Oxidative carboxylation of arylaldehydes with water by a Sulfoxylalkyl-substituted N-heterocyclic carbene catalyst

Article abstract of DOI:10.1039/b910187j

The N-Heterocyclic carbene-catalysed oxidative carboxylation of arylaldehydes with water successfully proceeded when a sulfoxylalkyl-substituted imidazolium salt was used as the catalyst. The reactions can be run in the absence of oxidant, and a variety of arylaldehydes having an electron-withdrawing group were converted to the corresponding carboxylic acids.

Full text of DOI:10.1039/b910187j

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Oxidative carboxylation of arylaldehydes with water by a
sulfoxylalkyl-substituted N-heterocyclic carbene catalyst
Masahiro Yoshida,* Yuki Katagiri, Wen-Bin Zhu and Kozo Shishido
Received 26th May 2009, Accepted 3rd July 2009
First published as an Advance Article on the web 31st July 2009
DOI: 10.1039/b910187j
The N-Heterocyclic carbene-catalysed oxidative carboxylation of arylaldehydes with water successfully
proceeded when a sulfoxylalkyl-substituted imidazolium salt was used as the catalyst. The reactions can
be run in the absence of oxidant, and a variety of arylaldehydes having an electron-withdrawing group
were converted to the corresponding carboxylic acids.
Introduction
The development of mild and efficient oxidation methods is
an attractive research area in organic chemistry.1 A potentially
valuable methodology is the oxidative transformation of aldehydes
using N-heterocyclic carbene (NHC) catalysts.2–5 For example,
it is known that an NHC catalyst derived from thiazolium or
imidazolium ions catalyzes the oxidative esterification of aldehydes
with alcohols.3 The addition of the NHC catalyst to arylaldehydes
in the presence of a suitable oxidant gives the acyl cation
intermediates, which are capable of transferring their acyl group
to an alcohol nucleophile to produce the corresponding esters
(Scheme 1).
Scheme 2 Oxidative carboxylation utilizing the imidazolium salt 1a.
Table 1 Initial attempts using imidazolium salt 1a
Entry
Solvent
Solvent/H2O
Yield (%)
1
2
3
4
5
6
7
8
9
THF
THF
THF
THF
CH3CN
CH2Cl2
NMP
DMSO
DMA
DMF
1/2
2/1
3/1
10/1
10/1
10/1
10/1
10/1
10/1
10/1
46
70
73
74
74
72
66
81
89
93
Scheme 1 NHC-catalysed oxidative esterifications.
We have been interested in the development of a new NHC
catalyst which promotes the oxidation of aldehydes more ef-
ficiently, and focused on the zwitterionic imidazolium salt 1a
(Scheme 2). Compound 1a, having a 2,6-diisopropylphenyl and
3-sulfoxylpropyl group on the imidazole ring, was first synthesized
by Schanz and Shaughnessy et al. as a ligand for water-soluble
metal-NHC complexes,6 but there are no reports of it being used
as a catalyst itself. We expected that the sulfoxylalkyl moiety in 1a
could influence the activity of NHC-catalysed reactions, and thus
we applied this compound in the oxidation of aldehydes. Herein,
the oxidative carboxylation of arylaldehydes5 with water utilizing
the imidazolium salt 1a is described (Scheme 2).
10
(3a) was produced in 46% yield (entry 1, Table 1). Further attempts
revealed that the ratio of THF to water influenced the reactivity
(entries 2–4). Thus, the yield of 3a was increased to 74% when
the reaction was carried out in THF/H2O (10/1) (entry 4). The
reaction also proceeded in a mixture of various aqueous solvents
to give 2a in good yields (entries 5–10), and the best result was
obtained by carrying out the reaction in DMF/H2O (10/1) (93%
yield, entry 10).
Results and discussion
Table 2 shows our attempts using various substituted imida-
zolium salts 1b–1i. When the imidazolium salt 1b containing a
methyl group on the imidazole ring was subjected to the reaction
with 2a, the carboxylic acid 3a was obtained in 72% yield (entry 1).
