2-Thiabicyclo[3.2.0]hepta-3,6-Dienes. 1. Aluminum Chloride Catalyzed Thermal [2 + 2] Cycloaddition of 2-Butynedinitrile with Alkyl-Substituted Thiophenes

Tetra- and Trialkylthiophenes 4–6 React with 2-Butynedinitrile in the Presence of 1 Equiv of Aluminum Chloride in Dichloromethane at 0 °C Via [2 + 2] Cycloaddition to 2-3.2.0] hepta-3,6-Diene-6,7-Dicarbonitriles 7–10 as the Major Products. Reaction of 4 and 5 Also Gave Small Amounts (4% and 6%) of 1,2-Benzenedicarbonitriles 11 and 12 Which Result from [4 + 2] Cycloaddition and Subsequent Extrusion of Sulfur. 4,5,6,7-Tetrahydro-1,3-Dimethylbenzo[C]thiophene (6) Reacts with 2-Butynedinitrile under the Same Conditions to Give an Ene Adduct (13) in Addition to the [2 + 2] Cycloadduct 10. 2,5-Dimethylthiophene (3) Gave a 2:1 Reaction Product 14, the Formation of Which Can Be Explained Via Friedel-Crafts Alkylation of 3 by the Initially Formed [2 + 2] Cycloadduct. the 2-3.2.0] hepta-3,6-Dienes 7–10 and the Ene Adduct 13 Show Intramolecular Charge-Transfer Absorptions in the UV Spectra. © 1982, American Chemical Society. All Rights Reserved

2-Thiabicyclo[3.2.0]-hepta-3,6-dienes. 2. Thermal and photochemical isomerization of 2-thiabicyclo[3.2.0]hepta-3,6-dienes. An example of the antarafacial-antarafacial Cope rearrangement

Thermal Rearrangement of 2-3.2.0] hepta-3,6-Diene-6,7-Dicarbonitrile8 1–3 to their 4,5-Dicarbonitrile Isomers 4–6 Takes Place at 110–140 °C in Yields of 82–84%. the Reactions Are First Order with Rates Almost Independent of the Polarity of the Solvent. Activation Parameters (ΔH‡ = 120–140 KJ Mol−1 and ΔS‡ ≈ 0 J K−1 Mol−1) Are in Agreement with a Concerted Symmetry-Allowed Antarafacial-Antarafacial Cope Rearrangement. Compounds 1–3 Rearrange Photochemically Via Cleavage of the C-1-S Bond to the 1,7-Dicarbonitrile Isomers 7–9. Isomer 9 Rearranges Further on Prolonged Irradiation to Give 10. a Tentative Mechanism for This Rearrangement is Given. © 1982, American Chemical Society. All Rights Reserved