Immobilised chiral inducer on Pt-based mesoporous titanate nanotubes as heterogeneous catalysts for enantioselective hydrogenation

A new series of chiral porous titanate nanotubes (TNTs) containing (−)-11-trimethoxysilyl-cinchonidine (TMS-CD) moieties were synthesized via direct surface anchorage. These hybrid solids were used as supports for Pt (1 wt%) catalysts and evaluated in heterogeneous enantioselective hydrogenation of 1-phenyl-1,2-propanodione. The supports and catalysts were characterized by a variety of techniques including TGA, N2 adsorption isotherms, solid state NMR (13C and 29Si), XRD, HR-TEM, DRIFTs and XPS. For the best system the hydrogen pressure, solvent effects and recycles were studied. The maximum enantiomeric excess (ee) and the turnover frequency obtained under optimized conditions with 1% Pt/TNT-CD were 37% (at 15 wt% nominal content of TMS-CD) and 165 min−1 (at 10 wt% nominal content of TMS-CD), respectively. The variation of H2 pressure and solvent affects both the activity and enantioselectivity due to the substrate adsorption on the metal active sites. Additionally, these heterogeneous catalysts can also be easily recycled and reused for three times without the loss of activity and enantioselectivity. After the 3rd cycle, catalyst deactivation due to the leaching of Pt nanoparticles and modifier hydrogenation in the feed was detected.The authors thank CONICYT for the financial support (FONDECYT GRANT 1061001, FONDECYT Initiation GRANT 11121631, FONDECYT postdoctoral 3140130 and 3140157) and REDOC.CTA doctoral network. P.A. Osorio-Vargas thanks MECESUP for doctoral fellowship.Peer Reviewe

A new non-cinchona chiral modifier immobilized on Pt/SiO2 catalysts for enantioselective heterogeneous hydrogenation

Pt heterogeneous catalysts were prepared by the covalent immobilization of (4′R,5′S)-4′,5′-dihydro-4′,5′-diphenyl-2-(6-cyanopyridyl)imidazoline (PyIm) as the asymmetric modifier. These novel series of catalysts were studied in the 1-phenyl-1,2-propanodione (PPD) hydrogenation. The effects of the PyIm surface concentration, hydrogen pressure, solvent nature and recycles on the reaction were studied. These modified catalysts represent the first effective immobilized chiral non-Cinchona-type modifier of Pt for the enantioselective hydrogenation. The enantio-differentiation was attributed to the substrate–modifier interactions involving hydrogen bonding between the keto-carbonyl O atom and the NH moiety PyIm. The results confirmed that the variations in the H2 pressure and the solvent affect the activity and the enantioselectivity due to the substrate adsorption on the active sites of the metal. Additionally, this heterogeneous catalyst can be conveniently reused at least five times without loss of its catalytic efficiency, but the enantioselectivity decreased, which may be due to the leaching of the modifier.The authors would like to thank CONICYT for their financial support (FONDECYT GRANT 1061001, FONDECYT Initiation GRANT 11121631, FONDECYT postdoctoral 3140130 and 3140157) and the REDOC.CTA doctoral network.Peer Reviewe