CdS Quantum Dot Gels as a Direct Hydrogen Atom Transfer Photocatalyst for C-H Activation

Here, we report CdS quantum dot (QD) gels, a three-dimensional network of interconnected CdS QDs, as a new type of direct hydrogen atom transfer (d-HAT) photocatalyst for C-H activation. We discovered that the photoexcited CdS QD gel could generate various neutral radicals, including α-amido, heterocyclic, acyl, and benzylic radicals, from their corresponding stable molecular substrates, including amides, thio/ethers, aldehydes, and benzylic compounds. Its C-H activation ability imparts a broad substrate and reaction scope. The mechanistic study reveals that this reactivity is intrinsic to CdS materials, and the neutral radical generation did not proceed via the conventional sequential electron transfer and proton transfer pathway. Instead, the C-H bonds are activated by the photoexcited CdS QD gel via a d-HAT mechanism. This d-HAT mechanism is supported by the linear correlation between the logarithm of the C-H bond activation rate constant and the C-H bond dissociation energy (BDE) with a Brønsted slope α=0.5. Our findings expand the currently limited direct hydrogen atom transfer photocatalysis toolbox and provide new possibilities for photocatalytic C-H activation.

Photocatalyzed Direct C(sp3)-H Alkenylation of Unactivated Alkanes via Tandem C-C Activation of Cyclopropenes

A highly adaptable method has been developed for the alkenylation of a broad spectrum of inert alkanes, employing milder reaction conditions. Tetrabutylammonium decatungstate (TBADT) serves as a photocatalyst for hydrogen atom transfer (HAT), instigating the formation of transient alkyl radicals through C(sp3)-H functionalization. These radicals exhibit regioselective addition to cyclopropenes, followed by the subsequent activation of C-C bonds, forming the corresponding vinylated derivatives. This methodology accommodates diverse unreactive C(sp3)-H bond motifs and multisubstituted cyclopropenes, enabling the efficient synthesis of highly functionalized olefins with high diastereoselectivity.

Guest-Regulated Generation of Reactive Oxygen Species from Porphyrin-Based Multicomponent Metallacages for Selective Photocatalysis

The development of novel materials for highly efficient and selective photocatalysis is crucial for their practical applications. Herein, we employ the host-guest chemistry of porphyrin-based metallacages to regulate the generation of reactive oxygen species and further use them for the selective photocatalytic oxidation of benzyl alcohols. Upon irradiation, the sole metallacage (6) can generate singlet oxygen (1O2) effectively via excited energy transfer, while its complex with C70 (6⊃C70) opens a pathway for electron transfer to promote the formation of superoxide anion (O2⋅-), producing both 1O2 and O2⋅-. The addition of 4,4'-bipyridine (BPY) to complex 6⊃C70 forms a more stable complex (6⊃BPY) via the coordination of the Zn-porphyrin faces of 6 and BPY, which drives fullerenes out of the cavities and restores the ability of 1O2 generation. Therefore, benzyl alcohols are oxidized into benzyl aldehydes upon irradiation in the presence of 6 or 6⊃BPY, while they are oxidized into benzoic acids when 6⊃C70 is employed as the photosensitizing agent. This study demonstrates a highly efficient strategy that utilizes the host-guest chemistry of metallacages to regulate the generation of reactive oxygen species for selective photooxidation reactions, which could promote the utilization of metallacages and their related host-guest complexes for photocatalytic applications.

Heterogeneous Photocatalytic Strategies for C(sp3 )-H Activation

Activation of ubiquitous C(sp3 )-H bonds is extremely attractive but remains a great challenge. Heterogeneous photocatalysis offers a promising and sustainable approach for C(sp3 )-H activation and has been fast developing in the past decade. This Minireview focuses on mechanism and strategies for heterogeneous photocatalytic C(sp3 )-H activation. After introducing mechanistic insights, heterogeneous photocatalytic strategies for C(sp3 )-H activation including precise design of active sites, regulation of reactive radical species, improving charge separation and reactor innovations are discussed. In addition, recent advances in C(sp3 )-H activation of hydrocarbons, alcohols, ethers, amines and amides by heterogeneous photocatalysis are summarized. Lastly, challenges and opportunities are outlined to encourage more efforts for the development of this exciting and promising field.

