HYDROGEN-BOND BASICITY OF THIOAMIDES AND THIOUREAS

The thermodynamic hydrogen-bond basicity scale pK(HB) (logarithm of the formation constant of 4-fluorophenol-base complexes in CCl4) has been determined for 18 thiocarbonyl bases, mainly thioamides and thioureas, and correlated to a spectroscopic basicity scale, the nu(OH) frequency shifts of methanol hydrogen bonded to the same basks. 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 Me(2)NC(Me)=NCSNMe(2) is the most basic thiocarbonyl base presently known. The apparently high basicity of secondary cyclic thioamides and thioureas is discussed.