Synthesis of functionalized polycyclic compounds: Rhodium(I)-catalyzed intramolecular cycloaddition of yne and ene vinylidenecyclopropanes

Article abstract of DOI:10.1002/anie.201105292

Three ring circus: The title reaction can efficiently provide functionalized polycyclic compounds containing cyclobutene (see scheme; PG=protecting group) or aza-cyclooctene moieties in a highly regio- and diastereoselective manner with moderate to good yields under mild reaction conditions. The scope and limitations are disclosed and plausible reaction mechanisms are discussed. Copyright

Full text of DOI:10.1002/anie.201105292

DOI: 10.1002/anie.201105292
Synthetic Methods
Synthesis of Functionalized Polycyclic Compounds: Rhodium(I)-
Catalyzed Intramolecular Cycloaddition of Yne and Ene
Vinylidenecyclopropanes**
Bei-Li Lu and Min Shi*
Herein we report that rhodium(I)-catalyzed intramolecular
cycloadditions of yne and ene vinylidenecyclopropanes
(VDCPs) can efficiently provide functionalized polycyclic
compounds containing cyclobutene or aza-cyclooctene moi-
eties in a highly regio- and diastereoselective manner with
moderate to good yields under mild reaction conditions. The
scope and limitations are disclosed and the plausible mech-
anisms are discussed given the results of a deuterium labeling
experiment.
taining cyclobutene units and medium-sized ring systems
À
using rhodium-catalyzed [2+2] cycloadditions and C H bond
activations is a very attractive option.[8]
VDCPs are highly strained but readily accessible mole-
cules that serve as useful building blocks in organic synthesis.
In the past several years, the chemistry of VDCPs has been
extensively studied because of its unique structural and
electronic properties.[9,10] During our ongoing studies of
VDCP chemistry, we designed a series of novel vinylidene-
cyclopropanes tethered to alkyne and alkene moieties
through a Mitsunobu reaction, and have successfully synthe-
sized them in moderate to good yields (for the preparation of
them, please see the Supporting Information). Herein, we
report the rhodium(I)-catalyzed intramolecular cycloaddi-
tions of these yne and ene VDCPs through [2+2] cyclo-
Organorhodium chemistry has now emerged as an
important synthetic tool in organic chemistry and in other
related areas.[1] Particularly, rhodium-catalyzed cycloaddi-
[3]
tions[2] and C H bond activation reactions have received
À
considerable attention from the synthetic community because
of their convenience, reduced environmental impact, and
good atom economy. Polycyclic compounds containing cyclo-
butene units are often encountered in biologically significant
molecules as well as in drug candidates,[4] such as 1-cyclo-
butenylmethylguanine, (+)-b-lumicolchicine, etc. The most
efficient way of preparing these compounds is to employ a
[2+2] cycloaddition between an alkene and alkyne directly.
However, there have been only a few reports, mainly
involving thermal, photochemical, and microwave reaction
conditions, thus implying the difficulties and challenges of this
strategy.[5] Recently, the rhodium-catalyzed [2+2] cycloaddi-
tion has been recognized as one of the most powerful and
promising methodologies for the construction of cyclobutene
units.[6] Meanwhile, the construction of polycyclic molecules
containing a seven- or eight-membered ring (medium ring)
À
addition and allylic C H bond activation for the synthesis of
polycyclic compounds having cyclobutene and aza-cyclooc-
tene units, respectively.
We started our work by examining the intramolecular
[2+2] cycloaddition of the yne VDCP 2a in the presence of a
rhodium(I) complex and found that the tricyclic adduct 3a
containing a cyclobutene unit was obtained (Scheme 1). The
Scheme 1. The optimal reaction conditions for the intramolecular
[2+2] cycloaddition of the yne VDCP 2a. Ts=4-toluenesulfonyl.
À
system by using selective C H bond activation is very difficult
À
because of the numerous C H bonds existing in the starting
molecules. Therefore, the diastereoselective construction of
such molecules is also a big challenge and must be considered
carefully.[7] Because of these reasons, developing a mild and
efficient way to synthesize the polycyclic compounds con-
structure of 3a has been unambiguously determined by the X-
ray diffraction and its CIF data are presented in the
Supporting Information.[11] Screening of the reaction condi-
tions revealed that using 10 mol% of [RhCl(CO)(PPh3)2] as
the catalyst and carrying out the reaction in toluene at 1008C
for 6 hours with a substrate concentration of 0.025m were the
best reaction conditions for this transformation, and the
product 3a was obtained in 71% yield (Scheme 1; see
Table SI-1 in the Supporting Information for the details).
