Rhodium-Catalyzed ON-OFF Switchable Hydrogenation Using a Molecular Shuttle Based on a [2]Rotaxane with a Phosphine Ligand.

A novel pH-responsive molecular shuttle based on a [2]rotaxane with a phosphine ligand has been designed and synthesized. In the rhodium-catalyzed hydrogenation of α,ß-dehydroamino acid esters and aryl enamides, ON/OFF-switchable catalysis was accomplished with high ON/OFF ratios by adjusting the movements of the rotaxane wheels located at the catalyst terminals with acid/base. Mechanistic studies using NMR spectroscopy and quasi in situ X-ray photoelectron spectroscopy revealed that RhIII -hydride species are possibly formed in a H2 atmosphere when the catalyst is in the OFF state. During the reaction, a heterolytic activation of dihydrogen occurs by the interlocked rotaxane dibenzylamine and RhI catalytic center acting as a frustrated Lewis pair. Subsequent homolytic splitting of dihydrogen with the newly formed RhI -hydride species generates RhIII -hydride species. These findings show that a substrate-selective hydrogenation can be achieved by using the OFF-state catalyst.

Asymmetric Hydrogenation of Quinoline Derivatives Catalyzed by Cationic Transition Metal Complexes of Chiral Diamine Ligands: Scope, Mechanism and Catalyst Recycling.

This personal account is focused on the asymmetric hydrogenation of quinolines and their analogues recently developed by using phosphorus-free chiral cationic ruthenium(II)/η6 -arene-N-monosulfonylated diamine complexes. In our initial study, the chiral Ru-diamine complexes were found to be highly effective catalysts for the asymmetric hydrogenation of difficult quinoline substrates in room temperature ionic liquids (RTILs) with unprecedentedly excellent enantioselectivity. Our further systematic study revealed that a wide range of quinoline derivatives could be efficiently hydrogenated in alcoholic solvents, or under solvent-free and concentrated conditions with good to excellent stereoselectivity. Complexes of iridium analogues could also efficiently catalyze the asymmetric hydrogenation of quinolines in undegassed solvent. Asymmetric tandem reduction of various 2-(aroylmethyl)quinolines was achieved in high yield with excellent enantioselectivity and good diastereoselectivity. More challenging substrates, alkyl- and aryl-substituted 1,5- and 1,8-naphthyridine derivatives were successfully hydrogenated with these chiral ruthenium catalysts to give 1,2,3,4-tetrahydronaphthyridines with good to excellent enantioselectivity. Unlike the asymmetric hydrogenation of ketones, quinoline is reduced via a stepwise H+ /H- transfer process outside the coordination sphere rather than a concerted mechanism. The enantioselectivity originates from the CH/π attraction between the η6 -arene ligand in the Ru-complex and the fused phenyl ring of dihydroquinoline via a 10-membered ring transition state with the participation of TfO- anion. In addition, the Ru-catalyzed asymmetric hydrogenation of quinolines could be carried out in some environmentally benign reaction media, such as undegassed water, RTILs and oligo(ethylene glycol)s (OEGs). In the latter two cases, unique chemoselectivity and/or reactivity were observed. Catalyst recycling could also be realized by using [BMIM]PF6 and OEGs as solvents, as well as via magnetic nanoparticles. Applications of this catalytic protocol were also exemplified by the employments of the reduced products for the syntheses of some important natural alkaloids, pharmaceutical intermediates, as well as chiral diamine ligands.