Diethyloxalate as “CO” Source for Palladium-Catalyzed Ethoxycarbonylation of Bromo- and Chloroarene Derivatives

Article abstract of DOI:10.1002/adsc.201700307

Palladium(II)-catalyzed ethoxycarbonylation of aryl bromides with diethyloxalate oxalate is described. Functionalized aromatic esters can be efficiently synthesized with only 0.65 mol % PdCl₂(PPh₃)₂ catalyst under microwav

Full text of DOI:10.1002/adsc.201700307

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DOI: 10.1002/adsc.201700307
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Diethyloxalate as “CO” Source for Palladium-Catalyzed
Ethoxycarbonylation of Bromo- and Chloroarene Derivatives
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Amandine Monrose,a Helori Salembier,a Till Bousquet,a Sylvain Pellegrini,a,* and
a,
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Lydie Pelinski *
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a
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University of Lille, CNRS, ENSCL, Centrale Lille, Univ. Artois,UMR 8181 – UCCS – Unite de Catalyse et Chimie du
Solide, F-59000 Lille, France
Fax: (+33)320436585; e-mail: sylvain.pellegrini@univ-lille1.fr; lydie.pelinski@univ-lille1.fr
Received: May 13, 2017; Revised: May 3, 2017; Published online: && &&, 0000
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.201700307
aryl halides was developed by Liu (Scheme 1, method
Abstract: Palladium(II)-catalyzed ethoxycarbony-
lation of aryl bromides with diethyloxalate oxalate
a).[12] However, this procedure requires the presence
of a bidentate phosphine ligand (dppp or dCypp). It is
is described. Functionalized aromatic esters can be
worth noting that this reaction, starting from iodoben-
efficiently synthesized with only 0.65 mol % PdCl2
zene and some activated bromobenzene derivatives,
(PPh3)2 catalyst under microwave irradiation and
was successfully catalyzed under ligand free condi-
without additional ligand. This method illustrates
tions.[13] In both protocols, the reaction was performed
an inexpensive and operationally simple method
for the preparation of aromatic esters.
at 1508C for 24 h in NMP solvent under nitrogen
conditions. In addition to these constraints, prior to
use, the potassium a-oxocarboxylate has to be first
Keywords: Esterification; carbonylation; diethylox-
synthesized from diethyloxalate. Gooßen has devel-
alate; palladium
oped a catalyst system (Pd(OAc)2/ dppp/DABCO)
which promotes the decarboxylation of benzyl oxa-
lates to give arylacetates from benzylic alcohols and
33 Carboxylic esters are found in many natural products dialkyl oxalates.[14]
34 and pharmaceutical compounds. Regarding their im-
35 portance, palladium-catalyzed carbonylation reactions
36 of aromatic halides employing CO and CO2 have been
37 investigated extensively for the synthesis of acids and
38 their derivatives such as esters and amides.[1] However,
39 the use of highly active CO gas is restricted due to its
40 toxicity and high pressure reaction conditions. For this
41 reason, the development of alternative ways to
42 produce CO in the reaction mixture is of considerable
43 interest in synthetic organic chemistry.[2] In this
44 context, carbonylation involving in situ generation of
45 CO from surrogates such as Mo(CO)6,[3] aldehydes,[4]
46 silacarboxylic acids,[5] formates,[6] formic acid[7] and
47 alcohols[8] was developed. More recently, a carbon-
48 ylative esterification reaction between aryl bromides
49 and alcohols in the presence of oxiranes as CO source
50 has been reported.[9]
Scheme 1. Palladium-catalyzed carboxylation reactions.
