Rhodium-catalyzed hydrosilylation of internal alkynes with silane reagents bearing heteroatom substituent. Studies on the regio-/stereochemistry and transformation of the produced alkenylsilanes by rhodium-catalyzed conjugate addition

Article abstract of DOI:10.1002/adsc.200505289

Rhodium-catalyzed hydrosilylation of internal alkynes furnished (E)-1,2-disubstituted alkenylsilanes. The obtained alkenylsilane was subjected to reaction with α,β-unsaturated carbonyl compounds in the presence of a rhodium catalyst to undergo conjugate a

Full text of DOI:10.1002/adsc.200505289

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DOI: 10.1002/adsc.200505289
Rhodium-Catalyzed Hydrosilylation of Internal Alkynes with
Silane Reagents bearing Heteroatom Substituents. Studies on the
Regio-/Stereochemistry and Transformation of the Produced
Alkenylsilanes by Rhodium-Catalyzed Conjugate Addition
Tomoyuki Sanadaa, Tsuyoshi Katoa, Makoto Mitania, Atsunori Moria, b,*
a
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-4 4259 Nagatsuta, Yokohama 226-8503, Japan
Fax: (þ81)-45-924-5224, e-mail: amori@res.titech.ac.jp
b
Present address: Department of Applied Chemistry, Kobe University, Rokkodai, Kobe 657-8501, Japan
E-mail: amori@kobe-u.ac.jp
Received: July 23, 2005; Accepted: Octobe 27, 2005
Abstract: Rhodium-catalyzed hydrosilylation of in-
ternal alkynes furnished (E)-1,2-disubstituted alke-
nylsilanes. The obtained alkenylsilane was subjected
to reaction with a,b-unsaturated carbonyl com-
pounds in the presence of a rhodium catalyst to un-
dergo conjugate addition. One-pot hydrosilylation-
conjugate addition with a rhodium catalyst was also
performed.
tive yield. The reaction proceeded efficiently without
solvent. Worthy of note is that the reaction took place
at room temperature with a small amount of the rhodi-
um catalyst (0.025–0.5 mol %). By contrast, the similar
reaction with a platinum catalyst, (n-Bu4N)2PtCl6, did
not proceed at room temperature.
Keywords: conjugate addition; 1,2-disubstituted al-
kenylsilanes; hydrosilylation; internal alkynes; rhodi-
um
ð1Þ
Hydrosilylation of alkynes serves as an important tool
for the synthesis of alkenylsilanes, which can be trans-
formed into a variety of organic molecules by transition
metal-catalyzed carbon-carbon bond-forming reactions
with organic electrophiles.[1] We have been studying the
use of rhodium complexes as a catalyst for the hydrosi-
lylation of alkynes and found that a rhodium catalyst
was effective for the regio-and stereoselective hydrosi-
lylation of terminal alkynes.[2] Our further interest has
turned to investigation of the reaction of internal al-
kynes, which form 1,2-disubstituted alkenylsilanes.[3]
We herein report that with the use of a rhodium complex
as a catalyst the hydrosilylation of internal alkynes took
place at room temperature. Further reactions of the thus
formed 1,2-disubstituted alkenylsilanes with several
a,b-unsaturated carbonyl compounds in the presence
of a rhodium complex were also studied.
The stereochemistry of the product was found to be the
E-form, which was confirmed by treatment of 3a with
tetra-n-butylammonium fluoride (TBAF) in the pres-
ence of CuI to give stilbene (4a) (Z/E¼9:1), suggesting
[4]
ꢀ
that cis-addition of H Si took place [Eq. (2)].(4) To
confirm the stereochemistry of 3b, whose desilylation
led to the rather volatile 3-hexene, hydrosilylation of do-
decyne (1c) was carried out and the desilylation of 3c
with TBAF afforded (Z)-6-dodecene (4c) in 81% yield.
The formation of 4c was also confirmed by comparison
with the authentic sample, which was synthesized by a
Wittig reaction of hexanal and the phosphonium salt
of 1-bromohexane. The stereochemical outcome of hy-
drosilylation contrasts to that with a ruthenium catalyst
reported by Trost to induce the trans-addition.[5,6]
The reaction of diphenylethyne (1a) with triethoxysi-
lane (2) was carried out in the presence of 0.5 mol % of
[RhCl(cod)]2 (cod¼1,5-cyclooctadiene) at room tem-
perature. The corresponding alkenylsilane (3a) was ob-
tained after stirring for 3 h [Eq. (1)]. Hydrosilylation of
3-hexyne (1b) with 2 also proceeded stereoselectively to
give (E)-3-triethoxysilyl-hex-3-ene (3b) in a quantita-
ð2Þ
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51

