Ullmann Coupling Research Articles

Long and isolated graphene nanoribbons by on-surface polymerization on Au(111)

Low electronic gap graphene nanoribbons (GNRs) are used for the fabrication of nanomaterial-based devices and, when isolated, for mono-molecular electronics experiences, for which a well-controlled length is crucial. Here, an on-surface chemistry protocol is monitored for producing long and well-isolated GNR molecular wires on an Au(111) surface. The two-step Ullmann coupling reaction is sequenced in temperature from 100 °C to 350 °C by steps of 50 °C, returning at room temperature between each step and remaining in ultrahigh vacuum conditions. After the first annealing step at 100 °C, the monomers self-organize into 2-monolayered nano-islands. Next, the Ullmann coupling reaction takes place in both 1st and 2nd layers of those nano-islands. The nano-island lateral size and shape are controlling the final GNR lengths. Respecting the above on-surface chemistry protocol, an optimal initial monomer coverage of ~1.5 monolayer produces isolated GNRs with a final length distribution reaching up to 50 nm and a low surface coverage of ~0.4 monolayer suitable for single molecule experiments.

A simple strategy for obtaining aggregation-induced delayed fluorescence material achieving nearly 20% external quantum efficiency for non-doped solution-processed OLEDs

The development of efficient non-doped organic luminescent materials is challenging work due to the existence of aggregation-caused quenching (ACQ) effect in most of emitters. Here, 1,10-phenanthroline (Phen) with a rigid structure was selected as acceptor unit, which could well limit intramolecular vibrations and rotations. According to the Ullmann coupling, the twisted tert-butyldiphenylamine-modified carbazole named N3,N3,N6,N6-tetrakis(4-(tert-butyl)phenyl)-9H-carbazole-3,6-diamine (tBuPhACz) was introduced into 2, 9-position of the Phen to afford tBuPhACz-Phen as target material, which not only can limit the π–π stacking of acceptor, but also could obtain the TADF and AIE effects due to the twisted D-A structure. The non-doped neat film of tBuPhACz-Phen shown high photoluminescence quantum yield (PLQY) of 76.5 %. The non-doped solution-processed electroluminescent devices obtained high performance with the maximum external quantum (EQEmax) up to 19.7 %. This result illustrated that the performance of this device is one of the best in the non-doped solution-processed OLEDs.

Theoretical Studies of the Mechanism of Ullmann Coupling of Naphthyl Halogen Derivatives to Binaphtyl on Coinage Metals

Theoretical Studies of the Mechanism of Ullmann Coupling of Naphthyl Halogen Derivatives to Binaphtyl on Coinage Metals

Copper-catalyzed one-pot synthesis of 1,4-benzothiazine-3-ones

An efficient potassium carbonate and copper-mediated CS and CN Ullmann coupling reaction between 2-bromothiophenol and 2-haloacetanilide is reported here. This protocol affords a simple and efficient approach for the construction of 1,4-benzothiazinone-3-one derivatives, avoiding the limitations of using noble metals and expensive ligands for the first time.

Cross-coupling by a noncanonical mechanism involving the addition of aryl halide to Cu(II).

Copper complexes are widely used in the synthesis of fine chemicals and materials to catalyze couplings of heteroatom nucleophiles with aryl halides. We show that cross-couplings catalyzed by some of the most active catalysts occur by a mechanism not previously considered. Copper(II) [Cu(II)] complexes of oxalamide ligands catalyze Ullmann coupling to form the C-O bond in aryl ethers by concerted oxidative addition of an aryl halide to Cu(II) to form a high-valent species that is stabilized by radical character on the oxalamide ligand. This mechanism diverges from those involving Cu(I) and Cu(III) intermediates that have been posited for other Ullmann-type couplings. The stability of the Cu(II) state leads to high turnover numbers, >1000 for the coupling of phenoxide with aryl chloride electrophiles, as well as an ability to run the reactions in air.

