Highly efficient green synthesis of the photochromic spironaphthoxazines using an eco-friendly choline hydroxide catalyst

Article abstract of DOI:10.1080/00397911.2019.1694690

A greener route for the synthesis of the photochromic spironaphthoxazines employing a highly efficient, biodegradable and recyclable choline hydroxide catalyst in an aqueous medium has been described. This methodology offers diverse advantages such as mild reaction conditions, short reaction time (1 h), excellent yields (up to 91%), and operational simplicity that make it a useful and attractive option for the synthesis of the spironaphthoxazines. The choline hydroxide showed excellent catalytic performance until five recycles without significant loss of activity. The structures of synthesized spironaphthoxazines were deduced by their ¹H NMR spectra and melting points.

Full text of DOI:10.1080/00397911.2019.1694690

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Highly efficient green synthesis of the
photochromic spironaphthoxazines using an eco-
friendly choline hydroxide catalyst
Shailesh N. Vajekar & Ganapati S. Shankarling
To cite this article: Shailesh N. Vajekar & Ganapati S. Shankarling (2019): Highly efficient green
synthesis of the photochromic spironaphthoxazines using an eco-friendly choline hydroxide
catalyst, Synthetic Communications, DOI: 10.1080/00397911.2019.1694690
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SYNTHETIC COMMUNICATIONS
https://doi.org/10.1080/00397911.2019.1694690
Highly efficient green synthesis of the photochromic
spironaphthoxazines using an eco-friendly choline
hydroxide catalyst
Shailesh N. Vajekar
and Ganapati S. Shankarling
Department of Dyestuff Technology, Institute of Chemical Technology, Mumbai, Maharashtra, India
ABSTRACT
ARTICLE HISTORY
A greener route for the synthesis of the photochromic spironaph-
thoxazines employing a highly efficient, biodegradable and recyc-
lable choline hydroxide catalyst in an aqueous medium has been
described. This methodology offers diverse advantages such as mild
reaction conditions, short reaction time (1 h), excellent yields (up to
Received 6 October 2019
KEYWORDS
Choline hydroxide;
biodegradable; recyclable;
spironaphthoxazines
9
1%), and operational simplicity that make it a useful and attractive
option for the synthesis of the spironaphthoxazines. The choline
hydroxide showed excellent catalytic performance until five recycles
without significant loss of activity. The structures of synthesized
1
spironaphthoxazines were deduced by their H NMR spectra and
melting points.
GRAPHICAL ABSTRACT
Introduction
The spironaphthoxazine molecules are well-known to display photochromic and acidi-
chromic properties. The spironaphthoxazines also exhibit high fatigue resistance and
[
1]
outstanding photo and pH stability. The reversible photochromism of these com-
pounds is based on the interconversion between a closed spiro form, that is pale yellow
[
2]
or colorless, and open planar merocyanine form that is deeply colored. This phenom-
enon is due to the photochemical cleavage of the C(spiro)–O bond that led to the co-
planarization of the two originally orthogonal heterocycles, leading to absorption at a
[
3]
longer wavelength owing to an extended p system in the merocyanine (MC) structure.
þ
In addition, the C(spiro)–O bond can also be broken by H to form a different type of
CONTACT Ganapati S. Shankarling
gsshankarling@gmail.com
Department of Dyestuff Technology, Institute of
Chemical Technology, N. P. Marg, Matunga, Mumbai, Maharashtra 400019, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
S. N. VAJEKAR AND G. S. SHANKARLING
(
a)
(
b)
Scheme 1. (a) Photo-induced and (b) proton-induced ring opening of spironaphthoxazine.
[
1,4]
open form (MCꢀH).
The open planar merocyanine structure returns back to the
closed spiro form by spontaneous fading in the dark or upon irradiation with visible
light (Scheme 1).
The spironaphthoxazines are T-type photo-switches in which the forward reaction is
compelled by light and the regression happens spontaneously in the dark. The struc-
tural modification of these compounds enhances their fatigue resistance than those of
spiropyrans. Spironaphthoxazines are employed in an extensive variety of applications
like photochromic molecular switches,
lenses, light filters, optical devices, optical memories, photo-control units for poly-
meric materials, in bipolar membranes and in liquid crystalline solvents.
