Palladium-Catalyzed H/D Exchange Reaction with 8-Aminoquinoline as the Directing Group: Access to ortho-Selective Deuterated Aromatic Acids and β-Selective Deuterated Aliphatic Acids

Copper(II)-Catalyzed Regioselective H/D Exchange Based on Reversible C-H Activation

Despite the increasing use of copper in C-H functionalizations, the Cu-catalyzed direct deuteration of C-H bonds remains a significant challenge due to its inherent low reactivity in inverse C-H bond reconstruction. In this paper, a novel strategy had been developed to reverse the copper-catalyzed concerted metalation-deprotonation process by inhibiting the unexpected disproportionation of Cu(II) to Cu(III). Picolinic acid was identified as a powerful ligand for facilitating this H/D exchange with D2O as deuterium source, and its inhibition activity was supported by preliminary control experiments and DFT studies.

2-Pyridone-Enhanced Mn-Catalysis for the Synthesis of ortho-Deuterated Aromatic Nitriles

We herein present a method for the synthesis of ortho-deuterated aryl nitrile using Mn(CO)5Br as catalyst with CH3OD as deuteratium source, where the structure of aryl imidates is used for interconversion with a cyanide group. This method features a broad substrate range and excellent functional group tolerance, allowing late modification of various complex molecules with good yields and deuterium incorporation. Mechanistic studies suggest that 2-pyridone is crucial to the success of this chemistry, serving as an endogenous base that enhances the rate of hydrogen isotope exchange.

Synthesis of β-Deuterated Amino Acids via Palladium-Catalyzed H/D Exchange

Despite several synthetic approaches that have been developed for α-deuterated amino acids, the synthesis of β-deuterated amino acids has remained a challenge. Herein, we disclose a palladium catalyzed H/D exchange protocol for a β-deuterated N-protected amino amide, which can be converted to a β-deuterated amino acid simply by removal of protecting groups. This protocol is highly efficient, simply manipulated, and appliable for deuterium-labeling of many amino amides. In addition, deuterium labeling of phenylalanine derivatives was also successful when pivalic acid served as an additive to promote the H/D exchange process.

C-H deuteration of organic compounds and potential drug candidates

C-H deuteration has been intricately developed to satisfy the urgent need for site-selectively deuterated organic frameworks. Deuteration has been primarily used to study kinetic isotope effects of reactions but recently its significance in pharmaceutical chemistry has been discovered. Deuterium labelled compounds have stolen the limelight since the inception of the first FDA-approved deuterated drug, for the treatment of chorea-associated Huntington's disease, and their pharmacological importance was realised by chemists, although surprisingly very late. Various approaches were developed to carry out site-selective deuteration. However, the most common and efficient method is hydrogen isotope exchange (HIE). This review summarises deuteration methods of various organic motifs containing C(sp²)-H and C(sp³)-H bonds utilizing C-H bond functionalisation as a key step along with a variety of catalysts, and exemplifies their biological relevance.

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.

Screening of anti-cancerous potential of classical Raudra rasa and modified Raudra rasa modified with hiraka bhasma (nanodiamond) through FTIR & LC-MS analysis

OBJECTIVES

Raudra rasa is an ayurvedic medicine explicitly prescribed for the treatment of arbuda (cancer), whereas hiraka bhasma has the potential to promote cancer healing properties. Together, these two medicines provide multifunction benefits. This paper analyses the functional groups of Raudra rasa modified with hiraka bhasma and compares it with the classically prepared raudra rasa. To identify the functional group, organic ligands, and active compounds present in samples of raudra rasa (CRR) and modified raudra rasa with hiraka bhasma (MRR) contributing to cancer alleviation by using Fourier transform infrared spectroscopy (FTIR) & LC-MS analysis.

METHODS

Classical raudra rasa (CRR), its ingredients, shadguna kajjali (SK); decoction of Piper betel Linn. (PBD); Amaranthus spinosus Linn. (ASD); Boerhaavia diffusa Linn. (BDD); Piper longum Linn. (PLD); cow urine (GM), & similarly modified raudra rasa (MRR), its ingredients, hiraka bhasma (HB); shadguna rasasindura (SHR); water-soluble extract of Piper betel Linn. (PBE); Amaranthus spinosus Linn. (ASE); Boerhaavia diffusa Linn. (BDE); cow urine ark (GA); Piper Longum Linn. (PLE) were subjected to FTIR and LC-MS analysis.