Highly dispersed Pd nanoclusters supported on covalent triazine-based frameworks for highly efficient aryl nitro reduction and Ullmann coupling

The feasible preparation of Pd catalysts, one of the most widely used catalysts, with high performance and easy recovery still remains the greatest challenge. Covalent triazine-based frameworks (CTFs), by virtue of strong covalent linkages and abundant nitrogen atoms, have been deemed as a central subclass of porous supports for nano-functional materials including metal nanocatalysts. Herein, a series of CTFs have been prepared by ionothermal synthesis followed by rearrangement at higher temperatures (500–900 °C). The CTF-immobilized nano-Pd(0) catalysts with different particle size distributions (<0.5, 1.35 ± 0.93, 2.21 ± 1.43, 3.80 ± 1.33, 6.10 ± 3.23 nm) were then prepared by simply suspension impregnation-in-situ reduction. Among them, the smallest and highly dispersed Pd nanoclusters (Pd/CTF-600) exhibited excellent activity and outstanding recyclability in aryl nitro reduction and Ullmann coupling. The study provides a new insight for the preparation of high-performance metal nano-catalysts that could enable efficient chemical transformations.

One-step synthesis of perfluorinated polyphenylenes using modified Ullmann coupling conditions

Fluorinated compounds have attracted attention in both the pharmaceutical industry and materials science due to the small size and strong electron-withdrawing property of the fluorine atom. In materials science, perfluorinated polyphenylenes play an important role as electron transport layers in organic electronics. While various strategies for the preparation of fluorinated arenes have been reported, the number of synthetic methods available for perfluorinated arenes remains limited, mainly due to the change in reactivity of reagents and substrates at the reaction site upon the introduction of additional fluorine atoms. Given the important applications of perfluorinated polyphenyl-based compounds, this article describes the one-step synthesis of the dendrimer perfluoro-3,3’5,5’-tetrakisphenyldiphenyl-1,1’, which to date no example of a targeted synthesis has been reported in literature, and the synthetic methodology for the direct preparation of linear perfluorinated para-sexiphenyl. Both strategies use starting materials that are either commercially available or can be easily accessed using standard literature methods.

Efficient Exploration of Reaction Space to Optimize Sustainable Cu-Mediated Ullmann Couplings

Efficient Exploration of Reaction Space to Optimize Sustainable Cu-Mediated Ullmann Couplings

Convenient synthesis of 6-Amino-2-naphthol by Copper-catalyzed Ullmann reaction

A strategic approach towards the synthesis of 6-amino-2-naphthol for energy efficient and consistent yield was exploited by two protocols: (a) a direct amination, and (b) a cascade synthetic approach involving methylation, amination, and demethylation of 2-bromo-6-naphthol by replacing naphthyl bromide with an –NH2 group. A modified copper-catalyzed Ullmann- type CN bond formation reaction at a controlled temperature (80–120 °C) was adopted for the amination reaction in both protocols. Amination reaction of two different substrates 2-bromo-6-methoxynapthalene and 2-bromo-6-naphthol were performed. Two different aminating reagents, namely, aq·NH3 (28–30%) and NaN3 were explored. Copper catalysts were elemental Cu, CuI, Cu2O, CuO, and CuSO4·5H2O. Extensive method optimization revealed that the combination of CuSO4•5H2O, l-proline, and Na-ascorbate is the superior catalyst when employed with NaN3 and NaOH at 120 °C affording the highest coupling yield of 61%. 6-amino-2-naphthol was obtained following both protocols however, the latter afforded a higher overall yield (53%). Thus, an Ullmann reaction strategy with a stable, environmentally friendly, earth-abundant, easy-to-handle, and inexpensive catalyst CuSO4•5H2O in combination with Na-ascorbate makes the amination reaction of bifunctional naphthyl bromide simple, sustainable, energy-efficient and convenient with consistent yield.

