Synthesis of ionic liquid-supported sulfonyl azide and its application in diazotransfer reaction

Article abstract of DOI:10.1021/jo301529b

The paper describes synthesis of a novel ionic liquid-supported sulfonyl azide and its applications as diazotransfer reagent of active methylene compounds as well as deformylative diazo transfer reagent. The diazo compounds were isolated in excellent yields (82-94%) and high purity. The method offers better separation of product and reagent. This method is experimentally simple and mild, and requires very short reaction time.

Full text of DOI:10.1021/jo301529b

Note
pubs.acs.org/joc
Synthesis of Ionic Liquid-Supported Sulfonyl Azide and Its
Application in Diazotransfer Reaction
Manoj Kumar Muthyala, Sunita Choudhary, and Anil Kumar*
Department of Chemistry, Birla Institute of Technology and Science, Pilani 333031, Rajasthan, India
S
* Supporting Information
ABSTRACT: The paper describes synthesis of a novel ionic
liquid-supported sulfonyl azide and its applications as
diazotransfer reagent of active methylene compounds as well
as deformylative diazo transfer reagent. The diazo compounds
were isolated in excellent yields (82−94%) and high purity.
The method offers better separation of product and reagent.
This method is experimentally simple and mild, and requires very short reaction time.
iazo compounds are useful industrial and pharmaceutical
improved the yield and the separation of products, and the
Dreagents since they can undergo a wide variety of solvent could be recycled.
In continuation of our interest in ionic liquid-supported
chemical transformations such as 1,3-dipolar cycloadditions and
insertion reactions under mild conditions.1,2 Commonly used
reagents in diazotransfer reaction are methanesulfonyl azide
(MsN3),3 p-tolylsulfonyl azide (TsN3),4 p-carboxybenzylsulfon-
yl azide (p-CBSA),5 imidazole-1-sulfonyl azide (Im-SO2N3),
trifluoromethanesulfonyl azide (TfN3),6 and triisopropylbenze-
nesulfonyl azide.7 Unfortunately, some of these reagents are
associated with purification problems due to formation of
sulfonamide coproducts, and some of them decompose at
higher temperature.3 To circumvent these problems, several
alternative reagents such as polymer-supported benzenesulfonyl
azide (PS-SO2N3),8 nonafluorobutanesulfonyl azide (NfN3),9
imidazole-1-sulfonyl azide hydrochloride,10,11 oligomeric sul-
fonyl azide,12 benzotriazol-1-yl-sulfonyl azide (Bt-SO2N3),13
and 2-azido-1,3-dimethylimidazolinium (ADM) salts14 have
been developed. Some of these reagents such as NfN3, ADMP
have improved yield in short reaction time, and the side
product is easily removed. The stability of imidazole-1-sulfonyl
azide salts has been improved by changing the counterion.11
Due to the wide importance of diazo compounds and problems
associated with the safety of reagents and the separation of
sulfonamides generated as side products in diazotransfer
reactions, the development of novel diazotransfer reagents is
of considerable interest.
Over the past few decades, ionic liquids have experienced
exponential growth in research activity.15,16 Now, ionic liquids
have marched far from their reaction media status.17 Task-
specific ionic liquids have been used as alternatively supported
tools,17,18 scavengers,19,20 catalysts,21 and reagents22,23 as well
in combinatorial chemistry (instead of polymers) due to their
unique properties. Task-specific ionic liquids with new
functionalities can be synthesized for application in many fields
of chemistry, especially in organic synthesis. Ramachary et al.
reported ionic liquid promoted organocatalyticall selective
diazotransfer reactions using some of the well-known diazo
transfer reagents.24 Use of ionic liquid as the solvent has
reagents in organic synthesis,18,19,23 herein we report ionic
liquid-supported sulfonyl azide (5) as a green reagent for
diazotransfer reaction under solvent-free conditions. To the
best of our knowledge, this is the first report on the synthesis
and application of ionic liquid-supported sulfonyl azide.
