Catalytic [2+2+1] cross-cyclotrimerization of silylacetylenes and two alkynyl esters to produce substituted silylfulvenes

Article abstract of DOI:10.1002/anie.201105517

Three become one: The cationic rhodium(I) complex [Rh(cod) ₂]BF₄ catalyzes the [2+2+1] cross-cyclotrimerization of silylacetylenes and two alkynyl esters, leading to substituted silylfulvenes (see scheme; cod=1,5-cyclooctadiene). The reductive complexation of the silylfulvene product with RhCl₃ in EtOH furnished the corresponding dinuclear electron-deficient cyclopentadienyl rhodium(III) complex.

Full text of DOI:10.1002/anie.201105517

DOI: 10.1002/anie.201105517
Fulvene Synthesis
Catalytic [2+2+1] Cross-Cyclotrimerization of Silylacetylenes and Two
Alkynyl Esters To Produce Substituted Silylfulvenes**
Yu Shibata and Ken Tanaka*
The transition-metal-catalyzed [2+2+2] cross-cyclotrimeriza-
electron-rich terminal alkynes and electron-deficient alkynyl
esters.[2a,b] In this catalyst system, the use of trimethylsilylace-
tylene and diethyl acetylenedicarboxylate furnished 2:1 and
1:2 cross-cyclotrimerization products in high combined yield
(Scheme 2).[2a,b]
tion of two or three different alkynes,[1] a transformation in
À
which three C C bonds form simultaneously, is a useful
method for the synthesis of substituted benzenes. A number
of successful examples have been reported (Scheme 1).[2] In
Scheme 2. Rhodium-catalyzed [2+2+2] cross-cyclotrimerization.
cod=1,5-cyclooctadiene.
We examined different combinations of electron-rich
terminal alkynes, electron-deficient internal alkynes, and
rhodium(I) catalysts in detail. As a result, we were pleased
to find that the [2+2+1] cross-cyclotrimerization of triiso-
propylsilylacetylene (1a) and two alkynyl esters 2a pro-
ceeded at 808C and gave substituted silylfulvene 3aa in high
yield using [Rh(cod)2]BF4 (5 mol%) as the catalyst and 1,4-
dioxane as the solvent (Scheme 3).[9]
Scheme 1. Transition-metal-catalyzed cross-cyclotrimerizations.
sharp contrast, the synthesis of substituted fulvenes by a
transition-metal-catalyzed [2+2+1] cross-cyclotrimerization
of two different alkynes, a transformation in which one C H
À
À
bond as well as three C C bonds form simultaneously, has not
been reported (Scheme 1). There have been only a few
reported examples of the transition-metal-catalyzed [2+2+1]
homo-cyclotrimerization of alkynes.[3–7] Rothwell and co-
workers reported the titanium(IV)-catalyzed homo-cyclotri-
merization of tert-butylacetylene, leading to a 1,3,6-substi-
tuted fulvene.[3] Yamamoto and co-workers reported the
palladium(II)-catalyzed homo-cyclotrimerization of terminal
alkynes, leading to 1,2,4-substituted fulvenes.[4,5] Herein, we
disclose the first catalytic [2+2+1] cross-cyclotrimerization of
two different alkynes to produce substituted silylfulvenes.[8]
Our research group previously reported that a cationic
rhodium(I)/H8-binap (2,2’-bis(diphenylphosphino)-5,5’,6,6’,-
7,7’,8,8’-octahydro-1,1’-binaphthyl) complex is a highly effec-
tive catalyst for the [2+2+2] cross-cyclotrimerization of
Scheme 3. Rhodium-catalyzed [2+2+1] cross-cyclotrimerization of 1a
and 2a.
