Published on Web 07/19/2008
Efficient Catalytic Oxidation of Alkanes by Lewis Acid/ [OsVI(N)Cl4]- Using Peroxides as Terminal Oxidants. Evidence for a Metal-Based Active Intermediate Shek-Man Yiu, Wai-Lun Man, and Tai-Chu Lau* Department of Biology and Chemistry, City UniVersity of Hong Kong, Tat Chee AVenue, Kowloon Tong, Hong Kong, China Received April 10, 2008; E-mail:
[email protected] Abstract: The oxidation of alkanes by various peroxides (tBuOOH, H2O2, PhCH2C(CH3)2OOH) is efficiently catalyzed by [OsVI(N)Cl4]-/Lewis acid (FeCl3 or Sc(OTf)3) in CH2Cl2/CH3CO2H to give alcohols and ketones. Oxidations occur rapidly at ambient conditions, and excellent yields and turnover numbers of over 7500 and 1000 can be achieved in the oxidation of cyclohexane with tBuOOH and H2O2, respectively. In particular, this catalytic system can utilize PhCH2C(CH3)2OOH (MPPH) efficiently as the terminal oxidant; good yields of cyclohexanol and cyclohexanone (>70%) and MPPOH (>90%) are obtained in the oxidation of cyclohexane. This suggests that the mechanism does not involve alkoxy radicals derived from homolytic cleavage of MPPH but is consistent with heterolytic cleavage of MPPH to produce a metal-based active intermediate. The following evidence also shows that no free alkyl radicals are produced in the catalytic oxidation of alkanes: (1) The product yields and distributions are only slightly affected by the presence of O2. (2) Addition of BrCCl3 does not affect the yields of cyclohexanol and cyclohexanone in the oxidation of cyclohexane. (3) A complete retention of stereochemistry occurs in the hydroxylation of cis- and trans1,2-dimethylcyclohexane. The proposed mechanism involves initial O-atom transfer from ROOH to [OsVI(N)Cl4]-/Lewis acid to generate [OsVIII(N)(O)Cl4]-/Lewis acid, which then oxidizes alkanes via H-atom abstraction.
Introduction
The selective oxidation of alkanes under mild conditions continues to be a challenge for chemists.1,2 A number of enzymes, such as cytochrome P-450 and methane monooxygenase, are able to make use of highly reactive iron-oxo species to oxidize alkanes,3–5 and this has inspired the development of synthetic models that can mimic the reactivity of these enzymes.4,6–12 Our approach to design catalytic systems for alkane oxidation is to make use of Lewis acids to activate metal-oxo species that are otherwise stable and rather unre(1) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer: Boston, 2000. (2) Goldberg, K. I., Goldman, A. S., Eds. ActiVation and Functionalization of C-H Bonds; ACS Symposium Series 885; American Chemical Society: Washington, DC, 2004. (3) Ortiz de Montellano, P. R., Ed. Cytochrome P450. Structure, Mechanism and Biochemistry; Plenum Press: New York, 1995. (4) McLain, J. L.; Lee, J.; Groves, J. T. In Biomimetic Oxidations Catalyzed by Transition Metal Complexes; Meunier, B., Ed.; Imperial College Press: London, 2000; pp 91-170. (5) Baik, M.-H.; Newcomb, M.; Friesner, R. A.; Lippard, S. J. Chem. ReV. 2003, 103, 2385–2420. (6) (a) Groves, J. T.; Bonchio, M.; Carofiglio, T.; Shalyaev, K. J. Am. Chem. Soc. 1996, 118, 8961–8962. (b) Groves, J. T.; Shalyaev, K.; Lee, J. Porphyrin Handb. 2000, 4, 17–40. (c) Wang, C.; Shalyaev, K. V.; Bonchio, M.; Carofiglio, T.; Groves, J. T. Inorg. Chem. 2006, 45, 4769–4782. (7) (a) Meunier, B. Chem. ReV. 1992, 92, 1411–1456. (b) Meunier, B.; Bernadou, J. In Metal-oxo and Metal-Peroxo Species in Catalytic Oxidations; Meunier, B., Ed.; Springer: Berlin, 2000. (8) Dolphin, D.; Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997, 30, 251–259. 10.1021/ja802625e CCC: $40.75 2008 American Chemical Society