RESULTS

Among all 15 samples studied, maximum numbers of peaks (21) were seen in MRR indicating a greater number of functional groups. Further, in MRR, a maximum peak in the double bond region is suggestive of its higher stability compared to CRR. Both the compound is preliminarily a mixture of the number of functional groups like; fluoro, methyl, amino, hydroxy, nitro, methylamino, carbonyl, and iodo groups, having known anti-proliferative activities. By the FT-IR analysis, the biologically active compounds in aqueous and methanol extract of CRR & MRR were identified that have anti-cancerous compounds. In the present study, a total of 40 major compounds like alkaloids, amino acid, carboxylic acid, Flavonoids, Nucleoside, Nucleotide, phenylpropanoid, Sphingosine, stilbenoid, sugar, phosphate, terpenoids, vitamin from aqueous & methanol extract of CRR & MRR were identified by LC-MS.

CONCLUSIONS

This research paper highlights the presence of different functional groups and bioactive compounds known to have anti-cancer activities. Thus, this review suggests future recommendations for the design and development of improved anticancer drugs with higher efficacy.

Silver salt enabled H/D exchange at the β-position of thiophene rings: synthesis of fully deuterated thiophene derivatives

We disclose a silver catalyzed H/D exchange reaction, which can introduce the deuterium atom at the β position of thiophene rings without the assistance of any coordinating groups. The advantages of this reaction include operation in open air, usage of D₂O as the deuterium source, good tolerance to a range of functional groups and obtaining high atom% deuterium incorporation. In addition, this H/D exchange reaction is employed for direct deuteration of a thiophene based monomer, which is usually prepared by multistep synthesis from expensive deuterated starting materials.

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.

Three-Component Visible-Light-Induced Palladium-Catalyzed 1,2-Alkyl Carbamoylation/Cyanation of Alkenes

A mild visible-light-induced Pd-catalyzed one-pot three-component alkyl-carbamoylation and cyanation of alkenes was developed. This general transformation, which proceeds via the in situ formation of a reactive ketenimine intermediate, allows for a rapid construction of a broad range of valuable amides and nitriles from readily available alkenes, alkyl iodides, and isocyanides. An efficient synthesis of tetrazole and amidine via this approach was also demonstrated.

Late-Stage β-C(sp3)-H Deuteration of Carboxylic Acids

Carboxylic acids are highly abundant in bioactive molecules. In this study, we describe the late-stage β-C(sp³)-H deuteration of free carboxylic acids. On the basis of the finding that C-H activation with our catalysts is reversible, the de-deuteration process was first optimized. The resulting method uses ethylenediamine-based ligands and can be used to achieve the desired deuteration when using a deuterated solvent. The reported method allows for the functionalization of a wide range of free carboxylic acids with diverse substitution patterns, as well as the late-stage deuteration of bioactive molecules and related frameworks and enables the functionalization of nonactivated methylene β-C(sp³)-H bonds for the first time.

A Guide to Directing Group Removal: 8-Aminoquinoline

The use of directing groups allows high levels of selectivity to be achieved in transition metal-catalyzed transformations. Efficient removal of these auxiliaries after successful functionalization, however, can be very challenging. This review provides a critical overview of strategies used for removal of Daugulis' 8-aminoquinoline (2005-2020), one of the most widely used N,N-bidentate directing groups. The limitations of these strategies are discussed and alternative approaches are suggested for challenging substrates. Our aim is to provide a comprehensive end-users' guide for chemists in academia and industry who want to harness the synthetic power of directing groups-and be able to remove them from their final products.

Palladium-catalyzed directed synthesis of ortho-deuterated phenylacetic acid and analogues

The synthesis of deuterium-labeled organic compounds is of increased interest, especially after the approval of deutetrabenazine by the Food and Drug Administration in 2014. The selective incorporation of deuterium in the place of hydrogen not only represents uniqueness in terms of a novel chemical class, but it also can improve the pharmacokinetic profiles of drug molecules while retaining potency and other parameters; thus, hydrogen-deuterium (H/D) exchange methods have been proven to be powerful additions in different areas of chemical science. In that regard, metal-catalyzed deuterium labeling via C-H activation mediated by a unique inbuilt directing group (DG) can play a significant role in the synthesis of novel deuterated chemical entities. In this context, herein, we divulge our results relating to Pd(ii)-catalyzed deuterium incorporation (>97%) at the γ C(sp2)-position of pyridone-containing phenylacetic acid derivatives, where 3-amino-1-methyl-1H-pyridin-2-one (AMP) not only acts as an efficient N,O-directing group, but it also constitutes a part of the target molecules of medicinal importance. Our methodology, which has been optimized based on the effects of temperature, catalyst, time, and substrate scope, shows advantages over existing protocols, with non-selectivity or meager deuteration or the use of an expensive metal (catalytic or super stoichiometric) and a deuterated solvent, reported previously for the deuteration of phenylacetic acid and its derivatives. Moreover, towards our aim of synthesizing deuterium-labeled biologically relevant compounds, the gram scale synthesis of a deuterated analogue of biphenyl acetic acid (3), known to have activity against epileptic seizures, has also been successfully accomplished in high yields and with excellent isotope enrichment via implementing this protocol.

