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

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 (kHkD) 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 SN2 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.