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Solvent- and metal-free regioselective halogenation of electron rich azobenzene derivatives

Organohalides are extremely valuable synthetic precursors, and the transformation of the carbon- halogen bond is on many occasions one of the synthetic steps in the preparation of numerous organic and natural compounds, pharmaceutically active substances, new materials, catalysts, etc. Traditional solvent-based methods often require reactions with elemental halogens, harsh reaction conditions, and metal catalysts which are in many cases additionally hampered by difficult-to- control selectivity. As a greener alternative to solvent- based reactions, chemical transformations in the solid-state have drawn increasing attention 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. In this work we present optimized conditions for solvent- and metal-free regioselective halogenation of 4’-(N, N dimethylamino)-4- nitroazobenzene and 4 methoxyazobenzene by mechanochemistry. This mild and environmentally friendly approach allowed the selective preparation of halogenated derivatives and led to an unexpected product. The reactions were monitored by in situ Raman spectroscopy which in combination with ex situ NMR, IR, and X-ray experiments allowed characterization of the involved reaction species and direct insight into the reaction dynamics.

azobenzene ; halogenation ; regioselective ; solvent-free ; metal-free

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