Chiral Mn salen,

Song and Roh investigated the epoxidation of compounds such as 2,2-dimethylchromene with a chiral Mn (salen) complex (Jacobsen catalyst) in a mixture of [BMIM][PFg] and CH2CI2 (1 4 v/v), using NaOCl as the oxidant (Scheme 5.2-12) [62]. 

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... 

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. 

Only a few years after the development of the homogeneous chiral Mn(salen) complexes by Jacobsen and Katsuki, several research groups began to study different immobiUzation methods in both liquid and soUd phases. Fluorinated organic solvents were the first type of Uquid supports studied for this purpose. The main problem in the appUcation of this methodology is the low solubility of the catalytic complex in the fluorous phase. Several papers were pubUshed by Pozzi and coworkers, who prepared a variety of salen ligands with perfluorinated chains in positions 3 and 5 of the saUcyUdene moiety (Fig. 2). 

Burrow et al. examined aziridination with chiral Mn(salen) in the presence of PhI=NTs, but no enantioselectivity was observed.160 However, Katsuki et al. reported that the aziridination of styrene with complex (52) showed moderate enantioselectivity, though the chemical yield was poor (Scheme 38).161 Remarkable improvements of both enantioselectivity (up to 94% ee) and chemical yield have been achieved by using a new type of Mn(salen) (53) as the catalyst.162... 

The synthesis of the first polymer-supported chiral Mn-salen derivatives was reported independently by Sivaram171 and Minutolo.171-173 Different monomeric Jacobsen-type units, containing two polymerizable vinyl groups, were copolymerized with styrene and divinylbenzene to yield the corresponding cross-linked polymers as a monolithic compact block.174-176 The less mobile system (Figure 19) with no spacer between the aromatic ring and the polymer backbone is less enantioselective. 

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... 

Reversal of the conformation of the chiral Mn-salen complex forces the substituents on the ethylenediamine moiety to take the disfavored axial position. This disfavored conformation (Fig. 4-8A) should be stabilized by the co-... 

A study of electro-assisted biomimetic activation of molecular oxygen by a chiral Mn(salen) complex in [BMIMJPFs showed that a highly reactive oxomanganese(V) intermediate could transfer its oxygen to an alkene (229). 

Chiral Mn-salen complex 173a in the presence of A-methylimidazole, which serves as axial ligand and H2O2 as oxygen source, has been employed by Pietikainen for the... 

SCHEME 91. Asymmetric epoxidation of 1,2-dihydronaphthalene using chiral Mn-salen complexes 173b and 173c... 

Chiral (salen) Mn(III) complexes have been found to be highly enantioselective for the asymmetric epoxidation of conjugated cis-disubstituted and trisubstituted oleftns[10]. The increasing interest towards this reaction brought some authors to develop the heterogeneous chiral salen catalysts. However, to date three kinds of approach have been adopted for the immobilization of chiral salens (1) Chiral Mn salen complexes were supported on polymers[l 1], (2) The encapsulation of salen complex using ship-in-bottle method was... 

PhlO, cat chiral Mn salen complex, pyridine N-oxide (enantioselective)... 

The syntheses of most chiral Mn-salen complexes are simple, and thus their recovery and reuse have not been studied extensively. Only a few studies of epoxidation with a polymer-bound Mn-salen complex or a Mn-salen complex embedded in nano-porous materials as catalyst has been performed. It has been disclosed, however, that the microenvironment provided by the macromolecule adversely affects the asymmetric induction by the Mn-salen catalyst to a considerable extent although reuse of the catalysts for several cycles are realized [68]. 

The Jacobsen Epoxidation allows the enantioselective formation of epoxides from various -substituted olefins by using a chiral Mn-salen catalyst and a stoichiometric oxidant such as bleach. 

Sun, W., H. Wang, C. Xia, J. Li and P. Zhao, Chiral-Mn(Salen)-Complex-Catalyzed Kinetic Resolution of Secondary Alcohols in Water, Angewandte Chemie International Edition, 42, 1042-1044 (2003). 

Cyclic and acyclic enol derivatives 480 can be asymmetrically aziridinated with (A -tosylimino)iodobenzene 481 using a chiral copper catalyst prepared in situ from [Cu(MeCN)4]PF6 and the optically active ligand 479. Collapse of the aminal (i.e., 482) leads to the formation of enantiomerically enriched Q-amino carbonyl compounds 483, although ee s to date are modest <2000EJ0557>. Similarly, dienes can be selectively aziridinated using the chiral Mn-salen complex 484 to give vinyl aziridines 486 in scalemic form (Scheme 124) <2000TL7089>. 

The report by Kochi and co-workers in 1986 that a (salen)manganese(lll) complex (Mn(salen) complex) was an efficient epoxidation catalyst for simple olefins <1986JA2309> quickly led to independent reports from the groups of Jacobsen <1990JA2801> and Katsuki <1990TL7345> that chiral Mn(salen) complexes could catalyze asymmetric epoxidation reactions. The reaction requires the use of a stoichiometric oxidant initially iodosylarenes were utilized, but it was quickly found that NaOCl was also successful. 

Kureshy, R. I., Khan, N.-u. H., Abdi, S. H. R., Singh, S., Ahmed, I., Shukla, R. S., Jasra, R. V. Chiral Mn " salen complex-catalyzed enantioselective epoxidation of nonfunctionalized aikenes using urea-H202 adduct as oxidant. J. Catal. 2003, 219,1-7. 

Katsuki has studied asymmetric epoxidation of non-functionalized olefins catalyzed by chiral Mn(salen) complex. Recently they proposed that the ligands of Mn(salen) complexes take non-planar stepped conformation and the direction of the folding ligands is strongly related to the sense of chirality in the asymmetric epoxidation (Eq. (7.26)) [71]. On the basis of this proposal, conformational con-... 

An alternative method for the immobihzation of chiral Mn(salen) complexes is to use crosslinked PS having hydroxyphenyl groups or sulfonate groups. Chiral Mn(salen) complexes were immobihzed axially onto these polymers by phenoxy or phenyl sulfonic groups, as shown in Scheme 3.41 [78]. In the presence of these polymeric complexes 140 the asymmetric epoxidahon of 141 and 132 occurred smoothly to give the product with comparable or even higher enanhoselectivihes as compared to those obtained with homogeneous catalysts. 

Sahoo S, Kumar P, Lefebvre F, Halligudi SB (2(X)9) Oxidative kinetic resolution of alcohols using chiral Mn-salen complex immobilized onto ionic hquid modified silica. Appl Catal A Gen 354 17-25... 

A series of five- and six-coordinate chiral [Mn (salen)] complexes known to be effective AE catalysts has been characterized by X-ray crystallography [56]. Comparison of some of these structures revealed that the ligand geometry around the metal center and the chiral diimine backbone remained remarkably constant, while the salicylidene regions of the complexes adopted a wide range of conformations. The conformational variations observed in the solid state likely reflect accessible solution conformations of the [Mn (salen)] complexes and possibly their oxo counterparts. 

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