Oxidative Homocoupling Reaction Research Articles

Efficient P, O chelate palladium(II)/AgNO3 cocatalyzed homocoupling of aromatic terminal alkynes in aqueous media under ambient atmosphere

GRAPHICAL ABSTRACT

Mono- and tetranuclear cyclopalladated complexes with N′-(9-anthracenylidene)benzothiohydrazide: Syntheses, structures and catalytic applications

Reaction of PdCl2, LiCl, N′-(9-anthracenylidene)benzothiohydrazide (H2L, 2 Hs represent the thioamide NH proton and the 9-anthracenylidene peri proton) and NaOAc·3H2O in 1:2:1:1 mole ratio in methanol produced [Pd4(L)4] (1) in 75% yield. Treatment of 1 with PPh3 (1:4.5 mole ratio) in acetone provided [Pd(L)(PPh3)] (2) in 72% yield. The molecular formulas of the diamagnetic and non-electrolytic 1 and 2 were established by elemental analyses. Molecular structures of 1 and 2 were determined by single crystal X-ray diffraction studies. In the tetranuclear 1, the palladium(II) centers are in distorted square-planar CNS2 coordination environments created by four (L)2−, each of which acts as 5,6-membered fused chelate rings forming thioamidate-S, azomethine-N and 9-anthracenylidene peri-C donor to one metal center and uses the thioamidate-S atom to bridge a second metal center. In the mononuclear 2, (L)2− and PPh3 assemble a distorted square-planar CNSP coordination environment around the palladium(II) center. Spectroscopic (IR, NMR and UV-Vis) measurements were also used to characterize 1 and 2. The catalytic properties of both complexes in the oxidative phenylacetylene homocoupling reaction were examined.

A mechanistic insights into manganese-catalyzed oxidative homocoupling reactions of Grignard reagents: A computational DFT investigation

We have carried out a DFT computational investigation on the mechanism of the manganese-catalyzed homocoupling reaction of aryl Grignard reagents RMgX using atmospheric oxygen as an oxidant. The oxidative addition gives an oxo-complex MnR2O2 where the oxygen molecule is η2-bonded to the metal. The free energy barrier for the subsequent reductive elimination (key-step of the cycle) is 9.4 kcal mol−1 when R = p-anisyl, which indicates a rather fast reduction step. A comparative analysis of various systems (R = p-anisyl, o- and p-nitrophenyl, pentafluorophenyl, mesityl, naphtyl and phenylethynyl) suggests that, in general, the kinetics of the reductive elimination step is the result of a complex interplay between electronic and steric effects and, in the case of aryl groups, strongly depends on the nature of the substituent and its position on the phenyl ring. The reduction is favored when the electron density on the coupling carbons is large enough to interact with the metal and form the intermediate oxo-complex. However it cannot be so large to prevent the coupling because of a too strong electron repulsion or too strong Mn-R bonds.

An Elastic Monolithic Catalyst: A Microporous Metalloporphyrin-Containing Framework-Wrapped Melamine Foam for Process-Intensified Acyl Transfer.

The advent of conjugated microporous polymers (CMPs) has had significant impact in catalysis. However, the presence of only micropores in these polymers often imposes diffusion limitations, which has resulted in the low utilization of CMPs in catalytic reactions. Herein, the preparation of a foam-supporting CMP composite with interconnective micropores and macropores and elastic properties is reported. Metalloporphyrin-based CMP organogels are synthesized within the melamine foam by a room-temperature oxidative homocoupling reaction of terminal alkynes. Upon drying, the CMP-based xerogels tightly wrap the framework skeletons of the foam, while the foam cells are still open to allow for the preservation of elasticity and macroporosity. Such a hierarchical structure is efficient for acyl transfer, facilitates substrate diffusion within interpenetrative macropores and micropores, and could be used to intensify catalytic processes.

