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

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.

Dehalogenative deuteration of alkyl and aryl bromides by thiyl radical catalysis under visible-light irradiation

Herein, we report a mild and practical method for the deuteration of alkyl and aryl bromides by a thiyl radical catalyst and halogen-atom transfer (XAT) using disulfides and silanes under visible-light irradiation. In this study, various organic bromides such as 1°, 2°, and 3°-alkyl bromides and aryl bromides were converted to deuterated products in good to excellent yields and D-incorporation.

Remote Radical Azidation of Unactivated C(sp3)-H Bonds in Sulfamoyl Azides

An efficient method for remote radical C(sp3)-H azidation at unactivated sites is described. C-H functionalization proceeds via intramolecular 1,5-hydrogen atom transfer to N-centered radicals that are generated via azido group transfer and/or fragmentation. The readily installed sulfamoyl azide serves as both an amidyl radical precursor and an azido source. This reaction features excellent site selectivity for tertiary, secondary, primary, and benzylic C(sp3)-H bonds and exhibits broad functional group compatibility.

Graphene Oxide Membrane Reactor for Electrochemical Deuteration Reactions

The deuteration of organic molecules is considerably important in organic and medicinal chemistry. An electrochemical membrane reactor using proton-conducting graphene oxide (GO) nanosheets was developed to synthesize valuable deuterium-labeled products via an efficient hydrogen-to-deuterium (H/D) exchange under mild conditions at ambient temperature and atmospheric pressure. Deuterons (D+) formed by the anodic oxidation of heavy water (D2O) at the Pt/C anode permeate through the GO membrane to the Pt/C cathode, where organic molecules with functional groups (C≡C and C═O) are deuterated with adsorbed atomic D species. Deuteration occurs in outstanding yields with high levels of D incorporation. We also achieved the electrodeuteration of a drug molecule, ibuprofen, demonstrating the promising feasibility of the GO membrane reactor in the pharmaceutical industry.

Electrocatalytic Hydrogenation Using Palladium Membrane Reactors

Hydrogenation is a crucial chemical process employed in a myriad of industries, often facilitated by metals such as Pd, Pt, and Ni as catalysts. Traditional thermocatalytic hydrogenation usually necessitates high temperature and elevated pressure, making the process energy intensive. Electrocatalytic hydrogenation offers an alternative but suffers from issues such as competing H2 evolution, electrolyte separation, and limited solvent selection. This Perspective introduces the evolution and advantages of the electrocatalytic Pd membrane reactor (ePMR) as a solution to these challenges. ePMR utilizes a Pd membrane to physically separate the electrochemical chamber from the hydrogenation chamber, permitting the use of water as the hydrogen source and eliminating the need for H2 gas. This setup allows for greater control over reaction conditions, such as solvent and electrolyte selection, while mitigating issues such as low Faradaic efficiency and complex product separation. Several representative hydrogenation reactions (e.g., hydrogenation of C=C, C≡C, C=O, C≡N, and O=O bonds) achieved via ePMR over the past 30 years were concisely discussed to highlight the unique advantages of ePMR. Promising research directions along with the advancement of ePMR for more challenging hydrogenation reactions are also proposed. Finally, we provide a prospect for future development of this distinctive hydrogenation strategy using hydrogen-permeable membrane electrodes.

Radical Reactions in Organic Synthesis: Exploring in-, on-, and with-Water Methods

Radical reactions in water or aqueous media are important for organic synthesis, realizing high-yielding processes under non-toxic and environmentally friendly conditions. This overview includes (i) a general introduction to organic chemistry in water and aqueous media, (ii) synthetic approaches in, on, and with water as well as in heterogeneous phases, (iii) reactions of carbon-centered radicals with water (or deuterium oxide) activated through coordination with various Lewis acids, (iv) photocatalysis in water and aqueous media, and (v) synthetic applications bioinspired by naturally occurring processes. A wide range of chemical processes and synthetic strategies under different experimental conditions have been reviewed that lead to important functional group translocation and transformation reactions, leading to the preparation of complex molecules. These results reveal how water as a solvent/medium/reagent in radical chemistry has matured over the last two decades, with further discoveries anticipated in the near future.

