Heterogeneous Asymmetric Hydrogenation of Activated Ketones: Mechanistic Insight into the Role of Alcohol Products by in Situ Modulation-Excitation IR Spectroscopy

Abstract

Present mechanistic models for the rationalization of enantiodifferentiation on cinchona-modified Pt focus on the activated ketone–modifier interaction, while the possible role of the product alcohol is largely ignored. Here we used in situ attenuated total reflection infrared (ATR-IR) spectroscopy combined with modulation-excitation spectroscopy (MES) and catalytic (kinetic) study to clarify the role of the two enantiomers of the alcohol products at the surface of chirally modified Pt/Al2O3. In situ monitoring of the solid–liquid interface proved that chiral modification of Pt with cinchonidine (CD) significantly reduced the amount of adsorbed (R)-methyl mandelate ((R)-MM), which is the major enantiomer in the asymmetric hydrogenation of methyl benzoylformate (MBF). Trace amounts of (R)-MM product on the surface were found to decrease significantly the hydrogenation rate of MBF. In situ ATR-IR spectroscopy with absolute configuration modulation indicated that an N–H–O type H bonding forms between CD and (R)-MM, whose structure is analogous to that of the diastereomeric CD–MBF complex. The rate deceleration is, therefore, considered to arise from competitive adsorption of the prochiral ketone and the product alcohol at the chirally modified surface. This conclusion is further supported by extending the spectroscopic study to (R)-ethyl lactate, (R)-pantolactone, and (R)-α-(trifluoromethyl)benzyl alcohol.