- Revised interpretation for N-cyclohexylmaleimide polymerization in the presence of an optically active cobalt(II) complex: Polymerization mediated by anionic species formed through monomer-Co(II) complex-O2 interaction
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This report revises the interpretations of the reaction mechanism in the polymerization of N-cyclohexylmaleimide (CHMI) in the presence of α,α′-azobis(isobutyronitrile) (AIBN) and (R,R)-N,N′-bis (3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminatocobalt(II) (1) [Macromolecules 2000, 33, 1489]. Although we proposed that the reaction was mediated by a radical species and that the interaction between growing polymer radical with 1 induced chirality in the main chain in the earlier report, the results in the present article indicate that the systems reported in the earlier report were most probably contaminated by a small amount of air and that the polymerization proceeds via an anionic species that was produced by the interaction of the monomer, 1, and O2. The effect of the contamination was overlooked due to the lack of a control experiment using O2 in the absence of AIBN in our earlier study. In this work, the polymerization of CHMI was performed in the presence and absence of 1, O2, αα′-azobis-(isobutyronitrile) (AIBN), and protonic additives (methanol and acetic acid). The results indicated the following: (1) the polymerization in a tetrahydrofuran-pyridine mixture occurs in the presence of 1 and a small amount of oxygen even without using AIBN; (2) the reaction does not take place in the presence of 1 alone or in the presence of 1 and AIBN under a strictly controlled N2 atmosphere; (3) a small amount of AcOH completely inhibits the polymerization with 1 and O2 while 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) does not kill the reaction; and (4) the polymer was likely to possess -OH or -OOH groups at the chain terminals (a telechelic structure). On the basis of these observations, we withdraw our earlier, wrong interpretation that the polymerization is mediated by a radical species and conclude that the polymerization and the observed stereochemistry are effected by an anionic mechanism.
- Nakano, Tamaki,Yade, Tohru,Okamoto, Yoshio
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- Vanadyl salen complexes covalently anchored to single-wall carbon nanotubes as heterogeneous catalysts for the cyanosilylation of aldehydes
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Single-wall carbon nanotubes (SWNT) have a special structure and morphology consisting of long tubes (μm scale) of less than 2 nm in diameter. We have taken advantage of this geometric feature of SWNT to use them as supports for the preparation of a heter
- Baleizao, Carlos,Gigante, Barbara,Garcia, Hermenegildo,Corma, Avelino
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- Olefin polymerization promoted by a stereorigid bridged diiminobis(phenolate) zirconium complex
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Two diiminobis(phenolate) zirconium complexes [C6H 10-{N=CH-(3,5-tBu2C6H 2-2-O)-kO}2]-ZrX2 [X = CH2Ph (1), Cl (3)] have been synthesized and characterized by NMR solution spectroscopy. Complex 1 was found conformationally stable and isolated in the octahedral C2 symmetric form. When exposed to the light in hydrocarbon solution, 1 readily undergoes to 1,2 benzyl migration converting the diiminobis(phenolate) ligand in the corresponding benzylamidoiminobis(phenolate) . Complex 3 is more thermally stable and found in slow equilibrium between the cis-α and cis-β forms with the former largely prevalent (9:1 molar ratio). The rate of interconversion between the two diastereoisomers was estimated in 3.6 × 10-1 s-1 by NMR exchange spectroscopy (EXSY). 1 and 3 are active ethylene polymerization catalysts after reaction with MAO or [CPh3] [B(C6F5) 4]/Al-iBu3. The polydispersity index of polyethylene produced by 1 is about 2, suggesting the presence of a single active species and that the symmetry of the precatalyst is retained during polymerization. The catalyst is stable over 3 h, and the polymerization activity linearly increases with time. Copolymers of ethylene with propylene or 1-hexene were synthesized, and the 13C NMR analysis of their microstructure suggested the highly regioregular 1,2 insertion of the 1-olefin and the inability of the ligand environment to express stereochemical control during monomer insertion.
