Magnesium hydrogensulfate [Mg(HSO4)2] as an efficient catalyst for the preparation of silyl ethers, dibenzo[a,j]xanthenes, and octahydroxanthene derivatives

Article abstract of DOI:10.1080/10426500902758329

Magnesium hydrogensulfate [Mg(HSO₄)₂], as a heterogeneous solid acid catalyst, has been used for the mild formation of trimethylsilyl (TMS) ethers from various primary, secondary, and tertiary aliphatic alcohols; aromatic alcohols; a

Full text of DOI:10.1080/10426500902758329

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Magnesium Hydrogensulfate
[Mg(HSO4)2] as an Efficient Catalyst
for the Preparation of Silyl Ethers,
Dibenzo[a,j]xanthenes, and
Octahydroxanthene Derivatives
Hamid Reza Shaterian a , Razieh Doostmohammadi a , Fahimeh
Khorami a & Majid Ghashang a
a Department of Chemistry, Faculty of Sciences , University of Sistan
and Baluchestan , Zahedan, Iran
Published online: 28 Dec 2009.
To cite this article: Hamid Reza Shaterian , Razieh Doostmohammadi , Fahimeh Khorami & Majid
Ghashang (2009) Magnesium Hydrogensulfate [Mg(HSO4)2] as an Efficient Catalyst for the Preparation
of Silyl Ethers, Dibenzo[a,j]xanthenes, and Octahydroxanthene Derivatives, Phosphorus, Sulfur, and
Silicon and the Related Elements, 185:1, 171-180
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Phosphorus, Sulfur, and Silicon, 185:171–180, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902758329
MAGNESIUM HYDROGENSULFATE [Mg(HSO4)2] AS AN
EFFICIENT CATALYST FOR THE PREPARATION OF SILYL
ETHERS, DIBENZO[a,j]XANTHENES, AND
OCTAHYDROXANTHENE DERIVATIVES
Hamid Reza Shaterian, Razieh Doostmohammadi,
Fahimeh Khorami, and Majid Ghashang
Department of Chemistry, Faculty of Sciences, University of Sistan
and Baluchestan, Zahedan, Iran
Magnesium hydrogensulfate [Mg(HSO4)2], as a heterogeneous solid acid catalyst, has been
used for the mild formation of trimethylsilyl (TMS) ethers from various primary, secondary,
and tertiary aliphatic alcohols; aromatic alcohols; and oximes using hexamethyldisilazane
(HMDS) under ambient conditions. In addition, 14-aryl-14H-dibenzo[a,j]xanthenes and 1,8-
dioxo-octahydroxanthene derivatives were synthesized in the presence of Mg(HSO4)2 with
short reaction times in high to excellent yield under solvent-free conditions.
Keywords Dibenzo[a,j]xanthene; 1,8-dioxo-octahydroxanthene; heterogeneous catalyst; hex-
amethyldisilazane; Mg(HSO4)2; trimethylsilyl ether
INTRODUCTION
One of the most important objectives in chemistry now is to adapt classical processes
so that pollution effects are kept to a minimum, with both a reduction in energy and
consumption of raw materials.1 In this respect, heterogeneous systems are promising, and
a new approach has been undertaken using solid acid chemistry. Heterogeneous systems
have many advantages, such as simple experimental procedures, mild reaction conditions,
and minimal chemical waste, when compared to analogous liquid-phase reactions.2 Solid
acid catalysts are easier to handle because they hold the acidity internally and are easily
separated from products by simple filtration. Moreover, constraining a reaction to the
surface of a solid habitually allows one to apply milder conditions. So far, a number of
solid acid–catalyzed reactions have been reported.1–3
Inorganic solid acidic salts, such as metal hydrogensulfates or solid acids, play a
prominent role in organic synthesis under heterogeneous conditions. In order to promote
environmental safety and due to their safe and ecofriendly nature, the importance of these
Received 17 October 2008; accepted 19 January 2009.
We are thankful to the Sistan and Baluchestan University Research Council for the partial support of this
research.
Address correspondence to Hamid Reza Shaterian, Department of Chemistry, Faculty of Sciences, University
of Sistan and Baluchestan, PO Box 98135-674, Zahedan, Iran. E-mail: hrshaterian@hamoon.usb.ac.ir
171

