π-Bonding in B–O ring species: Lewis acidity of Me 3B 3O 3, synthesis of amine Me 3B 3O 3 adducts, and the crystal and molecular structure of Me 3B 3O 3.NH 2iBu·MeB(OH) 2

Abstract

The synthesis and characterisation (m.p., elemental analysis, IR, NMR) of some previously unreported Me 3B 3O 3·L adducts (L=piperidine, isobutylamine, morpholine, 3-picoline, benzylamine) are described. The crystal and molecular structure of Me 3B 3O 3 .NH 2 i Bu·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 B 3O 3 ring of the trimethylboroxine adduct, Me 3B 3O 3·NH 2 i Bu. The direction of the H-bond strongly suggests sp 2 hybridisation for this O atom. The Lewis acidity of Me 3B 3O 3, Ph 3B 3O 3, 1,3,5-Ph 3B 3N 3Me 3 and 1,3,5-Ph 3C 6H 3 have been estimated by Gutmann’s method and data interpreted in terms of relative π-bonding strength within these ring systems.