Tautomeric preferences of phthalones and related compounds

Article abstract of DOI:10.1016/j.tet.2007.06.064

Multinuclear magnetic resonance and IR spectra prove that although 2-(diacylmethyl)pyridines and 2-(diacylmethyl)quinolines are β-diketones, their proton transfer product present in chloroform solution is not ketoenol but enaminone (earlier opinions were contradictory). Quinoline derivatives are less zwitterionic by character than the respective pyridyl congeners. The β-diketone form itself may also be rarely present in the solution. X-ray data show that 2-(2(1H)-pyridinylidene)-1H-indene-1,3(2H)-dione, i.e., enaminone tautomer of 2-(pyridin-2-yl)-2H-indene-1,3-dione, is also the only form present in crystal. Ab initio calculations show that the enaminone is usually more stable than other tautomeric forms. Values of geometry based aromaticity index HOMA (harmonic oscillator model of aromaticity) confirm that the zwitterionic structure really contributes to the enaminone forms detected.

Full text of DOI:10.1016/j.tet.2007.06.064

Tetrahedron 63 (2007) 9172–9178
Tautomeric preferences of phthalones and related compounds
a
b
b
a
ꢀ
Robert Dobosz, Erkki Kolehmainen, Arto Valkonen, Borys Osmia1owski and
Ryszard Gawineckia,
*
aDepartment of Chemistry, University of Technology and Life Sciences, Seminaryjna 3, PL-85-326 Bydgoszcz, Poland
bDepartment of Chemistry, University of Jyva€skyla€, PO Box 35, FIN-40014 Jyva€skyla€, Finland
Received 5 March 2007; revised 5 June 2007; accepted 21 June 2007
Available online 27 June 2007
Abstract—Multinuclear magnetic resonance and IR spectra prove that although 2-(diacylmethyl)pyridines and 2-(diacylmethyl)quinolines
are b-diketones, their proton transfer product present in chloroform solution is not ketoenol but enaminone (earlier opinions were contradic-
tory). Quinoline derivatives are less zwitterionic by character than the respective pyridyl congeners. The b-diketone form itself may also be
rarely present in the solution. X-ray data show that 2-(2(1H)-pyridinylidene)-1H-indene-1,3(2H)-dione, i.e., enaminone tautomer of 2-(pyr-
idin-2-yl)-2H-indene-1,3-dione, is also the only form present in crystal. Ab initio calculations show that the enaminone is usually more stable
than other tautomeric forms. Values of geometry based aromaticity index HOMA (harmonic oscillator model of aromaticity) confirm that the
zwitterionic structure really contributes to the enaminone forms detected.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
that only 1E, 1Z or even 1O is present in the crystalline
state.10
In chloroform solution, 2-phenacylquinolines (K) are in
equilibrium with (Z)-1,2-dihydro-2-benzoylmethylenequi-
nolines (E) (Scheme 1).1 Since proton exchange between
K and E is relatively slow, one may observe separate signals
for each tautomer in the NMR spectra.1,2
The principal aim of the present paper is to clarify which
forms are present in solution and in the solid state (crystal)
of phthalones and some related compounds, the widely
used yellow to red pigments15,16 that also have interesting
biological activities.15
O
N
2. Results and discussion
N
Ar
H
O
E
Ar
K
Attempts to prepare the desired compound may result in for-
mation of its more stable tautomer (species of comparable
stability equilibrate with each other). The compounds listed
in Scheme 2 were obtained by condensation of 2-methylpyr-
idine or 2-methylquinoline with the respective anhydrides.
Their melting points were generally in agreement with the
literature values. Beckett et al.17 found that compound 7
melted at 140 and 118 ꢀC when crystallized from ethanol
and from the hydrocarbon, respectively. Although these
two crystal forms have different IR spectra, their solution
UV–vis spectra are identical.17 These two forms are present
not only in the solid state but also in solution, where the pre-
ferred form depends upon the solvent.17 Absorptions at 325
and 370 nm are ascribed to the enaminone (7E) and ketoenol
(7O0) forms, respectively (Scheme 3), but no other data sup-
porting the nature of these tautomers are available.17
Scheme 1.
