Zur Kenntnis des chinoiden Zustandes-IX. Untersuchungen zur innermolekularen Beweglichkeit in Para-Chinonmethiden und in den Entsprechenden Phenoxylvorstuffen

The ESR spectra of p-RCH₂-substituted phenoxy radicals (1) can be interpreted in terms of a radical mechanism involving CH and (for 1g) CP hyperconjugation. Evidence for a hindered rotation around the CP bond is furnished by the temperature dependence of the ESR spectrum of 1g at -85 to +100°. The phenoxyls are dehydrogenated to p-quinone methides (2). Temperature dependent NMR spectra are only observed with either the directly or vinylogueously bound p-bis-quinone methides 2k and 21. The free enthalpy of activation for rotation around the exocyclic CC-double bond is calculated to be 26 and 20 kcal/mole for 2k and 21, respectively. Because of the structural preconditions for the rotation, a biradical transition state is thought to be involved.