Visible light enables catalytic formation of weak chemical bonds with molecular hydrogen

Park, Yoonsu; Kim, Sangmin; Tian, Long; Zhong, Hongyu; Scholes, Gregory D.; Chirik, Paul J.

doi:10.1038/s41557-021-00732-z

Cited by 33 publications

(49 citation statements)

References 44 publications

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“…Computational spin state calculations of the triplet excited state of Ir2 also supported this CT assignment, best described as a triplet excited state with metal-to-ligand (ML) and partial ligand-to-ligand (LL) CT character. 34 Castellano, Miller, and co-workers assigned similar MLCT/LLCT character with cartionic iridium methyl complexes bearing neutral bipyridine donors, 40 compounds distinct from those reported here.…”

Section: Resultssupporting

confidence: 75%

“…Our studies commenced with comprehensive evaluation of the photophysical properties of piano-stool iridium hydride complexes bearing “L-X” type supporting ligands. 34 Electronic absorption spectra of Ir1 – Ir4 exhibit absorption features in the visible region, which are red-shifted with increased π-conjugation on the bidentate ligand from Ir1 to Ir3 in tetrahydrofuran (THF) ( Figure 1 ). Absorptions at vertical excitations over 400 nm were tentatively assigned to metal-to-ligand charge transfer (MLCT) transitions, as examined by time-dependent density functional theory (TD-DFT) calculations, followed by natural transition orbital analysis at the cPCM (THF) ZORA/B3LYP/{ZORA-def2-TZVP, SARC-ZORA-TZVP (Ir)} level of theory ( Figure S118–S126 ).…”

Section: Resultsmentioning

confidence: 99%

“…For example, Ir1 , Ir3 , and Ir4 gave 19%, <5%, and 9% yields under otherwise identical catalytic conditions, respectively. 34 …”

Section: Resultsmentioning

confidence: 99%

“…At the outset, the excited-state dynamics of Ir2 were probed by TA spectroscopy with 450 nm photoexcitation in THF at room temperature. Although toluene and THF solvents were both effective for the catalytic reduction reactions ( Scheme 3 ), 34 THF was ideal for photophysical studies due to high solubility of the reaction components. The TA spectra displayed a ground-state bleaching (GSB) signal centered at 460 nm and two excited-state absorption (ESA) signals at 390 and 540 nm ( Figure 2 a and Figure S37 ), with the baseline recovered on a nanosecond time scale.…”

Section: Resultsmentioning

confidence: 99%

“…Harnessing photonic energy in conjunction with HAT is an attractive strategy for the synthesis of element–hydrogen bonds with BDFEs less than 55 kcal/mol using H 2 . Recently, our laboratories reported the application of piano-stool type iridium hydrides 34 to the photodriven hydrogenation of unsaturated organic compounds titanium-amido 29 and cobalt-imido 35 complexes. Although catalytic hydrogenation activity was minimal under dark thermal conditions likely due to strong ground-state Ir–H BDFEs (∼61 kcal/mol), visible light absorption enabled access to reactive excited states and initiated stoichiometric and catalytic reductions ( Scheme 1 ).…”

Section: Introductionmentioning

confidence: 99%

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Visible-Light-Driven, Iridium-Catalyzed Hydrogen Atom Transfer: Mechanistic Studies, Identification of Intermediates, and Catalyst Improvements

Park

Tian

Kim

et al. 2022

JACS Au

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The harvesting of visible light is a powerful strategy for the synthesis of weak chemical bonds involving hydrogen that are below the thermodynamic threshold for spontaneous H 2 evolution. Piano-stool iridium hydride complexes are effective for the blue-light-driven hydrogenation of organic substrates and contra-thermodynamic dearomative isomerization. In this work, a combination of spectroscopic measurements, isotopic labeling, structure–reactivity relationships, and computational studies has been used to explore the mechanism of these stoichiometric and catalytic reactions. Photophysical measurements on the iridium hydride catalysts demonstrated the generation of long-lived excited states with principally metal-to-ligand charge transfer (MLCT) character. Transient absorption spectroscopic studies with a representative substrate, anthracene revealed a diffusion-controlled dynamic quenching of the MLCT state. The triplet state of anthracene was detected immediately after the quenching events, suggesting that triplet–triplet energy transfer initiated the photocatalytic process. The key role of triplet anthracene on the post-energy transfer step was further demonstrated by employing photocatalytic hydrogenation with a triplet photosensitizer and a HAT agent, hydroquinone. DFT calculations support a concerted hydrogen atom transfer mechanism in lieu of stepwise electron/proton or proton/electron transfer pathways. Kinetic monitoring of the deactivation channel established an inverse kinetic isotope effect, supporting reversible C(sp 2 )–H reductive coupling followed by rate-limiting ligand dissociation. Mechanistic insights enabled design of a piano-stool iridium hydride catalyst with a rationally modified supporting ligand that exhibited improved photostability under blue light irradiation. The complex also provided improved catalytic performance toward photoinduced hydrogenation with H 2 and contra-thermodynamic isomerization.

