Reactivity studies of 1-propyl-2-(thiophen-2-yl)-1H-pyrrole

Abstract

The aim of this work is to study the reactivity of a thienylpyrrole heterocyclic system having in mind the selective functionalization of the pyrrole ring through electrophilic aromatic substitution (Vilsmeier-Haack formylation and azo coupling) or the selective functionalization of the thiophene heterocycle through metalation followed by electrophilic aromatic substitution. The formylation of 1-propyl-2-thiophen-2-ylpyrrole via two different methods of synthesis selectively yields two different products: 1-propyl-5-(thiophen-2-yl)-1H-pyrrole-2-carbaldehyde or 5-(1-propyl-1H-pyrrol-2-yl)thiophene-2-carbaldehyde. The azo coupling reaction of the thienylpyrrole derivative in the presence of an aryldiazonium salt yields the corresponding azo dye bearing the diazene moiety linked to the pyrrole heterocycle. These results motivate a further discussion on the typical reactivity of each of these heterocycles (thiophene and pyrrole) as well as the factors that determine the selectivity of these reactions. Several experimental techniques will be considered such as heating at reflux under anhydrous conditions and in an inert atmosphere, liquid-liquid extraction, gravity and vacuum filtration, thin layer chromatography (TLC), column chromatography using silica gel and melting point. The determination of the structure of all compounds synthesized will be also performed through 1H NMR, 13C NMR, IR and UV spectroscopic technique and mass spectrometry. The duration of the experiment will be 6 sessions: session 1: preparation of 1-propyl-5-(thiophen-2-yl)-1H-pyrrole-2-carbaldehyde; session 2: purification of 1-propyl-5-(thiophen-2-yl)-1H-pyrrole-2-carbaldehyde using column chromatography with a silica gel, session 3: preparation of 5-(1-propyl-1H-pyrrol-2-yl)thiophene-2-carbaldehyde, session 4: purification of 5-(1-propyl-1H-pyrrol-2-yl)thiophene-2-carbaldehyde using column chromatography with a silica gel; session 5: preparation of (E)-2-(4-nitrophenyl)-1-(1-propyl-5-(thiophen-2-yl)-1H-pyrrol-2-yl)diazene; session 6: Purification of (E)-2-(4-nitrophenyl)-1-(1-propyl-5-(thiophen-2-yl)-1H-pyrrol-2-yl)diazene using column chromatography on silica gel. The difficulty level of this experiment is high and the level of study is advanced.