Application of a novel, efficient and recyclable photo redox catalyst (Zn–Al layered double hydroxide/eosin) for the synthesis of substituted pyridine derivatives under visible light irradiation

Article abstract of DOI:10.1002/aoc.4585

Layered double hydroxides (LDHs) are a class of anionic clays with brucite-like layers and interlayer anions and varying in composition and morphology. LDHs show potential as supports for the immobilization of catalytically active species, to synthesize recyclable catalysts, in which catalytic sites can be preferentially orientated, highly dispersed and stabilized to afford high catalytic efficiency and recyclability in reaction media. Redox active organic dyes such as eosin Y with strong absorption in the visible region of the spectrum have been successfully used. In this work, an intercalated photo redox catalyst (Zn–Al LDH/eosin) was synthesized by co-precipitation of an aqueous solution of zinc nitrate and aluminium nitrate simultaneously with eosin Y as anionic dye. Zn–Al LDH/eosin as a recyclable catalyst was used in the reaction of various aryl ketones and benzylamines under molecular oxygen and visible light, giving good yields of substituted pyridines at room temperature. Also, the Zn–Al LDH/eosin catalyst could be reused three times without any significant changes in the reaction yields.

Full text of DOI:10.1002/aoc.4585

Received: 22 June 2018
DOI: 10.1002/aoc.4585
Revised: 11 August 2018
Accepted: 13 August 2018
F U L L P A P E R
Application of a novel, efficient and recyclable photo redox
catalyst (Zn–Al layered double hydroxide/eosin) for the
synthesis of substituted pyridine derivatives under visible
light irradiation
Nahid Rasouli
Department of Chemistry, Payame Noor
Layered double hydroxides (LDHs) are a class of anionic clays with brucite‐like
layers and interlayer anions and varying in composition and morphology.
LDHs show potential as supports for the immobilization of catalytically active
species, to synthesize recyclable catalysts, in which catalytic sites can be prefer-
entially orientated, highly dispersed and stabilized to afford high catalytic
efficiency and recyclability in reaction media. Redox active organic dyes such
as eosin Y with strong absorption in the visible region of the spectrum have
been successfully used. In this work, an intercalated photo redox catalyst
(Zn–Al LDH/eosin) was synthesized by co‐precipitation of an aqueous solution
of zinc nitrate and aluminium nitrate simultaneously with eosin Y as anionic
dye. Zn–Al LDH/eosin as a recyclable catalyst was used in the reaction of
various aryl ketones and benzylamines under molecular oxygen and visible
light, giving good yields of substituted pyridines at room temperature. Also,
the Zn–Al LDH/eosin catalyst could be reused three times without any
significant changes in the reaction yields.
University, PO Box 19395‐3697, Tehran,
Iran
Correspondence
Nahid Rasouli, Department of Chemistry,
Payame Noor University, PO Box 19395‐
3697, Tehran, Iran.
Email: n.rasouli@pnu.ac.ir; n.
rasooli55@yahoo.com
Funding information
Payame Noor University
KEYWORDS
eosin, layered double hydroxide, recyclable photo redox catalyst, visible light
1
| INTRODUCTION
Layered double hydroxides (LDHs) or hydrotalcite‐like
compounds, owing to their lamellar‐type crystal structure
[
5,6]
Homogeneous catalysts are most commonly used for
organic reactions and show high accessibility of catalytic
sites, regio‐ and enantio‐selectivity of products and high
and unique anion exchange ability,
are widely applied
in catalysis, electrochemistry, separation technology and
[
7–10]
drug delivery.
LDHs consist of positively charged
[1–3]
catalytic activity.
However, the difficulty in catalytic
metal hydroxide sheets with anions located between the
layers to compensate for the positive layer charges. The
separation and recyclability leads to significant economic
and environmental problems, which restrict the commer-
cial applications of homogeneous catalysts. To overcome
these problems, the use of a solid‐state catalyst would
be an attractive solution owing to facile separation and
recycling. Therefore, the development of efficient recy-
clable catalysts instead of their homogeneous counter-
parts is one of the main tendencies in the catalysis field.
compositions
are
generally
represented
as
2+
3+
n−
2+
3+
[M1−x Mx (OH) ][A ]x/n⋅mH O, where M and M
2
2
are di‐ and trivalent cations, respectively, x is the ratio
3
+
2+
3+
n−
M /(M + M ) and A
is an anionic species with
x/n
−
[4]
2−
a charge of n (such as NO , CO ). There has been little
3 3
work done on LDH materials as catalysts under condi-
tions where the crystalline layer structure remains intact.
Appl Organometal Chem. 2018;e4585.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4585

