Catalytic dechlorination of chlorobenzenes: effect of solvent on efficiency and selectivity

Polychlorinated benzenes were dechlorinated with NaBH4, using PdCl2(dppf) ( dppf = 1,1'-bis(diphenylphosphino)ferrocene) as the catalyst. A variety of solvents, namely THF, CH,CN, DMSO, diglyme, DMF and DMA (dimethylacetamide) were examined. A supporting base, TMEDA (N,N,N,N-tetramethyl-1,2-ethylened iamine) was used in some cases. Catalytic activity was strongly solvent-dep endent with DMA and DMF showing the best performance. Addition of TMEDA imp roved the yields in all cases, except for DMSO. In DMA, when TMEDA was used , the catalyst showed appreciable activity even after 2 weeks of reaction. Both solvent and the presence/absence of a base had a strong influence on t he selectivity of the reaction. For example, the dechlorination of pentachl orobenzene led to 90% 1,2,3,4-tetrachlorobenzene in DMA/TMEDA, but to 80% 1 ,2,4,5-tetrachlorobenzene in DMSO/TMEDA. (C) 1999 Elsevier Science B.V. All rights reserved.