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Heterogeneous Catalytic Application of COFs
Published in Atsushi Nagai, Covalent Organic Frameworks, 2019
A Heck cross-coupling reaction is the Pd-catalyzed coupling of aromatic or vinylic halides with an unsaturated olefinic C=C bond, as shown in Scheme 4.1 [12]. The catalytic cycle comprises oxidative addition of aromatic halides, coordination with an olefinic C=C bond, cis-insertion, cis-β-hydride elimination, and reductive elimination. This reaction requires a Pd(0) active species. The advantage of a Heck reaction is its high regio- and stereoselectivity. In contrast, the disadvantage is the costly Pd catalyst. The Sonogashira coupling reaction is the Pd/Cu cocatalyzed coupling of a terminal alkyne with aryl or vinylic halides to a Pd(0) center. Then, a Cu-amine complex mediates the transmetalation reaction. The last step is a reductive elimination, releasing the coupling product and regenerating the Pd(0) catalyst. A Cu-amine complex acts as a cocatalyst to assist in deprotonating the alkyne substrate. Pd-catalyzed Heck reaction and its catalytic cycle [12].
Sustainable Organic Synthesis Using Ionic Liquids
Published in Pedro Lozano, Sustainable Catalysis in Ionic Liquids, 2018
Toshiyuki Itoh, Toshiki Nokami
Ryu and Fukuyama reported the copper-free Sonogashira coupling reaction using an IL solvent system in 2002 (Figure 6.4) [20]. Although the Sonogashira coupling reaction is well respected as the useful means for preparing alkynylated aromatic compounds, the method has a serious weak point from the standpoint of sustainable chemistry that a large excess of copper powder is necessary to achieve the reaction. The authors solved this problem using ILs, they obtained the desired alkynylated benzene derivatives in an excellent yield in the absence of copper metal using the [C4mim][PF6] solvent system (Figure 6.4) [20].
Metal-Catalyzed Condensation Polymerization
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Carbon–carbon bond formation between a terminal alkyne and aryl/vinyl halide in the presence of a Pd(0) and Cu(I) halide as catalyst takes place in Sonogashira coupling, sometimes also referred to as Sonogashira–Hagihara coupling. The catalytic cycle involves reaction of Pd(0) complex with aryl or vinyl halide in oxidative addition to form Pd(II) complex, which undergoes transmetallation with copper acetylide formed in the presence of a base so that the acetylenic ligand is introduced into Pd(II) complex. When applied to conjugated polymer synthesis it gives poly(aryleneethynylene)s, where the aryl group is mainly thiophene or phenyl. Alternating porphyrin-diethynyldithienothiophene copolymers were synthesized by Sonogashira coupling using Pd(0) and CuI.31 Organometallic platinum alkyne units have been incorporated into conjugated polymers because d-orbital of the Pt can overlap with the p-orbital of the alkyne unit, leading to an increase in the π-electron delocalization along the polymer chain, and the probability of efficient charge separation is enhanced due to formation of triplet excited states. Copolymers of platinum diacetylene and electron-poor bithiazole with different numbers of electron-donating thiophene units have been synthesized.32 Multicomponent tandem polymerization involving multicomponent reactions was developed using Sonogashira coupling between alkynes and carbonyl chlorides to give a polymer containing alkynone groups. The alkynyl groups in the polymer backbone underwent hydrothiolation to yield unsaturated polymers with high sulfur content. The polymerization was conducted at 30°C in the presence of [Pd(PPh3)2Cl2], [CuI] and triethylamine (Figure 7.5). Polymers with high molecular weights (38–44 kDa) could be obtained under optimized conditions only since the ratio between [Pd(PPh3)2Cl2] and [CuI] seemed to influence the molecular weight.33
Mesomorphic properties improved via lateral fluorine substituent on benzoxazole-terminated mesogenic compounds
Published in Liquid Crystals, 2020
Mengting Zhang, Yiming Sun, Shenghua Du, Diao Yuan, Pei Chen, Guoqing Liu, Jiazhen Dang, Xinbing Chen, Zhongwei An
A synthetic route is illustrated in Scheme 1 for preparing the benzoxazole-based mesogens nPEFPBx and corresponding schiff base intermediates nPEFPSx, where a six-step synthesis consisted of nucleophilic addition, sonogashira-coupling, nucleophilic substitution and intramolecular cyclisation reactions was performed in turn. Overall yield of the synthetic method is 63%–95%, where the objective compounds are obtained with different terminal moieties (H, CH3 and NO2, coded as nPEFPBH, nPEFPBM and nPEFPBN, respectively) on benzoxazole side and bearing alkoxy chain of 5–12 carbon atoms on the other phenyl side. Incorporation of carbon-carbon triple bond into the molecule is achieved using sonogashira-coupling reaction with a good yield.