Microwave-assisted synthesis of photochromic fulgides

Article abstract of DOI:10.1007/s12039-010-0020-0

The oxazole and indole based heterocyclic photochromic fulgides were synthesized from their corresponding fulgenic acid derivatives by clay catalysed microwave irradiation methodology. Improved yields of fulgides were observed by the microwave irradiation method as compared other chemical methods employed so far. The proportions of clay (montmorillonite KSF) and isopropenyl acetate play a key role in increasing the yields of fulgides. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-010-0020-0

J. Chem. Sci., Vol. 122, No. 2, March 2010, pp. 183–188. © Indian Academy of Sciences.
Microwave-assisted synthesis of photochromic fulgides
SIVASANKARAN NITHYANANDAN, CHINNUSAMY SARAVANAN,
SENGODAN SENTHIL and PALANINATHAN KANNAN*
Department of Chemistry, Anna University, Chennai 600 025
e-mail: pakannan@annauniv.edu
MS received 27 January 2009; revised 10 August 2009; accepted 19 August 2009
Abstract. The oxazole and indole based heterocyclic photochromic fulgides were synthesized from
their corresponding fulgenic acid derivatives by clay catalysed microwave irradiation methodology.
Improved yields of fulgides were observed by the microwave irradiation method as compared other
chemical methods employed so far. The proportions of clay (montmorillonite KSF) and isopropenyl ace-
tate play a key role in increasing the yields of fulgides.
Keywords. Photochromism; fulgenic acid; fulgides; microwave-assisted synthesis; anhydride; cyclization.
1. Introduction
guished photochromic properties attributed to their
high thermal stability, strong fluorescence and
Microwave heating has taken as incontestable place
in analytical and organic laboratories practice as a
very effective and non-polluting method of activa-
tion. The organic reactions were carried out under
solvent-free reaction conditions and microwave irra-
diation lead to large reductions in reaction times,
enhancement in conversions, and sometime in selec-
tivity with several advantages of environmental
enhanced fatigue resistance compared to other ful-
6–9
gides.
The research articles regarding fulgides
mainly focused on their thermal irreversibility, pho-
tofatigue resistance and high efficiency in a rapid
photoconversion processes. The fulgide synthesis
involves Stobbe condensation of carbonyl compounds
with isopropylidene diethyl succinate followed by
cyclization of fulgenic acid using DCC and acetyl
chloride does not provide high yield of desired prod-
uct, particularly when the reactants are highly sub-
1,2
approach termed green chemistry. The design and
development of highly sensitive and reliable meth-
ods for growth of molecular level device is of scien-
tific interest, wherein the yield restrict and suppress
the development of new concepts and a challenging
task in confronting chemistry. Fulgide is one of the
most promising organic photochromic candidates for
optical device application. Fulgides undergo reversi-
ble photo-cyclization reaction between their two
isomers at well-separated absorption maximum upon
10
stituted. To increase the yield of fulgide, Thomas
et al attempted to synthesize indole substituted ful-
gide through base catalysed ring-opening reaction of
lactones followed by dehydration using acetyl chlo-
11
ride provide below 30% of yield. Lee et al recently
prepared fulgimide from fulgides through micro-
wave-assisted reaction with good yield at low reac-
12
tion time. This work deals with hitherto unreported
3
irradiation with appropriate wavelength of light.
synthesis and characterization of oxazole and indole
based heterocyclic photochromic fulgides through
clay catalysed microwave irradiation method from
their corresponding fulgenic acid. In addition, prepa-
ration of new vanillin containing oxazolyl fulgenic
acid is discussed.
This unique property made these materials consid-
ered to utilize in optical switches and optical data
4
storage devices. These devices should be capable of
ultrafast parallel access of stored information, eras-
able, good thermal stability and good fatigue resis-
5
tance with proficient in non-destructible read-out.
