W(CO)5(L)-catalyzed formal Cope rearrangement of allenyl silyl enol ethers

Article abstract of DOI:10.1021/ol0473694

(Chemical Equation Presented) On treatment of 5-siloxyhexa-1,2,5-trienes with a catalytic amount of W(CO)₆ under photoirradiation, formal Cope rearrangement proceeded to give 2-siloxyhex-1-en-5-ynes in good yield. The electrophilic activation o

Full text of DOI:10.1021/ol0473694

ORGANIC
LETTERS
2005
Vol. 7, No. 8
1445-1447
W(CO)5(L)-Catalyzed Formal Cope
Rearrangement of Allenyl Silyl Enol
Ethers
Tomoya Miura,† Koichi Kiyota, Hiroyuki Kusama, and Nobuharu Iwasawa*
Department of Chemistry, Tokyo Institute of Technology, Meguro-ku,
Tokyo 152-8551, Japan
niwasawa@chem.titech.ac.jp
Received December 22, 2004
ABSTRACT
On treatment of 5-siloxyhexa-1,2,5-trienes with a catalytic amount of W(CO)6 under photoirradiation, formal Cope rearrangement proceeded to
give 2-siloxyhex-1-en-5-ynes in good yield. The electrophilic activation of the allenyl moiety by W(CO)5 triggers the intramolecular attack of the
silyl enol ether in a 6-endo manner to produce a cyclohexenyl tungsten species. Carbon−carbon bond cleavage occurs by electron donation
from the anionic W(CO)5 into the silyloxonium moiety to afford the products with regeneration of the W(CO)5(L).
Low-valent carbonyl complexes of group 6 metals such as
M(CO)5(L) (M ) Cr, Mo, W; L ) THF, Et3N, etc.) have
emerged as useful catalysts for the electrophilic activation
of unsaturated carbon-carbon bonds, and a variety of new
reactions have been developed recently.1 In the course of
our studies on W(CO)5(L)-catalyzed cyclization reactions of
silyl enol ethers,2 we found that W(CO)5 can activate the
allenyl moiety effectively, and the endo-selective cyclization
of allenyl silyl enol ethers was found to proceed smoothly,2f
where 5-siloxyhexa-1,2,5-triene 1a gave six-membered â,γ-
unsaturated ketone 2a in 68% yield by carrying out the
reaction using a catalytic amount of W(CO)6 (0.2 equiv)
under photoirradiation in the presence of H2O (3.0 equiv)
(eq 1).3
During these studies, we found that the reaction of the
same substrate 1a in the absence of H2O under similar
conditions proceeded by a different pathway to give a formal
Cope rearrangement product, 2-siloxyhex-1-en-5-yne 3a, in
good yield. While there has been an abundant study of
thermal or transition-metal-catalyzed Cope rearrangements
of hexa-1,5-dienes,4 that of hex-1-en-5-ynes or hexa-1,2,5-
trienes has not been studied extensively despite their synthetic
potential.5-8 In this paper is described a novel W(CO)5(L)-
catalyzed formal Cope rearrangement of 5-siloxyhexa-1,2,5-
trienes leading to 2-siloxyhex-1-en-5-yne derivatives.
† Present address: Department of Synthetic Chemistry and Biological
Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan.
(1) For selected examples of catalytic reactions using low-valent group
6 metals, see: (a) Alcazar, E.; Pletcher, J. M.; McDonald, F. E. Org. Lett.
2004, 6, 3877. (b) Ohe, K.; Yokoi, T.; Miki, K.; Nishino, F.; Uemura, S.
J. Am. Chem. Soc. 2002, 124, 526. (c) Sangu, K.; Fuchibe, K.; Akiyama,
T. Org. Lett. 2004, 6, 353 and references therein.
(2) (a) Maeyama, K.; Iwasawa, N. J. Am. Chem. Soc. 1998, 120, 1928.
(b) Iwasawa, N.; Maeyama, K.; Kusama, H. Org. Lett. 2001, 3, 3871. (c)
Miura, T.; Iwasawa, N. J. Am. Chem. Soc. 2002, 124, 518. (d) Kusama,
H.; Yamabe, H.; Iwasawa, N. Org. Lett. 2002, 4, 2569. (e) Iwasawa, N.;
Miura, T.; Kiyota, K.; Kusama, H.; Lee, K.; Lee, P. H. Org. Lett. 2002, 4,
4463. (f) Miura, T.; Kiyota, K.; Kusama, H.; Lee, K.; Kim, H.; Kim, S.;
Lee, P. H.; Iwasawa, N. Org. Lett. 2003, 5, 1725. (g) Miura, T.; Murata,
H.; Kiyota, K.; Kusama, H.; Iwasawa, N. J. Mol. Cat. A 2004, 213, 59.
(3) The reaction using an equimolar amount of W(CO)6 was reported in
ref 2f. After further examinations, we found the catalytic conditions shown