[
5]
[
6]
[
7]
[
6,8]
photo-responsive sunglasses and ophthalmic
[
9]
[2]
[
10]
A substantial amount of research has been undertaken for the synthesis of spironaph-
thoxazines. The most frequently used methods for the synthesis of spironaphthoxazine
[
8,11]
includes the application of diverse organic or inorganic bases like triethylamine,
piperidine, NaOH,
[
8]
[12]
[6]
and KOH. All aforementioned hazardous bases produce spi-
ronaphthoxazines in little to reasonable yields as well as these procedures employed
volatile, flammable and toxic organic solvents like ethyl alcohol, isopropyl alcohol,
methyl alcohol, DMF, and DMSO. In the past few years, the growing environmental
awareness in chemical research and industry led to practicing eco-friendly and sustain-
able techniques involving environmentally benign catalysts and evading hazardous
organic solvents. Ionic liquids (ILs) are receiving extensive global attention and are
being engaged widely in chemical conversions as green solvents and/or catalysts due to
their exceptional properties such as solvation power, good catalytic activity, excellent
[
13–16]
recyclability, non-flammability, tunable solubility, and low volatility.
Choline
chloride (ChCl), a naturally occurring biocompatible compound belongs to the vitamin
B family. It is well known commercially manufactured chicken feed additive. Choline
hydroxide (ChOH) is an ionic liquid derived from Choline chloride.
ChOH has numerous advantages, viz. nontoxic, easily available, water soluble, bio-
degradable, reusable, strongly basic, good solvation power, inexpensive, and task-specific
[
17–21]
in various organic conversions.
In continuation of our endeavors in developing environmentally benign practices in
[
22–27]
organic synthesis,
we herein present a highly efficient green synthesis of the

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3
Scheme 2. General reaction for the synthesis of spironaphthoxazine.
Table 1. Yields of the synthesized spironaphthoxazine derivatives.
a
Notation
SNO-1
Product
Yield (%)
89
SNO-2
SNO-3
SNO-4
87
83
84
SNO-5
91
SNO-6
SNO-7
88
87
a
Isolated yield.
photochromic spironaphthoxazines using an eco-friendly ChOH catalyst in an aque-
ous medium.
Results and discussion
A series of spironaphthoxazine derivatives (SNO-1 to SNO-7) have been synthesized
using indolium iodide (1) and 1-nitroso-2-naphthol (2) in presence of choline hydrox-
ide as a catalyst (Scheme 2 and Table 1).
In order to optimize the reaction conditions, a series of experiments employing varied
conditions in terms of base catalyst, catalyst loading, reaction temperature and solvent
for a model reaction of 1,2,3,3-Tetramethyl-3H-inol-1-ium iodide and 5-nitro-1-nitroso-
2-naphthol were explored in detail as illustrated in Tables 2 and 3.
To ascertain the effect of the catalyst, the model reaction was first performed in
ꢁ
water without any catalyst, which lead to no reaction progress, even after 24 h at 80 C
(Table 2, entry 1). After that, we tried the model reaction in the presence of different

4
S. N. VAJEKAR AND G. S. SHANKARLING
Table 2. Effect of various bases on the synthesis of spironaphthoxazine.
Catalyst loading
Entry
Catalyst
(mol%)
Reaction time (h)
Isolated yield (%)
1
2
3
4
5
6
7
8
9
Without catalyst
KOH
–
10
10
10
10
10
10
10
10
10
10
10
04
06
08
10
12
24
3
3
3.5
3.5
3.5
3.5
6
6
4
4
5
No reaction
69
67
59
63
58
55
59
52
55
53
45
51
67
81
91
91
NaOH
þ
-
(CH
Cs CO
CO
Na CO
3
)
4
N OH
3
2
K
2
3
2
3
DABCO
DBU
Piperidine
10
11
12
13
14
15
16
17
Et
3
N
Pyridine
ChOH
ChOH
ChOH
ChOH
ChOH
3
2
1.5
1
1
Reaction conditions: Reaction of 1,2,3,3-tetramethyl-3H-indol-1-ium iodide (1 mmol) and 5-nitro-1-nitroso-2-naphthol
ꢁ
(
1 mmol), catalyst and water (3 mL) at 80 C.
