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 (71.5 kJ/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.
|Number of pages||4|
|Journal||Journal of the Chemical Society, Perkin Transactions 2|
|State||Published - 1 Jan 1979|
ASJC Scopus subject areas
- Chemistry (all)