Reductive Homocoupling Research Articles

Catalytic Reductive Homocoupling of Benzyl Chlorides Enabled by Zirconocene and Photoredox Catalysis

Catalytic Reductive Homocoupling of Benzyl Chlorides Enabled by Zirconocene and Photoredox Catalysis

Harnessing redox-inactive rare-earth metals for photocatalytic reductive coupling of benzyl bromides

Compared with rare and expensive late-transition metals, rare-earth photocatalysts are much less investigated in synthetic chemistry, particularly concerning redox-inactive rare-earth metals. Herein, we describe a general strategy to realize the redox-inactive rare-earth photocatalysis by grafting a light-absorbing scaffold onto common ligands in rare-earth organometallics. Three guanidinate rare-earth complexes with photocatalytic functions were synthesized and found to exhibit higher catalytic efficiency than phenothiazine in the reductive homocoupling of benzyl bromides. These preliminary results illustrated that our “grafting” strategy could serve as a facile methodology for the construction of redox-inactive rare-earth photocatalysis systems.

Eosin Y-catalyzed reductive homocoupling of para-quinone methides under visible-light.

In this manuscript, we demonstrate a visible-light driven dimerization of para-quinone methides using eosin Y catalyst via a reductive homocoupling process. This mild and operationally simple methodology was found to be compatible with a variety of differently substituted para-quinone methides and a broad range of tetra-arylethane derivatives were obtained in moderate to good yields (47%-87%).

Nickel/α‐diimine catalysts in C (sp2)–C (sp2) homocoupling: An underestimated and versatile ligand family

Application of α‐diimine ligands in Ni‐catalyzed reductive homocoupling of haloarenes is reported. The Ni/α‐diimine catalysts are shown to be highly active and allow for preparation of a broad scope of biaryls, including practically important cyclization and polycondensation products. The synthetic availability and versatility of α‐diimines allows for easy tuning of their structure aimed at achieving higher yields of each specific homocoupling product.

Pd/NHC catalyzed reduction and coupling of nitroaromatics for the synthesis of diarylamines.

Herein, we report a one-pot approach to diarylamines through the reductive homocoupling of nitroaromatics, employing triethylsilane as the reducing agent and Pd/NHC as the catalyst. This method enables nitroaromatics to serve both as electrophilic reagents and as precursors of nucleophilic reagents, allowing for the direct preparation of diarylamines without the need to isolate aromatic primary amines.

Air Tolerant Cadiot-Chodkiewicz and Sonogashira Cross-Couplings.

Cadiot-Chodkiewicz cross-couplings generate an unsymmetric buta-1,3-diyne by way of a Cu(I)-catalyzed coupling between a terminal alkyne and a 1-haloalkyne. Despite their widespread use, Cadiot-Chodkiewicz reactions are plagued by the generation of symmetric buta-1,3-diyne side products, formed through competing: (a) formal reductive homo-coupling of the 1-haloalkyne and (b) oxidative (Glaser-Hay/Eglinton) homo-coupling of the terminal alkyne. To overcome this issue, a large excess of one of the two reacting alkynes is commonly deployed, and difficult separations of cross- and homo-coupled products are often encountered. Here, we demonstrate that the use of ascorbate as a reductant leads to a suppression of these unwanted side reactions, hence permitting excellent yields with a roughly stoichiometric ratio of reactants. The procedure also avoids an inert gas atmosphere and uses a sustainable solvent. A similar approach is effective for cross-couplings involving a Pd(0)/Pd(II) catalytic cycle, with air tolerant Sonogashira couplings also established.

Template-OrientedPolyaniline-Supported Palladium Nanoclusters for Reductive Homocoupling of Furfural Derivatives.

Developing well-defined structures and desired properties for porous organic polymer (POP) supported catalysts by controlling their composition, size, and morphology is of great significance. Herein, we report a preparation of polyaniline (PANI) supported Pd nanoparticles (NPs) with controllable of the structure and morphology. The protocol involves the introduction of MnO2 with different crystal structures (α, β, γ, δ, ε) serving as both reaction templates and oxidants. MnO2 selectively oxidizes the aniline to polyaniline with diverse regular distribution of benzene and quinone, thus leading to the Pd/PANI catalyst growth mode which prominently impacts the charge transfer between Pd and PANI, as well as the dispersion of metal NPs. In particular, Pd/ε-PANI greatly improves the turnover frequency (TOF), up to 88.3 h-1,in the reductive coupling of the furfural derivatives to potential bio-based plasticizers. Systematic characterizations reveal the unique oxidation state of the support in the Pd/ε-PANI catalyst and coordination mode of Pd that drives the formation of highly dispersed Pd nanoclusters. Density functional theory (DFT) calculations show the more electron rich Pd/PANI catalyst has the lower energy barrier in the oxidative addition step, which favors the C-C coupling reaction.

