ChemInform Abstract: Carbanion Rearrangements by Intramolecular 1,ω-Proton Transfer. Part 5. Carbanion Rearrangements of ω-Phenyl-ω-(trimethylsilyl)alkyllithium Compounds: Intramolecular Reactions of Benzyltrimethylsilanes with a Carbon-Lithium Bond.
Abstract
ω-Phenyl-ω-(trimethylsilyl)alkyllithium compounds such as (I) show form out of five theoretically conceivable possibilities for intramolecular stabilization depending on the solvent and on the chain length n. While transmetalation of a methyl group at the Si atom by a 1,(n+2) proton transfer is observed in any case, the intramolecular 1,n shift of the benzylic proton does only take place with n ≫ 4. The main reaction, however for n = 3 and 4 only, is represented by the 1,n trimethyl silyl shift via a cyclic ate complex as an intermediate which partly splits off MeLi yielding the corresponding silacycloalkane derivatives.
ChemInform Abstract
ω-Phenyl-ω-(trimethylsilyl)alkyllithium compounds such as (I) show form out of five theoretically conceivable possibilities for intramolecular stabilization depending on the solvent and on the chain length n. While transmetalation of a methyl group at the Si atom by a 1,(n+2) proton transfer is observed in any case, the intramolecular 1,n shift of the benzylic proton does only take place with n ≫ 4. The main reaction, however for n = 3 and 4 only, is represented by the 1,n trimethyl silyl shift via a cyclic ate complex as an intermediate which partly splits off MeLi yielding the corresponding silacycloalkane derivatives. In going from Et2O to THF as the solvent, the silyl shifts are more accelerated than the proton shifts. In no case, however, a Grovenstein-Zimmerman rearrangement involving phenyl migration took place. Degenrate silyl shifts starting from α-deuterated ω-(trimethylsilyl)alkyllithium compounds could not be detected either. Only by introduction of a second trimethylsilyl group into the 3 position a 1,3-(C → C)-trimethylsilyl shift is initiated again.
