Photoredox-catalyzed deuteration and tritiation of pharmaceutical compounds

Chemical-Reductant-Free Electrochemical Deuteration Reaction using Deuterium Oxide

We report a method for the electrochemical deuteration of α,β-unsaturated carbonyl compounds under catalyst- and external-reductant-free conditions, with deuteration rates as high as 99 % and yields up to 91 % in 2 h. The use of graphite felt for both the cathode and the anode was key to ensuring chemoselectivity and high deuterium incorporation under neutral conditions without the need for an external reductant. This method has a number of advantages over previously reported deuteration reactions that use stoichiometric metallic reductants. Mechanistic experiments showed that O₂ evolution at the anode not only eliminates the need for an external reductant but also regulates the pH of the reaction mixture, keeping it approximately neutral.

Photocatalysis with organic dyes: facile access to reactive intermediates for synthesis

Organic dyes have emerged as a reliable class of photoredox catalysts. Their great structural variety combined with the easy fine-tuning of their electronic properties has unlocked new possibilities for the generation of reactive intermediates. In this review, we provide an overview of the available approaches to access reactive intermediates that employ organophotocatalysis. Our contribution is not a comprehensive description of the work in the area but rather focuses on key concepts, accompanied by a few selected illustrative examples. The review is organized along the type of reactive intermediates formed in the reaction, including C(sp3) and C(sp 2 ) carbon-, nitrogen-, oxygen-, and sulfur-centered radicals, open-shell charged species, and sensitized organic compounds.

Transition-Metal-Free Carbon Isotope Exchange of Phenyl Acetic Acids

A transition‐metal‐free carbon isotope exchange procedure on phenyl acetic acids is described. Utilizing the universal precursor CO₂, this protocol allows the carbon isotope to be inserted into the carboxylic acid position, with no need of precursor synthesis. This procedure enabled the labeling of 15 pharmaceuticals and was compatible with carbon isotopes [¹⁴C] and [¹³C]. A proof of concept with [¹¹C] was also obtained with low molar activity valuable for distribution studies.

Preparation of cyclohexene isotopologues and stereoisotopomers from benzene

The hydrogen isotopes deuterium (D) and tritium (T) have become essential tools of chemistry, biology, and medicine.¹ Beyond their widespread use in spectroscopy, mass spectrometry, and mechanistic and pharmacokinetic studies, there has been considerable interest in incorporating deuterium into drug molecules.¹ The deuterium kinetic isotope effect (DKIE), which compares the rate of a chemical reaction for a compound to its deuterated counterpart, can be dramatic.^1–3 The strategic replacement of hydrogen with deuterium can affect both the rate of metabolism and distribution of metabolites for a compound,⁴ improving the efficacy and safety of the drug. Deutetrabenazine, a promising treatment for Huntington’s disease,⁵ recently became the first deuterated drug to win FDA-approval.The pharmacokinetics of a deuterated compound depend on the location(s) of D. While methods currently exist for deuterium incorporation at both early and late stages of a drug’s synthesis,^6–7 these processes are often unselective and the stereoisotopic purity can be difficult to measure.^7–8 Here, we describe the preparation of stereoselectively deuterated building blocks for pharmaceutical research. As a proof of concept, we demonstrate a four-step conversion of benzene to cyclohexene with varying degrees of D incorporation, as bound to a tungsten complex. Using different combinations of deuterated and proteated acid and hydride reagents, the deuterated positions can be precisely controlled on the cyclohexene ring. In total, 52 unique stereoisotopomers of cyclohexene are available, in the form of ten different isotopologues. This concept can be extended to prepare discrete stereoisotopomers of functionalized cyclohexenes. Such systematic methods for the preparation of pharmacologically active compounds as discrete stereoisotopomers could improve pharmacological and toxicological properties of drugs and provide new mechanistic information related to their distribution and metabolism in the body.

Base-Directed Photoredox Activation of C-H Bonds by PCET

Photoredox catalysis using proton-coupled electron transfer (PCET) has emerged as a powerful method for bond transformations. We previously employed traditional chemical oxidants to achieve multiple-site concerted proton-electron transfer (MS-CPET) activation of a C-H bond in a proof-of-concept fluorenyl-benzoate substrate. As described here, photoredox oxidation of the fluorenyl-benzoate follows the same rate constant vs driving force trend determined for thermal MS-CPET. Analogous photoredox catalysis enables C-H activation and H/D exchange in a number of additional substrates with favorably positioned bases. Mechanistic studies support our hypothesis that MS-CPET is a viable pathway for bond activation for substrates in which the C-H bond is weak, while stepwise carboxylate oxidation and hydrogen atom transfer likely predominate for stronger C-H bonds.

