Imidazolide anions of imidazole, 2- and 4-methylimidazoles, 4,5-dimethyl and 4,5-diphenylimidazoles react with 1,3,5-trinitrobenzene (TNB) in dimethylsulfoxide (DMSO) to yield, initially, N-bonded σ-adducts under kinetic control. These undergo a subsequent conversion to C-adducts, which are the thermodynamically stable products. The formation of N,C-diadducts from the reversible attack of a second TNB molecule on the imidazolide moiety of the C-adducts is also observed. Provided that the corresponding position of the parent imidazolide anion is free, the formation of C-adducts occurs preferentially at the C-4 or C-5 carbon atom. However, C-2 substitution is observed in 4,5-disubstituted derivatives. Structures and preferred conformations of some adducts and diadducts are deduced from nOe experiments. The mechanisms of the various reactions are discussed. Evidence is presented that adduct formation results in a low-field shift of methyl groups located in a position adjacent to the substitution site, while all the other protons move upfield. This unusual deshielding presumably reflects a proximity effect between a methyl group and the hydrogen atom attached to the sp3 carbon of the adducts.
Unconventional vortex dynamics in the low-field superconducting phases of
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