1,8-Diazabicyclo [5.4.0] undec-7-ene functionalized cellulose nanofibers as an efficient and reusable nanocatalyst for the synthesis of tetraketones in aqueous medium

Article abstract of DOI:10.1007/s11164-020-04167-y

Abstract: 1,8-Diazabicyclo [5.4.0] undec-7-ene functionalized cellulose nanofibers (CNF@DBU[Cl]) were simply prepared and characterized by analytical techniques. Scanning electron microscope confirmed that CNF@DBU[Cl] was formed with average size of 30–60?nm. X-ray diffraction of CNF@DBU[Cl] showed that the crystalline structure of the cellulose nanofibers was remained unchanged after functionalization. According to elemental analyses and thermal gravimetric analysis, the loading amount of organic group on cellulose nanofibers was found to be 1.46?mmol/g. The catalytic activity of DBU functionalized cellulose nanofibers was studied for the synthesis of biologically important tetraketone derivatives in the reaction of aldehydes with 1,3-dicarbonyl compounds. In the presence of CNF@DBU[Cl], the corresponding tetraketones were obtained in 80–96% yields under very mild reaction conditions. The catalyst was recovered and reused four successive runs without significant loss of catalytic activity. This method showed several significant advantages including short reaction times, high yields of products, use of various substrates, convenient work-up, no necessity of extraction or chromatographic purification steps, environmentally friendly conditions and lack of use of any harmful solvent. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11164-020-04167-y

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04167-y
1,8‑Diazabicyclo [5.4.0] undec‑7‑ene functionalized
cellulose nanofbers as an efcient and reusable
nanocatalyst for the synthesis of tetraketones in aqueous
medium
Zahra Lasemi, et al. [full author details at the end of the article]
Received: 1 February 2020 / Accepted: 24 April 2020
© Springer Nature B.V. 2020
Abstract
1,8-Diazabicyclo [5.4.0] undec-7-ene functionalized cellulose nanofbers (CNF@
DBU[Cl]) were simply prepared and characterized by analytical techniques. Scan-
ning electron microscope confrmed that CNF@DBU[Cl] was formed with average
size of 30–60 nm. X-ray difraction of CNF@DBU[Cl] showed that the crystalline
structure of the cellulose nanofbers was remained unchanged after functionaliza-
tion. According to elemental analyses and thermal gravimetric analysis, the load-
ing amount of organic group on cellulose nanofbers was found to be 1.46 mmol/g.
The catalytic activity of DBU functionalized cellulose nanofbers was studied for the
synthesis of biologically important tetraketone derivatives in the reaction of alde-
hydes with 1,3-dicarbonyl compounds. In the presence of CNF@DBU[Cl], the cor-
responding tetraketones were obtained in 80–96% yields under very mild reaction
conditions. The catalyst was recovered and reused four successive runs without sig-
nifcant loss of catalytic activity. This method showed several signifcant advantages
including short reaction times, high yields of products, use of various substrates,
convenient work-up, no necessity of extraction or chromatographic purifcation
steps, environmentally friendly conditions and lack of use of any harmful solvent.
Graphic abstract
Vol.:(0123456789)
1 3