The Future of (Radio)-Labeled Compounds in Research and Development within the Life Science Industry

In this review the applications of isotopically labeled compounds are discussed and put into the context of their future impact in the life sciences. Especially discussing their use in the pharma and crop science industries to follow their fate in the environment, in vivo or in complex matrices to understand the potential harm of new chemical structures and to increase the safety of human society.

Assignment of the absolute configuration of molecules that are chiral by virtue of deuterium substitution using chiral tag molecular rotational resonance spectroscopy

Chiral tag molecular rotational resonance (MRR) spectroscopy is used to assign the absolute configuration of molecules that are chiral by virtue of deuterium substitution. Interest in the improved performance of deuterated active pharmaceutical ingredients has led to the development of precision deuteration reactions. These reactions often generate enantioisotopomer reaction products that pose challenges for chiral analysis. Chiral tag rotational spectroscopy uses noncovalent derivatization of the enantioisotopomer to create the diastereomers of the 1:1 molecular complexes of the analyte and a small, chiral molecule. Assignment of the absolute configuration requires high-confidence determinations of the structures of these weakly bound complexes. A general search method, CREST, is used to identify candidate geometries. Subsequent geometry optimization using dispersion corrected density functional theory gives equilibrium geometries with sufficient accuracy to identify the isomers of the chiral tag complexes produced in the pulsed jet expansion used to introduce the sample into the MRR spectrometer. Rotational constant scaling based on the fact that the diastereomers have the same equilibrium geometry gives accurate predictions allowing identification of the homochiral and heterochiral tag complexes and, therefore, assignment of absolute configuration. The method is successfully applied to three oxygenated substrates from enantioselective Cu-catalyzed alkene transfer hydrodeuteration reaction chemistry.

External Flash Generation of Carbenoids Enables Monodeuteration of Dihalomethanes

In this study, incorporation of one deuterium atom was achieved by H-D exchange of one of the two identical methylene protons in various dihalomethanes (halogen=Cl, Br, and I) through a rapid-mixing microflow reaction of lithium diisopropylamide as a strong base and deuterated methanol as a deuteration reagent. Generation of highly unstable carbenoid intermediate and suppression of its decomposition were successfully controlled under high flow-rate conditions. Monofunctionalization of diiodomethane afforded various building blocks composed of boryl, stannyl, and silyl groups. The monodeuterated diiodomethane, which served as a deuterated C1 source, was subsequently subjected to diverted functionalization methods to afford various products including biologically important molecules bearing isotope labelling at specific positions and homologation products with monodeuteration.

Carbon Dioxide Radical Anion by Photoinduced Equilibration between Formate Salts and [11C, 13C, 14C]CO2: Application to Carbon Isotope Radiolabeling

The need for carbon-labeled radiotracers is increasingly higher in drug discovery and development (carbon-14, β^-, t_1/2 = 5730 years) as well as in positron emission tomography (PET) for in vivo molecular imaging applications (carbon-11, β⁺, t_1/2 = 20.4 min). However, the structural diversity of radiotracers is still systematically driven by the narrow available labeled sources and methodologies. In this context, the emergence of carbon dioxide radical anion chemistry might set forth potential unexplored opportunities. Based on a dynamic isotopic equilibration between formate salts and [¹³C, ¹⁴C, ¹¹C]CO₂, C-labeled radical anion CO₂^•- could be accessed under extremely mild conditions within seconds. This methodology was successfully applied to hydrocarboxylation and dicarboxylation reactions in late-stage carbon isotope labeling of pharmaceutically relevant compounds. The relevance of the method in applied radiochemistry was showcased by the whole-body PET biodistribution profile of [¹¹C]oxaprozin in mice.

