Activation of Dioxygen Mediated by Metal Ions, and Synthetic Application.

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The main object of this work is placed on developing a synthetic method for one-step oxidation of benzene to phenols. In earlier works, we reported that Cu^+ ions can activate molecular xoygen. When benzene is present in the system, both phenol and hydroquinone are produced. The primary steps are estimated to be the reactions expressed by eqs. (1)-(3), but most of mechanistic details were left ambiguous.iCu^+ + O_2 + 2H^+ -> 2Cu^<2+> + H_2O_2 (1) ; Cu^+ + H_2O_2 + H^+ -> Cu^<2+> + OH + H_2O (2) ;OH + C_6H_6 -----> C_6H_5OH (3)In this work, mechanistic detail and synthetic feasibility of air oxidation of benzene catalyzed by Copper(1) ions were discussed. Conclusions are as follows.1. [Mechanism]. (1) By kinetic study, it is confirmed that active species is OH; (2) Deactivation occurs at the stage of OH or H_2O_2; (3) Efficiency for H_2O_2 production is quantitative; (4) The formation rate of H_2O_2 is much gaster than the decomposition rate.2. [Comparison with Other Systems]. ^<18>O-Tracer experiment indicates that hydroxy- cyclohexadienyl and its peroxy radicals are the intermediates. The formation of hydroquinone is characteristic feature of the present system, which is never observed in the other systems, such as the Fenton's reaction.3. [Synthetic Feasibility]. The relative ratio of phenol to hydroquinone strongly depends on solution pH. The former dominates at lower pHs (<3), and the latter at higher pHs (>3). When both continuous extraction and electrolytic regeneration of Cu^+ were applied to the system at pH 3, p-benzoquinone was obtained in a 50% current efficiency.The present system has several merits and, therefore, provides much synthetic feasibility. Namely, (1) Hydrogen peroxide is unnecessary; (2) The reaction is effective and mild; (3) Either phenol or hydroquinone is obtained selectively.