Insertion of CO2 Mediated by a (Xantphos)NiI -Alkyl Species.

The incorporation of CO2 into organometallic and organic molecules represents a sustainable way to prepare carboxylates. The mechanism of reductive carboxylation of alkyl halides has been proposed to proceed through the reduction of NiII to NiI by either Zn or Mn, followed by CO2 insertion into NiI -alkyl species. No experimental evidence has been previously established to support the two proposed steps. Demonstrated herein is that the direct reduction of (tBu-Xantphos)NiII Br2 by Zn affords NiI species. (tBu-Xantphos)NiI -Me and (tBu-Xantphos)NiI -Et complexes undergo fast insertion of CO2 at 22 °C. The substantially faster rate, relative to that of NiII complexes, serves as the long-sought-after experimental support for the proposed mechanisms of Ni-catalyzed carboxylation reactions.

Mechanistic Characterization of (Xantphos)Ni(I)-Mediated Alkyl Bromide Activation: Oxidative Addition, Electron Transfer, or Halogen-Atom Abstraction.

Ni(I)-mediated single-electron oxidative activation of alkyl halides has been extensively proposed as a key step in Ni-catalyzed cross-coupling reactions to generate radical intermediates. There are four mechanisms through which this step could take place: oxidative addition, outer-sphere electron transfer, inner-sphere electron transfer, and concerted halogen-atom abstraction. Despite considerable computational studies, there is no experimental study to evaluate all four pathways for Ni(I)-mediated alkyl radical formation. Herein, we report the isolation of a series of (Xantphos)Ni(I)-Ar complexes that selectively activate alkyl halides over aryl halides to eject radicals and form Ni(II) complexes. This observation allows the application of kinetic studies on the steric, electronic, and solvent effects, in combination with DFT calculations, to systematically assess the four possible pathways. Our data reveal that (Xantphos)Ni(I)-mediated alkyl halide activation proceeds via a concerted halogen-atom abstraction mechanism. This result corroborates previous DFT studies on (terpy)Ni(I)- and (py)Ni(I)-mediated alkyl radical formation, and contrasts with the outer-sphere electron transfer pathway observed for (PPh3)4Ni(0)-mediated aryl halide activation. This study of a model system provides insight into the overall mechanism of Ni-catalyzed cross-coupling reactions and offers a basis for differentiating electrophiles in cross-electrophile coupling reactions.

Nickel-Catalyzed Reductive Cycloisomerization of Enynes with CO2.

Carboxylate groups are ubiquitous in bioactive molecules. The syntheses of carboxylates from petroleum feedstock require a series of oxidation reactions. CO2 represents a cheap and sustainable, preoxidized C1 source. Herein, we describe a simple, selective, and mild procedure for the construction of (hetero)cyclic α,ß-unsaturated carboxylic acids from 1,6- and 1,7-enyes and CO2. Terminal 1,7-enynes and sterically hindered alkenes experience a change in regioselectivity and form unconjugated carboxylic acids. Mechanistic studies of the reductive cyclization suggest a hydride insertion pathway, explaining the change in regioselectivity caused by steric effects and distinguishing this work from previous reactions involving CO2.

Binuclear, High-Valent Nickel Complexes: Ni-Ni Bonds in Aryl-Halogen Bond Formation.

Metal-metal bonds play a vital role in stabilizing key intermediates in bond-formation reactions. We report that binuclear benzo[h]quinoline-ligated NiII complexes, upon oxidation, undergo reductive elimination to form carbon-halogen bonds. A mixed-valent Ni(2.5+)-Ni(2.5+) intermediate is isolated. Further oxidation to NiIII , however, is required to trigger reductive elimination. The binuclear NiIII -NiIII intermediate lacks a Ni-Ni bond. Each NiIII undergoes separate, but fast reductive elimination, giving rise to NiI species. The reactivity of these binuclear Ni complexes highlights the fundamental difference between Ni and Pd in mediating bond-formation processes.

N-N Bond Forming Reductive Elimination via a Mixed-Valent Nickel(II)-Nickel(III) Intermediate.

Natural products containing N-N bonds exhibit important biological activity. Current methods for constructing N-N bonds have limited scope. An advanced understanding of the fundamental N-N bond formation/cleavage processes occurring at the transition-metal center would facilitate the development of catalytic reactions. Herein we present an N-N bond-forming reductive elimination, which proceeds via a mixed-valent Ni(II) -Ni(III) intermediate with a Ni-Ni bond order of zero. The discrete Ni(II) -Ni(III) oxidation states contrast with the cationic dimeric Ni analogue, in which both Ni centers are equivalent with an oxidation state of 2.5. The electronic structures of these mixed-valent complexes have implications for the fundamental understanding of metal-metal bonding interactions.

Bimetallic C-C Bond-Forming Reductive Elimination from Nickel.

Ni-catalyzed cross-coupling reactions have found important applications in organic synthesis. The fundamental characterization of the key steps in cross-coupling reactions, including C-C bond-forming reductive elimination, represents a significant challenge. Bimolecular pathways were invoked in early proposals, but the experimental evidence was limited. We present the preparation of well-defined (pyridine-pyrrolyl)Ni monomethyl and monophenyl complexes that allow the direct observation of bimolecular reductive elimination to generate ethane and biphenyl, respectively. The sp(3)-sp(3) and sp(2)-sp(2) couplings proceed via two distinct pathways. Oxidants promote the fast formation of Ni(III) from (pyridine-pyrrolyl)Ni-methyl, which dimerizes to afford a bimetallic Ni(III) intermediate. Our data are most consistent with the subsequent methyl coupling from the bimetallic Ni(III) to generate ethane as the rate-determining step. In contrast, the formation of biphenyl is facilitated by the coordination of a bidentate donor ligand.