New type of prototropic tautomerism involving carbon, hydrogen, and oxygen atoms

Article abstract of DOI:10.1023/B:RUJO.0000036062.82821.ab

A low-molecular weight analog of the vinyl chloride-maleic anhydride copolymer, chloromethylsuccinic anhydride gives rise to a new type of prototropic tautomerism, cyclic anhydride-enol, with formation of quasiaromatic enol and dienol derivatives. The ratio of the anhydride and enol tautomers in solution at room temperature is 7:1.

Full text of DOI:10.1023/B:RUJO.0000036062.82821.ab

Russian Journal of Organic Chemistry, Vol. 40, No. 4, 2004, pp. 462–466. Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 4, 2004,
pp. 494–497.
Original Russian Text Copyright © 2004 by Filimoshkin, Kosolapova, Petrenko, Aksenov, Poleshchuk.
New Type of Prototropic Tautomerism
Involving Carbon, Hydrogen, and Oxygen Atoms
1
2
2
A. G. Filimoshkin , V. F. Kosolapova , T. V. Petrenko ,
3
4
V. S. Aksenov , and O. Kh. Poleshchuk
1
Tomsk State University, pr. Lenina 36, Tomsk, 634050 Russia
e-mail: poly@ipc.tsc.ru
2
Institute of Petroleum Chemistry, Siberian Division, Russian Academy of Sciences, Tomsk, Russia
3
Kursk State Technical University, Kursk, Russia
4
Tomsk Pedagogical University, Tomsk, Russia
Received April 27, 2003
Abstract—A low-molecular weight analog of the vinyl chloride–maleic anhydride copolymer, chloro-
methylsuccinic anhydride gives rise to a new type of prototropic tautomerism, cyclic anhydride–enol, with
formation of quasiaromatic enol and dienol derivatives. The ratio of the anhydride and enol tautomers in
solution at room temperature is 7:1.
While studying the properties of vinyl chloride–
maleic anhydride copolymer, we have revealed and
described a new type of prototropic reactions, namely
cyclic anhydride–enol tautomerism, which spontane-
ously occurs in the macromolecules in organic solvents
We presumed that, like vinyl chloride–maleic
anhydride copolymer [4], compound I in solution gives
rise to cyclic anhydride–enol tautomerism and that
the tautomeric equilibrium involves anhydride form
Ia, enol Ib, and dienol Ic, as shown in Scheme 1.
Tautomers Ib and Ic are stabilized via intramolecular
hydrogen bond O–H···Cl. The second O –H group in
tautomers Ic is involved in intermolecular hydrogen
bond with the solvent molecules.
[
1]. By physical and chemical methods we have found
1
that the copolymer molecules in the condensed state
consist of 67 mol % of alternating monomer units and
3
3 mol % of the corresponding enol tautomers [2]. Just
the presence of such tautomeric ensembles in the
polymeric chains is responsible for the anomalous
reactivity of the copolymer [3]. It was important to
elucidate whether the observed pattern originates from
conformation–configuration effects in the macromole-
cules or it is also intrinsic to the respective low-
molecular analogs. Therefore, the goal of the present
study was to synthesize chloromethylsuccinic anhy-
dride (I) as a low-molecular model of the regular unit
of the vinyl chloride–maleic anhydride copolymer and
examine its ability to undergo prototropic tautomerism
of the above type.
In order to estimate the possibility for such
tautomeric equilibrium, we performed quantum-
chemical calculations of the geometric structure and
energies of formation of tautomers Ia–Ic (see table).
The results showed that the formation of tautomers Ib
and Ic is accompanied by reduction of the electronic
energy by 9 kJ/mol, which corresponds to the energy
of stabilization due to intramolecular hydrogen
bonding H···Cl. The H···Cl distances in Ib and Ic
conform to the structural criterion for closure of such
bonds [5]: these distances are shorter than the sum of
the corresponding van der Waals radii (0.30 nm) [6].
Scheme 1.
