β-Cyclodextrin-TiO2: Green Nest for reduction of nitroaromatic compounds

Article abstract of DOI:10.1039/c4ra08059a

In this paper, highly efficient photocatalytic reduction of nitroaromatic compounds was investigated using β-cyclodextrin (β-CD) and commercial nano-TiO₂ as a host-guest system in water under sunlight irradiation. The nitroaromatic compounds solubilized in water through encapsulation in β-CD formed an inclusion complex which was attached to TiO₂ under sunlight irradiation. This 'guest-host system' that we call 'Green Nest' showed high efficiency for the reduction of nitro compounds to the corresponding amines, and more interestingly, one pot reductive N-formylation and N-acylation from nitroaromatic compounds can be carried out in the presence of triethyl orthoformate, acetic and benzoic anhydride. The β-CD-TiO₂ was characterized by transmission electron microscopy, UV-visible spectra, thermogravimetric analysis, Brunauer-Emmett-Teller measurements, UV-visible diffuse reflectance spectroscopy, and Raman spectroscopy.

Full text of DOI:10.1039/c4ra08059a

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b-Cyclodextrin–TiO : Green Nest for reduction of
2
nitroaromatic compounds†
Cite this: RSC Adv., 2014, 4, 52762
a
ab
a
Mazaher Abdollahi Kakroudi, Foad Kazemi* and Babak Kaboudin
In this paper, highly efficient photocatalytic reduction of nitroaromatic compounds was investigated using
b-cyclodextrin (b-CD) and commercial nano-TiO as a host–guest system in water under sunlight
irradiation. The nitroaromatic compounds solubilized in water through encapsulation in b-CD formed an
inclusion complex which was attached to TiO under sunlight irradiation. This ‘guest–host system’ that
2
2
we call ‘Green Nest’ showed high efficiency for the reduction of nitro compounds to the corresponding
amines, and more interestingly, one pot reductive N-formylation and N-acylation from nitroaromatic
compounds can be carried out in the presence of triethyl orthoformate, acetic and benzoic anhydride.
Received 3rd August 2014
Accepted 2nd October 2014
The b-CD–TiO
2
was characterized by transmission electron microscopy, UV-visible spectra,
DOI: 10.1039/c4ra08059a
thermogravimetric analysis, Brunauer–Emmett–Teller measurements, UV-visible diffuse reflectance
www.rsc.org/advances
spectroscopy, and Raman spectroscopy.
and environmentally acceptable photocatalytic reductive
methods for the nitro group without degrading the conjugated
Introduction
The concept of photochemistry as a green technology is more structure in a process that is direct and mild and could also
than a century old. Sunlight as an unending and natural source allow transformation to other functional groups. In recent
could be a key component of industry in the future. This dele- years, a great deal of attention has been paid to the photo-
gates a key role to photochemistry in future industrial catalytic transformation of nitro compounds into amine, imine
6
processes. Despite the advent of articial light sources and and benzimidazole functionalities based on TiO
other leaps in technology, it is not unexpected that mankind
turns to the sun as a free energy and light source for organic able, and more importantly cheap and safe solvent and has
2
.
Compared to other organic solvents, water is a green, avail-
1
7
photochemistry and a true ‘green’ route in organic synthesis.
Selective transformation of organic compounds using metal contrast to the popularity of ‘TiO
therefore attracted a lot of attention in organic synthesis. In
nanoparticles’ in photo-
2
oxide semiconductors as mediators is one of the current chal- catalytic transformations in organic solvents, their use in
lenging methods in organic synthesis from the viewpoints of selective organic synthesis in water is still not quite as wide-
2
renewable energy and environmental applications. Owing to its spread because of some limitations or disadvantages associated
biochemical compatibility, low cost, non-toxicity, optical and with the low solubility of substrates and over-oxidation of
electronic properties and high photo-activity, the main interest organic compounds which are a result of the oxidant species
À
among the various metal oxide semiconductors has been shif- produced (such as cOH and O
c/HO
c) in a photocatalytic
2
2
3
8
ted to TiO
2
.
