Kinetics of chromium(V)-oxo and chromium(IV)-oxo porphyrins: Reactivity and mechanism for sulfoxidation reactions.
Kinetics of chromium(V)-oxo and chromium(IV)-oxo porphyrins: Reactivity and mechanism for sulfoxidation reactions.
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DOI：
10.2139/ssrn.4163383
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发表时间：
2022-09
影响因子：
3.9
通讯作者：
Tristan Skipworth;Mardan Khashimov;Iyanu Ojo;Rui Zhang
中科院分区：
文献类型：
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作者：
Tristan Skipworth;Mardan Khashimov;Iyanu Ojo;Rui Zhang
在这项工作中，铬（IV）-氧代卟啉[CrIV（Por）（O）]（2）（Por =卟啉）通过[CrIII（Por）Cl]（1）与碘苯二乙酸盐的氧化或卟啉-铬（III）氯酸盐的可见光光解产生。随后用高氯酸银氧化2得到三种卟啉配体的铬（V）-氧代卟啉[CrV（Por）（O）]（ClO 4）（3），包括5，10，15，20-四（2，6-二氟苯基）卟啉（TMP，a），5，10，15，20-四（2，6-二氟苯基）卟啉（TDFPP，B）和5，10，15，20-四（五氟苯基）卟啉（TPFPP，c）.配合物2和3与茴香硫醚反应生成相应的亚砜，并研究了它们与一系列芳甲基硫醚（茴香硫醚）在有机溶液中的亚砜化反应动力学。铬（V）-氧代卟啉比铬（IV）-氧代物质的反应性高几个数量级，并且硫代安索氧化的代表性二级速率常数（kox）为（0.40 ± 0.01）M ~（-1）s ~（-1）（3a），（2.82 ± 0.20）× 102 M ~（-1）s ~（-1）（3b），（2.20 ± 0.01）× 103 M ~（-1）s ~（-1）（3c）。2和3的反应性顺序为TPFPP > TDFPP > TMP，与金属-氧代络合物的亲电性质一致。Hammett分析表明，[CrV（Por）（O）]+氧化对位取代的茴香硫醚的过渡态中存在显著的电荷转移。3a的ρ+常数为-1.69，3b为-2.63，3c为-2.89，这是先前发现的相关金属氧物种的镜像值。一个机制，涉及亲电攻击的CrV-氧代硫化物形成硫阳离子中间体的速率决定步骤的建议。竞争研究与铬（III）卟啉氯化物和PhI（乙酸乙酯）2给出了相对速率常数的氧化竞争thioanisoles密切匹配的绝对速率常数的比率从铬（V）-氧物种，这是真正的氧化剂在催化条件下。
In this work, chromium(IV)-oxo porphyrins [CrIV(Por)(O)] (2) (Por = porphyrin) were produced either by oxidation of [CrIII(Por)Cl] (1) with iodobenzene diacetate or visible light photolysis of porphyrin‑chromium(III) chlorates. Subsequent oxidation of 2 with silver perchlorate gave chromium(V)-oxo porphyrins [CrV(Por)(O)](ClO4) (3) in three porphyrin ligands, including 5,10,15,20-tetramesitylporphyrin(TMP, a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin(TDFPP, b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP, c). Complexes 2 and 3 reacted with thioanisoles to produce the corresponding sulfoxides, and their kinetics of sulfoxidation reactions with a series of aryl methyl sulfides(thioanisoles) were studied in organic solutions. Chromium(V)-oxo porphyrins are several orders of magnitudes more reactive than chromium(IV)-oxo species, and representative second-order rate constants (kox) for the oxidation of thioansole are (0.40 ± 0.01) M-1 s-1 (3a), and (2.82 ± 0.20) × 102 M-1 s-1 (3b), and (2.20 ± 0.01) × 103 M-1 s-1 (3c). The order of reactivity for 2 and 3 follows TPFPP > TDFPP > TMP, in agreement with the electrophilic nature of metal-oxo complexes. Hammett analyses indicate significant charge transfer in the transition states for oxidation of para-substituted thioanisoles by [CrV(Por)(O)]+. The ρ+ constants are -1.69 for 3a, -2.63 for 3b, and - 2.89 for 3c, respectively, mirror values found previously for related metal-oxo species. A mechanism involving the electrophilic attack of the CrV-oxo at sulfides to form a sulfur cation intermediate in the rate-determining step is suggested. Competition studies with chromium(III) porphyrin chloride and PhI(OAc)2 gave relative rate constants for oxidations of competing thioanisoles that closely match ratios of absolute rate constants from chromium(V)-oxo species, which are true oxidants under catalytic conditions.