A computational study of the formation of 1,3,2-dioxaborolane from the reaction of dihydroxy borane with 1,2-ethanediol

Boronic acids are well known to form covalent linkages with compounds containing 1,2- or 1,3-diol moieties that lead to 5- or 6-membered cyclic boronate esters, respectively. In the present study, we use density functional theory at the B3LYP/6-311++G** computational level to discuss formation of the representative 5-membered cyclic ester 1,3,2-dioxaborolane, HB(-O-CH2-CH-O-), from the dehydration reaction of dihydroxy borane, HB(OH)(2), and 1,2-ethanediol, HOCH2-CH2OH. Calculations support a stepwise mechanism in which the monoester HB(OH)-O-CH-CH2OH is formed in an initial bimolecular dehydration; this is followed by a unimolecular elimination in which HB(-O-CH2-CH2-O-) is formed. Since diol complexations with boronic acids are typically carried out either in aprotic solvents using a dehydrating agent or in basic aqueous media, the effects of acetonitrile and various Lewis bases on the thermodynamic and kinetic parameters of these dehydration and elimination reactions are also discussed. (C) 2003 Elsevier B.V. All rights reserved.