Ronny Wirz | PubMed Health

PubMed2006

ATR-IR spectroscopy of pendant NH2 groups on silica involved in the Knoevenagel condensation.

The liquid-phase Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate catalyzed by aminopropyl-modified silica has been investigated using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. The aim of the work was to demonstrate the different levels of information on the reaction mechanism that can be achieved by operating the spectroscopic cell in the absence and in the presence of a solvent, in flow-through and stop-flow modes and in combination with concentration modulation spectroscopy. The reaction mechanism involves the formation of an imine intermediate whose existence has been verified in situ by combining in one experiment continuous and stop-flow operations. Identical information has been gained more elegantly using concentration modulation spectroscopy, which additionally provided information on the possible origin of the solvent effect observed in the Knoevenagel reaction. Faster production and consumption of the imine intermediate was observed in cyclohexane solvent than in toluene. Identification of other species evolving on the catalyst surface and monitoring of the effluents of the spectroscopic cell provided some insight in possible catalyst deactivation.

Acetates↗

PubMed2004

Absolute configuration modulation attenuated total reflection ir spectroscopy: an in situ method for probing chiral recognition in liquid chromatography.

A method to selectively probe the different adsorption of enantiomers at chiral solid-liquid interfaces is applied, which combines attenuated total reflection infrared spectroscopy and modulation spectroscopy. The spectral changes on the surface are followed while the absolute configuration of the adsorbate is changed periodically. Demodulated spectra are calculated by performing a subsequent digital phase-sensitive data analysis. The method is sensitive solely to the difference of the interaction of the two enantiomers with the chiral surface, and the small spectral changes are amplified by the phase-sensitive data analysis. Its potential is demonstrated by investigating an already well-studied system in liquid chromatography, namely, the enantiomer separation of N-3,5-dinitrobenzoyl-(R,S)-leucine (DNB-(R,S)-Leu) using tert-butylcarbamoyl quinine (tBuCQN) as the chiral selector immobilized on the surface of porous silica particles. The performed experiments and density functional theory calculations confirm an interaction model that was proposed earlier based on solution NMR and XRD in the solid state. It emerges that the ionic interaction is the strongest one, but the main reason for the potential for enantioseparation of the chiral stationary phase (CSP) is the distinct formation of a hydrogen bond of the (S)-enantiomer with the chiral selector. This H-bond is established between the amide N-H of DNB-(S)-Leu with the carbamate C=O of the CSP. The (R)-enantiomer instead shows no specific hydrogen bonds. Only the unspecific ionic bonding between the protonated quinine part of the tBuCQN and the carboxylate of the DNB-(R)-Leu (holds also for DNB-(S)-Leu) is observed.

Journal Article↗