Anna C.
Brezny
*a,
James H.
Lyke
a,
Sigrid E.
Olsen
a,
Emma R.
Straton
b and
Olha
Zubarieva
a
aDepartment of Chemistry, St. Olaf College, Northfield, MN 55057, USA. E-mail: brezny1@stolaf.edu
bDepartment of Chemistry, Skidmore College, Saratoga Springs, NY 12866, USA
First published on 1st July 2025
Ferric porphyrins catalyse the aerobic oxidative cleavage of 2,3-dimethylindole, mimicking dioxygenase enzymes. The data herein suggest that the indole reduces the FeIII(porphyrin) to FeII(porphyrin) via a proton-coupled electron transfer mechanism. Subsequently, FeII(porphyrin) binds O2 and generates the critical superoxo intermediate, which oxidatively cleaves a second equivalent of substrate.
Common biomimics of heme-containing enzymes use iron porphyrin complexes such as Fe(TPP) (TPP = meso-tetraphenylporphyrin).4 Synthetic metalloporphyrins are known to activate O2, and are often used in the context of the oxygen reduction reaction.5–9 Stahl recently reported the use of O2-derived metal–oxos to achieve dioxygenase reactivity.10
Fe(TPP) has been used with indole substrates in order to mimic TDO and IDO. Both dioxygenase enzymes are thought to oxidize the substrate via an iron superoxo intermediate (FeIII(O2˙−)).11–14 In order to provide more evidence that the active oxidant is FeIII(O2˙−), recent work from the Goldberg and Wijeratne labs demonstrated that synthetic iron porphyrin superoxo complexes can oxidatively cleave indoles in a stoichiometric fashion.15,16 These elegant studies showed that at low temperatures the FeIII(O2˙−)(TPP) complexes react with 2,3-dimethyl-1H-indole (2,3-DMI, 1) (Scheme 2). More specifically, the superoxo complex oxidatively cleaves the CC and generates N-(2-acetyl-phenyl)-acetamide (2) in moderate (49%15 and 31%16) yields. Wijeratne showed that the cleavage reactions occur faster with electron-poor iron porphyrins such as Fe(PFPP)(L) (PFPP = meso-pentafluorophenylporphyrin).
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Scheme 2 Goldberg and Wijeratne demonstrated the stoichiometric oxidation of indoles with FeIII(O2˙−)(TPP) complexes. |
Work from Dey and coworkers explored a catalytic oxidative cleavage of 2,3-dimethylindole by an iron picket fence porphyrin attached to a self-assembled monolayer-modified gold electrode.17,18 Using in situ resonance Raman spectroscopy, they demonstrated that under these electrochemical conditions, the FeIII(O2˙−)(porphyrin) is the active oxidant as well. In all of these aforementioned biomimetic systems, 2,3-dimethylindole serves as a model substrate due to its rapid reaction with the iron superoxo species.
We were interested in further exploring the reactivity of FePFPP with indole substrates. Surprisingly, upon combining FeIII(PFPP)Cl with 2,3-dimethylindole in air, we observed formation of the cleavage product 2 in 35% yield assuming 1:
1 stoichiometry (Scheme 3). This reactivity was unexpected because the iron porphyrin was added to the reaction in the +3 oxidation state, but superoxo complexes are generated from reaction of O2 with an iron site in the +2 oxidation state. Intrigued by this result, we sought to understand how this unexpected reactivity could occur.
One key observation was that during the reaction, the solution visually changed from a green-brown color to a bright red, suggesting a change in metal oxidation state. In order to investigate this observation quantitatively, in situ UV-Vis spectra were used to interrogate the catalyst resting state. We compared the catalytic reaction spectra to those of independently prepared FeIII(PFPP)Cl and FeIII(O2˙−)(PFPP)(L) complexes, where L is presumably acetonitrile solvent. The in situ spectra revealed that the majority of iron during catalysis is in the form of the superoxo complex (Fig. 1). The presence of the FeIII(O2˙−)(PFPP)L complex is consistent with the observed red color. This is a surprising result because only iron(III) was used and no reductant was added to the reaction mixture, yet only iron(II) binds dioxygen.
