Ring transformations of heterocyclic compounds. XXIII [1]. The UV irradiation product of photochromic 2,4,6-triaryl-1 - (spiro[2H-1-benzopyran-2,2′-indoline]-6-yl)pyridinium salts - A phenolate betaine or a pyridinium substituted merocyanine dye?

Article abstract of DOI:10.1002/jhet.5570400409

The synthesis of the novel 2,4,6-triaryl-1-(spiro [2H- 1-benzopyran-2,2′-indoline]-6-yl)pyridinium perchlorates 4 by reaction of 5-nitrosalicylaldehydes 6 with 1,3,3-trimethyl-2-methyleneindoline (7) to 6-nitrospiro [2H-1-benzopyran-2,2′-indolines] 1, the

Full text of DOI:10.1002/jhet.5570400409

Ring Transformations of Heterocyclic Compounds. XXIII [1].
The UV Irradiation Product of Photochromic 2,4,6-Triaryl-1-
(spiro[2H-1-benzopyran-2,2'-indoline]-6-yl)pyridinium Salts - A
Phenolate Betaine or a Pyridinium Substituted Merocyanine Dye?
611
Jul-Aug 2003
*
Thomas Zimmermann and Ortwin Brede
Interdisziplinäre Arbeitsgruppe Zeitaufgelöste Spektroskopie, Fakultät für Chemie und Mineralogie,
Universität Leipzig, Permoserstraße 15, D-04303 Leipzig, Germany
Received March 14, 2003
The synthesis of the novel 2,4,6-triaryl-1-(spiro[2H-1-benzopyran-2,2'-indoline]-6-yl)pyridinium per-
chlorates 4 by reaction of 5-nitrosalicylaldehydes 6 with 1,3,3-trimethyl-2-methyleneindoline (7) to 6-nitro-
spiro[2H-1-benzopyran-2,2'-indolines] 1, their stannous chloride reduction to the 6-amino derivatives 8,
followed by a 2,6-[C5+N] ring transformation with 2,4,6-triarylpyrylium perchlorates 9, is reported. UV
irradiation experiments in twenty solvents of different polarity prove their photochromic properties and
show that the photochemically generated negative solvatochromic dyes 5, formed by ring opening of the
benzopyran moiety of 4, are rather merocyanine than pyridinium phenolate betaine dyes.
J. Heterocyclic Chem., 40, 611(2003).
In recent years, photochromism, i.e. the reversible trans-
with the benzopyran. The uv irradiation of the colourless 1
leads to the merocyanine 2 which can be recyclized by heat
or visible light to the starting spirobenzopyran 1. For an
efficient formation of the photoproduct 2 the presence of
the electron attracting nitro group at C-6, stabilizing the
negative charge at the oxygen, is essential [2]. The mero-
cyanine 2 shows a slightly negative solvatochromism, i.e.
a shift of the longest absorption maximum to shorter wave-
lengths by enhancing the polarity of the solvent, which can
formation of chemical species, induced in one or both
directions by electromagnetic radition between two states
showing observable light absorptions in different regions
has been intensively studied. The aim of these studies has
been to improve our knowledge of this phenomenon and to
obtain new photochromic systems for practical applica-
tions [2].
The 2H-1-benzopyrans, possessing a second ring, usu-
ally heterocyclic, attached to the 2-carbon atom in a spiro
manner (“spirochromenes”) are one of the main classes of
photochromic compounds [2]. In the well established com-
pounds of the type 1, an indoline moiety is spiro connected
be characterized by Reichardt's E (30) empirical parame-
T
ters based on the solvatochromism of the pyridinium phe-
nolate betaine dye 3 [3].
Substitution of the nitro group in the spirobenzopyran 1
by a 2,4,6-triarylpyridinium moiety, having also a high
electron attracting capability, would result in the pyri-
dinium derivatives of the type 4. Their uv irradiation should
give rise to the ring-opened isomers 5 possessing a pheno-
late betaine as well as a merocyanine substructure. On the
basis of investigations on the solvatochromism of the dyes
5 it can be decided which type of substructure preferentially
determines the properties of 5. If it is the phenolate betaine
moiety, a high degree of solvatochromism as in the case of
the structurally related Reichardt's dye 3 should be found
[3] whereas if the merocyanine unit predominates a much
lower solvatochromism should be detectable. In this paper
we wish to report on the synthesis of the 2,4,6-triaryl-1-
(spiro[2H-1-benzopyran-2,2'-indoline]-6-yl)pyridinium
salts 4, on their photochromic properties and on the nature
of the photochemically obtained dyes 5, based on studies of
their solvatochromism.

