Rhodium(II)-catalyzed intramolecular annulation of 1-sulfonyl-1,2,3- triazoles with pyrrole and indole rings: Facile synthesis of N-bridgehead azepine skeletons

Article abstract of DOI:10.1002/anie.201400881

A convenient and efficient synthetic method has been developed to construct highly functionalized N-bridgehead azepine skeletons, which are of great importance in biological and pharmaceutical industry. The reaction proceeds through a rhodium(II) azavinyl carbene intermediate, which initiated the intramolecular C-H functionalization with pyrrolyl and indolyl rings. A variety of azepine derivatives were obtained in moderate to good yields under mild reaction conditions with high chemoselectivity. Several interesting derivatizations of the resulting products demonstrate that this method is synthetically valuable and useful. Heads up: A convenient and efficient synthetic method of highly functionalized N-bridgehead azepine skeletons was developed using a rhodium(II)-catalyzed intramolecular annulation of pyrrolyl and indolyl triazoles. Several interesting transformations of the products into poly-heterocyclic products and the reaction mechanism are disclosed. Ts=4-toluenesulfonyl.

Full text of DOI:10.1002/anie.201400881

Angewandte
Chemie
DOI: 10.1002/anie.201400881
Heterocycle Synthesis
Rhodium(II)-Catalyzed Intramolecular Annulation of 1-Sulfonyl-1,2,3-
Triazoles with Pyrrole and Indole Rings: Facile Synthesis of
N-Bridgehead Azepine Skeletons**
Jin-Ming Yang, Cheng-Zhi Zhu, Xiang-Ying Tang,* and Min Shi*
Abstract: A convenient and efficient synthetic method has
been developed to construct highly functionalized N-bridge-
head azepine skeletons, which are of great importance in
biological and pharmaceutical industry. The reaction proceeds
through a rhodium(II) azavinyl carbene intermediate, which
À
initiated the intramolecular C H functionalization with pyr-
rolyl and indolyl rings. A variety of azepine derivatives were
obtained in moderate to good yields under mild reaction
conditions with high chemoselectivity. Several interesting
derivatizations of the resulting products demonstrate that this
method is synthetically valuable and useful.
T
he azepine skeleton is a privileged structural motif in many
biologically active and medicinally valuable molecules.[1] In
addition, polyheterocyclic frameworks built on the azepine
backbone lead to relatively rigid structures which might be
expected to show substantial selectivity in their interactions
with enzymes or receptors.[2] Among these poly-heterocycles,
the structurally diverse and biologically interesting N-bridge-
head azepine skeletons are of great importance because of
their well-represented and wide distribution in nature.
Representative examples, such as Cephalotaxus alkaloids,[3]
Stemona alkaloids,[4] ant venom/frog alkaloids,[5a] and the
antitumor antibiotics anthramycin,[5b] all have N-bridgehead
azepine skeletons (Figure 1). Construction of N-bridgehead
azepine units usually requires multistep approaches.[3–5]
Therefore, it is not surprising that much attention has to be
paid to developing a more general and efficient strategy to
afford such azacyclic skeletons.
Figure 1. Selected examples of naturally occurring compounds and
drugs containing N-bridgehead azepine unit.
1-Sulfonyl-1,2,3-triazoles, stable precursors of rhod-
ium(II) azavinyl carbenes, provide opportunities for a series
of novel carbene-induced transformations such as transannu-
À
lation with unsaturated compounds, X H (X = C, N, or O)
bond functionalization, 1,2-migration, and so on.[6] To date,
the research groups of Fokin,[7] Gevorgyan,[8] Davies,[9]
Murakami,[10] and others[11] have intensively investigated the
diverse reactivities of this carbenoid intermediate. In the case
of a rhodium imino carbene intermediate, it has enabled the
development of many useful transformations, including cyclo-
[7d]
propanation,[7b] cycloaddition,[7a,h,8a] C H insertion, O H/
À
À
[7i,10b,d]
À
N H insertion,
and arylation with boronic acids.[7e] In
2013, the groups of Sarpong[11b] and Gevorgyan[8b] independ-
ently reported intramolecular transannulation of allenyl and
alkynyl triazoles to form 3,4-fused pyrroles. Very recently,
Murakami and co-workers[10h] synthetized tricyclic 3,4-fused
dihydroindoles by a rhodium-catalyzed dearomatizing [3+2]
annulation reaction of 4-(3-arylpropyl)-1,2,3-triazoles. Nota-
bly, Davies and co-workers recently also reported a catalytic
enantioselective formal [3+2] cycloaddition of 1-sulfonyl-
1,2,3-triazoles with C3-substituted indoles (Scheme 1a).[9c]
Encouraged by the finding of Davies and co-workers, and
on the basis of our previous work of gold-catalyzed cyclo-
isomerization of 1,6-diynes[12a] and 1,1-bis(indolyl)-5-alky-
nes,[12b] we successfully the prepared stable pyrrolyl and
indolyl triazoles A through copper(I)-catalyzed azide–alkyne
cycloaddition (CuAAC),[13] and investigated its performance
in rhodium(II)-catalyzed annulation. One major challenge
that needed to be tackled in the desired reaction was the
carbene-induced 1,2 H migration. Given the intramolecular
[*] J.-M. Yang, C.-Z. Zhu, Prof. Dr. M. Shi
Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science and Technology
Meilong Road No. 130, Shanghai, 200237 (China)
Dr. X.-Y. Tang, Prof. Dr. M. Shi
State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences
354 Fenglin Road, Shanghai 200032 (China)
E-mail: siocxiangying@mail.sioc.ac.cn
mshi@mail.sioc.ac.cn
[**] We are grateful for the financial support from the Shanghai
Municipal Committee of Science and Technology (11JC1402600),
the National Natural Science Foundation of China (20472096,
20872162, 20672127, 21372241, 21302203, 21361140350,
21121062, 20732008, and 20902019), the National Basic Research
Program of China (973)-2010CB833302, and the Fundamental
Research Funds for the Central Universities.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400881.
