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

Abstract

Polychlorinated benzenes were dechlorinated with NaBH4, using PdCl2(dppf) (dppf = 1,1'-bis(diphenylphosphino)ferrocene) as the catalyst. A variety of solvents, namely THF, CH3CN, DMSO, diglyme, DMF and DMA (dimethylacetamide) were examined. A supporting base, TMEDA (N,N,N,N- tetramethyl-1,2-ethylenediamine) was used in some cases. Catalytic activity was strongly solvent-dependent with DMA and DMF showing the best performance. Addition of TMEDA improved 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 the selectivity of the reaction. For example, the dechlorination of pentachlorobenzene led to 90% 1,2,3,4-tetrachlorobenzene in DMA/TMEDA, but to 80% 1,2,4,5-tetrachlorobenzene in DMSO/TMEDA.