Rhodium-catalyzed ring expansion of cyclopropanes to seven-membered rings by 1,5 C-C bond migration

Article abstract of DOI:10.1002/anie.201104861

The golden side of rhodium: Highly functionalized seven-membered rings were prepared from substituted cyclopropanes by a Rh^I-catalyzed tandem isomerization reaction. The π-acidic Rh^I catalyst promoted the formation of an allene inter

Full text of DOI:10.1002/anie.201104861

DOI: 10.1002/anie.201104861
Ring Expansion
Rhodium-Catalyzed Ring Expansion of Cyclopropanes to
À
Seven-membered Rings by 1,5 C C Bond Migration**
Xiaoxun Li, Min Zhang, Dongxu Shu, Patrick J. Robichaux, Suyu Huang, and Weiping Tang*
Selective cleavage and subsequent elaboration of carbon–
carbon s bonds into complex molecules represents a funda-
complementary functionalities would be highly desirable. We
herein describe the synthesis of functionalized alkylidene
cycloheptadiene 3 by a RhI-catalyzed tandem isomerization
of the monoactivated cyclopropane 1 via an allene inter-
mediate 2 [Eq. (2)].
À
mental challenge in chemistry. The C C s bonds of cyclo-
propanes are activated owing to ring strain, and the ring
expansion of cyclopropanes is an attractive route to other ring
systems because of the well-documented stereoselective
methods for cyclopropanation.[1] Indeed, ring expansion of
cyclopropanes to four- or five-membered rings by 1,2 or 1,3
migration is routinely practiced in organic synthesis
[Eq. (1)].[2] In contrast, ring expansion of cyclopropanes to
À
seven-membered rings by 1,5 C C bond migration has not
Based on the high reactivity of allenes with respect to
transition metals,[7] we hypothesized that a 1,5 migration of a
À
cyclopropane C C bond in intermediate 2 may become
favored over a 1,3 migration, which would generate an allenyl
cyclopentene without the participation of the allene. We
recently discovered that the [{Rh(CO)2Cl}2] catalyst was able
to promote 1,3 acyloxy migration of propargyl esters to form
allenes;[8] this transformation was previously realized mainly
by p-acidic metal-based catalysts such as silver,[9,10]
copper,[9–11] platinum,[11,12] and gold[10,13] in various cascade
reactions.[14,15] Given this novel reactivity of [{Rh(CO)2Cl}2]
for the promotion of 1,3 acyloxy migration and its well-known
capability to undergo oxidative addition and reductive
elimination, we envisioned that alkylidene cycloheptadiene
3 could be prepared directly from readily available cyclo-
propane 1[16] in the presence of the [{Rh(CO)2Cl}2] catalyst.[17]
The acyloxy substituent on propargyl ester 1 not only eases
the preparation of allene 2 but also differentiates the three
double bonds in product 3.
To test the proposed transformation in Equation (2),
substrate 1a was prepared in four steps from commercially
available cyclopropanecarboxaldehyde.[16] Treatment of
cyclopropane 1a with 3 mol% of [{Rh(CO)2Cl}2] at 608C
provided a significant amount of product 3a (Table 1, entry 1)
after 12 hours and some starting materials were also recov-
ered. Complete conversion was realized after increasing the
catalyst loading to 5 mol% (Table 1, entry 2). Under these
reaction conditions, however, a small amount of eight-
membered-ring product 4a was also observed. Product 4a
was isolated in 22% yield when the catalyst loading was
increased to 10 mol% (Table 1, entry 3). We speculated that
increasing the CO pressure would provide more CO-insertion
product 4a. Surprisingly, a complex mixture was observed
with more CO (Table 1, entries 4 and 5). No reaction occurred
in the presence of several other rhodium catalysts (Table 1,
entries 6–8). Changing the solvent from DCE to dioxane led
been developed as a general method, in spite of the
prevalence of cycloheptane skeletons in natural products
and pharmaceutical agents.[3] The 1,5 migration of a cyclo-
À
propane C C bond has been mainly studied in bicyclo-
[4.1.0]heptadienes and a few other conformationally con-
strained bicyclic compounds under thermal conditions.[4] In
fact, it was reported that simple 1,3-dienylcyclopropanes
À
underwent 1,3 C C bond migration to form vinylcyclopen-
tenes in the presence of Ni or Pd catalysts.[5]
The most well-known example of the direct expansion of
three-membered rings to seven-membered rings is the [3,3]-
sigmatropic Cope rearrangement of divinylcyclopropanes to
1,4-cycloheptadienes.[6] Although this process requires the
double activation of cyclopropanes by two vinyl groups, it is
still one of the most important methods for the preparation of
seven-membered rings. Methods that can promote ring
expansion of cyclopropanes to seven-membered rings with
[*] X. Li, Dr. M. Zhang, S. Huang, Prof. Dr. W. Tang
The School of Pharmacy, University of Wisconsin
Madison, WI 53705-2222 (USA)
E-mail: wtang@pharmacy.wisc.edu
D. Shu, P. J. Robichaux
Department of Chemistry, University of Wisconsin
Madison, WI 53706-1322 (USA)
[**] We thank the NIH (R01GM088285) and University of Wisconsin for
funding. S.H. was partially supported by a fellowship from the
Chinese Scholarship Council.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104861.
Angew. Chem. Int. Ed. 2011, 50, 10421 –10424
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10421

