3,3′‐ and 4,4′‐Dimethoxy‐2,2′‐bipyrroles: Highly Electron‐Rich Model Compounds for Polypyrrole Formation.

3,3′‐Dimethoxy‐2,2′‐bipyrrole ( 1 ) and 4,4′‐dimethoxy‐2,2′‐bipyrrole ( 2 ) were obtained in short sequences and good yields from N ‐benzyl‐3‐hydroxypyrrole‐2,4‐dicarboxylic acid. The key intermediate leading to 1 is an N ‐benzyl‐3‐methoxypyrrole, which is dimerized by lithiation and oxidation with NiCl 2 . The formation of 2 is achieved by a classical Ullmann coupling of diethyl 1‐benzyl‐2‐bromo‐4‐methoxypyrrole‐3,5‐dicarboxylate. The N ‐benzyl protection groups of 1 and 2 are cleaved under reducing conditions with sodium in liquid ammonia. Both isomeric bipyrroles are extremely sensitive toward air. Compound 1 has a very low oxidation potential of 0.09 V against AgCl but film formation hardly occurs. On the other hand, compound 2 with a potential of 0.35 V readily forms stable polypyrrole films with anodic waves at −0.51 and −0.35 V and a cathodic wave at −0.77 V, the lowest potential ever observed for a p‐doped polymer.