Deuterium in drug discovery: progress, opportunities and challenges

Substitution of a hydrogen atom with its heavy isotope deuterium entails the addition of one neutron to a molecule. Despite being a subtle change, this structural modification, known as deuteration, may improve the pharmacokinetic and/or toxicity profile of drugs, potentially translating into improvements in efficacy and safety compared with the non-deuterated counterparts. Initially, efforts to exploit this potential primarily led to the development of deuterated analogues of marketed drugs through a 'deuterium switch' approach, such as deutetrabenazine, which became the first deuterated drug to receive FDA approval in 2017. In the past few years, the focus has shifted to applying deuteration in novel drug discovery, and the FDA approved the pioneering de novo deuterated drug deucravacitinib in 2022. In this Review, we highlight key milestones in the field of deuteration in drug discovery and development, emphasizing recent and instructive medicinal chemistry programmes and discussing the opportunities and hurdles for drug developers, as well as the questions that remain to be addressed.

C-H Bond Activation Relay (CHAR) of Proline Ester Derivatives Promoted by In Situ Triarylamine Radical Cation: Selective Synthesis of 4-Bromopyrrole Derivatives

Using the in situ generated triarylamine radical cation as an initiator, the sp³ C-H bond of proline esters was smoothly oxidized and brominated through C-H activation relay (CHAR), giving a series of 4-bromopyrroles in good yields with high regioselectivity. The mechanistic study revealed that the oxidation of the active C-H bond initiated the followed 1,5-HAT and bromination, which provides a new method to realize the functionalization of the remote C-H bond.

Photocatalytic Deuterium Atom Transfer Deuteration of Electron-Deficient Alkenes with High Functional Group Tolerance

Due to its mild reaction conditions and unique chemoselectivity, hydrogen atom transfer (HAT) hydrogenation represents an indispensable method for the synthesis of complex molecules. Its analog using deuterium, deuterium atom transfer (DAT) deuteration, is expected to enable access to complex deuterium-labeled compounds. However, DAT deuteration has been scarcely studied for synthetic purposes, and a method that possesses the favorable characteristics of HAT hydrogenations has remained elusive. Herein, we report a protocol for the photocatalytic DAT deuteration of electron-deficient alkenes. In contrast to the previous DAT deuteration, this method tolerates a variety of synthetically useful functional groups including haloarenes. The late-stage deuteration also allows access to deuterated amino acids as well as donepezil-d₂ . Thus, this work demonstrates the potential of DAT chemistry to become the alternative method of choice for preparing deuterium-containing molecules.

Catalytic asymmetric deuterosilylation of exocyclic olefins with mannose-derived thiols and deuterium oxide

Metal-free, photoinduced asymmetric deuterosilylation of exocyclic olefins has been achieved using a mannose-derived thiol catalyst.

Catalytic Vilsmeier-Haack Reactions for C1-Deuterated Formylation of Indoles

The Vilsmeier-Haack reaction is a powerful tool to introduce formyl groups into electron-rich arenes, but its wide application is significantly restricted by stoichiometric employment of caustic POCl₃. Herein, we reported a catalytic version of the Vilsmeier-Haack reaction enabled by a P(III)/P(V)═O cycle. This catalytic reaction provides a facile and efficient route for the direct construction of C1-deuterated indol-3-carboxaldehyde under mild conditions with stoichiometric DMF-d7 as the deuterium source. The products feature a remarkably higher deuteration level (>99%) than previously reported ones and are not contaminated by the likely unselective deuteration at other sites. The present transformation can also be used to transfer other carbonyl groups. Further downstream derivatizations of these deuterated products manifested their potential applications in the synthesis of deuterated bioactive molecules. Mechanistic insight was disclosed from studies of kinetics and intermediates.

Radical deuteration

Deuterated chemicals are becoming irreplaceable in pharmaceutical engineering, material science and synthetic chemistry. Many excellent reviews have discussed acid/base-dependent or metal-catalyzed deuteration reactions, but radical deuterations have been discussed less. With the development of radical chemistry, there has been a rapid growth in radical deuterium-labelling technology. Diverse mild, cheap and efficient strategies for deuterium atom installation have been reported, and this review summarizes the recent achievements of radical deuteration classified by the reaction types.

