- Formation and kinetic studies of manganese(IV)-oxo porphyrins: Oxygen atom transfer mechanism of sulfide oxidations
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Visible light irradiation of photo-labile porphyrin-manganese(III) chlorates or bromates (2) produced manganese(IV)-oxo porphyrins [MnIV(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-difluorophenyl)porphyrin?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[sbnd]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 σ values, indicating no significant positive charge developed at the sulfur during the oxidation process. The mechanistic results strongly suggest [MnIV(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.
- Klaine, Seth,Bratcher, Fox,Winchester, Charles M.,Zhang, Rui
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- A biomimetic oxidation catalyzed by manganese(III) porphyrins and iodobenzene diacetate: Synthetic and mechanistic investigations
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Abstract With iodobenzene diacetate [PhI(OAc)2] as the oxygen source, manganese(III) porphyrin complexes exhibit remarkable catalytic activity toward the selective oxidation of alkenes and activated hydrocarbons. Conspicuous is the fact that the readily soluble PhI(OAc)2 in the presence of a small amount of water is more efficient than the commonly used PhIO and other oxygen sources under same catalytic conditions. High selectivity for epoxides and excellent catalytic efficiency with up to 10,000 TON have been achieved in alkene epoxidations. It was found that the reactivity of manganese(III) porphyrin catalysts was greatly affected by axial ligand and the weakly binding perchlorate gave the highest catalytic activity in the epoxidation of alkenes. A manganese(IV)-oxo porphyrin was detected in the reaction of the manganese(III) porphyrin and PhI(OAc)2. However, our catalytic competition and Hammett studies have suggested that the more reactive manganese(V)-oxo intermediate was favored as the premier active oxidant, even it is too short-lived to be produced in detectable concentrations.
- Kwong, Ka Wai,Chen, Tse-Hong,Luo, Weilong,Jeddi, Haleh,Zhang, Rui
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- Enthalpy-Entropy Compensation Effect in Oxidation Reactions by Manganese(IV)-Oxo Porphyrins and Nonheme Iron(IV)-Oxo Models
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"Enthalpy-Entropy Compensation Effect"(EECE) is ubiquitous in chemical reactions; however, such an EECE has been rarely explored in biomimetic oxidation reactions. In this study, six manganese(IV)-oxo complexes bearing electron-rich and -deficient porphyrins are synthesized and investigated in various oxidation reactions, such as hydrogen atom transfer (HAT), oxygen atom transfer (OAT), and electron-transfer (ET) reactions. First, all of the six Mn(IV)-oxo porphyrins are highly reactive in the HAT, OAT, and ET reactions. Interestingly, we have observed a reversed reactivity in the HAT and OAT reactions by the electron-rich and -deficient Mn(IV)-oxo porphyrins, depending on reaction temperatures, but not in the ET reactions; the electron-rich Mn(IV)-oxo porphyrins are more reactive than the electron-deficient Mn(IV)-oxo porphyrins at high temperature (e.g., 0 °C), whereas at low temperature (e.g., -60 °C), the electron-deficient Mn(IV)-oxo porphyrins are more reactive than the electron-rich Mn(IV)-oxo porphyrins. Such a reversed reactivity between the electron-rich and -deficient Mn(IV)-oxo porphyrins depending on reaction temperatures is rationalized with EECE; that is, the lower is the activation enthalpy, the more negative is the activation entropy, and vice versa. Interestingly, a unified linear correlation between the activation enthalpies and the activation entropies is observed in the HAT and OAT reactions of the Mn(IV)-oxo porphyrins. Moreover, from the previously reported HAT reactions of nonheme Fe(IV)-oxo complexes, a linear correlation between the activation enthalpies and the activation entropies is also observed. To the best of our knowledge, we report the first detailed mechanistic study of EECE in the oxidation reactions by synthetic high-valent metal-oxo complexes.
