Electronic absorption spectra of pyrylium and benzodihydrochromenylium salts

Article abstract of DOI:10.1007/s10593-008-0134-1

The electronic absorption spectra of benzodihydrochromenylium, tetrahydrochromenylium, and dichloropyrylium salts were studied for the first time. It was noticed that intramolecular charge transfer takes place mainly between the substituent at position 4 of the pyrylium ring and the π-system of the cation as a whole, while the bathochromic shift depends substantially on the geometry of the molecule. It was shown that the chlorine atoms have an effect on the form of the spectra when they are introduced into the ring of the pyrylium cation and into the aryl substituents.

Full text of DOI:10.1007/s10593-008-0134-1

Chemistry of Heterocyclic Compounds, Vol. 44, No. 8, 2008
ELECTRONIC ABSORPTION SPECTRA
OF PYRYLIUM AND BENZODIHYDRO-
CHROMENYLIUM SALTS
A. M. Burov, N. V. Pchelintseva, and O. V. Fedotova
The electronic absorption spectra of benzodihydrochromenylium, tetrahydrochromenylium, and
dichloropyrylium salts were studied for the first time. It was noticed that intramolecular charge transfer
takes place mainly between the substituent at position 4 of the pyrylium ring and the π-system of the
cation as a whole, while the bathochromic shift depends substantially on the geometry of the molecule.
It was shown that the chlorine atoms have an effect on the form of the spectra when they are introduced
into the ring of the pyrylium cation and into the aryl substituents.
Keywords: 1,5-diketones, benzodihydrochromenylium salts, chlorine-substituted pyrylium salts,
intramolecular charge transfer.
Pyrylium salts exhibit extremely interesting optical characteristics, due mainly to the structure of the
cation, which contains an electronegative oxygen atom. This opens up wide possibilities for the use of pyrylium
salts as fluorescent materials, photosensitizers, organic photoconductors, and semiconductors [1].
It is known that the absorption bands in the spectra of the unsubstituted pyrylium cation at λmax 269 and
219 nm correspond to π → π* transitions. The introduction of aryl substituents at positions 2,4,6 of the pyrylium
ring complicates the form of the spectrum with strong absorption bands in its long-wave region (λmax
350-450 nm) [2, 3]. Here it is considered that these bands correspond to transitions with intramolecular charge
transfer from the aryl substituent to the cation. The substituent at the C-4 atom of the pyrylium ring has the
greatest effect on these transitions, since there are no symmetry restrictions for change of charge density at the
atoms of the π−system and electron density accumulates at the heteroatom [4, 5].
In order to determine the effect of the nature of the absorption bands and the effect of the molecular
geometry (the extent of the π−system and its nature) on their parameters we studied the electronic spectra of a
series of pyrylium, tetrahydrochromenylium, and benzodihydrochromenylium salts.
We obtained 2-phenyl-4-R-7,8-dihydrochromenylium fluoroborates 3a-c in the reaction of
propanonyltetrahydronaphthalenones 1a-c with boron trifluoride etherate in various solvents (glacial acetic acid,
acetic anhydride, and toluene).
It was found that the yield of the fluoroborates 3a-c in acetic anhydride increases to 65-67% (Table 1).
This is determined by the accepting characteristics of the acetyl cation, which promotes removal of a hydride ion
according to the general-theoretical concepts [6] from position 4 of the 2,4-R-7,8-benzo-5,6-dihydro-chromene,
as intermediate, during heteroaromatization to the corresponding fluoroborate 3. At 140°C
__________________________________________________________________________________________
N. G. Chernyshevskii Saratov State University, Saratov 410012, Russia; e-mail:
Burov_AM@rambler.ru. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1152-1159,
August, 2008. Original article submitted May 16, 2008.
924
0009-3122/08/4408-0924©2008 Springer Science+Business Media, Inc.

