RhIII-Catalyzed Synthesis of Highly Substituted 2-Pyridones using Fluorinated Diazomalonate

Article abstract of DOI:10.1002/asia.201901620

A Rh^III-catalyzed strategy was developed for the rapid construction of highly substituted 2-pyridone scaffolds using α,β-unsaturated oximes and fluorinated diazomalonate. The reaction proceeds through direct, site-selective alkylation based on migratory insertion and subsequent cyclocondensation. A wide substrate scope with different functional groups was explored. The requirement of fluorinated diazomalonate was explored for this transformation. The developed methodology was further extended with the synthesis of the bioactive compound.

Full text of DOI:10.1002/asia.201901620

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Rh(III)-Catalyzed Straightforward Synthesis of Highly Substituted
2-Pyridones using Fluorinated Diazomalonate
Authors: Debapratim Das, Gopal Sahoo, Aniruddha Biswas, and
Rajarshi Samanta
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To be cited as: Chem. Asian J. 10.1002/asia.201901620
Link to VoR: http://dx.doi.org/10.1002/asia.201901620
A Journal of
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Rh(III)-Catalyzed Straightforward Synthesis of Highly Substituted
2-Pyridones using Fluorinated Diazomalonate
Debapratim Das, Gopal Sahoo, Aniruddha Biswas and Rajarshi Samanta*
Dedication ((optional))
Abstract.
A
straightforward Rh(III)-catalyzed strategy was
site-selective C-H bond functionalization based on carbene
migratory insertion has been developed widely for the
construction of various nitrogen-containing heterocycles.[12-14]
However, several developed methods based on migratory
insertion dealt with isoquinolone derivatives[15], whereas rare
effort was observed for the accomplishment of the highly
substituted 2-pyridone core via this protocol. Further,
construction of fluorinated compounds is extremely important
developed for the rapid construction of highly substituted 2-pyridone
scaffolds using the α, β-unsaturated oximes and fluorinated
diazomalonate. The reaction proceeds through direct, site-selective
alkylation based on migratory insertion and subsequent
cyclocondensation. A wide substrate scope with different functional
groups was explored. The requirement of fluorinated diazomalonate
was explored for this transformation. The developed methodology
was further extended with the synthesis of the bioactive compound.
due to its unique properties[16]
.
Especially, fluorinated
diazomalonates have its own recognition as important synthons
in different organic transformations.[17]
Highly substituted 2-pyridones are considered as one of the
most important classes of electron-deficient nitrogen-containing
heterocycles due to its often occurrence as pharmaceuticals,
bioactive natural products, organic-materials (Figure 1).[1] Since
the development of classical synthetic strategies with their
specific limitations[2], transition metal catalysis played a crucial
role in the development of step-economic strategies for the
construction of this core.[3]
Scheme 1. Construction of highly substituted 2-pyridone using fluorinated
diazomalonate.
Based on our ongoing interest in the directed functionalization of
C-H bonds via migratory insertion,[18]we hypothesized that
Rh(III)-catalyzed alkylation at the β–position of the α, β–
unsaturated oxime using sufficiently electrophilic diazo malonate
followed by nucleophilic attack of oxime N-center might trigger to
our desired highly substituted 2-pyridone moiety. Herein, we
disclose an efficient Rh(III)-catalyzed synthetic method for the
preparation of highly substituted 2-pyridones via cascade
reaction of α, β–unsaturated oxime with fluorinated-
diazomalonate via site-selective alkylation and its subsequent
cyclocondensation (Scheme 1iii). We began our optimization
using methyl oxime (1a) and the diazo compound, bis(2,2,2-
trifluoroethyl) diazomalonate (2a) for the coupling purpose.
