Rh(III)-Catalyzed [3 + 2] Annulation via C-H Activation: Direct Access to Trifluoromethyl-Substituted Indenamines and Aminoindanes

Article abstract of DOI:10.1021/acs.orglett.9b00720

The rhodium(III)-catalyzed direct C-H addition and annulation of benzimidates and aldimines with β-(trifluoromethyl)-α,β-unsaturated ketones is described. This protocol provides the facile and efficient formation of various trifluoromethyl-containing inde

Full text of DOI:10.1021/acs.orglett.9b00720

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rh(III)-Catalyzed [3 + 2] Annulation via C−H Activation: Direct
Access to Trifluoromethyl-Substituted Indenamines and
Aminoindanes
Bharatkumar Chaudhary, Prashant Auti, Suchita Dattatray Shinde, Prasanna Anjaneyulu Yakkala,
Deepesh Giri, and Satyasheel Sharma*
Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research - Ahmedabad, Gandhinagar,
Gujarat 382355, India
S
* Supporting Information
ABSTRACT: The rhodium(III)-catalyzed direct C−H addition and annulation of benzimidates and aldimines with β-
(trifluoromethyl)-α,β-unsaturated ketones is described. This protocol provides the facile and efficient formation of various
trifluoromethyl-containing indenamines or aminoindanes in moderate to high yields.
ndenes and indanes are very important carbocyclic
enones and allyl alcohols.8 Recently, Feng and co-workers have
Iderivatives that are found in various natural products and demonstrated Pd-catalyzed [3 + 2] annulation of (2,2-
pharmaceutically active molecules.1 Moreover, they find
application in organometallics and material science.2 In
particular, aminoindene/indane derivatives have shown im-
portant biological activities,3 such as glutamate receptor
antagonist, calcium antagonist, and anti-Parkinson. In addition,
trifluoromethyl-containing indane acts as a selective CB2 ligand
(Figure 1).4
difluorovinyl)-2-iodoarenes with internal alkynes triggered by
a fluoride nucleophilic addition to produce 1-(trifluoromethyl)-
1H-indenes (Scheme1).9 Thus, development of an efficient,
straightforward route to the 1-(trifluoromethyl)-1H-indenes
framework is highly desired.
Transition-metal-catalyzed C−H annulation reactions have
been of considerable interest for the synthesis of various
heterocycles and carbocycles of biological and material
importance.10 To this end, various indanes/indenes derivatives
have been synthesized using C−H annulation reactions,11
particularly aminoindane derivatives.12 However, synthesis of
1H-inden-3-amines has been scarcely reported using a C−H
activation strategy; for example, in 2005, Kuninobu and Takai
described the first synthesis of 1H-inden-3-amine derivatives
using a C−H annulation strategy with aromatic aldimines and
internal alkynes under Re(I) catalysis.12a The construction of a
1H-inden-3-amine motif from the cross coupling of benzimidate
with alkene was recently reported by Zhang13a under Rh(III)
catalysis and Jeganmohan13b under Ru(II) catalysis. Both
groups have reported the synthesis of 3-aminoindenone as
shown in Scheme 1. In addition, transition-metal-catalyzed C−
H annulation with a benzimidate directing group has been
utilized for the construction of diverse heterocycles recently.14
To the best of our knowledge, there is no report for the construction of
1-(trifluoromethyl)-1H-inden-3-amine scaffolds. We herein report
the first synthesis of trifluoromethyl-containing 1H-inden-3-
amines under Rh(III) catalysis involving the C−H annulation
Figure 1. Selected examples of bioactive aminoindanes and
(trifluoromethyl)indanes.
Introduction of perfluoroalkyl groups brings stimulating
changes in the electronics of the parent molecule; in addition,
incorporation of a trifluoromethyl group into the bioactive
molecules can have a variety of overall positive effects, such as
making them more selective, potent, highly efficacious, or easier
to administer.5 Despite many synthetic advances for the
synthesis of indene derivatives,6 the synthetic approach for 1-
(trifluoromethyl)-1H-indenes is scarcely reported.7 For exam-
ple, it can be synthesized by nucleophilic trifluoromethylation of
indenones or by using Grignard reagents with 2,2,2-trifluor-
oacetophenone followed by intramolecular cyclization7c or by
using intramolecular cyclization of trifluoromethyl-containing
Received: February 26, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00720
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Letter
afford the corresponding derivative of 1-(trifluoromethyl)-1H-
inden-3-amine 3a in 40% yield (Table 1, entry 1). Screening of
additives showed that the combination of AgSbF6 and AgOAc
was the most effective in this catalytic reaction to afford our
desired product 3a in 54% yield (Table 1, entries 2−6).
