Synthesis of novel photochromic 6-benzyloxo-spirobenzopyran compounds

Article abstract of DOI:10.1080/00397910500281044

The synthesis of four novel derivatives of 1′,3′,3′- trimethylspiro-[2H-1]-benzopyran-6-benzyloxo-2,2′-indoline is reported. The synthetic approach to these compounds involves the preparation of the key compound, 3-formyl-4-hydroxybenzophenone, by means o

Full text of DOI:10.1080/00397910500281044

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Synthesis of Novel Photochromic
6‐Benzyloxo‐spirobenzopyran Compounds
Luis E. Elizalde a , Gladys de los Santos a , Aída E. García a , Diana I.
Medellín a & Ricardo Acosta a
a Centro de Investigación en Química Aplicada , Saltillo, Coahuila, México
Published online: 20 Aug 2006.
To cite this article: Luis E. Elizalde , Gladys de los Santos , Aída E. García , Diana I. Medellín & Ricardo
Acosta (2005) Synthesis of Novel Photochromic 6‐Benzyloxo‐spirobenzopyran Compounds, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 35:24,
3087-3097
To link to this article: http://dx.doi.org/10.1080/00397910500281044
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Synthetic Communicationsw, 35: 3087–3097, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500281044
Synthesis of Novel Photochromic
6-Benzyloxo-spirobenzopyran Compounds
´
´
Luis E. Elizalde, Gladys de los Santos, Aıda E. Garcıa,
´
Diana I. Medellın, and Ricardo Acosta
´ ´ ´
Centro de Investigacion en Quımica Aplicada,Saltillo, Coahuila, Mexico
Abstract: The synthesis of four novel derivatives of 10,30,30-trimethylspiro-[2H-1]-
benzopyran-6-benzyloxo-2,20-indoline is reported. The synthetic approach to these
compounds involves the preparation of the key compound, 3-formyl-4-hydroxybenzo-
phenone, by means of the Reimer–Tiemann reaction. The photochromic compounds
1
were isolated and characterized by H and 13C NMR as well as FT-IR and UV-VIS
spectrometry. Comparative studies performed in these compounds showed a higher
molar extinction coefficient and red shift of maximum absorption wavelength when
the benzyloxo group is introduced in the spirobenzopyran molecule.
Keywords: 3-Formyl-4-hydroxybenzophenone, optical materials, photochromism,
spiropyrans
INTRODUCTION
The application of photochromic compounds in areas such as data storage,
ophthalmic lenses, and security devices, among others, have attracted the
interest of many researchers throughout the world. The high commercial
value of these compounds has resulted in the publication of a plethora of
patents and papers dealing with either new synthetic methods for the prep-
aration of these compounds or determination of their photochemical and
photophysical properties.
Received in the USA June 30, 2005
Address correspondence to Luis E. Elizalde, Centro de Investigacion en Quımica
´
´
´
Aplicada, Blvd. Enrique Reyna #140, Saltillo, Coahuila, Mexico. E-mail: elizalde@
ciqa.mx
3087

