Transition metal-catalyzed carbocyclization of nitrogen and oxygen-tethered 1,n-enynes and diynes: Synthesis of five or six-membered heterocyclic compounds

Article abstract of DOI:10.1039/c2cc34739c

Cycloisomerization of 1,n-enynes and diynes is a powerful method in organic synthesis to access heterocyclic compounds and has drawn increasing attention from organic chemists. In this paper, we attempted to summarize our recent results on the transition

Full text of DOI:10.1039/c2cc34739c

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FEATURE ARTICLE
Transition metal-catalyzed carbocyclization of nitrogen and
oxygen-tethered 1,n-enynes and diynes: synthesis of five or
six-membered heterocyclic compounds
Di-Han Zhang, Zhen Zhang and Min Shi*
Received 3rd July 2012, Accepted 31st July 2012
DOI: 10.1039/c2cc34739c
Cycloisomerization of 1,n-enynes and diynes is a powerful method in organic synthesis to access
heterocyclic compounds and has drawn increasing attention from organic chemists. In this paper,
we attempted to summarize our recent results on the transition metal-catalyzed cycloisomerization
to synthesize five or six-membered heterocyclic compounds using 1,n-enynes and diynes having a
propargylic ester moiety. First, we will describe the synthesis of 2,3-disubstituted 3-pyrrolines via
gold catalyzed cycloisomerization of 1,6-diynes. In addition, we will also disclose a novel silver
catalyzed tandem 1,3-acyloxy migration/Mannich-type addition/elimination of the sulfonyl
group of N-sulfonylhydrazone-propargylic esters to 5,6-dihydropyridazin-4-one derivatives.
Furthermore, we will introduce three interesting examples of the synthesis of bicyclic compounds
via titanium or rhodium catalyzed carbocyclization of enynes. In this context, we have presented
that 1,n-enynes and diynes containing propargylic esters are highly reactive and useful starting
materials for the cycloisomerization catalyzed by a transition metal catalyst.
1. Introduction
some of the most attractive methodologies for synthesizing
heterocyclic compounds,3 since a transition metal-catalyzed
reaction can directly construct complicated molecules from
readily accessible starting materials under mild conditions.4 As
Nakamura and Yamamoto proposed,3d the construction of
heterocyclic skeletons is classified into two major processes:
(1) C–C bond formation from the corresponding acyclic
precursors; (2) C–Y bond formation from the corresponding
acyclic precursors (Scheme 1). The synthesis of heterocycles
via the cycloisomerization of enynes belongs to category 1.
The transition metal-catalyzed enyne cycloisomerization
is among the most important strategies for the synthesis of
Heterocyclic compounds are worth our attention for their
biological properties, and many drugs are heterocycles.1 Although
a tremendous number of synthetic methods to approach hetero-
cyclic compounds have been known, the development of newer
and more efficient protocols remains an area of ongoing interest
in organic synthesis.2 Transition metal-catalyzed reactions are
State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, P. R. China.
E-mail: Mshi@mail.sioc.ac.cn
Di-Han Zhang was born in
1987 in Jiangxi (P. R. China).
He received his BS degree in
2008 from the East China
Normal University, before
joining a PhD program in
Shanghai Institute of Organic
Chemistry, Chinese Academy
of Sciences (SIOC, CAS). He
is currently a PhD candidate
Zhen Zhang was born in 1986
in Jiangsu (P. R. China). He
received his BS degree in 2008
from
Shanxi
University,
before joining a PhD program
in the East China University
of Science & Technology. He
is currently a PhD candidate
under
Professor Min Shi.
the
direction
of
under
Professor Min Shi.
the
direction
of
Di-Han Zhang
Zhen Zhang
Chem. Commun., 2012, 48, 10271–10279 10271
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Scheme 1 Two major processes of heterocycle synthesis.
functionalized cyclic structures.5 The chemical behavior of
enynes may be influenced by other functional groups, such
as alcohols, aldehydes, ethers, alkenes, or alkynes, thus
enhancing the molecular complexity of the synthesized products.
