2,2’‐Biquinoline‐Based Recyclable Electroauxiliaries for the Generation of Alkyl Radicals via C−C Bond Cleavage

2,2’-Biquinoline-based electroauxiliaries have successfully been developed as suitable reagents for the site-selective generation of alkyl radicals. Primary, secondary and tertiary alkyl radicals could be generated and several free radical reactions could be studied. The platform has the advantage of recyclability, which makes these alkyl radical precursors superior compared to previous N-heterocyclic electroauxiliaries.

Abstract

Alkyl radical precursors are essential for a wide variety of photocatalytic and 3d-metal-catalyzed C−C bond forming reactions. Neutral organic heterocycles as electroauxiliaries such as 4-alkyl Hantzsch esters have become reliable tools for alkyl radical formation. Here we show that 2,2’-biquinoline-derived alkyl-substituted dihydroquinolines act as competent radical precursors with the ability to form primary, secondary and tertiary alkyl radicals. Hydroalkylation of benzalmalononitriles and N-Boc protected diazenes has been achieved through copper catalysis under mild conditions of 50 °C with good to very good yields of up to 85 %. Furthermore, the dihydroquinolines’ reactivity towards a denitrative alkylation of nitroolefins such as β-nitrostyrene was discovered. Most importantly, the released biquinoline can be recycled, which greatly improves the overall atom-economy of these alkyl radical precursors in comparison to previous N-heterocyclic electroauxiliaries.

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