In this rhodium(I)-catalyzed silylfulvene synthesis, the
choice of catalyst and solvent is important. The effect of the
catalyst and solvent is summarized in Table 1, starting with
the best catalyst and solvent combination with a higher
catalyst loading (10 mol%) and at a lower temperature
(608C; entry 1) than used in the initial reaction conditions
shown in Scheme 3. With regard to the catalyst, [Rh-
(cod)OAc]2 (entry 2) and an in situ generated cationic
rhodium(I)/cod complex (entry 3) catalyzed the fulvene
formation. However, the use of a cationic rhodium(I)/nbd
[*] Y. Shibata, Prof. Dr. K. Tanaka
Department of Applied Chemistry, Graduate School of Engineering
Tokyo University of Agriculture and Technology
Koganei, Tokyo 184-8588 (Japan)
E-mail: tanaka-k@cc.tuat.ac.jp
Homepage: http://www.tuat.ac.jp/~tanaka-k/
[**] This work was supported partly by Grants-in-Aid for Scientific
Research (Nos. 23105512, 20675002, and 21906) from MEXT
(Japan). We thank Umicore for generous support in supplying
rhodium complexes.
complex (entry 4),
a neutral rhodium(I)/cod complex
(entry 5), and a cationic rhodium(I)/binap (2,2’-bis(diphenyl-
phosphino)-1,1’-binaphthyl) complex (entry 6) led to poor
substrate conversions. The use of a rhodium catalyst is
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105517.
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Communications
Table 1: Effect of catalysts and solvents.[a]
Entry
Catalyst
[Rh(cod)2]BF4
[{RhOAc(cod)}2]
[{RhCl(cod)}2]/2AgBF4
[Rh(nbd)2]BF4
Solvent
Yield[b][%]
E/Z
1
2
3
4
5
6
7
8
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
DME
THF
acetone
toluene
CH3CN
86
67
40
<5
<5
0
59:41
80:20
95:5
–
–
–
[{RhCl(cod)}2]
[Rh(cod)2]BF4/rac-binap
[{IrCl(cod)}2]/2AgBF4
Pd(OAc)2
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
0
0
–
–
9
71
67
39
23
<5
0
77:23
79:21
60:40
82:18
–
10
11
12
13
14
CH2Cl2
–
[a] Catalyst (0.010 mmol), 1a (0.10 mmol), 2a (0.20 mmol), and solvent
(1.0 mL) were used. [b] Determined by 1H NMR spectroscopy.
DME=1,2-dimethoxyethane, nbd=norbornadiene, THF=tetrahydro-
furan.
essential to promote this reaction; both a cationic iridium(I)/
cod complex (entry 7) and Pd(OAc)2 (entry 8) failed to
catalyze the fulvene formation. With regard to the solvent, the
fulvene formation proceeded in moderately coordinating
solvents (entries 9–12). However, the use of a highly coordi-
nating solvent (entry 13) and a less-coordinating solvent
(entry 14) led to poor substrate conversions.
The scope of this rhodium(I)-catalyzed silylfulvene syn-
thesis is shown in Scheme 4. With respect to the alkynyl ester,
not only the methyl-substituted alkynyl ester 2a but also a
variety of primary and secondary alkyl-substituted esters (2b–
f) reacted with 1a to give the corresponding silylfulvenes in
high yields. The reactions of 1a and haloalkyl-substituted
alkynyl esters 2g and 2h also proceeded to give the
corresponding silylfulvenes in good yields. Alkynyl esters
2i–k, which have oxygen-containing functional groups (R1),
could also participate in this reaction, although the product
yields from propargyl alcohol derivatives 2i and 2j were
moderate owing to their partial decomposition during iso-
lation by chromatography on silica gel. Phenyl-substituted
alkynyl ester 2l and isopropyl 2-butynoate (2m) could be
employed, although high catalyst loadings were required.
Unfortunately, the reaction of 1a and alkynyl amide 2n
furnished the corresponding silylfulvene 3an in < 5%
yield.[10] Importantly, when alkynyl ester 2j and alkynyl
amide 2n were used, cross-dimerization products 4aj and 4an
were generated in 13% and 78% yields, respectively;[11] the
reason for this result is not clear. The scope of the
silylacetylenes was also examined. The reactions of various
bulky silylacetylenes 1b–d with 2a were attempted, but (tert-
butyldimethylsilyl)acetylene (1b) was the only silylacetylene
that could be employed for this reaction.[12]
Scheme 4. Scope of silylacetylenes and alkynyl esters (amide). [Rh-
(cod)2]BF4 (0.015 mmol), 1 (0.30 mmol), 2 (0.66 or 0.30 mmol), and
1,4-dioxane (3.0 mL) were used. The cited yields are of the isolated
products. [a] Cross-dimerization product 4aj or 4an was also isolated.
[b] 10 mol% of catalyst. [c] Yield is based on 2l. [d] 20 mol% of
catalyst.
