Ionic and Neutral Mechanisms for C–H Bond Silylation of Aromatic Heterocycles Catalyzed by Potassium t-Butoxide
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.
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.
Supplemental Material - ja6b13032_si_001.pdf