Initially, there was no desired product formation when the
reaction was done with [Cp*RhCl2]2 and AgSbF6 catalyzed
conditions in acetonitrile solvent (table 1, entry 1). Gratifyingly,
the desired cyclized product 3a was obtained in 66% yield in
DCE solvent (Table 1, entry 2). In the search of higher yield, we
screened several other common solvents. Importantly, the
isolated yield of 3a improved up to 89% in 1,4-dioxane (table 1,
entry 3). Among other screened solvents, only 2,2,2-
trifluoroethanol provided a acceptable yield of desired product
3a (Table 1, entries 4-5).
Figure 1.Biologically relevant highly substituted C4-arylated 2-pyridones.
In general, transition metal-catalyzed cycloaddition based
strategies were used extensively for the rapid development of 2-
pyridone scaffold by Padwa and others.[4,5] Cycloisomerization of
N-alkenylalkynylamides was developed to form 2-pyridone by
Tanaka’s group under Au(I) or Pd(II) catalysts.[6] In another
leading approach, oxidative coupling of internal alkynes with
suitable coupling partners was explored under Rh(III)[7], Ru(II)[8],
Pd(II)[9] and Fe(III)[10] catalysis for the preparation of this core
(Scheme 1i). In an elegant advancement, Wang group employed
a Rh(III)-catalyzed strategy to furnish 2-pyridones using N-
methoxymethacrylamide and diazo compounds (Scheme 1ii).[11]
Notably, all these efforts initiated with acrylamide derivatives.
Recently, transition metal-catalyzed, especially Rh(III)-catalyzed
Dr. D. Das, Mr. G. Sahoo, Mr. A. Biswas, Dr. R. Samanta
Department of Chemistry
Indian Institute of Technology Kharagpur
Kharagpur 721302, India
E-mail: rsamanta@chem.iitkgp.ac.in
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
Table 1. Optimization Table [a]
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10.1002/asia.201901620
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desired products in moderate to excellent yields (Scheme 2, 3g-
3i). Electron withdrawing groups at the aryl ring also furnished
the desired products in moderate to good yields (Scheme 2, 3j-
3l).
Entry
Catalyst
Ag salt
Solvent
Yield (%)[b]
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
(Cp*RhCl2)2
-
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgNTf2
AgNO3
AgOAc
AgSbF6
AgSbF6
-
MeCN
DCE
n.d.
66
1
2
3
dioxane
TFE
89
4
30
5
toluene
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
n.d.
87
6
7
n.d.
trace
85
8
9[c]
10[d]
11
12
13
14
15[e]
71
n.d.
n.d.
trace
trace
n.d.
AgSbF6
AgSbF6
AgSbF6
AgSbF6
[(Cp*IrCl2)2]
[(p-cym)RuCl2]2
[Cp*Co(CO)I2]
[a]Reaction Conditions: 1a (0.1 mmol), 2a (0.12 mmol), [Cp*RhCl2]2 catalyst (2
mol%), Ag salt (8 mol%), 110 °C, solvent (0.1 M), 12 h. [b]Isolated yields. n.d. =
not detected. [c]at 130 °C. [d][Cp*RhCl2]2 catalyst (1 mol%) used.
[e][Cp*Co(CO)I2] catalyst (5 mol%) used.
Further, the other silver additives were examined (table 1,
entries 6-8) under the [Cp*RhCl2]2 catalyzed conditions.
Followingly, AgNTf2 provided a comparable yield of desired
product 3a (Table 1, entry 6). However, other tested silver salts
were not able to deliver the desired product in isolable amount
(table 1, entries 7-8). Increment in the temperature did not
improve the desired product yield (table 1, entry 9). Further, to
check the method’s efficiency, the reaction was carried out with
1 mol% [Cp*RhCl2]2 catalyst with a satisfying yield of desired
product 3a (Table 1, entry 10). There was no desired product
formation when the reaction was carried out without silver salt or
Rh(III)-catalyst (table 1, entries 11-12). Other related transition
metal catalysts like (Cp*IrCl2)2, [(p-cym)RuCl2]2 or Cp*Co(CO)I2
were unable to provide the desired product under the optimized
conditions (Table 1, entries 13-15). Importantly, diazo
compounds having one methyl ester and other electron
withdrawing groups like -COCH3 or –COPh did not provide any
desired product. However, when dimethyl 2-diazomalonate was
used as a coupling partner, only trace amount of product (<5%)
was formed.
