Boron difluoride β-diketonates: IV. Reaction of p-alkyl-substituted benzoylacetonates with N-bromosuccinimide

Article abstract of DOI:10.1023/B:RUGC.0000016026.56938.f2

Bromination of p-alkyl-substituted boron difluoride and chromium(in) benzoylacetonates with N-bromosuccinimide was studied. It was found that that the boron difluoride complexes are selectively brominated by the alkyl radical, whereas chromium toluoylacet

Full text of DOI:10.1023/B:RUGC.0000016026.56938.f2

Russian Journal of General Chemistry, Vol. 73, No. 10, 2003, pp. 1578 1580. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 10, 2003,
pp. 1671 1673.
Original Russian Text Copyright
2003 by Gukhman, Reutov.
Boron Difluoride -Diketonates:
IV.1 Reaction of p-Alkyl-substituted Benzoylacetonates
with N-Bromosuccinimide
E. V. Gukhman and V. A. Reutov
Institute of Chemistry, Far-East Division, Russian Academy of Sciences, Vladivostok, Russia
Institute of Chemistry and Applied Ecology, Far-East State University, Vladivostok, Russia
Received November 13, 2001
Abstract Bromination of p-alkyl-substituted boron difluoride and chromium(III) benzoylacetonates with
N-bromosuccinimide was studied. It was found that that the boron difluoride complexes are selectively
brominated by the alkyl radical, whereas chromium toluoylacetonate, by the central carbon atom of the
chelate ring.
1
Electrophilic substitution reactions in metal -di-
ketonates has been studied in sufficient detail. A wide
variety of electrophilic reagents has been found,
whose reactions with metal -diketonates occur under
mild conditions, involving no rupture of the chelate
rings and resulting in substitution of the hydrogen
atom at the central carbon atom of the chelate ring (
position of the chelate ring) by various functional
groups [2 9]. At the same time, electrophilic substi-
tution reactions in the closest analogs of metal -di-
Ib diplays only one strong band (1530 cm ) in the
1
range 1600 1500 cm , whereas in the spectrum of
compound Ia we observe three bands in this range, at
1558, 1523, and 1501 cm . These findings are indi-
cative of selective substitution of the -hydrogen atom
of the chelate ring.
1
CH3
CH3
NBS
O
ketonates, boron difluoride
-diketonattes, has
O
1/3C
(PhCOO)2
scarcely been explored [10]. The aim of the present
work to compare the behavior of boron difluoride and
chromium(III) -diketonates in electrophilic substi-
tuion reactions.
1/3C
H
Br
O
O
Ia
Ib
Compound Ia has two reaction centers: position
of the chelate ring and the methyl group in the
benzene ring. For substitution of the -hydrogen atom
of the chelate ring, Kluiber [6] proposed an electro-
philic mechanism. Bromination of methyl group is
known to occur by a radical mechanism [12]. The re-
action of compound Ia with N-bromosuccinimide
even under classical radical bromination conditions
(excess N-bromosuccinimide in the presence of
benzoyl peroxide, route b) is slow and yields com-
pound Ib.
One of the most thoroughly studied reactions of
metal -diketonates is halogenation. Singh and Sahai
[4] showed that metal benzoylacetonates and diben-
zoylmethanates react with N-bromosuccinimide (NBS)
by way of substitution of the -hydrogen atom of the
chelate ring. Therewith, the IR spectra of the chelates
characteristically change according to Druden’s rule
[11]: The two absorption bands in the range 1600
1
1500 cm , corresponding to stretching vibrations of
the -diketonate ring, coalesce to form a single strong
band.
Unlike chromium chelate Ia, boron toluoylaceto-
nate (IIa) and boron difluoride p-ethylbenzoylaceto-
nate (IIIa) react with N-bromosuccinimide only in the
presence of benzoyl peroxide. Therewith, even with a
great excess of N-bromosuccinimide, selective substi-
tution of alkyl -hydrogen by bromine occurs.
The bromination of chromium toluoylacetonate (Ia)
with N-bromosuccinimide in carbon tetrachloride
(route a) gives a single reaction product which, ac-
cording to the elemental analysis, contains one bromine
atom in each chelate ring. The IR spectrum of product
The structure of boron difluoride p-(bromomethyl)-
benzoylacetonate (IIb) p-( -bromoethyl)benzoylace-
1
For communication III, see [1].
1070-3632/03/7310-1578$25.00 2003 MAIK Nauka/Interperiodica

