First [4 + 2] Cycloaddition of 1-Phenyl-1-benzothiophenium Salts with Dienes

Abstract

The reaction of 1-phenyl-1-benzothiophenium triflates 1 (R ) H, Me, and Ph) with cyclopentadiene successfully proceeded to give the corresponding cycloadducts, respectively, in good yields. The single-crystal X-ray analysis of 1 (R ) H) showed the endo isomer. Similarly, 1,3-diphenylisobenzofuran reacted with 1 (R ) H) to give the corresponding cycloadduct. These novel [4 + 2] cycloadditions of 1-phenyl-1-benzothiophenium triflates as the first example clearly indicate the olefinic nature of the thiophene ring arising from the lack of aromaticity.Hypervalent or polycoordinated main-group compounds have attracted much current interest because of their unique structure and chemical properties. Synthesis and reactions of thiophenes and benzothiophenes have been extensively investigated, 1 but those of 1-thiophenium and 1-benzothiophenium salts have received much less attention. 2 We have previously found that 1-phenyl-1-benzothiophenium salts can be prepared by intramolecular cyclization of o-(phenylsulfanyl)phenyl-substituted ethenes 3 and alkynes 4 and by direct S-phenylation of benzothiophenes. 5 The most important and notable property is a high stability of the 1-phenyl-1-benzothiophenium salts, which enables a wide variety of investigations and may disclose unknown chemistry of the 1-benzothiophenium ion.We have found that the 1-phenyl-1-benzothiophenium salts undergo nucleophilic substitution to lead to ring-opened alkenes or dephenylation products. 6 The crystal structure of 1-phenyl-1-benzothiophenium triflate 5 reveals that the stereochemistry around the sulfur atom is tetrahedral and the short CdC double bond length (1.308 Å) of the thiophene ring is within the observed range of the CdC double bond † Kyushu University. Kawasato, H.; Kobayashi, S.; Taniguchi, H. Chem. Lett. 1986, 339-342. (b) Kitamura, T.; Takachi, T.; Kawasato, H.; Taniguchi, H.; Shiro, M. Tetrahedron 1993, 49, 5055-5066. (4) (a) Kitamura, T.; Takachi, T.; Kawasato, H.; Kobayashi, S.; Taniguchi, H. Tetrahedron Lett. 1989, 30, 7445-7446. (b) Kitamura, T.; Takachi, T.; Miyaji, M.; Kawasato, H.; Taniguchi, H. Yamane, M.; Furuki, R.; Taniguchi, H.; Shiro, M.

Novel [4 + 2]-Cycloaddition of 1-Phenyl-1-benzothiophenium Salts with Dienes. Experimental Evidence for a Lack of Aromaticity in the Thiophene Ring

“…Accordingly, adduct 2a is considered to be the endo adduct. The assignment of the structure of 2a gave good agreement with the results of the single-crystal X-ray structural analysis . The formation of cycloadduct 2a experimentally indicates that the aromaticity of the thiophene ring is destroyed and the C2−C3 bond has an olefinic character.…”

Tungsten(0) η2-Thiophene Complexes: Dearomatization of Thiophene and Its Facile Oxidation, Protonation, and Hydrogenation

“…When benzaldehyde was added to a mixture of base and 10 , compound 1 was recovered in good yield. 1-Phenyl-1-benzothiophenium salts undergo cycloaddition with both cyclopentadiene and 1,3-diphenylisobenzofuran in refluxing methylene chloride . Each of the dienes were combined with the S -methylthiophenium complex 10 , but cycloaddition was not observed (16 h, 40 °C).…”

Aromaticity as a Cornerstone of Heterocyclic Chemistry

“…On converting 1-benzothiophene into 1-phenyl-1benzothiophenium triflate (95), this salt becomes a dienophile and reacts readily with cyclopentadiene or 1,3-diphenylisobenzofuran to give the adduct 96 (Scheme 45). 143 This example of the dienophilic nature of the double bond in the benzothiophene ring arises from reduced aromaticity.…”