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Fig. 1 UV-Vis spectrum of Q-bands of catalyst during the oxidative cleavage of 2,3-dimethyl indole (black) compared to genuine Fe(PFPP)Cl (green) and Fe(O2˙−)(PFPP) (red). |
In order to understand how the superoxo species was being generated during catalysis, we considered several possible mechanisms. The first was a photocatalytic reaction for dioxygen activation and substrate oxidation. Several studies with metalloporphyrins have proposed the formation of a superoxo where the MIII(P) undergoes photoexcitation, followed by intersystem crossing to form a relatively long-lived excited state. This complex then binds O2 to generate a MIV-superoxo complex, which is proposed to oxidize substrates.19–24 We compared the yields when the reaction vessel was exposed to ambient light, blue LEDs, white LEDs, and kept dark. The reactions all gave full conversion and 35% yield of the oxidative cleavage product in 5 hours.
After ruling out a photocatalytic mechanism, we next considered a mechanism in which 2,3-DMI was serving as the reductant. In order to test this hypothesis, we reacted the FeIII(PFPP)Cl with excess 2,3-DMI in the absence of O2. The resulting UV-Vis spectrum gave a spectroscopic signature in the Q-band region that matched independently prepared ferrous porphyrin (Fig. 2). Essentially, the reaction of FeIII(PFPP)Cl and 2,3-DMI leads to the formation of FeII(PFPP)(L) and an indole-derived byproduct. This result demonstrates that the first equivalent of 2,3-DMI sacrificially reduces the ferric porphyrin. Then under catalytic conditions, the resulting ferrous porphyrin can bind O2 to generate the superoxo complex resting state. This in turn oxidizes the second equivalent of 2,3-DMI.
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Fig. 2 UV-Vis spectrum of Q-bands of FeIII(PFPP)Cl reacted with excess 2,3-DMI in the absence of O2 (black) compared to genuine FeIII(PFPP)Cl (green) and FeII(PFPP)(L) (blue). |
However, simple outer-sphere reduction of the metal by the indole is thermodynamically unfavourable based on the reduction potential of the oxidized indole (1.1 V vs. SCE)25 compared to the iron porphyrin (−0.37 V vs. Fc+/0 or 0.01 V vs. SCE). Moreover, we observed the same catalytic activity when using iron tetraphenylporphyrin chloride (Fe(TPP)Cl) which is even harder to reduce.26 In either case, the minimum barrier (25 kcal mol−1) is inconsistent with the observed reaction rate (full conversion in 5 hours).
Although outer-sphere electron transfer is infeasible, it is possible that the reduction instead occurs through a proton-coupled electron transfer (PCET) mechanism, generating a neutral indole radical (3) (Scheme 4). Consistent with this hypothesis, we isolated the major byproduct 4 from a large-scale reaction. This compound is a known dimerization product from neutral indole radicals reacting in the presence of dioxygen.27–30 The formation of 3 is tentatively proposed based on the isolation of 4 from the reaction mixture.
In order to provide evidence for the proposed PCET mechanism, we synthesized 1-methyl-2,3-dimethyl-1H-indole (N-methyl-DMI), a substrate in which the N–H has been replaced with an N–Me. Consistent with our hypothesis, mixing N-methyl-DMI with FeIII(PFPP)Cl in an N2 atmosphere showed no reaction by UV-Vis (Fig. 3). In contrast to the reaction with 1, we did not observe the formation of the ferrous FeII(PFPP)(L). Additionally, under catalytic conditions in the presence of O2, no reaction of the N-methyl-DMI was observed—only starting material was recovered (100% NMR yield).