active toward alkanes. We have reported that the oxidation of alkanes by anionic oxo species of ruthenium,13 iron,14 chromium,15 and manganese15,16 is greatly enhanced by just a few equivalents of Lewis acids. In particular, we have recently studied the activation of [Os(N)(O)3]- by FeCl3 in detail.17 Although [Os(N)(O)3]- is a very stable osmium-oxo species, (9) (a) Nam, W.; Goh, Y. M.; Lee, Y. J.; Lim, M. H.; Kim, C. Inorg. Chem. 1999, 38, 3238–3240. (b) Nam, W.; Lee, H. J.; Oh, S.-Y.; Kim, C.; Jang, H. G. J. Inorg. Biochem. 2000, 80, 219–225. (c) Nam, W.; Lim, M. H.; Oh, S.-Y.; Lee, J. H.; Lee, H. J.; Woo, S. K.; Kim, C.; Shin, W. Angew. Chem., Int. Ed. 2000, 39, 3646–3649. (d) Nam, W.; Oh, S.-Y.; Sun, Y. J.; Kim, J.; Kim, W.-K.; Woo, S. K.; Shin, W. J. Org. Chem. 2003, 68, 7903–7906. (e) Nam, W. Acc. Chem. Res. 2007, 40, 522–531. (10) (a) Costas, M.; Chen, K.; Que, L., Jr. Coord. Chem. ReV. 2000, 200202, 517–544. (b) Chen, K.; Que, L., Jr. J. Am. Chem. Soc. 2001, 123, 6327–6337. (c) Company, A.; Gomez, L.; Gueell, M.; Ribas, X.; Luis, J. M.; Que, L., Jr.; Costas, M. J. Am. Chem. Soc. 2007, 129, 15766–15767. (d) Que, L., Jr Acc. Chem. Res. 2007, 40, 493–500. (11) Britovsek, G. J. P.; England, J.; White, A. J. P. Inorg. Chem. 2005, 44, 8125–8134. (12) Mekmouche, Y.; Me´nage, S.; Toia-Duboc, C.; Fontecave, M.; Galey, J.-B.; Lebrun, C.; Pe´caut, J. Angew. Chem., Int. Ed. 2001, 40, 949– 952. (13) Lau, T. C.; Mak, C. K. J. Chem. Soc., Chem. Commun. 1993, 766– 767. (14) Ho, C. M.; Lau, T. C. New J. Chem. 2000, 24, 587–590. (15) Lau, T. C.; Wu, Z. B.; Bai, Z. L.; Mak, C. K. J. Chem. Soc., Dalton Trans. 1995, 695–696. (16) Lam, W. W. Y.; Yiu, S. M.; Lee, J. M. N.; Yau, S. K. Y.; Kwong, H. K.; Lau, T. C.; Liu, D.; Lin, Z. J. Am. Chem. Soc. 2006, 128, 2851–2858. (17) Yiu, S. M.; Wu, Z. B.; Mak, C. K.; Lau, T. C. J. Am. Chem. Soc. 2004, 126, 14921–14929. J. AM. CHEM. SOC. 2008, 130, 10821–10827
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Scheme 1
in the presence of FeCl3 it is able to oxidize alkanes efficiently at room temperature in CH2Cl2/CH3COOH to give alcohols and carbonyl products. The active intermediate is proposed to be a bimetallic [(O)3OsVIIItN-FeIII] species. This oxidation can be made catalytic by using 2,6-dichloropyridine N-oxide (Cl2pyO) as the terminal oxidant.17 However, 18O-labeling studies suggest that the active oxidant in catalytic oxidation is different from that in stoichiometric oxidation, and [Cl4(O)OsVIIItN-FeIII] is proposed to be the active intermediate (scheme 1). Although yields of >80% are obtained in catalytic oxidation, the rates are rather slow (ca. 4 turnovers/h at 23 °C). Kinetic studies suggest that the rate-limiting step is oxygen-atom transfer from Cl2pyO to [Os(N)Cl4]-. Herein we report that the MIII/ [Os(N)Cl4]- (MIII ) FeCl3 or Sc(OTf)3) system is a remarkably efficient catalyst for the