HClO4
Ph
Ar
Ar
Et2O·BF3
+
+
O
Ph
O
Ph
Ph
_
O O
X
2
1a–c
3a–c, 4a,b,
5a–c
Br2 / AcOH
1, 3−5 a Ar = Ph, b Ar = 4-MeOC6H4; 1,3 c Ar = 4-Me2NC6H4; 3 X = BF4, 4 X = ClO4,
5a,b X = Br; 5c Ar = C6H4−N+HMe2Br−, X = Br·5H2O
2,4-diphenyl-7,8-benzo-5,6-dihydrochromene (2) (59%) is formed together with the salt 3a, and this is probably
due to the concurrent simple dehydration of the product from semi-ketalization of the diketone 1a according to
the usual mechanism [7]. It is not possible to isolate benzohexahydrochromene, which makes it possible to link
salt formation to oxidative aromatization by the action of atmospheric oxygen according to the following
scheme:
Ph
Ph
2
1a
..
–H2O
O
Ph
O
Ph
O
OH
H
H
O
Ph
Ph
OH
Ph
O
O
H
Ph
O2
2
..
O
Ph
O
Ph
Ph
Et2O · BF3
–H2O
+
O
Ph
–
BF4
3a
The possibility of such a transformation is supported by the formation of the fluoroborates 3a,b (up to
55%) in toluene (Table 1) in the absence of hydride ion acceptors.
The respective perchlorates 4a,b were isolated during the reaction of the diketones 1a,b with perchloric
acid in acetic acid.
The bromides 5a,b and 4-(4-dimethylaminophenyl)-2-phenyl-7,8-benzo-5,6-dihydrochromenylium
dibromide 5c were synthesized by the action of bromine on propanonyltetrahydronaphthalenones 1a-c in glacial
acetic acid. According to the data from thermogravimetric analysis, compound 5c was obtained in the form of a
crystallohydrate with five molecules of water, the content of which amounts to 12% and is removed in the range
of 60-180°C. The anhydrous dibromide 5c is unstable under polythermal conditions – there is no horizontal area
indicating a constant composition on the thermogravimetric curve. In the range of 200-480°C there is a slow
925

TABLE 1. The Yields of the Products from the Reaction of the Diketones
1a-c with Boron Trifluoride Etherate and Bromine
Benzohydrochro-
Reaction
menylium Salts
Diketone
Solvent
Reagent
Yield, %
temperature,°C
3 and 5
1a
AcOH
Ac2O
Ac2O
Et2O·BF3
Et2O·BF3
Et2O·BF3
Et2O·BF3
Br2
Et2O·BF3
Et2O·BF3
Et2O·BF3
Br2
20
20
140
110
118
20
3a
3a
3a
3a
5а
3b
3b
3b
5b
3c
3c
5c
54
67
7*
51*
40
57
65
55
7
1a
1a
1a
PhMe
AcOH
AcOH
Ac2O
1a
1b
1b
1b*2
1b
1c
1c*2
1c
20
PhMe
AcOH
AcOH
PhMe
AcOH
110
118
20
Et2O·BF3
Et2O·BF3
Br2
52
55
76
110
20
_______
* Benzodihydrochromene 2 is formed in addition to the salt 3a with yields of
59 and 13% respectively.
*2 The corresponding benzodihydrochromenes were detected by chromato-
graphy.
decrease of weight as a result of the destruction of the substance and removal of the decomposition products. In
this range of temperatures there are no thermal effects on the differential thermal curve. At 480-810° a
significant exo effect for the oxidation of the free carbon and the products from decomposition of the
investigated substance is observed on the differential thermal curve.
During comparison of the absorption spectra of 2,4-diphenyl-7,8-benzo-5,6-dihydrochromenylium (4a)
and 2-phenyl-7,8-benzo-5,6-dihydrochromenylium (6) perchlorates [4], which have only one band in the long-
wave region at λmax 437 nm, it was found that the introduction of a phenyl substituent at position 4 does not
substantially affect the parameters of the band, but a new band appears at λmax 356 nm. The latter disappears in
the transition to the electronic spectrum of 4-(4-methoxyphenyl)-2-phenyl-7,8-benzo-5,6-dihydrochromenylium
perchlorate (4b), and strong absorption is only observed at λmax 430 nm. A similar pattern is observed for the
fluoroborates 3a and 3b (Table 2).
Thus, the absorption bands at λmax 356-358 nm in the salts 3a and 4a can be interpreted as bands for
intramolecular charge transfer from the π-system of the phenyl at position 4 to the cation. The absence of such
bands in the spectra of compounds 3b and 4b indicates that the methoxyphenyl fragment is not coplanar in
relation to the plane of the ring in the cation. The form of the electronic spectra of the salts 3a,b, 4a,b, and 5a,b
with various anions indicates that the counter-ion has little effect.
Theoretical calculations of the absorption bands for the investigated compounds by the semiempirical
AM1 method confirm the relationships that we discovered. It is seen that the resolved band at λmax 374 nm
(intensity 0.671) in the gas phase for compound 3b in acetic acid and methylene chloride disappears on account,
apparently, of the effect of the solvent.
A distinguishing feature of the absorption spectra of the difluoroborate 3c and dibromide 5c is the
presence of two absorption bands in the long-wave region (569 and 416 nm respectively), which belong to the
charge transfer bands and, in our opinion, reflect their structure as bications.
926

