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Palladium-mediated regioselective C–H bond halogenation of azobenzene substrates by mechanochemistry

Development of methods for the direct and selective replacement of inert C–H bond by C–X, C–N, C–S, C–O, or C–C bonds is of immense importance for organic synthesis. The transition metal-catalyzed direct transformations of inactive C–H bond into other functional groups has emerged as a superior approach, which significantly simplifies the synthesis and reduces the generation of byproducts. Currently, these processes still rely predominantly on solvent-based protocols. During the past decade, chemical transformations in the solid-state have become increasingly popular among chemists due to the unique benefits which the solid-state methods provide in terms of sustainability, reaction times, yields, solubility, selectivity, and chemical reactivity. As a continuation of our previous comparative mechanistic study of the mechanochemical C–H bond activation in an unsymmetrical azobenzene by common Pd(II) catalysts, here we present results for palladium-mediated regioselective mechanochemical C–H bond halogenation of azobenzene and its substrates containing electron-donating and electron-withdrawing groups in 4, 4’-positions of azobenzene. Careful selection of liquid and/or solid additives can direct the reaction towards different halogenated products which depend on the electronic properties of substituents, and utilization of ex situ 1H NMR spectroscopy provided detailed insight into the monitored reactions and revealed possible intermediates.

C-H bond halogenation ; azobenzene ; palladium ; mechanochemistry

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