Rhodium(I)-catalyzed acylation of alkyne with acylsilane: Transformation of 5- or 6-alkynoylsilane to α-alkylidenecycloalkanone

Article abstract of DOI:10.1246/cl.2001.1210

Rhodium(I)-catalyzed intramolecular acylation of alkynyl group proceeds by the use of acylsilanes having an alkynyl moiety. 5-and 6-Alkynoylsilanes are converted to α-alkylidenecycloalkanones by treatment with [RhCl(CO)₂]₂ in the presence of acetic acid via transmetallation between the acylsilane moiety and a rhodium(I) complex.

Full text of DOI:10.1246/cl.2001.1210

1210
Chemistry Letters 2001
Rhodium(I)-Catalyzed Acylation of Alkyne with Acylsilane:
Transformation of 5- or 6-Alkynoylsilane to α-Alkylidenecycloalkanone
Motoki Yamane, Takuma Amemiya, and Koichi Narasaka*
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received September 10, 2001; CL-010891)
Rhodium(I)-catalyzed intramolecular acylation of alkynyl
Group 9 metals are frequently employed in the reactions of
alkynes such as the Pauson–Khand type reaction.8 Accordingly,
we tried the reaction of acylsilane 3a with a rhodium(I) catalyst,
[RhCl(CO)2]2. That is, when acylsilane 3a was treated with 5
mol% of [RhCl(CO)2]2, the desired product, 2-benzylidenecyclo-
pentanone 5a, was obtained in 22% yield along with recovery of
the starting material (eq 2).
group proceeds by the use of acylsilanes having an alkynyl
moiety. 5- and 6-Alkynoylsilanes are converted to α -
alkylidenecycloalkanones by treatment with [RhCl(CO)2]2 in
the presence of acetic acid via transmetallation between the
acylsilane moiety and a rhodium(I) complex.
We previously reported palladium(0)-catalyzed disilylation
of alkyne with disilyl ketone 1, in which the oxidative addition
of the carbonyl carbon–silicon bond of 1 toward a low valent
palladium complex was proposed as the key step of the catalyt-
ic cycle (Scheme 1).1
The product 5a seemed to be generated via cleavage of the
carbonyl carbon–silicon bond by the rhodium catalyst. In this
reaction, none of the acyl-silylation product 6 was detected, and
dimethylphenylsilyl group of the consumed acylsilane 3a was
recovered as disiloxane 7 after work up. These facts indicated a
mechanism depicted in Scheme 2. After the coordination of the
alkynyl moiety to the rhodium center, transmetallation takes
place to give acyl rhodium(I) species 8.9 Then intramolecular
acyl-rhodation of alkynyl group proceeds to generate vinylrhodi-
um(I) intermediate 9. The cyclized product 5a seems to be
formed by protonation of vinyl rhodium(I) species 9 at the last
step of the catalytic cycle with a small amount of water contami-
nant in the solvent.
In the above catalytic reaction, carbonyl group of disilyl
ketone 1 was not introduced to alkyne because of the rapid elimi-
nation of carbon monoxide from unstable silylcarbonyl-
palladium(II) species 2. Although it was expected that catalytic
acylation might be achieved by employing acylsilane in stead of
disilyl ketone, simple acylsilanes such as acetylsilane could not
be used for the metal-catalyzed acylation because of its low reac-
tivity toward the metal catalyst. The neighboring effect of coor-
dination groups is widely known in the activation of
carbon–hydrogen bonds2 and carbon–carbon bonds3 with transi-
tion-metal complexes. We, therefore, examined intramolecular
acylation of alkynoylsilane, expecting that the coordination of the
alkynyl moiety might accelerate the activation of carbonyl car-
bon–silicon bond.4,5
5-Alkynoylsilane 3a was synthesized by Katritzky’s method
with a slight modification of the last hydrolysis step.6 Treatment
of acylsilane 3a with Pd(PPh3)4, however, resulted in the recov-
ery of the starting material. Only when bis(diphenylphosphino)-
butane (dppb) was added as the ligand of palladium catalysts (eq
1), the isomerization product, cyclic vinyl ether 4, was obtained,
whereas acylation of the alkynyl moiety could not be detected.7
Because it was considered that the last protonation step
