Gold-catalysed intramolecular trans-allylsilylation of alkynes forming 3-allyl-1-silaindenes

Article abstract of DOI:10.1039/b804721a

3-Allyl-1-silaindenes are synthesised from alkynes having an allylsilane moiety by a gold-catalysed intramolecular trans-allylsilylation reaction. The Royal Society of Chemistry.

Full text of DOI:10.1039/b804721a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Gold-catalysed intramolecular trans-allylsilylation of alkynes forming
3-allyl-1-silaindenesw
Takanori Matsuda, Sho Kadowaki, Yoshiyuki Yamaguchi and
Masahiro Murakami*
Received (in Cambridge, UK) 25th March 2008, Accepted 23rd April 2008
First published as an Advance Article on the web 20th May 2008
DOI: 10.1039/b804721a
3-Allyl-1-silaindenes are synthesised from alkynes having
giving 2a in 97% yield.11z A gold complex bearing IPr12 ligand
exhibited moderate catalytic activity for the reaction.
The reaction of various sila-enynes 1 having a range of the
alkyne substituents (R) was examined using 3 as the catalyst,
and the results are listed in Table 1. Methyl, sec-alkyl,
methoxycarbonyl, alkynyl, and aryl groups were all suitable
substituents to give the corresponding 2-substituted 3-allyl-1-
silaindenes 1b–j in moderate to good yield (entries 1–9).
Higher catalyst loadings (6–12 mol%) were required to com-
plete the reaction in the case of the cyclopentyl and aryl
groups. Notably, a variety of functional groups such as a
boryl group and a thienyl group were tolerated in the gold-
catalysed reaction. Chloro- and bromoalkyne moieties could
be contained in the substituents, producing 2-halo-1-silain-
denes 2k and 2l, respectively (entries 10 and 11).13 On the other
hand, the substrates possessing a hydrogen (i.e., a terminal
alkyne), a bulky tert-butyl group, and 2-pyridyl group failed to
afford the corresponding 1-silaindenes.
an allylsilane moiety by
a gold-catalysed intramolecular
trans-allylsilylation reaction.
Enyne cycloisomerisation reactions catalysed by electrophilic
transition metal complexes have emerged as a powerful and
efficient method for the construction of complex cyclic
structures.1 Yamamoto and his co-workers reported an inter-
esting example using an alkyne tethered to an allylsilane
moiety.2 The intramolecular trans-allylsilylation reaction is
catalysed by a hafnium complex to afford carbocyclic com-
pounds (eqn (1)). Our interest in the synthesis of silole
(silacyclopentadiene) derivatives, which possess unique photo-
physical and electronic properties,3 led us to examine the
reaction of enynes linked by a slicon-containing tether (eqn
(2)). Herein, we report a gold(I)-catalysed intramolecular
trans-allylsilylation reaction of silicon-tethered enynes
furnishing silole cores.4–6
Next, structural variation in the allylsilane moiety was
investigated (Table 2). Whereas methylphenylsilylene-bridged
sila-enyne 1m provided 1-silaindene 2m in good yield (entry 1),
diethylsilylene-bridged substrate 2n was less reactive (entry 2).
Methallyl-, crotyl-, and cyclohexenylsilanes 1o–q also gave the
corresponding 1-silaindenes 2o–q (entries 3–5). The result with
1p indicated that the allylation took place regioselectively at
the g position. Reaction of allylgermane 1r occurred in a
similar manner to afford 3-allyl-1-germaindene 2r in 85%
yield (entry 6).
ð1Þ
ð2Þ
The synthesis of 1-silaindene derivatives possessing
extended p-conjugation is exemplified in eqn (3) and (4).
m-Phenylenebis(1-silaindene) 5a was obtained by two-fold
trans-allylsilylation of 4a. An analogous two-fold allylsilyla-
tion successfully proceeded with substrate 4b to afford
1,5-disila-s-indacene 5b.
Gold complexes possess a highly electrophilic character which
often works specifically on alkynes.7 Thus, we examined the
use of a gold(I) complex to catalyse a reaction of allyl-
dimethyl[2-(pent-1-ynyl)phenyl]silane (1a) (Scheme 1).8
Whereas a neutral chlorogold(I) complex was ineffective,
cationic gold complexes having a bistriflimide counter anion9
catalysed an intramolecular trans-allylsilylation reaction at
room temperature to produce 3-allyl-1-silaindene 2a.10 The
yield of 2a was significantly affected by the ligand. 2-Di-tert-
butylphosphinobiphenyl was found to be the ligand of choice,
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 Electronic supplementary information (ESI) available: Experimental
procedures and characterisation data for new compounds. See DOI:
10.1039/b804721a
Scheme 1 Gold-catalysed trans-allylsilylation of sila-enyne 1a.
ꢀc
This journal is The Royal Society of Chemistry 2008
2744 | Chem. Commun., 2008, 2744–2746

