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.