One-pot esterification and amide formation via acid-catalyzed dehydration and ritter reactions

Article abstract of DOI:10.1080/00397911.2013.837485

Esterification of carboxylic acid is achieved using acetonitrile as a water trap. Water liberated during esterification is consumed in cyanide hydrolysis, thereby driving the esterification to completion. Substrates having carboxylic acid and nitrile groups undergo intramolecular dehydration and rehydration to amido esters in the absence of acetonitrile. Cyano acids also undergo esterification and Ritter reaction in one pot when excess alcohol is used. For the first time, we have observed an interesting Ritter reaction of primary alcohols, leading to ester amide product in one pot. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications for the following free supplemental resource(s): Full experimental and spectral details.]

Full text of DOI:10.1080/00397911.2013.837485

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One-Pot Esterification and Amide
Formation via Acid-Catalyzed
Dehydration and Ritter Reactions
Pankaj Dawar a , M. Bagavan Raju b & Ramesha Andagar Ramakrishna
a
a R L Fine Chem , Bangalore , India
b Gland Institute of Pharmaceutical Sciences , Naraspur , India
Published online: 20 Feb 2014.
To cite this article: Pankaj Dawar , M. Bagavan Raju & Ramesha Andagar Ramakrishna (2014) One-Pot
Esterification and Amide Formation via Acid-Catalyzed Dehydration and Ritter Reactions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
44:6, 836-846, DOI: 10.1080/00397911.2013.837485
To link to this article: http://dx.doi.org/10.1080/00397911.2013.837485
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Synthetic Communications1, 44: 836–846, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2013.837485
ONE-POT ESTERIFICATION AND AMIDE FORMATION
VIA ACID-CATALYZED DEHYDRATION AND RITTER
REACTIONS
Pankaj Dawar,1 M. Bagavan Raju,2 and
Ramesha Andagar Ramakrishna1
1R L Fine Chem, Bangalore, India
2Gland Institute of Pharmaceutical Sciences, Naraspur, India
GRAPHICAL ABSTRACT
Abstract Esterification of carboxylic acid is achieved using acetonitrile as a water trap.
Water liberated during esterification is consumed in cyanide hydrolysis, thereby driving
the esterification to completion. Substrates having carboxylic acid and nitrile groups
undergo intramolecular dehydration and rehydration to amido esters in the absence of acet-
onitrile. Cyano acids also undergo esterification and Ritter reaction in one pot when excess
alcohol is used. For the first time, we have observed an interesting Ritter reaction of primary
alcohols, leading to ester amide product in one pot.
[Supplementary materials are available for this article. Go to the publisher’s online edi-
tion of Synthetic Communications1 for the following free supplemental resource(s): Full
experimental and spectral details.]
Keywords Amide; dehydration; esterification; hydrolysis; rehydration
INTRODUCTION
Esterification and Ritter reactions are important and useful synthetic transfor-
mations in both industry and academic research.[1] Many esters and amides are impor-
tant intermediates in various industries such as pharmaceuticals, fine chemicals, aroma
chemicals, plasticizers, and food.[2] There is always a need for new scientific methods to
Received January 14, 2013.
Address correspondence to Ramesha Andagar Ramakrishna, R L Fine Chem, No. 15, KHB
Industrial Area, Yelahanka New Town, Bangalore 560064, India. E-mail: ramesha63@hotmail.com
836