Bold values indicates the finalized reaction conditions i.e. optimized conditions which gave best results.
organic bases (viz. tetramethylammonium hydroxide, 1,4-diazabicyclo[2.2.2]octane
(DABCO), 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU), piperidine, triethylamine, pyri-
dine) and inorganic bases (viz. KOH, NaOH, Cs CO , K CO , Na CO ). The listed bases
2
3
2
3
2
3
gave the desired product yields maximum up to 69% (Table 2, entry 2 to 12). In litera-
ture, a stoichiometric amount of these bases are used at high temperature. Further, they
required longer reaction times and are non-recyclable. In order to overcome aforesaid
difficulties, we turned our consideration toward recyclable ionic liquids. To our delight,
ChOH as a catalyst gave the desired product with excellent yield of 91% within a short
time (Table 2, entry 16). An excellent yield of the product was accomplished with choline
hydroxide catalyst since it performs as a homogeneous catalyst in an aqueous medium
which helps to increase the solubility of the reactants via hydrogen bonding and conse-
quently increases the speed of the reaction. To decide the optimum catalyst loading, the
ꢁ
model reaction was performed with varied mol % of ChOH at 80 C (Table 2, entries
13–17). The reaction happened efficiently with 10 mol % of ChOH giving a single product
in 91% yield (Table 2, entry 16). Increasing the quantity of ChOH further, displayed no
significant enhancement in the yield or reaction rate.
Different solvents were screened for the model reaction, wherein reaction perform-
ance was found well in an aqueous medium (Table 3, entry 11). Later on the model
ꢁ
reaction was carried at various temperatures ranging from 50 to 90 C (Table 3, entries
ꢁ
8
–12) and 80 C was found to be the optimum temperature. Eventually, the optimized
parameters for the model reaction are 1,2,3,3-Tetramethyl-3H-indol-1-ium iodide
ꢁ
(
1.0 mmol), 5-nitro-1-nitroso-2-naphthol (1.0 mmol) and ChOH (10 mol %) at 80 C for
1
h in an aqueous medium.
Recyclability studies
In order to determine the industrial applicability of this procedure, the reaction of
1,2,3,3-Tetramethyl-3H-indol-1-ium iodide and 5-nitro-1-nitroso-2-naphthol catalyzed

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a
Table 3. Influence of solvent and temperature on model reaction .
Temperature
Reaction time
(h)
Isolated yield
(%)
ꢁ
Entry
Solvent
Methanol
Ethanol
1,2-dichloroethane
Dichloromethane
Chloroform
Tetrahydrofuran
toluene
Water
Water
Water
Water
( C)
1
2
3
4
5
6
7
8
9
65
79
84
40
61
66
111
50
60
70
80
90
4
4
5
5
5
5
6
4
3
2
1
1
64
61
43
35
33
41
30
37
49
61
91
91
10
11
12
a
Water
Reaction conditions: Reaction of 1,2,3,3-tetramethyl-3H-inol-1-ium iodide (1 mmol), 5-nitro-1-nitroso-2-naphthol
1 mmol), ChOH (10 mol %) and solvent (3 mL).
Bold values indicates the finalized reaction conditions i.e. optimized conditions which gave best results.
(
100
91
8
7
8
5
8
1
8
0
7
7
6
0
4
0
2
0
0
1
2
3
4
5
Run
Figure 1. Study of choline hydroxide.
by choline hydroxide was carried out on a larger scale (100 mmol) for the duration of
1
9
h. To our pleasure, the results were reproducible to that of a small scale (1 mmol) and
1% yield of the product was achieved. The recovery and reuse of a catalyst are highly
desirable for a greener method. Thus, the recyclability of ChOH was investigated by
using the above model reaction. After the accomplishment of the reaction (checked by
TLC), the product was obtained by filtration. The catalyst in the form of filtrate reused
in the next run without additional purification. In a test of five cycles, the catalyst could
be reused without any significant loss of catalytic activity (Fig. 1).
