A hydrophilic inorganic framework based on a sandwich polyoxometalate: Unusual chemoselectivity for aldehydes/ketones with in situ generated hydroxylamine

Article abstract of DOI:10.1039/c7dt02411h

A hydrophilic inorganic porous catalyst was prepared via the hydrothermal method. The combination of [WZn₃(H₂O)₂(ZnW₉O₃₄)₂]^12- and Co(ii) provides a synergistical catalytic way to promote oximation of aldehyde/ketone with in situ generated hydroxylamine that initially produces an oxime, which further either dehydrates into a nitrile or undergoes a Beckmann rearrangement to form an amide.

Full text of DOI:10.1039/c7dt02411h

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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A Hydrophilic Inorganic Framework Based on Sandwich
Polyoxometalate: Unusual Chemoselectivity for Aldehydes/
Ketones with in-situ Generated Hydroxylamine
Received 00th January 20xx,
Accepted 00th January 20xx
Songzhu Xing,a† Qiuxia Han, *a,b† Zhuolin Shi,a Shugai Wang,a PeiPei Yang,a Qiang Wuc and Mingxue
Li*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A hydrophilic inorganic porous catalyst was prepared by processes the formation of hydroxylamine from ammonia and
hydrothermal method. The combination of aqueous hydrogen peroxide and further in situ oximation of
-
[WZn3(H2O)2(ZnW9O34)2]12 and Co(II) provides a synergistical ketones and aldehydes using a single workup stage is highly
catalytic way to promote oximation of aldehyde/ketone with desirable for the environmentally benign preparation fine
in-situ generated hydroxylamine, and further either chemicals. Nowadays, increasing attention is being devoted to
dehydrate into nitrile or undergo Beckmann rearrangement develop highly selective and efficient catalysts for
into amide.
implementing the catalytic oximation by a one-pot procedure
in a solvent-free manner under mild conditions.7
Oximes and amides are highly useful pharmaceutical
intermediates with important industrial applications and
versatile building blocks in organic transformations.1 The high
chemoselectivity and efficient conversion of aldehydes and
ketones with hydroxylamine into value-added oximes and
amides is indispensable in the modern organic synthesis.2
However, there are still two questions to be addressed at
present in this synthetic procedure. One of them is that the
hydroxylamine has the risk for being explosive on heating and
rapidly decomposed on absorption of water vapor and CO2 at
room temperature.3 The other is the special chemoselectivity,
because the reaction of ketone and aldehyde with
hydroxylamine initially produce oxime, which can further
either dehydrate into nitrile or undergo an acid catalyzed
Beckmann rearrangement to form amide.4
Polyoxometalates (POMs), with their elaborately fine-tuned
abilities, excellent thermal and oxidative stability toward
oxygen donors, have revealed broad prospect in applications
as solid acids and oxidation catalysts and become attractive
candidates as homogeneous/heterogeneous catalysts.8
Thereinto, [WZn3(H2O)2(ZnW9O34)2]12-, with a sandwich like
structure (abbreviated as {Zn5W19}), displays its tremendous
advantages for oxidative catalysis of organic compounds,
which has been extensively explored.9 In 2006, Neumann’s
group reported the pioneer work with a self-assembled
Na12[WZn3(H2O)2(ZnW9O34)2]⋅46H2O (Na-Zn5W19) as catalyst in
aqueous biphasic media for the reaction of NH3 and H2O2 to
synthesize hydroxylamine and further produce the
corresponding oxime through the nucleophilic addition of
aldehyde/ ketone in situ.10
Although {Zn5W19} as homogeneous catalyst generally
exhibits the superiority of high activity and selectivity in
synthetic reactions, their practical applications remain limited
in scope due to catalyst instability and difficulty in catalyst/
product separation. The design of active, environmentally
benign, and recyclable heterogeneous catalysts is expected to
have a major impact on oxidation of ketones and aldehydes
applications. Porous coordination networks are ideally suited
for heterogeneous catalytic conversions, because they can
impose size and shape selective restriction through readily
fine-tuned channels or pores and furnish accessible catalytic
sites to interact with substrates. 11 The combination of oxidant
catalyst {Zn5W19} and Lewis acid catalyst metal ions in
To solve the first question, hydroxylamine is often replaced
by a salt NH2OSO3H or (NH2OH)x⋅HxB, where B is sulphate,
nitrate, chloride and phosphate.5 However, the traditional
synthetic routes require complex reaction steps, complicated
operation, harsh reaction conditions, many by-products,
serious pollution and so on.6 A one-pot procedure that
inorganic porous materials possibly provide
a promising
synergistical catalytic way, because metal ions can further
promote the conversion of aldoximes to nitriles and ketoximes
to amides.12
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Scheme 1. Synthetic procedure of Co4Zn5W19, showing the hydrophilic inorganic pores for the oximation of ketone/aldehyde by nucleophilic addition of in
situ generated hydroxylamine, and further promote conversion into nitrile/amide in hydrosolvent.
