ORGANIC LETTERS
Selective Organocatalytic Oxygenation of Hydrocarbons by Dioxygen Using Anthraquinones and N-Hydroxyphthalimide
2005 Vol. 7, No. 2 263-266
Guanyu Yang,†,‡ Qiaohong Zhang,† Hong Miao,† Xinli Tong,† and Jie Xu*,† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, 116023, P. R. China, and Department of Chemistry, Zhengzhou UniVersity, 75 Daxue Road, Zhengzhou, 450052, P. R. China
[email protected] Received November 3, 2004
ABSTRACT
Purely organic and catalytic systems of anthraquinones and N-hydroxyphthalimide efficiently promote oxygenation of hydrocarbons with dioxygen under mild conditions, e.g., fluorene can be converted completely to fluorenone with 85% yield at 80 °C.
Selective catalytic oxygenation of hydrocarbons with dioxygen as the terminal oxidant is a fundamental industrial technology; its economical and environmentally benign nature attracts much attention.1 The usual oxidation catalysts contain transition metals. Recently, metal-free organic molecule catalysts have been widely used because of their stability and nontoxicity compared to traditional metallic ones.2 Metal-free organocatalytic oxygenations of C-H bonds with dioxygen have also been reported;3 however, these processes often need light irradiation or occur under higher temperatures. N-Hydroxyphthalimide (NHPI) in com†
Dalian Institute of Chemical Physics. Zhengzhou University. (1) (a) Jacobson, E. N. AdV. Synth. Catal. 2004, 346, 109. (b) Schreiner, P. R.; Fokin, A. A. Chem. Rec. 2004, 3, 247. (c) Schuchardt, U.; Cardoso, D.; Sercheli, R.; Pereira, R.; da Cruz, R. S.; Guerreiro, M. C.; Mandelli, D.; Spinace´, E. V.; Pires, E. L. Appl. Catal., A 2001, 211, 1. (2) (a) Shi, Y. Acc. Chem. Res. 2004, 37, 488. (b) Yang, D. Acc. Chem. Res. 2004, 37, 497. (c) Brown, S. P.; Brochu, M. P.; Sinz, C. J.; MacMillan, D. W. C. J. Am. Chem. Soc. 2003, 125, 10808. (d) Kumar, V. S.; Aubele, D. L.; Floreancig, P. E. Org. Lett. 2001, 3, 4123. (e) Bolm, C.; Magnus, A. S.; Hildebrand, J. P. Org. Lett. 2000, 2, 1173. ‡
10.1021/ol047749p CCC: $30.25 Published on Web 12/17/2004
© 2005 American Chemical Society
bination with some cocatalysts has been shown to be an organocatalyst for efficient oxygenation of hydrocarbons with dioxygen4 in recent research. It is believed that its active catalytic species, phthalimide N-oxyl radical (PINO), which is formed in situ from NHPI via one electron transfer, initiates the radical propagation of autoxidation. However, even the best known NHPI-based combined catalyst for oxidation, called the Ishii system, still needs metal cocatalysts,5 such as Co, Mn, etc. Some nonmetallic compounds, such as alkyl hydroperoxide,6 R,R′-azobisisobutyronitrile,7 (3) (a) Bjørsvik, H.-R.; Liguori, L.; Merinero, J. A. V. J. Org. Chem. 2002, 67, 7493. (b) Co´rdova, A.; Sunde´n, H.; Engqvist, M.; Ibrahem, I.; Casas, J. J. Am. Chem. Soc. 2004, 126, 8914. (c) Ohkubo, K.; Fukuzumi, S. Org. Lett. 2000, 2, 3647. (4) (a) Ishii, Y.; Sakaguchi, S.; Iwahama, T. AdV. Synth. Catal. 2001, 343, 393. (b) Saha, B.; Koshino, N.; Espenson, J. H. J. Phys. Chem. A 2004, 108, 425. (c) Baucherel, X.; Gonsalvi, L.; Arends, I. W. C. E.; Ellwood, S.; Sheldon, R. A. AdV. Synth. Catal. 2004, 346, 286. (d) Minisci, F.; Punta, C.; Recupero, F.; Fontana, F.; Pedulli, G. F. J. Org. Chem. 2002, 67, 2671. (e) Koshino, N.; Cai, Y.; Espenson, J. H. J. Phys. Chem. A 2003, 107, 4262. (f) Arnaud, R.; Milet, A.; Adamo, C.; Einhorn, C.; Einhorn, J. J. Chem. Soc., Perkin Trans. 2 2002, 1967.