612
T. Zimmermann and O. Brede
Vol. 40
For the synthesis of the pyridinium salts 4 a strategy was
C-3' are responsible in 4a for one and in 4b,c for two sin-
glets at 0.89-1.03 ppm. Another singlet at 2.30-2.38 ppm
can be attributed to the protons of the N-bonded methyl
group. The olefinic proton at C-3 of the benzopyran moi-
ety resonates at 5.72-5.90 ppm as a doublet with a cou-
pling constant of 9.8-10.4 Hz. The protons of the benzene
rings cause the multiplet at 6.38-8.60 ppm that usually
masks the doublet of the olefinic proton at C-4 of the ben-
zopyran. A singlet at 8.56-8.78 ppm shows the presence of
the two protons bonded at the pyridinium ring [10]. In the
FAB mass spectra, recorded for 4a-c, the mass peaks of the
corresponding cations were found. A characteristic feature
of the uv spectra, recorded in acetonitrile, are strong
absorption bands at 242-249 nm and 296-352 nm.
To study the photochromic properties of the 2,4,6-tri-
aryl-1-(spiro[2H-1-benzopyran-2,2'-indoline]-6-yl)pyri-
dinium perchlorates 4 solutions of 4a-k in methanol were
uv irradiated and the changes monitored by running the
uv-vis spectra (details see Experimental part). In all cases
the formation of a blue dye was observed, the longest
wavelength absorption maxima of which are compiled in
Table 3. The figures show that the substitution of the
hydrogen atom at C-8 in4a by a methyl group (4b) or by a
phenyl ring (4c) is accompanied by a shift of this absorp-
tion maximum to longer wavelengths whereas the varia-
chosen based on the commercially available or easily
accessible salicylaldehydes 6a-c. They were reacted in
ethanol with 1,3,3-trimethyl-2-methyleneindoline (7)
(Fischer base) [4] according to known methods to give the
6-nitro substituted spiro[2H-1-benzopyran-2,2'-indolines]
1a-c [5,6]. A critical step of the envisaged reaction
sequence was their reduction to the 6-amino derivatives
8a-c, since it has to be performed chemoselectively at the
nitro group under neutral conditions. By testing various
reducing agents and solvents it was found that stannous
chloride dihydrate in boiling ethanol [7] gave the best
results. Applying this method, the 6-aminospiro[2H-1-
benzopyran-2,2'-indolines] 8a-c were obtained in good
yield as air-sensitive oils. Hence, they were immediately
reacted with the 2,4,6-triarylpyrylium salts 9a-i [8] and
anhydrous sodium acetate in ethanol to afford by the well
known 2,6-[C +N] [9] pyrylium ring transformation,
5
where the oxygen atom is replaced by nitrogen, the desired
2,4,6-triaryl-1-(spiro[2H-1-benzopyran-2,2'-indoline]-6-
yl)pyridinium perchlorates 4a-k in 53-64% yield.
The results of the elemental analyses and the spectro-
scopic data are in agreement with the structure of the pyri-
1
dinium salts 4a-k (cf. Tables 1 and 2). In the H nmr spec-
tra the diastereotopic protons of the two methyl groups at

