Photoredox catalytic transformations of pyrazoles

Redoks reakcije, ki potečejo po absorpciji fotonov, omogočajo pripravo in funkcionalizacijo heterocikličnih spojin pod blagimi reakcijskimi pogoji. Od teh smo se osredotočili na pretvorbe spojin s pirazolnim skeletom. Pod vidno svetlobo v prisotnosti fotoredoks katalizatorja eozina ter kisika kot oksidanta smo uspešno oksidirali 1-substituirane pirazolidin-3-one. Nastali azometin imini so uporabni gradniki v sintezi heterocikličnih spojin, kar smo pokazali z enolončno reakcijo fotokatalizirane oksidacije ter cikloadicije nastalih dipolov z inoni. Po optimizaciji reakcijskih pogojev smo tako sintetizirali nabor azometin iminov ter pirazolo[1,2-a]pirazolov. Uspešno smo oksidirali tudi 1-arilpirazolidin-3-one do pirazol-3-onov. Kot stranske produkte oksidacije pirazolidin-3-onov smo identificirali aldehide ter ketone, ki nastanejo iz substituenta na mestu N1 tvorjenih azometin iminov. Pirazolo[1,2-a]pirazoli absorbirajo vidno svetlobo, pri čemer tudi fluorescirajo. Izmerili smo nekatere optične lastnosti teh bicikličnih spojin ter prvič pripisali izpostavljenost svetlobi kot vzrok za njihovo nestabilnost v raztopini, kjer pride do odpiranja enega od pirazolnih obročev. Z optimizacijo reakcijskih pogojev smo dosegli kar 6 selektivnih pretvorb v različno substituirane pirazole. Potek reakcij je odvisen od substituentov na pirazolo[1,2-a]pirazolnem obroču ter dodanih reagentov. Predstavljene raziskave so ključnega pomena za uporabo pirazolo[1,2-a]pirazolov kot fluorescentnih označevalcev. Pirazole smo poskusili sulfonirati s fotoredoks katalizirano radikalsko adicijo. Pri tem smo uporabili sulfonil kloride, prav tako pridobljene pod fotoredoks katalitskimi pogoji. Izbranih heterocikličnih spojin nismo uspeli funkcionalizirati. Raziskavo smo zaključili z nepričakovano odkrito dvostopenjsko tvorbo dialkenil sulfonov z zaporednima radikalskima adicijama med triflil kloridom ter arilacetileni. Transformacija kljub znanim sorodnim reakcijam med triflil kloridom ter alkeni še ni bila raziskana.Redox reactions, which take place after photon absorption, enable preparation and functionalization of heterocyclic compounds under mild reaction conditions. Out of those, we focused our attention on the transformations of pyrazole-based compounds. Under visible light irradiation in the presence of eosin, a photoredox catalyst, and oxygen, an oxidant, 1-substituted pyrazolidin-3-ones were successfully oxidized. So formed azomethine imines are useful building blocks in the synthesis of heterocyclic compounds, as demonstrated by a one-pot photocatalyzed oxidation and cycloaddition of ensuing dipoles with ynones. After optimization of reaction conditions, a plethora of azomethine imines and pyrazolo[1,2-a]pyrazoles were synthesized. 1-Arylpyrazolidin-3-ones were also successfully oxidized to 1-arylpyrazol-3-ones. Aldehydes and ketones, which are formed from N1 substituent of azomethine imines, were identified as side products of pyrazolidin-3-one oxidation. Pyrazolo[1,2-a]pyrazoles are fluorescent compounds, which can absorb visible light. Some of the optical properties of these bicyclic compounds were measured. Their instability in solutions was for the first time attributed to exposure to visible light as pyrazole ring opening takes place. With optimization of reaction conditions, 6 distinct transformations into substituted pyrazoles were achieved. The selectivity of the reactions is dependent on the substituents on the pyrazolo[1,2-a]pyrazole and the added reagents. The combined research proved crucial in the use of pyrazolo[1,2-a]pyrazoles as fluorescent markers. Attempts to sulfonate pyrazoles with photoredox catalyzed radical addition with sulfonyl chlorides, which were synthesized under photoredox catalyzed conditions as well, were unsuccessful. The research was concluded with an unexpected two-step formation of dialkenyl sulfones with sequential radical additions between triflyl chloride and arylacetylenes. This transformation has not yet been explored despite already investigated reactions involving alkenes

Photoinduced Ring Opening of Methyl 1-Aryl-5-oxo-6,7-dihydro-1<i>H</i>,5<i>H</i>-pyrazolo[1,2-<i>a</i>]pyrazole-2-carboxylates in the Presence of Diaryl Disulfides

Among the methods used for the synthesis of functionalized heterocyclic compounds, photochemistry has gained immense popularity due to the reactivity of intermediates in photoinduced reactions. In this study, we report on the effect of diaryl disulfides as hydrogen atom transfer catalysts on the photoinduced transformations of pyrazolo[1,2-a]pyrazolones. After excitation with visible light, these compounds are susceptible to C–N bond cleavage, followed by intermolecular hydrogen atom abstraction. By modifying the reaction conditions, we have developed two novel methods for the synthesis of highly substituted pyrazoles

