Reductive Homocoupling Reaction Research Articles

Reduction of Li+ within a borate anion

Group 1 elements exhibit the lowest electronegativity values in the Periodic Table. The chemical reduction of Group 1 metal cations M+ to M(0) is extremely challenging. Common tetraaryl borates demonstrate limited redox properties and are prone to decomposition upon oxidation. In this study, by employing simple yet versatile bipyridines as ligands, we synthesized a series of redox-active borate anions characterized by NMR and X-ray single-crystal diffraction. Notably, the borate anion can realize the reduction of Li+, generating elemental lithium metal and boron radical, thereby demonstrating its potent reducing ability. Furthermore, it can serve as a powerful two-electron-reducing reagent and be readily applied in various reductive homo-coupling reactions and Birch reduction of acridine. Additionally, this borate anion demonstrates its catalytic ability in the selective two-electron reduction of CO2 into CO.

Improved 2-pyridyl reductive homocoupling reaction using biorenewable solvent Cyrene™ (dihydrolevoglucosenone).

The synthesis of 5,5'-bis(trifluoromethyl)-2,2'-bipyridine using 2-bromo-5-(trifluoromethyl) pyridine was achieved at 50 °C using palladium acetate, tetrabutylammonium iodide (TBAI), potassium carbonate, and isopropanol in Cyrene™ (dihydrolevoglucosenone), a bio-renewable "green" solvent formed by a two-step process from cellulose. Improvements were achieved with 50% of γ-valerolactone (GVL) in Cyrene™ resulting in a 95% yield and 99% product purity without the use of column chromatography or recrystallization. At 80 °C, the reaction was completed within 1 h. Full conversion with 1 mol% instead of 15 mol% of palladium acetate was observed within 10 h. We showed that the formed 2,2'-bipyridine product significantly accelerated the reaction probably due to the stabilization of the catalytic species. The addition of TBAI was essential for the rapid homocoupling, however, 20 mol% of TBAI was sufficient to reach full conversion of 2-bromo-5-(trifluoromethyl) pyridine within 6 h at 80 °C. Another improvement was observed with the substitution of isopropanol by 1,4-butanediol achieving full conversion within 6 h. 2-Bromopyridines with electron withdrawing substituents in the 6, 5, 4 ring position reacted under these conditions. 2-Bromopyridines with an electron donating substituent reacted slower. Overall, we demonstrated that the 50% GVL in Cyrene™ blend is a superior "green" and less toxic alternative to dimethylformamide for the reductive homocoupling reaction. Using a quantitative scoring for twelve principles of green chemistry (DOZN™), we found significant improvements that were mediated by higher yield (atom economy), shorter heating time and lower reaction temperature (energy efficiency), safer solvent (hazardous chemical synthesis), and safer chemistry (accident prevention).

π-Conjugated Hexaazatrinaphthylene-Based Azo Polymer Cathode Material Synthesized by a Reductive Homocoupling Reaction for Organic Lithium-Ion Batteries.

A novel hexaazatrinaphthylene-based (HATN) azo polymer (PAH) was synthesized from a newly designed tri-nitro compound trinitrodiquinoxalino[2,3-a:2',3'-c]phenazine (HATNTN) through a Zn-induced reductive homocoupling reaction and used as a cathode material for lithium-ion batteries (LIBs). The integration of redox-active HATN units and azo linkages can improve the specific capacity, rate performance, and cycling stability of the PAH cathode. The control LIBs were assembled from HATNTN, in which HATNTN can be electrochemically reduced to an HATN-based azo polymer. Compared with the HATNTN cathode, the PAH cathode delivers higher specific capacities with much-improved cycling stability (97 mA h g-1 capacity retention after 1500 cycles at 500 mA g-1, which is around 28 times that of the HATNTN cathode) and considerably better rate performance (118 mA h g-1 at 2000 mA g-1, which is around 90 times that of the HATNTN cathode), simultaneously. This work provides a chemical polymerization strategy to construct extended π-conjugated azo polymers with multiple redox centers from nitro compounds for developing high-performance LIBs.

Decarbonylative Reductive Coupling of Aromatic Esters by Nickel and Palladium Catalyst

Abstract Ni or Pd-catalyzed decarbonylative reductive couplings of aromatic esters were developed. A range of (hetero)aromatic esters was applicable to these reductive homocoupling reactions. Moreover, it was found that Pd-catalysis enables a reductive decarbonylative cross-coupling between two different aromatic esters to afford a nonsymmetrical biaryls system.

