Synthesis and characterization of phosphate anchored MnO2 catalyzed solvent free synthesis of xanthene laser dyes

Article abstract of DOI:10.1016/j.molcata.2014.04.035

A highly stable and environmentally benign phosphate anchored MnO ₂ (P-MnO₂) catalyst has been prepared. The catalyst has been characterized by using TGA, FTIR, Raman, XRD, UV-DRS, XPS, FESEM, and EDAX techniques. The catalytic behav

Full text of DOI:10.1016/j.molcata.2014.04.035

Accepted Manuscript
Title: Synthesis and Characterization of Phosphate Anchored
MnO2 Catalyzed Solvent free Synthesis of Xanthene Laser
Dyes
Author: G. Harichandran S. David Amalraj P. Shanmugam
PII:
S1381-1169(14)00173-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.04.035
MOLCAA 9097
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
1-8-2013
22-4-2014
27-4-2014
Please cite this article as: G. Harichandran, S.D. Amalraj, P. Shanmugam, Synthesis
and Characterization of Phosphate Anchored MnO2 Catalyzed Solvent free Synthesis
of Xanthene Laser Dyes, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.04.035
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Synthesis and Characterization of Phosphate Anchored MnO2 Catalyzed
Solvent free Synthesis of Xanthene Laser Dyes
G. Harichandran,*a S. David Amalraja and P. Shanmugam*b
a Department of Polymer Science, University of Madras, Guindy Campus, Chennai-600 025,
INDIA; E-mail: umghari@gmail.com
b Organic Chemistry Division, CSIR-Central Leather Research Institute, Adyar, Chennai-600
020, INDIA
Abstract
A highly stable and environmentally benign phosphate anchored MnO2 (P-MnO2) catalyst
has been prepared. The catalyst has been characterized by TGA, FTIR, Raman, XRD, UV-
DRS, XPS, FESEM and EDAX. The catalytic behavior of P-MnO2 has been demonstrated for
the synthesis of 1, 8-dioxooctahydroxanthene and 14-aryl (alkyl)-14H-dibenzo[a, j] xanthene
laser dye derivatives under solvent free conditions. The catalyst is recyclable, high yield,
short reaction time, relative non-toxicity and easy workup are the advantages of the present
catalyst.
Keywords: P-MnO2 catalyst; 1, 8-dioxooctahydroxanthene; 14-aryl(alkyl)-14H-dibenzo[a, j]
xanthene; Laser dye; Solvent-free synthesis.
1. Introduction
Development of newer synthetic methods based on green chemistry processes are
increasingly of interest in organic synthesis due to economic and environmental concerns.
Owing to the growing environmental concern, advancement of green processes using
heterogeneous catalysts under solvent free condition has awakened great interest in recent
1
Page 1 of 26

years. Solvent free heterogeneous catalysis makes organic synthesis simpler, saves energy,
and prevents wastage of solvent, chemical hazards and toxicity [1]. Since heterogeneous
catalysts play a key role in organic transformations a number of catalysts have been
developed. Solid acid catalysts have advantage over homogeneous acid catalysts as they can
be easily recovered from reaction mixture by filtration and can be reused several times,
without loss of active sites, thereby making the process more economically and
environmentally viable. Heterogeneous catalysts anchored or impregnated with phosphoric
acid over the metal oxide phases have been widely used in industry for decades e.g. olefin
hydration to alcohols, propylene oligomerisation, alkylation and isomerisation of 1-butene [2-
4]. Thus, there are efforts to develop new solid acid catalysts and to demonstrate their
catalytic application in organic transformations [5].
In the current investigation, a phosphate anchored manganese oxide (Mn(H2PO4)2·2H2O)
with enhanced catalytic property, increased surface acidity, stabilized surface area and crystal
phase has been prepared [6]. Xanthenes and benzoxanthenes are important classes of
heterocyclic compounds used as leuco dyes in laser technology [7] and pH sensitive
fluorescent materials for visualization of biomolecules [8]. Moreover, these compounds have
received great attention due to their therapeutic and biological properties such as antibacterial
[9], antiviral [10] and anti-inflammatory [11] properties; their use in photodynamic therapy
[12] and inantagonism of the paralyzing action of zoxazolamine [13]. Several methods have
been reported for the syntheses of 1,8-dioxooctahydroxanthene and 14-aryl-14H-dibenzo[a,j]
xanthene using a number of catalysts [14-37]. However, application of the new catalyst,
Mn(H2PO4)2·2H2O (P-MnO2) in the synthesis of these compounds has not been reported so
far. We demonstrate the efficacy of this catalyst in the synthesis of xanthene derivatives via a
multicomponent reaction under solvent-free condition.
2
Page 2 of 26

