Intramolecular cyclization to 1-phenyl-1-benzothiophenium salts by electrophilic addition of o-(phenylsulfanyl)phenylalkynes

Abstract

Electrophilic addition of 1 -[o- (phenylsulfanyl)phenyI] -2-(p-methoxypheny1)ethyne with electrophiles such as perchloric acid, tetrafluoroboric acid, bromine, and benzenesulfenyl chloride gave 1 -phenyl-I -benzothiophenium salts exclusively. The substituent effect on the intramolecular cyclization with electrophiles has been examined. The aryl-substituted alkynes afforded predominantly the cyclized 1 -phenyl-I -benzothiophenium salts but methyl-substituted alkynes yielded a mixture of the cyclized salt and the 1,2-addition product. In addition to the intramolecular cyclization at the intermediate vinyl cation or bridged ion, it is proposed, on the basis of the product from the reaction of methyl-substituted alkyne, that the 71 complex partly participates in the i n t ra m ol ec u I a r cyc I izat i o n .

Efficient Access to Extended Yagupolskii–Umemoto‐Type Reagents: Triflic Acid Catalyzed Intramolecular Cyclization of ortho‐Ethynylaryltrifluoromethylsulfanes

“…Metal salts or acids have been used to induce the cyclization of alkynes to obtain five-membered rings and heterocycles. [14] However, under the same conditions, o-SMe-substituted alkynes gave de-methylated benzo[b]thiophenes, and S-methyl salts were not detected. [11,12] Although recent rapid progress in the coinage-metal-catalyzed cyclization [13] of ortho-thio-substituted aryl alkynes has allowed easy access to benzothio-phenes, this method is not suitable for the synthesis of benzothiophenium salts.…”

“…In 1994, Kitamura and co-workers reported the cyclization of o-SPh-substituted aryl alkynes using HBF 4 or HClO 4 that afforded cyclized 1-phenyl-1benzothiophenium salts. [14] However, under the same conditions, o-SMe-substituted alkynes gave de-methylated benzo[b]thiophenes, and S-methyl salts were not detected. Electrophilic cyclization using reagents such as I 2 , Br 2 , and PhSeCl also gave the corresponding benzo[b]thiophenes through a demethylation pathway.…”

“…In contrast, triflic acid (CF 3 SO 3 H, TfOH) was extremely efficient in affording the desired product 2 a in 79 % yield (Table 1, entry 7). No CF 3 migration, often observed for the cyclization to S-phenyl, methyl, or related alkylthiophenium salts, [10][11][12][13][14] was observed within the ring. The annulation of a broad range of o-SCF 3 -substituted aryl alkynes that have functionalized aromatic rings afforded their corresponding S-(trifluoromethyl)thiophenium salts in good to excellent yields (64-94 %; Table 1, entries 8-14).…”

“…The annulation of a broad range of o-SCF 3 -substituted aryl alkynes that have functionalized aromatic rings afforded their corresponding S-(trifluoromethyl)thiophenium salts in good to excellent yields (64-94 %; Table 1 , entries 8-14). No CF 3 migration, often observed for the cyclization to S-phenyl, methyl, or related alkylthiophenium salts, [10][11][12][13][14] was observed within the ring. Annulation of benzyl-substituted alkyne 1 i with TfOH gave a complex mixture of products, presumably owing to the deprotonation/de-trifluoromethylation of 1 i into A (Table 1, entry 15; Scheme 2).…”

“…Cyclic and acyclic bketo esters afforded their corresponding trifluoromethylated compounds, which had a quaternary carbon center, in good to high yields in the presence of 1,8-diazabicyclo [5.4.0]undec-7ene (DBU) or tert-butyliminotri(pyrrolidino)phosphorane (P 1 ) as the base (59-95 %; Table 2, entries 1-25). Cyclopropyl-substituted reagent 1 j afforded slightly improved yields compared to when phenyl-substituted 1 a was used ( Table 2, entries 1-6 versus [7][8][9][10][11][12][13][14]. Cyclopropyl reagent 1 j presents a clear advantage of higher yields over both the aromatic analogue (1 a) and the commercially available Umemoto reagent (S-(trifluoromethyl)dibenzothiophenium tetrafluoroborate) in the trifluoromethylation of acyclic bketo esters ( Table 2, entries 15-17).…”