Formation and Structural Characterization of Benzoxa- and Thienoxaphosphaborinines

Benzoxa- and thienoxaphosphaborinines, a new group of six-membered boron-phosphorus heterocycles, are reported. Their synthesis involved aromatic lithiation of appropriate 2-aryl-6-butyl[1.3.6.2]dioxazaborocans, followed by treatment with PhPCl₂ and subsequent hydrolysis giving rise to (2-(phenylhydrophosphoryl)aryl)boronic acids. These intermediates exhibit nucleophilic character and undergo readily condensation reactions with aldehydes and ketones giving rise to target products. The mechanism of the latter reaction was studied by means of DFT calculations showing that activation of the carbonyl partner occurs through the formation of the hydrogen bond with the B(OH)₂ group rather than through an interaction with the Lewis acid boron atom due to a plausible FLP-like behavior. The molecular structures of a few derivatives were determined by X-ray crystallography showing that in all cases, the molecules assemble through intermolecular BOH···O═P hydrogen bonds.