in eq 1.
10.1021/ol0473694 CCC: $30.25
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When 5-siloxyhexa-1,2,5-triene 1a was treated with a
catalytic amount of W(CO)6 (0.2 equiv) in THF at 40 °C
under photoirradiation for 1 day in the absence of H2O, the
cyclized product 2a was obtained in a trace amount and
2-siloxyhex-1-en-5-yne 3a, a formal Cope rearranged prod-
uct, was obtained in 81% yield (Scheme 1).
into the silyloxonium moiety induces the carbon-carbon
bond cleavage to give 2-siloxyhex-1-en-5-yne 3a with
regeneration of W(CO)5(L) (path a). Six-membered â,γ-
unsaturated ketone 2a is obtained by the protonation of the
carbon-tungsten bond with a trace amount of H2O present
in the reaction mixture (path b).
Examinations of several reaction conditions revealed that
the reaction time was greatly diminished from 1 day to 2 h
by changing the reaction solvent from THF to toluene.
Furthermore, by the addition of a catalytic amount of
DABCO (0.1 equiv), the reaction proceeded cleanly to give
the product 3a in 90% yield as a sole product.11
Scheme 1
Under these optimized conditions, reactions of a variety
of 2-siloxyhex-1-en-5-ynes were carried out, and the results
are summarized in Table 1.
Table 1. Formal Cope Rearrangement of
5-Siloxyhexa-1,2,5-trienes 1 with a Catalytic Amount of
W(CO)6 in Toluenea
No reaction occurred when the substrate 1a was irradiated
at ambient temperature in the absence of W(CO)6,9 and thus,
activation of the substrate 1a by W(CO)5(L) is essential for
this transformation.10 The reaction pathway is proposed as
follows: Coordination of W(CO)5, generated in situ from
W(CO)6 under photoirradiation, onto the allenyl moiety gives
the allene-W(CO)5 η2-complex A. Then, intramolecular
attack of the silyl enol ether occurs on the distal carbon of
the electrophilic allene moiety to give the vinylmetallic
intermediate B. Electron donation from the W(CO)5 anion
(4) For reviews on the Cope rearrangement reaction, see: (a) Hill, R.
K. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Oxford, 1991; Vol. 5, pp 785-826. (b) Lutz, R. P. Chem.
ReV. 1984, 84, 205. (c) Overman, L. E. Angew. Chem., Int. Ed. Engl. 1984,
23, 579.
(5) For the first example, see: Black, D. K.; Landor, S. R. J. Chem.
Soc. 1965, 6784.
(6) For reviews dealing with the acetylenic or allenic Cope rearrangement,
see: (a) Viola, A.; Collins, J. J.; Filipp, N. Tetrahedron 1981, 37, 3765.
(b) Huntsman, W. D. In The Chemistry of Ketenes, Allenes and Related
Compounds; Patai, S., Ed.; J. Wiley and Sons: Chichester, 1980; Part 2,
pp 582-643.
(7) For recent representative examples, see; (a) Owens, K. A.; Berson,
J. A. J. Am. Chem. Soc. 1990, 112, 5973. (b) Black, K. A.; Wilsey, S.;
Houk, K. N. J. Am. Chem. Soc. 1998, 120, 5622. (c) Hopf, H.; Wolff, J.
Eur. J. Org. Chem. 2001, 4009 and references therein.
(8) Metal-free Cope rearrangements of hexa-1,2,5-trienes normally require
high reaction temperature (>250 °C).
(9) When the substrate 1a was heated at 250 °C in the absence of the
catalyst, a thermal Cope rearrangement gradually proceeded to give a 3:1
mixture of 1a and 3a after 4 h.
a The reaction was carried out with 1 and DABCO (0.1 equiv) in toluene
(0.1 M) in the presence of W(CO)6 (0.2 equiv) at 40 °C under photoirra-
diation, unless otherwise noted. b The reaction was carried out in toluene
(1.0 M). c syn:anti)1:1
The catalytic process worked well either with substrates
containing a trisubstituted silyl enol ether moiety or a
(10) For an example of the transition-metal-catalyzed cycloisomerizations
of allenynes, see: Cadran, N.; Cariou, K.; Herve´, G.; Aubert, C.;
Fensterbank, L.; Malacria, M.; Marco-Contelles, J. J. Am. Chem. Soc. 2004,
126, 3408 and references therein.
(11) No additive (71%). Other amines: Et3N (62%), i-Pr2NEt (62%),
n-Bu3N (72%), and DBU (66%).
1446
Org. Lett., Vol. 7, No. 8, 2005

