Ni/Pd-catalyzed Suzuki–Miyaura cross-coupling of alcohols and aldehydes and C–N cross-coupling of nitro and amines via domino redox reactions: base-free,hydride acceptor-free

Domino oxidation-Suzuki–Miyaura cross-coupling of benzyl alcohols with phenylboronic acid and domino reduction-C–N cross-coupling of the nitro compounds with aryl halides were carried out using a strong Ni/Pd bimetallic redox catalyst. The catalyst bearing a copolymer with two Ni/Pd coordinated metals in porphyrin (derived from demetalated chlorophyll b) and salen-type ligands, and pyridine moiety as a base functionality all immobilized on magnetite NPs was synthesised and characterized. The domino oxidation cross-coupling reaction was accomplished under molecular O₂ in the absence of any hydride acceptor or/and base. Also, the domino reduction C–N cross-coupling reaction was performed in the presence of NaBH₄ without the need for any base and co-reductant. This multifunctional catalyst gave moderate to good yields for both coupling reactions with high chemoselectivity. A wide investigation was conducted to determine its mechanism and chemoselectivity.

A new Ni/Pd bimetallic multifunctional catalytic system has been developed for the domino Suzuki–Miyaura cross-coupling of benzyl alcohols with phenyl boronic acid and domino reduction C–N cross-coupling of the nitro compounds with aryl halides.     

C(acyl)–C(sp2) and C(sp2)–C(sp2) Suzuki–Miyaura cross-coupling reactions using nitrile-functionalized NHC palladium complexes

Application of N-heterocyclic carbene (NHC) palladium complexes has been successful for the modulation of C–C coupling reactions. For this purpose, a series of azolium salts (1a–f) including benzothiazolium, benzimidazolium, and imidazolium, bearing a CN-substituted benzyl moiety, and their (NHC)₂PdBr₂ (2a–c) and PEPPSI-type palladium (3b–f) complexes have been systematically prepared to catalyse acylative Suzuki–Miyaura coupling reaction of acyl chlorides with arylboronic acids to form benzophenone derivatives in the presence of potassium carbonate as a base and to catalyse the traditional Suzuki–Miyaura coupling reaction of bromobenzene with arylboronic acids to form biaryls. All the synthesized compounds were fully characterized by Fourier Transform Infrared (FTIR), and ¹H and ¹³C NMR spectroscopies. X-ray diffraction studies on single crystals of 3c, 3e and 3f prove the square planar geometry. Scanning Electron Microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), metal mapping analyses and thermal gravimetric analysis (TGA) were performed to get further insights into the mechanism of the Suzuki–Miyaura cross coupling reactions. Mechanistic studies have revealed that the stability and coordination of the complexes by the CN group are achieved by the removal of pyridine from the complex in catalytic cycles. The presence of the CN group in the (NHC)Pd complexes significantly increased the catalytic activities for both reactions.

Nitrile-functionalized Pd(ii) complexes have evaluated for the Suzuki–Miyaura cross-coupling reactions. The highest TON value was reached for the acylative Suzuki–Miyaura cross-coupling reaction of acyl chlorides with phenylboronic acids.     

C–C and C–H coupling reactions by Fe3O4/KCC-1/APTPOSS supported palladium-salen-bridged ionic networks as a reusable catalyst

This study investigates the potential application of an efficient, easily recoverable and reusable magnetically separable Fe₃O₄/KCC-1/APTPOSS nanoparticle-supported salen/Pd(ii) catalyst for C–C and C–H cross-couplings. The Fe₃O₄/KCC-1/APTPOSS/salen/Pd(ii) MNPs were thoroughly characterized by using TEM, FE-SEM, TGA, XRD, VSM, FT-IR, ICP-MS, and BET. This observation was exploited in the direct and selective chemical reaction of 2-acetyl-benzaldehyde with cyclopentadiene for the synthesis of pentafulvene. The recycled catalyst has been analyzed by ICP-MS showing only minor changes in the morphology after the reaction, thus confirming the robustness of the catalyst.

This study investigates the potential application of an efficient, easily recoverable and reusable magnetically separable Fe₃O₄/KCC-1/APTPOSS nanoparticle-supported salen/Pd(ii) catalyst for C–C and C–H cross-couplings.     

