A Neutral Ru Hydride Complex for the Regio- and Chemoselective Reduction of N-Silylpyridinium Ions.

A detailed experimental analysis of the 1,4-selective reduction of pyridine with hydrosilanes catalyzed by a coordinatively unsaturated Ru thiolate complex is reported. The previously suggested intermediates, N-silylpyridinium ions and a neutral Ru hydride, have been independently synthesized and do indeed participate in the catalytic cycle. The resting state is not the cationic Ru complex initially used as the catalyst but its pyridine-coordinated congener. All Ru complexes, including the one resulting from hydrosilane activation, are in equilibrium with each other. The N-silylated 1,4-dihydropyridine together with the cationic Ru complex convert back into the corresponding N-silylpyridinium ion and the neutral Ru hydride (retro-hydrosilylation), followed by further backward reaction into the hydrosilane and the pyridine adduct of the cationic complex. These steps prove the overall reversibility of the transformation.