One-pot oximation-Beckmann rearrangement of ketones and aldehydes to amides of industrial interest: Acetanilide, caprolactam and acetaminophen

Article abstract of DOI:10.1016/j.catcom.2014.02.007

High yielding one-pot oximation-Beckmann rearrangement of ketones to amides in ktrifluoroacetic acid has been conducted on several ketones and aldehydes. The substrate reactivity showed to depend on both oximation and Beckmann rearrangement reaction rate. In this synthetic procedure, trifluoroacetic acid acts as solvent, acid catalyst and organocatalyst and can be easily recycled.

Full text of DOI:10.1016/j.catcom.2014.02.007

Catalysis Communications 49 (2014) 47–51
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
One-pot oximation–Beckmann rearrangement of ketones and aldehydes
to amides of industrial interest: Acetanilide, caprolactam
and acetaminophen
Fabio Aricò b,c, Giuseppe Quartarone a,b, Elia Rancan a,b, Lucio Ronchin a,b, , Pietro Tundo b,c, Andrea Vavasori a,b
⁎
a
Department of Molecular Science and Nanosystems, Ca' Foscari University, 2137 Dorsoduro, 30123 Venice, Italy
Green Chemistry Group, 2137 Dorsoduro, 30123 Venice, Italy
Department of Environmental Science, Informatics and Statistics, Ca' Foscari University, 2137 Dorsoduro, 30123 Venice, Italy
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
High yielding one-pot oximation–Beckmann rearrangement of ketones to amides in ktrifluoroacetic acid has
been conducted on several ketones and aldehydes. The substrate reactivity showed to depend on both oximation
and Beckmann rearrangement reaction rate. In this synthetic procedure, trifluoroacetic acid acts as solvent, acid
catalyst and organocatalyst and can be easily recycled.
Received 24 December 2013
Received in revised form 3 February 2014
Accepted 4 February 2014
Available online 12 February 2014
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Amides
Oximation
Beckmann rearrangement
Trifluoroacetic acid
Ketones
1. Introduction
of the free hydroxylamine that then attacks the carbonyl group of the
ketone.
Amides are important building blocks in organic and material chem-
istry as they are widely employed not only in plastic, rubber, paper and
color industry (crayons, pencils and inks), but also in water and sewage
treatment[1–3]. Furthermore, numerous pharmaceutical molecules
incorporate amides as core unit; N-acetyl-4-aminophenol and local
anesthetic lidocaine and dibucaine are just few examples [1–4]. Thus,
considering their importance as intermediate in the industry and as
precursor in drug formulation, the development of simpler and more
economical process for amide synthesis has been of great interest over
the last twenty years [5].
Among the most commonly used synthetic approaches for these
compounds many involve the reaction of amine with anhydrides, acyl
chlorides or, in some cases, with the acid itself [1]. This latter approach
results in the complete conversion of the substrate only when the
water, formed during the reaction, is continuously removed [1].
Amides can also be synthesized by a two step reaction: oximation of
ketones, a quite facile reaction generally carried out with hydroxyl-
amine hydrochloride or sulfate in an aqueous or water–ethanol solu-
tion, followed by Beckmann rearrangement in mineral acids [4–10].
The first step proceeds in the presence of a base to allow the formation
On the other hand, Beckmann rearrangement is generally carried
out in mineral acid i.e. H2SO4 or oleum [5–11], for which safety and/or
disposal problems must be taken into account especially in the industrial
practice [12–17]. Furthermore, the resulting amides are protonated,
thus, dilution with water or neutralization of the acid (typically with
aqueous ammonia) is required to recover the pure product [12–17].
A commodity produced according to this synthetic approach is
caprolactam, the monomer of nylon [12–17]. Nowadays the oximation–
rearrangement sequence in oleum is superseded by the Enichem–
Sumitomo processes consisting of liquid phase ammoximation [13]
followed by a gas phase Beckmann rearrangement [14–17].
Another interesting approach to amide synthesis uses ionic liquids in
combination with Lewis acids. This procedure results in a high yielding
Beckmann rearrangement for some activated oximes [18,19]. The pro-
cess, however, requires a preliminary oximation stage and a tedious
work-up for both oximation and Beckmann rearrangement.
