Asymmetric resolution in ester reduction by NaBH4 at the interface of aqueous aggregates of amino acid, peptide, and chiral-counterion-based cationic surfactants

This study provides insight into the physicochemical aspects of aqueous aggregates that comprise amino acid, peptide, and chiral-counterion-based cationic surfactants and their correlation with the proficiency of asymmetric resolution in ester reduction. The effects of the structural differences in the naturally occurring amino acid based and synthetic chiral-counterion-containing gemini surfactants on the surface properties as well as on other microstructural parameters were studied and correlated to the varied head groups of the surfactants. The supramolecular chirality induced from the head-group region of chiral amphiphiles in aqueous self-aggregates is evident from circular dichroism, scanning electron microscopy, and transmission electron microscopy studies. This large-scale chirality at the interface of self-aggregates was exploited towards asymmetric resolution in ester reduction by NaBH4. An enantiomeric excess of 53% ((R)-2-phenylpropan-1-ol) was found in the case of the n-hexyl ester of 2-phenylpropionic acid as substrate in the aqueous aggregate of N,N'-dihexadecyl-N,N,N',N'-tetramethyl-N,N'-ethanediyldiammonium diquinate. Thus, a simple and environmentally benign pathway for asymmetric resolution in ester reduction by sodium borohydride alone is reported, which utilizes the varied spatial asymmetry at the interface of aqueous aggregates of cationic chiral amphiphiles.