Dehalogenative Coupling Research Articles

Linear‐ and Cyclo‐Hexa(amino)hexaboranes: New Routes to Synthesis of B6 Chain and B6 Ring

AbstractIn this work, a linear‐hexaborane(8) B6(NMe2)6(CH2SiMe3)2 was synthesized using a new method. By replacing one chlorine atom of Cl2B3(NMe2)3 with CH2SiMe3 group, compound ClB3(NMe2)3CH2SiMe3 was prepared. Reductive dehalogenative coupling of ClB3(NMe2)3CH2SiMe3 led to linear‐hexaborane(8). Moreover, the six‐membered boron ring, cyclo‐hexaborane B6(NEt2)6 was synthesized by reductive dehalogenation of Br2BNEt2. The first linear‐hexaborane(8) and cyclo‐hexaborane B6(NEt2)6 crystal structures were fully characterized by X‐ray structural analysis and 11B, 1H, and 13C NMR spectroscopy. While the B1⋅⋅⋅B6 chain in linear‐hexaborane(8) is strongly twisted at all B−B bonds, the B6 ring cyclo‐hexaborane B6(NEt2)6 has a chair conformation similar to cyclohexane.

Revealing the Influence of Molecular Chemisorption Direction on the Reaction Selectivity of Dehalogenative Coupling on Au(111): Polymerization versus Cyclization.

The control of reaction selectivity is of great interest in chemistry and depends crucially on the revelation of key influencing factors. Based on well-defined molecule-substrate model systems, various influencing factors have been elucidated, focusing primarily on the molecular precursors and the underlying substrates themselves, while interfacial properties have recently been shown to be essential as well. However, the influence of molecular chemisorption direction on reaction selectivity, as a subtle interplay between molecules and underlying substrates, remains elusive. In this work, by a combination of scanning tunneling microscopy imaging and density functional theory calculations, we report the influence of molecular chemisorption direction on the reaction selectivity of two types of dehalogenative coupling on Au(111), i.e., polymerization and cyclization, at the atomic level. The diffusion step of a reactive dehalogenated intermediate in two different chemisorption directions was theoretically revealed to be the key to determining the corresponding reaction selectivity. Our results highlight the important role of molecular chemisorption directions in regulating the on-surface dehalogenative coupling reaction pathways and products, which provides fundamental insights into the control of reaction selectivity by exploiting some subtle interfacial parameters in on-surface reactions for the fabrication of target low-dimensional carbon nanostructures.

Theoretical insights into the interplay between metal-organic and covalent bonding in single-layer molecular networks formed by halogen dissociation.

Synthesis via dehalogenative coupling due to thermal annealing is one of the most common routes of growing metal-organic and covalent polymer networks on catalytic metal surfaces. We present a computational approach taking into account both metal-coordinated and covalent C-C bonding interactions, which drive the self-assembly of tetrabrominated polyarene molecules into single-layer ordered and disordered nanostructures. The proposed coarse-grained lattice model is simulated using the Monte Carlo method. We investigate the annealing effect in ensembles of nearly and fully dehalogenated molecules, accordingly decreasing the concentration of dissociated (chemisorbed) halogen atoms, to account for the desorption process. The results suggest that dissociated halogens may be at least partially responsible for fragmentation of metal-organic networks on the Cu and Au surfaces. The simulations also show that fragmented covalent networks are mostly disordered or characterized by short-range glass-like order, but larger domains of these phases can be obtained after removing the split off Br atoms. We additionally examine the potential formation of fragments with a hybrid structure consisting of oligomer chains linked side-to-side by metal adatoms.

Visualizing the Hierarchical Evolution of Aryl-Metal Bonding in Organometallic Nanostructures on Ag(111).

