The four-component coupling reaction of aldehydes, amines, dialkyl acetylenedicarboxylates, and nitromethane in the presence of molecular iodine as a catalyst furnished the corresponding 1,2,3,4-tetrasubstituted pyrroles under reflux. The products are formed in high yields (65-88%) within 8 hours. The method is simple, efficient, cost-effective, and metal-free.
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Das et al. (2011) studied this question.