Powering Artificial Enzymatic Cascades with Electrical Energy

We have developed a scalable platform that employs electrolysis for an in vitro synthetic enzymatic cascade in a continuous flow reactor. Both H₂ and O₂ were produced by electrolysis and transferred through a gas-permeable membrane into the flow system. The membrane enabled the separation of the electrolyte from the biocatalysts in the flow system, where H₂ and O₂ served as electron mediators for the biocatalysts. We demonstrate the production of methylated N-heterocycles from diamines with up to 99 % product formation as well as excellent regioselective labeling with stable isotopes. Our platform can be applied for a broad panel of oxidoreductases to exploit electrical energy for the synthesis of fine chemicals.

Lightdriven deracemization enabled by excitedstate electron transfer

Deracemization is an attractive strategy for asymmetric synthesis, but intrinsic energetic challenges have limited its development. Here, we report a deracemization method in which amine derivatives undergo spontaneous optical enrichment upon exposure to visible light in the presence of three distinct molecular catalysts. Initiated by an excited-state iridium chromophore, this reaction proceeds through a sequence of favorable electron, proton, and hydrogen-atom transfer steps that serve to break and reform a stereogenic C-H bond. The enantioselectivity in these reactions is jointly determined by two independent stereoselective steps that occur in sequence within the catalytic cycle, giving rise to a composite selectivity that is higher than that of either step individually. These reactions represent a distinct approach to creating out-of-equilibrium product distributions between substrate enantiomers using excited-state redox events.

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.

Bringing biocatalytic deuteration into the toolbox of asymmetric isotopic labelling techniques

Enzymes dependent on nicotinamide cofactors are important components of the expanding range of asymmetric synthetic techniques. New challenges in asymmetric catalysis are arising in the field of deuterium labelling, where compounds bearing deuterium (²H) atoms at chiral centres are becoming increasingly desirable targets for pharmaceutical and analytical chemists. However, utilisation of NADH-dependent enzymes for ²H-labelling is not straightforward, owing to difficulties in supplying a suitably isotopically-labelled cofactor ([4-²H]-NADH). Here we report on a strategy that combines a clean reductant (H₂) with a cheap source of ²H-atoms (²H₂O) to generate and recycle [4-²H]-NADH. By coupling [4-²H]-NADH-recycling to an array of C=O, C=N, and C=C bond reductases, we demonstrate asymmetric deuteration across a range of organic molecules under ambient conditions with near-perfect chemo-, stereo- and isotopic selectivity. We demonstrate the synthetic utility of the system by applying it in the isolation of the heavy drug (1S,3'R)-[2',2',3'-²H₃]-solifenacin fumarate on a preparative scale.

Water Compatible Hypophosphites-d2 Reagents: Deuteration Reaction via Deutero-deiodination in Aqueous Solution

Contrary to conventional deuteration approaches which typically entail deuterated solvents and/or moisture exclusion, an unprecedented deutero-deiodination reaction attainable in aqueous (H₂O) solution is presented herein. By utilizing the stability of inorganic deuterated calcium/sodium hypophosphites against wayward H/D isotopic exchange within pH 2.5-11.7, these shelf-stable, nontoxic, cost-effective, and environmentally benign deuteration reagents mediate deuteration of a broad range alkyl and aryl iodides with ample isotopic incorporation in aqueous (H₂O) solution.

Aminoalkyl radicals as halogen-atom transfer agents for activation of alkyl and aryl halides

Organic halides are important building blocks in synthesis, but their use in (photo)redox chemistry is limited by their low reduction potentials. Halogen-atom transfer remains the most reliable approach to exploit these substrates in radical processes despite its requirement for hazardous reagents and initiators such as tributyltin hydride. In this study, we demonstrate that α-aminoalkyl radicals, easily accessible from simple amines, promote the homolytic activation of carbon-halogen bonds with a reactivity profile mirroring that of classical tin radicals. This strategy conveniently engages alkyl and aryl halides in a wide range of redox transformations to construct sp3-sp3, sp3-sp2, and sp2-sp2 carbon-carbon bonds under mild conditions with high chemoselectivity.

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.

Synthesis of Enantioenriched α-Deuterated α-Amino Acids Enabled by an Organophotocatalytic Radical Approach

A mild, versatile organophotoredox protocol has been developed for the preparation of diverse, enantioenriched α-deuterated α-amino acids. Distinct from the well-established two-electron transformations, this radical-based strategy offers the unrivaled capacity of the convergent unification of readily accessible feedstock carboxylic acids and a chiral methyleneoxazolidinone fragment and the simultaneous highly diastereo-, chemo-, and regioselective incorporation of deuterium. Furthermore, the approach has addressed the long-standing challenge of the installation of sterically demanding side chains into α-amino acids.

