A Highly Efficient NHC-Catalyzed Aerobic Oxidation of Aldehydes to Carboxylic Acids

Article abstract of DOI:10.1055/s-0037-1610069

An N-heterocyclic carbene (NHC) organocatalytic aerobic oxidation of aldehydes to the corresponding carboxylic acids is explored. Remarkably, this method allows for efficient conversion of different classes of aldehydes including highly challenging electron-rich aryl aldehydes, ortho -substituted aryl aldehydes, various heteroaromatic aldehydes and α,β-unsaturated aldehydes under mild reaction conditions. These substrates, under previously reported NHC-catalyzed methods, are typically unreactive or give poor yields, require high reaction temperatures and reaction times of several days.

Full text of DOI:10.1055/s-0037-1610069

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2018, 50, A–E
special topic
A
en
A. K. Khatana et al.
Special Topic
Synthesis
A Highly Efficient NHC-Catalyzed Aerobic Oxidation of Aldehydes
to Carboxylic Acids
Anil Kumar Khatanaa,b ‡
Vikram Singha ‡
Manoj Kumar Guptab
Bhoopendra Tiwari*a
NHC cat. (5 mol%)
O
O
N
BF4
Mes
N
N
DABCO (50 mol%)
O2, THF, r.t., 16 h
R
OH
R
H
NHC catalyst
0
0
0
0
0-
0
0
2
4-
1
8
7
3-
3
1
3
Electron-deficient as well as electron-rich (hetero)aromatic aldehydes, enals
20 Examples, up to 96% yield; oxygen as the sole oxidant
Transition-metal-free, mild reaction conditions with shorter reaction times
Demonstrated on a 1 g scale synthesis
a Division of Molecular Synthesis & Drug Discovery, Centre of
Biomedical Research, SGPGIMS-Campus, Raebareli Road,
Lucknow-226014, India
btiwari@cbmr.res.in
b Department of Chemistry, Central University of Haryana,
Mahendergarh-123031, Haryana, India
‡ These authors contributed equally to this work.
Dedicated to Dr. Srivari Chandrasekhar, IICT, Hyderabad, India
on his 54th birthday
Published as part of the Special Topic Heterocycles as Catalysts,
Ligands, and Targets
Received: 31.03.2018
Accepted after revision: 27.04.2018
Published online: 16.07.2018
and an attractive area of research in organic chemistry. To-
ward this objective, several metal-based catalytic aerobic
oxidation methods have been developed.2
DOI: 10.1055/s-0037-1610069; Art ID: ss-2018-c0219-st
Metal-free organocatalysis has been extensively ex-
plored as an alternative mode of activation for a variety of
transformations previously known to be catalyzed only by a
metal complex.3 This process offers several distinct advan-
tages over metal-based approaches, including robustness in
operation, ready availability, and improved environmental
and economic aspects. Among all the organic-molecule-
based catalysts, N-heterocyclic carbenes (NHCs) have
evolved as the most promising catalysts for the oxidation of
aldehydes to carboxylic acids.4–6 To date, several methods
using NHC catalysts have been developed. In 2009, Yoshi-
da7a reported the oxidation of aldehydes using a sulfoxylal-
kyl-substituted imidazolium NHC catalyst. This was fol-
lowed by independent reports from the groups of Zhang7b
and Nair7c using CO2 as the oxidant. In contrast to the re-
ports of Zhang and Nair, Bode and Chiang proposed O2 as
the actual oxidant, and not CO2, under their conditions.8 In
2013, Fu reported an abnormal bis-NHC-mediated oxida-
tion.9 In spite of the remarkable progress realized in these
reports, the methods suffer from one or more limitations
such as limited substrate scope [primarily suitable for acti-
vated electron-deficient (hetero)aryl aldehydes], require re-
action times of several days and/or higher temperatures. In
a more recent publication, Blechert described the oxidation
of a variety of electron-rich aryl aldehydes (along with oth-
er aldehydes) having para/meta-benzylic hydroxy function-
ality.10 However, a more electron-rich para-hydroxybenzal-
dehyde required several days and a higher catalyst loading.