With the optimized reaction conditions being identified,
we next examined the substrate scope of this highly regiose-
lective intramolecular [2+2] cycloaddition of yne VDCPs and
the results are summarized in Table 1. For substrates 2b–2 f,
wherein R2 = CH3, R3 = H, X = NTs, and R1 is a either a
noncyclic substituent (ethyl or butyl) or a cyclic substituent
(cyclohexyl, cyclopentyl, or cycloheptyl), this rhodium(I)-
[*] B.-L. Lu, Prof. Dr. M. Shi
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (China)
E-mail: Mshi@mail.sioc.ac.cn
[**] 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).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105292.
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Communications
Table 1: Rhodium(I)-catalyzed [2+2] cycloaddition of the yne VDCPs 2
under the optimal reaction conditions.
a mixed solvent of toluene and MeCN (2:1) at 1008C for
2.5 hours could produce the aza-cyclooctene derivative 6a in
78% yield (see Table SI-2 in the Supporting Information for
the details). These reaction conditions are the best for this
rhodium(I)-catalyzed intramolecular cycloaddition and the
generality of this reaction was examined using a variety of ene
VDCPs (5). The results are shown in Table 2. Most of the
reactions proceeded smoothly to give the corresponding aza-
Entry[a] 2: R1, R1
R2
R3
X
Yield [%][b]
1
2
3
4
5
6
7
8
2b: C2H5, C2H5 CH3
H
H
H
H
H
H
H
H
H
H
H
CH3
CH3
NTs
NTs
NTs
NTs
NTs
NTs
NTs
NTs
NBs
NBs
NBs
3b: 74
3c: 71
3d: 77
3e: 65
3 f: 68
3g: 78
3h: 69
3i: 73
3j: 64
3k: 60
3l: 67
2c: C4H9, C4H9
2d: –(CH2)5–
2e: –(CH2)4–
2 f: –(CH2)6–
2g: CH3, CH3
2h: CH3, CH3
2i: –(CH2)5–
2j: CH3, CH3
2k: C4H9, C4H9
2l: –(CH2)5–
2m: CH3, CH3
2n: CH3, CH3
2o: CH3, CH3
CH3
CH3
CH3
CH3
C6H5
p-CH3C6H4
p-ClC6H4
CH3
CH3
CH3
Table 2: Rhodium(I)-catalyzed intramolecular cycloaddition of the ene
VDCPs 5 under the optimal reaction conditions.
9[c]
10[c]
11[c]
12[c]
13[d,e]
14[f]
Entry[a]
5, R1, R1
X
t [h]
Yield [%][b]
CH3
CH3
CH3
NNs 3m: 55
NNs 3g+4a: 52
1
2
3
4
5
6
7
8
9
5b: –(CH2)5–
5c: –(CH2)4–
5d: –(CH2)6–
5e: CH3, CH3
5 f: –(CH2)5–
5g: C4H9, C4H9
5h: CH3, CH3
5i: –(CH2)5–
5j: –(CH2)5–
NTs
NTs
NTs
NBs
NBs
NBs
NNs
NNs
O
2.5
2.5
2.5
5
5
5
8
8
15
6b: 77
6c: 69
6d: 80
6e: 73
6 f: 78
6g: 70
6h: 71
6i: 74
C6H5 NTs
complex
mixture
complex
mixture
complex
mixture
15[f]
16[f]
2p: CH3, CH3
2q: –(CH2)5–
CH3
CH3
TMS
H
NTs
O
complex
mixture
[a] All reactions were carried out using 2 (0.15 mmol) in the presence of
[RhClCO(PPh3)2] (10 mol%) in toluene (6.0 mL) at 1008C. [b] Yield of
isolated product. [c] The reaction time is 10 h. [d] The reaction time is
14 h. [e] 3n/4a=1.2:1.0. [f] The reaction time is 24 h. Bs=4-bromo-
benzenesulfonyl, Ns=4-nitrobenzenesulfonyl, TMS=trimethylsilyl.
[a] All reactions were carried out using 5 (0.2 mmol) in the presence of
[{RhCl(CO)2}2] (5 mol%) in toluene/MeCN (2.0 mL/1.0 mL) under CO
(1.0 atm). [b] Yield of isolated product.