On-surface dehalogenative coupling reactions are promising for constructing nanostructures with diverse properties and functionalities. Extensive efforts have been devoted to single aryl-halogen (C-X) substituents and substitutions at various functionalization sites (typically including meta- and para-substitutions) to generate aryl-aryl single bonds. Moreover, multiple C-X substituents at the ortho-site and the peri- and bay-regions have been applied to create a variety of ring scaffolds. However, for multiple C-X substituents, the hierarchy of aryl-metal bond formation and dissociation remains elusive. Herein, by combining scanning tunneling microscopy imaging and density functional theory calculations, we have visualized and demonstrated the hierarchical evolution of aryl-metal bonding in organometallic intermediates involved in a dehalogenative coupling reaction on Ag(111), using a molecular precursor with both para-substitution and potential bay-region substitution. Our results elucidate how metal atoms are progressively embedded into and removed from organometallic intermediates, enhancing the understanding of on-surface dehalogenative coupling reactions for the controlled construction of the desired nanostructures.

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.

On-Surface Synthesis of Polyphenylene Wires Comprising Rigid Aliphatic Bicyclo[1.1.1]Pentane Isolator Units.

Bicyclo[1.1.1]pentane (BCP) motifs are of growing importance to the pharmaceutical industry as sp3 -rich bioisosteres of benzene rings and as molecular building blocks in materials science. Herein we explore the behavior of 1,3-disubstituted BCP moieties on metal surfaces by combining low-temperature scanning tunneling microscopy / non-contact atomic force microscopy studies with density functional theory modeling. We examine the configuration of individual BCP-containing precursors on Au(111), their supramolecular assembly and thermally activated dehalogenative coupling reactions, affording polymeric chains with incorporated electronically isolating units. Our studies not only provide the first sub-molecular insights of the BCP scaffold behavior on surfaces, but also extend the potential application of BCP derivatives towards integration in custom-designed surface architectures.

On‐Surface Synthesis of Polyphenylene Wires Comprising Rigid Aliphatic Bicyclo[1.1.1]Pentane Isolator Units

AbstractBicyclo[1.1.1]pentane (BCP) motifs are of growing importance to the pharmaceutical industry as sp3‐rich bioisosteres of benzene rings and as molecular building blocks in materials science. Herein we explore the behavior of 1,3‐disubstituted BCP moieties on metal surfaces by combining low‐temperature scanning tunneling microscopy / non‐contact atomic force microscopy studies with density functional theory modeling. We examine the configuration of individual BCP‐containing precursors on Au(111), their supramolecular assembly and thermally activated dehalogenative coupling reactions, affording polymeric chains with incorporated electronically isolating units. Our studies not only provide the first sub‐molecular insights of the BCP scaffold behavior on surfaces, but also extend the potential application of BCP derivatives towards integration in custom‐designed surface architectures.

Modulation on the Iron Centers by Selective Synthesis of Organic Ligands with Stereo-Specific Conformations.

Advanced fabrication of surface metal-organic complexes with specific coordination configuration and metal centers will facilitate to exploit novel nanomaterials with attractive electronic/magnetic properties. The precise on-surface synthesis provides an appealing strategy for in situ construction of complex organic ligands from simple precursors autonomously. In this paper, distinct organic ligands with stereo-specific conformation are separately synthesized through the well-known dehalogenative coupling. More interestingly, the exo-bent ligands promote the mono-iron chelated complexes with the Fe center significantly decoupled from the surface and of high spin, while the endo-bent ligands lead to bi-iron chelated ones instead with ferromagnetic properties.

Initiating Ullmann-like coupling of Br2Py by a semimetal surface

Intensive efforts have been devoted to surface Ullmann-like coupling in recent years, due to its appealing success towards on-surface synthesis of tailor-made nanostructures. While attentions were mostly drawn on metallic substrates, however, Ullmann dehalogenation and coupling reaction on semimetal surfaces has been seldom addressed. Herein, we demonstrate the self-assembly of 2, 7-dibromopyrene (Br2Py) and the well controllable dehalogenation reaction of Br2Py on the Bi(111)–Ag substrate with a combination of scanning tunnelling microscopy (STM) and density functional theory calculations (DFT). By elaborately investigating the reaction path and formed organic nanostructures, it is revealed that the pristinely inert bismuth layer supported on the silver substrate can initiate Ullmann-like coupling in a desired manner by getting alloyed with Ag atoms underneath, while side products have not been discovered. By clarifying the pristine nature of Bi–Ag(111) and Ullmann-like reaction mechanisms, our report proposes an ideal template for thoroughly exploring dehalogenative coupling reaction mechanisms with atomic insights and on-surface synthesis of carbon-based architectures.

