Evaluation of a P,N-ligated iridium(I) catalyst in hydrogen isotope exchange reactions of aryl and heteroaryl compounds

Pegylated Phosphine Ligands in Iridium(I) Catalyzed Hydrogen Isotope Exchange Reactions in Aqueous Buffers

The synthesis of a water-soluble, phosphine-pegylated iridium(I) catalyst and its application in hydrogen isotope exchange (HIE) reactions in buffer is reported. The longer polyethylene glycol side chains on the phosphine increased the water solubility independently from the pH. HIE reactions of polar substrates in protic solvents were studied. DFT calculations gave further insights into the catalytic processes. The scope and limitation of the pegylated catalyst was studied in HIE reactions of several complex compounds in borax buffer at pH 9 and the best conditions were applied in a tritium experiment with the drug telmisartan.

Direct Multi-Deuterium Labelling of Pirtobrutinib

Herein, we demonstrate an efficient method for multi-deuterium labelling of pirtobrutinib-a Bruton's tyrosine kinase inhibitor recently approved by the FDA-using a straightforward hydrogen isotope exchange (HIE) reaction. A remarkably high level of deuterium incorporation was achieved using an excess of a Kerr-type iridium catalyst. The key factor in the significant deuterium labelling was the decision to employ a deuterium uniformly labelled solvent, chlorobenzene-d5, at an elevated temperature. Virtually, no d0-d3 species were detected, with only traces of d4-d5 isotopomers (< 5%) observable in the mass spectrum of pirtobrutinib-d8, fulfilling requirements for stable isotope-labelled internal standard. The labelled compound-mainly consisting of isotopomers d6-d9 at 82.4% of the total abundance-was isolated in a high yield (73%) and purity (99%). Noteworthy, fluorine group acting as a directing group was observed for the first time. Significant incorporation of deuterium in ortho-positions, exceeding 87%, was observed. Interestingly, chlorinated solvent used in the HIE reactions was non-specifically deuterated yielding up to 0.42 deuterium per chlorobenzene molecule even at an exceptionally low iridium catalyst loading of 4.17 × 10-2 mol%.

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.

Ortho-Deuteration of Aromatic Aldehydes via a Transient Directing Group-Enabled Pd-Catalyzed Hydrogen Isotope Exchange

A practical and scalable ortho-selective deuteration of aromatic aldehydes was accomplished by Pd-catalyzed hydrogen isotope exchange with deuterium oxide as an inexpensive deuterium source. The use of tert-leucine as a transient directing group facilitates the exchange, affording a wide range of ortho-deuterated aromatic aldehydes with deuterium incorporation up to 97%. The control experiments suggest that the addition of silver trifluoroacetate resists the unexpected reduction of Pd(II), while the theoretical study indicates a rapid reversible concerted metalation-deprotonation process.

Highlights of C (sp2 )-H hydrogen isotope exchange reactions

The highlights of C (sp2 )-H hydrogen isotope exchange (HIE) methods developed over the past 10 years are summarized in this review. Major developments include improved Ir(I) catalysts with greater functional group and solvent compatibility and the development of novel base metal catalysts for HIE. In addition, a number of novel Ru-based catalysts have been developed with promising activity. In the area of Pt- and Pd-catalysed exchange, in addition to new advances on heterogeneous Pt- and Pd-catalysed HIE by Sajiki and Shevchenko, a number of groups have reported on homogenous catalysts of Pt and Pd that show an interesting activity and selectivity.

Recent advances in iridium(I) catalysis towards directed hydrogen isotope exchange

The initial discovery and establishment of a family of novel iridium catalysts possessing N-heterocyclic carbene units alongside bulky phosphine ligands allowed selected substrates to be labelled using deuterium or tritium gas at desirably low catalyst loadings via an ortho-directed C-H insertion process. Such a method has broad applicability and offers distinct advantages within the pharmaceutical industry, directly facilitating the ability to carefully monitor a potential drug molecule's biological fate. Over the past decade since these initial protocols were divulged, many additional advances have been made in terms of catalyst design and substrate scope. This review describes the broadened array of new iridium catalysts and associated protocols for direct and selective C-H activation and hydrogen isotope insertion within a number of new chemical entities of direct relevance to the pharmaceutical industry.

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.

Comparison of Iridium(I) Catalysts in Temperature Mediated Hydrogen Isotope Exchange Reactions

The reactivity and selectivity of iridium(I) catalysed hydrogen isotope exchange (HIE) reactions can be varied by using wide range of reaction temperatures. Herein, we have done a detailed comparison study with common iridium(I) catalysts (1–6) which will help us to understand and optimize the approaches of either high selectivity or maximum deuterium incorporation. We have demonstrated that the temperature window for these studied iridium(I) catalysts is surprisingly very broad. This principle was further proven in some HIE reactions on complex drug molecules.

Enabling Two-Electron Pathways with Iron and Cobalt: From Ligand Design to Catalytic Applications

Homogeneous catalysis with Earth-abundant, first-row transition metals, including iron and cobalt, has gained considerable recent attention as a potentially cost-effective and sustainable alternative to more commonly and historically used precious metals. Because fundamental organometallic transformations, such as oxidative addition and reductive elimination, are two-electron processes and essential steps in many important catalytic cycles, controlling redox chemistry-in particular overcoming one-electron chemistry-has been as a central challenge with Earth-abundant metals. This Perspective focuses on approaches to impart sufficiently strong ligand fields to generate electron-rich metal complexes able to promote oxidative addition reactions where the redox changes are exclusively metal-based. Emphasis is placed on how ligand design and exploration of fundamental organometallic chemistry coupled with mechanistic understanding have been used to discover iron catalysts for the hydrogen isotope exchange in pharmaceuticals and cobalt catalysts for C(sp²)-H borylation reactions. A pervasive theme is that first-row metal complexes often promote unique chemistry from their precious-metal counterparts, demonstrating that these elements offer a host of new opportunities for reaction discovery and for more sustainable catalysis.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Highly Selective Directed Iridium-Catalyzed Hydrogen Isotope Exchange Reactions of Aliphatic Amides

For the first time, we describe highly selective homogeneous iridium-catalyzed hydrogen isotope exchange (HIE) of unactivated C(sp3 ) centers in aliphatic amides. When using the commercially available Kerr catalyst, the HIE with a series of common antibody-drug conjugate (ADC) linker side chains proceeds with high yields, high regioselectivity, and with deuterium incorporation up to 99 %. The method is fully translatable to the specific requirements of tritium chemistry and its effectiveness was demonstrated by direct tritium labelling of a maytansinoid. The scope of the method can be extended to simple amino acids, with high HIE activity observed for glycine and alanine. In di- and tripeptides, a very interesting protecting-group-dependent tunable selectivity was observed. DFT calculations gave insight into the energies of the transition states, thereby explaining the observed selectivity and the influence of the amino acid protecting groups.