Ullmann homocoupling of trichlorobenzenes: Transformation of lindane wastes into biphenyl and oligophenylenes

The Ullmann homocoupling of 1,2,4-trichlorobenzene (1,2,4-TCB) in a biphasic medium, catalysed by Pd/C under reductive conditions and phase transfer catalysis, produces mixtures of chlorinated biphenyls, terphenyls and quaterphenyls. Meanwhile, competitive hydrodechlorination transforms the chlorinated compounds into halogen-free aromatics through a complex reaction pathway. By tuning the reaction conditions, a maximum selectivity for polyaromatics of around 75% is achieved, while 25% of the starting TCB is transformed into benzene. The catalyst recovery study demonstrates the ability of Pd/C to be reused in at least four consecutive cycles without significant loss of activity. Chloroarenes with different chlorine contents—from tetrachloro- to monochlorobenzene—and different substitution patterns lead to variable reactivity and selectivity due to stereoelectronic factors. The reaction with the mixture of chloroarenes derived from dehydrochlorination of lindane production waste leads to slightly lower homocoupling selectivity. Microwave-assisted activation opens the way to improved homocoupling and one-pot dehydrochlorination+homocoupling, with up to 90% selectivity toward homocoupling products.