Catalytic Carboxylation of Heteroaromatic Compounds: Double and Single Carboxylation with CO 2

Article abstract of DOI:10.1055/s-0037-1612414

In the presence of PdCl ₂ [P(n -Bu) ₃ ] ₂ (10 mol%) and ZnEt ₂, 2-furyl and 2-pyrrolylmethyl acetate were carboxylated with CO ₂ (1 atm), affording doubly carboxylated products in good yields. In this dearomative transformation, α,?-dicarboxylic acids were obtained selectively, in contrast to our previous report in which α,γ-dicarboxylic acids were selectively produced from 2-indolylmethyl acetates. In contrast, 5-thiazolylmethyl acetate and naphthylmethyl acetates predominantly underwent single carboxylation.

Full text of DOI:10.1055/s-0037-1612414

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–D
letter
A
en
T. Mita et al.
Letter
Synlett
Catalytic Carboxylation of Heteroaromatic Compounds:
Double and Single Carboxylation with CO2
Tsuyoshi Mita*
Hiroki Masutani
Sho Ishii
0
0
0
0-0
0
0
2-6
6
5
5-
3
4
3
9
CO2 (1 atm)
PdCl2[P(n-Bu)3]2
(10 mol%)
PdIIEt
γ
CO2Me
OAc
δ
ε
α
α
X
O
β
X
ZnEt2
DMA, 40 °C
then TMSCHN2
EtZnO2C
X
ε
X
C
O
MeO2C
O
C
Yoshihiro Sato*
0
0
0
0-
0
0
0
3-2
5
4
0-5
5
2
5
X = O, NBoc
NC(O)OMe
O
up to 50% yield
nucleophilic
nucleophilic
Faculty of Pharmaceutical Sciences, Hokkaido University,
Sapporo 060-0812, Japan
tmita@pharm.hokudai.ac.jp
biyo@pharm.hokudai.ac.jp
Received: 05.02.2019
Accepted after revision: 19.02.2019
Published online: 19.03.2019
al groups,7 is in high demand. We herein describe a catalyt-
ic double carboxylation of furan and pyrrole derivatives by
using our established Pd/ZnEt2 system5,8 through a dearo-
mative process.9 In cases in which dearomatization did not
proceed, which depended on the substrate employed, single
carboxylation proceeded predominantly. We explain herein
both catalytic transformations in detail.
DOI: 10.1055/s-0037-1612414; Art ID: st-2019-u0071-l
Abstract In the presence of PdCl2[P(n-Bu)3]2 (10 mol%) and ZnEt2, 2-
furyl and 2-pyrrolylmethyl acetate were carboxylated with CO2 (1 atm),
affording doubly carboxylated products in good yields. In this dearoma-
tive transformation, ,-dicarboxylic acids were obtained selectively, in
contrast to our previous report in which ,-dicarboxylic acids were se-
lectively produced from 2-indolylmethyl acetates. In contrast, 5-thi-
azolylmethyl acetate and naphthylmethyl acetates predominantly un-
