Efficient and ecofriendly options for the chemoselective oxidation of alkenes using manganese porphyrin and dioxygen

Selective oxidation of styrene to benzaldehyde was carried out for the first time in homogeneous system by using a green and easily prepared manganese porphyrin as an efficient catalyst under elevated oxygen pressure in the absence of additive and co-reductant. Effect of temperature (80-130 degrees C), pressure (0.6-1.0 MPa), the amount of catalyst (2-15 mg) and reaction media was studied to obtain the optimum conditions for the reaction. The best results were obtained at 110 degrees C and 0.8 MPa pressure using manganese deuteroporphyrin dimethyl ester (Mn-DPDME) with 98% styrene conversion and 82% selectivity of benzaldehyde in benzonitrile solvent. In order to elaborate the effect of meso substituents on porphyrin catalytic activity and stability, four different Mn-porphyrins were selected and compared under the identical condition. It was found that the natural Mn-DPDME which has no substituents in the meso-sites but does have the methyl and propionate groups in the beta-sites exhibited the higher activity than the synthetic manganese tetraphenylporphyrin, however, the later showed the superior stability and high selectivity of benzaldehyde. Furthermore, the catalytic method was widely used in the oxidation of various cycloalkenes, which exhibited excellent selectivity of epoxide. An alternative mechanism for the aerobic oxidation of styrene has been proposed and discussed. (C) 2013 Elsevier B.V. All rights reserved.