Intramolecular Halo Stabilization of Silyl Cations-Silylated Halonium- and Bis-Halo-Substituted Siliconium Borates.

The stabilizing neighboring effect of halo substituents on silyl cations was tested for a series of peri-halo substituted acenaphthyl-based silyl cations 3. The chloro- (3 b), bromo- (3 c), and iodo- (3 d) stabilized cations were synthesized by the Corey protocol. Structural and NMR spectroscopic investigations for cations 3 b-d supported by the results of density functional calculations, which indicate their halonium ion nature. According to the fluorobenzonitrile (FBN) method, the silyl Lewis acidity decreases along the series of halonium ions 3, the fluoronium ion 3 a being a very strong and the iodonium ion 3 d a moderate Lewis acid. Halonium ions 3 b and 3 c react with starting silanes in a substituent redistribution reaction and form siliconium ions 4 b and 4 c. The structure of siliconium borate 4 c [B Br ] reveals the trigonal bipyramidal coordination environment of the silicon atom with the two bromo substituents in the apical positions.