Silver as a Lewis Acid Reagent

On this basis, silver-catalyzed aldolization and allylation reactions have been developed, expanding the technology for creating a C-C bond using silver (see Chapter 9). Asymmetric versions have also been developed.74,37 

In related reactions, Snapper and Hoveyda et al. reported an asymmetric version of Mannich-type reactions catalyzed by silver acetate.38,39 Propargyl imines proved to be susceptible to nucleophilic addition of sily lenol ethers derived from acetate esters in the 

TABLE 10.3. Asymmetric Mannich Reactions of Enol Ethers with Alkynyl Imines 

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While the silver and zinc salts were effective Lewis acids for these cyclizations, Kikugawa and coworkers reported that the alkoxynitrenium ions could be generated directly from hydroxamic esters (4) using hypervalent iodine oxidants such as hydroxy(tosyloxy) iodobenzene (HUB) and phenyliodine(lll)bis(trifluoroacetate) (PIFA) . Presumably, with such reagents the reactions proceed through A-(oxoiodobenzene) intermediates (54), which can themselves be regarded as anomeric hydroxamic esters and sources of alkoxynitrenium ions (55) (Scheme 11). 

Earlier mention has been made of the use of Lewis acid and Friedel-Crafts reagents as initiators of carbenium ion formation. Another versatile device is to employ a metal to assist in generation of the carbenium ion. In its simplest form, addition of silver(I) ion to an alkyl halide is an excellent technique for encouraging reaction by means of the 5n 1 pathway. This process was first applied to the Ritter reaction by Cast and Stevens, but yields obtained were modest. A recent elegant application of this technique is the two-step conversion of dodecahedrane into its acetamide derivative (Scheme 45). ... 

My first contact with Meerwein was in 1954 when I received, while still working in isolated Hungary, a letter from him. He had read a paper of ours, and offered useful comments, even pointing out that we had missed a relevant reference. We subsequently kept up correspondence. He continued to be interested in our early efforts to obtain long lived carbocations from alkyl (and acyl) fluorides in liquid boron trifluoride (and other Lewis acid halides) at low temperature, as well as via metathetic reactions of organic halides with silver tetrafluoroborate (a reagent we introduced in 1955 for the ionization of bromocyclohexadienes to form benzenium ions and which Meerwein developed to a wide utility akylation method). 

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