ORGANIC LETTERS
Co(III)−Alkyl Complex- and Co(III)−Alkylperoxo Complex-Catalyzed Triethylsilylperoxidation of Alkenes with Molecular Oxygen and Triethylsilane
2002 Vol. 4, No. 21 3595-3598
Takahiro Tokuyasu, Shigeki Kunikawa, Araki Masuyama, and Masatomo Nojima* Department of Materials Chemistry & Frontier Research Center, Graduate School of Engineering, Osaka UniVersity, Suita, Osaka 565-0871, Japan
[email protected] Received July 9, 2002
ABSTRACT
Both a Co(III)−alkyl complex and a Co(III)−alkylperoxo complex were found to catalyze triethylsilylperoxidation of alkenes with O2 and Et3SiH. On this basis, together with the nonstereoselectivity in the Co(II)-catalyzed peroxidation of 3-phenylindene and the formation of the corresponding 1,2-dioxolane from 2-phenyl-1-vinylcyclopropane (a radical clock), we propose a reasonable mechanism for the Co(II)-catalyzed novel autoxidation of alkenes with Et3SiH discovered by Isayama and Mukaiyama.
Much attention has been focused on selective oxygenations of the CdC double-bond moiety by molecular oxygen.1 Of these, cobalt complex-catalyzed autoxidation in the presence of a hydrogen donor such as 2-propanol, hydrogen peroxide, or BH4- seems to be unique, since the products are alcohols and ketones instead of allylic alcohols and epoxides as expected in the conventional autoxidation reactions.2,3 In connection with this, Isayama and Mukaiyama have reported a novel peroxidation reaction of alkenes catalyzed by bis(1,3-diketonato)cobalt(II) in the presence of triethylsilane (Et3SiH), in which the corresponding triethylsilyl peroxides * Corresponding author. (1) (a) Mukaiyama, T.; Yamada, T. Bull. Chem. Soc. Jpn. 1995, 68, 17. (b) Hayashi, T.; Okazaki, K.; Urakawa, N.; Shimakoshi, H.; Sessler, J. L.; Vogel E.; Hisaeda, Y. Organometallics 2001, 20, 3074. (c) Hirano, K.; Sakaguchi, S.; Ishii, Y. Tetrahedron Lett. 2002, 43, 3617. (d) Kato, K.; Yamada, T.; Takai, T.; Inoki, S.; Isayama, S. Bull. Chem. Soc. Jpn. 1990, 63, 179. (e) Nishimura, T.; Kakiuchi, N.; Onoue, T.; Ohe, K.; Uemura, S. J. Chem. Soc., Perkin Trans. 1 2000, 1915. (f) Meunier, B. Biomimetic Oxidations Catalyzed by Transition Metal Complexes; Imperial College Press: London, 2000. (g) Barf, G. A.; Sheldon, R. A. J. Mol. Catal. 1995, 102, 23. (h) Katsuki, T. J. Mol. Catal. 1996, 113, 87. (i) Yu, H.-B.; Zheng, X.-F.; Lin, Z.-M.; Hu, Q.-S.; Huang, W.-S.; Pu, L. J. Org. Chem. 1999, 64, 8149. (2) (a) Zombeck, A.; Hamilton, D. E.; Drago, R. S. J. Am. Chem. Soc. 1982, 104, 6782. (b) Hamilton, D. E.; Drago, R. S.; Zombeck, A. J. Am. Chem. Soc. 1987, 109, 374. (3) (a) Okamoto, T.; Oka, S. J. Org. Chem. 1984, 49, 1589. (b) Nishinaga, A.; Yamada, T.; Fujisawa, H.; Ishizaki, K.; Ihara, H.; Matuura, T. J. Mol. Catal. 1988, 48, 24. 10.1021/ol0201299 CCC: $22.00 Published on Web 09/24/2002
© 2002 American Chemical Society
