Chiral salen catalysts, olefins asymmetric

Figure 4.13 Chiral polymeric salen catalysts for asymmetric epoxidation of olefins.

Ten years after Sharpless s discovery of the asymmetric epoxidation of allylic alcohols, Jacobsen and Katsuki independently reported asymmetric epoxidations of unfunctionalized olefins by use of chiral Mn-salen catalysts such as 9 (Scheme 9.3) [14, 15]. The reaction works best on (Z)-disubstituted alkenes, although several tri-and tetrasubstituted olefins have been successfully epoxidized [16]. The reaction often requires ligand optimization for each substrate for high enantioselectivity to be achieved. 

Many efforts have been made to develop salen catalysts for the epoxidation of unfunctionalized olefins, and such work has been well documented.93 Very recently, Ito and Katsuki94 proposed that the ligand of the oxo salen species is not planar, but folded as shown in Figure 4-7 (R/ / H, R2 = H, L = achiral axial ligand). This folded chiral structure amplifies asymmetric induction by the Mn-salen complex. This transition state proposed by Ito and Katsuki is not compatible with the proposal by Palucki et al.95 that the salen ligands of oxo species are planar. 

In conclusion, the chiral salen Co(III) complexes immobilized on Si-MCM-41 colud be synthesized by multi-grafting method. The asymmetric synthesis of diols from terminal olefins was applied with success using a hybrid catalyst of Ti-MCM-41/chiral Co(III) salen complexes. The olefins are readily oxidized to racemic epoxides over Ti-MCM-41 in the presence of oxidants such as TBHP, and then these synthesized diols are generated sequentially by epoxide hydrolysis on the salen Co(lll) complexes. This catalytic system may provide a direct approach to the synthesis of enantioselective diols from olefins. 

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. 

Jacobsen reported in 1990 that Mnm complexes of chiral salen ligands (41) were the most efficient catalysts available for the enantioselective epoxidation of alkyl- and aryl-substituted olefins.118 This stimulated a rapid development in the chemistry and applications of chiral SB complexes, which offer promising catalytic applications to several organic reactions, such as enantioselective cyclopropanation of styrenes, asymmetric aziridination of olefins, asymmetric Diels-Alder cycloaddition, and enantioselective ring opening of epoxides.4,119... 

Polymer-supported salen catalysts were also developed by employing poly (norbornene)-immobihzed salen complexes 139 of manganese and cobalt (Scheme 3.40) [77]. The poly(norbornene) complexes are highly active and selective catalysts for the epoxidation of olefins. The asymmetric epoxidation of cis-P-methylstyrene 132 occurred smoothly at -20 °C to give the chiral epoxide 133 in 100% conversion with 92% ee. Under the same reaction conditions, Jacobsen s catalyst (an unsupported salen complex) afforded the same product with 93% ee. 

The development of chiral [Mn (salen)] complexes for asymmetric epoxidation of unfunctionalized olefins has been reviewed extensively [1,2,57,58]. Systematic variation of the steric and electronic environment of the complexes has led to the discovery of catalysts that are particularly effective for the epoxidation of several important classes of olefins. 

Asymmetric reactions that can exhibit this type of behavior include atom and group transfer reactions, such as the asymmetric oxidation of sulfides, some asymmetric epoxidations of olefins, " asymmetric aziridination of olefins, - and as)rmmetric cyclo-propanation of olefins. In the asymmetric oxidation of sulfides, a non-racemic, cliiral, low-valent metal complex is oxidized, in this case by iodosobenzene, to generate a highly reactive 0x0 intermediate. The 0x0 is then transferred directly to the sulfur to form the sulfoxide in the enantioselectivity-determining step. A representative example is illustrated in Equation 14.12 that involves a chiral salen-based catalyst. ... 

The ability of the catalyst to form asymmetric epoxides led Jacobsen to ask whether the same chiral salen ligands that discriminate between the enantiotopic faces of an approaching olefin also create an effective dissymmetric environment for nucleophilic attack at a bound epoxide. Indeed, Jacobsen et al. were also able to use very similar salen catalysts for the asymmetric ring opening of epoxides by nucleophilic attack on an epoxide activated by binding to a chiral, Lewis acidic Cr(III) metal salen complex. 

