SYNTHESIS OF BORON-CONTAINING COMPLEXES
1563
Thus, the IR spectrum contains a band at
o-hydroxyacetophenone complex (III) with benzal-
dehyde under reflux in acetic anhydride. Both pro-
ducts had the same physicochemical characteristics.
1
1655 cm , characteristic of chalcone (C=O) and
1
(CH=CH) stretching vibrations [8]. The H NMR
spectrum of compound V displays at 6.40 and
6.88 ppm doublets of olefin protons with a coupling
constant of 18.2 Hz, which implies an E configuration
of this compound [9].
PhCHO
III
VI
Ac2O
EXPERIMENTAL
Compound V was brought into condensation with
boron trifluoride etherate without additional purifica-
tion. The complex formation was effected by boiling
equimolar amounts of V and boron trifluoride etherate
in toluene for 4 5 h. The reaction mixture was filtered
while hot, after which the solvent was removed in a
vacuum, and the product was recrystallized from
benzene.
The IR spectra were obtained on a Specord IR
spectrophotometer for suspensions in Vaseline oil.
The electronic absorption spectra were measured on
4
an M-80 spectrophotometer in ethanol for 10 solu-
1
tions. The H NMR spectra were taken on a Bruker
spectrometer (200 MHz) in deuterochloroform;
internal reference TMS. Thin-layer chromatography
was performed on Silufol plates.
F
F
O
BF3 OEt2
B
O
Boron fluoride o-hydroxyacetophenone com-
plex (III). Boron trifluoride etherate, 4.40 g, was
added to a solution of 3.20 g of o-hydroxyaceto-
phenone in 25 ml of dry benzene, and the mixture was
heated for 60 min at 40 50 C. After cooling, the
precipitate was filtered off and washed with benzene
to obtain 3.13 g (71%) of complex III, mp 141
143 C {published data [4]: mp 142.5 145.0 C}.
V
C
CH=CH
VI
We assigned structure VI to the resulting complex,
relying on the following evidence. The complex turns
the flame to an intense green color, which is charac-
teristic of boron-containing compounds. The chalcone
(2-Hydroxyphenyl)-3-phenyl-2-propen-1-one
sodium salt (V). Sodium hydroxide, 3.85 g (30%
solution, 9 ml), was added to 3.20 g of 2-hydroxy-
acetophenone and 2.50 g of benzaldehyde in 30 ml of
ethanol at 40 50 C. The mixture was stirred for
20 min and cooled to room temperature. The pre-
cipitate that formed was filtered off, washed with
benzene, and dried over calcium chloride to obtain
5.20 g (97%) of compound V that was used in sub-
sequent reactions without additional purification. IR
1
(C=O) band is shifted red by 15 cm , suggesting
involvement of the oxygen atom in complex forma-
tion. Similar shift is observed in going from 2-hydr-
oxyacetophenone to its boron fluoride complex III.
Moreover, the IR spectrum of complex VI shows new,
compared with ligand V, bands at 1150 1200 and
1
1050 1100 cm , characteristic of stretching vibra-
tions of B O and B F bonds in the chelate entity.
Similar bands are observed in the IR spectra of boron
fluoride -diketone complexes [10]. All signals in the
1H NMR spectrum of complex VI are shifted down-
field compared with ligand V, which is associated
with the electron-acceptor nature of the chelate entity.
Therewith, the olefin proton signals get closer to-
gether but preserve their pattern (doublets at 7.07
and 7.10 ppm, J 18.0 Hz), implying that the olefin
moiety has preserved its E configuration.
1
spectrum, , cm : 1655 (C=O + CH=CH), 1595,
1538, 1320, 1187, 987, 880. Electronic absorption
1
spectrum, max, nm ( ): 319 (7200), 251 (9300). H
NMR spectrum, , ppm: 6.02 d (1H), 6.40 d (1H),
6.88 d (1H), 6.91 t (1H), 7.00 7.20 m (5H), 7.48 t
(1H), 7.87 d (1H).
Complex formation of chalcone V with boron
trifluoride etherate. Boron trifluoride etherate, 9.50 g,
was added with vigorous stirring to 30 ml of toluene
in 5.20 g of compound V. The mixture was heated at
75 80 C for 4 h, after which it was filtered while hot,
cooled, filtered once more, the solvent was removed
in a vacuum, and the residue was recrystallized from
benzene to obtain 1.28 g (81%) of compound VI, mp
242 243 C {published data [5]: mp 242 244 C}. IR
The electronic absorption spectrum of complex VI
differs from the spectrum of ligand V in that it con-
tains a new band as an inflection at 360 390 nm.
Graphical resolution of the experimental absorption
curve shows that this band has its maximum at 376
1
nm with an intensity of 4000 mol l cm .
1
spectrum, , cm : 1630 (C=O + CH=CH), 1575,
Further evidence for the structure of complex VI
was obtained by the independent synthesis of this
compound by the condensation of the boron fluoride
1520, 1250, 1180 (BF2), 1130 (BF2), 1080 (BF2),
1050 (BF2). Electronic absorption spectrum, max, nm
( ): 360 390 (4200), 318 (11300). H NMR spectrum
1
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001