DETERMINATION OF REACTIVITY BY MO THEORY .62. THEORETICAL-STUDIES ON THE GAS-PHASE PYROLYSIS OF ESTERS

The gas-phase thermal decomposition reactions of alkanoic esters, Y=C(R)-X-CH2CH3 where X = Y = O or S and R = H, CH3, or NH2, have been studied theoretically by the semiempirical MO methods of MNDO and AM1. The experimental reactivity order is reproduced correctly by AM1 but not with MNDO. The decomposition proceeds through a six-membered cyclic transition state (TS) conforming to a concerted process of the retro-ene reaction. The reactivity can be conveniently interpreted with the frontier orbital theory of three-species interactions; the TS is comprised of three fragments, a donor and two acceptors, whose frontier orbitals overlap all in-phase with one another. The two most important factors controlling the reactivity are the jr-donating ability of the Tr-HOMO of the donor (Y=C) and the accepting ability of the σ*-LUMO of the acceptor, (Cβ-H), i.e., the ease of nucleophilic attack of Y upon β-hydrogen. Any substituent increasing any of these abilities enhances the reactivity, one example being an increasing reactivity found with an increasing π-donating power of the substituent R, H < CH3 < NH2. The cleavage of the Cα-X bond is found to be of little importance in the rate-determining step. © 1990 American Chemical Society.