Rapid one pot synthesis of xanthene derivatives by an efficient and reusable nano-ZnAl2O4 - An insight into a new process

Article abstract of DOI:10.1016/j.molcata.2014.04.030

Nano-ZnAl₂O₄ was synthesized by blending metal-citrate complex-acrylamide polymer using the precursor method. Powder X-ray diffraction analysis of the catalyst affirmed that the spinel oxide was phase pure. Elemental analysis was confirmed by Energy Dispersive X-ray analysis. Transmission Electron Microscopic (TEM) analysis and Scherrer's formula revealed that the particles were found to be in the order of 20 nm in size. Thermal stability of the catalyst was analyzed by thermogravimetric and differential thermal analysis (TG-DTA). Total acidity of the nano-ZnAl ₂O₄ evaluated by NH₃-TPD was 8.8 mmol NH ₃/g. Diversity in the synthesis of xanthene derivatives was investigated using nanozinc aluminate as a catalyst. Nanoform of the catalyst showed better activity than the bulk form due to its large surface area 50 m²/g. The 2,2′-arylmethylene bis (3-hydroxy-2-cyclohexene-1- one), 2,2′-arylmethylene bis (3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one) and 1-Oxo-hexahydroxanthenes were synthesized rapidly using ZnAl ₂O₄ and the catalyst was successfully recycled without any loss of activity.

Full text of DOI:10.1016/j.molcata.2014.04.030

Accepted Manuscript
Title: Rapid one pot synthesis of Xanthene derivatives by an
efficient and reusable nano ZnAl2O4 - an insight into a new
process
Author: Triveni Rajashekhar Mandlimath Balijapalli
Umamahesh Kulathu I. Sathiyanarayanan
PII:
S1381-1169(14)00168-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.04.030
MOLCAA 9092
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
6-2-2014
21-4-2014
22-4-2014
Please cite this article as: T.R. Mandlimath, B. Umamahesh, K.I. Sathiyanarayanan,
Rapid one pot synthesis of Xanthene derivatives by an efficient and reusable nano
ZnAl2O4 - an insight into a new process, Journal of Molecular Catalysis A: Chemical
(2014), http://dx.doi.org/10.1016/j.molcata.2014.04.030
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Rapid one pot synthesis of Xanthene derivatives by an efficient and reusable nano ZnAl2O4
- an insight into a new process
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Triveni Rajashekhar Mandlimath, Balijapalli Umamahesh and Kulathu I.
Sathiyanarayanan*
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Chemistry Division – School of Advanced Sciences
VIT University, Vellore – 632014, Tamil Nadu, India
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Highlights
 Synthesis of nano ZnAl2O4 by [metal citrate]-acrylamide polymer precursor
method.
 The nano ZnAl2O4 nanoparticles were hexagonal-like with 20 nm in size.
 Nano ZnAl2O4 had 50 m2/g surface area and total surface acidity of 8.8 mmol
NH3/g.

The nano spinel oxide was an excellent catalyst for the synthesis of
xanthenes.
We recycled the catalyst for 10 cycles with consistent activity.

