Facile and efficient method for the synthesis of 14-substituted-14-h- dibenzo[a,j]xanthenes catalyzed by ruthenium chloride hydrate as a homogeneous catalyst

Article abstract of DOI:10.1080/00397911.2010.486507

Synthesis of dibenzoxanthenes through condensation of β-naphthol with various aromatic and aliphatic aldehydes in ethanol as an ecofriendly solvent using Ru^III as catalyst is reported.

Full text of DOI:10.1080/00397911.2010.486507

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Facile and Efficient Method for the
Synthesis of 14-Substituted-14-H-
dibenzo[a,j]xanthenes Catalyzed
by Ruthenium Chloride Hydrate as a
Homogeneous Catalyst
Khalil Tabatabaeian a , Alireza Khorshidi a , Manouchehr Mamaghani a
& Ali Dadashi a
a Department of Chemistry, Faculty of Sciences, University of
Guilan, Rasht, Iran
Version of record first published: 30 Mar 2011.
To cite this article: Khalil Tabatabaeian, Alireza Khorshidi, Manouchehr Mamaghani & Ali Dadashi
(2011): Facile and Efficient Method for the Synthesis of 14-Substituted-14-H-dibenzo[a,j]xanthenes
Catalyzed by Ruthenium Chloride Hydrate as a Homogeneous Catalyst, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 41:10, 1427-1434
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Synthetic Communications1, 41: 1427–1434, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.486507
FACILE AND EFFICIENT METHOD FOR THE SYNTHESIS
OF 14-SUBSTITUTED-14-H-DIBENZO[a,j]XANTHENES
CATALYZED BY RUTHENIUM CHLORIDE HYDRATE AS
A HOMOGENEOUS CATALYST
Khalil Tabatabaeian, Alireza Khorshidi,
Manouchehr Mamaghani, and Ali Dadashi
Department of Chemistry, Faculty of Sciences, University of Guilan,
Rasht, Iran
GRAPHICAL ABSTRACT
Abstract Synthesis of dibenzoxanthenes through condensation of b-naphthol with various
aromatic and aliphatic aldehydes in ethanol as an ecofriendly solvent using RuIII as catalyst
is reported.
Keywords Aldehyde; homogeneous catalyst; b-naphthol; ruthenium; xanthene
INTRODUCTION
Xanthene framework is featured in a wide variety of compounds that show
pharmacological and biological activities such as antibacterials, antivirals, and
anti-inflammatory agents.[1] Xanthene-based compounds are also being developed
to act as new clinical agents in cancer therapy.[2] It is also noteworthy that upon
oxidation, xanthene derivatives can be converted to corresponding xanthylium salts,
which are candidates as dyes and fluorescent materials.[3] Xanthene dyes are
extracted from natural sources.[4] Therefore, significant effort has been directed
toward synthesis of xanthene derivatives. Synthetic 14-H-dibenzo[a,j]xanthenes have
been prepared by different methods such as the reaction of 2-naphthol with
2-naphthol-1-methanol,[5] aldehyde acetals,[6] and carbon monoxide.[7] However,
these methods suffer from long reaction times, unsatisfactory yields, tedious experi-
mental procedures, and excess reagents. Recently, the synthesis of xanthenes has
Received January 24, 2010.
Address correspondence to Khalil Tabatabaeian, Department of Chemistry, Faculty of Sciences,
University of Guilan, P.O. Box 41335-1914, Rasht, Iran. E-mail: Taba@guilan.ac.ir
1427

1428
K. TABATABAEIAN ET AL.
Scheme 1. RuCl3 ꢀ nH2O-catalyzed synthesis of 14-substituted-14-H-dibenzo[a,j]xanthenes.
been improved by mixing b-naphthol with aldehydes in the presence of a catalyst
such as para-toluenesulfonic acid (p-TSA),[8] sulfamic acid,[9] AcOH=H2SO4,[10]
amberlyst 15,[11] LiBr,[12] I2,[13] cyanuric chloride,[14] K5CoW12O40 ꢀ 3H2O,[15] HClO4-
SiO2,[16] silica sulfuric acid,[17] Yb(OTf)3,[18] KAl(SO4)2 ꢀ 12H2O (alum),[19] tetrabutyl-
ammonium bromide,[20] H14[NaP5W30O110],[21] and Dowex-50w.[22]. In spite of their
potential utility, some of these methods have their own drawbacks such as use of
toxic solvents[8] and catalysts,[13] cumbersome product isolation procedures, strong
acidic conditions,[10] long reaction times,[8,9] use of expensive reagents, poor yields of
products,[8] special apparatus, requirement of a large or stoichiometric amount of
catalysts,[19] and drastic conditions for catalyst preparation.[16] Thus, development
of new benign protocols using simple operating conditions is still required.
Recently, we have been involved in the study of the catalytic activity of
ruthenium in organic reactions such as condensation of indoles and aldehydes,[23]
oxidation of aromatic and heteroaromatic compounds,[24] double-conjugate
1,4-addition to enones,[25] nucleophilic addition to epoxides,[26] oxidative trimeriza-
tion of indoles,[27] and cross-aldol reactions.[28] As a matter of fact, many organic
transformations, which involve ruthenium species as catalyst, are known and well
documented.[29–31]
We found that ruthenium chloride hydrate smoothly acts as a homogeneous
catalyst in the double addition of b-naphthol to aldehydes, providing the desired
xanthene derivatives in excellent yields under mild reaction conditions and using a
relatively easy workup (Scheme 1).
RESULTS AND DISCUSSION
Typical results of the ruthenium-catalyzed condensation of b-naphthol and
aldehydes are shown in Table 1. In procedure optimized with respect to the catalyst
concentration, choice of solvent, temperature, and time, the treatment of benzal-
dehyde (1 mmol) with b-naphthol (2 mmol) in the presence of RuCl3 ꢀ nH2O catalyst
(10 mol%) in ethanol (5 mL) at 100 ꢁC for 2 h gave the corresponding dibenzox-
anthene as a solid precipitate, which was easily purified (product 3a, 90% yield).
As shown in Table 1, the present protocol works with a wide range of aromatic
aldehydes bearing electron-withdrawing and electron-donating groups. All reactions
were completed within 1–4 h, as indicated. Although much effort has been made to
exclude the reaction solvent or to use environmentally friendly solvents in the reaction
step, less attention has been paid to the high amounts of organic solvent waste in the
purification step in the reported methods.[13,16,22] The unique feature of the reaction is
that most of the corresponding dibenzoxanthene derivatives are insoluble in the reac-
tion solvent, and this leads to an easy workup along with excellent yields (80–94%).