Template‐Oriented Polyaniline‐Supported Palladium Nanoclusters for Reductive Homocoupling of Furfural Derivatives

AbstractDeveloping well‐defined structures and desired properties for porous organic polymer (POP) supported catalysts by controlling their composition, size, and morphology is of great significance. Herein, we report a preparation of polyaniline (PANI) supported Pd nanoparticles (NPs) with controllable structure and morphology. The protocol involves the introduction of MnO2 with different crystal structures (α, β, γ, δ, ϵ) serving as both the reaction template and the oxidant. The different forms of MnO2 each convert aniline to a PANI that contains a unique regular distribution of benzene and quinone. This leads to the Pd/PANI catalysts with different charge transfer properties between Pd and PANI, as well as different dispersions of the metal NPs. In this case, the Pd/ϵ‐PANI catalyst greatly improves the turnover frequency (TOF; to 88.3 h−1), in the reductive coupling of furfural derivatives to potential bio‐based plasticizers. Systematic characterizations reveal the unique oxidation state of the support in the Pd/ϵ‐PANI catalyst and coordination mode of Pd that drives the formation of highly dispersed Pd nanoclusters. Density functional theory (DFT) calculations show the more electron rich Pd/PANI catalyst has the lower energy barrier in the oxidative addition step, which favors the C−C coupling reaction.

Rearrangements of Internally‐bridged Octaphyrin(1.1.1.1.1.1.1.1)s to Porphyrin Dimers

AbstractInternally‐bridged octaphyrin(1.1.1.1.1.1.1.1) underwent a rearrangement to meso‐meso linked porphyrin dimer as a new metamorphosis. Extensive examination suggests that hydride‐addition is a key step for the rearrangements to afford thermodynamically more stable porphyrin framework. Further, β‐tetrabromo[36]octaphyrin was transformed to meso‐meso, β‐β, β‐β triply linked porphyrin dimer via a similar mechanism combined with Pd‐mediated reductive homocoupling.

Rearrangements of Internally-bridged Octaphyrin(1.1.1.1.1.1.1.1)s to Porphyrin Dimers.

Internally-bridged octaphyrin(1.1.1.1.1.1.1.1) underwent a rearrangement to meso-meso linked porphyrin dimer as a new metamorphosis. Extensive examination suggests that hydride-addition is a key step for the rearrangements to afford thermodynamically more stable porphyrin framework. Further, β-tetrabromo[36]octaphyrin was transformed to meso-meso, β-β, β-β triply linked porphyrin dimer via a similar mechanism combined with Pd-mediated reductive homocoupling.

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).

Synthesis and Characterization of Benzene- and Triazine-Based Azo-Bridged Porous Organic Polymers.

Porous organic polymers incorporating nitrogen-rich functionalities have recently emerged as promising materials for efficient and highly selective CO2 capture and separation. Herein, we report synthesis and characterization of new two-dimensional (2D) benzene- and triazine-based azo-bridged porous organic polymers. Different synthetic approaches towards the porous azo-bridged polymers were tested, including reductive homocoupling of aromatic nitro monomers, oxidative homocoupling of aromatic amino monomers and heterocoupling of aromatic nitro monomers and a series of aromatic diamines of different lengths and rigidity. IR spectroscopy, 13C CP/MAS NMR spectroscopy, powder X-ray diffraction, elemental analysis, thermogravimetric analysis, nitrogen adsorption-desorption experiments and computational study were used to characterize structures and properties of the resulting polymers. The synthesized azo-bridged polymers are all amorphous solids of good thermal stability, exhibiting various surface areas (up to 351 m2 g-1). The obtained results indicated that the synthetic methods and building units have a pronounced effect on the porosity of the final materials. Reductive and oxidative homocoupling of aromatic nitro and amino building units, respectively, lead to 2D azo-bridged polymers of substantially higher porosity when compared to those produced by heterocoupling reactions. Periodic DFT calculations and Grand-canonical Monte Carlo (GCMC) simulations suggested that, within the used approximations, linear linkers of different lengths do not significantly affect CO2 adsorption properties of model azo-bridged polymers.

Geländer Molecules with Orthogonal Joints: Synthesis of Macrocyclic Dimers.