ESTERIFICATION AND AMIDE FORMATION
837
improve the yield of esters and amides. Conventionally esters are synthesized by
refluxing the carboxylic acids with alcohol in the presence of a catalytic amount of
acid.[3] However, there are several methods for the esterification under neutral,
acidic, and alkaline conditions in the literature. Reagents such as diazomethane,[4]
trimethylsilyl diazomethane,[5] methyl triazine,[6] and dialkyl formamidedimethyl
acetal[7] are known to convert carboxylic acids to esters under neutral conditions.
Besides these reagents, catalysts such as polymer-bonded dimethylaminopyridines
also used for facile esterification by N,N0-dicyclohexylcarbodiimide (DCC).[8] Ion
exchange resin–facilitated esterification has also been reported.[9] Haworth methyla-
tion, which employs dimethylsulfate (DMS) and LiOH in tetrahydrofuran (THF),[10]
proceeds well to furnish almost quantitative yield of esters without any side reac-
tions. Under alkaline conditions, strong electrophiles such as alkyl iodides[11] or
the corresponding mesylates or tosylates[12–15] are used for the esterification. These
hard electrophiles such as methyl iodide and alkyl sulfonates are carcinogens and
genotoxic, and dimethyl sulfate is extremely hazardous.[16–18] Salts such as Appel’s
salt ((4,3-dichloro-1,2,3-dithiazolium chloride) and 2,6-lutidine are documented in
literature for esterification under milder conditions.[19] Graphite bisulfate has been
reported to play a dual role as acid catalyst and dehydrating agent.[20] Phosphonium
anhydrides are also reported for esterifications.[21] Cyclic imides such as O-methyl
caprolactam,[22] isourea ethers,[23] modified thiazynes, and S-propargyl xanthates[24]
are a few more reagents that are used successfully for the esterification. Further,
asymmetric esterification by organo catalysts[25] and enzymes for optically active
esters was also reported in the literature.[26] Transesterification mediated by 1,
3-disubstituted tetraalkyldistannoxanes is also documented.[27] Ritter reactions
provide an easy access for the synthesis of amides. This reaction is complementary
to esterification. Here water is one of the substrates, whereas in esterification water
is liberated as a side product. This reaction is normally carried out with alkyl=aryl cya-
nides and tert-alcohol=ester or an activated secondary alcohols in the presence of acid
catalyst.[28] There is a recent review on the Ritter reaction that describes
the importance of the Ritter reaction and the catalysts that bring about this
transformation.[29a] Some of the classical methods involve catalysts like H2SO4,[29b]
phosphotungstic acid (PTA,H3PW12O40),[30,31] Lewis acid, boron trifluoride,[32]
and cobalt(II).[33] There are many alternative acidic reagents such as MeSO3H=
Al2O3,[34] (CF3SO2),[35] and zeolites.[36] Many modified methods include
Ph2C þ ClSbCl6,[37] tertiary esters,[38] and trimethylsily cyanides.[39] Recently we have
published an interesting one-pot reaction of esterification and Ritter reaction using
tert-butyl methyl ether (TBME) as solvent and reagent.[40] To the best of our knowledge,
there are no reports on the Ritter reaction of an unactivated primary alcohols so far.
RESULTS AND DISCUSSION
Acid-catalyzed esterification is equilibrium controlled, therefore a large excess
of alcohol is used and the reaction is not quantitative. If water is removed as azeo-
trope, then this equilibrium can be shifted toward product formation. We decided to
see whether if water is removed from the reaction by the way of Ritter reaction, is it
possible to do two reactions in one pot? If so, this would also drive the esterification

