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Synthesis and properties of allylic, benzylic, propargylic and allenylic oxonium ions

Nature Synthesis (2026)Cite this article

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Abstract

Despite numerous studies of trialkyloxonium ions in the literature, investigations into the chemistry of allylic, benzylic, propargylic and allenylic oxonium ions are rare. Existing reports on well-characterized allylic and benzylic oxonium ions invariably construct these species based on constrained tricyclic oxatriquinane or oxatriquinacene scaffolds, with only limited studies reported on unconstrained benzylic oxonium ions. Here we report an investigation on a collection of allylic, benzylic and hitherto unknown propargylic and allenylic oxonium ions prepared on unconstrained scaffolds by a general, modular and unified strategy. Permutation of the substitution pattern of these oxonium ions allowed the extension of the strategy for the syntheses of various doubly substituted oxonium ions. Most of these oxonium ions could be characterized at room temperature by NMR spectroscopy, and a series of unexpected reactions and chemical behaviours pertinent to these species are briefly described.

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Fig. 1: Literature examples of trialkyl and (tri)aryloxonium ions and oxonium ions from this study.
Fig. 2: Synthesis and NMR spectroscopy characterization, and a comparison of oxonium ions bearing an unsaturated substituent.
Fig. 3: NMR spectroscopy characterization of oxonium ions bearing unsaturation at two positions.
Fig. 4: Oxonium ion equilibration.
Fig. 5: Characterization and reactivity of oxonium ion 24v·Al(pftb)4 showing fluxionality by NMR spectroscopy.
Fig. 6: Rearrangement of a benzylic oxonium ion.
Fig. 7: Rearrangement of propargylic oxonium ions.
Fig. 8: Generation of vinyl oxonium ion 62·Al(pftb)4 from 23x.

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Data availability

The data (experimental procedures, characterization data, NMR spectra, and computational methods and results for oxonium ion characterization) supporting the findings of this study are available within the Article and its Supplementary Information.

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Acknowledgements

J.L.S. is grateful to the Royal Commission for the Exhibition of 1851 for an Industrial Fellowship and the generous support provided by that fellowship, and to the EPSRC Centre for Doctoral Training in Synthesis for Biology and Medicine (EP/L015838/1) for a studentship, generously supported by AstraZeneca, Diamond Light Source, Defence Science and Technology Laboratory, Evotec, GlaxoSmithKline, Janssen, Novartis, Pfizer, Syngenta, Takeda, UCB and Vertex. D.S.B. is grateful to the Biotechnology and Biological Sciences Research Council and Syngenta for the award of an iCASE (BB/P504890/1). R.S.P. acknowledges support from the National Science Foundation (NSF CHE2400056) and the Alpine HPC resource, which is jointly funded by the University of Colorado Boulder, the University of Colorado Anschutz and Colorado State University, and the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) through allocation TG-CHE180056. We thank M. Mascal and N. Hafezi for productive discussions regarding NMR assignment of oxonium ions.

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Authors and Affiliations

  1. Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Oxford, UK

    Hau Sun Sam Chan, Jack L. Sutro, Daniel S. Brown & Jonathan W. Burton

  2. Department of Chemistry, Colorado State University, Fort Collins, CO, USA

    Yingzi Li & Robert S. Paton

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Contributions

J.W.B. and H.S.S.C. designed the project. H.S.S.C., J.L.S. and D.S.B. conducted the synthetic experiments. Y.L. and R.S.P. designed and performed the computational study. H.S.S.C., J.W.B. and R.S.P. wrote the paper with contributions from all the authors. All authors interpreted the results in the paper.

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Correspondence to Robert S. Paton or Jonathan W. Burton.

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Extended data

Extended Data Fig. 1 Formation of Friedel–Crafts products 54 and 55 from oxonium ions 24m·SbF6 and 24l·SbF6.

Reagents and conditions: For 24m·SbF6: 1) CD2Cl2, r.t., 1.5 hrs. 2) 2.0 eq. Et3N, CD2Cl2, r.t., 61% over 2 steps. For 24l·SbF6: 1) CD2Cl2, −10 °C to r.t., 2 hrs. 2) 2.0 eq. Et3N, CD2Cl2, r.t. 68% over 2 steps.

Extended Data Fig. 2 Formation of 1,4-aryl transfer product 60 from oxonium ion 24l·Al(pftb)4.

Reagents and conditions: 1) CD2Cl2, 0 °C to r.t., 30 minutes. 2) Filter, then 5.0 eq. Tetrabutylammonium bromide, CD2Cl2, r.t. 60: 21% over 2 steps, 61: 42% over 2 steps (d.r. ~ 4:1).

Supplementary information

Supplementary Information (download PDF )

Experimental details, Supplementary Figs. 1–28, computational methods and Cartesian coordinates.

Supplementary Data 1 (download PDF )

Supplementary NMR spectra.

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Chan, H.S.S., Li, Y., Sutro, J.L. et al. Synthesis and properties of allylic, benzylic, propargylic and allenylic oxonium ions. Nat. Synth (2026). https://doi.org/10.1038/s44160-025-00964-8

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