Bromodimethylsulfonium bromide-ZnCl2: A mild and efficient catalytic system for beckmann rearrangement

Article abstract of DOI:10.1055/s-0029-1218730

Bromodimethylsulfonium bromide, in combination with zinc chloride, has been shown to be an excellent catalytic system for liquid-phase Beckmann rearrangement of various ketoximes into the corresponding amides/lactams in acetonitrile at reflux temperature with good to excellent yields. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0029-1218730

PAPER
1771
Bromodimethylsulfonium Bromide–ZnCl2: A Mild and Efficient Catalytic
System for Beckmann Rearrangement
B
romodimethyl
a
sulfonium
B
romide–ZnC
D
l
-MediatedBec
k
h
mannRearra
a
ngement r S. Yadav,* Rajesh Patel, Vishnu P. Srivastava
2
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211002, India
Fax +91(532)2460533; E-mail: ldsyadav@hotmail.com
Received 23 February 2010; revised 3 March 2010
hydroxyapatite, metal ilerite, supported oxide, and zeo-
lites have been reported.10 However, these processes suf-
fer from high reaction temperature (300 °C), low
selectivity and can suffer from a rapid decay in the activity
of the catalyst.10 The liquid-phase catalytic BKR usually
proceeds under milder conditions, and has become a topic
of current interest due to its advantages such as ease of
work-up, and its practical application in industry. Various
inorganic catalysts, such as P2O5,5g HSO3Cl,7b
HSO3NH2,6h metallic Lewis acid-Yb(OTf)3,6a Ga(OTf)3,6c
RuCl3,5c HgCl2,7e AlCl3·6H2O/KI/H2O/MeCN,6e and
Abstract: Bromodimethylsulfonium bromide, in combination with
zinc chloride, has been shown to be an excellent catalytic system for
liquid-phase Beckmann rearrangement of various ketoximes into
the corresponding amides/lactams in acetonitrile at reflux tempera-
ture with good to excellent yields.
Key words: Beckmann rearrangement, ketoximes, amides, bro-
modimethylsulfonium bromide (BDMS), catalytic process, Lewis
acids
The exploitation of reagents or catalysts for developing
new synthetic protocols is an art and constitutes a chal-
lenging process in organic chemistry. Consequentially,
intense effort has been made over the years to design and
develop reagents or catalysts that minimise the drawbacks
of those presently in use. In this context, bromodimethyl-
sulfonium bromide (BDMS),1 a commercially available,
light-orange solid compound, has gained considerable
interest in organic synthesis after its development by
Meerwein,1c due to its ease of handling, its low cost, as
well as its ready access and varied applications both as a
catalyst2 and as an effective reagent.3
The Beckmann rearrangement (BKR)4 remains of prime
synthetic importance even today, essentially because it al-
lows easy access to a variety of N-substituted carboxylic
amides from ketoximes. It accomplishes, in one stroke,
both the cleavage of a carbon–carbon bond and the forma-
tion of a carbon–nitrogen bond, and represents a powerful
method, particularly for manufacturing e-caprolactam, in
the chemical industry. Conventional Beckmann rear-
rangement usually requires relatively high temperature
(~130 °C), and a large amount of strongly acidic and de-
hydrating media such as concentrated sulfuric acid, poly-
phosphoric acid, phosphorus pentachloride in diethyl
ether, thionyl chloride, or hydrogen chloride in a mixture
of acetic acid and acetic anhydride. Thus, this reaction can
lead to large amounts of waste and serious corrosion prob-
lems, which can preclude its application to sensitive sub-
strates. To avoid these requisite harsh conditions, several
methodologies in the liquid phase,5–7 supercritical water,8
ionic liquids,9 and vapour phase10 have been developed.
Needless to say, the development of vapour-phase BKR
was significant, and many catalyst systems such as boria-
6d
[RhCl(cod)]2 have been developed as new variants for
liquid-phase processes. However, due to drawbacks such
as toxicity, high cost, and corrosiveness, most of these cat-
alysts have not yet been industrially utilised. Therefore,
development of a simple, clean and inexpensive catalytic
system for liquid-phase BKR is still in demand.
Since reports from Chandrasekhar and Gopalaiah11 fo-
cused on small organic molecule (oxalic acid and chloral)
catalysed BKR, organocatalytic liquid-phase processes
have attracted researchers’ attention because of its effi-
cient catalytic activity and its ease of handling during the
rearrangement. Cyanuric chloride (CNC) – the first orga-
nocatalyst – was reported by Ishihara and co-workers to
be a highly efficient catalyst for the BKR.6g Recently,
bis(2-oxo-3-oxazolidinyl)phosphinic chloride (BOP-
Cl),7c 1,3,5-triazo-2,4,6-triphosphorine-2,2,4,4,6,6-chlo-
ride (TAPC)7a and p-toluenesulfonyl chloride (TsCl)7g
have all been employed as organocatalysts for BKR.