838
P. DAWAR, M. B. RAJU, AND R. A. RAMAKRISHNA
Table 1. Optimization of esterification of benzoic acid
Entrya
H2SO4 (mmol)
Temp. (ꢀC)
Time (h)
Yield (%)b
1
2
3
4
5
6
18
38
57
82
94
94
80–85 ꢀC
80–85 ꢀC
80–85 ꢀC
80–85 ꢀC
80–85 ꢀC
80–85 ꢀC
18
18
18
18
18
18
68
75
79
89
98
58c
aBenzoic acid (81 mmol), alcohol (90 mmol), and acetonitrile (162 mmol) are used for the experiments.
bIsolated yield. All reactions were carried out for 16–18 h.
cIn the absence of acetonitrile.
to completion, leading to dehydration and rehydration reaction in one pot. To
achieve this, easily available acetonitrile was chosen as the dehydrating agent for
the dehydration and rehydration reaction. When benzoic acid 1a is reacted with
1.1 equivalent of butyl alcohol (Table 1, entry 1) and acetonitrile (2 eq.) in the pres-
ence of 20% of concentrated sulfuric acid, butyl benzoate 1b is formed in 68%
yield. Yield improved to 98% when 1.16 equivalent of sulfuric acid is used
(Table 1, entry 5). Interestingly, when compared with the normal method of esteri-
fication, the presence of acetonitrile in reaction gives quantitative conversion of
acid to ester (Table 1, entry 6). It is interesting to see that excess alcohol is not
required for the completion of the reaction like in the conventional method. In
the optimized conditions, several substrates were converted to their corresponding
esters in almost quantitative yield (Table 2, entries 1–23). Many functional groups
such as olefinic (5a), amido (6a), tert-hydroxyl (benzillic acid) (7a), halo (8a), and
nitro (9a) esters are not affected during the reaction condition. Tryptophan gives a
moderate yield of the corresponding ester (Table 2, entry 13). This is probably
because of the strong acidic condition during the reaction. Even anhydrides are
converted to corresponding esters under similar reaction conditions (Table 2, entry
14). Products 1b–21b (Table 2) are all for intermolecular dehydration and rehydra-
tion where acid is converted to ester and the acetonitrile group is converted to
amide. When alcohol quantity is increased to about 2 equivalents, we obtained
ester amide as the major product (entries 22b–23b) in the absence of acetonitrile.
When alcohol quantity is increased to about 4 equivalents for cyano substrates,
we observed an interesting esterification and Ritter reaction product. When n-butyl
alcohol is reacted with cyanoacetic acid, it gave interesting n-butyl ester and 2-ami-
nobutane Ritter product in moderate yield (entry 24, Table 3). To the best of our
knowledge and available literature, this is the first example of Ritter reaction of an
unactivated primary alcohol. When a secondary alcohol like isopropanol or cyclo-
hexanol is used, it gave cyclohexyl ester and amide product (entries 25, 27, Table 3).

839

840

ESTERIFICATION AND AMIDE FORMATION
841
Table 3. Esterification and Ritter reaction in one pot
Yield
(%)a
Entry
24
Substrate
Alcohol
Product
16a
16a
24b
53
25b
25
62
16a
26b
26
60
16a
27b
27
50
28a
28b
28
29
58
61
29a
29b
aAll the reactions were carried with 80 mmol of substrate, 360 mmol of alcohol, and 94 mmol of sulfuric
acid (98%). Yield refers to isolated yield.
Very interestingly, when isobutyl alcohol is used for the reaction, tert-amido isobu-
tyl ester is formed in one pot (entries 26, 28, 29, Table 3). A tentative mechanism
for the acid-catalyzed reaction is given in Scheme 1.

842
P. DAWAR, M. B. RAJU, AND R. A. RAMAKRISHNA
Scheme 1. Tentative mechanism for the dehydration, rehydration–esterification, and Ritter reaction.
CONCLUSIONS
Almost quantitative conversion of carboxylic acid to ester is achieved using
acetonitrile as a water trap. Substrates having a nitrile group undergo intramolecu-
lar dehydration and rehydration to give amido ester in good yield. We have been
able to show for the first time the Ritter reaction with unactivated primary alcohol.
To sum up, we have developed a new reaction condition for esterification and
hydrolysis in one pot in good yield, which should have great application in organic
synthesis.
EXPERIMENTAL
Reagents and solvents were obtained from commercial sources and used with-
out further purification. Thin-layer chromatography (TLC) was performed on pre-
coated Merck silica-gel 60 F254 plates. Compounds were visualized using
ultraviolet (UV) light (254 nm). Column chromatography refers to flash column
chromatography on silica gel (200–300 mesh, Merck). NMR spectra were recorded
on Jeol LA-300, Bruker AV400 spectrometer in CDCl3. Tetramethylsilane [TMS
1
(Me4Si); d ¼ 0.00 ppm] served as internal standard for H NMR. Chemical shifts
were determined relative to the residual solvent peaks (CHCl3, d ¼ 7.26 ppm for
hydrogen atoms, d ¼ 77.0 for carbon atoms). The following abbreviations are used
to indicate signal multiplicity: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet;
and bs, broad singlet. Mass spectra were measured with Micromass Q-Tof
(ESI-HRMS) and GC=MS were measured with GCMSQP 2010 Plus gas chromato-
graphy mass spectrometer (Shimadzu).