51
At the same time, Gooßen and co-workers discov-
52 ered that the Pd/Cu-catalyzed decarboxylative cross-
53 coupling of a-oxocarboxylates gave rise to ketones.[10]
54 This method has been applied to realize ortho-
More recently, polystyrene-supported Palladium(0)
55 acylation of acetanilides and C2-acylation of in- (Pd@PS) nanoparticles were explored as a heteroge-
56 doles.[11] Inspired by this work, a Pd-catalyzed decar- neous catalyst for hydroxycarboxylation of aryl halides
57 boxylative coupling of oxalate monoester salts with
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in the presence of oxalic acid under microwave Unlike previous studies,[12,13] the use of NMP as
irradiation (Scheme 1, method b).[15]
solvent led to a very limited reactivity (entry 4). In
In this context, we wish to describe herein an addition, it appeared that decreasing the amount of
efficient microwave-promoted ethoxycarbonylation of diethyl oxalate or the loading of catalyst was slightly
aromatic and heteroaromatic bromides or chlorides beneficial to the yield (entry 5 vs 6 and entry 6 vs 7
via Pd-catalyzed with the commercial available dieth- respectively). It has to be noted that no reaction
yloxalate as CO source.
occurred and the vial pressure did not increased when
Our first attempt to evaluate the feasibility of the the reaction was performed at 1208C (entry 9). This
reaction was realized starting from bromobenzene 1a result supported the assumption that a higher temper-
10 in the presence of diethyl oxalate 2 (2 equiv.), ature was then necessary for the required decomposi-
11 1.3 mol% of PdCl2(PPh3)2 and dimethylaminopyridine tion of diethyloxalate. Furthermore, a lower yield was
12 DMAP (1.2 equiv.) and ethanol (1.5 equiv.) (Table 1). obtained when the reaction was carried out for 10 min
13 The reaction was then irradiated by microwave for (entry 10). Replacing the bromine atom by chlorine or
14 20 min at 1408C. It is important to note that the iodine atoms led to an absence of reactivity (entry 11)
15 pressure increased rapidly due to gas release when the or a slight decrease in yield (entry 12).
16 temperature reached 1308C. The expected ethyl
The reaction was then carried out with various
17 benzoate 3 was obtained in 79% yield (entry 1, bases (entries 1 to 5, Table 2) and Pd sources (entries 7
18 Table 1). Hence, encouraged by this first result, to 10, Table 2). It appeared that the presence of a
19 investigations were performed to determine the mesomeric effect in the base is necessary to obtain
20 optimal conditions. The amount of ethanol as addic- good yields. Indeed, higher yields were obtained using
21 tive was first evaluated. The results revealed that the dimethylaminopyridine (DMAP), 1,8-diazabicycloun-
22 presence of ethanol was crucial for the reaction and dec-7-ene (DBU) (entries 1–2).
23 increasing its amount to 3 equiv. improved the yield to
24 97% (46% vs 97% yield, entry 3 vs entry 2, Table 1).
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Table 2. Screening of base and catalyst starting from bromo-
benzene 1a.[a]
Entry
[Pd]
Base[b]
Yield (%)[c]
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Table 1. Optimization of the reaction conditions starting
from halogenobenzene 1a–c.[a]
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2
3
4
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2
Pd/C
Pd(OAc)2
Pd(TFA)2
DMAP
DBU
TMG
DIPEA
TEA
DMAP
DMAP
DMAP
DMAP
DMAP
97
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0
22
48
70
8
5
6[d]
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Entry
1
X
[Pd] mol%
2 (eq)
Yield
(%)[b]
8
9
10
95
95
1[c]
2
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
I
1.3
1.3
1.3
1.3
1.3
1.3
0.65
0.3
0.65
0.65
0.65
0.65
6.8
6.8
6.8
6.8
2.1
1.2
1.2
1.2
1.2
1.2
1.2
1.2
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97
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[a] Reaction conditions: 1a (1 mmol), 2 (1.2 mmol), [Pd]
(0.6 mol%), base (1.2 mmol), EtOH (3 mmol) at 1408C for
20 min under microwave irradiation.
3[d]
4[e]
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[b] DBU=1,8-diazabicyclo undec-7-ene, TMG=tetramethyl-
guanidine, DIPEA = di isopropylethylamine, TEA =
triethylamine.