Photoredox Catalysis with Visible Light for Synthesis of Thioxanthones Derivatives

A visible-light-promoted and transition-metal-free photoredox-catalysis strategy is developed for the synthesis of thioxanthone derivatives (TXs). The mechanistic study and density functional theory calculations suggest that visible-light-promoted intramolecular cyclization can be divided into hydrogen atom transfer, C-C formation, and oxidative dehydrogenation with high regioselectivity and reactivity. Significantly, this C-C formation strategy can be used in TXs-based commercial photoinitiators and drugs at the gram level.

Visible-Light-Active Coumarin- and Quinolinone-Based Photocatalysts and Their Applications in Chemical Transformations

Organic dyes have been actively studied as useful photocatalysts because they allow access to versatile structural flexibility and green synthetic applications. The identification of a new class of robust organic chromophores is, therefore, in high demand to increase structural diversity and variability. Although coumarins and quinolinones have long been acknowledged as organic chromophores, their ability to participate in photoinduced transformations is somewhat less familiar. Fascinated by their chromophoric features and adaptable platform, our group is interested in the identification of fluorescent bioactive molecules and in the development of new photoinduced synthetic methods using coumarins and quinolinones as photocatalysts. This account provides an overview of our recent progress in the discovery and application of light-absorbing coumarin and quinolinone derivatives in photochemistry and medicinal chemistry.

A Vitamin B2 -Photocatalysed Approach to Methionine Analogues

Access to new non-canonical amino acid residues is crucial for medicinal chemistry and chemical biology. Analogues of the amino acid methionine have been far less explored-despite their use in biochemistry, pharmacology and peptide bioconjugation. This is largely due to limited synthetic access. Herein, we exploit a new disconnection to access non-natural methionines through the development of a photochemical method for the radical α-C-H functionalization of sulfides with alkenes, in water, using inexpensive and commercially-available riboflavin (vitamin B2 ) as a photocatalyst. Our photochemical conditions allow the two-step synthesis of novel methionine analogues-by radical addition to unsaturated amino acid derivatives-and the chemoselective modification of peptide side-chains to yield non-natural methionine residues within small peptides. The mechanism of the bio-inspired flavin photocatalysis has been probed by experimental, DFT and TDDFT studies.

A Vitamin B2-Photocatalysed Approach to Methionine Analogues

Access to new non-canonical amino acid residues is crucial for medicinal chemistry and chemical biology. Analogues of the amino acid methionine have been far less explored-despite their use in biochemistry, pharmacology and peptide bioconjugation. This is largely due to limited synthetic access. Herein, we exploit a new disconnection to access non-natural methionines through the development of a photochemical method for the radical α-C-H functionalization of sulfides with alkenes, in water, using inexpensive and commercially-available riboflavin (vitamin B2) as a photocatalyst. Our photochemical conditions allow the two-step synthesis of novel methionine analogues-by radical addition to unsaturated amino acid derivatives-and the chemoselective modification of peptide side-chains to yield non-natural methionine residues within small peptides. The mechanism of the bio-inspired flavin photocatalysis has been probed by experimental, DFT and TDDFT studies.

Synthesis of New Morphinan Opioids by TBADT-Catalyzed Photochemical Functionalization at the Carbon Skeleton

The synthesis of new morphinan opioids by the addition of photochemically generated carbon-centered radicals to substrates containing an enone in the morphinan C-ring, is described. Using tetrabutylammonium decatungstate (TBADT) as a hydrogen atom transfer photocatalyst, diverse radical donors can be used to prepare a variety of C8-functionalized morphinan opioids. This work demonstrates the late-stage modification of complex, highly functionalized substrates.

Photoinduced Chemo-, Site- and Stereoselective α-C(sp3 )-H Functionalization of Sulfides

The ubiquity of sulfur-containing molecules in biologically active natural products and pharmaceuticals has long attracted synthetic chemists to develop efficient strategies towards their synthesis. The strategy of direct α-C(sp³ )-H modification of sulfides provides a streamlining access to complex sulfur-containing molecules. Herein, we report a photoinduced chemo-, site- and stereoselective α-C(sp³ )-H functionalization of sulfides using isatins as the photoredox reagent and coupling partner catalyzed by a chiral gallium(III)-N,N'-dioxide complex. The reaction proceeds through a verified single-electron transfer (SET) mechanism with high efficiency, excellent functional group tolerance, as well as a broad substrate scope. Importantly, this cross-coupling protocol is highly selective for the direct late-stage functionalization of methionine-related peptides, regardless of the inherent structural similarity and complexity of diverse residues.

