Transition states in Ei reactions

Abstract

AbstractThe pyrolysis of amine oxides, sulfoxides, selenoxides, and esters to form alkenes is believed to be a concerted reaction with a cyclic transition state. Phosphine oxides, sulfones, and nitro compounds are unreactive. This study seeks to identify reasons for the lack of reactivity of the latter. Transition states were located for all substrates progressing from RHF/3‐21G* to the MP2/6‐31+G(d,p) level (in certain cases). For sulfones and nitro compounds, two possible reasons for lack of reactivity were considered: (1) Atoms approaching one another in the transition state may be considered to participate in a local nO → σ*CH interaction. The second oxygen in the sulfone or nitro compound lowers the energy of the ‘non‐bonded electrons’ at oxygen leading to a greater mismatch in energy with the antibonding CH orbital (in comparison to the sulfoxide or amine oxide). (2) The sulfone and nitro ‘lone pairs’ are not really ‘alone’, and available to react. In fact, complex bonding arrangements exist in the SO2 and NO2 groups. The situation is less clear for phosphine oxides, although ground state stability of bonds is important. Implications concerning the Hammond postulate are covered. Copyright © 2009 John Wiley & Sons, Ltd.