An expedient and straightforward solvent-free synthesis of 1,8-dioxo-octahydroxanthenes using eco-friendly aluminized polyborate catalyst

Article abstract of DOI:10.1002/jhet.4090

The present work deals with a catalytic application of aluminized polyborate as a mild solid acid catalyst for multicomponent synthesis of 1,8-dioxo-octahydroxanthenes under solvent-free condition at 100°C-110°C. It involves selective domino Knoevenagel-M

Full text of DOI:10.1002/jhet.4090

An expedient and straightforward solvent-free synthesis of 1,8-dioxo-
octahydroxanthenes using eco-friendly aluminized polyborate catalyst
Dilip Aute,1,2 Akshay Kshirsagar,1 Bhagwat Uphade1 and Anil Gadhave1*
1Department of Chemistry and Research Centre, Padmashri Vikhe Patil College,
Pravaranagar, Dist-Ahmednagar-413713 (MS), India.
(Affiliated to Savitribai Phule Pune University, Pune)
2Department of Chemistry, Arts, Commerce & Science College, Satral.
Email-anilgadhave@gmail.com
Abstract
The present work deals with a catalytic application of aluminized polyborate as a mild
solid acid catalyst for multicomponent synthesis of 1,8-dioxo-octahydroxanthenes under
solvent-free condition at 100-110oC. It involves selective domino Knoevenagel-Michael
reaction of one mole of aromatic aldehyde with two moles dimedone. The aluminized
polyborate is simply synthesized by using boric acid and aluminium trichloride and
characterized by FT-IR, XRD, SEM and EDAX techniques. The mild Lewis acidity,
crystalline solid nature, stability and recyclability are significant features of the catalyst. The
solvent-free condition, clean reaction profile, inexpensive and non-toxic catalyst, operational
simplicity, good to excellent product yields, reduced reaction time and applicability to wide
range of substrate are crucial features of this protocol.
Keywords: 1,8-dioxo-octahydroxanthene, aluminized polyborate, solvent-free condition,
green protocol.
This article has been accepted for publication and undergone full peer review but has
not been through the copyediting, typesetting, pagination and proofreading process
which may lead to differences between this version and the Version of Record. Please
cite this article as doi: 10.1002/jhet.4090
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1 | INTRODUCTION
Xanthene is annulated pyran heterocyclic compounds found as an important core of
natural products, compounds of biological importance and also in fluorescent dyes.[1] Due to
presence of pyran framework they are associated with biological functions such as antiviral,[2]
anticancer,[3] antioxidant,[4] antimicrobial,[5] analgesic,[6] anti-inflammatory[7] and
antimalarial.[8] Xanthene is privileged scaffold found in compounds having therapeutic
importance (Figure 1).
R2
O
R3
O
O
R1
N
O
O
O
O
O
O
O
O
O
allosteric modulators of the
d-opioid receptor
osteogenic agents
estrogen receptor modulators
FIGURE 1 Structure of some biologically important xanthene nucleus
Beside this, due to their high molecular extinction coefficient as well as high quantum yield
they also find application in dyes. They are widely used as pH-sensitive substance for the
visualization of biomolecules and dyes.[9] The fluorescent dyes such as fluorescein and
rhodamine are utilized as a synthetic electrostatic chemosensors and super resolutions
bioimaging of different biological samples.[10] They are also extensively used as
photodynamic sensitizers[11] and also acts as corrosion inhibitors.[12]
Nowadays, there is tremendous demand for synthetic approaches which reduces use
of toxic material and produce products which find application for well being of society. It is a
key challenge for chemist to follow the green chemistry principles for development of
environmentally sustainable process.[13] In this context, the multicomponent reactions
(MCRs) have emerged as powerful tool for investigation of novel methodologies which
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involves three or more substrate components.[14] The MCRs have significant contribution in
synthesis of multi-functionalized complex organic compounds in single step with faster
