BENZYLIC LITHIUM COMPOUNDS - THE MISSING LINK IN CARBON-LITHIUM COVALENCY - DYNAMICS OF ION REORIENTATION, ROTATION AROUND THE RING-BENZYL BOND, AND BIMOLECULAR C-LI EXCHANGE
BENZYLIC LITHIUM COMPOUNDS - THE MISSING LINK IN CARBON-LITHIUM COVALENCY - DYNAMICS OF ION REORIENTATION, ROTATION AROUND THE RING-BENZYL BOND, AND BIMOLECULAR C-LI EXCHANGE
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DOI：
10.1021/ja00146a018
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发表时间：
1995-10-18
影响因子：
15
通讯作者：
MARTIN, KV
中科院分区：
文献类型：
--
作者：
FRAENKEL, G;MARTIN, KV
苄基锂化合物，迄今为止从NMR数据假定由离子对组成，已经发现在双分子碳-锂键交换太慢而不能平均耦合常数的条件下，在C-13和直接结合的Li-6之间显示出自旋耦合。这些条件包括使用锂内部溶剂化的物质或在低温下使用苄基锂-C-13-Li-6（在C-α处富集）的稀溶液（0.005 M）。（1）J（C-13-Li-6）的低值（3-4 Hz）意味着C-Li共价的可检测程度很小，C-α周围的排列偏离了共面性。苄基锂中的C-Li键位于许多单体物种之间的C-Li共价连续体中，其中（1）J（C-13-Li-6）为16 +/- 1 Hz和分离的离子对。对内部溶剂化的苄基锂2b的数据的NMR线形分析提供了对配位的锂相对于苄基平面的分子内重新取向、围绕环-C-α键的旋转和双分子碳-锂键交换的动力学的定量洞察，这些动力学以广泛不同的增加速率的顺序列出，活化参数为相同顺序的Δ H-双匕首（kcal/mol）和Δ S-双匕首（eu）：14和6.6; 6.4和-14; 10.8和-21。
Benzyllithium compounds, hitherto assumed from NMR data to consist of ion pairs, have been found to exhibit spin coupling between C-13 and directly bound Li-6 under conditions wherein bimolecular carbon-lithium bond exchange is too slow to average the coupling constants. These conditions involved the use of species in which lithium is internally solvated or of dilute solutions (0.005 M) of benzyllithium-C-13-Li-6 (enriched at C-alpha) at low temperature. The low values of (1)J(C-13-Li-6), 3-4 Hz, imply a small detectable degree of C-Li covalence with the arrangement around C-alpha distorted from coplanarity. The C-Li bonds in benzyllithium are concluded to lie in a continuum of C-Li covalency between the many monomeric species in which (1)J(C-13-Li-6) is 16 +/- 1 Hz and separated ion pairs. NMR line shape analysis of data for internally solvated benzyllithium 2b provides quantitative insight into the dynamics of intramolecular reorientation of coordinated lithium with respect to the benzyl plane, rotation around the ring-C-alpha bond, and bimolecular carbon-lithium bond exchange, listed in order of widely different increasing rates, the activation parameters being Delta H-double dagger (kcal/mol) and Delta S-double dagger (eu) in the same order: 14 and 6.6; 6.4 and -14; 10.8 and -21.