Herein, by utilizing the readily available sandwich
polyoxometalate K12[WZn3(H2O)2(ZnW9O34)2] and Co(NO3)2
under hydrothermal conditions, we report the design and
four edge-sharing ZnO6 octahedra. W(10) and Zn(3) share the
same position with an equal occupancy (Fig. S2).
The potassium ion adopts a distorted dicapped trigonal
prismatic and coordinates to two terminal oxygen atoms from
two adjacent {Zn5W19} moieties (Fig. 1b). Each of the
crystallographically independent cobalt(II) ion is coordinated in
octahedral geometry with five oxygen atoms from coordinated
water and one terminal oxygen atom from {Zn5W19}, which is
suspended on the surface of {Zn5W19}. The removability of the
coordinated water molecules indicates the potential of
Co4Zn5W19 to act as an active Lewis acid catalyst. The {Zn5W19}
anion acts as a four connector to connect with four potassium
ions through the terminal oxygen atoms forming a 2D square
grid sheet (Fig. 1). The coexistence of oxidant catalyst {Zn5W19}
and Lewis acid catalyst Co(II) in one framework would provide
synthesis of
[K(H2O)6]2[Co(H2O)5]4
Co4Zn5W19). Co4Zn5W19 displays
a
new inorganic porous material, [H3O]2
[WZn3(H2O)2(ZnW9O34)2] 19H2O
two-dimensional (2D)
⋅
(
a
structure and possesses hydrophilic pores, which is conducive
to the reaction of NH3 and H2O2 for generating hydroxylamine
in hydrosolvent and further nucleophilic addition of ketone
and aldehyde to yield the corresponding oxime. And the Co(II)-
complexes potentially act as cooperative catalytic sites to
enhance the rearrangement of intermediate oxime to nitrile or
amide (Scheme 1).
Single-crystal X-ray diffraction analysis indicates that
Co4Zn5W19 crystallizes in
a
space group C2/c. The
crystallographic data and structural refinement parameters are
summarized in Table S1. Elemental analyses and powder X-ray
analysis (XRD) indicated the pure phase of its bulk sample (Fig.
S3). As shown in Fig. S1, the asymmetric unit of Co4Zn5W19
a
synergistical catalytic way to synthesize oxime from
aldehyde/ketone by nucleophilic addition of in situ generated
hydroxylamine, and further promote rearrangement into
nitrile/amide in hydrosolvent.
contains
half
a
Zn-substituted
sandwich-type
[WZn3(H2O)2(ZnW9O34)2]12 unit, two hexa-coordinate cobalt
cations ([Co(H2O)5(Ot)]2+), one eight-coordinate potassium
cations ([K(H2O)6(Ot)2]+), one protonated water and nine and a
half crystallization water molecules. (Fig. s1) Two asymmetric
units can form a molecule unit by symmetry operations. The
crystal structure indicates that the {Zn5W19} contains two
trivacant Keggin [B-α-ZnW9O34]12– fragments in a staggered
−
These 2D sheets are parallel staggered each other, giving
rise to 1D channels with both the active sites of the Lewis acid
Co(II) and the oxidation catalysts {Zn5W19} aligned (Fig.1d).
