Oxidative cleavage of alkenes catalyzed by a water/organic soluble
manganese porphyrin complex
Shiuh-Tzung Liu,
*
K. Venugopal Reddy and Rung-Yi Lai
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Received 11 August 2006; revised 12 December 2006; accepted 13 December 2006
Available online 8 January 2007
Abstract—Tetrakis(4-hydroxyphenyl)porphyrin [TPP–(OH)4] was modified with poly(ethylene glycol) chain as four side arms, such that this
compound is soluble in both organic and water solutions. Complexation of this porphyrin with manganese metal ions resulted in the formation of MnCl–TPP–(PEO750)4. This complex proved to be an excellent catalyst for the oxidative cleavage of C]C bonds, yielding the
correspond-ing carbonyl compounds with sodium periodate as an oxidant. Mechanistic pathway for this cleavage is discussed. Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
Metalloporphyrin species exist ubiquitously in nature and have found a broad spectrum of applications. Amongst, for example, the modeling of cytochrome P-450 for oxy-genation of hydrocarbons has received much attention over decades.1–24 As for the chemical reaction, great efforts
have been made to develop new metalloporphyrin-based cat-alytic systems for the oxidation process, aiming at a higher degree of chemo- and stereoselectivities as well as efficiency under mild conditions.1–5In this context, epoxidation and hydroxylation of alkenes catalyzed by metal porphyrin com-plexes have been a major focus.6–24
Recently, porphyrin–Fe and porphyrin–Mn complexes and their corresponding oxo species,15–17heterogeneously sup-ported manganese porphyrins18–20and poly(ethylene glycol)
(PEO) side-chain manganese porphyrins21have been widely used as catalysts for either epoxidation or hydroxylation of alkenes under ambient temperatures. However, to our knowl-edge, there is no precedence concerning the use of manga-nese porphyrin complexes in the catalytic cleavage of C]C bonds.22,23To obtain aldehydes from olefins that are
not fully substituted, two common methods are ozonization of olefins followed by a reductive workup and oxidative cleavage with osmium tetraoxide–sodium periodate, both of which, however, required stringent reaction conditions.22,23 For the safety concern, chemists have been developing alternative methods for the cleavage of C]C, especially on the basis of catalytic approach.23 Herein we report the
preparation of a water/organic soluble metal porphyrin com-plex MnCl–TPP–(PEO750)4, and its high-catalytic activity for the oxidative cleavage of alkenes, resulting in the corre-sponding carbonyl compounds.
2. Results and discussion
Preparation of MnCl–TPP–(PEO750)4is depicted inScheme 1. Incorporation of PEO side chain was achieved by direct alkylation of [TPP–(OH)4] with CH3(OCH2CH2)16OTs in 88% yield.1H NMR spectra of TPP–(PEO
750)4showed char-acteristic shifts at 8.82 (8H) for the pyrrole methine protons and a singlet at 2.79 ppm for the NH protons in the pyrrole unit. In addition, the integration ratio of ethylene protons for –OCH2CH2– versus pyrrole methine protons confirmed the tetraalkylation on the porphyrin moiety. Treatment of TPP–(PEO750)4with MnCl2under basic conditions afforded the desired complex in 93% yield. Characterization of this complex was performed via UV–vis and elemental analysis. The characteristics UV–vis spectrum of MnCl– TPP–(PEO750)4in CH2Cl2is similar to that of MnCl–TPP, proving the formation of manganese complex.21,25This
com-plex is soluble in water as well as in most organic solvents, and its UV–vis spectrum in water is essentially identical to that in dichloromethane.
To examine the catalytic activity, oxidation of alkenes with oxidants over the prepared metal complex was investigated (Table 1). The oxidation reaction of styrene, in the presence of MnCl–TPP–(PEO750)4as the catalyst, was first studied under air or H2O2. In this reaction, however, trace of benz-aldehyde was obtained as the product even with the addition of imidazole, which acted as an axial ligand for the metal Keywords: Oxidative cleavage; Porphyrin; Manganese; Olefin.
