Direct Evaluation of the Reactivity of Nonheme Iron(V)âOxo

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Direct Evaluation of the Reactivity of Nonheme Iron(V)-Oxo Intermediates Toward Arenes Oleg Y. Lyakin, Alexandra M. Zima, Nikolay V. Tkachenko, Konstantin P. Bryliakov, and Evgenii P. Talsi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00661 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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Direct Evaluation of the Reactivity of Nonheme Iron(V)-Oxo Intermediates Toward Arenes Oleg Y. Lyakin,a,b Alexandra M. Zima,a,b Nikolay V. Tkachenko,a,b Konstantin P. Bryliakov,a,b and Evgenii P. Talsi a,b* a

Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation

b

Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russian Federation

ABSTRACT: The reactivity of the nonheme iron(V)-oxo intermediates toward aromatic C–H oxidation at −70 °C has been directly evaluated. The intermediates were generated upon the interaction of ferric complex

[(PDP*)FeIII(µ-OH)2FeIII(PDP*)](OTf)4

(4◊),

PDP*

=

N,N′-bis(3,5-dimethyl-4-

methoxypyridyl-2-methyl)-(S,S)-2,2′-bipyrrolidine), with peracetic acid in the presence of acetic or 2ethylhexanoic acid. The second-order rate constants (k2) for the reaction of substituted benzenes with iron-oxo intermediates [(PDP*)FeV=O(OC(O)R)]2+ at −70 °C were determined (R = CH3, 3-heptyl). For more electron-rich arenes, the much higher k2 values were observed, increasing in the following order: nitrobenzene < acetophenone < chlorobenzene < benzene < toluene, in accordance with the electrophilic aromatic substitution mechanism. The catalytic oxidation of mono- and dialkylbenzenes with H2O2 proceeded with good efficiency (up to 36.5 TN per Fe atom) and high selectivity toward aromatic oxidation products (up to 91%).

KEYWORDS: aromatic C–H oxidation, enzyme models, EPR, iron, mechanism 1 ACS Paragon Plus Environment

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INTRODUCTION Direct selective oxidation of aromatic compounds is among the most challenging reactions in organic synthesis. In nature, these reactions are mediated by heme and nonheme iron enzymes.1 For heme enzymes, the active intermediate is the so called Compound I – a high-valent low-spin (S = 1/2) iron(IV)-oxo heme cation radical (formally oxoiron(V) species). For a number of nonheme enzymes, the active intermediate is the high-spin (S = 2) iron(IV)-oxo species.2 Very recently, the second-order rate constants of aromatic hydroxylation by 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin iron based Compound I model have been measured directly by low pressure mass spectrometry.2c The selective hydroxylation of aromatic substrates catalyzed by synthetic nonheme iron complexes is known, but the nature of the active intermediates has not been established.3 Several examples of FeIV=O mediated intramolecular aromatic C–H and C–F hydroxylation have been reported.4 However, all known synthetic FeIV=O complexes are inert toward intermolecular oxidation of arenes;3 the only exception has been the hydroxylation of anthracene with FeIV=O species.5 Iron complexes 1-3 (Chart 1) are among the most extensively studied bioinspired catalysts for the selective oxidation of hydrocarbons with H2O2. Previously, it was found that the treatment of complex 1 with perbenzoic acids and complex 2 with H2O2/benzoic acid results in the formation of high-valent iron-oxo intermediates undergoing self-hydroxylation of the aromatic ring to form the corresponding iron(III)-salicylate complexes through intramolecular oxo-transfer process.3a,b The iron-assisted hydroxylation of benzoic acid to salicylic acid by 1,2/H2O2 has been achieved under mild conditions.3e Importantly, the system 2/H2O2 conducts intermolecular hydroxylation of benzene and substituted benzenes.3f The system 3/CH3CO3H mediates the aromatic ring hydroxylation of benzyl alcohol to afford iron(III) complex with 2-hydroxybenzyl alcohol.3d Que, Nam and coworkers have shown that independently prepared (TPA)FeIV=O species is unreactive toward benzoic acids. Therefore, it was assumed that (TPA)FeV=O species conducts intramolecular hydroxylation of aromatic ring in the systems 1/perbenzoic acids.3a Later, Rybak-Akimova and 2 ACS Paragon Plus Environment

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ACS Catalysis

coworkers provided strong indirect evidence in support of the involvement of (BPMEN)FeV=O species in the aromatic hydroxylation by the catalyst system 2/H2O2, while (BPMEN)FeIV=O was inert toward arenes.3f

Chart 1. Bioinspired Iron Catalysts for Selective Oxidation of Organic Compounds with H2O2

