Metal oxo-catalyzed epoxidations

Metalloporphyrin complexes have been shown to catalyze epoxidation of alkenes (Fig. 12), and in this case metal oxo intermediates have been implicated [19-21]. In most cases strong oxidants are required however, it has recently been reported that a ruthenium trans-dioxo complex catalyses direct epoxidation of an alkene with molecular oxygen as the oxidant [22]. It is believed that porphyrinato metal complex-catalyzed epoxidations are related to alkene epoxidations catalyzed by cytochrome P-A50 or other active enzymatic systems. It has recently been shown that creating site-isolation by anchoring (tetra-phenylporphinato)manganese(lll) acetate to a rigid polymer support considerably enhances the rate of epoxidation of cyclohexene by sodium hypochlorite [23]. 

Hi. Cr, Mo, W. In contrast to group IV and V transition metals, the catalytic active oxidant is of another type for group VI transition metal-catalyzed epoxidations The transition-metal-oxo complexes, in which the oxygen that is transferred is bonded to the metal via a double bond, are the active oxidizing species. 

In the last decade, transition metal complexes (e.g. metalloporphyrins) have been used to catalyze epoxidation. These entities can reproduce and mimic all reactions catalyzed by heme-enzymes (cytochromes P-450)54. Synthetic metalloporphyrins are analogous to the prosthetic group of heme-containing enzymes which selectively catalyze various oxidation reactions. The metallo complexes of Fe, Co, Cr, Mn, Al, Zn, Ru, etc. possessing porphyrin ligands have been mostly studied55 -57. Porphyrin ligands (4) are planar and can possess several redox states of the central metallic ions and hence they can exist as oxo metals. 

J. Bernadou, A. Fabiano, A. Robert, B. Meunier, Redox tautomerism in high-valent metal-oxo-aquo complexes.Origin of the oxygen atom in epoxidation reactions catalyzed by water-soluble metalloporphyrins,/. Am. Chem. Soc. 116 (1994) 9375. 

In many cases, the addition of Lewis bases capable of coordinating to the metal center during epoxidation catalysis has been found to have a beneficial effect on catalyst turnover rate and number as well as epoxide yield. Commonly used additives include pyridine, imidazole, and pyridine N-oxide derivatives. The proposed roles of N-oxide derivatives in [Cr(salen)] -catalyzed and [Mn(salen)] -catalyzed epoxidation reactions include activation of the intermediate metal-0X0 complex [15,50], dissociation of umeactive p-oxo dimer complexes to reactive monomeric species [19,25], and/or solubilization of the active oxidant in bi-phasic reaction media [51]. 

In short, the experimental results presented above collectively form a more coherent understanding of the [Mn(salen)] -catalyzed epoxidation of unfunctionalized olefins. Side-on approach of the substrate at the metal-oxo species leading to stepwise C-0 bond formation offers a straightforward explanation for product selectivity and additive effects. The degree of C-0 bond formation reflects the position of the transition state along the reaction coordinate, and it is this position that is critical to the level of asymmetric induction in the [Mn(salen)]-catalyzed epoxidation. 

Two successful strategies for the enantioselective synthesis of trans-epoxides by means of oxo-metal catalysis have been discovered. The stereospecific epoxidation of frans-olefins offers a direct route to trans-epoxides, although progress in this area has been limited (see Sect. 2.2.1). Alternatively, the [Mn(salen)]-catalyzed epoxidation of cis-disubstituted olefins in the presence of alkaloid-derived phase-transfer catalysts such as 24 resulted in the formation of the transepoxide as the major, and in some cases nearly exclusive, product (Scheme 6)... 

Two kinds of systems based on Fe(III) or Mn(III) porphyrins are available now for the oxidation of hydrocarbons. The first ones involve such a metalloporphyrin catalyst and an oxygen atom donor like PhIO, H2O2 or O2 and a reducing agent 2-10 They reproduce quite well the reactions catalyzed by cytochrome P450-dependent monooxygenases and involve a high-valent metal-oxo active species which is able to epoxidize alkenes, hydroxylate alkanes and aromatic compounds and perform N- or S- oxidations. 

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