Jul-Aug 2003
Ring Transformations of Heterocyclic Compounds. XXIII
613

614
T. Zimmermann and O. Brede
Vol. 40
Table 2 (continued)
tion of the substitution pattern of Ar and Ar' in 4c-k, i.e.
the introduction of an electron attracting or releasing sub-
stituent in p-position, has practically no influence. Storing
in the dark, the blue colour of the solutions vanishes and it
could be regenerated by repeated uv irradiation. After the
five coloration/decoloration cycles tested no significant
decomposition was detected. The observations made are
characteristic behaviour of a photochromic compound and
can be explained by a uv initiated electrocyclic ring open-
ing of the benzopyran moiety of the pyridinium perchlo-
rates 4 to give the coloured isomers 5 and their thermal
back reaction to the starting salts 4 [2].
empirical E (30) parameters. In addition, the molar transi-
T
tion energies E were calculated from the absorption max-
T
ima according to equation (1) [2].
-1
ET/(kcal·mol ) = h · c · NA/l (1)
h: Planck's constant, c: velocity of the light, N :
A
Avogadro's number, l : wavelengths of the longest absorp-
tion maximum.
From figures it can be seen that the enhancement of the
solvent polarity on going from toluene to methanol causes
a shift of the absorption maxima to shorter wavelengths.
Hence, compounds 5a-c must be classified as negative
solvatochromic dyes. Another interesting observation was
that only in alcohols was the dye formation found to be a
reversible process. In all the other solvents used, after the
first irradiation a more-or-less decomposition caused by a
thermal reaction was observed.
Besides the qualitative interpretation of the data a linear
regression analysis on the dependence of the transition
energies E of 5a-c from the solvent polarity E (30) was
T
T
performed giving rise to the following equations and
correlation coefficients (R) (2) - (4):
-1
-1
5a: E /(kcal·mol ) = 0.286·E (30)/(kcal·mol ) + 35.809 R = 0.990 (2)
T
T
-1
-1
5b: E /(kcal·mol ) = 0.226·E (30)/(kcal·mol ) + 37.753 R = 0.982 (3)
T
T
-1
-1
5c: E /(kcal·mol ) = 0.230·E (30)/(kcal·mol ) + 36.510 R = 0.989 (4)
T
T
The data show a good quantitative correlation between
E and E (30). The slope does not differ so much for 5a
T
T
(R = H), 5b (R = Me) and 5c (R = Ph) and its magnitude
indicates that the susceptibility to changes of the solvent
polarity is much lower for 5a-c than for the classical pyri-
dinium phenolate betaine dye 3. On the other hand, the
shift of the absorption maximum to shorter wavelengths on
going from the less polar toluene to the more polar ethanol
(5a: 57 nm, 5b: 45 nm, 5c: 53 nm) is comparable to that
determined for the 6-nitrospiro[2H-1-benzopyran-2,2'-
indoline] (1a) (65 nm) [2] in which instead of the 2,4,6-tri-
To get information on the nature of the photochemically
generated blue dyes 5 their solvatochromism, i.e. the
dependence of the position of the longest wavelength
absorption maximum from the polarity of the solvent, in
which it is generated, was studied. Table 4 contains
maxima of these dyes 5a-c obtained by uv irradiation of
the pyridinium perchlorates 4a-c in twenty selected
solvents of different polarity characterized by Reichardt's