aldehyde,8 and NO2,9 have been used as mediators for NHPIbased oxidation; they are consumed during the reactions and are not recoverable. These nonmetallic mediators play the sole role of radical initiator, to induce the formation of PINO.10 Recently, we reported11 a biomimetic model encompassing anthraquinones, NHPI, and zeolite HY for efficient selective oxygenation of ethylbenzene. In the catalysis of this model, it was confirmed that the one-electron-transfer interaction of anthraquinones and NHPI resulted in the formation of PINO and then efficiently promoted the oxygenation of hydrocarbons. The catalytic redox cycle is shown in Scheme 1. As far as we are aware, it is the first demonstration of
Scheme 1.
Catalytic Cycle of Anthraquinone and NHPI
organocatalytic hydrocarbon oxygenation by dioxygen under mild conditions without photo- and radical-initiations. It is known that the surrounding organic environment of an enzyme is critical for high selectivities of products in biological processes, and the product orientation of reported organocatalytic reactions comes from the actions of substituents on the backbones of organocatalysts toward the reactants.2 Herein, by modifying the substituents on anthraquinone and then combining with NHPI, we report the potential of this organocatalytic system for selective oxygenation of different hydrocarbons. (5) (a) Vondervoort, L. S.-van de; Bouttemy, S.; Heu, F.; Weissenbo¨ck, K.; Alsters, P. L. Eur. J. Org. Chem. 2003, 578. (b) Ishii, Y.; Iwahama, T.; Sakaguchi, S.; Nakayama, K.; Nishiyama, Y. J. Org. Chem. 1996, 61, 4520. (c) Iwahama, T.; Syojyo, K.; Sakaguchi, S.; Ishii, Y. Org. Process Res. DeV. 1998, 2, 255. (d) Sawatari, N.; Yokota, T.; Sakaguchi, S.; Ishii, Y. J. Org. Chem. 2001, 66, 7889. (6) (a)Amorati, R.; Lucarini, M.; Mugnaini, V.; Pedulli, G. F. J. Org. Chem. 2003, 68, 1747. (b) Arends, I. W. C. E.; Sasidharan, M.; Ku¨hnle, A.; Duda, M.; Jost, C.; Sheldon, R. A. Tetrahedron 2002, 58, 9055. (7) Aoki, Y.; Sakaguchi, S.; Ishii, Y. AdV. Synth. Catal. 2004, 346, 199. (8) (a) Einhorn, C.; Einhorn, J.; Marcadal, C.; Pierre, J.-L. Chem. Commun. 1997, 447. (b) Tsujimoto, S.; Sakaguchi, S.; Ishii, Y. Tetrahedron Lett. 2003, 44, 5601. (9) (a) Eikawa, M.; Sakaguchi, S.; Ishii, Y. J. Org. Chem. 1999, 64, 4676. (b) Nishiwaki, Y.; Sakaguchi, S.; Ishii, Y. J. Org. Chem. 2002, 67, 5663. (10) (a) Sheldon, R. A.; Arends, I. W. C. E. AdV. Synth. Catal. 2004, 346, 1051. (b) Hermans, I.; Vereecken, L.; Jacobs, P. A.; Peeters, J. Chem. Commun. 2004, 1140. (11) Yang, G.; Ma, Y.; Xu, J. J. Am. Chem. Soc. 2004, 126, 10542. 264
Anthraquinone (1.25 mol %) was first used in combination with NHPI (5.0 mol %) to catalyze the oxygenation of fluorene (1), which is shown in Table 1 as a model reaction;
Table 1. Oxygenation of Fluorene Catalyzed by NHPI and Different Anthraquinone Derivativesa
entry
X
convb (%)
yield of 2c (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14e 15f 16g 17h
H 2-chloro 2-ethyl 1-amino 2-amino 1-nitro 1,4-diamino 1,4-dihydroxyl dinitrod 1-amino-2,4-dibromo 1-amino-2-bromo-4-hydroxyl 1,4-dihydroxyl-2-sulfonic acid 1,4-diamino-2,3-dichloro 1,4-diamino-2,3-dichloro 1,4-diamino-2,3-dichloro
50 53 50 46 41 56 42 43 47 39 31 16 59 85 0 25 2
34 45 43 29 25 45 27 24 28 23 15 6 57 (54)i 85 (82)i 0 11 0.4
