7766
13
have compared our results with the activity of mushroom tyrosinase (Table 1) and two
observations emerge: (i) for 2,6-di-tert-butylphenol the dioxygenated product of complex 3
provides a better isolated yield of the diphenoquinone than that obtained with tyrosinase. For
2
,6-dimethylphenol the yields obtained with tyrosinase and our system are comparable. (ii)
Compared to tyrosinase the turnover numbers obtained with the present system are not
impressive. However, it is beyond doubt that the product obtained from our binuclear complex
3
and dioxygen is a catalyst in promoting the oxidation of hindered phenols and the yields of
the diphenoquinones are good. It is worth noting here that well-characterized copper(II)-peroxo
complexes also show similar reactivity behavior.
In summary, we have demonstrated that under suitable reaction conditions, CH Cl solutions
14,15
2
2
of a dicopper(I) complex catalytically convert 2,4-di-tert-butylphenol to a CꢀC coupled bisphen-
ol product and 2,6-dimethylphenol or 2,6-di-tert-butylphenol to diphenoquinones. This is the
first time that a m-xylyl-based open-chain ligand system, providing only two nitrogens coordi-
nated to each copper center, that exhibits aromatic ring hydroxylation (CꢀH to CꢀOH
transformation) has been modified by replacement of the CꢀH bond activating site by a CꢀF
bond, exhibits exogenous substrate reactivity. In essence, replacement of the m-xylyl CꢀH bond
by CꢀF inhibits the endogenous ligand hydroxylation and this is a necessary condition to
perform the present catalytic oxidations.** In this work hindered phenols have been used as the
substrates and a comparison has been made regarding the efficiency of this model system to that
of a naturally occurring tyrosinase. Structural characterization of the irreversibly oxidized
dihydroxy-bridged copper(II) compound 6 is currently underway and will be published else-
where.
Acknowledgements
We thank the Council of Scientific & Industrial Research and Department of Science &
Technology for financial support of this research. R.G. gratefully acknowledges the award of a
fellowship (SRF) by CSIR.
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**
10
It should be noted here that although Karlin et al. synthesized a closely similar ligand with tridentate arms
containing bis[(2-pyridyl)ethyl]amine units, the use of such a system for externally added substrate oxidation studies
has so far not been reported.