Oxidation of 2, 6-Dialkylphenols Catalyzed by Cobalt (II) Schiff Base Complexes

Abstract

[N, N'-Disalicylidene-2, 3-dimethyl-2, 3-butanediamine] cobalt (II)/[1b] and [N, N'-bis (3-methoxysalicylidene) -2, 3-dimethyl-2, 3-butanediamine] cobalt(II) [1d] were synthesized and their activity as oxidation catalyst for 2, 6-dialkylphenols was evaluated in comparison with conventional salcomine complexes, [1a] and [1c]. For the O₂-oxidation of 2, 6-dimethylphenol to 2, 6-dimethyl-p-benzoquinone, [1b] and [1d] showed lower activity than [1a]and [1c]. This is attributed to the steric hindrance due to the four methyl groups of [1b] and [1d]. The reaction catalyzed by [1a] ceased before the conversion reached 100 % (e. g., 27% in pyridine, 69% in chloroform +pyridine). On the other hand, the conversion of the reaction catalyzed by [1b] approximated closely to 100% (e. g., 95% in pyridine, 98% in chloroform +pyridine). [1b] and [1d] gave the selectivity (for 2, 6-dimethyl-p-benzoquinone>90%) much higher than [1a] and [1c]. In particular, both the conversion and the selectivity for [1d] in DMF were 100%. The addition of pyridine to [1a] in chloroform slightly enhanced the selectivity (35%>48%) but significantly lowered the conversion (97%>69%). In [1b] in chloroform, in contrast, the selectivity was improved (37%>64%) without lowering the conversion. These results consistently suggest that conventional salcomine complexes are decomposed by activated oxygen species or bases, while tetramethyl-substituted [1b] and [1d] are stable enough to mediate the oxidation in a turnover manner. This is a new method to design stable catalysts using cobalt (II) for oxidation reactions of 2, 6-dialkylphenols.