2-(Diacylmethyl)pyridines may be considered as 2-(pyridin-
2-yl)-1,3-diketones. Although 1,3-dicarbonyl compounds
(K) are usually in equilibrium with their ketoenol forms
(O),3 solutions of their pyridyl derivatives may also contain
the enaminone tautomer E (Scheme 2).
Opinions about the structure of the condensation product of
phthalic anhydride with 2-methylpyridine and 2-methylqui-
noline are quite divergent. Thus, in solution the former (1 in
Scheme 2) is believed to have a structure O,4–8 Z (a reso-
nance form of 1E)9–11 or O being in tautomeric equilibrium
with E12–14 (Scheme 2). On the other hand, X-ray data prove
Crystals of 2-methyl-2-quinolin-2-ylpropiophenones (K in
Scheme 4), the fixed tautomers of 2-phenacyquinolines,
are colorless.18 On the other hand, (Z)-1,2-dihydro-2-
Keywords: Phthalones; Tautomerism; Enaminones; Molecular structure.
* Corresponding author. Tel.: +48 52 3749070; fax: +48 52 3749005;
e-mail: gawiner@utp.edu.pl
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.06.064

R. Dobosz et al. / Tetrahedron 63 (2007) 9172–9178
9173
R2
R1
R2
R2
R1
4
R2
R1
5
6
9'
3
O
O
O
O
2
1
N
H
R3
N
R3
R1
N
H
R3
N
H
R3
8'
7
8
R4
O
R4
O
R4
O
R4
O
9
K
E
O
Z
R1 R2
R3
R4
1
2
3
4
H
H
1,2-phenylene
1,2-phenylene
1,8-naphthylene
benzo
H
H
benzo 1,8-naphthylene
2,2'-biphenylene
benzo 2,2'-biphenylene
5
6
7
8
H
H
H
H
Ph
Ph
Ph
Ph
benzo
Scheme 2.
tautomer in an unsymmetric case are always visible in the
spectrum.23–25 Due to fast proton exchange between the ke-
toenol forms, only two average signals are seen in the spec-
trum of unsymmetric b-diketone.24 It is obvious that only
one ‘diketo’ and only two ‘ketoenol’ signals should be pres-
ent in the spectrum of a symmetric b-diketone. One signal in
180–210 ppm range for a symmetric b-diketone proves that
its solution contains practically one tautomer: diketo or ke-
toenol. Although cyclic b-diketones cannot be stabilized
by the intramolecular hydrogen bond, four signals can still
be seen in the spectrum of unsymmetric cyclic b-diketone:
two of them can be assigned to the carbonyl carbon atoms
in the diketo form and two to enol and carbonyl carbon
atoms in two different ketoenol forms (fast proton exchange
takes place between them).24,26 Unique signal in 180–
210 ppm range for symmetric cyclic b-diketone proves
that solution contains practically only one tautomer: diketo
or ketoenol.26
Ph
O
N
O
H
N
H
Ph
Ph
O
O
Ph
7E
7O'
Scheme 3.
benzoylmethylenequinolines (E in Scheme 1), (Z)-1-meth-
yl-1,2-dihydro-2-benzoyl-methylenequinolines (E in
Scheme 4), and (Z) 2-(2-hydroxy-2-phenyl vinyl)pyridines
(O in Scheme 4) are brightly colored.1,2,19 Since crystals
of quinophthalone are yellow, this compound is expected
to have the 2-(2(1H)-quinolidene)-1H-indene-1,3(2H)-di-
one (2E) or 3-hydroxy-2-(quinolin-2-yl)-1H-inden-1-one
(2O in Scheme 4) structure (its tautomeric 2-(quinolin-2-
yl)-2H-indene-1,3-dione (2K in Scheme 4) form is expected
to be colorless).
In addition to the C1 and C3 signals, C2 should also be con-
sidered when discussing the proton transfer phenomena in
1,3-diketones. Three signals of C2 are expected to be present
in the 13C NMR spectrum of unsymmetric b-diketone being
in equilibrium with two different ketoenol forms: one for di-
keto form and two for two different ketoenol forms. Slow
proton exchange between diketo form and the ketoenol
forms is responsible for the presence of the C2 signal at
52–68 ppm for the diketo form.23–25 On the other hand, pro-
ton exchange between two ketoenol forms is fast on the
NMR timescale and this results in the presence of only
one average C2 signal at 96–115 ppm in the spectrum of un-
symmetric b-diketone.24,25 One C2 signal for the diketo
form23,25 and one signal for C2 of the ketoenol form25 are
characteristic for the spectra of symmetric b-diketones.
