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1. Mechanistic Evidence of a Ni(0/II/III) Cycle for Nickel Photoredox Amide Arylation

   https://doi.org/10.1002/anie.202310753  

   Bradley, Robert D.; McManus, Brennan D.; Yam, Jessalyn G.; Carta, Veronica; Bahamonde, Ana (September 2023, Angewandte Chemie International Edition)

   Abstract This work demonstrates the dominance of a Ni(0/II/III) cycle for Ni‐photoredox amide arylation, which contrasts with other Ni‐photoredox C‐heteroatom couplings that operate via Ni(I/III) self‐sustained cycles. The kinetic data gathered when using different Ni precatalysts supports an initial Ni(0)‐mediated oxidative addition into the aryl bromide. Using NiCl₂as the precatalyst resulted in an observable induction period, which was found to arise from a photochemical activation event to generate Ni(0) and to be prolonged by unproductive comproportionation between the Ni(II) precatalyst and the in situ generated Ni(0) active species. Ligand exchange after oxidative addition yields a Ni(II) aryl amido complex, which was identified as the catalyst resting state for the reaction. Stoichiometric experiments showed that oxidation of this Ni(II) aryl amido intermediate was required to yield functionalized amide products. The kinetic data presented supports a rate‐limiting photochemically‐mediated Ni(II/III) oxidation to enable C−N reductive elimination. An alternative Ni(I/III) self‐sustained manifold was discarded based on EPR and kinetic measurements. The mechanistic insights uncovered herein will inform the community on how subtle changes in Ni‐photoredox reaction conditions may impact the reaction pathway, and have enabled us to include aryl chlorides as coupling partners and to reduce the Ni loading by 20‐fold without any reactivity loss. 

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2. Identification of key functionalization species in the Cp*Ir( iii )-catalyzed- ortho halogenation of benzamides

   https://doi.org/10.1039/d0dt00565g  

   Guzmán Santiago, Alexis J.; Brown, Caleb A.; Sommer, Roger D.; Ison, Elon A. (January 2020, Dalton Transactions)

   Cp*Ir( iii ) complexes have been shown to be effective for the halogenation of N , N -diisopropylbenzamides with N -halosuccinimide as a suitable halogen source. The optimized conditions for the iodination reaction consist of 0.5 mol% [Cp*IrCl 2 ] 2 in 1,2-dichloroethane at 60 °C for 1 h to form a variety of iodinated benzamides in high yields. Increasing the catalyst loading to 6 mol% and the time to 4 h enabled the bromination reaction of the same substrates. Reactivity was not observed for the chlorination of these substrates. A variety of functional groups on the para -position of the benzamide were well tolerated. Kinetic studies showed the reaction dependence is first order in iridium, positive order in benzamide, and zero order in N -iodosuccinimide. A KIE of 2.5 was obtained from an independent H/D kinetic isotope effect study. Computational studies (DFT-BP3PW91) indicate that a CMD mechanism is more likely than an oxidative addition pathway for the C–H bond activation step. The calculated functionalization step involves an Ir( v ) species that is the result of oxidative addition of acetate hypoiodite that is generated in situ from N -iodosuccinimide and acetic acid. 

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3. Mechanistic details of the cobalt-mediated dehydrogenative dimerization of aminoquinoline-directed benzamides

   https://doi.org/10.1039/D0SC02066D  

   Xu, Li-Ping; Liu, Elaine E.; Bacsa, John; MacBeth, Cora E.; Musaev, Djamaladdin G. (June 2020, Chemical Science)