derwent single carboxylation.
Our previous work
CO2 (1 atm)
PdCl2[P(n-Bu)3]2
(10 mol%)
CO2Me
γ
γ
OAc
CO2Me
R
R
α
α
ZnEt2 (3.0 equiv)
DMF, 40 °C, <4 h
then TMSCHN2
N
N
Bn
Bn
up to 65% yield
Key words carboxylation, CO2, dearomatization, palladium, furan,
pyrrole
Catalytic carboxylation of stable heteroaromatics with
CO2 is a powerful tool for the synthesis of heteroaromatic
CO2
CO2ZnEt
1
γ
γ
PdIIEtLn
carboxylic acids.2–4 These carboxylic acids can be employed
as synthetic precursors for many biologically active mole-
cules because the carboxylic group is readily manipulated
into pharmacologically important amidine, lactam, and lac-
tone units. Among the reported routes for CO2 fixation into
heterocyclic compounds, one molecule of CO2 is generally
incorporated.2–4 However, we recently discovered that two
CO2 molecules could be incorporated into indole derivatives
simultaneously;5 2-indolylmethyl acetates were doubly car-
boxylated at the -and -positions via nucleophilic η1-allyl-
palladium and enamine species in the presence of the elec-
tron-rich palladium complex, PdCl2[P(n-Bu)3]2, and ZnEt2
(Scheme 1).5 Encouraged by this double carboxylation, we
then considered that the extended -position might also be
carboxylated when furan and pyrrole derivatives lacking
the aryl moiety were employed. Given that catalytic double
carboxylation using CO2 has been very limited,6 a new type
of catalytic double carboxylation of diverse substrates,
without using highly nucleophilic arylmagnesium or lithi-
um reagents, which are incompatible with several function-
α
R
R
α
N
CO2
N
Bn
Bn
nucleophilic
nucleophilic
Our working hypothesis
CO2
CO2ZnEt
γ
PdIIEtLn
γ
α
ε
α
and/or
α
ε
X
CO2
CO2
EtZnO2C
X
X
CO2
nucleophilic
nucleophilic
Scheme 1 Our previous and current studies on double carboxylation
We first investigated the carboxylation of furan deriva-
tives (Table 1). 2-Furylmethyl acetate 1, which can be readi-
ly prepared from the corresponding 2-furfuryl alcohol, was
employed as a standard substrate. By following the previ-
ously established conditions,5 we utilized PdCl2[P(n-Bu)3]2
(10 mol%), an air- and moisture-stable Pd complex, and
ZnEt2 (3.0 equiv) in DMF under 1 atm of CO2 atmosphere.
When the reaction was conducted at room temperature,
doubly carboxylated compound 2 was obtained in 8% yield
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–D