Recently, examples of direct oximation–Beckmann rearrangement
of cyclohexanone to ε-caprolactam has also been reported in liquid
phase reaction starting from cyclohexanone, ammonia and air in the
presence of bifunctional catalysts [20,21]. Similar results have been
claimed by Uhde/Inventa-Fischer in the heterogeneously catalyzed liq-
uid phase ammoximation–Beckmann rearrangement of cyclohexanone
to caprolactam [22]. These processes are complex one-pot three-step
reactions: ammonia oxidation to hydroxylamine, oximation of cyclo-
hexanone and Beckmann rearrangement of the cyclohexanone oxime.
⁎
Corresponding author at: Department of Molecular Science and Nanosystems,
Ca' Foscari University, 2137 Dorsoduro, 30123 Venice, Italy. Tel.: +39 041 2348626.
E-mail address: ronchin@unive.it (L. Ronchin).
1566-7367/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2014.02.007

48
F. Aricò et al. / Catalysis Communications 49 (2014) 47–51
However, the final product can be isolated only in moderate yield (20–
50%) and the procedure has not been investigated on different ketones.
Several metal-catalyzed one-pot syntheses of amides from aldehyde
have also been reported although these processes require either long
reaction time, high temperature or toxic solvents [23,24]. In additions,
these reactions do not occur with ketones which limit their synthetic
interest.
2. Experimental
2.1. Materials
All the solvent and products were employed as received without
further purification. Acetophenone ≥98%, acetone ≥99.9%, 2-hydroxy-
acetophenone ≥98%, 4-methylacetophenone ≥95%, 2-methylaceto-
phenone ≥98%, 2,4,6 trimethylacetophenone ≥98%, 4-bromo
acetophenone 98%, 2-bromo acetophenone 98%, propiophenone 99%,
butyrophenone ≥99%, 2,2-dimethylpropiophenone 98%, isobutyro-
phenone 97%, benzophenone ≥99%, 4-phenyl-2-butanone 98%,
ethyl benzoylacetate 97%, 4-nitrobenzaldehyde 98%, 4-isopropyl-
benzaldehyde 98%, 2-hydroxy benzaldehyde ≥98%, hexanal 98%,
trifluoroacetic acid 99%, and hydroxylamine hydrochloride 99% were
all Aldrich products; 4-hydroxiacethophenone ≥98% (HPLC) was a
HPLC grade Fluka product. Cycloexanone 99.8% was an ACROS reagent.
Benzaldehyde 99% was a Carlo Erba reagent. Deuterated chloroform
and deuterated DMSO-d6 were EurisoTop products.
The use of trifluoroacetic acid (TFA) as catalyst in the presence of
CH2Cl2 as solvent was firstly reported by Cossy and co-workers in the
Beckmann rearrangement of oxime carbonate [25]. In this synthetic
approach the key step is the formation of the oxime carbonate being
more activated than the naked oxime in the Beckmann rearrangement
due to the electron withdrawing effect of the carbonate group.
TFA has been also employed as catalyst for the Beckmann rearrange-
ment of cyclohexanone oxime to ε-caprolactam [26–28]. In particular, a
TFA/CH3CN mixture was used for a practical and high yielding synthesis
of amides [26–28]. The proposed reaction mechanism envisages the
formation of the oxime ester of the trifluoroacetic acid, which, after
rearrangement, forms a trifluoroacetyl amide. This compound is the
key intermediate of the trifluoroacetylation process of the oxime as it
continuously reforms sustaining the catalytic cycle (Scheme 1).
In our previous work we outlined that the mechanism of the
Beckmann rearrangement in TFA occurs via esterification of the oxime
also for the acetophenone oxime and for the 4-hydroxyacetophenone
oxime [29].
Recently, Luo and co-workers reported the Beckmann rearrange-
ment of cyclohexanone oxime to caprolactam in TFA/CH3CN optimizing
the caprolactam yield by using conditions similar to those employed in
our previous papers [26–30]. The same authors reported an oximation–
Beckmann rearrangement of cyclohexanone to caprolactam using a
TFA/CH3CN system [31]. This synthetic procedure, although interesting,
focuses only on one substrate i.e. caprolactam without discussing the
general applicability of the system.
