Rhodium-catalysed intramolecular trans-bis-silylation of alkynes to synthesise 3-silyl-1-benzosiloles

Article abstract of DOI:10.1039/c2ob25242b

Intramolecular addition of a Si-Si bond across a C-C triple bond occurs in a trans fashion in the presence of rhodium(i) catalysts. The trans-bis-silylation reaction of (2-alkynylphenyl)disilanes affords 3-silyl-1-benzosiloles.

Full text of DOI:10.1039/c2ob25242b

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Rhodium-catalysed intramolecular trans-bis-silylation of alkynes to synthesise
3-silyl-1-benzosiloles†
Takanori Matsuda* and Yoichiro Ichioka
Received 2nd February 2012, Accepted 1st March 2012
DOI: 10.1039/c2ob25242b
Intramolecular addition of a Si–Si bond across a C–C triple
bond occurs in a trans fashion in the presence of rhodium(I)
catalysts. The trans-bis-silylation reaction of (2-alkynylphe-
nyl)disilanes affords 3-silyl-1-benzosiloles.
Table 1 Bis-silylation of disilanyl ether 1a
Addition of intermetallic σ-bonds (E–E′; E, E′ = Si, B, Sn etc.)
to alkynes provides a simple and efficient route for the prep-
aration of stereodefined vicinally bismetallated alkenes, which
are useful precursors to multisubstituted alkenes.1 Usually, the
1,2-addition reaction is catalysed by group 10 metal complexes
and proceeds via a mechanism consisting of oxidative addition,
insertion and reductive elimination.2,3 As a result, cis-adducts
form stereoselectively in most cases. A few transition metal cata-
lysed bismetallation reactions afford trans-adducts as major pro-
ducts; however, these adducts usually result from cis–trans
isomerisation processes independent of the catalytic addition
process.4
Entry
Catalyst (mol%)
Conditions
2aa
3aa
—
1
2
3
[RhCl(nbd)]2 (2.5)
110 °C, 6 h
110 °C, 4 h
80 °C, 2.5 h
20%
20%
—
RhCl(PPh3)3 (5)
Pd(OAc)2–t-OcNCb (2/33)
84%
a Isolated yield. b 1,1,3,3-Tetramethylbutyl isocyanide.
Extensive studies on the catalytic synthesis of silole (silacyclo-
pentadiene) derivatives have been conducted,5 because com-
pounds possessing the silole skeleton exhibit unique properties
owing to their low-lying LUMO.6 Recently, we reported that the
intermolecular reaction of hexamethyldisilane with internal
alkynes catalysed by a rhodium(I) complex produces silole deri-
vatives.5i Subsequently, our attention was drawn to the intramo-
lecular variant of this rhodium-catalysed reaction. Herein, we
show the first example of a genuine trans-selective bis-silylation
reaction of alkynes catalysed by rhodium(I) complexes. The reac-
tion enables the synthesis of 3-silyl-1-benzosiloles from (2-alky-
nylphenyl)disilanes.
When the disilanyl ether of propargylic alcohol 1a was treated
with [RhCl(nbd)]2 (2.5 mol%, 5 mol% Rh, nbd = norborna-2,5-
diene), which is an effective catalyst for the intermolecular reac-
tion of alkynes with disilanes, in toluene at 110 °C for 6 h, 4-
silyl-2,5-dihydro-1,2-oxasilole 2a was produced in 20% yield
(Table 1, entry 1). The same reaction performed with RhCl
(PPh3)3 also resulted in the selective formation of the five-
Table 2 Rhodium-catalysed trans-bis-silylation of 4a
Entry
Catalyst
Time
Isolated yield
1
2
3
4
RhCl(PPh3)3
[RhCl(nbd)]2
[RhCl(CO)2]2
[RhCl(cod)]2
20 h
16 h
7 h
40%
51%
66%
(Low conv.)
