Visible-light-promoted aerobic oxidative cyclization to access 1,3,4-oxadiazoles from aldehydes and acylhydrazides

Smartphone-Based Quantitative Measurement of Cu2+: Fluorescent Turn-on Chemosensor via Radical Cation Formation

We report a unique radical cation formation-based fluorescent chemosensor (E)-N'-(4-(diphenylamino)benzylidene)thiophene-2-carbohydrazide (DBTC) that quantitatively determines Cu2+ based on the RGB model using a smartphone. DBTC exhibited a weak turquoise fluorescence due to fluorescence suppression by amide isomerization. When Cu2+ was added into DBTC, it showed strong light blue fluorescence with a high quantum yield ([Formula: see text] = 0.470). The detection limit of Cu2+ was determined to be 0.40 µM at the concentration range of 0-7.5 µM. In addition, the detection mechanism of DBTC for Cu2+ was demonstrated to be an oxidative cyclization reaction through 1H NMR titration, ESI-MS analysis, and DFT calculation. Remarkably, DBTC could be applied to the quantitative measurement of Cu2+ using a smartphone and RGB analysis. The detection limit was calculated to be 0.05 µM, which is the lowest detection limit among chemosensors that could detect Cu2+ through smartphone-based fluorescence measurements. Additionally, spike and recovery experiments conducted with different concentrations of Cu2+ showed good recovery values. DBTC exhibited its potential as a chemosensor for determining Cu2+ through the application of a smartphone-based platform capable of real-time monitoring.

Photoredox/Cobalt-Catalyzed Cascade Oxidative Synthesis of 2,5-Disubstituted 1,3,4-Oxadiazoles under Oxidant-Free Conditions

An efficient oxidant-free, photoredox-mediated cascade cyclization strategy for the synthesis of 1,3,4-oxadiazoles by using an organo acridinium photocatalyst and a cobaloxime catalyst has been developed. Various acylhydrazones have been transformed into the corresponding 1,3,4-oxadiazole products in up to 96% yield, and H2 is the only byproduct. Mechanistic experiments and density functional theory (DFT) calculation studies indicate carbon-centered radicals rather than oxygen-centered radicals as π-radicals produced by the oxidation of photoexcited Mes-Acr+* along with deprotonation, which is responsible for this transformation. The practical utility of this method is highlighted by the one-pot gram-scale synthesis starting directly from commercially available aldehydes and acylhydrazides.

Flow synthesis of oxadiazoles coupled with sequential in-line extraction and chromatography

An efficient continuous flow process is reported for the synthesis of various 1,3,4-oxadiazoles via an iodine-mediated oxidative cyclisation approach. This entails the use of a heated packed-bed reactor filled with solid K2CO3 as a base. Using DMSO as solvent, this flow method generates the target heterocycles within short residence times of 10 minutes and in yields up to 93%. Scale-up of this flow process was achieved (34 mmol/h) and featured an integrated quenching and extraction step. Lastly, the use of an automated in-line chromatography system was exploited to realise a powerful flow platform for the generation of the heterocyclic targets.

Photocatalytic cross-dehydrogenative coupling reaction toward the synthesis of N,N-disubstituted hydrazides and their bromides

An efficient strategy for the divergent synthesis of N,N-disubstituted hydrazides and their bromides is reported through photoredox-catalytic cross-dehydrogenative coupling of N,N-disubstituted hydrazines and aldehydes.

Retraction: Eosin Y catalysed photoredox synthesis: a review

Retraction of ‘Eosin Y catalysed photoredox synthesis: a review’ by Vishal Srivastava et al., RSC Adv., 2017, 7, 31377–31392, https://doi.org/10.1039/C7RA05444K.

Cobalt(II) nitrate promoted cyclization of benzoyl hydrazone for the synthesis of 2,5-diphenyl-1,3,4-oxadiazole derivatives

A Co(NO3)2 method to promote cyclization of benzoyl hydrazone for the formation of 2,5-diphenyl-1,3,4-oxadiazoles has been developed. The reaction proceeded smoothly and was promoted by Co(NO3)2 under air at 110 °C in DCE; 16 examples of products were obtained.

Additive-free photo-mediated oxidative cyclization of pyridinium acylhydrazones to 1,3,4-oxadiazoles: solid-state conversion in a microporous organic polymer and supramolecular energy-level engineering

We discovered the efficient catalyst-free, photo-mediated oxidative cyclization reaction of bis-p-pyridinium benzoyl hydrazone (BH1) to 2-pyridinium-5-phenyl-1,3,4-oxadiazoles.

Polymer-supported eosin Y as a reusable photocatalyst for visible light mediated organic transformations

Rapid synthesis of highly stable polymer-supported eosin Y for visible light-driven photoxidation of thioethers to sulfoxides and phenylboronic acids to phenols.

Photocatalysis in organic and polymer synthesis

This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.

Synthesis of 2,2-difluoro-homoallylic alcohols via ring-opening of gem-difluorocyclopropane and aerobic oxidation by photo-irradiation in the presence of an organic pigment

The aerobic oxidation took place after the visible light-mediated ring-opening reaction of gem-difluorocyclopropane in the presence of an organic dye and amine to furnish 2,2-difluoro-homoallylic alcohols in good yields.

Visible light photocatalysis with CBr4: a highly selective aerobic photooxidation of methylarenes to aldehydes

An efficient, operationally simple, metal-free strategy for the selective oxidation of methylarenes to aromatic aldehydes through visible light photocatalysis employing CBr₄ and molecular oxygen is reported.

N-Hydroxyphthalimide: a new photoredox catalyst for [4+1] radical cyclization of N-methylanilines with isocyanides

A highly regioselective synthesis of 3-iminodihydroindoles using N-methylanilines and isocyanides at room temperature is described. The protocol utilizes N-hydroxyphthalimide as a new single electron transfer photoredox catalyst.

Recent synthetic additions to the visible light photoredox catalysis toolbox

The boom in visible light photoredox catalysis (VLPC) research has demonstrated that this novel synthetic approach is here to stay. VLPC enables reactive radical intermediates to be catalytically generated at ambient temperature, a feat not generally allowed through traditional pyrolysis- or radical initiator-based methodologies. VLPC has vastly extended the range of substrates and reaction schemes that have been traditionally the domain of radical reactions. In this review the photophysics background of VLPC will be briefly discussed, followed by a report on recent inroads of VLPC into decarboxylative couplings and radical C-H functionalization of aromatic compounds. The bulk of the review will be dedicated to advances in synergistic catalysis involving VLPC, namely the combination of photoredox catalysis with organocatalysis, including β-functionalization of carbonyl groups, functionalization of weak aliphatic C-H bonds, and anti-Markovnikov hydrofunctionalization of alkenes; dual catalysis with gold or with nickel, photoredox catalysis as an oxidation promoter in transition metal catalysis, and acid-catalyzed enantioselective radical addition to π systems.

Visible-light initiated oxidative cyclization of phenyl propiolates with sulfinic acids to coumarin derivatives under metal-free conditions

A visible-light initiated oxidative cyclization of phenyl propiolates with sulfinic acids has been developed at room temperature under metal-free conditions.