Hydrogen-bond basicity of thioamides and thioureas

Abstract

The thermodynamic hydrogen-bond basicity scale pK_HB(logarithm of the formation constant of 4-flouorophenol–base complexes in CCl₄) has been determined for 18 thiocarbonyl bases, mainly thioamides and thioureas, and correlated to a spectroscopic basicity scale, the v(OH) frequency shifts of methanol hydrogen bonded to the same bases. We confirm that the major site for hydrogen bonding is the sulfur atom and that the thiocarbonyl bases are always weaker than the corresponding carbonyl bases on the pK_HB scale. Iminology strongly increases hydrogen-bond basicity and the thiourea iminologue Me₂NC(Me)NCSNMe₂ is the most basic thiorcarbonyl base presently known. The apparently high basicity of secondary cyclic thioamides and thioureas is discussed.