A green, chemoselective, and efficient protocol for Paal–Knorr pyrrole and bispyrrole synthesis using biodegradable polymeric catalyst PEG‐SO₃H in water

Abstract

In water and in the presence of 1 mol % of Poly(ethylene glycol)-bounded sulfonic acid, amines undergo smooth condensation with 2,5-hexadione at room temperature to afford the corresponding pyrroles in good to excellent yields. The sole products were solidified as the reaction proceeded and separated by simple filtration. In addition, bispyrroles were synthesized in excellent yields via this eco-friendly protocol. The method is easy, simple, cost-effective, chemoselective, and environmentally benign that introduces a beautiful example of click chemistry in water.

Sulfonated polyethylene glycol (PEG-OSO3H) as a polymer supported biodegradable and recyclable catalyst in green organic synthesis: recent advances

“…The pyrrole ring system is a useful structural element in medicinal chemistry18 and has found broad application in drug development as an antibacterial, antiviral, anti-inammatory, antitumor, and antioxidant agent. 132 In addition, bispyrroles were synthesized in excellent yields via this eco-friendly and green methodology. The reaction was carried out in water as greener solvent at room temperature (Scheme 17a and b).…”

Biomimetic Clauson‐Kass and Paal‐Knorr Pyrrole Synthesis Using β‐Cyclodextrin‐SO3H under Aqueous and Neat Conditions ‐ Application to Formal Synthesis of Polygonatine†

“…Amongst these, Clauson‐Kaas and Paal–Knorr are most commonly used reactions for the synthesis of pyrroles as they use simple and readily available precursors. These reactions were reported with several catalysts such as Ionic liquid, zeolite, Sc(OTf) 3 , Ti(OiPr) 4 , Al 2 O 3 , Bi(NO 3 ) 3 , Yb(OTf) 3 , CoCl 2 , InCl 3 , RuCl 3 , montmorillonite‐KSF, sulfamic acid, SnCl 2 ⋅2H 2 O, silica sulfuric acid, microwave, P 2 O 5 , CuCl 2 , FeCl 3 .7H 2 O, montmorillonite K‐10, CeCl 3 , Bi(NO 3 ) 3 .5H 2 O, MgI 2 Et 2 O, I 2 , Cyclodextrin, ultrasound irradiation, nano‐organocatalyst, Amberlite IR 120, Pr(OTf) 3 , and PEG‐SO 3 H . Despite the advances, many of them suffer from certain drawbacks such as harsh reaction conditions, use of expensive reagents, toxic solvents, strongly acidic conditions, costly catalysts, longer reaction times, non‐recyclability, involve tedious workup procedures and use extra energy sources such as microwaves or ultrasound.…”

Unique chemoselective Paal-Knorr reaction catalyzed by MgI2 etherate under solvent-free conditions

“…It is worthy to be noted that the highly selective conversion of aniline was observed versus benzyl amine, which is just different from the silimar reaction catalyzed by PEG-SO 3 H reported by Jafari. 27 These results suggest that the relative reactivity of aromatic amines in the MgI 2 etherate-catalyzed process is determined almost solely by nucleophilicity of nitrogen lone pair upon the aromatic amines themselves. The reaction using MgI 2 etherate as catalyst has shown an important feature that is the ability to tolerate various amines including aliphatic, aromatic, and heterocyclic amines.…”