Perturbation of conjugation in internally solvated allylic lithium compounds: Variation of ligand structure. NMR and X-ray crystallography
Perturbation of conjugation in internally solvated allylic lithium compounds: Variation of ligand structure. NMR and X-ray crystallography
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DOI：
10.1021/ja060272n
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发表时间：
2006-06-28
影响因子：
15
通讯作者：
Liu, Hua
中科院分区：
文献类型：
--
作者：
Fraenkel, Gideon;Gallucci, Judith;Liu, Hua
制备了几种烯丙基锂化合物，其具有在C-2处栓系的不同潜在配体。它们是CH3OCH2CH2NCH3CH2-，5和1-TMS 6，（CH3）（2）NCH2CH2NCH3CH2-，1-TMS 7，和（（CH3）（2）NCH2CH2）（2）NCH2-，8和1-TMS 9。在所有这些化合物中，Li与侧配体完全配位，并且如X射线晶体学和NMR光谱的组合所示，在烯丙基末端之一处偏离垂直于烯丙基平面的轴。化合物5和8是锂桥二聚体，如通过X射线晶体学所示，并且在苯溶液中也是二聚体，如通过凝固点测定所确定的。化合物6、7和9在THF-d（8）或乙醚-d（10）溶液中均为单体，低温下表现为一个C-13（1），Li-6标量偶合键。两者合计的晶体学和NMR数据表明，所有这些化合物纳入部分离域烯丙基部分。化合物5和8经历快速的1，3-Li-σ迁移，其被提出在低浓度的单体内与普遍的二聚体快速平衡。随着温度的升高，将单键C-13、Li-6耦合常数平均为6、7和13，得到双分子C-Li交换动力学，Δ H-双匕首值分别为6.7、12和13 kcal中心点mol（-1）。非对映异构体N（CH 3）（2）C-13共振的平均值为7表明配位配体在烯丙基平面的面之间的快速转移Δ H-双匕首= 5.3 kcal中心点mol（-1），结合在氮气下的较慢反转。化合物8表现出类似的效果。结果表明，配体结构的变化改变了化合物的动力学行为，但对化合物的离域程度影响不大。
Several allyic lithium compounds were prepared with different potential ligands tethered at C-2. These are with CH3OCH2CH2NCH3CH2-, 5 and 1-TMS 6, with (CH3)(2)NCH2CH2NCH3CH2-, 1-TMS 7, and with ((CH3)(2)NCH2CH2)(2)NCH2-, 8 and 1-TMS 9. In all these compounds Li is fully coordinated to the pendant ligand and is sited off the axis perpendicular to the allyl plane at one of the allyl termini as indicated by a combination of X-ray crystallography and NMR spectra. Compounds 5 and 8 are Li-bridged dimers as shown by X-ray crystallography and also dimeric in benzene solution as determined from freezing point determinations. Compounds 6, 7, and 9 are monomeric in THF-d(8) or diethyl ether-d(10) solution and exhibit one bond C-13(1), Li-6 scalar coupling at low temperature. Taken together the crystallographic and NMR data indicate that all of these compounds incorporate partially delocalized allylic moieties. Compounds 5 and 8 undergo fast 1,3-Li-sigmatropic shifts that are proposed to take place within low concentrations of monomers in fast equilibrium with prevalent dimers. Averaging with increasing temperature of the one-bond C-13, Li-6 coupling constant in 6, 7, and 13 provided the dynamics of bimolecular C-Li exchange with Delta H-double dagger values of 6.7, 12, and 13 kcal center dot mol(-1), respectively. Averaging of the diastereotopic N(CH3)(2) C-13 resonances of 7 is indicative of fast transfer of coordinated ligand between faces of the allyl plane Delta H-double dagger = 5.3 kcal center dot mol(-1) combined with slower inversion at nitrogen. Compound 8 exhibits similar effects. It is concluded that variation of the ligand structure changes dynamic behavior of the compounds but has little influence of their degrees of delocalization.