A new simple method for the pKa evaluation of Bronsted acids

Over recent years, chiral Brønsted acid catalysis has emerged as a fast growing branch of asymmetric organocatalysis. Phosphoric acids (1), N-triflyl phosphoric amides (2) and bis (sulfonyl)imides (3) were introduced aiming at significantly higher acidity in order to access to the activation of a wider range of substrates (Figure 1). A quantitative determination of the pKa of these compounds has been achieved only through UV-VIS techniques since 2011;[1,2] however a more straightforward method for a quick determination of the relative acidity of these compounds is desirable. Figure 1. In the ‘70s Gutmann[3] reported the establishment of a Lewis acidity scale through the measurement of the 31P-NMR shifts of the complexes between of several Lewis acids with and a Lewis base (tributylphosphine oxide); more recently, Rueping and Gshwind reported the observation of the formation of an ion pair/H-bond complex between a phosphoric acid and an imine through NMR techniques.[4] Starting from these observations, we have found that the 1H-NMR shifts of complexes of several Brønsted acids A-H with an imine (4), present a linear correlation with respect to the Brønsted acids’ pKa values (Figure 2). We report here preliminary results in the establishment of a new method for the relative pKa evaluation of Brønsted acids in different solvents. Figure 2. Bibliography: 1. Berkessel et al. Chem. Eur. J. 2011, 17, 8524. 2. Rueping et al. Angew. Chem. Int. Ed. 2013, 52, 11569. 3. Gutmann et al. Coord. Chem. Rev. 1976, 18, 225. 4. Rueping, Gschwind et al. Angew. Chem. Int. Ed. 2011, 123, 6488.