Mauksch Michael, Tsogoeva Svetlana B
Department of Chemistry and Pharmacy, Institute of Theoretical Chemistry, Computer Chemistry Center, Nägelsbachstrasse 25a, 91052, Erlangen, Germany.
Department of Chemistry and Pharmacy, Organic Chemistry Chair I and Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Henkestrasse 42, 91054, Erlangen, Germany.
Chemistry. 2017 Aug 1;23(43):10264-10269. doi: 10.1002/chem.201702276. Epub 2017 Jul 6.
Inspired by recent reports of low-valent iron-complex-catalyzed formal [2+2] cycloaddition of olefins, we demonstrate computationally that with such low-valent iron complexes and with "strong" ligands, the olefin metathesis is also preferred over the undesired cyclopropanation side-reaction, competition already studied by Hoffmann and co-workers almost 40 years ago (J. Am. Chem. Soc. 1981, 103, 5582). The [2+2] cycloaddition step in metathesis propagation, which gives a Chauvin-type metallacyclobutane intermediate, is proposed to proceed either via a planar four-electron Craig-Möbius aromatic [π2 +π2 ] transition-state structure with a low barrier of 4.7 kcal mol or, alternatively, via a twisted Zimmerman-Möbius aromatic [π2 +π2 ] transition state with a 5.5 kcal mol activation-energy barrier, with respect to an "encounter" π-complex minimum obtained from an Fe alkylidene and the entering olefin, while the corresponding triplet pathways are all disfavored.
受近期关于低价铁配合物催化烯烃的形式[2+2]环加成反应报道的启发，我们通过计算证明，使用此类低价铁配合物和“强”配体时，烯烃复分解反应也比不期望的环丙烷化副反应更占优势，近40年前Hoffmann及其同事就已研究过这种竞争情况（《美国化学会志》，1981年，第103卷，5582页）。复分解反应传播过程中的[2+2]环加成步骤会生成一个肖万型金属环丁烷中间体，该步骤被认为要么通过平面四电子克雷格 - 莫比乌斯芳香性[π2 +π2 ]过渡态结构进行，其势垒较低，为4.7千卡/摩尔，要么通过扭曲的齐默尔曼 - 莫比乌斯芳香性[π2 +π2 ]过渡态进行，活化能势垒为5.5千卡/摩尔，这是相对于从亚烷基铁和进入的烯烃得到的“相遇”π配合物最小值而言的，而相应的三重态途径均不占优势。