Synthesis of 5 was achieved following the reaction sequences
shown in Scheme 1. Initially, the reaction of 1-methylimidazole
(1) with 1,3-propanesultone (2) at 0 °C followed by reaction
with trifluoromethanesulfonic acid (TfOH) at room temper-
ature for 2 h gave 3. Reaction of 3 with thionyl chloride under
reflux conditions gave 4, which on reaction with sodium azide
1
afforded 5. The structure of 5 was confirmed by IR, H NMR,
and high-resolution mass spectrometry. The IR spectrum of 5
1
showed a strong band for the N3 group at 2130 cm−1. The H
NMR spectrum showed a triplet and a multiplet at δ 4.28 and δ
3.80 for aliphatic protons adjacent to sulfonyl group and 1-
methylimidazole, respectively. Peaks for protons of imidazole
moiety were observed at δ 9.07, 7.79, and 7.65. In HRMS, the
peak for m/z of [M-CF3SO3]+ ion appeared at 230.0714, which
confirmed the structure of 5.
The differential scanning calorimetry (DSC) experiment
showed that the exothermic decomposition temperature of
pure ionic liquid-supported sulfonyl azide (5) is above 150 °C
(Figure 1) with an initiation temperature of 159.69 °C and end
point at 213.72 °C. Therefore, it should work without any
problem below 100 °C. We recommend the use of 5 well below
its decomposition temperature, preferably at room temperature.
It is worth mentioning that 5 has not shown any sign of
decomposition or loss of reactivity even after storing for one
month at room temperature.
After successfully synthesizing 5, we studied its reaction as
diazotransfer reagent for active methylene compounds (Scheme
Received: July 26, 2012
Published: September 7, 2012
© 2012 American Chemical Society
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The Journal of Organic Chemistry
Note
Scheme 1. Synthesis of Ionic Liquid-Supported Sulfonyl Azide (5)
ketosulfone for the synthesis of sulfonylpyrazoles by reacting
with nitroalkenes.25 To further broaden the scope of our
reagent 5, we explored the reaction of β-ketosulfones. As
expected, the reaction went very smooth in a short time with
excellent yield (82−87%) to give corresponding diazo
derivatives of β-ketosulfones (Table 1, entries k−n). It is
worth mentioning that β-ketosulfones have been reported as
poor substrates with several diazotransfer reagents such as Im-
13
SO2N3·HCl10 and Bt-SO2N3 (Table 2, entries 7,8).
The reactivity and stability of 5 was compared with other
traditional diazotransfer reagents (Table 2). As is apparent from
Table 1 and Table 2, higher yields of diazo products were
obtained with 5 in shorter reaction time under optimized
reaction conditions in comparison to other traditional reagents
such as oligomeric-SO2N3, PS-SO2N3, p-CBSA, Im-SO2N3·HCl,
ADMP, and Bt-SO2N3 on similar substrates. Presset et al.26
reported that TsN3 is a better reagent for multigram scale than
other reagents such as MsN3, Im-SO2N3·HCl, ADMC, and p-
CBSA. However, it becomes very difficult to remove tosylamide
generated during the reaction, and repetitive column
chromatography first over silica gel and then over basic
alumina is required. We can overcome this problem by using 5.
Simple extraction with a hexane−ethyl acetate mixture followed
by a water wash gave pure product, except in the case of 8g,
where purification was performed over a silica-gel column.
Comparing the DSC experimental data with some of the
traditional diazotransfer reagents, it was observed that the
initiation temperature at which decomposition started in DSC
for 5 (159 °C) was higher or comparable to these diazotransfer
reagents (Table 2). Only for AMDP was the initiation
temperature at which decomposition started in DSC (200
°C) higher than that for 5.
Figure 1. DSC curve for pure ionic liquid-supported sulfonyl azide
(5).
2). Reaction of dimedone (6h) with 5 was taken as model
reaction to optimize the reaction conditions. In our initial
Scheme 2. Diazotransfer to Active Methylene Compounds
Using 5
Encouraged by these results, we then decided to investigate
the suitability of 5 for synthesis of diazo compounds from
acetophenone. The reaction did not work under these
conditions, and we recovered the acetophenone. We also
screened this reaction using different strong bases such as
sodium hydride and potassium tert-butoxide instead of
triethylamine but did not get the desired product. It is also
reported that diazotransfer to the simple carbonyl compound
usually fails when the methylene group is linked to a single
carbonyl group only.27 The diazo derivative of such compounds
can be prepared by employing the deacylating diazotransfer
strategy.28 We studied the use of 5 for detrifluoroacetylative
diazo group transfer for commercially available 4,4,4-trifluoro-1-
phenylbutane-1,3-dione (6o) and 4,4,4-trifluoro-1-(2-thienyl)-
butane-1,3-dione (6p) (Scheme 3). Treating 1 equiv of 6o and
6p with 5 in the presence of triethyl amine under the above
conditions gave corresponding α-diazo derivatives 8o and 8p in
82% and 80%, respectively.
