Inverted potential-energy surfaces in the radical-cation Cope rearrangements of hexa-1,5-diene and semibullvalene

Abstract

Comparison of the radical-cation transformations of hexa-1,5-diene and semibullvalene with the degenerate rearrangements of these neutral compounds reveals that the equilibrium structure of the radical cation in each case corresponds to a symmetrical transition-state structure for the neutral molecule. Thus, the cyclohexane-1,4-diyl radical cation is formed initially by the one-electron oxidation of hexa-1,5-diene, spectroscopic proof of its delocalized chair structure being obtained by matrix-isolation studies. Similarly, it is found that the ionization of semibullvalene generates the bicyclo[3.3.0]octa-2,6-diene-4,8-diyl radical cation as a delocalized mesovalent species corresponding to the case of strong interaction between two allylic groups held in a boat conformation. Spectroscopic studies reveal that the singly occupied molecular orbitals of these diyl and diallyl radical cations are non-bonding and correspond to the respective HOMOs of the neutral transition states. Inversion of the potential-energy surface for the radical cation comes about because the difference between the ionization potentials of the neutral molecule and its transition state exceeds the activation enthalpy for the degenerate rearrangement of the neutral molecule, the low ionization potential of the transition state being attributable to the non-bonding character of its HOMO. The radical-cation rearrangements are therefore termed ‘half-Cope’ reactions since the degeneracy of the reaction coordinate is lifted in going from the neutral molecule to its radical cation. A secondary rearrangement of the cyclohexane-1,4-diyl radical cation to the cyclohexene radical cation by hydrogen transfer is accessible on account of the potential-energy minimum for the intermediate diyl radical cation. In contrast, the lack of cyclohexene formation as a side product of the neutral Cope reaction suggests that in this case there is no intermediate species with a significant potential-energy well along the reaction surface to allow this competitive path to occur.