In recent years, propargylic esters have received extensive
attention for their rich reactivities and easy availabilities,6,7
and have been transformed into various synthetically valuable
products via three routes: 1,2-acyloxy migration,8 1,3-acyloxy
migration9 and nucleophilic substitution10 (Scheme 2). Herein,
we wish to summarize our efforts on the synthesis of five or six-
membered heterocyclic compounds during the recent years,
and report some novel transition metal-catalyzed cycloisomeri-
zations of 1,n-enynes and diynes containing a propargylic ester
moiety.
Scheme 2 Transition metal-catalyzed reactions of propargylic esters.
Scheme 3 Optimal reaction conditions for gold(I)-catalyzed intra-
molecular cyclization.
propargylic ester and arenyne toward nitrogen-containing
five-membered heterocyclic rings, such as 2,3-disubstituted
3-pyrrolines,6k which have been extensively used as synthetic
building blocks in organic synthesis and appear as structural
motifs in many natural products, exhibiting interesting bio-
logical activities.13
2. Synthesis of five or six-membered heterocyclic
compounds
2.1 Synthesis of 2,3-disubstituted 3-pyrrolines by a gold(I)
catalyst
Homogeneous catalysis by gold complexes has received con-
siderable attention in recent years.11 Among these interesting
reactions, gold-catalyzed cycloisomerization of 1,6-enynes and
1,6-diynes12 is one of the most important strategies for the
construction of functionalized cyclic structures. In the context
of our ongoing efforts to develop gold-catalyzed tandem
reactions, we realized that gold-catalyzed cascade transforma-
tion of 1,6-diynes into abnormal five-membered cycloadducts
has been less explored. Thus far, only one example has been
reported for the gold–phosphine catalyzed cycloisomerization
of terminal 1,6-diynes to give cyclopentene products in less than
43% yield.12o Hence, we reported a novel C–C bond formation
along with cycloisomerization from 1,6-diyne containing
Initial studies using propargylic acetate-arenyne 1a (0.2 mmol)
as the substrate were aimed at determining the reaction outcome
and subsequently optimizing the reaction conditions. As shown
in Scheme 3, the optimal reaction conditions have been identified
to carry out the reaction in DCE at 80 1C using (tBu3P)AuCl/
AgOTf (5 mol%) as the catalyst in the presence of H2O
(1.0 equiv.). We next examined the substrate generality of
the reaction under the optimized conditions and the results are
shown in Scheme 4. All of the reactions proceeded smoothly to
afford the corresponding cycloadducts 2 in 35–88% yields.
When the terminal of an alkyne moiety is a hydrogen atom
(1s), the corresponding enone 3a could be formed in 71% yield
rather than the cyclized product (Scheme 5).
On the other hand, in the case of oxygen-tethered 1,6-diynes
containing propargylic ester and arenyne such as substrates 1t
Dr Min Shi was born in 1963
in Shanghai, China. He
received his BS in 1984
(Institute
of
Chemical
Engineering of East China)
and PhD in 1991 (Osaka
University, Japan). He had
his postdoctoral research
experience with Prof. Kenneth
M. Nicholas at University of
Oklahoma (1995–1996) and
worked as an ERATO
Researcher in Japan Science
and Technology Corporation
(JST) (1996–1998). He is
currently a group leader of
Scheme 4 Gold(I)-catalyzed cycloisomerization of 1,6-diynes 1.
Min Shi
the State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry (SIOC).
Scheme 5 Gold(I)-catalyzed cycloisomerization of 1s.
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Scheme 6 Gold(I)-catalyzed cycloisomerization of 1t and 1u.
Piv = pivaloyl, Tf = triflate.
Scheme 8 The aromatization of products 2c and 2k.
Scheme 9 Optimal reaction conditions for the formation of 7a.
highly efficient Mannich reaction, various C–H bond activated
compounds including carboxylic acid derivatives, nitroalkanes
and terminal alkynes have been applied as nucleophiles to achieve
different classes of amines.17 However, employing novel nucleo-
philes without activated C–H bonds such as internal alkynes and
allenic esters is limited when using metal catalysts.18 Hence, we
reported a novel addition of allenic esters to CQN bonds initiated
by a silver catalyzed 1,3-migration of propargylic esters.19
Initially, we started the optimization of the reaction conditions
by using (E)-5-(2-benzylidene-1-tosylhydrazinyl)-2-methylpent-
3-yn-2-yl acetate 6a. As shown in Scheme 9, the use of AgSbF6
(10 mol%) as the catalyst and water (1.0 equiv.) as an additive
in dry dichloromethane at room temperature have been identi-
fied as the optimal reaction conditions.
