Silver catalyzed

Silver Catalyst Process. In early formaldehyde plants methanol was oxidized over a copper catalyst, but this has been almost completely replaced with silver (75). The silver-catalyzed reactions occur at essentially atmospheric pressure and 600 to 650°C (76) and can be represented by two simultaneous reactions ... 

Ethylene oxide (qv) was once produced by the chlorohydrin process, but this process was slowly abandoned starting in 1937 when Union Carbide Corp. developed and commercialized the silver-catalyzed air oxidation of ethylene process patented in 1931 (67). Union Carbide Corp. is stiU. the world s largest ethylene oxide producer, but most other manufacturers Hcense either the Shell or Scientific Design process. Shell has the dominant patent position in ethylene oxide catalysts, which is the result of the development of highly effective methods of silver deposition on alumina (29), and the discovery of the importance of estabUshing precise parts per million levels of the higher alkaU metal elements on the catalyst surface (68). The most recent patents describe the addition of trace amounts of rhenium and various Group (VI) elements (69). 

There are many ways to produce acetaldehyde. Historically, it was produced either hy the silver-catalyzed oxidation or hy the chromium activated copper-catalyzed dehydrogenation of ethanol. Currently, acetaldehyde is obtained from ethylene hy using a homogeneous catalyst (Wacker catalyst). The catalyst allows the reaction to occur at much lower temperatures (typically 130°) than those used for the oxidation or the dehydrogenation of ethanol (approximately 500°C for the oxidation and 250°C for the dehydrogenation). 

R.B. Grant, and R.M. Lambert, A single crystal study of the silver-catalyzed selective oxidation and total oxidation of ethylene, 7. Catal. 92, 364-375 (1985). 

Scheme 2. Scheme for growing silver catalyzed redox reaction. (Reprinted from Ref [28], 1999, with permission from American Chemical Society.)... 

Scheme 6.7 shows some other examples of enantioselective catalysts. Entry 1 illustrates the use of a Co(III) complex, with the chirality derived from the diamine ligand. Entry 2 is a silver-catalyzed cycloaddition involving generation of an azome-thine ylide. The ferrocenylphosphine groups provide a chiral environment by coordination of the catalytic Ag+ ion. Entries 3 and 4 show typical Lewis acid catalysts in reactions in which nitrones are the electrophilic component. 

Liu W, Jiang H, Huang L (2010) One-pot silver-catalyzed and PIDA-mediated sequential reactions Synthesis of polysubstituted pyrroles directly from alkynoates and amines. Org Lett 12(2) 312—315... 

Several reviews on the synthesis of aziridines have been published in the previous year. These publications include a review on the silver catalyzed addition of nitrenes (among other intermediates such as carbene) across a double bond <06EJOC4313> a review on sulfur ylide addition to imines to form aziridines <06SL181> a review on nitrogen addition across double bonds <06ACR194> a general review on functionalization of a,p-unsaturated esters with some discussion of aziridination <06TA1465>... 

For the bicarbonate reaction, gently mix for 20 hours at 4°C. For the silver-catalyzed reaction, continue the reaction for 1 hour or until the silver complex has fully dissolved. 

The latter mode of reaction has even been reported to proceed in presence of sil-ver(I) ions [127], which is not easy to understand in the context of Marshall s silver-catalyzed cycloisomerization of allenyl ketones (see Chapter 15). 

Marshall and co-workers used the silver-catalyzed version of this cycloisomerization as the final step in the synthesis of (-)-kallolide B from precursor 115 (Scheme 15.33) [51, 54]. Again, the reaction is stereospecific, as has also been demonstrated in the synthesis of kallolide A [55] and other examples [77]. 

Application of silver-catalyzed cydization is a key step in the synthesis of clavepic-tines A and B, a synthesis which also established the absolute configuration for these compounds. With regard to the allene unit and the heterocycle, enantiomeri-cally pure precursor 129 was prepared and then cyclized to the quinolizidine 130 with AgN03 in a diastereoselective manner (Scheme 15.40) [89, 90]. The synthesis was conducted with an inseparable 1 1 mixture of diastereomers at C-14 from the diastereomerically pure allene with regard to the axial chirality of the allene a 7 1 mixture of diastereomers (at C-10) was formed. 

Kinetic experiments have been performed on a copper-catalyzed substitution reaction of an alkyl halide, and the reaction rate was found to be first order in the copper salt, the halide, and the Grignard reagent [121]. This was not the case for a silver-catalyzed substitution reaction with a primary bromide, in which the reaction was found to be zero order in Grignard reagents [122]. A radical mechanism might be operative in the case of the silver-catalyzed reaction, whereas a nucleophilic substitution mechanism is suggested in the copper-catalyzed reaction [122]. The same behavior was also observed in the stoichiometric conjugate addition (Sect. 10.2.1) [30]. 

