Photochemical Conditions Research Articles

Regioselective synthesis of 4-arylamino-1,2-naphthoquinones in eutectogel as a confined reaction medium using LED light.

Predicting selectivity and conversion in a confined reaction medium under photochemical conditions is highly challenging as compared to the corresponding conventional synthesis. Herein, we report the use of a simple carbohydrate-derived eutectogel to facilitate LED-light-induced regioselective synthesis of 4-arylamino-1,2-naphthoquinones in good yield. This methodology, by including a reusable reaction medium, proved to have the potential of affording the regioselective formation of various desired products in good yields.

Electrochemical and photochemical reaction of isatins: a decade update.

This review presents the latest progress in photochemical and electrochemical reactions involving isatins. Isatin and its functionalized scaffolds e.g., oxindoles, spirooxindoles, and quinolines are privileged heterocycles as they are largely present in several agrochemical, natural products, and pharmaceuticals. Thus, the functionalization of isatins using sustainable approaches, i.e., electro- and photochemical methods is of recent research interest worldwide. In this review, we have discussed most of the important reactions of isatins based on types of bond formation involved under electro- and photochemical conditions over the last decade. The reaction mechanism for each reaction has been discussed in detail to offer an inclusive guide to readers. Lastly, a summary of current challenges and the future outlook toward the development of effective electrochemical and photochemical methods for the reaction of isatins is also presented.

Photocatalysis as a green alternative toolkit for the construction of nitrogen-enriched heterocycles via the direct and indirect activation of alkynes/alkenes

By harnessing the reactivity of alkynes and alkenes under photochemical conditions, novel pathways have been unlocked to construct N-heterocyclic compounds, allowing for more efficient and sustainable synthesis.

Photochemical H2 activation by an Zn-Fe heterometallic: a mechanistic investigation.

Addition of H2 to a Zn-Fe complex was observed to occur under photochemical conditions (390 or 428 nm LED) and leads to the formation of a heterometallic dihydride complex. The reaction does not occur under thermal conditions and DFT calculations suggest this is an endergonic, light driven process. Through a combined experimental and computational approach, the plausible mechanisms for H2 activation were investigated. Inhibition experiments, double-label cross-over experiments, radical trapping experiments, EPR spectroscopy and DFT calculations were used to gain insight into this system. The combined data are consistent with two plausible mechanisms, the first involving ligand dissociation followed by oxidative addition of H2 at the Fe centre, the second involving homolytic fragmentation of the Zn-Fe heterometallic and formation of radical intermediates.

Hydrogen production by a fully de novo enzyme.

Molecular catalysts based on abundant elements that function in neutral water represent an essential component of sustainable hydrogen production. Artificial hydrogenases based on protein-inorganic hybrids have emerged as an intriguing class of catalysts for this purpose. We have prepared a novel artificial hydrogenase based on cobaloxime bound to a de novo three alpha-helical protein, α3C, via a pyridyl-based unnatural amino acid. The functionalized de novo protein was characterised by UV-visible, CD, and EPR spectroscopy, as well as MALDI spectrometry, which confirmed the presence and ligation of cobaloxime to the protein. The new de novo enzyme produced hydrogen under electrochemical, photochemical and reductive chemical conditions in neutral water solution. A change in hydrogen evolution capability of the de novo enzyme compared with native cobaloxime was observed, with turnover numbers around 80% of that of cobaloxime, and hydrogen evolution rates of 40% of that of cobaloxime. We discuss these findings in the context of existing literature, how our study contributes important information about the functionality of cobaloximes as hydrogen evolving catalysts in protein environments, and the feasibility of using de novo proteins for development into artificial metalloenzymes. Small de novo proteins as enzyme scaffolds have the potential to function as upscalable bioinspired catalysts thanks to their efficient atom economy, and the findings presented here show that these types of novel enzymes are a possible product.

Catalytic CO2 Reduction with Heptacoordinated Polypyridine Complexes: Switching the Selectivity via Metal Replacement.

