Antimony(V) chloride as an efficient reagent for deprotection of methyl ethers

Article abstract of DOI:10.1055/s-0033-1339310

This paper proposes a new and efficient method for the deprotection of methyl ethers using antimony pentachloride at ambient temperature. The procedure described here is a facile and practical method for the removal of the methyl group from aryl and benzyl methyl ethers. High selectivity was observed for the removal of the methyl group from dimethoxyarenes. The notable advantages of this protocol are mild reaction conditions, high yields, and the facility of workup procedure. The mechanism of the ether cleavage is proposed to explain this new reaction. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1339310

1702▌
LETTER
lAettner timony(V) Chloride as an Efficient Reagent for Deprotection of Methyl
Ethers
Antimony(V) Chloride as an Efficient Reagent for Deprotection
Fariba Saadati,* Hamid Meftah-Booshehri
Department of Chemistry, Faculty of Science, University of Zanjan, P.O. Box 45195-313, Zanjan, Iran
Fax +98(241)2283203; E-mail: saadati@znu.ac.ir
Received: 21.05.2013; Accepted after revision: 29.05.2013
Therefore, more efficient methods are desired to deprotect
Abstract: This paper proposes a new and efficient method for the
methyl ethers under ambient conditions.
deprotection of methyl ethers using antimony pentachloride at am-
bient temperature. The procedure described here is a facile and
practical method for the removal of the methyl group from aryl and
benzyl methyl ethers. High selectivity was observed for the removal
of the methyl group from dimethoxyarenes. The notable advantages
of this protocol are mild reaction conditions, high yields, and the
facility of workup procedure. The mechanism of the ether cleavage
is proposed to explain this new reaction.
Antimony pentachloride is a commercially available and
inexpensive reagent that exhibits a variety of interesting
and useful properties. It has been successfully used as a
standard Lewis acid22 and a strong oxidant in organic syn-
thesis.23 This led researchers to develop this reagent in
several organic process, such as O-desilylation reaction,24
Friedel–Crafts alkylation,25 polymerization,26 electrophil-
ic additions to alkenes,27 aromatization of enamines,28 and
chlorination of organic compounds.29
Key words: demethylation, methyl ether, antimony pentachloride,
selectivity, dimethoxyarenes
We report herein a general method for the effective de-
protection of methyl ethers under mild reaction conditions
using antimony pentachloride as a high selective and effi-
cient reagent.
Ethers as a masked hydroxyl group are versatile inter-
mediates of valuable compounds in organic chemistry
from both academic and industrial perspectives,1 so the
preparation or deprotection of ether groups is an important
organic transformation.
To our knowledge, selective deprotection of ethers has
scarcely been investigated. Demethylation of poly-
methoxyarene derivatives is a very attractive route for the
preparation of fine chemicals and intermediates. A disap-
pointingly high proportion of the prior works focuses on
high-temperature conditions on which deprotections are
relatively easy to promote.
A useful method for the protection of hydroxyl groups in
aliphatic and aromatic compounds is O-methylation with
dimethyl sulfate (DMS),2 methyl halides,3 dimethyl car-
bonate (DMC),4 and methanol in the presence of strong
acid5 or zeolite6 as catalyst.
We now wish to report our development of selective and
room-temperature methodology with minimal waste
streams and good yields. Reactions are possible with aryl
methyl ethers, benzyl methyl ether, and dimethoxyarenes.
This method is also superior to the synthesis of 2-
methoxy-2′-hydroxy-1,1′-binaphtyl, monodentate BINOL
monoether ligand.
The ether groups have been traditionally considered as in-
ert and highly stable to a variety of reagents and experi-
mental conditions. Hence, deblocking of ethers is
laborious and requires highly challenging transformation
and deep study. Numerous cleavage methods are avail-
able,7 such as using bases,8 oxidative deprotection,9 re-
ductive cleavages,10 and alkali metal agents.11 Another
well-documented ones are the use of microwave
irradiation12 and improving biological methods in this ar-
ea.13 The most practical and widely employed strategy is
the deprotection of aryl methyl ethers using Lewis acids
and strong acids. Examples include BBr3,14 the combina-
tion of AlCl3/pyridine,15 as well as AlBr3/NaI,16 BeCl2,17
NiCl2/Zn,18 ionic liquids (IL) such as 3-methylimidazoli-
um bromohydrogenate (HmimBr–HBr),19 pyridine–
hydrochloride system,20 and iodocyclohexane.21
A series of methoxy-substituted compounds were conve-
niently prepared by O-methylation of di- and monohy-
droxybenzene and naphthol derivatives 1b–j, 1,1′-bi-
naphthyl-2,2′-diol (1k, BINOL), and phenyl methanol
(1g). The purification was performed according to the
procedure described in the experimental section below
without using column chromatography. Of all the protec-
tion systems tested, the most effective turned out to be the
couple of potassium carbonate (base)/dimethyl sulfate
(DMS) in anhydrous acetone at reflux conditions. Once
the optimal reaction conditions were established, the cor-
However, some of these methods restricted by low reac-
tion yields and high temperatures, which are not suitable
for the synthesis of thermolabile compounds, result in un-
desired products because of the harsh reaction conditions.
Me2SO4
ROH
ROMe
K2CO3
acetone
1
2
SYNLETT 2013, 24, 1702–1706
Advanced online publication: 16.07.2013
DOI: 10.1055/s-0033-1339310; Art ID: ST-2013-D0470-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
Scheme 1 Synthesis of methyl ether derivatives

