Photochemical Transformations Research Articles

Unlocking Photocycloaddition Reactivity of Tropolone by Cage-Confined Visible-Light Photocatalysis for Multilevel Selective Transformation.

The precise asymmetric photochemical transformation of organic compounds containing multiple reactive sites presents significant progress in synthetic chemistry. Herein, we report an unprecedented visible-light-induced cascade transformation of tropolone cyclic triene derivatives by using chiral photoactive metal-organic cages (cPMOCs) as enzyme-mimicking multipocket photocatalysts. The cage-confined photocatalysis promotes three successive elementary steps, i.e., enantioselective [2 + 2] photocycloaddition with chalcone, regio-, and diastereoselective α-ketol rearrangement, and a stereoselective 1,3-acyl shift, resulting in bicyclo[3.2.2]nonane skeleton with multichiral-centers unattainable by other methods. This study demonstrates how complex synthetic challenges of peri-, chemo-, and stereoselectivities could be subtly manipulated by cage-confined supramolecular catalysis for exploration of new reactivities.

Chemodivergent Phototransformations of Green Fluorescent Protein Related Chromophores: Wavelength‐Dependent Photodimerization and Spirocyclization

Benzylidene imidazolones with an ortho‐sec‐amino group undergo chemodivergent photochemical transformations, based on the wavelength of irradiation. Irradiation at 525 nm causes dimerization and the formation of cyclobutanes with two spiroatoms, related to dictazoles in 87‐98% yields. In turn, irradiation at 365 nm provides access to spirocyclic indolines in 67‐92% yields. Each of the processes proceeds with exclusive atom economy and chemoselectivity.

Concise Synthesis of Norzoanthamine Enabled by a Set of Photochemical Transformations.

Norzoanthamine is a structurally complex polycyclic natural product that expresses a broad range of biological activities, rendering facile access to it and its analogs of considerable importance in drug discovery and development. However, strategies for efficient access to this class of marine alkaloids remain lacking. Here, we report a strategy, characterized by three key photochemical reactions, that we used to synthesize norzoanthamine in 16 steps. A photoinduced dearomative-6π-desymmetrization was developed for facile access to the ABC-tricyclic core of the alkaloid. This was supplemented by a [2 + 2]-photocycloaddition, a visible-light-induced decarboxylative borylation, and a retro-aldol process, constituting an effective solution to the challenging problem of establishing the C9-C22 vicinal all-carbon quaternary stereogenic centers. Finally, a one-pot cascade transformation, involving global deprotection, cyclization, and epimerization reaction, was designed for efficient and stereocontrolled assembly of the core framework of norzoanthamine.

Icelandic glacial dissolved organic carbon fluxes, composition and variability - relevance for the global glacial carbon budget

Despite their relatively large size, organic carbon (OC) fluxes from Icelandic glaciers have not been explicitly considered in current global glacial OC flux calculations. Most global glacial OC estimates are based on limited individual flux estimates, which show considerable spatial differences, are often based on melt season fluxes, and rarely account for seasonal and diurnal variability of glacial dissolved organic matter (DOM). Using an annual dataset of 25 Icelandic glaciers (and their glacial streams) we investigate DOM concentration and composition, calculating an estimate for downstream OC fluxes from Icelandic glaciers, considering diurnal and seasonal variability. DOM source and composition distinctly changed from a terrestrial character toward a more proteinaceous character as melt increased, both on a seasonal and diurnal basis, likely reflecting the flow path of the meltwater. While DOC concentration did not change on a diurnal basis, DOM composition was more labile in the afternoons, possibly indicating photochemical or biological transformation processes. Overall, the glacial streams predominantly acted as CO2 sinks. However, higher DOC concentrations, along with contributions of more proteinaceous DOM in proglacial streams, led to a decrease in the uptake potential for CO2. Finally, we estimated an export flux of 0.0026 ± 0.0029 Gg C yr−1 km−2 of DOC, and 0.011 ± 0.007 Tg C yr−1 km−2 of POC, from Icelandic glaciers. We reveal larger than previously assumed DOC and POC fluxes from Icelandic glaciers, with higher-than-global-average areal fluxes (~3 % and 9 % of global glacial C flux respectively). Our findings underscore the importance of revising current global estimates to include the not-fully-accounted-for contribution of Icelandic, and other glaciers. This is particularly important considering ongoing climatic changes will likely affect glacial meltwater discharge and sources, leading to altered DOM composition and DOC concentration, having potentially considerable consequences for glacial OC export and CO2 uptake potentials of glacial streams.