In short, there is an urgent need for an efficient, metal-free
catalytic method for challenging substrates like ortho-sub-
Abstract An N-heterocyclic carbene (NHC) organocatalytic aerobic
oxidation of aldehydes to the corresponding carboxylic acids is ex-
plored. Remarkably, this method allows for efficient conversion of dif-
ferent classes of aldehydes including highly challenging electron-rich
aryl aldehydes, ortho-substituted aryl aldehydes, various heteroaromat-
ic aldehydes and α,β-unsaturated aldehydes under mild reaction condi-
tions. These substrates, under previously reported NHC-catalyzed
methods, are typically unreactive or give poor yields, require high reac-
tion temperatures and reaction times of several days.
Key words carboxylic acids, aldehydes, N-heterocyclic carbenes,
aerobic oxidation, organocatalysis
Carboxylic acids are one of the most encountered func-
tionalities in organic compounds used in pharmaceuticals,
agrochemicals and industrial chemicals. In general, this
class of compound is prepared via oxidation of the corre-
sponding alcohol or aldehyde. Therefore, the oxidation of
aldehydes to their carboxylic acid counterparts is a funda-
mentally significant organic manipulation with huge indus-
trial application. Numerous metal-based oxidants have
been developed, e.g., chromates, permanganates, perchlo-
rates, peroxides, etc.1 Oxidation reactions based on these
hazardous oxidants utilize stoichiometric amounts of the
oxidant and produce toxic by-products. The use of molecu-
lar oxygen as the oxidant offers several advantages over
other reagents due to operational simplicity, higher atom
economy, and produces water as the only by-product.
Therefore, the development of catalytic, environmentally
benign aerobic oxidation methods is of increasing interest
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–E

B
A. K. Khatana et al.
Special Topic
Synthesis
stituted aryl aldehydes, highly electron-rich aryl aldehydes
(e.g., methoxybenzaldehydes) and indole-3-carboxalde-
hydes. Herein, we a report a highly efficient triazolium
NHC-catalyzed aerobic oxidation of aryl aldehydes and
enals at room temperature employing a much shorter reac-
tion time.11
Experimentally, we set out to optimize the reaction con-
ditions using benzaldehyde as a model substrate under an
oxygen atmosphere and the key results are summarized in
Table 1. In the absence of an NHC catalyst, no formation of
product 2a was observed (entry 1). Imidazolium NHC prec-
atalysts A–C (Figure 1), with either N-isopropyl and N-Mes
substituents, produced the desired acid 2a in poor yields in
the presence of DABCO as the base and THF as the solvent
(entries 2–4). Thiazolium precatalyst D was not suitable for
this reaction (entry 5).
Cl
Cl
Cl
I
N
N
N
N
N
O
N
Mes
Mes
Mes
Mes
N
S
Me
A
B
C
D
BF4
N
H
BF4
BF4
Ph
N
N
N
N
N
Mes
Mes
N
N
N
E
F
G
Figure 1 Catalysts A–G
We next examined triazolium NHCs. Pyrrolidinone-de-
rived precatalyst E with an N-phenyl substituent gave the
desired product in a slightly improved yield of 32% (Table 1,
entry 6). Replacing the N-phenyl group on this precatalyst
with a more electron-rich N-Mes group (precatalyst F) had
a dramatic influence on the conversion and the product 2a
could be isolated in an excellent yield of 92% (entry 7). The
use of aminoindanol-based precatalyst G produced a com-
parable result (entry 8). Taking the cost, availability and
atom economy into consideration, the precatalyst F was
utilized further for the optimization study. Different bases
such as DBU, Cs2CO3, K2CO3 and t-BuOK in the presence of
precatalyst F in THF were not effective and led to either
poor or no product formation (entries 9–12). With precata-
lyst F as the optimum NHC catalyst and DABCO as the base,
we also investigated the solvent effect. Among all the other
solvents screened, the desired product was only formed in
toluene with a reduced yield of 73% (entries 13–17). A high-
er catalyst loading or an elevated reaction temperature had
no noticeable improvement on the reaction yield (entries
18 and 19). Lower catalyst loadings resulted in reduced
yields of product 2a (entries 20 and 21), whilst the oxida-
tion under an air atmosphere gave acid 2a in a lower 76%
yield (entry 22).
