Microwave-assisted palladium-catalyzed heterogeneous vinylation of 2(1H)-pyridones

Article abstract of DOI:10.1002/ejoc.200900677

A. mild and efficient microwave-assisted vinylation of 2(1H)-pyridones has been developed using potassium, vinyltrifluoroborate with palladium on carbon as the catalyst. Various vinylating agents, Pd sources and solvents have been screened. Wiley-VCH Verl

Full text of DOI:10.1002/ejoc.200900677

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200900677
Microwave-Assisted Palladium-Catalyzed Heterogeneous Vinylation of
2(1H)-Pyridones
Brajendra K. Singh,[a,b] Claudia Cavalluzzo,[a] Marc De Maeyer,[c] Zeger Debyser,[d]
Virinder S. Parmar,[b] and Erik Van der Eycken*[a]
Keywords: Microwave chemistry / Heterogeneous catalysis / Nitrogen heterocyles / Natural products
A mild and efficient microwave-assisted vinylation of 2(1H)-
pyridones has been developed using potassium vinyltri-
fluoroborate with palladium on carbon as the catalyst. Vari-
ous vinylating agents, Pd sources and solvents have been
screened.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
The vast number of bioactive natural products and drugs
based on the 2(1H)-pyridone ring system exemplifies the
importance of this class of compounds.[1] Functionalized
2(1H)-pyridones have been used as versatile intermediates
in the synthesis of a wide range of nitrogen-containing het-
erocycles, such as pyridine, piperidine, quinolizidine, and in-
dolizidine alkaloids.[2]
Vinylated 2(1H)-pyridones are extremely interesting as
they can be easily converted into various key intermediates
for the synthesis of various natural products[3] and pharma-
ceuticals[4] via reduction (hydrogenation, hydroboration,
hydrosilylation etc.), oxidation (epoxidation, aziridination,
dihydroxylation, halogenation, etc.), hydroamination, hy-
dration, hydroformylation, cycloaddition etc.[5] (Scheme 1).
Although several methods are described for the synthesis
of various functionalized 2(1H)-pyridones,[6] however many
of the established approaches are still severely limited in
their use by the lack of generality, the harsh reaction condi-
tions involved, or the multistep procedures required.
Transition-metal-catalyzed couplings with vinylmetallic
donors are extremely attractive methods for the introduc-
tion of a vinyl group into aromatic and heteroaromatic sys-
tems. Among possible vinylmetallics, tin (Stille coupling),
silicon (Hiyama coupling) and boron (Suzuki coupling) de-
Scheme 1. Vinylated 2(1H)-pyridones as versatile synthons for syn-
thesis of various bioactive derivatives.
rivatives are frequently used donors for such cross-cou-
plings because of their tolerance towards a broad range of
functional groups.[7] The majority of investigations em-
ploying these transition-metal-catalyzed reactions have fo-
cused on aromatic and heteroaromatic systems.[7,8] How-
ever, to the best of our knowledge, aromatic imidate systems
[a] Laboratory for Organic & Microwave-Assisted Chemistry
(LOMAC), Katholieke Universiteit Leuven,
Celestijnenlaan 200F, 3001 Leuven, Belgium
[b] Bioorganic Laboratory, Department of Chemistry, University have never been applied in this regard. Therefore, we have
of Delhi,
Delhi 110007, India
elaborated a novel and convenient method for the vinyl-
ation of 2(1H)-pyridones.
[c] Laboratory for Biomolecular Modelling, Katholieke Universiteit
Leuven,
At the outset, the Pd-catalyzed Stille[9] cross-coupling of
6-chloro-3-nitro-2(1H)-pyridone (1) as aromatic imidate
system with vinyltributyltin (2)[10] in the presence of
Pd(PPh3)4 was examined. When a mixture of 6-chloro-3-
nitro-2(1H)-pyridone (1) with 1.1 equiv. of vinyltributyltin
in the presence of 5 mol-% of Pd(PPh3)4 in toluene was re-
Celestijnenlaan 200G, 3001 Leuven, Belgium
[d] Molecular Medicine, Katholieke Universiteit Leuven,
Kapucijnenvoer 33, 3000 Leuven, Belgium
Fax: +32-16327990
E-mail: erik.vandereycken@chem.kuleuven.be
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200900677.
