Mechanistic Insights into Electrocatalytic Carbon−Bromine Bond Cleavage in Polybrominated Phenols

Carbon−halogen bond cleavage has been studied extensively for many years as a simple electrosynthesis step in the formation of more complex natural products. Reduction of halogenated phenols has received less attention, in part, due to the lowered faradaic efficiency resulting from the competing hydrogen evolution reaction. Herein, we report the electroreduction of a series of brominated phenols through a homogeneous electrocatalytic (EC′) mechanism. Beginning with the structurally simple 2-bromophenol, we use foot-of-the-wave analysis to determine optimal catalysts. Nickel(II) salen requires the lowest overpotential for C−Br reduction and was used across all substrates. Chronoamperometric studies and density functional theory calculations were carried out to contribute to our understanding of the reduction mechanism. Next, the more complex 2,6-dibromophenol and tetrabromobisphenol-A are studied by means of cyclic voltammetry, chronoamperometry, and density functional theory. Through analysis of molecular orbitals diagrams, the more complex brominated phenols are found to undergo sequential carbon−bromine bond reduction, wherein the electrogenerated radical species accepts a second electron to form a carbanion before second carbon−bromine bond cleavage occurs.