The protonation of n-phenylpyrroles

Five‐Membered Heterocycles: Pyrrole and Related Systems

“…This is supported by the fact that the dipole moment of pyrrole is directed into the ring, as opposed to its tetrahydro derivative pyrrolidine, which displays a dipole moment direction towards the nitrogen atom [4]. Based on observations during protonation studies involving various substituted pyrrole derivatives [215,217], it is clear that the basicity is markedly increased by introduction of alkyl groups by stabilizing the corresponding cations. The presence of tert-butyl groups even allows isolation of stable 2H-pyrrolium salts, for example 145, which was obtained in quantitative yield as a crystalline solid from pyrrole 146 (Scheme 4.45) [218].…”

Isotopic Labeling Study of the Formation of Calix[5]pyrroles Catalyzed by Bi(NO3)3

“…The result was interesting because it suggested the involvement of the Brønsted acid in the medium. The basicity of the 3-and 4-positions of pyrrole has been demonstrated in 2,5-disubstituted pyrroles (Scheme 4) [16].…”

Electrophilic Substitution of the Pyrrole Ring

“…From these data and the observed changes in the UV spectrum of pyrrole in acid as the pH is decreased, it can be estimated that the pK a for ß-protonation of pyrrole is -5-9. (12) Steric inhibition of solvation applies for both a-and ß-protonation since the two rings are almost at right angles to each other. From protonation studies of carbazole, the pK a of pyrrole for protonation at the nitrogen atom has been estimated to be ca.…”

“…Thus, the protonation of pyrrole occurs to the extent of less than 1% at the ßposition. (12) The IR spectra of the hydrochloride salts of 2-formyl and 2-and 3-acetyl pyrroles indicate that protonation occurs on the carbonyl oxygen atom thereby increasing the electron-withdrawing effect of the substituent upon the pyrrole ring. -10, indicating that N-protonation would occur to the extent of only one molecule in a million compared with protonation at either the a-or ß-carbon atom.…”

“…However, the 4-sulphonic acid, isolated as the potassium salt, was obtained by Ciamician from the reaction of 2-acetylpyrrole with sulphur trioxide, (440) whilst Terentev found that sulphonation of pyrrole and alkylpyrroles was best effected with the pyridine . Reaction with formaldehyde converts the sulphonic acids into dipyrrylmethanes(ll)-> (12). (441) Using this method, 2-chloropyrrole reacts at 60° to give the 5-sulphonic acid in a 40% yield and 1-phenylpyrrole is sulphonated on both rings to give the 2,4'-disulphonic acid.…”