Iridium-catalyzed reduction of C-X bonds (X = F, Cl, Br, I, O) bonds with triethylsilane

Yang, Jian

Iridium-catalyzed reduction of C-X bonds (X = F, Cl, Br, I, O) bonds with triethylsilane Public Deposited

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March 21, 2019

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Yang, Jian
- Affiliation: College of Arts and Sciences, Department of Chemistry

Abstract

Alkyl halides are generally reduced with trialkyl tin hydrides via a radical chain mechanism. Alternative reduction procedures are desired owing to the toxicity of the tin reagents and problems separating tin byproducts from reduction products. We have discovered and developed a highly efficient and environmental friendly procedure for the reduction of a broad range of alkyl halides by triethylsilane reagents with cationic iridium pincer catalysts. In-depth mechanistic studies have been carried out which have revealed a unique catalytic cycle. The electrophilic iridium hydride complex binds and activates the silane. This complex transfers "Et3Si+" to the halide forming a highly active bridged halonium ion which is rapidly reduced by the iridium dihydride remaining following silyl transfer and the cationic iridium hydride complex is thus regenerated. All key intermediates have been identified by in situ NMR monitoring and kinetic studies have been completed. The key Ir(silane)intermediate has even been isolated and fully characterized by NMR spectroscopy and X-ray crystallography, which shows an unprecedented example of a cationic transition metal eta1-silane complex. In application of this novel chemistry to other organic functional groups, we have been particularly drawn to the cleavage and reduction of alkyl ethers, due to its several potential applications. We have found that these cationic iridium pincer catalysts are highly active for the room-temperature cleavage and reduction of a wide variety of unactivated alkyl ethers with triethylsilane. For example, diethyl ether can be readily converted to two equivalents of ethane and Et3SiOSiEt3. Poly(ethylene glycol) can be readily degraded to Et3SiOCH2CH2OSiEt3 and ethane. Mechanistic studies have revealed the full details of the catalytic cycle with the resting state(s) depending on the basicity of the alkyl ether. The key intermediate diethyl(triethylsilyl)oxonium ion has been isolated and fully characterized by low temperature NMR spectroscopies and X-ray crystallography.

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