Homolytic C−H Bond Activation by Phosphine−Quinone‐Based Radical Ion Pairs

Abstract

Herein, we present the formation of transient radical ion pairs (RIPs) by single‐electron transfer (SET) in phosphine–quinone systems and explore their potential for the activation of C–H bonds. PMes3 (Mes = 2,4,6‐Me3C6H2) reacts with DDQ (2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone) with formation of the P–O bonded zwitterionic adduct Mes3P–DDQ (1), while the reaction with the sterically more crowded PTip3 (Tip = 2,4,6‐iPr3C6H2) afforded C–H bond activation product Tip2P(H)(2‐[CMe2(DDQ)]‐4,6‐iPr2‐C6H2) (2). UV‐Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip3]•+[DDQ]•–, and subsequent homolytic C–H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip2P(H)(2‐[CMe2{TCQ–B(C6F5)3}]‐4,6‐iPr2‐C6H2) (4, TCQ = tetrachloro‐1,4‐benzoquinone) and Tip2P(H)(2‐[CMe2{oQtBu–B(C6F5)3}]‐4,6‐iPr2‐C6H2) (8, oQtBu = 3,5‐di‐tert‐butyl‐1,2‐benzoquinone), from reactions of PTip3 with Lewis‐acid activated quinones, TCQ–B(C6F5)3 and oQtBu–B(C6F5)3, respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C–H bond activation by the synergistic action of radical ion pairs.