Syntheses and UV-visible spectroscopic properties of two new hydrophilic 2,6-di(carbamoyl)-substituted solvatochromic pyridinium N-phenolate betaine dyes

Article abstract of DOI:10.1002/poc.623

Synthesis and negative solvatochromism of two 2,6-di(carbamoyl) -substituted pyridinium N-phenolate betaines dyes were investigated. The exceptionally large negative solvatochromism of the standard betaine dye was used to establish a UV-Vis spectroscopically derived scale of solvent polarity. It was found that the primary indicator dye was sufficiently soluble in most organic solvents.

Full text of DOI:10.1002/poc.623

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2003; 16: 682–690
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.623
Syntheses and UV–visible spectroscopic properties
of two new hydrophilic 2,6-di(carbamoyl)-substituted
solvatochromic pyridinium N-phenolate betaine dyesy
,
z
Christian Reichardt,* Andreas Ro¨ chling§ and Gerhard Scha¨fer
Department of Chemistry and Scientific Center of Material Sciences, Philipps University, Hans-Meerwein-Strasse,
D-35032 Marburg, Germany
Received 6 December 2002; revised 4 February 2003; accepted 4 February 2003
ABSTRACT: Syntheses and negative solvatochromism of two 2,6-di(carbamoyl)-substituted pyridinium N-phenolate
betaines dyes, 3 and 4, are described for the purpose of obtaining more hydrophilic zwitterionic dyes which are better
soluble in water and other aqueous media (such as binary water–solvent mixtures or aqueous ionophore solutions)
than the rather hydrophobic standard betaine dyes 1 and 2, which have been used to establish an empirical scale of
solvent polarity, called the ET(30) or ETN scale. Copyright # 2003 John Wiley & Sons, Ltd.
KEYWORDS: betaine dyes; ETð30Þ values; solvatochromism; solvent effects; UV–visible spectroscopy
INTRODUCTION
ETð30Þ values are simply defined as the molar transition
energy (in kcal molꢀ1; 1 kcal ¼ 4.184 kJ) of the standard
betaine dye 1, measuredꢁin solvents of different polarity
at room temperature (25 C) and normal pressure (1 bar),
according to the equation
Solutions of the pyridinium N-phenolate betaine
dye 1 (Scheme 1) are solvatochromic, thermochromic,
piezochromic and halochromic.3–5 This means that the
longest-wavelength intramolecular charge-transfer (CT)
absorption band in the visible spectrum of dye 1 depends
on solvent polarity,3–5 solution temperature,6 external
pressure7 and the nature and concentration of added
ionophores.8 Ionophores are ionic compounds (such as
Naþ Clꢀ ) which, in contrast to ionogens, consist of ions
in the crystal, in the melt, and in solution. Ionogens (such
as H—Cl) are compounds with molecular crystal lattices
which produce ions only when dissolved in appropriate
solvents, inducing the ionization of the covalent com-
pound.9 By definition, negative (positive) solvatochro-
mism means that solvent-influenced UV–Vis absorption
bands of the solute chromophore are shifted hypso-
chromically (bathochromically) with increasing solvent
polarity.5 The exceptionally large negative solvatochro-
mism of the standard betaine dye 1 has been used to
establish a UV–Vis spectroscopically derived scale of
solvent polarity, called the ETð30Þ or ETN scale.3,4,10 The
ETð30Þðkcal molꢀ1Þ ¼ h ꢂ c ꢂ ꢀ~max ꢂ NA
¼ ð2:8951 ꢃ 10ꢀ3Þ ꢂ ꢀ~maxðcmꢀ1
¼ 28591=ꢁmaxðnmÞ
Þ
ð1Þ
where ꢀmax is the wavenumber and ꢁmax the wavelength
of the maximum of the long-wavelength solvatochromic
absorption band of betaine dye 1 in the visible spectral
region.
~
High ETð30Þ values correspond to high solvent polarity,
here defined as the overall solvation capability of a sol-
vent.4,5,11 The ETð30Þ solvent polarity scale ranges from
63.1 kcal molꢀ1 for water to 30.7 kcal molꢀ1 for tetra-
methylsilane (TMS) as the most and least polar solvents,
respectively. In order to avoid use of the non-SI unit kcal
molꢀ1, the dimensionless, normalized ETN scale was later
introduced, using water ðETN ¼ 1:000Þ and TMS ðETN ¼
0:000Þ as extreme reference solvents to fix the scale.12
The primary indicator dye 1 is sufficiently soluble in
most organic solvents, although insoluble or only spar-
ingly soluble in non-polar solvents such as aliphatic
hydrocarbons, perfluoroalkanes and TMS, and also in
water. For the indirect determination of ETð30Þ values of
such non-polar solvents, the lipophilic penta-tert-butyl-
substituted betaine dye 2 was introduced as a secondary
indicator dye, the ETð2Þ values of which correlate linearly
with that of the primary indicator dye 1 for those solvents
in which both betaine dyes are soluble.12 Thus, based on
the excellent linear correlation between the ET values of 1
*Correspondence to: C. Reichardt, Department of Chemistry, Philipps
University, Hans-Meerwein-Strasse, D-35032 Marburg, Germany.
E-mail: reichardt-marburg@t-online.de
yPart XXVII in the series Pyridinium N-Phenoxide Betaine Dyes and
Their Application to the Determination of Solvent Polarities. For Part
XXVI, see Ref. 1.
zDedicated to Professor Dr T. M. Krygowski, Warsaw, in recognition of
his remarkable contributions in the areas of physical organic chemistry,
on the occasion of his 65th birthday.
§
2
¨
Based on the Diploma and PhD Thesis of A. Rochling.
Contract/grant sponsor: Deutsche Forschungsgemeinschaft.
Contract/grant sponsor: Fonds der Chemischen Industrie.