Photoinduced Radical Borylation of Robust Carbon–Heteroatom Bonds

AbstractPhotoinduced borylation has emerged as a valuable strategy for synthesizing arylboronic esters. However, photochemical transformations involving inert bonds such as C(sp2)−F bonds are still challenging. Herein, we report a straightforward and operationally simple method for the activation of various inert carbon–heteroatom bonds, enabling the synthesis of diverse arylboronic esters without the need for transition metals or catalysts. Mechanistic investigations reveal that the deprotonation of DMSO plays a pivotal role in the reaction, and the excited DMSO anion can undergo electron transfer to the aryl substrates activated by anionic sp2–sp3 diboron compounds, i. e., [FB2pin2]−, thereby facilitating the cleavage of carbon–heteroatom bonds. The reaction allows the conversion of diverse Caryl–hetero bonds in batch and flow reactors into the corresponding arylboronic esters. The products can be used in a subsequent Suzuki‐Miyaura cross‐coupling without isolation.

Intramolecular Photochemical C−H Insertions of α‐Diazocarbonyl Compounds

AbstractIntramolecular photochemical C−H insertion reactions of α‐diazocarbonyl compounds are powerful synthetic transformations leading to carbon‐carbon bond formation. While there are limitations in the scope of C−H bonds amenable to C−H insertion under photochemical conditions relative to metal catalysed C−H insertions, the benefits of conducting these synthetically powerful transformations in a metal‐free approach offers “green” advantages. Recent advances in technology have revolutionised the potential for use of photochemical transformations in synthesis, including the wavelength control enabled by LEDs and the benefits associated with continuous flow processing which overcome many of the traditional challenges associated with synthetic photochemistry including scale‐up. The potential for telescoping the synthesis of α‐diazocarbonyl compounds and downstream photochemical reactions is clear. This review summarises the development of intramolecular photochemical C−H insertions and highlights the benefits of recent technological advances.

Mechanism for the Indirect Photo‐transformation of Molecular Switches using a Pyrrolo‐pyrrole Two‐photon Absorbing Antenna

Electron‐rich symmetric pull‐push‐pull molecules can act as efficient two‐photon absorbing units. When these chromophores are bonded directly to photo‐switchable molecules they can function as antenna systems to indirectly induce photochemical transformations in isomerizable groups after energy transfer. Recently, we developed an antenna–molecular switch system based on a pyrrolo–pyrrole two‐photon active chromophore. When this antenna section is functionalized with symmetrically situated azo‐sections as N,N’‐pyrrolic‐substituents, these azo molecular switches can be efficiently transformed from their E to their Z isomers after non‐linear light absorption by the antenna, followed by indirect excitation of the azo‐sections. In this contribution we present a case‐study of one of these systems through femtosecond‐resolved fluorescence to observe the dynamics involved in the excitation and relaxation steps within the antenna section, as well as the energy transfer pathways. By time‐resolving the emission signals we observed that the energy transfer can occur in parallel with the relaxation within the first singlet vibronic states localized at the pyrrolo–pyrrole antenna.

