Michael additions chiral ionic liquids

Ni, B., Zhang, Q. and Headley, A.D. (2007) Functionalized chiral ionic liquid as recyclable organocatalyst for asymmetric Michael addition to nitrostyrenes. Green Chem., 9 (7), 737-739. 

In terms of chiral ionic liquids, the discussion will focus on Michael addition reactions (Figure 6.8). This reaction has been performed using lactate as... 

Figure 6.8 Asymmetric Michael addition reactions using chiral ionic liquids (a) derived from lactate, (b) derived from proline.

Wang Z M, Wang Q, Zhang Y, et al. Synthesis of new chiral ionic liquids from natural acids and their applications in enantioselective michael addition. Tetrahedron Lett. 2005. 46, 4657-4660. 

In addition, several supported chiral ionic liquids have been investigated for these reactions. Therefore, Wang et al. have developed a new type of polymer-immobilised pyrrolidine-based chiral ionic liquids, which were capable of inducing the Michael addition of ketones to nitrostyrenes in high yields. 

A chiral phase transfer catalyst was dissolved in ionic liquid media for the enantioselective Michael reaction of dimethyl malonate with l,3-diphenylprop-2-en-l-one with K2CO3 203). The phase-transfer catalyst was a chiral quininium bromide (Scheme 20). The reaction proceeded rapidly with good yield and good enantioselectivity at room temperature in all three ionic liquids investigated, [BMIM]PF6, [BMIM]BF4 and [BPy]BF4. In the asymmetric Michael addition, the enantioselectivity or the reaction in [BPy]Bp4 was the same as in conventional organic solvents. 

Scheme 19. Synthesis of camphor based ionic liquids by Michael addition and asymmetric hydrogenation. An example of transfer of the chiral infor-mation through ion-pairing effects.

Table 7.14 Asymmetric Michael addition of cyclohexanone to P-nitrostyrene using ionic liquid-supported chiral pyrrolidines as a catalyst.

However, despite the rapid design of news CILs, successful applications in synthesis remained elusive for some time. Only in the last 2-3 years have some significant results been obtained. Leitner and co-workers in 2006 reported a high enantiomeric excess (84% ee) by using a chiral anion containing ionic liquid for an aza-Baylis-Hillman reaction (Scheme 4.11), whereas CILs with an imidazolium or a benzimidazolium unit attached to (5)-pyrrolidine have been used with success as solvents or catalysts for asymmetric aldol reactions and Michael additions to nitroolefins (ee up to 99%). ... 

Kitazume et al. examined the first example of Michael additions via the MBH-type reaction that used 3-fluoromethylprop-2-enamide as a chiral auxiliary electrophile towards activated olefins in the DABCO ionic liquid system. The reaction of (45)-3-[( )-4,4,4-trifluorobut-2-enoyl]-4-isopropyl-2-oxazolidinone (297) with activated vinyl moiety proceeded smoothly at 80 °C to give the corresponding adducts 298 in moderate yields, albeit with low diastereoselectivity (Scheme 1.109). 

Instead of using chloramine-T (pKa 13.5), the employment of more nucleophilic chloramine salt, A-chloro-A-sodiobenzyloxycarbamate (pKa 15.3), allows for an efficient aziridination of electron-deficient olefins (Michael acceptors) in the presence of a solid-liquid phase-transfer catalyst (Scheme 2.38) [57]. The reaction would involve an ionic pathway where the Michael-addition of chloramine salt to alkenes and the following back-attack of the resulting enolate at the electrophilic N-center to cyclize. This reaction was successfully extended to the asymmetric aziridination of the enones that have an auxiliary, to produce chiral aziridines with good enantioselectivities up to 87% ee. Another option to aziridinate electron-deficient alkenes is the utilization of... 

Truong and Vo-Thanh (2010) developed an efficient method for the synthesis of functionalized chiral ammonium imidazolium and pyridinium based ionic liquids derived from (IR, 2S) ephedrine by solvent-free asymmetric Michael addition under microwave irradiation with good yields in very short reaction time. 

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