Electron Transfer and Polar Reactions of Cyclizable Anion Radicals: Structural Consequences of Orbital Selection Rules and Chain-Length Constraints

摘要

To be or not to be bonded is a fundamental question for the transition state (TS) of an election transfer (ET) reaction of an electrophile-nucleophile combination that also has a classical polar (bond-forming) pathway available to it. Valence bond configuration mixing (VBCM) analysis predicts that when either the nucleophile or the electrophile is an ion radical, both the ET and polar pathways may have TS structures with strong bonding interactions governed by reaction-specific orbital selection rules. These orbital selection rules determine the resonance energies of the respective transition states and thereby exert stereo- and regiospecific requirements. According to the VBCM analysis, when the reactive fragments involve a car-bonyl anion radicalmoiety and a carbon-halide moiety (C-X), the orbital selection rule for the ET-TS dictates a structure that optimizes the overlap between the singly occupied pi* orbital of the anion radical moiety and the sigma* orbital of the C-X moiety. In contrast, the TS structure for the polar (substitution) mechanism requites the optimum overlap between the doubly occupied oxygen lone-pair orbital of the anion radical and the a* orbital of C-X. The reaction-specific orbital interactions provide structural rules for the dichotomy of ET and polar mechanisms in ion radical chemistry. The role of the orbital selection rules has been corroborated in a recent ab initio study of the interrnolecular ET and substitution (SUB) reactions in a simple model system: the formaldehyde anion radical and methyl chloride. It was found that the structures of both the ET and SUB transition states have strong bonding interactions and possess distinct regio- and orientational selectivity in accord with the selection rules.