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Relevant academic research and scientific papers

Nematic thiophenes for STN-LCDS and flexoelectric LCDS

We report anew class of liquid crystals incorporating a 2,5-disubstituted thiophene ring and one or two conjugated trans-carbon-carbon double bonds in the terminal chain for use as components of nematic mixtures for TN-LCDs and STN-LCDs. The combination of these molecular elements contribute towards generating a high birefringence, a high nematic clearing point as well as a low viscosity for these materials. Several of these compounds exhibit a melting point below room temperature. This is the first time a room temperature nematic phase has been reported for phenyl thiophene derivatives. The new thiophene compounds can be used to induce a high birefringence in nematic mixtures for LCDs with short response times.

Well-Defined Palladium N-Heterocyclic Carbene Complexes: Direct C-H Bond Arylation of Heteroarenes

A series of palladium N-heterocyclic carbene (NHC) complexes of type trans-{(NHC)PdCl2L} (L = C5H5N, 3-ClC5H4N, and PPh3) (3-5) have been developed as efficient precatalysts for direct C-H bond arylation of various heteroarenes. In particular, an in situ generated new NHC ligand derived from {1,3-di-(2,6-diethylphenyl)acenaphtho[1,2-d] imidazolium} chloride (2) is used for the stabilization of the palladium metal center. Among the screened palladium precatalysts (3-5), the most active PEPPSI themed complex (3) was successfully employed toward direct C-H bond arylation of various heteroarenes and aryl bromides. A range of functional groups on aryl bromides as well as on heteroarenes sustained throughout the standard reaction conditions for easy access of various arylated heterocyclic compounds. Significantly, the utility of the protocol was demonstrated by the effective synthesis of a precursor of raloxifene, a selective estrogen receptor modulator.

A Convoluted Polyvinylpyridine-Palladium Catalyst for Suzuki-Miyaura Coupling and C?H Arylation

The development of highly active and reusable supported catalysts for Suzuki-Miyaura coupling and catalytic C?H arylation is important for fundamental and applied chemistry, with these reactions being used to produce medical compounds and functional materials. Herein, we found that a mesoporous composite made of a linear poly(4-vinylpyridine) and tetrachloropalladate acted as a dual-mode catalyst for a variety of cross-coupling reactions, with both Pd nanoparticles and a Pd complex catalyst being observed under different conditions. The polyvinylpyridine-palladium composite 1 was readily prepared via the molecular convolution of poly(4-vinylpyridine) and sodium tetrachloropalladate to provide a hardly soluble polymer-metal composite. The Suzuki-Miyaura coupling and the C?H arylation of aryl chlorides and bromides with arylboronic acids, thiophenes, furans, benzene, and anisole proceeded in the presence of 0.004 mol% (40 mol ppm) to 1 mol% Pd of 1 to afford the corresponding coupling products in high yields. Furthermore, the catalyst was reused without an appreciable loss of activity. Pharmaceutical compounds and functional materials were synthesized via the coupling reactions. N2 gas adsorption/desorption analysis indicated that the catalyst had a mesoporous nature, which played a crucial role in the catalysis. In the Suzuki-Miyaura couplings, in situ generated palladium nanoparticles in the polymer matrix were catalytically active, while a polymeric Pd(II) complex was crucial in the C?H arylations. These catalytic species were investigated via XAFS, XPS, far-infrared absorption, and Raman spectroscopies, as well as DFT calculations. (Figure presented.).

Microwave-assisted, ligand-free, direct C-H arylation of thiophenes in biomass-derived γ-valerolactone

The Pd-catalyzed C-H arylation of heterocycles with aryl halides is a straightforward and more environmentally-friendly route to the synthesis of well-defined, pi-conjugated polymers for challenging applications in electronic devices. Although this type of transformation is more atom efficient than cross-couplings, it still poses environmental issues in the form of reaction media and the use of phosphine ligands. A procedure for the C-H arylation of thiophenes, with a substantially improved environmental impact, and a promising application of this protocol to the synthesis of regioregular polythiophenes are reported in this work. γ-Valerolactone (GVL), a non-toxic biomass-derived solvent, has been used in this phosphine-free microwave-assisted process and replaces commonly used dipolar aprotic media. Activated aryl bromides gave arylthiophenes in good yields, while iodides were used and pivalic acid was added when electron-withdrawing groups were present. The reaction was also extended to C-H (hetero)arylation polycondensation and a high molecular weight poly(3-hexyl)thiophene was obtained, using low Pd loading, in high yields and good regioregularity in short reaction times. Computational studies have shed light on GVL's role as a ligand in the catalytic system.