In this work, following our previous investigation on the Beckmann
rearrangement of ketoximes, we account on a general approach for the
synthesis of amides starting from ketones or aldehydes via a one-pot
oximation–Beckmann rearrangement. Hydroxylamine is used in indus-
trial processes as oximating agent after neutralization of its sulfate salt
being hydroxylamine itself an unstable reagent [1–4]. Here we used
hydroxylamine hydrochloride, which is a stable and soluble salt, as
oximation agent and TFA as catalyst and solvent. The reaction is of
general application and results in the high yielding preparation of
amides. The use of TFA as catalyst and solvent renders the processes
both sustainable and highly efficient. In fact, the reaction does not
require any work-up operations as TFA can be removed by low temper-
ature vacuum distillation and recycled. Furthermore, due to the TFA low
protonation ability, the resulting amides can be easily recovered as pure
compounds.
2.2. Instruments and analysis
Reaction products were analyzed by Gas Chromatography (GC) and
Gas Chromatography coupled to Mass Spectroscopy (GC–MS), using an
Agilent model 5975C interfaced with a GC Agilent model 7890 a HP5
capillary column (300 μm i.d. 30 m long, 95% methyl, 5% phenyl silicone
phase).
The samples were also checked by a high performance liquid chro-
matography (HPLC). The instrument employed was a Perkin Elmer
binary LC pump 250 with phenomenex Luna, 5 μm C18 100 Å, LC
column 30 mm × 4.6 mm (detector: Perkin Elmer LC 235 C Diode
Array), wavelengths: 255 nm and 220 nm; eluent: water–acetonitrile
with a concentration gradient 60% water (9 min), 50% water (5 min)
and 30% water (1 min).
The 1H Nuclear Magnetic Resonance (NMR) spectra were recorded
on a Bruker AC 200 spectrometer operating at 200.13 MHz, and the
sample temperature was maintained at 298 K. All the chemical shifts
were referred to internal tetramethylsilane.
2.3. Ketones reactivity
All the reactions were carried out in a well stirred pressurized glass
reactor thermostated at 70 °C temperature and containing weighed
samples of the solvent and reagents.
In a typical experiment a glass reactor equipped with magnetic
bar was charge with 1.5 mmol of the selected ketone or aldehyde,
4.4 mmol of hydroxylamine hydrochloride and 22 mmol of
trifluoroacetic acid under inert atmosphere of nitrogen. The reaction
time was computed after the heating fluid starts to circulate in the
OH
N
OCOCF3
O
N
COCF3
N
-H2O
CF3COOH
+
OH
OCOCF3
O
O
N
N
COCF3
+
NH
N
+
Scheme 1. Reaction mechanism of TFA catalyzed Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam [26–28].

F. Aricò et al. / Catalysis Communications 49 (2014) 47–51
49
Table 1
reactor jacket. The reaction mixture was heated at 70 °C for 16 h, and
then cooled down.
Oximation–Beckmann rearrangement in trifluoroacetic acid of selected ketones. Run
conditions: reaction time = 16 h, substrate = 2 mmol, T = 70 °C, and substrate/
NH2OH·HCl/CF3COOH = 1/3/15.
Products were isolated after distillation of the TFA (at 40 °C and
250 Pa of pressure) and in a rotary evaporator, washed with cold
water and recrystallized from hexane or light petroleum ether.
In selected cases, the product was purified by column chromatogra-
phy employing dichloromethane/diethyl ether 9:1 as elution mixture.
In all the conducted experiments, analysis of the isolated products was
consistent to the data reported in the literature (see Supplementary
materials).
Entry Substrate
Conversion Selectivity
(%)
(%)
1
80
Other
4
96
2
93
Other
1
3. Results and discussion
99
84
Table 1 reports the results achieved for the one-pot oximation–
Beckman rearrangement reaction on some selected aromatic and
aliphatic ketones (Scheme 2). In all the experiments the amide resulted
the main product formed with selectivity up to 99%; the formation of
oxime intermediate has also been sometime observed.
3
4
99
71
11
Other
11
It is evident that, being a multistep process, the outcome of the reac-
tion depends on the reactivity of the substrate toward both oximation
reaction and Beckmann rearrangement. Consequently, acetophenone,
acetone and cyclohexanone (entries 1–4, Table 1), whose structures
allow an easy oximation, are converted into amides in high yield (70–
90%). This result also implies a facile Beckmann rearrangement of the
corresponding oximes.