   Key mechanistic features of the cobalt-mediated and aminoquinoline-directed dehydrogenative aryl–aryl coupling were investigated computationally and experimentally. A series of Co II and Co III complexes relevant to the proposed reaction cycle have been synthesized and characterized. Stoichiometric reactions and electrochemical studies were used to probe the role of different additives in the reaction pathway. Computationally, three different mechanisms, such as charge neutral , anionic , and dimetallic were explored. It is shown that the mono-metallic anionic and charge neutral mechanisms are the most favorable ones, among which the former mechanism is slightly more encouraging and proceeds via the: (a) concerted-metalation-deprotonation (CMD) of the first benzamide C–H bond, (b) PivOH-to-PivO − rearrangement, (c) CMD of the second benzamide C–H bond, (d) C–C coupling, (e) product formation facilitated by the amide nitrogen re-protonation, and (f) catalyst regeneration. The rate-determining step of this multi-step process is the C–C coupling step. The computational studies suggest that the electronics of both the aryl-benzamide and pyridine fragments of the aminoquinoline-benzamide ligand control the efficiency of the reaction. 

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4. Carbonyl and ester C–O bond hydrosilylation using κ ⁴ -diimine nickel catalysts

   https://doi.org/10.1039/C8DT01857J  

   Rock, Christopher L.; Groy, Thomas L.; Trovitch, Ryan J. (January 2018, Dalton Transactions)

   The synthesis of alkylphosphine-substituted α-diimine (DI) ligands and their subsequent addition to Ni(COD) 2 allowed for the preparation of ( iPr2PPr DI)Ni and ( tBu2PPr DI)Ni . The solid state structures of both compounds were found to feature a distorted tetrahedral geometry that is largely consistent with the reported structure of the diphenylphosphine-substituted variant, ( Ph2PPr DI)Ni . To explore and optimize the synthetic utility of this catalyst class, all three compounds were screened for benzaldehyde hydrosilylation activity at 1.0 mol% loading over 3 h at 25 °C. Notably, ( Ph2PPr DI)Ni was found to be the most efficient catalyst while phenyl silane was the most effective reductant. A broad scope of aldehydes and ketones were then hydrosilylated, and the silyl ether products were hydrolyzed to afford alcohols in good yield. When attempts were made to explore ester reduction, inefficient dihydrosilylation was noted for ethyl acetate and no reaction was observed for several additional substrates. However, when an equimolar solution of allyl acetate and phenyl silane was added to 1.0 mol% ( Ph2PPr DI)Ni , complete ester C–O bond hydrosilylation was observed within 30 min at 25 °C to generate propylene and PhSi(OAc) 3 . The scope of this reaction was expanded to include six additional allyl esters, and under neat conditions, turnover frequencies of up to 990 h −1 were achieved. This activity is believed to be the highest reported for transition metal-catalyzed ester C–O bond hydrosilylation. Proposed mechanisms for ( Ph2PPr DI)Ni -mediated carbonyl and allyl ester C–O bond hydrosilylation are also discussed. 

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5. Transamidation of Amides and Amidation of Esters by Selective N–C(O)/O–C(O) Cleavage Mediated by Air- and Moisture-Stable Half-Sandwich Nickel(II)–NHC Complexes

   https://doi.org/10.3390/molecules26010188  

   Buchspies, Jonathan; Rahman, Md. Mahbubur; Szostak, Michal (January 2021, Molecules)

   null (Ed.)

   The formation of amide bonds represents one of the most fundamental processes in organic synthesis. Transition-metal-catalyzed activation of acyclic twisted amides has emerged as an increasingly powerful platform in synthesis. Herein, we report the transamidation of N-activated twisted amides by selective N–C(O) cleavage mediated by air- and moisture-stable half-sandwich Ni(II)–NHC (NHC = N-heterocyclic carbenes) complexes. We demonstrate that the readily available cyclopentadienyl complex, [CpNi(IPr)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), promotes highly selective transamidation of the N–C(O) bond in twisted N-Boc amides with non-nucleophilic anilines. The reaction provides access to secondary anilides via the non-conventional amide bond-forming pathway. Furthermore, the amidation of activated phenolic and unactivated methyl esters mediated by [CpNi(IPr)Cl] is reported. This study sets the stage for the broad utilization of well-defined, air- and moisture-stable Ni(II)–NHC complexes in catalytic amide bond-forming protocols by unconventional C(acyl)–N and C(acyl)–O bond cleavage reactions. 

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