The aromatic heterocyclic ring structures containing
photochromic fulgides are excellent candidates;
especially indolyl and oxazolyl fulgides have distin-
2. Experimental
2.1 Reagents
Diethylsuccinate, acetone, t-butyl alcohol, potassium
*For correspondence
tertiarybutoxide, sodium hydride, 4-methoxyaniline,
183

184
Sivasankaran Nithyanandan et al
sodium hydroxide, hydroquinone, and 1-bromobutane condition for 6 h. After completion of reaction, flask
(Merck, Germany) were used as received. Methanol, was cooled to room temperature; dipotassium salt was
ethanol, phenol, THF, diethyl ether, chloroform, filtered and washed with excess of ethanol. Then salt
triethylamine, dimethylformamide (DMF) (SRL, was dissolved in water and acidified with dilute
India) and other solvents were purified by the hydrochloric acid to afford crude diacid. The obtained
13
reported procedures.
diacid was filtered, washed with excess of water and
dried. The crude product was further purified by
column chromatography (silica gel, ethyl acetate and
chloroform) to afford bright yellow coloured pow-
der, which was used as such for subsequent reaction.
2.2 Instrumentation
The infrared spectra were obtained on a Bruker IFS
66 V Fourier transform spectrometer using KBr pel-
1
13
lets. High-resolution H and C-NMR spectra were
recorded on a Bruker 300 MHz spectrometer in
CDCl3 with TMS as an internal standard. The absorp-
tion spectra of fulgides in spectroscopic grade chlo-
2.4 Synthesis of 2-[1,3-dimethyl-2-indolyl-
methylene]-3-isopropylidenesuccinic anhydride
(IFO)
roform solution were measured on a Shimadzu UV- The isopropenyl acetate (6⋅6 mL, 60 mmol) was added
260 spectrophotometer. The photocoloration– into blended mixture of IFA (1⋅57 g, 7.0 mmol) and
decolaration reaction of fulgides was carried out us- montmorillonite KSF (1 g) in a flat-bottomed flask
ing a 500-W high-pressure Hg lamp (Ushio SX-UI and stirred. It was irradiated under focused micro-
5000) equipped with a cut filter (Sigma, UTVAF- wave (2450 MHz) for 10 min. After completion of
35U) for UV irradiation (360 nm) and with a cut fil- reaction, mixture was cooled to room temperature
ter (Sigma, DIF-GRE) for visible light (560 nm).
and poured with excess of chloroform and stirred.
After 30 min of stirring, the clay was filtered off and
removed the chloroform under vacuum. The crude
product was further purified by column chromatog-
raphy to afford IFO as dark brown coloured powder
(1⋅23 g, 84%). M.p.: 123–125°C. FT–IR (KBr,
2.3 Synthesis of 2-[1,3-dimethyl-2-indolyl-
methylene]-3-isopropylidenesuccinic acid
(IFA)
–1
cm ): 1852, 1768 (–C=O), 1635 (–CH=C–), 1606
The IFA was synthesized according to the reported
10
procedure. A flask charged with 60% sodium hy-
(–C=C(CH3)2), 1371 (N–CH3), 856 (substituted aro-
1
matic). H-NMR (400 MHz, CDCl3) δ: 7⋅70 (s, 1H,
dride in paraffin oil (0⋅06 mol, 3⋅45 g) and washed
with hexane to remove the oil completely, to this,
benzene (120 mL) was added at room temperature and
stirred. A solution of 1,3-dimethylindole-2-carboxy-
aldehyde (0⋅04 mol, 7 g) (1) and isopropylidenedi-
ethyl succinate (2) (0⋅04 mol, 8⋅6 g) in benzene
(50 mL) were added drop-wise to the reaction mix-
ture at ambient temperature. After 1 mL of above
solution was added, ethanol (2 drops) was added to
initiate the reaction, followed by addition of remain-
ing solution in a period of 3 h. After completion of
addition, the reaction mixture was allowed to stir for
another 16 h. The reaction mixture was subsequently
quenched with ethanol and poured into crushed ice.