Also, because of its excellent photochemical process. Thus, overcoming these impediments in water is
plays a crucial role in a wide range of important worth highlighting in photocatalytic synthesis. Recently,
properties, TiO
2
redox organic transformations, especially in selective reduction molecular host–guest complexes have provided one of the
4
of nitro compounds. Up to now, a wide variety of traditional promising routes to adsorption of organic molecules on the
methods has been developed for the reduction of nitro surface of metal oxide semiconductors in order to improve the
compounds to corresponding amines using hazardous reagents interaction of organic substrates with photocatalytically active
5
but they show poor efficiency. To overcome these limitations, sites when water is used as the reaction medium. Quite recently,
many attempts have been made at designing new sustainable Nichols et al. demonstrated that cyclodextrin–TiO nanowires
2
with an open and porous structure show self-assembly capa-
9
bility as a result of simulated sunlight irradiation in water.
a
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
Their marine sponge-like structure, with high porosity and
surface area, played a role as host for easy encapsulation of
organic compounds. Recent studies indicate an increased
4
9195-1159, Zanjan, Iran. E-mail: kazemi_f@iasbs.ac.ir
b
Center for Climate and Global Warming (CCGW), Institute for Advanced Studies in
Basic Sciences (IASBS), Gava Zang, Zanjan 45137-66731, Iran
1
†
Electronic supplementary information (ESI) available: Experimental details,
H
2
photocatalytic performance when the surface of TiO particles is
13
NMR and C NMR spectra of the synthesized compounds, UV-vis spectra, TGA,
Raman and BET analysis. See DOI: 10.1039/c4ra08059a
10
modied with host molecules such as b-cyclodextrin (b-CD). In
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other words, the modication of TiO
2
with cyclodextrin through Table 1 Photoreduction of 3-nitroacetophenone using b-CD–TiO
2
for various conditions under sunlight irradiation
interactions between the hydroxyl groups of b-CD and TiO2
draws the organic molecules near the surface of the photo-
catalyst to enhance their interaction and also results in an
11
increase in the photocatalytic redox ability of TiO
2
.
In
continuation of our effort to design novel systems for photo-
12
catalytic organic transformations, in this work we present a
highly efficient and green photocatalytic system for selective
reduction of the nitro group under sunlight irradiation based
on in situ modication of TiO2 P25 with b-CD in water. We
a
Entry
Additive
Time (h)
Yield (%)
1
2
3
4
5
6
7
—b
Oxalic acid
4
1
3
3
3
6
6
49
100
46
31
95
believe that b-CD can enhance the dispersion of TiO in water,
2
and together produce a ‘host nano-reactor system’, that we call
the ‘Green Nest’, in order to compensate for the weak interac-
Potassium iodide
Sodium sulfate
Ammonium formate
tion of organic nitro compounds with the surface of TiO
aqueous medium. Also, b-CD can promote the charge transfer
rate from the photo-excited TiO to the electron acceptor guest
2
in an
c
—
—
—
d
—
2
a
3
b
c
GC yield. Luximetry 10–80 Â 10 Lux. Without additive. Without
molecule. Thus, it is assumed that the hydrophobic inner cavity
of b-CD is a nano-photo-reactor in this system. To investigate
this, rst nitro compounds were added to an aqueous solution
of b-CD, then this inclusion complex of b-CD-nitro compound
was adsorbed on the surface of titania by sunlight irradiation,
and the reduction of the nitro group to amine function was
carried out in the hydrophobic cavity of b-CD. Since amide
derivatives such as formanilides are valuable intermediates in
d
2
TiO P25. In the dark.
13
organic synthesis, we attempted to expand this proposed
photocatalytic system to one-pot reductive N-formylation and N-
acylation.
Results and discussion
b-CD is a well-known host molecule for organic compounds due
to its hydrophobic cavity. The hydroxyl groups in an inclusion
14
complex of organic compound/b-CD make it soluble in water.
Besides, recent research in the development of high-efficiency
photocatalysis processes shows that the presence of a host
À4
Fig. 1 UV-visible absorption spectra of p-nitrophenol (1 Â 10 M)
molecule such as b-CD is very protable. With this in mind, in aqueous media and titration of this solution by b-cyclodextrin
À3
nitro compounds were added to a solution of b-CD, and then (1 Â 10 M).
the resulting inclusion complex of b-CD-nitro compound was
adsorbed on the surface of titania by sunlight irradiation. Initial
experiments were carried out with a water solution containing of p-nitrophenol increases with the addition of a b-CD solution
b-CD (0.1 mmol), 3-nitroacetophenone (0.1 mmol), and resulting in the formation of a host–guest complex.
