Silicon-carbon bond cleavage of pentacoordinate anionic silicates by N-bromosuccinimide

Silicon-carbon bonds in diaryltrifluorosilicates [ArPhSiF 3 ] - ,K + .18-crown-6 are readily cleaved by N-bromosuccinimide (NBS) in THF to give the corresponding aryl bromides. The reactivity of silicon-aryl bonds towards the NBS cleavage decreases in the following order: 4-MeOC 6 H 4 > 4-MeC 6 H 4 > C 6 H 5 > 4-ClC 6 H 4 > 4-CF 3 C 6 H 4 , with ρ = -3.0 in the Hammett plots. The reactivity of 2,6-xylyl group is 10 times greater than that of phenyl group in the intramolecular competition, indicative of no steric hindrance effect. The NBS cleavage of pentacoordinate silicates seems to proceed through the direct electrophilic attack by the positive bromine atom on the Si-ipso carbon not via any hexacoordinate species in the rate-determining step.