Metal hydrogen sulfates catalyzed methoxymethylation of alcohols under solvent-free conditions

Article abstract of DOI:10.1002/jccs.200700153

Methoxymethylation of a variety of alcohols was performed by using formaldehyde dimethoxy acetal in the presence of metal hydrogen sulfate M(HSO₄)_n at room temperature and solvent-free conditions. The methoxymethyl ethers (MOM-ethers

Full text of DOI:10.1002/jccs.200700153

Journal of the Chinese Chemical Society, 2007, 54, 1067-1073
1067
Metal Hydrogen Sulfates Catalyzed Methoxymethylation of Alcohols under
Solvent-Free Conditions
Khodabakhsh Niknam,a* Mohammad Ali Zolfigol,b* Mohsen Shayeghb and Reza Zareb
aChemistry Department, Faculty of Sciences, Persian Gulf University, Bushehr, Zip Code 75169, Iran
bFaculty of Chemistry, Bu-Ali Sina University, Hamedan, Zip Code 65174, Post Box No. 4135, Iran
Methoxymethylation of a variety of alcohols was performed by using formaldehyde dimethoxy acetal
in the presence of metal hydrogen sulfate M(HSO4)n at room temperature and solvent-free conditions. The
methoxymethyl ethers (MOM-ethers) were obtained with high yields and purity.
Keywords: Metal hydrogen sulfates; M(HSO4)n; Methoxymethylation; Alcohols; Formaldehyde
dimethoxy acetal; Solvent-free conditions.
INTRODUCTION
Acids are widely used as catalysts in industry for pro-
eco-friendly nature as attention is directed toward the de-
velopment of clean and green technologies for important
organic molecules to promote environmental safety.
ducing more than 1 ´ 108 mt/year of products.1 The most
commonly used are HF, H2SO4 , HClO4 , and H3PO4 (in liq-
uid form or supported on Keiselguhr). Solid acids have
many advantages such as simplicity in handling, decreased
reactor and plant corrosion problems, and more environ-
mentally safe disposal in different chemical processes. Also,
wastes and by-products can be minimized or avoided by us-
ing solid acids in developing cleaner synthesis routes. On
the other hand, any reduction in the amount of liquid acid
needed and/or any simplification in handling procedures is
required for risk reduction, economic advantage and envi-
ronmental protection.2 On the other hand, using an applica-
ble industrial catalyst that is safe and eco-friendly, green
and simply recycled in the reaction mixtures, has received
attention. Thus, green chemistry has been defined as a set
of principles that reduces or eliminate the use or generation
of hazardous substances throughout the entire life of chem-
ical materials. Inorganic solid acidic salts such as metal hy-
drogen sulfates play a prominent role in organic synthesis
under heterogeneous conditions.3 Along this line, using
Al(HSO4)3,4 Fe(HSO4)3,5 Mg(HSO4)2,6 NaHSO4.H2O,7
Oxone® (2KHSO5.KHSO4.K2SO4)8 which are low in toxic-
ity, highly stable towards humidity, recyclable and air sta-
ble have received more attention. The importance of these
solid acid catalysts is growing because of their safe and
Protection of the hydroxyl functional group is an im-
portant process in multi-step syntheses. One of the popular
methods for this purpose is to transform hydroxyl groups to
their corresponding methoxymethyl ethers (MOM-ethers).
The methoxymethyl protecting group is thus superior to the
tetrahydropyranyl group, especially in the cases of polyols,
because it is easier to distinguish mono-protection from
multi-protection and to estimate impurity by NMR spec-
troscopy.9 Methoxymethylation has usually been done with
chloromethyl methyl ether under basic conditions.10 The
formaldehyde dimethoxy acetal (FDMA) is a cheap and
commercially available compound that can be used for the
preparation of MOM-ethers from hydroxyl compounds.
Even though handling this reagent is easy, its main draw-
back is its poor methoxymethylating power that needs force-
ful conditions.11 For the activation of FDMA, a variety of
catalysts have been reported, but these methods have their
own limitations as well as merits. For example, MoO2(acac)2
has been reported as an excellent catalyst for the methoxy-
methylation of different kinds of alcohols, but it is not a
cheap and commercial catalyst for this purpose.12 Patney
has also reported anhydrous iron(III) chloride dispersed on
molecular sieves (3 Å) for the methoxymethylation of pri-
mary and secondary alcohols by using FDMA at room tem-
perature and with excellent yields. However, this procedure
* Corresponding author. Fax: +98-771-4545188; E-mail: khniknam@gmail.com (Khodabakhsh Niknam); Fax: +98-811-8257407;
E-mail: zolfi@basu.ac.ir and mzolfigol@yahoo.com (Mohammad Ali Zolfigol)

