Division of Chemical and Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, 1 Nanyang Link, Singapore 637371, Singapore.
Chemistry. 2010 Jan 11;16(2):588-94. doi: 10.1002/chem.200901727.
We carried out a principle study on the reaction mechanism of rhodium-catalyzed intramolecular aziridination and aziridine ring opening at a sugar template. A sulfamate ester group was introduced at different positions of glycal to act as a nitrene source and, moreover, to allow the study of the relative reactivity of the nitrene transfer from different sites of the glycal molecule. The structural optimization of each intermediate along the reaction pathway was extensively done by using BPW91 functional. The crucial step in the reaction is the Rh-catalyzed nitrene transfer to the double bond of the glycal. We found that the reaction could proceed in a stepwise manner, whereby the N atom initially induced a single-bond formation with C1 on the triplet surface or in a single step through intersystem crossing (ISC) of the triplet excited state of the rhodium-nitrene transition state to the singlet ground state of the aziridine complexes. The relative reactivity for the conversion of the nitrene species to the aziridine obtained from the computed potential energy surface (PES) agrees well with the reaction time gained from experimental observation. The aziridine ring opening is a spontaneous process because the energy barrier for the formation of the transition state is very small and disappears in the solution calculations. The regio- and stereoselectivity of the reaction product is controlled by the electronic property of the anomeric carbon as well as the facial preference for the nitrene insertion, and the nucleophilic addition.
我们在糖模板上进行了铑催化的分子内氮丙啶化和氮丙啶环开环反应机理的基础研究。在糖醛的不同位置引入磺酰胺酯基作为氮宾源，并允许研究氮宾从糖醛分子不同位置转移的相对反应性。通过使用 BPW91 函数，对反应途径中每个中间体的结构进行了广泛的优化。反应的关键步骤是铑催化的氮宾向糖醛双键的转移。我们发现反应可以分步进行，其中 N 原子最初在三重态表面上与 C1 诱导单键形成，或者通过三重态铑-氮宾过渡态的系间穿越（ISC）到氮丙啶络合物的单线态基态一步进行。从计算得到的势能面（PES）获得的氮宾转化为氮丙啶的相对反应性与从实验观察获得的反应时间很好地吻合。氮丙啶环的开环是一个自发的过程，因为形成过渡态的能垒非常小，并且在溶液计算中消失。反应产物的区域和立体选择性由端基碳原子的电子性质以及氮宾插入、亲核加成的面选择性控制。
