Epoxidation of trans-cyclooctene by methyltrioxorhenium/H2O2: Reaction of trans-epoxide with the monoperoxo complex

Article abstract of DOI:10.1021/jo000564l

The epoxidation of trans-cyclooctene (trans-1) with the MTO/H₂O₂, MTO/UHP, and NaY/MTO/H₂O₂ oxidants leads to a mixture of trans/cis-olefins 1, trans/cis-epoxides 2, and the cis-diol 3. While the oxygen transfer proceeds stereoselectively, the monoperoxo rhenium complex A, which is generated in situ during the catalytic cycle, is responsible for the facile deoxygenation, isomerization, and hydrolysis of the trans-epoxide. In the case of the homogeneous MTO/H₂O₂ system, rapid decomposition of the catalytically active rhenium species into HReO₄ circumvents the formation of such side products. In contrast, for the heterogeneous oxidants MTO/UHP and NaY/MTO/H₂O₂, the catalytically active rhenium species are sufficiently stabilized and survive long enough to promote the observed side reactions.

Full text of DOI:10.1021/jo000564l

J . Org. Chem. 2000, 65, 5001-5004
5001
2 2
Ep oxid a tion of tr a n s-Cycloocten e by Meth yltr ioxor h en iu m /H O :
Rea ction of tr a n s-Ep oxid e w ith th e Mon op er oxo Com p lex
Waldemar Adam,* Chantu R. Saha-M o¨ ller, and Oliver Weichold
Institut f u¨ r Organische Chemie der Universit a¨ t W u¨ rzburg, Am Hubland, D-97074 W u¨ rzburg, Germany
adam@chemie.uni-wuerzburg.de
Received April 13, 2000
The epoxidation of trans-cyclooctene (trans-1) with the MTO/H
2 2
O , MTO/UHP, and NaY/MTO/
2 2
H O
oxidants leads to a mixture of trans/cis-olefins 1, trans/cis-epoxides 2, and the cis-diol 3. While
the oxygen transfer proceeds stereoselectively, the monoperoxo rhenium complex A, which is
generated in situ during the catalytic cycle, is responsible for the facile deoxygenation, isomerization,
and hydrolysis of the trans-epoxide. In the case of the homogeneous MTO/H
decomposition of the catalytically active rhenium species into HReO circumvents the formation of
such side products. In contrast, for the heterogeneous oxidants MTO/UHP and NaY/MTO/H
2 2
O system, rapid
4
2
2
O ,
the catalytically active rhenium species are sufficiently stabilized and survive long enough to
promote the observed side reactions.
In tr od u ction
Resu lts a n d Discu ssion
2 2
A variety of oxidation reactions with H O are cata-
trans-Cyclooctene (trans-1) was prepared from its cis
lyzed by methyltrioxorhenium (MTO), of which the
6
isomer according to the literature. The authentic refer-
1
epoxidation has been investigated intensively. For most
5
ence epoxides cis- and trans-2 were obtained by DMD
olefins, this preparatively valuable catalytic oxidation
proceeds cleanly in high conversion and excellent yields,
with high product selectivity and diastereoselectivity.
Undesirable side reactions, the most prominent is the
hydrolytic opening of the epoxide ring and subsequent
pinacol-type rearrangements and C-C bond cleavages,
epoxidation of the corresponding cyclooctene diastereo-
mers. The diols cis- and trans-3 were obtained from the
7
cyclooctene cis-1 by hydroxylation with KMnO
4
(cis-3)
and epoxidation with HCO H/H O , followed by basic
2 2 2
hydrolysis (trans-3).
The catalytic oxidation of trans-cyclooctene (trans-1)
2 2
are known since the first reports on MTO/H O -mediated
was conducted with the known MTO-based systems
2
2b
epoxidations. Several additives, e.g., urea and pyri-
8
2b
9
MTO/85% H
in methylene chloride at ambient temperature (ca. 20 °C).
For the standard MTO/H oxidant, the reaction ini-