TABLE 2. The Electronic Absorption Spectra* of the Salts 3a-c, 4a,b,
5a-c, 6, 8a,b, and 10a-d
Com-
CH2Cl2
AcOH
Quantum-chemical calculation
pound*2
λ, nm
OD, А
λ, nm
OD, А
λ, nm
OD, А
3a
3b
3c
4a
437
358
291
233
1.458
1.001
0.897
1.120
430
347
287
235
1.001
0.599
0.560
0.978
402
344
288
239
0.634
0.653
0.292
0.109
443
287
236
1.776
0.990
1.229
425
285
238
1.083
0.455
1.068
397
374
285
241
0.657
0.671
0.350
0.169
566
413
293
247
1.019
0.495
0.662
1.503
429
306
257
0.663
0.503
0.123
—
—
434
356
0.695
1.200
431
355
0.725
1.240
434
356
0.695
1.200
4b
5a
430
2.525
427
2.130
—
—
438
356
292
232
0.905
0.687
0.978
1.755
438
356
292
232
0.905
0.687
0.978
1.755
—
—
5b
5c
435
288
236
1.031
0.479
0.555
435
288
236
1.031
0.479
0.555
—
—
569
416
295
240
1.020
0.557
0.756
1.480
554
412
287
236
1.017
0.705
0.930
1.615
569
416
295
240
1.020
0.557
0.756
1.480
6
437
269
219
1.625
405
296
250
0.675
0.477
0.237
—
—
8a
8b
365
256
1.738
0.845
360
250
1.913
1.088
372
333
266
0.816
0.359
0.314
416
370
258
1.413
1.423
1.206
395
365
252
1.137
1.286
0.964
392
344
262
0.876
0.394
0.296
10a
10b
409
0.226
397
0.255
408
377
0.532
0.429
467
374
320
0.478
0.280
0.495
453
359
305
0.323
0.289
0.474
415
376
327
0.515
0.540
0.179
10c
430
380
300
0.517
0.429
0.366
422
360
0.202
0.233
422
382
332
0.607
0.456
0.230
10d
420
347
0.111
0.188
357
0.342
428
399
0.648
0.423
_______
* λ = wavelength, OD = optical density.
*2 For compounds 4a,b and 5a-c the wavelengths of the salts with various
anions calculated by the quantum-chemical method are identical.
It is known that 2,6-diphenylpyrylium perchlorate has only one band in the long-wave region of the
spectrum at λmax 398 nm [1]. By comparing the spectra of the benzodihydrochromenylium salts 3a,b and 4a,b
with those for 2,4,6-triphenylpyrylium 7a (λmax 416, 362 nm) and 4-(4-methoxyphenyl)-2,6-diphenylpyrylium
(7b) [1] it is possible to observe a similar steric effect from the methoxyphenyl substituent at the C-4 atom
(λmax 422 nm).
927

