Greener route for the synthesis of photo- and thermochromic spiropyrans using a highly efficient, reusable, and biocompatible choline hydroxide in an aqueous medium

Article abstract of DOI:10.1080/00397911.2017.1395053

Here, we report the synthesis of photo- and thermochromic spiropyrans promoted by a highly efficient, biocompatible, and reusable choline hydroxide (ChOH) in greener solvent water. This procedure provides several advantages such as simple workup, mild reaction conditions, short reaction time, and high yields of the products. Furthermore, the ChOH could be reused at least five times without significantly losing its catalytic activity. The structures of the synthesized spiropyran derivatives were confirmed by several characterization methods such as ¹H NMR, ¹³C NMR, and mass spectra.

Full text of DOI:10.1080/00397911.2017.1395053

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
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Greener route for the synthesis of photo- and
thermochromic spiropyrans using a highly
efficient, reusable, and biocompatible choline
hydroxide in an aqueous medium
Jyotsna G. Pargaonkar, Sanjay K. Patil & Shailesh N. Vajekar
To cite this article: Jyotsna G. Pargaonkar, Sanjay K. Patil & Shailesh N. Vajekar (2017): Greener
route for the synthesis of photo- and thermochromic spiropyrans using a highly efficient, reusable,
and biocompatible choline hydroxide in an aqueous medium, Synthetic Communications, DOI:
10.1080/00397911.2017.1395053
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Date: 19 December 2017, At: 13:04

SYNTHETIC COMMUNICATIONS®
https://doi.org/10.1080/00397911.2017.1395053
Greener route for the synthesis of photo- and thermochromic
spiropyrans using a highly efficient, reusable, and
biocompatible choline hydroxide in an aqueous medium
Jyotsna G. Pargaonkar, Sanjay K. Patil, and Shailesh N. Vajekar
Department of Chemistry, Changu Kana Thakur Arts, Commerce and Science College, New Panvel,
Maharashtra, India
ABSTRACT
ARTICLE HISTORY
Received 22 August 2017
Here, we report the synthesis of photo- and thermochromic
spiropyrans promoted by
a highly efficient, biocompatible, and
KEYWORDS
reusable choline hydroxide (ChOH) in greener solvent water. This
procedure provides several advantages such as simple workup, mild
reaction conditions, short reaction time, and high yields of the
products. Furthermore, the ChOH could be reused at least five times
without significantly losing its catalytic activity. The structures of the
synthesized spiropyran derivatives were confirmed by several
characterization methods such as 1H NMR, 13C NMR, and mass spectra.
Biocompatible; choline
hydroxide; photo- and
thermochromic; reusable;
spiropyran
GRAPHICAL ABSTRACT
Introduction
The spiropyrans are popular classes of photo- and thermochromic compounds because
they exhibit two forms viz. colorless closed spiro form and colored open merocyanine
form. The mechanism of photo- and thermochromic transformations of spiropyrans
involves reversible heterolytic cleavage of the spiro C–O bond followed by Z–E isomeriza-
tion to colored merocyanine structure. The latter may be converted into the initial spiro
isomer on cooling or irradiation with visible light (Fig. 1).[1] Spiropyrans play a special role
since their photoinduced and reversible ring-opening to the corresponding merocyanines
is accompanied not only by a significant structural change from a nonplanar to a planar
structure but also by a large polarity increase.[2,3] It is easy to control physical and chemical
properties of spiropyrans by modifying their molecular structures, so they are used for a
wide range of applications in areas such as data storage, electronic devices, optical filters,
optical switching, imaging systems, etc.[3–8]
CONTACT Shailesh N. Vajekar
shaileshchem81@gmail.com
Department of Chemistry, Changu Kana Thakur Arts,
Commerce and Science College, New Panvel, Maharashtra 410206, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data (full experimental detail, IR, 1H-NMR, 13C-NMR, mass spectra and microanalysis) can be accessed on
the publisher’s website.
© 2017 Taylor & Francis

