The mechanistic aspects of iron(III) porphyrin catalyzed oxidation reactions in mixed solvents

Inorganica Chimica Acta 372 (2011) 295–303

Contents lists available at ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

The mechanistic aspects of iron(III) porphyrin catalyzed oxidation reactions in mixed solvents Amit Singh, Arunava Agarwala, Kaliappan Kamaraj, Debkumar Bandyopadhyay ⇑ Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110 016, India

a r t i c l e

i n f o

Article history: Available online 27 February 2011 Dedicated to Prof. S.S. Krishnamurthy. Keywords: Organopalladium compound Iron(III) porphyrin F20TPPFeCl t-BuOOH 2,4,6-Tri tert-butyl phenol (TTBP) Oxidation

a b s t r a c t In the iron(III) porphyrin catalyzed oxidation reactions, the formation of various reactive intermediates have been observed to depend upon the nature of the catalyst, the oxidant and the solvent used for the study. The various iron(III) porphyrin catalysts such as F20TPPFeCl, F16TPPFeCl, F12TPPFeCl and F8TPPFeCl have been used in the present study to understand the effect of solvent system in the activation of the catalysts. As the terminal oxidant t-BuOOH has been used. It has been observed that acetonitrile contaminated with water activates all the catalysts. It has been noted that 9% of water in acetonitrile is the best solvent system for the activation of all the catalysts. The results obtained have been applied to successfully oxidize cyclohexene and cyclohexane by these oxidizing systems. It has also been observed that CH3OH mixed with CH2Cl2 play a very important role in the activation of catalyst in hydroperoxide oxidizing system. The 33 ± 3% ratio of CH3OH in CH2Cl2 acts as the most suitable solvent system to convert organopalladium compound 1a–c to 2a–c. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Cytochrome P450 catalyzed monooxygenation of organic substrates has been a subject of intensive research for several decades [1]. This enzyme catalyzes several reactions in biological systems for example: the hydrocarbon hydroxylation, olefin epoxidation, carbon–carbon bond cleavage reaction, heteroatom dealkylation, heteroatom oxidation and oxidative dehalogenation. The active site of this enzyme contains an iron(III) porphyrin and the metal ion is covalently bound with the single protein chain through the anionic sulfur of a cysteine residue. The crystal structure of this enzyme has been resolved and it is now known that the sixth coordination site of iron(III) is occupied by at least one water molecule and it is believed that these water molecules assist in dioxygen activation by providing proton to directly bound dioxygen [2]. There has been enormous in vitro study with this enzyme to achieve high yield transformation of several industrially important substrates [3]. In many such studies dioxygen has been used as the terminal oxidant but the use of other terminal oxidants such as iodosylarenes, peracids, hydroperoxides, hydrogen peroxide has also been reported [4]. From these studies it has been understood that the oxygenation reaction proceeds through several intermediates as depicted in Scheme 1. Among the proposed intermediates,

⇑ Corresponding author. Tel.: +91 11 26591509; fax: +91 11 26581102. E-mail address: [email protected] (D. Bandyopadhyay). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.02.054

1–6 are spectroscopically well characterized. The involvement of 7 has been proposed in the final step of oxygen atom transfer to the organic substrates, but it is so reactive that its isolation and spectroscopic identification remained a formidable task [5]. In modeling studies, two reactions in particular: the hydroxylation and epoxidation of several hydrocarbons have been very thoroughly investigated [6]. In these studies metalloporphyrins are found to be very efficient catalysts and several terminal oxidants such as iodosylarenes [7], peracids [8], hydroperoxides [9], hydrogen peroxide [10], persulfate [11], pyridine N-oxide [12] and sodium hypochlorite [13], have been used extensively. Among the metal ions, iron followed by manganese, has been the most popular choice and from these studies it has been clearly established in the first phase that meso substituted metalloporphyrins are the most stable catalysts to perform monooxygenase reactions [14]. Thus by using TPPFe(III)Cl and TMPFe(III)Cl, selective high yield epoxidation of several organic substrates were possible in early days. However the selective hydroxylation of the unactivated C– H bonds remained a challenge [15]. Thus, the development of new catalysts were worked out and it has been established that electronegatively substituted metalloporphyrins were very good because they are more robust under the oxidizing environments used for the high yield oxidation of the substrates in particular [16]. Studies of the last decade however have indicated that there should be an optimization in putting electronegative substituents on the porphyrin periphery and this has been clearly demonstrated that the metalloporphyrin with all eight nitro substituted pyrroles

