meso-Tetraphenylporphine as a prochiral solvating agent (pro-CSA): A physicochemical study

Abstract

Meso-tetraphenylporphine (TPP) can be used as a prochiral chiral solvating agent (pro-CSA) for NMR detection of enantiomeric excess (ee) of chiral organic acids, in this study 2-phenoxypropionic acid (PPA). The pro-CSA sensing mechanism is unique in that it does not depend on formation of diastereomers. Here we discuss the factors affecting construction of the calibration curves ([Formula: see text] observed linearity between induced chemical shift non-equivalency and ee) including temperature, guest exchange rate and binding strength. We have employed various NMR techniques including variable temperature NMR, titration experiments and analyses of NMR spectral line shapes. It is confirmed that optimization of ee sensing conditions is required for each TPP/acid guest pair. For the TPP/PPA host–guest system, an optimum operating temperature is -32[Formula: see text] C with an acceptable range from -28[Formula: see text] C to -50[Formula: see text] C. The upper bound is due to complete diprotonation of TPP by PPA and the lower bound is due to the limit of PPA guest exchange rate at TPP’s binding site [Formula: see text] s[Formula: see text]. These findings, typical for porphyrin-type diprotonated pro-CSAs, yields valuable insight for design of the next generation of ee sensing molecules.