Isomerisation of decatetraenes and dimethyldecatetraenes via pentadienyl and 3-methylpentadienyl radicals

The thermal isomerisations of E,E-deca-l,3,7,9-tetraene and E,E-3,7-dimethyldeca-l,3,7,9-tetraene take place via the intermediacy of pentadienyl and 3-methylpentadienyl radicals, respectively, rather than by concerted Cope type rearrangements. The pentadienyl radicals isomerise to pairs of E- and Z-pentadienyl radicals which recombine by end to end and end to centre, but not centre to centre, coupling to give mixtures of isomeric decatetraenes. The relative free energies of formation of these decatetraenes were derived from their equilibrium proportions and compared with relative enthalpies of formation calculated by the empirical MM2(87) method and the semiempirical AM1 and PM3 SCF MO methods. None of these theories was successful at predicting and rationalizing the experimentally observed enthalpy changes. However, they all were somewhat better in dealing with branching than with cis/trans isomerisation. Frontal strain in the decatetraene plays an important part in influencing their stability.