Photoaccelerated energy transfer catalysis of the Suzuki–Miyaura coupling through ligand regulation on Ir(iii)–Pd(ii) bimetallic complexes

Three bimetallic Ir(iii)–Pd(ii) complexes [Ir(ppy)₂(bpm)PdCl₂](PF₆) (ppy = 2-phenylpyridine, 1), [Ir(dfppy)₂(bpm)PdCl₂](PF₆) (dfppy = (4,6-difluorophenyl)pyridine, 2), and [Ir(pq)₂(bpm)PdCl₂](PF₆) (pq = 2-phenylquinoline, 3) were synthesized by using 2,2′-bipyrimidine (bpm) as a bridging ligand. The influences of the cyclometalated ligand at the Ir(iii) center on the photophysical and electrochemical properties as well as photocatalytic activity for the Suzuki–Miyaura coupling reaction under mild conditions were evaluated. The results revealed that complex 3 enables dramatically accelerating the Suzuki–Miyaura coupling reaction under visible light irradiation at room temperature, due to the effective absorption of visible light and appropriate locus of the excited chromophore. Mechanism studies showed that the chromophore [Ir(pq)₂(bpm)] fragment absorbs visible light to produce the triplet excited state centering on the bridging ligand which boosts the formation of electron rich Pd(ii) units and facilitates the oxidative addition step of the catalytic cycle. Simultaneously, the excited chromophore undergoes energy transfer efficiently to the Pd(ii) reaction site to form the excited Pd(ii) species, resulting in enhancement of Pd(ii) reduction steps of the Suzuki–Miyaura coupling reaction and increasing the reactivity of the catalyst. This provides a new strategy for designing photocatalysts for coupling reaction through altering the cyclometalated ligand to modulate the photophysical properties and the cooperation between two metal units.

A series bimetallic catalysts were synthesized. Relationship between the structure of catalysts and catalytic reactivities were studied and improvement of the catalytic efficiency for Suzuki–Miyaura coupling was accomplished by regulating their chromophores.     

Microflowers formed by complexation-driven self-assembly between palladium(ii) and bis-theophyllines: immortal catalyst for C–C cross-coupling reactions

The Pd catalyst for Suzuki–Miyaura or the other C–C coupling reactions is one of the central tools in organic synthesis related to medicine, agricultural chemicals and advanced materials. However, recycling palladium is a bottleneck for developing the extreme potential of Pd in chemistry. Herein, we established a new heterogeneous Pd catalytic system in which the catalyst is a nanopetal-gathered flower-like microsphere self-assembled from PdCl₂ and alkyl-linked bis-theophyllines. The microflowers catalyzed quantitatively the reaction of aryl bromides and phenylboronic acid in aqueous media at room temperature. It was found that the reaction proceeds better in an air atmosphere than in nitrogen gas even though the Pd(ii) species employed was lowered to 0.001 mol% in the substance. Very interestingly, the microflowers could be recycled 20 times without deactivation in the C–C coupling reaction between bromobenzene and phenylboronic acid in the presence of sodium chloride. We found that the sodium chloride added played an important role in maintaining the morphology of microflowers and preventing the formation of metallic Pd particles.

Bis-theophylline-palladium complex exhibit high catalytic activity in the C–C coupling reaction with excellent recyclability in the presence of NaCl.     

Synthesis and characterization of a supported Pd complex on volcanic pumice laminates textured by cellulose for facilitating Suzuki–Miyaura cross-coupling reactions

Herein, a novel high-performance heterogeneous catalytic system made of volcanic pumice magnetic particles (VPMP), cellulose (CLS) natural polymeric texture, and palladium nanoparticles (Pd NPs) is presented. The introduced VPMP@CLS-Pd composite has been designed based on the principles of green chemistry, and suitably applied in the Suzuki–Miyaura cross-coupling reactions, as an efficient heterogeneous catalytic system. Concisely, the inherent magnetic property of VPMP (30 emu g⁻¹) provides a great possibility for separation of the catalyst particles from the reaction mixture with great ease. In addition, high heterogeneity and high structural stability are obtained by this composition resulting in remarkable recyclability (ten times successive use). As the main catalytic sites, palladium nanoparticles (Pd NPs) are finely distributed onto the VPMP@CLS structure. To catalyze the Suzuki–Miyaura cross-coupling reactions producing biphenyl pharmaceutical derivatives, the present Pd NPs were reduced from chemical state Pd²⁺ to Pd⁰. In this regard, a plausible mechanism is submitted in the context as well. As the main result of the performed analytical methods (including FT-IR, EDX, VSM, TGA, FESEM, TEM, BTE, and XPS), it is shown that the spherical-shaped nanoscale Pd particles have been well distributed onto the surfaces of the porous laminate-shaped VPMP. However, the novel designed VPMP@CLS-Pd catalyst is used for facilitating the synthetic reactions of biphenyls, and high reaction yields (∼98%) are obtained in a short reaction time (10 min) by using a small amount of catalytic system (0.01 g), under mild conditions (room temperature).

An efficient natural-based catalyst constructed of volcanic pumice, cellulose polymeric chains, and palladium nanoparticles is presented for Suzuki–Miyaura coupling reaction.     

Ultrafine Pd nanoparticles loaded benzothiazole-linked covalent organic framework for efficient photocatalytic C–C cross-coupling reactions

We proposed a strategy that a benzothiazole-linked covalent organic framework (TTT-COF) was used as a substrate to prepare metal composite photocatalyst Pd NPs@TTT-COF. Firstly, benzothiazole linked TTT-COF exhibited superior chemical stability and photoresponse. Moreover, a finer particle size (2.01 nm) and more uniform distribution of Pd NPs were observed in Pd NPs@TTT-COF owing to the binding interaction between Pd NPs and S in benzothiazole groups. Pd NPs@TTT-COF exhibited superior efficiency and reusability in photocatalytic C–C cross-coupling reactions. Mechanism study suggested that photogenerated electrons and holes on TTT-COF played important roles in these reactions.

