Department of Chemistry, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States.
Department of Chemistry, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
J Am Chem Soc. 2016 Feb 24;138(7):2411-25. doi: 10.1021/jacs.5b13455. Epub 2016 Feb 12.
Photoinduced metal-free atom transfer radical polymerization (ATRP) of methyl methacrylate was investigated using several phenothiazine derivatives and other related compounds as photoredox catalysts. The experiments show that all selected catalysts can be involved in the activation step, but not all of them participated efficiently in the deactivation step. The redox properties and the stability of radical cations derived from the catalysts were evaluated by cyclic voltammetry. Laser flash photolysis (LFP) was used to determine the lifetime and activity of photoexcited catalysts. Kinetic analysis of the activation reaction according to dissociative electron-transfer (DET) theory suggests that the activation occurs only with an excited state of catalyst. Density functional theory (DFT) calculations revealed the structures and stabilities of the radical cation intermediates as well as the reaction energy profiles of deactivation pathways with different photoredox catalysts. Both experiments and calculations suggest that the activation process undergoes a DET mechanism, while an associative electron transfer involving a termolecular encounter (the exact reverse of DET pathway) is favored in the deactivation process. This detailed study provides a deeper understanding of the chemical processes of metal-free ATRP that can aid the design of better catalytic systems. Additionally, this work elucidates several important common pathways involved in synthetically useful organic reactions catalyzed by photoredox catalysts.
采用几种吩噻嗪衍生物和其他相关化合物作为光氧化还原催化剂，研究了甲基丙烯酸甲酯的光诱导无金属原子转移自由基聚合（ATRP）。实验表明，所有选定的催化剂都可以参与激活步骤，但并非所有催化剂都能有效地参与失活步骤。通过循环伏安法评估了催化剂衍生的自由基阳离子的氧化还原性质和稳定性。激光闪光光解（LFP）用于确定光激发催化剂的寿命和活性。根据离解电子转移（DET）理论对激活反应的动力学分析表明，只有催化剂的激发态才能发生激活。密度泛函理论（DFT）计算揭示了自由基阳离子中间体的结构和稳定性以及具有不同光氧化还原催化剂的失活途径的反应能量曲线。实验和计算都表明，激活过程经历 DET 机制，而在失活过程中，涉及三分子碰撞的缔合电子转移（与 DET 途径相反）是有利的。这项详细的研究提供了对无金属 ATRP 化学过程的更深入理解，有助于设计更好的催化体系。此外，这项工作阐明了光氧化还原催化剂催化的几种在合成上有用的有机反应中涉及的重要共同途径。
