Ph3P/I2-Catalyzed beckmann rearrangement of ketoximes into amides

Article abstract of DOI:10.1080/00397911.2011.585270

A Beckmann rearrangement of ketoximes reaction using triphenylphosphine/ iodine as an effective catalyst in acetonitrile at reflux temperature is reported. The results indicate that conversion of oximes to amides can be reached in good to excellent yields under optimal reaction conditions within several minutes. The reaction mechanism is also proposed. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2011.585270

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Ph3P/I2-Catalyzed Beckmann
Rearrangement of Ketoximes into
Amides
Feng Xu a , Ni-Ge Wang a , You-Ping Tian a , Yong-Mei Chen b & Wei-
Chun Liu a
a Key Laboratory of Macromolecular Science of Shaanxi Province,
School of Chemistry and Materials Science, Shaanxi Normal
University, Xi'an, Shaanxi, China
b Department of Chemistry, School of Science, Xi'an Jiaotong
University, Xi'an, China
Accepted author version posted online: 12 Jan 2012. Version of
record first published: 02 Aug 2012
To cite this article: Feng Xu, Ni-Ge Wang, You-Ping Tian, Yong-Mei Chen & Wei-Chun Liu (2012): Ph3P/
I2-Catalyzed Beckmann Rearrangement of Ketoximes into Amides, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 42:23, 3532-3539
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Synthetic Communications1, 42: 3532–3539, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.585270
Ph3P/I2-CATALYZED BECKMANN REARRANGEMENT
OF KETOXIMES INTO AMIDES
Feng Xu,1 Ni-Ge Wang,1 You-Ping Tian,1 Yong-Mei Chen,2
and Wei-Chun Liu1
1Key Laboratory of Macromolecular Science of Shaanxi Province,
School of Chemistry and Materials Science, Shaanxi Normal University,
Xi’an, Shaanxi, China
2Department of Chemistry, School of Science, Xi’an Jiaotong University,
Xi’an, China
GRAPHICAL ABSTRACT
Abstract A Beckmann rearrangement of ketoximes reaction using triphenylphosphine/
iodine as an effective catalyst in acetonitrile at reflux temperature is reported. The results
indicate that conversion of oximes to amides can be reached in good to excellent yields
under optimal reaction conditions within several minutes. The reaction mechanism is also
proposed. Supplemental materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications to view the free supplemental file.
Keywords Amide; Beckman rearrangement; iodine; ketoximes; triphenylphosphine
INTRODUCTION
Amides are important compounds for chemicals. The classical method for prepa-
ration of these compounds is conversion of ketoxime into corresponding amide,
known as the Beckmann rearrangement,[1,2] which is of tremendous interest to all
practicing organic chemists because the reaction insert a nitrogen into a carbon
framework. It is a powerful method, particularly for manufacturing e-caprolactam
in the chemical industry. However, the conventional Beckmann rearrangement
usually requires harsh conditions such as high temperature and use of an excess
amount of strongly Brønsted acid such as concentrated H2SO4 or polyphosphoric
acid (PPA), which cause serious problems such as corrosion of equipment and
Received March 31, 2011.
Address correspondence to Feng Xu, Key Laboratory of Macromolecular Science of Shaanxi
Province, School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an, Shaanxi
710062, China, or Yong-Mei, Chen, Department of Chemistry, School of Science, Xi’an Jiaotong
University, Xi’an 710049, China. E-mail: fengxu@snnu.edu.cn; chenym@mail.xjtu.edu.cn
3532