Unfortunately, for the catalyst systems 1-3/H2O2/arene, efficient catalysis is inhibited by strong coordination of the resulting phenols to the iron(III) center, so that very few catalytic cycles can be realized. Nevertheless, the high reaction rate suggests the existence of highly reactive intermediate, hypothetically, a FeV=O species.3a,b,d-f However, until now there have been no direct reactivity studies of the nonheme iron(V)-oxo intermediates toward arenes. Recently, we have found that the iron-oxo intermediates 4aAA and 4aEHA are formed in the highly efficient catalyst systems 4◊/H2O2/RCOOH and 4◊/CH3CO3H/RCOOH for the selective oxidation of alkanes and alkenes (RCOOH = acetic acid, AA, or 2-ethylhexanoic acid, EHA).6 Initially, 4aAA and 4aEHA

were

assigned

to

the

structures

analogous

to

that

of

Compound

I,

[(PDP*)•+FeIV=O(OC(O)CH3)]2+ and [(PDP*)•+FeIV=O(OC(O)R)]2+ (R = 3-heptyl), respectively.6a,b Very recently, Que, Lipscomb and co-workers have presented EPR spectroscopic data supporting the assignment of the related iron-oxo intermediate 1aAA formed in the catalyst system 1/CH3CO3H to the 3 ACS Paragon Plus Environment

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(L)FeV=O species rather than to its electronic isomer (L•+)FeIV=O.7 Therefore, in this study, we have used the [(PDP*)FeV=O(OC(O)CH3)]2+ and [(PDP*)FeV=O(OC(O)R)]2+ presentation for 4aAA and 4aEHA, respectively (Chart 2), leaving the unambiguous discrimination between (L)FeV=O and (L•+)FeIV=O electronic structures to further investigations.

Chart 2. Proposed Structures of the Iron-Oxo Intermediates

RESULTS AND DISCUSIION In this work, we have studied the reactivity of the intermediates 4aAA and 4aEHA toward substituted benzenes at −70 °C. The intermediates 4aAA and 4aEHA display virtually identical EPR spectra (Figure 1; Table 1, entries 1 and 2). One can conclude that the replacement of acetic acid additive with 2ethylhexanoic acid does not significantly affect the electronic structure of the considered intermediates. The concentrations of intermediates 4aAA and 4aEHA, formed in the samples 4◊/CH3CO3H/AA and 4/CH3CO3H/EHA, do not exceed 10% of the total iron concentration. 4aAA and 4aEHA decay at −70 °C with the first order rate constants k1 = (1.8 ± 0.2) × 10−3 s−1 and (1.5 ± 0.2) × 10−3 s−1, respectively. It is worth noting that the catalyst systems 4◊/H2O2/AA6a and 4◊/CH3CO3H/AA6c display the same intermediate 4aAA at −70 °C. However, in the latter system, the maximum concentration of 4aAA was much larger (by an order of magnitude). The reason of this difference is still unclear. Previously, it was shown that benzene hydroxylation by the system 2/H2O2 resulted in significant inverse KIE (kH/kD = 0.8),3f which is consistent with an electrophilic addition to the aromatic ring.9,10 The electrophilic oxidant responsible for this transformation is proposed to be the oxoiron(V) species.3f 4 ACS Paragon Plus Environment

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For the related catalyst systems 4◊/H2O2/RCOOH and 4◊/CH3CO3H/RCOOH, a similar mechanism of the aromatic ring hydroxylation is expected; in accordance with this prediction, the value of KIE determined for benzene hydroxylation by the catalyst systems 4◊/H2O2/AA and 4◊/CH3CO3H/AA (kH/kD = 0.9, see Supporting Information, SI, for the experimental details) was close to that for the system 2/H2O2. Therefore, one could expect that the iron-oxo intermediates 4aAA and 4aEHA would display higher reactivities toward more electron-rich arenes. The reactivities of 4aAA and 4aEHA were determined from the analysis of their decay in the presence and in the absence of arene. Accurate second order rate constant k2 values can only be obtained if the decay rate of 4aAA and 4aEHA in the presence of the arene is not too fast for the studies by the continuous wave EPR technique. Otherwise, only a rough estimate of k2 is possible (for more details see below). 2.071

4aAA

4aAA 2.008

1.960

A)

4aAA 4aEHA

EHA

4a

B) 4aEHA

3200

3280

3360

3440

3520

H/G

Figure 1. EPR spectra (−196 °C) of the samples (A) 4◊/CH3CO3H/AA and (B) 4◊/CH3CO3H/EHA, frozen 1 min after mixing the reagents at −70 °C. [Fe]:[CH3CO3H]:[carboxylic acid] = 1:3:10, [Fe] = 0.04 M in both samples. Mixed solvent (CH2Cl2/CH3CN, 1.8:1 v/v) was used.

