Preparation of seven- and eight-membered boron heterocycles from differentsalen ligands and arylboronic acids

Arylboronic acids and different salen ligands have been brought to reaction in a 2:1 stoichiometry in ethanol, toluene, and acetonitrile. In all cases bimetallic boronates with chiral boron atoms could be isolated with the di fference that in ethanol mostly open bimetallic boronic esters are obtained , while in toluene or acetonitrile closed bimetallic complexes with a centr al seven- or eight-membered heterocyclic ring are formed. Both structural t ypes have been characterized by spectroscopic techniques and X-ray crystall ography showing that the reactions are diastereoselective in the case of th e bimetallic heterocyclic structures. The configurations and conformations of the seven- and eight-membered rings are different, and this may depend o n steric effects and/or repulsive intramolecular pi-pi interactions between the two salicylidene moieties attached to the central ring.