Heterocyclic analogs of pleiadiene 76. Synthesis and tautomeric conversions of mono- and disubstituted perimidines with electron-withdrawing substituents in the naphthalene fragment

Article abstract of DOI:10.1007/s10593-010-0506-1

A series of new mono- and disubstituted perimidines with electron-withdrawing substituents in the naphthalene fragment has been synthesized. Their prototropic annular tautomerism has been investigated by 1H NMR spectroscopy.

Full text of DOI:10.1007/s10593-010-0506-1

Chemistry of Heterocyclic Compounds, Vol. 46, No. 3, 2010
HETEROCYCLIC ANALOGS OF PLEIADIENE
76.* SYNTHESIS AND TAUTOMERIC CONVERSIONS
OF MONO- AND DISUBSTITUTED PERIMIDINES WITH
ELECTRON-WITHDRAWING SUBSTITUENTS IN THE
NAPHTHALENE FRAGMENT**
I. V. Borovlev1***, A. F. Pozharskii2, E. A. Filatova2, and O. P. Demidov1
A series of new mono- and disubstituted perimidines with electron-withdrawing substituents in the
naphthalene fragment has been synthesized. Their prototropic annular tautomerism has been
investigated by 1H NMR spectroscopy.
Keywords: perimidine, PPA (polyphosphoric acid), annular prototropy, arenesulfonation, acylation.
We reported previously on the extremely slow annular prototropic tautomerism of derivatives of
2-trifluoromethylperimidine containing a formyl, acetyl, or p-toluenesulfonyl group in position 6(7) [2, 3]. This
1
19
phenomenon is expressed in the H and F NMR spectra of these compounds by the simultaneous observation of
signals of both tautomers in low polarity solvents at room temperature and even on heating to 120-130oC. It is
assumed that the reason for this is the low basicity of these compounds in combination with spatial screening of both
heteroatoms not only from the side of the 2-CF3 substituent but also from the hydrogen atoms in positions 4 and 9.
In the light of this it seemed of interest to investigate the prototropic tautomerism of 2-tert-
butylperimidine (1) and also of the product of its 6(7)-acylation. It was expected that, not being an acceptor but
being large, the tert-butyl group may hinder transfer of proton.
We obtained compound 1 by the action of pivaloyl chloride on 1,8-naphthalenediamine. On acylation
with acetic acid in PPA at 70-75oC the isomeric 6(7)- and 4(9)-diacetyl derivatives of 2-tert-butylperimidine 2
and 3 were synthesized in 66 and 11% yield respectively.
1
The H NMR spectrum of 2-tert-butylperimidine (1) in CDCl3 was unresolved. A strongly broadened
peak of the ortho protons (H-4 and H-9) and a multiplet signal of the remaining aromatic protons indicated a
somewhat slow prototropy. However, unlike other perimidines, the picture of the spectrum is practically
_______
* For Communication 75 see [1].
** Dedicated to Professor M. A. Yurovskaya on her birthday.
*** To whom correspondence should be addressed, e-mail: k-biochem-gcs@stavsu.ru.
1Stavropol State University, Faculty of Medicine, Biology, and Chemistry, Stavropol 355009, Russia.
2Southern Federal University, Faculty of Chemistry, Rostov-on-Don 344090, Russia; e-mail: apozharskii@rsu.ru.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 392-404, March, 2010. Original
article submitted September 18, 2009.
0009-3122/10/4603-0307©2010 Springer Science+Business Media, Inc.
307

