501 research outputs found

    Molecular catalysis for precise olefin polymerization and ROP 2015

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    The growth of emerging markets, particularly in the Far East, has fuelled the demand for new plastic materials. This in turn has stimulated both academic and industrial interest in the design of catalyst systems for which new Intellectual Property (IP) can be generated, and in new polymeric materials possessing desirable properties such as biodegradabilit

    Metallocalixarene catalysts: α-olefin polymerization and ROP of cyclic esters

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    This perspective review discusses metallocalix[n]arene complexes that have been employed in either α-olefin polymerization or in the ring opening polymerization (ROP) of cyclic esters over the last 5 years. Synthesis, molecular structure and catalytic potential are discussed. For α-olefin polymerization, systems based on early transition metals in combination with calix[n]arenes (n = 4, 6 or 8), depleted calix[4]arenes or thia/sulfinyl/sulfonyl calix[4]arenes have been reported, and in some cases, are highly active. For the ROP studies, a number of the systems, typically of the early transition metals, only exhibit activity under robust conditions, whereas other systems, for example those of magnesium, demonstrate exceptional activity, immortal behaviour and intriguing stereoselectivity

    Use of metal catalysts bearing Schiff base macrocycles for the ring opening polymerization (ROP) of cyclic esters

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    © 2017 by the authors. Schiff base macrocycles are emerging as useful scaffolds for binding two or more catalytic metals in close proximity. Such coordination chemistry allows for the evaluation of potentially beneficial catalytic cooperative effects. In the field of ring opening polymerization (ROP) of cyclic esters, only a handful of metal systems bound by Schiff base [2 + 2] type macrocycles have been studied. Nevertheless, results to date have, for certain metals, identified some interesting structure activity relationships, whilst for other systems results have revealed particular combinations of metals and macrocycles to be virtually inactive. This perspective review takes a look at two types of recently-reported Schiff base macrocycles that have been employed as pro-ligands in the metal-catalyzed ROP of cyclic esters, specifically ε-caprolactone and rac-lactide

    Reduction of diphenylacetylene using Al powder in the presence of noble metal catalysts in water

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    Diphenylacetylenes can be reduced to the corresponding diphenylethanes (2) in water in excellent yield using Al powder and Pd/C at 60 °C for 3 h in a sealed tube. In addition, the complete reduction of both aromatic rings required 80 °C for 15 h with Al powder in the presence of Pt/C. However, the nature of hydrogenated product formed was found to be strongly influenced by the reaction temperature, time, volume of water and the amount of catalyst being employed

    Direct evidence of a blocking heavy atom effect on the water-assisted fluorescence enhancement detection of Hg²⁺ based on a ratiometric chemosensor

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    At the current stage of chemosensor chemistry, the critical question now is whether the heavy atom effect caused by HTM ions can be blocked or avoided. In the present work, we provide unequivocal evidence to confirm that the heavy atom effect of Hg²⁺ is inhibited by water and other solvent molecules based on results using the chemosensor L. Most importantly, the heavy atom effect and blocking thereof were monitored within the same system by the use of ratiometric fluorescence signal changes of the pyrene motif. These observations not only serve as the foundation for the design of new ‘turn-on’ chemosensors for HTM ions, but also open up new opportunities for the monitoring of organic reactions

    Molybdenum (VI) imido complexes derived from chelating phenols : Synthesis, characterization and ɛ-Caprolactone ROP capability

