Catalytic Generation of Vinylthionium Ions. (4 + 3)-Cycloadditions and Friedel–Crafts Alkylations

Abstract

A 3-phenylsulfanyl-substituted allylic alcohol and an ester thereof were treated with Brønsted acids or a gold catalyst, respectively, to generate vinylthionium ions. These species react with dienes, primarily substituted furans, to give products of either (4 + 3)-cycloaddition or Friedel-Crafts alkylation. The results are rationalized on the basis of a stepwise mechanism in which the relative rates of ring closure versus proton loss in the intermediate σ-complex determine the course of the reaction.

Access to Diosgenyl Glycoconjugates via Gold(I)-Catalyzed Etherification of Diosgen-3-yl ortho-Hexynylbenzoate

“…This combination stood out as the optimal choice of conditions for the gold-catalyzed etherification of 1 , and the result also suggests that a homoallylic carbocation is involved in the transformation. Based on this observation and literature results, ,,, a plausible mechanism is illustrated in Scheme . Coordination of the gold­(I) catalyst to the triple bond of 1 leads to intermediate A , which enhances the electrophilicity of the alkyne group, and the subsequent nucleophilic attack of the carbonyl oxygen to the activated alkyne generates the homoallylic carbocation intermediate B and metallic isocoumarin C .…”

Fun with (4+3)-Cycloadditions

“…In the case of 2,5-dimethylfuran an 85% yield of cycloadduct was obtained (Scheme 7). 24 We performed experiments demonstrating that this reaction was indeed catalyzed by gold(I) and not by adventitious protic acid that might be present in the reaction mixture.…”

Gold‐Catalyzed Allylation Reactions

“…Interestingly, the combined use of PPh 3 AuOTf (5 mol %) and functionalized allylic alcohols was utilized by Harmata and co‐workers in the FC‐type alkylation of furan and the subsequent cycloaddition reaction . Vinylthionium ions were considered to be key intermediates in the cascade reaction.…”