933-11-9Relevant academic research and scientific papers
Enantiodifferentiating photoisomerization of 1-methycyclooct-1-ene sensitized by chiral alkyl benzenecarboxylates: steric effects upon stereodifferentiation
Tsuneishi, Hiroshi,Hakushi, Tadao,Tai, Akira,Inoue, Yoshihisa
, p. 2057 - 2062 (2007/10/03)
Enantiodifferentiating Z-to-E photoisomerizations of 1-methylcyclooct-1-ene (2) sensitized by (-)-menthylbenzene(poly)carboxylates were performed at varying temperatures and the steric effects of the methyl group introduced to the parent cyclooctene (1) upon both isomerization and enantiodifferentiation processes were studied.The photostationary state 2E:2Z ratio, (E/Z)pss, was shown to decrease dramatically with decreasing irradiation temperature and increasing steric hindrance in the sensitizer.Kinetic analyses of the sensitized photoisomerization demonstrated that the temperature- and sensitize-dependent (E/Z)pss ratios originate solely from the quenching process.The steric effect on the (E/Z)pss values of the introduced methyl group is much greater for the (Z)-isomer (2Z) than for the (E)-isomer (2E), as compared with the cyclooctene case reported previously.The optical purities (percentop) of photoproduct 2E were found to be relatively low () and entropy (TΔΔS) as functions of the number of chiral groups in the benzene(poly)carboxylate sensitizer give a quite similar profile to those parameters obtained for parent 1, showing uniform increases for 2 of 0.4 and 0.15 kJ mol-1 in ΔΔH and TΔΔS, respectively, for all chiral sensitizers.
Direct and Sensitized Geometrical Photoisomerization of 1-Methylcyclooctene
Tsuneishi, Hiroshi,Inoue, Yoshihisa,Hakushi, Tadao,Tai, Akira
, p. 457 - 462 (2007/10/02)
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 pss = 0.30> and the excitation ratio (εZ/εE = 0.305) calculated from the extinction coefficients (ε) at 214 nm of both isomers, we obtain the decay ratio (kdE/kdZ) of 0.98 for the singlet-excited 1-methylcyclooctene (1p).This ratio, being close to that of cyclooctene (kdE/kdZ = 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)pss values 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 raio, 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 mutal 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.
Influence of strain on chemical reactivity. Relative reactivity of torsionally distorted double bonds in MCPBA epoxidations
Shea, Kenneth J.,Kim, Jang-Seob
, p. 3044 - 3051 (2007/10/02)
The second-order reaction rates were measured for the MCPBA epoxidation in CH2Cl2 for a series of cyclic olefins including bridgehead olefms and trans-cycloalkenes. As expected, strained bridgehead alkenes and trans-cycloalkenes showed faster reaction rates than nonstrained cis-cycloalkenes. The MM-2 steric energies of alkenes, alkanes, and their corresponding epoxides were calculated to evaluate the strain energy released in each reaction (ΔSE). Plots of log krel vs olefin strain did not show a good correlation. However, the plot of log krel vs ΔSE (which is defined as the steric energy difference between olefin and the corresponding epoxide) showed a good correlation for each set of di- and trisubstituted olefins. This result suggests that ΔSE directly reflects strain energy relief in the transition state. From the slope for the plot log krel vs ΔSE, it was thought that approximately 42% of strain (ΔSE) was released in the transition state for the MCPBA epoxidation. Also, trialkyl-subtituted alkenes were found to be about 50 times more reactive than dialkyl-substituted alkenes in cases where the strain energy relief (ΔSE) is the same. The reaction rate is also plotted versus ionization potential of the olefin, assuming that the major orbital interaction lies between the LUMO of the peracid and the HOMO of the olefin. Although, in some cases, a rough correlation of the reaction rate with the ionization potential of the olefin exists, the frontier orbital interaction is not viewed as the dominant factor since conjugated alkenes, which have higher HOMO energies than simple olefins, are not more reactive in MCPBA epoxidation.
