Determination of the pKaH of Established Isothiourea Catalysts

Isothioureas (ITUs) represent a powerful family of (chiral) Lewis base organocatalysts. Interestingly, the Brønsted basicity of these frequently used compounds has so far not systematically been investigated. Thus, we have now determined the pKaH values of the most privileged (chiral) ITUs in acetonitrile (ACN) and DMSO by using NMR. Employing Wallace’s chemical shift imaging NMR method, the herein investigated ITUs were found to be weak Brønsted bases with pKaH values in the range of 16.8 – 17.9 in ACN and 6.3 – 7.8 in DMSO.