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Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

Nature Chemistry volume 6, pages 983–988 (2014)Cite this article

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Abstract

Perfluorarylborane Lewis acids catalyse the addition of silicon–hydrogen bonds across C=C, C=N and C=O double bonds. This ‘metal-free’ hydrosilylation has been proposed to occur via borane activation of the silane Si–H bond, rather than through classical Lewis acid/base adducts with the substrate. However, the key borane/silane adduct had not been observed experimentally. Here it is shown that the strongly Lewis acidic, antiaromatic 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene forms an observable, isolable adduct with triethylsilane. The equilibrium for adduct formation was studied quantitatively through variable-temperature NMR spectroscopic investigations. The interaction of the silane with the borane occurs through the Si–H bond, as evidenced by trends in the Si–H coupling constant and the infrared stretching frequency of the Si–H bond, as well as by X-ray crystallography and theoretical calculations. The adduct's reactivity with nucleophiles demonstrates conclusively the role of this species in metal-free ‘frustrated-Lewis-pair’ hydrosilylation reactions.

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Figure 1: The mechanism of FLP activation of Si–H and H–H bonds.
Figure 2: Equilibrium formation of borane–silane adduct 2.
Figure 3: Partial proton NMR spectrum of solutions of 1 and Et3Si–H.
Figure 4: Crystal structure of boraindene–silane adduct 2 (50% probability ellipsoids).
Figure 5: Reactivity of boraindene silane adduct 2.

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Acknowledgements

Funding for the experimental work described was provided by the Natural Sciences and Engineering Research Council of Canada in the form of a Discovery Grant to W.E.P. Funding for the computational work described was provided by the Academy of Finland in the form of a Research Grant and Fellowship to H.M.T.

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Authors and Affiliations

  1. Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, T2N 1N4, Alberta, Canada

    Adrian Y. Houghton & Warren E. Piers

  2. Department of Chemistry, Nanoscience Center, University of Jyväskylä, PO Box 35, Jyväskylä, FI-40014, Finland

    Juha Hurmalainen, Akseli Mansikkamäki & Heikki M. Tuononen

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Contributions

A.Y.H. and W.E.P. conceived and designed the experiments, A.Y.H. performed the experiments and determined the X-ray structure, H.M.T. and A.M. conceived and designed the computational work, J.H. and A.M. executed the calculations and performed data analyses. A.Y.H., W.E.P. and A.M. co-wrote the paper with input from J.H. and H.M.T.

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Correspondence to Warren E. Piers or Heikki M. Tuononen.

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Houghton, A., Hurmalainen, J., Mansikkamäki, A. et al. Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations. Nature Chem 6, 983–988 (2014). https://doi.org/10.1038/nchem.2063

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