2 2 2 2
O , MTO/UHP, and NaY/MTO/85% H O
3
dine, have been found to circumvent these problematic
side reactions, but the responsible rhenium species has
2
O
2
to date not been defined with certainty.2
a,4
tially proceeds diastereoselectively; i.e., only the trans-
epoxide is observed (Table 1, entries 1 and 2). However,
once the substrate is completely consumed after ca. 1 h
In this context, we have observed complex oxidation
chemistry in the MTO-catalyzed epoxidation of trans-
cyclooctene as a substrate. Besides the expected ring-
(entry 2), complex chemistry ensues on prolonged
5
opening of the strained trans-epoxide, also cis-epoxide
reaction time, in which the trans-epoxide is not only
hydrolyzed to the cis-3 diol, but also deoxygenated back
to the olefin, as suggested by the lower extent of conver-
sion (entry 3). Thus, for the highly diastereoselective
preparation of the trans-2 epoxide, the reaction must be
terminated after 1 h. Mechanistically significant, some
cis-epoxide is formed, as evidenced by the drop in the
trans/cis-epoxide ratio, which signifies loss of stereo-
selectivity; moreover, the trans/cis ratio drops further on
still longer reaction time (data not shown). The latter is
even more pronounced for the heterogeneous oxidants
and cis-cyclooctene were observed, which required mecha-
nistic clarification. Herein we report our unusual results
on the epoxidation of trans-cyclooctene 1 with the homo-
2 2
geneous MTO/H O and the heterogeneous MTO/UHP
and NaY/MTO/UHP oxidants. The in-situ-generated
monoperoxo rhenium species has been identified as the
active intermediate responsible for the complex chemis-
try.
*
To whom correspondence should be addressed. Fax: +49 931 888
756. Internet: http://www-organik.chemie.uni-wuerzburg.de.
1) (a) Vassell, K. A.; Espenson, J . H. Inorg. Chem. 1994, 33, 5491-
498. (b) Adam, W.; Herrmann, W. A.; Lin, J . Saha-M o¨ ller, C. R. J .
4
5
(
MTO/UHP and NaY/MTO/H
cis-epoxide ratios even at low conversions (compare
entries 4 and 6 with 1). As for the MTO/H oxidant, in
2 2
O , which display low trans/
Org. Chem. 1994, 59, 8281-8283. (c) Herrmann, W. A. J . Organomet.
Chem. 1995, 500, 149-174. (d) Murray, R. W.; Iyanar, K.; Chen, J .;
Wearing, J . T. Tetrahedron 1996, 37, 805-808. (e) Gable, K. P. Adv.
Organomet. Chem. 1997, 41, 127-161. (f) Adam, W.; Saha-M o¨ ller, C.
R.; Mitchell, C. M.; Weichold, O. In Structure and Bonding; Meunier,
B., Ed.; Springer, Berlin, 2000, in press.
2
O
2
heterogeneous systems, the stereoselectivity deteriorates
(6) Inoue, Y.; Tsuneishi, H.; Hakushi, T.; Tai, A. In Photochemical
Key Steps in Organic Synthesis, An Experimental Course Book; Mattay,
J ., Griesbeck, A., Eds.; VCH: Weinheim, 1994; p 207.
(7) Cope, A. C.; Fenton, S. W.; Spencer, C. F. J . Am. Chem. Soc.
1952, 74, 5884-5888.
(8) Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 1638-1641.
(9) Adam, W.; Saha-M o¨ ller, C. M.; Weichold, O. J . Org. Chem. 2000,
65, 2897-2899.
(2) (a) Herrmann, W. A.; Fischer, R. W.; Scherer, W.; Rauch, M. H.
J . Mol. Catal. A 1994, 86, 243-266. (b) Adam, W.; Mitchell, C. M.
Angew. Chem., Int. Ed. Engl. 1996, 35, 533-534.
(3) Rudolph, J .; Reddy, K. L.; Chiang, J . P.; Sharpless, K. B. J . Am.
Chem. Soc. 1997, 119, 6189-6190.
4) Al-Ajlouni, A. M.; Espenson, J . H. J . Org. Chem. 1996, 61, 3969-
976.
5) Shea, K. J .; Kim, J .-S. J . Am. Chem. Soc. 1992, 114, 3044-3051.
(
3
(
1
0.1021/jo000564l CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/11/2000