Benzothiazole-linked covalent organic framework with characteristic properties loaded Pd nanoparticles for photocatalytic C–C cross-coupling reactions.     

Palladium nanoparticles decorated into a biguanidine modified-KIT-5 mesoporous structure: a recoverable nanocatalyst for ultrasound-assisted Suzuki–Miyaura cross-coupling

In the current research work, a new KIT-5-biguanidine-Pd(0) catalyst was prepared and applied to ultrasound-assisted Suzuki–Miyaura cross-coupling reactions using ultrasound waves at ambient temperature. The ultrasound-assisted method is a green and efficient method for C–C coupling. Many parameters of the Suzuki coupling reaction were examined, such as the irradiation time, the types of organic and inorganic bases, the types of aprotic and protic solvents, and the dosage (mol%) of catalyst. Also, the results showed that the yields from the ultrasound-assisted coupling reactions were higher than from non-irradiated reactions. The prepared catalyst was characterized via HR-TEM, SEM-EDX-mapping, FT-IR, ICP-AAS, BET-BJH, and XRD studies. The stability and catalytic performance of the prepared catalyst were good, and it could be reused 6 times without catalytic activity loss for the Suzuki–Miyaura cross-coupling reaction.

In the current research work, a new KIT-5-biguanidine-Pd(0) catalyst was prepared and applied to ultrasound-assisted Suzuki–Miyaura cross-coupling reactions using ultrasound waves at ambient temperature.     

Evaluation of P-bridged biaryl phosphine ligands in palladium-catalysed Suzuki–Miyaura cross-coupling reactions

A family of biaryl phosphacyclic ligands derived from phobane and phosphatrioxa-adamantane frameworks is described. The rigid biaryl phosphacycles are efficient for Suzuki–Miyaura cross-coupling of aryl bromides and chlorides. In particular, coupling reactions of the challenging sterically hindered and heterocyclic substrates were viable at room temperature.

Efficient palladium catalyst systems consisting of bench-stable biaryl phosphacycles and Pd(OAc)₂ are described for Suzuki–Miyaura cross-coupling reactions of a diverse array of aryl halides and arylboronic acids.     

1,2,3-Triazole framework: a strategic structure for C–H⋯X hydrogen bonding and practical design of an effective Pd-catalyst for carbonylation and carbon–carbon bond formation

1,2,3-Triazole is an interesting N-heterocyclic framework which can act as both a hydrogen bond donor and metal chelator. In the present study, C–H hydrogen bonding of the 1,2,3-triazole ring was surveyed theoretically and the results showed a good agreement with the experimental observations. The click-modified magnetic nanocatalyst Pd@click-Fe₃O₄/chitosan was successfully prepared, in which the triazole moiety plays a dual role as both a strong linker and an excellent ligand and immobilizes the palladium species in the catalyst matrix. This nanostructure was well characterized and found to be an efficient catalyst for the CO gas-free formylation of aryl halides using formic acid (HCOOH) as the most convenient, inexpensive and environmentally friendly CO source. Here, the aryl halides are selectively converted to the corresponding aromatic aldehydes under mild reaction conditions and low Pd loading. The activity of this catalyst was also excellent in the Suzuki cross-coupling reaction of various aryl halides with phenylboronic acids in EtOH/H₂O (1 : 1) at room temperature. In addition, this catalyst was stable in the reaction media and could be magnetically separated and recovered several times.

The C–H hydrogen bonding of a 1,2,3-triazole framework was studied. An Fe₃O₄–chitosan core–shell incorporating a triazole/Pd complex was investigated as a nanocatalyst in carbonylation and C–C coupling.     

Fabrication of a novel magnetic metal–organic framework functionalized with 2-aminothiophenol for preconcentration of trace silver amounts in water and wastewater

Herein, a novel magnetic metal–organic framework functionalized (MMOF) with 2-aminothiophenol (2-ATP) was fabricated and employed for separation/preconcentration of trace silver amounts. At first magnetite nanoparticles (Fe₃O₄ NPs) were synthesized and then coated with SiO₂. Thereafter, the Fe₃O₄@SiO₂ nanoparticles were modified with 2-ATP. Finally, the functionalized MMOF was prepared by the fabrication of MIL-101(Cr) in the presence of Fe₃O₄@SiO₂@2-ATP NPs. MIL-101(Cr)/Fe₃O₄@SiO₂@2-ATP nanocomposite was characterized with FT-IR, SEM, elemental analysis, XRD and VSM and then utilized in the separation/determination of silver ions in various real samples. The effects of diverse experimental variables such as pH, uptake time, adsorbent amount, desorption time, eluent concentration and volume were studied comprehensively employing experimental design methodology. After optimization, LOD and linearity were 0.05 ng mL⁻¹ and 0.2–200 ng mL⁻¹, respectively. Repeatability of the new method was determined based on RSD value for 5, 50, 150 ng mL⁻¹ (n = 5) concentrations which was 9.3%, 6.8% and 4.5%, respectively. Ultimately, the outlined method was utilized in the separation/determination of silver ions in various water and wastewater samples satisfactorily.