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Abstract
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Nano ZnAl2O4 was synthesized by blending metal-citrate complex-acrylamide polymer using the
precursor method. Powder X- ray diffraction analysis of the catalyst affirmed that the spinel
oxide was phase pure. Elemental analysis was confirmed by Energy Dispersive X-ray analysis.
Transmission Electron Microscopic (TEM) analysis and Scherrer’s formula revealed that the
particles were found to be in the order of 20 nm in size. Thermal stability of the catalyst was
analyzed by thermogravimetric and differential thermal analysis (TG-DTA). Total acidity of the
nano ZnAl2O4 evaluated by NH3-TPD was 8.8 mmol NH3/g. Diversity in the synthesis of
xanthene derivatives was investigated using nano zinc aluminate as a catalyst. Nanoform of the
catalyst showed better activity than the bulk form due to its large surface area 50 m2/g. The 2,2'-
arylmethylene bis (3-hydroxy-2cyclohexene-1-one), 2,2'-arylmethylene bis (3-hydroxy-5,5-
dimethyl-2cyclohexene-1-one) and 1-Oxo-hexahydroxanthenes were synthesized rapidly using
ZnAl2O4 and the catalyst was successfully recycled without any loss of activity.
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Key words: nano ZnAl2O4, TEM, 1-Oxo-hexahydroxanthene
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*
Corresponding author
-
Kulathu I. Sathiyanarayanan Fax: +914162243092
E-mail: sathiya_kuna@hotmail.com
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1. Introduction
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ZnAl2O4 (Ghanite), a normal spinel oxide with AB2O4 structure, is one of the most important
aluminates investigated in literature due to its charismatic properties such as high thermal and
chemical stability, high mechanical resistance, ductility, low sintering temperature. The structure
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crystallizes in cubic system in which Zn2+ ion occupies tetrahedral sites and octahedral sites by
Al3+ ion. This has been widely studied by several researchers for diverse applications. For
instance, cobalt doped ZnAl2O4 proved as a blue pigment with good chromaticity [1]. Ghanite
acts as a host lattice for several transition metals and rare earth ions, enhancing the
photoluminescent properties such as green light emitting ZnAl2O4: Mn, red light emitting Eu3+
doped ZnAl2O4, green light emitting Tb3+ doped ZnAl2O4 and yellow light emitting Dy3+ doped
ZnAl2O4 [2- 4].
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The catalytic behavior of ZnAl2O4 has been extensively investigated for a variety of
organic reactions: ZnAl2O4 nanoparticle is an excellent catalyst for the acetylation of amines,
alcohols and phenols at room temperature [5]. Sr(II)-ZnAl2O4 acts as an efficient catalyst for the
oxidation of various alcohols [6]. Palladium nanoparticles immobilized on microporous ZnAl2O4
has been studied for Suzuki-Miyaura coupling reaction to achieve high yield within short period
[7]. Apart from the liquid phase reactions, Ghanite has also been used for gas phase
hydroxylation of 2-hydroxypyridine, gas phase transformation of anisole to cresol and gas phase
alkylation of m-cresol [8-10]. In addition to these, ZnAl2O4 has been used as a potential catalyst
for photodegradation of gaseous toluene, photooxidation of toluene and lanthanum doped
Ghanite for biodiesel preparation from soyabean oil. Oxidation of light alkanes has been
catalyzed using Ru/ZnAl2O4 and Pt/ZnAl2O4, Ni/ZnAl2O4-CeO2 for hydrogen production from
ethanol steam and Pt-Sn/ ZnAl2O4 for Propane dehydrogenation [11- 18].
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Xanthene derivatives are fascinating organic compounds with vast applications in
pharmacology because of their anti-inflammatory, antibacterial, antifungal, anti-depressant and
antimalarial activity [19-21]. They are thrombin inhibitors, and also act as stimulants of the
central nervous system [22, 23]. These compounds are potassium channel openers [24] and
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possess anticancer activity [25]. Apart from its biological applications, xanthene dyes have been
used for diagnostic and imaging applications [26]. Benzopyran derivatives possess anti-
inflammatory, anti-analgesic and antimicrobial properties [27]. They act as insulin sensitizers
[28]. The compounds having benzopyran rings function as potential potassium channel activators
[29], and also active towards post-coital contraceptives [30].
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Condensation of aromatic aldehydes with cyclic 1,3-diketones is one of the methods to
synthesize 2,2'-arylmethylene bis (3-hydroxy-2-cyclohexene-1-one), 2,2'-arylmethylene bis (3-
hydroxy-5,5-dimethyl-2-cyclohexene-1-one) and 1-Oxo-hexahydroxanthenes. Traditional
method for synthesizing these compounds involves the use of piperidine as catalyst [31]. This
method has a major drawback because piperidine cannot be recycled. Synthesis of 2,2'-
arylmethylene bis (3-hydroxy-2-cyclohexene-1-one) and 2,2'-arylmethylene bis (3-hydroxy-5,5-
dimethyl-2-cyclohexene-1-one) derivatives have been reported using NaOH under high intensity
ultrasound radiation [32] and urea under ultrasound radiation [33]. These methods may not be
applicable for large scale production, and further the catalysts cannot be reused. Several
homogeneous catalysts such as ZnCl2, ethylenediamine diacetate (EDDA), SmCl3, SDS, CsF and
heterogeneous catalysts like ZnO, Yb(OTf)3-SiO2, FeCl3.6H2O/TMSCl/[bmim][BF4], HClO4-
SiO2 and L-Lysine have been reported by various authors [34-42]. However, many of these
catalysts are expensive, require toxic solvents, cannot be recycled and require prolonged
duration.
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1-Oxo-hexahydroxanthene has been synthesized by Sabitha et al. using CeCl3.7H2O [43].
Although this protocol tends to give better yields, the catalyst is expensive, non-reusable, and
needs long reaction time. Pesyan et al. have used acetic acid for the synthesis of 1-Oxo-
hexahydroxanthenes [44]. Kuarm et al. attempted the use of cellulose sulfuric acid as catalyst
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[45]. The catalyst requires acid for its preparation. Fei He et al. have performed the synthesis of
1-Oxo-hexahydroxanthenes using glycerol [46]. Even though it is a catalyst-free synthesis, it
requires prolonged refluxing to achieve the products. Apart from these, various homogeneous
catalysts such as cationic surfactant triethylbenzylammoium chloride (TEBA), TCT and tetra-n-
butylammonium fluoride (TBAF) and PTSA have been reported for the synthesis [47-50]. In
spite of showing catalytic activity, few of these catalysts are halogen containing, active only in
the presence of toxic solvents, and also need prolonged reaction time.
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In the present work, we report the synthesis of nano ZnAl2O4 using [metal citrate
complex]-acrylamide polymer precursor method and the investigation of its catalytic activity for
the synthesis of 2,2'-arylmethylene bis (3-hydroxy-2cyclohexene-1-one), 2,2'-arylmethylene bis
(3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one) and 1-Oxo-hexahydroxanthenes by Knoevenagel
condensation followed by Micheal addition of aromatic aldehydes with cyclic 1,3-diketones.
Diversity in the synthesis of xanthenes derivatives has been studied using nano ZnAl2O4.
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2. Experimental Work
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Zinc nitrate (Zn(NO3)2.6H2O, 99%), Aluminium nitrate (Al(NO3)3.9H2O, 99%) were purchased
from Himedia. Citric acid monohydrate (C6H10O8, 99.99%), acrylamide (C3H5NO, 99%), N, N’-
methylenebisacrylamide (C7H10N2O2, 99%), aldehydes and diketones were purchased from
Sigma Aldrich.
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2.1. Preparation of bulk ZnAl2O4
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Stoichiometric quantities of Zn(NO3)2.6H2O and Al(NO3)3.9H2O were separately dissolved in
distilled water. The metal nitrate solutions were mixed together under constant stirring. The
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homogenized solution mixture was dried at 80 °C overnight to get the solid mass. The obtained
precursor was then calcined at 700 °C for 12 h with a heating rate of 10 °C/ min.
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2.2. Preparation of nano ZnAl2O4
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Stoichiometric quantities of Zn(NO3)2. 6H2O and Al(NO3)3.9H2O were separately dissolved in
distilled water. The metal nitrate solutions were added drop wise to the citric acid solution under
constant stirring. The ratio of [Zn+Al] and citric acid was 1. To the above homogeneous
solution, 3 g of acrylamide and N, N’- methylenbisacrylamide were added. The temperature was
raised to 80 °C followed by the addition of a pinch of ammonium peroxodisulphate. The
obtained white gel was dried overnight at 80 °C to get the solid mass. The solid precursor was
later calcined at 300 °C, 500 °C and 700 °C for 6 h at each stage with a heating rate of 10 °C/
min.
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2.3. Catalytic investigation of ZnAl2O4
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In a typical procedure, aromatic aldehyde (1 mmol), 1,3-diketone (1,3-cyclohexanedion/5,5-
dimethyl-1,3-cyclohexanedione) (2 mmol) and calculated amount of catalyst bulk /nano ZnAl2O4
along with 5 mL solvent were placed in a round bottom flask fitted with reflux condenser. The
mixture was then heated at 80 °C. Completion of the reaction was monitored by thin layer
chromatography (TLC). The catalyst was removed by filtration, and was washed twice with
acetone. It was dried at 60 °C in oven for 3 h and used for further cycle. Xanthene derivatives
were obtained by re-crystallization in ethanol-THF. Schemes 1 and 2 represent the synthesis of
xanthene derivatives using ZnAl2O4.
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R1
CHO
O
O
O
ZnAl2O4
+
6
2
Ethanol, reflux
R
O
R1
R
R
R
R
R
OH
OH
3a-3l
1
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Scheme 1. Synthesis of 2,2'-arylmethylene bis (3-hydroxy-2-cyclohexene-1-one) and 2,2'-
arylmethylene bis (3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one)
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R
R
OH
O
CHO
O
O
HO
ZnAl2O4
+
Ethanol, reflux
R
R
O
R2
R2
O
R
R
5a-5j
4
1
Scheme 2. Synthesis of 1-Oxo-hexahydroxanthene
2.4. Characterization
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Phase identification of the catalyst was obtained by powder X-ray diffractometer (Bruker, D8
Advanced) using Cu Kα radiation (λ = 1.5406 Å) in the angle range of 2θ = 10 ° - 70 ° at room
temperature. The surface area of the powder samples was analyzed by BET method on
Micromeritics ASAP 2020 V3.00 H instrument. Fourier Transformed Infrared Spectra (FTIR) of
the prepared samples was recorded on Shimadzu IR affinity – 1 FTIR spectrometer by KBr disk
method. The morphology and particle size were affirmed by TEM (JEOL 3010 instrument with
UHR pole piece). The particles’ size was calculated by using image J software. Elemental
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analysis of the catalyst was confirmed by Field Emission Scanning Electron Microscope coupled
with Energy Dispersive X-ray Analysis (FESEM-EDX, JEOL JSM 7001F with BRUKER-
QUNTAX Version 1.8.2). Thermal analysis of the powder precursor was performed on TA
Instrument (SDT Q600) in the temperature range between room temperature and 1000 ºC with
the heating rate of 20 ºC per minute. Total surface acidity of the nano ZnAl2O4 was evaluated by
NH3 temperature-programmed desorption, TPD technique (Autochem 2910, Micromeritics
instrument). 0.74 g of the sample was heated to 120 ºC at 10 ºC/min in 30 mL high pure Helium
flow, kept at 120 ºC for 30 min. 10% NH3 in Helium gas at 30 mL/min flow was passed through
the sample for 30 min. Pure Helium was purged (30 mL/min) and then desorption of NH3 was
analyzed from 120 ºC to 700 ºC at 10 ºC/min using thermal conductivity detector. Surface
basicity of the nano ZnAl2O4 was analyzed by CO2 temperature-programmed desorption, TPD
technique on the same instrument. In a typical procedure, 1.00 g of the dried sample was
pretreated at 200 ºC in 50 mL high pure Helium flow for 30 min. After pretreatment the sample
was saturated with 10% CO2 in Helium gas at 30 ºC (with a flow rate of 75 mL/min). Later
Helium was flushed at 105 ºC over the sample for 2 h in order to remove physisorbed CO2. TPD
analysis was done from ambient temperature to 750 ºC at 10 ºC/min. Concentration of leached
metal ions from the catalyst during recycle was tested by Atomic Absorption Spectroscopic
technique using Varian AA240 instrument. The synthesized Xanthene derivatives were analyzed
by 1H NMR and 13C NMR spectra (Bruker FT NMR – 400 MHz).
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3. Results and discussion
3.1. Characterization of the catalyst
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3.1.1. Powder XRD
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Powder XRD patterns of the calcined bulk and nano ZnAl2O4 are depicted in Fig. 1a. and 1b.
Both bulk and nano ZnAl2O4 are crystallized in face centered cubic structure. The powder XRD
patterns were indexed based on ICDD data (# 821043). No impurity peaks were found. Fig 1a
consists of narrow peaks, which is apparently bulk ZnAl2O4. Whereas, the broadening of XRD
peaks in Fig. 1b. indicates the formation of nano ZnAl2O4. The average crystallite size of both
bulk and nano ZnAl2O4 were obtained from the Scherrer’s formula (D = 0.9λ/B Cos θ) by
introducing full width half maximum (FWHM) values of all the peaks in the XRD pattern. ‘D’ is
the average particle size, ‘λ’ is wavelength of X-ray beam used, ‘B’ is full width half maximum
of the peak and ‘θ’ is the Bragg angle. The average crystallite size of bulk and nano ZnAl2O4 was
found to be 5 µm and 16 nm respectively. It is apparent from Fig. 2a. and 2b. that the nano
ZnAl2O4 exists even after the 10th cycle.
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3.1.2. Mechanism of formation of nanosize ZnAl2O4
There are 3 different steps involved in the formation of smaller size particles.
i) Metal salts form metal-citrate complex with citric acid.
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ii) Monomer acrylamide and cross linker N, N’- methylenbisacrylamide undergo polymerization
in the presence of radical initiator ammonium peroxodisulphate giving rise to a polymer network.
Thus, Metal citrate persists inside the polymer matrix as shown in the scheme 3.
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iii) The [metal-citrate complex]-polymer matrix undergoes decomposition after calcinations and
the polymer matrix hinder the growth of particles to larger size and also avoids agglomeration of
particles.
H2
C
H2
C
H
C
H
C
CH2
CH
C
HC
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O
C
C
O
C
O
O
NH
NH2
NH2
NH
700 ºC
nano ZnAl2O4
HN
C
NH2
NH
C
NH2
C
O
C
O
O
O
H2
C
H2
C
HC
CH
CH
H2C
CH
Scheme 3. Mechanism of formation of nano ZnAl2O4
3.1.3. BET Surface area
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Surface area of bulk and nano ZnAl2O4 from Nitrogen adsorption desorption isotherms obtained
by BET method was found to be 10.4 m2/g and 50 m2/g respectively.
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3.1.4. FTIR Analysis
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Figures 3a. and 3b. show FTIR spectrum of bulk and nano ZnAl2O4. In both the spectra, the
bands at 663 cm-1, 559 cm-1 and 501 cm-1 were assigned to stretching and bending mode of Al-O
of octahedral AlO6 units respectively. Since the characteristic stretching vibration bands of
inverse spinel units (AlO4) in the range 700-850 cm-1 were absent, it was clear that the prepared
ZnAl2O4 was pure with normal spinel structure [51]. In case of nano ZnAl2O4, the peaks at 1630
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cm-1 and 1122 cm-1 were assigned to OH stretching of the adsorbed water molecule and
asymmetric stretching vibrations of AlO-OH respectively. These two peaks were absent in the
FTIR spectrum of bulk ZnAl2O4. This is because nano ZnAl2O4 possesses relatively large surface
area (50 m2/g) inclined to adsorb larger number of water molecules over its surface. Also, the
possibility of greater number of low coordination site (O2-) over the surface of ZnAl2O4 cannot
be ruled out.
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3.1.5. TEM analysis
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Figure 4. indicates the TEM image of the prepared nano ZnAl2O4, which reveals that the nano
ZnAl2O4 particles exhibit hexagonal - like morphology, and are well dispersed. TEM image
shows that the particles have a uniform morphology. It is clear that the particles’ size is in the
range of 5 nm - 20 nm, which corroborates the average particle size obtained from the Scherrer’s
formula (16 nm). Figure 4 shows the electron diffraction rings, which are indexed as cubic
ZnAl2O4 structure. Particle size distribution is represented in Fig. 5. Maximum size of the
particle ranged from 14 to 16 nm.
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3.1.6. Energy Dispersive X-ray Analysis (EDX)
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The presence of elements Zn, Al and O are confirmed by EDX spectrum (Fig. 6). It was found
that theoretical wt % of the elements closely matched with the experimental values.
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3.1.7. Thermal Analysis
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The compiled TGA and DTA curves of ZnAl2O4 precursor are shown in Figure. 7. Thermo
gravimetric profile revealed that the precursor underwent thermal degradation in two stages. The
first weight loss occurred between 50 °C and 120 °C corresponding to the dehydration of
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precursor. The second stage of mass loss between 340 °C and 460 °C can be attributed to the
decomposition of organic molecules from the precursor. The mass loss from 460 °C to 1000 °C
was due to the evolution of entrapped organic molecules in the internal cavities, channels [52].
The weight losses observed from TGA curve was confirmed by the appearance of two
endothermic peaks in the same region of the DTA profile.
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3.1.8. NH3-TPD and CO2-TPD Analysis
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Total surface acidity of the nano ZnAl2O4 evaluated by NH3-TPD analysis was 8.8 mmol NH3/g.
It was very clear from the NH3-TPD pattern (Fig. 8) that the broad peaks at 239 °C and 370 °C
were associated with weak acidic and medium acidic sites of the material respectively, whereas,
the peak at 599 °C was related to strong acidic site. Intensity of broad peaks indicates that the
larger distribution of weak and medium acidic sites than the strong acidic sites.
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Surface basicity was analyzed by CO2-TPD. From CO2-TPD analysis (Fig. 9) it was found that
there were no prominent CO2 desorption peaks and thus the presence of basic sites was ruled out.
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3.2. Catalytic Studies of ZnAl2O4
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3.2.1. Synthesis of 2,2’-arylmethylene bis(3-hydroxy-2-cyclohexene-1-one) and 2,2′-
arylmethylene bis(3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one)
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A mixture of benzaldehyde (1 mmol), 1,3-diketone (1,3-cyclohexanedion/5,5-dimethyl-
1,3-cyclohexanedione) (2 mmol) and appropriate amount of catalyst along with 5 mL ethanol
were refluxed for few minutes. In general, the mechanism of the formation of xanthenediones
involves 2 steps: Knoevenagel condensation followed by Michael addition reaction with
cyclization. But surprisingly, the reaction is terminated at the Michael addition step and ended up
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with uncylized products (3) within few minutes of using nano ZnAl2O4 (Scheme 4). Irrespective
of the derivatives of benzaldehyde, similar observations were noticed. There was no formation of
xanthenediones (3a) even after refluxing the reaction mixture for few hours. Formations of
similar products were noticed using both bulk and nano ZnAl2O4. But nano was found to be
more active than bulk catalyst. Since these compounds are pharmacologically very significant,
we carried out the reaction with benzaldehyde derivatives and the findings are tabulated in
Table.1. Similar observation was made by Malek et al. using ZnO [38]. It is very important to
note that the authors achieved the product in 8-15 h of refluxing in CH3CN solvent.
R
R
H
H
H
ZnAl2O4
O
Ar
O
O
O
O
O
O
OH
O
Ar
O
O
ZnAl2O4
Michael
addition
Ar
Ar
H
Knovenagel
condensation
-H2O
R
R
R
ZnAl2O4
R
OH
OH
O
R
R
R
R
R
R
1A
3
2A
2B
O
O
Ar
O
R
O
R
1
O
R
R
R
R
3a
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Scheme 4. Plausible mechanism of the formation of uncyclized products.
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3.2.2. Synthesis of 1-Oxo-hexahydroxanthene
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A mixture of salicylaldehyde (1 mmol), 1,3-diketone (1,3-cyclohexanedion/5,5-dimethyl-
1,3-cyclohexanedione) (2 mmol) and appropriate amount of catalyst along with 5 mL ethanol
were refluxed for few minutes. Surprisingly, 1-Oxo-hexahydroxanthene formed within few
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minutes (Scheme 5). We performed reactions for the other derivatives of salicylaldehyde (Table
2) and obtained 1-Oxo-hexahydroxanthene.
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We carried out the synthesis of uncylized products as well as 1-Oxo-hexahydroxanthene
without using a catalyst, under similar reaction conditions, and found that the uncyclized product
formed after 1 h whereas the latter was formed after 2 h. Consequently, the requirement of
catalyst for the synthesis played a crucial role. Here, ZnAl2O4 accelerated the reaction by
bringing both the reactants together over its surface. Table 3. describes the advantage of using
nano ZnAl2O4 over bulk ZnAl2O4. Due to the small size particles possessing larger surface area
(50 m2/g), nano exhibited better activity than the bulk catalyst whose surface area was 10.4 m2/g.
A Plausible mechanism of 1-Oxo-hexahydroxanthene formation is shown in scheme 5.
R
R
OH
O4Al2 Zn
H
O
H
H
H
HO
O
O
O
O
O
O
Zn
H
Al2
O4
4
O
O
R
R
-H2O
1A
Knovenagel
condensation
R
O
H
R
O
R
O
R
O
4A
ZnAl2O4
4B
Michael
addition
R
R
1
H
Zn
Al2O4
O
O
O
H
O
O
-H2O
R
R
R
R
OH
O
4C
R
R
R
OH
O
R
5
292
293
Scheme 5. Plausible mechanism of formation of 1-Oxo-hexahydroxanthene
14
Page 14 of 48

