To estimation of pKa for spiropyrans of the indoline series

Article abstract of DOI:10.1023/A:1021650501830

A mathematically substantiated approach to estimating acid-base characteristics of spiropyrans of the indoline series with account for the ring-chain tautomerism of these compounds was proposed. The pK_a values of spiropyrans of various basicity were estimated.

Full text of DOI:10.1023/A:1021650501830

Russian Journal of General Chemistry, Vol. 72, No. 9, 2002, pp. 1468 1472. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 9, 2002,
pp. 1557 1561.
Original Russian Text Copyright
2002 by Chernyshev, Chernov’yants, Voloshina, Voloshin.
To Estimation of pKa for Spiropyrans of the Indoline Series
A. V. Chernyshev, M. S. Chernov’yants, E. N. Voloshina, and N. A. Voloshin
Research Institute of Physical and Organic Chemistry, Rostov State University, Rostov-on-Don, Russia
Rostov State University, Rostov-on-Don, Russia
Received November 30, 2000
Abstract A mathematically substantiated approach to estimating acid base characteristics of spiropyrans
of the indoline series with account for the ring chain tautomerism of these compounds was proposed. The
pKa values of spiropyrans of various basicity were estimated.
Spiropyrans of the indoline series are among the
most effective and, therefore, sufficiently thoroughly
studied class of organic photochromic compounds
capable of forming, under UV irradiation, colored
quinoid zwitter-ionic structures whose reversible
rearrangement into the initial spiro form occurs spon-
taneously or under visible light [1 3]. Recently
complex formation of the colored merocyanine form
of spiropyran with metal ions in aprotic or aqueous-
organic media have received researcher’s attention.
Spiropyrans are considered as promising organic
reagents for photometric and fluorescent analysis of
metal ions [4 7]. The existence in solutions of the
analytically active merocyanine form of spiropyrans
is associated with the basicity of the donor oxygen
atom, which depends on substituents and can be
measured by the protolytic equilibrium constant.
There have been only a few works on this topic.
Dzhaparidze [8] has reported pKa values for spiro-
pyrans to illustrate a correlation between the stability
of their open-chain form and the basicity of the pyran
oxygen atom. Drumond and Furlog [9] have studied
the photochromic and protolytic properties of spiro-
pyrans in aqueous-organic media.
Spiropyrans I V were obtained by reactions of
1-R1-2,3,3-trimethyl-5-R2-3H-indolium iodide with
2-hydroxy-3-methoxy-5-R3-benzaldehydes in the
presence of a base.
Scheme 1.
CHO
Me
Me
Me
Me
R2
R2
HO
Me
+
+
R3
R3
MeO
N
O
N
I
R1
R1
MeO
I V
I, R1 = Me, R2 = R3 = H; II, R1 = Me, R2 = H, R3 = CHO; III, R1 = Me, R2 = OMe, R3 = CHO; IV, R1 = CH2Ph, R2 = H,
R3 = Br; V, R1 = CH2Ph, R2 = H, R3 = NO2.
In the present work we performed a first potentio-
metric and spectrophotometric study of the protolytic
properties of spiropyrans of the indoline series (com-
pounds I V) with account for the existence in solu-
tions of an equilibrium between the cyclic (A), mero-
cyanine (B), and protonated (C) forms of these com-
pounds. This equilibrium in aqueous-organic media
can be represented by Scheme 2.
Thermodynamic stability of the open-chain mero-
cyanine form B depends on solvent polarity, pos-
sibility of hydrogen bonding between the charged
centers of the molecule (formation of solvate bridge),
and electronic effects of substituents in the indoline
and pyrane moieties [2].
In view of Scheme 2, the protolytic equilibrium of
1070-3632/02/7209-1468$27.00 2002 MAIK Nauka/Interperiodica

TO ESTIMATION OF pKa FOR SPIROPYRANS OF THE INDOLINE SERIES
1469
Scheme 2.