a Fluorene (5 mmol) was oxygenated by O (e 0.3 MPa) in the presence 2 of NHPI (5 mol %) and various anthraquinone derivatives (1.25 mol %) at 80 °C for 5 h in 10 mL of acetonitrile. b Conversions of 1 were calculated from GC measurement using 1,2-dichlorobenzene as internal standard. c GC measurement after treatments of reaction mixture with Ph3P. d A mixture of 1,5- and 1,8-dinitro anthraquinone. e For 25 h. f NHPI-free. g In the absence of any anthraquinone derivatives. h In the absence of both NHPI and anthraquinones. i Isolated yield.
50% of 1 (entry 1) was converted when the reaction was carried out under 0.3 MPa of O2 at 80 °C for 5 h. GC and GC-MS measurements indicated that the products contained 91% of fluorenone (2) and 9% of 9-hydroxyfluorene (3) without any appreciable overoxidation products. It is wellknown that alkyl hydroperoxide is the initial intermediate of the autoxidation of a hydrocarbon and always remains in the reaction mixture. However, its yield cannot be measured directly by GC analysis because of its thermolability. Unfortunately, this fact was rarely mentioned in the literature and hydroperoxides were often directly measured as their decomposed products, i.e. ketone and alcohol. To verify the formation of 9-fluorenyl hydroperoxide (4) during studies, the mixture was treated initially with excessive Ph3P for 1 h and analyzed again by GC measurement. Because alkyl hydroperoxide could be reduced quantitatively to the corresponding alcohol by Ph3P at room temperature, a second GC measurement could accurately quantify 2. In the second analysis, 68% selectivity of 2 and 32% of 3 were obtained. Org. Lett., Vol. 7, No. 2, 2005
The difference between the two selectivities of 2 inferred that over 23% of 4 existed in the products. In the absence of anthraquinone and NHPI, 2% of 1 was directly oxygenated at the same conditions as a result of autoxidation (Table 1, entry 17), giving only 22% of 2 but 78% of 4. This slight autoxidation was somewhat promoted by NHPI alone, which realized 25% conversion (Table 1, entry 16), but over 42% of products as 4. This roughly agreed with Ishii’s result,12 which gave 15% isolated yield of 2 at 80 °C and 80% at 100 °C (but no data for 3 available) in the presence of NHPI (10 mol %) when the reaction was operated in PhCN under a dioxygen atmosphere for 20 h. Such promotion was believed to be that the fluorenyl peroxide radical, one of the first intermediates of autoxidation of 1, reacted with NHPI to form PINO, so that NHPI was indicated as carbon radical chain promoter.10a In the case of employing anthraquinone and NHPI together, the reaction of fluorenyl peroxide radical with semihydroquinone radical (in Scheme 1) was more facile than with NHPI as a result of formation of the π-bond in the resulting carbonyl, although the latter also occurred. Notwithstanding, it was clearly shown that anthraquinone and NHPI could be coupled to form a two-component catalytic system for oxygenation of a hydrocarbon under modest conditions, as we expected. Then different anthraquinone derivatives were used in combination with NHPI in the oxygenation of 1. The results are displayed in Table 1, which shows that many substituted anthraquinones exhibited considerable activity. On the basis of these results, it could be concluded that the groups with an acidic proton decreased the catalytic activity of anthraquinone. The electronic effect of substituents on the anthraquinone seems nonessential, since there were not clearly different actions between electron-withdrawing and electron-donating groups. However, it was notable and interesting to find that 1,4-diamino-2,3-dichloro-anthraquinone (5) gives 57% yield (54% isolated yield) in 59% conversion of 1 (Table 1), showing an excellent selectivity to 2. The rate of the present catalyst sysytem was examined