294
295
296
297
298
299
300
301
302
During the synthesis of 2,2’-arylmethylene bis(3-hydroxy-2-cyclohexene-1-one), 2,2′-
arylmethylene bis(3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one) and 1-Oxo-
hexahydroxanthene, carbonyl group of the aldehyde interacted with the acidic sites (Zn2+ and
Al3+) of the catalyst thereby activated the carbonyl carbon for the nucleophilic attack and thus
accelerated the reaction. Formation of xanthenediones (3a) generally required high acidic
catalyst for the cyclization of product (3). But due to the larger number of weak and medium
acidic sites on the surface of nano ZnAl2O4, the reaction was terminated at uncyclized product
(3). The reaction proceeded due to the presence of only acidic sites as the catalyst did not
exhibited basic sites.
303
304
305
306
307
308
309
310
311
312
313
It was observed that both benzaldehyde and salicylaldehyde derivatives containing
electron withdrawing group gave relatively higher yield in short duration (Table 1 and 2).
Several authors have reported the synthesis of 1-Oxo-hexahydroxanthene from salicylaldehyde
in which they utilized hazardous solvents [49], expensive homogeneous catalysts such
CeCl3.7H2O and TEBA [43][47], TBAF [49] and it also required longer duration to obtain the
products [47-49]. We have made a comparison of the activity of the several catalysts for the
synthesis 1-Oxo-hexahydroxanthene with nano ZnAl2O4 (Table 4). Although the reported
homogeneous catalysts produced good yields (Table 4), they contain halogen, very expensive,
could not be reused and also took hours for the completion of reaction. Hence nano ZnAl2O4
could be the best alternative catalyst for the synthesis of 1-Oxo-hexahydroxanthene over the
reported homogeneous catalysts.
314
315
316
Next, the effect of solvents for 1-Oxo-hexahydroxanthene synthesis was studied (shown
in Table 5). The reaction is found to be faster in protic solvents with high yield. A mere 50 mg of
the catalyst gave 86% yield. By increasing the amount of catalyst, the yield obtained was more
15
Page 15 of 48

