Epoxidation, manganese complex applications

The applicability of the Sharpless asymmetric epoxidation is however limited to functionalized alcohols, i.e. allylic alcohols (see Table 4.11). The best method for non-functionalized olefins is the Jacobsen-Kaksuki method. Only a few years after the key publication of Kochi and coworkers on salen-manganese complexes as catalysts for epoxidations, Jacobsen and Kaksuki independently described, in 1990, the use of chiral salen manganese (111) catalysts for the synthesis of optically active epoxides [276, 277] (Fig. 4.99). Epoxidations can be carried out using commercial bleach (NaOCl) or iodosylbenzene as terminal oxidants and as little as 0.5 mol% of catalyst. The active oxidant is an oxomanganese(V) species. 

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. 

The application of fluorinated media seems to be a flourishing new area in homogeneous catalysis (103). However, until now enantioselective catalytic reactions in this alternative solvent are very rare. Chiral perfluoroalkylated SALEN-manganese complexes have been used for asymmetric epoxidation (104). 

A similar strategy was developed by Kureshy et al. [45]. In 1998, Sherrington et al. [46] reported the application of polymer-bound Mn(lll)-salen catalyst in the epoxidation of 1-phenylcyclohex-l-ene, resulting in 49% yield and 91% ee, which are comparable to results obtained with the nonimmobilized Jacobsen catalyst. The synthesis of these catalysts started from polymer-bound salicy-laldehyde derivatives, which were first treated with 1,2-diamino cyclohexane and then with a second salicylaldehyde derivative. In the last step, the manganese complex was formed. Polymethacrylate was used as polymeric backbone. A similar approach was employed by Peukert and Jacobsen [47] to immobilize this catalyst to polystyrene. 

Many transition-metal complexes have been widely studied in their application as catalysts in alkene epoxidation. Nickel is unique in the respect that its simple soluble salts such as Ni(N03)2 6H20 are completely ineffective in the catalytic epoxidation of alkenes, whereas soluble manganese, iron, cobalt, or copper salts in acetonitrile catalyze the epoxidation of stilbene or substituted alkenes with iodosylbenzene as oxidant. However, the Ni(II) complexes of tetraaza macrocycles as well as other chelating ligands dramatically enhance the reactivity of epoxidation of olefins (90, 91). 

Transition metal carbenes constitute a very important class of molecules that have found a multitude of applications. With reference to manganese, a number of new carbene complexes have been reported recently. The ) -alkyne complex (18) was found to undergo oxidation by dimethyldioxirane to afford the a-keto carbene (19). The fascinating aspect of this reaction is the likely existence of an oxirene intermediate (20). Stable oxirene complexes have never been reported, imdoubtedly because of the extreme instability of the antiaromatic oxirene ring. The possibility of trapping an oxirene by epoxidation of a coordinated alkyne is intriguing. Scheme 10 summarizes the chemistry involved. 

However, 1,2-diamino-l,2-di-(cr(-butylethane (3) holds particular interest because of its increased steric bulk and the absence of benzylic protons. Its recent ready availability should render it as attractive as the frequently used vicinal diamines 7 and 8. To our knowledge, only one application of this diamine has been previously described in the literature (eq 5), where the regio- and enantioselective epoxidation of conjugated aliphatic dienes were studied using the chiral manganese salen complex (9). 

Other Applications. Other (/ ,/ )-stilbenediamine derivatives have been used to direct the stereochemical course of alkene dihydroxylation (with stoichiometric quantities of Osmium Tetroxide and epoxidation of simple alkenes with Sodium Hypochlorite and manganese(III) complexes. ... 

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