Synthesis of Benzofuranones via Palladium-Catalyzed Intramolecular Alkoxycarbonylation of Alkenylphenols

Article abstract of DOI:10.1002/chem.201705808

Herein, a new catalytic system to synthesize benzofuranones is reported. A palladium-catalyzed intramolecular alkoxycarbonylation is employed to generate 3-substituted-benzofuran-2(3H)-ones from alkenylphenols under mild reaction conditions, linked to an ex situ formation of CO from N-formylsaccharin. The carefully chosen catalytic system enables an efficient reaction with a novel functional group tolerance, despite the high polymerization tendency of the starting material.

Full text of DOI:10.1002/chem.201705808

A Journal of
Accepted Article
Title: Synthesis of Benzofuranones via Palladium-Catalyzed
Intramolecular Alkoxycarbonylation of Alkenylphenols
Authors: Vera Hirschbeck and Ivana Fleischer
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To be cited as: Chem. Eur. J. 10.1002/chem.201705808
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COMMUNICATION
Synthesis of Benzofuranones via Palladium-Catalyzed
Intramolecular Alkoxycarbonylation of Alkenylphenols
[
b]
[a]
Vera Hirschbeck, and Ivana Fleischer*
Abstract: Herein,
a
new catalytic system to synthesize
some substrates and the employed high temperature (90 °C) in
combination with an acid. Protonation of the substrate leads to the
protonated quinone methide intermediate 3, which undergoes
cationic polymerization. Herein we report the first
cyclocarbonylation of 2-vinylphenols proceeding at room
temperature, which allows for the expansion of the substrate
scope to vinyl phenols with substituted aryl ring. In order to avoid
working with gaseous carbon monoxide, N-formylsaccharin (NFS)
benzofuranones is reported. A palladium-catalyzed intramolecular
alkoxycarbonylation is employed to generate 3-substituted-
benzofuran-2(3H)-ones from alkenylphenols under mild reaction
conditions, linked to an ex situ formation of CO from N-
formylsaccharin. The carefully chosen catalytic system enables an
efficient reaction with a novel functional group tolerance, despite the
high polymerization tendency of the starting material.
[
9]
-
originally described by Manabe and co-workers - was used as
a CO surrogate, in combination with a two-chamber pressure tube
Benzofuran-2(3H)-ones, which can often be found in natural
[10]
developed by Skrydstrup (Scheme 1, bottom).
products, constitute interesting structural motives with unique
[
1]
Cyclocarbonylation:
pharmaceutical importance. Moreover, substituted derivatives
play an important role in polymer chemistry, where they are used
as antioxidants, in order to inhibit oxidative degradation of
polymers. Due to the weak benzylic C–H bond, benzofuranyl
radicals can easily be formed, which are able to trap oxygen and
R
R
CO
R
O
R
O
OH
2
[
2]
1
therefore prevent autoxidation.
Many different synthetic
R
strategies to synthesize benzofuran-2(3H)-ones are known in the
and/or
R
[
3]
literature. A convenient and atom economic way to generate
benzo-fused 5-, 6- or 7-membered lactones is the intramolecular
O
O
[
4]
Reppe-type carbonylation of alkenyl- or allylphenols. The
control of regioselectivity is one of the challenges of this
transformation. Several approaches to the synthesis of 6-
Challenges:
•
•
•
regioselectivity
reaction conditions
R-substituents
• competing polymerization
R
[
5]
membered lactones were reported. A general method for the
synthesis of benzofuran-2(3H)-ones (2) from alkenylphenols (1)
proved more difficult (Scheme 1). Alper employed a catalyst
system based on a Pd(0) precursor and bidentate ligand dppb
via:
R
3
OH
This work:
(
1,4-bis(diphenylphosphino)butane) in a ionic liquid to obtain a
series of 5-membered lactones with no or moderate
CO from NFS
R
R
O
[
6]
regioselectivity. Better results were achieved by Manabe et al.,
OH
[Pd], ligand,
H
O
who developed a cyclization of 2-vinyl aryl formates using
1
2
Ru
Ru]). In 2014, Shi and co-workers reported a hydroesterification
of alkenylphenols with phenyl formate as CO surrogate
3
(CO)12 under forcing reaction conditions (135 °C, 15 mol% of
room temperature
[
7]
[
O
[
8]
O
• mild conditions
generating benzofuran-2(3H)-ones. They described a catalytic
system consisting of Pd(OAc) (5 mol%) and PPh (20 mol%),
S
•
•
CO surrogate
highly regioselective
N
2
3
O
which showed a good functional group tolerance for α-, β- or non-
substituted alkenylphenols.
