Homolytic C-H Bond Activation by Phosphine-Quinone-Based Radical Ion Pairs.

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. PMes (Mes=2,4,6-Me C H ) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P-O bonded zwitterionic adduct Mes P-DDQ (1), while the reaction with the sterically more crowded PTip (Tip=2,4,6-iPr C H ) afforded C-H bond activation product Tip P(H)(2-[CMe (DDQ)]-4,6-iPr -C H ) (2). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip ]⋅ [DDQ]⋅ , and subsequent homolytic C-H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip P(H)(2-[CMe {TCQ-B(C F ) }]-4,6-iPr -C H ) (4, TCQ=tetrachloro-1,4-benzoquinone) and Tip P(H)(2-[CMe {oQ -B(C F ) }]-4,6-iPr -C H ) (8, oQ =3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip with Lewis-acid activated quinones, TCQ-B(C F ) and oQ -B(C F ) , 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.