(Ar-CH=C), 7⋅55 (d, 1H, Ar), 7⋅05–7⋅29 (m, 3H,
Ar), 3⋅67 (s, 3H, N-CH3), 2⋅45 (s, 3H, Ar-CH3), 1⋅83
(s, 3H, –C=C(CH3)2), 1⋅24 (s, 3H, –C=C(CH3)2. C-
NMR (400 MHz, CDCl3) δ: 165⋅5, 163⋅0 (carbonyl),
159⋅3 (Ar-CH=C), 139⋅0, 132⋅2, 128⋅9, 127⋅4, 125⋅4,
124⋅7, 120⋅5, 119⋅9, 117⋅5, 109⋅4 (aromatic), 30⋅85,
27⋅46, 25⋅9, 23⋅41, 11⋅02. Anal. Calcd. for C18H17NO3 :
C, 73⋅26%, H, 5⋅88%, N, 4⋅75%. Found C, 73⋅24%,
H, 5⋅88%, N, 4⋅70%.
13
2.5 Synthesis of 4-butyloxy-3-methoxybenzaldehyde
(3)
The organic layer was separated and extracted with A suspension of anhydrous K2CO3 (36⋅29 g,
10% sodium hydroxide solution (3 × 150 mL). All 0⋅268 mol), pinch of KI and vanillin (10 g, 0⋅066 mol)
the aqueous layers were combined and acidified in dry DMF (260 mL) were stirred at 90°C for 1 h.
with dilute hydrochloric acid. Liberated product was 4-Bromobutane (11⋅19 mL, 0⋅06 mol) was added
extracted with ethyl acetate and dried over anhy- drop-wise to the reaction mixture and stirred addi-
drous sodium sulphate. Solvent was removed under tionally for 48 h at the same temperature. After
vacuum to afford diacid, which was hydrolysed in completion of reaction, mixture was cooled to room
potassium hydroxide ethanol solution under reflux temperature, poured into ice/water mixture and neu-

Microwave-assisted synthesis of photochromic fulgides
185
tralized with 10% hydrochloric acid. The aqueous starting materials. The settled product at the bottom
solution was extracted with diethyl ether (3 × of beaker was filtered, dried and recrystallized by
200 mL) and combined organic layer washed with acetone to afford N-oxide hydrochloride of com-
10% KOH solution (3 × 250 mL), saturated brine pound 5. It was dissolved in glacial acetic acid
solution (3 × 250 mL) and dried over anhydrous (50 mL) and activated zinc dust (20 g) was added
sodium sulphate. Solvent was removed under vac- and stirred well. After stirring, the reaction mixture
uum to afford compound 3 as brown coloured oil was poured into water (500 mL) to separate out the
–1
(7⋅5 g, 60%). FT–IR (KBr, cm ): 2942 and 2865 desired product and dried. The obtained product was
(–CH2–), 1603 and 1514 (aromatic C–H), 1681 (Ar- recrystallized from methanol to afford compound 5
1
C=O), 1164 and 1084 (C–O–C). H-NMR (400 MHz, as light yellow coloured powder (42 g, 69⋅10%).
–1
–1
CDCl3) δ: 9⋅67 (s, 1H, –CHO), 7⋅21 (s, 1H, Ar), FT–IR (KBr, cm ): FT-IR (KBr, cm ): 2942 and
6⋅82–7⋅04 (m, 2H, Ar), 3⋅72 (s, 3H, Ar–O–CH3), 2865 (–CH2–), 1611 and 1518 (aromatic –C–H),
1
3⋅56 (t, 2H, Ar–O–CH2–), 0⋅82–1⋅52 (m, 7H, –CH2– 1678 (Ar–C=O), 1294 and 1264 (C–O–C). H-NMR
13
1
CH2–CH3). C-NMR (400 MHz, CDCl3) δ: 190⋅4 (400 MHz, CDCl3) δ: H-NMR (400 MHz, CDCl3)
(carbonyl), 153⋅9, 149⋅5, 129⋅5, 126⋅4, 111⋅1, 108⋅9 δ: 7⋅47 (s, 1H, Ar), 6⋅85–7⋅08 (m, 2H, Ar), 3⋅78 (s,
(aromatic), 68⋅4, 55⋅6 (methoxy), 30⋅6, 18⋅8, 13⋅4 3H, Ar–O–CH3), 3⋅86 (t, 2H, Ar–O–CH2–), 2⋅70 (s,
(aliphatic).