Scheme 7 A plausible reaction mechanism for the formation of 2a.
and 1u, the reactions produced the corresponding 2,5-dihydro-
furan derivative 4a (normal cyclization) in 62% and 61% yield
at room temperature (20 1C), respectively (Scheme 6).
Under the optimized conditions, we next investigated the
tolerance of silver(I)-catalyzed tandem cyclization of various
hydrazones 6 and the results are shown in Scheme 10. All of
the reactions proceeded smoothly to afford the corresponding
products 7 in 30–88% yields.
A plausible mechanism for this reaction is outlined in Scheme 7
on the basis of above control experiments. Two cationic Au(I)
complexes A1 first coordinate to the two alkyne moieties of 1a,
respectively, affording intermediate B1, which undergoes a
Au-catalyzed 3,3-sigmatropic rearrangement to give carboxyallene
intermediates C1. The resulting oxonium intermediate D1 under-
goes hydrolysis to give enone E1. The enone E1 undergoes a
Lewis acid-catalyzed enolization to give intermediate F1. Activa-
tion of the remaining alkyne moiety by the gold complex induces
a 5-endo-dig cycloaddition to give intermediate G1. The hydro-
lysis of intermediate G1 produces cycloadduct 2a and regene-
rates Au(I) complex A1 to complete the catalytic cycle.
A study on the aromatization of products 2 has been
undertaken to synthesize 2,3-disubstituted pyrrole derivatives.
The results are shown in Scheme 8. The benzenesulfonyl group
and the 4-nitrobenzenesulfonyl group could be easily removed
by treatment with sodium naphthalene14 or thiophenol in the
presence of K2CO3,15 respectively, giving the corresponding
pyrrole derivatives 5a and 5b in good yields.
On the other hand, for styrenyl hydrazone 6u, the pyrazole
8a was formed in 73% yield without the formation of the
5,6-dihydropyridazin-4-one product (Scheme 11).20
A plausible reaction mechanism is outlined in Scheme 12.
The cationic silver(I) first coordinates to the alkyne moiety of 6
to afford intermediate A2, which undergoes a 3,3-sigmatropic
Scheme 10 Substrate scope of the silver-catalyzed tandem reaction.
2.2 Synthesis of 5,6-dihydropyridazin-4-ones by a silver(I)
catalyst
The addition of nucleophiles to CQN bonds offers a highly
efficient synthetic strategy for accessing nitrogen-containing
molecules.16 In the well-developed addition reactions, such as the
Scheme 11 Silver-catalyzed tandem reaction 6u.
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Scheme 14 Optimal reaction conditions for the formation of 12a
and 13a.
cyclization of enynes has been carried out using TiCl4 as a
promoter. Only one notable example is the TiCl4–Et3N mediated
intramolecular carbocyclization of active methine compounds
with unactivated alkyne groups.22 Herein, we reported a novel
carbocyclization of 1,6-enynes in which easily available and
inexpensive metal halides can serve as effective promoters to
achieve the selective synthesis of 3-azabicyclo[3.1.0]hexanes and
functionalized allenes by controlling the reaction temperature.23
These 3-azabicyclo[3.1.0]hexanes are core structures of a variety
of biologically active natural products24 and the examples of
direct transformation of enynes into allenes accompanied with
ring formation are rare.25
Scheme 12 A plausible reaction mechanism for the formation of 7.
rearrangement to give carboxyallene intermediate B2. Activa-
tion of the hydrazone sp2 nitrogen atom of B2 by silver(I)
produces intermediate C2, which induces a Mannich-type
addition of the allenic acetate to the CQN bond to give
intermediate D2. In the presence of water, intermediate D2
undergoes hydrolysis to give intermediate E2 and regenerates
the silver(I) catalyst. Then the carbonyl group of intermediate
E2 is activated by cationic silver(I), affording intermediate F2
which undergoes a soft enolization to give intermediate G2. An
elimination of tolylsulfinate of G2 produces product 7.