2. Experimental
2.1.
Characterization
Melting points were determined in open capillaries and are uncorrected. Chromatography
purification was conducted by column chromatography using silica gel. Solvents used for
purification were of commercial grade and were purified before use. FT-IR spectra were
recorded on a Thermo Mattson Satellite - 3000 spectrophotometer by Nujol mull and KBr
pellet methods. 1H and 13C NMR spectra were recorded on a Bruker ultra shield spectrometer
(300 and 75 MHz) or a JEOL-500MHz spectrometer (500 and 125 MHz) in CDCl3 solvent
using TMS as internal standard. Mass spectra were recorded on JEOL GCMATE II GC-MS
Mass spectrometer. The TG experiments were carried out in air on a NETZSCH STA 490C-
CD apparatus by increasing temperature at a rate of 10˚C/min from 50 to 800˚C with calcined
Al2O3 powder as the standard reference. FE-SEM/EDAX analyses were carried out on a
HITACHI S-3400N microscope. The phase composition and crystalline nature of the catalyst
was analyzed using PXRD. Bruker D8 ADVANCE MODEL diffractometer was used for
recording PXRD data between 2 theta values ranging from 20 and 70 deg using CuKα as
source operating at 40 kV and 30 mA. XPS measurements were performed with Omicron
Nanotechnology, GmBH, Germany XM1000-monochromator with Al Kα radiation of 1483
eV operated at 300 W (20 mA emission current, 15 kV) and a base pressure of 5x10-5 mbar.
The survey scan was performed with a step size of 0.5 eV along with 50 eV as the pass
energy. The high-resolution scan was done with 0.03 eV as the step size and 20 eV as the
pass energy with three sweep segments. Raman spectra were recorded usinga BRUKER RFS
27: Stand-alone FT-Raman Spectrometer equipped with Nd: YAG 1064 nm as excitation
source.
2.2.
Spectroscopic data for selected compounds
2.2.1. 9-(2-thienyl)-3,3,6,6-tetramethyl-1,2,3,4,5,6,7,8-octahydroxanthene-1,8-dione
5h
3
Page 3 of 26

1
White solid, m.p. 145 °C; FT-IR (KBr) νmax: 2900, 1643, 1373, 1249, 995, 648 cm-1; H
NMR (500 MHz, CDCl3) δH: 7.03-7.02 (m, 1H, ArH), 6.97-6.96 (m, 1H), 6.84-6.82 (m, 1H),
5.15 (s, 1H, CH), 2.46 (s, 4H, (CH2)2), 2.26 (s, 4H, (CH2)2), 1.11 (s, 6H, (CH3)2), 1.06 (s, 6H,
(CH3)2); 13C NMR (125 MHz, CDCl3) δC: 196.4, 162.8, 148.2, 126.9, 125.4, 123.5, 115.3,
50.8, 40.9, 32.3, 29.4, 27.4, 26.4.
2.2.2. 9-(2-ferrocenyl)-3,3,6,6-tetramethyl-1,2,3,4,5,6,7,8-octahydroxanthene-1,8-dione 5i
Brown solid; m.p. 220 °C; FTIR (KBr) νmax: 2962, 2869, 1658, 1365, 1195, 1134, 026, 802,
671, 570 cm-1; 1H NMR (500 MHz, CDCl3) δH: 4.65 (s, 1H, CH), 4.08-3.99 (m, 7H), 3.82 (s,
2H), 2.45-2.32 (m, 8H, (CH2)4), 1.16 (s, 6H, (CH3)2), 1.14 (s, 6H, (CH3)2); 13C NMR (125
MHz, CDCl3) δC: 196.8, 164.1, 116.3, 68.9, 67.0, 66.5, 51.0, 41.1, 32.0, 29.8, 27.2, 23.5.
Mass (ESI) calcd for C27H30FeO3: 458.3703; found: 458.4198
3. Results and discussion
3.1.
Thermal analysis
The TG curve for MnO2 showed weight loss in the range between 100 Ԩ to 310 Ԩ, and is
about 4.1 wt %, which is attributed to the loss of the adsorbed water molecules. The TG
curves of P-MnO2 are shown in Fig. 1. The TG curve shows the mass losses between 30 and
800 ˚C, which are related to the elimination of water molecules and the phase
transformations. The eliminations of water were observed in three ranges: 25–120, 120–197
and 197–293 ˚C. The corresponding observed mass losses are 19.88%, 5.71% and 6.69%
respectively. The thermal decomposition of Mn(H2PO4)2·2H2O involves the dehydration of
the co-ordinated water molecules and an intramolecular dehydration of the protonated
4
Page 4 of 26

