Bromodimethylsulfonium bromide (BDMS) in ionic liquid: a mild and efficient catalyst for Beckmann rearrangement

Article abstract of DOI:10.1016/j.tetlet.2009.11.128

Bromodimethylsulfonium bromide (BDMS)-catalyzed Beckmann rearrangement of a variety of ketoximes has been carried out in the imidazolium-based ionic liquid [bmim]PF₆ under mild conditions without using any additional cocatalyst or solvent to afford excellent conversion and selectivity. The ionic liquid is recovered and reused for up to three runs without any loss of efficiency.

Full text of DOI:10.1016/j.tetlet.2009.11.128

Tetrahedron Letters 51 (2010) 739–743
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Bromodimethylsulfonium bromide (BDMS) in ionic liquid: a mild
and efficient catalyst for Beckmann rearrangement
*
Lal Dhar S. Yadav , Garima, Vishnu P. Srivastava
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Bromodimethylsulfonium bromide (BDMS)-catalyzed Beckmann rearrangement of a variety of ketoximes
Received 19 October 2009
Revised 23 November 2009
Accepted 27 November 2009
Available online 2 December 2009
6
has been carried out in the imidazolium-based ionic liquid [bmim]PF under mild conditions without
using any additional cocatalyst or solvent to afford excellent conversion and selectivity. The ionic liquid
is recovered and reused for up to three runs without any loss of efficiency.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Beckmann rearrangement
Ketoximes
Amides
Bromodimethylsulfonium bromide (BDMS)
Ionic liquids
In general, amides are potential precursors for the synthesis of
various natural products as well as synthetic intermediates for
Among the various possibilities for realizing these reactions un-
der milder conditions, the use of room-temperature ionic liquids
(ILs) instead of the more common polar media has been reported.
1
10
medicinal compounds and materials. The Beckmann rearrange-
2
ment (BKR) is an important reaction for transformation of ketox-
Ionic liquids have negligible vapour pressure and excellent thermal
stability that combined with easy preparation, recycling, and good
solvating ability of a wide range of substrates and catalysts, make
them viable ecosustainable solvents for a number of stoichiometric
imes into amides, which has been successfully utilized to produce
e
-caprolactam and laurolactam in industry. This reaction generally
requires high reaction temperature and a large amount of a strong
3
11
Brønsted acid and dehydrating media. Thus, the reaction leads to
and catalytic processes. This class of novel solvents is attracting
large amounts of by-product precluding its application to sensitive
substrates. To avoid these requisite harsh conditions, several meth-
odologies under vapour phase, solvent-free, supercritical water,
increasing popularity as a potential green alternative to conven-
tional volatile organic media (DMF, CH CN, etc.) in view of the pro-
3
4
5
6
jected advantages such as environmental compatibility,
reusability, greater selectivity, operational simplicity, non-corro-
7
–9
or liquid phase
conditions have been developed. Of the liquid
8f
11
phase processes, cyanuric chloride (CNC), 1,3,5-triazo-2,4,6-trip-
sive nature, and ease of isolation.
9a
8g
12
hosphorine-2,2,4,4,6,6-chloride (TAPC), sulfamic acid, chloro-
Bromodimethylsulfonium bromide (BDMS), a commercially
9
b
sulfonic acid,
bis(2-oxo-3-oxazolidinyl) phosphinic chloride
available light orange solid compound, has attracted considerable
interest in the field of organic chemistry after the discovery by
9
c
9d
9e
(
BOP-Cl), diethyl chlorophosphate, HgCl
romethyl) carbonate/DMF, p-toluenesulfonyl chloride (TsCl),
trifluoroacetic acid,
2
/CH
3
CN, bis(trichlo-
9f
9g
12c
Meerwein, due to its easy handling, low cost, as well as its easy
9
h
9i
13
p-toluenesulfonic acid (TsOH),
and
access and varied applications both as a catalyst and as an effec-
9j
14
mesitylenesulfonyl chloride catalyzed BKR are elegant and recent
approaches. However, some of these variants suffer from draw-
backs such as toxicity, the use of toxic solvents, expensive reagents,
production of considerable amounts of by-products, long reaction
times and low yields. Therefore, the development of a simple, mild,
inexpensive catalyst for highly efficient and selective catalytic BKR
process is still in demand.
tive reagent.
However, its synthetic utility as a catalyst for the BKR has not
been explored until now. Very recently, we have demonstrated
the virtue of this reagent for the conversion of aldoximes and pri-
1
5a
mary carboxamides to the corresponding nitriles.
In continua-
tion of the research directed towards the development of new
1
5
and efficient synthetic methodologies, we sought to explore the
advantages of this reagent further for other important transforma-
tions. In this report, we present our results on the first highly effec-
tive BDMS-catalyzed BKR of ketoximes to the corresponding
amides in which the molar ratio of BDMS and oxime was 0.20:1.
*
Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533.
E-mail address: ldsyadav@hotmail.com (Lal Dhar S. Yadav).
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.128

