Gold-catalysed alkenyl- and arylsilylation reactions forming 1-silaindenes

Article abstract of DOI:10.1039/c1cc12457a

In the presence of gold(i)-phosphine catalysts, alkenyl- and arylsilanes undergo intramolecular cyclisation reactions onto appendant alkyne moieties to afford 1-silaindene derivatives. The reaction pathways vary depending on the substituent on silicon. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc12457a

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COMMUNICATION
Gold-catalysed alkenyl- and arylsilylation reactions forming
1-silaindeneswz
Takanori Matsuda,*a Yoshiyuki Yamaguchi,b Masanori Shigeno,b Shinya Satoa and
Masahiro Murakami*b
Received 27th April 2011, Accepted 21st June 2011
DOI: 10.1039/c1cc12457a
In the presence of gold(I)–phosphine catalysts, alkenyl- and
arylsilanes undergo intramolecular cyclisation reactions onto
appendant alkyne moieties to afford 1-silaindene derivatives. The
reaction pathways vary depending on the substituent on silicon.
Allylsilanes, alkenylsilanes and arylsilanes undergo carbo-
silylation across a carbon–carbon triple bond in the presence
of various promoters such as Lewis acids and transition metal
complexes.1 We recently reported the gold(I)-catalysed intra-
molecular trans-allylsilylation reaction forming 3-allyl-1-
silaindenes.2 Our continuing interest in the synthesis of silole
derivatives3,4 led us to extend the gold catalysis5 to alkenyl-
silylation and arylsilylation reactions. Herein, we report the
synthesis of 1-silaindene derivatives6 by the gold(I)-catalysed
Scheme 1 Gold(I)-catalysed trans-alkenylsilylation of 2a.
intramolecular alkenyl- and arylsilylation reactions.
First, substrates having an ethynyl group were examined.
When (2-ethynylphenyl)isobutenyldimethylsilane (2a) was treated
with a catalytic amount of gold(I) complex 1 bearing a bulky
biaryl phosphine ligand (t-BuXPhos) at room temperature in
dichloromethane for 2 h, intramolecular trans-alkenylsilyla-
tion occurred across the ethynyl group to afford 3-isobutenyl-
1-silaindene 3a in 73% yield,y as shown in Scheme 1.7 We
propose a possible mechanistic pathway as depicted therein for
the formation of 3a from 2a; (i) the cationic gold(I) species
of p-acidic character activates the ethynyl group to induce
intramolecular nucleophilic attack of the alkene moiety in a
6-endo fashion, resulting in the formation of the six-membered
ring intermediate A having a carbocationic centre b to silicon, (ii)
the carbocationic intermediate A undergoes skeletal rearrange-
ment to five-membered gold-stabilised carbocation B,8 (iii) the
cationic centre in B electrophilically attacks the regenerated
isobutenyl group to generate tertiary cyclopropylmethyl
cation C and (iv) the cyclopropyl ring opens with release of
the cationic gold(I) species to form 1-silaindene 3a. Thus,
intramolecular trans-alkenylsilylation is completed.
A deuterium-labelling experiment was carried out using 2a-d
which incorporated a deuterium atom at the terminal position
of the ethynyl group (eqn (1)). With the resulting 1-silaindene
3a-d, the deuterium atom was found at the 2-position, staying
on the carbon on which it originally resided. This labelling
experiment supported that the reaction of 2a proceeded via the
trans-alkenylsilylation mechanism rather than via an enyne
metathesis-type mechanism (vide infra).
ð1Þ
Cyclopentenylsilane 2b underwent the trans-alkenylsilyla-
tion reaction to afford the corresponding silaindene 3b in 50%
yield (eqn (2)).
a Department of Applied Chemistry, Tokyo University of Science,
1-3 Kagurazaka, Tokyo 162-8601, Japan.
E-mail: matta@rs.kagu.tus.ac.jp; Fax: +81 3 5261 4631
b Department of Synthetic Chemistry and Biological Chemistry,
Kyoto University, Katsura, Kyoto 615-8510, Japan.
