Cu(I)-Catalyzed Intramolecular Tandem Cyclization of N-Indole-Tethered Cyclopropenes: Synthesis of Functionalized Hydrogenated Diazabenzo[ a]cyclopenta[ cd]azulene Derivatives

Article abstract of DOI:10.1021/acs.orglett.9b00864

A Cu(I)-catalyzed [3 + 2] intramolecular cycloaddition reaction of N-indole-tethered cyclopropenes is presented in this paper. This reaction starts from the formation of ?-allyl cationic intermediate or its resonance-stabilized metal carbenoid intermediate upon activation of cyclopropene with Cu(I) catalyst and a Friedel-Crafts-type cyclization to give functionalized hydrogenated diazabenzo[a]cyclopenta[cd]azulenes in good to excellent yields along with moderate to good dr values. The asymmetric variant of this cycloaddition reaction can be realized, giving the desired products with moderate ee values.

Full text of DOI:10.1021/acs.orglett.9b00864

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cu(I)-Catalyzed Intramolecular Tandem Cyclization of N‑Indole-
Tethered Cyclopropenes: Synthesis of Functionalized Hydrogenated
Diazabenzo[a]cyclopenta[cd]azulene Derivatives
Peng-Hua Li,† Song Yang,† Tong-Gang Hao,† Qin Xu,† and Min Shi*,†,‡
†Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China
University of Science and Technology, 130 Mei Long Road, Shanghai 200237, People’s Republic of China
‡State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, University of Chinese Academy
of Sciences, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, People’s
Republic of China
S
* Supporting Information
ABSTRACT: A Cu(I)-catalyzed [3 + 2] intramolecular
cycloaddition reaction of N-indole-tethered cyclopropenes is
presented in this paper. This reaction starts from the formation
of π-allyl cationic intermediate or its resonance-stabilized metal
carbenoid intermediate upon activation of cyclopropene with
Cu(I) catalyst and a Friedel−Crafts-type cyclization to give
functionalized hydrogenated diazabenzo[a]cyclopenta[cd]-
azulenes in good to excellent yields along with moderate to good dr values. The asymmetric variant of this cycloaddition
reaction can be realized, giving the desired products with moderate ee values.
highly strained carbocycles are readily accessible and can be
involved in transition-metal-catalyzed reactions through
various reaction modes due to their unique characteristics.
First, vinyl carbenoid metal complex, resulting from the ring-
opening process of cyclopropenes, can be formed upon
transition-metal catalysis, which further undergo intramolecu-
lar 1,5- or 1,6-C(sp3)-H insertion reactions or the nucleophilic
additions by oxygen atom to afford the corresponding cyclized
products.4 In addition, cycloaddition reactions with unsatu-
rated moieties are the other important reaction mode. Namely,
these vinyl carbenoid metal complexes can undergo intra-
molecular [1 + 2] cyclopropanation with olefins to achieve
cyclizations (Scheme 1, eq 1).5 Moreover, cyclopropenes can
act as the three-carbon synthon to undergo [3 + 2]
cycloaddition reactions with olefins or [3 + 4] cycloaddition
reactions with 2H-pyran-2-ones in the presence of metal
catalysts or under heating conditions (Scheme 1, eq 1).6 On
the other hand, highly substituted cyclopropane derivatives can
be produced when the ring-opening process of cyclopropenes
is not involved in these cycloaddition reactions. For instance,
substituted cyclopropenes can be used as dipolarophiles in the
cycloaddition reactions with azomethine ylides.7 In 2018,
Stepakov and coworkers disclosed such an example in the
cycloaddition reaction of 1,2-diphenylcyclopropene with
azomethine ylide generated from ketone and proline,
producing formal [3 + 2] cycloadduct without cyclopropene
ring-opening (Scheme 1, eq 2).7b Furthermore, as for
ultiple parallel cyclic structures containing seven-
Mmembered aza-heterocyclic rings are valuable inter-
mediates in organic synthesis. As its subunits, these frame-
works, containing aza-azulene or cyclopenta-azepine motifs
and existing in a great number of bioactive natural products
and pharmaceuticals, frequently displayed significant activity
against rubrum and subtilis and have been used in traditional
medicine to treat carbuncle abscess in folk medicine and others
and as an insecticide (Figure 1).1 Thus, the exploration of new
synthetic approaches to directly construct this pivotal skeleton
and their derivatives is an important objective in the area of
synthetic organic chemistry and medicinal chemistry.2
Cyclopropenes, which have a fascinating versatile reactivity,
have captured the attention of organic chemists in the area of
transition-metal catalysis for more than half a century.3 These
