Potential energy surface for the acid- and BF3-catalyzed rearrangement of methylpropene oxide

The acid- and BF3-catalyzed rearrangement of methylpropene oxide to methylpropanal has been investigated by density functional methods. Isotope effects are calculated and are consistent with the reaction occurring via a carbocation intermediate followed by a 1,2-hydride shift. Inverse secondary isotope effects for hydride migration reflect a combination of changes in C1-H(D) stretching and out-of-plane bending frequencies with a stronger force constant of the nonmigrating C1-H bond at the transition structure than in the carbocation. Gas-phase DFT(B3LYP)/6-31G* level calculations favor a geometry of the carbocation with the boron of BF3 below the plane of the carbon framework while B3LYP/B-31G*(SCI-PCM) single point solvation calculations favor a geometry with the boron in the plane. From this latter conformation hydride migration to either face of the carbocation is equally favored. The transition structure for hydride (deuteride) migration is early. A calculated correction applied to the experimentally observed result (M-H/M-D = 1.92) gives a primary isotope effect for the reaction (k(H)/k(D) = 1.56) close to the theoretically calculated value (k(H)/k(D) = 1.68).