Silylsilver intermediate

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... 

The electronic nature of silylsilver intermediate was interrogated through inter-molecular competition experiments between substituted styrenes and the silylsilver intermediate (77).83 The product ratios from these experiments correlated well with the Hammett equation to provide a p value of —0.62 using op constants (Scheme 7.19). Woerpel and coworkers interpreted this p value to suggest that this silylsilver species is electrophilic. Smaller p values were obtained when the temperature of the intermolecular competition reactions was reduced [p = — 0.71 (8°C) and —0.79 (—8°C)]. From these experiments, the isokinetic temperature was estimated to be 129°C, which meant that the product-determining step of silver-catalyzed silylene transfer was under enthalpic control. In contrast, related intermolecular competition reactions under metal-free thermal conditions indicated the product-determining step of free silylene transfer to be under entropic control. The combination of the observed catalytically active silylsilver intermediate and the Hammett correlation data led Woerpel and colleagues to conclude that the silver functions to both decompose the sacrificial cyclohexene silacyclopropane as well as transfer the di-terf-butylsilylene to the olefin substrate. 

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