Thiophenium salts, aromaticity

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... 

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). 

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. 

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. 

This structural outcome suggests a lack of aromaticity of the thiophene ring in the l-phenylbenzo[/)]thiophenium ion. A novel [4+2] cycloaddition reaction of 1-phenyl-l-benzothiophenium salts with cyclopentadiene or 1,3-diphe-nylbenzo[r]furan has been reported which experimentally confirmed the olefinic nature of the thiophene ring arising from a lack of aromaticity <2003JOC731>. 

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