Rhodium(I)-catalyzed [3+2] intramolecular cycloaddition of alkylidenecyclopropane-propargylic esters

Article abstract of DOI:10.1016/j.tetlet.2011.11.084

An interesting rhodium(I)-catalyzed [3+2] intramolecular cycloaddition of alkylidenecyclopropanes 1 containing propargylic esters has been described in this context. A variety of bicyclo[3.3.0]octene derivatives were obtained in moderate to good yields un

Full text of DOI:10.1016/j.tetlet.2011.11.084

Tetrahedron Letters 53 (2012) 487–490
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Tetrahedron Letters
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Rhodium(I)-catalyzed [3+2] intramolecular cycloaddition of
alkylidenecyclopropane–propargylic esters
⇑
Di-Han Zhang, Min Shi
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An interesting rhodium(I)-catalyzed [3+2] intramolecular cycloaddition of alkylidenecyclopropanes 1
containing propargylic esters has been described in this context. A variety of bicyclo[3.3.0]octene deriv-
atives were obtained in moderate to good yields under mild conditions. The alkylidenecyclopropane-eny-
nes 2 could be also converted to the corresponding cycloadducts 3 in good yields.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 22 September 2011
Revised 3 November 2011
Accepted 4 November 2011
Available online 25 November 2011
There are a variety of methods for the generation of polycyclic
structures, but metal-catalyzed carbocyclization reactions have
proven among the most attractive for their construction.1,2 Methyl-
enecyclopropanes (MCPs) and alkylidenecyclopropanes (ACPs),
containing a coordinating double bond and a strained carbocycle,
can undergo a number of interesting metal-assisted transforma-
tions.3 Rhodium,4 nickel,5 ruthenium,6 as well as palladium,7 all
can catalyze intermolecular and intramolecular cycloaddition of al-
kene- or alkyne-tethered ACPs, constructing a variety of interesting
bicycles or tricycles, which are useful building blocks for the syn-
thesis of natural products and medicinally important substances.
In recent years, propargylic esters have received extensive atten-
tion as a special class of alkynes for their rich reactivities and easy
availabilities.8,9 Transition-metal catalyst, for example, rhodium
has been identified as an effective promoter in its transformations
to a variety of valuable substances.10
OAc
cat. Rh(I)
TsN
TsN
ð3Þ
3a
1a
Although the intermolecular rhodium(I)-catalyzed [3+2+2]
carbocyclization reaction of ACPs with activated alkynes has been
reported (Eq. 1), the corresponding intramolecular carbocycliza-
tion of ACPs has not been forthcoming and complex product mix-
tures were formed (Eq. 2). Herein, we wish to describe the first
intramolecular rhodium(I)-catalyzed [3+2] cycloaddition of ACPs 1
containing propargylic esters, for the construction of the bicy-
clo[3.3.0]octene derivatives 3 (Eq. 3). 11
4
Initial studies using alkylidenecyclopropane–propargylic ester
1a (0.2 mmol) as the substrate were aimed at determining the reac-
tion outcomes and subsequently optimizing the reaction conditions.
The results of these experiments are summarized in Table 1. We
found that an interesting cycloadduct 3a was obtained in a 36% yield
using RhCl(CO)(PPh3)2 as the catalyst (5 mol %) in toluene at 80 °C
(Table 1, entry 1). In the presence of [Rh(COD)Cl]2, RhCl(PPh3)3 or
[RhCp⁄Cl2]2, 3a could not be formed and [RhCl(CO)2]2, RhH(CO)
(PPh3)3 as well as Rh(COD)2BF4 were not effective catalysts in this
reaction (Table 1, entries 2–7). Further examination of solvent ef-
fects revealed that toluene was the solvent of choice, and other
organic solvents such as 1,2-dichloroethane (DCE), CH3NO2 or 1,4-
dioxane did not facilitate the formation of 3a (Table 1, entries
8–10). Using RhCl(CO)(PPh3)2 (10 mol %) afforded 3a in a 45% yield
(Table 1, entry 11). Decreasing the concentration of reactant to
0.025 M or 0.0125 M, 3a were obtained in a 60% yield and a 54%
yield, respectively under identical conditions (Table 1, entries 12
and 13). Carrying out the reaction under reflux (110 °C), 3a was ob-
tained in a 50% yield (Table 1, entry 14). Adding base, acid, and silver
salts did not improve the yield (Table 1, entries 15–20). Therefore,
Intermolecular
R2
R1
cat. Rh(I)
EWG
ð1Þ
ð2Þ
X
R1
X
EWG
major
R2
II
I
Intramolecular
TsN
cat. Rh(I)
messy
III
⇑
Corresponding author. Fax: +86 21 64166128.
E-mail address: mshi@mail.sioc.ac.cn (M. Shi).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.084

