Direct Pd(II)-Catalyzed Site-Selective C5-Arylation of 2-Pyridone Using Aryl Iodides

Article abstract of DOI:10.1021/acs.orglett.8b02112

A straightforward Pd(II)-catalyzed general strategy was developed for the C5-selective arylation of the 2-pyridone core with easily available aryl iodides. The transformation was highly regioselective and accomplished with a wide scope and functional group tolerance. Silver nitrate played a crucial role in this direct site-selective arylation. The method was extended to synthesize biologically active molecules.

Full text of DOI:10.1021/acs.orglett.8b02112

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Direct Pd(II)-Catalyzed Site-Selective C5-Arylation of 2‑Pyridone
Using Aryl Iodides
Saurabh Maity,‡ Debapratim Das,‡ Souradip Sarkar, and Rajarshi Samanta*
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
S
* Supporting Information
ABSTRACT: A straightforward Pd(II)-catalyzed general
strategy was developed for the C5-selective arylation of the
2-pyridone core with easily available aryl iodides. The
transformation was highly regioselective and accomplished
with a wide scope and functional group tolerance. Silver
nitrate played a crucial role in this direct site-selective arylation. The method was extended to synthesize biologically active
molecules.
Scheme 1. Regioselective Direct Arylation of 2-Pyridone
5-Aryl-2-pyridone alkaloids represent a class of natural
products with extremely rich biological profiles and pharma-
ceutical applications alike. They often show promising
antifungal, antibacterial, and insecticidal activities against the
deleterious effects of neurodegenerative diseases (Figure 1).1,2
selective dehydrogenative heteroarylation of 2-pyridone with
1,3-azoles (Scheme 1iv).7a
Figure 1. Biologically relevant natural products containing C5-
arylated 2-pyridone scaffold.
Following this, Liu’s group and our group also showed
Rh(III) catalyzed C6-arylation of the 2-pyridone scaffold using
a directing group strategy (Scheme 1v).7b,c The method was
further extended by the Das group under a Ru(II)-catalyzed
oxidative arylation strategy (Scheme 1vi).7d Despite the
elegance of the above-mentioned approaches, direct C5-
arylation is quite rare.8 In a seminal work, Li’s group astutely
developed a palladium-catalyzed dehydrogenative C5-arylation
with highly electron-deficient polyfluoroarenes (Scheme 1vii).8
However, direct site-selective arylation of substrates with a
blocked functionalization site like cyclic enaminone,9 quino-
lone scaffolds10 is known in the literature. Nevertheless, one of
the major problems behind general C5-arylation could be
getting control of the C5-position over the C3-position. Often
the aryl group introduces selectively at the C3-position or
diarylation happens at both the C3- and C5-position of the 2-
pyridone core. To overcome this challenge, some in situ
generated cationic palladium catalysts especially Pd−nitrate
catalytic systems11 with enhanced electrophilicity were often
used to obtain the kinetically favored C−H functionalization.12
Intrigued by our recent studies on direct pyridone7c,13a−c
and other heterocycle functionalizations,13d we hypothesized
In addition to these remarkable biological applications, the
structural complexity associated with these arylated pyridones,
often required the use of multiple steps for their preparation.3
Arguably, one of the straightforward methods to construct
such a scaffold is direct site-selective arylation at the C5-
position via C−H functionalization. Recently, straightforward
regioselective functionalizations of the 2-pyridone moiety have
garnered significant attention;4,5 arylation especially becomes
an important challenge due to the omnipresence of aryl groups
in various bioactive 2-pyridone scaffolds.6,7 Recently, Hirano
and Miura’s group reported Mn(III) mediated C3-arylation of
2-pyridones using aryl boronic acid (Scheme 1i).6a Later they
improved their regioselective method under visible-light-
promoted Ir(III) based photoredox catalysis.6b Since the
pioneering work by Hirano and Miura’s group, Maiti and co-
workers found an alternative strategy using an Fe(III) catalytic
system (Scheme 1ii) for this C3-arylation.6c In a different
approach, Zografos group showed Pd(II)-catalyzed C3-
arylation of 4-hydroxy-2-pyridone scaffold utilizing its enol-
like character (Scheme 1iii).6d In a pioneering study, the
Hirano and Miura group developed a copper-mediated C6-
Received: July 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02112
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
that a similar concept might lead to our desired site-selective
arylation on 2-pyridone core. Herein, we report a silver nitrate
promoted direct palladium-catalyzed highly site-selective C5-
arylation of 2-pyridone with aryl iodide as a coupling partner
(Scheme 1viii).