LETTER
Antimony(V) Chloride as an Efficient Reagent for Deprotection
1703
responding methyl ethers were successfully obtained in Table 2 shows that the reaction was not very sensitive to
quantitative yields (Table 1, Scheme 1).
the amount of SbCl5. Under the same conditions, the vari-
ation of the molar ratio of anisole to SbCl5 (1:0.5, 1:1.5,
1:2, and 1:5) in acetonitrile produced different yields (65–
92%).
In order to find a suitable procedure for the selective de-
methylation, a model reaction of anisole (2a) was initially
performed. In the presence of anisole, the solvent, temper-
ature, molar ratio, and atmosphere conditions were exam- The reaction under air was compared with that under an
ined as the common factors affecting the reaction. The inert atmosphere (Table 2, entry 13 vs. entry 5) and indi-
results are summarized in Table 2.
cates that air is responsible for lowering the yield. This is
probably due to the fact that antimony pentachloride is a
hygroscopic agent which reacts explosively with H2O.
As for the solvent effect, the cleavage efficiency was
highly solvent-dependent (Table 2, entries 1–5). The reac-
tion was carried out using as low as 1.5 equivalents SbCl5 The above-described investigations prompted us to use
in anhydrous tetrahydrofuran, toluene, dichloromethane, SbCl5 (1.5 equiv) in anhydrous acetonitrile at ambient
acetonitrile, and wet acetonitrile (5% w/v) at ambient tem- temperature under nitrogen atmosphere as the optimized
perature under inert atmosphere. In the presence of tolu- reaction conditions to probe the scope of deprotection of
ene and THF, low yields of 29% and 35% were produced various types of methyl ethers (Scheme 2).
with several byproducts within 48 and 24 hours, respec-
tively. Since acetonitrile produced the best conversion and
SbCl5
ROMe
ROH
yield, it was chosen as the preferred solvent for the reac-
tion. It is worth noting that the reaction consistently pro-
duced low yields under reflux conditions with the above-
mentioned solvents.
MeCN
r.t.
2
1
Scheme 2 Deprotection of methyl ether derivatives
The corresponding phenol was obtained in good yields at
ambient temperature (Table 2, entries 5–8). The reaction
continued with the dropwise addition of SbCl5 to the
stirred mixture of methyl ether and anhydrous acetonitrile
at 0 °C under nitrogen atmosphere, and the mixed temper-
ature became ambient. This method result in minimal
waste streams and higher yields (Table 2, entries 5 and 6).
As can be seen in Table 3, deprotection of various types
of aryl methyl ethers is completed by SbCl5 under desired
reaction conditions with good to excellent yields. In addi-
tion, the reaction of methoxy methyl benzene 2g in the
presence of SbCl5 gave the benzyl alcohol in good yield
(85%). An important point in this method was the selec-
tive monodeprotection of dimethoxy aryl compounds.
Table 1 Methylation of Alcohol and Phenol Derivatives with Potassium Carbonate/Dimethyl Sulfate in Acetone
Entry
1
Substrate
Product
Time (h)
12
Yield (%)a
95
4-bromophenol
1-bromo-4-methoxybenzene
1b
2b
2
3-bromophenol
1c
1-bromo-3-methoxybenzene
2c
10
10
8
95
96
90
97
97
90
91
92
95
3
2-chlorophenol
1d
1-chloro-2-methoxybenzene
2d
4
1-naphthol
1e
1-methoxynaphthalene
2e
5
2-naphthol
1f
2-methoxynaphthalene
2f
7
6
phenyl methanol
1g
1-(methoxymethyl)benzene
2g
15
24
24
24
18
7
benzene 1,2-diol
1h
1,2-dimethoxybenzene
2h
8
benzene-1,3-diol
1i
1,3-dimethoxybenzene
2i
9
benzene-1,4-diol
1j
1,4-dimethoxybenzene
2j
10
(S)-1,1′-bi-2-naphthol
(S)-2,2′-dimethoxy-1,1′-binaphthyl
1k
2k
a Isolated yield.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1702–1706

