Chiral Boron Receptors

The boronic acid 20 reported by Shinkai is a rare example of an enantiopme chiral redox-active receptor." It was found to bind linear and cyclic sugars in aqueous solution at neutral pH due to the reversible reaction between the boronic acid moiety of the protonated ferrocene receptor and the sugar to form a boronate ester. Electrochemical studies revealed that complexation imparted cathodic (ca. - 50 mV) shifts in the Fc /Fc redox couple. Interestingly, moderate enantioselectivity was observed in that (+ ) — 1 bound the linear saccharide D-sorbitol more strongly than L-sorbitol (ATd/AIl = 1-4). However, differences in the electrochemical response to the binding of these enantiomers by 20 were not apparent. 

Our first elDA was developed for chiral or-hydroxy-carboxylates (Figure 35) and vicinal diols using chiral aryl-boronic acids as receptors. The reversible interaction between arylboronic acids and a-hydroxycarboxylates or diols has been described in the previons section. Our original design is based on the hypothesis that the incorporated stereocenters near the postulated B-N dative bond in the receptors would induce enantioselectivity for the binding between the receptor and two enantiomers of the analyte. However, extensive studies reveal that the proposed B-N dative bond does not exist to an appropriate extent in protic sol vents.A small library of achiral and chiral phenylboronic acids (66, 78-80) was synthesized, and... 

Zhao and co-workers describe boronic acid based systems for enantioselec-tive fluorescent recognition. The scaffold used for the chiral sensors employs a fluorophore, arylboronic acid binding sites (two are best) and a linker (of appropriate size for the guest) between the two receptors. 

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