Substituent effects on the barrier to carbamate C–N rotation

Seven aryl-substituted t-butyl N-methyl-N-aryl carbamates were prepared, and in each case, the barrier to C–N bond rotation, ΔG‡, was determined in CDCl3 solution using variable temperature NMR. A linear free energy relationship is observed between ΔG‡ and the electronic stabilization effect of the substituent on the N-aryl ring. More specifically, electron donating groups increase ΔG‡, whereas withdrawing groups decrease ΔG‡. A plot against σ– was more linear (r2=0.96), than a plot against σ (r2=0.90) or σ+ (r2=0.88) and a value of ρ=1.76 was obtained at 243 K. Thus, rotation about the carbamate C–N bond in weakly polar chloroform involves a decrease in positive charge on the nitrogen.