Jordan Robert W, Khoury Peter R, Goddard John D, Tam William
Guelph-Waterloo Centre for Graduate Work in Chemistry and Biochemistry, Department of Chemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1.
J Org Chem. 2004 Nov 26;69(24):8467-74. doi: 10.1021/jo048590x.
The ruthenium-catalyzed [2 + 2] cycloadditions of 7-substituted norbornadienes with an alkyne have been investigated. The cycloadditions were found to be highly regio- and stereoselective, giving only the anti-exo cycloadducts as the single regio- and stereoisomers in good yields. The results on the relative rate of different 7-substituted norbornadienes in the Ru-catalyzed [2 + 2] cycloadditions with an alkyne indicated that the reactivity of the alkene component decreases dramatically as the alkene becomes more electron deficient. Ab initio computational studies on the ruthenium-catalyzed [2 + 2] cycloadditions provided important information about the geometries and the arrangements of the four different groups on the Ru in the initial Ru-alkene-alkyne pi-complex, 14, and in the metallacyclopentene 15. Based on our computational studies, we also found that the first carbon-carbon bond formed in the [2 + 2] cycloaddition is between the C(5) of the alkene and the C(b) (the acetylenic carbon attached to the ester group) of the alkyne 8. Our computational studies on the potential energy profiles of the cycloadditions showed that the activation energy relative to the reactants for the oxidative addition step is in the range of 9.3-9.8 kcal/mol. The activation energy relative to the metallacyclopentene for the reductive elimination step is much higher than for the oxidative addition step (in the range of 25.9-27.6 kcal/mol).
对钌催化的7-取代降冰片二烯与炔烃的[2 + 2]环加成反应进行了研究。发现该环加成反应具有高度的区域选择性和立体选择性，仅以良好的产率得到反式外型环加成产物作为单一的区域和立体异构体。关于不同7-取代降冰片二烯在钌催化的与炔烃的[2 + 2]环加成反应中的相对速率的结果表明，随着烯烃电子缺乏程度增加，烯烃组分的反应活性显著降低。对钌催化的[2 + 2]环加成反应的从头算计算研究提供了有关初始钌-烯烃-炔烃π-络合物14和金属环戊烯15中钌上四个不同基团的几何结构和排列的重要信息。基于我们的计算研究，我们还发现[2 + 2]环加成反应中形成的第一个碳-碳键是在烯烃的C(5)与炔烃8的C(b)（连接到酯基的炔碳）之间。我们对环加成反应势能面的计算研究表明，氧化加成步骤相对于反应物的活化能在9.3 - 9.8 kcal/mol范围内。还原消除步骤相对于金属环戊烯的活化能远高于氧化加成步骤（在25.9 - 27.6 kcal/mol范围内）。
