On water: iodine-mediated direct construction of 1,3-benzothiazines from ortho-alkynylanilines by regioselective 6-exo-dig cyclization

Article abstract of DOI:10.1039/C9OB00128J

Herein, we report the 6-exo-dig ring closure of ortho-alkynylanilines with readily available aroyl isothiocyanate. An environmentally benign, metal- and base-free, iodine promoted cascade synthesis of highly functionalized (benzo[1,3]thiazin-2-yl)benzimid

Full text of DOI:10.1039/C9OB00128J

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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DOI: 10.1039/C9OB00128J
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On Water: Iodine-Mediated Direct Construction of 1, 3-
Benzothiazines from ortho-Alkynylanilines by Regioselective 6-
Exo-dig Cyclization
Received 00th January 20xx,
Accepted 00th January 20xx
Kapil Mohan Saini, Rakesh K. Saunthwal, Shiv Kumar and Akhilesh K. Verma*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Herein, we report the 6-exo-dig ring closure of ortho-alkynylanilines with readily available aroyl isothiocyanate. An
environmentally benign, metal- and base-free, iodine promoted cascade synthesis of highly functionalized
benzo[1,3]thiazin-2-yl)benzimidic acids has been accomplished via in situ generated ortho-alkynylthiourea. The established
methodology employs the abundant chemical feed stocks of ortho-alkynylanilines and aroyl isothiocyanates and could be
applied in the late-stage synthesis of pharmaceutically active 1,3-benzothiazine containing molecules. Furthermore, the
discovered protocol exclusively delivers the bis (benzo[1,3]thiazin-2-yl)dibenzimidic acid products and preserves the iodo-
olefin substitution pattern which can be exploited by further derivatization.
Nitrogen and sulphur-containing compounds, widely used as
organic electroluminescent devices3 and pharmaceutical drug
candidates,4 play an important role in modern organic
Introduction
Iodine-mediated transformations have become ubiquitous in
the functionalizations and assembly of biological and
pharmaceuticals molecules.1 Most of the reported methods
converts carbon-carbon triple bond into the carbon-carbon
double bond using organic solvents2 in the presence of base
and iodine due to the high stability of the resulting iodonium
ions in organic solvents and the presence of soft-soft
interactions between iodine and alkyne bond compared to the
C-C double bond counterparts. Carbon-carbon triple bond
activation in water has become a subject of intensive research
due to the unprecedented reactivity of alkyne bonds in water
and the development of new environment-friendly methods
synthesis. 1,3-benzothiazines derivatives have shown
significant antiproliferative and anticancer activity (Fig. 1, i,
and ii).5 The benzothiazine core moiety has found many
applications, such as it required for the protein N-acetylation
in Aspergillusnidulans (Fig. 1, iii). Due to the biological
importance of these compounds, over the past several years,
significant effort has been devoted6 and are still demanding for
the development of an efficient approach for the synthesis.
Previous work
2015
a) Our group,
H
N
S
H2O/ 80 o
C
NH2
for the synthesis of biologically active molecules.
