A simple and highly efficient one-pot procedure for the synthesis of amides via beckmann rearrangements using 1-tosylimidazole (TsIm)

Article abstract of DOI:10.1080/10426500903176554

A facile and highly efficient method for one-pot Beckmann rearrangement of ketoximes into N-substituted amides using N-(p-toluenesulfonyl)imidazole (TsIm) is described. In this method, ketoximes are refluxed with TsIm and Cs2CO3 in the presence of SiO2 as a recyclable catalyst in DMF affording the corresponding amides in high yields. This methodology is highly efficient and regioselective for various structurally diverse ketoximes including symmetrical and unsymmetrical as well as cyclic oximes. The results of quantum mechanical studies used to rationalize the experimental outcomes are discussed. Copyright Taylor & Francis Group, LLC.

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Phosphorus, Sulfur, and Silicon and the
Related Elements
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A Simple and Highly Efficient One-Pot
Procedure for the Synthesis of Amides
via Beckmann Rearrangements Using 1-
Tosylimidazole (TsIm)
a
b
Mohammad Navid Soltani Rad , Ali Khalafi-Nezhad , Somayeh
b
a
b
Behrouz , Zohreh Amini & Marzieh Behrouz
a
Department of Chemistry, Faculty of Basic Sciences, Shiraz
University of Technology, Shiraz, Iran
b
Department of Chemistry, College of Sciences, Shiraz University,
Shiraz, Iran
Version of record first published: 02 Aug 2010.
To cite this article: Mohammad Navid Soltani Rad, Ali Khalafi-Nezhad, Somayeh Behrouz, Zohreh
Amini & Marzieh Behrouz (2010): A Simple and Highly Efficient One-Pot Procedure for the Synthesis of
Amides via Beckmann Rearrangements Using 1-Tosylimidazole (TsIm), Phosphorus, Sulfur, and Silicon
and the Related Elements, 185:8, 1658-1671
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Phosphorus, Sulfur, and Silicon, 185:1658–1671, 2010
Copyright ꢀ
Taylor & Francis Group, LLC
C
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500903176554
A SIMPLE AND HIGHLY EFFICIENT ONE-POT PROCEDURE
FOR THE SYNTHESIS OF AMIDES VIA BECKMANN
REARRANGEMENTS USING 1-TOSYLIMIDAZOLE (TsIm)
1
2
Mohammad Navid Soltani Rad, Ali Khalafi-Nezhad,
2
1
2
Somayeh Behrouz, Zohreh Amini, and Marzieh Behrouz
Department of Chemistry, Faculty of Basic Sciences, Shiraz University
1
of Technology, Shiraz, Iran
2
Department of Chemistry, College of Sciences, Shiraz University, Shiraz, Iran
A facile and highly efficient method for one-pot Beckmann rearrangement of ketoximes
into N-substituted amides using N-(p-toluenesulfonyl)imidazole (TsIm) is described. In this
method, ketoximes are refluxed with TsIm and Cs2CO3 in the presence of SiO2 as a recyclable
catalyst in DMF affording the corresponding amides in high yields. This methodology
is highly efficient and regioselective for various structurally diverse ketoximes including
symmetrical and unsymmetrical as well as cyclic oximes. The results of quantum mechanical
studies used to rationalize the experimental outcomes are discussed.
Keywords Amide; Beckmannrearrangement; ketoxime;SiO2;N-(p-toluenesulfonyl)imidazole
(TsIm)
INTRODUCTION
Amides and lactams are potential precursors for the synthesis of various natural
1
products as well as synthetic intermediates for medicinal drugs and materials. There are
2
numerous general methods for accessing amides. Among these, perhaps the best known
transformation of ketoximes into N-substituted amides is the Beckmann rearrangement
2,3
(
BR). BR has been extensively used and is a fundamental reaction in organic synthe-
sis, which has led to numerous applications due to the ease with which nitrogen can be
inserted into carbon chains starting from readily available ketones. It also represents a
powerful method particularly for manufacturing ε-caprolactam as a precursor of nylon-6
in the chemical industry. The conventional BR, however, generally requires relatively high
reaction temperature, large amounts of strong Lewis or Brønsted acids, dehydrating media,
and harsh reaction conditions, and causes a large number of byproducts and wastes that
can not be used with sensitive substrates. On this basis, mild conditions were explored and
Received 16 May 2009; accepted 10 July 2009.
We wish to thank the Shiraz University of Technology and Shiraz University Research Councils for partial
support of this work.
Address correspondence to Mohammad Navid Soltani Rad, Department of Chemistry, Faculty of Ba-
sic Sciences, Shiraz University of Technology, Shiraz 71555-313, Iran. E-mail: soltani@sutech.ac.ir; nsoltani-
rad@gmail.com
1
658

