Formation and kinetic studies of manganese(IV)-oxo porphyrins: Oxygen atom transfer mechanism of sulfide oxidations
Formation and kinetic studies of manganese(IV)-oxo porphyrins: Oxygen atom transfer mechanism of sulfide oxidations
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DOI：
10.1016/j.jinorgbio.2019.110986
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发表时间：
2020-03-01
影响因子：
3.9
通讯作者：
Zhang, Rui
中科院分区：
文献类型：
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作者：
Klaine, Seth;Bratcher, Fox;Zhang, Rui
可见光照射光不稳定的卟啉-锰（III）氯酸盐或溴酸盐(2)产生三种卟啉配体中的锰(IV)-氧卟啉[Mn-IV(Por)(O)] （Por =卟啉）(3)。相应的锰（III）前体(1)与二乙酸碘苯即PhI(OAc)(2)的化学氧化也形成了相同的氧物种3。所研究的体系包括5,10,15,20-四（五氟苯基）卟啉-锰(IV)-氧（3a）， 5,10,15,20-四(2,6-二氟苯卟啉-锰(IV)-氧（3b）和5,10,15,20-四酰基卟啉-锰(IV)-氧（3c）。正如预期的那样，配合物3与硫苯甲醚反应生成相应的亚砜和过氧化砜。在CH3CN溶液中研究了这些生成的3与芳基硫化物的氧原子转移反应动力学。硫化物氧化反应的二级速率常数与烯烃环氧化反应和活化的C-H键氧化反应的二级速率常数相当。对于给定的底物，锰(IV)-氧的反应顺序为3a > 3b > 3c，与卟啉大环的吸电子能力相一致。自由能Hammett分析与o值呈近线性相关，表明在氧化过程中硫没有产生显著的正电荷。机理结果强烈表明[Mn-IV(Por)(O)]作为直接OAT剂通过在本研究中检测到的锰（II）中间体与硫化物底物发生反应。然而，当反应性较差的底物存在时，涉及3歧化形成更高氧化锰(V)-氧的替代途径可能是显著的。Hammett相关图的竞争产物研究证实，锰（III）卟啉与PhI（OAc）催化的硫化物氧化不是真正的氧化剂(2)。
Visible light irradiation of photo-labile porphyrin-manganese(III) chlorates or bromates (2) produced manganese (IV)-oxo porphyrins [Mn-IV(Por)(O)] (Por = porphyrin) (3) in three porphyrin ligands. The same oxo species 3 were also formed by chemical oxidation of the corresponding manganese(III) precursors (1) with iodobenzene diacetate, i.e. PhI(OAc)(2). The systems under study include 5,10,15,20-tetra(pentafluorophenyl)porphyrin-manganese(IV)-oxo (3a), 5,10,15,20-tetra(2,6-difluorophenyeporphyrin-manganese(IV)-oxo (3b), and 5,10,15,20-tetramesitylporphyrin-manganese(IV)-oxo (3c). As expected, complexes 3 reacted with thioanisoles to produce the corresponding sulfoxides and over-oxidized sulfones. The kinetics of oxygen atom transfer (OAT) reactions of these generated 3 with aryl sulfides were studied in CH3CN solutions. Second-order rate constants for sulfide oxidation reactions are comparable to those of alkene epoxidations and activated C-H bond oxidations by the same oxo species 3. For a given substrate, the reactivity order for the manganese(IV)-oxo species was 3a > 3b > 3c, consistent with expectations on the basis of the electron-withdrawing capacity of the porphyrin macrocycles. Free-energy Hammett analyses gave near-linear correlations with o values, indicating no significant positive charge developed at the sulfur during the oxidation process. The mechanistic results strongly suggest [Mn-IV(Por)(O)] reacts as a direct OAT agent towards sulfide substrates through a manganese(II) intermediate that was detected in this work. However, an alternative pathway that involves a disproportionation of 3 to form a higher oxidized manganese(V)-oxo species may be significant when less reactive substrates are present. The competition product studies with the Hammett correlation plot confirmed that the observed manganese(IV)-oxo species is not the true oxidant for the sulfide oxidations catalyzed by manganese(III) porphyrins with PhI(OAc)(2).