Direct esterification of aromatic aldehydes with tetraphenylphosphonium bromide under oxidative N-heterocyclic carbene catalysis

Article abstract of DOI:10.1002/hlca.201300369

An unconventional reagent, tetraphenylphosphonium bromide, was employed as a phenyl source in the direct transformation of aromatic aldehydes to the corresponding phenyl esters under oxidative N-heterocyclic carbene (NHC) catalysis. The phenyl esters were obtained in moderate yields under mild and organocatalytic conditions. Copyright

Full text of DOI:10.1002/hlca.201300369

Helvetica Chimica Acta – Vol. 97 (2014)
431
Direct Esterification of Aromatic Aldehydes with Tetraphenylphosphonium
Bromide under Oxidative N-Heterocyclic Carbene Catalysis
by Bandaru T. Ramanjaneyulu, Manish Pareek, Virsinha Reddy, and R. Vijaya Anand*
Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali,
Sector 81, Knowledge City, S. A. S. Nagar, Mohali, Punjab – 140306, India
(
fax: þ 91-172-2240266; e-mail: rvijayan@iisermohali.ac.in)
An unconventional reagent, tetraphenylphosphonium bromide, was employed as a phenyl source in
the direct transformation of aromatic aldehydes to the corresponding phenyl esters under oxidative N-
heterocyclic carbene (NHC) catalysis. The phenyl esters were obtained in moderate yields under mild
and organocatalytic conditions.
Introduction. – Tetraphenylphosphonium salts have gained momentous attention
due to their eclectic range of applications as ion-pair extractants for the heavy metals
[
1], phase-transfer catalysts [2][3], herbicides [4], ionic liquids [5], and as conducting
materials [6]. Some of the tetraphenylphosphonium salts have been utilized as
molecular probes for imaging tumors [7]. Apart from these applications, tetraphenyl-
phosphonium bromide (Ph PBr) has also been used as an additive in metal-catalyzed
4
cross-coupling reactions [8 – 11]. However, very few reports are available in the
literature, where tetraphenylphosphonium salts served as an aryl-transfer reagents.
Yamamoto and co-workers reported Pd-catalyzed arylation of electron-deficient
olefins with Ph PCl [12]. Recently, Chang and co-workers have shown that Ph PCl
4
4
could be effectively utilized as a phenyl source for Pd-catalyzed Heck, Suzuki, and
Sonagashira coupling reactions [13].
Oxidative N-heterocyclic carbene catalysis [14][15], a type of N-heterocyclic
carbene (NHC) catalysis [16 – 25], has been emerging as a powerful method for the
construction of Cꢀheteroatom bonds. This potential has been explored in a few
important transformations, including aerobic oxidation of aldehydes [26][27], ester-
ification [28 – 36], lactone formation [37 – 39], and amidation reactions [40]. Recently,
we reported an efficient and environmentally friendly synthesis of aryl esters from
aromatic aldehydes and aryl boronic acids under oxidative N-heterocyclic carbene
catalysis [41]. We also reported that a combination of oxidative N-heterocyclic carbene
catalysis and ꢀclick chemistryꢁ was very effective for the one-pot synthesis of ester
containing 1,2,3-triazoles from aldehydes [42]. Herein, we report an alternative method
for the synthesis of phenyl esters from aromatic aldehydes and Ph PBr using N-
4
heterocyclic carbene as a catalyst under aerobic conditions.
Results and Discussion. – While developing an organocatalytic method for the
preparation of aryl esters [41], we came across a few alternative aryl sources, especially
phenyl-transfer agents, which include tetraphenylphosphonium salts. Since Ph PBr is
4
ꢂ 2014 Verlag Helvetica Chimica Acta AG, Zꢃrich

