Phosphinorhodium-catalyzed dehalogenation of chlorinated and fluorinated ethylenes: Distinct mechanisms with triethylsilane and dihydrogen

Catalytic dehalogenation of chlorinated and fluorinated ethylenes by (PR₃)₃RhCl complexes is described. The C-Cl and C-F bonds are activated by the catalyst in the presence of triethylsilane (Et ₃SiH) or dihydrogen (H₂). Spectroscopic studies, in addition to substrate preference, indicate that rhodium hydride species are important intermediates. Kinetic parameters and product distribution for dehalogenation reactions were determined using NMR spectroscopy. Evidence for sequential chlorine removal was obtained, and the rates of dehalogenation were found to increase with decreasing halogen content. It was also shown that this catalytic system has a preference for sp²over sp³- hybridized carbon-halogen bonds. Dechlorination using (PPh₃) ₃RhCl and either H₂ or Et₃SiH supports an insertion/β-chloride elimination mechanism; however, the two systems display distinct differences. On the basis of these differences, the dominant pathway for Et₃SiH is proposed to involve rhodium(I), while the H₂ system is proposed to primarily involve rhodium(III). This is supported with isotopic labeling studies using D₂, Et₃SiD, and (PPh₃)₃RhD, which yield different stereochemistry of dechlorinated products. With D₂, only products consistent with Syn-β-chloride elimination were observed, while with Et₃SiD and (PPh₃)₃RhD both syn- and anti-β-chloride elimination products were observed. In addition, NMR spectroscopic evidence of different hydride intermediates in the H₂ and Et₃SiH systems was obtained. Different pathways for dehalogenation with Et₃SiH and H₂ is further supported by the observation of 1,2-addition (hydrogenation) products using H2 and the lack of 1,2-addition (hydrosilylation) products using Et₃SiH.