Fressigné Catherine, Lautrette Anne, Maddaluno Jacques
Institut de Recherche en Chimie Organique Fine, UMR 6014 CNRS, Université de Rouen, 76821 - Mont St Aignan Cedex (France).
J Org Chem. 2005 Sep 30;70(20):7816-28. doi: 10.1021/jo050524n.
[Chemical reaction: See text] The interaction between three different sp2 organolithium compounds (vinyllithium, 2-methoxyvinyllithium and phenyllithium) and formaldehyde has been investigated using DFT theoretical methods. The unsolvated monomers and dimers have been considered and compared to the 1:1 mixed aggregates formed with lithium dimethylamide. In all cases, the separate entities, their docking complexes, the transition states, and the condensation products have been characterized and compared to the corresponding situations involving methyllithium, taken as a prototypic sp3 nucleophile. Regarding the monomers, this study shows that, in the three cases considered, formaldehyde forms a pretransition state complex in which the oxygen of the carbonyl interacts with the lithium cation along one of its lone pair. A small energy barrier (< or =2.1 kcal.mol(-1)) brings to the transition state, then to the lithium alcoholate resulting from the largely exothermic condensation (approximately 40 kcal.mol(-1)). The structure of the homogeneous dimers considered in a second step has been optimized and lead to arrangements in which a planar quadrilateral C-Li-C-Li is always obtained. In the presence of formaldehyde, these entities provide complexes exhibiting lithium-oxygen interaction similar to those occurring with the monomers. For the dimers, the geometry at the TS evokes a pi-complex between the C=O and the lithium cation, particularly pronounced in the case of phenyllithium. The resulting alcoholates are obtained following a larger exothermic reaction (approximately 55 kcal.mol(-1)). The heterogeneous dimers with lithium dimethylamide have been finally examined. In these cases, the aldehyde can orientate toward either the carbon or the nitrogen, leading to the expected lithium alcoholate or alpha-amino alcoholate, respectively. Whatever the orientation, the complexes present characteristics close to those calculated for the homogeneous dimer complexes. These similarities are conserved at the transition state.
[化学反应：见正文] 运用密度泛函理论方法研究了三种不同的sp²有机锂化合物（乙烯基锂、2-甲氧基乙烯基锂和苯基锂）与甲醛之间的相互作用。研究考虑了未溶剂化的单体和二聚体，并与由二甲基锂酰胺形成的1:1混合聚集体进行了比较。在所有情况下，对单独的实体、它们的对接复合物、过渡态和缩合产物进行了表征，并与涉及甲基锂（作为典型的sp³亲核试剂）的相应情况进行了比较。关于单体，本研究表明，在所考虑的三种情况下，甲醛形成了一种预过渡态复合物，其中羰基的氧沿着其孤对之一与锂阳离子相互作用。一个小的能垒（≤2.1千卡·摩尔⁻¹）导致形成过渡态，进而形成由大量放热缩合反应（约40千卡·摩尔⁻¹）产生的醇锂盐。第二步中考虑的均相二聚体的结构已得到优化，并得到了总是形成平面四边形C-Li-C-Li排列的结构。在甲醛存在的情况下，这些实体提供了与单体类似的具有锂-氧相互作用的复合物。对于二聚体，过渡态的几何结构引发了C=O与锂阳离子之间的π-复合物，在苯基锂的情况下尤为明显。由此产生的醇锂盐是通过更大的放热反应（约55千卡·摩尔⁻¹）得到的。最后研究了与二甲基锂酰胺形成的非均相二聚体。在这些情况下，醛可以分别朝向碳或氮定向，分别导致预期的醇锂盐或α-氨基醇锂盐。无论何种取向，这些复合物的特征都与为均相二聚体复合物计算的特征相近。这些相似性在过渡态时得以保留。