Unique signal for C2 in the spectrum of symmetric
b-diketone proves that only one tautomeric form (diketo or
ketoenol) is practically present in solution.25 On the other
hand, there are still two signals of C2 present in the spectrum
of unsymmetric cyclic b-diketones: one for the diketo (at
ca.59 ppm24) and one for two different ketoenol forms (at
ca. 109 ppm24,26). If the solution of a symmetric cyclic
b-diketone contains practically one tautomer, only one sig-
nal (either at 44–60 ppm for the diketo or at 107–112 ppm
for the ketoenol form26) is present in the spectrum. One
O
O
N
H
N
Ar
N
Me
Ar
O
O
Ar
O
K
E
O
O
N
N
N
H
H
O
O
O
2E
2O
2K
Scheme 4.
Numerous b-dicarbonyl compounds are susceptible to the
proton transfer.3 The NMR spectroscopy is very useful for
identifying which species are present in the tautomeric mix-
ture.1,2,20–22 Six signals are expected to be present in the
180–210 ppm range of the 13C NMR spectrum of the unsym-
metric 1,3-diketone being in equilibrium with two different
ketoenol forms. Two of them can be easily assigned to car-
bonyl C1 and C3 in the diketo form and four to the enol
and carbonyl C1 and C3 in two ketoenol forms. Since proton
exchange between diketo and ketoenol forms is relatively
slow (on the NMR timescale), two signals of the former

9174
R. Dobosz et al. / Tetrahedron 63 (2007) 9172–9178
should bear in mind that chemical shift of C7 (for numbering
of the positions in the molecule, see Scheme 2) for the
enolimine and enaminone tautomeric forms of 2-phenacyl-
pyridines or 2-phenacylquinolines, respectively, vary in the
range of 91–97 ppm2 and 89–90 ppm,1 respectively.
are comparable with those typical for (Z)-1,2-dihydro-2-
benzoylmethylenequinolines (E in Scheme 1): d(13C7)¼
88.9–90.3 ppm, d(13C8)¼181.7–184.9 ppm.1 Thus, one
may conclude that only 1E form (Scheme 2) is present in so-
lution. It is noteworthy that the same tautomeric form is pres-
ent in solution of the respective quinoline derivative 2
(99.0 ppm (C7), 194.3 ppm (C8), and 190.2 ppm (C80)
(Table 1)). Two carbonyl carbon signals in the 13C NMR
spectra of compounds 1 and 2 (Table 1) confirm that their
molecules have the E character. Yavari et al.29 reported
that the 13C NMR spectra of 2 recorded at various tempera-
tures show an interesting behavior: some signals coalesce as
a result of rotation around the C2–C7 ‘double’ bond in 2E.
