Pyrrole alkali metal salts

Pyrroles do not react with alkyl halides in a simple fashion polyalkylated products are obtained from reaction with methyl iodide at elevated temperatures and also from the more reactive allyl and benzyl halides under milder conditions in the presence of weak bases. Alkylation of pyrrole Grignard reagents gives mainly 2-alkylated pyrroles whereas N-alkylated pyrroles are obtained by alkylation of pyrrole alkali-metal salts in ionizing solvents. 

Pyrrole and alkylpyrroles can be acylated by heating with acid anhydrides at temperatures above 100 °C. Pyrrole itself gives a mixture of 2-acetyl- and 2,5-diacetyl-pyrrole on heating with acetic anhydride at 150-200 °C. iV-Acylpyrroles are obtained by reaction of the alkali-metal salts of pyrrole with an acyl halide. AC-Acetylimidazole efficiently acetylates pyrrole on nitrogen (65CI(L)1426). Pyrrole-2-carbaldehyde is acetylated on nitrogen in 80% yield by reaction with acetic anhydride in methylene chloride and in the presence of triethylamine and 4-dimethylaminopyridine (80CB2036). 

Pyrroles can be converted into alkali metal salts (with NaNH2/NH3 or K/toluene) (see also Section 2.3.3.1). The use of pyrrolyl or indolyl sodium or potassium salts under ionizing conditions favors the formation of A-acyl or IV-alkyl derivatives (Scheme 4) (740S(54)58,74OS(54)60). 

In addition to Grignard reactions and those occurring through complete transformation into alkali metal salts, there are reactions which take place under conditions of only partial conversion into anions. In some cases, 1-substituted compounds are formed, e.g. pyrrole is benzoylated in the presence of NaOH-H20 to yield 1-benzoylpyrrole. 

Specific acylation on the nitrogen atom of pyrrole and its benzologues is generally accomplished via their alkali metal salts (see Section 3.05.1.2.9). 

The apparent abnormal formation of ethyl pyrrole-2-carboxylate from the reaction of ethyl chloroformate and alkali metal salts of pyrrole could be accounted for by a thermal rearrangement during the isolation of the products. However, whilst this may be partly correct, an analysis of the reaction products prior to their isolation has established (77CJC4103) the formation of the thermodynamically more stable 2-carboxylic ester from the 1-isomer by a reaction sequence shown in Scheme 36. 

Potassium salts of pyrrole, 2-acetylpyrrole, 1,2,3,4-tetrahydrocarbazole, 9-carbazole, indole, imidazole, and 1,2,4-triazole react with [Tr-CjHjFe ( 0)2 ] to form ct-N derivatives, e.g., [7r-C5H5Fe(CO)2(CT-N-pyrrolyl)] (III) (400, 421), shown to be intermediates in the formation of the n-complexes e.g., (IV) (400). Anions [M(CO)5L] (M = Cr, Mo, W) were similarly prepared from the hexacarbonyls and alkali metal derivatives of succinimide, phthalimidine, and saccharin (49). 

Like secondary amines, pyrrole proves to be a (weak) NH acid with pK about 17.5 (for comparison pyrrolidine pfC, about 25). Therefore, pyrrole is N-deprotonated by alkali metals (Na, K), strong bases (like NaH and NaNH2), or organometallics (like lithium organyls or Grignard compounds) to give salt-like pyrrolyl metal compounds 6, for example ... 

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