16-Electron Nickel(0)-Olefin Complexes in Low-Temperature C(sp<sup>2</sup>)–C(sp<sup>3</sup>) Kumada Cross-Couplings
Sigrid Lutz, Lukas Nattmann, Nils Nöthling, Josep Cornellà
Abstract
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.