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Table 1: Optimization of the reaction conditions.
Entry[a]
[Rh]
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
[Rh2(Oct)4]
[Rh2(Piv)4]
[Rh2(esp)4]
[Rh2(OAc)4]
[Rh2(Adc)4]
[Rh2(tfa)4]
[Rh2(S-NTTL)4]
[Rh2(Oct)4]
[Rh2(Oct)4]
[Rh2(Oct)4]
DCE
DCE
DCE
DCE
DCE
DCE
DCE
toluene
cyclohexane
CHCl3
86
80
77
78
80
0
70
78
[c]
–
[c]
–
[a] Reaction conditions: 1a (0.1 mmol), cat. (5 mol%), dry solvent
(1.0 mL). [b] Yield of isolated product. [c] Not determined. DCE=1,2-
dichloroethane.
Scheme 1. Previous work and our work on the annulation of pyrroles
and indoles.
Information).[15] To our delight, a one-pot treatment of the
formed enamine with 2.0 equivalents of NaBH3CN resulted in
a smooth reduction of 2a’ to afford the alkylamine 2a in 86%
yield (entry 1). Other catalysts, such as [Rh2(Piv)4], [Rh2-
(esp)2], [Rh2(OAc)4], and [Rh2(Adc)4] gave no better results
than that of [Rh2(Oct)4] (entries 2–5). When 1a was treated
with [Rh2(tfa)4] no reaction occurred (entry 6). The use of
chiral the rhodium(II) tetracarboxylate catalyst [Rh2(S-
NTTL)4][16] afforded the racemic product 2a in 70% yield
(entry 7). Next we turned our attention to examining the
solvent effects. Toluene was substantially less effective than
1,2-dichloroethane (entry 8), and the use of cyclohexane and
chloroform, which have been reported as the optimum
solvents for carbenoid transformations from triazoles,[7c,h,9c]
provided poor results (entries 9 and 10).
With these optimized one-pot, reaction conditions in
hand, we turned our attention to determining the scope and
limitations of the reaction in the presence of [Rh2(Oct)4]. As
summarized in Table 2, all of the reactions proceeded
smoothly to afford the corresponding products 2b–f in 78–
84% yields when sulfonyl group was p-methylbenzenesul-
fonyl (Ts), o-methylbenzenesulfonyl, m-methylbenzenesul-
fonyl, benzenesulfonyl, and mesitylenesulfonyl (Mes), thus
indicating that the electronic and steric effects of the sulfonyl
group did not have significant impact on the reaction
outcome. Next, the scope of this transformation with respect
to the sulfonyl moiety (R2) of the triazole was examined. The
annulations reported by the groups of Murakami,[10h] Gevor-
gyan,[8b] and Sarpong,[11b] the fused ring systems are all
restricted to five- and six-membered rings. The seven-
membered ring has not yet been achieved, presumably
because the longer chain will greatly reduce the chance for
intramolecular reaction, and the 1,2 H migration will become
dominant. To our great delight, upon treatment of A with
a dirhodium catalyst, an interesting N-bridgehead azepine
skeleton product (D) was obtained as the sole product
(Scheme 1b), thus stemming from tandem triazole ring
opening and denitrogenation to generate the key imino
carbene intermediate B[7] and the subsequent annulation[14]
with regeneration of the dirhodium catalyst. We envisaged
that as a result of the highly electrophilic pyrrole or indole,
the 1,2 H migration was suppressed. This transformation
results in important skeletal units, azepines, which are found
in numerous natural products, and in compounds having
important chemical, biological, and medicinal properties
(Figure 1).