In the spectra of 2,4-diphenyl- (8a) and 4-(4-methoxyphenyl)-2-phenyl-5,6,7,8-tetrahydrochromenylium
(8b) perchlorates that we have described long-wave bands are observed at 365-370 nm, but in the spectrum of
the latter a new band appears at 416 nm. Taking account of the fact that there is only one band at 376 nm in the
spectrum of 2-phenyl-5,6,7,8-tetrahydrochromenylium perchlorate [8], the band in the low-frequency part of the
spectrum of the salt 8b can be assigned to intramolecular transfer of the charge of the substituent at the C-4
atom, and the absence of this in the spectrum of 8a suggests that the phenyl substituent at position 4 is not
coplanar. Thus, the introduction of an alicyclic fragment into the hetero system alters the geometry of the
molecule and has a significant effect on the form of the spectra.
During the action of perchloric acid on the dichloropentenediones 9a-d [9] in a mixture of acetic acid
and acetic anhydride (1:1) the chlorine-substituted pyrylium salts 10a-d were isolated with yields of 74-77%:
Ar1
Ar1
HClO4
Cl
Cl
Ar
Cl
Cl
+
AcOH, Ac2O
Ar
_
Ar
O
Ar
O O
ClO4
10a–d
9a–d
9,10 a Ar = Ar1 = Ph, b Ar = Ph, Ar1 = 4-ClC6H4, c Ar = 4-ClC6H4, Ar1 = Ph,
d Ar = Ph, Ar1 = 4-MeOC6H4
It was shown that the introduction of chlorine into the pyrylium ring changes the character of the
spectrum. The long-wave band at 409 nm in 3,5-dichloro-2,4,6-triphenylpyrylium perchlorate (10a) remains in
the same position, but the band at 362 nm characteristic of the unsubstituted analog 7a disappears. Its presence
in the gas phase, according to quantum-chemical calculations (377 nm), indicates that the phenyl substituent at
the C-4 atom is not coplanar in relation to the plane of the cation on account of the steric effect of the chlorine
atoms, ruling out intramolecular charge transfer from the substituent to the cation.
The opposite behavior is observed in the spectra of 4-(4-methoxyphenyl)-2,6-diphenylpyrylium (7b)
and 3,5-dichloro-4-(4-methoxyphenyl)-2,6-diphenylpyrylium (10d) perchlorates. The retention of the long-wave
band (420 nm) and the appearance of a new band at 347 nm in the spectrum of the salt 10d favor the existence
of intramolecular charge transfer from the substituent at the C-4 atom to the cation. Here it is worth mentioning
the contribution from the chlorine atoms to conjugation, since the long-wave band in the spectrum observed in
methylene chloride and belonging to the n→π transition disappears if glacial acetic acid is used as solvent.
The introduction of a chlorine atom in the substituents at positions 2, 6, or 4 in the salts 10b,c
complicates the spectrum by the appearance of new bands in their long-wave parts (Table 2). Here, in the case of
compound 10c a low-energy transition of the n→π type can be observed in the transition to acetic acid. (The
band at 300 nm disappears.)
EXPERIMENTAL
The electronic absorption spectra of the compounds were recorded on a Akvilon SF 201 spectrometer
with c = 5·10−4 M. The IR spectra were obtained on a Specord M-80 spectrophotometer in tablets with KBr, and
the 1H NMR spectra were obtained on a Bruker AM 300 instrument (300 MHz) in deuterochloroform with TMS
as internal standard. The course of the reaction and the individuality of the compounds were monitored by TLC
928

