Directed Lithiation of Pentadienylsilanes
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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.
