Chiral Lewis Acids with Two Boron Centers

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A chiral bimetallic Lewis acid, 2,2'-(1,2-phenylene)bis[(4R,5R)-4,5-diphenyl-1,3,2-dioxaborolane](1), has been synthesized. The exceptionally strong binding of 1 with benzylamine was demonstrated by titrations. The complex formation ratio of 1 : 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<<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 1 ; 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 1-phenylethylamine has been obtained with 1, the clear separation of the peaks of the amine provides the possibility to use 1 as an NMR chiral-shift reagent.In the presence of another chiral bimetallic Lewis acid, 2,2'-{(1R,2R)-1,2-ethylene}bis(4-aryl-1,3,2-dioxaborolan-5-one)(2), acetophenone and its derivatives were reduced by LiBH_4, in moderate to good ee's(up to 99% ee). Amino group in the substrates affected on the enantioselectivities.