24 h
membered ring product 2a (entry 2). Contrary to the palladium-
catalysed bis-silylation of 1 that selectively gave 3-silylmethy-
lene-1,2-oxasiletane 3a via cis-addition (entry 3),7 the present
rhodium-catalysed bis-silylation proceeded exclusively in a trans
fashion.8
As part of our ongoing studies on the synthesis of siloles by
trans-addition,5b,d,k the trans-bis-silylation reaction of (2-alky-
nylphenyl)disilanes 4 was carried out to synthesise 3-silyl-1-ben-
zosiloles. Reaction conditions were optimised using 2-[2-(p-
tolylethynyl)phenyl]disilane 4a as the substrate, and the results
are shown in Table 2. The intramolecular trans-bis-silylation of
Department of Applied Chemistry, Tokyo University of Science,
1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. E-mail: mtd@
rs.tus.ac.jp; Fax: +81 3 5261 4631
†Electronic supplementary information (ESI) available: Experimental
procedures and characterisation data for new compounds. See DOI:
10.1039/c2ob25242b
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Table 3 Synthesis of 3-silyl-1-benzosiloles 5 by rhodium-catalysed
trans-bis-silylationa
Isolated yield with
Entry
4 (R)
[RhCl(CO)2]2
RhCl(PPh3)3
1
2
3
4
5
6
7
8
9
4b (Ph)
55%
41%
36%
38%
63%d
10%b
56%
46%b
—
Scheme 1 Crossover experiment.
4c (3,5-Me2C6H3)
4d (2-MeC6H4)
4e (4-MeOC6H4)
4f (4-O2NC6H4)
4g (3-AcC6H4)
4h (5-Me-2-thienyl)
4i (Me)
64%b
23%bc
42%
55%b
54%b
59%b
37%cd
52%bc
16%c
4j (SiMe3)
4k (H)
—
—
10
a Unless otherwise noted, 2-(alkynylphenyl)disilanes (4, 0.20 mmol)
were reacted in toluene (1.0 mL) at 110 °C for 3–32 h in the presence of
rhodium catalysts (5 mol%). b 10 mol% Rh. c [RhCl(nbd)]2 was used
instead of [RhCl(CO)2]2. d Xylene, 130 °C.
4a proceeded both in the presence of RhCl(PPh3)3 and [RhCl-
(nbd)]2 to afford 3-silylbenzosilole 5a in 40% and 51% yields,
respectively (entries 1 and 2). The highest yield of 5a was
obtained when [RhCl(CO)2]2 was used as the catalyst (entry 3),
whereas the reaction was quite sluggish in the presence of [RhCl
(cod)]2 (entry 4).
A variety of (2-alkynylphenyl)disilanes 4 were converted into
3-silyl-1-benzosiloles 5 using rhodium catalysts (Table 3).9 The
intramolecular trans-bis-silylation of phenyl-, 3,5-xylyl- and o-
tolyl-substituted derivatives 4b–d, respectively, gave the corre-
sponding benzosiloles 5b–d in 23–64% yields (entries 1–3).
Substrates 4e–g bearing an electron-donating or electron-with-
drawing group on the terminal phenyl substituent were utilised
(entries 4–6), and a thienyl substituent was also tolerated (entry
7). Unlike the aryl- and heteroaryl-substituted substrates, alkyl-
and silyl-substituted alkynes 4i and 4j, respectively, and the sub-
strate bearing a terminal alkyne moiety 4k failed to yield the
desired products in the presence of RhCl(PPh3)3; however, they
gave the corresponding benzosiloles 5i–k in 16–52% yields
when [RhCl(nbd)]2 was used as the catalyst (entries 8–10).
The rhodium-catalysed trans-bis-silylation of (2-alkynylphe-
nyl)disilanes tolerated isobutyl (4l) and phenyl (4m) groups at
the terminal silicon atom (eqn (1)).
Scheme 2
catalyst to produce a four-membered silacycle 6n in 86% yield
(eqn (2)).
