C O M M U N I C A T I O N S
Table 1. Ruthenium-Catalyzed Cyclization of 1-Ethynyl-3-ols and
cis-Enynes
a reaction intermediate, which equilibrates with its σ-allyl species
F and would ultimately generate diene d3-5 bearing a deuterium
distribution inconsistent with our observation.
Although cis-3-en-1-yne is a common and practical functional-
ity,14 cycloisomerization of this moiety into a cyclopentadiene or
related framework is unprecedented before our findings. Here we
report that TpRuPPh3(CH3CN)2PF6 implements the cycloisomer-
ization of unactivated cis-3-en-1-ynes and efficiently produces stable
cyclopentadiene and related derivatives. The mechanism of this
cyclization is proposed to involve a [1,5]- sigmatropic hydrogen
shift of ruthenium-vinylidene intermediates on the basis of
deuterium-labeling experiments.
Acknowledgment. We thank the National Science Council,
Taiwan, for supporting this work.
Supporting Information Available: NMR spectra, spectral data
2
1
of compounds 1-39, NMR spectra of H-labeled d3-3 and d3-5, H
NOE spectra of 3, 5, 37, and 39, and X-ray structural data of cyclized
products 29 and 33. This material is available free of charge via the
References
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a 10 mol % catalyst, [substrate] ) 0.15 M, benzene, 80 °C, 12 h. b Product
yields were given after separation from a silica column. c Diene 38 was
obtained in a 10:1 mixture of two isomers, and only the major isomer was
shown.
Scheme 3
(3) (a) Hudlicky, T.; Reed, J. W. In ComprehensiVe Organic Synthesis; Trost,
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Scheme 4
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(8) The 1H NOE spectra of compounds 3, 5, 37, and 39 and X-ray diffraction
studies of compounds 29 and 33 are provided in Supporting Information.
(9) For formation of metal-vinylidene intermediates using this catalyst, see:
Lian, J.-J.; Odedra, A.; Wu, C.-J.; Liu, R.-S. J. Am. Chem. Soc. 2005,
127, 4186 and our related work cited therein.
(10) Substituted cyclopentadienes readily undergo a [1,5]-hydrogen shift and
form several regioisomers. In this study, the aromatic substituent of
cyclized products tends to conjugate with diene functionality to give one
single regioisomer, which is inactive toward intramolecular [4+2]-
cycloaddition under catalytic conditions.
(11) The d1-5 and d3-5 samples were obtained at catalytic reactions at 30%
conversion level (80 °C, 3 h). In the case of sample d1-5, a loss of 23%
deuterium content is caused by the proton exchange of the alkynyl proton
of species d1-5 with residual water. This is a common phenomenon for
metal-vinylidene chemistry; see our related work.9
(12) Heating diene d3-5 in hot benzene (80 °C, 16 h) led to a 96% and 91%
recovery of this sample in the absence and presence of ruthenium catalyst,
respectively. Its Cb-H proton content was decreased to 0.55H and 0.46H,
respectively.
(13) (a) Maier, G. Angew. Chem., Int. Ed. Engl. 1967, 6, 402. (b) Tessier, P.
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Rautenstrauch, V. J. Org. Chem. 1984, 49, 950.
d3-3 relocates to the CD2 fragment of diene d3-5, and the other
deuterium is present at the Ca-carbon of d3-5. In this transformation,
the alkynyl proton of d3-3 undergoes a 1,2-shift to relocate to the
Cb-carbon of d3-5.
Scheme 4 shows a plausible mechanism to rationalize the
deuterium-labeling experiments. The 1,2-shift of the alkynyl
hydrogen of d3-3 indicates the formation of ruthenium-vinylidene
intermediate5 A, which undergoes a subsequent 1,5-sigmatropic shift
to generate ruthenahaxa-3,5-triene B. A subsequent 6π-electro-
cyclization13 of species B gives ruthenacyclohexa-2,4-diene species
C. Reductive elimination of this Ru(IV)-triene species produces
cyclopentadiene D and ultimately yields the most stable regioisomer
d3-5 via a 1,5-hydrogen shift. The deuterium distribution of d3-5
in Scheme 3 precludes an involvement of ruthenium-π-allyl E as
(14) For metal-catalyzed reactions of 3-en-1-ynes, see (a) Saito, S.; Yamamoto,
Y. Chem. ReV. 2000, 100, 2901. (b) Nieto-Oberhuber, C.; Lopez, S.;
Echavarren, A. M. J. Am. Chem. Soc. 2005, 127, 6178. (c) Saito, S.;
Ohmori, O.; Yamamoto, Y. Org. Lett. 2000, 2, 3853.
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