CHIRAL BIMETALLIC BORONIC ESTERS - A DONOR-ACCEPTOR COEXISTING RECEPTOR FOR AMINES

A chiral bimetallic Lewis acid, lene)bis[(4R,5R)-4,5-diphenyl-1,3,2-di oxaborolane] (1a), has been synthesized. The exceptionally strong bind ing of la with benzylamine was demonstrated by titrations. The complex formation ratio of 1a:amine=1:2 was determined by a Job plot. The bin ding constants, K-1 and K-2, were determined by non-linear curve fitti ng 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 on e of the two boron atoms of la; at the same time, one NH proton intera cts with one of the two oxygen atoms in the other dioxaborolane ring t o form a hydrogen bond. As a result, the two dioxaborolane rings are c onformationally fixed by two-point binding to provide a preferable bin ding 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.