are produced by a formal addition of Et3SiOOH.4,5 By treatment with 1 drop of concentrated HCl in MeOH, the triethylsilyl peroxides are easily desilylated to give the corresponding hydroperoxides. Thus, by using this IsayamaMukaiyama reaction as the key step, a variety of biologically active cyclic peroxides such as analogues of Yingzhaosu C,6 Yingzhaosu A,7 and Artemisinin8 have been prepared from alkenes via the corresponding hydroperoxides. By virtue of the facts that (i) peroxides are generally labile to transition metals9 and, nevertheless, under the conditions developed by Isayama and Mukaiyama, the protected hydroperoxides are produced in good yield and (ii) reliable synthetic methods for sec- and tert-alkyl hydroperoxides (4) (a) Isayama, S. Bull. Chem. Soc. Jpn. 1990, 63, 1305. (b) Isayama, S.; Mukaiyama, T. Chem. Lett. 1989, 573. (5) A similar Co(II)-porphyrin complex-catalyzed peroxygenation in 2-propanol leading to the corresponding hydroperoxide has been also reported by Sugamoto and co-workers: Sugamoto, K.; Matsushita, Y.; Matsui, T. J. Chem. Soc., Perkin Trans. 1 1998, 3989. Furthermore, Magnus and co-workers reported the Mn(III) complex-catalyzed hydroperoxidation of alkenes in the presence of phenylsilane under an oxygen atmosphere: Magnus, P.; Scott, D. A.; Fielding, M. R. Tetrahedron Lett. 2001, 42, 4127. (6) (a) Xu, X.-X.; Dong, H.-Q. J. Org. Chem. 1995, 60, 3039. (7) (a) Tokuyasu, T.; Masuyama, A.; Nojima, M.; McCullough, K. J.; Kim, H.-S.; Wataya Y. Tetrahedron 2001, 57, 5979. (b) Tokuyasu, T.; Masuyama, A.; Nojima, M.; Kim, H.-S.; Wataya, Y. Tetrahedron Lett. 2000, 41, 3145. (8) (a) Oh, C. H.; Kang, J. H. Tetrahedron Lett. 1998, 39, 2771. (b) Oh, C. H.; Kim, H. J.; Wu, S. H.; Won, H. S. Tetrahedron Lett. 1999, 40, 8391.
particularly are quite limited,10 any mechanistic insight will be quite interesting. We report herein a novel catalytic role of Co(III)-alkylperoxo and Co(III)-alkyl complexes in the peroxidation of alkene with molecular oxygen and Et3SiH. As test reactions, we first conducted peroxidation of alkene 1 with three types of cobalt(II) complexes as catalysts in the presence of Et3SiH in 1,2-dichloroethane (DCE) at room temperature (Table 1).11 Produced was a mixture of triethyl-
Table 1. Co(III)-Catalyzed Triethylsilylperoxidation of Alkene 1
To elucidate the reaction mechanism, it is important to obtain information about the reactive complexes involved in the catalytic cycle. In this respect, the result reported by Isayama4a provides an important clue to the investigation of the reaction mechanism. He has found that Co(acac)2catalyzed triethylsilylperoxidation of styrene, having a poor reactivity, is very slow. However, by addition of a small amount of tert-butyl hydroperoxide (TBHP), the induction period is significantly shortened. This result would be interpreted in terms of the contribution of the Haber-Weiss mechanism.13 That is, the possible role of TBHP is generation of Co(III)-alkoxo and Co(III)-alkylperoxo complexes, which would promote the present peroxidation. To confirm the possibility of participation of Co(III)-alkylperoxo complex, we prepared the Co(III)-cumylperoxo complex 414 and investigated the peroxidation of alkene 1 with complex 4 (16 mol %) and Et3SiH (2 equiv) (Scheme 1). After the
Scheme 1. Catalytic Effect of CumylOOCo(acac)2Py 4 in the Peroxidation of Alkene 1
% yields catalysta
reaction time, h
% conversion
2a
2b
3
Co(modp)2 Co(acac)2 Co(SB)
3.5 3.5 6.5
100 95 33
89 80 57
5 12
9 27
a Reaction was carried out in the presence of 5 mol % catalyst at room temperature.
