H/D scrambling in a chromium-catalyzed dehydrocoupling reaction of a borane-dimethylamine adduct

H/D scrambling took place in a chromium-catalyzed dehydrocoupling reaction of a deuterium-labeled borane-dimethylamine adduct. In the hydrogen elimination of BH3.NDMe2 (1a-d(N)), H-2, HD and D-2 were generated in 65 : 30 : 5 ratio, and 62% of deuterium atoms were incorporated into the major product, the dimethylaminoborane dimer. Proton and deuteron nuclei were thus concentrated into the evolved dihydrogen and aminoborane dimer, respectively. The mechanism of H/D scrambling is understood based on the reaction pathway of the dehydrocoupling of 1a, which was previously proposed based on DFT calculations. The H/D distribution in the products is explained by the energy difference according to the deuterated position in an intermediate of the dehydrocoupling reaction.