FT-IR analysis
FT-IR is a great tool for determining forms of chemical bonds in a molecule by creating
an infrared absorption spectrum resembling a molecular “fingerprint”. For every chem-
ical bond, there is a specific value of the wavelength of light absorbed (Fig. 2). With the
aid of FT-IR spectroscopy, the fresh and recycled (after a 5th run) ChOH have been
investigated. The relative study of the fresh and recycled ChOH demonstrates that there
is no substantial alteration witnessed in the IR spectra of choline hydroxide. The protu-
ꢂ1
berance at 3311 cm is typical of O–H stretching vibration. The aliphatic C-H stretch-
ꢂ1
ing vibration detected at 2957 cm . The methyl deformation was witnessed at

6
S. N. VAJEKAR AND G. S. SHANKARLING
Figure 2. FT-IR spectra of fresh and recycled ChOH.
Scheme 3. Proposed mechanism for formation of spironaphthoxazine catalyzed by ChOH.

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Scheme 4. Synthesis of choline hydroxide (ChOH).
ꢂ1
ꢂ1
1
477 cm . The peak near 1359 cm matches to stretching vibration of the C–N bond.
ꢂ1
The band at 1077 cm agrees to C–O stretch in a primary alcoholic group. The C-H
deformation of –CH2– group was detected around 951 and 881 cm .
ꢂ1
[
17,28]
Based on the present investigational observations and literature reports,
a pos-
sible mechanism for the reaction between 1,2,3,3-tetramethyl-3H-indol-1-ium iodide (i)
and 1-nitroso-2-naphthol (iii) in presence of ChOH is shown in Scheme 3. The reaction
began with the generation of 1,3,3-trimethyl-2-methyleneindolenine intermediate (ii) by
deprotonation of (i) using choline hydroxide. An intermediate (v) was formed by an
intermolecular cyclization of (ii) and (iii) catalyzed by ChOH. Finally, the catalyst will
help an intermediate (v) to go through dehydration to yield spironaphthoxazine deriva-
tive. The significance of the alcoholic –OH group of choline hydroxide, we consider
that it will form hydrogen bonding with nitroso group of 1-nitroso-2-naphthol (iii).
This makes the nitroso group further electrophilic and prone to the nucleophilic attack
by (ii).
Conclusion
In summary, a simple, time saving, efficient, economic and ecofriendly protocol for the
synthesis of photochromic and thermochromic spironaphthoxazine was developed. This
report first time claims the application of choline hydroxide for the synthesis of spiro-
naphthoxazine derivatives. The catalyst shows good catalytic activity for the synthesis of
spironaphthoxazine derivatives at mild reaction conditions. The catalyst could be reused
for four consecutive cycles.
Experimental section
Materials and methods
All the required chemicals were purchased from s d Fine-Chem Limited and utilized without
additional purification. The advancement of all reactions was checked by aluminum TLC
plate, silica gel coated with fluorescent indicator F254 (Merck) identifying the spots with the
1
help of UV light as a visualizing agent. H NMR spectra were obtained on a 500 MHz instru-
ment (Agilent NMR spectrometer) and 400 MHz (Bruker Ultrashield Plus NMR spectrom-
eter) in CDCl or DMSO-d solvent at ambient temperature. We have mentioned the
3
6
chemical shifts values on d scale (ppm) relative to an internal standard tetramethylsilane
Si(CH ) ¼0.00ppm]. Infrared spectra were obtained with a JASCO-FT/IR 4100 LE ATR
[
3
4
1
PRO450-S spectrometer. All the synthesized derivatives were analyzed by H NMR spectra

8
S. N. VAJEKAR AND G. S. SHANKARLING
and melting points. Melting points were recorded on standard melting point instrument
bought from Sunder Industrial Product, Mumbai and are uncorrected.
General method for the preparation of choline hydroxide (ChOH)
[
17]
The Choline hydroxide was prepared as per the literature.
The solution of choline
ꢁ
chloride (10 mmol), KOH (10 mmol) and methyl alcohol (15 mL) was heated at 60 C
for 12 h using continuous stirring. Solution was then cooled to rt and filtered later on
to remove precipitated KCl. Eventually, the filtrate was concentrated by means of vac-
uum to get rid of methyl alcohol. The residue obtained was utilized without additional
purification (Scheme 4).