An Elastic Monolithic Catalyst: A Microporous Metalloporphyrin‐Containing Framework‐Wrapped Melamine Foam for Process‐Intensified Acyl Transfer

AbstractThe advent of conjugated microporous polymers (CMPs) has had significant impact in catalysis. However, the presence of only micropores in these polymers often imposes diffusion limitations, which has resulted in the low utilization of CMPs in catalytic reactions. Herein, the preparation of a foam‐supporting CMP composite with interconnective micropores and macropores and elastic properties is reported. Metalloporphyrin‐based CMP organogels are synthesized within the melamine foam by a room‐temperature oxidative homocoupling reaction of terminal alkynes. Upon drying, the CMP‐based xerogels tightly wrap the framework skeletons of the foam, while the foam cells are still open to allow for the preservation of elasticity and macroporosity. Such a hierarchical structure is efficient for acyl transfer, facilitates substrate diffusion within interpenetrative macropores and micropores, and could be used to intensify catalytic processes.

The 5th Molecular Materials Meeting (M3) @ Singapore

On 14–16 January 2013, the 3rd Molecular Materials Meeting (M3) @ Singapore was successfully held at Biopolis, a biomedical hub of Singapore hosting key research institutes of A*STAR and research and development laboratories of multinational corporations.With the theme of ‘Frontiers inMaterials Science, Chemistry and Physics’, the conference featured six thematic sessions, namely NewCatalysts and Catalytic Science, Soft Materials for Cosmetic and Health Care, Electrochemical Energy Conversion, Storage Materials and Technology, Materials for Advanced Sensors and Transducers, Lanthanidedoped Luminescence Nanomaterials, and Interface Engineering for Devices. The 3-day conference enabled lively discussions geared at cross-discipline collaborations in molecular materials (Fig. 1). This year, the M3 @ Singapore follows the past practice to publish a commemorative edition in the Australian Journal of Chemistry on selected papers presented at the conference. In the first issue [Aust. J. Chem. 2011, 64 (9)], the volume covered diverse topics that reflected the contemporary interests in the broad field of molecular materials, e.g. from temperature-driven molecular self-organisation to atom-transfer radical polymerisation based functional amphiphilic polymers synthesis, from dye-sensitised solar cells to nanoparticle-catalysed water-splitting, from metal-organic frameworks to aggregation-induced emission bioimaging, etc. The second edition [Aust. J. Chem. 2012, 65 (9)] is however relatively focussed. It featured mainly on contributions in research of nanoparticles and organic energy materials. The nanoparticle papers typified the harness of nanotechnology to enable new applications, e.g. using electro-responsive core-shell nanoparticles for rheology tuning, carbon nanotube-basedmaterials as catalysts in fuel cells, using chitosan-functionalised Au/Pd alloy nanoparticles/nanoclusters for catalysis of aerobic oxidative homocoupling reactions, and CdTe based hybrid fibres enabled low-voltage-driven electroluminescence devices, etc; papers on organic semiconductor research focussed on the structure–property relationship that leads to design and synthesis of high performance molecular materials, e.g. heterocyclic dyes for efficient dye-sensitised solar cells, phenyl-1Hpyrrole end-capped thiophenes for organic field-effect transistors, and pyridine incorporated dihexylquaterthiophene as blue emitter in organic light emitting diode applications. In this commemorative edition, seven papers are selected as highlights of the meeting. Bai et al. (Institute of Materials Research and Engineering, Singapore) introduced a new dinuclear Cu(II) complex (Fig. 2) with 1,2,3-triazoles of a chelatebridging mode. Using rare azole-bridged ferromagnetically coupled dinuclear Cu(II) complexes as examples, the paper demonstrates the potential of using heterocyclic triazoles to support magnetically active dinuclear Cu(II) complexes. The finding marks a step closer to the establishment of magnetostructural relationship and understanding of magnetic exchange

ChemInform Abstract: Metal‐Free‐Catalyzed Oxidative Trimerization of Indoles Using NaNO2 to Construct Quaternary Carbon Centers: Synthesis of 2‐(1H‐Indol‐3‐yl)‐2,3′‐biindolin‐3‐ones.

