2014
DOI: 10.1002/ejoc.201402871
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31P NMR Spectroscopically Quantified Hydrogen‐Bonding Strength of Thioureas and Their Catalytic Activity in Diels–Alder Reactions

Abstract: 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 fu… Show more

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Cited by 32 publications
(28 citation statements)
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“…Phosphine oxide 20a , which resembles a carbonyl group, was explored as 31 P-NMR signal detection-based probe (Figure 5) [29]. Upon formation of the hydrogen bond between a thiourea and the oxygen atom of 20a , the electron density is shifted towards the thiourea, resulting in the downfield shift of the 31 P-NMR signal.…”
Section: Experimental Techniquesmentioning
confidence: 99%
“…Phosphine oxide 20a , which resembles a carbonyl group, was explored as 31 P-NMR signal detection-based probe (Figure 5) [29]. Upon formation of the hydrogen bond between a thiourea and the oxygen atom of 20a , the electron density is shifted towards the thiourea, resulting in the downfield shift of the 31 P-NMR signal.…”
Section: Experimental Techniquesmentioning
confidence: 99%
“…Notably, this line of thought is counter to the prevailing wisdom encountered with many Brønsted acid catalyzed reactions. 22 , 23 , 24 In the event, the analogous catalyst 7b bearing a methyl group at the quinoline 7-position led to higher yield (31%) and enantioselection (81% ee) (Table 1, entry 5) that improved further with the C7-methoxy catalyst 7c (75% yield, 89% ee) (Table 1, entry 6). 25 Interestingly, higher substrate concentrations led to diminished yield and selectivity (Table 1, entries 7–8), concomitant with signs that the reaction mixtures were more heterogeneous due to insolubility of one or more reagents.…”
Section: Resultsmentioning
confidence: 98%
“…For the thiourea derivatives investigated, the affinity constants were moderate but varied with the highest K value for the most acidic 5 a ( K =121 m −1 ) and the lowest K value ( K =0.67 m −1 ) for the least acidic 2 a . Hence, 1 H NMR titrations provide a useful tool to confirm the p K a determination obtained by titration, so as to predict catalytic activity …”
Section: Resultsmentioning
confidence: 99%