Electronic structure of π-systems. Studies of electronic structures and tautomeric transformations of a series of 4-methyl-8-(R-phenylazo)dihydrofuro[2, 3-h]coumarin-9-ones

Article abstract of DOI:10.1007/s11172-010-0190-4

Tautomeric transformations of a series of 4-methyl-8-(R-phenylazo) dihydrofuro[2,3-h]-coumarin-9-ones were studied by ¹H NMR, IR, electronic absorption spectroscopy, mass spectrometry, and quantum chemistry methods. The obtained results suggest that (p-R-phenyl)-derivatives exist in the quinone hydrazone form, while (o-R-phenyl)derivatives exist as a mixture of quinone hydrazone and azo enol tautomers with domination of the former. The anti-quinone hydrazone tautomers with a strong intramolecular hydrogen bond are more favorable than the syn-quinone hydrazone tautomers.

Full text of DOI:10.1007/s11172-010-0190-4

Russian Chemical Bulletin, International Edition, Vol. 59, No. 5, pp. 960—966, May, 2010
960
Electronic structure of πꢀsystems. Studies of electronic
structures and tautomeric transformations of a series of
4ꢀmethylꢀ8ꢀ(Rꢀphenylazo)dihydrofuro[2,3ꢀh]coumarinꢀ9ꢀones
O. B. Safronova, N. L. Filippova, I. I. Saharuk†, N. A. Kondratova, and V. F. Traven
D. I. Mendeleev University of Chemical Technology of Russia,
9 Miusskaya pl., 125047 Moscow, Russian Federation.
Eꢀmail: traven@muctr.ru
Tautomeric transformations of a series of 4ꢀmethylꢀ8ꢀ(Rꢀphenylazo)dihydrofuro[2,3ꢀh]ꢀ
coumarinꢀ9ꢀones were studied by 1H NMR, IR, electronic absorption spectroscopy, mass
spectrometry, and quantum chemistry methods. The obtained results suggest that (pꢀRꢀphenyl)ꢀ
derivatives exist in the quinone hydrazone form, while (oꢀRꢀphenyl)derivatives exist as a
mixture of quinone hydrazone and azo enol tautomers with domination of the former. The
antiꢀquinone hydrazone tautomers with a strong intramolecular hydrogen bond are more
favorable than the synꢀquinone hydrazone tautomers.
Key words: coumarin derivatives, dihydrofurocoumarinones, azo coupling, tautomerism,
azo enol, azo ketone, quinone hydrazone, solvatochromism.
Scheme 1
Coumarins are widespread in nature. Many coumarin
derivatives exhibit biological activities. Particularly,
furocoumarins are used to treat the skin diseases: alopecia
circumscripta, psoriasis, mycosis, vitiligo, and a number
of other diseases.1,2 Psoralens are efficient in the photoꢀ
pheresis therapy of such diseases as dermal lymphoma,
progressive systemic sclerosis (sclerodermatitis), lupus
erythematosus, and rheumatoid arthritis.3 Here, furocouꢀ
marins can serve as both monoꢀ and bifunctional reagents
with respect to DNA of viruses and microorganisms.
Earlier,4,5 with the aim of search for new biologically
active compounds in the furocoumarin family we have
performed azo coupling of 4ꢀmethyldihydrofuroꢀ
[2,3ꢀh]coumarinꢀ9ꢀone (1) with arenediazonium salts
[RC6H4N+≡N]Cl– to synthesize 4ꢀmethylꢀ8ꢀ(Rꢀphenylꢀ
azo)dihydrofuro[2,3ꢀh]coumarinꢀ9ꢀones 2—14.
In studies of reactions and tautomeric transformations
of 4ꢀmethyldihydrofuro[2,3ꢀh]coumarinꢀ9ꢀone (1), we
found6—8 that it is smoothly halogenated to αꢀhalo derivaꢀ
tives and Oꢀacetylated, undergoes dimerization and
coupling with aldehydes and ketones. All the aboveꢀmenꢀ
tioned reactions involve both the oxygen and the carbon
atoms of the dihydrofuranone ring and in line with the
ketoꢀenol tautomerism of compound 1 (Scheme 1).
The ketoꢀenol equilibrium in the solutions of comꢀ
pound 1 was detected by means of electronic absorption
spectra (EAS). Thus, in solutions of compound 1 with
gradual change in their composition from CCl4 to EtOH,
the ketone 1A and enol 1B forms undergo interconversions
as evidenced by the existence of an isosbestic point in
EAS of the solutions with different percentage of EtOH.