* Corresponding author. Tel.: +886 2 2366 0352; fax: +886 2 2363 6359; e-mail:[email protected]
0040–4020/$ - see front matterÓ 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2006.12.029
complex.26Alternatively, using sodium periodate as the
ox-idant in CH2Cl2/H2O (1:1 v/v), a similar catalytic reaction at room temperature revealed better conversion efficiency of turning styrene into a mixture of benzaldehdye and phenyl-acetaldehyde (seeTable 1, entry 1). It is believed that the production of phenylacetaldehyde is mainly via the epoxida-tion of styrene, followed by the isomerizaepoxida-tion.11
Further-more, as shown in entry 2, the selectivity of the reaction improved significantly by using acetonitrile/water (2:1 v/v) as solvent. For elevating temperatures (see entries 3 and 4), benzaldehyde became the exclusive product under simi-lar conditions.
In view of the above results, the manganese catalytic system with NaIO4and imidazole in acetonitrile/water rendered the best yield for oxidative cleavage of olefins and was followed in the following studies. Subsequently, various alkenes were tested under this catalytic system and the results are summa-rized in Table 1. Except for simple 1-alkenes, olefinic substrates underwent the oxidative cleavage to give the cor-responding carbonyl compounds in excellent isolated yields. For the phenyl-disubstituted alkene such as 1-diphenyl-ethene, benzophenone (ketones) was observed to be the major product (entry 6). Oxidative cleavage of cycloalkenes proceeded smoothly at room temperature to give the R = H, TPP–(OH)4 R = CH3, TPP–(OMe)4 NaOH THF OH O O 15 OTs O O 15 TsCl TPP–(PEO750)4 MnCl–TPP–(PEO750)4 N NH N HN OR RO OR OR MnCl2 PEO750OH
Scheme 1. Preparation of the manganese porphyrin complex.
Table 1. Catalytic oxidation of various alkenes catalyzed by MnCl–TPP–(PEO750)4a
Entry Substrate Cat.b NaIO4
b
Imidazoleb Tempc Product Yieldd(%)
1 Styrene (in CH2Cl2/water) 0.003 2.5 1.0 rt PhCHO, PhCH2CHO 44, 30
2 Styrene 0.003 1.5 1.0 rt PhCHO, styrene oxide 51, 11
3 Styrene 0.003 4.0 1.0 60 PhCHO 66 4 Styrene 0.003 4.0 1.0 80 PhCHO 97 5 m-ClC6H4CH]CH2 0.003 4.0 1.0 80 m-ClC6H4CHO 96 6 1,1-Diphenylethene 0.01 7.0 3.0 80 Benzophenone 99 7 1-Methylstyrene 0.003 4.0 1.0 rt Acetophenone 99 8 trans-Stilbene 0.01 7.0 3.0 80 PhCHO 87 9 2-Vinylpyridine 0.01 7.0 3.0 80 2-Vinylpyridineoxide, 2-Pyridinecarbaldehyde 70, 25 10 Cyclohexene 0.0013 3.0 0.5 rt 1,6-Hexanedial 97
11 Styrene oxide 0.0013 3.0 0.5 rt PhCHO 99
12 Cyclohexene oxide 0.0013 3.0 0.5 rt 1,6-Hexanedial 96
13 Cyclopentene 0.0013 3.0 0.5 rt 1,5-Pentanedial 99
14 Cyclooctene 0.01 4.0 0.5 rt 1,8-Octanedial 65
15 Benzyl alcohol 0.03 7.0 3.0 80 PhCHO 60
16 Diphenylacetylene 0.01 7.0 3.0 80 — — 17 1-Phenylcyclohexene 0.01 4.0 0.5 rt 6-Phenyl-6-oxohexanal 77 18 Tartaric acid 0.01 4.0 1.0 rt — — 19 Norbornylene 0.01 7.0 1.0 rt Cyclopentane-1,3-dicarbaldehyde 33 20 ()-Limonene 0.01 3.0 1.0 rt (+)-4-Methyl-3-(3-oxobutyl)-4-pentenal 89 21 a-Pinene 0.01 4.0 1.0 rt (3-Acetyl-2,2-dimethylcyclobutyl)-acetaldehyde 44 22e Styrene 0.003 4.0 1.0 80 PhCHO 97 a
Reaction conditions: substrate (1 mmol) in CH3CN (2 ml)/H2O (1 ml) for 24 h. b In mmol.