The reactions using phenyl- and 2,4,6-trimethylphenyl-substituted
imidazolium salts 1c and 1d afforded 3a in 67% and 72% yield,
respectively (entries 2 and 3). These results imply that the presence
The initial reactions were carried out using p-nitrobenzaldehyde
(2a). When 2a was treated with 5 mol% of the imidazolium salt
1a and 2 equiv DBU in THF/H2O (1/2) at rt, p-nitrobenzoic acid
Graduate School of Pharmaceutical Sciences, The University of Tokushima,
1-78-1 Sho-machi, Tokushima, Japan. E-mail: yoshida@ph.tokushima-
u.ac.jp; Tel: +81 88 633 7294
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Table 2 Reactions of 2a to 3a using various imidazolium salts 1b–1ia
Entry
1
Catalyst
Yield of 3a (%)
1b
1c
72
2
3
67
72
1d
4
1e
66
81
45
83
60
5
1f
6b
1g
7
1h
8
1i
Scheme 3 Proposed reaction mechanism.
acid IV and regenerates the catalyst I. Compound IV would be
immediately subjected to oxidative aromatisation with dissolved
oxygen in the reaction mixture8,9 to produce the product 3a.
Although it is not clear why the presence of the sulfoxylalkyl
moiety increases the yield of 3a, one possible explanation is that
presumably there is an intermolecular interaction between the
sulfonate and water to form the intermediate III¢, which could
enhance the nucleophilicity of water.
a Reactions were carried out in the presence of 5 mol % 1 and 2 equiv. DBU
in DMF/H2O (10/1) at rt for 4–24 h. b The corresponding HBr salt of 1h
was used.
of bulky substituents on the phenyl ring is important for the
reaction. The yield was decreased to 66% when a sulfonate-free
imidazolium salt 1e was used (entry 4). This result indicates that
the presence of the sulfonate moiety in 1a enhances the reactivity
in the oxidation process. The imidazolium salt 1f, containing
a 4-sulfoxylbutyl group, uneventfully catalyzed the reaction to
produce 3a in 81% yield (entry 5). In contrast, the reaction
using a 2-sulfoxylethyl-substituted compound 1g gave the poorest
result (45% yield) because of the decomposition of the catalyst
observed under basic conditions (entry 6).7 When 1h, which has
a carboxylate moiety on the alkyl side chain, was used, 3a was
obtained in 83% yield (entry 7). When the reaction was attempted
using thiamine (1i) to compare the reactivity, the production of
3a decreased to 60% (entry 8). From these results, it was found
that the imidazolium salt 1a is the most suitable catalyst for this
reaction.
A plausible mechanism for the reaction is shown in Scheme 3.
The deprotonation of the imidazolium salt 1 generates the
carbene I, which adds to the carbonyl moiety in 2a to give the
Breslow intermediate II. The likely equilibrium between II and
the dearomatised acyl imidazolium intermediate III, facilitated by
the electron-withdrawing nitro group, followed by nucleophilic
addition of water to the activated III affords the carboxylic
The results of the reactions of various arylaldehydes 2b–2i in the
presence of the imidazolium salt 1a are summarized in Table 3.
The substrate 2b, having a nitro group at the ortho position,
reacted to afford the carboxylic acid 3b in 53% yield (entry 1).
Surprisingly, 3-nitrobenzaldehyde (2c), which was expected to
not be a suitable substrate, was uneventfully transformed to the
product 3c in 71% yield (entry 2). When the reactions of the acetyl-
, fluoro- and chloro-substituted substrates 2d–2f were carried out,
the corresponding products 3d–3f were obtained in moderate to
good yields (entries 3–5). The reaction of the a,b-unsaturated
aldehyde 2g also proceeded to afford 3g in 69% yield (entry 6).
The aldehydes 2h and 2i having a 2-pyridyl and 2-quinolinyl group
were successfully converted to the corresponding carboxylic acids
3h and 3i in 90% and 92% yield, respectively (entries 7 and 8).
On the other hand, benzoic acid (3j) was produced in less than
10% yield from the reaction of benzaldehyde (2j) (entry 9). The
result supports our proposal of the dearomatization pathway being
triggered by the electron-withdrawing group as shown in Scheme 3.
