Reactions of dicobalt octacarbonyl and related compounds with gem-dihalides

Dicobalt octacarbonyl and some of its derivatives (NaCo(CO)₄, Co₄(CO)₁₂, HgCo(CO)₄]₂, Co(CO)₃PPh₃]₂, NaCo(CO)₃PPh₃) react with activated gem-dihalides, R₂CX₂, such as dichlorodiphenylmethane, 9,9-dihalofluorenes and dimethyl dibromomalonate, to give the ‘dimer’ olefin, R₂CCR₂. The course of this conversion involves formation of the coupling product, R₂XCCXR₂, followed by dehalogenation of the latter. These separate steps have been confirmed for activated monohalides (bromodiphenylmethane, 9-bromofluorene, dimethyl bromomalonate) which were readily coupled by cobalt carbonyls, and for activated vicinal dihalides (D,L and meso-dibromostilbene, 9,9′-dichlorobisfluorenyl) which cobalt carbonyls readily dehalogenated. A radical mechanism is favored for these processes, and indirect evidence in its favor is presented.