by employing different amounts of 5 and a constant amount of NHPI (5 mol %) in the oxygenation of 1, and the results are presented in Figure 1. It was found that even 0.32 mol % of 5 exhibit obvious activation, giving 44% of conversion, but selectivity of 2 was as low as 73%. Increase of 5 accounted for an increase of both conversion and selectivity. However, at 1.25 mol % and beyond of 5, the reaction reached a plateau at the level of about 60% conversion and 96% selectivity after 5 h. This phenomenon may be because the reaction becomes slower with a decrease of concentrations both of 1 and 4 but does not stop. It was further illustrated by longer term reaction. When the reaction was prolonged for 25 h in the loading of 1.25 mol % of 5 and 5.0 mol % of NHPI, the conversion increased to as high as 85% and 2 was the sole product (Table 1, entry 14). The isolation of 2 gave 82% yield. (12) Ishii, Y.; Nakayama, K.; Takeno, M.; Sakaguchi, S.; Iwahama, T.; Nishiyama, Y. J. Org. Chem. 1995, 60, 3934.
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Figure 1. Catalyzed oxygenation of 1 (5 mmol) with different amounts of 5 in combination with NHPI (5.0 mol %) under 0.3 MPa of O2 at 80 °C for 5 h. Effect of feed amounts of 5 on conversion (9) and on selectivity of 2 (2).
The optimized amounts of 5 and NHPI were further employed to catalyze oxygenations of various hydrocarbons under the same conditions except for reaction time (Table 2). Tetralin was oxygenated with high conversion (91%) and converted selectively to tetralone with 88% isolated yield. Acenaphthene, ethylbenzene, adamantane, and indane were
Table 2. Oxygenation of Different Hydrocarbons Catalyzed by 5 and NHPIa
a The reactions were carried out under 0.3 MPa of O at 80 °C in 10 mL 2 of CH3CN, using 5 mmol of solid substrates and 2 mL of liquid; 1.25 mol % of 5 and 5.0 mol % of NHPI were used in all cases. b Reaction at 100 °C. c Isolated yield.
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oxidized at good conversions (59-94%). The major oxidation product of toluene was benzoic acid, because as demonstrated before aldehyde can used as radical-initiator for NHPI-based oxygenation8 and is not stable in such radical reaction. At 80 °C for 5 h, 2% cyclohexane was converted, but a significant conversion (15%) of cyclohexane was obtained at 100 °C during 4 h. In comparison with the current industrial process (about 3-7% conversion at 160 °C),1c the results for cyclohexane oxygenation indicate that the catalytic activity of this system increased as the temperature was raised and was applicable even to inactive alkanes. In conclusion, we have developed a new metal-free catalytic system for effective oxygenation of hydrocarbons with dioxygen using anthraquinone derivatives and NHPI. Further studies on the cause for the obvious decrease of
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hydroperoxides when some anthraquinone derivatives, especially 5, are used are currently underway. Acknowledgment. This work was supported financially by National Natural Science Foundation of China (project 20473088) and The National High Technology Research and Development Program of China (project 2004AA32G020). We also thank Dr. Jian-Min Fu from Xenon Pharmaceutical, Inc. for his assistance with the preparation of this manuscript. Supporting Information Available: Detailed experimental procedures and GC measurement method. This material is available free of charge via the Internet at http://pubs.acs.org. OL047749P
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