might be accelerated by the addition of protic acids, the reactions
were investigated in the presence of some protic acids (Table 1).
The strong acid such as p-toluenesulfonic acid gave only
cation cyclization10 product 10 in 40% yield (Entry 1). The yield
of 2-benzylidenecyclopentanone (5a) was increased in the pres-
ence of carboxylic acids such as trifluoroacetic acid and acetic
acid (Entries 2–5). Particularly, when the acylation was carried
out at 70 °C in the presence of 10 molar amounts of acetic acid,
5a was obtained in 77% yield after complete consumption of
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1211
in 22% yield (Entry 3). The acylation was not observed in the
reaction of acylsilane 15 with a pentamethylene tether, and 15 was
recovered completely even after a long reaction time (Entry 4).
As mentioned above, it was found that the transmetallation
between acylsilane and rhodium(I) complex proceeds, in the case
that the acylsilanes have a coordination site at appropriate posi-
tion. This finding would provide the possibility to utilize acylsi-
lanes for transition metal-catalyzed acylation reaction.
Dedicated to Prof. Hideki Sakurai on the occasion of his
70th birthday.
References and Notes
1
H. Sakurai, M. Yamane, M. Iwata, N. Saito, and K. Narasaka, Chem.
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“Activation of Unreactive Bonds and Organic Synthesis,” ed. by S.
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2
3
4
C.-H. Jun, D.-Y. Lee, Y.-H. Kim, and H. Lee, Organometallics, 20,
2928 (2001); C.-H. Jun, H. Lee, and S.-G. Lim, J. Am. Chem. Soc.,
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Very recently palladium(0)-catalyzed reaction of acylsilane with
allylic ester was reported; Y. Obora, Y. Ogawa, Y. Imai, T.
Kawamura, and Y. Tsuji, the 47th Symposium on Organometallic
Chemistry, Japan, Nagoya, October, 2000, Abstr., No. B101.
For transition metal-catalyzed acylation reaction with acylstannane,
see: E. Shirakawa, Y. Nakao, H. Yoshida, and T. Hiyama, J. Am.
Chem. Soc., 122, 9030 (2000); E. Shirakawa, K. Yamasaki, H.
Yoshida, and T. Hiyama, J. Am. Chem. Soc., 121, 10221 (1999).
A. R. Katritzky, H. Lang, Z. Wang, and Z. Lie, J. Org. Chem., 61,
7551 (1996). Acetic acid (80% in water) was used for the last hydrol-
ysis step instead of 5 M HCl.
Cyclic vinyl ether was considered to be formed by the isomerization
of alkyne to diene as reported: D. Ma and X. Lu, Tetrahedron, 46,
6319 (1990). In fact, when acylsilane 13 was treated with a catalytic
amount of palladium diacetate and bis(diphenylphosphino)butane,
dienoylsilane 16 was obtained in quantitative yield (eq 3).
5
acylsilane 3a (Entry 5). The use of smaller amounts of acetic
acid (2 molar amounts) at higher temperature (100 °C), however,
decreased the yield of 2-benzylidenecyclopentanone (5a) to 40%
and 2-diphenylmethylidenecyclopentanone (11) was formed in
8% yield.
6
7
8
For the Rh(I)-catalyzed Pauson–Khand type reaction, see: T.
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9
This rhodium(I)-catalyzed cyclization was applied to the
intramolecular acylation of some other alkynoylsilanes (Table 2).
The cyclization of acylsilane 3b, which involves o-phenylene
tether, was converted to 1-benzylidene-2-indanone (6b) in 52%
yield with 6% of indenyl phenyl ketone 12 as a side product.
Thus, in the formation of cyclopentanone derivatives, the starting
materials 3a and 3b were consumed completely and the cyclized
products were obtained in moderate yield (Entries 1 and 2).11
The cyclization of alkynoylsilane 13 having a tetramethylene
tether required higher temperature (100 °C) and longer reaction
time (24 h), and α-benzylidenecyclohexanone (14) was obtained
10 For acid-catalyzed reaction of aldehyde with alkyne, see: A. Hayashi,
M. Yamaguchi, and M. Hirama, Synlett, 1995, 195.
11 For transition metal-catalyzed hydro-acylation of alkyne, see: K.
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