Transition Metal-Free Supramolecular Photoredox Catalysis in Water: A Phenoxazine Photocatalyst Encapsulated in V-Shaped Aromatic Amphiphiles

Tunable C-H functionalization and dearomatization enabled by an organic photocatalyst

C-H functionalization and dearomatization constitute fundamental transformations of aromatic compounds, which find wide applications in various research areas. However, achieving both transformations from the same substrates with a single catalyst by operating a distinct mechanism remains challenging. Here, we report a photocatalytic strategy to modulate the reaction pathways that can be directed toward either C-H functionalization or dearomatization under redox-neutral or net-reductive conditions, respectively. Two sets of indoles and indolines bearing tertiary alcohols are divergently furnished with good yields and high selectivity. The key to success is the introduction of isoazatruxene ITN-2 as a novel photocatalyst (PC), which outperforms the commonly used PCs. The ready synthesis and high modulability of isoazatruxene type PCs indicate their great application potential.

Substrate Selectivity Imparted by Self-Assembled Molecular Containers and Catalysts

Recent trends in catalysis are devoted to mimicking some peculiar features of enzymes like site selectivity, through functional group recognition, and substrate selectivity, through recognition of the entire surface of the substrate. The latter is a specific feature of enzymes that is seldomly present in homogeneous catalysis. Supramolecular catalysis, thanks to the self-assembly of simple subunits, enables the creation of cavities and surfaces whose confinement effects drive the preferential binding of a substrate among others with consequent substrate selectivity. The topic is an emerging field that exploits recognition phenomena to discriminate the reagents based on their size and shape. This review deals this cutting-edge field of research covering examples of supramolecular self-assembled molecular containers and catalysts operating in organic as well as aqueous media, with special emphasis for catalytic systems dealing with direct competitive experiments involving two or more substrates.

Visible light-mediated C (sp3)-H bond functionalization inside an all-organic nanocavity

Ultrafast C-H bond activation and functionalization in confinement using visible light will enable engineering chemical reactions with extraordinary speed and selectivity. To provide a transition metal-free route, here we demonstrate C-H bond activation reactions on poly-aromatic hydrocarbons (PAH) in all-organic cationic nanocage ExBox4+ for the first time. Visible light excitation in the host-guest charge transfer (CT) state allows the formation of oxidized photoproducts with high selectivity. Mechanistic understanding of this CT-mediated photoreaction using femtosecond broadband transient absorption revealed a few ∼100 ps timescale for C-H bond breaking on the attached -CH3 group via sequential electron transfer and proton transfer steps. We envision that our photosensitizer-free method will open up new avenues to pursue organic reactions using cavities that could serve both as photoredox catalysts and hosts for reactive reaction intermediates.

Visible-Light-Driven α-Substituted Amines Enabled by In Situ Formation of Amine Substrate Aggregates

A visible-light-driven, photocatalyst-free, air-promoted, α-substituted reaction of amines with varying nucleophiles is described. The amine substrate aggregates formed in situ through physical π-π stacking by H2O regulation in organic solvent can absorb visible light and then generate iminium ion intermediates, which undergo nucleophilic substitution reactions with varying nucleophiles to afford α-substituted amines. This reaction features catalyst-free, good functional group tolerance, simple operation procedure, and green reaction conditions.

Reductive photoredox transformations of carbonyl derivatives enabled by strongly reducing photosensitizers

Visible-light photoredox catalysis is well-established as a powerful and versatile organic synthesis strategy. However, some substrate classes, despite being attractive precursors, are recalcitrant to single-electron redox chemistry and thus not very amenable to photoredox approaches. Among these are carbonyl derivatives, e.g. ketones, aldehydes, and imines, which in most cases require Lewis or Brønsted acidic additives to activate via photoinduced electron transfer. In this work, we unveil a range of photoredox transformations on ketones and imines, enabled by strongly reducing photosensitizers and operating under simple, general conditions with a single sacrificial reductant and no additives. Specific reactions described here are umpolung C-C bond forming reactions between aromatic ketones or imines and electron-poor alkenes, imino-pinacol homocoupling reactions of challenging alkyl-aryl imine substrates, and γ-lactonization reactions of aromatic ketones with methyl acrylate. The reactions are all initiated by photoinduced electron transfer to form a ketyl or iminyl that is subsequently trapped.

Unravelling the Effect of Water Addition in Consecutive Photocatalysis with Naphthalene Diimide

We describe the effect of water addition in the catalyst performance for a C-H functionalization of benzene. Improved yields and selectivity were achieved in a consecutive photoredox catalysis in contrast to the reaction without water. Mechanistic analysis demonstrated this is due to a better catalyst stability and faster kinetic rather than a change in the different steps of the mechanism. The addition of water constitutes a convenient approach to improve catalyst performance, and it was also observed with other catalysts.

Development of Carbazole-Cored Organo-Photocatalyst for Visible Light-Driven Reductive Pinacol/Imino-Pinacol Coupling

Benzoperylenocarbazole (BPC), a unique carbazole-based organophotocatalyst, is reported herein as a potent organo-photoreductant. Lower excited state oxidation potential (-2.0 V vs SCE) and reasonable excited state lifetime (4.61 ns) render BPC an effective photosensitizer. Under irradiation of blue light employing low catalyst loading (0.5 mol %), a plethora of vicinal diols and diamines were synthesized in excellent yields through reductive coupling of carbonyls and imines, respectively. Insight about the electronic structure of BPC was obtained by DFT calculations.