CMe3
N
CMe3
NH
CMe3
NH
H
NH2 NH2
N
N
O
N
Me3CCOCl
AcOH
PPA
Me
+
1 (89 %)
2
3
O
Me
unchanged on transferring to DMSO-d6. Only on heating to 50oC was the signal of the ortho protons converted into
a broadened doublet and the fine structure of the signals of the other aromatic protons displayed (see Table 1). This
means that the nonpolar tert-butyl group hinders substantially the formation of a hydrogen bond with the NH
group proton and a solvent molecule.
1
The H NMR spectrum of 6(7)-acetyl-2-tert-butylperimidine (2) in CDCl3 in the region of 6-9 ppm is a
set of 11 strongly broadened singlets. Only the most low-field of them was double the remainder in intensity
(Table 1 and Fig. 1, a). Judging by the chemical shift this combined signal of the peri protons of tautomers 2a
and 2b is deshielded by the carbonyl group.
CMe3
CMe3
2
H
2
H
3
N
N
N 3
1 N
1
4
9
4
5
9
8
8
5
6
7
7
6
Me
O
Me
O
2a
2b
It follows from the relative intensities of the remaining singlets that in tautomeric equilibrium 2a and 2b
are in a ~1:1 ratio. Consequently, for 6(7)-acetyl-2-tert-butylperimidine in low-polarity CDCl3 the rate of
tautomeric conversion is somewhat greater than in the case of the trifluoromethyl analogs. But unlike the latter
the addition of a drop of D2O to a solution of ketone 2 in CDCl3 does not lead to acceleration of prototropy
(Fig. 1, b). Only two singlets at 8.0 and 8.1 ppm disappear from the spectrum, which permits their assignment to
the NH proton of the two tautomers.
It was somewhat unexpected that on changing from CDCl3 to DMSO-d6 the rate of proton transfer was
not increased but fell (Fig. 1, c), in difference to their 2-CF3 analogs [3], and also other derivatives of perimidine
[4]. In this case the ratio of tautomers 2a and 2b in the equilibrium mixture was 38:62, i.e. the more basic 7-
tautomer 2b predominates. In our view this is scarcely whether this is not the first stage of slow prototropy in so
polar a proton-withdrawing solvent. The relatively high basicity and low NH-acidity of 6(7)-acetyl-2-tert-
butylperimidine explains its insensitivity in the scheme of tautomerism to the presence of water and, on the other
hand, its sensitivity to the known contaminant HCl in CDCl3, which also leads, in our opinion, to some
acceleration of proton exchange (in comparison with DMSO-d6).
Since 4(9)-isomeric aldehydes, ketones, and sulfones are formed together with 6(7) derivatives on
acylation, formylation, and arene sulfonation [5-7], it seemed of interest to investigate the possibility of their
prototropic conversions, and also the degenerate tautomerism of 4,9-disubstituted analogs.
308

309

Fig. 1. 1H NMR spectra of 6(7)-acetyl-2-tert-butylperimidine (2) in the aromatic proton region: a – in
CDCl3, 25oC; b – in CDCl3 + D2O, 25oC; c – in DMSO-d6, 25oC.
Due to the formation of an extremely stable intramolecular hydrogen bond between the NH proton and
the oxygen of the carbonyl group, the 4(9)-formyl- (4, 6) and 4(9)-acetylperimidines (5, 7) in nonpolar solvents,
and judging overall, in the solid state exist exclusively in the chelated 9-CO form. This indicates that the
tautomerism in them is frozen under these conditions. A characteristic feature of such compounds is the presence
in their 1H NMR spectra in nonpolar solvents of a low-field signal of the chelated NH proton at 12-13 ppm. For
example, in the spectrum of 9-formyl-2-trifluoromethylperimidine (6) in CDCl3 it is found at 12.34 against 8.23
ppm for the peak of the nonchelated NH proton in the 6(7) isomeric analog [6].
R
R
R
H
2
1
2
2
Me
H
O
3
N
N
3
N 3
N
N
N
O
O
1
1
R1
R1
9
R1
4
5
9
9
4
4
8
8
8
5
5
7
7
6
7
6
6
8–10
a
b
4–7
4, 8 R = R1 = H; 5, 9 R = H, R1 = Me; 6, 10 R = CF3, R1 = H; 7 R = CF3, R1 = Me
The fixed (N-methylated) 4-CO forms of the same aldehydes and ketones 8-10 have a series of
characteristic special features. Firstly, the 4-CHO group proton of aldehydes 8 and 10 in CDCl3, in comparison with
the 9-tautomers 4 and 6 with fixed intramolecular hydrogen bonds, is found at far weaker field (~10.75 ppm). This
indicates that in their stable conformation the aldehydic proton is oriented to the side of the pyridine nitrogen
atom, by which it is also deshielded. In other words, if the 9-tautomer exists in the (Z)-configuration then for the
4-tautomers the (E)-isomer, in which repulsion between the unshared electron pairs of the oxygen atom and the
nitrogen atom is absent, is more beneficial.
310

