Silver, catalysis

Silver catalysis of the reduction of silver ions appears to be a necessary condition for normal development. The reaction of developing agents including several types of chemical compounds, e.g., hydroquinone, p-aminophenol, hydroxylamine, catechol, and p-phenylenediamine, are known to exhibit this catalysis to a high degree. 

Thus, the available evidence indicates that little or no adsorption of hydroquinone by silver occurs. Rabinovich s data are unacceptable because of the large experimental errors involved. The possible amount of adsorption indicated by the data of Perry, Ballard, and Sheppard does not exceed the limits of error in their analytical determination of hydroquinone and could not under any circumstances cover more than a small fraction of the silver surface. The kinetics of the reaction between hydroquinone and silver ions do not indicate adsorption of the reducing agent, although the first-order dependence of rate on concentration is not incompatible with weak adsorption. It seems unlikely, accordingly, that adsorption of hydroquinone by silver plays a role of any consequence in the silver catalysis of the reaction between hydroquinone and silver ion. 

The kinetics of the reduction of silver ions by p-phenylenediamine differ in important respects from those of the reduction by hydroquinone and hydroxylamine. Once more, the silver catalysis is marked and the reaction rate varies directly as the area of the catalyst surface, but the rate is directly proportional to the silver ion concentration (James, 7). 

Intramolecular radical cyclization of an aryl bromide and an alkyne can be used to produce dihydroquinolines (Equation 57) <1998TL2965>. An analogous reaction setup utilizes a Lewis acid-catalyzed novel one-pot domino pathway using silver catalysis in high regioselectivity (Scheme 26) <2005OL2675>. Three mole equivalents of the alkyne are used with the final cyclization step arising from alkynic addition. 

In 2004, Gevorgyan et al. reported that a phosphatyloxy alkynone was also able to smoothly give a phosphatyloxy allenone under silver catalysis. The authors reported only a single example (Scheme 3.43).67... 

Although acyloxy, phosphatyloxy, and tosyl allenones were obtained from the corresponding propargyl alcohol derivatives via silver catalysis through an overall process that resembled [3,3]-sigmatropic rearrangement (see Sections 3.41 and 3.43), the mechanism was not fully proved and is still in question. 

The chemical reactions possible with silver catalysis are multiple and cover cycloadditions, cycloisomerizations, allylations, aldol reactions, and even C-H bond activation. Also, asymmetric versions are known, even though they still need to be improved.3-10... 

Dake s group84 published an interesting report in which AgOTf and cationic gold (I) complexes were compared for their use in the synthesis of the pyrrole scaffold. From (3-alkynyl ketones 66 (Scheme 5.29) and various primary amines 67, the imine intermediates JJ were formed in situ and the intramolecular cyclization produced various pyrroles 68. Both catalysts AgOTf (5 mol%) or Au(PPh3)OTf (5 mol%) were efficient, but the reaction proceeded more rapidly with silver catalysis. 

Silver salts or reagents have received much attention in preparative organic chemistry because they are useful catalysts for various transformations involving C-G and C-heteroatom bond formation.309 Especially, the silver(i)/ BINAP (2,2 -bis(diphenylphosphino)-l,T-binaphthalene) system is a very effective catalyst for a variety of enantio-selective reactions, including aldol, nitroso aldol, allylation, Mannich, and ene reactions. Moreover, silver salts are known to efficiently catalyze cycloisomerization and cycloaddition reactions of various unsaturated substrates. Recently, new directions in silver catalysis were opened by the development of unique silver complexes that catalyze aza-Diels-Alder reactions, as well as carbene insertions into C-H bonds. 

Besides copper, rhodium, and ruthenium were able to perform similar chemistry (119). However, silver is an improvement versus these metals due to its lower toxicity to humans and cheaper price. More work is needed in order to understand these intriguing silver catalysis systems, specifically mechanistic studies. 

The use of transition metal species can lower appreciably the decomposition temperature of ot-diazo-carbonyl compounds they can also alter the reactivity of the carbene intermediate (resulting from the initial nitrogen elimination see Section 3.9.2.1) by complex formation. Hence, the Wolff rearrangement may occur with difficulty or, usually, not at all. Thus, some copper species (excepting, for example, Cul), or Rh and Pd catalysts are inappropriate. Freshly prepared silver(I) oxide has been used most frequently, but silver salts (especially silver benzoate) are sometimes preferred.Silver-based catalysts are usually employed in combination with an alkaline reagent e.g. sodium carbonate or a tertiary amine). Even under silver catalysis competing reactions may be observed, and sometimes the products of Wolff rearrangement may not be obtained (see Section 3.9.2.3). 

The intramolecular cycloaddition of the Simmons-Smith cyclopropanation product 30 of nor-born-5-en-2-one trimethylsilyl enol ether 29 is promoted by donor substitution and/or silver catalysis. ... 

If, instead of an imine, an oxime is used, the result is an isoquinoline A -oxide. Such closures can be effected with silver catalysis, " or with iodine, 4-iodoisoquinoline A -oxides being the products in the latter cases. "... 

Although much attention has been paid to silver catalysis in the past several years and significant progress has been achieved in this field of silver-mediated sp C-H bond transformations, compared to the catalysis with other transition metals, the reports in this field are still very limited. In this chapter, we have reviewed a variety of important silver-catalyzed sp C-H transformations, including C-C bond formation, C-N bond formation, C-F bond formation, and C-S bond formation. A... 

Scheme 26.2 Selected examples of phosphine catalysis in cooperation with palladium and silver catalysis.

With Silver Mahindra and Jain [36] reported some nice work on the arylation of Af-trifloromethyl-acetyl i-histidine methyl ester in the 2-position, using arylboronic acids and a silver catalysis (Figure 4.16). The scope was very broad for the transferred aryl group and the yields were generally moderate. AgNOj was used as the catalyst. A radicd-based mechanism was mooted. 

Domino Michael-cyclisation reaction catalysed by chiral cinchona alkaloid catalysis and silver catalysis. 

The decarboxylative fluorination of a wide range of aliphatic carboxylic acids with Selectfluor is achievable under silver catalysis (Scheme 4.83) [87]. 

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