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RASOULI
On the other hand, their thermal stability is relatively
limited. Nevertheless, many useful catalytic reactions
can be carried out on the surfaces and/or in the layers
of crystalline LDH materials. Various organic and inor-
ganic anions can be immobilized into such layers by ion
were obtained using a FT‐IR spectrometer (JASCO FT/
IR‐4200, Japan) with KBr discs, in the range
−
1
400–4000 cm . The particle size, external morphology
and analysis of elements in the samples were character-
ized using a scanning electron microscopy (SEM)
[11]
1
exchange or co‐precipitation.
Among various organic
instrument (JEOL JEM‐3010). H NMR spectra were
substances used as interlayer species, redox active organic
dyes such as eosin Y (Scheme 1) with strong absorption
ability in the visible region have been widely used in pho-
recorded with a Bruker Avance AQS 400 MHz. The
chemical shifts were measured with reference to
tetramethylsilane as the internal standard. TLC was per-
formed using commercially available 100–400 mesh silica
gel plates. The green light source applied consisted of
high‐power LEDs (λ = 530 nm).
[12–15]
tochemical organic transformations.
Visible light
photo redox catalysis has become an attractive alternative
not only for classical synthetic methods but also for pho-
tochemical reactions since these methods suffer from lack
of cost‐effectiveness and by‐product formation. Eosin Y as
a photo redox catalyst is less toxic and cheaper compared
2.2 | Synthesis of Zn–Al LDH/eosin
[16,17]
to organometallic and inorganic catalysts.
A mixture of Zn(NO ) ⋅6H O (6.44 g, 0.034 mol) and
3
2
2
Here, for the first time, we report the immobilization
of eosin Y into Zn–Al LDH as a recyclable photo redox
catalyst for the synthesis of substituted pyridines under
molecular oxygen and visible light irradiation at room
temperature (Scheme 2). Functionalized arylpyridines
have been extensively applied as chemosensors, catalysts,
photosensitizers and intermediates in the synthesis of
therapeutic drugs, insecticides, herbicides and surfac-
Al(NO ) ⋅9H O (14.48 g, 0.068 mol) with an Al to Zn
3
2
2
molar ratio of 2:1 in 100 ml of deionized water was slowly
added to 50 ml of a solution of NaOH (16 g, 0.4 mol) and
eosin Y (17.29 g, 0.025 mol) under constant stirring until
reaching pH = 9 to complete the precipitation process.
Then, the obtained product was stirred for 24 h at 65°C.
After filtration and washing with distilled water several
times, the solid product was dried in an oven at 60°C
overnight.
[18–22]
tants.
2
2
| EXPERIMENTAL
2.3 | Catalytic Experiments
In a typical experiment, to a round‐bottom flask (10 ml)
equipped with a magnetic stirring bar were added Zn–Al
LDH/eosin (10 mol%), aromatic ketone (1.0 mmol) and
benzylamine (3.0 mmol) dissolved in dimethylformamide
(DMF; 5 ml) and BF ⋅Et O (50 mol%). The round‐bottom
.1 | Instrumentation and Reagents
All materials were of commercial reagent grade and
purchased from Merck or Sigma Aldrich. The analysis
of the samples was performed using powder X‐ray diffrac-
tion (XRD; Philips X‐pert X‐ray diffractometer with Cu
Kα radiation). Fourier transform infrared (FT‐IR) spectra
3
2
flask was filled with oxygen using an oxygen balloon. The
reaction mixture was irradiated using 530 nm LEDs at
room temperature for 3 h. After completion of the reac-
tion, the DMF was removed under vacuum conditions
and the residue purified on a silica‐gel plate using ethyl
acetate and petroleum ether as eluent to afford pure
products. Identities of the products were confirmed using
1
13
FT‐IR, H NMR and C NMR spectral data. Blank exper-
iments were also performed under the same conditions
without the catalyst.
SCHEME 1 Chemical structure of eosin Y.
SCHEME 2 Synthesis of 2,4,6‐
triphenylpyridine catalysed by Zn–Al
LDH/eosin.