reaction rate and excellent yield.[15] Along with this, solvent-free condition or use of green
solvent, use of non-toxic reagents, reduce waste, clean and simple reaction profile and good
atom economy are additional advantages of MCR approach.[16] It is known that solvent itself
is a key basis of waste, thus solvent-free reaction becomes one of the important way to
minimize waste and make the process more greener.[17]
Boric acid has made its place in development of newer catalytic system due to its
ready availability, cheap and non-toxic nature.[18] But the reactions in which temperature is
above 100oC, boric acid converts into polymeric borate through elimination of water
molecule. This water molecule may hamper progress of some reactions.[19] This limitation
gave insights for development of polymeric boric acid catalyst such as sulfated and
aluminized polyborate with enhanced catalytic activity.[20] The presence of aluminium and
boron in aluminized polyborate generates mild Lewis acidic character and makes it suitable
for catalytic applications.[21]
In general, the straightforward synthesis of 1,8-dioxooctahydroxanthene involves one-
pot domino Knoevenagel-Michael reaction of two moles of dimedone with one mole of
aromatic aldehydes. In literature several catalytic systems are reported for their synthesis
such as [DDPA][HSO4],[22] Ni NPs@N-doped titania,[23] C/TiO2-SO3H,[24] ZnO,[25] sulfated
zirconia,[26] Fe3O4@SiO2 Imid-PMA,[27] β-cyclodextrin,[28] Bronsted acidic ionic liquids,[29]
Fe–Cr–Ni alloy nano-belts,[30] ZnO-NPs,[31] alumina-sulfuric acid,[32] MSrGO NCs,[33] Choline
chloride,[34] SmCl3,[35] PDNES,[36] PVPP-BF3,[37] PSA,[38] natural phosphate,[39] n-TSA,[40]
Fe2(SO4)3.7H2O,[41] barium perchlorate,[42] Ag@CDNS-N/PMelamine,[43] sawdust sulphonic
acid,[44] nano-WO3-supported sulfonic acid,[45] CoNP@SBA-15,[46] PFPA,[47] MnFe2O4,[48]
-
Fe3O4@PS@His[HSO4 ],[49] FSM-16/AEPC-SO3H [50] and zeolite nanoparticles.[51]
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All these reported methods have some beneficial features but at the same time lacks
with one or more vital synthetic parameters such as use of costly and toxic catalyst, lengthy
work-up, extended reaction time, low yield and use of non-green solvents. To overcome such
problems there is still demand to develop novel methodologies with greener conditions. The
biological importance of xanthene, need of newer environmental benign protocol and in
continuation to our work[52-54] herein, we report aluminized polyborate as mild solid acid
catalysed expedient and efficient synthesis of 1,8-dioxo-octahydroxanthenes using two moles
of dimedone and one mole of aromatic aldehyde under solvent-free condition at 100-110oC
with excellent product yield in shorter time (Scheme 1).
O
CHO
+
O
R
2mmol
2
1mmol
1
HO
OH
O
O
R
B
B
Al
Cl
OH OH
100-110oC
2
O
O
aluminized polyborate
solvent-free
O
3
SCHEME 1 General scheme for the synthesis of 1, 8-dioxo-octahydroxanthenes
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2 | RESULTS AND DISCUSSION
2.1 | Synthesis of aluminized polyborate
Boric acid and aluminium chloride were used for the synthesis of aluminized polyborate
catalyst following the procedure reported in the literature (Figure 2).[21] The catalyst obtained
is solid crystalline in nature having good stability with mild Lewis acidity. The fresh as well
as recovered catalyst has been characterized by FT-IR, XRD, SEM and EDAX techniques.
The results obtained specify formation of aluminized polyborate. The characterization data is
provided in supplementary material associated with this article.
FIGURE 2 Outline for synthesis of aluminized polyborate
2.2 | Application of aluminized polyborate
The successful synthesis and characterization of aluminized polyborate motivated us to
explore its catalytic function for the synthesis of 1,8-dioxo-octahydroxanthenes. Initially we
started our study with choosing a reaction of one mole of benzaldehyde (1a) and two mole of
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dimedone (2) as a model reaction for optimizing influence of solvent, temperature and
amount of catalyst for the synthesis of 1,8-dioxo-octahydroxanthenes (Scheme 2).