Our efforts began by examining a possible Pd(II)-catalyzed
arylation of N-methyl-2-pyridone (1a) with iodobenzene (2a)
as a coupling partner (Table 1). The reaction was initially
role of the nitrate anion,14 other relevant nitrates were tested.
But none of these nitrates were able to provide the desired
product (Table 1, entries 15−17). Importantly, use of AgNO3
in a substoichiometric amount (30 mol %) afforded only 28%
of the desired product (Table 1, entry 18). This result indicates
the stoichiometric requirement of AgNO3 salt for successful
transformation. In general, the examined silver salts provided
C5 selective arylation with some decomposition of starting
material. However, screening other additives like K2CO3 and
Cs2CO3 in place of the silver salt did not provide the isolable
desired product (Table 1, entries 19−20). Finally, the structure
of compound 3a with C5 phenyl group was firmly confirmed in
comparison with the literature known analytical data.15
Table 1. Optimization of C5-Selective Arylation of 2-
a
Pyridones
With the optimized conditions in hand, the scope study of 2-
pyridones was carried out (Scheme 2). As expected, the
yield
a
b
Scheme 2. Scope Using Various 2-Pyridones
entry
catalyst
solvent
additive (equiv)
(%)
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
Pd(TFA)2
Pd(PPh3)4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
DME
DME
DME
DME
DME
DME
DME
toluene
dioxane
DMF
−
n.d.
n.d.
n.d.
45
50
50
63
30
42
56
73
17
35
n.d.
n.d.
n.d.
n.d.
28
Ag2CO3 (1)
AgOAc (1)
AgPF6 (1)
AgClO4 (1)
AgNO3 (1)
AgNO3 (1.5)
AgNO3 (1.5)
AgNO3 (1.5)
AgNO3 (1.5)
9
10
11
12
13
14
15
16
17
18
19
20
DME/DMF (5:1) AgNO3 (1.5)
DME
DME
DME
AgNO3 (1.5)
AgNO3 (1.5)
AgNO3 (1.5)
DME/DMF (5:1) NaNO3 (1.5)
DME/DMF (5:1) KNO3 (1.5)
DME/DMF (5:1) [Me4N]NO3 (1.5)
DME/DMF (5:1) AgNO3 (0.3)
DME/DMF (5:1) K2CO3 (1.5)
DME/DMF (5:1) Cs2CO3 (1.5)
a
Reaction conditions: 1 (0.2 mmol), 2a (0.6 mmol), Pd(OAc)2 (10
n.d.
n.d.
mol %), AgNO3 (0.3 mmol), DME/DMF (5:1; 0.2 M), 110 °C, 12 h.
b
AgClO4 was used. Bn = benzyl, PMB = p-methoxybenzyl, Mes =
a
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), Pd catalyst (10
mesityl.
b
mol %), additive (0.3 mmol), 110 °C, 0.2 M, 12 h. Isolated yields.
n.d. = not detected. DME = 1,2-dimethoxyethane.
reaction was smoothly proceeded for various N-alkylated 2-
pyridones (Scheme 2, 3a−3d) with good to moderate yields.
