Base‐Mediated Remote Deuteration of N‐Heteroarenes – Broad Scope and Mechanism

Base Promoted Hydrogenation of N-Heteroarenes with Ammonia Borane and DMSO

Herein, we report a sodium tert-butoxide-promoted reduction of N-heteroarenes using ammonia borane and dimethyl sulfoxide (DMSO) under mild reaction conditions. This method demonstrates broad functional group compatibility across diverse N-heteroarene substrates. Notably, substituting DMSO with deuterated DMSO-d6 enables the synthesis of C3-deuterated 1,2,3,4-tetrahydroquinolines with remarkable positional selectivity. Mechanistic investigations indicate that the protons are derived from both ammonia borane and DMSO. This strategy establishes a novel and environmentally benign approach for the synthesis of (deuterated) N-heterocycles, offering practical advantages in terms of operational simplicity and sustainable reaction conditions.

Selective Hydrogen Isotope Exchange Catalysed by Simple Alkali-Metal Bases in DMSO

Dedicated to Proferssor Robert E. Mulvey on the occasion of his 65th birthday. Isotope Exchange processes are becoming the preferred way to prepare isotopically labelled molecules, avoiding the redesign of multistep synthetic protocols. In the case of deuterium incorporation, the most used strategy has employed transition metals, that offer high reactivity under mild reaction conditions. Despite their success, the trade-off is that these metals are precious, so expensive, and often exhibit high toxicity. Therefore, alternative transition-metal-free protocols would be a welcome addition to this field. In this report we show how the simple bases NaHMDS (HMDS=hexamethyldisilazide) and NaCH2SiMe3 can efficiently and selectively promote deuteration of a wide range of C(sp2)-H and C(sp3)-H bonds in DMSO-d6, providing an easy and direct access to deuterated compounds. Heterocycles, fluoroarenes, N-heterocyclic carbenes, amides and other aromatic molecules could be deuterated under mild conditions using catalytic amounts of base. Mechanistic studies along with the isolation and characterisation of reaction intermediates have flagged up the importance of the metalated substrate and metalated solvent in solution, establishing an equilibrium between these compounds is crucial for the success of this approach. An alkali-metal effect was observed, with heavier alkali-metal amides being more reactive at room temperature, but their lower stability at higher temperatures made sodium bases the optimal reagents for Hydrogen Isotope Exchange.

Mechanistic insights into the base-mediated deuteration of pyridyl phosphonium and ammonium salts

Pyridines can be deuterated at the remote sites by treatment with KOtBu in DMSO-d 6, although without discrimination between the meta- and para-position. Herein, base-catalyzed deuterations have been studied, computationally and experimentally, using a series of pyridyl phosphonium salts with a temporary electron-withdrawing group to block the para-position while increasing the acidity in the other positions.

Divergent and chemoselective deuteration of N-unsubstituted imidazoles enabled by precise acid/base control

Herein, we report acid/base-controlled and divergent deuteration of N-unsubstituted imidazoles in an imidazole-selective manner. This protocol enabled the deuteration of not only the 4-arylimidazoles but also the 2-arylimidazoles without labelling the aromatic rings. We demonstrated the advantages of this protocol by the synthesis of deuterated pharmaceuticals, which is difficult to achieve by means of transition metals.

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.

Direct C-H Hydroxylation of N-Heteroarenes and Benzenes via Base-Catalyzed Halogen Transfer

Hydroxylated (hetero)arenes are valued in many industries as both key constituents of end products and diversifiable synthetic building blocks. Accordingly, the development of reactions that complement and address the limitations of existing methods for the introduction of aromatic hydroxyl groups is an important goal. To this end, we apply base-catalyzed halogen transfer (X-transfer) to enable the direct C-H hydroxylation of mildly acidic N-heteroarenes and benzenes. This protocol employs an alkoxide base to catalyze X-transfer from sacrificial 2-halothiophene oxidants to aryl substrates, forming SNAr-active intermediates that undergo nucleophilic hydroxylation. Key to this process is the use of 2-phenylethanol as an inexpensive hydroxide surrogate that, after aromatic substitution and rapid elimination, provides the hydroxylated arene and styrene byproduct. Use of simple 2-halothiophenes allows for C-H hydroxylation of 6-membered N-heteroarenes and 1,3-azole derivatives, while a rationally designed 2-halobenzothiophene oxidant extends the scope to electron-deficient benzene substrates. Mechanistic studies indicate that aromatic X-transfer is reversible, suggesting that the deprotonation, halogenation, and substitution steps operate in synergy, manifesting in unique selectivity trends that are not necessarily dependent on the most acidic aryl position. The utility of this method is further demonstrated through streamlined target molecule syntheses, examples of regioselectivity that contrast alternative C-H hydroxylation methods, and the scalable recycling of the thiophene oxidants.

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.

K2CO3/18-Crown-6-Catalyzed Selective H/D Exchange of Heteroarenes with Bromide as a Removable Directing Group

The K₂CO₃/18-crown-6-catalyzed H/D exchange of heretoarenes in high atom % deuterium incorporation is disclosed. The use of a weak base as a catalyst leads to excellent site selectivity and broad functional group tolerance. Control experiments indicated that the use of bromide, which enhances the adjacent C-H bond reactivity, as a removable directing group is essential. Moreover, conversion of bromide to other functional groups is also performed to construct other useful deuterated compounds.