Highly selective and efficient lewis acid-base catalysts based on lanthanide-containing polyoxometalates for oximation of aldehydes and ketones

Article abstract of DOI:10.1002/ejic.201200901

Two lanthanide-containing polyoxometalates (POMs) have been developed as Lewis acid-base catalysts for highly selective and efficient oximation of various aldehydes and ketones under mild conditions. Copyright

Full text of DOI:10.1002/ejic.201200901

Job/Unit: I20901
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SHORT COMMUNICATION
DOI: 10.1002/ejic.201200901
Highly Selective and Efficient Lewis Acid–Base Catalysts Based on
Lanthanide-Containing Polyoxometalates for Oximation of Aldehydes and
Ketones
Shen Zhao,[a] Lujiang Huang,[a] and Yu-Fei Song*[a]
Keywords: Polyoxometalates / Lanthanides / Oximation / Lewis acids / Homogeneous catalysis
Two lanthanide-containing polyoxometalates (POMs) have
been developed as Lewis acid–base catalysts for highly se-
lective and efficient oximation of various aldehydes and
ketones under mild conditions.
to in situ synthesize hydroxylamine but also activates the
nucleophilic surfaces of the resulting hydroxylamine to pro-
mote the reaction.[8] The bare Lewis base nucleophilic sur-
faces result from the external oxygen atoms of W–O–W and
W=O species, and they can act as nucleophilic sites as well
as stabilizers of cationic intermediates.[9–12] However, the
application of Na-Zn5W19 for oximation led to relatively
low yields of the corresponding aromatic aldoximes, be-
cause of the formation of byproducts (amides and nitriles)
and because they solely afford the corresponding aliphatic
carboxylic acids with aliphatic aldehydes as substrates.
Therefore, the search for highly selective and efficient cata-
lysts to solve the above problems is very important. Herein,
lanthanide-containing POMs K11[Ln(PW11O39)2] {K-Ln-
(PW11)2; Ln = La and Ce},[13] which introduce lanthanide
ions as Lewis acidic metal centers into the POMs,[14,15] have
been developed as Lewis acid–base catalysts for the oxim-
ation of aldehydes and ketones. It should be mentioned that
the ions La3+ and Ce3+ have been selected for economic
reasons. The catalytic results suggest that the highly selec-
tive and efficient oximation of various aldehydes and
ketones can be realized by using K-Ln(PW11)2 as catalyst
under mild conditions.
Introduction
Aldoximes and ketoximes have been widely utilized in
medicine, industry, and analytical chemistry, and they are
very useful and versatile intermediates in synthetic organic
chemistry. For example, aldoximes and ketoximes can be
reduced to amines or oxidized to nitrile oxides, which are
important precursors.[1,2] In general, these oximes can be
industrially synthesized by nucleophilic addition of various
mineral salts of hydroxylamine [(NH2OH)x·HxB, B =
SO42–, PO43–, NO3 , and Cl–; x = 1, 2, and 3] to aldehydes
–
or ketones.[3,4] However, three serious disadvantages of the
above method largely restrict its further application in in-
dustry: (1) the relatively high cost of hydroxylamine, (2) the
large amounts of inorganic salt byproducts,[5] and (3) the
poor yields of the corresponding oximes because the re-
sulting oximes can undergo either acid-catalyzed Beckmann
rearrangement to amides or dehydration to nitriles. From
the industrial viewpoint, it is significant to develop recycla-
ble, highly selective and efficient catalysts that can suppress
the formation of byproducts and increase the yields of the
corresponding oximes as much as possible through in situ
synthesis of hydroxylamine for oximation of aldehydes and
ketones.