Abstract A simple, convenient, and efficient synthesis of 2-(1H-indol-3-yl)-2,3′-biindolin-3-one derivatives via a transition-metal-free-catalyzed oxidative trimeric reaction of indoles has been developed. This transformation may have occurred through a tandem oxidative homocoupling reaction by using NaNO2 in pyridine as oxidant. This methodology provides an alternative approach for the direct generation of all-carbon quaternary centers at the C2 position of indoles.

ChemInform Abstract: Oxidative Coupling of Organoboron Compounds

AbstractReview: [summary on both the seminal early work and recent developments in the area of transition‐metal‐catalyzed oxidative homocoupling reactions of organoboron compounds and mechanistic details; 71 refs.

Metal-Free–Catalyzed Oxidative Trimerization of Indoles Using NaNO2 to Construct Quaternary Carbon Centers: Synthesis of 2-(1H-Indol-3-yl)-2,3′-biindolin-3-ones

A simple, convenient, and efficient synthesis of 2-(1H-indol-3-yl)-2,3′-biindolin-3-one derivatives via a transition-metal-free-catalyzed oxidative trimeric reaction of indoles has been developed. This transformation may have occurred through a tandem oxidative homocoupling reaction by using NaNO2 in pyridine as oxidant. This methodology provides an alternative approach for the direct generation of all-carbon quaternary centers at the C2 position of indoles.

Site-Selective Sequential Coupling Reactions Controlled By “Electrochemical Reaction Site Switching”: A Straightforward Approach to 1,4-Bis(diaryl)Buta-1,3-Diynes

Recently, π-conjugated compounds have received considerable attention because they are known to be key building blocks for pharmaceuticals and candidates for electrochemical and optical materials. One of the most straightforward and powerful ways to construct π-skeletons is through transition-metal catalysed cross-coupling reactions using aryl halides or triflates and organometallic species such as in Suzuki–Miyaura coupling. Alternatively, transition metal-catalysed oxidative homo-coupling reactions of aryl metal species (e.g., arylboronic acids and esters) or terminal alkynes have also been shown to be convenient methods for building symmetrical π-structures. In these oxidative homo-coupling reactions, a stoichiometric amount of an oxidant is usually required. One way to overcome this drawback is to use electro-oxidation instead of a stoichiometric oxidant. The electrochemical method is advantageous, not only because it does not require excess oxidant, but also because the system can be switched between oxidative and neutral conditions by the on/off application of electricity. We considered that this feature could make it possible to realize a site selective coupling reaction controlled by the on/off application of electricity, and we call this strategy “Electrochemical Reaction Site Switching (e-RSS)”. This idea prompted us to design a novel integrated sequential reaction that consisted of an oxidative homo-coupling reaction and a typical coupling reaction. As the first example of the construction of π-conjugated compounds by a sequential coupling reaction controlled by e-RSS, we chose π-conjugated compounds that include a diyne moiety as a target; they are of interest due to their unique optical and electrochemical characteristics. The sequential reactions consist of two coupling reactions (Scheme 1). The first step is a Pd/Cu-catalysed electrochemical homo-coupling of p-bromophenylacetylene to give bis(p-bromophenyl)butadiyne (1st step: electricity ON), and the second step is a sequential Suzuki–Miyaura coupling reaction to build up 1,4-bis(diaryl)butadiynes (2nd step: electricity OFF). First, we optimized the reaction conditions of Pd/Cu-catalyzed electro-oxidative homo-coupling of p-bromophenylacetylene (1) (1st step) and we obtained diyne 2 in good yield. With the optimized conditions for the first step in hand, we next carried out the sequential reactions, including the electrooxidative homo-coupling of 1 and subsequent Suzuki–Miyaura coupling with several arylboronic acids on the basis of the e-RSS strategy. After the electro-oxidative homo-coupling of p-bromophenylacetylene, the electricity was turned off and to the resulting mixture were added p-tolylboronic acid, P(t-Bu)3·HBF4, Cs2CO3, and DME, then refluxed to give 1,4-bis(p-tolylphenyl)butadiyne 3a in 95% yield. In a similar manner, i-Pr- or t-Bu-substituted 1,4-bis(diaryl)butadiyne 3b and 3c were obtained in 79 and 86% yields, respectively. Anisole and aniline skeletons can also be introduced (3d and 3e). Arylboronic acids bearing electronwithdrawing functional groups could also be used in the reactions. 1,4-Bis-(diaryl)butadiynes with fluoro (3f), acetyl (3g), and cyano (3h) groups at the p-position can be synthesized by the sequential coupling reactions in respective yields of 68%, 65%, and 47%. The synthesis of aryl-substituted diynes with much greater π-conjugation was achieved by the use of 2-naphthylboronic acid or 2-phenylethenylboronic acid to afford 3i and 3j in 72% and 70% yields, respectively. We will discuss the details of the reactions and further transformations of 1,4-bis(diaryl)buta-1,3-diynes.