This phenomenon is possible due to increased contribution
of the zwitterionic structure 2Z (Scheme 2).29 As a result
of fast rotation and high contribution of the zwitterionic
structure, the signals of carbonyl carbon atoms may not be
observed in the 13C NMR spectra of quinophthalone re-
corded at higher temperatures.29 The presence of only one
13C80 signal in the NMR spectra of compounds 3, 4, and 7
(Table 1) proves that coalescence temperature for them is
lower than 303 K (the spectra were recorded just at that tem-
perature). Of course, there are no H7 signals in the 1H NMR
spectra of both 1 and 2. As shown by the chemical shift of
H1, 3 and 4 contain the strongest and 1 and 2 the weakest hy-
drogen bonds, respectively. One may see that 1H1 and 15N1
NMR signals in the spectra of pyridine derivatives 1, 3, and 7
are deshielded with respect to those in the spectra of quino-
line derivatives 2, 4, 6, and 8. This suggests that electron
density at N1 in 1, 3, and 7 is lower than that in 2, 4, and
8, respectively. Thus, one may see that 1, 3, and 7 are
more zwitterionic by character than 2, 4, and 8 (quantitative
effect). It seems worthy to mention that benzo annulation of
similar compounds may have a qualitative effect on the spe-
cies present in solution: 2-phenacylpyridines and 2-phen-
acylquinolines equilibriate with (Z) 2-(2-hydroxy-2-phenyl
vinyl)pyridines and (Z) 1,2-dihydro-2-benzoylmethylene-
quinolines, respectively.1,2
The chemical shift of the enol carbon atom (C1 or C3) in the
13C NMR spectrum of unsymmetric cyclic b-diketone
(6,7,8,9-tetrahydro-5H-benzocyclohept-5,7-dione) is at
163.70 ppm.24 It is noteworthy, that a similar valuewas found
for the enol carbon atom (ca. 162 ppm) for the enolimine
tautomeric form of 2-phenacylpyridines.2 The chemical shift
of the enol carbon atom for acyclic ketoenol forms of
b-diketones is >180 ppm,24 which is very similar to those
of the carbonyl carbon atoms in the spectra of enaminone tau-
tomeric forms of 2-phenacylquinoline (ca. 183 ppm).1
The ketoenol form of acyclic 1,3-diketone such as O0 in
Scheme 5 may be stabilized by intramolecular hydrogen
bond of the OH/O type. On the other hand, OH/N hydro-
gen bond can be present in 2-pyridyl substituted ketoenol
forms of both cyclic and acyclic 1,3-diketones (see O, i.e.,
enolimine form in Scheme 2). Among the compounds stud-
ied in the present paper, only 7 and 8 can have the O0 form.
The presence of a broad singlet at ca. 18 ppm in the 1H NMR
spectrum of 4-(quinolin-2-yl)dimedone (R1/R2¼benzo, R3/
R4¼CH2CMe2CH2 in Scheme 2) detected by Yousif et al.6
shows that E is the only form present in chloroform solution.
R2
R1
R3
N
O
H
R4
O'
O
Scheme 5.
The literature6 1H NMR spectrum of 2-phenyl-1,3-cyclohex-
anedione 9 (Scheme 6) shows that its chloroform solution
contains practically only the ketoenol form. The presence
4
9'
3
2
O
5
6
1
O
of the H7 singlet at 4.34 ppm in the H NMR spectrum
7
8'
and integration of the signals shows that solution of 2-phen-
ylindanedione 10 (Scheme 6) in CDCl3 contains only the
diketo form.27 Earlier studies26 show that there are two im-
portant 13C signals in the NMR spectrum of 10 in chloro-
form: 59.8 ppm (C7) and 198.1 ppm (C8 and C80). Thus,
there is no doubt that only diketo form 10 is present in solu-
tion. On the other hand, positions of C7, C8, and C80 signals
at 97.5 ppm, 194.0 ppm, and 190.2 ppm, respectively, pres-
ent in 13C NMR spectrum of 1 in the same solvent (Table 1)
1
8
O
9 O
9
10
Scheme 6.
Only one or none C80 signal is seen in the spectra of com-
pounds 3, 4, 6, and 7 (Table 1). One may suppose that in-
creased contribution of the zwitterionic structure such as
Table 1. Selected experimental NMR chemical shifts (d) for 0.1–0.2 M solutions of 1–8 in CDCl3 at 303 K
N1
C2
C7
97.5
99.0
104.4
104.9
112.9
105.7
106.2
C8
C80
C3
H1b
1
2
3
4
6
7
8
ꢁ221.6
151.3
150.7
155.9
156.2
194.0
194.3
190.2
190.2
182.1
120.8
118.9
124.6
122.5
120.1
14.72
14.13
19.80
19.07
18.10
18.04
17.08
ꢁ240.0
a
ꢁ184.9
a
a
a
ꢁ204.6
183.6
a
ꢁ220.0
152.8
a
ꢁ179.5, ꢁ67.0
193.4
192.7
120.7
a
ꢁ230.8
154.5
197.6
a
Not observed.
Singlet.
b

R. Dobosz et al. / Tetrahedron 63 (2007) 9172–9178
9175
2Z (Scheme 2), that enables fast rotation around the C2–C7
bond in their molecules,29 is responsible for the observed
feature of their NMR spectra.
at 1735 cmꢁ1 shows that this solution also contains low
amount of the diketo form.31 On the other hand, Gren
et al.31 found that intensive bands at 1705 and 1732 cmꢁ1
and weak absorption at 1609 cmꢁ1 in the IR spectrum of 2-
phenyl-1,3-cycloheptanedione 12 prove that its 1,2-dichloro-
ethane solution contains the diketo form and an insignificant
amount of ketoenol.