Free protonated water molecules as Brønsted acid catalytic
site are located in the channels.13 TGA revealed that the guest
water molecules could be readily removed in the temperature
range 100−200 °C, and the frameworks are stable up to
fashion linked via
a rhomb-like Zn3O16 group in a
∼300 °C. It was further confirmed by the variable temperature
infrared. As can be seen from Fig. S4, the typical vibration
centrosymmetric arrangement (C2h symmetry). The Zn(1) is
coordinated to three bridge oxygen atoms and one µ4-oxygen
atoms from one [B-α-ZnW9O34]12– units. The Zn(2) atom is
coordinated to one water oxygen atom O(1W), five oxygen
atoms of two [B-α-ZnW9O34]12– units. The µ4-oxygen (O(24) of
the Zn3O16 group in each [B-α-ZnW9O34]12– simultaneously
coordinates to three Zn atoms on each side of the plane,
resulting in the formation of central Zn3O16 cluster through
1
bands of Zn-polyoxometalate near 925 and 876, 769 cm−
basically remain unchanged at 300°C until heated to 400°C.
Most importantly, the guest water molecules can be
expediently removed by soaking the crystals of Co4Zn5W19 in a
methanol solution and then thermally assisted evacuation. As
a result, channels of ex-Co4Zn5W19 were enlarged with the
accessible pores about 1942 Å3 calculated from PLATON
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19% of porosity).14 It's worth mentioning that these channels are consolidated by the
COMMUNICATION
analysis (
∼
Fig. 1. a) The coordination mode of the {Zn5W19} clusters. b) The coordination mode of the potassium ions. c) Perspective view of the two-dimensional sheet
connected by {Zn5W19} clusters and the potassium ions, the Co-complexes and H2O molecules were omitted for clarity. d) 3D open network of Co4Zn5W19
showing the stacking pattern of the grid-like sheets and the 1D channels viewed down the a-axis. Solvent molecules are omitted for clarity.
In comparison, we performed a series of catalytic reactions
between aliphatic ketones and aromatic ketones with the
catalyst under similar conditions. For aliphatic ketones, the
main product were amides and little oximes. For example,
when the substrates were acetone and butanone, the yields
were up to 90% and amide compounds accounted for 88%.
Especially for cyclohexanone, the yield was 96% and the
chemoselectivity of amide product was up to 99%. In contrast
to the smooth reaction of substrates 9-12, the oximation
catalytic reaction in the presence of more large size and
hydrophobic ketones gave negligible conversion under the
same reaction conditions, presumably due to poorer contact in
the hydrophilic pores between the water-soluble
hydroxylamine and the ketone substrates (Table 1, entries 13-
15). It is suggested that the reactions indeed occur in the
channels of ex-Co4Zn5W19, not on the external surface.
hydrophilic {Zn5W19} anions and metal-water complexes.
Consequently, both NH3 and H2O2 are able to ingress and
egress through these hydrophilic channels to interact with
each other as well as the catalytic active sites in the channels.
Meanwhile the chemoselectivity for the synthesis of nitriles
from hydrophilic aldehydes can be improved.
