C(sp3)-H bond activation by the carboxylate-adduct of osmium tetroxide (OsO4)
C(sp3)-H bond activation by the carboxylate-adduct of osmium tetroxide (OsO4)
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四氧化锇 (OsO4) 的羧酸盐加合物活化 C(sp3)-H 键
DOI：
10.1039/d1dt03819b
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发表时间：
2022
影响因子：
4
通讯作者：
Shinobu Itoh
中科院分区：
文献类型：
--
作者：
Tomohiro Fujimoto;Yuka Hirata;Hideki Sugimoto;Mayuko Miyanishi;Yoshihito Shiota;Kazunari Yoshizawa;Shinobu Itoh
四氧化锇（OsO 4）与羧酸根阴离子（乙酸根：X− = AcO−和苯甲酸根：X− = BzO−）反应生成1：1的加合物[OsO 4（X）]−（1X），其结构通过X射线晶体学分析确定。 在这两种情况下，羧酸根阴离子X与锇中心配位，生成扭曲的三角双锥锇（VIII）络合物。羧酸盐加合物的氧化还原电位（E1/2）比四面体OsO 4本身负移，νOsO伸缩峰红移.尽管E1/2负移，这些加合物1X的反应性增强相比，OsO 4在苄基C（sp3）-H键氧化。该反应在1X和底物上均服从一级动力学，给出了二级速率常数（k2），其与底物的C-H键离解能（BDEC-H）呈线性相关（咕吨，9，10-二氢蒽，芴和1，2，3，4-四氢萘）和9.7（k2（咕吨-h2）/k2（咕吨-d2））的动力学氘同位素效应（KIE）。这些动力学数据的基础上，连同DFT计算结果，我们提出了一个逐步的反应机制，涉及限速苄基氢原子提取和随后反弹的生成的有机自由基中间体上的锇中心的剩余oxido基团。
The reaction of osmium tetroxide (OsO4) and carboxylate anions (acetate: X− = AcO− and benzoate: X− = BzO−) gave 1 : 1 adducts, [OsO4(X)]− (1X), the structures of which were determined by X-ray crystallographic analysis. In both cases, the carboxylate anion X coordinates to the osmium centre to generate a distorted trigonal bipyramidal osmium(VIII) complex. The carboxylate adducts show a negative shift of the redox potentials (E1/2) and a red shift of the νOsO stretches as compared to those of tetrahedral OsO4 itself. Despite the negative shift of E1/2, the reactivity of these adduct complexes 1X was enhanced compared to that of OsO4 in benzylic C(sp3)–H bond oxidation. The reaction obeyed the first-order kinetics on both 1X and the substrates, giving the second-order rate constant (k2), which exhibits a linear correlation with the C–H bond dissociation energy (BDEC–H) of the substrates (xanthene, 9,10-dihydroanthracene, fluorene and 1,2,3,4-tetrahydronaphthalene) and a kinetic deuterium isotope effect (KIE) of 9.7 (k2(xanthene-h2)/k2(xanthene-d2)). On the basis of these kinetic data together with the DFT calculation results, we propose a stepwise reaction mechanism involving rate-limiting benzylic hydrogen atom abstraction and subsequent rebound of the generated organic radical intermediate to a remaining oxido group on the osmium centre.