5.38 Discussion of the Case
The investigation, essentially made by ESR, begun from the spectrum of CoSalen
in the presence of E-methyl ferulate (1) (Fig. 5.27) in deaerated chloroform
solution.
The contact with oxygen in the presence of (1) modifies the resonances of
CoSalen and gives rise to the formation of the superoxo derivative of CoSalen and
of a radical species (Fig. 5.28).
It has been suggested that in the presence of O 2 the following processes take
place
Co
II
ðsalenÞ
Â
à þ ROH þ O 2 Co
III
ðsalenÞðROHÞ O 2
ð Þ
Â
à Co
III
ðsalenÞðROHÞ O 2
ð Þ
Â
à À þ ROH Co
III
ðsalenÞðROHÞ RO
Ã
ð
Þ
Â
à þ HO 2
À
Co
III
ðsalenÞðROHÞ RO
Ã
ð
Þ
Â
à þ HO 2
À
Co
III
ðsalenÞ RO
À
ð
Þ RO
Ã
ð
Þ
Â
à þ H 2 O 2
The formation of the phenoxy radical Co
III (Salen)(ROH)(RO
Á
) is the determining step of the whole oxidative process, and it is mediated by the superoxide anion.
The free coordination of (1) to the cobalt is needed to the process.
Fig. 5.27 a ESR spectrum of
CoSalen in CHCl 3 in the
presence of (1) under inert
atmosphere and b under
oxygen atmosphere [20]
120
5 The Symmetry Properties Describe the Electronic Structure …