Synthesis and luminescence of 14-aryl-or alkyl-14h-dibenzo[a,j]xanthenes catalyzed by 2-1'-methylimidazolium-3-yl-1-ethyl sulfate

Article abstract of DOI:10.1080/00397910902838854

A facile and efficient synthesis of 14-aryl or alkyl-14H-dibenzo[a,j] xanthenes has been accomplished by treatment of-naphthol with aryl or alkyl aldehydes catalyzed by 2-1'-methylimidazolium-3-yl-1-ethyl sulfate in ionic liquid [BMIM][BF4] at 80C. The lu

Full text of DOI:10.1080/00397910902838854

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Synthesis and Luminescence
of 14-Aryl- or Alkyl-14H-
dibenzo[a,j]xanthenes
Catalyzed by 2-1′-
Methylimidazolium-3-yl-1-ethyl
Sulfate
Hui Wu a b , Xiu-Mei Chen a , Yu Wan a b , Hai-Qiang
Xin a , Hua-Hong Xu a , Cai-Hui Yue a , Li-Ling Pang a
& Rui Ma a
a School of Chemistry and Chemical Engineering,
Xuzhou Normal University, Jiangsu, China
b Key Laboratory of Biotechnology on Medical Plant
of Jiangsu Province, Jiangsu, China
Published online: 07 Oct 2009.
To cite this article: Hui Wu , Xiu-Mei Chen , Yu Wan , Hai-Qiang Xin , Hua-Hong Xu ,
Cai-Hui Yue , Li-Ling Pang & Rui Ma (2009) Synthesis and Luminescence of 14-Aryl- or
Alkyl-14H-dibenzo[a,j]xanthenes Catalyzed by 2-1′-Methylimidazolium-3-yl-1-ethyl
Sulfate, Synthetic Communications: An International Journal for Rapid Communication
of Synthetic Organic Chemistry, 39:21, 3762-3771, DOI: 10.1080/00397910902838854
To link to this article: http://dx.doi.org/10.1080/00397910902838854
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Synthetic Communications1, 39: 3762–3771, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902838854
Synthesis and Luminescence of 14-Aryl-
or Alkyl-14H-dibenzo[a,j]xanthenes Catalyzed by
2-10-Methylimidazolium-3-yl-1-ethyl Sulfate
Hui Wu,1,2 Xiu-mei Chen,1 Yu Wan,1,2 Hai-qiang Xin,1
Hua-hong Xu,1 Cai-hui Yue,1 Li-ling Pang,1 and Rui Ma1
1School of Chemistry and Chemical Engineering,
Xuzhou Normal University, Jiangsu, China
2Key Laboratory of Biotechnology on Medical Plant of Jiangsu
Province, Jiangsu, China
Abstract: A facile and efficient synthesis of 14-aryl or alkyl-14H-dibenzo[a,j]
xanthenes has been accomplished by treatment of b-naphthol with aryl or alkyl
aldehydes catalyzed by 2-10-methylimidazolium-3-yl-1-ethyl sulfate in ionic liquid
[BMIM][BF4] at 80ꢀC. The luminescence of products was studied in detail.
Keywords: Luminescence, 2-10-methylimidazolium-3-yl-1-ethyl sulfate, synthesis,
xanthenes
INTRODUCTION
The synthesis of xanthenes, especially benzoxanthenes, has received
significant attention because they possess diverse biological and
therapeutic properties, such as antiviral,[1] antibacterial,[2] and anti-
inflammatory[3] activities. Furthermore, these compounds have emerged
as sensitizers in photodynamic therapy[4] and are used as leuco-dyes[5]
in laser technology and pH-sensitive fluorescent materials for the
visualization of biomolecular assemblies.[6]
Received October 26, 2008.
Address correspondence to Hui Wu, School of Chemistry and Chemical
Engineering, Xuzhou Normal University, Jiangsu 221116, China. E-mail: wuhui72@
yahoo.com.cn
3762