1068 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Niknam et al.
is not suitable for methoxymethylation of tertiary alco-
hols.13 Very recently Sc(OTf)3 and Bi(OTf)3 have also been
used as effective catalysts for methoxymethylation of alco-
hols.14
the times and yields of four consecutive protections leading
to MOM-ethers. In these experiments the product was iso-
lated by filtration and washing the solid residues with
CH2Cl2 and the remaining catalyst being reloaded with
fresh reagents for further runs. No decrease in the yield was
observed; therefore, the described metal hydrogen sulfates
[Al(HSO4)3 and Ca(HSO4)2] could be reused as a catalyst in
methoxymethylation reactions. It should be mentioned that
Fe(HSO4)3 and Mg(HSO4)2 could not be recycled and re-
used again.
RESULTS AND DISCUSSION
In connection with our recent interest in the direct
methoxymethylation of alcohols with FDMA in the pres-
ence H3PMo12O40.xH2O15 and silica sulfuric acid16 as solid
acid catalysts, and also in continuation of our studies on the
use of inorganic hydrogen sulfate acidic salts in organic
transformation,4-8 we have found that metal hydrogen sul-
fates [M(HSO4)n] are suitable for direct methoxymethyla-
tion of alcohols with FDMA at room temperature under
solvent-free conditions (Scheme I).
Both primary and secondary alcohols can be smoothly
converted into the corresponding MOM-ethers in excellent
yields. Benzylic alcohols bearing electron withdrawing
groups such as nitro and halogens (Table 2, entry 2-5) or
electron releasing groups (Table 2, entry 6) are converted
into the corresponding methoxymethylated products with
excellent yields.
During our investigation, we also found that hindered
tertiary alcohols such as 1-adamantanol and 1-methyl-2-
phenyl-2-propanol were also methoxymethylated in re-
fluxing CH3CN with good to excellent yields. Fortunately,
in the case of tertiary alcohols, an elimination reaction was
not observed (Table 2, entries 14 and15). Results are tabu-
lated in Table 2. Furthermore, our examination showed that
this method is not applicable for protection of phenolic
hydroxyl groups, and the starting material was recovered
intact after 24 h. Therefore we observed the competitive
methoxymethylation of alcohols in the presence of phenol.
In a control experiment, when an equimolar mixture of
benzyl alcohol (1 mmol) and phenol (1 mmol) with FDMA
(10 mmol) in the presence of metal hydrogen sulfates [for
Al(HSO4)3 (0.2 mmol) and another hydrogen sulfates Fe,
Mg, and Ca (0.25 mmol)] under solvent-free conditions,
the benzyl alcohol underwent chemoselectively methoxy-
methylation and gives benzyl methoxymethyl ether (80-
86%), whereas the phenol remained intact (Scheme II).
In this regard, we decided to use a molecule contain-
ing both phenolic and alcoholic hydroxyl groups such as
2,6-bis-hydroxymethyl-4-chloro-phenol. Therefore, in an-
other experiment we also refluxed 2,6-bis-hydroxymeth-
yl-4-chloro-phenol, FDMA(60 mmol) and Al(HSO4)3 (0.5
mmol) in acetonitrile for 18 h (Table 3). As shown in
Scheme III, only the two non-phenolic hydroxy groups
were converted into their corresponding methoxymeth-
ylated forms. The low yield of the obtained product con-
cerned the sensitive two non-phenolic hydroxyl groups of
Scheme I
M(HSO4)n
RR'R"COH
RR'R"COCH2OMe
(CH3O)2CH2 (as excess)
r.t. neat
R, R', R"= Alkyl, Aryl, H
M: Al, Fe, Mg, Ca
Methoxymethylation reaction of 2-phenylethanol
was performed with different molar ratios of solid acids in
order to optimize the reaction conditions. In a preliminary
study, the effect of various solid acids on the yields and
times of methoxymethylation was investigated with the
above mentioned substrate as a model compound. As shown
in Table 1, Al(HSO4)3 is the best solid acid catalyst for this
purpose under solvent-free conditions.
Also, Mg(HSO4)2, Ca(HSO4)2, and Fe(HSO4)3 cata-
lyze this protection with a longer reaction time. Such a re-
action did not proceed in the presence of oxone®, KHSO4,
HIO3, H5IO6, and silica zirconium hydrogen phosphate
(Table 1, entries 7-11). The required molar ratio, 1:10:0.2,
was applied for alcohol, FDMA, and catalyst Al(HSO4)3,
respectively, under the abovementioned conditions. The
molar ratio of other metal hydrogen sulfates (Fe, Mg, Ca) is
0.25 (Table 1, entries 15, 18, 20). Although ethyl acetate,
acetonitrile, and CH2Cl2 afforded the product in high yields,
we chose the solvent-free conditions for green and environ-
mental acceptability. Entries 6 and 20 of Table 1, describe