Among the catalysts developed for the oximation reac-
tion,[6–8] most studies are focused on the ammoximation of
cyclohexanone, and a very limited range of substrates have
been investigated. Interestingly, Neumann and co-workers
reported a sandwich-type polyoxometalate (POM) cluster,
Results and Discussion
Initially, cyclohexanone ammoximation catalyzed by lan-
thanide-containing POMs K-Ln(PW11)2 (Ln = La and Ce)
was performed as a model reaction under optimized condi-
tions, and the catalytic results indicate that the correspond-
ing oximes could be obtained in high yields of approxi-
mately 97% (Table 1, Entries 1 and 2). In contrast, the
monolacunary Keggin POM K7[PW11O39] (K-PW11), the
Keggin POM Na3PW12O40 (Na-PW12), and the peroxomet-
alate K2[{WO(O2)2(H2O)}2(μ-O)]·2H2O (K-W2) provide
yields of 78, 77, and 76%, respectively (Entries 3–5). Com-
monly used catalysts such as LaCl3 and La2O3 are not effec-
Na12[WZn3(H2O)2(ZnW9O34)2]·46H2O
(Na-Zn5W19),
which not only catalyzes the reaction of NH3 and H2O2
[a] State Key Laboratory of Chemical Resource Engineering,
Beijing University of Chemical Technology
Beijing 100029, P. R. China
Fax: /-Tel.: +86-10-64431832
E-mail: songyufei@hotmail.com
songyf@mail.buct.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200901.
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S. Zhao, L. Huang, Y. F. Song
SHORT COMMUNICATION
tive (Entries 6 and 7). The physical mixture of LaCl3 and the biphasic system.[8] The above results indicate that the
Na-PW12 gives a yield of 77% (Entry 8). Tungsten oxide catalytic oximation of electron-donating substituted aro-
WO3 is not effective for this oximation reaction (Entry 9). matic aldehydes gives excellent yields. In the case of the
In the absence of catalysts, the oximation reaction does not electron-withdrawing substituted aromatic aldehydes in-
proceed (Entry 10). It is worth noting that further increase cluding 2-chlorobenzaldehyde, 3-chlorobenzaldehyde, 4-
of the oximation time from 6 hours to 12 hours, 24 hours, chlorobenzaldehyde, 2-bromobenzaldehyde, 3-bromobenz-
or 48 hours does not increase the yield when Na-PW12, K- aldehyde, and 4-bromobenzaldehyde, good yields in the
PW11, or K-W2 are applied as catalysts. It is clear that, al- range 75–80% could be achieved by using Lewis acid–base
though the monolacunary Keggin POM K-PW11, Keggin catalysts K-La(PW11)2 (Table 2, Entries 8–13).
POM Na-PW12, and peroxometalate POM K-W2 are ef-
ficient as catalysts for this oximation reaction, they cannot
meet the requirement for industrial catalysts with yields of
Table 2. Oximation of various aldehydes and ketones catalyzed by
K-La(PW11)2.[a]
approximately 77%. In contrast, K-Ln(PW11)2 (Ln = La
and Ce) functions as a highly efficient catalyst for the am-
moximation of cyclohexanone with yields of approximately
97% at room temperature. Therefore, the above results re-
veal that the La3+ and Ce3+ ions play significant roles in
increasing the yields of the oximes from 77 to 97%. More
importantly, no byproducts were observed when Ln-
(PW11)2 (Ln = La and Ce) were used as catalysts.
Table 1. The effect of catalysts on the ammoximation of cyclohexa-
none.[a]
Entry
Catalyst
Yield (%)
1
2
3
4
5
6
7
8
9
10
K-La(PW11)2
K-Ce(PW11)2
K-PW11
Na-PW12
K-W2
LaCl3
La2O3
mixture of LaCl3 and Na-PW12
WO3
none
98
97
78
77
76
1.1
0.2
77
8
0
[a] Reaction conditions: cyclohexanone (1 mmol), catalyst
(0.25 mol-% with respect to cyclohexanone), H2O2 (30% aq. solu-
tion, 5 mmol), NH3 (25% aq. solution, 2 mmol), 1-butanol
(0.2 mL), room temperature, 6 h. Yields were determined by GC
analysis with comparison to reference standards. Assignments of
1
cyclohexanone oximes were performed by H NMR spectroscopy.
