A mechanistic investigation of carbon–hydrogen bond stannylation: synthesis and characterization of nickel catalysts

Abstract

The complex (ⁱPr₃P)Ni(η²-Bu₃SnCHCH₂)₂ (1a) was characterized by NMR spectroscopy and was identified as the active species for catalytic C–H bond stannylation of partially fluorinated aromatics, for example in the reaction between pentafluorobenzene and Bu₃SnCHCH₂, which generates C₆F₅SnBu₃ and ethylene. The crystalline complex (ⁱPr₃P)Ni(η²-Ph₃SnCHCH₂)₂ (1b) provides a more easily handled analogue, and is also capable of catalytic stannylation with added Ph₃SnCHCH₂ and C₆F₅H. Mechanistic studies on 1b show that the catalytically active species remains mononuclear. The rate of catalytic stannylation is proportional to [C₆F₅H] and inversely proportional to [Ph₃SnCHCH₂]. This is consistent with a mechanism where reversible Ph₃SnCHCH₂ dissociation provides (ⁱPr₃P)Ni(η²-Ph₃SnCHCH₂), followed by a rate-determining reaction with C₆F₅H to generate the stannylation products. Kinetic competition reactions between the fluorinated aromatics pentafluorobenzene, 1,2,4,5-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4-trifluorobenzene, 1,3,5-trifluorobenzene and 1,3-difluorobenzene all suggest significant Ni–aryl bond formation in the rate-determining step under catalytic conditions. Labelling studies are consistent with an insertion of the hydrogen of the arene into the vinyl group, followed by β-elimination or β-abstraction of the SnPh₃ moiety.