Catalysis of Radical Cyclizations from Alkyl Iodides under H2: Evidence for Electron Transfer from [CpV(CO)3H]−

Kuo, Jonathan L.; Lorenc, Chris; Abuyuan, Janine M.; Norton, Jack R.

doi:10.1021/jacs.8b02119

Cited by 31 publications

(17 citation statements)

References 63 publications

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“…While it is well-established that these open-shell intermediates are readily accessed via halogen atom transfer from alkyl halide (RX) feedstocks, [1][2][3][4] the toxicity of tin, silicon, or trialkylborane reagents traditionally employed to initiate this reactivity motivates the development of complementary synthetic methods with improved sustainability. Among the numerous alternatives pursued, 5,6,[7][8][9][10][11][12][13][14]15,16 strategies that enable RX activation via single electron transfer (SET) present an opportunity for radical generation to be interfaced with SET catalysts, where energy input from visible light 17 or electricity 18,19 enables turnover under mild conditions. RX reduction under photoredox conditions typically employ coordinatively-saturated Rh or Ir polypyridyl catalysts.…”

Section: Introductionmentioning

confidence: 99%

Controlled Single-Electron Transfer via Metal–Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways

et al. 2021

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Electrocatalysis enables the construction of C-C bonds under mild conditions via controlled formation of carboncentered radicals. For sequences initiated by alkyl halide reduction, coordinatively-unsaturated Ni complexes commonly serve as single electron transfer agents, giving rise to the foundational question of whether outer-or inner-sphere electron transfer oxidative addition prevails in redox mediation. Indeed, rational design of electrochemical processes requires the discrimination of these two electron transfer pathways, as they can have outsized effects on the rate of substrate bond activation and thus impact radical generation rates and downstream product selectivities. We present results from combined synthetic, electroanalytical, and computational studies that examine the mechanistic differences of single electron transfer to alkyl halides imparted by Ni metal-ligand cooperativity. Electrogenerated reduced Ni species, stabilized by delocalized spin density onto a redox-active tpyPY2Me polypyridyl ligand, activates alkyl iodides via outer-sphere electron transfer, allowing for the selective activation of alkyl iodide substrates over halogen atom donors and the controlled generation and sequestration of electrogenerated radicals. In contrast, the Ni complex possessing a redoxinnocent pentapyridine congener activates the substrates in an inner-sphere fashion owning to a purely metal-localized spin, thereby activating both substrates and halogen atom donors in an indiscriminate fashion, generating a high concentration of radicals and leading to unproductive dimerization. Our data establish that controlled electron transfer via Ni-ligand cooperativity can be used to limit undesired radical recombination products and promote selective radical processes in electrochemical environments, providing a generalizable framework for designing redox mediators with distinct rate and potential requirements.

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Section: Introductionmentioning

confidence: 99%

Controlled Single-Electron Transfer via Metal–Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways

et al. 2021

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“…The current synthetic gap is especially evident in the case of unactivated alkyl halides, where only dehalogenation and intramolecular cyclization of iodides have been reported (7)(8)(9)(10). The difficulties in engaging these feedstocks in redox chemistry arise from their highly negative reduction potentials (Ered < -2 V vs SCE for unactivated alkyl and aryl iodides), which in turn necessitate the use of strongly reducing systems (11,12) (Fig. 1A).…”

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confidence: 99%

“…After optimization, we achieved efficient deuteration of primary, secondary and tertiary alkyl iodides in nearly quantitively yields (5)(6)(7)(8)(9)(10)(11)(12). The mild reaction conditions tolerated multiple functional groups showcasing the strong chemoselectivity of this XAT approach.…”

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confidence: 99%

Aminoalkyl radicals as halogen-atom transfer agents for activation of alkyl and aryl halides

Constantin

Zanini

Regni

et al. 2020

Science

424

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Organic halides are important building blocks in synthesis, but their use in (photo)redox chemistry is limited by their low reduction potentials. Halogen-atom transfer remains the most reliable approach to exploit these substrates in radical processes despite its requirement for hazardous reagents and initiators such as tributyltin hydride. In this study, we demonstrate that α-aminoalkyl radicals, easily accessible from simple amines, promote the homolytic activation of carbon-halogen bonds with a reactivity profile mirroring that of classical tin radicals. This strategy conveniently engages alkyl and aryl halides in a wide range of redox transformations to construct sp3-sp3, sp3-sp2, and sp2-sp2 carbon-carbon bonds under mild conditions with high chemoselectivity.

