1,720,974 research outputs found
Mechanistic studies on the selective oxidative carbonylation of MeOH to dimethyl oxalate catalyzed by [Pd(COOMe)n(TsO)2−n(PPh3)2] (n = 0, 1, 2) using p-benzoquinone as a stoichiometric oxidant
No full textOxidative carbonylation of alkanols catalyzed by Pd(II)-phosphine complexes
No full textThe complexes trans–[Pd(COOR)(H2O)(PPh3)2](TsO) (R = Me, Et, nPr, iPr, nBu, iBu, secBu) and trans–[Pd(COOR)nX2–n(PPh3)] (R = Me, n = 1, 2; X = ONO2, ONO, OTs, Br. R = iPr, n = 1, X = Cl, Br) have been synthesised and characterised. Most of them have been used as catalyst precursors for the oxidative carbonylation of MeOH, selective to dimethyloxalate, using p–benzoquinone (BQ) as an oxidant. BQ changes properties of the reaction centre. When using iPrOH in place of MeOH, high activity and selectivity toward diisopropyloxalate have been achieved using trans–[PdX2(PAr3)2] with strongly coordinating X (Br), in presence of a base (NEt3 or PAr3–BQ adduct) and of an excess of LiBr. The slow step of the catalysis (RDS) might be the reoxidation of Pd(0), which is formed in the product–forming step. Using [PdX2(P-P)] the best performance has been achieved with weakly coordinating X (TsO) and using P-P of relatively wide bite angle but with cis–geometry. The RDS is related to the nature of P-P.Sono stati sintetizzati e caratterizzati i complessi trans–[Pd(COOR)(H2O)(PPh3)2](TsO) (R = Me, Et, nPr, iPr, nBu, iBu, secBu) e trans–[Pd(COOR)nX2–n(PPh3)] (R = Me, n = 1, 2; X = ONO2, ONO, OTs, Br. R = iPr, n = 1, X = Cl, Br). Molti sono stati utilizzati come precursori catalitici nella carbonilazione ossidativa di MeOH, selettiva a dimetiloxalato, impiegando p–benzochinone (BQ) come ossidante. Il BQ modifica le proprietà del centro di reazione. Con iPrOH al posto di MeOH si ottengono elevate attività e selettività a diisopropyloxalato utilizzando trans–[PdX2(PAr3)2] con X fortemente coordinanti (Br), in presenza di base (NEt3 o PAr3–BQ) e di LiBr in eccesso. Lo stadio lento della catalisi (RDS) potrebbe essere la riossidazione del Pd(0) che si forma nello stadio di formazione dell’ossalato. Con [PdX2(P-P)] le migliori prestazioni sono state ottenute con X debolmente coordinanti (TsO) e con P-P ad angolo di morso relativamente ampio ma di geometria cis. L’ RDS dipende da P-P
Vanadium catalyzed aerobic carbon-carbon cleavage
No full textIn recent years, the use of vanadium complexes as catalysts and air or oxygen as oxidants has emerged as a new method for the effective oxidative carbon-carbon bond cleavage in a series of substrates, including 1,2-diols and α-hydroxy ethers. In addition to the advantageous use as a synthetic tool, vanadium catalyzed aerobic C. C bond cleavage becomes even more appealing if applied to the degradation of lignocellulosic biomass and in particular in the oxidative depolymerization/degradation of lignin or cellulose. In this review, this chemistry, the most recent advances and the role played by homogeneous vanadium complexes are presented and discussed.SCOPUS: re.jinfo:eu-repo/semantics/publishe
High-Temperature Batch and Continuous-Flow Transesterification of Alkyl and Enol Esters with Glycerol and Its Acetal Derivatives
No full textA new procedure for the transesterification of alkyl acetates and formates with model glycerol acetals (GAs: Solketal and glycerol formal) was explored in the absence of any catalysts at 180-275 °C. Highly selective transformations occurred in both batch and continuous-flow (CF) modes; particularly, the enol derivative isopropenyl acetate (iPAc) was the best performing reactant by which quantitative acetylation reactions were achieved with yields on GAs acetates >95%. An excess acylating agent was necessary (2-20 molar equivs), but the unconverted ester was fully recovered and could be reused. The reaction plausibly involved multiple mechanisms where either the electrophilic and the nucleophilic activation of reagents took place through both traces of acetic acid (formed in situ by the hydrolysis of esters) and the autoprotolysis of GAs. iPAc confirmed a superior performance than other esters also for the high-temperature conversion of glycerol; in this case, although acylation and acetalization processes were simultaneously possible, conditions were optimized to achieve the exhaustive transesterification of glycerol to triacetin, in both batch and CF modes. Triacetin was isolated in 99% yield
Supercritical CO2 extraction of natural antibacterials from low value weeds and agro-waste
Full text linkThe suitability of supercritical carbon dioxide (scCO2) to extract cosmetic-friendly preservatives from low value weeds and agro-waste was investigated. The essential oils extracted from Humulus Lupulus and Datura Stramonium (leaves and flowers) were analysed in search for potential antibacterial compounds. Their antibacterial activity was evaluated against Escherichia coli in view of using the extracts as preservatives in cosmetic formulations.
