Catalysis of oxidation of di-t-butylphenol by μ-peroxodicobalt(III) complexes

Abstract

The oxidation of 2,6-di-t-butylphenol (dbp) by molecular oxygen is catalyzed by the μ-peroxodicobalt(III) complexes formed from tetraethylenepentamine (tetren), bis(picolyl)diethylenetriamine (pydien), and 1,4,10,13-tetraaza-7-thiatridecane (tattd). The oxygenation equilibrium constants of the cobalt(II) complexes of these ligands, and the observed rate constants, are presented in Table I. The rates of formation of the oxidative coupling product 3,3′,5,5′-tetra-t-butyldiphenoquinone (dpq) and the partial oxygen insertion product 2,6-tetra-t-butylbenzoquinone (bq) are shown to be first order with respect to the concentrations of both the cobalt–dioxygen complex (designated ‘cat’ for catalyst) and the phenol. The following rate expressions apply to this system: ▪ ▪ ▪ It is interesting to note that the observed rate was lowest for the most stable dioxygen complex. t001 Oxygenation Constants and 2,6-Di-t-butylphenol Oxidation Rate Constants. Ligand, L a log K O 2 b k obs b k 1 c k 2 d tetren 15.8 e 0.67 0.24 0.43 pydien 11.4 e 3.7 1.6 2.1 tattd 8.4 f 1.1 0.65 0.45 a Ligand abbreviations defined in text. b K O 2 = [M 2L 2O 2]/(ML) 2[O 2]. c Defined by eqn. (2). d Defined by eqn. (3). e Ref. 1. f Ref. 2. The oxidation reactions were run with a steady state concentration of dioxygen complex. It was noted, however, that when the supply of dioxygen was cut off, the reaction rate for bq formation was slowed considerably even though most of the dioxygen complex was still present. On the other hand, the oxygen insertion reaction does not occur at all in the absence of the dioxygen complex. These observations, and temperature coefficient data, indicate that the reactive dioxygen complex is probably the mononuclear superoxo species governed by the equilibrium: ▪The results lend support to the concept that the mechanism of oxygen insertion involves the intermediate 1, in accordance with suggestions of Nishinaga [3] and Drago [4] for other dioxygen complex systems. ▪