O
H
O
O
O
2
+
O
O
3
2
1
SCHEME 2 Model reaction for synthesis of 1,8-dioxo-octahydroxanthenes (3a)
In primary study, we performed series of experiments on model reaction with aluminized
polyborate catalyst in various solvent like EtOH, H2O, MeOH, THF, DMF and solvent-free at
different temperatures (Table 1). The reaction carried out at room temperature in various
systems does not forward the reaction. This shows that heat energy is essential to carry the
reaction. The outcome indicates, solvent-free reaction at 100-110oC is the most appropriate
condition for synthesis of 1,8-dioxo-octahydroxanthene (Table 1, entry 13,14) in terms of
clean reaction, excellent yield in short time.
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TABLE 1 Optimization of effect of solvent and temperature for synthesis of (3a)a
Entry
1
Solvent
H2O
Catalyst, mg
Temperature, oC
Time, min
Yield,b %
75
75
75
75
75
75
75
75
75
75
75
75
75
75
r.t.
60
30
30
30
30
30
30
30
30
30
30
30
25
15
15
--
25
59
--
2
H2O
3
H2O
reflux
r.t.
4
EtOH
5
EtOH
reflux
r.t.
84
--
6
H2O:EtOH
H2O:EtOH
MeOH
7
reflux
reflux
reflux
80
68
66
55
62
--
8
9
THF
10
11
12
13
14
DMF
solvent-free
solvent-free
solvent-free
solvent-free
r.t.
80
75
93
93
100
110
aReaction conditions: benzaldehyde (1 mmol), dimedone (2 mmol) and aluminized
polyborate (75 mg).
bIsolated yield.
With the model reaction, our next aim was to investigate the most suited amount of catalyst
for conversion into product. The results obtained shows that in absence of catalyst there was
no formation of product while with increase in amount of catalyst from 15 mg, 25 mg, 50 mg
to 75 mg there is significant increase in product yields (Table 2). The 75 mg of aluminized
polyborate is the optimistic amount of catalyst required for prompt synthesis of 3a in
excellent yield (Table 2, entry 5).
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TABLE 2 Optimization of amount of catalyst for synthesis of (3a)a
Entry
Catalyst, mg
Temperature
100-110oC
100-110oC
100-110oC
100-110oC
100-110oC
100-110oC
Time, min
Yield,b %
catalyst-free
60
60
40
25
15
15
trace
34
1
2
3
4
5
6
15
25
53
50
69
75
93
100
93
aReaction conditions: benzaldehyde (1 mmol), dimedone (2 mmol) and aluminized
polyborate.
bIsolated yield.
It is important to state that solvent-free reaction at 100-110oC with 75 mg aluminized
polyborate is the optimum condition for synthesis of 3a. With this optimized results, we
further thought to extend our study to ensure scope, versatility and selectivity of this MCR for
synthesis of 1,8-dioxo-octahydroxanthene derivatives using aromatic aldehydes having
electron donar as well as electron withdrawing groups like -Me, -OMe, -Cl, -Br, -F, -OH and
-NO2 (Table 3). It is noteworthy to mention that reactions with all substrates resulted with
good to excellent yields in short time. The sterically hindered substrates as well as
heteroaromatic aldehyde also gave product with good yields.
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TABLE 3 One-pot synthesis of 1,8-dioxo-octahydroxanthene derivativesa 3 (a-o)
R
O
H
O
O
O
Aluminized polyborate
2
+
Solvent-free,
100-110oC
O
O
3
R
2
1
Entry
Product
Time,
min
15
Yieldb,
%
Melting point (oC)
Observed
Reported [ref]
1
93
204-206
227-229
254-256
167-168
204-206 [29]
O
O
O
3a
2
3
4
18
17
14
90
85
88
226-228 [33]
254-256 [29]
168-170 [29]
Cl
O
O
O
3b
NO2
O
O
O
3c
NO2
O
O
O
3d
Cl
O
5
16
93
185-186
184-186 [31]
O
O
3e
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OMe
O
6
7
18
22
87
89
159-160
182-184
160-162 [31]
181-183 [33]
O
O
O
O
O
O
3f
OMe
OMe
O
O
3g
OMe
8
9
20
14
18
22
93
94
91
86
246-248
228-229
240-241
220-222
246-248 [29]
227-228 [31]
240-242 [29]
220-222 [33]
O
O
O
O
3h
NO2
O
3i
Br
10
11
O
3j
N
O
O
O
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3k
OH
12
13
25
17
88
94
246-248
231-233
247-249 [31]
O
O
3l
Cl
232-234 [33]
O
O
O
3m
14
15
18
25
92
87
213-215
164-165
215-217 [31]
O
O
O
O
O
3n
163-165 [31]
S
O
3o
aReaction conditions: aromatic aldehydes (1 mmol), dimedone (2 mmol) and aluminized
polyborate (75 mg) at 100-110oC under solvent-free condition.
bIsolated yield.