Figure 1. Selected examples of nature products with aza-azulene or
cyclopenta-azepine motifs
Received: March 10, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00864
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Scheme 1. General Reaction Modes of Cyclopropene
Derivatives and This Work
Table 1. Optimization of the Reaction Conditions for
Transition-Metal-Catalyzed Cycloaddition Reactions
a
b
entry
catalyst
solvent yield (%)/(2a:2a′)
1
2
3
4
5
6
7
8
AgOTf
PtCl2
Rh2(OAc)4
PPh3AuNTf2
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCE
25 (1.5:1)
74 (9.9:1)
25 (5.0:1)
88 (1.5:1)
93 (1.3:1)
98 (3.0:1)
88 (2.0:1)
78 (1.7:1)
71 (2.0:1)
25 (1.0:1)
12 (1.4:1)
63 (6.0:1)
IPrAuNTf2
[JohnPhosAu](MeCN)SbF6
DppmAu(MeCN)SbF6
[JackiePhosAu](MeCN)OTf
[tBuXPhosAu](MeCN)NTf2
[tBuBrettPhosAu](MeCN)OTf
9
10
11
12
13
14
15
16
t
[Me4 BuXPhosAu](MeCN)OTf
Cu(MeCN)4BF4
Cu(MeCN)4PF6
Cu(MeCN)4PF6
Cu(MeCN)4PF6
Cu(MeCN)4PF6
c
(86 )(12:1)
74 (10:1)
82 (1.1:1)
64 (3.2:1)
CHCl3
THF
a
Reaction was run under the following conditions: a solution of 1a
cyclopropenes containing an ester group at C3 position, the
coordination between metal catalyst and cyclopropene can
generate metal-activated cyclopropenes, which can be attacked
by the tethered nucleophiles to give functionalized cyclo-
propanes (Scheme 1, eq 3).8
(0.05 mmol) and catalyst (0.005 mmol) in dry solvent (1.0 mL) was
stirred at room temperature under nitrogen atmosphere. Yields and
dr values were determined by H NMR spectroscopic analysis of the
crude mixture. Isolated yield of 2a after purification by flash
b
1
c
chromatography.
In the field of chemical transformations of cyclopropenes, we
investigated a series of reactions of functionalized cyclo-
propenes with indoles or propargylic esters.9 As our ongoing
efforts on the exploration of transition-metal-catalyzed or
mediated new reactions of indoles10 and cyclopropenes, we
designed a new type of N-indole-tethered cyclopropenes to
realize the rapid construction of polyheterocyclic frameworks.
Herein, we report a copper(I)-catalyzed transformation of
these functionalized cyclopropenes to hydrogenated
diazabenzo[a]cyclopenta[cd] azulene derivatives via a tandem
cycloaddition accompanied by a cyclopropene ring-opening
process (Scheme 1, eq 4).
To clarify the feasibility of our working hypothesis, we
initially investigated the reaction of N-(2-(1H-indol-1-yl)-
ethyl)-N-((2-butylcycloprop-2-en-1-yl)methyl)-4-methylben-
zenesulfonamide 1a in the presence of various metal catalysts
(10 mol %), and the results are summarized in Table 1 (for
more details, see Table S1 in the Supporting Information). To
our delight, the desired formal [3 + 2] cycloaddition took place
smoothly in dichloromethane (DCM) at room temperature in
the presence of AgOTf for 20 h, affording the cycloadducts 2a
and 2a′ in total 25% yield along with 1.5:1 dr value (Table 1,
entry 1). Next, we optimized the reaction conditions by
screening various transition-metal catalysts. When PtCl2 was
employed as the catalyst, the diazabenzo[a]cyclopenta[cd]-
azulene 2a was obtained in 74% yield along with the 9.9:1 dr
value (Table 1, entry 2). When Rh2(OAc)4 was used as the
catalyst, 2a was produced in only 25% yield along with a
moderate dr value (5.0:1) (Table 1, entry 3). Notably, the use
of PPh3AuNTf2 as the catalyst gave the desired product 2a in
good yield up to 88% but with lower dr value (1.5:1) (Table 1,
entry 4). On the basis of this finding, several other gold
catalysts coordinated with different ligands were investigated,
and we found that when IPrAuNTf2, [JohnPhosAu](MeCN)-
SbF6, or DppmAu(MeCN)SbF6 was used as the catalyst, the
desired product 2a was given in excellent yields but with lower
dr values (>88% yields and <3.0:1 dr values) (Table 1, entries
5−7). Using sterically bulky phosphine coordinated gold
catalysts such as [JackiePhosAu](MeCN)OTf and
[tBuXPhosAu](MeCN)NTf2 as the catalysts afforded the
desired products in 71−78% yields but still with lower dr
values (<2.0:1 dr values) (Table 1, entries 8 and 9). The other
sterically hindered phosphine ligated gold catalysts such as
[tBuBrettPhosAu](MeCN)OTf and [Me4 BuXPhosAu]-
t
(MeCN)OTf were not efficient catalysts in this transformation,
both giving 2a/2a′ in <25% yields and <1.5:1 dr values (Table
1, entries 9 and 10). Furthermore, we found that when
Cu(MeCN)4BF4 was employed as the catalyst, the desired
product 2a was obtained in 63% yield along with 6.0:1 dr value
(Table 1, entry 12). Gratifyingly, the desired product 2a was
produced in 86% isolated yield and 12:1 dr value when
Cu(MeCN)4PF6 was used as the catalyst (Table 1, entry 13).