172
H. R. SHATERIAN ET AL.
solid acid catalysts is growing as attention is directed toward the development of clean and
green technologies for important organic molecules.3
In this article, we show that magnesium hydrogensulfate, as a solid heterogeneous
acid catalyst, can be used for the efficient synthesis of silyl ethers under ambient conditions
(Scheme 1).
Scheme 1 Preparation of silyl ethers.
In addition, 14-aryl-14H-dibenzo[a,j]xanthenes and 1,8-dioxo-octahydroxanthene
derivatives were synthesized in the presence of the mentioned catalyst (Scheme 2)
with short reaction times in high to excellent yield. This catalyst is safe, easy to han-
dle, environmentally benign, presents fewer disposal problems, and is stable in reaction
media.4
Scheme 2 Preparation of 14-aryl-14H-dibenzo[a,j]xanthene and 1,8-dioxo-octahydroxanthene derivatives.
RESULTS AND DISCUSSION
Preparation of Silyl Ethers
Silyl ethers are a group of chemical compounds that contain a silicon atom covalently
bonded to an alkoxy group and are usually used as protecting groups for alcohols in organic
synthesis. One of the silyl ethers is trimethylsilyl (TMS) ether. This group of chemical
compounds provides a wide spectrum of selectivity for protecting group chemistry. This
group is particularly useful because it can be introduced and removed very selectively under
mild conditions.5
Although many methods are available for forming trimethylsilyl ethers, there are
two common strategies: reaction of the alcohol with a silyl chloride and an amine base
and reaction of the alcohol with a 1,1,1,3,3,3-hexamethyldisilazane (HMDS) with a Lewis

MAGNESIUM HYDROGENSULFATE AS AN EFFICIENT CATALYST
173
Table I Preparation of benzyl trimethylsilyl ether using Mg(HSO4)2 (0.05 g) as catalyst under
solvent and solvent-free conditions at room temperature
Entry
Solvent
Time (min)
GC yield (%)
Yield (%)a
1
2
3
4
5
6
7
Dichloromethane
Chloroform
Ethyl acetate
n-Hexane
Solvent-free
Diethyl ether
Acetonitrile
91
120
200
105
115
191
3
100
100
100
100
100
100
100
83
85
81
89
87
83
93
aIsolated yield, and the product gave satisfactory IR and NMR spectra.
or Brønsted acid catalyst. HMDS is a stable, commercially available, cheap reagent and
gives ammonia as the only byproduct. In addition, silylation with HMDS is nearly neutral
and does not need special precautions. The low silylation power of HMDS is the main
drawback to its application. Therefore, there are varieties of catalysts for activating this
reagent.5 Now we show that magnesium hydrogensulfate can be added to this library
collection. This catalyst can be used for the efficient synthesis of silyl ethers under ambient
conditions.
To optimize the reaction conditions, initially we converted benzyl alcohol (1mmol)
to its corresponding benzylsilylether with a predetermined amount of Mg(HSO4)2 (0.05 g,
0.23 mmol) as catalyst and HMDS (0.75 mmol) in the presence of various solvents and
also solvent-free conditions at room temperature (Table I). The results in Table I show that
among these solvents, acetonitrile was the solvent of choice in terms of reaction time.
To find out the optimum quantity of the catalyst, the reaction of benzyl alcohol
(1 mmol) with HMDS (0.75 mmol) was carried out using different quantities of Mg(HSO4)2
under ambient conditions in acetonitrile as solvent (Table II). As can be seen from Table
II, the best results were obtained by using 0.05 g of the catalyst.
Thus, we prepared a range of silyl ethers under the optimized reaction conditions:
hydroxyl compound (1 equiv), HMDS (0.75 equiv), Mg(HSO4)2 (0.05 g, 0.23 mmol), and
acetonitrile (2 mL) (Schemes 3–8).
A wide range of structurally diverse and functionalized phenols, alcohols, and oximes
underwent silylation by this procedure to provide the corresponding TMS ethers in high
to excellent isolated yields (Schemes 3–8). Primary alcohols mostly reacted faster than
secondary and tertiary alcohols. Generally, in the all cases of benzyl and primary, secondary,
and tertiary alcohols, the reactions were completed in less than 20 min at ambient conditions
accompanied by evolution of NH3 gas from the reaction mixture (Schemes 3–6).
Amines remained unaffected under the reaction conditions (Scheme 7).
Table II Optimization of catalyst in the synthesis of benzyl trimethylsilyl ether in acetonitrile as solvent
Entry
Catalyst (g)
Time (min)
Isolated yield (%)
1
2
3
0.05
0.025
0.0125
3
18
25
93
91
89

