Practical, metal-free remote heteroarylation of amides via unactivated C(sp3)-H bond functionalization

Sulfonamide-directed site-selective functionalization of unactivated C(sp3)-H enabled by photocatalytic sequential electron/proton transfer

The generation of alkyl radical from C(sp3)-H substrates via hydrogen atom abstraction represents a desirable yet underexplored strategy in alkylation reaction since involving common concerns remain adequately unaddressed, such as the harsh reaction conditions, limited substrate scope, and the employment of noble metal- or photo-catalysts and stoichiometric oxidants. Here, we utilize the synergistic strategy of photoredox and hydrogen atom transfer (HAT) catalysis to accomplish a general and practical functionalization of unactived C(sp3)-H centers with broad reaction scope, high functional group compatibility, and operational simplicity. A combination of validation experiments and density functional theory reveals that the N-centered radicals, generated from free N - H bond in a stepwise electron/proton transfer event, are the key intermediates that enable an intramolecular 1,5-HAT or intermolecular HAT process for nucleophilic carbon-centered radicals formation to achieve heteroarylation, alkylation, amination, cyanation, azidation, trifluoromethylthiolation, halogenation and deuteration. The practical value of this protocol is further demonstrated by the gram-scale synthesis and the late-stage functionalization of natural products and drug derivatives.

Regioselective synthesis of isoquinolinonediones through remote unactivated C(sp3)-H bonds

Herein, a general strategy for the remote-site-selective cascade addition/cyclization of unactivated C(sp3)-H bonds in free alcohols and sulfonamides to build isoquinolinonedione skeletons is developed. The site selectivity occurs predominantly via a 1,5-hydrogen atom transfer (HAT) process, triggered by heteroatom-centred radicals generated directly under silver catalysis. A broad substrate scope and excellent regio-/chemo-selective control are demonstrated in this method.

Iodine-Mediated δ-Amination of sp3 C-H Bonds

We present a direct δ-amination reaction of sp3 C-H bonds, employing molecular iodine (I2) as the sole oxidant under transition-metal-free conditions. This remote C-H functionalization approach is operationally simple and provides facile, efficient access to pyrrolidines and related heterocyclic derivatives from readily accessible substrates.

Photo-Induced C-H Methylation Reactions

Direct C-H methylation is a highly valuable approach for introducing methyl groups into organic molecules, particularly in pharmaceutical chemistry. Among the various methodologies available, photo-induced methylation stands out as an exceptional choice due to its mild reaction conditions, energy efficiency, and compatibility with functional groups. This article offers a comprehensive review of photochemical strategies employed for the direct and selective methylation of C(sp3 )-H, C(sp2 )-H, and C(sp)-H bonds in various organic molecules. The discussed methodologies encompass transition-metal-based photocatalysis, organophotocatalysis, as well as other metal-free approaches, including electron donor-acceptor (EDA)-enabled transformations. Importantly, a wide range of easily accessible agents such as tert-butyl peroxide, methanol, DMSO, methyl tert-butyl ether, TsOMe, N-(acetoxy)phthalimide, acetic acid, methyl halides, and even methane can serve as effective methylating reagents for modifying diverse targets. These advancements in photochemical C-H methylation are anticipated to drive further progress in the fields of organic synthesis, photocatalysis, and pharmaceutical development, opening up exciting avenues for creating novel organic molecules and discovering new drug compounds.

Metal-Free Thiocarbamation of Quinolinones: Direct Access to 3,4-Difunctionalized Quinolines and Quinolinonyl Thiocarbamates at Room Temperature

A novel and practical method for the preparation of difunctionalized quinolines, bearing a thiocarbamate group at the C3-position and an acyloxyl group at the C4-position, and quinolinonyl thiocarbamates from quinolinones, tetraalkylthiuram disulfides, and hypervalent iodine(III) reagents has been developed via thiocarbamation of quinolinones at room temperature. The present method features mild reaction conditions, good tolerance with diverse functional groups, and a wide substrate scope, providing the desired products in good yields. Furthermore, this transformation is easy to scale up, and the desired products can be readily converted to heterocyclic thiols. Most importantly, this protocol allows for the late-stage thiocarbamation of bioactive compounds. Mechanistic studies show that radicals may be involved in this transformation, water is probably the oxygen source of thiocarbamates, and difunctionalized quinolines are possibly formed via nucleophilic attack of carboxylic anions, which derive from hypervalent iodine(III) reagents.

