Salen-derived ligands epoxidation

N. J. Kerrigan, H. Muller-Bunz, D. G. Gilheany, Salen ligands derived from trans-l,2-dimethyl-1,2-cycIohexanediamine preparation and application in oxo-chromium salen mediated asymmetric epoxidation of alkenes, J. Mol. Catal. A Chem. 227 (2005) 163. 

Kureshy developed a polymer-based chiral Mn-salen complex (Figure 21). Copolymerization of styrene, divinylbenzene, and 4-vinylpyridine generated highly cross-linked (50%) porous beads loaded with pyridine ligands at 3.8 mmol g-1. Once the polymer was charged with the metal complex catalyst, enantioselective epoxidation of styrene derivatives was achieved with ee values in the range 16 46%. 79... 

Various other chromene derivatives 176a-d could be epoxidized with Katsuki s Mn-salen catalyst 173d using either H2O2 or TMS2O2 as oxidant. With this catalytic system several axial ligands (none, 7V-methylimidazole, pyridine TV-oxide) and additives (none. 

These catalysts, 11-13, show good enantioselectivity ranging from 80 to 95% ee in the epoxidation of conjugated cfs-di- and tri-substituted olefins. Epoxidation of "good substrates such as 2,2-dimethylchromene derivatives proceeds with excellent enantioselectivity (>95% ee). Since the results obtained with these first-generation Mn-salen catalysts have been reviewed [21,33], only typical examples are shown in Table 6B.1. These reactions are usually carried out in the presence of donor ligand [34] such as 4-phenylpyridine A -oxide with terminal oxidants such as iodosylbenzene and sodium hypochlorite as described above. However, the use of some other terminal oxidants under well-optimized conditions expands the scope of the Mn-salen-... 

The mechanistic picture obtained from the ESMS studies fully explain the solution-phase observations. It was early recognized by Kochi et al. that (salen)Mn complexes with electron-donating substituents such as the 5,5 -dimethoxy derivative effect only poor yields of epoxide, whereas the catalyst with 5,5 -dinitro substituents gave the best product yields [102]. While the reactivity differences seen with differently substituted achiral salens led to the conclusion that the more electron-deficient ligand will give the more effective catalyst, the interplay between epoxidation efficiency and selectivity is much more subtle and less predictable for asymmetric epoxidation. In 1991, Jacobsen reported the dramatic... 

Tridentate salen ligands (10) derived from 1 have given excellent results in the enantiocontrol of the hetero Diels-Alder addition reaction of dienes with aldehydes (eq 7) and in the asymmetric additions of TMS-azide to mc5o-epoxide and trimethylsilyl cyanide to benzaldehyde (up to 85% ee). Phosphino-oxazolines derived from 1 have been employed for the asymmetric control of palladium-catalyzed allylic substitution reactions products of 70-90% ee were obtained. Photolysis of crystalline adducts of enantiomerically pure 1 with prochiral alcohols results in asymmetric inductions of up to 79% in a rare example of a solid-state enantioselective reaction. ... 

The enantioselective ring opening of epoxides with salen-Cr complexes yields intermediates for the manufacture of ( )-9-[2-(phosphonomethoxy)propyl]ade-nine [67] (a prophylactic against SIV infection). Os-catalyzed asymmetric amino-hydroxylation (ligand modified by cinchona alkaloids) leads to a-hydroxy-j6-phenylalanine, a derivative for the C13 chain of taxol [68]. 

In simplest terms, the incorporation of only two structural properties into the Hgand system is required for attainment of good enantioselectivity in olefin epoxidation by [Mn (salen)] complexes (i) a dissymmetric diimine bridge derived from a C2-symmetric 1,2-diamine and (ii) bulky substituents on the 3-and 3 -positions of the salicylide ligand. This is illustrated in Table 1 for the epoxidation of czs-P-methylstyrene by various [Mn (salen)] catalysts. 

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