studies, we investigated the effect of different solvents. It was
observed that the best yield of 8h (94%) was obtained under
solvent-free conditions. Among different solvents studied, the
maximum yield of 8h was obtained in ethanol (92%), whereas
good to moderate yield of 8h was obtained in other solvents
such as acetonitrile (70%), ionic liquid [bmim][BF4] (65%),
THF (60%), and toluene (35%). No product formation was
observed in water.
To assess the scope of 5 as diazotransfer reagent, structurally
diverse 1,3-diketones, β-ketoester, and 1,3-diesters were reacted
with 5 under the optimized reaction conditions to afford
corresponding diazo compounds in excellent yields (Table 1).
There was no appreciable difference in the yield and rate of
reaction with different substrates. All the synthesized diazo
compounds were well-characterized by 1H NMR and 13C NMR
(see Supporting Information).
In conclusion, we have developed a green method for
diazotransfer reaction using a novel ionic liquid-supported
sulfonyl azide under solvent free conditions. Different diazo
compounds were synthesized from active methylene com-
pounds in excellent yield (82−94%) and high purity. The
reagent is versatile and can be used for detrifluoroacetylative
α-Diazo-β-ketosulfones have been employed as carbene
source in intramolecular cyclopropanation, insertion reactions.
Recently, Kumar and Namboothiri has utilized α-diazo-β-
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The Journal of Organic Chemistry
Note
Table 1. Synthesis of Diazo Compounds Using 5
a
b
Isolated yields. Product purified by column chromatography over silica-gel.
methane sulfonic acid (1.1 mmol) was added dropwise to zwitter ion
at 0 °C. The solution was stirred at 40 °C after completion of the
addition until a thick liquid was obtained. The resulting liquid was
washed with diethyl ether to remove excess of triflic acid. The
compound was dried under reduced pressure to give 3 as a thick liquid
in 98% yield.
Procedure for Synthesis of Ionic Liquid-Supported Sulfonyl
Chloride (4). Thionyl chloride (SOCl2) (3 equiv) was added
dropwise to the ionic liquid sulfonic acid 3 (1 equiv) at 0 °C. The
resulting mixture was stirred at room temperature for 8 h and finally
heated up to 80 °C for 2 h. The excess of thionyl chloride was
removed by rotatory evaporator under reduced pressure with nitrogen
atmosphere to get ionic liquid sulfonyl chloride 4 as a thick yellow
liquid in 96% yield.
diazotransfer as well. The method offers better and simple
purification and high purity of product.
EXPERIMENTAL SECTION
■
Procedure for Synthesis of Ionic Liquid-Supported Sulfonic
acid (3). Ionic liquid-supported sulfonic acid (3) was prepared
according to earlier procedures with slight modifications. Propane
sultone (1 equiv) was added dropwise to 1-methylimidazole (1 equiv)
at 0 °C. The resulting solution was stirred at room temperature until a
solid was obtained. After completion of the reaction, the product was
washed with toluene (3 × 15 mL) and finally with diethyl ether (3 ×
15 mL) to remove unreacted starting materials. The compound was
dried under reduced pressure to get the zwitterionic form. Trifluoro-
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The Journal of Organic Chemistry
Note
1
(diazo), 1643, 1317, 1176 cm−1; H NMR (400 MHz, CDCl3) δ 7.57
(d, J = 7.5 Hz, 4H), 7.44 (t, J = 7.4 Hz, 2H), 7.32 (t, J = 7.6 Hz, 4H);
13C NMR (101 MHz, CDCl3) δ 186.4, 136.9, 132.6, 128.3, 128.3.
