C-H Activation Reactions Mediated by Pd Based Heterogeneous Catalysts : Regioselective Formation of C-O and C-X Bonds (original) (raw)
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European Journal of Inorganic Chemistry, 2018
An N-heterocyclic carbene palladium(II) complex containing two anthracene side arms was immobilized on the surface of reduced graphene oxide (rGO) by π-stacking. The activity of the homogeneous analogue and the supported complex in undirected C-H acetoxylation reaction of arenes was studied. The results show that the catalytic efficiency in acetoxylation of benzene is improved in the immobilized materials compared to the homogeneous analogue. According to XPS analysis, the immobilized catalyst maintains the original oxidation state of Pd(II) after the catalytic reaction.
Chemistry (Weinheim an der Bergstrasse, Germany), 2017
Heterogeneous catalysts for selective oxidation of C-H bonds were synthesized by co-polymerization of new N-heterocyclic carbene-palladium(II) (NHC-Pd(II) ) monomers with divinylbenzene. The polymer-supported NHC-Pd(II) -catalysed undirected C-H acetoxylation of simple and methylated arenes as well as polyarenes, with similar or superior efficiency compared to their homogeneous analogues. In particular, the regioselectivity has been improved in the acetoxylation of biphenyl and naphthalene compared to the best homogeneous catalysts. The new polymer-supported catalysts maintain the original oxidation state of Pd(II) after repeated catalytic reactions, and exhibit no significant leaching of palladium. In addition, the new catalysts have been successfully recovered and reused without loss of activity over several cycles of reactions.
Catalysis Letters, 2020
A new and efficient N-heterocyclic carbene (NHC)-palladium(II) complex immobilized on graphene oxide (NHC-Pd@ GO) has been successfully designed and synthesized. The prepared NHC-Pd@GO heterogeneous catalyst was fully characterized using a combination of fourier transform infrared spectroscopy (FTIR), inductively coupled plasma-optical emission spectroscopy (ICP-OES), energy-dispersive X-ray spectroscopy (EDS), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA) and Brunauer-Emmett-Teller surface area analysis (BET). This new air-and moisture-stable NHC-Pd@GO heterogeneous catalytic system was found to be a highly active catalyst in the Suzuki-Miyaura cross-coupling between phenylboronic acid and various aryl halides (bromides/chlorides/iodides) and in the reduction of nitroarenes. These organic transformations were best performed in an aqueous ethanol and aqueous methanol solvent system respectively with low catalyst loading under mild reaction conditions. Furthermore, NHC-Pd@GO heterogeneous catalyst could be recovered easily and reused at least eleven times in Suzuki-Miyaura cross-coupling and nine times in reduction of nitroarenes without any considerable loss of its catalytic activity. The stability and good selectivity of the NHC-Pd@GO heterogeneous catalyst in recycling experiments signify that it could be useful for practical application in various organic transformations. Vishal Kandathil and Bhakti Kulkarni have contributed equally.
Organic letters, 2015
N-Chelation-directed C-H activation reactions that utilize the Pd(II)/Pd(IV) catalytic cycle have been previously reported. To date, these reactions employ only homogeneous palladium catalysts. The first use of a solid-supported Pd(II) catalyst [Pd(II) nanoparticles on multiwalled carbon nanotubes, Pd(II)/MWCNT] to carry out N-chelation-directed C-H to C-O, C-Cl, and C-Br transformations is reported. The results presented demonstrate that the solid-supported Pd(II)/MWCNT catalyst can effectively catalyze C-H activation reactions using the Pd(II)/Pd(IV) catalytic cycle.
Chemistry – A European Journal, 2019
A new generation of N‐heterocyclic carbene palladium(II) complexes containing vinyl groups in different positions in the backbone of the N‐heterocycle have been developed. The fully characterised monomers were copolymerised with divinylbenzene to fabricate robust polymer supported NHC‐PdII complexes and these polymers were applied as heterogeneous catalysts in directed C−H halogenation of arenes with a pyridine‐type directing group. The catalysts demonstrated medium‐high catalytic activity with up to 90 % conversion and 100 % selectivity in chlorination. They are heterogeneous and recyclable (at least six times) with no significant leaching of palladium in batch mode catalysis. The best catalyst was also applied under continuous flow conditions where it disclosed an exceptional activity (90 % conversion) and 100 % selectivity for the mono‐halogenated product for at least six days, with no leaching of palladium, no loss of activity and an ability to maintain the original oxidation st...