5
4
5
5
Cl
Cl
Cl
3
4
3
4
3
2
H
2
H
2
3
O
1
O
1
OH
O1
O
1
O
1
1
O2
O2
Ib
O2
3
3
Ia
Ic
1
070-4280/04/4004-0462 © 2004 MAIK “Nauka/Interperiodica”

NEW TYPE OF PROTOTROPIC TAUTOMERISM
463
It is known that H-chelate ring in enol forms of
β-diketones is a quasiaromatic system [7]. We believe
that tautomer Ib contains such six-membered quasi-
aromatic chelate ring which involves two π-electrons
Calculated energy parameters, bond lengths d, and bond
angles ω of tautomers Ia–Ic
Parameter
Ia
Ib
Ic
1
4
3
–Etot, kJ/mol
355.6333
354.2576
371.0927
of the C =C bond, four p-electrons on the O and Cl
3
5
∆
H
f
, kJ/mol
912.7036 1644.7220 1591.9433
1327.1355 1336.2227 1335.3282
atoms, and two electron holes (O –H proton and C ).
For example, delocalization of electron density in
tropone and tropolone involves the carbonyl group and
is enhanced due to intramolecular hydrogen bonding,
while in the tropylium cation electron hole participates
in the delocalization [8]. The ten-electron system in
tautomer Ic is delocalized over a combination of five-
and six-membered rings.
–E , kJ/mol
El
Bond
d, nm
1
2
C –O
0.1394
0.1390
0.1380
0.1389
0.1369
0.1369
2
2
O –C
1
4
3
2
3
C –C
0.1531
0.1537
0.1520
0.1194
0.1529
0.1806
–
0.1356
0.1509
0.1526
0.1332
0.1491
0.1832
0.0976
0.2318
0.1362
0.1458
0.1352
0.1345
0.1494
0.1833
0.0974
0.2352
4
C –C
3
The total energies E of all tautomers are approxi-
tot
C –C
mately similar. We cannot estimate the thermodynamic
stability of anhydride Ia relative to tautomers Ib and Ic
1
C –O
4
5
C –C
on the basis of their enthalpies of formation ∆H , for
f
5
C –Cl
the corresponding entropies should differ considerably
due to negative contribution of intramolecular hydro-
gen bonding. Moreover, the energy of stabilization via
electron density delocalization over the six-membered
H-chelate ring may be comparable with the energy
of conjugation between p- and π-electrons in the
O=C–O–C=O fragment in anhydride Ia (26 kcal/mol
in acetic anhydride [9]). However, the heat of forma-
tion of tautomer Ic is lower by 52.8 kJ/mol than that of
Ib; this difference is likely to reflect the energy gain
due to conjugation in the ten-electron system.
3
1
O –H
1
H –Cl
–
Angle
ω, deg
108.1631
1
2
2
C O C
111.4372
108.7871
106.5096
111.2770
2
2
3
O C C
107.9509
2
3
4
1
2
C C C
104.0270
102.7158
105.7285
3
4
C C C
102.6853
108.7884
129.6182
114.1254
110.6927
106.7805
114.3896
134.6774
132.1701
114.2746
105.7292
110.7557
136.6700
131.3158
113.9925
4
1
C C O
4
1
3
C C O
1
4
5
Let us consider structural criteria of aromaticity.
The contiguous five- and six-membered rings in
C C C
4
5
l
C C C
4
5
tautomers Ib and Ic are coplanar, the angle C C Cl is
5
l
1
5
C C H
–
–
–
088.8462
141.9191
108.1126
089.8251
140.8491
107.3475
equal to ~114°, and the C atom has trigonal rather
l
1
3
2
.15
C H O
than tetragonal configuration (sp ), ensuring co-
planar arrangement of the rings. The furan ring is
characterized by leveled bond lengths and bond angles.
1
3
1
H O C
1
4
The C =C bond in enol Ib is slightly longer, while
should be fairly stable. Below are given experimental
proofs for the existence of anhydride–enol tautomer-
ism for compound I.
4
5
1
3
the C –C and C –C bonds are slightly shorter than
might be expected [6]. An analogous pattern is ob-
served for dienol Ic where intramolecular hydrogen
bond is somewhat weaker than in enol Ib, but the
Identification of particular tautomers by spectral
methods is often difficult because of their low concen-
tration. For example, acetylacetone (which is a clas-
sical compound capable of keto–enol tautomerism)
exists as a 4:1 mixture of the ketone and enol forms
1
4
C =C bond is even longer than in Ib, indicating
a stronger conjugation.
Thus, the results of ab initio calculations performed
for the isolated molecules support our assumption that
enol (Ib) and dienol (Ic) tautomers can be formed from
anhydride Ia. Obviously, in going to real systems
interaction with the solvent should be taken into
account, specifically formation of intermolecular
hydrogen bonds. Therefore, solvated enol tautomers
1
[10]. As applied to compound I, IR and H NMR
spectra turned out to be informative. The IR spectrum
of I (see Experimental) contains absorption bands due
to stretching vibrations of three energetically non-
equivalent C=O groups and also from O–H and C=C
bonds, indicating the presence of enol tautomers.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 4 2004

4
64
FILIMOSHKIN et al.
