Cu(CH3CN)4PF6 immobilized on halloysite as efficient heterogeneous catalyst for oxidation of allylic C–H bonds in olefins under mild reaction condition

Article abstract of DOI:10.1007/s11164-019-03745-z

Considering the importance of oxidation of allylic C–H bonds in olefins and the unresolved challenging issues, such as long reaction time and the large quantity of catalyst required, and encouraged by the excellent performance of halloysite as a catalyst

Full text of DOI:10.1007/s11164-019-03745-z

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03745-z
Cu(CH3CN)4PF6 immobilized on halloysite as efcient
heterogeneous catalyst for oxidation of allylic C–H bonds
in olefns under mild reaction condition
Samahe Sadjadi1 · Saadi Samadi2 · Mojgan Samadi2
Received: 30 October 2018 / Accepted: 12 January 2019
© Springer Nature B.V. 2019
Abstract
Considering the importance of oxidation of allylic C–H bonds in olefns and the
unresolved challenging issues, such as long reaction time and the large quantity of
catalyst required, and encouraged by the excellent performance of halloysite as a
catalyst support, a novel catalytic system was developed to promote this reaction
efciently. To prepare the catalyst, halloysite was frst functionalized with ionic liq-
uid then reacted with 2-aminopyrimidine to aford a potential support, Hal-P. The
latter was then used for in situ immobilization of Cu(CH3CN)4PF6 and applied to
promote the reaction of cycloolefn and tert-butyl p-nitrobenzoperoxoate under
mild reaction condition. The results showed that the nature of copper could play an
important role in the catalytic activity. Moreover, the presence of ionic liquid and
2-aminopyrimidine in the structure could improve the activity of the fnal catalyst.
Notably, low amounts of catalyst could catalyze the reaction to aford corresponding
allylic esters in good yield. It was also found that the reaction was size selective and
that cyclic olefns with lower strain could undergo this reaction more efectively.
Study of the recyclability of the catalyst confrmed that it was recyclable and could
be recovered and recycled for fve consecutive reaction runs.
Keywords Halloysite · Allylic C–H bond oxidation · Heterogeneous catalyst ·
Olefns · Cu(CH3CN)4PF6
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-019-03745-z) contains supplementary material, which is available to authorized users.
*
*
Samahe Sadjadi
samahesadjadi@yahoo.com; s.sadjadi@ippi.ac.ir
Saadi Samadi
s.samadi@uok.ac.ir
1
2
Gas Conversion Department, Faculty of Petrochemicals, Iran Polymer and Petrochemicals
Institute, PO Box 14975-112, Tehran, Iran
Laboratory of Asymmetric Synthesis, Department of Chemistry, Faculty of Science, University
of Kurdistan, Sanandaj 66177-15175, Iran
Vol.:(0123456789)
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S. Sadjadi et al.
Introduction
Oxidation of allylic C–H bonds in olefns to obtain allylic alcohols and allylic
esters is a valuable synthetic route in organic chemistry to generate additional
functionality at the allylic position while leaving the C=C bond intact, which dis-
tinguishes allylic C–H bond oxidation from hydroxylation or epoxidation. The
functionalized olefns obtained via this reaction can be further functionalized to
furnish a variety of products. More importantly, this process is considered to be a
simple and cost-efective protocol to introduce oxygen atom. This reaction is pro-
moted in the presence of various types of metal salts, particularly those of copper
[1–13].
Heterogenization of catalytic species by immobilization on clay is a well-
established methodology [14]. One of the most attractive clays, which has
received tremendous attention in recent years, is halloysite (mostly abbreviated as
Hal), a biocompatible, kaolin-like, naturally available, nontoxic clay with formula
Al2(OH)4Si2O5·2H2O (Al:Si ratio of 1:1) [15]. This type of clay benefts from
some unique features such as tubular morphology, chemical and mechanical sta-
bility, high surface area, tunable surface chemistry, and oppositely charged inner
and outer surface [14, 15–19], rendering it a potent candidate for use in a diverse
range of applications [20–29], especially in catalysis. Work to date has confrmed
the utility of Hal for immobilization of catalytically active species and the devel-
opment of heterogeneous catalysts. To achieve efcient immobilization and con-
trol the leaching issue, pristine Hal is mostly modifed by surface functionaliza-
tion on one or both sides [16, 17, 30–39].
In continuation of our research on development of heterogeneous catalysts for
promoting efcient protocols for chemical transformations, we recently disclosed
use of CuO encapsulated in SBA-15 for oxidation of allylic C–H bonds in olefns
[40]. Although that catalyst could promote the reaction efciently, the reaction time
was very long. Moreover, the amount of catalyst required was very high. On the
other hand, our recent focus on the utility of Hal as a catalyst support confrmed the
excellent performance of this clay [41–43]. Inspired by the efciency of Hal and in
attempt to develop a more efcient strategy for oxidation of allylic C–H bonds in
olefns, in the work presented herein, we designed and prepared a novel catalyst for
oxidation of allylic C–H bonds in olefns by functionalizing Hal with ionic liquid
(IL) and 2-aminopyrimidine (P) followed by immobilization of Cu(CH3CN)4PF6.
The role of IL and P in the catalytic activity as well as the efect of copper salt were
investigated. Moreover, the recyclability of the catalyst was studied.
Experimental
All chemicals used in this work were provided by Sigma-Aldrich and used as
received. The structure of the catalyst was confrmed by Fourier-transform infra-
red (FTIR) spectra obtained using a PerkinElmer Spectrum 65 instrument after
1 3