Oxidative Coupling of Organoboron Compounds

AbstractAn increasing number of boron‐mediated reactions have become leading synthetic tools in organic chemistry. The transition‐metal‐catalyzed oxidative coupling reactions of aryl‐ and alkylboron compounds have been intensively studied over recent decades. Palladium‐based catalysts are often used in the presence of an appropriate oxidant, and the reaction mechanism has been fully elucidated. Efficient homocoupling reactions that are catalyzed by nanosized (<2 nm) gold particles were introduced in 2004, which allow the use of oxygen as a sustainable oxidant. Besides palladium and gold, other transition metals and bimetallic combinations can also serve as excellent catalysts. This review summarizes both the seminal early work and recent developments in the area of transition‐metal‐catalyzed oxidative homocoupling reactions of organoboron compounds and discusses the mechanistic details.

Ferrocenyl end capped molecular rods: synthesis, structure, and properties

A series of ferrocenyl phenylethynyl rods 1–11 were designed, and synthesized by a Pd catalysed Sonogashira cross-coupling, and oxidative homo-coupling reactions. The ferrocenyl moieties in the rods 1–11 are attached at the para/meta position of phenylethynyl unit or at the acetylenic unit. The effect of enhancing the conjugation, and connectivity (meta/para) on the photonic and electrochemical properties were explored. The para linkage, and butadiynyl linkage lower the band gap in the rods. The meta linked ferrocenyl phenylethynyl rods 2, 4, and 10 show good HOMO–LUMO separation compared to para linked rods 1, 3, 9. The single crystal X-ray structures of rods 2, 4, 10, 14 and 19 are reported. The ferrocene units in rods 2, 4, and 10 are in a trans configuration with respect to the phenyl ring.

Laccase‐Catalysed Homocoupling of Primary Aromatic Amines towards the Biosynthesis of Dyes

AbstractColoured disubstituted benzoquinonimine trimeric structures are obtained as main reaction products of the oxidation of p‐electron donor primary aromatic amines using two different laccases, CotA‐laccase from Baccilus subtilus and TvL from Trametes versicolor. These orange‐red to purple products, presenting high molar extinction coefficients, presumably result from oxidative homocoupling reactions, through the formation of NC bonds at positions 2 and 5, of the laccase oxidised intermediate as showed in the proposed oxidative pathway. The product of 1,4‐phenylenediamine is shown to be the trimer known as Bandrowski’s base which has an established role in hair and fur dyeing. Our results also show that the occurrence and/or rates of oxidation of aromatic amines are strongly dependent on the presence of p‐electron releasing substituents in the aromatic ring and are independent on the properties of the enzyme used. Overall our data contribute for (i) understanding key features of laccase reactivity with p‐substituted aromatic amines and (ii) establishing enzymatic processes that lead to the synthesis of coloured bio‐products under mild conditions with potential impact in the cosmetic and dye industries.magnified image

Reaction pathways over copper and cerium oxide catalysts for direct synthesis of imines from amines under aerobic conditions

