Torsional barriers in substituted NN-dimethylcarbamates. A probe for perturbational molecular orbital analyses of amide rotation

Abstract

Free energies of activation for torsion about the N–CO bond in four aryl NN-dimethylcarbamates, p-YC₆H₄X·CO·NMe₂(1)–(4), were measured using dynamic n.m.r. spectroscopy, and utilizing a lanthanide shift reagent to increase resolution between the diastereotopic N-methyl groups. Barriers were: (1; X = O, Y = H) 16.5 kcal/mol)(69.0 kJ/mol); (2; X = O, Y = NO₂) 17.1 kcal/mol (71.5kJ/mol); (3; X = S, Y = H) 14.9 kcal/mol; (62.3 kJ/mol) and (4; X = S, Y = NO₂) 15.0 kcal/mol (62.8 kJ/mol). The results are discussed in terms of two recently proposed PMO analyses of amide rotation, one focusing on effects of the substituents at the torsional ground state and the other one on effects at the transition state. The higher barriers obtained for the para-nitro-substituted compounds relative to the unsubstituted ones is evidence for the dominance of ground-state effects in this system.