Jul-Aug 2003
Ring Transformations of Heterocyclic Compounds. XXIII
615
1,3,3-trimethyl-2-methyleneindoline (7) in boiling ethanol as
described in the literature [5,6].
arylpyridinium moiety a nitro group is bonded at C-6.
These results clearly indicate that the dyes 5a-c formed by
uv irradiation of the pyridinium salts 4a-c should be classi-
fied not as pyridinium phenolate betaine but as merocya-
nine type dyes.
Reduction of the 6-Nitrospiro[2H-1-benzopyran-2,2'-indolines]
1a-c to the 6-Amino Derivatives 8a-c [7].
To 40 ml ethanol 6 mmoles 6-nitrospiro[2H-1-benzopyran-2,2'-
indoline] 1 and 6.77 g (30 mmoles) stannous dichloride dihydrate
were added. The reaction mixture was then refluxed and magneti-
cally stirred under nitrogen for one hour. A dark brown precipitate
formed in some cases and was separated by suction and the resulting
solution was poured into a stirred mixture of 100 ml sodium hydrox-
ide in water (5%) and of 50 ml chloroform. After transfer into a sep-
aratory funnel the chloroform layer was separated, the aqueous layer
extracted with chloroform (30 ml), the combined chloroform solu-
tions washed with water (100 ml) and dried over anhydrous sodium
sulfate. The solvent was removed in vacuo on a water bath at 40° to
give the 6-aminospiro[2H-1-benzopyran-2,2'-indoline] (8a) (80%),
the 6-amino-8-methylspiro[2H-1-benzopyran-2,2'-indoline] (8b)
(60%) and the 6-amino-8-phenylspiro[2H-1-benzopyran-2,2'-indo-
line] (8c) (85%), respectively, as dark brown air sensitive oils which
were reacted immediately with the pyrylium salts 9. A thin layer
chromatographic analysis carried out on silica gel with acetone/n-
hexane (v:v = 3:1) as eluent showed that the reduction was complete
and only traces of by-products were formed.
EXPERIMENTAL
The melting points were measured on a Boëtius hot stage
apparatus. The H nmr and 13C nmr spectra were recorded on a
1
Varian Gemini 200 spectrometer (1H: 199.975 MHz, 13C: 50.289
MHz) and on a Varian Gemini 2000 spectrometer (1H: 200.041
MHz, 13C: 50.305 MHz) in dimethyl-d6 sulfoxide at 25° with
hexamethyl disiloxane as internal standard. The uv spectra were
obtained on a Zeiss M 40 instrument (acetonitrile, 25°) or on a
Shimadzu UV-2101 PC scanning spectrophotometer (various
solvents, 25°). Mass spectra were determined on a VG ZAB HSQ
Analytical Instruments spectrometer (FAB, matrix: 3-nitrobenzyl
alcohol). For the irradiation experiments a Müller
Elektronic/Optik ELL 1000 apparatus (mercury high pressure
lamp) was used. The salicylaldehydes 6b [11], 6c [6] and the
pyrylium perchlorates 9a [12], 9b [13], 9c [14], 9d,f [15], 9e
[16], 9g-i [17] were synthesized according to literature proce-
dures. 5-Nitrosalicylaldehyde (6a), 1,3,3-trimethyl-2-methyl-
eneindoline (7) and stannous chloride dihydrate were purchased
from Aldrich. All solvents used were of analytical grade and
anhydrous.
Synthesis of the 2,4,6-Triaryl-1-spiro[2H-1-benzopyran-2,2'-
indoline]-6-yl)pyridinium Perchlorates 4a-k from the 6-
Aminospiro[2H-1-benzopyran-2,2'-indolines] 8a-c and the 2,4,6-
Triarylpyrylium Perchlorates 9a-i [8].
General Procedure (cf. Tables 1 and 2).
Preparation of the 6-Nitrospiro[2H-1-benzopyran-2,2'-indolines]
1a-c.
To absolute ethanol (30 ml) 5 mmoles 6-aminospiro[2H-1-
benzopyran-2,2'-indoline] 8, 5 mmoles 2,4,6-triarylpyrylium per-
chlorate 9 and 0.82 g (10 mmoles) anhydrous sodium acetate
The 6-nitrospiro[2H-1-benzopyran-2,2'-indolines] 1a-c were
prepared by the reaction of the 6-nitrosalicylaldehydes 6a-c with