Expression of Reaction Selectivity and the Substituent Effect in Ullmann, Suzuki, Hiyama, and Allylic Arylation Reactions Caused by Reducing Catalyst Loading

The control of stereo-, regio-, and chemo-selectivity in transition-metal-catalyzed coupling reactions is a key topic in organic synthesis. Several methods for controlling selectivity have been reported thus far. On the other hand, the reduction of catalyst loading during reactions is one of the most important issues in organic synthesis from the standpoint of green sustainable chemistry. As another advantage of reducing catalyst loading, the expression of reaction selectivity and the substituent effect caused by the reduction of catalyst loading to the parts-per-million (ppm) level in various catalytic reactions is presented herein.

On-surface Ullmann coupling of halo-derivatives of arenes: Monte Carlo simulations for tetracene

On-surface synthesis of C–C covalent low-dimensional nanomaterials is a promising method of obtaining structures with tailored and novel physicochemical and electric properties. In this contribution, the Monte Carlo simulation approach was proposed to predict the topology of metal–organic (MO) intermediates formed in the Ullmann homocoupling of halogenated isomers of tetracene. The coarse-grained model of polyaromatic hydrocarbons (PAH) haloderivatives and divalent copper adatoms on a metallic crystal surface (111) was used, where locations of substituents in the molecules were encoded as active centres with directional C–Cu interactions. The computations were performed for various structural isomers of tetracene, from disubstituted to tetrasubstituted units. As a result, diverse superstructures were obtained, such as dimers, trimers, and other oligomers, chains and ladders, and metal–organic (MO) networks, both chiral and achiral. Additionally, for the prochiral linkers, simulations of the racemic mixtures were performed. Our study provided useful insight into the influence of substituents’ position and the carbon backbone’s size on the topology of the modelled precursor architectures.

Toward Two-Dimensional Tessellation through Halogen Bonding between Molecules and On-Surface-Synthesized Covalent Multimers.

The ability to engineer sophisticated two-dimensional tessellation organic nanoarchitectures based on triangular molecules and on-surface-synthesized covalent multimers is investigated using scanning tunneling microscopy. 1,3,5-Tris(3,5-dibromophenyl)benzene molecules are deposited on high-temperature Au(111) surfaces to trigger Ullmann coupling. The self-assembly into a semi-regular rhombitrihexagonal tiling superstructure not only depends on the synthesis of the required covalent building blocks but also depends on their ratio. The organic tessellation nanoarchitecture is achieved when the molecules are deposited on a Au(111) surface at 145 °C. This halogen-bonded structure is composed of triangular domains of intact molecules separated by rectangular rows of covalent dimers. The nearly hexagonal vertices are composed of covalent multimers. The experimental observations reveal that the perfect semi-regular rhombitrihexagonal tiling cannot be engineered because it requires, in addition to the dimers and intact molecules, the synthesis of covalent hexagons. This building block is only observed above 165 °C and does not coexist with the other required organic buildings blocks.

Synthesis of Functionalized ABAC- and ABCD-Type Inherently Chiral Heteracalix[4]aromatics.

Despite their interesting stereochemistry and potential applications in (supramolecular) chemistry and chiroptical materials, inherently chiral macrocyclic compounds remain rare and are largely unexplored. We report herein a fragment coupling method to construct ABAC- and ABCD-type inherently chiral heteracalix[4]aromatics. The synthesis involves SNAr CuI-catalyzed Ullmann coupling and aliphatic nucleophilic substitution reactions as key steps using readily available starting materials. Postmacrocyclization functionalization reactions enabled the production of amino-substituted and (benzo[d])imidazole-2-(thi)one-bearing heteracalix[4]aromatics. Enantiopure ABCD-type macrocycles were obtained from resolution.

Copper-Catalyzed Consecutive Ullmann, Decarboxylation, Oxidation, and Dehydration Reaction for Synthesis of Pyrrolo or Pyrido[1,2-a]imidazo[1,2-c]quinazolines.

