ORGANIC
LETTERS
2011
Vol. 13, No. 19
4992–4995
Platinum-Catalyzed Cycloisomerization of
1,4-Enynes via 1,2-Alkenyl Rearrangement
Takuma Sato,† Toshiki Onuma,† Itaru Nakamura,*,†,‡ and Masahiro Terada†,‡
Department of Chemistry and Research and Analytical Center for Giant Molecules,
Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Received June 6, 2011
ABSTRACT
The cycloisomerization of 1,4-enyne 1 in the presence of platinum(II) catalyst afforded 1,2,3-trisubstituted 1H-indene 2 and 20 in good to excellent
yields. The reaction proceeded through an unprecedented 1,2-alkenyl rearrangement that afforded a novel reaction topology of 1,4-enynes.
The cycloisomerizations of enynes catalyzed by π-electro-
philic transition metals such as platinum and gold complexes
offer unique chemo- and stereoselectivities and production
of unique structures, such as multisubstituted carbocycles
and heterocycles, via mild reaction conditions, and thus
have been indispensible for the construction of multifunctio-
nalized carbon frameworks. Several researchers have recently
expanded this cycloisomerization methodology to include
short, tethered 1,4-enynes, which exhibited unique reactivities
that differ from those of long-tethered enynes such as
1,5-, 1,6-, and 1,7-enynes.1 The pioneering work of the
gold-catalyzed cycloisomerization of 1,4-enynes was reported
in 2005 by Toste and co-workers for their synthesis of
enantioselective cyclopentenones.2a In the same year, Sarpong
and co-workers reported an efficient pentannulation of
1,4-enynes.2b In both cases, the key intermediary step was
the π-activation of alkynes followed by 1,2-acyloxy rearran-
gement, known as the Rautenstrauch rearrangement3
(Scheme 1, a). In 2006, Nolan and co-workers reported a
gold-catalyzed cycloisomerization of propargyl acetates con-
taining an aryl moiety at the propargylic position (Scheme 1,
b).2c A bicyclo[3.1.0]hexene synthesis that involves a 1,3-
acetoxy migration was reported by Gagosz and co-workers.2d
Although most reactions of 1,4-enynes involve the reactivity
of the propargylic esters, Liu and co-workers have very
recently reported the platinum-catalyzed cycloisomerization
of 1,4-enynes that do not possess propargylic ester function-
alities (Scheme 1, c).2e To the best of our knowledge, however,
direct rearrangement of a carbon substituent on alkyne
carbons of 1,4-enynes has yet to be reported.4 To achieve
this, we envisioned the rearrangement of carbon substituents
involving 1,4-enynes as substrates; specifically, a “bulky” 1,
4-enyne that favors a slipped η1-alkyne complex (Scheme 2),
which facilitates the behavior of an uncoordinated
alkyne carbon as the electrophile. Herein we report the
† Department of Chemistry.
‡ Research and Analytical Center for Giant Molecules.
(1) Selected recent reviews on platinum- and gold-catalyzed enyne
cycloisomerizations: (a) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem.
(3) Rautenstrauch, V. J. Org. Chem. 1984, 49, 950–952.
ꢀ
ꢀꢁ
Rev. 2008, 108, 3351–3378. (b) Jimenez-Nunez, E.; Echavarren, A. M.
Chem. Rev. 2008, 108, 3326–3350. (c) Hashmi, A. S. K. Chem. Rev. 2007,
107, 3180–3211. (d) Gorin, D. J.; Toste, F. D. Nature 2007, 446, 395–403.
(4) Recent examples on catalytic skeletal rearrangement involving
CꢀC bond cleavage at the propargylic position: (a) Markham, J. P.;
Staben, S. T.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 9708–9709.
ꢀ
ꢀꢁ
(e) Jimenez-Nunez, E.; Echavarren, A. M. Chem. Commun. 2007, 333–
346. (f) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006,
45, 7896–7936. (g) Zhang, L.; Sun, J.; Kozmin, S. A. Adv. Synth. Catal.
2006, 348, 2271–2296.
ꢀ
(b) Kirsch, S. F.; Binder, J. T.; Liebert, C.; Menz, H. Angew. Chem., Int.
Ed. 2006, 45, 5878. (c) Li, G.; Huang, X.; Zhang, L. J. Am. Chem. Soc.
2008, 130, 6944.
(5) WagnerꢀMeerwein type of 1,2-alkenyl rearrangement in a gold-
catalyzed reaction: Zhao, X.; Zhang, E.; Tu, Y.-Q.; Zhang, Y.-Q.; Yuan,
D.-Y.; Cao, K.; Fan, C.-A.; Zhang, F.-M. Org. Lett. 2009, 11, 4002–
4004.
(6) 1,3-Alkenyl shift in a silver-catalyzed reaction: Oh, C. H.;
Karmakar, S.; Park, H. S.; Ahn, Y. C.; Kim, J. W. J. Am. Chem. Soc.
2010, 132, 1792–1792.
(2) (a) Shi, X.; Gorin, D. J.; Toste, F. D. J. Am. Chem. Soc. 2005, 127,
5802–5803. (b) Prasad, B. A. B.; Yoshimoto, F. K.; Sarpong, R. J. Am.
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Chem. Soc. 2005, 127, 12468–12469. (c) Marion, N.; Dıez-Gonzalez, S.;
´
ꢀ
De Fremont, P.; Noble, A. R.; Nolan, S. P. Angew. Chem., Int. Ed. 2006,
45, 3647–3650. (d) Buzas, A.; Gagosz, F. J. Am. Chem. Soc. 2006, 128,
12614–12615. (e) Vasu, D.; Das, A.; Liu, R.-S. Chem. Commun. 2010, 46,
4115–4117.
r
10.1021/ol202104c
Published on Web 09/13/2011
2011 American Chemical Society