Pyrrole cations

Schmuck, C., Lex, J. (1999) Acetate binding within a supramolecular network formed by a guanidiniocarbonyl pyrrole cation in the solid state, Org. Lett. 3, 1253-1256. 

Like other aromatic systems, pyrroles reveal their unsaturated condition in certain addition reactions. The most obvious of these are partial or complete reduction to pyrrolines or pyrrolidines. Sodium and alcohol are without effect on pyrrole s but zinc and acid produce A -pyrro-lines 39, 233, 303 This reaction may involve attack of hydride ion upon the a-protonated pyrrole cation. Many pyrroles have been reduced by a variety of catalytic procedures to the fully saturated pyrrolidines and electrolytic reduction also converts pyrrole into pyrrolidine i. 

The formation of 5-(2 -pyrrolyl)-l-pyrrolines (54) by dimerization has not been observed to occur, and indeed could not occur without substituent elimination, with 2,5-disubstituted pyrroles [see the example of the dimerization of (56) above]. The dimerizatipn involves electrophilic attack by a j8-protonated pyrrole cation upon the conjugate base, as for example... 

The mechanism of the ring opening by hydroxylamine is interesting cause for speculation. As with the addition of bisulphite to pyrrole (p. 82), the reaction could satisfactorily be represented as a nucleophilic attack by hydroxylamine upon a pyrrole cation ... 

Such a reaction would be more rapid with the more basic (p. 60) dimethyl-pyrrole, but under the recommended conditions the concentrations of pyrrole cations must be extremely small. 

With substitutions effected in acid media, the possible role of pyrrole cations has not been elucidated. However, in electrophilic substitutions it seems most improbable that any entity other than the neutral molecule should be involved. Not only must it be more reactive to electrophilic attack than are the cations, but also it is difficult to formulate any other mechanism such as, for example, nucleophilic attack upon the cation, followed by electrophilic attack and elimination. The consequences of nucleophilic attack upon the j8-protonated pyrrole cation are seen in the trimerization of pyrrole. 

Of radical and nucleophilic substitutions, very little is known. In the one or two reactions which look like nucleophilic attacks (addition of bisulphite, attack by hydroxylamine and related compounds), the question of the intervention of pyrrole cation needs investigation. 

The question of pyrrole cations was mentioned above, and the view expressed that an active role for them in electrophilic substitutions was improbable. Other workers have been impressed by the catalytic influence of acids upon many pyrrole substitutions, and have tried to represent that influence as arising from some form of interaction between the acid and the... 

Polypyrrole (PPy) is one of the most studied conducting polymers due to its aqueous compatibility, low oxidation potential of the monomer, ease of preparation, and high conductivity [61-62]. The structure of PPy is shown in Fig. 1. Electropolymerizaion of polypyrrole involves the formation of pyrrole cation radicals, dimer radicals, and... 

Unsymmetrically substituted dipyrromethanes are obtained from n-unsubstitued pyrroles and fl(-(bromomethyl)pyiToIes in hot acetic acid within a few minutes. These reaction conditions are relatively mild and the o-unsubstituted pyrrole may even bear an electron withdrawing carboxylic ester function. It is still sufficiently nucleophilic to substitute bromine or acetoxy groups on an a-pyrrolic methyl group. Hetero atoms in this position are extremely reactive leaving groups since the a-pyrrolylmethenium( = azafulvenium ) cation formed as an intermediate is highly resonance-stabilized. 

Many methine cationic dyes, styrylic (141), pyrrolic. or amino-substituted (142) derivatives of thiazolium, possess interesting anthelmintic properties (143). This last class has been used as accelerators of the catabolism and activators of cellular exchanges (144). 

Silver fluorocomplexes are also used ia the separation of olefin—paraffin mixtures (33), nitration (qv) of aromatic compounds (34), ia the synthesis of (9-bridged bicycHcs (35), pyrroles (36), cyclo-addition of vinylbromides to olefins (37), and ia the generation of thioben2oyl cations (38). 

N-Alkylpyrroles may be obtained by the Knorr synthesis or by the reaction of the pyrrolyl metallates, ie, Na, K, and Tl, with alkyl haUdes such as iodomethane, eg, 1-methylpyrrole [96-54-8]. Alkylation of pyrroles at the other ring positions can be carried out under mild conditions with allyhc or hensylic hahdes or under more stringent conditions (100—150°C) with CH I. However, unless most of the other ring positions are blocked, poly alkylation and polymerisation tend to occur. N-Alkylation of pyrroles is favored by polar solvents and weakly coordinating cations (Na", K" ). More strongly coordinating cations (Li", Mg " ) lead to more C-alkylation. 

