Interception of a rautenstrauch intermediate by alkynes for [5+2] cycloaddition: Rhodium-catalyzed cycloisomerization of 3-acyloxy-4-ene-1,9- diynes to bicyclo[5.3.0]decatrienes

Article abstract of DOI:10.1002/anie.201103136

Rholling in the bicycles: A rhodium(I)-catalyzed cycloisomerization for the synthesis of bicyclic compounds containing a cycloheptatriene ring from linear alkenynes (see scheme; cod=1,5-cyclooctadiene) is proposed to proceed through 1,2-acyloxy migration, 6 π electrocyclization, migratory insertion, and reductive elimination. The overall process can be viewed as a novel intramolecular [5+2] cycloaddition with concomitant 1,2-acyloxy migration.

Full text of DOI:10.1002/anie.201103136

DOI: 10.1002/anie.201103136
Cycloisomerization
Interception of a Rautenstrauch Intermediate by Alkynes for [5+2]
Cycloaddition: Rhodium-Catalyzed Cycloisomerization of 3-Acyloxy-
4-ene-1,9-diynes to Bicyclo[5.3.0]decatrienes**
Xing-zhong Shu, Suyu Huang, Dongxu Shu, Ilia A. Guzei, and Weiping Tang*
Dedicated to Professor Barry M. Trost on the occasion of his 70th birthday
In 1984, Rautenstrauch reported that the 3-acyloxy-1,4-enyne
1 could undergo cyclization to form cyclopentadiene 2 and
cyclopentenone 3 in the presence of a palladium catalyst
through 1,2-acyloxy migration (Scheme 1).[1] The vinyl metal
complex 4, metal carbene 5, and metallacyclohexadiene 6
were proposed as intermediates in this transformation.[1,2] The
scope of this rearrangement reaction has been expanded
significantly by the use of p-acidic metals,[3] such as gold- and
platinum-based catalysts, for the synthesis of functionalized
five-membered rings.[4] The 1,2-acyloxy migration of prop-
argyl esters has also been employed in other synthetically
useful transformations catalyzed by gold,[5,6] platinum,[6,7]
ruthenium,[8,9] copper,[6] and more recently rhodium.[10]
tethered alkyne in a [5+2] cycloaddition under rhodium
catalysis.[12–16] We herein report a new atom-economical[17]
synthesis of a bicyclo[5.3.0]decatriene 8 through a rho-
dium(I)-catalyzed cycloisomerization[18] of a 3-acyloxy-4-
ene-1,9-diyne 7 [Eq. (1)]. The net result of this reaction is
an intramolecular [5+2] cycloaddition[14–16] with concomitant
1,2-acyloxy migration. The resulting complex bicyclo-
[5.3.0]decane skeletons are present in many natural prod-
ucts.[19]
We recently found that [{Rh(CO)2Cl}2] was able to
catalyze the 1,3-acyloxy migration of propargyl esters in the
synthesis of functionalized cyclohexenones.[11] The combina-
tion of this novel reactivity of RhI in promoting acyloxy
migration and its well-known capability to undergo facile
oxidative addition, migratory insertion, and reductive elim-
ination may offer many opportunities for the design of new
reactions. We envisioned that a conceptually new approach to
seven-membered rings was possible if intermediate 6 in the
Rautenstrauch rearrangement could be intercepted by a
Besides the Rautenstrauch rearrangement to form five-
membered rings, a number of other pathways may also
compete with the desired cycloisomerization of enyne 7 to the
bicyclic compound 8. For example, if a carbene intermediate
similar to 5 is generated, it may undergo cyclopropanation or
cyclopropenation with alkenes or alkynes in the system.
However, when substrate 7a, available in four steps from 2-
butene-1,4-diol,[20] was treated with a catalytic amount of
[{Rh(CO)2Cl}2], cycloisomerization occurred to give the
bicyclic product 8a in 19 and 48% yield in toluene and
dichloroethane (DCE), respectively (Table 1, entries 1 and 2).
