Rulísek Lubomír, Sebek Pavel, Havlas Zdenek, Hrabal Richard, Capek Pavel, Svatos Ales
Max-Planck-Institute for Chemical Ecology, Hans-Knöll-Str. 8, 07745, Jena, Germany.
J Org Chem. 2005 Aug 5;70(16):6295-302. doi: 10.1021/jo050759z.
The stereoselectivity of the reaction of furan (1) with maleic anhydride (2) and maleimide (3) was studied experimentally and theoretically. Although the two reactions are highly similar with regard to their preference for endo and exo steroisomers, notable differences were experimentally observed and explained on the basis of calculated reaction-free energies and transition-state barriers. The experimental values of rate constants (k(1+2endo) = (1.75 +/- 0.48) x 10(-5); mol(-1) l s(-1); k(1+2exo) = (3.10 +/- 0.55) x 10(-5); mol(-1) l s(-1); k(1+3endo) = (1.93 +/- 0.082) x 10(-5); mol(-1) l s(-1), k(1+3exo) = (1.38 +/- 0.055) x 10(-5); mol(-1) l s(-1) all at 300 K) and the observed reaction course clearly confirm that neither of these reactions are prototypical examples of Diels-Alder [4 + 2] cycloadditions, whose dominant preference is for endo isomers. However, only by comparing their energetics-calculated at the CCSD(T) level of theory-with the analogous reactions involving cyclopentadiene (8) as a diene can these observations be understood. The low thermodynamic stability of furan [4 + 2] adducts opens retro-Diels-Alder reaction channels and overrules the very small kinetic preference (calculated and measured here) of initial formation for endo stereoisomers. On a macroscopic scale "an irregular"-thermodynamically more stable-exo stereoisomer was consequently observed as a dominant species.
对呋喃（1）与马来酸酐（2）及马来酰亚胺（3）反应的立体选择性进行了实验和理论研究。尽管这两个反应在对endo和exo立体异构体的偏好方面高度相似，但实验观察到了显著差异，并基于计算得到的反应自由能和过渡态能垒进行了解释。速率常数的实验值（在300 K时，k(1 + 2endo) = (1.75 ± 0.48) × 10⁻⁵；mol⁻¹ l s⁻¹；k(1 + 2exo) = (3.10 ± 0.55) × 10⁻⁵；mol⁻¹ l s⁻¹；k(1 + 3endo) = (1.93 ± 0.082) × 10⁻⁵；mol⁻¹ l s⁻¹，k(1 + 3exo) = (1.38 ± 0.055) × 10⁻⁵；mol⁻¹ l s⁻¹）以及观察到的反应过程清楚地证实，这些反应都不是狄尔斯-阿尔德[4 + 2]环加成反应的典型例子，狄尔斯-阿尔德[4 + 2]环加成反应主要偏好endo异构体。然而，只有将它们在CCSD(T)理论水平下计算得到的能量与涉及环戊二烯（8）作为二烯的类似反应进行比较，才能理解这些观察结果。呋喃[4 + 2]加合物的低热力学稳定性打开了逆狄尔斯-阿尔德反应通道，并推翻了endo立体异构体初始形成时非常小的动力学偏好（此处计算和测量得到）。因此，在宏观尺度上观察到“不规则的”——热力学上更稳定的——exo立体异构体作为主要产物。
