C–O Bond Cleavage of Alcohols via Visible Light Activation of Cobalt Alkoxycarbonyls

A strategy for the activation of C–O bonds in
alcohols via
a carbonylation–homolysis–decarboxylation process is
described. Using readily available cobalt­(II) porphyrin precursors,
carbonylation of simple alcohols provides access to alkoxycarbonyl
cobalt­(III) complexes. Spectroscopic, crystallographic, and computational
methods are used to provide structural details and an estimate for
the Co–C bond dissociation energy of an alkoxycarbonylcobalt­(III)
complex of 39.8 kcal/mol for the first time. Visible light irradiation
in the presence of the radical trapping agent TEMPO and a thiol reducing
agent demonstrates the cleavage of the alcohol C–O bond under
oxidative and reductive conditions, respectively. Addition of a stoichiometric
reducing agent allows the use of a catalytic amount of hindered thiol
for the reduction of a benzylic alcohol to the corresponding hydrocarbon.