On the mechanism of the palladium(0)-catalyzed isomerization of epoxides to carbonyl compounds

Article abstract of DOI:10.1016/S0040-4020(97)10363-5

The isomerization of trans-3-methyl-2-(2-naphthyl)oxirane to 2-naphthylpropanone by Pd(OAc)₂/PBu₃ in C₆H₆ is first order in both epoxide and in palladium catalyst, with activation parameters of ΔH = 12.5 kcal/mol and ΔS = -35.7 e.u. A comparison of the isomerization rates of trans-3-methyl-2-(2-naphthyl)oxirane and trans-3-deuterio-3-methyl-2-(2-naphthyl)oxirane reveals a kinetic isotope effect (k(H)/k(D)) of 1.01 (± 0.09). The reaction rates in benzene and acetonitrile are very similar; however, changing from benzene to a hydrogen bond-donating solvent (t-BuOH) increases the reaction rate substantially. These observations are consistent with a mechanism involving: (1) turnover-limiting S(N)2 attack of Pd(0) at the benzylic position of the epoxide; (b) rapid β-hydride elimination to form a Pd(II) hydrido-enolate complex; and (c) fast reductive elimination to afford the ketone, with concomitant regeneration of the Pd(0) catalyst.