N-Heterocyclic carbene-Pd(II)-1-methylimidazole complex catalyzed C[sbnd]H bond arylation of (benzo)thiophenes with aryl chlorides

NHC-Pd(II)-Im complex showed efficient catalytic activity toward the direct C[sbnd]H bond arylation of (benzo)thiophenes with the challenging aryl chlorides. Under the suitable conditions, all reactions proceeded smoothly to give the desired C[sbnd]H bond

Bulky α-diimine palladium complexes: Highly efficient for direct C-H bond arylation of heteroarenes under aerobic conditions

Through the strategy to enhance the bulkiness on both the backbone and the N-aryl moieties, we designed and synthesized a type of bulky α-diimine palladium complex (i.e., {[Ar-NC(R)-C(R)N-Ar]PdCl2, (Ar = 2-benzhydryl-4,6-dimethylphenyl)}, C1, R = H; C2, R = An; C3, R = Ph). The structures of these palladium complexes were well characterized, while C1 and C3 were further characterized by X-ray diffraction. The catalytic performances of the precatalysts were screened for direct C-H bond arylation of heteroarenes. The bidentate N,N-palladium complex C3 with both a backbone and N-aryl bulkiness was found to be a highly efficient precatalyst under aerobic conditions. With a low palladium loading of 0.5-0.1 mol%, a variety of heteroarenes with challenging bulky steric aryl bromides as well as heteroaryl bromides are all applicable for this cross-coupling reaction.

Aerobic and Efficient Direct Arylation of Five-Membered Heteroarenes and Their Benzocondensed Derivatives with Aryl Bromides by Bulky α-Hydroxyimine Palladium Complexes

In the present work, a series of α-hydroxyimine palladium complexes with bulky substituents (i.e., {[Ar-N=C(R)-C(R)2-OH]PdCl2} (C1, R = Me, Ar = 2-diphenylmethyl-4,6-dimethylphenyl; C2, R = Me, Ar = 2,6-bis(diphenylmethyl)-4-methylphenyl; C3, R = Me, Ar = 2,6-bis(diphenylmethyl)-4-methyoxylphenyl; C4, R = Me, Ar = 2,6-bis(diphenylmethyl)-4-chlorophenyl; C5, R = Ph, Ar = 2,6-dimethylphenyl; C6, R = Ph, Ar = 2,6-diisopropylphenyl)) were synthesized and characterized. The structures of palladium complexes C1 and C2 were determined by X-ray diffraction. These bidentate N,O-palladium complexes were applied for direct arylation under aerobic conditions. The effects of the reaction conditions and ligand substitution on the catalytic activity were evaluated. Upon a low palladium loading of 0.5 mol %, the bulky palladium complex C6 was successfully used to catalyze the cross-coupling of a variety of five-membered heteroarenes and their benzo-condensed derivatives with (hetero)aryl bromides. The mechanistic investigation on the direct arylation supported the involvement of a Pd(0)/Pd(II) CMD process.

Rhodium-catalyzed borylation of aryl 2-pyridyl ethers through cleavage of the carbon-oxygen bond: Borylative removal of the directing group

The rhodium-catalyzed reaction of aryl 2-pyridyl ethers with a diboron reagent results in the formation of arylboronic acid derivatives via activation of the C(aryl)-O bonds. The straightforward synthesis of 1,2-disubstituted arenes was enabled through catalytic ortho C-H bond functionalization directed by the 2-pyridyloxy group followed by substitution of this group with a boryl group. Several control experiments revealed that the presence of a sp2 nitrogen atom at the 2-position of the substrate and the use of a boron-based reagent were crucial for the activation of the relatively inert C(aryl)-O bond of aryl 2-pyridyl ethers.

Direct C-H arylation of thiophenes at low catalyst loading of a phosphine-free bis(alkoxo)palladium complex

An efficient phosphine-free direct C-H arylation of thiophenes at the α-position has been developed at low catalyst loading of bis(alkoxo)palladium complex (Cat.I, 0.1-0.2 mol %). The developed synthetic method can be applied to the synthesis of α-aryl/heteroaryl thiophenes from aryl or heteroaryl bromides in good to excellent yields and is compatible with the substrates bearing electron-donating or electron-withdrawing groups. The reactivities of the 2- and 5-positions of thiophenes are equivalent and not dependent on steric hindrance under optimal conditions. This condition can also be applied to other heterocyclic moieties such as benzothiophene, benzofuran, and pyrrole with high conversion yields.

Scope of the two-step, one-pot palladium-catalyzed borylation/Suzuki cross-coupling reaction utilizing bis-boronic acid

The use of bis-boronic acid for the direct synthesis of boronic acids has greatly facilitated the two-step, one-pot borylation/Suzuki cross-coupling reaction between aryl and heteroaryl halides. With use of Buchwald's second-generation XPhos preformed catalyst, high yields of cross-coupled products were obtained for most substrates. The method also allows an efficient two-step, one-pot synthesis, providing access to three distinct cross-coupled products after column chromatography. The method also provides a rapid and convenient route to teraryl compounds.