2
J. G. PARGAONKAR ET AL.
Figure 1. Ring opening and closing of photochromic and thermochromic spiropyran.
Scheme 1. Synthesis of choline hydroxide.
The synthesis of spiropyrans using a wide range of organic or inorganic bases such as
Et3N, piperidine, pyridine, and NaOH are reported. All these hazardous bases produced
spiropyrans in low to moderate yields and these synthetic methods used organic solvents
like ethanol, methanol, isopropyl alcohol, DMF, etc.[8]
As a nontoxic and naturally abundant derivative of choline and a water soluble essential
nutrient, choline hydroxide (ChOH) has shown excellent catalysis performance for aldol reac-
tion and trans-esterification reaction for biodiesel production. This suggests that choline
hydroxide is a suitable basic catalyst in organic transformations.[9–12] Ionic liquids with the
unique properties of low volatility, good and tunable solubility, nonflammability, and excellent
recyclability are receiving considerable global attention and used extensively in organic
synthesis as green reaction media or designable catalysts.[13–19] Choline chloride (ChCl), a
member of the vitamin B family is a very cheap, commercially available, biodegradable, and
nontoxic quaternary ammonium salt, which can be simply produced by gas-phase reaction
or directly extracted from biomass.[20,21] In the present work, we have used a nontoxic, cheap,
and biodegradable ChCl-derived ionic liquid, i.e., choline hydroxide, as a catalyst for the syn-
thesis of spiropyrans in aqueous medium. Choline hydroxide can be readily prepared through
anion exchange reaction (Scheme 1) and it is liquid at room temperature (25 °C).[22–24]
Results and discussion
With the objective of optimization of the reaction conditions to synthesize spiropyrans, the
effect of different organic bases such as piperidine (20%), Et3N (20%), pyridine (20%),
DABCO (20%), DBU (20%), tetramethyl ammonium hydroxide (40%), and inorganic bases
such as NaOH (40%) and K2CO3 (40%) were investigated in detail for the model reaction
of 1,2,3,3-tetramethyl-3H-indol-1-ium iodide and 2-hydroxy benzaldehyde (or salicylalde-
hyde). These bases gave the desired product yields around 40–61% (Table 1, Entries 2–8).
Whereas, in the presence of 40% ChOH as a catalyst in an aqueous medium we obtained
the desired product in good yield 81% (Table 1, Entry 1). The higher yield of the product
was achieved with ChOH because it acts as a homogeneous catalyst in an aqueous medium
which assist to increase the solubility of the reactants through hydrogen bonding and there-
fore speed up the rate of the reaction.