Enantioselective photochemical reactions within the confined cavities of supramolecular assemblies

Compared to bulk solvents, reactions in the confined spaces of supramolecular self-assemblies feature rate acceleration, high efficiency and substrate selectivity. These advantages lead to efficient catalytic efficiency and excellent selectivity in enantioselective supramolecular photochemical transformations. During the last few years, enantioselective supramolecular photocatalysis has developed into one of the most powerful strategies to construct enantioenriched chiral compounds. In this review, the recent advances of enantioselective photochemical reactions taking place within the confined spaces of supramolecular assemblies are summarized, with an emphasis on the specific catalytic modes and chemical transformations. Organization of the data follows a subdivision according to supramolecular host and reaction type. At last, the current limitations and the future research orientation of this research field are discussed.

CsPbX3 Nanocrystal Incorporating Transition Metal Cocatalyst for Photocatalytic Organic Transformation by Single Electron Transfer

ABSTRACTThe construction of composite photocatalysts is a promising strategy for improving the overall catalytic efficiency in organic photochemical transformation. In this study, we synthesize a series of inorganic metal perovskite CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) with excellent chemical stability and wide‐range visible‐light absorption. The Pd cocatalyst is then induced to the surfaces of CsPbX3 fabricating Pd/CsPbX3 composites via visible‐light reduction method. Photoelectrochemical measurements show the introduction of Pd cocatalyst promotes efficiently the charge separation and transfer of CsPbX3. The prepared Pd/CsPbX3 composites exhibit remarkable photocatalytic efficiency in visible‐light‐induced C–C cross‐coupling reaction. The reaction is characterized by mild reaction conditions, good recyclability, and high yield from a broad variety of substrates. Mechanism investigations show the reaction relies on the synergy of CsPbX3 photocatalysis and Pd catalysis. The valence band of CsPbX3 oxidizes the phenylboronic acid into active radical species via single electron transfer. On another hand, Pd(0) as active sites occurs oxidative addition with aryl halides for achieving the C–C cross‐coupling reaction. This study displays a feasible approach for the introduction of Pd catalyst in CsPbX3 to achieve the photocatalytic organic transformation via photoinduced electron transfer.

Photochemical Transformation of Ibuprofen and Chlorophene Induced by Dissolved Organic Matter.

Both ibuprofen (IBP) and chlorophene (CP) are frequently detected contaminants in surface aqueous environment. Dissolved organic matter (DOM) is an important component in water with high photo-reactivity, playing an important role in the transformation processes of various organic pollutants. This study systematically studied the influence of DOM on the photochemical transformation of IBP and CP by using humic acid as model DOM. In addition, the effect of inorganic salts on this process is also considered due to the high salt content in the ocean. Further quenching experiments and reactive oxygen species (ROSs) detection were also conducted to explore the reactive species acting on the IBP and CP transformation. Based on the products analysis and theoretical calculation, we proposed the IBP and CP transformation mechanism. Overall, this study provides some new insights into the transformation of organic pollutants in natural surface water, which is significant for assessing the fate of pollutants.

Photodegradation of polychlorinated biphenyls (PCBs) on suspended particles from the Yellow River under sunlight irradiation: QSAR model and mechanism analysis

Polychlorinated biphenyls (PCBs), as a class of hydrophobic organic pollutants, are widely found in river sediments and suspended particles, and the environmental fate of different PCBs can be better understood by investigating their photochemical transformation process. In this study, the quantitative structure-activity relationship (QSAR) model between the photodegradation rate constants of 17 PCBs adsorbed on Yellow River suspended particles in water (as a typical heterogeneous photodegradation system) and the physicochemical parameters of PCBs was constructed by SPSS and machine learning. The model showed that the more hydrophobicity of the molecule, the more positive charge carried by the aromatic C atoms, and the presence of chlorine atoms adjacent to the carbon bridge could all enhance the photochemical activity of PCBs. From the combined analysis of rate constants, quenching experiments and theoretical calculations, it was revealed for the first time that in natural suspended particle containing organic matter, the higher concentration of •O2- and 1O2 in the hydrophobic zone contributed more to the more hydrophobic PCBs, while •OH in the hydrophilic zone played a major role in the degradation of the less hydrophobic PCBs. Findings of this study would deepen the understanding of the degradation mechanism of hydrophobic pollutants by active species in complex environments.

