A novel synthesis of carboxylic acid oxime esters catalyzed by 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate

Article abstract of DOI:10.14233/ajchem.2014.16522

We describe a facile synthesis of carboxylic acid oxime esters in acetonitrile from the corresponding carboxylic acids and oximes catalyzed by 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate in the presence of triethylamine at room temperature under mild conditions.

Full text of DOI:10.14233/ajchem.2014.16522

Asian Journal of Chemistry; Vol. 26, No. 21 (2014), 7125-7128
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.16522
A Novel Synthesis of Carboxylic Acid Oxime Esters Catalyzed by
2-(1H-Benzotriazole-1-yl)-1,1,3,3-tetramethyluronium Tetrafluoroborate
*
WEI SHI, JIMING ZHANG , QIN ZHOU, XINGJIAN WANG, XIAOWEI QI, LIANYONG GUO, WANYONG MA and JIANHUA ZHOU
School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, P.R. China
*Corresponding author: E-mail: zhangjiming@gmail.com; slzshiwei@163.com
Received: 14 October 2013;
Accepted: 30 January 2014;
Published online: 30 September 2014;
AJC-16085
We describe a facile synthesis of carboxylic acid oxime esters in acetonitrile from the corresponding carboxylic acids and oximes catalyzed
by 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate in the presence of triethylamine at room temperature under
mild conditions.
Keywords: Oxime ester, Carboxylic acid, 2-(1H-Benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate, Oxime esterification.
1,1,3,3-tetramethyluronium tetrafluoroborate is a mild and
effective coupling reagent that has some favorable charac-
teristics from an industrial or environmental standpoint, such
as high efficiency, need for only stochiometric quantities and
solubility in currently used solvents. Herein, we report that
2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium
tetrafluoroborate can be used as a reactive coupling reagent to
promote carboxylic acid oxime ester formation at room tempe-
rature (Scheme-I).
INTRODUCTION
Oxime esters and their derivatives are valuable products
or intermediates in the synthesis of medicinal compounds1,2
and bioactive agents3-6. They are also important starting
materials in the preparation of photosensitive compositions7,8.
Oxime esters are generally synthesized from the corresponding
oximes reacting with the high sterically hindered acyl halide9-11
,
anhydride12, benzoyl peroxide13 and potassium methanecarbo-
telluroate14. However, these methods have not been entirely
satisfactory owing to such drawbacks as low yield, long
reaction time, difficulty in obtaining reagents, high temperature
media, harsh reaction conditions, tedious work-up procedures
and so on.
R3
R2
O
O
R3
R2
OH
TBTU, TEA
CH3CN, r.t.
O
+
N
R1
OH
N
R1
1
2
3
Generally the formation of carboxylic acid oxime esters
requires activation of the related carboxyl groups. In the last
decade, many peptide coupling reagents have been designed,
synthesized and commonly used in peptide synthesis for
activating carboxyl groups, such as DCC/DMAP15, phosphorus
oxychloride16, COMU17, etc. The most successful approaches
known today involve activation of the carboxylic group with
uronium/guanidinium salts. The most popular members of this
family are coupling reagents based on benzotriazole derivatives
such as HOBt or HOAt, both of which are also commonly used
as additives in carbodiimide-mediated peptide coupling. There-
fore, we wish to choose a coupling reagent in this family with
a high performance-to-cost ratio.
Scheme-I: Synthesis of carboxylic acid oxime esters
EXPERIMENTAL
All reagents and solvents were purchased from commer-
cial sources, unless otherwise noted and were used without
purification. Melting points were determined on a SGW X-4
melting-point apparatus with microscope and were uncor-
rected. Infrared spectra were recorded on an IRPrestige-21
FTIR spectrophotometer (KBr). 1H and 13C NMR spectra were
recorded on a BrukerAdvance-II spectrometer at 400 and 100
MHz with TMS and referenced to DMSO-d6; δ values are
reported in ppm and J values in Hz.