Fluoroamide-Directed Regiodivergent C-Alkylation of Nitroalkanes

Herein, by exploiting different activation modes of fluoroamides, we achieved α- and δ-C(sp³)-H alkylation of nitroalkanes with switchable regioselectivity. Cu catalysis enabled the interception of a distal C-centered radical by a N-centered radical to couple nitroalkanes and unactivated δ-C-H bonds. In addition, imines generated in situ by fluoroamides were trapped by nitroalkanes to realize the α-C-H alkylation of amides. Both of those scalable protocols have broad substrate scopes and good functional group tolerance.

Thiochromane Formation via Visible-Light-Mediated Intramolecular δ-C(sp3)-H Bond Arylation of Sulfonamides

Visible-light-mediated intramolecular site-selective δ-C(sp³)-H bond arylation of aliphatic trifluoromethanesulfonamides was developed. The reaction proceeds through a radical cascade, including the generation of a sulfonamidyl radical, which triggers a 1,5-hydrogen atom transfer, affording a δ-C-centered radical, which finally cyclized onto a neighboring thiopolyfluoroaryl moiety to deliver a range of synthetically useful thiochromanes. The cyclization process occurs through two distinct pathways depending upon the nature of the substituent X ortho to the native C-S bond.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

Fluorosulfonamide-Directed Heteroarylation of Aliphatic C(sp3)-H Bonds

Herein, we describe a formal dehydrogenative cross coupling of heterocycles with unactivated aliphatic amines. The resulting transformation enables the direct alkylation of common heterocycles by merging N-F-directed 1,5-HAT with Minisci chemistry, leading to predictable site selectivity. The reaction provides a direct route for the transformation of simple alkyl amines to value-added products under mild reaction conditions, making this an attractive option for C(sp³)-H heteroarylation.

C-H Diselenation and Monoselenation of Electron-Deficient Alkenes via Radical Coupling at Room Temperature

A new, simple, and metal-free route for the diselenation of maleimides has been first developed employing (bis(trifluoroacetoxy)iodo)benzene (PIFA) at room temperature. The present method is compatible with different functional groups, and various diselenyl maleimides were obtained in moderate to excellent yields. Moreover, this protocol further highlights the unique practical application for the functionalization of biologically relevant molecules and amino acid derivatives. Preliminary mechanism studies suggest that radicals may be involved in this novel transformation. Additionally, this protocol is also applicable for the monoselenation of maleimides by switching the reaction conditions and selenation of other electron-deficient alkenes.

Mechanochemistry: New Tools to Navigate the Uncharted Territory of "Impossible" Reactions

Mechanochemical transformations have made chemists enter unknown territories, forcing a different chemistry perspective. While questioning or revisiting familiar concepts belonging to solution chemistry, mechanochemistry has broken new ground, especially in the panorama of organic synthesis. Not only does it foster new "thinking outside the box", but it also has opened new reaction paths, allowing to overcome the weaknesses of traditional chemistry exactly where the use of well-established solution-based methodologies rules out progress. In this Review, the reader is introduced to an intriguing research subject not yet fully explored and waiting for improved understanding. Indeed, the study is mainly focused on organic transformations that, although impossible in solution, become possible under mechanochemical processing conditions, simultaneously entailing innovation and expanding the chemical space.

Directed Ni-Catalyzed Reductive Arylation of Aliphatic C–H Bonds

Herein, we describe the nickel-catalyzed reductive arylation of remote C(sp³)-H bonds with aryl electrophiles. The reaction targets secondary and tertiary C(sp³)-H bonds to deliver all-carbon quaternary centers. The success of this method relies on a novel amidyl radical precursor that tolerates reducing conditions, namely O-oxalate hydroxamic acid esters.

