Catalytic dehalogenation of highly chlorinated benzenes and aroclors using PdCl2(dppf) and NaBH4:: Efficiency, selectivity, and base support

Reported herein is a convenient one-pot system that can dehalogenate highly chlorinated benzenes at room temperature with reasonable conversion rates using PdCl2(dppf) (dppf = 1,1'-bis(diphenylphosphino)ferrocene) as catalyst, NaBH4 as reducing agent, TMEDA ((N,N,N',N'-tetramethyl-1,2-ethylenediamine) as supporting base, and THF as solvent. Total conversion of substrate to less chlorinated isomers is achieved within 200 h when hexachloro-, pentachloro-, and tetrachlorobenzenes are used. Degradation to benzene is not achievable, but the efficiency shown in the partial dechlorination is encouraging. A pronounced selectivity is accomplished with removal of meta-substituted chlorines being preferred over ortho- or para-substituted Cl atoms. The sequence in which reagents are added is also critical, thus indicating a protective role of the base. The effectiveness of the method was tested on the PCB mixtures Aroclor 1242, 1248, and 1254. Dechlorination efficiency at 67 degrees C is satisfactory.