These reports, and our continued quest for the develop-
ment of environmentally friendly new synthetic method-
ologies,12 have prompted us to investigate the possible use
of BDMS as a organocatalyst for BKR in view of its ready
availability, its ease of handling and its known electro-
philic character. Herein, we present our results on the
highly effective BDMS-catalysed BKR of ketoximes to
the corresponding amides (Scheme 1).
As liquid phase BKR usually requires electrophilic activa-
tion of the oxime hydroxy group, we hypothesised that
BDMS might facilitate the transformation of ketoximes
Br
S+
Br–
Me
OH
Me
H
N
N
R2
BDMS
R1
SYNTHESIS 2010, No. 11, pp 1771–1776
x
x.
x
x
.
2
0
1
0
R1
R2
ZnCl2, MeCN, reflux
O
Advanced online publication: 09.04.2010
DOI: 10.1055/s-0029-1218730; Art ID: Z04310SS
© Georg Thieme Verlag Stuttgart · New York
Scheme 1 BDMS-ZnCl2-catalysed Beckmann rearrangement

1772
L. D. S. Yadav et al.
PAPER
into amides or lactams by forming an active O-dimethyl- of an acid HBr (generated in situ by the reaction of BDMS
sulfonium intermediate (a better leaving group) with the with ketoxime) catalysed reaction (Table 1, entry 11).
hydroxy group of the ketoxime, thus alleviating the need
The effect of solvent was also investigated. As evident
for traditional harsh conditions. A preliminary examina-
from Table 1, polar solvents such as acetonitrile (MeCN)
tion of the feasibility of BDMS-mediated BKR using ben-
and nitromethane (MeNO2) were both suitable for
zophenone oxime as a model substrate under various
BDMS-catalysed BKR (Table 1, entries 4 and 7). In tet-
conditions is compiled in Table 1. Initially, benzophenone
rahydrofuran (THF) and in 1,4-dioxane (Table 1, entries 8
oxime was treated with 25 mol% of BDMS in acetonitrile
and 10), the conversion of substrate (72–84%) proved to
at reflux temperature for two hours. Although the reaction
be satisfactory, but no selectivity for benzanilide was ob-
was found to occur, the substrate conversion rate and se-
served and rapid deoximation processes took place to give
lectivity were not ideal (Table 1, entry 1). In order to en-
the parent ketone, benzophenone. The use of non-polar
hance the catalytic activity of BDMS, we conducted the
1,2-dichloroethane (DCE; Table 1, entry 9) as solvent
same reaction in the presence of zinc(II) chloride whilst
gave unsatisfactory results in terms of both conversion
varying the mol% of catalyst and the temperature
and selectivity, which shows the important role of the sol-
(Table 1, entries 2–6). To our amazement, we observed a
vent in this reaction. Use of other Lewis acids as co-cata-
rapid and selective conversion of benzophenone oxime
lysts, namely: InCl3, FeCl3, SnCl4, MgCl2, and CuCl2, did
into benzanilide using 12 mol% each of BDMS and ZnCl2
not show any significant superiority over ZnCl2 (Table 1,
in acetonitrile at reflux temperature (Table 1, entry 3). A
entries 12–16). The generality and scope of the BDMS-
competitive rearrangement experiment of benzophenone
catalysed BKR was demonstrated across a range of ke-
oxime using 12 mol% HBr in conjugation with ZnCl2
toximes as substrates using BDMS (12 mol%) and ZnCl2
gave unsatisfactory results, thus ruling out the possibility
(12 mol%) in MeCN at reflux temperature (Table 2). The
results showed that excellent yields (81–96%) were ob-
Table 1 Optimisation of the BDMS–ZnCl2-Catalysed Beckmann Rearrangement of Benzophenone Oximea
OH
O
N
BDMS
N
H
solvent, 2 h
Entry
1
Solvent
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeNO2
THF
BDMS (mol%)
Cocatalyst (mol%)
–
Temp (°C)
reflux
reflux
reflux
50
Conversion (%)b Selectivity (%)c
25
25
12
12
8
60
100
100
60
80
–
94
98
98
98
98
–
2
ZnCl2 (25)
ZnCl2 (12)
ZnCl2 (12)
ZnCl2 (12)
ZnCl2 (12)
ZnCl2 (12)
ZnCl2 (12)
ZnCl2 (12)
ZnCl2 (12)
ZnCl2 (12)
InCl3 (12)
FeCl3 (12)
SnCl4 (12)
MgCl2 (12)
CuCl2 (12)
3
4
5
reflux
reflux
82
6
–
7
12
12
12
12
96
72
60
84
30
94
80
20
40
30
88
–
8
reflux
reflux
82
9
DCE
25
–
10
11
12
13
14
15
16
1,4-dioxane
MeCN
d
–
reflux
reflux
reflux
reflux
reflux
reflux
42
80
96
70
68
80
MeCN
12
12
12
12
12
MeCN
MeCN
MeCN
MeCN
a Reaction conditions: benzophenone oxime (197 mg, 1 mmol), solvent (3 mL).
