MECHANISTIC CROSSOVER INDUCED BY STERIC HINDRANCE - A THEORETICAL-STUDY OF ELECTRON-TRANSFER AND SUBSTITUTION MECHANISMS OF CYANOFORMALDEHYDE ANION-RADICAL AND ALKYL-HALIDES

This paper describes a mechanistic crossover driven by steric hindranc e, from C-alkylation (SUB(C)) to dissociative electron transfer (ET), in the reactions between cyanoformaldehyde anion radical and alkyl chl orides of variable steric size (alkyl = Me, Et, i-Pr, t-Bu). The compu tations provide structural details on the transition state (TS) struct ures which undergo this mechanistic transformation, and thereby enable links to experimental investigations on the relationship between clas sical substitution mechanisms and their ET counterparts to be drawn. T he TS's of the interchanging mechanisms possess the C---C---Cl structu re, where the first C is the carbon atom of the formyl group. It is fo und that the TS's for the less hindered substrates (Me, Et), with R(CC ) = 2.35 and 2.45 Angstrom, collapse to C-alkylation product, hence a SUB(C) mechanism. As steric hindrance increases (i-Pr, t-Bu) and the C ---C distance increases to 2.57 Angstrom and then to 2.96 Angstrom, th e TS falls apart to dissociated ET products, hence an ET mechanism. Tl ;is is therefore an isostructural mechanistic transformation within a narrow range of change in the C---C distance. A third mechanism of O-a lkylation (SUB(O)) is also observed, but while its TS undergoes O---C loosening by the steric hindrance, no mechanistic transformation occur s. This dichotomy of the steric hindrance is analyzed with use of the valence bond configuration mixing (VBCM) method and shown to originate in the parity (odd vs even) of the number of electrons which particip ate in the bond reorganization. The VBCM method projects that ET and S UB(C) mechanisms are nascent from the VB mixing of the same set of con figurations, and as such the two mechanisms are ''entangled'' and thei r corresponding TS's involve hybrid characters. Near the changeover zo ne (e.g., where the TS for the i-PrCl substrate is located in Figure 6 ), the degree of entanglement is strong, and may lead to surface bifur cation. The origins of the experimentally observed residual stereosele ctivity of ET reactions are discussed in this respect and as a result of radical collapse. The ET-TS which emerges from the computations pos sesses significant and variable bonding which conforms to simple orbit al selection rules (refs 1, 10, and 11). The importance of probing the bonding is discussed along with potential strategies thereof.