It is interesting to observe that the highest conversion was obtained
when cyclohexanone was used as substrate, suggesting an easy
oximation stage. However, results collected indicate that the rearrange-
ment was only partially completed as demonstrated by the presence of
cyclohexanone oxime (11%). The presence of oxime depends on the
relative rate of reaction of the two reaction steps, but also on the solvent,
acidity and steric hindrance phenomena [6–10].
The reactivity of mono-substituted aryl ketones was then investigated
(entries 4–8, Table 1). It is noteworthy that in our reaction conditions,
N-acetyl-4-aminophenol, the widely used drug acetaminophen, can be
easily synthesized starting from 4-hydroxy-acetophenone (entry 4,
Table 1). In this case, the pure product can be isolated by recrystalliza-
tion after TFA removal by a rotary evaporator.
On the other hand, when the substrate incorporates a sterically
hindered carbonyl group, the oximation resulted more difficult (entries
6 and 8, Table 1) [6,18]. This result also explains the different reactivities
of 4-methyl acetophenone (entry 5) and 2-methyl acetophenone (entry
6) as the conversion of the latter substrate in the related amide is only
moderate (entries 5 and 6 respectively, Table 1).
89
85
91
5
6
7
90
53
91
Other
10
Other
9
13
85
96
8
9
70
79
Other
4
Other
6
94
92
10
11
75
68
Other
8
78
18
An analogous trend was also observed for the 4- and 2-
bromoacetophenone (entries 7 and 8, Table 1) as the ortho substituted
substrates resulted less reactive than the para ones. It is noteworthy
that when 2-substituted acetophenones were employed as substrates
the formation of oximes intermediate was never observed. This is in
agreement with Beckmann rearrangement being easier with 2- or 6-
substituted aryl ketones than with unsubstituted or 4-substituted
ones [10,32]. This result might also suggest that, when hindered aryl
ketones are employed, the nucleophilic substitution is the determining
step of the reaction instead of the Beckmann rearrangement of the
ketoxime.
12
13
44
26
20
99
8
Other
1
Table 1) was used as substrate the anilide/benzamide ratio detected
was bigger than the one observed in the reaction involving isopropyl-
phenylketone (entry 11, Table 1) (78/18 and 20/8, respectively). This
is an obvious consequence of the relative rate of formation of the sin
and anti-ketoxime, which is determined by the steric hindrance of the
alkyl group.
Finally, the extremely hindered benzophenone (entry 13, Table 1) is
only scarcely converted into the N-phenylbenzamide although with an
almost total selectivity to the rearranged amide. Most probably the
low conversion can be ascribed to a less efficient oximation reaction
due to the steric hindrance of the phenyl groups, while the high
Linear alkyl substrates methyl, ethyl and n-propyl phenylketones
(entries 1, 9 and 10) have also been investigated, showing a conversion
decreasing with the increasing of the aliphatic chain length, although
the related amide is, in any case, the main product observed (entries
1, 9, 10, Table 1).
The presence of a branched substituent, i.e. an isopropyl- or tert-
butyl-moiety, influences the reaction conversion resulting in a non-
selective formation of anilides (entries 11 and 12, Table 1). This is
due to the competitive formation of the sin and anti-ketoximes that
can be ascribed to the similar steric hindrance of phenyl or branched
aliphatic groups [33]. In fact, when tert-butyl phenylketone (entry 12,

50
F. Aricò et al. / Catalysis Communications 49 (2014) 47–51
OH
O
C
O
C
N
C
CF3COOH
CF3COOH
- HCl
R'
NH2OH.HCl
R'
R
NH
R'
R
R
Scheme 2. Oximation–Beckmann rearrangement of ketones to amides.
selectivity can be ascribed to the stability of the starting ketone, which
does not give side reactions.
In order to investigate the eventual limits of this synthetic proce-
dure, the reactivity of complex ketones, incorporating different func-
tional groups, has also been investigated (Table 2).
were acetanilide and benzonitrile, but several products of fragmenta-
tion rearrangement and condensation have been also observed.