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32
Helvetica Chimica Acta – Vol. 97 (2014)
commercially available and has not been utilized for esterification reactions so far, we
decided to study it as a phenyl source in the oxidative esterification reaction.
The optimization studies were carried out with 4-chlorobenzaldehyde (1) as a
model substrate and Ph PBr (2) as a phenyl source. A variety of NHC precursors, 4 – 8,
4
were screened for this oxidative esterification under different conditions (Table 1). All
these reactions were carried out under aerobic conditions. Our initial attempt in the
optimization studies using NHC precursor 4 was disappointing, as the anticipated
product 3 was not observed even after 24 h (Entry 1, Table 1). However, when the
experiment was conducted with 5 as a catalyst, the expected ester 3 was obtained in
20% yield (Entry 2). Encouraged by this result, we performed the optimization studies
with a few more NHC precursors and different bases, and the results are compiled in
Table 1. Of the few conditions tried, the best was found as indicated in Entry 3
(
Table 1); NHC precursor 6 was used as a catalyst, and the phenyl ester 3 was obtained
in 67% yield after 4 h at room temperature.
After having found an appropriate reaction condition, we focused our attention on
the scope of this transformation. Consequently, a variety of aromatic aldehydes, 9a – 9o,
were subjected to the oxidative esterification reaction with Ph PBr under optimized
4
conditions, and the results are collected in Table 2. In most of the cases, the required
esters were obtained in moderate yields. Halogenated aromatic aldehydes such as 4-
Table 1. Optimization Studiesa)
Entry
NHC
Base
Solvent
Time [h]
Yield [%]b)
1
2
3
4
5
6
7
8
9
4
5
6
7
8
6
6
6
6
6
6
6
Cs CO
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Toluene
THF
24
26
4
36
24
36
24
24
14
4
0
20
67
56
Trace
23
0
Trace
44
56
2
3
3
3
3
3
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
K CO
2
3
i
Pr NEt
2
t
KO Bu
Cs CO
2
3
1
1
1
0
2
3
Cs CO
2
3
3
3
Cs CO
DME
DCM
5
30
33
24
2
Cs CO
2
a) Reaction conditions: 1/2 1.3 : 1 equiv. in 0.15m solution. b) Yield of isolated 3; r.t., 32 – 358.

Helvetica Chimica Acta – Vol. 97 (2014)
433
bromobenzaldehyde and 4-fluorobenzaldehyde gave the corresponding esters, 10a and
0b, in 47 and 55%, respectively. This methodology worked relatively better in the cases
of electron-rich aromatic aldehydes, i.e., 10g, 10j, and 10k. In the case of benzaldehyde
9c), phenyl benzoate (10c) was obtained in 41% yield after 14 h. Electron-poor
1
(
aromatic aldehydes such as 4-formylbenzonitrile (9d) and 3-nitrobenzaldehyde (9f)
gave the esters 10d and 10f in 33 and 32% yields, respectively. 3-Fluorobenzaldehyde
(
9o) gave the corresponding ester, 10o, in 40% yield in 5 h. The yield of the ester 10e is
slightly better in the case of methyl 4-formylbenzoate. 4-(Trifluoromethyl)-benzalde-
hyde (9h) was also converted to the product 10h in 55% yield. In the case of alkyl- and
aryl-substituted benzaldehydes such as 4-ethylbenzaldehyde (9i) and 4-phenylbenzal-
dehyde (9n), the reaction was very slow, and the esters 10i and 10n were obtained in 38
and 39% yield, respectively, after 48 h. A few heteroaromatic aldehydes, such as
furfural (9l) and thiophene-2-carbaldehyde (9m) provided the corresponding esters 10l
and 10m in 48 and 50% yields, respectively. Aliphatic aldehydes, such as dihydrocin-
namaldehyde and cyclohexanecarboxaldehyde failed to give the desired products;
instead the starting materials were decomposed, since the reaction medium was highly
basic. Almost in all the cases, a considerable amount of the corresponding acid was also
formed along with ester. This could be due to the competitive oxidation of aldehydes to
acids under oxidative NHC-catalyzed conditions [26][27]. It is also known that Ph PBr
4
decomposes under strong basic conditions [43]. These are probably the reasons why
lower yields were obtained in most of the cases.
Table 2. Scope with Respect to the Substratea)
Entry
Product
R
Time [h]
Yield [%]
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
10a
10b
10c
10d
10e
10f
10g
10h
10i
10j
10k
10l
10m
10n
10o
4-Br
4-F
H
4-CN
36
5
14
12
5
6
28
9
48
36
48
3
48
48
5
47
55
41
33
43
32
58
55
38
48
43
48
50
39
40
4-COOMe
3-NO2
3,4-(Methylenedioxy)
4-CF3
4-Et
t
1
1
1
1
1
1
4- Bu
4-MeO
Furan-2-yl )
Thiophene-2-yl )
4-Ph
3-F
b
b
a) Reaction conditions: 9/2 1.3 : 1 equiv. in 0.15m solution in 1,4-dioxane; r.t., 32 – 358. b) Instead of
RꢀC H .
6
4

