Synthesis and rearrangement of cage [4.3.2]propellanes that contain a spiro linkage

Article abstract of DOI:10.1002/ejoc.201700617

Rearranged cage ketones 6a and 6b are reported in eight linear synthetic steps from 2,5-dimethoxybenzaldehyde 9 without the use of protecting groups. In this regard, Diels–Alder reaction, [2+2]photocycloaddition and Lewis acid promoted rearrangement with BF₃·OEt₂ were used as key steps. Surprisingly, during the ring expansion process with Lewis acid, solvent in-corporation occurred. This rearrangement approach has provided difficult complex targets through non-obvious synthetic routes. The rearrangement process demonstrated here opens up a new synthetic strategy to interesting and unusual cage molecules.

Full text of DOI:10.1002/ejoc.201700617

A Journal of
Accepted Article
Title: Synthesis and Rearrangement of New Cage [4.3.2]Propellanes
Containing Spiro Linkage
Authors: Sambasivarao Kotha, Subba Rao Cheekatla, and Binita
Mandal
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700617
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10.1002/ejoc.201700617
European Journal of Organic Chemistry
Synthesis and Rearrangement of New Cage [4.3.2]Propellanes Containing
Spiro Linkage
Sambasivarao Kotha*, Subba Rao Cheekatla and Binita Mandal
Department of Chemistry, Indian Institute of Technology-Bombay, Powai, India
Fax: 022-25727152; Phone: +91(22)-2572 7160, E-mail: srk@chem.iitb.ac.in
Abstract
Rearranged cage ketones 6a and 6b are reported in eight linear synthetic steps starting with
2,5-dimethoxybenzaldehyde 9 without the involvement of protecting groups. In this regard,
Diels‒Alder reaction, [2+2] photocycloaddition and Lewis acid promoted rearrangement with
BF3OEt2 has been used as key steps. Surprisingly, during the ring expansion process with
Lewis acid, solvent incorporation occurred. This rearrangement approach has provided difficult
complex targets via non-obvious synthetic routes. Rearrangement process demonstrated here
opens up a new synthetic strategy to interesting and unusual cage molecules.
Keywords:
Cage compounds, CAN oxidation, DielsAlder reaction, [2+2]photocycloaddition, Lewis
acid catalyzed rearrangement.
Introduction
Carbocyclic cage compounds1-5 have received the attention of synthetic chemists during the
past four decades because of their applications in diverse areas of chemistry and these include:
natural product synthesis,3b,6 high energy materials,5a,7 medicinal chemistry,8 pharmaceutical
applications,9 polymers,10 thermostable oils11 and supramolecular chemistry.12 Some of these
molecules are also of theoretical interest because of their molecular symmetry, compact rigid
1
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carbon framework, ready tendency to undergo skeletal rearrangements, unusual strain energy
and deviation from normal C‒C bond angles.13
Lewis acid-promoted rearrangement of various polycyclic cage diones as well as propellanes
has received considerable interest due to their synthetic importance as well as mechanistic
scrutiny.14-16 Ring-rearrangement approach provide a non-conventional access to a variety of
new and unusual cage systems.17 As part of our ongoing programme, we are interested in
developing novel strategies for the synthesis of new cage molecules which are suitable for
rearrangement approach that provide intricate structures which are difficult to obtain by direct
synthetic routes. In view of our current efforts in the synthesis of propellane derivatives,23 here
we report the synthesis and rearrangement of a rigid cage [4.3.2]propellanediones. Carbocation
.
mediated rearrangement using BF3 OEt2 gave solvent incorporated product. The structure of
the product 6a was unambiguously established by single-crystal X-ray diffraction studies.
Results and Discussion
Scheme 1. Synthesis of cage dione, rearranged cage ketones and polyquinane
Previous work14d
Present work
Previously (Scheme 1, eq. 1),14d carbocation mediated rearrangement of cage system 1 using
.
BF3 OEt2 was reported. In this regard, cage [4.4.2]propellanedione 1 has been prepared by
DielsAlder reaction of cyclopentadiene and 1,4-napthaquinone followed by [2+2]
2
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.
photocycloaddition and hydrogenation sequence which undergo BF3 OEt2 mediated
rearrangement in benzene to deliver 2, 3 and 4 in 15%, 25% and 20% selective yields,
respectively. In our present strategy (Scheme 1, eq. 2), we disclose ring rearrangement of cage
[4.3.2]propellanes such as 5a-b with the aid of Lewis acid.