The oximation of various aldehydes and ketones cata-
lyzed by K-La(PW11)2 under optimized conditions has been
investigated. All the corresponding oximes can be obtained
in good yields. For aromatic aldehydes, benzaldehyde can
be converted into the corresponding oxime in an excellent
yield of 99% (Table 2, Entry 1). Methyl-substituted aro-
matic aldehydes including 2-methylbenzaldehyde, 3-methyl-
benzaldehyde, and 4-methylbenzaldehyde could be con-
verted into the corresponding oximes with yields of 97, 96,
and 97%, respectively (Table 2, Entries 2–4). In terms of
methoxy-substituted aromatic aldehydes such as 2-meth-
oxybenzaldehyde, 3-methoxybenzaldehyde, and 4-methoxy-
[a] Reaction conditions: substrate (1 mmol), K-La(PW11)2
(0.25 mol-% with respect to substrate), H2O2 (30% aq. solution,
5 mmol), NH3 (25% aq. solution, 2 mmol), 1-butanol (0.2 mL),
room temperature, 6 h. Yields were determined by GC analysis
with use of reference standards. Assignments of the corresponding
1
oximes were performed by H NMR spectroscopy.
In the case of the aliphatic aldehydes, the oximation of
benzaldehyde, the corresponding oximes can be obtained in hexanal, heptanal, octanal, nonanal, and decanal gives the
high yields of 94, 87, and 86%, respectively (Table 2, Entries corresponding oximes with yields of 94, 99, 92, 97, and
5–7). In contrast, previous experiments showed that only 95%, respectively (Table 2, Entries 14–18). These results are
carboxylic acids of the methoxy-substituted aromatic alde- quite different from those of reactions catalyzed by Na-
hydes can be prepared by using Na-Zn5W19 as catalyst in Zn5W19, which oxidize the substrates to the corresponding
2
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Highly Selective and Efficient Lewis Acid–Base Catalysts
carboxylic acids.[8] The oximation of the cyclic aliphatic al- Table 3. Oximation of cyclohexanone with different catalysts.[a]
dehyde cyclohexanecarboxaldehyde proceeds efficiently
with a yield of 98% (Table 2, Entry 19). The oximation
yield of alkyl ketone 2-heptanone is 77% without the
formation of amides or nitriles (Table 2, Entry 20). It
should be pointed out that cyclohexanone ammoximation
can proceed efficiently with 90% yield at room temperature
in 12 hours with only 0.05 mol-% of K-La(PW11)2, and the
turnover number (TON) can reach values as high as 1808
(Scheme 1). To the best of our knowledge, K-La(PW11)2
is one of the most efficient catalysts for cyclohexanone
ammoximation reported so far in the literature.
Entry Catalyst[b]
Cat./Sub.
(g/mmol) (°C) (h)
T
t
Yield Ref.
(%)
[6]
1
2
3
4
5
6
7
Ti-Meso-MOR
Ti-MOR
TS-1
clay-Based TS-1
clay-Based TS-1
Na-Zn5W19
Mg3Al-LDHs-
Zn5W19
0.005
0.005
0.015
0.330
0.330
0.012
0.003
60
60
60
50
80
25
25
2
2
2
2.5
2.5
6
97
99
96
14
97
90
90
[6]
[6]
[7]
[7]
[8]
[16]
6
8
9
10
K-La(PW11)2
K-La(PW11)2
K-Ce(PW11)2
0.015
0.015
0.015
25
50
25
6
1.5
6
97
97
97
this work
this work
this work
[a] Solvents: Entries 1–5 in a 1:6 mixture of tert-butyl alcohol and
water; Entry 6 in water; Entries 7–10 in 1-butanol. [b] Ti-Meso-
MOR: mesoporous titanosilicate with the MOR topology; Ti-
MOR: titanosilicate with MOR topology; TS-1: titanium silicalite-
1; LDHs: layered double hydroxides.
Scheme 1. The ammoximation of cyclohexanone with 0.05 mol-%
K-La(PW11)2.