To further highlight the potential of this process, we
next attempted the oxidative esterification3 and amidation4 of
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Table 3 Reactions of various aldehydes 2b–2j to give carboxylic acids
3b–3j using NHC catalyst 1aa
best carried out in the presence of a sulfoxylalkyl-substituted
imidazolium salt 1a. A variety of arylaldehydes having an
electron-withdrawing group can be converted to the corresponding
carboxylic acids. The reaction was successfully applied to the
synthesis of esters and amides by the use of alcohols and amines
as the nucleophiles. Application of this catalyst system to other
NHC-catalysed reactions is now in progress.
Entry Aldehyde
1
Product
Yield (%)
53
2b
3b
2
3
2c
2d
3c
3d
71
45
Experimental
General experimental
Materials were obtained from commercial suppliers and used with-
out further purification except when otherwise noted. Solvents
were dried and distilled according to standard protocol. Imida-
zolium salts 1a, 1d, 1h, N-(2,6-diisopropylphenyl)imidazole and
N-phenylimidazoles were prepared according to the procedures
described in the literature.6,10,11
4
5
6
2e
3e
92
55
69
2f
3f
2g
3g
Preparation and spectral data of imidazolium salts
3-(1-Methyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate (1b).
To a stirred solution of N-methylimidazole (1.03 g, 12.5 mmol)
in acetone (20 mL) was added 1,3-propanesultone (1.53 g,
12.5 mmol) in acetone (20 mL) at 0 ◦C, and the reaction mixture
was stirred at rt for 5 days. After filtration of the reaction mixture
through a glass filter, the resulting solids were washed by acetone
7
8
9
2h
2j
3h
3j
90
92
2i
3i
◦
twice and dried in vacuo at 60 C to afford the imidazolium salt
1b (1.65 g, 65%) as a white solid: mp 182–183 ◦C; IR (KBr) 3111,
2104, 1566, 667 cm-1; 1H-NMR (400 MHz, d6-DMSO) d 9.10 (1H,
s), 7.77 (1H, s), 7.68 (1H, s), 4.29 (2H, t, J = 6.8 Hz), 3.84 (3H, s),
2.40 (2H, t, J = 6.8 Hz), 2.07 (2H, quint, J = 6.8 Hz); 13C-NMR
(100 MHz, d6-DMSO) d 136.7 (Cq), 123.5 (CH), 122.3 (CH), 47.7
(CH2), 47.2 (CH2), 35.7 (CH), 26.2 (CH2); HRMS (ESI) m/z calcd
for C7H12N2O3SNa [M + Na]+ 227.0466, found 227.0469.
< 10
a Reactions were carried out in the presence of 5 mol% 1a and 2 equiv.
DBU in DMF/H2O (10/1) at rt for 4–24 h.
3-(1-Phenyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate (1c).
By following the same procedure described for 1b, the imida-
zolium salt 1c was obtained from N-phenylimidazole and 1,3-
propanesultone in 36% yield as a white solid: mp 263–264 ◦C;
1
IR (KBr) 3451, 3093, 1556, 1203 cm-1; H-NMR (400 MHz, d6-
DMSO) d 9.81 (1H, s), 8.31 (1H, s), 8.06 (1H, s), 7.81 (2H, d, J =
7.2 Hz), 7.66 (2H, t, J = 7.2 Hz), 7.58 (1H, t, J = 7.2 Hz), 4.40
(2H, t, J = 7.2 Hz), 2.52 (2H, t, J = 7.2 Hz), 2.21 (2H, quint,
J = 7.2 Hz); 13C-NMR (100 MHz, d6-DMSO) d 135.5 (Cq), 134.8
(CH), 130.1 (CH), 129.6 (CH), 123.4 (CH), 121.9 (CH), 121.1
(CH), 48.5 (CH2), 47.5 (CH2) 25.9 (CH2); HRMS (ESI) m/z calcd
for C12H15N2O3S [M + H]+ 267.0803, found 267.0802.
Scheme 4 Oxidative esterification and amidation using 1a.
