Photocatalysis remains a crucial field to investigate. It is seen as a powerful tool which allows more efficient, relatively greener and economical chemical reactions. Nevertheless, catalysts usually include rare and pricy metals, with supplies being highly dependent on geopolitics. Thus, chemists turned their attention to alternatives such as natural products or organic molecules, conciliating performances, polyvalence and easy access. Among the promising candidates, polyaromatics rise as robust species and precursors of stable radicals which could be used as photocatalysts. Triangulenes are polycyclic hydrocarbons which are expected to shine in multiple fields such as spintronics, energy and data storage or semiconductors. This work reports the studies of photocatalytic reactions including the trioxotriangulene (TOT). Through the studies of three reactions ranging from sulfoxidation to dehalogenation of 4-bromoacetophenone, including the reduction of nitroaromatics, we discuss the performances, the polyvalence and the mechanism of the systems including this triangulene. As a proof of concept, we conclude this work with a description of "a sequential RBG photoredox catalytic system" where the three reactions are successively and selectively performed in a "one-pot" fashion.

A detailed characterization of sulfur compounds in crude oil samples is essential for optimizing their downstream processing. Traditional analytical techniques for this purpose often face challenges such as coelution with other compounds typically present in crude oils, including polycyclic aromatic hydrocarbons, aromatic oxygen heterocycles, aromatic nitrogen heterocycles, and others. In this study, we present a novel method for the group-type analysis and pre-separation of polycyclic aromatic sulfur heterocycles (PASHs) and sulfidic-type polycyclic aromatic compounds (S-PACs) from various crude oil samples. The method utilizes liquid chromatography, incorporating a novel stationary phase in combination with a previously reported stationary phase containing immobilized palladium cations. The system incorporates a Cu-modified mercaptopropyl silica gel stationary phase, used to protect the Pd-based phase from irreversible deactivation while efficiently separating the S-PACs in asingle chromatographic run. The separation process was monitored using ultraviolet and visible spectroscopy, andthe appropriate fractions were collected for further analysis. Real samples, including atmospheric gas oil (AGO), hydrotreated gas oil (HGO), and vacuum gas oil (VGO), were evaluated, with new structures detected in VGO using both gas and liquid chromatography techniques. The work is intended as a proof-of-concept analytical study, focusing on the functional performance of the system rather than providing a detailed physicochemical characterization of the stationary phases.

In recent years, the catalytic activity of scandium triflate Sc(OTf)3 has attracted significant attention due to its robust Lewis acidity and the oxophilicity of Sc3+. These features have led to impressive progress in developing diverse organic reactions, including C-C bond formation. The Sc3+ also facilitates single electron transfer in photoinduced reactions either by coordination to an organophotoredox catalyst, which modifies its redox reactivity, or by the formation of a scandium-superoxide anion complex after electron transfer from a light-absorbing redox-active compound. The prior consideration of Sc3+ as a redox-inactive/innocent metal ion initially hampered the investigation of the possibility of using Sc(OTf)3 as a sole visible light photoredox catalyst. This work demonstrates the use of Sc(OTf)3 as a visible light photocatalyst capable of direct and mild aerobic oxidative C-H functionalisation of aromatic substrates by oxidation of the benzylic position and direct cyanation of the aromatic ring.

Flavins have been established as effective catalysts in oxidative photoredox catalysis. Conversely, their use in reductive photocatalysis remains limited, mainly due to the relatively low stability of the transient flavin radicals (semiquinones), which are used in photoreductions. The fully reduced forms of flavins are also disadvantaged in photocatalysis because they absorb light in the UV rather than in the visible region. In this work, we present a new approach for reductive flavin photocatalysis that utilises a flavin (isoalloxazine) anion derived from the elusive 10-unsubstituted 3,7,8-trimethylisoalloxazine, an unstable tautomer of 3-methyllumichrome. We found the conditions under which this isoalloxazine anion is formed by in situ deprotonation/isomerisation from the readily available 3-methyllumichrome and we subsequently used it as a photoredox catalyst in the reductive dehalogenation of activated bromoarenes and their C-P coupling reaction with trimethyl phosphite to form an arylphosphonate. Steady-state and transient absorption spectroscopy, NMR and cyclic voltammetry investigations, together with quantum chemical calculations, showed that the anion of oxidised isoalloxazine has several advantages, compared to other forms of flavins used in photoreductions, such as high stability, even in the presence of oxygen, an absorption maximum in the visible region, thereby allowing the use of excitation light between 470 and 505 nm, and a relatively long-lived singlet excited-state.

