Isolation and characterisation of a stable 2-azaphenalenyl azomethine ylide

Abstract

Although azomethine ylides have been fully exploited as versatile reactive intermediates in dipolar cycloaddition reactions to construct a variety of heterocyclic compounds involving a nitrogen atom, little is known about their structural and electronic properties. Here a method is developed for the preparation, isolation and characterization of a stable 2-azaphenalenyl based azomethine ylide. N-Phenyl-5,8-di-t-butyl-2-azaphenalenyl cannot be isolated because it undergoes rapid dimerization by C–C bond formation at the 1 and 3 positions. In contrast, sterically bulkier N-2,6-di(isopropyl)phenyl-5,8-di-t-butyl-2-azaphenalenyl can be generated and isolated as deep green crystals under deoxygenated conditions. X-ray crystal analysis of this stable azomethine ylide reveals that its azaphenalenyl skeleton has very small bond alternation and structural deformation. The results of spectroscopic and electrochemical theoretical studies indicate that N-2,6-di(isopropyl)phenyl-5,8-di-t-butyl-2-azaphenalenyl has electronic features that are similar to those of phenalenyl anion and it possesses an extremely high HOMO energy.