Catalytic oxidations using cobalt(II) complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 313

and involve species as a key intermediate. Turnover numbers
in excess of 100 and ca. 10% conversion of CyH to CyOH and CyO are
achieved in 4 h in most catalytic oxidations.

12. OXIDATIONS WITH Co-CYCLIDENE COMPLEXES

usch and coworkers 173-175 have synthesized a series of remarkable
vaulted and lacunar cobalt(II) cyclidene complexes, which reversibly bind
dioxygen and undergo autoxidation to cobalt(III) species. In some cases they
exhibit catalytic properties in the oxidation of phenol derivatives.
The totally synthetic superstructured cobalt(II) cyclidene complexes
(Figure 61) CoA (vaulted), CoB (unbridged) and CoC (lacunar), function as
both oxidase and oxygenase models in oxygenation of substituted phenols 176 .
The vaulted complex CoA catalyzes the oxidation
of 2,6-di- tert -butylphenol in acetonitrile solution. Typically, at 25°C and 1
atm a conversion of 37% is obtained in 24 hours, the products being 5%
2,6-di- tert -butyl-

B

-tert -
butyl-4,4'-diphenoquinone (TTBDQ ). Irreversible loss of the catalyst occurs
due to autoxidation. The product distribution is determined by the competing
reactions shown in Figure 62.
Previous studies on the catalytic oxidation of phenols led to the
conclusion that the phenoxyl free radical required for interpretation of the
observed product pattern and kinetic behavior is formed via

1 ,4-benzoquinone (DTBQ) and 22% 3,5,3',5'- tetra

H-atom

abstraction by the this work an

alternative mechanism is proposed, involving electron transfer between the
same pair of reactants, followed by the loss of a proton, as described in
Figure 63. Electron transfer is pictured to occur through the delocalized
system of the phenol, which is supported by the results of molecular
mechanics studies 173 .

omnipresent superoxocobalt species .In