title: Selective homogeneous hydrogenation of biogenic carboxylic acids with [Ru(TriPhos)H]+: a mechanistic study.
abstract: Selective hydrogenation of biogenic carboxylic acids is an important transformation for biorefinery concepts based on platform chemicals. We herein report a mechanistic study on the homogeneously ruthenium/phosphine catalyzed transformations of levulinic acid (LA) and itaconic acid (IA) to the corresponding lactones, diols, and cyclic ethers. A density functional theory (DFT) study was performed and corroborated with experimental data from catalytic processes and NMR investigations. For [Ru(TriPhos)H](+) as the catalytically active unit, a common mechanistic pathway for the reduction of the C═O functionality in aldehydes, ketones, lactones, and even free carboxylic acids could be identified. Hydride transfer from the Ru-H group to the carbonyl or carboxyl carbon is followed by protonation of the resulting Ru-O unit via σ-bond metathesis from a coordinated dihydrogen molecule. The energetic spans for the reduction of the different functional groups increase in the order aldehyde < ketone < lactone ≈ carboxylic acid. This reactivity pattern as well as the absolute values are in full agreement with experimentally observed activities and selectivities, forming a rational basis for further catalyst development.
authors.affiliation: Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany.
authors.forename: Frank M A
authors.initials: FM
authors.lastname: Geilen
authors.forename: Barthel
authors.initials: B
authors.lastname: Engendahl
authors.forename: Markus
authors.initials: M
authors.lastname: Hölscher
authors.forename: Jürgen
authors.initials: J
authors.lastname: Klankermayer
authors.forename: Walter
authors.initials: W
authors.lastname: Leitner
mesh_terms: D002384:Catalysis; D004988:Ethers, Cyclic / Q000138:chemical synthesis; D006865:Hydrogenation; D007783:Lactones / Q000138:chemical synthesis; D007982:Levulinic Acids / Q000737:chemistry*; D017975:Ruthenium Compounds / Q000737:chemistry*; D013386:Succinates / Q000737:chemistry*
chemical_list: D004988:Ethers, Cyclic; D007783:Lactones; D007982:Levulinic Acids; D017975:Ruthenium Compounds; D013386:Succinates; C037556:gamma-valerolactone; C005229:itaconic acid; C032246:levulinic acid