Generation and Trapping of Triafulvene

Substituted methylidenecyclopropanes 12a–d, being easily available from 1,1‐dibromo‐2‐(phenylthio)‐cyclopropane (9a), are attractive precursors of triafulvene (2‐methylidene‐1‐cyclopropene; 1). Both the sulfoxide 12b and the sulfone 12c react with an excess of alkoxides (t‐BuOK and NaOMe) to give 12e and 12f, respectively, while the sulfinyl group of 12b may be replaced by the PhCH2S substituent in the presence of PhCH2SH/t‐BuOK. These reactions (Scheme 4) may be explained by assuming 1 as a reactive intermediate, although an alternative sequence including carbene 20 (Scheme 6) is not completely ruled out. D‐labelling experiments (Scheme 5) do not give conclusive evidence due to D scrambling, but deprotonation/methylation sequences show that HC(2) of 12a–c is the most acidic proton. Final evidence for 1 results from the reaction of 12d with cyclopentadienide (Scheme 7): the reaction of 1 with cyclopentadiene produces the expected [4 + 2]‐cycloaddition product 23, while some mechanistic insight results from the sequence 12d → 24 → 25.