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Alkyl Aryl Ether

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Narayanaswamy Jayaraman – One of the best experts on this subject based on the ideXlab platform.

  • Multicolor Reversible Thermochromic Properties of Gallic Acid-Cored Polydiacetylenes Appended with Poly(Alkyl Aryl Ether) Dendrons
    Macromolecular Chemistry and Physics, 2016
    Co-Authors: Yashapal Singh, Narayanaswamy Jayaraman

    Abstract:

    This study describes synthesis of zero and first generation poly(Alkyl Aryl Ether) dendrons onto a gallic acid core, tEthered with three diacetylene moieties and their photopolymerization. Resulting polydiacetylenes are studied by (i) UV-vis, IR, Raman spectroscopies, and chromoisomerism by naked eye visualization; (ii) their stabilities by differential scanning calorimetry, thermogravimetric method, and (iii) structure, morphology, and emission behavior by scanning electron, transmission electron, atomic force microscopies, powder X-ray diffraction method, and fluorescence spectroscopy. The dendron-appended polydiacetylenes show robust nanoribbon morphologies and exhibit multicolor, reversible chromoisomerism at temperatures up to 300 degrees C. Reversibility of amide bond IR frequencies, as a function of temperature, shows the polymer stabilities, in addition to the results of thermal and powder X-ray diffraction studies. Temperature annealed polymers having red color phase exhibit emission maxima at 572 and 633 nm, that shift to higher wavelength upon heating and revert to the initial emission wavelengths on cooling.

  • Dynamic internal cavities of dendrimers as constrained media. A study of photochemical isomerizations of stilbene and azobenzene using poly(Alkyl Aryl Ether) dendrimers.
    The Journal of organic chemistry, 2012
    Co-Authors: Baskar Natarajan, Shipra Gupta, Nithyanandhan Jayaraj, Vaidhyanathan Ramamurthy, Narayanaswamy Jayaraman

    Abstract:

    Dendritic microenvironments defined by dynamic internal cavities of a dendrimer were probed through geometric isomerization of stilbene and azobenzene. A third-generation poly(Alkyl Aryl Ether) dendrimer with hydrophilic exterior and hydrophobic interior was used as a reaction cavity in aqueous medium. The dynamic inner cavity sizes were varied by utilizing Alkyl linkers that connect the branch junctures from ethyl to n-pentyl moiety (C2G3–C5G3). Dendrimers constituted with n-pentyl linker were found to afford higher solubilitiesof stilbene and azobenzene. Direct irradiation of trans-stilbene showed that C5G3 and C4G3 dendrimers afforded considerable phenanthrene formation, in addition to cis-stilbene, whereas C3G3 and C2G3 gave only cis-stilbene. An electron-transfer sensitized trans–cis isomerization, using cresyl violet perchlorate as the sensitizer, also led to similar results. Thermal isomerization of cis-azobenzene to trans-azobenzene within dendritic microenvironments revealed that the activation e…

  • Interfacial Regions Governing Internal Cavities of Dendrimers. Studies of Poly(Alkyl Aryl Ether) Dendrimers Constituted with Linkers of Varying Alkyl Chain Length
    The Journal of organic chemistry, 2011
    Co-Authors: Baskar Natarajan, Shipra Gupta, Vaidhyanathan Ramamurthy, Narayanaswamy Jayaraman

    Abstract:

    This report deals with a study of the properties of internal cavities of dendritic macromolecules that are capable of encapsulating and mediating photoreactions of guest molecules. The internal cavity structures of dendrimers are determined by the interfacial regions between the aqueous exterior and hydrocarbon like interior constituted by the linkers that connect symmetrically sited branch points constituting the dendrimer and head groups that cap the dendrimers. Phloroglucinol-based poly(Alkyl Aryl Ether) dendrimers constituted with a homologous series of Alkyl linkers were undertaken for the current study. Twelve dendrimers within first, second, and third generations, having ethyl, n-propyl, n-butyl, and n-pentyl groups as the linkers and hydroxyl groups at peripheries in each generation, were synthesized. Encapsulation of pyrene and coumarins by aqueous basic solutions of dendrimers were monitored by UV-vis and fluorescence spectroscopies, which showed that a lower generation dendrimer with an optimal Alkyl linker presented better encapsulation abilities than a higher generation dendrimer. Norrish type I photoreaction of dibenzyl ketone was carried out within the above series of dendrimers to probe their abilities to hold guests and reactive intermediate radical pairs within themselves. The extent of cage effect from the series of third generation dendrimers was observed to be higher with dendrimers having an n-pentyl group as the linker.

