Highlights of aliphatic C(sp3 )-H hydrogen isotope exchange reactions

Synthesis of Deuterated Compounds by Flow Chemistry

Development of the efficient and practical method for the synthesis of deuterated compounds which occupies the broadest area among stable isotopes is one of the most essential issues toward the industrial advance and building a sustainable society. This review describes recent advances in deuteration reactions, where the continuous flow chemistry plays pivotal roles for the successful installation of deuterium atom into diverse organic frameworks, opening new fields of isotope-based synthetic chemistry.

Deuterated Alkyl Sulfonium Salt Reagents; Importance of H/D Exchange Methods in Drug Discovery

Deuterated drugs (heavy drugs) have recently been spotlighted as a new modality for small-molecule drugs because the pharmacokinetics of pharmaceutical drugs can be enhanced by replacing C-H bonds with more stable C-D bonds at metabolic positions. Therefore, deuteration methods for drug candidates are a hot topic in medicinal chemistry. Among them, the H/D exchange reaction (direct transformation of C-H bonds to C-D bonds) is a useful and straightforward method for creating novel deuterated target molecules, and over 20 reviews on the synthetic methods related to H/D exchange reactions have been published in recent years. Although various deuterated drug candidates undergo clinical trials, approved deuterated drugs possess CD3 groups in the same molecule. However, less diversification, except for the CD3 group, is a problem for future medicinal chemistry. Recently, we developed various deuterated alkyl (dn-alkyl) sulfonium salts based on the H/D exchange reaction of the corresponding hydrogen form using D2O as an inexpensive deuterium source to introduce CD3, CH3CD2, and ArCH2CD2 groups into drug candidates. This concept summarises recent reviews related to H/D exchange reactions and novel reagents that introduce the CD3 group, and our newly developed electrophilic dn-alkyl reagents are discussed.

In situ Generated Iridium Nanoparticles as Hydride Donors in Photoredox-Catalyzed Hydrogen Isotope Exchange Reactions with Deuterium and Tritium Gas

We have studied the photoredox-catalyzed hydrogen isotope exchange (HIE) reaction with deuterium or tritium gas as isotope sources and in situ formed transition metal nanoparticles as hydrogen atom transfer pre-catalysts. By this means we have found synergistic reactivities applying two different HIE mechanisms, namely photoredox-catalyzed and CH-functionalization HIE leading to the synthesis of highly deuterated complex molecules. Finally, we adopted these findings successfully to tritium 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.

Deuterodehalogenation Under Net Reductive or Redox-Neutral Conditions Enabled by Paired Electrolysis

Interest in deuterated active pharmaceutical ingredients (APIs) is increasing as deuteration holds promise for kinetic isotope effect (KIE) regulated fine-tuning of API performance. Moreover, deuterium isotope labeling is frequently carried out to study organic and bioorganic reaction mechanisms and to facilitate complex target synthesis. As such, methods for highly selective deuteration of organic molecules are highly desirable. Herein, we present an electrochemical method for the selective deuterodehalogenation of benzylic halides via a radical-polar crossover mechanism, using inexpensive deuterium oxide (D2 O) as the deuterium source. We demonstrate broad functional group compatibility across a range of aryl and heteroaryl benzylic halides. Furthermore, we uncover a sequential paired electrolysis regime, which permits switching between net reductive and overall redox-neutral reactions of sulfur-containing substrates simply by changing the identity of the sacrificial reductant employed.

Cesium Amide-Catalyzed Selective Deuteration of Benzylic C-H Bonds with D2 and Application for Tritiation of Pharmaceuticals

Hydrogen isotope exchange (HIE) represents one of the most attractive labeling methods to synthesize deuterium- and tritium-labeled compounds. Catalytic HIE methods that enable site-selective C-H bond activation and exchange labeling with gaseous isotopes D₂ and T₂ are of vital importance, in particular for high-specific-activity tritiation of pharmaceuticals. As part of our interest in exploring s-block metals for catalytic transformations, we found CsN(SiMe₃ )₂ to be an efficient catalyst for selective HIE of benzylic C-H bonds with D₂ gas. The reaction proceeds through a kinetic deprotonative equilibrium that establishes an exchange pathway between C-H bonds and D₂ gas. By virtue of multiple C-H bonds activation and high activity (isotope enrichment up to 99 %), the simple cesium amide catalyst provided a very powerful and practically convenient labeling protocol for synthesis of highly deuterated compounds and high-specific-activity tritiation of pharmaceuticals.

Synthesis and analysis of isotopically stable labeled nitrosamines as mass spectrometry standards for drug impurity quality control

We describe a simple and easy pathway to synthesize nitrosamine mass spectrometry standards in good to moderate yields. N-alkylation of Boc-protected primary or secondary amines using stable isotope labeled alkyl halides yielded the key intermediates that were deprotected, and then, the nitrosamine was formed with sodium nitrite and sodium hydrogensulfate. Special attention to safety, disposal of waste, and surface cleaning was carried throughout.

