Iridium-Catalyzed Propenylation Reactions for the Synthesis of 4-Pyridone Derivatives

Article abstract of DOI:10.1002/adsc.201801177

Herein we report an iridium-catalyzed propenylation reaction of allylic carbonates with 4-hydroxypyridine derivatives. The process efficiently provides 4-pyridone derivatives with high stereoselectivities under mild conditions. The products could constitute valuable building blocks for the synthesis of natural products and other bioactive molecules. Preliminary mechanistic studies indicated that a tandem allylic substitution/isomerization reaction occurs to afford the propenylation products. (Figure presented.).

Full text of DOI:10.1002/adsc.201801177

Title: Iridium catalyzed propenylation reactions for the synthesis of 4-
pyridone derivatives
Authors: Xue-dan Bai, Jie Wang, and Ying He
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801177
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10.1002/adsc.201801177
Advanced Synthesis & Catalysis
COMMUNICATION
Iridium-Catalyzed Propenylation Reactions for the Synthesis of 4-
Pyridone Derivatives
Xue-dan Bai, Jie Wang, and Ying He*
Abstract: Herein we report an iridium-catalyzed propenylation
reaction of allylic carbonates with 4-hydroxypyridine derivatives. The
process efficiently provides 4-pyridone derivatives with high
stereoselectivities under mild conditions. The products could
constitute valuable building blocks for the synthesis of natural
products and other bioactive molecules. Preliminary mechanistic
studies indicated that a tandem allylic substitution/isomerization
reaction occurs to afford the propenylation products.
controlled isomerization of olefins using an iridium-pincer
complex as the catalyst (Scheme 1, a).[8] On the other hand,
iridium catalyzed allylic substitution for C-N formation is a well-
established strategy for allylic amines and N-heterocycles
10]
(Scheme 1, b).[9,
Inspired by these reports, we wondered
whether we could combine these two systems in one-pot, as a
means of achieving the direct propenylation of nucleophiles. On
the basis of its utility in the preparation of bioactive compounds,
we selected 4-hydroxypyridine as the nucleophile and
anticipated that our studies would lead to a synthesis of
propenylated 4-pyridones (Scheme 1, c).
Keywords: Iridium catalysis • propenylation reactions • 4-pyridone
derivatives • allylic substitution • isomerization
4-Pyridones are versatile building blocks in organic synthesis
and their derivatives constitute a large class of drugs, natural
products, and other bioactive molecules (Figure 1).[1]
Consequently, substantial effort has been devoted to the
development of methods for the diversification and elaboration
of this scaffold. As an inexpensive and commercially available
starting material, 4-hydroxypyridine is an attractive precursor for
the preparation of 4-pyridone derivatives, and it has been
employed as a nucleophilic reagent in a number of synthetic
methods.[2,3]
Me
F
N
H
N
O
N
O
O
Cl
OH
N
O
HO
N
MeO
C6H13
Me
(S)-Levofloxacin
OMe
(S)-Lasubine II
Cylindricine B
Figure 1. Selective bioactive molecules containing 4-pyridone motifs.
Scheme 1. Iridium catalyzed isomerization, allylic substitution, and
propenylation reactions.
Alkenylation reactions have emerged as an area of
considerable interest during the past few decades. In addition to
the classic Heck reaction,[4] a number of other strategies have
been reported including C-H activation, alkene rearrangement
reactions, and coupling reactions for vinyl species.[5] While
alkenylation chemistry provides a means for the direct formation
of compounds containing C=C bonds. Transition metal complex
catalyzed olefin isomerization offers another powerful technique
for the preparation of olefins and their derivatives.[6,7] Very
recently, the Miller group reported a new efficient strategy of the
We began our studies by examining the reaction of
cinnamyl carbonate (1a) and 4-hydroxypyridine (2a). The
iridacycle catalyst was generated in situ by the treatment of
[Ir(cod)Cl]2 and Feringa phosphoramidites L1 with 1,5,7-
triazabicyclo[4.4.0]dec-5-ene
(TBD).[11]
Using
1,8-
Diazabicyclo[5.4.0]undec-7-ene (DBU) as a base, we were
pleased to find that the reaction proceeded smoothly to afford
the desired propenylation product rather than allylic substitution
product in 94% yield and ≥30:1 selectivity for the (Z)-isomer
(Table 1, entry 1). Different phosphoramidite ligands were also
studied. However, the yield of the product decreased
dramatically when other phosphoramidite ligands were used
(Table 1, entries 2-4). No desired product was obtained when
using PPh3 or P(OPh)3 as the ligand (Table 1, entries 5 and 6).