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Table 1. EPR Spectroscopic Data for S = 1/2 Aminopyridine Iron-Oxo Intermediates observed in the catalyst systems 4◊/CH3CO3H/RCOOH and 5/CH3CO3H

no

compound

1

[(PDP*)FeV=O(OC(O)CH3)]2+ (4aAA) a

2 3

V

2+

g2

g3

ref

2.071

2.008

1.960

6a

EHA a

2.070

2.008

1.958

6b

2+

2.07

2.01

1.95

8

[(PDP*)Fe =O(OC(O)R)] (4a V

g1

)

AA b

[(PyNMe3)Fe =O(OC(O)CH3)] (5a )

a

Solvent CH3CN/CH2Cl2 (v/v = 1:1.8). RCOOH = 2-ethylhexanoic acid. b Solvent CH3CN/(CH3)2CO (v/v = 1:3).

The electron-deficiency of substituted benzenes may be qualitatively characterized by Hammett substituent constants σm and σp, or Brown-Okamoto polar substituent constants σm+ and σp+. The lower the σ and σ + values, the more electron-rich the substituted benzene is. According to the σ and σ + values in Table 2, the electron-deficiency of substituted benzenes decreases in the following order: nitrobenzene > acetophenone > chlorobenzene > benzene > toluene. The addition of chlorobenzene (to make its concentration 0.02 M) to the solutions containing 4aAA or 4aEHA ([4aAA] ≈ [4aEHA] ≈ 2 × 10−3 M) at −70 °C resulted in an almost immediate (within 30 s) drop of the concentration of 4aAA and 4aEHA (50-fold for 4aAA and 12-fold for 4aEHA). This allowed the evaluation of the second order rate constant k2 for the reaction of 4aAA and 4aEHA with chlorobenzene at −70 °C (∼6 and ∼2 M−1s−1, respectively, Table 2, entry 3, see Supporting Information for the details). Benzene and toluene are more electron-rich than chlorobenzene. Therefore, 4aAA and 4aEHA should react with these substrates more rapidly. Indeed, when chlorobenzene was replaced by benzene or toluene in the previous samples, no traces of 4aAA and 4aEHA were observed after mixing the reagents during 30 s at −70 °C. So, the second-order rate constants k2 for the reaction of 4aAA and 4aEHA with benzene and toluene (0.02 M) at −70 °C are larger than 10 M−1s−1.


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Table 2. Second-Order Rate Constants for the Reaction of Intermediates 4aAA and 4aEHA with Substituted Benzenes at −70 °C a

entry substrate

a

k2 (M−1s−1)

σm

σp

σm+

0.23 0.50

0.40 n/a b

0.11 n/a b

∼6 0.25

0.78

0.67

0.79

3.4 × 10−3 ≤ 1 × 10−3

σp

+

4aAA −0.07 −0.17 −0.07 −0.31 >10 0 0 0 0 >10

1 2

toluene benzene

3

chlorobenzene

4

acetophenone

0.37 0.38

5

nitrobenzene

0.71

4aEHA >10 >10

∼2 0.16

Solvent CH3CN/CH2Cl2 (v/v = 1:1.8). b Data are not available.

For more electron-deficient substrates (acetophenone or nitrobenzene), the k2 values can be determined more precisely, since the decay rate of 4aAA and 4aEHA at −70 °C in the presence of these substrates is within the range suitable for monitoring the kinetics by continuous-wave EPR spectroscopy. As an example, the self-decay of 4aEHA in the sample 4◊/CH3CO3H/EHA ([Fe]:[CH3CO3H]:[carboxylic acid] = 1:3:10, [Fe] = 0.04 M) at −70 °C, corresponding to the first-order kinetics with the rate constant k1 = (1.5 ± 0.2) × 10−3 s−1, is presented in Figure 2A. In the presence of acetophenone (0.08 M), the decay of 4aEHA is substantially accelerated (Figure 2B) and is described by the effective rate constant kobs = k1 + k2[acetophenone], where k2 = 0.16 ± 0.02 M−1s−1 (Figure 3). The same approach was used for the evaluation of k2 for the reaction of 4aAA with acetophenone. In this case, a somewhat larger value of k2 = 0.25 ± 0.03 M−1s−1 was revealed (Table 2, entry 4; Figure S1, SI).