Reductive transformations of substances that are difficult to reduce continue to pose challenges for photoredox catalysis. Promising photoreduction catalysts include flavin and deazaflavin derivatives; however, even their reductive abilities are limited for the range of substrates considered “inert”. In this work, we present 5‐deazaalloxazines, a new group of deazaflavin analogues that are predisposed to catalyze reductions due to their low reduction potential (down to −1.65 V vs. SCE) even in the ground state. We studied three series of 5‐deazaalloxazines ([i] 5‐unsubstituted, [ii] 5‐aryldeazaalloxazines, and [iii] 5‐trifluoromethyl‐5‐deazaalloxazines) to determine their photophysical and electrochemical properties and their ability to participate in model photoreduction reactions. From 31 compounds, we selected 1,3‐dimethyl‐7,8‐dimethoxy‐5‐( o ‐tolyl)‐5‐deazaalloxazine [ 3a( o ‐MePh) ], as it showed, among other things, the highest efficiency in photodehalogenation of p ‐fluoroanisole and was photostable and absorbed in the visible light region, thereby allowing photoreactions using a 400 nm LED. Practical applicability was demonstrated in the C─P coupling reaction of electron‐rich aryl halides (including chloroanisoles and p ‐fluoroanisole) with trimethyl phosphite, providing an arylation reaction to form dimethyl arylphosphonates, and in the release/deprotection of amines from the corresponding tosyl and triflylamides.

Three series of novel deazaflavinum salts differing in their substitutions at positions 5 (R = H, phenyl, or mesityl), 7, and 8 (R = OMe, Me, H, or Cl) were synthesized as potential catalysts of a novel chemoselective visible light-mediated anaerobic oxidation of primary and secondary alcohols to carbonyl compounds. This mild procedure uses acetonitrile as a solvent, which acts simultaneously as a sacrificial electron acceptor (in place of the oxygen usually used in photooxidation reactions), and therefore the reaction does not need any additives. Structure and properties-versus-catalytic activity studies identified 5-mesityl-7,8-dimethoxy-3-methyldeazaflavinium chloride (3a-Cl) as the most potent catalyst. 3a-Cl was effective in non-deuterated acetonitrile (CH3CN), unlike its original 5-phenyl analogue 2a-Cl, which is efficient only in deuterated solvent (CD3CN). This difference arises because the regeneration of the 2a-Cl catalyst is slower in CH3CN than in CD3CN. Our method using the optimized 3a-Cl photocatalyst and CH3CN as a sacrificial oxidant and solvent in one is a useful addition to synthetic organic chemistry. Anaerobic conditions prevent side oxygenation reactions and overoxidations that usually occur in air or oxygen. This property makes this method suitable for dehydrogenations of alcohols that possess additional group(s) sensitive to oxygenation.

The drawbacks commonly observed in synthetic methods for alcohol oxidation often stem from the utilization of complex, toxic, hazardous, or waste-producing oxidants. When sensitive or complex substrates bearing several functional groups are to be transformed, the selectivity of oxidation becomes another significant challenge. Herein, a chemoselective and operationally simple catalyst-free and additive-free method is presented for the aerial oxidation of 1-phenylpropargyl and 1-phenylallyl alcohols to their corresponding ketones, requiring only a solvent and visible light irradiation. The crucial role of dimethylsulfoxide (DMSO) as the solvent lies in achieving high chemoselectivity. Singlet oxygen, whose formation is photosensitized by the substrate and the product, is captured by DMSO, thereby preventing the undesired over-oxidation that occurs in other solvents. The application of DMSO to protect the substrate against singlet oxygen represents a novel approach that is potentially applicable to other aerobic photocatalytic processes. Irradiation of a solution of an unsaturated benzyl alcohol in DMSO under oxygen leads to ketone formation. Side-oxidations are minimized, thanks to the protective role of the DMSO solvent against singlet oxygen. No catalyst or additives are required.

