Transition-Metal-Free Reverse Reactivity of (2-Alkynyl)-Arylaldimines: Assembly of Functionalized Amino-Indinones

Article abstract of DOI:10.1002/chem.201904294

Transition-metal-free regioselective synthesis of functionalized amino-indenones by the reaction of ortho-alkynylaldehydes with a broad range of primary amines by intramolecular cyclization has been described. The designed reaction proceeds through KOH-mediated Prins-type cyclization and DMSO-promoted oxidation of the cyclopent-2-en-1-one system. The proposed mechanism and role of solvent were well supported by control experiments. For the first time, we have disclosed the reverse reactivity of (2-alkynyl)-arylaldimines in a super basic system.

Full text of DOI:10.1002/chem.201904294

A Journal of
Accepted Article
Title: Transition-Metal-Free Reverse Reactivity of (2-alkynyl)-
Arylaldimines: Assembly of Functionalized Amino-Indinones
Authors: Akhilesh Kumar Verma, Kapil Mohan Saini, Rakesh K.
saunthwal, and Sushmita .
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To be cited as: Chem. Eur. J. 10.1002/chem.201904294
Link to VoR: http://dx.doi.org/10.1002/chem.201904294
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Transition-Metal-Free Reverse Reactivity of (2-alkynyl)-
Arylaldimines: Assembly of Functionalized Amino-Indinones
Kapil Mohan Saini,a Rakesh K. Saunthwal,a,b Sushmita,a and Akhilesh K. Verma*a
Dedication ((optional))
Abstract: Transition-metal-free regioselective synthesis of
functionalized amino-indenones by the reaction of ortho-
involved the activation of the alkyne followed by nucleophilic
attack onto the alkyne. However, our methodology describes a
KOH/DMSO promoted the regioselective direct synthesis of
functionlized amino- indenones using a wide range of ortho-
alkynylaldehydes and primary amines to generate (2-alkynyl)-
arylaldimines species, which could give rise to a Prins-type24 ring
closure onto the carbonyl carbon center followed by DMSO
promoted oxidation (Figure 2).
alkynylaldehydes with
a broad range of primary amines via
intramolecular cyclization has been described. The designed
reaction proceeds via KOH-mediated Prins-type cyclization and
DMSO-promoted oxidation of cyclopent-2-en-1-one system. The
proposed mechanism and role of solvent were well supported by
control experiments. For the first time, we have disclosed the
reverse reactivity of (2-alkynyl)-arylaldimines in a super basic system.
The synthesis of highly substituted indenones during the
past decades has received significant attention.1 These
heterocycles are a crucial structural motif of euplectin,2 and are
also essential starting materials for the π-stacked
polybenzofulvene hosts.3 Indenone framework also found in
many natural products and bioactive molecules (Figure 1).4
Methods for the synthesis of substituted indenones include the
traditional
Friedel−Crafts
cyclizations,5-6
photochemical
reactions,7 Nazarov cyclization,8 and Aldol condensation.9
Transition-metal-catalyzed10-13 approaches have also been
reported, including direct C−H annulations,14 Heck–Larock
annulations,15 alkyne annulation reactions,16 and Rh-catalyzed
C-H activation.17 Indenones have also been synthesized by
metal-free approaches via radical18 or ionic pathways,19 and also
by MeOTf induced carboannulated cyclization.20
Figure 2. A route for base-mediated Prins-type cyclizations.
Various research groups have made significant progress in
the transition-metal-free synthesis of heterocycles and
carbocycles.25 Our group also involved in the metal-free
synthesis of biologically active heterocyclic and carbocyclic
cores.26 Along with our ongoing metal-free research in tandem
synthesis, herein we report a transition-metal-free tandem
approach for the synthesis of amino-1H-indenone by the
reaction of ortho-alkynylaldehydes with a broad range of primary
amines.
Figure 1. Biologically active indenones
We started by exploring the aldehyde-amine coupling
step with ortho-alkynylaldehyde and primary amine to allow
us to study the viability of the regioselective hydroxylation
and intramolecular ring-closure. Metal-free regioselective
hydration of ortho-alkynylaldehyde was already known
under mild reaction conditions; therefore ortho-
Nevertheless, such methods make use of available
alkynes as starting materials and transition-metal-based
catalysts and are less relevant to the synthesis of indenones
bearing arylamino or long/short alkylamino side chains.
Therefore, the development of transition-metal-free approaches
for the synthesis of highly functionalized amino-indenones is of
immense importance.
