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Isolation of a pentadienyl-type radical featuring a central secondary carbon

Abstract

Stable tertiary R3C· carbon radicals have been known since Gomberg’s pioneering discovery of the triphenylmethyl radical more than a century ago. In stark contrast, secondary R2CH· and primary RCH2· carbon radicals are elusive species only observed spectroscopically. Herein, we describe the isolation of a crystalline pentadienyl-type radical, featuring a central secondary carbon, prepared by single-electron reduction of a bis(imino)carbene conjugate acid. The key to its stability is the presence of two N-heterocyclic imine substituents, which impart both steric protection and electronic stabilization. Density functional theory calculations confirm that the central secondary carbon atom is the principal spin carrier. Accordingly, electron paramagnetic resonance spectroscopy reveals that the hydrogen atom attached to the central carbon atom exhibits an exceptionally large hyperfine coupling constant (>10 G), which we believe is the largest recorded for an isolated organic radical. In the presence of an H· donor, hydrogen atom abstraction occurs exclusively at the central carbon to form a methylene unit. Furthermore, this radical can participate in a radical–radical cross-coupling reaction with [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl], providing an example of cross-coupling between two stable organic radicals.

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Fig. 1: Carbon radicals.
Fig. 2: Electrochemical studies.
Fig. 3: Synthesis and characterization of radical 1H∙.
Fig. 4: EPR and DFT studies of radical 1H∙.
Fig. 5: Syntheses and solid-state structures of 1H-TEMPO and 1H2.

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Data availability

The data supporting the findings of this study are available within the Article and its Supplementary Information. Crystallographic data for the structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 2245614 (1H+[TfO]), 2267005 (1H), 2267006 (1H2) and 2312276 (1H-TEMPO). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/.

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Acknowledgements

This work was supported by the NSF (grant no. CHE-2246948). Y.K.L. thanks A*STAR for the award of a postdoctoral fellowship. We gratefully acknowledge the scientific support and high-performance computing resources provided by the Erlangen National High Performance Computing Center (NHR@FAU) of the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU). The hardware is funded by the German Research Foundation (DFG). EPR work was funded by NSF MRI grant no. CHE 2019066.

Author information

Authors and Affiliations

Authors

Contributions

Y.K.L. conceived and performed the synthetic experiments. L.G. carried out the UV–Vis analysis and additional synthetic experiments during revision. M.M. performed the EPR and electrochemical studies. M.M. and M.G. performed the X-ray crystallographic analyses. D.M. performed the computational work. G.B. supervised the project. The manuscript was written by Y.K.L., M.M. and G.B.

Corresponding authors

Correspondence to Ying Kai Loh or Guy Bertrand.

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The authors declare no competing interests.

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Peer review information

Nature Synthesis thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editor: Peter Seavill, in collaboration with the Nature Synthesis team.

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Supplementary information

Supplementary Information

Experimental details, Supplementary Figs. 1–27 and Tables 1–10.

Supplementary Data 1

Crystallographic data for compound 1H+[TfO]; CCDC 2245614.

Supplementary Data 2

Crystallographic data for compound 1H·; CCDC 2267005.

Supplementary Data 3

Crystallographic data for compound 1H2; CCDC 2267006.

Supplementary Data 4

Crystallographic data for compound 1H-TEMPO; CCDC 2312276.

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Loh, Y.K., Gojiashvili, L., Melaimi, M. et al. Isolation of a pentadienyl-type radical featuring a central secondary carbon. Nat. Synth 3, 727–731 (2024). https://doi.org/10.1038/s44160-024-00516-6

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