Biomimetic oxidation of cyclic and linear alkanes: high

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Cite this: New J. Chem., 2017, 41, 997

Biomimetic oxidation of cyclic and linear alkanes: high alcohol selectivity promoted by a novel manganese porphyrin catalyst† Vinicius Santos da Silva,a Warleson Caˆndido dos Santos Vieira,a Alexandre Moreira Meireles,a Geani Maria Ucoski,b Shirley Nakagaki,b Ynara Marina Idemoria and Gilson DeFreitas-Silva*a A novel third-generation metalloporphyrin, chloro-5,10,15,20-tetrakis-(3 0 -bromine-4 0 -methoxyphenyl)2,3,7,8,12,13,17,18-octabromoporphyrinatomanganese(III), [MnIIIBr12T4(-OMe)PPCl], designated as MnBr12Por, was synthesized and characterized. The catalytic activity of the new manganese porphyrin was investigated in the oxidation of cyclohexane, adamantane and n-hexane by iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)2) in comparison to the catalytic activity of the second-generation catalyst [MnIIIT4(-OMe)PPCl], designated as MnPor. All yields were based on oxidants. The MnBr12Por/PhIO system led to higher yields of

Received (in Nottingham, UK) 30th September 2016, Accepted 11th December 2016

cyclohexane oxidation products (45%) with high selectivity for cyclohexanol (80%) as compared to the MnPor/ PhIO system (23% and 77%, respectively). Addition of imidazole to MnPor/PhIO increased the total product yield from 23 to 43%; addition of imidazole to MnBr12Por/PhIO did not alter the total product yield at all. For the

DOI: 10.1039/c6nj03072f

MnPor/PhI(OAc)2 and MnBr12Por/PhI(OAc)2 systems, addition of imidazole increased the product yields from 19 to 45% and from 35 to 66%, respectively. Addition of water increased the total product yields during cyclohexane

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and adamantane oxidation for all the studied systems. In all cases, MnBr12Por performed better than MnPor.

Introduction Synthetic metalloporphyrins have been extensively investigated as catalysts in the oxidation of organic substrates – they can act as biomimetic models of cytochromes P450 during these reactions.1–4 Cytochromes P450 consist of a protein matrix and a prosthetic group (heme) in which iron protoporphyrin IX is axially coordinated to the sulfur of a cysteine residue.5–7 In vivo, cytochromes P450 catalyze the oxidation of xenobiotics with wide specificity and regio- and stereoselectivity.6,7 Groves8 designed the first cytochrome P450 biomimetic model, the first-generation metalloporphyrin chloro(5,10,15,20-tetraphenylporphyrinato)iron(III), or [FeIIITPPCl], and used it to catalyze the oxidation of cyclohexane, adamantane, and cis-stilbene by iodosylbenzene (PhIO). Since then, researchers have adopted various strategies such as (1) introduction of bulky electronwithdrawing substituents at the peripheral positions of the porphyrin macrocycle;9–12 (2) use of additives during the oxidation a

Departamento de Quı´mica, Instituto de Cieˆncias Exatas, Universidade Federal de Minas Gerais (UFMG), 31.270-901, Belo Horizonte, MG, Brazil. E-mail: [email protected] b ´rio de Bioinorga ˆnica e Cata ´lise, Departamento de Quı´mica – Centro Politećnico, Laborato ´ (UFPR), 81.531-980, Curitiba, PR, Brazil Universidade Federal do Parana † Electronic supplementary information (ESI) available: FTIR and NMR spectra, and tables with the catalytic results. See DOI: 10.1039/c6nj03072f

reactions, such as pyridine and imidazole;12–15 (3) use of different oxidants (PhIO, iodobenzene diacetate (PhI(OAc)2) and H2O2);4,16–18 and (4) metalloporphyrin immobilization on different supports (galactodendritic conjugated porphyrin, layered double hydroxide (LDH) and silica)19,20 to obtain biomimetic systems that are as efficient as cytochromes P450. About 30 years ago, Traylor and Tsuchyia21 were the first to lower the electron density over the porphyrin ring by introducing electronwithdrawing groups at the meso-aryl and/or b-pyrrole positions of the porphyrin ring, to obtain second- and third-generation metalloporphyrins, respectively.1 In their work,21 the authors used the extremely stable 5,10,15,20-tetrakis(2,6-dichlorophenyl)2,3,7,8,12,13,17,18-octabromoporphyrinatoiron(III) chloride to catalyze the hydroxylation and epoxidation of organic substrates. In the following years, many authors described the use of thirdgeneration iron and manganese porphyrins to catalyze the oxidation of organic substrates by different oxidants.22–24 For instance, in 1990, Ellis and Lions23 used 5,10,15,20-tetrakis(2,3,4,5,6pentafluorophenyl)-2,3,7,8,12,13,17,18-octabromoporphyrinatoiron(III) chloride to catalyze the oxidation of isobutane by molecular oxygen, to obtain over 90% selectivity toward the alcohol product and a turnover number higher than 13 000. Additives can boost the catalytic efficiency of metalloporphyrinbased biomimetic systems. For example, nitrogen (imidazole and pyridine)15,25 and oxygen (H2O and OH) bases10,14,26 can positively