Gratifyingly, electron withdrawing groups like CF3, ester or
nitro at the C3-position of 2-pyridone provided very good
yields of the desired products (Scheme 2, 3e−3g). A relatively
lower yield was obtained when a CN group was tethered at the
C3-position of 2-pyridone ring (Scheme 2, 3h). Further,
substrates bearing electron-donating groups at various
positions worked uninterruptedly with moderate yields
(Scheme 2, 3i−3k). Moreover, this site-selective arylation
was further extended with highly substituted 2-pyridone
(Scheme 2, 3l). Interestingly, our attempt to explore
isoquinolone remained successful under the developed
conditions, albeit in reduced yield (Scheme 2, 3m). To our
pleasure, the reaction was smoothly carried out with various
aryl iodides having electronically and sterically variable
functional groups (Scheme 3, 4b−4n). Gratifyingly, electron-
rich methoxy or acetoxy phenyl iodides provided satisfying
yields of desired products under the optimized reaction
conditions (Scheme 3, 4b−4d). Notably, aryl iodide having
para-amine functionality also worked uninterruptedly (Scheme
3, 4e). Aryl iodides with a methyl group at the para or meta
position gave desired products with synthetically acceptable
screened in 1,2-dimethoxyethane (DME) with 10 mol %
Pd(OAc)2 catalyst and in the absence of any Ag salt or in the
presence of Ag2CO3 or AgOAc to obtain no desired product
formation (Table 1, entries 1−3). Gratifyingly, 45% of the
desired product (3a) was isolated when the reaction was
performed with the AgPF6 additive (Table 1, entry 4). The
isolated yield of product 3a was marginally increased due to a
change in silver salt to AgClO4 (Table 1, entry 5).
Unfortunately, formation of some complex mixture of products
along with the desired product made this condition less
effective. Furthermore, in a cleaner reaction, a comparable
amount of desired product 3a was obtained under AgNO3
mediated conditions. For further improvement of reaction
efficacy, the amount of AgNO3 salt was increased. As expected,
the yield of desired product improved to 63% (Table 1, entry
7). However, the change of solvents did not improve the
desired product yields (Table 1, entries 8−10). Gladly, the
efficacy of the reaction was further improved to 73% by fine-
tuning the solvent mixture with DME/DMF in a 5:1 ratio
(Table 1, entry 11). Next, screening of other Pd(II) and Pd(0)
salts offered either a poor yield or no desired product
formation (Table 1, entries 12−14). Assuming the important
B
DOI: 10.1021/acs.orglett.8b02112
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
In another known procedure, 4-phenylisoquinolone derivative
3m could be easily converted into a pharmaceutically active
known compound 9 (Scheme 4b).17 In a separate application,
biologically active compound 11 was achieved from 2-pyridone
derivative 1n (Scheme 4c). Strategically, site-selective C5-
arylation of 1n followed by ester hydrolysis of 3n and coupling
with glycine methyl ester afforded compound 10. Furthermore,
cleavage of methyl ether and subsequent ester hydrolysis
provided our desired compound 11 in synthetically acceptable
yield.18
Scheme 3. Scope Using Various Aryl Iodides
To reveal the mechanistic insight of the reaction, various
control experiments were performed (Scheme 5). To under-
Scheme 5. Control Experiments
a
Reaction conditions: 1a (0.2 mmol), 2 (0.6 mmol), Pd(OAc)2 (10
mol %), AgNO3 (0.3 mmol), DME/DMF (5:1; 0.2 M), 110 °C, 12 h.
b
c
1
Reaction temperature: 125 °C. C3 isomer detected via crude H
NMR of reaction mixture.
yields (Scheme 3, 4f−4g). Further, halogen-containing
trifluoromethoxy or fluoro substituted aryl iodides were well
survived during the transformation (Scheme 3, 4h−4i).
Moreover, the C5-arylation was extended with electron-
deficient aryl iodides having an ester, a trifluoromethyl, or a
nitro group at its different positions (Scheme 3, 4j−4n).
To demonstrate the general applicability of the developed
method, compound 4b was explored with post operation
modifications (Scheme 4a). Initially, it was treated with phenyl
stand the preference of electronically different aryl iodides (2b
and 2j), they were treated separately with 2a and 1a. The
outcome of this competition experiment illustrated that the
rate of the reaction was marginally varied (Scheme 5i−ii).
