Intramolecular Hydride Transfer Reactions in (Formazanate)Boron Dihydride Complexes

The
solution-phase thermolysis of (formazanate)boron dihydrides
(LBH2; 2) results in the formation of aminoborane
compounds (4) via a series of boron-to-ligand intramolecular
hydride transfer reactions. Monitoring the reactions by NMR spectroscopy
allowed identification of several intermediates, and a reaction mechanism
is proposed. In the case of a ligand with an N-Mes substituent it
was possible to characterize an intermediate (7b-i) during this transformation that shows an unexpected cyclohexadiene
moiety, which results from hydride transfer to the ortho-position of the mesityl substituent. Two consecutive boron-to-ligand
hydride transfers eventually result in reductive N–N bond cleavage
to give triazaboroles as the final product.