Visible-light mediated catalytic asymmetric radical deuteration at non-benzylic positions

Site- and enantioselective incorporation of deuterium into organic compounds is of broad interest in organic synthesis, especially within the pharmaceutical industry. While catalytic approaches relying on two-electron reaction manifolds have allowed for stereoselective delivery of a formal deuteride (D^–) or deuteron (D⁺) at benzylic positions, complementary strategies that make use of one-electron deuterium atom transfer and target non-benzylic positions remain elusive. Here we report a photochemical approach for asymmetric radical deuteration by utilizing readily available peptide- or sugar-derived thiols as the catalyst and inexpensive deuterium oxide as the deuterium source. This metal-free platform enables four types of deuterofunctionalization reactions of exocyclic olefins and allows deuteration at non-benzylic positions with high levels of enantioselectivity and deuterium incorporation. Computational studies reveal that attractive non-covalent interactions are responsible for stereocontrol. We anticipate that our findings will open up new avenues for asymmetric deuteration.

Catalytic asymmetric deuterations rely largely on two-electron reaction manifolds and are mostly limited to benzylic positions. Here, a metal-free platform using peptide- or sugar-derived chiral thiols and deuterium oxide allows for asymmetric open-shell deuteration at non-benzylic positions under visible-light irradiation.

Highly selective single and multiple deuteration of unactivated C(sp3)-H bonds

Selective deuteration of unactivated C(sp3)-H bonds is a highly attractive but challenging subject of research in pharmaceutical chemistry, material science and synthetic chemistry. Reported herein is a practical, highly selective and economical efficient hydrogen/deuterium (H/D) exchange of unactivated C(sp3)-H bonds by synergistic photocatalysis and hydrogen atom transfer (HAT) catalysis. With the easily prepared PMP-substituted amides as nitrogen-centered radical precursors, a wide range of structurally diverse amides can undergo predictable radical H/D exchange smoothly with inexpensive D2O as the sole deuterium source, giving rise to the distal tertiary, secondary and primary C(sp3)-H bonds selectively deuterated products in yields of up to 99% and excellent D-incorporations. In addition to precise monodeuteration, this strategy can also achieve multideuteration of the substrates contain more than one remote C(sp3)-H bond, which opens a method to address multi-functionalization of distal unactivated C(sp3)-H bonds.

Site-Specific and Degree-Controlled Alkyl Deuteration via Cu-Catalyzed Redox-Neutral Deacylation

Deuterated organic compounds have become increasingly important in many areas; however, it remains challenging to install deuterium site-selectively to unactivated aliphatic positions with control of the degree of deuteration. Here, we report a Cu-catalyzed degree-controlled deacylative deuteration of diverse alkyl groups with the methylketone (acetyl) moiety as a traceless activating group. The use of N-methylpicolino-hydrazonamide (MPHA) promotes efficient aromatization-driven C-C cleavage. Mono-, di-, and trideuteration at specific sites can be selectively achieved. The reaction is redox-neutral with broad functional group tolerance. The utility of this method has been demonstrated in forming a complete set of deuterated ethyl groups, merging with the Diels-Alder reaction, a net devinylative deuteration, and the synthesis of the d₂-analogue of Austedo.

Easy-to-Implement Hydrogen Isotope Exchange for the Labeling of N-Heterocycles, Alkylkamines, Benzylic Scaffolds, and Pharmaceuticals

Facilitating access to deuterated and tritiated complex molecules is of paramount importance due to the fundamental role of isotopically labeled compounds in drug discovery and development. Deuterated analogues of drugs are extensively used as internal standards for quantification purposes or as active pharmaceutical ingredients, whereas tritiated drugs are essential for preclinical ADME studies. In this report, we describe the labeling of prevalent substructures in FDA-approved drugs such as azines, indoles, alkylamine moieties, or benzylic carbons by the in situ generation of Rh nanoparticles able to catalyze both C(sp²)-H and C(sp³)-H activation processes. In this easy-to-implement labeling process, Rh nanocatalysts are formed by decomposition of a commercially available rhodium dimer under a deuterium or tritium gas atmosphere (1 bar or less), using the substrate itself as a surface ligand to control the aggregation state of the resulting metallic clusters. It is noteworthy that the size of the nanoparticles observed is surprisingly independent of the substrate used and is homogeneous, as evidenced by transmission electron microscopy experiments. This method has been successfully applied to the one-step synthesis of (1) deuterated pharmaceuticals usable as internal standards for MS quantification and (2) tritiated drug analogues with very high molar activities (up to 113 Ci/mmol).

