Synthesis of polysubstituted 3-aminoindenes: Via rhodium-catalysed [3+2] cascade annulations of benzimidates with alkenes

Article abstract of DOI:10.1039/c9cc00567f

A novel Rh-catalysed intermolecular [3+2] cascade cyclization of benzimidates and alkenes has been developed to assemble polysubstituted 3-aminoindenes. The target products are important building blocks for organic synthesis and drug discovery. Cheap and easily available α,β-unsaturated ketones or nitroalkenes are applied as coupling partners. Acyl or nitro groups are easily introduced to 3-aminoindenes at the C-2 position, which makes this reaction particularly attractive for further transformation to synthesize various important building blocks.

Full text of DOI:10.1039/c9cc00567f

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C9CC00567F
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Synthesis of polysubstituted 3-aminoindenes via rhodium-
catalysed [3 + 2] cascade annulations of benzimidates with
alkenes†
Received 00th January 20xx,
Accepted 00th January 20xx
Binjie Wu, Zi Yang, Hong Zhang, Lianhui Wang, and Xiuling Cui*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel Rh-catalysed intermolecular [3 + 2] cascade cyclization of
benzimidates and alkenes has been developed to assemble
polysubstituted 3-aminoindenes. The target products are
important building blocks for organic synthesis and drug discovery.
Cheap and easily available α,β-unsaturated ketones or nitroalkene
are applied as coupling partner. Acyl or nitro groups are easily
introduced to 3-aminoindenes at the C-2 position, which makes this
reaction particularly attractive for further transformation to
synthesize various important building blocks.
a) Takai (2005):
H
R2
R2
HN
R3
Ph
HN
Re
NR2
R3
Ph
R1
or
+
touene
reflux
R1
R1
Ph
R3
b) Zhao (2010), Cramer (2011), Li (2012, 2013), Wang (2016) and Yi (2018):
R2
R2 H
N
R4
R3
R5
Rh, Ru, or Re
NR3
R1
R1
+
R5
R4
c) This work:
OR2
Indenamines are key structural frameworks embedded in a
large number of biologically active molecules and exhibit a wide
range of interesting pharmacological properties, such as
compound A and B (Fig. 1).1 As a result, these molecules provide
an attractive platform for structure-activity relationship studies
and discovery in drug development.2 Therefore, novel efficient
strategies to access such a skeleton are highly desirable.
NH2
R4
Rh(III)
R4
R1
NH
+
R1
1-Ada-CO2H
DCE
R3
R3
Scheme 1 Synthesis of indenamines.
3-aminoindenes via Re catalysed cyclization of aldimines with
phenyl acetylenes in 2005 (Scheme 1a).5 Afterwards, Zhao and
co-workers reported a Rh-catalysed [3+2] annulation of N-
unsubstituted ketimines and internal alkynes to afford
substituted 1-aminoindene derivatives (Scheme 1b).6
Subsequently, Cramer and co-workers reported an
enantioselective synthesis of 1-aminoindenes via rhodium(I)-
catalysed C-H functionalization of unsubstituted ketimines with
internal alkynes (Scheme 1b).7 Later, Li and co-workers
reported a catalytic annulation of N-sulfonyl imines or
azomethine ylides with alkynes to form indenamines (Scheme
1b).8 Wang and co-workers reported a rhenium catalysed [3 +
2] carbocyclization of ketimines and alkynes to approach the
unprotected tertiary indenamines in 2016 (Scheme 1b).9 More
recently, Yi and co-workers reported an efficient rhodium(III)-
catalysed C-H transformation of oximes for the synthesis of
indenamine skeletons (Scheme 1b).10 These strategies all
focused on the cycloaddition of arylimines with alkynes, which
is not beneficial to expand the substrate scope. Herein, we
report a protocol to polysubstituted 3-aminoindenes from
benzimidates and alkenes through rhodium-catalysed C-H
activation/carbocyclization (Scheme 1c). Cheap and easily
O
HN
O
HN
O
O
NH Cl
A
B
(potent antitussive activity)
(calcium antagonists)
Fig. 1 Examples illustrating the importance of indenamines.