ð2Þ
The reaction mechanism of the present trans-bis-silylation is a
subject of speculation. In our previous study on the intermolecular
reaction of alkynes with hexamethyldisilane,5i a silylrhodium(I)
species was expected to be involved in the catalytic cycle. To
determine whether the products result from an intermolecular
mechanism involving a silylrhodium(I) species, a crossover
experiment was conducted. The rhodium(I)-catalysed reaction of
a 1 : 1 mixture of 4e and 4l afforded 5e and 5l without any
detectable scrambling (Scheme 1). These results strongly imply
that the bis-silylation reaction proceeds via an intramolecular
process rather than an intermolecular one.
ð1Þ
In contrast to (2-alkynylphenyl)disilanes 4a–l that gave ben-
zosiloles by trans-bis-silylation, ynenyldisilane 4n exclusively
underwent cis-bis-silylation in the presence of the rhodium
Unlike the bis-silylation of alkynes, hydrosilylation often
occurs in a trans fashion with various transition metal cata-
lysts.10 Crabtree claimed that trans-hydrosilylation can be
3176 | Org. Biomol. Chem., 2012, 10, 3175–3177
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considered by assuming that η2-vinyl transition metal (1-metalla-
cyclopropene) intermediates possess electrophilic carbene char-
acter and that a 1,2-silyl shift to the electron-deficient centres
readily occurs.11 Indeed, the η2-vinyl pathway can explain the
mechanism of trans-bis-silylation, albeit with a lack of exper-
imental evidence for the intermediate species (Scheme 2).
Initially, the Si–Si bond of 4 undergoes oxidative addition to a
Rh(I) complex to generate bis(silyl)rhodium(III) species A,12 and
subsequently, the C–C triple bond inserts intramolecularly into
the Rh–Si bond to form (Z)-β-silylalkenyl(silyl)rhodium(III) B
that has a four-membered ring. After isomerisation from B to η2-
vinylrhodium (1-rhodacyclopropene) species C, the dimethyl-
silylene group migrates to the electrophilic carbene carbon to
afford intermediate D, which then rearranges to another β-silyl-
alkenyl(silyl)rhodium(III) species E. Finally, reductive elimin-
Chem. Lett., 1991, 20, 245; (c) T. Ohmura, K. Oshima and M. Suginome,
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M. Murakami, Org. Lett., 2007, 9, 133; (b) T. Matsuda, S. Kadowaki and
M. Murakami, Chem. Commun., 2007, 2627; (c) T. Ohmura, K. Masuda
and M. Suginome, J. Am. Chem. Soc., 2008, 130, 1526; (d) T. Matsuda,
S. Kadowaki, Y. Yamaguchi and M. Murakami, Chem. Commun., 2008,
5744; (e) M. Shimizu, K. Mochida and T. Hiyama, Angew. Chem., Int.
Ed., 2008, 47, 9760; (f) M. Tobisu, M. Onoe, Y. Kita and N. Chatani, J.
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ation from
E furnishes 3-silyl-1-benzosilole 5 with the
regeneration of the Rh(I) catalyst.
In conclusion, we have developed an intramolecular trans-bis-
silylation of alkynes catalysed by rhodium(I) complexes, which
affords 3-silyl-1-benzosiloles with different functionalities at the
2-position. Although we have suggested a possible mechanism
for the trans-bis-silylation, further work needs to be directed
towards validating the mechanistic hypothesis.
7 (a) M. Suginome, A. Matsumoto and Y. Ito, J. Org. Chem., 1996, 61,
4884; (b) M. Suginome, A. Takama and Y. Ito, J. Am. Chem. Soc., 1998,
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8 trans-Bis-silylation failed to occur when the tether length was increased
by one carbon. For example, 2-[2-(phenylethynyl)phenoxy]disilane (1b)
led to the formation of the identical five-membered cis-bis-silylation
product 3b in both rhodium and palladium catalysts
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research for Young Scientist (B) (No. 23750115) from the Min-
istry of Education, Culture, Sports, Science and Technology,
Japan.
.
Notes and references
9 Palladium-catalyzed reaction of 4 gave four-membered ring products via
cis-bis-silylation. See ESI.†.
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This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 3175–3177 | 3177