Directed Lithiation of Pentadienylsilanes

Deprotonation
of dimethyl­(pyrrolidylmethyl)­silylpentadiene
(5) and bis­(2-methoxyethyl)­aminomethyl­(dimethyl)­silylpentadiene
(7) with nBuLi/tmeda and nBuLi, respectively, results in their corresponding
lithium complexes, 6 and 8. The lithium
cation in 6 is coordinated by an η1-pentadienyl
ligand via the α-carbon and by the pendant pyrrolidyl group.
The lithium cation in 8 is η2 coordinated
by the pentadienyl α- and β-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 α-carbon,
although a series of electrophile quenching reactions with 6 show that regioselectivity does not depend on the electronic structure
of the pentadienyl carbanion.