Orthogonal joints, understood as connections with an angle of 90°, were introduced in the design of the "Geländer" model compounds 1 and 2. The banister, consisting of a conjugated carbazole dimer linked by either 1,3-butadiyne (2) or a single thiophene (1), wraps around an axis composed of a phthalimide dimer due to the dimensional mismatch of both subunits, which are interconnected by phenylene rungs. The "Geländer" structure was assembled from a monomer comprising the 1,4-diaminobenzene rung with one amino substituent as part of a 4-bromo phthalimide subunit forming the orthogonal junction to the axis, and the other as part of a masked 2-ethynyl carbazole as orthogonal joint to the banister. The macrocycle was obtained by two sequential homocoupling steps. A first dimerization by a reductive homocoupling assembled the axis, while an oxidative acetylene coupling served as ring-closing reaction. The formed butadiyne was further derivatized to a thiophene, rendering all carbons of the model compound sp2 hybridized. Both helical structures were fully characterized and chirally resolved. Assignment of the enantiomers was achieved by simulation of chiroptical properties and enantiopure synthesis.

One Pot Synthesis of 1,2‐Disubstituted Ethanones by Base‐Mediated Reductive Homocoupling of Aldehydes

AbstractReductive homocoupling transformations mediated by bases and TMSCN have been used as a strategy for synthesis of 1,2‐disubstituted ethanones. This protocol provides a method for synthesizing functionalized ethanones from aldehydes in one pot without transition metals.magnified image

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.

Molecular Catalysts for the Reductive Homocoupling of CO2 towards C2+ Compounds.

The conversion of CO2 into multicarbon (C2+) compounds by reductive homocoupling offers the possibility to transform renewable energy into chemical energy carriers and thereby create “carbon‐neutral” fuels or other valuable products. Most available studies have employed heterogeneous metallic catalysts, but the use of molecular catalysts is still underexplored. However, several studies have already demonstrated the great potential of the molecular approach, namely, the possibility to gain a deep mechanistic understanding and a more precise control of the product selectivity. This Minireview summarizes recent progress in both the thermo‐ and electrochemical reductive homocoupling of CO2 toward C2+ products mediated by molecular catalysts. In addition, reductive CO homocoupling is discussed as a model for the further conversion of intermediates obtained from CO2 reduction, which may serve as a source of inspiration for developing novel molecular catalysts in the future.

Molecular Catalysts for the Reductive Homocoupling of CO2 towards C2+ Compounds

AbstractThe conversion of CO2 into multicarbon (C2+) compounds by reductive homocoupling offers the possibility to transform renewable energy into chemical energy carriers and thereby create “carbon‐neutral” fuels or other valuable products. Most available studies have employed heterogeneous metallic catalysts, but the use of molecular catalysts is still underexplored. However, several studies have already demonstrated the great potential of the molecular approach, namely, the possibility to gain a deep mechanistic understanding and a more precise control of the product selectivity. This Minireview summarizes recent progress in both the thermo‐ and electrochemical reductive homocoupling of CO2 toward C2+ products mediated by molecular catalysts. In addition, reductive CO homocoupling is discussed as a model for the further conversion of intermediates obtained from CO2 reduction, which may serve as a source of inspiration for developing novel molecular catalysts in the future.

Synthesis of Symmetrical Thiosulfonates via Cu(OTf)2‐Catalyzed Reductive Homocoupling of Arenesulfonyl Chlorides

AbstractA variety of symmetrical thiosulfonates are synthesized via Cu‐catalyzed reductive homocoupling of aryl sulfonyl chlorides. This protocol uses organic amine acting as a mild reductant and low‐cost copper as an effective catalyst. Such a reductive coupling process features a broad substrate scope and good functional group tolerance. Related thiosulfonate products can also be converted into diverse functional molecules.

Enantioselective Reductive Homocoupling of Allylic Acetates Enabled by Dual Photoredox/Palladium Catalysis: Access to C2-Symmetrical 1,5-Dienes

Transition-metal-catalyzed reductive coupling reactions have emerged as powerful protocols to construct C-C bonds. However, the development of enantioselective C(sp3)-C(sp3) reductive coupling remains challenging. Herein, we report a highly regio-, diastereo-, and enantioselective reductive homocoupling of allylic acetates through cooperative palladium and photoredox catalysis using diisopropylethylamine or Hantzsch ester as a homogeneous organic reductant. This straightforward protocol enables the stereoselective construction of C(sp3)-C(sp3) bonds under mild reaction conditions. A series of C2-symmetrical chiral 1,5-dienes were easily prepared with excellent enantioselectivities (up to >99% ee), diastereoselectivities (up to >95:5 dr), and regioselectivities (up to >95:5 rr). The resultant chiral 1,5-dienes can be directly used as chiral ligands in asymmetric synthesis, and they can be also transformed into other valuable chiral ligands.