silyl peroxide 2a, hydroperoxide 2b, and alcohol 3. To avoid decomposition of the derived peroxides by Co(II), the reaction was usually quenched before the alkene was consumed completely. Thus, the yield of the peroxides was calculated on the basis of the consumed alkene.12 Consistent with the result of Isayama and Mukaiyama, Co(modp)2- or Co(acac)2-catalyzed peroxidation of 1 proceeded smoothly and regioselectively, affording the corresponding triethylsilyl peroxide 2a in high yield. Although less efficient, cobalt(II) Schiff base complex (Co(SB)) also catalyzed the same reaction. (9) Dussault, P. H. In ActiVe Oxygen in Chemistry; Foote, S., Valentine, J. S., Greenberg, A., Liebman, J. F., Eds.; Chapman & Hall: New York, 1995; Chapter 5. (10) Porter, N. A. In Organic Peroxides; Ando, W., Ed.; Wiley: New York, 1992; Chapter 2. (11) Representative Reaction Conditions. Into a two-neck 50 mL flask, charged with dioxygen, was added a mixture of alkene (2 mmol) and cobalt(II) complex (0.10 mmol) in 1,2-dichloroethane (DCE) (5 mL). Then, triethylsilane (4 mmol) was added via a 1.0 mL gastight syringe, and the reaction mixture was stirred vigorously under an oxygen atmosphere at room temperature. (12) In all cases, the resulting triethylsilyl peroxide could not be separated from the remaining alkene. Therefore, after treatment of this mixture with one portion of concentrated HCl in MeOH, the peroxide was isolated in the form of the corresponding hydroperoxide. 3596
mixture was stirred for 3 h, triethylsilyl peroxide 2a was obtained in 71% yield, together with 2b (14%) and 3 (11%) (the recovery of alkene 1 was 45%).15 Thus, it was evident that the Co(III)-cumylperoxo complex 4 also could catalyze the triethylsilylperoxidation of an alkene with molecular oxygen and Et3SiH. This characteristic behavior of the Co(III)-cumylperoxo complex in the presence of Et3SiH is in marked contrast to the fact that, in the absence of Et3SiH, the complex catalyzes autoxidation of alkenes to provide products such as allylic alcohols and R,β-unsaturated ketones.13,16 It has been well established that under the latter conditions, homolytic fission of the O-O bond (or the Co-O bond) yielding an alkoxy radical (or an alkylperoxy radical) is the predominant mode of decomposition of the Co(III)alkylperoxo complex. (13) Saussine, L.; Brazi, E.; Robine, A.; Mimoun, H.; Fischer, J.; Weiss, R. J. Am. Chem. Soc. 1985, 107, 3534. (14) Talsi, E. P.; Chinakoc, V. D.; Babenko, V. P.; Sidelnikov, V. N.; Zamaraev, K. I. J. Mol. Catal. 1993, 81, 215. (15) Conversion in the complex 4-catalyzed peroxidation was much lower than that in the conventional Co(II)-catalyzed reaction, which would be due to the stabilization of complex 4 by the coordinated pyridine as a sixth ligand. (16) Chavez, F. A.; Mascharak, P. K. Acc. Chem. Res. 2000, 33, 539 and references therein. Org. Lett., Vol. 4, No. 21, 2002
An interesting finding, illustrated in Scheme 1, is that most of the cumylperoxy moiety in 4 was incorporated into the corresponding triethylsilyl peroxide 5 (71%) and the alcohol 6 (16%).17 This led us to deduce that metal exchange between Co(III) complex 4 and Et3SiH occurs very rapidly and efficiently, thereby providing cobalt(III)-hydride complex 7, together with triethylsilyl peroxide 5.18 In accordance with this notion, the reaction of 4 with 23 equiv of Et3SiH under an Ar atmosphere in the absence of alkene 1 gave peroxide 5 in an isolated yield of 60%, together with alcohol 6 (33%) (Scheme 2). Moreover, in the 1H NMR spectrum of a solution
Scheme 3. Proposed Mechanism
Scheme 2. Reaction of Co(III) Complex 4 with Et3SiH