A Metal-Organic Framework as a Multiphoton Excitation Regulator for the Activation of Inert C(sp3 )-H Bonds and Oxygen

The activation and oxidization of inert C(sp³ )-H bonds into value-added chemicals affords attractively economic and ecological benefits as well as central challenge in modern chemistry. Inspired by the natural enzymatic transformation, herein, we report a new multiphoton excitation approach to activate the inert C(sp³ )-H bonds and oxygen by integrating the photoinduced electron transfer (PET), ligand-to-metal charge transfer (LMCT) and hydrogen atom transfer (HAT) events together into one metal-organic framework. The well-modified nicotinamide adenine dinucleotide (NAD⁺ ) mimics oxidized Ce^III -OEt moieties to generate Ce^IV -OEt chromophore and its reduced state mimics NAD^. via PET. The in situ formed Ce^IV -OEt moiety triggers a LMCT excitation to form the alkoxy radical EtO^. , abstracts a hydrogen atom from the C(sp³ )-H bond, accompanying the recovery of Ce^III -OEt and the formation of alkyl radicals. The formed NAD^. activates oxygen to regenerate the NAD⁺ for next recycle, wherein, the activated oxygen species interacts with the intermediates for the oxidization functionalization, paving a catalytic avenue for developing scalable and sustainable synthetic strategy.

Direct, Site-Selective and Redox-Neutral α-C-H Bond Functionalization of Tetrahydrofurans via Quantum Dots Photocatalysis

As one of the most ubiquitous bulk reagents available, the intrinsic chemical inertness of tetrahydrofuran (THF) makes direct and site-selective C(sp³ )-H bond activation difficult, especially under redox neutral condition. Here, we demonstrate that semiconductor quantum dots (QDs) can activate α-C-H bond of THF via forming QDs/THF conjugates. Under visible light irradiation, the resultant alkoxyalkyl radical directly engages in radical cross-coupling with α-amino radical from amino C-H bonds or radical addition with alkene or phenylacetylene, respectively. In contrast to stoichiometric oxidant or hydrogen atom transfer reagents required in previous studies, the scalable benchtop approach can execute α-C-H bond activation of THF only by a QD photocatalyst under redox-neutral condition, thus providing a broad of value added chemicals starting from bulk THFs reagent.

A Visible-Light-Harvesting Covalent Organic Framework Bearing Single Nickel Sites as a Highly Efficient Sulfur-Carbon Cross-Coupling Dual Catalyst

Covalent Organic Frameworks (COFs) have recently emerged as light-harvesting devices, as well as elegant heterogeneous catalysts. The combination of these two properties into a dual catalyst has not yet been explored. We report a new photosensitive triazine-based COF, decorated with single Ni sites to form a dual catalyst. This crystalline and highly porous catalyst shows excellent catalytic performance in the visible-light-driven catalytic sulfur-carbon cross-coupling reaction. Incorporation of single transition metal sites in a photosensitive COF scaffold with two-component synergistic catalyst in organic transformation is demonstrated for the first time.

C-H Activation by an Iron-Nitrido Bis-Pocket Porphyrin Species

High-valent iron-nitrido species are nitrogen analogues of iron-oxo species which are versatile reagents for C-H oxidation. Nonetheless, C-H activation by iron-nitrido species has been scarcely explored, as this is often hampered by their instability and short lifetime in solutions. Herein, the hydrogen atom transfer (HAT) reactivity of an Fe porphyrin nitrido species (2 c) toward C-H substrates was studied in solutions at room temperature, which was achieved by nanosecond laser flash photolysis (LFP) of its Fe^III -azido precursor (1 c) supported by a bulky bis-pocket porphyrin ligand. C-H bonds with bond dissociation enthalpies (BDEs) of up to ≈84 kcal mol^-1 could be activated, and the second-order rate constants (k₂ ) are on the order of 10² -10⁴  s^-1  m^-1 . The Fe-amido product formed after HAT could further release ammonia upon protonation.