In the last decades, many groups reported the exo and
endo-dig cyclization of (2-alkynyl)-arylaldimines21 using iodine,
22a a Lewis acid, 22b and transition-metal-catalyst.23 These reports
alkynylaldehyde
1 were made by the Sonogashira cross-
coupling of 2-bromobenzaldehyde. After extensive
examination of the array of reaction parameters, a system
comprising KOH (2.0 equiv) in dimethyl sulfoxide (2.0 mL)
o
at 100 C was found to be the most favorable condition for
the reaction to afford amino-1H-indenone 3a in 85% yield
(Table 1, entry 1). Further elevation in reaction temperature
provides the same yield in 0.5 h (entry 2). Afterwards, on
decreasing the reaction temperature in an effort to increase
the product yield, rendered the reaction sluggish and had a
negative effect on the yield (entry 3). Lowering the amount
of KOH provided the desired product in 60% yield (entry 4).
[a]
[b]
Kapil Mohan Saini, Rakesh K. Saunthwal, Sushmita, and Akhilesh
K. Verma*
Department of Chemistry, University of Delhi, Delhi 110007, India
E-mail: : averma@acbr.du.ac.in
School of Chemistry, University of Bristol, UK, BS8 1TS
1
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No product was observed upon attempting the reaction with
other solvents like DMF, THF, and toluene, which
decomposed the starting material (entries 5-7). In contrast
to KOH, sodium hydroxide and lithium hydroxide provided
the desired product in 40% and 8% yield respectively
(entries 8 and 9). It is noteworthy that no desired product 3a
was obtained when NaH and K2CO3 were used in the place
of KOH (entries 10 and 11). KOH (2.0 equiv) in 2.0 mL
chain aliphatic amines (2w), gave the corresponding amino-
1H-indenone (3t-w) in 52% - 68% yields in 3 h.
o
DMSO at 100 C was found to be the best for the overall
reaction efficiency and yield.
Table 1. Optimization of reaction conditions.a
entry
Modification
Yield (%)b
3a
85
85
45c
60d
-
1
None
2
KOH (2.0 equiv), DMSO, 120 oC, 0.5 h
KOH (2.0 equiv), DMSO, 70 oC, 3 h
KOH (1.5 equiv), DMSO, 100 oC, 3 h
KOH (2.0equiv), DMF, 100 oC, 3 h
KOH (2.0 equiv), THF, 65oC, 3 h
3
4
5
6
-
7
KOH (2.0 equiv), Toluene, 100oC, 3 h
NaOH (2.0 equiv), DMSO, 120 oC, 3 h
LiOH (2.0 equiv), DMSO, 120 oC, 3h
NaH (2.0 equiv), DMSO, 120 oC, 3h
K2CO3 (2.0 equiv), DMSO, 70 oC, 3 h
-
8
40c
8c
9
10
11
NR
NR
[a] Reaction performed using 0.5 mmol of 1a and 0.5 mmol of amine,
2a in 2 mL solvent. [b] isolated yield. [c] along with starting material. [d]
complex mixture.
Having established the optimum reaction conditions, we
next explored the scope and generality of the tandem
reaction using our metal-free approach. The scope of the
primary amine for Prins-type cyclization was explored in
Scheme 1. Initially, a range of primary amines (2a
-i) were
examined. Variation of the substitution on benzene ring
2
indicated a broad substrate scope, including H (2a),
electron-donating 4-methyl (2b); 4-amino (2c), sterically
hindered ortho-iodo (2d), 4-iodo (2e), electron-withdrawing
4-fluoro (2f); 4-nitro (2g); 2, 5-dichloro (2h) and 2-pyrrolyl
Scheme 1. Scope of primary amine
(
2i) groups was observed. Fairly good yields of 3a-i were
obtained. It is noteworthy that the reaction was efficient with
electron-neutral, electron-donating and electron-
withdrawing group bearing pyridyl amines such as 2-pyridyl
Next, the generality of ortho-alkynylaldehyde for the Prins-
type cyclization reaction was explored with a range of
arylalkynylaldehydes (Scheme 2, 1b
various terminal alkynes, including electron-donating methyl
1b); bulky 4-tert-butyl (1c) and 4-OMe (1d) are capable of
giving the desired products (4b ) in 75-80% yields.
