Selective and Efficient Catalytic Oxygenation of Alkyl Aromatics Employing H2O2 Catalyzed by Simple Porphyrin Iron(II) under Mild Conditions

Abstract

The excessive utilization of additives in chemical reactions is a troublesome problem in industrial processes, due to their adverse effects on equipment and processes. To acquire oxidative functionalization of alkyl aromatics under additive-free and mild conditions, a large library of metalloporphyrins was applied to the oxygenation of alkyl aromatics as catalysts with H2O2 as an oxidant. On the basis of systematic investigation of the catalytic performance of metalloporphyrins, it was discovered that, surprisingly, only porphyrin irons(II) possessed the ability to catalyze the oxygenation of alkyl aromatics with H2O2 under additive-free conditions and with satisfying substrate scope. Especially with 5,10,15,20-tetrakis(2,6-dichlorophenyl) porphyrin iron(II) (T(2,6-diCl)PPFe) as the catalyst, the substrate conversion reached up to 27%, with the selectivity of 85% to the aromatic ketone in the representative oxygenation of ethylbenzene with H2O2 as oxidant and without any additive used. The study of apparent kinetics and mechanisms in the optimal oxygenation system was also conducted in detail. Based on thorough exploration and characterization, the source of the superior catalytic performance of T(2,6-diCl)PPFe was acquired mainly as its planar structure, the low positive charge in the metal center, and better solubility in the oxygenation mixture, which favored the approach of reactants to the catalytic center, and the interaction between the metal center and H2O2. The beneficial interaction between T(2,6-diCl)PPFe and H2O2 was verified through cyclic voltammetry measurements and UV–vis absorption spectra. In comparison to previous studies, in this work, an efficient, selective, and additive-free means was developed for the oxygenation of alkyl aromatics under mild conditions, which could act as a representative example and a valuable reference for industrial processes in oxygenation of alkyl aromatics, and a great advance in the realization of oxygenation of alkyl aromatics under additive-free and mild conditions.