Tunneling Effect That Changes the Reaction Pathway from

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Tunneling Effect That Changes the Reaction Pathway from Epoxidation to Hydroxylation in the Oxidation of Cyclohexene by a Compound I Model of Cytochrome P450 Ranjana Gupta, Xiao-Xi Li, Kyung-Bin Cho, Mian Guo, YongMin Lee, Yong Wang, Shunichi Fukuzumi, and Wonwoo Nam J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00461 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 18, 2017

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The Journal of Physical Chemistry Letters

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Tunneling Effect That Changes the Reaction Pathway from 6

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Epoxidation to Hydroxylation in the Oxidation of Cyclohexene 7 9

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by a Compound I Model of Cytochrome P450 10 12

1

Ranjana Gupta,† Xiao-Xi Li,†,‡ Kyung-Bin Cho,† Mian Guo,† Yong-Min Lee,† Yong 14

Wang,*,‡ Shunichi Fukuzumi,*,†,§ and Wonwoo Nam*,†,‡ 15

13

16 17 †

18 20

Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea 21

‡

19

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, China § Faculty of Science and Engineering, Meijo University, SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi 468-8502 28

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24

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2

29 30 31 3

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E-mail: [email protected], [email protected], [email protected] 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57

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1 ACS Paragon Plus Environment


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ABSTRACT: The rate constants of the C=C epoxidation and the C-H hydroxylation (i.e., 5

allylic C-H bond activation) in the oxidation of cyclohexene by a high-valent iron(IV)-oxo 7

6

4

porphyrin 8

-cation

radical

complex,

[(TMP•+)FeIV(O)(Cl)]

(1,

TMP

=

meso-tetramesitylporphyrin dianion), were determined at various temperatures by analyzing 10

the overall rate constants and the products obtained in the cyclohexene oxidation by 1, 12

1

9

leading us to conclude that the reaction pathway changes from the C=C epoxidation to the 13

C-H hydroxylation by decreasing reaction temperature. When cyclohexene was replaced by 15

14

deuterated cyclohexene (cyclohexene-d10), the epoxidation pathway dominated irrespective 17

of the reaction temperature. The temperature dependence of the rate constant of the C-H 19

18

16

hydroxylation pathway in the reactions of cyclohexene and cyclohexene-d10 by 1 suggests 20

that there is a significant tunneling effect on the hydrogen atom abstraction of allylic C-H 2

21

bonds of cyclohexene by 1, leading us to propose that the tunneling effect is a determining 24

23

factor for the switchover of the reaction pathway from the C=C epoxidation pathway to the 25

C-H hydroxylation pathway by decreasing reaction temperature. By performing density 27

26

functional theory (DFT) calculations, the reaction energy barriers of the C=C epoxidation 29

and C-H bond activation reactions by 1 were found to be similar, supporting the notion that 30

28

small environmental changes, such as the reaction temperature, can flip the preference for 32

31

one reaction to another. 3 34 36

35

TOC graphic: 37 38 39 40 41 42 43 4 45 46 47 48 49 51

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Keywords: Compound I, Epoxidation, Hydroxylation, Cyclohexene, Tunneling Effect 52 53 54 5 56 57

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High-valent iron(IV)-oxo porphyrin -cation radical species, called Compound I (Cpd I), 6

5

4

have been spectroscopically characterized and well accepted as the reactive intermediates 8

7

in the catalytic cycles of heme enzymes, such as cytochrome P450 (P450), horseradish 10

9

peroxidase (HRP), catalase, and chloroperoxidase.1-8 Biomimetic studies using synthetic 12

1

iron porphyrin complexes have provided valuable insights into the reactivities and reaction 14

13

mechanisms of the intermediates in various oxidation reactions as well as in 16

15

electron-transfer reactions.9-22 For example, it has been reported that the regioselectivity of 18

17

C=C epoxidation versus C–H hydroxylation in the oxidation of cyclohexene by Cpd I 20

19

models changes dramatically depending on reaction temperatures, substrates, and the 2

21

electronic nature of iron porphyrins.23,24 However, activation parameters of the rate 24

