Palladium-Catalyzed Intermolecular Heck-Type Reaction of Epoxides

The
palladium-catalyzed intermolecular Heck-type reaction of both
cyclic and acyclic epoxides is reported with tolerance of typical
polar groups and acidic protons. Suitable alkenes include styrenes,
conjugate dienes, and some electron-deficient olefins. In reactions
of aliphatic terminal epoxides, ring opening occurs selectively at
terminal positions, and stereocenters of epoxides are fully retained.
Mechanistic studies provide evidence for in situ conversion of epoxides
to β-halohydrins, generation of alkyl radicals,
and radical addition to alkenes as key steps. Cyclovoltammetric determination
of reduction potentials suggests that during activation of alkyl iodides
by palladium(0) complexes, inner-sphere halogen abstraction is more
likely than outer-sphere single electron transfer.