With optimized reaction conditions in hand (Table 1,
entry 7), the substrate scope was investigated (Scheme 1).
To our delight, even highly electron-rich aryl aldehydes af-
forded the desired products 2b–h in good to excellent
yields. A clear effect of the substitution pattern was ob-
served. The meta- and para-substituted aryl aldehydes per-
formed better than the corresponding ortho-substituted al-
dehydes (2b vs 2c and 2d; 2f vs 2g and 2h). Electron-defi-
cient aryl aldehydes also produced the corresponding
carboxylic acids 2i–l in good to excellent yields. Other aryl
aldehydes such as 1-naphthaldehyde and anthracene-9-
carboxaldehyde were well tolerated under the optimized
conditions giving the products 2m and 2n, respectively, in
good yields. Heteroaryl aldehydes were also found to be
suitable substrates for this methodology leading to the cor-
responding acids 2o–r. It is worth mentioning here that we
were initially interested in preparing indole-3-carboxylic
Table 1 Optimization of the Reaction Conditionsa
O
O
NHC, base
H
OH
solvent, O2, r.t., 16 h
1a
2a
Entry
Catalyst
Base
Solvent
Yield (%)b
1
2
–
A
B
C
D
E
F
G
F
F
F
F
F
F
F
F
F
F
F
F
F
F
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DBU
THF
–
THF
<5
16
25
<5
32
92
94
36
–
3
THF
4
THF
5
THF
6
THF
7
THF
8
THF
9
THF
10
11
12
13
14
15
16
17
18c
19d
20e
21f
22g
Cs2CO3
K2CO3
THF
THF
–
t-BuOK
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
THF
–
DMF
DMSO
CH2Cl2
toluene
MeCN
THF
–
–
–
73
–
93
89
72
54
76
THF
THF
THF
THF
a Reaction conditions: 1a (0.5 mmol), catalyst A–F (5 mol%), base (50
mol%), O2, solvent (3.0 mL), r.t.; unless otherwise specified.
b Yield of isolated product 2a.
c 10 mol% of F was used.
d Reaction performed at 50 °C.
e 2 mol% of F was used.
f 1 mol% of F was used.
g Reaction under an air atmosphere.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–E

C
A. K. Khatana et al.
Special Topic
Synthesis
acids (2q and 2r, which are highly useful synthons) before
embarking on this study, with most of the NHC-catalyzed
protocols reported in the literature failing to produce a sat-
isfactory result. We also examined the generality of several
enals under our conditions. Neutral or electron-rich aryl
substituents at the β-position of the enals produced the
products 2s and 2t in excellent yields, whereas the presence
of an electron-withdrawing substituent gave the product
2u in good yield, but as a mixture with the corresponding
saturated analogue in a 75:25 ratio. For this substrate, the
use of catalyst G under similar reaction conditions slightly
improved the yield and the ratio of the desired product
(80% yield, 86:14). An enal substituted at the α-position
also worked well affording acid 2v in 75% yield. Even though
partial conversion into the corresponding acid was ob-
served with an aliphatic aldehyde (1-pentanal) and a β-al-
kyl-substituted enal (crotonaldehyde), the reaction was not
clean, and the product could not be isolated in pure form
(inseparable mixtures containing an unidentified impuri-
ty).
