1928
J. Am. Chem. Soc. 1998, 120, 1928-1929
the alkyne part becomes electron deficient due to the electron-
withdrawing nature of W(CO)5 and intramolecular attack of the
silyl enol ether occurs to give the vinyl metallic intermediate C
or D, which is finally protonated to give the cyclopentene 27,16
(Scheme 1).
W(CO)5‚THF-Catalyzed Endo-Selective Cyclization
of ω-Acetylenic Silyl Enol Ethers
Katsuya Maeyama and Nobuharu Iwasawa*
Although transition-metal-promoted, intramolecular attack of
silyl enol ethers on alkenes or alkynes has several precedents,8
in most cases a stoichiometric amount of palladium(II)9 or
mercuric(II)10 compounds has been employed. On the other hand,
this W(CO)5‚THF-promoted reaction proceeds in the presence of
H2O, and hence, it was expected that W(CO)5‚THF would be
regenerated on protonation of the vinyl metallic intermediate C
or D. In practice, the reaction was found to proceed even with
a catalytic amount of W(CO)5‚THF. For example, treatment of
1 with 30 mol % of W(CO)5‚THF11 in the presence of 2 equiv of
H2O at room temperature for 3 days gave the same product 2 in
73% yield. Even 10 mol % of W(CO)5‚THF gave the product in
56% yield, although in this case about 40% of the starting material
1 was recovered. Reactions of representative substrates are
summarized in Table 1. Several features of the reaction are as
follows:
(1) In every case, the reaction proceeds at room temperature
in the presence of H2O or methanol to give cyclized â,γ-
unsaturated ketones in good yield without isomerization of the
double bond,12 even using only 10 mol % of W(CO)5‚THF.
(2) At present, the reaction is specific for terminal alkynes.
Although the reaction of o-ethynylacetophenone derivative 3a
proceeds cleanly to give silylated 1-naphthol derivative 4 in high
yield, the corresponding reaction of 3b containing an internal
alkyne gave no cyclized product under similar reaction conditions.
(3) The most characteristic feature of this reaction is that the
endo mode of cyclization occurs more readily than in other related
Department of Chemistry, Graduate School of Science
The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113
ReceiVed September 22, 1997
The development of new types of cyclization reaction for the
construction of carbocycles is an important focus of synthetic
organic chemistry.1 One problem requiring fresh approaches is
the achievement of reactions proceeding selectively through the
endo mode, which remains relatively unexplored compared with
exo cyclizations.1,2 It is known that terminal alkynes and carbonyl
complexes of low-valent group 6 metals such as M(CO)5‚L (M
) Cr, Mo, W; L ) THF, Et3N, etc.) are in equilibrium with their
η2-alkyne complexes and/or their vinylidene complexes.3 Al-
though these complexes are expected to have electrophilic
character, their use in synthetic reactions has been quite limited.4,5
We thought of employing a substrate having both a terminal
alkyne and a nucleophilic group in the molecule, expecting that
a formal endo mode of cyclization could be achieved via its
vinylidene complex, the central carbon of which has high
electrophilicity.4 In this paper, we report an endo-selective
cyclization of ω-acetylenic silyl enol ethers by the catalytic use
of W(CO)5‚THF or related species.
First, the reaction was examined employing silyl enol ether 1
as substrate and using a stoichiometric amount of W(CO)5‚THF.
When 1 was treated with 1.5 equiv of preformed W(CO)5‚THF6
at room temperature for 2 days, it was completely consumed and
a cyclopentene derivative 2, produced by intramolecular attack
of the silyl enol ether on the terminal alkyne in an endo manner,
was obtained in moderate yield. Other group 6 metal complexes
such as Cr(CO)5‚THF and Mo(CO)5‚THF failed to give better
results, but the reaction did proceed in better yield in the presence
of a proton source such as methanol or H2O. Thus, the
cyclopentene derivative 2 was obtained in 80% yield when the
same reaction was carried out at room temperature in the presence
of 2 equiv of H2O. At present the reaction is assumed to proceed
as follows: treatment of the silyl enol ether 1 with W(CO)5‚-
THF generates a small amount of the η2-alkyne complexes A
and/or the vinylidene complex B. When this complex is formed,
(7) We monitored the reaction by NMR by carrying it out in THF-d8. During
the course of the reaction, only the starting material and the cyclized product
were observed and none of the possible intermediates could be detected.
(8) For the reaction of transition-metal-alkene or -alkyne complexes with
nucleophiles, see: (a) McDaniel, K. F. In ComprehensiVe Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon:
Oxford, 1995; Vol. 12, p 601. (b) Hegedus, L. S. In ComprehensiVe Organic
Synthesis; Trost, B. M., Ed.; Pergamon: Oxford, 1991; Vol. 4, p 571.
(9) For the reactions of silyl enol ethers with alkenes promoted by
palladium(II) complexes, see: (a) Ito, Y.; Aoyama, H.; Hirao, T.; Mochizuki,
A.; Saegusa, T. J. Am. Chem. Soc. 1979, 101, 494. (b) Ito, Y.; Aoyama, H.;
Saegusa, T. J. Am. Chem. Soc. 1980, 102, 4519. (c) Kende, A. S.; Roth, B.;
Sanfilippo, P. J. J. Am. Chem. Soc. 1982, 104, 1784. (d) Kende, A. S.; Roth,
B.; Sanfilippo, P. J.; Blacklock, T. J. J. Am. Chem. Soc. 1982, 104, 5808.
