Facile montmorillonite K-10-supported synthesis of xanthene derivatives under microwave and thermal conditions

Article abstract of DOI:10.1080/00397910802005299

Microwave-assisted and thermal condensations of aromatic and aliphatic aldehydes with β-naphthol adsorbed onto montmorillonite K-10 were conducted in a one-pot procedure leading to the formation of a variety of xanthene derivatives. Under microwave irradi

Full text of DOI:10.1080/00397910802005299

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Facile Montmorillonite K-10-
Supported Synthesis of
Xanthene Derivatives Under
Microwave and Thermal
Conditions
Ali Sharifi a , M. Saeed Abaee a , Azam Tavakkoli a
Mojtaba Mirzaei a & Alireza Zolfaghari a
,
a Faculty of Organic Chemistry and Natural Products,
Chemistry and Chemical Engineering Research
Center of Iran, Tehran, Iran
Published online: 28 Aug 2008.
To cite this article: Ali Sharifi , M. Saeed Abaee , Azam Tavakkoli , Mojtaba Mirzaei &
Alireza Zolfaghari (2008) Facile Montmorillonite K-10-Supported Synthesis of Xanthene
Derivatives Under Microwave and Thermal Conditions, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 38:17,
2958-2966, DOI: 10.1080/00397910802005299
To link to this article: http://dx.doi.org/10.1080/00397910802005299
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Synthetic Communications1, 38: 2958–2966, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802005299
Facile Montmorillonite K-10-Supported
Synthesis of Xanthene Derivatives Under
Microwave and Thermal Conditions
Ali Sharifi, M. Saeed Abaee, Azam Tavakkoli, Mojtaba Mirzaei,
and Alireza Zolfaghari
Faculty of Organic Chemistry and Natural Products, Chemistry and Chemical
Engineering Research Center of Iran, Tehran, Iran
Abstract: Microwave-assisted and thermal condensations of aromatic and
aliphatic aldehydes with b-naphthol adsorbed onto montmorillonite K-10 were
conducted in a one-pot procedure leading to the formation of a variety of
xanthene derivatives. Under microwave irradiation, reactions completed within
4–6 min. The procedure is applicable to both aromatic and aliphatic aldehydes,
and the catalyst was recycled and reused efficiently in later reactions.
Keywords: Microwave, montmorillonite, solvent-free, xanthenes
Synthesis of xanthene derivatives are of particular importance in organic
chemistry because of their numerous biological and pharmaceutical
applications.[1] In addition, they are useful precursors for the preparation
of many natural and synthetic targets such as pH-sensitive fluorescent
products,[2] photodynamic therapy sensitizers,[3] and dyes.[4] Because of
these applications, development of efficient and novel procedures for
the synthesis of xanthenes is of particular interest. The traditional
approach for this synthesis usually involves cyclodehydration of suitably
substituted bis-naphthyl substrates in the presence of acidic reagents.[5]
Many nonclassical procedures are also developed so far for the synthe-
sis of these compounds including cyclocondensation of hydroxyl substi-
tuted aromatic aldehydes with 2-tetralone,[6] heteropolyacid-mediated
Received January 2, 2008.
Address correspondence to Ali Sharifi, Chemistry and Chemical Engineering
Research Center of Iran, P.O. Box 14335-186, Tehran, Iran. E-mail: sharifi@
ccerci.ac.ir
2958

K-10-Supported Synthesis of Xanthene Derivatives
2959
reactions,[7] heterogeneous catalysis,[8] Lewis acid catalysis,[9] solvent-free
reactions,[10] and microwave irradiation (MW) techniques.[11] However,
in many of these methods, either a combination of several additives
and catalysts is employed or long time periods are required for reactions
to complete. Therefore, development of new methods capable of inducing
rapid and efficient formation of the title compounds would be of interest
to synthetic chemists.
In line with recent increasing global interests to develop green chemi-
cal processes, solid catalysts have emerged as powerful and environmen-
tally compatible means to induce various organic reactions.[12] In this
context, montmorillonite has been used as a clean, recyclable, and envir-
onmentally safe catalyst in many synthetic transformations such as the
Friedel–Crafts reaction,[13] synthesis of nucleosides,[14] conjugate addi-
tions,[15] aminolysis of aziridines,[16] synthesis of benzodioxoles,[17] and
thiolysis of epoxides.[18] Very recently, this solid acidic catalyst was
reported to facilitate smooth preparation of xanthenediones.[19] However,
the reaction was limited to aromatic aldehydes, required the presence of
additional Feþ3 ions, and reached completion after several hours. In con-
tinuation of our investigations to develop efficient and environmentally
friendly procedures for the synthesis of various organic structures,[20]
the application of montmorillonite K-10 is hereby reported to induce
efficient condensation of various aldehydes with b-naphthol using micro-
wave (MW) irradiation. Under these circumstances, reactions complete
within a few minutes, the catalyst is recyclable, and the methodology is
applicable to both enolizable and nonenolizable aldehydes (Scheme 1).
We first examined the reactions of benzaldehyde with b-naphthol
under various sets of conditions to evaluate the effects of the catalyst
and MW irradiation on the progress of the reaction. The results summar-
ized in Table 1 clearly illustrate that the combined presence of both the
catalyst and MW irradiation effectively facilitates the condensation so
that the starting substrates could completely convert to the desired
product 1a (R ¼ C6H5) within a few minutes (entry 1). Neither montmor-
illonite (entry 2) nor MW irradiation (entry 3) alone can induce signifi-
cant condensation between the aldehyde and b-naphthol. Alternatively,
the thermal reaction of the same heterogeneous mixture at elevated
Scheme 1. Montmorillonite-supported synthesis of xanthenes.

