The Friedel-Crafts isopropylation of 2-substituted pyrrole derivatives

Hopkins, Lloyd Cameron (1965) The Friedel-Crafts isopropylation of 2-substituted pyrrole derivatives. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Priedel-Crafts isopropylations of methyl 2-pyrrolecarboxylate, 2-acetylpyrrole, and 2-pyrrolecarboxaldehyde have been investigated. Various Lewis acids were employed as catalysts and solvents included carbon disulfide, ethylene chloride, and nitromethane. The changes in product distribution and reaction rate with variations in substrate, solvent, catalyst and conditions of reaction are discussed. -- Under normal reaction conditions (carbon disulfide solvent, aluminum chloride catalyst, 50°0.) the alkylation of methyl 2-pyrrolecarboxylate gave the 4-isopropyl, 5-isopropyl and 4,5-diisopropyl-2-esters, all of which have been isolated and identified. It was found that while the 4-isopropyl-2-ester rearranged to the 5-isopropyl-2-esters under the reaction conditions, neither the latter nor the 4,5-diisopropyl-2-ester underwent any change. The behaviour of 2-acetylpyrrole under Friedel-Crafts conditions was similar to that of the ester but alkylation was more complete and occurred predominantly at the 4-position giving much less of the other products. Also, the 4-isopropyl group did not rearrange as readily as that of the ester. In marked contrast to the other substrates, 2-pyrrolecarboxaldehyde underwent reaction very rapidly to give only the 4-isopropyl-2-carboxaldehyde which was shown not to rearrange. -- Ultraviolet, infrared, and nuclear magnetic resonance spectra of the various pyrrole derivatives are discussed. N. m. r. studies indicate that substantial changes in the electron density pattern of the pyrrole ring are brought about by donor-acceptor complexing between the Lewis acid and the carbonyl oxygen of the substrate molecules. The relative strength of complexing along the series: aldehyde>ketone>ester, is considered to provide a partial explanation of the product distributions observed in Friedel-Crafts isopropylation. -- Molecular orbital calculations of localization energies for electrophilic attack on 2-pyrrolecarboxaldehyde have been found to account satisfactorily for the results which have been observed in nitration. As well, in agreement with experiment, the calculations indicate that complexing of the carbonyl oxygen with Lewis acids, as in the Friedel-Crafts reaction, should greatly favor substitution at the 4-position.

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