R3
R3
Ka
Me
Me
Me
Me
Me
Me
H+
R2
R2
R2
K
H+
+
N
+
N
R3
N
O
OMe
HO
OMe
O
R1
R1
R1
MeO
A
B
C
spiropyrans (L) in the solution can be represented by
Eq. (1) and quantitatively described by the apparent
equilibrium constant Ka [Eq. (2)].
signals of indicator groups (formyl and/or gem-di-
methyl). Spiropyrans I and IV, according to H NMR
and UV spectral data, are present in the cyclic form
1
exclusively. The K value for spiropyran V could not
1
LH+ + H2O
Lm + Lc + H3O+,
(1)
(2)
be determined by H NMR spectroscopy because of
the poor solubility of this compound. The ring chain
equilibrium constants K of spiropyrans II and III are
0.87 and 6.25, respectively.
Ka = [H3O+][L ]/[LH+].
Here [LH+], [Lm], and [Lc] are the equilibrium con-
centrations of the protonated, merocyanine, and cyclic
forms, and [L ] = [Lm] + [Lc] is the sum of the equi-
librium concentrations of nonprotonated forms of
spiropyrans. The Ka value reported by Drumond and
Furlog [9] is considered apparent in view of the fact
that in determining [L ] the authors did not estimate
the equilibrium concentrations of forms A and B.
Equation (10) for estimating pKa from potentio-
metric titration data (it is this value which is deter-
mined experimentally) was derived from the material
balance equation (8) and the solution electroneutrality
equation (9).
cL = [LH+] + [L ],
[Na+] + [H+] + [LH+] = [OH ] + [Cl ],
(8)
(9)
The protonation constant of the merocyanine form
(Ka) can be determined is this form is present in
appreciable amounts [Eq. (3)].
(1 a)cL [H+]
(10)
pKa = pH + log
.
acL + [H+]
Ka = [H3O+][Lm]/[LH+].
(3)
Here [Cl ] = cL and [Na+] = acL (a = cNaOH/cL is the
neutralization degree of the solution).
Based of the material balance equation (4), the law
of mass action [Eq. (2)], and the ring chain equilib-
rium equation (5), we come to Eq. (6) which relates
Ka to Ka in normal or logarithmic forms [Eq. (7)].
The pKa values were calculated by Eq. (7).
The potentiometric technique, while being univer-
sal, involves a number of limitations. In titration of
dyes, for instance, the latter may well be adsorbed on
the membrane of the glass electrode, thus affecting
the potential measured [11]. At concentrations of
cL = [LH+] + [Lm] + [Lc],
K = [Lm]/[Lc],
(4)
(5)
(6)
(7)
2
3
about 10 10 M (working concentrations in poten-
tiomeric titration), dimeric structures may form.
Ka = Ka(1 + 1/K),
pKa = pKa
log (1 + 1/K).
Preliminary analysis of the electronic absorption
spectra of spiropyrans in neutral and acidic aqueous-
acetone (1:1) solutions showed that spiropirans II,
III, and V are present in all the three forms A C
(Fig. 1). The merocyanine form selectively absorbs in
the long-wave region ( 545, 546, and 570 nm,
respectively). Spiropyrans I and IV (Fig. 2) are
present only in forms A and B, and the latter selec-
tively absorbs at 400 nm. In view of the specific
behavioral features of spiropyrans in solutions, the
approaches to estimating pKa from the absorption of
Here cL is the total concentration of the spiropyran
forms in the solution.
The ring chain equilibrium constants K were
1
determined by a known procedure [10] from the H
NMR spectra of spiropyrans I V in a mixture of
(CD3)2CO and D2O, deuterated analogs of the media
where the pKa values were estimated. The molar frac-
tions of the open-chain and cyclic forms in the solu-
tion were found from the intensity ratio of the proton
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 9 2002

1470
CHERNYSHEV et al.
1.0
0.8
0.6
0.4
1
1.4
D
D
1
2
1.2
1.0
0.8
0.6
0.4
2
3
3
4
4
5
5
6
0.2
0
6
0.2
0
7
300
400
500
600
700
, nm
350
400
450
500
550
, nm
Fig. 1. Absorption spectra of spiropyran III at various
pHs of the aqueous-acetone (1:1) solutions. pH: (1) 7.0,
Fig. 2. Absorption spectra of spiropyran I at varios pHs
of the aqueous-acetone (1:1) solution. pH: (1) 1.40,
(2) 2.05, (3) 5.20, (4) 6.60, (5) 7.20, and (6) 7.70
(2) 6.4, (3) 5.3, (4) 4.9, (5) 4.4, (6) 3.7, and (7) 2.0
5
5
(c
2.5 10 M).