c
Oil bath temperature inC.
d
Isolated yield, except entries 1 and 2 by NMR integration.
corresponding di-aldehyde in excellent yields (entries 10, 13, and 14). Interestingly, a mixture of 2-pyridine epoxide (70%) and 2-pyridinecarbaldehdye (25%) was obtained from 2-vinylpyridine under similar reaction conditions. It was also found that simple a-olefins or terminal alkenes such as 1-octene, 1-hexene, carvone, b-pinene, and 2-methyl-hexene did not undergo the oxygenation, as supported by the starting material that is quantitatively recovered. In addi-tion, the oxidation did not take place on alkyne or a,b-unsat-urated carbonyl compounds, the results of which are in sharp difference from the traditional methods. We thus tentatively concluded that for the MnCl–TPP–(PEO750)4catalytic reac-tion, the internal double bond of limonene was cleaved to yield the corresponding carbonyl functions, while the termi-nal one remained intact (entry 20), demonstrating its remark-able selectivity.
Oxidation of styrene oxide and cyclohexene oxide with a low loading of catalyst (0.13%) rendered benzaldehyde and 1,6-hexanedial as the desired products, respectively (entries 11 and 12) at room temperature. These results provided a support to illustrate this oxygenation involving the epoxide intermediate, which was subsequently hydrolyzed to yield the 1,2-diol and then C–C cleavage (Scheme 2). In addition, it was found that the cleavage of epoxide was much faster than that of olefin, suggesting that the rate limiting step of this oxidative cleavage is the formation of epoxide.
R R HO OH O R IO4 -R R R R O [O] r.d.s
Scheme 2. Pathway for oxidative cleavage.
Figure 1shows the difference of MnCl–TPP–(PEO750)4and MnCl–TPP–(OMe)4as catalysts for the oxidative cleavage of styrene. It is noticed that the formation of styrene oxide
is not observed during this study, presumably its highly activ-ity toward cleavage. As illustrated, styrene was completely converted into benzaldehyde with the use of MnCl–TPP– (PEO750)4as the catalyst within 24 h, but the referenced cat-alyst MnCl–TPP–(OMe)4only reached up to 55% under the similar reaction conditions. It appears that the activity of the polyether attached catalyst is much better than the un-modi-fied one, indicating the neediness of polyether linkage on this catalyst for good activity.
To investigate the active oxidizing species in this catalysis, we performed a reaction of MnCl–TPP–(PEO750)4 with equal molar amount of methylimidazole and NaIO4in water. The UV–vis spectrum of the resulting solution showed the initial formation of a periodate–Mn species (lmax¼476 nm), and then the conversion into a high valent Mn–oxo species (Soret lmax¼405 nm),26but this species was slowly dimin-ished. By the addition of styrene to this solution, the oxida-tion product was formed. This observaoxida-tion suggests that the oxidation process might involve either a periodate–Mn com-plex or a Mn–oxo species.
The re-use of catalyst is also possible for this catalytic system. After the completion of the reaction, the organic products were removed from the water layer. The aqueous portion with the catalyst is ready for the further re-use. In order to investigate the stability and duration of active cata-lyst, the leaching of metal into the product mixture was determined after three reaction cycles. From analysis, no detectable leaching of manganese could be found, suggesting that the catalytic species remains in aqueous solution. The re-sults of the oxidation of styrene into benzaldehyde over the catalyst with different reaction cycles are listed inTable 2. It is noticed that the activity of catalyst slowly decreases along with the repeated cycles, presumably due to the accu-mulation of salt in aqueous phase. Under the similar condi-tions, the replacement of catalyst MnCl–TPP–(PEO750)4 with MnCl–TPP–(OMe)4 provided less than 60% of the cleavage product, and the catalyst was mixed with organic products, which was not easily separated, i.e. the re-use of MnCl–TPP–(OMe)4as a catalyst is not practical.