New Analytical Paradigm to Determine Concentration of Brown Carbon and Its Sample-by-Sample Mass Absorption Efficiency.

Brown carbon (BrC) has a substantial direct radiative effect, but current estimates of its impact on radiative balance are highly uncertain due to a lack of measurements of its light-absorbing properties, such as mass absorption efficiency (MAE). Here, we present a new analytical paradigm based on a Bayesian inference (BI) model that takes multiwavelength aethalometer measurements and total carbon data to resolve the concentrations of black carbon and BrC, and MAEs of BrC on a sample-by-sample basis. Hourly MAEs, unattainable in previous studies, can now be calculated, enabling the first-time observation of the darkening-bleaching dynamics of BrC in response to photochemical transformation. We demonstrate the application of this BI model to analyze measurements collected over one year (2021-2022) in Hong Kong. Diel variations in MAE370nm of BrC reveal a darkening-to-bleaching transition occurring between 8 and 10 O'clock when the solar irradiance ranges from 30 to 400 W m-2. Furthermore, we consistently observe an increase in MAE370nm of BrC with nitrogen oxide concentrations, suggesting the enhanced formation of nitrogenous organics. This BI model-based data analysis would bring forth a breakthrough in amassing observation data of BrC and its MAEs in diverse ambient environments and with high time resolution.

Photoinduced Evolutions of Permafrost-Derived Carbon in Subarctic Thermokarst Pond Surface Waters.

In subarctic regions, rising temperature and permafrost thaw lead to the formation of thermokarst ponds, where organics from eroding permafrost accumulate. Despite its environmental significance, limited knowledge exists regarding the photosensitivity of permafrost-derived carbon in these ponds. In this study, laboratory experiments were conducted to explore the photochemical transformations of organic matter in surface water samples from thermokarst ponds from different environments in northern Quebec, Canada. One pond near Kuujjuarapik is characterized by the presence of a collapsing palsa and is therefore organically rich, while the other pond near Umiujaq is adjacent to a collapsing lithalsa and thus contains fewer organic matters. Photobleaching occurred in the Umiujaq sample upon irradiation, whereas the Kuujjuarapik sample exhibited an increase in light absorbance at wavelength related to aromatic functionalities, indicating different photochemical aging processes. Ultrahigh-resolution mass spectrometry analysis reveals that the Kuujjuarapik sample preferentially photoproduced highly unsaturated CHO compounds with great aromaticity, while the irradiated Umiujaq sample produced a higher proportion of CHON aromatics with reduced nitrogen functionalities. Overall, this study illustrates that the photochemical reactivity of thermokarst pond water varies with the source of organic matter. The observed differences in reactivity contribute to an improved understanding of the photochemical emission of volatile organic compounds discovered earlier. Further insights into the photoinduced evolutions in thermokarst ponds may require the classification of permafrost-derived carbon therein.

Simulation of the non-adiabatic dynamics of an enone-Lewis acid complex in an explicit solvent.

Unlocking the full potential of Lewis acid catalysis for photochemical transformations requires a comprehensive understanding of the ultrafast dynamics of substrate-Lewis acid complexes. In a previous article [Peschel et al., Angew. Chem. Int. Ed., 2021, 60, 10155], time-resolved spectroscopy supported by static calculations revealed that the Lewis acid remains attached during the relaxation of the model complex cyclohexenone-BF3. In contrast to the experimental observation, surface-hopping dynamics in the gas phase predicted ultrafast heterolytic dissociation. We attributed the discrepancy to missing solvent interactions. Thus, in this work, we present an interface between the SHARC and FermiONs++ program packages, which enables us to investigate the ultrafast dynamics of cyclohexenone-BF3 in an explicit solvent environment. Our simulations demonstrate that the solvent prevents the dissociation of the complex, leading to an intriguing dissociation-reassociation mechanism. Comparing the dynamics with and without triplet states highlights their role in the relaxation process and shows that the Lewis acid inhibits intersystem crossing. These findings provide a clear picture of the relaxation process, which may aid in designing future Lewis acid catalysts for photochemical applications. They underscore that an explicit solvent model is required to describe relaxation processes in weakly bound states, as energy transfer to the solvent is crucial for the system to reach its minimum geometries.