O
• good FG-tolerance
NFS
However, only one example with substituent on aryl ring was
shown, probably because of the high polymerization tendency of
Scheme 1. Regioselectivity of cyclocarbonylation and our new system.
[
a]
b]
Prof. Dr. Ivana Fleischer
Department of Organic Chemistry
University of Tuebingen
Auf der Morgenstelle 18, 72076 Tuebingen, Germany
E-mail: ivana.fleischer@uni-tuebingen.de
Vera Hirschbeck
Based on our earlier reports on the alkoxy- and the
thiocarbonylation of alkenes at ambient temperature, a modified
[
11]
Pd(0)-based catalytic system for lactonization was identified.
We found that the combination of Pd(dba) , ligand L1 (dppdtbpf;
-diphenylphosphino-1´-(di-tertbutylphosphino)-ferrocene) and
2
[
[
12]
1
Department of Organic Chemistry
University of Regensburg
diphenylphosphoric acid (DPPA) that was previously used in the
thiocarbonylation led to 81% of 2a (Table 1, entry 1). Whereas,
the original alkoxycarbonylation conditions (L2, (dtbpx; bis(di-tert-
Universitaetsstrasse 31, 93053 Regensburg, Germany
Supporting information for this article is given via a link at the end of
the document.
[
13]
butylphosphinomethyl)benzene) /BNPA
(1,1´-bi-2-naphthol
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Chemistry - A European Journal
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COMMUNICATION
phosphoric acid)) generated only 73% yield (entry 2). A transpose
of the respective ligand/acid pair led to decreased activity, which
Also, few additives were tested. In the thiocarbonylation
project we suggested that the used thiol forms the Pd-H via oxida-
[
11b]
n
is in accordance to our earlier report (entries 3,4).
A slightly
tive addition, therefore the same amount of C
7
H
15SH was added.
increased yield was observed by using TFA (entry 5), but because
of its volatility and in order to circumvent possible reproducibility
issues in the two-chamber pressure tube, we decided to use
DPPA in further studies. The acid is essential for the generation
of the catalytically active Pd-hydride, as confirmed by the
In addition, it also may form the reactive thioester first and this
[
15]
would undergo fast transesterification. However, this led to a
reduction of catalytic activity in case of intramolecular
alkoxycarbonylation resulting in 70% yield (entry 7). Then, we
examined if there may be a positive influence by the addition of
methanol (entries 8, 9). Beside the intramolecular reaction, also
an intermolecular carbonylation (generation of the branched
methylester) followed by trans-esterification to generate 2a is
conceivable. Unfortunately, no positive influence was observed
by the addition of MeOH. By increasing the amount of MeOH
more of the undesired branched methylester was observed. LiCl
was tested as Lewis acid additives since it might accelerate the
final rate-determining alcoholysis by coordination to the carbonyl
group (entry 10; see SI for the commonly accepted mechanism of
[
13c, 14]
corresponding control experiment without any acid (entry 6).
[
a]
Table 1. Initial optimizations.
CO (2.5 bar,
from NFS)
O
OH Pd(dba) , acid,
O
2
ligand, solvent,
RT, 24 h
1
a
2a
acid
[
16]
O
alkoxycarbonylations). However, a white precipitate and no lac-
tone were observed. The corresponding intermolecular transfor-
mation of styrene with phenol under this reaction conditions was
not successful, which might be reasoned with steric effects.
O
O
P
O
O
OH
P
OH
O
Furthermore, a solvent screening was performed considering
DPPA
BNPA
[
17]
Kamlet-Taft parameters (Figure 1). The choice of the solvent
depends on its properties and other factors, such as waste
management. On one side the solvent has to be polar in order to
dissolve the acid and other reaction components, on the other
side the solvent should not be too basic, because otherwise it can
coordinate to the catalyst. Other important factors influencing the
search of a suitable solvent, for example the viscosity plays an
important role if one reaction partner is a gas, such as CO in
ligand
P(tBu)2
P(tBu)2
P(tBu)2
Fe
PPh2
dppdtbpf, L1
dtbpx, L2
Prior Thiocarbonylation Prior Alkoxycarbonylation
conditions conditions
[
18]
carbonylations.
Entry
ligand
acid
solvent
additive
Yield
[
c]
[
%]
1
2
3
4
5
6
7
8
9
L1
L2
L2
L1
L1
L1
L1
L1
L1
L1
DPPA
BNPA
DPPA
BNPA
TFA
CH
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
2
2
2
2
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
2
2
-
-
-
-
-
81
73
75
69
83
0
-
-
[
[
d]
e]
n
[d]
DPPA
DPPA
DPPA
DPPA
C
7
H
15SH
[e]
70
27
[f]
MeOH
[
g]
MeOH
CH Cl
-
9
0
[
h]
1
0
2
2
LiCl
[
17]
Figure 1. Solvent screening considering Kamlet-Taft parameters.
