Thiopheniums

The principal electrophiles to attack ring sulfur are either oxidants or alkylating reagents. Thiophene sulfoxide and sulfone formation is discussed in Section 3.02.2.6. Alkylating agents capable of forming thiophenium salts include trimethyloxonium tetrafluoroborate (MeaO BF ) and alkyl fluorosulfonates (ROSO2F). The salts e.g. 87) are conveniently isolated as hexafluorophosphates (88). 

Benzo[i]thiophenium, 1-methyl- H NMR, 4, 733 <70JCS(C)1764) (Benzotriazenido)iridium(I),... 

Benzo[b]thiophenium, 1,2,3,5-tetramethyl-theoretical methods, 4, 3 Benzo[b]thiophenium salts preparation, 4, 723-724 Ben2o[b]thiophen-4-one, 4,5,6,7-tetrahydro-synthesis, 4, 905-906 5H-Benzo[b]thiophen-4-one, 6,7-dihydro-synthesis, 4, 900 Benzo[b]thiophyne, 4, 750 1H- 1,3,4-Benzotriazepine, 2,3-dihydro-synthesis, 7, 637 1H-1,2,4-Benzotriazepines synthesis, 7, 638 1,2,5-Benzotriazepines synthesis, 7, 640... 

Thiophenium fluorosulfonate, 1,2,3,4,5-tetramethy 1-ylide, 4, 724 Thiophenium salts aromaticity, 4, 724 proton abstraction, 4, 766 pyramidal inversion barrier, 4, 724 structure, 4, 715 synthesis, 4, 723-724 Thiophenium salts, 1-alkyl-solvolysis, 4, 766 UV spectra, 4, 766 Thiophenium salts, aryl-synthesis, 4, 726... 

Thiophenium ions were prepared with AICI3/HQ in CD2Q2 ... 

Rhodium(II) acetate was found to be much more superior to copper catalysts in catalyzing reactions between thiophenes and diazoesters or diazoketones 246 K The outcome of the reaction depends on the particular diazo compound 246> With /-butyl diazoacetate, high-yield cydopropanation takes place, yielding 6-eco-substituted thiabicyclohexene 262. Dimethyl or diethyl diazomalonate, upon Rh2(OAc)4-catalysis at room temperature, furnish stable thiophenium bis(alkoxycarbonyl)methanides 263, but exclusively the corresponding carbene dimer upon heating. In contrast, only 2-thienylmalonate (36 %) and carbene dimer were obtained upon heating the reactants for 8 days in the presence of Cul P(OEt)3. The Rh(II)-promoted ylide formation... 

Thiophenium salt derivatives, 23 715 Thiophilic adsorption, 6 405 Thiophosgene, 23 625-626, 4 837 reactions of, 23 625-626 Thiophthalimide vulcanization retarders, 21 800... 

Thiophene-1-oxide and 1 -substituted thiophenium salts present reduced aromaticity.144 A variety of aromaticity criteria were used in order to assess which of the 1,1-dioxide isomers of thiophene, thiazole, isothiazole, and thiadiazole was the most delocalized (Scheme 46).145 The relative aromaticity of those molecules is determined by the proximity of the nitrogen atoms to the sulfur, which actually accounts for its ability to participate in a push-pull system with the oxygen atoms of the sulfone moiety. The relative aromaticity decreases in the series isothiazole-1,1-dioxide (97) > thiazole-1,1 -dioxide (98) > thiophene-1-dioxide (99) then, one has the series 1,2,5 -thiadiazole-1,1 -dioxide (100) > 1, 2,4-thiadiaz-ole-1,1-dioxide (101) > 1,2,3-thiadiazole-1,1 -dioxide (102) > 1,3,4-thiadiazole-l,1-dioxide (103) in the order of decreasing aromaticity. As 1,2,5-thiadiazole-1,1-dioxide (100) was not synthesized, the approximations used extrapolations of data obtained for its 3,4-dimethyl-substituted analogue 104 (Scheme 46). 

