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

• DOI: 10.1002/ejoc.201402871
• Publication Date: 2014-09-10T14:47:40Z
• Authors: Alexander R. Nödling; Gergely Jakab; Peter R. Schreiner; Gerhard Hilt
• Keywords: 01 natural sciences; 0104 chemical sciences

AbstractThe hydrogen‐bonding strength of a variety of commonly employed thiourea catalysts was quantified by using a trialkylphosphine oxide as a 31P NMR probe. Simple diarylthioureas and more complex bifunctional amine‐ and hydroxy‐substituted 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 31P NMR chemical shifts. A linear correlation between both variables was observed throughout the functionalized thioureas. The 31P NMR probe correlation fared better in comparison to a pKa correlation. Accordingly, the quantification presented herein by using a 31P 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.