Pentachlorophenylmagnesium chloride

We were able to prepare pentachlorophenylmagnesium chloride 5>, pentachlorophenyl-lithium 7 9>, and tetrachloroanthranilic acid 19>, relatively easily. However, we found that pentabromophenyl-lithium could not be formed in an acceptable yield from hexabromobenzene and that tetrabromoanthranilic acid was not readily available by the published methods. The preparation of tetrabromoanthranilic acid, in high yield (>85%), has now been achieved, and we find that all of the tetra-halogenbenzynes react with benzene with the exclusive formation of the 1,4-cyclo-adduct in good yield. The adduct of tetrabromobenzyne with benzene has been prepared very recently in 3% yield from pentabromophenyl-lithium 58>. 

Although pentachlorophenylmagnesium chloride can be made in tetrahydrofuran without the use of the entrainment method, the Grignard reagent in this solvent does not react with carbon dioxide to give pentachlorobenzoic acid in good yield.10... 

A solution of pentachlorophenylmagnesium chloride" is prepared from hexachlorobenzene (66 g, 0.23 mol) and magnesium (8.44 g, 0.35 mol) in a mixture of THF (150 ml) and benzene (150 ml) (HAZARD toluene would be preferable). The solution is heated to reflux, and ethyl formate (40 ml, excess) is added by syringe at such a rate that the exothermic reaction maintains refluxing. The mixture is stirred as it is allowed to cool to room temperature during 1 h. Methanol (20 ml) is added, and the solvents are evaporated. The resulting dark-brown residue is washed with hydrochloric acid, methanol and ethyl acetate, and then recrystallized from 4 1 toluene-pyridine, to give bis(pentachlorophenyl)methanol (35 g, 29%). 

The conditions for the reaction with pentachlorophenylmagnesium chloride are noteworthy. In this case carbon dioxide is passed into the solution without special precautions steric hindrance slows the reaction with carbon dioxide, but also the side-reactions. 

The reactions of silicon tetrahalides with polyhalometal organic compounds may also be used preparatively. For example, the reactions of silicon tetrachloride (Eq. 3.71) or silicon tetrafluoride (Eq. 3.72) with pentachlorophenylmagnesium chloride [185], silicon tetrachloride with pentafluorophenyllithium [186] (Eq. 3.73) and silicon tetrabromide with phenylmercury trichloromethane [187] (Eq. 3.74) have been reported. 

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