LiBr‐Promoted Photoredox Minisci‐Type Alkylations of Quinolines with Ethers

Dehydrogenative Coupling of Alkylamines with Primary Alcohols Forming α-Amino Ketones

Acceptorless dehydrogenative coupling reactions between C-H bonds offer straightforward and atom-economical methods connecting readily available materials while liberating gaseous hydrogen as the sole byproduct. Despite the growing interest in such transformations, their realization still poses a significant challenge. Here we report a photoinduced dehydrogenative coupling reaction of alkylamines with primary alcohols. C-H bonds adjacent to nitrogen and oxygen are site-selectively cleaved, and a C-C bond is created between the carbon atoms in a cross-selective manner to produce α-amino ketones. Diverse polar functionalities such as esters, amides, and carboxylic acids survived, demonstrating the broad applicability of the present method.

Unraveling the Potential of Vinyl Ether as an Ethylene Surrogate in Heteroarene C─H Functionalization via the Spin-Center Shift

Despite the simplicity and abundance of ethylene, its practical application presents significant hurdles due to its nature as a highly flammable gas. Herein, a strategic use of easily handled vinyl ether is reported as a latent ethylene surrogate achieved via a spin-center shift (SCS) pathway, enabling the successful three-component reaction that bridges heteroarenes and various coupling partners, including sulfinates, thiols, and phosphine oxides. Through a photoredox catalytic process, α-oxy radicals are generated by combining various radicals with phenyl vinyl ether, which are subsequently added to N-heteroarenes. Subsequently, the radical-mediated SCS pathway serves as the driving force for C─O bond cleavage, effectively engaging the phenoxy group as a leaving group. In addition, by broadening the utility of the method, a valuable synthon is provided for efficient C─H vinylation of N-heteroarenes following sulfonyl group elimination. This approach not only enriches the toolbox of synthetic methodology but also provides a more streamlined alternative, circumventing the challenges associated with direct ethylene gas usage. The versatility of the method, particularly evident in late-stage functionalizations of medicinally relevant molecules and peptides, underscores its capability to produce invaluable three-component compounds and vinylated N-heteroarene derivatives.

Visible Light-Induced Cross-Dehydrocoupling of 3-Cyanocoumarins with Unactivated Aliphatic Aldehydes Enables Access to 4-Acylated Coumarins

We report visible light-induced cross-dehydrocoupling of 3-cyanocoumarins with unactivated aliphatic aldehydes for the direct synthesis of 4-acylated coumarins. Inexpensive and readily available (n-Bu)₄NBr was used as a precursor of the hydrogen atom transfer (HAT) catalyst and [Ir(ppy)₂(dtbbpy)PF₆] as a photocatalyst. A variety of aldehydes including linear, branched, cyclic, and α,β-unsaturated aliphatic aldehydes were suitable for this reaction yielding the desired acylated coumarin derivatives in 41-98% yields. This method provides a mild, atom-economical, and environmentally friendly synthetic route for the direct acylation of 3-cyanocoumarins. This is the first example so far of C-4 aliphatic acylation of coumarins.

1,2-Radical Shifts in Photoinduced Synthetic Organic Transformations: A Guide to the Reactivity of Useful Radical Synthons

The exploration of 1,2-radical shift (RS) mechanisms in photoinduced organic reactions has provided efficient routes for the generation of important radical synthons in many chemical transformations. In this Review, the basic concepts involved in the traditional 1,2-spin-center shift (SCS) mechanisms in recently reported studies are discussed. In addition, other useful 1,2-RSs are addressed, such as those proceeding through 1,2-group migrations in carbohydrate chemistry, via 1,2-boron shifts, and by the generation of α-amino radicals. The discussion begins with a general overview of the basic aspects of 1,2-RS mechanisms, followed by a demonstration of their applicability in photoinduced transformations. The sections that follow are organized according to the mechanisms operating in combination with the 1,2-radical migration event. This contribution is not a comprehensive review but rather aims to provide an understanding of the topic, focused on the more recent advances in the field, and establishes a definition for the nomenclature that has been used to describe such mechanisms.

General electrochemical Minisci alkylation of N-heteroarenes with alkyl halides

Herein, we report, a general, facile and environmentally friendly Minisci-type alkylation of N-heteroarenes under simple and straightforward electrochemical conditions using widely available alkyl halides as radical precursors. Primary, secondary and tertiary alkyl radicals have been shown to be efficiently generated and coupled with a large variety of N-heteroarenes. The method presents a very high functional group tolerance, including various heterocyclic-based natural products, which highlights the robustness of the methodology. This applicability has been further proved in the synthesis of various interesting biologically valuable building blocks. In addition, we have proposed a mechanism based on different proofs and pieces of electrochemical evidence.

Application of the Spin-Center Shift in Organic Synthesis

Spin-center shift (SCS) is a radical process involving 1,2-radical translocation along with a two-electron ionic movement, such as elimination of an adjacent leaving group. Such a process was initially observed in some important biochemical transformations, and the unique property has also attracted considerable interest in synthetic chemistry. Experimental, kinetic, as well as computational studies have been performed, and a series of useful radical transformations have been developed and applied in organic synthesis based on SCS processes in the last 20 years. This Perspective is an overview of radical transformations involving the SCS mechanism.

