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  • Chiral organocatalysts based on lipopeptide micelles for aldol reactions in water.

Chiral organocatalysts based on lipopeptide micelles for aldol reactions in water.

Physical chemistry chemical physics : PCCP (2016-12-13)
B M Soares, A M Aguilar, E R Silva, M D Coutinho-Neto, I W Hamley, M Reza, J Ruokolainen, W A Alves
ABSTRACT

A comprehensive study of the self-assembly in water of a lipopeptide consisting of a sequence of l-proline, l-arginine and l-tryptophan with a hydrocarbon chain has been performed. Fluorescence assays were used to determine the critical aggregation concentration. In situ small-angle X-ray scattering (SAXS) and molecular dynamics simulations showed the presence of spherical micelles with diameters around 6 nm. In agreement with these results, cryo-TEM images showed globular aggregates with diameters ranging from ≈4 nm up to ≈9 nm. Furthermore, the lipopeptide catalytic activity has been tested for the direct aldol reaction between cyclohexanone and p-nitrobenzaldehyde, and we have observed that the self-association of the organocatalyst played a critical role in the enhanced activity. Water affects the selectivity, and poor results are obtained under neat reaction conditions. The location of the catalytic groups at the lipopetide/water solvent interface also endowed unusual selectivity in the catalyzed aldol reactions. Under optimized reaction conditions, high yields (up to >99%), good enantioselectivity (ee up to 85%) and high diastereoselectivity (ds up to 92 : 8) were obtained.