Photo- and thermochromic spirans. 38. New (1-alkyl-4,5-diphenyl)imidazolyl- substituted spirobenzopyrans

Article abstract of DOI:10.1007/s10593-013-1169-5

3-Diphenylimidazolyl-substituted 2-hydroxybenzaldehyde has been obtained by the formylation of 2-diphenylimidazolyl-substituted phenol. From the obtained compound, new photochromic spiroindolinebenzopyrans containing a 4,5-diphenylimidazole group at the position 8 of the benzopyran fragment were synthesized. The obtained compounds possess photochromic properties in solution.

Full text of DOI:10.1007/s10593-013-1169-5

Chemistry of Heterocyclic Compounds, Vol. 48, No. 10, January, 2013 (Russian Original Vol. 48, No. 10, October, 2012)
PHOTO- AND THERMOCHROMIC SPIRANS.
38*. NEW (1-ALKYL-4,5-DIPHENYL)IMIDAZOLYL-
SUBSTITUTED SPIROBENZOPYRANS
E. V. Solov'eva1, A. V. Chernyshev1**, N. A. Voloshin2, A. V. Metelitsa1, and V. I. Minkin2
3-Diphenylimidazolyl-substituted 2-hydroxybenzaldehyde has been obtained by the formylation of
2-diphenylimidazolyl-substituted phenol. From the obtained compound, new photochromic
spiroindolinebenzopyrans containing a 4,5-diphenylimidazole group at the position 8 of the benzopyran
fragment were synthesized. The obtained compounds possess photochromic properties in solution.
Keywords: merocyanins, spiropyrans, triarylimidazole, photochromism.
Spiropyrans are a widely known class of organic photochromic compounds. The possibility of a
directed change of the spectral-kinetic characteristics of compounds over a wide range by variation of chemical
structure is an important factor stimulating the interest of investigators towards these derivatives [1-4].
Functionalization of spiropyran molecules by introducing substituents of various nature also raises the
possibility of obtaining a broad series of polyfunctional photochromic molecular systems displaying magnetic
[5], fluorescent [6-9], and chelating [9-13] properties reversible with the aid of light.
One of the methods of such modification is the introduction of heterocyclic substituents into the
spiropyran molecule at the ortho position relative to the chromene oxygen atom, which leads to a modification
of the spectral [14-16] and complex-forming [13, 16, 17] properties of the initial compounds.
As a continuation of these investigations, the present work is devoted to the synthesis and study of the
photochromic properties of a series of spirobenzopyrans containing a 1-benzyl-4,5-diphenylimidazole group in
the position 8 of the benzopyran fragment.
1,2,4,5-Tetrasubstituted imidazoles are usually obtained by a four-component cyclocondensation of a
1,2-diketone with an aldehyde, a primary amine, and ammonium acetate [18]. 3-(1-Benzyl-4,5-diphenyl-
imidazolyl)-containing 2-hydroxybenzaldehyde 2 was obtained by cyclocondensation of 5-bromosalicylic
aldehyde, benzil, ammonium acetate, and benzylamine with subsequent formylation according to Duff of the
resultant imidazolyl-substituted phenol 1. Spirobenzopyrans 4a-d were formed as a result of the condensation of
3H-indolium salts 3a-d with 2-hydroxybenzaldehyde 2 in the presence of triethylamine.
_______
*For Communication 37, see [1].
**To whom correspondence should be addressed, e-mail: photo@ipoc.sfedu.ru.
1Scientific-Research Institute of Physical and Organic Chemistry, Southern Federal University, 194/2 Stachki
Ave., Rostov-on-the-Don 344090, Russia.
2Southern Scientific Center, Russian Academy of Sciences, 41 Chekhova St.. Rostov-on-the-Don 344006,
Russia; e-mail: ssc-ras@ssc-ras.ru
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1645-1650, October, 2012.
Original article submitted October 27, 2011.
0009-3122/13/4810-1533©2013 Springer Science+Business Media New York
1533