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 682–690

SOLVATOCHROMISM OF DICARBAMOYL-SUBSTITUTED PYRIDINIUM N-PHENOLATE BETAINE DYES
683
the pyridinium moiety, but with limited success.15,16
However, replacement of at least two of the five periph-
eral phenyl groups by 2-, 3- or 4-pyridyl rings leads to
comparatively good water-soluble pyridinium N-pheno-
late betaine dyes, as has been shown recently.17 This
increased solubility in water is obviously due to the
possibility of forming intermolecular hydrogen bonds
between water and the pyridyl-nitrogen atoms, thus pull-
ing the whole dye into the aqueous solution. Other
hydrophilic groups which should lead to better water
solubility of 1 are carbamoyl groups of the type —CO—
NH—CH3 and —CO—N(CH3)2. Here, we report on the
synthesis and solvatochromism of the new pyridinium
N-phenolate betaine dyes 3 and 4 with two methyl-
substituted carbamoyl groups in 2,6-positions of the
phenolate moiety as hydrogen-bond acceptors (Scheme 1).
Scheme 1. Molecular structure of the betaine dyes 1–4
RESULTS AND DISCUSSION
and 2, ETð30Þ values can be calculated for such non-polar
solvents in which the primary probe dye is not soluble
enough for UV–Vis spectroscopic measurements.4,12,13
Unfortunately, the primary indicator dye 1 is only
sparingly soluble in water (its solubility in water is only
about 2 ꢃ 10ꢀ6 mol lꢀ1),14 just enough to determine its
ETð30Þ values directly with a saturated solution in a
10 cm quartz cell. This low solubility of 1 in water is
obviously caused by the core of five hydrophobic phenyl
groups around its zwitterionic pyridinium N-phenolate
chromophore. In order to obtain better water-soluble
betaine dyes, the peripheral phenyl rings of 1 should be
modified either by introduction of hydrophilic substitu-
ents into these phenyl groups or through replacement by
other more hydrophilic groups, but without changing the
essential solvatochromic chromophore. For this reason,
we synthesized pyridinium N-phenolates with hydro-
Syntheses of betaine dyes 3 and 4
The key step in the synthesis of such betaine dyes is the
condensation reaction between 2,4,6-triaryl-substituted
pyrylium salts and 2,6-disubstituted 4-aminophenols,
producing N-(4-hydroxyphenyl)pyridinium salts, which
are eventually deprotonated to give the corresponding
betaine dyes (Schemes 2 and 3). For the synthesis of 3
and 4, the known 2,4,6-triphenylpyrylium tetrafluorobo-
rate18 has to be reacted with the unknown 2,6-disubsti-
tuted 4-aminophenols 7 (Scheme 2) and 12 (Scheme 3),
which were prepared as follows.
The base-catalyzed, two-fold aldol condensation reac-
tion of sodium nitromalonaldehyde monohydrate19 with
dimethyl 3-oxoglutarate yielded 4-nitrophenol 5 (Scheme
2), which was converted into the 2,6-dicarbamoyl-sub-
stituted 4-nitrophenol 6 by reaction with methylamine.
Palladium-catalyzed reduction of 6 with dihydrogen
15
philic carboxylate (—COꢀ2 NaþÞ and methanesulfonate
(—SO2CH3) substituents16 in the 2,4,6-phenyl rings of
Scheme 2. Synthesis of the 2,6-di(N-methylaminocarbonyl)-substituted betaine dye 3
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 682–690

¨
C. REICHARDT, A. ROCHLING AND G. SCHAFER
¨
684
Scheme 3. Synthesis of the 2,6-di(N,N-dimethylaminocarbonyl)-substituted betaine dye 4
gave the oxygen-sensitive 4-aminophenol 7, which was
immediately reacted with the 2,4,6-triphenylpyrylium
salt18 to give N-(4-hydroxyphenyl)pyridinium salt 8.
NMR shift of the OH signal of 11 and 13 is in agreement
with the participation of O—H in an intramolecular
hydrogen bond (ꢂ ¼ 11.7 and 11.3 ppm in CDCl3).
The molecular structures of all new compounds were
confirmed by elemental analysis and mass, UV–Vis, IR
and 1H and 13C NMR spectra (see Experimental section).
1
The downfield H NMR shift of the NH and OH signal
of 8 indicates participation of these groups in intramole-
cular hydrogen bonds as schematically indicated by the
dotted lines in formulas 6–8 [ꢂ(NH) ¼ 7.9 and
ꢂ(OH) ¼ 15.6 ppm for 8 in CDCl3]. In analogy with the
—
intramolecular O—Hꢂ ꢂ ꢂO C hydrogen bond found
—
UV–Vis spectra and solvatochromism of betaine
dyes 3 and 4
experimentally in salicylic acid derivatives,20 the most
probable molecular fine structure of 6–8 is that given in
Scheme 2, with one N—Hꢂ ꢂ ꢂO—H and one O—
In acetonitrile, a solvent of intermediate polarity, the
UV–Vis spectra of betaine dyes 3 and 4 exhibit two
—
Hꢂ ꢂ ꢂO C intramolecular hydrogen bond.
—
Deprotonation of 8 with a suspension of the basic
anion-exchange resin Amberlyst A-21 in dichloro-
methane gave eventually the orange betaine dye 3 as
sesquihydrate. Betaine dye 3 can now form two intramo-
lecular N—Hꢂ ꢂ ꢂꢀO— hydrogen bonds directed to the
same phenolate oxygen atom, in competition with the
intermolecular hydrogen bonds formed between N—H
and —Oꢀ and the water of crystallization.
An analogous two-fold aldol condensation of sodium
nitromalonaldehyde monohydrate19 with 3-oxoglutaric
acid yielded the dicarboxylic acid 9 (Scheme 3), which
was converted into its dichloride 10 with thionyl chloride.
