A New Access to 2-(Alkylamino)- and 2-(Arylamino)pyrroles by Addition of Isocyanides to Protonated 1-Azabutadienes

Abstract

A number of 5-(alkylamino)- or 5-(arylamino)-2H-pyrrolium salts 3 or 5 have been obtained by treating the 1-aza-1,3-diene hydrochlorides 2 with isocyanides R4NC in refluxing acetonitrile or chloroform for a few hours. Depending on the experimental conditions, deprotonation of these species can occur in the reaction medium to furnish the corresponding 2-aminopyrroles 4 and 6. Insertion of isocyanide into a carbon–hydrogen bond of the pyrrolium salts can also lead to the generation of the (pyrrol-2-yl)methyleneiminium chlorides 7–9. Under similar conditions, treatment with an excess of tert-butyl isocyanide converts the protonated α-chlorocinnamaldimines 2j,k into the 5-(tert-butylamino)pyrrole-2-carbonitriles 13. Structural assignments of all the cycloadducts have been made on the basis of their NMR-spectroscopic properties, particularly the effects observed in NOEDIFF experiments. Mechanisms are suggested to account for the ring-closure reactions and autoxidation of pyrroles 4 and 6 under atmospheric oxygen to give the 5-imino-2-pyrrolinones 17 and 18.