High Level ab Initio Study of Thermal 1,3-Sigmatropic Shift in CH2CHCH2X with X = BH2, NH2, and CH3

Abstract

The thermal 1,3-sigmatropic migrations in X-CH2−CHCH2 with X = BH2, NH2, and CH3 have been investigated at the RHF, MP2, B3LYP, G3, and CBS-APNO levels using basis sets with added diffuse and polarization functions. In all three cases, the suprafacial allowed path proceeds through a TS in which the CC π electron is delocalized into the 2p AO in the migrating X. For X = BH2 with a vacant 2p orbital, the activation enthalpy is less than 1.0 kcal mol-1 (CBS-APNO) so that the 1,3-shift is almost barrierless. For X = NH2 there are two suprafacial pathways, one of which (higher energy path a) proceeds by participation of the lone pair on N. The lower energy pathway has a barrier height (ΔH⧧ = 68.9 kcal mol-1) of only 1.0 kcal mol-1 below the bond dissociation energy of the C1−N bond, so that a diradical pathway can compete with the concerted sigmatropic shift. For X = CH3, the activation barrier (ΔH⧧ = 76.1 kcal mol-1) is higher (by ca. 1 kcal mol-1 at the CBS-APNO level) than the bond dissociation energy of the C1−X(CH3) bond, so that the diradical reaction path is favored. The antrafacial 1,3-shifts are forbidden in all cases as evidenced by the second-order saddle-points (with two imaginary frequencies) and much higher barrier heights than those for the corresponding suprafacial shifts.