II Schiff-base complexes as catalysts

The cobalt(II) Schiff-base complexes Co(salen) and Co(salpr), as well as their 5-coordinate derivatives LCo(salen) and LCo(salpr) catalyze the oxidation of substituted phenols by dioxygen. This propensity is due to the formation of dioxygen complexes upon contact with 0, which is usually reversible, especially in nonprotic solvents. 

Of specific interest is that while Co(salen) complexes form both superoxo and peroxo species, Co(salpr) gives only the 1 1 superoxo complex [16]. 

Co(salen) has long been known to be a homogeneous catalyst for the oxidation of substituted phenols to p-benzoquinones by molecular oxygen 

The oxidation of 2,6-disubstituted phenols catalyzed by Co(salen) and its derivatives has gradually achieved considerable importance due to the feasibility of studies on the relationship between the mode of oxidation and the type of dioxygen adduct [19-22, 37]. 

The selectivity of 2,6-di-t-butylphenol (2,6-DTBP) oxidation catalyzed by Co(salen) derivatives depends on the nature of axially coordinated (5th) ligand as well as on the solvent used. The relevant data of Kothari and Tazuma [23] are listed in Table I. These authors have found that DMF is an effective and selective solvent for benzoquinone (BQ) formation. With Co(salen)py as catalyst, up to 100 % selectivities to BQ can be reached relative to diphenoquinone (DPQ) formation. Mn(salen) in DMF, however, steers the reaction with 99.9 % selectivity to DPQ. The proper choice of reaction conditions permits even monoalkylphenols (2-t-butylphenol and 2-methylphenol) and phenol itself to be oxidized, mostly to the corresponding BQ. Phenol oxidation occurs at 6 bar 0 and ca. 70 °C in DMF. A recent report describes a 

---