A scheme for the synthesis of MIL-101/Fe₃O₄@SiO₂@2-ATP nanocomposite.     

Extension and functionalization of an encapsulating macrobicyclic ligand using palladium-catalyzed Suzuki–Miyaura and Sonogashira reactions of iron(ii) dihalogenoclathrochelates with inherent halogen substituents

A new approach for performing Suzuki–Miyaura and Sonogashira reactions of iron(ii) dihalogenoclathrochelates, optimizing their reaction conditions (such as temperature, solvent and a palladium-containing catalyst) and the nature of other reagents (such as arylboron components) is elaborated. These palladium-catalyzed reactions are very sensitive to the nature of the macrobicyclic substrates. The reactivity of the leaving halogen atoms correlates with their ability to undergo an oxidative addition, decreasing in the order: I > Br > Cl, and iron(ii) diiodoclathrochelate underwent these C–C cross-couplings under their “classical” conditions. Phenylboronic, 4-carboxyphenylboronic and 6-ethoxy-2-naphthylboronic acids, and the diethyl ether of 4-(ethoxycarbonyl)boronic acid were tested as components of Suzuki–Miyaura reactions in DMF and in THF. The highest yields of the target products were obtained in DMF, while the highest activation was observed with sodium and potassium carbonates. The Suzuki–Miyaura reaction of a diiodoclathrochelate with 6-ethoxy-2-naphthylboronic acid gave the mono- and difunctionalized clathrochelates resulting from the tandem hydrodeiodination – C–C cross-coupling and double C–C cross-coupling reactions, respectively. Its Sonogashira reactions with trimethylsilylacetylene and acetylenecarboxylic acid in THF and in DMF were tested. This palladium-catalyzed reaction with a (CH₃)₃Si-containing active component gave the target products in a high total yield. The complexes obtained were characterized using elemental analysis, MALDI-TOF, UV-Vis, ¹H and ¹³C{¹H} NMR spectroscopy, and by single crystal XRD. Despite the non-equivalence of the ribbed α-dioximate fragments of their molecules, the encapsulated iron(ii) ion is situated almost in the centre of its FeN₆-coordination polyhedron, the geometry of which is almost intermediate between a trigonal prism and a trigonal antiprism.

A new approach for performing Suzuki and Sonogashira reactions of iron(ii) dihalogenoclathrochelates, optimizing their reaction conditions, is elaborated.     

Synthesis of benzoxazoles via an iron-catalyzed domino C–N/C–O cross-coupling reaction

An eco-friendly and efficient method has been developed for the synthesis of 2-arylbenzoxazoles via a domino iron-catalyzed C–N/C–O cross-coupling reaction. Some of the issues typically encountered during the synthesis of 2-arylbenzoxazoles in the presence of palladium and copper catalysts, including poor substrate scope and long reaction times have been addressed using this newly developed iron-catalyzed method.

The synthesis of benzoxazoles via an iron-catalyzed cascade C–N and C–O coupling is described.

2-Arylbenzoxazoles are an important class of structures in natural products, and pharmaceuticals and has shown a wide range of biological activities, such as antitumor, antiviral, and antimicrobial activities.¹ In particular, they show a marvellous efficacy in the treatment of duchenne muscular dystrophy (DMD) which is one of the most common of the muscular dystrophies that is caused by a mutation in the gene DMD, located in humans on the X chromosome (Xp21).² So the synthesis of 2-arylbenzoxazoles has been intensively studied for use in organic and medicinal chemistry over the past few years.Numerous methods have been reported to synthesise this motif, one of the common methods is transition-metal-catalyzed (like Pd,³ Ni,⁴ Cu,⁵ Mn⁶etc.) cross-coupling from pre-existing benzoxazoles with aryl halide or arylboronic acid. And another method is the classic one employing a cyclocondensation approach between an aminophenol and either a carboxylic acid⁷ or benzaldehyde⁸ (Scheme 1, path a). In 2004, Frank Glorius'' group reported a domino copper-catalyzed C–N and C–O cross-coupling for the conversion of primary amides into benzoxazoles⁹ (Scheme 1, path b) which is a new reaction type for the synthesis of benzoxazoles. Bunch et al. apply this domino reaction in the synthesis of planar heterocycles in 2014.¹⁰ In addition the cyclization of o-halobenzenamides to benzoxazoles has been reported several times.^11,12 Nevertheless, some limitations in the reported methods need to be overcome, such as the use of palladium complexes and narrow substrate range.Open in a separate windowScheme 1Classic method of benzoxazole formation.In the last few years, there has been a significant increase in the number of reports pertaining to the development of iron-catalyzed reactions in organic synthesis, where iron has shown several significant advantages over other metals, such as being more abundant, commercially inexpensive, environmentally friendly and drug safety.¹³ Compared with palladium and copper, the use of iron is particularly suitable for reactions involving the preparation of therapeutic agents for human consumption. With this in mind, it was envisaged that an new method should be developed for the synthesis of benzoxazoles via an iron-catalyzed domino C–N/C–O cross-coupling reaction.The reaction of benzamide (1a) with 1-bromo-2-iodobenzene was used as model transformation to identify the optimum reaction conditions by screening a variety of different iron salts, bases, ligands and solvents (l-proline provided no product (14 high-purity Fe₂O₃ (99.999%) and K₂CO₃ (99.999%) were applied in the reaction (