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L. E. Elizalde et al.
Photochromic compounds undergo a reversible transformation induced
by ultraviolet light, which causes a change in the molecular structure. This
is detected by a change in the coloration of the original molecule. When the
excited molecule is subjected again to white light or heat, it returns to its
original state. Therefore, a photochromic system has two different
molecular structures, each of them with different properties, namely, absorp-
tion spectra, refractive indexes, dielectric constants, oxidation/reduction
potentials, and molecular geometries.
However, the photochromism can be affected by side reactions that result
in the formation of products unable to return to the original molecular
structure. This effect is called fatigue, and it reduces the half-life of the
active compound. Another drawback of these molecules is the color
stability of the photochromic transition. In some applications (e.g., optical
storage) it is very important that the secondary state has a definite transition
period. Thus, many studies have focused on improving the stability and the
half-life of photochromic compounds.
Although there are several families of compounds that exhibit photochro-
mism, such as spiropyrans, spirooxazines, fulgides, diarylethenes, viologens,
and azo compounds, the spiropyrans derived from 10,30,30-trimethylspiro[2H-1]
benzopyran-2,20-indoline are especially attractive because they have appli-
cations in electronic storage media as well as in optoelectronic devices.[1–3]
These molecules were first studied by Hirshberg and Fisher[4] and basically
consist of two orthogonal systems linked by a tetrahedral spiro carbon. These
compounds, commonly called BIPS, are numbered according to the Inter-
national Union of Pure and Applied Chemistry (IUPAC), as shown in Scheme 1.
In optical device applications, the merocyanine requires a reasonable
thermal stability at room temperature once the material is no longer irradiated
(back thermal reaction on Scheme 1). By changing the electronic character of
the substituents at the benzopyran ring, the thermal stability can be modified.[5]
We now report the synthesis of novel spirobenzopyrans derivatives 1
of 10,30,30-trimethylspiro-[2H-1]benzopyran-6-benzyloxo-2,20-indoline. These
compounds can develop a colored form with maximum absorption wavelength
beyond 560 nm. Such novel molecules 1a–d contain a benzyloxo group in
position 6 (Scheme 2) to improve the thermal stability on the merocyanine,
which results from increase of the resonance effect.
Scheme 1. Isomerization reaction for spirobenzopyrane indoline molecules.

6-Benzyloxo-spirobenzopyrans
3089
Scheme 2. Synthesized derivatives of 6-benzyloxospirobenzopyrane molecules.
EXPERIMENTAL
All inorganic reagents and solvents were analytical grade and used as
purchased (Aldrich, Merck). Sulfuryl chloride, 2-methylene-1,3,3-trimethyl-
indoline 97%, and 4-hydroxybenzophenone 98% were purchased from
Aldrich. The reference photochromic compounds 3a–d were synthesized
and purified according to reported methods.[6,7] 1H and 13C NMR spectra
were obtained using a 300-MHz Jeol-JNM-ECP300 spectrometer, using
CDCl3 as solvent and tetramethyl silane as standard, at room temperature.
Routine infrared spectra were obtained on a Magna Nicolet 550 infrared spec-
trometer. Elemental analyses were determined on a Perkin Elmer 2400 series
analyser and UV-Vis spectra on an HP 8452 spectrometer. The chromato-
grams were run on a GC-MS Hewlett Packard HP-5971.
Synthesis of 3-Formyl-4-hydroxybenzophenone (2a)
A mixture of 1 g (5 mmol) of 4-hydroxybenzophenone and 20 mL of an
aqueous solution containing 3.2 g (80 mmol) of sodium hydroxide was
heated under inert atmosphere in a three-necked round-bottomed flask. Once
the solution reached 708C, 0.87 mL (10.86 mmol) of chloroform was added
dropwise over 30 min. This temperature was maintained for at least 2 h.
After this time, the reaction mixture was acidified by addition of 10% HCl
aqueous solution and extracted with ether. KOH aqueous solution (50 mL,
7.5% w/w) was added to the organic layer and extracted twice with 50 mL
of chloroform. The basic phase was acidified and extracted with ethyl ether.
The extract was dried over MgSO4. After the solvent evaporation, 0.74 g of a
white powder was recovered (65% yield). 1H NMR (300 MHz, CDCl3) d: 7.08
(d, 1H, J ¼ 9 Hz), 7.53 (m, 2H), 7.61 (tt, 1H, J ¼ 6 Hz, J ¼ 1.38 Hz), 7.76
(m, 2H), 8.06 (dd, 1H, J ¼ 2 Hz, J ¼ 9 Hz), 8.13 (d, 1H, J ¼ 2 Hz),
9.95 (s, 1H), 11.48 (s, 1H). 13C (50 MHz, CDCl3) d: 196, 165, 161, 138, 136,
133, 132.65, 132, 129.78, 129.6, 128.5, 128.2, 117.97, 115. FTIR (KBr) nmax
(cm21): 1642, 1600; EI-MS m/z 226 (Mþ). Anal. calcd. for C14H10O3:
C, 74.03%; H, 4.45%. Found C, 74.09%; H, 4.55%.