catalyst F (5 mol%)
O
O
DABCO (50 mol%)
THF, O2, r.t., 16 h
R
H
O
R
OH
1
2
O
O
Me
OH
O
OH
OH
Me
2b, 89%
2a, 92%
2c, 95%
O
O
OH
OH
OH
Me
OMe
Me
2d, 95%
Me
2f, 70%
2e, 87%
O
O
O
MeO
O2N
OH
OH
OH
OH
MeO
2g, 90%
2i, 92%
2h, 81%
O
O
O
OH
OH
NC
F
O2N
We next examined the reaction on a 1 grams scale using
benzaldehyde. With 2 mol% and 5 mol% of catalyst loading,
the expected product 2a was obtained in 71% and 87% yield,
respectively, over a reaction time of 24 hours.
2j, 96%
2k, 85%
2l, 93%
O
OH
O
OH
O
In conclusion, we have developed a highly efficient tri-
azolium-NHC-catalyzed method for the aerobic oxidation
of aldehydes to the corresponding carboxylic acids under
mild conditions in a short reaction time. More significantly,
this method is suitable for several classes of challenging al-
dehydes such as ortho-substituted aryl aldehydes, highly
electron-rich aryl aldehydes and indole-3-carboxaldehydes.
We have also demonstrated this method for a gram-scale
synthesis.
OH
S
2m, 69%
2n, 64%
2o, 83%
O
O
O
OH
OH
OH
N
N
O
Boc
Me
2p, 96%
2q, 84%
2r, 66%
O
O
O
OH
OH
OH
MeO
2u, 64% (75:25)a
O2N
Unless otherwise stated, all reactions were performed under an O2 at-
mosphere. THF was distilled from Na using benzophenone as indica-
tor. All aldehydes are commercially available and were used as sup-
plied. TLC was carried out on precoated plates (Merck silica gel 60,
80% (86:14)b
2s, 94%
2t, 85%
O
OH
F
254), and the spots were visualized with UV light or by dipping in
Me
2v, 75%
PMA/KMnO4 solution and charring the plates. 1H NMR spectra were
recorded on a Bruker Avance III 400 MHz spectrometer using CDCl3 or
DMSO-d6 as the solvent. The 1H NMR data of all the isolated products
were in agreement with those reported previously in the literature.
Scheme 1 Substrate scope of aldehydes 1. Reagents and conditions: 1
(0.5 mmol), precatalyst F (5 mol%), DABCO (50 mol%), O2, THF (3.0
mL), r.t., 16 h; unless otherwise specified. Yields are those of isolated
products 2. a Ratio of 2u and its saturated analogue. b Precatalyst G was
employed; ratio of 2u and its saturated analogue.
Oxidation; General Procedure
To a dry, two-neck 25 mL round-bottom flask equipped with a mag-
netic stir bar was added NHC catalyst F (0.025 mmol) and aldehyde 1
(0.5 mmol). The reaction vessel was charged with anhydrous THF (3
mL), followed by flushing with O2 gas. DABCO (0.25 mmol) was added
and the flask was again flushed with O2 gas. The reaction mixture was
stirred for 16 h at r.t. under an O2 atmosphere (1 atm, O2 balloon). Af-
ter completion of the reaction, as monitored by TLC, the mixture was
diluted with EtOAc (10 mL) and aqueous 1.0 M NaOH solution was
added. The aqueous layer was separated, washed with EtOAc (10 mL)
and acidified using 3.0 M aqueous HCl solution (10 ml). This aqueous
layer was extracted with EtOAc (10 mL) twice and the combined or-
ganic layers were washed with brine, dried over anhydrous Na2SO4
and concentrated under reduced pressure to give the pure desired
product.
Benzoic Acid (2a)12a
Pale yellow solid; yield: 56 mg (92%).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–E

D
A. K. Khatana et al.
Special Topic
Synthesis
1H NMR (400 MHz, CDCl3): δ = 8.17–8.10 (m, 2 H, Ar-H), 7.66–7.59
(m, 1 H, Ar-H), 7.52–7.45 (m, 2 H, Ar-H).