View Article Online
Table 1 Gold-catalysed trans-allylsilylation of sila-enynes 1b–la
Table 2 Reaction of 1m–r in the presence of 3a
Entry
1
1
2
Yieldb
83%
2c
53%
83%
69%
46%
Entry
1 (R)
mol% Au
2
Yieldb
1
2
3
4
5
6
7
8
1b (Me)
1c (cyclopentyl)
1d (CO2Me)
3
6
3
3
6
6
6
12
6
3
2b
2c
2d
2e
2f
2g
2h
2i
58%
71%c
89%
84%
75%e
81%
71%
77%
42%
79%
77%
1e (CRCTIPSd)
1f (4-MeC6H4)
3
f
1g (4-(pin)BC6H4 )
1h (3-MeOC6H4)
1i (2-FC6H4)
1j (5-methyl-2-thienyl)
1k (Cl)
9
2j
2k
2l
10g
11g
a
1l (Br)
3
4
Sila-enynes 1b–l (0.20 mmol) were reacted in the presence of the
catalyst 3 (3–12 mol%) in CH2Cl2 (2.5 mL) at room temperature for
b
9 h. Isolated yield by preparative TLC. 33% yield with 3 mol% of
c
d
3. TIPS = triisopropylsilyl. 13% conversion after 1 h. Pin =
e
f
g
pinacolato (–OCMe2CMe2O–). Reaction time: 2 h.
5
6
85%
ð3Þ
a
Unless otherwise noted, all reactions were carried out in the presence
b
of 3 mol% of 3 in CH2Cl2 at room temperature for 9 h. Isolated yield
c
d
by preparative TLC. 6 mol% of 3 was used. cis : trans = 93 : 7.
ð4Þ
A mechanism conceivable for the production of 2 from 1 is
shown in Scheme 2.14 Initially, the alkyne moiety of 1 acti-
vated by electrophilic Au+ (in A) induces intramolecular
addition of the pendant allyl group to produce seven-mem-
bered ring intermediate B. Subsequent ring opening accom-
plishes allylation to give alkenylgold species C, to which the
cationic silyl moiety undergoes electrophilic ring closure.
Finally, demetalation from alkylgold species D furnishes sila-
cycle 2 and regenerates Au+.15
The produced 3-allyl-1-silaindenes 2 exhibited fluorescence
upon ultraviolet excitation with 250 nm in hexane solution.
Silaindene 2a emitted fluorescence at 361 nm with quantum
efficiency of 0.07, and 5b at 408 nm with 0.33.
Scheme 2 Mechanism of gold-catalysed intramolecular trans-allyl-
silylation.
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 2744–2746 | 2745

