catalysts (e.g., Pd(PPh3)(4)). The reactivity of catalytic Pd(OAc)(2)/PR3 toward multiply-substituted epoxides is compared to traditional Lewis acid catalysts; the former is found to be much more selective for isomerization without skeletal rearrangement. A mechanistic rationale involving turnover-limiting S(N)2-like attack of Pd(O) at the benzylic carbon is proposed.