Pd/PTABS: Low Temperature Etherification of Chloroheteroarenes

CuSO4/N-(9H-carbazol-9-yl)picolinamide-Catalyzed C-O Coupling of (Hetero)Aryl Chlorides with Phenols on Water

A Cu-catalyzed C-O coupling of (hetero)aryl chlorides with phenols at 120 °C on water was developed with a designed ligand, N-(9H-carbazol-9-yl)picolinamide (L2). This method features a good substrate scope (both electron-donating and electron-withdrawing), low catalyst/ligand loadings (down to 1 mol %), and excellent scalability and practicability.

Accelerated photochemical reactions at oil-water interface exploiting melting point depression

Water can accelerate a variety of organic reactions far beyond the rates observed in classical organic solvents. However, using pure water as a solvent introduces solubility constraints that have limited the applicability of efficient photochemistry in particular. We report here the formation of aggregates between pairs of arenes, heteroarenes, enamines, or esters with different electron affinities in an aqueous medium, leading to an oil-water phase boundary through substrate melting point depression. The active hydrogen atoms in the reactants engage in hydrogen bonds with water, thereby accelerating photochemical reactions. This methodology realizes appealingly simple conditions for aqueous coupling reactions of complex solid molecules, including complex drug molecules that are poorly soluble in water.

DMSO-Assisted K3 PO4 -Catalyzed Cooperative Metal-Free, Base-Free Etherification of Chloroheteroarenes at Low Temperature

Etherification of chloroheteroarenes was performed at low temperatures under metal-free, ligand-free and base-free conditions, that is, the reaction is promoted by the cooperative effect of DMSO (solvent) as a promoter and K3 PO4 providing the catalytic surface (rather than a base). The protocol exhibits good substrate scope under mild reaction conditions and has also been explored mechanistically.

Rapid plugged flow synthesis of nucleoside analogues via Suzuki-Miyaura coupling and heck Alkenylation of 5-Iodo-2'-deoxyuridine (or cytidine)

Nucleosides modification via conventional cross-coupling has been performed using different catalytic systems and found to take place via long reaction times. However, since the pandemic, nucleoside-based antivirals and vaccines have received widespread attention and the requirement for rapid modification and synthesis of these moieties has become a major objective for researchers. To address this challenge, we describe the development of a rapid flow-based cross-coupling synthesis protocol for a variety of C5-pyrimidine substituted nucleosides. The protocol allows for facile access to multiple nucleoside analogues in very good yields in a few minutes compared to conventional batch chemistry. To highlight the utility of our approach, the synthesis of an anti-HSV drug, BVDU was also achieved in an efficient manner using our new protocol.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s41981-023-00265-1.

Novel 1,2,3-triazolyl phosphine with a pyridyl functionality: synthesis, coinage metal complexes, photophysical studies and Cu(I) catalyzed C-O coupling of phenols with aryl bromides

This manuscript describes the synthesis and coinage metal complexes of pyridine appended 1,2,3-triazolyl-phosphine [2-{(C₆H₄N)(C₂(PPh₂)N₃C₆H₅)}] (1), photophysical studies and their catalytic application. The reactions of 1 with copper salts afforded dimeric complexes [{Cu(μ₂-X)}₂{2-(C₆H₄N)(C₂(PPh₂)N₃C₆H₅)}₂] (2, X = Cl; 3, X = Br; and 4, X = I). The crystal structure indicates that the Cu⋯Cu distance in 4 (2.694 Å) is significantly shorter than that in complexes 3 (3.0387 Å) and 2 (3.104 Å), indicating strong cuprophilic interactions which is also supported by NBO calculations, signifying the involvement of 3d_z² orbitals from each Cu atom contributing to the bonding interaction. The fluorescence studies on complexes 2-4 carried out in the solid state showed broad emission bands around 560 nm on excitation at λ_ex = 420 nm. Complex 4 on treatment with two equivalents of 1,10-phenanthroline yielded a mononuclear complex 5 which showed almost complete quenching of fluorescence in the solid state, clearly indicating that the emissive properties of 4 are mainly due to the Cu⋯Cu interaction, along with (M + X)LCT. The reactions of 1 with silver salts led to the isolation of dimeric complexes [{Ag(μ₂-X)}₂{2-(C₆H₄N)(C₂(PPh₂)N₃C₆H₅)}₂] (6, X = Cl; 7, X = Br; and 8, X = I) in good yield. The reaction between 1 and [AuCl(SMe₂)] yielded [{AuCl}{2-(C₆H₄N)(C₂(PPh₂)N₃C₆H₅)}] (9). The molecular structures of 2-5 and 7-9 were confirmed by single crystal X-ray analysis. The complex 4 is found to be an excellent catalyst for C-O coupling under mild conditions.