296

A. Singh et al. / Inorganica Chimica Acta 372 (2011) 295–303

2. Experimental III

2.1. Materials

IV

2

2 +

+

III

2 2

-

III

+ II

+ 2

III

-

III

Cyclohexene (>99%), cyclohexane (>99%), acetonitrile-D3 (CD3CN, 99.8% atom D), triphenylphosphine (99%), triphenylphosphine oxide (98%) and tertiary butyl-hydroperoxide (t-BuOOH, 70% in water), were purchased from Aldrich Chemical Co. HPLC grade acetonitrile (CH3CN) was obtained from Qualigens Chemicals (Glaxo, India); extra pure dichloromethane (CH2Cl2, 99%) from Merck (India) Ltd. Dichloromethane and acetonitrile were dried over CaCl2 and freshly distilled over P4O10. CH3OH was dried over CaO and was purified further as reported by Perrin [27]. Acetonitrile was further distilled using 1:1 mixture of KMnO4 and Li2CO3. Cyclohexene was purified by passing through a silica gel (60–120 mesh) column until it was nearly 100% pure (as analyzed by GC) prior to use. TTBP was recrystallized four times from ethanol– water (95:5), dried in air and then over CaCl2. The exact oxygen content of the t-BuOOH oxidant was determined iodometrically prior to use. The porphyrins such as 5,10,15,20-tetrakis (pentafluorophenyl)porphinato iron(III) chloride (F20TPPFe(III)Cl); the 5,10,15,20-tetrakis (2,3,5,6-tetrafluorophenyl)porphinato iron(III) chloride (F16TPPFe(III)Cl); the 5,10,15,20-tetrakis (2,4,6-triflourophenyl)porphinato iron(III) chloride (F12TPPFe(III)Cl) and the 5,10,15,20-tetrakis (2,6-difluorophenyl)porphinato iron(III) chloride (F8TPPFe(III)Cl) were prepared according to literature procedure [28]. The H2O used in all the reactions was doubly distilled. 2.2. Instrumental methods

Scheme 1. Proposed reactive intermediates of cytochrome P450. The substrate takes up the position of water in all the intermediates are not shown for clarity.

is not a good catalyst and the five nitro substituted one is the best one in oxygenation of alkanes to alcohols [17]. Several groups have used various kinds of solvents such as benzene [18], toluene [19], dichloromethane [20], and also various mixed solvents [21]. Some studies have indicated that solvents containing water has given more promising results. Thus by using CH2Cl2–CH3OH–H2O (80:18:2) [22], CH3CN–CH2Cl2–H2O (3:2:0.03) [23] and CH3CN– H2O (9:1) [24] the hydroxylation of most difficult hydroxylating substrate cyclohexane has been successful. It has also been noted that cyclohexane was hydroxylated exclusively to cyclohexanol by using a metalloporphyrin as the catalyst and C6F5IO as the terminal oxidant in 93% yields in dichloromethane solvent [7]. Secondly by using F20TPPFe(III)Cl as the catalyst and m-CPBA as the terminal oxidant the conversion of cyclohexane to cyclohexanol was achieved in 89% yields [25]. However, there is very little success in the hydroxylation of C–H bonds by hydroperoxides or by H2O2 with any of the metalloporphyrin catalyst. Later studies were thus focused to achieve high yield hydroxylations by hydroperoxides and hydrogen peroxide in particular. In our sustained effort for last few years we have realized the definitive solvent effect in the oxygenation reactions of several hydrocarbons and organometallic compounds [26]. Herein, we address why a certain percentage of CH3OH in CH2Cl2 or H2O in CH3CN provides the most favorable condition for oxidation reactions. The systematic variation of CH3OH in CH2Cl2 by using F20TPPFeCl and H2O in CH3CN by using F20TPPFeCl, F16TPPFeCl, F12TPPFeCl and F8TPPFeCl as catalysts for the selective oxidation of organopalladium compound 1a–c and TTBP as substrates is described. The results obtained for the oxidation of cyclohexene and cyclohexane have also been summarized.

The UV–Vis spectra were taken by using Perkin–Elmer Lambda (2S) Spectrophotometer. The cell holder of the spectrophotometer was connected to a Julabo F-30 temperature regulator and kinetic data were collected at 25 ± 1 °C. The 1H NMR was obtained on Bruker DPX-300 (300 MHz) NMR spectrometer in CDCl3 or CD3CN. The product analysis was performed on Perkin–Elmer Autosystem XL gas chromatograph equipped with flame ionization detector (FID) and Perkin–Elmer carbowax capillary column of 30 m length. The products were quantified with respect to the internal standard (C6F5I). The identification and quantitative analysis of the products were done from the response factors of standard samples. Oxygen was removed from solvents and substrates by bubbling with argon prior to use. Reaction mixture containing solvent, internal standard, catalyst and substrate were prepared in air-tight, sealed 4.0 mL reaction vials with red PTFE silicone septum. The oxidant was injected into the reaction mixture with a gas-tight microliter syringe through the septum. Samples for GC analysis were withdrawn using a gas-tight microliter syringe to prevent exposure to oxygen. 3. Results and discussion It has been noted that all the catalysts used in this study are reactive in mixed solvents in CH2Cl2–CH3OH or CH3CN–H2O. The nature of all four catalysts was very similar in CH3CN–H2O solvent system. Thus the behavior of only one: the F20TPPFeCl have been under taken for detail investigation. 3.1. Conductometric titration of F20TPPFeCl in CH2Cl2 with CH3OH It has been observed that the catalyst F20TPPFeCl was not conducting in pure CH2Cl2 but the conductance varies with the increase in percentage of CH3OH up to a certain limit. In this experiment the F20TPPFeCl (10.27 mg, 9.6 lM) was dissolved in

297

A. Singh et al. / Inorganica Chimica Acta 372 (2011) 295–303 1

F20 TPPFeCl þCH3 OH ! ½F20 TPPFeðCH3 OHÞx þ þCl

ð1Þ

B þ t-BuOOH ! ½F20 TPPFeðt-BuOOHÞðCH3 OHþ

ð2Þ

A

B x¼1 or 2

C

Fig. 1. A plot of conductivity of a micromolar solution of F20TPPFeCl vs. percentage of methanol in dichloromethane.