2-Diazo-3-oxo-butyric Acid Ethyl Ester (8c). Pale yellow liquid29
(139 mg, 89%); IR (neat) νmax 2136 (diazo), 1720 (−CO), 1658
Table 2. Comparison of Stability and Yield for Selected
Substrates with Other Reagents
S.
no.
stability
time
(h)
yield
a,b
c
,
d
reagent
(°C)
substrate product
(%)
e
7512
(O−CO), 1319, 1072 cm−1; H NMR (400 MHz, CDCl3) δ 4.29
1
1
Oligomeric-
SO2N3
−
6h
8h
0.5
(q, J = 7.1 Hz, 2H), 2.47 (s, 3H), 1.32 (t, J = 7.1 Hz, 3H); 13C NMR
(101 MHz, CDCl3) δ 190.4, 161.6, 61.6, 28.3, 14.5.
2
3
4
5
NfN3
120
130
6a
6d
6d
6d
8a
8d
8d
8d
0.25
16
899
638
765
6510
PS-SO2N3
p-CBSA
2-Diazomalonic Acid Diethyl Ester (8d). Yellow liquid29 (162 mg,
87%); IR (neat) νmax 2144 (diazo), 1720, 1373, 1319, 1095 cm−1; H
1
f
−
∼85
16
NMR (400 MHz, CDCl3) δ 4.29 (q, J = 7.1 Hz, 4H), 1.31 (t, J = 7.1
Hz, 6H); 13C NMR (101 MHz, CDCl3) δ 161.2, 61.7, 14.5.
2-Diazo-3-oxo-butyric Acid Methyl Ester (8e). Yellow liquid30 (128
mg, 90%); IR (neat) νmax 2142 (diazo), 1687, 1365, 752, 642 cm−1; 1H
NMR (400 MHz, CDCl3) δ 3.85 (s, 3H), 2.49 (s, 3H); 13C NMR
(101 MHz, CDCl3) δ 190.1, 161.8, 52.2, 28.2.
Im-
SO2N3·HCl
ADMP
16
6
7
8
200
85
6g
6n
6n
8g
8n
8n
0.15
14
7814
5613
Bt-SO2N3
10
Im-
SO2N3·HCl
85
48
−
3-Diazo-2,4-pentanedione (8f). Colorless liquid29 (112 mg, 89%);
a
b
Yields are as reported in the literature. Yields for 8a, 8d, 8g, 8h, and
8n using 5 are 94%, 87%, 87%, 94%, and 84%, respectively (Table 1).
IR (neat) νmax 2140 (diazo), 1687, 1365, 752, 642 cm−1; H NMR
1
(400 MHz, CDCl3) δ 2.37 (s, 6H); 13C NMR (101 MHz, CDCl3) δ
188.4, 28.5.
c
d
Initiation temperature at which decomposition started in DSC. For
e
reagent 5, initiation temperature is 159 °C. Slowly decomposes at
room temperature. Stable at room temperature after two days.
2-Diazoindan-1,3-dione (8g). Solid (150 mg, 87%); mp 146−149
f
°C (Lit.31 148−149 °C); IR (neat) νmax 2121 (diazo), 1697, 1357,
1095, cm−1; H NMR (400 MHz, CDCl3) δ 7.86−7.82 (m, 2H),
1
7.77−7.74 (m, 2H); 13C NMR (101 MHz, CDCl3) δ 182.3, 137.3,
135.0, 122.9.
Scheme 3. Detrifluoroacetylative Diazotransfer
2-Diazo-5,5-dimethylcyclohexane-1,3-dione (8h). Yellow solid
(156 mg, 94%); mp 103−106 °C (Lit.12 105−107 °C); IR (neat)
1
νmax 2138, 1738, 1693 cm−1; H NMR (400 MHz, CDCl3) δ 2.44 (s,
4H), 1.12 (s, 6H); 13C NMR (101 MHz, CDCl3) δ 189.9, 50.5, 31.1,
28.4.
2-Diazocyclohexane-1,3-dione (8i). Yellow solid (130 mg, 94%);
mp 47−49 °C (Lit.26 48−49 °C); IR (neat) νmax 2136, 1737, 1695
cm−1; 1H NMR (400 MHz, CDCl3) δ 2.58−2.53 (m, 4H), 2.08−1.99
(m, 2H); 13C NMR (101 MHz, CDCl3) δ 190.5, 37.0, 18.7.
