The divergent synthesis of nitrogen heterocycles by rhodium(II)-catalyzed cycloadditions of 1-sulfonyl 1,2,3-triazoles with 1,3-dienes

Article abstract of DOI:10.1002/anie.201400426

The first rhodium(II)-catalyzed aza-[4+3] cycloadditions of 1-sulfonyl 1,2,3-triazoles with 1,3-dienes have been developed, and enable the efficient synthesis of highly functionalized 2,5-dihydroazepines from readily available precursors. In some cases, the reaction pathway could divert to formal aza-[3+2] cycloadditions, thus leading to 2,3-dihydropyrroles. In this context, the titled reaction represents a capable tool for the divergent synthesis of two types of synthetically valuable aza-heterocycles from common rhodium(II) iminocarbene intermediates. On the (di)verge: Rhodium(II)-catalyzed cycloadditions of 1-sulfonyl 1,2,3-triazoles with 1,3-dienes have been developed and enable the efficient and divergent synthesis of two types of synthetically valuable nitrogen heterocycles, 2,5-dihydroazepines and 2,3-dihydropyrroles, by formal [4+3] and [3+2] cycloadditions, respectively. Ts=4-toluenesulfonyl.

Full text of DOI:10.1002/anie.201400426

Angewandte
Chemie
DOI: 10.1002/anie.201400426
Cycloaddition
The Divergent Synthesis of Nitrogen Heterocycles by Rhodium(II)-
Catalyzed Cycloadditions of 1-Sulfonyl 1,2,3-Triazoles with 1,3-
Dienes**
Hai Shang, Yuanhao Wang, Yu Tian, Juan Feng, and Yefeng Tang*
Abstract: The first rhodium(II)-catalyzed aza-[4+3] cyclo-
additions of 1-sulfonyl 1,2,3-triazoles with 1,3-dienes have
been developed, and enable the efficient synthesis of highly
functionalized 2,5-dihydroazepines from readily available
precursors. In some cases, the reaction pathway could divert
to formal aza-[3+2] cycloadditions, thus leading to 2,3-
dihydropyrroles. In this context, the titled reaction represents
a capable tool for the divergent synthesis of two types of
synthetically valuable aza-heterocycles from common rhodi-
um(II) iminocarbene intermediates.
I
nvention of new reactions that enable the divergent syn-
thesis of different aza-heterocycles from common intermedi-
ates has been a subject of intensive research in organic
synthesis. As a paradigm, rhodium(II) iminocarbenes,[1] which
readily generated from 1-sulfonyl 1,2,3-triazoles upon treat-
ment with rhodium(II) catalysts, have recently emerged as
capable intermediates in the synthesis of various aza-hetero-
cycles, including pyrrole,[2] imidazole,[3] oxazoline,[3b] pyrro-
loindoline[4] and others[5] (Figure 1a), as described in the
seminal contributions from the groups of Fokin, Gevorgyan,
Murakami, and Davies. Formally, rhodium(II) iminocarbenes
Figure 1. Cycloadditions of 1-sulfonyl 1,2,3-triazoles with unsaturated
chemical bonds. Ms=methanesulfonyl, Ts=4-toluenesulfonyl.
could serve either as a [1C] synthon to promote [2+1]
[7]
cycloaddition,[6] C H insertion, and others,[8] or as an aza-
[4+3] cycloadditions of 4-alkenyl-1-sulfonyl-1,2,3-triazoles
with 1,3-dienes (Figure 1b).[10] However, the putative alkenyl
rhodium(II) iminocarbene intermediates served as [3C]
synthon instead of aza-[3C] synthon. More recently, the
rhodium(II)-catalyzed cycloadditions of 1-sulfonyl 1,2,3-tri-
azoles with a,b-unsaturated aldehydes were disclosed by
Murakami and co-workers, however, only one case of an aza-
[4+3] cycloaddition was documented as side-reaction.[5d] In
this context, the aza-[4+3] cycloadditions of 1-sulfonyl 1,2,3-
triazoles with 1,3-dienes remain to be explored.
À
[3C] synthon in various formal [3+2] cycloadditions.[2–5] So
far, a wide range of unsaturated chemical bonds, including
alkyne,[2a–c,5a,b] alkene,[6] allene,[2d,e] aldehyde,[3b] imine,[3b]
nitrile,[3a] isocyanate, and isothiocyanate,[5c] have been
employed as [2C] components in formal [3+2] cycloadditions.