R2
R3-NCS
R2
N
OH
R3
COOR1
2017
HO
Me
R1OOC
OH
HN
b) Patel group,
NH2
O
S
S
N
S
HO
O
H
N
H
O
Me
N
Ar2
Cs2CO3
N
Ar2
NCS
HO
HO
OH
O
Ar1
CH3CN, air
80 o
Ar1
OH
O
C
S
Antiproliferative
(i)
Anticancer
(ii)
Antifungal
(iii)
c) This work
Iodine-Mediated C-S bond formation
Fig 1.Biologically active benzothiazines cores
N
N
Ar
OH
NH2
I2, (2.0 equiv)
O
R1
R1
S
H2O, 25 o
20-22 h
C
Ar
NCS
R2
Mr. Kapil Mohan Saini, Dr. Rakesh K. Saunthwal, Mr. Shiv Kumar and Prof
Akhilesh K. Verma
Synthetic Organic Chemistry Research Laboratory, Department of Chemistry,
University of Delhi, Delhi–110007, India
I
R2
Scheme 1 Synthetic approaches for utilizing isothiocyanate
E-mail:averma@acbr.du.ac.in
In recent years, the iodocyclization of C-C triple bonds using
base and organic solvents has received widespread attention, 7
yet the use of water as a solvent in this context remains
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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ARTICLE
underdeveloped. In pioneering studies, Katritzky8 reported Table 1. Optimization of the reaction conditiona
Journal Name
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DOI: 10.1039/C9OB00128J
the synthesis of [1,3]benzothiazineviaortho-lithiation of
thiophenols by the using N,N-bis[(benzotriazol-1-yl)
methyl]amines as 1,3-biselectrophile synthons. Later Wu and
co-workers9 reported the tandem synthesis of 1,3-
benzothiazines from 2-alkynylbenzenamines under silver
catalysis. Valliribera,10 Reboul11a, and our group12 have also
been involved in developing synthetic approaches towards the
N
N
Ph
NH2
O
I2 (X equiv)
S
OH
Ph
NCS
base, solvent,
25 oC, time
I
Ph
Ph
1a
2a
3a
reagent
(equiv)
I2 (1.0)
I2 (2.0)
I2 (2.0)
I2 (2.0)
I2 (2.0)
I2 (2.0)
I2 (2.0)
I2 (2.0)
I2 (2.0)
I2 (2.0)
I2 (2.0)
I2 (2.0)
I2 (2.0)
I2 (1.0)
I2 (1.0)
ICl (1.0)
KI
base (2.0
equiv)
K2CO3
K2CO3
Cs2CO3
KHCO3
NaHCO3
Et3N
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
-
time
(h)
10
10
10
10
10
10
10
10
10
15
20
30
20
20
20
20
20
20
yield
(%)b3a
00
00
00
00
00
00
00
00
65
82
90
88
90
50c
50d
70
entry
solvent
synthesis of
1,3-benzothiazines. Sashida group also
demonstrated the synthesis of 1,3-benzothiazines using
iodine-base system in organic solvent, however in the absence
of Cs2CO3 the reaction gave the trace amount of product.13
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DCE
EtOH
H2O
H2O
In
2015,
we
reported
the
synthesis
of
tetrahydroquinazolines from 2-aminophenylacrylate and aryl
isothiocyantes in presence of water under heating (Scheme
1a).14 Most of the existing synthetic strategies do not rely on
the use of iodine in water as activating reagent. In 2017, Patel
group describe the Cs2CO3-CH3CN-promoted synthesis of
quinoline-thiones from ortho
‑
alkynylanilines and
aroylisothiocyanates (Scheme 1b).15 Interestingly, replacement
of Cs2CO3-CH3CN with iodine-water completely changed the
course of the reaction and the product outcome, giving a
benzo[1,3]thiazin-2-yl)benzimidic acid via formation of the
iodonium ion and followed by 6-exo-dig ring closure at
ambient temperature (Scheme 1b vs 1c).
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
-
-
-
-
00
00
Results and discussion, Experimental
NaI
-
Based on this initial concept,16 we started screening coupling
partners 2-(phenylethynyl)aniline 1a with aroylisothiocyanate
2a as the standard substrates. When 1a and
aroylisothiocyanates 2a employed with iodine (1.0 equiv) as
the activating reagent in the presence of K2CO3 in
dichloromethane at room temperature, no 6-exo-dig ring
closure product (3a) was detected (Table 1, entry 1). Increase
of iodine loading did not provide the product 3a (entry 2). We
screened the other bases such as Cs2CO3, KHCO3, NaHCO3 and
organic base Et3N, which were found to be ineffective for the
reaction (entries 36).
aUnless otherwise noted all reactions were carried out using 2-
(phenylethynyl)aniline 1a (0.50 mmol), aroylisothiocyanate 2a (0.52
mmol), and I2 in 2 mL solvent, Isolated yield, 50% conversion of
substrate into the product. dUnder oxygen atmosphere at 80 oC.
b
c
Having established the optimum reaction condition in
hand, we next explored the scope and generality of cascade
reaction using an eco-friendly solvent (Scheme 2). The reaction
of electron-neutral and electron-donating substrate 1ad
bearing functional groups, such as 2-Me, 3-Me and 4-Me on
the benzene ring of the alkyne, afforded the corresponding
products 3ad in 8493% yield. Alkynes bearing n-butyl and
tert-butyl group on the para position of benzenering, provided
the desired products 3ef in 88% and 86% yield respectively.