317
318
319
320
321
322
323
324
325
and also the reaction time was shortened (Table 6). The reaction was completed in 15 min in the
presence of 90 mg of nano ZnAl2O4, whereas, it completed in 90 min in the presence of 50 mg
catalyst. After every cycle, the catalyst was collected by filtration and washed with acetone to
remove organic molecules. The catalyst was dried at 60 °C in oven before using it for the next
cycle. Nano ZnAl2O4 was found to be consistently active for 10 cycles as shown in Fig. 10. The
yield was reduced after 5 cycles due to the loss of negligible amount of catalyst during filtration.
The filtrate collected after each cycle was subjected to AAS analysis for leachability.
Leachability of Zn was nil, and hence same was true for Al too. This could be the reason for the
consistent performance of catalyst in every cycle.
326
4. Conclusion
327
328
329
330
331
332
333
334
Pure phase of nano ZnAl2O4 was successfully synthesized by using metal-citrate by polymer
precursor method. Particles were found to be in the order of 20 nm size. Nano ZnAl2O4 was
utilized as catalyst for the synthesis of 2,2’-arylmethylene bis(3-hydroxy-2-cyclohexene-1-one),
2,2′-arylmethylene
bis(3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one)
and
1-Oxo-
hexahydroxanthenes. Non-toxic ethanol was used as solvent. We achieved xanthene derivatives
with high yields within few minutes. No column chromatography was required. The catalyst was
recycled for 5 times without loss of activity. Being non-toxic, inexpensive, highly stable,
ZnAl2O4 emerged as an efficient catalyst, exhibiting diversity in its activity.
335
336
4.1. Spectral data of selected xanthene derivatives
4.1.1. Compound 3g (Sl. No. 7 in table 1)
337
338
White solid; Melting point: 192 °C, 1H NMR (400 MHz, CDCl3) ppm: 1.10 (s, 6H), 1.23 (s,
6H), 2.48-2.29 (m, 8H), 5.54 (s, 1H), 7.10-7.09 (d, J = 4 Hz, 2H), 7.19-7.15 (t, J = 8 Hz, 1H),
16
Page 16 of 48

