Notes and references
1 For reviews, see: (a) N. Shindoh, Y. Takemoto and K. Takasu,
Chem.–Eur. J., 2009, 15, 12168; (b) D. E. Fogg and E. N. dos
Santos, Coord. Chem. Rev., 2004, 248, 2365; (c) J.-C. Wasilke,
S. J. Obrey, R. T. Baker and G. C. Bazan, Chem. Rev., 2005, 105,
1001; (d) Y. Yamamoto, J. Org. Chem., 2007, 72, 7817;
(e) L. F. Tietze, Chem. Rev., 1996, 96, 115; (f) A. De Meijere,
P. V. Zezschwitz and S. Brase, Acc. Chem. Res., 2005, 38, 413.
¨
Scheme 4 The deuterium experiment.
2 Some selected recent examples of auto-tandem catalysis:
(a) J.-R. Chen, C.-F. Li, X.-L. An, J.-J. Zhang, X.-Y. Zhu and
W.-J. Xiao, Angew. Chem., Int. Ed., 2008, 47, 2489; (b) H. Fuwa,
K. Noto and M. Sasaki, Org. Lett., 2010, 12, 1636;
(c) O. A. Attanasi, G. Favi, P. Filippone, F. Mantellini,
G. Moscatelli and F. R. Perrulli, Org. Lett., 2010, 12, 468;
(d) T. Toyoshima, Y. Mikano, T. Miura and M. Murakami, Org.
Lett., 2010, 12, 4584.
3 (a) N. Asao, T. Nogami, K. Takahashi and Y. Yamamoto, J. Am.
Chem. Soc., 2002, 124, 764; (b) S. Mondal, T. Nogami, N. Asao and
Y. Yamamoto, J. Org. Chem., 2003, 68, 9496; (c) S. Kamijo and
Y. Yamamoto, J. Org. Chem., 2003, 68, 4764.
4 G. B. Bajracharya, I. Nakamura and Y. Yamamoto, J. Org. Chem.,
2005, 70, 892.
5 (a) T. Jin and Y. Yamamoto, Org. Lett., 2008, 10, 3137;
(b) G. Abbiati, A. Arcadi, G. Bianchi, S. D. Giuseppe,
F. Marinelli and E. Rossi, J. Org. Chem., 2003, 68, 6959.
6 X. Feng, Z. Tan, D. Chen, Y. Shen, C.-C. Guo, J. Xiang and
C. Zhu, Tetrahedron Lett., 2008, 49, 4110.
7 For reviews on silver-catalyzed organic transformations, see:
(a) U. Halbes-Letinois, J.-M. Weibel and P. Pale, Chem. Soc.
Rev., 2007, 36, 759; (b) J.-M. Weibel, A. Blanc and P. Pale,
Chem. Rev., 2008, 108, 3149; (c) M. Alvarez-Corral, M. Munoz-
´ ´
Dorado and I. Rodrıguez-Garcıa, Chem. Rev., 2008, 108, 3174;
Scheme 5 The proposed reaction mechanism.
3-alkylidene-3H-indolium cation I.12 Cation I is trapped by
another indole to give intermediate II, where the coordination
of alkyne to silver salt promotes 5-exo–dig cyclization at the
nucleophilic C-3 position of indole to give the spirocyclic
iminium ion IV.13 The intermediate IV undergoes 1,2-shift14
to give intermediate V which undergoes aromatization by
deprotonation and protonolysis of the resulting silver–carbon
bond to produce intermediate VI and with regeneration of
silver catalyst. Finally, silver salt promotes hydroarylation of
intermediate VI with the third indole to give the final
product.15 The postulated 1,2-migration of intermediate IV is
consistent with the structure of the final products.
´
(d) M. Naodovic and H. Yamamoto, Chem. Rev., 2008, 108, 3132;
(e) N. T. Patil and Y. Yamamoto, Chem. Rev., 2008, 108, 3199.
