Dichloromethylation of enones by carbon nitride photocatalysis

Small organic radicals are ubiquitous intermediates in photocatalysis and are used in organic synthesis to install functional groups and to tune electronic properties and pharmacokinetic parameters of the final molecule. Development of new methods to generate small organic radicals with added functionality can further extend the utility of photocatalysis for synthetic needs. Herein, we present a method to generate dichloromethyl radicals from chloroform using a heterogeneous potassium poly(heptazine imide) (K-PHI) photocatalyst under visible light irradiation for C1-extension of the enone backbone. The method is applied on 15 enones, with γ,γ-dichloroketones yields of 18–89%. Due to negative zeta-potential (−40 mV) and small particle size (100 nm) K-PHI suspension is used in quasi-homogeneous flow-photoreactor increasing the productivity by 19 times compared to the batch approach. The resulting γ,γ-dichloroketones, are used as bifunctional building blocks to access value-added organic compounds such as substituted furans and pyrroles

Visible Light Driven Reductive (Cyclo)Dimerization of Chalcones Over Heterogeneous Carbon Nitride Photocatalyst

Single-electron reduction of chalcones to the respective radical anions is a useful technique to activate these molecules toward subsequent transformations. Herein, a metal-free photocatalytic version of chalcones reduction in the presence of triethanolamine as a convenient electron donor and using heterogeneous carbon nitride visible-light photocatalyst is presented. The reaction proceeds via a long-lived radical species of the heterogeneous organic semiconductor. The scope of the reaction was studied, and regioselectivity of the chalcone radicals coupling was investigated. (1) Ten chalcones gave selectively polysubstituted cyclopentanoles with 31–73% isolated yield; (2) Two chalcones bearing electron-donor groups, 4-MeOC6H4 and 2-thienyl, gave selectively the β-ketodienes in 42% and 53% isolated yield, respectively; (3) Pentafluorophenyl substituted chalcone gave exclusively the product of the radicals coupling followed by hydrogen transfer from triethanolamine–hexane-1,6-dione in 65% isolated yield. Reductive cross cyclodimerization of a mixture of two different chalcones proceeded regioselectively with the formation of one product out of four possible. The mechanism was investigated by cyclic voltammetry and linear sweep voltammetry and suggests that the reaction proceeds through proton-coupled electron transfer

Controlling the strength of interaction between carbon dioxide and nitrogen-rich carbon materials by molecular design

Thermal treatment of hexaazatriphenylene-hexacarbonitrile (HAT-CN) in the temperature range from 500 °C to 700 °C leads to precise control over the degree of condensation, and thus atomic construction and porosity of the resulting C2N-type materials. Depending on the condensation temperature of HAT-CN, nitrogen contents of more than 30 at% can be reached. In general, these carbons show adsorption properties which are comparable to those known for zeolites but their pore size can be adjusted over a wider range. At condensation temperatures of 525 °C and below, the uptake of nitrogen gas remains negligible due to size exclusion, but the internal pores are large and polarizing enough that CO2 can still adsorb on part of the internal surface. This leads to surprisingly high CO2 adsorption capacities and isosteric heat of adsorption of up to 52 kJ mol−1. Theoretical calculations show that this high binding enthalpy arises from collective stabilization effects from the nitrogen atoms in the C2N layers surrounding the carbon atom in the CO2 molecule and from the electron acceptor properties of the carbon atoms from C2N which are in close proximity to the oxygen atoms in CO2. A true CO2 molecular sieving effect is achieved for the first time in such a metal-free organic material with zeolite-like properties, showing an IAST CO2/N2 selectivity of up to 121 at 298 K and a N2/CO2 ratio of 90/10 without notable changes in the CO2 adsorption properities over 80 cycles