Electron-withdrawing and sterically hindered groups, such as
4-F and 2-F, were also well-suited, leading to 3g and 3h in good
yields. Electron-rich thienyl alkyne 1i gave the cyclized product
3i in 82% yield. Isothiocyanates bearing electron-donating
group 2b and 2c, afforded the corresponding products 3jo in
82-90% yield. Electron-withdrawing groups bearing
aroylisothiocyanates 2d, was also compatible, leading to 3p in
84% yields.
A similar result was observed using dichloroethane and
ethanol as a solvent (entries 78); nevertheless, superior
results were observed when the reaction was performed in
water (entry 9). Based on these facts, we increased the
reaction time, and formation of benzo[1,3]thiazin-2-
yl)benzimidic acid product was observed in good yields (entries
1012). Surprisingly, when the reaction was performed in the
absence of the base under iodine (2.0 equiv) and water as a
solvent, the desired product 3a was obtained in 90% yield
(entry 13). Lowering of iodine loading leads to the incomplete
conversion of substrate (entries 14 and 15). Use of other
electrophile like ICl provided the cyclized product
comparatively in lower yield (entry 16). KI and NaI gave the
isothiourea intermediate instead of the final product (entries
17 and 18). Reaction of benzoyl isocyanate with 2a failed to
provide the desired product.
2 | J. Name., 2012, 00, 1-3
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ARTICLE
N
N
Ar2
OH
N
N
Ar2
View Article Online
I2, (2.0 equiv)
I2, (2.0 equiv)
NH2
NH2
O
O
R
R
DOI: 10.1039/C9OB00128J
H2O, 25 o
20 h
C
H2O, 25 o
20 h
C
S
Ar2
NCS
S
OH
Ar2
NCS
Ar1
Ar1
Ar1
I
Ar1
I
1
2
3
1
2
4
Me
N
Me
N
N
N
N
N
S
N
Me
N
N
Me
N
N
Me
N
N
S
OH
S
OH
OH
S
OH
S
OH
S
OH
I
I
I
I
I
I
Me
CF3
Me
N
b
3a
, 90%
3c
, 87%
3b
, 84%
b
4a
, 91%
4b
4c
, 87%
, 83%
Me
Cl
Me
Me
N
N
N
N
S
Me
N
N
N
N
Me
N
N
S
OH
S
OH
OH
S
OH
S
OH
S
OH
I
I
I
I
I
I
nBu
Me
tBu
Me
b
3d
, 93%
b
3e
, 88%
b
b
3f
, 86%
4d
, 90%
4e
, 87%
4f
, 86%
N
N
N
N
N
N
N
N
S
N
N
N
N
S
S
OH
S
OH
S
OH
OH
S
OH
OH
OMe
Cl
F3C
F
I
I
I
I
I
I
,
S
F
F
b
b
3g,
85%
N
4g
84%
4i, 80%b
b
3h,
80%
4h,
82%
3i
, 82%
Me
Me
Me
N
N
N
N
N
S
OH
Me
S
OH
S
OH
N
N
I
I
I
S
OH
Cl
Me
3l
I
3k
, 88%
b
3j
, 86%
, 87%
Me
Me
N
N
N
N
N
N
S
R1
4h
S
OH
S
OH
OH
I
I
I
Scheme 3 Scope of aniline
F
tBu
, R1 = m-Me, 86%b
, R1 = m-Cl, 84%b
3o
3p
b
3m
, 90%
3n
, 82%
Next, the generality of aliphatic alkyne bearing aniline was
evaluated (Scheme 4). ortho-Alkynylaniline 1p having long
aliphatic chain, afforded the desired product 5a in good yield
with excellent regioselectivity. 5-Chloropentyne substituted
aniline 1q, provided the cyclized product 5b in 80% yield. tert-
Butyl group substituted alkyne 1r, was well tolerated to give
the desired product in good yields. ortho-Alkynylanilines
having aliphatic carbocyclic group such as cyclopropyl (1s),
cyclohexane (1t) and cyclohexene (1u) gave the desired
products 5df in 6578% yields.