1H NMR (400 MHz, DMSO-d6): δ = 13.00 (br s, 1 H, COOH), 8.04–7.95
(m, 2 H, Ar-H), 7.30 (t, J = 8.8 Hz, 2 H, Ar-H).
2-Methylbenzoic Acid (2b)12b
4-Cyanobenzoic Acid (2l)12a
Pale yellow solid; yield: 60 mg (89%).
Pale yellow solid; yield: 68 mg (93%).
1H NMR (400 MHz, CDCl3): δ = 8.07 (d, J = 8.0 Hz, 1 H, Ar-H), 7.45 (t,
J = 8.0 Hz, 1 H, Ar-H), 7.28 (t, J = 7.2 Hz, 2 H, Ar-H), 2.67 (s, 3 H, CH3).
1H NMR (400 MHz, DMSO-d6): δ = 13.52 (br s, 1 H, COOH), 8.07 (d,
J = 8.0 Hz, 2 H, Ar-H), 7.96 (d, J = 8.0 Hz, 2 H, Ar-H).
3-Methylbenzoic Acid (2c)12c
1-Napthoic Acid (2m)2a
Pale yellow solid; yield: 65 mg (95%).
Light yellow solid; yield: 59 mg (69%).
1H NMR (400 MHz, CDCl3): δ = 12.04 (br s, 1 H, COOH), 7.95–7.93 (m,
2 H, Ar-H), 7.43 (d, J = 8.0 Hz, 1 H, Ar-H), 7.37 (t, J = 8.0 Hz, 1 H, Ar-H),
2.43 (s, 3 H, CH3).
1H NMR (400 MHz, CDCl3): δ = 9.12 (d, J = 8.8 Hz, 1 H, Ar-H), 8.44 (dd,
J1 = 7.2 Hz, J2 = 1.2 Hz, 1 H, Ar-H), 8.11 (d, J = 8.0 Hz, 1 H, Ar-H), 7.93
(d, J = 8.0 Hz, 1 H, Ar-H), 7.72–7.64 (m, 1 H, Ar-H), 7.62–7.53 (m, 2 H,
Ar-H).
4-Methylbenzoic Acid (2d)12a
Anthracene-9-carboxylic Acid (2n)12d
Pale yellow solid; yield: 65 mg (95%).
Yellow solid; yield: 71 mg (64%).
1H NMR (400 MHz, CDCl3): δ = 8.02 (d, J = 8.0 Hz, 2 H, Ar-H), 7.28 (d,
J = 8.0 Hz, 2 H, Ar-H), 2.44 (s, 3 H, CH3).
1H NMR (400 MHz, DMSO-d6): δ = 13.90 (br s, 1 H, COOH), 8.72 (s, 1 H,
Ar-H), 8.15 (d, J = 8.4 Hz, 2 H, Ar-H), 8.08 (d, J = 8.0 Hz, 2 H, Ar-H),
7.67–7.53 (m, 4 H, Ar-H).
4-Isopropylbenzoic Acid (2e)2a
Off-white solid; yield: 71 mg (87%).
Thiophene-2-carboxylic Acid (2o)2a
1H NMR (400 MHz, CDCl3): δ = 8.05 (d, J = 8.0 Hz, 2 H, Ar-H), 7.34 (d,
J = 8.0 Hz, 2 H, Ar-H), 2.99 (sept, J = 7.2 Hz, 1 H, Ar-CH), 1.29 [d, J = 7.2
Hz, 6 H, (CH3)2].
White solid; yield: 53 mg (83%).
