Epoxidation and catechol oxidation catalytic processes promoted by manganese(III) complexes of salen-type ligands

Abstract

Four new mononuclear manganese(III) complexes namely 1, 2, 3 and 4 of salen-type ligands H₂L¹-H₂L⁴ (ligands were obtained in situ via Schiff-base condensation of  2-formyl-6-hydroxymethyl-4-methylphenol and amines cyclohexane-1,2-diamine, 2-methylpropane-1,2-diamine, propane-1,2-diamine and ethane-1,2-diamine respectively) have been synthesised and characterised by routine physicochemical techniques. 1 is further characterised by X-Ray single crystal structure analysis. Catalytic efficiencies of the complexes as epoxidation catalysts (substrates: styrene and (E)-stilbene; terminal oxidants: PhIO/NaOCl; solvent: MeCN /dicholorometane) and as catalysts for oxidation of catachol (substrates: 3,5-di-tert-butylcatechol (3,5-DTBC); solvent: methanol) have been evaluated. In both cases the catalytic efficiency increases on going from 4-1 although the actual mechanisms in those two catalytic reactions are completely different. However, the observations have been rationalized on the basis of steric and electronic factors exerted by the alkyl substituents present on the imine back-bone of the salen-type ligands. This study also verifies that there is at least another active epoxidizing species, [Cl–O–Mn(III)(salen)X] in addition to the discrete  Mn(V)=O(salen) species in epoxidation of olefins depending upon the terminal oxidants employed.