Scheme 2 Scope of acyl isothiacynate, bTime 22 h
Additionally, we explored the scope of ortho-
alkynylanilines for the green cascade approach (Scheme 3). 5-
Me substituted ortho-alkynylanilines analogues 1jl reacted
smoothly with aroylisothiocyanates bearing electron-neutral
and electron-donating group 2ac, provided the corresponding
products 4af in 83-91% yields. The regioselectivity of the
products
was
unambiguously
assigned
by
X-ray
crystallographic study17 of product 4h. Substrates (1mo)
having electron-withdrawing functional groups, such as fluoro,
chloro, and trifluoromethyl at the 4-positon of the aniline
moiety, afforded the desired product 4gi in 80-84% yield.
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DOI: 10.1039/C9OB00128J
N
N
O
I2, (2.0 equiv)
NH2
R1
R1
S
OH
H2O, 25 o
22 h
C
NCS
R2
I
R2
1
2a
5
N
N
N
N
N
N
S
OH
S
OH
S
OH
Cl
F3C
Me
Me
I
5a
I
I
Me
, 77%
, 79%
5b
, 80%
5c
N
N
N
N
S
N
N
Fig 2 Stability of enol over keto due to hydrogen bonding (4h)
S
OH
OH
S
OH
F3C
I
I
I
The structure and enol conformation of the
benzo[1,3]thiazin-2-yl)benzimidic
acid
products
were
5d
, 78%
5e
5f
, 65%
, 78%
investigated in the solid state. Crystals appropriate for X-ray
analysis were obtained for structure and enol confirmation,
thus disclosing the solid-state confirmation of 4h (Figure 2).
The clear intra and intermolecular hydrogen bonding was
observed in the molecule. The O1-H-S1 distances 1.958 Ǻ,
shows strong intramolecular hydrogen bonding, therefore, the
hydrogen of O1 is tilt towards the S1 and O1-H-N1 distance
2.904 Ǻ, shows moderate intermolecular hydrogen-bonding.
This crystallography study suggested an intermolecular
Scheme 4. Scope of aliphatic alkyne
Encouraged by above results, we pushed the scope of 1,3-
dialkyne further and found that the double 6-exo-dig ring
closures of 6a could be performed with aroylisothiocyanate2a
to produce 7a in the 71% yield. With 1, 3-dialkyne, the
reaction of 2b using the 3.5 equiv of iodine in water afforded
the cyclized products 7b in 75% yield. Finally, electron-
withdrawing 4-F substituent at the benzene ring of 1,3-
dialkyne, was also effectively employed in this protocol and
gave the desired products 7c and 7d in good yields (Scheme 5).
transannular
benzo[1,3]thiazin-2-yl)benzimidic
hydrogen-bonding
between
acid molecule,
the
two
a
characteristic of attention for the development of biologically
active molecules with “hidden hydrophilicity”.17b
R
N
8
p-OMeC6H4
N
N
N
N
Ph
OH
NH2
H2
N
I2, (3.5 equiv)
I
PdCl2(PPh3)2 (5 mol%)
S
OH
O
R
N
OH
S
N
R
i)
S
H2O, 25 o
22 h
C
Ar2
NCS
N
S
HO
Et3N (2.0 equiv)
CH3CN, 70 oC
I
O-MeC6H4
C6H4Mep-O
Me
I
2
6
R
7
3b
9
, 79%
Me
Me
Me
(HO)2B
OMe
10
N
N
N
N
S
OH
N
PdCl2(PPh3)2 (5 mol%)
ii)
S
OH
p-tBuC6H4
3m
N
I
N
OH
S
N
I
K2CO3 (2.0 equiv)
DMF:EtOH
N
OH
S
N
I
N
S
HO
MeO
tBu
80 o
C
N
S
HO
11
, 75%
Me
I
, 71%
I
7b
, 75%
Scheme 6. Synthetic utility of products
7a
Me
F
F
I
Because of the synthetic utility of alkyne and aryl groups, a
mild and selective method for late-stage introduction of alkyne
and aryl group in benzothiazine molecules will be highly
desirable in synthetic chemistry. Thus, we employed the
product 3b and 3m under Sonogashira (scheme 6i) and Suzuki
coupling (scheme 6ii) reaction conditions, affording the
coupling products 9 and 11 in good yields (Scheme 6).