SYNTHESIS OF 14-SUBSTITUTED-14-H-DIBENZO[a,j]XANTHENES
1429
Table 1. RuCl3 ꢀ nH2O-catalyzed synthesis of 14-substituted-14-H-dibenzo[a,j]xanthenes
Mp (ꢁC)
Entry
1
Aldehyde
Time (h)
2
Product
3a
Yield (%)a
90b
Found
Reported
181–183
180–182
2
3
4
1
2
3b
3c
3d
92c
92b
94d
326–328
308–310
298–300
324
300–302
297–299
1.5
5
6
7
2
3e
3f
86e
90b
86c
218–220
255–258
212–214
214–216
253–255
212–220
1.5
1.5
3g
8
9
2.5
3
3h
3i
87f
90b
85f
230–232
215–218
140–142
229
213–215
140
10
2.5
3j
11
3
3k
89
208–210
–
–
12
13
4
3l
85
260–265
150–152
CH3CH2CHO
3.5
3m
82g
152
(Continued )

1430
K. TABATABAEIAN ET AL.
Table 1. Continued
Mp (ꢁC)
Entry
Aldehyde
Time (h)
Product
Yield (%)a
Found
Reported
14
15
16
CH3CH2CH2CHO
(CH3)2CHCHO
(CH3)2CHCH2CHO
4
3
4
3n
3o
3p
85e
80h
84i
153–154
154–156
110–112
152–154
155–157
112–113
Note. All products were characterized by 1H NMR, 13C NMR, and IR data.
aIsolated yields.
bIdentified by comparison with authentic samples.[20]
cIdentified by comparison with authentic samples.[21]
dIdentified by comparison with authentic samples.[22]
eIdentified by comparison with authentic samples.[16]
fIdentified by comparison with authentic samples.[17]
gIdentified by comparison with authentic samples.[8]
hIdentified by comparison with authentic samples.[14]
iIdentified by comparison with authentic samples.[19]
Scheme 2. Plausible mechanism for the synthesis of xanthene in the presence of RuCl3 ꢀ nH2O as catalyst.

SYNTHESIS OF 14-SUBSTITUTED-14-H-DIBENZO[a,j]XANTHENES
1431
In many cases, simple filtration of the reaction mixture and washing with cold ethanol
provided spectroscopically pure products. It was also observed that the electronic
properties of the aromatic ring have an effect on the rate of this nucleophilic addition
reaction. The rate is accelerated if an electron-withdrawing group is present on the
aromatic ring. With regard to the aliphatic aldehydes, however, the present protocol
showed longer reaction times and lower yields (Table 1, entries 13–16).
A plausible mechanism for the reaction under ruthenium catalysis is outlined in
Scheme 2. As it is shown, prior activation of the carbonyl group of aldehyde by RuIII
to give intermediate 1, followed by a nucleophilic attack from C1 of b-naphthol,
provides intermediate 2, which serves as an electrophile, ready for attack by a second
molecule of b-naphthol. After loss of water, bis(naphtholyl)methane species (3)
undergo dehydration to afford the desired product (4).
CONCLUSION
In summary, we have developed a simple, convenient, and efficient synthetic
protocol for the double addition of b-naphthol to aldehydes. To the best of our
knowledge, this is the first report on the use of RuIII salts as catalyst for the prep-
aration of 14-substituted-14-H-dibenzo[a,j]xanthene derivatives.
This method offers several significant advantages such as simplicity of the reac-
tion procedure, short reaction times, low catalyst loading, exellent yields of products,
use of ethanol as an ecofriendly solvent, efficiency, and mild reaction conditions.
Further manipulation of this reaction is under way.
EXPERIMENTAL
All products were characterized by physical data (mp), spectral data [Infrared
(IR), 1H NMR, 13C NMR] and elemental analysis. IR spectra were obtained on a Shi-
1
madzu Fourier transform (FT)–IR-8400S spectrometer. H NMR spectra were mea-
sured on a Bruker DRX-500 Avance spectrometer, and 13C NMR spectra were
obtained on a Bruker DRX-125 Avance spectrometer. Chemical shifts of 1H and 13C
NMR spectra were expressed in parts per million (ppm) downfield from tetramethyl-
silane (TMS) as internal standard. Melting points were measured on a Buchi melting-
¨
point B-540 instrument and are uncorrected. Elemental analyses were made by a
Carlo-Erba EA1110 CNNO-S analyzer and results agreed with the calculated values.
Materials
b-Naphthol, aldehydes, RuCl3 ꢀ nH2O, and solvents were purchased from
Merck and used without further purification.
Ruthenium-Catalyzed Condensation of b-Naphthol and Benzaldehyde
A mixture of b-naphthol (288.34 mg, 2 mmol), benzaldehyde (106.13 mg,
1 mmol), and RuCl3 ꢀ nH2O (21.43 mg, 0.1 mmol) in ethanol (5 mL) was refluxed in
an oil bath at 100 ꢁC for 2 h and yielded a solid precipitate. The progress of the reaction
was monitored by thin-layer chromatography (TLC). After completion of the