CH2Cl2 (10 mL) and methanol was added to this solution systematically and the change in conductivity of the porphyrin solution was observed. It was observed that F20TPPFeCl was completely nonconducting in dichloromethane. However, the conductivity was increased steadily with the increase of CH3OH in CH2Cl2 and it reached a maximum of 50 ± 4 lS in 33 ± 3% CH3OH in CH2Cl2. The measured conductances at different percentage of methanol in dichloromethane are presented graphically (Fig. 1). The observation can be explained by the dissociation of the iron(III) bound chloride with the formation of CH3OH adduct with the catalyst. 3.2. Spectral feature of F20TPPFeCl in CH2Cl2 and CH3OH It has been observed that catalytic ability of F20TPPFeCl has been enhanced in 33 ± 3% CH3OH in CH2Cl2. The Soret maximum of F20TPPFeCl in pure CH2Cl2 was observed at 410 nm. When it was treated with CH3OH this band was shifted from 410 to 404 nm. It was interesting to observe that the shift was completed with 33 ± 3% CH3OH in CH2Cl2 (Fig. 2 inset plot). On addition of t-BuOOH to this solution the Soret was shifted from 404 to 410 nm and the species thus generated has played a vital role in the oxygenation reactions. The 410 nm band was finally shifted to 390 nm after 5 min. The conductometric and spectroscopic behaviors of F20TPPFeCl in CH2Cl2–CH3OH medium can be explained by Eqs. (1) and (2). The formation of B could be highly feasible in polar solvent such as CH3OH and this could

be the reason of observed conductance of the catalyst in CH3OH– CH2Cl2 and not in pure CH2Cl2. We believe this species absorbs at 404 nm. The formation of t-BuOOH adduct is feasible from B and not from A could be the reason of catalysis from methanolic-CH2Cl2 to give the reactive intermediate of type C with transient Soret at 410 nm. Thus, we believe that CH3OH helps dissociation of iron(III) bound chloride so that the solvato species (B) can easily react with t-BuOOH.

3.3. Spectral features of iron(III) porphyrin in CH3CN–H2O It has been noted that the catalysts are not that active in pure acetonitrile solvent but their activity was increased in wet solvent. Therefore in order to quantify the solvent composition the spectroscopic behavior of the catalysts in presence of the known quantities of the H2O was undertaken. In all the measurements, 15–16 lM of the catalyst solution in 1 mL of CH3CN was taken in low-volume quartz cuvette. Double distilled water was gradually added to this catalyst solution and the changes in the UV–Vis spectra were recorded. The Soret maxima of F20TPPFeCl at 407 nm in CH3CN was shifted isosbestically to 402 nm as the percentage of H2O in the medium was increased (Fig. 3). The complete shift of the Soret peak was observed in 8–10% of H2O in CH3CN. Similar observation was noted for F16TPPFeCl (Fig. 4) where it was observed that the Soret peak was shifted from 408 to 402 nm in same amount of H2O in CH3CN as in case of F20TPPFeCl. For F12TPPFeCl the shift of Soret peak was observed from 409 to 405 nm (Fig. 5). In case of F8TPPFeCl the shift was from 409 to 406 nm (Fig. 6). In all these four cases it has been clearly observed that the broad band at 360 nm was disappeared, which support that the iron(III) bound chloride dissociate out with possible coordination of OH2 or OH, which ultimately assists the terminal oxidant (t-BuOOH) to coordinate with Fe(III) easily as has been proposed in case of MeOH in CH2Cl2 in the previous section.

Fig. 2. Change of F20TPPFe(III)Cl spectrum (14.9 lM) in presence of 2 mM of t-BuOOH in MeOH–CH2Cl2 (1:2) at 25 °C (Inset plot of Soret peak shift with % of CH3OH in CH2Cl2).

298

A. Singh et al. / Inorganica Chimica Acta 372 (2011) 295–303

Fig. 3. Spectral change of a solution of F20TPPFe(III)Cl (15.0 lM) in CH3CN with addition of H2O (Inset: plot of Soret peak shift with % of H2O in CH3CN).

Fig. 4. Spectral change of a solution of F16TPPFe(III)Cl (15.0 lM) in CH3CN with addition of H2O (Inset: plot of Soret peak shift with % of H2O in CH3CN).

3.4. Determination of binding constant of different iron(III) porphyrin catalysts with water in CH3CN medium The spectroscopic titrations of the different iron(III) porphyrin catalysts with H2O in CH3CN have been described in Section 3.3. We believed that H2O is replacing chloride bound to iron(III) and there is an equilibrium of the type as shown in Eq. (3). Assuming this, we have determined the binding constant. In calculating these we have used Benesi–Hildebrand equation (Eq. (4)) [29].

varied as function of 1/[H2O] with a linear relationship indicating the 1:1 stoichiometry between H2O and iron(III) porphyrin catalyst. A representative plot for F20TPPFeCl catalyst and H2O is presented in Fig. 7. Thus it has been concluded from these spectroscopic measurements that 33 ± 3% CH3OH in CH2Cl2 and 9% H2O in CH3CN could be the most suitable solvents for the oxidation work and in both cases the chloride dissociation is very critical.

