Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 139, Issue 20, Pages 6867-6879Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jacs.6b13031
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Funding
- NSF under the CCI Center for Selective C-H Functionalization (CCHF) [CHE-1205646, CHE-1212767]
- NSF
- CCI Center for Selective C-H Functionalization
- Bristol-Myers Squibb
- Resnick Sustainability Institute at Caltech
- Dow Chemical
- NSERC
- Direct For Mathematical & Physical Scien [1361104] Funding Source: National Science Foundation
- Division Of Chemistry [1361104] Funding Source: National Science Foundation
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [1205646] Funding Source: National Science Foundation
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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 beta-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 H-2 gas. In situ NMR experiments with deuterated substrates show that H-2 is formed by a cross-dehydrogenative mechanism The stereochemical course at the silicon center was investigated utilizing a H-2-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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