Pеакції N-алкілгідразонів аліфатичних кетонів

For the first time data concerning the reactions of N-alkylhydrazones of aliphatic ketones have been systematized. The possible reaction centres of hydrazones and the main lines of attack for electrophilic and nucleophilic reagents have been distinguished. Differences in the reactivity of N-alkyl and N-arylhydrazones have been studied. The types of tautomerism of hydrazones and their impact on the reactivity of these compounds have been considered. Most reactions occurring on the sp2-hybridized carbon atom lead to formation of azocompounds, which are generally aliphatic radical sources or initiators of radical processes. The interaction of hydrazones with oxidising reagents leading to the corresponding bisazocompounds has been studied. For a number of N-acylation hydrazones the ring-chain tautomerism has been described, as a result these compounds exist in cyclic and acyclic forms. The general methods for construction of heterocyclic compounds based on N-alkylhydrazones of aliphatic ketones due to cyclization of hydrazones with bis-functional reagents on both nitrogen atoms, as well as the nitrogen atom and the carbon atom have been considered. Hydrazone-enhyidrazine tautomerism is inherent to hydrazones of aliphatic ketones with the α-methylene group; it causes a simultaneous attack of dielectrophiles on the sp3-hybridized carbon atom and the nitrogen atom of hydrazone and can be used to construct derivatives of pyridazine and pyrazole, and the use of compounds of phosphorus and of silicon as dielectrophiles results in formation of phosphodiazoles and silapyrazolines, respectively. Reduction reactions for C=N bond of N-alkylhydrazones of aliphatic ketones have been studied.Впервые систематизированы литературные данные, касающиеся реакций N-алкилгидразонов алифа- тических кетонов. Выделены возможные реакционные центры гидразонов и определены основные на- правления атаки электрофильных и нуклеофильных реагентов. Исследованы различия в реакционной способности N-алкил и арилгидразонов. Рассмотрены виды таутомерии гидразонов и их влияние на реакционную способность. Большинство реакций, проходящих по sp2-гибридизированному атому углерода, приводят к образованию азосоединений, которые являются источниками алифатических радикалов или инициаторами радикальных процессов. Изучено взаимодействие гидразонов с окислительными реагентами, которое приводит к соответствующим бисазосоединениям. Для ряда N-ацилированных гидразонов описана кольчато-цепная таутомерия, результатом которой является их существование в циклических и ациклических формах. Рассмотрены общие методы построения гетероциклических соединений на основе N-алкилгидразонов алифатических кетонов за счет циклизации гидразонов с бифункциональными реагентами как по обоим атомам азота, так и по атому азота и углерода. Для гидразонов алифатических кетонов, в которых имеется α-метиленовая группа, присуща гидразон-енгидразинная таутомерия, что обусловливает одновременную атаку биэлектрофилов по sp3-гибридизованому атому углерода и атому азота гидразона и может быть использована для построения производных пиридазина и пиразола, а применение в качестве биэлектрофилов соединений фосфора и кремния приводит к образованию фосфодиазолов и силапиразолинов соответственно. Рассмотрены реакции восстановления связи С=N N-алкилгидразонов кетонов алифатического ряда.Вперше систематизовані літературні дані, що стосуються реакцій N-алкілгідразонів аліфатичних кетонів. Виділені можливі реакційні центри гідразонів та визначені основні напрямки атаки електрофільних та нуклеофільних реагентів. Досліджені відмінності в реакційній здатності N-алкіл- та арилгідразонів. Розглянуті види таутомерії гідразонів та їх вплив на реакційну здатність. Більшість реакцій, що перебігають по sp2-гібридизованому атому вуглецю, приводять до утворення азосполук, які здебільшого є джерелом аліфатичних радикалів або ініціаторами радикальних процесів. Вивчено взаємодію гідразонів з окиснювальними реагентами, яка приводить до відповідних бісазосполук. Для ряду N-ацильованих гідразонів описана кільчато-ланцюгова таутомерія, наслідком якої є їх існування в циклічних та ациклічних формах. Розглянуті загальні методи побудови гетероциклічних сполук на основі N-алкілгідразонів аліфатичних кетонів за рахунок циклізації гідразонів з біфунціональними реагентами як по обох атомах азоту, так і по атому азоту та вуглецю. Для гідразонів аліфатичних кетонів, у яких наявна α-метиленова група, притаманна гідразон-єнгідразинна таутомерія, що зумовлює одночасну атаку біелектрофілів на sp3-гібридизований атом вуглецю та атом азоту гідразону і може бути використана для побудови похідних піридазинів та піразолів, а застосування в ролі біелектрофілів сполук фосфору та силіцію приводить до утворення фосфодіазолів та силапіразолінів відповідно. Розглянуті реакції відновлення С=N зв’язку N-алкілгідразонів кетонів аліфатичного ряду

Highly crystalline poly(heptazine imides) by mechanochemical synthesis for photooxidation of various organic substrates using an intriguing electron acceptor – Elemental sulfur

Low-defect potassium poly(heptazine imide) (PHIK-BM) was engineered for application in photocatalytic oxidation of organic substrates. Mechanochemical pretreatment of a mixture of 5-aminotetrazole in LiCl/KCl eutectics using high-energy ball milling afforded a highly homogeneous mixture that, upon sequential thermolysis at 600 °C, gave nanosized particles of PHIK–BM. The photocatalytic activity of the free-standing PHIK–BM plates was assessed in the oxidation of benzyl alcohol to benzaldehyde under visible light irradiation using elemental sulfur as an electron acceptor. Both quantitative conversion (gt;99) of benzyl alcohol and selectivity (gt;98) with respect to benzaldehyde were achieved. The developed method was extended to aliphatic alcohol oxidation coupled with multicomponent Hantzsch 1,4-dihydropyridine synthesis. These 1,4-dihydropyridines were also photocatalytically oxidized by PHIK–BM to the corresponding substituted pyridines, with very good yields and under mild metal-free conditions

A "waiting" carbon nitride radical anion: a charge storage material and key intermediate in direct C-H thiolation of methylarenes using elemental sulfur as the "S"-source

Potassium poly(heptazine imide), a carbon nitride based semiconductor with high structural order and a valence band potential of +2.2 V vs. NHE, in the presence of hole scavengers and under visible light irradiation gives the corresponding polymeric radical anion, in which the specific density of unpaired electrons reaches 112 [small mu ]mol g-1. The obtained polymeric radical anion is stable under anaerobic conditions for several hours. It was characterized using UV-vis absorption, time resolved and steady state photoluminescence spectra. The electronic structure of the polymeric radical anion was confirmed by DFT cluster modelling. The unique properties of potassium poly(heptazine imide) for storing charges were employed in visible light photocatalysis. A series of substituted dibenzyldisulfanes was synthesized in 41-67% yield from the corresponding methylarenes via cleavage of the methyl C-H bond under visible light irradiation and metal-free conditions

Visible-light-Driven Photochemical Activation of sp³ C-H Bond for Hemiaminal Formation

Photochemical synthesis of organic chemicals has attracted a massive revitalized research interest. A metal-free photochemical system with carbon tetrabromide (CBr4) is here developed for the synthesis of hemiaminal compounds. In this reaction system, the sp3 C-H bond of the N-alkyl group in the amide is photochemically activated, producing an ionic iminium species. The hemiaminal compounds are produced by nucleophilic attack of the iminium ion by alcohol in the presence of base. This photochemical conversion system shows high yield and broad substrate scope