RASOULI
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3
3
| RESULTS AND DISCUSSION
.1 | Characterization
peaks in the XRD pattern of Zn–Al LDH/eosin are similar
[
24]
to those in the XRD pattern of Zn–Al LDH.
However,
compared to Zn–Al LDH, the (003) diffraction peak of
Zn–Al LDH/eosin moves to a greater 2θ angle of 11.77°
and the basal spacing decreases to 0.75 nm. This result
can confirm intercalation of eosin anion by anion
The crystal structures and phase purity of the synthesized
Zn–Al LDH/eosin were investigated using XRD. Figure 1
shows the XRD pattern of Zn–Al LDH/eosin. The pattern
showed that the synthesized Zn–Al LDH/eosin exhibits
[
24]
exchange in the Zn–Al LDH compound. The crystallite
sizes for the synthesized Zn–Al LDH/eosin were calcu-
lated using the Scherer formula:
[23]
typical characteristics of LDH phase (Figure 1).
The
FIGURE 1 XRD patterns for (a) Zn–Al
LDH and (b) Zn–Al LDH/eosin.
FIGURE 2 SEM images of heterogeneous photo redox catalyst Zn–Al LDH/eosin.

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FIGURE 3 EDX spectrum of Zn–Al
LDH/eosin.
0
:9λ
and molecular oxygen in DMF under green light
irradiation (530 nm) for 180 min at room temperature.
The results are summarized in Table 1. The results
showed that this reaction was strongly influenced by the
amount of catalyst. In the absence of catalyst, no product
was obtained even after 12 h (Table 1, entry 1; Figure 4)
indicating that the catalyst is necessary for the reaction.
Excellent yield in short reaction time was obtained when
the amount of the catalyst was 10 mol% (Table 1, entry 5).
Purification of the crude product gave the substituted
D ¼
(1)
β cos θ
where λ is the wavelength of X‐rays and β is full width at
half maximum of the peak at diffraction angle of θ. The
calculated crystallite size was 24 nm. The morphology
and chemical composition of the synthesized sample were
investigated using SEM and energy‐dispersive X‐ray
(
EDX) analysis. The SEM images of the synthesized sam-
ple are shown in Figure 2. The original sheet and lamellar
structure of Zn–Al LDH changed on intercalation of eosin
Y anions. Figure 3 depicts the EDX spectrum of the
prepared sample showing presence of relevant elements,
which demonstrates the purity of the synthesized Zn–Al
LDH/eosin.
3.2 | Catalytic Activity
In order to determine the optimal conditions, the reaction
of acetophenone (1.0 mmol) and benzylamine (3.0 mmol)
as a model reaction was investigated using various
amounts of recyclable photo redox catalyst Zn–Al LDH/
eosin in the presence of BF ⋅Et O (50 mol%) as additive
3
2
TABLE 1 Effect of catalyst amount on reaction between
acetophenone (1.0 mmol) and benzylamine (3.0 mmol) in the
3 2 2
presence of BF ⋅Et O (50 mol%), O and green light at room
temperature
Catalyst amount
(mol%)
a
Entry
Time (min)
Yield (%)
1
2
3
4
5
None
720
600
300
240
180
Trace
10
1
5
40
7
70
10
90
13
1
FIGURE 4 (a) C NMR spectrum and (b) H NMR spectrum of
2,4,6‐triphenylpyridine.
a
Yield of isolated products.

RASOULI
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triphenylpyridine in 90% yield (Scheme 2). Moreover, the
product yield was not changed by increasing the amount
of catalyst. The reaction between acetophenone
(1.0 mmol) and benzylamine (3.0 mmol) in the absence
of BF ⋅Et O, LED light or molecular oxygen failed to
3
2
give triarylpyridine. To improve the yield of the synthe-
sized product, the reaction between acetophenone
(1.0 mmol) and benzylamine (3.0 mmol) using Zn–Al
LDH/eosin (10 mol%) was investigated in various
solvents at room temperature, and the results are pre-
sented in Table 2. Using acetonitrile (entry 4; 75%),
TABLE 2 Effect of various solvents on reaction of between
acetophenone (1.0 mmol) and benzylamine (3.0 mmol) in the
3 2 2
presence of Zn–Al LDH/eosin (10 mol%), BF ⋅Et O (50 mol%), O
and green light at room temperature
a
Entry
Solvent
Time (min)
Yield (%)
1
2
3
4
5
DMF
180
180
180
180
180
90
80
65
75
58
DMSO
3
CH OH
Acetonitrile
Dichloromethane
1
3
1
a
FIGURE 6 (a) C NMR spectrum of and (b) H NMR spectrum
of 2,6‐bis(4‐chlorophenyl)‐4‐phenylpyridine.
Yield of isolated products.
1
3
1
13
1
FIGURE 5 (a) C NMR spectrum and (b) H NMR spectrum of
2
FIGURE 7 (a) C NMR spectrum and (b) H NMR spectrum of
2,6‐bis(4‐bromophenyl)‐4‐phenylpyridine.
,6‐bis(4‐fluorophenyl)‐4‐phenylpyridine.

