Equilibrium acidities of BINOL type chiral phenolic hydrogen bonding donors in DMSO

The pK_a values of fifteen BINOL type chiral phenolic catalysts were determined by the overlapping indicator method in DMSO via UV spectrophotometric titrations. The acidities cover the range from 9.30 to 16.43. The pK_a gap between BINOL and 2-naphthol was explained by IR spectrum data, crystal structure and ¹H NMR. The relationship between the pK_a values of 3,3′-modified BINOLs and Hammett ortho substituent constants (σ_o ^-) was investigated and a good correlation (R² = 0.984) was obtained. The results may be helpful for the rational design and development of novel BINOL type phenolic catalysts, sensors and ligands.