Pyrrolidine-Sourced Ionic Organocatalysts

1 Catalysts with the Ionic Appendix at Position 2 oTthe Pyrrolidine Ring 

In parallel, Xu and coworkers discovered that in the presence of similar I L-tagged pyrrolidines 84c,d the reaction proceeded nearly as efficiently in the [bmimJIPFs] medium, so there is no need to dehver the acid co-catalyst (TFA) to the system because this role is obviously played by the IL fragment [96]. Catalysts 84a,b were recovered from the reactant medium by precipitation with ether, while the 84c,d/ IL systems were reused after product extraction without further purification. Four reaction cycles of catalysts 84 did not reduce reaction diastereo- and enantioselec-tivities however, the recovered catalysts gradually became less active with each cycle. Surfactant-type IL-supported asymmetric organocatalyst 84e synthesized by Luo and Cheng and coworkers in 2006 catalyzed Michael addition to nitroalkenes with high stereoselectivities in water without any additives [97]. 

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]. 

In 2011, Hermeke and Toy synthesized phosphonium ion-tagged chiral phosphoric acids 117 and 118 bearing a BINAP core and examined them as recoverable organocatalysts in Friedel-Crafts alkylation of indoles [55], The phosphonium tag position in the catalysts was cmcial for the reaction outcome. Compound 117 with the tags at positions 3 and 3 of phosphoric acid, where they served as spaceblocking groups, failed to catalyze the reactions. However, a shift of the phosphonium ion groups to positions 6 and 6 produced efficient and enantioselective 

Schema 22.28 Pyrrolidine derivatives tagged to alkylimidazolium cations and their use as organocatalysts in asymmetric Michael reactions of carbonyl compounds with a-nitroalkenes. 

---