Fixed stereochemical control in the synthesis of new mono- and disubstituted 2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononenes

Five new boronates of the type 2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononenes (6a-e) were prepared from substituted 2-[(2-hydroxyethylamino)methyl]phenols (4a-e) and phenylboronic acid (5) in benzene-EtOH (4:1) mixtures. Tridentate ligands 4a-a and boronates 6a-a were characterized by H-1, C-13, N-15, and 2D-NMR (HETCOR, NOESY, and COLOC) experiments, FT-IR, mass spectra, and elemental analysis, as well as B-11 NMR for the boron derivatives. Suitable monocrystals of 2-[(2-hydroxyethylamino)methyl]phenol hydrochloride (4a), cis-2-phenyl-6-aza-1,3-dioxa-2-borabenzocyclononene (6a), (2S,5R,6S)-2,5-diphenyl-6-aza-1,3-dioxa-2-borabenzocyclononene (6b), and (2S,4R,5R,6S)-2,4-diphenyl-5-methyl-6-aza-1,3-dioxa-2-borabenzocyclononene (6e) were obtained and their structures are discussed. The X-ray structures of 6a, 6b, and 6e, as well as the NMR data established that the configurations at the nitrogen and boron atoms are both "S" and the transannular fusion is cis. A semi-empirical (SAM1) study was used to calculate 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 and previous work related to amino acid boronates.