Metalation of toluene and cumene with alkali metal-crown ether complexes

Article abstract of DOI:10.1134/S1070428009010047

Metalation of toluene and isopropylbenzene with alkali metal-crown ether complexes led to the corresponding α-metalated alkylbenzenes. Treatment of the latter in succession with solid carbon dioxide, water, and hydrochloric acid gave carboxylic or dicarboxylic acids in 65-78% yield. Metalation of isopropylbenzene with sodium or potassium crown ether complexes above 90°C was accompanied by cleavage of the polyether ring with formation of organometallic compounds which then reacted with isopropylbenzene to produce 2-sodio(potassio)-2-phenylpropane and open-chain oligoether.

Full text of DOI:10.1134/S1070428009010047

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 1, pp. 26–29. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © A.L. Shabanov, N.M. Seidov, U.A. Gasanova, Z.O. Kakhramanova, M.M. Gasanova, 2009, published in Zhurnal Organicheskoi
Khimii, 2009, Vol. 45, No. 1, pp. 33–37.
Metalation of Toluene and Cumene
with Alkali Metal–Crown Ether Complexes
A. L. Shabanova, N. M. Seidova, U. A. Gasanovab, Z. O. Kakhramanovaa, and M. M. Gasanovaa
a Research Institute of Petroleum and Gas Geotechnology Problems and Chemistry, Azerbaidjan State Petroleum Academy,
pr. Azadlyg 20, Baku, Azerbaidjan 1010
e-mail: a.shabanov_36@mail.ru
a Azerbaidjan Medical University, Baku, Azerbaidjan
Received May 25, 2007
Abstract—Metalation of toluene and isopropylbenzene with alkali metal–crown ether complexes led to the
corresponding α-metalated alkylbenzenes. Treatment of the latter in succession with solid carbon dioxide,
water, and hydrochloric acid gave carboxylic or dicarboxylic acids in 65–78% yield. Metalation of isopropyl-
benzene with sodium or potassium crown ether complexes above 90°C was accompanied by cleavage of the
polyether ring with formation of organometallic compounds which then reacted with isopropylbenzene to
produce 2-sodio(potassio)-2-phenylpropane and open-chain oligoether.
DOI: 10.1134/S1070428009010047
Replacement of a benzylic hydrogen atom via met-
alation of alkylaromatic hydrocarbons with sodium or
potassium is undoubtedly very important from the
synthetic viewpoint. However, there are almost no
published data on the direct substitution of hydrogen in
benzylic or other positions by the action of alkali
metals. The most widely known classical procedure for
metalation of alkylaromatic hydrocarbons is based on
reactions with alkali metal hydrocarbyls [1]; it was
applied more or less successfully to hydrocarbons and
their derivatives. A disadvantage of this procedure is
that it requires preliminary preparation of alkyl- or
arylsodium or -potassium. On the other hand, the
yields are often not high. A limited number of relevant
examples have been reported.
We studied substitution of benzylic hydrogen atom
in alkylaromatic hydrocarbons by alkali metals in the
presence of crown ethers (CE). It is known that CEs
react with sodium and potassium in hydrocarbons to
produce ultradispersed colloidal systems [2–8]. Such
systems are formed as follows. At a temperature below
0°C, macrocyclic ligand knocks out an alkali metal
cation from the crystal lattice to give complex
[KE·M+] M– [2], where M– is alkali metal anion. Fur-
ther studies have shown that, apart from [KE·M+] M–
complexes, complexes with the [KE·M+] M7– composi-
Scheme 1.
O
(1) Solid CO2
(2) H2O
(3) concd. HCl
Me
Me
Me
O
O
O
O
50°C
Me
Me
COOH
+
M+
M–
Me
M
O
I, II
III
Me
Me
Me
Na
PhNa
(1) Solid CO2; (2) H2O; (3) concd. HCl
Me
III
I, M = Na; II, M = K.
26

