Synthesis and comparative study of three monomeric boronates by spectroscopic methods and X-ray crystallography

Three boronates have been synthesized by reaction of tridentate azomethine ligands derived from salicylaldehyde with phenylboronic acid. The [5.4.0], [4.4.0], and [4.3.0]heterobicyclic structures obtained have been characterized by NMR spectroscopy and X-ray crystallography, whereby it could be proved that the [4.4.0]heterobicycle is the most stable one. Unexpectedly the salicylideneaminoethanol derivative does not lead to a macrocyclic structure, although molecular modeling reveals high ring strain for the monomeric product that is actually obtained.