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Study of Unusually High Rotational Barriers about SN Bonds in Nonafluorobutane‐1‐sulfonamides: The Electronic Nature of the Torsional Effect
Author(s) -
Lyapkalo Ilya M.,
Reissig HansUlrich,
Schäfer Andreas,
Wagner Armin
Publication year - 2002
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200290006
Subject(s) - chemistry , geminal , sulfonamide , crystallography , ground state , stereochemistry , solid state , crystal structure , bond length , computational chemistry , physics , quantum mechanics
Slow rotation about the SN bond in N , N ‐disubstituted nonafluorobutane‐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 be the highest ever observed for sulfonamide 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 )‐ π and of n N σ 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 SN bond (1.59 Å) compared to the mean value in sulfonamides (1.63 Å) are confirmed by single‐crystal X‐ray study of N , N ‐dibenzylnonafluorobutane‐1‐sulfonamide ( 1c ).