Manganese asymmetric epoxidation

Asymmetric epoxidation is another important area of activity, initially pioneered by Sharpless, using catalysts based on titanium tetraisoprop-oxide and either (+) or (—) dialkyl tartrate. The enantiomer formed depends on the tartrate used. Whilst this process has been widely used for the synthesis of complex carbohydrates it is limited to allylic alcohols, the hydroxyl group bonding the substrate to the catalyst. Jacobson catalysts (Formula 4.3) based on manganese complexes with chiral Shiff bases have been shown to be efficient in epoxidation of a wide range of alkenes. 

These reports sparked off an extensive study of metalloporphyrin-catalyzed asymmetric epoxidation, and various optically active porphyrin ligands have been synthesized. Although porphyrin ligands can make complexes with many metal ions, mainly iron, manganese, and ruthenium complexes have been examined as the epoxidation catalysts. These chiral metallopor-phyrins are classified into four groups, on the basis of the shape and the location of the chiral auxiliary. Class 1 are C2-symmetric metalloporphyrins bearing the chiral auxiliary at the... 

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. 

ASYMMETRIC EPOXIDATION OF DISUBSTITUTED Z-ALKENES USING A CHIRAL SALEN-MANGANESE COMPLEX111... 

There are now many examples of the industrial use of manganese(lll) salen catalyzed asymmetric epoxidations. For example, the as5mmetric epoxidation of a chromene derivative was central to the S5mthesis of the potassium channel activator BRL 55834 (Figure 11.5). ... 

SCHEME 33. Asymmetric epoxidation of olefins catalyzed by salen manganese complexes. 

The use of Mn-salen catalysts for asymmetric epoxidation has been reviewed.30 Oxo(salen)manganese(V) complexes, generated by the action of PhIO on the corresponding Mn(III) complexes, have been used to oxidize aryl methyl sulfides to sulfoxides.31 The first example of C—H bond oxidation by a (/i-oxo)mangancsc complex has been reported.32 The rate constants for the abstraction of H from dihydroanthracene correlate roughly with O—H bond strengths. 

Asymmetric epoxidation of alkenes. Two groups have prepared chiral (salen)-manganese complexes such as 2 from ( + )- or (-)-l and salicylaldehyde derivatives... 

The facial selectivity required for an asymmetric epoxidation can be achieved with manganese complexes to provide sufficient induction for synthetic utility (Scheme 9.10).98-103 This manga-nese(III) salen complex 5 can also use bleach as the oxidant rather than an iodosylarene.104,105 The best selectivities are seen with cA-alkenes. 

Catalytic, asymmetric epoxidations are one of the most important asymmetric processes. In 1980 Katsuki and Sharpless reported a stoichiometric asymmetric epoxidation of allylic alcohols, a method that was later improved to become a catalytic process.9 Moreover, catalytic asymmetric epoxidations of unfunctionalized olefins using salen-manganese complexes have been reported independently by several groups.10-12 In striking contrast to these successful achievements, an efficient catalytic asymmetric epoxidation of enones with broad generality has not been developed.13-22... 

As a solvent for the asymmetric epoxidation of 2,2-dimethylchromene mediated by Jacobsen s chiral (salen)-manganese catalyst.49... 

In order to increase the yield and/or the enantioselectivity of the reaction, the reaction temperature and additives were examined. Although aziridination was found to proceed smoothly at 0 °C, the product was not obtained at lower temperatures. Katsuki and co-workers have reported that pyridine /V-oxide is an effective additive for the asymmetric epoxidation catalyzed by salen-manganese(IH) complexes [24], and applied these findings to the asymmetric aziridination of olefins with Phi = NTs [9f]. Thus, the addition of pyridine /V-oxide at 0°C improved the enantioselectivity and allowed the reaction to proceed even at -20 °C (Table 6.1). Other additives, such as 4-phenylpyridine IV-oxide, 4-methylmorphorine N-oxide and 1-methylimidazole were used in the place of pyridine JV-oxide, but positive effects were not observed. 

---