环戊烯与四氧化锇环加成反应的机理研究

The mechanism of cycloaddition reaction between osmium tetraoxide and cyclopentene

环加成反应是合成杂环化合物的常用方法之一,四氧化锇与直链烯烃的环加成反应是四氧化锇氧化烯烃双羟化中的关键步骤.结合相对论赝势和自然键轨道(NBO)分析,运用M06-2X方法研究了四氧化锇与环戊烯环加成生成含五元锇杂环的锇酸酯反应的机理.结果表明该反应存在[3+2]一步协同环加成和[2+2]环加成-扩环的分步方式两种历程.其中,[2+2]加成-扩环方式两步的能垒分别高达181.9 kJ/mol和109.9 kJ/mol,而经历[3+2]一步协同加成方式的能垒仅为10.8 kJ/mol,因此[2+2]环加成-扩环分步路径在动力学上非常不利,反应主要以[3+2]的协同方式进行.一方面,形成[3+2]的过渡态时有较小的形变能.另一方面,相较于[2+2]过渡态,在[3+2]过渡态中存在更多的由金属向氧配体再到环烯双键的反键轨道的电子转移,使得在[3+2]方式中更容易活化烯烃的双键,能垒较小.

Cycloaddition reaction is one of the common methods for the synthesis of heterocyclic compounds. The cycloaddition reaction between osmium tetroxide and linear olefins is a key step in the dihydroxylation of olefins by osmium tetroxide. Combined with relativistic pseudopotential and the analysis of natural bond orbital(NBO), the mechanism of the cycloaddition of osmium tetroxide and cyclopentene to osmate containing five-membered osmium heterocycle was studied by M06-2 X method. The results show that there are two processes of [3+2] one-step concerted cycloaddition and [2+2] cycloaddition-ring expansion step-by-step way. Among these, the energy barriers of the two steps in [2+2] addition-ring expansion pathway are as high as 181.9 kJ/mol and 109.9 kJ/mol respectively, while the energy barrier of the one-step concerted [3+2] addition is only 10.8 kJ/mol. Therefore, the step-by-step pathway of [2+2] cycloaddition-ring expansion is very unfavorable kinetically, and the reaction mainly proceeded in the concerted way of [3+2] cycloaddition. On the one hand, the transition state of [3+2] addition has smaller deformation energy. On the other hand, compared to the [2+2] transition state, there is more electron transfer from metal to oxygen ligand and then to the antibonding orbital of cycloolefin double bond in the [3+2] transition state, which makes it easier to activate double bond of olefin in the [3+2] mode and has a smaller energy barrier.