SYNTHETIC COMMUNICATIONS®
3
Table 1. Catalytic performance of various bases on the synthesis of spiropyran.
Entry
Catalyst
Reaction time (h)
Isolated yield (%)
1
2
3
4
5
6
7
8
9
ChOH in water (40%)
Piperidine in ethanol (20%)
Et3N in ethanol (20%)
1
4
4
3
4
4
4
3
4
81
44
40
43
40
37
39
41
61
NaOH in water (40%)
Pyridine in THF (20%)
DABCO in ethanol (20%)
DBU in ethanol (20%)
K2CO3 in water (40%)
Tetramethyl ammonium hydroxide in water (40%)
Table 2. Effect of solvent on model reaction.
Entry
Catalyst
Solvent
Temperature (°C)
Reaction time (h)
Isolated yield (%)
1
2
3
4
5
ChOH (40%)
ChOH (40%)
ChOH (40%)
ChOH (40%)
ChOH (40%)
Water
80
80
80
80
80
1
3
3
8
7
81
77
76
59
67
Methanol
Ethanol
Toluene
Dioxane
To screen the effect of solvent, the model reaction was performed using choline
hydroxide (30%) in various solvents among which water was the most effective solvent.
(Table 2, Entry 1). To determine the catalyst loading, a model reaction of 1,2,3,3-
tetramethyl-3H-indol-1-ium iodide and 2-hydroxy benzaldehyde (or salicylaldehyde) with
the different percentage of ChOH in water was performed. The reaction occurred smoothly
in the presence of 40% ChOH as a catalyst and water as a solvent, affording a single
product in 81% yield (Table 3, Entry 5). Increasing the amount of catalyst more than
40% showed no substantial improvement in the yield. In the optimization of temperature
and time, the suitable temperature and time for the reaction was found to be 80 °C and 1 h.
(Table 4, Entries 4 and 5). Therefore, optimized reaction parameters include 1.0 mmol of
1,2,3,3-tetramethyl-3H-indol-1-ium iodide and 1.0 mmol of 2-hydroxy benzaldehyde (or
salicylaldehyde), 40% of ChOH as a catalyst and water as a solvent at 80 °C and 1 h.
Table 3. Optimization of catalyst loading using the model reaction.
Entry
Catalyst
Temperature (°C)
Reaction time (h)
Isolated yield (%)
1
2
3
4
5
6
Without ChOH
ChOH (10%)
ChOH (20%)
ChOH (30%)
ChOH (40%)
ChOH (50%)
80
80
80
80
80
80
24
08
06
04
1
No reaction
67
73
78
81
82
1
Table 4. Optimization of temperature and time on model reaction.
Entry
Catalyst
Temperature (°C)
Reaction time (h)
Isolated yield %
1
2
3
4
5
6
ChOH (40%)
ChOH (40%)
ChOH (40%)
ChOH (40%)
ChOH (40%)
ChOH (40%)
r.t.
40
1
1
No reaction
63
72
81
81
83
60
1
80
80
100
1
1.5
1

4
J. G. PARGAONKAR ET AL.
Table 5. Yields of the synthesized spiropyran derivatives.
Entry
Notation
Product
Yield (%)a
81
1
SP-1
2
3
4
5
6
7
8
9
SP-2
SP-3
SP-4
SP-5
SP-6
SP-7
SP-8
SP-9
83
87
93
77
89
91
90
74
10
SP-10
SP-11
78
88
11
aIsolated yields.

SYNTHETIC COMMUNICATIONS®
5
Figure 2. Recyclability study of ChOH. Note: ChOH, choline hydroxide.
The recyclability of the ChOH was important which helps to deal with both economical
and environmental outlook for large-scale operations. To tackle these issues, recyclability of
ChOH was examined for the model reaction of 1,2,3,3-tetramethyl-3H-indol-1-ium iodide
and 2-hydroxy benzaldehyde (or salicylaldehyde). After completion of the reaction, the
crude product was filtered off and filtrate containing choline hydroxide can be reused
directly for the next cycle under the same conditions. The results shown in Fig. 2 reveal
that ChOH can be recycled for at least five consecutive runs without any significant
decrease in its activity.
Scheme 2. Proposed mechanism for spiropyran formation in the presence of ChOH. Note: ChOH,
choline hydroxide.