Identification of a Self-Photosensitizing Hydrogen Atom Transfer Organocatalyst System

We developed organocatalyst systems to promote the cleavage of stable C-H bonds, such as formyl, α-hydroxy, and benzylic C-H bonds, through a hydrogen atom transfer (HAT) process without the use of exogenous photosensitizers. An electronically tuned thiophosphoric acid, 7,7'-OMe-TPA, was assembled with substrate or co-catalyst N-heteroaromatics through hydrogen bonding and π-π interactions to form electron donor-acceptor (EDA) complexes. Photoirradiation of the EDA complex induced stepwise, sequential single-electron transfer (SET) processes to generate a HAT-active thiyl radical. The first SET was from the electron-rich naphthyl group of 7,7'-OMe-TPA to the protonated N-heteroaromatics and the second proton-coupled SET (PCET) from the thiophosphoric acid moiety of 7,7'-OMe-TPA to the resulting naphthyl radical cation. Spectroscopic studies and theoretical calculations characterized the stepwise SET process mediated by short-lived intermediates. This organocatalytic HAT system was applied to four different carbon-hydrogen (C-H) functionalization reactions, hydroxyalkylation and alkylation of N-heteroaromatics, acceptorless dehydrogenation of alcohols, and benzylation of imines, with high functional group tolerance.

Photoredox Neutral Decarboxylative Hydroxyalkylations of Heteroarenes with α-Keto Acids

Photoredox neutral decarboxylative hydroxyalkylations of heteroarenes with α-keto acids under mild conditions are described. Stable and readily available α-keto acids were employed as hydroxyalkylating reagents with only CO₂ released as the byproduct. A range of aromatic and aliphatic α-keto acids were successfully converted into hydroxyalkylated products with various heteroarenes. This transformation proceeded through a decarboxylation/Minisci addition/SCS sequence, generating a variety of valuable hydroxyalkylated heteroarenes.

Organic-acid catalysed Minisci-type arylation of heterocycles with aryl acyl peroxides

A metal-free method for the Minisci-type arylation of heterocycles with aryl acyl peroxides has been reported. This strategy enables the rapid and simple synthesis of a series of Minisci-type adducts...

Investigations into mechanism and origin of regioselectivity in the metallaphotoredox-catalyzed α-arylation of N-alkylbenzamides

A mechanistic study on the α-arylation of N-alkylbenzamides catalyzed by a dual nickel/photoredox system using aryl bromides is reported herein. This study elucidates the origins of site-selectivity of the transformation, which is controlled by the generation of a hydrogen atom transfer (HAT) agent by a photocatalyst and bromide ions in solution. Tetrabutylammonium bromide was identified as a crucial additive and source of a potent HAT agent, which led to increases in yields and a lowering of the stoichiometries of the aryl bromide coupling partner. NMR titration experiments and Stern–Volmer quenching studies provide evidence for complexation to and oxidation of bromide by the photocatalyst, while elementary steps involving deprotonation of the N-alkylbenzamide or 1,5-HAT were ruled out through mechanistic probes and kinetic isotope effect analysis. This study serves as a valuable tool to better understand the α-arylation of N-alkylbenzamides, and has broader implications in halide-mediated C–H functionalization reactions.

A mechanistic study of the α-arylation of N-alkylbenzamides catalyzed by a dual nickel/photoredox system using aryl bromides elucidates the origins of site-selectivity of the transformation and identifies the hydrogen atom transfer agent.

Bromine-radical-induced Csp2–H difluoroalkylation of quinoxalinones and hydrazones through visible-light-promoted Csp3–Br bond homolysis

Bromine radicals derived from photo-induced Csp3–Br bond homolysis can mediate H abstraction/imine radical formation from quinoxalinones and hydrazones, which in turn quench the in situ-generated difluoroalkyl radicals to furnish the products.

Visible-light- and bromide-mediated photoredox Minisci alkylation of N-heteroarenes with ester acetates

A visible-light-induced photoredox Minisci alkylation reaction of N-heteroarenes with ethyl acetate has been reported. The low-toxic ethyl acetate was used for the first time as an alkylation reagent. Hence, 4-quinazolinones, quinolines and pyridines reacted smoothly in the current reaction system. Mechanistic studies indicate that LiBr plays a key role to dramatically improve the efficiency of the reaction by the mediation of hydrogen atom transfer.

Visible-Light-Driven Dehydrogenative Coupling of Primary Alcohols with Phenols Forming Aryl Carboxylates

A preparative method for obtaining aryl esters from aliphatic primary alcohols and phenols was developed. The reaction proceeds under the irradiation of visible light at ambient temperature, dispensing with any oxidant or hydrogen acceptor. Primary alcohols having a variety of functional groups are successfully esterified with phenols. The produced esters can be utilized as the precursor of various carbonyl compounds.

