Chiral salen ligands

Non-covalently Immobilized Catalysts Based on Chiral Salen Ligands. . 152... 

Chiral salen ligands are diimines of salicylaldehydes with chiral diamines, usually cyclohexane-1,2-diamine (salen Hgands 1) or 1,2-diphenylethylene-diamine (salen ligands 2). The most widely used salen ligand in homogeneous catalysis is probably Jacobsen s ligand (la. Fig. 2), which is commercially available and hence has been used as reference to compare the results of im-... 

Fig. 5 Chiral salen ligand with improved solubility in ionic liquids...

Fig. 8 Symmetrical chiral salen ligands used in solid phase immobilization...

Fig. 14 Chiral salen ligands with charged substituents...

The chiral diol 17 derived from tartaric acid is exploited in the titanium-catalyzed asymmetric pinacol coupling in the presence of Zn and MesSiCl to give the corresponding diol in 11-71 ee % [44], The chiral salen ligands 18-20 are used in the titanium-catalyzed enantioselective coupling reaction, which achieves the higher selectivity [45-47]. The chromium complex with TBOxH (21) efficiently catalyzes the asymmetric coupling reaction of both aromatic and aliphatic aldehydes [48]. 

Schaus et al.41 have also reported an asymmetric hetero Diels-Alder reaction of Danishefsky s diene 10042 with aldehyde 101 catalyzed by chromium(III) complex 99 bearing a similar chiral salen ligand. Product 102 is obtained in moderate to good yield and stereoselectivity (Scheme 5-31 and Table 5-5). 

Figure 5.18 Styryl-functionalized chiral salen ligands used in the crosslinking radical suspension polymerization with styrene to generate Mn epoxidation catalysts [71].

Fig. 6 shows the diffuse reflectance UV-visible spectra of typical salen Co(IH) complexes immobilized on MCM-41, the homogeneous salen complex of the same structure, and a pure Si-MCM-41. The chiral salen ligands of Co(IU) form showed the bands at near 250 and 370 nm on the UV spectra. But the pure siliceous MCM-41 no absorption peak at all. This broad band is probably due to the charge-transfer transitions between metal and ligand. This result indicates that the successful immobilization of chiral salen ligands was achieved. 

Mn(III) complexes with C2 chiral salen ligands (1-8 mol %) also... 

Chiral (salen)Mn(III)Cl complexes are useful catalysts for the asymmetric epoxidation of isolated bonds. Jacobsen et al. used these catalysts for the asymmetric oxidation of aryl alkyl sulfides with unbuffered 30% hydrogen peroxide in acetonitrile [74]. The catalytic activity of these complexes was high (2-3 mol %), but the maximum enantioselectivity achieved was rather modest (68% ee for methyl o-bromophenyl sulfoxide). The chiral salen ligands used for the catalysts were based on 23 (Scheme 6C.9) bearing substituents at the ortho and meta positions of the phenol moiety. Because the structures of these ligands can easily be modified, substantia] improvements may well be made by changing the steric and electronic properties of the substituents. Katsuki et al. reported that cationic chiral (salen)Mn(III) complexes 24 and 25 were excellent catalysts (1 mol %) for the oxidation of sulfides with iodosylbenzene, which achieved excellent enantioselectivity [75,76]. The best result in this catalyst system was given by complex 24 in the formation of orthonitrophenyl methyl sulfoxide that was isolated in 94% yield and 94% ee [76]. 

In conclusion, mention should be made of dendronised chiral salen ligands and their Co2+ and Ni2+ complexes, which were prepared for use as Jacobsen-type catalysts by diaza-Cope rearrangement [24]. However, instead of the meso compounds produced, the enantiomers will have to be synthesised directly in enantioselective manner (Fig. 4.69). 

Fig. 4.69 Chiral salen ligands (according to Vogtle, Portner)...

However, Jacobsen was able to show that, after addition of a pyridine JV-oxide derivative, trisubstituted alkenes are in fact excellent substrates. The dissymmetry of the chiral salen ligand can effectively orient the radical selectivity ... 