174
H. R. SHATERIAN ET AL.
Scheme 3
Scheme 4
In order to examine the functional group compatibility, we tested some more alcohols
having other functional groups such as carbonyl group, amino group, alkene, and ethers.
Alcohols were successfully converted to the corresponding silyl ethers, whereas other
functional groups were intact (Scheme 3, entry 5; Schemes 7 and 8).
We also investigated selective silylation of different binary mixtures of alcohols
(Table III). This method was shown to be highly selective for primary alcohols such as
benzyl alcohols compare to secondary and tertiary alcohols (Table III, entries 1–3). The

MAGNESIUM HYDROGENSULFATE AS AN EFFICIENT CATALYST
175
Scheme 5
Scheme 6
Scheme 7
primary alcohols were completely converted to the corresponding silyl ethers, while the
secondary and tertiary alcohols were converted to the corresponding silylated product with
0–15% yield. It is noteworthy that this method can be used for chemoselective silylation of
primary alcohols in the presence of secondary and tertiary alcohols.
Preparation of 14-Aryl-14H-dibenzo[a,j]xanthene
and 1,8-Dioxo-octahydroxanthene Derivatives
Derivatives of xanthene are commonly referred to collectively as xanthenes, and
among these are benzoxanthenes6 and octahydroxanthene7 derivatives. This group of chem-
ical compounds provides a wide spectrum of selectivity for the basis of a class of dyes
and biological and therapeutic properties.6a–e Many methods are available for forming

176
H. R. SHATERIAN ET AL.
Scheme 8
benzoxanthenes, and there are two common strategies: i) dehydration of bis(2-hydroxy-
1-naphthyl) methane using POCl3 or by boiling acetic acid diester of bis(2-hydroxy-1-
naphthyl) methane6f–g and ii) condensation of β-naphthol with aliphatic and aromatic
aldehydes in the presence Lewis or Brønsted acid catalyst.6h–o
In our research, we tried to prepare a range of 14-aryl-14H-dibenzo[a,j]xanthene
(Scheme 9) and 1,8-dioxo-octahydroxanthene derivatives (Scheme 10) using Mg(HSO4)2
as a Brønsted acid catalyst.
In order to carry out the condensation in a more efficient way by minimizing the
time, temperature, and amount of catalyst, the reaction of benzaldehyde and 2-naphthol
Table III Selective silylation of different binary mixture of alcohols
Substrate
Product
Molar Ratio
Substrate 1/Substrate
Time
(min)
GC yield
(%)
Entry
Binary mixture
2/HMDS/Catalyst (g)
100
1
1/1/0.75/0.05
5
5
4
100
1/1/0.75/0.05
2
3
15
100
1/1/0.75/0.05
5
0

MAGNESIUM HYDROGENSULFATE AS AN EFFICIENT CATALYST
177
Scheme 9
Scheme 10
(1:2 molar ratio) was selected as model to investigate the effects of the catalyst at different
reaction temperatures (50, 100, 120, and 150◦C) and different amounts of catalyst (0.1,
0.05, 0.025, and 0.01 g), The best result was obtained by carrying out the reaction using
0.05 g of Mg(HSO4)2 (0.23 mmol) at 120◦C under thermal solvent-free conditions.
Using these optimized reaction conditions, the scope and efficiency of these pro-
cedures were explored for the synthesis of a wide variety of substituted 14-aryl-14H-
dibenzo[a,j]xanthene and 1,8-dioxo-octahydroxanthene derivatives. The results are sum-
marized in Schemes 9 and 10.

