Iron-Catalyzed Annulation of 1,2-Diamines and Diazodicarbonyls for Diverse and Polyfunctionalized Quinoxalines, Pyrazines, and Benzoquinoxalines in Water

Activation of enamine by photoexcited organocatalyst assisted singlet oxygen: synthesis of oxazoles and quinoxalines

Herein, a novel transition-metal-free thiol-based donor-acceptor organophotocatalyst-assisted, singlet-oxygen-mediated tandem oxidative cyclization for the synthesis of substituted oxazoles in moderate-to-good yields is described. The developed method demonstrates applicability for the synthesis of various substituted quinoxalines in good-to-excellent yields. The metal-free methodology shows a practical route for the synthesis of oxazole and quinoxaline derivatives, which are privileged moieties prevalent in various biologically active compounds and natural products. To the best of our knowledge, both the thiol photocatalyst and synthesis of oxazoles by visible-light irradiation are reported for the first time.

Rhodium(II)-Catalyzed Alkynyl Carbene Insertion into N-H Bonds

The first insertion of an alkynyl carbene into N-H bonds under Rh-catalysis is developed. Alkynyl hydrazone carboxylates are used as donor-acceptor carbene precursors and are exquisitely inserted into the N-H bonds of various amines, amides, and 1,2-diamines. A wide variety of 3-alkynyl 3,4-dihydroquinoxalin-2(1H)-ones and densely functionalized α-alkynyl α-amino esters are obtained in good to excellent yields. Further, chemoselective N-H insertion reactions, mechanistic studies, and various synthetic transformations for obtaining valuable heterocycles are demonstrated.

Facile access to C-substituted piperazin-2-ones and mianserin derivative enabled by chemoselective carbene insertion and cyclization cascade

The chemoselective N-H insertion of unsymmetrical diamines into carbene is a longstanding challenge. A simple copper-catalyzed strategy for synthesizing C-substituted piperazinones is described, employing easily accessible diazo compounds and 1,2-diamines. The reaction proceeded via chemo-selective carbene insertion at the comparatively less nucleophilic amine, followed by instantaneous cyclization. The protocol was further extended to access NH-free piperazinone, and the synthesis of a Mianserin derivative.

Amphiphilic polymeric nanoparticles enable homogenous rhodium-catalysed NH insertion reactions in living cells

Rh-catalysed NH carbene insertion reactions were exported to living cells with help of amphiphilic polymeric nanoparticles. Hereto, hydrophobic dirhodium carboxylate catalysts were efficiently encapsulated in amphiphilic polymeric nanoparticles comprising dodecyl and Jeffamine as side grafts. The developed catalytic nanoparticles promoted NH carbene insertions between α-keto diazocarbenes and 2,3-diaminonaphthalene, followed by intramolecular cyclisation to form fluorescent or biologically active benzoquinoxalines. These reactions were studied in reaction media of varying complexity. The best-performing catalyst was exported to HeLa cells, where fluorescent and cytotoxic benzoquinoxalines were synthesized in situ at low catalyst loading within a short time. Most of the developed bioorthogonal transition metal catalysts reported to date are easily deactivated by the reactive biomolecules in living cells, limiting their applications. The high catalytic efficiency of the Rh-based polymeric nanoparticles reported here opens the door to expanding the repertoire of bioorthogonal reactions and is therefore promising for biomedical applications.

Reaching strong absorption up to 700 nm with new benzo[g]quinoxaline-based heteroleptic copper(I) complexes for light-harvesting applications

Heteroleptic copper(I) complexes, with a diimine as a chromophoric unit and a bulky diphosphine as an ancillary ligand, have the advantage of a reduced pseudo Jahn-Teller effect in their excited state over the corresponding homoleptic bis(diimine) complexes. Nevertheless, their lowest absorption lies generally between 350 to 500 nm. Aiming at a strong absorption in the visible by stable heteroleptic Cu(I) complexes, we designed a novel diimine based on 4-(benzo[g]quinoxal-2'-yl)-1,2,3-triazole derivatives. The large π-conjugation of the benzoquinoxaline moiety shifted bathochromically the absorption with regard to other diimine-based Cu(I) complexes. Adding another Cu(I) core broadened the absorption and extended it to considerably longer wavelengths. Moreover, by fine-tuning the structure of the dichelating ligand, we achieved a panchromatic absorption up to 700 nm with a high molar extinction coefficient of 8000 M^-1 cm^-1 at maximum (λ = 570 nm), making this compound attractive for light-harvesting antennae.

