Three concomitant C–C dissociation pathways during the mechanical activation of an N-heterocyclic carbene precursor

Abstract

Chemical reactions usually proceed through a radical, concerted or ionic mechanism; transformations in which all three mechanisms occur are rare. In polymer mechanochemistry, a mechanical force, transduced along polymer chains, is used to activate covalent bonds in mechanosensitive molecules (mechanophores). Cleavage of a C–C bond often follows a homolytic pathway, but some mechanophores have also been designed that react in a concerted or, more rarely, a heterolytic manner. Here, using 1H- and 19F-nuclear magnetic resonance spectroscopy in combination with deuterium labelling, we show that the dissociation of a mechanophore built around an N-heterocyclic carbene precursor proceeds with the rupture of a C–C bond through concomitant heterolytic, concerted and homolytic pathways. The distribution of products probably arises from a post-transition-state bifurcation in the reaction pathway, and their relative proportion is dictated by the polarization of the scissile C–C bond.

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Fig. 1: The ultrasound activation of NHC precursor 1H proceeds via three concomitant dissociation pathways.
Fig. 2: Synthesis and CoGEF simulation of mechanophore-containing polymer 1.
Fig. 3: Mechanical activation of polymer 1H leads to the scission of the central C–C bond.
Fig. 4: Deuterium labelling experiments reveal three dissociation pathways.
Fig. 5: Scissile bond polarization controls the relative importance of each dissociation pathway.
Fig. 6: The main dissociation pathways are probably the result of a post-TS bifurcation on the force-modified PES.

Data availability

Crystallographic data for the compound 2H have been deposited at the Cambridge Crystallographic Data Centre under deposition number CCDC 1991781. The data that support the findings of this study are available within the paper and its Supplementary Information, or are available from the figshare data repository (https://doi.org/10.6084/m9.figshare.12156378.v1). Source data are provided with this paper.

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Acknowledgements

We thank the EPSRC for giving a studentship to R.N. and the Royal Society for giving a University Research Fellowship to G.D.B.

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G.D.B. conceived the project. R.N. and G.D.B. designed the experiments. R.N. carried out the experimental work. G.D.B performed the calculations. All the authors contributed to the analysis of the results and the writing of the manuscript.

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Correspondence to Guillaume De Bo.

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Supplementary information

Supplementary Information

Experimental procedures, NMR spectra and computational details.

Supplementary Data

Crystallographic data for the compound 2H, CCDC 1991781.

Source data

Source Data Fig. 6

Source data of PES in Fig. 6b.

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Nixon, R., De Bo, G. Three concomitant C–C dissociation pathways during the mechanical activation of an N-heterocyclic carbene precursor. Nat. Chem. (2020). https://doi.org/10.1038/s41557-020-0509-1

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