Palladium-Catalyzed C–H/C–H Cross-Coupling by Mechanochemistry: Direct Alkenylation and Heteroarylation of N1-Protected 1H-Indazoles

Mechanochemical C-H Arylation and Alkylation of Indoles Using 3 d Transition Metal and Zero-Valent Magnesium

Although the 3 d transition-metal catalyzed C-H functionalization have been extensively employed to promote the formation of valuable carbon-carbon bonds, the persistent problems, including the use of sensitive Grignard reagents and the rigorous operations (solvent-drying, inert gas protection, metal pre-activation and RMgX addition rate control), still leave great room for further development of sustainable methodologies. Herein, we report a mechanochemical technology toward in-situ preparation of highly sensitive organomagnesium reagents, and thus building two general 3 d transition-metal catalytic platforms that enables regioselective arylation and alkylation of indoles with a wide variety of halides (including those containing post transformable functionalities and heteroaromatic rings). This mechanochemical strategy also brings unique reactivity and high step-economy in producing functionalized N-free indole products.

Temperature-induced formation of Pd nanoparticles in heterogeneous nanobiohybrids: application in C-H activation catalysis

The effect of the temperature in the synthesis of Pd nanoparticles in the metal-enzyme biohybrids is evaluated. The effect on the formation, size, and morphology of nanoparticles was evaluated using C. antarctica B lipase as the protein scaffold. XRD analyses confirmed the formation of crystalline Pd(0) as the metal species in all cases. TEM analyses revealed spherical crystalline nanoparticles with average diameter size from 2 nm at 4 °C synthesis to 10 nm obtained at 50 °C synthesis. The thermal phenomenon was also critical in the final hybrid formation using more complex enzymes, where the relation of the protein structure and temperature and the influence of the latter has been demonstrated to be critical in the reducing efficiency of the enzyme in the final Pd nanoparticle formation, in the metal species, or even in the final size of the nanoparticles. Different Pd biohybrids were evaluated as catalysts in the C-H activation of protected l-tryptophan under mild conditions. Pd@CALB4 showed the best results, with >99% conversion for C-2 arylation in methanol at room temperature with a TOF value of 64 min^-1, being 2 or 4 times higher than that of the other synthesized hybrids. This catalyst showed a very high stability and recyclability, maintaining >95% activity after three cycles of use.

Mechanochemical halogenation of unsymmetrically substituted azobenzenes

The direct and selective mechanochemical halogenation of C–H bonds in unsymmetrically substituted azobenzenes using N-halosuccinimides as the halogen source under neat grinding or liquid-assisted grinding conditions in a ball mill has been described. Depending on the azobenzene substrate used, halogenation of the C–H bonds occurs in the absence or only in the presence of Pd^II catalysts. Insight into the reaction dynamics and characterization of the products was achieved by in situ Raman and ex situ NMR spectroscopy and PXRD analysis. A strong influence of the different 4,4’-substituents of azobenzene on the halogenation time and mechanism was found.

Palladium-catalyzed cross-coupling of unreactive C(sp3)-H bonds with azole C(sp2)-H bonds by using bromide as a traceless directing group

A palladium-catalyzed intermolecular cross-coupling of unreactive C(sp³)-H bonds and azole C(sp²)-H bonds with bromide as a traceless directing group is described. The judicious selection of the bulky and electron-rich phosphine ligand is the key for the success of this cascade process. The protocol features a broad substrate scope, excellent regioselectivity, and good functional group tolerance.

Traditional and sustainable approaches for the construction of C–C bonds by harnessing C–H arylation

Biaryl scaffolds are found in natural products and drug molecules and exhibit a wide range of biological activities. In past decade, the transition metal-catalyzed C–H arylation reaction came out as an effective tool for the construction of biaryl motifs. However, traditional transition metal-catalyzed C–H arylation reactions have limitations like harsh reaction conditions, narrow substrate scope, use of additives etc. and therefore encouraged synthetic chemists to look for alternate greener approaches. This review aims to draw a general overview on C–H bond arylation reactions for the formation of C–C bonds with the aid of different methodologies, majorly highlighting on greener and sustainable approaches.

