Olefin complexes unfunctionalized, asymmetric

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. 

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. 

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... 

Following the success with the titanium-mediated asymmetric epoxidation reactions of allylic alcohols, work was intensified to seek a similar general method that does not rely on allylic alcohols for substrate recognition. A particularly interesting challenge was the development of catalysts for enantioselective oxidation of unfunctionalized olefins. These alkenes cannot form conformationally restricted chelate complexes, and consequently the differentiation of the enan-tiotropic sides of the substrate is considerably more difficult. 

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. 

Historically, reaction of simple olefins in the presence of chiral phosphine-Rh complexes in 1968 marked the first examples of homogeneous asymmetric hydrogenation [6], However, only a few successful results have been reported for asymmetric hydrogenation of unfunctionalized olefins. Some examples with late and early transition-metal complexes are illustrated in Schemes 1.27-28 and Schemes 1.29-30, respectively. 

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... 

The rigid, chiral salen complexes of Mn(III) shown below catalyze the asymmetric epox-idation of alkenes when treated with commercial bleach (NaOCl). This synthesis of enan-tio-enriched epoxides is particularly powerful since the method is applicable to unfunctionalized olefins. In general, (Z)-l,2-disubstituted alkenes afford higher enantioselectiv-ities than do the ( )-isomers or trisubstituted alkenes. The reaction mechanism is com-plex and proceeds via the formation of a Mn(III,IV) dinuclear species. ... 

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. 

Irie, R., Noda, K., Ito, Y., Matsumoto, N., Katsuki, T. Catalytic asymmetric epoxidation of unfunctionalized olefins using chiral (salen)manganese(lll) complexes. Tetrahedron Asymmetry 1991,2, 481-494. 

The 7 -symmetric complex ethylene bis(tetrahydroindenyl) titanium l,T-binaphth-2,2 -dithiolate has been used to catalyze the asymmetric hydrogenation of unfunctionalized trisubstituted olefins.1680 The kinetic resolution of racemic disubstituted 1-pyrrolidines via asymmetric reduction has been described.1681... 

M. Palucki, G. J. McCormick, E. N. Jacobsen, Low temperature asymmetric epoxidation of unfunctionalized olefins catalyzed by (salen)Mn(III) complexes. Tetrahedron Lett. 36 (1995) 5457. 

V. B. Valodkar, G. L. Tembe, R. N. Ram, H. S. Rama, Catalytic asymmetric epoxidation of unfunctionalized olefins by supported Cu(II)-amino acid complexes, Catal. Lett. 90 (2003) 91. 

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