Pyrrolidines ionic organocatalysts

For the six-membered-ring cyclic ketones, many effective organocatalysts [36] have been developed in the past decade (as shown in Scheme 5.17 for selected examples). These catalysts proved to be effective for a broad range of aryl nitroalkenes, and some of the reactions could be conducted using alcohol or even water as solvent. Interestingly, alkyl nitroalkene was also suitable electrophile when 29 [36b] was used as the catalyst. On the other hand, the lluorous (S)-pyrrolidine sulfonamide 30 [36c] and functionalized chiral ionic liguids 31 [36d] were reusable. 

Chiral 1,2,3-triazolium ionic liquid tethered pyrrolidine catalysts built from (S)-proline and its derivatives have been successfully ap>plied in various catalytic reactions (Khan, Shah et al. 2010 Khan, Shah et al. 2010 Maltsev, Kucherenko et al. 2010 Yacob, Shah et al. 2008). The 1,2,3-triazolium ionic liquid-tagged organocatalysts derived from proline and its derivatives are mostly viscous liquids at room temperature and are completely miscible with polar solvents such as methanol, chloroform, acetonitrile, dimethylsulfoxide, dimethylformamide and water. They are insoluble in less-polar solvents such as n-hexane and diethyl ether. In some cases, the ionic liquid sub-unit serves not only as a phase tag for efficient recycling but also as an effective chiral amplifier through polar interactions and steric shielding. 

Triazolium ionic liquid tethered pyrrolidine organocatalysts 25, 26, 30 - 32, 38, 44, 48-50, 58 were synthesised in a simple emd versatile way from (S)-proline or trans 4-hydroxy-(S)-proline. The synthesis of these catalysts involves primarily the preparation of the corresponding azide and terminal alkyne derivatives for dick cydoaddition reaction. The ionic liquid products were obtained quantitatively with a straightforward two-step procedure by N-alkylation of the click reaction products. The protecting group are finally removed to liberate the target catalyst. 

Yacob, Z. J. Shah J. Leistner and J. Liebscher (2008). "(S)-pyrrolidin-2-ylmethyl-l,2,3-tiiazolium salts - Ionic liquid suprported organocatalysts for enantioselective Michael additions to beta-nitrostyrenes." Synlett, No.l 5, (Sep 15, 2008), p>p. 2342-2344, ISSN 0936-5214... 

The O-TMS-diphenylprolinol 103/BzOH catalytic system is also applicable as an organocatalyst of Michael reactions of nitroalkanes with a, i-enals in aqueous medium [117]. However, functionalized task-specific ionic liquids incorporated in the chiral-pyrrolidine unit, apart from being very efficient and versatile organo-catalysts of Michael and some other asymmetric reactions, show much worse behavior in asymmetric aldol reactions, where their performance is inferior to IL-supported catalysts bearing the a-amino acid fragment [118]. 

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