Unexpectedly, the position of the N1 signal in 15N NMR
spectrum of (Z)-1,2-dihydro-2-benzoylmethylenequinoline
(Scheme 7, R¼H, d(15N)¼ꢁ228.7 ppm1) is comparable
with that of 1,3-diphenyl-2-(quinolin-2(1H)-ylidene)pro-
pane-1,3-dione (Scheme 7, R¼COPh, d(15N)¼ꢁ230.8 ppm
(Table 1.)). This shows that (i) no ketimine (K) or enolimine
(O) forms are present in solutions of compounds 1 and 2 (see
Scheme 2 to identify the tautomers) and (ii) enaminone (E)
structure of the tautomer detected is unquestionable. On the
other hand, 15N1 signals of 3 and 4 are deshielded when
compared with those of 1 and 2, which shows that electron
density at N1 in former compounds is low. Significant zwit-
terionic character of the former compounds is responsible
for the disappearance of one signal of carbonyl carbon
atom and shielding of the signal of another carbonyl carbon
atom in their spectra (Table 1). The contribution of the zwit-
terionic structure is extremely high for 3. When comparing
the d(15N1) values, one may also see that contribution of
the E structure in solutions of 2 and 4 is higher than that
in solutions of 1 and 3, respectively. d(1H1) values also
prove high contribution of the Z structure in 3 and 4
(d(1H1)>18 ppm shows that zwitterionic character of the
molecule is significant: positive charge at N1 deshields
this proton). d(1H1)<15 ppm suggests that the molecule
has the enaminone character (it is the case for 1 and 2).
O
O
O
11
O
12
Scheme 8.
The presence of the bands at 1600, 1700, and 1720 cmꢁ1 in
the IR spectrum of chloroform solution of 2-phenylindane-
dione 1028 shows that only diketo form is present there. Chlo-
roform solution of 4-(pyridin-2-yl)dimedone (R1¼H¼R2,
R3/R4¼CH2CMe2CH2 in Scheme 2) absorbs at 1625, 1650,
1708, and 1741 cm .
ꢁ1 32 The presence of the two latter bands
shows that the diketo form is present in solution.32
Irrespective of O (Scheme 2) or O0 (Scheme 5) character of
the ketoenol, similar bands can also be seen in the IR spectra
of compounds 1–4 and 6–8 (Table 2). Conjugation of the
ketone carbonyl group with the aromatic part of the mole-
cule is responsible for the absorption at 1670 and
1677 cmꢁ1 for 1 and 2, respectively.30 The bands at ca.
1710 cmꢁ1 for 7 and 6, and at 1740 cmꢁ1 for 3 are the results
of symmetric and antisymmetric stretching vibrations in
dicarbonyl compounds.33,34
R
N
H
O
Ph
X-ray diffraction allows the determination of the molecular
structure in the crystal state. The ORTEP plot (Fig. 1) shows
that the 1E tautomer is preferred not only in solution. The
bond lengths as well as valence and dihedral angles of the
molecule are listed in Table 3. One may see that our data, es-
pecially bond lengths, are more or less different from those
Scheme 7.
15N1 chemical shifts (ꢁ179.5 ppm) and 13C7 (ꢁ105.7 ppm)
of 7 show that the E form is present in solution. On the other
hand, signals at ꢁ67.0 ppm (15N1) and 65.9 ppm (13C7)
prove that it contains also the K form.1,2
Table 2. Positions [cmꢁ1] of C]O stretching bands in the IR spectra of
compounds 1–4 and 6–8 (solutions in chloroform)
Exceptionally large deshielding of N1 in compounds 3 and 7
(see the respective 15N NMR shifts in Table 1) suggests
a very low electron density at the nitrogen atom. This signif-
icant deshielding of N1 signals in the spectra of 3 and 7 is
indicative of a high contribution of the zwitterionic structure
to the molecular character of these compounds.