3-Butyl-1-(2,6-diisopropylphenyl)-1H-imidazol-3-ium bromide
aldehydes with alcohols and amines (Scheme 4). When aldehyde
2a was subjected to reaction in the presence of 1a with DBU in
THF/MeOH solution, the methyl ester 3k was produced in 68%
yield. Similarly, the reaction of 2a with dimethylamine in DMSO
gave the corresponding dimethyl amide 3l in 60% yield.
(1e). To
a stirred solution of N-(2,6-diisopropylphenyl)-
imidazole (300 mg, 1.31 mmol) in toluene (3.5 mL) was added 1-
bromobutane (0.70 mL, 6.55 mmol) at rt, and the reaction mixture
was stirred at 80 ◦C for 3 days. During the reaction, further 1-
bromobutane (0.70 mL, 6.55 mmol) was added four times. The
reaction mixture was filtered through a glass filter, and the resulting
solid was extracted with water and ether at 0 ◦C. The product
was dried in vacuo for 2 h ◦to afford the catalyst as a white solid
(395 mg, 82%): mp 81–82 C; IR (KBr) 3600, 3517, 3070, 2965,
1542, 1213 cm-1; 1H-NMR (400 MHz, d6-DMSO) d 9.76 (1H, s),
8.24 (1H, s), 8.14 (1H, s), 7.64 (1H, t, J = 8.0 Hz), 7.46 (2H, d,
Conclusions
In conclusion, we have developed a NHC-catalysed oxidative
carboxylation of arylaldehydes. The reactions can be run in the
absence of oxidant, and it was found that the reactions are
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J = 8.4 Hz), 4.36 (2H, t, J = 6.8 Hz), 2.25 (2H, septet, J = 6.4 Hz),
1.90 (2H, quint, J = 6.8 Hz), 1.26 (2H, quint, J = 6.8 Hz), 1.15
(12H, d, J = 6.4 Hz) 0.93 (3H, t, J = 6.8 Hz); 13C-NMR (100 MHz,
d6-DMSO) d 145.0 (Cq), 137.6 (CH), 131.4 (CH), 130.4 (Cq), 125.1
(CH), 124.3 (CH), 123.4 (CH), 49.0 (CH2), 30.9 (CH2), 28.0 (CH),
23.7 (CH3), 18.5 (CH2), 13.1 (CH3); HRMS (ESI) m/z calcd for
C19H28BrN2 [M - H]+ 363.1436, found 363.1429.
d6-DMSO) d 8.30 (2H, d, J = 8.8 Hz), 8.15 (2H, d, J = 8.8 Hz); 13C-
NMR (100 MHz, d6-DMSO) d 165.8 (Cq), 150.0 (Cq), 136.4 (Cq),
130.7 (CH), 123.7 (CH); HRMS (ESI) m/z calcd for C7H4NO4
[M - H]+ 166.0140, found 166.0140.
2-Nitrobenzoic acid (3b). White solid; mp 145–146 ◦C; IR
(KBr) 2888, 1683, 1490, 1365 cm-1; 1H-NMR (400 MHz, d6-
DMSO) d 7.97 (1H, d, J = 8.0 Hz), 7.85 (1H, d, J = 6.8 Hz),
7.74–7.81 (2H, m); 13C-NMR (100 MHz, d6-DMSO) d 165.9 (Cq),
148.4 (Cq), 133.1 (CH), 132.4 (CH), 129.9 (CH), 127.3 (Cq), 123.7
(CH); HRMS (ESI) m/z calcd for C7H4NO4 [M - H]+ 166.0140,
found 166.0138.