7
40
Lal Dhar S. Yadav et al. / Tetrahedron Letters 51 (2010) 739–743
Thus, it is a green process for preparation of amides from ketoxi-
mes precluding the use of any additional Lewis or Bronsted acids
as a cocatalyst, toxic organic solvents and without producing any
significant corrosive waste (Scheme 1).
To optimize the reaction conditions and find the right solvent
using acetophenone oxime as a model substrate, few experiments
were carried out with different solvents at varied reaction temper-
ature and mol % of catalyst as illustrated in Table 1. BDMS was
found to be an effective reagent for BKR in polar nucleophilic or-
6
in [bmim]PF for 1–6 h (Table 2). Moderate to good results were
obtained over BKR of acetone, 2,4-dimethylpentanone, cyclo-
pentanone, and cyclohexanone oximes (entries 1–4). The conver-
sion of all these oximes was nearly 100%, however, the
selectivities were only 80%, 76%, 72%, and 69%, respectively. Based
on the qualitative analysis by GC–MS, it could be known that their
main by-products were the corresponding ketones and only trace
amounts of dimeric oximes were observed (0.2%). Much better re-
sults could be obtained if aryl ketoximes were used as substrates
in BKR.
3 3 2
ganic solvents (CH CN and CH NO ) as indicated in Table 1 (entries
5
and 6). However, in order to make BKR catalytic we chose to per-
The conversions of aromatic ketoximes, for example, 2-, 3-, 4-
methoxyacetophenone oximes, 2-, 4-haloacetophenone oximes,
4-nitroacetonephenone oxime and symmetrical and unsymmetrical
benzophenone oximes were 94–100% with selectivities >94% (entries
5–15). Usual migratory aptitude of BKR is followed with the listed
substrates. For example, in all cases of substituted acetophenone oxi-
mes (entries 5 and 7–12), only migration of the aryl group is observed
without any product from migration of the methyl group. In the
reactions of the unsymmetrical benzophenone oxime (4-fluoro-
phenyl)-phenylmethanone oxime gave a mixture of isomeric amides
N-(4-fluorophenyl)benzamide and 4-fluoro-N-phenylbenzamide in
the ratio of 0.8:1.0 (entry 14). Similarly, (4-methoxyphenyl)-phenyl-
methanone oxime produced N-(4-methoxyphenyl)benzamide and
4-methoxy-N-phenylbenzamide as an isomeric mixture in the ratio
of 1.0:0.7 (entry 15). However, if the migratory aptitudes of the two
substituents are close, mixture of products was obtained. Pivalophe-
noneoxime(entry6)asanticipatedproducesbothN-tert-butylbenza-
mide and pivalanilide, but the migration of phenyl group is still
favored over that of the tert-butyl group by a factor of 4. These results
imply that electron-rich aryl groups have better migrating aptitude
than the alkylgroupstowardthe oximino nitrogenterminus andthus,
cationic species of the oximino nitrogen terminus is involved (vide in-
fra). In conformity with the earlier observations on the BKR, we also
found that electron-donating groups on the aromatic ring (entries
7–9, 15) facilitate the reaction, and electron-withdrawing groups (en-
tries 10–12, 14) retard it. It has been recognized that BKR is stereospe-
cific and generally the group anti to the hydroxyl group on the
ketoxime selectively migrates. This behaviour has also been used to
assign syn or anti configuration of oximes. 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 single amide isomer
was obtained as the product, which is favoured based on the migra-
tory aptitude. This indicates that BDMS is also capable of catalyzing
the syn–anti isomerization of oximes under the conditions of BKR.
The equilibrium among involved isomers is faster than the rearrange-
ment so that the product composition is determined by the relative
rates of migration of the involved groups and is independent of the
stereochemistry of the starting oximes.
form the same reaction in ionic liquid and to our delight, it was
found that the treatment of acetophenone oxime with catalytic
amount of BDMS (20 mol %) in [bmim]PF
ilide in a selectivity of 99% with almost complete conversion (Table
, entry 9), whereas the same reaction in other ionic liquids such as
bmim]Br, [bmim]Cl, and [bmim]BF accomplished the rearrange-
ment far less effectively with 0–30% conversion (Table 1, entries
, 8 and 10). The above reaction condition was appropriate for
6
at 80 °C afforded acetan-
1
[
4
7
the transformation because lowering the reaction temperature
from 80 to 50 °C and decreasing the amount of catalyst from 20
to 10 mol % lowered the substrate conversion rate (Table 1, entries
9
, 11 and 12). No conversion was observed in the absence of BDMS
(Table 1, entry 13).
To explore the generality and scope of the BKR catalyzed by
BDMS, various ketoximes as substrates were examined at 80 °C
Br
Br
S
BDMS
OH
H
N
R2
N
R1
o
R1 R2
[bmim]PF , 80 C
6
O
1
2
R , R = phenyl, alkyl, cycloalkyl
Scheme 1. Beckmann rearrangement.
Table 1
BDMS-catalyzed Beckmann rearrangement of acetophenone oxime in different
solventsa
OH
H
N
BDMS
N
Solvent, 2h
O
Entry Solvent
Reaction temp (°C) Conversionb (%) Selectivityc (%)
After isolation of products (amides/lactams), the ionic liquid
1
2
3
4
5
6
7
8
9
0
1
2
3
THF
CH
DCE
Reflux
80
Reflux
Reflux
Reflux
80
80
80
80
80
20
30
25
39
100
96
30
20
100
—
0
86
0
98
98
86
54
0
99
—
96
98
—
16
[
bmim]PF
6
was easily recovered, and reused without any signif-
3
NO
2
icant loss of activity. For example, the acetophenone oxime rear-
ranged to the corresponding amide in similar yields and purity of
the first run over three cycles (91%, 88%, 90%, respectively). Based
on the fact that BDMS can be in situ generated from DMSO and
CH
3
CH
3
CH
3
CN
CN
NO
d
d
2
[bmim]Br
[bmim]Cl
[bmim]PF
17
HBr, a plausible catalytic cycle for BDMS-mediated Beckmann
rearrangement is outlined in Scheme 2.
6
1
1
1
1
[bmim]BF
4
In conclusion, we have disclosed a mild and green procedure for
obtaining amides/lactams from the corresponding ketoximes via
Beckmann rearrangement employing BDMS in the ionic liquid
[bmim]PF
[bmim]PF
[bmim]PF
6
6
6
50
80
80
40
61
—
e
f
[
6
bmim]PF under catalytic conditions in the absence of any addi-
a
Reaction condition: 135.0 mg (1 mmol) acetophenone oxime, 44.4 mg
tional cocatalyst or solvent. The operational simplicity, general
applicability, use of inexpensive and commercially available cata-
lyst, relatively fast reaction rate, high yields and recyclability of
the ionic liquid makes this protocol an attractive and valid alterna-
tive to existing methodologies. The present work has opened up a
new aspect of the synthetic utility of BDMS.
(
0.2 mmol) BDMS, solvent (2 mL).
b
Conversion (%) of acetophenone oxime as determined by GC analysis.
Selectivity for acetanilide.
c
d
2
2
22 mg (1 mmol) BDMS was used.
2.2 mg (0.1 mmol) BDMS was used.
e
f
In the absence of BDMS.

