Selective C–H activation via hydrogen atom transfer (HAT)
has gained significant interest recently. One industrial application for
HAT can be found in the tertiary benzylic C–H bond of cumene for producing phenol, a versatile chemical feedstock for various industries.
However, the overall phenol yield remains low using the existing Hock
process due to the poor selectivity towards the key reaction intermediate, cumene hydroperoxide (CHP). Here, we demonstrate an efficient
strategy for phenol production through photooxidation of cumene to
CHP using iron bromide under blue light irradiation, where bromine
radicals (generated in situ via ligand-to-metal charge transfer) are used
as hydrogen atom abstractors. Mechanistic studies revealed the cumene-to-CHP conversion occurred via a tertiary C–H bond abstraction
by a radical mechanism, and the acetic acid and water additives increased CHP selectivity through stabilizing peroxides. The cumene-phenol process achieved up to 88% yield of CHP in 1 hour and could be used
with a wide range of substrates. We thus developed a selective, mild,
and efficient phenol production method using iron bromide under photooxidative conditions.