In summary, modification of the porphyrin moiety with four poly(ethylene glycol) chains allows TPP–(PEO750)4 to be soluble in both organic and water solutions. Using this unique property, we have developed the manganese-based catalytic oxidation methods for the cleavage of a wide range of olefins to produce aldehydes. As this catalyst is easy to handle and is much less toxic than OsO4, this cleavage method is green-chemistry oriented and is expected to be useful in a variety of organic syntheses. Works focusing on detailed mechanistic pathway and the extension of this sys-tem to other oxygenation reactions are currently in progress.
4 8 12 16 20 24 h 20 40 60 80 100 %
Figure 1. Plot of yields (benzaldehyde) versus time for the oxidative cleav-age of styrene by MnCl–TPP–(PEO750)4(-) and MnCl–TPP–(OMe)4(C).
Reaction conditions: styrene (1 mmol), NaIO4 (4 mmol), and imidazol
(1 mmol) in CH3CN (2 ml)/H2O (1 ml) at 80C.
Table 2. Oxidation of styrene by the re-used catalysta
Run 1 2 3 4
Yieldb(%) 97 85 83 75
a
Reaction conditions: catalyst (0.003 mmol) and imidazole (1 mmol) in CH3CN (2 ml)/H2O (1 ml) for 24 h. For each run, styrene (1 mmol),
NaIO4(4 mmol), and CH3CN (2 ml) were added. b
3. Experimental 3.1. General
Nuclear magnetic resonance spectra were recorded in CDCl3 or acetone-d6on either a Bruker AM-300 or AVANCE 400 spectrometer. Chemical shifts are given in parts per million relative to Me4S for 1H NMR. Infrared spectra were mea-sured on a Nicolet Magna-IR 550 spectrometer (Series-II) as KBr pellets, unless otherwise noted. Chemicals and sol-vents were of analytical grade and used as received unless otherwise stated. TPP–(OMe)4was prepared according to the reported methods.27
3.1.1. Preparation of 5,10,15,20-tetrakis(4-hydroxy-phenyl)porphyrin [TPP–(OH)4]. The porphyrin TPP– (OMe)4(1 g, 1.3 mmol) was dissolved in 30 mL of dichloro-methane with stirring. To this, borontribromide (8.1 g, 32.6 mmol) in dichloromethane (5 mL) was added slowly. The resulting mixture was stirred for 12 h at room tempera-ture and then was quenched with 5 mL of methanol and neu-tralized with ammonia solution till the solution turned from green to dark red color. The mixture was evaporated till dry-ness, extracted with ethyl acetate and the organic layer was washed several times with water. It was separated, dried over anhydrous magnesium sulfate, filtered, and concentrated. The residue was chromatographed on silica gel with ace-tone/light petroleum (1:1) to yield pure compound [TPP– (OH)4] (0.8 g, 87% yield).27 IR (neat) 3330.4, 1613.3, 1513.8, 1248.6, 1182.3, 804.4 cm1.1H NMR (400 MHz, acetone-d6) d 8.91 (s, 8H), 8.86 (s, 4H), 8.05 (d, J¼8.4 Hz, 8H), 7.29 (d, J¼6.5 Hz, 8H), 2.70 (s, 2H).