commercial TiO
2
(30 mg) under sunlight irradiation for 4 h
Also, using polyethylene glycol/water, EtOH/water and
(
Table 1, entry 1) resulting in a 49% conversion according to gas prototype polyhydroxyl carbohydrates such as glucose showed
chromatography (GC). Addition of ammonium formate or oxalic that although the solubility of nitrobenzene increased, the
acid as a hole scavenger under the same conditions was inves- conversion and product selectivity decreased. An increase in the
tigated (Table 1, entries 5 and 2). No conversion of 3-nitro- visible-light activity of TiO
can be achieved in combination
2
acetophenone was observed without TiO or in the dark (Table
2
1, entries 6 and 7).
b-CD can play a more crucial role in the above mentioned
reaction in water. First, b-CD is the host molecule for the nitro
compounds and increases their solubility. This fact can be
easily observed through the disappearance of nitrobenzene in
water in a separate test.
Also, it was reported that the formation of host–guest
complexes between b-CD and organic compounds in water
affects the UV-visible properties. Therefore, as can be seen in
Scheme 1 pH enhancement of a b-CD–TiO
irradiation results in deprotonation from a hydroxyl group of the CD
2
solution during sunlight
15
2
Fig. 1, the intensity of UV-visible spectra of an aqueous solution molecules on TiO .
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Table 2 Reduction of nitro compounds using b-CD–TiO
2
in water under sunlight irradiation
a
Entry
Substrate
Product
Time [h]
3
Yield [%]
b
1
100 (83)
2
3
1
2
100
100
4
5
1.5
92
3
95
95
6
7
3.5
5
c
88
8
9
2
3
100
96
1
1
0
1
4
89
1.5
100
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Table 2 (Contd.)
a
Entry
Substrate
Product
Time [h]
1.5
Yield [%]
1
2
3
100
1
3.5
95
a
2
GC yield using an internal standard (biphenyl) method unless otherwise stated. Reaction conditions: nitro compounds (0.1 mmol), TiO (30 mg),
b
b-CD (0.1 mmol), water (15 mL) and oxalic acid (15 mg) or ammonium formate (25 mg) was degassed by Ar gas and irradiated with solar light. The
reaction was carried out in 2 mmol of nitrobenzene with the following reaction conditions: nitro compounds (2 mmol), TiO (0.5 g), b-CD (0.8
mmol), water (60 mL) and ammonium formate (0.1 g), 12 h, under solar light irradiation, isolated yield. For synthesis of benzimidazole: with
the same conditions using a mixture of ethanol (2 mL) and water (13 mL). Daily sunlight (9 am–3 pm; sunlight intensity of 10–80 Â 10 Lux).
2
c
3
+
with b-CD because of the hydroxyl group's interaction with h
intact during the photocatalytic reduction reaction (Table 2,
16
on the surface of photocatalyst. To show this fact, the above entries 4 and 11). Also, reduction of 1,2-dinitrobenzene to the
reaction was carried out in the absence of b-CD. It was observed corresponding diamine was carried out (Table 2, entry 6). When
that the reduction of nitrobenzene in water without b-CD was the same reaction was carried out in a solution of ethanol–water
not a selective and clean reaction with a conversion of 30% aer (2–13 mL), benzimidazole was obtained with 88% yield (Table 2,
3
h. On the other hand, we used a mixture of water and ethanol entry 7). When the reaction was carried out on a greater scale, 2
or polyethylene glycol instead of b-CD to increase the solubility mmol of nitrobenzene, the product (aniline) was obtained in
of the nitro compounds in water. Nevertheless, a mixture of 83% isolated yield aer 12 h under solar light irradiation (Table
b
products for this condition was observed. b-CD molecule is built 2, entry 1 ).
up from glucose units, which can also reduce the electron–hole
The mole ratio of nitro to ammonium formate is 1 to 3.2 and
recombination rate. To show the importance of the hydro- nitro to oxalic acid is 1 to 1.7. The ammonium formate and
phobic cavity in b-CD, glucose/TiO was used in a similar oxalic acid convert to ammonia, carbon dioxide or carbon
manner to b-CD–TiO . The low yield of arylamine conrmed the dioxide during photocatalytic reaction respectively. We found
17
2
2
crucial role of the b-CD cavity in the high performance of pho- that, in an experimental control test, when the reaction was
tocatalytic reduction of nitroaromatic compounds in water. carried out in the presence of excess amount of ammonium
Also, in a model experiment, changing the acidity of the mixture formate, amine product was obtained in quantitative yield aer
through attachment of b-CD to TiO2 was measured and an separation by extraction with EtOAc. The aqueous phase
increase of acidity from 7 (in the absence of b-CD) to 4 (in the including excess ammonium formate can be used for the next
presence of b-CD under sunlight irradiation) was observed. reduction cycle without loss of the conversion yield (see ESI,
These pH changes can be due to the surface adsorption of b-CD Scheme S1†).
to TiO
2
particles and formation of the b-CD–TiO
2
composite in
The important factors in photocatalytic applications in
aqueous medium are the stability and reusability of the nano-
18
water under sunlight irradiation (Scheme 1).