Flow Photochemistry as a Tool in Organic Synthesis

Photochemical transformations of molecular building blocks have become an important and widely recognized research field in the past decade. Detailed and deep understanding of novel photochemical catalysts and reaction concepts with visible light as the energy source has enabled a broad application portfolio for synthetic organic chemistry. In parallel, continuous-flow chemistry and microreaction technology have become the basis for thinking and doing chemistry in a novel fashion with clear focus on improved process control for higher conversion and selectivity. As can be seen by the large number of scientific publications on flow photochemistry in the recent past, both research topics have found each other as exceptionally well-suited counterparts with high synergy by combining chemistry and technology. This review will give an overview on selected reaction classes, which represent important photochemical transformations in synthetic organic chemistry, and which benefit from mild and defined process conditions by the transfer from batch to continuous-flow mode.

Visible-Light-Enabled C-H Functionalization by a Direct Hydrogen Atom Transfer Uranyl Photocatalyst

The development of the uranyl cation as a powerful photocatalyst is seriously delayed in comparison with the advances in its fundamental and structural chemistry. However, its characteristic high oxidative capability in the excited state ([UO₂ ]²⁺ * (+2.6 V vs. SHE; SHE=standard hydrogen electrode) combined with blue-light absorption (hv=380-500 nm) and a long-lived fluorescence lifetime up to microseconds have reveals that the uranyl cation approaches an ideal photocatalyst for visible-light-driven organic transformations. Described herein is the successful use of uranyl nitrate as a photocatalyst to enable C(sp³ )-H activation and C-C bond formation through hydrogen atom transfer (HAT) under blue-light irradiation. In particular, this operationally simple strategy provides an appropriate approach to the synthesis of diverse and valuable diarylmethane motifs. Mechanistic studies and DFT calculations have provided insights into the detailed mechanism of the photoinduced HAT pathway. This research suggests a general platform that could popularize promising uranyl photocatalytic performance.

Phenylglyoxylic Acid: An Efficient Initiator for the Photochemical Hydrogen Atom Transfer C-H Functionalization of Heterocycles

C-H functionalization at the α-position of heterocycles has become a rapidly growing area of research. Herein, a cheap and efficient photochemical method was developed for the C-H functionalization of heterocycles. Phenylglyoxylic acid (PhCOCOOH) could behave as an alternative to metal-based catalysts and organic dyes and provided a very general and wide array of photochemical C-H alkylation, alkenylation, and alkynylation, as well as C-N bond forming reaction methodologies. This novel, mild, and metal-free protocol was successfully employed in the functionalization of a wide range of C-H bonds, utilizing not only O- or N-heterocycles, but also the less studied S-heterocycles.

Golden Jubilee of Singapore National Institute of Chemistry (1970-2020): Celebrating its Partnership with Wiley-VCH

This year Singapore National Institute of Chemistry (SNIC) is celebrating its golden jubilee (1970-2020). Wiley-VCH has been a steadfast partner accompanying the rapid rise of chemistry research in Singapore. In celebration of this golden jubilee, we highlight 50 significant papers published in Angewandte Chemie by scholars currently based in Singapore, covering the widest possible spectrum of chemistry research.

Photocatalytic Reductive Radical-Polar Crossover for a Base-Free Corey-Seebach Reaction

A metal-free generation of carbanion nucleophiles is of prime importance in organic synthesis. Herein we report a photocatalytic approach to the Corey-Seebach reaction. The presented method operates under mild redox-neutral and base-free conditions giving the desired product with high functional group tolerance. The reaction is enabled by the combination of photo- and hydrogen atom transfer (HAT) catalysis. This catalytic merger allows a C-H to carbanion activation by the abstraction of a hydrogen atom followed by radical reduction. The generated nucleophilic intermediate is then capable of adding to carbonyl electrophiles. The obtained dithiane can be easily converted to the valuable α-hydroxy carbonyl in a subsequent step. The proposed reaction mechanism is supported by emission quenching, radical-radical homocoupling and deuterium labeling studies as well as by calculated redox-potentials and bond strengths.

Photoredox-Catalyzed Isomerization of Highly Substituted Allylic Alcohols by C-H Bond Activation

Photoredox-catalyzed isomerization of γ-carbonyl-substituted allylic alcohols to their corresponding carbonyl compounds was achieved for the first time by C-H bond activation. This catalytic redox-neutral process resulted in the synthesis of 1,4-dicarbonyl compounds. Notably, allylic alcohols bearing tetrasubstituted olefins can also be transformed into their corresponding carbonyl compounds. Density functional theory calculations show that the carbonyl group at the γ-position of allylic alcohols are beneficial to the formation of their corresponding allylic alcohol radicals with high vertical electron affinity, which contributes to the completion of the photoredox catalytic cycle.

