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  • Review Article
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Modern applications of low-valent early transition metals in synthesis and catalysis

Abstract

Low-valent early transition metals are often intrinsically highly reactive as a result of their strong propensity toward oxidation to more stable high-valent states. Harnessing these highly reducing complexes for productive reactivity is potentially powerful for C–C bond construction, organic reductions, small-molecule activation and many other reactions that offer orthogonal chemoselectivity and/or regioselectivity patterns to processes promoted by late transition metals. Recent years have seen many exciting new applications of low-valent metals through building new catalytic and/or multicomponent reaction manifolds out of classical reactivity patterns. In this Review, we survey new methods that employ early transition metals and invoke low-valent precursors or intermediates in order to identify common themes and strategies in synthesis and catalysis.

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Fig. 1: Relative abundance of the valence states of early transition metal organometallic complexes.
Fig. 2: Examples of synthetic routes to low-valent early transition metal complexes.
Fig. 3: Recent advances in two-electron reductive coupling reactions proceeding through titanacyclopropane and titanacyclopropene intermediates from Ti(OiPr)4/RM and related species.
Fig. 4: Examples of new reductive coupling reactions.
Fig. 5: Catalytic (de)hydrofunctionalizations.
Fig. 6: Single-electron processes catalysed by TiIII complexes.
Fig. 7: Single-electron strategies in small-molecule activation chemistry.
Fig. 8: Group-transfer catalysis mediated by early transition metal complexes.

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Acknowledgements

Financial support was provided by the National Institutes of Health (1R35GM119457) and the Alfred P. Sloan Foundation (I.A.T. is a 2017 Sloan Fellow).

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E.P.B., A.J.P. and X.Y.S. contributed equally. E.P.B., A.J.P. and X.Y.S. researched data for the article. All authors made substantial contributions to discussion of content, writing, reviewing and editing of the manuscript before submission.

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Correspondence to Ian A. Tonks.

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Glossary

Inner-sphere reductants

Reagents that act as reductants after first forming a bond to the complex of interest.

Umpolung reactions

Reactions that proceed through polarity inversion of the given functional group.

Redox non-innocent ligands

Ligands bound to a metal complex in which the oxidation state is ambiguous, wherein oxidation or reduction of the ligands may occur in tandem with or instead of metal oxidation or reduction.

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Beaumier, E.P., Pearce, A.J., See, X.Y. et al. Modern applications of low-valent early transition metals in synthesis and catalysis. Nat Rev Chem 3, 15–34 (2019). https://doi.org/10.1038/s41570-018-0059-x

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