Layered double hydroxide-supported l-methionine-catalyzed chemoselective o-methylation of phenols and esterification of carboxylic acids with dimethyl carbonate: A "green" protocol

Article abstract of DOI:10.1002/chem.200902626

" Chemical equation presented" Simple methodology: A layered double hydroxide-supported L-methionine (LDH-Met) catalyst is designed in a simple methodology to explore its synthetic utility in biologically relevant reactions. The organocatalyst is characterized by FT-IR, TGA/DTA, powder XRD, and EDX spectroscopic techniques. This material has been successfully utilized for the preparation of aryl methyl ethers and esters from the corresponding phenols and carboxylic acids, respectively, in moderate to high yields (see scheme).

Full text of DOI:10.1002/chem.200902626

DOI: 10.1002/chem.200902626
Layered Double Hydroxide-Supported l-Methionine-Catalyzed
Chemoselective O-Methylation of Phenols and Esterification of Carboxylic
Acids with Dimethyl Carbonate: A “Green” Protocol
[a]
Amarajothi Dhakshinamoorthy, Alagarsamy Sharmila, and Kasi Pitchumani*
[
45–50]
Alkylation of aromatics, an important reaction in organic
chemistry, is widely used in the synthesis of petrochemicals,
proaches have been reported
with different catalytic
systems to obtain selective O-methylation of phenol with
DMC as methylating agent. For example, O-methylation of
phenol is achieved using alkyl methyl carbonates as methyl-
ating agent with a limitation of having bulky linear alkyl
group (at least 3 carbon atoms) and a polar aprotic sol-
[1–4]
fine chemicals and pharmaceuticals.
Methyl halides, di-
methyl sulfate and diazomethane are commonly used in
[5]
methylation reactions. The alkylation of a carboxylic acid
to the corresponding methyl ester is also another fundamen-
[
6–7]
[45]
tal transformation in organic chemistry.
techniques including microwave applications
lite-based catalysts
A number of
vent. 2-Naphthol is methylated with DMC and methanol
[
8–10]
and zeo-
have been developed for the esterifi-
and has resulted in a mixture of products (O-methylation
and C-methylation) which mainly depends on the differen-
ces in the acid strengths of the zeolite and activation of 2-
naphthol as well as the alkylating molecule. As the ionic
liquid, [BMIm]Cl is used for O-methylation of phenol selec-
tively to anisole and the percentage conversion mainly de-
pends on the imidazolium moiety of the ionic liquid.
Phenol is O-methylated by a continuous-flow method which
requires poly(ethyleneglycol) 1000 as an anion activator
Fluorine-modified Mg-Al mixed
oxides have been used for O-methylation of phenol and the
percentage conversion depends on F/Mg ratio. Ouk et al.
have demonstrated O-methylation of phenol with DMC
using potassium ion based bases.
[11–17]
cation process to prepare fine chemicals used in the synthe-
sis of drugs, food preservatives, solvents, perfumes, pharma-
[46]
[18–19]
ceuticals, plasticizers and cosmetics.
Most of the report-
ed procedures for esterification/etherification require use of
sulfuric acid, hydrochloric acid and other toxic chemicals
which are environmentally hazardous. Considering the
impact of these chemicals on the environment, there is an
urgent need to develop more ecofriendly methods for the
production of methyl ethers/esters. During the past two de-
cades, the non-toxic dimethyl carbonate (DMC) has
emerged as a green alternative to replace the above toxic re-
agents and a number of methods have been reported for
DMC-mediated processes with homogeneous and heteroge-
[47]
[48]
along with K CO .
2
3
[49]
[50]
In a continuous flow
process use of alumina or alumina/silica as heterogeneous
catalysts leads to the formation of aryl methyl ethers in
good yield but byproducts due to C-methylation are also
[
20–36]
[37–40]
neous catalysts,
uids,
zeolite-based catalysts,
ionic liq-
[
41–42]
[43]
[44]
[51]
pentaalkylguanidines and DBU.
Generally, O-methylation of phenols is very slow at low
temperatures as it requires a higher energy of activation and
hence normally higher temperatures are necessary. Though
it is a safe and efficient alternative, use of DMC in O-meth-
ylation needs rigorous conditions, as the major reaction is
only methoxycarbonylation. The procedure is also not suita-
ble for high boiling-point phenols. Consequently, various ap-
observed. These results have prompted us to develop a pro-
cess which can avoid the use of solvent, does not require an
additional anion activator, selective towards O-methylation
and applicable to a wide range of substrates with different
functional groups.
Layered materials such as hydrotalcite-like layered
double hydroxides (LDH) possess properties such as large
surface area, high ion exchange capacity, swelling and inter-
calation which make them ideal supports for immobiliza-
[
52]
[53]
[
a] Dr. A. Dhakshinamoorthy, A. Sharmila, Prof. Dr. K. Pitchumani
School of Chemistry, Madurai Kamaraj University
Madurai, 625021 Tamil Nadu (India)
tion of homogeneous catalysts such as chiral catalysts,
[54]
[55]
metal–salen complex, Pd metal and metal nanoparticles
namely Pd ,
0
[56]
0[57]
0[58]
Rh
and Pt
in their interlayer space.
Fax : (+91)0452-2459181
E-mail: pit12399@yahoo.com
LDH has also been used to prepare highly oriented organic-
[59]
LDH films. In this work, we report the preparation of a
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902626.
novel LDH-supported l-methionine (based on a reported
1128
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 1128 – 1132