2960
A. Sharifi et al.
Table 1. Condensation of b-naphthol with benzaldehyde under various
conditions
Entry
Adsorbent
Conditions
Solvent
Time
Yield (%)
1
2
3
4
5
6
7
Montmorillonite K-10
Montmorillonite K-10
—
Montmorillonite K-10
Montmorillonite K-10
—
MW
RT
MW
Reflux
Reflux
Reflux
80ꢀC
—
—
—
4 min
24 h
8 min
24 h
24 h
24 h
18 h
92
2
3
<1
85
<1
90
THF
CH2Cl2
CH2Cl2
—
Montmorillonite K-10
temperatures does not proceed in THF (entry 4), perhaps because of
deactivation of the catalyst via co-ordination of the solvent molecules
to Lewis acidic sites of the surface, or takes several hours in CH2Cl2 to
complete (entry 5).
Based on these observations, we next subjected solvent-free mixtures
of b-naphthol with various aldehydes to condensation using the opti-
mized conditions. The results are summarized in Table 2. Under thermal
conditions, mixtures of montmorillonite K-10 with b-naphthol and
aromatic aldehydes bearing various electron-releasing and electron-
withdrawing groups gave products 1b–h in high yields within 16–20 h.
Using MW irradiation, the same mixtures gave 1b–h in considerably
shorter time periods (entries 2–8). The methodology was employed for
aliphatic aldehydes as well. Again, formaldehyde (entry 9), propionalde-
hyde (entry 10), isobutyraldehyde (entry 11), and nonanal (entry 12) gave
high yields of their respective products when subjected to reaction with
b-naphthol.
Interestingly, terephthalaldehyde reacted chemoselectively under
both sets of conditions, leading to the sole formation of product 1m.
Under no conditions could the second carbonyl group be persuaded to
undergo condensation with b-naphthol (Scheme 2). All products were
easily separated from the reaction mixtures via simple extraction by ethyl
acetate and were characterized by spectroscopic methods.
To illustrate the environmental compatibility of the reaction, the
catalyst was isolated from the reaction mixtures by simple filtration
and was used in the next reactions to evaluate its activity. As a result,
when the recycled catalyst was used in four consecutive condensations
of b-naphthol with benzaldehyde, yields of 92, 90, 87, and 85% were
obtained, respectively, showing high reusability of the recycled catalyst.
To investigate the recycling effect on the catalyst activity, texture and
structure properties of the catalyst were studied. The catalyst texture was

K-10-Supported Synthesis of Xanthene Derivatives
2961
Table 2. Thermal and MW-assisted condensation of various aldehydes with
b-naphthol on montmorillonite K-10 surface
Time
Isolated yield (%)
Entry
Aldehyde
Product
Ref.
80ꢀC (h)a MW (min) 80ꢀC
MW
92
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
18
18
20
18
18
16
18
16
4
4
4
6
6
4
6
6
90
92
83
99
96
82
89
90
7
7
95
80
10
99
11a
11a
9b
8c
99
80
1g
1h
95
87
10
9
10
11
12
HCHO
1i
1j
18
24
24
24
4
4
4
6
85
79
77
95
85
80
82
90
11a
10
C2H5CHO
(CH3)2CHCHO
C8H17CHO
1k
1l
11a
—
aSolventless.
Scheme 2. Condensation of b-naphthol with terephthalaldehyde.

2962
A. Sharifi et al.
Table 3. Effect of recycling on catalyst activity
No. of
recycles
Yield
(%)
SBET
(m2=g)
Total pore
volume (cm3=g)
0
1
2
92
90
87
173.5
76.2
62.0
0.31
0.14
0.12
examined using Brunauer-Emmett-Teller (BET) and Barrett-Joiner-
Halenda (BJH) methods. The BET surface area and BJH pore volume
of the catalyst before and after each recycling run are presented in Table
3. It could be deduced from the results that the surface area and the pore
volume of the catalyst after the first experiment decrease by factors of 44
and 45%, respectively. However, because of availability of enough sur-
face area, the activity does not drop significantly as reflected by the yield
of the second experiment. The decrease in BET and BJH parameters are
much less in the next runs and consequently the catalyst could be effec-
tively reused in several experiments in a row.
To further evaluate the catalyst performance, X-ray diffractograms
of montmorillonite K-10 samples before the reaction and after the first
and second runs were prepared at 2h between 2ꢀ and 70ꢀ. As depicted
in Fig. 1, all three diffractograms are nearly identical, indicating negli-
gible changes in the lattice parameters of the catalyst after recycling.
In conclusion, successful development of a facile and efficient
method for the preparation of a variety of xanthene derivatives is
Figure 1. X-ray diffractograms of the original (a), the first recycled (b), and the
second recycled montmorillonite samples.