Initially, we tested the carbocyclization of enyne 11a using
various Lewis acids such as BiCl3, TiCl4, BCl3 and TiBr4. We
found that only TiCl4 gave good results, affording 12a in 73%
yield within 20 minutes at room temperature (25 1C). However,
it was found that allene 13a could be obtained in 69% yield at
À20 1C after 30 minutes in the presence of 1.70 equiv. TiCl4
(Scheme 14).
The transformation of the product 5,6-dihydropyridazin-4-one
7 into other pyridazin derivatives was briefly examined
(Scheme 13). Compound 7e was readily converted to N-acetylated
product 9a in the presence of acetic anhydride when pyridine was
used as the base. However, when triethylamine was used instead
of pyridine, the base could deprotonate the C–H bond of the
methyl group and then enolization occurred which underwent
acetylation to form product 9b. By a conventional O3 oxida-
tion procedure, compound 9a could be readily converted to
5-hydroxypyridazin-4-one 10a and compound 9b could be
easily transformed into 1,6-dihydropyridazin 10b, respectively.
With these optimized conditions in hand, we next investigated
the tolerance of TiCl4-mediated carbocyclization with various
enynes 11 to construct the corresponding 3-azabicyclo[3.1.0]-
hexanes. As depicted in Scheme 15, all of the reactions proceeded
smoothly to give the desired products 12 in 54–77% yields along
with excellent diastereoselectivities (4 : 1 to 47 : 1) at 25 1C.
To obtain allene derivatives 13, the reaction should be
conducted at lower temperature (À20 or À30 1C) and the results
are shown in Scheme 16. For various alkyl groups substituted 11,
2.3 Synthesis of 3-azabicyclo[3.1.0]hexanes and functionalized
allenes by a titanium(IV) complex
TiCl4 has been widely used in many carbon–carbon bond-forming
reactions,21 to the best of our knowledge, no investigation of
Scheme 15 TiCI4-mediated synthesis of 3-azabicyclo[3.1.0]hexanes 12.
Scheme 13 The transformation of 7 into pyridazin derivatives 9 and 10.
10274 Chem. Commun., 2012, 48, 10271–10279
Scheme 16 TiCl4-mediated synthesis of allenes 13.
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Scheme 18 Optimal reaction conditions for the formation of 16a.
Scheme 17 A plausible reaction mechanism for the formation of 12
and 13.
the corresponding allene derivatives 13 could be obtained in
50–91% yields along with good diastereoselectivities (4 : 1 to
26 : 1). In the case of substrate 11b with a cyclohexyl group, the
product (E)-3-(chloro(phenyl)methyl)-4-(cyclohexenylmethylene)-
1-tosylpyrrolidine 13b was obtained in 68% yield without the
formation of the corresponding allene derivative.
Scheme 19 Rhodium(I)-catalyzed [3+2] intramolecular cycloaddition
of alkylidenecyclopropane-propargylic esters.
We proposed a mechanism for the TiCl4-mediated carbo-
cyclization in Scheme 17. Coordination of the ester group to
TiCl4 gives intermediate A3. The nucleophilic intramolecular
addition of the pendant olefin to the alkyne along with leaving
of the acyloxy group affords carbocation B3, which contains a
vinylidene moiety. At low temperature, the chloride ion from
the metal is transferred to an incipient benzylic carbocation to
produce chlorinated allene 13 (path a). Due to the stability of
the benzylic cation, the intermediate B3 could be isomerized to
vinyl cation C3 at elevated temperature, which then undergoes
chloride ion transfer from the metal to generate the chlori-
nated 3-azabicyclo[3.1.0]hexane 12 (path b).
Scheme 20 Rhodium(I)-catalyzed [3+2] intramolecular cycloaddition
of alkylidenecyclopropane-enyne.
all of the reactions proceeded smoothly to give the corre-
sponding cycloadducts 16 in 36–60% yields.
On the other hand, we found that alkylidenecyclopropane-
enyne 15 can also produce the corresponding bicyclo[3.3.0]-
octene derivatives 16 in moderate to good yields under similar
conditions, and the results are summarized in Scheme 20.
A plausible mechanism for the formation of these bicyclo-
[3.3.0]octene derivatives 16 is tentatively outlined in Scheme 21.