ESTERIFICATION AND AMIDE FORMATION
843
Procedure for Ester (Table 2, entries 1–12)
Sulfuric acid (98%, 94 mmol, 5 ml) was added to a well-stirred mixture of acid
(80 mmol), and alcohol (90 mmol), and acetonitrile (162 mmol), at room temperature.
The mixture was slowly heated to 80–85 ꢀC and maintained at 80–85 ꢀC for 16–18 h.
The reaction mixture was cooled and added to 20% sodium carbonate solution
(100 mL). The reaction mass was extracted in CH2Cl2 (50 mL ꢁ 2). The combined
organic layer was washed with water (100 mL), dried over sodium sufate, and concen-
trated under reduced pressure to obtain corresponding ester (97–99%) as the only pro-
duct. Most examples gave NMR pure product. Where minor impurities are formed,
the product was purified over silica gel using ethyl acetate–petroleum ether (1:9).
Procedure for Amino Acid/Anhydrides (Table 2, entries 13–15)
Sulfuric acid (98%, 94 mmol, 5 ml) was added to a well-stirred mixture of
amino acid (80 mmol, or anhydride), alcohol (180 mmol), and acetonitrile
(162 mmol) at room temperature, and the temperature was maintained at 80–85 ꢀC
for 16–18 h. The reaction mixture was cooled and added to 20% sodium carbonate
solution (100 mL). The reaction mass was extracted in CH2Cl2 (50 mL ꢁ 2). The
combined organic layer was washed with water (100 mL), dried over sodium sufate,
and concentrated under reduced pressure to obtain the product (60–99%). Most
examples gave the pure product. Where minor impurities are formed, the product
was purified over silica gel using ethyl acetate–petroleum ether (1:9).
Procedure for Cyano Acid (Table 2, entries 16b–21b)
Sulfuric acid (98%, 94 mmol, 5 ml) was added to a well-stirred mixture of cyano
acid (80 mmol) in alcohol (90 mmol) and acetonitrile (162 mmol) at room tempera-
ture, and temperature was maintained at 80–85 ꢀC for 16–18 h. The reaction mixture
was cooled and added to 20% sodium carbonate solution (100 mL). The reaction
mass was extracted in CH2Cl2 (50 mL ꢁ 2). The combined organic layer was washed
with water (100 mL), dried over sodium sufate, and concentrated under reduced
pressure to obtain the product (85–95%). Most examples gave NMR pure product.
Where minor impurities are formed, the product was purified over silica gel using
ethyl acetate–petroleum ether (1:9).
Butyl 2-Cyano-3-(3,4-dimethoxyphenyl)acrylate[41] (17b)
1
=
H NMR (400 MHz, CDCl3) d (ppm): 8.14 (s, 1H, C CH), 7.80 (s, 1H, Ar-H),
7.47 (d, J ¼ 8.4 Hz, 1H, Ar-H), 7.28 (s, 1H, Ar-H), 4.31 (t, J ¼ 6.6 Hz, 2H, OCH2),
3.96 [s, 6H, (OCH3)2], 1.76–1.71 (m, 2H, -CH2), 1.49–1.43 (m, 2H, -CH2), 0.98 (t,
J ¼ 7.4 Hz, 3H, CH3); 13C NMR (100 MHz, CDCl3) d (ppm): 163.1, 154.5, 153.6,
149.2, 127.8, 124.5, 116.2, 56.0, 55.9, 30.5, 19.0, 13.6. MS (m=z, %): 289 (100%);
HRMS calc. for C16H19NO4Na: 312.1212; found: 312.1212(M þ Na).
Procedure for Cyano Acid (Table 2, entries 22b–23b)
Sulfuric acid (98%, 94 mmol, 5 ml) was added to a well-stirred mixture of cyano
acid (80 mmol) in alcohol (190 mmol) at room temperature, and temperature was