93%, 5th: 93%, and 6th: 91%) do not show a significant
decrease after six cycles (Figure S2). The slight loss of ac-
tivity can be accounted for some minor loss of the POM
catalyst K-La(PW11)2. The strong peak appearing at
–12.0 ppm in the 31P NMR spectrum and indicating the
structure of [La(PW11O39)2]11– is the same for both fresh
and reused K-La(PW11)2 samples (Figure 1A). In addition,
FTIR spectra of the fresh and reused K-La(PW11)2 sample
also exhibit the same peaks at 1092, 1050, 951, 890, 835,
771, 593, and 515 cm–1 (Figure 1B), which could be as-
It is instructive to compare cyclohexanone ammoxim-
ation through in situ synthesis of hydroxylamine catalyzed
by different compounds reported so far (Table 3). Though
Ti-Meso-MOR (Entry 1), Ti-MOR (Entry 2), and TS-1
(Entry 3) are highly efficient for cyclohexanone ammoxim-
ation,[6] these catalytic systems require the cumbersome
synthesis of Ti-Meso-MOR, Ti-MOR, and TS-1 and a rela-
tively high reaction temperature of 60 °C. In the case of
clay-based TS-1, a high reaction temperature of 80 °C is
necessary (Entry 5), as a much lower yield of cyclohexanone
oxime is obtained at 50 °C (Entry 4).[7] Another problem
with the above catalytic systems is the quite limited scopes
of the substrates. The POM catalyst Na-Zn5W19 was re-
ported to catalyze oximation of some substrates, but the
resulting oximes underwent acid-catalyzed Beckmann re-
arrangement or dehydration, resulting in the low yields. As
a result, Na-Zn5W19 was intercalated into layered double
hydroxides (LDHs) to overcome its shortcomings (En-
try 7).[16] Nevertheless, the relatively difficult preparation of
Mg3Al-LDHs-Zn5W19 and the unsatisfying yields of aldox-
imes restrict the further application of this intercalated sys-
tem. In contrast, K-Ln(PW11)2 (Ln = La and Ce) show high
yields of 97 and 98% at room temperature (Entries 8 and
10). It should be noted that the yield of cyclohexanone ox-
ime can reach 97% in 1.5 h at 50 °C (Entry 9). Additionally,
POM catalysts K-Ln(PW11)2 (Ln = La and Ce) were dem-
onstrated to be highly efficient not only for the ammoxim-
ation of cyclohexanone but also for that of various other
aldehydes and ketones.
Recycling experiments were performed for K-La(PW11)2
to check its stability. In this catalytic system, K-La(PW11)2
can be recovered simply by extracting the obtained cyclo-
hexanone oxime, and the recycling experiments were carried
out by re-addition of cyclohexanone, 30% H2O2 and 25%
NH3·H2O over 6 hours at room temperature. The yields of
Figure 1. (A) The 31P NMR spectra of fresh and reused K-
La(PW11)2; (B) the FTIR spectra of fresh and reused K-La-
cyclohexanone oxime (1st: 96%, 2nd: 94%, 3rd: 93%, 4th: (PW11)2.
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S. Zhao, L. Huang, Y. F. Song
SHORT COMMUNICATION
signed to the vibration associated with the [La(PW11- corresponding oxime products can be extracted from the
O39)2]11– cluster.[14] All these results reveal that the structure system, and the catalysts can then be directly used for the
of K-La(PW11)2 is not affected by the oximation reaction.
next run over six times without a significant decrease in
Combining the nucleophilic Lewis base surface of POMs catalytic activity. In addition, the structures of K-Ln-
with Lewis acid metal centers, these two POMs of K- (PW11)2 remain unchanged after the oximation reaction, as
Ln(PW11)2 (Ln = La and Ce) could function as Lewis acid– evidenced by the 31P NMR and FTIR spectra. The easy
base catalysts[14,15] and effectively promote the oximation of preparation, recyclability, high selectivity, and efficiency of
various aldehydes and ketones. As shown in Scheme 2, the the lanthanide-containing POM catalysts provide great po-
lanthanide-containing POMs K-Ln(PW11)2 (Ln = La and tential for their further application.