Room-Temperature Dialkylamination of Chloroheteroarenes Using a Cu(II)/PTABS Catalytic System

The dimethylamino functionality has significant importance in industrially relevant molecules and methodologies to install these efficiently are highly desirable. We report herein a highly efficient, room-temperature dimethylamination of chloroheteroarenes performed via the in-situ generation of dimethylamine using N,N-dimethylformamide (DMF) as precursor wiith a large substrate scope that includes various heteroarenes, purines as well as commercially relevant drugs such as altretamine, ampyzine and puromycin precursor.

Water-Soluble Pd-Imidate Complexes as Versatile Catalysts for the Modification of Unprotected Halonucleosides

Modification of unprotected nucleosides has been attracting continuous interest, since these building blocks themselves and their phosphate-upgraded corresponding nucleotides have shown a plethora of uses in fields like biochemistry or pharmacy. Pd-catalyzed cross-coupling reactions, conducted in water or its mixtures with polar organic solvents, have frequently been the researchers' choice for the functionalization of the purine/pyrimidine base of the unprotected nucleosides. In this scenario, the availability of hydrophilic ligands and its water-soluble palladium complexes has markedly set the pace of the advances. The approach of our group to the synthesis of such complexes, Pd-imidates specifically, has faced critical stages, namely the jump to synthesize water soluble complexes from our experience working in conventional solvents, the preparation of phosphine free complexes and the overall goal of getting catalytic systems able to work close to room temperature. The continuous feedback with Kapdi's group, experienced in the chemistry of nucleosides, has produced over the last decade the interesting results in both fields presented here.

Quadrol-Pd(II) complexes: phosphine-free precatalysts for the room-temperature Suzuki-Miyaura synthesis of nucleoside analogues in aqueous media

Commercially available Quadrol, N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine (THPEN), has been used for the first time as a N^N-donor neutral hydrophilic ligand in the synthesis and characterization of new water soluble palladium(II) complexes containing chloride, phthalimidate or saccharinate as co-ligands. [PdCl2(THPEN)] (1) [Pd(phthal)2(THPEN)] (2), [Pd(sacc)2(THPEN)] (3) and the analogous complex with the closely related N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (THEEN) [Pd(sacc)2(THEEN)] (4) were efficiently prepared in a one-pot reaction from [PdCl2(CH3CN)2] or Pd(OAc)2. Structural characterization of 1 and 3 by single crystal X-ray diffraction produced the first structures reported to date of palladium complexes with Quadrol. The resultant palladium complexes are highly soluble in water and were found to be effective as phosphine-free catalysts for the synthesis of functionalized nucleoside analogues under room-temperature Suzuki-Miyaura cross-coupling conditions between 5-iodo-2'-deoxyuridine (& 5-iodo-2'-deoxycytidine) with different aryl boronic acids in neat water. This is the first report of the coupling process performed on nucleosides in water at room temperature.

