Unusually efficient deformylative synthesis of 1,2,8,9-tetrasubstituted dipyrrins from 4,5-disubstituted pyrrole-2-carbaldehydes

Upon heating in a mixture of hydrobromic acid and acetic acid, 4-arylmethyl-5-(4-methoxyphenyl)pyrrole-2-carbaldehydes (9a-c) react to give 2,8-bis(arylmethyl)-1,9-bis(4-methoxyphenyl)dipyrrins (10a-c) in high yields, demonstrating the first example of an unusually efficient deformylative transformation of pyrrole-2-carbaldehyde to dipyrrin. Dipyrrins 10 show a clear color change from red to blue, when exposed to Brønsted acid. Structure of 10a·H+ was determined by X-ray crystallographic analysis. The absorption change of 10a in the presence of a metal ion was also studied.