B
T. Mita et al.
Letter
Synlett
together with recovery of 1 in 23% yield (entry 1). The ob-
tained product was the ,-dicarboxylate 2, which high-
lighted the different reactivity compared with the double
carboxylation of 2-indolylmethyl acetate, in which ,-di-
carboxylates were selectively produced (see above).5 When
the reaction temperature was raised to 40 °C, the yield in-
creased to 44% (entry 2). However, further increasing the
temperature did not give any improvement (entries 3 and
4). In addition, a shorter reaction time (4 h) gave a lower
yield (entry 5). A higher pressure of CO2 (10 atm) resulted in
a similar yield (entry 6). Without the Pd catalyst, the car-
boxylation did not proceed and 1 was recovered in 56%
yield (entry 7). Changing the solvent from DMF to DMA in-
creased the yield and 2 was obtained in 50% isolated yield
(entry 8). Neither a higher temperature with DMA (60 °C) nor
changing the solvent to THF was effective (entries 9 and 10).
Furans are known to react with electrophiles at the 5-
position rather than the 3-position through electrophilic
substitution (i.e., Friedel–Crafts reaction). However, in our
catalytic system, the first carboxylation was unlikely to be
an electrophilic substitution because the carboxylation did
not proceed without the Pd catalyst (Table 1, entry 7). Addi-
tionally, we believe that the reaction proceeded through a
first carboxylation of η1-allylpalladium species followed by
a second carboxylation of the resulting enol intermediate
(see below) (Scheme 2). The first carboxylation has gained
much interest because the unprecedented nucleophilic ad-
dition of η1-allylpalladium species takes place at the 5-posi-
tion through -conjugation.
Table 1 Screening the Conditions for Double Carboxylation of 2-Furyl-
methyl Acetate
CO2 (1 atm)
PdCl2[P(n-Bu)3]2
(10 mol%)
CO2Me
CO2Me
OAc
γ
CO2Me
α
+
ε
O
α
ZnEt2 (3.0 equiv)
solvent, temp, time
then TMSCHN2
O
MeO2C
O
1
2
2'
Entry Solvent
Temp. (°C) Time (h) Yield (%)a
2
2′
Recovered 1
1
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMA
DMA
THF
r.t.
40
60
80
40
40
40
40
60
40
16
16
17
18
4
8
trace
23
0
2
40 (44) 3 (5)
3
21
0
0
0
4
0
0
5
19
44
0
trace
trace
0
trace
2
6b
7c
8
16
16
18
16
18
56
0
49 (50) trace
9
39
9
2
0
0
10
0
a Yields were determined by 1H NMR analysis by using 1,1,2,2-tetrachlo-
roethane as an internal standard. Isolated yields are given in parentheses.
b The reaction was conducted under 10 atm of CO2.
c Without PdCl2[P(n-Bu)3]2.
zylpyrrole derivative 3 was initially employed as a sub-
strate. However, product 4 was obtained in only 2% yield,
even at longer reaction times, without recovery of 3, which
was probably due to its instability, and this promoted the
elimination of the acetate moiety.
Therefore, different protecting groups with electron-
withdrawing ability were investigated under the optimal
conditions (Scheme 4). Fortunately, the carbamate protec-
tion with C(O)Ot-Bu (Boc) and C(O)OMe increased the yield
of 6 up to 43% with ,-selectivity, but these protecting
groups were partially eliminated under the purification
PdIIEtLn
OAc
Pd0Ln
ZnEt2
CO2, ZnEt2
H
O
Friedel-Crafts
5
O
O
EtZnO2C
CO2
EtZnO2C
3
OAc
O
5
PdIIEtLn
Pd0Ln, ZnEt2
CO2
1
EtZnO2C
CO2
O
5
Scheme 2 Exclusion of Friedel–Crafts type carboxylation
CO2 (1 atm)
PdCl2[P(n-Bu)3]2
(10 mol%)
CO2Me
OAc
This catalytic system has been expanded to the double
carboxylation of pyrrole derivatives that also have 6 elec-
tron systems (Scheme 3). According to our previous report,5
an electron-donating substituent at the nitrogen atom is
expected to be preferred due to the enhancement of the nu-
cleophilicity of the enamine intermediate. Thus, N-ben-
ZnEt2 (3.0 equiv)
DMA, 40 °C, 16 h
then TMSCHN2
N
MeO2C
N
R
R
5
6
R = C(O)Ot-Bu (Boc)
CO2Me
CO2Me
+
N
Boc
N
H
MeO2C
MeO2C
6a
6'
43% yield (6a/6' = 30%/13%)
R = C(O)OMe
CO2 (1 atm)
CO2Me
CO2Me
PdCl2[P(n-Bu)3]2
(10 mol%)
CO2Me
OAc
OAc
+
N
N
H
MeO2C
MeO2C
N
ZnEt2 (3.0 equiv)
DMA, 40 °C, 47 h
then TMSCHN2
N
Bn
CO2Me
6b
N
Bn
MeO2C
Bn
6'
3
4
3
40% yield (6b/6' = trace/40%)
(unstable)
2% yield
(isolated)
Scheme 4 Examination of protecting group variation of pyrrole 5. Iso-
Scheme 3 Carboxylation of N-benzylpyrrole derivative 3
lated yields are shown.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–D