Tomoyuki Sanada et al.
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Table 1. Rhodium-catalyzed hydrosilylation of internal alkynes.[a]
Alkyne (R)
Silane
Catalyst
Time [h]
3
Product
Yield [%]
99
Ph (1a)
HSi(OEt)3 (2)
[Rh(cod)Cl]2 (0.25)
2
2
(0.025)
[Rh(cod)Cl]2 (0.25)
24
3
3a
84
83
Et (1b)
HSiMe2Ph (5)
[Rh(cod)Cl]2 (0.25)
1
99
5
5
RhCl(PPh3)3 (0.5)
RhI(PPh3)3 (0.5)
3
3
6b
6b
94
84
[a]
Unless noted, the reaction was carried out with an internal alkyne (1.0 mmol) and silane (1.0 mmol) at room temperature
for 3 h.
Table 1 summarizes the hydrosilylation of internal al- Table 2. Rhodium-catalyzed hydrosilylation of unsymmetri-
cal internal alkynes.[a]
kynes with several silane reagents. In contrast with our
previous findings on the hydrosilylation of terminal al-
kynes, there is little advantage of the use of rhodium io-
dide concerning the stereoselectivity as well as the reac-
tivity of rhodium catalyst.[2] Although other rhodium
catalysts such as RhCl(PPh3)3 and RhI(PPh3)3 also
affected the hydrosilylation, the reactions were slightly
slower than with [RhCl(cod)]2. In addition to alkoxysi-
lane 2, dimethylphenylsilane (5) underwent the hydrosi-
lylation to give 6.
We next investigated the hydrosilylation of unsym-
metrical internal alkynes (7: R1CꢁCR2). As shown in
Table 2, several internal alkynes with different substitu-
ents were examined. It was not easy to control the regio-
selectivity with the differences of steric bulkiness. Al-
though the regiochemistry in the reaction of phenyl(tri-
methylsilyl)ethyne (7a) with 2 was highly controlled to
afford (E)-1-triethoxysilyl-1-trimethylsilyl-2-phenyle-
thene (8a) exclusively, hydrosilylation of 1-trimethylsil-
yl-1-propyne (7b) or 1-trimethylsilyl-1-octyne (7c) af-
Yield Ratio[b] of 8:9
[%]
of
(8þ9)
R1
R2
TMS
TMS
TMS
C6H5
99
100
24
60
62
42
11
0
84
40
38
58
89
n-C6H13 99
Me 99
4-MeOC6H4 n-C6H13 92[b]
C6H5
n-C6H13 86[b]
4-O2NC6H4 n-C6H13 65[b]
[a]
The reaction was carried out with 1.0 mmol of alkyne,
1.0 mmol of silane reagent, and 0.0025 mmol of
[RhCl(cod)]2 at room temperature.
[b]
1
The ratio was estimated by H NMR analysis.
Since the obtained 1,2-disubstituted alkenylsilanes
forded a mixture of regioisomers. On the other hand, possess heteroatom substituents on the silicon atom,
it should be pointed out that the regioselectivity of the these alkenylsilanes exhibit potential synthetic utility
rhodium-catalyzed hydrosilylation was strongly influ- for further transformations to various organic molecules
enced by the electronic effect of the substituent on the by transition metal-catalyzed reactions. As a representa-
aromatic ring. In the reaction of triethoxysilane (2) tive application we studied the rhodium-catalyzed con-
with the 1-aryl-1-octyne 7d bearing an electron-donat- jugate addition of 3 to a,b-unsaturated carbonyl com-
ing methoxy group the hydrosilylation afforded 8d and pounds [Eq. (3)]. Rhodium-catalyzed conjugate addi-
9d in a ratio of 62:38. The reaction with unsubstituted tion of organosilicon reagents was recently demonstrat-
7e resulted in a 42:58 ratio of 8e and 9e. By contrast, a ed by Oi and Inoue, Murata and Masuda, Li, and our
drastic change of the regioselectivity was observed group.(7) [7] However, the reaction with 1,2-disubstitut-
when an alkyne bearing an electron-withdrawing nitro ed alkenylsilanes has not been examined so far. Alkenyl-
group, e.g., 7f, was employed as the substrate. Gevorgy- silane 3b was subjected to reaction with dimethyl fuma-
an recently reported regiochemical studies on the palla- rate (10) in the presence of TBAF and 5 mol % of
dium-catalyzed hydrostannylation of unsymmetrical in- [RhCl(cod)]2 in THF. The reaction took place to yield
