Catalytic oxidations using cobalt(II) complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 271

These 5-coordinate cobalt complexes form mononuclear superoxo and
bridged dinuclear complexes, which are reactive toward phenolic
substrates, thereby initiating catalytic oxidation cycles 23 :

Typical reaction conditions are shown in Figure 5 for syringyl alcohol as
substrate. In the presence of the catalysts in Figure 4,

2,6-dimethoxy-

benzoquinone is formed in 71 and 88% yield, respectively.

The p -substituted phenolic substrates and the yields of their oxidation

products are listed in Figure 6.
The proposed mechanism of oxidation is shown in Figure 7 25,26 .
The superoxo complex abstracts the phenolic hydrogen,
affording a phenoxy radical, which is trapped by a second complex
or dioxygen, giving intermediate ( I ) . When phenol oxidation is carried out
stoichiometrically with type systems, structural analogs of I
have been isolated and characterized 27 . Elimination of formaldehyde from I
produces quinone and, when a catalytically active Co-hydroxy
species 28 . The complex formed in the H-abstraction step breaks
down to regenerate the starting catalyst 26 . The catalytic effect depend on the
ease of removal of the phenolic hydrogen, the rate decreasing in the
following order of p -substituents: MeO (123), Me (28) CN (1).
The propenoidic phenols, E -methyl ferulate ( 1 ), E -4-hydroxycinnamic
acid methyl ester ( 4 ) and E -3-chloro-4-hydroxycinnamic acid methyl ester
( 7 ), can be catalytically oxidized with dioxygen in the presence of
[Co(salen)] (Figure 8) 29 . The yields depend on the solvent and the phenyl
substituents. EPR and electronic spectra suggest the involvement of a
coordinated

-benzosemiquinone type radical as the active intermediate.
Oxidative processes are involved in both the polymerization of
phenylpropendioic phenols to lignin and the degradation of lignin in the
environment 30,31 . Model systems for the oxidative degradation of phenolic
phenylpropenoids provide mechanistic information about the biological
cycle of lignin 32 . Co(salen) was found to catalyze the oxidation of the lignin

o