Copper(II) oxide, cerium oxide, and several copper(II) oxide supported on cerium oxide catalysts are explored in the aerobic oxidative homocoupling of benzylamine to form N-benzylidenebenzylamine in DMSO at 110°C. Although both CuO and CeO2 alone are shown to catalyze the reaction, CuOCeO2 catalysts are shown to most efficiently catalyze the reaction, providing higher rates (per g Cu) due to the presence of both copper and ceria species in the reactor. Catalysts with lower copper loadings and increased ceria content reduce the product yield, as ceria domains can catalyze the decomposition of the desired product as well. The amine conversion occurs with a significant induction period, associated with the putative formation of an initial benzylimine intermediate in the case of catalysis with CeO2 alone or from slow copper solubilization in cases where supported or unsupported copper catalysts are used, followed by rapid conversion to the N-benzylidenebenzylamine product. Copper leaching studies clearly demonstrate that catalysis using copper-containing catalysts is primarily associated with turnover by soluble copper species. A series of experiments targeted at elucidating the reaction pathway suggests that copper oxide domains promote the coupling of the initial intermediate, benzylimine, with benzylamine to produce the N-benzylidenebenzylamine product (path A), with a maximum production rate of 888μmol/m2h (13.3mmol/gCuh) over the pure CuO catalyst or 22.9μmol/m2h (26.1mmol/gCuh) over the CuOCeO2 catalyst. In contrast, cerium oxide domains are suggested to primarily convert the benzylimine to benzaldehyde, followed by condensation of the benzaldehyde with benzylamine in a rapid step to yield the N-benzylidenebenzylamine product (path B), with a maximum amine production rate of 2.74μmol/m2h. Both ceria and copper(II) oxide domains promote the initial benzylimine formation at comparable rates. Although the CuOCeO2 catalyst leaches about 11% of the copper during the reaction, and these soluble copper species are largely responsible for the catalytic turnover, the recovered solid can be recycled until the copper is depleted, catalyzing the reaction with an identical rate per g Cu in a second cycle, after calcination. The copper–ceria family of catalysts offers an alternative, potentially lower cost composition for the target oxidative homocoupling reaction than previously studied precious metal catalysts, although copper leaching is a distinct drawback to the catalyst composition.

Ligand‐Free Synthesis of 1,4‐Disubstituted‐1,3‐diynes by Iron/Copper Cocatalyzed Homocoupling of Terminal Alkynes

AbstractA simple and efficient protocol for Fe/Cu cocatalyzed oxidative homocoupling reaction of terminal alkynes to symmetrical 1,4‐disubstituted‐1,3‐diynes was presented. The results showed that both CuBr and FeCl3 played crucial roles in the reaction. It is noteworthy that this protocol employs mild, efficient, aerobic and ligand free conditions. The alkynes, including aromatic, heteroaromatic and aliphatic alkynes, were transformed into the corresponding 1,3‐diynes in good to excellent yields.

Cross-linked Polymers with Exceptionally High Ru(bipy)32+ Loadings for Efficient Heterogeneous Photocatalysis

Phosphorescent cross-linked polymers 1 and 2 were synthesized via oxidative homocoupling reactions of tetra(ethynyl) derivatives of Ru(bpy)32+ (with the alkynyl groups located at 4,4′- or 5,5′- positions of two substituted bipyridines). These cross-linked polymer particles contain exceptionally high Ru(bpy)32+ loadings and serve as highly efficient and reusable heterogeneous photocatalysts for a range of organic transformations, presumably owing to the ability of the Ru(bpy)32+ moieties in the polymer network to transport triplet excited states to particle surfaces to initiate the organic reactions. This work illustrates the potential of developing photocatalytic cross-linked polymers from photoactive molecular building blocks for solar energy utilization.

