(Thio)urea Organocatalyst Equilibrium Acidities in DMSO

Bordwell's method of overlapping indicators was used to determine the pKa values of some of the most popular (thio)urea organocatalysts via UV spectrophotometric titrations. The incremental effect of CF3 groups on acidic strength was also investigated. The pKa's are in the range of 8.5–19.6. The results may lead to a better understanding of noncovalent organocatalysis and may aid in future catalyst development.