3H, –CO–CH3), 2⋅48 (s, 3H, –CH3), 0⋅82–1⋅53 (m,
7H, –CH2–CH2–CH3). C-NMR (400 MHz, CDCl3)
13
δ: 201⋅09 (carbonyl), 156⋅3, 153⋅5, 150⋅5, 148⋅8,
135⋅7, 124⋅3, 119⋅4, 118⋅8 (aromatic), 68⋅8, 56⋅1
(methoxy), 28⋅1, 23⋅4, 12⋅3, 10⋅4, 9⋅8 (aliphatic).
2.6 Synthesis of N-hydroxyiminoacetyl acetone (4)
A solution of acetyl acetone (32⋅5 mL, 0⋅317 mol) in
50 mL of glacial acetic acid was stirred at 5–10°C
and then the cooled solution of sodium nitrite (25 g,
0⋅362 mol) in water (500 mL) was added in portion
wise over 30 min maintaining at the same tempera-
ture. After completion of addition, solution was
2.8 Synthesis of 5-methyl-2-phenyl(3-methoxy-4-
butyloxy)oxazolylethylidene(isopropylidene)
succinic acid (OFA)
slowly allowed to come up room temperature and A solution of potassium-t-butoxide (3⋅6 g, 0⋅032 mol)
additionally stirred for 5 h. The reaction mixture ex- in 40 mL of t-butanol under nitrogen was stirred at
tracted with diethyl ether, washed with 10% sodium room temperature. To this, solution of compound 5
bicarbonate solution (3 × 150 mL) to remove unre- (7⋅7 g, 0⋅0359 mol) and compound 6 (5⋅8 g, 0⋅019 mol)
acted acetic acid and dried over anhydrous sodium in t-butanol (40 mL) were drop-wise added. After
sulphate. The solvent was removed under vacuum to completion of addition, the reaction mixture was
afford white powder of compound 4 (19⋅8 g, 48%) heated under reflux condition for 6 h. Then, the mix-
–1
FT–IR (KBr, cm ): 3282 (C=N), 3065 (–N–OH), ture was cooled to room temperature; solvent con-
1
1698 (–C=O). H-NMR (400 MHz, CDCl3) δ: 2⋅38 centrated under vacuum, poured into water (200 mL)
13
(s, 3H, CH3), C-NMR (400 MHz, CDCl3) δ: 195⋅8, and neutralized with 10% HCl solution. The product
156⋅1, 28.
was extracted with diethyl ether (3 × 150 mL) and
washed with 10% sodium bicarbonate solution
(3 × 100 mL) to remove pure half ester from organic
layer. The combined bicarbonate solution was neu-
tralized by 10% HCl solution, extracted with diethyl
2.7 Synthesis of 4-acetyl-5-methyl-2-phenyl
(3-methoxy-4-butyloxy)oxazole (5)
To a solution of 4-butyloxy-3-methoxybenzaldehyde ether again and dried over anhydrous sodium sul-
(33 × 48 g 0⋅161 mol) in glacial acetic acid (16 mL) phate. The solvent was removed under vacuum to
was added N-hydroxyiminoacetyl acetone (27⋅3 g, afford half acid, which was dissolved in ethanol
0⋅21 mol) by one portion at room temperature. The (100 mL) and refluxed overnight with KOH (3 g).
reaction mixture was cooled to 0–5°C and subjected After completion of reaction, solvent was concen-
to flow of hydrogen chloride gas for 4 h with main- trated under vacuum and poured into water (200 mL)
taining the same temperature. After the completion and neutralized with 10% HCl solution. The sepa-
of flow of hydrogen chloride, the reaction mixture rated product was extracted with diethyl ether and
was poured into a beaker containing diethyl ether dried over anhydrous sodium sulphate. The solvent
(500 mL) and stirred for 1 h to remove unreacted was removed under vacuum to afford OFA as color-

186
Sivasankaran Nithyanandan et al
14,15
less powder, which was used in situ for succeeding respectively.