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92
97 (85)[f]
95
0
75
0
[c] GC yields with diphenylmethane as internal standard.
[d] Reaction performed with 0.6 mmol of DMAP.
9[g]
10[h]
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With the optimal reaction conditions in hand, we
next turned our effort to examine the substrate scope
of the ethoxycarbonylation with a good number of
diversely substituted aryl bromides and some aryl
iodides (Table 3). As shown in Table 3, a broad range
of aryl bromides carrying electron-donating and elec-
tron-attracting substituents reacted smoothly with
diethyl oxalate in good to excellent isolated yields.
Lower yields observed for starting compounds 4f, 4h
and 4i arised from the nucleophilic substitution of
[a] Reaction conditions: 1 (1 mmol), 2, PdCl2(PPh3)2, DMAP
(1.2 mmol), EtOH (3 mmol) at 1408C for 20 min under
microwave irradiation.
[b] GC yield with diphenylmethane as internal standard.
[c] EtOH (1.5 mmol).
[d] Reaction performed without EtOH.
[e] Reaction performed using NMP as solvent (3 mmol).
[f] Isolated yield.
[g] Reaction performed at 1208C.
[h] Reaction performed for 10 min.
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Table 3. Scope of the ethoxycarbonylation of bromo- and iodoarenes under optimal conditions.[a]
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[a] Reaction conditions: 4 (1 mmol), 2 (1.2 mmol), PdCl2(PPh3)2 (0.6 mol%), DMAP (1.2 mmol), EtOH (3 mmol) at 1408C for
20 min under microwave irradiation.
[b] Isolated yields.
[d]
[c] Reaction performed using dimethyloxalate (1.2 mmol) in MeOH (3 mmol). Reaction performed using H2O as solvent
(3 mmol).
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dimethylamino, chloro and nitro substituents by conditions were then applied to some aryl chlorides
ethanol. Halogenated heterocycles such as quinoline leading to the corresponding aryl esters in good yields
and pyridine gave moderate to good yields of the ranging from 50 to 78% (Table 5).
corresponding esters 5q–s (entries 17–19). A wide
range of functional groups including ester, ketone,
amide were tolerated. The esters 5k–o were obtained
in good yields ranging from 60 to 87% (entries 11–15).
Table 5. Scope of the ethoxycarbonylation of chloroarenes
under optimal conditions.[a]
The chemoselective ethoxycarbonylation was ob-
served for 4h to produce 5h (entry 8). A lactonization
10 was occurred in the presence of an alcohol function in
11 the ortho position in 4t and 4u. The isobenzofuranone
12 5t and 5u were obtained in excellent yields of 77 and
13 88% respectively (entries 20 and 21). When the
14 reaction was performed in the presence of dimethylox-
15 alate starting from 1a, methylbenzoate 5w was
16 obtained in 88% yield (entry 23). Besides, the use of
17 water as solvent led to the formation of benzoic acid
18 5x in excellent yield of 95% (entry 24).
19
Even though the above conditions could not be
20 applied from chlorobenzene 1b (Table 1, entry 11), we
21 found that the addition of a ligand such as dppp was
22 beneficial for its alkoxycarbonylation (Table 4). Fur-
23 ther optimization of the conditions revealed that the
24 reaction has to be conducted at 1508C for 30 min, in
25 the presence of 3 mol% of PdCl2(PPh3)2 to furnish
26 ester 3a in 85% yield (Table 4, entry 6). These
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Table 4. Optimization of the reaction conditions starting
from chlorobenzene.[a]
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[a] Reaction conditions: 6 (1 mmol), 2 (1.2 mmol), DMAP
(1.2 mmol), PdCl2(PPh3)2 (0.03 mmol), dppp (0.06 mmol),
EtOH (3 mmol) at 1508C for 30 min under microwave
irradiation.