Gregg T. Beckham – One of the best experts on this subject based on the ideXlab platform.

  • Mechanistic Study of a Ru-Xantphos Catalyst for Tandem
    Alcohol Dehydrogenation and Reductive ArylEther Cleavage
    , 2016
    Co-Authors: Stephen C. Chmely, Seonah Kim, Peter N. Ciesielski, Robert S. Paton, Gonzalo Jiménez-osés, Gregg T. Beckham

    Abstract:

    We employ density functional theory
    (DFT) calculations and kinetics measurements to understand the mechanism
    of a xantphos-containing molecular ruthenium catalyst acting on an
    Alkyl Aryl Ether linkage similar to that found in lignin to produce
    acetophenone and phenol. The most favorable reaction pathway suggested
    from DFT is compared to kinetics measurements, and good agreement
    is found between the predicted and the measured activation barriers.
    The DFT calculations reveal several interesting features, including
    an unusual 5-membered transition state structure for oxidative insertion
    in contrast to the typically proposed 3-membered transition state,
    a preference for an O-bound over a C-bound Ru–enolate, and
    a significant kinetic preference for the order of product release
    from the catalyst. The experimental measurements confirm that the
    reaction proceeds via a free ketone intermediate, but also suggest
    that the conversion of the intermediate ketone to acetophenone and
    phenol does not necessarily require ketone dissociation from the catalyst.
    Overall, this work elucidates the kinetically and thermodynamically
    preferred reaction pathways for tandem alcohol dehydrogenation and
    reductive Ether bond cleavage by the ruthenium-xantphos catalyst

  • Mechanistic Study of a Ru-Xantphos Catalyst for Tandem Alcohol Dehydrogenation and Reductive ArylEther Cleavage
    ACS Catalysis, 2013
    Co-Authors: Stephen C. Chmely, Seonah Kim, Peter N. Ciesielski, Gonzalo Jiménez-osés, Robert S. Paton, Gregg T. Beckham

    Abstract:

    We employ density functional theory (DFT) calculations and kinetics measurements to understand the mechanism of a xantphos-containing molecular ruthenium catalyst acting on an Alkyl Aryl Ether linkage similar to that found in lignin to produce acetophenone and phenol. The most favorable reaction pathway suggested from DFT is compared to kinetics measurements, and good agreement is found between the predicted and the measured activation barriers. The DFT calculations reveal several interesting features, including an unusual 5-membered transition state structure for oxidative insertion in contrast to the typically proposed 3-membered transition state, a preference for an O-bound over a C-bound Ru–enolate, and a significant kinetic preference for the order of product release from the catalyst. The experimental measurements confirm that the reaction proceeds via a free ketone intermediate, but also suggest that the conversion of the intermediate ketone to acetophenone and phenol does not necessarily require k…

Jorge Rencoret – One of the best experts on this subject based on the ideXlab platform.

  • CHANGES IN THE COMPOSITION AND STRUCTURE OF LIGNIN DURING THE GROWTH OF A EUCALYPTUS GLOBULUS CLONE
    , 2016
    Co-Authors: José C. Del Río, Jorge Rencoret, Ana Gutierrez, Lidia Nieto, J. Jiménez-barbero, Angel T Martinez

    Abstract:

    The content, composition and structure of lignin in a Eucalyptus globulus clone were studied at different growth stages (1 month, 18 months, and 9 years). The lignin content in the eucalypt woods increase during growth and the changes in its composition and structure were studied “in situ ” by Py-GC/MS and 2D-NMR of the whole wood. In addition, milled-wood lignins were isolated and analyzed by Py-GC/MS, 2D-NMR and thioacidolysis followed by Raney Nickel desulfurization. The data obtained indicated that H and G lignin units are deposited first at the earlier stages, whereas the woods are enriched in S lignin during late lignification. The main linkages present were β-O-4 ' AlkylAryl Ether and resinol β-β', whereas other substructures (such as β-5 ' phenylcoumaran and β-1 ' spirodienones) were present in lower abundances during all stages of lignification. The differences in monomer deposition affect the distribution of the different linkages during lignification. I