E-mail: murakami@sbchem.kyoto-u.ac.jp; Fax: +81 75 383 2748
w This article is part of the ChemComm ‘Advances in catalytic C–C
bond formation via late transition metals’ web themed issue
z Electronic supplementary information (ESI) available: Experimental
procedures and characterisation data for new compounds. See DOI:
10.1039/c1cc12457a
ð2Þ
The reaction of a substrate having a substituted alkynyl
group was also examined (eqn (3)). In the case of alkenylsilane
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8697–8699 8697

Scheme 3 Gold(I)-catalysed 1,1-arylsilylation of 2d.
were swapped, and its reactivity was examined in comparison
with 2a (Scheme 2). When 4 was subjected to the reaction
using (Ph3P)AuNTf2 as the catalyst, different types of cyclisation
reactions took place to give two enyne metathesis-type
products 5 and 6 in 76% combined yield. The formation of
the two products is attributed to different modes of initial
cyclisation. The major product 5 having a 1-silanaphthalene
skeleton was formed via a skeletal rearrangement initiated
with 6-endo cyclisation. Alkenylgold intermediate D thus
formed rearranges to gold-stabilised cyclopropylmethyl cation
E, which undergoes a further skeletal rearrangement to give
cyclopropylgold F. Final demetallation furnishes six-membered
silacycle 5 having an isopropylidene moiety. On the other
hand, the initial cyclisation in a 5-exo fashion led to the
formation of the minor 1-silaindene product 6 via a pathway
similar to that mentioned above from D to 5.
Scheme 2 Gold(I)-catalysed skeletal rearrangement of 4.
2c possessing a hex-1-ynyl moiety, an analogous alkenylsilyla-
tion reaction occurred to produce 1-silaindene 3c in 44% yield.
ð3Þ
We then examined the gold(I)-catalysed reaction of aryl-
(2-ethynylphenyl)silanes. In marked contrast to the case of alkenyl-
silanes, phenylsilane 2d underwent exclusive 1,1-arylsilylation
Ethynyl(2-isobutenylphenyl)silane 4 is a constitutional isomer
of 2a in which the ethynyl group and the isobutenyl group
Table 1 Gold(I)-catalysed 1,1-arylsilylationa
Entry
2
7
Yieldb (%)
1
2
Ar = 2-MeC6H4 (2e)
Ar = 3,5-Me2C6H3 (2f)
Ar = 4-PhC6H4 (2g)
Ar = 4-MeOC6H4 (2h)
Ar = 4-CF3C6H4 (2i)
7e
7f
7g
7h
7i
52
64
42
24
3
4c
5
Trace
6
60
7d
22e
a
b
Unless otherwise noted, all reactions were carried out in the presence of 5 mol% of 1 in CH2Cl2 at 40 1C for 9–18 h. Isolated yield by
preparative TLC. Reaction was carried out at rt. 10 mol% of 1 was used. Purified by preparative GPC.
c
d
e
c
8698 Chem. Commun., 2011, 47, 8697–8699
This journal is The Royal Society of Chemistry 2011

in refluxing dichloromethane in the presence of 5 mol% of
1 to give 2-phenyl-1-silaindene 7d in 63% yield (Scheme 3).9
We assume that 7d was formed by the reaction initiated with
6-endo cyclisation, as is the case with 2a–c. The resulting
intermediate G undergoes skeletal rearrangement to five-
membered gold-stabilised carbocation H, which corresponds to
B in Scheme 1. Finally, the hydride rather than the phenyl
group of H shifts onto the next carbon10 with release of the
cationic gold(I) species to afford 7d. Labelled substrate 2d-d
having a deuterium atom on the terminal carbon of the
ethynyl group was prepared and the gold(I)-catalysed reaction
was carried out. A deuterium atom was found at the C(3)
position of the product to support the mechanism shown in
Scheme 3.
the residue was subjected to preparative thin-layer chromatography on
silica gel (hexane) to afford 1,1-dimethyl-3-(2-methylprop-1-enyl)-1-
silaindene (3a, 15.8 mg, 73%).
