Epoxidation manganese -catalysed

Non-heme iron- and manganese-catalysed selective oxidations of alkanes, as well as chemo- and stereo-selective epoxidations and cix-dihydroxylations of alkenes, using... 

The requirement for the presence of an adjacent alcohol group can be regarded as quite a severe limitation to the substrate range undergoing asymmetric epoxidation using the Katsuki-Sharpless method. To overcome this limitation new chiral metal complexes have been discovered which catalyse the epoxidation of nonfunctionalized alkenes. The work of Katsuki and Jacobsen in this area has been extremely important. Their development of chiral manganese (Ill)-salen complexes for asymmetric epoxidation of unfunctionalized olefins has been reviewed1881. 

A typical manganese-salen complex (27)[89] is capable of catalysing the asymmetric epoxidation of (Z)-alkenes (Scheme 18) using sodium hypochlorite (NaOCl) as the principle oxidant. Cyclic alkenes and a, (3-unsaturated esters are also excellent starting materials for example indene may be transformed into the corresponding epoxide (28) with good enantiomeric excess1901. The epoxidation of such alkenes can be improved by the addition of ammonium acetate to the catalyst system 911. 

The epoxidation of alkenes using iodosylbenzene, with tetra-n-butylammonium bromide and a manganese or cobalt polytungstate as co-catalysts [24], appears to have little advantage as a synthetic procedure over other methods. n-Hexene produces the oxirane (58%), when catalysed by the manganese salt, whereas norbornene is more readily converted (96%) into the oxirane with the cobalt salt. 

Although (salen)manganese(III) complexes are widely studied, those with other metals are largely unexplored. The stereochemistry of Cr-salen-catalysed epoxidation of dimethylchromenes and styrenes was found to be highly solvent-dependent, with polar solvents giving the opposite sense of induction to non-polar.17 This may be explained by competitive collapse of diastereomeric metalloxetanes either directly to epoxide or via a cationic intermediate. 

The two-phase epoxidation of alkenes by hydrogen peroxide in water-dichloromethane system, catalysed by manganese(III)-porphyrin, is strongly accelerated by addition of catalytic amounts of a carboxylic acid and lipophilic imidazole or pyridine axial ligand365,366. Manganese(III)-porphyrin bound to colloidal anion-exchange particles is more active in the selective epoxidation of styrene by aqueous hypochlorite than the same catalyst in aqueous solution367. 

Figure 2.18 Dualfunction of imidazole in the epoxidation of alkenes by hydrogen peroxide catalysed by manganese (III) porphyrins.

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. 

The details of the manganese(salen) catalysed epoxidation remain a topic of some debate. It has been established that the epoxidation proceeds via formation of an 0X0 Mn(V) complex by reaction of the Mn(lll) complex with the oxidant. However, a number of other oxidising species have also been... 

Racemic substrates have been employed in Mn(salen)-catalysed epoxidation reactions. Reasonable to good kinetic resolution of racemic chromanes (selectivity factor, S = 2.7-9.3) and racemic allenes have been achieved using enantiomer-ically pure manganese(salen) complexes to catalyse epoxidation. 

Gilbert, B.C., Smith, J.R.L., Mairata i Payeras, A., Oakes, J., Pons i Prats, R. (2004) A Mechanistic Study of the Epoxidation of Cinnamic Acid by Hydrogen Peroxide Catalysed by Manganese l,4,7-Trimethyl-l,4,7-triazacyclononaneComplexes. J. Mol. Catal. A Chem. 219 265-272. 

Preparation of (1S,2/7)-1,2-epoxy-1,2,3,4 tetrahycb onaphthaiene (Structure 16). Asymmetric epoxidation using NaOCl catalysed by Jacobsen s chiral manganese salen complex (Structure 17),... 

Oxidation of aliphatic C-H groups with HP is efficiently catalysed by cis-a-aminopyridine manganese complexes in the presence of acetic acid. The reaction demonstrated excellent efficiency (up to TON = 970), site selectivity, and stereospecificity (up to >99%). Manganese(II) and iron(II) complexes based on ligands with a rigid, chiral diamine derived from proline and two benzimidazoles (2) were synthesized and applied in epoxidation reaction with aqueous HP. Mn-complex catalyses the epoxidation of olefins. Isolated yields of 60-99% and up to 95% ee were obtained with 0.01-0.2mol% catalyst loading. The turnover frequencies and turnover numbers reached 59,000 h and 9600, respectively. Iron(II) complex exhibited a... 

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]. 

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