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Fig. 3 UV-Vis spectrum of Q-bands of FeIII(PFPP)Cl reacted with excess N-methyl-DMI in the absence of O2 (black) compared to genuine FeIII(PFPP)Cl (green) and FeII(PFPP)(L) (blue). |
Cyclic voltammetry studies were used to estimate the oxidation potential of 2,3-DMI and N-methyl-DMI. Although the voltammograms are chemically irreversible, the onset potential for the methylated indole is similar to that of 2,3-DMI (Fig. 4). If a simple outer-sphere electron transfer were operative, N-methyl-DMI would be able reduce FeIII(PFPP)Cl. Instead, the fact that the methylated indole cannot reduce the ferric porphyrin provides evidence that the N–H proton is critical, consistent with a PCET mechanism.
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Fig. 4 Cyclic voltammograms of 2,3-DMI (blue) and N-methyl-DMI (red) in 0.1 M [NBu4][PF6] in acetonitrile with 100 mV s−1 scan rate. |
A deuterated substrate provided further evidence supporting the N–H being critical in the reduction of the ferric porphyrin. The deuterated analogue of the substrate, 2,3-dimethyl-1H-indole-1-d (1-D), was synthesized and subjected to the catalytic reaction conditions. The deuterium was observed to scramble into the methylene and methyl group of the dimer product (4-D) (Scheme 5). This observation could not be explained by simple acid/base exchange with solvent. Instead, it is more consistent with a PCET mechanism in which the H/D-atom from the nitrogen is lost as part of the reduction of the metal (Scheme 6).
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Scheme 5 The oxidation of N-D-2,3-DMI (1D) with air catalysed by FeIII(PFPP)Cl in acetonitrile leads to the formation of 2-D and dimer 4-D. |
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Scheme 6 Proposed mechanism for the formation of deuterated dimer (4-D) from a PCET oxidation of 1-D based on literature reports for formation of 4.27–30 See ESI† for a more detailed mechanism. |
The combination of these results helps to explain the moderate (35%) yield observed in the oxidative cleavage of 2,3-dimethylindole. Because the dimer 4 is a requisite byproduct of the balanced reaction, the formation of this species accounts for two equivalents of every indole substrate added to the reaction. Therefore, the theoretical yield of 2 is actually only 33% of the original indole. Taking this theoretical maximum into account, the isolated yield of 2 (30%, see ESI†) is in fact a 91% yield.
By exploiting our understanding of the catalytic mechanism, we imagined being able to “turn on” reactivity for substrates that do not intrinsically undergo this reaction with ferric porphyrin. As an example, 3-methylindole does not exhibit reactivity under the same reaction conditions as 2,3-DMI (Scheme 7, top). This can possibly be explained by the slightly higher bond dissociation free energy (BDFE) of the N–H in 3-methylindole versus 2,3-DMI.31 Because 3-methylindole is unable to reduce the iron porphyrin, it is unable to generate the iron superoxo intermediate necessary to enable oxidative cleavage. However, if we include an exogenous chemical reductant that can reduce the metal from iron(III) to iron(II), we should be able to enable reactivity with this substrate. Experimentally, in the presence of decamethylferrocene (Fc*) as the reductant, we observe partial conversion of 3-methylindole to the oxidative cleavage product (N-(2-acetylphenyl)formamide) in 49% NMR yield (Scheme 7, bottom). This finding highlights that we can enable reactivity of the ferric porphyrin by adding a reductant, allowing us to perform the oxidative cleavage of indoles that would otherwise be unreactive with the ferric porphyrin.
The exploration of this mechanism allowed us to enable catalytic reactivity of 3-methylindole, a substrate that does not react with the iron(III) porphyrin. This mechanistic insight also highlights the importance of careful substrate selection in biomimetic systems. If this PCET reactivity is not accounted for, deleterious reactivity could occur and complicate the kinetics and conclusions of biomimetic studies.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5dt01271f |
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