A mechanistic proposal for this rhodium(I)-catalyzed
fulvene synthesis is shown in Scheme 5.[13] Carborhodation of
alkynyl ester 2 with rhodium acetylide A, which was
generated through the reaction of silylacetylene 1[11b–e] and
the cationic rhodium(I) complex, affords (alkenyl)rhodium B
and this compound reacts with another 2 to generate
(dienyl)rhodium C. Subsequent intramolecular carborhoda-
tion to the silyl-bound triple bond affords (fulvenyl)rhodium
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Scheme 8. Reductive complexation of 3aa with RhCl3.
reductive complexation[15,16] of 3aa with RhCl3 smoothly
proceeded in EtOH at 808C to give the corresponding
dinuclear electron-deficient cyclopentadienyl rhodium(III) 5
in high yield.[6e,f,17,18]
A dicationic rhodium(III)/Cp* complex, derived from
[{Cp*RhCl2}2], is known to be a highly effective catalyst for a
number of C H bond activation reactions.[19] To improve the
À
selectivity and reactivity, sterically tuned cyclopentadienyl
ligands have been investigated recently,[20,21] but an electroni-
cally tuned cyclopentadienyl ligand has not been investigated.
We anticipated that an in situ generated dicationic complex of
5, bearing the electron-deficient cyclopentadienyl ligand,
would show higher catalytic activity than that of
Scheme 5. A mechanistic proposal for the formation of fulvenes 3.
À
D. Protonation of intermediate D affords the cross-trimeriza-
tion product (E)-3 and regenerates the rhodium(I) catalyst.
As the cationic rhodium(I) complexes are effective catalysts
for the E/Z isomerization of conjugated carbonyl com-
pounds,[14] the isomerization of the initially formed (E)-3
would furnish (Z)-3. Alternatively, protonation of intermedi-
ate B affords cross-dimerization product 4.
Consistent with the above pathway, partially deuterated
3aa was generated in the presence of an external deuterium
source (CD3OD, Scheme 6), and the E/Z isomerization of the
isolated (E)- and (Z)-3 proceeded under the same reaction
conditions as described in Scheme 3 (Scheme 7).
[{Cp*RhCl2}2] in the electrophilic C H bond activation
reaction. Indeed, the dicationic complex derived from 5 had
significantly higher activity and selectivity than the complex
derived from the commonly employed [{Cp*RhCl2}2], in the
oxidative coupling of both electron-rich and electron-defi-
cient acetanilides, 6a and 6b, respectively, with alkyne 7, thus
complete substrate conversion and quantitative product
yields were achieved at room temperature (Scheme 9).[22]
Scheme 9. Comparison of catalytic activity between two cationic
rhodium(III) complexes, generated in situ from [{Cp*RhCl2}2] and 5.
Scheme 6. Rhodium-catalyzed [2+2+1] cross-cyclotrimerization of 1a
and 2a in the presence of CD3OD.
In conclusion, we have developed the [2+2+1] cross-
cyclotrimerization of silylacetylenes and two alkynyl esters
catalyzed by the cationic rhodium(I)/cod complex to produce
substituted silylfulvenes. The unprecedented reductive com-
plexation of the silylfulvene product with RhCl3 in EtOH
furnished the corresponding dinuclear electron-deficient
cyclopentadienyl rhodium(III) complex. This rhodium(III)
complex is a highly active and selective precatalyst for the
oxidative coupling of acetanilides and alkynes. Future work
will focus on further utilization of the novel electron-deficient
cyclopentadienyl rhodium(III) complexes in various electro-
Scheme 7. Rhodium-catalyzed E/Z isomerization of (E)- and (Z)-3aa.
À
philic C H bond activation reactions.
The E/Z mixture of silylfulvene 3aa can serve as a useful
precursor for the preparation of a h5-cyclopentadienyl
rhodium(III) complex as shown in Scheme 8. The direct
Received: August 4, 2011
Published online: September 16, 2011
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Communications
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Keywords: [2+2+1] cross-cyclotrimerization · alkynes ·
cyclopentadienyl complexes · fulvenes · rhodium
.
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dimethyl-1-butyne) with 1a.
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a
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rhodium(III)/Cp* complex could improve the catalytic activity
À
and change the reaction pathway in the oxidative C H bond
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