Lal Dhar S. Yadav et al. / Tetrahedron Letters 51 (2010) 739–743
741
Table 2
Beckmann rearrangement of ketoximes/lactams to amides in [bmim]PF
6
using 20 mol % BDMS as the catalysta
OH
N
H
R2
BDMS
N
R1
[bmim]PF , 80 0C
R1
R2
6
O
R 1, R 2 = Phenyl, alkyl and cycloalkyl
Entry
1
Ketoxime
Amide/lactam
Time (h)
2
Conversionb (%)
99
Selectivityc (%)
80
Yieldd,e (%)
OH
N
O
71
68
N
H
O
OH
N
2
3
2.5
3
99
99
76
72
N
H
OH
O
N
N
64
60
NH
OH
O
NH
4
5
3
2
99
69
99
OH
OH
H
N
N
100
94
O
O
1
H
N
N
N
H
6
+
2
98
97
81f
O
5.3
:
OH
MeO
H
N
MeO
MeO
N
7
8
9
1
100
100
100
99
99
99
91
88
90
O
OH
OH
H
N
MeO
MeO
N
1.2
1
O
O
H
N
N
M
eO
OH
H
N
N
1
0
1
3
98
98
98
96
86
84
O
F
F
OH
Br
H
Br
N
N
1
2.7
O
OH
H
N
N
1
2
3
6
2
97
94
98
81
95
O
O N
2
O N
2
OH
N
H
N
1
100
O
(
continued on next page)

