1,721,150 research outputs found
Mechanistic study of the phase-transfer-catalyzed reduction of nitrobenzene to aniline by iron carbonyl complexes. Role of the radical anion [Fe-3(CO)(11)](center dot-)
No full textContrary to what was earlier believed, the phase-transfer-catalyzed reaction of Fe-3(CO)(12) with OH- affords the radical anion cluster [Fe-3(CO)(11)](.-) and not [HFe3(CO)(11)](-). When a nitro compound is also present, it is the radical anion that is mainly responsible for its reduction to aniline, not the hydride cluster, which is completely unreactive under the same experimental conditions
Palladium/Phenanthrolines Catalyzed Carbonylation of Nitroarenes : the Key Role of the Nitrogen Ligand
No full textNitroarene carbonylation is the most direct alternative to the currently employed phosgene-based strategy for the industrial production of aryl isocyanates such as toluendiisocyanate (TDI) and 4,4'-methylendiphenyildiisocyanate (MDI). Despite the high toxicity of phosgene, several million tons of this product are still produced each year. Catalytic systems based on the palladium/phenanthroline combinations are by far the most promising for an industrial application of the carbonylation strategy. Phenantrolines are unique in their ability to stabilize the catalyst under the forcing conditions and, unlike phosphines, are not oxidized by nitroarenes
Schiff Bases of the BIAN Family : from Symmetrical Biaryl Derivatives to Mixed, Alkyl, Chiral or Reduced Ligands and Heterogeneous Catalysts
Full text linkThe number of applications of bis-imines as ligands in homogeneous catalysis has much increased in
recent years. Derivatives of acenaphthenequinone (R-BIANs) are especially useful to this purpose
because the rigidity of the acenaphthene skeleton strengthen coordination to a metal and add stability
against the rupture of the central C-C bond of the diimine moiety.
During the years, we have expanded the available range of Ar-BIANs to ligands where the aryl group
bears strongly electronwithdrawing substituents, two different aryl groups are present, and
first prepared usually unstable Alkyl-BIAN compounds.
The key principles which allowed these products to be
obtained will be illustrated. Solubility of the products and the
right control of ring strain for alkyl derivatives are essential
points.
The coordinating strength of a series of Ar-BIAN ligands to
several palladium complexes has been measured and varies
linearly with the log of Hammett constant. The slope of the
correlation depends on the metal complex and can be
regarded as measure of its Lewis acidity.
The family of R-BIAN ligands was then further expanded to
chiral derivatives and finally to reduced ligands, Ar-BIANH2. The latter
are air sensitive compounds, but are stable under in the solid state an inert atmosphere and can be
employed to synthesize catalytically active complexes without resorting to the use of reduced metal
precursors or alkaline-metal reduced intermediates
At the end of this evolution, the last application
was to decompose the coordinated ligand at
high temperatures to generate
heterogeneous nitrogen- enriched graphitic cobalt
catalysts, effective in hydrogenation reactions for
which traditional cobalt catalysts are inactiv
Reductive carbonylation of nitro compounds
No full textThis chapter describes the progress made in the field of reductive carbonylation of nitroarenes to give isocyanates, carbamates and ureas. Isocyanates are intermediates in the production of many chemicals among which are polyurethanes. Carbamates and urea are of interest themselves, but can also be thermally cracked to afford the corresponding isocyanates. However, the techniques employable for this thermolysis step are not discussed here. The most effective catalytic systems are all homogeneous and based on palladium, rhodium or ruthenium complexes or on selenium compounds. The chapter include all significant contributions from the early reports in the '960 to October 2015. More attention is given to the systems which are more promising in view of an industrial application. Several mechanistic studies are also considered in a critical manner
Gaseous CO-Free Palladium-Catalyzed Reductive Cyclization Reactions of Nitroarenes: An Endless Story Manar ahmed fouad abdellatif, doaa reda mohamed Ramadan, Simone Galié, F. Ferretti, F. Ragaini
No full textRecent progresses on Gaseous CO-Free Palladium-Catalyzed Reductive Cyclization Reactions of Nitroarenes are discussed
Method of Establishing the Lewis Acidity of a Metal Fragment Based on the Relative Binding Strength of Ar-BIAN Ligands (Ar-BIAN = Bis(aryl)acenaphthenequinonediimine).