Synthesis of N-containing heterocycles in water

An organic reaction with water as a medium has numerous benefits, like improvement in reactivities and selectivities, simple workup techniques, possibility of recycling the catalyst with milder reaction conditions and eco-friendly synthesis. Further, exploring of water as a reaction medium gives rise to unusual reactivities and selectivities, supplementing the organic chemist’s necessity for reaction media. This review focus on the use of water for the synthesis of Nitrogen-containing heterocycles covering from 2011 to 2021.

Organometallic catalysis in aqueous and biological environments: harnessing the power of metal carbenes

Translating the power of transition metal catalysis to the native habitats of enzymes can significantly expand the possibilities of interrogating or manipulating natural biological systems, including living cells and organisms. This is especially relevant for organometallic reactions that have shown great potential in the field of organic synthesis, like the metal-catalyzed transfer of carbenes. While, at first sight, performing metal carbene chemistry in aqueous solvents, and especially in biologically relevant mixtures, does not seem obvious, in recent years there has been a growing number of reports demonstrating the feasibility of the task. Either using small molecule metal catalysts or artificial metalloenzymes, a number of carbene transfer reactions that tolerate aqueous and biorelevant media are being developed. This review intends to summarize the most relevant contributions, and establish the state of the art in this emerging research field.

Regioselective Synthesis of Trisubstituted Quinoxalines Mediated by Hypervalent Iodine Reagents

A facile and regioselective synthesis of quinoxalines, an important motif in medicinal chemistry and materials sciences, was developed. Despite their prospective utility, the regioselective preparation of trisubstituted quinoxalines has not been previously established. In the reported system, hypervalent iodine reagents catalyzed the annulation between α-iminoethanones and o-phenylenediamines in a chemo/regioselective manner to afford trisubstituted quinoxalines. Excellent regioselectivities (6:1 to 1:0) were achieved using [bis(trifluoroacetoxy)iodo]benzene and [bis(trifluoroacetoxy)iodo]pentafluorobenzene as annulation catalysts.

Exporting Metal-Carbene Chemistry to Live Mammalian Cells: Copper-Catalyzed Intracellular Synthesis of Quinoxalines Enabled by N-H Carbene Insertions

Implementing catalytic organometallic transformations in living settings can offer unprecedented opportunities in chemical biology and medicine. Unfortunately, the number of biocompatible reactions so far discovered is very limited, and essentially restricted to uncaging processes. Here, we demonstrate the viability of performing metal carbene transfer reactions in live mammalian cells. In particular, we show that copper (II) catalysts can promote the intracellular annulation of alpha-keto diazocarbenes with ortho-amino arylamines, in a process that is initiated by an N-H carbene insertion. The potential of this transformation is underscored by the in cellulo synthesis of a product that alters mitochondrial functions, and by demonstrating cell selective biological responses using targeted copper catalysts. Considering the wide reactivity spectrum of metal carbenes, this work opens the door to significantly expanding the repertoire of life-compatible abiotic reactions.

NaOH-Mediated Direct Synthesis of Quinoxalines from o-Nitroanilines and Alcohols via a Hydrogen-Transfer Strategy

A NaOH-mediated sustainable synthesis of functionalized quinoxalines is disclosed via redox condensation of o-nitroamines with diols and α-hydroxy ketones. Under optimized conditions, various o-nitroamines and alcohols are well tolerated to generate the desired products in 44-99% yields without transition metals and external redox additives.

Green Approach for Visible-Light-Induced Direct Functionalization of 2-Methylquinolines

A transition metal- and oxidant-free visible light-photoinduced protocol for direct functionalization of 2-methylquinolines has been developed. This protocol enabled the C-H functionalization of substituted 2-methylquinolines with diacetyl or ethyl pyruvate, under environmentally friendly conditions. A mechanistic investigation based on density functional theory (DFT) calculations provided details about the origins of reactivity and selectivity.

The Power of Iron Catalysis in Diazo Chemistry

The use of iron catalysis to enable reactions with diazo compounds has emerged as a valuable tool to forge carbon–carbon or carbon–heteroatom bonds. While diazo compounds are often encountered with toxic and expensive metal catalysts, such as Rh, Ru, Pd, Ir, and Cu, a resurgence of Fe catalysis has been observed. This short review will showcase and highlight the recent advances in iron-mediated reactions of diazo compounds.