View Article Online
In summary, we have developed a new method for the
synthesis of 3-allyl-1-silaindenes by a gold-catalysed intra-
molecular trans-allylsilylation reaction of alkynes.
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 Techno-
logy, Japan.
11863; (b) A. Furstner and P. W. Davies, J. Am. Chem. Soc., 2005,
¨
127, 15024; (c) S. Cacchi, J. Organomet. Chem., 1999, 576, 42.
6 For cycloisomerisations of 1,7-enynes, see: (a) A. Ochida, H. Ito
and M. Sawamura, J. Am. Chem. Soc., 2006, 128, 16486; (b) S.
Porcel and A. M. Echavarren, Angew. Chem., Int. Ed., 2007, 46,
2672; (c) N. Cabello, C. Rodrı
2007, 1753.
7 Reviews: (a) A. Hoffmann-Roder and N. Krause, Org. Biomol.
´
guez and A. M. Echavarren, Synlett,
¨
´
Chem., 2005, 3, 387; (b) E. Jimenez-Nu´ nez and A. M. Echavarren,
Chem. Commun., 2007, 333; (c) D. J. Gorin and F. D. Toste,
Nature, 2007, 446, 395; (d) A. Furstner and P. W. Davies, Angew.
¨
Notes and references
Chem., Int. Ed., 2007, 46, 3410; (e) Y. Yamamoto, J. Org. Chem.,
2007, 72, 7817; (f) A. S. K. Hashmi, Chem. Rev., 2007, 107, 3180.
8 A reaction in the presence of HfCl4 (CH2Cl2, 0 1C) resulted in a
complex mixture. No reaction occurred in the presence of PtCl2
(toluene, 80 1C).
´
9 N. Mezailles, L. Ricard and F. Gagosz, Org. Lett., 2005, 7, 4133
10 Siloxane 6 was formed in 23% yield in addition to 2a (61%) in the
case of (Ph3P)AuNTf2
z General procedure: to a Schlenk tube containing [2-PhC6H4P-
(t-Bu)2]AuNTf2 (3, 7.0 mg, 9.0 mmol, 3 mol%) under an argon
atmosphere was added a solution of allyldimethyl[2-(pent-1-ynyl)-
phenyl]silane (1a, 72.7 mg, 0.30 mmol) in dichloromethane
(0.37 mL). After stirring for 9 h at room temperature, the volatile
materials were removed under reduced pressure. The residue was
purified by preparative thin-layer chromatography (hexane) to give
3-allyl-1,1-dimethyl-2-propyl-1-silaindene (2a, 70.2 mg, 97%).
1 Reviews: (a) M. Mendez, V. Mamane and A. Furstner, Chem-
´
¨
tracts, 2003, 16, 397; (b) A. M. Echavarren and C. Nevado, Chem.
Soc. Rev., 2004, 33, 431; (c) C. Bruneau, Angew. Chem., Int. Ed.,
2005, 44, 2328; (d) S. Ma, S. Yu and Z. Gu, Angew. Chem., Int. Ed.,
2006, 45, 200; (e) C. Nieto-Oberhuber, S. Lopez, E. Jimenez-Nu´ nez
´ ´
11 Unlike the previous Al- and Hf-catalysed trans-allylsilylation
reaction of alkynes, the complex 3 failed to promote an inter-
molecular reaction of allyldimethyl(phenyl)silane with pent-1-ynyl-
benzene.
and A. M. Echavarren, Chem.–Eur. J., 2006, 12, 5916; (f) L.
Zhang, J. Sun and S. A. Kozmin, Adv. Synth. Catal., 2006, 348,
2271.
2 (a) K.-i. Imamura, E. Yoshikawa, V. Gevorgyan and Y. Yama-
moto, J. Am. Chem. Soc., 1998, 120, 5339. For the intermolecular
variants, see; (b) S. H. Yeon, J. S. Han, E. Hong, Y. Do and I. N.
Jung, J. Organomet. Chem., 1995, 499, 159; (c) N. Asao, E.
Yoshikawa and Y. Yamamoto, J. Org. Chem., 1996, 61, 4874;
(d) E. Yoshikawa, V. Gevorgyan, N. Asao and Y. Yamamoto, J.
Am. Chem. Soc., 1997, 119, 6781; (e) N. Asao, H. Tomeba and Y.
Yamamoto, Tetrahedron Lett., 2005, 46, 27.
12 IPr = Bis(2,6-diisopropylphenyl)imidazol-2-ylidene.
13 Dehalogenated 1-silaindene 7 (ca. 14%) was also isolated as the
side product. Performing the reaction for a longer period of time
than 2 h increased the formation of 7
3 (a) T. Matsuda, S. Kadowaki, T. Goya and M. Murakami, Org.
Lett., 2007, 9, 133; (b) T. Matsuda, S. Kadowaki and M. Mura-
kami, Chem. Commun., 2007, 2627; (c) T. Matsuda, Y. Yamaguchi
and M. Murakami, Synlett, 2008, 561.
4 For recent examples of a gold-catalysed intramolecular cis-allylsi-
lylation reaction of 4-sila-1,5-enynes, see: (a) S. Park and D. Lee, J.
Am. Chem. Soc., 2006, 128, 10664; (b) Y. Horino, M. R. Luzung
and F. D. Toste, J. Am. Chem. Soc., 2006, 128, 11364.
14 It is premature for us to argue about the mechanism in more detail.
A different mechanism can be also assumed to operate depending
on the substrates employed.
5 For related intramolecular allyl migration reactions, see: (a) A.
Furstner, F. Stelzer and H. Szillat, J. Am. Chem. Soc., 2001, 123,
15 For a related silyldeauration process, see: I. Nakamura, T. Sato,
M. Terada and Y. Yamamoto, Org. Lett., 2007, 9, 4081.
¨
ꢀc
This journal is The Royal Society of Chemistry 2008
2746 | Chem. Commun., 2008, 2744–2746