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Review
. 2021 Dec 10;4(1):173.
doi: 10.1038/s42004-021-00611-1.

The continuum of carbon-hydrogen (C-H) activation mechanisms and terminology

Kristof M Altus  1   2 , Jennifer A Love  3
Affiliations
Review

The continuum of carbon-hydrogen (C-H) activation mechanisms and terminology

Kristof M Altus et al. Commun Chem. .

Abstract

As a rapidly growing field across all areas of chemistry, C-H activation/functionalisation is being used to access a wide range of important molecular targets. Of particular interest is the development of a sustainable methodology for alkane functionalisation as a means for reducing hydrocarbon emissions. This Perspective aims to give an outline to the community with respect to commonly used terminology in C-H activation, as well as the mechanisms that are currently understood to operate for (cyclo)alkane activation/functionalisation.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Continuum of charge transfer.
Representation of the continuum of charge transfer in C-H activation reaction pathways.
Fig. 2
Fig. 2. Examples of sigma and agostic complexes.
A First methane sigma complex fully characterised in situ. B Agostic complex featuring two agostic interactions.
Fig. 3
Fig. 3. Electrophilic mechanisms.
A Three most common transition states are found in electrophilic C-H activation. B Simplified depiction of the AMLA(6)/CMD mechanism. C DFT calculated mechanism for the C-H activation of methane at (NHC)2Pd(II) via the AMLA(6)/CMD transition state.
Fig. 4
Fig. 4. Calculated transition states for the C-H activation of methane at Pd(II).
All energies are relative to sigma complex 2.1, Fig. 2. Pd-H red dashed lines in A and B do not represent an interaction, instead are representative of distance to the metal, whereas the dashed line in C does represent a Pd-H interaction.
Fig. 5
Fig. 5. Oxidative addition mechanism.
A Simplified depiction of the oxidative addition mechanism. B Concerted oxidative addition of an alkane to an unsaturated Iridium(I) PCP complex.
Fig. 6
Fig. 6. Sigma bond metathesis mechanisms.
A Transition states encountered during the different sigma bond metathesis mechanisms; green dots represent the ring critical points of the interaction which show the point of minimum electron density in the interaction. B Simplified reaction mechanism for MAσBM reaction. C Hartwig’s alkane borylation reaction pathway.
Fig. 7
Fig. 7. 1,2-Addition mechanism.
A Simplified reaction pathway. B Reaction pathway showing the transition state of the 1,2-addition of a C-H bond to a titanium alkylidyne complex. Alkylidene and imido complexes follow the same transition state.
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