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

The first cyclic pi -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(pentafluoropfienyl)borate (TPFPB) as a counter anion in CD2Cl2 at -50 degreesC by hydride abstraction from the corresponding silacycloheptatriene (silepin). The observed chemical shifts of H-1, C-13, and Si-29 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. (C) 2001 Elsevier Science Ltd. All rights reserved.