Stoichiometric and Catalytic Deuteration of Pyridine and Methylpyridines by H/D Exchange with Benzene-d6 Promoted by an Unsaturated Osmium Tetrahydride Species

Treatment of toluene solutions of the hexahydride complex OsH 6 (P i Pr 3 ) 2 ( 1 ) with pyridine, 3-methylpyridine, and 4-methylpyridine affords the tetrahydride derivatives OsH 4 (Rpy)(P i Pr 3 ) 2 (Rpy = py ( 3 ), 3-methylpyridine ( 4 ), and 4-methylpyridine ( 5 )). In benzene- d 6, these compounds release the heterocycles, and the resulting unsaturated tetrahydride OsH 4 (P i Pr 3 ) 2 ( 2 ) promotes the stoichiometric and catalytic deuteration of pyridine, 3-methylpyridine, and 4-methylpyridine by means of H/D exchanges between the heterocycles and the solvent. The deuteration rates of the pyridinic C−H bonds depend upon their positions in the heterocycles. For pyridine, they increase as the C−H bonds are separated from the heteroatom. A methyl substituent has a marked negative effect on the deuteration of its adjacent C−H bonds. The kinetic analysis of the deuteration reveals that the rate-determining step for the H/D exchanges is the C−H activation of the bond that is deuterated. DFT calculations show that this step is formed by two elemental stages: the direct coordination of the C−H bond and its subsequent rupture.