Anion-π interactions influence pKa values

Five 8-(4-R-phenyl)-1-naphthol derivatives were prepared by PdCl2-catalysed electrophilic aromatic substitution. The pK(a)' values for these 1,8-disubstituted arene naphthols have been measured in acetonitrile/water (R = NO2, 8.42; R = Cl, 8.52; R = H, 8.56; R = Me 8.68; and R = OMe, 8.71) and indicate a correlation with the electronic nature of the arene substituent, as determined through LFER analysis. Contributions to the relative pK(a)' values have been interpreted, using M06-2X DFT calculations, as consisting of two components: A small contribution from initial OH-pi bonding in the starting materials and a larger contribution from anion-pi interactions in the products. Such effects have implications for a range of other systems.