ABSTRACT
Although there are limited data pertinent to the details of arene hydroxylation by tyrosinase, substantial mechanistic advances have been made in alkane and arene monooxygenation reactions by synthetic model dicopper(II)/peroxo compounds [10,197]. From this work, it is probable that phenol monooxygenation in tyrosinase proceeds by an electrophilic attack of the active copper/peroxo oxidant at the sub strate C-H bond. This is consistent with the electron-deficient nature of tyrosinasebound dioxygen, as inferred from the spectroscopic properties of oxy-tyrosinase [192]. What is unclear is whether 0 -0 bond cleavage takes place before or after this electro philic attack, since examples of both scenarios are known in copper chemistry. This is exemplified by the reaction in Fig. 10, whereby low-temperature oxygenation of the dicopper(I) precursor affords a concentration-dependent mixture of [Cu2(|i-02)]2+ and [Cu2(|!-0)2]2+ species, both of which undergo intramolecular hydroxylation upon warming [198]. These two different hydroxylation reactions give rise to very different kinetic isotope effects and Hammett constants [197,198], so that if these data were determined for tyrosinase it would be possible to distinguish between these two mechanisms.
