Germylium Catalyzed Dehydrofluorination Reactions of Fluorinated Alkanes
Julie Borel1, Morten Lehmann2, Martin Kaupp2
1Department of Chemistry, Humboldt-Universität zu Berlin, Berlin, Germany.
Abstract:
Lewis acidic compounds such as germylium ions can be applied to C─F bond activations to access fluorinated building blocks or degrade fluorinated alkyl compounds in order to address environmental concerns and to promote a circular economy. Germylium ions [R3Ge][B(C6F5)4] (R = Et, nBu, and Ph) were isolated after treatment of [Ph3C][B(C6F5)4] with tertiary germanes. Derivatization reactions include the formation of binuclear fluoronium ions [R3Ge-F-GeR3][B(C6F5)4] (R = Et, nBu, Ph) or fluorogermanes R3GeF (R = Et, nBu, Ph) after reaction with CsF. Various Lewis pairs such as [nBu3Ge-PPh3][B(C6F5)4] were also generated. The strong Lewis acidity of the germylium ions enables the C─F bond dehydrofluorination of mono- and polyfluorinated substrates. Thus, fluorocyclohexane was selectively converted into cyclohexene along with the production of H2 and fluorogermane. DFT calculations and experimental investigations revealed a concerted E2-type mechanism where the tertiary germane reacts as a base to generate the olefinic products. The reaction pathway is in kinetic competition with an also concerted SN2-type hydrodefluorination step. In contrast, the analogous silylium-based reactions occur via a step-wise SN1 mechanism, with the silane acting as a hydride donor to a fluorosilane-stabilized carbenium ion leading exclusively to the thermodynamically favoured hydrodefluorination product.
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