The quantum chemical calculations using the semiꢀ
empirical methods MNDO, AM1, and PM3 confirm the
existence of the equilibrium between ketone 1A and enol
1B tautomeric forms; the ketone form A being more stable.
In the present work, we studied the electronic strucꢀ
tures and tautomeric transformations of substituted
4ꢀmethylꢀ8ꢀ(Rꢀphenylazo)dihydrofuro[2,3ꢀh]coumarinꢀ
1
9ꢀones 2—14 using methods of H NMR, IR, and UV
spectroscopy, mass spectrometry, their quantum chemiꢀ
cal calculations were also carried out by both the semiꢀ
empirical (the Pariser—Parr—Pople configuration interꢀ
action (PPPꢀCI) method, AM1, PM3) and nonempirical
(the selfꢀconsistent field method using the 6ꢀ31G*
(6ꢀ31GF*) basis set) methods.
Since compounds 2—14 are able to undergo both ketoꢀ
enol and azo enolꢀquinone hydrazone tautomerisms, the
†
Deceased.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 939—945, May, 2010.
1066ꢀ5285/10/5905ꢀ0960 © 2010 Springer Science+Business Media, Inc.

Dihydrofuro[2,3ꢀh]coumarinꢀ9ꢀone tautomers
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 5, May, 2010
961
Scheme 2
R = Н (2), 2ꢀМе (3), 4ꢀMе, (4), 4ꢀOМe (5), 4ꢀCl (6), 4ꢀBr (7), 2ꢀNO2 (8), 3ꢀNO2 (9), 4ꢀNO2 (10), 2,4ꢀ(NO2)2 (11), 4ꢀEt (12), 3ꢀEt (13), 2ꢀEt (14).
number of possible tautomeric forms is increased as comꢀ
pared with the starting compound 1 (Scheme 2). Theoꢀ
retically, compounds 2—14 can exist in azo keto (A),
quinone hydrazone (B) and azo enol (C) tautomeric forms.
According to 1H NMR spectra, compounds 2—14
exist only as tautomers B and C. The absence of the azo
ketone form (A) is confirmed by the absence of signals for
H(8) in the range of δ 5.35—6.56 in the 1H NMR spectra
of all examined compounds.9
fragment has the singlets at δ 2.49 and 6.41 and the
doublets at δ 8.13 and 7.36 (3J = 8.6 Hz) assigned to
the protons of the methyl group and protons H(3), H(5),
and H(6), respectively. In the lowꢀfield region, the singlet
at δ 11.05 was observed, which was assigned to proton
H(12) of the hydrazone group. It confirms that comꢀ
pound 2 exists in solution in DMSOꢀd6 in the quinone
hydrazone form B. The protons of the phenyl group
appear as a multiplet in the range of δ 6.98—7.35.
1
In the H NMR spectrum of 4ꢀmethylꢀ8ꢀphenylazoꢀ
dihydrofuro[2,3ꢀh]coumarinꢀ9ꢀone (2), the coumarin
Studies of 1H NMR spectrum of 8ꢀ(2,4ꢀdinitrophenylꢀ
azo)dihydrofurocoumarinone (11) show that the quinone
hydrazone form of this compound is in the tautomeric
equilibrium with the azo enol form (11B : 11C = 70 : 30).
1
Table 1. H NMR (DMSOꢀd6) spectroscopic data of
1
The H NMR spectrum of compound 11 contains a lowꢀ
compounds 2—14*
field signal for the proton of the hydrogenꢀbonded OH
group of the azo enol form 11C (δ 13.65) along with the
signal for the NHꢀproton of the quinone hydrazone form
11B (δ 11.20), which is characteristic of this family of
compounds. Notably, the transition from the quinone
hydrazone form 11B to the azo enol form 11C considerꢀ
ably affects only the chemical shifts of protons H(5) and
H(6) of the coumarin fragment. Thus, two sets of signals
are observed for protons H(5) and H(6) assigned to the
quinone hydrazone B and azo enol C forms of compound
11, the signals of tautomer C being shifted upꢀfield
as compared with the signals of tautomer B (Table 1). The
chemical shift of proton H(3) of the coumarin fragment
is not changed, and the chemical shifts of the protons
of the benzene fragment of tautomers B and C are almost
the same.
