Pyrrole-2 carbodithioates, reaction

Both pyrrole itself, as well as 4,5,6,7-tetrahydroindole underwent N-vinylation with 2-cyano-l-phenylacetylene in DMSO in the presence of KOH, while the treatment of 4,5,6,7-tetrahydroindole with 2-benzoyl-1-phenylacetylene under the same conditions furnished predominantly a C-vinylated product <03S1272>, Several functionalized 3-vinylpyrroles were prepared by reaction of pyrrole-3-carbodithioates with malonitrile, cyanacetamide, or ethyl cyanoacetate induced by KOH in DMSO <03TL3501>. 

Systematic investigations of the reaction of pyrroles with carbon disulfide in the superbase system KOH/DMSO [537-541] have shown that pyrrole anions, generated in this system, attack CS2 (20°C-25°C, 2 h) exclusively or mainly by the position 2 to afford pyrrole-2-carbodithioates. The latter, after alkylation with alkylhalides (20°C-25°C, 2 h), give the corresponding pyrrole-2-carbodithionic acid esters in 46%-75% yields (Scheme 2.71, Table 2.9) [537-540]. The only exception is unsubstituted pyrrole, which gives only pyrrole-l-carbodithioate [540]. 

The ratio between substitution. When only one methyl group is introduced into the a-position of the pyrrole ring, pyrrole-2-carbodithioate becomes the single reaction product (46% yield), while N-isomer is not detected in the reaction mixture [542]. Any other combinations of alkyl substituents in the pyrrole moiety lead to selective formation of pyrrole-2-carbodithioates in up to 71% yield (Table 2.9) [537-542]. 

Contrary to data [533,534], it has been established that if the pyrrole contains substituents at both a-positions, the reaction exclusively delivers pyrrole-3-carbodi-thioates in 36%-61% yield (Scheme 2.73, Table 2.9) [542]. Pyrrole-l-carbodithioates have not been found among the reaction products. 

The energy of unsubstituted pyrrole-3-carbodithioate anion is by 5.76 kcal/mol higher than that of its 1-isomer, while 3-carbodithioate anions of 2-methyl- and 2,3-dimethylpyrroles slightly differ in energy from the 1-carbodithioate isomers 2-methylpyrrole-3-carbodithioate and 2,3-dimethylpyrrole-1-carbodithioate are more preferable by 0.32 kcal/mol and 1.57 kcal/mol, respectively [543,544,546]. Relying on the calculation results, one can expect that 2,5-dimethylpyrrole anion will attack CS2 by 3(4) position (at thermodynamic stage of the reaction) to afford... 

SCHEME 2.77 Formation of pyrrolyldisulfide in the attempted reaction of pyrrole-l-carbodithioate with acrylonitrile and methyl acrylate. 

With electron-deficient acetylenes, the dithioate anions can be involved in the reactions of both 1,3-anionic cyclo- and nucleophilic additions [550], The reactions of 1,3-anionic cycloaddition are typical provided that the central atom of [S-C-S] has aromatic substituents ensuring the anion stabilization [551,552], For instance, potassium pyrrole-l-carbodithioate selectively reacts (acetonitrile, -30°C, CH3COOH) with dimethyl ester of acetylenedicarboxylic acid to form rapidly polymerizing 2-(pyrrol-l-yl)-4,5-dimethoxycarbonyl-l,3-dithiol (Scheme 2.80) [552,553]. 

The reaction of pyrrole-l-carbodithioate with ethyl propynoate and benzoylacet-ylene (aqueous DMSO, KOH, room temperature, 2 h) leads to normal adducts, S-vinylpyrrole-l-carbodithioates in a low yield (10%-18%, Scheme 2.81, Table 2.9) [549,554]. Among the major reaction products are substituted divinyl sulfides as a mixture of E,E-, Z,E-, and Z,Z-isomers. 

SCHEME 2.80 Reaction of pyrrole-l-carbodithioates with dimethyl ester of acetylenedicarboxylic acid. 

SCHEME 2.82 Reaction of pyrrole-1-carbodithioate with disubstituted acetylenes. 

The reaction of pyrrole-l-carbodithioate with disubstituted electron-deficient acetylenes exclusively gives the corresponding divinyl sulfides, which are also formed in the presence of acetic acid (Scheme 2.82). 

Unlike pyrrole-l-carbodithioate, 4,5,6,7-tetrahydroindole-2-carbodithioate reacts with acylacetylenes giving rise to pyrrolothiazolidines (46%-47%, Scheme 2.83, Table 2.9), the products of the intramolecular cyclization of the intermediate 2-acylvinylpyrrole-2-carbodithioates. The last are detected in the reaction mixture only in trace amounts [549,554]. The reaction proceeds readily in a two-phase system (aqueous DMSO/diethyl ether) at 20°C-25°C. 

The reaction of pyrrole-2-carbodithioates with malononitrile and cyanoacetamide leads to, along with 2-vinylpyrroles, a small amount (5%) of their intramolecular... 

The reaction of ethylcyanoacetate, ethylacetoacetate, or diethylmalonate as CH acids with pyrrole-2-carbodithioates leads to vinylpyrroles bearing the ester fnnc-tion (Scheme 2.90) [563,564]. 

Trofimov, B.A., L.N. Sobenina, A.I. Mikhaleva et al. 2000. Reaction of pyrrole anions with carbon disulfide. Synthesis of pyrrole-3-carbodithioates. Tetrahedron 56 7325-7329. 

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