Chiral Chalcogenyl-Substituted Naphthyl- and Acenaphthyl-silanes and their Cations.

Cyclic silylated chalconium borates 13 [B(C 6 F 5 ) 4 ] and 14 [B(C 6 F 5 ) 4 ] with peri -acenaphthyl and peri -naphthyl skeletons were synthesized from unsymmetrically substituted silanes 3 , 4 , 6 , 7 , 9 and 10 using the standard Corey protocol (Chalcogen Ch = O, S, Se, Te). The configuration at the chalcogen atom is trigonal pyramidal for Ch = S, Se, Te, leading to the formation of cis - and trans -isomers in the case of phenylmethylsilyl cations. With the more bulky tert -butyl group at silicon, the configuration at the chalcogen atoms is predetermined to give almost exclusively the trans -configurated cyclic silylchalconium ions. The barriers for the inversion of the configuration at the sulfur atoms of sulfonium ions 13c and 14a are substantial (72- 74 kJ mol -1 ) as shown by variable temperature NMR spectroscopy. The neighboring effect of the thiophenyl group is sufficiently strong to preserve chiral information at the silicon atom at low temperatures.