(η6-Arene)ruthenium(N-heterocyclic carbene) Complexes for the Chelation-Assisted Arylation and Deuteration of Arylpyridines: Catalytic Studies and Mechanistic Insights

LED-induced Ru-photoredox Pd-catalyzed C-H arylation of (6-phenylpyridin-2-yl)pyrimidines and heteroaryl counterparts

N-heterocycles are essential building blocks and scaffolds in medicinal chemistry. A Pd-catalyzed, Ru-photoredox-mediated C-H arylation is applied herein, for converting a series of functionality-inclusive (6-phenylpyridin-2-yl)pyrimidines to single arylated derivatives, using phenyldiazonium tetrafluoroborate as aryl source. This green chemistry-compliant transformation is induced by LED light. The drug-like modular substrates are constructed via combination of Biginelli multi-component condensation and Suzuki C-C cross-coupling, in order to strategically install, adjacent to the Ph-ring intended to undergo C-H arylation, a (6-pyridin-2-yl)pyrimidine that plays the role of a chelating directing moiety for the C-H arylation catalyst. The scope has been demonstrated on a series of 26 substrates, comprising diverse Ph-ring substituents and substitution patterns, as well as with 13 different aryl donors. Substrates in which the Ph-ring (arylation acceptor) was replaced by an electron-rich heteroaryl counterpart (2-/3-thiophene or -benzofuran) have also been examined and found to undergo arylation regioselectively. End-product conformations afford interesting motifs for occupying 3D chemical space, as implied by single-crystal X-ray diffraction, which has allowed the elucidation of six structures of aryl derivatives and one of an unprecedented pyrimidine-pyridine-benzofuran carbopalladated complex, believed to be a C-H activation derivative.

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.

RhIII-Catalyzed heteroarylation of N-2,6-difluorophenyl arylamides with heteroaryl boronate esters

An efficient strategy to aryl-heteroaryl formation via RhIII-catalyzed C–H heteroarylation of arenes with N-heterocyclic boronates has been disclosed.

The synthesis of aryl-heteroaryl derivatives via the RhIII-catalyzed heteroarylation of arenes and heteroaromatic boronates

An efficient Rh^III-catalyzed strategy for constructing aryl-heteroaryl derivatives with removable ketoxime ether auxiliaries via direct C-H heteroarylation based on arenes and heteroaromatic boronates has been disclosed. This protocol could tolerate various pyridine, pyrimidine, pyrazole, thiophene, and furan heteroaromatic boronates well, providing the desired products with high reactivities and excellent regioselectivity. The easy synthetic accessibility may offer potential for application in the synthesis of heterocyclic drug molecules containing aryl-heteroaryl motifs.

Ortho-Selective Hydrogen Isotope Exchange of Phenols and Benzyl Alcohols by Mesoionic Carbene-Iridium Catalyst

Hydrogen isotope exchange reactions of phenols and benzyl alcohols have been achieved by a mesoionic carbene-iridium catalyst with high ortho selectivity and high functional group tolerance. Control experiments indicated that acetate is crucial to realize the ortho selectivity, whereas density functional theory calculations supported an outer-sphere direction with hydrogen bonding between acetate and the hydroxyl group.

RhIII-Catalyzed Direct Heteroarylation of C(sp3)-H and C(sp2)-H Bonds in Heterocycles with N-Heteroaromatic Boronates

Herein, we disclose a Rh^III-catalyzed heteroarylation of C(sp³)-H and C(sp²)-H bonds in heterocycles with organoboron reagents. This protocol displays high efficiency and excellent functional group tolerance. A range of heterocyclic boronates with strong coordinating atoms, including pyridine, pyrimidine, pyrazole, thiophene, and furan derivatives, can be extensively served as the coupling reagents. The direct heteroarylation method could supply potential application in terms of the synthesis of drug molecules with multiple heterocycles.

