Pyrrole trimer, formation

Physically this account of the acid catalysis is obscure. The clue to the true nature of the acid catalysis is contained in the mechanism of pyrrole trimer formation, discussed above the catalyst functions by activating the entity which is to attack the pyrole nucleus. Thus, the acid catalysis of the reactions of pyrroles with carbonyl compounds is caused by protonation of the carbonyl compound. 

Protonation of pyrrole, furan and thiophene derivatives generates reactive electrophilic intermediates which participate in polymerization, rearrangement and ring-opening reactions. Pyrrole itself gives a mixture of polymers (pyrrole red) on treatment with mineral acid and a trimer (146) under carefully controlled conditions. Trimer formation involves attack on the neutral pyrrole molecule by the less thermodynamically favored, but more reactive, (3-protonated pyrrole (145). The trimer (147) formed on treatment of thiophene with phosphoric acid also involves the generation of an a-protonated species. 

A novel intermolecular type of trimer formation occurs with 2-hydroxy-5,10,15,20-tetraphenylporphyrinatoiron(lll). Deprotonation of the 2-hydroxy group creates an anionic aryloxo ligand for another Fe center, and the cyclic trimer thus forms. The metal ions are weakly coupled high-spin Fe centers (/Xeff = 5.4 0.2/xb), and the 3-H NMR resonances of this low-symmetry trimer are uniquely found at very negative chemical shifts (-89.8, -94.7 and -99.3 ppm at 291 K) and are very broad (670-690 Hz), while all other /3-pyrrole protons resonate in the +82 to +62 ppm region typical of high-spin Fe in monomeric complexes.The structure of the trimeiic complex has also been reported. ... 

Ni(II)-meso-tetramethylporphine ( 3a) was converted by two ways into polymeric porphyrins Firstly, bromation of 83a) in CCI4 in the presence of AIBN gave the expected monobrommethylporphyrin 83 b). This reactive intermediate (detected as methoxymethyiderivate 83 c) is reacting easily with a /8-pyrrol position of another porphyrin (Eq. 38). Beside dimer and trimer formation the polymer 84) was obtained (yield 21%). 83 c) is also converted into 84) with HCl (51% yield). In the electronic spectra the broader Soret band at 425 nm of 84) is shifted to the bathochrome side due to connection in /8-position of the pyrrole ring compared with starting compound 83 a). 

D. Discussion of a Possible Free-Radical Mechanism for the Formation of Pyrrole Trimer... 

The mineral-acid-catalysed polymerisation of pyrrole involves a series of Mannich reactions, but under controlled conditions, pyrrole can be converted into an isolable trimer, which is probably an intermediate in the polymerisation. The key to understanding the formation of the observed trimer is that the less stable, therefore more reactive, P-protonated pyrrolium cation is the electrophile that initiates the sequence, attacking a second mole equivalent of the heterocycle. The dimer , an enamine, is too reactive to be isolable, however pyrrole trimer , relatively protected as its salt, reacts further only slowly. ... 

Reactions involving primary amines have been successfully carried out via Schiff s bases derived from a number of aldehydes. Thus, the reaction of pyrrole with the protonated enamine (1), which results in the formation of the pyrrole trimer (2 equation 1), constitutes a good example of a Mannich reaction involving an aldehyde other than formaldehyde. ... 

Pyrrole forms with sodium bisulphite an addition compound, C4H7N0eS2Na2.2H20, which has been formulated as the dihydrate of the disulphonate (49). The reaction has been represented as a nucleophilic attack upon the polarized pyrrole molecule. Mechanistically it could be very satisfactorily represented as a nucleophilic attack by bisulphite anion upon the highly reactive -protonated cation of pyrrole in a manner analogous to the formation of pyrrole trimer (p. 86). However, under the conditions used 4, the concentration of cation must be extremely small. 

Basicity and Acidity Measurements The formation of salts, or adducts, in the reactions between pyrroles and proton acids or Lewis acids has been described by numerous investigators.z4,178-197 In many instances it has been shown these products are the salts of the pyrrole dimer and in the case of the unsubstituted pyrrole it is known that a trimer is readily formed in dilute aqueous acid.178,179,198 Higher polymeric products have also been reported.199 Relatively few monomeric pyrrole salts have been isolated,186,188 but proof of their existence in solution has been reported.189-198 200-202... 

Treatment of 2-methylpyrrole with HCl produces a dimer, not a trimer as does pyrrole itself. Suggest a structure for the dimer, C10H14N2, and explain the non-formation of a trimer. 

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