Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States of America.
J Am Chem Soc. 2020 Mar 18;142(11):5314-5330. doi: 10.1021/jacs.0c00250. Epub 2020 Mar 4.
Mechanistic studies are reported on the inter- and intramolecular [2 + 2] alkene cycloadditions to form cyclobutanes promoted by (PDI)Fe(N) (PDI = 2,6-(2,4,6-tricyclopentyl)CHN = CMe)CHN). A combination of kinetic measurements, freeze-quench Fe Mössbauer and infrared spectroscopic measurements, deuterium labeling studies, natural abundance C KIE studies, and isolation and characterization of catalytically relevant intermediates were used to gain insight into the mechanism of both inter- and intramolecular [2 + 2] cycloaddition reactions. For the stereo- and regioselective [2 + 2] cycloaddition of 1-octene to form -1,2-dihexylcyclobutane, a first-order dependence on both iron complex and alkene was measured as well as an inverse dependence on N pressure. Both Fe Mössbauer and infrared spectroscopic measurements identified (PDI)Fe(N)(η-1-octene) as the catalyst resting state. Rate-determining association of 1-octene to (PDI)Fe(η-1-octene) accounts for the first order dependence of alkene and the inverse dependence on N. Heavy atom C/C kinetic isotope effects near unity also support post rate-determining C-C bond formation. By contrast, the intramolecular iron-catalyzed [2 + 2] cycloaddition of 1,7-octadiene yielded -bicyclo[4.2.0]octane in 92:8 d.r. and a first order dependence on the iron precursor and zeroth order behavior in both diene and N pressure were measured. A pyridine(diimine) iron -bimetallacycle was identified as the catalyst resting state and was isolated and characterized by X-ray diffraction and H NMR and Fe Mössbauer spectroscopies. Dissolution of the iron -bimetallacycle in benzene- produced predominantly the -cyclobutane product, establishing interconversion between the and metallacycles during the catalytic reaction and consistent with a Curtin-Hammett kinetic regime. A primary C/C kinetic isotope effect of 1.022(4) was measured at 23 °C, consistent with irreversible unimolecular reductive elimination to form the cyclobutane product. Despite complications from competing cyclometalation of chelate aryl substituents, deuterium labeling experiments were consistent with unimolecular C-C reductive elimination that occurred either by a concerted pathway or a radical rebound sequence that is faster than C-C bond rotation.
报告了（PDI）Fe（N）（PDI=2,6-（2,4,6-三环戊基）CHN=CM e）CHN）促进的烯键[2+2]环加成反应的分子间和分子内[2+2]环加成反应的机理。通过动力学测量、冷冻淬火 Fe Mössbauer 和红外光谱测量、氘标记研究、天然丰度 C KIE 研究以及催化相关中间体的分离和表征，深入了解了分子间和分子内[2+2]环加成反应的机理。对于 1-辛烯形成-1,2-二己基环丁烷的立体和区域选择性[2+2]环加成反应，测量了铁配合物和烯烃的一级依赖性以及 N 压力的反比依赖性。Fe Mössbauer 和红外光谱测量均确定（PDI）Fe（N）（η-1-辛烯）为催化剂的静止状态。1-辛烯与（PDI）Fe（η-1-辛烯）的速率决定缔合解释了烯烃的一级依赖性和 N 的反比依赖性。接近单位的重原子 C/C 动力学同位素效应也支持决定速率后 C-C 键的形成。相比之下，1,7-辛二烯的分子内铁催化[2+2]环加成反应以 92:8 d.r.生成-bicyclo[4.2.0]octane，并且测量了铁前体的一级依赖性以及二烯和 N 压力的零级行为。鉴定出吡啶（二亚胺）铁双金属环为催化剂的静止状态，并通过 X 射线衍射、H NMR 和 Fe Mössbauer 光谱对其进行了分离和表征。铁双金属环在苯中的溶解主要生成-环丁烷产物，在催化反应过程中建立了-和-金属环之间的相互转换，与 Curtin-Hammett 动力学范围一致。在 23°C 时测量到的初级 C/C 动力学同位素效应为 1.022（4），与形成环丁烷产物的不可逆单分子还原消除一致。尽管螯合芳基取代基的竞争环金属化存在复杂性，但氘标记实验与单分子 C-C 还原消除一致，该反应要么通过协同途径发生，要么通过比 C-C 键旋转更快的自由基回弹序列发生。
