Nucleophilic vinylic substitution with transition metal carbonyl anions—a rare case of a halophilic reaction mechanism: Formation of halo(acyl)rhenate complexes and X-ray structure of cis-[CF2CF(CO)Re(CO)4Br]Na

Reactions of polyfluorinated alkenyl halides Z-(CF3)3CCFCFHal (Hal=Cl, I-Cl, Hal=Br, I-Br) and CF2CFBr (II-Br) with [CpFe(CO)2]K (FpK) and [Re(CO)5]Na proceed through the initial attack of metal carbonyl anion on halogen. Reaction with FpK gives minor amounts of σ-alkenyl complexes Z-RfCFCFFe(CO)2Cp (I-Fe, II-Fe) (3–30%), but primarily leads to dimer [CpFe(CO)2]2. Reaction with [Re(CO)5]Na produces anionic halo(acyl)rhenates cis-[Z-RfCFCF(CO)Re(CO)4Hal]Na (V-Cl, V-Br, VI) (70–90%) which were isolated, and halo(acyl)rhenate VI (Rf=F) was characterized by X-ray structure analysis. Halo(acyl)rhenates result from the attack of the intermediate carbanion [RfCFCF]− on the carbonyl ligand of Re(CO)5Hal. The involvement of [RfCFCF]− is demonstrated by their trapping with t-BuOH or CH-acid, which gives the protodehalogenated alkenes I-H and II-H, and suppresses the nucleophilic substitution reaction leading to I-Fe, II-Fe or V-Cl. Arguments against a radical/SET mechanism for the substitution reaction are also advanced.