General procedure for the synthesis of compound 3a:
To a solution of benzoic acid (244 mg, 2 mmol) and 2-(1H-
benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoro-
borate (706 mg, 2.2 mmol) in 40 mL acetonitrile, triethylamine
(0.56 mL, 4 mmol) was added and the mixture stirred at room
A uronium-based peptide coupling agent, namely, 2-(1H-
benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate
is one of the most common coupling reagents and additives
used in peptide chemistry18-20. 2-(1H-Benzotriazole-1-yl)-

7126 Shi et al.
Asian J. Chem.
temperature for 0.5 h. Then, benzophenone oxime (394 mg, 2
mmol) was added and the reaction mixture stirred at room
temperature for 0.5 h. The progress of the reaction was moni-
tored by TLC (petroleum ether/ethyl acetate 3:1). After comp-
letion of the reaction, the reaction mixture was washed with
distilled water and extracted with CH2Cl2 (3 × 10 mL). The
combined organic layers were then washed with saturated brine
solution (5 mL), dried over anhydrous magnesium sulfate,
filtered, concentrated in vacuo and purified by silica gel column
chromatography to obtain pure product.
Compound 3a: White solid, m.p. 100-102 °C (lit.20
101 °C). 1H NMR (400 MHz, DMSO-d6): δ H 7.69 (d, 2H, J
7.6 Hz, Ar-H), 7.64 (t, 1H, J 7.4 Hz, Ar-H), 7.57 (d, 3H, J 7.1
Hz, Ar-H), 7.46-7.52 ( m, 12 H, Ar-H). 13C NMR (100 MHz,
DMSO-d6): δ 165.5, 163.8, 134.6, 133.2, 132.8, 131.0, 129.7,
(m, 6H, CH2). 13C NMR (100 MHz, DMSO-d6): δ 169.3, 164.3,
144.5, 143.8, 130.2, 129.6, 129.1, 126.6, 32.2, 27.1, 26.7, 25.8.
FT-IR (KBr, νmax, cm-1): 2933, 1730, 1606, 1448, 1267, 1169,
1076, 918, 833, 750, 478.
Compound 3h: White solid, m.p. 117-119 °C (lit.25 116-
118 °C). 1H NMR (400 MHz, DMSO-d6): δ H 7.98 (d, 2H, J 8
Hz, Ar-H), 7.82 (d, 2H, J 8 Hz, Ar-H), 7.48-7.56 (m, 3H, Ar-
H), 7.40 (d, 2H, J 8 Hz, Ar-H), 2.51 (s, 3H, CH3), 2.41 (s, 3H,
CH3). 13C NMR (100 MHz, DMSO-d6): δ 163.8, 162.9, 134.5,
130.7, 129.5, 129.3, 128.7, 126.9, 125.8, 21.2, 14.3. FT-IR
(KBr, νmax, cm-1): 3032, 2922, 1732, 1607, 1443, 1312, 1250,
1068, 899, 748, 696, 567.
Compound 3i: White solid, m.p. 91-93 °C. 1H NMR (400
MHz, DMSO-d6): δ H 8.93 (s, 1H, N=CH), 7.95 (d, 2H, J 8.1
Hz, Ar-H), 7.83 (d, 2H, J 8.4 Hz, Ar-H), 7.61 (d, 2H, J 8.4 Hz,
Ar-H), 7.40 (d, 2H, J 8.0 Hz, Ar-H), 2.41 (s, 3H, CH3). 13C
NMR (100 MHz, DMSO-d6): δ 163.0, 156.8, 144.4, 136.4,
134.1, 129.9, 129.5, 129.3, 129.1, 125.2, 21.2. FT-IR (KBr,
νmax, cm-1): 3031, 2924, 1737, 1612, 1489, 1261, 1080, 937,
817, 740, 513.
129.6, 129.1, 128.8, 128.7, 128.4, 128.2. FT-IR (KBr, νmax
,
cm-1): 3061, 1740, 1446, 1244, 1087, 982, 868, 702.
Compound 3b: White solid, m.p. 98-100 °C (lit.20 98-
100 °C). 1H NMR (400 MHz, DMSO-d6): δ H 8.09 (d, 2H, J
7.5 Hz, Ar-H), 7.83 (d, 2H, J 6.9 Hz, Ar-H), 7.73 (t, 1H, J 7.4
Hz, Ar-H), 7.60 (t, 2H, J 7.6Hz, Ar-H), 7.49-7.56 (m, 3H,
Ar-H), 2.52 (s, 3H, CH3). 13C NMR (100 MHz, DMSO-d6): δ
164.2, 164.1, 134.7, 133.8, 131.1, 130.2, 130.1, 129.3, 129.0,
128.8, 127.6, 15.1. FT-IR (KBr, νmax, cm-1): 3061, 2924, 1746,
1598, 1446, 1313, 1242, 1058, 908, 764, 700, 567.
Compound 3c: White solid, m.p. 95-96 °C (lit.13 98 °C).
