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

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 ¹²C/¹³C 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.