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

Abstract

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 H(2) 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 H(2) pressure has no effect on either the ee or the turnover frequency. Step (1), H(-) transfer, can be carried out stoichiometrically if the H(2) 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 CpRu(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(H(2)) cation (i.e., [CpRu(dppm)(eta(2)-H(2))](+)) is used as the catalyst and the enamine is more basic than the product amine. Ionic hydrogenation of an alpha,beta-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=CH(2)) the latter may compete with H(2) for a coordination site and decrease the turnover frequency.

Mechanistic Investigations of Imine Hydrogenation Catalyzed by Dinuclear Iridium Complexes

“…This mechanism is similar to that recently reported by Norton et al for the hydrogenation of iminium to ammonium cations catalyzed by [CpRu(diphosphine)H] complexes, in which H À transfer was found to be slow step. [12] A similar cycle has also been recently proposed for high-pressure imine hydrogenations catalyzed by rhenium complexes. [13] The catalytic cycle in Equation (7) is likely to be interfered by the basic N-benzylaniline hydrogenation product, Figure 5.…”

The kinetics and mechanism of the organo-iridium-catalysed enantioselective reduction of imines

“…There have been fewer investigations into the imine/amine reaction using organometallic catalysts, although kinetic and isotope labelling studies using a cyclopentadienone ruthenium catalyst 27,28 have attempted to differentiate an inner sphere mechanism, involving direct coordination of the substrate to the metal, and an outer sphere process in which the amine/imine nitrogen does not bind directly to the ruthenium. Distinguishing between stepwise and concerted hydride and proton transfer steps is also controversial 29 . It has been suggested 30 that the asymmetric transfer hydrogenation of imines with formic acid-triethylamine mixtures using Rh-chiral diamine catalysts involves the neutral imine as the reactive species, whereas prior imine protonation has been proposed because the isolated ruthenium hydride reacts faster with an imine substrate than a corresponding ketone due to the greater basicity of the imine.…”

Molybdenum Nitrosyl Complexes and Their Application in Catalytic Imine Hydrogenation Reactions

“…[3,4] It involves the transfer of a hydride (H -) or proton (H + ) as a H 2 equivalent to the substrate. [5] Various catalytic systems enable the efficient hydrogenation of ketones [6][7][8] and imines [9][10][11][12][13] by Wilkinson/Osborn-type or ionic hydrogenation. However, most of these processes are presently based on precious metals, which require tedious catalyst recycling for economic and toxicity reasons.…”