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impact the total product yield and selectivity as well as diminish the degree of catalyst destruction. Gunter and Tuner25 used imidazole to improve the iron and manganese porphyrin-catalyzed oxidation and epoxidation of organic substrates. Strategies that increase reaction efficiency and selectivity and generate less environmentally harmful oxidant byproducts are desirable. Therefore, deciding which oxidant to employ during the development of biomimetic cytochrome P450 systems is extremely important. In the context of cytochrome P450 mimics, PhIO is a classic oxidant for in vitro reactions. It usually replaces the oxidants O2/NAD(P)H and H2O2 employed by cytochrome P450 systems in vivo.27 Groves et al.8 introduced the use of PhIO in biomimetic systems in 1979. Since then, PhIO has been the oxidant of choice in most studies on metalloporphyrins. Although this classic O-donor leads to a direct formation of the active species, its use has caused inconveniences, such as: it is poorly soluble in most organic solvents, it is potentially explosive and it undergoes slow but progressive disproportionation.28 In this scenario, PhI(OAc)2 has emerged as an alternative to PhIO: basic PhI(OAc)2 hydrolysis generates PhIO, not to mention that PhI(OAc)2 meets the principles of green chemistry because it requires fewer steps to generate PhI(OAc)2 in comparison to PhIO.28 In addition, many research groups have used PhI(OAc)2 without noting significant alterations in the total product yield.11,15,29 This study has evaluated the catalytic performance of two manganese porphyrins (MnPs, Fig. 1) derived from 5,10,15,20tetrakis(4-methoxyphenyl)porphyrin H2T4-(OMe)PP (H2P, Fig. 1), namely second- and third-generation MnPs designated as MnPor and MnBr12Por, respectively, in the oxidation of alkanes (cyclohexane, adamantane, or n-hexane) by PhIO or PhI(OAc)2. Based on the literature reports,13,25,30 we have also added imidazole or water to the reactions in an attempt to improve the catalytic efficiency of the target systems.

Experimental A.

Reagents

5,10,15,20-Tetrakis(4-methoxyphenyl)porphyrin (H2P, Fig. 1) was purchased from Aldrich Chemical Co, purified by column chromatography (Al2O3:CH2Cl2), and characterized by UV-Vis,

Fig. 1

FTIR and 1H NMR spectroscopy (ESI,† Fig. S2 and S3). Analytical grade CH3OH, CH2Cl2, and CHCl3 were obtained from Vetec and distilled prior to use. Dimethylformamide (DMF) was acquired from Vetec. PhIO was prepared according to a literature procedure,31 stored at 20 1C in a freezer and periodically controlled for purity by iodometric titration. All the other reagents and solvents were of analytical grade and were used without further purification, unless stated otherwise. B.

Equipment

Ultraviolet-visible (UV-Vis) spectra were registered in the 200– 800 nm range on an HP-8453A diode-array spectrophotometer. Infrared (IR) spectra were registered on a Perkin Elmer BXFTIR spectrometer; using KBr pellets. KBr was crushed with a small amount of samples. Room-temperature (25 1C) 1H NMR spectra were obtained in CDCl3 on a Bruker DPX-400 Advance spectrometer operating at 400 MHz; tetramethylsilane (TMS) was used as the internal standard. The ESI-MS analyses were conducted on an LCQ Fleet (Thermo Scientific, San Jose, CA, USA) mass spectrometer equipped with an electrospray ionization (ESI) source operating in the positive ion mode; CH3OH was used as solvent. Electron paramagnetic resonance (EPR) measurements of the powder materials were accomplished on an EPR BRUKER EMX microX spectrometer (frequency X, band 9.5 GHz), at room temperature and 77 K (liquid N2); a perpendicular microwave polarization X-band was employed. The ultrasound equipment Uniques MaxiClean 1400, 40 kHz, was also employed in the experiments. Cyclic voltammetry was carried out on an Eco Chemie l-Autolab potentiostat; the GPES software was used. The electrochemical cell contained a homemade glassy carbon working electrode, a platinum wire counter electrode, and a homemade Ag/AgCl reference electrode. These analyses were carried out according to the procedure used by Silva et al.,13 as recommended by IUPAC for measurements in non-aqueous solvents.32 All the products from the catalytic oxidation reactions were analyzed on a Shimadzu GC-17A chromatograph equipped with a flame ionization detector and a Carbowax capillary column (measuring 30.0 m 0.32 mm, with a film thickness

Structural representation of the metalloporphyrins synthesized in this work.

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of 0.25 mm). The oven temperature program employed for the determination of the oxidation products from cyclohexane oxidation started at 80 1C. Then, the temperature was increased to 150 1C at 5 1C min1, which was maintained for 1 min. For analysis of the products originating from adamantane, the temperature program started at 100 1C, which was maintained for 1 min, followed by a temperature elevation to 130 1C at 15 1C min1, maintained for 5 min, and a further temperature increase to 180 1C at 15 1C min1, kept for 1 min. For analysis of the products originating from n-hexane, the temperature program started at 45 1C, which was maintained for 4 min, followed by a temperature elevation to 100 1C at 7 1C min1, kept for 5 min. C.

