Study of unusually high rotational barriers about S-N bonds in nonafluorobutane-1-sulfonamides:: The electronic nature of the torsional effect

Slow rotation about the S-N bond in N,N-disubstituted nonaflurobutane-1-sulfonamides 1 can easily be detected by NMR measurements at room temperature. This effect causes magnetic nonequivalence of otherwise identical geminal substituents in symmetrical staggered ground-state conformation A. The torsional barriers determined (62-71 kJ (.) mol(-1)) proved to he the highest ever observed for suffonamide moieties. They are comparable to the values routinely measured for carboxylic acid amides or carbamates. The restricted rotation is interpreted as result (n(N)-d(S) )-pi and of n(N)-sigma(SC)* interactions, which develop substantial S.N double-bond character in sulfonamides 1. The S.N binding interaction is increased by the highly electron-withdrawing effect of the perfluorobutyl group. The anticipated symmetry of the ground-state conformation A and the considerable shortening of the S-N bond (1.59 Angstrom)compared to the mean value in sulfonamides (1.63 Angstrom) are confirmed by single-crystal X-ray study of N,N-dibenzylnonaflurobutane-1-sulfonamide (1c).