(10) For the reactions of silyl enol ethers with alkynes promoted by
mercury(II) compounds, see: (a) Drouin, J.; Boaventura, M. A.; Conia,
J. M. J. Am. Chem. Soc. 1985, 107, 1726. (b) Huang, H.; Forsyth, C. J. J.
Org. Chem. 1995, 60, 2773. (c) Huang, H.; Forsyth, C. J. J. Org. Chem. 1995,
60, 5746. (d) Frontier, A. J.; Raghavan, S.; Danishefsky, S. J. J. Am. Chem.
Soc. 1997, 119, 6686.
(1) (a) Thebtaranonth, C.; Thebtaranonth, Y. Cyclization Reactions; CRC
Press: Boca Raton, FL, 1994. (b) Giese, B.; Kopping, B.; Go¨bel, T.; Dickhaut,
J.; Thoma, G.; Kulicke, K. J.; Trach, F. Org. React. 1996, 48, 301. (c) Grigg,
R.; Sridharan, V. In ComprehensiVe Organometallic Chemistry II; Abel, E.
W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol. 12,
p 299.
(11) Based on the amount of W(CO)6 employed.
(2) For recent examples of endo cyclization, see: (a) Albrecht, U.;
Wartchow, R.; Hoffmann, H. M. R. Angew. Chem., Int. Ed. Engl. 1992, 31,
910. (b) Hoffmann, H. M. R.; Herden, U.; Breithor, M.; Rhode, O. Tetrahedron
1997, 53, 8383. (c) Bruyere, D.; Gaignard, G.; Bouyssi, D.; Balme, G.
Tetrahedron Lett. 1997, 38, 827.
(3) (a) Landon, S. J.; Shulman, P. M.; Geoffroy, G. L. J. Am. Chem. Soc.
1985, 107, 6739. (b) Szyman´ska-Buzar, T.; Downs, A. J.; Greene, T. M.;
Marshall, A. S. J. Organomet. Chem. 1995, 495, 163.
(4) (a) McDonald, F. E.; Zhu, H. Y. H. Tetrahedron 1997, 53, 11061. (b)
Cosset, C.; Del R´ıo, I.; Pe´ron, V.; Windmu¨ller, B.; Le Bozec, H. Synlett 1996,
435. (c) Hoye, T. R.; Suriano, J. A. J. Am. Chem. Soc. 1993, 115, 1154. (d)
Parlier, A.; Rudler, H. J. Chem. Soc., Chem. Commun. 1986, 514.
(5) For the review of vinylidene complexes, see: Bruce, M. I. Chem. ReV.
1991, 91, 197. Bruce, M. I.; Swincer, A. G. AdV. Organomet. Chem. 1983,
22, 59. For synthetic use of vinylidene or η2-alkyne intermediates of other
transition metals, see: Trost, B. M. Chem. Ber. 1996, 129, 1313. Bruneau,
C.; Dixneuf, P. H. J. Chem. Soc., Chem. Commun. 1997, 507. Merlic, C. A.;
Pauly, M. E. J. Am. Chem. Soc. 1996, 118, 11319. Ohe, K.; Kojima, M.;
Yonehara, K.; Uemura, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 1823.
(6) W(CO)5‚THF was prepared just before use by irradiating a slurry of
W(CO)6 in dry THF (concentration about 0.1 mol/L) under Ar for 4 h with a
high-pressure Hg lamp.
(12) Products were isolated by SiO2 (deactivated with 10% H2O) column
chromatography without a detectable isomerization of the double bond.
(13) It has been reported that the HgCl2-promoted reaction of 5 gave no
cyclized products. See ref 10a.
(14) Conia, J. M.; Le Perchec, P. Synthesis 1975, 1.
(15) At present we have no explanation for this solvent effect. The
difference of the reaction procedure (the reaction mixture was directly irradiated
due to the instability of W(CO)5‚L (L ) Et2O or CH2Cl2)) is not the main
reason for this effect, because the reaction in THF with direct irradiation still
gave the exo-cyclized product with high selectively.
(16) To obtain information on the mechanism of this reaction, we have
carried out several D2O experiments and obtained the following results. The
reaction of 1 and 5 in the presence of 2 equiv of D2O revealed that both of
the olefinic protons are partially deuterated with the degree of deuteration
dependent on the substrate. Furthermore the reaction of 1 and 5 in the presence
of 10 equiv of D2O revealed that the degree of deuterium incorporation at the
two olefinic protons was total of about 50%. This result removes the possibility
that H-D exchange occurs rapidly during the reaction. On the basis of these
results, we at present propose that the reaction proceeds in both ways (through
η2-complex and vinylidene complex) and that the ratio of the two reaction
pathways is substrate dependent (for 1, η2-complex vs vinylidene complex
about 1:1; for 5, about 3:7 based on the deuterium incorporation).
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