BPO-promoted [4 + 2] cyclization of enaminones and o-phenylenediamines to 2-acyl quinoxalines via a cascade transamination and C-H amination

Rapid assembly of quinoxalines in a single step from readily available precursors has been recognized as an ideal platform in terms of efficiency and operation. Herein, we report a BPO-promoted metal-free approach to 2-acyl quinoxalines from enaminones and o-phenylenediamines via cascade transamination and C-H amination. This methodology demonstrates excellent compatibility with various substrates, including o-hydroxy enaminones, drug derivatives and natural products under mild reaction conditions.

A visible-light-induced photosensitizer-free decarbonylative Minisci-type reaction

This study presents a green and practical visible-light-induced photosensitizer-free decarbonylative Minisci-type reaction using aldehydes as alkyl radical precursors. The photocatalytic system exhibits a broad substrate scope and synthetically useful yields. Mechanistic experiments revealed that alkyl radicals could be generated through auto-oxidation of aldehydes under irradiation, which is a mild and effective method for achieving late-stage functionalization of N-heteroarenes. Some biologically active N-heteroarenes could be alkylated using this photocatalytic system smoothly.

Photocatalyst-free visible-light-induced highly selective acylation of purine nucleosides at the C6 position

A protocol for visible-light-induced C-H acylation selectively at the C6 position of purine nucleosides with aldehydes under photocatalyst-free conditions was established herein. This protocol allows the green, mild, and efficient functionalization of various purine nucleosides with a broad range of alkyl and aryl aldehydes.

Metal-, Photocatalyst-, and Light-Free Minisci C-H Acetylation of N-Heteroarenes with Vinyl Ethers

Herein, we report a mild, operationally simple method for Minisci C-H acetylation of N-heteroarenes using vinyl ethers as robust, inexpensive acetyl sources. The reactions do not require a conventional photocatalysis, electrocatalysis, metal catalysis, light activation, or high temperature. This method is thus significantly more sustainable than previously reported methods in terms of cost, reagent toxicity, and waste generation. This protocol can be expected to obtain medically relevant molecules from abundant feedstock materials.

Photo-redox catalyzed dehydrazinative acylation of N-heterocycles via Minisci reaction

Visible light-induced acylation of heteroaromatic compounds have been achieved using benzoyl hydrazides as an efficient acyl source under mild reaction conditions. The photo-redox catalyzed oxidative cleavage of hydrazides leads to in situ formation of acyl radicals, which subsequently couple with various N-heterocycles to produce acylated products. This synthetic strategy performs the classic Minisci reaction in an eco-friendly and greener way with functional group tolerance and regioselectivity. Control experiments confirm the radical pathway for this transformation.

Visible light-induced acylation of heteroaromatic compounds have been achieved using benzoyl hydrazides as an efficient acyl source under mild reaction conditions.

Minisci aroylation of N-heterocycles using choline persulfate in water under mild conditions

A modified Minisci aroylation of isoquinoline using arylglyoxylic acids in the presence of choline persulfate or pre-composition (choline acetate and K2S2O8) in water at ambient temperature affording aroylated isoquinolines is reported.

Recent advances in the synthesis and reactivity of quinoxaline

Quinoxalines are observed in several bioactive molecules and have been widely employed in designing molecules for DSSC's, optoelectronics, and sensing applications. Therefore, developing newer synthetic routes as well as novel ways for their functionalization is apparent.

Photocatalytic redox-neutral hydroxyalkylation of N-heteroaromatics with aldehydes

Hydroxyalkylation of N-heteroaromatics with aldehydes was achieved using a binary hybrid catalyst system comprising an acridinium photoredox catalyst and a thiophosphoric acid organocatalyst. The reaction proceeded through the following sequence: (1) photoredox-catalyzed single-electron oxidation of a thiophosphoric acid catalyst to generate a thiyl radical, (2) cleavage of the formyl C-H bond of the aldehyde substrates by a thiyl radical acting as a hydrogen atom transfer catalyst to generate acyl radicals, (3) Minisci-type addition of the resulting acyl radicals to N-heteroaromatics, and (4) a spin-center shift, photoredox-catalyzed single-electron reduction, and protonation to produce secondary alcohol products. This metal-free hybrid catalysis proceeded under mild conditions for a wide range of substrates, including isoquinolines, quinolines, and pyridines as N-heteroaromatics, as well as both aromatic and aliphatic aldehydes, and tolerated various functional groups. The reaction was applicable to late-stage derivatization of drugs and their leads.

Anodically Triggered Aldehyde Cation Autocatalysis for Alkylation of Heteroarenes

Alkylation of heteroarenes by using aldehydes is a direct approach to increase molecular complexity, which however often involves the use of stochiometric oxidant, strong acid, and high temperature. This study concerns an energy-efficient electrochemical alkylation of heteroarenes by using aldehydes under mild conditions without mediators. Interestingly, the graphite anode can trigger aldehyde cationic species, which act as the effective autocatalysts to react with a range of heteroarenes to produce the corresponding products with excellent regioselectivity and in high yields. Compared to the traditional electro-synthesis approaches, this electro-triggered reaction provides an electricity-saving and eco-friendly route to high-value chemicals.