COMMUNICATION
[60]
procedure ) and its successful use as a reusable catalyst for
methylation of phenols and carboxylic acids with DMC
with electron-donating groups and/bulky substituents retard
[47]
O-methylation (Table 2, entries 11–13).
This O-methyla-
(
Scheme 1). The products are obtained by simple filtration
tion protocol is also found to be suitable for ketones having
hydroxyl groups (Table 2, entries 16–19). o-Hydroxyaceto-
phenone results in low yield
of the catalyst after extracting with dichloromethane, which
and this may be due to the in-
tramolecular hydrogen bonding.
This elegant methodology is
also extended to prepare p-me-
thoxystyrene in good yield
(
Table 2, entry 20).
To generalize this method
further, S- and N-methylation
are also carried out under iden-
tical conditions. S-Methylation
is very efficient giving 97%
Scheme 1. Chemoselective reactions of DMC in the presence of LDH-supported l-methionine catalyst.
makes this protocol suitable for various synthetic purposes.
Table 1. O-Methylation of phenol to anisole with DMC in the presence
of different catalysts.
The prepared (LDH-Met) catalyst is characterized by
powder XRD, FT-IR, TGA/DTA and EDX analysis (Sup-
porting information).
[a]
[b]
Run
Catalyst
t [h]
Yield [%]
Phenol is used as the substrate to optimize the reaction
conditions. Without catalyst, O-methylation of phenol with
DMC has afforded no product, while in the presence of cat-
alyst, anisole is obtained in quantitative yield with 100% se-
lectivity (Table 1). It is noteworthy that no C-methylated
1
2
3
4
5
6
7
8
9
none
l-methionine
hydrotalcite
4
4
6
6
6
4
6
4
8
0
0
23
39
25
90
100
0
K
2
CO
3
Na
2
CO
3
LDH-Met
LDH-Met
LDH-Met
LDH-Met
1
products are observed as evident from its H NMR spectrum
[
[
c]
(
Supporting Information). DMC acts as a solvent as well as
d]
8
methylating agent. Control experiments show that, the reac-
tion fails at reflux temperature (908C) and is very slow at
[a] Phenol (0.1 mL), DMC (1.2 mL), catalyst (100 mg), temperature
1808C) in an autoclave. [b] Determined by GC. [c] Reaction carried out
at reflux temperature (908C). [d] Methanol is used as methylating agent.
(
1408C. However, at 1808C the reaction leads to completion
with 100% selectivity towards O-methylation. Also it is ap-
propriate to note that the percentage conversion gradually
increases as the reaction time increases and optimum con-
version is realized in 6 h. It has been well established that
the methyl group attached to the methionine sulfur atom in
S-adenosyl methionine (SAM) is chemically reactive and
this group gets readily transferred to an acceptor substrate
in transmethylation reactions. More than 40 metabolic reac-
tions involve this transfer of a methyl group from SAM to
various substrates such as nucleic acids, proteins, and
yield with very little amount of diphenyl disulfide as byprod-
uct. On the other hand, aniline reacts slowly and yields
mono- as well as dimethylated products which is in accord-
[62]
ance with previous report.
Though aniline is generally
more nucleophilic than phenol, the decrease in reactivity in
methylation observed here is attributed to a) facile ioniza-
tion to more nucleophilic phenolate ion taking place readily
under basic conditions and b) greater steric hindrance to
methylation with the former. Based on the observed results,
the following order of reactivity is (O-methylation ꢀ S-
methylation > N-methylation) is rationalized.
[61]
lipids.
In view of the importance of this methyl group
transfer in SAM, we wish to study the effect of LDH-Met in
methylation reactions involving DMC.
Further studies show that substituted phenols also found
to be readily converted to the corresponding ethers under
the present experimental conditions in the presence of
LDH-Met as catalyst (Table 2, entries 1–4). Substituted poly-
phenolic compounds are also satisfactorily converted to its
corresponding ethers (Table 2, entries 5,7,8), though low
yields were observed with catechol. Resorcinol gives exclu-
sively dimethylated product whereas quinol gives a mixture
of mono- and dimethylated products (Table 2, entries 7 and
Recently, Selva et al. have reported zeolite-based chemo-
selective esterification of indolecarboxylic acids with DMC
[63]
as methylating agent. These observations have prompted
us to extend the scope of this protocol for esterification of
various carboxylic acids under identical conditions and it
has been observed that simple carboxylic acids (Table 3, en-
tries 1–4) are alkylated in good yields and excellent purities.
When phenolic and amino groups are present, methylation
8). Under the present experimental conditions, 4-aminophe-
occurs at both OH as well as at NH group and the ratio of
2
nol yields both O- and N-methylated products (Table 2,
entry 9). The reactivity of various phenols is found to
depend on the substituents in the benzene ring. Phenols
the products are determined by NMR spectroscopy
(Table 3, entries 5–6). N-Protected amino acids such as N-
acetylated p-aminobenzoic acid (Table 3, entry 3) can also
Chem. Eur. J. 2010, 16, 1128 – 1132
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1129