Comꢀ
pound
δ
Н(3)
Н(5)
Н(6)
NН
OH
2
3**
4
5
6
7
8
9
10
11
6.41
6.41
6.35
6.40
6.42
6.35
6.40
6.35
6.40
6.45
8.13
8.12
8.07
8.10
8.13
8.10
8.15
8.15
8.15
8.25
(8.05)
8.12
8.00
8.12
7.36
7.20
7.32
7.34
7.37
7.20
7.40
7.35
7.40
7.55
(7.45)
7.30
7.30
7.21
11.05
9.76
—
12.02
—
—
—
—
—
—
—
10.90
10.99
11.15
11.05
11.60
11.35
11.60
11.20
13.65
12
13
14**
6.40
6.40
6.40
11.02
11.00
9.80
—
—
12.20
The azo enolꢀquinone hydrazone tautomerism is also
observed in the case of (2ꢀmethylꢀ and 2ꢀethylphenylazo)ꢀ
* The chemical shifts of the protons of the azo enol
form are shown in parentheses.
** The chemical shifts of protons H(5) and H(6) of
the azo enol form are not shown because of low specꢀ
tral resolution (low solubility of sample).
1
dihydrofurocoumarinones (3 and 14). In the H NMR
spectra of compounds 3 and 14, the signals for the proꢀ
tons of the OH groups of the azo enol form C appear in
the lowꢀfield at δ 12.02 and 12.20, respectively, while the

962
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 5, May, 2010
Safronova et al.
signals for the NH protons are shifted upꢀfield as comꢀ
pared with the same signals of azo compounds that can
existed only in the quinone hydrazone form B and appear
at δ 9.76 and 9.80, respectively (see Table 1). Apparently,
this shift is connected with the introduction of electronꢀ
donating substituents in position 2 of the benzene ring.
tern in the electron impact (EI) mass spectra. The peaks
assigned to the R—Ph—NH—N=C fragment were present,
while in the EI mass spectra of 6ꢀ(Rꢀphenylazo)dihydroꢀ
furocoumarinones 15a—c, knowingly existing in the azo
form,5 only R—Ph—N=N fragments were found.
The quinone hydrazone form of compounds 2—14
was also confirmed by the data from IR spectroscopy. For
example, in the IR spectrum of compound 2 the band of
moderate intensity is observed at 3230 cm–1 assigned to
stretching vibrations of the Hꢀbonded NH group. The
vibration bands of the carbonyl groups of the lactone and
1
According to the H NMR data, the quinone hydrazone
tautomers 3B and 14B are prevalent and exist in equilibꢀ
rium with the azo enol forms, the ratios B : C = 70 : 30
and 60 : 40, respectively.
1
According to the data from the H NMR spectra of
compounds 4—10, 12, and 13 in DMSOꢀd6, they exist only
in quinone hydrazone form B. The characteristic signal of
these compounds is the signal for the proton of the hydraꢀ
zone group in the range of δ 10.90—11.60 (see Table 1).
furanone fragments are observed at 1690 and 1735 cm–1
,
respectively. In the IR spectrum of compound 2, no
stretching vibrations band of the hydroxyl group is present.
The unambiguous assignment of the bands observed in
the IR spectrum of compound 2 was made on the base of
a comparison with the positions of the characteristic bands
in the IR spectra of compound 2, 6ꢀ(phenylazo)derivative
15a, and 6ꢀazidoꢀ9ꢀhydroxyꢀ4ꢀmethylꢀ8,9ꢀdihydrofuroꢀ
[2,3ꢀh]chromenꢀ2ꢀone (16) (see Ref. 10).
