Isomerization of allylic silyl ethers catalyzed by ReO3(OSiR3) complexes

Article abstract of DOI:10.1016/S0040-4039(99)02352-7

Efficient isomerization of allylic alcohols and allylic ethers at room temperature by rhenium (VII) oxo complexes is described. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(99)02352-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1549–1552
Isomerization of allylic silyl ethers catalyzed by ReO (OSiR )
3
3
complexes
∗
Stéphane Bellemin-Laponnaz, Jean Pierre Le Ny and John A. Osborn
Laboratoire de Chimie des Métaux de Transition et de Catalyse, U.M.R. 7513 au C.N.R.S., Institut Le Bel,
Université Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France
Received 9 November 1999; accepted 15 December 1999
Abstract
Efficient isomerization of allylic alcohols and allylic ethers at room temperature by rhenium (VII) oxo complexes
is described. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: rhenium; catalysis; rearrangements; isomerization.
The 1,3-transposition of a hydroxy group on an allyl moiety using a metal oxo catalyst is an important
transformation in organic synthesis. The isomerization of allyl alcohols can be catalyzed at high
temperatures (ca. 130–200°C) by complexes such as VO(OR) or WO(OR) , or at room temperature by
3
4
t
1–3
VO(acac) or MoO (acac) when activated by Me SiOOSiMe or MoO X (X=Cl, O Bu). Recently,
2
2
2
3
3
2
2
we have reported rhenium(VII) catalysts that are very active at room temperature or below for this
4
process. The use of these catalysts leads to an equilibrium between the substrate and the product. We
would like to determine whether the isomerization of alcohols can be extended to other substrates and
whether this equilibrium is then modified. Therefore, we decided to apply the isomerization reaction to
allyl silyl ethers. We report here some reaction studies using the trioxorhenium complexes ReO (OSiR )
3
3
5
,6
(
R=Me, Ph) as catalysts and the trialkylsilyl allylic ethers as substrates (Scheme 1).
Wilkinson et al. reported that ReO (OSiMe ) can react with the protected alcohol ROSiMe (R=Me
3
3
3
t
7
or R= Bu) to give the corresponding complex ReO (OR) in good yield. We found that this reaction can
3
be applied to the catalytic isomerization of trialkylsilyl allyl ethers. For example, using ReO (OSiMe )
3
3
as a catalyst in dichloromethane at 25°C, 100 equivalents of trans-1-(trimethylsiloxy)hex-2-ene were
Scheme 1.
∗
Corresponding author. E-mail: osborn@chimie.u-strasbg.fr (J. A. Osborn)
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02352-7
tetl 16280

1
550
Fig. 1. Conversion of trans-1-(trimethylsiloxy)hex-2-ene into 3-(trimethylsiloxy)hex-1-ene in CH
2
Cl
2
at 25°C,
−3
[
ReO
3
(OSiMe
3
)]=4.4×10 M, 100 and 50 equivalents of substrate
Table 1
2 2 3 3
Equilibrium of different allylic substrates in CH Cl at room temperature, catalyst: ReO (OSiMe )

1
551
isomerized at an initial rate of 8 turnovers per minute, reaching equilibrium in less than 60 minutes
(
(
Fig. 1). If a coordinating solvent like acetonitrile or THF was used, the rate of the reaction was reduced
respectively, 0.8 and 0.4 maximum turnovers per minute vs 8 in CH Cl ). When only 50 equivalents
2
2
of the substrate were used, we observed an initiation period at the beginning of the reaction, and
approximately 2.5 hours were required to reach equilibrium. The analogous triphenylsiloxy derivative
ReO (OSiPh ) or rhenium oxide Re O also catalyzed the reaction. With ReO (OSiPh ) and 100
3
3
2
7
3
3
equivalents of substrate, a small induction period was needed and a maximum rate of 3.5 turnovers
per minute was measured. With Re O , and in the same conditions, the rate of the reaction could not be
2
7
quantified because Re O is not soluble in dichloromethane.
2
7
Table 1 shows the result of the isomerization of different substrates in dichloromethane at room
temperature with ReO (OSiMe ) as catalyst. In each case, when the alcohol was replaced by the
3
3
trimethylsiloxy group, the proportion of primary allylic ether increased by about 15% at equilibrium.
For instance, the equilibrium with hexenol as substrate was composed of 40% of the primary alcohol
(hex-2-en-1-ol) and 60% of the secondary alcohol (hex-1-en-3-ol) (entry 2). If the hydroxy was replaced
by a trimethylsiloxy group, the ratio was inverted (55% of the primary ether and 45% of the secondary
ether).
The equilibrium between the two isomers could be displaced by simply varying the substituents on
the silicon. When the triethylsiloxy group was used, the ratio between the primary and the secondary
hexenyl ethers was 62:38, but the catalytic process was very slow (with 100 equivalents of substrate,
several hours were required to reach equilibrium) (entry 2). Curiously, with the tert-butyldimethylsiloxy
group the ratio was not improved (57% of the primary ether and 43% of the secondary ether).
Fig. 2. Proposed mechanism of the rhenium-catalyzed isomerization of allyl silyl ethers

1
552
We propose a mechanism for the isomerization process which is inspired by the one by Chabardes et
al.1,8 The first step is an exchange of the trialkylsiloxy ligand or tetraoxorhenium unit of the precursor
with the substrate to form the active species (Fig. 2, step I).9,10 A migration of the allyl group to a metal
oxo unit then takes place (step II). The product is obtained after an exchange step with the substrate (step
III). The low reaction rate in coordinating solvents (vide supra) could be explained by the competition
for coordination of the Lewis-acidic rhenium center between the substrate and the solvent in step III.
17
Furthermore, the exchange between ReO (OSiMe ) and ROSiMe has recently been evidenced by
NMR studies.
O
3
3
3
1
1
In conclusion, we have shown that the reactivity of a silylated alcohol toward a high oxidation state
transition metal can be applied towards an efficient catalytic isomerization process. This process may be
applied in the future to other kinds of allyl ethers, and to organic synthesis in general.
Acknowledgements
We thank Dr. L. Barloy, Dr M.-T. Youinou and Professor G. C. Fu for critical reading of the manuscript
and the C.N.R.S. for funding.
References
1
2
3
. (a) Chabardes, P.; Kuntz, E.; Varagnat, J. Tetrahedron 1977, 33, 1775. (b) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis,
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5
, 57–64.
9
. The reaction of high-valent metal halides (or alkoxides) with a silylated alcohol ROSiMe to form M-OR and XSiMe is a
3
3
well-established preparative method to introduce these functionalities on a metal atom. See: Chisholm, M. H.; Rothwell, I.
P. Comprehensive Coordination Chemistry; Pergamon: Oxford, 1987; Vol. 2, p. 343.
1
1
0. The initiation period observed when 50 equiv. were used could be explained by a slower formation of the active species due
to a higher order in substrate in step I.
1. Herrmann, W. A.; Wojtczack, W. A.; Artus, G. R.; Kühn, F. E.; Mattner, M. R. Inorg. Chem. 1997, 36, 465–471.