The first silatropylium ion stabilized by rigid σ-frameworks: preparation, properties, and some reactions

The first cyclic π-conjugated silylium ion, the silatropylium ion 5 annelated with three bicyclo[2.2.2]octene units having a mesityl group on the silicon atom, was prepared with tetrakis(pentafluorophenyl)borate (TPFPB) as a counter anion in CD2Cl2 at −50°C by hydride abstraction from the corresponding silacycloheptatriene (silepin). The observed chemical shifts of 1H, 13C, and 29Si NMR for cation 5 were in agreement with the calculated values. These results and NICS calculations indicated that cation 5 has the aromaticity approaching that of the tropylium ion. When cation 4 having a methyl group instead of a mesityl group and cation 5 were generated in the presence of two equivalents of acetonitrile, the acetonitrile complexes 16 and 17 with considerable covalent-bond character were formed. For the complexes 16 and 17, the exchange process of acetonitrile was observed for the first time as an acetonitrile complex of a silyl cation, and the intermediacy of a pentavalent silicon was demonstrated. When cations 4 and 5 were generated by the use of triphenylmethyl perchlorate instead of triphenylmethyl TPFPB, covalent perchlorate esters 18 and 19 having the O–Si covalent bond were formed instead of ionic salts. By treatment with water, the perchlorate ester 18 was found to undergo a novel rearrangement to give a cyclopentene derivative 20a spiro-connected with bicyclo[2.2.2]octane at the 3-position.