Effect of phosphine and phosphine sulfide ligands on the cobalt-catalyzed reductive coupling of 2-iodobutane with n-butyl acrylate

Tris[2-(diphenylphosphino)ethyl]phosphine disulfide (pp3S2), in which two terminal phosphino groups are selectively sulfidated, was prepared by utilizing the selective sulfidation reaction of [PdI(pp3)]I. Co(II) complexes with bidentate, tridentate and tetradentate phosphines and pp3S2 were prepared from anhydrous CoI2. X-ray crystal analyses revealed that the reaction of CoI2 with 1 equivalent of 1,2-bis(diphenylphosphino)ethane (p2) gave rise to partial oxidation of p2 to give the dicationic octahedral [Co(p2O2)2(CH3CN)2]2+ (p2O2 = p2 dioxide) and dianionic p2O-bridged tetrahedral dinuclear [CoI3(p2O)CoI3]2− (p2O = p2 monooxide) complexes, while the reaction with 2 equivalents of p2 gave the square-pyramidal [CoI(p2)2]+ complex. The catalytic activity for the Co-catalyzed coupling reaction of 2-iodobutane with n-butyl acrylate was compared using multidentate phosphines and phosphine sulfides as ligands, and the efficiency of the phosphine sulfides was shown. The tendency for multidentate phosphine to deactivate the Co-catalysis can substantiate an oxidative addition driven mechanism in which the multidentate ligand should interfere with the formation of the alkyl halide Co(III) adduct and subsequent coordination of an alkene.

Graphical abstractSteric and electronic effects of phosphine and phosphine sulfide ligands on the cobalt-catalyzed reductive coupling of an alkyl halide, 2-iodobutane, with an activated alkene, n-butyl acrylate, are reported. The observed differences in the catalytic activity provide insight into the reaction mechanism of the catalytic cycle and knowledge for choosing the ligand of the cobalt catalyst.Figure optionsDownload full-size imageDownload as PowerPoint slide