R
R
H
2
2
H
R1
3
N
N 1
N
3
O
N
1
R1
4
5
9
O
9
4
8
8
5
7
6
6
7
a
b
(Z)-configuration
(E)-configuration
Secondly, the furthest low field of the aromatic protons is not the proton in the electron-deficient
position 2, but proton H-5. Finally, thirdly, the distinguishing special feature of compounds 8-10 is the high field
signal of the proton in position 9 (see EXPERIMENTAL). All this enables the 4- and 9-tautomeric forms of
compounds 4-7 to be identified reliably.
Since the proton-accepting properties of DMSO are extremely significant (pKa 0 [8]), and allowing for
the fact that it is in a large excess, this solvent may successfully compete with the carbonyl group for the NH
group proton. The truth of ruptures of the IHB assumes a preliminary bifurcational interaction of the solvent
with this proton with the formation of a three-center hydrogen bond [9, 10]. It is also known that conjugation
between the substituents in one benzene ring of the naphthalene is more efficient in comparison with
substituents located in different nuclei [11], since the 9-tautomer, even without allowing for the IHB, must
possess a high thermodynamic stability. All these factors, it might seem, do not assist the display of prototropy
for the given compounds.
In reality, a comparative analysis of the spectral data of compounds 4, 5, and 7 and their fixed 4-CO
forms 8-10 shows that if a rapid equilibrium also exists, and the absence of a meta constant of the spin-spin
interaction, then in DMSO-d6 it is strongly displaced towards the 9-tautomer (see EXPERIMENTAL). It is
interesting that even heating 4(9)-acetylperimidine (5) in DMSO-d6 to 110oC does not change the picture of the
spectrum significantly. Attention is drawn by the fact that on going from CDCl3 to DMSO-d6 the chemical shift
of the NH···O signal is changed insignificantly, but the proton in position 2 is split into a doublet in both
solvents (J2H-NH = 3.1 and 3.3 Hz respectively). The signs of annular prototropy are also absent from the
spectrum of 4(9)-acetyl-2-tert-butylperimidine (compound 3, see Table 1).
The sole exception, indicating the presence of a tautomeric equilibrium, proved to be 4(9)-formyl-
1
2-trifluoromethylperimidine (6), the H NMR spectrum of which on going from CDCl3 to DMSO-d6 became
averaged for two tautomers and unresolved due to strong broadening of all signals. Two signals were observed
for the CHO group protons at 9.90 and 10.45 ppm. If the first of them to all appearances belongs to the chelated
9-CHO group and the second to the 4-CHO group in tautomer 6b, then the ratio of tautomers 6a:6b estimated by
integral intensity is 1:3. This unexpected result may be partially explained by steric hindrance of the reverse
conversion of tautomer 6b into tautomer 6a, both from the side of CF3 and from the side of the hydrogen atom
of the CHO group in the (E)-configuration. On heating to 95oC both signals of the CHO group protons melted
into one at 10.20 ppm and the signal of the NH proton disappeared, which indicates the rapid conversion of the
tautomers into one another. It is interesting that even for the closest analog of aldehyde 6 – ketone 7 – similar
conversions were not a characteristic (see EXPERIMENTAL).
1
Previously [2] we reported that on going from CDCl3 to DMSO-d6 the H NMR spectrum of 4,9-di-
formyl-2-trifluoromethylperimidine (11) becomes symmetrical even at 25oC, which indicates the facile cleavage
of the IHB between the NH proton and the CHO group under the action of the polar solvent and to a rapid
equilibrium between the degenerate forms of aldehyde 11.
However the ease of fission of the IHB in dialdehyde 11 might have been predicted from an estimate of
the enhanced NH-acidity of this compound in comparison with aldehyde 6. It seemed of interest to compare the
prototropy of dialdehyde 11 and of 4,9-diformyl-, 4,9-diacetyl-, and 4,9-di(p-toluenesulfonyl)perimidines.
311