The IFA and OFA were prepared
step.
via Stobbe condensation of one mole of isopro-
pylidenediethyl succinate and one mole of corre-
sponding carbonyl compounds using sodium hydride
and potassium tert-butoxide as base at room tem-
perature respectively. The resultant products were
hydrolysed using alcoholic KOH at reflux tempera-
ture for overnight. The poor yield of fulgides limits
the large-scale utilization of fulgides for commercial
and scientific investigations. To improve the yield of
fulgides, IFO and OFO were prepared from corre-
sponding fulgenic acid with varying proportions of
Montmorillonite KSF and isopropenyl acetate (table
1) according to reported procedure for synthesis of
2.9 Synthesis of 5-methyl-2-phenyl(3-methoxy-4-
butyloxy)oxazolylethylidene(isopropylidene)
succinic acid (OFO)
The isopropenyl acetate (6⋅6 mL, 60 mmol) was added
into blended mixture of OFA (2⋅2 g, 4⋅97 mmol) and
montmorillonite KSF (1 g) in a flat-bottomed flask
and stirred. It was irradiated under focused micro-
wave (2450 MHz) for 10 min. After completion of
reaction, mixture was cooled to room temperature
and poured excess of chloroform and stirred. After
30 min of stirring, the clay was filtered off and chlo-
roform removed under vacuum. The crude product
was further purified by column chromatography to
afford OFO as dark brown coloured powder (1⋅52 g,
16
anhydrides from corresponding dicarboxylic acids
(schemes 1 and 2). To optimize the conditions for
good yield, proportions of montmorillonite KSF and
isopropenyl acetate were varied as shown in table 1.
Both IFO and OFO show maximum yield when
60 mmol of isopropenyl acetate and 1g of mont-
morillonite KSF proportions (table 1) employed
respectively. It revealed that the proportions of clay
and isopropenyl acetate play a key role in the yield
of fulgides. Specifically in the presence of excess
isopropenyl acetate, non-ionizing radiation increases
the molecular motion of ions, rotation of dipoles in
the reaction site of clay and increase the penetration
–1
72⋅2%,). M.p.: 145–149°C. FT–IR (KBr, cm ):
2942 and 2865 (–CH2–), 1834, 1756 (–C=O), 1610
(oxazole C=N), 1294 and 1264 (C–O–C), 1041 (Ar–
1
C=C). H-NMR (400 MHz, CDCl3) δ: 7⋅83 (s, 1H,
Ar), 7⋅42–7⋅53 (m, 1H, Ar), 6⋅96–7⋅03 (m, 1H, Ar),
4⋅01 (t, 2H, –O–CH2–), 3⋅78 (s, 3H, Ar-OCH3), 2⋅49
(s, 3H, CH3), 2.03 (s, 3H, –C=C(CH3)), 0⋅99–1⋅79
13
(m, 13H, –CH2–CH2–CH3, CH3–C–CH3). C-NMR
(400 MHz, CDCl3) δ: 172⋅8, 168⋅9 (carbonyl),
163⋅5, 159⋅8, 152⋅1, 148⋅4, 146⋅9, 142⋅3, 123⋅1,
122⋅7, 121⋅6, 113⋅3, 112⋅0, 111⋅8, 67⋅9, 55⋅5, 48⋅2,
35⋅2, 35⋅0, 30⋅6, 22⋅8, 22⋅7, 20⋅7, 18⋅7, 13⋅6. Anal.
Calcd. for C24H27NO6: C, 67⋅76%, H, 6⋅35%, N,
3⋅29% found C, 66⋅48%, H, 5⋅82%, N, 2⋅79.
16
depth of microwave radiation. Here, both IFO and
OFO demonstrated good improvement in yield
when compared with that of conventional chemical
method used for synthesis of fulgides. The percent-
age yield of isomers of products was calculated from
NMR spectral values of corresponding compounds.
