Kalra Anshika, Bagchi Vivek, Paraskevopoulou Patrina, Das Purak, Ai Lin, Sanakis Yiannis, Raptopoulos Grigorios, Mohapatra Sudip, Choudhury Amitava, Sun Zhicheng, Cundari Thomas R, Stavropoulos Pericles
Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri 65409, United States.
Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri 65409, United States; Institute of Nano Science and Technology, Mohali, Punjab 160062, India.
Organometallics. 2021 Jun 4;40(12):1974-1996. doi: 10.1021/acs.organomet.1c00267.
Recent research has highlighted the key role played by the electron affinity of the active metal-nitrene/imido oxidant as the driving force in nitrene additions to olefins to afford valuable aziridines. The present work showcases a library of Co(II) reagents that, unlike the previously examined Mn(II) and Fe(II) analogues, demonstrate reactivity trends in olefin aziridinations that cannot be solely explained by the electron affinity criterion. A family of Co(II) catalysts (17 members) has been synthesized with the assistance of a trisphenylamido-amine scaffold decorated by various alkyl, aryl, and acyl groups attached to the equatorial amidos. Single-crystal X-ray diffraction analysis, cyclic voltammetry and EPR data reveal that the high-spin Co(II) sites ( = 3/2) feature a minimal [NN] coordination and span a range of 1.4 V in redox potentials. Surprisingly, the Co(II)-mediated aziridination of styrene demonstrates reactivity patterns that deviate from those anticipated by the relevant electrophilicities of the putative metal nitrenes. The representative LCo catalyst (-COCMe arm) is operating faster than the LCo analogue (-COCF arm), in spite of diminished metal-nitrene electrophilicity. Mechanistic data (Hammett plots, KIE, stereocontrol studies) reveal that although both reagents follow a two-step reactivity path (turnover-limiting metal-nitrene addition to the C atom of styrene, followed by product-determining ring-closure), the LCo catalyst is associated with lower energy barriers in both steps. DFT calculations indicate that the putative [LCo]NTs and [LCo]NTs species are electronically distinct, inasmuch as the former exhibits a single-electron oxidized ligand arm. In addition, DFT calculations suggest that including London dispersion corrections for LCo (due to the polarizability of the -Bu substituent) can provide significant stabilization of the turnover-limiting transition state. This study highlights how small ligand modifications can generate stereoelectronic variants that in certain cases are even capable of overriding the preponderance of the metal-nitrene electrophilicity as a driving force.
最近的研究强调了活性金属氮烯/亚胺基氧化剂的电子亲合性在氮烯加成到烯烃以生成有价值的氮杂环丙烷反应中所起的关键作用。目前的工作展示了一系列钴（II）试剂，与之前研究的锰（II）和铁（II）类似物不同，这些钴（II）试剂在烯烃氮杂环丙烷化反应中表现出的反应趋势不能仅用电子亲合性标准来解释。借助于由连接在赤道酰胺基上的各种烷基、芳基和酰基修饰的三苯基氨基胺支架，合成了一族钴（II）催化剂（17个成员）。单晶X射线衍射分析、循环伏安法和电子顺磁共振数据表明，高自旋钴（II）位点（S = 3/2）具有最小的[NN]配位，氧化还原电位范围为1.4 V。令人惊讶的是，钴（II）介导的苯乙烯氮杂环丙烷化反应表现出的反应模式与假定的金属氮烯的相关亲电性所预期的不同。尽管金属氮烯亲电性降低，但代表性的LCo催化剂（-COCMe臂）的反应速度比LCo类似物（-COCF臂）快。机理数据（哈米特图、动力学同位素效应、立体控制研究）表明，尽管两种试剂都遵循两步反应路径（限速的金属氮烯加成到苯乙烯的C原子上，然后是决定产物的闭环反应），但LCo催化剂在这两个步骤中都具有较低的能垒。密度泛函理论计算表明，假定的[LCo]NTs和[LCo]NTs物种在电子性质上是不同的，因为前者表现出单电子氧化的配体臂。此外，密度泛函理论计算表明，对LCo包含伦敦色散校正（由于-Bu取代基的极化率）可以显著稳定限速过渡态。这项研究突出了小的配体修饰如何能够产生立体电子变体，在某些情况下，这些变体甚至能够超越金属氮烯亲电性作为驱动力的主导地位。