Polypyrrole–Palladium Nanocomposite Coating of Micrometer-Sized Polymer Particles Toward a Recyclable Catalyst

A range of near-monodisperse, multimicrometer-sized polymer particles has been coated with ultrathin overlayers of polypyrrole-palladium (PPy-Pd) nanocomposite by chemical oxidative polymerization of pyrrole using PdCl(2) as an oxidant in aqueous media. Good control over the targeted PPy-Pd nanocomposite loading is achieved for 5.2 μm diameter polystyrene (PS) particles, and PS particles of up to 84 μm diameter can also be efficiently coated with the PPy-Pd nanocomposite. The seed polymer particles and resulting composite particles were extensively characterized with respect to particle size and size distribution, morphology, surface/bulk chemical compositions, and conductivity. Laser diffraction studies of dilute aqueous suspensions indicate that the polymer particles disperse stably before and after nanocoating with the PPy-Pd nanocomposite. The Fourier transform infrared (FT-IR) spectrum of the PS particles coated with the PPy-Pd nanocomposite overlayer is dominated by the underlying particle, since this is the major component (>96% by mass). Thermogravimetric and elemental analysis indicated that PPy-Pd nanocomposite loadings were below 6 wt %. The conductivity of pressed pellets prepared with the nanocomposite-coated particles increased with a decrease of particle diameter because of higher PPy-Pd nanocomposite loading. "Flattened ball" morphologies were observed by scanning/transmission electron microscopy after extraction of the PS component from the composite particles, which confirmed a PS core and a PPy-Pd nanocomposite shell morphology. X-ray diffraction confirmed the production of elemental Pd and X-ray photoelectron spectroscopy studies indicated the existence of elemental Pd on the surface of the composite particles. Transmission electron microscopy confirmed that nanometer-sized Pd particles were distributed in the shell. Near-monodisperse poly(methyl methacrylate) particles with diameters ranging between 10 and 19 μm have been also successfully coated with PPy-Pd nanocomposite, and stable aqueous dispersions were obtained. The nanocomposite particles functioned as an efficient catalyst for the aerobic oxidative homocoupling reaction of 4-carboxyphenylboronic acid in aqueous media for the formation of carbon-carbon bonds. The composite particles sediment in a short time (<several tens of minutes) by gravity alone and hence recycling of this catalyst is easy.

A new procedure for the synthesis of π-conjugated polymers via ligand-free iron(iii)-catalyzed oxidative homocoupling reaction of grignard reagents

The FeCl3-catalyzed oxidative homocoupling reaction of Grignard reagents was used for the synthesis of π-conjugated polymers in this study. In the presence of an oxidant (1,2-dichloroethane), three polymers were successfully prepared, namely, poly(9,9-dioctylfluorene-2,7-diyl), poly(N-butylcarbazol-3,6-diyl) and poly(2,5-dihexyloxyphenylene-1,4-diyl). In comparison with the polymers obtained by Suzuki coupling, the polymers in this work showed no obvious difference in chemical structures or optical properties. On the basis of the relatively low cost of iron(III)-catalysts and Grignard reagents, this route would be an attractive alternative for the synthesis of π-conjugated polymers.

Metal-free catalyzed oxidative trimerization of indoles by using TEMPO in air: a biomimetic approach to 2-(1H-indol-3-yl)-2,3′-biindolin-3-ones

A simple, convenient and efficient metal-free catalyzed oxidative trimeric reaction of indoles toward a variety of 2-(1H-indol-3-yl)-2,3'-biindolin-3-one derivatives in moderate to excellent yields has been developed. This transformation proceeds via a tandem oxidative homocoupling reaction by using TEMPO in air as an environmentally benign oxidant. This methodology provides an alternative approach for the direct generation of all-carbon quaternary centers at the C3 position of indoles.

Synthesis of Dendrimers from Alkyne-focal Dendrons by Oxidative Homo-coupling of Terminal Acetylene

General, fast, and efficient fusion methods for the synthesis of dendrimers with 1,3-diynes at a core were developed. The synthetic strategy was employed the oxidative homo-coupling of terminal alkyne. The oxidative homo-coupling reaction of the alkyne-functionalized Frechet-type dendrons 1-Dm was allowed to provide first through fourth generation dendrimers 2-Gm with 1,3-diynes at core. The fusion of the propargylfunctionalized PAMAM dendrons 3-Dm by homo-coupling of terminal alkyne lead to the formation of symmetric PAMAM dendrimers 4-Gm. Their structure of dendrimers was confirmed by 1 H and 13 C NMR