In the IR spectrum of compound
16, a broad band of stretching vibraꢀ
tions of the hydroxyl group linked
by the intermolecular hydrogen bond
is observed at 3340 cm–1, a narrow
vibration band of the azido group is
at 2050 cm–1, and a vibration band
R = H (a), 4ꢀMe (b), 4ꢀCl (c)
of the carbonyl group of the lactone
fragment is at 1680 cm–1. The IR
That compounds 2—14 exist in the quinone hydraꢀ
zone form B is also confirmed by the fragmentation patꢀ
Table 2. The enthalpies of formation (Hf°) and relative energies (Erel) of the tautomeric forms of compounds 2—14 according to the
calculations by the AM1 and PM3 methods
Tautomer
АМ1
РМ3
Tautomer
АМ1
РМ3
—Нf°
Erel
—Нf°
ΔE
—Нf°
Erel
—Нf°
ΔE
kcal mol–1
kcal mol–1
8C
9A
9B
9C
0—
0—
0—
0—
0—
0—
0—
0—
0—
0—
0—
0—
0—
0—
0—
0—
0—
0—
38.52
45.32
46.48
40.22
46.62
46.53
40.26
49.57
50.01
44.00
51.42
51.35
47.06
52.22
51.32
46.47
47.25
47.22
41.75
6.01
1.16
0.00
6.26
0.00
0.09
6.36
0.44
0.00
6.01
0.00
0.07
4.36
0.00
0.90
5.75
0.00
0.03
5.50
2A
2B
2C
3A
3B
3C
4A
4B
4C
5A
5B
5C
6A
6B
6C
7A
7B
7C
8A
8B
4.94
3.48
1.22
15.42
10.96
3.88
12.83
11.35
6.32
43.45
41.09
36.92
11.78
10.54
3.81
0—
0.00
1.46
6.16
0.00
4.46
11.54
0.00
1.48
6.51
0.00
2.36
6.53
0.00
1.24
7.97
0—
38.92
37.01
27.20
46.82
45.63
40.14
48.51
46.65
41.86
77.50
74.94
70.73
45.48
43.82
38.82
30.80
29.35
24.29
44.53
43.29
0.00
1.91
11.72
0.00
1.19
6.68
0.00
1.86
6.65
0.00
2.56
6.77
0.00
1.66
6.66
0.00
1.45
6.51
0.00
1.24
10A
10B
10C
11A
11B
11C
12A
12B
12C
13A
13B
13C
14A
14B
14C
0—
0—
17.22
17.07
12.07
20.59
17.09
11.35
20.13
15.18
7.46
0.00
0.15
5.15
0.00
3.50
9.24
0.00
4.95
12.67
0—
0—
0—
0—
0—
0—
0—
0—

Dihydrofuro[2,3ꢀh]coumarinꢀ9ꢀone tautomers
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 5, May, 2010
963
Table 3. The enthalpies of formation (Hf°) and energies of
hydrogen bonds (EH...O) of the synꢀ and antiꢀisomers
of quinone hydrazone tautomeric form B of compounds
2—14 according to the calculations by the PM3 method
spectrum of compound 15a has the vibration bands of the
carbonyl groups of the lactone and furanone fragments
that strongly overlap and are observed at 1725 cm–1
.
A comparison of the IR spectra of compounds 2, 15a, and
16 suggests that compound 2 exists in the quinone hydraꢀ
zone form B.
The fact that compounds 2 and 4—7 easily undergo
Nꢀacetylation with acetic anhydride in the presence of
pyridine to give respective Nꢀacetyl derivatives also corꢀ
roborates the quinone hydrazone form B.
The enthalpies of formation of three possible tautoꢀ
meric forms of 4ꢀmethylꢀ8ꢀ(Rꢀphenylazo)dihydrofuroꢀ
[2,3ꢀh]coumarinꢀ9ꢀones 2—14 were calculated by the
AM1 and PM3 methods (Table 2).
The semiempirical quantum chemical calculations that
describe the state of a substance in the gas phase predict,
for compounds 2—14, higher stabilities of azo ketone A
and quinone hydrazone B tautomers. The difference in
the enthalpy of formation between tautomers A and B is
less then 3.2 kcal mol–1, and for 4´ꢀnitroꢀ (10) and 2´ꢀ
ethyl derivatives (14) it is less than 0.1 kcal mol–1, which
makes ambiguous the conclusion on the dominant tautoꢀ
meric form. In the case of 3´ꢀnitroꢀ (9) and 2´,4´ꢀdinitroꢀ
substituted (11) compounds, the quinone hydrazone tauꢀ
tomeric form B is the most stable, which is in accord with
the spectroscopic data for compounds 2—14 in solution.
At the same time, the calculations show that the enol
forms C for all compounds 1—13 are less stable (relative
energy ΔE is 6.01—11.72 kcal mol–1).
The stability of tautomeric forms is known to be largely
determined by the existence of intramolecular hydrogen
bonds.11 The energy of these bonds in tautomers 2B—14B
was calculated using the PM3 method by comparing the
enthalpies of formation of the quinone hydrazones with
(antiꢀisomers) and without (synꢀisomer) the hydrogen
bonds (Scheme 3, Table 3).