Synthesis of metalloporphyrin catalysts

C.1. Chloro-5,10,15,20-tetrakis(40 -methoxyphenyl)porphyrinatomanganese(III) – [MnIIIT4(-OMe)PPCl] or MnPor. MnPor (Fig. 1) was synthesized according to the procedure used to prepare [MnIIIAPTPPCl],13 with some modifications. The free-base porphyrin, H2P, (99.66 mg; 0.136 mmol) was dissolved in a minimum volume of CHCl3. Then, Mn(OAc)24H2O (333.0 mg; 1.359 mmol) dissolved in CH3OH was added, and the system was kept under magnetic stirring and reflux for 12 h. The porphyrin complex was washed with distilled water, extracted with CHCl3, and purified by chromatography on an SiO2 column; CHCl3/CH3OH (10 : 1) was used as the eluent. Yield 92% (102.1 mg; 0.124 mmol). UV-Vis (CHCl3), lmax (nm) (log e): 382 (4.69); 480 (4.98); 587 (3.91); 627 (4.09). FTIR (ESI,† Fig. S5) in KBr (cm1): (1604) dCQC; (1294) dporphyrin skeleton; (1249) nC–O–C, (1173) nOCH3; (1004) dMn–N. ESI-TOF [MnIIIT4(-OMe)PP]+ m/z 787.21 (100%). C.2. Chloro-5,10,15,20-tetrakis(3 0 -bromo-4 0 -methoxyphenyl)2,3,7,8,12,13,17,18-octabromoporphyrinatomanganese(III) – [MnIIIBr12T4(-OMe)PPCl] or MnBr12Por. The new metalloporphyrin MnBr12Por (Fig. 1) was synthesized according to the procedure used to prepare [MnIIIBr9APTPPCl].13 Briefly, MnPor (101.35 mg; 0.12316 mmol) and liquid bromine (B0.3 mL, 5.6 mmol, 40-fold molar excess) were dissolved in DMF and kept under magnetic stirring at ambient temperature for 12 h. Then, distilled water was added to the system, which led to the MnP precipitation. MnBr12Por was purified by column chromatography on SiO2; CHCl3/CH3OH (10 : 1) was used as the eluent. Subsequently, MnBr12Por was precipitated with n-hexane, percolated on a Sephadex column, and eluted with CHCl3. The yield was 65% (141.69 mg; 0.080054 mmol). Elemental analysis. Found: C, 35.5; H, 1.9; N, 2.9%. Calc. for C48H24Br12ClMnN4O4.1C6H14: C, 35.0; H, 2.1, N, 3.0%. UV-Vis (CHCl3), lmax (nm) (log e): 371 (4.71); 508 (5.09); 623 (4.11); 670 (4.30). FTIR (ESI,† Fig. S5) in KBr (cm1): (1594) dCQC; (1289) dporphyrin skeleton; (1272) nCb–Br; (1255) nC–O–C; (1175) nOCH3; (1004) dMn–N. ESI-TOF [MnIIIBr12T4(-OMe)PP]+ (ESI,† Fig. S6) m/z 1734.07 (100%). C.3. Alkane oxidation reactions. All the catalytic reactions were realized according to the method described by da Silva et al.26 All the catalytic reactions were performed in 2 mL Wheatons vials sealed with Teflon-faced silicon septa. The reactions were conducted under magnetic stirring, at 25 1C, for 90 min. Oxidation was carried out in air; either PhI(OAc)2 or PhIO were used as the oxygen donor. Reaction mixtures comprised 2.0 104 mmol of

the catalyst (MnP); 2.0 103 mmol of the oxidant (PhIO or PhI(OAc)2); 100 mL of cyclohexane (0.93 mmol), or 100 mL of n-hexane (0.76 mmol), or 100 mL of adamantane solution in CH2Cl2 (0.2 mol L1; 0.02 mmol); and 200 mL of CH2Cl2. The catalyst/oxidant/substrate molar ratio was 1 : 10 : 4650 for cyclohexane, and 1 : 10 : 3800 for n-hexane. As for adamantane, the catalyst/oxidant/substrate molar ratio was 1 : 10 : 100, because adamantane was poorly soluble in CH2Cl2. When deemed necessary, the reaction was quenched by addition of sulfite and borax solution.44 The reaction mixtures were analyzed by capillary gas chromatography; bromobenzene was the internal standard. The retention times of the products were confirmed by comparison with the retention times of authentic product samples. The yields were based on either initial PhIO or PhI(OAc)2. Each reaction was accomplished at least three times, and the reported data represent the average of the results of these reactions. Errors in yields and selectivity were calculated on the basis of reaction reproducibility. The degree of MnP destruction (bleaching) was determined by UV-Vis spectroscopy at the end of the catalytic run. Control reactions were conducted in three ways: (1) in the absence of the catalyst, (2) in the absence of the oxidant and (3) by substitution of the MnP for a manganese salt (Mn(OAc)24H2O). The effect of imidazole was studied by adding a 10 mL aliquot of an imidazole (Im) 1.0 102 mol L1 solution in CH2Cl2 to the reaction medium. To test the effect of water on cyclohexane and adamantane oxidation, 0.5 mL of water was added to the reaction mixture. The MnP : Im molar ratio was 1 : 0.5, whereas for the MnP : H2O molar ratio was 1 : 139.

Results and discussion A.

Obtainment of metalloporphyrin catalysts

The second-generation catalyst MnPor was obtained by the chloroform/methanol method33 and characterized by thin layer chromatography. When dichloromethane was the eluent, MnPor remained close to the application point (Rf B 0.04), whereas the free-base porphyrin ran along the plate (Rf B 0.6). Another evidence of metal insertion into H2P was the loss of fluorescence (red) typical of the free-base porphyrin under irradiation with ultraviolet light.34 The novel third-generation catalyst MnBr12Por was prepared according to the method described by Richards et al.,35 with modifications. The use of dimethylformamide and excess bromine favored complete polybromination of the porphyrin macrocycle.35 The activating character of the methoxy (–OCH3) substituents favored addition of a bromine atom at the ortho-position relative to –OCH3. Sankar et al.36 observed the same outcome when they added 15 or 16 bromine atoms (at the b-pyrrole positions and the meso-aryl groups) to [ZnIIT3,5DMPP]. In a recent paper by our group, [MnIIIT3,5(-OMe)PPCl] bromination also introduced bromine atoms at the ortho-position relative to the –OCH3 groups.29 The UV-Vis spectra of MnPor and MnBr12Por (Fig. 2) agreed with the spectra of other MnPs described in the literature.9,10,26 Comparison of the spectra of H2P and MnPor revealed alterations


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Fig. 2 UV-Vis absorption spectra of H2Por (3.15 106 mol L1), MnPor (1.05 105 mol L1) and MnBr12Por (4.79 106 mol L1) in CH2Cl2.