Dendritic structured palladium complexes: magnetically retrievable,highly efficient heterogeneous nanocatalyst for Suzuki and Heck cross-coupling reactions

The recyclable nanomagnetic Pd-complex PAMAM G₀-Pd@γ-Fe₂O₃ is reported for catalytic C–C cross-coupling reactions of challenging substrates. Mainly, a great variety of aryl chlorides can be used as substrates for Suzuki–Miyaura and Mizoroki–Heck reactions under mild reaction conditions (60–90 °C) and low catalyst loading (<1 mol% Pd) in aqueous media. The presence of numerous polar groups in the polymer matrix increases the solubility of the catalyst in water, thus facilitating its operation in aqueous environments. The immobilization of the catalyst on the surface of a magnetic platform allows its effective recovery and reuse without significant loss of catalytic activity for at least six cycles with total leaching of <1% palladium metal, meeting the requirements for acceptable metal residues in the pharmaceutical industry.

The recyclable nanomagnetic Pd-complex PAMAM G₀-Pd@γ-Fe₂O₃ is reported for catalytic C–C cross-coupling reactions of challenging substrates.     

New iron(iii) complex of bis-bidentate-anchored diacyl resorcinol on a Fe3O4 nanomagnet: C–H bond oxygenation,oxidative cleavage of alkenes and benzoxazole synthesis

We have synthesized a novel, bis-bidentate, covalently anchored, 4,6-diacetyl resorcinol (DAR) ligand on silica-coated magnetic Fe₃O₄ nanoparticles and the corresponding bi-metallic iron(iii) complex (Fe₃O₄@SiO₂-APTESFe₂L^DAR). Both the chemical nature and the structure of the homogeneously heterogenized catalyst were investigated using physico-chemical techniques. The results obtained by XPS, XRD, FT-IR, TGA, VSM, SEM, TEM, EDX, ICP and AAS revealed a magnetic core, a silica layer and the grafting of a binuclear iron complex on the Fe₃O₄@SiO₂-APTES, as well as its thermodynamic stability. Despite many reports of metal complexes on different supports, there are no reports of anchored, bi-metallic complexes. To the best of our knowledge, this is the first report of a bi-active site catalyst covalently attached to a support. This study focuses on the catalytic activity of an as-synthesized, bi-active site catalyst for C–H bond oxygenation, the oxidative cleavage of alkenes, and the multicomponent, one-pot synthesis of benzoxazole derivatives with excellent yields from readily available starting materials. Our results indicated high conversion rates and selectivity under mild reaction conditions and simple separation using a magnetic field. The leaching and recyclability tests of the catalyst were investigated for the above processes, which indicated that all the reactions proceed via a heterogeneous pathway and that the catalyst is recyclable without any tangible loss in catalytic activity for at least 8, 5 and 5 cycles for C–H bond oxygenation, C C bond cleavage and benzoxazole synthesis, respectively.

A novel, bi-metallic iron(iii) complex of covalently anchored, diacetyl resorcinol ligand on silica-coated magnetic Fe₃O₄ nanoparticles has been synthesized and applied in organic transformations.     

Fe3O4@SiO2@Au nanoparticles for MRI-guided chemo/NIR photothermal therapy of cancer cells

Novel functionalized (biofunctionalization followed by cisplatin immobilization) Fe₃O₄@SiO₂@Au nanoparticles (NPs) were designed. The encapsulation of Fe₃O₄ cores inside continuous SiO₂ shells preserves their initial structure and strong magnetic properties, while the shell surface can be decorated by small Au NPs, and then cisplatin (cPt) can be successfully immobilized on their surface. The fabricated NPs exhibit very strong T₂ contrasting properties for magnetic resonance imaging (MRI). The functionalized Fe₃O₄@SiO₂@Au NPs are tested for a potential application in photothermal cancer therapy, which is simulated by irradiation of two colon cancer cell lines (SW480 and SW620) with a laser (λ = 808 nm, W = 100 mW cm⁻²). It is found that the functionalized NPs possess low toxicity towards cancer cells (∼10–15%), which however could be drastically increased by laser irradiation, leading to a mortality of the cells of ∼43–50%. This increase of the cytotoxic properties of the Fe₃O₄@SiO₂@Au NPs, due to the synergic effect between the presence of cPt plus Au NPs and laser irradiation, makes these NPs perspective agents for potential (MRI)-guided stimulated chemo-photothermal treatment of cancer.

