Pyrroles conjugate acids

Calculated by Terrier and coworkers104 from data of Alexander and Butler105 by assuming a kinetic isotope effect, kyJkT) = 18, for dehydrogenation of the pyrrole conjugate acids. No statistical correction is made here. 

Pyrroles react with the conjugate acids of aldehydes and ketones to give carbinols (e.g. 67) which cannot normally be isolated but which undergo proton-catalyzed loss of water to give reactive electrophiles (e.g. 68). Subsequent reaction may lead to polymeric products, but in the case of reaction of pyrrole and acetone a cyclic tetramer (69) is formed in high yield. 

Examine pyrrole s highest-occupied molecular orbital (HOMO) to see if your can predict the most favorable protonation site. Which of the pyrrole s conjugate acids (N protonated, C2 proto noted, C3 proto noted pyrrole) is lowest in energy Examine electrostatic potential maps to see if the lowest-energy form is also that in which the positive charged is best delocalized. Rationalize your result using resonance arguments. What should be the favored substitution product ... 

B. Conjugate Acids 1. Furan, Pyrrole, and Thiophene Cations... 

Chemical evidence led to the conclusion that the conjugate acids of pyrrole probably exist predominantly as 16 or 17 rather than as X52,4,5 Although infrared spectra were initially interpreted on the... 

This is reflected in the basicity of pyrrole. Pyrrole is a particularly weak base, with pA a of the conjugate acid —3.8. First, we should realize that protonation of pyrrole will not occin on nitrogen nitrogen has... 

It is possible to protonate pyrrole using a strong acid, but even then the protonation occurs on C-2 and not on the nitrogen. Although this still destroys aromaticity, there is some favourable resonance stabilization in the conjugate acid. Protonation on C-3 is not as favourable, in that there is less resonance stabilization in the conjugate acid. It turns out that, as opposed to acting as a base, pyrrole is potentially an... 

Indoles, like pyrroles, are very weak bases. The conjugate acid of indole has pATa — 3.5 that of pyrrole has pATa - 3.8. As in the case of pyrrole (see Section 11.3), nitrogen has already contributed its lone pair to the aromatic sextet, so A-protonation... 

Predictably, isoindole is a stronger base than pyrrole, the conjugate acid of which has a pKa of -3.8 on the H0 scale (-4.4 on the Hr scale). From the few measurements that are available, isoindoles having phenyl groups at the 1,3-positions are protonated at the... 

Examine pyrrole s highest-occupied molecular orbital (HOMO) to see if your can predict the most favorable protonation site. Which of the pyrrole s conjugate acids (N protonated, C2 proto noted, C3 protonated pyrrole) is... 

UV Absorption of Alkyl and Aryl Pyrroles and Their Conjugate Acids... 

Pyrroles having electron-withdrawing substituents are considerably less basic than alkyl pyrroles. Whereas there appears to be strong UV spectral evidence for the existence of the conjugate acids of... 

The spectra of conjugate acids of pyrrole and of pyrrolyl anions have also been investigated (see Section III, A, 1). Protonation of the... 

We have presented evidence that pyrrole-2-carboxylic acid decarboxylates in acid via the addition of water to the carboxyl group, rather than by direct formation of C02.73 This leads to the formation of the conjugate acid of carbonic acid, C(OH)3+, which rapidly dissociates into protonated water and carbon dioxide (Scheme 9). The pKA for protonation of the a-carbon acid of pyrrole is —3.8.74 Although this mechanism of decarboxylation is more complex than the typical dissociative mechanism generating carbon dioxide, the weak carbanion formed will be a poor nucleophile and will not be subject to internal return. However, this leads to a point of interest, in that an enzyme catalyzes the decarboxylation and carboxylation of pyrrole-2-carboxylic acid and pyrrole respectively.75 In the decarboxylation reaction, unlike the case of 2-ketoacids, the enzyme cannot access the potential catalysis available from preventing the internal return from a highly basic carbanion, which could be the reason that the rates of decarboxylation are more comparable to those in solution. Therefore, the enzyme cannot achieve further acceleration of decarboxylation. In the carboxylation of pyrrole, the absence of a reactive carbanion will also make the reaction more difficult however, in this case it occurs more readily than with other aromatic acid decarboxylases. 

Pyrrole is a 7r-electron excessive compound and reacts at the 2-position with electrophiles more rapidly than benzene. However, depending on the acid strength, either the free base or the conjugate acid is the reacting species. 

The nitration of indole (14a) in acetic anhydride occurs in both the pyrrole and the benzene rings to produce 3,6-dinitroindole (15), whereas in sulfuric acid, in which the conjugate acid is the reactant, nitration occurs to yield 5-nitroindole (16) (63JOC2262 64TL803). 

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