Electrochemical stability and degradation mechanisms of commercial carbon-supported gold nanoparticles in acidic media

Electrochemical stability of a commercial Au/C catalyst in an acidic electrolyte has been investigated by an accelerated stress test (AST), which consisted of 10,000 voltammetric scans (1 V/s) in the potential range between 0.58 and 1.41 VRHE. Loss of Au electrochemical surface area (ESA) during the AST pointed out to the degradation of Au/C. Coupling of an electrochemical flow cell with ICP-MS showed that only a minor amount of gold is dissolved despite the substantial loss of gold ESA during the AST (∼35% of initial value remains at the end of the AST). According to the electrochemical mass spectrometry experiments, carbon corrosion occurs during the AST but to a minor extent. By using identical location scanning electron microscopy and identical location transmission electron microscopy, it was possible to discern that the dissolution of small Au particles (<5 nm) within the polydisperse Au/C sample is the main degradation mechanism. The mass of such particles gives only a minor contribution to the overall Au mass of the polydisperse sample while giving a major contribution to the overall ESA, which explains a significant loss of ESA and minor loss of mass during the AST. The addition of low amounts of chloride anions (10-4 M) substantially promoted the degradation of gold nanoparticles. At an even higher concentration of chlorides (10-2 M), the dissolution of gold was rather effective, which is useful from the recycling point of view when rapid leaching of gold is desirable. © 2021 American Chemical Society

Eosin Y-catalyzed visible-light-mediated aerobic transformation of pyrazolidine-3-one derivatives

By utilizing an underexplored reactivity of N1-substituted pyrazolidine-3-ones, we developed a visible-light-induced aerobic oxidation of N1-substituted pyrazolidine-3-one derivatives yielding the corresponding azomethine imines. The resulting azomethine imines can be further reacted with ynones in situ under copper catalyzed [3 + 2] cycloaddition reaction conditions yielding the corresponding pyrazolo[1,2-a]pyrazoles in good yields. The methodology can be extended to other 1-aryl-substituted pyrazolidinones which undergo endocyclic oxidation deriving the corresponding pyrazolones as single products

Eosin Y-Catalyzed Visible-Light-Mediated Aerobic Transformation of Pyrazolidine-3-One Derivatives

By utilizing an underexplored reactivity of N1-substituted pyrazolidine-3-ones, we developed a visible-light-induced aerobic oxidation of N1-substituted pyrazolidine-3-one derivatives yielding the corresponding azomethine imines. The resulting azomethine imines can be further reacted with ynones in situ under copper catalyzed [3 + 2] cycloaddition reaction conditions yielding the corresponding pyrazolo[1,2-a]pyrazoles in good yields. The methodology can be extended to other 1-aryl-substituted pyrazolidinones which undergo endocyclic oxidation deriving the corresponding pyrazolones as single products

Copper- and Photoredox-Catalyzed Cascade to Trifluoromethylated Divinyl Sulfones

A Cu(I)-photoredox-catalyzed trifluoromethylchlorosulfonylation reaction of terminal alkynes under visible light conditions was developed, giving rise to trifluoromethyl-substituted vinylsulfonyl chlorides, which can subsequently be coupled to a second alkyne under photocatalytic conditions. The transformation proceeds with high regio- and stereoselectivity and can be applied to aliphatic and aromatic alkynes with various functional groups. Trifluoromethyl-substituted divinyl sulfones prepared by this protocol can be readily used as synthetically valuable intermediates as demonstrated with various postmodification examples

Catalytic photoredox C–H arylation of 4-oxo-4H-pyrido[1,2-a]pyrimidine-3-diazonium tetrafluoroborates and related heteroaryl diazonium salts

Irradiation of mixtures of title diazonium salts and heteroarenes with green light (510 nm) in the presence of eosin Y disodium salt (EY-Na$_2$) as a photocatalyst furnished the corresponding arylation products in 8−63% yields. The proposed photocatalytic cycle is analogous to that proposed previously for closely related photoredox C−H arylations with aryl diazonium salts as aryl radical sources. This method has a broad substrate scope and represents a metal-free alternative for the synthesis of 3-heteroaryl-substituted 4H-quinolizin-4-ones and azino- and azolo-fused pyrimidones with a bridgehead nitrogen atom

2-acyl-1-aryl-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazole derivatives

A series of thirteen 2-acyl-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazole derivatives were synthesized by using a copper-catalyzed azomethine-imine-alkyne cycloaddition as a key step. These bimane-related fluorescent compounds with λex = 338–400 nm and λem = 522–644 nm exhibit interesting and promising optical properties, including a remarkably large Stokes shift (150–268 nm), positive solvatochromism, and strong dependence of emission intensity on pH in a range of pH 2–10. The benzotriazolide- and N-succinimidyl ester-functionalized compounds were successfully used as fluorescent probes for protein labelling