Comparably, cycloadditions of 1-sulfonyl 1,2,3-triazoles with
conjugate dienes have been rarely explored.[9] In 2013, Parr
and Davies reported the first rhodium(II)-catalyzed formal
Azepines are fundamental structural elements widely
distributed in natural products and pharmaceuticals.[11]
Among the many strategic bond disconnections of azepine
frameworks,[12] [4+3] cycloadditions are particularly attrac-
tive for their remarkable efficiency and convergency. Despite
some advances on this subject,[13] the development of a new
variant of [4+3] cycloadditions to assemble azepines remains
highly desirable. Given that rhodium(II) iminocarbenes have
demonstrated their versatile reactivities in the [2+1] and
[3+2] cycloadditions, we anticipated that they could also be
applied to aza-[4+3] cycloadditions. As a result, we report
herein the first rhodium(II)-catalyzed aza-[4+3] cycloaddi-
tions of 1-sulfonyl 1,2,3-triazoles with 1,3-dienes, a reaction
enabling the efficient synthesis of highly functionalized 2,5-
dihydroazepines from readily available precursors (Fig-
ure 1c). Notably, in some cases the reaction pathway could
[*] Dr. H. Shang,[+] Y. Wang,[+] J. Feng, Prof. Dr. Y. Tang
The Comprehensive AIDS Research Center, Department of
Pharmacology & Pharmaceutical Sciences, School of Medicine
Tsinghua University, Beijing 100084 (China)
E-mail: yefengtang@tsinghua.edu.cn
Dr. Y. Tian
Institute of Medicinal Plant Development, Chinese Academy of
Medical Sciences, Peking Union Medical College
Beijing 100193 (China)
[+] These authors contributed equally to this work.
[**] We acknowledge the financial support from the National Science
Foundation of China (21102081, 21272133), New Teachers’ Fund for
Doctor Stations, Ministry of Education (20110002120011), and
Beijing Natural Science Foundation (2132037).We also thank J. K.
Fu of Peking University for the initiation of this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400426.
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Table 1: Screening for rhodium(II)-catalyzed cycloadditions of 1,2,3-
divert to formal [3+2] cycloadditions, thus mainly leading to
2,3-dihydropyrroles, another important class of aza-hetero-
cycles which are widely employed as key intermediates in
organic synthesis.[14]
triazoles with 1,3-dienes.[a]
We initiated our study by treatment of the readily
accessible triazole 1a[15] and (E)-1-phenyl-1,3-butadiene
(2a)[16] with 1 mol% [Rh2(oct)4] in 1,2-DCE at 1208C for
12 hours. Fortunately, two major products formed, and one of
them, isolated in 37% yield, was unambiguously confirmed as
the aza-[4+3] cycloadduct 3a by X-ray crystallography
(Scheme 1).[17] The other one, isolated in 55% yield, turned
Entry Cat.
Solvent
Other conditions
Yield [%][b]
3a
4a
1
[Rh2(oct)4]
1,2-DCE 2a, 1408C, 12 h
1,2-DCE 2a, 1008C, 2 h
1,2-DCE 2a, 808C, 12 h
1,2-DCE 2a, 1408C, 12 h
1,2-DCE 2a, 1408C, 12 h
1,2-DCE 2a, 1408C, 12 h
–
85
25
trace
51
trace
80
79
67
61
55
64
5
2
[Rh2(oct)4]
47
15
–
–
–
–
–
–
–
3
[Rh2(oct)4]
4
5
6
[Rh2(OAc)4]
[Rh2(TFA)4]
[Rh2(S-ptad)4]
7
[Rh2(S-dosp)4] 1,2-DCE 2a, 1408C, 12 h
8
9
10
11
12
13
[Rh2(oct)4]
[Rh2(oct)4]
[Rh2(oct)4]
[Rh2(oct)4]
[Rh2(oct)4]
[Rh2(oct)4]
CHCl3
toluene
xylene
PhCl
2a, 1408C, 12 h
2a, 1408C, 12 h
2a, 1408C, 12 h
2a, 1408C, 12 h
–
61
1,2-DCE 5a, 1408C, 0.5 h
1,2-DCE 5a, 4 ꢀ M.S., MW 81
1208C, 5 min
–
[a] Reaction conditions: 1a (0.20 mmol), 2a (0.3 mmol) or 5a
(0.4 mmol), and rhodium(II) catalyst (0.002 mmol) in DCE (1.0 mL).
[b] Yield of isolated product. DCE=dichloroethane, (S)-dosp=4-(do-
decyl-phenyl)sulfonyl-(2S)-prolinate, M.S.=molecular sieves, MW=mi-
crowave, oct=octanoate, (S)-ptad=N-phthaloyl-(S)-adamantylglycine,
TFA=trifluoroacetate.
