A Chromium(III)–Superoxo Complex in Oxygen Atom Transfer Reactions as a Chemical Model of Cysteine Dioxygenase

Metal–superoxo species are believed to play key
roles in
oxygenation reactions by metalloenzymes. One example is cysteine dioxygenase
(CDO) that catalyzes the oxidation of cysteine with O2,
and an iron­(III)–superoxo species is proposed as an intermediate
that effects the sulfoxidation reaction. We now report the first biomimetic
example showing that a chromium­(III)–superoxo complex bearing
a macrocyclic TMC ligand, [CrIII(O2)­(TMC)­(Cl)]+, is an active oxidant in oxygen atom transfer (OAT) reactions,
such as the oxidation of phosphine and sulfides. The electrophilic
character of the Cr­(III)–superoxo complex is demonstrated unambiguously
in the sulfoxidation of para-substituted thioanisoles.
A Cr­(IV)–oxo complex, [CrIV(O)­(TMC)­(Cl)]+, formed in the OAT reactions by the chromium­(III)–superoxo
complex, is characterized by X-ray crystallography and various spectroscopic
methods. The present results support the proposed oxidant and mechanism
in CDO, such as an iron­(III)–superoxo species is an active
oxidant that attacks the sulfur atom of the cysteine ligand by the
terminal oxygen atom of the superoxo group, followed by the formation
of a sulfoxide and an iron­(IV)–oxo species via an O–O
bond cleavage.