178
H. R. SHATERIAN ET AL.
The process tolerates aromatic aldehydes containing both electron-donating and
electron-withdrawing substituents. It can be observed that all the aldehydes have reacted in
short reaction times under thermal solvent-free conditions to afford the desired xanthenes
in high isolated yields (Scheme 9 and 10).
CONCLUSION
In conclusion, Mg(HSO4)2 showed an efficient catalytic activity for the preparation of
silyl ethers, 14-aryl-14H-dibenzo[a,j]xanthenes, and 1,8-dioxo-octahydroxanthene deriva-
tives. Simple procedures, easy workup, low cost, ease of preparation, high efficiency, and
eco-friendliness of the catalyst are some advantages of this method.
EXPERIMENTAL
All reagents were purchased from Merck and Aldrich and were used without further
purification. Mg(HSO4)2 was prepared according to the reported procedure.4 All yields
refer to isolated products after purification. Products were characterized by comparison
with authentic samples and by spectroscopy data (IR, 1H NMR spectra). The NMR spectra
were recorded on a Bruker Avance DPX 300 and 500 MHz instrument. The spectra were
measured in CDCl3 relative to TMS (0.00 ppm). GC analysis was run with Shimadzu
GC-14A. IR spectra were recorded on a Jasco FT-IR 460plus spectrophotometer. TLC was
performed on silica-gel polygram SIL G/UV 254 plates.
General Procedure for the Trimethylsilylation of Hydroxyl Compounds
To a stirred solution of alcohols or oximes (1 mmol), HMDS (0.75 mmol) and acetoni-
trile as solvent (2 mL), Mg(HSO4)2 (0.05 g, 0.23 mmol) was added at room temperature,
and the mixture was stirred for the appropriate time (Schemes 3–8). The reaction was
followed by TLC (n-hexane:EtOAc, 9:1). After completion of the reaction, the solvent
was evaporated. Then n-hexane was added, and the catalyst was easily isolated by simple
filtration. The resulting mixture was adsorbed on silica gel, passed through a short pad
column of silica gel, and washed with n-hexane (2 × 10 mL). Evaporation of the solvent
under reduced pressure gave pure product(s) (Schemes 3–8). The desired pure product(s)
was characterized by comparison of its physical data with those of known compounds.5 The
isolated catalyst was washed several times with n-hexane, dried, and reused. The recovered
catalyst was reused three times subsequently for the same reaction without any loss in
activity (Scheme 3, entry 1).
General Procedure for Preparation of 14H-Dibenzo[a,j]xanthene
Derivatives
To a mixture of aldehyde (1 mmol) and β-naphthol (2 mmol), magnesium hydrogen-
sulfate (0.05 g, 0.23 mmol) was added, and the mixture was inserted in an oil bath and heated
for 120◦C for the appropriate time (Scheme 9). Completion of the reaction was indicated by
TLC. After the reaction was completed, ethyl acetate was added and the reaction mixture
was stirred until solid crude product was dissolved. Then, the heterogeneous catalyst was
isolated from the mixture of the reaction by simple filtration. In continuation of workup,
the filtrate organic solution was evaporated. The crude product was recrystallized using

MAGNESIUM HYDROGENSULFATE AS AN EFFICIENT CATALYST
179
aqueous ethanol 15% twice. The desired pure product(s) were characterized by comparison
of their physical data with those of known benzoxanthenes.6
General Procedure for Preparation of 1,8-Dioxo-octahydroxanthene
Derivatives
To a mixture of aryl aldehyde (1 mmol) and dimedone (2 mmol), Mg(HSO4)2
(0.05 g, 0.23 mmol) was added, and the mixture was heated at 125◦C for the appropriate
time (Scheme 10). Completion of the reaction was indicated by TLC. After completion,
the reaction mass was cooled to 25◦C, then ethyl acetate was added until the solid crude
product was dissolved. Then, the heterogeneous catalyst was isolated from the mixture of
the reaction by simple filtration. In continuation of workup, the filtrate organic solution was
evaporated. The crude product was recrystallized using aqueous ethanol 15% twice. The
desired pure product(s) was characterized by comparison of its physical data with those of
known 1,8-dioxo-octahydroxanthenes.7
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