3.1.2. Preparation of CH3(OCH2CH2)16OTs. Sodium hydroxide (3.65 g, 91.3 mmol) and poly(ethylene glycol) methylether CH3(OCH2CH2)16OH (45.70 g, 60.9 mmol) were dissolved in a mixture of THF (140 mL) and water (20 mL). A solution of p-toluenesulfonyl chloride (12.76 g, 67.0 mmol) in 20 mL of THF was added slowly to the above solution at ice-cooled temperature. The solution was stirred at 0C for 3 h and then at room temperature for 12 h and then poured into 50 mL of ice water. The mixture was extracted several times with methylene chloride. The com-bined organic layers were washed with dilute HCl followed by brine and then dried over magnesium sulfate. After filtra-tion, the solvent was removed under reduced pressure to give the desired tosylate (41 g, 75% yield).1H NMR (400 MHz, CDCl3) d 7.77 (d, J¼7.6 Hz, 2H), 7.32 (d, J¼7.6 Hz, 2H), 4.14 (t, J¼4.8 Hz, 2H), 3.67 (t, J¼4.0 Hz, 2H), 3.65–3.56 (m, 60H), 3.54–3.51 (m, 2H), 3.35 (s, 3H), 2.43 (s, 3H). These data are similar to those reported.28
3.1.3. Preparation of TPP–(PEO750)4. A mixture of [TPP–(OH)4] (1 g, 1.47 mmol) and CH3(OCH2CH2)16OTs (5.32 g, 5.89 mmol) [Ts¼p-CH3C6H4SO2–] was dissolved in 50 mL of dimethylformamide. To this, potassium carbon-ate (0.81 g, 5.9 mmol) was added and the solution was stirred at 80C for 16 h. After cooling to room temperature, the solution was poured into water and extracted thrice with methylene chloride. The combined organic layers were washed with water and then with brine, dried over magne-sium sulfate, filtered, and the solvent was removed at reduced pressure. Chromatography (silica, ether/acetone,
1:3) was employed to furnish the pure product (4.80 g, 88% yield). UV–vis (3, CH2Cl2): lmax 421 (3.6105), 517 (2.3104), 555 (1.6104), 596 (9.2103), 649 (1.0104). 1H NMR (400 MHz, CDCl 3) d 8.82 (s, 8H), 8.08 (d, J¼8.8 Hz, 8H), 7.27 (d, J¼8.8 Hz, 8H), 4.41 (t, J¼5.2 Hz, 8H), 4.04 (t, J¼4.8 Hz, 8H), 3.87–3.61 (m, 232H), 3.54– 3.51 (m, 8H), 3.34 (s, 12H), 2.79 (s, 2H). Anal. Calcd for C176H294N4O68: C, 59.48; H, 8.34; N, 1.58. Found: C, 59.00; H, 8.09; N, 2.02.
3.1.4. Preparation of MnCl–TPP–(PEO750)4.The porphy-rin (1 g, 0.28 mmol) was dissolved in 50 mL of refluxing DMF for several minutes and MnCl2$4H2O (0.16 g, 0.84 mmol) was added to the solution. The solution was al-lowed to reflux for 30 min. The reaction was monitored by TLC. DMF was distilled off and the resulting syrupy oil was chromatographed on silica gel with CH2Cl2–MeOH (10:1) as an eluent to give the desired complex as green solid (0.93 g, 93% yield). UV–vis (3, CH2Cl2): lmax 380 (8.7104), 407 (8.2104), 481 (1.7104), 533 (1.2104), 587 (1.7104), 627 (2.5104). Anal. Calcd for C176H292ClMnN4O68$2H2O: C, 57.46; H, 8.11; N, 1.52. Found: C, 57.86; H, 8.04; N, 1.76.
