Silver silylenoid intermediate

Woerpel and Calad tested for the formation of the silacarbonyl ylide by interrogating the behavior of the electrophilic silver silylenoid intermediate 115 toward a,(3-unsaturated carbonyl compounds (Scheme 7.37).82 They hypothesized that formation of silacarbonyl ylide 131 might trigger a 6jt-electrocyclization to form oxasilacyclopentene 132. As anticipated, exposure of cyclohexene silacyclopropane 58 to substoichiometric amounts of silver trifluoroacetate in the presence of a,(3-unsaturated carbonyl compounds 130 produced oxasilacyclopentenes 132. The reaction tolerated a substitution at the a and/or (3 position and was general for both esters and ketones. 

Exploration of the reactivity of cyclohexene silacyclopropane led Woerpel and coworkers to discover that the inclusion of metal salts enabled silylene transfer to monosubstituted olefins at reduced temperatures (Table 7.1).11,74 A dramatic reduction in the temperature of transfer was observed when cyclohexene silacyclopropane was exposed to copper, silver, or gold salts. Silver salts were particularly effective at decomposing 58 (entries 6-11). The use of substoichiometric quantities of silver triflate enabled ra-hexene silacyclopropane 61 to be formed quantitatively at —27°C (entry 6). The identity of the counterion did affect the reactivity of the silver salt. In general, better conversions were observed when noncoordinating anions were employed. While the reactivity differences could be attributed to the solubility of the silver salt in toluene, spectroscopic experiments suggested that the anion played a larger role in stabilizing the silylenoid intermediate. 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

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. 

A catalytically active silylsilver intermediate was observed using low-temperature 29Si NMR and IR spectroscopy (Scheme 7.18).83 Exposure of cyclohexene silacyclopropane 58 to the silver complex produced cyclohexene as well as a new species, which exhibited two doublets at 97 ppm (./Agsi = 260 and 225 Hz) in the 29 Si 1H NMR spectmm. The 29 Si H NMR spectra of this species are consistent with a Lewis base-stabilized metal silylenoid. Tilley and coworkers have reported that (Et3P)2Pt(H)-Si(Sf-Bu)2(OTf) appears at 52 ppm,39 and Jiitzi et al. observed... 

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