PFeIII -Cl þ H2 O PFeIII -OH þ HCl

ð3Þ

  1 a 1 ¼ þ1 A  A0 a  b K½H2 O

3.5. 1H NMR spectral features of iron(III) porphyrin in CD3CN and CD3CN–D2O

ð4Þ

The 1H NMR spectra of all four iron(III) porphyrin catalysts were taken in pure CD3CN solvent and in CD3CN–D2O solvent mixture. In CD3CN, the b-pyrrole signals were observed at 80 ppm but in CD3CN–D2O solvent the b-pyrrole signals were upfield shifted. The NMR data for all the catalysts are given in Table 2.

The binding constant values for all the four iron porphyrin catalysts are given in Table 1. According to the linear Benesi–Hildebrand expression, the measured absorbance [1/A  A0] at 353 nm

A. Singh et al. / Inorganica Chimica Acta 372 (2011) 295–303

299

Fig. 5. Spectral change of a solution of F12TPPFe(III)Cl (15.5 lM) in CH3CN with addition of H2O (Inset: plot of Soret peak shift with % of H2O in CH3CN).

Fig. 6. Spectral change of a solution of F8TPPFe(III)Cl (15.5 lM) in CH3CN with addition of H2O (Inset: plot of Soret peak shift with % of H2O in CH3CN).

Table 1 The binding constant of H2O for four iron (III) porphyrin catalysts at 25 ± 1 °C.a

a

Entry

Catalyst

Solvent

K (M1)

1 2 3 4

F20TPPFe(III)Cl F16TPPFe(III)Cl F12TPPFe(III)Cl F8TPPFe(III)Cl

CH3CN CH3CN CH3CN CH3CN

0.024 0.052 0.152 0.122

Concentration of catalysts: 15–16 lM.

3.6. Catalytic oxygenation of C–Pd bond by t-BuOOH in CH2Cl2:CH3OH (2:1) It has been observed from our earlier studies that F20TPPFeCl– t-BuOOH oxidizing system efficiently oxidizes various organic substrates in CH2Cl2–CH3OH system. Thus this solvent system is chosen to address oxidation of C–Pd bonded few compounds. All the organopalladium compounds shown in Scheme 2 were

Fig. 7. Benesi–Hildebrand plot of F20TPPFe(III)Cl with H2O. Concentration of catalyst: 15.0 lM; solvent: CH3CN.

300

A. Singh et al. / Inorganica Chimica Acta 372 (2011) 295–303

Table 2 b-Pyrrole proton signal in CD3CN and CD3CN–D2O solvent.

100

Solvent

b-Pyrrole proton signal

F20TPPFe(III)Cl

CD3CN CD3CN–D2O CD3CN CD3CN–D2O CD3CN CD3CN–D2O* CD3CN CD3CN–D2O⁄

83.12 ppm 70.14 ppm 81.68 ppm 67.39 ppm 81.99 ppm – 81.69 ppm –

F16TPPFe(III)Cl F12TPPFe(III)Cl F8TPPFe(III)Cl *

These catalysts were very less soluble in acetonitrile–water (CD3CN–D2O) solvent and we could take the spectra.

% yield of TTBP radical

90

Catalyst

80

F20TPPFeCl

70

F16TPPFeCl

60

F12TPPFeCl

50

F8TPPFeCl

40 30 20 10 0 0

2

4

6

8

10

12

14

16

18

% of H2O in CH3CN R

4

4

R

R

4

5

5

Fig. 8. Variation of the yield of TTBP radical in CH3CN–H2O at 25 ± 1 °C. The concentrations of TTBP = 100 mM, catalyst = 15 ± 1 lM and t-BuOOH = 1 mM in all the experiments. Averages of duplicate sets of experiments are used in all the plots.

5

O Cl

Cl

Pd

N

Pd

N

N

N

15

N

13.5

S

S

S CH3

CH3

CH3 O

1a= R=4Me 1b= R=5Me 1c= R= H

2a= R=4Me 2b= R=5Me 2c= R= H

3a= R=4Me 3b= R=5Me 3c= R= H

Scheme 2. Organopalladium compounds.

kobs value of TTBP radical formation (x103 s-1)

Cl Pd

N

F20TPPFeCl

12

F16TPPFeCl

10.5

F12TPPFeCl

9

F8TPPFeCl

7.5 6 4.5 3 1.5 0

0

2

4

6

8

10

12

14

16

18

% of H 2 O in CH 3 CN Table 3 Percentage yield of products and catalyst survival for the reaction of 1a–c with F20TPPFeCl + t-BuOOH at 33 ± 2 °C. S. no.

Substrate

Solvent

Reaction time

% Yield products 2 3

% Catalyst survival

1

1a

10 min

50

0

87

2

1b

10 min

52

0

78

3.

1c

10 min

56

0

74

4

1a

CH2Cl2:CH3OH (2:1) CH2Cl2:CH3OH (2:1) CH2Cl2:CH3OH (2:1) CH2Cl2

12 h

<2

0

73

independently prepared and spectroscopically characterized as described in the literature [26,30]. The organopalladium compounds 1a, 1b and 1c were selectively oxidized to 2a, 2b and 2c, respectively by t-BuOOH in presence of F20TPPFeCl as a catalyst and dichloromethane–methanol (2:1) mixture was used as a solvent. However, when 1a was attempted to oxidize directly with t-BuOOH in CH2Cl2 or CH2Cl2:CH3OH (2:1) mixture in absence of iron(III) porphyrin as a catalyst, traces of 2a were identified. The reaction conditions and product yields are given in Table 3. All the products were separated by column chromatography and the catalysts were recovered (about 70–87%) after the reactions.

