Thiophenium salts

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[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 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). 

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... 

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). 

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). 

Thiophenium salts and thiophenium ylids, chemistry of, 45, 151 Thiopyrans, 34, 145 59, 179 Thiopyrones (monocyclic sulfur-containing pyrones), 8, 219... 

On converting l-phenylbenzo[ ]thiophene into 1-phenyl-1-benzo[ ]thiophenium triflate 123, this salt becomes a dienophile and reacts readily with cyclopentadiene or l,3-diphenylbenzo[r]furan to give the adduct 124 (Scheme 9) <19990L257>. This example of the dienophilic nature of the double bond in the benzo[/ ]thiophene ring arises from reduced aromaticity. Thiophene 1-oxide and 1-substituted thiophenium salts present reduced aromaticity <1970JA7610>. 

There are a few reports on the theoretical study of thiophenium salts <1995JP0753, 1995JHC483, 1995J(P2)455>, demonstrating that the aromaticity of the thiophene ring is destroyed. However, there is no experimental evidence for the lack of aromaticity. 

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