Reactions of arylcarbamoyl chlorides—II : Reaction with aromatic amines

The reaction of N-arylcarbamoyl chlorides with anilines in acetonitrile initially gives isocyanates, which can react with primary or secondary amines to give N,N-diaryl ureas. The rate of HCl elimination fits the Bronsted catalysis law with β ≈ 1, suggesting that there is extensive proton transfer in the transition state. Plots of log k against σ have ? ≈ 0 when the aryl substituent has a negative σ and ? ≈ 1 when it has a positive σ value. These observations and the kinetic deuterium isotope effect suggest that with electron attracting substituents, an initial proton transfer gives an ion pair which then loses chloride ion, but with electron releasing substituents, these steps become more concerted.