A mechanistic investigation of carbon-hydrogen bond stannylation: Synthesis, characterization of nickel catalysts

摘要

The complex (~iPr _3P)Ni(η ~2-Bu _3SnCHCH _2) _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 Bu _3SnCHCH _2, which generates C _6F _5SnBu _3 and ethylene. The crystalline complex (~iPr _3P)Ni(η ~2-Ph _3SnCHCH _2) _2 (1b) provides a more easily handled analogue, and is also capable of catalytic stannylation with added Ph _3SnCHCH _2 and C _6F _5H. Mechanistic studies on 1b show that the catalytically active species remains mononuclear. The rate of catalytic stannylation is proportional to [C _6F _5H] and inversely proportional to [Ph _3SnCHCH _2]. This is consistent with a mechanism where reversible Ph _3SnCHCH _2 dissociation provides (iPr _3P) Ni(η ~2-Ph _3SnCHCH _2), followed by a rate-determining reaction with C _6F _5H 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 SnPh _3 moiety.