Ph3P/I2-CATALYZED BECKMANN REARRANGEMENT
3533
environmental pollution. To overcome these requisite harsh problems, chemists
search for many methods such as vapor-phase,[3–6] solvent-free,[7–9] supercritical
water,[10–13] and liquid-phase processes, such as Beckmann rearrangement catalyzed
by cyanuric chloride (CNC),[14] sulfamic acid (NH2SO3H),[15] chlorosulfomic acid,[16]
bis(2-oxo-3- oxazolidinyl)phosphinic chloride (BOP-Cl),[17] diethyl chlorophosphate
(Et2PO3Cl),[18] HgCl2,[19] bis(tri-chloromethyl) carbonate,[20] p-toluenesulfonic chloride
(TsCl),[21] trifluoro acetic acid,[22] p-toluenesulfonic acid (TsOH),[2] and mesitylenesulfo-
nyl chloride.[23,24] However, some of these methods still have disadvantages, such as
toxicity of catalyst, tedious workup, vigorous reaction conditions, expensive reagents,
difficulty in handing, long reaction time, and unsatisfactory yields. More recently, I2-
mediated Beckmann rearrangement[25] has been reported, which required 0.5 equivalent
I2 for optimum yields. PPh3=NCS[26] has been reported as reaction intermediate in the
Beckmann rearrangement reaction, but it needed 1.2 equivalents of PPh3 and
N-chlorosuccinimide (NCS). Therefore, it is necessary to develop a simple, clean, inex-
pensive catalyst system for highly efficient and selective catalytic Beckmann rearrange-
ment process. In continuation of our previous work on applications of PPh3 as a catalyst
in an organocatalytic reaction,[27] here we report a facile method for Beckmann
rearrangement of ketoximes to amides using triphenylphoshine=iodine (Ph3P=I2) as
an efficient catalyst under mild reaction conditions.
Triphenylphosphine (Ph3P) and iodine (I2) are commercially available, cheap che-
micals with outstanding physical properties and stability. Recently, it has been shown
that Ph3P has the prospect to be used as a substitute for conventional basic catalytic
materials.[28–31] Also, I2 is a useful acidic catalytic material. In homogeneous liquid reac-
tions, Ph3P=I2 is easy to handle as a catalyst owing to its miscibility with water and
unique catalytic features. Intrinsic zwitterionic property of Ph3P=I2 is very different from
that of conventional catalysts.[32,33] All those features prompted us to explore further
applications of Ph3P=I2 as a special catalyst in carbon–nitrogen bond-forming reactions.
We report herein, for the first time, Ph3P=I2-catalyzed Beckmann rearrangement of
ketoximes to produce corresponding amides (Scheme 1) without producing any waste.
Initially, I2 was chosen for the catalytic Beckmann rearrangement. As shown in
Table 1, the transformation cannot proceed in various solvents and different tem-
peratures using only I2. We also used Ph3P as catalyst, but the results were disap-
pointing. We studied the reaction of acetophenone oximes with Ph3P in the
presence of various electrophilic halogen sources such as N-bromosuccinimide
(NBS), iodine (I2), and CCl4. The results are shown in Table 1. Results in Table 1
show that Ph3P=I2 is more efficient than Ph3P=NBS and Ph3P=CCl4. The reaction
of acetophenone oximes with PPh3=I2 occurs at reflux temperature in CH3CN to
produce phenyl acetamide in excellent yields (entries 11 and 12); however, the same
reaction with Ph3P=CCl4 as catalyst produces phenyl acetamide only in 10% yield,
Scheme 1. Beckmann rearrangement catalyzed by PPh3=I2.