Room-Temperature Amination of Chloroheteroarenes in Water by a Recyclable Copper(II)-Phosphaadamantanium Sulfonate System

Buchwald-Hartwig amination of chloroheteroarenes has been a challenging synthetic process, with very few protocols promoting this important transformation at ambient temperature. The current report discusses about an efficient copper-based catalytic system (Cu/PTABS) for the amination of chloroheteroarenes at ambient temperature in water as the sole reaction solvent, a combination that is first to be reported. A wide variety of chloroheteroarenes could be coupled efficiently with primary and secondary amines as well as selected amino acid esters under mild reaction conditions. Catalytic efficiency of the developed protocol also promotes late-stage functionalization of active pharmaceutical ingredients (APIs) such as antibiotics (floxacins) and anticancer drugs. The catalytic system also performs efficiently at a very low concentration of 0.0001 mol % (TON = 980,000) and can be recycled 12 times without any appreciable loss in activity. Theoretical calculations reveal that the π-acceptor ability of the ligand PTABS is the main reason for the appreciably high reactivity of the catalytic system. Preliminary characterization of the catalytic species in the reaction was carried out using UV-VIS and ESR spectroscopy, providing evidence for the Cu(II) oxidation state.

Oxalohydrazide Ligands for Copper-Catalyzed C-O Coupling Reactions with High Turnover Numbers

Here, we report a class of ligands based on oxalohydrazide cores and N-amino pyrrole and N-amino indole units that generates long-lived copper catalysts for couplings that form the C-O bonds in biaryl ethers. These Cu-catalyzed coupling of phenols with aryl bromides occurred with turnovers up to 8000, a value which is nearly two orders of magnitude higher than those of prior couplings to form biaryl ethers and nearly an order of magnitude higher than those of any prior copper-catalyzed coupling of aryl bromides and chlorides. This ligand also led to copper systems that catalyze the coupling of aryl chlorides with phenols and the coupling of aryl bromides and iodides with primary benzylic and aliphatic alcohols. A wide variety of functional groups including nitriles, halides, ethers, ketones, amines, esters, amides, vinylarenes, alcohols and boronic acid esters were tolerated, and reactions occurred with aryl bromides in pharmaceutically related structures.

A comprehensive review of caged phosphines: synthesis, catalytic applications, and future perspectives

Caged phosphines are versatile ligands due to their rigid backbones, exhibiting a range of catalytic activities, as depicted through the given pictorial representation.

1,3,5-Triaza-7-phosphaadamantane (PTA) Derived Caged Phosphines for Palladium-Catalyzed Selective Functionalization of Nucleosides and Heteroarenes

Phosphines have, in combination with transition metals, played a pivotal role in the rapid development of efficient catalytic processes. Caged phosphines constitute a class of three-dimensional scaffolds providing unique control over steric and electronic properties. The versatility of the caged phosphine ligands has been demonstrated elegantly by the groups of Verkade, Gonzalvi as well as Stradiotto. Our research group has also been working extensively for the past several years in the development of 1,3,5-triaza-7-phosphaadamantane-based caged ligands and in this personal note we have summarized these applications pertaining to the modification of biologically useful nucleosides and heteroarenes.

Scale-Up of a Heck Alkenylation Reaction: Application to the Synthesis of an Amino-Modifier Nucleoside ‘Ruth Linker’

Ruth linker is a C5 pyrimidine modified nucleoside analogue widely utilized for the incorporation of a primary amine in a synthetic oligonucleotide. The increasing demand for non-radioactive labeling, detection of biomolecules, and assembly of COVID-19 test kits has triggered a need for scale-up of Ruth linker. Herein, an efficient protocol involving a palladium-catalyzed Heck alkenylation is described. The synthesis has been optimized with a goal of low catalyst concentration, column-free isolation, high product purity, reproducibility, and shorter reaction time. The scalability and utility of the process have been demonstrated successfully on a 100 g scale (starting material). Additionally, for scale-up of the Heck alkenylation protocol, 7-phospha-1,3,5-triaza-adamantanebutane sulfonate (PTABS) as the coordinating caged phosphine ligand was also synthesized on a multigram scale after careful optimization of the conditions.