in the spectral profile: MnPor displayed fewer Q bands, and there was a bathochromic shift of the Soret band. Strong interaction between the orbitals of the porphyrin macrocycle and the orbitals of the metal could account for these alterations.37 Comparison of the spectra of MnPor and MnBr12Por showed a further bathochromic shift of the Soret band, which agreed with the characterization of other b-octabrominated MnPs as compared to their non-brominated counterparts.10,38–40 The bromine atoms at the b-pyrrole positions promoted the bathochromic shift of the Soret band: bromine stabilized the HOMOs and LUMOs of the porphyrin ring by an inductive effect and destabilized the HOMOs by conformational changes (planar to saddle shape), which diminished the conjugation of the p orbitals. Consequently, the energy between the frontier orbitals decreased.39,41,42 The infrared vibrational spectra (ESI,† Fig. S5) helped to identify the –OCH3 groups in the structure of the porphyrin macrocycle and to confirm the metal insertion into the porphyrin ring: the band due to N–H (pyrrole) bond deformation (966 cm1) disappeared, and a new band (1004 cm1) due to the axial deformation of the Mn–Npyrrole bond appeared.43 The analysis of the mass spectra of MnPor and MnBr12Por (ESI,† Fig. S6) confirms the proposed structure because it presents the peaks at m/z 787.21 (100%) and 1734.07 (100%), respectively, associated with the loss of the chloride ion. The 1 H NMR (ESI,† Fig. S4) spectrum of the brominated free-base porphyrin H2Br12Por confirmed that eight and four bromine atoms were introduced at the b-pyrrole positions and at one ortho-position relative to the –OCH3 groups in the meso-aryl substituents of the porphyrin macrocycle, respectively. This is observed because the introduction of bromine atoms at two ortho-positions, relative to the –OCH3 groups, should be hampered by steric hindrance. In addition, since the aryl group is less reactive than the b-pyrrole positions, a longer reaction time may be required for the substitution to take place at the two ortho-positions. The oxidation of organic substrates catalyzed by metalloporphyrins depends on the oxidation state and redox potential

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Fig. 3 EPR spectra of MnBr12Por in the solid state (black line) and DMF solution (red line).

of the metal ion, which will determine the formation, stability, and reactivity of the high-valence active species and will account for oxygen transfer to the substrate.44 Therefore, we subjected MnBr12Por in the solid state as well as in DMF solution to electron paramagnetic resonance (EPR) at 77 K (Fig. 3, frequency X, band 9.5 GHz) by perpendicular polarization with the microwave X-band. Metalloporphyrins bearing electron-withdrawing bromine atoms at the b-pyrrole positions should favor lower metal ion oxidation states (e.g., Mn(II)).45–47 According to the EPR spectra illustrated in Fig. 3, Mn(II) EPR signals did not emerge in the spectrum of solid-sate MnBr12Por, suggesting that MnBr12Por only contained Mn(III), an EPR silent species, under the measurement conditions.48,49 This behavior has been observed for other b-halogenated MnPs.29 Indeed, the Mn(III) ion is paramagnetic and has four unpaired d electrons (S = 2). Mn(III) typically exhibits a pronounced Jahn–Teller distortion, which results in a substantial spin–orbit coupling.50 The final outcome of this coupling and rapid Mn(III) spin relaxation processes is the absence of signals in the conventional Mn(III) EPR spectra recorded with the X-band (microwave field (H1) perpendicular to the static magnetic field (H0)) even at 77 K,50 even though spin transitions may arise during measurements in the parallel mode (H1 J H0).51 The voltammograms in Fig. 4 helped to evaluate the reversibility of the process MnIII + e " MnII for the brominated MnBr12Por and its non-brominated counterpart MnPor. The process was quasi-reversible for both MnPs – it did not meet some parameters that characterize reversibility, such as the difference between the anodic and cathodic potentials, which was not constant, and the ratio between the anodic and cathodic peaks and the scanning rate, which were not close to one.52 Comparison of the half-wave potentials of the MnIII/MnII process (Table 1 and Fig. 4) revealed a 367 mV anodic shift for MnBr12Por as compared to MnPor. The introduction of bromine atoms at the b-pyrrole positions of the porphyrin ring destabilized HOMOs and LUMOs via an inductive effect and HOMOs via conformational changes. As a consequence, the energy of the LUMOs decreased, favoring reduction. Thus, brominated


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Fig. 4 Cyclic voltammograms of MnPor and MnBr12Por (CH2Cl2, [MnIIIP] = 5.0 104 mol L1, [TBABF4] = 0.1 mol L1, scan rate 0.1 V s1).

Table 1 Potentials of the anodic peak (Epa) and cathodic peak (Epc) and the half-wave potential (E1/2) vs. Fc+/Fc for Mn(III)/Mn(II) in MnPs (in CH2Cl2, 100 mV s1)

Complex

Epa (mV)

Epc (mV)

E1/2 (mV)

DE1/2 (mV)

MnPor MnBr12Por

750 368

930 578

840 473

— 367

MnPs are systematically easier to reduce than their corresponding non-brominated analogues.13,39,41,46 B.

Catalytic studies

Selective oxidation of inert organic molecules, like alkanes, under mild conditions has been one of the most investigated topics over the last decades for both practical and economic reasons.2 Oxidation of cyclic alkanes, such as adamantane and cyclohexane, is important because their oxidation products are precursors in various syntheses (1-adamantanol (1-adol), 2-adamantanol (2-adol), and 2-adamantanone (2-adone))53 and constitute raw materials for the production of nylon-6 and nylon-66 (cyclohexanol and cyclohexanone).54 Nevertheless, the oxidation of cyclohexane is one of the least efficient industrial processes.55 As for the oxidation of n-hexane, interest in this process stems from the difficulty in transforming highly stable linear alkanes into high-value products. B.1. Oxidation of cyclohexane. The oxidation of cyclohexane by different oxidants catalyzed by metalloporphyrins generally produces cyclohexanol (Cy-ol) and cyclohexanone (Cy-one) as majority products and adipic acid may arise in some systems.56 The catalytic systems studied here generated Cy-ol and Cy-one only. Monitoring of the oxidation of cyclohexane along time (Fig. 5) suggests that the MnBr12Por/PhIO system produced Cy-ol faster than the MnPor/PhIO system. The bromine atoms at the b-pyrrole positions of MnBr12Por withdrew electron density from the porphyrin ring and the metal ion, not to mention that they modified the macrocycle conformation. Consequently, the energy level of the LUMOs (eg) and HOMOs (a2u) decreased and increased, respectively, favoring the oxidation reaction. Indeed, the