Advanced Synthesis & Catalysis, 2009
While developing novel catalysts for carbon-carbon or carbon-heteroatom coupling (nitrogen, oxygen, or fluorine), we were able to introduce tridentate N-heterocyclic carbene (NHC)-amidate-alkoxide palladium(II) complexes. In aqueous solution, these NHC-Pd(II) complexes showed high ability for C À H activation of various hydrocarbons (cyclohexane, cyclopentane, dimethyl ether, tetrahydrofuran, acetone, and toluene) under mild conditions.
The use of functional properties of native cyclodextrins in palladium nanoparticle–b-cyclodextrin– graphene nanosheet (Pd@CD–GNS) catalyzed carbon–carbon (C–C) coupling reactions have been investigated under green reaction conditions. The supramolecular catalyst was prepared by deposition of Pd nanoparticles (Pd NPs) on CD–GNS using ethanol as the greener solvent and in situ reducing agent. The catalyst was characterised by FTIR, XRD, RAMAN, UV-Vis spectroscopy, TEM, SAED, XPS and ICP-AES. The catalytic activity of these catalysts is investigated in C–C coupling reactions such as Suzuki–Miyaura and Heck–Mizoroki reactions of aryl bromides and aryl chlorides containing functional groups under green reaction conditions i.e. in water, under phosphine free and aerobic conditions. This catalyst afforded excellent selectivities for the products in good to excellent yields under low Pd loadings (0.2–0.05 mol%), while ensuring the recovery and reusability of the catalysts. The reused catalyst was characterized by FTIR, TEM, XPS and ICP-AES. The CD supramolecular mediators loaded on GNS act as stabilising agents for the Pd NPs. The excellent catalytic activity of this system was attributed to the presence of CDs, excellent dispersibility in water, hydrophobic nature of the GNS support for the accumulation of organic substrates in water, " Breslow effect " , the presence of PTC to overcome the mass transfer limitation onto the surface of GNS and formation of ternary CD/substrate/additive complexes on the Pd–GNS surface.
RSC Adv., 2014
The use of functional properties of native cyclodextrins in palladium nanoparticle-b-cyclodextringraphene nanosheet (Pd@CD-GNS) catalyzed carbon-carbon (C-C) coupling reactions have been investigated under green reaction conditions. The supramolecular catalyst was prepared by deposition of Pd nanoparticles (Pd NPs) on CD-GNS using ethanol as the greener solvent and in situ reducing agent. The catalyst was characterised by FTIR, XRD, RAMAN, UV-Vis spectroscopy, TEM, SAED, XPS and ICP-AES. The catalytic activity of these catalysts is investigated in C-C coupling reactions such as Suzuki-Miyaura and Heck-Mizoroki reactions of aryl bromides and aryl chlorides containing functional groups under green reaction conditions i.e. in water, under phosphine free and aerobic conditions. This catalyst afforded excellent selectivities for the products in good to excellent yields under low Pd loadings (0.2-0.05 mol%), while ensuring the recovery and reusability of the catalysts. The reused catalyst was characterized by FTIR, TEM, XPS and ICP-AES. The CD supramolecular mediators loaded on GNS act as stabilising agents for the Pd NPs. The excellent catalytic activity of this system was attributed to the presence of CDs, excellent dispersibility in water, hydrophobic nature of the GNS support for the accumulation of organic substrates in water, "Breslow effect", the presence of PTC to overcome the mass transfer limitation onto the surface of GNS and formation of ternary CD/substrate/additive complexes on the Pd-GNS surface.
Dissimilar catalytic behavior of molecular or colloidal palladium systems with a new NHC ligand
Dalton transactions (Cambridge, England : 2003), 2017
In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a better stabilizing effect than its pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields aggregated ones. Additionally, molecular Pd(ii) coordination compounds of L and HLCl were synthesized and characterized to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further tested in catalytic C-C coupling processes. Three different types of reactions have been observed depending on the catalytic system: (i) the Suzuki-Miyaura reaction takes place with Pd molecular complexes; (ii) a secondary reaction, the dehalogenation of the substrate, ...