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c /c0

c /c0

4aEHA

1.0 0.9

k1 = 1.5 × 10−3 s−1 (−70 °C)

0.8 0.7

4aEHA +

1.0 0.8

kobs = 0.013 s−1 (−70 °C)

0.6 0.4

0.6 0.5

0.2

0.4

0

2

4

6

8

10

12

14

0

1

t / min

A

B 2.070

2

3

4

t / min

2.008

4aEHA

4aEHA

4aEHA 1.958 0 min

0 min

3 min

1 min

6 min

2 min

9 min

3 min

12 min

* *

15 min

4 min

* 3230

3310

3390 H/G

3470

3550

3200

3280

3360

3440

3520

H/G

Figure 2. EPR spectra (−196 °C) (A) of the sample 4◊/CH3CO3H/EHA ([Fe]:[CH3CO3H]:[EHA] = 1:3:10, [Fe] = 0.04 M), frozen after mixing the reagents during 2 min at −70 °C in a CH2Cl2/CH3CN mixture (v/v = 1.8:1) and storing the sample at −70 °C for various times, and (B) of the same sample with acetophenone (0.08 M). (Top) Relative concentration of 4aEHA (c/c0) vs. time calculated from the decay of its EPR signal. Asterisks denote resonances of unknown iron complex raised upon the decay of 4aEHA in the presence of acetophenone.


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ACS Catalysis kobs / s−1 0.020

0.015

0.010

−1 −1

k2 = 0.16 M s

0.005

(−70 °C)

0 0

0.02

0.04

0.06

0.08

0.10

0.12

[Acetophenone] / M

Figure 3. Concentration dependence of kobs for the reaction of 4aEHA with acetophenone at −70 °C monitored by in situ EPR spectroscopy. Solvent: CH3CN/CH2Cl2 (v/v = 1:1.8).

The reactivity of intermediates 4aAA and 4aEHA toward nitrobenzene is lower compared with that toward acetophenone. The corresponding dependence of the effective rate constant kobs for 4aAA decay vs. nitrobenzene concentration is presented in Figure S2 (SI). The determined value of k2 = (3.4 ± 0.3) × 10−3 M−1s−1 was >70 times smaller than the corresponding value for acetophenone (Table 2, compare entries 4 and 5). 4aEHA is less reactive toward nitrobenzene, than 4aAA, so for 4aEHA the value of k2 was only roughly evaluated (Table 2, entry 5). The analysis of the rate constants k2 for the reactions of 4aAA and 4aEHA with various substituted benzenes shows that the more electron-rich is the substituted benzene, the larger is the k2 value (Table 2). The reactivity of 4aAA toward substituted benzenes is somewhat higher than that of 4aEHA. The observed difference in the reactivities of 4aAA and 4aEHA toward acetophenone at −70 °C (k2 of 0.25 and 0.16 M−1s−1, respectively) corresponds to only 0.2 kcal/mol difference in activation energy. Therefore, we assume that the electronic structures of 4aAA and 4aEHA are very similar, thus it is not surprising that they exhibit identical EPR spectra. A slightly lower reactivity of 4aEHA may be caused by the more sterically hindered 2-ethylhexanoate moiety, in comparison with the acetate moiety of 4aAA.


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To establish that intermediate 4aAA does drive benzene hydroxylation, we compared the yield of phenol formed in the system 4◊/CH3CO3H/AA/C6H6 at −70 °C ([Fe] = 0.04 M, [CH3CO3H] = 0.4 M, [AA] = 0.4 M, [C6H6] = 1.2 M) with that expected from the kinetic data for 4aAA. The EPR spectra of the catalyst system 4◊/CH3CO3H/AA6c in the absence of phenol show that the concentration of 4aAA during the first 10 min after mixing the reagents changes from 2 × 10−3 to 0.5 × 10−3 M (Figure S3, SI). The average concentration of 4aAA in this interval is about 10−3 M. During the first 10 min after the reaction onset in the system 4◊/CH3CO3H/AA, the rate of 4aAA decay WD = k1 [4aAA] is comparable with its rate of formation WF. In opposite case (WD >> WF), 4aAA would not be detected. Indeed, in the presence of benzene, 4aAA is not observed in agreement with its high reactivity toward benzene at −70 °C, suggesting that 4aAA should be almost quantitatively consumed by benzene before its self-decay. Thus, after t seconds of the reaction onset, phenol concentration can be evaluated from the equation: [phenol] = k1 [4aAA] t

(1)

Values of k1 = 1.8 × 10−3 s−1 and [4aAA] ≈ 10−3 M derived from our EPR measurements would thus predict (eq. 1) that the phenol formed during 10 min of the reaction at −70 °C should reach the concentration of 10−3 M. Phenol (1.8 × 10−3 M) was the only oxidation product detected by GC (see SI for details). Close values of the predicted and experimental phenol yield support the key role of 4aAA in benzene hydroxylation by the catalyst system 4◊/CH3CO3H/AA. The above results, supporting the key role of the iron(V)-oxo intermediate in benzene hydroxylation, encouraged us to test complex 4◊ in benzene oxidation under catalytic conditions that we typically used for alkene epoxidations.11 Initially, peracetic acid was used as oxidant. Unfortunately, a brown precipitate of iron(III) hydroxide compounds formed 10 minutes after the reaction onset, pointing to complete destruction of the iron catalyst by CH3CO3H. As a result, the phenol yield and selectivity were low (Table 3, entries 1 and 2).


