984-09-8Relevant articles and documents
31P NMR Spectroscopically Quantified Hydrogen-Bonding Strength of Thioureas and Their Catalytic Activity in Diels-Alder Reactions
N?dling, Alexander R.,Jakab, Gergely,Schreiner, Peter R.,Hilt, Gerhard
, p. 6394 - 6398 (2014)
The 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 pK a 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. The correlation of 31P NMR shift values [Δδ(31P)] 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 pK a values of the thiourea derivatives.
Quantification of electrophilic activation by hydrogen-bonding organocatalysts
Walvoord, Ryan R.,Huynh, Phuong N. H.,Kozlowski, Marisa C.
supporting information, p. 16055 - 16065 (2015/02/19)
A spectrophotometric sensor is described that provides a useful assessment of the LUMO-lowering provided by catalysts in Diels-Alder and Friedel-Crafts reactions. A broad range of 33 hydrogen-bonding catalysts was assessed with the sensor, and the relative rates in the above reactions spanned 5 orders of magnitude as determined via 1H- and 2H NMR spectroscopic measurements, respectively. The differences between the maximum wavelength shift of the sensor with and without catalyst (Δλmax-1) were found to correlate linearly with ln(krel) values for both reactions, even though the substrate feature that interacts with the catalyst differs significantly (ketone vs nitro). The sensor provides an assessment of both the inherent reactivity of a catalyst architecture as well as the sensitivity of the reaction to changes within an architecture. In contrast, catalyst pKa values are a poor measure of reactivity, although correlations have been identified within catalyst classes.