Microwave-assisted modified synthesis of C8-analogues of naturally occurring methylxanthines: Synthesis, biological evaluation and their practical applications

An efficient, microwave-assisted, oxidant-interceded, transition-metal-free, cross-dehydrogenative C_sp²-C_sp³ coupling of C₈-Caffeine 2/Theobromine 3/theophylline 4 with substituted aliphatic alcohols 11a-lvia CH bond activation for the preparation of series of substituted C₈-(hydroxymethyl) Caffeine 12a-l/theobromine 13a-c/theophylline 14a-b has been developed using microwave irradiation upto 98% yield. The reaction proceeds smoothly in the presence of tert-butyl hydroperoxide (TBHP) under solvolysis condition at 120 °C for 20 min to corresponding substituted C₈-(hydroxymethyl)-methylxanthine derivatives in good to excellent yields. The good substrate scope, control experiments, gram-scale synthesis, and practical synthetic transformations further highlights the practicality of this methodology. These C₈-(hydroxymethyl) Caffeine 12a-l, 13a-c and 14a-b have been found to show promising in vitro antioxidant as well as antiplatelet activities.

Transition-Metal-Free One-Pot Tandem Synthesis of 3-Ketoisoquinolines from Aldehydes and Phenacyl Azides

An efficient and transition-metal-free strategy for the synthesis of 3-keto-isoquinolines in one pot has been developed from the easily accessible 2-(formylphenyl)acrylates and phenacyl azides. Various substituted aldehydes and phenacyl azides were successfully employed in this transformation to furnish a variety 3-keto-isoquinolines in very good yields. A number of controlled experiments were conducted to postulate the reaction mechanism. Secondary functionalizations of 2-keto-isoquinolins were also performed to showcase the synthetic utility.

Visible Light-Mediated Direct C-H Aroylation and Alkylation of Heteroarenes

Herein, we describe the photocatalytic generation of nucleophilic aroyl radicals from simple aroyl chlorides as a universal and efficient cross-coupling strategy for the direct aroylation of heteroarenes. Furthermore, visible light-mediated direct alkylation of heteroarenes has also been achieved using unactivated bromoalkanes as radical precursors. These two strategies feature high functional group tolerance, exclusive regioselectivity for reaction at the more electrophilic position of heteroarenes, easily accessible substrates, and mild reaction conditions. Moreover, mechanism studies of two reactions are carried out to support our hypotheses.

Visible-Light-Mediated Direct Decarboxylative Acylation of Electron-Deficient Heteroarenes Using α-Ketoacids

Acylation of electron-deficient heteroaromatic compounds has been developed using visible light. α-Ketoacids have been used as an efficient source of acyl radicals under photoredox conditions. The in situ generated acyl radicals from α-ketoacids have been coupled to a wide variety of electron-deficient heteroaromatic compounds in a Minisci type reaction. This method would be attractive to access biologically attractive molecules.

Versatile cross-dehydrogenative coupling of heteroaromatics and hydrogen donors via decatungstate photocatalysis

A facile sunlight-induced derivatization of heteroaromatics via photocatalyzed C-H functionalization in amides, ethers, alkanes and aldehydes is described. Tetrabutylammonium decatungstate (TBADT) was used as the photocatalyst and allowed to carry out the process under mild conditions.

Metal-free oxidative coupling of quinoxalin-2(1H)-ones with arylaldehydes leading to 3-acylated quinoxalin-2(1H)-ones

An efficient protocol for the synthesis of 3-aroylquinoxalin-2(1H)-ones has been developed via a metal-free oxidative cross-coupling reaction of quinoxalin-2(1H)-ones with aryl aldehydes under mild conditions with good yields.

Oxidant-Controlled C-sp2/sp3-H Cross-Dehydrogenative Coupling of N-Heterocycles with Benzylamines

Oxidant controlled ionic liquid mediated cross-dehydrogenative coupling (CDC) of benzylamines with N-heterocycles having sp² or sp³ carbon resulted in the formation of C-benzoylated or alkenylated products. Benzoylation of N-heterocycles occurs via (NH₄)₂S₂O₈ catalyzed benzoyl radical formation. An oxidative alkenylation of N-heterocycles having C-sp³ carbon (2-methylaza-arenes) occurs via deamination of benzylamine followed by C-sp³-H bond activation in high stereoselectivity. Both benzoylation and alkenylation protocols are metal-free, green, simple, efficient, and tolerate a wide variety of functional groups.

Recent Advances in Radical C-H Activation/Radical Cross-Coupling

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).

Use of unprotected amino acids in metal-free tandem radical cyclization reactions: divergent synthesis of 6-alkyl/acyl phenanthridines

We demonstrated a metal-free protocol for the construction of C–C bonds utilizing unprotected amino acids as radical sources.

Transition metal-free cross-dehydrogenative coupling acylation of coumarins by the K2S2O8/Aliquat 336 catalytic system: a versatile strategy towards 4-aroylcoumarin derivatives

A new and efficient transition metal-free oxidative cross-dehydrogenative coupling (CDC) reaction is described for the preparation of 4-aroylcoumarin derivatives.

Copper-catalyzed oxidative coupling between quinazoline 3-oxides and unactivated aldehydes: an efficient approach to functionalized quinazolines

The first copper-catalyzed oxidative coupling between quinazoline 3-oxides and unactivated aldehydes was described.