Amino acid intercalated layered double hydroxide catalyzed chemoselective methylation of phenols and thiophenols with dimethyl carbonate

Article abstract of DOI:10.1016/j.tetlet.2013.10.098

Sixteen different amino acids are intercalated into Mg-Al layered double hydroxides (LDHs) by the reconstruction method and are characterized by powder XRD and FT-IR. The intercalated amino acid-LDHs (AA-LDHs) are used as catalysts for chemoselective O-methylation of phenol and S-methylation of thiophenol with dimethyl carbonate (DMC) as a green methylating agent. The intercalation behavior of various amino acids is influenced by various structural features of amino acids, namely, carbon chain length, structure, and physicochemical properties. In particular, amino acids possessing a hydrophobic side-chain show higher catalytic activity. A suitable reaction mechanism is proposed. The catalyst can also be recycled.

Full text of DOI:10.1016/j.tetlet.2013.10.098

Tetrahedron Letters 54 (2013) 7167–7170
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Tetrahedron Letters
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Amino acid intercalated layered double hydroxide catalyzed
chemoselective methylation of phenols and thiophenols with
dimethyl carbonate
Thirumeni Subramanian a, Amarajothi Dhakshinamoorthy a,b, Kasi Pitchumani a,b,
⇑
a School of Chemistry, Madurai Kamaraj University, Madurai-21, Tamil Nadu, India
b Centre for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj University, Madurai-21, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Sixteen different amino acids are intercalated into Mg–Al layered double hydroxides (LDHs) by the recon-
struction method and are characterized by powder XRD and FT-IR. The intercalated amino acid–LDHs
(AA-LDHs) are used as catalysts for chemoselective O-methylation of phenol and S-methylation of thio-
phenol with dimethyl carbonate (DMC) as a green methylating agent. The intercalation behavior of var-
ious amino acids is influenced by various structural features of amino acids, namely, carbon chain length,
structure, and physicochemical properties. In particular, amino acids possessing a hydrophobic side-
chain show higher catalytic activity. A suitable reaction mechanism is proposed. The catalyst can also
be recycled.
Received 20 September 2013
Revised 21 October 2013
Accepted 22 October 2013
Available online 28 October 2013
Keywords:
Amino acids
Layered double hydroxides
Dimethyl carbonate
Methylation
Ó 2013 Elsevier Ltd. All rights reserved.
Chemoselectivity
Green methylating agent
In recent years, clay minerals have received considerable atten-
tion as support for the synthesis of new organic–inorganic nanohy-
brid materials.1 Among the different types of clay minerals, layered
double hydroxides (LDHs) are considered to be highly promising
due to their easy preparation and broad uses as adsorbents.2 In
addition, LDHs are biocompatible and find application in pharma-
ceuticals such as non-viral vectors for delivery of antisense oligo-
nucleotides, drug stabilizer, anticancer drugs in cancer treatment,
therapy of digestive disorders, and support for controlled release
formulations of pharmaceuticals.3,4 Owing to the intercalation
property of LDHs, many LDH composites with intercalated benefi-
cial organic anions, such as DNA,5 pesticide,6 plant growth regula-
tors,7 and drugs8 have been reported. Among them nucleotide,9
deoxyribonucleic acid,10 and amino acid11 intercalated LDHs exhi-
bit good catalytic activity and excellent chemoselectivity. Recently,
Nakayama et al.12 reported intercalated amino acids and oligopep-
tides over LDHs by the reconstruction method (memory effect)
which proved the ability of LDHs to regenerate the layered struc-
ture when exposed to water and anions.
dyes, and agricultural chemicals and are widely used as antioxidants
in oils and stabilizers for polymers.14 Protection of thiol groups is yet
another important reaction in organic synthesis and especially in
peptide, protein, and b-lactam synthesis.15 Various methylating
agents such as iodomethane,16 methanol,17 methyl halide,18 di-
methyl sulfate,19 trimethyl phosphate,20 and tetramethylammo-
nium chloride21 have been used for methylation reaction. The
traditional synthetic methodologies using some of these toxic alkyl-
ating reagents generate large quantities of waste. Safer, clean, and
selective methylation protocols can be conceived with the use of
non-toxic DMC.22,23 This safe, green methylating agent enjoys
advantages such as environmentally benign, cost-effective, can act
as solvent, and above all produces CO2 and methanol as by-products
(which can be recycled back to DMC). Consequently, various ap-
proaches have been reported with different catalytic systems to ob-
tain selective O-methylation of phenol with DMC, for example,
fluorine-modified mesoporous Mg–Al mixed oxides,22b ZnCl2 modi-
fied –Al2O3,24 Cs-loaded MCM-41,25 and [BMIM]ClÀ.26
Recently, we have reported L-methionine intercalated LDHs as
Protection and deprotection of fine chemicals through environ-
mentally clean and economical processes is an emerging area with
high commercial importance.13 The protected O-methylated phe-
nols (anisoles) are important intermediates in the field of fragrances,
catalysts for chemoselective O-methylation of phenol and esterifi-
cation of aromatic carboxylic acids using DMC as methylating
reagent.27 In this context, we report the synthesis and character-
ization of sixteen different AA-LDHs with an aim to identify an
ideal catalyst based on their activity derived through their struc-
tural interaction with LDHs in chemoselective methylation of phe-
nol and thiophenol. Although, intercalation of amino acids onto the
⇑
Corresponding author.
E-mail address: pit12399@yahoo.com (K. Pitchumani).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.10.098