- Cuomo, Cinzia,Strianese, Maria,Cuenca, Tomas,Sanz, Martial,Grassi, Alfonso
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- Salen manganese (III) complexes as catalysts for R-(+)-limonene oxidation
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The epoxidation of R-(+)-limonene using in situ generated dimethyldioxirane (DMD) as the oxidizing agent and four Jacobsen-type catalysts ((R,R)-Jacobsen, (S,S)-Jacobsen, racemic Jacobsen and achiral Jacobsen) was examined. The effect of the amount of KHSO5 and acetone in the catalyzed and un-catalyzed reaction was also assessed. The main reaction products were diepoxide and endocyclic monoepoxide. In the absence of catalyst, the amount of KHSO5 did not significantly influence conversion and selectivity. The catalyst can be segregated to a different phase and separated from the reaction media when the amount of KHSO5 is above the stoichiometric ratio, R-(+)-limonene/KHSO5 = 0.5 mmol/mmol, and acetone/mmol R-(+)-limonene = 2 mL/mmol. However, when the amount of KHSO5 is below the stoichiometric ratio (R-(+)-limonene/KHSO5 = 1.5 mmol/mmol) the catalyst is difficult to separate. Under the reaction conditions of this study, when the catalyst is segregated, no effect of the catalyst chiral center, (R,R)-Jacobsen or (S,S)-Jacobsen, was found on conversion and selectivity. Additionally, the (R,R)-Jacobsen's catalyst proved to be very stable to oxidative degradation.
- Cubillos, Jairo,Vásquez, Santiago,Montes de Correa, Consuelo
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experimental part
p. 57 - 65
(2010/10/02)
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- Diastereomerically-specific zirconium complexes of chiral salan ligands: Isospecific polymerization of 1-hexene and 4-methyl-1-pentene and cyclopolymerization of 1,5-hexadiene
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Chiral Salan ligands were found to wrap in a highly diastereoselective manner around zirconium leading to C2-symmetric complexes of predetermined chirality at the metal. These complexes led to active polymerization of higher olefins, their activity and is
- Yeori, Adi,Goldberg, Israel,Shuster, Michael,Kol, Moshe
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p. 13062 - 13063
(2008/02/08)
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- A practical one-pot synthesis of enantiopure unsymmetrical salen ligands
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A practical, one-pot synthesis of enantiopure unsymmetrical salen ligands is described, using a 1:1:1 molar ratio of a chiral diamine and two different salicylaldehydes. The new synthetic protocol can be readily performed in good yields (60-85%) on a mult
- Holbach, Michael,Zheng, Xiaolai,Burd, Caroline,Jones, Christopher W.,Weck, Marcus
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p. 2903 - 2906
(2007/10/03)
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- Titanium-salen complexes as initiators for the ring opening polymerisation of rac-lactide
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A family of bis(iso-propoxide) titanium(iv) complexes supported by tetradentate Schiff base (salen) ligands has been synthesised and characterised, including a structural determination of N,N′-bis(6′-methylenimino- 2′,4′-di-tert-butylphenoxy)cyclohexyl-(1
- Gregson, Charlotte K.A.,Blackmore, Ian J.,Gibson, Vernon C.,Long, Nicholas J.,Marshall, Edward L.,White, Andrew J.P.
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p. 3134 - 3140
(2007/10/03)
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- METHODS FOR PRODUCING NANOPARTICULATE METAL COMPLEXES AND ALTERING NANOPARTICLE MORPHOLOGY
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Nanoparticulate metal complexes, such as those involving ruthenium, iron, cobalt, and nickel salens, are formed using precipitation with compressed antisolvent technology. The nanoparticle morphology may be altered by altering the planarity of molecular structure of the metal complex starting material.