[
a] reaction conditions: The reaction was carried out in a 2-chamber system.
Chamber A: CO generation (2.5 bar): NFS (2.13 mmol, 450 mg), Na CO (3.20
mmol, 339 mg) in DMF (1 mL); Chamber B: 2-vinylphenol (115 µL, 1.0 mmol),
Pd(dba) (5.8 mg, 10 μmol, 1 mol%), ligand (40 μmol, 4 mol%), acid (150 μmol,
5 mol%), solvent (1 mL), RT, 24 h. [c] determined by quant. NMR using
2
3
2
Various solvents were tested and the yield could be increased
to 88% by using Et O or EtOAc, instead of CH Cl (81%). As
1
n
2
2
2
OHCNPh
mg, 1.3 mmol). [e] CH
2
as an internal standard; [d] CH
2 2 7
Cl (790 µL), C H15SH (210 µL, 177
2
Cl (790 µL), MeOH (210 µL, 166 mg, 5.2 mmol). [f]
expected no yield was observed by using hexane, because of its
poor ability to solubilize reaction components. The lower yield in
propylene carbonate can be traced back to the high viscosity and
acetonitrile can coordinate to the catalyst and therefore deactivate
2
Further 47% of branched methylester (determined by GC-FID). [g] Further 62%
of branched methylester (determined by NMR). [h] LiCl (200 µmol, 8.5 mg,
0
.2 eq).
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Chemistry - A European Journal
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COMMUNICATION
[
a]
Table 3. Carbonylation of different aryl-, α- or β- substituted 2-vinylphenols.
it (acetonitrile is a better ligand because of π-backbonding). We
2
decided to use Et O for further studies, since it can be easily
R
3
CO (2.5 bar,
R2
from NFS)
2
removed after the reaction also in case of more volatile products.
For the final optimizations, the temperature was increased to
4
5
R
R1
O
1
1
Pd(dba)2
L1, DPPA
Et O, RT, 48 h
2
O
OH
3
0 °C, which led to higher yield of 95% (Table 2).
6
1a-m
2a-m
[
a]
Table 2. Final optimizations for the lactonization of 2-vinylphenol.
[
b]
Entry
1
Substrate
Product
Yield [%]
CO (2.5 bar,
from NFS)
O
95
1
mol% Pd(dba)2
mol% L1
OH
O
O
O
O
1
1
a
b
2a
4
O
O
O
OH
1
a
15 mol% DPPA
Et2O
2a
2
87
83
[
b]
Entry
Time [h]
24
Temp [°C]
Yield [%]
2b
2c
OH
OH
1
2
3
RT
30
88
95
94
3
4
24
1c
1d
48
RT
[
c]
11
88
O
[
a] reaction conditions: The reaction was carried out in a 2-chamber system.
Chamber A: CO generation (2.5 bar): NFS (2.13 mmol, 450 mg), Na CO
3.20 mmol, 339 mg) in DMF (1 mL); Chamber B: 2-vinylphenol (115 µL,
.0 mmol), Pd(dba) (5.8 mg, 10 μmol, 1 mol%), L1 (21 mg, 40 μmol, 4 mol%),
DPPA (38 mg, 150 μmol, 15 mol%), Et O (1 mL). [b] determined by quantitative
as an internal standard. .No 6-membered lactone was
2d
O
OH
2
3
(
1
2
5
O
O
O
O
O
O
O
2
1
e
2e
2f
O
O
O
NMR using OHCNPh
detected.
2
OH
OH
OH
[
d]
6
7
0
Almost the same result was observed by performing the
reaction for 48 h at room temperature. Since most of the
envisaged substrates show a high polymerization tendency,
which is significantly influenced by the temperature, 48 h and RT
were chosen as the optimized reaction conditions. Subsequently,
the substrate scope for the lactonization of 2-vinylphenol
derivatives was evaluated (Table 3). Therefore, 1 mol% of
1f
90
85
1g
2g
O
O
8
O
O
2 2
Pd(dba) , 4 mol% of L1, 15 mol% of DPPA in Et O for 48 h at RT
1
1
h
i
2h
2i
O
O
OH
OH
OH
were applied in order to carbonylate aryl-, α- or β- substituted
substrates by using 2.5 bar of ex situ generated CO. Methyl
substituents in 3´ and 5´ positions are well tolerated, whereas
there is breakdown in activity for double 3´,6´-substitution (entries
O
O
O
9
O
78
22
2
-4). The steric influence of the methyl groups next to both
reaction sites seems to be too strong. Another challenge is the
carbonylation of vinyl phenols containing electron donating
methoxy groups, due to polymerization issues. Under the mild
reaction conditions, we were able to convert 4´-and 6´-MeO-
substituted substrates in excellent yields (88%, 90%, entries 5, 7).