The low basicity of pyrrole is a consequence of the loss of aromaticity which accompanies protonation on the ring nitrogen or on carbon 2 or carbon 3 of the ring. The thermodynamically most stable cation is the 2H-pyrrolium ion, and the p/sTa for protonation at C-2 has been recorded as -3.8 the corresponding pK values for protonation at C-3 and at nitrogen are -5.9 and ca. -10 (Scheme 7). 

It is of interest to compare the stability of the protonated forms of benzo[u], benzo[Z>] and benzo[c] fused pyrroles, i.e. the cations derived from indolizines, indoles and isoindoles. Indolizine gives a stable pyridinium ion and does not polymerize in the presence of acid. 

By using an aromatic aldehyde carrying an electron-releasing group the intermediate cation can be stabilized. This is the basis of the widely-used Ehrlich colour reaction for pyrroles, indoles and furans which have a free reactive nuclear position (Scheme 21). 

A mild and effective method for obtaining N- acyl- and N- alkyl-pyrroles and -indoles is to carry out these reactions under phase-transfer conditions (80JOC3172). For example, A-benzenesulfonylpyrrole is best prepared from pyrrole under phase-transfer conditions rather than by intermediate generation of the potassium salt (81TL4901). In this case the softer nature of the tetraalkylammonium cation facilitates reaction on nitrogen. The thallium salts of indoles prepared by reaction with thallium(I) ethoxide, a benzene-soluble liquid. 

The general discussion (Section 4.02.1.4.1) on reactivity and orientation in azoles should be consulted as some of the conclusions reported therein are germane to this discussion. Pyrazole is less reactive towards electrophiles than pyrrole. As a neutral molecule it reacts as readily as benzene and, as an anion, as readily as phenol (diazo coupling, nitrosation, etc.). Pyrazole cations, formed in strong acidic media, show a pronounced deactivation (nitration, sulfonation, Friedel-Crafts reactions, etc.). For the same reasons quaternary pyrazolium salts normally do not react with electrophiles. 

Two general types of heterocyclic cations may be recognized those in which the charge results from an increase in the bonding number of an atom by the addition of a proton (or equivalent), and those in which the charge results from a decrease in the bonding number by removal of hydride. These two possibilities are shown for pyrrole in (167) and (168). 

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

Reaction between [W(RC=C)Cl(CO)2(py)2] (R = Ph, Me) with the anionic chelating Schiff base pyrrole-2-carboxaldehyde methylimine yields the cationic complexes [NEt4][W(RCCO)(NN)2(CO)] (where NN is the dianion of the pyrrole ligand). These complexes react with methyltriflate, forming the neutral acetylenic complexes [W(NN)2(CO)(RC=COMe)] (87OM1503). One of the pyrrolic Schiff bases is coordinated via the pyrrole and imino nitrogen atoms, and another one only via the imino nitrogen atom. 

Azaferrocene reacts with aromatic hydrocarbons in the presence of aluminium chloride, giving rise to the cationic complexes of the type (Ti -arene)(Ti -cyclopenta-dienyl)iron(l+) isolated as BF4 salts [87JOM(333)71]. The complex 28 is obtained by reaction of the sulfane compound [Cp(SMc2)3Fe]BF4 with pentamethyl-pyrrole [88AG(E)579 88AG(E)1468 90ICA(170)155]. The metallic site in this center reveals expressed Lewis acidity (89CB1891). 

Hydroxymethylation and alkylation of furans, thiophenes, and pyrroles in the presence of H" " cations 98KGS3. 

Allen et al mainly on the evaluation of data reported by previous workers, advanced structure (5) for 2-methylpyrrole dimer. These data were (a) the monobasic nature of the dimer and (b) the conversion by aqueous acid into an indole which they showed by elimination (i.e., by the synthesis of 2,5-, 2,6-, and 2,7-dimethylindoIes) to be 2,4-dimethyIindole (this compound has since been synthesized by Marion and Oldfield ). Structure (5) for the dimer was confirmed by Edwards and Smith by conversion, by way of the methiodide (6), into the pyrrolidinopyrrole (7), the structure of which was proved by synthesis l,5-dimethylpyrrolid-2-one was condensed with 2-methyI-pyrrole by means of POCI3 to give the cation (8), isomeric with the... 

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