4-[1-(2,6-Diisopropylphenyl)-1H-imidazol-3-ium-3-yl]butane-
1-sulfonate (1f). To
a
stirred solution of N-(2,6-
diisopropylphenyl)imidazole (200 mg, 0.875 mmol) in toluene
(2◦.0 mL) was added 1,4-butanesultone (0.179 mL, ◦1.75 mmol) at
0 C, and the reaction mixture was stirred at 100 C for 3 days.
After filtration of the reaction mixture through a glass filter, the
resulting solids were washed by acetone twice and dried in vacuo
3-Nitrobenzoic acid (3c). White solid; mp 139–140 ◦C; IR
(KBr) 2925, 1710, 1482, 1351 cm-1; 1H-NMR (400 MHz, d6-
DMSO) d 8.61 (1H, s), 8.46 (1H, d, J = 8.0 Hz), 8.34 (1H, d,
J = 8.0 Hz), 7.80 (1H, t, J = 8.0 Hz); 13C-NMR (100 MHz,
d6-DMSO) d 165.9 (Cq), 148.4 (Cq), 133.1 (CH), 132.4 (CH),
129.9 (CH), 127.3 (Cq), 123.7 (CH); HRMS (ESI) m/z calcd for
C7H4NO4 [M - H]+ 166.0140, found 166.0138.
◦
at 65 C for 1 h to afford the imidazolium salt 1f (229 mg, 75%)
as a white solid: mp 310–311 ◦C; IR (KBr) 3519, 2962, 1560,
1
1190 cm-1; H-NMR (400 MHz, d6-DMSO) d 9.61 (1H, s), 8.17
(1H, s), 8.11 (1H, s,), 7.62 (1H, t, J = 7.6 Hz), 7.44 (2H, d, J =
7.6 Hz), 4.35 (2H, t, J = 6.8 Hz), 2.45–2.53 (2H, m), 2.25 (2H,
septet, J = 6.8 Hz), 1.99 (2H, quint, J = 6.8 Hz), 1.56 (2H, quint,
J = 6.8 Hz), 1.14 (12H, d, J = 6.8 Hz); 13C-NMR (100 MHz,
d6-DMSO) d 145.1 (Cq), 137.7 (CH), 131.4 (CH), 130.5 (Cq),
125.1 (CH), 124.3 (CH), 123.4 (CH), 50.3 (CH2), 49.1 (CH2) 28.4
(CH2) 28.1 (CH) 23.8 (CH3) 21.6 (CH2); HRMS (ESI) m/z calcd
for C19H29N2O3S [M + H]+ 365.1899, found 365.1902.
4-Acetylbenzoic acid (3d). White solid; mp 208–210 ◦C; IR
1
(KBr): 2925, 1681 cm-1; H-NMR (400 MHz, d6-DMSO) d 8.04
(4H, s), 2.61 (3H, s); 13C-NMR (100 MHz, d6-DMSO) d 197.8
(Cq), 166.7 (Cq), 139.8 (Cq), 134.7 (Cq), 129.5 (CH), 128.3 (CH),
27.0(CH3); HRMS (ESI) m/z calcd for C9H7O3 [M - H]+ 163.0395,
found 163.0392.
2-[1-(2,6-Diisopropylphenyl)-1H-imidazol-3-ium-3-yl]ethane-
1-sulfonate hydrogen bromide (1g). To a solution of 2,6-
diisopropylphenyl-imidazole (237 mg, 1.04 mmol) in toluene
(3.0 mL), 2-bromoethanesulfonic acid (190 mg, 1.04 mmol) was
added at rt. Then the mixture was stirred at 100 ◦C for 3 days.
The reaction mixture was filtered through a glass filter, and the
resulting solid was washed by ether at 0 ◦C. The product was dried
in vacuo at 70 ◦C for 2 h to afford the imidazolium salt 1g as a white
solid (308 mg, 71%): mp 168–170 ◦C IR (KBr) 3525, 3102, 2964,
1540, 1236 cm-1; 1H-NMR (400 MHz, d6-DMSO) d 9.48 (1H, s),
8.06 (1H, s), 8.00 (1H, s), 7.62 (1H, t, J = 7.6 Hz), 7.44 (2H, d,
J = 7.6 Hz), 3.55 (2H, t, J = 8.0 Hz), 2.93 (2H, t, J = 8.0 Hz),
2.20 (2H, septet, J = 6.8 Hz), 1.14 (12H, d, J = 6.8 Hz); 13C-NMR
(100 MHz, d6-DMSO) d 145.1 (Cq), 137.0 (CH), 131.2 (CH), 130.6
(Cq), 124.7 (CH), 124.2 (CH), 121.0 (CH), 54.5 (CH2), 28.0 (CH),
23.7 (CH3); HRMS (ESI) m/z calcd for C17H24BrN2O3S [M - H]+
415.0691, found 415.0695.
