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(PiPr3)2 (1) with pyridine, 3-methylpyridine, and 4-methylpyridine affords the tetrahydride derivatives OsH4(Rpy)(PiPr3)2 (Rpy = py (3), 3-methylpyridine (4), and 4-methylpyridine (5)). In benzene-d6, these compounds release the heterocycles, and the resulting unsaturated tetrahydride OsH4(PiPr3)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. © 2009 American Chemical Society.Financial support from the MICINN(Projects CTQ2008-00810 and Consolider Ingenio 2010 CSD2007-00006) and Diputación General de Aragón (E35) is acknowledged.Peer Reviewe