beta-Silyl and beta-Germyl Carbocations Stable at Room Temperature.

Stable carbocations have been prepared at room temperature with benzene as solvent and tetrakis(pentafluorophenyl)borate (TPFPB) as anion by the addition of solvated triethylsilylium TPFPB or of tributylgermylium TPFPB to 1,1-diphenylethene. These carbocations are stabilized by sigma conjugation with nonadjacent group 14 atoms and respectively constitute stable saturated beta-silyl and beta-germyl carbocations. NMR parameters, including the (29)Si chemical shift, the (13)C chemical shifts of the cationic, aryl, and methylene carbons, and the one-bond (13)C-(1)H coupling constant of the methylene group adjacent to the group 14 element, define the distribution of positive charge between carbon and (through sigma conjugation) silicon or germanium. The extent of hyperconjugation is estimated quantitatively by comparison with models. The NMR parameters indicate that the cations are open rather than bridged.