BORON DIFLUORIDE -DIKETONATES: IV.
1579
IR and 1H NMR spectral data for boron difluoride -diketonates F2B(CH3COCHCOC6H4R)
1
, cm
, ppm (J, Hz)
Comp.
no.
Ar
C=O,
C=C
11B F,
11B O,
10B O
chel CH3,a chel H,a
Ar
R
10B F
s (3H)
s (1H)
IIa
1610,
1592
1610,
1600
1611,
1579
1550,
1510
1563,
1510
1553,
1539
1541,
1510
1164,
1143
1160,
1140
1156,
1136
1150,
1135
1085,
1050
1090,
1063
1061,
1050
1091,
1060
2.40
6.54
7.33 d (2H, 8.5), 2.46 s (3H)
7.97 d (2H, 8.5)
7.55 d (2H, 8.6), 4.51 s (2H)
8.04 d (2H, 8.6)
7.37 d (2H, 9.0), 2.88 q (2H, 3.3),
7.99 d (2H, 9.0) 1.30 t (3H, 3.1)
7.59 d (2H, 9.0), 5.20 q (1H, 3.3),
8.04 d (2H, 9.0) 2.07 d (3H, 2.5)
IIb
IIIa
2.44
6.57
2.43
6.57
IIIb 1608,
2.43
6.59
1589
a
chel CH and chel H denote the methyl group and the hydrogen atom attached to the chelate ring, respectively.
3
tonate (IIIb) was proved by elemental analysis and
effect of the bromine substituent, bromination into the
second reaction center. Therefore, in both cases we
observe selective bromination into one reaction center.
1
1H NMR spectroscopy (see table). The H NMR spec-
tra contain singlets at 6.6 ppm, assignable to the -
hydrogen atom of the -diketonate ring. The IR spectra
of boron difluoride -diketonates almost do not change
on bromination of the alkyl group, unlike what we
observe in the case of -substitution of the chelate ring.
The IR spectra were assigned empirically with re-
ference to [13, 14] (see table).
EXPERIMENTAL
The IR spectra were recorded on Impakt-400 and
1
Specord IR-75 instruments in KBr. The H NMR
spectra were measured on a Bruker WH-50 spectro-
meter (250 MHz) in CDCl3 relative to TMS.
CH2R
CHBrR
Analysis for chromine and boron was performed by
means of atomic absorption and flame emission spec-
troscopy, respectively [15].
NBS
O
F2B
O
F2B
(PhCOO)2
H
Boron difluoride toluoylacetonate and p-ethylben-
zoylacetonate were prepared according to [16] and
chromium toluoylacetonate, according to [17].
H
O
O
IIa, IIIa
IIb, IIIb
Chromium(III) -bromotoluoylacetonate (Ib). a.
A mixture of 0.001 mol of chromium toluoylacetonate
(Ia), 0.004 mol of N-bromosuccinimide, and 30 ml of
carbon tetrachloride was heated under reflux for
30 min, cooled in an ice bath, and the succinimide
precipitate was filtered off. The filtrate was reduced
and passed through a bed of alumina. The solvent was
removed by distillation, and the residue was recrystal-
lized from benzene petroleum ether. Yield 0.41 g
(50%), mp 108 110 C. Found, %: C 48.24, 48.86; Cr
6.73, 6.91. C33H30Br3CrO6. Calculated, %: C 48.41;
Cr 6.85.
R = H (IIa, IIb), CH3 (IIIa, IIIb).
Probably, the nature of the complex-forming agent
strongly affects the direction of bromination of ben-
zoylacetylacetonates with alkyl substituents in the
benzene ring. In the metal complexes, -hydrogen of
the chelate ring is selectively substituted, whereas in
boron difluoride -diketonates, -hydrogen of the
alkyl group. Such a strongly reduced reactivity of the
-hydrogen atom in boron difluoride -diketonates
compared with metal chelates is readily explained in
view of the fact that fluorine is a strong -acceptor
and, as a result, the negative charge on the -carbon
atom in boron difluoride diketonates is lower than in
metal -diketonates.
b. A mixture of 0.001 mol of chromium toluoyl-
acetonate (Ia), 0.007 mol of N-bromosuccinimide,
0.1 g of benzoyl peroxide, and 30 ml of carbon tetra-
chloride was heated under reflux for 30 min. The re-
action product was isolated as described in procedure
a. Yield 0.36 g (44%), mp 108 110 C.
Bromination into the position of chromium to-
luoylacetonate and into the alkyl group of compounds
IIb and IIIb hinders, because of the electron-acceptor
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 10 2003