1 (a) S. H. Yeon, J. S. Han, E. Hong, Y. Do and I. N. Jung,
J. Organomet. Chem., 1995, 499, 159; (b) N. Asao, E. Yoshikawa
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E. Yoshikawa, V. Gevorgyan and Y. Yamamoto, J. Am. Chem.
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Y. Yamamoto, J. Am. Chem. Soc., 2001, 123, 10899; (e) K. Miura,
N. Fujisawa, S. Toyohara and A. Hosomi, Synlett, 2006, 1883;
(f) S. Park and D. Lee, J. Am. Chem. Soc., 2006, 128, 10664;
(g) Y. Horino, M. R. Luzung and F. D. Toste, J. Am. Chem. Soc.,
2006, 128, 11364.
2 T. Matsuda, S. Kadowaki, Y. Yamaguchi and M. Murakami,
Chem. Commun., 2008, 2744.
3 For reviews on silole derivatives, see: (a) J. Dubac, A. Laporterie
and G. Manuel, Chem. Rev., 1990, 90, 215; (b) S. Yamaguchi and
K. Tamao, J. Chem. Soc., Dalton Trans., 1998, 3693;
Other results of the intramolecular 1,1-arylsilylation reaction
are summarised in Table 1. 2-Tolyl, 3,5-xylyl and 4-biphenyl
derivatives (2e–g) gave the corresponding 2-aryl-1-silaindenes
(7e–g) in yields ranging from 42% to 64% (entries 1–3). However,
substitution with a methoxy group at the 4-position of the phenyl
ring decreased the yield of 7h to 24% (entry 4), and only a trace
amount of the product was obtained with (4-trifluoromethyl-
phenyl)silane 2i (entry 5). The reaction of triarylsilane 2j gave
the corresponding silaindene 7j in 60% yield (entry 6). A 2,20-(1,4-
phenylene)bis(1-silaindene) skeleton was constructed by the
gold(I)-catalysed reaction of 1,4-phenylenebis[(ethynylphenyl)-
silane] 2k (entry 7). On the other hand, arylsilanes equipped with
an internal alkyne moiety failed to undergo arylsilylation even at
elevated temperatures.
(c) M. Hissler, P. W. Dyer and R. Reau, Coord. Chem. Rev.,
´
2003, 244, 1; (d) S. Yamaguchi and K. Tamao, Chem. Lett., 2005,
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ch. 17, pp. 1181–1223; (g) X. Zhan, S. Barlow and S. R. Marder,
Chem. Commun., 2009, 1948.
4 For our previous reports on transition metal catalysed synthesis of
silole derivatives, see: (a) T. Matsuda, S. Kadowaki, T. Goya and
M. Murakami, Org. Lett., 2007, 9, 133; (b) T. Matsuda,
S. Kadowaki and M. Murakami, Chem. Commun., 2007, 2627;
(c) T. Matsuda, Y. Yamaguchi and M. Murakami, Synlett, 2008,
561; (d) T. Matsuda, Y. Yamaguchi, N. Ishida and M. Murakami,
Synlett, 2010, 2743. See also: (e) T. Matsuda, S. Kadowaki,
Y. Yamaguchi and M. Murakami, Org. Lett., 2010, 12, 1056.
5 For recent reviews on homogeneous gold-catalysed reactions, see:
(a) N. Bongers and N. Krause, Angew. Chem., Int. Ed., 2008,
47, 2178; (b) R. Skouta and C.-J. Li, Tetrahedron, 2008, 64, 4917;
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Chem.–Eur. J., 2008, 14, 4555; (e) Z. Li, C. Brouwer and C. He, Chem.