Cu(CH3CN)4PF6 immobilized on halloysite as efcient…
degassing at 423 K for 2 h using a BELSORP Mini II. Thermogravimetric analy-
sis was carried out using a Mettler Toledo instrument under N2 atmosphere at
heating rate of 10 °C min−1. Inductively coupled plasma (ICP) analysis (Varian,
Vista-pro), feld-emission transmission electron microscopy (CM30300Kv), and
X-ray difraction (XRD) analysis (Siemens D5000) were also carried out, after
degassing of the sample at 423 K for 3 h using a BELSORP Mini II.
Functionalization of Hal surface with Cl to synthesize Hal‑Cl
The frst step in the synthesis of the catalyst was surface functionalization of Hal with
Cl. For this purpose, Hal (1.2 g) was dispersed in dry toluene (50 mL) using ultra-
sonic irradiation at power of 12 W. Next, (3-chloropropyl)trimethoxysilane (4 mL) was
slowly introduced into the Hal suspension. The resulting mixture was then refuxed at
110 °C under inert (N2) atmosphere for 24 h. Upon reaction completion, the precipitate
was simply separated by fltration. To obtain pure Hal-Cl, the precipitate was washed
with dry toluene several times, then dried at 90 °C for 14 h.
Incorporation of imidazole to synthesize Hal‑I
In the next step, imidazole was incorporated into the structure of the catalyst. Briefy,
Hal-Cl (1.5 g) was suspended in 60 mL ethanol. Then, imidazole (1.36 g) was added
to the aforementioned suspension, and the resulting mixture was refuxed at 80 °C for
48 h in presence of catalytic amount of triethylamine. Upon reaction completion, the
obtained solid was fltrated, washed with diethyl ether (3×10 mL) repeatedly, then
dried at 90 °C in an oven for 10 h.
Conjugation of epichlorohydrin and synthesis of IL‑decorated Hal (Hal‑E)
Introduction of epoxy group on the surface of Hal was achieved by using a previously
reported procedure [44]. Typically, Hal-I (1.2 g) was bathed with excess amounts of
epichlorohydrin at 90 °C for 2 days. At the end of the reaction, the obtained precipitate
was washed with acetonitrile repeatedly, then dried under vacuum at 50 °C for 20 h.
Reaction with 2‑aminopyrimidine to synthesize Hal‑P
In the next step, to a suspension of Hal-E (1.2 g), dry toluene 2-aminopyrimidine
(0.5 g) was added, and the resulting mixture was refuxed at 110 °C overnight. After
that, the solid was fltered, washed with dry toluene for several times, and dried in an
oven at 60 °C overnight (Fig. 1).
1 3

