Mechanism Research Of Radical Initiation In Dioxygen Oxidation Of Cyclohexane Catalyzed By Metalloporphyrin

The oxygenation mechanism operating in CYP450 and in synthetic metalloporphyrin models has received much attention in recent decades.One of metalloporphyrin-catalyzed biomimetic oxidation systems using O2 as the oxidant, Lyons system, is analogous to the catalytic cycle of CYP450 systems except that it does not require reductants or photochemical and electrochemical technical assistance. There are three main reaction mechanisms for this system. One is high-value metal-oxo species mechanism, one is radical-chain autoxidation mechanism, and the last is a metal-bound alkylperoxide mechanism. Besides, our group proposed a novel mechanism that is the"biological-chemical-cycle coupling mechanism"based on the industrial experiment of cyclohexane oxidation.In this paper, we studied the radical initiation mechanism of dioxygen oxidation of cyclohexane catalyzed by metalloporphyrin based on the mechanism proposed by our group and others.1. Using cyclohexane and cyclohexae-d12, we studied the kinetic isotope effect in this oxidation system. The KIE value obtained is 2.85±0.21 (KIE>2) which suggests that the rate determining step involves the removal of an hydrogen atom during C-H bond cleavages. And it suggests that the high-valent iron-oxo complex react with cyclohexane under"oxygen rebound mechanism"not"concerted mechanism".2. Isotopically labeled water, H218O, was used as a probe for exploring high-valent metala-oxo species. The GC-MS analysis of the products showed that no 18O incorporates into the cyc1ohexanol. Although the incorporation of 18O from H218O into the oxygenated products has often been considered as sufficient evidence for the intermediacy of high-valent metal-oxo species in catalytic oxygenation reactions, the absence of 18O in the cyclohexanol product formed does not imply that high-valent iron-xo species can be ruled out as a potent oxidizing agent since studies showed that the rate of oxygen exchange between high-valent metal-oxo complex and water is slow relative to the rate of substrate oxidation in some cases.3. When the oxidation of alkanes by molecular oxygen was carried out at relatively low temperature (100℃) in the presence of CBrCl3, cyclohexyl bromide was found in the products, but no reaction took place under the same conditions in the absence of Tp-ClPPFeCl. Besides, the radical reaction product cyclohexyl bromide was found under the N2 atmosphere. These results above clearly indicate that Tp-ClPPFeCl initates the radical reaction of cyclohexane, and it doesn't need the participation of dioxygen.4. We proposed a radical initiation mechanism based on the research in our paper and others, which begins with the direct reaction of metalloporphyrin and cyclohexane to give a Fe(II) complex and a cyclohexyl radical. The Fe(II) complex could react with oxygen molecular and obtained the high-valent iron-oxo complex, which can transfer the oxygen atom to cyclohexane and itself was reduced to Fe(II) complex to join in the next cycle.