phosphate groups. There is a gradual mass loss (about 7.31%) in the range of 293–800˚C.
This is possibly related to the reaction between Mn2P4O12 and air to form manganese
pyrophosphate, Mn2P2O7 as indicated in Eq (1) [38]. The overall reaction could be presented
as:
Mn(H2PO4)2.2H2O → Mn2P2O7 + P2O5 + 4H2O …..(1)
Fig.1. TGA curves of the synthesized (a) MnO2 and (b) P-MnO2
FTIR analysis
3.2.
The FT-IR spectra of MnO2 and P-MnO2 are shown in Fig. 2. The FT-IR spectrum of the
P-MnO2 catalyst shows bands at 3394 and 1641 cm−1, corresponding to the OH stretching
and bending vibrations, respectively. Two bands (3394 and 3217 cm−1) that appear in the
region of the hydroxyl stretching are assigned to the strongly H-bonded O–H species and the
residual surface hydroxyl groups and ligated H2O in the catalyst surface. The absorption band
at 3217 cm−1 is attributed to PO–H stretching (hydrogen phosphates). The absorption bands at
1149 and 1049 cm-1 is assigned to antisymmetric and symmetric stretching mode vibrations
5
Page 5 of 26

3-
of the isolated PO4 tetrahedra. The absorption bands at 802, 648 and 516 cm-1 are assigned
3-
to the ν(P–OH) stretching, δ(P–O–Mn) bending and ν4(PO4 ), respectively [39-40].
Fig.2. FT-IR spectra of (a) MnO2 and (b) P- MnO2 catalyst
3.3.
Temperature Programmed Desorption (TPD) analysis
The quantification of acidity for the P-MnO2 catalyst was established from an
ammonia TPD analysis shown in Fig. 3. The results confirm the presence of 0.5 mmol/g
of acidity of the catalyst.
15.05
P-MnO2
15.00
14.95
14.90
14.85
14.80
14.75
6
Page 6 of 26
50
100
150
200
250
300
350
400

Fig. 3. TPD analysis of P- MnO2 catalyst
3.4.
Raman spectra
The strong Raman band at 646 cm−1 of commercial MnO2 can be attributed to the
symmetric stretching vibration (Mn–O) of the MnO2 (Fig. 4). The Raman spectrum of P-
MnO2 catalyst showed a peak at 1158 cm-1 corresponds to the in-plane bending of P–O–H,
and the peaks at 1099 and 939 cm-1 can be assigned to ν1 and ν3 modes of symmetric and
antisymmetric stretching of PO43- ions. The libration band for water molecules is seen at 684
cm-1. The band observed in the 560–500 cm-1 regions of the IR and Raman spectra of P-
MnO2 were assigned to ν4 mode of antisymmetric bending of PO43- ion. The band at 322 cm-1
corresponds to ν2 mode of PO43- ion and/or Mn–O stretching vibration [41].
MnO2
P-MnO2
200
400
600
800
1000
1200
1400
Wavenumber cm-1
Fig.4. Raman spectrum of the (a) MnO2 and (b) P-MnO2 catalyst
3.5.
XRD analysis
The X-ray powder pattern of the precursor MnO2 shows peaks at 2θ = 28.75, 37.53
7
Page 7 of 26

