13C nuclear magnetic resonance spectroscopy of nitrogen heterocycles. Part 4. intra-extra Configuration of the N-acetyl group in phenothiazine and related systems with a ‘butterfly’ shape

Abstract

Nitrogen lone-pair delocalization into aromatic rings has been studied for phenothiazine, N-acetylphenothiazine, and related tricyclic systems with a ‘butterfly’ conformation, by using ¹³C n.m.r. spectroscopy. Pyridazo[4,5-d][1,4]benzothiazines, phenoxazines, acridane, and simple model compounds such as diphenylamine, 2-methylthioaniline, and their corresponding N-acetyl derivatives were examined. The preferred configuration of the N-acetyl group has been determined for CDCl₃ and Me₂SO solutions. Nitrogen lone-pair delocalization of the N-acetyl group into the aromatic system is strongly hindered for all tricyclic compounds, and greatly reduced for N-acetyldiphenylamine and also for 2-methylthioacetanilide in Me₂SO. For this last compound, the decrease of conjugation is due to a solvent-induced change of conformation; for tricyclic systems it is a consequence of the preferred extra-configuration of the N-acetyl group.