Chiral bimetallic boronic esters: A donor-acceptor coexisting receptor for amines

A chiral bimetallic Lewis acid, 2,2'-(1,2-phenylene)bis[(4R,5R)-4,5-diphenyl-1,3,2-dioxaborolane] (1a), has been synthesized. The exceptionally strong binding of la with benzylamine was demonstrated by titrations. The complex formation ratio of 1a:amine=1:2 was determined by a Job plot. The binding constants, K-1 and K-2, were determined by non-linear curve fitting to be K-1 much less than K-2. The results can be explained in terms of an allosteric effect. The first amine molecule coordinates with one of the two boron atoms of la; at the same time, one NH proton interacts with one of the two oxygen atoms in the other dioxaborolane ring to form a hydrogen bond. As a result, the two dioxaborolane rings are conformationally fixed by two-point binding to provide a preferable binding site for the second amine molecule. Although only a small chiral recognition of l-phenylethylamine has been obtained with 1a, the clear separation of the peaks of the amine provides the possibility to use 1a as an NMR chiral-shift reagent.