Z. Lasemi et al.
Keywords 1,8-Diazabicyclo [5.4.0] undec-7-ene functionalized cellulose
nanofbers · Tetraketone · 1,3-dicarbonyl compounds · Aldehydes · Reusable
heterogeneous catalyst
Introduction
Recently, the investigations of supported heterogeneous catalysts became one of the
main focuses of modern worldwide research in catalysis. Many researches have been
directed toward fnding of sustainable and ecological resources, in other words green
chemistry. To attain an environmentally friendly and sustainable chemical process,
the use of renewable materials as solid supports has found many attention. In this
regard, biopolymer is an attractive candidate as a solid support material in prepa-
ration of heterogeneous catalysts. Biopolymers such as cellulose [1–6], chitosan
[7–10] and chitin [11–13] have attracted remarkable attention as support materials
for catalytic systems owing to their unique properties, renewability, non-toxicity,
low cost and abundance in nature.
Today, nanotechnology has become a top research feld in the world due to difer-
ent applications of nanomaterials in diverse sectors including nanomedicine, catal-
ysis, sensors, environment, water treatment and energy storage [14–28]. In recent
years, the application of nanomaterials has been attracted as catalysts supports.
The development of nano-sized systems as supported heterogeneous cataly-
sis makes it possible to improve the reactivity and the selectivity of the catalysts
[29–31] compared to the solid materials.
Cellulose nanofbers (CNFs) have received much attention because of their
unbeatable properties such as large surface area-to-volume ratio, biocompatibility,
simple surface functionalization, easy handling and outstanding mechanical proper-
ties [32–34]. The CNFs and their composites have been utilized for various potential
applications such as sensors, energy production, catalysis, biomedical, separators
and flters [35–38]. It is well known that cellulose is widely used and easily modi-
fed by functionalization, which makes it a suitable polymeric support for heteroge-
neous catalysts. The presence of hydroxyl groups on the surface of cellulose fbers
provides a stable matrix for anchoring a variety of adsorbed compounds to prepare
the biomaterials for targeted applications [39, 40]. These interesting properties make
this green material an attractive alternative for conventional synthetic organic or
inorganic supports in catalytic applications [41, 42].
2,2′-Arylmethylene bis(3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one (tetrak-
etone) derivatives are an important class of organic compounds containing oxygen
that have attracted a lot of interest due to their various uses in pharmacy, biology
and material sciences. Tetraketone derivatives show biological and therapeutic prop-
erties such as lipoxygenase inhibitory [43], tyrosinase inhibitory in dermatological
disorders including hyperpigmentation and skin melanoma [44], antibacterial and
antiviral activities [43]. Moreover, they are important synthetic intermediates that
can act as useful precursors in the synthesis of xanthene and polyhydroacridine
derivatives [45, 46] that display biological and therapeutic activities.
1 3

1,8‑Diazabicyclo [5.4.0] undec‑7‑ene functionalized…
The conventional method for the preparation of tetraktones is the condensation
of aromatic aldehydes with cyclic 1,3-diketones in the presence of various cata-
lysts such as natural phosphate [47], KF/Al2O3 [48], alumina–sulfuric acid [49],
Yb(OTf)3–SiO2 [50], HClO4–SiO2 [51], mesoporous silica nanoparticles (MSN)-
adsorbed HBF4 [52], copper nanoparticles supported onto silica [53], SmCl3 [54],
Fe3+-montmorillonite [55], HY zeolite [56], nano-Fe/NaY [57], cerium-impreg-
nated-MCM-41 [58] and CoFe2O4 [59]. Although most of these methods for syn-
thesis of tetraketones are efective, some of them sufer from drawbacks such as low
yield and long reaction times [46], leaving harmful waste on work-up [45], tedious
work-up procedure [49], expensive reagents, use of organic solvents, hazardous and
harsh reaction conditions [48] and using unrecyclable catalysts. The development of
simple, efcient and environmentally friendly methods for the synthesis of tetrak-
etones in the presence of novel catalysts is still in great demand.
In continuation of our work on the development of new methods for synthesis of
heterocyclic compounds [60–64], herein, we report the preparation and characteri-
zation of DBU functionalized cellulose nanofbers (CNF@DBU[Cl]) and its appli-
cation as a recyclable heterogeneous green catalyst for the synthesis of tetraketone
derivatives in aqueous medium (Scheme 1).
Experimental
Chemicals and materials
Chemicals (except CNF) were purchased from Merck and Sigma Chemical Compa-
nies. CNF was procured from Nano Novin Polymer Company. The known products
were identifed by comparison of their melting points and spectral data with those
reported in the literature. 1H and 13C NMR spectra were recorded on a Bruker DRX-
400 AVANCE spectrometer in CDCl3 as solvent. Fourier transform infrared (FT-IR)
Scheme 1 Synthesis of tetraketone derivatives using CNF@DBU[Cl]
1 3

