Selective conversion of primary amides to esters promoted by KHSO4

Article abstract of DOI:10.24820/ark.5550190.p010.392

Primary amides, either aliphatic or aromatic, are easily converted to the corresponding esters via reflux in lower primary alcohols in the presence of KHSO₄. Secondary amides lead to complicated mixtures under analogous conditions, whereastertiary amides were inert. Use of isopropyl alcohol resulted inthe formation of product atslower rate and lower yieldalong withside products, whereas, use of tertiary alcoholsdid not give successful conversion andallyl and benzyl alcohol provided complex mixtures.

Full text of DOI:10.24820/ark.5550190.p010.392

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Selective conversion of primary amides to esters promoted by KHSO4
Narsimha Sattenapally, Jhanvi Sharma, and Yuqing Hou*
Meyers Institute for Interdisciplinary Research in Organic and Medicinal Chemistry, and Department of
Chemistry and Biochemistry, Southern Illinois University, Carbondale, IL 62901 USA
Email: houyq@siu.edu
Received 11-13-2017
Accepted 04-15-2018
Published on line 06-17-2018
Abstract
Primary amides, either aliphatic or aromatic, are easily converted to the corresponding esters via reflux in
lower primary alcohols in the presence of KHSO4. Secondary amides lead to complicated mixtures under
analogous conditions, whereas tertiary amides were inert. Use of isopropyl alcohol resulted in the formation
of product at slower rate and lower yield along with side products, whereas, use of tertiary alcohols did not
give successful conversion and allyl and benzyl alcohol provided complex mixtures.
Keywords: Amide, ester, alcoholysis, KHSO4, selective conversion
DOI: https://doi.org/10.24820/ark.5550190.p010.392
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Introduction
Carboxylic acids and their derivatives, such as amides and esters, play important roles in nature as well as in
artificial chemicals. The interconversion of these derivatives is a frequently encountered transformation by
chemists. Not surprisingly, the conversion of carboxylic acids or esters to amides proceeds usually much
easier than the reverse process, due to the high stability of the amide functionality, which has been well
utilized by nature to build the back bone of protein structures. The hydrolysis of amides to carboxylic acids
usually requires strongly acidic or basic conditions,1,2 enzymatic,3 or metal catalysis,4 except in special cases
where N-C=O conjugation is prevented because of constraint5 or where the nitrogen is a part of a heterocyclic
structure such as imidazole.6 Similarly, strongly acidic conditions are usually required to effect the alcoholysis
of amides to esters. Examples include the use of HCl gas,7,8 BF3 gas,9 SOCl2,10 Me3SiCl,11 nitrite/Me3SiCl,12 or
TsOH.H2O13 for such transformations. Additionally, TiCl4, in combination with one equiv of aqueous HCl, was
found to catalyze conversion of amides to esters.14 More recently, Zn(OTf)2 has been reported to catalyze the
esterification of a special type of amides, -hydroxyethylamides.15 Acidic Amberlyst resins16 have been
reported as milder reagents to convert amides and hydrazides to esters, which required up to 168 h of heating
to achieve good yields in some instances. Truly mild reaction and selective conversion has been only achieved
with enzymatic method.17 Thus, milder, economical, selective, and convenient methods to covert amides to
esters are still in need.
Previously, it has been reported that carboxamides go through Hofmann rearrangement in the presence
of Oxone® and catalytic amount of o-t-butyliodobenzene in refluxing methanol.18 Interestingly, when 2-
phenoxyacetamide (1a) was treated under such conditions, methyl 2-phenoxyacetate (2a) was also obtained
in 24% yield as a side product (Scheme 1). Further experiments showed that this was a not a specific reaction
that only occurred with 1a. It also occurred with other aliphatic amides, but not with aromatic amides under
analogous reaction conditions. We decided to explore the scope and limit of the reaction and wish to report
the results of this study herein.
Scheme 1. Treatment of 1a with Oxone/o-t-butyliodobenzene (cat.) in MeOH.
Results and Discussion
It was at first questioned how this reaction proceeded and the role of each reagent. Tests showed that o-t-
butyliodobenzene was not needed for the esterification. Furthermore, replacing Oxone with KHSO4 led to the
formation of the product in higher yield. Thus, it was concluded that this was an acid promoted alcoholysis of
the amides, which became the focus of the study. Optimization of the reaction was carried out with 2-
(naphthalen-1-yl)acetamide (1b) and 4, 6, 8, and 10 equivalents of KHSO4 in refluxing methanol (Table 1). The
reaction was complete in 12 h when 8 equiv of KHSO4 were used. Higher excess of KHSO4 did not increase the
o
yield, nor shorten the reaction time. Microwave irradiation at 90 C did not seem to accelerate the reaction
much. Remarkably, with KHSO4, the methyl ester was the only product and the pure product can be obtained
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by trituration of the semi-solid residue with a suitable organic solvent after removal of methanol in vacuo,
whereas with Oxone, a few side products, although in tiny quantities, were also produced, which necessitates
a column chromatography purification process to obtain the pure product.