on Silufol UV-254 plates with 3:1:1 hexane–ether–acetone as eluent. The standards used during the AM1
quantum-chemical calculations were the data from X-ray crystallographic analysis for the dihedral angles of
rotation of the substituents to the plane of the cation in 2,4,6-triphenylpyrylium perchlorate: α(C-2) = 10.4,
γ(C-4) = 18, α'(C-6) = 2.3° [10]. The differential thermal curve was recorded on the Derivatograph system of
F. Paulik, J. Paulik, and L. Erdey.
The initial diketones 1a-c [11] and the dichloropentenediones 9a-c [12] and 9d [13] were obtained by
the known methods. The fluoroborates 3a-c and bromides 5a,b were prepared in acetic acid by the method in
[11, 14], and the perchlorates 4a,b, 8a,b, and 10a-c by the method in [8, 9, 15].
4-Aryl-2-phenyl-7,8-benzo-5,6-dihydrochromenylium Fluoroborates 3a-c. A. The compounds were
prepared in acetic acid. Compound 3a, yield 54%, mp 265-267°C (reprecipitation from Me2CO with i-Pr2O)
[14]; compound 3b, yield 57%, mp 246-248°C (reprecipitation from Me2CO with i-Pr2O). [11].
1
Compound 3c. Yield 52%; mp 222-223°C (reprecipitation from Me2CO with i-Pr2O). H NMR
spectrum, δ, ppm: 2.1 (4H, s, 2CH2); 2.8 (6H, s, N(CH3)2); 6.6 (1H, s, H-3); 7.0-7.9 (13H, m, H-2'3',4',5',6' + H-
2",3",5",6" + H-11,12,13,14). Found, %: C 59.02; H 4.76; N 2.48. C27H24BF4NO. Calculated, %: C 58.63;
H 4.56; N 2.53.
B. To a solution of the diketones 1a,b (28 mmol) in acetic anhydride (40 ml) with stirring we added
drop by drop boron trifluoride etherate (5.4 ml, 42 mmol). After two days the precipitate was filtered off,
washed with i-Pr2O, and dried. We obtained compound 3a (8.04 g, 67%), mp 265-267°C (reprecipitation from
Me2CO with i-Pr2O) [14] and compound 3b (8.00 g, 65%), mp 246-248°C (reprecipitation from Me2CO with
i-Pr2O) [11].
C. To a solution of the diketones 1a-c (2.8 mmol) in toluene (10 ml) we added drop by drop boron
trifluoride etherate (0.5 ml, 4.2 mmol,), and we boiled the mixture for 3 h. The salts were precipitated with ether,
filtered off, washed with ether, and dried. From the ether mother solution we obtained compound 2 (0.12 g,
13%); mp 103-104°C (alcohol). IR spectrum (thin layer), ν, cm−1: 1247 (C–O–C). 1H NMR spectrum, δ, ppm (J,
Hz): 2.22 (2H, m, H-5); 2.69 (2H, m, H-6); 4.16 (1H, d, J3,4 = 13.2, H-4); 5.48 (1H, d, J3,4 = 13.2, H-3);
7.25-7.90 (14H, m, H–Ar). Found, %: С 88.07; Н 6.21. С25Н20О. Calculated, %: С 88.29; Н 5.95. Salt 3a, yield
0.6 g (51%), 3b 0.48 g (40%), 3c 0.64 g (55%).
4-(4-N,N-Dimethylaminophenyl)-2-phenyl-7,8-benzo-5,6-dihydrochromenylium Dibromide (5c).
To a boiling solution of the diketone 1c (7.58 g, 191 mmol) in acetic acid (40 ml) we added drop by drop over
30 min a solution of bromine (4.5 g, 1.5 ml, 28 mmol,) in acetic acid (20 ml). The solvent was distilled, the
mixture was treated with boiling ethanol and evaporated, and the crystals were filtered off and dried. We
obtained the dibromide 5c (8.3 g, 76%); mp 239-240°C (reprecipitation from chloroform with ether). According
to data from thermogravimetric analysis, the compound was isolated from the reaction mixture in the form of the
pentahydrate. IR spectrum (thin layer), ν, cm−1: 1570 (pyrylium cation), 1590 (Ar), 3400 (Н2O). Found, %:
С 51.75; Н 5.56; Вr 25.03; N 2.25. C27H24Br2NO6. Calculated, %: С 51.51; Н 5.56; Br 25.43; N 2.23.
3,5-Dichloro-4-(4-methoxyphenyl)-2,6-diphenylpyrylium Perchlorate (10d). A solution of the
diketone 9d (1.06 g, 2.5 mmol) in glacial acetic acid (15 ml), Ac2O (5 ml), and 70% perchloric acid 0.75 g (0.5
ml, 7.5 mmol,) were heated at 100°C, cooled, and diluted with ether (150 ml). The crystalline precipitate was
filtered off, washed with ether, and dried. We obtained the perchlorate 10d (0.89 g, 70%), mp 230-232°C (acetic
acid). IR spectrum (thin layer), ν, cm−1: 1610 (pyrylium cation), 1590 (Ar), 1150 (С–О–С), 1070 (С1O4), 650
(С–Сl). For analysis the perchlorate 10d was converted into the corresponding tetrachloroferrate. Found, %:
С 48.10; Н 2.61; Сl 35.09. C24H17Cl6FeO2. Calculated, %: С 47.61; Н 2.76; Сl 35.14.
The work was carried out with financial support from the Russian Fundamental Research Foundation
(grant 06-03-32667a).
929

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