Nickel-catalyzed reductive carboxylation reactions of aryl electrophiles typically require the use of metallic reducing agents. At present, the prevailing perception is that these serve as both a source of electrons and as sources of Lewis acids that may aid CO2 insertion into the Ni-C bond. Herein, we provide evidence for the in situ formation of organometallic species from the metallic reductant, a step that has either been ruled out or has been unexplored in catalytic carboxylation reactions with metal powder reductants. Specifically, we demonstrate that Zn(0) acts as a reductant and that Zn(II) generates arylzinc species that might play a role in the C(sp 2)-S carboxylation of arylsulfonium salts. Overall, the reductive Ni-catalyzed C(sp 2)-S carboxylation reaction proceeds under mild conditions in a non-amide solvent, displays a wide substrate scope, and can be applied to a formal para C-H carboxylation of arenes.

Monodentate phosphine ligands are frequently employed in the Ni-catalyzed C-O functionalization of aryl esters. However, the extensive body of preparative work on such reactions contrasts with the lack of information concerning the structure and reactivity of the relevant nickel intermediates. In fact, experimental evidence for a seemingly trivial oxidative addition into the C-O bond of aryl esters with monodentate phosphines and low-valent nickel complexes still remains elusive. Herein, we report a combined experimental and theoretical study on the Ni(0)/PCy-catalyzed silylation of aryl pivalates with CuF/CsF additives that reveals the involvement of unorthodox dinickel oxidative addition complexes in C-O bond cleavage and their relevance in C-Si bond formation. We have obtained a mechanistic picture that clarifies the role of the additives and demonstrates that dinickel complexes act as reservoirs of the propagating monomeric nickel complexes by disproportionation. We believe this study will serve as a useful entry point to unravelling the mechanistic underpinnings of other related Ni-catalyzed C-O functionalization reactions employing monodentate phosphines.

Although the catalytic carboxylation of unactivated alkyl electrophiles has reached remarkable levels of sophistication, the intermediacy of (phenanthroline)Ni(I)–alkyl species—complexes proposed in numerous Ni-catalyzed reductive cross-coupling reactions—has been subject to speculation. Herein we report the synthesis of such elusive (phenanthroline)Ni(I) species and their reactivity with CO 2 , allowing us to address a long-standing question related to Ni-catalyzed carboxylation reactions.