Manganese chiral Schiff base complex

Figure 9.8 Catalytic cycle for the epoxidation of unfunctionalized alkenes with a chiral Schiff base complex of manganese as the catalyst.

A. Martinez, C. Hemmert, B. Mcunicr, A macrocyclic chiral manganese(III) Schiff base complex as an efficient catalyst for the asymmetric epoxidation of olefins, /. Catal. 235 (2005) 250. 

Epoxidation of alkenes with iodosylbenzene can be effectively catalyzed by the analogous salen or chiral Schiff base complexes of manganese(in), ruthenium(II), or ruthenium(III). For example, the oxidation of indene with iodosylbenzene in the presence of (/ ,5)-Mn-salen complexes as catalysts affords the respective (15,2/ )-epoxyindane in good yield with 91-96% ee [704]. Additional examples include epoxidation of alkenes with iodosylbenzene catalyzed by various metalloporphyrins [705-709], corrole metal complexes, ruthenium-pyridinedicarboxylate complexes of terpyridine and chiral bis(oxazoUnyl)pyridine [710,711]. 

Silva AR, Budarin V, Clark JH, Castro B de, Freire C. Chiral manganese(IIl) schiff base complexes anchored onto activated carbon as enantioselective heterogeneous catalysts for alkene epoxidation. Carbon 2005 43(10) 2096-2105. 

The main drawback in Sharpless epoxidation is that the substrate must bear a functional group to achieve the precoordination required for high enantioselec-tivity (as in the case of allyl alcohol). This restriction is not applicable to the epoxidation of alkyl- and aryl-substituted olefins with manganese complexes of chiral Schiffs bases as catalysts. Very high enantioselectivities can be obtained in these reactions (Jacobsen, 1993). The most widely used catalysts that give high enantioselectivity are those derived from the Schiff bases of chiral diamines such as [SiS] and [RR] 1,2-diphenylethylenediamine and [SS] and [RR] cyclohexyl-1,2-diamine. An example is the synthesis of cromakalim. 

C-H oxidation (150). The Jacobsen s ligand has been used for the modification of the manganese exchanged Al-MCM-41, that is the immobilized catalyst has been investigated in epoxidation of styrene (151). Chiral manganese-Schiff base complexes were immobilized on a glassy carbon electrode surface (152). Enan-tioselective epoxidation reactions were also catalyzed with Mn(III) complexes of chiral porphirines (97,153). 

Whilst the chiral manganese complexes can epoxidize alkenes with high enantioselectivity (> 90% e.e.), they are not particularly stable. This instability is probably due to the easily oxidizable imine and phenoxide ligands on the complex. Attempts are currently being made to immobilize Schiff-bases in order to increase their stability in a similar manner to the metalloporphyrins discussed earlier. 

In this context it is worth noting that neither the titanium(IV) tartrate catalyst nor other metal catalyst-alkyl hydroperoxide reagents are effective for the asymmetric epoxidation of unfunctionalized olefins. The only system that affords high enantioselectivities with unfunctionalized olefins is the manganese(III) chiral Schiff s base complex/NaOCl combination developed by Jacobsen [42]. There is still a definite need, therefore, for the development of an efficient chiral catalyst for asymmetric epoxidation of unfunctionalized olefins with alkyl hydroperoxides or hydrogen peroxide. 

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