Phenanthrenequinone (PQ) catalyzed cross-dehydrogenative coupling of alkanes with quinoxalin-2(1H)-ones and simple N-heteroarenes under visible light irradiation

A direct and convenient strategy to 3-alkylquinoxalin-2(1H)-ones and other alkyl N-heteroarenes via a photocatalyzed alkylation of quinoxalin-2(1H)-ones and other N-heterocycles with commercially available, low-cost alkanes under ambient conditions using phenanthrenequinone (PQ) as a photocatalyst was developed. This transformation has advantages of environment-friendly protocol, mild conditions, good functional-group tolerance, and high yields of products.

Cu(II)-Catalyzed N-Directed Distal C(sp3)-H Heteroarylation of Aliphatic N-Fluorosulfonamides

A copper-catalyzed δ-regioselective C(sp³)-H heteroarylation of N-fluorosulfonamides has been developed. A broad range of heteroarenes were well tolerated and reacted with various N-fluorosulfonamides to give the corresponding heteroarylated amides in good yields. Notably, all types (1°, 2°, and 3°) of δ-C(sp³)-H bonds in the N-fluorosulfonamides could be regioselectively activated through the 1,5-HAT process. This protocol provides a practical strategy for the functionalization of heteroarenes and amides via forging a C(sp³)-C(sp²) bond.

Site-Selective Desaturation of C(sp3)-C(sp3) Bond via Photoinduced Ruthenium Catalysis

Ruthenium(II) photocatalysis has emerged as one of the most advanced tools amongst modern synthetic chemistry whereas its catalytic mode is generally limited to single electron transfer and triplet energy transfer...

Remote C-H Pyridylation of Hydroxamates through Direct Photoexcitation of O-Aryl Oxime Pyridinium Intermediates

Herein, we report an efficient strategy for the remote C-H pyridylation of hydroxamates with excellent ortho-selectivity by designing a new class of photon-absorbing O-aryl oxime pyridinium salts generated in situ from the corresponding pyridines and hydroxamates. When irradiated by visible light, the photoexcitation of oxime pyridinium intermediates generates iminyl radicals via the photolytic N-O bond cleavage, which does not require an external photocatalyst. The efficiency of light absorption and N-O bond cleavage of the oxime pyridinium salts can be modulated through the electronic effect of substitution on the O-aryl ring. The resultant iminyl radicals enable the installation of pyridyl rings at the γ-CN position, which yields synthetically valuable C2-substituted pyridyl derivatives. This novel synthetic approach provides significant advantages in terms of both efficiency and simplicity and exhibits broad functional group tolerance in complex settings under mild and metal-free conditions.

Copper-Catalyzed Oxidative Dehydrogenative Reaction of Quinoline-N-Oxides with Donor-Acceptor Cyclopropanes: Installation of a Tertiary Alkyl Motif at C2 Position

A copper-catalyzed oxidative dehydrogenative reaction of quinoline N-oxides with donor-acceptor cyclopropanes has been demonstrated to construct C2-alkylated quinolines containing a γ-keto diester motif. The use of molecular oxygen as an oxidant, excellent site-selectivity, and good functional group tolerance are the important features in this process. The preliminary mechanistic studies demonstrate that the catalyst plays a dual role as a Lewis acid and a redox catalyst.

Remote Radical Desaturation of Unactivated C-H Bonds in Amides

Desaturation of inert aliphatic C-H bonds in alkanes to form the corresponding alkenes is challenging. In this communication, a new and practical strategy for remote site-selective desaturation of amides via radical chemistry is reported. The readily installed N-allylsulfonylamide moiety serves as an N radical precursor. Intramolecular 1,5-hydrogen atom transfer from an inert C-H bond to the N-radical generates a translocated C-radical which is subsequently oxidized and deprotonated to give the corresponding alkene. The commercially available methanesulfonyl chloride is used as reagent and a Cu/Ag-couple as oxidant. The remote desaturation is realized on different types of unactivated sp³ -C-H bonds. The potential synthetic utility of this method is further demonstrated by the dehydrogenation of natural product derivatives and drugs.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class-either sp² or sp³ C-H functionalization-lends perspective and tactical strategies for use of these methods in synthetic applications.