The use of other cinnamyl electrophiles such as –Cl, –OBoc, –
[a]
Xue-dan Bai, Jie Wang, Prof. Dr. Ying He
School of Chemical Engineering, Nanjing University of Science &
Technology, Nanjing, 210094, China
E-mail: yhe@njust.edu.cn
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10.1002/adsc.201801177
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OBz, –OP(OEt)2 and –OH led to diminished yield (Table 1,
entries 7-11), as did the use of Et3N as the base (Table 1, entry
13). Changing to toluene as the solvent result in a comparable
yield of 93% with high stereoselectivity (Table 1, entry 12).
the reaction was not limited to cinnamyl methyl carbonates; a
vinylogous substrate also furnished the desired product in high
yield (Table 2, compound 3s). However, when using methyl
pent-2-en-1-yl carbonate as the substrate, propenylation product
was not observed; and allylic product 3t was obtained with 86%
yield instead (Table 2, compound 3t).
Table 1. Optimization of reaction conditions.a
Table 2. Substrate scope.a
a
Standard conditions：1 (0.1 mmol), 2a (0.2 mmol), [Ir(cod)Cl]2 (2 mol%),
Ligand 1 (4 mol%), TBD (10 mol%), DBU (0.2 mmol), THF (1.0 ml), 50oC for
8h, argon atmosphere. b Isolated yields; LG = leaving group.
a Reaction conditions: 1 (0.1 mmol), 2 (0.2 mmol), [Ir(cod)Cl]2 (2 mol%), L1 (4
mol%), TBD (10 mol%), DBU (0.2 mmol), THF (1.0 ml), 50oC for 8h, argon
atmosphere, isolated yields; Z/E ratio was calculated by 1H-NMR, 3b was
With the optimized conditions in hand, we next explored
substrate scope for the reaction. Various substituted group in the
para-positions of arenes were well tolerated under optimized
conditions. High to excellent yields were obtained using most
electron-neutral, electron-donating and electron-withdrawing
cinnamyl methyl carbonates as substrates (Table 2, compounds
3a-3f). However, only a moderate yield of 50% was obtained
when strong electron-deficient p-nitrocinnamyl carbonate 1g was
used (Table 2, compound 3g). A range of ortho-, meta- and
para-substituted, electron-poor, and electron-rich analogues
gave high yield of propenylation products utilizing 4-
hydroxypyridine as a nucleophile (Table 2, compounds 3h-3l).
The reaction was also amenable to electron-poor and electron-
rich heterocycles, and high yields were obtained, albeit with
moderate stereoselectivities in the case of electron-rich
heterocycles (Table 2, compounds 3o-3r). Finally, we note that
b
selected as an example for the NOE experiment, see SI for details; The
reaction was reacted for 24 h at 50 oC.
With respect to the 4-pyridone partner, 3-methyl
substituted and 3,5-dichloro-substituted 4-hydroxypridines
reacted smoothly under reaction conditions, forming the desired
pyridone products in quantitative and 61% yield, respectively
(Table 2, compounds 3u and 3v).[12] Additionally, the quinolone
bearing several functional groups, including an iodo substituent,
could also prepared in moderate to good yields (Table 2,
compounds 3w-3y).
In an effort to gain more insight into the reaction
mechanism, deuterium labeling and control experiments were
carried out. As shown in eq 1 and eq 2, no D/H scrambling of the
H1 position was observed in propenylation products 5a and 5b.
In addition, 93% deuterium atom incorporation has been found
in H1 of 5c (eq 3), which corresponds to nearly quantitative
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10.1002/adsc.201801177
Advanced Synthesis & Catalysis
COMMUNICATION
retention of deuterium at the H1 position in the transformation
from 4c to 5c. However, using substrate 4d labeled with
deuterium next to the aryl ring led to the product 5d with 73% D
incorporation at the methyl group, indicating the incomplete D-
migration in the course of the transformation (eq 4). The lower
than expected 99% D might be due to trace water in the reaction
medium. When the same reaction was performed with 20 eq
D2O or MeOD added, the products 5e and 5f with 14% and 6%
deuterium incorporation in the methyl group were obtained,
respectively (eq 5). We also observed that the use of bulky
substrate 1n resulted in desired product 3n in 69% yield, along
with allylic substituted product 6 in 30% isolated yield under the
standard conditions. Longer reaction time was required to obtain
the desired propenylation product 3n in 98% yield (eq 6).[13]
When allylic product 6 and DBU was heated in THF overnight,
product of 3n was obtained. This result suggest that the
isomerization of intermediate (I) was attributed the base DBU
(See SI for details). Based on these results, we proposed that
the reaction proceeded via two steps: allylic substitution,
followed by isomerization. The allylic products (I) would be first
formed under the classic allylic substitution conditions, and then
the isomerization of terminal alkene (I) would result in the
desired product. (Scheme 3).[7j, 14]
Scheme 3. Proposed Mechanism.
To explore the functionalization of the alkene moiety and
the pyridone scaffold, synthetic elaborations were performed on
the pyridone products (Scheme 4). The reduction of 3a furnished
the product 1-(1-phenylpropyl)pyridin-4(1H)-one in 75% isolated
yield. Treatment of 3a with Lawesson reagent afforded the
pyridine-4-thione derivative in moderate yield.[15] Compound 9
and 10 could be prepared through a Suzuki coupling and
Sonogashira coupling, affording the product in 99% and 57%
yield, respectively, leaving the double bond intact for further
transformations.