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Next, we tried to oxidize benzene with hydrogen peroxide that is known to be a milder oxidant. Indeed, when H2O2 was applied, the formation of iron(III) hydroxide compounds was not observed. Three times higher phenol yields were achieved (up to 2.3 TN at −30 °C, Table 3, entries 3–8), in comparison with yields for CH3CO3H-based systems. Conducting the reaction at +25 °C increased the benzene conversion, but reduced the phenol yield and selectivity (Table 3).

Table 3. Benzene Oxidations with H2O2 and Peracetic Acid in the Presence of Complex 4◊ a

entry T (°C) 1 2 3 4 5 6 7 8 9 10

0 0 0 0 0 0 −30 −30 25 25

oxidant

additive (µmol)

CH3CO3H CH3CO3H H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2 H2O2

AA (100) EHA (100) AA (30) AA (100) AA (300) EHA (100) AA (100) EHA (100) AA (100) EHA (100)

conversion phenol yield hydroquinone selectivity (TN) b (TN) b yield (TN) b (%) c 1.8 1.2 2.4 2.4 2.4 1.6 2.6 2.3 4.0 3.1

0.7 0.5 1.6 1.5 1.5 1.1 2.3 1.7 1.0 0.8

1.1 0.7 0.8 0.9 0.9 0.5 0.3 0.6 3.0 2.3

39 42 67 63 63 69 88 74 25 26

a

Reaction conditions: benzene (100 µmol), oxidants, H2O2 (200 µmol), CH3CO3H (100 µmol), carboxylic acid (30–300 µmol), solvent CH3CN, catalyst loading 0.5 µmol 4◊ (1 µmol Fe), oxidant was added by a syringe pump over 30 min, and the mixture was stirred for 2.5 h, followed by GC analysis. For the detailed procedure, see Supporting Information. b TN = turnover number, moles of products/mole of iron. c Selectivity toward the main product phenol. Hydroquinone was the only detected byproduct.

The TN numbers of biomimetic aminopyridine-iron catalyzed benzene oxidation with hydrogen peroxide obtained herein were several times lower than those obtained for iron complexes with pentadentate and hexadentate aminopyridine ligands,3c,j and for iron complex bearing tetradentate Nheterocyclic carbene ligand.3k Gratifyingly, under optimized reaction conditions, much higher catalytic 11 ACS Paragon Plus Environment

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efficiencies were documented. In particular, catalyst 4◊ performed 12.6 TN in the oxidation of benzene (Table 4, entry 1) and 13.7–36.5 TN in the oxidation of mono- and dialkylbenzenes with H2O2 in the presence of 10 equiv. of acetic acid (Table 4, entries 2–10). In all cases, good to high selectivity for aromatic oxidation was documented: 79–91% for the oxidation of monoalkylbenzenes and 66–88% for dialkylbenzenes. In the latter case, overall yields of oxidized products were higher, since dialkylbenzenes are more electron-rich than monoalkylbenzenes. Detailed information on the oxidation product structures is provided in Table S1 (Supporting Information).

Table 4. Catalytic Oxidation of Substituted Benzenes with H2O2 in the Presence of Complex 4◊ a

entry

substrate

conversion (TN) b

aromatic oxidation products (TN) b

aliphatic oxidation products (TN) b

other (TN) b

selectivity toward aromatic oxidation (%)

1 2 3 4 5 6 7 8 9 10

benzene toluene ethylbenzene cumene isobutylbenzene o-xylene p-xylene 2-ethyltoluene 3-ethyltoluene 4-ethyltoluene

12.6 16.4 16.5 13.7 13.6 26.0 36.5 21.7 33.8 29.4

12.6 15.0 13.5 10.9 10.8 23.1 30.8 14.4 28.4 25.1

– 1.4 3.0 2.8 1.8 1.9 3.2 7.3 5.4 4.1

– – – – 1.0 1.0 2.5 – – 0.2

– 91 82 80 79 88 84 66 84 85

a

Reaction conditions: 0 °C; substrate (100 µmol), H2O2 (400 µmol), CH3COOH (1000 µmol), diferric catalyst 4◊ 0.62 mol.% (1.24 µmol Fe), solvent CH3CN (400 µL), oxidant was added by a syringe pump over 60 min, and the mixture was stirred for 1.5 h, followed by GC-MS analysis. b TN = turnover number, moles of products/mole of Fe.