The oximations of aldehydes with NH3 and H2O2 in
hydrosolvent were initially examined along with ex
(0.1% mol ratio) in a heterogeneous mixture by a single
workup stage at 20°C for h. The conversions and
-Co4Zn5W19
6
chemoselectivities were shown in Table 1. The results exhibit
excellent reaction efficiencies, which revealed the successful
execution of our design. Some phenomena may be noted. In
terms of aliphatic aldehyde, for example, acetaldehyde
generated the corresponding acetonitrile with the yields
almost up to 100% (Table 1, entry 1). However, the different
chemoselectivities were found corresponding to the different
hydrophobic and hydrophilic performance of aromatic aldehydes
(Table 1, entries 2-8). Besides generated nitriles, oximes and amides
were also produced. The more hydrophilic aromatic aldehydes
tended to form oximes and nitriles, such as o-phthalaldehyde
entirely convert into the nitrile (Table 1, entry 2). However, the
more hydrophobic aromatic aldehydes tended to less selective,
with formation of most oximes and rare amides. For substrates 5-8,
it gave ~88% oximes and ~12% amides. The high efficiency with
nitriles for hydrophilic aldehydes presumably ascribed to the
convenient ingress into the hydrophilic channels to interact with
the catalyst sites, as well as the cooperative catalysis of Lewis acid
Co(II) centers.12 One broad C-O stretching vibration at 1687 cm-1
was presented in the IR spectrum of the catalyst ex-Co4Zn5W19
impregnated with benzaldehyde in water solution, which had a red
shift of 14 cm-1 from 1701 cm-1 of the free benzaldehyde (Fig. S5).
The result unambiguously demonstrated the absorbance of
In order to verify the heterogeneity, the removal of ex-
Co4Zn5W19 by centrifugal separation after 3 hours shut down
the reaction of cyclohexanone, and the filtrate not afforded
o
additional Ɛ-Caprolactam after stirring at 20 C for another 3
hours. The observation suggested that ex-Co4Zn5W19 was a
true heterogeneous catalyst. Solids of Co4Zn5W19 could be
isolated from the reaction suspension by simple filtration
alone and be reused at least six times with moderated loss of
activity (from 96% to 92% of yield for Ɛ-Caprolactam, 100% to
97% of yield for acetonitrile). The index of XRD patterns of the
Co4Zn5W19 bulky sample recycling from the catalytic reaction
indicated the maintenance of the crystallinity (Figure S3). It's
worth
chemoselectivity and efficiency compared with the self-
assembled Na-Zn5W19 and K11[Ln(PW11O39)2]
10, 8d For instance,
noting
that
Co4Zn5W19
displays
different
.
the application of Na-Zn5W19 and K11[Ln(PW11O39)2] for
benzaldehyde and cyclohexanone led to high yield of the
corresponding oximes as main products with no further
Beckmann rearrangement to yield amides. In the case of
benzaldehyde and possible activation of the substrate by the
15
aliphatic aldehyde substrates, Na-Zn5W19
gave the
unsaturated Co(II) in the channel of ex-Co4Zn5W19
.
The higher
corresponding carboxylic acid products. In addition, compared
with K11[Ln(PW11O39)2], Co4Zn5W19 can proceed efficiently by
using water as solvent rather than organic solvent and give
more higher yields in shorter time.
conversion in the case of ex-Co4Zn5W19 system is possibly
attributed to the synergy catalysis of {Zn5W19} oxidant, Lewis
acid catalytic Co(II) centers and Brønsted acid in a confined
space.
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Table 1. Conversion and chemoselectivity in the reaction of
Journal Name
We have developed
a
new strategy to construct
aldehyde/ketone with in-situ generated hydroxylamine multifunctional polyoxometalate-based catalyst and explored
a
catalyzed by Co4Zn5W19
.
it in the cascade reactions for oximation of aldehyde/ketone
with in-situ generated hydroxylamine, and further either
dehydrate into nitriles or undergo Beckmann rearrangement
into amides. In the crystal structure of Co4Zn5W19, the
Brønsted acid, Lewis acid and oxidation catalyst coexist within
a confined space, which provided a particular hydrophilic
environment for the efficient chemoselectivity conversion of
aldehyde/ketone into the oxime, amide and nitrile,
respectively. The results demonstrated that Co4Zn5W19 was an
effective, stable, and recyclable catalyst for aldehyde/ ketone
oximation with NH3 and H2O2 in aqueous media under mild
conditions.
