Furanyl bis(indolyl)methane as a palladium ion-selective chromogenic agent

The colorless solution of furan-2-yl bis(indolyl)methane (BIM) is newly revealed to work as a palladium (Pd2+) ion-selective chromogenic agent by turning orange. 5-(N-Methyl-N-phenyl-aminomethyl)-furan-2-yl BIM could be synthesized from 5-chloromethylfurfural as a biorenewable feedstock via one-pot and double functionalization, and a mixture of its solution and Pd2+ ions showed the highest absorbance at 465 nm in UV-Vis analysis. On the other hand, other metal ions (Cu2+, Cr2+, Cr3+, Fe2+, Fe3+, Ni2+, Zn2+, In2+, Pt2+, or Ce3+) exhibited no response.

Visible-Light-Driven Regioselective Decarboxylative Acylation of N-Methyl-3-phenylquinoxalin-2(1H)-one by Dual Palladium-Photoredox Catalysis Through C-H Activation

We report herein an efficient visible-light-promoted approach for the regioselective decarboxylative C-H acylation of N-methyl-3-phenylquinoxalin-2(1H)-ones using α-oxo-2-phenylacetic acids via dual palladium-photoredox catalysis. The reactions were carried out at room temperature in the presence of 24 W blue LEDs. The established protocol tolerated a wide range of functional groups and enabled the synthesis of several acylated N-methyl-3-phenylquinoxalin-2(1H)-ones in good to excellent yields. The proposed mechanism for this transformation was supported by control experiments.

Reversible Single-Electron-Transfer to Oxygen in a Stable N-Heterocyclic Carbene Palladium(I) Metalloradical

The chemical and electrochemical one-electron oxidation of [Pd(IPr)2] (1) leads to the formation of mononuclear palladium(I) complex [Pd(IPr)2][PF6] (2). This bench-stable metalloradical has been fully characterized and its structure confirmed by X-ray diffraction analysis. EPR and DFT studies confirm the localization of the unpaired electron onto the metal center. Low temperature NMR and EPR measurements reveal the ability of complex 2 to reversibly coordinate and reduce the dioxygen molecule, leading to the formation of a three-coordinate complex, [PdII(IPr)2(η1-O2)]+ (4), in which the unpaired electron has been transferred to the superoxido ligand.

Palladium-catalyzed cascade cyclization of allenamide with 2-iodoanilines to access functionalized indoloquinolines

A novel and efficient palladium-catalyzed cascade cyclization to indoloquinoline derivatives in one pot has been developed by using allenamide derivatives and 2-iodoanilines as the key building blocks. The process involved two cyclizations: intramolecular cyclization/π-allylic substitution and intramolecular 6-endo Heck cyclization. Furthermore, dihydrobenzofuro[2,3-b]quinoline derivatives could also be achieved via this strategy using allenyl ethers instead of allenamides. The readily available substrates, mild conditions, high efficiency and step economy make this strategy a promising method in the synthesis of polycyclic motifs.

Palladium-Catalyzed and Photoinduced Benzylic C-H Carbonylation/Annulation under Mild Conditions

A Pd-catalyzed and photoinduced benzylic cascade benzylic C-H carbonylation/annulation reaction is realized under mild conditions (35 °C, 2 bar CO). The use of a catalytic amount of base is crucial for the reaction to achieve high yields. The reaction consists of a Pd-catalyzed generation of amidyl radical from O-benzyl hydroxylamide substrates and 1,5-HAT to give a benzylic radical, followed by carbonylation and annulation. Various homophthalimides, which could be readily converted to a number of bioactive compounds, could be obtained with up to 96% yield.

