Efficient Organocatalytic Dehydrogenation of Ammonia Borane.

Dehydrogenation of ammonia borane by sterically encumbered pyridones as organocatalysts is reported. With 6-tert-butyl-2-thiopyridone as the catalyst, a turnover frequency (TOF) of 88 h-1 was achieved. Experimental mechanistic investigations, substantiated by DLPNO-CCSD(T) computations, indicate a mechanistic scenario that commences with the protonation of a B-H bond by the mercaptopyridine form of the catalyst. The reactive intermediate formed by this initial protonation was observed by NMR spectroscopy and the molecular structure of a surrogate determined by SCXRD. An intramolecular proton transfer in this intermediate from the NH3 group to the pyridine ring with concomitant breaking of the S-B bond regenerates the thiopyridone and closes the catalytic cycle. This step can be described as an inorganic retro-ene reaction.