Ruthenium-Catalyzed Ionic Hydrogenation of Iminium Cations. Scope and Mechanism

Catalysis by CpRu(P−P)H (where P−P is a chelating diphosphine) of the ionic hydrogenation of an iminium cation inolves (1) the transfer of H- to form an amine, (2) the coordination of H2 to the resulting Ru cation, and (3) the transfer of H+ from the coordinated dihydrogen to the amine formed in (1). With CpRu(dppe)H the principal Ru species during catalysis remains the hydride complex, and H2 pressure has no effect on either the ee or the turnover frequency. Step (1), H- transfer, can be carried out stoichiometrically if the H2 is replaced by a coordinating solvent. A methyl substituent on the Cp ring decreases the H- transfer rate and the turnover frequency slightly. Electron-donating substituents on the phosphine increase the H- transfer rate and increase the turnover frequency up to a point: eventually the hydride ligand (i.e., the one in Cp*Ru(dmpe)H) becomes sufficiently basic to deprotonate the iminium cation to the corresponding enamine, and this pre-equilibrium competes with H- transfer. Ionic hydrogenation of enamines is possible when a Ru(H2) cation (i.e., [CpRu(dppm)(η2-H2)]+) is used as the catalyst and the enamine is more basic than the product amine. Ionic hydrogenation of an α,β-unsaturated iminium cation saturates both the CC and the CN bonds. A CN bond is more reactive toward ionic hydrogenation than a CC one, but in some cases (i.e., CHCH2) the latter may compete with H2 for a coordination site and decrease the turnover frequency.