3.7. Catalytic oxidation and product analysis It is known that all the reactive intermediates formed from PFeCl-Ox are very short lived. In order to trap all such reactive intermediates we have used first a very hypersensitive substrate 2,4,6-tri-tert-butyl phenol (TTBP). In a set of experiments it has

Fig. 9. Variation of rate of formation of TTBP radical in CH3CN–H2O at 25 ± 1 °C. The concentrations of TTBP = 100 mM, catalyst = 15 ± 1 lM and t-BuOOH = 1 mM in all the experiments. Averages of duplicate sets of experiments are used in all the plots.

been noted that 100 mM of TTBP is most suitable to trap all oxidants. Thus, in an experiment, TTBP (100 mM) was taken in a cuvette fitted with silicon rubber septum. The cuvette was degassed by blowing argon over it for 25 min. Acetonitrile was taken in a 5 mL gas-tight syringe and was degassed by bubbling argon through the solvent for 25 min. This degassed acetonitrile (1 mL) was used to dissolve the TTBP in the cuvette. In a separate vial H2O was also degassed by bubbling argon through it for 25 min and required amount of H2O was added to TTBP solution in the cuvette. Standard solutions of the iron–porphyrin catalysts in acetonitrile were prepared in vial and from these stock solutions aliquot volume was added to the cuvette so that the final catalyst concentration was 15 ± 1 lM in all the experiments. The t-BuOOH solution was prepared in degassed acetonitrile in a small (4 mL) screw capped vial. An aliquot volume of this stock solution was added to the cuvette to initiate the oxidation reaction. The cell was shaken vigorously and was placed immediately in a thermostated cell holder of the spectrophotometer and the absorbance data at 680 nm were collected at 10-s intervals. The quantification of the radical was done by taking e680 = 278 M1 cm1. It has been observed that TTBP was not oxidized by t-BuOOH alone either in dry or in moist acetonitrile at room temperature. In presence of catalytic quantity of F20TPPFeCl this oxidation was observed to be very slow in dry acetonitrile and this is true for all the other three catalysts used in this study. Interestingly the yields of this oxidation of TTBP were systematically increased when the water content in acetonitrile was increased up to a certain limit. These results are given in Fig. 8. These results have clearly indicated that oxidant has definitely activated by the cata-

301

A. Singh et al. / Inorganica Chimica Acta 372 (2011) 295–303

lyst to evolve some reactive intermediates which should be utilized to oxidize organics in this particular medium. It is indeed interesting to observe also that the variation of the rates of production of the TTBP radical with the variation of the solvent composition for different catalyst were not similar. In case of F20TPPFeCl and F16TPPFeCl the rates were increased with the increase of the water content in acetonitrile, but for F12TPPFeCl the rate was decreased after 9% of water and for F8TPPFeCl maximum rate was observed with 13% of water in acetonitrile and afterwards it was almost constant. These results were shown in Fig. 9. It has also been observed that in all cases there is almost no catalyst activation until the water content of the medium was more than 0–2%. The plots in Fig. 8 indicated that within this 0–2% of water content in acetonitrile the catalyst activation of F20TPPFeCl is appreciable and it is not very good for F12TPPFeCl and F8TPPFeCl. Combining these observations from these two Figs. 8 and 9, we conclude that F20TPPFeCl and F16TPPFeCl behave in a very similar way and the behaviors of F12TPPFeCl and F8TPPFeCl are again very similar especially in the range of 7.5–10.5% of water in acetonitrile. In case of F20TPPFeCl we have done detailed analysis of the structural change of the catalyst in this solvent system and we realized that the major change that takes place is the exchange of the iron(III) bound chloro by hydroxo ligand [31]. This same structural change is predicted to be responsible for the other catalysts too. The results are presented in Fig. 9. The additional point to be noted in Fig. 8 is that with 2–4% of H2O in CH3CN always the yield of TTBP radical is more in case of F20TPPFeCl followed by F16TPPFeCl, and F12TPPFeCl. There is marginal difference between F12TPPFeCl and F8TPPFeCl. This data supports that among the electronegatively substituted catalysts F8TPPFeCl and F12TPPFeCl are fine, however F16TPPFeCl is better and F20TPPFeCl is the best.

3.8. Catalytic oxygenation of cyclohexane by t-BuOOH in acetonitrile water

Cyclohexene (23 lL, 200 mM) was reacted with 2 mM of tBuOOH in presence of F20TPPFeCl (50.0 lM) in 9% H2O in CH3CN (total volume of solvent 1.1 mL). The reaction was continued for 10 min. In this reaction the maximum conversion of cyclohexene to 2-cyclohexen-1-ol was obtained (Fig. 11). For all the catalysts 2-cyclohexen-1-ol was the major product between 72% and 97% in CH3CN–H2O solvent system. 3.10. Nature of reactive intermediates The results support that role of MeOH in CH2Cl2 and that of H2O in CH3CN is to dissociate the iron(III) bound chloride and to evolve solvato species. In case of CH3OH–CH2Cl2 this species absorbs at 404 nm and in case of CH3CN–H2O it absorbs at 402 nm. Afterwards, in methanolic medium we observe that the Soret moves to 390 nm on addition of t-BuOOH, but with a transient Soret at 410 nm (Fig. 2). The transient Soret at 410 nm could be due to the formation of catalyst-oxidant adduct of type F20TPPFe(III)–OOtBu. The 390 nm Soret has been observed due to the HCOOH adduct on F20TPPFe(III) by an independent observation on reaction of HCOOH with F20TPPFe(III)Cl without any oxidant. The formation of HCOOH in the medium is due to the oxidation of CH3OH because such phenomenon is absent in CH3CN–H2O media. In CH3CN–H2O the Soret at 410 nm is very stable and along with that we observe a band at 547 nm which is characteristic of PFe(IV)@O species. Thus, we believe that in CH3CN–H2O Eq. (5)