1432
K. TABATABAEIAN ET AL.
reaction, as indicated by TLC, simple filtration of the reaction mixture and washing
with ethanol provided a pure product (322.62 mg, 90%). The same procedure was also
used for the other products listed in Table 1. In the case of entries 10–12 in Table 1,
purification of the reaction mixture by preparative TLC (petroleum ether–ethyl acet-
ate, 10:3) provided pure products.
Selected Characterization Data
14-(3,4-Dimethoxyphenyl)-14-H-dibenzo[a,j]xanthene (3k). Solid; mp
208–210 ꢁC, IR (KBr): n (cmꢂ1); 3060, 2920, 1620, 1595, 1505, 1457, 1260, 1245,
1
1210, 1130, 1020, 810, 740; H NMR [500 MHz, (CD3)2SO, 25 ꢁC] d ¼ 8.1 (d, 2H,
J ¼ 8.5 Hz), 7.72 (d, 2H, J ¼ 7.8 Hz), 7.65 (d, 2H, J ¼ 8.6 Hz), 7.23 (t, 2H, J ¼ 7.1 Hz),
7.17 (t, 2H, J ¼ 7.2 Hz), 7.10 (d, 2H, J ¼ 8.6 Hz), 6.96 (s, 1H), 6.73 (d, 1H,
J ¼ 8.1 Hz), 6.72 (s, 1H), 6.44 (d, 1H, J ¼ 8.1 Hz), 3.68 (s, 3H), 3.50 (s, 3H) ppm;
13C NMR [125 MHz, (CD3)2SO, 25 ꢁC] d ¼ 149.30, 147.69, 137.14, 135.11, 129.34,
129.25, 129.22, 126.91, 126.63, 124.48, 122.73, 121.21, 120.79, 120.28, 113.58,
112.20, 56.34, 56.27, 41.92 ppm. Anal. calcd. for C29H22O3: C, 83.23; H, 5.30. Found:
C, 83.29; H, 5.32.
14-(Fluorene-2-yl)-14-H-dibenzo[a,j]xanthene (3l). Solid; mp 260–265 ꢁC,
IR (KBr): n (cmꢂ1); 3070, 2900, 1590, 1510, 1457, 1430, 1400, 1250, 1240, 815,
1
765, 740; H NMR [500 MHz, (CD3)2SO, 25 ꢁC] d ¼ 8.72 (d, 2H, J ¼ 8.4 Hz),
7.91–7.93 (m, 4H), 7.83 (d, 1H, J ¼ 7.7 Hz), 7.61–7.65 (m, 5H), 7.59 (d, 2H,
J ¼ 8.8 Hz), 7.44 (t, 2H, J ¼ 7.3 Hz), 7.37 (d, 1H, J ¼ 7.2 Hz), 7.22 (t, 1H, J ¼ 7.2 Hz),
7.16 (t, 1H, J ¼ 7.2 Hz), 6.77 (s, 1H), 3.63 (s, 2H) ppm; 13C NMR [125 MHz,
(CD3)2SO, 25 ꢁC] d ¼ 148.85, 145.20, 144.21, 143.80, 141.28, 140.27, 131.82,
131.54, 129.87, 129.45, 127.78, 127.72, 127.40, 125.73, 125.38, 124.34, 120.55,
120.42, 118.63, 118.38, 37.55, 37.02 ppm. Anal. calcd. for C34H22O: C, 91.45; H,
4.97. Found: C, 91.40; H, 5.0.
Supplementary data are also available for the other products listed in Table 1.
ACKNOWLEDGEMENT
Support of this study by Research Council of Guilan University is gratefully
acknowledged.
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