Reaction of 10 with dimethylammonium dimethylcarba-
mate (Dimcarb) as dimethylamine precursor21 led to the
2,6-bis(dimethylaminocarbonyl)-substituted 4-nitrophe-
nol 11. Reduction of 11 to the 4-aminophenol 12 and
its immediate reaction with 2,4,6-triphenylpyrylium tet-
rafluoroborate18 gave the N-(4-hydroxyphenyl)pyridi-
nium salt 13, which was deprotonated by means of a
suspension of Amberlyst A-26 (OH form) in dichloro-
methane to give the blue–violet betaine dye 4 as the ca
dihydrate. The most probable molecular fine structure of
main absorption bands of different intensity at ꢁmax
¼
451 and 544 nm (" ꢄ 2600–2800 1 molꢀ1 cmꢀ1) and
ꢁmax ¼ 303 nm (" ꢄ 35 000–38 000 1 molꢀ1 cmꢀ1). The
spectrum of betaine dye 3 shows an additional inter-
mediate absorption band at ꢁmax ¼ 369 nm (" ꢄ
8800 l molꢀ1 cm1). The corresponding intermediate
absorption band of 4 can only be seen as shoulder
because of overlap with the intensive short-wavelength
band (see Table 1 and Experimental section).
Only the position of the long-wavelength band in the
visible spectral region is strongly solvent (and substitu-
ent) dependent; the others are not. On going from non-
polar benzene to polar ethanol as solvent, the visible
absorption bands of 3 and 4 are shifted hypsochromically
by ꢀꢁ ¼ ꢀ128 and ꢀ216 nm, respectively, which cor-
responds to a solvent-induced increase in the molar
electronic transition energies of ꢀET ¼ þ15.4 and
þ19.1 kcal molꢀ1, respectively (see Table 1).
Various quantum-chemical calculations and different
spectroscopic measurements22–33 as well as direct evi-
dence by terahertz (THz) spectroscopy34 have shown that
the long-wavelength Vis absorption band of betaine dyes
such as 1 is related to an intramolecular charge transfer
(CT) from the HOMO of the phenolate to the LUMO of
the pyridinium moiety. Therefore, the position of this CT
intermediates 11–13 is described with one intramolecular
—
O—Hꢂ ꢂ ꢂO C hydrogen bond, in analogy with corre-
—
1
sponding salicylic acid derivatives.20 The downfield H
Copyright # 2003 John Wiley & Sons, Ltd.
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SOLVATOCHROMISM OF DICARBAMOYL-SUBSTITUTED PYRIDINIUM N-PHENOLATE BETAINE DYES
685
Table 1. Long-wavelength, solvent-dependent CT absorption maxima, ꢁmax (nm) and the corresponding ET(X) values
(kcal molꢀ1) (in parentheses) of betaine dyes X ¼ 3 and 4, measured in up to 19 solvents of different polarity at 25 ꢁC and
4
normal pressure, ordered according to decreasing ET(30) values
No.
Solvent
ET(30)a
ꢁ
max(3) [ET(3)]b
ꢁ
max(4) [ET(4)]b
c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Water
Methanol
N-Methylformamide
Ethanol
1-Propanol
1-Butanol
2-Propanol
Acetonitrile
Dimethyl sulfoxide
N,N-Dimethylformamide
Acetone
1,2-Dichloroethane
Dichloromethane
Pyridine
Trichloromethane
Ethyl acetate
Tetrahydrofuran
1,4-Dioxane
Benzene
63.1
55.4
54.1
51.9
50.7
49.7
48.4
45.6
45.1
43.2
42.2
41.3
40.7
40.5
39.1
38.1
37.4
36.0
34.3
–
–
415 (68.9)
448 (63.8)
463 (61.7)
471 (60.7)
479 (59.7)
482 (59.3)
506 (56.5)
544 (52.6)
551 (51.9)
568 (50.3)
584 (48.9)
594 (48.1)
595 (48.1)
601 (47.6)
597 (47.9)
624 (45.8)
645 (44.3)
662 (43.2)
687 (41.6)
d
–
428 (66.8)
431 (66.3)
435 (65.7)
434 (65.9)
451 (63.4)
449 (63.7)
459 (62.3)
470 (60.8)
489 (58.5)
492 (58.1)
486 (58.8)
498 (57.4)
501 (57.1)
511 (56.0)
528 (54.1)
556 (51.4)
ꢀꢁ ¼ ꢀ 128 nme
ꢀꢁ ¼ ꢀ 216 nme
ꢀET ¼ 15.4e
ꢀET ¼ 19.1 kcal molꢀ1 e
a
Taken from Ref. 4.
The ET(X) values given in parentheses are calculated from the long-wavelength absorption maxima of X ¼ 3 and 4 according to Eqn (1).
b
c
d
e
Not soluble in this solvent.
Not measurable because of overlap with the intense UV absorption band.
ꢀꢁ ¼ ꢁmax(EtOH) ꢀ ꢁmax(C6H6); ꢀET ¼ ET(EtOH) ꢀ ET(C6H6).
band should depend on the ionization energy of the
electron donor (i.e. the phenolate part) and on the
electron affinity of the electron acceptor (i.e. the pyridi-
nium part). On protonation of the phenolate part of
betaine dyes 1–4, the solvent-dependent CT band dis-
appears and the corresponding N-(4-hydroxyphenyl)pyr-
idinium salts formed (e.g. 8 and 13) absorb only at
ꢁmax ꢄ 300–310 nm, corresponding to the ꢃ ! ꢃ* absorp-
tion of the 2,4,6-triphenylpyridinium chromophore.
Replacement of the two 2,6-phenyl groups in the
the ꢀꢁ or ꢀET values observed experimentally, the
electron-withdrawing effect increases in the order —
CO—N(CH3)2 < —CO—NH—CH3, which seems rea-
sonable. The greater hypsochromic band shift observed
for betaine dye 3 reflects an increased ionization energy
of the phenolate moiety of 3. This increase is presumably
caused by the formation of intramolecular hydrogen
bonds of the two —CO—NH—CH3 groups with the
phenolate oxygen atom, as shown in Scheme 2.