After a quick solvent screening in 1,2-dichloroethane (DCE),
CHCl3, and THF, it was found that DCM was the solvent of
choice, giving 2a in higher yield along with better dr value
(Table 1, entries 14−16). The structures of 2a and 2a′ have
both been assigned by X-ray diffraction. Their ORTEP
drawings were shown in Figure 2, and the CIF data were
deposited as CCDC 1401260 and 1574677.
With the optimized reaction conditions in hand, we then
turned our attention toward the substrate scope, and the
reaction limitations and results were summarized in Scheme 2.
B
DOI: 10.1021/acs.orglett.9b00864
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C6H5, none of the desired product could be formed. Next, we
synthesized a series of indole substituted substrates to
investigate the electronic or steric effect on the indole moiety
in this Cu(I)-catalyzed cascade cycloaddition reaction. For
substrates 1g−1m, regardless of whether they had an electron-
rich or electron-poor substituent at indole′s 5-position, the
reactions all proceeded efficiently to afford the corresponding
products 2g−2k and 2m in good to excellent yields ranging
from 76 to 90% along with moderate to good dr values ranging
from 86:14 to 92:8. Only in the case of substrate 1l bearing a
cyano group at the 5-position, the corresponding product 2l
was afforded in 66% yield along with 48:52 d.r. value, perhaps
due to the coordination between cyano group and the
copper(I) catalyst. The methyl substituent could be introduced
at the 4-, 6-, or 7-positions of the indole moiety (substrates
1n−1p), giving the corresponding diazabenzo[a]cyclopenta-
[cd]azulene derivatives 2n−2p in 81, 84, and 90% yields along
with 90:10, 89:11, and 81:19 dr values, respectively. Moreover,
this cycloaddition reaction also tolerated substrates 1q and 1r,
in which the methyl group was introduced at indole′ C2- and
C3-position, also giving the desired products 2q and 2r in 88
and 94% yields along with 86:14 and 88:12 dr values under the
standard reaction conditions, respectively. To further demon-
strate the substrate scope of this reaction, we replaced the
indole moiety with a pyrrole unit (substrate 1s) or a 7-
azaindole moiety (substrate 1t) and found that different
reaction outcomes were obtained. For substrate 1s, the
reaction did not take place under the standard conditions;
however, if IPrAuNTf2 was used as the catalyst, 1-(hex-1-en-2-
yl)-3-tosyl-2,3,4,5-tetrahydro-1H-pyrrolo[1,2-d][1,4]diazepine
2s was obtained in 88% yield at room temperature for 20 h.
This is because upon gold catalysis, the reaction tends to
undergo aromatization and protodeauration to produce a
relatively stable aromatization product instead of the expected
cycloaddition.11 On the other hand, for substrate 1t, the
cyclopropene ring-opening reaction took place exclusively,
presumably due to the coordination of nitrogen atom in the
tethered 7-azaindole moiety with the Cu catalyst, giving a 1,3-
diene derivative 2t in 64% yield under the standard reaction
condition (see Schemes S1 and S2 in the Supporting
Information for their reaction details and the plausible
mechanisms).12
Figure 2. ORTEP drawings of 2a and 2a′.
Scheme 2. Substrate Scope of Cu(I)-Catalyzed [3 + 2]
a,b
Cycloaddition Reactions
To investigate this [3 + 2] cycloaddition in an
intermolecular manner, we synthesized indole derivative 3
and cyclopropene 4 to examine their reaction outcome and
identified that no reaction could take place under the standard
reaction conditions (Scheme 3, eq 1). To further demonstrate
the synthetic usefulness of this [3 + 2] cycloaddition reaction,
a scale-up experiment was performed with 2.0 mmol of
substrate 1a, affording the desired product 2a in 80% yield
(681.3 mg) along with 7:1 dr value after 48 h (Scheme 3, eq
2). Furthermore, its asymmetric variant was investigated by
using chiral phosphine or bisoxazoline ligands (see Table S2 in
the Supporting Information for more details). After condition
screening, we found that using bisoxazoline L34 as a chiral
ligand produced 2a in 80% yield and 14:1 dr value along with
86:13 er value (Scheme 3, eq 3). In addition, for Bs-protected
substrate 1b, the desired product 2b was obtained in 84% yield
and 10:1 dr value along with 66:34 er value (Scheme 3, eq 3).
