Pyrrole mechanism

Polyheterocycles. Heterocychc monomers such as pyrrole and thiophene form hiUy conjugated polymers (4) with the potential for doped conductivity when polymerization occurs in the 2, 5 positions as shown in equation 6. The heterocycle monomers can be polymerized by an oxidative coupling mechanism, which can be initiated by either chemical or electrochemical means. Similar methods have been used to synthesize poly(p-phenylenes). 

The light-induced rearrangement of 2-phenyl- to 3-phenyl-thiophene may occur by a similar mechanism an equilibrium between the bicyclic intermediate (26) and the cyclopro-penylthioaldehyde (27) has been suggested (Scheme 2). The formation of IV-substituted pyrroles on irradiation of either furans or thiophenes in the presence of a primary amine supports this suggestion (Scheme 3). Irradiation of 2-phenylselenophene yields, in addition to 3-phenylselenophene, the enyne PhC=C—CH=CH2 and selenium. Photolysis of 2-phenyltellurophene furnishes solely the enyne and tellurium (76JOM(108)183). 

Pyrroles, furans and thiophenes undergo photoinduced alkylation with diarylalkenes provided that the alkene and the heteroaromatic compound have similar oxidation potentials, indicating that alkylation can occur by a non-ionic mechanism (Scheme 20) (81JA5570). 

Acyl-pyrroles, -furans and -thiophenes in general have a similar pattern of reactivity to benzenoid ketones. Acyl groups in 2,5-disubstituted derivatives are sometimes displaced during the course of electrophilic substitution reactions. iV-Alkyl-2-acylpyrroles are converted by strong anhydrous acid to A-alkyl-3-acylpyrroles. Similar treatment of N-unsubstituted 2- or 3-acyIpyrroles yields an equilibrium mixture of 2- and 3-acylpyrroles pyrrolecarbaldehydes also afford isomeric mixtures 81JOC839). The probable mechanism of these rearrangements is shown in Scheme 65. A similar mechanism has been proposed for the isomerization of acetylindoles. 

The mechanism is presumed to involve a pathway related to those proposed for other base-catalyzed reactions of isocyanoacetates with Michael acceptors. Thus base-induced formation of enolate 9 is followed by Michael addition to the nitroalkene and cyclization of nitronate 10 to furnish 11 after protonation. Loss of nitrous acid and aromatization affords pyrrole ester 12. 

The mechanism of the original Knorr pyrrole synthesis entails in situ reduction of the oxime moiety to an amine, condensation with the second carbonyl compound, and cyclization with loss of a second molecule of water to give a pyrrole for example, 10 + 11 to 12. Several studies have demonstrated that different pathways and pyrrole products obtain depending on the substrates. 

Despite its apparent simplicity, the PK pyrrole synthesis has retained its mystique since being discovered. Several investigations into the PK mechanism have been reported, including a gas phase study. Current evidence (intermediate isolation, kinetics, isotope effects) suggests the following (abbreviated) mechanism for the formation of pyrrole 13. However, the specific PK mechanism is often dependent on pH, solvent, and amine and dicarbonyl structure, especially with regard to the ringclosing step. 

Carbon tetrachloride was also found to react with pyrryl potassium to give 3-chloropyridine, however the mechanism is obscure and would justify further investigation. In a preparatively useful reaction, pyrrole and chloroform in the vapor phase at 500-550° gave 3-chloro-pyridine (33%) and a little 2-chloropyridine (2-5%). No interconversion of the isomers occurred under these conditions, though pyrolytic rearrangement of N-alkylpyrrole to 3-substituted pyridines is considered to involve 2-alkylpyrroles as intermediates. There is some independent evidence that dichlorocarbene is formed in the vapor phase decomposition of chloroform. ... 

All these data seem to be in agreement with a mechanism depicted in Scheme 16, where the thermal-activated step is the 1,2-sigmatropic shift between the Dewar pyrroles. 

A mechanism has been formulated, starting with a condensation to give the imine 4, that can tautomerize to the corresponding enamine 5. The latter can be isolated in some cases, thus supporting the formulated mechanism. A cyclization and subsequent dehydration leads to the imine 6, which tautomerizes to yield the aromatic pyrrole 3 ... 

Substituted pyrroles are often prepared by treatment of a 1,4-dikelone with ammonia. Propose a mechanism. 

Fischer alkenylcarbene complexes undergo cyclopentannulation to alkenyl AT,AT-dimethylhydrazones (1-amino-1-azadienes) to furnish [3C+2S] substituted cyclopentenes in a regio- and diastereoselective way along with minor amounts of [4S+1C] pyrrole derivatives. Enantiopure carbene complexes derived from (-)-8-(2-naphthyl)menthol afford mixtures of trans,trans-cycloipentenes and ds,ds-cyclopentenes with excellent face selectivity [75]. The mechanism proposed for the formation of these cyclopentene derivatives is outlined in Scheme 28. The process is initiated by nucleophilic 1,2-attack of the carbon... 

The most widely accepted mechanism for the anodic polymerization of pyrroles and thiophenes involves the coupling of radical cations produced at the electrode (Scheme l).5 The oligomers so produced, which are more easily oxidized than the monomer, are rapidly oxidized and couple with each other and with monomer radical cations. Coupling occurs predominantly at the a-positions (i.e., 2- and 5-position),5 and so pyrroles and thiophenes with substituents in either of these positions do not undergo anodic polymerization. The reaction is stoichiometric in that two... 

Anionic polymerization of lactams was shown to proceed according to what is called the activated monomer mechanism. With bischloroformates of hydroxy-terminated poly(tetramethyleneglycol) and poly(styrene glycol) as precursors for a polymeric initiator containing N-acyl lactam ends, block copolymers with n-pyrrol-idone and e-caprolactam were obtained by bulk polymerizations in vacuum at 30 and 80 °C, respectively361. ... 

One also obtains analogous findings with trace-crossing effects for the electropolymerization of thiophene and pyrrole. This cannot be explained by a simple linear reaction sequence, as presented in Scheme I, because it indicates competing homogeneous and heterogeneous electron transfer processes. Measurements carried out in a diluted solution of JV-phenylcarbazole provide a more accurate insight into the reaction mechanism (Fig. 2). 

Whereas pyrroles normally undergo substitution at the C-2 position, 1-arylsulfonylpyrroles display a tunable reactivity in Friedel-Crafts acylations, wherein substitution occurs at C-3 in the presence of "hard acids" such as aluminum chloride but mainly at C-2 when catalyzed by weaker acids <81TL4899,81TL4901>. An alternative route to 3-aroylpyrroles 38 has been introduced via irradiation of 1-acetyl- or 1-phenylsulfonylpyrrole (36) with arenethiocarboxamides <96H(43)463>. The proposed mechanism involves formation and scission of a thietane intermediate 37, followed by hydrolysis of the resultant imine. 

Polyacetylene in the doped state is sensitive to air and moisture. Other polymers (e.g., those of pyrrole, thiophene, and benzene) are stable in air and/or toward humidity in their doped and undoped states. Generally, when stored in the doped state, the polymers lose doping level by mechanisms not fully understood in most cases the loss is reversible. 

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