C-H deuteration of organic compounds and potential drug candidates

C-H deuteration has been intricately developed to satisfy the urgent need for site-selectively deuterated organic frameworks. Deuteration has been primarily used to study kinetic isotope effects of reactions but recently its significance in pharmaceutical chemistry has been discovered. Deuterium labelled compounds have stolen the limelight since the inception of the first FDA-approved deuterated drug, for the treatment of chorea-associated Huntington's disease, and their pharmacological importance was realised by chemists, although surprisingly very late. Various approaches were developed to carry out site-selective deuteration. However, the most common and efficient method is hydrogen isotope exchange (HIE). This review summarises deuteration methods of various organic motifs containing C(sp²)-H and C(sp³)-H bonds utilizing C-H bond functionalisation as a key step along with a variety of catalysts, and exemplifies their biological relevance.

Convergent Synthesis of 1,4-Dicarbonyl Z-Alkenes through Three-Component Coupling of Alkynes, α-Diazo Sulfonium Triflate, and Water

We report a general protocol for the convergent synthesis of 1,4-dicarbonyl Z-alkenes form alkynes using α-diazo sulfonium triflate and water. The C═O, C═C, and C-H bonds are formed under mild conditions with a wide range of functional groups tolerated. The reaction exhibits excellent Z-selectivity and complete regioselectivity. The resulting 1,4-dicarbonyl Z-alkenes can smoothly undergo follow-up conversion to a variety of heteroaromatic scaffolds. Moreover, the reaction also provides a facile access to the corresponding deuterated Z-alkenes and deuterated heteroarenes with a high level of deuterium incorporation (90-97% D-inc.) by directly using D₂O, thus rendering the method highly valuable. The comprehensive mechanistic studies indicate that a free carbyne radical intermediate is formed via the photocatalytic single electron transfer process, and KH₂PO₄ plays a crucial role in significant improvements on yield and selectivity based on density-functional theory calculations, providing a new direction for radical coupling reactions of diazo compounds.

Recent Developments for the Deuterium and Tritium Labeling of Organic Molecules

Organic compounds labeled with hydrogen isotopes play a crucial role in numerous areas, from materials science to medicinal chemistry. Indeed, while the replacement of hydrogen by deuterium gives rise to improved absorption, distribution, metabolism, and excretion (ADME) properties in drugs and enables the preparation of internal standards for analytical mass spectrometry, the use of tritium-labeled compounds is a key technique all along drug discovery and development in the pharmaceutical industry. For these reasons, the interest in new methodologies for the isotopic enrichment of organic molecules and the extent of their applications are equally rising. In this regard, this Review intends to comprehensively discuss the new developments in this area over the last years (2017-2021). Notably, besides the fundamental hydrogen isotope exchange (HIE) reactions and the use of isotopically labeled analogues of common organic reagents, a plethora of reductive and dehalogenative deuteration techniques and other transformations with isotope incorporation are emerging and are now part of the labeling toolkit.

Site-Selective Coupling of Remote C(sp3)−H/meta-C(sp2)−H Bonds Enabled by Ru/Photoredox Dual Catalysis and Mechanistic Studies

Construction of C(sp2)−C(sp3) bonds via regioselective coupling of C(sp2)−H/C(sp3)−H bonds is challenging due to the low reactivity and regioselectivity of C−H bonds. Here, a novel photoinduced Ru/photocatalyst-cocatalyzed regioselective cross-dehydrogenative coupling...

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.

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.