844
P. DAWAR, M. B. RAJU, AND R. A. RAMAKRISHNA
maintained at 80–85 ꢀC for 16–18 h. The reaction mixture was cooled and added to
20% sodium carbonate solution (100 mL). The reaction mass was extracted in
CH2Cl2 (50 mL ꢁ 2). The combined organic layer was washed with water
(100 mL), dried over sodium sufate, and concentrated under reduced pressure to
obtain the product (95–98%). Most examples gave NMR pure product. Where minor
impurities are formed, the product was purified over silica gel using ethyl acetate–
petroleum ether (1:9).
Methyl 3-Amino-2-cyclohexylidene-3-oxo-propanoate[41] (22b)
1H NMR (400 MHz, CDCl3) d (ppm): 5.89 (d, J ¼ 32.9 Hz, 2H, NH2), 3.76 (s,
3H, OCH3), 2.61 [t, J ¼ 5.8 Hz, 2H, (CH2) (Cyclic)], 2.44 [t, J ¼ 5.9 Hz, 2H, (CH2)
(Cyclic)], 1.72–1.59 [m, 6H, (CH2)3]; 13C NMR (100 MHz, CDCl3) d (ppm): 168.4,
166.2, 160.1, 123.1, 51.9, 33.4, 31.8, 28.4, 28.1, 26.0. HRMS (m=z): calc. for
C10H15NO3Na: 220.0950. Found: 220.0949 (M þ Na).
Procedure for Cyano Acid (Table 3, entries 24b–29b)
Sulfuric acid (98%, 94 mmol, 5 ml) was added to a well-stirred mixture of cyano
acid (80 mmol) in alcohol (360 mmol) at room temperature, and the temperature was
maintained at 80–85 ꢀC for 16–18 h. The reaction mixture was cooled and added to
20% sodium carbonate solution (100 mL). The reaction mass was extracted in
CH2Cl2 (50 mL ꢁ 2). The combined organic layer was washed with water
(100 mL), dried over sodium sufate, and concentrated under reduced pressure to
obtain the product (50–62%). Most examples gave NMR pure product. Where
minor impurities are formed, the product was purified over silica gel using ethyl
acetate–petroleum ether (1:9).
Butyl 3-(Sec-butylamino)-3-oxopropanoate:[41] (24b)
1H NMR (400 MHz, CDCl3) d (ppm): 4.14 [t, 2H, (OCH2)], 3.97–3.90 (m, 1H,
CH), 3.29 (s, 2H, CO-CH2), 1.67–1.60 (m, 2H, CH2), 1.53–1.43 (m, 2H, CH2),
1.41–1.33 (m, 2H, CH3), 1.25 (s, 1H, NH), 1.13 (s, 2H, CH2), 0.95–0.89 (m, 3H,
CH3); 13C NMR (100 MHz, CDCl3) d (ppm): 169.8, 164.2, 65.3, 46.7, 41.2, 30.4,
29.4, 20.2, 19.0, 13.6, 10.2. HRMS calc. for C11H21NO3 Na: 238.1419. Found:
238.1415 (M þ Na).
SUPPORTING INFORMATION
Full experimental details, 1H and 13C NMR spectra, GC=MS, and HRMS (entries
1b–29b) can be found via the Supplementary Content section of this article’s Web page.
ACKNOWLEDGMENTS
The authors thank the director of R. L. Fine Chemicals, Anjan K. Roy,
for financial support, and K. R. Prabhu, Indian Institute of Science, for useful
discussion.

ESTERIFICATION AND AMIDE FORMATION
845
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