Acyl radicals have been obtained from a-keto acids by silver-catalyzed decarboxylation with peroxydisulfate. Decarboxylation takes place easily and can be interpreted according to Scheme 10. 

The usual sources used for the homolytic aromatic arylation have been utilized also in the heterocyclic series. They are essentially azo- and diazocompounds, aroyl peroxides, and sometimes pyrolysis and photolysis of a variety of aryl derivatives. Most of these radical sources have been described in the previous review concerning this subject, and in other reviews concerning the general aspects of homolytic aromatic arylation. A new source of aryl radicals is the silver-catalyzed decarboxylation of carboxylic acids by peroxydisulfate, which allows to work in aqueous solution of protonated heteroaromatic bases, as for the alkyl radicals. 

The most extensive work on the catalyzed reaction was carried out with a nuclear gold sol (phosphorus reduction) added as the initial catalyst. The gold particles soon become coated with silver, so the major portion of the measured reaction is actually silver-catalyzed. The reaction rate is proportional to the surface area of the catalyst. This is shown by the following experimental results. 

Fig. 2. Kinetics of the silver-catalyzed reduction of silver ions by p-phenylenediamine 1, variation of rate with p-phenylenediamine concentration in presence of gum arabic 2, same in presence of gelatin 3, variation with silver ion concentration 4, variation with sulfite ion concentration.

The reduction of silver ions by catechol (a good developer) is silver-catalyzed. This reaction has been studied to only a limited extent (James, 7, 35) but the mechanism appears to be quite similar to that of the hydroquinone reaction. The rate is directly proportional to the catechol concentration at a pH of 7.58. 

The autoacceleration of the silver-catalyzed reaction in the early stages is greater than would be expected from a simple increase in catalyst surface by growth of the silver nuclei orginally present. Apparently new nuclei are readily formed during the reaction. A possible mechanism which could lead to this result is ... 

The kinetics of the silver-catalyzed reduction of silver chloride were studied on two types of preparations. In the first the nuclei were... 

The effect of the cyanine dye and of gelatin on the reaction rate shows that reduction of silver ions from solution is not the rate-controlling process. These influences of adsorbed components on the reaction rate speak against the concept that solution of the silver halide is the rate controlling process. Hence, the silver catalyzed reduction of silver chloride by hydroxylamine takes place substantially at the solid silver/ silver halide interface. 

A reactive intermediate may be responsible for the copper catalysis of the hydroxylamine reaction. The intermediate formed in the silver-catalyzed reaction, if it has any real existence, is not further oxidized but breaks down into nitrogen and water. Oxidation of hydroxylamine by cupric ion, on the other hand, yields predominately nitrous oxide. The intermediate formed by the removal of a single electron from the hydroxylamine in this reaction must be further oxidized to yield the final product. Such an intermediate may react readily with silver ions in solution. 

Minisci-type substitution is one of the most useful reactions for the synthesis of alkyl- and acyl-substituted heteroaromatics. The acyl radicals are formed by the redox decomposition from aldehyde and /-butyl hydroperoxide or by silver-catalyzed decarboxylation of a a-keto acid with persulfate. Synthesis of acylpyrazines 70 as ant pheromones are achieved by this methodology using trialkyl-substituted pyrazines 69 with the acyl radicals generated from aldehydes or a-keto acids (Equation 10) <1996J(P1)2345>. The latter radicals are highly effective for the acylation. Homolytic alkylation of 6-chloro-2-cyanopyrazine 71 is performed by silver-catalyzed decarboxylation of alkanoic acids to provide 5-alkyl-substituted pyrazines 72 (Scheme 18) <1996CCC1109>. 

Similar results were obtained by Dake et al. with silver catalyzed reactions, showing that the thermal stability of gold was higher than silver but silver catalyzed reactions were faster [138]. 

Numerous papers and several review articles889,899-907 deal with adsorption studies and discuss the kinetics and mechanism of the silver-catalyzed epoxidation of ethylene. A simple triangular kinetic scheme of first-order reactions satisfies the experimental observations (Scheme 9.23). On the best industrial catalysts fci/ 2 is 6, and k2/ 3 is 2.5. 

Propylene Oxide. Unlike ethylene, propylene cannot be selectively transformed to propylene oxide by silver-catalyzed oxidation. Instead, indirect oxidations (the peracid and the hydroperoxide routes) are employed.912-915... 

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