SPECTROMAGNETIC INVESTIGATION OF THE ACTIVE SPECIES IN THE OXIDATION OF PROPENOIDIC PHENOLS CATALYZED BY -BIS(SALICYLIDENE)-ETHANE-1,2-DIAMINATO]COBALT(II)

The oxidation of propenoidic phenols by molecular oxygen, catalysed by '-bis(salicylidene)ethane-1,2-diaminato]cobalt(II) [Co(salen)], was s tudied in different solvents and for variously substituted phenols to find processes alternative to those reported for the removal of polyph enols from waste waters in the paper industry. The reaction of methyl E-4-hydroxy-3-methoxycinnamate (E-methyl ferulate) selectively gave th e corresponding 4-hydroxybenzoic acid and 4-hydroxybenzaldehyde; yield s are highest in chloroform, good in methanol and very low in pyridine . Conversion was high with E-methyl ferulate, lower with methyl E-4-hy droxycinnamate, while the other phenols, methyl E-3-hydroxy-4-methoxyc innamate (E-methyl isoferulate) and methyl E-3-chloro-4-hydroxycinnama te, did not react. An EPR investigation of the reaction mixtures, perf ormed on samples taken at different reaction times, demonstrated that the most probable mechanism involves reactions (i)-(iii). The (i) [Co- II(salen)] + ROH + O-2 reversible arrow [Co-III(salen)(ROH)(O-2)](-) ( ii) [Co-III(salen)(ROH)O-2)](-) + ROH reversible arrow [Co-III(salen)( ROH)(RO.)] + HO2- (iii) [Co-III(salen)(ROH)(RO.)] + HO2- reversible ar row [Co-III(salen)(RO-)(RO.)] + H2O2 superoxocobalt radical, [Co-III(s alen)(ROH)(O-2,)](-), and the phenoxy cobalt radical, [Co-III(salen)(R O-)(RO .)], are the EPR-active species; RO . is suggested easily to di ssociate from [Co-III(salen)(RO-)(RO.)] and, reacting with O, at the b eta-carbon of the allyl substituent, probably gives a dioxethane. This decomposes to aldehyde and acid. The phenol electron-donor properties promote the formation of the superoxocobalt derivative and consequent ly favour the formation of the phenoxy cobalt radical; a too high stab ility of this radical does not favour its evolution into oxidation pro ducts.