and 41.01º confirming the presence of β-MnO2 [JCPDS# 81-2261] (Fig. 5a).
Fig.5. XRD Spectrum of the (a) MnO2 and (b) P- MnO2 catalyst
On anchoring phosphate ion, over the crystalline MnO2, all the reflections
corresponding to MnO2 show decreased intensity with the emergence of reflections due to
manganese dihydrogen phosphate [JCPDS # 75-0740] which was formed by the reaction of
crystalline MnO2 with phosphoric acid. This confirms that the phosphate ion has been
anchored over the crystalline MnO2.
3.6.
UV- Diffuse reflectance spectroscopy (DRS) analysis
The UV-DRS absorption spectrum of the P-MnO2 recorded in the range of 200–800 nm
(Fig. 6) showed two absorption edges in the UV region at around 251 nm (39840 cm-1) and
310 nm (32258 cm-1) due to oxygen to Mn(III) charge transfer bands. The appearance of two
bands in the visible region at around 478 nm (20921 cm-1) and 577 nm (17331 cm-1)
5
5
correspond to the d-d transitions B1g→5Eg and B1g→5B2g respectively, can be explained by
8
Page 8 of 26

the presence of a strong Jahn-Teller effect. The ground state splitting coincides with the fifth
band and the excited state splitting is the difference between the third and the fourth bands, a
value of 3590 cm-1. Both values are of the same order but slightly lower than those found in
literature [40].
Fig. 6. Diffuse reflectance spectrum of (a) MnO2 and (b) P- MnO2 catalyst
3.7.
X-ray photoelectron spectroscopy (XPS)
Figure 7a presents the Mn 2p XPS of P-MnO2 catalyst with a Gauss–Lorentz
distribution function after Shirley integrated background subtraction. The binding energies of
642.2 and 654.2 eV were attributed to the Mn 2p lines with spin orbit splitting of 12 eV
between Mn 2p3/2 and Mn 2p1/2 matching with the previously reported values [42,43],
indicating that the oxidation state of manganese is +4. The satellite peak at 646.2 eV shows
the presence of Mn+3. The deconvoluted core level O 1s spectrum (Fig. 7b) showed peaks at
531.4 and 532.8 eV corresponding to P–O–M [44] and P-OH [45], respectively. These peaks
can alternatively be assigned to other oxygen-containing chemical bonds such as a water
9
Page 9 of 26

molecule (H–O–H) at 532.8 eV, and hydroxide (Mn–O–H) at 531.4 eV [46]. The P 2p
photoelectron peak (Fig. 7c) can be found at 133.7 eV, and the peak is ascribed to the P5+ in
Mn-PO4. Similar BE was reported for transition metal phosphates [47]. The P 2p
photoelectron peak can also be fitted adequately with a Gaussian peak centered at 134.5 eV,
again indicating that P is present in the P–O bonding only [48].
Fig. 7. (a) Mn 2p, (b) O 1s and (c) P 2p core level XPS spectra
3.8.
FE-SEM Micrograph Analysis
The FE-SEM micrograph of the bare MnO2 (Fig. 8(a)) showed particle size ranging from
10 μm to 0.5 μm. On supporting the phosphate species over the MnO2, the particle size
drastically decreases to a range of 2.5 μm to 250 nm, which can be clearly visualized from
the FE-SEM image (Fig. 8(b)). The EDAX analysis confirms the presence of MnO2 and
anchored phosphate ion over the MnO2, by the presence of phosphorus peak at 2 Kev (Fig.
8(d)).
10
Page 10 of 26

a
c
b
d
Fig. 8. FESEM micrographs of: (a) MnO2 and (b) P-MnO2 catalyst; EDAX analysis of: (c)
MnO2 and (d) P-MnO2 catalyst
3.9.
Conclusion drawn from characterization of P-MnO2 catalyst
1. The FT-IR spectrum of the P-MnO2 catalyst showed absorption bands at 3394 and 1641
cm−1, corresponding to the OH stretching and bending vibrations, respectively. The
absorption band at 3217 cm−1 is attributed to PO–H stretching (hydrogen phosphate).
Absorption bands at 1149 and 1049 cm-1 is assigned to antisymmetric and symmetric
3-
stretching mode vibrations of the isolated PO4 tetrahedra. The absorption bands at 802,
11
Page 11 of 26