488
D.-H. Zhang, M. Shi / Tetrahedron Letters 53 (2012) 487–490
Table 1
Table 2
Optimization of the reaction conditions
Rhodium(I)-catalyzed [3+2] intramolecular cycloaddition of alkylidenecyclopropane–
propargylic esters 1 under optimized conditions
OAc
R2
catalyst (x mol%)
R3
additive (y equiv)
TsN
OR1
R2
TsN
toluene, 80 °C, 24 h
RhCl[CO](PPh3)2 (10 mol%)
toluene, 80 °C, 24 h
R3
X
N
X
N
3a
1a
Entrya
Catalyst (ꢀ mol %)
Additive (y equiv)
Yieldb (%) 3a
1
3
1
2
3
RhCl(CO)(PPh3)2 (5 mol %)
[Rh(COD)Cl]2 (2.5 mol %)
[Rh(CO)2]2 (2.5 mol %)
—
—
—
36
NR
15
Entrya,c Substrate 1
Product
Yieldb (%) 3
OCOCF3
4
5
6
RhCl(PPh3)3 (5 mol %)
—
—
—
—
—
—
—
—
NR
NR
12
Trace
31
Trace
10
45
60
54
50
NR
19
25
Complex
Complex
Complex
[RhCp⁄Cl2]2 (2.5 mol %)
TsN
1
36
TsN
TsN
TsN
RhH(CO)(PPh3)3 (5 mol %)
Rh(COD2)BF4 (5 mol %)
3a
3a
3d
7
1b
8c
RhCl(CO)(PPh3)2 (5 mol %)
RhCl(CO)(PPh3)2 (5 mol %)
RhCl(CO)(PPh3)2 (5 mol %)
RhCl(CO)(PPh3)2 (10 mol %)
RhCl(CO)(PPh3)2 (10 mol %)
RhCl(CO)(PPh3)2 (10 mol %)
RhCl(CO)(PPh3)2 (10 mol %)
RhCl(CO)(PPh3)2 (10 mol %)
RhCl(CO)(PPh3)2 (10 mol %)
RhCl(CO)(PPh3)2 (10 mol %)
RhCl(CO)(PPh3)2 (10 mol %)
RhCl(CO)(PPh3)2 (10 mol %)
RhCl(CO)(PPh3)2 (10 mol %)
OBz
OAc
OAc
9d
10e
11
12f
13g
14f,h
15f
16f
17f
18f
19f
20f
TsN
2
46
—
—
—
1c
K2CO3 (1.5 equiv)
DABCO (1.0 equiv)
HOAc (1.0 equiv)
AgSbF6 (0.1 equiv)
AgOTf (0.1 equiv)
AgBF4 (0.1 equiv)
TsN
3
46d
1d
1e
TsN
4
38
41
a
All reactions were carried out using 1a (0.1 mmol), additive (y equiv) in the
TsN
presence of catalyst (ꢀ mol %) in toluene (2.0 mL) otherwise specified. The reaction
3e
3f
concentration was 0.05 M.
b
OAc
Isolated yield.
In DCE.
In CH3NO2.
In 1,4-dioxane.
The reaction concentration was 0.025 M.
The reaction concentration was 0.0125 M.
At refluxing temperature.
c
TsN
d
5
e
TsN
TsN
TsN
f
1f
g
h
OAc
OAc
OAc
Br
TsN
6
41
39
Br
1g
3g
3h
the optimal reaction conditions were identified to carry out the reac-
tion in toluene at 80 °C using RhCl(CO)(PPh3)2 (10 mol %) as the
catalyst.
Cl
TsN
7
Cl
1h
We next examined the substrate generality of the reaction un-
der optimized conditions and the results are shown in Table 2.
As can be seen from Table 2, as for substrates 1b (R1 = CF3CO)
and 1c (R1 = C6H5CO), the reactions proceeded smoothly to give
the desired products 3a in a 36% yield and a 46% yield, respectively
(Table 2, entries 1 and 2). When both R2 and R3 were methyl
groups, the corresponding cycloadduct 3d was formed in a 46%
yield (Table 2, entry 3). For the cycloalkyl group substituted prop-
argylic esters 1e and 1f, the corresponding bicyclo[3.3.0]octene
derivatives 3e and 3f were obtained in 38% and 41% yields, respec-
tively (Table 2, entries 4 and 5). For various propargylic esters in
which R3 was aromatic groups, the desired [3+2] adducts could
be obtained in 39–41% yields under standard conditions (Table 2,
entries 6–8). In the case of other N-sulfonated amines (X = Bs or
Ns), the reactions also proceeded smoothly to give the desired cyc-
loadducts 3j and 3k in 56–60% yields (Table 2, entries 9 and 10). As
for the substrate 1l (R1 = H), no reaction occurred under standard
conditions (Table 2, entry 11). The product structures of 3a–3k
were determined by NMR spectroscopic analysis, mass spectrome-
try (MS), and HRMS (see Supplementary data).
TsN
Cl
8
39
60
56
TsN
BsN
3i
Cl 1i
OAc
BsN
9
3j
1j
OAc
OH
NsN
10
NsN
3k
1k
TsN
11
NR
1l
a
On the other hand, we found that alkylidenecyclopropane-eny-
ne 2 can also produce the corresponding bicyclo[3.3.0]octene
derivatives 3 in moderate to good yields, and the results are sum-
marized in Table 3. As for substrate 2a having an isopropenyl group
at the terminal of alkyne moiety, the corresponding cycloadduct 3a
was formed in a 69% yield (Table 3, entry 1). Substrate 2b having a
All reactions were carried out using
1 (0.2 mmol) in the presence of
RhCl[CO](PPh3)2 (10 mol %) in toluene (8.0 mL) at 80 °C for 24 h. The reaction
concentration was 0.025 M.
b
Isolated yield.
c
Ts = 4-toluenesulfonyl; Ns = 4-nitrobenzenesulfonyl Bs = 4-bromobenzene-
sulfonyl.
d
The products are isomers.