1
The structures of the obtained compounds 1, 2, 4a-d were established by H NMR spectroscopy and
1
confirmed by data of elemental analysis. The H NMR spectra of spiropyrans 4a-d contain two signals of
magnetically nonequivalent geminal methyl groups, signals of N-alkyl substituents of the indoline and
imidazole fragments, and a methoxy group (spiropyran 4d) in the high field region, and also several groups of
mutually linked signals in the low field region of the spectrum belonging to indoline, pyran, and imidazole
fragments. The signals of the phenyl ring protons form a complex picture of multiplets. The prochirality of the
methylene group protons of the N-benzyl substituent of the imidazole fragment of spiropyrans 4a-d leads to a
diastereotopic splitting of the proton signals displayed as two doublets at 4.22-4.25 and 4.73-4.74 ppm,
respectively.
Br
Br
CH2NH2
O
Ph
O
HO
NH4OAc
AcOH
+
+
CH2Ph
N
N
Ph
HO
CHO
Ph
Ph
1
Me
Me
O
Me
Me
R
Br
R
OHC
HO
Me
X–
+
N
N
Br
(CH2)6N4
TFA
Me
3a–d
Me
CH2Ph
Et3N
N
N
N
N
CH2Ph
Ph
4a–d
3, 4 a R = H, b R = Br, c R = Cl, d R = OMe; 3 a,b,d X = I, c X = ClO4
Ph
Ph
2
Ph
1
All the H NMR spectroscopic data enumerated above confirm unequivocally the structure of the
obtained spiropyrans. The absence of proton signals for the indoline and benzopyran fragments in the regions of
the spectrum characterized for the open merocyanin form MC [19-21], indicates that the obtained compounds in
CDCl3 solution exist in the spirocyclic SP form.
Me
Me
Br
R
Me
+
Me
R
N
O
Br
h
Me
–
CH2Ph
Ph
N
O

N
N
Me
N
N
CH2Ph
Ph
Ph
MC
Ph
SP
In toluene solution the investigated compounds were also found to exist exclusively in the spirocyclic
SP form. The absorption spectra of the spirocyclic forms were characterized by two diffuse bands without
distinct maxima in the region of 289-296 and 336-344 nm with molar extinction coefficients of 20800-24050 and
7100-8630 l·mol-1·cm-1, respectively (Table 1). Substituents in the indoline portion of the molecule did not affect
the intensity of the absorption bands, but led to an insignificant bathochromic shift of the long-wave component
of the band. Irradiation of colorless solutions of spiropyrans with 365 nm wavelength UV light causes their
coloration, linked to a photochemical reaction of ring opening and the formation of merocyanin forms. In the
absorption spectra, this is displayed as the absorption characteristic for merocyanines [2] in the 500-700 nm
region with band maxima at 625-634 nm (Fig. 1, Table 1).
1534

TABLE 1. Spectral and Kinetic Characteristics of Spiropyrans 4a-d in
Toluene, T 293 K
Com-
pound
λmaxabs (SP), nm
λmaxabs (MC), nm
kMC–SP, sec–1
Ea, kJ·mol–1
(ε·10–3, l·mol–1 ·cm–1)
292 (24.05) sh.
336 (8.63) sh.
296 (20.80) sh.
338 (7.10) sh.
289 (23.57) sh.
339 (7.46) sh.
293 (22.62) sh.
344 (8.01) sh.
625
627
628
634
0.1032
0.3592
0.3329
0.0365
72.2
89.1
80.6
79.6
4a
4b
4c
4d
Fig. 1. Absorption spectra of compound 4b in toluene (c 1.54.10-4 M) on irradiation with light of λ 365 nm,
T 293 K, 2-sec intervals between spectra.
After the end of irradiation a spontaneous decolorization of the solutions occurred due to the reverse
thermal recyclization of the merocyanine forms into the initial spirocyclic forms. The kinetic curves of the dark
relaxation process were satisfactorily described by a monoexponential function (Fig. 2). The lifetime of the
colored isomers at T 293 K lies in the range of 0.9-27.0 sec and essentially depends on the substituent at the
position 5 of the indoline fragment of the molecule. In the series from the unsubstituted compound 4a to the
halo-substituted derivatives 4b,c an increase in the thermal recyclization reaction rate constant was observed,
while the introduction of a methoxy group (compound 4d) increased the kinetic stability of the merocyanine
isomer in comparison with the unsubstituted spiro compound 4a by almost one order of magnitude (Table 1).
The activation energy values for the recyclization reactions were determined from the temperature dependencies
of the thermal decoloration rate constants (Fig. 2, inset). These values lie in the region of 72.2-80.6 kJ·mol-1.
Unlike the benzothiazolyl-substituted spirobenzopyrans studied previously [17], the compounds investigated
were characterized by higher rate constants for the thermal reaction.
1535

