Direct and sensitized geometrical photoisomerization of 1-methylcyclooctene

Abstract

Effects of methylation at C(1) of cyclooctene on the ground-state structure, potential energy surfaces, and photochemical behaviour in the singlet and triplet manifolds have been investigated. Molecular mechanics calculations using the MM2 force field reveal that, as compared with the parent (E)-cyclooctene, methylation increases the strain of the (E)-isomer by 8.9 kJ mol^–1, which is however released in part through the minimized steric repulsion between one of the ring methylenes and the introduced methyl by increasing the dihedral angle C(Me)–C(1)C(2)–C(3).

Direct excitation at 214 nm of 1-methylcyclooctene does not lead to the Rydberg state-derived rearrangement products, but results in efficient geometrical isomerization, affording a photostationary Z–E mixture upon prolonged irradiation. From the photostationary state E/Z ratio observed [(E/Z)_pss= 0.30] and the excitation ratio (ε_z/ε_E= 0.305) calculated from the extinction coefficients (ε) at 214 nm of both isomers, we obtain the decay ratio (K_dE/K_dz)of 0.98 for the singlet-excited 1-methylcyclooctene (¹P). This ratio, being close to that of cyclooctene (k_dE/k_dz= 0.88), means equal-probability decay from ¹P to highly strained (E)-isomer and less-strained (Z)-isomer. Somewhat unexpectedly, triplet sensitization with aromatic hydrocarbons gave only slightly higher (E/Z)_pssvalues of 0.03–0.26 than the corresponding values for cyclooctene, indicating that the crossing profiles of the triplet and ground-state surfaces of 1-methylcyclooctene resemble each other, in spite of the increased strain in the (E)-isomer. By contrast, the singlet sensitization was shown to be very sensitive to the steric hindrance of the sensitizer. As compared with the cyclooctene case, the less congested benzoate gives almost the same (E/Z)_pss ratio, whereas the use of more congested benzenepolycarboxylates results in much decreased ratios. This drastic difference between triplet and singlet sensitizations may be rationalized in terms of the intervention of an exciplex in the singlet sensitization, which provides stronger and longer mutual interaction between sensitizer and substrate. The singlet-sensitized photoisomerization has been used as a convenient one-step route to the highly constrained (E)-1 -methylcyclooctene.