Silver-assisted

The Ritter reaction with unsaturated carbenium ions under either silver-assisted solvolysis or photolytic conditions leads to excellent yields of isoquiaolines (173). The ease of preparation of the requited vinyl bromides makes an attractive route to highly substituted isoquiaolines. 

Release by Silver-Assisted Cleavag e. A soluble silver complex formed imagewise in the undeveloped areas of the silver haUde layer may be used to effect a cleavage reaction that releases a dye or a dye precursor. The process yields positive dye transfer images directiy with negative-working emulsions (46). An example is the silver-assisted cleavage of a dye-substituted thiazolidine compound, as shown in equation 7. 

Discrimination between exposed and unexposed areas in this process requires the selection of thia zolidine compounds that do not readily undergo alkaline hydrolysis in the absence of silver ions. In a study of model compounds, the rates of hydrolysis of model /V-methyl thia zolidine and A/-octadecyl thiazolidine compounds were compared (47). An alkaline hydrolysis half-life of 33 min was reported for the /V-methyl compound, a half-life of 5525 min (3.8 days) was reported for the corresponding V/-octadecyl compound. Other factors affecting the kinetics include the particular silver ligand chosen and its concentration (48). Polaroid Spectra film introduced silver-assisted thiazolidine cleavage to produce the yellow dye image (49), a system subsequentiy used in 600 Plus and Polacolor Pro 100 films. 

Fig. 14. Schematic cross section of Spectra integral film. The 600 Plus film has a similar stmcture. In these films the yellow image is formed by silver-assisted cleavage of a yellow dye releaser. A colorless developer reduces exposed silver hahde in the blue-sensitive emulsion in unexposed areas dissolved silver diffuses to the dye releaser layer and triggers the release of the yellow image dye.

Figure 2-24. Silver-assisted reaction of an alkyl halide with a nucleophile.

The silver-induced reactions of a-haloimines have been compared to those of a-haloketones (Scheme 3.6).13 These silver-assisted reactions of a-haloimines have been interpreted in terms of the a-imidoylcarbenium intermediates, which rearrange in a fashion analogous to that of the corresponding ketones. 

Rearrangements of propargyl esters with silver salts were first mentioned by Zakharova in the mid-1940s.49 He described the conversion of 3-chloro-3-methyl-but-l-yne into a mixture of acetates in which the allenic acetate, l-acetoxy-3-methylbut-1,2-diene, was the major compound (Scheme 3.30). Although this product could arise from a silver assisted SN2 reaction, it could also be produced from the substitution product through rearrangement, probably catalyzed by silver ions. 

Figure 4.4. Silver assistance in the ring opening of halocyclopropanes.

Under these conditions, silver-assisted electrocyclic ring opening provided the haloallyl cation, which was subsequently trapped by isocyanate anion. Interception of the cationic species with isocyanate was successful since bromide was removed from the reaction mixture as a precipitate (AgBr). Finally, treatment of intermediate 9 with methanol furnished the desired carbamate in 96% yield. This example demonstrates the usefulness of the silver(I)-mediated process. Removal of free halide from the reaction mixture affords a long-lived cationic species that can be captured by a different nucleophile, such as solvent, the silver(I) counteranion, or an intramolecular nucleophile. This reactivity has been exploited in many different ways throughout the years and is examined in greater detail later in this chapter. 

Halides that do not undergo SN2 reactions readily (tertiary, cyclopropyl and bridgehead halides) react by silver-assisted substitution in the presence of silver salts89. Tertiary halides have also been reduced using the in situ generation of cyanoborohydride reagents90. Since primary and secondary halides are apparently unaffected by this reagent, a selective reaction has been developed. 

Lastly, silver-assisted removal of halide has been used to facilitate a Favorskii rearrangement in the key step of the synthesis of the natural sesquiterpene ( )-sirenin (equation 190)989. Fused bicyclic cyclopropane derivatives have also been made by a similar... 

As with Favorskii rearrangements, the semipinacol process may also be facilitated by silver-assisted halide removal1012"1014. 

