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1. Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

   https://doi.org/10.1002/anie.201903638  

   Roberts, Courtney C. ; Camasso, Nicole M. ; Bowes, Eric G. ; Sanford, Melanie S. ( May 2019 , Angewandte Chemie International Edition)

   Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp³)−C and C(sp³)−O bond‐forming reactions at high‐valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral Ni^IVcomplex ligated by tris(pyrazolyl)borate and a cationic octahedral Ni^IIIcomplex ligated by tris(pyrazolyl)methane. Key features of reactivity/selectivity are revealed: 1) C(sp³)−C(sp²) bond‐forming reductive elimination occurs from both centers, but the Ni^IIIcomplex reacts up to 300‐fold faster than the Ni^IV, depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO⁻), the Ni^IVcomplex exclusively undergoes C(sp³)−OAc bond formation, while the Ni^IIIanalogue forms the C(sp³)−C(sp²) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M−C(sp³) bonds, and thus their relative reactivity towards outer‐sphere S_N2‐type bond‐forming reactions.

    

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2. Binuclear, High‐Valent Nickel Complexes: Ni−Ni Bonds in Aryl–Halogen Bond Formation

   https://doi.org/10.1002/ange.201611572  

   Diccianni, Justin B. ; Hu, Chunhua ; Diao, Tianning ( February 2017 , Angewandte Chemie)

   Metal–metal bonds play a vital role in stabilizing key intermediates in bond‐formation reactions. We report that binuclear benzo[h]quinoline‐ligated Ni^IIcomplexes, 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 Ni^III, however, is required to trigger reductive elimination. The binuclear Ni^III–Ni^IIIintermediate lacks a Ni−Ni bond. Each Ni^IIIundergoes separate, but fast reductive elimination, giving rise to Ni^Ispecies. The reactivity of these binuclear Ni complexes highlights the fundamental difference between Ni and Pd in mediating bond‐formation processes.

    

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3. Aerobic C–C and C–O bond formation reactions mediated by high-valent nickel species

   https://doi.org/10.1039/c9sc03758f  

   Smith, Sofia M. ; Planas, Oriol ; Gómez, Laura ; Rath, Nigam P. ; Ribas, Xavi ; Mirica, Liviu M. ( November 2019 , Chemical Science)

   Nickel complexes have been widely employed as catalysts in C–C and C–heteroatom bond formation reactions. While Ni(0), Ni( i ), and Ni( ii ) intermediates are most relevant in these transformations, recently Ni( iii ) and Ni( iv ) species have also been proposed to play a role in catalysis. Reported herein is the synthesis, detailed characterization, and reactivity of a series of Ni( ii ) and Ni( iii ) metallacycle complexes stabilized by tetradentate pyridinophane ligands with various N-substituents. Interestingly, while the oxidation of the Ni( ii ) complexes with various other oxidants led to exclusive C–C bond formation in very good yields, the use of O 2 or H 2 O 2 as oxidants led to formation of appreciable amounts of C–O bond formation products, especially for the Ni( ii ) complex supported by an asymmetric pyridinophane ligand containing one tosyl N-substituent. Moreover, cryo-ESI-MS studies support the formation of several high-valent Ni species as key intermediates in this uncommon Ni-mediated oxygenase-type chemistry. 

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4. N 3 -Ligated nickel( ii ) diketonate complexes: synthesis, characterization and evaluation of O 2 reactivity

   https://doi.org/10.1039/d0dt01338b  

   Elsberg, Josiah G. ; Peterson, Austin ; Fuller, Amy L. ; Berreau, Lisa M. ( January 2020 , Dalton Transactions)