of Co(III)-cumylperoxo complex 4 and Et3SiH in CD2Cl2, a broad signal was observed at -2.8 ppm, which is probably attributable to the hydrogen in the Co(III)-hydride complex (see Supporting Information, Figure S1).19 In the case of Et3SiD, this signal was not observed. This metal exchange would be reasonable, since both the Co-O bond in 416 and the Si-H bond in Et3SiH are very weak20 and the affinity of silicon toward oxygen is very high. On the basis of these results, together with the suggestion of the contribution of a H-Co(III) complex in the Co(II) complex-catalyzed autoxidation in the presence of a hydrogen donor such as 2-propanol or BH4-,3 we considered that the mechanism illustrated in Scheme 3 would best rationalize the Isayama-Mukaiyama reaction. The first step in this catalytic cycle involves insertion of alkene into the H-Co bond of Co(III)-hydride complex I to give Co(III)-alkyl complex II.21 The second step is homolytic cleavage of the Co-C bond of complex II, which is followed by reaction with molecular oxygen to give Co(III)-alkylperoxo complex III.22 Finally, complex III undergoes transmetalation with Et3SiH, resulting in the formation of triethylsilyl peroxide and regeneration of Co(III)-hydride complex I.23 (17) Drago and co-workers suggested that alkene would react directly with cobalt(III)-peroxy complex to cause insertion of alkene into the Co-O bond.2 In the present reaction, however, no product derived from such an insertion reaction was observed. (18) Analogous metal-exchange reactions of tin alkoxides and copper(I) alkoxides with silane are known: (a) Hays, D. S.; Scholl, M.; Fu, G. C. J. Org. Chem. 1996, 61, 6751 and references therein. (b) Lipshutz, B. H.; Chrisman, W.; Noson, K. J. Organomet. Chem. 2001, 624, 367. (c) Jurkauskas, V.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124, 2892. (19) (a) Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; Dubois, M. R. J. Am. Chem. Soc. 2002, 124, 2984. (b) Schrauzer, G. N.; Holland, R. J. J. Am. Chem. Soc. 1971, 93, 1505. (20) Wu, Y.-D.; Wong, C.-L. J. Org. Chem. 1995, 60, 821. (21) (a) Schrauzer, G. N. Angew. Chem., Int. Ed. Engl. 1976, 15, 417. (b) Derenne, S.; Gaudemer, A.; Johnson, M. D. J. Organomet. Chem. 1987, 322, 229. (c) Kemmitt, R. D. W.; Russel, D. R. In ComprehensiVe Organometallic Chemistry; Wilkinson, G., Eds.; Pergamon: Oxford, 1982; Vol. 5, p 81. Org. Lett., Vol. 4, No. 21, 2002
As the first approach to confirm this mechanism, we next investigated whether the Co(III)-alkyl complex II can catalyze the peroxidation. Since a variety of Co(III)(salen)alkyl complexes are known to be easy to handle, we prepared PhCH2CH2CH2Co(SB) complex 8 from Co(I)(SB)- nucleophile and 3-phenylpropyl bromide by the reported procedure.24a The reaction of alkene 1 in the presence of the derived complex 8 (9 mol %) and Et3SiH under an oxygen atmosphere gave triethylsilyl peroxide 9 (34% based on 8), together with 2a, 2b, and 3 in yields of 81, 2, and 12%, respectively (based on the consumed alkene; 83%) (Scheme 4). This suggests that the Co(III)-alkyl complex II is also
Scheme 4. Co(III)-Alkyl Complex 8-Catalyzed Peroxidation of Alkene 1
one of the key intermediates in the Isayama-Mukaiyama reaction. In connection with this, it is well-known that under an oxygen atmosphere, Co(III)-alkyl complex II transforms into the corresponding Co(III)-alkylperoxy complex III by homolytic cleavage of the C-Co bond and the subsequent trap by molecular oxygen.22 To confirm whether or not a distinct alkyl radical participates in the Isayama-Mukaiyama (22) (a) Iqbal, J.; Bhatia, B.; Nayyar, N. K. Chem. ReV. 1994, 94, 519. (b) Iqbal, J.; Sanghi, R.; Nandy, J. P. In Radicals in Organic Synthesis; Renaud, P., Sibi, M. P., Eds.; Willey-VCH: Weinheim, Germany, 2001; Chapter 1.8. (c) Jensen, F. R.; Kiskis, R. C. J. Am. Chem. Soc. 1975, 97, 5825. (23) Since the present peroxidation is a formal addition reaction of Et3SiOOH, the reaction of 1-dodecene with Et3SiOOH (2 equi) was conducted in the presence of a catalytic amount of Co(modp)2 under an Ar atmosphere. After the mixture was stirred for 16 h, 2-dodecanone was obtained as the sole identifiable product in 73% yield (based on the consumed alkene; 20%); no formation of the expected dodecan-2-yl triethylsilyl peroxide was observed. However, peroxidation of 1-dodecene with Et3SiH and O2 proceeded more smoothly (6.5 h, 69% conversion), affording dodecan-23597
reaction, Co(modp)2-catalyzed triethylsilylperoxidation of vinylcyclopropane 10 (an efficient radical clock25) with O2 and Et3SiH was conducted (Scheme 5). After the mixture
