Determination of pKaValues for Diether Derivatives of 7,7′-Dihydroxy-8,8′-biquinolyl: Dependence of Basicity on Interannular Dihedral Angle

7,7′-Dihydroxy-8,8′-biquinolyl and 2,2′-di-tert-butyl-7,7′-dihydroxy-8,8′-biquinolyl were O,O′-dialkylated with MeI or dihaloalkanes [X(CH2)nX′, n = 1, 3-6]. The resulting diethers were characterized by UV spectroscopy and (wherever possible) single-crystal X-ray diffraction analysis. Biquinolyl basicity was found to positively correlate with interannular dihedral angle (IDA). Two pK a points could be located for all biquinolyl diethers with IDA above ca. 65˚ (pK a1 2.3-2.7; pK a2 4.5-5.3 in aq MeOH). By contrast, methylene diethers (IDA ≤ 60˚) were only very weakly basic (pK a 2.7-3.1) and could not be doubly protonated. Lone-pair/lone-pair proximity effects are invoked to account for the observations.