Wang Junpeng, Ong Mitchell T, Kouznetsova Tatiana B, Lenhardt Jeremy M, Martínez Todd J, Craig Stephen L
Department of Chemistry, Duke University , Durham, North Carolina 27708, United States.
Materials Science Division, Lawrence Livermore National Laboratory , Livermore, California 94550, United States.
J Org Chem. 2015 Dec 4;80(23):11773-8. doi: 10.1021/acs.joc.5b01493. Epub 2015 Sep 14.
The dynamics of reactions at or in the immediate vicinity of transition states are critical to reaction rates and product distributions, but direct experimental probes of those dynamics are rare. Here, s-trans, s-trans 1,3-diradicaloid transition states are trapped by tension along the backbone of purely cis-substituted gem-difluorocyclopropanated polybutadiene using the extensional forces generated by pulsed sonication of dilute polymer solutions. Once released, the branching ratio between symmetry-allowed disrotatory ring closing (of which the trapped diradicaloid structure is the transition state) and symmetry-forbidden conrotatory ring closing (whose transition state is nearby) can be inferred. Net conrotatory ring closing occurred in 5.0 ± 0.5% of the released transition states, in excellent agreement with ab initio molecular dynamics simulations.
过渡态或其紧邻区域的反应动力学对反应速率和产物分布至关重要，但对这些动力学的直接实验探测却很少见。在此，利用稀聚合物溶液脉冲超声产生的拉伸力，沿着纯顺式取代偕二氟环丙烷化聚丁二烯的主链施加张力，捕获了s-反式，s-反式1,3-双自由基过渡态。一旦释放，就可以推断出对称允许的顺旋环化（捕获的双自由基结构是其过渡态）和对称禁阻的对旋环化（其过渡态在附近）之间的分支比。在5.0±0.5%的释放过渡态中发生了净对旋环化，这与从头算分子动力学模拟结果非常吻合。
