<i>meso</i>-Triaryl-Substituted Smaragdyrins: Facile Aromaticity Switching

Abstract

All-aza smaragdyrin, a historic but elusive pentapyrrolic macrocycle, was successfully synthesized and characterized for the first time. [22]Smaragdyrin BF₂-complex 3 was smoothly synthesized by 2-fold S_NAr reaction of 1,9-dibromo-5-mesityldipyrrin with 5,10-dimesityltripyrrane. Treatment of 3 with methanesulfonic acid gave [20]smaragdyrin 5 as a stable antiaromatic compound. Reduction of 5 with NaBH₄ under inert conditions gave [22]smaragdyrin 6 as an aromatic congener, which was easily oxidized back to 5 in the air. Complex 3 was interconvertible with its 20π-congener 4 via oxidation with MnO₂ and reduction with NaBH₄. Treatment of 3 or 4 with CuCl₂ in the presence of NaOAc gave hetero bimetal complex 8.