Base-Mediated Deuteration of Organic Molecules: A Mechanistic Insight

H/D Exchange of Aromatic Sulfones via Base Promotion and Silver Catalysis

Aromatic sulfones are the prevailing scaffolds in pharmaceutical and material sciences. However, compared to their widespread application, the selective deuterium labeling of these structures is restricted due to their electron-deficient properties. This study presents two comprehensive strategies for the deuteration of aromatic sulfones. The base-promoted deuteration uses DMSO-d6 as the deuterium source, resulting in a rapid H/D exchange within 2 h. Meanwhile, a silver-catalyzed protocol offers a much milder option by using economical D2O to furnish the labeled sulfones.

Catalyst-Free α-trans-Selective Hydroboration and (E)-Selective Deuterated Semihydrogenation of Alkynyl Sulfones

Here, we present a straightforward α-trans-selective hydroboration of alkynyl sulfones with NHC-boranes without the need for a catalyst. This reaction is compatible with a wide range of substrates for efficiently producing structurally diverse α-borylated vinyl sulfones in satisfactory yields. The hydride transfer from NHC-borane 2a to alkynyl triflone 1b is studied by density functional theory (DFT) calculations for trans-hydroboration. Moreover, a regiodivergent deuterated semihydrogenation of alkynyl triflones has also been developed using D2O as the deuterium source. A variety of diversity-oriented D-containing vinyl triflones were prepared in good to excellent yields with excellent deuterium incorporation ratios. Synthetic manipulations of the deuterated products are achieved for the conversion into valuable deuterated molecules, indicating the utility of this protocol.

Alkali-metal bases in catalytic hydrogen isotope exchange processes

The preparation of compounds labelled with deuterium or tritium has become an essential tool in a range of research fields. Hydrogen isotope exchange (HIE) offers direct access to said compounds, introducing these isotopes in a late stage. Even though the field has rapidly advanced with the use of transition metal catalysis, alkali-metal bases, used as catalysts or under stoichiometric conditions, have also emerged as a viable alternative. In this minireview we describe the latest advances in the use of alkali-metal bases in HIE processes, showcasing their synthetic potential as well as current challenges in the field. It is divided in different sections based on the isotope source used, emphasizing their benefits, disadvantages and limitations. The influence on the choice of alkali-metal in these processes as well as their possible mechanistic pathways are also discussed.

Programmable Deuteration of Indoles via Reverse Deuterium Exchange

Methods for selective deuterium incorporation into drug-like molecules have become extremely valuable due to the commercial, mechanistic, and biological importance of deuterated compounds. Herein, we report a programmable labeling platform that allows access to C2, C3, or C2- and C3-deuterated indoles under mild, user-friendly conditions. The C2-deuterated indoles are accessed using a reverse hydrogen isotope exchange strategy which represents the first non-directed C2-deuteration of indoles.

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.

Practical approaches to labelling terminal alkynes with deuterium

Base catalysed exchange with sodium hydroxide, calcium oxide or N,N,N,N‐tetramethylguanidine in deuterium oxide is a viable procedure for the preparation of terminally deuterated alkynes for those alkynes stable to strong base. The use of silver perchlorate as a catalyst is an alternative practical option when labelling alkynes which are sensitive to base or contain functionalities which would lead to labelling elsewhere in the molecule. Labelling with this catalyst takes place smoothly at ambient temperature in a mixture of N,N‐dimethylformamide and deuterium oxide.

Hydrogen/Deuterium Transfer from Anisole to Methoxy Radicals: A Theoretical Study of a Deuterium-Labeled Drug Model

Recently, deuterium-labeled drugs, such as deutetrabenazine, have attracted considerable attention. Consequently, understanding the reaction mechanisms of deuterium-labeled drugs is crucial, both fundamentally and for real applications. To understand the mechanisms of H- and D-transfer reactions, in this study, we used deuterated anisole as a deutetrabenazine model and computationally considered the nuclear quantum effects of protons, deuterons, and electrons. We demonstrated that geometrical differences exist in the partially and fully deuterated methoxy groups and hydrogen-bonded structures of intermediates and transition states due to the H/D isotope effect. The observed geometrical features and electronic structures are ascribable to the different nuclear quantum effects of protons and deuterons. Primary and secondary kinetic isotope effects (KIEs) were calculated for H- and D-transfer reactions from deuterated and undeuterated anisole, with the calculated primary KIEs in good agreement with the corresponding experimental data. These results reveal that the nuclear quantum effects of protons and deuterons need to be considered when analyzing the reaction mechanisms of H- and D-transfer reactions and that a theoretical approach that directly includes nuclear quantum effects is a powerful tool for the analysis of H/D isotope effects in H- and D-transfer reactions.