ternal alkynes.[6] The course of the regiochemistry was the conjugate addition product 11 in 75% yield after stir-
shown to be similar to that of the present rhodium-cata- ring at 708C for 24 h. As shown in Table 3, the reaction
lyzed hydrosilylaton.
of 3b with dimethyl maleate (12) also afforded 11 in 78%
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Rhodium-Catalyzed Hydrosilylation of Internal Alkynes
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yield. Ethyl cinnamate (13) also reacted to afford the
corresponding conjugate addition product 14 in 63%
yield. The reaction of a,b-unsaturated ketone 15 also
took place to yield 16. In addition, 1,2-diphenyl-
ethenylsilane 3a reacted with diethyl fumarate 17 to fur-
nish 18 in 61% yield.
Scheme 1.
lane was subjected to further reaction with a,b-unsatu-
rated carbonyl compounds leading to the conjugate ad-
dition products. The reactions were also carried out in
ð3Þ
A one-pot procedure for the sequential rhodium-cata- one-pot successfully with a rhodium-catalyzed hydrosi-
lyzed hydrosilylation and conjugate addition was found lylation-conjugate addition sequence.
to occur effectively. The reaction of 3-hexyne (1b) and
triethoxysilane was carried out in the presence of
5 mol % of [RhCl(cod)]2 at room temperature. After
stirring for 24 h, TBAFand diethyl fumarate were added
Experimental Section
to the resulting reaction mixture, which was then heated
at 708C and stirring was continued for further 24 h.
Overall yield of the hydrosilylation-conjugate addition
to afford 19 was 83% as shown in Scheme 1.
In conclusion, the hydrosilylation of internal alkynes
was found to proceed at room temperature with a rhodi-
um complex. The obtained 1,2-disubstituted alkenylsi-
One-Pot Hydrosilylation-Conjugate Addition of 1b
and 2
To a solution of 3-hexyne (1b, 0.17 mL, 1.5 mmol) and
[RhCl(cod)]2 (12.4 mg, 0.025 mmol) in THF (4.5 mL) was add-
ed triethoxysilane (2, 0.28 mL, 1.5 mmol) under an argon at-
mosphere. The mixture was stirred at room temperature for
24 h. Diethyl fumarate (17, 0.082 mL, 0.5 mmol), TBAF
(1.5 mL of 1 M THF solution, 1.5 mmol), and 2 mL of water
were added and the resulting mixture was heated at 708C for
24 h. After cooling to room temperature, the mixture was pour-
ed into diethyl ether (20 mL) and 1 M hydrochloric acid
(20 mL) and the two phases were separated. The aqueous layer
was extracted with diethyl ether twice and the combined organ-
ic layers were dried over anhydrous magnesium sulfate. Re-
moval of the solvent under reduced pressure left a crude oil,
which was subjected to silica gel column chromatography to af-
ford 19 as a colorless oil; overall yield: 105 mg (83%). 1H NMR
(300 MHz, CDCl3): d¼0.93 (t, J¼6.8 Hz, 3H), 0.97 (t, J¼
6.8 Hz, 3H), 1.23 (t, J¼5.7 Hz, 6H), 1.96–2.19 (m, 4H), 2.45
(dd, J¼16.8, 5.4 Hz, 1H), 2.90 (dd, J¼16.8, 10.2 Hz, 1H),
3.42 (dd, J¼10.2, 5.4 Hz, 1H), 4.10 (q, J¼5.7 Hz, 2H), 4.13
(q, J¼5.7 Hz, 2H), 5.28 (t, J¼7.2 Hz, 1H); 13C NMR
(75 MHz, CDCl3): d¼13.7, 14.3, 14.4, 21.2, 23.2, 36.6, 47.8,
60.7, 60.9, 130.3, 137.2, 172.3, 173.7; anal. calcd. for C14H24O4:
C 65.60, H 9.44; found: C 65.16, H 9.32.
Table 3. Rhodium-catalyzed conjugate addition of 1,2-disub-
stituted alkenylsilanes.[a]
Silane Substrate
Product
Yield [%]
75
3b
78
63
78
61
3a
Acknowledgements
[a]
The reaction was carried out with alkenylsilane
(1.5 mmol), a,b-unsaturated carbonyl compound
(0.5 mmol) and TBAF (1.5 mmol) with [RhCl(cod)]2
The authors thank Toagosei Co., Ltd. for kind donation of trie-
thoxysilane. This work was partially supported by a Grant-in-
Aid for Scientific Research (No. 13650915) from Ministryof Ed-
ucation, Culture, Sports, Science, and Technology, Japan.
(5 mol %) in THF (6 mL) and H2O (2 mL) at 70 8C .
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