N
N
I
N
OH
S
N
N
OH
S
N
N
F
S
HO
N
S
HO
Me
I
I
F
7d
, 72%
7c
, 70%
Scheme 5. Scope of dialkynes
4 | J. Name., 2012, 00, 1-3
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ARTICLE
R
thiourea group on the more electron deficient carbon of
NH2
O
View Article Online
H2O
S
Ph
i)
iodonium intermediate, in the favouDreOdI: 106.1-0e3x9o/-Cd9igOB0m01o2d8eJ
provided the species B, which after keto-enoltautomerisation,
give the final products. The enol form would be stabilized by
intermolecular transannular hydrogen bonding.
Ph
NCS
25 oC, 30 min
N
N
O
R
H
H
1a
1b:
1d:
2a
R= o-MeC6H4
R= p-nBuC6H4
12a,
12b
R= o-MeC6H4, 95%
, R= p-nBuC6H4, 93%
R
N
N
Ph
OH
I2 (2.0 equiv)
S
Ph
O
ii)
S
R
H2O
N
N
25 oC, 20 h
Conclusions
H
H
I
3b,
R= o-MeC6H4, 87%
12a 12b
&
3e
, R= p-nBuC6H4, 90%
In summary, we have disclosed a mild, one-pot, organic
solvent-free and base-free methodology for the regioselective
ring closure of ortho-alkynylanilines with aroylisothiocyanate
enabled by the carbon-carbon triple bond activation using
iodine. A variety of readily available ortho-alkynylanilines were
reactive, including electron-withdrawing and electron-
donating alkynylanilines. Additionally, the substrate scope was
extended for the double 6-exo-dig cyclization of 1,3-
dialkynylanilines. Aromatic and aliphatic alkynylanilines are
equally reactive and selective and can be cyclized in the
presence of iodine. We anticipate that this protocol is useful
for the synthesis of highly functionalized benzothiazine
derivatives, which could find further application in the
synthesis of biologically active compound. To the best of our
knowledge, this work represents the first example of iodine-
mediated alkyne activation without using any base on water.
iii)
Deuterium Labeling Study
NH2
C4H9
O
No D incorporation
Ph
D2O
S
Ph
NCS
25 oC, 30 min
N
N
O
C4H9
1p
2a
H
H
12c
, 95 %
N
S
N
Ph
O
NH2
I2 (2.0 equiv)
OH
No D incorporation
C4H9
Ph
NCS
2a
D2O
25 oC, 22 h
C4H9
1p
I
5a,
82 %
Scheme 7. Mechanistic control experiments
To validate the mechanism of iodine-mediated 6-exo-dig
ring closure several control experiments has been performed
in Scheme 7. The reaction of 1b and 1d with
o
aroylisothiocyanate 2b in water at 25 C, provided the urea
intermediate 12 (Scheme 7i), further we employed these
intermediate with optimized reaction condition to afford the
final product 3b and 3e in good yield (Scheme 7ii). Moreover,
intermolecular cyclization experiments indicated that reaction
would compete in a cascade manner. In deuterium labeling
study, H-D exchange was not observed which infer that the
solvent is not responsible for protonation (Scheme 7 iii).