b Conversion (%) of benzophenone oxime as determined by GC analysis.
c Selectivity for benzanilide.
d HBr (48%, 12 mol%, 0.012 mL) was used in place of BDMS.
Synthesis 2010, No. 11, 1771–1776 © Thieme Stuttgart · New York

PAPER
Bromodimethylsulfonium Bromide–ZnCl2-Mediated Beckmann Rearrangement
1773
tained for aromatic ketoximes (Table 2, entries 1–11), found. In the reactions of unsymmetrical benzophenone
with 96–100% substrate conversion and up to 97–99% se- oximes, (4-fluorophenyl)phenylmethanone oxime gave a
lectivity for the amide being observed. Cyclododecanone mixture of isomeric amides N-(4-fluorophenyl)benz-
oxime was also very reactive and was converted into the amide and 4-fluoro-N-phenylbenzamide in a ratio of
corresponding amide (a starting material for nylon) in 0.9:1.0 (Table 2, entry 2). Similarly, (4-methoxyphe-
91% yield (Table 2, entry 14). As evident from Table 2, nyl)phenylmethanone oxime produced N-(4-methoxyphe-
moderate to good results were obtained from the BKR of nyl)benzamide and 4-methoxy-N-phenylbenzamide as an
cyclopentanone, cyclohexanone and acetone oximes isomeric mixture in a ratio of 1.0:0.8 (Table 2, entry 3).
(Table 2, entries 12, 13 and 15). The conversion of all However, when the migratory aptitudes of the two substit-
these oximes was nearly 100%, however, the selectivities uents were close, a mixture of products was obtained
were only 71%, 64%, and 68%, respectively. Based on the (Table 2, entry 4). Pivalophenone oxime (Table 2, entry
qualitative analysis by GC-MS, it could be seen that the 5), as anticipated, produced both N-tert-butylbenzamide
main by-products were the corresponding ketones and and pivalanilide, but the migration of phenyl group was
only trace amounts of dimeric oximes were observed still favoured over that of the tert-butyl group by a factor
(0.2%). The usual migratory aptitude of BKR was fol- of four. These results imply that electron-rich aryl groups
lowed with the listed substrates. For example, in all the have better migrating aptitude than the alkyl group toward
substituted acetophenone oximes (Table 2, entries 4 and the oximino nitrogen terminus and, thus, a cationic spe-
6–11), only migration of the aryl group was observed with cies on the oximino nitrogen terminus is involved (vide
no product from migration of the methyl group being infra).
a
Table 2 Generality and Scope of the Beckmann Rearrangement Catalysed by BDMS–ZnCl2
Entry Ketoxime
Amide/lactam
Time (h) Conversion (%)b Selectivity (%)c Yield (%)d,e
OH
O
N
1
2
3
100
98
94
N
H
F
O
0.9
1.0
OH
N
H
N
2
99
97
83f
O
F
N
H
F
OMe
O
1.0
OH
N
H
N
3
1.5
100
97
89f
O
0.8
MeO
N
H
MeO
OH
H
N
N
4
2
2
100
99
97
96
O
H
N
OH
5.3
1.0
N
O
O
5
98
81f
N
H
Synthesis 2010, No. 11, 1771–1776 © Thieme Stuttgart · New York

1774
L. D. S. Yadav et al.
PAPER
a
Table 2 Generality and Scope of the Beckmann Rearrangement Catalysed by BDMS–ZnCl2 (continued)
Entry Ketoxime
Amide/lactam
Time (h) Conversion (%)b Selectivity (%)c Yield (%)d,e
OH
MeO
H
N
MeO
N
6
7
1
100
100
99
99
91
88
O
OH
OH
H
N
N
MeO
1.3
MeO
O
O
H
N
N
8
9
1
100
98
99
97
98
97
90
86
84
82
MeO
MeO
OH
H
N
N
3
O
F
F
OH
Br
H
N
Br
N
10
11
2.8
98
O
OH
H
N
N
5
96
O
O2N
O2N
N
O
OH
OH
12
13
3
3
99
99
71
64
64
59
NH
O
N
NH
O
NH
14
15
2.5
2
99
99
97
68
91
60
OH
N
O
OH
N
Me
N
H
a See experimental section for general procedure.