A protocol for a copper-catalyzed intermolecular cross-coupling cascade between 2-(2-bromoaryl)-1H-benzo[d]imidazole analogues and proline or pipecolic acid has been developed. The developed protocol allows access to a variety of synthetically useful N-fused pyrrolo or pyrido[1,2-a]imidazo[1,2-c]quinazoline scaffolds with high efficiency and good functional group compatibility. Proline or pipecolic acid plays a dual role in the reaction: as ligand and reactants. A mechanistically consecutive approach for the Ullmann coupling, decarboxylation, oxidation, and dehydration reaction process was presented.

Facile Synthesis Hyper-Crosslinked PdFe Bimetallic Polymer as Highly Active Catalyst for Ullmann Coupling Reaction of Chlorobenzene.

The synthesis of efficient and sustainable heterogeneous Pd-based catalysts has been an active field of research due to their crucial role in carbon-carbon coupling reactions. In this study, we developed a facile and eco-friendly in situ assembly technique to produce a PdFe bimetallic hyper-crosslinked polymer (HCP@Pd/Fe) to use as a highly active and durable catalyst in the Ullmann reaction. The HCP@Pd/Fe catalyst exhibits a hierarchical pore structure, high specific surface area, and uniform distribution of active sites, which promote catalytic activity and stability. Under mild conditions, the HCP@Pd/Fe catalyst is capable of efficiently catalyzing the Ullmann reaction of aryl chlorides in aqueous media. The exceptional catalytic performance of HCP@Pd/Fe is attributed to its robust absorption capability, high dispersion, and strong interaction between Fe and Pd, as confirmed by various material characterizations and control experiments. Furthermore, the coated structure of a hyper-crosslinked polymer enables easy recycling and reuse of the catalyst for at least 10 cycles without any significant loss of activity.

Ion-imprinted magnetic adsorbents for the selective capture of Cu(II) and their cascade application as heterogeneous recyclable catalysts for Ullmann and Glaser coupling reactions

The selective capture and efficient reuse of Cu(II) from contaminated seawater requires a multifunctional adsorbent with a high Cu(II) selectivity and magnetic properties to facilitate recycling. Moreover, the cascade application of the recycled adsorbent is important but has not been extensively investigated. Accordingly, in this study, ion-imprinted magnetic adsorbents, particularly, copper ion-imprinted polymers (Fe3O4@mSiO2@CIIP) were prepared using magnetic mesoporous silica microspheres (Fe3O4@mSiO2) as carriers, with organosilicon containing hydroxyl and Schiff bases as the polymer backbone to selectively collect Cu(II) from aqueous media. The adsorption isotherms, kinetics, selectivity, and mechanisms were examined under various adsorption conditions. Fe3O4@mSiO2@CIIP-2 shows high selectivity and fast adsorption kinetics, and the equilibrium adsorption amount of Cu(II) is 263.2 mg/g within 30 min. The pseudo-second-order model and the Langmuir model are supported by the adsorption kinetics and isotherms, respectively. Particularly, the adsorption amount of Fe3O4@mSiO2@CIIP-2 could be reused eight times without substantial loss. Additionally, specific functional monomers bound to Fe3O4@mSiO2@CIIP-2 could both selectively adsorb Cu and serve as functional ligands for catalysts that can heterogeneously catalyze the Ullmann and Glaser coupling reactions after adsorbing Cu(II).

Construction of 2D metal-organic precursor networks with tunable porosity: A Monte Carlo simulation approach

