13C NMR Spectroscopic Study of 2-[1-(trimethylsilyl)vinyl]-2-adamantyl Cation, the First ß-silyl-substituted Carbocation
2-[1-(Trimethylsilyl)vinyl]-2-adamantyl cation (2) was prepared by the ionization of 2-[1-(trimethylsilyl)vinyl]-2-adamantanol in FSO3H/SO2CF at -130°C and was characterized by 13C NMR spectroscopy. For comparison, the 2-vinyl-2-adamantyl cation (4) was similarly prepared and characterized. The cationic center of 2 is deshielded by 11.5 ppm over that of 4, indicating no ß-silyl stabilization effect (either inductive or hyperconjugative). However, the low barrier for rotation across the C2-C1' bond in 2 indeed supports the intermediate perpendicular cation being stabilized by ß-silyl effect. The results are rationalized by MM2, ab initio, and IGLO calculations.
G. K. Prakash et al., "13C NMR Spectroscopic Study of 2-[1-(trimethylsilyl)vinyl]-2-adamantyl Cation, the First ß-silyl-substituted Carbocation," Journal of the American Chemical Society, vol. 114, no. 8, pp. 3076-3078, American Chemical Society (ACS), Apr 1992.
The definitive version is available at http://dx.doi.org/10.1021/ja00034a047
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