Helix-to-Disc Conversion of Thia[6]helicenes into Coronenes Facilitated by Sulfur Oxidation and Fluorination.

Helicenes, with their unique helical structures, have long captured the interest of synthetic chemists, not only as end products, but also as versatile platforms for further chemical transformations. However, transforming [6]helicene into planar coronene typically requires harsh conditions and poses significant challenges. Herein, we demonstrate that replacing the terminal benzene ring of [6]helicene with a thiophene ring enables its photochemical transformation into coronene. Sulfur oxidation of the thiophene ring enables the corresponding thermal transformation, and the terminal tetrafluorination of the opposite benzene ring further accelerates this process, yielding 1,2-difluorocoronene, as confirmed by X-ray crystallography. The transformation begins with an intramolecular Diels-Alder reaction, whose activation energy is significantly lowered by these structural changes. Our findings underscore the utility of strategic modifications such as sulfur oxidation and fluorination in promoting this "helix-to-disc" conversion and opening new avenues for synthesizing functional polycyclic aromatics.

Oxidative Synthesis of 1,3‐Diarylphenanthro[9,10‐c]thiophenes and their Aerobic Photoconversion to 9,10‐Diaroylphenanthrenes

AbstractOur study introduces a streamlined oxidative method for synthesizing 1,3‐diarylphenanthro[9,10‐c]thiophenes, and their subsequent oxidative cleavage to yield 9,10‐diaroylphenanthrenes. Harnessing an optimized aerobic photochemical transformation, we showcase broad substrate scope and tolerance to various photosensitive functional groups. Our mechanistic investigations unveil the pivotal role of singlet oxygen, providing intriguing insights into the reaction pathway. The synthesized 9,10‐diaroylphenanthrenes serve as versatile building blocks for nitrogen‐containing heterocycles, dibenzoannulated phthalazines, that are promising compounds with diverse functionalities. Offering mild conditions, and broad applicability, our synthetic strategy is useful for organic synthesis and chemical biology exploration.

Novel insights into the photochemical transformation of neonicotinoid insecticides: Potential involvement of adjuvants

Commercial pesticide formulations often contain both the active ingredient and different adjuvants, the potential impacts of adjuvants on pesticides’ transformation have been barely considered. In this study, effects of typical adjuvants (i.e. hexadecyl trimethyl ammonium bromide (CTAB), hexadecyl trimethyl ammonium chloride (CTAC) and p-Octyl polyethylene glycol phenyl ether (TX-100)) on the photodegradation of four selected neonicotinoid insecticides (NIs), including thiamethoxam (TMX), dinotefuran (DIN), imidacloprid (IMP) and acetamiprid (ACP) were systematically investigated. Specifically, CTAB and CTAC exhibited minimal or no impact on the photolysis of four NIs, while TX-100 significantly inhibited their decay. Such inhibitory effect could mainly be attributed weakening the formation of key transient intermediates, such as the corresponding radical cation (NI•+), radical anion (NI•-) as well as hydrated electron (eaq-). Photoproducts identification revealed a novel transformation pathway of NIs in the presence of TX-100, characterized by de-chain reactions, resulting in the formation of photostable dimeric photoproducts. These dimers exhibited varying toxicity: for TMX, DIN and IMP, their corresponding dimers showed increased toxicity compared to their parent compounds, whereas for ACP, they displayed decreased toxicity. These provide important information for the evaluation the environmental fates and toxicology of formulaic NIs.