1
2
3
4
6
7
8
1630, 1670
1627, 1677
1637, 1740, 1779
1636
1634, 1710
1620, 1695, 1789
1636
The IR spectra may also help to distinguish the species pres-
ent in the tautomeric mixture. The 1580–1800 cmꢁ1 region
seems to be very important in studies on keto–enol tauto-
merism of b-diketones.30 Strong bands at 1639 and
1695 cmꢁ1 and weak absorption at 1735 cmꢁ1 in the IR spec-
trum of 2-phenyl-1,3-cyclopentanedione 11 (Scheme 8) in
1,2-dichloroethane (there are no other bands in the 1580–
1800 cmꢁ1 region) prove that ketoenol predominates in solu-
tion.31 On the other hand, only ketoenol form is present in
chloroform solution of this compound (it absorbs at
1607 cmꢁ1 only).27 The same tautomer is a major species
present in 1,2-dichloroethane solution of 2-phenyl-1,3-
cyclohexanedione 9 (Scheme 6) (proved by strong bands at
1630 and 1655 cmꢁ1 in its IR spectrum).31 Weak absorption
Figure 1. ORTEP-3 plot of the crystal structure of 1E. The thermal
ellipsoids are drawn by 50% probability level.

9176
R. Dobosz et al. / Tetrahedron 63 (2007) 9172–9178
Table 3. Selected X-ray bond lengths, interatomic distances [pm] and valence and torsion angles [deg] in 1E
N1–C2
C2–C3
C3–C4
C4–C5
C5–C6
N1–C6
C2–C7
C7–C8
C7–C80
C8–O9
C80–O90
N1–H1
H1/O9
N1/O9
136.4(2)
140.8(3)
137.2(3)
139.9(3)
136.0(3)
134.4(3)
141.9(3)
143.3(3)
144.4(3)
125.0(2)
123.6(2)
91.1(15)
199.5(18)
273.7(2)
N1C2C3
C2C3C4
C3C4C5
C4C5C6
C5C6N1
C6N1C2
N1C2C7
C3C2C7
C2C7C8
C2C7C80
C7C8O9
C7C80O90
C8C7C80
N1H1O9
116.09(18)
120.60(18)
120.45(19)
118.32(19)
120.50(19)
124.00(17)
118.05(16)
125.85(16)
123.96(16)
126.97(17)
127.85(18)
129.45(18)
108.96(17)
137.50(17)
N1C2C3C4
N1C2C7C8
N1C2C7C80
C3C2C7C8
C3C2C7C80
C6N1C2C3
C6N1C2C7
C2N1C6C5
N1C6C5C4
C7C2C3C4
O9C8C7C2
O90C80C7C2
O9C8C7C80
O90C80C7C8
2.2(3)
ꢁ1.1(3)
ꢁ176.8(2)
177.4(2)
1.8(3)
ꢁ1.9(3)
176.8(2)
0.3(3)
1.0(3)
ꢁ176.3(2)
3.9(3)
ꢁ0.9(3)
ꢁ179.8(2)
ꢁ177.1(2)
measured by Kemme.10 As expected, the molecule is practi-
cally planar, the angle between the planes of the pyridine and
indandione ring fragments being 5.75(9)ꢀ. C80–O90 bond is
shorter (it has more double bond character) than C8–O9.
The C7–C8 and C7–C80 bonds are significantly shorter
than typical C–C bonds. On the other hand, C2–C7 bond is
much longer than typical C]C bonds. Thus, Z structure
contributes considerably to enaminone form present in the
crystal (it is stabilized by the strong intramolecular hydrogen
bond).
delocalization of p electrons in the quasiring B in the pyri-
dine and quinoline tautomers E (HOMA¼0.62–0.73 and
0.61–0.70, respectively) prove that intramolecular hydrogen
bond, which stabilizes this form, is of RAHB type (reso-
nance assisted hydrogen bond).35–39 As proved by the
HOMA values, p electrons are also effectively delocalized
in the quasiring C of the same tautomers. One may see
that HOMA values for rings A, B, and C in 1E based on
X-ray geometries are higher than those based on optimized
geometries (Table 4). Thus, the intramolecular interactions
in crystal increase the aromatic character of 1E molecule.