In the next step, studies were extended to the reduction of material at the end of the reaction. To study this, the photo-
different types of nitroaromatic compounds including func- catalytic performance of b-CD–TiO was investigated aer four
2
tional groups such as carbonyl, nitrile and halide using the b- usages (Fig. 2). At the end of each run, ethyl acetate was added
CD–TiO system in water under sunlight irradiation (Table 2). to the aqueous mixture and sonicated for 5 min. The aqueous
2
2
Several aromatic nitro compounds with electron-donating or solution containing b-CD–TiO nanoparticles was reused in the
electron-withdrawing functional groups were reduced to the next run. A decrease in the photocatalytic efficiency can be due
corresponding amines (Table 2, entries 8–13). Consequently, a to the partial loss of photocatalyst during product separation.
higher reduction conversion of the nitroaromatic compounds These results are consistent with the retention of good photo-
in the presence of electron-withdrawing groups was observed. catalytic activities of b-CD–TiO
High chemoselectivity of the reaction was shown with reduction Various characterizations of b-CD–TiO
2
aer four usages.
conrm the pres-
2
of nitro functional group in the presence of other sensitive ence of b-CD in the titania surface aer 3 h sunlight irradiation
functional groups such as carbonyl and nitrile that remained of the solution and b-CD. Fig. S5† shows the thermogravimetric
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attributed to the attachment of b-CD to the titania surface under
sunlight irradiation (Table S1 in the ESI†).
FTIR spectral analysis of b-CD–TiO2 hybrid nanoparticles
using TiO as a reference was carried out (Fig. 3). In comparison
2
with P25 (Fig. S11 in the ESI†), in the spectrum of TiO
intensity increases at 3400 cm . This result can be due to the
2
–b-CD the
À1
presence of the O–H groups of b-CD. The peak centered at
À1
1
676 cm was assigned as the stretching of the C]C bonds.
À1
Also, the absorption at 1414 cm was assigned to the O–H
in-plane bending, while the antisymmetric C–O–C stretch was
assigned at 1156 cm . The absorption at 1030 cm
À1
À1
was
20
assigned as the C–O stretching vibrations.
Fig. 2 Reusability and photocatalytic efficiency of the b-CD–TiO
aqueous solution for reduction of nitrobenzene under sunlight irra-
diation after 3 h.
2
In continuation, we studied a green, one-pot protocol for
further transformation of photocatalytic-produced amines into
valuable N-formylated and N-acylated amines in water
(Scheme 3). Carboxylic anhydrides and triethyl orthoformate
analysis (TGA) of b-CD–TiO2 aer separation and washing
several times with water. From the TGA curve it was observed
that there was around 4% CD in b-CD–TiO
2
À1
. Raman spectra
(C–H) which is
show a band located that 2800–3000 cm
evidence of b-CD in b-CD–TiO
However, as presented in Scheme 2, there were no differences in
the lattice structure in TEM images of b-CD–TiO and TiO
2
nanocomposite (Fig. S9 in ESI†).
2
2
.
Considering the UV-visible diffuse reectance spectroscopy
analysis of b-CD–TiO (Fig. S6 in the ESI†), the latter displayed a
2
1
9
higher absorption in the visible light region than TiO
2
.
Also, a
2
based
decrease in surface area of b-CD–TiO compared to TiO
2
on the Brunauer–Emmett–Teller (BET) method can be
2
Fig. 3 FTIR spectrum of b-CD–TiO hybrid nanoparticles.
Scheme 3 One-pot N-formylation and N-acylation of photocatalytic
produced amines through encapsulated nitrobenzene in b-CD under
sunlight.
2 2
Scheme 2 TEM images of b-CD–TiO (a) and TiO (b).