Several other RhI catalysts also promoted this reaction
(Table 1, entries 4–6). The cationic RhI catalyst [Rh-
(cod)2]BF4 promoted the tandem cycloisomerization even at
room temperature (Table 1, entry 6). The reaction is solvent-
dependent (Table 1, entries 7 and 8), and higher yields were
generally observed with chlorinated solvents (entries 9 and
10). A complex 5,7-fused bicyclic compound can thus be
prepared in a single step from a readily available linear 3-
acyloxy-4-ene-1,9-diyne under rhodium catalysis. AuI, PtII, or
Brønsted acid catalysts did not provide any of the desired
product (Table 1, entries 11–13).
Scheme 1. Rautenstrauch rearrangement.
[*] Dr. X-z. Shu, S. Huang, Prof. Dr. W. Tang
The School of Pharmacy, University of Wisconsin
Madison, WI 53705-2222 (USA)
We next examined the scope of this tandem cycloisome-
rization under conditions A (Table 2). The reaction remained
efficient when the ester was changed from a pivalate to an
acetate or benzoate (Table 2, entries 1–3). Substrates with a
nitrogen or a gem-diester linker in the 1,6-enyne yielded
bicyclic compounds 8d and 8e successfully (Table 2, entries 4
and 5). The structure of bicyclic product 8d was assigned
unambiguously by X-ray crystallographic analysis.[21]
E-mail: wtang@pharmacy.wisc.edu
Homepage: https://mywebspace.wisc.edu/wtang5/web/
D. Shu, I. A. Guzei
Department of Chemistry, University of Wisconsin (USA)
[**] We thank the NIH (R01GM088285) and the University of Wisconsin
for funding. S.H. was partially supported by a fellowship from the
Chinese Scholarship Council.
We systematically examined the scope of this rhodium(I)-
catalyzed cycloisomerization by placing substituents at differ-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103136.
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Table 1: Screening of catalysts and conditions for rhodium(I)-catalyzed
Table 2: Scope of the rhodium(I)-catalyzed cycloisomerization.
cycloisomerization.
Entry Substrate
Product
Yield [%][a]
(cond.[b])
Entry Conditions
Yield [%][a]
1
2
3
7a, R=Piv
7b, R=Ac
7c, R=Bz
8a, R=Piv
8b, R=Ac
8c, R=Bz
85 (A)
81 (A)
83 (A)
1
2
3
4
5
6
7
8
9
[{Rh(CO)2Cl}2] (5 mol%), toluene, 908C, 8 h
19
48
43
21
77
70
n.r.
n.r.
81
83
0
[{Rh(CO)2Cl}2] (5 mol%), DCE, 908C, 8 h
[{Rh(CO)2Cl}2] (5 mol%), TCE, 908C, 1.5 h
[{Rh(cod)Cl}2] (5 mol%), TCE, 908C, 8 h
[Rh(PPh3)3Cl] (5 mol%), TCE, 908C, 8 h
[Rh(cod)2]BF4 (5 mol%), DCE, RT, 8 h
[Rh(cod)2]BF4 (5 mol%), toluene, RT, 8 h
[Rh(cod)2]BF4 (5 mol%), dioxane, RT, 8 h
[Rh(cod)2]BF4 (5 mol%), TCE, 508C, 20 h
[Rh(COD)2]BF4 (5 mol%), CH2Cl2, RT, 8 h
[AuCl(PPh3)] (5 mol%), AgOTf (5 mol%), MeCN, RT,
20 h
4
96 (A)
7d
7e
8d
8e
5
75 (A)
10
11
6
7
8
9
88 (B)
82 (B)
70 (B)
60 (B)
12
13
PtCl2 (10 mol%), DCE, 808C, 20 h
HNTf2 (10 mol%), CH2Cl2, RT, 20 h
0
0
7 f
7g
7h
8 f
8g
8h
[a] The yield was calculated on the basis of 1H NMR spectroscopy with an
internal standard. cod=1,5-cyclooctadiene, n.r. =no reaction, Piv=pi-
valoyl, TCE=tetrachloroethane, Tf=trifluoromethanesulfonyl.