Water-soluble NHC Pd/Ni bimetallic nanoparticles for H/D exchange in aromatic amino-acids

Labelling of amino-acids is important for the production of deuterated proteins. However, aromatic amino-acid reduction is a common undesired process with noble-metal nanocatalysts. In this work, we describe a new NHC-stabilized water-soluble Pd/Ni system able to perform H/D exchange reactions in an enantiospecific fashion without reducing the aromatic ring of phenylalanine and tyrosine thanks to a synergetic Pd-Ni effect.

Benzylic deuteration of alkylnitroaromatics via amine-base catalysed exchange with deuterium oxide

This paper describes the deuterium-labelling of alkylnitroaromatics by base-catalysed exchange with deuterium oxide. As the alkyl protons alpha to the aromatic ring are the most acidic sites in the molecule, regioselective hydrogen isotope exchange at this benzylic location leads to a regiospecifically deuterated product. The exchange labelling takes place in good yields and with high atom% abundance in the presence of an appropriate nitrogen base. Alkylated 2,4-dinitrobenzenes deuterate at room temperature under catalysis by triethylamine, whilst alkylated 2-nitro- or 4-nitrobenzenes and related mono-nitroaromatics require higher temperatures and catalysis by 1,5-diazobicyclo[4.3.0]non-5-ene (DBN). The labelling reactions require an inert gas atmosphere, but otherwise are simple and high yielding with no obvious byproducts. Those compounds in which the benzylic protons are in an ortho-orientation with respect to the nitro group label somewhat more slowly than the analogues where there is a para relationship. In addition, higher alkyl homologues undergo benzylic deuteration at slower rates than methyl.

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).

Efficient Aliphatic Hydrogen-Isotope Exchange with Tritium Gas through the Merger of Photoredox and Hydrogenation Catalysts

Employment of a combination of an organophotoredox catalyst with Wilkinson's catalyst (Rh(PPh₃)₃Cl) has given rise to an unprecedented method for hydrogen-isotope exchange (HIE) of aliphatic C(sp³)-H bonds of complex pharmaceuticals using T₂ gas directly. Wilkinson's catalyst, commonly used for catalytic hydrogenations, was exploited as a precatalyst for activation of D₂ or T₂ and hydrogen atom transfer. In this combined methodology and mechanistic study, we demonstrate that by coupling photocatalysis with Rh catalysis, carbon-centered radicals generated via photoredox catalysis can be intercepted by Rh-hydride intermediates to deliver an effective hydrogen atom donor for hydrogen-isotope labeling of complex molecules in one step. By optimizing the ratio of the photocatalyst and Wilkinson's catalyst to balance the rate of the dual catalytic cycles, we can achieve efficient HIE and high recovery yield. This protocol was readily applied to direct HIE of C(sp³)-H bonds in 10 complex drug molecules, showing high isotope incorporation efficiency and exceptionally good functional group tolerance and demonstrating this approach as a practical and attractive labeling method for deuteration and tritiation.

Rhodium-Catalyzed Stereoselective Deuteration of Benzylic C-H Bonds via Reversible η6 -Coordination

We report a convenient method for benzylic H/D exchange of a wide variety of substrates bearing primary, secondary, or tertiary C-H bonds via a reversible η⁶ -coordination strategy. A doubly cationic [Cp^CF3 Rh^III ]²⁺ catalyst that serves as an arenophile facilitates deprotonation of inert benzylic hydrogen atoms (pK_a >40 in DMSO) without affecting other hydrogen atoms, such as those on aromatic rings or in α-positions of carboxylate groups. Notably, the H/D exchange reactions feature high stereoretention. We demonstrated the potential utility of this method by using it for deuterium labeling of ten pharmaceuticals and their analogues.

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.

Tritium hydrogen-isotope exchange with electron-poor tertiary benzenesulfonamide moiety; application in late-stage labeling of T0901317

The multifunctional radioligand [3 H]T0901317 ([3 H]1) has been employed as a powerful autoradiographic tool to target several receptors, such as liver X, farnesoid X, and retinoic acid-related orphan receptor alpha and gamma subtypes at nanomolar concentrations. Although [3 H]1 is commercially available and its synthesis via tritiodebromination has been reported, the market price of this radioligand and the laborious synthesis of corresponding bromo-intermediate potentially preclude its widespread use in biochemical, pharmacological, and pathological studies in research lab settings. We exploit recent reports on hydrogen-isotope exchange (HIE) reactions in tertiary benzenesulfonamides where the sulfonamide represents an ortho-directing group that facilitates CH activation in the presence of homogenous iridium(I) catalysts. Herein, we report a time- and cost-efficient method for the tritium late-stage labeling of compound 1-a remarkably electron-poor substrate owing to the tertiary trifluoroethylsulfonamide moiety. Under a straightforward HIE condition using a commercially available Kerr-type NHC Ir(I) complex, [(cod)Ir (NHC)Cl], the reaction with 1 afforded a specific activity of 10.8 Ci/mmol. Additionally, alternative HIE conditions using the heterogeneous catalyst of Ir-black provided sufficient 0.72 D-enrichment of 1 but unexpectedly failed while repeating with tritium gas.