In Table 1, the molar transition energies ET(3) and
ET(4) of the new betaine dyes 3 and 4, measured for up to
19 HBD and non-HBD solvents, are also given (in
parentheses), together with the corresponding ET(30)
values of the primary indicator dye 1. In both cases,
there exists a good linear correlation between the ET
values of the new dyes 3 and 4 and the ET(30) values of
dye 1 with correlation coefficients r ꢄ 0.98–0.99 for all
HBD and non-HBD solvents, as shown by the following
regression equations and Fig. 1:
phenolate part of the standard betaine dye 1 (ꢁmax
¼
627 nm in acetonitrile) by the two N-methyl-substituted
aminocarbonyl groups results in both cases in a
hypsochromic shift of the long-wavelength CT band,
amounting to ꢀꢁ ¼ 451 ꢀ 627 ¼ ꢀ176 nm (ꢀET ¼
þ17.8 kcal molꢀ1) for 3 (with two —CO—NH—CH3
groups) and to ꢀꢁ ¼ 544 ꢀ 627 ¼ ꢀ83 nm (ꢀET ¼ þ
7.0 kcal molꢀ1
) for 4 [with two —CO—N(CH3)2
groups] in acetonitrile as solvent. The replacement of
the two 2,6-phenyl groups [Hammett substituent
constant ꢄp ¼ 0.02 for —C6H5 (For comprehensive col-
lections of Hammett constants, see Ref. 35)] by the two
electron-withdrawing aminocarbonyl groups (ꢄp¼0.31
for —CO—NH235,36) increases the ionization energy of
the phenolate moiety, with the experimentally observed
hypsochromic CT band shift as a consequence. The ꢄp
value of —CO—NH—CH3 is 0.36,35,37 similar to that of
the —CO—NH2 group;36 the corresponding ꢄp value for
—CO—N(CH3)2 seems not to be known.35 According to
ETð3Þ=ðkcal molꢀ1Þ ¼ 0:854 ꢂ ETð30Þ=ðkcal molꢀ1
þ 23:89
Þ
ð2Þ
(n ¼ 16 ; r ¼ 0.979; ꢄ ¼ 0.982 kcal molꢀ1
)
ETð4Þ ðkcal molꢀ1Þ ¼ 1:016 ꢂ ETð30Þ=ðkcal molꢀ1
þ 6:86
Þ
ð3Þ
(n ¼ 19; r ¼ 0.992; ꢄ ¼ 0.983 kcal molꢀ1); see also Fig. 1)
J. Phys. Org. Chem. 2003; 16: 682–690
Copyright # 2003 John Wiley & Sons, Ltd.

¨ ¨
C. REICHARDT, A. ROCHLING AND G. SCHAFER
686
Figure 1. Linear correlation between the ET(30) values of standard betaine dye 1 and the ET(4) values of betaine dye 4,
measured in seven HBD solvents (open circles) and 12 non-HBD solvents (full circles), according to Eqn (3). The separate dashed
line represents the linear correlation for the seven HBD solvents only, according to Eqn (4). Solvent numbering as in Table 1
The slope of correlation Eqn (2) is distinctly smaller
than unity, indicating that betaine dye 3 is less sensitive to
a change in solvent polarity than standard dye 1, whereas
Eqn (3) exhibits a slope of almost unity, demonstrating
the high susceptibility of betaine dye 4 against changes in
solvent polarity, comparable to that of dye 1. The good
quality of both correlations allows the calculation of
ET(30) values for such media for which a direct experi-
mental determination is not possible for reasons of
limited solubility or strong solvent acidity, thus using
betaines 3 and 4 as additional standard dyes.
(n ¼ 7 HBD solvents; r ¼ 0.986; ꢄ ¼ 0.716 kcal molꢀ1
;
see also Fig. 1)
Analogous HBD/non-HBD solvent dispersions of lin-
ear ET/ET correlations have also been found for some
other pyridinium N-phenolate betaine dyes.17 Surpris-
ingly, the specific solvation of betaine dye 4 in HBD sol-
vents leads to an altogether smaller susceptibility (slope
only 0.789) of 4 against a solvent change. Nevertheless,
Eqn (3) can be used for the calculation of ET(30) values for
aqueous media, with the aid of ET(4) values of the more
hydrophilic betaine dye 4, with sufficient precision.
A closer look at the correlation line given by Eqn (3)
and illustrated in Fig. 1 reveals some family-dependent
behaviour for betaine dye 4. The group of seven HBD
solvents (i.e. water, N-methylformamide and the alco-
hols; the very weak C—H hydrogen-bond donors CHCl3,
CH2Cl2 and CH3CN are considered as non-HBD sol-
vents) follow a regression line with a slope smaller than
the regression line given by Eqn (3), which can be
described by the equation
The new betaine dyes 3 and 4 are sufficiently soluble in
most organic solvents, including benzene as non-polar
hydrocarbon, but both dyes are insoluble in text-butyl
methyl ether, tetrachloromethane and cyclohexane.
Betaine dye 4 is suitably soluble in water, whereas dye
3 is surprisingly and disappointingly not water-soluble.
Dissolution of a solid in a solvent means that the solvent
gradually destroys the crystal lattice by non-specific
and specific solvation of the molecules located on the
crystal surface. In the crystal lattice of 3, intra- and
intermolecular hydrogen bonds between the zwitterionic
dye molecules are presumably formed, which obviously
ETð4Þ=ðkcal molꢀ1Þ ¼ 0:789 ꢂ ETð30Þ=ðkcal molꢀ1
þ 19:47
Þ
ð4Þ
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 682–690

SOLVATOCHROMISM OF DICARBAMOYL-SUBSTITUTED PYRIDINIUM N-PHENOLATE BETAINE DYES
687
prevent the interaction of the dye molecules with water, a
solvent which can usually act as a hydrogen-bond donor
(HBD) and hydrogen-bond acceptor (HBA) solvent simul-
taneously. In solutions of 3 in non-HBD and non-HBA
solvents, intramolecular N—Hꢂ ꢂ ꢂO hydrogen bonds are
probably formed to such an extent, that the formation of
intermolecular hydrogen bonds with added HBD solvents
seems to be reduced. In the case of betaine dye 4 as solid,
intra- and intermolecular hydrogen bonding between the
dye molecules is not possible, and the dimethylaminocar-
bonyl and phenolate oxygen atoms are now prone to
specific intermolecular interaction with HBD solvents
such as water, leading to the desired water solubility.
