Reductive Elimination from Sterically Encumbered Ni–Polypyridine Complexes

Day, Craig S.; Ton, Stephanie J.; McGuire, Ryan T.; Foroutan‐Nejad, Cina; Martı́n, Rubén

doi:10.1021/acs.organomet.2c00362

Cited by 7 publications

(4 citation statements)

References 57 publications

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“…This is attributed to the steric demand of the methyl substituents in the ortho position to the N coordination sites. A recently reported example of a distorted square planar Ni(II) complex containing a 2,9-dmphen derivative [Ni(CH 2 TMS) 2 (N ∧ N)] (N ∧ N = 2,9-dimethyl-4,7-dimethoxy-1,10-phenanthroline; TMS = trimethylsilyl) features the same 35° tilt of the sterically hindered ligand and even stronger distortion toward a tetrahedral geometry . Furthermore, numerous reported examples of [Ni(X) n (2,9-dmphen)] (L = Cl – , SO 4 2– , N 3 – ) derivatives actually feature an expanded coordination sphere around nickel, which serves to avoid this geometric strain, i.e., a pentagonal bipyramidal or distorted octahedral geometry. − We observed significantly lower stability in solution for 3 than for all other compounds, which we attribute to the geometric strain and poor shielding of the Ni center in the compound.…”

Section: Resultsmentioning

confidence: 99%

Assessing the Character of the C₆F₅ Ligand from the Electrochemical and Photophysical Properties of [Ni(C₆F₅)₂(N^∧N)] Complexes

Jordan,

Schäfer,

Sander

et al. 2024

Inorg. Chem.

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Organonickel complexes containing α-diimine ligands [Ni(C 6 F 5 ) 2 (N ∧ N)] (N ∧ N = 2,2′-bipyridine (bpy), 2,9-dimethyl-1,10-phenanthroline (dmphen), 3,4,7,8tetramethyl-1,10-phenanthroline (tmphen), dipyrido[3,2-a:2′,3′-c]phenazine (dppz), 1,4bis(isopropyl)-1,4-diazabutadiene (iPr-DAB), and 1,4-bis(2,6-dimethylphenyl)-1,4-diazabutadiene (Xyl-DAB) were prepared and studied structurally, spectroscopically, and electrochemically. Their molecular structures from single-crystal X-ray diffraction show near-perfect square planar Ni(II) coordination except in the case of dmphen. Primary reversible electrochemical reductions in the range from −1 to −2 V vs ferrocene/ ferrocenium couple lead to mainly diimine-localized radical anion complexes, while secondary reductions in the range from −2 to −2.5 V lead to dianion complexes, as shown through spectroelectrochemistry. Irreversible metal-centered oxidations at around 0.7 V result in rapid aryl−aryl reductive elimination and formation of decafluorobiphenyl. No photoluminescence was detected for the complexes containing chromophoric α-diimine ligands at room temperature. At 77 K in frozen glassy 2-Me-THF matrices, weak photoluminescence was detected for the dmphen and tmphen derivatives, with broad emission bands peaking around 570 nm. All results are rationalized with the support of (TD-)DFT calculations, highlighting the role of the C 6 F 5 ligand in different systems.

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Section: Resultsmentioning

confidence: 99%

Assessing the Character of the C₆F₅ Ligand from the Electrochemical and Photophysical Properties of [Ni(C₆F₅)₂(N^∧N)] Complexes

Jordan,

Schäfer,

Sander

et al. 2024

Inorg. Chem.

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“…Additionally, we discerned that oxidative addition with methyl iodide generated a high-valent nickel intermediate that rapidly proceeded to reductive elimination. A concurrent study demonstrated that increasing the steric effect of the ligand can accelerate reductive elimination from a bis-alkyl nickel complex …”

Section: Introductionmentioning

confidence: 99%

“…A concurrent study demonstrated that increasing the steric effect of the ligand can accelerate reductive elimination from a bis-alkyl nickel complex. 49 …”

Section: Introductionmentioning

confidence: 99%

Nickel-Catalyzed Radical Mechanisms: Informing Cross-Coupling for Synthesizing Non-Canonical Biomolecules

Dawson,

Spielvogel,

Diao

2023

Acc. Chem. Res.

