Cationic rhodium(I) complex-catalyzed cotrimerization of propargyl esters and arylacetylenes leading to substituted dihydropentalenes

Article abstract of DOI:10.1021/ol1025266

It has been established that a cationic rhodium(I)/cod complex catalyzes the cotrimerization of propargyl esters and arylacetylenes, leading to substituted dihydropentalenes, in the presence of excess cod through elimination of carboxylic acids.

Full text of DOI:10.1021/ol1025266

ORGANIC
LETTERS
2010
Vol. 12, No. 23
5596-5599
Cationic Rhodium(I) Complex-Catalyzed
Cotrimerization of Propargyl Esters and
Arylacetylenes Leading to Substituted
Dihydropentalenes
Yu Shibata,† Keiichi Noguchi,‡ and Ken Tanaka*,†
Department of Applied Chemistry, Graduate School of Engineering, and
Instrumentation Analysis Center, Tokyo UniVersity of Agriculture and Technology,
Koganei, Tokyo 184-8588, Japan
tanaka-k@cc.tuat.ac.jp
Received October 19, 2010
ABSTRACT
It has been established that a cationic rhodium(I)/cod complex catalyzes the cotrimerization of propargyl esters and arylacetylenes, leading
to substituted dihydropentalenes, in the presence of excess cod through elimination of carboxylic acids.
Transition-metal carbenoids are known to be valuable key
intermediates for various synthetic transformations, such as
cycloaddition reactions, olefin metathesis, reactions via
reactive ylides, and insertion reactions into σ bonds.1 To
avoid the use of hazardous diazo compounds as metal
carbenoid precursors, facile methods for generation of
transition-metal carbenoids from alkynes have been devel-
oped.2-8 In most cases, intramolecular attack of a nucleo-
phile on the alkyne carbon through transition-metal-catalyzed
activation of the alkyne triple bond affords a (vinyl)metal
species that is subsequently transformed into a metal car-
benoid.3 Although a large number of such intramolecular
reactions have been reported,3 intermolecular variants using
external nucleophiles have been reported in a limited
number.4-7 For example, Iwasawa, Kusama, and co-workers
reported the platinum(II)-catalyzed cycloaddition of allenes
(3) For recent reviews of catalyses via metal carbenoids generated from
ꢀ-akynylcarbonyl compounds and propargyl esters, see: (a) Ohe, K.; Miki,
K. J. Synth. Org. Chem. Jpn. 2009, 67, 1161. (b) Marion, N.; Nolan, S. P.
Angew. Chem., Int. Ed. 2007, 46, 2750. (c) Gorin, D. J.; Toste, F. D. Nature
2007, 446, 395. (d) Kusama, H.; Iwasawa, N. Chem. Lett. 2006, 35, 1082.
(e) Miki, K.; Uemura, S.; Ohe, K. Chem. Lett. 2005, 34, 1068.
(4) Kusama, H.; Ebisawa, M.; Funami, H.; Iwasawa, N. J. Am. Chem.
Soc. 2009, 131, 16352.
† Department of Applied Chemistry.
‡ Instrumentation Analysis Center.
(1) For recent reviews, see: (a) Herndon, J. W. Coord. Chem. ReV. 2009,
253, 1517. (b) Wu, Y.-T.; Kurahashi, T.; De Meijere, A. J. Organomet.
Chem. 2005, 690, 5900. (c) Do¨tz, K. H.; Koch, A.; Werner, M. In Handbook
of Functionalized Organometallics; Knochel, P., Ed.; Wiley-VCH: Wein-
heim, 2005; p 451. (d) Metal Carbenes in Organic Synthesis; Do¨tz, K. H.,
Ed.; Topics in Organometallic Chemistry; Springer-Verlag: Berlin, 2004;
Vol. 13. (e) Zaragoza Do¨rwald, F. Metal Carbenes in Organic Synthesis;
Wiley-VHC: Weinheim, 1998. (f) Doyle, M. P.; McKervey, M. A.; Ye, T.
Modern Catalytic Methods for Organic Synthesis with Diazo Compounds:
From Cyclopropanes to Ylides; Wiley-Interscience: New York, 1998.
(2) For recent reviews of catalyses via metal carbenoids generated from
R,ω-enynes, see: (a) Lee, S. I.; Chatani, N. Chem. Commun. 2009, 371. (b)
Jime´nez-Nu´n˜ez, E.; Echavarren, A. M. Chem. ReV. 2008, 108, 3326. (c)
(5) (a) Ye, L.; Cui, L.; Zhang, G.; Zhang, L. J. Am. Chem. Soc. 2010,
132, 3258. (b) Ye, L.; He, W.; Zhang, L. J. Am. Chem. Soc. 2010, 132,
8550. (c) Lu, B.; Li, C.; Zhang, L. J. Am. Chem. Soc. 2010, 132, 14070.
(6) Our research group reported the cationic rhodium(I) complex-
catalyzed asymmetric cotrimerization of acetylenedicarboxylates and alkenes
via cationic rhodium(I) carbenoids generated through ring contraction of
rhodacyclopentenes. See: Shibata, Y.; Noguchi, K.; Hirano, M.; Tanaka,
K. Org. Lett. 2008, 10, 2825.
(7) The palladium(II)-catalyzed cotrimerization of diarylacetylenes,
leading to dihydrocyclopenta[a]indenes, via palladium(II) carbenoids gener-
ated through ring contraction of palladacycloheptatrienes was reported. See
:Wu, Y.-T.; Kuo, M.-Y.; Chang, Y.-T.; Shin, C.-C.; Wu, T.-C.; Tai, C.-C.;
Cheng, T.-H.; Liu, W.-S. Angew. Chem., Int. Ed. 2008, 47, 9891.
Zhang, L.; Sun, J.; Kozmin, S. A. AdV. Synth. Catal. 2006, 348, 2271
.
10.1021/ol1025266  2010 American Chemical Society
Published on Web 11/11/2010