14-Aryl- or Alkyl-14H-dibenzo[a,j]xanthenes
3763
Scheme 1. Synthesis of 14-aryl or alkyl-14H-dibenzo[a,j]xanthenes.
Various methods are available for the preparation of benzoxanthenes
involving the reaction of b-naphthol with formamide, 2-naphthol-1-
methanol,[7] or carbon monoxide.[8] However, these methods have some
drawbacks such as poor yields, prolonged reaction times, toxic organic
solvents, excess reagents=catalysts, and harsh reaction conditions. Some
catalysts such as p-toluenesulfonic acid (p-TSA),[9] sulfamic acid,[10] AcOH=
H2SO4,[11] iodine,[12,13] K5CoW12O40 ꢁ 3H2O,[14] cyanuric chloride,[15]
LiBr,[16] HClO4 ꢁ SiO2,[17] KAl(SO4)2 ꢁ 12H2O,[18] silica sulfuric acid,[19]
[20]
and Yb(OTf)3
have been used to improve the condensation of
b-naphthol with aldehydes. Even now, searching for new catalysts and
green solvents for this reaction is still highly desirable.
2-10-Methylimidazolium-3-yl-1-ethyl sulfate, which was synthesized
in our laboratory, is a stable, nonvolatile, cheap, and safe reagent.
Here we report a simple and facile synthesis of 14-aryl- or alkyl-
14H-dibenzo[a,j]xanthenes by treatment of b-naphthol with aryl or
alkyl aldehydes in ionic liquid 1-n-butyl-3-methyl-imidazolium tetrafluor-
oborate ([BMIM][BF4]) catalyzed by 2-10-methylimidazolium-3-yl-1-ethyl
sulfate (Scheme 1).
RESULTS AND DISCUSSION
Synthesis of 14-Aryl or Alkyl-14H-dibenzo[a,j]xanthenes
In our initial endeavor, the reaction of 4-chlorobenzaldehyde
(1.0 mmol), b-naphthol (2.0 mmol), and catalyst 2-10-methylimidazolium-
3-yl-1-ethyl sulfate were carried out in [BMIM][BF4] at 80ꢀC for the given
time to screen optimum reaction conditions (Table 1). Among the eight
tested solvents, such as methanol, ethanol, dimethylformamide (DMF),
CH3CN, tetrahydrofuren (THF), 1,2-dichloroethane, [BMIM][BF4], and
[BPY][BF4], the best result was obtained in [BMIM][BF4], which may
be attributed to its strong basicity and solubility (entries 1–8). An increase

3764
H. Wu et al.
Table 1. Optimization of the reaction between 4-chlorobenzal-
dehyde and b-naphthola
Amount of
catalyst (mol %)
Time
(h)
Yieldb
(%)
Entry
Solvent
1
2
3
4
5
6
7
8
9
MeOH
EtOH
DMF
CH3CN
THF
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.04
0.06
0.10
0.12
12
12
16
12
12
12
8
6
6
6
6
68
56
Nrc
37
58
72
79
89
63
76
82
78
ClCH2CH2Cl
[BPY][BF4]d
[BMIM][BF4]
[BMIM][BF4]
[BMIM][BF4]
[BMIM][BF4]
[BMIM][BF4]
10
11
12
6
aReaction conditions: 4-chlorobenzaldehyde (1.0 mmol), b-
naphthol (2.0 mmol), and catalyst 2-10- methylimidazolium-
3-yl-1-ethyl sulfate (0.08 mol%) in [BMIM][BF4] at 80ꢀC.
bYield of isolated products.
cNo reaction.
d[BPY] ¼ 1-n-butylpridinum.
in the quantity of catalyst from 0.04mol% to 0.08mol% improved the
product yield from 63% to 89% (entries 8–10), which showed that the
catalyst concentration plays a major role in the optimization of the pro-
duct yield. However, when an excess amount (0.12mol%) of catalyst was
used in the reaction, the product yields unexpectedly decreased to 72%
(entries 11 and 12); thus 0.08mol% catalyst is the best choice for opti-
mum yield of compound 3.
In our initial experiments, we found that aldehydes 1 (1.0 mmol)
and b-naphthol 2 (2.0 mmol) in [BMIM][BF4] were stirred in the
presence of a catalytic amount (0.08 mol%) of 2-10-methylimidazolium-
3-yl-1-ethyl sulfate at 80ꢀC for 5–8 h to give compound 3 with good
yields. As shown in Table 2, compared with the previous meth-
ods,[11,16,20] this method takes less reaction time with about equal yields,
which indicated the efficient catalysis of this new catalyst as a strong
acid. Aromatic aldehydes bearing both electron-rich and electron-
deficient groups worked well. However, fatty aldehydes gave some-
what lower yields than aromatic aldehydes as a result of their higher
volatility.

3765

3766
H. Wu et al.
Figure 1. Single-crystal x-ray structure of 2-10-methylimidazolium-3-yl-1-ethyl sulfate.
Luminescence Properties of 14-Aryl or Alkyl-14H-dibenzo[a,j]xanthenes
The luminescence properties of products are shown in Fig. 2 and Table 3.
From Table 3, we can clearly see that (1) compared with the fluorescence
emission of b-naphthol at 352 nm, the emission of all products moved to
Figure 2. Fluorescent emission spectrum of 14-aryl or alkyl-14H-dibenzo[a,j]
xanthenes. (A-b-naphthol; 1–14: 3b, 3a, 3c, 3d, 3e 3n, 3h, 3f, 3j, 3o, 3g, 3i, 3m, 3k).

