4,6-Dinitro-benzofuroxan and -benzofurazan adducts of 1,8-bis(dimethylamino)naphthalene. A conformational study by low temperature nuclear magnetic resonance

Low temperature 1H nuclear magnetic resonance spectra of the zwitterionic and anionic carbon-bonded adducts resulting from the addition of 4,6-dinitrobenzofuroxan (DNBF) or 4,6-dinitrobenzofurazan (DNBZ) to 1,8-bis(dimethylamino)naphthalene, the proton sponge PS, have been recorded at 500 MHz inDMF-d7. They reveal the presence of two conformers due to restricted rotation around the bond between C-4 of PS and the sp3 carbon C-7′ of the negatively charged DNBF− or DNBZ− moiety. The geometry of these conformers was deduced from nuclear Overhauser enhancement difference spectra. The results show that in all systems there is a large preference for the less strained s-trans type of conformation, whose population lies between 80 and 90%. An interesting feature is the dependence of the chemical shift of the peri H5 proton on the conformation. In accordance with previously reported peri effects, a significant deshielding of H5 is observed in the major conformers, where this proton is spatially close to H7′. In con...