1H NMR (400 MHz, DMSO-d6): δ H 8.92 (s, 1H, N=CH),
8.07 (d, 2H, J 7.5 Hz, Ar-H), 7.82 (d, 2H, J 6.8 Hz, Ar-H),
7.73 (t, 1H, J 7.4 Hz, Ar-H), 7.52-7.62 (m, 5H, Ar-H). 13C
NMR (100 MHz, DMSO-d6): δ 171.8, 156.7, 134.2, 133.7,
133.4, 131.8, 130.2, 129.8, 129.7, 128.9, 128.8, 128.5, 128.4.
FT-IR (KBr, νmax, cm-1): 3061, 2924, 1744, 1599, 1448, 1244,
1061, 864, 760, 704, 513.
Compound 3d: White solid, m.p. 55-56 °C (lit.21 56-
57 °C). 1H NMR (400 MHz, DMSO-d6): δ H 8.08 (d, 2H, J 7.4
Hz, Ar-H), 7.60 (t, 1H, J 7.4 Hz, Ar-H), 7.48 (t, 2H, J 7.7 Hz,
Ar-H), 2.69 (t, 2H, J 6.4 Hz, CH2), 2.49 (t, J 6.4 Hz, 2H, CH2),
1.65-1.85 (m, 6H, CH2). 13C NMR (100 MHz, DMSO-d6): δ
164.4, 164.1, 132.9, 129.8, 129.3, 129.1, 128.3, 128.1, 32.0,
26.9, 26.6, 25.7, 25.5. FT-IR ( KBr, νmax, cm-1): 3066, 2935,
1738, 1637, 1450, 1263, 1062, 921, 864, 702.
Compound 3j: Light yellow solid, m.p. 110-111 °C (lit.26
1
111-112 °C). H NMR (400 MHz, DMSO-d6): δ H 8.35 (d,
2H, J 8.8 Hz, Ar-H), 8.23 (d, 2H, J 8.8 Hz, Ar-H), 2.66 (t, 2H,
J 6.2 Hz, CH2), 2.38 (t, 2H, J 6 Hz, CH2), 1.60-1.74 (m, 6H,
CH2). 13C NMR (100 MHz, DMSO-d6): δ 170.7, 161.8, 150.3,
134.3, 130.6, 123.9, 31.2, 26.5, 26.3, 25.4, 24.6. FT-IR (KBr,
νmax, cm-1): 3109, 2949, 1730, 1629, 1525, 1346, 1267, 1093,
854, 711.
Compound 3k: Light yellow solid, m.p. 197-198 °C. 1H
NMR (400 MHz, DMSO-d6): δ H 9.00 (s, 1H, N=CH), 8.41
(d, 2H, J 8.8 Hz, Ar-H), 8.29 (d, 2H, J 8.8 Hz, Ar-H), 7.85 (d,
2H, J 8.8 Hz, Ar-H), 7.62 (d, 2H, J 8.4 Hz, Ar-H). 13C NMR
(100 MHz, DMSO-d6): δ 161.7, 157.8, 150.5, 136.8, 133.6,
130.8, 130, 129.3, 128.7, 124.0. FT-IR (KBr, νmax, cm-1): 3084,
1757, 1610, 1527, 1346, 1257, 1082, 931, 831, 707, 518.
RESULTS AND DISCUSSION
Our optimization of the reaction conditions involved us
studying the oxime esterification of benzoic acid with
benzophenone oxime. Initially, when benzoic acid was treated
with 1 equiv of benzophenone oxime, 1 equiv of 2-(1H-benzo-
triazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate
and 1 equiv of triethylamine at room temperature for 0.5 h in
acetonitrile, 71.6 % of product was obtained. It is worth
pointing out that lengthening the reaction time to 5 h did not
affect the yield of the reaction. We attempted to use DIPEA,
DBU, methylamine, or pyridine as the base instead of triethyl-
amine, nevertheless, they gave a lower yield. Dichloromethane
and tetrahydrofuran were also evaluated as reaction solvents
and the yields were 62.1 and 62.7 %, respectively. However,
acetonitrile was the most effective solvent. Further, the
influence of 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyl-
uronium tetrafluoroborate and triethylamine in reaction of
benzoic acid and cyclohexanone oxime was also investigated
and the results are recorded in Table-1. Table-1 reveals, in the
absence of 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyl-
uronium tetrafluoroborate, no product was observed (Table-1,
Entry-1). Therefore, the results indicated that the best condi-
tions for obtaining oxime ester entailed us using 1 equiv of
Compound 3e: Colorless liquid (lit. colorless liquid22).