with alkenyl ethers via generation of platinum(II) carbenoids
through intermolecular nucleophilic attack of alkenyl ethers
on allenes.4 Recently, Zhang and co-workers reported the
gold(I)-catalyzed oxidative cyclization via generation of
gold(I) carbenoids through intermolecular nucleophilic attack
of pyridine N-oxides on alkynes.5 In this paper, we describe
the cationic rhodium(I)/cod complex-catalyzed cotrimeriza-
tion of propargyl esters and arylacetylenes, leading to
substituted dihydropentalenes,9 presumably via generation
of a cationic rhodium(I) carbenoid10,11 through intermolecu-
lar attack of a rhodium arylacetylide on the propargyl ester
followed by the reaction of another arylacetylene and
elimination of a carboxylic acid.
Scheme 2
Our research group has recently reported the cationic
rhodium(I) complex-catalyzed [3 + 2] and [2 + 1] cycload-
ditions of alkoxycarbonyl-substituted propargyl esters 1 with
electron-deficient alkynes and alkenes presumably via gen-
eration of carbonyl-stabilized cationic rhodium(I) vinylcar-
benoids A through the intramolecular 1,2-acyloxy rearrange-
ment (Scheme 1).8
generate (vinyl)rhodium intermediate B. Subsequent elimina-
tion of a carboxylic acid would generate carbonyl-stabilized
cationic rhodium(I) vinylcarbenoid C, which reacts with an
acrylamide derivative to give a trisubstituted cyclopropane.
We first examined the reaction of methoxycarbonyl-
substituted propargyl ester 1a, phenylacetylene (2a), and N,N-
dimethylacrylamide (3) at 40 °C in CH2Cl2 in the presence
of 10 mol % of [Rh(cod)2]BF4, which was effective for the
reaction shown in Scheme 1. However, an unexpected
cotrimerization product between 1a and 2a, dihydropentalene
4aa, was obtained in low yield instead of the expected
cyclopropanation product (Scheme 3).
Scheme 1
Scheme 3
On the other hand, it is well-known that the cationic
rhodium(I) complex is able to catalyze the cross-dimerization
reactions between terminal alkynes and internal alkynes.12
In these reactions, the cationic rhodium(I) complex first reacts
with the terminal alkyne to generate the corresponding
rhodium acetylide, which subsequently reacts with the
internal alkyne to generate a (vinyl)rhodium intermediate.12
Thus we designed a new method for the generation of the
cationic rhodium(I) carbenoids as shown in Scheme 2.
Intermolecular nucleophilic attack of the rhodium arylacetyl-
ide on alkoxycarbonyl-substituted propargyl ester 1 would
We then focused our attention on the synthesis of
dihydropentalene 4aa and examined the reaction conditions
in the absence of acrylamide 3 to improve the yield of 4aa
(Table 1). However, the reaction of 1a and 2a in the absence
of 3 did not furnish 4aa at all (entry 1). It was anticipated
that coordination of 3 to the cationic rhodium plays an
(10) For examples of catalyses via rhodium(I) carbenoids, see: (a)
Nishimura, T.; Maeda, Y.; Hayashi, T. Angew. Chem., Int. Ed. 2010, 49,
7324. (b) Ref.8a (c) Ref 6. (d) Barluenga, J.; Vicente, R.; Lo´pez, L. A.;
Toma´s, M. J. Am. Chem. Soc. 2006, 128, 7050. (e) Barluenga, J.; Vicente,
R.; Lo´pez, L. A.; Toma´s, M. J. Organomet. Chem. 2006, 691, 5642. (f)
Barluenga, J.; Vicente, R.; Lo´pez, L. A.; Toma´s, M. Tetrahedron 2005,
61, 11327. (g) Barluenga, J.; Vicente, R.; Lo´pez, L. A.; Rubio, E.; Toma´s,
(8) (a) Shibata, Y.; Noguchi, K.; Tanaka, K. J. Am. Chem. Soc. 2010,
132, 7896. For examples of catalyses via metal carbenoids through the
intramolecular 1,2-acyloxy rearrangement of propargyl esters, see: (b)
Rautenstrauch, V. Tetrahedron Lett. 1984, 25, 3845. (c) Rautenstrauch, V.
J. Org. Chem. 1984, 49, 950. (d) Mainetti, E.; Mourie`s, V.; Fensterbank,
L.; Malacria, M.; Marco-Contelles, J. Angew. Chem., Int. Ed. 2002, 41,
2132. (e) Miki, K.; Ohe, K.; Uemura, S. Tetrahedron Lett. 2003, 44, 2019.
(f) Miki, K.; Ohe, K.; Uemura, S. J. Org. Chem. 2003, 68, 8505. (g)
Johansson, M. J.; Gorin, D. J.; Staben, S. T.; Toste, F. D. J. Am. Chem.
Soc. 2005, 127, 18002.
´
M.; Alvarez-Ru´a, C. J. Am. Chem. Soc. 2004, 126, 470.
(11) For reviews of catalyses via rhodium(II) carbenoids, see: (a) Davies,
H. M. L.; Walji, A. M. In Modern Rhodium-Catalyzed Organic Reactions;
Evans, P. A., Tsuji, J., Eds.; Wiley-VCH: Weinheim, Germany, 2005; p
301. (b) Doyle, M. P. In Modern Rhodium-Catalyzed Organic Reactions;
Evans, P. A., Tsuji, J., Eds.; Wiley-VCH: Weinheim, Germany, 2005; p
341.
(12) For selected recent examples, see :(a) Matsuyama, N.; Hirano, K.;
Satoh, T.; Miura, M. J. Org. Chem. 2009, 74, 3576. (b) Ito, J.; Kitase, M.;
Nishiyama, H. Organometallics 2007, 26, 6412. (c) Nishimura, T.; Guo,
X.-X.; Ohnishi, K.; Hayashi, T. AdV. Synth. Catal. 2007, 349, 2669. (d)
Katagiri, T.; Tsurugi, H.; Funayama, A.; Satoh, T.; Miura, M. Chem. Lett.
2007, 36, 830.
(9) Recent examples of the efficient synthesis of substituted dihydro-
pentalenes, see: (a) Gotoh, H.; Ogino, H.; Ishikawa, H.; Hayashi, Y.
Tetrahedron 2010, 66, 4894. (b) Hong, B.-C.; Shr, Y.-J.; Wu, J.-L.; Gupta,
A. K.; Lin, K.-J. Org. Lett. 2002, 4, 2249. (c) Ref 7.
Org. Lett., Vol. 12, No. 23, 2010
5597