Subsequently, the reaction of various amides with methanol was explored under optimized conditions
(Table 2). Generally, the reaction went faster with aliphatic amides (Table 2, entries 1 - 4), and slower with
aromatic amides (Table 2, entries 5 - 14). It should also be noted that the reaction was very sluggish with
secondary amides, and did not proceed at all with the tertiary amide tested. Additionally, no reaction was
observed when sulfonamide or benzonitrile was used instead of carboxamides.
Table 1. Optimization of reaction conditions with 1b and KHSO4 in refluxing methanol
Entry
Equiv of KHSO4
Reaction time (h)
Isolated yield (%)
1
2
3
4
6
8
10
8
24
17
12
12
5
71
100
100
91
4
5a
33
aMicrowave reactor was used for this reaction at 90 oC.
Table 2. Conversion of amides to methyl esters with methanol/KHSO4 at 65 oC
Entry
Amide
Structure
Reaction time Yield (%)
(h)
86
14
1
1a
2
100
1b
1c
1d
1e
12
3
4
5
14
16
48
93
93
88
6
1f
24
86
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Table 2. Continued
Entry
Amide
1g
Structure
Reaction time Yield (%)
(h)
60
7
8
78
86
1g
60
36
9
1i
90
10
11
1j
48
72
91
89
1k
12
13
14
1l
60
72
48
92
90
83
1m
1n
15
16
1o
1p
48
48
Trace
Trace
17
1q
48
0
18
19
1r
1s
16
24
0
0
It was then tested whether this method works with other alcohols with 1f (Table 3). The method provided
the corresponding esters in high yields for primary alcohols, and a moderate yield with isopropyl alcohol in 48
h. No product was observed with t-butyl alcohol, while allyl alcohol and benzyl alcohol provided complex
mixture. A possible explanation for this was due to water formation from these alcohols, via either
intramolecular or intermolecular dehydration (vide infra), in the presence of KHSO4. Surprisingly, ethylene
glycol also produced only a trace of ester. Unlike other primary alcohols, however, ethylene glycol (15 mL)
dissolved all KHSO4 (1.1 g) when heated to 85 oC, which prompted us that the reaction occurred at the surface
of the solid KHSO4, not in solution. Indeed, adding water to the reaction system significantly slowed down the
reaction (entry 12, table 3 vs. entry 1, table 3). It should be noted that 1f did not dissolve well in MeOH/H2O
(2:1). To exclude the possibility that low solubility of 1f in MeOH/H2O caused the low yield, we also tested 1n
with MeOH/H2O (2:1), where all 1n dissolved in the mixed solvent. Again, lower conversion rate was observed.
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When treated with water, 1n was hydrolyzed to the corresponding carboxylic acid only in 34% in 48 h. Similar
treatment of 1a, an aliphatic amide with good water solubility, produced the carboxylic acid in 100% yield in
48 h, confirming our observation that aliphatic amides reacted faster than aromatic amides (Table 2). These
observations support the explanation that water formation from alcohols was a possible cause for the lower
yield or no product formation when dehydration of alcohols is significant.
Table 3. Alcoholysis (hydrolysis) of selected amides in the presence of solid KHSO4
Entry Amide Solvent
Temp.(oC) Product 2
Yield (%)
1
2
3
4
5
6
1f
Methanol
Ethanol
2-Chloroethanol
Isopropyl alcohol
t-Butyl alcohol
Allyl alcohol
65
78
127
83
82
97
2f, R’=Me
2fa, R’=Et
2fb, R’=2-ClEt
2fc, R’=i-Pr
0
86
92
83
34
0
Complex
mixture
7
8
9
10
11
Benzyl alcohol
n-Propanol
85
97
118
85
Complex mixture
2fd R’=1-Pr
2fe R’=1-Bu
trace
77
75
n-Butanol
Ethylene glycola
Ethylene
glycol- 85
trace
acetonitrile (1:5)
12
13
14
15
Methanol-Water (2:1)
Methanol-Water (2:1)
Waterb
65
65
100
100
2f 15a
2n
2n
2a
41b
60b
0b
1n
1a
Waterb
0b
aAll KHSO4 was dissolved during the reaction. bThe corresponding carboxylic acids, 2-naphthoic acid,
m-toluic acid, and phenoxyacetic acid, were obtained in 15%, 25%, 34%, and 100% respectively.
