For best experience please turn on javascript and use a modern browser!
You are using a browser that is no longer supported by Microsoft. Please upgrade your browser. The site may not present itself correctly if you continue browsing.
Van 't Hoff Institute for Molecular Sciences Van 't Hoff Institute for Molecular Sciences

Homolytic C–H bond activation by phosphine–quinone-based radical ion pairs

16 October 2023

In a paper just accepted by Angewandte Chemie, researchers led by Dr Chris Slootweg of the research group Synthetic Organic Chemistry at the Van ‘t Hoff Institute for Molecular Sciences show how phosphine–quinone-based radical ion pairs can be used for C-H bond activation. Their study presents key mechanistic insights, underpinning the synergistic action of the radical ion pairs.
Image: HIMS.

In their paper, Slootweg and co-workers describe the single-electron transfer processes between sterically demanding triarylphosphines and quinones of appropriate electron-accepting character. They report how they were able to obtain either P–O coupled adducts or C–H bond activation products, depending on the steric demand of the phosphine. Experimental and theoretical studies showed that the C–H bond activation proceeds stepwise via transient radical ion pairs. Currently the researchers are exploring the steric and electronic tuning of electron donor–acceptor complexes to enable the C–H bond activation of external substrates by in situ generated radical ion pairs.

Abstract of the paper

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.

Paper details

Christoph Helling, Lars J. C. van der Zee, Jelle Hofman, Felix J. de Zwart, Simon Mathew, Martin Nieger, Chris Slootweg: Homolytic C–H Bond Activation by Phosphine–Quinone-Based Radical Ion Pairs. Angewandte Chemie International Edition, First published: 13 October 2023. DOI: 10.1002/anie.202313397