3534
F. XU ET AL.
Table 1. Different catalysts catalyze Beckmann rearrangement of acetophenone oxime in different
solvents
Entry
Catalyst (amount mol%)
Solvent (temp.)
Time (min)
Yielda (%)
1
2
I2 (5)
I2 (10)
I2 (10)
DMF (rt)
DMF (rt)
180
180
180
180
180
180
180
180
180
240
5
—
—
—
—
—
—
—
—
20
20
96
90
10
90
3
CH3CN (rt)
CH3CN (reflux)
Acetone (rt)
4
I2 (10)
I2 (10)
I2 (10)
5
6
Acetone (reflux)
DMF (rt)
7
PPh3=I2 (10=10)
PPh3=I2 (10=10)
PPh3=I2 (10=10)
PPh3=I2 (10=10)
PPh3=I2 (10=10)
PPh3=I2 (5=5)
PPh3=CCl4 (10=10)
PPh3=NBS (10=10)
8
DMF (reflux)
CH3CN (rt)
9
10
11
12
13
14
Toluene (reflux)
CH3CN (reflux)
CH3CN (reflux)
CH3CN (reflux)
CH3CN (reflux)
60
60
30
aIsolated yields.
even prolonging reaction time to 60 min. For the case of Ph3P=NBS, the yield is
good, but it needs longer reaction time.
To improve the catalytic efficiency of the Ph3P=I2 system in this reaction, the
solvent effect was also inspected in the reaction of acetophenone oxime rearrange-
ment. The results are summarized in Table 1 (entries 7–12). Acetonitrile as solvent
gives the best result (entry 11). Considering all these results together, 10 mol%
Ph3P and 10 mol% I2 are chosen as the optimal catalytic system and acetonitrile as
solvent for Beckmann rearrangement in our study.
The generality and scope of the Ph3P=I2 system–catalyzed Beckmann
rearrangement of ketoximes to amides were also investigated (Table 2). One can
Table 2. Beckmann rearrangement of ketoximes (1a–1m) to amides (2a–2m)a
Entry
R1
R2
Product
Time (min)
Yieldb (%)
1
2
Phenyl
Methyl
Methyl
Methyl
Methyl
Methyl
Methyl
Methyl
Methyl
Phenyl
Phenyl
Phenyl
Methyl
Methyl
2a
2b
2c
2d
2e
2f
5
10
10
10
5
96
96
4-Methoxylphenyl
4-Methylphenyl
3-Methoxylphenyl
2,4-Dimethylphenyl
2,5-Dimethoxylphenyl
1-Naphthyl
3
95
86
4
5
93
94
6
5
7
2g
2h
2i
10
10
10
90
90
90
90
95
93
8
2-Naphthyl
Phenyl
9
95
88
10
11
12
13
2-Chlorophenyl
4-Chlorophenyl
4-Aminophenyl
2,4-Dihydroxyl
2j
2k
2l
90
Trace
Trace
2m
aIn CH3CN, reflux, 10 mol% Ph3P and 10 mol% I2 as catalyst.
bIsolated yields.

Ph3P/I2-CATALYZED BECKMANN REARRANGEMENT
3535
see that excellent yields are obtained for all of ketoximes. The conversions of
acetophenone oxime and substituted acetophenone oximes, for example, p-methyl
acetophenone oxime, p-methoxyl acetophenone oxime, o-methoxyl acetophenone
oxime, 2,4-dimethyl acetophenone oxime, and 2,5-dimethoxyl acetophenone oxime,
to amides reach more than 90% yields (entries 1–6) within a short reaction time, even
as short as 5 min. (entries 5 and 6). The results also suggest that the structures of the
oximes have very little effect on the reaction. In the case of 4-chlorobenzophenone
oxime and 2-chlorobenzo-phenone oxime (entries 10 and 11), they take much longer
time than others. The reason for the chloride substitute reducing the reaction rate
may be that the electron-withdrawing group on the aromatic ring, which has stron-
ger electron-attracting ability, influences the migration of the aryl group. Hydroxyl
and amino groups are electron-donating groups, but they are not able to rearrange in
this catalytic system (entries 12 and 13), probably because the amino group in the
aromatic ring can react with iodine of the Ph3P=I2 system, which causes the catalyst
invalidation and influences the rearrangement reaction. While 2,4-dihydroxyl aceto-
phenone oxime has acidity, it can react with triphenylphosphine, which sabotages
the rearrangement.
Recently, an organocatalyst for the Beckmann rearrangement has attracted
researchers, attention for its efficiency in catalytic activity and easy handling during
the rearrangement.[34] Organophosphrus catalyst triphosphazene (TAPC)[35] was
reported as a highly efficient catalyst for the Beckmann rearrangement by Ishihara
and coworkers, and a novel mechanism was proposed to account for the new genera-
tion of organocatalytic Beckmann rearrangement. Based on this hypothesis, a simi-
lar mechanism is also described for the PPh3=I2-catalyzed Beckmann rearrangement,
as shown in Scheme 2.
Reaction of Ph3P with iodine gives the known electrophilic Ph3P ꢀ I2 adduct,[33]
which is attacked by the hydroxyl group to liberate hydrogen iodide[36,37] and inter-
mediate (1). Hydrogen iodide can protonate the oxygen to give intermediate (2).[38]
Scheme 2. Plausible reaction path for the Beckmann rearrangement catalyzed by PPh3=I2.