Transition-metal-catalyzed C–H arylations are an effective tool for the construction of biaryl motifs in an efficient and selective manner. Here the authors provide an overview of the state-of-the-art of the field and perspectives on emerging directions toward increased sustainability.

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.

Palladium-catalyzed oxidative C-H/C-H cross-coupling of pyrazolo[1,5-a]azines with five-membered heteroarenes

The use of Pd(OAc)₂ as the catalyst and AgOAc as the oxidant enabled the direct regioselective oxidative C-H/C-H cross-coupling of pyrazolo[1,5-a]pyrimidines or pyrazolo[1,5-a]pyridines with various five-membered heteroarenes without the need of pre-activation and/or directing groups. Successful coupling partners include thiophenes, benzothiophenes, thiazoles, furans, oxazoles, indoles and imidazo[1,2-a]pyridines.

Synthetic strategy and structure-activity relationship (SAR) studies of 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1, Lificiguat): a review

Since 1994, YC-1 (Lificiguat, 3-(5′-hydroxymethyl-2′-furyl)-1-benzylindazole) has been synthesized, and many targets for special bioactivities have been explored, such as stimulation of platelet-soluble guanylate cyclase, indirect elevation of platelet cGMP levels, and inhibition of hypoxia-inducible factor-1 (HIF-1) and NF-κB. Recently, Riociguat®, the first soluble guanylate cyclase (sGC) stimulator drug used to treat pulmonary hypertension and pulmonary arterial hypertension, was derived from the YC-1 structure. In this review, we aim to highlight the synthesis and structure–activity relationships in the development of YC-1 analogs and their possible indications.

Since 1994, YC-1 (Lificiguat) has been synthesized, and many targets for special bioactivities have been explored, such as stimulation of platelet-soluble guanylate cyclase, indirect elevation of platelet cGMP levels, and inhibition of HIF-1 and NF-κB.

Infrared Irradiation-Assisted Solvent-Free Pd-Catalyzed (Hetero)aryl-aryl Coupling via C-H Bond Activation

The increasing attention towards environmentally friendly synthetic protocols has boosted studies directed to the development of green and sustainable methods for direct C-H bond arylation of (hetero)arenes. In this context, here the infrared (IR) irradiation-assisted solvent-free Pd-catalyzed direct C-H bond arylation of (hetero)arenes was achieved. Several heteroaryl-aryl coupling reactions were described, also involving heterocycles commonly used as building blocks for the synthesis of organic semiconductors. The reaction tolerated many functional groups on the aromatic nuclei. The IR-irradiation as the energy source compared favorably with thermal heating and, in combination with solvent-free conditions, provided an important contribution to the development of protocols fitting with the principles of green chemistry.

Mechanochemical Magnesium-Mediated Minisci C-H Alkylation of Pyrimidines with Alkyl Bromides and Chlorides

A novel method to synthesize 4-alkylpyrimidines by the mechanochemical magnesium-mediated Minisci reaction of pyrimidine derivatives and alkyl halides has been reported. The reaction process operates with a broad substrate scope and excellent regioselectivity under mild conditions with no requirement of transition-metal catalysts, solvents, and inert gas protection. The practicality of this protocol has been demonstrated by the up-scale synthesis, mechanochemical product derivatization, and antimalarial drug pyrimethamine preparation.

Pd-Catalyzed Intermolecular Dehydrogenative Heck Reactions of Five-Membered Heteroarenes

The Pd-mediated cross-coupling of (hetero)arenes with alkenes may be an effective method for the formation of a C–C bond from two C–H bonds. Discovered by Fujiwara and co-workers in 1967, this reaction led to a number of reports that we firstly highlighted in 2011 (review with references till June 2010) and for which, we retained the name “dehydrogenative Heck reaction”. The topic, especially the reactions of five-membered heteroarenes, has been the subject of intensive research over the last ten years. The present review is limited to these dehydrogenative Heck reactions published since 2010, underlining the progress of the procedures.