Fig. 5 Monitoring of cyclohexanol production during the oxidation of cyclohexane by PhIO catalyzed by MnPs in CH2Cl2 under aerobic conditions, MnP/oxidant/cyclohexane/CH2Cl2 molar ratio = 1 : 10 : 4650 : 15 550, 25 1C, magnetic stirring, 90 min. of reaction. The yields are based on PhIO.

increased energy of the HOMOs (a2u) favors the formation of the high-valence active species MnV(O)P.49 Table 2 lists the results concerning the oxidation of cyclohexane by PhIO catalyzed by the MnPs prepared in this work, all yields were based on the oxidant (PhIO or PhI(OAc)2). MnPor (entry 1) was slightly more efficient than [MnIIITPPCl], a first-generation metalloporphyrin, that is the classic reference in studies on the catalytic activity of MnPs8 and gives a Cy-ol yield of 14% and a selectivity of 56% under the same reaction conditions.13 The –OCH3 group at the para–meso-aryl positions of MnPor must have led to an anodic shift in the MnIII/MnII redox potential and increased the reactivity of the high-valence active species. A similar behavior emerged after comparison of the catalytic activity of MnPor with the activity of [MnIIIT3,5DMPPCl] (21% total yield), which bears a –OCH3 group at the meta–meso-aryl positions of the porphyrin ring.29 Compared to [MnIIITCMPPCl], the MnP containing groups like carbomethoxy (–COOCH3) in the para–meso-aryl positions,9 MnPor gave practically the same total product yield. However, MnPor was less efficient than metalloporphyrins bearing groups like –NH2 in the para–mesoaryl positions.13,26,57 Substituents containing nitrogen atoms may coordinate with the metal ion of another MnP complex and weaken the MnQO bond.25 In an attempt to develop more robust catalytic systems that abide by the principles of green chemistry, we evaluated the efficiency of MnPor and MnBr12Por in the oxidation of cyclohexane by PhI(OAc)2 (Table 2, entries 7 and 8). This oxidant is commercially available and it does not undergo disproportionation, in contrast to PhIO. Some literature papers have described that PhI(OAc)2 can efficiently substitute PhIO.11,13,29 Here, MnP/PhI(OAc)2 systems (Table 2, entries 7 and 8) gave slightly lower product yields and selectivity for Cy-ol, and higher bleaching than MnP/PhIO systems (Table 2, entries 1 and 2). Distinct active species may have emerged during the oxidation reactions conducted in the presence of different


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Table 2 Product yields obtained during the oxidation of cyclohexane by PhIO or PhI(OAc)2 catalyzed by MnPs in CH2Cl2 under aerobic conditions, with or without water or imidazole as an additive

Product yield (%) Entry

Catalyst

System

Cy-ola,d

Cy-onea

Total yield (%)

Select.b (%)

TONe

Bleac.c (%)

1 2 3 4 5 6 7 8 9 10 11 12

MnPor MnBr12Por MnPor MnBr12Por MnPor MnBr12Por MnPor MnBr12Por MnPor MnBr12Por MnPor MnBr12Por

PhIO PhIO PhIO/Im PhIO/Im PhIO/H2O PhIO/H2O PhI(OAc)2 PhI(OAc)2 PhI(OAc)2/Im PhI(OAc)2/Im PhI(OAc)2/H2O PhI(OAc)2/H2O

18 37 28 34 22 39 14 27 32 50 30 32

5 9 12 11 12 14 5 8 13 16 15 10

23 46 40 45 34 53 19 35 45 66 45 42

77 80 70 75 65 74 74 77 71 76 67 77

2.3 4.6 5.9 4.5 3.4 5.3 1.9 3.5 4.5 6.6 4.5 4.2

61 44 44 38 43 40 68 74 67 52 67 55

Reaction conditions: [MnP] = 5 104 mol L1, [PhIO or PhI(OAc)2] = 5 103 mol L1, MnP/oxidant/cyclohexane/CH2Cl2 molar ratio = 1 : 10 : 4650 : 15 550, 25 1C, magnetic stirring, 90 min. of reaction. MnP/Im molar ratio in reactions 3, 4, 9, and 10 = 1 : 0.5. MnP/H2O molar ratio in reactions 5, 6, 11, and 12 = 1 : 139 (0.5 mL). a Yields based on the oxidant. Cy-ol: cyclohexanol and Cy-one: cyclohexanone. b Selectivity for the alcohol was determined from the relation: selectivity = 100 [%Cy-ol/(%Cy-ol + %Cy-one)]. c The degree of catalyst destruction was calculated from the UV-Vis absorption spectra recorded at the end of the reaction. d The maximum error is 3% with a confidence level of 95%. e TON = total moles of products per mole of catalyst.

