31P NMR Spectroscopically Quantified Hydrogen-Bonding Strength of Thioureas and Their Catalytic Activity in Diels-Alder Reactions

The hydrogen-bonding strength of a variety of commonly employed thiourea catalysts was quantified by using a trialkylphosphine oxide as a P-31 NMR probe. Simple diarylthioureas and more complex bifunctional amine-and hydroxysubstituted thiourea derivatives were examined. Their catalytic activity was determined in a Diels-Alder reaction, and the obtained pseudo-first-order rate constants were correlated with the P-31 NMR chemical shifts. A linear correlation between both variables was observed throughout the functionalized thioureas. The P-31 NMR probe correlation fared better in comparison to a pK(a) correlation. Accordingly, the quantification presented herein by using a P-31 NMR probe offers an elegant way to estimate the catalytic activity of thiourea catalysts in hydrogen-bond-activated reactions such as the Diels-Alder reaction.