C
T. Mita et al.
Letter
Synlett
process using basic silica gel column chromatography (NH
silica gel). Even though the yields were moderate, we suc-
cessfully demonstrated a new type of double carboxylation
of nitrogen-containing heteroaromatics.
We next examined other heteroaromatics in this car-
boxylation with CO2. The use of 2-thiophenylmethyl acetate
afforded the doubly carboxylated product in only 2% yield
(data not shown), whereas carboxylation of 5-thiazolyl-
methyl acetate 7 afforded the singly carboxylated product
in 38% yield at slightly higher temperature (60 °C) without
generation of the doubly carboxylated product 9 (Scheme 5).
philic enol or the enamine can then undergo the second
carboxylation with CO2 after transmetalation with ZnEt2,13
giving oxocarbenium or iminium intermediate V, which
could undergo rearomatization followed by protonation by
acidic work-up and methyl esterification to afford 2 or 6.
Et
CO2ZnEt
ZnEt2
H
H
2 or 6
then
TMSCHN2
X
X
CO2
EtZnO2C
EtZnO2C
IV
V
1 or 5
Pd0Ln
Et
H
ZnEt2
CO2 (1 atm)
AcO
PdCl2[P(n-Bu)3]2
CO2Me
PdIILn
OAc
CO2Me
(10 mol%)
N
N
N
X = O, NBoc, NC(O)OMe
X
LnEtPdIIO2C
α
ZnEt2 (3.0 equiv)
DMA, 60 °C, 16 h
then TMSCHN2
S
S
S
MeO2C
X
III
dearomative
process
I
7
8
9
38% yield
(isolated)
γ
PdIIEtLn
δ
α
β
ε
ZnEt2
X
CO2
Scheme 5 Carboxylation of 5-thiazolylmethyl acetate 7
ZnEtOAc
II
CO2ZnEt
H
N
8
then
TMSCHN2
The observed mono-selectivity was attributed to the
difficulty of breaking the aromaticity of the thiazole ring.
The terminal η1-allylpalladium species did not react at ei-
ther the - or -position through a dearomative process. In
fact, 1- and 2-naphthalenemethyl acetates, which are stable
aromatics but susceptible to the initial oxidative addition to
Pd(0), underwent single carboxylation predominantly, af-
fording -carboxylated products 11a and 11b in 51% and
44% yield, respectively, without forming the dearomative
product 11a′ (Scheme 6). Although the yields were moder-
ate due to the generation of methylnaphthalenes 12a and
12b, this is the first example of palladium-catalyzed car-
boxylation of benzylic carboxylates.10,11
S
IX
Et
H
7
ZnEt2
Pd0Ln
AcO
PdIILn
CO2PdIIEtLn
N
N
double allylic
position
S
S
VIII
VI
PdIIEtLn
PdIIEtLn
α
ZnEt2
N
N
ZnEtOAc
γ
CO2
β
S
S
VII'
VII
Et
CO2
PdIILn
N
S
CO2Me
OAc
VII"
CO2 (1 atm)
PdCl2[P(n-Bu)3]2
(10 mol%)
α
CO2Me
Scheme 7 Proposed catalytic cycle
+
ZnEt2 (3.0 equiv)
DMA, 60 °C, 16 h
then TMSCHN2
11a'
11a
51% yield
(isolated)
12a
10a
36% yield
(by 1H NMR)
In a similar manner, 5-thiazolylmethyl acetate 7 can ox-
idatively add to Pd(0)Ln to afford VI, which is transmetalat-
ed with ZnEt2, affording VII. Intermediate VII would be in
equilibrium with VII′ through η3-allylpalladium VII′′. Of
two η1-allylethylpalladium intermediates (VII and VII′), ter-
minal VII seemed to be more stable than internal VII′, but
- or -carboxylation would not proceed from VII because
braking the aromaticity of the thiazole nucleus would be
difficult. Therefore, η1-allylethylpalladium VII′, stabilized
by two double bonds (alkene and imine), would attack CO2
at the -position to give palladium carboxylate VIII, which
is reduced by ZnEt2 to regenerate Pd(0)Ln together with re-
lease of zinc carboxylate IX. It would then be hydrolyzed,
affording singly carboxylated compound 8.
α
as above
OAc
CO2Me
+
10b
11b
44% yield
(isolated)
12b
35% yield
(by 1H NMR)
Scheme 6 Single carboxylation of naphthalene derivatives
We next propose two reasonable catalytic cycles
(Scheme 7). In the carboxylation of 2-furylmethyl acetate 1
and 2-pyrrolylmethyl acetate 5, initially, the oxidative addi-
tion of 1 or 5 to Pd(0)Ln would result in the formation of η3-
allylpalladium I. Transmetalation between I and ZnEt2 gives
the nucleophilic η1-allylethylpalladium II,12 which reacts
with CO2 at the -position through dearomatization of the
furan or the pyrrole ring, giving palladium carboxylate III,
bearing either an enol or an enamine moiety. The nucleo-
In summary, we have developed a catalytic carboxyl-
ation of heteroaromatic compounds with CO2. Double or
single carboxylation occurred, depending on the substrate
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–D

D
T. Mita et al.
Letter
Synlett
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competing -hydride elimination, this methodology could
open new routes to the synthesis of carboxylated heterocy-
cles from CO2.
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This work was financially supported by the Japan Society for the Pro-
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1612414.
S
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S
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gInformati
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–D