648 and 516 cm-1 are assigned to the ν(P–OH) stretching, δ(P–O–Mn) bending and
3-
ν4(PO4 ), respectively.
This result is further supported by Raman spectrum which showed a peak at 1158
cm-1 corresponds to the in-plane bending of P–O–H, and the peaks at 1099 and 939 cm-1
3-
can be assigned to ν1 and ν3 modes of symmetric and antisymmetric stretching of PO4
3-
ions. The band at 322 cm-1 corresponds to ν2 mode of PO4 ion and/or Mn–O stretching
vibration.
2. The NH3 TPD analysis proved the acidity of the catalyst.
3. From XRD analysis, anchoring phosphate ion over the crystalline MnO2, all the
reflections corresponding to MnO2 showed decreased intensity. This confirms that the
phosphate ion has been anchored over the crystalline MnO2.
4. The XPS studies confirm the presence of P-OH species on the catalyst surface.
5. The EDAX analysis confirms the presence of MnO2 and anchored phosphate ion over the
MnO2, by the presence of phosphorus peak at 2 Kev.
Based on the FTIR, Raman, XPS analyses discussed in manuscript, the surface
structure of the catalyst is arrived as follows (Scheme 1).
Mn
O
H
Mn
O
P
O
O
Mn
Scheme 1.
3.10. Catalytic activity study: Synthesis of xanthene derivatives
The catalytic activity of P-MnO2 was explored for the synthesis of 1,8-
dioxooctahydroxanthene and 14-aryl(alkyl)-14H-dibenzo[a,j]xanthene derivatives under
solvent free conditions. Initially, we examined the condensation of benzaldehyde 1a with 5,
5-dimethyl-1,3-cyclohexanedione 2a under solvent and catalyst free conditions for the
synthesis of 1,8-
dioxo-
O
O
O
O
O
12
O
O
1a
a
b or c
OH
4a
O
OH
O
Page 12 of 26
O
2a
2a
5a
a. MnO2, Neat, 100 ºC, 98%; b. MnO2-AcCl, Neat, 100 ºC, 40%;
c. P-MnO2, Neat, 100 ºC, 98%

octahydro-xanthenes (Table 1, entry 1). However, the reaction afforded only 2,2’-
phenylmethylenebis (3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one) 4a in 40% yield and the
cyclized xanthene product 5a was not observed. Interestingly, the use of commercially MnO2
as catalyst under solvent-free condition afforded only 2,2’- phenylmethylenebis (3-hydroxy-
5,5-dimethyl-2-cyclohexene-1-one) 4a in execellent yield (98%). Longer reaction time and
increased mol% of the catalyst MnO2 (40 mol %) did not yield the cyclized product 5a (Table
1, entries 2 and 3, Scheme 2). However, when the combination of MnO2–AcCl is used,
compound 5a was obtained only in 40% yield (Table 1, entry 4) [49]. On the other hand, the
reaction with 10 mol % of P-MnO2 catalyst under solvent-free conditions at 80 °C provided
the desired compound 5a in 65% yield (Table 1, entry 5). When reaction was carried out at
100 °C the desired compound 5a was obtained in execellent yield (98%) and the results are
summarized in Table 1. It has been found that 10 mol % of P-MnO2 is the optimum amount
of catalyst (Table 1, entry 7). Increasing mol % of catalyst beyond 10% did not alter the yield
while reducing the mol % of catalyst decreased the yield (entries 6 and 8).
Scheme 2. Synthesis and optimization of compound 5a
Table 1. Optimization of catalyst for the one-pot condensation of dimedone 2a with
benzaldehyde 1a
Entry
Catalyst
(mole %)
Temperature (°C) Time (min) Product Yielda (%)
1
2
3
4
5
-
100
100
100
80
15
15
15
220
25
4a
4a
4a
5a
5a
40
98
98
40
65
MnO2 (10)
MnO2 (40)
MnO2(10)–AcClb,c
P-MnO2 (10)
80
13
Page 13 of 26