ent positions on the 1,9-diyne. For substrates 7 f–7i with an
internal alkyne on the left-hand side, either no reaction or
only a trace amount of the product was observed under
conditions A. We then explored the effect of ligands on the
cycloisomerization of substrate 7 f with the [Rh(cod)2]BF4
catalyst. The addition of PPh3, iBu3P, or 1,2-bis(diphenyl-
phosphanyl)ethane (dppe) had no effect. Triethyl phosphite
improved the conversion of substrate 7 f to 21% according to
1H NMR spectroscopy. Similar conversion was also observed
with the electron-poor phosphine ligands (C6F5)3P and (p-
7i
7j
8i
8j
10
11
76 (B)
90 (A)
CF3C6H4)3P.
The
electron-poor
phosphite
ligand
(CF3CH2O)3P significantly improved the conversion: sub-
strate 7 f was completely consumed within 8 hours, and
product 8 f was isolated in 88% yield (Table 2, entry 6). A
novel catalytic system composed of cationic RhI and
tris(2,2,2-trifluoroethyl) phosphite was thus developed (con-
ditions B).
Dramatic improvements were also observed for other
substrates with internal alkynes when a combination of the
catalyst [Rh(cod)2]BF4 and the ligand (CF3CH2O)3P was used
(Table 2, entries 7–9). The all-carbon tether was not limited to
substrates with gem-diester substituents. Moderate conver-
sion (40–50%) was observed for substrate 7j when the
catalyst [Rh(cod)2]BF4 (3–10 mol%) was used alone. Again,
the addition of the ligand (CF3CH2O)3P improved the yield of
product 8j (Table 2, entry 10).
7k[c]
8k[c]
12
13
14
90 (A)
76 (B)
80 (B)
7l
8l
7m[c]
8m[c]
7n[c]
8n[c]
We then examined the effects of substituents in the tether
region. Substituents adjacent to the left-hand alkyne had no
apparent effect, and the cycloisomerization proceeded effi-
ciently under conditions A (Table 2, entries 11 and 12). We
were very pleased to find that the reaction even tolerated the
quaternary carbon center adjacent to the reacting alkyne in
substrate 7l. Substituents adjacent to the alkene, however,
lowered the conversion, and the addition of (CF3CH2O)3P as
a ligand was necessary for the formation of the product in
good yield (Table 2, entries 13 and 14). A trisubstituted olefin
was also tolerated: the bicyclic product 8o was obtained in
15
16
80 (B)
7o
7p
8o
complex mixture
(A or B)
[a] Yield of the isolated product. [b] Conditions A: [Rh(cod)2]BF4 (3–
5 mol%), CH2Cl2 (0.05m), RT or 508C, 8–48 h; conditions B: [Rh-
(cod)2]BF4 (5–10 mol%), (CF3CH2O)3P (10–20 mol%), CH2Cl2 (0.025–
0.05m), 508C, 8–24 h. [c] The diastereomeric ratio is 1:1. Bz=benzoyl,
Ts =p-toluenesulfonyl.
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good yield (Table 2, entry 15). However, when a substrate
with a tertiary ester was subjected to conditions A or B, a
complex mixture was formed (Table 2, entry 16).
