Effects of Substituents on the Barriers to Rotation in Phenyl and Benzyl Carbamates

Abstract

The free energy of activation for rotation about the C-N bond (ΔG^‡_Tc) in substituted phenyl and benzyl N,N- dimethylcarbamates was determined by the ¹³C dynamic n.m.r. method. The ΔG^‡_Tc value depended linearly on Hammett's σ with positive p. We have separated the substituent effects into inductive and resonance effects by using dual substituent parameter ( d.s.p .) analysis, and found that the resonance effect is more important than the inductive effect for para derivatives, and vice versa for meta derivatives. General trends of electronic substituent effects on barriers to rotation in carbamates and structurally related amides are also discussed. The ortho steric effect was small.