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L. E. Elizalde et al.
Synthesis of 2-Chloro-3-formyl-4-hydroxybenzophenone (2b)
3-Formyl-4-hydroxybenzophenone (2a) (1.51 g, 6.7 mmol) dissolved in
20 mL of dry CH2Cl2 was placed in a reactor. Then, carefully and under
vigorous stirring, 0.65 mL (8 mmol) of sulfuryl chloride was added. The
reaction mixture was kept at 258C for 24 h. Then, it was washed with
20 mL of distilled water and 10% NaHCO3 aqueous solution. The organic
layer was finally dried over MgSO4. The product was recovered by precipi-
1
tation in ether. Yield 30%, mp 1568C, H NMR (300 MHz, CDCl3) d: 7.53
(m, 2H), 7.61 (tt, 1H, J ¼ 6 Hz, J ¼ 1.38 Hz), 7.76 (m, 2H), 8.06 (d, 1H,
J ¼ 2 Hz,), 8.13 (d, 1H, J ¼ 2 Hz), 9.95 (s, 1H), 11.92 (s, 1H).
13C (50 MHz, CDCl3) d: 198.17, 198.5, 161.51, 141.83, 138.56, 137.65,
136.42, 130.32, 129.12, 127.63, 120.1, 112.96. FTIR (KBr) nmax (cm21):
1631, 1605; EI-MS m/z 260 (100), 262 (30) (Mþ, Mþ þ 2). Anal. calcd.
for C14H9ClO3: C, 64.5%; H, 3.5%, Cl, 13.6%. Found C, 64.75%; H,
3.43%, Cl, 13.57%.
Synthesis of 2-Bromo-3-formyl-4-hydroxybenzophenone (2c)
Dry CH2Cl2 (15 mL) and 1.51 g (6.74 mmol) of 3-formyl-4-hydroxybenzo-
phenone (2a) were placed in a three-neck round-bottom flask under inert
atmosphere. The flask was cooled in an acetone–CO2 bath at –708C. Then,
8.11 mmol of bromine were added through a dropping funnel. As soon as
the addition was completed, some crystals of anhydrous FeCl3 were added.
After 30 min, the temperature was allowed to reach room temperature, and
the mixture was stirred for 6 h. The reaction mixture was then washed with
distilled water. The organic phase was extracted with 10% NaOH aqueous
solution to remove the product as the sodium salt. The aqueous phase was
acidified and extracted with chloroform, and then dried over anhydrous
MgSO4. After evaporation of the solvent, the final product was crystallized
in ethyl ether yielding 0.78 g (38%) of a white solid with a melting point of
1
1668C. H NMR (300 MHz, CDCl3) d: 7.53 (m, 2H), 7.61 (tt, 1H, J ¼ 6 Hz,
J ¼ 1.38 Hz), 7.76 (m, 2H), 8.06 (d, 1H, J ¼ 2 Hz), 8.32 (d, 1H, J ¼ 2 Hz),
9.95 (s, 1H), 12.08 (s, 1H). 13C (50 MHz, CDCl3) d: 196.07, 193.15,
151.51, 141.33, 135.32, 135.55, 132.07, 130.70, 128.84, 128.5, 120.49,
111.32. FTIR (KBr) nmax (cm21): 1655, 1618 EI-MS m/z 304 (50), 306
(50). Anal. calcd. for C14H9BrO3 C, 55.1%; H, 3.0%; Br, 26.2%. Found C,
54.37%; H, 2.98%; Br, 26.37%.
Synthesis of 2-Nitro-3-formyl-4-hydroxybenzophenone (2d)
A mixture of 0.3 g of ammonium nitrate (3.7 mmol), 0.42 g of 3-formyl-4-
hydroxybenzophenone (1.85 mmol), and 25 mL of dry chloroform was