Hutchings el al. (102,103) synthesized enantioselective Mn catalysts by adsorption of chiral salen ligands on Mn-exchanged Al-MCM-41 and used the materials in selective epoxidation of dv-stilbcnc with PhlO. Although the simplicity of this approach is appealing, reuse of the material in a second run led to a dramatic decline of the epoxide yield from 69 to 18%, and of the ee of the trans epoxide from 70 to 30%. 

These experiments demonstrate that the reaction occurring with Mn-exchanged Al-MCM-41 salen is wholly heterogeneously catalysed. At this stage we have made no attempt to optimise the catalytic performance, but we anticipate that appropriate modification of the chiral salen ligand and the reaction conditions will lead to enhanced reactivity and enantioselection. 

The catalytic asymmetric epoxidation of alkenes offers a powerful strategy for the synthesis of enantiomerically enriched epoxides and enantioselective oxidation reactions in ionic liquids have been summarised previously.[39] Complexes based on chiral salen ligands - usually with manganese(III) as the coordinated metal - often afford excellent yields and enantioselectivities and the catalytic cycle for the reaction is depicted in Scheme 5.5 J40 ... 

Epoxidation of alkenes with complex of a chiral salen ligand and manganese(III), 1.69 or 1.70, is known as Jacobsen epoxidation ... 

Asymmetric aziridination can also be accomplished via chiral salen ligands. Shi has synthesized a number of axially dissymmetric binaphthyldiimine salen complexes that have shown excellent facility in catalytic asymmetric aziridination reactions <2001TA3105>. Although yields were generally good with acyclic electron-deficient olefins, the chemical yield with electron-rich olefin indene was relatively low (25%). A reasonable enantiomeric excess of 73% was achieved at —20°C over a 24h reaction period (Equation 9). 

Jacobsen reported in 1990 that Mnm complexes of chiral salen ligands (41) were the most efficient catalysts available for the enantioselective epoxidation of alkyl- and aryl-substituted olefins.118 This stimulated a rapid development in the chemistry and applications of chiral SB complexes, which offer promising catalytic applications to several organic reactions, such as enantioselective cyclopropanation of styrenes, asymmetric aziridination of olefins, asymmetric Diels-Alder cycloaddition, and enantioselective ring opening of epoxides.4,119... 

Metalated chiral salen ligands were first introduced during the 1990s by Jacobsen and Katsuki as highly enantioselective catalysts for the asymmetric... 

A catalytic asymmetric cyanosilylation of carbonyl compounds with Me SiCN using either a carbohydrate-ba.sed phosphine oxide (54) " or the monolithium salt of a chiral salen ligand " has been studied. Comparing to the BINOL analogues, the reaction involving 54 does not require phosphine oxide additives to attain high levels of asymmetric induction, the catalytic activity is higher, and Me SiCN can be introduced rapidly. 

Meunier has reviewed recent advances in asymmetric oxidation. Jacobsen s asymmetric epoxidation catalysts are some of the most successful. These use Mn(III) in a chiral salen ligand with NaOCl as primary oxidant. The intermediacy of Mn(V) 0x0 species has been proposed as the active species formed after O atom transfer from the hypochlorite. Enantiomeric excesses of 97-98% are seen in the epoxide product on a consistent basis across a wide variety of alkene substrates. 

The efficiency of new unsymmetrical chiral salen ligands was examined in the asymmetric trimethylsilylcyanation of benzaldehyde. A very high level of enantioselectivity was attainable over chiral Ti(IV) salen complexes prepared from salicylaldehyde and 3,5-Di-/ert-butylsalicylaldehyde derivative as compared to the conventional salen catalyst. Enantiomeric excess of the corresponding reaction product was generally more than 70% over unsymmetric chiral salen catalysts. The chiral Titanium(IV) salen complexes immobilized on a mesoporous MCM-41 by multi grafting method showed a relatively high enantioselectivity for the addition of trimethylsilyl cyanide to the benzaldehyde. 

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