F20 FeðIIIÞ-OOt Bu ! F20 PFeðIVÞ@O þ t-BuO

50

100

45

90

40

80

35 30 25 20 15

70 60 50 40 30 20

10

10

5

0

0 0

10

20

30

40

50

60

ð5Þ

might be more reasonable and all these reactive intermediates, could be the reason for the oxidation of the substrates studied in this media. In these two media (CH3OH–CH2Cl2 and CH3CN–H2O) we also have reacted F20TPPFeCl with t-BuOOH under the catalytic conditions used in this study in presence of stoichiometric quantity of PPh3 (1 eq w.r.t. oxidant) and we observed that in both the cases P(O)Ph3 was quantitatively formed (Fig. 12). In a typical experiment 10.4 mg (80 mM) of PPh3 was dissolved in 0.5 mL of MeOH–CH2Cl2 (33 ± 3%) or CH3CN–H2O (9%) containing 100 lM of F20TPPFeCl and to this solution a strong solution of t-BuOOH was added so that the final concentration of the oxidant was 80 mM. The mixture was stirred under argon and evaporated to dryness. The solid was redissolved in CDCl3 (0.5 mL) and 31P NMR was recorded (Fig. 12). It was observed that all PPh3 was disappeared and it was quantitatively transformed to P(O)Ph3. In an-

% Yield of 2-cyclohexen-1-ol

% Yield of cyclohexanol

Cyclohexane (48 lL, 400 mM) was reacted with 2 mM of tBuOOH in presence of F20TPPFeCl (50.0 lM) in 9.09% H2O in CH3CN (total volume of solvent 1.1 mL). The reaction was continued for 30 min. In this reaction the only product was cyclohexanol with 47% yield (with respect to oxidant concentration). The formation of cyclohexanol was monitored in different time interval and the profile of product formation with time is given in Fig. 10. The oxidation reactions of cyclohexane were carried out with all four catalysts, i.e. F20TPPFeCl, F16TPPFeCl, F12TPPFeCl and F8TPPFeCl. For all the catalysts the yields in the hydroxylation reactions were similar, between 42% and 47% in CH3CN–H2O medium.

3.9. Catalytic oxygenation of cyclohexene by t-BuOOH in acetonitrile water

0

2

4

6

8

10

12

14

% of H2O in CH3CN

Time (min) Fig. 10. The % yield of cyclohexanol vs. time plot. Concentration of F20TPPFe(III)Cl = 50.0 lM; concentration of t-BuOOH = 2 mM. Yields are based on total oxidants.

Fig. 11. Variation of the yield of 2-cyclohexen-1-ol in CH3CN–H2O at 25 ± 1 °C. The concentrations of cyclohexene = 200 mM, catalyst = 50 ± 1 lM and tBuOOH = 2 mM in all the experiments. Averages of duplicate sets of experiments are used in all the plots.

302

A. Singh et al. / Inorganica Chimica Acta 372 (2011) 295–303

Fig. 12. 31P NMR spectra of the reaction mixture from the oxidation of PPh3 (80 mM) by t-BuOOH (80 mM) in 33 ± 3% CH3OH–CH2Cl2 or from 9% H2O–CH3CN. (a) In absence of catalyst and (b) in presence of F20TPPFeCl (100 lM). The inset plots show the 31P NMR spectra of pure PPh3 and that of P(O)Ph3 in CDCl3. Notice the complete conversion of PPh3 into P(O)Ph3 at 29.4 ppm in both the reactions.

other blank experiment under similar conditions without the catalyst when the experiment was conducted PPh3 was also converted to P(O)Ph3 (Fig. 12). Thus we believe that catalyst-oxidant adduct which has very similar structure the oxidant alone (ROOH) could be one of the major reactive intermediate in these media. In CH3CN–H2O media, the evolution of clear UV–Vis spectrum of PFe(IV)@O supports, PFe(IV)@O also to be the potential oxidant besides the catalyst-oxidant adduct.

4. Conclusions In an appropriate solvent system the activation of iron(III) porphyrin catalysts to react with a hydroperoxide to evolve reactive intermediates has been demonstrated. Both the yields and rates of the catalytic oxidation of TTBP by t-BuOOH in acetonitrile were significantly improved simply by increasing the percentage composition of water in the solvent and the reaction condition is used for selective hydroxylation of an alkene. Using this medium the efficient oxidation of cyclohexane and cyclohexene has also been achieved. We have reported one of the best ways to oxidize C–Pd bond selectively in 33 ± 3% ratio of methanol in dichloromethane. Usefulness of this method for the activation of other catalysts and to hydroxylate other hydrocarbons with hydroperoxides and hydrogen peroxide is under progress.