Potassium tert-Butoxide Promoted Regioselective Deuteration of Pyridines

A regioselective deuteration at β- and γ-position of pyridines is reported. Efficient deuteration occurred with a combination of KOtBu and DMSO-d6, replenishing the prevailing α-deuteration of pyridine systems. Preliminary mechanistic...

Transition-Metal-Free Regioselective Hydroamination of Styrenes with Amino-Heteroarenes

Base-mediated anti-Markovnikov hydroamination of functionally varied styrenes with amino-substituted pyridine, quinoline, pyrimidine, pyrazine, and phenanthridine with excellent regioselectivity has been described. Double hydroamination was observed chemoselectively on secondary amine leaving...

Broadening of horizons in the synthesis of CD3-labeled molecules

In the light of the recent potentials of deuterated molecules as pharmaceuticals or even in mechanistic understanding, efficient methods for their synthesis are continually desired. CD₃-containing molecules are prominent amongst these motifs due to the parallel of the "magic methyl effect": introducing a methyl group into pharmaceuticals could positively affect biological activities. The trideuteromethyl group is bound to molecules either by C, N, O, or S atom. For a long time, the preparation methods of such labeled compounds were underestimated and involved multi-step syntheses. More recently, specific approaches dealing with the direct incorporation of the CD₃ group have been developed. This Review gives an overview of the methods for the preparation of CD₃-labeled molecules from conventional functional group interconversion techniques to catalytic approaches and include radical strategy. Detailed reaction mechanisms are also discussed.

3D Printing to Increase the Flexibility of the Chemical Synthesis of Biologically Active Molecules: Design of On-Demand Gas Generation Reactors

The development of new drugs is accelerated by rapid access to functionalized and D-labeled molecules with improved activity and pharmacokinetic profiles. Diverse synthetic procedures often involve the usage of gaseous reagents, which can be a difficult task due to the requirement of a dedicated laboratory setup. Here, we developed a special reactor for the on-demand production of gases actively utilized in organic synthesis (C2H2, H2, C2D2, D2, and CO2) that completely eliminates the need for high-pressure equipment and allows for integrating gas generation into advanced laboratory practice. The reactor was developed by computer-aided design and manufactured using a conventional 3D printer with polypropylene and nylon filled with carbon fibers as materials. The implementation of the reactor was demonstrated in representative reactions with acetylene, such as atom-economic nucleophilic addition (conversions of 19-99%) and nickel-catalyzed S-functionalization (yields 74-99%). One of the most important advantages of the reactor is the ability to generate deuterated acetylene (C2D2) and deuterium gas (D2), which was used for highly significant, atom-economic and cost-efficient deuterium labeling of S,O-vinyl derivatives (yield 68-94%). Successful examples of their use in organic synthesis are provided to synthesize building blocks of heteroatom-functionalized and D-labeled biologically active organic molecules.

Site-Selective Silver-Catalyzed C-H Bond Deuteration of Five-Membered Aromatic Heterocycles and Pharmaceuticals

Catalytic methods for the direct introduction of hydrogen isotopes into organic molecules are essential to the development of improved pharmaceuticals and to the alteration of their absorption, distribution, metabolism, and excretion (ADME) properties. However, the development of homogeneous catalysts for selective incorporation of isotopes in the absence of directing groups under practical conditions remains a long-standing challenge. Here, we show that a phosphine-ligated, silver-carbonate complex catalyzes the site-selective deuteration of C-H bonds in five-membered aromatic heterocycles and active pharmaceutical ingredients that have been resistant to catalytic H/D exchange. The reactions occur with CH₃OD as a low-cost source of the isotope. The silver catalysts react with five-membered heteroarenes lacking directing groups, tolerate a wide range of functional groups, and react in both polar and nonpolar solvents. Mechanistic experiments, including deuterium kinetic isotope effects, determination of kinetic orders, and identification of the catalyst resting state, support C-H bond cleavage from a phosphine-ligated, silver-carbonate intermediate as the rate-determining step of the catalytic cycle.