S. Sadjadi et al.
Fig. 1 Schematic procedure for synthesis of the catalyst
In situ immobilization of copper species and oxidation of allylic C–H bonds
in cyclic olefns
Initially, Hal-P (15 mg) was preheated under vacuum at 50 °C for 2 h, then
tetrakis(acetonitrile)copper(Ι) hexafuorophosphate (0.04 mmol, 15 mg) and
anhydrous acetonitrile (3 mL) were added (Fig. 1). Then, the reaction mixture
was stirred for 1.5 h at ambient temperature under N2. Next, cycloolefn (5 mmol)
and tert-butyl p-nitrobenzoperoxoate (0.85 mmol, 0.203 g) were added to the
reaction mixture. When the reaction had completed, as monitored by thin-layer
chromatography (TLC), the heterogeneous catalyst was collected by fltration,
then washed with methylene chloride (3 × 8 mL) and air-dried. The fltrate, on
the other hand, was washed with saturated aqueous NH4Cl (10 mL) then satu-
rated aqueous NaHCO3 (10 mL), and dried over MgSO4. Finally, the solvent was
evaporated under vacuum, and the product was purifed by column chromatogra-
phy (EtOAc/n-hexane 2–10 %) to aford white solid at yield of up to 90 %. The
products were characterized by FTIR and 1H and 13C nuclear magnetic resonance
(NMR) (Supporting Information) [45].
1 3

Cu(CH3CN)4PF6 immobilized on halloysite as efcient…
Fig. 2 TEM images of catalyst
Hal
Hal-E
Catalyst
Hal-I
1.2
1
0.8
0.6
0.4
0.2
0
3400
2900
2400
1900
1400
900
400
Wavenumber (cm-1)
Fig. 3 FTIR spectra of Hal, Hal-Cl, Hal-I, Hal-E, Hal-P, and catalyst
Results and discussion
Catalyst characterization
First, the morphology of the catalyst was studied by TEM. As shown in Fig. 2,
the catalyst exhibited tubular morphology, confrming that surface functionaliza-
tion and incorporation of IL and copper species did not cause the tubular mor-
phology to collapse.
1 3

S. Sadjadi et al.
To further characterize the catalyst and confrm the success of each step in its
preparation, FTIR spectra of Hal, Hal-Cl, Hal-I, Hal-E, Hal-P, and the catalyst were
recorded and are compared in Fig. 3. According to literature [46], the most impor-
tant characteristic bands of Hal can be assigned to Si–O stretching (1037 cm−1),
inner –OH (3599 and 3717 cm−1), and Al–O–Si vibration (509 cm−1) [36]. These
characteristic bands could be detected in all the recorded FTIR spectra, confrming
that the structure of Hal was preserved upon decoration with IL and P and immo-
bilization of copper. The FTIR spectrum of Hal-I was very similar to that of Hal.
Notably, the characteristic band of imidazole ring, C=N (1650 cm−1) overlapped
with the bands of Hal. Similarly, in the FTIR spectrum of Hal-E, the characteristic
band of epoxy functional group (912 cm−1) overlapped with the bands of Hal. In the
FTIR spectrum of the catalyst, the intensities of the bands at 1649 cm−1, 1559 cm−1,
and 1479 cm−1, which can be attributed to –C=N, –C=C, and –C–N functionality,
respectively, were more pronounced.
To further investigate the structure of the catalyst, its XRD pattern was recorded
(Fig. 4). The XRD pattern of the catalyst exhibited the characteristic peaks of pris-
tine Hal at 2θ=11.8°, 19.9°, 24.8°, 26.5°, 36°, 38.5°, 55.3°, and 62.5° in agreement
with Joint Committee on Powder Difraction Standards (JCPDS) card no. 29-1487
[38, 47] (for better comparison, the XRD pattern of pristine Hal is also shown in
Fig. 4). It is worth noting that the characteristic peaks of Hal were not shifted in the
spectrum of the catalyst, implying that the functionalities and copper were located
on the exterior surface of Hal. Comparison of the two XRD patterns reveals that,
apart from the characteristic bands of Hal, some additional peaks can be observed in
the XRD pattern of the catalyst, being attributable to the copper species.
To elucidate the efect of conjugation of IL and immobilization of copper spe-
cies on the textural features, the N2 adsorption–desorption isotherm of the catalyst
was recorded (Fig. 5). The isotherm of the catalyst is similar to that of pristine Hal
[48] (Fig. S21), corresponding to type II with H3 hysteresis loops [38]. Moreover, it
was found that, upon surface modifcation of Hal, the specifc surface area decreased
Hal
Catalyst
6000
3000
0
7
17
27
37
47
57
67
77
2 Theta angle (degree)
Fig. 4 XRD patterns of pristine Hal and catalyst
1 3

