Vibrational circular dichroism of 2,6-di-sec-butyl-4-methylpyridine and 2,6-di-sec-butyl-4-methylpyridine-N-oxide: theoretical evidence on the existence of multiple –CH, –CH_2, and –CH_3...O intramolecular hydrogen bonds on the nitroxide oxygen

摘要

Enantiopure (±)-2,6-di-sec-butyl-4-methylpyridine and its oxidized form (±)-2,6-di-sec-butyl-4-methylpyridine- N-oxide were prepared and then separated by chiral HPLC. Their stereochemical structures and their conformational distribution in solution were investigated using vibrational circular dichroism (VCD) and infrared spectroscopy (IR) combined with density functional theory (DFT). The experimental spectra have been compared with theoretical data. This comparison indicated that (-)-2,6-di-secbutyl- 4-methylpyridine and its oxidized form (+)-2,6-di-sec-butyl-4-methylpyridine-N-oxide have an (S,S)-configuration and exist in several conformations. The good fit confirms the reliability of the conformational analysis. Our results indicated that going from the reduced form to the oxidized one strongly influences the type of conformers in solution. Moreover, DFT calculations showed that for all of the conformers of (S,S)-2,6-di-sec-butyl-4-methylpyridine-N-oxide, the formation of four centered intramolecular hydrogen bonds between the hydrogen of the –CH, –CH_2, and –CH_3 groups and the nitroxide oxygen is possible. Moreover the stability of the conformers of both compounds is influenced by the all-trans structure of the sec-butyl moieties.