Nitroalkane oxidases (NAOs) are flavoenzymes that catalyse the oxidation of nitroalkanes to their corresponding carbonyl compounds while producing nitrite anions. Herein, we present an artificial catalytic system using flavins or ethylene-bridged flavinium salts that works via an NAO-like process. Under conditions optimised in terms of solvent, base, temperature and oxygen pressure, primary nitroalkanes were transformed to aldehydes. In our system, aldehydes immediately reacted with other nitroalkane molecules to form β-nitroalcohols. The reduced flavin catalyst was re-oxidised by oxygen. An alternative mechanism towards β-nitroalcohols via 5-(2-nitrobutyl)-1,5-dihydroflavin was suggested through quantum chemical calculations and by trapping and characterising this dihydroflavin intermediate. Interestingly, 5-(2-nitrobutyl)-1,5-dihydroflavin is an analogue of the flavin adenine dinucleotide adduct previously observed in an NAO X-ray structure. In both mechanistic pathways, flavin-5-iminium species is formed by nitroalkanide addition to flavin. This process represents flavin-based umpolung of an original donor to an acceptor.

Catalyst recovery is an integral part of photoredox catalysis. It is often solved by adding another component-a sacrificial agent-whose role is to convert the catalyst back into its original oxidation state. However, an additive may cause a side reaction thus decreasing the selectivity and overall efficiency. Herein, we present a novel approach towards chemoselective photooxidation reactions based on suitable solvent-acetonitrile acting simultaneously as an electron acceptor for catalyst recovery, and on anaerobic conditions. This is allowed by the unique properties of the catalyst, 7,8-dimethoxy-3-methyl-5-phenyl-5-deazaflavinium chloride existing in both strongly oxidizing and reducing forms, whose strength is increased by excitation with visible light. Usefulness of this system is demonstrated in chemoselective dehydrogenations of 4-methoxy- and 4-chlorobenzyl alcohols to aldehydes without over-oxidation to benzoic acids achieving yields up to 70 %. 4-Substituted 1-phenylethanols were oxidized to ketones with yields 80-100 % and, moreover, with yields 31-98 % in the presence of benzylic methyl group, diphenylmethane or thioanisole which are readily oxidized in the presence of oxygen but these were untouched with our system. Mechanistic studies based on UV-Vis spectro-electrochemistry, EPR and time-resolved spectroscopy measurements showed that the process involving an electron release from an excited deazaflavin radical to acetonitrile under formation of solvated electron is crucial for the catalyst recovery.

We report an effective, operationally simple, and environmentally friendly system for the synthesis of tertiary amides by the oxidative coupling of aromatic or aliphatic aldehydes with amines mediated by riboflavin tetraacetate (RFTA), an inexpensive organic photocatalyst, and visible light using oxygen as the sole oxidant. The method is based on the oxidative power of an excited flavin catalyst and the relatively low oxidation potential of the hemiaminal formed by amine to aldehyde addition.

The Cover Feature shows the photocatalytic oxidative cycloelimination of a coumarin dimer by a flavinium salt inspired by the light-induced photolyase DNA repair reaction in nature. Light absorption and subsequent intersystem crossing (isc) enable diffusion-controlled electron transfer (eT) from the coumarin dimer to the excited triplet flavinium salt. The oxidized coumarin dimer dissociates and back electron transfer (beT) completes the photocatalytic cycle. More information can be found in the Full Paper by R. Cibulka, R. Kutta, and co-workers.

Flavinium salts are frequently used in organocatalysis but their application in photoredox catalysis has not been systematically investigated to date. We synthesized a series of 5-ethyl-1,3-dimethylalloxazinium salts with different substituents in the positions 7 and 8 and investigated their application in light-dependent oxidative cycloelimination of cyclobutanes. Detailed mechanistic investigations with a coumarin dimer as a model substrate reveal that the reaction preferentially occurs via the triplet-born radical pair after electron transfer from the substrate to the triplet state of an alloxazinium salt. The very photostable 7,8-dimethoxy derivative is a superior catalyst with a sufficiently high oxidation power (E*=2.26 V) allowing the conversion of various cyclobutanes (with E-ox up to 2.05 V) in high yields. Even compounds such as all-trans dimethyl 3,4-bis(4-methoxyphenyl)cyclobutane-1,2-dicarboxylate can be converted, whose opening requires a high activation energy due to a missing pre-activation caused by bulky adjacent substituents in cis-position.

