Strategies for Promoting Reductive Elimination of Bi- and Bis-Oxazoline Ligated Organonickel Complexes

Nickel-mediated carbon-carbon bond formation is an essential step in the catalytic cycle of cross-coupling reactions. Bi- and bis-oxazoline (biOx and box) ligands are commonly applied to nickel catalyzed reactions as they confer ideal redox properties for substrate activation and new bond formation. The synthesis of square-planar biOx and (box)Ni(II)-dineosilyl complexes allowed for the direct comparison of their redox properties. CV studies reveal that (biOx)Ni(II) complexes can be oxidized more easily than (box)Ni(II) complexes and that biOx is better at stabilizing Ni(I) states than box. Several factors promote reductive elimination from d8-organonickel complexes, including the coordination of L- and X-type ligands, the inner-sphere transfer of one or two electrons to alkyl halides, and the outer-sphere electron transfer to oxidants. Analysis of the product distribution utilizing an alkyl iodide as the oxidant unveiled the mechanism for this reductive elimination process by distinguishing the one-electron halogen atom abstraction pathway from the competing two-electron oxidative addition pathway. Moreover, comparing the rates and the product distributions between the biOx and box ligands reveals the effect of steric bulk on these associative pathways.