METALATION OF TOLUENE AND CUMENE
27
tion are also formed, where M7– ion is a nanoparticle
[3, 4, 7]. Taking into account strong basicity of alkali
metal anions [5–7], we examined replacement of the
α-hydrogen atom in toluene and cumene (isopropyl-
benzene) by an alkali metal via generation of the above
complexes using these hydrocarbons as reaction me-
dium. Dispersion of alkali metal in isopropylbenzene
in the presence of dicyclohexyl-18-crown-6, dibenzo-
15-crown-5, or dibenzo-18-crown-6 on heating result-
ed in replacement of hydrogen at the tertiary carbon
atom by alkali metal as shown in Scheme 1.
organosodium derivative, and treatment of the latter in
succession with solid carbon dioxide, water, and con-
centrated hydrochloric acid afforded acid III which
was identical to a sample obtained by metalation of
isopropylbenzene with sodium in the presence of
crown ether. The melting point of acid III coincided
with that given in [1], and no depression of the melting
point was observed on mixing samples prepared by
different methods. The structure of III was also con-
1
firmed by the IR and H NMR spectra. The IR spec-
trum contained a strong absorption band at 1701 cm–1
due to carboxy group; the phenyl group gave rise to
absorption bands at 3032 (C–Harom) and 1589 cm–1
(C=Carom), and bands at 758 and 698 cm–1 indicated the
presence of a monosubstituted benzene ring. In the
1H NMR spectrum of III, protons in the two methyl
groups resonated as a six-proton narrow multiplet cen-
tered at δ 1.19 ppm, multiplet at δ 7.1 ppm (5H) was
assigned to aromatic protons, and carboxy proton ap-
peared as a singlet at δ 8.6 ppm (1H).
The formation of α-substituted sodium and potas-
sium derivatives I and II is confirmed by the isolation
of 2-methyl-2-phenylpropionic acid III after treatment
of the reaction mixture with solid carbon dioxide,
followed by careful dilution with water under nitrogen
and acidification with concentrated hydrochloric acid
(pH 2). Recrystallization of the brown–grey precipitate
from petroleum ether gave a dark grey sample with
mp 76–77°C, which was additionally purified by re-
precipitation from aqueous alkali with concentrated
hydrochloric acid. The pure product had mp 78°C
which coincided with that reported for 2-methyl-2-
phenylpropionic acid (III) [9]. No replacement of
α-hydrogen atom was observed in the absence of CE
even when alkali metal was dispersed using a high-
speed stirrer (up to 10000 rpm) on heating.
We found that dibenzo-18-crown-6 and dibenzo-15-
crown-5 are optimal crown ethers for the metalation
with potassium and sodium, respectively. The reaction
direction depends on the temperature. The benzylic
hydrogen atom is replaced by potassium or sodium in
the presence of crown compounds below 60°C. Above
90°C, the predominant process is exchange metalation
of alkylaromatic hydrocarbons involving cleavage of
the polyether ring in complex IV. Presumably, organo-
potassium compound V is formed as intermediate
which metalates isopropylbenzene (Scheme 2). This
follows from the result of successive treatment of the
Acid III was also synthesized by independent
method from isopropylbenzene and preliminarily pre-
pared phenylsodium [9]. As might be expected, met-
alation of isopropylbenzene with phenyl sodium gave
Scheme 2.
OK
K
O
O
O
O
O
O
O
O
O
K+
K–
O
O
IV
V
OH
O
Et
O
Me
OH
Et
O
(1) Solid CO2
(2) H2O
(3) concd. HCl
O
O
O
O
Me
PhCHMe2
K
+
+ III
O
O
O
VI
VII
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 1 2009