The discovery of molecular catalysts for the CO2 reduction reaction (CO2 RR) in the presence of water, which are both effective and selective towards the generation of carbon-based products, is a critical task. Herein we report the catalytic activity towards the CO2 RR in acetonitrile/water mixtures by a cobalt complex and its iron analog both featuring the same redox-active ligand and an unusual seven-coordination environment. Bulk electrolysis experiments show that the cobalt complex mainly yields formate (52 % selectivity at an applied potential of -2.0 V vs Fc+ /Fc and 1 % H2 O) or H2 (up to 86 % selectivity at higher applied bias and water content), while the iron complex always delivers CO as the major product (selectivity >74 %). The different catalytic behavior is further confirmed under photochemical conditions with the [Ru(bpy)3 ]2+ sensitizer (bpy=2,2'-bipyridine) and N,N-diisopropylethylamine as electron donor, where the cobalt complex leads to preferential H2 formation (up to 89 % selectivity), while the iron analog quantitatively generates CO (up to 88 % selectivity). This is ascribed to a preference towards a metal-hydride vs. a metal-carboxyl pathway for the cobalt and the iron complex, respectively, and highlights how metal replacement may effectively impact on the reactivity of transition metal complexes towards solar fuel formation.

Decomposition of a manganese complex loaded on TiO2 nanoparticles under photochemical reaction

Water splitting, through electrochemical and photochemical methods, is a promising approach for large-scale hydrogen production. However, the efficiency of these methods is hindered by the water-oxidation reaction (WOR). A variety of Mn complexes have been explored as potential models for water-oxidizing complex in Photosystem II. Among different Mn complexes, [H2O(terpy)MnIII(μ-O)2MnIV(terpy)H2O](NO3)3 (1, terpy = 2,2’:6′2″-terpyridine) has been extensively studied under WOR conditions. In this study, the reactivity of 1 is investigated when loaded onto TiO2 nanoparticles and subjected to photochemical conditions. Our main goal was to identify possible products that could arise from the conversion of 1 given the specified conditions. After conducting various analytical methods, it was determined that the reaction yielded MnOx and Mn(II) species as the resulting reaction products. These findings offer valuable insights into the progress of advanced WOR catalysts and the underlying mechanism of efficient water splitting for energy storage.

Energy-efficient Approach to Multicomponent Reaction for the Synthesis of Therapeutically Relevant Heterocycles

Abstract: Multi-component reactions have been used as an important synthetic strategy for the synthesis of diverse varieties of therapeutically useful heterocyclic scaffolds. High atom economy, one-pot reaction, and involvement of synthetically simple steps are some of the interesting features that make MCRs greener compared to conventional methods. The development of environmentally benign and eco-friendly synthetic methods has been a very demanding area of research in the past few decades. In particular, the development of energyefficient methods has attracted the attention of the research community due to heavy dependence on nonrenewable energy resources, which is depleting fast. Therefore, the present review has highlighted the multicomponent reactions developed under the energy efficient protocol, which mainly include the reactions developed under the microwave, ultra-sonication, mechano-chemical, and photochemical reaction conditions for the synthesis of therapeutically relevant heterocycles.

Elucidating the Impacts of Various Atmospheric Ventilation Conditions on Local and Transboundary Ozone Pollution Patterns: A Case Study of Beijing, China

AbstractThe ventilation condition in the atmospheric boundary layer, which varies with the synoptic pattern, is a crucial factor affecting the transport and dispersion of air pollutants. In this study, taking Beijing as an example, local ozone (O3) accumulation and the transboundary O3 pollution (TOP) patterns during the warm season were explored under different boundary layer ventilation conditions by using integrated objective weather classification, non‐negative matrix factorization, and backward trajectory model. Results show that local sources are a major contributor to O3 pollution in the whole of Beijing, accounting for ∼73.36% of the O3 concentration on average. The local accumulation is mainly facilitated by poor ventilation conditions with weak wind speeds (<3 m/s) under favorable photochemical reaction conditions and abundant precursors, resulting in local O3 events. In contrast, the occurrence of synoptic patterns associated with TOP is more frequent, so the TOP contribution cannot be ignored, especially for the northern regions, where it may exceed 50%. Horizontal wind vector variations play a marked role in driving the TOP, affecting not only the location of the source region and transport pathway but also the transport distance and volume. In addition, a strong nocturnal low‐level jet (LLJ) results in horizontal transport and a temporary rise in O3 concentration, while a daytime LLJ stimulates a peak in O3 concentration over the downwind area, lagging by about 1 hr. Our findings provide new perspectives on the effects of boundary layer ventilation on the regulation of O3 pollution, as well as other air pollutants.

