Kinetic study of thermolysis of diarylhomonaphthoquinones. Endo/exo substituent and solvent effects

Abstract

The kinetics of thermal cyclopropane ring-opening of a series of m- and p-substituted endo/exo diphenylbromohomonaphthoquinones 1a–i and the unsubstituted diphenylchlorohomonaphthoquinone 1j have been investigated and compared with biphenyl-2,2′-diylhalogenohomonaphthoquinones 2a,b. The first-order rate constants k/s⁻¹ for 1a–i at 100 °C in toluene increased with the electron-donating ability of the substituents. The kinetic substituent effects were much more pronounced for the exo family than for the endo one and revealed the crucial role of the resonance contribution of diaryl groups; log (k/k_o)_exo = −1.99σ⁺ + 0.086 and log (k/k_o)_endo = −0.784σ⁺ + 0.002, respectively. The compounds 2a,b thermolyzed very quickly as compared with the corresponding diphenylhalogenohomonaphthoquinones 1e,j. The kinetic solvent effects on the thermolysis of representative compound 1e were so minute that the rates tended to slightly increase with the solvent polarity but decrease with the solvent basicity. These kinetic results were interpreted in terms of a concerted disrotatory ring opening of the incorporated cyclopropane ring.