Novel functionalized nanoparticles, with toxicity controlled by laser irradiation, are perspective agents for potential (MRI)-guided stimulated chemo-photothermal treatment of cancer.     

Direct phosphorylation of benzylic C–H bonds under transition metal-free conditions forming sp3C–P bonds

Direct phosphorylation of benzylic C–H bonds was achieved in a biphasic system under transition metal-free conditions. A selective radical/radical sp³C–H/P(O)–H cross coupling was proposed, and various substituted toluenes were applicable. The transformation provided a promising method for constructing sp³C–P bonds.

Direct phosphorylation of benzylic C–H bonds with secondary phosphine oxides was first realized. The reaction was performed in organo/aqueous biphasic system and under transition metal-free conditions, proceeding via the cross dehydrogenative coupling.

To construct C–P bonds is of great significance in modern organic synthesis,¹ because organophosphorus compounds play varied roles in medical,^2,3 materials,⁴ and synthetic chemistry fields.⁵ Traditionally, the C–P bonds were formed from P–Cl species via nucleophilic substitutions with organometallic reagents,⁶ P–OR species via Michaelis–Arbusov reactions,⁷ or P–H species via the alkylation in the present of a base or a transition metal.⁸Over the past decades, cross dehydrogenative coupling reactions (CDC reactions) have become a powerful and atom-economic methodology for constructing chemical bonds.⁹ By using this strategy, C–H bonds can couple with Z–H bonds without prefunctionalization and thus short-cut the synthetic procedures (Scheme 1a).Open in a separate windowScheme 1Cross dehydrogenative coupling reactions and direct phosphorylation of benzylic C–H bonds.A similar construction of C–P bonds via CDC was also realized.^10,11 Among these methods, the phosphorylation of sp³C–H having an adjacent N or O atom, or the carbonyl group was well-developed.¹¹ Relatively, the formation of benzylic sp³C–P bond was less reported,^11w which was mainly limited to the sp³C–H of xanthene or 8-methylquinoline (Scheme 1b).¹² In these reported processes, transition metal catalysts or photo-, electro-catalysts were usually involved,^11v and an excess of P(O)–H compounds was usually employed.^11,13 To the best of our knowledge, the phosphorylation of non-active benzylic C–H bonds has scarcely been reported.Considering both benzylic and phosphorus radicals could be generated by oxidation,¹⁴ which might subsequently couple, the phosphorylation of benzylic sp³C–H bonds would be achieved (Scheme 1c). Herein, we disclosed the construction of benzylic C–P bonds from toluene and P–H species. The reaction was carried out under transition metal-free reaction conditions,¹⁵ and exhibited high regio-selectivity. The aromatic C–H remained intact during the reaction.We began our investigation by exploring the reaction of toluene 1a and diphenylphosphine oxide 2a in the presence of an oxidant (16 Other persulfates could also be used as an oxidant albeit with decreasing yields ( EntryOxidantToluene/water (v/v)Additive3a yield^b (%)3a/4^c1K₂S₂O₈1 : 0—Trace—2K₂S₂O₈1 : 1—1216 : 843K₂S₂O₈1 : 1SDS3726 : 744Na₂S₂O₈1 : 1SDS3127 : 735(NH₄)₂S₂O₈1 : 1SDS2723 : 776Oxone1 : 1SDSNone—7—1 : 1SDSNone—8—1 : 1—None—9K₂S₂O₈1 : 1SDBS4136 : 6410K₂S₂O₈1 : 2SDBS4628 : 7211K₂S₂O₈1 : 3SDBS3524 : 7612^dK₂S₂O₈1 : 2SDBS4632 : 6813^eK₂S₂O₈1 : 2SDBS2621 : 7914^fK₂S₂O₈1 : 2SDBS2926 : 7415^gK₂S₂O₈1 : 2SDBS4838 : 6216^g,hK₂S₂O₈1 : 2SDBS0—Open in a separate window^aReaction condition: 1a (1 mL), 2a (0.2 mmol), oxidant (2 equiv.), additive (1 equiv.) and H₂O, 120 °C, 3 h. under N₂.^bGC yields using n-dodecane as an internal standard.^cThe ratio of 3a/4 was determined by GC analysis.^d50 mol% SDBS was used.^e20 mol% SDBS was used.^fAt 100 °C.^g1 (0.8 mL) and H₂O (1.6 mL), 3 equiv. K₂S₂O₈ was used, 120 °C for 15 min.^h3 equiv. TEMPO was added.Based on the above results, we can easily find that a serious amount of 1,2-diphenylethane 4 was formed. These results suggest that the homocoupling rate of 1a was very quick. Thus, the choice of the phase transfer reagent and oxidant is the key to cross-coupling of toluene and diphenylphosphine oxide in this biphasic solvent system.Excessive P–H species were usually employed in reported CDC reactions to form C–P bonds, because of their facile oxidation.¹³ In our procedure, toluene was excessive, thus the yields were calculated based on 2a. The yields looked like low, which did not indicate the poorer conversion rate of P–H species. With the optimized conditions in hand, the substrate scope of the CDC reactions was explored ( Open in a separate window^aReaction conditions: 1 (0.8 mL), 2 (0.2 mmol), K₂S₂O₈ (3 equiv.), SDBS (50%) and H₂O (1.6 mL), 120 °C, under N₂, the reactions were monitored by TLC and/or GC until 2 work out.^b1 mmol scale, 30 min.^c130 °C.^d100 °C.In addition to toluene, o-xylene, m-xylene, p-xylene, mesitylene, and 1,2,4,5-tetramethylbenzene all coupled with 2a to give the expected 3b–f in moderate yields. Methoxy substituted toluene gave relatively lower yields of 3g and 3h. para-Halo substituted toluene exhibited good reactivity, furnishing the coupling products 3i and 3j in moderate yields. Comparing to toluene, a decreasing order of reactivity was observed for ethyl benzene (3l, 30% yield), isobutyl benzene (3m, 27% yield), isopropyl benzene (3n, <10% yield), and diphenyl methane (3o, trace). The order was probably controlled by the steric hindrance around the benzylic carbon. 1-Methylnaphthalene and 2-methylnaphthalene also gave low yields (3p and 3q). However, 2-methylquinoline served well and coupled with 2a, affording the product 3r in 49% yield, which could be ascribed to the activation of the nitrogen atom. Besides of 2a, diaryl phosphine oxides having methyl, F, and Cl substituents could also be employed as the substrate, producing 3s–3u in moderate yields under similar reaction conditions.Although the mechanism of the direct phosphorylation of benzyl C–H bond in aqueous solution is not quite clear, some aspects could be grasped based on experimental results. Firstly, the desired product 3a was not detected when the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was added, implying that a radical pathway might be possible in this reaction (entry 16). Secondly, the reaction only occurred in aqueous phase, and relied on the presence of PTC (phase transfer catalyst), as seen in that 3a was difficultly formed in entries 1 and 2 of 17We supposed 1 is converted to benzyl radical 5 by SO₄⁻ radical that is generated via hemolytic cleavage of potassium persulfates.¹⁸ Meanwhile, phosphorus radical 6 is similarly formed from 2. Because potassium persulfate was water soluble, both 1 and 2 had to be transferred into aqueous solution to react with potassium persulfates.¹⁹ This proposal is also in accord with the experimental results that no products of sp²C–H phosphorylation are detected, which are the main products in the previous radical systems.²⁰ Finally, cross couplings between 5 and 6 produce 3 (Scheme 2).Open in a separate windowScheme 2Proposed mechanism for the direct phosphorylation of benzylic C–H bonds under transition metal-free reaction conditions.     

