NITRATION MECHANISM OF DIBENZO-18-CROWN-6
1515
reaction mixture indicates that in this case the reac-
tion is irreversible and results in a mixture of dinitro
derivatives of DB18C6 (III).
signal with poorly resolved superfine structure
(g 2.0094). The position of the signal and its super-
fine structure remain unchanged for weeks.
At n 1.2 the green color also appears and further
becomes brown. The visible spectrum contains a band
of dinitro derivatives of DB18C6 III ( max 440 nm),
a stronger band of mononitro derivative II with a
maximum at 430 nm, and a band at 630 nm arising
presumably from one-electron transition from the
Cation-radical of DB18C6 (V) also forms at treating
compounds of DB18C6 in acetonitrile with a solution
of SbCl in this solvent. In 5 min after mixing the
5
reagents the reaction mixture gets dark-green, and
then precipitate dark crystals of hexachloroantimonate
of DB18C6 cation-radical. The study of this crystal-
line product by ESR showed that it is 100% radical
species. However no well-resolved superfine structure
was observed in the ESR spectrum of the compound.
-electron system of the benzene ring in the substrate
to nitronium cation that in this case operates as an
oxidant.
At the above reagents ratio the electronic spectrum
contains also a wide absorption band at 505 nm
A fundamental possibility of formation of cation-
radicals from benzo-crown ethers in solution was
revealed in the ESR study of photoinduced one-elec-
tron transfer from substrates to dichlorocycloquinone
[5]. However we have not found published examples
on cation-radicals formation from benzo-crown ethers
as intermediates in electrophilic substitution reactions.
*
*
alongside the classical
and n
bands of an
aromatic ring with nitro group substituent. The same
band was observed in the spectrum of mononitro
DB18C6 derivative (II) recorded in the polyphos-
phoric acid. This fact indicates that in the solution in
the polyphosphoric acid of mononitro-DB18C6 (II)
exists an equilibrium between the initial substrate and
a structure with quinoid-like nitro-substituted benzene
ring IV which arises from protonation of the system.
Formation of such structure was proved for the
monoacetyl-DB18C6 [4].
The formation of dinitro derivative of DB18C6
(III) since the beginning of the reaction disregarding
the deficit of the nitrating agent evidences a competi0
tion for NO -radical between the cation-radical of
2
DB18C6 (V) and cation-radical of mononitro-
DB18C6, and the latter wins the competition due its
higher reactivity.
At n the reaction mixture becomes green and thus
remains for several weeks. In the electronic spectrum
is observed a strong band of cation-radical of
DB18C6 (V). The lack of bands corresponding to
mono and dinitro derivatives indicates that under
these conditions the cation-radical is the only reaction
product as shows also the TLC analysis of reaction
products obtained after quenching of the reaction
mixture.
Hexachloroantimonate of cation-radical of
DB18C6. A solution of 0.6 g (1.67 mmol) of di-
benzo-18-crown-6 in 14 ml of dry acetonitrile pre-
liminary distilled on P O was treated with 1.2 g
2
5
(
4.17 mmol) of SbCl in 14 ml of acetonitrile. The
5
precipitate was filtered off. washed with dry aceto-
nitrile, and dried, Yield 0.8 g (69%).
Electron absorption spectra were registered on
SF4A device. ESR spectra were measured of spectro-
meter Bruker ER 200D-SRC. Polyphosphoric acid
was prepared along procedure [6]. SbCl was obtain-
5
ed as in [7].
Spectroscopic measurements were performed on
solutions prepared as follows: In 2 g of solution
of 0.05 g (0.13 mmol) of DB18C6 in polyphosphoric
acid was dissolved an appropriate weight of potas-
sium nitrate: 0.035 g (n 2.5), 0.017 g (n 1.2), and
0
.006 g (n 0.4).
ESR spectra of reaction mixtures were recorded in
thin capillaries. The ESR spectrum of hexachloro-
antimonate of cation-radical of DB18C6 was
measured both in crystalline state and in acetonitrile
solution.
In order to prove the radical nature of the reaction
product we recorded ESR spectra of the reaction
mixture obtained at n 0.4. The spectra contain a
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001
Andreev
