Preparation and NMR Study of Silylated Carboxonium Ions¹

A series of silylated carboxonium ions, 2a-6a, were prepared as long-lived species by treating triethylsilane and triphenylmethyl tetrakis(pentafluorophenyl)borate (Ph3C(+)B(C6F5)4-) with ketones, enones, carbonates, amides, and urea in CD2Cl2 solution. They were characterized by 13C and 29Si NMR spectroscopy at -78 degrees C. The NMR study indicates that the silylated carbonyl compounds are resonance hybrids of oxocarbenium and carboxonium ions, while the latter are the major contributors to the overall structures. The structure and 13C and 29Si NMR chemical shifts of the model trimethylsilylated carboxonium ions were also calculated by density functional theory/IGLO methods. The calculated results agree well with the experimental data.