Alkyl Radical Coupling with Phenoxy(imine)–Nickel(II)–Aryl Complexes: Evidence for a Multistep Process in C–C Bond Formation

High Resolution Image Download MS PowerPoint Slide The reaction of well-defined phenoxy(imine)–nickel(II)–aryl complexes with alkyl radicals was investigated en route to C–C bond formation. A 4-dimethylaminopyridine–titanium tris(anilide) complex was identified as a selective reagent for halogen-atom abstraction of unactivated alkyl halides, generating alkyl radicals that were subsequently captured by phenoxy(imine)–nickel(II)–aryl complexes. A combination of radical clock and competition experiments provided insight into ligand effects on both the phenoxy(imine) and the nickel–aryl in radical capture and subsequent C–C bond formation. Differences between parallel and competition radical clock experiments support a multistep process for C–C bond formation. Results from 12 C/ 13 C kinetic isotope effect measurements and computational studies are also consistent with this pathway. Treatment of phenoxy(imine)–nickel(II)–aryl complexes with various oxidants generated transient nickel(III) complexes that were detected by electron paramagnetic resonance (EPR) spectroscopy and intercepted by organozinc reagents to generate C–C bond formation products, supporting the intermediacy of this oxidation state in the overall process.