posted on 2023-08-24, 21:29authored byEric C.
R. McKenzie, Seyyedamirhossein Hosseini, Mayank Tanwar, Matthew Neurock, Shelley D. Minteer, Stephen C. Jacobson
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.