Palladium Nanoparticles for the Deuteration and Tritiation of Benzylic Positions on Complex Molecules

Palladium nanoparticles (PdNp) were revealed as an efficient hydrogen isotope exchange catalyst for the deuterium and tritium labeling of benzylic positions of complex molecules. A practical way to obtain small palladium nanoparticles and to apply them as a catalyst for hydrogen isotope exchange (HIE) is presented. Several model compounds and popular bioactive molecules were submitted to HIE reactions catalyzed by the PdNp. Benzylic positions situated far away from heteroatoms were labeled with high isotopic enrichments. The observed non-directed HIE gave rise to regioselectivities complementary to those obtained with other methods, which typically require specific directing groups. For this reason, the successful deuteration of a broad variety of benzylic positions created a helpful tool to produce internal LC-MS standards of complex drugs. Furthermore, this nanocatalyst paved the way for the radiolabeling of drug molecules with high specific activities by using low pressures of tritium gas.

Late-Stage β-C(sp3)-H Deuteration of Carboxylic Acids

Carboxylic acids are highly abundant in bioactive molecules. In this study, we describe the late-stage β-C(sp³)-H deuteration of free carboxylic acids. On the basis of the finding that C-H activation with our catalysts is reversible, the de-deuteration process was first optimized. The resulting method uses ethylenediamine-based ligands and can be used to achieve the desired deuteration when using a deuterated solvent. The reported method allows for the functionalization of a wide range of free carboxylic acids with diverse substitution patterns, as well as the late-stage deuteration of bioactive molecules and related frameworks and enables the functionalization of nonactivated methylene β-C(sp³)-H bonds for the first time.

A highly selective decarboxylative deuteration of carboxylic acids

In this paper, we report a mild and practical method for precise deuteration of aliphatic carboxylic acids by synergistic photoredox and HAT catalysis. The reaction delivers excellent D-incorporation (up to 99%) at predicted sites even in substrates bearing reactive C-H bonds or versatile functional groups. The use of a recirculation reactor with a peristaltic pump supports a scalable preparative ability (up to 50 mmol) under very mild reaction conditions. The practical and precise deuteration of readily available complex carboxylic acids makes this protocol promising for the preparation of deuterium-labelled compounds.

Recent advances in visible-light photocatalytic deuteration reactions

The recent advances in visible-light photocatalytic deuteration of X–H, C–halogen, CC, and other bonds for the synthesis of deuterium-labeled organic molecules have been summarized according to the type of bond deuterated in the reactions.

C-H Functionalization-Prediction of Selectivity in Iridium(I)-Catalyzed Hydrogen Isotope Exchange Competition Reactions

An assessment of the C-H activation catalyst [(COD)Ir(IMes)(PPh3 )]PF6 (COD=1,5-cyclooctadiene, IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) in the deuteration of phenyl rings containing different functional directing groups is divulged. Competition experiments have revealed a clear order of the directing groups in the hydrogen isotope exchange (HIE) with an iridium (I) catalyst. Through DFT calculations the iridium-substrate coordination complex has been identified to be the main trigger for reactivity and selectivity in the competition situation with two or more directing groups. We postulate that the competition concept found in this HIE reaction can be used to explain regioselectivities in other transition-metal-catalyzed functionalization reactions of complex drug-type molecules as long as a C-H activation mechanism is involved.

NHC-Stabilized Iridium Nanoparticles as Catalysts in Hydrogen Isotope Exchange Reactions of Anilines

The preparation of N-heterocyclic carbene-stabilized iridium nanoparticles and their application in hydrogen isotope exchange reactions is reported. These air-stable and easy-to-handle iridium nanoparticles showed a unique catalytic activity, allowing selective and efficient hydrogen isotope incorporation on anilines using D2 or T2 as isotopic source. The usefulness of this transformation has been demonstrated by the deuterium and tritium labeling of diverse complex pharmaceuticals.

The Natural Product Lepidiline A as an N-Heterocyclic Carbene Ligand Precursor in Complexes of the Type [Ir(cod)(NHC)PPh3)]X: Synthesis, Characterisation, and Application in Hydrogen Isotope Exchange Catalysis

A range of iridium(I) complexes of the type [Ir(cod)(NHC)PPh3)]X are reported, where the N-heterocyclic carbene (NHC) is derived from the naturally-occurring imidaozlium salt, Lepidiline A (1,3-dibenzyl-4,5-dimethylimidazolium chloride). A range of complexes were prepared, with a number of NHC ligands and counter-ions, and various steric and electronic parameters of these complexes were evaluated. The activity of the [Ir(cod)(NHC)PPh3)]X complexes in hydrogen isotope exchange reactions was then studied, and compared to established iridium(I) complexes.