Z. Lasemi et al.
spectra were recorded on a Bruker Vector 22 spectrometer in KBr matrix. Scan-
ning electron microscopy (SEM) images were observed using a VEGA/TESCAN-
XMU instrument. X-ray difraction (XRD) was performed using a Philips PW 1830
difractometer with Cu Kα radiation (λ=1.54 Å) in the Bragg angle range of 5-80º
at room temperature. Thermogravimetric analysis was performed using TGA 951
Dupont. Elemental analyses were carried out on an Elemental Analyzer CHNLECO
600. Melting points were determined with an Electrothermal 9100 apparatus.
Preparation of modifed cellulose nanofber (CNF@[Cl])
CNF (1.0 g) was suspended in 30 mL of dry toluene for 40 min, and then, 3-chloro-
propyltrimethoxysilane (1.5 mL) was added to it. The suspension was stirred under
refux for 48 h. Then, the reaction was stopped and cooled to room temperature. The
reaction mixture fltered and washed with toluene, ethanol and fnally diethyl ether.
The modifed cellulose nanofber (CNF@[Cl]) was dried in an oven at 70 °C for 6 h.
Preparation of of CNF@DBU[Cl]
To 1 g of CNF@[Cl] was added acetone (50 mL) and DBU (0.7 mL). The mix-
ture was refuxed with stirring under argon for 36 h. Then the reaction mixture was
cooled to room temperature and fltered to separate the white powder. The solid
was washed with acetone and dichloromethane respectively and dried in an oven at
70 °C for 6 h.
General procedure for the synthesis of tetraketone derivatives (3)
A mixture of aldehyde (1 mmol), 1,3-dicarbonyl compound (2 mmol) and CNF@
DBU[Cl] (0.08 g) in water (5 mL) was stirred at room temperature. After comple-
tion of the reaction (as monitored by TLC), acetone was added to the reaction mix-
ture. Then, the catalyst was separated by centrifugation. Pure products were aforded
by evaporation of the fltrate. The obtained products were characterized by their
melting points as well as 1H and 13C NMR spectral data.
Selected spectral data
2,2′-((2-nitrophenyl)methylene)bis(3-hydroxy-5,5-dimethylcyclohex-2-enone) (3ia):
1
mp 187–189 °C (188–189 °C [65]); H NMR (400 MHz, CDCl3) δ=11.62 (br s,
1H, OH), 7.25-7.57 (m, 4H, ArH), 6.05 (s, 1H, CH), 2.21-2.53 (m, 8H, 4CH2), 1.17
(s, 6H, 2CH3), 1.03 (s, 6H, 2CH3) ppm; 13C NMR (100 MHz, CDCl3) δ=190.9,
189.4, 149.7, 132.1, 131.4, 129.6, 127.2, 124.3, 114.6, 46.9, 46.30, 31.9, 30.0, 28.5,
28.2 ppm.
2,2′-((3-bromophenyl)methylene)bis(3-hydroxycyclohex-2-enone) (3fb): mp 246–
1
248 °C (248–249 °C [66]); HNMR (400 MHz, CDCl3) δ=12.35 (brs, 2H, OH),
1 3

1,8‑Diazabicyclo [5.4.0] undec‑7‑ene functionalized…
7.31(d, J=7.6 Hz, 1H, ArH), 7.22 (s, 1H, ArH), 7.14 (t, J=7.6 Hz, ArH), 7.04 (d,
J=8.0 Hz, 1H, ArH), 5.43 (s, 1H, CH), 2.55-2.70 (m, 4H, 2CH2), 2.35-2.52 (m,
4H, 2CH2), 2.00-2.09 (m, 4H, 2CH2) ppm; 13CNMR (100 MHz, CDCl3) δ=192.2,
190.9, 140.6, 129.7, 129.6, 129.0, 125.2, 122.5, 115.9, 33.5, 33.0, 32.8, 20.1 ppm.
3,3′-((4-chlorophenyl)methylene)bis(4-hydroxy-2H-chromen-2-one) (3dc): mp 250–
252 °C (250 °C [67]); 1H NMR (400 MHz, CDCl3) δ=11.2 (brs, 2H, OH), 7.88 (d,
J=12 Hz, 2H, ArH), 7.58 (t, J=20 Hz, 2H, ArH), 7.34 (d, J=8 Hz, 2H, ArH), 7.30
(t, J=16 Hz, 2H, ArH), 7.26 (d, J=8 Hz, 2H, ArH), 7.16 (d, J=8 Hz, 2H, ArH),
6.30 (s, 1H, CH) ppm; 13C NMR (100 MHz, CDCl3) δ=166.4, 165.1, 152.8, 140.3,
132.2, 130.4, 129.1, 128.3, 124.4, 124.0, 118.8, 116.3, 103.6, 36.2 ppm.
Results and discussion
According to various research reports on composite materials and their diferent
applications [68–77], we decided to prepare the nanocatalyst based on cellulose
nanofbers as an abundant and natural material and to examine its catalytic activity
for the synthesis of tetraketones.
Initially, CNF@DBU[Cl] was synthesized in two steps. In the frst step, the cel-
lulose nanofber was reacted with 3-chloropropyltrimethoxysilane (CPTMS) in
dry toluene under refux condition, and modifed cellulose nanofber (CNF@[Cl])
was prepared. Next, CNF@[Cl] and DBU were refuxed in acetone for 36 h. The
obtained catalyst (CNF@DBU[Cl]) was characterized by FT-IR, SEM, XRD, TGA
and CHN analyses.
Characterization of CNF@DBU[Cl]
The FT-IR transmission spectra for CNF and CNF@DBU[Cl] are shown in Fig. 1.
For CNF, the strong band at 3425 cm−1 is related to stretching vibration of hydroxyl
group. The absorption band at 2905 cm−1 is corresponding to stretching vibration
of the C-H bond in the CH2 and CH3 groups. The appeared band at 1430 cm−1
shows the bending vibrations of the CH2 group. The characteristic vibration peak of
C–O–C appears at 1090 cm−1. The observed band at 1680 cm−1 is attributed to the
bending vibration of H–O–H, and eventually, a small sharp band at 898 cm−1 refers
to beta-glycoside bonds in cellulose (Fig. 1a) [78].
For CNF@DBU[Cl] (Fig. 1b), the absorption bands of Si–O functional group and
stretching vibration of Si–O–C group are appeared at regions of 575–750 cm−1 and
1100–1200 cm−1. The observed band at 1000–1350 cm−1 is related to the stretching
vibration of the C–N bond, and absorption band at 1665 cm−1 is in accordance with
the stretching vibration of the C=N bond [79]. This demonstrates that the compound
containing silicon has been successfully attached to the oxygen of the cellulose’s
hydroxyl groups.
The thermal gravimetric analysis (TGA) of CNF and CNF@DBU[Cl] is shown in
Fig. 2. In TGA of CNF, it was observed that weight loss occurred in two stages. The
1 3