oxygen donors for the same substrate, as proposed in a kinetic study reported by Collman et al.58 In the presence of PhIO or PhI(OAc)2, the third-generation catalyst MnBr12Por was much more efficient than its secondgeneration counterpart, MnPor (Table 2). However, in the presence of PhI(OAc)2 the catalyst MnBr12Por was more destroyed. As MnBr12Por is in a saddle-shaped conformation, it is more distorted, which due to steric hindrance makes it more resistant to oxidative destruction (the high-valence active species react with the catalyst). However, this distortion of the macrocycle causes a smaller overlap of the p orbitals, and consequently, this compound is more susceptible to be oxidized. PhI(OAc)2 may form different oxidant species in the reaction medium,59 thus the third-generation metalloporphyrin in the presence of this oxidant shows a higher destruction (bleaching). This behavior is not observed in the presence of PhIO, which is polymeric and insoluble in the solvent used; this oxidant only favors the formation of the high valence active species. The higher efficiency of MnBr12Por systems was attributed to the presence of the bromine atoms at the b-pyrrole positions of the macrocycle. These substituents withdrew electron density from the metal center and modified the macrocycle conformation, increasing the energy level of the HOMOs (a2u) and making the high-valence active species more reactive.22,49,59 Comparison of the catalytic activity of MnBr12Por to the activity of other third-generation MnPs revealed that MnBr12Por provided higher product yields than [MnIIIBr8T4CMPPCl],9 similar product yields to [MnIIIBr9APTPPCl],13 and lower product yields than cis-[MnIIIBr12DAPDPPCl][26] and [MnIIIBr12T3,5DMPPCl].29 Overall, third-generation brominated MnPs are more efficient than second-generation MnsP, so the synthesis of third-generation MnPs via a direct bromination of second-generation MnPs should constitute an attractive strategy to develop novel metalloporphyrin catalysts. In attempt to develop more efficient catalytic systems to oxidize cyclohexane, we investigated how additives such as imidazole and

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water affected the catalytic systems evaluated here. The additive should coordinate to the metal center in the metalloporphyrin catalyst, weakening the MnQO bond and boosting the reactivity of the high-valence active species MnV(O)P toward the substrate.14 Moreover, the added ligand should prevent metal ion displacement from the porphyrin macrocycle plane, reducing catalyst destruction.15,25 Addition of imidazole to the MnPor/PhIO (entry 3 in comparison to entry 1) system increased the total product yield (Cy-ol + Cy-one) from 23 to 40% and diminished the degree of catalyst destruction, indicating that imidazole made the high-valence species more reactive. Addition of imidazole to the MnBr12Por/PhIO system did not alter significantly the total product yield, selectivity and bleaching (entry 4 in comparison to entry 2). The saddle-shaped conformation of MnBr12Por probably made it difficult for the additive (Im) or the substrate to approach the metal ion.15 Addition of imidazole increased the total product yield for both the MnPor/PhI(OAc)2 from 19 to 45% (entry 9 in comparison to entry 7), and MnBr12Por/ PhI(OAc)2 from 35 to 66% (entry 10 in comparison to entry 8) systems. The fact that the MnBr12Por/PhIO and MnBr12Por/ PhI(OAc)2 systems behaved differently upon addition of imidazole corroborated the proposal of Collman et al., who suggested that different oxidants generate distinct high-valence active species.58 Besides that, addition of imidazole diminished the degree of catalyst destruction, showing that small quantities of this additive generated more efficient catalytic systems.15 Apart from imidazole, other additives such as water can be introduced into catalytic systems to enhance catalyst efficiency. As proposed in a theoretical study by Balcells et al. (2008),14 water as an axial ligand could act like imidazole by coordinating to the central metal ion and weakening the MQO bond. Furthermore, in systems that employ PhI(OAc)2 as the oxidant, water could hydrolyze PhI(OAc)2, to generate PhIO in situ.60–62 In agreement with the results of Silva et al.13 and Balcells et al.,14 addition of water (0.5 mL) to the MnPor/PhIO and MnBr12Por/PhIO


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systems (entries 5 and 6) enhanced the total product yield from 23 to 43% and from 44 to 53%, respectively and decreased the degree of catalyst destruction. Water must have coordinated to an axial position in the metalloporphyrin, weakening the MnQO bond and activating the high-valence species (MnV(O)P), as described by Balcells et al.14 Addition of water to the MnPor/PhI(OAc)2 and MnBr12Por/PhI(OAc)2 systems increased the total product yield from 19 to 45% and 35 to 42% and diminished bleaching (entries 11 and 12, respectively), evidencing that water may have partially hydrolyzed PhI(OAc)2, to generate PhIO in situ.60,62 In contrast, addition of 5 mL of H2O to the MnPor/PhIO and MnBr12Por/PhIO systems reduced Cy-ol yields by 10% and 20%, respectively, as compared to the systems without added water. These results corroborated the hypothesis that water acted as an axial ligand, but its excess can promote hexacoordination in the metalloporphyrin, leaving no positions available for reaction with the oxidant for consequent substrate oxidation. The values of the turnover number, TON (total moles of products per mole of catalyst), are small for all systems. This is because the oxidant was used in a small amount relative to the substrate. As an attempt to increase the TON values, using our best catalyst (MnBr12Por), two strategies were used: (1) we increased the oxidant: MnP : oxidant : cyclohexane molar ratio to 1 : 50 : 4650 (entries 47 and 48, Table S2, ESI†). It was observed that the yields and selectivity for alcohol were diminished (entries 2 and 8, Table 2). The smaller yields can be explained by the formation of MnIVP(oxo), which is little reactive63 or by increased oxidative destruction. However, there was an increase in the TON. This shows that increasing the amount of oxidant can generate better catalytic systems, but this increment leads to catalyst destruction. (2) We diminished the amount of cyclohexane and increased the amount of oxidant, leading to a MnP : oxidant : cyclohexane molar ratio of 1 : 50 : 50. In this system, it