Retrosynthetic analysis of hexacyclic dione 5a was depicted in Scheme 2. In this context the
key building block, indane-based quinone 7 on Diels‒Alder reaction may produce cycloadduct
which on [2+2] photocycloaddition can generate the dione 5a. The known indane derivative 7
was assembled from dimethoxy indanone 8 involving reduction of carbonyl group and CAN
oxidation. The required indanone 8 was prepared from a readily available aldehyde 9 via
unsaturated acid in two step sequence involving Fridel-Crafts cyclization as key step.
Scheme 2. Retrosynthetic Analysis of target molecule 5a
Our journey towards the synthesis of cage dione 5a begin with the preparation of the key
building block, quinone derivative 7. Commercially available 2,5-dimethoxybenzaldehyde 9,
on treatment with malonic acid 10 under Knoevenagel reaction conditions afforded α, β-
unsaturated acid 1118 in 75% yield (Scheme 3). Later the acid derivative 11 was smoothly
transformed into saturated acid 1219 (92%) via hydrogenation with Pd-C. Next, 2,5-
dimethoxybenzenepropanoic acid 12 was subjected to cyclization in the presence of P2O5 and
MeSO3H afforded the corresponding indanone 820 in 60% yield. Subsequently, carbonyl group
was converted to methylene in the presence of trialkylsilane in CF3CO3H media to deliver the
4,7-dimethoxy-indane 1321 in 55% yield. Oxidation of dimethoxy indane 13 was performed
with CAN oxidation at 0 °C to afford the key building block 7,22 a useful dienophile in Diels‒
Alder sequence with various dienes like cyclopentadiene 14 as well as spirodienes 15 and 16
(Scheme 3). The required spirodienes15 and 16 were prepared by adopting the known literature
procedures.24-25
3
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Scheme 3. Synthesis of key building block 7
Having prepared the building block 7, next we directed our efforts to prepare the cage diones
(Scheme 3). [4+2] Cycloaddition20d,23 of quinone 7 with a freshly cracked 1,3-cyclopentadiene
14 at 0 °C delivered the DA adduct 17 (84%). Along similar lines, cycloaddition26,27 of the
quinone 7 with freshly generated spirodienes 15 and 16 under thermal conditions produced DA
adducts 18 and 19 in good yields (Scheme 4). The stereochemistry of the DA adducts 17, 18
and 19 has been expected to be endo and this observation found to be correct, when we realized
that these DA adducts underwent a smooth intramolecular [2+2] photocycloaddition23,26 upon
exposure to UV light within 2 h. The formation of DA adducts 17, 18 and 19 was strongly
supported on the basis of 1H NMR, 13C NMR and HRMS data.
The DA adducts 17, 18 and 19 derived from indanedione 7 undergo [2+2]
photocycloaddition in dry ethyl acetate when they were irradiated with 125W UV lamp through
a pyrex immersion well for 2 h under N2 atmosphere to furnish the required cage diones 5a, 5b
and 5c in excellent yields (Scheme 4). The structure of the cage propellanes 5a, 5b and 5c were
1
13
fully established on the basis of H NMR, C NMR, APT and further supported by HRMS
data. The structure of the dione 5c was further supported by single-crystal X-ray diffraction
studies (Figure 1).28
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Scheme 4. Synthesis of rearranged cage ketone’s 6a, 6b and spiro cage dione 5c
5c (CCDC 1481200)
6a (CCDC 1481201)
Figure 1. Crystal structures of 5c and 6a with thermal ellipsoids drawn at the 50% probability
level.
Based on previous literature reports,14-16 we envisioned that hexa- as well as heptacyclic cage
propellanediones 5a and 5b are useful candidates for carbocation rearrangements. Therefore,
.