23

constants of the C=C epoxidation versus the C-H hydroxylation of the same substrate, 26

25

including deuterium kinetic isotope effect (KIE), has yet to be clarified in comparison with 28

27

density functional theory (DFT) calculations. In addition, factors that determine the 30

29

reaction pathways (e.g., C=C epoxidation versus C-H hydroxylation) in the oxidation of 31 32

cyclohexene by Cpd I models need to be clarified, although we have discussed recently on 3 34

the regioselectivity switch (e.g., C=C epoxidation versus C-H hydroxylation) in the 36

35

oxidation of cyclic olefins by nonheme metal(IV)-oxo complexes25-29 and Shaik and 37 38

co-workers have shown the tunneling effect on the counterintuitive hydrogen atom 39 40

(H-atom) abstraction reactivity of nonheme iron(IV)-oxo complexes.30,31 41 43

42

We report herein the temperature effect on the rate constants of the C=C epoxidation 45

4

and C-H hydroxylation pathways in the oxidation of cyclohexene and deuterated 47

46

cyclohexene (cyclohexene-d10) by a Cpd I model compound, by determining both the 49

48

overall rate constants and products at various reaction temperatures. The switchover of the 51

50

reaction pathway from the C=C epoxidation to the C-H hydroxylation in the oxidation of 53

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cyclohexene by a Cpd I model compound was observed by decreasing reaction temperature. 5

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The temperature dependence of the deuterium KIE revealed the significance of tunneling 57

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effect on the H-atom abstraction step in the hydroxylation of cyclohexene, resulting in the 60

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1 2 3

switchover of the reaction pathway from the C=C epoxidation to the C-H hydroxylation by 4 6

5

decreasing the reaction temperature. This experimental observation was corroborated by 8

7

DFT calculations. 9

An iron(IV)-oxo porphyrin -cation radical complex, [(TMP•+)FeIV(O)(Cl)] (1, TMP = 12

1

10

meso-tetramesitylporphyrin dianion), was prepared by reacting [(TMP)FeIII(Cl)] with two 14

13

equiv of m-chloroperbenzoic acid (m-CPBA) in butyronitrile (C3H7CN) at 193 K (see 16

15

Supporting Information (SI), Figures S1 – S2 for the UV-vis absorption and EPR spectra of 18

17

1).23-24,29 Upon addition of cyclohexene to a C3H7CN solution of 1 at 193 K, the absorption 20

19

band at 666 nm due to 1 decayed with an increase in the absorption band at 507 nm due to 2

21

[(TMP)FeIII(Cl)] (Figure 1a). The decay time profile of absorbance at 666 nm due to 1 24

23

obeyed first-order kinetics (Figure 1a, inset). The first-order rate constant (kobs) was 26

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proportional to the concentration of cyclohexene (Figure 1b), and a second-order rate 28

constant (k2(H)) was determined to be 0.42 M–1 s–1 at 193 K from the slope of the linear plot 30

29

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of the first-order rate constants vs concentrations of cyclohexene (Figure 1b). Similarly, the 31 32

k2(H) values at different temperatures were determined from the slopes of the linear plots of 3 34

the first-order rate constants vs concentrations of cyclohexene (Table 1; Figure 1b and SI, 36

35

Figures S3 – S6). When cyclohexene (10 mM) was replaced by cyclohexene-d10 (10 mM), 38

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the decay rate constant of 1 with cyclohexene-d10 (1.7 × 10–3 s–1 at 193 K) became smaller 40

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than that of 1 with cyclohexene (4.3 × 10–3 s–1 at 193 K) (Figure 1a, inset). The k2(D) values 41 43

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in the oxidation of cyclohexene-d10 by 1 at various temperatures were also determined from 45

4

the slopes of the linear plots of the first-order rate constants vs concentrations of 46

cyclohexene-d10 (Table 1; Figure 1b and SI, Figures S3 – S6). 49

48

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The products in the oxidation of cyclohexene and cyclohexene-d10 by 1 were analyzed 51

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to be cyclohexene oxide and cyclohex-2-enol (see SI, Experimental Section), and the 53