4-Fluorobenzoic acid (3e). White solid; mp 184–186 ◦C; IR
1
(neat) 2923, 1678, 1234 cm-1; H-NMR (400 MHz, d6-DMSO) d
7.97–8.01 (2H, m), 7.29–7.33 (2H, m); 13C-NMR (100 MHz, d6-
DMSO) d 166.3 (Cq), 132.1 (CH), 132.0 (Cq), 115.7 (CH), 115.5
(Cq); HRMS (EI) m/z calcd for C7H4O2F [M - H]+ 139.0195,
found 139.0193.
4-Chlorobenzoic acid (3f). White solid; mp 242–243 ◦C; IR
1
(KBr): 2981, 1685, 1016 cm-1; H-NMR (400 MHz, d6-DMSO)
d 7.93 (2H, d, J = 8.8 Hz), 7.56 (2H, d, J = 8.8 Hz); 13C-NMR
(100 MHz, d6-DMSO) d 166.4 (Cq), 137.8 (Cq), 131.1 (CH), 129.6
(Cq), 128.7 (CH); HRMS (ESI) m/z calcd for C7H4O2Cl [M - H]+
154.9900, found 154.9900.
4-Nitrocinnamic acid (3g). White solid; mp 246–248 ◦C (de-
1
comp); IR (KBr) 3000, 1687, 1629, 1529, 1348 cm-1; H-NMR
(400 MHz, d6-DMSO) d 8.23 (2H, d, J = 8.6 Hz), 7.97 (2H, d,
J = 8.6 Hz), 7.68 (1H, d, J = 16.0 Hz), 6.74 (1H, d, J = 16.0 Hz);
13C-NMR (100 MHz, d6-DMSO) d 167.0 (Cq), 148.0 (Cq), 141.3
(CH), 140.7 (Cq), 129.3 (CH), 123.9 (CH), 123.6 (CH); HRMS
(ESI) m/z calcd for C9H6NO4 [M - H]+ 192.0297, found 192.0291.
General procedure for the oxidative carboxylation of arylaldehydes
with water (Table 1, entry 10)
To a stirred solution of 4-nitrobenzaldehyde (2a) (57.0 mg,
0.377 mmol) and the imidazolium catalyst 1a (6.6 mg,
0.0189 mmol) in DMF (1.0 mL) and H2O (0.1 mL) was added
DBU (0.112 mL, 0.756 mmol) at rt. After stirring at rt for 10 h,
the reaction mixture was added to 10% aq. NaOH and extracted
with AcOEt. 10% Aq. HCl was added to the water phase and it
was carefully extracted with AcOEt again. The separated organic
layer was dried over anhydrous MgSO4 and the solvent was
evaporated under reduced pressure to provide 4-nitrobenzoic acid
(3a) (58.6 mg, 0.351 mmol) in 93% yield.
2-Picolinic acid (3h). Yellow solid; mp 137–138 ◦C; IR (KBr)
1
2709, 1774 cm-1; H-NMR (400 MHz, CDCl3) d 8.46–8.48 (1H,
m), 7.82–7.91 (3H, m), 7.17–7.26 (1H, m); 13C-NMR (100 MHz,
d6-DMSO) d 156.6 (Cq), 145.6 (CH), 137.5 (CH), 135.9 (Cq),
121.1 (CH), 119.5 (CH); HRMS (ESI) m/z calcd for C6H4NO2
[M - H]+ 122.0242, found 122.0238.