Cu(CH3CN)4PF6 immobilized on halloysite as efcient…
ADS
DES
100
50
0
0
0.5
1
p/p0
Fig. 5 N2 adsorption–desorption isotherm of the catalyst
100
80
60
40
20
0
Hal
Catalyst
20
120
220
320
420
520
620
720
Temperature (°C)
Fig. 6 TGA results for Hal and catalyst
signifcantly (from 51 m2 g−1 for pristine Hal to 20 m2 g−1 for the catalyst), indicat-
ing that the functional groups and copper species covered the surface of Hal.
Next, the copper loading on the catalyst was investigated using ICP analysis. The
ICP sample was prepared by digesting the organic moiety according to previous
reports. ICP revealed that the content of copper on the catalyst was 4 mol%.
TG analysis of the catalyst (Fig. 6) revealed that the isotherm of the catalyst was
diferent from that of pristine Hal. More precisely, Hal exhibited only two weight
losses at 150 and 560 °C, assigned to loss of water and dehydroxylation, respectively
1 3

S. Sadjadi et al.
[41, 49]. The thermogram of the catalyst, however, showed additional weight loss
stages, which can be attributed to loss of IL and P.
Catalytic activity
As mentioned in “Introduction” section, considering the importance of oxidation of
allylic C–H bonds in olefns and in an attempt to furnish a solution to the challeng-
ing issues related to this reaction, such as long reaction time and the large amount of
catalyst required, a novel Hal-based catalyst was developed in this work. To achieve
this, Hal was frst surface functionalized with IL and P, then used for immobilization
of copper species. As the main catalytically active species in this reaction is copper,
it was assumed that the nature of copper could efectively afect the reaction. Moreo-
ver, considering the fact that IL in its individual form can exert a catalytic efect, it
was postulated that the presence of IL in the structure of the catalyst could infuence
its catalytic activity. On the other hand, the presence of P on the backbone of the cat-
alyst could provide multiple nitrogen groups that could efectively play a role in pre-
serving the copper species and prevent its leaching. To verify these assumptions, the
role of the catalyst support and nature of copper was frst studied. For this purpose,
the catalytic performance of several copper species, including Cu(OAc)2, Cu(NO3)2,
Table 1 Role of copper salts and catalyst support in oxidation of allylic C–H bonds of cyclohexene
Entry
Catalyst
Support
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
Cu(OAc)2
Cu(NO3)2
CuI
Cu(CH3CN)4PF6
Cu(OAc)2
Cu(NO3)2
CuI
Cu(CH3CN)4PF6
Cu(OAc)2
Cu(NO3)2,
CuI
–
–
–
–
120
110
150
30
70
64
83
7
100
90
96
62
65
60
75
70
72
82
90
50
60
55
80
Hal-P
Hal-P
Hal-P
Hal-P
Hal-E
Hal-E
Hal-E
Hal-E
9
10
11
12
Cu(CH3CN)4PF6
24
1 3