Ce) possess both electrophilic centers (lanthanide ions) and
bare nucleophilic surfaces (surface oxygen atoms). Alde-
hydes or ketones could be activated by the lanthanide cen-
ters, and the in situ synthesized hydroxylamine is also acti- Experimental Section
vated by surface oxygen atoms (Step 1). In other words, co-
activation of both aldehyde or ketone and in situ synthe-
sized hydroxylamine can be achieved by using K-Ln-
Synthesis of K11[Ln(PW11O39)2] {K-Ln(PW11)2, Ln = La and
Ce}:[13] H3PW12O40·xH2O (10.0 g, 3.47 mmol) in H2O (20 mL) was
added dropwise to warm concentrated solutions of LaCl3·7H2O
(PW11)2 as catalyst, and the nucleophilic addition of in situ
hydroxylamine to aldehydes or ketones leads to the forma-
tion of the corresponding oximes, as indicated by Step 2.
(0.6 g, 1.85 mmol) or CeCl3·6H2O (0.7 g, 1.85 mmol) and
CH3COOK (8.0 g, 0.08 mol, pH adjusted to 7.0 with CH3COOH).
The mixture was stirred at 90 °C until a clear solution was ob-
tained. The clear solution was cooled to room temperature and
then kept in the refrigerator (ca. 4 °C) overnight. The obtained pre-
cipitate was filtered and recrystallized twice. From the TGA and
ICP analysis, the formulas of K-Ln(PW11)2 (Ln = La and Ce)
should be K11[La(PW11O39)2]·13H2O and K11[Ce(PW11O39)2]·
14H2O.
Procedure for Oximation of Aldehydes and Ketones: In a typical
experiment, ketones or aldehydes (1 mmol), aqueous H2O2 solution
(30%), aqueous NH3 solution (25%), K-Ln(PW11)2 (Ln = La and
Ce) as catalyst (0.25 mol-%), and solvents (0.2 mL) were placed in
a 20 mL glass bottle at room temperature, and the reaction mixture
was stirred vigorously. The reaction was effectively quenched after
6 h. The resulting oily products were extracted with diethyl ether,
analyzed by GC, and identified by 1H NMR spectroscopy to deter-
mine the yield by comparison to reference standards.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details and catalytic results.
Acknowledgments
This research was supported by the National Natural Science
Foundation of China (21076020), the 973 Program
(2011CBA00504), the 863 Program (2010AA03A403), the program
for New Century Excellent Talents of the Ministry of Education of
China (NCET-09-0201), the Beijing Nova Program (2009B12), the
Beijing Natural Science Foundation (2113049), the Fundamental
Research Funds for the Central Universities (QN0908, ZZ1227).
Scheme 2. The proposed mechanism of the oximation reaction.
[Step 1: co-activation of aldehydes or ketones and the hydroxyl-
amine in situ synthesized by using K-Ln(PW11)2 as catalyst; Step
2: the nucleophilic addition of hydroxylamine to aldehydes or
ketones leads to the formation of the oximes.].
[1] B. A. Mendelsohn, S. Lee, S. Kim, F. Teyssier, V. S. Aulakh,
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5096–5099.
Conclusions
In summary, the application of the Lewis acid–base cata-
lysts K-Ln(PW11)2 (Ln = La and Ce) with lanthanide ions
as electrophilic centers and nucleophilic surfaces to the ox-
imation reaction leads to the co-activation of both alde-
hydes or ketones and hydroxylamine. As a result, K-Ln-
(PW11)2 are able to realize highly selective and efficient ox-
imation of various substrates under mild conditions. The
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4
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Highly Selective and Efficient Lewis Acid–Base Catalysts
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Published Online: ᭿
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S. Zhao, L. Huang, Y. F. Song
SHORT COMMUNICATION
Lanthanide-Containing POMs
Two lanthanide-containing polyoxometal-
ates (POMs) have been developed as Lewis
acid–base catalysts for highly selective and
efficient oximation of various aldehydes
and ketones under mild conditions.
S. Zhao, L. Huang, Y. F. Song* ........ 1–6
Highly Selective and Efficient Lewis Acid–
Base Catalysts Based on Lanthanide-Con-
taining Polyoxometalates for Oximation of
Aldehydes and Ketones
Keywords: Polyoxometalates
/ Lanthan-
ides / Oximation / Lewis acids / Homo-
geneous catalysis
6
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