X-ray analysis and structural characterization of 2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononenones

Nine new monomeric boronates of the type 2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononen-4-ones (3a–3i) were prepared from N-(2-hydroxybenzyl)-α-aminoacids 1a–1i and phenylboronic acid 2 using 20:1 benzene–DMSO mixtures. The compounds were characterized by 1H-, 13C-, 11B-, 15N- and 2D-NMR (HETCOR, NOESY and COLOC) experiments, FT infrared, mass spectra and elemental analysis. Suitable monocrystals of cis-2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononen-4-one (3a), (2S,5S,6R)-2-phenyl-6-aza-1,3-dioxa-5-secbutyl-2-borabenzocyclononen-4-one (3e) and 2,5-diphenyl-6-aza-1,3-dioxa-2-borabenzocyclononen-4-one (3h) were obtained and their structures are discussed. The X-ray structures, as well as the NMR data established that the configurations at the nitrogen and boron atoms are ‘R’ and ‘S’, respectively and the transannular fusion is cis. A semi-empirical (SAM1) study allowed calculation of the energy for all possible stereoisomers, showing that the stabilization increases as the THC (TetraHedral Character of the boron atom) increases and also as the N→B bond distance decreases, in agreement with the experimental results.