catalyzed [2+2] cycloaddition proceeded smoothly to give the
corresponding products 3b–3 f in yields within the range of
65–77% (Table 1, entries 1–5). Using the yne VDCP 2g
wherein R2 is a phenyl group, the corresponding product 3g
was obtained in 78% yield (Table 1, entry 6). Moreover,
adding moderately electron-donating or electron-withdraw-
ing substituents onto the aromatic rings (substrates 2h and 2i)
afforded the corresponding products 3h and 3i in 69% and
73% yields, respectively (Table 1, entries 7 and 8). Using the
yne VDCPs 2j–2m wherein X = NBs or NNs, the corre-
sponding tricyclic adducts 3j–3m containing cyclobutene
units were formed in 55–67% yields a single regioisomer after
a prolonged reaction time (10 h; Table 1, entries 9–12). The
introduction of a methyl group at the terminus of the alkyne
leads to a longer reaction time for this cycloaddition to go to
completion, and affords the products 3n and 4a in 52%
overall yield as a 1.2:1.0 regioisomeric mixture (Table 1,
entry 13). However, when R3 = C6H5 or trimethylsilyl (TMS),
only complex product mixtures were formed, presumably as a
result of the steric bulk (Table 1, entries 14 and 15). In the
case of the oxygen-tethered yne VDCP 2q, the reaction also
afforded complex product mixtures (Table 1, entry 16).
We also attempted to use the ene VDCP 5a as a substrate
to examine the reaction outcome under the above standard
reaction conditions, but only complex product mixtures were
produced. Further investigations revealed that using 5 mol%
[{RhCl(CO)2}2] as the catalyst and carrying out the reaction in
cyclooctene derivatives 6 as single diastereoisomers in 69–
80% yields, and the reaction outcomes are not influenced by
the substituent on either the allene moiety or the nitrogen
atom (Table 2, entries 1–8). The structure and the configu-
ration of 6b were unambiguously determined by the X-ray
diffraction and its CIF data are presented in the Supporting
Information.[12] Unfortunately, when using the oxygen-teth-
ered ene VDCP 5j as a substrate, the reaction system only
gave complex product mixtures and no pure product could be
isolated (Table 2, entry 9). Attempts to synthesize both ene
VDCPs with two aryl groups on the cyclopropane and the
carbon-tethered substrates failed to give the desired products
(see the Supporting Information for the details).
Plausible mechanisms for the formation of the polycyclic
compounds 3 and 6 are outlined in Scheme 2. In the
rhodium(I)-catalyzed [2 + 2] cycloaddition of the yne
VDCPs 2,[6a–c,f] selective coordination of the internal double
bond of the allene and alkyne moiety by the rhodium(I)
complex gives intermediate A (Scheme 2a). The correspond-
ing polycyclic derivatives 3 containing a cyclobutene moiety
can be afforded after cyclometalation/reductive elimination.
In contrast, for the rhodium(I)-catalyzed intramolecular
cycloaddition of the ene VDCPs 5, acetonitrile and the
tethered terminal alkene can coordinate to the rhodium(I)
metal center to give intermediate C,[8k,13] which undergoes
oxidative addition of the rhodium(I) catalyst onto the
À
neighboring allylic C H bond to give the p-allyl rhodium
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Angew. Chem. Int. Ed. 2011, 50, 12027 –12031

sponding deuterated product [D2]-6a was exclusively formed
in 73% yield (> 99% deuterium incorporation at both C1 and
C5 positions). This observation clearly suggested that one
hydrogen atom at C5 position of [D2]-5a was completely
transferred to the C1 position of aza-cyclooctene derivative
[D2]-6a under the standard reaction conditions by the
À
rhodium(I)-catalyzed C H bond activation.
Unexpectedly, in the case of the ene VDCPs 5k and 5m,
which have a methyl group at the terminal position of the
alkene, and the ene VDCPs 5l and 5n, which bear a methyl
group at the internal position of the alkene, different aza-
cyclooctene derivatives (7a–7d) were obtained in yields
within the range of 29–38% through the rhodium(I)-cata-
3
À
lyzed sp C H bond activation along with a cyclopropane
ring-opening process (Scheme 4). Some unidentified products
were also obtained in the above reactions. The structures and
3
À
Scheme 4. Rhodium(I)-catalyzed sp C H bond activation of the ene
VDCPs 5k–5n.
configurations of 7b and 7c have been unambiguously
determined by the X-ray diffraction and their CIF data, as
well as a plausible reaction mechanism are presented in the
Supporting Information.[16,17]
Scheme 2. Plausible mechanisms for the formation of a) 3 and b) 6.
In summary, we have developed two interesting rho-
dium(I)-catalyzed intramolecular cycloadditions of yne and
ene VDCPs which provide various functionalized polycyclic
compounds in moderate to good yields and with high regio-
and diastereoselectivity under mild reaction conditions. The
polycyclic systems containing either a cyclobutene or an aza-
cyclooctene are difficult to synthesize by other methods.
Furthermore, a deuterium labeling experiment was con-
ducted to confirm the mechanistic hypothesis of the rho-
hydrogen species D (Scheme 2b).[8,14] Next, intramolecular
insertion to the internal double bond of the allene moiety
leads to the rhodium hydrogen species E. Products 6 can be
formed from intermediate E through reductive elimination
along with the regeneration of rhodium(I) catalyst.