SHABANOV et al.
28
Scheme 3.
O
Me
O
O
O
O
CH2K
CH2COOK
CO2 (solid)
+
K+
K–
O
IV
COOK
K
COOK
COOH
COOH
PhCH2K
Δ
CO2 (solid)
HCl
COOK
COOH
–CO2
VIII
IX
reaction mixture with solid carbon dioxide, water, and
hydrochloric acid, which gave a mixture of 2-methyl-
2-phenylpropionic acid (III) and 2-(2-{2-[2-(2-ethoxy-
phenoxy)ethoxy]ethoxy}phenoxy)ethanol (VII). Acid
III was isolated by neutralization with aqueous alkali
and subsequent reprecipitation by acidification. The
white crystalline substance which did not dissolve in
alkaline solution was recrystallized from petroleum
ether (mp 65–67°C). Its structure was determined on
the basis of its elemental composition and IR and
1H NMR spectra. The IR spectrum contained an ab-
sorption band at 3437 cm–1, corresponding to stretch-
chemically pure grade (Merck), dibenzo-18-crown-6,
and benzo-15-crown-5 (Cherkassy, Ukraine) were com-
mercial products.
Phenylmalonic acid (VIII). A mixture of 0.39 g
(0.01 mol) of potassium and 50 ml of toluene was
heated to 75°C in a nitrogen atmosphere, and the
resulting potassium melt was dispersed using a high-
speed stirrer (10 000 rpm) so that the size of metal
particles did not exceed 25 μm. The fine suspension of
potassium in toluene thus obtained was cooled to
–5°C, and 3.6 g (0.01 mol) of dibenzo-18-crown-6 was
added. The mixture spontaneously warmed up under
stirring at room temperature. The blue solution turned
dark brown at 55–60°C. The mixture was stirred for
3 h at 75°C, cooled, and poured onto solid carbon
dioxide taken in a large excess. It was then carefully
quenched with water, the aqueous phase was acidified
with hydrochloric acid, and the precipitate was filtered
off and dried. The product was additionally purified by
dissolution in aqueous alkali and subsequent precipita-
tion with concentrated hydrochloric acid; the precip-
itate was filtered off and dried. Yield 0.67 g (75%),
mp 153°C [10]. On heating above 160°C, compound
VIII was converted almost quantitatively into phenyl-
acetic acid with mp 76°C [1].
1
ing vibration of hydroxy group, and in the H NMR
spectrum we observed signals typical of hydroxy, aro-
matic, and methylene protons. These data were con-
sistent with structure VII.
Unlike isopropylbenzene, metalation of toluene
with potassium in the presence of dibenzo-18-crown-6
and subsequent treatment of the reaction mixture in
succession with solid carbon dioxide, water, and con-
centrated hydrochloric acid gave phenylmalonic acid
(VIII) as the major product. Its formation may be
illustrated by Scheme 3. The physical constants of
phenylmalonic acid (VIII) obtained according to
Scheme 3 fully coincided with those reported in [10].
On heating above 155°C acid VIII underwent decar-
boxylation to give phenylacetic acid (IX).
2-Methyl-2-phenylpropionic acid (III). a. Acid
III was synthesized as described above for compound
VIII using 0.78 g (0.02 mol) of metallic potassium and
7.2 g (0.02 mol) of dibenzo-18-crown-6 in 100 ml of
isopropylbenzene. Acid III was purified by reprecipi-
tation from an alkaline solution with hydrochloric acid.
Yield 1.15 g (78%), mp 78°C [1]. IR spectrum, ν, cm–1:
1589 (C=Carom); 3032 (C–Harom); 1701 (C=O); 758,
EXPERIMENTAL
The IR spectra were recorded in the range from
4000 to 400 cm–1 on a Specord M80 spectrometer from
samples prepared as KBr pellets. The 1H NMR spectra
were measured on a Bruker-300 instrument (300 MHz)
using D2O and DMSO as solvents and tetramethyl-
silane as internal reference. Toluene and cumene of
1
698 (δC–Harom). H NMR spectrum, δ, ppm: 1.19 m
(6H, CH3), 7.1 m (5H, C6H5), 8.6 s (1H, COOH).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 1 2009

METALATION OF TOLUENE AND CUMENE
29
b. Cumene, 50 ml, was added under stirring to
a black suspension of 10.05 g (0.1 mol) of phenylso-
dium (prepared preliminarily from chlorobenzene and
metallic sodium according to the procedure described
in [1]), maintaining the temperature at 25–30°C. The
mixture was stirred for 1.5 h at 112°C, cooled to room
temperature, and poured onto solid carbon dioxide
taken in a large excess. Excess carbon dioxide was re-
moved, the residue was carefully quenched with water,
the aqueous phase was neutralized with concentrated
hydrochloric acid, and the precipitate was filtered off.
The product was additionally purified by dissolution in
aqueous alkali and subsequent precipitation with con-
centrated hydrochloric acid; the precipitate was filtered
off and dried. Yield 5.7 g (35%), mp 78°C. No depres-
sion of the melting point was observed on mixing with
a sample of III prepared as described in a.
perature and slowly poured onto solid carbon dioxide
taken in a large excess. It was then carefully quenched
with water under nitrogen, the aqueous phase was
acidified with hydrochloric acid, and the grey–brown
precipitate was filtered off. Recrystallization of the
product from petroleum ether gave 4.5 g (28%) of acid
III as brown crystals. Compound VII (it did not dis-
solve in aqueous alkali) was purified by recrystalliza-
tion from petroleum ether. Yield 0.64 g (36%), white
1
crystals, mp 65°C. H NMR spectrum, δ, ppm: 7.12 m
(8H, Harom), 1.3 t (3H, CH3CH2O), 3.5–3.8 m (14H,
OCH2), 4.5 s (1H, OH). Found, %: C 66.21; H 7.24.
C20H26O6. Calculated, %: C 66.3; H 7.18.
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sodium and 75 ml of isopropylbenzene was heated to
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