Initially, we started our investigation by using the model
substrate 1a and 5 mol% [Rh2(OAc)4] in dry 1,2-DCE (1,2-
dichloroethane) at 808C, and the desired enamine 2a’ was
obtained as two isomers in high overall yield (90%) after
2 hours (Table 1). The structure of E-2a’ has been unequiv-
ocally confirmed by X-ray diffraction (see the Supporting
2
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Table 2: Reaction scope: N-pyrrolyl substrates.
Table 3: Reaction scope: N-indolyl substrates.[a]
[a] Reaction conditions: 3 (0.1 mmol); [Rh2(Oct)4] (5 mol%), anhydrous
DCE (1.0 mL). Yields are those of the isolated products. [b] Substrates
were used only in the first step, and the two resulting isomers were not
reduced. [c] 4l’’ was obtained in 43% yield.
Reaction conditions: 1 (0.1 mmol), [Rh2(Oct)4] (5 mol%), anhydrous
DCE (1.0 mL). Yields are those of the isolated products.
p-bromobenzenesulfonyl (Bs), mesyl (Ms), and mesitylene-
sulfonyl (Mes) derivatives 2g–i could be obtained, using this
method, in moderate to good yields. Furthermore, substituted
pyrrole derivatives bearing methyl, p-methylphenyl, and p-
bromophenyl groups were also suitable for this reaction, thus
giving the corresponding cyclized products 2j–l in reasonable
yields. Thus, the reaction was highly general with respect to
the R1, R2, and R3 groups.
Next, the further examinations were performed to extend
the scope of this cyclization to indolyl triazoles (3; Table 3),
thus affording the N-bridgehead azepines 4 under the
optimized reaction conditions (Table 1, entry 1). As can be
seen from Table 3, the substrate bearing no substituent gave
the desired product 4a in 77% yield. Meanwhile, substrates
bearing either electron-donating (5-Me, 6-Me, 7-Me, 5-OMe)
or electron-withdrawing (6-F, 5-Cl, 4-Br, 5-Br, 6-Br) groups
on the indole core all afforded the corresponding products
4b–e and 4 f–j in moderate to good yields. Moreover, the
carbon-tethered substrate 3k was also suitable for this
cyclization reaction, thus providing the desired isomers 4k’
in 67% overall yield.[17] Only when the substrate was tethered
by an oxygen atom, were the (E)-isomer 4l’ obtained in low
yield along with the b-hydride elimination byproduct 4l’’
(trans only, J = 12.4 Hz, see 1H NMR spectroscopy in the
Supporting Information) in 43% yield. The structures of 4h
and 4l’ have been unequivocally confirmed by X-ray diffrac-
tion.[15]
Considering the easy-to-handle functional groups of the
product, further transformations of 4a to construct polycyclic
azepines were investigated (Scheme 2). The propargyl-sub-
stituted product 5a could be obtained in 69% yield by
treatment with potassium carbonate and propargyl bromide,
and subsequently gave the polycyclic indole 6a in 85% yield
in the presence of [Au(tBuXPhos)(NCMe)][SbF6] (XPhos =
2-dicyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl)
(5 mol%).[18] Moreover, the corresponding tetracyclic
[1,4]diazepino[4,5-a]indole skeleton 7a could be synthesized
in dichloromethane at room temperature by the Pictet–
Spengler reaction in 71% yield.[19]
A kinetic isotope effect (KIE) study suggested that the
À
C H bond cleavage was not involved in the rate-determining
step (for more details, see the Supporting Information).
In conclusion, we have developed a highly efficient and
accessible rhodium(II)-catalyzed intramolecular annulation
of pyrrolyl and indolyl triazoles. A variety of N-bridgehead
azepine derivatives are obtained in moderate to good yields
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Scheme 2. Transformations of 4a into 5a, 6a, and 7a. TFA=trifluoro-
acetic acid.
under mild reaction conditions with high chemoselectivity
and wide scope with respect to the azepine ring (N, O, C). The
reaction mechanism is proposed on the basis of isotopic
labeling and control experiments. Further applications of this
chemistry and more detailed mechanistic investigations are
underway in our laboratory.
Received: January 27, 2014
Published online: && &&, &&&&
Keywords: annulation · heterocycles · reaction mechanisms ·
.
rhodium · synthetic methods
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4
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Communications
Heterocycle Synthesis
J.-M. Yang, C.-Z. Zhu, X.-Y. Tang,*
M. Shi*
&&&& — &&&&
Rhodium(II)-Catalyzed Intramolecular
Annulation of 1-Sulfonyl-1,2,3-Triazoles
with Pyrrole and Indole Rings: Facile
Synthesis of N-Bridgehead Azepine
Skeletons
Heads up: A convenient and efficient
synthetic method of highly functionalized
N-bridgehead azepine skeletons was
developed using a rhodium(II)-catalyzed
intramolecular annulation of pyrrolyl and
indolyl triazoles. Several interesting
transformations of the products into poly-
heterocyclic products and the reaction
mechanism are disclosed. Ts=4-tolue-
nesulfonyl.
6
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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