On the α,α'‐Oxidative Coupling of β‐Alkoxy‐Substituted 2‐Pyrrolecarboxylates

Abstract

The electrochemical and chemical oxidative α,α'‐coupling of 3‐methoxy‐4‐methyl‐2‐pyrrolecarboxylic acid (2) and its ethyl ester 4 is compared with that of the corresponding 4‐methoxy‐2‐pyrrolecarboxylates (1 and 3 respectively). The β‐alkoxy substituent exerts a strong directing influence on its vicinal α‐position, which in the case of compounds 2 and 4 determines the 2,5'‐coupling and allows the isolation of the corresponding 1H,2'H‐2,2'‐bipyrrole 6 in the case of 4. For 2 the polypyrrole can be obtained by exhaustive electrochemical or chemical coupling, whereas in the case of 1 only the corresponding tetramers are formed. The structures of these tetrapyrroles, already described in the literature, here obtained by oxidation of 4‐alkoxy‐2‐pyrrolecarboxylic acid (1), have been reexamined: they are mixtures of tetramers with zwitterionic structure, have at least two substitution patterns and are partially oxidized to the corresponding bipyrrolylidenes.