1H NMR (400 MHz, DMSO-d6): δ H 8 (d, 2H, J 7.4 Hz,Ar-H),
7.68 (t, 1H, J 7.4 Hz, Ar-H), 7.55 (t, 2H, J 7.7 Hz, Ar-H), 2.08
(s, 3H, CH3), 2.03 (s, 3H, CH3). 13C NMR (100 MHz, DMSO-
d6): δ 164.5, 163.8, 133.1, 129.4, 129.2, 128.4, 21.9, 16.9.
FT-IR (KBr, νmax, cm-1): 3068, 2926, 1741, 1627, 1450, 1373,
1274, 1247, 1085, 1062, 1022, 908, 864, 709.
Compound 3f: White solid, m.p. 143-145 °C (lit.23 143-
1
144 °C). H NMR (400 MHz, DMSO-d6): δ H 8.95 (s, 1H,
N=CH), 8.06 (d, 2H, J 7.6 Hz, Ar-H), 7.84 (d, 2H, J 8.1 Hz,
Ar-H), 7.73 (t, 1H, J 7.2 Hz, Ar-H), 7.60 (t, 4H, J 7.9 Hz, Ar-
H). 13C NMR (100 MHz, DMSO-d6): δ 163.0, 157.0, 136.5,
133.8, 130.0, 129.2, 129.0, 128.0. FT-IR ( KBr, νmax, cm-1):
3055, 1742, 1595, 1489, 1255, 1082, 934, 831, 702, 514.
Compound 3g: White solid, m.p. 69-70 °C (lit.24 69-
70 °C). 1H NMR (400 MHz, DMSO-d6): δ H 7.96 (d, 2H, J 8
Hz, Ar-H), 7.26 (d, 2H, J 6 Hz, Ar-H), 2.68 (t, 2H, J 6 Hz,
CH2), 2.48 (t, 2H, J 6 Hz, CH2), 2.43 (s, 3H, CH3), 1.63-1.84

Vol. 26, No. 21 (2014)
A Novel Synthesis of Carboxylic Acid Oxime Esters 7127
TABLE 2
REACTIONS OF CARBOXYLIC ACIDS AND
OXIMES AT ROOM TEMPERATURE
benzoic acid, 1 equiv of benzophenone oxime, 1.1 equiv of 2-
(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetra-
fluoroborate and 2 equiv of triethylamine at room temperature
for 0.5 h in acetonitrile and benzophenone O-benzoyloxime
was obtained in 89.5 % yield (Table-1, Entry-11).
Time Yield
Entry
R1
R2
R3
Product
(h)
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
5.0
5.0
5.0
(%)
89.5
88.6
83.6
85.2
65.5
92.1
86.6
87.4
90.8
87.0
77.5
-
1
2
C6H5
C6H5
C6H5
C6H5
C6H5
CH3
H
3a
3b
3c
3d
3e
3f
Table-1
3
C6H5
C6H5
OPTIMIZATION OF 2-(1H-BENZOTRIAZOLE-1-YL)-1,1,3,3-
TETRAMETHYLURONIUM TETRAFLUOROBORATE
(TBTU) LOADING AND TRIETHYLAMINE (TEA)
AMOUNT IN THE REACTION OF BENZOIC ACID
WITH BENZOPHENONE OXIMEa
4
C6H5
(CH2)5
CH3
5
C6H5
CH3
H
6
C6H5
4-ClC6H5
(CH2)5
C6H5
7
4-CH3C6H4
4-CH3C6H4
3g
3h
3i
8
CH3
H
Entry
1
TBTU (equiv.)
TEA (equiv.)
Yield (%)
-
9
4-CH3C6H4 4-ClC6H5
4-NO2C6H4 (CH2)5
4-NO2C6H4 4-ClC6H5
0.0
0.5
0.75
1.0
1.1
1.2
1.1
1.1
1.1
1.1
1.1
1.1
1.0
1.0
1.0
1.0
1.0
1.0
1.2
1.4
1.6
1.8
2.0
2.2
10
11
12
13
14
3j
2
61.7
69.6
71.6
72.2
72.2
74.6
79.5
82.3
86.8
89.5
89.5
H
3k
3l
3
CH3
Cl-CH2
C5H11
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
4
-
3m
3n
5
-
6
7
8
We also investigated the mechanism of the reaction
(Scheme-II). According to the literature27,28, the reaction
mechanism for oxime ester bond formation through 2-(1H-
benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoro-
borate activation mechanism has been proposed for this
coupling reaction. The proposed mechanism may consist of
the following steps: Firstly, addition of the carboxylate anion
to 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium
tetrafluoroborate. Secondly, decomposition of the resulting
intermediate acyloxy-aminium/uronium salt. After that,
addition of the released anion to the carbonyl center, followed
by loss of water-miscible tetramethylurea. Finally, oximes react
with the activated carbonyl group to give the corresponding
oxime esters and 1-hydroxybenzotriazole (HOBt) as a water-
soluble by-product.