The acidic nature of aluminized polyborate catalyst was studied by potentiometric
titration.[21] An exactly weighed aluminized polyborate (0.1 g) was dissolved in water and
glycerin (2:1) mixture and titrated against standard 0.1 N NaOH solution. The graph of
titration data ΔmV/ΔV was plotted against the volume of NaOH added. The Figure 3 shows
that, the H+ ion concentration was found to be 19 mmol/g at equivalence point which
supports the acidic character of aluminized polyborate catalyst.
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80
70
60
50
40
30
20
10
0
1
6
11
16
21
26
Volume of NaOH added
FIGURE 3 Potentiometric titration curve of aluminized polyborate
In further study we explored the reusability of aluminized polyborate catalyst for
synthesis of diverse 1,8-dioxo-octahydroxanthene derivatives. The catalyst recovered
(mention in general procedure) was reused over four cycles in model reaction 3a under
optimized condition. It was found that there was no significant loss in catalytic activity in
terms of product yield and reaction time (Figure 4). The recovered catalyst was characterized
by FT-IR, XRD, SEM and EDAX techniques which indicates that there are no major changes
in catalyst composition as compared with fresh catalyst.
100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
Time (min)
Yield (%)
15
93
17
91
20
89
22
88
Reaction cycles
FIGURE 4 Recyclability of aluminized polyborate catalyst
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To evaluate the merits of this protocol, we compared with literature reported methods for the
synthesis of 1,8-dioxo-octahydroxanthenes (Table 4). The results illustrate that, faster
reaction rate, excellent product yields, low-cost catalyst and solvent-free conditions are some
values of this method.
TABLE 4 The comparison of various literature reported methods with present protocol for
the synthesis of 1, 8-dioxo-octahydroxanthenes
Entry
Catalyst and solvent
Catalyst Temperatu
Time
Yield
(%)
Reference
amount
re
[DDPA][HSO4], solvent-free 0.5 mmol
100oC
120°C
70-80oC
1-2 h
0.5-2.5 h
8-12 h
87-97
85-94
84-95
85-95
22
24
26
29
1
2
3
4
C/TiO2-SO3H , solvent-free
sulfated zirconia, EtOH
0.2 g
15 wt%
Bronsted acidic ionic liquids,
solvent-free
10 mol% 95-100°C 30-45 min
MSrGO-NCs, solvent-free
PDNES, solvent-free
PSA, solvent-free
15 mg
5 mol%
0.05 g
1.5 g
80°C
90°C
80°C
reflux
90°C
120°C
60°C
120°C
0.8-3 h
80-97
33
36
38
39
40
41
46
50
5
6
30-120 min 80-97
40-110 min 75-89
7
natural phosphate, EtOH
n-TSA, solvent-free
6.5-8.5 h
63-90 min
1.0-4.5 h
0.5-1.5 h
15-95 min
84-95
83-96
71-94
88-98
64-89
8
0.013 g
9
Fe2(SO4)3.7H2O, solvent-free 10 mol%
10
11
12
CoNP@SBA-15, H2O
0.1 mol%
FSM-16/AEPC-SO3H,
solvent-free
0.45
mol%
aluminized polyborate,
solvent-free
75 mg
100-
110°C
14-25 min
85-94
Present
work
13
3 | EXPERIMENTAL
3.1 | Materials and methods
The chemicals used in this study were of analytical grade and used without further
purification. The progress of reaction was checked by TLC using silica G60 F254 plates
(Merck). The melting points were measured in open capillary tube and are uncorrected. FT-
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IR spectra were obtained on Perkin-Elmer, 400 FT-IR spectrometer. XRD analysis is
performed on Panalyticals X Pert Pro, X-Ray Diffractometer. The SEM images were
1
recorded on JSM6100, (Jeol) Scanning Electron Microscope. The H-NMR spectra were
recorded on Bruker Avance 500MHz NMR spectrometer using CDCl3 as solvent and TMS as
internal standard. Mass spectra were recorded on Waters Q-ToF Micromass spectrometer.
