Basicity of 1,8-bis(dimethylamino)naphthalene and 1,4-diazabicyclo[2.2.2]octane in water and dimethylsulfoxide

Abstract

The ionization constants of the conjugate acids [Formula: see text] of two bases widely used in synthesis, 1,8-bis(dimethylamino)naphthalene (PS), and 1,4-diazabicyclo[2.2.2]octane (DABCO), have been obtained in Me2SO and water. The partition coefficients of both bases B and their heats of solution and protonation have also been determined. The basicity of PS is dramatically reduced when passing from water to Me2SO while the basicity of DABCO remains unchanged, thus making DABCO the stronger base in Me2SO. The solvent effect on the base strength is analyzed in terms of free energy and enthalpy of transfer for B and H+, and for BH+, from water to Me2SO. The entropy-controlled much stronger solvation of PS in Me2SO is in line with that of aromatic compounds while the increased solvation of DABCO in water is due to H-bonding of its two nitrogens. PSH+, like H+, is better solvated by Me2SO while DABCOH+ does not show a definite preference for one solvent. The calculated enthalpy of solution of gaseous PSH+ is abnormally low when compared to that of DABCOH+ and of related BH+ ions, thus indicating that the proton of PSH+, in a strong intramolecular H-bond, does not normally H-bond to the solvent. Our results suggest that the proton sponge acid-base system could be used as a convenient reference to determine the free energy and enthalpy of transfer of H+ between solvents.