Sugar ‘Chirality’ Sensing Using a ‘Prochiral’ Salen–Co(II) Complex

A prochiral salen–Co(II) complex (6) bearing two boronic acid groups was synthesized. The saccharide-binding event was conveniently monitored by a circular dichroism (CD) spectral change. The exciton-coupling-type CD spectra thus obtained were compared with those of chiral salen–Co(II) complexes, (R)-4 and (R)-5. Very interestingly, monosaccharides which are selectively bound to (R)-4 and (R)-5 with P helicity generate the P helicity CD spectra in prochiral 6. This means that the two boronic acid groups in (R)-4 and (R)-5 are chirally preorganized suitable to the binding of these monosaccharides. Reflecting the short distance between the two boronic acid groups, 6 shows the very high selectivity for talose which has all OH groups arranged on the same side of the pyranose ring. These results indicate that 6 is useful for sugar ‘chirality’ sensing at visible wavelength region and prediction of d/l selectivity for (R)-4 and (R)-5.