One-pot synthesis of N, N, N, N-tetraethyl-9H-xanthene-3,6-diamine and its conversion to pyronine B

Article abstract of DOI:10.1016/j.dyepig.2012.03.034

N, N, N, N-tetraethyl-9H-xanthene-3,6-diamine (9H-xanthene) was prepared by one-pot reaction of 3-diethylaminophenol with (CH₃O)₃CH using pyridinium para-toluenesulfonate (PPTS) as catalyst. It was found that 9H-xanthene is photo-active and could be efficiently photo-converted to pyronine B. Treatment of 9H-xanthene with acid (HCl) produced protonated 9H-xanthene (9H-xanthene·HCl), and 9H-xanthene·HCl was spontaneously converted to pyronine B in aqueous solution. Besides, both distinct color change and turn on fluorescence were observed during the conversion. These findings not only provide a new and simple approach to the preparation of 9H-xanthene and pyronine B but also benefit the applications such as indicator and probe.

Full text of DOI:10.1016/j.dyepig.2012.03.034

Dyes and Pigments 96 (2013) 59e62
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
One-pot synthesis of N, N, N, N-tetraethyl-9H-xanthene-3,6-diamine and its
conversion to pyronine B
1
*
Shulei Han , Yi Chen
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
N, N, N, N-tetraethyl-9H-xanthene-3,6-diamine (9H-xanthene) was prepared by one-pot reaction of 3-
diethylaminophenol with (CH3O)3CH using pyridinium para-toluenesulfonate (PPTS) as catalyst. It was
found that 9H-xanthene is photo-active and could be efficiently photo-converted to pyronine B.
Treatment of 9H-xanthene with acid (HCl) produced protonated 9H-xanthene (9H-xanthene$HCl), and
9H-xanthene$HCl was spontaneously converted to pyronine B in aqueous solution. Besides, both
distinct color change and turn on fluorescence were observed during the conversion. These findings not
only provide a new and simple approach to the preparation of 9H-xanthene and pyronine B but also
benefit the applications such as indicator and probe.
Received 14 November 2011
Received in revised form
19 January 2012
Accepted 12 March 2012
Available online 1 August 2012
Keywords:
N, N, N, N-tetraethyl-9H-xanthene-3,6-
diamine
Ó 2012 Elsevier Ltd. All rights reserved.
Pyronine B
Synthesis
One-pot
Photo-conversion
Fluorescent dye
1. Introduction
diethylaminophenol with formaldehyde solution in the presence
of concentrated hydrochloric acids (HCl and H2SO4) and the
Photo-active molecules have currently attracted considerable
attention because of wide potentials in chemistry [1e5],
biochemistry [6e10], environmental science [11e14] and material
science [15e19]. Integrating an external stimulus to regulate targets
affords a means to control chemical processes and modulate the
behavior of functional materials containing them. Light is a partic-
ularly effective stimulus to spatially and temporally trigger changes
in structure and function of molecules and materials [20e24].
Xanthene dyes are one class of important organic compounds
[25e29] and are extensively employed in both foundational
research [30e35] and applications [36e39]. Pyronines, best known
as pyronine B and pyronine Y, are commercial and widely used in
functional dye industry [40e43]. Recently, a class of N, N, N, N-
tetrasubstituted xanthylium salts (pyronines) has been reported
[44] by Clunas et al., they found that these compounds showed
biological properties and could be used as drugs in the treatment of
tauopathies such as Alzheimer’s disease.
oxidation of 9H-xanthene to pyronine B in acidic condition with
oxidizing reagent. A large amount of strong acids (H2SO4 and HCl)
and strict reaction condition (temperature of reaction is over
140 ꢀC) are the chief drawback of the approach.
In this paper, we report a new approach to the preparation of
9H-xanthene and pyronine B. 9H-xanthene is obtained in 25%
isolated yield by one-pot reaction of 3-diethylaminophenol with
trimethyl orthoformate using PPTS (0.1 equiv) as catalyst, and
pyronine B is obtained in 80% isolated yield by the photo-
conversion of 9H-xanthene in acidic EtOH solution. Besides, it is
found that treatment of 9H-xanthene with acid (HCl) produces
protonated 9H-xanthene (9H-xanthene$HCl), which can be
spontaneously converted to pyronine B in aqueous solution. The
one-pot synthesis of 9H-xanthene and its conversion to pyronine
B are outlined in Fig. 1.
2. Experimental
9H-xanthene and pyronine B are generally synthesized [44] by
a
two-step reaction including the condensation of 3-
2.1. General
1H and 13C NMR spectra were recorded at 400 and 100 MHz,
respectively, with TMS as an internal reference. MS spectra were
recorded with TOC-MS and MALDI-MS spectrometer, respectively.
* Corresponding author. Tel.: þ86 10 8254 3595; fax: þ86 10 6487 9375.
E-mail address: yichencas@yahoo.com.cn (Y. Chen).
Tel.: þ86 10 8254 3595; fax: þ86 10 6487 9375.
1
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.03.034