Scheme 1. Initial results of cycloaddition of 1-sulfonyl 1,2,3-triazole
with (E)-1-phenyl-1,3-diene.
out to be 4a, a formal [3+2] cycloadduct. Interestingly, we
found that extending the reaction time resulted in the
formation of 4a as the sole product in 65% yield, and thus
indicated that 3a could gradually be converted into 4a under
the reaction conditions. This speculation was subsequently
proved by the conversion of the isolated 3a into 4a under
thermal conditions (1,2-DCE, 1408C, 12 h), presumably by
a metal-free, thermo-promoted allylic amine 1,3-migration.[18]
Encouraged by the preliminary results, we conducted
a systematic condition screening to improve both the
efficiency and selectivity of the reaction. A simple evaluation
of the effect of reaction temperature showed that high
temperature (1408C) and long reaction times gave 4a as
sole product in 85% yield (entry 1, Table 1), whereas low
temperature (1008C) and short reaction times favored the
formation of 3a as major product (entry 2). A further
decrease in the temperature to 808C only resulted in poor
conversion (entry 3). Next, various rhodium(II) catalysts and
solvents were evaluated in the reaction, however, none of
them led to satisfying results (entries 4–11). Most of the cases
afforded a mixture of 3a and 4a in the early stage of the
reaction, and then 4a as dominant product in the end.
Serendipitously, we found the geometry of 1,3-diene partner
had a profound influence on the outcome: when (Z)-1-
phenyl-1,3-diene (5a) was employed, 3a was isolated in 61%
yield, along with only small amount of 4a (entry 12). More-
over, no Z isomer of 4a (structure not shown) was observed in
this reaction. Following this route, we finally identified the
optimal reaction conditions (5a, 4 ꢀ M.S., MW, 1208C, 5 min)
which afforded 3a in excellent yield (entry 13). Notably, it was
crucial to control the reaction temperature and time for
obtaining good results.
With the optimal reaction conditions secured, we turned
to evaluate their generality. First of all, the scope of the [3+2]
cycloadditions was examined. It was found that various (E)-1-
aryl-1,3-dienes underwent the desired reactions with 1a to
afford the 2,3-dihydropyrroles 4a–h in good to excellent
yields (Scheme 2). Generally, the electron-rich dienes showed
better reactivity than the electron-deficient ones. An array of
4-aryl-1-sulfonyl-1,2,3-triazoles were also examined with 2a
as the diene partner. It turned out that all of them gave
satisfying outcomes, although the electron-deficient triazoles
usually performed better than the electron-rich ones. In
alignment with the previous observations, the reactions with
E-1,3-dienes generally afforded a mixture of [4+3] and [3+2]
cycloadducts at the early stage of the reaction, and then the
latter as dominant product in the end.
In parallel, the scope of [4+3] cycloadditions was also
evaluated by employing various (Z)-1-aryl-1,3-dienes and 4-
aryl-1-sulfonyl-1,2,3-triazoles. It was found that most of the
reactions worked well to give the corresponding 2,5-dihy-
droazepines 3a–g and 3i–l in good to excellent yields
(Scheme 2). The only exception was the reaction with 2-
methoxyphenyl-1,3-diene, which gave a mixture of 3h and 4 f
in excellent combined yields but poor selectivity.
In addition to 1-aryl-1,3-dienes, we also employed 2-aryl-
1,3-dienes 6 in the reactions (Scheme 3). Interestingly, it was
shown that the 2-aryl-1,3-dienes displayed properties distinct
from the 1-aryl-1,3-dienes. Firstly, all of the reactions with 2-
aryl-1,3-dienes afforded the corresponding [4+3] cycload-
ducts 7a–f in good to excellent yields, thus showing little
substituent effects with respect to both the diene and triazole
partners. Notably, although we did isolate a small amount of
the [3+2] adducts (e.g. 8c, 8i, 8k and 8l) in some cases, the
2
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Scheme 4. Cycloadditions of 1a with other 1,3-dienes. Conditions A
and B are the same as those described in Scheme 2. Yields are those
of the isolated products. TBS=tert-butyldimethylsilyl, TIPS=triisopro-
pylsilyl.
Scheme 2. Scope of [3+2] and [4+3] cycloadditions. Conditions A:
1 (0.20 mmol), 2 (0.30 mmol), and [Rh2(oct)4] (0.002 mmol) in DCE
(1.0 mL) at 1408C for 12 h. Conditions B: 1 (0.20 mmol), 5
(0.40 mmol), 4 ꢀ M.S., and [Rh2(oct)4] (0.002 mmol) in DCE (1.0 mL)
at 1208C with microwave irridation for 5-10 min. Yields are those of
the isolated products.
