Griffiths Emily F, Dixon Jay A, Caffyn Andrew J M, Langley Stuart K, Maciá Beatriz, Caprio Vittorio, Mewis Ryan E
Faculty of Science and Engineering, Department of Natural Sciences, Manchester Metropolitan University, Manchester, UK.
Magn Reson Chem. 2025 Jan;63(1):17-23. doi: 10.1002/mrc.5485. Epub 2024 Oct 16.
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 pK values provides insight into their catalytic activity. Herein, a F{H} NMR methodology is detailed to determine the pK 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 ΔpK (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 P-Bu 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 ΔpK were compared. Values of pK 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 pK of rac-1 being determined to be 8.47-8.71.
布朗斯特酸，例如源自手性1,1'-联二萘酚（BINOL）的磷酸，是碳-碳键和碳-杂原子键形成过程中的重要催化剂。例如，这些布朗斯特酸的催化活性与其酸度密切相关，因此，对化合物进行评估以确定其pK值有助于深入了解其催化活性。本文详细介绍了一种利用氟核磁共振（F{H} NMR）方法来测定氟化联萘衍生次膦酸rac-1在乙腈中以及在氟化磺酰胺参考化合物（2-4）存在下的pK值。该方法最初使用2和3进行测试，其ΔpK（0.08）与先前报道的值（2为6.6，3为6.68/6.73）高度一致。归一化化学位移变化（Δδ）相对于添加的碱磷腈P-Bu当量的S形曲线在2和3的情况下重叠，但在rac-1与2、3或4的情况下，存在明显分离。比较了多种不同的测定ΔpK的方法。当归一化Δδ为90%时测定的pK值对2和3是最佳的，而对于4，归一化Δδ为75%是最佳的，由此确定rac-1的pK为8.47 - 8.71。