6-Benzyloxo-spirobenzopyrans
3091
placed in an inert atmosphere. After complete dissolution of the reactants,
3.25 g (15.5 mmol) of trifluoroacetic anhydride were added. The mixture
was refluxed in chloroform for 2 h. Then, it was washed with distilled
water. The organic phase was separated and dried over anhydrous
magnesium sulfate, after purification by column chromatography (hexanes–
ethyl acetate 7:3), and 0.29 g of a yellow powder was recovered. Mp 968C
1
(57% yield). H NMR (300 MHz, CDCl3) d: 7.53 (m, 2H), 7.61 (tt, 1H,
J ¼ 6 Hz, J ¼ 1.38 Hz), 7.76 (m, 2H), 8.55 (d, 1H, J ¼ 2 Hz), 8.83 (d, 1H,
J ¼ 2 Hz), 9.95 (s, 1H), 12.08 (s, 1H). 13C (50 MHz, CDCl3) d: 189.67,
188.15, 158.51, 138.47, 136.32, 133.64, 133.07, 130.70, 129.92, 129.15,
125.49, 116.32. FTIR (KBr) nmax (cm21): 1650, 1595, 1534; EI-MS m/z;
271 (Mþ). Anal. calcd. for C14H9NO5 C, 62.0%; H, 3.3%; N, 5.2%. Found
C, 61.01%; H, 3.29%; N, 5.48%.
General Procedure for Preparation of Derivatives of 10,30,30-
Trimethylspiro-[2H-1] Benzopyrane-8-benzyloxo-2,20-indoline
(Benzyloxo-BIPS)
A mixture of 3 g of 3-formyl-4-hydroxybenzophenone (2a), 2.475 mL of
2-methylene-1,3,3-trimethylindoline, and 50 mL of distilled ethanol was
boiled for 4 h. The reaction mixture was filtrated and washed with cold
ethanol. The precipitate was recovered, washed with cold ethanol, and then
recrystallized in ethanol.
Characterization of 10,30,30-Trimethylspiro-[2H-1] Benzopyran-
6-benzyloxo-2,20-indoline (1a)
1H NMR (300 MHz, CDCl3) d: 8.23 (d, 1H, J ¼ 2 Hz), 8.059 (dd, 1H,
J ¼ 9 Hz, J ¼ 2 Hz), 7.66 (tt, 1H, J ¼ 6 Hz, J ¼ 1.38 Hz), 7.55 (m, 2H),
7.78 (m, 2H), 7.28 (m, 2H), 7.15 (d, 1H, J ¼ 8 Hz), 7.01 (d, 1H, J ¼ 9 Hz),
6.82 (d, 1H, J ¼ 12 Hz), 6.38 (d, 1H, J ¼ 8 Hz), 5.75 (d, 1H, 12H), 3.05
(s, 3H), 1.25 (s, 3H), 1.15 (s, 3H). 13C (50 MHz, CDCl3) d: 190.54, 156.15,
149.8, 143.47, 138.52, 136.64, 134.07, 131.70, 130.92, 129.15, 129.04,
122.49, 122.13, 119.32, 118.95, 109.53, 108.32, 97.351.8, 30.41, 18.63.
EI-MS m/z 159 (100), 381 (1). FT-IR (KBr film) nmax cm21: 1648, 1285,
885, 732. Anal. calcd. for C26H23NO2: C, 81.9%; H, 6.1%; N, 3.7%. Found
C, 82.10%; H, 6.23%; N, 3.25%.
Characterization of 10,30,30-Trimethylspiro-[2H-1] Benzopyran-
6-benzyloxo-8-chloro-2,20-indoline (1b)
1H NMR (300 MHz, CDCl3) d: 8.18 (d, 1H, J ¼ 2 Hz), 8.05 (d, 1H, J ¼ 2 Hz),
7.66 (tt, 1H, J ¼ 6 Hz, J ¼ 1.38 Hz), 7.55 (m, 2H), 7.78(m, 2H), 7.28 (m, 2H),

