Palladium(0)-catalyzed reaction of cyclopropylidenecycloalkanes with carbon dioxide

Article abstract of DOI:10.1002/ejoc.201101192

Cyclopropylidenecycloalkanes, which are highly strained methylenecyclopropane (MCPs) containing a cycloalkane moiety, react with carbon dioxide smoothly to give the corresponding five-membered lactone derivatives in moderate to good yields through a cyclo

Full text of DOI:10.1002/ejoc.201101192

FULL PAPER
DOI: 10.1002/ejoc.201101192
Palladium(0)-Catalyzed Reaction of Cyclopropylidenecycloalkanes with Carbon
Dioxide
Kai Chen,[a] Min Jiang,[a] Zhen Zhang,[a] Yin Wei,[a] and Min Shi*[a]
Keywords: Alkenes / Palladium / Strained molecules / Carbon dioxide fixation / Lactones / Small ring systems
Cyclopropylidenecycloalkanes, which are highly strained
methylenecyclopropane (MCPs) containing a cycloalkane
moiety, react with carbon dioxide smoothly to give the corre-
sponding five-membered lactone derivatives in moderate to
good yields through a cyclopropane ring-opening process in
the presence of Pd0 catalyst and PCy3 upon heating under
40 atm of CO2. The relative configuration of the major dia-
stereomers has been determined and a plausible reaction
mechanism has also been proposed.
small rings with CO2 is an important subject. For example,
Inoue and co-workers have disclosed that two kinds of lact-
ones could be formed from the reaction of simple MCPs
with carbon dioxide (40 atm) upon heating in the presence
of palladium(0) complex, affording the corresponding prod-
ucts in 6–77% total yields (Scheme 1).[8] As part of our
studies on the catalytic fixation of CO2 by transition metal
complexes, we attempted to use these cyclopropylidenecy-
cloalkanes as substrates to investigate their reactions with
CO2 catalyzed by palladium(0) complexes because these
methylenecyclopropane derivatives are more strained than
simple MCPs, providing an additional driving force to in-
corporate CO2, which is an extremely inert and unreactive
gas.
Introduction
Methylenecyclopropanes (MCPs), as highly strained but
readily accessible molecules, can undergo a variety of ring-
opening reactions in the presence of transition metals or
Lewis acids because the relief of ring strain can provide a
potent thermodynamic driving force.[1,2] However, it should
also be noted that release of such strain energy (27 kcal/
mol)[3] is not sufficient for high reactivity. The π character
of the ring bonds of a cyclopropane provides the kinetic
opportunity to initiate the unleashing of the strain.[4] It is
well-known that cyclopropane-containing compounds such
as MCPs can easily undergo ring-opening reactions with
other substrates in the presence of various transition metal
catalysts, giving efficient access to enhanced molecular
complexity in organic syntheses.[5] More recently, we have
also reported that (cyclopropylidenecyclohexyl)benzene de-
rivatives can undergo interesting dehydrogenative re-
arrangement along with the cyclopropane ring opening pro-
cess in the presence of AuPPh3Cl/AgOTf or Pd(OAc)2 at
high temperature in toluene to give the corresponding bi-
aryl or isopropenylbiaryl derivatives in moderate to good
yields.[6] These interesting findings have encouraged us to
continue to discover more useful transformations with these
special MCPs.
Scheme 1. Palladium-catalyzed reactions of MCPs with CO2.
Results and Discussion
Recently, much attention has been focused on the fixa-
tion of carbon dioxide because CO2, the earth’s most abun-
dant carbon resource, is remarkably little used as a chemical
feedstock.[7] In this area, catalytic fixation of CO2 into or-
ganic compounds using transition metal catalysts to form a
new C–C bond from the incorporation of highly strained
Initial studies using (cyclopropylidenecyclohexyl)benzene
(1a) as the substrate were aimed at determining the optimal
reaction conditions for the palladium(0)-catalyzed reac-
tions; the results of these experiments are summarized in
the Supporting Information (Table SI-1). On the basis of
the examination of palladium sources, solvents, additives,
and phosphane ligands, we found that utilizing 0.2 equiv.
of [Pd2(dba)3] as the catalyst, 0.2 mL of dimethyl sulfoxide
(DMSO) as the additive, and PCy3 as the ligand (0.8 equiv.)
at 120 °C in toluene for 36 h, provided 2a and 2Јa as a pair
of diastereoisomers (5:1) in 76% yield, which served as the
best conditions for this transformation (Scheme 2).