60
S. Han, Y. Chen / Dyes and Pigments 96 (2013) 59e62
J ¼ 9.3 Hz, 2H, ArH), 7.21 (dd, J ¼ 9.3, 2.2 Hz, 2H, ArH), 6.91 (d,
J ¼ 1.8 Hz, 2H, ArH), 3.67 (q, J ¼ 7.0 Hz, 8H, CH2), 1.22 (t, J ¼ 7.0 Hz,
12H, CH3). 13C NMR (100 MHz, DMSO):
d
¼ 159.3, 157.4, 147.4, 135.3,
116.1, 115.5, 97.6, 47.2, 14.2. HRMS (EI) calcd for C21H27ClN2O (Mþ):
358.18. Found: 358.18.
Method B. The conversion of 9H-xanthene$HCl to pyronine B
was carried out in a glass beaker of 500 ml. The solution of 9H-
xanthene$HCl (60 mg) in water (100 ml, 1.5 ꢁ 10ꢂ3 mol/L) was kept
in the dark at ambient temperature for 3 h, TLC test showed that
starting material 9H-xanthene$HCl was completely converted to
pyronine B. After evaporation of the water under reduced pressure,
the product was purified by flash column (elute: dichloromethane/
methol ¼ 20: 1, v/v) to afford pyronine B (44 mg) in 78% isolated
yield. The structure of pyronine B was confirmed by both 1H NMR,
13C NMR and HRMS spectra.
3. Results and discussion
3.1. Synthesis of 9H-xanthene
9H-Xanthene was prepared by one-pot reaction of 3-
diethylaminophenol with (CH3O)3CH (Fig. 1) using the experi-
mental conditions described in Experimental Section. The reaction
was performed at 80 ꢀC (refluxed) under atmosphere of N2 in the
presence of PPTS as catalyst, proceeding smoothly to give 9H-
xanthene in 25% isolated yield after column chromatography with
about 30% starting material 3-diethylaminophenol recovered. It is
worth noting that PPTS plays a key role in the synthesis of 9H-
xanthene. As presented in Table 1, no 9H-xanthene was obtained
without PPTS or the amount of PPTS was more than 0.5 equiv. and
the optimum condition for the reaction was with 0.1 equiv. of PPTS,
in this case, 9H-xanthene was obtained in best yield (25%). Besides,
it was found that the amount of (CH3O)3CH had little affected the
synthesis of 9H-xanthene when the amount of (CH3O)3CH is over
2.0 equiv.
Fig. 1. One-pot synthesis of 9H-xanthene and its conversion to pyronine B.
UV absorption spectra and fluorescence spectra were measured
with an absorption spectrophotometer (Hitachi U-3010) and
a
fluorescence spectrophotometer (F-2500), respectively. All
chemicals for synthesis were purchased from commercial
suppliers, and solvents were purified according to standard
procedures. Reaction monitored by TLC silica gel plates (60F-254).
Column chromatography was performed on silica gel (Merck,
70e230 mesh). A 365 nm lamp (36 W) and a xenon light (500 W)
were used as light sources for detection and preparation,
respectively.
3.2. Photo-conversion of 9H-xanthene to pyronine B
2.2. Chemical
Photo-conversion of 9H-xanthene to pyronine B was per-
formed in EtOHeHCl solution. Irradiating the solution of 9H-
xanthene in EtOHeHCl (1.5 ꢁ 10ꢂ3 mol/L, EtOH/HCl ¼ 100: 2, v/v)
under a high-pressure Hg lamp (500 W, exposure: 0.12 W) for
30 min, the photo-conversion reaction was completed, pyronine B
was obtained in 80% isolated yield after column chromatography.
The structure of pyronine B was confirmed by both NMR (1H NMR
and 13C NMR) and HRMS spectroscopy. Control experiment
confirmed that the photo-conversion was readily performed in
polar solvents such as MeOH, EtOH, CH2Cl2, CH3CN, but was
inhibited in non-polar solvents such as benzene, toluene, and
cyclohexane. Besides, control experiment also confirmed that the
Synthesis
of
9H-xanthene.
To
a
solution
of
3-
diethylaminophenol (1.65 g, 10 mmol) in 1,2-dichloroethane
(70 ml) was added trimethyl orthoformate (2.0 g, 2.1 mmol) and
pyridinium para-toluenesulfonate (PPTS) (0.25 g, 1.0 mmol). The
mixture was refluxed under N2 for 12 h and then cooled down to
ambient temperature. After evaporation of the solvent, the crude
product was purified by flash column chromatography (elute:
petroleum ether/ethyl acetate ¼ 35: 1, v/v) to afford 9H-xanthene
(0.4 g) in 25% isolated yield. 1H NMR (400 MHz, CDCl3,
d): 6.97 (d,
J ¼ 8.3 Hz, 2H, ArH), 6.42e6.33 (m, 4H, ArH), 3.83 (s, 2H, eCH2e),
3.33 (q, J ¼ 7.1 Hz, 8H, CH2), 1.16 (t, J ¼ 7.1 Hz, 12H, CH3). 13C NMR
(100 MHz, CDCl3):
d
¼ 153.0, 147.8, 129.6, 108.1, 107.4, 99.7, 44.7,
26.1, 12.8. HRMS (EI) calcd for C21H28N2O (Mþ): 324.2274. Found:
324.2271.
Table 1
Optimum reaction for the synthesis of 9H-xanthene.a
2.3. Preparation of pyronine B
PPTS (equiv)
(CH3O)3CH (equiv)
Yield of 9H-xanthene (%)
0
2
2
2
2
2
2
1
4
0
5
25
20
5
0
4
24
Method A. Photo-synthesis of pyronine B was carried out with
a 500-W high-pressure mercury lamp (exposure: 39. 5 mW/cm2) in
a glass beaker of 500 ml. The solution of 9H-xanthene (50 mg) in
EtOHeHCl (100 ml, 1.5 ꢁ 10ꢂ3 mol/L, EtOH/HCl ¼ 100: 2, v/v) was
stirred under the light irradiation till no starting material was
detected by TLC plate (irradiation time about 1 h). After evaporation
of the solvent, the product was purified by flash column (elute:
dichloromethane/methol ¼ 20: 1, v/v) to afford pyronine B (45 mg,
0.05
0.1
0.2
0.5
1.0
0.1
0.1
a
The reaction condition for the synthesis of 9H-xanthene: 3-diethylaminophenol
(1.0 equiv) with (CH3O)3CH using PPTS as catalyst in 1,2-dichloroethane, the
mixture solution was refluxed for 10 h.
80%). 1H NMR (400 MHz, DMSO,
d): 8.75 (s, 1H, ]CH), 7.87 (d,

