Mechanochemical, solvent free, palladium-catalyzed hydrodechlorination of chloroaromatic hydrocarbons

Palladium-catalyzed, mechanochemical hydrodechlorination (HDC) of solid hexachlorobenzene (HCB) with various solid hydrogen donors was carried out at ambient temperature in a planetary ball-mill to give benzene and a mixture of partially chlorinated benzenes. The mechanochemical hydrogen transfer dechlorination is effected in the presence of Pd on carbon, or palladium(II) acetate, and in the absence of a solvent, while no such reaction took place in the absence of Pd. The presence of a base (NaOH) and mmol amounts of water (introduced in the form of sodium phosphate heptahydrate) were needed to improve HDC efficiency. Of the H-donors studied: sodium hypophosphite, sodium borohydride and calcium hydride, the last one was the most efficient, realizing complete loss of HCB over 20 h. The chemoselectivity of the HDC depends on the hydrogen donor, e.g., sodium hypophosphite (SHP) gives 1,2-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,3,4-tetrachlorobenzene and pentachlorobenzene as the major intermediates in the dechlorination process. A chemoselectivity similar to that determined in mechanochemical HDC was found in a liquid phase hydrogen transfer HDC using a Pd/C catalyst and SHP in aqueous acetonitrile.