Study of Unusually High Rotational Barriers about SN Bonds in Nonafluorobutane‐1‐sulfonamides: The Electronic Nature of the Torsional Effect

Abstract

Dedicated to Professor Dieter Seebach on the occasion of his 65th birthday Slow rotation about the SÀN 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 À1 ) 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 Àd S )-p and of n N Às* S;C 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 SÀN bond (1.59 ä) compared to the mean value in sulfonamides (1.63 ä) are confirmed by single-crystal X-ray study of N,N-dibenzylnonafluorobutane-1-sulfonamide (1c).Restricted rotation [1] is a well-known phenomenon for compounds I with single bonds YÀA that adopt considerable double-bond character due to the binding interactions of the electron-withdrawing p-system XY with the lone pair at A, as expressed by the dipolar mesomeric formula II (Scheme 1). This has been described for Y and A elements of the second row of the periodic table. Not surprisingly, the highest values of barriers to rotation are normally observed for compounds containing an amino group (A N), due to the very strong p-donating character of the N-atom. Hindered rotation of this type is well-documented, and the barriers to rotation have been measured for many classes of compounds [1] [2], including esters, nitrites, enamines, hydrazones, thio-and selenoamides, nitrosoamines, and triazenes [3]. Undoubtedly, carbamates and amides proved to be the most frequently occurring substances exhibiting atropisomerism about their CÀN bonds. Reviews on dynamic effects in amides and related compounds [2] [4] reflect the fundamental character and significance of this topic. Elements of the third (e.g., Si, P, S) and higher periods exhibit much lower tendencies to p-bond. As a consequence, the barriers to rotation in the

Synthesis and structure of environmentally relevant perfluorinated sulfonamides

“…At ambient temperature several of the -N-CH 2 -systems show magnetically nonequivalent protons and at temperatures below 0°C typical geminal coupling constant of approximately 14.3 to 18.5 Hz were observed ( Table 2). The free energy of activation ΔG ≠ for several -N-CH 2 -systems was estimated using variable temperature NMR spectroscopy to be approximately 62-71 kJ mol −1 [16,25]. These values are in good agreement with literature values [16].…”

“…A similar conformation has been reported for N,Ndibenzyl-perfluorobutane-1-sulfonamide in both the solid state and in solution [16]. The S-N bonds of 4b (1.574(5) and 1.576(5) Å), 7e (1.581(5) Å) and 12 (1.590(9) Å) are short due to hyperconjugation with the perfluorooctyl chain [16]. For comparison, the unweighted sample mean for C-SO 2 -N(H)C and C-SO 2 -N(C)C bonds is 1.633±0.019 and 1.642 ±0.024, respectively [24].…”

“…The ethyl groups in both structures are on the site opposite to the perfluorooctyl group to minimize steric crowding around the S-N bond. A similar conformation has been reported for N,Ndibenzyl-perfluorobutane-1-sulfonamide in both the solid state and in solution [16]. The S-N bonds of 4b (1.574(5) and 1.576(5) Å), 7e (1.581(5) Å) and 12 (1.590(9) Å) are short due to hyperconjugation with the perfluorooctyl chain [16].…”

“…The dialkyl perfluoroalkanesulfonamides 7d-g and 8g, compounds of interest as analytical standards, were prepared from fluoride 1 or 2 and the respective dialkyl amines 6d-g using the same strategies employed for the monoalkyl derivatives 4a and 4b (Method D and E, Entries [15][16][17][18]. The best results with regard to purity were obtained using an excess of the amine as a base (Method D, entries 15 and 17).…”

“…Following these earlier (patent) reports we initially investigated the synthesis of perfluorooctanesulfonamide 7f by alkylation of Nmethyl perfluorooctanesulfonamide 4a with iodoethane. The desired perfluorooctanesulfonamide 7f was obtained in 78% yield when sodium methoxide was employed as a base [16], but the reaction was more straightforward and the yields significantly improved when potassium carbonate was used as a base.…”