Nucleophilic substitution at the pyrrole ring. Comparison with furan, thiophene, and benzene rings in piperidinodenitration

The reactivity of the pyrrole ring in nucleophilic aromatic substitution has been evaluated for the first time by rate measurements for the piperidinodenitration of 1-methyl-2,5-dinitropyrrole (1) at varying temperatures. Relative rates (krel) have been established at 25°C and show that 1 is markedly less reactive than 2,5-dinitrofuran and 2,5-dinitrothiophene, the krel values being 1, 2.4 × 106, and 4.4 × 103, respectively. A less significant difference is found with the rate of 1,4-dinitrobenzene (krel = 9.6). The factors affecting the observed reactivities and activation parameters are discussed.