(c 4.2 10 M).
III
I
+
form B (spiropyrans II, III, and V) or form C (spiro-
pyrans I and IV) should be different.
([LH+] + [Lc]) =
[LH ] + c[Lc].
(12)
+
LH
With [Lm] = 0, the apparent rate constant Ka can
only be determined [Eq. 13].
The calculations were based on the optical density
additivity equation (11) and the material balance
equation (4).
+
(
)
LH
Ka = [H3O+]
.
(13)
(
)
c
A = cLl,
(11)
Optical density measurements are convenient to
perform at the max of the protonated form. In view of
the fact that spiropyrans completely pass from the
cyclic to LH+ form at the maximum acidity of the
+
+
+
A = ( m[Lm] + c[Lc] +
[LLH ])l.
LH
Here l = 1 cm is the absorbing layer thickness,
=
+
LH+
is the mean molar extinction
+
+
m
c
cmoefficienct,
= L[HLi]/cL are the molar fractions of the
solution, Amax
=
cL. Then Eq. (13) can be brought
LH
i
LH+
correponding spiropyran forms,
,
,
are the
into form (14) which is convenient for calculations.
molar extinction coefficients of mthec forms at the
(Amax Ai)
working wavelength, and A is the optical density.
(14)
pKa = pH log
.
(Ai Amin
)
For spiropyrans I and IV, [Lm] = 0, and Eq. (11)
takes form (12).
Here Amax, Amin, and Ai are the maximal, minimal, and
current optical densities for a given spectral series.
The absorption spectra of compound I at varied
pHs of the solution are presented in Fig. 2. The pKa
values calculated by Eq. (14) are listed in the table.
Protolytic equilibrium constants for spiropyrans I V in
aqueous acetone (1: 1)
To calculate pKa values for the merocyanine forms
of spiropyrans II and III in cases where the concentra-
tions of the open-chain and cyclic forms are com-
parable (Fig. 1), the optical density additivity equation
(11) can be transformed into Eq. (15).
Potentiometric
Spectrophotometry
Comp.
no.
titration
pKa
5.52 0.06
pKa
pKa
pKa
I
II
5.72 0.08
([LH+] + [Lm] + [Lc]) =
4.62 0.03 4.95 0.03 4.37 0.08 4.70 0.08a
4.64 0.08 4.97 0.08b
+
+
=
[LH ] + m[Lm] + c[Lc].
(15)
LH
III
5.00 0.10 5.06 0.10a
4.98 0.20 5.04 0.23b
Substituting Eq. (15) into Eq. (3) and taking the
logarithm, we obtain Eq. (16).
IV
V
3.61 0.20
3.31 0.10
3.94 0.04
3.30 0.05
+
)
(
LH
pKa = pH log
.
(16)
[( /K +
)
(1 + 1/K)]
a
b
c
m
Calculated by Eq. (17). Calculated by Eq. (18).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 9 2002

TO ESTIMATION OF pKa FOR SPIROPYRANS OF THE INDOLINE SERIES
1471
When one works at the absorption maximum of the
c
LH+
= 0), Eq. (16) trans-
i
merocyanine form ( = 0,
1.0
0.8
0.6
0.4
0.2
LH+
fors into Eq. (17).
L?
mcL Ai(1 + 1/K)
(17)
.
pKa = pH + log
Ai
The molar extinction coefficients of the mero-
cyanine forms of spiropyrans II and III (4.66 104
and 3.85 104 l mol 1 cm , respectively) were deter-
1
L?
10 pH
mined by the formula
= Amax(1 + K)/(cLK).
m
0
2
4
6
8
When one works at the absorption maximum of
the LH+ form, Eq. (16) transforms into Eq. (18).
Fig. 3. Distribution of various forms of spiropyran III
vs. pH of the aqueous-acetone (1:1) solution.