Benzophenone assisted UV-activated synthesis of unique Pd-nanodendrite embedded reduced graphene oxide nanocomposite: a catalyst for C–C coupling reaction and fuel cell

In this work we report the use of benzophenone (BP) for the synthesis of a palladium (Pd) embedded on reduced graphene oxide (rGO) nanocomposite (Pd/rGO) using a simple aqueous solution and UV irradiation. The simple and facile evolution of thermodynamically unstable branched Pd(0) nanodendrites was achieved by BP photoactivation, circumventing the growth of more stable nanomorphologies. The synthesis of Pd(0)-embedded rGO nanosheets (PRGO-nd) was made possible by the simultaneous reduction of both the GO scaffold and PdCl₂ by introducing BP into the photoactivation reaction. The nanocomposites obtained in the absence of BP were common triangular and twinned Pd(0) structures which were also implanted on the rGO scaffold (PRGO-nt). The disparity in morphologies presumably occurs due to the difference in the kinetics of the reduction of Pd²⁺ to Pd⁰ in the presence and absence of the BP photoinitiator. It was observed that the PRGO-nd was composed of dense arrays of multiple Pd branches around nucleation site which exhibited (111) facet, whereas PRGO-nt showed a mixture of (100) and (111) facets. On comparing the catalytic efficiencies of the as-synthesized nanocatalysts, we observed a superiority in efficiency of the thermodynamically unstable PRGO-nd nanocomposite. This is due to the evolved active facets of the dendritic Pd(0) morphology with its higher surface area, as testified by Brunauer–Emmett–Teller (BET) analysis. Since both PRGO-nd and PRGO-nt contain particles of similar size, the dents and grooves in the structure are the cause of the increase in the effective surface area in the case of nanodendrites. The unique dendritic morphology of the PRGO-nd nanostructures makes them a promising material for superior catalysis, due to their high surface area, and the high density of surface atoms at their edges, corners, and stepped regions. We investigated the efficiency of the as-prepared PRGO-nd catalyst in the Suzuki–Miyaura coupling reaction and showed its proficiency in a 2 h reaction at 60 °C using 2 mol% catalyst containing 0.06 mol% active Pd. Moreover, the electrochemical efficiency for the catalytic hydrogen evolution reaction (HER) was demonstrated, in which PRGO-nd provided a decreased overpotential of 68 mV for a current density of 10 mA cm⁻², a small Tafel slope of 57 mV dec⁻¹ and commendable stability during chronoamperometric testing for 5 h.

