A Ligand-Dissociation-Involved Mechanism in Amide Formation of Monofluoroacylboronates with Hydroxylamines

Abstract

Acylborons, as a growing class of boron reagents, were successfully applied to amide ligation and showed potential in chemoselective bioconjugation reactions in recent years. In this manuscript, a density functional theory (DFT) study was performed to investigate the mechanism of the amide formation between monofluoroacylboronates and hydroxylamines. An updated pathway was clarified herein, including water-assisted hemiaminal formation, pyridine ligand dissociation, elimination via a six-membered-ring transition state, and water-assisted tautomerization. The proposed mechanism was further examined by applying it to investigate the activation barriers of other monofluoroacylboronates, and the related calculations well reproduced the experimentally reported relative reactivities. On the basis of these results, we found that the ortho substitution of the pyridine ligand destabilizes the acylboron substrates and the hemiaminal intermediates by steric effects and thus lowers the energy demand of the ligand dissociation and elimination steps. By contrast, the para substitution of the pyridine ligand with an electron-donating group enhances the coordination of the ligand by electronic effects, which is a disadvantage to the ligand dissociation and elimination steps. The ligand bearing a rigid linkage blocks the rotation of the pyridine ligand and makes ligand dissociation difficult.