General method for the synthesis of spironaphthoxazines
A mixture of indolium iodide 1 (1.0 mmol) and choline hydroxide (10 mol %) in water
ꢁ
(
3 mL) was stirred at 80 C for 20 min. Then 1-nitroso-2-naphthol 2 (1.0 mmol) was
ꢁ
added and the resulting mixture stirred at 80 C for another 40 min. Then reaction mass
was cooled to rt. The solid product was then filtered and washed with water. The syn-
thesized product was finally purified using column chromatography technique (silica gel
100–200 mesh size) with ethyl acetate:hexane (05:95) as an eluent to give the desired
spironaphthoxazine (Scheme 2 and Table 1).
Characterization data for selected spironaphthoxazine
0
SNO-1: 1,1,3-trimethyl-1,3-dihydrospiro[benzo[e]indole-2,3 -naphtho[2,1-b][1,4]oxazine]
ꢁ
ꢁ
0
Yield: 89%; Colorless solid; mp 123–124 C. Reported mp 125 C.
1
0
0
H NMR (500 MHz, DMSO-d ) d 8.499 (1H, d, J¼ 8.5 Hz, C10 –H), 7.968 (1H, s, C2 –H),
6
0
0
0
7
.912 (1H, d, J ¼ 8.6 Hz, C7 –H), 7.870–7.825 (3H, m, C6 –H, C8 –H, & C9 –H),
0
7
.590 (1H, t, J ¼ 8.3 Hz, C6–H), 7.418 (2 H, m, C4 & C5 –H), 7.243 (1H, t, J ¼ 8.0 Hz,
C5–H), 7.075 (1 H, d, J ¼ 8.5 Hz, C7–H), 2.778 (3H, s, N–CH ), 1.594 (3H, s, C1–CH ),
3
3
1
.472 (3H, s, C1–CH ).
3
Full experimental details and data pertaining to the characterization of the com-
1
pounds SNO-1 to SNO-7 using H NMR and melting points are given in the
Supplementary information.
Acknowledgments
The authors cordially thank the Department of Dyestuff Technology, Institute of Chemical
1
Technology, Mumbai, India for HNMR and FT-IR analysis. The authors are also grateful to
1
SAIF IIT Bombay, India for HNMR analysis.
Disclosure statement
No potential conflict of interest was reported by the authors.

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SYNTHETIC COMMUNICATIONS
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Funding
The authors acknowledge their sincere gratitude to the University Grant Commission (UGC),
New Delhi for funding this research work under the scheme of Minor Research Project.
ORCID
Shailesh N. Vajekar
Ganapati S. Shankarling
http://orcid.org/0000-0003-0321-3864
http://orcid.org/0000-0002-1727-7941
References
[
1] Li, Y.; Wong, K. M. C.; Tam, A. Y. Y.; Wu, L.; Yam, V. W. W. Thermo- and Acid-
Responsive Photochromic Spironaphthoxazine Containing Organogelators. Chem. Eur. J.
010, 16, 8690–8698. DOI: 10.1002/chem.201000150.
2
[2] Berkovic, G.; Krongauz, V.; Weiss, V. Spiropyrans and Spirooxazines for Memories and
Switches. Chem. Rev. 2000, 100, 1741–1754. DOI: 10.1021/cr9800715.
[3] Azizi, N.; Edrisi, M. Biodegradable Choline Hydroxide Promoted Environmentally Benign
Thiolysis of Epoxides. Tetrahedron Lett. 2016, 57, 525–528. DOI: 10.1016/j.tetlet.2015.12.
080.
[
[
[
4] Kol’tsova, L. S.; Zaichenko, N. L.; Shiyonok, A. I.; Marevtsev, V. S. Merocyanine Form of
Photochromic Spirooxazines in Acid Solutions. Russ. Chem. Bull. 2001, 50, 1214–1217.
DOI: 10.1023/A:1014098505092.
5] Harada, J.; Kawazoe, Y.; Ogawa, K. Photochromism of Spiropyrans and Spirooxazines in
the Solid State: Low Temperature Enhances Photocoloration. Chem. Commun. 2010, 46,
2593–2595. DOI: 10.1039/b925514a.