compounds 5a and 5b were treated with BF3 OEt2 in dry benzene under reflux conditions to
afford novel rearranged cage ketones 6a and 6b in a moderate to good yields. Along with
5
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similar lines, compound 5c was subjected to BF3OEt2 in dry benzene under reflux conditions
and unfortunately, a complex mixture of products was obtained which could not be separated
by column chromatography (Scheme 4). The structures of both these cage ketones 6a and 6b
1
13
have been strongly confirmed by H NMR, C NMR, APT and HRMS data. Finally, the
structure of ring-rearranged ketone 6a was well established by single-crystal X-ray diffraction
studies (Figure 1).28
Scheme 5. Plausible mechanism for the rearrangement of cage dione 5a to 6a
The proposed mechanism for the Lewis acid catalyzed rearrangement of cage dione 5a with
BF3.OEt2 is shown in Scheme 5. Initially, the Lewis acid attacks on the carbonyl group present
in cage dione 5a, and further migration of the cyclobutane bond in 5a will generate carbocation
intermediate A2 which is trapped by the solvent (benzene) forms C2. Later aromatization
followed by hydrogen transfer gives intermediate D2. Finally, elimination of BF3OH from D2
will produce carbocation E2 which on hydride transfer from 5a (or from solvent) generates the
rearranged product 6a (Scheme 5).
Variation in results obtained from 1 and 5a-b (Scheme 1) is attributed to the difference in
flexibility and strain of the fused rings such as cyclohexane and cyclopentane rings. The
migratory modes of propellane frame works might be influenced by their ring size as well as
the presence of an incoming nucleophile. Since the six-membered ring in 1 is more flexible
than the five-membered ring in 5a-b and less strain involved in the cyclohexane ring more
6
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favourable to formation of different products obtained from 1 such as 2, 3 and 4 (Scheme 1,
eq. 1).
In case of 5a-b such feasibility is not involved due to size of the fused ring. Because of less
flexibility as well as more strain in 5a-b causes the formation of exclusive rearranged product
6a-b where solvent has been incorporated (Scheme 1, eq. 2). This is due to the reason where
carbocation has formed immediately trapped by the solvent and does not accesses to the any
other rearranged products. This approach has provided a new synthetic strategy to interesting
and unusual cage systems.
Conclusion
In summary, we have successfully demonstrated a concise and efficient synthetic route to
various functionalized cage [4.3.2]propellanediones 5a, 5b, 5c and rearranged cage ketones 6a
and 6b from commercially available 2,5-dimethoxybenzaldehyde. Interestingly, solvent
incorporation was observed in cage propellane framework during Lewis acid promoted
carbocation rearrangement. This rearrangement strategy opens up new opportunities to create
novel cage frameworks that are difficult to prepare via direct synthetic methodologies. Further
studies to assemble other interesting functionalized caged systems will be reported in due
course.
Experimental Section
General Experimental Details
All the required reagents, chemicals and solvents were purchased from the commercial grade
and used as without any further purification. Analytical TLC was performed on (10×5) glass
plates coated with Acme’s silica gel (GF-254) containing 13% calcium sulfate as a binder. All
the reactions were monitored by TLC using suitable solvent system and visualization was done
under UV light, exposure to iodine vapour and by dipping in to a solution of KMnO4. Dry
reactions were performed in oven dried glassware under nitrogen atmosphere by using standard
syringe-septum techniques. Acme’s silica gel (100-200 mesh size) and neutral alumina was
used for column chromatography and solvents were concentrated at vacuo on rotary evaporator.
Benzene, Toluene, DCM were distilled from P2O5, CaH2 and ethyl acetate was dried by using
K2CO3.
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IR spectra were collected on a Nicolet Impact-400 FTIR spectrometer and
samples were prepared as a thin film between CsCl plates by dissolving the compound in DCM
and chloroform and then evaporating the solvent. 1H NMR (400 and 500 MHz), 13C, 13C-APT
NMR (100 and 125 MHz) spectra were recorded on Bruker spectrometer and samples were
made in chloroform-d solvent. The chemical shifts are reported in parts per million (ppm) on
delta scale with TMS as an internal standard and values for the coupling constants (J) are given
in Hz. The standard abbreviations for 1H NMR spin couplings are given as s, d, t, q, dd, dt, td,
and m for singlet, doublet, triplet, quartet, doublet of doublet, doublet of triplet, triplet of
doublet and multiplet respectively. High-resolution mass spectra (HRMS) were recorded in a
positive ion electrospray ionization (ESI). All melting points were recorded on veego VMP-
CMP melting point apparatus and are uncorrected. Whatever all reported yields are isolated
yields of the products after column purification. X-ray data were recorded on diffractometer
equipped with graphite monochromated Mo Kα radiation and structure was solved by direct
methods shelxl-97 and refined by full-matrix least-squares against F2 using shelxl-97 software.
Dihydroindanedione 722 and spiro dienes 1525 and 1624 were prepared according to known
literature procedures.