3.2. Catalysis—general procedure
A mixture of MnCl–TPP–(PEO750)4, NaIO4, imidazole, and the substrate was placed in a round bottomed flask. Solvents were added to this mixture. The reaction was heated with stirring at 80–100C for a certain period of hours under aerobic conditions. The progress of the reaction was moni-tored by1H NMR spectrometer. For various substrates, the reaction mixture was extracted with ether, which was then filtered through a short column of silica gel. The filtrate was evaporated under reduced pressure or distilled off to give the desired product. Oxidative cleavage products obtained in this work were characterized by spectral methods particularly with1H NMR and the data were consistent with those reported. 3.2.1. p-Chlorobenzaldehdye.29 1H NMR (400 MHz, CDCl3) d 9.96 (s, 1H), 7.80 (d, J¼8.0 Hz, 2H), 7.49 (d, J¼8.1 Hz, 2H). 3.2.2. Benzophenone.30 1H NMR (400 MHz, CDCl 3) d 7.79–7.76 (m, 4H), 7.56 (t, J¼7.0 Hz, 2H), 7.45 (tt, J¼7.0, 1.4 Hz, 4H). 3.2.3. Acetophenone.301H NMR (400 MHz, CDCl3) d 7.92 (d, J¼7.6 Hz, 2H), 7.50 (t, J¼7.6 Hz, 2H), 7.42 (dd, J¼7.6, 8.0 Hz, 2H), 2.55 (s, 3H). 3.2.4. 2-Vinylpyridineoxide.31 1H NMR (400 MHz, CDCl3) d 8.52 (d, J¼4.8 Hz, 1H), 7.51–7.48 (m, 1H), 7.22–7.19 (m, 2H), 3.97 (t, J¼4.0 Hz, 1H), 3.14 (dd, J¼10.3 Hz, 1H), 2.90 (dd, J¼8.2 Hz, 1H). 3.2.5. 2-Pyridinecarbaldehyde.31 1H NMR (400 MHz, CDCl3) d 10.05 (s, 1H), 8.80 (d, J¼4.8 Hz, 1H), 7.90 (d, J¼5.2 Hz, 1H), 7.82 (t, J¼3.6 Hz, 1H), 7.51–7.48 (m, 1H). 3.2.6. 1,6-Hexanedial.321H NMR (400 MHz, CDCl 3) d 9.74 (t, J¼3.4 Hz, 2H), 2.45–2.43 (m, 4H), 1.64–1.61 (m, 4H).
3.2.7. 1,5-Pentanedial.32 1H NMR (400 MHz, CDCl3) d 9.75 (t, J¼2.4 Hz, 2H), 2.50 (m, 4H), 1.96–1.91 (m, 2H). 3.2.8. 1,8-Octanedial.331H NMR (400 MHz, CDCl 3) d 9.75 (t, J¼3.4 Hz, 2H), 2.41–2.38 (m, 4H), 1.59–1.55 (m, 4H), 1.34–1.30 (m, 4H). 3.2.9. 6-Phenyl-6-oxohexanal.34 1H NMR (400 MHz, CDCl3) d 9.74 (t, J¼1.6 Hz, 1H), 7.93–7.91 (m, 2H), 7.52 (t, J¼7.6 Hz, 1H), 7.44–7.43 (m, 2H), 2.96 (t, J¼6.8 Hz, 2H), 2.48–2.45 (m, 2H), 1.75–1.70 (m, 4H). 3.2.10. Cyclopentane-1,3-dicarbaldehyde.35 1H NMR (400 MHz, CDCl3) d 9.63 (d, J¼2.0 Hz, 2H), 2.83–2.74 (m, 2H), 2.30–2.14 (m, 2H), 1.98–1.85 (m, 2H), 1.81–1.74 (m, 2H). 3.2.11. (±)-4-Methyl-3-(3-oxobutyl)-4-pentenal.36 1H NMR (400 MHz, CDCl3) d 9.64 (t, J¼4.5 Hz, 1H), 4.82– 4.74 (m, 2H), 2.65–2.60 (m, 1H), 2.56–2.30 (m, 4H), 2.10 (s, 3H), 1.68–1.60 (m, 5H). 3.2.12. (3-Acetyl-2,2-dimethylcyclobutyl)acetaldehyde.37 1H NMR (400 MHz, CDCl 3) d 9.70 (t, J¼2.8 Hz, 1H), 2.90–2.87 (m, 1H), 2.44–2.40 (m, 3H), 2.03 (s, 3H), 1.95– 1.93 (m, 2H), 1.36 (s, 3H), 0.87 (s, 3H). Acknowledgements
This work was partially supported by the National Science Council, Taiwan, ROC for the financial support (NSC94-2113-M002-035).
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