Pyrrole-2-carbodithioate anions

Electrophilic alkynes are also very active acceptors of pyrrole-2-carbodithioate-anions. Thus, the addition of 4,5,6,7-tetrahydroindole-2-carbodithioate 772b to acylalkynes occurs to furnish stereoselectively the (Z)-adducts 774 (Scheme 152). The sterically overcrowded double bond of adducts 774 still remains active enough to participate... 

Pyrrole anions generated in KOH-DMSO attacked carbon disulfide either exclusively or preferably at position 2 to afford pyrrole-2-carbodithioate anions, which after alkylation gave the corresponding esters of pyrrole-2-carbodithioic acids 134 (Equation (38)) (03M1127, 05M197). 

As highly nucleophilic species, pyrrole-2-carbodithioate anions added smoothly to elecrtophilic alkenes such as acrylonitrile, acrylamide or methyl acrylate to furnish the corresponding derivatives of propionic acid (99ZOR1534, 01S293, 03M1127, 05M197). 

The calculated values of energy difference (the same basis) show that the unsubstituted pyrrole-2-carbodithioate anion is by 5.47 kcal/mol more stable than the... 

The methyl group in a-position increases the stability of pyrrole-2-carbodithio-ate anion by 11.12 kcal/mol as compared to the corresponding 1-isomer. The second methyl substituent in p-position augments this value only negligibly (to 11.95 kcal/mol). High energy preference of the methyl-substituted pyrrole-2-carbodithioate anions rationalizes their observed regioselective formation, which is most likely a thermodynamic result, while 1- or 3-isomers can be kinetic products. 

In contrast to pyrrole-l-carbodithioates, pyrrole-2-carbodithioate anions easily add to acrylic acid derivatives to afford the corresponding pyrrole-2-carbodithioate in up to 62% yield (Scheme 2.78, Table 2.9) [548,549],... 

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... 

Obviously, due to the steric hindrances in the disubstituted acetylenes, the competing solvolysis of pyrrole-l-carbodithioate anions results in sulfide ions that react with acetylenes to furnish divinyl sulfides. 

Previously, it was assumed that the addition of pyrrole anions to carbon disulfide leads mainly to pyrrole-l-carbodithioates [533-536]. 

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]. 

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]. 

Pyrrole-2-vinylthiolates, formed from pyrrole-2-carbodithioates and CH-acid anions, are highly reactive mild nucleophiles that readily (room temperature) substitute halogen even at the ip-carbon atom (in bromobenzoyl- or chloroethylthioacety-lenes) to afford ethynyl sulfides, which further cyclize to functionally substituted pyrrolothiazolidines (37%-85% yield. Scheme 2.89, Table 2.11) [561,562]. 

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. 

---