Film formation from plasma-enabled surface-catalyzed dehalogenative coupling of a small organic molecule.

This work demonstrates a new pathway to the direct on-surface fabrication of surface coatings by showing that application of a plasma can lead to dehalogenative coupling of small aromatic molecules at a catalytic surface. Specifically, we show that a room temperature, atmospheric pressure plasma can be used to fabricate a coating through a surface-confined dehalogenation reaction. Plasma treatments were performed using a dielectric barrier discharge (DBD) technique under pure nitrogen with a variety of power levels and durations. Samples were analysed by optical and helium ion microscopy (HIM), X-ray photoelectron spectroscopy (XPS), optical profilometry, and contact angle measurement. By varying the plasma parameters we could control the chemistry, morphology and roughness of the film. Surface wettability also varied with the plasma parameters, with high-dose plasmas leading to a hydrophobic surface with water contact angles up to 130°.

Flash vacuum pyrolysis of benzylidene halides, benzotrihalides and aryl halides over magnesium

Flash vacuum pyrolysis at 600°C through glass wool coated with freshly sublimed magnesium is examined as a preparative method for dehalogenative coupling in organic synthesis. Substituted benzylidene chlorides give predominantly the corresponding stilbenes and in some cases these are readily isolated in pure form. With an ortho-halogen substituent, additional cyclisation gives phenanthrenes but the method is not compatible with the presence of several reactive groups. An ortho-methoxy substituent leads to unexpected formation of mono- and dimethyl products. With 1,4-bis(dihalomethyl)benzenes, halogenated polymers are deposited directly from the gas phase via generation of halogenated p-xylylenes. The 1,2- and 1,3-isomers lead respectively to benzocyclobutadiene, isolated as a dimer, and to pyrene. The 1,4-bis(trihalomethyl)benzenes give more highly halogenated polymers directly from the gas phase via halogenated p-xylylenes. While halobenzenes generally give the corresponding benzenes and biphenyls, 1,2-dihalobenzenes additionally produce triphenylene in preparatively useful yield by a process not involving intermediacy of free benzyne.

Substrates as Electron-Donor Precursors: Synthesis of Naphtho-Fused Oxindoles via Benzannulation of 2-Halobenzaldehydes and Indolin-2-ones.

An unusual benzannulation reaction has been realized by integrating intermolecular adol condensation with subsequent intramolercular base-promoted homolytic aromatic substitution. This novel cascade reaction provides a straightforward approach toward various naphtho-fused oxindoles from 2-halobenzaldehydes and indolin-2-ones in the presence of Cs2CO3 in DMSO. The enolates of indolin-2-ones as new and internal electron donors have been demonstrated to initiate intramolecular radical dehalogenative coupling.

Unexpected rearrangement during the gas-phase dehalogenation approach to benzodithiophenes

As well as the expected dehalogenative coupling products, isomers are formed via a novel radical process.

Synthesis of Inclusion Compounds of Br–Th–Py–Br and (RO)2B–Th–Py–Br (Th: Thiophene-2,5-diyl; Py: Pyridine-2,5-diyl) with γ-Cyclodextrin

Abstract Inclusion compounds of Br–Th–Py–Br and (RO)2B–Th–Py–Br ((OR)2: –OCH2C(CH3)2CH2O–) with γ-cyclodextrin (CD), denoted by Br–ThPy–Br–CD and (RO)2B–ThPy–Br–CD, respectively, were prepared. Ni(0)-promoted dehalogenative coupling of Br–ThPy–Br–CD gave an oligomeric inclusion product, Oligo(ThPy–CD). Oligo(ThPy–CD) was soluble in organic solvents, and 1H- and CP/MAS 13C{1H} NMR spectra as well as the powder XRD pattern of Oligo(ThPy–CD) supported the CD-encapsulated structure.