trisubstituted allene moiety to give the corresponding prod-
ucts 3b-d in good yield (entries 2-4). In the case of
substrate 1e possessing both of these trisubstituted moieties,
the reaction gave the product 3e in moderate yield as a 1:1
mixture of syn and anti isomers (entry 5). It was noted that
the reaction in the presence of H2O gave the cyclized product
2e in 71% yield stereoselectively (Scheme 2). Thus, isomer-
ization probably occurred at the vinylmetallic intermediate
B and/or C by DABCO.
mixture of E and Z isomers14 together with a 26% yield of
silyl enol ether 6 (eq 2).
Scheme 2
In summary, we have developed a formal Cope rearrange-
ment of 5-siloxyhexa-1,2,5-trienes catalyzed by W(CO)5(L).
We can prepare two types of synthetically useful compounds,
that is, 6-endo cyclization products or the Cope rearrange-
ment products, from the same starting materials Via the same
intermediates simply by changing reaction conditions. Further
studies to expand the utility of this reaction are in progress
in our laboratory.
Acknowledgment. This research was partly supported by
a Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science, and Technology of
Japan. We also thank Central Glass Co., Ltd. for a generous
gift of TfOH. T. M. was a recipient of a Research Fellowship
of the Japan Society for the Promotion of Science for Young
Scientists.
Condition A: W(CO)6 (0.2 equiv) and DABCO (0.1 equiv) in
toluene (1.0 M) under photoirradiation at 40 °C. Condition B:
W(CO)6 (0.2 equiv) and H2O (3.0 equiv) in THF (1.0 M) under
photoirradiation at 40 °C.
Supporting Information Available: Experimental details
and spectral data for new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL0473694
Next we examined a ring-expansion reaction by formal
Cope rearrangement of a cyclic 5-siloxyhexa-1,2,5-triene.12
When cyclooctane derivative 4 was irradiated in the presence
of W(CO)6 (1.0 equiv) and DABCO (1.1 equiv) in toluene,
the reaction proceeded as expected to give the ring-expanded
12-membered cyclic product 513 in 53% yield as an 86:14
(12) For an example of a ring-expansion reaction by thermal allenyl Cope
rearrangement of hepta-1,2,6-triene, see: Vedejs, E.; Cammers-Goodwin,
A. J. Org. Chem. 1994, 59, 7541.
(13) For some examples of synthesis of cyclic alkynes, see: (a) Gordon,
D. M.; Danishefsky, S. J.; Schulte, G. K. J. Org. Chem. 1992, 57, 7052.
(b) Sugai, M.; Tanino, K.; Kuwajima, I. Synlett 1997, 461. (c) Young, D.
G. J.; Burlison, J. A.; Peters, U. J. Org. Chem. 2003, 68, 3494.
(14) The geometry of 5 was not determined.
Org. Lett., Vol. 7, No. 8, 2005
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