6] Xiong, Y.; Vargas Jentzsch, A.; Osterrieth, J. W. M.; Sezgin, E.; Sazanovich, I.; V;
Reglinski, K.; Galiani, S.; Parker, A. W.; Eggeling, C.; Anderson, H. L. Spironaphthoxazine
Switchable Dyes for Biological Imaging. Chem. Sci. 2018, 9, 3029–3040. DOI: 10.1039/
C8SC00130H.
[
[
[
7] Salemi-Delvaux, C.; Luccioni-Houze, B.; Baillet, G.; Giusti, G.; Guglielmetti, R. Effect of
Photodegradation on the Thermal Bleaching Rate Constant of Photochromic Compounds
in Spiro[Indoline-Pyran] and Spiro[Indoline-Oxazine] Series. J. Photochem. Photobiol. A
Chem. 1995, 91, 223–232. DOI: 10.1016/1010-6030(95)04113-X.
8] Balmond, E. I.; Tautges, B. K.; Faulkner, A. L.; Or, V. W.; Hodur, B. M.; Shaw, J. T.;
Louie, A. Y. Comparative Evaluation of Substituent Effect on the Photochromic Properties
of Spiropyrans and Spirooxazines. J. Org. Chem. 2016, 81, 8744–8758. DOI: 10.1021/acs.
joc.6b01193.
9] Nigel Corns, S.; Partington, S. M.; Towns, A. D. Industrial Organic Photochromic Dyes.
Color. Technol. 2009, 125, 249–261. DOI: 10.1111/j.1478-4408.2009.00204.x.
[
10] Fedorova, O. A.; Koshkin, A. V.; Gromov, S. P.; Strokach, Y. P.; Valova, T. M.; Alfimov,
M. V.; Feofanov, A. V.; Alaverdian, I. S.; Lokshin, V. A.; Samat, A. Transformation of
0
6
-Aminosubstituted Spironaphthoxazines Induced by Pb(II) and Eu(III) Cations. J. Phys.
Org. Chem. 2005, 18, 504–512. DOI: 10.1002/poc.890.
[
[
[
11] Lokshin, V.; Samat, A.; Guglielmetti, R. Synthesis of Photochromic Spirooxazines from
1-Amino-2-Naphthols. Tetrahedron. 1997, 53, 9669–9678. DOI: 10.1016/S0040-
4020(97)00640-6.
12] Zhou, J.; Zhao, F.; Li, Y.; Zhang, F.; Song, X. A. Novel Chelation of Photochromic
Spironaphthoxazines to Divalent Metal Ions. J. Photochem. Photobio. A: Chem. 1995, 92,
193–199. DOI: 10.1016/1010-6030(95)04136-0.
13] Mehraban, J. A.; Azizi, K.; Jalali, M. S.; Heydari, A. Choline Azide: New Reagent and
Ionic Liquid in Catalyst-Free and Solvent-Free Synthesis of 5-Substituted-1H-Tetrazoles: A
Triple Function Reagent. ChemistrySelect. 2018, 3, 116–121. DOI: 10.1002/slct.201702427.

1
0
S. N. VAJEKAR AND G. S. SHANKARLING
[
[
[
[
14] Kalla, R. M. N.; Zhang, Y.; Kim, I. Highly Efficient Green Synthesis of
a-Hydroxyphosphonates Using a Recyclable Choline Hydroxide Catalyst. New J. Chem.
2
017, 41, 5373–5379. DOI: 10.1039/C6NJ03948K.
15] Siddalingamurthy, E.; Mahadevan, K. M.; Shrungesh Kumar, T. O. Choline Chloride/Urea
Ionic Liquid Catalyzed a Convenient One-Pot Synthesis of Indole-3-Propanamide
Derivatives. Synth. Commun. 2013, 43, 3153–3162. DOI: 10.1080/00397911.2013.769601.
16] Garc ꢀı a-Su ꢀa rez, E. J.; Men ꢀe ndez-V ꢀa zquez, C.; Garc ꢀı a, A. B. Chemical Stability of Choline-
Based Ionic Liquids Supported on Carbon Materials. J. Mol. Liq. 2012, 169, 37–42. DOI:
10.1016/j.molliq.2012.02.022.