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Section: Resultssupporting

confidence: 75%

Section: Resultsmentioning

confidence: 99%

“…For example, Ir1 , Ir3 , and Ir4 gave 19%, <5%, and 9% yields under otherwise identical catalytic conditions, respectively. 34 …”

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Visible-Light-Driven, Iridium-Catalyzed Hydrogen Atom Transfer: Mechanistic Studies, Identification of Intermediates, and Catalyst Improvements

Park

Tian

Kim

et al. 2022

JACS Au

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Discovery of a Thioxanthone–TfOH Complex as a Photoredox Catalyst for Hydrogenation of Alkenes Using p‐Xylene as both Electron and Hydrogen Sources

Kang

Duan

et al. 2022

Angewandte Chemie

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Hydrogenation of alkenes is one of the most fundamental transformations in organic synthesis, and widely used in the petrochemical, pharmaceutical, and food industries. Although numerous hydrogenation methods have been developed, novel types of catalysis with new mechanisms and new hydrogen sources are still desirable. Thioxanthone (TX) is widely used in energy‐transfer photoreactions, but rarely in photoredox processes. Herein we show that a catalytic amount of TfOH as a co‐catalyst can tune the properties of TX to make it a photoredox catalyst with highly enhanced oxidative capability in the hydrogenation of carbonylated alkenes with the cheap petroleum industrial product p‐xylene serving as the hydrogen source. Deuterium can also be introduced by this method by using D2O as the D source. To the best of our knowledge, this is the first example of using p‐xylene as a hydrogen source.

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Chemoselective Quinoline and Isoquinoline Reduction by Energy Transfer Catalysis Enabled Hydrogen Atom Transfer

Liu,

Nagashima,

Liang

et al. 2023

Angewandte Chemie

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(Hetero)arene reduction is one of the key avenues for synthesizing related cyclic alkenes and alkanes. While catalytic hydrogenation and Birch reduction are the two broadly utilized approaches for (hetero)arene reduction across academia and industry over the last century, both methods have encountered significant chemoselectivity challenges. We hereby introduce a highly chemoselective quinoline and isoquinoline reduction protocol operating through selective energy transfer (EnT) catalysis, which enables subsequent hydrogen atom transfer (HAT). The design of this protocol intelligently bypasses the conventional metric of reduction reaction, i.e., the reductive potential, but instead, relies on chemical moieties’ triplet energies and kinetic barriers of energy and hydrogen atom transfer events. Many reducing labile functional groups which were incompatible with previous (hetero)arene reductions, are retained in this work. We anticipate this protocol to trigger further advancement of chemoselective arene reductions and enable the current arene‐rich drug space escaping from flatland.

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Visible light enables catalytic formation of weak chemical bonds with molecular hydrogen

Cited by 33 publications

References 44 publications

Visible-Light-Driven, Iridium-Catalyzed Hydrogen Atom Transfer: Mechanistic Studies, Identification of Intermediates, and Catalyst Improvements

Visible-Light-Driven, Iridium-Catalyzed Hydrogen Atom Transfer: Mechanistic Studies, Identification of Intermediates, and Catalyst Improvements

Discovery of a Thioxanthone–TfOH Complex as a Photoredox Catalyst for Hydrogenation of Alkenes Using p‐Xylene as both Electron and Hydrogen Sources

Chemoselective Quinoline and Isoquinoline Reduction by Energy Transfer Catalysis Enabled Hydrogen Atom Transfer

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