(1-Selenocyanatoethyl)benzene: A Selenocyanation Reagent for Site-Selective Selenocyanation of Inert Alkyl C(sp3)-H Bonds

A new, simple, yet easily accessible, (1-selenocyanatoethyl)benzene has been designed and applied as a SeCN group transfer reagent for selenocyanation of aliphatic C(sp³)-H bonds for the first time. This protocol is featured with mild reaction conditions and wide substrate scope. Control experiments reveal that a radical-group transfer mechanism might be involved.

Regioselective N-F and α C(sp3)-H Arylation of Aliphatic N-Fluorosulfonamides with Imidazopyridines

A regioselective arylation of aliphatic N-fluorosulfonamides with imidazopyridines enabled by breaking of N-F and α C(sp³)-H bond to form C-N and C-C bonds was described. With CuCl as the catalyst, a radical mechanism was proposed to produce N-arylated aliphatic sulfonamides via a N radical intermediate. Importantly, under acidic conditions, an in situ generated imine was the possible intermediate, which was trapped by imidazopyridines to form α C(sp³)-H arylated aliphatic sulfonamides. The current protocol featured a broad substrate scope, tunable reaction conditions, operational convenience, and good regioselectivity.

Mechanochemical Magnesium-Mediated Minisci C-H Alkylation of Pyrimidines with Alkyl Bromides and Chlorides

A novel method to synthesize 4-alkylpyrimidines by the mechanochemical magnesium-mediated Minisci reaction of pyrimidine derivatives and alkyl halides has been reported. The reaction process operates with a broad substrate scope and excellent regioselectivity under mild conditions with no requirement of transition-metal catalysts, solvents, and inert gas protection. The practicality of this protocol has been demonstrated by the up-scale synthesis, mechanochemical product derivatization, and antimalarial drug pyrimethamine preparation.

Remote Site-Selective Radical C(sp3 )-H Monodeuteration of Amides using D2 O

Site-selective incorporation of deuterium into biologically active compounds is of high interest in pharmaceutical industry. We present a mild and environmentally benign metal-free method for the remote selective radical C-H monodeuteration of aliphatic C-H bonds in various amides with inexpensive heavy water (D2 O) as the deuterium source. The method uses the easily installed N-allylsulfonyl moiety as an N-radical precursor that generates the remote C-radical via site-selective 1,5- or 1,6-hydrogen atom transfer (HAT). Methyl thioglycolate, that readily exchanges its proton with D2 O, serves as the radical deuteration reagent and as a chain-carrier. The highly site-selective monodeuteration has been applied to different types of unactivated sp3 -C-H bonds and also to the deuteration of C-H bonds next to heteroatoms. The potential utility of this method is further demonstrated by the site-selective incorporation of deuterium into natural product derivatives and drugs.

Green chemistry meets medicinal chemistry: a perspective on modern metal-free late-stage functionalization reactions

The progress of drug discovery and development is paced by milestones reached in organic synthesis. In the last decade, the advent of late-stage functionalization (LSF) reactions has represented a valuable breakthrough. Recent literature has defined these reactions as the chemoselective modification of complex molecules by means of C-H functionalization or the manipulation of endogenous functional groups. Traditionally, these diversifications have been accomplished by organometallic means. However, the presence of metals carries disadvantages related to their cost, environmental hazard and health risks. Fundamentally, green chemistry directives can help minimize such hazards through the development of metal-free LSF methodologies. In this review, we expand the current discussion on metal-free LSF reactions by providing an overview of C(sp2)-H, and C(sp3)-H functionalizations, as well as the utilization of heteroatom-containing functional groups as chemical handles. Selected topics such as metal-free cross-dehydrogenative coupling (CDC) reactions, organocatalysis, electrochemistry and photochemistry are also discussed. By writing the first review on metal-free LSF methodologies, we aim to highlight current advances in the field with examples that reveal specific challenges and solutions, as well as future research opportunities.