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“…Last year we reported that [CpV(CO) 3 H] − catalyzes radical cyclizations from alkyl iodides under H 2 in the presence of the organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). 22 Some of the observations we made in the catalytic studies were more compatible with an electron transfer mechanism (e.g., the slow conversion of aryl iodides or alkyl bromides). For catalytic turnover, we proposed that H 2 coordinates to the coordinatively unsaturated CpV(CO) 3 and generates the dihydrogen complex CpV(CO) 3 (H 2 ), whose deprotonation regenerates the anionic hydride [CpV(CO) 3 H] − .…”

Section: ■ Introductionmentioning

confidence: 99%

Thermodynamics of H⁺/H^•/H^–/e^– Transfer from [CpV(CO)₃H]⁻: Comparisons to the Isoelectronic CpCr(CO)₃H

et al. 2019

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Hydrogen atom (H•) donors generated from H2 facilitate the atom efficient reduction of small molecule substrates. However, generating H• donors with X–H bond dissociation free energies (BDFEs) below 52 kcal mol–1 is especially challenging because they thermodynamically favor the bimolecular evolution of H2. We have recently proposed that [CpV(CO)3H]− catalyzes the conversion of H2 into a proton, an electron, and a hydrogen atom in the presence of a sacrificial base. In order to understand the driving force for H• transfer, the free energies of H+/H•/H–/e– transfer from [CpV(CO)3H]− have been evaluated using solution phase techniques and state-of-the-art quantum chemical calculations. Thermochemical cycles have been constructed in order to anchor the computational values against experimental observations. This facilitates a quantitative comparison of the thermodynamic driving force for H+/H•/H–/e– transfer between isoelectronic anionic/neutral hydrides of the same row (the corresponding values are already available for CpCr(CO)3H). The overall charge greatly influences the thermodynamics of transferring H+, H–, and e– (i.e., [CpV(CO)3H]− is a much weaker acid, a stronger hydride donor, and a stronger reductant than CpCr(CO)3H); there is almost no change in the thermodynamics of H• transfer (V–H BDFE 54.7 kcal mol, Cr–H BDFE 57.0 kcal mol–1). In MeCN, the one electron oxidation of [CpV(CO)3H]− (−0.83 V vs Fc/Fc+) generates CpV(CO)3H, which spontaneously evolves H2. The resulting CpV(CO)3 is trapped as the solvent adduct CpV(CO)3(MeCN). Because H• transfer is now coupled to metal–solvent binding, the V–H bond is substantially weakened for CpV(CO)3H (V–H BDFE 36.1 kcal mol–1), amounting to a strategy for obtaining very reactive H atoms from H2.

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Catalysis of Radical Cyclizations from Alkyl Iodides under H₂: Evidence for Electron Transfer from [CpV(CO)₃H]⁻

Cited by 31 publications

References 63 publications

Controlled Single-Electron Transfer via Metal–Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways

Controlled Single-Electron Transfer via Metal–Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways

Aminoalkyl radicals as halogen-atom transfer agents for activation of alkyl and aryl halides

Thermodynamics of H⁺/H^•/H^–/e^– Transfer from [CpV(CO)₃H]⁻: Comparisons to the Isoelectronic CpCr(CO)₃H

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Catalysis of Radical Cyclizations from Alkyl Iodides under H2: Evidence for Electron Transfer from [CpV(CO)3H]−

Cited by 31 publications

References 63 publications

Controlled Single-Electron Transfer via Metal–Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways

Controlled Single-Electron Transfer via Metal–Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways

Aminoalkyl radicals as halogen-atom transfer agents for activation of alkyl and aryl halides

Thermodynamics of H+/H•/H–/e– Transfer from [CpV(CO)3H]−: Comparisons to the Isoelectronic CpCr(CO)3H

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Catalysis of Radical Cyclizations from Alkyl Iodides under H₂: Evidence for Electron Transfer from [CpV(CO)₃H]⁻

Thermodynamics of H⁺/H^•/H^–/e^– Transfer from [CpV(CO)₃H]⁻: Comparisons to the Isoelectronic CpCr(CO)₃H