Table 1. Optimization of Reaction Conditionsa
Table 2. Rhodium-Catalyzed Cotrimerization of Propargyl
Esters 1a-f and Terminal Alkynes 2a-fa
catalyst
(mol %
metal)
2a
yield
entry
catalyst
additive solvent (equiv) (%)a
1
2
3
4
5
6
7
8
9
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(nbd)2]BF4
[Rh(cod)2]BF4/
BINAP
[Rh(cod)2]SbF6
[RhOAc(cod)]2
[RhCl(cod)]2
10
10
10
10
10
10
10
10
10
-
-
-
-
-
-
-
-
-
CH2Cl2
toluene
Et2O
DME
dioxane
THF
acetone
CH3CN
THF
2
2
2
2
2
2
2
2
2
0
<5
5
15
14
20
7
<5
<5
10
11
12
13
10
10
10
10
-
-
-
-
THF
THF
THF
THF
2
2
2
2
5
<5
0
0
[RhCl(cod)]2/
14
15
16
17
18
2AgBF4
10
10
10
10
20
20
-
THF
2
2
5
2
5
5
11
30
33
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
cod (5 equiv) THF
cod (5 equiv) THF
cod (5 equiv) THF
cod (5 equiv) THF
cod (5 equiv) THF
38
59b
42b
19c [Rh(cod)2]BF4
[IrCl(cod)]2/
20
2AgBF4
20
cod (5 equiv) THF
5
0
a NMR yield. b Isolated yield. c At 25 °C.
important role to promote the formation of 4aa. Thus, the
effect of solvents, possessing diverse coordination ability,
on the present catalysis was investigated (entries 2-8), which
revealed that moderately coordinative solvents were effective
(entries 3-7) and the use of THF furnished 4aa in the highest
yield (entry 6). The effect of ligands and counteranions of
the rhodium catalysts was also examined (entries 9-14). The
use of nbd (norbornadiene) or BINAP as a ligand signifi-
cantly lowered the product yield (entries 9 and 10). The use
of SbF6 as a counteranion also significantly lowered the
product yield (entry 11), and neutral rhodium(I) complexes
failed to catalyze this reaction (entries 12 and 13). In situ
generation of the cationic rhodium(I)/monocod complex,
[Rh(cod)]BF4, by mixing [Rh(cod)Cl]2 and AgBF4 resulted
in low yield (entry 14). On the contrary, the addition of
excess cod increased the yield of 4aa to 30% (entry 15). In
this reaction, oligomerization of 2a proceeded as a major
side reaction. Therefore, increasing the amount of 2a to 5
equiv was attempted, while only a slight increase of the yield
was observed (entry 16). Fortunately, the use of excess 2a
was effective when the catalyst loading was increased to 20
mol %, and the yield of 4aa was improved to 59% (entries
17 vs 18). Under these optimized reaction conditions, the
reaction proceeded even at room temperature to give 4aa in
42% yield (entry 19). Finally, we have determined that the
present dihydropentalene formation could not be catalyzed
by a cationic iridium(I)/cod complex (entry 20).
a Reactions were conducted using [Rh(cod)2]BF4 (20 mol %), cod (5
equiv), 1a-f (1 equiv), and 2a-f (5 equiv) in THF at 40 °C for 16 h. b An
unidentified dihydropentalene isomer was generated in ca. 7% yield,
although it was not isolated in a pure form.
phenylacetylene (2a, entry 1) but also various arylacetylenes
2b-e (entries 2-5) reacted with 1a to give the corresponding
dihydropentalenes in moderate yields. The structure of 4ab
was unambiguously confirmed by X-ray crystallographic
analysis (Figure 1). However, alkylacetylene 2f, possessing
the less acidic sp C-H bond, failed to react with 1a (entry
Next, the scope of this reaction was examined as shown
in Table 2. With respect to terminal alkynes, not only
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Org. Lett., Vol. 12, No. 23, 2010