Scheme 6. Peroxidation of 3-Phenylindene 16
Scheme 5. Peroxidation of Vinylcylopropane 10
isolable product (Scheme 6). This result is clearly consistent with the proposed mechanism illustrated in Scheme 3. Exclusive deuterium atom transfer to indene 16 at C-2 gives the corresponding alkyl radical, which is trapped by an oxygen molecule from both faces, and as a result, a 1:1 mixture of two stereoisomers of 17-d is produced. A brief comment should be made on the initiation step leading to the formation of Co(III)-hydride complex: it is well-known that Co(II) complexes such as Co(salen) and Co(acac)2 are oxidized by molecular oxygen to afford the corresponding µ-peroxocobalt(III) and superoxocobalt(III).27 Subsequent transmetalation of superoxocobalt(III) 18 or µ-peroxocobalt(III) 19 complex with Et3SiH would give the corresponding Co(III)-hydride complex I (Scheme 7), as
Scheme 7. Most Likely Mechanism of Initation
was stirred for 7 h, the 1,2-dioxolane 14 and the keto alcohol 15 were obtained in yields of 38 and 16%, respectively (based on the consumed alkene; 67%). The formation of the 1,2-dioxolane 14 clearly indicates that the alkyl radical 11 is formed first by the transfer of a hydrogen atom from H-Co(III) complex to the terminal carbon of the CdC bond, which is followed by immediate ring opening to give the alternative carbon radical 12. Then, trapping by an oxygen molecule and subsequent intramolecular cyclization of the derived peroxy radical occurs to give the carbon radical 13, which in turn leads to the formation of 1,2-dioxolane 14 as the final product. It should be noticed that a similar mechanism has been proposed by Feldman to explain the elegant production of 3-phenyl-5-vinyl-1,2-dioxolane from the phenylthio radical-initiated autoxidation of the same vinylcyclopropane 10.26 Finally, we tried to obtain information on the stereochemistry of the formal addition of Et3SiOOH. Treatment of 3-phenylindene 16 with Et3SiD under an oxygen atmosphere gave a 1:1 mixture of two stereoisomers of 17-d as the sole yl triethylsilyl peroxide in 74% yield. The significant difference between the two reactions suggests that a mechanism involving the formation of Et3SiOOH at the first step and the following addition to the alkene is not important in the Isayama-Mukaiyama reaction. (24) (a) Schrauzer, G. N.; Sibert, H. W.; Windgassen, R. J. J. Am. Chem. Soc. 1968, 90, 6681. (b) Arkel, B.; Baan, J. L.; Balt, S.; Bickelhaupt, F.; Bolster, M. W. G.; Kingma, I. E.; Klumpp, G. W.; Moos, J. W. E.; Spek, A. L. J. Chem. Soc., Perkin Trans. 1 1993, 3023. (25) (a) Nonhebel, D. C. Chem. Soc. ReV. 1993, 347. (b) Stevenson, J. P.; Jackson, W. F.; Tanko, J. M. J. Am. Chem. Soc. 2002, 124, 4271. (26) (a) Feldman, K. S. Synlett 1995, 217. (27) (a) Talsi, E. P.; Zimin, Y. S.; Nekipelov, V. M. React. Kinet. Catal. Lett. 1985, 27, 361. (b) Yoshino, Y.; Hayashi, Y.; Iwahama, T.; Sakaguchi, S.; Ishii, Y. J. Org. Chem. 1997, 62, 6810 and references cited therein. (c) Bianchini, C.; Zoellner, R. W. AdV. Inorg. Chem. 1997, 44, 263. 3598
is postulated in the case of the Co(III)-alkylperoxo complex. Perhaps in accordance with this, treatment of Co(modp)2 with O2 and Et3SiH (38 equiv) for 18 h gave Et3SiOOSiEt3 20 and Et3SiOH 21 in yields of 200 and 350%, respectively (based on the Co(modp)2) (Scheme 7). In conclusion, we have discovered that both a Co(III)alkyl complex and a Co(III)-alkylperoxo complex can catalyze triethylsilylperoxidation of alkenes with O2 and Et3SiH. This provides further insight into the mechanism of the reaction developed by Isayama and Mukaiyama and, moreover, can open new applications of these important Co(III) complexes. Acknowledgment. This work was supported by the Ministry of Education, Science, Sports and Culture of Japan (14021066). Supporting Information Available: Full detail of experimental procedures and physical properties of all products. This material is available free of charge via the Internet at http://pubs.acs.org. OL0201299 Org. Lett., Vol. 4, No. 21, 2002
