Amine hemilability enables boron to mechanistically resemble either hydride or proton

Tetracoordinate MIDA (N-methyliminodiacetic acid) boronates have found broad utility in chemical synthesis. Here, we describe mechanistic insights into the migratory aptitude of the MIDA boryl group in boron transfer processes, and show that the hemilability of the nitrogen atom on the MIDA ligand enables boron to mechanistically resemble either a hydride or a proton. The first case involves a 1,2-boryl shift, in which boron migrates as a nucleophile in its tetracoordinate form. The second case involves a neighbouring atom-promoted 1,4-boryl shift, in which boron migrates as an electrophile in its pseudo-tricoordinate form. Density functional theory studies and in situ NMR measurements all suggest that MIDA can act as a dynamic switch. These findings encouraged the development of novel migration processes involving boron that exploit the chameleonic behaviour of boron by acting as both a nucleophile and an electrophile, including the first report of a compound with a boronate functionality bound to carbon in the carboxylic acid oxidation state.