Chiral manganese salen catalysts have been widely used for the asymmetric oxidation of unactivated olefins. The dendritic polyglycerol-supported Mn-salen catalyst (44) was developed for the asymmetric epoxidation of the chromene derivative in a continuous membrane fiow reactor. This fiow system involves the continuous removal of the product (and unreacted substrate) from the high-molecular-weight dendritic catalyst (44) by filtration through a nanomembrane (Scheme 7.33). Under optimal conditions, 70% conversion with up to 92% ee was achieved [133]. In this system, however, the dendritic catalyst (44) worked as a homogeneous catalyst rather than a heterogeneous one. 

Irie, R., Noda, K., Ito, Y., Matsumoto, N. and Katsuki, T. Catalytic Asymmetric Epoxidation of Unfunctionalized Olefins. Tetrahedron Lett 31,7345-7348 (1990). Minutolo, F, Pini, D. and Salvador , P. Polymer-Bound Chiral (Salen)Mn(III) Complex as Heterogeneous Catalyst in Rapid and Clean Enantioselective Epoxidation of Unfunctionalised Olefins. Tetrahedron Lett 37, 3375-3378 (1996). 

For investigating the pore confinement effect, the chiral Mn(Salen) catalyst was immobilized in MCM-41 and MCM-48 with different pore sizes [84]. In the asymmetric epoxidation of unfunctionalized olefins with m-chloroperoxybenzoic acid as oxidant, it was found that the conversion and enantioselectivity were closely correlated with the pore size of the supports. The catalysts immobilized on MS with large pore sizes exhibited higher conversion. For the MCM-41-supported catalyst, the enantioselectivity increased with increasing pore size. However, for MCM-48-supported catalysts, the compatible pore size of the support with the substrate was found to be beneficial for obtaining higher enantioselectivity in olefin epoxidation. 

A breakthrough in the area of asymmetric epoxidation came at the beginning of the 1990s, when the groups of Jacobsen and Katsuki more or less simultaneously discovered that chiral Mn-salen complexes (15) catalyzed the enantioselective formation of epoxides [71, 72, 73], The discovery that simple achiral Mn-salen complexes could be used as catalysts for olefin epoxidation had already been made... 

In 2005, Nguyen et al. reported the first example of asymmetric cyclopropa-nation of olefins with EDA mediated by a combination of a (salen) ruthenium(II) catalyst and a catalytic amount of a chiral sulfoxide (Scheme 6.7). These authors proposed that the mechanism explaining the asymmetric induction involved the axial coordination of the chiral sulfoxide to the ruthenium centre as the key induction step in the reaction stereoselectivity. 

The insoluble polymer-supported Rh complexes were the first immobilized chiral catalysts.174,175 In most cases, however, the immobilization of chiral complexes caused severe reduction of the catalytic activity. Only a few investigations of possible causes have been made. The pore size of the insoluble support and the solvent may play important roles. Polymer-bound chiral Mn(III)Salen complexes were also used for asymmetric epoxidation of unfunctionalized olefins.176,177... 

Spectacular achievements in catalytic asymmetric epoxidation of olefins using chiral Mnm-salen complexes have stimulated a great deal of interest in designing polymeric analogs of these complexes and in their use as recyclable chiral catalysts. Techniques of copolymerization of appropriate functional monomers have been utilized to prepare these polymers, and both organic and inorganic polymers have been used as the carriers to immobilize these metal complexes.103... 

In 1990, Jacobsen et al. and Katsuki et al. independently reported asymmetric epoxidation of conjugated olefins by using complexes 9 and 10, respectively, as catalysts [29], These Mn-salen complexes were further improved to complexes 11 [30], 12 [31], and 13 [32]. The common features of these first-generation Mn-salen complexes are i) they possess C2-symmetry, ii) two sp3 carbons at the ethylenediamine moiety are replaced with chiral ones, and iii) they have tert-butyl groups or enantiopure 1-phenylpropyl groups at the C3 and C3 positions. 

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