D.-H. Zhang, M. Shi / Tetrahedron Letters 53 (2012) 487–490
489
Table 3
cyclopentenyl group at the terminal of alkyne moiety gave the
desired product 3e in a 44% yield (Table 3, entry 2). Further exam-
ination of substrate 2c (R3 = 4-BrC6H4) revealed that the corre-
Rhodium(I)-catalyzed [3+2] intramolecular cycloaddition of alkylidenecyclopropane-
enyne 2 under optimized conditions
sponding cycloadduct 3g could be obtained in
a 50% yield
R2
R2
R3
(Table 3, entry 3). In the case of other N-sulfonated amine
(X = Bs), the reaction also proceeded smoothly to give the desired
[3+2] adduct 3j in a 76% yield (Table 3, entry 4).
In order to obtain more mechanistic information, we performed
1H NMR tracing experiments to detect the active species (see Sup-
plementary data). We confirmed that alkylidenecyclopropane-eny-
ne 2a could not be detected in the NMR tracing experiments.
Meanwhile, we also synthesized propargylic ester 4. In the control
RhCl[CO](PPh3)2 (10 mol%)
toluene, 80 °C, 18 h
X
N
R3
X
N
3
2
experiment, we found that upon treatment of
RhCl[CO](PPh3)2 (10 mol %) in toluene at 110 °C for 24 h, enyne 5
4
with
Entrya,c
Substrate 2
Product
Yieldb (%) 3
could not be formed (Eq. 4).
TsN
OAc
1
69
44
TsN
cat. Rh(I)
2a
TsN
3a
3e
TsN
toluene, 110 °C, 24 h
ð4Þ
TsN
4
5
2
2b
TsN
A plausible mechanism for the formation of these bicyclo
[3.3.0]octene derivatives 3 is tentatively outlined in Scheme 1 on
the basis of above results. Cycle L involves initial insertion of the me-
tal at the distal position of the alkylidenecyclopropane 1a to give
metallacyclobutene B, followed by isomerization to intermediate C
through a TMM-like transition state and carbometalation to afford
intermediate D.7e Reductive elimination of intermediate D provides
the final adduct 3a along with the release of HOAc. In Cycle R, prop-
argylic ester 1a produces enyne 2a in the presence of RhI complex A.
Oxidative addition into the distal bond of the cyclopropane should
afford the metallacyclobutene E, which can presumably rearrange
to intermediate F.4 Intermediate F undergoes carbometalation to
give intermediate G. Reductive elimination of intermediate G pro-
duces the corresponding cycloadduct 3a and regenerates the RhI
complex A to complete the catalytic cycle.
Br
TsN
BsN
3
4
50
76
TsN
2c
Br
3g
BsN
2d
3j
a
All reactions were carried out using
2 (0.2 mmol) in the presence of
RhCl[CO](PPh3)2 (10 mol %) in toluene (8.0 mL) at 80 °C. The reaction concentration
was 0.025 M.
b
Isolated yield.
c
Ts = 4-toluenesulfonyl; Ns = 4-nitrobenzenesulfonyl Bs = 4-bromobenzene-
sulfonyl.
OAc
TsN
1a
OAc
TsN
RhI
TsN
2a
TsN
B
RhIII
E
RhIII
OAc
RhIII
TsN
L
RhI
R
A
C
F
TsN
OAc
RhIII
RhIII
D
G
RhIII
TsN
TsN
TsN
3a
Scheme 1. A plausible reaction mechanism.

490
D.-H. Zhang, M. Shi / Tetrahedron Letters 53 (2012) 487–490
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catalyzed intramolecular [3+2] carbocyclization of alkylidenecy-
clopropane containing propargylic esters for the construction of
bicyclo[3.3.0]octene derivatives. The alkylidenecyclopropane-eny-
ne 2 could also be converted to the corresponding cycloadducts
in good yields. More detailed mechanistic investigation and further
application of this chemistry are underway in our laboratory.
Acknowledgments
We thank the Shanghai Municipal Committee of Science and
Technology (08dj1400100-2), National Basic Research Program of
China (973)-2009CB825300, and the National Natural Science
Foundation of China for financial support (21072206, 20472096,
20872162, 20672127, 20821002 and 20732008).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.11.084.
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