Carboxonium ions silylated

Recently, a variety of (3-silylated carboxonium ions have been prepared and characterized by NMR spectroscopy.541 Kira et al.631 used the Corey hydride transfer method, whereas Olah, Prakash, and co-workers applied triphenylmethyl tetrakis (pentafluorophenyl)borate to silylate esters,632 ketones, enones, and carbonates633 [Eq. (3.91)]. The ions thus produced are resonance hybrids of oxocarbenium (327b) and carboxonium (327a) ions with the latter as the major contributors. Calculated (DFT/IGLO) 29 Si NMR chemical shifts agree well with the experimental data. 

All the substrates A E introduced here are alkenes that are electron-rich. This is why they react with electrophiles. Enols (A), enol ethers (C), silyl enol ethers (D) and silylketene acetals (E) react electrophiles to form oxocarbenium or carboxonium ions, whereas the reaction of enamines (B) with electrophiles gives iminium ions ... 

Fig. 14.62. Polar hydrogenation/ hydrogenolysis of an aromatic ketone (meta-nitroacetophenone). CF3COOH causes a reversible protonation of the ketone to the carboxonium ion A. The reducing agent triethylsilane then transfers a hydride ion onto A to form a benzylic alcohol. This alcohol presumably is silylated, protonated, and converted into the benzyl cation B. A second hydride transfer yields the final product.

CARBOCATIONS, CARBANIONS, FREE RADICALS, CARBENES, AND NITRENES nitrogen. A silylated carboxonium ion, such as 19, has been reported. ... 

Carboxonium/oxocarbenium ions that are produced from a silyl enol ether or a silylketene acetal and an electrophile are desilylated in the second step of the reaction. This also produces an a-functionalized carbonyl or carboxyl compound. 

---