Apart from signals belonging to anhydride structure
afforded a mixture of methyl ester III and methyl ether
IV. The intensities of the methyl proton signals (δ 3.56
and 3.62 ppm for III and IV, respectively) indicate
approximately equimolar amounts of the products. In
1
Ia, compound I shows in the H NMR spectrum a nar-
row singlet whose position depends on the concen-
tration. Obviously, this signal corresponds to an acidic
proton. The spectral pattern is complex, and over-
lapped signals of tautomers Ia and Ib can be divided
1
the H NMR spectrum of the product obtained at 40°C
we also observed signals from dimethyl ether V whose
fraction did not exceed 10%. Probably, the etherifica-
tion is faster than esterification, and the tautomeric
equilibrium is displaced to the right. This displacement
is also favored by elevated temperature.
into three groups: (1) ClCH in Ia and Ib, (2) CH in Ia
2
and CH in Ib, and (3) CH in Ia. The intensity ratio
2
2
of these signals is 2.28:1.28:2. No signals were
present in the region typical of olefinic protons (δ 4.5–
7
.0 ppm), presumably due to low concentration of
Compound I also undergoes other chemical trans-
formations which are not typical of anhydride form Ia.
Dissolution of compound I in water at room tempera-
ture is accompanied by evolution of HCl (4.4 mol % in
dienol Ic. Thus the ratio of anhydride tautomer Ia and
enol form Ib is 7:1. It is interesting that signals from
the ClCH protons of tautomers Ia and Ib overlap each
2
other, though the signal from enol Ib should appear in
a weaker field due to effect of the double C=C bond in
5
min and 21.6 mol % in 24 h after dissolution) as
a result of S reaction at the CH Cl group. It is known
N
2
the α-position. Presumably, the ClCH protons in enol
2
that dissociation of the C–Cl bond in alkyl halides
requires a lot of energy (400–600 kJ/mol). The ease
of HCl evolution implies that the C–Cl bond is
cleaved in the enol form Ib which gives rise to
hydrogen bond H· · · Cl. Presumably, the reaction in-
volves transition state with a low energy of activation,
as in the dehydrochlorination of 2-chloroethanol where
the chlorine atom is more labile than in common
alkyl halides [12].
Ib are arranged at angle of 120° with respect to the six-
membered quasiaromatic ring, so that they suffer its
shielding effect.
We also performed hydrolysis of anhydride I in
aqueous solution at 40°C (reaction time 5 days). The
hydrolysis product was hydroxymethylsuccinic acid
(
II), which showed the following signals in the
1
H NMR spectrum, δ, ppm: 4.36 q (H , J = 9, J =
a
ab
ac
8
xc
.5 Hz), 4.46 q (H , J = 9, J = 5.5 Hz), 2.7 d (H ,
b ab bc x
In addition, solutions of compound I in DMF and
DMSO, as well as solutions of mixtures of equimolar
amounts of compound I and NaN , NaNO , and
J = 8 Hz), 3.54 t (H , J = 8, J = 8.5, J = 5.5 Hz),
c
xc
ac
bc
9
s (OH). As in the spectrum of initial anhydride I, the
3
2
HOCH protons in acid II give rise to AB part of an
2
KCNO, are red–brown. Their electron absorption
spectra contain new overlapping bands belonging to
molecular complexes between the enol (Ib) and dienol
ABC spin system, while the CH protons (former H )
2
x
become magnetically equivalent. The signal from the
hydroxy protons is broader and more diffuse than
the OH signal in the spectrum of I. This means that the
forms (Ic) and the above reagents, λ 376, 395, 415,
max
and 467 nm. Analogous complexes with the same
reagents are formed by the vinyl chloride–maleic
anhydride copolymer [3]. After storage for more than
1
OH groups in I, identified by the IR and H NMR
spectra, cannot be assigned to carboxy groups.
6
months, compound I no longer forms such com-
Treatment of compound I with diazomethane
according to the procedure described in [11] at 0°C
plexes because of spontaneous dehydrochlorination.
Finally, we failed to effect esterification of compound
I with methanol under standard conditions for esterif-
ication of succinic anhydride and its derivatives [13].