1H NMR (400 MHz, CDCl3): δ = 7.91 (dd, J1 = 3.6 Hz, J2 = 1.2 Hz, 1 H, Ar-
H), 7.65 (dd, J1 = 5.2 Hz, J2 = 1.2 Hz, 1 H, Ar-H), 7.15 (dd, J1 = 5.2 Hz,
J2 = 1.2 Hz, 1 H, Ar-H).
2-Methoxybenzoic Acid (2f)12a
Pale yellow solid; yield: 53 mg (70%).
Furoic Acid (2p)2a
1H NMR (400 MHz, CDCl3): δ = 8.13 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1 H, Ar-
H), 7.57–7.52 (m, 1 H, Ar-H), 7.12–7.03 (m, 2 H, Ar-H), 4.05 (s, 3 H,
CH3).
White solid; yield: 54 mg (96%).
1H NMR (400 MHz, CDCl3): δ = 9.75 (br s, 1 H, COOH), 7.64 (s, 1 H, Ar-
H), 7.33 (d, J = 3.6 Hz, 1 H, Ar-H), 6.56 (dd, J1 = 3.6 Hz, J2 = 1.6 Hz, 1 H,
Ar-H).
3-Methoxybenzoic Acid (2g)12a
Pale yellow solid; yield: 68 mg (90%).
1-(tert-Butoxycarbonyl)-1H-indole-3-carboxylic Acid (2q)12e
1H NMR (400 MHz, CDCl3): δ = 7.73 (d, J = 8.0 Hz, 1 H, Ar-H), 7.64–
7.63 (m, 1 H, Ar-H), 7.39 (t, J = 8.0 Hz, 1 H, Ar-H), 7.18–7.15 (m, 1 H,
Ar-H), 3.87 (s, 3 H, CH3).
White solid; yield: 110 mg (84%).
1H NMR (400 MHz, CDCl3): δ = 8.40 (s, 1 H, Ar-H), 8.25–8.16 (m, 2 H,
Ar-H), 7.44–7.33 (m, 2 H, Ar-H), 1.71 (s, 9 H, Boc).
4-Methoxybenzoic Acid (2h)12a
N-Methyl-1H-indole-3-carboxylic Acid (2r)12f
Pale yellow solid; yield: 62 mg (81%).
Light brown solid; yield: 63 mg (66%).
1H NMR (400 MHz, CDCl3): δ = 8.25–8.21 (m, 1 H, Ar-H), 7.89 (s, 1 H,
1H NMR (400 MHz, CDCl3): δ = 12.58 (br s, 1 H, COOH), 7.89 (d, J = 8.8
Hz, 2 H, Ar-H), 7.00 (d, J = 8.8 Hz, 2 H, Ar-H), 3.82 (s, 3 H, CH3).
Ar-H), 7.40–7.30 (m, 3 H, Ar-H), 3.87 (s, 3 H, CH3).
3-Nitrobenzoic Acid (2i)12a
trans-Cinnamic Acid (2s)12g
Pale yellow solid; yield: 77 mg (92%).
White solid; yield: 70 mg (94%).
1H NMR (400 MHz, DMSO-d6): δ = 8.59 (s, 1 H, Ar-H), 8.45 (d, J = 8.0
Hz, 1 H, Ar-H), 8.33 (d, J = 8.0 Hz, 1 H, Ar-H), 7.80 (t, J = 8.0 Hz, 1 H, Ar-
H).
1H NMR (400 MHz, CDCl3): δ = 11.10 (br s, 1 H, COOH), 7.71 (d, J = 16
Hz, 1 H, Alkene-H), 7.51–7.41 (m, 2 H, Ar-H), 7.37–7.27 (m, 3 H, Ar-H),
6.37 (d, J = 16 Hz, 1 H, Alkene-H).
4-Nitrobenzoic Acid (2j)12a
trans-4-Methoxycinnamic Acid (2t)12g
Light yellow solid; yield: 80 mg (96%).
Off white solid; yield: 76 mg (85%).