Z. Lasemi et al.
Fig. 1 FT-IR spectrum of a CNF and b CNF@DBU[Cl]
Fig. 2 TGA curves of a CNF and b CNF@DBU[Cl]
frst weight decrease was assigned to the catalyst surface moisture (at below 100 °C,
8% weight loss), while the second decrease at 250–380 °C showed the decompo-
sition of hemicellulose polymeric chains of cellulose (Fig. 2a) [80]. According to
TGA diagram, weight loss was also seen in two steps for the CNF@DBU[Cl]. At
the frst step, there is a weight loss about 2% below 100 °C which is related to sur-
face moisture. The second weight loss occurred at 250–350 °C, which is less than
that for CNF (Fig. 1b). By comparing thermal gravimetric analysis of CNF and
CNF@DBU[Cl] together, it is determined that the presence of the created functional
1 3

1,8‑Diazabicyclo [5.4.0] undec‑7‑ene functionalized…
Table 1 CHN analysis of CNF
Catalyst
C (%)
H (%)
N (%)
and CNF@DBU[Cl]
CNF
38.01
39.81
5.87
6.15
0.2
0.49
CNF@DBU [Cl]
Fig. 3 SEM micrograph of a CNF@DBU[Cl]) and b CNF
group in the CNF@DBU[Cl] catalyst led to increase in the thermal stability of cata-
lyst than CNF. It is necessary to mention that this catalyst is stable between 100 and
250 °C.
Elemental analyses (CHN) were carried out on CNF and CNF@DBU[Cl].
According to the obtained results from CHN (Table 1), it is observed that the per-
centage of carbon, hydrogen and nitrogen elements is increased in CNF@DBU[Cl],
which can confrm the loading of organic group on cellulose nanofbers. The organic
group loading of CNF@DBU[Cl] was found to be 1.46 mmol/g.
The morphology and particle size of CNF and CNF@DBU[Cl] were studied by
scanning electron microscope (SEM) images (Fig. 3). According to SEM images of
CNF, the average diameter of the cellulose nanofber is 20–50 nm [81]. As shown in
Fig. 3a, in CNF@DBU[Cl] catalyst, spherical particles were appeared on cellulosic
nanofbers with average size of 30–60 nm, which led to the increase in the diameter
of cellulosic fbers.
CNF@DBU[Cl] was analyzed by X-ray difraction to compare its crystalline state
with CNF. In XRD analyses of cellulose nanofber, the presence of characteristic
peaks at 2θ values of 14.9°, 16.7°, 20.36°, 52.22° and 34.7° shows the crystalline
nature of the cellulose nanofbers structure [82]. In XRD of CNF@DBU[Cl], no
sharp peak was observed, other than those related to CNF, and this shows that the
crystalline structure of the cellulose nanofber was retained after functionalization
(Fig. 4). With respect to the XRD analyses of CNF@DBU[Cl] and using the Sherer
equation, cellulose nanofber’s diameter is about 25 nm.
1 3

