Separation of enantiomers of chiral fullerene derivatives through enantioselective encapsulation within an adaptable helical cavity of syndiotactic poly(methyl methacrylate) with helicity memory

Abstract

Optically active left (M)- and right (P)-handed helical syndiotactic poly(methyl methacrylate)s (M- and P-st-PMMAs) with a helicity memory enantioselectively encapsulated the racemic C₆₀ derivatives, such as 3,4-fulleroproline tert-butyl ester (rac-1) and tetraallylated C₆₀ (rac-2), as well as the C₆₀-bound racemic 3₁₀-helical peptides (rac-3) within their helical cavities to form peapod-like inclusion complexes and a unique “helix-in-helix” superstructure, respectively. The enantiomeric excess (ee) and separation factor (enantioselectivity) (α) of the analyte 1 (ee = 23%–25% and α = 2.35–2.50) encapsulated within the helical cavities of the M- and P-st-PMMAs were higher than those of the analytes 2 and 3 (ee = 4.3%–6.0% and α = 1.28–1.50). The optically pure (S)- and (R)-1 were found to more efficiently induce an excess one-handed helical conformation in the st-PMMA backbone than the optically pure (S)- and (R)-1-phenylethylamine, resulting in intense mirror-image vibrational circular dichroism (VCD) spectra in the PMMA IR regions. The excess one-handed helices induced in the st-PMMAs complexed with (S)- and (R)-1 were memorized after replacement with the achiral C₆₀, and the complexes exhibited induced electric CDs in the achiral C₆₀ chromophore regions.