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Deacylative transformations of ketones via aromatization-promoted C-C bond activation.

Nature 2019 January 31
Carbon-hydrogen (C-H) and carbon-carbon (C-C) bonds are the main constituents of organic matter. The recent advancement of C-H functionalization technology has vastly expanded our toolbox for organic synthesis1 . In contrast, C-C activation methods that allow editing of the molecular skeleton remain limited2- . So far, a number of methods have appeared for catalytic C-C activation, particularly with ketone substrates, which are typically promoted either by ring-strain release as a thermodynamic driving force4,6 or using directing groups5,7 to control the reaction outcome. Although effective, these strategies require highly strained ketone substrates or those containing a pre-installed directing group, or are limited to more specialist substrate classes5 . Here we report a general C-C activation mode driven by aromatization of a pre-aromatic intermediate formed in situ. This reaction is suitable for various ketone substrates, is catalysed by an iridium/phosphine combination, and is promoted by a hydrazine reagent and 1,3-dienes. Specifically, the acyl group is removed from the ketone, transformed to a pyrazole, and the resulting alkyl fragment undergoes various transformations. These include the deacetylation of methyl ketones, carbenoid-free formal homologation of aliphatic linear ketones and deconstructive pyrazole synthesis from cyclic ketones. Given that ketones are prevalent in feedstock chemicals, natural products and pharmaceuticals, these transformations could offer strategic bond disconnections in the synthesis of complex bioactive molecules.

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