3.2 | General procedure for synthesis of 1, 8-dioxo-octahydroxanthenes (3a-o)
A mixture of substituted aromatic aldehyde (1a-o, 1 mmol) and dimedone (2, 2 mmol) were
heated under solvent-free condition at 100-110oC in presence of 75 mg of aluminized
polyborate catalyst (for time, Table 3). The progress and completion of reaction was checked
by thin layer chromatography using n-hexane and ethyl acetate (7:3) as mobile phase. After
completion of reaction, 5 mL of ethanol was added and resulting mixture was warmed. The
hot solution was filtered to remove any insoluble material. The pure solid products were
obtained from mother liquor by filtration at room temperature. The filtrate was evaporated to
recover the solid catalyst. This recovered catalyst was dried at 100oC for one hour and used
1
for next cycle. The physical as well as IR, H-NMR and mass spectral data are in good
agreement with that reported in literature.
3.3 | Spectral data of some representative 1, 8-dioxooctahydroxanthenes
3,3,6,6-tetramethyl-9-phenyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8-(2H)-dione (3a)
1
White solid, m.p. 204-206°C; IR (KBr): 3083, 3032, 2915, 1662, 1491, 1235, 841 cm-1; H-
NMR (500 MHz, CDCl3) δ: 0.98 (s, 6H, 2 x CH3), 1.09 (s, 6H, 2 x CH3), 2.14-2.24 (m, 4H, 2
x -CH2), 2.46 (s, 4H, 2 x -CH2), 4.74 (s, 1H, -CH), 7.08-7.29 (m, 5H, Ar-H) ppm; MS m/z =
351 (M+1).
9-(3,4-dimethoxyphenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8-
(2H)-dione (3g)
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White solid, m.p. 182-184°C; IR (KBr): 3083, 3005, 2955, 1667, 1624, 1466, 1263, 1139 cm-
1; 1H-NMR (500 MHz, CDCl3) δ: 1.00 (s, 6H, 2 x CH3), 1.10 (s, 6H, 2 x CH3), 2.17-2.25 (m,
4H, 2 x –CH2), 2.45 (s, 4H, 2 x –CH2), 3.79 (s, 3H, OCH3), 3.85 (s, 3H, OCH3), 4.70 (s, 1H, -
CH), 6.70 (d, 1H, J=8 Hz, Ar-H), 6.75 (dd, 1H, J=8 Hz & 2 Hz, Ar-H), 6.90 (d, 1H, J=2 Hz,
Ar-H); MS m/z = 412 (M+1)..
9-(4-nitrophenyl)-3, 3, 6, 6-tetramethyl-3, 4, 5, 6, 7, 9-hexahydro-1H-xanthene-1, 8-(2H)-
dione (3i)
White solid, m.p. 228-229°C; IR (KBr): 3082, 3039, 2960, 1659, 1620, 1468, 1361, 1234,
866 cm-1; 1H-NMR (500 MHz, CDCl3) δ: 0.99 (s, 6H, 2 x CH3), 1.12 (s, 6H, 2 x CH3), 2.15-
2.27 (m, 4H, 2 x –CH2), 2.49 (s, 4H, 2 x –CH2), 4.82 (s, 1H, -CH), 7.46 (d, 2H, J=7.5 Hz, Ar-
H), 8.08 (d, 2H, J=7.5 Hz, Ar-H) ppm; MS m/z = 396 (M+1).
9-(4-bromophenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8-(2H)-
dione (3j)
White solid, m.p. 240-241°C; IR (KBr): 3080, 3026, 2953, 1660, 1625, 1468, 1260, 1003,
849 cm-1; 1H-NMR (500 MHz, CDCl3) δ: 0.98 (s, 6H, 2 x CH3), 1.10 (s, 6H, 2 x CH3), 2.14-
2.25 (m, 4H, 2 x –CH2), 2.42 (s, 4H, 2 x –CH2), 4.69 (s, 1H, -CH), 7.15 (d, 2H, J=9 Hz, Ar-
H), 7.32 (d, 2H, J=9 Hz, Ar-H) ppm; MS m/z = 430 (M+1).