b Conversion of ketoxime as determined by GC analysis.
c Selectivity for amides/lactams.
d All the products are known compounds6g,7e and were characterised by comparison of their mp or bp, TLC, IR and 1H NMR data with those of
authentic samples.
e Yields of the isolated pure compounds.
f Overall yield of isomeric mixture.
Synthesis 2010, No. 11, 1771–1776 © Thieme Stuttgart · New York

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Bromodimethylsulfonium Bromide–ZnCl2-Mediated Beckmann Rearrangement
1775
1
corded with a Perkin–Elmer 993 IR spectrophotometer. H NMR
spectra were recorded with a Bruker AVII 400 spectrometer in
CDCl3/DMSO-d6 using TMS as internal reference. GC-MS were
obtained on a Hewlett–Packard model GCD-HP1800A.
R1
R2
N
O
OH
R1
R2
Me
Br
Me
N
H
S Br
BDMS–ZnCl2-2Catalysed Beckmann Rearrangement; General
Procedure
BDMS
To a solution of ketoxime (1 mmol) in MeCN (3 mL), BDMS (12
mol%) and ZnCl2 (12 mol%) were added at r.t. and the reaction mix-
ture was heated at reflux for 1–5 h (Table 2). After completion of
the reaction, as indicated by TLC, it was quenched with sat. aq
NaHCO3 (10 mL). The organic layer was extracted with EtOAc
(3 × 10 mL), dried over anhydrous MgSO4, and concentrated in
vacuo to yield the crude product, which was purified by silica gel
column chromatography (EtOAc–hexane) to give the correspond-
ing amide. The structure of the products were confirmed by compar-
R1
R2
Me
N
Br
S
O
H
S
Br
Me
Me
Me
Br
R1
Br
S
H
O
Me
Me
N
Br
R2
OH
1
ison of their mp or bp, TLC, IR or H NMR data with authentic
R1
samples obtained commercially or prepared by literature meth-
R2
Br
ods.6g,7e
N
Since all the products are known, the spectral data of only some typ-
ical compounds are given below:
Scheme
2
A
plausible catalytic cycle for BDMS-catalysed
Beckmann rearrangement
N-Phenylbenzamide (Table 2, Entry 1)
IR (KBr): 3344, 1655, 1599, 1551, 1430, 1374, 1318, 760, 692
cm–1.
In agreement with the earlier observations on the BKR,
we also found that electron-donating groups on the aro-
matic ring (Table 2, entries 3 and 6–8) facilitate the reac-
tion, whereas electron-withdrawing groups (Table 2,
entries 2 and 9–11) retard it. The starting oximes used in
the present study were mixtures of syn and anti isomers
(where applicable) but, in most cases, a majority of a sin-
gle amide isomer – depending on the migratory aptitude –
was obtained as the product. This indicates that BDMS is
also capable of catalysing the syn–anti isomerisation of
oximes under the conditions of the BKR. The equilibrium
between the isomers is apparently established faster than
the rearrangement, so that the product composition is de-
1H NMR (400 MHz, CDCl3): d = 7.91 (br s, 1 H), 7.85–7.83 (m,
2 H), 7.62 (d, J = 7.9 Hz, 2 H), 7.54–7.49 (m, 1 H), 7.46–7.43 (m,
2 H), 7.37–7.31 (m, 2 H), 7.14–711 (m, 1 H).
N-Phenylacetamide (Table 2, Entry 4)
IR (KBr): 3293, 3155, 1657, 1597, 1555, 1500, 1434, 1369, 1322,
1264, 755, 695 cm–1.
1H NMR (400 MHz, CDCl3): d = 8.44 (br s, 1 H), 7.51 (d, J = 8.5
Hz, 2 H), 7.25 (t, J = 7.8 Hz, 2 H), 7.03 (t, J = 7.2 Hz, 1 H), 2.09 (s,
3 H).
N-(2-Methoxyphenyl)acetamide (Table 2, Entry 6)
IR (KBr): 3250, 2924, 1654, 1541, 1496, 1458, 1252, 1117, 1025,
termined by the relative rates of migration of the involved 750 cm–1.