Recently, transition-metal-catalysed directly intermolecular
cyclization via C-H bond activation3 have been established as a
straightforward method to construct such a core.4 For example,
Takai and co-workers reported the first catalytic synthesis of
Engineering Research Center of Molecular Medicine of Ministry of Education, Key
Laboratory of Fujian Molecular Medicine, Key Laboratory of Xiamen Marine and
Gene Drugs, School of Biomedical Sciences, Huaqiao University, Xiamen 361021, P.
R. China. E-mail: cuixl@hqu.edu.cn
† Electronic Supplementary Information (ESI) available. CCDC 1889341. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Table 2 Scope of the substratesa,b
available α,β-unsaturated ketones11 or nitroalkene12 are
applied as coupling partner. The acyl or nitro groups could be
smoothly introduced to 3-aminoindenes at the C-2 position,
which makes the products of this reaction particularly attractive
for further transformation to synthesize various important
molecules. Additional, this protocol exhibits good functional-
group tolerance and excellent regioselectivity.
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DOI: 10.1039/C9CC00567F
NH2
OR1
R4
[Cp*Rh(MeCN)3](SbF6)2 (5 mol%)
O
NH
+
R2
R3
R4
O
1-Ada-CO2H (1 equiv)
DCE
R2
R3
2
1
3
NH2
NH2
NH2
R2
The condensation of ethyl benzimidate (1ab) and 1,3-
diphenyl-2-propen-1-one (2a) was initially chosen as a model
reaction to optimize the various reaction parameters (Table 1).
To our great delight, the desired product 3a was obtained in 39%
yield when [Cp*RhCl2]2/AgSbF6 was used as a catalyst and AcOH
(1 equiv) as an additive in DCE at 120 °C under air atmosphere
(entry 1). Other metals, such as Pd(OAc)2, [Ru(p-cymene)Cl2]2,
and [Cp*IrCl2]2 complexes, were valuated as catalyst in our
system. Unfortunately, none of them worked for this
transformation (entries 2−4). 42% Yield of 3a was obtained
when [Cp*RhCl2]2/AgNTf2 was employed as a catalyst (entry 5).
Switching the catalyst to [Cp*Rh(MeCN)3](SbF6)2 gave a higher
yield of 47% (entry 6). The reaction did not proceed smoothly
without Rh catalyst (entry 7). Then, various additives, including
PivOH, 1-Ada-CO2H, NaOAc, and LiOAc, were screened, showing
that 1-Ada-CO2H gave the highest yield (entries 8−11). The yield
of 3a was decreased to 33% in the absence of additive (entry
12). Other solvents, such as toluene, 1,4-dioxane, and TFE, were
less effective for this catalytic reaction (entries 13−15). The
reaction was not significantly affected when carried out under
O2 or N2 atmosphere (entries 16−17). Changing the reaction
temperature or decreasing the reaction time to 24 hours did not
favor this reaction (entries 18−20).
R2
O
O
O
3a
R1=Me, 70%
2
2
3b
3c
3d
3e
, R =Cl, 77%
3f
, R =CH3, 76%
2
2
R1=Et, 88% (1 mmol, 57%)
R1=n-Pr, 88%
, R =Br, 87%
3g
, R =OMe, 57%
2
, R =Me, 88%
2
R1=i-Bu, 84%
, R =OMe, 68%
NH2
R4
O
NH2
NH2
O
O
R3
R3
4
3
3o
3p
3q
3
, R =4-FC6H4, 75%
3l
, R =3-MeC6H4, 81%
3h
, R =4-FC6H4, 85%
4
3
3
, R =4-ClC6H4, 82%
3m
, R =3-OMeC6H4, 53%
3i
3j
, R =4-ClC6H4, 75%
4
3
3
, R =4-MeC6H4, 67%
3n
, R =3-NO2C6H4, 74%
, R =4-NO2C6H4, 76%
4
3r
3
, R =4-OMeC6H4, 64%
3k
, R =4-MeC6H4, 74%
NH2
NO2
NH2
NH2
O
NH2
O
O
3s
3t, 56%
, 62%
3v
3u
, 71%
, 66%
aReaction conditions: 1 (0.2 mmol), 2 (2.0 equiv), [Cp*Rh(MeCN)3](SbF6)2 (5.0 mol%), 1-
Ada-CO2H (1.0 equiv), DCE (2.0 mL), 120 °C, 36h, under air. bIsolated yields.