Visible-Light-Induced Selective Defluoroborylation of Polyfluoroarenes, gem-Difluoroalkenes, and Trifluoromethylalkenes

Fluorinated organoboranes serve as versatile synthetic precursors for the preparation of value-added fluorinated organic compounds. Recent progress has been mainly focused on the transition-metal catalyzed defluoroborylation. Herein, we report a photocatalytic defluoroborylation platform through direct B-H activation of N-heterocyclic carbene boranes, through the synergistic merger of a photoredox catalyst and a hydrogen atom transfer catalyst. This atom-economic and operationally simple protocol has enabled defluoroborylation of an extremely broad scope of multifluorinated substrates including polyfluoroarenes, gem-difluoroalkenes, and trifluoromethylalkenes in a highly selective fashion. Intriguingly, the defluoroborylation protocol can be transition-metal free, and the regioselectivity obtained is complementary to the reported transition-metal-catalysis in many cases.

Dual Benzophenone/Copper-Photocatalyzed Giese-Type Alkylation of C(sp3 )-H Bonds

Photocatalyzed Giese-type alkylations of C(sp³ )-H bonds are very attractive reactions in the context of atom-economy in C-C bond formation. The main limitation of such reactions is that when using highly polymerizable olefin acceptors, such as unsubstituted acrylates, acrylonitrile, or methyl vinyl ketone, radical polymerization often becomes the dominant or exclusive reaction pathway. Herein, we report that the polymerization of such olefins is strongly limited or suppressed when combining the photocatalytic activity of benzophenone (BP) with a catalytic amount of Cu(OAc)₂ . Under mild and operationally simple conditions, the Giese adducts resulting from the C(sp³ )-H functionalization of amines, alcohols, ethers, and cycloalkanes could be synthesized. Preliminary mechanistic studies have revealed that the reaction does not proceed through a radical chain, but through a dual BP/Cu photocatalytic process, in which both Cu^II and low-valent Cu^I/0 species, generated in situ by reduction by the BP ketyl radical, may react with α-keto or α-cyano intermediate radicals, thus preventing polymerization.

Asymmetric Synthesis of 1,4-Dicarbonyl Compounds from Aldehydes by Hydrogen Atom Transfer Photocatalysis and Chiral Lewis Acid Catalysis

Enantioenriched 1,4-dicarbonyl compounds are versatile synthons in natural product and pharmaceutical drug synthesis. We herein report a mild pathway for the efficient enantioselective synthesis of these compounds directly from aldehydes through synergistic cooperation between a neutral eosin Y hydrogen atom transfer photocatalyst and a chiral rhodium Lewis acid catalyst. This method is distinguished by its operational simplicity, abundant feedstocks, atom economy, and ability to generate products in high yields (up to 99 %) and high enantioselectivity (up to 99 % ee).

Photochemical Hydroacylation of Michael Acceptors Utilizing an Aldehyde as Photoinitiator

The hydroacylation of Michael acceptors constitutes a useful tool for the formation of new C-C bonds. In this work, an environmentally friendly procedure was developed, utilizing 4cyanobenzaldehyde as the photoinitiator and household bulbs as the irradiation source. A great variety of substrates was well-tolerated, leading to good yields, and mechanistic experiments were performed to elucidate the catalyst's possible mechanistic pathway. Moreover, the inherent selectivity challenge regarding α,α-disubstituted aldehydes (decarbonylation problem) was studied and addressed.

Light-Driven Intramolecular C-N Cross-Coupling via a Long-Lived Photoactive Photoisomer Complex

Reported herein is a visible-light-driven intramolecular C-N cross-coupling reaction under mild reaction conditions (metal- and photocatalyst-free, at room temperature) via a long-lived photoactive photoisomer complex. This strategy was used to rapidly prepare the N-substituted polycyclic quinazolinone derivatives with a broad substrate scope (>50 examples) and further exploited to synthesize the natural products tryptanthrin, rutaecarpine, and their analogues. The success of gram-scale synthesis and solar-driven transformation, as well as promising tumor-suppressing biological activity, proves the potential of this strategy for practical applications. Mechanistic investigations, including control experiments, DFT calculations, UV-vis spectroscopy, EPR, and X-ray single-crystal structure of the key intermediate, provides insight into the mechanism.