Efficient Brønsted-Acid-Catalyzed Deuteration of Arenes and Their Transformation to Functionalized Deuterated Products

Programmable iodization/deuterolysis sequences of phosphonium ylides to access deuterated benzyl iodides and aromatic aldehydes

Herein, a tunable iodization/deuterolysis protocol for phosphonium ylides by employing D₂O as the deuterium source was designed. Notably, this process could be manipulated by tuning the base, thus leading to two valuable deuterated building blocks - benzyl iodides and aromatic aldehydes with broad substrate scope, good functional group compatibility and excellent deuteration degree. Concise syntheses of a series of deuterated drug analogues have been achieved based on the developed deuteration reaction platform.

Perdeuteration of Deactivated Aryl Halides by H/D Exchange under Superelectrophile Catalysis

Superelectrophilic silylium/arenium ions are shown to be highly effective H/D exchange promoters for the exhaustive deuteration of electron-deficient aryl halides. Several of the resulting perdeuterated aryl halides have been previously inaccessible with existing deuterium-labeling procedures. Using inexpensive C₆D₆ as the deuterium source, excellent degrees of deuterium incorporation were achieved under ambient reaction conditions. Importantly, the perdeuteration remains unaffected on multigram scale, even at a reduced catalyst loading of 0.1 mol %. By this method, otherwise expensive or noncommercially available NMR solvents such as 1,2-dichloro- and 1,2-difluorobenzene can be prepared.

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.

On-surface cyclodehydrogenation reaction pathway determined by selective molecular deuterations

Understanding the reaction mechanisms of dehydrogenative C_aryl–C_aryl coupling is the key to directed formation of π-extended polycyclic aromatic hydrocarbons. Here we utilize isotopic labeling to identify the exact pathway of cyclodehydrogenation reaction in the on-surface synthesis of model atomically precise graphene nanoribbons (GNRs). Using selectively deuterated molecular precursors, we grow seven-atom-wide armchair GNRs on a Au(111) surface that display a specific hydrogen/deuterium (H/D) pattern with characteristic Raman modes. A distinct hydrogen shift across the fjord of C_aryl–C_aryl coupling is revealed by monitoring the ratios of gas-phase by-products of H₂, HD, and D₂ with in situ mass spectrometry. The identified reaction pathway consists of a conrotatory electrocyclization and a distinct [1,9]-sigmatropic D shift followed by H/D eliminations, which is further substantiated by nudged elastic band simulations. Our results not only clarify the cyclodehydrogenation process in GNR synthesis but also present a rational strategy for designing on-surface reactions towards nanographene structures with precise hydrogen/deuterium isotope labeling patterns.

Selective deuterations were exploited to synthesize graphene nanoribbons on Au(111) surface with a specific H/D pattern on edges, allowing the determination of cyclodehydrogenation reaction pathway within the framework of pericyclic reactions.

The Power of the Proton: From Superacidic Media to Superelectrophile Catalysis

Superacidic media became famous in connection with carbocations. Yet not all reactive intermediates can be generated, characterized, and eventually isolated from these Brønsted acid/Lewis acid cocktails. The counteranion, that is the conjugate base, in these systems is often too nucleophilic and/or engages in redox chemistry with the newly formed cation. The Brønsted acidity, especially superacidity, is in fact often not even crucial unless protonation of extremely weak bases needs to be achieved. Instead, it is the chemical robustness of the aforementioned counteranion that determines the success of the protolysis. The advent of molecular Brønsted superacids derived from weakly coordinating, redox-inactive counteranions that do withstand the enormous reactivity of superelectrophiles such as silicon cations completely changed the whole field. This Perspective summarizes general aspects of medium and molecular Brønsted acidity and shows how applications of molecular Brønsted superacids have advanced from stoichiometric reactions to catalytic processes involving protons and in situ generated superelectrophiles.

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%).

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.