For substrates with an internal alkyne at the right-hand
end (e.g. 7q, Scheme 2), the formation of benzene derivatives
(e.g. 9) in the presence of a PtCl2 catalyst has been reported.[22]
A 1,3-acyloxy migration followed by a Diels–Alder-type
reaction was proposed for this transformation. When we
treated 7q with the cationic RhI catalyst, a trace amount of
product 9 was observed, and the starting material was mainly
recovered. We have previously shown that [{Rh(CO)2Cl}2] is
an efficient catalyst for the 1,3-acyloxy migration of propargyl
esters.[11] Indeed, our preliminary study showed that product 9
could be obtained in 30–40% yield with the catalyst
[{Rh(CO)2Cl}2]. Since this transformation has been carried
out with the catalyst PtCl2, no further optimization was
conducted. These results, however, did show that the 1,2- and
1,3-acyloxy migration of propargyl esters is dependent on the
nature of the substrate and the RhI catalyst, and are thus
consistent with observations made with other metal cata-
lysts.[3]
Substrates with six-atom or longer tethers between the
two reactive p systems are often challenging in transition-
metal-catalyzed intramolecular cycloaddition and cycloiso-
merization reactions.[18] Substrate 10 (Scheme 2) was pre-
pared to test the limits of the present cycloisomerization.
Under standard conditions A or B, no reaction occurred, and
the starting material was recovered. A tethered alkene also
failed to intercept the Rautenstrauch intermediate: when the
3-acyloxy-substituted dienyne 11 was subjected to condi-
tions A or B, the starting material was recovered.
Scheme 3. Proposed mechanism for the rhodium(I)-catalyzed cyclo-
isomerization and evidence for the involvement of a rhodium(I)
carbene.
(2.0 equiv) was added to the reaction mixture. However,
when the external cyclooctadiene was replaced by the same
amount of styrene, no cyclopropanation product derived from
styrene was observed. This difference may be attributed to the
bidentate nature of cyclooctadiene. When we treated pro-
pargyl ester 18 with the different RhI catalysts in Table 1 in
the presence of styrene, the known cyclopropane 19[8] was
isolated in several cases. This outcome again suggested the
formation of a RhI carbene from the propargyl ester.
Although there are other potential mechanisms, the above
results are consistent with the mechanism proposed in
Scheme 3 based on the interception of a Rautenstrauch
intermediate by an alkyne.
We propose a mechanism involving a Rautenstrauch
intermediate for the formation of products 8 from enediynes
7 (Scheme 3): A rhodium(I)-promoted 1,2-acyloxy migration
of the propargyl ester in complex 12 provides a vinyl metal
species 13. The metallacyclohexadiene 15 may be formed
through the direct cyclization of intermediate 13, or via
carbene 14 through a 6 p electrocyclization. Insertion of the
tethered alkyne into the metallacycle 15, followed by
reductive elimination of the metallacyclooctatriene 16, then
produces product 8 with a seven-membered ring.[18] As the
yield for the transformation of substrate 7i into product 8i
was the lowest observed for the successful reactions in this
study (Table 2, entry 9), we carefully analyzed the by-
products of this reaction. We isolated a small amount of
cyclopropane 17 (Scheme 3), which was presumably derived
from the reaction between a RhI carbene and one of the
cyclooctadiene ligands in the catalyst. Compound 17 became
the major product when excess external cyclooctadiene
In summary, we have developed a conceptually novel
intramolecular [5+2] cycloaddition with concomitant 1,2-
acyloxy migration for the synthesis of highly functionalized
seven-membered rings. Various substituted bicyclo-
[5.3.0]decatrienes were synthesized in this way from readily
available linear starting materials. The cycloheptatriene in the
resulting bicyclic system has three well-differentiated double
bonds ready for further functionalization.[19] Cyclohepta-
trienes themselves are also widely present in polycyclic
natural products and pharmaceutical agents.[23] Further stud-
ies to uncover the details of the mechanism, expand the scope
of the reaction, and apply this novel cycloisomerization to the
synthesis of natural products and pharmaceutical agents are
currently in progress.
Received: May 6, 2011
Published online: July 11, 2011
Keywords: alkenynes · cycloaddition · cycloisomerization ·
.
polycycles · rhodium
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Communications
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[20] See the Supporting Information for details.
[21] CCDC 823148 (8d) 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.
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