Halochromism of pyridinium azomethine ylides stabilized by dicyanopyrazine group

Reactions of 2-bromomethyl-3-phenyl-5,6-dicyanopyrazine with methyl substituted pyridine gave dicyanopyrazylmethylene pyridinium bromides. These salts were changed to their corresponding methylides by the addition of a base, which were restored by the addition of an acid. The pyridinium methylides exhibited intense visible absorption. This study was attempted to apply the pyridinium halides with a dicyanopyrazine group to a reversibly colored material to gain an external response. The 1,3-dipolar cycloaddition reaction of such pyridinium azomethine ylides containing a stabilized dicyanopyrazine group with dimethyl acetylenedicarboxylate (DMAD) afforded the indolizine derivatives.