SYNTHESIS OF AMIDES VIA BECKMANN REARRANGEMENTS
1659
several variants were developed that essentially focused on the formation of activated oxime
derivatives that rearrange to the corresponding amides. For instance, cyanuric chloride in
4
a
4b
5
6
7
8
DMF or MeCN, chloral, solid metaboric acid, [RhCl(cod)2]/(p-tol)3P, sulfamic acid,
9
10
ethyl chloroformate/boron trifluoride etherate, O-alkyl-N,N-dimethyl formamidium salt,
11
12
and chlorosulfonic acid have been used for BR. Recently, BR in supercritical water and
ionic liquid have also been reported. The BR of oxime sulfonates14 is usually preferred
13
due to their high reactivity, ease of handling, and facile preparation from oxime using TsCl
1
4c,d,15
or MsCl.
Although this method possesses synthetic value, all reported procedures in-
volve the isolation of the starting oxime sulfonates, some of which lack stability, followed
with a tedious workup and cumbersome separation processes. Furthermore, working with
harmful and toxic sulfonyl halides remains a problem. Therefore, the in situ generation of
O-ketoxime-sulfonates with a cheap, nontoxic, and stable sulfonating reagent would seem
to be a suitable and attractive strategy, and indeed there are a few reports that have explained
16
the one-pot BR of ketoximes via the oxime sulfonate intermediates.
The aforementioned methods have several drawbacks such as non-generality for var-
ious types of ketoximes, the use of harmful and/or expensive reagents, undesirable side
reactions, formation of large amounts of byproducts and wastes, long reaction times, and
low yields. So, there is still a need to extend and improve a practical, efficient, and selec-
tive method for the Beckmann rearrangement. Recently, we reported N-(p-toluenesulfonyl)
imidazole (TsIm) as a highly efficient, cheap, and stable reagent for various organic transfor-
1
7a
17b
mations including one-pot conversion of alcohols to alkyl azides, alcohols to nitriles,
esterification of alcohols,1 and N-alkylation of nucleobases. In continuation of our in-
terest in application of TsIm in organic synthesis, in this article we report that symmetrical
and unsymmetrical ketoximes can be efficiently converted into their corresponding amides
using TsIm/SiO2 in the presence of Cs2CO3 in refluxing DMF (Scheme 1).
7c
17d
OH
N
O
TsIm/Cs CO /SiO
2
2
3
R
N
R'
R
R'
DMF, reflux,1-2 h
H
'
R, R= alkyl and aryl,...
O
N
TsIm: H3C
S
N
O
Scheme 1
RESULTS AND DISCUSSION
The first step of this synthetic approach involved the optimization of reaction condi-
tions. Initially, the effect of various solvents on the model reaction of acetophenone oxime
in the presence of freshly prepared TsIm1 (1.2 equiv) and SiO2 (1 g) as a catalyst was
studied. The results are depicted in Table I. As the data in Table I demonstrate, DMF (entry
7a
2) was the most efficient solvent; thus it was the solvent of choice for all reactions. Using
DMSO, MeCN, and HMPA (Table I, entries 1, 5, and 6) also afforded high yields of the
corresponding amide; however, the reaction time for completion was longer.

1660
M. N. S. RAD ET AL.
Table I Effect of various solvents on the conversion of acetophenone oxime into N-phenyl acetamide
NOH
O
TsIm/Cs CO / SiO
2
2
3
Solvent, reflux
N
H
a
Entry
Solvent
DMSO
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
3
1
12
48
4
90
96
NR
NR
84
86
Trace
30
Trace
DMF
DMF
THF
b
c
c
MeCN
HMPA
Toluene
Acetone/H2O
H2O
4
48
10
48
d
a
Isolated yield.
b
c
Anhydrous DMF.
No reaction.
d(1:1) ratio.
The choice of the base for activation of the ketoximes to react with TsIm and sub-
sequent conversion into amides had a great significance. In this case, we evaluated the
potency of several organic and inorganic bases on the model reaction (Table II). In absence
of the base, the reaction was not achieved at all or achieved in trace amounts even after
prolonged reaction time. As the results in Table II indicate, among the examined bases in
this experiment, Cs2CO3 (Table II, entry 6) proved to be the most efficient base for the
Table II Effect of various bases on the conversion of acetophenone oxime into N-phenyl acetamide
NOH
O
TsIm/Base/ SiO2
DMF, reflux
N
H
a
Entry
Base
None
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
24
7
12
18
10
1
4
10
4
trace
30
25
10
20
96
63
20
75
DBU
DABCO
DMAP
MgO
Cs2CO3
K2CO3
TEA
b
Al2O3
a
Isolated yield.
b
Basic alumina.