3536
F. XU ET AL.
Next, the loss of the Ph3POHI moiety with concomitant transformation R1 group to
nitrogen atom forms the imine cationic intermediate (3). Finally, electrophilic
Ph3P ꢀ I2 adduct can be regenerated by protonating hydroxyl group of the pentacoor-
dinated phosphorus and losing H2O, following by iodide attack to the phosphorus.
The intermediate 3 hydrated gives the final product amide and hydrogen iodide.
CONCLUSION
In conclusion, PPh3=I2 is a convenient catalyst system with respect to other
reported catalyst systems and can be used as a valid alternative catalyst in Beckmann
rearrangement to convert oximes into amides in good yields under optimal reaction
conditions within several minutes.
EXPERIMENTAL
All solvents and reagents were used as obtained from commercial sources. The
oximes were prepared according to standard methods, and their purities were estab-
1
lished before utilization by melting point. Standard H NMR and 13C NMR were
recorded at 300 and 75.4 MHz, from CDCl3 or dimethylsulfoxide (DMSO-d6) solu-
tions. Ultra-high-resolution MS were obtained on a MaXis UHR-TOP instrument.
All runs were conducted at least in duplicate.
A solution of ketoxime (1 mmol), 10 mol% PPh3, and 10 mol% I2 in MeCN was
refluxed for a determined time. After completion of the reaction as monitored by
thin-layer chromatography (TLC), the reaction was stopped, cooled to room tem-
perature, and concentrated in vacuo. The residue was dissolved in EtOAc and
washed with saturated sodium thiosulfate solution. The organic layer was separated,
washed with NaCl saturated aqueous, and then concentrated. The resulting crude
product was purified by column chromatography on silica gel to give the correspond-
ing amide.
All products are known, which were confirmed by the melting point, 1H NMR,
and UHR-MS. The representative compounds also were characterized by 13C NMR.
All the characterized data of products were identical with those described in the
literature.
N-(2,4-Dimethylphenyl)acetamide (2e)
Yield: 93%. Mp 132–133 ꢁC. 1H NMR (300 MHz, CDCl3): d 7.51 (d,
J ¼ 7.9 Hz, 1H), 7.08 (s, 1H), 7.00 (d, J ¼ 7.9 Hz, 1H), 2.32 (s, 3H), 2.20 (s, 3H),
2.16 (s, 3H). 13C NMR: d 169.13, 135.49, 132.74, 131.43, 130.57, 127.4, 124.32,
23.67, 20.65, 17.74. UHR-MS (ESI): m=z calc. for C10H13NO þ [Na] 186.0895; found
186.0884 (Mþ þ Na).
N-(p-Chlorophenyl)benzamide (2k)
1
Yield 90%, Mp 195–197 ꢁC. H NMR (300 MHz, d-DMSO): d 10.34 (s, 1H),
7.98 (d, J ¼ 8.5 Hz, 2H), 7.75 (d, J ¼ 8.3 Hz, 2H), 7.61 (d, J ¼ 8.4 Hz, 2H),
7.34–7.39, (m, 2H), 7.10–7.15(m, 1H). 13C NMR: d 165.00, 139.23, 136.88, 134.02,

Ph3P/I2-CATALYZED BECKMANN REARRANGEMENT
3537
130.04, 129.12, 128.92, 128.10, 124.38, 122.32, 120.95, 120.85. UHR-MS (ESI): m=z
calc. for C13H10ClNO þ [Na] 254.0349; found 254.0334 (Mþ þ Na).
ACKNOWLEDGMENTS
This work was financially supported by the National Natural Science
Foundation of China (20872085) and the Natural Science Foundation of Shaanxi
Province (No. SJ08B22).
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