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.

Addition of isocyanides to ?-(methylthio)-benzylidenamidinium iodides: A surprising access to 2-(dialkylamino)imidazoles and 3,5-diamino-2H-pyrrolium salts

“…Additionally, the salt 7a shows two intense absorption peaks about 1660 and 1610 cm ‫1מ‬ which may be assigned to conjugated five-membered ring iminium and C‫ס‬C groups, respectively. The structure 7 was also supported by the 13 C NMR data reported in Table 2, in satisfying agreement with values in the literature for related 2H-pyrrolium salts [11].…”

“…The reaction (b) conforms to a [1 ‫ם‬ 1 ‫ם‬ 2] cycloaddition pattern across the C‫ס‬N double bond of the starting salt 1. Similar 1:2 cycloadditions giving rise to fourmembered heterocyclic compounds have already been reported [5,11,18].…”

“…Recently [11], the formation of 5-amino-2H-pyrrolium salts from 1-azabutadiene hydrochlorides and isocyanides was rationalized by a nucleophilic addition to the terminal C-4 (path a, Scheme 1). Reactions on the 2-position were also observed.…”

A simple four-component synthesis of ferrocenyl amidodiesters and ferrocenyl triamides

“…Subsequent [1 þ 4] cycloaddition reaction with isocyanide 2 affords an iminolactone intermediate 7 [27,28], followed by reaction with alcohol leads to the vinylogous carbonate 8. Subsequently loss of acetone from 8 and the reaction with another molecule of alcohol to afford product 5 (Scheme 1) [24].…”

Four-membered rings from isocyanides: Developments since the mid 1980s

“…The experiment has been carried out along with studies on ,-unsaturated imines which, however, afforded aminopyrrole derivatives instead of analogs of 19. 15 The thioimidate unit of the salts 20a,b could be converted into an azetidine using tert-butyl and isopropyl isocyanide ( 21a-c). But a competing reaction gave the 2-aminoimidazoles 22a-e (by involvement of the HC=NR' 2 moiety of 20).…”