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34
Helvetica Chimica Acta – Vol. 97 (2014)
At this stage, our attention was shifted towards understanding the mechanism of
this reaction. Careful monitoring of the reaction between 4-chlorobenzaldehyde and
Ph PBr under the standard conditions revealed that triphenylphosphine oxide (Ph PO)
4
3
and 4-chlorobenzoic acid were the by-products. Since 4-chlorobenzoic acid was
observed in the reaction, we initially thought that the reaction proceeds via acid, which
then reacts with Ph PBr under basic conditions to give the product and Ph PO. To
4
3
confirm this, an experiment was performed by treating 4-chlorobenzoic acid with
Ph PBr in 1,4-dioxane using 3 equiv. of Cs CO as a base. However, the phenyl ester 3
4
2
3
was not observed even after 24 h at room temperature. This clearly indicates that the
reaction does not proceed via acid intermediate. Another possible intermediate for this
reaction could be PhOH, which might be formed by the decomposition of Ph PBr
4
under oxidative conditions. Although PhOH formation was not observed (by TLC) in
our experiments, we carried out an experiment, in which Ph PBr was exposed to NHC
4
and air (O ) in 1,4-dioxane under basic condition. However, PhOH was not detected
2
even after stirring the reaction mixture for a prolonged period at room temperature. It
is evident from the above mentioned experiments that the reaction involves neither
PhCOOH nor PhOH as an intermediate. Based on these observations, we propose a
concerted mechanism, which is depicted in the Scheme.
Scheme. Proposed Mechanism
We presume that the Breslow intermediate 11, formed by the reaction of PhCHO
(
9c) with NHC, reacts with O and Ph PBr in a concerted manner to give intermediate
2 4
1
2, which decomposes readily to intermediate 13 with the expulsion of Ph PO. On
3
deprotonation, intermediate 13 releases the product along with NHC.
Conclusions. – We have developed an alternative method for the direct synthesis of
phenyl esters from aromatic aldehydes under oxidative N-heterocyclic carbene

