Visible-Light-Driven Synthesis of 1,3,4-Trisubstituted Pyrroles from Aryl Azides

Abstract

The synthesis of 1,3,4-trisubstituted pyrroles via visible-light mediated photoredox catalyzed condensation of arylazides and aldehydes has been reported herein. The methodology avoids the use of stoichiometric oxidants and provides the corresponding N-containing arenes in good yields (up to 78%) and mild conditions. Mechanistic rationale is provided via a dedicated and combined spectroscopic/experimental investigations.

Pd-Catalyzed Triple-Fold C(sp2)–H Activation with Enaminones and Alkenes for Pyrrole Synthesis via Hydrogen Evolution

“…The classical Paal–Knorr reaction, Knorr reaction, and Hantzsch reaction, for example, have contributed enormously to the advances of pyrrole chemistry. However, following the daily increasing demand in molecular diversity-, sustainability-, and safety-oriented synthetic strategies, devising alterative and/or complementary methods for pyrrole synthesis has emerged as a significant goal in the science of synthesis . As one novel example of a newly emerged tool in pyrrole synthesis, the cascade annulation involving the key activation of one or more C–H bonds has brought an important breakthrough.…”

Recent Advancements in Pyrrole Synthesis

“…For instance, Bandini and Protti reported condensation of aryl azides and aldehydes promoted by Ru(II) catalysis and blue LED light for the regioselective preparation of 1,3,4-trisubstituted pyrroles 112 (Scheme 12). 62 In this reaction, aryl azides served both as a source of nitrogen atom for the pyrrole core as well as a formal stoichiometric oxidant. The proposed mechanism begins by the generation of photoexcited [Ru 2+ ]* complex, which undergoes oxidative quenching by aryl azide to produce anilino radical cation 113 upon protonation of the emerging nitrogen radical intermediate.…”

Redox-Neutral 1,3-Dipolar Cycloaddition of 2H-Azirines with 2,4,6-Triarylpyrylium Salts under Visible Light Irradiation

“…The reaction employed an Ir photocatalyst for the photocatalytic reduction of α-bromo ketones, generating alkyl radicals which reacted with enamines in Hantzsch fashion to provide disubstituted pyrroles. Recently, Protti, Bandini, and co-workers developed a visible light photoredox catalyzed synthesis of 1,3,4-trisubstituted pyrroles via condensation of aryl azides with aldehydes . The reaction employed a Ru photocatalyst and involved in situ generation of aryl amines and 1,4-dialdehydes which undergo Paal–Knorr-type condensation to afford the final product.…”

“…Recently, Protti, Bandini, and co-workers developed a visible light photoredox catalyzed synthesis of 1,3,4-trisubstituted pyrroles via condensation of aryl azides with aldehydes. 8 The reaction employed a Ru photocatalyst and involved in situ generation of aryl amines and 1,4-dialdehydes which undergo Paal−Knorr-type condensation to afford the final product. Another method developed by Maurya and co-workers utilized Ru-polypyridyl complexes to photocatalyze the coupling of 1/2naphthols and 2-hydroxy-1,4-naphthoquinones with 2H-azir-ines obtained by photodecomposition of azidochalcones to prepare a 2,3-fused pyrrole motif.…”