17] Pargaonkar, J. G.; Patil, S. K.; Vajekar, S. N. Greener Route for the Synthesis of Photo-
and Thermochromic Spiropyrans Using a Highly Efficient, Reusable, and Biocompatible
Choline Hydroxide in an Aqueous Medium. Synth. Commun. 2018, 48, 208–215. DOI: 10.
1080/00397911.2017.1395053.
[
18] Smith, E. L.; Abbott, A. P.; Ryder, K. S. Deep Eutectic Solvents (DESs) and Their
Applications. Chem. Rev. 2014, 114, 11060–11082. DOI: 10.1021/cr300162p.
[
19] Zhu, A.; Bai, S.; Li, L.; Wang, M.; Wang, J. Choline Hydroxide: An Efficient and
Biocompatible Basic Catalyst for the Synthesis of Biscoumarins under Mild Conditions.
Catal. Lett. 2015, 145, 1089–1093. DOI: 10.1007/s10562-015-1487-6.
[
[
[
[
[
[
20] Lu, W.; Ma, J.; Hu, J.; Zhang, Z.; Wu, C.; Han, B. Choline Hydroxide Promoted Chemical
Fixation of CO2 to Quinazoline-2,4(1H,3H)-Diones in Water. RSC Adv. 2014, 4,
50993–50997. DOI: 10.1039/C4RA08551E.
21] Abello, S.; Medina, F.; Rodriguez, X.; Cesteros, Y.; Salagre, P.; Sueiras, J. E.; Tichit, D.;
Coq, B. Supported Choline Hydroxide (Ionic Liquid) as Heterogeneous Catalyst for Aldol
Condensation Reactions. Chem. Commun. 2004, 9, 1096–1097. DOI: 10.1039/B401448K.
22] Vajekar, S. N.; Shankarling, G. S. Application of Fe O @Silica Sulfuric Acid as
3
4
AMagnetic Nanocatalyst for the Synthesis of Rhodamine Derivatives. ChemistrySelect
018, 3, 5848–5852. DOI: 10.1002/slct.201801127.
2
23] Gayakwad, E. M.; Patil, V. V.; Shankarling, G. S. Metal-Free Oxidation of Aldehydes to
Acids Using the 4Na2SO4ꢀ2H2O2ꢀNaCl Adduct. Environ. Chem. Lett. 2017, 15, 459–465.
DOI: 10.1007/s10311-017-0606-6.
24] Gayakwad, E. M.; Patil, V. V.; Shankarling, G. S. Amberlyst-15 Catalysed Oxidative
Esterification of Aldehydes Using a H2O2 Trapped Oxidant as a Terminal Oxidant. New
J. Chem. 2017, 41, 2695–2701. DOI: 10.1039/C6NJ03831J.
25] Gadilohar, B. L.; Deshpande, S. S.; Pinjari, D. V.; Shankarling, G. S. Concentrated Solar
Radiation Aided Energy Efficient Protocol for Oxidation of Alcohol Using Biodegradable
Task Specific Ionic Liquid-Choline Peroxydisulfate. Sol. Energy. 2016, 139, 328–336. DOI:
10.1016/j.solener.2016.09.044.
[
26] Ghorpade, P.; Gadilohar, B.; Pinjari, D.; Shinde, Y.; Shankarling, G. Concentrated Solar
Radiation Enhanced One Pot Synthesis of DES and N-Phenyl Phthalimide. Sol. Energy.
2
015, 122, 1354–1361. DOI: 10.1016/j.solener.2015.10.040.
[
27] Jarag, K. J.; Pinjari, D. V.; Pandit, A. B.; Shankarling, G. S. Synthesis of Chalcone (3-(4-
Fluorophenyl)-1-(4-Methoxyphenyl)Prop-2-En-1-One): Advantage of Sonochemical
Method over Conventional Method. Ultrason. Sonochem. 2011, 18, 617–623. DOI: 10.
1016/j.ultsonch.2010.09.010.
[
28] Sanap, A. K.; Shankarling, G. S. Environmentally Benign Synthesis of 4-Aminoquinoline-
2
1
-Ones Using Recyclable Choline Hydroxide. New J. Chem. 2015, 39, 206–212. DOI: 10.
039/C4NJ01281J.