[a] State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Lu, Shanghai 200032, China
Fax: +86-21-64166128
E-mail: mshi@mail.sioc.ac.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101192.
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K. Chen, M. Jiang, Z. Zhang, Y. Wei, M. Shi
FULL PAPER
the desired product 2l (and 2Јl) could also be obtained in
85% yield (Table 1, entry 11).
Changing the cyclohexyl group to a cyclopentyl or cy-
cloheptyl group afforded the corresponding products 2m
(and 2Јm) and 2n (and 2Јn) in 85 and 74% yields, respec-
tively, along with moderate diastereomeric ratios under the
standard conditions (Table 1, entries 12 and 13). Using ali-
phatic MCPs 1o and 1q as substrates (R2 = C4H9, R1 = R3
= H), the desired lactone products 2o (and 2Јo) and 2p (and
2Јp) were obtained in moderate yields with 3:1 and 1:1 dia-
stereomeric ratios, indicating the wide substrate generality
in this Pd0-catalyzed CO2 fixation process (Table 1, entries
14 and 15).
Scheme 2. Optimal reaction conditions for the Pd0-catalyzed reac-
tion of 1a with CO2.
Having identified the optimized reaction conditions, we
next carried out palladium(0)-catalyzed lactonization reac-
tions of a variety of MCPs 1 with CO2 to evaluate the scope
of this reaction. The results are summarized in Table 1. For
substrates 1b–g (R1 = aryl, R2 = R3 = H, and n = 1), the
corresponding products, 2b–g (and 2Јb, 2Јg), could be ob-
tained in moderate yields along with moderate dia-
stereomeric ratios (Table 1, entries 1–6). For MCPs 1h–k
(R2 = aryl, R1 = R3 = H and n = 1), the reaction also
proceeded efficiently to give the corresponding lactone de-
rivatives 2h–k (and 2Јh, 2Јk) in good yields (74–91%) and
moderate diastereomeric ratios (Table 1, entries 7–10). In
the case of MCP 1l (R3 = aryl, R1 = R2 = H and n = 1),
The diastereomeric ratios of these lactone products were
determined by separation with TLC, silica gel column
1
chromatography, and HPLC, or by H NMR spectroscopic
analysis. Furthermore, the structures of the major dia-
stereoisomers 2g and 2h have been determined by X-ray
diffraction of their single-crystals. The ORTEP drawings
are shown in Figures 1 and 2 and their CIF data are pre-
sented in the Supporting Information.[9] As a result of these
investigations, the products 2 were identified as the major
diastereoisomers in most cases.
Table 1. Reaction of 1 under the optimal conditions with CO2.
Figure 1. ORTEP drawing of 2g.
[a] Reaction conditions: 1 (0.2 mmol), Pd2(dba)3 (0.04 mmol), PCy3
(0.16 mmol), DMSO (0.2 mL), toluene (2.0 mL), and the reactions
were carried out at 120 °C. [b] Isolated yield. [c] The ratios were
Figure 2. ORTEP drawing of 2h.
Using (cyclopropylidenecyclobutyl)benzene (1q) as sub-
strate afforded the expected product 2s in very low yield
1
determined by H NMR spectroscopic data.
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Palladium(0)-Catalyzed Carbon Dioxide Fixation
(8%) under identical conditions, presumably due to its in- phenyl group is on the equatorial bond, is 17.82 kJ/mol
stability at high temperature (Scheme 3). For simple MCPs lower than that of conformer P-2, in which the phenyl
1r and 1s, complex product mixtures were obtained under group is on the axial bond (Scheme 5), indicating that con-
the standard conditions, indicating that cycloalkanes con- former P-1 is more stable.
taining methylenecyclopropanes are essential for this Pd0-
catalyzed fixation of CO2 (Scheme 4).
Scheme 5. Reaction of 1r and 1s with CO2.