Hb
Hx
CH2
CH2
O
Hc
Cl
N
CH
C
COOMe
C
C
Our results led us to conclude that chloromethyl-
succinic anhydride is capable of prototropic tauto-
merism involving the C, H, and O atoms, which we
called cyclic anhydride–enol tautomerism.
HO
C
COOH
Ha
Cl
Hx
N
C
COOH
H
II
III
Cl
Hb
Hc
1
Hx
Hy
Cl
Cl
2
MeO
O
MeO
OMe
H
a
O
O
O
O
HO
O
IV
V
O
O
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 4 2004

NEW TYPE OF PROTOTROPIC TAUTOMERISM
465
2
EXPERIMENTAL
3.19 m (H , J = 7.1, J = 5.9 Hz), 3.39 (C H ), 3.87
(
c
ac
bc
2
1
C H ), 9.97 s (OH). Found, %: C 40.32; H 3.21.
2
The IR spectra were recorded in KBr on a Specord
M-80 instrument. The H NMR spectra of solutions in
C H ClO . Calculated, %: C 40.40; H 3.37.
5 5 3
1
acetone-d were obtained on a Tesla BS-491 spec-
REFERENCES
6
trometer (100 MHz, HMDS, 24°C). Ab initio quantum-
chemical calculations were performed with the aid
of GAUSSIAN 98W software [14]. The geometric
parameters were optimized, and the electronic struc-
ture parameters were calculated, in terms of the density
functional theory B3LYP/6-31G(d) with the Becke B3
exchange functional [15] and Lee–Yang–Parr correla-
tion functional [16]. Carbon atom hybridizations were
calculated in terms of the natural bond orbital approx-
imation [17].
1
2
3
4
. Filimoshkin, A.G., Terentieva, G.A., Berezina, E.M.,
and Pavlova, T.V., J. Polym. Sci., Polym. Chem., 1993,
vol. 31, p. 1911.
. Pavlova, T.V., Terent’eva, G.A, Filimonov, V.D.,
Chernov, E.B, and Filimoshkin, A.G., Zh. Prikl. Khim.,
1
999, vol. 72, p. 1515.
. Filimoshkin, A.G., Chernov, E.B., Terent’eva, G.A.,
Berezina, E.M., and Telegin, A.G., Vysokomol. Soedin.,
1
997, vol. 39, p. 1986.
. Pribytkov, E.G., Berezina, E.M., Eremina, N.S.,
Terent’eva, G.A., Chernov, E.B., and Filimosh-
kin, A.G., Vysokomol. Soedin. 1997, vol. 39, p. 1365;
Filimoshkin, A.G., Chernov, E.B., Terent’eva, G.A.,
Berezina, E.M., and Safronova, M.S., Zh. Prikl. Khim.,
Chloromethylsuccinic anhydride (I) was syn-
thesized by hydrochlorination of itaconic anhydride
(
3
contrary to the Markownikoff rule [18]). A solution of
0 g (0.23 mol) of itaconic acid in 45 ml (0.48 mol) of
2
001, vol. 74, p. 292; Berezina, E.M., Pavlova, T.V.,
acetic anhydride containing 3 drops of concentrated
hydrochloric acid was heated for 1 h at 40°C. The
solvent was removed at room temperature, and the
residue was washed with carbon tetrachloride, recrys-
tallized from chloroform, and dried under reduced pres-
sure over P O . Yield of itaconic anhydride 23.727 g
Terent’eva, G.A., Chernov, E.B., and Filimosh-
kin, A.G., Vysokomol. Soedin., 1995, vol. 37, p. 2043.
5
6
7
8
. Khimicheskaya entsiklopediya (Chemical Encyclo-
pedia), Moscow: Sovetskaya Entsiklopediya, 1988,
vol. 1, p. 402.
2
5
. Gordon, A.J. and Ford, R.A., The Chemist’s Com-
panion, New York: Wiley, 1972. Translated under the
title Sputnik khimika, Moscow: Mir, 1976, p. 128.
(
92%); colorless needles, mp 69–70°C [18]. IR spec-
–1
trum, ν, cm : 2935 (CH ); 3120 (=C–H); 1780, 1850
2
1
(
C=O); 1660 (C=C); 1240 (C–O–C). H NMR spec-
. Ionin, B.I. and Ershov, B.A., YaMR-spektroskopiya v
organicheskoi khimii (NMR Spectroscopy in Organic
Chemistry), Leningrad: Khimiya, 1967, p. 297.
trum, δ, ppm: 3.66 d.d (2H, J = 2.5, 2.75 Hz), 5.88 t
(
1H, J = 2.5 Hz), 6.16 t (1H, J = 2.75 Hz).