1704
F. Saadati, H. Meftah-Booshehri
LETTER
a
Table 2 Optimization of the Reaction Conditions in the Demethylation of Anisole 2a with SbCl5
Entry
SbCl5 (equiv)
Solvent
Atmosphere
Temp (°C)
Time (h)
Yield (%)c
1
2
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
1
CH2Cl2
toluene
THF
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
air
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.b
55
16
48
24
24
16
16
16
19
16
16
16
16
16
65
29d
35d
75
90
82
80
70
65
75
90
92
75
3
4
MeCN (5% H2O)
MeCN
5
6
MeCN
7
MeCN
8
MeCN
30
9
MeCN
r.t.
r.t.
r.t.
r.t.
r.t.
10
11
12
13
MeCN
2
MeCN
5
MeCN
1.5
MeCN
a SbCl5 was added dropwise at 0 °C.
b SbCl5 was added dropwise at r.t.
c Isolated yield.
d Isolated yield by column chromatography.
1,1′-Bi-(2-naphthol) (1k, BINOL) derivatives, an axially lectively monodeprotected to provide the corresponding
chiral aromatic molecule, with C2-symmetry have been product, 2-methoxy-2′-hydroxy-1,1′-binaphtyl (3k,
found to be useful in various fields of chemistry, especial- BINOL-Me), in excellent yields (Table 3, Scheme 3). The
ly in stoichiometric and catalytic asymmetric reactions.30 monodentate BINOL monoether ligand is well demon-
In experiments involving O-methylated (S)-BINOL (2k) strated for a variety of asymmetric reactions.31
which contained dimethoxy groups, this compound is se-
a
Table 3 Demethylation of Methyl Ethers Using SbCl5
Entry
1
Substrate
Product
Time (h)
16
Yield (%)b
90
anisole
phenol
2a
1a
2
3
4
5
6
7
8
9
1-bromo-4-methoxybenzene
2b
4-bromophenol
1b
24
24
24
48
48
30
24
24
80
78
80
90
82
85
95
88
1-bromo-3-methoxybenzene
2c
3-bromophenol
1c
1-chloro-2-methoxybenzene
2d
2-chlorophenol
1d
1-methoxynaphthalene
2e
1-naphthol
1e
2-methoxynaphthalene
2f
2-naphthol
1f
1-(methoxymethyl)benzene
2g
phenylmethanol
1g
1,2-dimethoxybenzene
2h
2-methoxyphenol
3h
1,3-dimethoxybenzene
3-methoxyphenol
2i
3i
Synlett 2013, 24, 1702–1706
© Georg Thieme Verlag Stuttgart · New York

