Ethers Homogeneous catalysis

It is in the very nature of the catalytic process that the intermediate compound formed between catalyst and reactant is of extreme lability therefore not many cases are on record where the isolation by chemical means, or identification by physical methods, of intermediate compounds has been achieved concomitant with the evidence that these compounds are true intermediaries and not products of side reactions or artifacts. The formation of ethyl sulfuric acid in ether formation, catalyzed by HjSO , and of alkyl phosphates in olefin polymerization, catalyzed by liquid phosphoric acid, are examples of established intermediate compound formation in homogeneous catalysis. With regard to heterogeneous catalysis, where catalyst and reactant are not in the same... 

When catalysts are recycled as solid residues, it is important to exclude impurities that may piggyback —such as metal particles—as the active species. This was probed in two ways. First, the tape was removed after a first cycle, rinsed, and transferred to a new vessel. A second charge of 17 and dibutyl ether was added, but not the PhMe2SiH. The sample was warmed to 55 °C, the now off-white tape was fished out , and PhMe2SiH was added. The rate profile was similar to the first cycle (ca. 20% slower at higher conversions), consistent with predominant homogeneous catalysis by desorbed fiuorous species. Second, the second cycle of a sequence was conducted in the presence of elemental mercury, which inhibits catalysis by metal particles [57]. However, the rate profile was the same as a sequence in the absence of mercury. 

Kuntz, E., Amgoune, A., Lucas, C. and Godard, G. (2006) Palladium TPPTS catalyst in water C-allylation of phenol and guaiacol with allyl alcohol and novel isomerisation of allyl ethers of phenol and guaiacol./. Mol. Catal. A Chem., 244,124. Kuntz, E.G. (1987) Homogeneous catalysis. .. in water. Chemtech, 17, 570. Cornils, B. (1999) Bulk and fine chemicals via aqueous biphasic catalysis. /. Mol. Catal. A Chem., 143, 1. 

J. F. Hartwig, Transition Metal Catalyzed Synthesis of Arylamines and Aryl Ethers from Aryl Halides and Triflates Scope and Mechanism, Angew. Chem. Int. Ed. 1998,37,2047. P. G. Jessop, T. Ikariya, and R. Noyori, Homogeneous Catalysis in Supercritical C02, Chem. Rev. 1995, 95, 259. 

G Braca, C. Sbrana, G. Valentmi, C- Andrich and G. Gregorio Carbonytation and Homologation of Methanol, Methyl Ethers and Esters in the presence of Ruthenium Catalysis (Fundamental Research in Homogeneous Catalysis v. 3. ed.. M. Tsutsui), pp. 221 238. Plenum Prevs (1979). 

In homc meous catalysis the catalyst (e.g., an acid, a base, or a transition metal complex) is dissolved in a liquid phase. An example from the chemical industry is hydro-formylation, in which olefins react with synthesis gas (CO/Hj mixture) on cobalt or rhodium catalysts to give aldehydes [Weissermel 2003], The practical applications of homogeneous catalysis date back to the eighth century. At this time mineral acids were used to prepare ether by dehydration of ethanol. [Thomas 1994]. 

Among such oxidations, note that liquid-phase oxidations of solid paraffins in the presence of heterogeneous and colloidal forms of manganese are accompanied by a substantial increase (compared with homogeneous catalysis) in acid yield [3]. The effectiveness of n-paraffin oxidations by Co(III) macrocomplexes is high, but the selectivity is low the ratio between fatty acids, esters, ketones and alcohols is 3 3 3 1. Liquid-phase oxidations of paraffins proceed in the presence of Cu(II) and Mn(II) complexes boimd with copolymers of vinyl ether, P-pinene and maleic anhydride (Amberlite IRS-50) [130]. Oxidations of both linear and cyclic olefins have been studied more intensively. Oxidations of linear olefins proceed by a free-radical mechanism the accumulation of epoxides, ROOH, RCHO, ketones and RCOOH in the course of the reaction testifies to the chain character of these reactions. The main requirement for these processes is selectivity non-catalytic oxidation of propylene (at 423 K) results in the formation of more than 20 products. Acrylic acid is obtained by oxidation of propylene (in water at 338 K) in the presence of catalyst by two steps at first to acrolein, then to the acid with a selectivity up to 91%. Oxidation of ethylene by oxygen at 383 K in acetic acid in... 

The early development of aromatic phosphine chemistry took place during the last quarter of the nineteenth century mainly in Michaelis s school.He prepared dichloro(phenyl)phosphine by the passage of the mixed vapours of benzene and phosphorus trichloride through a red hot porcelain tube. This was quickly followed by the preparation of phenylphosphine, dialkylphenylphosphines, and triphenylphosphine. This latter phosphine was destined some 75 years later to become one of the most important phosphines in the development of homogeneous catalysis by phosphine metal complexes. Michaelis obtained it by the reaction of sodium with dichloro(phenyl)phosphine or phosphorus trichloride and bromobenzene in boiling ether.The development of aromatic phosphorus chemistry rapidly overtook that of their intractable aliphatic analogs. It was well established by the 1920s whereas the aliphatic had to await the... 

An enhancement of the selectivity in the dicarboxylation of butadiene has been claimed by Tkatchenko and coworkers [83]. When an ether such as tetrahydrofurane or dimethoxyethane is used as solvent and catalytic amounts of an iron carbonyl complex such as Cp2Fe2(C0)4 are added, the electrocarboxylation of butadiene is achieved with an overall current yield of 76 % and a selectivity of the cis- and trans 3-hexenedioic acid up to 75 %. Possibly, this interesting combination of electrochemistry and homogeneous catalysis will enable further enhancements in the C-C linkage of dienes and carbon dioxide. 

Among the industrially relevant cyclic ethers tetrahydrojuran (THF) is by far the most important. THF is produced by condensation of 1,4-butandiol that is obtained from several feedstock basis (acetylene, propene, butadiene, and butane, see Scheme 5.3.4). 1,4-Dioxane is obtained by condensation of diethylene glycol that itself is obtained by the addition of water to ethylene oxide. The reaction is catalyzed by sulfuric acid (homogeneous catalysis) or by acidic ion-exchange resins (heterogeneous catalysis) and proceeds at about 160 °C with continuous distillative removal of water. 

Habenicht, C., Kam, L.C., Wilschut, M.J. and Antal, M.J., Jr., Homogeneous catalysis of ethyl terf-butyl ether formation from tert butyl alcohol in hot, compressed liquid ethanol, Ind. Eng. Chem. Res., 1995, 34(11), 3784-3792. 

The characteristics of homogeneous crown-ether catalysis were nicely demonstrated by Thomassen et al. (1971) who studied the rate of alkylation of potassium phenoxide with 1-bromobutane in dioxan at 25°C. By measuring the initial consumption of phenoxide (r, in M s-1), any effect of the bromide ion was neglected. The results (Table 21) show hardly any effect of tetraglyme... 

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