5
002 J . Org. Chem., Vol. 65, No. 16, 2000
Adam et al.
Ta ble 1. Ma ss Ba la n ces, Con ver sion s, a n d Tr a n s/Cis Ra tios for MTO-Ca ta lyzed Ep oxid a tion s of th e tr a n s-Cycloocten e
(
tr a n s-1)
product distribution
oxidantb
time (h)
convn (%)
a
mb (%)
a
1 (trans/cis)a
2 (trans/cis)a
entry
1
2
3
4
5
6
7
MTO/85% H2O2
0.5
1
2
0.5
2
0.5
2
89
>95
85
42
60
82
91
88
91
85
87
74
14 (>95:5)
2 (>95:5)
17 (>95:5)
64 (>95:5)
47 (>95:5)
89 (>95:5)
76 (>95:5)
86 (>95:5)
98 (>95:5)
c
77 (89:11)
MTO/UHP
36 (79:21)
53 (77:23)
11 (61:39)
24 (45:55)
NaY/MTO/85% H2O2
23
43
a
Determined by 1H NMR spectroscopy on the reaction mixture against naphthalene as internal standard, reproducible within the
b
experimental error ((5% of the given value). Methyltrioxorhenium (MTO), urea/hydrogen peroxide adduct (UHP), zeolite (NaY); reaction
performed with 330 µmol trans-1 in 0.6 mL CDCl3, 1:100:100 MTO/H2O2/trans-1. c Also 6% of the cis-diol (cis-3) was formed.
Ta ble 2. Ma ss Ba la n ces (m b) a n d Tr a n s/Cis Ra tios of th e
Con tr ol Exp er im en ts for th e Isom er iza tion of
tr a n s-Cycloocten e (tr a n s-1) a n d tr a n s-Cycloocten e
Ep oxid e (tr a n s-2)
Sch em e 1. Ca ta lytic Cycle in MTO-Med ia ted
Oxid a tion s
alone, significant amounts of cyclooctene (10-15%) and
cis-diol 3 (15%) were formed.
It is generally accepted1e,8 that under normal oxidation
conditions, i.e., with a 100-fold excess of the oxygen donor
2 2
H O relative to the rhenium catalyst, no MTO is present
in solution, but rather the oxygen-transferring agent
diperoxo complex B. Thus, the MTO may hardly be the
culprit for the observed loss of diastereoselectivity during
the epoxidation of cyclooctene trans-1, but maybe the
monoperoxo complex A. This becomes evident from
inspection of the catalytic oxidation cycle in the MTO-
catalyzed epoxidation (Scheme 1): For every oxidized
olefin molecule, one molecule of the monoperoxo complex
A is formed, which subsequently regenerates the diperoxo
complex B with hydrogen peroxide.
a
1
Determined by H NMR spectroscopy directly on the reaction
mixture against naphthalene as internal standard, error (5% of
b
the given value. Besides ca. 70-75% epoxide, also trans/cis-
c
cyclooctene (10-15%) and cis-diol (15%) were observed. All
epoxide was consumed, and a 1:1 mixture of trans/cis-cyclooctene
(
20%) and cis-diol (29%) was observed.
on prolonged reaction times (compare entries 4, 5 and 6,
). Although the expected initial order of reactivity; i.e.,
MTO/H > MTO/UHP > NaY/MTO/H is obeyed, the
7
2
O
2
2 2
O
highest trans/cis-epoxide ratio was found for the most
Lewis-acidic of all catalytic oxidants used here, namely
the MTO/H O .
2 2
To gain further mechanistic insight into the complex
MTO-catalyzed oxidation process of trans-cyclooctene, the
formation and disappearance of the epoxides by NMR
spectroscopy was monitored with time. For this purpose,
trans-cyclooctene was treated with 1 equiv of in-situ-
generated diperoxo complex B (Scheme 2). Within the
first 5 min (convn >95%), a 90:10 trans/cis mixture of
the epoxides and the monoperoxo complex A is obtained.
Subsequently, after an additional 30 min, most of the
trans-epoxide is selectively consumed. At this point, not
only are the trans/cis-cyclooctenes 1 (45:55) and the cis-3
diol formed, but more cis-2 epoxide is observed after the
second step than produced in the first step. To substanti-
ate this result, the monoperoxo complex A was generated
in-situ by treating the diperoxo complex B with 1 equiv.
of cyclopentene (data not shown), whose epoxide is not
deoxygenated. To this reaction mixture was then added
an equimolar amount (based on rhenium complex) of
trans/cis-epoxides (1:1) and the time profile for epoxide
consumption was assessed (Figure 1). As anticipated, the
trans-epoxide was consumed within minutes after addi-
tion and 10-15% of the trans/cis-1 cyclooctenes were
The question arises as to the origin of the cis-2 epoxide.
To assess whether the observed decrease of the trans/
cis-epoxide ratio stems from trans-olefin isomerization
and subsequent epoxidation of the cis-cyclooctene or from
subsequent isomerization of the trans-epoxide, the per-
sistence of the authentic trans-1 cyclooctene and trans-2
epoxide was checked under various conditions as control
experiments (Table 2). Only marginal olefin isomerization
took place with MTO alone (entry 1) and none with its
decomposition product HReO
4
(entry 2). The control
experiments with the authentic trans-epoxide were more
instructive. Although no isomerization of the trans-
epoxide was observed with MTO alone (entry 3), or with
the diperoxo complex B (entry 5), under conditions at
which mainly the monoperoxo complex A is generated
2 2
(i.e., with 10 mol % of MTO and 8 mol % of 30% H O ,
entry 4), no epoxide was recovered after 24 h. Instead,
in a poor mass balance (entry 4), 29% of the cis diol and
2
0% of trans/cis-1 were found. Also in the case of MTO

