π-Bonding in B-O ring species:: Lewis acidity of Me3B3O3, synthesis of amine Me3B3O3 adducts, and the crystal and molecular structure of Me3B3O•3NH2iBu•MeB(OH)2

The synthesis and characterisation (m.p., elemental analysis, IR, NMR) of some previously unreported Me3B3O3. L adducts (L=piperidine, isobutylamine, morpholine, 3-picoline, benzylamine) are described. The crystal and molecular structure of (Me3B3O3NH2Bu)-Bu-i . MeB(OH)(2) has been determined by a single-crystal X-ray diffraction study. The structure reveals an unusual H-bond interaction between MeB(OH)(2) and a ring O atom of the six-membered B3O3 ring of the trimethylboroxine adduct, Me3B3O3. (NH2Bu)-Bu-i. The direction of the H-bond strongly suggests sp(2) hybridisation for this O atom. The Lewis acidity of Me3B3O3, Ph3B3O3, 1,3,5-Ph3B3N3Me3 and 1,3,5-Ph3C6H3 have been estimated by Gutmann's method and data interpreted in terms of relative K-bonding strength within these ring systems. (C) 1999 Elsevier Science S.A. All rights reserved.