616
T. Zimmermann and O. Brede
Vol. 40
[3a] C. Reichardt, Solvent and Solvent Effects in Organic
Chemistry, 2nd ed, VCH, Weinheim, 1990; [b] C. Reichardt, Chem.
Soc. Rev., 147 (1992); [c] C. Reichardt, Chem. Rev., 94, 2319 (1994);
[d] C. Reichardt, J. Phys. Org. Chem., 8, 761 (1995) and literature
cited therein.
[4] For a recent review on indoles and their derivatives see:
H. Döpp, D. Döpp, U. Langer and B. Gerding, in Houben-Weyl, Vol
E6b1/E6b2, R. P. Kreher, ed, Thieme, Stuttgart, 1994, pp 546-1354.
[5] A. Hinnen, C. Audic and R. Gautron, Bull. Soc. Chim. Fr.,
2066 (1968).
[6] E. V. Braude and M. A. Gal'bershtam, Khim. Geterotsikl.
Soedin., 943 (1974).
[7] F. D. Bellamy and K. Ou, Tetrahedron Lett., 25, 839
(1984).
[8a] A. T. Balaban, A. Dinculescu, G. N. Dorofeenko,
G. W. Fischer, A. V. Koblik, V. V. Mezheritskii and W. Schroth,
Pyrylium Salts. Syntheses, Reactions and Physical Properties,
Advances in Heterocyclic Chemistry, Suppl 2, Academic Press, New
York, 1982; [b] W. Schroth, W. Dölling and A. T. Balaban, in
Houben-Weyl,VolE7b, R. P. Kreher, ed, Thieme, Stuttgart, 1992, pp
755-1014.
were added. The resulting reaction mixture was then refluxed for
two hours under nitrogen. The pyridinium salts 4 formed crystal-
lized in some cases from the hot solutions. Otherwise their
crystallization was initiated by cooling. They were filtered by
suction and washed with water and ethanol. For purification the
compounds 4a and 4b were recrystallized from ethanol whereas
the 8-phenyl derivatives 4c-k were dissolved in a minimal
amount of chloroform (in the case of scarcely soluble compounds
acetonitrile was added) and precipitated by dropwise addition of
absolute ethanol.
UV Irradiation Experiments.
Solutions of the 2,4,6-triaryl-1-(spiro[2H-1-benzopyran-2,2'-
-1
indoline]-6-yl)pyridinium perchlorates 4 (2x10-5mol·l ) in the sol-
vents given in Table 4 were irradiated with a mercury high pressure
lamp in a quartz cell (1 cm) thermostated at 5°. After 5 min and
30 min formation of the blue dyes 5 was verified by transferring
the cell to the uv-vis spectrophotometer and running the spectra.
After the intensity of the longest wave length absorption of 5 had
reached a maximum the irradiation was stopped, the solutions were
stored for 30 min in the dark and another spectrum was recorded to
obtain information on the result of the thermal back reaction. Then,
the next irradiation was started. To elucidate the reversibility of the
coloration/decoloration up to five cycles were monitored.
[9] For the classification of pyrylium ring transformations see
ref [8a].
[10] M. Hesse, H. Meier and B. Zeeh, Spektroskopische
Methoden in der organischen Chemie, Thieme, Stuttgart, New York,
1997.
[11a] K. Auwers and R. Bondy, Ber. Dtsch. Chem. Ges., 37,
3915 (1904), [b] P. H. Vandewyer, J. Hoefnagels and G. Smets,
Tetrahedron,25, 3251 (1969).
[12] A. T. Balaban and C. Toma, Tetrahedron, Supplement 7, 1
(1966).
Acknowledgement.
Financial support by the Deutsche Forschungsgemeinschaft is
gratefully appreciated.
[13] A. Mistr, M. Vavra, J. Skoupy and R. Zahradnik, Collect.
Czech. Chem. Commun., 37, 1520 (1972).
REFERENCES AND NOTES
[14] R. Wizinger, S. Losinger and P. Ulrich, Helv. Chim. Acta.,
39, 5 (1956).
[15] K. Dimroth, C. Reichardt, T. Siepmann and F. Bohlmann,
Liebigs Ann. Chem., 661, 1 (1963).
[16] G. N. Dorofeenko, S. V. Krivun and V. V. Mezheritskii,
Zh. Obshch. Khim., 35, 632 (1963).
[17] G. W. Fischer and M. Herrmann, J. Prakt. Chem., 326, 287
(1984).
[1] Part XXII: T. Zimmermann and L. Hennig,
J. Heterocyclic Chem., 39, 263 (2002).
[2] Organic Photochromic and Thermochromic Compounds,
C. Crano and R. Guglielmetti, eds, Vol 1, Plenum Press, New York,
London, 1999, Vol 2, Kluwer Academic, Plenum Publishers, New
York, Boston, Dordrecht, London, Moscow, 1999, and literature
cited therein.