Kinetic evidence for dihapto (eta(2)) pi-aryl participation in acid-catalyzed ring opening of diarylhomobenzoquinone epoxides.

The BF(3)-catalyzed ring-opening reaction of variously endo/exo m- and p-substituted diarylhomobenzoquinone epoxides proceeded through a transannular S(E)2-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 CDCl(3) at 30 degrees C in order to elucidate the possible remote pi-aryl participation. The rates were significantly increased with increasing electron-donating ability of the endo-aryl substituents X (k(p-MeO)/k(p-CF3) = 8200) but only negligibly influenced by the distal exo-aryl substituents Y (k(p-MeO)/k(p-CF3) = 2.1). For the endo-X substituted series, an excellent linear free energy relationship, log k(rel)(endo) = -2.49sigma(ipso) - 1.62sigma(ortho) - 0.108 (R(2) = 0.98, n = 8), was attained using two modified site-dependent substituent parameters sigma(ipso) (using sigma(p)(+) for p-X and sigma(m) for m-X) and sigma(ortho) (using sigma(m) for p-X and sigma(p)(+) for m-X). This means that the dihapto(eta(2)) pi-coordination occurs in the pi-aryl participation, with the ipso pi-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 k(rel)(exo) = -0.912(sigma(0) + 0.237Deltasigma(R)(+)) (R(2) = 0.96, n = 8). These kinetic substituent effects were compared with those of the acid-catalyzed pi-aryl assisted transannular S(E)2-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 eta(2) pi-aryl participation.