Based on a series of stabilization investigations on the
substrates, a proposed reaction pathway is shown in
Scheme 6. Under the standard reaction conditions, the sub-
Scheme 3. Reaction of MCP 1q with CO2 under the standard con-
ditions.
strate exists predominantly as conformer P-1, which reacts
with palladium(0) catalyst to generate intermediate A.[11]
Intermediate A undergoes a ring-opening process to pro-
duce intermediate B, in which Ca position is negatively
charged. Therefore, carbon dioxide can approach Ca either
from the upside or the underside to produce intermediate
D-1 and intermediate D-2, respectively, via intermediate C.[8]
Scheme 4. Reactions of 1r and 1s with CO2 under the standard
The negatively charged oxygen atom attacks the allyl palla-
conditions.
dium moiety to produce the corresponding lactones 2 and
To shed some light on the stereoselectivity of this palla- 2Ј, respectively. The two axial hydrogen atoms (printed in
dium(0)-catalyzed reaction, density functional theory bold) partially impair the incoming CO2 from approaching
(DFT) studies have been performed to investigate the con- from the underside due to steric repulsion, resulting in the
formers of the substrate 1b.[10] The calculation results formation of lactones 2 as the major diastereomers in most
showed that the relative energy of conformer P-1, in which cases. Moreover, according to the previously reported
Scheme 6. A proposed mechanism.
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K. Chen, M. Jiang, Z. Zhang, Y. Wei, M. Shi
FULL PAPER
mechanism in the Pd0-catalyzed reaction of simple MCPs
with CO2,[8] intermediate C can also be transformed into
intermediate E, producing the other two regioisomers 3 and
3Ј through intermediates F-1 and F-2, respectively. How-
ever, the cycloalkanes bearing an aromatic group in cyclo-
propylidenecycloalkanes 1 sterically favor the formation of
intermediate C and disfavor the formation of intermediate
E at high temperature, therefore affording products 2 and
2Ј exclusively. This expains why only regioisomers 2 and 2Ј
were exclusively formed in the above catalytic system.
Under the standard conditions, we succeeded in the syn-
thesis of (Ϯ)-norbakkenolide A from MCP 3.[12] As shown
in Scheme 7, this interesting biologically active com-
pound,[13] which, for instance, shows cytotoxic and anti-
feedant activities, inhibition of the activity against platelet
activation factor, and activity as an antiplatelet aggregatory
agent, was obtained in 50% yield as a pair of diastereoiso-
mers (major/minor = 1:0.85) under the standard conditions.
The tube was placed in an autoclave and the pressure was raised
to 40 atm with a CO2 cylinder after purging the autoclave with CO2
three times. The reaction was heated to 110–120 °C (oil bath). After
36 h, the reaction was stopped and the pressure was reduced to
1 atm. The solvent was removed under reduced pressure and the
residue was purified by flash column chromatography (SiO2) to give
the corresponding products 2 and 2Ј.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed description of experimental procedures, the CIF data
of 2g and 2h, full characterization of new compounds.
Acknowledgments
We thank the Shanghai Municipal Committee of Science and Tech-
nology (08dj1400100-2), National Basic Research Program of
China [(973)-2009CB825300], and the National Natural Science
Foundation of China (NSFC) for financial support (grant numbers
21072206, 20472096, 20872162, 20672127, 20821002, and
20732008).
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In conclusion, we have developed an interesting Pd0-cat-
alyzed transformation of cyclopropylidenecycloalkanes
with carbon dioxide, giving the corresponding five-mem-
bered lactone derivatives in moderate to good yields (up
to 91%) along with high regioselectivities. Because lactones
constitute core structural units for a wide range of func-
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diffraction analysis. Efforts are in progress to further under-
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Experimental Section
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General Remarks: H and 13C NMR spectra were recorded with a
Bruker AM-300 spectrometer for solutions in CDCl3 with tet-
ramethylsilane (TMS) as an internal standard; J values are given
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General Procedure: A dried tube equipped with a magnetic stirring
bar was charged with MCP (0.2 mmol), [Pd2(dba)3] (0.04 mmol),
PCy3 (0.16 mmol), anhydrous toluene (2.0 mL), and anhydrous
DMSO (0.2 mL), and then CO2 was bubbled through for 2 min.
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Palladium(0)-Catalyzed Carbon Dioxide Fixation
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Received: August 12, 2011
Published Online: October 18, 2011
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