Spectroscopic Evidence for a Cobalt-Bound Peroxyhemiacetal Intermediate

Aldehyde deformylation
reactions by metal dioxygen adducts have
been proposed to involve peroxyhemiacetal species as key intermediates.
However, direct evidence of such intermediates has not been obtained
to date. We report the spectroscopic characterization of a mononuclear
cobalt­(III)-peroxyhemiacetal complex, [Co­(Me3-TPADP)­(O2CH­(O)­CH­(CH3)­C6H5)]+ (2), in the reaction of a cobalt­(III)-peroxo complex
(1) with 2-phenylpropionaldehyde (2-PPA). The formation
of 2 is also investigated by isotope labeling experiments
and kinetic studies. The conclusion that the peroxyhemiacetalcobalt­(III)
intermediate is responsible for the aldehyde deformylation is supported
by the product analyses. Furthermore, isotopic labeling suggests that
the reactivity of the cobalt­(III)-peroxo complex depends on the second
reactant. The aldehyde inserts between the oxygen atoms of 1, whereas the reaction with acyl chlorides proceeds by a nucleophilic
attack. The observation of the peroxyhemiacetal intermediate provides
significant insight into the initial step of aldehyde deformylation
by metalloenzymes.