R
R
2
2
1
H
H
O
X
3 N
N
O
X
N 3
O
X
N
O
X
1
4
9
4
9
8
8
5
5
6
7
6
7
11 R = CF3, X = CH; 12 R = H, X = CMe; 13 R = H, X = SOC6H4Me-p
We were unable to obtain the first compound under the conditions of the Vilsmeier reaction [1]. The synthesis of
the second and third was effected on interacting perimidine with an excess of acetic acid or p-toluenesulfonic
acid respectively, at 125-135oC in PPA. In the course of the acetylation a complex mixture of substances was
formed from which, by dry flash chromatography [12] on silica gel, only diketone 12 possessing the greatest
mobility was isolated in a pure state in 5% yield. 4,9-Di(p-toluenesulfonyl)perimidine (13) was obtained
analogously in the same yield together with the previously described 4(9)- and 6(7)-monosulfones [7].
1
Fig. 2. H NMR spectrum of 4,9-diacetylperimidine (12), a – in CDCl3; b – in DMSO-d6 at 25oC; c – in
DMSO-d6 at 100oC.
As is seen from Fig. 2 the 1H NMR spectrum of the diacetyl derivative 12 is not symmetrized on going
from CDCl3 to DMSO-d6, i.e. the IHB in this case is significantly more stable than in dialdehyde 11. The signal
of the H-2 proton is displayed as a doublet with J2H-NH = 2.75 Hz, which indicates the slow chemical NH
exchange. The signals of the H-5 and H-8 protons coalesce at 70oC, and of the methyl groups at 80oC, but even
at 100oC the signal of the IHB NH···O is displayed, but the H-6 and H-7 protons give only a broadened singlet.
Since sulfones form less stable hydrogen bonds than sulfoxides [13], we expected symmetrization of the
spectrum of 4,9-di-p-toluenesulfonylperimidine (13) on going from CDCl3 to DMSO-d6. It was made clear
however that it was also asymmetric in both solvents at room temperature which indicates a slow degenerate
tautomerism in the scale of NMR time (Fig. 3). On heating a solution in DMSO-d6 the signal of the NH proton
disappeared and the spectrum mainly was simplified. However even at 110oC complete symmetrization was not
established, and two of the aromatic protons seemed to disappear (the initial spectrum was restored on cooling).
312

Fig. 3. 1H NMR spectrum of 4,9-di-p-toluenesulfonylperimidine 13, a – in CDCl3; b – in DMSO-d6 at 25oC;
c – in DMSO-d6 at 110oC.
Such a phenomenon is usually observed at intermediate rates of chemical exchange of proton in the NMR time scale.
In the absence of the 2-CF3 group fission of the IHB in the molecules of 4(9)-mono- and 4,9-dicarbonyl
or 4,9-di-p-toluenesulfonyl derivatives of perimidine requires significant energy consumption, consequently in
both polar and nonpolar solvents their degenerate prototropy becomes noticeable only at enhanced temperature.
EXPERIMENTAL
1
The H NMR spectra were described on a Bruker-250 instrument (250 MHz), internal standard was
TMS. The IR spectra were recorded on a UR-20 instrument. A check on the progress of reactions and the
homogeneity of the synthesized compounds was effected on Silufol UV-254 plates, column chromatography
was carried out on Chemapol L 40/100 silica gel, and flash chromatography on Chemapol L 5/40 silica gel.
PPA with 86% P2O5 content was obtained by the standard procedure of [14] (we reported previously
[15] on its relatively special features in comparison with standard PPA).
2-tert-Butylperimidine (1). Pivaloyl chloride (6.03 g, 50 mmol) was added with stirring to a solution of
1,8-naphthalenediamine (1.58 g, 10 mmol) in benzene (30 ml) and the mixture was stirred at room temperature
for 1 h. The solvent was evaporated in vacuum, the residue was treated with cold water (100 ml), and made
alkaline with ammonia to pH ~8-9. The precipitated oil was extracted with ethyl acetate (3×10 ml), the solution
was boiled with a mixture of silica gel and anhydrous Na2SO4, filtered, and the solvent evaporated. The yellow
oil slowly crystallized on extended trituration and cooling. Yield of compound 1 was 2.0 g (89%). Yellow-green
crystals, mp 65-66oC (petroleum ether). Found, %: C 80.66; H 7.50; N 12.21. C15H16N2. Calculated, %: C 80.32;
H 7.19; N 12.49.
313

