Ionic organocatalysts

Figure 11.5 Ionic organocatalysts 31 for the asymmetric aldol and Michael reactions in ionic liquids.

Ionic Organocatalysts Incorporating a-Amino Acid Units... 

Ionic Organocatalysts Incorporating oc-Amino Amide Units... 

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. 

Zhou, L. and Wang, L. (2007) Chiral ionic liquid containing L-proline unit as a highly efficient and recyclable asymmetric organocatalyst for aldol reaction. Chem. Lett., 36 (5), 628-629. 

By using of a modified proline, L-prolinamide 47 (which is known to be a more reactive catalyst than L-proline in cross-aldol reactions [80]), the enantioselectivity of the direct aldol reactions in ionic liquid [bmim][BF4] was remarkably increased as compared with the reaction carried out in acetone (69% ee) (Scheme 7.26) [81]. However, the reusability of the recovered 47 when immobilized in the ionic liquid layer was somewhat inferior to that of the L-proline catalyst this effect could be ascribed to the increased solubility of the organocatalyst 47 in the extracting organic solvents (not provided in the literature), leading to an increased leaching of the catalyst. 

Table 7.15 Asymmetric Diels-Alder reaction between cyclohexadiene and acrolein using organocatalyst 51 in ionic liquids.

Zheng X, Qian YB, Wang Y (2010) 2-Pytrotidinecarboxyhc add ionic liquid as a highly efficient organocatalyst for the asymmetric one-pot Mannich reaction. Eur J Org Chem 515-522... 

Recently, the reactivity (base, nucleophile, catalyst) and the role in modem organic synthesis of N-heterocyclic carbenes (cychc carbenes bearing at least one amino substituent) has been set-up by Ender et al. [60,61]. The possible utilization of NHCs as ligands for transition-metal catalyst [62,63] and as organocatalyst has been emphasized [61]. The inversion of the normal reactivity (umpolung) induced via NHCs has been extensively discussed. Classical carbon-carbon-bond-forming reactions (benzoin condensation, Stetter reaction, etc.) have been re-examined using ionic liquids as pre-catalysts in the presence of bases. 

The recent development of polymers containing imidazolium, thiazolium and related structures as supported equivalents of ionic liquids (i.e. 148 and 149, Scheme 10.23) is an interesting development of this field [356-361], The corresponding salts containing basic counter-anions can act as basic catalysts but can also be transformed, depending on the exact structure, into N-heterocydic car-benes that can act as nucleophilic organocatalysts [362]. As an example, resins 148... 

Sugar-hased prolinamide 16m has also been employed as catalyst for the asymmetric Michael addition of cyclohexanones to p-nitroslyrenes. During optimisation of the reaction conditions, the authors found that the polarity of the solvent does not modify the yield or stereoselectivity, but the best ee was obtained under neat conditions at -20 °C. Ammonium ionic liquids 41a,b are also efficient organocatalysts for the asymmetric Michael addition of aldehydes to nitro-olefins giving the adducts with excellent yields and enantioselectivities and modest to high diastereoselectivities. 

Extensive efforts have been made in recent years to prepare polymer or ionic-liquid-supported 4-hydro3qq3rolinamide derivatives that would combine the high catalytic activity and efficient stereocontrol of unsupported prolinamides with the significant water tolerance and recyclability of immobilised organocatalysts in useful catalytic transformations (Figure 10.5). 

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