Polystyrene-supported phosphine oxide-catalysed Beckmann rearrangement of ketoximes in 1,1,1,3,3,3-hexafluoro-2-propanol

Article abstract of DOI:10.1080/10426507.2018.1539489

A polystyrene-supported phosphine oxide-catalysed Beckmann rearrangement of ketoximes in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) has been developed. Good substrate compatibility, mild reaction conditions, good yields as well as the reusability of the catalyst/solvent made this procedure more environmentally benign.

Full text of DOI:10.1080/10426507.2018.1539489

Phosphorus, Sulfur, and Silicon and the Related Elements
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Polystyrene-supported phosphine oxide-catalysed
Beckmann rearrangement of ketoximes in
1,1,1,3,3,3-hexafluoro-2-propanol
Yaoyao Wang, Qun Chen, Mingyang He & Liang Wang
To cite this article: Yaoyao Wang, Qun Chen, Mingyang He & Liang Wang (2018): Polystyrene-
supported phosphine oxide-catalysed Beckmann rearrangement of ketoximes in 1,1,1,3,3,3-
hexafluoro-2-propanol, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI:
10.1080/10426507.2018.1539489
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PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
https://doi.org/10.1080/10426507.2018.1539489
Polystyrene-supported phosphine oxide-catalysed Beckmann rearrangement
of ketoximes in 1,1,1,3,3,3-hexafluoro-2-propanol
Yaoyao Wang, Qun Chen, Mingyang He, and Liang Wang
School of Petrochemical Engineering, Jiangsu Key Laboratory of Advanced Catalytic Materials & Technology, Changzhou University,
Changzhou, P. R. China
ABSTRACT
ARTICLE HISTORY
Received 2 July 2018
Accepted 19 October 2018
A polystyrene-supported phosphine oxide-catalysed Beckmann rearrangement of ketoximes in
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) has been developed. Good substrate compatibility, mild
reaction conditions, good yields as well as the reusability of the catalyst/solvent made this proced-
ure more environmentally benign.
KEYWORDS
Supported phosphine oxide;
hexafluoroisopropanol;
Beckmann rearrange-
ment; ketoximes
GRAPHICAL ABSTRACT
1. Introduction
and efficient methods for the Beckmann rearrangement is still
highly desirable.
The Beckmann rearrangement is an important and useful
reaction, which provides an atom-economic approach to con-
struct amides and lactams from the corresponding
oximes.[1–4] A very important application, through Beckmann
rearrangement, in industry is the manufacture of e-caprolac-
tam, which is the precursor for the synthesis of the polyamide,
nylon-6.[5] However, traditional Beckmann rearrangement
requires the use of a strongly acidic catalyst and high tempera-
ture. Therefore, recent studies have focused on the develop-
ment of suitable procedures by using various metal catalysts
(Rh,[6] Ru,[7] Pd,[8] Zn[9] and calcium salts[10]), organocata-
lysts or promoters[11] (cyanuric chloride,[11a] bis(2-oxo-3-oxa-
zolidinyl)-phosphinic chloride,[11b] triphosphazene,[11c] tosyl
chloride,[11d] propylphosphonic anhydride,[11e] trifluoroacetic
acid,[11f] dichloroimidazolidinedione,[11g] cyclopropenium
salts,[11h] boronic acid,[11i] Mukaiyama Reagent,[11j] Vilsmeier
reagent[11k] and hypervalent iodine[11l]), heterogeneous cata-
lysts[12] as well as alternative reaction mediums (supercritical
water[13] and ionic acid[14]). While acknowledging these con-
tributions, there are still issues to be addressed in some instan-
ces. For example, most of the reported reactions were carried
out under refluxing condition; some acid catalysts were toxic
and corrosive;[11a,b,d,e,f] and additives were usually required to
promote the reactions when organocatalysts were employ-
ed.[11b,d] Moreover, noble catalysts, supercritical water and
Other approaches using activated triphenylphosphine
moieties for Beckmann-like rearrangements have also been
reported.[15] However, phosphine oxide was generated dur-
ing the reaction.[15a,d] Recently, the application of catalytic
phosphine oxide/oxalyl chloride combination in organic
transformations has received much attention. The in situ
generated [ClPPh3]þCl- can efficiently promote the chlorin-
ation of alcohols and dehydration of amides (Scheme
1).[16–20] Inspired by these works, we envisioned that inter-
mediate chlorophosphonium chloride A could react with a
ketoxime via oxygen to give intermediate B, which then
undergoes elimination to generate intermediate C (or the
imidoyl chloride D) and phosphine oxide. The intermediate
C may be attacked by water or proceed through a self-prop-
agating pathway to afford the Beckmann rearrangement
product. We also believed that utilization of a polymer-sup-
ported phosphine oxide catalyst could benefit the separation
and purification process since the catalyst could be readily
recovered by simple filtration. Moreover, the use of
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as the solvent may
favor the N-O bond cleavage of intermediate B and E via its
high hydrogen bonding as well as good substrate solubil-
ity.[21–27] Herein, we wish to report a facile and efficient het-
erogeneous
phosphine
oxide-catalysed
Beckmann
ionic acids were expensive. Thus, the development of facile rearrangement of ketoximes in HFIP (Scheme 1).
CONTACT Liang Wang
lwcczu@126.com; Mingyang He
hemingyangjpu@yahoo.com
School of Petrochemical Engineering, Jiangsu Key Laboratory of
Advanced Catalytic Materials & Technology, Changzhou University, Changzhou, 213164, P. R. China.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
ß 2018 Taylor & Francis Group, LLC