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Conspectus Nickel excels at facilitating selective radical chemistry, playing a pivotal role in metalloenzyme catalysis and modern cross-coupling reactions. Radicals, being nonpolar and neutral, exhibit orthogonal reactivity to nucleophilic and basic functional groups commonly present in biomolecules. Harnessing this compatibility, we delve into the application of nickel-catalyzed radical pathways in the synthesis of noncanonical peptides and carbohydrates, critical for chemical biology studies and drug discovery. We previously characterized a sequential reduction mechanism that accounts for chemoselectivity in cross-electrophile coupling reactions. This catalytic cycle begins with nickel(I)-mediated radical generation from alkyl halides, followed by carbon radical capture by nickel(II) complexes, and concludes with reductive elimination. These steps resonate with mechanistic proposals in nickel-catalyzed cross-coupling, photoredox, and electrocatalytic reactions. Herein, we present our insights into each step involving radicals, including initiation, propagation, termination, and the nuances of kinetics, origins of selectivity, and ligand effects. Radical generation from C(sp 3 ) electrophiles via one-electron oxidative addition with low-valent nickel radical intermediates provides the basis for stereoconvergent and cross-electrophile couplings. Our electroanalytical studies elucidate a concerted halogen atom abstraction mechanism, where electron transfer is coupled with halide dissociation. Using this pathway, we have developed a nickel-catalyzed stereoselective radical addition to dehydroalanine, facilitating the synthesis of noncanonical peptides. In this application, chiral ligands modulate the stereochemical outcome through the asymmetric protonation of a nickel-enolate intermediate. The capture of the alkyl radical by nickel(II) expands the scope of cross-coupling, promotes reductive elimination through the formation of high-valent nickel(III) species, and governs chemo- and stereoselectivity. We discovered that nickel(II)-aryl efficiently traps radicals with a barrier ranging from 7 to 9 kcal/mol, followed by fast reductive elimination. In contrast, nickel(II)-alkyl captures radicals to form a nickel(III) species, which was characterized by EPR spectroscopy. However, the subsequent slow reductive elimination resulted in minimal product formation. The observed high diastereoselectivity of radical capture inspired investigations into C- aryl and C- acyl glycosylation reactions. We developed a redox auxiliary that readily couples with natural carbohydrates and produces glycosyl radicals upon photoredox activation. Nickel-catalyzed cross-coupling of the glycosyl radical with bromoarenes and carboxylic acids leads to diverse non-natural glycosides that can facilitate drug discovery. Stoichiometric studies on well-defined d 8 -nickel complexes have showcased...

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“…Recent mechanistic studies have shown that CO 2 insertion predominantly occurs at well-defined Ni(I) centers . We anticipated that accessing alkyl–Ni(I) species from unactivated secondary alkyl halides and low-valent NiL n would be problematic with commonly employed heterogeneous metal reductants given (a) the low rates at which these entities promote single-electron transfer en route to alkyl–Ni(I) and (b) the propensity of alkyl–Ni(II) species toward β-hydride elimination . If successful, however, a study aimed at designing a retained Ni-catalyzed carboxylation of secondary alkyl halides would not only offer new opportunities in the carboxylation arena but also a starting point for understanding the intricacies of Ni speciation in cross-couplings of sp 3 electrophiles. , Herein, we report the successful realization of this goal, which culminates in the development of a light-induced Ni-catalyzed retained carboxylation that operates under mild conditions and with excellent chemo- and site-selectivity profiles (Scheme , right).…”

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confidence: 99%

Kinetically-Controlled Ni-Catalyzed Direct Carboxylation of Unactivated Secondary Alkyl Bromides without Chain Walking

Davies,

Lyonnet,

Carvalho

et al. 2024

J. Am. Chem. Soc.

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Herein, we report the direct carboxylation of unactivated secondary alkyl bromides enabled by the merger of photoredox and nickel catalysis, a previously inaccessible endeavor in the carboxylation arena. Site-selectivity is dictated by a kinetically controlled insertion of CO 2 at the initial C(sp 3 )−Br site by the rapid formation of Ni(I)−alkyl species, thus avoiding undesired β-hydride elimination and chain-walking processes. Preliminary mechanistic experiments reveal the subtleties of stereoelectronic effects for guiding the reactivity and site-selectivity.

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Reductive Elimination from Sterically Encumbered Ni–Polypyridine Complexes

Cited by 7 publications

References 57 publications

Assessing the Character of the C₆F₅ Ligand from the Electrochemical and Photophysical Properties of [Ni(C₆F₅)₂(N^∧N)] Complexes

Assessing the Character of the C₆F₅ Ligand from the Electrochemical and Photophysical Properties of [Ni(C₆F₅)₂(N^∧N)] Complexes

Nickel-Catalyzed Radical Mechanisms: Informing Cross-Coupling for Synthesizing Non-Canonical Biomolecules

Kinetically-Controlled Ni-Catalyzed Direct Carboxylation of Unactivated Secondary Alkyl Bromides without Chain Walking

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Reductive Elimination from Sterically Encumbered Ni–Polypyridine Complexes

Cited by 7 publications

References 57 publications

Assessing the Character of the C6F5 Ligand from the Electrochemical and Photophysical Properties of [Ni(C6F5)2(N∧N)] Complexes

Assessing the Character of the C6F5 Ligand from the Electrochemical and Photophysical Properties of [Ni(C6F5)2(N∧N)] Complexes

Nickel-Catalyzed Radical Mechanisms: Informing Cross-Coupling for Synthesizing Non-Canonical Biomolecules

Kinetically-Controlled Ni-Catalyzed Direct Carboxylation of Unactivated Secondary Alkyl Bromides without Chain Walking

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Assessing the Character of the C₆F₅ Ligand from the Electrochemical and Photophysical Properties of [Ni(C₆F₅)₂(N^∧N)] Complexes

Assessing the Character of the C₆F₅ Ligand from the Electrochemical and Photophysical Properties of [Ni(C₆F₅)₂(N^∧N)] Complexes