K. Pitchumani et al.
Table 2. O-Methylation of phenols with DMC in the presence of LDH-
Met.
Table 3. Synthesis of carboxylic esters using LDH-Met as heterogeneous
catalyst.
[
a]
[a]
[
b]
[
b]
Entry Starting acid
Product
Yield [%]
Entry
Substrate
Yield [%]
1
2
3
4
R
R
R
R
1
2
3
89
88
90
X=O
1
2
3
4
5
6
7
8
9
H
Cl
H
H
H
H
H
H
H
OH
H
H
H
H
H
H
H
H
H
Br
NO
NO
H
H
OH
NH
2
OCH
H
H
H
H
H
H
H
H
H
H
H
tBu
H
H
92
90
95
89
H
2
2
[c]
4
5
89
88
OH
OH
H
H
H
H
H
tBu
tBu
H
81
10
[
c]
89
88
90
[
d]
[
e]
10
11
12
13
14
15
16
17
18
19
20
3
82
10
05
06
89
94
87
CH
H
tBu
3
6
84
3 7
C H
7
8
90
89
2-naphthol
4-hydroxybenzophenone
[f]
COCH
3
H
H
H
H
H
H
H
23
H
H
H
COCH
3
86
89
80
H
H
COCH
3
CH=CH
9
85
89
2
X=S
2
1
2
H
H
H
H
H
H
H
H
97
50
1
0
X=NH
2
1
1
1
1
2
3
84
–
[
(
a] Reaction conditions: Substrate (0.1 mL), DMC (1.2 mL), catalyst
100 mg), temperature (1808C) in an autoclave, time (6 h). [b] Isolated
1
yield and are characterized by H NMR. [c] Both hydroxyl groups are
methylated. [d] Combined yield of mixture of products and mono/di
91
(
84:16) methylation ratio is determined by GC. [e] Combined yield of
mixture of products and O/N (52:48) methylation ratio is determined by
NMR. [f] Yield determined by GC.
14
91
91
1
1
5
6
84
81
be cleanly transformed to the corresponding esters. In addi-
tion indole derivatives are cleanly converted to the corre-
sponding esters (Table 3, entries 8–9). Dicarboxylic acids are
conveniently methylated at both carboxyl groups with the
present catalytic system (Table 3, entry 10). 1-Phenyl, 1-
naphthylacetic and also p-methylthiophenylacetic acids are
converted to the corresponding esters successfully (Table 3,
entries 11, 13 and 14). This protocol is also very much useful
17
[
a] Reaction conditions: Substrate (0.1 mL), DMC (1.2 mL), catalyst
(100 mg), temperature (1808C) in an autoclave, time (6 h).
Table 4. Recycling of LDH-Met. in the reaction of phenol with DMC.
for the synthesis of cinnamic acid ester and its p-NO substi-
2
Cycle
1
2
3
4
tuted esters in very good yield without affecting the olefinic
double bond (Table 3, entries 15 and 16). In all cases, prod-
ucts are obtained with >98% purity. The catalyst is also re-
cycled. In the reaction of phenol with DMC, LDH-Met is
recovered by filtration after extracting with dichlorome-
thane (10 mL) and it is activated in an air oven at 908C for
Selectivity [%]
Yield [%]
100
90
100
91
100
88
100
89
[36]
ple.
For example, oxygen nucleophiles such as alcohols
1
h. In all the runs, the mass balance was >90%. This is
give mainly transesterification products either under gas–
[64]
reused for further runs as shown in Table 4.
liquid phase-transfer catalysis at 1808C
or under batch
Tundo et al. have demonstrated that nucleophilic substitu-
tion on DMC by various anions having different soft/hard
character can be rationalized on the basis of HSAB princi-
conditions. At 2008C also, the reaction occurs at the carbon-
yl group only suggesting a hard acid/hard base interaction.
On the other hand, methylation of alcohols is reported to
1130
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 1128 – 1132