Unfortunately, the results of an x-ray structure determina-
tion of 3 and 4 are not yet available because of the
difficulty in obtaining suitable single crystals.
(FD). Analytical TLC: 60F-245 plates with silica gel and
fluorescence indicator on aluminium foil (Merck). Flash
chromatography: silica gel 60 (Merck), particle size
0.040–0.063 mm. Solvents: solvents for the solvatochro-
mic measurements were used as commercially available
in the highest available quality (analytical or spectro-
scopic grade) and were additionally dried and purified by
means of molecular sieves and, if necessary, by filtration
through a column of basic alumina, activity grade I, in
order to remove traces of acids. For these measurements
the solvents must be absolutely acid free, otherwise the
betaine dyes are protonated and the long-wavelength
solvatochromic CT absorption band disappears. Solvents
for synthetic work were purified according to the usual
standard methods (see Chapter A-2 and Table A-3 in
Ref. 5).
Synthesis of betaine dye 3 (Scheme 2). 2,6-Bis
(methoxycarbonyl)-4-nitrophenol (5). The preparation
analogous to Ref. 38. To a stirred solution of NaOH
(2.47 g, 61.8 mmol) in water (180 ml) and methanol
(190 ml) were added dimethyl 3-oxoglutarate (34.6 ml,
41.5 g, 238 mmol) and subsequently sodium nitromalo-
naldehyde monohydrate (37.4 g, 238 mmol).19 The mix-
ture was stirred at room temperature for ca 24 h. The
yellow precipitate formed was filtered off, washed with
ice-cold water and then suspended in cold water. The
suspension was acidified with ice-cold aqueous 2 M HCl
(pH ꢄ 1) and stirred until the yellow solid was dissolved
and a new colourless precipitate was formed. This pre-
cipitate was filtered off, washed with cold water, recrys-
tallized twice from ethanol–water (1:1) and dried in
vacuo, to affoꢁrd 5 (53.2 g, 88%) as colourless crystals
CONCLUSION
The new carbamoyl-substituted pyridinium N-phenolate
betaine dyes 3 and 4, synthesized according to the reac-
tion pathways given in Schemes 1 and 2, enlarge the
supply of highly solvatochromic indicator dyes for the
empirical determination of solvent polarities, in addition
to the already known standard betaine dyes 1 and 2.
In particular, betaine dye 4 should be useful for the
indirect determination of ET(30) values for all kinds of
aqueous media because of its water solubility, in addi-
tion to the hydrophilic, also water-soluble pyridyl-
substituted betaine dyes described recently.17 The advan-
tage of betaine dye 4 over these pyridyl-substituted
betaine dyes17 is their simpler and less expensive synth-
esis. Disappointingly, in spite of the introduction of two
hydrophilic N-methylaminocarbonyl groups, betaine dye
3 is not sufficiently soluble in water, presumably because
of the preferred formation of intramolecular hydrogen
bonds with the phenolate oxygen atom of 3 instead of
intermolecular hydrogen bonds with the solvent.
with m.p. 141 C. IR (KBr): ꢀ (cmꢀ1) ¼ 3456 (OH), 1736
1
—
(C O), 1528 and 1340 (NO ). H NMR (CDCl ): ꢂ
3
—
2
(ppm) ¼ 3.95 (s, 6 H, CH3), 8.85 (s, 2 H, aromatic H),
12.39 (s, 1 H, OH). 13C NMR (CDCl3): ꢂ (ppm) ¼ 53.1
(CH3), 117.1 and 131.2 (phenol-C-2 and C-3), 138.9
—
(C—NO ), 165.6 (C—OH), 166.3 (C O). MS (EI):
—
2
m/z (%) ¼ 255 (100) [Mþ ], 223 (34), 192 (68), 165
(66). C10H9NO7 (255.2): calcd C 47.07, H 3.55, N
5.49; found C 46.92, H 3.70, N 5.62%.
EXPERIMENTAL
2,6-Bis(N-methylaminocarbonyl)-4-nitrophenol (6). Gase-
ꢁ
General methods. Melting-points (not corrected):
Kofler–Mikroheiztisch (Reichert) and Mel-Temp appara-
tus. Elemental analyses: performed with a CHN-Automat
Vario EL (Heraeus) at the Analytik-Servicelabor of the
Department of Chemistry, Marburg. UV–Vis spectra:
double-beam UV–Vis–NIR U-3410 spectrophotometer
(Hitachi) with 1.00 cm quartz cells Suprasil (Hellma).
IR spectra: IFS 88 spectrophotometer (Bruker) with KBr
discs. 1H and 13C NMR: ARX 200 and AC-300 spectro-
meters (Bruker) in CDCl3 and CD3SOCD3 with tetra-
methylsilane as internal standard. Mass spectra: MAT
CH-7A spectrometer (Varian) with electron ionization
(EI, 70 eV) and MAT 711 (Varian) with field desorption
ous methylamine (b.p. ꢀ6.5 C), prepared by heating an
aqueous methylamine solution (c ¼ 40 cg gꢀ1) in a hot-
water bath, was passed through two wash-bottles (the first
empty and cooled, the second filled with KOH pellets)
into a solution of 5 (5.00 g, 19.6 mmol) in tetrahydrofuran
(350 ml) for ca 5 h. The excess of methylamine leaving
the reaction vessel was condensed in a cooling trap,
cooled with liquid nitrogen. After 5 h, the system reaction
vessel–cooling trap (with condensed methylamine) was
closed, provided with a bubble counter and the cooling
agent (liquid N2) was removed. Then, the reaction mix-
ture was stirred at room temperature for 4 days in the
closed system, filled with methylamine. Eventually, the
Copyright # 2003 John Wiley & Sons, Ltd.
