Chiral manganese salen complex

Asymmetric epoxidation of alkenes. Two groups have prepared chiral (salen)-manganese complexes such as 2 from ( + )- or (-)-l and salicylaldehyde derivatives... 

Sabater, M. J., Corma, A., Domenech, A., Fomes, V. and Garcia, H. Chiral salen manganese complex encapsulated within zeolite Y a heterogeneous enantioselective catalyst for the epoxidation of alkenes, Chem. Commun., 1997, 1285-1286. 

Manganese salen complex catalyzes C—H oxidation of organic molecules with NaOCl or PhIO, giving alcohols . Larrow and Jacobsen observed kinetic resolution in the benzylic hydroxylation . Katsuki and coworkers used the axis chiral salen manganese complexes for the benzyl hydroxylation and ether hydroxylation, and attained higher ee with the ligand possessing (/f,/f)-diamine and (R)-axis chirality (equation 84). ... 

Epoxidation of Alkenes Catalyzed by Chiral Salen Manganese Complexes... 

Katsuki and coworkers examined enantiotopic selective hydroxylation of pro-chiral substrates with chiral (salen)manganese complexes as catalysts [13]. This reaction also proceeds via a radical intermediate [13a]. The kinetic isotopic effect (kn/kD = 4.6) observed in the hydroxylation of ethylbenzene with complex lib supports the idea that hydrogen atom abstraction is the rate-determining step [13b]. In the reaction using chiral (salen)manganese complexes which have no chiral cavity, radical decay should occur less selectively and should deteriorate the enan-tioselectivity of hydroxylation. A solvent of intense viscosity constitutes a strong... 

Figure 1.6 Chiral Salen manganese complex of Katsuki [110].

Epoxidation of olefins was catalyzed by the ruthenium(II) complex of the above perfluorinated y3-diketone in the presence of 2-methylpropanal (Scheme 50). Unfunctionalized olefins were epoxidized with a cobalt-containing porphyrin complex, and epoxidation of styrene derivatives was catalyzed by chiral salen manganese complexes (248) (Scheme 50). In the latter case, chemical yields were generally high, however, the products showed low enantiomeric excess with the exception of indene (92% ee). [Pd(C7Fi5COCHCOC7Fi5)2] efficiently catalyzed the oxidation of terminal olefins to methyl ketones with f-butylhydroperoxide as oxidant in a benzene-bromoperfluoro-octane solvent system (Scheme 50). In all these reactions, the product isolation and efficient catalyst recycle was achieved by a simple phase separation. 

Asymmetric imidations of aryl alkyl sulfides with [(tosylimino)iodo]ben-zene, catalyzed by various chiral (salen)manganese(III) complexes, have been investigated in some detail [31,32]. The influence of catalyst structure, solvent, temperature, 3°-amine AT-oxides, and the presence of molecular sieves on product yields and the enantioselectivity of imidation with 17 was evaluated. Enan-tioselectivities as high as 90 % ee and 97 % ee with methyl 2-nitrophenyl sulfide and methyl 2,4-dinitrophenyl sulfide, respectively, were achieved. 

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. 

Irie, R., Noda, K., Ito, Y., Katsuki, T. Enantioselective epoxidation of unfunctionalized olefins using chiral (salen)manganese(lll) complexes. Tetrahedron Lett. 1991, 32, 1055-1058. 

Kinetic resolutions. A chiral alcohol is obtained on. selective removal of one enantiomer by acetylation using a chiral analog 1 of DMAP, or by oxidation based on hydrogen transfer to acetone mediated by a Ru complex 2. Benzylic secondary alcohols are resolved by selective pivaloylation with optically activeA-pivaloyl-4-t-butylthiazolidine-2-thione. A kinetic resolution of sulfoxides is based on asymmetric oxidation with (i-PrO)4Ti-cumyl hydroperoxide in the presence of a tartrate ester. Kinetic resolution of 1,3-diarylallenes is realized by selective oxidation with NaClO catalyzed by a chiral (salen)manganese(III) complex, whereas asymmetric hydrolysis of terminal epoxides with the aid of a chiral (salen)cobalt(II) catalyst solves the problem of their accessibility. 