A related reaction occurs on irradiation of a hypochlorite (Scheme 2). The alkoxyl radical can again abstract a hydrogen atom in a l,S-shift, and the final chloro alcohol can be cyclized to form a tetrahydro-furan. Some processes lead directly to the cyclic product For instance, an alcohol with an accessible 8-hydrogen can be directly converted to a tetrahydrofuran on refluxing with lead tetraacetate. In a related reaction, treatment of an alcohol with silver carbonate and bromine can lead to the cyclic ether by initial formation of a hypohalite. The cyclization occurs when the rearranged radical is converted to a cation, either by oxidation with Pb(OAc)4 or by silver-assisted loss of halide ion. Hypoiodites are also fiequent-ly used, generated in situ. ... 

In other work, which was too recent to be reviewed by Wiberg and coworkers ", Cristol and coworkers studied the silver-assisted acetolysis of the tricyclic cyclopropylcarbinyl... 

T.F. Walsh and co-workers synthesized two (S)- 3-methyl-2-aryltryptamine based gonadotropin hormone antagonists via a consecutive Larock indole synthesis and Suzuki cross-coupling. The required (S)-P-methyltryptophol derivatives were prepared by coupling 4-substituted o-iodoanilines with optically active internal alkynes under standard conditions. The resulting 2-trialkylsilyl substituted indoles were then subjected to a silver-assisted iododesilylation reaction to afford the 2-iodo-substituted indoles that served as coupling partners for the Suzuki cross-coupling step. 

Ganem, B., Boeckman, R. K., Jr. Silver-assisted dimethyl sulfoxide oxidations. Improved synthesis of aldehydes and ketones. Tetrahedron Lett. 1974, 917-920. 

The reaction of Cd(CF3)2DME with Mn(CO)5Br is very slow at ambient temperature, but if AgN03 is added in a 1 1 mole ratio with the cadmium reagent, the silver-assisted reaction is complete within 15 min. The addition of CuBr to Cd(CF3)2DME trifluoromethylations of Mn(CO)jBr, however, is ineffective (36). The results of the AgN03 comparison are... 

In order to better assess the efficiency of the silver-assisted trifluoromethylations described earlier, we elected to reexamine the reactions of the group 10 diligand dihalides with Cd(CF3)2DME. The reactions were all carried out in CH2C12 at ambient temperature with one equivalent of silver salt for each equivalent of the group 10 halide. The results of these reactions are... 

Very thorough studies have been carried out on the deamination of the epime-ric 4-t-butylcyclohexylamines (226)138 20S 208 the silver-assisted dehalodecarboxy-lation of the analogous chloroformates (22 7)201, and the solvolysis of the corresponding p-nitrobenzenesulfonates (22S)213. Although ion pairs play an important role in some of the deamination reactions (cf. Section 4.3.2), cis- and trans-(226) clearly... 

Hydride transfers are implicated in several acylations of alkynes by saturated acyl chlorides leading to cyclopentenes. Intramolecular [1,5] hydride shifts have been shown to be a common feature in silver-assisted reactions of cyclohexanylcarbonyl chloride with alkynes.The nature of the final product depends on the structure of the resulting cation. Capture of fluoride ion, ring contraction and acyl or alkyl migrations of axial substituents have been observed (Scheme 24). 

Both acid chlorides, including ethylsuccinyl chloride," and acid anhydrides "-" have been used as acylating agents, as have carbamoyl chlorides. Silver-assisted reactions of anhydrides of a-bromo-toluic acid also lead to alkynic ketones, probably via an intermediate acyloxyalkyloxycarbenium salt (Scheme 26). ... 

In general, virtually any process leading to 4-azetidinylthioesters from penams was put to its best use by the oxoamide-thioester coupling. Preparation of FCE-22101, FCE 22891 and related penems, could be achieved in fewer steps [81,82,92,96,153]. In particular, the silver-assisted 1,2-cleavage of penicillins is ideally suited to accompany the phosphite-mediated condensation a unique... 

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