Z. Lasemi et al.
Fig. 4 XRD patterns of a CNF
and b CNF@DBU[Cl]
Scheme 2 Synthesis of CNF@DBU[Cl]
Catalytic activity of CNF@DBU[Cl]
After synthesis and characterization of the catalyst, we investigated the catalytic
activity of nano-solid catalyst for synthesis of tetraketones, by the condensation of
various aldehydes and 1,3-dicarbonyl compounds (Scheme 2).
To optimize quantity of CNF@DBU[Cl] and reaction conditions, a model experi-
ment was carried out using 4-chloro benzaldehyde and dimedone. When the model
reaction was examined in the absence of catalyst at room temperature, no desired
product was formed even after 60 min (Table 2, entry 1), but in the presence of
1 3

1,8‑Diazabicyclo [5.4.0] undec‑7‑ene functionalized…
Table 2 Optimization of reaction conditions for the synthesis of tetraketone 3da
Entry
Catalyst (g)
Solvent/condition
Time (min)
Yield (%)a
1
2
3
4
5
6
7
8
–
H2O/25 °C
H2O/25 °C
H2O/25 °C
H2O/25 °C
H2O/25 °C
H2O/25 °C
EtOH/25 °C
CH2Cl2/25 °C
CH3CN/25 °C
PhCH3//25 °C
H2O/25 °C
H2O/25 °C
H2O/25 °C
60
4
–
CNF@DBU[Cl] (0.08)
MCC (0.08)
DBU(0.08)
MCC@DBU[Cl] (0.08)
CNF (0.08)
CNF@DBU[Cl] (0.08)
CNF@DBU[Cl] (0.08)
CNF@DBU[Cl] (0.08)
CNF@DBU[Cl] (0.08)
CNF@DBU[Cl] (0.04)
CNF@DBU[Cl] (0.06)
CNF@DBU[Cl] (0.1)
92
60
90
70
65
30
35
30
20
73
76
94
85
180
30
50
8
60
60
70
10
10
4
9
10
11
12
13
Reaction conditions: reactants 1d (1.0 mmol) and 2a (2.0 mmol) in 5 mL solvent
aYields refer to isolated products
CNF@DBU[Cl] (0.08 g) after 4 min a high yield of the expected tetraketone 3da
was obtained, showing the role of the catalyst in this reaction (Table 2, entry 2). The
model reaction was also studied in the presence of MCC, DBU, MCC@DBU[Cl]
and CNF, which no satisfactory result was achieved under these catalytic conditions
(Table 2, entries 3–6). The above reaction was also examined in various solvents
such as EtOH, CH2Cl2, CH3CN and toluene at room temperature (Table 2, entries
7–10). All the solvents required a longer reaction time and gave lower yields than in
water (Table 2, entry 2).
Similar reactions were then attempted using 0.04, 0.06, 0.08 and 0.1 g of CNF@
DBU[Cl] (Table 2, entries 2 and 11–13). It was found that utilizing 0.08 g CNF@
DBU[Cl] in water at 25 °C is sufcient to complete the reaction with maximum
yield (Table 2, entry 2). Increasing the amount of catalyst did not change the yield,
whereas reduction in it signifcantly decreased the product yield (Table 2, entries
11, 12). Therefore, the best result was obtained by the use of molar ratio of 1: 2 of
benzaldehyde with 1,3-dicarbonyl compound and 0.08 g CNF@DBU[Cl] as catalyst
in H2O at 25 °C.
After optimization studies, the scope and generality of this method were
further explored with diferent substrates. As shown in Table 3, we initially
1 3