9
10
11
12
a Reaction conditions: Reactions carried out using benzoic acid (1.0
equiv) and benzophenone oxime (1.0 equiv) in acetonitrile at room
temperature for 0.5 h.
Subsequently, we investigated the scope of the reaction
substrates under the optimized conditions, as shown in Table-2.
From Table-2, we can see, by this method, that the aromatic
carboxylic acids gave satisfactory results (Table-2, Entries 1-
11). Unfortunately, when acetic acid, chloroacetic acid or
hexanoic acid was used in this reaction, no corresponding
product was isolated (Table-2, Entries-12-14). Benzoic acid
reacted with a series of oximes, of which the non-cyclic
aliphatic oxime gave the lowest yield (Table-2, Entry-5). We
were also interested to study the electronic effect of substi-
tutions on aromatic ring of benzoic acids. The substitution of
an electron-donating methyl group and the substitution of an
electron-withdrawing nitro group did not affect the yields
(Table-2, Entry-7-11).
Geometric optimization of the products was carried out
at the b3lyp/6-311G(d,p) level along with analytic vibrational
frequency calculations with the Gaussian-03 software
package29. The frequencies were scaled by a factor of 0.967.
The calculated vibrational frequencies are consistent with the
experimental infrared data.
N
N
N
O
N
O
BF4
O
N
N
O
N
N
TEA
O
N
+
BF4
O
N
R1
R1
OH
O
N
BF4
N
R1
N
N
N
N
O
N
1
4
3
2
O
N
N
N
N
N
OH
O
N
R3
N
O
N
N
+
O
N
N
+
O
OH
N
R1
R2
N
O
R1
R2
R3
6
5
Scheme-II: Proposed reaction mechanism for oxime ester bond formation through 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate activation

7128 Shi et al.
Asian J. Chem.
13. S.K. Kundu, M. Rahman, P. Dhara, A. Hajra and A. Majee, Synth.
Commun., 42, 1848 (2012).
Conclusion
In summary, a practical method for the synthesis of carbo-
14. S. Nakaiida, S. Kato, O. Niyomura, M. Ishida, F. Ando and J. Koketsu,
Phosphorus Sulfur Rel. Elem., 185, 930 (2010).
xylic acid oxime ester using commercially available and
inexpensive carboxylic acid, oxime ester and 2-(1H-benzotria-
zole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate in
acetonitrile at room temperature under mild conditions has
been developed.Additionally, the novel method offers several
advantages, such as the generation of water-soluble and low-
toxicity by-products, ease of product separation and short
reaction time.
15. B. Kongkathip, S. Akkarasamiyo, K. Hasitapan, P. Sittikul, N.
Boonyalai and N. Kongkathip, Eur. J. Med. Chem., 60, 271 (2013).
16. H. Chen, X.F. Xu, L. Liu, G. Tang and Y.F. Zhao, RSC Adv., 3, 16247
(2013).
17. R. Subirós-Funosas, L. Nieto-Rodriguez, K.J. Jensen and F. Albericio,
J. Pept. Sci., 19, 408 (2013).
18. S. Balalaie, M. Mahdidoust and R. Eshaghi-Najafabadi, Chin. J. Chem.,
26, 1141 (2008).
19. S. Balalaie, M. Mahdidoust and R. Eshaghi-Najafabadi, J. Iran. Chem.
Soc., 4, 364 (2007).
ACKNOWLEDGEMENTS
20. M.Z.A. Badr, M.M. Aly, S.A. Mahgoub, A.M. Fahmy andA.A.Atallah,
Bull. Chem. Soc. Jpn., 61, 1779 (1988).
This work was supported by the Natural Science Founda-
tion of Shandong Province, China (No. ZR2012BM004) and
the Project of Shandong Province Higher Educational Science
and Technology Program (No. J12LD07).
21. E.A. Dikusar and N.A. Zhukovskaya, Russ. J. Org. Chem., 44, 1389
(2008).
22. S. Nakaiida, S. Kato, O. Niyomura, M. Ishida, F. Ando and J. Koketsu,
Phosphorus Sulfur Rel. Elem., 185, 930 (2010).
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