Pyrroles 2,2 -dimerization

Pyrrole, then, polymerizes very readily in acid. By the controlled use of dry HCl in ether or of 6iY aqueous HCl for a few seconds, variable yields of a homogeneous trimer may be obtained. The isolation of pyrrole dimer has not yet been reported, although what may be a fairly homogeneous complex of dimer and SnCU has been described. Attempts to liberate the free base from this complex were unsuccessful. 

This compound was prepared by Allen et who found it to be monobasic and to form a simple methiodide. Attempted Hofmann degradation of this methiodide failed to give any information. The dimer was found to be perfectly stable to hot aqueous sulfuric acid. With the foregoing data, and by analogy with the alkyl pyrrole dimers, they proposed structure (9) for the dimer. Later work confirmed this structure by conversion via the methiodide into base (10) which was synthesized by way of the metho salt (11), isomeric with... 

Cation (14) leads straightforwardly to tripyrrole, by way of protonated pyrrole dimer (18), the structure of which corresponds with the structures proved for 2-methyl- and 2-phenyl-pyrrole dimers. 

The first question one asks is why does the reaction with 2-methyl-pyrrole stop at the dimer stage, whereas the pyrrole dimer itself cannot be isolated, but reacts further to form the trimer. The answer probably lies in the greater electrophilic reactivity of... 

Nothing is known of the nature of krypto- and phyllo-pyrrole dimers and an investigation of their structure would be of great interest. 

As is seen from the previous section, autoassociation of pyrrole involves NH-77- bonding. Hydrogen bonding of this type can also occur between pyrrole and other aromatic and 77-electron systems. The strength of such bonds is less than that of the pyrrole dimer and considerably smaller than observed for NH-X bonds (vide infra), as shown by the frequency shift of the NH stretching vibration (Table IV). 

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... 

O. Picazo et al., Large chiral recognition in hydrogen-bonded complexes and proton transfer in pyrrolo[2, 3-b]pyrrole dimers as model compounds. J. Org. Chem. 68, 7485-7489 (2003)... 

The salts of methylene derivatives of 2H- and 3//-pyrroles and -indoles are produced in the Ehrlich reaction (Scheme 32, Section 3.05.1.2.8) and they are also intermediates in the Vilsmeier-Haack reaction (Scheme 24, Section 3.05.2.1.6). Although 6-fV,fV-dimethyl-amino-l-azafulvene, i.e. 2-(Ar,Ar-dimethylaminomethylene)-2//-pyrrole, dimerizes spontaneously (see Section 3.05.2.5), the 6-aryl-6-Ar)Ar-dimethylamino-l-azafulvenes (514 R = aryl) can be isolated (71JCS(B)1405) but, curiously, they are more susceptible to reactions with nucleophiles at the 6-position than are the corresponding salts (B-77MI30508). The benzo[6 ]-1 -azafulvenes, obtained from the reaction of 2-formylindoles with dialkylamines, also dimerize spontaneously, but the isomeric benzo[c]-2-azaf ulvenes, derived from 3-formylindoles, are thermally more stable, although they are extremely moisture sensitive... 

The data show that the quadmpole hyperfine patterns of the rotational transitions are different between the two states, due to changes of the relative positions of some of the hyperfine components within the multiplet. The rotational spectrum of a pyrrole dimer is consistent with essentially a T-shaped structure, in which the planes of the two pyrrole monomers form an angle of 55.4(4)° and the nitrogen side of one ring is directed to the 7t-electron system of the other ring establishing a weak H bond <1997JCP504>. 

HD as a required step in the induction of both testicular and nervous system injuries (5-8). Subsequent oxidation and cross-linking of these tissue-bound heterocyclic aromatic compounds result in a complex array of products, including pyrrole dimers (9, 10). 

Attempts to polymerize pyrrole dimers and trimers were not initially very successful, with evidence for the formation of short-chain oligomers rather than continuous films [119]. However, good quality films have been obtained from the dimer, particularly in the presence of moderately nucleophilic anions [107,120]. 

Further support for the coupling of pyrrole through the 2-,5-positions comes from solid-state nuclear magnetic resonance (NMR) [33], infrared (IR) spectroscopy [34,35], and x-ray photoelectron spectroscopy (XPS) [36] experiments. The presence of pyrrole units in both the oxidized and neutral forms of polypyrrole is inferred from NMR ( C) spectra [37]. After polymerization, a downfield shift is observed in the a-carbon resonance that is consistent with the formation of an a-substituted pyrrole. The IR spectra exhibit strong similarities between a pyrrole dimer, trimer, and polymer and polypyrrole in its neutral form. Finally, the XPS carbon h and nitrogen Is polypyrrole spectra have features similar to that of a pyrrole monomer [38,39]. 

An outline of the polymerization scheme for polypyrrole is given in Fig. 1, The mechanism for polymerization can be deduced from several observations. First, a radical cation produced by the initial oxidation of pyrrole can either react with another radical cation to produce a pyrrole dimer or make an electrophilic attack on a neutral molecule. The coupling of two radicals is favored over electrophilic attack, since the electrochemical polymerization reaction occurs only when a sufficient potential is applied to oxidize the polymer, and at the potentials normally applied the amount of neutral species present at the electrode surface is essentially zero. Second, the amount of charge consumed during polymer formation has an initial linear time dependence and is independent of pyrrole concentration during constant-potential electrolysis [20,40]. This evidence indicates that the entire electrochemical reaction occurs at the electrode surface. Third, the dimer (bipyrrole). 

Fig. 31.1 Structures of calix[4]arene-based molecular containers, cages, and capsules Cram s carcerand (1), tetraureacalix[4]arene dimer (2), Ballester s tetraureacalix[4]pyrrole dimer (3), Rebek s imide containing calix[4]resorcinarene (4), and thiourea-containing calix[4]resorcinarene...

The NH/tt interactions in other complexes were also reported. The MP2 calculations of aniUne complexes with ethylene, propene and 1-butene suggest an NH/tt interaction between the N - H bond of aniline and the double bond of the alkene for the propene and 1-butene complexes, with a CH/a-type interaction between the CH bond and the lone pair of nitrogen of anihne for the ethylene complex [103]. The MP2/6-311G -level optimized geometry of the pyrrole dimer shows that the dimer prefers the tilted T-shaped orientation, in which one monomer points toward the ring plane of the... 

SubstitutedA-protectedpyrroles could also be C2-phenylated with trace amounts (< 10%) of pyrrole dimerization (eq 31). 

Many of the physical characteristics of pyrrole indicate at least partial association. In particular, the boiling point is 98°C higher than that of furan. It has been postulated that various associated dimeric and higher stmctures occur because of hydrogen bonding (9,10). 

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