7168
T. Subramanian et al. / Tetrahedron Letters 54 (2013) 7167–7170
Table 1
positive charge occurring between the LDH basal layer and the
side-chain of Arg and Lys.12
Interlayer distance of amino acids intercalated LDHs
The catalytic activity of these AA-LDHs was tested in the
chemoselective methylation of phenol and thiophenol with
DMC31 and the observed results are summarized in Table 2. Phenol
and thiophenol were selected as model substrates to optimize the
reaction conditions using DMC as methylating reagent. LDHs re-
sulted in 26% and 10% yields for the methylation of phenol and
thiophenol respectively at 180 °C in 12 h. The amino acids possess-
Entry
Amino acid
d (nm)
Gallery height (nm)
a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
—
0.78
1.44
1.43
1.11
1.11
1.08
1.48
1.72
1.08
0.81
0.82
0.79
0.79
0.77
0.77
0.77
0.30
0.96
0.95
0.63
0.63
0.60
1.00
1.24
0.60
0.33
0.34
0.31
0.31
0.29
0.29
0.29
Leucine
Isoleucine
(Leu)
(Ile)
(a-Aba)
a
-Aminobutyric acid
Aspartic acid
Glutamic acid
Phenylalanine
Histidine
Tyrosine
Alanine
Serine
Glycine
Cysteine
Proline
Arginine
Lysine
(Asp)
(Glu)
(Phy)
(His)
(Tyr)
(Ala)
(Ser)
(Gly)
(Cys)
(Pro)
(Arg)
(Lys)
ing large hydrophobic pockets such as Leu-LDHs, Ile-LDHs, a-Aba-
LDHs, Asp-LDHs, Glu-LDHs, His-LDHs, and Phy-LDHs were tested
as catalysts for the O-methylation of phenol and afforded more
than 90% yield (Table 2, entries 2–7). It is noteworthy to mention
that no C-methylated products were observed as evidenced from
1H NMR and ESI-MS. On the other hand, amino acids intercalated
with short side chain such as Ala, Ser, Gly, Tyr, Cys, and Pro re-
sulted in low yield (Table 2, entries 9–14). However, complete con-
version of phenol was observed with Arg-LDHs and Lys-LDHs along
with high chemoselectivity (Table 2, entries 15 and 16). Although it
was quite difficult to achieve intercalation of Arg and Lys over
LDHs, the physical adsorption of these amino acids over LDHs
showed higher activity and this is believed to be due to the hydro-
phobic pocket present in these amino acids. The catalytic activity
of various amino acids was also tested and the observed results
a
LDHs.
layers of LDHs is a well known procedure to obtain inorganic–or-
ganic hybrid materials, their catalytic applications are quite lim-
ited. In this regard, we tried to intercalate various amino acids
such as arginine, lysine, leucine, isoleucine, phenylalanine, a-ami-
nobutyric acid, aspartic acid, glutamic acid, histidine, alanine, ser-
ine, glycine, tyrosine, cystine, and proline onto the layers of LDHs
and their catalytic activity is tested in methylation of phenol and
thiophenol with DMC.
The as prepared AA-LDHs are characterized by powder XRD and
FT-IR (see Supplementary data). Characteristic reflections for Mg–
Al LDHs are observed at 11.4°, 22.8°, and 34.8° with d003 = 0.78 nm
which indicates the existence of CO23À ions intercalated onto the
interlayer space of LDHs. The thickness of the LDH basal layer is
0.48 nm and the interlayer space is calculated as 0.30 nm (Table 1,
entry 1), and these values are in good agreement with previous
report.12 The diffraction peak Leu-LDHs (d003 = 1.44 nm) (Fig. 1),
Table 2
Chemoselective methylation of phenol and thiophenol with DMC in the presence of
various AA-LDHsa
XH
O
XCH
3
AA-LDHs
180°C
H3C
CH3
CO2
CH3OH
O
O
Entry
AA-LDHs
Time (h)
Yieldb (%)
X = O
X = S
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
LDHs
Leu-LDHs
Ile-LDHs
12
6
6
6
6
6
6
6
12
12
12
6
12
12
6
26
98
82
87
96
95
88
70
03
05
02
89
02
27
96
95
92
10
98
95
80
82
96
85
88
36
22
19
90
26
29
94
94
97
Ile-LDHs (d003 = 1.43 nm),
(d003 = 1.11 nm), and Glu-LDHs (d003 = 1.08 nm) with the distinct
expanded LDH structure (Table 1, entries 2–6) are observed. In
the hydrophobic amino acids such as
a-Aba-LDHs (d003 = 1.11 nm), Asp-LDHs
a-Aba-LDHs
L
-Phe-LDHs (d003 = 1.48 nm)
Asp-LDHs
Glu-LDHs
Phy-LDHs
His-LDHs
Ala-LDHs
Ser-LDHs
Gly-LDHs
Tyr-LDHs
Cys-LDHs
Pro-LDHs
Arg-LDHs
Lys-LDHs
Met-LDHc
and -His-LDHs (d003 = 1.72 nm) expanded basal spacing (Table 1,
L
entries 7 and 8) is observed. These values indicate that the amino
acids are intercalated vertically onto the LDHs through the interca-
lation with carboxylate groups (Fig. 1). The diffraction peaks with
unexpanded
(d003 = 0.81 nm),
(d003 = 0.79 nm), Tyr-LDHs (d003 = 0.80 nm), Cys-LDHs (d003 = 0.79
nm), and Pro-LDHs (d003 = 0.77 nm) (Table 1, entries 9–14) which
suggest the parallel arrangement over the interlayer due to the ab-
sence of the large hydrophobic group. The interlayer values after
intercalation with amino acids are slightly higher compared to car-
bonate ions indicating the effective intercalation of amino acids
into the interlayer space of LDHs. On the other hand, Arg and Lys
are difficult to be intercalated into the LDHs due to the repulsion
LDHs
are
also
noticed
(d003 = 0.82 nm),
with
Ala-LDHs
Gly-LDHs
Ser-LDHs
6
6
a
Reaction conditions: phenol/thiophenol (1 mmol), DMC (1.2 mL), AA-LDHs
(100 mg), 180 °C in an autoclave.
b
Determined by GC.
Data taken from Ref. 27.
c
+
+
+
+
+
+
+
O-
O
1.44nm
0.96nm
NH2
H2N
0.78 nm
0.30 nm
O
-O
+
+ + + +
+
+
Leu-LDHs
Figure 1. Schematic structural models of Mg–Al LDHs and Leu-LDHs.

