Potassium tert-Butoxide-Catalyzed Dehydrogenative C–H Silylation of Heteroaromatics: A Combined Experimental and Computational Mechanistic Study
2017
Liu, Wen-Bo | Schuman, David P. | Yang, Yunfang | Toutov, Anton A. | Liang, Yong | Klare, Hendrik F. T. | Nesnas, Nasri | Oestreich, Martin | Blackmond, Donna G. | Virgil, Scott C. | Banerjee, Shibdas | Zare, Richard N. | Grubbs, Robert H. | Houk, K. N. | Stoltz, Brian M.
We recently reported a new method for the direct dehydrogenative C–H silylation of heteroaromatics utilizing Earth-abundant potassium tert-butoxide. Herein we report a systematic experimental and computational mechanistic investigation of this transformation. Our experimental results are consistent with a radical chain mechanism. A trialkylsilyl radical may be initially generated by homolytic cleavage of a weakened Si–H bond of a hypercoordinated silicon species as detected by IR, or by traces of oxygen which can generate a reactive peroxide by reaction with [KOt-Bu]₄ as indicated by density functional theory (DFT) calculations. Radical clock and kinetic isotope experiments support a mechanism in which the C–Si bond is formed through silyl radical addition to the heterocycle followed by subsequent β-hydrogen scission. DFT calculations reveal a reasonable energy profile for a radical mechanism and support the experimentally observed regioselectivity. The silylation reaction is shown to be reversible, with an equilibrium favoring products due to the generation of H₂ gas. In situ NMR experiments with deuterated substrates show that H₂ is formed by a cross-dehydrogenative mechanism. The stereochemical course at the silicon center was investigated utilizing a ²H-labeled silolane probe; complete scrambling at the silicon center was observed, consistent with a number of possible radical intermediates or hypercoordinate silicates.
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