14-Aryl- or Alkyl-14H-dibenzo[a,j]xanthenes
Table 3. Luminescence of products
3767
Emax (abs) kmax(ex) kmax(em)
(nm)
Stokes
RFIa (nm) UF
b
Compound
(Mꢂ1ꢁ cmꢂ1) (nm)
9,10-Diphenylanthracence
b-naphthol (A)
2.221
0.067
0.056
0.085
0.055
2.236
1.978
2.294
2.291
1.653
2.067
2.353
2.077
2.235
2.173
2.231
261
231
254
254
254
254
254
254
253
254
254
255
252
255
253
254
408
9989 147 0.660
3191 121 0.006
352
307
308
308
307
308
308
307
307
308
307
308
307
308
308
3a
3b
3c
3d
3e
3f
8576
6714
8650
8838
9033
9397
9583
9265
9705
9530
10000
9929
9190
9535
53 0.014
54 0.018
54 0.014
53 0.588
54 0.532
54 0.641
54 0.653
53 0.456
54 0.597
52 0.634
56 0.618
52 0.660
55 0.594
54 0.633
3g
3h
3i
3j
3k
3m
3n
3o
aRelative fluorescence intensity.
bRelative fluorescence quantum yield.
about 308 nm (Fig. 2); the blue-shifted kmax(ex) of products means that
they can emit fluorescence easier than b-naphthol. Meanwhile, the OH
group of b-naphthol joined the reaction to give a different framework
with b-naphthol. The stronger fluorescence emission of products than
that of b-naphthol showed the formation of a longer conjugated system
in the skeleton of products. (2) The electro and steric effects of the sub-
stituted group on aromatic aldehydes have little influence on the fluores-
cence properties, which clearly showed that the luminescence of products
come from their framework directly. (3) Besides 3a, 3b, and 3c, other pro-
ducts exhibited stronger fluorescence in ethanol (EtOH) with the about
equal fluorescence quantum yield to that of 9,10-diphenylanthracence
(UF ¼ 0.660, EtOH), which showed their potential application as new
fluorescent probes or luminescence material.
EXPERIMENTAL
General Procedure
All reagents were purchased from commercial sources and used without
further purification. Thin-layer chromatographic (TLC) analysis was

3768
H. Wu et al.
performed with glass-backed plates precoated with silica gel and
examined under ultraviolet (UV) light (254 nm). NMR spectra were
measured in CDCl3 with Me4Si as the internal standard on a Bruker
Advance DPX-400 instrument at room temperature. Infared (IR) spectra
were recorded on a Bruker Fourier transform (FT)-IRspectrometer;
absorbance are reported in centimetersꢂ1. Fluorescence properties were
reported on
a F-4500 fluorescence spectrophotometer (Hitachi,
Japan). The absorption spectrum was measured with a Tu-1201 uv
spectrophotometer (Shimadzu).
General Procedure for the Preparation
of 2-10-Methylimidazolium-3-yl-1-ethyl Sulfate
1-Methylimidazole (0.3 mol) and chloroethanol (0.3 mol) were added in a
flask containing 10 mL of CHCl3; the mixture was refluxed for 8 h.
CHCl3 was removed under vacuum and the unreacted 1-methylimidazole
was extracted with ether to give 1-methyl-3-(2-hydroxylethyl)imidazo-
lium chloride. Then chlorosulfonic acid was added dropwise at 0–5ꢀC;
when the mixture became solid, a suitable single crystal was obtained
after 3 days by recrystallization with dimethylformamide (DMF). Its
structure was further confirmed by x-ray crystallographic analysis
(Fig. 1). This solid inner salt reacted with chlorosulfonic acid continu-
ously until it became a ropy liquid to afford acidic ionic liquid 2-10-
methylimidazolium-3-yl-1-ethyl sulfate.
General Synthesis of 14-Aryl or Alkyl-14H-dibenzo[a,j]xanthenes
The reaction mixture of aldehyde (1.0 mmol) and b-naphthol (2.0 mmol)
in [BMIM][BF4] (2.0 mL) and 2-10-methylimidazolium-3-yl-1-ethyl
sulfate (0.08 mol%) was stirred at 80ꢀC for the given time (Table 2). After
completion of the reaction, as indicated by TLC, the reaction mixture
was cooled to room temperature, filtered, washed with water, and
recystallized from EtOH and DMF to afford the pure product.
General Preparation Solution of 14-Aryl or Alkyl-14H-
dibenzo[a,j]xanthenes
The EtOH solution of 9,10-diphenylanthracence, b-naphthol, and 3a–o
(1.0 ꢃ 10ꢂ6 mol=L) were measured on an uv and fluorescence spectro-
photometer to give their absorption and emission spectra, respectively.