SYNTHESIS OF AMIDES VIA BECKMANN REARRANGEMENTS
1661
conversion of acetophenone oxime into N-phenyl acetamide. Other bases such as K2CO3
and Al2O3 (Table II, entries 7 and 9) afforded lower yields of amide.
The optimized amount of TsIm was found to be 1.2–2.0 equiv. per equivalent of
ketoxime. We also investigated other TsIm analogues (Table III). As the data in Table III
indicate, using TsIm (Table III, entry 3) increased the reaction rate and yield in comparison
Table III Comparison of TsIm reactivity with analogues on the conversion of acetophenone oxime into N-phenyl
acetamide
NOH
O
Reagent/Cs2CO3/ SiO2
DMF, reflux
N
H
a
Entry
1
Reagent
Time (h)
7
Yield (%)
65
58
96
60
54
2
3
4
5
6
1
7
12
6
7
12
48
49
NRb
a
Isolated yield.
No reaction.
b

1662
M. N. S. RAD ET AL.
Table IV Effect of various Lewis acids on the conversion of acetophenone oxime into N-phenyl acetamide
NOH
O
TsIm/Cs CO / Lewis Acid
2
3
DMF, reflux
N
H
a
Entry
Lewis acid
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
None
LiCl
SiO2
Al(OAc)3
MnCl2.4H2O
CdCl2.H2O
NiCl2.6H2O
FeCl2.4H2O
SnCl2.2H2O
ZnCl2
8
5
1
2
18
2
8
3
10
5
10
42
82
96
91
15
88
30
90
38
45
20
1
1
0
1
CuCl
aIsolated yield.
with other sulfonyl analogues. Replacing the tolyl in TsIm with methyl, trifluoromethyl,
and phenyl did not afford satisfactory results (Table III, entries 1, 2, and 4). Furthermore,
other azole analogues of TsIm were not as effective as imidazole (Table III, entries 5 and 6).
N-Tosyl phthalimide (Table III, entry 7) was inactive for the conversion of acetophenone
oxime into N-phenyl acetamide even after refluxing for 48 h.
We also evaluated the role of several Lewis acids as catalysts on the reaction model
(Table IV). In the absence of catalyst, the reaction occurred but only in moderate yield.
However, using SiO2 (Table IV, entry 3) shortened the reaction time and remarkably
increased the yield. The use of Al(OAc)3, CdCl2.H2O, and FeCl2.4H2O (Table IV, entries
4, 6, and 8) also afforded good results. However, SiO2 was preferred for use not only
because of higher yield and reasonable reaction rate but also because of it is cheap, easy to
handle, and readily available. Moreover, SiO2 is a heterogeneous and reusable catalyst that
19
can be removed from the reaction mixture easily by simple filtration.
The generality and versatility of this method was demonstrated by its application to
various structurally different ketoximes (Table V). As the results in Table V indicate, both
symmetrical and unsymmetrical ketoximes were successfully converted into the amides in
good yields. Most of the oximes used in this research are commercially available or can
20
be easily prepared by the procedure explained in the literature. By this method, both (E)-
and (Z)-isomers of ketoximes can be converted into amides and in most cases a mixture of
(
Z)- and (E)-isomers was used.
The BR mechanism of ketoximes and its O-sulfonate derivatives is well discussed
in many articles.1
4g,21
Both Z- and E-O-oxime sulfonates are prone to BR, which in most
1
4g,21
cases an alkyl or aryl residue anti to the leaving group is prior to migrate.
Since we
simultaneously have Z- and E-O-oxime sulfonates in our reaction mixture, the synthesis of
two isomer amides is expected. Furthermore, BR attempts of pure E- or Z-oxime isomers
22
or O-oximes sulfonates did not furnish the synthesis of an amide isomer exclusively.