Methoxymethylation of Alcohols
J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 1069
Table 1. The reaction of 2-phenylethanol (1 mmol) with formaldehyde dimethoxy acetal (10
mmol) in the presence of solid acids at room temperature under various conditions
Time (Conversion)
Entry
Catalyst
Amounts of cat.
Conditions
h (%)
1
2
3
4
5
6
Al(HSO4)3
Al(HSO4)3
Al(HSO4)3
Al(HSO4)3
Al(HSO4)3
Al(HSO4)3
0.2 mmol
0.2 mmol
0.2 mmol
0.2 mmol
0.1 mmol
0.2 mmol
Me3C-O-Me
CH2Cl2
CH3CN
CH3COOEt
Solvent-free
Solvent-free
24.0 (71)
7.0 (100)
3.5 (100)
2.75 (100)
5.5 (70)
0.42 (90), 2.5 (88),
3.33 (88), 4.5a (85)a,b
13.0 (-c)
7
8
9
Oxoneâ
SiO2-ZrPO4H
HIO3
H5IO6
KHSO4
NaHSO4.H2O
Fe(HSO4)3
Fe(HSO4)3
Fe(HSO4)3
Mg(HSO4)2
Mg(HSO4)2
Mg(HSO4)2
Ca(HSO4)2
Ca(HSO4)2
0.3 mmol
1.5 g
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
10.0 (-c)
0.25 mmol
0.4 mmol
0.4 mmol
0.5 mmol
0.1 mmol
0.2 mmol
0.25 mmol
0.15 mmol
0.2 mmol
0.25 mmol
0.2 mmol
0.25 mmol
10.0 (-c)
10
11
12
13
14
15
16
17
18
19
20
13.0 (-c)
16.0 (-c)
16.0 (30c)
8.0 (15)
4.0 (45)
2.0 (100)
5.0 (33)
3.5 (53)
2.0 (100)
4.0 (55)
2.0 (87), 4.0 (72),
4.45 (61), 6.0a (54)a,b
a The same recycled catalyst was used for each of the four runs. b Isolated yield. c No reaction.
Scheme II
OH
O
O
M(HSO4)n
OH
O
O
+
+
FDMA/ neat
rt/ 24 h
Alcohol
Phenol
protected
protected
M= Al
83%
0%
0%
M= Fe
M= Mg
M= Ca
80%
86%
0%
0%
83%
Scheme III
yields of reactions were increased (Table 3, entries 2 and 3).
In conclusion, we have demonstrated that M(HSO4)n
can be used for the methoxymethylatoin of primary, sec-
ondary, and tertiary alcohols by formaldehyde dimethoxy
acetal. We hope that Al(HSO4)3 will be superior to the other
inorganic acidic metal hydrogen sulfates analogues due to a
combination of Lewis and protic acidity.3 The notable ad-
vantages of this method are mild reaction conditions, high
yields, cheapness, safety and eco-friendliness, and recycla-
blity of the catalysts.
OH
OH
OH
M(HSO4)n
O
O
O
O
HO
FDMA/ Reflux
Cl
Cl
I
II
4-substituted-2,6-dihydroxymethylphenol under acidic me-
dia;19 when we used the protected phenolic hydroxyl group
analogues (anisole derivatives) as starting materials the