13
339
340
7.28-7.25 (m, 2H), 11.09 (bs s, 2H); C NMR (100 MHz, CDCl3) ppm: 27.4, 29.7, 32.8, 46.5,
47.1, 115.6, 115.9, 125.8, 126.8, 128.2, 138.1, 189.4, 190.5.
341
4.1.2. Compound 5b (Sl. No. 2 in table 2)
342
343
344
345
346
Yellow solid; Melting point: 229 °C, 1H NMR (400 MHz, CDCl3) ppm: 1.86-1.75 (m, 2H), 2.17-
2.00 (m, 4H), 2.86-2.37 (m, 6H), 3.89 (s, 3H), 4.63 (s, 1H), 6.61-6.59 (d, J = 8 Hz, 2H), 6.77-
6.75 (d, J = 8 Hz, 1H), 6.96-6.92 (t, J = 8 Hz, 1H), 10.80 (bs s, 1H); 13C NMR (100 MHz,
CDCl3) ppm: 19.7, 19.9, 28.0, 28.0, 29.7, 36.0, 37.0, 56.1, 110.3, 112.2, 119.7, 119.8, 124.3,
125.6, 140.5, 146.9, 170.9, 172.8, 197.1, 201.5.
347
4.1.3. Compound 5c (Sl. No. 3 in table 2)
348
349
350
351
352
Off white solid; Melting point: 221 °C, 1H NMR (400 MHz, CDCl3) ppm: 1.47-1.44 (t, J = 6 Hz,
3H), 2.17-1.82 (m, 6H), 2.85-2.37 (m, 6H), 4.14-4.09 (m, 2H), 4.62 (s, 1H), 6.60-6.58 (d, J = 8
13
Hz, 1H), 6.76-6.74 (d, J = 8 Hz, 1H), 6.93-6.89 (t, J = 8 Hz, 1H), 10.80 (bs s, 1H); C NMR
(100 MHz, CDCl3) ppm: 14.9, 19.6, 20.0, 28.0, 28.0, 29.7, 36.0, 37.0, 65.0, 112.1, 112.2, 119.7,
119.9, 124.2, 125.6, 140.9, 146.3, 171.0, 172.6, 197.0, 210.5.
353
4.1.4. Compound 5f (Sl. No. 6 in table 2)
354
355
356
357
358
359
White solid; Melting point: 203 °C, 1H NMR (400 MHz, CDCl3) ppm: 0.98 (s, 3H), 0.99 (s,
3H), 1.02 (s, 3H), 1.12 (s, 3H), 2.01-1.91 (q, J = 48.0, 16 Hz, 2H), 2.37-2.33 (d, J = 16.0 Hz,
4H), 2.62-2.45 (q, J = 48.0, 16.0 Hz, 2H), 4.67 (s, 1H), 7.03-6.98 (m, 3H), 7.26-7.13 (m, 1H),
10.48 (bs s, 1H); 13C NMR (100 MHz, CDCl3) ppm: 26.5, 27.2, 27.8, 29.2, 29.8, 30.9, 32.3,
41.6, 43.2, 49.9, 50.6, 111.0, 115.7, 118.3, 124.3, 124.6, 127.5, 128.0, 151.0, 169.1, 170.7, 196.6,
200.9.
360
4.1.5. Compound 5h (Sl. No. 8 in table 2)
17
Page 17 of 48