Eur. J. Org. Chem. 2009, 4589–4592
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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E. Van der Eycken et al.
SHORT COMMUNICATION
fluxed for 48 h, reaction took place and the desired vinyl- certain advantages compared to other vinylmetallic for pal-
ated product 3 was obtained in 23% yield along with 12% ladium-catalyzed cross-coupling reactions. In particular, or-
of the butylated product 4 next to the starting material ganosilicon reagents are nontoxic[12] and the silicon con-
(Scheme 2). However, when the same reaction was per- taining byproducts are readily removed by simple workup
formed in refluxing dioxane for 48 h,% product yield ratio procedures. Recently, Denmark and co-workers have inves-
changed to 9:6. Other investigated catalytic systems tigated palladium-catalyzed vinylations with commercially
[Pd2(dba)3/(tBu)3P, (η3-C3H5PdCl)2 and PdCl2(PPh3)] were available organosilicon reagents in the presence of inexpen-
ineffective in both refluxing toluene or dioxane, whereas sive silanolate activators.[13] For our purpose, we selected
Pd(OAc)2/PPh3 in boiling toluene for 48 h yielded the vinyl- commercially
available
divinyltetramethyldisiloxane
ated product 3 in 12% yield along with 7% of the butylated (DVDS, 5) and 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclo-
V
product 4, next to the unreacted starting material tetrasiloxane (D4 , 6) as potential vinyl donors.[13] When the
(Scheme 2).
reactions were carried out for 4 d at room temperature in
DMF or for 2 d under reflux in toluene or dioxane with
1.2 equiv. of DVDS (5) as vinyl donor in the presence of
5 mol-% of Pd(dba)2 as catalyst, 5mol-% of PPh3 as ligand
and 2 equiv. of KOSiMe3 as activator, no product formation
was observed. However, when the reactions were performed
under microwave irradiation at a ceiling temperature of
120 °C and 200 W maximum power in various solvents, a
different extent of product formation was observed
(Table 2). In order to achieve full conversion different cata-
lysts were screened applying DMF, dioxane or toluene un-
der microwave irradiation (120 °C, 200 W). However, none
of the screened conditions led to full conversion (Table 2,
Entries 1–6). Even when duplicating the aforementioned
conditions with D4V (6), full conversion was never achieved
(Table 2, entries 7–12).
Scheme 2. Vinylation of pyridone 1 under conventional heating.
The application of microwave irradiation is well known
for its beneficial effects on transition-metal-catalyzed reac-
tions.[11] This prompted us to investigate the vinylation un-
der microwave irradiation conditions. A series of experi-
ments were performed using dioxane or toluene as solvent
under various irradiation conditions (Scheme 2, Table 1).
This revealed that performing the reaction at a ceiling tem-
perature of 120 °C with a maximum power of 200 W was
most effective in terms of both yield and time. Full conver-
sion was accomplished in 30 min with exclusive formation
of the desired vinylated product 3 in 59% yields (Table 1,
entry 8) next to some unidentified compound. In all other
cases, with the exception of entry 4, the formation of butyl-
ated product 4 was observed (Table 1, entries 1–3 and 5–
7). Apparently, the high ceiling temperature of 120 °C and
maximum power of 200 W are required to suppress the side
reactions.
Table 2. Screening of vinylsilicon compounds as vinylating agents
applying microwave irradiation.[a,b]
Entries Vinyl donor Pd source[c]
Solvent
Time[d] Conv.[e]
[min]
(%)
Table 1. Screening of microwave conditions.[a]
1
2
3
4
5
6
7
8
DVDS (5)
Pd(dba)2/PPh3 DMF
Pd(dba)2/PPh3 dioxane
Pd(dba)2/PPh3 toluene
120
60
90
120
60
90
120
45
60
120
30
45
0
50
20
0
50
30
0
70
60
0
70
70
Entry
Solvent
Temp/Power
Time[b]
[min]
% Yield[c]
[°C/W]
3/4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
DMF
dioxane
toluene
1
2
3
4
5
6
7
8
dioxane
dioxane
dioxane
dioxane
toluene
toluene
toluene
toluene
100/100
100/200
120/100
120/200
100/100
100/200
120/100
120/200
90
60
45
20
90
60
60
30
33:17
37:11
40:7
37:0
41:13
46:9
v
D4 (6)
Pd(dba)2/PPh3 DMF
Pd(dba)2/PPh3 dioxane
Pd(dba)2/PPh3 toluene
9
10
11
12
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
DMF
dioxane
toluene
53:9
59:0
[a] Reactions were performed on a 0.1 mmol scale of 6-chloro-3-
nitro-2(1H)-pyridone (1) using tributylvinyltin (2) (1.1 equiv.) and
Pd(PPh3)4 (5 mol-%). [b] Time taken for full conversion. [c] Isolated
yields are given.
