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 PhCH 2 S substituent in the presence of PhCH 2 SH/ 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 HC(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 .