LETTER
Antimony(V) Chloride as an Efficient Reagent for Deprotection
1705
a
Table 3 Demethylation of Methyl Ethers Using SbCl5 (continued)
Entry
10
Substrate
Product
Time (h)
15
Yield (%)b
94
1,4-dimethoxybenzene
4-methoxyphenol
2j
3j
11
12
13
14
(S)-2,2′-dimethoxy-1,1′-binaphthyl
2k
(S)-2-methoxy-2′-hydroxy-1,1′-binaphthyl
3k
24
72
72
48
95
–
2-methoxyphenol
3h
benzene-1,2-diol
1h
3-methoxyphenol
3i
benzene-1,3-diol
1i
–
4-methoxyphenol
benzene-1,4-diol
47
3j
1j
a Reaction conditions: molar ratio substrate/SbCl5 = 1:1.5; MeCN as solvent; r.t.; N2 atmosphere.
b Isolated yield.
was dissociated after the addition of a saturated sodium
hydrogen carbonate solution during the isolation step
(Scheme 4).
SbCl5
OMe
OMe
OMe
OH
MeCN
r.t.
In conclusion, SbCl5 in acetonitrile was proposed as a
very efficient demethylating agent in the deprotection re-
action of methyl ethers. The generality of the procedure,
high selectivity, and mild reaction conditions, good to ex-
cellent yields, and purification with a very simple workup
procedure could make this method a very useful addition
to the present reported processes. In view of exploiting the
process, selective monodemethylation of poly aryl methyl
ethers was established.
2k
3k
Scheme 3 Synthesis of monodentate BINOL
An interesting feature of this method was that the selectiv-
ity results were the same in the presence of higher
amounts of SbCl5 in the ratio of 2, 3, and 5 mmol to the
substrate. Application of this new method to monode-
methylation of dimethoxybenzene derivatives was also
successful (Table 3).
All chemicals were purchased from commercial suppliers and were
distilled or recrystallized before use. NMR spectra (1H NMR, 13C
NMR) were obtained using a 250 Bruker Avance instrument with
the chemical shifts reported as δ (ppm) and the couplings expressed
in hertz. The IR spectra were recorded using an FTIR Bruker Vector
spectrophotometer with samples prepared as a KBr pellet or in a
mull matrix. Microanalysis of the products was carried out on a
CHN analyzer, PerkinElmer model 2400. Optical rotations were de-
termined with a JASCO-D1P-370 digital polarimeter. Column
chromatography was performed using Merck silica gel 60 (230–400
mesh). Merck silica gel 60 F254 plates (no. 5744) were used to
monitor the reaction progress on TLC.
It is worth noting that the demethylation of 2-methoxy-
phenol (3h) did not proceed after 72 hours. This observa-
tion can be ascribed to steric hindrance by the preferred
coordination of SbCl5 to the OH moiety of the substrate.
It is also interesting to note that the 3-methoxyphenol (3i)
produced similar results. It should be noted that 4-me-
thoxyphenol (3j) was deprotected and changed to the cor-
responding dihydroxy compound under these conditions
within 48 hours, and that no yield better than 47% was ob-
tained (Table 3, entries 11–13). It is worth mentioning that
the preferred coordination of SbCl5 to the OH moiety of 3j
can cause the low yield but it has less steric hindrance in
comparison with ortho and meta derivatives 3h and 3i, re-
spectively.
Preparation of Methyl Ether
A mixture of hydroxyl compound (1 mmol) and anhyd K2CO3 (0.21
g, 1.5 mmol) in anhyd acetone (2 mL) was stirred for 15 min. The
dimethyl sulfate (1 mmol for monohydroxy compound and 1.5
mmol for dihydroxy aryl compound) was added to the mixture and
refluxed to complete the reaction as monitored by TLC. Volatile
compounds were removed under vacuum, and then CH2Cl2 (15 mL)
was added to the reaction mixture under stirring for 5 min. The re-
sulting mixture was filtered, and the solution was extracted with dis-
tilled H2O (4 × 25 mL) and washed with brine solution. The
combined organic layers were dried over Na2SO4 and were evapo-
rated under reduced pressure to afford the corresponding methyl
ether. The analytical spectral data were identical with literature ref-
erences.32b
The mechanistic illustration of these reactions prompted
us to envision the formation of Sb(V) species of type 5 and
then removal of MeCl to obtain type 6. The Sb–O bond
Cl
Sb
Cl
Cl
Me
R
Cl
Cl
Cl
+
1
2
O
O
Sb
Cl
R
Cl
MeCl
Demethylation of Methyl Ethers
5
6
The distilled SbCl5 (0.2 mL, 1.5 mmol) was added dropwise to the
stirred mixture of methyl ether (1 mmol) and anhyd MeCN (2.0 mL)
at 0 °C under nitrogen atmosphere over 15 min. The temperature of
Scheme 4 Proposed mechanism for the deprotection in the presence
of SbCl5
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1702–1706

1706
F. Saadati, H. Meftah-Booshehri
LETTER
(9) Snyder, C. D.; Rapoport, H. J. Am. Chem. Soc. 1972, 94,
the resulting mixture was brought to the ambient temperature. Once
the reaction was complete, the solvent was evaporated under re-
duced pressure. The complex was treated with sat. NaHCO3 for 2 h
(CAUTION: Antimony pentachloride reacts explosively with H2O
and this operation should be performed in a well-ventilated fume
cupboard), and the mixture was extracted using CH2Cl2 (4 × 25
mL). The aqueous mixture was acidified to pH 2 by adding 1 M
HCl. Then, the extraction with CH2Cl2 (4 × 25 mL), drying with
Na2SO4, and evaporation under reduced pressure afforded the cor-
responding product (78–95% yield). Analytical and spectroscopic
data were identical with literature references for known demethyl-
ated products.32
227.
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(S)-2-Methoxy-2′-hydroxy-1,1′-binaphthyl (3k)
Yield 95%; white solid; mp 83–85 °C; [α]D27 –38.7 (c 0.67, THF).
IR (KBr): 3431.9 cm–1. 1H NMR (250 MHz, CDCl3): δ = 3.68 (1 H,
s), 5.05 (1 H, s), 7.11 (1 H, d, J = 8.5 Hz), 7.18–7.41 (6 H, m), 7.47
(1 H, d, J = 9.0 Hz), 7.87 (1 H, d, J = 8.2 Hz), 7.90 (2 H, d, J = 8.0
Hz), 7.98 (1 H, d, J = 9.0 Hz). 13C NMR (62.9 MHz, CDCl3): δ =
53.17, 107.06, 110.52, 114.00, 119.75, 120.30, 120.46, 121.51,
122.54, 123.75, 124.17, 124.67, 125.48, 125.64, 125.72, 127.70,
130.27, 149.01, 151.23. Anal. Calcd for C21H16O2: C,83.66; H,
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Synlett 2013, 24, 1702–1706
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