[a] Reactions were performed on a 0.1 mmol scale of 6-chloro-3-
nitro-2-(1H)pyridone (1) under microwave irradiation at a ceiling
temperature of 100 °C, and a maximum power of 200 W using 5
or 6 as vinyl donor (1.5 equiv.), KOSiMe3 as activator (2 equiv.),
Pd(dba)2 or Pd(PPh3)4 as Pd source (5 mol-%) in solvent (3 mL).
[b] When the same reactions were performed under conventional
conditions, no product formation was observed even after 4 d at
room temp. or 2 d of reflux. [c] In case of Pd(dba)2 as catalyst,
PPh3 (5 mol-%) was used as additional ligand. [d] Reactions were
run until no further conversion was observed by GC. [e] Conver-
Although these results were acceptable, there were still
improvements that could be made to render the process
more efficient. Recently different research groups have de-
scribed various vinyl sources based on silicon and boron.[7]
Therefore, we investigated vinylsilicon reagents, which offer sions were determined by GC.
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Eur. J. Org. Chem. 2009, 4589–4592

Palladium-Catalyzed Heterogeneous Vinylation of 2(1H)-Pyridones
Unfortunately, organosilicon vinyl donors were ineffec- Table 3. Screening of catalyst system and solvent using potassium
vinyltrifluoroborate (7) as potential vinylating agent under micro-
tive for the investigated pyridone system, so we diverted our
investigations to organoborane as vinyl donor, and in par-
wave irradiation.[a]
ticular potassium vinyltrifluoroborate (7), which appears to
be more nucleophilic[14] than other organoboron comounds.
Moreover, potassium vinyltrifluoroborate (7) has been
proven to be a particularly useful boron reagent that is air-
and moisture-stable, atom economical, resistant to protode-
boronation, thereby allowing stoichiometric quantities of
reagent to be used in cross-coupling protocols.[15] Addition-
ally, it is commercially available and numerous methods
have been developed for its synthesis which are environmen-
tally sound and byproducts are of low toxicity compared to
other organometallic reagents, particularly organo-
stannanes.[15] Recently, different groups have successfully
demonstrated the use of microwave irradiation for this kind
of cross-couplings applying potassium organotrifluorobor-
ate.[16] Therefore we decided to explore the palladium-cata-
lyzed reaction of 2(1H)-pyridones using potassium vinyltri-
fluoroborate (7) under microwave irradiation.
Entries Pd source
Solvent
Base
Time[b] % Yield[c]
[min]
1
2
3
4
5
6
7
8
9
10
PdCl2/PPh3
Pd(OAc)2/PPh3 THF/H2O (9:1) Cs2CO3
Pd(PPh3)4
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
THF/H2O (9:1) Cs2CO3
50
30
30
40
40
30
30
50
50
30
68
69
70
67
69
71
69
70
68
70
THF/H2O (9:1) Cs2CO3
THF/H2O (9:1) Cs2CO3
NMP/H2O (9:1) Cs2CO3
NMP
H2O
H2O
H2O
H2O
Cs2CO3
Cs2CO3
Na2CO3
K2CO3
KOAc
Pd/C
Initially a brief screening of various catalyst systems was [a] Reactions were performed on a 0.1 mmol scale of 6-chloro-3-
nitro-2-(1H)-pyridone (1) using potassium vinyltrifluoroborate (7)
performed. Adopting the described procedure, we evaluated
(1.1 equiv.), Pd source (2 mol-%) and 3 equiv. of base at a ceiling
the cross-coupling with PdCl2/PPh3, Pd(OAc)2/PPh3,
temperature of 100 °C and a maximum power of 200 W. [b] Time
taken for full conversion. [c] Isolated yields.