Room-Temperature Synthesis of Isoindolone Spirosuccinimides: Merger of Visible-Light Photocatalysis and Cobalt-Catalyzed C-H Activation

A room-temperature C-H bond functionalization of benzamides has been developed by merging a photocatalyst with a cobalt catalyst for the synthesis of isoindolone spirosuccinimides. The reaction proceeds in aerobic conditions and does not require any sacrificial external oxidants such as Ag(I) or Mn(III) salts. Visible light activates the photocatalyst, and it acts as an electron-transfer reagent and helps in the fundamental organometallic steps by modulating the oxidation state of the cobalt complex. This C-H bond functionalization and spirocyclization showed wide substrate scope and good functional group tolerance. A possible reaction mechanism was proposed from the experimental outcome, showing that C-H bond activation is irreversible and not the rate-determining step.

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.

Highlights of aliphatic C(sp3 )-H hydrogen isotope exchange reactions

This review summarizes the highlights of aliphatic C (sp3 )-H carbon hydrogen isotope exchange (HIE) methods developed in the last 10 years. In particular, new highly selective and reactive protocols in the areas of nanoparticle and metal-catalyzed homogeneous catalysis are reported.

Silver-catalyzed regioselective deuteration of (hetero)arenes and α-deuteration of 2-alkyl azaarenes

A simple silver-catalyzed regioselective deuteration of (hetero)arenes and α-deuteration of 2-alkyl azaarenes has been described. This strategy provides an efficient and practical avenue to access various deuterated electron-rich arenes, azaarenes and α-deuterated 2-alkyl azaarenes with good to excellent deuterium incorporation utilizing D₂O as the source of deuterium atoms.

A practical silver-catalyzed regioselective deuteration of (hetero)arenes and α-deuteration of 2-alkyl azaarenes utilizing D₂O as a deuterium source has been developed.

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.

A highly selective H/D exchange reaction of 1,4-dihydropyridines

Herein, we report a simple, economical, and effective acid-mediated method for the in situ deuteration of Hantzsch esters and their 4-substituted derivatives, including some drugs that constitute important calcium channel blockers which are effective for hypertension treatment. Hydrogen isotope exchange occurred selectively at α-alkyl C-H bonds in the 2,6-substituents.

Trideuteromethylation Enabled by a Sulfoxonium Metathesis Reaction

A conceptually novel sulfoxonium metathesis reaction between TMSOI and cost-effective DMSO- d6 is developed for the efficient generation of a new trideuteromethylation reagent TDMSOI. The new reagent TDMSOI is produced highly efficiently by simply heating a mixture of TMSOI and DMSO- d6 and directly used for subsequent trideuteromethylation in a "one-pot" operation. The preparative power of the new versatile reagent and the "one-pot" protocol is demonstrated by its use to install the -CD3 moiety into broad functionalities including phenols, thiophenols, acidic amines, and enolizable methylene units in high yield and at a useful level of deuteration (>87% D).

Model Complexes for the Palladium-Catalyzed Transannular C-H Functionalization of Alicyclic Amines

This report describes the synthesis of model complexes for key intermediates in the Pd-catalyzed transannular C(sp³)-H arylation of alicyclic amines. These complexes react stoichiometrically with phenyl iodide to afford C-H arylation products. Furthermore, they participate in H/D exchange with d ₁₀-tert-butanol at temperatures as low as 40 °C. Overall, these studies provide insights into the factors controlling transannular C(sp³)-H activation in these systems.

Protodepalladation as a Strategic Elementary Step in Catalysis

Protodepalladation is the redox-neutral conversion of a C-Pd(II) bond to a C-H bond promoted by a Brønsted acid. It can be viewed as the microscopic reserves of Pd(II)-mediated C-H cleavage. In the context of catalytic reaction development, protodepalladation offers a means of converting organopalladium(II) intermediates to organic products without a change in oxidation state at the metal center. Hence, when integrated into catalytic cycles, it can be a uniquely enabling elementary step. The goal of this Review is to provide an overview of protodepalladation, including exploration of different reactions types, discussion of literature examples, and analysis of mechanistic features. Our hope is that this review will stimulate other researchers in the field to pursue new applications of this underexploited step in catalysis.