Reactions of some metal derivatives of pyrrole

Abstract

Reaction of pynrcrolylmagnesium bromide with iodomethane in diethyl ether gave only C-substituted pyrroles. When the reaction was carried out in the presence of more than two parts of hexamethylphosphoramide (HMPA) N-methylpyrrole was also obtained. The ratio of N-substituted pyrrole to C-substituted pyrroles increased with increasing amount of the HMPA added and if the HMPX was used as solvent, the product was almost exclusively N-methylpyrrole. It was believed that in the presence of an excess of this strongly basic complexing agent, the bond between the pyrrolyl group and the magnesyl group became dissociated. The reactions of pyrrolylmagnesium bromide with derivatives of carboxylic acids were believed to involve two types of Lewis acid and Lewis base reaction: (a) the reaction of the carbonyl carbon atom with the pyrrolyl group of the undissociated Grignard reagent, (b) the reaction of the carbonyl oxygen atom with the magnesyl group of the) Grignard reagent. The carbonyl group of such compounds as acetyl chloride, acetic anhydride, phosgene and carbon dioxide where the electcrophilicity of the carbonyl carbon atom was greatly enhanced by the electron-withdrawing group preferred the first type of Lewis acid and Lewis base reaction. The carbonyl group of such compounds as diethyl carbonate and phenyl isocyanate where the nucleophilicity of the carbonyl oxygen atom was enhanced by electron-donating groups preferred the second type of Lewis acid and Lewis base reaction. The first type of reaction always gives C-substituted products while the second type of reaction always gives N-substituted products. The reaction of 2-methylpyrrolylmagnesium bromide with ethyl chloroformate with the Grignard reagent from ethyl 2-pyrrolecarboxylate gave only ethyl diethyl 1,2-pyrroledicarboxylate. The reaction, of pyrrolyllithium with diethyl carbonate in 1,4-dioxane at room temperature gave almost exclusively ethyl-pyrrolecarboxylate. At the boiling temperature of 1,4-dioxarve, the ethyl 1-pyrrolecarboxylate was converted slowly to the thermodynamically more stable products, ethyl 2-pyrrolecarboxylate and 1,2-dipyrrolyl ketone.