Salen Mn complexes

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... 

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). 

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... 

Figure 21 Kureshy s polymer-based Mn-salen complex.

Although salen complexes of chromium, nickel, iron, ruthenium, cobalt, and manganese ions are known to serve as catalysts for epoxidation of simple olefins, the cationic Mn-salen complex is the most efficient. 

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. 

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-... 

Deng and Jacobsen38 used Mn-salen complex for the asymmetric epox-idation of ethyl cinnamate. Over 95% ee was obtained for the epoxide compound (Scheme 7-81). 

The reagent PhI=NTs can be drawn in the resonance form Phi-NTs, where its resembalnce to CIO-becomes clear. Moreover, the issues of the square planar coordination sphere of the Mn(salen) complex don t exist with Cu(II), so a very simple mechanism can be drawn coordination of the N of the reagent to Cu(II), displacement of Phi by a lone pair on Cu to give a Cu(IV)=NTs reagent, [2 + 2] addition to the alkene, and reductive elimination. 

C. Bowers, and P. K. Dutta, Olefin oxidation by zeolite-encapsulated Mn(salen)" complexes under ambient conditions, J. Catal. 122, 271—279 (1990). 

Figure 11.3 Enantioselective epoxidation of alkenes by Mn salen complexes.

Katsuki and coworkers have developed a family of salen-metal complexes capable of effecting a C—H oxidation at activated positions. meso-Tetrahydrofurans may be oxidized to the lactol in good yield and excellent enantioselectivity using iodosylbenzene as the stoichiometric oxidant and a Mn-salen complex as catalyst [Eq. (10.45)]. " Meso acylpyrrolidines behave similarly, providing slightly lower enantioselectivities using a similar catalyst [Eq. (10.46)]d ... 

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). 

Without additives, radical formation is the main reaction in the manganese-catalyzed oxidation of alkenes and epoxide yields are poor. The heterolytic peroxide-bond-cleavage and therefore epoxide formation can be favored by using nitrogen heterocycles as cocatalysts (imidazoles, pyridines , tertiary amine Af-oxides ) acting as bases or as axial ligands on the metal catalyst. With the Mn-salen complex Mn-[AI,AI -ethylenebis(5,5 -dinitrosalicylideneaminato)], and in the presence of imidazole as cocatalyst and TBHP as oxidant, various alkenes could be epoxidized with yields between 6% and 90% (in some cases ionol was employed as additive), whereby the yields based on the amount of TBHP consumed were low (10-15%). Sterically hindered additives like 2,6-di-f-butylpyridine did not promote the epoxidation. 

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... 

Other studies on the chromium(in)-salen catalysts of type 10 have shown that the 3 -substituent (i.e., R) exhibits relatively little sensitivity with regard to chiral induction, in stark contrast to the analogous Mn-salen complexes, in which the 3 -position must bear a sterically bulky group for acceptable enantiomeric excesses. Thus, the 3 -chloro catalyst with a triphenyl-... 

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