Visible light harvesting alkyne hydrosilylation mediated by pincer platinum complexes

In this manuscript we assess the catalytic properties of pincer platinum complexes in alkyne hydrosilylation either under photo or thermal conditions. The visible light-induced hydrosilylation proved to be more efficient. It can be performed at room temperature and required lower catalyst loadings than that operating under thermal conditions. The platinum complexes play a dual role in photohydrosilylation as serve as a photosensitizer and a catalyst enabling species in bond breaking/forming transformations. In addition, alkyne hydrosilylation is achieved with moderate regio- and stereoselectivity but is enhanced under photocatalytic conditions and in the case of terminal alkynes we have observed the formation of β(Z) products not observable under thermal conditions. Such differences in selectivity constitute an example of stereodivergent catalysis dictated under photochemical or thermal conditions. The selectivity differences are ascribed to a distinctive reaction mechanism for the light- vs thermally-induced process that involve radical or organometallic intermediates, respectively.

Photochemical Oximesulfonylation of Alkenes Using Sulfonyl-oxime-ethers as Bifunctional Reagents.

Utilization of oxime ethers as bifunctional reagents remains unknown. Herein, we present a mechanistically distinct strategy that enables oximesulfonylation of olefins using sulfonyl-oxime-ethers as bifunctional reagents under metal-free photochemical conditions. Via concomitant C-S and C-C bond formation, the process permits incorporation of oxime and sulfonyl groups into olefins in a complete atom-economic fashion, providing rapid access to multi-functionalized β-sulfonyl oxime ethers with good yields and stereoselectivity. The method is amenable to functionalization of complex bioactive molecules and is shown to be scalable. A radical chain mechanism initiated via photochemical Hydrogen Atom Transfer (HAT) mediated N-O bond cleavage is suggested for the process, based on our results on mechanistic investigations.

Photocatalysis aided antibacterial properties of a green graphitic carbon-Cu2O-Ag2O composite: Towards an active non-persistent disinfectant

A novel green graphitic carbon (GGC)-supported Cu2O-Ag2O composite has been successfully synthesized via a facile co-precipitation technique. Its photocatalysis-aided antibacterial activity was evaluated under low-intensity light-emitting diode irradiation. The GGC not only served as an adsorbent matrix but also helped in restricting the rapid recombination of the electron-hole pairs, resulting in 100 % inactivation of 2.3 × 103 CFU mL−1Escherichia coli (E. coli) by the GGC-Cu2O-Ag2O composite within 15 min. In comparison to the composite, pristine GGC and Cu2O-Ag2O resulted in 13.8 and 61.5 % inactivation efficiency, respectively. Under optimal photo-chemical conditions (i.e., light wavelength: 440 to 625 nm; light intensity: 950 W m−2; pH: 6.4; catalyst dosage: 1 g L−1), 6 × 107 CFU mL−1 of E. coli and Enterococcus faecalis were completely inactivated within 45 min and 60 min of irradiation, respectively. The addition of scavengers decreased E. coli inactivation efficiency from 100 % to 61 % implying that holes are the main reactive oxygen species (ROS). The scanning electron micrographs depict prominent ROS-induced morphological changes in the bacterial cells. Further, there was no regrowth of bacteria via photoreactivation and the dark repair process. Moreover, the composite exhibited excellent stability as well as adequate inactivation activity even after three consecutive reuse cycles. The remnants of the composite after exhaustion are likely to be naturally sequestrated without any transport potential and persistence.

Dearomatization of C6 Aromatic Hydrocarbons by Main Group Complexes.

The dearomatization reaction is a powerful method for transformation of simple aromatic compounds to unique chemical architectures rapidly in synthetic chemistry. Over the past decades, the chemistry in this field has evolved significantly and various important organic compounds such as crucial bioactive molecules have been synthesized through dearomatization. In general, photochemical conditions or assistance by transition metals are required for dearomatization of rigid arenes. Recently, main-group elements, especially naturally abundant elements in the Earth's crust, have attracted attention as they have low toxicity and are cost-effective compared to the late transition metals. In recent decades, a variety of low-valent main-group molecules, which enable the activation of stable aromatic compounds under mild conditions, have been developed. This minireview highlights the developments in the chemistry of dearomatization of C6 aromatic hydrocarbons by main-group compounds leading to the formation of seven-membered EC6 (E = main-group elements) ring or cycloaddition products.