361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
Off white solid; Melting point: 170 °C, 1H NMR (400 MHz, CDCl3) ppm: 0.99 (s, 6H), 1.03 (s,
3H), 1.12 (s, 3H), 1.48-1.44 (t, J = 8 Hz, 3H), 2.02-1.92 (q, J = 44.0, 20.0 Hz, 2H), 2.37-2.33 (m,
4H), 2.69-2.65 (d, J = 16.0 Hz, 1H), 2.58-2.54 (d, J = 16.0 Hz, 1H), 4.14-4.08 (m, 2H), 4.66 (s,
1H), 6.59-6.57 (d, J = 8 Hz, 1H), 6.76-6.74 (d, J = 8 Hz, 1H), 6.93-6.89 (t, J = 8.0 Hz, 1H),
10.46 (bs s, 1H); 13C NMR (100 MHz, CDCl3) ppm: 14.9, 18.5, 26.5, 27.1, 27.8, 29.2, 29.9,
30.9, 32.3, 41.5, 43.2, 49.9, 50.6, 64.9, 110.8, 112.2, 118.2, 119.8, 124.2, 125.2, 141.1, 146.5,
169.0, 170.5, 196.6, 201.0.
Acknowledgements
Triveni Rajashekhar Mandlimath and B. Umamahesh thank CSIR for providing Senior Research
Fellowship. The DST-FIST NMR facility at VIT University is greatly acknowledged. The
authors thank Dr. D.Rajan Babu, Professor, SAS, VIT University India and Dr. P. Anandan, RIE
Shizuoka University, Hamamatsu, Japan for recording FESEM and EDX. The authors
acknowledge M. Sathish Kumar for his help and suggestions.
Appendix. Supplementary data
376
377
Supplementary data associated with this article can be found, in the online version at
http://dx.doi.org/xxxxxxxxxx
378
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464
465
466
Tables
467
468
Table 1. Synthesis of 2,2'-arylmethylene bis (3-hydroxy-2cyclohexene-1-one) and 2,2'-arylmethylene bis
(3-hydroxy-5,5-dimethyl-2cyclohexene-1-one)
- 2
469
470
471
Table 2. Synthesis of 1-Oxo-hexahydroxanthenes
-
2
Table 3.
Comparison of the effect of reaction conditions on the yield of 1-Oxo-hexahydroxanthenes
- 3
472
473
Table 4. Comparison of the activity of the catalysts for the synthesis of 1-Oxo-hexahydroxanthenes
- 3
474
475
Table 5. Effect of solvent on the synthesis of 1-Oxo-hexahydroxanthenes
- 3
- 4
Table 6. Effect of catalyst amount on the synthesis of 1-Oxo-hexahydroxanthenes
22
Page 22 of 48