T. Subramanian et al. / Tetrahedron Letters 54 (2013) 7167–7170
7169
Table 3
are given in Table 3. It is surprising to note that those amino acids
exhibiting higher activity within the interlayer space of LDHs
showed negligible activity without LDHs. This clearly indicates that
the intercalation of amino acids onto the layer of LDHs creates an
environment which can facilitate the reaction in a facile manner
with high conversion and selectivity.
These interesting preliminary results prompted us to expand
the scope of Leu-LDHs with various substituted phenols and thi-
ophenols and the observed results are given in Table 4. Phenols
with electron withdrawing and donating substituents were con-
verted to their corresponding anisoles in higher yields at short
reaction time (5–6 h). o-, m-, p-Nitrophenols gave higher yields un-
der the optimized condition. p-Bromo and p-iodophenols resulted
in 90% and 92% yields of the respective anisoles. Increasing the ste-
ric hindrance on the aromatic ring showed not much influence on
the reaction yield. For example, o-acetyl and p-t-butylphenol
exhibited 89% and 94% yields to their respective anisoles. 2-Naph-
thol also gave 98% yield to its corresponding ether.
Chemoselective O-methylation of phenol with amino acidsa
Entry
Amino acid
Time (h)
Yieldb (%)
1
2
3
4
5
Arginine
Glutamic acid
Lysine
Aspartic acid
Leucine
6
6
6
6
6
3
4
2
2
2
a
Reaction conditions: phenol (1 mmol), DMC (1.2 mL), amino acid (50 mg),
180 °C in an autoclave.
b
GC yield.
Table 4
Leu-LDHs catalyzed chemoselective O-methylation of substituted phenols and
thiophenols with DMCa
XH
O
XCH
3
Leu-LDHs
180°C
H3C
CH3
CO2
CH3OH
O
O
R1
R1
On the basis of the above results and also in accordance with
our previous report,27 a plausible reaction pathway is proposed
for Leu-LDHs catalyzed chemoselective O-methylation of phenol
with DMC as shown in Scheme 1. Leu is intercalated into anionic
LDHs through COOÀ group in such a way that the amine group is
exposed and the elongated side chain created a hydrophobic pock-
et. The free amine abstracts acidic hydrogen from phenol generat-
ing a softer phenolate anion, which in turn attacks DMC at the
methyl group (softer electrophile) forming a six membered transi-
tion state inside the hydrophobic pocket, which further rearranges
to give anisole and methanol as the end products.
Entry
R1
H–
o-NO2–
m-NO2–
p-NO2-
o-Cl–
p-Cl–
o-Br–
p-Br–
p-I–
X
T (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
6
5
5
5
6
5
6
6
6
6
6
6
6
6
6
6
6
6
6
6
98
89
99
98
86
82
87
90
92
91
89
94
91
98
95
92
98
87
81
89
p-OCH3–
o-COCH3–
p-tBu–
One of the main advantages of heterogeneous catalysis is the
recovery and reuse of the catalyst after the reaction. In this connec-
tion, the reusability of Leu-LDHs was tested in the O-methylation
p-COOH–
2-Naph–c
m-Isopropyl–
p-C6H5CO–
H–
p-Cl–
p-Br–
Table 5
Reusability of the catalysta
S
S
S
Reuse
First
98
Second
96
Third
95
Fourth
91
p-OCH3–
Yieldb (%)
a
Reaction conditions: substrate (1 mmol), DMC (1.2 mL), Leu-LDHs (100 mg),
a
Reaction conditions: phenol (1 mmol), DMC (1.2 mL), Leu-LDHs (100 mg),
180 °C in an autoclave.
b
180 °C, 6 h.
GC yield.
b
c
GC yield.
2-Naphthol.
NH2
O-
CH3
CH3
O
O
R
H2C
R =
NH2
O-
H2N
O
-O
Leu-LDHs
Leu-LDHs
OH
O
O
H3C
O
NH2
O
CH3
H
OCH3
O
NH2
R
O
H
H
O-
+
N
R
O-
O
R
+ CO2 + CH3OH
Leu-LDHs
(reusable)
O
Leu-LDHs
Leu-LDHs
Scheme 1. Plausible reaction mechanism for Leu-LDHs catalyzed O-methylation of phenol with DMC.