The aim of the present study is to describe the mechanism controlling heat-induced formation of sunflower oil triacylglycerol and fatty acid methyl ester oligomers. The unique combination of high-performance size-exclusion chromatography with hyphenated electrospray ionization mass spectrometry (MS), atmospheric pressure chemical ionization-MS, and high-temperature gas chromatography-MS techniques allows differentiating between radical coupling species and Diels-Alder cycloadducts. Targeted analysis of thermally degraded sunflower oils confirms the exact structures of various acyclic oligomers accompanied by less-abundant products of pericyclic transformations. A series of model experiments simulate the impact of dienophile nature on the course of Diels-Alder reactions. Thus, alpha-tocopherylquinone, delta-tocopherylquinone, and methyl-(E)-11-oxoundec-9-enoate are synthesized as naturally occurring dienophiles bearing electron-withdrawing groups. The geometry of poor dienophiles does not affect concerted cyclization, while the structure of electron deficient dienophiles can overcome low reactivity. Practical Application: In the absence of oxygen, heat-induced degradation of polyunsaturated triacylglycerols proceed predominantly via a radical pathway, whereas concerted reactions represent minor mechanisms. Sunflower oil triacylglycerol molecules in the system without propagation stage can be effectively protected by natural and/or synthetic antioxidants. Application of chelates is also recommended. However, antioxidant-derived quinones, such as alpha-tocopherylquinone, can enter the Diels-Alder reaction even more easily than dienophiles without electron-withdrawing groups. Unsaturated core aldehydes possess the same reactivity. Examination of the mechanism controlling high-temperature degradation of triacylglycerols is especially important for processing engineers in edible oil refineries and food technologists. New perspective may help them to minimize undesirable changes in polyunsaturated species.

New flavin‐based photocatalytic systems used for chemoselective aerobic visible‐light oxidations have been developed by tuning the flavin structure and reaction conditions. 1,3‐Dimethyl‐7‐trifluoromethylalloxazine ( 2 ) and 10‐butyl‐3‐methyl‐7‐trifluoromethylisoalloxazine ( 3 ) were shown to mediate the selective oxidation of benzyl alcohols to form aldehydes in the presence of Cs 2 CO 3 . Flavin 3 was superior in the oxidation of toluene derivatives to form aldehydes in the presence of trifluoroacetic acid. On the other hand, photooxidations provided by ethylene‐bridged quaternary flavinium salt 1 gave the corresponding carboxylic acids. The usefulness of the developed catalytic systems using 1 – 3 was also demonstrated in the oxidation of secondary benzylic and aliphatic alcohols, and benzylic methylene groups to form the corresponding ketones. The systems have the advantage of a broad substrate scope and metal‐free conditions, which distinguish them from the previously reported flavin photooxidation reactions.

Nitrosobenzene has been demonstrated to participate in the Mitsunobu reaction in an analogous manner to dialkyl azodicarboxylates. The protocol using nitrosobenzene and triphenylphosphine (1:1) under mild conditions (0 °C) provides the ester derivatives of aliphatic and aromatic acids using various alcohols in moderate yield and with good enantioselectivity, giving the desired products predominantly with an inversion of configuration. The proposed mechanism, which is analogous to that observed using dialkyl azodicarboxylates, involves a nitrosobenzene-triphenylphosphine adduct and an alkoxytriphenylphosphonium ion and was supported by density functional theory calculations, 31P NMR spectroscopy, and experiments conducted with isotopically labeled substrates.

Bio-oil upgrading through its hydrodeoxygenation (HDO) using sulfided catalysts has attracted significant attention because of its potential to provide advanced biofuels. Although many studies have been undertaken, a detailed understanding of the changes in the chemical composition on the molecular level that would allow the better design of catalysts for bio-oil upgrading is still insufficient. Therefore, we have subjected straw bio-oil and products obtained from its hydrotreatment over a broad range of experimental conditions to a detailed quantitative chemical analysis. Most of the volatile compounds were quantified by GC-MS. Among them, 115 compounds were quantified directly (i.e., using the appropriate standards) and more than 100 indirectly (i.e., based on their structural similarity with corresponding standards). Moreover, the total concentrations of carboxylic acids, carbonyls and phenols were quantified by the carboxylic acid number (CAN), Faix, and Folin-Ciocalteu methods, respectively, to obtain complementary and supporting information on the chemical composition to the GC-MS data. The detailed quantification of most volatile compounds in the feed and the products allowed us to create a reactivity order of the oxygen-containing functional groups present and to understand the origin of some of the compounds. On the basis of the results, the upgrading of straw bio-oil from ablative fast pyrolysis at 340 degrees C and 4 MPa seems to be optimal when evaluating the severity of the reaction conditions and hydrogen consumption, on the one hand, and the products quality, on the other hand. This provides a good starting point for further catalyst development and optimization allowing the long-term upgrading of the bio-oil for obtaining petroleum refinery-compatible feedstock.

