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Ionic and Neutral Mechanisms for C–H Bond Silylation of Aromatic Heterocycles Catalyzed by Potassium t-Butoxide

Banerjee, Shibdas and Yang, Yun-Fang and Jenkins, Ian D. and Liang, Yong and Toutov, Anton Alexandrovich and Liu, Wen-Bo and Schuman, David P. and Grubbs, Robert H. and Stoltz, Brian M. and Krenske, Elizabeth H. and Houk, Kendall N. and Zare, Richard N. (2017) Ionic and Neutral Mechanisms for C–H Bond Silylation of Aromatic Heterocycles Catalyzed by Potassium t-Butoxide. Journal of the American Chemical Society, 139 (20). pp. 6880-6887. ISSN 0002-7863.

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Exploiting C–H bond activation is difficult, although some success has been achieved using precious metal catalysts. Recently, it was reported that C–H bonds in aromatic heterocycles were converted to C–Si bonds by reaction with hydrosilanes under the catalytic action of potassium t-butoxide alone. The use of Earth-abundant potassium cation as a catalyst for C–H bond functionalization seems to be without precedent, and no mechanism for the process was established. Using ambient ionization mass spectrometry, we are able to identify crucial ionic intermediates present during the C–H silylation reaction. We propose a plausible catalytic cycle, which involves a pentacoordinate silicon intermediate consisting of silane reagent, substrate, and the t-butoxide catalyst. Heterolysis of the Si–H bond, deprotonation of the heteroarene, addition of the heteroarene carbanion to the silylether, and dissociation of t-butoxide from silicon lead to the silylated heteroarene product. The steps of the silylation mechanism may follow either an ionic route involving K^+ and tBuO^– ions or a neutral heterolytic route involving the [KOtBu]_4 tetramer. Both mechanisms are consistent with the ionic intermediates detected experimentally. We also present reasons why potassium t-butoxide is an active catalyst whereas sodium t-butoxide and lithium t-butoxide are not, and we explain the relative reactivities of different (hetero)arenes in the silylation reaction. The unique role of potassium t-butoxide is traced, in part, to the stabilization of crucial intermediates through cation-π interactions.

Item Type:Article
Related URLs:
URLURL TypeDescription Information
Banerjee, Shibdas0000-0002-3424-8157
Yang, Yun-Fang0000-0002-6287-1640
Jenkins, Ian D.0000-0003-3961-2277
Liang, Yong0000-0002-7225-7062
Toutov, Anton Alexandrovich0000-0002-6561-0462
Liu, Wen-Bo0000-0003-2687-557X
Grubbs, Robert H.0000-0002-0057-7817
Stoltz, Brian M.0000-0001-9837-1528
Krenske, Elizabeth H.0000-0003-1911-0501
Houk, Kendall N.0000-0002-8387-5261
Zare, Richard N.0000-0001-5266-4253
Additional Information:© 2017 American Chemical Society. Received: December 19, 2016; Published: May 2, 2017. Authors thank N. Dalleska (Caltech), R. Pfattner (Stanford), and M. R. Angell (Stanford) for their help. This work was supported by National Science Foundation under the CCI Center for Selective C–H Functionalization (CHE-1205646 and CHE-1361104), Air Force Office of Scientific Research through Basic Research Initiative grant (AFOSR FA9550-16-1-0113), and the Australian Research Council (FT120100632 to EHK). Calculations were performed on the Hoffman2 cluster at UCLA and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the NSF, and the National Facility of the Australian National Computational Infrastructure.
Funding AgencyGrant Number
Air Force Office of Scientific Research (AFOSR)FA9550-16-1-0113
Australian Research CouncilFT120100632
Issue or Number:20
Record Number:CaltechAUTHORS:20170502-104649589
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Official Citation:Ionic and Neutral Mechanisms for C–H Bond Silylation of Aromatic Heterocycles Catalyzed by Potassium tert-Butoxide Shibdas Banerjee, Yun-Fang Yang, Ian D. Jenkins, Yong Liang, Anton A. Toutov, Wen-Bo Liu, David P. Schuman, Robert H. Grubbs, Brian M. Stoltz, Elizabeth H. Krenske, Kendall N. Houk, and Richard N. Zare Journal of the American Chemical Society 2017 139 (20), 6880-6887 DOI: 10.1021/jacs.6b13032
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:77127
Deposited By: Tony Diaz
Deposited On:02 May 2017 17:54
Last Modified:07 Jul 2020 20:02

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