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5‐Bromofuran‐2‐carboxylates were conducted in a palladium‐catalyzed reductive homocoupling reactions in the presence of alcohols as the reductant to afford [2,2’]bifuran‐5,5’‐dicarboxylic acid esters, which are essential monomers of poly(ethylene bifuranoate), serving as an green and versatile alternative polymer for traditional poly(ethylene terephthalate). More details are discussed in the article by Yin and Jiang et al. on page 62—68.image

Synthesis of [2,2’]Bifuranyl‐5,5’‐dicarboxylic Acid Esters via Reductive Homocoupling of 5‐Bromofuran‐2‐carboxylates Using Alcohols as Reductants†

Herein, we describe an environmentally benign and cost‐effective protocol for the synthesis of valuable bifuranyl dicarboxylates, starting with α‐bromination of readily accessible furan‐2‐carboxylates by LiBr and K2S2O8. Furthermore, the bromination intermediate product 5‐bromofuran‐2‐carboxylates were then conducted in a palladium‐catalyzed reductive homocoupling reactions in the presence of alcohols to afford bifuranyl dicarboxylates. One of the final products in this protocol, [2,2’]bifuran‐5,5’‐dicarboxylic acid esters, are essential monomers of poly(ethylene bifuranoate), which can be served as an green and versatile alternative polymer for traditional poly(ethylene terephthalate) that is currently common in technical plastics.

Synthesis of Double-Bridged Cofacial Nickel Porphyrin Dimers with 2,2'-Bipyridyl Pillars and Their Restricted Coordination Space.

Double-bridged cofacial Ni porphyrin dimers 2 with 2,2'-bipyridyl pillars were effectively prepared by a one-step reductive homocoupling reaction of bis(chloropyridyl)-substituted Ni porphyrin derivatives followed by a specific separation of a cyanopropyl-modified silica gel column using pyridine eluent systems. The structural analyses of 2 and its Pd complex were carried out in their solid and solution states by means of X-ray single crystal analysis and NMR, respectively. The complexation of η3-allylpalladium chloride (Pd) with 2 on the spatially restricted 2,2-bipyridine moieties on 2 gave a 2:1 (Pd:2) complex, in which the 2,2'-bipyridine ligands only provided one of the N atoms on a 2,2'-bipyridine ligand to a Pd. Therefore, the 2,2-bipyridine moieties acted as a monodentate ligand.

Piperazine as an Inexpensive and Efficient Ligand for Pd-Catalyzed Homocoupling Reactions to Synthesize Bipyridines and Their Analogues.

The synthesis of bipyridines, especially 2, 2'-bipyridines, remains challenging because the catalytic cycle can be inhibited due to coordination of bipyridine to transition metal. Thus, the development of efficient methods for the synthesis of bipyridines is highly desirable. In the present work, we presented a promising approach for preparation of bipyridines via a Pd-catalyzed reductive homocoupling reaction with simple piperazine as a ligand. Simple and inexpensive piperazine was used as a ligand for Pd-catalyzed homocoupling reaction. The combination of Pd(OAc)2 and piperazine in dimethylformamide (DMF) was observed to form an excellent catalyst and efficiently catalyzed the homocoupling of azaarenyl halides, in which DMF was used as the solvent without excess reductants although stoichiometric reductant was generally required to generate the low-oxidation-state active metal species in the catalytic cycles. In this case, good to excellent yields of bipyridines and their (hetero) aromatic analogues were obtained in the presence of 2.5 mol% of Pd(OAc)2 and 5 mol% of piperazine, using K3PO4 as a base in DMF at 140°C. According to the results, piperazine as an inexpensive and efficient ligand was used in the Pd(OAc)2-catalyzed homocoupling reaction of heteroaryl and aryl halides. The coupling reaction was operationally simple and displayed good substrate compatibility.

Co-immobilization of Pd and Zn nanoparticles in chitosan/silica membranes for efficient, recyclable catalysts used in ullmann reaction

In this study, an efficient heterogeneous catalyst including palladium (Pd) and zinc (Zn) nanoparticles supported on chitosan/silica (CS/SiO2) composite membrane is synthesized using partially etching of SiO2 technique. N2 sorption isotherm results shows that the prepared Pd-Zn@CS/SiO2 (1/1) porous membrane had a BET surface area of 26.50m2/g. High-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS), characterization of the catalyst shows that the Pd0 nanoparticles (below 5nm), and Zn0 aggregates (about 10–15nm) dispersed well in the CS/SiO2 matrix. Zinc crystal was also detected by X-ray diffraction study and HR-TEM observation. The prepared Pd-Zn@CS/SiO2 membrane catalyst is highly active for Ullmann reductive homocoupling reactions of aryl iodides and aryl bromides, and can be recycled 5 times without significant loss of activities. This work supplies a successful approach to realize Pd catalysts and Zn reducing reagent co-immobilized in the same carrier material with excellent catalytic performances.

Palladium‐catalyzed reductive homocoupling of aryl sulfonates via cleavage of C─O bond at room temperature

Palladium‐catalyzed reductive homocoupling of aryl sulfonates has been successfully achieved under mild conditions. This transformation is a new method for the homocoupling reaction of aryl sulfonates at room temperature via the cleavage of C─O bonds, thus providing an alternative synthesis of symmetric biaryls. The reported reductive homocoupling reaction is tolerant of many common functional groups regardless of electron‐donating or electron‐withdrawing nature, making this newly developed transformation important for complementing Ullmann coupling.Experimental Section.Typical procedure for the products.