Communications
Table 1: Screening of catalysts and reaction conditions.
Table 2: Scope of the RhI-catalyzed ring expansion.[a]
Entry
Substrates
Products
Yield
[%][b]
Z/E[c]
Entry Catalyst
Conditions
Yield [%]
1
2
1a, R=Piv
1b, R=Ac
3a
3b
83
83
–
–
1
2
3
[{Rh(CO)2Cl}2] (3 mol%)
[{Rh(CO)2Cl}2] (5 mol%)
[{Rh(CO)2Cl}2] (10 mol%) 608C, DCE, 12 h
608C, DCE, 12 h
608C, DCE, 12 h
3a, 72[a]
3a, 82[b]
3a, 70[b]
4a, 22[b]
complex
mixture
complex
mixture
NR
4
5
[{Rh(CO)2Cl}2] (10 mol%) 608C, DCE,
3
4
5
1c, R=Ph
1d, R=nBu
1e, R=iPr
3c
3d
3e
76
82
93
14:1
1:1
2:1
CO (1 atm), 12 h
608C, DCE,
[{Rh(CO)2Cl}2] (5 mol%)
CO (5 atm), 12 h
608C, DCE, 3 h
608C, DCE, 3 h
608C, DCE, 3 h
608C, dioxane, 6 h
6
7
8
9
10
11
[{Rh(CO)2Cl}2] (5 mol%)
[Rh(PPh3)3Cl] (5 mol%)
[Rh(cod)2]BF4 (5 mol%)
[{Rh(CO)2Cl}2] (5 mol%)
[{Rh(CO)2Cl}2] (5 mol%)
[AuCl(PPh3)] (5 mol%)
AgOTf (5 mol%)
NR
NR
3a, >95[a]
1008C, dioxane, 2.5 h 3a, >95[a]
6
1 f
3 f
92
–
RT, MeCN, 20 h
0
12
13
PtCl2 (10 mol%)
HNTf2 (10 mol%)
808C, DCE, 20 h
RT, CH2Cl2, 20 h
0
0
[a] Yields were determined by 1H NMR spectroscopy using CH2Br2 as
internal standard. [b] Yields of the isolated product. cod=cycloocta-
diene, DCE=dichloroethane, NR=No reaction, Piv=pivaloyl.
7
8
9
10
1g, Ar=Ph
3g
3h
3i
87
87
70
83
7:1
9:1
8:1
3:1
1h, Ar=4-MeOC6H4
1i, Ar=4-CF3C6H4
1j, Ar=2,4-Cl2C6H3
3j
to quantitative production of seven-membered-ring 3a with-
out any eight-membered-ring 4a, as determined by 1H NMR
analysis of the crude product (Table 1, entry 9). The reaction
time was shortened from 6 hours to 2.5 hours at higher
temperature (Table 1, entry 10). Moreover, p-acidic metal-
based catalysts such has AuI and PtII or Brønsted acid catalyst
did not provide any desired product 3a (Table 1, entries 11–
13).
11
12
13
1k, R=CH2OAc
1l, R=CH2OH
1m, R=CHO
3k
3l
3m
95
81
66
8:1
7:1
5:1
We then set out to examine the scope of this tandem
isomerization process (Table 2). The reaction worked well
when the ester was changed from pivalate to acetate (Table 2,
entry 2). The gem-dimethyl group on the propargyl ester
could be replaced by other alkyl or aryl groups (Table 2,
entries 3–5). However, a stereoselectivity issue arises for the
exocyclic olefins in these cases. For propargyl ester 3c with a
phenyl substituent, we observed a 14:1 Z/E selectivity[16]
(Table 2, entry 3), which is much higher than the selectivity
that was obtained in the [5+1] cycloaddition of acyloxy-
substituted allenyl cyclopropanes with CO.[8] Lower selectiv-
ity was generally observed for substrates with alkyl groups
(Table 2, entries 4 and 5). The 1,1-disubstituted cyclopropane
1 f could provide product 3 f, which has an additional
substituent on the seven-membered ring (Table 2, entry 6).
The Z/E selectivity for the exo cyclic olefins varied slightly
with electron-rich or electron-poor aryl substituents (Table 2,
entries 7–9).[18] An aryl group with one ortho substituent
significantly lowered the Z/E selectivity (Table 2, entry 10).
Various functional groups, such as acetate, free alcohol, and
even aldehyde, were tolerated under the reaction conditions
(Table 2, entries 11–13). Ring expansion of cyclopropanes 1n
and 1o led to 6,7 and 5,7 fused bicyclic systems respectively
14
15
1n
1o
3n
3o
91
85
–
–
16
17
18
1p, R=Ph (92% ee)
1q, R=CH2OTIPS
1r, R=CH2OAc
3p (91% ee)
3q
3r
68
99
92
–
–
–
[a] Reaction conditions: [{Rh(CO)2Cl}2] (5 mol%), 1008C, dioxane, 2-3 h.
[b] Yields of the isolated products. [c] Estimated by H NMR spectros-
copy. TIPS=triisopropylsilyl.
1
(Table 2, entries 14 and 15); these fused bicyclic systems are
widely present in bioactive natural products.[19]
For unsymmetrically substituted cyclopropanes, the cleav-
À
age of different cyclopropane C C s bonds will lead to
10422 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 10421 –10424