1580
GUKHMAN, REUTOV
Boron difluoride -bromotoluoylacetonate (IIb).
4. Singh, P.R. and Sahai, R., Aust. J. Chem., 1967,
A mixture of 0.005 mol of boron difluoride toluoyl-
acetonate (IIa), 0.0075 mol of N-bromosuccinimide,
0.15 g of benzoyl peroxide, and 50 ml of anhydrous
carbon tetrachloride was refluxed with stirring for
30 min, after which it was quickly cooled to 0 C, the
succinimide precipitate was filtered off, and the
filtrate was diluted with 20 ml of petroleum ether left
to stand overnight at 10 C. The precipitate that
formed was filtered off and recrystallized from iso-
propyl alcohol. Yield 0.5 g (33%), mp 98 99 C.
Found, %: C 43.37, 43.56; B 4.15, 4.20. C11H10B
BrF2O2. Calculated, %: C 43.47; B 4.22.
vol. 20, no. 4, p. 639.
5. Singh, P.R. and Sahai, R., Indian J. Chem., 1969,
vol. 7, no. 6, p. 628.
6. Kluiber, R.W., J. Am. Chem. Soc., 1960, vol. 82,
p. 4839.
7. Collman, J.P., Angew. Chem., Int. Ed. Engl., 1965,
vol. 4, no. 2, p. 132.
8. Moreno-Manas, M., Marquet, J., and Vallribera, A.,
Tetrahedron, 1996, vol. 52, no. 10, p. 3377.
9. Samath, S.A., Raman, N., Jeyasubramannian, K.,
Thambidurai, S., and Ramalingam, S.K., Indian J.
Chem., Sect. A, 1993, vol. 32, no. 7, p. 623.
Boron difluoride p-( -bromoethyl)benzoyl-
acetonate (IIIb). A mixture of 0.01 mol boron di-
fluoride p-ethylbenzoylacetonate (IIIa), 0.013 mol of
N-bromosuccinimide, 0.15 g of benzoyl peroxide, and
70 ml of anhydrous carbon tetrachloride was refluxed
with stirring for 30 min, after which it was quickly
cooled to 0 C, the succinimide precipitate was filtered
off, the solution was reduced by half, diluted with
20 ml of petroleum ether, and left overnight at 10 C.
The precipitate that formed was filtered off and dried
in air. Yiled 2.3 g (72%), mp 114 115 C. Found, %:
C 45.24, 45.38; B 3.35, 3.44. C12H13BBrF2O2. Calcu-
lated, %: C 45.33, B 3.40.
10. Skoldinov, A.P., Medvedeva, V.G., Shigorin, D.I., and
Ryabchikova, T.S., Zh. Obshch. Khim., 1963, vol. 33,
no. 9, p. 3110.
11. Druden, R.P. and Winston, A., J. Phys. Chem., 1958,
vol. 62, no. 5, p. 635.
12. March, J., Advanced Organic Chemistry. Reactions,
Mechanisms, and Structure, New York: Wiley, 1985.
Translated under the title Organicheskaya khimiya,
Moscow: Mir, 1987, vol. 3.
13. Kopteva, T.S., Medvedeva, V.G., Rodionov, A.N.,
Rucheva, I.I., Skoldinov, A.P., and Shigorin, D.N.,
Zh. Obshch. Khim., 1973, vol. 43, no. 7, p. 1587.
14. Brown, N.M.D. and Bladon, P., J. Chem. Soc. A, 1969,
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