5-Diazo-1,3-dimethylpyrimidine-2,4,6-trione (8j). Yellow solid
(166 mg, 91%); mp 141−145 °C (Lit.32 158 °C solvent isopropanol);
Procedure for Synthesis of Ionic Liquid-Supported Sulfonyl
Azide (5). Ionic liquid-supported sulfonyl chloride (4, 30 mmol) was
dissolved in acetonitrile and then treated with NaN3 (36 mmol). The
resulting solution was stirred for 12 h at 60 °C. The reaction mixture
was filtered to remove sodium salts. The organic layers concentrated to
give a crude product, which was then washed with DCM and ethyl
acetate mixture (4 × 50 mL, 1: 1 v/v) to purify the ionic liquid.
Finally, the ionic liquid-supported sulfonyl azide (5) was dried under
reduced pressure.
1
IR (neat) νmax 2156 (diazo), 1650, 1303, 1265 cm−1; H NMR (400
MHz, CDCl3) δ 3.34 (s, 6H); 13C NMR (101 MHz, CDCl3) δ 158.2,
150.5, 28.6.
2-Diazo-1-(4-chlorophenyl)-2-tosylethanone (8k). Solid (274 mg,
82%); mp 125−127 °C (Lit.33 134 °C); IR (neat) νmax 2104 (diazo),
Ionic Liquid-Supported Sulfonyl Azide (5). Thick pale yellow
1
1680 (CO), 1381, 1286, 1072 cm−1; H NMR (400 MHz, CDCl3)
1
liquid; IR (neat) νmax 2145 (diazo), 1366, 1257, 1157, 1034 cm−1; H
δ 7.90 (d, J = 8.1 Hz, 2H), 7.51 (d, J = 8.3 Hz, 2H), 7.41 (d, J = 8.3
Hz, 2H), 7.35 (d, J = 7.9 Hz, 2H), 2.45 (s, 3H); 13C NMR (101 MHz,
CDCl3): δ 181.6, 145.6, 139.4, 138.5, 138.2, 129.8, 129.2, 129.0, 128.2,
21.7.
NMR (500 MHz, DMSO-d6) δ 9.06 (s, 1H), 7.75 (s, 1H), 7.69 (s,
1H), 4.28 (t, J = 6.9 Hz, 2H), 3.83 (s, 2H), 3.80 (d, J = 7.6 Hz, 2H),
2.36−2.28 (m, 2H); 13C NMR (126 MHz, DMSO-d6) δ 137.9, 124.2,
122.6, 122.4, 119.8, 52.2, 47.1, 36.2, 24.3; HRMS (ESI-qTOF) Calcd
for C7H12N5O2S+ 230.0706; Found 230.0714 [M-CF3SO3]+.
2-Diazo-1-(4-tolyl)-2-tosylethanone (8l). Solid (273 mg, 87%); mp
120−123 °C; IR (neat) νmax 2124 (diazo), 1675 (CO), 1361, 1276
General Procedure for Diazotransfer Using Ionic Liquid-
Supported Sulfonyl Azide (5). (CAUTION: Although we never had
any accident while using 5, it may be explosive.) Triethyl amine (1.5
mmol) was added to the mixture of active methylene compound (1
mmol) and ionic liquid-supported sulfonyl azide (5) (1.2 mmol). The
resulting mixture was stirred at room temperature for the time
indicated in Table 2. The progress of reaction was monitored by TLC.
After completion of reaction, the product was extracted with hexane/
ethyl acetate mixture (4 × 10 mL, 1: 1 v/v) and washed with water.
The organic layers were combined, dried with anhydrous sodium
sulfate, and concentrated to give crude product. In the case of 8g,
purification was performed over silica-gel column. The side product
ionic liquid-supported sulfonylamide (7) does not dissolve in organic
layer and thus can be easily removed. Physical and spectral data of
synthesized diazo compounds are given below.
1
cm−1; H NMR (400 MHz, CDCl3) δ 7.95 (d, J = 8.4 Hz, 2H), 7.47
(d, J = 8.2 Hz, 2H), 7.36 (d, J = 8.1 Hz, 2H), 7.23 (d, J = 7.9 Hz, 2H),
2.45 (s, 3H), 2.39 (s, 3H); 13C NMR (101 MHz, CDCl3) δ 182.3,
145.3, 144.0, 138.7, 133.7, 129.7, 129.5, 128.2, 127.6, 21.7, 21.6.
