Olefin isomerization by a ruthenium carbenoid complex. Cleavage of allyl and homoallyl groups

Article abstract of DOI:10.1016/S0040-4039(02)00141-7

Ruthenium-carbenoid catalysts such as Grubbs' complex I mediate efficiently the isomerization of β,γ-unsaturated ethers and amines to the corresponding vinyl ethers and enamines. This reaction can be useful in the deprotection of allyl and homoallyl ethers as well as amines.

Full text of DOI:10.1016/S0040-4039(02)00141-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1839–1841
Olefin isomerization by a ruthenium carbenoid complex.
Cleavage of allyl and homoallyl groups
Christine Cadot, Peter I. Dalko and Janine Cossy*
Laboratoire de Chimie Organique associe´ au CNRS, ESPCI, 10, rue Vauquelin, 750231 Paris Cedex 05, France
Received 20 November 2001; accepted 15 January 2002
Abstract—Ruthenium–carbenoid catalysts such as Grubbs’ complex I mediate efficiently the isomerization of b,g-unsaturated
ethers and amines to the corresponding vinyl ethers and enamines. This reaction can be useful in the deprotection of allyl and
homoallyl ethers as well as amines. © 2002 Elsevier Science Ltd. All rights reserved.
Various metal complexes act as efficient catalysts for
the isomerization of b,g-unsaturated oxygen and nitro-
gen containing compounds. These complexes can be
used for the isomerization and deprotection of allylic
ethers and amines.1,2 The most frequently used catalysts
for this purpose are palladium,3 iridium(I),4 ruthe-
nium(II)5,6 and rhodium complexes,7,8 including Wilkin-
son’s catalyst.7a The reaction is completed usually by
acid hydrolysis or by oxidation of the resulting enol
ether or enamine. Other methods are also known
including the use of t-BuOK9 or oxidative conditions
such as DDQ,10 SeO2,11 NBS/hw12 ozone13 or OsO4/
NaIO4 systems.14 Here, we report that a ruthenium–
carbenoid catalyst such as I (2nd generation Grubbs’
catalyst)15 is able to mediate isomerization of O- and
N-allyl as well as O-homoallyl groups to produce the
corresponding enol ethers and enamines. The resulting
enol ethers and enamines can be transformed to the
corresponding alcohols and amines by acidic work-up
(Scheme 1).
When allyl ether 1 was treated with catalyst I (3%) in
methylene chloride at room temperature for 12 h and
subsequently with an aqueous HCl solution (2N), the
corresponding alcohol 6 was obtained in 90% yield.
Similar results were obtained when 2, 3 and 5 were
treated under the same conditions. The corresponding
alcohols 7, 8 and 10 were isolated in a range of 75–95%
yield. The aryl vinyl ether 9 derived from 4 was shown
considerably more stable than the corresponding
aliphatic ones and was isolated in 78% yield when the
acidic treatment was omitted. The results are summa-
rized in Scheme 2.
The isomerization reaction (cat. I, 3–8%, rt, 12 h) also
took place when the allyl group was substituted at the
C-3 and C-2 positions as in compounds 11 and 12
which were transformed to the corresponding indanol
10 in 92 and 82% yield, respectively (Scheme 3). For
these compounds, the enol products were isolated in 30
and 78% yield, respectively, when the acidic treatment
was omitted (Scheme 3). However, when the double
bond was disubstituted at C-3, as in compound 13, the
starting material was recovered unchanged (Scheme 3).
When the allylic chain was substituted at C-1, a dra-
matic effect on the isomerization was observed as exem-
plified by ether 17 which was converted to the
corresponding homoallylic ether 18. Furthermore, alco-
hol 19a was converted to ketone 20 in 26% yield and to
dimer 21 (40% yield) which corresponds to the cross-
metathesis product. However, no isomerization was
observed when the corresponding benzyl ether 19b was
submitted to these reaction conditions and the dimer
was the only isolated product (Scheme 4).
Scheme 1.
Keywords: alkenes; isomerization; protecting groups.
* Corresponding author. Fax: +33 (0)1 40 79 46 60; e-mail:
janine.cossy@espci.fr
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00141-7

1840
C. Cadot et al. / Tetrahedron Letters 43 (2002) 1839–1841
conditions to the corresponding alcohol 24 in 63%
overall yield (Scheme 5).
The isomerization conditions were also applied to allyl-
amine derivatives. When tosyl allylamine 25 was
treated with the ruthenium catalyst I, without acidic
treatment, the dimer was formed, which corresponds to
the cross-metathesis product, in 41% yield, and the
enamine 28 in 21% yield. In the case of the benzyl
allylamine 26, the only product observed and isolated
was the amine 29 (46% yield). These results suggest that
the presence of basic nitrogen promotes rather the
isomerization than the dimerization of olefins (Scheme
6).
The isomerization may occur analogously to that of
related 16 electron Ru complexes, by hydrometallation
followed by b-elimination.6 The active catalyst is proba-
bly not complex I, but the corresponding hydrido
derivatives formed in situ under the reaction conditions.
In summary, we have shown that L2X2RuꢀCHR com-
plexes such as the Grubbs’ complex I are able to
mediate efficiently the isomerization of b,g-unsaturated
ethers and amines.16 This reaction can be useful in
preparing vinyl ethers and enamines, or to achieve the
Scheme 2.
Scheme 4.
Scheme 5.
Scheme 3.
It is worth noting that the isomerization of olefins was
not limited to allylic substrates. When the homoallylic
derivative 22 was treated with the ruthenium catalyst I,
a 1/1 mixture of Z/E isomers of vinyl ethers 23 was
obtained and subsequently transformed under acidic
Scheme 6.

C. Cadot et al. / Tetrahedron Letters 43 (2002) 1839–1841
1841
deprotection of allyl and homoallyl ethers and amines.
This process must be taken into consideration and can
explain, in certain cases, the modest or low yields of
cross-metathesis reactions as well as ring-closure
metathesis reactions.
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The Association pour la Recherche contre le Cancer
(ARC) (grant No. 7579) is gratefully acknowledged for
financial support.
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