Acknowledgements The financial support from the Department of Science and Technology, Govt. of India and fellowship to A.S. from IIT Delhi are acknowledged.

References [1] P.R. Ortiz de Montellano (Ed.), Cytochrome P450 Structure, Mechanism, and Biochemistry, third ed., Kluwer Academic/Plenum Publishers, New York, 2005. [2] (a) T.L. Poulos, B.C. Finzel, A.J. Howard, J. Mol. Biol. 195 (1987) 687; (b) M. Vidakovic, S.G. Sligar, H. Li, T.L. Poulos, Biochemistry 37 (1998) 9211; (c) D.E. Benson, K.S. Suslick, S.G. Sligar, Biochemistry 36 (1997) 5104; (d) I. Schlichting, J. Berendzen, K. Chu, A.M. Stock, S.A. Maves, D.E. Benson, Science 287 (2000) 1615; (e) J.R. Cupp-Vickery, O. Han, C.R. Hutchinson, T.L. Poulos, Nat. Struc. Biol. 3 (1996) 632. [3] (a) V. Stanjek, M. Mihsch, P. Lueer, U. Matern, W. Boland, Angew. Chem., Int. Ed. 38 (1999) 400; (b) B. Tudzynski, M.C. Rojas, P. Gaskin, P. Hedden, J. Biol. Chem. 277 (2002) 21246; (c) A.D.N. Vaz, D.F. McGinnity, M.J. Coon, Proc. Natl. Acad. Sci. USA 95 (1998) 3555; (d) M. Newcomb, P.F. Hollenberg, M.J. Coon, Arch. Biochem. Biophys. 409 (2003) 72; (e) C. Veeger, J. Inorg. Biochem. 9 (2002) 35. [4] (a) J.T. Groves, D.V. Subramanian, J. Am. Chem. Soc. 106 (1984) 2177; (b) J.T. Groves, G.E. Avaria-Neisser, K.M. Fish, M. Imachi, R.L. Kuczkowski, J. Am. Chem. Soc. 108 (1986) 3837.

A. Singh et al. / Inorganica Chimica Acta 372 (2011) 295–303 [5] (a) R. Devydov, T.M. Markis, V. Kofman, D.E. West, S.G. Sligar, B.M. Hoffman, J. Am. Chem. Soc. 123 (2001) 1413; (b) V. Schuemann, C. Jung, J. Terner, A.X. Trautwein, R. Weiss, J. Inorg. Biochem. 91 (2002) 586; (c) D.G. Kellner, S.C. Hung, K.E. Weiss, S.G. Sligar, J. Biol. Chem. 277 (2002) 9641. [6] (a) B. Meunier, Chem. Rev. 92 (1992) 1411; (b) D. Mansuy, Coord. Chem. Rev. 125 (1993) 129; R.A. Sheldon (Ed.), Metalloporphyrins in Catalytic Oxidations, Marcel Dekker, New York, 1994; (c) D. Dolphin, T.G. Traylor, L.Y. Xie, Acc. Chem. Res. 30 (1997) 251; J.L. Mclain, J. Lee, J.T. Groves, in: B. Meunier (Ed.), Biomimetic Oxidations Catalyzed by Transition Metal Complexes, Imperial College Press, London, 2000, pp. 91–169. [7] (a) T.G. Traylor, K.W. Hill, W.-P. Fann, S. Tsuchiya, B.E. Dunlap, J. Am. Chem. Soc. 114 (1992) 1308; (b) J.T. Groves, T.E. Nemo, J. Am. Chem. Soc. 105 (1983) 5786; (c) J.-F. Bartoli, O. Brigaud, P. Battioni, D. Mansuy, J. Chem. Soc. Chem. Commun. (1991) 440; (d) A.L. Balch, L. Latos-Grazynski, M.W. Renner, J. Am. Chem. Soc. 107 (1985) 2983; (e) M.W. Grinstaff, M.G. Hill, J.A. Labinger, H.B. Gray, Science 264 (1994) 1311; (f) J.P. Collman, A.S. Chien, T.A. Eberspacher, J.I. Brauman, J. Am. Chem. Soc. 122 (2000) 11098; (g) J.P. Collman, L. Zeng, R.A. Decerau, Chem. Commun. (2003) 2974. [8] (a) J.E. Penner-Hamm, T.J. McMurry, M. Renner, L. Latos-Grazynski, K.S. Eble, I.M. Davis, A.L. Balch, J.T. Groves, J.M. Dawson, K.D. Hodgson, J. Biol. Chem. 258 (1983) 12761; (b) T.G. Traylor, C. Kim, L.J. Richards, F. Xu, C.L. Perrin, J. Am. Chem. Soc. 117 (1995) 3468; (c) K. Kamaraj, D. Bandyopadhyay, J. Am. Chem. Soc. 119 (1997) 8099; (d) K. Machi, Y. Watanabe, I. Morishima, J. Am. Chem. Soc. 117 (1995) 6691; (e) A. Franke, M. Wolak, R. van Eldik, Chem. Eur. J. 15 (2009) 10182. [9] (a) P. Wadhwani, M. Mukherjee, D. Bandyopadhyay, J. Am. Chem. Soc. 123 (2001) 12430; (b) D. Mansuy, J.-F. Bartoli, J.-C. Chottard, M. Lange, Angew. Chem., Int. Ed. Engl. 19 (1980) 909. [10] (a) S.L.H. Rebelo, M.M. Pereira, M.M.Q. Simões, M.G.P.M.S. Naves, J.A.S. Cavaleiro, J. Catal. 234 (2005) 76; (b) N.A. Stephenson, A.T. Bell, J. Am. Chem. Soc. 127 (2005) 8635; (c) I.D. Cunningham, T.N. Danks, J.N. Hay, I. Hamerton, S. Gunathilagan, C. Janczak, J. Mol. Catal. A: Chem. 185 (2002) 25; (d) M. Pratt, T.I. Ridd, L.J. King, J. Chem. Soc., Chem. Commun. (1995) 2297; (e) T.G. Traylor, W.-P. Fann, D. Bandyopadhyay, J. Am. Chem. Soc. 111 (1989) 8009; (f) P. Battioni, J.P. Renaud, J.F. Bartoli, M. Reina-Artiles, M. Fort, D. Mansuy, J. Am. Chem. Soc. 110 (1988) 8462.

