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-Me₃ C₆ H₂ ) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P-O bonded zwitterionic adduct Mes₃ P-DDQ (1), while the reaction with the sterically more crowded PTip₃ (Tip=2,4,6-iPr₃ C₆ H₂ ) afforded C-H bond activation product Tip₂ P(H)(2-[CMe₂ (DDQ)]-4,6-iPr₂ -C₆ H₂ ) (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, Tip₂ P(H)(2-[CMe₂ {TCQ-B(C₆ F₅ )₃ }]-4,6-iPr₂ -C₆ H₂ ) (4, TCQ=tetrachloro-1,4-benzoquinone) and Tip₂ P(H)(2-[CMe₂ {oQ^tBu -B(C₆ F₅ )₃ }]-4,6-iPr₂ -C₆ H₂ ) (8, oQ^tBu =3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip₃ with Lewis-acid activated quinones, TCQ-B(C₆ F₅ )₃ and oQ^tBu -B(C₆ F₅ )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.

Keywords: C−H Activation; Frustrated Lewis Pairs; Radical-Ion-Pair; Radicals; Single-Electron Transfer.