Z. Lasemi et al.
Table 3 Synthesis of tetraketone derivatives 3
Entry
R
1,3-dicarbonyl Producta
compound
Time (min)
Yield (%)a
References
1
2
3
4
5
6
7
8
C6H5 (1a)
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2c
2c
2c
2c
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
15
4
2
4
10
4
12
6
5
7
8
60
20
45
30
35
5
15
20
8
5
4
12
7
16
9
90
95
96
92
90
95
93
96
95
95
90
90
87
93
92
90
40
30
90
92
94
95
90
92
95
95
93
94
92
92
90
95
94
90
35
40
90
92
80
85
[44]
[59]
[83]
[59]
[67]
[84]
[85]
[67]
[65]
[65]
[85]
[85]
[65]
[65]
[67]
[85]
[67]
[86]
[44]
[59]
[83]
[59]
[44]
[66]
[85]
[87]
[84]
[66]
[44]
[87]
[84]
[84]
[66]
[59]
[87]
[87]
[67]
[88]
[89]
[90]
2-Cl–C6H4 (1b)
3-Cl–C6H4 (1c)
4-Cl–C6H4 (1d)
2-Br–C6H4 (1e)
3-Br–C6H4 (1f)
4-Br–C6H4 (1g)
4-F–C6H4 (1h)
2-NO2–C6H4 (1i)
4-NO2–C6H4 (1j)
4-CN–C6H4 (1k)
4-CF3–C6H4 (1l)
4-MeO–C6H4 (1m)
4-Me–C6H4 (1n)
2-HO–C6H4 (1o)
4-HO–C6H4 (1p)
CH3CH2CH2 (1q)
CH3(CH2)3CH2 (1r)
C6H5 (1a)
2-Cl-C6H4 (1b)
3-Cl-C6H4 (1c)
4-Cl-C6H4 (1d)
2-Br–C6H4 (1e)
3-Br–C6H4 (1f)
4-Br–C6H4 (1g)
4-F–C6H4 (1h)
2-NO2–C6H4 (1i)
3-NO2–C6H4 (1s)
4-NO2–C6H4 (1j)
4-CN–C6H4 (1k)
4-MeO–C6H4 (1m)
4-Me–C6H4 (1n)
2-HO–C6H4 (1o)
4-HO–C6H4 (1p)
CH3CH2CH2 (1q)
CH3(CH2)3CH2 (1r)
4-Cl–C6H4 (1d)
4-Br–C6H4 (1g)
4-MeO–C6H4 (1m)
4-Me–C6H4 (1n)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
3ja
3ka
3la
3ma
3na
3oa
3pa
3qa
3ra
3ab
3bb
3cb
3db
3eb
3fb
3gb
3hb
3ib
3sb
3jb
3kb
3mb
3nb
3ob
3pb
3qb
3rb
3dc
3gc
3mc
3nc
8
7
10
10
25
60
45
45
13
30
100
130
180
240
All compounds are known, and their structures were established from their spectral data and melting
points as compared with literature values
aYields refer to isolated products
1 3

1,8‑Diazabicyclo [5.4.0] undec‑7‑ene functionalized…
investigated the reaction of dimedone with benzaldehyde which aforded the cor-
responding tetraketone in excellent yields (Table 3, entries 1). When dimedone
reacted with benzaldehydes including halogen group and electron-withdrawing
group on phenyl ring, the corresponding product was formed with 90-96% yields
(Table 3, entries 2–12). Aromatic aldehydes bearing electron-donating gave high
yields of the corresponding products (Table 3, entries 13–16), but within longer
reaction time and lower yields than benzaldehydes containing electron-withdraw-
ing group.
1,3-cyclohexane-dione was also treated successfully with diferent aromatic alde-
hydes and gave the target tetraketones in high yields (Table 3, entries 19–34). But
the reaction with dimedone was fnished faster than with 1,3-cyclohexane-dione
which may be owing to the diference of the activity between the two active methyl-
ene reagents.
Similarly, when dimedone and 1,3-cyclohexane-dione were treated with aliphatic
aldehydes in the presence of CNF@DBU[Cl] (0.08 g) in water, a moderate yield of
the corresponding products was obtained (Table 3, entries 17–18 and 35–36). The
obtained results showed aromatic aldehydes aforded to desired product with higher
yields in shorter reaction time than aliphatic aldehydes.
4-Hydroxy-2H-chromen-2-one (2c) was also reacted with various aromatic alde-
hydes, and the corresponding tetraketone products were obtained in good yields but
within longer times (Table 3, entries 37-40).
An interesting feature of this method is that the catalyst can be regenerated at the
end of the reaction and used several times without losing its activity. To recover the
catalyst after completion of the reaction, catalyst was separated by centrifuging fol-
lowed by fltration. The solid residue was washed several times with acetone. Finally,
the regenerated catalyst was dried at 60 °C. After activating the catalyst, it was used
for the reaction of 4-chloro benzaldehyde with dimedone. This process was repeated
for four cycles, and the yield of product 3da did not signifcantly change (Table 4).
These results clearly show the stability of CNF@DBU[Cl] in the reaction media as
well its recovery and recycling without any appreciable decrease in its activity.
The proposed mechanism for the formation of tetraketon derivatives has shown in
Scheme 3. First, the catalyst basic site (DBU[Cl]) absorbs the proton from 1,3-dicar-
bonyl compound 2 and enolate I is formed [91]. In next step, by the Knoevenagel
reaction of enolate I and the activated aldehyde 1 (activated with OH groups of
CNF), intermediate II is generated. Subsequently, dehydration took place to form
Table 4 Recycling studies for
Run
Cycle
Time (min)
Yield (%)a
the model reaction using CNF@
DBU[Cl] under optimal reaction
conditions
1
2
3
4
5
0
1
2
3
4
4
4
4
4
4
92
92
92
90
90
aIsolated yield
1 3

