Amine-catalyzed B-O-C bond formation: Mechanistic insights from density functional theory and second-order Moller-Plesset perturbation theory

Boronic acids react with compounds containing 1,2- or 1,3-diols to form five- or six-membered cyclic boronate esters, respectively, although many factors that influence these reactions are not well understood. In the present study, density functional theory and second-order Moller-Plesset (MP2) perturbation theory were employed to examine the mechanism in which a primary aliphatic amine acts as an internal Lewis base to catalyze the formation of a boron-oxygen-carbon linkage in the methanolysis of H2N-CH2-CH=CH-B(OH)(2) to afford H2N-CH2-CH=CH-B(OH)(OCH3); solvent effects were assessed using the polarized continuum model and explicit water molecules. In vacuo, the lowest-energy conformer of H2N-CH2-CH=CH-B(OH)(2) was a seven-membered, hydrogen-bonded ring structure in which the boronic acid moiety had a planar, trigonal geometry. The catalytic role of the primary amine group in the methanolysis of H2N-CH2-CH=CH-B(OH)2 results from facilitation of a proton transfer from an intermolecular B-O dative-bonded adduct between methanol and this boronic acid, rather than from the formation of an intramolecular B-N dative bond. In the absence of amine catalysis, transition states for the rate-determining proton-transfer step in this methanolysis are 12.8-17.3 kcal/mol higher in energy. In the reaction field of water, a five-membered B-N dative-bonded ring conformer of H2N-CH2-CH=CH-B(OH)(2) was lowest in energy at the MP2 level, but hydrated zwitterionic structures also appear to play an important role in this complex aminoboronic acid/methanol association and ether formation. In contrast to the PBE1PBE functional, B3LYP gave anomalous results for some steps in the methanolysis when compared with those from the more robust, albeit expensive, ab initio MP2 method.