1
663

1
664

1
665

1
666

SYNTHESIS OF AMIDES VIA BECKMANN REARRANGEMENTS
1667
It is well demonstrated and fully established that both Z- and E-oxime isomers can
23
be interconverted by nitroso-oxime tautomerization ([1,3]-H shift). In addition, isomeric
16a
interconversion can happen for Z and E-O-oxime sulfonates in strong acidic media. These
isomeric interconversions annihilate the effort for selective synthesis of one isomer amide
and often lead to cumbersome and tedious separation of isomers from a complicated reaction
mixture. However, comparing the ratio of synthesized amides from acetophenoxime using
our method that employs SiO2 with other effective Lewis acids (Scheme 2) shown in Table
IV, we experienced that ratios of two amide isomers can be altered by changing the Lewis
acid. The results are depicted in Table VI.
NOH
O
O
TsIm/Cs2CO3/ LewisAcid
+
N
DMF, reflux, 1-5 h
N
H
H
2
a
2b
Mixture of Z and E
isomers
Scheme 2
As the data in Table VI indicate, among the effective Lewis acids used in our exper-
iments (see Table IV), SiO2 is proven to exhibit the better selectivity for catalyzing BR
of E-acetophenoxime that affords N-phenyl acetamide (2a) mainly with a trace amount of
2b isomer. Although the better migratory aptitude is for anti phenyl moiety in E-isomer in
comparison with anti methyl in Z-isomer, however, because of nitroso-oxime tautomeric
interconvertion of the E- and Z-isomer, the generation of amide 2b is inevitable, and
this was indicated by TLC and HPLC analysis. One may assume the weaker Lewis acid
property of SiO2 compared to other Lewis acids in Table VI is the reason for the higher
selectivity. Nevertheless, using ab initio (Hartree-Fock; 6–31G, run on Gaussian 98 ver-
sion 9.2 software) and semi-empirical [Austin Model 1 (AM1) and Parameterized Model
3
(PM3) run on MOPAC in CS Chem 3D Ultra 8; 2004 Cambridge Soft and Hyperchem,
Table VI Effect of various Lewis acids on the ratio of N-phenyl acetamide (2a)/N-methyl benzamide (2b)
NOH
O
O
TsIm/Cs2CO3/ Lewis Acid
+
N
DMF, reflux
N
H
H
2
a
2b
Entry
Lewis acid
Time (h)
2a/2ba
1
2
3
4
5
LiCl
SiO2
Al(OAc)3
CdCl2.H2O
FeCl2.4H2O
5
1
2
2
3
7.0/3.0
9.5/0.5
6.5/3.5
7.5/2.5
8.0/2.0
aThe ratios were attained by HPLC analysis.

1668
M. N. S. RAD ET AL.
Figure 1 Binding of Z- and E-O-acetophenoxime sulfonate at surface of silica gel.
Hypercube Inc., version 7] quantum mechanical calculations24 to rationalize this result has
manifested more accurate standpoints. The studies indicate that higher reactivity of E-O-
acetophenoxime sulfonate does not merely depend on phenyl moiety anti to leaving group,
but also depends on differences in the geometrical and spatial orientation between Z- and
E-O-acetophenoxime sulfonate. In Figure 1, the approaches of the optimized geometry of
Z- and E-O- acetophenoxime sulfonate for binding to the surface of silica gel using ab
initio 6-31G is demonstrated. As is shown in Figure 1, it is supposed that the hydrogen
bonding between hydroxyl groups on silica surface and negative oxygens of sulfonate direct
the approach of Z- or E-O-acetophenoxime sulfonate to the electrophilic center of silicon.
To witness this claim, the charges of heteroatoms in both Z- and E-O-acetophenoxime
sulfonate were calculated, and the data are depicted in Table VII. As is indicated in Table
VII, the oxygen of the sulfonyl moiety has the most negative values, and it is assumed that
maximum interaction between these oxygens and the silica surface can happen. However,
Table VII Calculated charges of heteroatoms in Z- and E-O- acetophenoxime sulfonate using AM1 calculations
4
3
O
S
O
5
N O
1
2
Chargea
O(3)
Isomer
N(1)
O(2)
O(4)
S(5)
E-isomer
Z-isomer
0.08292
0.08599
−0.66618
−0.69705
−0.97670
−0.98678
−1.00095
−0.98369
3.12114
3.11724
aCharges in Mulliken unit.

