Regioselective formation of peroxyquinolatocobalt(III) complexes in the oxygenation of 2,6-di-t-butylphenols with cobalt(II) Schiff-base complexes

Abstract

The oxygenation of 2,6-di-t-butylphenols with five-co-ordinate cobalt(II) Schiff-base complexes in aprotic solvents, such as CH₂Cl₂, thf, and dmf (thf = tetrahydrofuran, dmf = dimethylformamide), has been found to result in regioselective formation of peroxyquinolatocobalt(III) complexes. The regioselectivity depends on the nature of the substituent at the 4-position of the phenol used: 4-alkyl-2,6-di-t-butylphenols (1) afford peroxy-p-quinolatocobalt(III) complexes, whereas peroxy-o-quinolato-complexes are formed from 4-aryl-2,6-di-t-butyl phenols (4). The initiation of the oxygenation is hydrogen abstraction by superoxocobalt(III) species from the phenols to give the corresponding phenoxy-radicals (10). Rapid reduction of (10) with cobalt(II) species follows giving rise to a phenolatocobalt(III) complex intermediate, within which dioxygen is incorporated. The regioselectivity of the oxygenation is attributable to the formation of the phenolatocobalt(III) complex intermediate. Crystals of the peroxy-p-quinolatocobalt(III) complex (2a) are orthorhombic, space group P2₁2₁2₁, with a= 33.749(11), b= 11.844(5), c= 9.329(4)Å, and Z= 4. The crystal structure has been refined from 3 018 diffractometer data to R= 0.067.