Quinoline-2-carboxylic acid (3i). Brown solid; mp 157–159 ◦C;
IR (KBr) 2925, 1710 cm-1; 1H-NMR (400 MHz, d6-DMSO) d 8.53
(1H, d, J = 8.4 Hz), 8.06–8.16 (3H, m), 7.86 (1H, t, J =7.6 Hz),
7.73 (1H, t, J = 7.6 Hz); 13C-NMR (100 MHz, d6-DMSO) d 166.4
(Cq), 148.7 (Cq), 146.7 (Cq), 137.6 (CH), 130.5 (CH), 129.7 (CH),
4-Nitrobenzoic acid (3a). White solid; mp 241–242 ◦C; IR
(KBr) 3116, 1695, 1540, 1351 cm-1; 1H-NMR (400 MHz,
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128.8 (CH), 128.5 (Cq), 128.0 (CH); HRMS (ESI) m/z calcd for
C10H6NO2 [M - H]+ 172.1602, found 172.1604.
Acknowledgements
This study was supported in part by a Grant-in-Aid for the
Encouragement of Young Scientists (B) from the Japan Society
for the Promotion of Science (JSPS) and the Program for the
Promotion of Basic and Applied Research for Innovations in the
Bio-oriented Industry (BRAIN).
Procedure for the oxidative esterification of an arylaldehyde with
an alcohol (Scheme 4)
To a stirred solution of 4-nitrobenzaldehyde (2a) (50.0 mg,
0.333 mmol) and the imidazolium salt 1a (5.8 mg, 0.0165 mmol)
in THF (0.5 mL) and methanol (0.05 mL) was added DBU
(0.112 mL, 0.756 mmol) at rt. After stirring for 12 h, the reaction
mixture was added to 10% aq. NaOH and extracted with AcOEt.
The organic layer was washed with brine and dried over anhydrous
MgSO4. The solvent was removed under reduced pressure, and the
crude material was chromatographed on silica gel with hexane–
AcOEt (70:30 v/v) as eluent to give the methyl ester 3k (41.1 mg,
0.227 mmol) in 68% yield.
Notes and references
1 K. Ekoue-Kovi and C. Wolf, Chem.–Eur. J., 2008, 14, 6302.
2 For reviews, see: (a) D. Enders and T. Balensiefer, Acc. Chem. Res.,
2004, 37, 534; (b) J. S. Johnson, Angew. Chem., Int. Ed., 2004, 43, 1326;
(c) V. Nair, S. Bindu and V. Sreekumar, Angew. Chem., Int. Ed., 2004,
43, 5130; (d) M. Christmann, Angew. Chem., Int. Ed., 2005, 44, 2632;
(e) K. Zeitler, Angew. Chem., Int. Ed., 2005, 44, 7506; (f) N. Marion,
S. D´ıez-Gonza´lez and S. P. Nolan, Angew. Chem., Int. Ed., 2007, 46,
2988; (g) D. Enders, O. Niemeier and A. Henseler, Chem. Rev., 2007,
107, 5606; (h) V. Nair, S. Vellalath and B. P. Babu, Chem. Soc. Rev.,
2008, 37, 2691.
3 For examples of oxidative esterifications, see: (a) J. Castells, F. Pujol,
H. Llitjo´s and M. Moreno-Man´as, Tetrahedron, 1982, 38, 337; (b) H.
Inoue and S. Tamura, J. Chem. Soc., Chem. Commun., 1985, 141; (c) S.
Shinkai, Y. Hara and O. Manabe, Bull. Chem. Soc. Jpn., 1983, 56, 770;
(d) Y. Yano and Y. Tsukagoshi, J. Chem. Res. (S), 1984, 406; (e) W. H.
Rastetter and J. Adams, J. Org. Chem., 1981, 46, 1882; (f) D. Hilvert and
R. Breslow, Bioorg. Chem., 1984, 12, 206; (g) S.-Ws. Tam, L. Jimenez
and F. Diederich, J. Am. Chem. Soc., 1992, 114, 1503; (h) B. E. Maki, A.
Chan, E. M. Phillips and K. A. Scheidt, Org. Lett., 2007, 9, 371; (i) C.
Noonan, L. Baragwanath and S. J. Connon, Tetrahedron Lett., 2008,
49, 4003; (j) B. E. Maki and K. A. Scheidt, Org. Lett., 2008, 10, 4331;
(k) J. Guin, S. D. Sarkar, S. Grimme and A. Studer, Angew. Chem., Int.
Ed., 2008, 47, 8727.