Cycle L involves initial insertion of the metal at the distal
position of the alkylidenecyclopropane 14a to give metalla-
cyclobutene B4, followed by isomerization to intermediate C4
through a TMM-like transition state and carbometalation to
afford intermediate D4. Reductive elimination of intermediate
D4 provides the final adduct 16a along with the release of
HOAc. In Cycle R, propargylic ester 14a produces enyne 15a
in the presence of RhI complex A4. Oxidative addition into the
distal bond of the cyclopropane should afford the metalla-
cyclobutene E4, which can presumably rearrange to inter-
mediate F4. Intermediate F4 undergoes a carbometalation to
give intermediate G4. Reductive elimination of intermediate
G4 produces the corresponding cycloadduct 16a and regenerates
the RhI complex A4 to complete the catalytic cycle.
2.4 Synthesis of 3-azabicyclo[3.3.0]octenes by a rhodium(I)
catalyst
Methylenecyclopropanes (MCPs) and alkylidenecyclopropanes
(ACPs), containing a coordinating double bond and a strained
carbocycle, can undergo a number of interesting metal-assisted
transformations.26 Rhodium,27 nickel,28 ruthenium,29 as well
as palladium,30 all can catalyze intermolecular and intra-
molecular cycloaddition of alkene- or alkyne-tethered ACPs,
constructing a variety of interesting bicycles or tricycles, which
are useful building blocks for the synthesis of natural products
and medicinally important substances. Although the inter-
molecular rhodium(I)-catalyzed [3+2+2] carbocyclization
reaction of ACPs with activated alkynes has been reported,27
the corresponding intramolecular carbocyclization of ACPs is
not forthcoming. Hence, we discovered the first intramolecular
rhodium(I)-catalyzed [3+2] cycloaddition of ACPs containing
propargylic esters, for the construction of the bicyclo[3.3.0]-
octene derivatives.31,32
2.5 Synthesis of bicyclo[4.2.0]oct-5-enes by a titanium(IV)
complex
Initial studies using alkylidenecyclopropane-propargylic
ester 14a as the substrate were aimed at determining the reaction
outcomes and subsequently optimizing the reaction conditions.
As shown in Scheme 18, the optimal reaction conditions were
identified to carry out the reaction in toluene (0.025 M) at 80 1C
using RhCl(CO)(PPh3)2 (10 mol%) as the catalyst.
Gold, platinum or palladium-catalyzed intramolecular cyclo-
isomerization of propargylic esters is an efficient route to
synthesize a variety of bicyclo[n.1.0]enol esters in good yields.33
More recently, we have found that titanium(IV) chloride and
bismuth(III) chloride can serve as effective promoters to achieve
the synthesis of chlorinated 3-azabicyclo[3.1.0]hexanes from the
We next examined the substrate generality of the reaction
under the optimized conditions. As can be seen from Scheme 19,
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Scheme 24 The transformation of Z-17a–c.
Scheme 21 A plausible reaction mechanism for the formation of 16a.
corresponding easily available propargylic esters under mild
conditions.23 These intriguing results promote us to examine
other novel intramolecular carbocyclization processes to access
other interesting bicyclic ring skeletons. Hence, we reported an
efficient route to accomplish the synthesis of bicyclo[4.2.0]oct-
5-ene derivatives34 which have been only synthesized by heating
allenenes at high temperature in dioxane or DMF through
thermal-induced intramolecular [2+2] cycloaddition reactions.35
Initially, we started the optimization of the reaction condi-
tions by using E-17a. As shown in Scheme 22, the optimal
reaction conditions were identified to carry out the reaction in
dichloromethane at room temperature using TiCl4 (1.2 equiv.)
as a Lewis acid promoter, and under these conditions, cyclo-
isomerization product 18a was formed in 56% yield along with
10 : 1 diastereoselectivity (cis : trans).
Scheme 25 A plausible reaction mechanism for the formation of 18.
the desired products 18 in moderate yields along with good
diastereoselectivities.
To gain more insight into this TiCl4-mediated transforma-
tion of 17 into 18, we investigated the stereochemical course of
ring enlargement of methylenecyclopropanes (MCPs) using
Z-17a and Z-17b as the substrates under the standard condi-
tions and found that the products 18a and 18b were obtained
in 52% yield and 42% yield, respectively (Scheme 24). Sub-
strate 17c having an alkyl group substituted methylenecyclo-
propane was also tested in this reaction, but the corresponding
nucleophilic displacement product 19 was afforded in 52%
yield without the formation of 3-azabicyclo[4.2.0]oct-5-ene
under the standard conditions (Scheme 24).