However, the AcOH as additive was found to be less effective
(Table 1, entry 7). Lowering the loading of AgOAc to 20 mol %
gives a similar yield of 3a (Table 1, entry 8). The effect of
solvents was then investigated (Table 1, entries 9−15); it was
found that DCE, ethyl acetate, toluene, and t-BuOH provided
similar yields. Being more green and cheaper, EtOAc was the
choice of solvent for further investigation. Further, an increase in
catalyst loading to 4 mol % provided the desired compound 3a in
80% yield. Further, without AgOAc, the reaction did proceed
with a slightly decreased yield (Table 1, entry 17); however, a
trace amount of product was observed in the absence of AgSbF6
(Table 1, entry 18).
Scheme 1
With the optimized C−H annulation conditions in hand,
various trifluoromethyl α,β-unsaturated ketones (2a−p) were
selected to realize this coupling with benzimidate (1a). As
depicted in Scheme 2, both electron-donating and electron-
a
Scheme 2. Scope of β-CF3-enones
between benzimidates and β-(trifluoromethyl)-α,β-unsaturated
ketones.
Our investigation was initiated by examining the coupling of
benzimidate (1a) and trifluoromethyl α,β-unsaturated ketone
(2a) under rhodium catalysis (Table 1). To our delight, the
cationic rhodium complex, derived from [Cp*RhCl2]2 (2.5 mol
%) and AgSbF6 (10 mol %), was found to promote the coupling
of 1a and 2a in 1,2-dichloroethane (DCE) at 80 °C for 24 h to
a
Table 1. Selected Optimization for Reaction Conditions
b
entry
additive (mol %)
AgSbF6 (10)
solvent
yield (%)
1
DCE
40
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
AgSbF6 (10) + AgOAc (50)
AgSbF6 (10) + NaOAc (50)
AgSbF6 (10) + CsOAc (50)
AgSbF6 (10) + Cu(OAc)2 (50)
AgSbF6 (10) + AgOTf (50)
AgSbF6 (10) + AcOH (50)
AgSbF6 (10) + AgOAc (20)
AgSbF6 (10) + AgOAc (20)
AgSbF6 (10) + AgOAc (20)
AgSbF6 (10) + AgOAc (20)
AgSbF6 (10) + AgOAc (20)
AgSbF6 (10) + AgOAc (20)
AgSbF6 (10) + AgOAc (20)
AgSbF6 (16) + AgOAc (20)
AgSbF6 (16) + AgOAc (20)
AgSbF6 (16)
DCE
DCE
DCE
DCE
DCE
DCE
DCE
EtOH
THF
toluene
TFE
EtOAc
H2O
t-BuOH
EtOAc
EtOAc
EtOAc
54
31
35
50
trace
22
55
50
44
54
47
57
NR
54
80
74
trace
a
Reaction conditions: 1a (0.3 mmol), 2a-2p (0.2 mmol),
[RhCp*Cl2]2 (4 mol %), AgSbF6 (16 mol %), AgOAc (20 mol %),
b
EtOAc (1 mL) at 80 °C for 24 h under air in reaction tubes. Isolated
yield by column chromatography.
withdrawing substituents at the para- and meta-positions in the
aryl ring of CF3-enone (2a−i) underwent smooth coupling to
furnish the corresponding product (3a−i) in good yields. In the
case of ortho-substituted enones, only moderate yields were
obtained (3j−m), presumably because of the steric bulk. Other
useful substrates such as 2,4-dichlorophenyl enones, piperonyl
enones, and thiophene enones also underwent the C−H
annulation reaction to give the corresponding 1-(trifluorometh-
yl)-1H-inden-3-amines (3n−p) in moderate to good yields.
c
c
c
AgOAc (20)
a
Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), [RhCp*Cl2]2
(2.5 mol %), additive (quantity noted, mol %), solvent (1 mL) at 80
°C for 24 h under air in reaction tubes. Isolated yield by column
chromatography. [RhCp*Cl2]2 (4 mol %) was used.
b
c
B
DOI: 10.1021/acs.orglett.9b00720
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Letter
Furthermore, the generality and compatibility of this coupling
reaction with a variety of benzimidates (1b−p) were
investigated under the optimized conditions (Scheme 3).
reaction to produce the aminoindane derivative 6aa in 84% yield
as shown in Scheme 4. Other aldimine directing groups such as,
a
Scheme 4. Synthesis of C3-Trifluoromethylindanes
a
Scheme 3. Scope of Benzimidates
a
Reaction conditions: 5a−g (0.2 mmol), 2 (0.2 mmol), [RhCp*Cl2]2
(2.5 mol %), AgSbF6 (10 mol %), AgOAc (20 mol %), and DCE (1
mL) under air at 80 °C for 24 h in reaction tubes. Isolated yield by
column chromatography.