Base-Mediated Anti-Markovnikov Hydroamidation of Vinyl Arenes with Arylamides

We investigated a base-promoted protocol for the intermolecular anti-Markovnikov hydroamidation of vinyl arenes with arylamides to furnish the arylethylbenzamides with excellent chemo- and regioselectivity. The reaction tolerates an extensive variety of functional groups and has been successfully extended with electronically varied handles, aminobenzamides, electron-rich/electron-deficient heterocyclic amides, and vinyl arenes to afford the hydroamidated products. Excellent chemoselectivity was observed for the amide group over amine. The proposed mechanism and vital role of the solvent was well supported by deuterium labeling studies and control experiments.

Nonmetal-catalyzed hydroamination of ynamides with amines

A TfOH-catalyzed hydroamination of ynamides with primary and secondary amines under mild reaction conditions is described for the efficient synthesis of N-arylimines and ethene-1,1-diamines.

A straightforward catalytic approach to obtain deuterated chloroform at room temperature

We report the catalytic activity for the complexes-cis-[RuCl₂ (dppb)(bipy)] (A), and [η⁶ -(p-cymene)Ru (dppb)Cl]PF₆ (B), wherein dppb = 1,4-bis(diphenylphosphine)butane, and bipy = 2,2'-bipyridine-for the synthesis of CDCl₃ from CHCl₃ using D₂ O as deuterium source. H/D exchange reactions were performed using a chloroform/D₂ O, 1:2 molar ratio, vigorously stirred, at room temperature. One mole of KOH was dissolved in D₂ O fraction and catalytic complexes from 0.002 to 0.05 mmol were dissolved in chloroform. The H/D exchange reactions were monitored using ¹³ C nuclear magnetic resonance sequences without proton decoupling. The reaction using 0.01 mmol of compound A reached approximately 55% of H/D conversion in 1 h. In the same time, the reactions with 0.002 mmol of compound A and without catalyst show approximately 28% and 3% H/D exchange, respectively. Without the catalysts, the H/D exchange was only 12.0% in 5 h. For compound B, 55% H/D conversion was observed in 1 h, only when 0.05 mmol was used, which is much higher catalyst concentration. After the isolation of the chloroform fraction and two more addition of D₂ O, it was possible to obtain 95.0% H/D exchange in approximately 3 h, using 0.01 mmol of the compound A. Therefore, compound A is an efficient catalyst for a rapid and straightforward synthesis of CDCl₃ from CHCl₃ at room temperature and using D₂ O as deuterium source.

Direct electrochemical reductive amination between aldehydes and amines with a H/D-donor solvent

A novel electrochemical synthesis protocol has been achieved for reductive amination between aldehydes and amines in undivided cells at room temperature. Under metal-free and external-reductant-free electrolysis conditions, various important secondary amine products are obtained in moderate-to-high yields. Deuterium-labeling experiments have demonstrated that low-toxicity DMSO acts both as a solvent and a H-donor in the reaction. On this basis, various deuterium-labeled products with good-to-excellent D-incorporation have been synthesized by using DMSO-d6 as a solvent. Furthermore, a molecule with GR-antagonistic activity has been synthesized through further sulfonylation.

Metal-free chemoselective hydrogenation of unsaturated carbon–carbon bonds via cathodic reduction

A straightforward protocol for efficient and highly selective hydrogenation of unsaturated carbon–carbon bonds via electrochemical reduction has been reported.

Catalytic α-Selective Deuteration of Styrene Derivatives

We report an operationally simple protocol for the catalytic α-deuteration of styrenes. This process proceeds via the base-catalyzed reversible addition of methanol to styrenes in DMSO -d₆ solvent. The concentration of methanol is shown to be critical for high yields and selectivities over multiple competing side reactions. The synthetic utility of α-deuterated styrenes for accessing deuterium-labeled chiral benzylic stereocenters is demonstrated.