A High Level Ab Lnitio Study of Thermal 1,3-Sigmatropic Shift in CH2-CHCH2X with X=BH2,,NH2 and CH3

초록

The thermal 1,3-sigmatropic migrations in X-CH2-CH=CH2 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 priceeds through the TS in which the C=C electron is delocalized into the 2p AO in the migrating X. for X=BH2 with a vacant 2p orbital, the activation enthalpy is les 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 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