14-Aryl- or Alkyl-14H-dibenzo[a,j]xanthenes
3769
Spectral Data of New Products
14-(Benzo[d][1,3]dioxol-5-yl)-14H-dibenzo[a,j]xanthene (3n)
1H NMR (400 MHz, CDCl3) d: 8.36 (d, J ¼ 8.4 Hz, 2H), 7.78–7.84 (m,
4H), 7.58 (t, J ¼ 7.6 Hz, 2H), 7.40–7.48 (m, 4H), 7.10 (d, J ¼ 8.0 Hz,
1H), 6.87 (s, 1H), 6.61 (d, J ¼ 8.4 Hz, 1H), 6.42 (s, 1H), 5.74 (s, 2H);
13C NMR (100 MHz, CDCl3) d: 148.67, 147.87, 145.95, 139.06, 131.36,
131.07, 128.85, 126.82, 124.28, 122.65, 121.23, 118.05, 117.27, 108.87,
107.79, 100.84, 37.64; IR (KBr, n, cmꢂ1): 3005, 1621, 1592, 1265.
14-(4-Iodophenyl)-14H-dibenzo[a,j]xanthene (3o)
1H NMR (400 MHz, CDCl3) d: 8.30 (d, J ¼ 8.4 Hz, 2H), 7.78–7.84 (m, 4H),
7.57 (t, J ¼ 8.0 Hz, 2H), 7.40–7.48 (m, 6H), 7.27 (d, J ¼ 2.0 Hz, 2H), 6.43 (s,
1H); 13C NMR (100 MHz, CDCl3) d: 148.68, 144.68, 137.55, 131.25,
131.06, 130.18, 129.12, 128.92, 126.94, 124.39, 122.39, 118.02, 116.62,
113.22, 91.81, 37.60; IR (KBr, n, cmꢂ1): 3071, 1620, 1591, 1239.
Crystal Data for 2-10-Methylimidazolium-3-yl-1-ethyl Sulfate
The single-crystal growth was carried out in DMF at room temperature.
Crystal data: Empirical formula C6H10N2O4S, yellow, crystal dimension
0.48 ꢃ 0.46 ꢃ 0.43 mm, orthorhombic, space group P2(1)2(1)2(1), a ¼ 7.4262
ꢀ
ꢀ
˚
˚
˚
˚
(13) A, b ¼ 10.0243 (15) A, c ¼ 12.0997 (18) A, a¼90.00 , b¼90.00 ,
3
3
ꢀ
c¼90.00 , V ¼ 900.7 (2) A , Mr ¼ 206.22, Z ¼ 4, Dc ¼ 1.521 Mg=m ,
(Mo Ka) ¼ 0.345 mmꢂ1
,
F(000) ¼ 432, S ¼ 1.089,
˚
k ¼ 0.71073 A,
m
R ¼ 0.0328, and wR ¼ 0.0795. Crystallographic data for the structures
of 2-10-methylimidazolium-3-yl-1-ethyl sulfate reported in this letter have
been deposited with the Cambridge Crystallographic Date Centre as sup-
plementary publication no. CCDC-655836.
CONCLUSION
In summary, we have discovered that a novel and highly efficient catalyst
2-10-methylimidazolium-3-yl-1-ethyl sulfate, can promote the condensa-
tion reaction of aldehydes and b-naphthol to construct 14-aryl- or
alkyl-14H-dibenzo[a,j]xanthenes in [BMIM][BF4]. Furthermore, the
screening for optical properties showed that some products may have a
good future as new luminescence material’s or fluorescence probes.

3770
H. Wu et al.
ACKNOWLEDGMENTS
We are grateful to the National Natural Sciences Foundation of China
(No. 20772103), the Natural Science Foundation in Jiangsu Province
(No. BK2007028), the Surpassing Project in Jiangsu Province (No.
QL200607), the Postgraduate Creative Project in Jiangsu Province (No.
CX07S-0162), and the Postgraduate Scientific Research Project
Foundation of Xuzhou Normal University (No. 08YLB029) for financial
support.
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