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L. E. Elizalde et al.
7.15 (d, 1H, J ¼ 8 Hz), 6.82 (d, 1H, J ¼ 12 Hz), 6.38 (d, 1H, J ¼ 8 Hz), 5.75
(d, 1H, 12H) 3.05 (s, 3H), 1.25 (s, 3H), 1.15 (s, 3H). 13C (50 MHz, CDCl3)
d: 191.50, 156.15, 149.8, 146.47, 139.32, 136.64, 134.07, 131.70, 130.92,
129.15, 129.04, 122.49, 122.13, 119.32, 115.92, 111.23, 108.32, 97.31,
51.83, 30.41, 18.63. EI-MS m/z 159 (100), 415 (5), 417 (2). FT-IR (KBr
film) nmax: 1649, 1286, 892, 733 cm21. Anal. calcd. for C26H22ClNO2: C,
75.1%; H, 5.3%; Cl, 8.5%; N, 3.4%. Found C, 75.31%; H, 5.45%; N,
3.27%; Cl, 8.35%.
Characterization of 10,30,30-Trimethylspiro-[2H-1] Benzopyran-
6-benzyloxo-8-bromo-2,20-indoline (1c)
1H NMR (300 MHz, CDCl3) d: 8.33 (d, 1H, J ¼ 2 Hz), 8.19 (d, 1H, J ¼ 2 Hz),
7.66 (tt, 1H, J ¼ 6 Hz, J ¼ 1.38 Hz), 7.55 (m, 2H), 7.78(m, 2H), 7.28 (m, 2H),
7.15 (d, 1H, J ¼ 8 Hz), 6.82(d, 1H, J ¼ 12 Hz), 6.38 (d, 1H, J ¼ 8 Hz), 5.75
(d, 1H, 12H), 3.05 (s, 3H), 1.25 (s, 3H), 1.15 (s, 3H). 13C (50 MHz, CDCl3)
d: 193.54, 156.15, 149.8, 146.47, 139.32, 136.64, 134.07, 131.70, 130.92,
129.15, 129.04, 122.49, 122.13, 119.32, 118.95, 111.23, 108.32, 97.31,
51.83, 30.41, 18.63. EI-MS m/z, 159 (100), 459 (2), 461 (2). FT-IR (KBr
Film) nmax cm21: 1648, 1285, 885, 732. Anal. calcd. for C26H22BrNO2: C,
67.8%; H, 4.8%; Br, 17.4%; N, 3.0%. Found C, 68.23%; H, 4.91%; N,
2.85%; Br, 17.36%.
Characterization of 10,30,30-Trimethylspiro-[2H-1] Benzopyran-
6-benzyloxo-8-nitro-2,20-indoline (2d)
1H NMR (300 MHz, CDCl3) d: 8.85 (d, 1H, J ¼ 2 Hz), 8.55 (d, 1H, J ¼ 2 Hz),
7.66 (tt, 1H, J ¼ 6 Hz, J ¼ 1.38 Hz), 7.55 (m, 2H), 7.78 (m, 2H), 7.28 (m, 2H),
7.15 (d, 1H, J ¼ 8 Hz), 6.82 (d, 1H, J ¼ 12 Hz), 6.38 (d, 1H, J ¼ 8 Hz), 5.75
(d, 1H, 12H), 3.05 (s, 3H), 1.25 (s, 3H), 1.15 (s, 3H). 13C (50 MHz, CDCl3) d:
190.54, 156.15, 149.8, 146.47, 139.32, 136.64, 134.07, 131.70, 130.92,
129.15, 129.04, 122.49, 122.13, 119.32, 118.95, 116.23, 108.32, 97.31,
51.83, 30.41, 18.63. EI-MS m/z 159 (100), 426 (2). FT-IR (KBr Film) nmax
cm21: 1650, 1595, 1279, 710. Anal calcd. for C26H22N2O2: C, 73.2%; H,
5.2%; N, 6.6%. Found C, 73.7%; H, 5.45%; N, 6.32%.
RESULTS AND DISCUSSION
Four substituted derivatives of 10,30,30-trimethylspiro-[2H-1]-benzopyran-6-
benzyloxo-2,20-indoline compounds (1a–d) were prepared according to
Scheme 3. Yields, melting points, and UV absorption data are depicted in
Table 1.

