A series of stable organosuperbases, N-alkyl- and N-aryl-1,3-dialkyl-4,5-dimethylimidazol-2-ylidene amines, were efficiently synthesized from N,N'-dialkylthioureas and 3-hydroxy-2-butanone and their basicities were measured in acetonitrile. The derivatives with tert-alkyl groups on the imino nitrogen were found to be more basic than the tBuP(1) (pyrr) phosphazene base in acetonitrile. The origin of the high basicity of these compounds is discussed. In acetonitrile and in the gas phase, the basicity of the alkylimino derivatives depends on the size of the substituent at the imino group, which influences the degree of aromatization of the imidazole ring, as measured by (13)C NMR chemical shifts or by the calculated ΔNICS(1) aromaticity parameters, as well as on solvation effects. If a wider range of imino-substituents, including electron-acceptor substituents, is treated in the analysis then the influence of aromatization is less predominant and the gas-phase basicity becomes more dependent on the field-inductive effect, polarizability, and resonance effects of the substituent.

A series of new N,N'-dialkyl-4,5-dimethylimidazolium cations possessing electron-rich 2-imidazolylidene- or phosphoranylidene-amino substituents has been efficiently synthesized from common precursors, N,N'-dialkyl-4,5-dimethylimidazol-2-ylidenes. The new lipophilic salts obtained have been found to be highly stable towards strong alkali under both biphasic and homogeneous conditions. Their exceptional aqueous base resistance, which has hitherto only been seen with peralkylated polyaminophosphazenium cations, may be attributed to three factors: aromatic stabilization, efficient resonance charge delocalization, and steric protection of the exocyclic nitrogen linkage due to bulky lipophilic N-alkyl substituents.

Both primary and secondary aliphatic nitro compounds 1 were found to react with two equivalents of bromotrimethylsilane in the presence of triethylamine followed by aqueous workup to give appropriate N,N-bis(trimethylsilyloxy)enamines 3 in good isolated yields. Products 3, starting from some secondary and/or sterically hindered compounds 1, are synthesized from the corresponding silyl nitronates 2.

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