Triphenylphosphine (Ph3P) activated by various electrophiles (e.g., alkyl diazocarboxylates) represents an effective mediator of esterification and other nucleophilic substitution reactions. We report herein an aza-reagent-free procedure using flavin catalyst (3-methyl riboflavin tetraacetate), triphenylphosphine, and visible light (448 nm), which allows effective esterification of aromatic and aliphatic carboxylic acids with alcohols. Mechanistic study confirmed that photoinduced electron transfer from triphenylphosphine to excited flavin with the formation of Ph3P˙+ is a crucial step in the catalytic cycle. This allows reactive alkoxyphosphonium species to be generated by reaction of an alcohol with Ph3P˙+ followed by single-electron oxidation. Unexpected stereoselectivity control by the solvent was observed, allowing switching from inversion to retention of configuration during esterification of (S)- or (R)-1-phenylethanol; for example with phenylacetic acid, the ratio shifting from 10 : 90 (retention : inversion) in trifluoromethylbenzene to 99.9 : 0.1 in acetonitrile. Our method uses nitrobenzene to regenerate the flavin photocatalyst. This new approach to flavin re-oxidation has also been successfully proved in benzyl alcohol oxidation, which is a "standard" process among flavin-mediated photooxidations.

Pyrolysis bio-oils could be used in the future as biofuels or as a source of valuable oxygen-containing chemicals. To facilitate efficient exploitation of bio-oils, a detailed understanding of their structure is necessary. Over the past decade, petroleomic analysis has been widely applied to characterize pyrolysis bio-oils from the lignocellulosic biomass. Typically, a petroleomic analysis has been performed using high-resolution mass spectrometry (HRMS). HRMS has enabled the researchers to determine the molecular weights and molecular formulas of thousands of less volatile and nonvolatile, high-molecular-weight bio-oil compounds to obtain structural information that cannot be obtained using any other method. Here, we discuss the theoretical principles of HRMS and present an overview of the investigations regarding the petroleomic characterization of pyrolysis bio-oils and their key findings. In addition, this review outlines the current knowledge of the structure of bio-oil compounds detectable by HRMS. This could help us to understand the chemical composition of bio-oils in more detail and facilitate the design of processes for bio-oil upgrading and further utilization.

Pyrolysis bio-oils have great potential for the future use as biofuels and source of oxygenated chemicals. To optimize a pyrolysis process, detailed knowledge about the chemical composition of bio-oils is necessary. In recent years, high-resolution mass spectrometry (HRMS) has successfully been used to the characterization of pyrolysis bio-oils from lignocellulosic biomass. This method enabled to detect thousands of semivolatile and nonvolatile, high-molecular-weight bio-oil compounds and provided partial information about their structure. In this work, we used high-resolution orbitrap mass spectrometry to characterize semivolatile and nonvolatile, high-molecular-weight compounds of four bio-oils obtained from the ablative flash pyrolysis of different biomass sources. Before the analyses of these bio-oils, we analyzed model bio-oil compounds and commercially available bio-oil from fast pyrolysis of wood using positive-ion and negative-ion electrospray (ESI) and positive-ion and negative-ion atmospheric pressure chemical ionization (APCI) orbitrap mass spectrometry and compared the results. Based on this comparison, a combination of negative-ion ESI and APCI was found to be well suited for the characterization of pyrolysis bio-oils; these techniques were thus used for the study of bio-oils from different biomass sources and the obtained results were compared. In the studied bio-oils, mostly compounds with 1-8 oxygen atoms per molecule were detected and their degree of unsaturation (DBE) was about 1-10 (negative ion ESI) and 1-17 (negative-ion APCI), respectively. Among the studied bio-oils, the differences were observed mostly in abundances of their major compounds (compound classes). The analyses of model bio-oil compounds brought valuable information about their behavior during the HRMS characterization of bio-oils. The presented results could help to improve the understanding of bio-oil composition and HRMS characterization of bio-oils and facilitate their further utilization. (C) 2017 The Authors. Published by Elsevier B.V.