Characterization of Matrix-Isolated Antiaromatic Three-Membered Heterocycles. Preparation of the Elusive Thiirene Molecule

Article abstract of DOI:10.1021/ja00393a002

The preparation and characterization of thiirene (4), a heterocyclic analogue of cyclobutadiene (1), which is derived from the photolysis of 1,2,3-thiadiazole (5), is described.The methodology exploits the C_2v symmetry of the species 4 and utilizes isotopic labeling to aid in the characterization and narrow band-pass filters to protect thiirene from reaction with light of λ > 2700 Angstroem, when it is generated photochemically.The evidence for 4 is based on (1) the fact that the same monolabeled species <13C>X is formed from distinctly labeled <13C>-1,2,3-thiadiazoles, (2) the photoisomerization of labeled X to ethynyl mercaptan and thioketene both with randomized label, and (3) the likelihood that the observed infrared bands attributed to 4 belong to a single species possessing cyclopropenoid character.In addition to thiirene, ethynyl mercaptan (14) and thioketene (15) are derived from 5.There are thus two paths (λ > 2900 Angstroem) originating from 5 which give rise to 14 and 15.One stems from thiadiazole without the intervention of 4 and does not scramble hydrogens or carbons; the other is mediated by thiirene, which necessarily makes the C-H bonds equivalent.Irradiation (λ ca. 3500 Angstroem) of doubly labeled <2H,13C> thiirene gives all four possible ethynyl mercaptans, indicating at least one exchange of hydrogens between carbon atoms must occur.The most reasonable explanation for this exchange presumes that both hydrogens reside on a single carbon atom at some point during the photoisomerization of thiirene to ethynyl mercaptan (14).