1H NMR (400 MHz, DMSO-d6): δ = 8.32 (d, J = 8.8 Hz, 2 H, Ar-H), 8.17
(d, J = 8.8 Hz, 2 H, Ar-H).
1H NMR (400 MHz, CDCl3): δ = 7.74 (d, J = 16 Hz, 1 H, Alkene-H), 7.50
(d, J = 8.8 Hz, 2 H, Ar-H), 6.92 (d, J = 8.8 Hz, 2 H, Ar-H), 6.31 (d, J = 16
Hz, 1 H, Alkene-H), 3.85 (s, 3 H, CH3).
4-Fluorobenzoic Acid (2k)2a
White solid; yield: 60 mg (85%).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–E

E
A. K. Khatana et al.
Special Topic
Synthesis
trans-4-Nitrocinnamic Acid (2u)12g
(3) For selected recent reviews on organocatalysis, see: (a) Qin, Y.;
Zhu, L.; Luo, S. Chem. Rev. 2017, 117, 9433. (b) James, T.;
Gemmeren, M.-V.; List, B. Chem. Rev. 2015, 115, 9388. (c) Volla,
C. M. R.; Atodiresei, I.; Rueping, M. Chem. Rev. 2014, 114, 2390.
(4) For selected recent reviews on NHC catalysis, see: (a) Murauski,
K. J. R.; Jaworski, A. A.; Scheidt, K. A. Chem. Soc. Rev. 2018, 47,
1773. (b) Menon, R. S.; Biju, A. T.; Nair, V. Beilstein J. Org. Chem.
2016, 12, 444. (c) Flanigan, D. M.; Romanov-Michailidis, F.;
White, N. A.; Rovis, T. Chem. Rev. 2015, 115, 9307.
(d) Mahatthananchai, J.; Bode, J. W. Acc. Chem. Res. 2014, 47,
696. (e) Ryan, S. J.; Candish, L.; Lupton, D. W. Chem. Soc. Rev.
2013, 42, 4906. (f) Izquierdo, J.; Hutson, G. E.; Cohen, D. T.;
Scheidt, K. A. Angew. Chem. Int. Ed. 2012, 51, 11686.
Off white solid; yield: 78 mg (80%).
1H NMR (400 MHz, DMSO-d6): δ = 12.50 (br s, 1 H, COOH), 8.22 (d,
J = 8.8 Hz, 2 H, Ar-H), 7.96 (d, J = 8.8 Hz, 2 H, Ar-H), 7.68 (d, J = 16 Hz, 1
H, Alkene-H), 6.73 (d, J = 16 Hz, 1 H, Alkene-H).
(E)-α-Methylcinnamic Acid (2v)12h
White solid; yield: 61 mg (75%).
1H NMR (400 MHz, CDCl3): δ = 11.60 (br s, 1 H, COOH), 7.75 (d, J = 1.2
Hz, 1 H, Alkene-H), 7.36–7.21 (m, 5 H, Ar-H), 2.05 (d, J = 1.2 Hz, 3 H, α-
CH3).
(5) For selected recent reviews on oxidative NHC catalysis, see:
(a) Albanese, D. C. M.; Gaggero, N. Eur. J. Org. Chem. 2014, 5631.
(b) De Sarkar, S.; Biswas, A.; Samanta, R. C.; Studer, A. Chem. Eur.
J. 2013, 19, 4664. (c) Knappke, C. E. I.; Imami, A.; Jacobi von
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Funding Information
B.T. thanks the Science & Engineering Research Board (SERB), New
Delhi, India, for a research grant (EMR/2015/00097).
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Acknowledgment
A.K.K. thanks CSIR, New Delhi, India for a fellowship.
(7) (a) Yoshida, M.; Katagiri, Y.; Zhu, W.-B.; Shishido, K. Org. Biomol.
Chem. 2009, 7, 4062. (b) Gu, L.; Zhang, Y. J. Am. Chem. Soc. 2010,
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1610069.
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