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.

以正丁基锂/四甲基乙二胺（nBuLi/tmeda）和正丁基锂（nBuLi）分别对二甲基(吡咯烷基甲基)硅基戊二烯（5）以及双(2-甲氧基乙基)氨基甲基(二甲基)硅基戊二烯（7）进行去质子化反应，可得到对应的锂配合物6与8。配合物6中的锂离子通过α-碳原子与η¹-戊二烯基配体配位，同时受到悬挂式吡咯烷基基团的配位作用。配合物8中的锂离子则通过戊二烯基的α-和β-碳原子以及双(甲氧基乙基)氨基基团以η²方式配位。配合物6的结构在苯溶液中得以保持，但在四氢呋喃（thf）溶剂中，其辅助配体tmeda会被溶剂置换。在苯溶剂中，可观察到配合物8的戊二烯基碳原子存在3.5:1的W型与S型构象混合物。对配合物6和8中戊二烯基碳原子的自然键轨道（NBO）电荷开展密度泛函理论（DFT）计算结果显示，锂离子会将电子密度极化至α-碳原子一侧；不过，针对配合物6开展的一系列亲电试剂淬灭反应表明，区域选择性并不依赖于戊二烯基碳负离子的电子结构。