Ylides other than acceptor-substituted diazomethanes have only occasionally been used as carbene-complex precursors. lodonium ylides (PhI=CZ Z ) [1017,1050-1056], sulfonium ylides [673], sulfoxonium ylides [1057] and thiophenium ylides [1058,1059] react with electrophilic transition metal complexes to yield intermediates capable of undergoing C-H or N-H insertions and olefin cyclopropanations. 

Thiophenium bis(alkoxycarbonyl)methylides (44) are obtained in high yield by rhodium(II) carboxylate-catalyzed reaction of diazomalonate esters with thiophene derivatives (88JCS(P1)1023). Likewise, ylides from benzo[b]thiophene and dibenzothiophene (e.g. 45) have also been reported by tram-ylidation using phenyliodonium bis(phenylsulfonyl)methylide (88JHC1599). 

Stable a-protonated pyrrolium salts have been obtained by treating di- and tri-t-butylpyrroles with tetrafluoroboric acid (81LA789) and stable a-protonated thiophenium salts result from the reaction of... 

S-Arylbenzo[fi]thiophenium ions (143) undergo ring opening by cleavage of the S - C2 bond when treated with NaOMe in MeOH (92CL1357). With 3-unsubstituted compounds, the primary process is abstraction of the proton attached to C-3 subsequent cleavage of the S-C2 bond results in the formation of acetylenes (144) in quantitative yield. 

The sulfur atom of thiophenes is weakly nucleophilic and can act in some cases as the site for attack of an electrophilic reagent. In strongly acidic solutions the a-carbon atom is protonated (66RTC1072) this could be a thermodynamically controlled product, however. Reaction of thiophene with powerful alkylating agents provides the thiophenium salt (56),... 

The question of aromaticity arises. Neither thiophenium salts nor thiophene sulfoxides are especially stable, making the classical reactivity test of electrophilic aromatic substitution difficult. The former dealkylate readily and the latter, at least for the case of thiophene sulfoxide, readily undergo self-dimerization (65CCC1158) (the bulky substituents of (57) impede this reaction). Aromaticity requires that the lone pair on sulfur participate in the aromatic sextet. If the lone pair, because of sp3 hybridization and improper symmetry, is not delocalized into the butadiene segment, the system will be antiaromatic. 

Various applications of 13C NMR spectroscopy have been made examples include analysis of the spectra of substituted thieno[2,3-6 ]thiophenes (76ACS(B)417> and studies of the relaxation of both proton and 13C nuclei (80JMR(40)197>. The 13C spectra of thiophenium salts have been examined and, again, a strong downfield shift of the 0 -carbon atoms is seen, indicative of considerable charge transfer from sulfur to the 0 -positions (74TL75). 

Thermolysis of the ylide (15) in thiophene results in ready rearrangement to dimethyl thiophene-2-malonate (16). The same product is obtained if the thermolysis is carried out in the presence of 2-methylthiophene or cyclohexene, proving that the rearrangement occurs by an intramolecular process (78CC85). However, when 2,5-disubstituted thiophenium ylides are thermolyzed, dissociation to carbenoid species seems to occur. This reaction is further discussed in Section 3.14.2.9. 

Various NMR studies have shown that protonation of thiophene occurs at position 2. Thus the thiophenium ions (48) were prepared (73TL3929) from (47) in quantitative yield by protonation in fluorosulfonic acid at -70 °C protonation occurred exclusively at position 2. 

Proton abstraction from the pentamethylthiophenium ion (13) gave the ylide (141) reaction of this with p-nitrobenzaldehyde gave the epoxide (142) (74TL75). Thiophenium salts, in contrast to thiophene 1-oxides (Section 3.14.4.1), do not seem to undergo Diels-Alder additions (74TL75). 

Thiophenes are best converted to the tetrahydro derivatives by the so-called ionic hydrogenation. This depends on the successive addition of a proton (from trifluoroacetic acid) and a hydride ion (from triethylsilane) (75T311). A subsequent improvement involved the use of HC1/A1C13 to form the thiophenium ion and then reaction with triethylsilane (78T1703) best results are obtained with the substrate/Et3SiH/AlCl3 ratio of 1 3 0.3. The mechanism of the reaction is shown in Scheme 43. Evidence for this has been provided by the use of Et3SiD, when D enters positions 3 and 5 in the product. 

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