Cu(CH3CN)4PF6 immobilized on halloysite as efcient…
CuI, and Cu(CH3CN)4PF6 in homogeneous form (without support), was investigated
in the model reaction of cyclohexene with perester as oxidant at room temperature
in acetonitrile in comparison with their heterogeneous counterparts (catalysts immo-
bilized on Hal-P). The results (Table 1) confrmed the importance of the copper salt
in the catalytic activity. More precisely, in both the homogeneous and heterogeneous
cases, Cu(CH3CN)4PF6 showed the best results. Moreover, it was found that, for all
the copper sources, use of homogeneous copper salts resulted in lower product yield
and longer reaction time, implying a role for the support in the catalytic activity.
Next, the role of P in the catalytic activity was studied. For this purpose, a control
catalyst Hal-E (without P) was synthesized and applied for immobilization of all
copper species. The results (Table 1) confrmed that, in all cases, immobilization of
copper on Hal-P was more efective than Hal-E, indicating a role for P in the cataly-
sis. It is worth noting that use of bare Hal as a catalyst support led to lower product
yield than with Hal-P or Hal-E, confrming the contribution of IL to the catalysis.
Armed with these data, Cu(CH3CN)4PF6 immobilized on Hal-P was selected as
the catalyst of choice. Then, the reaction variables, reaction temperature, catalyst
amount, and solvent were optimized (Table 2). The results showed that, among vari-
ous tested solvents (Table 2), acetonitrile led to the best results. Moreover, the yield
and reaction rate decreased dramatically at lower temperature. Notably, at elevated
temperature, formation of byproducts resulted in decreased yield of the desired
product. Regarding the amount of catalyst, it was found that the optimum value was
15 mg; Use of a lower amount of catalyst led to lower yield and reaction rate.
Table 2 Efect of solvent and temperature on oxidation of allylic C–H bonds of cyclohexene
Entry
Solvent
Temp. (°C)
Catalyst (mg)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
Acetonitrile
Acetone
Dichloromethane
Chloroform
Benzene
25
25
25
25
25
25
10
0
15
15
15
15
15
15
15
15
5
7
90
85
72
45
52
25
83
66
68
80
90
15
27
56
110
150
33
70
22
17
8.5
n-Hexane
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
9
10
11
25
25
25
10
20
1 3

S. Sadjadi et al.
Table 3 Synthesis of allylic esters in presence of Hal-P-supported Cu(CH3CN)4PF6
Catalyst
Yield
(%)
Starting
material
Time
(h)
Entry
Products
Cu(CH3CN)4PF6 on
Hal-P
1
45
7
67
90
Cu(CH3CN)4PF6 on
Hal-P
2
Cu(CH3CN)4PF6 on
Hal-P
3
4
35
16
80
95
Cu(CH3CN)4PF6 on
Hal-P
Next, the generality of the developed protocol was investigated by using various
cyclic olefns (Table 3). The results showed that the yield and reaction rate were
strongly dependent on the size of the cyclic olefn. In detail, among substrates with
one C=C bond, the best result was achieved in the case of cyclohexene, which is a
cyclic olefn with less strain. Study of cyclic olefns with two C=C bonds proved
that not only can this protocol be generalized to this group of olefns, but also that
they were more active than their counterparts with only one C=C bond. It is worth
mentioning that this protocol was not very successful for acyclic olefns, in most
cases showing low yield and several byproducts.
To study the merit of this protocol, the catalytic activity of the catalyst was
compared with our previously reported one [40], viz. CuO encapsulated in SBA
(Cu–B–N–SBA-15). This comparison confrmed that a very low amount of Hal-
based catalyst, compared with Cu–B–N–SBA-15 catalyst, could promote the reac-
tion in signifcantly shorter reaction time to furnish the desired products with com-
parable yield.
Reaction mechanism
According to literature [45, 50–57], a mechanistic rationale for oxidation of allylic
C–H bonds of cyclohexene in presence of the heterogeneous catalyst is proposed
in Scheme 1. Initially, oxygen–oxygen bond in tert-butyl 4-nitrobenzoperoxoate is
cleaved by Cu(I) species to aford Cu(II) p-nitrobenzoate and tert-butoxy radical.
Subsequently, the tert-butoxy radical attracts a hydrogen atom from allylic position
1 3

Cu(CH3CN)4PF6 immobilized on halloysite as efcient…
Scheme 1 Proposed pathway for allylic oxidation of cyclohexene in presence of catalyst
1 3