Table 1 Optimization of reaction conditionsa
NH2
Ph
Catalyst
Ag salt
OEt
NH
O
O
+
Additive
Solvent
Temperature
Ph
Ph
Ph
1ab
2a
3a
Entry
1
2
3
4
5
6
7
8
Catalyst
Additive
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
PivOH
Solvent
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Toluene
1,4-Dioxane
TFE
DCE
DCE
Yieldb(%)
39
N.R.
N.R.
N.R.
42
47
N.R.
48
88
42
81
33
51
Trace
21
83
89
70
63
72
[Cp*RhCl2]2 /AgSbF6
Pd(OAc)2
[Ru(p-cymene)Cl2]2
[Cp*IrCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[Cp*Rh(MeCN)3](SbF6)2
-
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
[Cp*Rh(MeCN)3](SbF6)2
Fig. 2 X-ray molecular structure of 3a.
With the optimal reaction conditions in hand (entry 6, table
1), we first studied the scope and generality with respect to aryl
imidates as shown in Table 2. Different alkyl groups as R1 were
used in the reaction and gave the intended product 3a in 70-88%
yields. Ethyl and n-propyl groups gave the superior results.
While introducing methyl group as R1 led to a relatively lower
yield. Therefore the ethyl group as R1 of aryl imidates was
chosen for the next study. Significantly, the transformation
could be carried out on a 1 mmol scale in reasonable yield (57%).
The molecular structure of 3a was confirmed by single-crystal
X-ray diffraction analysis (Fig. 2 and SI). Aryl imidates bearing
both electron-donating and electron-withdrawing groups at the
para position of the phenyl ring all coupled smoothly with
chalcone 2a to afford the annulated products (3b-3e) in 68-88%
yields. In the case of meta-substituted arylimidates, the C-H
activation occurred selec-tively and consistently at the less
hindered site, furnishing the products in good yields (3f, 3g).
Aryl imidates bearing a halogen atom (Cl, Br) were well
9
1-Ada-CO2H
NaOAc
LiOAc
10
11
12
13
14
15
16c
17d
18e
19f
20g
-
1-Ada-CO2H
1-Ada-CO2H
1-Ada-CO2H
1-Ada-CO2H
1-Ada-CO2H
1-Ada-CO2H
1-Ada-CO2H
1-Ada-CO2H
DCE
DCE
DCE
aReaction conditions: 1ab (0.2 mmol), 2a (2.0 equiv), [Cp*Rh(MeCN)3](SbF6)2 (5.0 mol %),
1-Ada-CO2H (1.0 equiv), DCE (2 mL), 120 °C, 36 h, under air. bIsolated yields. cunder N2.
dunder O2. e110 °C. f130 °C. g24 h. N.R. = no reaction. 1-Ada-CO2H = 1-adamantane
carboxylic acid.
2 | J. Name., 2012, 00, 1-3
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tolerated under the optimized conditions to deliver the target
products (3b, 3c).
To further investigate the reaction mecVhieawnAisrtmicle, Onwlinee
investigated the annulation of ethyl benzDimOIi:d1a0t.1e0(319a/Cb9)CaCn0d0516,37F-
Subsequently, substituted α,β-unsaturated ketones were diphenyl-2-propen-1-one (2a) under the standard reaction
then tested with model benzyl imidate 1a under the optimized conditions (Scheme 3). After 3 hours, the intermediate E was
conditions (Table 2). When R3 and R4 were substituted by generated and detected by HRMS. However, the intermediate E
phenyl groups, a variety of electron-donating or electron- could not be detected by HRMS after 36 hours. These results
withdrawing groups in R3 or R4 were well-tolerated in this suggested that the final product 3a might be generated from
transformation, producing the corresponding 3-aminoindenes the intermediate E by elimination of EtOH.