Epoxidation of trans-Cyclooctene by Methyltrioxorhenium/H
2
O
2
J . Org. Chem., Vol. 65, No. 16, 2000 5003
Sch em e 2. Stoich iom etr ic Oxid a tion of tr a n s-Cycloocten e (tr a n s-1) to th e tr a n s/cis-Ep oxycycloocta n es
90:10) by th e Dip er oxo Com p lex B (1 equ iv) for th e In Situ Gen er a tion of th e Mon op er oxo Com p lex A a n d
F u r th er Rea ction
(
reactive enough to deoxygenate epoxides, N-oxides, and
sulfoxides. In our case, however, no phosphine is present
and the reactions are performed with hydrogen peroxide.
As stated above (cf. Scheme 1), under these conditions
the monoperoxo complex A is formed and serves as the
deoxygenating species. The mechanistic details are as yet
obscure for this deoxygenation of the trans-cyclooctene
epoxide.
Now that the rhenium species responsible for the
unprecedented epoxide deoxygenation and loss of dia-
stereoselectivity has been identified, namely the mono-
peroxo complex A, the question remains, why the various
MTO-based oxidants examined herein (Table 1) display
such differences in their trans/cis selectivity for the
epoxidation of trans-cyclooctene 1. In the case of MTO/
2 2
H O (entry 1), the trans-1 cyclooctene is oxidized cleanly
F igu r e 1. Time profile for the decomposition of trans/cis-
epoxycyclooctanes 2 (1:1 mixture) by the monoperoxo complex
A (1 equiv), generated in situ through stoichiometric epoxi-
dation of cyclopentene by the diperoxo complex B.
trans-selectively by the catalytically in-situ-generated
diperoxo complex B. Rapid decomposition of the rhenium
species to catalytically inactive products11 such as HReO
4
circumvents the deoxygenation of the trans-epoxide, as
evidenced by the fact that the decomposition products do
not subsequently alter the trans/cis-epoxide ratio (Table
formed, while the amount of cis-epoxide remained es-
sentially unchanged (Figure 1). Low mass balances were
obtained in these stoichiometric reactions, which ac-
counts for the appreciable amount of unidentified de-
composition products compared to the catalytic runs
2
b
2, entry 2). For the MTO/UHP and NaY/MTO/85%
9
2 2
H O oxidants, we have recently demonstrated that the
oxidizing rhenium species are stabilized and persist
longer in the host interior. Therefore, in the heteroge-
neous oxidation systems the interaction between the
sensitive trans-epoxide and the monoperoxo complex A
(
Table 1, entries 1-3), for which such complex chemistry
is essentially negligible.
These results unequivocally establish that the mono-
peroxo complex A selectively reacts with the trans-
epoxide. As products thereof, the trans/cis-1 cyclooctenes
are formed, along with some cis-epoxide, which implicates
selective deoxygenation of the trans-epoxide. In this
context it is relevant to mention that the deoxygenation
of epoxides by MTO has been observed before in the
2 2
is more pronounced than in the MTO/H O case, and
more epoxide decomposition occurs (lower trans/cis-
epoxide ratios, entries 4-7, Table 1). An alternative
explanation may be that in the heterogeneous oxidations,
2 2
MTO faces a lower effective concentration of H O , which
favors the formation of the monoperoxo complex A and,
consequently, the degradation of the trans-epoxide is
more facile. However, no significant difference in the
amounts of the monoperoxo complex A was observed by
NMR and UV analyses in the heterogeneous versus
homogeneous systems.
1
0
absence of H
2
O
2
,
3
but required PPh as oxygen acceptor
and large amounts of the rhenium catalyst (10 mol %);
thus, the monoperoxo complex A cannot be the deoxy-
genating agent in this process. Rather, a mechanism was
proposed, in which the PPh
3
generates the methyldi-
oxorhenium (MDO) species from MTO, and MDO is
(
10) Zhu, Z.; Espenson, J . H. J . Mol. Catal., A: Chemical 1995, 103,
(11) Abu-Omar, M. M.; Hansen, P. J .; Espenson, J . H. J . Am. Chem.
Soc. 1996, 118, 4966-4974.
8
7-94.

5
004 J . Org. Chem., Vol. 65, No. 16, 2000
Adam et al.
Con clu sion
tionen Metall-aktivierter Molek u¨ le”) and the Fonds der
Chemischen Industrie for generous support.
We have demonstrated that the MTO-catalyzed oxygen
transfer to trans-cyclooctene proceeds initially stereo-
selectively, but the trans-epoxide does not persist under
the reaction conditions and is in part deoxygenated,
isomerized and hydrolyzed. The monoperoxo complex A,
which is generated during the oxygen transfer from the
diperoxo complex B, is responsible for the facile trans-
formation of the trans-epoxide to the cyclooctenes 1.
Su p p or tin g In for m a tion Ava ila ble: The experimental
procedures for the MTO-catalyzed oxidations of trans-cy-
clooctene as well as control experiments are reported. This
material is available free of charge on the Internet at
http://pubs.acs.org.
Ack n ow led gm en t. The authors thank the Deutsche
Forschungsgemeinschaft (SFB 347, “Selektive Reak-
J O000564L