Yang Xuejin, Gitter Sean R, Roessler Allison G, Zimmerman Paul M, Boydston Andrew J
Department of Chemistry, University of Wisconsin-Madison, Madison, WI, 53706, USA.
Department of Chemistry, University of Michigan, Ann Arbor, MI, 48109, USA.
Angew Chem Int Ed Engl. 2021 Jun 14;60(25):13952-13958. doi: 10.1002/anie.202016393. Epub 2021 May 7.
Stereochemistry can have a profound impact on polymer and materials properties. Unfortunately, straightforward methods for realizing high levels of stereocontrolled polymerizations are often challenging to achieve. In a departure from traditional metal-mediated ring-opening metathesis polymerization (ROMP), we discovered a remarkably simple method for controlling alkene stereochemistry in photoredox mediated metal-free ROMP. Ion-pairing, initiator sterics, and solvation effects each had profound impact on the stereochemistry of polynorbornene (PNB). Simple modifications to the reaction conditions produced PNB with trans alkene content of 25 to >98 %. High cis content was obtained from relatively larger counterions, toluene as solvent, low temperatures (-78 °C), and initiators with low Charton values. Conversely, smaller counterions, dichloromethane as solvent, and enol ethers with higher Charton values enabled production of PNB with high trans content. Data from a combined experimental and computational investigation are consistent with the stereocontrolling step of the radical cationic mechanism proceeding under thermodynamic control.
立体化学对聚合物和材料性能会产生深远影响。不幸的是，实现高水平立体控制聚合的直接方法往往具有挑战性。与传统的金属介导开环易位聚合（ROMP）不同，我们发现了一种在光氧化还原介导的无金属ROMP中控制烯烃立体化学的非常简单的方法。离子对、引发剂空间位阻和溶剂化效应均对聚降冰片烯（PNB）的立体化学产生了深远影响。对反应条件进行简单修改即可得到反式烯烃含量为25%至>98%的PNB。相对较大的抗衡离子、甲苯作为溶剂、低温（-78°C）以及查顿值较低的引发剂可得到高顺式含量。相反，较小的抗衡离子、二氯甲烷作为溶剂以及查顿值较高的烯醇醚能够制备出高反式含量的PNB。实验与计算相结合的研究数据与自由基阳离子机理在热力学控制下进行的立体控制步骤一致。