Kinetic isotope effects and synthetic strategies for deuterated carbon-11 and fluorine-18 labelled PET radiopharmaceuticals

The deuterium labelling of pharmaceuticals is a useful strategy for altering pharmacokinetic properties, particularly for improving metabolic resistance. The pharmacological effects of such metabolites are often assumed to be negligible during standard drug discovery and are factored in later at the clinical phases of development, where the risks and benefits of the treatment and side-effects can be wholly assessed. This paradigm does not translate to the discovery of radiopharmaceuticals, however, as the confounding effects of radiometabolites can inevitably show in preliminary positron emission tomography (PET) scans and thus complicate interpretation. Consequently, the formation of radiometabolites is crucial to take into consideration, compared to non-radioactive metabolites, and the application of deuterium labelling is a particularly attractive approach to minimise radiometabolite formation. Herein, we provide a comprehensive overview of the deuterated carbon-11 and fluorine-18 radiopharmaceuticals employed in PET imaging experiments. Specifically, we explore six categories of deuterated radiopharmaceuticals used to investigate the activities of monoamine oxygenase (MAO), choline, translocator protein (TSPO), vesicular monoamine transporter 2 (VMAT2), neurotransmission and the diagnosis of Alzheimer's disease; from which we derive four prominent deuteration strategies giving rise to a kinetic isotope effect (KIE) for reducing the rate of metabolism. Synthetic approaches for over thirty of these deuterated radiopharmaceuticals are discussed from the perspective of deuterium and radioisotope incorporation, alongside an evaluation of the deuterium labelling and radiolabelling efficacies across these independent studies. Clinical and manufacturing implications are also discussed to provide a more comprehensive overview of how deuterated radiopharmaceuticals may be introduced to routine practice.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

Ruthenium-Catalyzed ortho- and meta-H/D Exchange of Arenes

Ruthenium-catalyzed aromatic H/D exchange in [D₄]acetic acid has been developed. By using N-heteroarenes as directing groups, both ortho and meta positions are selectively deuterated with high levels of D incorporation. Moreover, this strategy provides an alternative way to achieve meta-C-H activation.

Palladium-Catalyzed Remote meta-C-H Bond Deuteration of Arenes Using a Pyridine Template

Palladium-catalyzed meta-selective C-H deuteration of a series of substrates, including phenylacetic acids, hydrocinnamic acid, benzylphosphonate, benzylsulfonate, and benzyl and phenyl ethyl alcohol ester, is developed by using a pyridine-based directing template. The template is installed into the substrate through a practical ester linkage. Under mild reaction conditions, a variety of phenylacetic acids containing alkyl, methoxyl, and halo substituents are compatible in the reaction, resulting in high levels of D-incorporation at the meta position.

Ruthenium(η⁶,η¹-arene-CH₂-NHC) Catalysts for Direct Arylation of 2-Phenylpyridine with (Hetero)Aryl Chlorides in Water

A series of new benzimidazolium halides were synthesized in good yields as unsymmetrical N-heterocyclic carbene (NHC) precursors containing the N–CH₂–arene group. The benzimidazolium halides were readily converted into ruthenium(II)–NHC complexes with the general formula [RuCl₂(η⁶,η¹–arene–CH₂–NHC)]. The structures of all new compounds were characterized by ¹H NMR (Nuclear Magnetic Resonance), ¹³C NMR, FT-IR (Fourier Transform Infrared) spectroscopy and elemental analysis techniques. The single crystal structure of one benzimidazole ruthenium complex, 2b, was determined. The complex is best thought of as containing an octahedrally coordinated Ru center with the arene residue occupying three sites, the remaining sites being occupied by a (carbene)C–Ru bond and two Ru–Cl bonds. The catalytic activity of [RuCl₂(η⁶,η¹–arene–CH₂–NHC)] complexes was evaluated in the direct (hetero)arylation of 2-phenylpyridine with (hetero)aryl chlorides in water as the nontoxic reaction medium. These results show that catalysts 2a and 2b were the best for monoarylation with simple phenyl and tolyl chlorides. For functional aryl chlorides, 2d, 2e, and 2c appeared to be the most efficient.

