REARRANGEMENTS DURING NITROSODECARBOXYLATION
691
In this event due to the decarboxylation of ipso-bromo
complexes [8] 2,4,6-tribromophenol (XIV) should be
present in the reaction mixtures.
dissolution of the salt, and then was added at stirring by
small portions 2.14 g (31 mmol) of NaNO within 2 h.
2
Then the reaction mixture was kept at room temperature
for 1h and afterwards poured into 200 ml of 10% solution
In the products of the reactions under investigation we
did not detect even traces of phenol XIV. It was found
however that 2,4,6-tribromophenol in the glacial acetic
acid at room temperature in the presence of sodium nitrite
quickly transformed into a mixture of 2,6-dibromo-4-
nitro- (III) and 4,6-dibromo-2-nitro- (IV) phenols in a
ratio 1:2.16. It was also established that under similar
conditions the nitrosodebromination rate of phenol XIV
is greater than the rate of the nitrosodecarboxylation of
the dibromohydroxybenzoic acids (conversion 100% in
of Na CO . The formed precipitate was 2-nitro-4,6-
2
3
dibromophenol (IV) (0.39 g) containing trace impurity
of phenol III (TLC data). After filtering off compound
IV the separated precipitate was filtered off and dried to
obtain 0.28 g of 4-nitro-2,6-dibromophenol (III). The
filtrate was diluted with water to the volume of 500 ml
and extracted with ether. The ether extract was washed
with water, dried, and the solvent was evaporated. The
residue (0.25 g) was a mixture of compound III (85%)
with initial acid II.
2
h). Thus the experimental data obtained do not permit
the choice between the possible rearrangement routes.
Reactionofsodium3,5-dibromo-2-hydroxybenzoate
with NaNO2. The reaction was carried out in a similar
way. The reaction mixture was diluted with 200 ml
of 10% solution of Na CO , the formed precipitate
Unsubstituted 2-hydroxy- and 4-hydroxybenzoic
acids and sodium 2-hydroxybenzoate did not undergo
rearrangements under the action of sodium nitrite at 20°C
both in the glacial and dilute (75%vol) acid. 4-Sodium
hydroxybenzoate was also stable in the glacial acetic
2
3
(0.69 g) was a mixture of the initial salt and 2-nitro-4,6-
dibromophenol (IV) that was separated from the salt by
extraction with ether (we obtained 0.06 g of compound
IV). The filtrate was acidified with concn. HCl to pH
1–2, the separated precipitate was filtered off and dried
to obtain 0.15 g of pure 3,5-dibromo-2-hydroxybenzoic
acid (I). The filtrate after isolating acid I was diluted with
water to the volume of 500 ml and extracted with ether.
The ether extract was dried, the ether was evaporated.
The residue (0.11 g according to TLC and IR spectrum
consisted of phenols III and IV, acid I, and nitro group
containing hydroxybenzoic acids [in the IR spectrum
additional absorption bands appeared at 1342, 1516,
acid, whereas in dilute CH COOH it formed insignificant
3
quantity of 4-nitrophenol (conversion of the initial salt
within 3 h reached 0.5%).
EXPERIMENTAL
IR spectra were recorded on a spectrophotometer
1
InfraLYUM FT-02 from pellets with KBr, H NMR
spectra, on a spectrometer Bruker AC-200 (200 ΜHz) in
CDCl , internal reference HMDS. TLC was carried out
3
on Silufol UV-254 plates, eluent hexane–acetone, 7:3,
development in iodine vapor or under UV irradiation.
–1
and 1524 cm , characteristic of aromatic nitro group,
Quantum-chemical calculations were performed
using software package HyperChem 7.0. The enthalpy
of formation of ipso-complexes was calculated by
semiempirical PΜ3 method [10] with optimization of
all geometrical parameters of all molecular systems
under consideration. According to the calculations
performed in most cases the complexes are characterized
by conformational nonuniformity (the existence of
local minima). The data presented in Schemes 3 and 4
correspond only to the most stable conformations.
and also at 1671, 2500–3100 (series of wide bands) and
–
1
3371 cm , characteristic of aromatic hydroxyacids].
The composition of the residue was established from
1
the H NMR spectrum that contained in the region of
aromatic protons pairs of doublets of equal intensity
at 7.59 and 7.74 ppm (J 2.3 Hz) (acid I), 8.35 and 8.53
ppm (J 2.3 Hz) (acid IX), 8.49 and 8.72 ppm (J 2.3 Hz)
(acid VIII), 7.77 and 7.99 ppm (J 2.3 Hz) (phenol IV),
and a singlet at 8.13 ppm (phenol III). From the ratio of
the overall integral intensity of aromatic protons in these
compounds their content in the residue was calculated as
18, 36, 9, 6, and 31% respectively.
Acids I [11], II [12], phenol III [13], IV [14], V [5],
and XIV [15] were prepared by published procedures.
Reactionofsodium3,5-dibromo-4-hydroxybenzoate
The reactions with NaNO of isomeric dibromo- and
2
with NaNO . A mixture of 1 g (3.1 mmol) of sodium
unsubstituted hydroxybenzoic acids and the workup of the
reaction mixture and isolation of products were carried
out as described above.
2
3
,5-dibromo-4-hydroxybenzoate and 50 ml of glacial
acetic acid was stirred at room temperature till complete
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 5 2008