Thermal isomerisation of substituted semibullvalenes and cyclooctatetraenes - A kinetic study

Whereas 1,5-dimethylsemibullvalene (1b) equilibrates with 1,5-dimethylcyclo octatetraene (2b), the semibullvalene-2,6-dicarbonitriles 13 rearrange irre versibly to afford mixtures of the isomeric cyclooctatetraene-1,5-dicarboni triles 14 and 15. Arrhenius and Eyring parameters of these thermal isomeris ation reactions have been determined for the gas phase (1b reversible arrow 2b) and [D-6]benzene solutions (1b reversible arrow 2b, 13 --> 14 + 15). F urthermore, the activation parameters of the known rearrangement of octamet hylcyclooctatetraene (3) to octamethylsemibullvalene (4) have been determin ed. - The data for these compounds, together with those for related compoun ds previously reported in the literature, show that substituents not only i nfluence the relative stabilities of the semibullvalene and cyclooctatetrae ne systems but also the height of the energy barrier for their interconvers ion. Those substituents that lower the barrier toward the degenerate Cope r earrangement of semibullvalenes simultaneously accelerate their rearrangeme nt to cyclooctatetraenes thus limiting the thermal stability of the former.