KHSO4 is easily available, inexpensive, safe, and non-toxic reagent and has been known to catalyze organic
reactions, such as acetal formation, Michael addition, heterocycle formation.20 Goswami et al.21 also reported
that KHSO4-SiO2-MeOH deprotects efficiently esters to alcohols, presumably through trans-esterification
-
reaction. HSO4 has a pKa of 1.9919 and is considered as a medium-strength acid. Thus, this is a milder method
to convert amides, particularly primary amides, to esters of primary alcohols, comparing to reported
methods.7-14,16 We tested this method on some compounds with two or more amide (carbamate)
functionalities (Table 4) to study the selectivity of this method. Boc and acetyl protection groups were
removed under the reaction condition (entries 1, 3, 4, table 4), while most of Cbz and benzoyl protection
groups remained (entries 2, 5 - 8, table 4). It is interesting to note that this method showed good selectivity
towards primary amides, which is understandable since the reaction probably occurred at the surface of the
solid KHSO4 and primary amides are more accessible due to less steric hindrance. We also tested our reaction
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on chiral amides. The ee value of 3g (Entry 7, Table 4) was determined via optical rotation to be 86%,
indicating a 7% racemization during the esterification process.
Table 4. Selectivity of solid KHSO4 promoted amide alcoholysis in refluxing alcohol
Entry
Amide
Alcohol Time
(h)
Product
code
3a
Product structure
Yield
(%)
1
MeOH
MeOH
MeOH
EOH
48
48
48
48
48
48
25
25a
2
3
4
5
6
7
3b
3c
3d
3e
3f
0
35b
93
39
55c
72
MeOH
EtOH
MeOH
3g
8
MeOH
24
3h
43
3ha
18
aThe Boc protection group was lost during the reaction and the product was recovered by treating the crude
b
c
residue with K2CO3/Boc2O in methanol. Yield after recrystallization from 1:1 hexanes/ethyl acetate. 29%
starting amide was recovered.
Conclusions
It has been demonstrated that primary amides, either aliphatic or aromatic, when treated with a primary
alcohol in the presence of solid KHSO4, are converted very easily the corresponding esters via rather simple
workup. This method also selectively converts primary amides into the corresponding esters in the presence
of secondary amides. Solid KHSO4 promotes the reaction faster than dissolved KHSO4. The observed selectivity
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is possibly due to steric hindrance.. Only slight racemization was observed when an optically active amide was
converted to the methyl ester.
Experimental Section
Typical alcoholysis procedure. A mixture of the amide (1, 1 mmol), alcohol (15 mL), and pulverized potassium
bisulfate (1.1 g, 8 mmol) was refluxed for the specified time. The alcohol was removed in vacuo and the
residue was triturated with hexanes (or other appropriate solvent such as DCM or ethyl acetate to dissolve the
product). Removal of hexanes in vacuo provided the following pure products.
2a.22 1H NMR: δ 3.81 (s, 3 H), 4.64 (s, 2 H), 6.91 (m, 2 H), 7.00 (m, 1 H), 7.30 (m, 2 H).
2b.10 1H NMR: δ 3.64 (s, 2 H), 3.70 (s, 3 H), 7.30 (m, 5 H).
2c.23 1H NMR: δ 3.68 (s, 3 H), 4.09 (s, 2 H), 7.42 (m, 2 H), 7.51 (m, 2 H), 7.80 (dd, J 2.0, 7.6 Hz, 1 H), 7.87 (dd, J
1.6, 8.0Hz, 1 H), 7.99 (d, J 8.4 Hz, 1 H).
2d.23 1H NMR: δ 3.57 (s, 2 H), 3.69 (s, 3 H), 3.79 (s, 3 H), 6.86 (d, J 8.8 Hz, 2 H), 7.20 (d, J 8.4 Hz, 2 H).
2e.25 1H NMR: δ 3.74 (s, 3 H), 5.03 (s, 1 H), 7.24-7.34 (m, 10 H).