Direct C−H Trifluoromethylation of (Hetero)Arenes in Water Enabled by Organic Photoredox‐Active Amphiphilic Nanoparticles

Photoredox‐catalyzed chemical conversions are predominantly operated in organic media to ensure good compatibility between substrates and catalysts. Yet, when conducted in aqueous media, they are an attractive, mild, and green way to introduce functional groups into organic molecules. We here show that trifluoromethyl groups can be readily installed into a broad range of organic compounds by using water as the reaction medium and light as the energy source. To bypass solubility obstacles, we developed robust water‐soluble polymeric nanoparticles that accommodate reagents and photocatalysts within their hydrophobic interior under high local concentrations. By taking advantage of the high excited state reduction potential of N‐phenylphenothiazine (PTH) through UV light illumination, the direct C−H trifluoromethylation of a wide array of small organic molecules is achieved selectively with high substrate conversion. Key to our approach is slowing down the production of CF₃ radicals during the chemical process by reducing the catalyst loading as well as the light intensity, thereby improving effectiveness and selectivity of this aqueous photocatalytic method. Furthermore, the catalyst system shows excellent recyclability and can be fueled by sunlight. The method we propose here is versatile, widely applicable, energy efficient, and attractive for late‐stage introduction of trifluoromethyl groups into biologically active molecules.

Assemblies of 1,4-Bis(diarylamino)naphthalenes and Aromatic Amphiphiles: Highly Reducing Photoredox Catalysis in Water

Host–guest assemblies of a designed 1,4-bis(diarylamino)naphthalene and V-shaped aromatic amphiphiles consisting of two pentamethylbenzene moieties bridged by an m-phenylene unit bearing two hydrophilic side chains emerged as highly reducing photoredox catalysis systems in water. An efficient demethoxylative hydrogen transfer of Weinreb amides has been developed. The present supramolecular strategy permits facile tuning of visible-light photoredox catalysis in water.

Visible-Light Photoredox Catalysis in Water

The use of water in organic synthesis draws attention to its green chemistry features and its unique ability to unveil unconventional reactivities. Herein, literature about the use of water as a reaction medium under visible-light photocatalytic conditions is summarized in order to highlight challenges and opportunities. Accordingly, this Synopsis has been divided into four different sections focused on (1) the unconventional role of water in photocatalytic reactions, (2) in-/on-water reactions, (3) water-soluble photocatalysts, and (4) photomicellar catalytic systems.

Ligand regulated metal–organic frameworks for synergistic photoredox and nickel catalysis

Synergistic photoredox and nickel catalytic cross-coupling systems have created a great attraction as a promising methodology to produce the aryl C−N bonds under mild conditions as well as extreme challenge,...

Recent advances in visible-light mediated functionalization of olefins and alkynes using copper catalysts

Over the past decade, visible-light photoredox catalysis has blossomed as a powerful strategy and offers a discrete activation mode complementary to thermal controlled reactions. Visible-light-mediated photoredox catalysis also offers exciting...

A cyanine dye based supramolecular photosensitizer enabling visible-light-driven organic reaction in water

We report the aggregation-induced photosensitizing activity of a cyanine dye in water and the mechanism. In addition, using the supramolecular assembly, visible-light-driven photooxidation of hydrophobic aromatic compounds in water was successfully performed.

Photocatalytic Homocoupling Transformations

Homocoupling reactions promoted by photocatalysts are not very abundant in the literature. However, the products generated from such processes are very interesting. In this review, we highlight the most relevant reports concerning photocatalyzed dimerizations covering the literature until the middle of 2020. Reactions will be classified according to the type of starting material employed, with an emphasis being placed on the corresponding mechanism.

1 Introduction

2 Arenes and Heteroarenes

3 Alkenes

4 Alkanes

5 Alkynes

6 Aldehydes, Ketones, Alcohols, Amines and Imines

7 Carboxylic Acids

8 Nitro Compounds

9 Conclusions

Converting p-terphenyl into a novel organo-catalyst for LED-driven energy and electron transfer photoreactions in water

p-Terphenyl is a potent photoredox catalyst under UV-irradiation. Aiming for more sustainable reaction conditions, we added two sulfonate groups to this key structure to achieve water solubility and incorporated an SO₂-bridge thereby shifting the absorption spectrum towards the visible. The resulting photocatalyst shows unexpected triplet reactivity in several test reactions.

De novo approach for the synthesis of water-soluble interlocked and non-interlocked organic cages

Research on self-assembled metallosupramolecular architectures has bloomed in recent times. Analogous metal-free organic architectures with water solubility are highly challenging. We report here a unique class of triazine based immidazolium water-soluble metal-free interlocked organic cage (1), which was synthesized in a one-pot reaction without using dynamic covalent chemistry and without any chromatographic separation. An analogous non-interlocked cage (2) was also successfully achieved by steric control using different positional isomers of the building blocks.

CBZ6 as a Recyclable Organic Photoreductant for Pinacol Coupling

A recyclable organic photoreductant (1 mol % CBZ6)-catalyzed reductive (pinacol) coupling of aldehydes, ketones, and imines has been developed. Irradiated by purple light (407 nm) using triethylamine as an electron donor, a variety of 1,2-diols and 1,2-diamines could be prepared. The oxidation potential of the excited state of CBZ6 is established as -1.92 V (vs saturated calomel electrode (SCE)). The relative high reductive potential enables the reductive coupling of carbonyl compounds and their derivatives. CBZ6 can be prepared in gram scale and is acid/base- or air-stable. It could be applied in large-scale photoreductive synthesis and recovered in high yield after the reaction.