S. Sadjadi et al.
of the cyclohexene to furnish tert-butyl alcohol and cyclohexenyl radical [56, 57].
Cu(III) p-nitrobenzoate is produced by addition of allylic radical to the Cu(II)
p-nitrobenzoate [58, 59], which can rearrange via a seven-membered cyclic transi-
tion state to give cyclohex-2-en-1-yl 4-nitrobenzoate and regenerate the Cu(I) cata-
lyst [56]. Notably, to confrm the role of the metallic species in the catalysis, the
reaction was also performed in absence of copper. This reaction did not proceed,
confrming the necessity for copper to promote the reaction.
Catalyst recyclability
Finally, the recovery and recyclability of the catalyst for allylic oxidation of
cyclohexene were investigated. To recycle the catalyst, it was simply separated from
the model reaction mixture via fltration, washed with methylene chloride, EtOH,
and distilled water, and dried at 50 °C for 3 h under vacuum. The recovered cata-
lyst was then applied in the next run of the same reaction under similar reaction
condition. The reactions were monitored by TLC, and at the end of each reaction,
the reaction time and yield of the product were obtained. This cycle was repeated
for fve consecutive reaction runs. Figure 7 compares the reaction times and prod-
uct yields of the recycling runs. When using the recycled catalyst, the reaction rate
decreased. However, this decreasing trend was not constant, with the third recycling
run showing a greater decrease in the reaction rate. However, the yield of the desired
product decreased slowly upon recycling, with a decrease of only 10 % in the ffth
run.
To further characterize the recycled catalyst, the FTIR spectrum of the catalyst
after fve recycling runs was recorded and compared with that of fresh catalyst
(Fig. 8). As depicted, after fve recycling runs, the intensities of the characteristic
bands of the catalyst decreased. To measure the leaching of copper upon recycling,
ICP analysis was applied. The results showed that, after fve recycling runs, 0.3 wt%
of the initial copper loading had been leached.
90
80
70
60
50
40
30
20
10
0
1
7
2
10
3
30
4
32
5
40
Time(h)
Yield(%)
90
87
90
84
80
Run
Fig. 7 Efect of catalyst recycling on the reaction time and yield of the model reaction
1 3

Cu(CH3CN)4PF6 immobilized on halloysite as efcient…
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Fresh catalyst
Recycled catalyst
3350
2850
2350
1850
1350
850
350
Wave number (cm-1)
Fig. 8 FTIR spectra of fresh and recycled catalysts
In the fnal part of this study, the nature of the catalysis was studied via hot
fltration test, carried out according to the standard protocol [60]. Briefy, the
reaction was stopped at short reaction time and the catalyst separated from the
reaction mixture. Then, the reaction of the fltrate was continued and its progress
traced. As no signifcant progress was observed in the absence of the catalyst, it
can be concluded that the catalysis was heterogeneous.
Conclusions
An efcient protocol for oxidation of allylic C–H bonds in olefns based on a
novel Hal-based catalytic system was developed. The catalyst was prepared by
prefunctionalization of Hal with ionic liquid (IL) and 2-aminopyrimidine (P) fol-
lowed by immobilization of Cu(CH3CN)4PF6. Study of the catalytic activity of
various copper salts and control supports confrmed that the nature of the cop-
per source could signifcantly afect the reaction. Moreover, both IL and P could
contribute to the catalysis. Study of the catalytic activity of the catalyst confrmed
that a low amount of catalyst could promote the target reactions under mild con-
ditions, afording the desired allylic esters in good yield and shorter reaction time
compared with previous reports. Investigation of the generality of this strategy
established that it could be applied to cyclic olefns with one or two C=C bonds.
However, the size of the cyclic substrate could efectively afect the yield and
reaction rate. Moreover, this protocol was not successful for acyclic olefns. The
results of recycling tests confrmed that the catalyst was recyclable and could be
recovered and recycled for fve consecutive reaction runs.
1 3

S. Sadjadi et al.
Acknowledgements S. Sadjadi is grateful to the Iran Polymer and Petrochemical Institute for partial
fnancial support. The support of Prof. Heravi is greatly appreciated. S. Samadi thanks the University
of Kurdistan Research Councils and Iranian National Science Foundation (proposal no. 95838133) for
partial support of this work.
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