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

[N, N'-Disalicyliderie-2, 3-dimethyl-2, 3-butanediamine]cobalt(II) [1 b] 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, [1 a] and [1 c]. For the O₂-oxidation of 2, 6-dimethyl-phenol to 2, 6-dimethyl-p-benzoquinone, [1 b] and [1 d] showed lower activity than [1 a] and [1 c]. This is attributed to the steric hindrance due to the four methyl groups of [1 b] and [1 d]. The reaction catalyzed by [1 a] 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 [1 b] approximated closely to 100% (e. g., 95% in pyridine, 98% in chloroform+pyridine). [1 b] and [1 d] gave the selectivity (for 2, 6-dimethyl-p-benzoquinone>90%) much higher than [1 a] and [1 c]. In particular, both the conversion and the selectivity for [1d] in DMF were 100%. The addition of pyridine to [2 a] in chloroform slightly enhanced the selectivity (35%→48%) but significantly lowered the conversion (97% →69%). In [1 b] 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 [2 b] and [2 d] 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.