Prediction of Antileishmanial Compounds: General Model, Preparation, and Evaluation of 2-Acylpyrrole Derivatives

In this work, the SOFT.PTML tool has been used to pre-process a ChEMBL dataset of pre-clinical assays of antileishmanial compound candidates. A comparative study of different ML algorithms, such as logistic regression (LOGR), support vector machine (SVM), and random forests (RF), has shown that the IFPTML-LOGR model presents excellent values of specificity and sensitivity (81-98%) in training and validation series. The use of this software has been illustrated with a practical case study focused on a series of 28 derivatives of 2-acylpyrroles 5a,b, obtained through a Pd(II)-catalyzed C-H radical acylation of pyrroles. Their in vitro leishmanicidal activity against visceral (L. donovani) and cutaneous (L. amazonensis) leishmaniasis was evaluated finding that compounds 5bc (IC₅₀ = 30.87 μM, SI > 10.17) and 5bd (IC₅₀ = 16.87 μM, SI > 10.67) were approximately 6-fold more selective than the drug of reference (miltefosine) in in vitro assays against L. amazonensis promastigotes. In addition, most of the compounds showed low cytotoxicity, CC₅₀ > 100 μg/mL in J774 cells. Interestingly, the IFPMTL-LOGR model predicts correctly the relative biological activity of these series of acylpyrroles. A computational high-throughput screening (cHTS) study of 2-acylpyrroles 5a,b has been performed calculating >20,700 activity scores vs a large space of 647 assays involving multiple Leishmania species, cell lines, and potential target proteins. Overall, the study demonstrates that the SOFT.PTML all-in-one strategy is useful to obtain IFPTML models in a friendly interface making the work easier and faster than before. The present work also points to 2-acylpyrroles as new lead compounds worthy of further optimization as antileishmanial hits.

Heterolytic carbon-iodine bond cleavage by a palladium(I) metalloradical

The well-defined Pd(I) metalloradical [Pd(PtBu₃)₂]⁺ reacts with aryl and alkyl iodides at room temperature, yielding [Pd(PtBu₃)(μ-I)]₂ and phosphonium salts. Pd(II) aryl/alkyl derivates, reflecting net radical oxidative addition of the substrate to the metalloradical, are generated during the reaction and two examples have been isolated and crystallographically characterised.

Pd/Et3N·HI-Catalyzed Intramolecular C-H Alkylation to Access [a]-Annulated Indoles via Highly Regioselective Ring-Opening of Epoxides

A Pd/Et₃N·HI-catalyzed intramolecular C-H alkylation of indoles with epoxides was achieved to furnish N-fused indole frameworks (5-, 6-, and 7-membered rings) bearing an alcohol group. The conversion proceeded smoothly in the presence of a catalytic amount of Et₃N·HI together with a palladium catalyst and exhibited great functional group tolerance. The employment of Et₃N·HI not only avoids the prior preparation of alkyl halide substrates but also is the key to the high chemoselective ring-opening of epoxides. Preliminary mechanism explorations strongly support a radical pathway, and the catalytic cycle was established tentatively according to the mechanism investigation experiments.

Visible-Light-Driven Palladium-Catalyzed Radical Tandem Dearomatization of Indoles with Unactivated Alkenes

A mild visible-light-driven palladium-catalyzed radical tandem dearomatization of indoles with unactivated alkenes is described with moderate to good yields and good to excellent diastereoselectivities. Under visible-light irradiation, the photoexcited state of the palladium complex was formed, which could transfer a single electron to N-(2-bromobenzoyl)indoles, leading to a hybrid palladium radical chemistry. This provides efficient and atom-economical access to diverse 2,3-disubstituted indoline derivatives.

Microwave-assisted palladium catalysed C-H acylation with aldehydes: synthesis and diversification of 3-acylthiophenes

The use of MW allows the efficient palladium(II)-catalysed C-3 acylation of thiophenes with aldehydes via C(sp²)-H activation for the synthesis of (cyclo)alkyl/aryl thienyl ketones (43 examples). Compared to standard thermal conditions, the use of MW reduces the reaction time (15 to 30 min vs. 1 to 3 hours), leading to improved yields of the ketones (up to 92%). The control of positional selectivity is achieved by 2-pyridinyl and 2-pyrimidyl ortho-directing groups at C-2 of the thiophene scaffold. To show the synthetic applicability, selected ketones were subjected to further transformations, including intramolecular reactions to directly embed the directing group in the core structure of the new molecule.