Deuteration of Formyl Groups via a Catalytic Radical H/D Exchange Approach

H/D exchange at formyl groups represents the straightforward approach to C-1 deuterated aldehydes. This transformation has been recently realized by transition metal and NHC carbene catalysis. Mechanistically, all these processes involve an ionic pathway. Herein we report a distinct photoredox catalytic, visible light mediated neutral radical approach. Selective control of highly reactive acyl radical in the energy barrier surmountable, reversible reaction enables driving the formation of deuterated products when an excess of D2O is employed. The power of the H/D exchange process has been demonstrated for not only aromatic aldehydes, but also aliphatic substrates, which have been difficult in transitional metal catalyzed H/D exchange reactions, and for selective late-stage deuterium incorporation into complex structures with uniformly high deuteration level (>90%).

Base metal-catalyzed hydrogen isotope exchange

Hydrogen isotope exchange (HIE) has played an increasingly important role in deuteration and tritiation of compounds in the pharmaceutical industry. Transition metal-catalyzed HIE methods have gained considerable attention in the past decades, and most of these methods were comprehensively reviewed in 2010 in a special JLCR issue. It covered a wide variety of HIE catalysis systems involving precious metal catalysts, and a relatively small percentage of base metal catalysts, with a major focus on heterogeneous nickel. While base metal catalysts have remained underdeveloped for HIE chemistry relative to second and third row transition metal catalysts, in recent years, the first examples of homogeneous iron, nickel, and cobalt catalysts have been introduced to the field. Hence, in this review, we describe the recent development of base metal catalysts for HIE and their applications in isotopic labeling of pharmaceutical compounds. These research efforts have resulted in the development of labeling approaches that complement traditional methods in terms of activity and selectivity, thus diversifying the methodologies available for isotope chemists.

Practical Method for Reductive Deuteration of Ketones with Magnesium and D2O

α-Deuterated alcohols have important applications in pharmaceuticals and mechanism studies. Here, we report a new and practical strategy for the reductive deuteration of ketones using a Mg/BrCH₂CH₂Br/D₂O system, which affords α-deuterated alcohols in good yields and with almost quantitative incorporation of deuterium. The synthetic value of this method has been demonstrated by the easy access to deuterated drugs or drug derivatives. This method may inspire the discovery of other deuterium-containing drugs.

Continuous-Flow Hydrogenation and Reductive Deuteration of Nitriles: a Simple Access to α,α-Dideutero Amines

A simple and efficient continuous flow methodology has been developed for hydrogenation and reductive deuteration of nitriles to yield primary amines and also valuable α,α-dideutero analogues. Raney nickel proved to be a useful catalyst for the transformation of a wide range of nitriles under reasonably mild conditions with excellent deuterium incorporation (>90 %) and quantitative conversion. Among known model compounds, three new deuterated primary amines were prepared. The large-scale synthesis of deuterated tryptamine was also carried out to deliver 1.1 g product under flow conditions.

Carbon isotope labeling of carbamates by late-stage [11C], [13C] and [14C]carbon dioxide incorporation

A procedure which allows labelling cyclic carbamates with all carbon isotopes has been developed. This protocol valorizes carbon dioxide, the universal building block for radiolabeling. A series of pharmaceuticals were obtained and a disconnection/reconnection strategy was implemented.

The unique catalytic role of water in aromatic C–H activation at neutral pH: a combined NMR and DFT study of polyphenolic compounds

NMR and DFT studies demonstrate the unique catalytic role of H₂O in aromatic C–H activation which results in a reduction of ΔG^# values by a factor of 20–30 kcal mol⁻¹.

Iron-mediated deuterium addition cascade cyano insertion/cyclization of N-arylacrylamides to access deuterium-labelled phenanthridines

Deuterated molecules feature important biological activities and pharmacokinetic properties for the design of pharmaceuticals.

Ag(i)-Mediated hydrogen isotope exchange of mono-fluorinated (hetero)arenes

An efficient approach to install deuterium into mono-fluorinated (hetero)arenes by a Ag₂CO₃/Sphos-mediated HIE protocol with D₂O as the deuterium source has been disclosed.

External-photocatalyst-free visible-light-mediated aerobic oxidation and 1,4-bisfunctionalization of N-alkyl isoquinolinium salts

Visible-light-induced aerobic alternate transformations of N-alkyl isoquinolinium/quinolinium salts in the absence of any external photocatalyst have been developed.