9-(4-chlorophenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1H-xanthene-1,8-(2H)-
dione (3m)
White solid, m.p. 231-233°C; IR (KBr): 3090, 3030, 2962, 1678, 1625, 1469, 1361, 849 cm-1;
1H-NMR (500 MHz, CDCl3) δ: 0.98 (s, 6H, 2 x CH3), 1.10 (s, 6H, 2 x CH3), 2.14-2.25 (m,
4H, 2 x –CH2), 2.46 (s, 4H, 2 x –CH2), 4.70 (s, 1H, -CH), 7.17 (d, 2H, J=8.7 Hz, Ar-H), 7.21
(d, 2H, J=8.7 Hz, Ar-H) ppm; MS m/z = 386 (M+1).
9-(p-tolylphenyl)-3, 3, 6, 6-tetramethyl-3, 4, 5, 6, 7, 9-hexahydro-1H-xanthene-1, 8-(2H)-
dione (3n)
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White solid, m.p. 213-215°C; IR (KBr): 3039, 3015, 2932, 1678, 1623, 1466, 1296 cm-1; 1H-
NMR (500 MHz, CDCl3) δ: 0.99 (s, 6H, 2 x CH3), 1.09 (s, 6H, 2 x CH3), 2.14-2.21 (m, 4H, 2
x –CH2), 2.23 (s, 3H), 2.45 (s, 4H, 2 x –CH2), 4.70 (s, 1H, -CH), 7.00 (d, 2H, J=8 Hz, Ar-H),
7.16 (d, 2H, J=8 Hz, Ar-H); MS m/z = 365 (M+1).
3.5 | Reaction mechanism
The plausible mechanism for the synthesis of 1,8-dioxo-octahydroxanthenes is depicted as
follows (Scheme 3). The aluminized polyborate activates the carbonyl group of dimedone as
well as aldehyde due to its Lewis acid character. Initially there is Knoevenagel condensation
reaction between aldehyde and first dimedone molecule to form Michael acceptor
intermediate. The activated intermediate undergoes Michael addition reaction with second
molecule of dimedone which undergoes cyclization with loss of water molecule to yield 1,8-
dioxo-octahydroxanthenes.
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Cl
HO
Ar
Al
O
O
OH O
O
H
O
H
Ar
OH
Cl
O H
O
Al
-H2O
Ar
O
Ar
O
O
O
Ar
O
H O
H
-H2O
O
O
O
O
OH
1,8-dioxo-octahydroxanthene
OH
Cl
Al
OH
HO
OH
O
O
Al
Cl
B
B
Al
Cl
aluminized polyborate
=
OH OH
2
SCHEME 3 Plausible mechanism for synthesis of 1,8-dioxo-octahydroxanthenes.
4 | CONCLUSION
In this work we have developed newer expedient, green and straightforward protocol for the
one-pot domino Knoevenagel-Michael synthesis of 1,8-dioxo-octahydroxanthenes prompted
by aluminized polyborate as a mild solid Lewis acid catalyst. The catalyst is characterized by
FT-IR, XRD, SEM and EDAX analyses. The catalyst is non-toxic and inexpensive with good
catalytic activity. The significant features of this protocol are faster reaction, solvent-free
condition, excellent yield, reusable catalyst, no side reactions, easy work-up and applicable to
wide range of substrates. This cost-effective and eco-sustainable methodology will be a good
contribution to green chemistry.
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ACKNOWLEDGEMENTS
We express our gratitude towards Management and Principal (P.V.P. College & A. C. S.
College, Satral) for providing all necessary facilities. We express our sincere thanks to SAIF
Punjab University, Chandigarh for providing spectral analysis. The authors are also thankful
to Dr. B. K. Karale for his valuable guidance.
ORCID
Anil Gadhave https://orcid.org/0000-0002-1733-132X
SUPPORTING INFORMATION
Additional information associated with this article may be found online in the supporting
information at the end of this article.
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