Acetylation of 2-tert-Butylperimidine (1). A mixture of 2–tert-butylperimidine (0.45 g, 2 mmol),
glacial acetic acid (0.18 ml, 3 mmol), and PPA (5 g) was stirred at 75-80oC for 3 h, then poured into cold water
(50 ml) with vigorous stirring, and made alkaline with ammonia to pH ~8-9. A semicrystalline solid separated.
The mixture was extracted with ethyl acetate (3×10 ml), and the solvent evaporated. The residue was treated
with benzene (10 ml) and chromatographed on a column of silica gel, eluting the first and second fractions with
benzene, and then a third with ethyl acetate (all fractions were yellow in color). After distilling off the solvent
4(9)-acetyl-2-tert-butylperimidine (3) (0.6 g, 11%) was obtained from the first fraction. Yellow crystals of mp
98-99oC (petroleum ether). Found, %: C 76.36; H 6.98; N 10.16. C17H18N2O. Calculated, %: C 76.66; H 6.81;
N 10.52.
The initial 2-tert-butylperimidine (0.04 g, 9%) was isolated from the second fraction.
6(7)-Acetyl-2-tert-butylperimidine (2) (0.35 g, 66%) was isolated from the third fraction. Yellow-
orange crystals of mp 204-205oC (benzene). IR spectrum (nujol mull), , cm-1: 3313 (NH); 1633 (C=O). Found,
%: C 76.28; H 6.52; N 10.36. C17H18N2O. Calculated, %: C 76.66; H 6.81; N 10.52.
4,9-Diacetylperimidine (12). Glacial acetic acid (0.5 ml, 8 mmol) was added with vigorous stirring to a
mixture of perimidine (0.34 g, 2 mmol) and 86% PPA (8 g) heated to 100oC. The temperature was increased to
130-135oC and the mixture was stirred at this temperature for 3 h. The mixture was cooled to 85oC, poured into
water (~70 ml), and made alkaline with ammonia to pH ~8. The solution together with the resinous solid was
then extracted with ethyl acetate (3×25 ml), the extract was dried over Na2SO4, and the solvent evaporated. The
residue was dissolved in the minimum volume of benzene–ethyl acetate, 4:1, and separated by dry flash
chromatography on silica gel [12] by washing with portions (about 4-5 ml) of the same mixture to give the first
fraction (check by TLC). Yield of compound 12 was 0.025 g (5%). Light-yellow crystals, mp 241-242oC
(benzene). Found, %: C 71.33; H 4.53; N 11.06. C15H12N2O2. Calculated, %: C 71.42; H 4.79; N 11.10.
4,9-Di(p-toluenesulfonyl)perimidine (13). A mixture of perimidine (0.34 g, 2 mmol), p-toluenesulfonic
acid monohydrate (0.57 g, 3 mmol), and 86% PPA (7 g) was stirred at 140-150oC for 1.5 h, cooled to 85oC, and
poured into water. The solution was made alkaline with ammonia to pH~8, the solid was filtered off, washed
with water, and dried. Separation was made by dry flash chromatography on silica gel 5/40 [12]. The first
fraction was washed off with benzene–ethyl acetate, 4:1, the second with benzene–ethyl acetate, 1:1, and the
third with ethyl acetate–alcohol, 10:1. Compound 13 (0.046 g, 5%) was obtained from the first fraction, 4(9)-p-
toluenesulfonylperimidine (0.3 g, 46%) from the second, and 6(7)-p-toluenesulfonylperimidine (0.2 g, 31%)
from the third. The compounds from the second and third fractions were identical with samples obtained
previously [7].
Compound 13 formed pale-yellow crystals, mp 163-164oC (benzene with petroleum ether). Found, %: C
63.18; H 4.35; N 5.70. C25H20N2O4S2. Calculated, %: C 63.01; H 4.23; N 5.88.
The 1H NMR spectra of compounds 1-3 and 12, 13 are given in Table 1, and of compounds 4 [1], 5 [5],
6 [6], 7 [3], 8 [1], 9 [16], and 10 [6] in the corresponding publications. New spectral data of compounds
discussed in the present work are given below.
1
4(9)-Acetylperimidine (tautomer 5a). H NMR spectrum (CDCl3), δ, ppm (J, Hz): 2.56 (3H, s,
3
CH3CO); 6.99, 7.44 (2H, two d, AB system, J = 9.1, H-7,8); 7.11, 7.26, 7.51 (3H, three dd, ABC system,
3
4
1
3J4,5 = 7.6, J6,5 = 8.1, J = 1.1, H-4,6,5); 7.68 (1H, d, J2-NH = 3.1, H-2); 12.60 (1H, br. s, NH···O). H NMR
spectrum (DMSO-d6, 20oC), δ, ppm (J, Hz): 2.55 (3H, s, CH3CO); 6.98 (1H, br. d, 3J4,5 = 7.6, H-4); 7.08, 7.65
3
3
(2H, two d, AB system, J = 9.1, H-7,8); 7.33 (1H, br. d, J6,5 = 7.2, H-6); 7.52 (1H, br. t, H-5); 7.86 (1H, d,
1
J
2-NH = 3.3, H-2); 12.40 (1H, br. s, NH···O). H NMR spectrum (DMSO-d6, 50oC), δ, ppm (J, Hz): 2.55 (3H, s,
3
CH3CO); 7.0 (1H, m, H-4); 7.08, 7.62 (2H, two d, AB system, J = 9.1, H-7,8); 7.33 (1H, m, H-6); 7.55 (1H,
br. t, H-5); 7.87 (1H, s, H-2); 12.37 (1H, br. s, N···O).
1
4-Acetyl-1-methylperimidine (9). H NMR spectrum (CDCl3), δ, ppm (J, Hz): 2.74 (3H, s, CH3CO);
3.26 (3H, s, N–CH3); 6.29 (1H, d, 3J9,8 = 7.7, H-9); 7.11, 7.72 (2H, two d, AB system, 3J = 8.8, H-6,5); 7.18 (1H,
d, 3J7,8 = 8.3, H-7); 7.30 (1H, dd, 3J8,7 = 8.3, 3J8,9 = 7.7, H-8); 7.46 (1H, s, H-2).
314