7170
T. Subramanian et al. / Tetrahedron Letters 54 (2013) 7167–7170
Table 6
Comparison of the present work with other reported catalytic systems for methylation of phenol
Entry
Catalyst
Methylating agent
(CH3)2SO4
Base
Solvent
Time (h)
Yield (%)
Reusability of catalyst
28
1
2
3
4
5
6
7
8
9
—
NaOH
K2CO3
—
—
—
KOH
—
—
—
—
EtOH
DMEb
—
—
—
—
—
—
—
1
89
—
—
—
5
—
—
—
—
—
4
Me4NCl21
—
25–60 min
1–3 min
19–91
72–99
>90
96
56–89
99
94
96
98
a
BF3/ether19b
P(OMe)3
a
[BMIm]Cl26
DMC
DMC
(CH3)2SO4
DMC
DMC
DBU29
10 min
3–12
8
2
1
6
Basic alumina20
F/Mg(Al)O22b
Calcined Mg–Al hydrotalcites30
Cs-MCM-4125a
Leu-LDHs (present work)
MeOH
DMC
10
—
a
Microwave mediated reaction.
1,2-dimethoxyethane.
b
of phenol with DMC. After completion of the reaction, Leu-LDHs
were recovered by simple filtration after extracting with acetoni-
trile (10 mL) and activated in an air oven at 90 °C for 1 h then used
for the next run. As it can be seen from Table 5, the catalyst can be
reused four times with no significant loss in its activity.
References and notes
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the increase in interlayer space of LDHs, characterized by powder
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Acknowledgements
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29. Tilstam, U. Org. Process Res. Dev. 2012, 16, 1150.
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31. General experimental procedure for AA-LDHs catalyzed methylation reactions: In
general, to 1 mmol of substrate, 100 mg of AA-LDHs and 1.2 mL of DMC were
added. This mixture was kept in an autoclave at 180 °C for 6 h. The reaction
mixture was extracted from the heterogeneous medium, upon stirring with
acetonitrile for about 8 h, and filtered. The organic layer was dried over
anhydrous sodium sulfate and the products were isolated and confirmed by
their 1H NMR data.
K.P. thanks the Department of Science and Technology, New
Delhi for the financial assistance and ADM thanks the University
Grants Commission, New Delhi for Assistant Professorship under
its Faculty Recharge Programme.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
10.098.