Highly Diastereoselective Synthesis of Octahydro-1H-cyclpenta[c]pyridine Skeleton via Pd/Au-Relay Catalyzed Cascade Reaction of (Z)-1-Iodo-1,6-diene and Alkyne

The Octahydro-1H-cyclopenta[c]pyridine Skeletons exist in a broad spectrum of bioactive natural products, and the development of efficient and convenient protocols to construct this skeleton remains a challenging task. Herein, we...

Synthesis of Unsymmetrical Biheteroarenes via Dehydrogenative and Decarboxylative Coupling: a Decade Update

The design and development of robust and efficient methods for installing one heterocycle with another is endowed as a ubiquitous and powerful synthetic strategy to access complex organic biheterocycles in recent days due to their pervasive applications in medicinal as well as material chemistry. This perspective presents an overview on the recent findings and developments for the synthesis of unsymmetrical biheteroarenes via dehydrogenative and decarboxylative couplings with literature coverage mainly extending from 2011 to 2021. For simplification of the readers, the article has been subcategorized based on the catalysts used in the reactions.

Selective functionalization of benzylic C-H bonds of two different benzylic ethers by bowl-shaped N-hydroxyimide derivatives as efficient organoradical catalysts

A highly efficient, site-selective benzylic C-H bond amination of two different benzylic ether substrates was described by using bowl-shaped N-hydroxyimide organoradical catalysts with diethyl azodicarboxylate. The synthetic utility of this approach is demonstrated by the subsequent transformation of the amination products into the corresponding aldehydes and alkylhydrazines.

Recent Advances in Copper-Catalyzed C-N Bond Formation Involving N-Centered Radicals

C-N bonds are pervasive throughout organic-based materials, natural products, pharmaceutical compounds, and agricultural chemicals. Considering the widespread importance of C-N bonds, the development of greener and more convenient ways to form C-N bonds, especially in late-stage synthesis, has become one of the hottest research goals in synthetic chemistry. Copper-catalyzed radical reactions involving N-centered radicals have emerged as a sustainable and promising approach to build C-N bonds. As a chemically popular and diverse radical species, N-centered radicals have been used for all kinds of reactions for C-N bond formation by taking advantage of their inherently incredible reactive flexibility. Copper is also the most abundant and economic catalyst with the most relevant activity for facilitating the synthesis of valuable compounds. Therefore, the aim of the present Review was to illustrate recent and significant advances in C-N bond formation methods and to understand the unique advantages of copper catalysis in the generation of N-centered radicals since 2016. To provide an ease of understanding for the readers, this Review was organized based on the types of nitrogen sources (amines, amides, sulfonamides, oximes, hydrazones, azides, and tert-butyl nitrite).

Photoinduced Palladium-Catalyzed Intermolecular Radical Cascade Cyclization of N-Arylacrylamides with Unactivated Alkyl Bromides

A mild visible-light-induced Pd-catalyzed intermolecular radical cascade reaction of N-arylacrylamides with unactivated alkyl bromides is disclosed. Photoexcited Pd complexes transfer a single electron in this protocol, and hybrid alkyl Pd-radical species are involved as the key reaction intermediates. Sophisticated bioactive oxindole derivatives bearing various substituents and substitution patterns can be efficiently afforded through this approach.

Palladium-catalyzed cascade 5-exo-trig radical cyclization/aromatic C-H alkylation with unactivated alkyl iodides

A novel and convenient palladium-catalyzed cascade 5-exo-trig radical cyclization/aromatic C-H alkylation with unactivated alkyl iodides has been described. This strategy provides an efficient access to a variety of 3a-methyl-1,2,3,3a,4,8b-hexahydroindeno[1,2-b]pyrrole derivatives, which facilitate access to a series of medically important heterocyclic bioactive molecules. This protocol involves mild catalytic reaction conditions and shows high functional group tolerance with high stereoselectivity. Mechanistic investigations reveal that an alkyl radical pathway is involved in this reaction.