À
To verify the allylic C H bond activation pathway from
the ene VDCPs 5 to products 6, we performed a deuterium
À
labeling experiment using [D2]-5a as the substrate
dium(I)-catalyzed allylic C H bond activation of ene VDCPs.
[15]
(Scheme 3). When the ene VDCP [D2]-5a
(> 99%
Notably, using the ene VDCPs 5k–5n, which have a methyl
group at either the terminus or the internal position of the
alkene, as the substrates produced aza-cyclooctene deriva-
tives 7 after a cyclopropane ring-opening step. These inter-
esting rhodoum(I)-catalyzed intramolecular cycloaddition
reactions may be useful synthetic tools in the preparation of
biologically active compounds. Additional efforts on the
scope and mechanistic details of this reaction are currently
ongoing in our laboratory.
deuterium incorporation at the allylic C5 position, see the
Supporting Information for the preparation) was used as the
substrate under the standard reaction conditions, the corre-
Experimental Section
2a: Under an argon atmosphere, triphenylphosphine (110 mg,
Scheme 3. Deuterium labeling experiment.
0.42 mmol)
and
4-methyl-N-(prop-2-ynyl)benzenesulfonamide
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Communications
(92 mg, 0.44 mmol) were added into a Schlenk tube. Then 3.0 mL
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THF was added and the reaction solution was cooled to 08C.
Vinylidenecyclopropane 1a (61 mg, 0.4 mmol) which was dissolved in
THF (1.0 mL) was added into the above Schlenk tube. Then diethyl
azodicarboxylate (DEAD) (0.066 mL, 0.42 mmol) was added drop-
wise at 08C and warmed up to room temperature naturally. On
reaction completion the solvent was removed under reduced pressure
and the residue was purified by a flash column chromatography (silica
gel. Eluent: EtOAc/petroleum ether= 1:30). Compound 2a was
isolated as a yellowish oil in 70% yield.
3a: Under an argon atmosphere, [RhClCO(PPh3)2] (10 mg,
0.015 mmol), substrate 2a (51 mg, 0.15 mmol), and toluene (6.0 mL)
were added into a Schlenk tube. The reaction mixture was stirred at
1008C until the reaction completed. Then, the solvent was removed
under reduced pressure and the residue was purified by a flash
column chromatography (silica gel. Eluent: EtOAc/petroleum
ether= 1:20). Compound 3a was isolated as a yellowish oil in 71%
yield.
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5a: Under an argon atmosphere, triphenylphosphine (110 mg,
0.42 mmol) and N-allyl-4-methylbenzenesulfonamide (93 mg,
0.44 mmol) were added into a Schlenk tube. Then 3.0 mL THF was
added and cooled to 08C. Vinylidenecyclopropane 1a (61 mg,
0.4 mmol) which dissolved in THF (1.0 mL) was added into the
above Schlenk tube. Then diisopropyl azodicarboxylate (DIAD)
(0.083 mL, 0.42 mmol) was added dropwise at 08C and warmed up to
rt naturally. On completion the solvent was removed under reduced
pressure and the residue was purified by a flash column chromato-
graphy (silica gel. Eluent: EtOAc/petroleum ether= 1:30). Com-
pound 5a was isolated as a yellowish oil in 59% yield.
6a: Under an argon atmosphere, [{RhCl(CO)2}2] (4 mg,
0.01 mmol) was added into a Schlenk tube. Then the atmosphere
was pumped off and a balloon filled with CO was connected to the
reaction flask. A solution of substrate 5a (70 mg, 0.2 mmol) in
toluene/MeCN (2.0 mL/1.0 mL) was added into the Schlenk tube. The
reaction mixture was stirred at 1008C until the reaction completed.
Then, the solvent was removed under reduced pressure and the
residue was purified by a flash column chromatography (silica gel.
Eluent: EtOAc/petroleum ether= 1:20). Compound 6a was isolated
as a colorless solid in 78% yield.
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was added into a Schlenk tube. Then the solution of substrate 5k
(72 mg, 0.2 mmol) in toluene/MeCN (2.0 mL/1.0 mL) was added into
the Schlenk tube. The reaction mixture was stirred at 808C until the
reaction completed. Then, the solvent was removed under reduced
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Received: July 28, 2011
Revised: September 17, 2011
Published online: October 25, 2011
À
Keywords: cycloadditions · C H bond activation · heterocycles ·
rhodium · synthetic methods
.
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[16] CCDC-830623 (7b) CCDC 6.. contains the supplementary
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[11] CCDC 791963 (3a) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
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[12] CCDC 816150 (6b) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
[17] CCDC-828941 (7c) CCDC 6.. contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Angew. Chem. Int. Ed. 2011, 50, 12027 –12031
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 12031