oxidation of alkanes using peroxides (including tBuOOH, H2O2, and PhCH2CMe2OOH) as terminal oxidants, with yields >90% and rates >15 turnovers/min. We also provide evidence that these catalytic reactions do not involve free radicals and that the active intermediate is the same as in oxidation by Cl2pyO, i.e., [Cl4(O)OsVIIItNsMIII]. Experimental Section Materials. All chemicals were of reagent grade unless otherwise specified. [nBu4N][Os(N)(O)3] and [nBu4N][Os(N)Cl4] were synthesized according to published procedures.18,19 The purity of these complexes was determined by CHN analysis and UV/vis spectrophotometry. Anhydrous ferric chloride (98%) and cis- and trans1,2-dimethylcyclohexane were purchased from Aldrich and used as received. Other alkanes and solvents were purified according to standard methods.20 Hydrogen peroxide (30%, Merck) and tertbutyl hydroperoxide (80%, in di-tert-butyl peroxide, Merck) were used as received and standardized by iodometry.21,22 2-Methyl-1phenyl-2-propyl hydroperoxide (MPPH) was prepared according to a literature method,23 and its purity was determined to be >99% by NMR. Active MPPH levels were further measured by iodometric titrations, which showed >99% peroxide activity. H218O (95 atom % 18O) and H218O2 (90 atom % 18O) were obtained from Aldrich and Icon Services Inc., respectively. Bromotrichloromethane was purchased from Aldrich and was purified by passing through a short column of neutral alumina prior to use. Instrumentation. Gas chromatographic analyses were performed on a HP 6890 GC/FID equipped with either a HP-5MS (30 m × 0.25 mm i.d.) or a HP-FFAP (25 m × 0.2 mm i.d.) column. GC/ MS measurements were carried out on a HP 6890 gas chromatograph interfaced to a HP 5975 mass-selective detector. Electrospray (18) Weber, R.; Dehnicke, K.; Schweda, E.; Stra¨hle, J. Z. Anorg. Allg. Chem. 1982, 490, 159–170. (19) Griffith, W. P.; Pawson, D. J. J. Chem. Soc., Dalton Trans. 1973, 1315–1320. (20) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; Pergamon: New York, 1988. (21) Jeffrey, G. H.; Bassett, J.; Mendham, J.; Denny, R. C. Vogel’s Textbook of QuantitatiVe Chemical Analysis, 5th ed.; Longman: Essex, 1989. (22) Traylor, T. G.; Ciccone, J. P. J. Am. Chem. Soc. 1989, 111, 8413– 8420. (23) Arends, I. W. C. E.; Ingold, K. U.; Wayner, D. D. M. J. Am. Chem. Soc. 1995, 117, 4710–4711. 10822
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ionization mass spectrometry (ESI/MS) was performed on a PE SCIEX API 365 mass spectrometer. 1H NMR spectra were recorded on a Varian (300 MHz) FT NMR spectrometer. Elemental analyses were done on an Elementar Vario EL analyzer. UV/vis spectra were recorded by using a Hewlett-Packard 8452A diode array spectrophotometer. Infrared spectra were recorded with sample as KBr pellets on a Nicolet Avatar 360 FTIR spectrophotometer. Catalytic Oxidation of Alkanes. All experiments were carried out under argon, unless otherwise specified. Strict exclusion of oxygenintheoxidationreactionswasachievedbyfourfreeze-pump-thaw cycles. In a typical catalytic oxidation reaction, a solution of peroxide (0.10 M) was added to a solution containing [nBu4N][Os(N)Cl4] (1.25 × 10-3 M), FeCl3 (1.0 × 10-2 M), and alkane (1.2 M) in CH2Cl2/CH3CO2H (5:2, v/v). The resulting