303

[11] E. Fouquet, G. Pratviel, J. Bernadou, B. Meunier, J. Chem. Soc., Chem. Commun. (1987) 1169 (and references therein). [12] (a) Z. Gross, S. Ini, Inorg. Chem. 38 (1999) 1446; (b) R. Ito, N. Umezawa, T. Higuchi, J. Am. Chem. Soc. 127 (2005) 834. [13] (a) A. Sorokin, A. Robert, B. Meunier, J. Am. Chem. Soc. 115 (1993) 7293; (b) Z. Fang, R. Breslow, Bioorg. Med. Chem. Lett. 15 (2005) 5463. [14] J.T. Groves, T.E. Nemo, R.S. Myers, J. Am. Chem. Soc. 101 (1979) 1032. [15] J.T. groves, T.E. Nemo, J. Am. Chem. Soc. 105 (1983) 5786. [16] (a) P.S. Traylor, D. Dolphin, T.G. Traylor, J. Chem. Soc., Chem. Commun. (1984) 279; (b) T.G. Traylor, Y.S. Byun, P.S. Traylor, P. Battioni, D. Mansuy, J. Am. Chem. Soc. 113 (1991) 7821. [17] J.-F. Bartoli, P. Battioni, W.R. De Foor, D. Mansuy, J. Chem. Soc., Chem. Commun. (1994) 23. [18] D. Mansuy, J.-F. Bartoli, M. Momenteau, Tetrahedron. Lett. 23 (1982) 2781. [19] L. Chen, Y. Yang, D. Jiang, J. Am. Chem. Soc. 132 (2010) 9138. [20] (a) J.T. Groves, Z. Gross, M.K. Stern, Inorg. Chem. 33 (1994) 5065; (b) T.G. Traylor, A.R. Miksztal, J. Am. Chem. Soc. 111 (1989) 7443. [21] (a) K.A. Lee, W. Nam, J. Am. Chem. Soc. 119 (1997) 1916; (b) T.G. Traylor, S. Tsuchiya, Y.-S. Byun, C. Kim, J. Am. Chem. Soc. 115 (1993) 2775. [22] T.G. Traylor, F. Xu, J. Am. Chem. Soc. 112 (1990) 178. [23] W. Nam, S.-E. Park, I.K. Lin, M.H. Lim, J. Hong, J. Kim, J. Am. Chem. Soc. 125 (2003) 14674. [24] J.-F. Bartoli, K.L. Barch, M. Palacio, P. Battioni, D. Mansuy, Chem. Commun. (2001) 1718. [25] M.H. Lim, Y.J. Lee, Y.M. Goh, W. Nam, C. Kim, Bull. Chem. Soc. 72 (1999) 707. [26] (a) K. Kamaraj, D. Bandyopadhyay, Organometallics 18 (1999) 438; (b) P. Wadhwani, D. Bandyopadhyay, Organometallics 19 (2000) 4435. [27] D.D. Perrin, W.L.F. Armarego, Purification of Laboratory Chemicals, third ed., Pergamon Press, Oxford, 1988, pp. 217. [28] (a) A.D. Alder, F.R. Longo, F. Kampas, J. Kim, J. Inorg. Nucl. Chem. 32 (1970) 2443; (b) J.S. Lindsey, R.W. Wagner, J. Org. Chem. 54 (1989) 828; (c) J.S. Lindsey, C.I. Schreiman, H.C. Hsu, P.C. Kearney, M.A. Marguerettez, J. Org. Chem. 52 (1987) 827; (d) H. Sharghi, A.H. Nejad, Tetrahedron 60 (2004) 1863. [29] (a) H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703; (b) M. Barra, C. Bohne, J.C. Scaiano, J. Am. Chem. Soc. 112 (1990) 8075; (c) M. Zhu, M. Yuan, X. Liu, J. Xu, J. Lv, C. Huang, H. Liu, Y. Li, S. Wang, D. Zhu, Org. Lett. 10 (2008) 1481. [30] (a) C.K. Pal, S. Chattopadhyay, C.R. Sinha, A. Chakravorty, J. Organomet. Chem. 439 (1992) 91. [31] A. Agarwala, D. Bandyopadhyay, Chem. Commun. (2006) 4823.