476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
23
Page 23 of 48

502
503
504
505
506
507
508
509
Table 1. Synthesis of 2,2'-arylmethylene bis (3-hydroxy-2cyclohexene-1-one) and 2,2'-arylmethylene bis
(3-hydroxy-5,5-dimethyl-2cyclohexene-1-one)
510
Sl. No.
R
R1
Product Reaction time
(min)/yield
M. P. (° C)
Observed
215
Reported
1
2
H
H
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
20/ 98
10/ 98
10/ 98
15/ 95
15/ 95
12/ 95
20/ 93
12/ 98
15/ 96
15/ 90
10/ 97
15/ 96
210 - 211 (Ref 36)
195 - 196 (Ref 36)
202 - 204 (Ref 39)
200 - 205 (Ref 53)
187 - 191 (Ref 53)
196 - 198 (Ref 53)
189 - 190 (Ref 33)
190 - 191 (Ref 33)
140 - 141 (Ref 33)
188 - 189 (Ref 33)
137 - 138 (Ref 33)
202 - 204 (Ref 38)
4 - NO2
4 - Cl
196
3
H
202
4
H
4 - OH
3 - Cl
202
5
H
190
6
H
3 - NO2
H
198
7
CH3
CH3
CH3
CH3
CH3
CH3
192
8
3 - NO2
4 - OCH3
4 - NO2
4 - Cl
190
9
142
10
11
12
3j
190
3k
3l
140
4 - OH
200
511
512
24
Page 24 of 48