Synthesis of Diels–Alder adduct 17
A solution of dihydroindanedione 7 (500 mg, 3.37 mmol) in methanol (10 mL) was cooled in
an ice-water bath at 0 ºC and freshly cracked cyclopentadiene 14 (0.3 mL, 3.54 mmol) was
added dropwise manner. Then, the resulting reaction mixture was stirred at same temperature
until all the starting material was consumed. After conclusion of the reaction (2 h, TLC
monitoring), the solvent was evaporated under reduced pressure and the crude residue was
directly purified by silica gel column chromatography without any further workup by using 5%
ethyl acetate/petroleum ether as an eluent which afforded DielsAlder adduct 17 as a pure pale
°
1
yellow solid. Yield 84% (610 mg); m.p. 97-99 C; IR (neat, cm-1) 2932,1660; H NMR (400
MHz, CDCl3): = 6.00 (s, 2H), 3.50 (s, 2H), 3.23 (s, 2H), 2.72-2.60 (m, 4H), 1.97-1.89 (m,
2H) 1.53, 1.43 (ABq, J = 8.0 Hz, 2H) ppm; 13C NMR (125 MHz, CDCl3): = 197.9, 155.2,
135.0, 50.7, 49.2, 48.6, 31.0, 21.5 ppm; HRMS (ESI) m/z calcd for C14H14NaO2 [M + Na]+
237.0888; found: 237.0886.
Synthesis of hexacyclic cage propellanedione 5a:
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The Diels–Alder adduct 17 (300 mg, 1.39 mmol) was dissolved in dry ethyl acetate (350 mL)
and irradiated in a pyrex immersion well by using 125W medium pressure UV mercury-vapour
lamp (Home-made) for 2 h under continuous flow of nitrogen at room temperature. After
completion of the reaction (TLC monitoring), then the solvent was evaporated under reduced
pressure and the crude residue was directly subjected to silica gel column chromatography by
using 10% ethyl acetate/petroleum ether as an eluent which afforded 5a as a pure white
°
crystalline solid. Yield 93% (280 mg); m.p. 142-144 C; IR (neat, cm-1) 2963, 2869, 1741,
1728, 1450, 1218, 1023; 1H NMR (500 MHz, CDCl3): = 2.92 (m, 2H), 2.67 (m, 4H), 2.02-
1.97 (m, 2H), 1.92-1.84 (m, 2H), 1.79-1.72 (m, 2H), 1.52 (d, J = 5.9 Hz, 1H), 1.50 (d, J = 6.1
Hz, 1H) ppm; 13C NMR (125 MHz, CDCl3): = 212.6, 60.7, 55.2, 44.2, 41.1, 40.7, 27.2, 26.6
ppm; HRMS (ESI) m/z calcd for C14H14NaO2 [M + Na]+ 237.0886; found: 237.0882.
Synthesis of Diels–Alder adduct 19
A solution of dihydroindanedione 7 (200 mg, 1.35 mmol) in dry benzene was added to freshly
made spiro[2.4]hepta-4,6-diene 16 (220 mg, 2.43 mmol) with stirring. Then the resulting
reaction mixture was refluxed for overnight. At the end of the reaction (TLC monitoring), the
solvent was removed under vacuo and the crude reaction mixture was directly subjected to
silica gel column chromatography without any further workup by using 5% ethyl
acetate/petroleum ether as an eluent to give the Diels–Alder adduct 19 as a pure light yellow
°
crystalline solid. Yield 67% (220 mg); m.p. 119-122 C; IR (neat, cm-1) 2989, 2955, 1657,
1
1427, 1391, 1320, 1103, 702 ; H NMR (500 MHz, CDCl3): = 6.09 (m, 2H), 3.40 (s, 2H),
13
2.86 (s, 2H), 2.70-2.64 (m, 4H),1.94-1.91 (m, 2H), 0.58-0.48 (m, 4H) ppm; C NMR (125
MHz, CDCl3): = 197.8, 155.6, 135.2, 53.6, 51.7, 45.1, 31.1, 21.6, 8.1, 7.1 ppm; HRMS (ESI)
m/z calcd for C16H16NaO2 [M + Na]+ 263.1043; found: 263.1047.