6
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3 2 2
TABLE 3 Synthesis of substituted pyridines using Zn–Al LDH/eosin (10 mol%), BF ⋅Et O (50 mol%), O and green light at room
temperature
Entry
Substrates
Product
Time (min)/Yield (%)
Ref.
[
25]
1
180/90
[
25]
2
180/78
[
25]
3
4
180/80
180/75
[
26]
5
6
180/—
[
22]
180/72
[
25]
7
180/80
(Continues)

RASOULI
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TABLE 3 (Continued)
Entry
Substrates
Product
Time (min)/Yield (%)
Ref.
[
25]
8
180/68
[
25]
9
180/68
DMSO (entry 2; 80%), dichloromethane (entry 5; 58%)
or methanol (entry 3; 65%) as solvents gave lower yields.
A better result was obtained using DMF providing a 90%
yield (entry 1). Reactions of electron‐withdrawing
substituted acetophenones such as p‐fluorophenylethanone
(Figure 5), p‐chlorophenylethanone (Figure 6) or p‐
bromophenylethanone (Figure 7) and benzylamine showed
good yields of products (78, 80 and 75%, respectively;
Table 3). However, p‐nitrophenylethanone was unreactive
under the reaction conditions (Figure 8). Next, we
studied the effect of substituents in the benzylamine.
The reaction between 4‐methoxybenzylamine and
1
3
1
13
1
FIGURE 8 (a) C NMR spectrum and (b) H NMR spectrum of
,6‐bis(4‐nitrophenyl)‐4‐phenylpyridine.
FIGURE 9 (a) C NMR spectrum and (b) H NMR spectrum of
2,6‐bis(4‐methylphenyl)‐4‐methoxyphenylpyridine.
2

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p‐methylphenylethanone (Figure 9), p‐chloropheny-
lethanone, p‐fluorophenylethanone (Figure 10) or p‐
methoxyphenylethanone, under the optimized reaction
conditions, gave corresponding triarylpyridines in good
yields (68–80%; Table 3). The obtained products were puri-
fied using silica gel column chromatography and character-
1
13
ized using H NMR, C NMR and FT‐IR spectroscopy.
The good reusability of a catalyst is an important
aspect of environmental chemistry. So the potential for
the recovery of Zn–Al LDH/eosin was investigated. The
activities of the recovered catalyst were carefully investi-
gated through the reaction between acetophenone
(
1.0 mmol) and benzylamine (3.0 mmol) using BF ⋅Et O
3
2
(
50 mol%), O and green light at room temperature. After
2
completion of the reaction, the product was extracted
with ethyl acetate and separated by simple filtration,
washed with acetonitrile, dried and reused. The catalyst
could be reused three times without any significant
changes in the reaction yields (Table 4).
To show the merit of the present work, the results
were compared with those reported in the literature. As
evident from Table 5, the present procedure offers several
advantages, such as excellent yields, short reaction times,
easy synthesis and simple work‐up in contrast with other
methods. Based on these observations, we propose the
following plausible mechanism for the synthesis of
substituted pyridines (Scheme 3). First, Zn–Al LDH/eosin
initiates the oxidation of benzylamine providing the cor-
responding imine and ammonia. Subsequently addition
of enol I, facilitated by BF ⋅Et O as an additive, to the
iminium ion results in formation of II that, upon photo-
catalytic oxidation, gives III. A second addition of enol I
to III leads to formation of IV. After that, condensation
of IV with ammonia gives V that, on aromatization, pro-
vides triarylpyridine.
1
3
1
FIGURE 10 (a) C NMR spectrum and (b) H NMR spectrum of
,6‐bis(4‐fluorophenyl)‐4‐methoxyphenylpyridine.
2
TABLE 4 Reusability of Zn–Al LDH/eosin (10 mol%) for prepa-
ration of triphenylpyridine in the presence of BF
and green light at room temperature
3
⋅Et
2
O (50 mol%),
3
2
O
2
Yield (%)
Cycle 1
Cycle 2
88
Cycle 3
85
9
0
TABLE 5 Comparison of Zn–Al LDH/eosin with previously reported catalysts
Entry
Catalyst
Conditions
Time (h)
Yield (%)
Ref.
[27]
1
Eosin
Aryl ketone (1.0 mmol), benzylamine (3.0 mmol),
catalyst (5 mol%), additive, solvent,
30–40
87
O
2
, green light, 30°C
Benzaldehydes (1.0 mmol), acetophenones
2.0 mmol), ammonium acetate (5.0 mmol)
and catalyst (4 mol%), solvent‐free,
20°C
Acetophenone (2 mmol), benzylamine
1.0 mmol), catalyst (1.0 mol%),
[
28]
2
Nanocrystalline MgAl
2
O
4
3
85
(
1
[29]
3
4
I
2
10
3
93
90
(
solvent, 140°C
Zn–Al LDH/eosin
Aryl ketone (1.0 mmol), benzylamine (3.0 mmol),
This work
catalyst (10 mol%), additive, solvent, O
green light, 25°C
2
,