With the help of the optimized conditions, we started our
substrate scope investigation. Aryl groups with electron and
sterically variable functional groups were well tolerated. Keto-
oxime having β–aryl and α–methyl reacted well to furnish fully
substituted 2-pyridone moiety (Scheme 2, 3a-3b) in excellent
yields. Bulkier 2-naphthyl substitution gave a moderate yield of
the desired product (Scheme 2, 3c).Next, only β-aryl-substituted
keto-oximes also worked well to produce C5-free 2-pyridone
moiety (Scheme 2, 3d-3t). Oximes containing electron-donating
groups at the aryl moiety gave desired products in good to
excellent yields (scheme 2, 3d-3f). Further, halogens at the
para-position of the aryl ring were also well-tolerated, giving the
Scheme 2: Scope of Highly Substituted 2-pyridone derivatives.Reaction
conditions: 1 (0.1 mmol), 2a (0.12 mmol), (Cp*RhCl2)2 (2 mol%), AgSbF6 (8
mol%), 1,4-dioxane (0.1 M), 110 °C, 12 h. Bn = benzyl. [a]reaction at 2.5 mmol
scale.
Notably, oximes containing meta-substituted aryl group at the β-
position produced the desired products in moderate yields
(Scheme 2, 3m-3n). Furthermore, ortho-fluoro-substituted aryl
moiety also furnished the desired product, albeit in lower yield
(Scheme 2, 3o).However, a further increase in the size of the
halogens like Cl, Br did not deliver any desired products under
the optimized conditions. Among other oxime derivatives, the
benzyl oxime survived to provide the desired 2-pyridone
(Scheme 2, 3p). Importantly, the aldoxime also reacted
uninterruptedly to provide the C5, C6-free 2-pyridone derivative
in moderate yield (Scheme 2, 3q). Next, keto-oximes containing
heterocycles provided desired C4-hetero arylated 2-pyridone
derivatives in excellent yields (Scheme 2, 3r-3s). Moreover, the
replacement of the aryl group with the styrene moiety produced
the desired product in very poor yield (Scheme 2, 3t). Further
screening of oximes having alkyl groups at the β-position did not
turn to be effective. To check the scalability, compound 1a was
used in 2.5 mmol scale under the developed protocol to furnish
the desired product 3a in 73% yield. Afterward, we want to
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10.1002/asia.201901620
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COMMUNICATION
extend our developed method for the construction of
pharmaceutically attractive 5,6-fused 2-pyridone core.[19] Various
carbocyclic keto-oximes (4) were explored for the construction of
carbocycle-fused highly substituted 2-pyridone moieties
(Scheme 3, 5a-5j).
To elucidate the synthetic applicability, compound 3a was
hydrolyzed under basic medium to produce corresponding
carboxylic acid 6 in good yield (Scheme 4i). Finally, its structure
was unequivocally determined by single-crystal x-ray analysis[20]
to further confirm the structure of compound 3a. The N-OMe
bond was cleaved by hydrogenation to give NH-free 2-pyridone
derivative 7 in good yield (Scheme 4ii). In another important
Ar
Ar
CO2R
O
[(Cp*RhCl2)2] (2 mol%)
AgSbF6 (8 mol%)
CO2R
CO2R
application, bioactive compound
9
was synthesized in
H
+
N2
synthetically acceptable yield using our developed protocol via
the formation of compound 8 followed by N-OME bond cleavage
(Scheme 4iii).[21] Next, the necessity of fluorinated
diazomalonate was examined through different control
experiments using mixed diazomalonates.