Design and application of aminoacridinium organophotoredox catalysts

Recent developments in preparative photocatalysis have reshaped synthetic strategies and now represent an integral part of current organic chemistry. Due to their favourable electrochemical and photophysical properties, the nowadays most frequently used photocatalysts are based on precious Ru- and Ir-polypyridyl complexes. Apart from that, organic catalysts such as the acridinium salts are now commonly employed to complement transition metals to provide potentially sustainable strategies amenable to large-scale synthesis. In this feature article, the design, synthesis and application of aminoacridinium photoredox catalysts as well as their exceptionally broad range of redox properties are highlighted. Due to their modularity, this burgeoning class of organophotocatalysts is anticipated to contribute significantly to synthetic methodology development and the translation to a wide range of innovative implementations.

Mild, Selective Ru-Catalyzed Deuteration Using D2 O as a Deuterium Source

A method for the selective deuteration of polyfunctional organic molecules using catalytic amounts of [RuCl2 (PPh3 )3 ] and D2 O as a deuterium source is presented. Through variation of additives like CuI, KOH, and various amounts of zinc powder, orthogonal chemoselectivities in the deuteration process are observed. Mechanistic investigation indicates the presence of different, defined Ru-complexes under the given specific conditions.

Formyl-selective deuteration of aldehydes with D2O via synergistic organic and photoredox catalysis

Formyl-selective deuteration of aldehydes is of high interest for labeling purposes and for optimizing properties of drug candidates. Herein, we report a mild general method for formyl-selective deuterium labeling of aldehydes with D₂O, an inexpensive deuterium source, via a synergistic combination of light-driven, polyoxometalate-facilitated hydrogen atom transfer and thiol catalysis. This highly efficient, scalable reaction showed excellent deuterium incorporation, a broad substrate scope, and excellent functional group tolerance and selectivity and is therefore a practical method for late-stage modification of synthetic intermediates in medicinal chemistry and for generating libraries of deuterated compounds.

Formyl-selective deuteration of aldehydes with D₂O mediated by the synergistic combination of light-driven, polyoxometalate-facilitated HAT and thiol catalysis is reported.

Palladium(0)/benzoic acid catalysis merges sequences with D2O-promoted labelling of C-H bonds

The combination of a Pd(0) complex with benzoic acid in the presence of D2O enables the synthesis of valuable families of highly deuterated organics through elaborate sequential reactions. The catalytic system can convert 2-butyne fragments into the corresponding d-dienamides, which can then readily deliver labeled polycyclic quinone motifs. Propargylated tryptamines lead to formation of highly enriched tetrahydrocarbolines through the C-H activation of their unprotected indole ring. Mechanistic studies reveal the ordered series of events that regulate the outcome of these complex reactions, which include multiple, sequential and selective H/D scrambling from the cheapest and safest deuterium source.

B(C6F5)3-Catalyzed α-Deuteration of Bioactive Carbonyl Compounds with D2O

An efficient deuteration process of α-C-H bonds in various carbonyl-based pharmaceutical compounds has been developed. Catalytic reactions are initiated by the action of Lewis acidic B(C₆F₅)₃ and D₂O, converting a drug molecule into the corresponding boron-enolate. Ensuing deuteration of the enolate by in situ-generated D₂O⁺-H then results in the formation of α-deuterated bioactive carbonyl compounds with up to >98% deuterium incorporation.

Practical Synthesis of C-1 Deuterated Aldehydes Enabled by NHC Catalysis

The recent surge in applications of deuterated pharmaceutical agents has created an urgent demand for synthetic methods that efficiently generate deuterated building blocks. Here we show that N-heterocyclic carbenes (NHC) promote a reversible hydrogen-deuterium exchange (HDE) reaction with simple aldehydes, which leads to a practical approach to synthetically valuable C-1 deuterated aldehydes. The reactivity of the well-established NHC catalysed formation of Breslow intermediates from aldehydes is reengineered to overcome the overwhelmingly kinetically favorable, irreversible benzoin condensation reaction and achieve the critical reversibility to drive the formation of desired deuterated products when an excess of D₂O is employed. Notably, this operationally simple and cost-effective protocol serves as a general and truly practical approach to all types of 1-D-aldehydes including aryl, -alkyl and -alkenyl aldehydes and enables chemoselective late-stage deuterium incorporation into complex, native therapeutic agents and natural products with uniformly high levels (>95%) of deuterium incorporation for a total of 104 substrates tested.