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

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Author(s)
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
Griffith University Author(s)
Year published
2017
Metadata
Show full item recordAbstract
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 ...
View more >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.
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View more >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.
View less >
Journal Title
Journal of the American Chemical Society
Volume
139
Issue
20
Copyright Statement
This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, copyright 2017 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/jacs.6b13032
Subject
Chemical sciences
Other chemical sciences not elsewhere classified