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¨ ¨
C. REICHARDT, A. ROCHLING AND G. SCHAFER
688
solvent was distilled off in a rotary evaporator to give the
yellow methylammonium salt of the product. This salt
was dissolved in a mixture of water, methanol, and
ethanol and cold 2 M HCl was added with stirring. The
colourless flocculent precipitate formed was filtered off,
washed with cold water, recrystallized twice from
ethanol and dried in vacuo to yield colourless flakes of
(%) ¼ 513 (100) [M þ ꢀ HBF4 ꢀ H]. C33H28BF4N3O3
(601.4): calcd C 65.91, H 4.69, N 6.99; found C 65.72, H
4.61, N 6.97%.
2,6-Bis(N-methylaminocarbonyl)-4-(2,4,6-triphenylpyri-
dinium-1-yl)phenolate (3). To a stirred suspension of
tetrafluoroborate 8 (0.51 g, 0.85 mmol) in dry dichloro-
methane (30 ml) was added the macroreticular basic
anion-exchange resin Amberlyst A-21 (2.27 g; Rohm
and Haas). The suspension was stirred for 6 h at room
temperature, the resin was filtered off and the solvent was
distilled off in a rotary evaporator. The solid residue was
washed several times with n-hexane and diethyl ether,
and dried in vacuo (10ꢀ3 mbar) at 50 ꢁC with P4O10, to
give 3 (0.28 g, 64%) asꢀo1range crystals with m.p. 360–
ꢁ
6 (3.31 g, 67%), which sublime at ca 268 C. IR (KBr):
—
ꢀ(cmꢀ1) ¼ 3379 (OH), 3321 (NH), 1670 (C O), 1344
—
1
(NO2). H NMR (CD3SOCD3): ꢂ (ppm) ¼ 2.76 (d, 6 H,
CH3), 8.75 (s, 2 H, aromatic H), 9.12 (broad m, 2 H, NH);
the OH signal is not discernible. 13C NMR (CD3SOCD3):
ꢂ (ppm) ¼ 26.2 (CH3), 119.1 (C—CO—NH—), 127.6
(phenol-C-3), 137.3 (C—NO2), 160.5 (C—OH), 165.9
þ
—
(C O). MS (EI): m/z (%) ¼ 253 (100) [M ], 223 (30),
—
361 ꢁC. IR (KBr): v (cm ) ¼ 3427 (OH), 1650 (C O).
—
192 (76), 146 (23). C10H11N3O5 (253.2): calcd C 47.44,
H 4.38, N 16.60; found C 47.60, H 4.23, N 16.90%.
—
UV–Vis (CH3CN): ꢁmax (log ") ¼ 451 (3.44), 369 (3.94),
303 nm (4.58). 1H NMR (CDCl3): ꢂ (ppm) ¼ 2.83 (d, 6 H,
CH3), 7.24–7.30 (aromatic H), 7.55–7.62 (m, 3 H, aro-
matic H), 7.66 (s, 2 H, phenolate-3-H), 7.80–7.83 (m, 2 H,
aromatic H), 8.03 (s, 2 H, pyridinium-3-H), 11.00 (m,
2 H, NH). 13C NMR (CDCl3): ꢂ (ppm) ¼ 25.5 (CH3),
120.6, 122.3, 125.3, 127.9, 129.0, 130.1, 130.6, 131.7,
4-Amino-2,6-bis(N-methylaminocarbonyl)phenol (7). A
suspension of a palladium catalyst (10% Pd on charcoal;
ca 100 mg) in a solution of 6 (2.03 g, 8.00 mmol) in
water–methanol (200 ml) was reduced with dihydrogen
at room temperature and normal pressure until the neces-
sary amount was absorbed (ca 530 ml H2; time ca 18 h).
The catalyst was filtered off under nitrogen, the solvent
was distilled off in vacuo and the residue was dried
in vacuo, to give 7 as yellow solid, which turned dark
on contact with air. Because of its sensitivity to oxygen,
7 was immediately converted into the pyridinium salt 8
without further characterization.
132.5, 132.7, 133.7, 156.0, 158.5, 167.7, and 172.1
—
(aromatic C and C O). MS (FD): m/z (%) ¼ 513 (100)
—
[Mþ ]. C33H27N3O3. 1.5 H2O (513.6 þ 27.0 ¼ 540.6):
calcd C 73.32, H 5.03, N 7.77; found C 73.50, H 5.32,
N 7.82%.