6-Benzyloxo-spirobenzopyrans
3093
Scheme 3. Preparation of 6-benzyloxospirobenzopyrane photochromic compounds.
The reaction involves the condensation of 2-methylene-1,3,3-trimethyl-
indoline with 3-formyl-4-hydroxy-benzophenone (2a), in ethyl alcohol as
solvent. The preparation of 2a was attempted by direct formylation of
4-hydroxybenzophenone using several methods.[8–10] The best result was
obtained when chloroform and sodium hydroxide were used as the formylat-
ing agent according to the Reimer–Tiemann reaction. This methodology
involves the interaction between the 4-hydroxybenzophenone and aqueous
sodium hydroxide at 708C, followed by the addition of chloroform as the for-
mylating agent for 0.5 h (see Scheme 4).
The synthesis of benzophenones 2b–d was achieved by further functio-
nalization of 3-formyl-4-hydroxy-benzophenone 2a, using halogenations
and nitration reactions (Scheme 5). The compounds 2b and 2c were
prepared according to the common aromatic electrophilic substitution using
sulfuryl chloride[11] or bromine[12] as electrophiles and dichloromethane as
solvent (the latter catalyzed with ferric trichloride). These compounds were
purified by fractional crystallization in ethyl ether, resulting in a 30% yield
for 2b (chloro substituted) and 38% yield 2c (bromo substituted).
We found that the nitration of 3-formyl-4-hydroxy benzophenone 2a can
be satisfactorily performed using trifluoro acetic anhydride and ammonium
nitrate in chloroform as solvent.[13] Compound 2d was obtained in 57%
yield after purification by column chromatography. All products were charac-
1
terized by common spectroscopic techniques such as IR, GC-MS, H NMR,
13C NMR, and C, H, N analysis.
Once 2a–d were obtained, the preparation of the photochromic
compounds 1a–d was completed by condensation with 2-methylene-1,3,3-tri-
methylindoline in ethanolic solution. Condensation occured rapidly, and the
product was removed from the reaction mixture by filtration and purified by
recrystallization from methanol. These novel spiropyrans 1a–b were also
characterized by the spectroscopic techniques previously mentioned.
The photochemical properties of the photochromic compounds 1a–d
were measured by ultraviolet spectroscopy from 1025 M solutions in
dimethylsulfoxide and compared with the absorption spectra of

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L. E. Elizalde et al.
l
l

6-Benzyloxo-spirobenzopyrans
3095

3096
L. E. Elizalde et al.
Scheme 4. Preparation of 3-formyl-4-hydroxybenzophenone.
Scheme 5. Preparation of 3-formyl-4-hydroxy-5-substituted-benzophenones.
10,30,30-trimethylspiro-[2H-1]-benzopyran-2,20-indoline, 6-chloro, 6-bromo,
and 6-nitro derivatives 3a–d in the same solvent and at the same concen-
tration. The latter compounds were synthesized according with reported
methods.[6,7] Dimethylsulfoxide was used as the solvent to prevent the solva-
tochromic effect.
The synthesized compounds showed absorptions in the 200–400-nm
interval (Table 1) and molar absorptions coefficients up to 10,000 when
they are in the closed form. When these compounds were irradiated with ultra-
violet light with a Hg lamp in a quartz cell, it was observed that chemical
changes resulted in a different UV pattern. We assumed that all molecules
reach the open form (right side of Scheme 1) and that changes can be
observed in the absorption spectra in the visible region.
The reference molecule 3a (which does not have the benzyloxo fraction in
position 6) does not develop any absorption in the visible region. However,
the irradiation with UV light we detected absorptions at 412 nm for the
molecule 1a.
Molecules bearing the benzyloxo group in the 6-position and groups with
strong electron-withdrawing character such as Cl, Br, and NO2 (1b–d)
showed a remarkable increase in photochromic properties when compared
with the reference molecules 3b–d evidenced by their ability to develop a
deep purple color, maximum wavelength absorption beyond 568 nm, and
molecular extinction coefficients higher than 2800 L/mol cm.
We can assume that these synthesized novel molecules 1a–d derivatives
of the 6-benzyloxospirobenzopyranindolines develop a deep purple color after
UV irradiation and can be applied in several optoelectronic devices.

6-Benzyloxo-spirobenzopyrans
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