Tautomer
–Нf°/kcal mol–1
synꢀisomer antiꢀisomer
EH
...
O
/kcal mol–1
2B
3B
4B
5B
6B
7B
8B
9B
10B
11B
12B
13B
14B
36.04
45.63
45.46
74.11
42.86
29.80
40.46
43.82
45.64
46.62
50.43
50.43
45.83
37.01
42.94
46.65
74.94
43.82
30.80
43.29
46.48
46.53
50.01
51.35
51.32
47.25
0.97
–2.69
1.19
0.83
0.96
1.00
2.83
2.66
0.89
3.39
0.92
0.99
1.42
Due to the intramolecular hydrogen bond, the antiꢀ
isomers of the quinone hydrazones are more stable than
the respective synꢀisomers (except for compound 3). Acꢀ
cording to the calculations, the strongest intramolecular
hydrogen bond is the hydrogen bond in compound 11.
Two nitro groups in the benzene ring considerably
increase the acidity of the NH group, strengthen the
intramolecular hydrogen bond, thereby stabilizing the
respective quinone hydrazone form.
For compound 2, the nonempirical calculations by
the 6ꢀ31GF* method were also performed. According to
the calculations (Table 4), the antiꢀform of the quinone
hydrazone is the most stable, which agrees well with the
experimental data. One should pay attention to the optiꢀ
mized values of angles and bond lengths calculated for
tautomers 2B and 2C. These parameters indicate that the
intramolecular hydrogen bonds in molecules 2B and 2C
are possible. Table 5 presents the lengths of O—H, N—H
bonds; lengths of intramolecular hydrogen bonds O...H,
N...H; angles O—H...N, N—H...O, C—O—H, N—N—H,
and C—O...H of the pseudoaromatic ring, and distances
between the atoms of nitrogen (hydrazone and azo groups)
and oxygen (carbonyl and hydroxyl groups).
Scheme 3
Table 4. Total (Etot) and relative (Erel) energies of the
tautomeric forms of compound 2 according to
the calculations with the 6ꢀ31GF* basis set
Tautomer
–Еtot/au
Еrel/kcal mol–1
2A
1096.935738
1096.941894
1096.940705
1096.921989
3.86
0.00
0.75
2B (anti)
2B (syn)
2C
12.49

964
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 5, May, 2010
Safronova et al.
Table 5. Several interatomic distances (d) and
bond angles (ω) in tautomers 2B,C according to
the calculation with the 6ꢀ31GF* basis set
lation of EAS of different classes of compounds and
for the analysis of the structures of electronic levels in
molecules of polycycloquinones,12,13 azo compounds,14
naphthoquinones,15 and anthraquinones.16 With accurate
selection of parameters of atoms and bonds, and with
allowance for intramolecular hydrogen bonding, the
results of calculations agree well with the experimental
data. The calculations of the tautomeric forms of azo
compounds by the PPP/CI method are of particular
interest. For example, the spectroscopic differences
between hydroxy azoꢀ (A) and quinone hydrazone (B)
tautomeric forms of compound 17 were adequately preꢀ
dicted by the PPP/CI method.13
Parameter
2B
2C
Bond
d/Å
O(13)...Н(12)
N(11)...О(13)
N(11)—Н(12)
Angle
2.138
2.862
0.998
0.954
2.860
2.141
ω/deg
N(10)—N(11)—Н(12) 119.07
N(11)—Н(12)...O(13) 109.87
104.40
131.12
108.53
С(9)—О(13)...Н(12)
114.03
In addition, the tautomeric transformations in the seꢀ
ries of azo compounds 2—14 were studied by the EAS
method using solvents with different polarity (methanol,
DMSO, chloroform, and binary mixtures with different
composition). Particularly, the electronic spectra were
recorded in mixtures MeOH—CCl4 with concentration
of methanol varying from 100% to 12%.
As the composition of the solvents changed, the EAS
of compounds possessing electronꢀdonating substituents
in the benzene ring, i.e., 3—5 and 12—14, show isosbestic
points, which implies the equilibrium between tautomeric
forms of these compounds (Fig. 1).