6
7
8
P-MnO2 (15)
P-MnO2 (10)
P-MnO2 (5)
100
100
100
15
15
15
5a
5a
5a
98
98
75
Reaction condition: benzaldehyde: 5, 5-dimethyl-1,3-cyclohexanedione 1:2, at different
temperature
a All the reactions carried without any solvent
b 2 mmol of AcCl
c Performed in acetonitrile under reflux
To demonstrate the method as general, under the optimized condition, a number of
aryl aldehydes 1a-i were reacted with 2a and the reactions preceded well to afford 1,8-
dioxooctahydro xanthens 5a-f in excellent yields (Table 2). The substitution at the 4-position
of the aromatic aldehyde had no significant effect and aldehydes with either electron-
donating group or electron-withdrawing group caused slightly lower yields compared to the
unsubstituted aldehyde. To further investigatethe scope of the reaction, the reaction of p-
phenylenedialdehyde 1g and dimedone 2a (1:4) was examined and it afforded the tetraketone
5g in good yield (Scheme 3; Table 2, entry 7). To demonstrate the diversity of the aldehydes,
experiments with heterocyclic aldehyde and aliphatic aldehyde such as 2-thiophene aldehyde
1h and butyraldehyde 1j were carried out to afford the desired product 5h and 5j in excellent
yield (Table 2, entries 8 and 10). Significantly, the reaction with ferrocenealdehyde 1i
afforded ferrocenyl xanthene derivative 5i (Scheme 3; Table 2, entry 9) in excellent yield. A
comparison of the present catalyst with recently demonstrated catalysts for the synthesis of
1,8-dioxo-octahydroxanthenes derivatives are summarised in Table 3. It should be noted that
the present catalyst has advantages over other catalysts in terms of high yield, short reaction
time, relative non-toxicity, easy workup and ability to perform reactions in solvent free
conditions. Recycling of catalyst was achieved for three times without loss of activity (Table
1
4). All the compounds were characterized by spectroscopic data (FT-IR, H NMR and 13C
14
Page 14 of 26

NMR). Final structure proof of representative compound 5e was obtained from single crystal
X-ray studies [50] (Figure 9).
Fe
O
O
O
O
O
1g
1i
O
O
P-MnO2
Neat, 120 oC
P-MnO2
O
O
o
Neat, 120 C
O
O
5i
2a
5g
Scheme 3. Synthesis of compounds 5g and 5i
Table 2. Synthesis of 1,8-Dioxo-octahydro-xanthenes 5a-j
Entry
Ar-CHO
C6H5, 1a
Diketone
2a
Time (min)
Product
5a
Yield (%)a
1
2
3
4
5
6
7
8
9
15
45
35
20
25
30
30
30
40
15
98
90
95
97
92
HO-4-C6H4,1b
H3CO-4-C6H4,1c
O2N-4-C6H4,1d
Cl-4-C6H4,1e
Br-4-C6H4,1f
OHC-4-C6H4,1g
2-Thienyl, 1h
2-Ferrocenyl, 1i
C4H8O 1j
2a
2a
2a
2a
2a
2a
2a
2a
5b
5c
5d
5e
5f
5g
5h
5i
96
90b
88b
85b
90
10
2a
5j
Reaction condition: aldehyde: 5, 5-dimethyl-1,3-cyclohexanedione 1:2, at 100 °C
a Isolated yield
b Reaction was done at 120 ˚C
Table 3. Comparison of the characteristic of the present catalyst for the synthesis of 1,8-
dioxo-octahydroxanthenes with some recently used catalysts
Entry
Catalyst
Solvent
Time (min)/
Yield%
Temperature
Ref.
(◦C)
85
reflux
100
1
2
3
CaCl2
Amberlyst 15
Fe3+-MMT
DMSO
CH3CN
Solvent free
240/85
300/90
360/92
16(b)
18(a)
18(c)
15
Page 15 of 26