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product yields at different temperatures are summarized in Table 2. The ratio of the epoxide 5

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to alcohol products in the oxidation of cyclohexene by 1 decreased with decreasing reaction 57

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temperature, such that the ratio became almost unity at 213 K and reversed at lower 60

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temperatures. In contrast to this, the product ratio of the epoxide to the alcohol products in 4 6

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the oxidation of cyclohexene-d10 slightly increased with decreasing reaction temperature. 8

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The cyclohexene oxide was the predominant product at the lower temperature (i.e., 193 K). 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 39

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Figure 1. (a) UV-vis absorption spectral changes observed in the oxidation of cyclohexene (10 mM) by 1 (0.10 mM) in C3H7CN at 193 K. Insets show the time courses (red circles for cyclohexene and blue circles for cyclohexene-d10) monitored the absorbance change at 666 nm due to 1. (b) Plots of pseudo-first-order rate constants (kobs) vs concentrations of cyclohexene (red) and cyclohexene-d10 (blue) in the oxidation of cyclohexene and cyclohexene-d10 by 1 (0.10 mM) in C3H7CN at 193 K. 47

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48 50

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Since the observed second-order rate constant (k2(H)) of the oxidation of cyclohexene by 52

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1 corresponds to the sum of the rate constants of epoxidation (kO(H)) and hydroxylation (kH), 54

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k2(H) = kO(H) + kH, the kO(H) and kH values were determined from the k2(H) values (Table 1) 56

5

and the product ratios of the epoxide to the alcohol products (Table 2). Similarly, the kO(D) 57

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and kD values of cyclohexene-d10 were determined form the k2(D) values (Table 1) and the 4 6

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product ratios (Table 2). It should be noted that the kO(H) values obtained from the reaction 8

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of cyclohexene were virtually identical to the kO(D) values obtained from the reaction of 10

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cyclohexene-d10 and thus, there is no deuterium kinetic isotope effect on the epoxidation of 12

1

cyclohexene by 1 (i.e., KIE = kO(H)/kO(D) = 1.0; see Table 1). In contrast to the C=C 14

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epoxidation, the KIE values obtained in the hydroxylation of cyclohexene and 16

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cyclohexene-d10 were different depending on reaction temperatures and the KIE values 18

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increased with decreasing reaction temperature (Table 1, column of kH/kD). 19 20 2

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Table 1. Rate Constants of Epoxidation (kO) and Hydroxylation of Cyclohexene (kH) and Cyclohexene-d10 (kD) with 1 in C3H7CN at Various Temperatures 24

23 25

cyclohexene-h10 27

26

cyclohexene-d10

KIE

temp. 29

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k2(H)

kO(H)

kH

k2(D)

kO(D)

kD

(M–1 s–1)

(M–1 s–1)

(M–1 s–1)

(M–1 s–1)

(M–1 s–1)

(M–1 s–1)

233

12(1)

8.5

3.5

11(1)

8.5

223

5.0(4)

2.9

2.1

3.5(3)

213

2.0(2)

0.98

1.0

203

0.70(5)

0.28

193

0.42(4)

0.15

(K) 30

kO(H)/kO(D)

kH/kD

3

32

2.5

1.0

1.4

2.9

0.64

1.0

3.3

1.1(1)

0.94

0.16

1.0

6.3

0.42

0.31(3)

0.27

0.040

1.0

11

0.27

0.17(2)

0.15

0.021

1.0

13

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38

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4 45 46 47 48 49 50 51 52 53 54 5 56 57

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Table 2. Product Yields in the Reactions of 1 (0.10 mM) with Cyclohexene and Cyclohexene-d10 in C3H7CN at Various Temperatures 6

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4

8

7

cyclohexene-h10

cyclohexene-d10

10

9

product yields (%)

product yields (%)

temp. 1

(K) cyclohexene oxide

cyclohex-2-enol

cyclohexene oxide

cyclohex-2-enol

18

17

16

15

14

13

233

65(5)

27(3)

64(4)

19(3)

223

54(5)

