from first principles PP-I-1

OY-III-2Platinum-Catalyzed Asymmetric Hydrogenation: Spectroscopic Evidencefor O-H-O Hydrogen Bond Interaction between Substrate and ModifierMeemken F., Maeda N., Hungerbühler K., Baiker A.Department of Chemistry and Applied Bioscience, ETH Zürich, Switzerlandfabian.meemken@chem.ethz.chSimple addition of catalytic amounts of cinchonidine (CD) induces remarkably highenantiomeric excess (ee) in the asymmetric hydrogenation of activated ketones on chirallymodified platinum (up to 98% ee). Understanding of the governing mechanism is a necessaryprerequisite for the rational design of such catalytic systems. Extensive studies have led to theproposal of several different mechanistic models which commonly focus on the role of thetertiary amine functionality of the modifier. [1] In contrast, little attention has been given to thesecondary alcohol group of CD, albeit blocking of this functional group leads to a significantdecrease in ee for hydrogenation of some ketones.Using in situ attenuated total reflection infrared (ATR-IR) spectroscopy in combination withmodulation excitation spectroscopy (MES) and phase-sensitive detection (PSD) [2] we uncovereda new hydrogen bonding between ketopantolactone (KPL) and modifier (CD) which so far hasbeen overlooked in the literature. This C 9 -O··H··O=C hydrogen bonding together with thepreviously postulated N-H··O=C bonding is crucial for the formation of the intermediateenantiodifferentiating diastereomeric surface complex. The phase-resolved surface spectra (seeFigure below) show adsorption-desorption cycles of KPL on Pt/Al 2 O 3 modified with CD and themethyl ether derivative, MeOCD. Interestingly, formation of the complex (φ = 60°), followinginitially adsorbed KPL (φ = 20°), is only observed on the CD-modified surface. Additionally,experimental evidence for the particular stereocontrol of the surface complex toward the majorproduct enantiomer indicates that for some ketones the C 9 -OH group of the modifier has to betaken into account for explaining enantiodifferentiation on chiral surfaces.References:[1] T. Mallat, E. Orglmeister, A. Baiker, Chem. Rev. 2007, 107, 4863-4890.[2] D. Baurecht, U. P. Fringeli, Rev. Sci. Instrum. 2001, 72, 3782-3792.95