The composition and antibacterial activity of the scCO2 extracts were compared to the ones obtained by Soxhlet extraction with ethanol. The two methods resulted very different in terms of yields, selectivity, composition, and antibacterial properties of the extracts. In general, scCO2 was better for the recovery of preservative compounds since it led to the selective extraction of volatile oils composed mainly by terpenes, terpenoids, fatty acids, and bitter acids
Carbon dots as photocatalysts for organic synthesis: metal-free methylene–oxygen-bond photocleavage
No full textWe report for the first time that irradiation of four different citric acid-derived carbon dots (CDs), in the absence of any other redox mediators, promotes an organic reaction. In this proof-of-concept study methylene-oxygen bond reductive photocleavage in N-methyl-4-picolinium esters is demonstrated. Cyclic voltammetry and UV-Vis spectra of the CDs and of the esters indicate that photocleavage reactivity correlates with the redox properties and the relative energies expressed in the Fermi scale. A photo-fragmentation mechanism is proposed. This study offers a new possibility to employ inexpensive and readily available CDs to promote photo-organic reactions
An NMR study on the mechanism of ethene hydromethoxycarbonylation catalyzed by cationic Pd(II)–PPh3 complexes
No full textThe reactivity of cis-[Pd(H2O)2(PPh3)2](TsO)2.2(H2O) (I,H2O), trans-[Pd(COEt)(TsO)(PPh3)2] (II) and trans-
[Pd(COOMe)(TsO)(PPh3)2] (III) has been studied by 1H and 31P{1H} NMR spectroscopy under conditions
that mime the catalytic ethene hydromethoxycarbonylation (EHMC), i.e. in the presence of PPh3, H2O and
TsOH. (I,H2O), in the presence of two equivalents of PPh3, reacts with MeOH and CO (0.3 MPa) at 193 K to
give [Pd(COOMe)(TsO)(PPh3)3] (III0), which reacts with H2O in the presence of TsOH at 293 K to generate
[PdH(PPh3)3](TsO) (IV) quantitatively. This hydride inserts ethene (0.3 MPa, 293 K) to give trans-
[Pd(Et)(TsO)(PPh3)2] (V), which reacts with CO (0.3 MPa, 223 K) giving [Pd(COEt)(PPh3)3](TsO) (II)0 and
initiates the catalytic EHMC at 293 K. II, in combination with PPh3 and TsOH, reacts at 293 K with MeOH
with quantitative formation of methyl propanoate (MP) and IV and promotes the catalysis starting from
this temperature, under 0.6 MPa of CO/ethene (1/1) when the ratio PPh3/TsOH/II is 2/6/1; upon
increasing the PPh3/II ratio, the catalytic activity passes through a maximum when the ratio is 4/1, even
though it initiates at a higher temperature. In the absence of added ligand, MP is formed in a stoichiometric
amount, catalysis is not observed and decomposition to Pd metal occurs. Therefore, PPh3 is
essential in order to stabilize hydride IV, though an excess of ligand is detrimental. III does not insert
ethene even at 343 K, a temperature well above that at which catalysis is observed. All these experimental
evidences support the PdeH cycle
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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