Kinetic Evidence for Dihapto (η²) π-Aryl Participation in Acid-Catalyzed Ring Opening of Diarylhomobenzoquinone Epoxides

The BF3-catalyzed ring-opening reaction of variously endo/exo m- and p-substituted diarylhomobenzoquinone epoxides proceeded through a transannular SE2-Ar cyclization of endo-aryl groups to give the tricyclic diketo-alcohols and cyclohexadienone spiro-linked tricyclic diketo-alcohols. Kinetics of these reactions has been investigated in CDCl3 at 30 °C in order to elucidate the possible remote π-aryl participation. The rates were significantly increased with increasing electron-donating ability of the endo-aryl substituents X (kp-MeO/kp-CF3 = 8200) but only negligibly influenced by the distal exo-aryl substituents Y (kp-MeO/kp-CF3 = 2.1). For the endo-X substituted series, an excellent linear free energy relationship, log krelendo = −2.49σipso − 1.62σortho − 0.108 (R2 = 0.98, n = 8), was attained using two modified site-dependent substituent parameters σipso (using σp+ for p-X and σm for m-X) and σortho (using σm for p-X and σp+ for m-X). This means that the dihapto(η2) π-coordination occurs in the π-aryl participation, with the ipso π-electron donation contributing 1.6 times more effectively than the ortho one. On the other hand, the distal exo-Y substituted series gave an acceptable Yukawa−Tsuno equation with small polar and resonance contributions; log krelexo = −0.912(σ0 + 0.237Δσ̅R+) (R2 = 0.96, n = 8). These kinetic substituent effects were compared with those of the acid-catalyzed π-aryl assisted transannular SE2-Ar cyclization of the cyclobutene-fused diarylhomobenzoquinones. It was found that the geometrical characteristics of the vacant oxirane Walsh orbital and the cyclobutene antibonding orbital play a crucial role in the topological features of η2 π-aryl participation.