(Amax Ai)
.
pKa = pH log
(18)
and additionally purified by recrystallization from
heptane.
[Ai(1 + 1/K) Amin
]
The pKa values were calculated by Eq. (7).
8-Methoxy-1 ,3 ,3 -trimethylspiro(2H-1-benzo-
pyran-2,2 -indoline) (I), yield 54%, mp 120 121 C
(mp 120 C [12]). 6-Formyl-8-methoxy-1 ,3 ,3 -tri-
Using the ring chain and protolytic equilibrium
constants, we constructed pH dependences of the
distrubution of the cyclic and open-chain (mero-
cyanine and protonated) forms of spiropyrans
(Fig. 3). The molar fractions of the forms ( i) were
calculated by Eq. (19).
methylspiro(2H-1-benzopyran-2,2 -indoline)
(II),
yield 75%, mp 150 151 C (mp 151 C [12]).
6-Formyl-5 ,8-dimethoxy-1 ,3 ,3 -trimethylspiro(2H-
1-benzopyran-2,2 -indoline) (III), yield 84%, mp
1
180.5 181.5 C. H NMR spectrum (CDCl3), , ppm:
Bi
=
.
1.16 s (3H, 3 -CH3), 1.26 s (3H, 3 -CH3), 2.68 s (3H,
1 -CH3), 3.70 s (3H, 8-OCH3), 3.77 s (3H, 5 -OCH3),
5.74 d (1H, 3-H, J 10.3 Hz), 6.42 d (1H, 7 -H, J
9.1 Hz), 6.66 6.69 m (2H, 4 -H, 6 -H), 6.86 d (1H,
4-H, J 10.3 Hz), 7.21 d (1H, 5-H, J 2.0 Hz), 7.25 d
(1H, 7-H, J 2.0 Hz), 9.77 s (1H, 6-CHO). Found, %:
C 72.24; H 6.47; N 3.72. C22H23NO4. Calculated, %:
C 72.31; H 6.34; N 3.83. 1 -Benzyl-6-bromo-8-
methoxy-3 ,3 -dimethylspiro(2H-1-benzopyran-2,2 -
i
(19)
Ka + KKa + K[H3O+]
Here Bi = KKa, Ka, and K[H3O+] for the merocyanine,
cyclic, and protonated forms.
The basicity of form B is higher than the estimate
from the apparent protolytic equilibrium constant.
Probably, the larger the fraction of form B (spiropyran
III), the closer the pKa and pKa values to each other.
1
indoline) (IV), yield 62%, mp 139 140 C. H NMR
EXPERIMENTAL
spectrum (CDCl3), , ppm: 1.24 s (3H, 3 -CH3),
1.32 s (3H, 3 -CH3), 3.67 s (3H, 8-OCH3), 4.22 d (1H,
1 -CH2Ph, J 16.7 Hz), 4.53 d (1H, 1 -CH2Ph, J
16.7 Hz), 5.75 d (1H, 3-H, J 10.2 Hz), 6.25 d (1H,
7 -H, J 7.7 Hz), 6.66 d (1H, 4-H, J 10.2 Hz), 6.78 d
(1H, 5-H, J 2.1 Hz), 6.80 t.d (1H, 5 -H, J 7.4 and
0.9 Hz), 6.82 d (1H, 7-H, J 2.1 Hz), 7.00 t.d (1H,
6 -H), 7.07 d.d (1H, 4 -H, J 7.3 and 1.1 Hz), 7.19
7.28 m (5H, 1-CH2Ph). Found, %: C 67.65; H 5.16;
N 3.12. C26H24BrNO2. Calculated, %: C 67.54; H
5.23; N 3.03. 1 -Benzyl-8-methoxy-3 ,3 -dimethyl-6-
nitrospiro(2H-benzopyran-2,2 -indoline) (V), yield
1
The H NMR spectra (300 MHz) we measured on a
2
Varian Unity-300 spectrometer in the H stabilization
mode. The electronic absorption spectra were obtained
on a Specord M-40 spectrophotomer in quartz cells
(l 1 cm). Potentiometric titration of 0.1 mmol of
spiropyran I V preliminarily protonated by treatment
with equivalent amount of hydrochloric acid was
performed with a 0.1 M solution of NaOH ( 0.1,
NaCl). The pHs of the solutions were measured on an
EV-74 pH-meter with a glass electrode. Acetone of
special purity and twice distilled water were used.