Next, with the optimized conditions in hand, we investigated
the substrate scope of this TiCl4-mediated carbocyclization
with various methylenecyclopropanes E-17 in the synthesis
of 3-azabicyclo[4.2.0]oct-5-enes 18. As can be seen from
Scheme 23, all of the reactions proceeded smoothly to give
We proposed a plausible reaction mechanism for this TiCl4-
mediated carbocyclization in Scheme 25. Coordination of the
ester group to TiCl4 gives intermediate A5. The nucleophilic
intramolecular addition of the pendant methylenecyclo-
propane to the alkyne moiety along with the release of an acyloxy
group affords carbocation B5, which contains a vinylidene
moiety. Subsequently, carbocationic intermediate B5 undergoes
intramolecular ring enlargement of cyclopropane via 1,2-carbon
migration giving intermediate C5 which can give vinyl cationic
intermediate D5 through isomerization. Then a chloride ion
is transferred to a vinyl cation from the in situ generated
metal complex, affording the corresponding chlorinated
bicyclo[4.2.0]oct-5-ene 18. It should be also noted that carbo-
cation B5 could be stabilized by the neighboring aryl unit
which can serve as a driving force for this transformation. While
alkyl group substituted methylenecyclopropane could not afford
the stabilized intermediate B5, only produces the corresponding
chlorinated nucleophilic displacement compound 19.
Scheme 22 Optimal reaction conditions for the formation of cis- and
trans-18a.
Scheme 23 TiCI4-mediated synthesis of bicyclo[4 2.0]oct-5-enes.
10276 Chem. Commun., 2012, 48, 10271–10279
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(b) J. Tsuji, in Transition Metal Reagents and Catalysts, John
Wiley & Sons Ltd., New York, 2000; (c) L. Yet, Chem. Rev.,
2000, 100, 2963; (d) I. Nakamura and Y. Yamamoto, Chem. Rev.,
2004, 104, 2127.
3. Conclusion
For a long time, carbocyclization of 1,n-enynes and diynes is a
powerful method in organic synthesis to access carbo- or
heterocyclic rings which are important structural motifs found
in many natural and pharmaceutical materials. Various transi-
tion metal catalysts, such as gold, rhodium, silver, and tita-
nium, have been identified as effective promoters in these
transformations to a variety of valuable substances. The
control experiments have indicated that the other metal cata-
lysts are totally ineffective in the corresponding transforma-
tion. Our group has made much more efforts for the synthesis
of five or six-membered heterocyclic compounds, and reported
some novel transition metal-catalyzed cycloisomerizations of
1,n-enynes and diynes containing a propargylic ester moiety.
Further applications of transition metal catalysts and the more
detailed investigation of the related reactions are underway in
our laboratory.
4 For the construction of heterocycles by metal-catalyzed skeletal
reorganization, see: (a) A. S. K. Hashmi, J. P. Weyrauch, W. Frey
and J. W. Bats, Org. Lett., 2004, 6, 4391; (b) A. W. Sromek,
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Acknowledgements
We thank the Shanghai Municipal Committee of Science and
Technology (11JC1402600), National Basic Research Program
of China (973)-2009CB825300, the Fundamental Research
Funds for the Central Universities and the National Natural
Science Foundation of China for financial support (21072206,
20472096, 20872162, 20672127, 21121062 and 20732008).
6 For selected papers on the gold-catalyzed reactions, see: (a) X. Shi,
D. J. Gorin and F. D. Toste, J. Am. Chem. Soc., 2005, 127, 5802;
(b) L. M. Zhang, J. Am. Chem. Soc., 2005, 127, 16804;
(c) S. Z. Wang and L. M. Zhang, J. Am. Chem. Soc., 2006,
128, 8414; (d) S. Z. Wang and L. M. Zhang, J. Am. Chem. Soc.,
2006, 128, 14274; (e) M. Yu, G. Z. Zhang and L. M. Zhang, Org.
Notes and references
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