CF3-groups critically enhance the binding of thiourea catalysts to ketones – a NMR and FT-IR study

Abstract

Substituted phenylthioureas have been established as efficient organocatalysts and substituents containing electron withdrawing CF3 groups have been shown to enhance catalytic efficiency. The effect of the CF3 groups on binding of catalysts to substrates in solution has however remained elusive. Here, we report on the effect of CF3 substituted diphenylthioureas on the association with the substrate 1,3-diphenyl-2-propenone in solution by using a combination of nuclear magnetic resonance (NMR) and Fourier-transform infrared (FT-IR) spectroscopy. We use the ensemble-averaged chemical shift of the thiourea proton as function of substrate concentration to determine the association constants between catalyst and substrate. To experimentally discriminate between free and bound catalyst we use infrared absorption spectra, which show a red-shift of thiourea's N–H stretching vibration upon association with the substrate. With both methods, we find the association constant K to increase from ∼1 L/mol to ∼20 L/mol with increasing number of CF3 substituents. This enhanced binding can explain the increased reaction rates observed for CF3 substituted diphenylthiourea catalysts. For the efficient catalyst containing four CF3-groups (Schreiner's catalyst), the strongest association is observed in toluene as a solvent, while the binding strength is somewhat weaker in dichloromethane, and association to the substrate is not detectable in acetonitrile. Our results thus demonstrate that even weak association between the thiourea catalysts and the ketone can facilitate efficient catalytic conversion. However, the association with the ketone substrates is very susceptible to competing interactions with the solvent.