Co(III)-alkyl complex- and Co(III)-alkylperoxo complex-catalyzed triethylsilylperoxidation of alkenes with molecular oxygen and triethylsilane

Article abstract of DOI:10.1021/ol0201299

ROOCo(III) + Et₃SiH → ROOSiEt₃ + [HCo(III)] cat. ROOCo(III) or cat. RCo(III) R'CH=CH₂ → R'CH(OOSiEt₃)CH₃ Et₃SiH, O₂ 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.

Full text of DOI:10.1021/ol0201299

ORGANIC
LETTERS
2002
Vol. 4, No. 21
3595-3598
Co(III)−Alkyl Complex- and
Co(III)−Alkylperoxo Complex-Catalyzed
Triethylsilylperoxidation of Alkenes with
Molecular Oxygen and Triethylsilane
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
nojima@chem.eng.osaka-u.ac.jp
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 O and Et3SiH.
2
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
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 Isayama-
Mukaiyama 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 hydro-
peroxides are produced in good yield and (ii) reliable
synthetic methods for sec- and tert-alkyl hydroperoxides
* 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.
(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.
10.1021/ol0201299 CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/24/2002

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-
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)2-
catalyzed 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 com-
plex, 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
Table 1. Co(III)-Catalyzed Triethylsilylperoxidation of Alkene 1
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.
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,â-unsaturated ke-
tones.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.
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.
(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.
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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
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)- nucleo-
phile 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 atmo-
sphere 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
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 Et3-
SiD, 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.
Scheme 4. Co(III)-Alkyl Complex 8-Catalyzed Peroxidation
of Alkene 1
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
- 3
donor such as 2-propanol or BH4 , 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
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
(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.
(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 Et3-
SiOOH, 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-2-
Org. Lett., Vol. 4, No. 21, 2002
3597

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
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 Et3-
SiH. This provides further insight into the mechanism of the
reaction developed by Isayama and Mukaiyama and, more-
over, can open new applications of these important Co(III)
complexes.
Finally, we tried to obtain information on the stereochem-
istry 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.
Acknowledgment. This work was supported by the
Ministry of Education, Science, Sports and Culture of Japan
(14021066).
Supporting Information Available: Full detail of ex-
perimental procedures and physical properties of all products.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(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.
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