Thermal Rearrangements of Perchlorohexatrienes–Structures and Experimental and Theoretical Evaluation of Pathways to Isomerization and Cyclization

We have prepared trans ‐ ( 1 ) and cis ‐octachloro‐1,3,5‐hexatriene ( 2 ) by known routes and studied their thermal behavior experimentally and theoretically by ab initio calculations. The three double bonds in 1 and 2 are completely decoupled due to steric hindrance by the eight Cls, as indicated by calculations as well as the single‐crystal X‐ray structure of 1 . The cis isomer 2 can be isomerized to the trans isomer 1 by heating it to 220–250 °C either neat or dissolved in high‐boiling solvents, leading to a roughly 2:1 mixture of trans and cis isomers. Calculations at several different levels of theory predict 1 and 2 to be isoenergetic within 2 kJ mol –1 . Unimolecular cis / trans isomerization is predicted to occur through an unusual vinylcyclobutene intermediate 7 , whose formation faces a barrier of more than 150 kJ mol –1 , but whose stability is comparable to that of 1 and 2 . The isomerization rate is strongly enhanced by the addition of small amounts of Br 2 or Cl 2 or by 3 and can be explained by a radical‐induced isomerization mechanism. The heating of trienes 1 and 2 to 250 °C leads to cyclization, yielding 71 % of the cyclopentene isomer 3 . Compound 3 can be dechlorinated by treatment with copper powder to give fulvene derivative 4 . Using flash vacuum pyrolysis, the thermal conversion of trienes 1 and 2 to hexachlorobenzene ( 5 ) occurs at higher temperatures between 600–1000 °C, likely via perchlorinated 1,3‐cyclohexadiene ( 6 ) as an intermediate. The elimination of molecular Cl 2 from 3 and 6 requires very high activitation energies in agreement with calculations.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)