At the same time, in the spectra of azo compound 2
and compounds 6 and 10 with electronꢀwithdrawing
groups in the pꢀposition of the benzene ring, isosbestic
points are absent, which implies the presence of only one
tautomeric form. This is confirmed by the data from
1H NMR spectra, which show that compounds 2, 6, and
10 exist in the quinone hydrazone form B. Unfortunately,
because of the extremely low solubility we failed to obtain
EAS of 2´,4´ꢀdinitroꢀsubstituted compound 11, for which
the data from H NMR spectroscopy suggest the existꢀ
ence of the tautomeric equilibrium.
The electronic absorption spectra of several tautomeric
forms of compounds 2, 5, 6, and 10 were calculated by
the PPP/CI method in the π approximation.
λ
λ
max(C6H12) = 425 nm
max = 409 nm
λ
λ
max(C6H12) = 503 nm
max = 506 nm
The results of calculations of EAS of 4ꢀmethylꢀ
8ꢀ(Rꢀphenylazo)dihydrofuro[2,3ꢀh]coumarinꢀ9ꢀones 2,
5, 6, and 10 are shown in Table 6. A comparison of the
experimental EAS and those calculated by the PPP/CI
method for antiꢀ and synꢀquinone hydrazone and azo
enol tautomers shows that these compounds exist as
quinone hydrazones in the antiꢀisomeric form. For this
tautomer, the calculated absorption maxima of comꢀ
pounds 2, 4, 6, and 10 are 429, 444, 433, and 433 nm,
respectively, which is close to the experimental data, 428,
451, 430, and 431 nm. The differences between the calcuꢀ
lated and experimental absorption maxima for the respecꢀ
tive azo enol and synꢀquinone hydrazone tautomeric forms
1
There are numerous data of the effective application
of the PPP/CI method for the quantum chemical calcuꢀ
A (arb. units)
Table 6. The experimental (λ exp) and calculated (PPP/CI)
max
0.8
(λ calc) values of the longꢀwave absorption maxima of quinone
max
hydrazone (B) and azo enol (C) tautomeric forms of compounds
2, 5, 6, and 10.
4
3
2
1
Comꢀ
pound
λ
exp/nm
(CHCl3)
λ
calc/nm
max
max
0.4
B (anti)
B (syn)
C
350
450
550
λ/nm
2
5
6
10
428.0
451.0
430.0
431.0
429.0
443.8
432.2
433.1
392.2
404.6
396.5
394.1
424.6
437.8
431.3
426.9
Fig. 1. Electronic absorption spectra of compound 12 recorded
in mixtures of MeOH—CCl4: 1 — MeOH, 2—4 — mixtures
MeOH—CCl4 with ratio 4 : 1 (2), 1 : 1 (3), 3 : 7 (4).

Dihydrofuro[2,3ꢀh]coumarinꢀ9ꢀone tautomers
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 5, May, 2010
965
f (arb. units)
HOMO
М1
A (arb. units)
424
a
392
429
1.0
0.5
0.5
М2
0.1
350
400
450
λ/nm
М1
М2
b
Fig. 2. Electronic absorption spectra of compound 2 in CHCl3.
Vertical lines show the positions of absorption maxima (λ
calc
)
max
and oscillator strength (f) for tautomers: quinone hydrazone
(B), antiꢀisomer (solid line), quinone hydrazone (B), synꢀisoꢀ
mer (dotted line), azo enol (C) (dashed line) calculated by the
PPP/CI method.
are considerably higher and, for example, in the case of
synꢀquinone hydrazones achieve 37—39 nm.
The experimental and calculated (PPP/CI) EAS of
4ꢀmethylꢀ8ꢀ(phenylazo)dihydrofuro[2,3ꢀh]coumarinꢀ
9ꢀone (2) are given in Fig. 2. This and data from Table 6
show that the quantum chemical calculations by the
PPP/CI method of the EAS of antiꢀquinone hydrazones
well reproduce both the absorption maxima and relative
band intensities.
The calculation by the PPP/CI method allows one to
make conclusion about the nature of bands in the EAS.
According to the calculations, the first band in EAS of
antiꢀquinone hydrazone tautomers B of compounds 2, 5,
6, and 10 relates to a practically singleꢀconfiguration
π—π*ꢀtransition from HOMO to LUMO. The contribuꢀ
tion of this configuration is determined by the coefficient
ϕ whose value reaches 97%. The sketches of the frontier
orbitals (HOMO and LUMO) of tautomers 2B 10B are
given in Fig. 3.
