Silver-catalyzed carbene transfer

Figure 25. Silver-catalyzed carbene transfer reactions.

The silyl group-transfer reaction, or the transfer of a silylene or a silylenoid intermediate to an unsaturated C—C bond, is analogous to nitrene and carbene transfers (136). Fewer methods were developed for the silylenoid transfer this is likely due to the difficulty of handling the substrates and products (137). Woerpl and co-workers (138) discovered several silver-catalyzed silylene-transfer reactions, which greatly enriched silylene-transfer chemistry and its applications. 

Following their investigations on nitrene, carbene, and oxo transfer reactions catalyzed by fluorinated silver tris(pyrazoyl)borate (see Chapter 6 on nitrene chemistry), Lovely et al. looked for a combined carbene transfer and [2,3]-sigmatropic rearrangement. On the basis of these mechanistic considerations, these authors showed that diazoacetates, indeed, reacted with allyl halides in the presence of this silver catalyst to give a-halo-y-unsaturated esters (Scheme 3.51).77... 

Silver can mediate oxidation reactions and has shown unique reactivity. In a few cases, namely, nitrene-, carbene-, and silylene-transfer reactions, novel reactivity was found with homogeneous silver catalysts. Some of these reactions are uniquely facilitated by silver, never having been reported with other metals. While ligand-supported silver catalysts were extensively utilized in enantioselective syntheses as Lewis acids, disappointingly few cases were reported with oxidation reactions. Silver-catalyzed oxidation reactions are still underrepresented. Silver-based catalysts are cheaper and less toxic versus other precious metal catalysts. With the input of additional effort, this field will undoubtedly give more promising results. 

Williams and coworkers have synthesized a small library of new chiral imi-noalkyl imidazolium salts from amino acids using a modular design approach (Figure 4.18) [58]. Deprotonation with silver oxide yields the corresponding silver carbene transfer reagents, which have been tested as ligand sources in the palladium-catalyzed allylic alkylation of l,3-diphenylprop-3-enyl acetate with dimethyl malonate. 

However, an encouraging result was obtained very recently for the 1,4-conjugate addition of dialkyl zinc to a variety of Michael acceptors catalyzed by copper. Alexakis, Roland and coworkers have investigated the addition of diethyl zinc to cycloheptenone and observed an enantiomeric excess of 93% (95% yield) in the presence of Cu(OAc)2 and the silver carbene derivative of imidazolium 1 (Scheme 3) [10]. Silver carbene complexes are efficient transfer agents to copper(II) and therefore the potentially harmful use of a base to generate the catalytic species is avoided. 

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