Directed Lithiation of Pentadienylsilanes

Deprotonation of dimethyl(pyrrolidylmethyl)silylpentadiene (5) and bis(2-rnethoxyethyl)aminomethyl(dimethyl)silylpentadiene (7) with (BuLi)-Bu-n/tmeda and (BuLi)-Bu-n, respectively, results in their corresponding lithium complexes, 6 and 8. The lithium cation in 6 is coordinated by an eta(1)-pentadienyl ligand via the alpha-carbon and by the pendant pyrrolidyl group. The lithium cation in 8 is eta(2) coordinated by the pentadienyl alpha- and beta-carbons and by the bis(methoxyethyl)amino group. The structure of 6 is retained in benzene solution, but in thf the tmeda coligand is displaced by the solvent. A 3.5:1 mixture of the W- and S-conformations of the pentadienyl carbons was observed for 8 in benzene. DFT calculations of NBO charges for the pentadienyl carbons in 6 and 8 show that lithium polarizes the electron density toward the alpha-carbon, although a series of electrophile quenching reactions with 6 show that regioselectivity does not depend on the electronic structure of the pentadienyl carbanion.