16-Electron Nickel(0)-Olefin Complexes in Low-Temperature C(sp²)–C(sp³) Kumada Cross-Couplings

Investigations into the mechanism of the low-temperature C­(sp²)–C­(sp³) Kumada cross-coupling catalyzed by highly reduced nickel-olefin-lithium complexes revealed that 16-electron tris­(olefin)­nickel(0) complexes are competent catalysts for this transformation. A survey of various nickel(0)-olefin complexes identified Ni­(nor)₃ as an active catalyst, with performance comparable to that of the previously described Ni-olefin-lithium precatalyst. We demonstrate that Ni­(nor)₃, however, is unable to undergo oxidative addition to the corresponding C­(sp²)–Br bond at low temperatures (<−40 °C), thus indicating that the canonical cross-coupling cycle is not operative under this condition. Instead, such binary nickel(0)-olefin complexes capitalize on the long-known Lewis acidity of the Ni(0) center when it is coordinated to olefins to accommodate the incoming charge from the Grignard reagent, forming a nickel(0)-alkylmagnesium complex. We demonstrate that this unique heterobimetallic complex is now primed for reactivity, thus cleaving the C­(sp²)–Br bond and ultimately delivering the C­(sp²)–C­(sp³) bond in high yields.