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ACS Catalysis

CONCLUSIONS The iron complex of the PDP family, [(PDP*)FeIII(µ-OH)2FeIII(PDP*)](OTf)4, catalyzes aromatic hydroxylation by H2O2 or peracetic acid in acetonitrile, performing up to 36.5 catalytic turnovers per Fe atom. For the oxidation of mono- and dialkylbenzenes with H2O2, high selectivity toward aromatic oxidation products (up to 91%) has been documented. The second-order rate constants k2 for the reaction of arenes with nonheme iron(V)-oxo intermediates, generated in the catalyst systems based on the iron complex studied family and peracetic acid as oxidant, have been evaluated at −70 °C. Close values of the predicted and experimental phenol yield support the key role of the iron(V)-oxo intermediates in aromatic hydroxylation by the studied catalyst systems. For more electron-rich arenes, the much higher k2 values have been observed, increasing in the following order: nitrobenzene < acetophenone < chlorobenzene < benzene < toluene. This observation, as well as sizeable inverse kinetic isotope effect (kH/kD = 0.9), is consistent with the electrophilic aromatic substitution mechanism of arene hydroxylation. We strongly believe that fine tuning of the ligand structure of iron complexes of the PDP family, as well as the amount and structure of the catalytic additive, can further improve their catalytic efficiency in aromatic oxidation reactions, as well as their selectivity toward aromatic oxidation products. Such studies are underway in our laboratory.

ASSOCIATED CONTENT Supporting Information. Experimental procedures, additional kinetic and catalytic data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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* E-mail: [email protected]

ORCID Oleg Y. Lyakin: 0000-0001-8540-8707 Alexandra M. Zima: 0000-0001-6871-223X Nikolay V. Tkachenko: 0000-0002-7296-4293 Konstantin P. Bryliakov: 0000-0002-7009-8950 Evgenii P. Talsi: 0000-0003-0756-1401

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial support from the Russian Science Foundation (#17-13-01117) is acknowledged. We thank Dr. M. V. Shashkov for the GC-MS measurements.

REFERENCES (1) (a) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, Jr., L. Dioxygen Activation at Mononuclear Nonheme Iron Active Sites: Enzymes, Models, and Intermediates. Chem. Rev. 2004, 104, 939–986. (b) Meunier, B.; de Visser, S. P.; Shaik, S. Mechanism of Oxidation Reactions Catalyzed by Cytochrome 14 ACS Paragon Plus Environment

Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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P450 Enzymes. Chem. Rev. 2004, 104, 3947–3980. (c) Abu-Omar, M. M.; Loaiza, A.; Hontzeas, N. Reaction Mechanisms of Mononuclear Non-Heme Iron Oxygenases. Chem. Rev. 2005, 105, 2227–2252. (d) Bruijnincx, P. C. A.; van Koten, G.; Klein Gebbink, R. J. M. Mononuclear Non-Heme Iron Enzymes with the 2-His-1-carboxylate Facial Triad: Recent Developments in Enzymology and Modeling Studies. Chem. Soc. Rev. 2008, 37, 2716–2744. (2) (a) Rittle, J.; Green, M. T. Cytochrome P450 Compound I: Capture, Characterization, and C-H Bond Activation Kinetics. Science 2010, 330, 933–937. (b) Crestoni, M. E.; Fornarini, S. Compound I of Naked Heme (Iron Protoporphyrin IX). Inorg. Chem. 2007, 46, 9018–9020. (c) Reinhard, F.G. C.; Sainna, M. A.; Upadhyay, P.; Balan, G. A.; Kumar, D.; Fornarini, S.; Crestoni, M. E.; de Visser, S. P. A Systematic Account on Aromatic Hydroxylation by a Cytochrome P450 Model Compound I: A LowPressure Mass Spectrometry and Computational Study. Chem. Eur. J. 2016, 22, 18608–18619. (d) Ye, S.; Riplinger, C.; Hansen, A.; Krebs, C.; Bollinger, Jr., J. M.; Neese, F. Electronic Structure Analysis of the Oxygen-Activation Mechanism by FeII- and α-Ketoglutarate (αKG)-Dependent Dioxygenases. Chem. Eur. J. 2012, 18, 6555–6567. (e) Nam, W. High-Valent Iron(IV)–Oxo Complexes of Heme and Non-Heme Ligands in Oxygenation Reactions. Acc. Chem. Res. 2007, 40, 522–531. (f) Nam,W.; Lee, Y.-M.; Fukuzumi, S. Tuning Reactivity and Mechanism in Oxidation Reactions by Mononuclear Nonheme Iron(IV)-Oxo Complexes. Acc. Chem. Res. 2014, 47, 1146–1154. (3) (a) Oh, N. Y.; Seo, M. S.; Lim, M. H.; Consugar, M. B.; Park, M. J.; Rohde, J.-U.; Han, J.; Kim, K. M.; Kim, J.; Que, Jr., L.; Nam, W. Self-Hydroxylation of Perbenzoic Acids at a Nonheme Iron(II) Center. Chem. Commun. 2005, 5644–5646. (b) Taktak, S.; Flook, M.; Foxman, B. M.; Que, Jr., L.; Rybak-Akimova, E. V. Ortho-Hydroxylation of Benzoic Acids with Hydrogen Peroxide at a Nonheme Iron Center. Chem. Commun. 2005, 5301–5303. (c) Thibon, A.; Bartoli, J.-F.; Guillot, R.; Sainton, J.; Martinho, M.; Mansuy, D.; Banse, F. Non-Heme Iron Polyazadentate Complexes as Catalysts for Aromatic Hydroxylation by H2O2: Particular Efficiency of Tetrakis(2-pyridylmethyl) Ethylenediamine– 15 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 19