6
J. G. PARGAONKAR ET AL.
Based on the present experimental observations, a plausible mechanistic pathway of
the reaction between 1,2,3,3-tetramethyl-3H-indol-1-ium iodide (i) and 2-hydroxy
benzaldehyde (iii) catalyzed by choline hydroxide is depicted in Scheme 2. The reaction
was initiated with the formation of 1,3,3-trimethyl-2-methyleneindolenine intermediate
(ii) upon deprotonation of (i) by ChOH. Intermolecular cyclization of (ii) and (iii) occurs
in presence of ChOH to yield an intermediate (v). Eventually ChOH will assist an inter-
mediate (v) to undergo dehydration to produce spiropyran. Importance of the alcoholic
–OH group of ChOH, we believe that it will form hydrogen bonding with carbonyl group
of (iii). This makes carbonyl carbon more electrophilic and susceptible for the nucleophilic
attack by (ii).
Experimental section
General information
All the solvents and reagents were procured fro m s d fine-chem limited and used without
further purification. Reaction progress was monitored by TLC using 0.25 mm E-Merck
silica gel 60 F254 plates, locating the spots using ultraviolet light as visualizing agent.
1H NMR spectra were recorded on 400 MHz (Bruker Ultrashield Plus NMR
spectrometer) and 200 MHz (Bruker AC-200 NMR spectrometer) in CDCl3 and 13C
NMR spectra were recorded on 101 MHz (Bruker NMR spectrometer) in CDCl3 at ambient
temperature. The chemical shifts values are expressed as δ value (ppm) down field with
1
residual protons of the solvent (CDCl3, H: δ ¼ 7.26 ppm, 13C: δ ¼ 77.0 ppm) as internal
1
standard. All synthesized compounds were characterized by LC-MS, H NMR, and 13C
NMR spectra. Melting points were determined on standard melting point apparatus from
Sunder Industrial Product, Mumbai and are uncorrected.
General procedure for the synthesis of choline hydroxide
Choline chloride (1 mmol) and KOH (1 mmol) in methanol (10 mL) were heated at 60 °C
for 12 h with constant stirring. After cooling to room temperature, the reaction mixture
was filtered to remove solid KCl salt and the filtrate was concentrated under vacuum to
remove methanol.[23,24] The obtained residue was dissolved in H2O to prepare 40 wt%
which was used without any further purification (Scheme 1).
General experimental procedure for the synthesis of spiropyrans
In a general procedure, a mixture of indolium iodide 1 (1.0 mmol), 2-hydroxy aryl
aldehyde 2 (1.0 mmol) and aqueous solution of 40 wt% choline hydroxide (1.5 mmol)
was stirred at 80 °C for 1 h. Then reaction mixture was cooled to room temperature, the
solid product was filtered off and washed with water. The crude product was purified by
column chromatography on silica gel with hexane:ethyl acetate (95:05) as the eluent to
afford the corresponding spiropyran 3 (Scheme 3).
Characterization data for selected spiropyran
(SP-1): 10,30,30-trimethylspiro[chromene-2,20-indoline]
81%; Colorless solid; M.P. 91–92 °C. Reported M.P. 92–94 °C.[25]

SYNTHETIC COMMUNICATIONS®
7
Scheme 3. General reaction for synthesis of spiropyrans.
1H NMR (200 MHz) δ 7.23 (1H, d, J ¼ 8.0 Hz, C5-H), 7.06-6.96 (5H, m, C4-H, C8-H,
C60-H, C40-H & C7-H), 6.83-6.70 (2H, m, C50-H & C6-H), 6.61 (1H, d, J ¼ 8.0 Hz,
C70-H), 5.69 (1H, d, J ¼ 10.0 Hz, C3-H), 2.57 (3H, s, N-CH3), 1.32 (3H, s, C30-CH3),
13
1.10 (3H, s, C30-CH3). C NMR: δ 154.64 (C8-C-O), 148.43 (C70-C-N), 136.28 (C30-C-
C40), 129.57 (C5), 129.40 (C7), 127.37 (C3 & C40), 126.65 (C60), 120.55 (C4-C-C5),
119.88 (C4), 119.55 (C6), 118.22 (C50), 114.79 (C8), 106.85 (C70), 105.18 (C2), 51.61
(C30), 29.66 (N-CH3), 25.69 (C30-CH3), 20.17 (C30-CH3).
LC-MS (m/z): Calculated for C19H20NO (MþH)þ 278.2, Observed 278.2
Conclusion
In summary, we have successfully synthesized photochromic and thermochromic
spiropyrans using choline hydroxide as a highly efficient green catalyst. The ready avail-
ability of this biocompatible and inexpensive reagent as well as the operational ease, good
yields, mild reaction conditions, and easy workup procedure make this methodology a
green alternative to the existing protocols.
Acknowledgments
The authors are grateful to Dr. S. T. Gadade, Principal, Changu Kana Thakur Arts, Commerce and
Science College, New Panvel, Maharashtra 410206, India for providing laboratory and other
facilities.
ORCID
Shailesh N. Vajekar
http://orcid.org/0000-0003-0321-3864
References
[1] Nikolaeva, O. G.; Gaeva, E. B.; Shepelenko, E. N.; Tsukanov, A. V.; Metelitsa, A. V.; Luk’yanov,
B. S.; Dubonosov, A. D.; Bren, V. A.; Minkina, V. I. Russ. J. Org. Chem. 2009, 45, 1091–1097.
[2] Beyer, C.; Wagenknecht, H. J. Org. Chem. 2010, 75, 2752–2755.
[3] Berkovic, G.; Krongauz, V.; Weizz, V. Chem. Rev. 2000, 100, 1741–1753.
[4] Chen, J. R.; Yang, D. Y. Org. Lett. 2009, 11, 1769–1772.
[5] Celani, P.; Bernardi, F.; Olivucci, M.; Robb, M. J. Am. Chem. Soc. 1997, 119, 10815–10820.
[6] Keum, S. R.; Choi, Y. K.; Kim, S. H.; Yoon, C. M. Dyes Pigments 1999, 41, 41–47.
[7] Cho, Y. J.; Rho, K. Y.; Kim, S. H.; Keum, S. R.; Yoon, C. M. Dyes Pigments 2000, 44, 19–25.
[8] Lukyanov, B.; Lukyanova, M. Chem. Heterocycl. Comp. 2005, 41, 281–311.
[9] Zhu, A.; Bai, S.; Li, L.; Wang, M.; Wang, J. Catal Lett. 2015, 145, 1089–1093.
[10] Abello, S.; Medina, F.; Rodriguez, X.; Cesteros, Y.; Salagre, P.; Sueiras, J. E.; Tichit, D.; Coq, B.
Chem. Commun. 2004, 9, 1096–1097.