1
64%, mp 165 166.5 C. H NMR spectrum (CDCl3),
Spiropyrans I V. A mixture of 1 mmol of 1-R1-
, ppm: 1.27 s (3H, 3 -CH3), 1.32 s (3H, 3 -CH3),
3.76 s (3H, 8-OCH3), 4.26 d (1H, 1 -CH2Ph, J
16.7 Hz), 4.52 d (1H, 1 -CH2Ph, J 16.7 Hz), 5.86 d
(1H, 3-H, J 10.3 Hz), 6.31 d (1H, 7 -H, J 7.7 Hz),
6.80 d (1H, 4-H, J 10.3 Hz,), 6.84 t.d (1H, 5 -H, J 7.4
and 0.9 Hz), 7.04 t.d (1H, 6 -H, J 7.7 and 1.1 Hz),
7.09 d.d (1H, 4 -H, J 7.3 and 1.1 Hz), 7.20 7.28 m
2,3,3-trimethyl-5-R2-3H-indolium iodide, 1 mmol of
2-hydroxy-3-methoxy-5-R3-benzaldehyde,
and
1 mmol of piperidine in 5 ml of 2-propanol was
heated under reflux for 2 h. The solvent was removed
in a vacuum, and the residue was subjected to chro-
matography on a column of Al2O3 (eluent chloroform)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 9 2002

1472
CHERNYSHEV et al.
(5H, 1-CH2Ph), 7.59 d (1H, 5-H, J 2.3 Hz), 7.65 d
(1H, 7-H, J 2.5 Hz). Found, %: C 72.73; H 5.74; N
6.70. C26H24N2O4. Calculated, %: C 72.88; H 5.65;
N 6.54.
4. Phillips, J.P., Mueller, A., and Przystal, F., J. Am.
Chem. Soc., 1965, vol. 87, no. 17, p. 4020.
5. Atabekyan, L.S., Chibisov, A.K., and Roitman, G.P.,
Zh. Anal. Khim., 1982, vol. 37, no. 3, p. 389.
6. Atabekyan, L.S. and Chibisov, A.K., Zh. Anal. Khim.,
ACKNOWLEDGMENTS
1988, vol. 43, no. 10, p. 1787.
7. Winkler, J.D., Bowen, C.M., and Michlet, V., J. Am.
Chem. Soc., 1998, vol. 120, no. 13, p. 3237.
The work was financially supported by CRDF and
the Ministry of Education of the Russian Federation
(grant REC-004).
8. Dzhaparidze, K.G., Spirokhromeny (Spirochromenes),
Tbilisi: Mitsniereba, 1979.
REFERENCES
9. Drumond, C.J. and Furlog, D.N., J. Chem. Soc.,
Faraday Trans., 1990, vol. 86, p. 3613.
1. Barachevskii, V.A., Lashkov, G.I., and Tsekhom-
skii, V., Fotokhromizm i ego primenenie (Photo-
chromism and Its Application), Moscow: Khimiya,
1977.
10. Pozzo, J.-L., Samat, A., Guglielmetti, R., and Keuke-
leire De, D., J. Chem. Soc., Perkin Trans. 2, 1993,
no. 7, p. 1327.
11. Ramenskaya, L.M., Borodin, V.A., Vasil’ev, V.P.,
and Kozlovskii, E.V., Zh. Fiz. Khim., 1999, vol. 73,
no. 6, p. 1029.
2. Photochromism. Molecules and Systems, Duerr, H.
and Bouas-Laurent, H., Eds., Amsterdam: Elsevier,
1990.
3. Photochromism.
Technology
of
Chemistry,
12. Hinnen, A., Audic, C., and Gautron, R., Bull. Soc.
Chim. Fr., 1968, no. 5, p. 2066.
Brown, G.B., Ed., New York: Wiley, 1971.
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