Helvetica Chimica Acta – Vol. 97 (2014)
435
catalysis. Although the yield of the esters was moderate in most cases, Ph PBr was
4
investigated, for the first time, as a Ph source in this methodology.
Experimental Part
General. Most of the reagents and starting materials used were purchased from commercial sources
and used as such. TLC: Merck silica gel 60 F254 plates with AcOEt/hexane as an eluent. Column
chromatography (CC): silica gel (SiO ; 100 – 200 mesh). IR Spectra: PerkinElmer FT-IR spectrometer;
2
ꢀ
1
1
13
with KBr, in cm . H- and C-NMR spectra: in CDCl on 400-MHz Brucker FT-NMR spectrometer;
3
chemical shifts (d) in ppm relative to TMS, and coupling constants (J) in Hz.
General Procedure for the Oxidative Esterification of Aromatic Aldehydes with Ph PBr. Aromatic
4
aldehyde (0.37 mmol) was added to a suspension of Ph PBr (0.29 mmol), 6 (0.029 mmol), and
4
Cs CO (0.86 mmol) in 1,4-dioxane (2 ml) at r.t. (32 – 358). After completion of the reaction, the reaction
2
3
mass was filtered, washed with AcOEt (10 ml), and dried (Na SO ). The solvent was removed under
2
4
reduced pressure, and the residue was purified by CC (SiO ; hexane/AcOEt (5%)) to give the pure
2
ester.
Phenyl 4-Chlorobenzoate (3) [41]. Yield: 67%. White solid. M.p. 104 – 1068. IR (KBr): 1732.
1
H-NMR: 8.15 (d, J ¼ 8.7, 2 H); 7.49 (d, J ¼ 8.7, 2 H); 7.47 – 7.42 (m, 2 H); 7.31 – 7.27 (m, 1 H); 7.23 – 7.19
13
(
m, 2 H). C-NMR: 164.5; 150.9; 140.3; 131.7; 129.7; 129.1; 128.2; 126.2; 121.8.
Phenyl 4-Bromobenzoate (10a) [41]. Yield: 47%. White solid. M.p. 117 – 1188. IR (KBr): 1731.
1
H-NMR: 8.07 (d, J ¼ 8.6, 2 H); 7.66 (d, J ¼ 8.6, 2 H); 7.46 – 7.41 (m, 2 H); 7.31 – 7.27 (m, 1 H); 7.22 – 7.20
13
(
m, 2 H). C-NMR: 164.7; 150.9; 132.1; 131.8; 129.7; 129.0; 128.6; 126.2; 121.8.
Phenyl 4-Fluorobenzoate (10b) [44]. Yield: 55%. White solid. M.p. 63 – 658. IR (KBr): 1734.
H-NMR: 8.26 – 8.21 (m, 2 H); 7.46 – 7.41 (m, 2 H); 7.30 – 7.26 (m, 1 H); 7.22 – 7.16 (m, 4 H). C-NMR:
1
13
1
66.3 (d, J ¼ 250.5); 164.4; 151.0; 132.9 (d, J ¼ 9.4); 129.7; 128.9; 126.0 (d, J ¼ 19.2); 121.8; 115.9 (d, J ¼
2
1.8).
1
Phenyl Benzoate (10c) [41]. Yield: 41%. White solid. M.p. 66 – 688. IR (KBr): 1731. H-NMR: 8.24 –
8
.21 (m, 2 H); 7.67 – 7.63 (m, 1 H); 7.54 – 7.51 (m, 2 H); 7.47 – 7.42 (m, 2 H); 7.31 – 7.27 (m, 1 H); 7.24 – 7.21
13
(
m, 2 H). C-NMR: 165.3; 151.1; 133.7; 130.3; 129.7; 129.6; 128.7; 126.0; 121.9.
Phenyl 4-Cyanobenzoate (10d) [41]. Yield: 33%. White solid. M.p. 94 – 968. IR (KBr): 1742, 2365.
1
H-NMR: 8.31 (d, J ¼ 8.7, 2 H); 7.83 (d, J ¼ 8.7, 2 H); 7.48 – 7.44 (m, 2 H); 7.33 – 7.29 (m, 1 H); 7.24 – 7.20
13
(
m, 2 H). C-NMR: 163.7; 150.7; 133.6; 132.6; 130.8; 129.8; 126.5; 121.6; 118.0; 117.6.
Methyl Phenyl Benzene-1,4-Dicarboxylate (10e) [41]. Yield: 43%. White solid. M.p. 106 – 1088. IR
1
(
1
1
KBr): 1734. H-NMR: 8.27 (d, J ¼ 8.6, 2 H); 8.18 (d, J ¼ 8.6, 2 H); 7.47 – 7.42 (m, 2 H); 7.32 – 7.28 (m,
13
H); 7.24 – 7.21 (m, 2 H); 4.0 (s, 3 H). C-NMR: 166.4; 164.6; 150.9; 134.6; 133.5; 130.3; 129.9;
29.7;126.3; 121.7; 52.7.
Phenyl 3-Nitrobenzoate (10f) [45]. Yield: 32%. Pale-yellow solid. M.p. 156 – 1588. IR (KBr): 1728,
1
2
7
923. H-NMR: 9.05 – 9.04 (m, 1 H); 8.55 – 8.49 (m, 2 H); 7.76 – 7.72 (m, 1 H); 7.49 – 7.44 (m, 2 H); 7.34 –
13
.3 ( m, 1 H); 7.26 – 7.22 (m, 2 H). C-NMR: 163.1; 150.6; 148.5; 135.9; 131.5; 130.3; 129.8; 128.1; 126.6;
1
25.3; 121.6.
Phenyl 1,3-Benzodioxole-5-carboxylate (10g) [41]. Yield: 58%. White solid. M.p. 80 – 828. IR (KBr):
1
1
715. H-NMR: 7.83 (dd, J ¼ 8.2, 1.7, 1 H); 7.62 (d, J ¼ 1.7, 1 H); 7.45 – 7.40 (m, 2 H); 7.29 – 7.24 (m, 1 H);
13
7
.21 – 7.18 (m, 2 H); 6.91 (d, J ¼ 8.1, 1 H); 6.08 (s, 2 H). C-NMR: 164.7; 152.3; 151.1; 148.1; 129.6; 126.4;
1
26.0; 123.6; 121.9; 110.1; 108.3; 102.1.
Phenyl 4-(Trifluoromethyl)benzoate (10h) [45]. Yield: 55%. Pale-yellow solid. M.p. 81 – 838. IR
1
(
1
1
KBr): 1733. H-NMR: 8.33 (d, J ¼ 8.04, 2 H); 7.79 (d, J ¼ 8.12, 2 H); 7.48 – 7.43 (m, 2 H); 7.33 – 7.29 (m,
13
H); 7.25 – 7.21 (m, 2 H). C-NMR: 164.2; 150.8; 135.2 (q, J ¼ 32.9); 133.0 (q, J ¼ 1.4); 130.7; 129.8;
26.4; 125.8 (q, J ¼ 3.66); 123.7 (q, J ¼ 270.6); 121.7.
1
Phenyl 4-Ethylbenzoate (10i) [41]. Yield: 38%. White solid. M.p. 62 – 638. IR (KBr): 1726. H-NMR:
8
2
1
.13 (d, J ¼ 8.1, 2 H); 7.45 – 7.41 (m, 2 H); 7.34 (d, J ¼ 8.0, 2 H); 7.29 – 7.26 (m, 1 H); 7.22 (d, J ¼ 8.0, 2 H);
13
.75 (q, J ¼ 7.6, 2 H); 1.29 (t, J ¼ 7.6, 3 H). C-NMR: 165.4; 151.2; 150.7; 130.5; 129.6; 128.2; 127.2; 125.9;
21.9; 29.2; 15.4.