A stream of thoroughly dried (concentrated sulfuric
acid, anhydrone) hydrogen chloride [19] was passed
over a period of 4 h through a solution of 13 g
. Becker, H., Einführung in die Elektronentheorie orga-
nisch–chemischer Reaktionen, Berlin: Wissenschaften,
1974, 3rd ed. Translated under the title Vvedenie v
elektronnuyu teoriyu organicheskikh reaktsii, Moscow:
Mir, 1977, p. 74.
(
0.117 mol) of itaconic anhydride in 100 ml of dry
chloroform, maintained at –5 to –10°C, and the reac-
tion flask was capped and left to stand for 12 h at room
temperature. This procedure was repeated three times,
and the mixture was kept for 24 h at –18°C until
it divided into layers. The yellow oily liquid was
separated and was kept under reduced pressure over
P O . The colorless crystals were thoroughly washed
9. Roberts, J.D. and Caserio, M., Basic Principles of
Organic Chemistry, New York: W.A. Benjamin, 1975,
2
nd ed. Translated under the title Osnovy organicheskoi
khimii, Moscow: Mir, 1968, vol. 1, p. 223.
1
0. Emsley, J.W., Feeney, J., and Sutcliffe, L.H., High-
Resolution Nuclear Magnetic Resonance Spectroscopy,
Oxford: Pergamon, 1966, vol. 2. Translated under the
title Spektroskopiya YaMR vysokogo razresheniya,
Moscow: Mir, 1968, vol. 1, p. 517.
2
5
with carbon tetrachloride, recrystallized from diethyl
ether, and dried under reduced pressure over P O .
2
5
Yield 8.247 g (48%), mp 126–130°C. IR spectrum, ν,
–1
11. Methoden der organischen Chemie (Houben–Weyl),
Müller, E., Bayer, O., Meerwein, H., and Ziegler, K.,
Eds., Stuttgart: Georg Thieme, 1953, 4th ed., vol. 2.
Translated under the title Metody organicheskoi khimii,
Moscow: Gos. Nauch.-Tekh. Izd. Khim. Lit., 1963,
vol. 2, pp. 270, 381.
cm : 3220–2800 br (O–H); 1850 w, 1776 m, 1708 s
1
(
C=O); 1640 (C=C); 1240 (C–O–C). H NMR spec-
trum, δ, ppm: 3.89 q (H , J = –17, J = 5.9 Hz),
b
ab
bc
3
–
.82 q (H , J = –17, J = 7.1 Hz), 2.79 q (H , J =
a ab ac y xy
17, J = 5.9 Hz), 2.58 q (H , J = –17, J = 7.1 Hz),
bc
x
xy
xc
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 4 2004

4
66
FILIMOSHKIN et al.
1
2. Khimicheskaya entsiklopediya (Chemical Encyclo-
pedia), Moscow: Sovetskaya Entsiklopediya, 1988,
vol. 2, p. 276.
3. Weygand–Hilgetag Organisch-chemische Experimen-
tierkunst, Hilgetag, G. and Martini, A., Eds., Leipzig:
Johann Ambrosius Barth, 1964, 3rd ed. Translated
under the title Metody eksperimenta v organicheskoi
khimii, Moscow: Khimiya, 1968, p. 349.
16. Lee, C., Yang, W., and Parr, R.G., Phys. Rev. B, 1988,
vol. 37, p. 785.
1
1
7. Reed, A.E., Weinstock, R.B., and Weinhold, F.,
1
J. Chem. Phys., 1985, vol. 83, p. 735.
8. Khimicheskaya entsiklopediya (Chemical Encyclo-
pedia), Moscow: Sovetskaya Entsiklopediya, 1988,
vol. 5, p. 501.
1
1
4. GAUSSIAN 98W. User’s Reference, Fritsch, E. and
Fritsch, M.J., Eds., Pittsburgh: Gaussian, 1998.
5. Becke, A.D., J. Chem. Phys., 1993, vol. 98, p. 5648.
1
9. Organikum. Organisch-chemisches Grundpraktikum,
Berlin: Wissenschaften, 1976, 15th ed. Translated under
the title Organikum, Moscow: Mir, 1979, vol. 2, p. 374.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 40 No. 4 2004