Sustainable Manganese-Catalyzed Solvent-Free Synthesis of Pyrroles from 1,4-Diols and Primary Amines

Abstract

A general and selective metal-catalyzed conversion of biomass-derived primary diols and amines to the highly valuable 2,5-unsubstituted pyrroles has been developed. The reaction is catalyzed by a stable nonprecious manganese complex (1 mol %) in the absence of organic solvents whereby water and molecular hydrogen are the only side products. The manganese catalyst shows unprecedented selectivity, avoiding the formation of pyrrolidines, cyclic imides, and lactones.

Cycloamination strategies for renewable N-heterocycles

“…[82][83][84][85] With hydrogen and water as co-products, catalytic acceptorless dehydrogenative coupling of 1,4-butanediol or 1,4-substituted 1,4-butanediols 5 and amines 2 over basemetal complexes (e.g., cobalt or manganese pincer complex) generate 1 or 1,2,5-substituted pyrroles 4, respectively, at 150°C for 24 h with 90% yield (Scheme 2). 86,87 In the catalytic process, an aldehyde or ketone intermediate 1 is initially formed by dehydrogenation of alcohol to liberate H 2 , followed by coupling with the primary amine 2 to produce the N-substituted pyrrole 4 and water via Paal-Knorr condensation (Scheme 2). 86,87 A synergic effect between the metal and ligand species is observed in the dehydrogenative coupling reaction, which mainly contributes to the enhanced selectivity towards the product pyrrole.…”

“…86,87 In the catalytic process, an aldehyde or ketone intermediate 1 is initially formed by dehydrogenation of alcohol to liberate H 2 , followed by coupling with the primary amine 2 to produce the N-substituted pyrrole 4 and water via Paal-Knorr condensation (Scheme 2). 86,87 A synergic effect between the metal and ligand species is observed in the dehydrogenative coupling reaction, which mainly contributes to the enhanced selectivity towards the product pyrrole. 87 Although these homogeneous catalytic systems exhibit pronounced performance in the Paal-Knorr condensation reaction, difficulty in the catalyst recovery will lead to additional cost and negative impact on the environment.…”

Manganese Catalyzed Acceptorless Dehydrogenative Coupling Reactions

“…Recently, Rueping and co‐workers had reported a solvent‐free methodology for the synthesis of pyrroles 33 from 1,4‐diols 34 and primary amines 2 in the presence of the complex Mn‐29 (Scheme ) …”

“…[44] Recently, Rueping and co-workers had reported a solventfree methodology for the synthesis of pyrroles 33 from 1,4-diols 34 and primary amines 2 in the presence of the complex Mn-29 (Scheme 25). [45] Later, a multi-component synthesis of highly substituted pyrroles 33 via the ADC of 1,2-diols 28, ketones 20, and primary amines 2 in the presence of lutidine derived pincer complex Mn-30, and a catalytic amount of KO t Bu was developed (Scheme 26). [46] Mechanistic studies showed that reaction takes place through a combined ADC and BH methodology (vide infra).…”

Manganese‐Catalysed Dehydrogenative Coupling – An Overview

“…A greener solvent‐free approach for pyrrole synthesis was introduced by Sepelgy et al. in recent years (Scheme ) . A well‐established Mn(I)‐PNP with K 2 CO 3 base jointly constituted the catalytic complex.…”