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 Me 2 SO 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 Me 2 SO while the basicity of DABCO remains unchanged, thus making DABCO the stronger base in Me 2 SO. 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 Me 2 SO. The entropy-controlled much stronger solvation of PS in Me 2 SO 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 Me 2 SO 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.