K-10-Supported Synthesis of Xanthene Derivatives
2963
presented using montmorillonite K-10 support under both thermal and
MW irradiation. Rapid completion of the reactions, sole formation of
single products, easy workup, and the generality of the method are the
advantages of this procedure. Efficient reusability of the solid support
and no solvent requirement make the present method attractive from
environmental points of view.
EXPERIMENTAL
Reactions were monitored by gas chromatography (GC) and thin-layer
chromatography (TLC). FT-IR spectra were recorded using KBr disks on
a Bruker Vector-22 infrared spectrometer, and absorptions were reported
1
as wave numbers (cm ꢁ 1). H NMR spectra were obtained on a FT-
NMR Bruker Ultra ShieldTM (500-MHz) instrument as CDCl3 solutions,
and the chemical shifts are expressed as d units with Me4Si as the internal
standard. GC experiments were carried out using a Fisons 8000 apparatus.
Elemental analysis was performed using a Thermo Finnigan Flash EA 1112
instrument. The BET surface areas and BJH pore volumes of montmorillon-
ite K-10 samples were measured on a Quantasorb, Quantachrom apparatus.
The crystallographic structure of the powder was investigated using a dif-
fractometer (Bruker D8 Advance with a Cu anode that generated Cu Ka
˚
radiation, k ¼ 1.5406 A). All chemicals and reagents were purchased from
commercial sources. Aldehydes were purified prior to use.
Typical Procedure for MW-Assisted Preparation of 1a–m
A mixture of benzaldehyde (1.11 g, 10.5 mmol), b-naphthol (2.88 g,
20.0 mmol), and activated montmorillonite K-10 (5 g) was irradiated in
a microwave oven for 4 min. (2 ꢂ 2 min with 1 min time interval). The
reaction mixture was extracted twice by 20-ml portions of EtOAc after
TLC and GC monitoring showed completion of the reaction. The com-
bined organic phases were separated from the catalyst by simple filtration
and dried over Na2SO4. The product was obtained (3.29 g, 92%) after
evaporation of the volatile portion and was characterized by mass, IR,
and NMR spectroscopy.
Data
14-Octyl-14H-dibenzo[a,j]xanthene (1l)
An oily product was obtained in 95% yield, IR (KBr, cmꢁ1) 2920, 2855,
1246; 1H NMR (500 MHz, CDCl3) d 8.31 (d, J ¼ 8.5 Hz, 2H), 7.93

2964
A. Sharifi et al.
(d, J ¼ 8.0 Hz, 2H), 7.82 (d, J ¼ 8.8 Hz, 2H), 7.69–7.66 (m, 2H), 7.53–
7.50 (m, 2H), 7.44 (d, J ¼ 8.8 Hz, 2H), 5.62 (dd, J ¼ 4.5, 4.5 Hz, 1H),
2.12–2.03 (m, 2H), 1.17 (m, 2H), 1.12–0.98 (m, 10H), 0.82 (dd, J ¼ 7.1,
7.3 Hz, 3H); 13CNMR (125 MHz, CDCl3) d 150.4, 131.9, 131.5, 129.3,
128.6, 127.0, 124.5, 122.9, 118.0, 117.2, 36.4, 32.1, 31.5, 30.2, 29.8,
29.6, 25.2, 29.9, 14.4; MS (70 eV) m=z (%): 394 (Mþ ), 281, 279, 181,
Calcd. for C29H30O: C, 88.28; H, 7.66. Found: C, 88.36; H, 7.59.
4-(14H-Dibenzo[a,j]xanthene-14-yl)benzaldehyde (1 m).
White crystals were obtained in 83% yield, mp308–312ꢀC (decomp.);
IR (KBr, cmꢁ1) 1686, 1589, 1242; 1H NMR (500 MHz, DMSO-d6) d 9.77
(s, 1H), 8.68 (d, J ¼ 8.5 Hz, 2H), 7.94 (dd, J ¼ 8.8, 7.6 Hz, 4H), 7.87 (d,
J ¼ 8.2 Hz, 2H), 7.69 (d, J ¼ 8.2 Hz, 2H), 7.64 (dd, J ¼ 7.3, 7.3, Hz, 2H),
7.58 (d, J ¼ 8.9 Hz, 2H), 7.47 (dd, J ¼ 7.3, 7.3, Hz, 2H), 6.85 (s, 1H); 13
C
NMR (125 MHz, DMSO-d6) d 193.0, 152.7, 148.9, 135.3, 131.7, 131.5,
130.6, 130.2, 129.5, 127.9, 125.5, 124.1, 118.6, 117.4, 37.6; MS (70 eV)
m=z (%): 386 (Mþ ), 282, 281, 252, 141, Calcd. for C28H18O2: C, 87.02;
H, 4.69. Found: C, 87.11; H, 4.75.
ACKNOWLEDGMENT
Partial financial support by the Ministry of Science, Research, and
Technology of Iran is greatly appreciated.
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