Scheme 5
Scheme 6
Figure 1. ORTEP drawing of 4ab with ellipsoids at 30% prob-
ability.
6). With respect to propargyl esters, not only methyl ester
1a but also ethyl and isopropyl esters 1b,c reacted with 2a
to give the corresponding dihydropentalenes in good yields
(entries 7 and 8). As a leaving group, the benzoxy group
could be employed instead of the acetoxy group, although
the yield of 4aa decreased (entry 9). However, the methoxy
group was ineffective (entry 10). Cyclohexane derivative 1f
could also participate in this reaction to give tricyclic
dihydropentalene 4fa (entry 11).
To elucidate a reaction mechanism, deuterium labeling
experiments were conducted. The reaction of 1a and 2a-d,
possessing deuterium at the alkyne terminus, led to selective
incorporation of deuterium at the vinylic position of product
4aa-d (Scheme 4). The reaction of 1a-d6, derived from
alkyne triple bond affords intermediate G. Elimination of
acetic acid generates carbonyl-stabilized cationic rhodium(I)
carbenoid H, which inserts into the C-H bond13 of the
methyl group to give 4aa.
In conclusion, we have established that a cationic rhod-
ium(I)/cod complex catalyzes the cotrimerization of prop-
argyl esters and arylacetylenes, leading to substituted dihy-
dropentalenes, in the presence of excess cod through
elimination of carboxylic acids. Future work will focus on
further utilization of the cationic rhodium(I) carbenoids in
organic synthesis.
Scheme 4
Acknowledgment. This work was supported partly by
Grants-in-Aid for Scientific Research (Nos. 20675002 and
21•906) from MEXT, Japan. We thank Umicore for generous
support in supplying rhodium complexes.
deuterated acetone [(CD3)2CO], and 2a led to selective
incorporation of one deuterium at the benzylic position of
product 4aa-d6 (Scheme 5).
On the basis of these observations, the formation of 4aa
from 1a and 2a might proceed through the mechanism shown
in Scheme 6. Carborhodation of 1a with rhodium pheny-
lacetylide D furnishes (vinyl)rhodium intermediate E, which
reacts with another 2a to generate (dienyl)rhodium interme-
diate F. Subsequent intramolecular carborhodation to the
Supporting Information Available: Experimental pro-
cedures, compound characterization data, and an X-ray
crystallographic information file. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL1025266
(13) For a recent review of catalytic insertion of metal carbenoids into
C-H bonds, see: Davies, H. M. L.; Manning, J. R. Nature 2008, 451, 417.
Org. Lett., Vol. 12, No. 23, 2010
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