Hydrogen-bond basicity of secondary and tertiary amides, carbamates, ureas and lactams

Abstract

The hydrogen-bond basicity scale pK_HB(logarithm of the formation constant of 4-fluorophenol–base complexes in CCl₄) has been measured for tertiary and secondary amides, carbonates, ureas and lactams. The hydrogen-bonding fixation site is the carbonyl group, even for the very hindered amide Bu^tCON(C₆H₁₁)₂. In the amides R¹CONR²R³ the hydrogen-bond basicity is decreased more by bulky R¹ substituents on the carbonyl carbon than by bulky R² and R³ substituents on nitrogen. The field effect of X substituents operates more effectively on hydrogen-bond basicity than the resonance effect in the XCONMe₂ series. The hydrogen-bond basicity is increased by six-membered cyclisation.