Entry
1
Sub.
Con.[b] Oxime[c] Amide[d] Nitrile[e]
100
99
0
0
0
0
0
100
100
12
O
H
H
2
3
O
O
H
99
88
4
5
6
7
89
91
88
85
87
89
86
89
0
13
0
O
H
11
14
11
O
We thank the National Natural Science Foundation of China
(21371048 and 21671055), the China Postdoctoral Science
Foundation (2015M580626), the Natural Science Foundation
of Henan Province (162102210171, 162300410012), the State
Key Laboratory of Fine Chemicals (KF1602).
H
0
0
8
9
86
92
90
88
12
12
12
88
88
0
0
0
Notes and references
10
1
J. S. Carey, D. Laffan, C. Thomsonc and M. T. Williamsd, Org.
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4
11
96
0
100
0
2
O
3405; (b) A. Rapeyko, M. J. Climent, A. Corma, P. Concepción
and S. Iborra, ACS Catal., 2016, 6, 4564; (c) S. Zhao, J. Xu, M.
12
13
14
15
89
-
19
-
81
-
0
-
O
Wei and Y. Song, Green Chem., 2011, 13, 384.
(a) N. Iranpoor and B. Zeynizadeh, Syn.Commun., 1999, 29
3
4
,
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2747; (b) Li. A. Long, Process Safety Process, 2004, 23, 114.
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Cat. Commun., 2014, 49, 47.
(a) H. Sharghi and M. H. Sarvari, J. Chem. Research (S), 2003,
[a] Reaction conditions: substate, 10 mmol; Co4Zn5W19
,
0.01 mmol;
H2O2(30%), 18 mmol; NH3
⋅
H2O (25%), 16 mmol, 2 mL H2O, 20°C, 6h. [b] The
yield was determined by 1H NMR spectroscopy of crude products (Table S2).
[c] Oxime product. [d] Beckmann rearrangement product. [e] Dehydration
product.
5
176; (b) A. Pohjakallio, P. M. Pihko, Chem. Eur. J., 2009, 15
,
3960; (c) C. L. Allen, S. Davulcu, J. M. J. Williams, Org. Lett.,
2010, 12,5096.
To further investigate the catalysis, a series of control
experiments were explored. For acetaldehyde and
cyclohexanone, in the absence of catalyst Co4Zn5W19, the
oximation reaction does not proceed. And Co(NO3)2 is also not
effective for the oximation reaction. The Beckmann
rearrangement experiments demonstrate that the loading 0.4
mol% of Co(NO3)2 led to the amide product in a conversion of
45% from the cyclohexanone-oxime. The Co4Zn5W19 performs
very well in consecutively converting ketones to amides in a
mild condition, which may be attributed to the following
factors: (i) hydroxylamine (NH2OH) is readily formed in situ
inside the pores from NH3 and H2O2 at the {Zn5W19} active sites,
(ii) the NH2OH in situ nucleophilic addition of activated ketone
by Co(II) Lewis acid sites to yield the corresponding oxime, and
(iii) the ketoximes was further Beckmann rearrangement to
yield amide and the aldoximes was dehydrated into nitrile with
the Brønsted acid and Co2+ Lewis acid in the channels,
respectively (Scheme S1).
6
7
J. Le Bars, J. Dakka, R. A. Sheldon, Appl. Catal. A, 1996, 136,
69.
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L. S. Long and W. B. Lin, J. Am. Chem. Soc., 2015, 137, 3197;
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4 | J. Name., 2012, 00, 1-3
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Adv. Synth. Cat., 2011, 353, 3262; (c) S, Zhao, L. Huang and
Y.F. Song, Eur. J. Inorg. Chem., 2013, 1659.
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Table of contents:
A green polyoxometalate catalyst provides a synergistical catalytic way to synthesize
oxime from aldehyde/ketone by nucleophilic addition of in situ generated hydroxylamine,
and further promote rearrangement into nitrile/amide in hydrosolvent.