7
42
Lal Dhar S. Yadav et al. / Tetrahedron Letters 51 (2010) 739–743
Table 2 (continued)
Entry Ketoxime
Amide/lactam
Time (h)
Conversionb (%)
Selectivityc (%)
Yieldd,e (%)
OH
F
N
H
N
H
N
1
4
5
3
99
97
84f
+
O
O
O
F
F
1.0
0
.8
:
OH
OMe
H
N
N
H
N
1
1.5
100
97
87f
+
O
MeO
MeO
1
.0
:
0.7
a
b
c
d
e
f
See Ref. 16 for general procedure.
Conversion (%) of ketoxime as determined by GC analysis.
Selectivity for amides/lactams.
All the products are known compounds
Yields of the isolated pure compounds.
Overall yield of isomeric mixture.
8c,9e
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rearrangement.
1
(
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Acknowledgements
We sincerely thank SAIF, CDRI, Lucknow, for providing microa-
nalyses and spectra. One of us (Garima) is grateful to the CSIR, New
Delhi, for the award of a Junior Research Fellowship.
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reaction progress was monitored by TLC. Upon completion, the reaction
mixture was cooled to rt and extracted with ether (4 Â 10 mL). The combined
4
1
4
organic phase was dried over MgSO , filtered and evaporated under reduced
pressure. The resulting crude product was purified by silica gel column
chromatography (EtOAc/hexane 3:7) to give the corresponding amide. The
structure of the products was confirmed by comparison of their mp, TLC, IR or
2
1
1
5. (a) Yadav, L. D. S.; Srivastava, V. P.; Patel, R. Tetrahedron Lett. 2009, 50, 5532; (b)
Yadav, L. D. S.; Patel, R.; Srivastava, V. P. Synlett 2008, 583; (c) Yadav, L. D. S.;
Patel, R.; Srivastava, V. P. Synlett 2008, 1789; (d) Yadav, L. D. S.; Patel, R.;
Srivastava, V. P. Tetrahedron Lett. 2007, 48, 7793; (e) Yadav, L. D. S.; Garima;
Kapoor, R. Tetrahedron Lett. 2009, 50, 5420; (f) Yadav, L. D. S.; Kapoor, R.;
Garima Synlett 2009, 1055.
H NMR data with authentic samples obtained commercially or prepared by
literature method. The residue of the ionic liquid was dissolved in CH
filtered on Celite. The filtrate was washed with water followed by saturated
aqueous K CO solution in order to remove residual acid and other impurities
2 2
Cl ,
2
3
and dried under vacuum to afford the ionic liquid [bmim]PF
in subsequent runs.
6
, which was used
1
6. General procedure for bromodimethylsulfonium bromide (BDMS)-catalyzed
Beckmann rearrangement: A stirred solution of ketoxime (1 mmol) and BDMS
17. (a) Majetich, G.; Hicks, R.; Reister, S. J. Org. Chem. 1997, 62, 4321; (b) Mislow,
K.; Simmons, T.; Melillo, J.; Ternay, A. J. Am. Chem. Soc. 1964, 86, 1452; (c)
Harrowven, D. C.; Dennison, S. T. Tetrahedron Lett. 1993, 34, 3323.
(
6
0.2 mmol) in 2 mL of [bmim]PF was heated at 80 °C for 1.5–6 h (Table 2). The