Key Laboratory of Photochemical Conversion and Optoelectronic Materials , Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences , Beijing 100190 , People's Republic of China.
School of Future Technology , University of Chinese Academy of Sciences , Beijing 100049 , People's Republic of China.
J Am Chem Soc. 2019 Sep 4;141(35):13941-13947. doi: 10.1021/jacs.9b06920. Epub 2019 Aug 23.
Direct activation of H-phosphine oxide to react with an unsaturated carbon-carbon bond is a straightforward approach for accessing alkenylphosphine oxides, which shows significant applications in both synthetic and material fields. However, expensive metals and strong oxidants are typically required to realize the transformation. Here, we demonstrate the utility of earth-abundant cobaloxime to convert H-phosphine oxide into its reactive radical species under visible light irradiation. The radical species thus generated can be utilized to functionalize alkenes and alkynes without any external photosensitizer and oxidant. The coupling with terminal alkene generates -alkenylphosphine oxide with excellent chemo- and stereoselectivity. The reaction with terminal alkyne yields linear -alkenylphosphine oxide via neutral radical addition, while addition with internal ones generates cyclic benzophosphine oxides and hydrogen. Mechanistic studies on radical trapping experiments, electron spin resonance studies, and spectroscopic measurements confirm the formation of phosphinoyl radical and cobalt intermediates that are from capturing the electron and proton eliminated from H-phosphine oxide. The highlight of our mechanistic investigation is the dual role played by cobaloxime, viz., both as the visible light absorber to activate the P(O)-H bond as well as a hydrogen transfer agent to influence the reaction pathway. This synergetic feature of the cobaloxime catalyst preforming multiple functions under ambient condition provides a convergent synthetic approach to vinylphosphine oxides directly from H-phosphine oxides and alkenes (or alkynes).
直接活化 H-膦氧化物与不饱和碳-碳键反应是一种直接合成烯基膦氧化物的方法，在合成和材料领域都有重要的应用。然而，通常需要昂贵的金属和强氧化剂来实现这种转化。在这里，我们展示了丰富的钴配合物在可见光照射下将 H-膦氧化物转化为其反应性自由基物种的用途。由此产生的自由基物种可用于在没有任何外部光敏剂和氧化剂的情况下官能化烯烃和炔烃。与末端烯烃的偶联生成具有优异的化学和立体选择性的 -烯基膦氧化物。与末端炔烃的反应通过中性自由基加成生成线性 -烯基膦氧化物，而与内部炔烃的加成则生成环状苯并膦氧化物和氢气。自由基捕获实验、电子顺磁共振研究和光谱测量的机理研究证实了磷酰基自由基和钴中间体的形成，这些中间体是从 H-膦氧化物中捕获电子和质子消除而来的。我们的机理研究的一个重点是钴配合物的双重作用，即作为可见光吸收剂激活 P(O)-H 键，以及作为氢转移剂影响反应途径。钴配合物催化剂在环境条件下协同地执行多种功能，为直接从 H-膦氧化物和烯烃（或炔烃）合成乙烯基膦氧化物提供了一种收敛的合成方法。