red solution was vigorously stirred for 5 min. Chlorobenzene was then added as an internal standard, and the mixture was analyzed by GC/FID and GC/MS. Kinetic isotope effects for catalytic oxidation of cyclohexane were investigated by using an equimolar mixture of cyclohexane and cyclohexane-d12 as substrate. The organic products were quantified and identified by GC and GC/MS. The KIE values were obtained by taking the ratio of the corresponding areas from GC measurements. Kinetic isotope effects were also determined by comparing the rates of single-substrate reactions. For 18O-labeling experiments, the 16O and 18O compositions of cyclohexanol were determined from the relative abundances of the peaks at m/z ) 100 (16O) and 102 (18O) in GC/MS (EI). Analysis of the Iron Species after Reaction. The solution after catalytic oxidation was evaporated to dryness at room temperature. The residue was dissolved in 0.25 M HCl and filtered. Excess potassium thiocyanate was then added, and the resulting intensely red-colored iron(III) thiocyanate solution was analyzed by UV/vis spectrophotometry at 480 nm.21 It was found that iron(III) thiocyanate was formed quantitatively, indicating that all the iron species remained in the 3+ oxidation state. Detection of Peracetic Acid. To investigate whether FeCl3 or [nBu4N][Os(N)Cl4] could catalyze the formation of peracetic acid from hydrogen peroxide and acetic acid, FeCl3 (0.01 M) or [nBu4N][Os(N)Cl4] (1.25 × 10-3 M) was added to a solution of H2O2 (0.17 M) in CH2Cl2/CH3CO2H (5:2, v/v). After 10 min, the CH2Cl2 was removed by rotary evaporation, and 50 µL of the residue was dissolved in D2O (0.5 mL) and then analyzed by 13C NMR. No peracetic acid was found in the FeCl3 or the [nBu4N][Os(N)Cl4] solution. Results and Discussion Catalytic Oxidation with tBuOOH. In a typical catalytic oxidation experiment, tBuOOH (0.10 M) was added to a solution containing [nBu4N][Os(N)Cl4] (1.25 × 10-3 M), FeCl3 (1.0 × 10-2 M), and cyclohexane (1.2 M) in CH2Cl2/CH3CO2H (5:2, v/v) at 23 °C under argon. After 5 min, analysis by GC and GC/MS indicated the formation of cyclohexanol (67%), cyclohexanone (21%), and chlorocyclohexane (3%) (Table 1, entry 5), with a total yield of 91% (turnover number, TON ) 73). No cyclohexyl tert-butyl peroxide was detected. A similar yield and product distribution are observed in cyclohexane oxidation by [Os(N)(O)3]-/FeCl3/Cl2pyO, but the reaction time is much longer (16 h at 23 °C).17 This catalytic system requires the presence of both [Os(N)Cl4]- and FeCl3 to function efficiently. The background catalytic oxidation by [Os(N)Cl4]- or FeCl3 alone was negligible under the same conditions, i.e., 5 min at 23 °C (Table 1, entries 1 and 2), although on a longer time scale, products of cyclohexane oxidation could be observed. If FeCl3 was used alone, 11% of cyclohexane-derived products could be detected after 24 h. On the other hand, if [Os(N)Cl4]- was used alone, 48% of products were observed after 8 h.
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Table 1. Catalytic Oxidation of Cyclohexane by [nBu4N][Os(N)Cl4]/ FeCl3/tBuOOH under Various Conditionsa product (% yield)b entry
total FeCl3 CH3CO2H CH2Cl2 c-C6H11OH c-C6H10O c-C6H11Cl yield (mL) (equiv) (mL) (%)
time
