Phosphinorhodium-Catalyzed Dehalogenation of Chlorinated and Fluorinated Ethylenes: Distinct Mechanisms with Triethylsilane and Dihydrogen

Catalytic dehalogenation of chlorinated and fluorinated ethylenes by (PR3)(3)RhCl complexes is described. The C-Cl and C-F bonds are activated by the catalyst in the presence of triethylsilane (Et3SiH) or dibydrogen (H-2). Spectroscopic studies, in addition to substrate preference, indicate that rhodium hydride species arc 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 sp(2)-over sp(3)-hybridized carbon-halogen bonds. Dechlorination using (PPh3)(3)RhCl and either H-2 or Et3SiH supports an insertion/beta-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 H-2 System is proposed to primarily involve rhodium(III). This is supported with isotopic labeling studies using D-2, Et3SiD, and (PPh3)(3)RhD, which yield different stereochemistry of dechlorinated products. With D-2, only products consistent with syn-beta-chloride elimination were observed, while with Et3SiD and (PPh3)(3)RhD both syn- and anti-beta-chloride elimination products were observed. In addition, NMR spectroscopic evidence of different hydride intermediates in the H-2 and Et3SiH systems was obtained. Different pathways for dehalogenation with Et3SiH and H-2 is further supported by the observation of 1,2-addition (hydrogenation) products using H-2 and the lack of 1,2-addition (hydrosilylation) products using Et3SiH.