Photoredox-Mediated Mono- and Difluorination of Remote Unactivated Methylene C(sp3)-H Bonds of N-Alkyl Sulfonamides

A photoredox-mediated δ-C(sp³)-H fluorination of sulfonyl-protected primary alkylamines with Selectfluor is developed. The reaction can proceed in excellent monofluorination selectivity for amine substrates without α substituent. For α-substituted substrates, a slightly modified reaction conditions with two rounds of operation gives the δ,δ-difluorination products in good yield. Mechanistic studies suggest SET oxidation of sulfonamide group directly generates the key sulfonamide N radical intermediate, which triggers a 1,5-HAT process to form the δ alkyl radical.

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.

Copper-Catalyzed Intramolecular Amination of C(sp3)-H Bond of Secondary Amines to Access Azacycles

The cross-coupling of C-N bond directly from inert C-H bonds is an ideal approach to synthesize saturated azacycles due to its high efficiency and atom economy. In this article, a copper-catalyzed intramolecular amination via the cross coupling of C(sp³)-H and N-H bonds of secondary amine has been reported, which exhibit excellent chemo- and regioselectivity, extensive substrate scope, and functional group tolerance in good to excellent yield, offering an efficient pathway to build nitrogen-containing heterocycle skeletons.

Radical heteroarylation of unactivated remote C(sp3)–H bonds via intramolecular heteroaryl migration

Described herein is the radical-mediated heteroarylation of unactivated remote C(sp3)–H bonds via intramolecular heteroaryl migration.

Copper-catalyzed, N-directed remote C(sp3)–H azidation and thiocyanation

A mild and practical protocol is developed for the synthesis of distal azido and thiocyanato alkylamines via N-directed remote C(sp³)–H functionalization.

Sulfonamide as Photoinduced Hydrogen-Atom Transfer Catalyst for Regioselective Alkylation of C(sp3)-H Bonds Adjacent to Heteroatoms

Based on the DFT calculations, the sulfonamide was explored as an efficient hydrogen-atom transfer catalyst for the C(sp³)-H alkylation. The combination of a metal-free photoredox catalyst and a sulfonamide catalyst enables highly regioselective alkylation of the C-H bonds adjacent to heteroatoms, which features broad substrate scope and excellent functional group compatibility. Remarkably, the sulfonamide catalyst was also applicable to the C(sp³)-C(sp³) couplings through the merger of photoredox, nickel, and HAT catalysis.

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

BI-OAc-Accelerated C3-H Alkylation of Quinoxalin-2(1H)-ones under Visible-Light Irradiation

An efficient, photoredox-catalyst-free radical alkylation of quinoxalin-2(1H)-ones has been described. This reaction utilizes 4-alkyl-1,4-dihydropyridines (R-DHPs) as alkyl radical precursors and acetoxybenziodoxole (BI-OAc) as an electron acceptor to undergo single-electron transfer with photoexcited R-DHPs. The benign conditions allow for good compatibility in the scope of both quinoxalin-2(1H)-ones and R-DHPs. The synthetic value of the protocol was also demonstrated by the successful functionalization of natural products and drug-based complex molecules.

Copper-Catalyzed Remote C(sp3)-H Amination of Carboxamides

Here we report a method for the site-selective intermolecular C(sp³)-H amination of carboxamides by merging transition-metal catalysis and the hydrogen atom transfer strategy. The reaction proceeds through a sequence of favorable single-electron transfer, 1,5-hydrogen atom transfer, and C-N cross-coupling steps, thus allowing access to a series of desired products. This reaction could accommodate a wide diversity of nitrogen nucleophiles as well as demonstrate excellent chemoselectivity and functional group compatibility.