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

Abstract

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.

Key words

8,8′-biquinolyl - lone-pair/lone-pair proximity effects - biaryl conformation - basicity - proton sponge

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Crystallographic data for all the compounds listed in Table  [²] have been submitted to the Cambridge Crystallographic Data Centre with deposition numbers CCDC 683590-683597. Copies of the data may be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: [phone] or e-mail: [e-mail]).