Geometrical requirements of the five-membered ring in 1E,
rather than number of carbonyl groups in the molecule, are
responsible for the length of intramolecular hydrogen
bond: H1/O9 distance in 1E is 5.5–33.5 pm longer than
that in (Z)-1,2-dihydro-2-benzoylmethylenequinoline (E in
Scheme 1).1 The molecules in the dimer are attracted to
each other by intermolecular NH/O]C hydrogen bonds.
HOMA values for the K tautomers based on optimized geo-
metries are also listed in Table 4. The ring A in respective
pyridine derivatives is fully aromatic (HOMA¼1.00) and
this in quinoline derivatives has HOMA¼0.84. Quasirings
B and C are, of course, non-aromatic both in pyridine and
quinoline derivatives K.
The bond lengths of tautomeric forms can be used to esti-
mate the geometry-based aromaticity index HOMA (har-
monic oscillator model of aromaticity)35 defined as in Eq. 1:
The calculations show that 5O and 7O are slightly more sta-
ble than 5E and 7E, respectively, and 3O is only slightly less
stable than 3E (Table 5). The NMR spectral data (Table 1)
show, however, that no O forms are present in solutions of
n
X
ꢀ
ꢁ
1
2
Table 4. HOMA values (B3LYP/6-311G(2d,p)) based on optimized geo-
metries for compounds 1–8
HOMA ¼ 1 ꢁ
ai Ropt;i ꢁ Rj
ð1Þ
n
j¼1
H
O
A
N
H
C
where n represents the total number of bonds in the molecule,
ai is a normalization constant (for CC, CO, and CN bonds
aCC¼257.7, aCO¼157.38, and aCN¼93.52, respectively). It
is fixed to give HOMA¼0 for a model non-aromatic system,
B
O
Ring A
Ring B
0.73 (0.78)a
Ring C
0.52 (0.67)a
ꢀ
e.g., Kekule structure of benzene and HOMA¼1 for the sys-
tem with all bonds equal to the optimal value Ropt,i, assumed
to be realized for fully aromatic systems. For C–C bonds,
Ropt,C–C¼138.8 pm, for CN bonds Ropt,C–N¼133.4 and for
C–O is Ropt,C–O¼126.5 pm. The higher the HOMA value,
the more aromatic is the ring in question, and hence, more
delocalized the p electrons of the system.
1
2
3
4
5
6
7
8
E
K
E
K
E
K
E
K
E
K
E
K
E
K
E
K
0.86 (0.92)a
1.00
0.72
0.84
0.90
1.00
0.76
0.84
0.88
1.00
0.74
0.84
0.86
1.00
0.72
0.83
ꢁ1.70
ꢁ1.69
0.70
ꢁ1.79
0.62
0.42
ꢁ1.75
0.38
ꢁ1.54
ꢁ1.54
0.61
ꢁ1.71
0.64
0.28
ꢁ1.75
0.26
HOMA values based on optimized geometries for the ring A
(Table 4) in the pyridine and quinoline derivatives of the E
type are equal to 0.86–0.90 and 0.72–0.76, respectively. Sig-
nificant contribution of the zwitterionic structure probably
results in the relatively high aromaticity of the ring A. Pres-
ence of the benzene ring in the quinoline derivatives 2, 4, and
8 results in low contribution of the Z structure as compared
to this in the pyridine derivatives 1, 3, and 7. Effective
ꢁ1.72
ꢁ1.75
0.66
ꢁ1.73
0.68
0.17
ꢁ1.82
0.19
ꢁ1.76
ꢁ1.77
0.68
ꢁ1.78
0.08
ꢁ1.86
a
Values based on X-ray geometries.

R. Dobosz et al. / Tetrahedron 63 (2007) 9172–9178
9177
Table 5. Calculated (B3LYP/6-311g(2d,p)) relative energies [kJ/mol] in
vacuum for different tautomers 1–8 (with respect to the E form)
a 1 mm diameter capillary inserted coaxially inside the
5 mm NMR tube. All NMR acquisition and processing
parameters are available from E.K. on request.
O
K
1
2
3
4
5
6
7
8
15.82
54.83
70.94
74.58
78.59
48.47
59.64
5.12
IR spectra were recorded on a Bruker Vector 22 FTIR spec-
trophotometer with samples at room temperature as solu-
tions in chloroform using KBr cell of 0.25 mm thickness.