Iron(II) Complexes. J. Mol. Catal. A.: Chem. 2008, 287, 115–120. (d) Yoon, J.; Wilson, S. A.; Jang, Y. K.; Seo, M. S.; Nehru, K.; Hedman, B.; Hodgson, K. O.; Bill, E.; Solomon, E. I.; Nam, W. Reactive Intermediates in Oxygenation Reactions with Mononuclear Nonheme Iron Catalysts. Angew. Chem. Int. Ed. 2009, 48, 1257–1260. (e) Makhlynets, O. V.; Das, P.; Taktak, S.; Flook, M.; Mas-Ballesté, R.; Rybak-Akimova, E. V.; Que, Jr., L. Iron-Promoted ortho- and/or ipso-Hydroxylation of Benzoic Acids with H2O2. Chem. Eur. J. 2009, 15, 13171–13180. (f) Makhlynets, O. V.; Rybak-Akimova, E. V. Aromatic Hydroxylation at a Non-Heme Iron Center: Observed Intermediates and Insights into the Nature of the Active Species. Chem. Eur. J. 2010, 16, 13995–14006. (g) Raba, A.; Cokoja, M; Herrmann, W. A.; Kühn, F. E. Catalytic Hydroxylation of Benzene and Toluene by an Iron Complex Bearing a Chelating di-Pyridyl-di-NHC Ligand. Chem. Commun. 2014, 50, 11454–11457. (h) Kejriwal, A.; Bandyopadhyay, P.; Biswas, A. N. Aromatic Hydroxylation Using an Oxo-Bridged Diiron(III) Complex: a Bio-Inspired Functional Model of Toluene Monooxygenases. Dalton Trans. 2015, 44, 17261–17267. (i) Carneiro, L.; Silva, A. R. Selective Direct Hydroxylation of Benzene to Phenol with Hydrogen Peroxide by Iron and Vanadyl Based Homogeneous and Heterogeneous Catalysts. Catal. Sci. Technol. 2016, 6, 8166–8176. (j) Capocasa, G.; Olivo, G.; Barbieri, A.; Lanzalunga, O.; Di Stefano, S. Direct Hydroxylation of Benzene and Aromatics with H2O2 Catalyzed by a Self-Assembled Iron Complex: Evidence for a Metal-Based Mechanism. Catal. Sci. Technol. 2017, 7, 5677–5686. (k) Raba, A,; Cokoja, M.; Herrmann, W. A.; Kühn, F. E. Catalytic Hydroxylation of Benzene and Toluene by an Iron Complex Bearing a Chelating di-Pyridyl-di-NHC Ligand. Chem. Commun. 2014, 50, 11454– 11457. (4) (a) Jensen, M. P.; Lange, S. J.; Mehn, M. P.; Que, E. L.; Que, L. Biomimetic Aryl Hydroxylation Derived from Alkyl Hydroperoxide at a Nonheme Iron Center. Evidence for an FeIV=O Oxidant. J. Am. Chem. Soc. 2003, 125, 2113–2128. (b) Mehn, M. P.; Fujisawa, K.; Hegg, E. L.; Que, L. Oxygen Activation by Nonheme Iron(II) Complexes: α-Keto Carboxylate versus Carboxylate. J. Am. Chem. Soc. 2003, 125, 7828–7842. (c) Bigi, J. P.; Harman, W. H.; Lassalle-Kaiser, B.; Robles, D. M.; Stich, T. A.; 16 ACS Paragon Plus Environment

Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Yano, J.; Britt, R. D.; Chang, C. J. A High-Spin Iron(IV)–Oxo Complex Supported by a Trigonal Nonheme Pyrrolide Platform. J. Am. Chem. Soc. 2012, 134, 1536–1542. (d) Sahu, S.; Widger, L. R.; Quesne, M. G.; de Visser, S. P.; Matsumura, H.; Moënne-Loccoz, P.; Siegler, M. A.; Goldberg, D. P. Secondary Coordination Sphere Influence on the Reactivity of Nonheme Iron(II) Complexes: An Experimental and DFT Approach. J. Am. Chem. Soc., 2013, 135, 10590–10593. (e) Sahu, S.; Zhang, B.; Pollock, C. J.; Dürr, M.; Davies, C. G.; Confer, A. M.; Ivanović-Burmazović, I.; Siegler, M. A.; Jameson, G. N. L.; Krebs, C; Golberg, D. P. Aromatic C–F Hydroxylation by Nonheme Iron(IV)–Oxo Complexes: Structural, Spectroscopic, and Mechanistic Investigations. J. Am. Chem. Soc. 2016, 138, 12791–12802. (5) De Visser, S. P.; Oh, K.; Han, A.-R.; Nam, W. Combined Experimental and Theoretical Study on Aromatic Hydroxylation by Mononuclear Nonheme Iron(IV)−Oxo Complexes. Inorg. Chem. 2007, 46, 4632–4641. (6) (a) Lyakin, O. Y.; Zima, A. M.; Samsonenko, D. G.; Bryliakov, K. P.; Talsi, E. P. EPR Spectroscopic

Detection

of

the

Elusive

FeV=O

Intermediates

in

Selective

Catalytic

Oxofunctionalizations of Hydrocarbons Mediated by Biomimetic Ferric Complexes. ACS Catal. 2015, 5, 2702–2707. (b) Zima, A. M.; Lyakin, O. Y.; Ottenbacher, R. V.; Bryliakov, K. P.; Talsi, E. P. Dramatic Effect of Carboxylic Acid on the Electronic Structure of the Active Species in Fe(PDP)Catalyzed Asymmetric Epoxidation. ACS Catal. 2016, 6, 5399–5404. (c) Zima, A. M.; Lyakin, O. Y.; Bryliakov, K. P.; Talsi, E. P. Direct Reactivity Studies of Non-Heme Iron-Oxo Intermediates toward Alkane Oxidation. Catal. Commun. 2018, 108, 77–81. (7) Oloo, W. N.; Banerjee, R.; Lipscomb, J. D.; Que, Jr., L. Equilibrating (L)FeIII−OOAc and (L)FeV(O) Species in Hydrocarbon Oxidations by Bio-Inspired Nonheme Iron Catalysts Using H2O2 and AcOH. J. Am. Chem. Soc. 2017, 139, 17313–17326.


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Page 18 of 19

(8) Serrano-Plana, J.; Oloo, W. N.; Acosta-Rueda, L.; Meier, K. K.; Verdejo, B.; Garsía-España, E.; Bassallote, M. G.; Münck, E.; Que, Jr., L.; Company, A.; Costas, M. Trapping a Highly Reactive Nonheme Iron Intermediate That Oxygenates Strong C–H Bonds with Stereoretention. J. Am. Chem. Soc. 2015, 137, 15833–15842. (9) Korzekwa, K. R.; Swinney, D. C.; Trager, W. F. Isotopically Labeled Chlorobenzenes as Probes for the Mechanism of Cytochrome P-450 Catalyzed Aromatic Hydroxylation. Biochemistry 1989, 28, 9019–9027. (10) Mitchell, K. H.; Rogge, C. E.; Gierahn, T.; Fox, B. G. Insight into the Mechanism of Aromatic Hydroxylation by Toluene 4-Monooxygenase by Use of Specifically Deuterated Toluene and p-Xylene. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3784–3789. (11) Zima, A. M.; Lyakin, O. Y.; Ottenbacher, R. V.; Bryliakov, K. P.; Talsi, E. P. Iron-Catalyzed Enantioselective Epoxidations with Various Oxidants: Evidence for Different Active Species and Epoxidation Mechanisms. ACS Catal. 2017, 7, 60–69.


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Direct Evaluation of the Reactivity of Nonheme Iron(V)âOxo

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