With iodobenzene diacetate [PhI(OAc)2] as the oxygen source, manganese(III) porphyrin complexes exhibit remarkable catalytic activity towards the selective oxidation of hydrocarbons. The readily soluble PhI(OAc)2 in the presence of small amount of water is more efficient oxygen source than the commonly used PhIO under the same conditions, leading to a potent catalytic system for the efficient oxidation of alkenes and activated hydrocarbons. When the reactions were carried out in the presence of a small amount of water at 50 oC, high selectivity for epoxides and excellent catalytic efficiency with up to 10,000 TON have been achieved in alkene epoxidations. In the case of cyclohexene, for example, 100% conversion with a 93% selectivity for epoxide was achieved within 10 min under 50 oC, exhibiting a 40 TON/min. The reactivity of catalysts was greatly affected by axial ligand binding to the manganese metal and the weakly binding perchlorate gave the highest catalytic activity in the epoxidation of alkenes. A manganese(IV)-oxo porphyrin was detected in the reaction of the manganese(III) porphyrin and PhI(OAc)2. However, catalytic competition and Hammett correlation studies have suggested that the high-valent manganese(V)-oxo intermediate was favored as the premier active oxidant, though it is too short-lived and does not accumulate to detectable concentrations. Competitive catalytic oxidation of ethylbenzene and ethlybenzene-d10 revealed a kinetic isotope effect (KIE) of kH/kD = 2.8 ± 0.1 at 298 K, similar to the KIE reported for the reaction of ethylbenzene with an electron-deficient iii manganese(V)-oxo porphyrin species. On the basis of Hammett correlation (ρ+ = -0.33, R = 0.99) and competitive product analysis, a catalytic cycle was proposed in which the highly reactive porphyrin-manganese(V)-oxo species is favored as the active oxidant.
Advisor(s) or Committee Chair
Dr. Rui Zhang
Chemistry | Physical Sciences and Mathematics
Kwong, Kawai, "Synthetic and Mechanistic Studies of Catalytic Oxidations by Manganese(III) Porphyrin and Iodobenzene Diacetate" (2015). Honors College Capstone Experience/Thesis Projects. Paper 526.