was observed the inactivation of the catalyst since we did not observe the oxidation products (entries 45 and 46, Table S2, ESI†). Furthermore, the catalyst was completely destroyed during both strategies. The same behavior was noted in the attempt to identify active species (ESI,† Fig. S8 and S9) in the absence of substrate. This result can be justified by the prevalence of catalyst oxidative destruction. B.2. Adamantane oxidation. Groves et al. (1979)8 were the first to employ metalloporphyrins as catalysts for the oxidation of adamantane. They used [FeIIITPPCl], a first-generation metalloporphyrin, as the catalyst and observed the formation of 1-adamantanol without the production of ketone. Comparison between the MnPor (entry 13) and [FeIIITPPCl] catalytic results showed that MnPor provided slightly better product yields than [FeIIITPPCl].8 On the other hand, MnPor was less efficient than the second-generation catalyst [MnIIIPFTDCPP]Cl obtained by Doro et al. (2000) (entry 13 versus 35% 1-adol, 10% 2-adol and 5% 2-adone, respectively).45 Such a behavior might have been associated with the electron-withdrawing substituents (fluoro and/or chloro) in the meso-aryl positions of the macrocycle in [MnIIIPFTDCPP]Cl in comparison to MnPor, which made the high-valence active species more reactive. These results agreed with observations made by Rayati et al. when they compared the catalytic activities of two partially brominated MnPs, namely [MnIIIBr4TPPCl] and [MnIIIBr4T4(-OMe)PP]Cl: these authors found that electron-deficient MnPs were better catalysts than electron-rich MnPs.64 In comparison to the MnPor/PhIO (Table 3, entry 13) system, the MnPor/PhI(OAc)2 system (Table 3, entry 19) afforded higher product yields probably because the different oxidants generated distinct high-valence active species.58 On the other hand, the MnBr12Por/PhIO and MnBr12Por/PhI(OAc)2 systems gave practically the same efficiency (entries 14 and 20, respectively). Compared to

Table 3 Product yield obtained during the oxidation of adamantane by PhIO or PhI(OAc)2 catalyzed by MnPs in CH2Cl2 under aerobic conditions, with or without water or imidazole as an additive

Alcohol yield (%) Entry

Catalyst

System

1-Adola,d

2-Adola

TONe

Regiosel.b (%)

Bleac.c (%)

13 14 15 16 17 18 19 20 21 22 23 24 25 26

MnPor MnBr12Por MnPor MnBr12Por MnPor MnBr12Por MnPor MnBr12Por MnPor MnBr12Por MnPor MnBr12Por — —

PhIO PhIO PhIO/Im PhIO/Im PhIO/H2O PhIO/H2O PhI(OAc)2 PhI(OAc)2 PhI(OAc)2/Im PhI(OAc)2/Im PhI(OAc)2/H2O PhI(OAc)2/H2O PhIO PhI(OAc)2

17 39 19 39 22 41 22 38 25 44 28 43 0 0

2 4 2 3 2 4 2 4 1 2 2 3 0 0

1.9 4.3 2.1 4.2 2.4 4.5 2.4 4.2 2.6 4.6 3.0 4.7 — —

97 97 97 97 97 97 97 97 98 99 98 98 — —

52 40 57 39 61 42 52 48 57 33 61 39 — —

Reaction conditions: [MnP] = 5 104 mol L1, [PhIO or PhI(OAc)2] = 5 103 mol L1, MnP/oxidant/adamantane/CH2Cl2 molar ratio = 1 : 10 : 100 : 23 325, 25 1C, magnetic stirring, 90 min. of reaction. MnP/Im molar ratio in reactions 15, 16, 21, and 22 = 1 : 0.5. MnP/H2O molar ratio in reactions 17, 18, 23, and 24 = 1 : 139 (0.5 mL). a Yields based on the oxidant. 1-Adol: 1-adamantanol, 2-adol: 2-adamantanol. b Regioselectivity was determined from the relation 1-adol [=100 (%)1-adol/(%)1-adol + ((%)2-adol/3)]. c The degree of catalyst destruction was calculated from the UV-Vis absorption spectra recorded at the end of the reaction. d The maximum error is 3% with a confidence level of 95%. e TON = total moles of products per mole of catalyst.


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MnPor (entries 13 and 19), MnBr12Por performed better (entries 14 and 20), giving higher 1-adol and 2-adol yields and lower degree of catalyst destruction regardless of the oxidant. The superior efficiency of MnBr12Por can be associated with the presence of bromine atoms at the b-pyrrole positions of the macrocycle – these substituents withdrew electron density from the metal center and distorted the metalloporphyrin ring,65 increasing the reactivity of the high-valence active species (MnV(O)P) and diminishing the degree of catalyst destruction.9,10,13,26 Both MnPor and MnBr12Por exhibited over 95% regioselectivity for 1-adol because the tertiary carbon was more reactive than the secondary carbon.66 The ketone (2-adone) emerged in small quantities (B0.5%). Addition of imidazole to the MnPor/PhIO and MnBr12Por/ PhIO systems (entries 21 and 22, respectively) at a MnP/Im molar ratio of 1 : 0.5 did not improve the product (1-adol and 2-adol) yields or diminish the degree of catalyst destruction. Doro et al. (2000)45 obtained markedly superior yields of hydroxylated products (61% 1-adol and 14% 2-adol) when they used the third-generation metalloporphyrin [MnIICl8PFTDCPP] to catalyze the oxidation of adamantane by PhIO with addition of imidazole at a MnP/Im ratio of 1 : 30. The MnP/Im ratio used here (1 : 0.5) was much lower than that reported by Doro et al. and may not be sufficient to generate the pentacoordinated catalytic species that was necessary to improve the catalytic efficiency. The substituents present at the meso-aryl positions of the macrocycle may also have influenced the amount of additive required to form the pentacoordinated species.15 Moreover, [MnIICl8PFTDCPP] contains electron-withdrawing substituents (Cl and F) in the meso-aryl positions of the macrocycle, which greatly increased the reactivity of the high-valence active species as compared to the active species originating from the MnPs employed here. When PhI(OAc)2 was the oxidant, imidazole raised the product yields only slightly, and the degree of catalyst destruction decreased very little (entries 21 and 22). Addition of water during the oxidation of adamantane by PhIO or PhI(OAc)2 catalyzed by MnPor or MnBr12Por (entries 17, 23, 18, and 24, respectively) increased the product yields. The rise in product yields was more pronounced for systems involving PhI(OAc)2, which corroborated the hypothesis that water acts in two different ways: as an axial ligand in the MnPs, as proposed by Barcells et al.14 and Silva et al.,13,26 and in the hydrolysis of PhI(OAc)2, as described by In et al.60 and Kwong et al.61 B.3. n-Hexane oxidation. The high dissociation energy of the C–H bond in linear alkanes makes the functionalization of these species one of the most interesting and challenging chemical transformations. Many research groups have attempted to develop catalysts that can increase the reactivity of these chemical compounds. Therefore, we have evaluated the catalytic performance of MnPor and MnBr12Por in the oxidation of n-hexane, a linear alkane that gives 2-hexanol (2-ol), 3-hexanol (3-ol), 2-hexanone (2-one), and 3-hexanone (3-one), Table S1 (ESI†), as oxidation products. The product 1-hexanol (1-ol) did not emerge in any of the reactions probably because the primary carbon had a higher C–H bond dissociation energy as compared to secondary and tertiary carbons.66