2
Y. WANG ET AL.
Scheme 1. Phosphine oxide-catalyzed the Beckmann rearrangement.
Table 1. Optimization of reaction conditions[a]
.
room temperature for 12 h, no desired product 2a was
detected (entry 1). Switching the solvents to DMF or DMSO
provided 2a in 74% and 30% yields, respectively (entry 2
and 3). However, reactions in other solvents such as
dioxane, tetrahydrofuran, toluene and water led to the fail-
ure of the reaction (entries 4 ꢀ 7). Pleasingly, good to
excellent yields were obtained within only 2 h when 2,2,2-tri-
fluoroethanol or HFIP was used as the solvent (entry 8 and
9). It was assumed that 2,2,2-trifluoroethanol and HFIP not
only showed good substrate solubility but might favor the
N-O bond cleavage of intermediate B and E via its high
hydrogen bonding. The catalyst loading was also checked.
Lowering the amount of the phosphine oxide catalyst to
5 mol% or 2.5 mol% resulted in similar yields of 2a (entry
10 and 11). In continuing to decrease the catalyst loading to
1 mol %, it was observed that the yield of 2a was lowered to
72% even with a prolonged time (6 h, entry 12). As
expected, no reaction occurred in the absence of any catalyst
(entry 13). As shown in Scheme 1, since HCl could be gen-
erated in situ from phosphine oxide catalyst and oxalyl
chloride, a control experiment was also carried out by per-
forming the reaction in the presence of HCl (2.5 mol%,
entry 14). It was observed that 11% yield of 2a was obtained
in HFIP, while no reaction took place in DMF. These results
suggested that HFIP played a vital role in this transform-
ation. When Et3N or Na2CO3 was added to the reaction sys-
tem, only slight decrease in the yield of 2a was observed,
indicating that the in situ generated HCl was not the catalyst
but was helpful to the reaction (entry 15). Finally, the
amount of oxalyl chloride was also investigated, and 1
equivalent was enough for the reaction (entries 16 ꢀ 18).
With the optimized reaction conditions having been
established, a series of ketoximes were then evaluated to
Entry Catalyst (mol%) (COCl)2 (equiv)
Solvent
t (h) Yield (%)[b]
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
10
5
2.5
1
–
–
2.5
2.5
2.5
2.5
1
1
1
1
1
1
1
1
1
1
1
1
1
CH3CN
DMF
DMSO
dioxane
THF
toluene
H2O
CF3CH2OH
HFIP
HFIP
HFIP
HFIP
HFIP
12
12
12
12
12
12
12
2
0
74
30
0
0
0
0
84
9
2
99
10
11
12
13
14
15
16
17
18
2
99
2
6
99
72
12
2
2
0
–
1
1.5
2
–
HFIP
HFIP
HFIP
HFIP
11[c], 0[d]
95[e]
99
1.5
1
99
HFIP
12
0
[a] Reaction conditions: 1a (0.5 mmol), solvent (2 mL), catalyst (1.2–1.8 mmol/g,
on polystyrene cross-linked with 2% divinylbenzene), (COCl)2, room tempera-
[b]
[c]
ture.
Isolated Yields. HCl (36–38 wt %, 2.5 mol%) was used as the cata-
[d]
[e]
lyst. DMF as the solvent. Et3N or Na2CO3 was added.
2. Results and discussion
To test the hypothesis, initially, ketoxime 1a was selected as
the model substrate to optimize the reaction conditions
(Table 1). It was observed that when a mixture of 1a
(0.5 mmol), oxalyl chloride (1 equiv) and supported phos- probe the scope and generality of present protocol. The
phine oxide (10 mol%) was stirred in acetonitrile (2 mL) at results were summarized in Table 2. Aromatic ketoximes