Synthesis of heptacyclic cage propellanedione 5c
The Diels–Alder adduct 19 (200 mg, 0.83 mmol) was dissolved in dry ethyl acetate (350 mL)
and irradiated in a pyrex immersion well by using 125W medium pressure UV mercury-vapour
lamp (Home-made) for 3 h under continuous flow of nitrogen at room temperature. After
conclusion of the reaction (TLC monitoring, 2 h) the solvent was evaporated under reduced
pressure and the crude reaction mixture was purified by silica gel column chromatography
using 8% ethyl acetate/petroleum ether as an eluent which afforded 5c as a white crystalline
solid. Yield 89% (178 mg); m.p. 176-178 °C; IR (neat, cm-1) 2955, 1742, 1264, 1217, 1181; 1H
NMR (500 MHz, CDCl3): = 2.90-2.86 (m, 4H), 2.27-2.26 (m, 2H), 2.05-1.91 (m, 2H), 1.80
9
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13
(td, J = 13.0, 6.9 Hz, 2H), 1.56-1.53 (m, 2H), 0.73-0.63 (m, 4H) ppm; C NMR (125 MHz,
CDCl3): = 212.5, 61.3, 55.7, 49.9, 41.5, 37.3, 27.3, 26.8, 5.5, 4.2 ppm; HRMS (ESI) m/z
calcd for C16H16NaO2 [M + Na]+ 263.1043; found: 263.1043.
Synthesis of Diels–Alder adduct 18
A solution of dihydroindanedione 7 (300 mg, 2.14 mmol) in dry toluene was added to freshly
made spiro[4.4]nona-1,3-diene 15 (460 mg, 3.85 mmol) with stirring and this was kept refluxed
for overnight. At the end of the reaction (TLC monitoring), the solvent was removed under
vacuo and the crude reaction mixture was directly purified by silica gel column
chromatography without any further workup by using 5% ethyl acetate/petroleum ether as an
eluent to furnish the Diels–Alder adduct 18 as a pure pale yellow crystalline solid. Yield 61%
(350 mg); m.p. 122-124 °C; IR (neat, cm-1) 2949, 1659, 1429, 1391, 1319, 704; 1H NMR (500
MHz, CDCl3): = 5.97 (m, 2H), 3.30-3.29 (m, 2H), 3.06 (s, 2H), 2.66-2.62 (m, 4H),1.93-1.88
(m, 2H), 1.58-1.41 (m, 8H) ppm; 13C NMR (125 MHz, CDCl3): = 198.3, 155.4, 135.7, 69.1,
56.1, 50.8, 31.9, 31.6, 30.9, 25.8, 25.3, 21.4 ppm; HRMS (ESI) m/z calcd for C18H20NaO2 [M
+ Na]+ 291.1356; found: 291.1355.
Synthesis of heptacyclic cage propellanedione 5b
The Diels–Alder adduct 18 (300 mg, 1.11 mmol) was dissolved in dry ethyl acetate (350 mL)
and irradiated in a pyrex immersion well by using 125W medium pressure UV mercury-vapour
lamp (Home-made) for 2 h under continuous flow of nitrogen at room temperature. After
conclusion of the reaction (TLC monitoring, 2h), the solvent was evaporated under reduced
pressure and the crude residue was directly purified by silica gel column chromatography
without any workup by using 10% ethyl acetate/petroleum ether as an eluent which afforded
°
5b as a white crystalline solid. Yield 95% (285 mg); m.p. 157-159 C; IR (neat, cm-1) 2949,
2328, 1740, 1723, 1263, 1217; 1H NMR (500 MHz, CDCl3): = 2.81-2.77 (m, 4H), 2.47 (m,
13
2H), 2.02-1.85 (m, 2H), 1.78 (td, J = 12.3, 6.5 Hz, 2H), 1.66-1.50 (m, 10H) ppm; C NMR
(125 MHz, CDCl3): = 212.4, 64.6, 60.4, 55.0, 51.9, 41.3, 32.1, 28.6, 27.2, 26.8, 25.7,25.5
ppm; HRMS (ESI) m/z calcd for C18H20NaO2 [M + Na]+ 291.1356; found: 291.1351.