Stereoselective Preparation of Six Diastereomeric Quatercyclopropanes from Bicyclopropylidene and Some Derivatives

Diastereomeric meso- and d,l-bis(bicyclopropylidenyl) (5) were obtained upon oxidation with oxygen of a higher-order cuprate generated from lithiobicyclopropylidene (4) in 50 and 31 % yield, respectively. Their perdeuterated analogues meso-[D(14)]- and d,l-[D(14)]-5 were obtained along the same route from perdeuterated bicyclopropylidene [D(8)]-3 (synthesized in six steps in 7.4 % overall yield from [D(8)]-THF) in 20.5 % yield each. Dehalogenative coupling of 1,1-dibromo-2-cyclopropylcyclopropane (6) gave a mixture of all possible stereoisomers of 1,5-dicyclopropylbicyclopropylidene 16 in 69 % yield, from which (Z)-cis-16 was separated by preparative gas chromatography (26 % yield). The crystal structure of meso-5 looks like a superposition of the crystal structures of two outer bicyclopropylidene units (3) and one inner s-trans-bicyclopropyl unit, whereas the two outer cyclopropyl moieties adopt a gauche orientation with respect to the cyclopropane rings at the inner bicyclopropylidene units in (Z)-cis-16. Birch reduction with lithium in liquid ammonia of meso-5 and d,l-5 gave two pairs of diastereomeric quatercyclopropanes trans,trans-(R*,S*,R*, S*)-17/cis,trans-(R*,S*,R*,R*)-18 and trans,trans-(R*,S*,S*,R*)-19/cis,trans-(R*,S*,S*,S*)-20 in 97 and 76 % yield, respectively, in a ratio 9:1 for every pair. The latter diastereomer was also obtained as the sole product by Birch reduction of (Z)-cis-16 in 96 % yield. Under the same conditions, tetradecadeuterio analogues trans,trans-[D(14)]-(R*,S*,R*,S*)-17/cis,trans-[D(14)]-(R*, S*,R*,R*)-18 (8:1) and trans,trans-[D(14)]-(R*,S*,S*,R*)-19/cis,trans-[D(14)]-(R*,S*,S*,S*)-20 (12:1) were prepared from meso-[D(14)]-5 and d,l-[D(14)]-5 in 37 and 63 % yield, respectively. Reduction of meso-5 with diimine gave the cis,cis-quatercyclopropane (S*,S*,R*,R*)-21 as the main product (58 % yield) along with the cis,trans-diastereomer (S*,S*,R*,S*)-18 (29 % yield). Thus, five of the six possible diastereomeric quatercyclopropanes were obtained from meso-5, d,l-5, and (Z)-cis-16. The X-ray crystal structure analyses of trans,trans-(R*,S*,R*,S*)-17 and cis,cis-(S*,S*,R*,R*)-21 revealed for the both an unusual conformation in which the central bicyclopropyl unit adopts an s-trans-(antiperiplanar) orientation with phi=180.0 degrees , and the two terminal bicyclopropyl moieties adopt a synclinal conformation with phi=49.8 and 72.0 degrees , respectively. In solution the vicinal coupling constants (3)J(H,H) in trans,trans-(R*,S*,R*,S*)-[D(14)]-17, trans,trans-(R*,S*,S*,R*)-[D(14)]-19, trans,cis-(R*,S*,R*,R*)-[D(14)]-18 and trans,cis-(R*,S*,S*,S*)-[D(14)]-20 were found to be 4.1, 4.7, 5.9 and 5.9 Hz, respectively. This indicates a predominance of the all-gauche conformer in (R*,S*,R*,S*)-17 and a decreasing fraction of it in this sequence of the other diastereomers.