In the case of IFO, the E-isomer alone was formed
(see supplementary informations) but in the case of
OFO, Z-isomer was the major product. When com-
3. Results and discussion
The IFO and OFO were prepared from IFA and pared to IFO, the OFO indicates less amount of
OFA in the presence of acetyl chloride and acetic yield with all concentration of isopropenyl acetate.
anhydride in a relatively poor yield of 32% and 15% The concentration of isopropenyl acetate is varied
Table 1. Improved yields of Fulgides IFO and OFO with different proportions of KSF and isopropenyl acetate.
Fulgenic acid
Isopropenyl
acetate
IFO (yield %)
OFO (yield %)
Net yield
derivative
IFA and OFA (mmol)
(mmol)
KSF (g)
Net yield
E
Z
E
Z
Time (min)
5
5
5
5
5
5
20
30
40
50
60
60
5
4
3
2
1
0
51
60
68
76
84
20
51
60
68
76
84
20
0
0
0
0
0
0
39
1
38
42
10
10
10
10
10
10
44
2
52⋅7
65
2⋅4 52⋅3
64
1⋅2 71
0⋅7 9⋅8
1
72⋅2
10⋅6

Microwave-assisted synthesis of photochromic fulgides
187
from 20 to 60 mM. Accordingly, the yield of Z- four methyl carbon atoms. It indicated no character-
isomer increases proportionately with minor product istic peak for isomers in IFO. The chloroform solu-
of E-isomer (table 1). In the presence of 40 mM of tion of OFO was irradiated with 360 nm UV light
isopropenyl acetate and 3 g of KSF gives 54⋅7% for long time (30 min); all the Z-form is isomerized
yield with mixture of isomers. Methyl protons of E- to E-form. The methyleneoxy protons resonated as
isomer appeared at 2⋅46 ppm as singlet with integral triplet at 4⋅00 ppm and methoxy protons resonated
1
value of 0⋅07 and methyl protons of Z-isomer as singlet at 3⋅78 ppm in H-NMR spectrum of irra-
appeared at 2⋅41 ppm as singlet with integral value diated solution of OFO (see Supplementary infor-
1
of 1⋅64 in the H-NMR spectrum. Percentage yield mation, figure S3). The oxazolyl methyl protons
13
of isomers was calculated using the integral values appeared at 2⋅49 ppm as singlet. The C-NMR spec-
from the total yield (table 1). The H-NMR spectrum
1
of IFO (see supplementary information, figure S1,
see www.ias.ac.in) depicts four methyl groups reso-
nated at four different positions between 1⋅24 and
3⋅67 ppm. The methylene proton resonated at
13
7⋅70 ppm as singlet. The C-NMR spectrum (see
supplementary information, figure S2) also shows
four peaks in the aliphatic region corresponding to
Figure 1. The absorption spectral pattern of IFO in
chloroform solution upon irradiation 360 nm light.
Scheme 1. Synthesis of IFA and IFO.
Figure 2. The absorption spectral pattern of OFO in
Scheme 2. Synthesis of OFA and OFO.
chloroform solution upon irradiation with 360 nm light.

188
Sivasankaran Nithyanandan et al
trum (see Supplementary information, figure S4) small-scale preparation and can be transformed to
also shows two peaks at 67⋅9 and 57⋅4 ppm corre- wide range of other fulgides.
sponding to methyleneoxy and methoxy protons
1
respectively. After UV irradiation, the H-NMR spec-
Acknowledgement
trum of OFO confirms with no characteristic peaks
for isomers (see Supplementary information, figure
The authors thank the Board of Research in Nuclear
S3). The deviation in yield and E and Z isomeric
Sciences, Mumbai, Government of India (2006/37/
ratios were around 2–3%. The photochemical prop-
13/BRNS/205) for financial support.
erties of fulgides were evaluated in chloroform solu-
tion. The absorption spectral changes of IFO and
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fully by simple and efficient means with improved
yield over conventional methods. This methodology
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organic solvents. This method found suitable for
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