However, a polymerization was observed for 5-methoxy-2-
vinylphenol (1f) under the reaction conditions (entry 6), and also
10
1j
2j
O
O
O
[
e]
1
1
88
77
1
1
k
2k
2k
O
O
OH
OH
t
already during its synthesis. To our delight, Bu- and COOMe-
12
O
O
l
groups were tolerated in 4´ position, providing good yields of 85%
and 77% respectively (entries 8, 9). Then, we examined the
influence of substituents on the double bond. For α-substituted
substrates such as 1j, the formation of five membered lactone
was not observed. This was the only case were the six-membered
lactone 2j was generated albeit in low yield of 22% (entry 10). By
applying the β-substituted alkene 1k, 2k was observed in
excellent yield of 88%, whereas 1l led to the same product in 78%
yield via initial isomerization. As expected, carbonylation of
sterically demanding 1m generated 2m in low yield of 11%.
Double substitution in β-position seems to be a limitation of the
catalytic system.
1
3
11
1
m
2m
O
OH
[
a] reaction conditions: The reaction was carried out in a 2-chamber system.
Chamber A: CO generation (2.5 bar): NFS (2.13 mmol, 450 mg), Na
3.20 mmol, 339 mg) in DMF (1 mL); Chamber B: 1a - 1m (1.0 mmol), Pd(dba)
2
CO
3
(
2
(5.8 mg, 10 μmol, 1 mol%), L1 (21 mg, 40 μmol, 4 mol%), DPPA (38 mg,
150 μmol, 15 mol%), Et O (1 mL), RT, 48 h. [b] Isolated yields. [c] 31%
conversion. [d] Complete polymerization. [e] E/Z (77/33) mixture.
2
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Chemistry - A European Journal
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COMMUNICATION
Subsequently, we turned our attention back to the challenging
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[
a]
(
1d).
CO (2.5 bar,
from NFS)
O
+
Pd(dba) , L1,
O
2
OH
O
O
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[
b]
Yield
p(CO)
Temp
[°C]
Ratio
Conv.
[%]
Yield
[b]
Entry
+
[b]
2d’
%]
[
bar]
[Pd]/lig/H
2d [%]
[
1
2
3
4
2.5
5
RT
RT
35
35
2/8/15
1/4/15
1/4/15
2/8/15
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58
88
97
44
30
51
62
23
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34
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2.5
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[
a] reaction conditions: The reaction was carried out in a 2-chamber system.
Chamber A: CO generation: 2.5 bar à NFS (2.13 mmol, 450 mg), Na CO
3.20 mmol, 339 mg) in DMF (1 mL); 5 bar à NFS (4.26 mmol, 900 mg),
Na CO (6.40 mmol, 678 mg) in DMF (2 mL); Chamber B: 1d (165 µL,
.0 mmol), Pd(dba) (5.8 mg, 10 μmol, 1 mol%; 11.6 mg, 20 µmol, 2 mol%),
L1 (21 mg, 40 μmol, 4 mol%; 42 mg, 80 μmol, 8 mol%), DPPA (38 mg,
50 μmol, 15 mol%), Et O (1 mL), RT/35 °C, 48 h. [b] determined by
quantitative NMR using OHCNPh as an internal standard.
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[
[
7]
8]
In conclusion, we investigated a highly active catalytic system
for the lactonization of alkenylphenols, which proceeds at room
temperature and therefore allows to employ various substrates
with the polymerization tendency. This enabled a new substrate
scope of vinyl phenols with substituents on the aryl ring. In
addition, single substitution at both positions of the double bond
was tolerated. The avoidance of gaseous carbon monoxide and
heating render this transformation an environmentally friendly
way of generating lactones, with a good functional group
tolerance also for sterically demanding and electron donating
substituents.
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Acknowledgements
We are grateful to the Fonds der Chemischen Industrie (Liebig
fellowship, I.F.; Ph.D. fellowship, V.H.) and the Universities of
Tübingen (Institutional Strategy of the University of Tübingen;
Deutsche Forschungsgemeinchaft, ZUK 63) and Regensburg for
financial support.
[
Keywords: carbonylation • palladium • cyclization • CO
surrogate • lactones
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Carbonylative lactonization at
Vera Hirschbeck, Ivana Fleischer*
ambient reaction conditions provides
benzofuranones. Various vinyl
Page No. – Page No.
phenols with substituents on the aryl
ring were converted in good yields.
Synthesis of Benzofuranones via
Palladium-Catalyzed Intramolecular
Alkoxycarbonylation of
((Insert TOC Graphic here))
Alkenylphenols
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