1 Introduction

2 Insertion Reactions

2.1 Insertion into B–H Bonds

2.2 Insertion into Si–H Bonds

2.3 Insertion into N–H Bonds

2.4 Insertion into S–H bonds

3 Ylide Formation and Subsequent Reactions

3.1 Doyle–Kirmse Rearrangement

3.2 [1,2]-Stevens and Sommelet–Hauser Rearrangements

3.3 Olefination Reactions

3.4 Cycloaddition Reactions

3.5 gem-Difluoroalkenylation

4 Three-Component Reactions

5 Miscellaneous

6 Conclusion

PIDA-mediated intramolecular oxidative C-N bond formation for the direct synthesis of quinoxalines from enaminones

A intramolecular oxidative C(sp2)-N bond formation mediated by hypervalent iodine(iii) to obtain quinoxalines from readily available N-(2-acetaminophenyl)enaminones was developed. A tandem process involving PIDA-mediated intramolecular condensation cyclization and a subsequent elimination was postulated, which was highly efficient and metal-free under mild conditions. Moreover, flexible structural modifications of quinoxalines bearing carbonyl groups are of interest for further transformations as building blocks in organic synthesis.

Intramolecular trapping of ammonium ylides with N-benzoylbenzotriazoles in aqueous medium: direct access to the pseudoindoxyl scaffold

The present work documents an operationally simple, clean and practical method for accessing the 2,2-disubstituted indolin-3-one (pseudoindoxyl) scaffold. The rhodium carbenoid mediated reaction between N-o-alkylamino benzoylbenzotriazoles and aryl diazoacetates occurs smoothly in water and exploits the leaving group ability of the benzotriazole moiety to install the carbonyl function in the product. Other highlights of the methodology are a wide substrate scope and experimental practicality given the re-use of the benzotriazole byproduct for starting material preparation.

One-pot construction of diverse and functionalized isochromenoquinolinediones by Rh(iii)-catalyzed annulation of unprotected arylamides with 3-diazoquinolinediones and their application for fluorescence sensor

A facile and efficient Rh(iii)-catalyzed annulation of arylamides with 3-diazoquinolinediones for the construction of diverse and highly functionalized isochromenoquinolinediones is described. Furthermore, the methodology is applicable for delivering various relevant molecules such as pyridopyranoquinolindiones, thienopyranoquinolinones, and indolopyranoquinolinone. The reaction proceeds via cascade C–H activation, carbene insertion, and intramolecular lactonization. The reaction exhibits high atom economy, good functional group tolerance, and high regioselectivity. The synthesized compound can also behave as a potent fluorescence sensor for Fe³⁺ ion.

An efficient Rh(iii)-catalyzed annulation of arylamides with 3-diazoquinolinediones for the construction of diverse and highly functionalized isochromenoquinolinedione derivatives is described.

Intrinsically Safe and Shelf-Stable Diazo-Transfer Reagent for Fast Synthesis of Diazo Compounds

We report a crystalline compound 2-azido-4,6-dimethoxy-1,3,5-triazine (ADT) as an intrinsically safe, highly efficient, and shelf-stable diazo-transfer reagent. Because the decomposition of ADT is an endothermal process (Δ H = 30.3 kJ mol^-1), ADT is intrinsically nonexplosive, as proved by thermal, friction, and impact tests. The diazo-transfer reaction based on ADT gives diazo compounds in excellent yields within several minutes at room temperature. ADT is very stable upon >1 year storage under air at room temperature.

Oxidation of β-Ketoamides: The Synthesis of Vicinal Tricarbonyl Amides

A facile and direct oxidative reaction for the synthesis of vicinal tricarbonyl amides in moderate to excellent yields (53-88%) was developed starting from readily available β-ketoamides in the presence of phenyliodine(III) bis(trifluoroacetate). The resulting products possess significant synthetic potential, making this approach a valuable addition to the group of traditional methods already available for the preparation of these molecules.

Iron-catalyzed intermolecular cycloaddition of diazo surrogates with hexahydro-1,3,5-triazines

An iron-catalyzed formal [4 + 1]-cycloaddition of diazo compounds with hexahydro-1,3,5-triazines has been achieved.

Copper-catalyzed direct coupling of benzoxazin-2-ones with indoles for the synthesis of diverse 3-indolylbenzoxazin-2-ones: access to natural cephalandole A

Diverse and functionalized 3-indolyl benzo[b][1,4]oxazin-2-ones were synthesized via copper-catalyzed direct coupling of benzo[b][1,4]oxazin-2-one and indoles in air.