BF3.OEt2-Promoted rearrangement of cage dione 5a
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Cage dione 5a (150 mg, 0.69 mmol) was added to a stirred solution of distilled BF3OEt2 (1
mL) in dry benzene (10 mL) at room temperature. Then, the resulting reaction mixture was
refluxed for 36 h. After completion of the reaction by TLC analysis, the reaction mixture was
quenched with a saturated aqueous NaHCO3 solution and this was extracted with benzene. The
combined organic layer was washed with water, brine solution and dried over anhydrous
Na2SO4. After removal of the solvent under vacuo, the resulting crude residue was subjected to
silica gel column chromatography by using 3% ethyl acetate/petroleum ether as an eluent to
give the desired rearranged caged molecule with a phenyl group 6a as a colourless liquid, which
is solidified as a white crystalline solid in the refrigerator upon storage for overnight in hexane
solvent. Yield 47% (90 mg); m.p. 112-114 °C; IR (neat, cm-1) 2942, 1759, 1217, 702, 666, 614;
1H NMR (500 MHz, CDCl3): = 7.35-7.20 (m, 5H), 2.60-2.59 (m, 1H), 2.55 (q, J = 5.7 Hz,
1H), 2.49 (m, 1H), 2.38-2.36 (m, 1H), 2.21 (dd, J = 12.2, 4.7 Hz, 2H), 1.96-1.93 (m, 1H), 1.87
(dt, J = 4.0, 2.0 Hz, 1H), 1.72-1.66 (m, 2H), 1.59-1.44 (m, 4H), 1.31 (d, J = 10.5 Hz, 1H) ppm;
13C NMR (125 MHz, CDCl3): = 217.9, 147.4, 128.7, 126.7, 126.0, 58.2, 55.3, 53.1, 50.2,
49.3, 46.4, 44.0, 41.4, 40.4, 36.6, 35.2, 22.9, 21.2 ppm; HRMS (ESI) m/z calcd for C20H20NaO
[M + Na]+ 299.1406; found: 299.1400.
BF3.OEt2-Promoted rearrangement of cage dione 5b
Cage dione 5b (200mg, 0.74 mmol) was added to stirred solution of distilled BF3OEt2 (1 mL)
in dry benzene (10 mL) at room temperature and the resulting reaction mixture was stirred at
90 ºC for 48 h. At the end of the reaction by TLC evident, the reaction mixture was quenched
with a saturated aqueous NaHCO3 solution and this was extracted with benzene. The combined
organic layer was washed with water, brine solution and dried over anhydrous Na2SO4. After
removal of the solvent under vacuo, the resulting crude reaction mixture was subjected to silica
gel column chromatography by using 7 % ethyl acetate/petroleum ether as an eluent to afford
the desired rearranged cage ketone 6b as a pure colourless liquid, which is solidified as a white
crystalline solid in the refrigerator upon storage for overnight in hexane solvent. Yield 60%
(150 mg); m.p. 125-127 °C; IR (neat, cm-1) 2952, 2925, 2864, 1742, 1459, 1376, 1218, 700; 1H
NMR (500 MHz, CDCl3): = 7.32-7.18 (m, 5H), 2.77-2.75 (m, 1H), 2.59-2.55 (m, 1H), 2.40
(td, J = 5.7, 1.5 Hz, 1H), 2.24 (dt, J = 5.5, 1.6 Hz, 1H), 2.08-2.00 (m, 2H), 1.94-1.91 (m, 1H),
13
1.87, 1.75-1.74 (m, 1H), 1.69-1.67 (m, 2H), 1.53-1.25 (m, 11H) ppm; C NMR (125 MHz,
CDCl3): = 217.8, 147.5, 128.7, 126.9, 125.9, 58.1, 57.2, 55.8, 54.1, 53.0, 49.4, 49.1, 49.0,
44.8, 42.2, 36.8, 32.8, 26.3, 26.1, 22.9, 21.2 ppm; HRMS (ESI) m/z calcd for C24H26NaO [M
+ Na]+ 353.1876; found: 353.1871.
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Associated Content
Supporting Information
Copies of NMR (1H, 13C & APT) spectra for all new compounds; X-ray crystallographic data
(CIF files) for compounds 6a and 5c. This material is available free of charge via the internet
at ……
Author information
Corresponding Author
*E-mail: srk@chem.iitb.ac.in.
Acknowledgement
We thank Defence Research and Development Organisation (DRDO, NO.
ARDB/01/1041849/M/1), New Delhi for financial assistance. The authors also thank
Gaddamedi Sreevani and Darshan Mhatre for their help in collecting the X-ray data and
structure refinement. S. K. thanks Department of Science and Technology (DST, NO.
SR/S2/JCB-33/2010) for the award of a J. C. Bose fellowship and Praj industries for Chair
Professorship (Green Chemistry). S. R. C. thanks University Grants Commission (UGC), New
Delhi for the award of a research fellowship.
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