Quantitative structure-activity relationship studies on ironporphyrin-catalyzed cyclohexane oxidation with PhIO

Quantitative structure-activity relationship (QSAR) studies were performed oil the ironporphyrin-catalyzed biomimetic oxidation of cyclohexane. Through quantum chemical calculations, the molecular structures of nine different ironporphyrin catalysts have been optimized and their respective quantum chemical descriptors (FMO energies E-HOMO, E-LUMO, FMO energy gap DEHL, partition coefficient log P) have been obtained. The ironporphyrin-catalyzed cyclohexane hydroxylation with PhIO was chosen as the model reaction. The yield of cyclohexanol (yield (%)) and the reaction rate constant (Igk) were obtained experimentally, and the reaction kinetics was studied accordingly. 2D-QSAR studies for ironporphyrin catalysts were performed by using multiple linear regression (MLR) analysis. From the established QSAR model equations of lgk, yield (%) and the quantum chemical descriptors (Igk=-1.433+0.009 log P-0.406E(LUMO-b) R=0.968 any yield (%)=9.556+0.500 log P-8.997E(LUMO-b), R=0.821), we conclude that it is the frontier molecular orbital (FMO) energy level ELUMO-b which has the most significant effect or) the catalytic activity of the ironporphyrins. Further molecular graphics studies and Mulliken's electron population analysis indicated that the energy level of ELUMO-b can be altered by introducing peripheral substituting groups on the meso-phenyl ring. Since the electron withdrawing substituents could lower ELUMO-b and disperse the electron density of around the centro-metal core of porphyrin better, they call facilitate ironporphyrin's binding with the oxidant and, consequently increase the catalytic activity of ironporphyrin. We also notice that the partition coefficient log P of ironporphyrin molecule affects the reaction rate and the yield of the cyclohexane hydroxylation reaction as well. Our study may be beneficial for future catalyst design for the metalloporphyrin-catalyzed hydrocarbon oxidations. (C) 2009 Elsevier B.V. All rights reserved.