Entry T
Time
[Pd]
(mol%)
[dppp][b]
(mol%)
Yield
(%)[c]
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(8C) (min)
1
2
3
140 20
PdCl2
(PPh3)2
(0.7)
PdCl2
(PPh3)2
(0.7)
PdCl2
(PPh3)2
(1.5)
PdCl2 (1.5)
Pd(OAc)2
(1.5)
–
0
[b] Isolated yields.
140 20
150 30
1.5
3
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Although a detailed mechanistic picture requires
further studies, results described above allow us to
suggest a plausible reaction pathway presented in
Scheme 2. In a first step, monosubstituted oxalate was
obtained by reaction of conjugated bases such as
DMAP or DBU with diethyloxalate.[16] From theorical
calculations, Fu and Liu have proposed a mechanism
based on the coordination of oxalate with Pd(II)
which was then subjected to a decarboxylation to give
the ester.[12] In our experimental conditions, we can
suggest that the decomposition of oxalate to produce
CO and CO2 in closed vessel is mechanistically similar
to that reported earlier by Das and co-workers.[15] Gas
chromatographic analysis of the reaction medium
displayed characteristic peaks of CO and CO2 (see
supporting information). Indeed, ester 3 was obtained
4
5
150 30
150 30
3
3
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15
6
150 30
PdCl2
(PPh3)2
(3)
6
85
[a] Reaction conditions: 1b (1 mmol), 2 (1.2 mmol), DMAP
(1.2 mmol), Pd catalyst, dppp, EtOH (3 mmol) under
microwave irradiation.
[b] dppp=1,3-bis-(diphenylphosphine)propane.
[c] GC yield with diphenylmethane as internal standard.
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NMR (75 MHz, CDCl3) d 166.6, 132.7, 130.5, 129.5,128.3,
60.9, 14.3.
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in only 7% when the reaction was performed with
monosubstituted oxalate. CO is then inserted into the
Pd(II)-Ar bond via a coordination with the metal.
Ligand exchange of bromide by ethanolate or ethanol
and reductive elimination give the ester. It is worth
noting that, when the reaction was performed in the
presence of diethyloxalate and methanol as solvent, a
mixture of methyl- and ethylesters was obtained in a
1/3 ratio. This result confirms that both ethanol and
Acknowledgements
`
Chevreul institute (FR 2638), Ministere de l’Enseignement
´
´
Superieur et de la Recherche, Region Nord – Pas de Calais
and FEDER are acknowledged for supporting and funding
this work.
10 ethanolate could react in the ligand exchange step or
11 in a transesterification reaction.
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Scheme 2. Proposed mechanism.
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34 chlorides, no nitrogen atmosphere. In consequence,
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37 esters.
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Experimental Section
41 General Procedure
42
Bromobenzene 1a (157 mg, 1 mmol), diethyloxalate
2
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(180 mg, 1.2 mmol), DMAP (153 mg, 1.2 mmol), PdCl2
(PPh3)2 (4.7 mg, 6.10À3 mmol) were taken in an oven-dried
test tube equipped with a magnetic stir bar and a Teflon
screw-cap using EtOH (142 mg, 3 mmol) as solvent. The
reaction mixture was then irradiated in a closed vessel
48 monomode microwave at 1408C for 20 min. At 1308C, the
pressure reached 5–7 bar. After cooling to ambient temper-
ature, an aqueous solution HCl 1 M (5 mL) was added and
the reaction mixture was extracted with diethyl ether (33
10 mL). The combined organic layers were dried over
MgSO4 and evaporated under reduced pressure. The purifi-
cation by chromatography over silica gel afforded methyl
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benzoate ester 3a as oil (128 mg, 85%); H NMR (300 MHz,
CDCl3) d 8.05 (dd, J=8.4 and 1.5 Hz, 2H), 7.54 (m, 1H), 7.42
(m, 2H), 4.37 (t, J=7.2 Hz, 2H), 1.39 (t, J=7.2 Hz, 3H). 13C
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Diethyloxalate as “CO” Source for Palladium-Catalyzed
Ethoxycarbonylation of Bromo- and Chloroarene Deriv-
atives
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´
Pelinski*