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The MnBr12Por/PhIO system (Table S1, ESI†) was more efficient and more selective for the alcohol products (2-ol and 3-ol) than MnPor/PhIO (entries 28 and 27, respectively). As discussed previously,49,67–70 this resulted from the introduction of bromine atoms at the b-pyrrole positions of the MnBr12Por macrocycle. In addition, MnBr12Por had a lower degree of destruction than MnPor. Less catalyst destruction is associated with a decrease in oxidative processes, which preferentially occur with planar metalloporphyrins bearing few substituents at the meso-aryl positions of the macrocycle.71 Comparison between MnPor and MnBr12Por in terms of product distribution showed alcohol products amounting to 53–56% of the total product yield, regardless of the oxidant. These results contrasted with the findings of Silva et al.,26 who obtained B71% and B74% alcohol products for the secondand the third-generation catalysts cis-[MnIIIDAPDPP]Cl and cis[MnIIIBr12DAPDPP]Cl, respectively. The results achieved by Silva et al. could be attributed to the intermolecular coordination of the amino group of a metalloporphyrin molecule to the metal center of another metalloporphyrin molecule. The use of PhI(OAc)2 as the oxidant lowered the product yields (entries 29 and 30). This fact probably can be related to n-hexane being a more stable substrate in comparison to adamantane and cyclohexane. The degree of catalyst destruction increased in the presence of n-hexane and accounted for the decreased reaction products. It is worth noting that despite the strong nature of the oxidants used (PhIO or PhI(OAc)2), the formation of oxidation products was not detected for any of the studied substrates when the reactions were performed in the absence of the MnPs (Table S1 (ESI†), entries 33 and 34 for n-hexane; Table S2 (ESI†), entries 37 and 38 for cyclohexane; Table 3, entries 25 and 26 for adamantane). If we increase the amount of oxidant 5 times (oxidant : substrate = 50 : 4650), only small quantities of products are formed for cyclohexane oxidation (Table S2, entries 43 and 44, ESI†). In addition, the substitution of the MnPs by a manganese salt (Mn(OAc)24H2O) also does not lead to the oxidation of the substrates (Table S1, entries 35 and 36 for n-hexane; Table S2, entries 41 and 42 for cyclohexane; Table S3, entries 49 and 50 for adamantane, ESI†). Besides, the systems without oxidants (PhIO or PhI(OAc)2), which contain only the MnPs in the presence of molecular oxygen from the air, also did not promote the substrate oxidation (Table S1, entries 31 and 32 for n-hexane; Table S2, entries 39 and 40 for cyclohexane; Table S3, entries 51 and 52 for adamantane, ESI†). Therefore, the catalytic activity reported in all systems presented is due to the MnPs synthesized (MnPor and MnBr12Por) in the presence of the oxidants, in low molar ratios (1 : 10).

Conclusions The usual synthetic route to obtain brominated third-generation MnPs involves five steps.11,23 In this work, we obtain the brominated third-generation catalyst MnBr12Por in two steps, by a direct bromination of the second-generation catalyst MnPor.


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Hence, this methodology meets the requirements of green chemistry. The yields of products, which were based on oxidants, resulting from cyclohexane and adamantane oxidation catalyzed by MnPor and MnBr12Por are good, however for n-hexane the yields of products are lower than cyclohexane and adamantane, which attests to the high stability of this linear alkane. In the oxidation of adamantane catalyzed by both MnPs verify regioselective for 1-adol over 95%. The third-generation catalyst MnBr12Por presents a lower degree of destruction than the second-generation MnPor during the oxidation of cyclohexane, adamantane, and n-hexane by PhIO. When PhI(OAc)2 is the oxidant, the degree of destruction of MnBr12Por is higher as compared to MnPor, but MnBr12Por destruction is not complete, in contrast to the literature reports.9 Addition of imidazole increases the product yields obtained using MnPor regardless of the oxidant and using the MnBr12Por/PhI(OAc)2 system. Addition of water improves the product yields for most of the studied systems, regardless of the oxidant, which shows that water acts as an axial ligand and also hydrolyzes PhI(OAc)2, to generate PhIO in situ. Overall, MnBr12Por performs better than MnPor, leading to higher product yields, higher selectivity toward the main product, and a lower degree of catalyst destruction. Nevertheless, additives favor the activity of the second-generation catalyst to a larger extent as compared to the third-generation catalyst; the exception is the MnBr12Por/PhI(OAc)2 system in the oxidation of cyclohexane, which affords the highest product yields (66%) among the evaluated systems.

Acknowledgements Financial support from CNPq and FAPEMIG is gratefully acknowledged. We are grateful to Prof. Dr Paulo Jorge Sanches Barbeira for the use of the potentiostat Eco Chemie l-Autolab during cyclic voltammetry analysis. We are grateful to Prof. Guilherme Sippel Machado, UFPR – Campus Litoral for the elemental analysis.

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