Ionic and neutral mechanisms for C-H bond silylation of aromatic heterocycles catalyzed by potassium tert-butoxide

Banerjee, Shibdas, Yang, Yun-Fang, Jenkins, Ian D., Liang, Yong, Toutov, Anton A., Liu, Wen-Bo, Schuman, David P., Grubbs, Robert H., Stoltz, Brian M., Krenske, Elizabeth H., Houk, Kendall N. and Zare, Richard N. (2017) Ionic and neutral mechanisms for C-H bond silylation of aromatic heterocycles catalyzed by potassium tert-butoxide. Journal of the American Chemical Society, 139 20: 6880-6887. doi:10.1021/jacs.6b13032


Author Banerjee, Shibdas
Yang, Yun-Fang
Jenkins, Ian D.
Liang, Yong
Toutov, Anton A.
Liu, Wen-Bo
Schuman, David P.
Grubbs, Robert H.
Stoltz, Brian M.
Krenske, Elizabeth H.
Houk, Kendall N.
Zare, Richard N.
Title Ionic and neutral mechanisms for C-H bond silylation of aromatic heterocycles catalyzed by potassium tert-butoxide
Formatted title
Ionic and neutral mechanisms for C-H bond silylation of aromatic heterocycles catalyzed by potassium tert-butoxide
Journal name Journal of the American Chemical Society   Check publisher's open access policy
ISSN 1520-5126
0002-7863
Publication date 2017-05-24
Year available 2017
Sub-type Article (original research)
DOI 10.1021/jacs.6b13032
Open Access Status Not yet assessed
Volume 139
Issue 20
Start page 6880
End page 6887
Total pages 8
Place of publication Washington, DC, United States
Publisher American Chemical Society
Language eng
Subject 1503 Catalysis
1600 Chemistry
1303 Biochemistry
1505 Colloid and Surface Chemistry
Abstract 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 tert-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 tert-butoxide catalyst. Heterolysis of the Si-H bond, deprotonation of the heteroarene, addition of the heteroarene carbanion to the silyl ether, and dissociation of tert-butoxide from silicon lead to the silylated heteroarene product. The steps of the silylation mechanism may follow either an ionic route involving K and BuO ions or a neutral heterolytic route involving the [KOBu] tetramer. Both mechanisms are consistent with the ionic intermediates detected experimentally. We also present reasons why KOBu is an active catalyst whereas sodium tert-butoxide and lithium tert-butoxide are not, and we explain the relative reactivities of different (hetero)arenes in the silylation reaction. The unique role of KOBu is traced, in part, to the stabilization of crucial intermediates through cation-π interactions.
Formatted abstract
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 tert-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 tert-butoxide catalyst. Heterolysis of the Si–H bond, deprotonation of the heteroarene, addition of the heteroarene carbanion to the silyl ether, and dissociation of tert-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 KOtBu is an active catalyst whereas sodium tert-butoxide and lithium tert-butoxide are not, and we explain the relative reactivities of different (hetero)arenes in the silylation reaction. The unique role of KOtBu is traced, in part, to the stabilization of crucial intermediates through cation−π interactions.
Keyword Chemistry, Multidisciplinary
Chemistry
Q-Index Code C1
Q-Index Status Provisional Code
Grant ID CHE-1205646
AFOSR FA9550-16-1-0113
FT120100632
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: HERDC Pre-Audit
School of Chemistry and Molecular Biosciences
 
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