Synthesis of Octachlorobicyclo[3.3.0]octa-1,4,6-triene and Lewis Acid Catalyzed Isomerization of Hexachlorobicyclo[3.3.0]octa-3,5,8-trien-2-one

Abstract Treatment of decachloro(4-allylidenecyclopentene) with enneacarbonyldiiron(0) gave octachlorobicyclo[3.3.0]octa-1,3,7-triene (δ-C8Cl8) in 40% yield. Dechlorination of decachlorobicyclo[3.3.0]octa-2,6-diene with powdered selenium, sulfur, phosphorus, or titanium in the presence of anhydrous aluminium chloride without any added solvent at 90–100 °C afforded octachlorobicyclo[3.3.0]octa-1,4,6-triene (2) (ε-C8Cl8) in good yield. Hexachlorobicyclo[3.3.0]octa-3,5,8-trien-2-one obtained from 2 was chlorinated by a concentrated solution of antimony(V) chloride in refluxing dichloromethane to give octachlorobicyclo[3.3.0]octa-3,6(or 7)-dien-2-one, but in a dilute solution (less than 0.1 g of antimony(V) chloride/cm3 of dichloromethane) hexachlorobicyclo[3.3.0]octa-4,6,8-trien-2-one (14) was obtained. Mechanism of the isomerization is discussed in terms of antiaromaticity of cyclopentadienyl cation. Reaction of 14 with methanol afforded pentachloro-4-methoxybicyclo[3.3.0]octa-3,5,7-trien-2-one. Ethanol or 2-propanol reacted with 14 similarly giving pentachloro-4-ethoxybicyclo[3.3.0]octa-3,5,7-trien-2 -one or pentachloro-4-isopropoxybicyclo[3.3.0]octa-3,5,7-trien-2-one, respectively.