UNUSUAL STABILITY OF AN ALKYLLITHIUM DIMER - PREPARATION, PROPERTIES, AND DECOMPOSITION MECHANISM OF (TERT-BULI.2OET2)2
UNUSUAL STABILITY OF AN ALKYLLITHIUM DIMER - PREPARATION, PROPERTIES, AND DECOMPOSITION MECHANISM OF (TERT-BULI.2OET2)2
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DOI：
10.1021/ja00223a032
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发表时间：
1988-07-20
影响因子：
15
通讯作者：
THOMAS, RD
中科院分区：
文献类型：
--
作者：
BATES, TF;CLARKE, MT;THOMAS, RD
用变温~（13）C和~（6Li）NMR研究了乙醚与正丁基锂在环戊烷中的相互作用和反应。在正丁基锂的碳氢化合物溶液中加入乙醚，在非配位四聚体和四配位二聚体之间产生动态的、依赖于温度的平衡，AH=-36 3 kcal/mol，AS=-155 15 eu。令人惊讶的是，氧与锂之比为2或更小的溶液在-80 ℃下几小时后没有显示出分解的迹象，而氧与锂之比大于2的溶液开始立即分解.这是解释的四配位的二聚体的反应，只有游离醚作为反应中的速率决定步骤。这可能对合成过程有一定的影响，通常是在低温下用正丁基锂在乙醚中进行的。四聚烷基锂在乙醚溶剂中的许多反应可能涉及解聚的物种（即二聚体或单体）作为反应中间体。1 n-BuLi的二聚体最近已被证明存在于THF溶液中，2· 3而Z-BuLi在THF中是单体，在乙醚中是二聚体。[4]非常需要对这些较小的配位聚集体的性质和反应性有更详细的了解，但是它们的反应性增加导致寿命显著降低。其他研究人员通过设计在烃溶剂中稳定的分子内配位的烷基锂化合物或使用非常快速的光谱技术来规避这个问题。3我们现在报道了四配位的Z-BuLi二聚体（z-BuLi-20 Et 2）2可以制备，并且当使用少于化学计量量的醚时，在烃溶剂中在低于-35 ℃下是稳定的。我们已经利用这种改进的稳定性来研究四聚体-二聚体平衡以及研究Z-BuLi与乙醚的反应机理。
Variable-temperature 13C and 6Li NMR spectroscopy were used to study the interaction and reaction of ethyl ether with ZerZ-butyllithium-6/,/in cyclopentane. Addition of ethyl ether to hydrocarbon solutions of ZerZ-butyllithiumproduces a dynamic, temperature-dependent equilibrium between uncoordinated tetramers and tetracoordinated dimers with AH=-36±3 kcal/mol and AS=-155±15 eu. Surprisingly, solutions with oxygen to lithium ratios of 2 or less show no signs of decomposition after several hours at-80 C, whereas solutions with a ratio greater than 2 begin to decompose immediately. This is explained in terms of the reaction of the tetracoordinated dimer with only free ether as the rate-determining step in the reaction. This may have implicationsfor syntheticprocedures normally carried out with ZerZ-butyllithium in diethyl ether at low temperature.Many reactions of tetrameric alkyllithium in ether solvents presumably involve deaggregated species (ie dimers or monomers) as the reactive intermediates. 1 The dimer of «-BuLi has recently been shown to be presentin THF solutions, 2· 3 while Z-BuLi is monomeric in THF and dimeric in ethyl ether. 4 A more detailed understanding of the nature and reactivity of these smaller co-ordinated aggregates would be highly desirable, but their increased reactivity results in a significantly decreased lifetime. Other researchers have circumvented this problem by devising intra-molecularly coordinated alkyllithium compounds that are stable in hydrocarbon solvent5 6or by using very rapid spectroscopic techniques. 3 We now report that tetracoordinated Z-BuLi dimer (z-BuLi-20Et2) 2 can be prepared and is stable below-35 C in hydrocarbon solvent when less than a stoichiometric amount of ether is used. We have taken advantage of this improved stability to study the tetramer-dimer equilibrium as well as to study the mechanism of reaction of Z-BuLi with ethylether.