Moreover, electron-withdrawing groups such as 4-fluoro
1e) and 4-trifluoromethyl (1f) were well-tolerated to give
the final products (4e ) in good yield. Interestingly, ortho-
alkynylaldehyde bearing naphthalenes (1g) showed high
proficiency in the reaction and provided the product 4g in
-l). Aldehydes bearing
(
(
2j); 3-pyridyl (2k), electron-donating 4-methyl (2l), 5-methyl
2m); 6-methyl 2n), and electron-withdrawing 4-
(
(
-
d
trifluoromethyl (2o). Other heterocyclic amines such as
pyrimidine (2p); quinoline (2q); benzo[d]thiazole (2r) 6-
methoxybenzo [d] thiazole (2s) were also well-tolerated in
the reaction. Additionally, we explored the scope of aliphatic
amines for the KOH/DMSO promoted intramolecular
(
-
f
-
h
cyclization. The small chain (2t), cyclic (2u-v) and long-
72% and 74% yields, respectively. The heteroarenes such
2
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as 2-pyridyl (1h), 3-thienyl (1i) bearing aldehydes also
provided fruitful results. Electron-donating 5-methyl (1j),
The mechanism was initiated by the aldehyde-amine coupling,
which leads to the formation of species A. The regioselective
Prins-type25c cyclization followed by the hydroxylation would
generate the species C. DMSO promoted keto-enol tautomerism
would generate the species D. Base assisted elimination of
dimethyl sulfide,27 will produce the desired products 3 and 4.
electron-withdrawing 5-trifluoromethyl (1k) and 5-chloro (1l
benzaldehydes were also capable of affording the desired
products (4l ) in good yields. 3-Pyridinecarboxaldehyde
)
-
n
was also capable of giving the desired products 4o and 4p
in productive yields. The regioselectivity was also confirmed
by X-ray crystallography study.25d
Scheme 3. Synthetic application of amino-1H-indenone
Scheme 4. Mechanistic experiments
Scheme 5. Proposed mechanism
In conclusions, we have demonstrated the KOH-DMSO
promoted intramolecular ring closure of readily accessible ortho-
alkynylaldehyde with a broad range of primary amines for the
synthesis of substituted amino-1H-indenone with high functional
group tolerance and good yield. The salient feature of the
reaction was the reverse reactivity of (2-alkynyl)-arylaldimines in
a basic medium, which enhance the industrial utility. The (2-
alkynyl)-arylaldimines gave the regioselective ring closure rather
than 6-endo-dig cyclization. X-ray crystallographic studies
further supported the designed chemistry. We anticipate that this
protocol is useful for the synthesis of amino-1H-indenone
containing polycyclic heterocycles, which could find further
application, in the synthesis of biologically active compounds.
Scheme 2. Scope of ortho-alkynylaldehyde
Due to the synthetic efficacy of aryl and alkynyl functional
groups, a mild and selective approach for the late-stage
introduction of aryl and alkynyl functional groups in amino-1H-
indenones is highly desirable in synthetic organic chemistry.
Therefore, we engaged the substituted product amino-1H-
indenone 3d under Suzuki coupling conditions and 3e with the
Sonogashira reaction conditions to successfully attain the
coupling products 5a and 5b in good yields (Scheme 3).
In support of the proposed mechanistic pathway, two
isotopic labeling experiments were performed. H-D exchange
and deuterium incorporation were not observed in both reactions,
these experiments suggested that the anilinyl proton comes from
in-situ generated water (Scheme 4).
Acknowledgements
The research work was supported by SERB (EMR/2017/003218/
OC) and University of Delhi. K.M.S. and Sushmita is thankful to
CSIR and UGC for the fellowship respectively.
24
Based on the above results and literature reports,
a
plausible mechanistic pathway has been proposed in Scheme 5.
3
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Sreekenth, L. Balagopal, Acc. Chem. Res. 2003, 36, 899–907. (d) G.
Balme, E. Bossharth, N. Monteiro, Eur. J. Org. Chem. 2003, 4101–
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Keywords: Amino-Indinones • Transition-Metal-Free • primary
amines • ortho-alkynyl aldehyde • Regioselective
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4
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Entry for the Table of Contents COMMUNICATION
Kapil Mohan Saini, Rakesh K.
Saunthwal, Sushmita, and Akhilesh K.
Verma*
Page No. – Page No.
Transition-metal-free regioselective synthesis of functionalized amino-indenones by
the reaction of ortho-alkynylaldehydes with a broad range of primary amines via
intramolecular cyclization has been described. The designed reaction proceeds via
KOH-mediated Prins-type cyclization and DMSO-promoted oxidation of cyclopent-
2-en-1-one system. The proposed mechanism and role of solvent were well
supported by control experiments. For the first time, we have disclosed the reverse
reactivity of (2-alkynyl)-arylaldimines in a super basic system.
Transition-Metal-Free Reverse
Reactivity of (2-alkynyl)-
Arylaldimines: Assembly of
Functionalized Amino-Indinones
5
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