Aspects of the metallic halide catalysed reaction of Grignard reagents with organic halides. Part V. Application to dibromobenzenes

Abstract

Application to dibromobenzenes of the cobalt(II) chloride catalysed reaction of Grignard reagents with organic halides has shown that, with slow admixture of reagents, and in the presence of benzene as diluting solvent, the products from m- and p-dibromobenzene are 3- and 4-bromobiphenyl, respectively, together with some biphenyl. Biphenyl is the sole biaryl product from o-dibromobenzene under these conditions, and is also the sole biaryl product from all three dibromobenzenes when the reactants are mixed more rapidly in concentrated solutions. Bromobenzene is probably an intermediate in the conversion of dibromobenzenes into phenyl radicals, which on reaction with benzene afford biphenyl. Benzyne is not an intermediate in the conversion of o-dibromobenzene into biphenyl. m- and p-Dibromobenzene, and also bromobenzene, can be used to provide phenyl radicals, which give substantially the same isomer ratios for the methylbiphenyls produced in reactions with toluene.