4
5
6
7
ZnONp
Solvent free
Solvent free
Solvent free
Solvent free
22/91
20/94
180/85
15/98
80
100
80
20(c)
22(a)
24(b)
-
[Et3NH][HSO4]
[Bmim][HSO4]
Present catalyst
100
Table 4.Recyclization of the catalyst for the synthesis of 5d
Entry
Recovery
Time/min
Yield (%)
1
2
3
4
0
1
2
3
20
20
20
20
97
97
95
93
Reaction condition: 4-nitrobenzaldehyde: 5, 5-dimethyl-1,3-cyclohexanedione 1:2, at 100 °C
Fig. 9. ORTEP diagram of compound 5h [50]
3.10.1. Plausible mechanism for the synthesis of xanthenes
A plausible mechanism of the reaction is shown in Scheme 4. The formation of 1,8-
dioxooctahydroxanthene derivative 5 was believed to be formed via an initial Knoevenagel
condensation of 5, 5-dimethyl-1,3-cyclohexanedione 2a with benzaldehyde 1a to afford
intermediate I. Then the active methylene of 5, 5-dimethyl-1,3-cyclohexanedione 2a reacts
with intermediate I via a conjugate Michael addition to generate the intermediate II, which
16
Page 16 of 26

upon dehydration gives cyclic compound 5a. The dehydration was facilitated by the anchored
P-MnO2 catalyst, without this only compound 4a was obtained.
O
Mn
O
O
O
Ar
O
Ar
H
O
Ar
O
O
Ar
O
O
Mn
P
O
O
Ar
O
1a
+
H
O
-H2O
2a
Mn
H
HO
HO
OH OH
5a
I
4a
II
O
OH
2a
Scheme 4. Plausible mechanism for the formation of compound 5a
3.10.1.1.
Support experiment for Proposed mechanism
A reaction of benzaldehyde 1a and 5, 5-dimethyl-1,3-cyclohexanedione 2a (1:1 ratio)
under optimized condition was carried out (Scheme 5). The reaction stopped at Intermediate I
shown in scheme 4. On treatment of intermediate I with another mole of 2a afforded
xanthene 5a. The above observation supports that the reaction preceded via Intermediate I.
O
H
O
H
O
Ar
O
O
Ar
O
catalyst
2a (1 equiv)
catalyst
Neat, 100 oC
+
+
Neat, 100 oC
catalyst
O
O
Neat, 100 oC
O
O
O
1a
2a
1a
2a
I
5a
(1:2)
(1:1)
isolated
Scheme 5. Evidence for proposed mechanism of compound 5a
Successful syntheses of 1,8-dioxo-octahydroxanthens 5a-i prompted us to explore the
synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes 6a-f . Thus, reactions of β-naphthol 3a with a
number of aromatic aldehydes under solvent-free condition at 120 ºC were carried out to get
14-aryl -14H-dibenzo[a,j]xanthene derivatives 6a-f in good to high yield within a short
period of time ca. 60-120 min. (Scheme 6, Table 5). The reaction proceeds very efficiently
for aromatic aldehydes carrying either electron-donating or electron-withdrawing
substituents.
17
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X
X
H
O
OH
+
HO
P-MnO 2
1(a-f)
+
Neat,120 oC
O
6(a - f)
3a
3a
Scheme 6. Synthesis of aryl-14H-dibenzo [a.j] xanthenes
Table 5. Synthesis of aryl-14H-dibenzo [a.j] xanthenes 6(a-f)
Entry
Ar
C6H5,1a
2-Napthol
Time (min)
Product
Yield %
95
1
2
3
4
5
6
3a
3a
3a
3a
3a
3a
90
110
120
100
90
6a
6b
6c
6d
6e
6f
HO-4-C6H4,1b
H3CO-4-C6H4,1c
O2N-4-C6H4, 1d
Cl-4-C6H4,1e
Br-4-C6H4,1f
92
90
92
97
60
95
Reaction condition: aldehyde: β-naphthol 1:2, at 120 °C
To explore the versatility of for the reaction in the synthesis of 1,8-dioxooctahydro-