8 So far, only one example in which silver salt plays a dual role as a s
Lewis acid and a p Lewis acid has been reported: (a) C. H. Oh,
S. Karmakar, H. Park, Y. Ahn and J. W. Kim, J. Am. Chem. Soc.,
2010, 132, 1792; Other kinds of silver promoted auto-tandem
catalysis: (b) Y. Luo, Z. Li and C.-J. Li, Org. Lett., 2005, 7, 2675;
(c) Z. Chen, X. Yang and J. Wu, Chem. Commun., 2009, 3469;
(d) M. Yu, R. Skouta, L. Zhou, H.-F. Jiang, X. Yao and C.-J. Li,
J. Org. Chem., 2009, 74, 3378.
9 (a) J. Chen and C.-M. Che, Angew. Chem., Int. Ed., 2004, 43, 4950;
(b) X.-Y. Liu, P. Ding, J.-S. Huang and C.-M. Che, Org. Lett.,
2007, 9, 2645; (c) X.-Y. Liu and C.-M. Che, Angew. Chem., Int. Ed.,
2008, 47, 3805; (d) X.-Y. Liu and C.-M. Che, Angew. Chem., Int.
Ed., 2009, 48, 2367.
Based on the proposed mechanism and the crystal structure
of cis-3j, the preference for the formation of cis-isomer from
the spiro acetylenic aldehyde 1d can be rationalized as follows:
spiro substituent occupies the down face of cyclohexene so as
to avoid steric interaction with the indole moiety (see Fig. 2).
As a result, the hydroarylation with indole prefers to occur at
the less sterically hindered up face of cyclohexene to give cis-3j.
In summary, we have demonstrated that silver salts are a
class of effective catalysts for the synthesis of highly substituted
tetrahydrocarbazoles from simple starting materials under
mild conditions. Silver(I) salt acts as a s Lewis acid and as a
p Lewis acid to catalyze two mechanistically different reactions
in a one-pot reaction.
10 Reviews: (a) J. Sapi and G. Massiot, Monoterpenoid Indole Alkaloids,
in The Chemistry of Heterocyclic Compounds, Suppl. Vol. 25, Part 4,
ed. J. E. Saxton and E. C. Taylor, Wiley, Chichester, 1994, ch. 7;
´
(b) J. Bonjoch and D. Sole, Chem. Rev., 2000, 100, 3455;
(c) H.-J. Knolker and K. R. Reddy, Chem. Rev., 2002, 102, 4303;
¨
(d) M. Somei and F. Yamada, Nat. Prod. Rep., 2005, 22, 73.
11 For silver-catalyzed C–C bond formation via the activation of
alkyne: (a) S. Porcel and A. M. Echavarren, Angew. Chem., Int.
Ed., 2007, 46, 2672; (b) J. Zhao, C. O. Hughes and F. D. Toste,
J. Am. Chem. Soc., 2006, 128, 7436.
12 (a) V. Nair, K. G. Abhilash and N. Vidya, Org. Lett., 2005, 7, 5857;
(b) V. Nair, N. Vidya and K. G. Abhilash, Tetrahedron Lett., 2006,
47, 2871; (c) J. Hao, S. Taktak, K. Aikawa, Y. Yusa, M. Hatano
and K. Mikami, Synlett, 2001, 1443.
13 (a) C. Ferrer and A. M. Echavarren, Angew. Chem., Int. Ed., 2006,
45, 1105; (b) C. Ferrer, C. H. M. Amijs and A. M. Echavarren,
Chem.–Eur. J., 2007, 13, 1358.
14 (a) M. Gruit, D. Michalik, A. Tillack and M. Beller, Angew. Chem.,
Int. Ed., 2009, 48, 7212; (b) J. D. Trzupek, D. Lee, B. M. Crowley,
V. M. Marathias and S. J. Danishefsky, J. Am. Chem. Soc., 2010,
132, 8506.
We are thankful for the financial support of The University
of Hong Kong (University Development Fund), Hong Kong
Research Grant Council (HKU 7052/07P), and the Areas of
Excellence Scheme established under the University Grants
Committee of the Hong Kong Special Administrative Region,
China (AoE/P-10/01).
15 S. W. Youn and J. I. Eom, J. Org. Chem., 2006, 71, 6705.
c
1314 Chem. Commun., 2011, 47, 1312–1314
This journal is The Royal Society of Chemistry 2011