Pd(PPh3)4[17] and Pd/C.[18] The reaction proceeded well and
full conversion was achieved in all cases using 2 mol-% of
Pd source, 3 equiv. of Cs2CO3 as base and THF/H2O (9:1)
model compound 1, were unsuccessful. The reactivity of 1
as solvent under microwave irradiation at 100 °C and a
could probably be ascribed to the activating nitro group in
maximum power of 200 W (Table 3). Although all evalu-
position 3 of the pyridone system.
ated Pd sources gave satisfactory results (Table 3, Entries 1–
4), we preferred to proceed with the heterogeneous catalytic
system (Pd/C) as this is supposed to prevent leaching of Pd
and is relatively inexpensive.[19] Several solvent systems were
investigated [THF/H2O (9:1), NMP/H2O (9:1), NMP, H2O]
(Table 3, Entries 4–7) using 2 mol-% of Pd/C and 3 equiv.
Cs2CO3 as base. All of them were found to be equally effec-
tive and furnished the desired vinylated compound in 67–
71% yield within 30–50 min.
Water offers practical advantages as it is inexpensive,
readily available, and non-flammable and it renders the pro-
cess a “greener” aspect.[20] Also different bases [Cs2CO3,
Na2CO3, K2CO3, KOAc] were screened using 2 mol-% of
Pd/C as catalyst in water as solvent (Table 3, entries 7–10).
Although no significant differences in yield were observed,
the reaction proceeded slower when using Na2CO3 or
K2CO3, hence KOAc was the preferred base as it is less
expensive than Cs2CO3.
Table 4. Evaluation of scope of the vinylation using different 2(1H)-
pyridones.[a]
Entry
X
Time
Solvent
Product
% Yield[c]
[min][b]
1.
2.
3.
4.
5-I
4-I
5-Br
3-Br
10
10
20
20
H2O
H2O
NMP/H2O (8:2)
NMP/H2O (8:2)
13
14
13
15
91
84
78[d]
81[d]
[a] Reactions were performed on a 0.1 mmol scale of 2(1H)-pyr-
idone using potassium vinyltrifluoroborate (7) (1.1 equiv.), Pd/C
(2 mol-%) and 3 equiv. of KOAc under microwave irradiation at a
ceiling temperature of 100 °C and a maximum power of 200 W. [b]
Time taken for full conversion as indicated by GC. [c] Isolated
yields. [d] When H2O was used as solvent no full conversion was
achieved.
Having optimized the protocol, we investigated the vinyl-
ation of various 2(1H)-pyridone halides. When 4- or 5-iodo-
2(1H)-pyridone was subjected to vinylation, the reaction
proceeded smoothly yielding the desired vinylated products
In summary, we have developed a novel and hetero-
in excellent yields of 84% and 91% in mere 10 min (Table 4, geneous, palladium-catalyzed vinylation of 2(1H)-pyridones
Entries 1 and 2). However, when bromo pyridones were re- using potassium vinyltrifluoroborate as vinyl donor. This
acted under the aforementioned conditions, full conversion protocol is applicable to various iodo and bromo 2(1H)-
could not be achieved. Switching to NMP/H2O (8:2) as sol- pyridones. Reactions can be performed in various solvents
vent mixture under the optimized conditions, furnished the like H2O, NMP, THF, or in mixtures of THF/H2O, NMP/
desired vinylated products in excellent yields of 78–91% H2O. This procedure is an interesting complement of the
within 20 min (Table 4, Entries 3–4). Attempts to use existing Suzuki cross-coupling reaction. It provides a meth-
chloro-2(1H)-pyridones for this cross-coupling, other than odology for the synthesis of various compounds derived
Eur. J. Org. Chem. 2009, 4589–4592
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E. Van der Eycken et al.
SHORT COMMUNICATION
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Received: June 18, 2009
Published Online: August 12, 2009
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