Fig. 2. Optical density (A) at the maximum of the long-wave absorption band of compound 4d in merocyanine
form, as a function of thermal relaxation time (points are experimental, the continuous line is an approximation
by monoexponential function), T 278 K, the solvent is toluene. The dependence of the thermal recyclization
reaction rate constant logarithm on reciprocal temperature is shown in the inset.
New spirobenzopyranindolines have been obtained, containing a 1-benzyl-4,5-diphenylimidazole group
at the position 8 of the benzopyran fragment and displaying photochromic properties in solution.
EXPERIMENTAL
The 1H NMR spectra were recorded on a Varian Unity-300 spectrometer (300 MHz) in CDCl3, internal
standard was the residual signal of the solvent (δ 7.26 ppm). Electronic absorption spectra and kinetic curves of
the thermal recyclization reactions of the investigated compounds were recorded on an Agilent 8453
spectrophotometer with an attachment for thermostatting samples. Photolysis of solutions was carried out by the
Newport system based on a mercury lamp (200 W) with a set of interference light filters. Elemental analysis
was carried out on a KOVO CHN analyzer. Melting points were determined on a Boetius hot stage apparatus.
Spectral grade toluene (Aldrich) was used for preparing solutions. Compounds 3a-d were obtained by the
procedure described previously in [22].
2-(1-Benzyl-4,5-diphenyl-1H-imidazol-2-yl)-4-bromophenol (1). A mixture of 5-bromosalicylic
aldehyde (6.0 g, 30 mmol), benzil (6.3 g, 30 mmol), benzylamine (4.89 ml, 45 mmol), NH4OAc (4.8 g, 60
mmol), and AcOH (90 ml) was refluxed for 12 h. The reaction mixture was poured into H2O (500 ml),
neutralized with conc. aqueous NH3 to pH 6-7, the solid filtered off, washed with water, dried, and
recrystallized from a mixture EtOAc–2-PrOH, 1:1. Yield 7.95 g (54%). Pale-gray crystals; mp 159-160ºC.
1H NMR spectrum, δ, ppm (J, Hz): 5.27 (2H, s, NCH2Ph); 6.98 (1H, d, J = 8.8, H-6); 7.18-7.25 (3H, m, H Ph);
7.27-7.38 (8H, m, H-5, H Ph); 7.40-7.44 (2H, m, H Ph); 7.46-7.50 (3H, m, H Ph); 7.57 (1H, d, J = 2.4, H-3);
9.32 (1H, br. s, OH). Found, %: C 69.95; H 4.52; N 5.75. C28H21BrN2O. Calculated, %: C 69.86; H 4.40;
N 5.82.
3-(1-Benzyl-4,5-diphenyl-1H-imidazol-2-yl)-5-bromo-2-hydroxybenzaldehyde (2). A mixture of
phenol 1 (4.82 g, 10 mmol), hexamethylenetetramine (5.6 g, 40 mmol), and trifluoroacetic acid (30 ml) was
refluxed under an inert atmosphere for 12 h, cooled, and a mixture of conc. HCl (14 ml) and H2O (28 ml) was
added. The reaction mixture was poured into water (130 ml) and neutralized with conc. aqueous NH3 to pH 6-7.
1536