No full textThe relative coordination strength of a series of differently substituted Ar-BIAN ligands (Ar-BIAN = bis(aryl)acenaphthenequinonediimine) to a series of palladium complexes both in the formal 0 and 2 oxidation states has been determined. In all cases a good to excellent linearity of the log Keq with respect to the Hammet constants of the substituents on the aryl fragments of the ligands was observed. The resulting constant is proposed to be a good indication of the Lewis acidity of the metal fragment, a physical quantity for which experimental parameters have been determined only for a limited class of compounds. The obtained parameters allow a comparison not only of different olefin complexes among themselves, but even with respect to different metal fragment such as Pd(OAc)2, Pd(Me)Cl, or a -allyl complex. The Lewis acidity of the olefin complexes is extremely variable and ranges from the less acidic (Pd(Ar-BIAN)(DMFU), DMFU = dimethylfumarate) to two of the most acidic (Pd(Ar-BIAN)(TCNE) and Pd(Ar-BIAN)(FN); TCNE = tetracyanoethylene, FN = fumarodinitrile) complexes among those examined. A cationic -allyl complex has the highest Lewis acidity among the complexes examined. The importance of steric effects is examined in some cases
Mineral oil and phase selectively soluble phenanthrolines: a new protocol for catalyst recycle in the carbonylation of nitroarenes
No full textMineral oil and phase selectively soluble phenanthrolines: a new protocol for catalyst recycle in the carbonylation of nitroarene
Mechanistic studies of palladium-catalysed carbonylation reactions of nitro compounds to isocyanates, carbamates and ureas
No full textMany different palladium-based catalytic systems have been reported for the carbonylation reactions of organic nitro compounds to isocyanates, carbamates and ureas. Almost all of these can be roughly divided into three groups: (i) those containing a second (or even a third) metal (usually a Lewis acid or a metal oxo compound or both), (ii) those in which phenanthroline or similar chelating nitrogen ligands are used and (iii) those in which monodentate phosphines are employed as ligands. The systems in which chelating phosphines are used as ligands lie in between the last two groups. The reaction mechanisms for the catalytic systems in each group appear to be related. Most of the information available does not derive from strictly mechanistic studies, but rather from synthetic studies and it is here critically analysed and compared with the information obtained from other related fields
Mechanistic study of the Ru-3(CO)(12)/chloride catalyzed carbonylation reactions of nitroarenes to carbamates and ureas; the role of the alkylammonium cation
No full textThe effect of the chloride countercation on the mechanism of the Ru-3(CO)(12)/chloride catalyzed carbonylation of nitroarenes to carbamates has been investigated. The reason for the higher activity and selectivity obtained with tetraethylammonium chloride with respect to [PPN][Cl] is due to the higher igroscopicity of the former (only when no aniline is added) and to its ability do decompose to yield triethylamine. The role of this last compound is twofold. On one hand, it accelerates the alcoholysis of the intermediately formed diarylurea. On the other, it favors a reaction pathway that consumes aniline together with nitrobenzene, thus converting a by-product into the desired product. (C) 2000 Elsevier Science B.V. All rights reserved
Intermediate formation of anilines in the synthesis of Schiff bases from nitroarenes and aldehydes
No full textThe role of the intermediate formation of anilines in the synthesis of Schiff bases from nitroarenes, aldehydes and CO catalysed by metal complexes has been investigated. Contrary to earlier belief, the reaction does not proceed through the reaction of an intermediate nitrene with the carbonyl compound, but by reduction of the nitroarene by CO and traces of water to initially afford the aniline, which then reacts with the aldehyde to afford the Schiff base, regenerating the water necessary to reduce more nitroarene. (C) 1999 Elsevier Science B.V. All rights reserved
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