1070 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Niknam et al.
Table 2. Preparation of MOM-ethers from alcohols (1 mmol) and formaldehyde dimethoxy acetal (10
mmol) in the presence of M(HSO4)n (M = Al, 0.2 mmol; M = Fe, Mg, Ca, 0.25 mmol) at
room temperature and solvent-free conditions
Al(HSO4)3
Fe(HSO4)3
Mg(HSO4)2
Ca(HSO4)2
Time (Yield) Time (Yield) Time (Yield) Time (Yield)
Entry
Substrate
h (%)a,b
h (%)a,b
h (%)a,b
h (%)a,b
1
2
CH2OH
1.67 (83)
2.25 (80)
2.5 (86)
2.5 (83)
Br
CH2OH
Cl
CH2OH
4.0 (90)
3.5 (89)
4.5 (90)
4.0 (85)
5.0 (88)
2.5 (83)
2.5 (89)
3
0.67 (85)
Cl
O2N
F
4
5
6
7
CH2OH
CH2OH
12.0 (75c)
0.86 (85)
0.33 (89)
3.0 (88)
10.0 (78c)
3.5 (80)
11.0 (75c)
4.0 (75)
17.0 (75c)
3.5 (85)
MeO
CH2OH
2.0 (82)
2.0 (80)
0.75 (83)
4.0 (77)
CH(OH)CH3
3.75 (72)
2.75 (75)
CH
OH
8
0.33 (90)
2.5 (90)
2.5 (91)
2.5 (90)
9
CH(OH)CH3
OH
1.5 (73)
1.0 (78)
2.5 (74)
2.5 (81)
2.25 (70)
3.0 (83)
3.5 (72)
2.0 (80)
10
11
17.5 (85)
18 (77)
20.0 (75)
21.0 (72)
OH
CH3(CH2)6CH2OH
12
13
4.5 (61)
3.25 (63)
2.0 (91)
3.0 (57)
2.0 (85)
4.5 (60)
2.0 (87)
0.42 (95)
CH2CH2OH
CH3
14
14.0 (78)
8.5 (75d)
9.5 (80)
12.0 (80)
9.0 (75d)
13.0 (72)
9.5 (70d)
CH2CCH3
OH
15
9.0 (70d)
OH
a
b
Isolated yield. All of the isolated products are known compounds and their spectra and physical
data have been reported in the literature.17,18 c FDMA (30 mmol) was used. d Reaction conditions: to
the mixture of alcohol (1 mmol), FDMA (30 mmol), and CH3CN (3 mL) M(HSO4)n (0.3 mmol) were
added at reflux conditions.