( )n
1,4-dioxane, 110 °C, 12 h
R = CH2CF3
N
OR1
NOR1
4, n = 1-4
( )n
2a
5
Ph
Ph
Ph
Ph
CO2R
CO2R
O
CO2R
CO2R
O
Ph
Ph
N
O
N
N
O
N
standard
conditions
CO2Me
CO2CH2CF3
O
CO2CH2CF3
CO2Me
OMe
5c, 63%
5b, 84%
5d, 57%
OMe
OMe
F
OMe
5a, 66%
(i)
N2
1a
a)
+
OMe
N
N
O
2b
OMe
OMe
3u, 42%
3a, 14%
CO2R
Ph
Ph
CO2R
O
CO2R
O
CO2R
standard
conditions
CO2Et
CO2CH2CF3
CO2CH2CF3
CO2Et
O
N
N2
b)
c)
+
OH
N
1a
N
N
O
OMe
N
5e, 38%
N
O
OMe
5h
, 51%
5g, 53%
OMe
5f, 67%
2c
OMe
OMe
3v, 23%
OMe
3a, 6%
O
S
standard
conditions
CO2CH2CF3
CO2R
O
CO2R
no conversion
N2
1a
1e
t
CO2 Bu
CO2Me
2d
OMe
N
N
O
5i, 65%
5j, 82%
OMe
OMe
2a
(ii)
+
Scheme 3: Scope with oximes having cycloalkyl rings. Reaction conditions: 1
(0.1 mmol), 2a (0.12 mmol), [(Cp*RhCl2)2] (2 mol%), AgSbF6 (8 mol%), 1,4-
dioxane (0.1 M), 110 °C, 12 h.
1j
standard
conditions
3h
CO2CH2CF3
O
CO2CH2CF3
N
N
O
3e:3j= 2:1
OMe
OMe
Notably, reaction worked with keto-oximes attached to five, six,
seven and eight member carbocyclic rings. The yields of the
desired products were moderate for five, and seven member
carbocycles fused 2-pyridones (Scheme 3, 5a and 5d).
Interestingly, the best yield was found when the oxime was
containing a cyclohexyl ring (Scheme 2, 5b). However, the
methyl substituted cyclohexyl ring at the adjacent position of the
keto-oxime group provided the decreased yield of the desired
product (Scheme 3, 5c). Further, an increase in the ring size to
eight membered-carbocycle, provided poor yield of the desired
product (Scheme 3, 5e). Keto-oximes having electronically
variable aryl rings and heterocycles with fused carbocyclic ring
afforded the desired products in moderate to good yields
(Scheme 3, 5f-5j).
(iii)
66%
O
[(Cp*RhCl2)2] (2 mol%)
AgSbF6 (8 mol%)
H/D
N
1a
Ph
CD3CO2D (30 equiv.)
1,4-dioxane, 110 °C, 1h
H/D 50%
Scheme 5: Kinetic studies and control experiments.
Strikingly, 2b was used to obtain the desired selectivity in 8
during the probable nucleophilic attack of oxime to the more
electrophilic trifluoroethyl ester center over methyl ester center.
Further, to compare the nucleophilicity of the nitrogen center of
the oxime, the reaction was performed with the diazo
compounds having highly electron deficient 2,2,2-trifluoroethyl
ester group and relatively less electron deficient methyl, ethyl
and t-butyl group (2b, 2c, and 2d) (Scheme 5i).When diazo
compound 2b was reacted with oxime 1a under the optimized
conditions, the nucleophilic nitrogen center attacked majorly at
the electron-deficient 2,2,2-trifluoroethyl ester to produce 3u as
the major product in 42% yield with minor product 3a in 14%
(Scheme 5ia). Further, an increase in steric bulk with ethyl ester
(2c) decreased the overall yield of products formation to 29%
only. Incidentally, again the product 3v was formed as the major
product in 23% and 3a in 6% (Scheme 5ib). For bulkier tert-butyl
group, there was no conversion (Scheme 5ic). These results
prove here that the cyclo-condensation step majorly depends on
Scheme 4: Product modifications and applications.