1
4(9)-Formyl-2-trifluoromethylperimidine (tautomer 6a). H NMR spectrum (CDCl3), δ, ppm (J, Hz):
3
3
7.17, 7.38 (2H, d, AB system, J = 8.8, H-7,8); 7.27, 7.42, 7.62 (3H, three dd, ABC system, J4,5 = 7.2,
3J6,5 = 8.0, 4J <1, H-4,6,5); 9.83 (1H, s, 9-CHO); 2.32 (1H, br. s, NH).
1
4(9)-Formyl-2-trifluoromethylperimidine (tautomers 6a and 6b). H NMR spectrum (DMSO-d6,
25oC), δ, ppm (J, Hz); 6.80 (1H, br. s, H-4a,9b); 7.24 (2H, m, H-6a,b,7a,b); 7.49 (2H, m, 5a,b,8a,b); 9.90 (1H,
br. s, 9-CHO); 12.45 (1H, br. s, 4-CHO). 1H NMR spectrum (DMSO-d6, 95oC), δ, ppm (J, Hz): 7.00 (1H, br. s,
H-4a,9b); 7.22 (2H, br d, 3J = 8.8, H-7a,6b); 7.38 (2H, br. s, H-6a,7b); 7.52 (2H, m, H-5a,b,8a,b); 10.2 (1H, br.
s, 4,9-CHO).
1
(9)-Acetyl-2-trifluoromethylperimidine (7). H NMR spectrum (CDCl3), δ, ppm (J, Hz): 2.61 (3H, s,
3
3
3
CH3); 7.10, 7.52 (2H, two d, AB system, J = 9.1, H-7,8); 7.24, 7.38, 7.57 (3H, three dd, J4,5 = 7.6, J6,5 = 8.2,
4J = 0.8, H-4,6,5); 13.2 (1H, br. s, NH). 1H NMR spectrum (DMSO-d6, 20oC), δ, ppm (J, Hz); 2.62 (3H, s, CH3);
3
3
3
7.20 (1H, br. d, J4,5 = 7.3, H-4); 7.27, 7.77 (2H, two d, AB system, J = 9.1, H-7,8); 7.55 (H, br. d, J6,5 = 8.0,
H-6); 7.66 (1H, dd, 3J5,6 = 8.0, 3J5,4 = 7.3, H-5); 13.2 (1H, br. s, NH).
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