Methoxymethylation of Alcohols
J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 1071
Table 3. Preparation of bis-MOM-ethers from bis-alcohols (1 mmol) and formaldehyde dimethoxy
acetal (40 mmol) in the presence of M(HSO4)n (M = Al, 0.6 mmol; M = Fe, Mg, Ca, 0.75
mmol) under reflux conditions
Al(HSO4)3
Fe(HSO4)3
Mg(HSO4)2
Ca(HSO4)2
Entry
Substrate
Time (Yield) Time (Yield)
Time (Yield) Time (Yield)
h (%)a
h (%)a
h (%)a
h (%)a
OH
HO
OH
1
18 (30e)
17 (28e)
17.5 (25e)
16.5 (32e)
Cl
OMe
HO
OH
2
12.5 (76)
13 (77)
15.0 (85)
16.0 (70)
Br
OMe
HO
OH
3
13.0 (74)
13 (75)
15.5 (82)
16.5 (70)
Cl
a Isolated yield. e Reaction conditions: to the mixture of alcohol (1 mmol), FDMA (60 mmol), and
M(HSO4)n [Al: (0.5 mmol), Fe, Mg, Ca: (0.75 mmol)] were added in reflux conditions.
EXPERIMENTAL SECTION
Typical Procedure for the Preparation of 4-Chloro-
2,6-bis-methoxymethoxy-methyl-phenol (II)
General
Substrates were purchased from Fluka, Merck and
To a solution of 2,6-bis-hydroxymethyl-4-chloro-
phenol (I), (0.188 g, 1 mmol) and FDMA (4.56 g, 60
mmol), M(HSO4)n [Al: (0.159 g, 0.5 mmol), Fe, Mg, Ca:
(0.75 mmol, 0.260 g, 0.163 g, 0.175 g respectively)] was
added, and the suspension was refluxed. The progress of
the reaction was monitored by TLC; after completion of the
reaction (Table 3, entry 1) the mixture was filtered, and the
remaining metal hydrogen sulfate was washed with metha-
nol (2 ´ 10 mL) and distilled; the solvent afforded a resi-
due, which was passed through a column of silica gel using
a mixture of ethyl acetate and n-hexane as eluent to afford
white crystals (mp 99 °C), 0.132 g, 30% of the product (II).
IR (KBr): 3195, 2916, 2852, 1599, 1463, 1377, 1229,
1082, 992, 852, 737, 577 cm-1. 1H NMR (CDCl3, 90 MHs):
d 4.545 (s, 6H), 4.767 (s, 4H), 5.170 (s, 4H), 6.800 (s, 2H),
7.675 (b, 1H).
Aldrich chemical companies. Al(HSO4)3, Fe(HSO4)3,
Mg(HSO4)2, Ca(HSO4)2,4-6 and 2,6-bis-hydroxymethyl-
4-chloro-phenol and 4-substituted-2,6-dihydroxymethyl
anisole were prepared according to our previously reported
procedures.19,20
General Procedure
To a solution of alcohol (1 mmol) and FDMA (10
mmol), M(HSO4)n (M = Al, 0.2 mmol; M = Fe, Mg, Ca,
0.25 mmol) was added, and the suspension was stirred for
an appropriate time. After completion of the reaction, the
excess of FDMA was removed by distillation. Then, t-butyl
methyl ether (10 mL) was added and the mixture was fil-
tered off. Water (10 mL) was added and neutralized with
NaHCO3 (10%) and extracted with t-butyl methyl ether (2
´ 10 mL). The organic layer was dried over anhydrous
Na2SO4 (3 g) and evaporation of the solvent on a rotary
evaporator afforded a residue, which was passed through a
short pad of silica gel using a mixture of ethyl acetate and
n-hexane as eluent to afford the highly pure MOM-ether.
13C NMR (CDCl3, 22.5 MHz): d 65.782, 91.360,
95.983, 124.036, 127.301, 128.301, 152.525.
4-Chloro-2,6-bis-methoxymethoxy-methyl-anisol
To a solution of 2,6-bis-hydroxymethyl-4-chloro-
anisol (0.202 g, 1 mmol) and FDMA (3.04 g, 40 mmol),

1072 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Niknam et al.
M(HSO4)n [Al: (0.159 g, 0.5 mmol), Fe, Mg, Ca: (0.75
mmol, 0.260 g, 0.163 g, 0.175 g, respectively)] was added
and the suspension was refluxed. The progress of the reac-
tion was monitored by TLC; after completion of the reac-
tion (Table 3, entry 3) the excess of FDMA removed by dis-
tillation, and the remaining metal hydrogen sulfate washed
with methanol (2 ´ 10 mL) and distilled the solvent af-
forded a residue, which was passed through a column of sil-
ica gel using a mixture of ethyl acetate and n-hexane as
eluent to afford white crystals (mp 45-47 °C), of the prod-
uct.
Received October 12, 2006.
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Financial support for this work by the Research Coun-
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Methoxymethylation of Alcohols
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