Discovery, Synthesis, and Scale-up of Efficient Palladium Catalysts Useful for the Modification of Nucleosides and Heteroarenes

Nucleic acid derivatives are imperative biomolecules and are involved in life governing processes. The chemical modification of nucleic acid is a fascinating area for researchers due to the potential activity exhibited as antiviral and antitumor agents. In addition, these molecules are also of interest toward conducting useful biochemical, pharmaceutical, and mutagenic study. For accessing such synthetically useful structures and features, transition-metal catalyzed processes have been proven over the years to be an excellent tool for carrying out the various transformations with ease and under mild reaction conditions. Amidst various transition-metal catalyzed processes available for nucleoside modification, Pd-catalyzed cross-coupling reactions have proven to be perhaps the most efficient, successful, and broadly applicable reactions in both academia and industry. Pd-catalyzed C-C and C-heteroatom bond forming reactions have been widely used for the modification of the heterocyclic moiety in the nucleosides, although a single catalyst system that could address all the different requirements for nucleoside modifications isvery rare or non-existent. With this in mind, we present herein a review showcasing the recent developments and improvements from our research groups toward the development of Pd-catalyzed strategies including drug synthesis using a single efficient catalyst system for the modification of nucleosides and other heterocycles. The review also highlights the improvement in conditions or the yield of various bio-active nucleosides or commercial drugs possessing the nucleoside structural core. Scale ups wherever performed (up to 100 g) of molecules of commercial importance have also been disclosed.

Selective synthesis of pyridyl pyridones and oxydipyridines by transition-metal-free hydroxylation and arylation of 2-fluoropyridine derivatives

An efficient protocol for the construction of various pyridyl pyridone and oxydipyridine derivatives through a hydroxylation and arylation tandem reaction of 2-fluoropyridines is reported. Under simple transition-metal-free conditions, the reaction provided a series of products in good to excellent yields, and their structures were confirmed by crystal diffraction analysis. Furthermore, the controlling effect of 6-position substituents on the highly selective synthesis of pyridone and oxydipyridine was studied.

Base-Mediated O-Arylation of Alcohols and Phenols by Triarylsulfonium Triflates

A mild and efficient protocol for O-arylation of alcohols and phenols (ROH) by triarylsulfonium triflates was developed under transition-metal-free conditions. Various alcohols, including primary, secondary and tertiary, and phenols bearing either electron-donating or electron-withdrawing groups on the aryl rings were smoothly converted to form the corresponding aromatic ethers in moderate to excellent yields. The reactions were conducted at 50 or 80 °C for 24 h in the presence of a certain base and showed good functional group tolerance. The base-mediated arylation with asymmetric triarylsulfonium salts could selectively transfer the aryl groups of sulfoniums to ROH, depending on their inherent electronic nature. The mechanistic studies revealed that the reaction might proceed through the nucleophilic attack of the in situ formed alkoxy or phenoxy anions at the aromatic carbon atoms of the C-S bonds of triarylsulfonium cations to furnish the target products.

Novel Carbazole-Based N-Heterocyclic Carbene Ligands to Access Synthetically Relevant Stilbenes in Pd-Catalyzed Coupling Processes

A series of new carbazole-based N-heterocyclic carbene (NHC) ligands have been synthesized in a simple and facile synthetic route and subsequently used in a Pd/carbazole-based NHC catalytic system, which was found to be effective in catalyzing Heck reactions to provide substituted stilbene derivatives in good yields. Several bioactive stilbenes, including pterostilbene, pinosylvin, trimethoxy resveratrol, and resveratrol, were synthesized in good yields, and a 10 mmol scale-up was also performed for trimethoxy resveratrol. The synthetic application was also extended by performing a double-tandem chemoselective Heck reaction followed by Miyaura borylation in a one-pot procedure to give single-step access to synthetically useful stilbenyl boronate esters. Similarly, a unique triple-tandem protocol of a chemoselective Heck reaction/Miyaura borylation/Suzuki-Miyaura coupling reaction sequence was performed for the one-pot modification of biologically relevant molecules.