Benzophenone photoinitiator aided synthesis of Pd-nanodendrite embedded rGO nanocatalyst possessing superior potential in C–C coupling reaction and fuel cell application.     

Microstructure–mechanical properties of Ag0/Au0 doped K–Mg–Al–Si–O–F glass-ceramics

In understanding the catalytic efficacy of silver (Ag⁰) and gold (Au⁰) nanoparticles (NPs) on glass-ceramic (GC) crystallization, the microstructure–machinability correlation of a SiO₂–MgO–Al₂O₃–B₂O₃–K₂O–MgF₂ system is studied. The thermal parameters viz., glass transition temperature (T_g) and crystallization temperature (T_c) were extensively changed by varying NPs (in situ or ex situ). Tc was found to be increased (T_c = 870–875 °C) by 90–110 °C when ex situ NPs were present in the glass system. Under controlled heat-treatment at 950 ± 10 °C, the glasses were converted into glass-ceramics with the predominant presence of crystalline phase (XRD) fluorophlogopite mica, [KMg₃(AlSi₃O₁₀)F₂]. Along with the secondary phase enstatite (MgSiO₃), the presence of Ag and Au particles (FCC system) were identified by XRD. A microstructure containing spherical crystallite precipitates (∼50–400 nm) has been observed through FESEM in in situ doped GCs. An ex situ Ag doped GC matrix composed of rock-like and plate-like crystallites mostly of size 1–3 μm ensured its superior machinability. Vicker''s and Knoop microhardness of in situ doped GCs were estimated within the range 4.45–4.61 GPa which is reduced to 4.21–4.34 GPa in the ex situ Ag system. Machinability of GCs was found to be in the order, ex situ Ag > ex situ Au ∼ in situ Ag > in situ Au. Thus, the ex situ Ag/Au doped SiO₂–MgO–Al₂O₃–B₂O₃–K₂O–MgF₂ GC has potential for use as a machinable glass-ceramic.

In understanding the catalytic efficacy of silver (Ag⁰) and gold (Au⁰) nanoparticles (NPs) on glass-ceramic (GC) crystallization, the microstructure–machinability correlation of a SiO₂–MgO–Al₂O₃–B₂O₃–K₂O–MgF₂ system is studied.     

Preparation of a magnetic and recyclable superparamagnetic silica support with a boronic acid group for immobilizing Pd catalysts and its applications in Suzuki reactions

Palladium is one of the best metal catalysts for Suzuki cross-coupling reaction to synthesize unsymmetrical biaryl compounds. However, homogeneous palladium (Pd) is limited in an industrial scale due to the high cost, separation, removal, and recovery issues. In this paper, a novel, high activity magnetic nanoparticles (Fe₃O₄@SiO₂-APBA-Pd) catalyst was prepared by a simple, cost-effective procedure. The as-prepared functional nanoparticles (Fe₃O₄@SiO₂-APBA) with boric acid group immobilized Pd through adding Pd(OAc)₂ to Fe₃O₄@SiO₂-APBA in absolute ethanol and maintaining for a certain time under a nitrogen atmosphere. The as-prepared catalyst was characterized by FT-IR, SEM, EDX, TEM, ICP-MS, XPS, and XRD. The results showed that the Pd (0.2–0.6 nm) was successfully anchored on the magnetic silica material with boric acid group. The amount of Pd was 0.800 mmol g⁻¹. This magnetic nanostructure (8–15 nm) is especially beneficial as a nanocatalyst because each nanoparticle can catalyze a reaction in a certain time without steric restriction, which could effectively improve the reaction efficiency. The current nanoparticles with the Pd catalyst could be used as a novel, green, and efficient heterogeneous catalyst for Suzuki reactions. This catalyst showed promising catalytic activity and excellent yields toward 14 kinds of Suzuki coupling reactions under mild reaction conditions, which was similar to homogeneous Pd and many reported heterogeneous Pd catalysts. In addition, the turnover number (TON) and turnover frequency (TOF) for the Suzuki reaction were high. TOF and TON were 9048 h⁻¹ and 20 250 for the Suzuki reaction of bromobenzene and phenylboronic acid. Furthermore, the nanoparticles could be easily separated by a magnet, and could be used repeatedly seven times without any significant loss in activity.

A novel, high activity and magnetic nanoparticles (Fe₃O₄@SiO₂-APBA-Pd) catalyst was prepared. It is 8–15 nm with 0.2–0.6 nm Pd particles. It can be reused 7 runs and catalyze 14 kinds of Suzuki reactions.