38(4)

72(5)

16(2)

213

45(4)

47(5)

78(4)

13(3)

203

36(3)

55(4)

82(4)

12(3)

193

34(4)

62(4)

84(4)

12(2)

12

20

19

2

21 23 24 26

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Arrhenius plots of the rate constants of the epoxidation (kO(H) = kO(D)) and the 28

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hydroxylation of cyclohexene (kH) and cyclohexene-d10 (kD) by 1 in C3H7CN are shown in 30

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Figure 2, where the activation energy of the epoxidation of cyclohexene and 31

cyclohexene-d10 (EO(H) ≈ EO(D) = 9.3 kcal mol–1) is similar to that of hydroxylation of 3

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cyclohexene-d10 (ED = 11 kcal mol–1), but is significantly higher than that of hydroxylation 36

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of cyclohexene (EH = 6.0 kcal mol–1). As a result, there is a crossing point between kO(H) 38

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and kH at around 213 K, where kO(H) is the same as kH. The kO(H) value is larger than the 40

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corresponding kH value at the higher temperatures, above 213 K, whereas the kH value is 42

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larger than the corresponding kO(H) value at the lower temperatures, below 213 K. Thus, a 4

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switchover of the reaction pathways between C=C epoxidation and C-H hydroxylation of 46

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cyclohexene by 1 occurs depending on the reaction temperature. The pre-exponential factor 48

(AH) of hydroxylation of cyclohexene (1.5  106 M–1 s–1) is significantly smaller than the 50

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corresponding value of cyclohexene-d10 (AD = 3.5  1010 M–1 s–1). Such a large difference 51 52

in the pre-exponential factor (AD >> AH) together with a large difference in the activation 54

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energies (ED –EH = 5.0 kcal mol–1), which is larger than that expected from the difference 56

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in the bond dissociation energy between C-H and C-D (BDED – BDEH = 1.3 kcal mol–1), 57

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results from a tunneling effect.32-34 Since it is the curvature of the Arrhenius plot that 4 6

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demonstrates experimentally the incidence of tunneling, the tunneling has been detected 7

with two distinctive findings of ED >> EH and AD >> AH.32-34 9

8 10 1 12 13 14 15 16 17 18 19 20 21 2 23 25

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Figure 2. Arrhenius plots of the rate constants of epoxidation (kO(H)) and hydroxylation of 27

cyclohexene-h10 (kH) and cyclohexene-d10 (kD) by [(TMP•+)FeIV(O)(Cl)] (1) in C3H7CN. 28

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29 31

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The reaction rate constants in Table 1 can be translated into free energy barriers 3

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through the Eyring equation, resulting in the C=C epoxidation and C-H hydroxylation free 35

energy barriers being in the ranges of 11.8 – 12.5 kcal mol–1 and 11.6 – 12.9 kcal mol–1, 37

36

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respectively, depending on the reaction temperature. These energy ranges, which are within 39

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1.3 kcal mol–1, pose a challenge for DFT calculations, as the error margins are typically 40 41

larger than that. Therefore, a successful calculation would be if the results showed that the 42 43

epoxidation energy barrier is not too much different from the hydroxylation reaction barrier, 4 45

as this will support the experiments showing that the reaction pathway can be changed by 46 48

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changing external factors (i.e., the temperature). With this goal set, we performed the DFT 50

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calculations for both C=C epoxidation and C-H hydroxylation reactions in the oxidation of 51

cyclohexene by the Cpd I model (SI, Tables S1 – S5). 54

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We use here the electronic energies (including solvent and dispersion effects) here due 56

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to a presumed higher accuracy rather than free energies (see SI for discussion). The S = 1/2 57

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and 3/2 spin states of Cpd I are found to be energetically degenerate within round-off errors 6

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(Figure 3, center). The energies and spin state distribution are in accord with earlier 7

findings on Cpd I and feature an  or  unpaired electron on the porphyrin ligand, 10

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depending on the spin state.35 The calculated electronic energies show that the epoxidation 12

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S = 1/2 low-spin barrier is the lowest at 10.4 kcal mol–1 and involve an initial -electron 14