Synthesis and Study of Arylenevinylene and Aryleneimine-Type Polymers Containing N-Hexyl-3.6-Carbazolyl Groups

Two monomers with preformed Schiff base and arylenevinylene structure and final bromine substituents were synthesized by coupling of 3-formyl-6-bromo-N-hexyl-carbazole with 1,4-phenylenediamine and tetraethyl-p-xylylene diphosphonate, respectively. These compounds were used to synthesize arylenimine and arylenevinylene polymers containing N-hexyl 3,6-carbazolylydiyl units in the main chain by dehalogenative coupling reactions via organometallic reductive catalysis (Ni°.bipy). The polymers were characterized by spectral methods (Fourier transform-infrared, 1H NMR, UV-vis and fluorescence) as well as gel permeation chromatography and differential scanning calorimetry.

Polyindolo[3,2-b]carbazoles: A New Class of p-Channel Semiconductor Polymers for Organic Thin-Film Transistors

The synthesis and field-effect transistor (FET) properties of a novel class of indolocarbazole-based conjugated polymers, poly(indolo[3,2-b]carbazole)s, are described. Dehalogenative coupling polymerization of dichloroindolo[3,2-b]carbazoles with Zn/NiCl2/PPh3/2.2‘-dipyridil gave the corresponding poly(indolo[3,2-b]carbazole)s, while oxidative coupling polymerization of indolo[3,2-b]carbazoles with FeCl3 proceeded regioselectively, affording poly(indolo[3,2-b]carbazole-2,8-diyl)s. Crystallization and optical properties of these polymers were different, depending on the regiochemistry of backbone linkages and peripheral substitutions. Thin film transistor devices using these polymer semiconductors exhibited p-channel FET behavior, dictated predominantly by nature of substitution and backbone linkages (2,8- or 3,9-linkage). Solution-processed polyindolo[3,2-b]carbazole thin-film semiconductors obtained from FeCl3 polymerization generally provided better FET performance.

Synthesis and Thin-Film Transistor Performance of Poly(4,8-didodecylbenzo[1,2-b:4,5-b‘]dithiophene)

The synthesis of novel regioregular poly(4,8-didodecylbenzo[1,2-b:4,5-b‘]dithiophene) and its use as a semiconductor in organic thin-film transistors (OTFTs) are described. The polymer was prepared from 2,6-dibromo-4,8-didodecylbenzo[1,2-b:4,5-b‘]dithiophene by a dehalogenative coupling polymerization. A field-effect mobility of ∼0.012 cm2 V-1 s-1 and a current on/off ratio of ∼2.5 × 105 have been obtained with OTFTs using this polymer semiconductor.

Electrochemical Synthesis of some Polyazomethines by Organometallic Reductive Catalysis

Three monomers with preformed Schiff base structures and ~nal bromine atoms were synthesized: N, N°-bis(4-bromophenyl)-terephthaldiimine (I), N, N°-bis(4-bromobenzylidene)-benzene-1,4-dia-mine (II) and N-(4-bromobenzylidene)-4°-bromoaniline (III). All these compounds were used to prepare polyazomethines by dehalogenative coupling reactions via organometallic reductive catalysis (Ni0.bipy). The monomers and corresponding polymers were characterized by spectral methods (FT-IR, 1HNMR, UV-Vis) as well as by thermal analysis

Ruthenium-catalyzed carbon–carbon formation to synthesize tetraarylethanes and tetraarylxylylene through dechlorinative dimeric reaction

Dechlorinative coupling–dimerization reaction is studied to synthesize alkanes by using tris-triphenylphosphine ruthenium (II) in the presence of hydrogen atmosphere. Two types of halides (α-chlorodiarylmethanes 1a– d and bis(chlorophenylmethyl)-1,3-phenylene 6) are employed as substrates to form radical species and then dimerized to generate alkanes in high yields. To our knowledge, it is the first time that the formation of 1,1,2,2-tetraarylethanes and 1,2,9,10-tetraphenyl-di- m-xylylene over typical ruthenium-catalyzed dehalogenative coupling–dimerization conditions is reported.