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Sels de Pyrylium. XVIII. Etude par RMN du Carbone-13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N-Diméthylaminopyrylium
Authors:M. Th. Chenon  S. Sib  M. Simalty
Abstract:The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG, ΔH, ΔS parameters related to the corresponding hindered rotation have been determined by 13C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG values decreases from 19.1 kcal mol?1 (79.9 kJ mol?1) to 12.6 kcal mol?1 (52.7 kJ mol?1) according to the following sequence for the R-4 substituents: -C6H5, -CH3, -OCH3, -N(CH3)2.
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