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
3
Table 2. Substrate scopea.
aReaction conditions: 1 (0.5 mmol), HFIP (2 mL), catalyst (2.5 mol%), (COCl)2 (0.5 mmol), room tem-
perature. Isolated Yields.
with functional groups such as methyl, methoxyl and halo- No desired product was observed when nitro-substituted
gen were all well tolerated, affording the desired products in ketoxime 1e was used as the substrate. Ortho-substituted
good to excellent yields. The conversion of electron-rich ketoximes 1i–1k were found to react smoothly under this
ketoximes was complete within 2 h, while only moderate protocol to provide the corresponding amides in good to
yield was obtained for the electron-deficient ketoxime (2g). excellent yields by slightly prolonging the reaction time.

4
Y. WANG ET AL.
CDCl3 or DMSO-d6 at ambient temperature on a 300, 400
or 500 MHz NMR spectrometer. Chemical shifts are
reported in d units, parts per million (ppm). The coupling
constants J are given in Hz. All the products are known
compounds and were identified by comparing of their phys-
ical and spectra data with those reported in the literature
(Supplemental Materials, Figures S1–S32).
Scheme 2. Scale-up experiment.
4.2 Typical procedure for synthesis of compound 2a
Polystyrene-supported phosphine oxide (10.4 mg, 2.5 mol%),
(E)-1-Phenylethan-1-one oxime 1a (67.5 mg, 0.5 mmol) and
HFIP (2 mL) were added to a 10-mL glass vessel containing a
magnetic stirring bar. Then, oxalyl chloride (64.8 mg,
0.5 mmol) was added at 0 ꢁC. The mixture was stirred at room
temperature for 2 h. After completion of the reaction (indi-
cated by TLC), the catalyst was removed by filtration and the
solvent was removed under reduced pressure. The crude
material was purified by silica gel column using PE/EtOAc as
the eluent to afford the desired product 2a in 99% yield.
N-Phenylacetamide 2a.[11g] White solid, mp 113 ꢀ 115 ꢁC;
1H NMR (500 MHz, DMSO-d6) d 9.93 (s, 1H), 7.58 (d,
J ¼ 8.0 Hz, 2H), 7.28 (t, J ¼ 7.5 Hz, 2H), 7.01 (t, J ¼ 7.3 Hz,
1H), 2.04 (s, 3H); 13C NMR (125 MHz, DMSO-d6) d 168.3,
139.3, 128.6, 123.0, 119.0, 24.0.
Meta-substituted ketoxime 1l and 1-acetonaphthone oxime
1 m also showed good reactivity, giving the desired products
2l and 2m in 95% and 90% yields, respectively. In addition,
(E)-1-phenylpropan-1-one oxime 1n was efficiently engaged
in the Beckmann rearrangement (2n, 93%). Heteroaryl
ketoxime 1o also reacted efficiently to give the desired prod-
uct 2o in 75% yield. It was noteworthy that aliphatic oxime
1p and 1q survived the reaction conditions albeit in a rela-
tively lower yield (2p, 73%; 2q, 42%).
A scale-up experiment was also performed using (E)-1-
Phenylethan-1-one oxime 1a as the substrate (Scheme 2). It
was found that this reaction proceeded smoothly to provide
the desired product 2a in 95% yield (10 mmol scale), indi-
cating the practicability of the present protocol. Moreover,
the recyclability of the catalyst and solvent is the most
important advantages of this protocol. The supported phos-
phine oxide catalyst could be readily recovered by simple fil-
tration, and HFIP could also be readily recovered by
distillation. It should be pointed out that the catalyst could
be reused without obvious loss in activity in six consecutive
runs (99%, 99%, 99%, 98%, 95% and 92%, respectively).
Funding
This project was financially supported by the Jiangsu Key
Laboratory of Advanced Catalytic Materials and Technology
(BM2012110), the Advanced Catalysis and Green
Manufacturing Collaborative Innovation Center of
Changzhou University, and China Postdoctoral Science
Foundation (2018M632289).
3. Conclusion
In summary, a polystyrene-supported phosphine oxide-cata-
lysed Beckmann rearrangement of ketoximes in HFIP has
been developed. The reactions proceeded smoothly to give
the desired products in good to excellent yields except the
substrates with strong electron-withdrawing groups. Scale-up
experiment could also be achieved under the present proto-
col. Moreover, ready availability of the catalyst, good sub-
strate compatibility, mild reaction conditions, good reaction
efficiency as well as the reusability of the catalyst and the
solvent made this procedure complementary to the previ-
ous methods.
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