The reactivity of selected aldehydes in this reaction condition has
also been investigated (Table 3). As expected aldehydes resulted more
reactive than ketones giving generally a higher conversion (see for
comparison Table 1). In fact, benzaldehyde was easily converted into
benzamide in high yield; a small amount of benzonitrile was also
detected [35]. A similar reactivity was observed for 4-nitrobenzaldehyde
(entry 2), which formed 4-nitrobenzamide in high yields. 4-Isopropyl
benzaldehyde (entries 3) gave almost quantitative conversion and
high selectivity to the corresponding benzonitrile. Analogously, 2-
hydroxy benzaldehyde (entry 4) was mainly converted into benzonitrile
together with a negligible amount of benzamide (only traces were
observed at the GC–MS analysis).
The lower aldehyde conversion observed in this case (85%) can be
ascribed to the steric hindrance of the ortho hydroxyl group, which
lowers the rate of the nucleophilic attack of the hydroxylamine as
already pointed out for ketones (see Table 1).
Finally, hexanal (entry 5) gave a complex mixture of oxidation
condensation and decomposition products, probably due to fast acid
catalyzed unwanted reactions.
In particular, when 4-phenylbutan-2-one (entry 1, Table 2) was
used as substrate, the expected amide formed with 70% selectivity. A
significant amount of 4-phenylbutan-2-one oxime was also observed
(25%). This is in agreement with the results reported in Table 1 (entries
11–13) where the sterically hindered phenylalkyl moiety slowed down
the rearrangement rate, diminishing the amide selectivity.
It should be noted that, in this synthetic procedure, the formation of
amide might be affected by the NH2OH nucleophilic attack to the
carbonyl group of the ketone, which is influenced by the steric hin-
drance of the substituents on the phenyl ring. In fact, in our reaction
conditions, 1-mesytilethanone undergoes only deacylation and substi-
tution reaction. Most probably the methyl groups in 2- and 6-positions
hamper the nucleophilic substitution of the hydroxylamine to give the
corresponding oxime so that the molecule is subjected to deacylation
and substitution and the Beckmann rearrangement is completely
suppressed. Such behavior agrees with the reactivity of the mesitoic
acid toward esterification, which occurs only in the presence of very
strong acid and severe conditions [34].
4. Conclusions
The reactivity of the 2-hydroxyacetophenone oxime (entry 3,
Table 2) showed a low conversion and the main product was not the
expected 2′-hydroxyacetanilide but the heterocyclic compound methyl
benzoxazole, which is a useful intermediate employed in fine chemical
production [4].
The one-pot oximation–Beckmann rearrangement reaction has also
been tested on the ethyl benzoylacetate. This β-ketoester (entry 4,
Table 2) leads to a complex mixture of products, due to a non-selective
nucleophilic attack (NH2OH) and to a competition among rearrange-
ment and fragmentation reactions. The two major products formed
Herein we report on a one-pot oximation–Beckmann rearrange-
ment of ketones and aldehyde as a novel synthetic pathway to achieve
amides of industrial interest. The reaction has been conducted on several
ketones resulting in a highly selective synthesis of amide, i.e. N-acetyl-4-
aminophenol. The conversion of ketones into the final amide is strictly
related to the reactivity of the substrate with respect to the two different
steps of the reaction. When the ketone undergoes easy oximation then
the Beckmann rearrangement is the determining step of the reaction.
Similarly, if the steric hindrance of the substituent hampers the
Table 2
Reactivity of complex ketones. Run conditions: reaction time = 16 h, substrate = 2 mmol, T = 70 °C, and substrate/NH2OH·HCl/CF3COOH = 1/3/15.
Entry
Substrate
Conversion
(%)
Selectivity
(%)
1
90
70
71
25
28
2
88
3
4
14
90
5
8
5
100

F. Aricò et al. / Catalysis Communications 49 (2014) 47–51
51
Table 3
Reactivity of some aldehydes. Run conditions: reaction time = 16 h, substrate = 2 mmol, T = 70 °C, and substrate/NH2OH·HCl/CF3COOH = 1/3/15.
Entry
Substrate
Conversion
(%)
Selectivity
(%)
1
97
Various productsa
12
8
80
80
2
3
99
99
Various productsa
14
6
Various productsa
6
94
traces
4
85
99
Various productsa
15
78
/
17
/
5
Various productsa
100
a
Several products have been identified by GC–MS analysis; oxidation, condensation and decomposition products, and all the species are, however, in amount poorly interesting from a
synthetic point of view.
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Acknowledgments
Financial support by Ca' Foscari University of Venice is gratefully
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.02.007.
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