State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences , 345 Lingling Road, Shanghai 200032, P. R. China.
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, P. R. China.
J Am Chem Soc. 2017 Mar 15;139(10):3876-3888. doi: 10.1021/jacs.7b00484. Epub 2017 Mar 1.
Transition-metal alkylidenes are important reactive organometallic intermediates, and our current knowledge on them has been mainly restricted to those with closed-shell electronic configurations. In this study, we present an exploration on open-shell iron alkylidenes with a weak-field tripodal amido-phosphine-amido ligand. We found that a high-spin (amido-phosphine-amido)iron(II) complex can react with (p-tolyl)CN to afford a high-spin (amido-ylide-amido)iron(II) complex, 2, which could transfer its alkylidene moiety to a variety of alkenes, either the electron-rich or electron-deficient ones, to form cyclopropane derivatives. The reaction of 2 with cis-β-deuterio-styrene gave deuterated cyclopropane derivatives with partial loss of the stereochemical integrity with respect to the cis-styrene. Kinetic study on the cyclopropanation reaction of 2 with 4-fluoro-styrene disclosed the activation parameters of ΔH = 23 ± 1 kcal/mol and ΔS = -20 ± 3 cal/mol/K, which are comparable to those of the cyclopropanation reactions involving transition-metal alkylidenes. However, the cyclopropanation of para-substituted styrenes by 2 shows a nonlinear Hammett plot of log(k/k) vs σ. By introduction of a radical parameter, a linear plot of log(k/k) vs 0.59σ + 0.55σ was obtained, which suggests the "nucleophilic" radical nature of the transition state of the cyclopropanation step. In corroboration with the experimental observations, density functional theory calculation on the reaction of 2 with styrene suggests the involvement of an open-shell (amido-phosphine-amido)iron alkylidene intermediate that is higher in energy than its (amido-ylide-amido)iron(II) precursor and an "outer-sphere" radical-type mechanism for the cyclopropanation step. The negative charge distribution on the alkylidene carbon atoms of the open-shell states (S = 2 and 1) explains the high activity of the cyclopropanation reaction toward electron-deficient alkenes. The study demonstrates the unique activity of open-shell iron alkylidene species beyond its closed-shell analogues, thus pointing out their potential synthetic usage in catalysis.
过渡金属亚烷基是重要的反应性有机金属中间体，我们目前对它们的了解主要限于那些具有满壳电子构型的化合物。在这项研究中，我们探索了具有弱场三角架酰胺-膦-酰胺配体的开壳铁亚烷基。我们发现，高自旋（酰胺-膦-酰胺）铁（II）配合物可以与（对甲苯基）CN 反应，得到高自旋（酰胺-亚基-酰胺）铁（II）配合物 2，后者可以将其亚烷基部分转移到各种烯烃上，无论是富电子的还是缺电子的，形成环丙烷衍生物。2 与顺-β-氘代-苯乙烯的反应得到了部分失去顺式-苯乙烯立体化学完整性的氘代环丙烷衍生物。对 2 与 4-氟-苯乙烯的环丙烷化反应的动力学研究揭示了活化参数 ΔH = 23 ± 1 kcal/mol 和 ΔS = -20 ± 3 cal/mol/K，与涉及过渡金属亚烷基的环丙烷化反应相当。然而，2 对取代的苯乙烯的环丙烷化反应呈现出非线性的哈米特图 log(k/k) vs σ。通过引入自由基参数，得到了 log(k/k) vs 0.59σ + 0.55σ 的线性图，这表明环丙烷化步骤过渡态具有“亲核”自由基性质。与实验观察结果一致，对 2 与苯乙烯反应的密度泛函理论计算表明，涉及开壳（酰胺-膦-酰胺）铁亚烷基中间体，其能量高于其（酰胺-亚基-酰胺）铁（II）前体，并且环丙烷化步骤的“外球”自由基型机制。开壳态（S = 2 和 1）中亚烷基碳原子上的负电荷分布解释了其对缺电子烯烃的高环丙烷化反应活性。该研究展示了开壳铁亚烷基物种的独特活性，超越了其闭壳类似物，从而指出了它们在催化中的潜在合成用途。