C-H Functionalisation for Hydrogen Isotope Exchange

The various applications of hydrogen isotopes (deuterium, D, and tritium, T) in the physical and life sciences demand a range of methods for their installation in an array of molecular architectures. In this Review, we describe recent advances in synthetic C-H functionalisation for hydrogen isotope exchange.

Iridium-Catalyzed Formyl-Selective Deuteration of Aldehydes

We report the first direct catalytic method for formyl-selective deuterium labeling of aromatic aldehydes under mild conditions, using an iridium-based catalyst designed to favor formyl over aromatic C-H activation. A good range of aromatic aldehydes is selectively labeled, and a one-pot labeling/olefination method is also described. Computational studies support kinetic product control over competing aromatic labeling and decarbonylation pathways.

Synthesis and catalytic applications of palladium N-heterocyclic carbene complexes as efficient pre-catalysts for Suzuki–Miyaura and Sonogashira coupling reactions

A new palladium complex series with N-heterocyclic carbene (NHC), pyridine and phosphine ligands, PdCl₂(L)NHC (2a–c)(L = NHC), PdCl₂(L¹)NHC(3a–c)(L¹ = pyridine), PdCl₂(L²)NHC(4a–c)(L² = triphenylphosphine) was synthesised and fully characterized.

Mono and dimetallic pyrene-imidazolylidene complexes of iridium(iii) for the deuteration of organic substrates and the C–C coupling of alcohols

Two pyrene-NHC complexes of Ir(iii) and one di-iridium(iii) complex with a pyrene-di-NHC ligand have been prepared and characterized. Their catalytic activity in the deuteration of organic substrates, and in the β-alkylation of secondary alcohols with primary alcohols is described.

Regioselective ruthenium catalysed H–D exchange using D2O as the deuterium source

An efficient and convenient ruthenium catalysed method for a regiospecific H/D exchange using D₂O is described.

Rh(III) -catalyzed C-H activation: a versatile route towards various polycyclic pyridinium salts

An efficient and convenient method for the synthesis of highly substituted polycyclic pyridinium salts from the reaction of various 2-aryl-pyridines and 2-aryl-sp(2) -nitrogen-atom-containing heterocycles with alkynes through rhodium(III)-catalyzed CH activation and annulation under an O2 atmosphere is described. A possible mechanism that involves the chelation-assisted CH activation of the 2-aryl-pyridine substrate, insertion of the alkyne, and reductive elimination is proposed. This mechanism was supported by the isolation of a five-membered rhodacycle (I'). In addition, kinetic isotope studies were performed to understand the intimate reaction mechanism.

Autocatalysis for C-H bond activation by ruthenium(II) complexes in catalytic arylation of functional arenes

Kinetic data for the C-H bond activation of 2-phenylpyridine by Ru(II)(carboxylate)(2)(p-cymene) I (acetate) and I' (pivalate) are available for the first time. They reveal an irreversible autocatalytic process catalyzed by the coproduct HOAc or HOPiv (acetonitrile, 27 °C). The overall reaction is indeed accelerated by the carboxylic acid coproduct and water. It is retarded by a base, in agreement with an autocatalytic process induced by HOAc or HOPiv that favors the dissociation of one carboxylate ligand from I and I' and consequently the ensuing complexation of 2-phenylpyridine (2-PhPy). The C-H bond activation initially delivers Ru(O(2)CR)(o-C(6)H(4)-Py)(p-cymene) A or A', containing one carboxylate ligand (OAc or OPiv, respectively). The overall reaction is accelerated by added acetates. Consequently, C-H bond activation (faster for acetate I than for pivalate I') proceeds via an intermolecular deprotonation of the C-H bond of the ligated 2-PhPy by the acetate or pivalate anion released from I or I', respectively. The 18e complexes A and A' easily dissociate, by displacement of the carboxylate by the solvent (also favored by the carboxylic acid), to give the same cationic complex B(+) {[Ru(o-C(6)H(4)-Py)(p-cymene)(MeCN)](+)}. Complex B(+) is reactive toward oxidative addition of phenyl iodide, leading to the diphenylated 2-pyridylbenzene.