Fressigne C, Maddaluno J, Marquez A, Giessner-Prettre C
Laboratoire des Fonctions Azotees & Oxygenees Complexes de l'IRCOF, UMR 6014 CNRS, Universite de Rouen, F-76821 Mont St Aignan Cedex, France, Departamento de Quimica Fisica, Facultad de Quimica, Universitad de Sevilla, E-41012 Sevi.
J Org Chem. 2000 Dec 29;65(26):8899-907. doi: 10.1021/jo000648u.
A DFT analysis of the condensation of monomeric methyllithium and lithium dimethylamide (LMA), as well as their homo and hetero dimers, on formaldehyde and acetaldehyde is reported. A stable complex, exhibiting a directional interaction between a lone pair of the oxygen on the aldehyde and a lithium, is first found. At this stage, the aldehyde carbonyl and the Li-X (X = C or N) bonds lie in the same plane. To proceed, the condensation reaction has to go through a transition state that mainly consists of a rotation of the aldehyde plane, placing it perpendicular to the C-C or C-N forming bond. The reaction then leads, in a strongly exothermic final step, to the addition product that is a lithium alcoholate or alpha-amino alcoholate, associating into an hetero-aggregate with the remaining moiety of the initial dimer. From the relative heights of the activation barriers, it appears that, for the heterodimer MeLi-LMA, the formation of the C-N bond should be kinetically favored over the C-C one, while the lithium ethylate resulting from the C-C binding is the thermodynamic product. A decomposition of the activation energy barriers has been carried out in order to determine the physicochemical forces responsible for the variation of the condensation activation barriers with the structure of the final species formed. The results obtained are discussed in relation with corresponding experimental data.
本文报道了对单体甲基锂和二甲基锂酰胺（LMA）及其同二聚体和异二聚体在甲醛和乙醛上缩合反应的密度泛函理论（DFT）分析。首先发现了一种稳定的络合物，醛基上的氧孤对与锂之间存在定向相互作用。在此阶段，醛羰基和Li-X（X = C或N）键位于同一平面内。为了继续反应，缩合反应必须经过一个过渡态，该过渡态主要由醛平面的旋转组成，使其与形成的C-C或C-N键垂直。然后，反应在一个强烈放热的最终步骤中生成加成产物，即锂醇盐或α-氨基醇盐，并与初始二聚体的剩余部分缔合形成异聚体。从活化能垒的相对高度来看，对于异二聚体MeLi-LMA，C-N键的形成在动力学上应比C-C键更有利，而由C-C键结合产生的乙醇锂是热力学产物。为了确定导致缩合活化能垒随最终形成物种结构变化的物理化学力，对活化能垒进行了分解。所得结果与相应的实验数据进行了讨论。