Heterogeneous Catalysis
COMMUNICATION
[
65]
occur in the presence of tertiary amines (e.g., (dimethyla-
mino)pyridine, 1,4-diazabicyclo[2.2.2]octane). In this case,
and remarkable environmentally benign features: 1) it uses
cheap and non-toxic DMC; 2) DMC acts as a solvent and
methylating agent; 3) catalyst can be prepared readily;
4) aryl methyl ethers and esters are quantitatively obtained
and waste of substrates are avoided; 5) LDH-Met can be
easily recycled and reused without decreasing catalytic activ-
ity; 6) no hazardous waste is produced. Finally, the products
are obtained in high purity and this protocol does not re-
quire further purification.
AHCTUNGTRENNUNG
the catalyst modifies the hard–soft character of the two cen-
ters, thus allowing the nucleophilic displacement at the
methyl group (softer acid) by the phenoxide ion (softer
base) to occur. With p-substituted phenols, electron-with-
drawing substituents in the ring, by facilitating the formation
of softer phenoxide anion, give S 2 displacement yielding
once again methylation as the major reaction.
N
To find support for the observed selectivity in the present
study, the following mechanism is proposed. l-Methionine is
anchored onto anionic layered double hydroxide via its
Experimental Section
À
COO group in such a way that the amine group is exposed
In general, to phenol (0.1 mL), LDH-Met (100 mg) and DMC (1.2 mL)
were added. The mixture was kept in an autoclave at 1808C for 6 h. The
reaction mixture was extracted from the heterogeneous medium, upon
stirring with dichloromethane for about 8 h, and filtered. The filtrate was
washed with water and the organic layer was dried over anhydrous
sodium sulfate. Products were isolated and were confirmed by their
and the elongated side chain creates a hydrophobic pocket.
The free amine abstracts acidic hydrogen of phenol generat-
ing a softer phenolate anion, which in turn attacks DMC at
the methyl group (softer electrophile) forming a six-mem-
bered transition state inside the hydrophobic pocket, which
rearranges to give anisole and methanol as the end products
1
HNMR spectra.
(
Scheme 2). In control experiments, reaction of phenol with
Acknowledgements
K.P. thanks Department of Science and Technology, New Delhi for finan-
cial assistance. A.D.M. thanks Council of Scientific and Industrial Re-
search, New Delhi for the award of Senior Research Fellowship.
Keywords: dimethyl
heterogeneous catalysis
methylation
carbonate
·
esterification
·
·
·
layered double hydroxide
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[
Received: September 23, 2009
Published online: December 10, 2009
2
1132
www.chemeurj.org
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Chem. Eur. J. 2010, 16, 1128 – 1132