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    Reaction of the bulky bi-phenols 2,2′-RCH[4,6-(t-Bu)₂C₆H₂OH]₂ (R = Me L¹ᵐᵉH₂, Ph L¹ᵖʰH₂) with the bis(imido) molybdenum(VI) tert-butoxides [Mo(NR¹)(NR²)(Ot-Bu)₂] (R¹ = R² = 2,6-C₆H₃-i-Pr₂; R¹ = t-Bu, R² = C₆F₅) afforded, following the successive removal of tert-butanol, the complexes [Mo(NC₆H₃ᵢ-Pr₂-2,6)₂L¹ᵐᵉ] (1), [Mo(NC₆H₃i-Pr₂-2,6)₂L¹ᵖʰ] (2) and [Mo(Nt-Bu)(μ-NC₆F₅)(L¹ᵐᵉ)]₂ (3). Similar use of the tri-phenol 2,6-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-4-methylphenol (L²H₃) with [Mo(NC₆H₃ᵢ-Pr₂-2,6)₂(Ot-Bu)₂] afforded the oxo-bridged product [Mo(NC₆H₃i-Pr₂-2,6)(NCMe)(μ-O)L2H]₂ (4), whilst use of the tetra-phenols α,α,α′,α′-tetrakis(3,5-di-tert-butyl-2-hydroxyphenyl)-p- or -m-xylene L³ᵖH₄/L³ᵐH₄ led to {[Mo(NC₆H₃ᵢ-Pr₂-2,6)₂]₂(μ-L³ᵖ)} (5) or {[Mo(NC₆H₃ᵢ-Pr₂-2,6)₂]₂(μ-L³ᵐ)} (6), respectively. Similar use of [Mo(NC₆F₅)₂(Ot-Bu)₂] with L³ᵖH₄ afforded, after work-up, the complex {[Mo(NC₆F₅)(Ot-Bu)₂]₂(μ-L³ᵖ)}·6MeCN (7·6MeCN). Molecular structures of 1, 2·CH₂Cl₂, 3, 4·6MeCN, 6·2C₆H₁₄, and 7·6MeCN are reported and these complexes have been screened for their ability to ring open polymerize (ROP) ε-caprolactone; for comparative studies the precursor complex [Mo(NC₆H₃ᵢ-Pr₂-2,6)₂Cl₂(DME)] (DME = 1,2-dimethoxyethane) has also been screened. Results revealed that good activity is only achievable at temperatures of ≥100 °C over periods of 1 h or more. Polymer polydispersities were narrow, but observed molecular weights (Mn) were much lower than calculated values

    Catalysis in Plastics for the 21st Century

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    For this Special Issue, which is part of the Organic and Polymer Chemistry Section, we would like to present the following editorial message [...

    Dialkylaluminium 2-imidazolylphenolates: Synthesis, characterization and ring-opening polymerization behavior towards lactides

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    The stoichiometric reaction of the 2-imidazolylphenols (L1–L9) with the trialkylaluminium reagents AlR₃ (R = Me, Et and iBu), afforded the corresponding dialkylaluminium 2-imidazolylphenolate complexes [R₂Al(L1–L9)] (C1–C11), which were characterized by ¹H/¹³C NMR spectroscopy and by elemental analysis. The molecular structures of the representative complexes C1, C2, C4, C6 and C11 were determined by single-crystal X-Ray diffraction, and revealed a distorted tetrahedral geometry at aluminum. These dialkylaluminium 2-imidazolylphenolates (C1–C11) could efficiently catalyze the ring-opening polymerization of lactides to afford high molecular weight polylactide, both in the presence and absence of BnOH, and as such represent rare examples of the use of bi-dentate ligation at aluminum in such lactide polymerization systems. On the basis of the polymerization results for l-lactide, d-lactide and rac-lactide, the nature of the ligands and the aluminum bound alkyls were found to significantly affect the catalytic activity as well as the properties of the resultant polylactides

    Dichloropalladium complexes ligated by 4,5-bis(arylimino)pyrenylidenes: Synthesis, characterization, and catalytic behavior towards Heck-reaction

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    A series of 4,5-bis(arylimino)pyrenylidenylpalladium(II) chloride complexes (C1–C4) were synthesized and characterized by FT-IR and NMR spectroscopy, elemental analysis as well as by single crystal X-ray diffraction for the representative complexes C1 and C3, which revealed a square planar geometry at the palladium center. All palladium complexes exhibited high activity for the Heck cross-coupling reaction, which were effective when conducted in various solvents. Furthermore, the in-situ mixture of palladium dichloride and the ligand (L1) provided an effective catalytic system for the Heck-reaction

    Reduction of phenylacetylenes using Raney Ni-Al alloy, Al powder in the presence of noble metal catalysts in water

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    Dedicated to Prof. Kenneth Laali on the occasion of his 66th birthday Received mm-dd-yyyy Accepted mm-dd-yyyy Published on line mm-dd-yyyy Dates to be inserted by editorial office Abstract The chemoselective reduction is based on the reaction of Raney Ni–Al alloy with Al powder in water which produces in situ hydrogen to utilize the hydrogenation of the targeted functional groups. Raney Ni–Al alloy with Al powder can reduce phenylacetylenes to the corresponding ethylbenzene (3) in water in excellent yield at 120 °C for 6 h in a sealed tube. In addition, the complete reduction of aromatic ring to ethylcyclohexane (4) required 60 °C for 12 h with Raney Ni–Al alloy, Al powder in the presence of Pt/C. The appropriate selection of the reaction conditions allowed the selective preparation of ethylbenzene as well as ethylcyclohexane from phenylacetylene. 1a : R = H ,1b : R = CH 3 , 1c : R = OCH 3 ,1d : R = C(CH 3)
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