  • Lignin–carbohydrate complexes from sisal (Agave sisalana) and abaca (Musa textilis): chemical composition and structural modifications during the isolation process
    Planta, 2016
    Co-Authors: Pepijn Prinsen, Ana Gutierrez, Edith M Cadena, Angel T Martinez, Jorge Rencoret

    Abstract:

    Main conclusion Two types of lignins occurred in different lignin–carbohydrate fractions, a lignin enriched in syringyl units, less condensed, preferentially associated with xylans, and a lignin with more guaiacyl units, more condensed, associated with glucans. Lignin–carbohydrate complexes (LCC) were isolated from the fibers of sisal ( Agave sisalana ) and abaca ( Musa textilis ) according to a plant biomass fractionation procedure recently developed and which was termed as “universally” applicable to any type of lignocellulosic material. Two LCC fractions, namely glucan–lignin (GL) and xylan–lignin (XL), were isolated and differed in the content and composition of carbohydrates and lignin. In both cases, GL fractions were enriched in glucans and comparatively depleted in lignin, whereas XL fractions were depleted in glucans, but enriched in xylans and lignin. Analysis by two-dimensional Nuclear Magnetic Resonance (2D-NMR) and Derivatization Followed by Reductive Cleavage (DFRC) indicated that the XL fractions were enriched in syringyl (S)-lignin units and β- O -4′ AlkylAryl Ether linkages, whereas GL fractions have more guaiacyl (G)-lignin units and less β- O -4′ AlkylAryl Ether linkages per lignin unit. The data suggest that the structural characteristics of the lignin polymers are not homogeneously distributed within the same plant and that two different lignin polymers with different composition and structure might be present. The analyses also suggested that acetates from hemicelluloses and the acyl groups (acetates and p -coumarates) attached to the γ-OH of the lignin side chains were extensively hydrolyzed and removed during the LCC fractionation process. Therefore, caution must be paid when using this fractionation approach for the structural characterization of plants with acylated hemicelluloses and lignins. Finally, several chemical linkages (phenylglycosides and benzyl Ethers) could be observed to occur between lignin and xylans in these plants.

  • Ferulates and lignin structural composition in cork
    Holzforschung, 2016
    Co-Authors: António Velez Marques, Jorge Rencoret, Ana Gutierrez, Helena Pereira

    Abstract:

    The structure of lignin and suberin, and ferulic acid (FA) content in cork from Quercus suber L. were studied. Extractive-free cork (Cork), suberin, desuberized cork (Corkₛₐₚ), and milled-cork lignins (MCL) from Cork and Corkₛₐₚ were isolated. Suberin composition was determined by GC-MS/FID, whereas the polymers structure in Cork, Corksap, and MCL was studied by Py-TMAH and 2D-HSQC-NMR. Suberin contained 94.4% of aliphatics and 3.2% of phenolics, with 90% of ω-hydroxyacids and α,ω-diacids. FA represented 2.7% of the suberin monomers, overwhelmingly esterified to the cork matrix. Py-TMAH revealed significant FA amounts in all samples, with about 3% and 6% in cork and cork lignins, respectively. Py-TMAH and 2D-HSQC-NMR demonstrated that cork lignin is a G-lignin (>96% G units), with a structure dominated by β–O–4′ AlkylAryl Ether linkages (80% and 77% of all linkages in MCL and MCLₛₐₚ, respectively), followed by phenylcoumarans (18% and 20% in MCL and MCLₛₐₚ, respectively), and smaller amounts of resinols (ca. 2%) and dibenzodioxocins (1%). HSQC also revealed that cork lignin is heavily acylated (ca. 50%) exclusively at the side-chain γ-position. Ferulates possibly have an important function in the chemical assembly of cork cell walls with a cross-linking role between suberin, lignin and carbohydrates.