Computational studies of ene reactions between aminoborane (F3C)2BN(CH3)2 and substituted propenes: additive effects on barriers and reaction energies

Abstract

Systematic computational studies of concerted pericyclic ene-type reactions between aminoborane (F₃C)₂BN(CH₃)₂, 1, and substituted propenes (R^1a)(R^1e)CC(R²)–C(R^3a)(R^3e)H (R = Me, CF₃, F; a = axial position in transition state, e = equatorial position in transition state) show that in all cases but one the reactions are exothermic. The reactions proceed through six-membered cyclic envelope-like transition states except in the case of 1 + C₃H(CF₃)₅. The data allow isolation of substitutional effects on barrier heights; the effects appear to be additive in all cases. Substitution at positions 1a, 1e, and 3a increases barriers, while substitution at positions 2 and 3e has variable impacts. The former observation is ascribed to steric crowding in the transition states, and is particularly prevalent for substitution at positions 1a and 1e. Substitution at position 2 lowers barriers for R = Me, F, possibly due to electronic demands, while raising them for R = CF₃ because of 1,3-diaxial repulsions between boron- and carbon-bound CF₃ groups. Substitution at position 3e has little impact on the barriers for R = Me, CF₃, but significantly raises them for R = F. This seems to arise from charge effects on the position of the transition states.