8
J. G. PARGAONKAR ET AL.
[11] Kantam, M. L.; Reddy, R. S.; Pal, U.; Sreedhar, B.; Bhargava, S. Adv. Synth. Catal. 2008, 350,
2231–2235.
[12] Fan, M.; Huang, J.; Yang, J.; Zhang, P. Appl. Energy 2013, 108, 333–339.
[13] Zhang, Q.; Zhang, S.; Deng, Y. Green Chem. 2011, 13, 2619–2637.
[14] Niedermeyer, H.; Hallett, J. P.; Garcia, I. J. V.; Hunt, P. A.; Welton, T. Chem. Soc. Rev. 2012, 41,
7780–7802.
[15] Pereira, M. M. A. Curr. Org. Chem. 2012, 16, 1680–1710.
[16] Tang, S.; Baker, G. A.; Zhao, H. Chem. Soc. Rev. 2012, 41, 4030–4066.
[17] Payagala, T.; Armstrong, D. W. Chirality 2012, 24, 17–53.
[18] Isambert, N.; Duque, M. M. S.; Plaquevent, J. C.; Genisson, Y.; Rodriguez, J.; Constantieux, T.
Chem. Soc. Rev. 2011, 40, 1347–1357.
[19] Ying, A. G.; Ye, W. D.; Liu, L.; Wu, G. F.; Chen, X. Z.; Qian, S.; Zhang, Q. P. Chin. J. Org. Chem.
2008, 28, 2081–2094.
[20] Ruß, C.; König, B. Green Chem. 2012, 14, 2969–2982.
[21] Zhang, Q.; Vigier, K. D. O.; Royer, S.; Jerome, F. Chem. Soc. Rev. 2012, 41, 7108–7146.
[22] Hu, H.; Qiu, F.; Anguo, Y. A.; Yang, J.; Meng, H. Int. J. Mol. Sci. 2014, 15, 6897–6909.
[23] Azizi, N.; Edrisi, M. Tetrahedron Lett. 2016, 57, 525–528.
[24] Kalmode, H.; Vadagaonkar, K.; Murugan, K. RSC Adv. 2015, 5, 35166–35174.
[25] Asiri, A. M.; Ersanli, C. C.; Sahin, O.; Arshad, M. N.; Hameed, S. A. J. Mol. Struct. 2016, 1111,
108–117.