Polyfluoroaryl organometallic compounds. Part VIII. Synthesis of and nucleophilic substitution in octafluorodibenzofuran

Removal of sulphur from octafluorodibenzothiophen (I) with Raney nickel does not give octafluorobiphenylene (III); only biphenyls are obtained. Oxidation of the dibenzothiophen (I) or di(pentafluorophenyl) sulphide with 85% hydrogen peroxide in trifluoroacetic acid anhydride gives the corresponding dioxides (IV) and (V). The principal mode of decomposition of octafluorodibenzothiophen 5,5-dioxide (IV) on pyrolysis, and partly on electron bombardment, is by loss of SO and pyrolysis of the dioxide (IV) gives the new heterocycle octafluorodibenzofuran (VIII), while similar pyrolysis of di(pentafluorophenyl) sulphone (V) leads only to loss of SO2. Nucleophilic substitution by methoxide ion occurs in the 3- and then the 7-position in octafluorodibenzofuran (VIII) and this orientation is deduced from the 1H and 19F n.m.r. spectra of the product. Factors governing nucleophilic substitution in these systems are discussed.