2f.25 1H NMR: δ 3.99 (s, 3 H), 7.57 (m, 2 H), 7.88 (d, J 8.8 Hz, 2 H), 7.95 (d, J 7.6 Hz, 1 H), 8.06 (dd, J 1.6, 8.8 Hz,
1 H), 8.62 (s, 1 H).
2g.26 1H NMR: δ 3.94 (s, 3 H), 7.40 (m, 1 H), 7.47 (m, 2 H), 7.63 (m, 2 H), 7.67 (d, J 8.4 Hz, 2 H), 8.11 (d, J 8.4 Hz,
2 H).
2h.27 1H NMR: δ 3.93 (s, 3 H), 7.31 (m, 1 H), 7.63 (ddd, J 2.0, 7.6 Hz, 1 H), 7.68 (ddd, J 1.6, 7.6 Hz, 1 H), 7.75
(dd, J 2.0, 7.6 Hz, 1 H), 7.92 (dd, J 1.6, 7.6 Hz, 1 H).
2i.24 1H NMR: δ 3.96 (s, 3 H), 7.59 (m, J 1 H), 7.82 (m, 1 H), 8.23 (m, 1 H), 8.31 (m, 1 H).
2j.10 1H NMR: δ 2.41 (s, 3 H), 3.90 (s, 3 H), 7.23 (d, J 8.4 Hz, 2 H), 7.93 (d, J 8.4 Hz, 2 H).
2k.28 1H NMR: δ 3.94 (s, 3 H), 7.31 (m, 1 H), 7.43 (m, 2 H), 7.82 (dd, J 1.6, 8.0 Hz, 1 H).
2l.10 1H NMR: δ 3.91 (s, 3 H), 7.41 (d, J 8.8 Hz, 2 H), 7.97 (d, 2 H).
2m.25 1H NMR: δ 3.86 (s, 3 H), 3.88 (s, 3 H), 6.91 (d, J 9.2 Hz, 2 H), 7.99 (d, 2 H).
2n.10 1H NMR: δ 2.4 (s, 3 H), 3.91 (s, 3 H), 7.32 (dd, J 7.6 Hz, 1 H), 7.36 (m, 1 H), 7.84 (m, 1 H), 7.86 (m, 1 H).
2fa.29 1H NMR: δ 1.45 (t, J 7.2 Hz, 3 H), 4.45 (q, J 7.2 Hz, 2 H), 7.57 (m, 2 H), 7.88 (d, J 8.4 Hz, 2 H), 7.96 (d, J 8
Hz, 1 H), 8.07 (dd, J 2.0, 8.8 Hz, 1 H), 8.61 (s, 1 H).
o
1
2fb. White solid, mp: 27-29 C. H NMR: δ 3.87 (m, 2 H), 4.64 (m, 2H), 7.58 (m, 2 H), 7.89 (m, 2 H), 7.97 (d, J
8.0 Hz, 1 H), 8.08 (dd, J 1.6, 8.8 Hz, 1 H), 8.64(s, 1 H); 13C: δ 41.7, 64.6, 125.2, 126.7, 126.8, 127.8, 128.3, 128.4,
129.4, 131.4, 132.4, 135.7, 166.4; HRMS (EI) calcd. for C13H11ClO2: 234.0448; found: 234.0445.
1
2fc. H NMR: δ 1.43 (d, J 6.0 Hz, 6 H), 5.33 (hept, J 6.0 Hz, 2 H), 7.56 (m, 2 H), 7.87 (d, J 8.4 Hz, 2 H), 7.96 (d, J 8
Hz, 1 H), 8.07 (dd, J 2.0, 8.4 Hz, 1 H), 8.60 (s, 1 H); 13C: δ 22.0, 68.5, 125.3, 126.6, 127.7, 128.0, 128.10, 128.12,
129.3, 130.9, 132.5, 135.4, 166.3.
2fd.30 1H NMR: δ 1.08 (t, J 7.2 Hz, 3 H), 1.85 (m, 2 H), 4.35 (t, J 6.8 Hz, 2 H), 7.57 (m, 2 H), 7.88 (d, J 8.4 Hz, 2 H),
7.96 (d, J 8 Hz, 1 H), 8.07 (dd, J 2.0, 8.8 Hz, 1 H), 8.62 (s, 1 H).
2fe.31 1H NMR: δ 1.01 (t, J 7.2 Hz, 3 H), 1.53 (m, 2 H), 1.81 (m, 2 H), 4.40 (t, J 6.4 Hz, 2 H), 7.57 (m, 2 H), 7.88 (d,
J 8.4 Hz, 2 H), 7.96 (d, J 8 Hz, 1 H), 8.07 (dd, J 1.6, 8.8 Hz, 1 H), 8.61 (s, 1 H).