Two-dimensional network structures comprising functional organic building blocks have been recently recognized as promising platforms for various physio-chemical processes running in confined spaces. In this study, we present theoretical results of the self-assembly of metal-organic precursor networks on a crystalline surface and provide quantitative characteristics of these planar architectures. To that end, a coarse-grained model of the adsorbed overlayer comprising halogenated polyaromatic aromatic hydrocarbon (PAH) molecules and bivalent metal atoms was used in combination with the equilibrium Monte Carlo (MC) simulation method. Our calculations focused on the effect of molecular features, such as size, shape and distribution of halogen substituents in the PAH monomer, on the quality and morphology of the networks formed on a model (111) crystalline surface. For the simulated porous assemblies, representing the precursor stage of the surface-assisted Ullmann coupling reaction, such descriptors as the unit cell parameters, radial distribution functions, pore size distributions and structure factors were determined. The obtained results demonstrated that fine-tuning of the monomeric PAH units enables the creation of periodic and aperiodic networks having diverse porous properties, ranging from monoporous to heteroporous with bi- and multimodal pore-size distributions. The findings of our theoretical investigations, apart from the potential application in the optimization of 2D polymeric networks, can also be helpful in designing real confined nanoenvironments for selective capture and immobilization/transformation of guest species.

On-Surface Synthesis of Pentagon-Incorporated Graphene-Like Nanoribbons with Multiple Precursors

Graphene nanoribbons (GNRs) and their derivatives are receiving increasing attention as a result of their unique electronic and magnetic properties, and many novel derivative structures have been fabricated. The carbon pentagon plays a crucial role in determining both geometric structures and electronic properties of carbon-based materials. Here, we demonstrate that carbon-pentagon-incorporated graphene-like nanoribbons (GLNRs), which are an important class of GNR derivatives, are successfully fabricated via the Ullmann coupling and aromatic cyclodehydrogenation reaction on the surface by a suitable choice of multiple tailored molecular precursors. Our approach provides a basis for the impact of adatoms in the reaction and proves the steering function of the aryl-metal interaction in procedures of self-assembly and organometallic state. In addition, this study paves the way for on-surface synthesis of GNRs and their derivatives as well as the fine tuning of electronic properties of carbon nanoarchitectures by manipulating the edge structures and embedding carbon pentagon heterojunctions.

On-Surface Cross-Coupling Reactions

On-surface synthesis, as a bottom-up synthetic method, has been proven to be a powerful tool for atomically precise fabrication of low-dimensional carbon nanomaterials over the past 15 years. This method relies on covalent coupling reactions that occur on solid substrates such as metal or metal oxide surfaces under ultra-high-vacuum conditions, and the achievements with this method have greatly enriched fundamental science and technology. However, due to the complicated reactivity of organic groups, distinct diffusion of reactants and intermediates, and irreversibility of covalent bonds, achieving the high selectivity of covalent coupling reactions on surfaces remains a great challenge. As a result, only a few on-surface covalent coupling reactions, mainly involving dehalogenation and dehydrogenation homocoupling, are frequently used in the synthesis of low-dimensional carbon nanosystems. In this Perspective, we focus on the development and synthetic applications of on-surface cross-coupling reactions, mainly Ullmann, Sonogashira, Heck, and divergent cross-coupling reactions.

Hierarchical Activation of C–Br and Cyano ortho C–H to Form Chiral Organometallics on Ag(111)

The combination of dehalogenative coupling and dehydrogenative coupling represents a promising avenue toward the on-surface synthesis of sophisticated nanostructures. Previous works have largely focused on the sequential activations of C–Br and C–H on pristine aromatic rings. The activation of ortho C–H on functionalized aromatic rings as the second step is rarely reported. In this work, we employ a precursor molecule equipped with bromine and cyano substituents at both ends for dehalogenative and dehydrogenative couplings, respectively. With stepwise annealing from 250 to 365 K, C–Br activation proceeds progressively, accompanied by an increase in organometallic (OM) dimers and the formation of delicate self-assemblies on Ag(111). Surprisingly, upon further annealing, the C–C coupling for OM dimers fails to occur. Instead, OM dimer dissociation and cyano ortho C–H activation are initiated, giving birth to chiral OM multimers. Plausibly, the cyano modification significantly reduces the ortho C–H activation barrier and increases the C–C coupling barrier. In general, this work serves as a paradigm of the hindered second step of Ullmann coupling not due to steric reason. The principle may be extended to the surface reactions of other functionalized aryl halides.