IRON PORPHYRIN-CATALYZED OXIDATION OF 1,2-DIMETHOXYARENES - A DISCUSSION OF THE DIFFERENT REACTIONS INVOLVED AND THE COMPETITION BETWEEN THE FORMATION OF METHOXYQUINONES OR MUCONIC DIMETHYL ESTERS

This paper describes the oxidation of an alpha,beta-diarylpropane lignin dimer model and other dimethoxyarenes by several iron porphyrin-based biomimetic systems. From l-(3,4-dimethoxyphenyl)-2-phenylpropanol (1), three types of products are identified: the 3,4-dimethoxybenzaldehyde derived from the C(alpha)-C(beta) cleavage of the propyl side chain and either quinones or muconic dimethyl esters resulting from oxidations at the level of the dimethoxyaryl group. The selectivity of the reaction is discussed with respect to the nature and reactivity of the high-valent iron-oxo species formed upon reaction of the oxidants, H2O2 or magnesium monoperoxyphthalate (MMP), with the iron porphyrins. Fe(TF5PP)CI-catalyzed oxidation of 1 by H2O2 in an aprotic medium (CH3CN/CH2Cl2), yields a clean ''lignin peroxidase-like'' reaction with selective formation of the aldehyde. In an aqueous buffered solution, MMP oxidation of para-substituted 1,2-dimethoxyarenes catalyzed by an iron tetrakis(pentafluorophenyl)-beta-tetrasulfonatoporphyrin, Fe(TF5PS4P), clearly depends on the electronic properties of the para-substituent. The reaction is selective for para-quinone formation in the case of an electron-releasing group and for muconic dimethyl ester formation in the case of an electron-withdrawing group.