Manganese porphyrins alkene epoxidation

In the case of manganese porphyrin catalyzed epoxidations, the axial ligands have been used alone or together with other additives like carboxylic acids (Banfi and coworkers) and soluble bases (Johnstone and coworkers). For example, Mansny and coworkers showed that in the presence of imidazole, 2-methylimidazole or 4-imidazole chloromanganese(tetra-2,6-dichlorophenylporphyrin) catalyzes the epoxidation of varions aUtenes including 1-alkenes by Under these conditions alkene conversion... 

The mechanisms proposed for manganese-porphyrin-catalyzed epoxidation of olefins are similar in several respects to that proposed for salen systems (365, 400), and this was utilized initially to provide insight into the Mn-salen systems. In the case of porphyrins, a Mnv=0 moiety is also formed by reaction with the secondary oxidant, for example iodosyl benzene, and the olefin is then believed to approach in a side-on manner. Various intermediates have been proposed for the epoxidation of the olefin, including ones in which the alkene is polarized to give positive and radical ends, and those shown earlier for the salen systems. The concept of the metallaoxe-tane (371, 372) was employed to explain isomerization of some of the olefinic substrates during conversion to the epoxide products, but now... 

In the presence of ascorbate coreducing agent and under phase-transfer conditions, manganese porphyrins have recently been shown to activate oxygen by catalyzing epoxidation of alkenes and hydroxylation of alkanes to alkanol-one mixtures.484... 

In the catalytic epoxidation of alkenes by a manganese porphyrin with phase-transfer catalysis and hypochlorite, the yield of epoxide also decreases with decreasing alkene concentration363 dibenzo-18-crown-6 has been shown to have an effect on the reaction364. 

Apart from the catalytic properties of the Mn-porphyrin and Mn-phthalo-cyanine complexes, there is a rich catalytic chemistry of Mn with other ligands. This chemistry is largely bioinspired, and it involves mononuclear as well as bi- or oligonuclear complexes. For instance, in Photosystem II, a nonheme coordinated multinuclear Mn redox center oxidizes water the active center of catalase is a dinuclear manganese complex (75, 76). Models for these biological redox centers include ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes, and Schiff bases. Many Mn complexes are capable of heterolytically activating peroxides, with oxidations such as Mn(II) -> Mn(IV) or Mn(III) -> Mn(V). This chemistry opens some perspectives for alkene epoxidation. 

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

The epoxidation of alkenes by sodium hypochlorite in the presence of manganese porphyrins under phase-transfer conditions has been thoroughly studied. Kinetic studies of this reaction revealed a Michaelis-Menten rate equation. As in Scheme 12, the active oxidant is thought to be a high-valent manganese( V)-oxo-porphyrin complex which reversibly interacts with the alkene to form a metal oxo-alkene intermediate which decomposes in the rate determining step to the epoxide and the reduced Mn porphyrin. Shape selective epoxidation is achieved when the sterically hindered complex Mn(TMP)Cl is used as the catalyst in the hypochlorite oxidation. ... 

T. G. Traylor, A. R. Miksztal, Alkene epoxidations catalyzed by iron(III), manganese(ni), and chromium(III) porphyrins. Effects of metal and porphyrin substituents on selectivity and regiochemistry of epoxidation, ]. Am. Chem. Soc. 11 (1989) 7443. 

H. C. Sacco, Y. lamamoto, J. R. L. Smith, Alkene epoxidation with iodosylbenzene catalysed by polyionic manganese porphyrins electrostatically bound to counter-charged supports, /. Chem. Soc., Perkin Trans. 2 (2001) 181. 

E. Brule, Y. R. de Miguel, Supported manganese porphyrin catalysts as P450 enzyme mimics for alkene epoxidation. Tetrahedron Lett. 43 (2002) 8555. 

S. Tangestaninejad, M. H. Habibi, V. Mirkhani, M. Moghadam, Manganese(III) porphyrin supported on polystyrene as a heterogeneous alkene epoxidation and alkane hydroxylation catalyst, Synth. Commun. 32 (2002) 3331. 

M. Benaglia, T. Danelli, G. Pozzi, Synthesis of poly(ethylene glycol)-supported manganese porphyrins Efficient, recoverable and recyclable catalysts for epoxidation of alkenes, Org. Biomol. Chem. 1 (2003) 454. 

In pursuit of biomimetic catalysts, metaUoporphyrins have been extensively studied in attempts to mimic the active site of cytochrome P450, which is an enzyme that catalyzes oxidation reactions in organisms. In recent decades, catalysis of alkene epoxidation with metaUoporphyrins has received considerable attention. It has been found that iron [1-3], manganese [4,5], chromium [6], and cobalt porphyrins can be used as model compounds for the active site of cytochrome P450, and oxidants such as iodosylbenzene, sodium hypochlorite [7,8], hydrogen peroxide [9], and peracetic acid [10] have been shown to work for these systems at ambient temperature and pressure. While researchers have learned a great deal about these catalysts, several practical issues limit their applicability, especially deactivation. 

There is also debate about the nature of the intermediates involved in the second step in the catalytic cycle illustrated in Fig. 19.1, that is, the transfer of oxygen to the alkene to form the epoxide. No intermediates have been detected experimentally, but five different possibilities have been proposed in the literature for the alkene complexed to the oxidized porphyrin [11,25-29]. The five proposed intermediates are radical, cation, concerted, metallaoxetane, and pi-radical-cation species. The literature is rather complicated due to the lack of direct experimental observation, and it is not clear that conclusions from, say, iron and chromium porphyrins also apply to manganese porphyrins [28]. Arasasingham et al. claim unequivocal evidence for a radical intermediate being involved in the oxidation of alkenes by manganese porphyrins [28]. They also discuss a charge-transfer complex that is similar to the concerted intermediate. Recently, density functional theory (DFT) and quantum mechanics/molecular mechanics (QM/MM) calculations were applied to styrene epoxidation by Mn-porphyrins ... 

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