Experimental Section
General Procedure for the Synthesis of Starting Substrate
1a-o: To a solution of substituted 2-iodoaniline (0.5 mmol) in
MeCN (2 mL), 3 mol% of Pd(PPh3)2Cl2 was added. The reaction
vial was then sealed and flushed with nitrogen. Then, 1.5 equiv
of Et3N and 0.51 mmol of alkyne were added to the reaction
mixture. The reaction was then stirred at 70°C until TLC
revealed complete conversion of the starting material. The
reaction mixture was then allowed to cool, was diluted with
H2O, and was extracted with EtOAc (3 × 10 mL). The combined
organic layers were dried over Na2SO4, concentrated under
vacuum, and purified by column chromatography using
100−200 mesh size silica gels (hexane: ethyl acetate) to afford
the corresponding product. The structure and purity of known
starting materials 1a-o were confirmed by comparison of their
physical and NMR-spectral data (1H NMR and 13C NMR) with
those reported in the literature.18-21
I
H
R
R
N
N
Ph
6-exo-dig
ring closure
I2
S
O
S
Ph
O
S
R
O
N
H
N
H
Ph
N
a
N
H
b
A
H
I
a
b
I
I
B
12
keto-enol
tautomerism
O
Ph
NCS
2
N
N
Ph
OH
NH2
S
I
R
R
3 4
5
7
and
,
,
1
Scheme 8. Plausible reaction pathway
General Procedure for the Synthesis of Starting Substrate
1p-u: To a solution of substituted 2-iodoaniline (0.5 mmol) in
Et3N (3 mL), 3 mol% of Pd(PPh3)2Cl2 and 1 mol% of CuI were
added. The reaction vial was then sealed and flushed with
nitrogen. Then 0.51 mmol of alkyne was added to the reaction
mixture. The reaction was then stirred at 25 °C until TLC
revealed complete conversion of the starting material. The
reaction mixture was then allowed to cool, was diluted with
H2O, and was extracted with EtOAc (3 × 10 mL). The combined
organic layers were dried over Na2SO4, concentrated under
vacuum, and purified by column chromatography using
Based on the control experiments and literature studies,
we envisaged the plausible mechanism for above-mentioned
iodocyclization reactions in Scheme 8. First, this process
would be initiated by the reaction between alkynylanilines 1
and aroylisothiocyanate 2, which would form thiourea
intermediate 12. Further, alkyne bond coordinate with I+,
therefore enhancing the electrophilicity of the carbon-carbon
triple bond to generate iodonium species A and liberate an
iodine anion at the same time. With the support of the iodine
anion, intramolecular nucleophilic attack of sulfur in the
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Organic & Biomolecular Chemistry
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Journal Name
4.
5.
(a) S. Sabatini, F. Gosetto, S. Serritella, G. VMiewanAfrtrioclne iO, nliOne.
DOI: 10.1039/C9OB00128J
Tabarrini, N. Iraci, J. P.Brincat, E. Carosati, M. Villarini, G.
100−200 mesh size silica gels (hexane: ethyl acetate) to afford
the corresponding product. The structure and purity of known
starting materials 1p, 1q, 1t, 1u were confirmed by comparison
of their physical and NMR-spectral data (1H NMR and 13C NMR)
with those reported in the literature.18-21
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General experimental procedure for green one-pot
synthesis of benzo[1,3]thiazin-2-yl)benzimidic acid 3-5: To a
solution of ortho-alkynlanilines
1
(0.5 mmol), aroy
lisothiocyanates 2 (0.52 mmol) and 2.0 equiv of I2 was added in
water (2.0 mL). The reaction was then stirred at room
temperature until TLC revealed a complete conversion of the
starting material. After the completion of the reaction, the
reaction mixture was quenched with saturated aq sodium
thiosulfate solution and extracted with EtOAc (3X10 mL). The
combined organic layers were dried over Na2SO4, concentrated
under vacuum, and purified by column chromatography using
100−200 mesh size silica gels (EtOAc: hexane) to afford the
corresponding product.
6.
7.
General experimental procedure for green one-pot
synthesis of Bisbenzo[1,3]thiazin-2-yl)di benzimidic acid 7: To
a
solution of ortho-haloanilines
1
(0.5 mmol),
aroylisothiocyanates 2 (1.04 mmol) and 3.5 equiv of I2 were
added in water (2.0 mL). The reaction was then stirred at room
temperature until TLC revealed complete conversion of the
starting material. After the completion of the reaction, the
reaction mixture was quenched with saturated aq sodium
thiosulfate solution and extracted with EtOAc (3X10 mL). The
combined organic layers were dried over Na2SO4, concentrated
under vacuum, and purified by column chromatography using
100−200 mesh size silica gels (EtOAc:hexane) to afford the
corresponding product.
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Conflicts of interest
There are no conflicts of interest to declare.
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17. Crystallographic data for the compound 4h has been
deposited with the Cambridge Crystallographic Data
Centre. CCDC deposit number for compound 4h is
1872209, contains all crystallographic details of this
publication and is available free of charge at
Acknowledgements
We gratefully acknowledge the SERB for financial support and
USIC, the University of Delhi for providing instrumentation
facilities. K.M.S. and S.K. are thankful to CSIR for fellowship.
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