Although it is well known that cyclic ethers are readily oxidized to the corresponding lactols or lactones, their asymmetric desymmetrization was not examined until quite recently. However, desymmetrization of prochiral or meso-cydic ethers is expected to be a useful tool for organic synthesis, since many prochiral or raeso-cyclic ethers are available in bulk. Recently, Miyafuji and Katsuki have reported the desymmetrization of 4-terf-butylcyclotetrahydropyran and meso-tetrahydrofurans with the chiral (salen)manganese(III) complex 13 as catalyst (Scheme 9) [24,25]. The oxidation of the former shows only the modest enantioselectivity, while the reaction of the latter exhibits excellent enantioselectivity. The low enantioselectivity (48% ee) observed in the oxidation of 4-ferf-butyltet-rahydropyran has been attributed to the participation of enantiomeric twist-boat conformers. Although 4-ferf-butyltetrahydropyran exists in an equilibrium mixture of chair and enantiomeric twist-boat conformers and the equilibrium ratio of the latter is very small, the latter is considered to be more reactive than the former for stereo electronic reasons. One of a-C-H bonds in the twist-boat conformer almost eclipses the -orbital while those in the chair conformer are gauche or anti to the -orbital. 

The amidation of saturated C—H bonds can be effectively catalyzed by ruthenium or manganese complexes. Unfunctionalized hydrocarbons, such as adamantane, cyclohexene, ethylbenzene, cumene, indane, tetralin, diphenylmethane and others, are selectively amidated with PhINTs in the presence of ruthenium or manganese porphyrins or the ruthenium cyclic amine complexes to afford N-substituted sulfonamides in 80-93% yields with high selectivity [807]. The enantioselective amidation of a C—H bond can be catalyzed by chiral (salen)manganese(III) complexes (e.g., 660) [808], or by chiral ruthenium(II) and manganese(III) porphyrins (Scheme 3.264) [809]. 

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). 

Several catalysts that can effect enantioselective epoxidation of unfunctionalized alkenes have been developed, most notably manganese complexes of diimines derived from salicylaldehyde and chiral diamines (salens).62... 

An asymmetric C-H insertion using a chiral 3,3, 5,5 -tetrabromosubstituted (salen)manganese(m) complex 107 with TsN=IPh afforded insertion products with ee up to 89%.258 Che reported the first amidation of steroids such as cholesteryl acetate with (salen)ruthenium(n) complexes 108.259... 

Chiral manganese complexes have been used to perform the enantioselective amidation of saturated C-H bonds.256-258,262 Cationic Mn(salen) 107 showed good catalytic activity and moderate enantioselectivity. Typical examples are shown in Equations (86)-(88). High enantioselectivity of 89% ee was obtained in the reaction of 1,1-dimethylindan (Equation (88)).258 Chiral manganese(m) porphyrin 106 was used in the enantioselective amidation as well nevertheless, the best enantioselectivity was only 54% (Equation (89)).256,257... 

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]). 

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]. 

A titanium complex derived from chiral /V-arencsulfonyl-2-amino-1 -indanol [20], a cationic chiral iron complex [21], and a chiral oxo(salen)manganese(V) complex [22] have been developed for the asymmetric Diels-Alder reaction of oc,P-unsaturated aldehydes with high asymmetric induction (Eq. 8A.11). In addition, a stable, chiral diaquo titanocene complex is utilized for the enantioselective Diels-Alder reaction of cyclopentadiene and a series of a.P Unsaturated aldehydes at low temperature, where catalysis occurs at the metal center rather than through activation of the dienophile by protonation. The high endo/exo selectivity is observed for a-substituted aldehydes, but the asymmetric induction is only moderate [23] (Eq. 8A. 12). 

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