   Interest in O 2 -dependent aliphatic carbon–carbon (C–C) bond cleavage reactions of first row divalent metal diketonate complexes stems from the desire to further understand the reaction pathways of enzymes such as DKE1 and to extract information to develop applications in organic synthesis. A recent report of O 2 -dependent aliphatic C–C bond cleavage at ambient temperature in Ni( ii ) diketonate complexes supported by a tridentate nitrogen donor ligand [(MBBP)Ni(PhC(O)CHC(O)Ph)]Cl ( 7-Cl ; MBBP = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine) in the presence of NEt 3 spurred our interest in further examining the chemistry of such complexes. A series of new TERPY-ligated Ni( ii ) diketonate complexes of the general formula [(TERPY)Ni(R 2 -1,3-diketonate)]ClO 4 ( 1 : R = CH 3 ; 2 : R = C(CH 3 ) 3 ; 3 : R = Ph) was prepared under air and characterized using single crystal X-ray crystallography, elemental analysis, 1 H NMR, ESI-MS, FTIR, and UV-vis. Analysis of the reaction mixtures in which these complexes were generated using 1 H NMR and ESI-MS revealed the presence of both the desired diketonate complex and the bis-TERPY derivative [(TERPY) 2 Ni](ClO 4 ) 2 ( 4 ). Through selective crystallization 1–3 were isolated in analytically pure form. Analysis of reaction mixtures leading to the formation of the MBBP analogs [(MBBP)Ni(R 2 -1,3-diketonate)]X (X = ClO 4 : 5 : R = CH 3 ; 6 : R = C(CH 3 ) 3 ; 7-ClO4 : R = Ph; X = Cl: 7-Cl : R = Ph) using 1 H NMR and ESI-MS revealed the presence of [(MBBP) 2 Ni](ClO 4 ) 2 ( 8 ). Analysis of aerobic acetonitrile solutions of analytically pure 1–3 , 5 and 6 containing NEt 3 and in some cases H 2 O using 1 H NMR and UV-vis revealed evidence for the formation of additional bis-ligand complexes ( 4 and 8 ) but suggested no oxidative diketonate cleavage reactivity. Analysis of the organic products generated from 3 , 7-ClO4 and 7-Cl revealed unaltered dibenzoylmethane. Our results therefore indicate that N 3 -ligated Ni( ii ) complexes of unsubstituted diketonate ligands do not exhibit O 2 -dependent aliphatic C–C bond clevage at room temperature, including in the presence of NEt 3 and/or H 2 O. 

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5. Unconventional mechanism and selectivity of the Pd-catalyzed C–H bond lactonization in aromatic carboxylic acid

   https://doi.org/10.1038/s41467-022-27986-6  

   Xu, Li-Ping ; Qian, Shaoqun ; Zhuang, Zhe ; Yu, Jin-Quan ; Musaev, Djamaladdin G. ( January 2022 , Nature Communications)

   The search for more effective and highly selective C–H bond oxidation of accessible hydrocarbons and biomolecules is a greatly attractive research mission. The elucidating of mechanism and controlling factors will, undoubtedly, help to broaden scope of these synthetic protocols, and enable discovery of more efficient, environmentally benign, and highly practical new C–H oxidation reactions. Here, we reveal the stepwise intramolecular S_N2 nucleophilic substitution mechanism with the rate-limiting C–O bond formation step for the Pd(II)-catalyzed C(sp³)–H lactonization in aromatic 2,6-dimethylbenzoic acid. We show that for this reaction, the direct C–O reductive elimination from both Pd(II) and Pd(IV) (oxidized by O₂oxidant) intermediates is unfavorable. Critical factors controlling the outcome of this reaction are the presence of the η³-(π-benzylic)–Pd and K⁺–O(carboxylic) interactions. The controlling factors of the benzylic vs ortho site-selectivity of this reaction are the: (a) difference in the strains of the generated lactone rings; (b) difference in the strengths of the η³-(π-benzylic)–Pd and η²-(π-phenyl)–Pd interactions, and (c) more pronounced electrostatic interaction between the nucleophilic oxygen and K⁺cation in the ortho-C–H activation transition state. The presented data indicate the utmost importance of base, substrate, and ligand in the selective C(sp³)–H bond lactonization in the presence of C(sp²)–H.

    

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