Z. Lasemi et al.
Scheme 3 Suggested mechanism for synthesis of tetraketone derivatives catalyzed by CNF@DBU[Cl]
the intermediate III. This intermediate as a Michael accepter reacts with enolate I,
to give IV. After tautomerization, the product 3 is prepared [84].
In Table 5, the efciency of CNF@DBU[Cl] was compared with some of the pre-
viously introduced procedures to demonstrate its catalytic activity for the synthesis
of diferent tetraketones. Obviously, CNF@DBU[Cl] shows higher catalytic activity
1 3

1,8‑Diazabicyclo [5.4.0] undec‑7‑ene functionalized…
Table 5 Comparative synthesis of compounds 3da using diferent catalysts method
Entry Catalyst
Conditions
Time (min) Yield (%) References
1
2
3
4
5
CNF@DBU[Cl]
Fe3O4@SiO2–SO3H
HClO4–SiO2
H2O, rt, 80 mg
H2O, rt, 10 mg
H2O, 100 °C, 10 mol%
H2O, rt, 500 mg
EtOH, 80 °C, 100 mg
4
80
30
120
60
92
83
89
98
93
[This work]
[92]
[44]
[47]
[53]
Natural phosphate
Copper nanoparticles onto
silica
6
7
8
CoFe2O4 Nanoparticles
Nano-Fe/NaY zeolite
GO/ZnO
H2O: EtOH 60 °C, 11.7 mg
EtOH, 78 °C, 2.5 mg
H2O, 100 °C, 10 mg
4
70
10
89
98
99
[59]
[57]
[84]
in comparison with several of the others in terms of catalyst loading, product yield,
avoiding the use of toxic transition metals, toxic solvents and short reaction time.
One of the most important advantages of CNF@DBU[Cl] is that it is based on cel-
lulose as the most abundant renewable biomass.
Conclusion
In conclusion, DBU functionalized cellulose nanofber was introduced as a het-
erogeneous eco-friendly nanocatalyst for the synthesis of tetraketones in aqueous
medium. According to SEM image, average size of catalyst was 30-60 nm. Ther-
mogravimetric analysis of catalyst exhibited thermal stability between 100 and
250 °C. According to the results of TGA and CHN, the loading amount of catalyst
was 1.46 mmol/g. All the reactions worked easily for a variety of benzaldehydes
with both electron-donating and electron-withdrawing groups to give corresponding
products in high yields (80–96%). The excellent catalytic performance, easy prepara-
tion and separation of the catalyst make it to be a good heterogeneous natural-based
renewable nanocatalyst for organic synthesis and transformations. In addition, oper-
ational simplicity, good yields, low cost, short reaction times, non-use of toxic sol-
vent, increased safety, environmental friendly reaction condition, easy removal and
recyclability of the catalyst are supporting this method toward the green chemistry.
Acknowledgements We are grateful to University of Mazandaran for partial support of this work.
Compliance with ethical standards
Confict of interest The authors declare that they have no confict of interest.
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Z. Lasemi et al.
Afliations
Zahra Lasemi1 · Mahmood Tajbakhsh2 · Heshmatoallah Alinezhad2 ·
Forough Mehrparvar2
*
Zahra Lasemi
zlasemi.z@gmail.com
1
2
Department of Chemistry, South Tehran Branch, Islamic Azad University, Tehran, Iran
Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran, Babolsar,
Iran
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