1H NMR (400 MHz, CDCl3): d = 8.36 (dd, J = 1.8, 7.8 Hz, 1 H),
7.77 (br s, 1 H), 7.08–6.91 (m, 2 H), 6.87 (dd, J = 1.8, 8.1 Hz, 1 H),
3.88 (s, 3 H), 2.21 (s, 3 H).
groups and is independent of the stereochemistry of the
starting oximes. Based on the fact that BDMS can be gen-
erated from dimethylsulfoxide (DMSO) and HBr in situ,13
a plausible catalytic cycle for the BDMS-mediated BKR
is outlined in Scheme 2.
N-(3-Methoxyphenyl)acetamide (Table 2, Entry 7)
IR (KBr): 3257, 2924, 2853, 1664, 1607, 1562, 1494, 1417, 1282,
1155, 1051, 860, 766, 688 cm–1.
1H NMR (400 MHz, CDCl3): d = 7.77 (br s, 1 H), 7.29–7.17 (m,
2 H), 6.94 (d, J = 8.0 Hz, 1 H), 6.63 (dd, J = 2.1, 8.1 Hz, 1 H), 3.80
(s, 3 H), 2.17 (s, 3 H).
In conclusion, BDMS in combination with zinc(II) chlo-
ride has been employed here for the first time as a mild
and efficient catalytic system for the conversion of a wide
range of ketoximes into the corresponding amides/lac-
tams via Beckmann rearrangement. The operational sim-
plicity, general applicability, use of inexpensive and
commercially available catalyst, relatively fast reaction
rate, and high yields makes this protocol an attractive and
valid alternative to existing methodologies. The present
work has opened up a new aspect to the synthetic utility of
BDMS.
N-(2-Fluorophenyl)acetamide (Table 2, Entry 9)
IR (KBr): 3290, 3071, 2923, 2853, 1662, 1617, 1559, 1507, 1402,
1235, 836 cm–1.
1H NMR (400 MHz, CDCl3): d = 7.45 (dd, J = 4.8, 9.0 Hz, 2 H),
7.18 (br s, 1 H), 7.01 (d, J = 9.0 Hz, 2 H), 2.17 (s, 3 H).
Azepan-2-one (Table 2, Entry 13)
IR (KBr): 3211, 3075, 2928, 2856, 1654, 1486, 1438, 1417, 1365,
1198, 1125, 823 cm–1.
1H NMR (400 MHz, CDCl3): d = 7.46 (br s, 1 H), 3.21–3.16 (m,
2 H), 2.45–2.40 (m, 2 H), 1.75–1.71 (m, 2 H), 1.68–1.62 (m, 4 H).
All starting materials and authentic samples were purchased as re-
agent grade and were used without further purification unless other-
wise specified by a reference. All solvents were distilled prior to
use. Column chromatography was carried out with silica gel (Merck
100–200 mesh) and TLC was performed using silica gel GF254
(Merck) plates. Melting points were determined by the open glass
capillary method and are uncorrected. IR spectra in KBr were re-
Azacyclotridecan-2-one (Table 2, Entry 14)
IR (KBr): 3299, 2931, 2854, 1639, 1546, 1448, 1059 cm–1.
Synthesis 2010, No. 11, 1771–1776 © Thieme Stuttgart · New York

1776
L. D. S. Yadav et al.
PAPER
(7) (a) Hashimoto, M.; Obora, Y.; Sakaguchi, S.; Ishii, Y.
1H NMR (400 MHz, CDCl3): d = 7.10 (br s, 1 H), 3.31 (dd, J = 6.0,
10.8 Hz, 2 H), 2.18–2.22 (m, 2 H), 1.64–1.70 (m, 2 H), 1.48–1.51
(m, 2 H), 1.32 (br, 14 H).
J. Org. Chem. 2008, 73, 2894. (b) Li, D.; Shi, F.; Guo, S.;
Deng, Y. Tetrahedron Lett. 2005, 46, 671. (c) Zhu, M.;
Cha, C.; Deng, W.; Shi, X. Tetrahedron Lett. 2006, 47,
4861. (d) Sardarian, A. R.; Shahsavari-Fard, Z.; Shahsavari,
H. R.; Ebrahimi, Z. Tetrahedron Lett. 2007, 48, 2639.
(e) Ramalingan, C.; Park, Y.-T. J. Org. Chem. 2007, 72,
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Acknowledgment
We sincerely thank SAIF, Punjab University, Chandigarh, for pro-
viding microanalyses and spectra. One of us (R.P.) is grateful to the
CSIR, New Delhi, for the award of a Junior Research Fellowship.
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