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36
Helvetica Chimica Acta – Vol. 97 (2014)
Phenyl 4-(tert-Butyl)benzoate (10j) [46]. Yield: 48%. White solid. M.p. 142 – 1468. IR (KBr): 1729.
1
H-NMR: 8.18 (d, J ¼ 8.4, 2 H); 7.57 (d, J ¼ 8.4, 2 H); 7.49 – 7.45 (m, 2 H); 7.33 – 7.24 (m, 3 H); 1.41 (s,
13
9
H). C-NMR: 165.3; 157.5; 151.2; 130.2; 129.6; 126.9; 125.9; 125.7; 121.9; 35.3; 31.3.
Phenyl 4-Methoxybenzoate (10k) [41]. Yield: 43%. White solid. M.p. 75 – 778. IR (KBr): 1727.
1
H-NMR: 8.14 (d, J ¼ 8.8, 2 H); 7.42 – 7.39 (m, 2 H); 7.26 – 7.22 (m, 1 H); 7.20 – 7.18 (m, 2 H); 6.97 (d, J ¼
13
8
.8, 2 H); 3.88 (s, 3 H). C-NMR: 165.9; 164.0; 151.2; 132.4; 129.6; 125.9; 122.0; 121.9; 114.0; 55.7.
Phenyl Furan-2-carboxylate (10l) [41]. Yield: 48%. White solid. M.p. 54 – 568. IR (KBr): 1736.
1
H-NMR: 7.68 (dd, J ¼ 1.7, 0.8, 1 H); 7.45 – 7.40 (m, 2 H); 7.39 (dd, J ¼ 3.5, 0.8, 1 H); 7.30 – 7.25 (m, 1 H);
13
7
1
.23 – 7.20 (m, 2 H); 7.60 (dd, J ¼ 3.5, 1.8, 1 H). C-NMR: 157.1; 150.3; 147.3; 144.2; 129.7; 126.2; 121.8;
19.6; 112.3.
Phenyl Thiophene-2-carboxylate (10m) [41]. Yield: 50%. Semisolid. IR (KBr): 1738. H-NMR: 7.99
1
(
2
dd, J ¼ 3.8, 1.3, 1 H); 7.67 (dd, J ¼ 5.0, 1.3, 1 H); 7.45 – 7.40 (m, 2 H); 7.30 – 7.25 (m, 1 H); 7.24 – 7.21 (m,
13
H); 7.18 (dd, J ¼ 5.0, 3.8, 1 H). C-NMR: 160.7; 150.7; 134.8; 133.6; 133.1; 129.6; 128.2; 126.1; 121.8.
Phenyl 1,1’-Biphenyl-4-carboxylate (10n) [41]. Yield: 39%. White solid. M.p. 158 – 1608. IR (KBr):
1
1
731. H-NMR: 8.28 (d, J ¼ 8.2, 2 H); 7.74 (d, J ¼ 8.2, 2 H); 7.67 (d, J ¼ 7.7, 2 H); 7.52 – 7.41 (m, 5 H);
13
7.31 – 7.24 (m, 3 H). C-NMR: 165.2; 151.2; 146.5; 140.0; 130.9; 129.7; 129.1; 128.5; 128.4; 127.5; 127.4;
1
26.0; 121.9.
Phenyl 3-Fluorobenzoate (10o) [41]. Yield: 40%. White solid. M.p. 58 – 598. IR (KBr): 1736.
H-NMR: 8.02 – 8.00 (m, 1 H); 7.91 – 7.87 (m, 1 H); 7.37 – 7.27 (m, 2 H); 7.23 – 7.20 (m, 2 H); 7.53 – 7.42 (m,
H). C-NMR: 164.1 (d, J ¼ 25.4); 161.5; 150.9; 131.9 (d, J ¼ 3.8); 130.4 (d, J ¼ 7.8); 129.7; 126.3; 126.1
1
13
3
(d, J ¼ 3.1); 121.7; 120.9 (d, J ¼ 21.2); 117.2 (d, J ¼ 22.9).
The authors sincerely thank the Department of Science and Technology (DST), New Delhi, for
financial support, and IISER Mohali for providing infrastructure. B. T. R. and V. R. thank CSIR, New
Delhi, for a research fellowship. The authors also acknowledge the NMR Facility of IISER Mohali for the
support.
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Received October 10, 2013