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Page/Page column 10
(2010/11/25)
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- A high yielding one-pot method for the preparation of salen ligands
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Phenols are converted to salicylaldehydes with paraformaldehyde, MgCl 2-Et3N in THF, and subsequently treated with (+)-(R,R)-1,2-diammoniumcyclohexane mono-(+)-tartrate salt affording the corresponding salen ligands in high yields. The reactions are conveniently carried out as a one-pot procedure.
- Hansen, Trond Vidar,Skatteb?l, Lars
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p. 3829 - 3830
(2007/10/03)
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- Chiral vanadyl salen complex anchored on supports as recoverable catalysts for the enantioselective cyanosilylation of aldehydes. Comparison among silica, single wall carbon nanotube, activated carbon and imidazolium ion as support
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The activity and enantiomeric excess (ee) (in some cases >85%) obtained for the asymmetric addition of trimethylsilyl cyanide to aldehydes using different heterogeneous chiral catalysts are compared. A library of recoverable catalysts was developed by imm
- Baleiz?o, Carlos,Gigante, Bárbara,García, Hermenegildo,Corma, Avelino
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p. 10461 - 10468
(2007/10/03)
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- Asymmetric cycloaddition reactions
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The present invention relates to a process for stereoselective cycloaddition reactions which generally comprises a cycloaddition reaction between a pair of substrates, each either chiral or prochiral, that contain reactive π-systems, in the presence of a non-racemic chiral catalyst, to produce a stereoisomerically enriched product. The present invention also relates to novel asymmetric catalyst complexes comprising a metal and an asymmetric tridentate ligand.
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(2010/01/31)
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- Catalytic asymmetric synthesis of O-acetylcyanohydrins from potassium cyanide, acetic anhydride, and aldehydes, promoted by chiral salen complexes of titanium(IV) and vanadium(V)
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The utility of the chiral [Ti(μ-O)(salen)]2 complexes (R)- and (S)-1 (H2salen was prepared from (R,R)- or (S,S)-cyclohexane-1,2-diamine and 3.5-di(tert-butyl)-2-hydroxybenzaldehyde) as catalysts for the asymmetric addition of KCN and Ac2O to aldehydes to produce O-acetylcyanohydrins was investigated. It was shown that the complexes were active at a substrate/catalyst ratio of 100:1 and produced the O-protected cyanohydrins with ee in the range of 60-92% at -40°. Other complexes, [Ti2(AcO)2(μ-O)(salen)2] ((R)-4) and [Ti(CF3COO)2(salen)] ((R)-5), were prepared from (R)-1 by treatment with different amounts of Ac2O and (CF3CO)2O, and their catalytic activities were tested under the same conditions. The efficiency of (R)-4 was found to be even greater than that of (R)-1, whereas (R)-5 was inactive. The synthesis of the corresponding salen complexes of VIV and VV,[V(O)(salen)] ((R)-2) and [V(O)(salen)(H2O)][S(O)3OEt] ((R)-3), was elaborated. and their X-ray crystal structures were determined. The efficiency of (R)-3 was sufficient to produce O-acetyl derivatives of aromatic cyanohydrins with ee in the range of 80-91% at -40°.
- Belokon, Yuri N.,Carta, Paola,Gutnov, Andrey V.,Maleev, Viktor,Moskalenko, Margarita A.,Yashkina, Lidia V.,Ikonnikov, Nicolai S.,Voskoboev, Nikolai V.,Khrustalev, Viktor N.,North, Michael
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p. 3301 - 3312
(2007/10/03)
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- Substituted salen-Ru(II) complexes as catalysts in the asymmetric cyclopropanation of styrene by ethyl diazoacetate: The influence of substituents and achiral additives on activity and enantioselectivity
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A series of alkyl-, halogen- and nitro-substituted salen ligands, 1, have been employed in the asymmetric cyclopropanation of styrene with ethyl diazoacetate by its ruthenium(II) complex with [RuCl2(p-cymene)]2 or RuCl2(PP
- Yao, Xiaoquan,Qiu, Min,Lue, Weiran,Chen, Huilin,Zheng, Zhuo
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p. 197 - 204
(2007/10/03)
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