31P NMR Spectroscopically Quantified Hydrogen‐Bonding Strength of Thioureas and Their Catalytic Activity in Diels–Alder Reactions. Issue 29 (10th September 2014)

• Record Type: Journal Article
• Authors: Nödling, Alexander R.; Jakab, Gergely; Schreiner, Peter R.; Hilt, Gerhard
• Is Part Of: European journal of organic chemistry. Issue 29(2014)
• Page Start: 6394
• Page End: 6398
• Publication Date: 2014-09-10
• Subjects: Hydrogen bonds -- Cycloaddition -- Kinetics -- NMR spectroscopy -- Lewis acids -- Thioureas; Chemistry, Organic -- Periodicals; Organic compounds -- Synthesis -- Periodicals; Bioorganic chemistry -- Periodicals; Chemistry, Physical organic -- Periodicals; 547
• DOI: 10.1002/ejoc.201402871
• Languages: English
• ISSNs: 1434-193X

Abstract

Abstract: The hydrogen‐bonding strength of a variety of commonly employed thiourea catalysts was quantified by using a trialkylphosphine oxide as a 31 P 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 31 P NMR chemical shifts. A linear correlation between both variables was observed throughout the functionalized thioureas. The 31 P NMR probe correlation fared better in comparison to a p K a correlation. Accordingly, the quantification presented herein by using a 31 P 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. Abstract : The correlation of 31 P NMR shift values [Δ δ ( 31 P)] for the determination of hydrogen‐bond strengths of thiourea derivatives with kinetic data [ln ( k rel )]of thiourea‐catalyzed Diels–Alder reactions is evaluated. A Hammett‐type plot provides a more reliable correlation than the correlation of kinetic data with the p K a values of the thiourea derivatives.