Pyrrole-3-carbodithioates

When both the pyrrole a-positions are substituted only pyrrole-3-carbodithioates 771 have been isolated, the yield being 36-61% (Equation 183). No pyrrole-1-carbodithioates have been detected among the products <2000T7325, B-2003MI127, 2005MI97>. 

When both a-positions were substituted, only pyrrole-3-carbodithio-ates 136 were isolated (Equation (39)) (00T7325, 03M1127, 05M197). 

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>. 

SCHEME 2.73 Synthesis of pyrrole-3-carbodithioates from pyrroles and CSj in the KOH/ DMSO system. 

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... 

Under the same conditions, pyrrole-3-carbodithioate reacts with acrylonitrile delivering the corresponding adduct in a lower yield (18%, Scheme 2.79, Table 2.9) [549],... 

Pyrrole-3-carbodithioate reacts with ethyl propynoate in the presence of acetic acid to give a mixture of divinyl sulfide and the adduct of Z-configurations (Scheme 2.85, Table 2.9), while with disubstituted electron-deficient acetylenes, only divinyl sulfides are formed [549]. 

Condensation of pyrrole-3-carbodithioates with CH-acids is chemoselective (in this case, intramolecular cyclization involving the NH function is impossible) and stereoselective 3-vinylpyrroles are always formed as single isomer, which probably have less sterically strained -configuration with i yn-orientation of the NH pyrrole and CN group. 

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. 

A comparative study of the electronic structures of A,A-diethyldithiocarbamate and pyrrole-A-carbodithioate has been undertaken.961 The enthalpy of formation of [Ni(S2CNMe2)2] (—146.1 10.9 kJmol-1) has been measured.962 The square planar dithiocarbamate complexes can be oxidized to the corresponding five-coordinate Ni111 dithiocarbamate complexes [Ni(S2CNR2)2X] (X = I, Br, C104) using Br2, I2, or (N0)C104.963,964... 

Molybdenum bisalkyne complexes form more readily in the pyrrole-/V-carbodithioate ligand system ( pyrroledithiocarbamate ) than in the corresponding dialkyldithiocarbamate systems (88). The pyrrole nitrogen is reluctant to share electron density with the attached CS2 moiety since the aromatic stabilization of the five-membered NC4 ring is lost in resonance form ii. As a result of decreased electron donation from the... 

Bisalkyne d4 monomers, with N = 3 by symmetry, exhibit proton and carbon chemical shifts at higher fields than those of monoalkynes with N = 4. The proton chemical shift of 10.45 ppm for Mo(PhC=CH)2-(S2CNEt2)2 (52) falls nicely between the four-electron donor Mo(CO)-(PhC=CH)(S2CNEt2)2 case (12.6 ppm) and the two-electron donor (7r-C5H5)2Mo(HC=CH) case [7.68 ppm (Table II)]. Additional data for bisalkyne complexes, including pyrrole-N-carbodithioate derivatives, support a correlation of H chemical shifts with alkyne ttj donation, with three-electron donors typically near 10.0 0.5 ppm. Similar H values are found for cyclopentadienyl bisalkyne complexes with terminal alkyne ligands. Chemical shifts between 8.5 and 10.5 ppm characterize all the neutral and cationic bisalkynes listed in Table V except for [CpMo-(RC=CH)2(MeCN)]+ where one isomer has S near 11 ppm for the acetylenic proton (72). 

Pyrrole-ZV-carbodithioate bisalkyne complexes display two distinct flux-ional processes. Rotation around the C—N bond of the S2C—NC4H4 ligand equilibrates both halves of the pseudoaromatic NC4H4 ring (AG = 10.7 kcal/mol) (88). Alkyne rotation exchanges both ends of the alkyne ligands at somewhat higher temperatures (AG =13.7 kcal/mol for MeC CMe, 13.8 kcal/mol for HC=CH). 

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... 

Preparation of 3-Chloro-2-Butenyl-lH-Pyrrole-l-Carbodithioate Regulator... 

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). 

Substituents in the pyrrole ring affected drastically the ratio of pyrrole-1- to pyrrole-2-carbodithioate isomers. With unsubstituted pyrrole, only the 1-isomer 135 (R = R = R = H, R = Et) was formed (63%) and practically no 2-isomer was detected. When just one methyl group was introduced into pyrrole s a-position, the pyrrole-2-carbo-dithioates 134 became the only product (46%) (Equation (38)). Any combinations of alkyl substituents on the pyrrole ring selectively gave pyrrole-2-carbodithioates 134 in a yield of up to 71%. With an aryl substituent at the a-position, along with the major pyrrole-2-carbodithio-ates 134 (44-59%), the 1-isomers 135 were also formed in 24—33% yields (Equation (38)). 

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). 

X = halide) bis(pyrrole-N-carbodithioate) chelate ccanplexes have been prepared. The 16-electron fragment (M(C0)2(S2CNC H )2] appears to be more electrophilic than [M(C0)2(S2CNEt2>2I the latter also forming many seven-coordinate adducts. 

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]. 

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. 

In the case of 2-aryl(hetaryl)-5-methylpyrroles, from two possible isomers, pyrrole-3- and pyrrole-4-carbodithioates, only the latter are formed (44%-61% yields. Table 2.9), that is, the isomers having the dithioate function adjacent to the methyl group (Scheme 2.74) [542]. Possibly, such regiospecificity is caused by steric shielding of the pyrrole position 3 by ortho-hydrogen atom of the aromatic or heteroaromatic substituent. 

SCHEME 2.72 Formation of pyrrole-1- and pyrrole-2-carbodithioates from 2-arylpyrroles and CS2 in the KOH/DMSO system. 

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. 

Pyrrole-l-carbodithioate, generated in situ from pyrrole and carbon disulfide in the system KOH/DMSO, does not almost add to electrophilic alkenes, such as acrylonitrile, acrylamide, or methyl acrylate [548,549], However, its treatment with excess HCl in the specified system leads to the acrylamide adduct in quantitative yield (Scheme 2.76, Table 2.9). 

Under the same conditions, pyrrole-l-carbodithioate does not react with acrylonitrile and methyl acrylate only pyrrolyldisulfide is isolated in both cases (Scheme 2.77). 

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],... 

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). 

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. 

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. 

With ethyl propynoate, pyrrole-2-carbodithioate reacts selectively under the aforementioned conditions to produce divinyl sulfide (Scheme 2.84,42% yield). 

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