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transfer from the substrate to the FeIVO moiety.36 The S = 3/2 epoxidation barrier is slightly 16

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higher at 13.2 kcal mol–1. The hydroxylation barriers on the other hand are even higher, at 18

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13.6 and 14.4 kcal mol–1 for S = 1/2 and S = 3/2 spin states, respectively. Both the high-spin 20

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pathways feature an intermediate state after the transition state where the substrate is in a 2

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radical state (see SI, “High-Spin Intermediates in the Reactions” for a more in-depth 24

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discussion about these intermediates), before the intermediate proceeds to the product with 26

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a low or no energy barrier. Such a stable intermediate state was not found for the low-spin 27 28

pathway. 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47

Figure 3. DFT calculated reaction energy barrier for the oxidation reaction of cyclohexene 49

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by Cpd I at the B3LYP/LACV3P+*//LACVP level including dispersion and solvent effects. 50 51 52 53

The calculation results imply that the C=C epoxidation pathway is preferred to the 5

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hydroxylation pathway by 3.2 kcal mol–1, which is in agreement with the experimental 56 57

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1 2 4

3

results described above except at the very low temperature. The difference in the value of 5

3.2 kcal mol–1 is sufficiently small, and implies that a hydroxylation reaction can be 8

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achieved if conditions are favorable for it, such as at a low temperature in which tunneling 10

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effect becomes relatively more important than other effects due to its insensitivity to 12

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changing temperature. 14

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In conclusion, a switchover of the reaction pathway from the C=C epoxidation to the 16

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C-H hydroxylation was observed in the oxidation of cyclohexene by a Cpd I model 18

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compound, [(TMP•+)FeIV(O)(Cl)] (1), by only lowering reaction temperature due to the 20

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tunneling effect on the H-atom abstraction for the hydroxylation pathway; the KIE values 2

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in the hydroxylation pathway increased with decreasing reaction temperature. Such a 24

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switchover of the reaction pathways between the C=C epoxidation and the C-H 26

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hydroxylation in the oxidation of cyclohexene by the Cpd I model compound depending on 28

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reaction temperature was further supported by DFT calculations, in which the C=C 30

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epoxidation reaction has a lower energy barrier than the H-atom transfer step in the C-H 31 32

hydroxylation reaction at ambient temperature, but the H-atom transfer step can become the 3 34

preferable pathway at low temperature resulting from the tunneling effect. Such tunneling 35 36

effect has never been considered as a factor to switch the reaction pathway previously. 37 38

Thus, the present study has demonstrated for the first time the importance of the tunneling 39 41

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effect, especially at low reaction temperature, in the H-atom transfer step in the C-H 43

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hydroxylation pathway, which is in competition with the epoxidation pathway, in the 4

oxidation of cyclohexene by high-valent iron(IV)-oxo porphyrin -cation radical species 47

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(Cpd I). 48 49 51

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ASSOCIATED CONTENT 53

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Supporting Information. Figures S1 – S6, Tables S1 – S5, Experimental and DFT 54

calculation sections, and DFT coordinates. This material is available free of charge via the 56

5

Internet at http://pubs.acs.org. 57

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AUTHOR INFORMATION 4 5

Corresponding Author 7

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* E-mail: [email protected], [email protected], 9

8

[email protected] 10 12

1

ACKNOWLEDGMENT 14

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The authors acknowledge financial support from the NRF of Korea through the CRI 15

(NRF-2012R1A3A2048842 to W.N.), GRL (NRF-2010-00353 to W.N.) and MSIP 17

16

(NRF-2013R1A1A2062737 to K.-B.C.) and from JSPS KAKENHI (No. 16H02268 to 19

S.F.). 20

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Conventional wisdom dictates that a Cpd I reduction reaction occurs preferentially through an -electron transfer to the porphyrin ligand. In this case, however, after a

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thorough investigation, we are confident that this is a -electron transfer to the FeIVO 18

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moiety instead, and details will be communicated separately in the near future. 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57

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Tunneling Effect That Changes the Reaction Pathway from

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