Determination of the pKa Value of a Brønsted Acid by 19F NMR Spectroscopy

Brønsted acids, such as phosphoric acids derived from chiral 1,1′‐bi‐2‐naphthol (BINOL), are important catalysts in the formation of carbon–carbon and carbon–heteroatom bonds, for example. The catalytic activity of these Brønsted acids is strongly linked to their acidity, and as such, the evaluation of compounds to determine pKa values provides insight into their catalytic activity. Herein, a 19F{1H} NMR methodology is detailed to determine the pKa of a fluorinated binaphthyl‐derived phosphinic acid, rac‐1, in acetonitrile and in the presence of a fluorinated sulfonamide reference compound (2–4). The approach was tested initially using 2 and 3, with the ΔpKa (0.08) in strong agreement with previously reported values (6.6 for 2 and 6.68/6.73 for 3). Sigmoidal curves of normalised chemical shift change (Δδ) against equivalents of the base phosphazene P1‐tBu added overlapped for 2 and 3, but in the case of rac‐1 and either 2, 3 or 4, there was significant separation. A variety of different approaches for determining the ΔpKa were compared. Values of pKa determined when the normalised Δδ was 90% were optimal for 2 and 3, whereas a normalised Δδ of 75% was optimal for 4, resulting in the pKa of rac‐1 being determined to be 8.47–8.71.