Rev., 2008, 108, 3239; (f) A. Arcadi, Chem. Rev., 2008, 108, 3266;
We carried out the gold(I)-catalysed reaction of 2-thienyl-
silane 2l, which exhibited an intermediary reactivity between
alkenylsilanes and arylsilanes (eqn (4)). The major product
was 3-(2-thienyl)-1-silaindene 3l (50%), which was formed via
the trans-alkenylsilylation mechanism shown in Scheme 1.
2-(2-Thienyl)-1-silaindene 7l was also isolated in 17% yield
as the minor product, which was formed via the 1,1-arylsilylation
mechanism shown in Scheme 3.
(g) E. Jimenez-Nu´ nez and A. M. Echavarren, Chem. Rev., 2008,
´
108, 3326; (h) D. J. Gorin, B. D. Sherry and F. D. Toste, Chem.
Rev., 2008, 108, 3351; (i) H. C. Chen, Tetrahedron, 2008, 64, 7847; (j)
A. S. K. Hashmi and M. Rudolph, Chem. Soc. Rev., 2008, 37, 1766;
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T. Jin, Chem. Commun., 2009, 5075; (n) A. Furstner, Chem. Soc. Rev.,
¨
2009, 38, 3208; (o) P. Belmont and E. Parker, Eur. J. Org. Chem., 2009,
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111, 1657; (t) M. Rudolph and A. S. K. Hashmi, Chem. Commun.,
2011, 47, 6536.
ð4Þ
6 For recent reports on the synthesis of 1-silaindene derivatives, see:
(a) S. Yamaguchi, C. Xu, H. Yamada and A. Wakamiya,
J. Organomet. Chem., 2005, 690, 5365; (b) L. Ilies, H. Tsuji, Y. Sato
and E. Nakamura, J. Am. Chem. Soc., 2008, 130, 4240; (c) M. Tobisu,
M. Onoe, Y. Kita and N. Chatani, J. Am. Chem. Soc., 2009, 131, 7506;
(d) L. Ilies, H. Tsuji and E. Nakamura, Org. Lett., 2009, 11, 3966.
7 Ruthenium-catalyzed skeletal rearrangement of the carbon
analogues of 2a giving 3-isobutenyl-1H-indenes has been reported.
See: R. J. Madhushaw, C.-Y. Lo, C.-W. Huang, M.-D. Su,
H.-C. Shen, S. Pal, S. I. R. Shaikh and R.-S. Liu, J. Am. Chem.
Soc., 2004, 126, 15560.
In conclusion, we have developed the gold(I)-catalysed
alkenyl- and arylsilylation reactions to synthesise 1-silaindene
derivatives. The substituent on silicon dictates the partitioning
between trans-1,2-addition and 1,1-addition pathways.
This work was supported by a Grant-in-Aid for Scientific
Research for Young Scientist (B) (No. 19750074) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan.
8 (a) G. Seidel, R. Mynott and A. Furstner, Angew. Chem., Int. Ed.,
¨
Notes and references
2009, 48, 2510; (b) D. Benitez, N. D. Shapiro, E. Tkatchouk, Y. Wang,
W. A. Goddard and F. D. Toste, Nat. Chem., 2009, 1, 482.
9 Gold complexes having JohnPhos (55%), XPhos (53%) and SPhos
(53%) also worked well, whereas almost no reaction occurred with
(Ph3P)AuNTf2.
y General procedure: to a Schlenk tube containing gold(I) complex 1
(4.8 mg, 5.3 mmol, 5 mol%) was added a solution of (2-ethynyl-
phenyl)dimethyl(2-methylprop-1-enyl)silane (2a, 21.6 mg, 0.10 mmol)
in dichloromethane (1.0 mL), and the mixture was stirred at rt for 2 h.
The reaction mixture was passed through a column of Florisils
(hexane : AcOEt = 10 : 1). After removal of the volatile materials,
10 Y. Xia, A. S. Dudnik, Y. Li and V. Gevorgyan, Org. Lett., 2010,
12, 5538.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8697–8699 8699