The solid was filtered off, washed with water, dried, purified by column chromatography on Al2O3 (eluent
CHCl3), and recrystalized from a 1:1 mixture of PhMe–2-PrOH. Yield 1.78 g (35%). Lemon-yellow crystals;
1
mp 233-235ºC. H NMR spectrum, δ, ppm (J, Hz): 5.24 (2H, s, NCH2Ph); 6.95-6.98 (2H, m, H Ph); 7.18-7.24
(3H, m, H Ph); 7.27-7.51 (10H, m, H Ph); 7.68 (1H, d, J = 2.5, H-4); 7.81 (1H, d, J = 2.5, H-6); 10.43 (1H, s,
CHO). Found, %: C 68.31; H 4.02; N 5.45. C29H21BrN2O2. Calculated, %: C 68.38; H 4.16; N 5.50.
8-(1-Benzyl-4,5-diphenyl-1H-imidazol-2-yl)-6-bromo-1',3',3'-trimethylspiro-[2H-1-benzopyran-
2,2'-indolines] 4a-d (General Method). A mixture of 3H-indolium salt 3a-d (1 mmol), aldehyde 2 (1 mmol),
and Et3N (0.14 ml, 1 mmol) in PhMe (10 ml) and 2-PrOH (4 ml) was refluxed for 12 h, the solvent was
evaporated, the residue purified by column chromatography on Al2O3 (eluent benzene) and by recrystallization.
8-(1-Benzyl-4,5-diphenyl-1H-imidazol-2-yl)-6-bromo-1',3',3'-trimethylspiro[2H-1-benzopyran-
2,2'-indoline] (4a). Yield 0.27 g (41%). Pale-pink crystals; mp 122-124ºC (hexane). 1H NMR spectrum, δ, ppm
(J, Hz): 1.15 (6H, s, C(CH3)2); 2.78 (3H, s, 1'-CH3); 4.22 (1H, d, J = 15.8) and 4.74 (1H, d, J = 15.8, NCH2Ph);
5.78 (1H, d, J = 10.3, H-3); 6.36-6.39 (2H, m, H Ph); 6.50-6.54 (3H, m, H -7', H Ph); 6.87 (1H, d, J = 10.3,
H-4); 6.91-7.19 (10H, m, H-5,4',5',6', H Ph); 7.23-7.30 (3H, m, H Ph); 7.41-7.44 (2H, m, H Ph); 7.68 (1H, d,
J = 2.4, H-7). Found, %: C 73.95; H 5.10; N 6.22. C41H34BrN3O. Calculated, %: C 74.09; H 5.16; N 6.32.
8-(1-Benzyl-4,5-diphenyl-1H-imidazol-2-yl)-5',6-dibromo-1',3',3'-trimethylspiro[2H-1-benzopyran-
1
2,2'-indoline] (4b). Yield 0.37 g (50%). Pale-gray crystals; mp 129-131ºC (heptane). H NMR spectrum, δ,
ppm (J, Hz): 1.15 (3H, s, 3'-CH3); 1.16 (3H, s, 3'-CH3); 2.73 (3H, s, 1'-CH3); 4.25 (1H, d, J = 15.8) and 4.73
(1H, d, J = 15.8, NCH2Ph); 5.76 (1H, d, J = 10.3, H-3); 6.36-6.40 (3H, m, H-7', H Ph); 6.57-6.60 (2H, m, H Ph);
6.89 (1H, d, J = 10.3, H-4); 6.96-7.22 (9H, m, H-5,4',6', H Ph); 7.26-7.31 (3H, m, H Ph); 7.41-7.45 (2H, m,
H Ph); 7.69 (1H, d, J = 2.4, H-7). Found, %: C 66.35; H 4.58; N 5.47. C41H33Br2N3O. Calculated, %: C 66.23;
H 4.47; N 5.65.
8-(1-Benzyl-4,5-diphenyl-1H-imidazol-2-yl)-6-bromo-5'-chloro-1',3',3'-trimethylspiro[2H-1-benzo-
pyran-2,2'-indoline] (4c). Yield 0.30 g (43%). Pale-gray crystals; mp 120-121ºC (hexane). 1H NMR spectrum,
δ, ppm (J, Hz): 1.15 (3H, s, 3'-CH3); 1.16 (3H, s, 3'-CH3); 2.73 (3H, s, 1'-CH3); 4.24 (1H, d, J = 15.8) and 4.73
(1H, d, J = 15.8, NCH2Ph); 5.76 (1H, d, J = 10.3, H-3); 6.37-6.40 (2H, m, H Ph); 6.41 (1H, d, J = 8.1, H-7');
6.56-6.60 (2H, m, H Ph); 6.89 (1H, d, J = 10.3, H-4); 6.98-7.03 (3H, m, H Ph); 7.04 (1H, d, J = 2.1, H-4');
7.06-7.16 (4H, m, H-5, H Ph); 7.20 (1H, dd, J = 8.2, J = 2.1, H-6'); 7.25-7.30 (3H, m, H Ph); 7.41-7.45 (2H, m,
H Ph); 7.69 (1H, d, J = 2.4, H-7). Found, %: C 70.62; H 4.90; N 5.85. C41H33BrClN3O. Calculated, %: C 70.44;
H 4.76; N 6.01.
8-(1-Benzyl-4,5-diphenyl-1H-imidazol-2-yl)-6-bromo-5'-methoxy-1',3',3'-trimethylspiro[2H-1-benzo-
pyran-2,2'-indoline] (4d). Yield 0.33 g (47%). Pale-lilac crystals; mp 221-223ºC (heptane). 1H NMR, δ, ppm
(J, Hz): 1.14 (3H, s, 3'-CH3); 1.16 (3H, s, 3'-CH3); 2.71 (3H, s, 1'-CH3); 3.80 (3H, s, OCH3); 4.22 (1H, d,
J = 15.8) and 4.74 (1H, d, J = 15.8, NCH2Ph); 5.78 (1H, d, J = 10.3, H-3); 6.37-6.40 (2H, m, H Ph); 6.41 (1H,
d, J = 8.2, H-7'); 6.57-6.60 (2H. m, H Ph); 6.71 (1H, d, J = 2.5, H-4'); 6.77 (1H, dd, J = 8.2, J = 2.5, H-6'); 6.86
(1H, d, J = 10.3, H-4); 6.96-7.22 (9H, m, H Ph); 7.26 (1H, d, J = 2.4, H-5); 7.41-7.45 (2H, m, H Ph); 7.68 (1H,
d, J = 2.4, H-7). Found, %: C 72.47; H 5.10; N 5.98. C42H36BrN3O2. Calculated, %: C 72.62; H 5.22; N 6.05.
The work was carried out with the financial support of the Ministry of Education and Science of the
Russian Federation for 2009-2013, state contract P2346), the Russian Foundation for Basic Research (project
09-03-93115), and the Grants Council of the President of the Russian Federation (grant NSh-927.2012.3).
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