Styrene epoxidation with iodosylbenzene catalyzed

FIGURE 19.4 Mechanism of styrene epoxidation with iodosylbenzene catalyzed by Mn-porphyrins proposed based on transition state structures identified. 

Gross et al. reported the first use of a chiral ruthenium porphyrin Ru"(L,)(CO) as a catalyst for styrene epoxidation . Chiral ruthenium porphyrin systems have also been reported by Che et a/. . The utilization of another chiral ruthenium porphyrin, Ru"(L2)(CO) as a catalyst for enantioselective epoxidation of olefins with 2,6-dichloropyridine A-oxide has been described by Berkessel and Frauenkron ". The highest enantiomeric excesses of the oxiranes were obtained in the epoxidation of tetrahydronaphthalene and styrene, 77% and 70% ee, respectively, with high jdelds (up to 88%). Terminal aliphatic olefins and tra 5-disubstituted olefins, represented by 1-octene and rraw-stilbene, were sluggish substrates and gave low ee s. The epoxidation of tetrahydronaphthalene with iodosylbenzene catalyzed by Ru(II)(L2)(CO) produced only 52% ee... 

Hammett correlations for styrene epoxidations have been reported for a variety of metal-catalyzed oxidations. In iron porphyrin systems, p values between -0.83 and -0.94 have been reported for PhIO reactions.Groves and Watanabe l reported a p value of -1.9 for styrene epoxidations where the intermediate is presumed to be the iron(IV) 0x0 porphyrin cation radical, generated by reaction of Fe(TMP)Cl with MCPBA at -50 °C. It is noteworthy that p values for reactions of iodosylbenzene and hypochlorite catalyzed by Fe(TDCPP)Cl were found by Collman et al. not to be identical. The p values for PFIB (pentafluoroiodosylbenzene) and PhIO were -0.86 and -0.91, respectively, whereas the p value for LiOCl plus a phase transfer catalyst was -0.57. Those authors suggested that either different oxidants were formed in the iodosylbenzene and hypochlorite reactions or differences in the reaction conditions were responsible for the different p values. In our reactions, the smaller p value for the H2O2 reaction relative to those for the PhIO and MCPBA reactions suggests that different intermediates are responsible for the oxygenations. 

The controlled oxygenation of alkanes, alkenes, and aromatic hydrocarbons is one of the most important technologies for the conversion of crude oil and natural gas to valuable commodity chemicals. Biomimetic studies of metalloporpltyrins have led to important advances in practical catalysis, especially with ruthenium porphyrins. Reaction of wj-CPBA, periodate, or iodosylbenzene with Ru(II)(TMP)(CO) produced RuCVIjfTMPXOjj . Remarkably, Ru(VI)(TMP)(0)2 was found to catalyze the aerobic epoxidation of olefins under mild conditions. Thus, for a number of olefins including cyclooctene, norbomene, cis-, and trans- -methyl styrene 16-45 equivalents of epoxide were... 

In 2004, Collman examined the impact of various donor ligands (Ph3P=0, pyridine A-oxide and 1-methylimidazole) on the [(salen )MnX] (X=C1, BF4, or OAc) catalyzed epoxidation of olefins [51]. In the presence of some of these donors, the particular iodosylarene (PhIO, CeFsIO, or MesIO) chosen made a significant impact on the enantioselectivities of styrene oxide produced. Unlike the previous related study by Roschmann [48], oxidation of Cl was judged not to be the key explanation for different results with different iodosylarenes. The results suggested that there are two active oxidants for styrene, one of which is a manganese iodosylbenzene complex. 

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