Department of Chemistry and Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455-0431, USA.
J Am Chem Soc. 2010 Aug 18;132(32):11071-82. doi: 10.1021/ja101104q.
Distinguishing between the concerted second-order mechanism for beta-eliminations and nonconcerted mechanisms with discrete carbanion intermediates is very difficult experimentally, but the ability of quantum chemistry to find stationary points of the free-energy surface in liquid-phase solutions, even for complex reagents, provides a new tool for elucidating such mechanisms. Here we use liquid-phase density functional theory calculations to find transition states and intermediates on the free-energy surfaces of four base-initiated alpha,beta-eliminations of acetoxy and mesyloxy esters and their analogous thioesters. The geometries, free energies, and charge distributions of these structures support a stepwise irreversible first-order elimination from a conjugate base (E1cB(I)) mechanism with acetoxy ester 3, acetoxy thioester 4, and mesyloxy thioester 6. However, mesyloxy ester 5, which has an excellent nucleofuge and a less-acidic proton, follows a concerted but asynchronous E2 mechanism with an E1cB-like transition state. The anti transition state is more favorable than the syn one, even for the poorer nucleofuge and more-acidic thioesters. The article includes a general scheme for describing liquid-phase reactions in terms of free-energy surfaces.
区分协同二级机制和具有离散碳负离子中间体的非协同机制在实验上非常困难，但量子化学能够在液相溶液中找到自由能表面的稳定点，即使对于复杂的试剂，也为阐明这些机制提供了新的工具。在这里，我们使用液相密度泛函理论计算来寻找四种基于碱基引发的乙酰氧基和甲氧基酯及其类似硫代酯的α，β-消除的自由能表面上的过渡态和中间体。这些结构的几何形状、自由能和电荷分布支持从共轭碱（E1cB（I））机制逐步不可逆的一级消除，其中包括乙酰氧基酯 3、乙酰氧基硫代酯 4 和甲氧基硫代酯 6。然而，具有极好的离去基团和较弱酸性质子的甲氧基酯 5 遵循协同但异步的 E2 机制，具有类似于 E1cB 的过渡态。反过渡态比顺过渡态更有利，即使对于较差的离去基团和更酸性的硫代酯也是如此。本文包括了一种根据自由能表面描述液相反应的一般方案。