isomeric products. Our group[8,20] and other groups[21] have
and treated it with RhI catalyst. In the absence of CO, triene
10 was isolated in 77% yield under the standard reaction
conditions in Table 2.[26] No reaction occurred in the presence
of CO.[27] This confirmed the importance of the allene group
À
found that the regioselectivity for the cleavage of C C
s bonds in cyclopropanes depends on the stereochemistry of
the cyclopropane ring. Only one regioisomer was obtained for
aryl- or alkyl-substituted trans cyclopropanes (Table 2,
entries 16–18). Interestingly, the corresponding cis cyclopro-
panes with either aryl or alkyl substituents provided a mixture
of seven-membered rings with low regioselectivity under
identical reaction conditions.[16] The chirality in substrate 1p
was successfully transferred to the seven-membered-ring
product 3p (Table 2, entry 16). This paved the way for the
preparation of optically pure seven-membered rings from
chiral cyclopropanes.
À
in intermediate 2 for the net 1,5 C C bond migration.
We also prepared allene 11 by treating propargyl ester 1q
with excess methyl cuprate reagents.[28] Under our standard
reaction conditions in Table 2, seven-membered-ring product
12 could be obtained in 62% yield from allene 11, thus
indicating that the acyloxy substituent in intermediate 2 is not
required for the ring expansion. However, the acyloxy
substituent is important for differentiating the three double
bonds in product 3 for selective functionalization.[16]
The mechanism for the ring expansion of cyclopropane 1
to alkylidene cycloheptadiene 3 is proposed in Scheme 1. A
RhI-catalyzed 1,3 acyloxy migration may occur first and
generate allene intermediate 2. Formation of h4-(vinylal-
lene)rhodium complex 5 can initiate an oxidative cyclization
to yield alkylidene metallacyclopentene 6. This transforma-
In summary, we developed a novel method for the
preparation of highly functionalized seven-membered rings
À
directly from substituted cyclopropanes through a 1,5 C C
migration. This method may provide efficient access to mono-
and polycyclic bioactive sesquiterpenoids that contain iso-
propylidene cycloheptanones.[29] The p-acidic rhodium cata-
lyst is essential for the conversion of readily available
cyclopropanes with propargyl ester group to alkylidene
cycloheptadienes with three well-differentiated double
bonds. Further studies to understand the details of the 1,5
À
C C bond migration and apply this tandem isomerization
reaction to the synthesis of bioactive compounds are currently
in progress.
Received: July 12, 2011
Published online: September 9, 2011
À
Keywords: C C migration · cyclopropane · isomerization ·
rhodium · ring expansion
.
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À
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