According to our experimental results and the previously
reported literature,6,9 a plausible mechanism for this [3 + 2]
cycloaddition reaction has been outlined in Scheme 4.
Activation of cyclopropene 1a by Cu(I)-catalyst forms
a
Unless otherwise specified, all reactions were carried out using 1 (0.1
mmol), Cu(MeCN)4PF6 (10 mol %) in DCM (1.0 mL) at room
temperature for 20 h. Yields and dr values were determined by H
NMR spectroscopic analysis of the crude mixture. IPrAuNTf2 (10
mol %) was used instead of Cu(MeCN)4PF6. No desired product
was obtained, and a conjugate diene was formed. N.R. = No reaction.
N.D. = No desired product.
b
1
c
d
Most of the reactions proceeded smoothly under the optimal
conditions, giving the desired products in good yields with
moderate to good diastereoselectivities. Substrates 1b−1d
having different N-protecting groups resulted in the desired
products 2b, 2c, and 2d with results similar to those of 2a,
suggesting that the N-protecting group did not have impact on
the reaction outcome. For substrate 1e, in which R1 and R2 =
C4H9, the reaction also proceeded efficiently to afford the
corresponding product 2e′ as the major diasteromer in 86%
total yield along with 21:79 dr value, presumably due to the
steric effect. In the case of substrate 1f, in which R1 and R2 =
C
DOI: 10.1021/acs.orglett.9b00864
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thermodynamically stable seven-membered heterocycle 2a is
obtained. For substrate 1s, in which the indole moiety is
replaced by a pyrrole unit, the corresponding product 2s can
be formed after protodeauration and aromatization through
Au-intermediate D′ under gold(I) catalysis.
Scheme 3. Further Investigations and Preliminary
Asymmetric Studies
In summary, we disclosed a novel Cu(I)-catalyzed [3 + 2]
cycloaddition reaction of N-indole-tethered cyclopropenes for
the divergent synthesis of functionalized hydrogenated
diazabenzo[a]cyclopenta[cd]azulene derivatives in moderate
to excellent yields under mild conditions along with moderate
to good dr values. The preliminary studies on the use of chiral
ligands demonstrated that the asymmetric version of this
cascade cyclization is also feasible. A plausible mechanism was
proposed on the basis of our experimental results and the
previous literature. Remarkably, this new synthetic method-
ology represents a novel example of transition-metal-catalyzed
[3 + 2] intramolecular cycloaddition reaction for the rapid
construction of complex polyheterocyclic motifs starting from
easily available substrates and subsequently enriches the
chemical transformation of cyclopropenes. Further investiga-
tions on the asymmetric version of this cycloaddition and
application of this protocol to the synthesis of biologically
active substances are underway.
Scheme 4. Plausible Mechanism for the Formation of 2a
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00864.
Experimental procedures and characterization data for
all compounds (PDF)
Accession Codes
CCDC 1401260 and 1574677 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*Fax: (+86)-21-6416-6128; E-mail: mshi@mail.sioc.ac.cn.
ORCID
Min Shi: 0000-0003-0016-5211
Notes
intermediate A, which produces intermediate B via the
addition of Cu+ to the double bond of cyclopropene. Then,
intermediate B undergoes a C−C bond cleavage to give a π-
allyl cationic intermediate C-1 or Cu-carbenoid intermediate
C-2 as well as its resonance-stabilized cationic intermediate C-
3. The key transition states are proposed in Scheme 4 to
account for the observed diastereoselectivity. Presumably, the
si-face attack is more favored because of a π−π stacking
interaction between the aromatic ring of indole and Cu-
carbenoid moiety. From intermediate C, a Friedel−Crafts-type
reaction takes place at indole’s C2 position to generate
intermediate D, which undergoes cyclization to afford
intermediate E. After release of copper(I) catalyst, the more
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Basic Research Program of China [(973)-2015CB856603], the
Strategic Priority Research Program of the Chinese Academy
of Sciences (Grant XDB20000000 and sioczz201808), the
National Natural Science Foundation of China (Grants
220472096, 21372241, 21572052, 20672127, 21421091,
21372250, 21121062, 21302203, 20732008, 21772037,
21772226, and 21861132014), and the Fundamental Research
Funds for the Central Universities Grant 222201717003.
D
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Tang, X. Y.; Shi, M. Angew. Chem., Int. Ed. 2014, 53, 5142. (c) Mei, L.
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(12) Gong, J.; Zhao, Z.; Zhang, F.; Wu, S.; Yan, G.; Quan, Y.; Ma, B.
Org. Lett. 2014, 16, 5524.
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