To further extend the substrate scope, we next examined
several other types of 1,3-dienes. As depicted in Scheme 4, the
1,1-diphenyl-, 1-phenyl-2-methyl-, and 1-TBSO-substituted
1,3-dienes displayed similar propensity to that of 1-aryl-1,3-
dienes, mainly leading to 2,3-dihydropyrroles (11a–c) under
the reaction conditions A (Scheme 4). Comparably, for the 1-
alkyl-substituted 1,3-dienes 9d and 9e, although the reactions
worked well, the selectivities (10d/11d and 10e/11e) were
only modest (ca. 1:2). Not surprisingly, in alignment with the
previous observations for 2-aryl-1,3-dienes, the 2-methyl-, 2,3-
dimethyl- and 2-TIPSO-1,3-dienes predominantly gave the
[4+3] cycloadducts (10 f–h) under the reaction conditions B
(Scheme 4).
Notably, in the cycloadditions with 9d and 9e, some
intermediates could be monitored at the early stage of the
reaction. To gain insight into the mechanism, the intermedi-
ates derived from 9d and 9e were isolated, and were
respectively proven to be the cyclopropylaldimines 12 and
13 (Scheme 5a,b).[19] We also found that 12 and 13 could be
converted into the corresponding [3+2]- and [4+3]-cyclo-
adducts under thermal conditions (1,2-DCE, 1408C, 12 h),
while the resulting 10d or 10e failed to advance to 11d or 11e,
respectively, under the same reaction conditions. A similar
phenomenon was also observed in the cycloaddition of 2a
with 1l (Scheme 5c). However, we failed to identify such
intermediates in most of the other cycloadditions with aryl-
substituted 1,3-dienes. We speculated that it could be
attributed to the fleeting nature of the cyclopropylaldimine
intermediates involved in these cases. This assumption was
supported by the following experiments: upon treatment of 13
with styrene in the presence of catalytic amount of the
Hoveyda–Grubbs second-generation catalyst in CH2Cl2 at
408C for 12 hours, a mixture of 3a and 4a (3a/4a = 3:1) were
obtained, apparently via the putative intermediate 16
(Scheme 5d). While the yield of the transformation was
modest without further optimization, we did not identify 10e
Scheme 3. Scope of [4+3] cycloadditions with 2-aryl-1,3-dienes. Con-
ditions B are the same as those described in Scheme 2. Yields are
those of the isolated products.
selectivities of the cycloadditions were generally good.
Secondly, in contrast of the previous observations, all of the
2,5-dihydroazepines resulting from 2-aryl-dienes were rather
stable and did not evolve into the corresponding 2,3-
dihydropyrroles, even with long reaction times.
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dihydropyrroles, respectively through formal aza-[4+3] and
[3+2] cycloadditions. The easy availability of the starting
materials, high synthetic value of the resulting products, and
intriguing mechanism of the titled reactions render it not only
conceptually novel, but also synthetically useful. Further
mechanistic studies and applications of the method to the
syntheses of bioactive molecules are in progress.
Received: January 15, 2014
Revised: March 10, 2014
Published online: && &&, &&&&
Keywords: carbenes · cycloaddition · nitrogen heterocycles ·
.
rhodium · synthetic methods
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Scheme 6. Proposed mechanism of cycloadditions of 1-sulfonyl 1,2,3-
triazoles with 1,3-dienes.
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In summary, the novel rhodium(II)-catalyzed cycloaddi-
tions of 1-sulfonyl 1,2,3-triazoles with 1,3-dienes has been
developed, and enable the efficient and divergent synthesis of
two types of aza-heterocycles, 2,5-dihydroazepines and 2,3-
4
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[17] CCDC 979317 (3a) contains the supplementary crystallographic
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ccdc.cam.ac.uk/data_request/cif.
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[19] 12 and 13 are not very stable and partially hydrolyzed into the
corresponding aldehydes when chromatographed. Thus, their
structures were assigned based on the aldehyde derivatives (for
details, see the Supporting Information). A similar strategy was
also applied to 14.
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

.
Angewandte
Communications
Communications
Cycloaddition
H. Shang, Y. Wang, Y. Tian, J. Feng,
Y. Tang*
&&&& — &&&&
The Divergent Synthesis of Nitrogen
Heterocycles by Rhodium(II)-Catalyzed
Cycloadditions of 1-Sulfonyl 1,2,3-
Triazoles with 1,3-Dienes
On the (di)verge: Rhodium(II)-catalyzed
cycloadditions of 1-sulfonyl 1,2,3-tri-
azoles with 1,3-dienes have been devel-
oped and enable the efficient and diver-
gent synthesis of two types of syntheti-
cally valuable nitrogen heterocycles, 2,5-
dihydroazepines and 2,3-dihydropyrroles,
by formal [4+3] and [3+2] cycloadditions,
respectively. Ts=4-toluenesulfonyl.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!