Catalysis by synthetic hydrotalcite-like materials in halide exchange between alkyl halides

Synthetic hydrotalcite-like materials, {A figure is presented}, were used as catalysts for the halide-exchange reaction between benzyl chloride and alkyl bromides in DMF or toluene. A material with M²⁺ = Mg²⁺, M³⁺ = Al³⁺ and x = 0.25 showed the highest activity. In the presence of 1 g of the material, containing 3.3 mmol of exchangeable interlayer Cl^- anions, reaction between benzyl chloride (33 mmol) and butyl bromide (33 mmol) in toluene at 373 K for 4 h gave benzyl bromide in 31% yield; in the absence of the material the reaction does not proceed. A 90% yield of benzyl bromide was attained in the reaction in DMF at 343 K for 20 h when 195 mmol of butyl bromide and 33 mmol of benzyl chloride were used. Interlayer Cl^- anions of the hydrotalcite-like materials are capable of migrating to external edge surfaces at which halide exchange reactions proceed. Ethyl bromide, propyl bromide and 1,4-dibromobutane are also effective bromination agents. The material is also an effective catalyst for a halide exchange reaction between benzyl chloride and butyl iodide. The halide exchange between a hydrotalcite-like material containing Cl^- anions and organic bromides is also examined and the reaction mechanism of the halide exchange is discussed.