Preparation of seven- and eight-membered boron heterocycles from different salen 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 difference that in ethanol mostly open bimetallic boronic esters are obtained, while in toluene or acetonitrile closed bimetallic complexes with a central seven- or eight-membered heterocyclic ring are formed. Both structural types have been characterized by spectroscopic techniques and X-ray crystallography showing that the reactions are diastereoselective in the case of the bimetallic heterocyclic structures. The configurations and conformations of the seven- and eight-membered rings are different, and this may depend on steric effects and/or repulsive intramolecular pi-pi interactions between the two salicylidene moieties attached to the central ring.
Sánchez
M
M
Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, C.P. 62210 Cuernavaca, Morelos, México.
Höpfl
H
H
Ochoa
M E
ME
Farfán
N
N
Santillan
R
R
Rojas
S
S