Palladium-catalyzed synthesis of 2,1′-disubstituted tetrahydrofurans from γ-hydroxy internal alkenes.: Evidence for alkene insertion into a Pd-O bond and stereochemical scrambling via β-hydride elimination

Palladium-catalyzed reactions of gamma-hydroxy internal acyclic alkenes with aryl bromides afford 2,1'-disubstituted tetrahydrofurans in good yields with cliastereoselectivities of 3-5:1. The analogous transformations of substrates bearing internal cyclic alkenes afford fused bicyclic and spirocyclic tetrahydrofuran derivatives in good yields with excellent diastereoselectivities (>20:1). A series of deuterium labeling experiments indicate that the origin of the modest diastereoselectivity in reactions of acyclic internal alkene substrates likely derives from a series of reversible P-hydride elimination and sigma-bond rotation processes that occur following a rare intramolecular alkene syn-insertion into an intermediate Pd(Ar)(OR) complex. In addition, these studies shed light on the chemoselectivity of insertion, suggesting that the alkene inserts into the Pd-O bond in preference to the Pd-C bond.