Potential of ionic liquids as co-modifiers in asymmetric hydrogenation on platinum

• Published in: Journal of molecular catalysis. A, Chemical Vol. 357; pp. 117 - 124
• Main Authors: Sano, Shogo, Beier, Matthias Josef, Mallat, Tamas, Baiker, Alfons
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.05.2012; Elsevier
• Subjects: Catalysis; Chemistry; Cinchona alkaloids; Enantioselective hydrogenation; Exact sciences and technology; General and physical chemistry; Ionic liquids; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; α-Ketoester; Ionic liquids; Cinchona alkaloids; α-Ketoester; Enantioselective hydrogenation; Solvent effect; Cinchona; Enantioselectivity; Transition metal; Asymmetric catalysis; Hydrogenation; Supported catalyst; Ionic liquid; Alkaloid; Platinum; Dicotyledones; Angiospermae; Rubiaceae; Spermatophyta; Ketoester; Alumina; Comparative study
• ISSN: 1381-1169; 1873-314X
• DOI: 10.1016/j.molcata.2012.01.029

[Display omitted] ► Enantioselective hydrogenation of methyl benzoylformate on Pt/alumina. ► Chiral modifiers: cinchonidine and O-phenyl or O-naphthyl derivatives. ► Ionic liquids as solvent, supported layer on the catalyst, or solvent additive. ► Enantioselectivity may be tuned with ionic liquids possessing an aromatic cation. ► Strong catalyst deactivation by ionic liquids, also in cyclohexene hydrogenation. The enantioselective hydrogenation of methyl benzoylformate on Pt/Al2O3 has been studied in the presence of various ionic liquids (ILs) and three structurally related chiral modifiers: cinchonidine (CD), O-phenyl-cinchonidine (PhOCD), and O-naphthyl-cinchonidine (NaphOCD). Addition of ca. 1% IL to a polar organic solvent – particularly alcohols – improved the enantioselectivity by up to 12% (to 92–93%) in the presence of CD. On the other hand, ILs diminished the reaction rate sometimes dramatically, by two orders of magnitude, and the poisoning effect was even stronger in cyclohexene hydrogenation under identical conditions. Comparative studies revealed that only ILs possessing a heteroaromatic cation interact strongly with Pt or the O-aryl function of PhOCD and NaphOCD. A tentative explanation is the strong π-bonding interaction of ILs with the Pt surface (leading to site blocking and deactivation) and with the O-aryl function of the modifier not involved in the adsorption onto Pt (resulting in a shift in enantioselectivity).