The concentration of solutions was chosen to give absorp-
tion in the 75–85% range.
29.34
0.93
9.78
ꢁ0.25
8.83
ꢁ0.97
10.27
18.75
Crystals of compound 1 were obtained by slow evaporation
of CDCl3 from NMR tube. The X-ray crystallographic data
were collected with Nonius Kappa CCD diffractometer,
using graphite monochromatised Mo Ka radiation (l¼
71.073 pm) and temperature of 173ꢂ0.1 K. The CCD data
were processed with DENZO-SMN v0.93.048 and all struc-
tures were solved by direct methods, using SHELXS-97,49
and refined on F2 by full-matrix least-squares techniques
with SHELXL-97.50 The hydrogen atoms, except N–H,
were calculated to their idealized positions with isotropic
temperature factors, Uiso(H)¼1.2Ueq(C), and refined as rid-
ing atoms. H bonded to N was found from the difference
Fourier map and fixed to its ideal distance from the parent
3 and 7. Presence of the low field signal at ꢁ67.0 ppm in the
15N NMR spectrum of 7 is not unexpected: since 7K is only
5.12 kJ/mol less stable than 7E (Table 5), the former tauto-
mer is present in low amount in chloroform solution.
3. Conclusions
2-(2(1H)-Pyridinylidene)-1H-indene-1,3(2H)-dione with
zwitterionic character is the only species present in
chloroform solution (it is not contaminated by 2-(pyridin-
2-yl)-2H-indene-1,3-dione and 3-hydroxy-2-(pyridin-2-yl)-
1H-inden-1-one). The enaminone form is also the only
tautomer found in solutions of the related condensation
products of 2-methylpyridine and 2-methylquinoline with
1,8-naphthalic, 2,20-diphenic and benzoic anhydrides (low
amount of 1,3-diphenyl-2-(pyridin-2-yl)propane-1,3-dione
was detected only in solution of 1,3-diphenyl-2-(pyridin-
2(1H)-ylidene)-propane-1,3-dione). The contribution of the
zwitterionic structure in 2-(2(1H)-pyridinylidene)-1,3(2H)-
diones is more distinct than the respective quinoline deriva-
tives. 2-(2(1H)-Pyridinylidene)-1H-indene-1,3(2H)-dione is
the only tautomer present also in the crystal state. Values of
the geometry-based aromaticity index HOMA show that the
zwitterionic structure contributes significantly to the charac-
ter of the enaminone form.
˚
atom (0.91 A at 173 K), with isotropic temperature factor
Uiso(H)¼1.2Ueq(N). Absorption correction was not used.
The figure was drawn with ORTEP-351 and MERCURY.52
Crystallographic data have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary pub-
lication number CCDC-638269. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK [fax: +44 1223 336033
or e-mail: deposit@ccdc.cam.ac.uk].
Full geometry optimizations have been carried out using the
GAUSSIAN software package.53 Structural computations
were performed using the B3LYP theory. The 6-
311G(2d,p) basis set was used. The frequency calculations
were performed to make sure that geometry is in the global
minimum.
4. Experimental
Acknowledgements
Anhydrous zinc chloride (0.2 g) was added to a solution of
2-methylpyridine or 2-methylquinoline (0.02 mol) and
phthalic, 1,8-naphthalic, diphenic or benzoic anhydride
(0.02 mol) in nitrobenzene (25 mL) and the obtained mix-
ture was refluxed for 4–6 h. The contents of the flask were
cooled down and the precipitated product recrystallized
from ethanol (yield: 36–39%). Melting points are as follows
[ꢀC]—1: 325–326 (lit. 290–292,4 295–296,40 286–28841), 2:
241–242 (lit. 242–243,40 239–241,41 241–242,42 238–23943),
3: 262–265 (lit. 268–26944), 4: 254–255 (lit. 254–255,44
265–26642), 6: 233–235 (lit. 228–22947), 7: 135–137 (lit.
140,17 141.545), 8: 189–191 (lit. 189–19046).
The authors are grateful to Spec. Lab. technician Reijo
Kauppinen for his help in running the NMR spectra. We
are very much indebted to the Academic Computer Centre,
ꢀ
CI TASK, Gdansk, for supply of computer time and provid-
ing programs.
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