Catalytic oxidations using cobalt(II) complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 301

7. OXIDATIONS CATALYZED BY COBALT(II) AMINE
COMPLEXES

7.1 Catalytic oxidation of substituted anilines

-phenylenediamine (OPD) undergoes
facile and selective catalytic dehydrogenation by dioxygen 137 , 138 . The
organic products are 2,3-diaminophenazine in methanol, and 2 H-
benzimidazoles in acetone and other aldehyde or ketone type solvents
(Figure 42).

In the presence of cobalt(II) salts, o

Kinetic studies have shown that the catalytic activity is due to the
species, which is reactive toward dioxygen 139 .
Under oxidative conditions, the square-planar o-benzosemiquinone-
diimine (s-BQDI) complex and the square-pyramidal
are formed; they have been characterized by X-ray
diffraction. 140 The complexes

(X = As, Sb, P) have

been isolated from the system and analyzed by X/ray

diffraction 141 .
Triphenylphosphine added to the system is oxidized to

indicating that there is an active superoxo or intermediate in the

142
reacting mixture .
Solutions of 2-aminothiophenol (HAT) in methanol or THF, containing
cobalt(II) perchlorate rapidly absorb dioxygen at room temperature and
atmospheric pressure 143 , producing 2,2'-diaminodiphenyl disulfide (DADS)
in 90% yield (Figure 43).
The rate of the catalytic reaction as a function of the 2-aminothio-
phenol concentration shows a maximum at a cobalt(II) to HAT ratio of 1:2,
indicating that a complex of the composition is the major
catalytic species. The proposed reaction mechanism for HAT oxidation is
shown in Figure 44.