b
a
Reaction conditions: 1b−p (0.3 mmol), 2a (0.2 mmol),
[RhCp*Cl2]2 (4 mol %), AgSbF6 (16 mol %), AgOAc (20 mol %),
b
EtOAc (1 mL) at 80 °C for 24 h under air in reaction tubes. Isolated
N-p-methoxyphenyl aldimine (5b) did provide the correspond-
ing product 6ba in 42% yield. However, tosylhydrazone (5c) did
not deliver the desired coupling reaction. Further, we have
carried out coupling of few selected substituted tosylimines
(5d−g) and β-CF3-enones (2a, 2d, 2h, 2l, and 2o). The
corresponding 3-(trifluoromethyl)-1-aminoindanes were ob-
tained in moderate to good yield. Notably, 2-chloro-substituted
β-CF3-enone provided the desired product, albeit in low yield. It
is to be noted that in all the CF3-aminoindanes mixtures of
inseparable diastereomers were obtained. At this stage, we could
not find any appropriate conditions for higher diasteroselectivity
(see selective optimization table in the Supporting Information),
which was observed with cyclic ketimines by Li and co-
workers.15 This could be attributed to the use of sterically bulky
cyclic ketimines for the formation of distereoselective
spiroindane derivatives in previous reports.
Next, we carried out the scaleup experiment of 1d with 2a
under 3 mol % and 1.5 mol % of Rh(III) catalyst; to our delight,
3 mol % of Rh (III) catalyst provided the corresponding product
4d in 78% yield, but with 1.5 mol % of Rh(III) the yield drops to
54% (Scheme 5). Further, we investigated the intramolecular
regioselectivity of the two ortho-C−H bonds of the imidate and
benzthiazole directing group of compound 7a under optimal
reaction conditions. We were delighted to see the formation of
regioselective C−H annulations with an imidate directing group
to afford CF3-indenamine product 8a in 59% yield. This
outcome of regioselectivity could be attributed to the strong
coordinating capability of the imidate moiety. In addition, when
the reactions of a few other olefins (9a−d) with 1a were carried
out, chalcone (9a) did provide the corresponding compound
(10a) in 44% yield. However, the enoates (9b and 9c) and N,N-
dimethylacrylamide (9d) did not undergo the desired coupling
under our optimized conditions.
yield by column chromatography.
Electron-donating (Me, OMe, NMe2) or electron-withdrawing
groups (F, Br, Cl, and CF3) at the meta- or para-positions of
benzimidates were found to undergo the C−H annulation
reaction in moderate to good yield. Notably, with m-OMe
benzimidate (1k), an inseparable regioisomeric mixture of CF3-
indenamine products (4k and 4k′) in a combined yield of 79%
was obtained, and 4k was found to be the major compound
based on the NOE data (see the Supporting Information).
Furthermore, 3,4-disubstituted benzimidates 1l and 1m
provided only one regioisomer of 4l and 4m in 55% and 31%
yield, respectively. We were delighted to see that an aldehyde
group in benzimidate (1n) was compatible during the reaction,
furnishing the formyl product 4n in 52% yield. It is noteworthy
that 2-thienyl imidate (1o) was also found to couple with CF3-
enone (2a) providing an access to the CF3-containing fused five
membered thiophene product (4o) in moderate yield. Ortho-
substituted benzimidate 1p afforded the corresponding product
4p, albeit in low yield.
Diastereodivergent spiroannulation of cyclic N-sulfonyl or N-
acyl ketimines with β-CF3-substituted enones under Rh catalysis
has been reported by Li and co-workers.15 However, the scope of
acyclic aldimines with β-CF3-substituted enones was not yet
explored. Thus, having established the C−H annulation of
benzimidates for 1-(trifluoromethyl)-1H-inden-3-amines syn-
thesis, we set out to investigate the feasibility of C−H annulation
of aldimines using a similar protocol. Initial screening of
tosylaldimine (5a) was carried out under otherwise identical
conditions. Unfortunately, no reaction was observed due to the
degradation of aldimine in ethyl acetate. Further changing the
solvent from ethyl acetate to DCE provided the desired coupling
C
DOI: 10.1021/acs.orglett.9b00720
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Letter
Scheme 5. Scale-up and Synthetic Utility
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b00720.
1
Experimental procedures, characterization data, and H
and 13C NMR spectra for all compounds (PDF)
Accession Codes
CCDC 1889903 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or 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
*E-mail: sharma.satyasheel@gmail.com.
ORCID
On the basis of the previous literature12 and our own
understanding, a plausible mechanistic pathway is depicted in
Scheme 6. In the presence of AgSbF6, cationic [Cp*Rh(III)] is
Bharatkumar Chaudhary: 0000-0001-6182-1273
Satyasheel Sharma: 0000-0002-4983-1572
Notes
Scheme 6. Plausible Reaction Mechanism
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by Department of Science and
Technology (DST) INSPIRE faculty award [DST/INSPIRE/
04/2016/000414]. We also thank the Department of
Pharmaceutical, Ministry of Chemicals and Fertilizers, India,
and NIPER-Ahmedabad for providing the necessary support.
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DOI: 10.1021/acs.orglett.9b00720
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