Continuous flow room temperature reductive aqueous homo-coupling of aryl halides using supported Pd catalysts.

A convenient and environmentally friendly protocol for the preparation of biaryls at room temperature under continuous flow conditions is reported. A simple reductive homo-coupling Ullmann-type reaction was performed in an H-Cube mini using commercially available supported Pd catalysts under mild reaction conditions, with quantitative conversion to target products. Commercial Pd catalysts were found to be highly stable under the investigated reaction conditions, with a minimum Pd leaching into solution after several reaction runs (ca. 20 h on stream).

ChemInform Abstract: Nickel‐Catalyzed One‐Pot Deoxygenation and Reductive Homocoupling of Phenols via C—O Activation Using TCT Reagent.

A new method for C–O bond activation of phenolic compounds has been achieved using 2,4,6-trichloro-1,3,5-triazine to utilize in one-pot Ni-catalyzed deoxygenation and reductive homocoupling reactions. With this simple method, phenolic compounds were converted to their corresponding arenes or biaryl compounds under mild conditions. The introduced methodology has a broad scope and demonstrates good functional group compatibility.

ChemInform Abstract: Direct Reductive Coupling of Secondary Amides: Chemoselective Formation of Vicinal Diamines and Vicinal Amino Alcohols.

AbstractThe first one‐pot reductive homocoupling reaction of secondary amides and cross‐coupling reaction of secondary amides with ketones to give secondary vicinal diamines and amino alcohols are reported.

Direct reductive coupling of secondary amides: chemoselective formation of vicinal diamines and vicinal amino alcohols.

We report the first one-pot reductive homocoupling reaction of secondary amides and cross-coupling reaction of secondary amides with ketones to give secondary vicinal diamines and amino alcohols. This method relies on the direct generation of α-amino carbon radicals from secondary amides by activation with trifluoromethanesulfonic anhydride, partial reduction with triethylsilane and samarium diiodide-mediated single-electron transfer. The reactions were run under mild conditions and tolerated several functional groups.

Nickel-catalyzed one-pot deoxygenation and reductive homocoupling of phenols via C-O activation using TCT reagent.

A new method for C-O bond activation of phenolic compounds has been achieved using 2,4,6-trichloro-1,3,5-triazine to utilize in one-pot Ni-catalyzed deoxygenation and reductive homocoupling reactions. With this simple method, phenolic compounds were converted to their corresponding arenes or biaryl compounds under mild conditions. The introduced methodology has a broad scope and demonstrates good functional group compatibility.

Copper-promoted reductive homocoupling of quasi-aromatic iron(ii) clathrochelates: boosting the inhibitory activity in a transcription assay

A copper-promoted reductive homocoupling reaction, for the first time used for a metal complex, allowed obtaining a new kind of complexes with the encapsulated metal ions, C-C conjugated bis-clathrochelates. These compounds demonstrate extremely high transcription inhibition activity in the T7 RNA polymerase system with values of IC50 reaching as low as the submicromolar range, which places them among the most potent metal-based transcription inhibitors.

Facile synthesis of azo-linked porous organic frameworks via reductive homocoupling for selective CO2 capture

A facile Zn-induced reductive homocoupling reaction was used to synthesize azo-linked porous organic frameworks (azo-POFs) from easily accessed nitro monomers.

Polymerized functional ionic liquid supported Pd nanoparticle catalyst for reductive homocoupling of aryl halides

Homopolymer of 3-(cyanomethyl)-1-vinylimidazolium hexafluorophosphate was synthesized and used as a support for Pd nanoparticles. The Pd@poly-CN-PF6 catalyst (2 mol % Pd) selectively catalyzes the reductive homocoupling reaction of aryl iodides and bromides at 100 °C in water, and the catalyst can be recycled and reused several times with slight loss of activity.

Facile and Efficient Reductive Homocoupling of Benzyl and Aryl Halides Catalyzed by Ionic Liquid [C12mim][CuCl2] in the Presence of Metallic Zinc and Copper

A facile and efficient synthesis of bibenzyl and biaryl derivatives by reductive homocoupling reaction is described. Treatment of benzyl and aryl halides with metallic zinc and copper powder in the presence of a catalytic amount of [C12mim][CuCl2] under ligand- and base-free conditions gives the corresponding bibenzyls and biaryls in good to high yields. The product can be isolated by a simple extraction with organic solvent, and the catalytic system can be recycled or reused without any significant loss of catalytic activity.

ChemInform Abstract: Ethanol‐Promoted Reductive Homocoupling Reactions of Aryl Halides Catalyzed by Palladium on Carbon (Pd/C).

AbstractFor the first time, ethanol is used as reducing agent in the presence of a base for the homocoupling reaction of aryl/pyridyl bromides or iodides to afford symmetric biaryls.