HRMS (ESI-qTOF) Calcd for C16H15N2O3S+ 315.0798; Found
315.0803 [M + H]+.
2-Diazo-1-(4-methoxyphenyl)-2-tosylethanone (8m). Pale yellow
solid (274 mg, 83%); mp 141−145 °C (Lit.33 124 °C); IR (neat) νmax
1
2121 (diazo), 1633 (CO), 1208, 1024, 642 cm−1; H NMR (400
MHz, CDCl3) δ 7.95 (d, J = 8.4 Hz, 2H), 7.61−7.54 (m, 2H), 7.35 (d,
J = 8.1 Hz, 2H), 6.95−6.88 (m, 2H), 3.85 (s, 3H), 2.44 (s, 3H); 13C
NMR (101 MHz, CDCl3) δ 181.3, 163.5, 145.3, 138.7, 129.9, 129.7,
128.4, 128.2, 114.1, 55.6, 21.7.
Ethyl 2-Diazo-2-(phenylsulfonyl)acetate (8n). Yellow solid (213
mg, 84%); mp 48−49 °C (Lit.13 52−54 °C); IR (neat) νmax 2129
2-Diazo-1-phenyl-butane-1,3-dione (8a). White solid (165 mg,
1
(diazo), 1727 (CO), 1371, 1286, 1072 cm−1; H NMR (300 MHz,
88%); mp 62−63 °C (Lit.29 60−62 °C); IR (neat) νmax 2177 (diazo),
1
1651 (CO), 1059, 752, 642 cm−1; H NMR (500 MHz, CDCl3) δ
CDCl3) δ 8.04 (d, J = 7.5 Hz, 2H), 7.69 (t, J = 7.6 Hz, 2H), 7.59 (t, J =
7.6 Hz, 2H), 4.25−4.18 (q, J = 14.1 Hz, 2H), 1.27 (t, J = 6.9 Hz, 3H);
13C NMR (101 MHz, CDCl3) δ 159.6, 141.7, 134.1, 129.2, 127.9, 62.4,
14.2.
7.64 (dd, J = 5.5, 4.0 Hz, 2H), 7.61−7.55 (m, 1H), 7.53−7.47 (m,
2H), 2.59 (s, 3H); 13C NMR (126 MHz, CDCl3) δ 190.9, 185.1,
137.3, 132.7, 128.9, 127.3, 29.2.
2-Diazo-1,3-diphenylpropane-1,3-dione (8b). Solid (212.6 mg,
2-Diazo-1-(phenyl)ethanone (8o). Yellow liquid (120 mg, 82%);
85%); mp 106−109 °C (Lit.5 107−109 °C); IR (neat) νmax 2119
(Lit.34 27−28 °C); IR (neat) νmax 2106 (diazo), 1725, 1371, 1228,
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The Journal of Organic Chemistry
Note
1180 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.79−7.72 (m, 2H), 7.57−
7.50 (m, 1H), 7.43 (dd, J = 6.8, 4.5 Hz, 2H), 5.93 (s, 1H); 13C NMR
(101 MHz, CDCl3) δ 186.4, 136.6, 132.7, 128.7, 126.7, 54.2.
2-Diazo-1-(2-thienyl)ethanone (8p). Solid (122.7 mg, 80%); mp
61−64 °C (Lit.35 60 °C); IR (neat) νmax 2108 (diazo), 1705, 1415,
1259, 1035 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.60 (dd, J = 5.0, 1.1
Hz, 1H), 7.50 (dd, J = 3.8, 1.1 Hz, 1H), 7.11 (dd, J = 4.9, 3.8 Hz, 1H),
5.81 (s, 1H); 13C NMR (101 MHz, CDCl3) δ 178.9, 142.5, 132.2,
129.0, 128.0, 54.3.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Copies of 1H NMR, 13C NMR, and mass spectra for all
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
(35) Regitz, M.; Tawfik, A. M.; Heydt, H. Synthesis 1979, 805.
AUTHOR INFORMATION
Corresponding Author
*E-mail: anilkumar@bits-pilani.ac.in
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Council of Scientific
and Industrial Research (CSIR), New Delhi (01(2214)/08/
EMR-II). M.K.M. and S.C. thank CSIR, New Delhi, for Senior
Research Fellowship (SRF) and Junior Research Fellowship
(JRF), respectively.
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