RASOULI
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SCHEME 3 Plausible reaction mechanism.
4
| CONCLUSIONS
5.3 | 2,6‐Bis(4‐chlorophenyl)‐4‐
phenylpyridine (Entry 3)
We showed the application of Zn–Al LDH/eosin as photo
redox catalyst for the reaction between aryl ketones and
benzylamines under visible light irradiation for the syn-
thesis of substituted arylpyridines at room temperature
in high yields. The method is mild and effective for the
synthesis of substituted arylpyridines. Investigation of
further applications of Zn–Al LDH/eosin as a recyclable
photo redox catalyst for the synthesis of heteroaromatic
compounds is in progress.
−
1
White solid; m.p. 173–177°C. FT‐IR (KBr, cm ): 3050,
15948, 1540, 1493, 1445, 1415, 1382, 1238, 11714, 1093,
015, 835, 765, 692. H NMR (CDCl , 400 MHz, δ, ppm):
.34–8.41 (d, 4H, ArH), 8.23 (s, 2H, ArH), 8.03–8.06
(d, 2H, ArH), 7.50–7.61 (m, 7H, ArH) (Figure 6).
1
1
3
8
5
.4 | 2,6‐Bis(4‐bromophenyl)‐4‐
phenylpyridine (Entry 4)
1
H NMR (CDCl , 400 MHz, δ, ppm): 7.38–7.48 (6H, 7.73
3
5
5
| SELECTED DATA
(
(
dddd), 7.44(ddd)), 7.53 (1H, tdd), 7.72 (4H, ddd), 7.83
2H, dddd), 8.38 (2H, d) (Figure 7).
.1 | 2,4,6‐Triphenylpyridine (Entry 1)
−
1
Colourless solid; m.p. 131–135°C. FT‐IR (KBr, cm ):
5
.5 | 2,6‐Bis(4‐nitrophenyl)‐4‐
3
8
062, 3033, 1613, 1591, 1547, 1395, 1215, 1034, 1016,
phenylpyridine (Entry 5)
1
75, 709, 646. H NMR (CDCl , 400 MHz, δ, ppm): 8.28
3
1
(
(
brd, J = 8 Hz, 4H, ArH), 7.95 (s, 2H, pyridine), 7.79
brd, J = 8 Hz, 4H, ArH), 7.61–7.57 (m, 6H, ArH),
H NMR (CDCl , 400 MHz, δ, ppm): 7.46 (2H, dddd),
3
7.5(1H, tdd), 7.75 (4H, ddd), 7.91 (2H, dddd), 8.20 (4H,
7
.54–7.47 (m, 3H, ArH) (Figure 4).
ddd), 8.43–8.48 (3H, 8.43) (Figure 8).
5
.2 | 2,6‐Bis(4‐fluorophenyl)‐4‐
5.6 | 2,6‐Bis(4‐methylphenyl)‐4‐
methoxyphenylpyridine (Entry 7)
phenylpyridine (Entry 2)
1
1
H NMR (CDCl , 400 MHz, δ, ppm): 2.28 (3H, s), 7.40
H NMR (CDCl , 400 MHz, δ, ppm): 2.29–2.30 (9H, 2.30
3
3
(
(
4H, ddd), 7.5 (2H, ddd), 7.72–7.84 ((6H, 7.81 (ddd), 7.76
ddd)), 8.40 (2H, d) (Figure 5).
(s), 2.3 (s)), 7.40–7.5 (6H, 7.4 (ddd), 7.5 (ddd)), 7.9
(4H, ddd, 8.03 (2H, ddd)), 8.48 (2H, d) (Figure 9).

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RASOULI
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.7 | 2,6‐Bis(4‐fluorophenyl)‐4‐
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of a novel, efficient and recyclable photo redox
catalyst (Zn–Al layered double hydroxide/eosin) for
the synthesis of substituted pyridine derivatives
under visible light irradiation. Appl Organometal
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