A cross-dehydrogenative C(sp3)-H heteroarylation via photo-induced catalytic chlorine radical generation

Hydrogen atom abstraction (HAT) from C(sp³)–H bonds of naturally abundant alkanes for alkyl radical generation represents a promising yet underexplored strategy in the alkylation reaction designs since involving stoichiometric oxidants, excessive alkane loading, and limited scope are common drawbacks. Here we report a photo-induced and chemical oxidant-free cross-dehydrogenative coupling (CDC) between alkanes and heteroarenes using catalytic chloride and cobalt catalyst. Couplings of strong C(sp³)–H bond-containing substrates and complex heteroarenes, have been achieved with satisfactory yields. This dual catalytic platform features the in situ engendered chlorine radical for alkyl radical generation and exploits the cobaloxime catalyst to enable the hydrogen evolution for catalytic turnover. The practical value of this protocol was demonstrated by the gram-scale synthesis of alkylated heteroarene with merely 3 equiv. alkane loading.

Hydrogen atom abstraction from C(sp³)–H bonds of naturally abundant alkanes for alkyl radical generation represents a promising yet underexplored strategy in the alkylation reaction designs. Here the authors show a photo-induced and chemical oxidant-free cross-dehydrogenative coupling between alkanes and heteroarenes using catalytic chloride and cobalt catalyst.

Ag-Catalyzed Remote Unactivated C(sp3)-H Heteroarylation of Free Alcohols in Water

Catalyzed by silver salt, the unactivated C(sp³)-H heteroarylation of free alcohol at the δ position is realized under gentle thermal conditions in water through a radical procedure. Both protonic acids and Lewis acids are found to be efficient for activating pyridines for this Minisci-type reaction. The reaction enjoys a good functional group tolerance and substrate scope. Terminal secondary and tertiary alcohols are suitable substrates. With either electron-donating or -withdrawing groups, the electron-deficient heteroarene substrates generate the target products in moderate to good yields. A gram-scale experiment can be successfully operated. A radical blocking experiment and a radical clock experiment are studied to support the radical mechanism.

Benzylic C(sp3)-C(sp2) cross-coupling of indoles enabled by oxidative radical generation and nickel catalysis

A mechanistically unique functionalization strategy for a benzylic C(sp3)-H bond has been developed based on the facile oxidation event of indole substrates. This novel pathway was initiated by efficient radical generation at the benzylic position of the substrate, with subsequent transition metal catalysis to complete the overall transformation. Ultimately, an aryl or an acyl group could be effectively delivered from an aryl (pseudo)halide or an acid anhydride coupling partner, respectively. The developed method utilizes mild conditions and exhibits a wide substrate scope for both substituted indoles and C(sp2)-based reaction counterparts. Mechanistic studies have shown that competitive hydrogen atom transfer (HAT) processes, which are frequently encountered in conventional methods, are not involved in the product formation process of the developed strategy.

Unexpected ortho C–H bond activation in coordinated 7,8-benzoquinoline: synthesis and characterisation of heteroleptic Ir(iii)-7,8-benzoquinoline complexes

Unusual ortho C–H activation observed in a 7,8 benzoquinoline ring coordinated to Ir(iii) centre.

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.

Photoinduced Specific Acylation of Phenolic Hydroxy Groups with Aldehydes

A convenient method is reported to specifically acylate phenolic hydroxyl groups through a radical pathway. When a mixture of an aldehyde and a phenol in ethyl acetate is irradiated with blue light in the presence of iridium and nickel bromide catalysts at ambient temperature, phenoxyl and acyl radicals are transiently generated in situ and cross-couple to furnish an ester. Aliphatic hydroxy groups remain untouched under the reaction conditions.

Silver-catalyzed direct C-H oxidative carbamoylation of quinolines with oxamic acids

A silver-catalyzed efficient and direct C-H carbamoylation of quinolines with oxamic acids to access carbamoylated quinolines has been developed through oxidative decarboxylation reaction. The reaction proceeds smoothly over a broad range of substrates with excellent functional group tolerance and excellent yields under mild conditions.

Dehydrogenative Coupling of Benzylic and Aldehydic C-H Bonds

A photoinduced dehydrogenative coupling reaction between benzylic and aldehydic C-H bonds is reported. When a solution of an alkylbenzene and an aldehyde in ethyl acetate is irradiated with visible light in the presence of iridium and nickel catalysts, a coupled α-aryl ketone is formed with evolution of dihydrogen. An analogous C-C bond forming reaction occurs between a C-H bond next to the nitrogen of an N-methylamide and an aldehydic C-H bond to produce an α-amino ketone. These reactions provide a straightforward pathway from readily available materials leading to valued structural motifs of pharmacological relevance.

Visible-light-mediated photoredox minisci C–H alkylation with alkyl boronic acids using molecular oxygen as an oxidant

Direct visible-light-mediated Minisci C–H alkylation reactions of N-heteroarenes with alkyl boronic acids at room temperature with molecular oxygen as an oxidant and boronic acid activation reagent were reported.