Manganese salen complexes, alkene epoxidation

Ordinary alkenes (without an allylic OH group) have been enantioselectively epoxidized with sodium hypochlorite (commercial bleach) and an optically active manganese-complex catalyst. Variations of this oxidation use a manganese-salen complex with various oxidizing agents, in what is called the Jacobsen-Katsuki... 

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. 

Epoxidation using manganese salen complexes is very easy to carry out it occurs under aqueous conditions and commercial house bleach can be used as the oxidant. The results are similar to those reported in the literature Table 6.1 gives other examples of alkenes which can be epoxidized using the same procedure. This method gives good results, especially for disubstituted Z-alkenes but trisubstituted alkenes can be epoxidized as well. 

Although the Sharpless catalyst was extremely useful and efficient for allylic alcohols, the results with ordinary alkenes were very poor. Therefore the search for catalysts that would be enantioselective for non-alcoholic substrates continued. In 1990, the groups of Jacobsen and Katsuki reported on the enantioselective epoxidation of simple alkenes both using catalysts based on chiral manganese salen complexes [8,9], Since then the use of chiral salen complexes has been explored in a large number of reactions, which all utilise the Lewis acid character or the capacity of oxene, nitrene, or carbene transfer of the salen complexes (for a review see [10]). 

In the same year (1990) that Jacobsen reported his asymmetric epoxidation, a group led by Tsutomu Katsuki at the University of Kyushu in Japan reported a closely related asymmetric epoxidation. The chiral catalyst is also a salen and the metal manganese. The oxidant is iodosobenzene (Phl=0) but this method works best for E-alkenes. It is no coincidence that Katsuki and Jacobsen both worked for Sharpless. It is not unusual for similar discoveries to be made independently in different parts of the world, the Katsuki manganese salen complex... 

Asymmetric epoxidation The catalytic asymmetric epoxidation of alkenes has been the focus of many research efforts over the past two decades. The non-racemic epoxides are prepared either by enantioselective oxidation of a prochiral carbon-carbon double bond or by enantioselective alkylidenation of a prochiral C=0 bond (e.g. via a ylide, carbene or the Darzen reaction). The Sharpless asymmetric epoxidation (SAE) requires allylic alcohols. The Jacobsen epoxidation (using manganese-salen complex and NaOCl) works well with ds-alkenes and dioxirane method is good for some trans-alkenes (see Chapter 1, section 1.5.3). 

Dendritic and nondendritic polystyrene-boimd manganese-salen complexes were described by Seebach and coworkers [30]. The supported catalysts were prepared by suspension copolymerization of styrene with the vinyl-substituted complexes and employed in the epoxidation of phenyl-substituted alkenes by m-CPBA/NMO. Activities and selectivities were similar to those obtained with the monomeric complexes. High catalyst stabilities were observed and it was demonstrated that the immobilized catalysts can be recycled up to 10 times without loss of performance. Laser ablation inductively coupled plasma mass spectrometry was used to monitor the manganese content in repeatedly used polystyrene beads and a correlation between metal leaching from the support and catalytic activity was disclosed [31]. 

From the work of Jacobsen and Katsuki, it is known that chiral manganese salen complexes are excellent catalysts for the asymmetric epoxidation of alkenes... 

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 new stereoselective epoxidation catalyst based on a novel chiral sulfonato-salen manganese(III) complex intercalated in Zn/Al LDH was used successfully by Bhattacharjee et al. [125]. The catalyst gave high conversion, selectivity, and enantiomeric excess in the oxidation of (i )-limonene using elevated pressures of molecular oxygen. Details of the catalytic activities with other alkenes using both molecular oxygen and other oxidants have also been reported [126]. 

Without additives, radical formation is the main reaction in the manganese-catalyzed oxidation of alkenes and epoxide yields are poor. The heterolytic peroxide-bond-cleavage and therefore epoxide formation can be favored by using nitrogen heterocycles as cocatalysts (imidazoles, pyridines , tertiary amine Af-oxides ) acting as bases or as axial ligands on the metal catalyst. With the Mn-salen complex Mn-[AI,AI -ethylenebis(5,5 -dinitrosalicylideneaminato)], and in the presence of imidazole as cocatalyst and TBHP as oxidant, various alkenes could be epoxidized with yields between 6% and 90% (in some cases ionol was employed as additive), whereby the yields based on the amount of TBHP consumed were low (10-15%). Sterically hindered additives like 2,6-di-f-butylpyridine did not promote the epoxidation. 

Unfunctionalized alkenes have been epoxidized in 40-80% yield by an aerobic process that is catalyzed by salen-manganese(m) complexes at room temperature (equation 3). 

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. 

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 catalyhc asymmetric epoxidation of alkenes offers a powerful strategy for the synthesis of enantiomerically enriched epoxides. Among the several existing catalyhc methods, the asymmetric epoxidahon of unfunctionalized alkenes catalyzed by chiral Mn(lll)(salen) complexes such as homochiral [( N.N )-bis(3,5-di-tert-butylsalicylidene)-l,2-cyclohexanediamine]manganese(lll) chloride (22) (Figure 7.7), as developed by Jacobsen and coworkers, represents one ofthe most reliable methods [39]. 

Metal complexes of enantiomericaUy pure N,N -ethylenebis(salicylideneaminato) (salen) complexes in combination with stoichiometric oxidants currently provide the most selective method for the catalytic asymmetric epoxidation of unfunctionalised alkenes. The use of C2-symmetric salen complexes of manganese(lll) were reported independently in 1990 by Jacobsen and coworkers and Katsuki and coworkers. The first generation catalysts are represented by the general structure (4.33). The complex with R = Bu is known as Jacobsen s catalyst. All of the first generation catalysts are composed of a enantiopure diamine core and possess large substituents at the 3/3 and 5/5 positions. Subsequently Katsuki and coworkers developed second generation catalysts such as (4.34) with axially chiral groups at the 3/3 positions. 

Most manganese(III)(salen) complexes have generally been found to be less effective for aziridination than for epoxidation, both in terms of enantioselectivity and yield. However, the salen complex (4.149) has been shown to give high enantioselectivity for some simple alkenes such as styrene (4.75) that undergo... 

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]. 

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