O
95% D
OH
D
D
Standard conditions
(1)
N
H
OCOOMe
D
H1
N
D
H1
No D-incorporation
4a
2a
5a
O
99% D
D
OH
Standard conditions
N
H
OCOOMe
(2)
H
H1
4b
N
D
No D-incorporation
H1
2a
5b
O
OH
Standard conditions
N
H
OCOOMe
94% D
H
(3)
D
H
N
93% D
D
2a
4c
5c
O
>99% D
D
OH
Standard conditions
N
H
OCOOMe
(4)
H
N
D
73% D
H
4d
2a
5d
O
N
OH
Standard conditions
with 20eq D2O or MeOD
OCOOMe
H
contain 14% D, with D2O
contain 6% D, with MeOD
(5)
H
N
H
1a
2a
5e and 5f
Scheme 4. Synthetic transformations. (a) Pd/C, H2, MeOH, rt; (b) Lawesson
reagent, toluene, reflux; (c) PhB(OH)2, (dppf)PdCl2, CsF, DME, 80oC; (d)
Phenylacetylene, (Ph3P)2PdCl2, CuI, Et3N, 50oC.
O
N
O
N
[Ir(cod)Cl]2 (2 mol%)
L1 (4 mol%)
OCOOMe
(6)
Me
TBD (10 mol%)
DBU (2.0 eq)
THF, 50 o
C
1n
3n, 69% yield
3n, 98% yield
6, 30% yield
6, not observed
reaction time: 8h
reaction time: 24h
In summary, we have disclosed a mild and efficient method
of Iridium-catalyzed propenylation reactions of pyridin-4(1H)-one
analogues. The reaction afforded the 4-pyridone derivatives in
excellent to high yields with high stereoselectivities. Furthermore,
the preliminary mechanism was proposed to proceed through
two catalytic cycles: allylic substitution and isomerization. We
Scheme 2. Deuterium labeling and control experiments.
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10.1002/adsc.201801177
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COMMUNICATION
[4]
[5]
For selective recent reviews on Heck reactions, see: a) F.-X. Felpin, L.
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suggests a new general strategy for alkenylation reactions.
Experimental Section
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General procedure for the synthesis of 3. The reaction was
carried out in
a glovebox under an argon atmosphere.
[Ir(cod)Cl]2 (1.4 mg, 0.002 mmol), L1 (2.5 mg, 0.004 mmol), and
TBD (1.4 mg, 0.01 mmol) were added to a 2 dram scintillation
vial (vial A) equipped with a magnetic stirring bar. Vial A was
[6]
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o
then charged with THF (0.5 mL) and stirred at 50 C for 30 min.
To another 2 dram scintillation vial (vial B) was added propenyl
carbonate (0.1 mmol), 4-hydroxypyridine analogues (0.2 mmol),
DBU (0.2 mmol) and THF (0.5 mL). The pre-formed catalyst
solution (vial A) was then transferred to vial B. Then vial B was
sealed and stirred at 50 °C for a certain time. Upon completion
of the reaction, vial B was removed from the glovebox and
uncapped. Saturated NH4Cl aqueous solution was added and
the mixture was extracted with DCM (10 mL x 3). The combined
organic phase was washed with brine, dried over Na2SO4,
filtered and concentrated in vacuo. The residue was purified by
column chromatography over silica gel to afford the desired
product.
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[8]
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We gratefully acknowledge the Natural Science Foundation of
Jiangsu Province (BK20180447) and the Fundamental Research
Funds for the Central Universities (30918011313) for financial
support. We thank Dr. Lei Wang for assistance with the NOE
experiment. We also thank Prof. Hongmiao Wu from Nanjing
University of Technology for helpful discussions. We specially
thank Prof. Yi-ming Wang from University of Pittsburgh for
language polishing of the manuscript.
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[12] For more bulky substrate 2-methylpyridin-4(1H)-one, less than 5% yield
of the desired product was obtained. The allylic substitution product
was obtained in 50% yield when using 2,6-dimethylpyridin-4(1H)-one as
a substrate. See SI for the details.
[15] The linear product 1-cinnamylpyridine-4(1H)-thione was obtained when
utilizing pyridine-4-thiol as a substrate. See SI for the details.
[13] Compound 3n was obtained in quantitative yield when 6 was subjected to
the propenylation reaction system under the optimized conditions. See
SI for the details.
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Entry for the Table of Contents (Please choose one layout)
Layout : Alkenylation by iridium catalysis
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Xue-dan Bai, Jie Wang, Ying He*
Page No. – Page No.
Iridium catalyzed propenylation
reactions for the synthesis of 4-
pyridone derivatives
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