S. Han, Y. Chen / Dyes and Pigments 96 (2013) 59e62
61
oxygen in the air had not significantly influenced on the photo-
conversion, and the photo-conversion of 9H-xanthene to pyro-
nine B could be performed in both air-saturated solution and
degassed solution.
1000
800
600
400
200
0
3.3. Conversion of 9H-xanthene to pyronine B in aqueous solution
Pyronine B was also obtained by the conversion of 9H-xan-
thene$HCl in aqueous solution. Treatment of 9H-xanthene with
HCl in DCM yielded protonated 9H-xanthene (9H-xanthene$HCl)
which could be spontaneously converted to pyronine B at ambient
temperature (Fig. 1). It confirmed that 78% isolated yield of pyro-
nine B was obtained from the conversion of 9H-xanthene$HCl in
water (1.5 ꢁ 10ꢂ3 mol/L). Control experiment demonstrated that
the conversion of 9H-xanthene$HCl to pyronine B could also be
performed in buffer solution such as TriseHCl (pH ¼ 7.4) and
citrateephosphate buffer (pH ¼ 7.4).
540
560
580
600
620
640
wavelength (nm)
Fig. 3. Fluorescence changes of 9H-xanthene (5
UV light irradiation (periods: 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300 s).
m
M, in EtOH: HCl ¼ 100: 1, v/v) with
3.4. Color change
Accompanying the conversion of 9H-xanthene to pyronine B,
a distinct color change was observed. It was found that the colorless
solution was changed to purple when 9H-xanthene was converted
to pyronine B. Further study found that the absorption of 235 nm,
which attributes to 9H-xanthene, decreased in intensity, and a new
absorption at lmax ¼ 552 nm, which corresponds to pyronine B,
appeared upon irradiation. The new absorption of 552 nm
increased in intensity with the irradiation time till conversion was
completed (Fig. 2). Similar absorption spectral change was also
detected when 9H-xanthene$HCl was converted to pyronine B in
aqueous solution.
conversion was completed. Similarly, the fluorescence spectral
change was also detected when 9H-xanthene$HCl was converted
to pyronine B in aqueous solution. Both distinct color change and
turn on fluorescence during the process of conversion reaction is
probably beneficial to applications such as indicator and probe.
4. Conclusions
In summary, a new approach for one-pot synthesis of 9H-
xanthene has been developed. It has demonstrated that 9H-
xanthene is efficiently photo-converted to pyronine B with UV
light irradiation, and the protonated 9H-xanthene is also readily
converted to pyronine B in aqueous solution, which provides a new
approach for the preparation of pyronine B. Accompanying the
conversion of 9H-xanthene to pyronine B, both distinct color
change and turn on fluorescence are observed. Further applications
such as indicator and probe will be reported in due course.
3.5. Fluorescence change
The turn on fluorescence was also observed when 9H-xanthene
was converted to pyronine B. As presented in Fig. 3, no fluores-
cence emission was detected when the solution of 9H-xanthene
was excited with a 550 nm excitation wavelength. Upon irradiation
with UV light, a strong fluorescence at lmax ¼ 570 nm, attributes to
pyronine B, was detected when the above solution was excited
with a 550 nm excitation wavelength, the fluorescence of pyronine
B was increased in intensity with increase of irradiation time till the
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (No 21073214) and the Beijing Natural Science
Foundation (No 2112041, Synthesis and Properties of Recording
Material for Multi-level Memory).
1.8
1.5
1.2
0.9
0.6
0.3
0.0
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