in moderate to excellent yields (3h−3r). These results revealed
On the basis of the results obtained and previous reports,13 a
that the electron density on the moiety of R3 or R4 did not plausible reaction pathway for this Rh(III)-catalysed cascade
significantly influence on the efficiency of the reaction. cyclization reaction is proposed and shown in Scheme 4. Firstly,
Moreover, the corresponding 3-aminoindene could be obtained coordination of the rhodium atom to the NH and the
in good yield when R3 or R4 was alkyl group (3s, 3t). The desired subsequent C-H activation result in the formation of the five-
product was afforded in 66% yield when R4 was 1-naphthyl (3u). membered rhodacyclic intermediate A. Subsequently, the
It is worth mentioning that nitroalkene could also be easily double bond of α,β-unsaturated ketone coordinates with Rh
converted into the corresponding product in 71% yield (3v). atom in intermediate A to produce species B, which undergoes
While the terminal olefin, such as 1-hexene or styrene and migratory insertion to generate seven-membered ring
olefins with ester, amide, sulfone, or other electron- intermediate C. Then, an intramolecular nucleophilic addition
withdrawing groups do not work under the standard reaction leads to complex D. Protonation of intermediate D delivered the
conditions.
active Rh(III) species for next catalytic cycle and the
Furthermore, the resultant products obtained via our intermediate E. Finally, elimination of EtOH as the only
protocol are amenable to further chemical transformations. For byproduct from E affords the final cyclization product 3a.
example, compound 3a could be condensed with
In summary, we have successfully developed a novel and
phenylhydrazine, leading to the corresponding pyrazole in efficient synthetic route for straightforward access to 1,2-
excellent yield (Scheme 2).
disubstituted 3-aminoindenes in moderate to excellent yields
via rhodium(III)-catalysed intermolecular cyclization of readily
available benzimidates and α,β-unsaturated ketones or
nitroalkene. This operationally simple protocol features good
functional-group tolerance and excellent regioselectivity. We
believe that this methodology will be widely applied in the
synthesis of complex molecules bearing an aminoindene motif.
Further mechanistic studies are currently underway in our
laboratory.
We thank the NSF of China (21572072), the Xiamen Southern
Oceanographic Center (15PYY052SF01), Science and the
Technology Bureau of Xiamen City (3502Z20150054), 111
project (BC2018061) and Subsidized Project for Cultivating
Postgraduates’ Innovative Ability in Scientific Research of
Huaqiao University for financial support.
NH2
O
Ph
Ph
N
N
EtOH
PhNHNH2·HCl
+
Ph
Ph
Ph
, 95%
3a
4
Scheme 2 Conversion of 3a.
EtO
NH2
OEt
NH
[Cp*Rh(MeCN)3](SbF6)2 (5 mol%)
1-Ada-CO2H (1 equiv)
O
O
3a
+
+
DCE
3h
Ph
Ph
Ph
Ph
1ab
E
2a
(M + H) + m/z 358.1802
found 358.1803
OEt
NH
[Cp*Rh(MeCN)3](SbF6)2 (5 mol%)
1-Ada-CO2H (1 equiv)
O
3a
E
did not find
+
DCE
Ph
Ph
36h
1ab
2a
Scheme 3 Control Experiments.
Conflicts of interest
There are no conflicts to declare.
EtO
NH2
3a
O
1a
Xiuling Cui: 0000-0001-5759-766X
Ph
()
Cp*Rh
H
Ph
H+
E
OEt
NH
Rh()
NH
O
EtO
Notes and references
Rh()
A
H
Ph
1
For selected examples, see: (a) T. George, R. Tahilramani, C. L.
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NH
EtO
NH
Rh()
Rh()
Ph
O
O
Ph
Ph
C
B
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Scheme 4 Proposed reaction mechanism.
This journal is © The Royal Society of Chemistry 20xx
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