Photocatalyst‐Free Wavelength‐Dependant Sequential Ring Transformations of Pyrazolo[1,2‐a]pyrazolones

We report new sequential wavelength‐selective photochemical transformations of 1‐alkenylpyrazolo[1,2‐a]pyrazolones to pyrazolo[1,2‐a][1,2]diazepines or cyclobuta[c]pyrazolo[1,2‐a]pyrazolones. Irradiation of 1‐alkenylpyrazolo[1,2‐a]pyrazolones with visible‐light (blue LED, 457 nm) induced selective ‘ring switching’ transformation into pyrazolo[1,2‐a][1,2]diazepines, which, upon irradiation with UV A light (black LED, 365 nm) underwent electrocyclisation into cyclobuta[c]pyrazolo[1,2‐a]pyrazolones. Due to the very narrow irradiation wavelength now available from OLED sources, the selective formation of either 5,7‐bicyclic or 5,5,4‐tricyclic ring systems from the 5,5‐bicyclic starting material is possible simply by changing the wavelength of the irradiation source. The transformations took place under mild conditions in the absence of additives or photocatalysts. Mechanistic studies indicate that these transformations proceed through the formation of an excited triplet state of the substrate, followed by selective homolytic C(1)–N(8) bond cleavage, intersystem crossing, and cyclization of the zwitterionic intermediate on the ground potential energy surface. Subsequent photoinduced disrotatory stereospecific 4‐π‐electrocyclization of the 5,7‐bicyclic systems leads to 3D‐rich 5,5,4‐tricyclic products.

Cyclization Through Dual C(sp3)−H Functionalization

AbstractC(sp3)−H functionalization methods have been widely employed in many organic transformations such as cyclization reactions, heterocycle synthesis, cross‐coupling protocols, and photochemical transformations. Among these transformations, cyclization reaction through C(sp3)−H functionalization offers a direct route to convert simple linear substrates to complex products. There are three common modes of utilizing C(sp3)−H bonds in cyclization reactions including single, double, and dual C(sp3)−H functionalization. As the most challenging mode, dual C(sp3)−H functionalization refers to converting two separate C(sp3)−H bonds in one molecule into desired C−Z bonds which can be employed in cyclization reactions. Cyclization reaction via dual functionalization of C(sp3)−H bonds can be classified based on the C−H reactivities. Therefore, these reactions can be categorized into three classes based on the types of C(sp3)−H bonds including activated‐activated, activated‐unactivated, and unactivated‐unactivated C(sp3)−H bonds. Most published reports for cyclization reactions through dual C(sp3)−H functionalization involve activated‐activated C(sp3)−H bonds. However, the number of reported papers on the other two classes has been growing. This review focuses on the dual C(sp3)−H functionalization protocols used for cyclization reactions and categorizes the published papers based on the types of C(sp3)−H bonds.

Enantioselective [2 + 2] Photocycloreversion Enables De Novo Deracemization Synthesis of Cyclobutanes.

While photochemical deracemization significantly enhances atom economy by eliminating the necessity for additional oxidants or reductants, the laborious presynthesis of substrates from feedstock chemicals is often required, thereby compromising the practicality of this method. In this study, we propose a novel approach known as de novo deracemization synthesis, which involves direct utilization of simple substrates undergoing both photochemical transformation and reversible photochemical transformation. The efficient enantiocontrol of chiral catalysts in the latter process establishes an effective platform for deracemization. This alternative and practical approach to address the challenges of asymmetric photocatalysis has been successfully demonstrated in the photosensitized de novo deracemization synthesis of azaarene-functionalized cyclobutanes featuring three stereocenters, including an all-carbon quaternary center. By exclusively employing a suitable chiral catalyst to enable kinetically controlled [2 + 2] photocycloreversion, we pave a creative path toward achieving more cost-effective photochemical deracemization.