Interplay between Structure and Mechanism in Reductive Dissociative Electron Transfers to α,β ‐Epoxyketones

The electrochemical reduction of several α,β ‐epoxyketones was studied using cyclic (linear sweep) voltammetry, convolution voltammetry, and homogeneous redox catalysis. The results were reconciled to pertinent theories of electron transfer. α,β ‐Epoxyketones undergo dissociative electron‐transfer reactions with C−O bond cleavage, via both stepwise and concerted mechanisms, depending on their structure. For aliphatic ketones, the preferred mechanism of reduction is consistent with the “sticky” concerted model for dissociative electron transfer. Bond cleavage occurs simultaneously with electron transfer, and there is a residual, electrostatic interaction in the ring‐opened (distonic) radical anion. In contrast, for aromatic ketones, because the ring‐closed radical anions are resonance‐stabilized and exist at energy minima, a stepwise mechanism operates (electron transfer and bond cleavage occur in discrete steps). The rate constants for ring opening are on the order of 108 s−1, and not significantly affected by substituents on the 3‐membered ring (consistent with C−O bond cleavage). These results and conclusions were fully supported and augmented by molecular orbital calculations. Concerted or stepwise? α,β‐Epoxyketones undergo dissociative electron transfer; the mechanism depends on their structure. The preferred mechanism is concerted for aliphatic ketones (electron transfer and bond cleavage occur simultaneously). Aromatic ketones follow a stepwise mechanism because the ring closed forms are resonance‐stabilized.