However, the reaction rate for electron-deficient 2-pyridone
(1f) was strongly favored over electron-rich 2-pyridone (1i)
under the optimized conditions (Scheme 5iii). Presumably, the
electron-withdrawing group (at C3) in 1a guided the product
ratio. Next, under Pd(II)/AgNO3 conditions, treatment of N-
methyl 2-pyridone (1a) with CD3CO2D as the cosolvent in the
absence of phenyl iodide offered 22% deuterium incorporation
at the C5-position exclusively (Scheme 5iv). Additionally,
another H/D scrambling experiment under AgClO4 mediated
conditions revealed 10% deuterium content at the C5-position
and a trace amount of deuterium incorporation at the C3-
position (see the Supporting Information). These outcomes
reveal that the C−H metalation at the C5-position is
reversible. Although C3-metalation is more unlikely here, the
possibility of irreversible C3-metalation cannot be totally ruled
out at this stage due to the observation of some amount of
starting material decomposition. Based on the control
experiments and previous literature,11,12 a tentative mechanism
involving a Pd(II)/Pd(IV) catalytic cycle is proposed (Scheme
6). Though the specific role of AgNO3 is not fully understood,
presumably it helps in the formation of an active catalyst and
regenerates the active palladium catalyst via AgI formation.
Due to presence of a nitrate anion, this palladium−nitrate
species is now more electrophilic and compatible for the
kinetically controlled C5-metalation to generate species B
through eletrophilic aromatic substitution (SEAr). Next, the
oxidative addition of aryl iodide to the Pd(II)-intermediate B
readily provides Pd(IV)-species C. Subsequently, it undergoes
reductive elimination to generate compound 3 or 4 with
Pd(II)-iodo species D. Finally, ligand exchange with the aid of
Scheme 4. Product Modifications and Applications
hydrazine to obtain C3 and C5 unsymmetrical diaryl
substituted 2-pyridone derivative 5.6e Next, C3-amidation of
4b was done under Cu(I) catalyzed conditions to provide
compound 6.7c Further, 4b was regioselectively C3-alkenylated
under Pd(II)-catalyzed conditions to offer compound 7.8
Importantly, a pharmaceutically active compound 8 could be
obtained from compound 4b via a literature known method.16
C
DOI: 10.1021/acs.orglett.8b02112
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Scheme 6. Plausible Mechanism for C5-Arylation of 2-
Pyridone
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AgNO3 regenerates the active Pd(II)-nitrate species A for
continuing the catalytic cycle. Here, the possibility of a Pd(0)/
Pd(II) catalytic cycle can be ruled out, as no catalytic turnover
was observed under Pd(0) conditions (Table 1, entry 14).
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reaction worked well with electron-deficient 2-pyridones.19
In conclusion, we have developed unprecedented direct
Pd(II)-catalyzed site-selective C5-arylation of 2-pyridones with
easily available aryl iodides. The developed method exhibited a
wide scope and functional group tolerance and extended to
synthesize biologically important scaffolds. The admirable role
of silver nitrate for the crucial site selectivity and catalytic cycle
was revealed. Currently, application of this method toward the
total synthesis of a related natural product is underway.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02112.
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Experimental procedures and characterization of new
compounds (1H, 13C, and 19F NMR spectra) (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rsamanta@chem.iitkgp.ac.in.
ORCID
Saurabh Maity: 0000-0002-5413-2135
Rajarshi Samanta: 0000-0002-7925-6601
Author Contributions
‡S.M. and D.D. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the SERB, India (YSS/2014/000383 and
PDF/2016/000242) is acknowledged. Authors thank DST,
India (SR/FST/CSII-026/2013) for 500 MHz NMR facility as
well as CSIR (D.D.) and UGC (S.S.) for fellowships. We thank
Dr. S. K. Patra, IIT Kharagpur for helpful mechanistic
discussions and CRF, IIT Kharagpur for analytical facilities.
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