Mechanistic crossover induced by steric hindrance: a theoretical study of electron transfer and substitution mechanisms of cyanoformaldehyde anion radical and alkyl halides

Abstract

This paper describes a mechanistic crossover driven by steric hindrance, from C-alkylation (SUB(C)) to dissociative electron transfer (ET), in the reactions between cyanoformaldehyde anion radical and alkyl chlorides of variable steric size (alkyl = Me, Et, i-Pr, t-Bu). The computations provide structural details on the transition state (TS) structures which undergo this mechanistic transformation, and thereby enable links to experimental investigations on the relationship between classical substitution mechanisms and their ET counterparts to be drawn. The TS's of the interchanging mechanisms possess the C- - -C- - -Cl structure, where the first C is the carbon atom of the formyl group. It is found that the TS's for the less hindered substrates (Me, Et), with R(CC) = 2.35 and 2.45 Å, 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 Å and then to 2.96 Å, the TS falls apart to dissociated ET products, hence an ET mechanism. This is therefore an isostructural mechanistic transformation within a narrow range of change in the C- - -C distance. A third mechanism of O-alkylation (SUB(O)) is also observed, but while its TS undergoes O- - -C loosening by the steric hindrance, no mechanistic transformation occurs. 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 participate in the bond reorganization. The VBCM method projects that ET and SUB(C) mechanisms are nascent from the VB mixing of the same set of configurations, and as such the two mechanisms are “entangled” and their corresponding TS's involve hybrid characters. Near the changeover zone (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 bifurcation. The origins of the experimentally observed residual stereoselectivity of ET reactions are discussed in this respect and as a result of radical collapse. The ET-TS which emerges from the computations possesses significant and variable bonding which conforms to simple orbital selection rules (refs 1, 10, and 11). The importance of probing the bonding is discussed along with potential strategies thereof.