The analysis of eigenvalues of the frontier orbitals of
antiꢀquinone hydrazones 2B and 5B shows that they are
delocalized over all atoms of the molecule (Fig. 3, a). At
the same time, for tautomers 6B and 10B the most longꢀ
wave transition is accompanied by transfer of electron
density from the coumarin to arylazofuranone fragment
of the molecule (see Fig. 3, b). Figure 3 also shows the
polarization directions of the first and the second electron
transition. For all discussed antiꢀquinone hydrazone tautoꢀ
mers calculated by the PPP/CI method, the polarization
direction of the first longꢀwave transition in EAS is
almost parallel to the long axis of the molecule.
Fig. 3. The sketches of the frontier orbitals (HOMO and LUMO)
of the quinone hydrazone tautomers B of compound 2 (a) and
10 (b). The arrows show the polarization direction of the first
(M1) and second (M2) longꢀwave transition.
Thus, the obtained spectral data and results of quanꢀ
tum chemical calculations altogether allow us to state that
(pꢀRꢀphenyl)derivatives exist in the quinone hydrazone
form, while (oꢀRꢀphenyl)derivatives exist in the tautoꢀ
meric equilibrium of the quinone hydrazone and azo enol
forms (the ratio of the tautomers is 70 : 30). The nonꢀ
empirical quantum chemical calculations by the 6ꢀ31GF*
method also predict higher stability of the quinone hydrꢀ
azone form.
Experimental
4ꢀMethylꢀ8ꢀ(Rꢀphenylazo)dihydrofuro[2,3ꢀh]coumarinꢀ
9ꢀones 2—14 were prepared according to the known method.5
1
The H NMR spectra of compounds 2—14 were recorded on a
Brukerꢀ200 spectrometer in DMSOꢀd6 using Me4Si as the
internal standard. The electronic absorption spectra were
obtained on a Specord UV/VIS spectrophotometer in methanol,

966
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 5, May, 2010
Safronova et al.
DMSO, and chloroform. The tautomeric transitions were studied
in a mixture MeOН—CCl4. The IR spectra were recorded on a
URꢀ20d spectrometer in KBr pellets. Mass spectra (EI, 70 eV)
were recorded on a SSQꢀ720 (Finnigan MAT) mass spectroꢀ
meter.
The semiempirical (AM1, PM3) quantum chemical calculaꢀ
tions were carried out using MOPAC (see Ref. 17). Preliminary
geometry optimization was performed by molecular mechanics
MM+ method. The nonempirical quantum chemical calculaꢀ
tions were carried using GAUSSIANꢀ94 with the 6ꢀ31 GF*
basis set.18 The geometrical characteristics of all studied strucꢀ
tures were fully optimized by the gradient procedure.19 The
quantum chemical calculations of the EAS were performed by
the PPP/CI method in the π approximation with standard atomic
and bond parameters including intramolecular hydrogen bonding
for carbonyl, hydroxyl and amino groups.13
9. B. I. Ionin, B. A. Ershov, A. I. Kol´tsov, YaMR spektroꢀ
skopiya v organicheskoj khimii, Leningrad [NMR Spectroꢀ
scopy in Organic Chemistry], Khimiya, Leningrad, 1983,
270 pp. (in Russian).
10. I. I. Saharuk, Ph. D. (Chem.) Thesis, D. Mendeleev Univ.
of Chemical Technology of Russia, Moscow, 1999, 126 pp.
(in Russian).
11. J. March, Advanced Organic Chemistry, John Wiley and Sons,
New York—Chichester—Brisbane—Toronto—Singapore,
1985.
12. A. I. Safronov, O. B. Safronova, T. A. Chibisova, V. F.
Traven, Zh. Obshch. Khim., 1992, 62, 2616 [Russ. J. Gen.
Chem. (Engl. Transl.), 1992, 62].
13. O. B. Safronova, A. I. Safronov, T. A. Chibisova, V. F.
Traven, Zh. Obshch. Khim., 1995, 65, 439 [Russ. J. Gen.
Chem. (Engl. Transl.), 1995, 65].
14. J. Griffiths, Dyes Pigm., 1982, 3, 211.
15. M. Matsuoka, K. Takagi, H. Obayashi, K. Wakasugi,
T. Kitao, J. Soc. Dyers Colour., 1983, 99, 257.
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Received July 8, 2009;
in revised form March 19, 2010