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-[CF 2CF(CO)Re(CO) 4Br]Na

Abstract

Reactions of polyfluorinated alkenyl halides Z-(CF 3) 3CCFCFHal (Hal=Cl, I- Cl, Hal=Br, I- Br) and CF 2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-R f CF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-R f CFCF(CO)Re(CO) 4Hal]Na ( V- Cl, V- Br, VI) (70–90%) which were isolated, and halo(acyl)rhenate VI (R f =F) was characterized by X-ray structure analysis. Halo(acyl)rhenates result from the attack of the intermediate carbanion [R f CFCF] − on the carbonyl ligand of Re(CO) 5Hal. The involvement of [R f CF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.