933-11-9Relevant articles and documents
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.
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.