Methyl 4-nitrobenzoate (3k). Colourless crystals; mp: 94–
◦
96 C IR (KBr) 3113, 3079, 1718, 1608, 1597, 1524, 1347 cm-1;
1H-NMR (400 MHz, CDCl3) d 8.29 (2H, d, J = 9.2 Hz), 8.21 (2H,
d, J = 9.2 Hz), 3.98 (3H, s); 13C-NMR (100 MHz, CDCl3) d 165.1
(Cq), 150.6 (Cq), 135.5 (Cq), 130.7 (CH), 123.5 (CH), 52.8 (CH3);
HRMS (ESI) m/z calcd for C8H6NO4 [M - H]+ 180.0297, found
180.0300.
Procedure for the oxidative amidation of an arylaldehyde with an
amine (Scheme 4)
To a stirred solution of 4-nitrobenzaldehyde (2a) (57.0 mg,
0.377 mmol) and the imidazolium salt 1a (6.6 mg, 0.0189 mmol)
and dimethylamine (102 mL of 50% MeOH solution, 1.13 mmol)
in DMSO (1.0 mL) was added DBU (0.112 mL, 0.756 mmol) at rt.
After stirring for 5 h, the reaction mixture was added to water and
extracted with AcOEt. The organic layer was washed with brine
and dried over anhydrous MgSO4. After removal of the solvent
under reduced pressure, the crude material was chromatographed
on silica gel with hexane–AcOEt (70:30 v/v) as eluent to give the
dimethyl amide 3l (44.0 mg, 0.227 mmol) in 60% yield.
4 For examples of oxidative amidations, see: (a) H. U. Vora and T. Rovis,
J. Am. Chem. Soc., 2007, 129, 13796; (b) J. W. Bode and S. S. Sohn,
J. Am. Chem. Soc., 2007, 129, 13798; (c) F. T. Wong, P. K. Patra, J.
Seayad, Y. Zhang and J. Y. Ying, Org. Lett., 2008, 10, 2333.
5 For example of oxidative carboxylation, see: J. Castells, H. Llitjos and
M. Moreno-Man˜as, Tetrahedron Lett., 1977, 18, 205.
6 L. R. Moore, S. M. Cooks, M. S. Anderson, H.-J. Schanz, S. T. Griffin,
R. D. Rogers, M. C. Kirk and K. H. Shaughnessy, Organometallics,
2006, 25, 5151.
7 As the decomposed compound, 1-(2,6-diisopropylphenyl)-1H-
imidazole, which would be eliminated from the 2-carboxyethyl moiety,
was obtained.
8 Y.-K. Liu, R. Li, L. Yue, B.-J. Li, Y.-C. Chen, Y. Wu and L.-S. Ding,
Org. Lett., 2006, 8, 1521.
9 When the reaction of 2a was carried out under deoxygenated condi-
tions, the yield of 3a was dramatically decreased. This result supports
that the dissolved oxygen acts as the oxidant.
10 M. C. Perry, X. Cui, M. T. Powell, D.-R. Hou, J. H. Reibenspies and
K. Burgess, J. Am. Chem. Soc., 2003, 125, 113.
11 G. Occhipinti, H.-R. Bjørsvik, K. W. To¨rnroos, A. Fu¨rstner and V. R.
Jensen, Organometallics, 2007, 26, 4383.
N,N -Dimethyl-4-nitrobenzamide (3l). Yellow solid; mp: 96–
◦
97 C; IR (KBr) 1635 cm-1; H-NMR (400 MHz, CDCl3) d 8.28
(2H, d, J = 8.2 Hz), 7.59 (2H, d, J = 8.2 Hz,), 3.15 (3H, s),
2.97 (3H, s); 13C-NMR (100 MHz, CDCl3) d 136.2 (Cq), 115.2
(Cq), 109.4 (Cq), 95.0 (CH), 90.7 (CH), 6.22 (CH3), 2.3 (CH3);
HRMS (ESI) m/z calcd for C9H11N2O3 [M + H]+ 195.1953, found
195.1953.
1
4066 | Org. Biomol. Chem., 2009, 7, 4062–4066
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