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Cobalt(II)–tetraphenylporphyrin-catalysed carbene transfer from acceptor–acceptor iodonium ylides via N-enolate–carbene radicals

Abstract

Square-planar cobalt(II) systems have emerged as powerful carbene transfer catalysts for the synthesis of numerous (hetero)cyclic compounds via cobalt(III)–carbene radical intermediates. Spectroscopic detection and characterization of reactive carbene radical intermediates is limited to a few scattered experiments, centered around monosubstituted carbenes. Here, we reveal the formation of disubstituted cobalt(III)–carbene radicals derived from a cobalt(II)–tetraphenylporphyrin complex and acceptor–acceptor λ3-iodaneylidenes (iodonium ylides) as carbene precursors and their catalytic application. Iodonium ylides generate biscarbenoid species via reversible ligand modification of the paramagnetic cobalt(II)–tetraphenylporphyrin complex catalyst. Two interconnected catalytic cycles are involved in the overall mechanism, with a monocarbene radical and an N-enolate–carbene radical intermediate at the heart of each respective cycle. Notably, N-enolate formation is not a deactivation pathway but a reversible process, enabling transfer of two carbene moieties from a single N-enolate–carbene radical intermediate. The findings are supported by extensive experimental and computational studies.

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Fig. 1: Applications and challenges of cobalt(III)–carbene radicals.
Fig. 2: Spectroscopic detection and characterization of the N-enolate–carbene radical IE–T.
Fig. 3: Schematic overview of spectroscopically detected catalytic intermediates and deactivation products.
Fig. 4: Trapping and reactivity studies.
Fig. 5: Full catalytic cycle for the [Co(TPP)]-catalysed reaction of DMM•IY and styrene.

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Data availability

All processed data that support the findings of this study are available within the article and its Supplementary Information (experimental details; synthetic procedures; catalytic studies; X-ray diffraction, EPR, NMR, UV/Vis, in situ attenuated total reflection Fourier-transform infrared spectroscopy and CSI–HR–MS data; and (additional) DFT and NEVPT2–CASSCF calculations). All raw data that support the findings of this study have been deposited at the FigShare repository53 with https://doi.org/10.6084/m9.figshare.17528879. The X-ray crystallographic data for IE–A was deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition number CCDC 2091203.

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Acknowledgements

Financial support from The Netherlands Organization for Scientific Research (NWO TOP–Grant 716.015.001 to B.d.B.) is gratefully acknowledged. We thank D. Vesseur and N. P. van Leest for fruitful discussions and F. J. de Zwart for assistance with the EPR measurements.

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R.F.J.E. and B.d.B. conceived and designed the project. R.F.J.E. and M.M.H. performed the experimental work, and R.F.J.E. and B.d.B., the spectroscopic work. Quantum chemical calculations were carried out by R.F.J.E., M.M.H. and B.d.B. Mass spectrometry measurements were performed by E.O.B. and R.F.J.E. Crystallographic measurements and characterization were carried out by S.M. The authors discussed all results and the manuscript in detail.

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Correspondence to Bas de Bruin.

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Supplementary Data 1

Crystallographic data for compound IEA. CCDC reference no. 2091203.

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Epping, R.F.J., Hoeksma, M.M., Bobylev, E.O. et al. Cobalt(II)–tetraphenylporphyrin-catalysed carbene transfer from acceptor–acceptor iodonium ylides via N-enolate–carbene radicals. Nat. Chem. 14, 550–557 (2022). https://doi.org/10.1038/s41557-022-00905-4

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