Onium-tagged proline

More recently, greater chemical yields, higher enantioselectivities and more efficient recycles were obtained on performing the same reaction in ILs. Two onium-tagged proline catalysts were employed (Scheme 4.5), evidencing the superior catalytic efficiency of ionic media for this kind of reaction. 

Scheme 4.5 Asymmetric aldol reaction using onium-tagged proline as catalyst.

Figure 4. Straightforward synthesis of onium ion-tagged prolines.

Onium ion-tagged proline catalysts 7 and 8 in [bmini][Tf2N] proved to be an excellent catalytic system for the direct asymmetric aldol reaction. The catalytic protocol developed makes use of use of a 6-fold lower amount of catalyst with respect to the proceeding reports based on the use of proline 12 and affords greater chemical yields and higher enantioselectivity. In particular, 8 in [bmim][Tf2N] gave better results compared both to the use of proline in the same IL and of 7 in DMSO. ... 

For example, onium ion-tagged prolines have been synthesized and their catalytic activity in direct asymmetric aldol condensation was initially studied in molecular solvents.The observed superior performance of IL-proline relative to Peg-proline or free proline, in terms of yields and enantiomeric excess, evidenced that the ionic moiety in IL-proline plays more than a silent or simply supporting role in the reaction (Scheme 4.4). ... 

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