Synthesis of betaine dye 4 (Scheme 3). 2-Hydroxy-
5-nitrobenzene-1,3-dicarboxylic acid (9). The prepara-
tion is analogous to Ref. 38. To a solution of NaOH
(16.1 g, 403 mmol) in water (200 ml) were added 3-
oxoglutaric acid (26.2 g, 179 mmol) and subsequently
sodium nitromalonaldehyde monohydrate (28.2 g,
179 mmol).19 The mixture was stirred at room tempera-
ture for ca 12 h. The dark-red solution was acidified with
2 M HCl (pH ꢄ 1). The colourless precipitate formed was
filtered off, recrystallized from water and dried in vacuo
(with P4O10), to give 9 (29.3 g, 72%) as colourless
crystals with m.p. 213–214 ꢁC (lit.39 213–214 ꢁC). IR
1-[4-Hydroxy-3,5-bis(N-methylaminocarbonyl)phenyl]-
2,4,6-triphenylpyridinium tetrafluoroborate (8). A solu-
tion of freshly prepared 7 (see above) and 2,4,6-triphe-
nylpyrylium tetrafluoroborate (2.70 g, 6.80 mmol)18 in
dry dichloromethane (75 ml) and dry methanol (75 ml)
was stirred at room temperature for 2 h. Then, three drops
of acetic acid were added and stirring was continued for 2
days. The dark reddish-brown solution was filtered and
the solvents were nearly completely distilled off in a
rotary evaporator. The liquid residue was dissolved in
acetone (20 ml) and this solution was slowly dropped
with stirring into diethyl ether (800 ml). The precipitate
formed was filtered off, washed with diethyl ether,
recrystallized twice from methanol, and dried in vacuo
(KBr): v (cmꢀ1) ¼ 3532 (carboxylic OH), 3485 (phenolic
1
OH), 1714 (C O), 1531 and 1344 (NO ). H NMR
—
—
2
(CD3SOCD3): ꢂ (ppm) ¼ 8.68 (s, 2 H, aromatic H), 9.69
(broad s, 3 H, OH). 13C NMR (CD3SOCD3): ꢂ
(40 C, 10ꢀ3 mbar, P4O10), to afford 8 (1.30 g, 27%) as an
(ppm) ¼ 118.1 (C—CO2H), 130.8 (phenol-C-3), 136.4
ꢁ
orange solid with m.p. 211–214 ꢁC. IR (KBr): ꢀ
(cmꢀ1) ¼ 3404 (OH), 1084 (BF4). UV–Vis (CH3CN):
ꢁmax (log ") ¼ 309 nm (4.57). 1H NMR (CDCl3): ꢂ
(ppm) ¼ 2.85 (d, 6 H, CH3), 7.27–7.30 (m, 6 H, aromatic
H), 7.37–7.40 (m, 4 H, aromatic H), 7.51–7.58 (m, 4 H,
aromatic H), 7.81–7.84 (m, 3 H, aromatic H), 7.93 (s, 2 H,
NH), 8.05 (s, 2 H, pyridinium-3-H), 15.62 (s, 1 H, OH).13
C NMR (CDCl3): ꢂ (ppm) ¼ 26.6 (CH3), 125.8, 128.3,
128.8, 129.3, 129.6, 129.9, 130.6, 132.4, 132.7, 133.9,
(C—NO ), 167.2 (C—OH), 170.5 (C O). C H NO
—
7
—
2
8
5
(227.1): calcd C 42.31 H 2.22, N 6.17; found C 41.90,
H 2.45, N 6.23%.
2-Hydroxy-5-nitrobenzene-1,3-dicarbonyl dichloride(10).
To a stirred suspension of 9 (4.00 g, 17.6 mmol) in dry
toluene (100 ml) was added thionyl chloride (25.0 ml,
41.0 g, 340 mmol) and stirring was continued at 80 ꢁC for
ca 8 h, until the generation of gas stopped and a light-
yellow solution was formed. Toluene and the excess
thionyl chloride were distilled off in vacuo (at 10 mbar)
—
157.4, and 157.6 (aromatic C), 161.2 (C O); not all
—
expected 17 signals are discernible. MS (FD): m/z
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SOLVATOCHROMISM OF DICARBAMOYL-SUBSTITUTED PYRIDINIUM N-PHENOLATE BETAINE DYES
689
and the residue was dried in vacuo (10ꢀ3 mbar, with
KOH) to yield 10 (4.65 g, ca 100%) as colourless solid
with m.p. 186–191 ꢁC, which should be stored under dry
dry dichloromethane (50 ml) and dry methanol (50 ml)
was added 2,4,6-triphenylpyrylium tetrafluoroborate
(2.96 g, 7.46 mmol)18 and stirring was continued at
room temperature for 2 h. Then, five drops of acetic
acid were added and stirring was continued for 5 days,
until a clear red solution was formed. The red solution
was filtered and the solvents were nearly completely
distilled off in a rotary evaporator. The liquid residue
was dissolved in acetone (20 ml) and this solution was
slowly dropped with stirring into diethyl ether (1000 ml).
The precipitate formed was filtered off and washed with
diethyl ether. The solid residue was then dissolved in
methanol, and this solution was again dropped with
stirring into diethyl ether (1000 ml). The precipitate
formed was filtered off, washed with diethyl ether and
dried. This purification procedure was repeated until a
nearly colourless, light-brown solid was observed, to
yield eventually 13 as monohydrate (1.78 g, 32%) as
fine crystals with m.p. 167–170 ꢁC. IR (KBr): v
N2 or Ar. IR (KBr): v (cmꢀ1) ¼ 3520 (OH), 1776 (C O),
—
—
1535 and 1350 (NO2).1H NMR (CD3SOCD3):
ꢂ
(ppm) ¼ 8.64 (s, 2 H, aromatic H), 14.27 (s, 1 H, OH).
13C NMR (CD3SOCD3): ꢂ (ppm) ¼ 117.6 (C—COCl),
130.3 (phenol-C-3), 136.4 (C—NO2), 163.2 (COCl),
166.7 (C—OH). MS (FD): m/z (%) ¼ 264 (100) [Mþ ].
C8H3Cl2NO5 (264.0): calcd C 36.40, H 1.15, N 5.31;
found C 36.24, H 1.34, N 5.29%.