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were used as acylation and formylation agents. In this area, Lou acylation agents also efficiently produced the corresponding
et al. have reported the reductive N-formylation of nitroarene amides. As summarized in Table 3, various types of nitro-
compounds using Au–TiO2 in the presence of ammonium aromatics including other functional groups were selectively
formate at two temperatures (room temperature and reux transformed into their corresponding amide compounds in
21
condition) in CH
ndings, when an excess amount of ammonium formate was selective N-formylation of p-nitrophenol with very high yield
used in our method, no N-formylated product was detected aer (Table 3, entry 10). As expected, when the reaction was carried
h. Then, in the next attempt, triethyl orthoformate was used as out in 2 mmol of nitrobenzene, the anilide was obtained in
a more active formylating agent. Interestingly, in the presence notable decreased isolated yield aer 12 h.
3
CN as an aprotic solvent. According to these water. The photocatalytic system also showed excellent regio-

5
of triethyl orthoformate, high yield of the corresponding amide
To study the formylation step, in control reactions, the same
was achieved. Moreover, using carboxylic anhydrides as conditions were used for the amine as the starting material, and
Table 3 One-pot synthesis of amides from the photocatalytic reduction of nitro compounds using triethyl orthoformate or anhydride
a
Entry
1
Substrate
Product
Time (h)
2.5
Yield (%)
100
2
3
4
1.5
3
91
97
2
94
5
6
7
3.5
2
100
b
CH(OEt)
3
3
100 (66)
CH(OEt)
3
94
91
8
9
CH(OEt)
3
3
1
CH(OEt)
2.5
90
89
1
0
CH(OEt)
3
4
a
Isolated yields using column chromatography for N-acylation products (1–5) and GC yields for N-formylation products (6–10). Reaction conditions:
P25 (30 mg), b-CD (0.1 mmol), water (15 mL), ammonium formate (25 mg) and triethyl orthoformate (0.2 mL) or
nitro compounds (0.1 mmol), TiO
2
3
b
anhydride (0.12 mmol) under sunlight. Daily sunlight (9 am–3 pm; sunlight intensity of 10–80 Â 10 Lux). The reaction was carried out in 2 mmol
of nitrobenzene with the following reaction conditions: nitro compounds (2 mmol), TiO
formate (0.1 g), and triethyl orthoformate (4 mL), 12 h, under solar light irradiation.
2
(0.5 g), b-CD (0.8 mmol), water (60 mL), ammonium
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excellent yield of the corresponding formamide was obtained anhydride (0.12 mmol) were added into a round-bottom Pyrex
even in dark conditions, and in the absence of b-CD. This result ask (25 mL). The reaction mixture was degassed by Ar gas
demonstrated a high nonphotocatalytic activity of the for- (20 min) and sealed with a septum. Aerwards, the ask was
mylation of amines. It seems that the formylation step had been irradiated under stirring with sunlight according to the data in
carried out on the non-encapsulated amine in b-CD. Therefore, Table 3. The organic material was extracted with ethyl acetate.
4
to clarify this issue, the titration of o-nitrophenol and o-amino- The organic phase was then dried (anhydrous MgSO ), ltered,
phenol with b-CD was carried out using UV-visible data. As and the solvent was removed under vacuum. Pure products were
shown in Fig. S3 and S4 (see ESI†), with the addition of b-CD obtained aer recrystallization or by column chromatography
aqueous solution to o-nitrophenol and o-aminophenol, the UV- on silica using n-hexane and ethyl acetate mixture as an eluent
visible intensity of o-nitrophenol increases more than o-ami- (SiO ; n-hexane–EtOAc 5 : 1).
2
nophenol because of the formation of the host–guest complex
with b-CD in water. Thus, we suggest that the intrinsic physi-
cochemical properties between amines (miscible) and nitro
Acknowledgements
compounds (nonmiscible) caused the reduction of the nitro
The authors acknowledge the support by IASBS Research
group in the hydrophobic cavity of b-CD and then N-formylation
Council of this work.
or N-acylation was performed in water.
Notes and references
Conclusions
1
2
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2
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2
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ꢀ
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C until a clear solution was formed. Nitro compounds
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4
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2
7
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sealed with a septum. Aerwards, the ask was irradiated under
stirring with sunlight according to the data in Table 1. Aer the
3
completion of the reaction, a suitable amount of NaHCO was
5
6
E. S. Chan-Shing, D. Boucher and J. Lessard, Can. J. Chem.,
added to control the pH at around 7 and the mixture was stirred
at room temperature. The organic material was extracted with
ethyl acetate. The organic phase was then dried using anhy-
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4
chromatograph.
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8
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ꢀ
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C until a clear solution was formed. Nitro compounds
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0 min. Commercial TiO
2
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