SYNTHESIS OF AMIDES VIA BECKMANN REARRANGEMENTS
1669
comparing the geometrical differences between two isomers is a good indication that the
E-isomer has more room for approaching the silica surface than Z-isomer does.
CONCLUSIONS
A facile and highly efficient method for Beckmann rearrangement of ketoximes
into N-substituted amides using N-(p-toluenesulfonyl)imidazole (TsIm) is described. This
methodology is highly efficient and regioselective for various structurally diverse ketoximes
including symmetrical and unsymmetrical as well as cyclic oximes. In this experiment, SiO2
as a mild Lewis acid catalyst affects the ease of reaction as well as regioselectivity. Several
quantum mechanical reasons were discussed for rationalizing the effect of SiO2 in this BR.
EXPERIMENTAL
All chemicals were purchased from Fluka or Merck chemical companies except for
TsIm17a and some of the oximes, which were prepared according to published methods.
20
25
Solvents were purified and dried according to reported methods and stored over 3Å
molecular sieves. The progress of the reactions was followed with TLC using silica gel
SILG/UV 254 plates. Silica gel 60, 0.063–0.200 mm (70–230 mesh ASTM) was used for
column chromatography. IR spectra were run on a Shimadzu FTIR-8300 spectrophotometer.
1
13
The H NMR (250 MHz) and C NMR (62.5 MHz) were run on a Br u¨ ker Avance DPX-250
FT-NMR spectrometer; δ in parts per million, J in hertz. Mass spectra were recorded on
a Shimadzu GCeMS-QP 1000 EX apparatus. Microanalyses were performed on a Perkin
Elmer 240-B microanalyzer. Melting points (mp) were recorded on a B u¨ chi 510 apparatus
in open capillary tubes and are uncorrected.
GENERAL PROCEDURE FOR THE BECKMANN REARRANGEMENT
OF KETOXIMES INTO AMIDES
To a double-necked round bottom flask (100 mL) equipped with a condenser, a
17a
mixture of ketoxime (0.01 mol), freshly prepared TsIm (0.012 mol), SiO2 (1 g), and
Cs2CO3 (0.01 mol) in DMF (20 mL) was added. The mixture was refluxed, and in most
cases, darkening occurred. Reflux was continued until TLC monitoring indicated no further
improvement in the conversion (Table V). The catalyst was filtered off, and the filtrate was
evaporated under vacuum to remove the solvent. The remaining foam was dissolved in
CHCl3 (100 mL) and subsequently washed with water (2 × 100 mL). The organic layer was
dried (Na2SO4) and evaporated. The crude product was purified by column chromatography
on silica gel and eluted with a mixture of n-hexane/EtOAc.
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(
4
Tetrahedron Lett., 49, 1115 (2008); (d) M. N. Soltani Rad, A. Khalafi-Nezhad, S. Behrouz, M.
A. Faghihi, A. Zare, and A. Parhami, Tetrahedron, 64, 1778 (2008).
18. (a) F. Matloubi Moghaddam, A. A. Rastegar Rad, and H. Zali-Boinee, Synth. Commun., 34, 2071
(
2004); (b) M. K. Dongare, V. V. Bhagwat, C. V. Ramana, and M. K. Gurjar, Tetrahedron Lett.,
5, 4759 (2004); (c) H. Eshghi and Z. Gordi, Synth. Commun., 33, 2971 (2003); (d) B. M.
Khadikar and D. J. Upadhyaya, Synth. Commun., 32, 1867 (2002).
9. The reusability of SiO was checked by elaborating the recycled SiO
4
1
2
2
for ten reactions without
remarkable losses in reactivity.
2
0. J. S. Buck and W. S. Ide, Org. Syn. Coll. Vol. II, 622 (1943).
2
1. (a) S. Yamabe, N. Tsuchida, and S. Yamazaki, J. Org. Chem., 70, 10638 (2005); (b) M. T.
Nguyen, G. Raspoet, and L. G. Vanquickenborne, J. Am. Chem. Soc., 119, 2552 (1997); (c) W.
Z. Heldt, J. Org. Chem., 26, 1695 (1961).

SYNTHESIS OF AMIDES VIA BECKMANN REARRANGEMENTS
1671
2
2. The separation of E- and Z-oximes was achieved following a procedure in the literature: J. H.
Paul, U.S. Patent, 4158015 (1979); Chem. Abstr., 91, P107810v (1979) and also visit http//www.
freepatentsonline.com/4158015.html.
2
3. To endorse such interconversion, AM1 calculation indicated that the barrier energy is 4.0505
Kcal/mol for conversion of Z- and E-acetophenoxime to each other. Also see E. D. Raczy n´ ska,
T. M. Krygowski, J. E. Zachara, B. O s´ miałowski, and R. Gawinecki, J. Phys. Org. Chem., 18,
8
92 (2005).
4. A. Hinchliffe, Molecular Modeling for Beginners (John Wiley & Sons, Ltd., Chichester, England,
005).
5. A. I. Vogel, Practical Organic Chemistry (Longmans, Green, London, England, 1954), Chapter
, pp. 161–176.
2
2
2
2