Photochemical Oximesulfonylation of Alkenes Using Sulfonyl‐oxime‐ethers as Bifunctional Reagents

Utilization of oxime ethers as bifunctional reagents remains unknown. Herein, we present a mechanistically distinct strategy that enables oximesulfonylation of olefins using sulfonyl‐oxime‐ethers as bifunctional reagents under metal‐free photochemical conditions. Via concomitant C‐S and C‐C bond formation, the process permits incorporation of oxime and sulfonyl groups into olefins in a complete atom‐economic fashion, providing rapid access to multi‐functionalized β‐sulfonyl oxime ethers with good yields and stereoselectivity. The method is amenable to functionalization of complex bioactive molecules and is shown to be scalable. A radical chain mechanism initiated via photochemical Hydrogen Atom Transfer (HAT) mediated N‐O bond cleavage is suggested for the process, based on our results on mechanistic investigations.

Photochemical Aminochlorination of Alkenes without the Use of an External Catalyst.

The niche field of photochemistry offers opportunities that are not found in "traditional" ground state chemical pathways. Aminochlorinated derivatives are an interesting family of 1,2-difunctionalised compounds that provide access to a variety of natural products and pharmaceutical active substances. A practical, catalyst-free chloroamination protocol is described herein, providing access to intermediates of great importance, utilising mild and photochemical reaction conditions (370 nm), where N-chlorosulfonamides are used as both nitrogen and chlorine sources. A wide variety of olefins, decorated with a plethora of functional groups, was tested providing excellent results (28 examples, 18-88 % yield). Mechanistic studies (UV-Vis, control experiments and quantum yield measurement) were also performed.

Hydrosilylation of alkenes with acylhydrosilanes.

Acylsilanes are an important class of organosilicon compounds that have gained increased attention recently. Their various reaction modes in particularly their ability to generate carbenes under thermal or photochemical conditions render them valuable reagents in organic synthesis. In this communication, an unprecedented route towards acylsilanes is reported by introducing the acylsilane moiety directly in a single step through alkene hydrosilylation. In this context, acylhydrosilanes representing a novel class of acylsilanes are introduced and applied as silylation reagents in rhodium catalyzed alkene hydrosilylations to access a great variety of acylsilanes. These reactions occur in good to excellent yields under mild conditions and show high functional group tolerance and atom economy.

Amino Acid Sulfinate Salts as Alkyl Radical Precursors

A general approach to the synthesis of amino acid sulfinate salts from commercially available α-chiral hydroxylated amino acids is reported. These reagents are shown to be valuable precursors to alkyl radicals under mild photochemical oxidation conditions. The photochemically generated amino acid radicals engage readily with alkyl and aryl disulfide radical traps to afford a diverse suite of modified amino acids.

Direct 2-Pyridyl-Alkylation of Benzyne with N-Alkylpyridinium Salts

AbstractA 2-pyridyl-alkylation of benzyne using N-alkylpyridinium salts has been developed under either photochemical or thermal conditions. This metal-free dicarbofunctionalization of benzyne undergoes a de Mayo-type process including cascade dearomative [3+2] cycloaddition and rearomative ring-opening reaction. An electron-deficient N-cyclic alkyl group was necessary. A formal ortho-C–H arylation of pyridine has been achieved.

Frontispiece: Isocyanide‐Based Multicomponent Reactions Promoted by Visible Light Photoredox Catalysis

Isocyanides, thanks to their multiple reactivities, are the main players in multicomponent reactions. While traditional approaches exploit their two-electrons reactivity, they are currently “on stage”, shining in a new light thanks to the development of visible light photochemical reaction conditions. For more information, see the Review by Gian Cesare Tron and Mariateresa Giustiniano et al. (DOI: 10.1002/chem.202203150).

Photoinduced Remote C(sp3)-H Imination Enabled by Vinyl Radical-Mediated 1,5-Hydrogen Atom Transfer.

A vinyl radical-mediated 1,5-hydrogen atom transfer strategy for remote C(sp3)-H imination under visible-light-induced photochemical metal-free conditions afforded diverse γ-imino alkenes with excellent stereoselectivity. Oxime ester-based bifunctional reagents provided not only nucleophilic alkyl radicals for radical addition reactions with electron-deficient alkynes but also long-lived steady-state imine radicals for trapping alkyl radicals following the intramolecular 1,5-hydrogen migration of unstable olefin radicals.