513
514
515
516
517
Table 2. Synthesis of 1-Oxo-hexahydroxanthenes
Sl. No.
R
R2
Product
Reaction
time
M. P. (° C)
(min)/yield
Observed
230
Reported
1
2
H
H
H
3- OCH3
3- OCH2CH3
5 - Br
5a
5b
5c
5d
5e
5f
15/ 98
14/ 93
15/ 92
10/ 98
13/98
15/ 92
14/ 89
15/ 88
14/ 99
10/98
226 (Ref 43)
-
229
3
H
221
-
4
H
235
236 - 238 (Ref 50)
245 - 246 (Ref 47)
204 (Ref 43)
229 - 231 (Ref 48)
-
5
H
5 - NO2
H
242
6
CH3
CH3
CH3
CH3
CH3
203
7
3- OCH3
3- OCH2CH3
5 - Br
5g
5h
5i
230
8
170
9
252
252 - 254 (Ref 50)
205 - 207 (Ref 47)
10
5 - NO2
5j
206
518
519
520
521
522
523
Table 3. Comparison of the effect of reaction conditions on the yield of 1-Oxo-hexahydroxanthenes
Sl. No.
Catalyst
Reaction
Yield (%)
time (min)
25
Page 25 of 48

1
2
3
Without catalyst
125
90
90
89
98
Bulk ZnAl2O4 (90 mg)
Nano ZnAl2O4 (90 mg)
15
524
525
526
Reaction Conditions: salicylaldehyde+1,3-cyclohexanedione (1:2 mole ratio),
ethanol, temperature = 80 °C.
solvent = 5 mL
527
528
529
530
Table 4. Comparison of the activity of the catalysts for the synthesis of 1-Oxo-hexahydroxanthenes
Sl. No.
Catalyst
Amount of catalyst
Reaction
time
Yield (%)
Reference
1
2
3
4
CeCl3.7H2O
TEBAa
Catalytic amount*
100 mg
1.5 – 5 h
3 – 5 h
82 - 96
85 - 95
89 - 95
88 - 98
43
47
TCTb
40 mg
2 – 3 h
48
Nano ZnAl2O4
90 mg
10 -15 min
Our work
a triethylbenzylammoium chloride, b 2,4,6-trichloro-1,3,5-triazine
* Amount is not given
531
532
533
534
535
Table 5. Effect of solvents on the synthesis of 1-Oxo-hexahydroxanthenes
Sl. No.
Solvent
Ethanol
THF
Yield (%)
1
2
98
75
26
Page 26 of 48

3
4
Acetonitrile
Toluene
74
70
Sl. No.
Catalyst amount (mg)
Yield (%)
1
2
3
90
60
50
98 (15 min)
89 (30 min)
86 (90 min)
5
Water
60
536
537
538
Reaction Conditions: salicylaldehyde+1,3-cyclohexanedione (1:2 mole ratio),
90 mg, solvent = 5 mL ethanol, temperature = 80 °C. Reaction time = 15 min.
nano ZnAl2O4 =
539
540
541
542
543
544
545
546
547
548
549
550
551
Table 6. Effect of catalyst amount on the synthesis of 1-Oxo-hexahydroxanthenes
552
553
Reaction Conditions: salicylaldehyde+1,3-cyclohexanedione (1:2 mole ratio),
ZnAl2O4, solvent = 5 mL ethanol, temperature = 80 °C. Reaction time = 15 min - 90 min.
catalyst = nano
554
27
Page 27 of 48

555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
28
Page 28 of 48

581
582
583
584
585
586
587
588
Figures
589
Fig. 1. Powder XRD pattern of bulk ZnAl2O4 (a) and nano ZnAl2O4 (b)
- 2
590
591
Fig. 2. Powder XRD pattern of fresh nano ZnAl2O4 (a) and nano ZnAl2O4 after 10 cycles (b)
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592
593
594
595
596
597
598
599
Fig. 3. FTIR spectra of bulk ZnAl2O4 (a) and nano ZnAl2O4 (b)
Fig. 4. TEM images of nano ZnAl2O4 taken and the inset is electron diffraction pattern
Fig. 5. Particle size distribution of nano ZnAl2O4 obtained by TEM
Fig. 6. Energy Dispersive X-ray Analysis (EDX) spectrum of nano ZnAl2O4
Fig. 7. TG-DTA curves of Metal nitrate-citrate with acrylamide precursor
Fig. 8. NH3-TPD Pattern of nano ZnAl2O4
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Fig. 9. CO2-TPD Pattern of nano ZnAl2O4
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Fig. 10. Reusability of ZnAl2O4
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600
601
602
603
604
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