Phenoxyacetic acid.32 1H NMR: δ 4.70 (s, 2 H), 6.93(dd, J 0.8, 8.4 Hz, 2 H), 7.03(dd, J 7.6 Hz, 1 H), 7.32(dd, J 7.6,
8.4 Hz, 2 H), 8.89 (br s, 1 H).
2-Naphthoic acid.33 1H NMR (DMSO-d6): δ 7.61 (m, 2 H), 7.97 (m, 3 H), 8.10 (d, J 8.0 Hz, 1 H), 8.59 (s, 1 H),
13.06 (br s, 1 H).
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m-Toluic acid.34 1H NMR (DMSO-d6): δ 2.34 (s, 3 H), 7.35 (dd, J 7.6 Hz, 1 H), 7.41 (d, J 7.6 Hz, 1H), 7.72 (d, J 7.2
Hz), 1 H), 7.75 (s, 1 H), 12.86 (s, 1 H); 13C: 21.3, 126.9, 128.9, 130.2, 131.1, 133.9, 138.3, 167.8.
3a.35 1H NMR: δ 1.23 (m, 1 H), 1.44 (m, 9 H), 1.65 (m, 4 H), 2.19 (m, 1 H), 2.93 (m, 1 H), 3.71 (s, 3 H), 3.96 (m, 1
H), 4.89 (m, 1 H).
3c.36 1H NMR: δ 3.85 (s, 3 H), 4.03 (br. s, 2 H), 6.64 (d, J 8.4 Hz, 2 H), 7.85 (d, J 8.4 Hz, 2 H); 13C: δ 51.6, 113.8,
119.7, 131.6, 150.8, 167.2
3d.37 1H NMR: δ 1.36 (t, 3 H), 4.09 (br. s, 2 H), 4.31 (q, J 7.2 Hz, 2 H), 6.64 (d, J 8.4 Hz, 2 H), 7.85 (d, J 8.4 Hz, 2
H).
3e.38 1H NMR: δ 8.066 (d, J= 8.4 Hz, 2H), 7.96 (bs, 1 H), 7.88 (d, J 8.4 Hz, 2 H), 7.74 (d, J 8.4 Hz, 2 H), 7.61-7.56
(m, 1 H), 7.53-7.49 (m, 2 H),), 3.92 (s, 3 H)
13C NMR: δ 52.1, 119.2, 125.8, 127.1, 128.9, 130.9, 132.2, 134.5, 142.1, 165.8, 166.6
3f.39 1H NMR: δ 1.31 (t, J 7.2 Hz, 3 H), 4.28 (q, J 7.2 Hz, 2 H), 7.54 (m, 2 H), 7.59 (m, 1 H), 7.94 (m, 6 H), 10.55
(s, 1 H); 13C (DMSO): 14.7, 60.9, 120.0, 125.0, 128.2, 128.9, 130.5, 132.3, 135.0, 144.1, 165.8, 166.4.
3g.40 1H NMR: δ 7.78 (d, J= 8.4 Hz, 2 H), 7.51-7.47 (m, 1 H), 7.44-7.39 (m, 2 H), 6.54 (d, J 7.6 Hz, 1 H), 4.90-4.85
(m, 1 H), 3.77 (s, 3 H), 1.80-1.63 (m, 2 H), 0.97(t, J= 6.8 Hz, 6 H); 13C NMR: δ 22.0, 22.8, 24.9, 51.2, 52.4, 127.1,
128.6, 131.7, 133.9, 167.2, 173.8. ee, 85.6%.
3h.41 1H NMR δ 0.89 (d, J 5.6, 6 H), 1.61-1.48 (m, 3 H), 3.66 (s,3H), 3.89 (m, 2 H), 4.62-4.56 (m, 1 H), 5.08 (s, 2
H), 5.90 (bs, 1 H), 6.97 (bs, 1 H), 7.26-7.31 (m,5 H); 13C NMR (100 MHz, CDCl3) δ 21.8, 22.75, 24.76, 25.0, 41.2,
44.3, 50.7, 52.3, 67.1, 128.1, 128.3, 128.5, 136.2, 156.7, 169.3,173.4
3ha.25 1H NMR: δ 7.32-7.30 (m, 5 H), 5.31 (bs, 1 H), 5.13 (s, 2 H), 3.98 (d, J 5.6 Hz, 2 H), 3.8 (s, 3 H). 13C: δ 42.6,
52.3, 67.1, 128.1, 128.2, 128.5, 136.2, 156.3, 170.5.
Acknowledgements
Financial support for this work was provided by Meyers Institute for Interdisciplinary Research in Organic and
Medicinal Chemistry, Southern Illinois University Carbondale. We thank Dr. Duy H. Hua and Jianyu Lu at
Department of Chemistry, Kansas State University for measuring the optical rotation of 3g.
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