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10.1002/asia.201901620
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The reaction mixture was stirred at 110 º C for 12 h. After completion of
the reaction, the reaction mixture was directly loaded to the column and
desired compound (3) was purified using hexane-ethyl acetate mixture
(1:3 to 1:1) as the eluent.
the electrophilicity of the ester moieties. To understand the
reaction rate, electron-rich oxime 1e and electron-deficient
oxime 1j were allowed to react with compound 2a. It was
observed that 1e reacted faster than 1j to produce 3e over 3j in
2:1 ratio (Scheme 5ii). This result suggests that the β–CH
metallation is kinetically favored with electron-rich aryl derivative
over the electron-poor one. Furthermore, H/D-scrambling
experiment of compound 1a in the solvent mixture of 1,4-
Acknowledgments
Financial support by the SERB, India (CRG/2018/000630) and
DST, India (SR/FST/CSII-026/2013; for 500 MHz NMR facility)
are thankfully acknowledged. DD and AB thank CSIR for their
fellowships.
dioxane/CD3COOD showed
cleavage at the β-position (Scheme 5iii).
a highly reversible C-H bond
OMe
(Cp*RhCl2)2
AgSbF6
O
C
OMe
N
N
RO
O
Keywords: 2-Pyridone; Rh(III)-catalyzed; C-H Functionalization;
AgCl
[Cp*RhIII]
D
Ph
1a
C-H metalation
H+
RO2C
RO2C
RO
metal-carbene; migratory-insertion; diazo
H
active species
Ph
proto demetalation
H+
+
[1]
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]
+
]
MeO
N
*Cp Rh
-ROH
N
MeO
Ph
A
3a
Rh
Ph
RO2C
RO2C
C
Cp*
N2 2a
[2]
migratory
insertion
N
Rh
CO2R
RO2C
MeO
RO2C
Ph
B
R= CF3CH2
2 metalocarbene
formation
Cp*
-N
CO2R
Scheme 6: Plausible Mechanism
Based on former literatures[11-13] and mechanistic studies, a
plausible reaction mechanism is proposed (Scheme 6). Initially,
Rh(III)-precatalyst forms an active Rh(III)-catalyst in the
presence of silver salt via halide exchange. Next, the five-
membered rhodacycle intermediate A forms via the directed C-H
bond metallation at the β-position of α, β-unsaturated oxime.
Then the diazo compound 2a reacts with A to furnish metal-
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carbene intermediate
migratory insertion produces six-membered rhodacycle C and its
subsequent protodemetalation furnishes β-alkylated
B via the extrusion of N2. Further,
intermediate D with the regeneration of active Rh(III)-catalyst.
Finally, the nucleophilic attack of oxime N-atom to the highly
electrophilic carbonyl center of the trifluoroethyl ester and
deprotonation provides the desired cyclized 2-pyridone
compound 3a.
In conclusion, we have developed a Rh(III)-catalyzed step-
economic strategy for the rapid construction of highly substituted
2-pyridone scaffold using the simple α,β-unsaturated oximes,
and fluorinated diazomalonate. Broad substrate scope with wide
functional groups was explored. The requirement of fluorinated
diazomalonate was accomplished. The developed protocol was
further extended for the synthesis of the bioactive compound.
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in a 10 mL screw cap vial. Then [(Cp*RhCl2)2] (2 mol%), AgSbF6 (8
mol%) and diazo compound (2) (0.12 mmol) were added to the mixture.
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10.1002/asia.201901620
Chemistry - An Asian Journal
COMMUNICATION
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10.1002/asia.201901620
Chemistry - An Asian Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Key Topic*
Debapratim Das, Gopal Sahoo,
Aniruddha Biswas and Rajarshi
Samanta*
Page No. – Page No.
Rh(III)-Catalyzed Straightforward
Synthesis of Highly Substituted 2-
Pyridones using Fluorinated
Diazomalonate
Highly substituted C4-arylated 2-pyridones were synthesized using oximes and
fluorinated diazomalonate under Rh(III)-catalyzed conditions. The utility of
bis(2,2,2-trifluoroethyl) 2-diazomalonate was explored for this straightforward
tandem method.
This article is protected by copyright. All rights reserved.