xanthenes, a reaction of benzaldehyde 1a, dimedone 2a, and β-naphthol 3a in 1:1:1 ratio at
120 ˚C for 20 min was carried out. The reaction afforded unsymmetrical
tetrahydrobenzo[a]xanthene-11-one 7a in 85% yield (scheme 7).
H
O
1a
O
O
P-MnO2
OH
+
o
Neat, 120 C
O
O
2a
3a
7a
Scheme 7. Synthesis of aryl-tetrahydrobenzo[a]xanthene-11-one 7a
4. Conclusion
In summary, a highly stable and environmentally benign phosphate anchored MnO2 (P-
18
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MnO2) catalyst was prepared and thoroughly characterized by TGA, FTIR, Raman, XRD,
UV-DRS, XPS, FESEM and EDAX techniques. The catalytic activity of P-MnO2 has been
demonstrated for the synthesis of 1,8-dioxooctahydroxanthene and 14-aryl(alkyl)-14H-
dibenzo[a, j] xanthenes under solvent free conditions. The catalyst was found to be recyclable
without loss of activity. Further work utilizing the catalysts for other synthetic
transformations is in progress in this laboratory.
5. Supplementary data
Electronic Supplementary Information (ESI) available: Detailed expreimental
procedure, compiled spectroscopic data and scanned copies of spectra.
Acknowledgements
SDA thanks CSIR (New Delhi) for award of Senior Research Fellowship (SRF). GH
thanks University of Madras for providing infrastructure facilities. Thanks are due to
Director, National Centre for Nanoscience and Nanotechnology (NCNSNT), University of
Madras for Raman, UV-DRS, XPS, and FE-SEM/EDAX measurements and Department of
Chemistry, St. Joseph College, Bangalore for XRD measurements.
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Figure Captions
Figure 1: TGA curves of the MnO2 and P-MnO2
23
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Figure 2: FT-IR spectra of (a) MnO2 and (b) P- MnO2 catalyst
Figure 3: TPD analysis of P-MnO2 catalyst
Figure 4: Raman spectrum of the (a) MnO2 and (b) P-MnO2 catalyst
Figure 5: XRD spectrum of the (a) MnO2 and (b) P- MnO2 catalyst
Figure 6: Diffuse reflectance spectrum of (a) MnO2 and (b) P- MnO2 catalyst
Figure 7: (a) Mn 2p, (b) O 1s and (c) P 2p core level XPS spectra
Figure 8: FESEM micrographs of (a) MnO2 and (b) P-MnO2 catalyst; EDAX analysis of: (c)
MnO2 and (d) P-MnO2 catalyst
Figure 9: ORTEP diagram of compound 5h [50]
Graphical Abstract
24
Page 24 of 26

Synthesis and Characterization of Phosphate Anchored MnO2 Catalyzed
Solvent free Synthesis of Xanthene Laser Dyes
G. Harichandran,*a S. David Amalraja and P. Shanmugam*b
R
O
O
H
1a-j
(or)
OH
O
2a
3a
200-220 oC
Sand bath
MnO2
H3PO4
P-MnO2
N
e
a
P-MnO2
C
t
º
,
1
0
2
0
0
1
º
R
C
,
t
a
R
O
e
O
N
O
O
5 a-j
6 a-f
25
Page 25 of 26

Highlights
ꢀP-MnO2 catalyst were used in the synthesis of 1,8-dioxooctahydroxanthene and 14-
aryl(alkyl)-14H-dibenzo[a,j] xanthene laser dye derivatives.
ꢀP-MnO2 catalyst has higher activity and selectivity than commercial MnO2 and
MnO2–AcCl.
ꢀReactions could be carried out under solvent free reaction conditions.
ꢀA plausible mechanism for the synthesis of xanthenes presented.
26
Page 26 of 26