- Guo, Mian,Zhang, Jisheng,Zhang, Lina,Lee, Yong-Min,Fukuzumi, Shunichi,Nam, Wonwoo
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supporting information
p. 18559 - 18570
(2021/11/22)
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- Biomimetic oxidation reactions of a naked manganese(V)-Oxo porphyrin complex
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The intrinsic reactivity of a manganese(V)-oxo porphyrin complex, a typically fleeting intermediate in catalytic oxidation reactions in solution, has been elucidated in a study focused on its gas-phase ion-chemistry. The naked high-valent MnV-oxo porphyrin intermediate 1 ([(tpfpp)Mn VO]+; tpfpp=meso-tetrakis(pentafluorophenyl)porphinato dianion), has been obtained by controlled treatment of [(tpfpp)Mn III]Cl (2-Cl) with iodosylbenzene in methanol, delivered in the gas phase by electrospray ionization and assayed by FT-ICR mass spectrometry. A direct kinetic study of the reaction with selected substrates, each containing a heteroatom X (X=S, N, P) including amines, sulfides, and phosphites, was thus performed. Ionic products arising from electron transfer (ET), hydride transfer (HT), oxygen-atom transfer (OAT), and formal addition (Add) may be observed, with a predominance of two-electron processes, whereas the product of hydrogen-atom transfer (HAT), [(tpfpp)MnIVOH]+, is never detected. A thermochemical threshold for the formation of the product radical cation allows an evaluation of the electron-transfer ability of a Mn V-oxo complex, yielding a lower limit of 7.85 eV for the ionization energy of gaseous [(tpfpp)MnIVO]. Linear free-energy analyses of the reactions of para-substituted N,N-dimethylanilines and thioanisoles indicate that a considerable amount of positive charge is developed on the heteroatom in the oxidation transition state. Substrates endowed with different heteroatoms, but similar ionization energy display a comparable reaction efficiency, consistent with a mechanism initiated by ET. For the first time, the kinetic acidity of putative hydroxo intermediates playing a role in catalytic oxidations, [(tpfpp)FeIVOH]+ and [(tpfpp)Mn IVOH]+, has been investigated with selected reference bases, revealing a comparatively higher basicity for the ferryl, [(tpfpp)Fe IVO], with respect to the manganyl, [(tpfpp)MnIVO], unit. Finally, the neat association reaction of 2 has been studied with various ligands showing that harder ligands are more strongly bound.
- Lanucara, Francesco,Crestoni, Maria Elisa
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experimental part
p. 12092 - 12100
(2011/11/28)
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- Laser flash photolysis generation and kinetic studies of porphyrin-manganese-oxo intermediates. Rate constants for oxidations effected by porphyrin-MnV-oxo species and apparent disproportionation equilibrium constants for porphyrin-MnIV-oxo species
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Porphyrin-manganese(V)-oxo and porphyrin-manganese(IV)-oxo species were produced in organic solvents by laser flash photolysis (LFP) of the corresponding porphyrin-manganese(III) perchlorate and chlorate complexes, respectively, permitting direct kinetic studies. The porphyrin systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20- tetrakis(pentafluorophenyl)porphyrin (TPFPP), and 5,10,15,20-tetrakis(4- methylpyridinium)porphyrin (TMPyP). The order of reactivity for (porphyrin)MnV(O) derivatives in self-decay reactions in acetonitrile and in oxidations of substrates was (TPFPP) > (TMPyP) > (TPP). Representative rate constants for reaction of (TPFPP)MnV(O) in acetonitrile are k = 6.1 ×105 M-1 s-1 for cis-stilbene and k = 1.4 × 105 M-1 s-1 for diphenylmethane, and the kinetic isotope effect in oxidation of ethylbenzene and ethylbenzene-d10 is kH/kD = 2.3. Competitive oxidation reactions conducted under catalytic conditions display approximately the same relative rate constants as were found in the LFP studies of (porphyrin)MnV(O) derivatives. The apparent rate constants for reactions of (porphyrin)MnIV(O) species show inverted reactivity order with (TPFPP) IV(O) disproportionates to (porphyrin)MnIIIX and (porphyrin)MnV(O), which is the primary oxidant, and the equilibrium constants for disproportionation of (porphyrin)MnIV(O) are in the order (TPFPP) V(O) with (TPFPP)MnIIICl to give (TPFPP)MnIV-(O) (k = 5 × 108 M-1 s -1) and disproportionation reaction of (TPP)MnIV(O) to give (TPP)MnV(O) and (TPP)-MnIIIX (k ≈ 2.5 × 109 M-1 s-1) were observed. The relative populations of (porphyrin)MnV(O) and (porphyrin)MnIV(O) were determined from the ratios of observed rate constants for self-decay reactions in acetonitrile and oxidation reactions of cis-stilbene by the two oxo derivatives, and apparent disproportionation equilibrium constants for the three systems in acetonitrile were estimated. A model for oxidations under catalytic conditions is presented.
- Zhang, Rui,Horner, John H.,Newcomb, Martin
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p. 6573 - 6582
(2007/10/03)
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