2,6-Bis(dimethylaminocarbonyl)-4-nitrophenol (11). To a
stirred solution of 10 (3.49 g, 13.2 mmol) in dry tetra-
hydrofuran (100 ml) was added dropwise at ca 0 ꢁC
(cooling with an ice-bath) a solution of dimethylam-
monium dimethylcarbamate (Dimcarb;21 15.0 ml,
15.8 g, 120 mmol) in dry tetrahydrofuran (50 ml). Stirring
was continued for 16 h, during which the temperature
gradually rose to room temperature. The solvent was
distilled off in a rotary evaporator and the residue
was dissolved in water (ca 100 ml). The stirred solution
was acidified with HCl (c ¼ 10 cg ꢂ gꢀ1; pH ꢄ 1) and the
aqueous mixture was extracted four times with dichlor-
omethane (4 ꢃ 50 ml). The combined dichloromethane
extracts were dried with MgSO4 and the solvent was
distilled off in a rotary evaporator. To the foamy yellow
residue diethyl ether (ca 100 ml) was added and the
suspension was heated under reflux for 2 h. After cooling
to room temperature, the solid was filtered off, and dried
in vacuo (10ꢀ3 mbar), to afford 11 (2.50 g, 67%) as
colourless fine crystals with m.p. 142–144 ꢁC. IR
(cmꢀ1) ¼ 3428 (OH), 1625 (C O), 1057 (BF ). UV–
—
—
4
1
Vis (CH3CN): ꢁmax (log ") ¼ 309 nm (4.54). H NMR
(CDCl3): ꢂ (ppm) ¼ 2.41 (broad s, 6 H, CH3), 2.93 (broad
s, 6 H, CH3), 7.31 (m, 8 H, aromatic H), 7.42 (m, 4 H,
aromatic H), 7.48–7.56 (m, 3 H, aromatic H), 7.79–7.85
(m, 2 H, aromatic H), 8.02 (s, 2 H, pyridinium-3-H),
11.32 (s, 1 H, OH). 13C NMR (CD3SOCD3): ꢂ
(ppm) ¼ 34.6 (CH3), 37.5 (CH3), 124.9, 125.2, 128.2,
128.5, 128.8, 129.7, 130.0, 130.4, 132.6, 133.0, 133.2,
—
150.6, 155.6, and 156.6 (aromatic C), 166.0 (C O); not
—
all expected 18 signals were discernible. MS (FD): m/z
(%) ¼ 542 (100) [Mþ ꢀ BF4]. C35H32BF4N3O3ꢂ H2O
(629.5 þ 18.0 ¼ 647.5): calcd C 64.92, H 5.29, N 6.49;
(KBr): ꢀ (cmꢀ1) ¼ 3430 (OH), 1643 (C O), 1528 and
found C 65.14, H 5.24,
N 6.45%.
—
—
1336 (NO2). 1H NMR (CDCl3): ꢂ (ppm) ¼ 3.08 (s, 12 H,
CH3), 8.25 (s, 2 H, aromatic H), 11.7 (broad s, 1 H, OH).
13C NMR (CDCl3): ꢂ (ppm) ¼ 37.6 (CH3), 121.9 (C—
CONMe2), 125.9 (phenol-C-3), 138.8 (C—NO2), 161.2
(C—OH). MS (FD): m/z (%) ¼ 281 (100) [Mþ ].
C12H15N3O5 (281.3): calcd C 51.24, H 5.37, N 14.94;
found C 51.23, H 5.44, N 14.54%.
2,6-Bis(dimethylaminocarbonyl)-4-(2,4,6-triphenylpyridi-
nium-1-yl)phenolate (4). To a stirred suspension of
tetrafluoroborate 13 (0.90 g, 1.43 mmol) in dry dichlo-
romethane (60 ml) was added the macroreticular basic
anion-exchange resin Amberlyst A-26 (4.40 g; OH form;
Rohm and Haas). The suspension was stirred for 18 h at
room temperature, then the resin was filtered off from the
dark-violet solution and washed with dichloromethane.
The filtrate and the washings were combined and the
solvent was distilled off in a rotary evaporator. The solid
residue was washed several times with n-hexane and
diethyl ether and dried in vacuo (10ꢀ3 mbar) with
P4O10, to afford 4 as hydrate (0.58 g, 75%) as blue–violet
4-Amino-2,6-bis(dimethylaminocarbonyl)phenol (12). A
suspension of a palladium catalyst (10% Pd on charcoal;
ca 100 mg) in a solution of 11 (2.47 g, 8.78 mmol) in dry
methanol (100 ml) was reduced with dihydrogen at room
temperature and normal pressure until the necessary
amount was absorbed (ca 600 ml H2; time ca 18 h). The
catalyst was filtered off under nitrogen, the solvent was
distilled off in vacuo and the residue was dried in vacuo
(10ꢀ3 mbar), to yield 12 as a light-brown solid, which
turned dark on contact with air. Because of its sensitivity
to oxygen, 12 was immediately converted into the pyr-
idinium salt 13 without further characterization.
crystals with m.p. 272–277 ꢁC. IR (KBr): v (cmꢀ1) ¼
—
3419 (OH), 1620 (C O). UV–Vis (CH CN): ꢁ
(log
max
—
3
1
") ¼ 544 (3.42), 303 nm (4.55). H NMR (CDCl3): ꢂ
(ppm) ¼ 2.60 (s, 6 H, CH3), 2.84 (s, 6 H, CH3), 6.59 (s,
2 H, phenolate-3-H), 7.29–7.35 (m, 8 H, aromatic H),
7.55–7.60 (m, 4 H, aromatic H), 7.79–7.83 (m, 3 H,
aromatic H), 8.02 (s, 2 H, pyridinium-3-H). MS (FD):
m/z (%) ¼ 541 (100) [Mþ ]. C35H31N3O3ꢂ 2.5 H2O
(541.6 þ 45.0 ¼ 586.6): calcd C 71.66, H 6.19, N 7.16,
found C 71.75, H 5.92, N 6.91%.
1-[4-Hydroxy-3,5-bis(dimethylaminocarbonyl)phenyl]-
2,4,6-triphenylpyridinium tetrafluoroborate (13). To a
stirred solution of freshly prepared 12 (see above) in
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 682–690

¨ ¨
C. REICHARDT, A. ROCHLING AND G. SCHAFER
690
¨
17. Reichardt C, Che D, Heckenkemper G, Schafer G. Eur. J. Org.
Chem. 2001; 2343–2361.
Acknowledgements
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For financial support of this work we thank the Deutsche
Forschungsgemeinschaft, Bonn, and the Fonds der Che-
mischen Industrie, Frankfurt/Main.
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Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 682–690