P.L. Coe et al. / Journal of Fluorine Chemistry 92 (1998) 27±32
29
5
methylsilane (1.86 g, 0.01 mol.) in acetic anhydride (1,8 g,
0.018 mol) at room temperature. The mixture was then
heated at 458C for 5 h and the progress of the reaction
was monitored by TLC. Since starting material was still
present more nitric acid (1.43 g) was added and heating was
continued for further 17 h. Work up as above afforded an oil
(1.62 g). Puri®cation by column chromatography (hexane/
ether [5:1] on silica) gave 2,4-di¯uoronitrobenzene (0.52 g,
32.7%), 1,3-di¯uorobenzene (0.03 g, 2.3%) both identical
to authentic samples and 3-¯uoro-4,6- dinitrophenol
(0.14 g, 6.8%) mp 82±838C ꢀH7.10 (d, 1H, 3JHF11.2 Hz,
Hz, 2-H), 9.06 (d, 1H, 4JHF7.8 Hz, 5-H) and 11.10 (s, 1H,
11.7%); ꢀH0.42 (t, 9H, JHF1.5 Hz, SiMe3), 6.95
3 3 5
(ddd,1H, JHF9 Hz, JHH7 Hz, JHF1.5 Hz, 5-H),
8.08 (td, 1H,4JHF9 Hz, JHH6 Hz, 4-H), ꢀF 85.8 (m,
3
1F, 6-F ), 100.2 (m, 1F, 2-F) MS (EI) m/z 231 (M) , 216
(M±Me) , 171 (M±MeNO) . The 13C NMR spectrum was
consistent with the proposed structure.
2.8. Preparation of 3-bromo-2,6-
difluorophenyltrimethylsilane
A
solution of 1-bromo-2,4-di¯uorobenzene (10 g,
0.052 mol) in dry THF (30 cm3) was added dropwise to a
solution of LDA (0.052 mol in THF (25 cm3) at 788C. The
mixture was stirred for 1 h and chlorotrimethylsilane (10 g
0.092 mol) was added over 30 min. The mixture was stirred
overnight as it was gradually warmed to room temperature.
The lithium chloride which formed was ®ltered off and the
®ltrate was washed successively with 5% NaHCO3 and
water. The aqueous washings were extracted with ether
and the combined organic layers were dried (MgSO4)
and the solvent evaporated to leave an oil (11.3 g) which
on distillation in vacuo afforded 3-bromo-2,6-di¯uorophe-
nyltrimethylsilane (6.3 g, 46%) bp 80±858C @ 5.5 mm Hg.
OH); [MS (EI) m/z 202(M) , 186 (M±O) , 172 (M±NO) ].
2.6.2. (b) By Eaborn's method
Nitric acid (70%, 2.7 g, 0.03 mol) was added dropwise
over 5 min. to acetic anhydride (8.4 g, 0.082 mol) with
stirring in a conical ¯ask cooled in an ice bath (CARE).
This mixture was then added dropwise with stirring over
15 min to 2,4-di¯uorophenyltrimethylsilane (2.8 g,
0.015 mol) in acetic acid (9.6 g, 0.16 mol) at room tem-
perature. The reaction mixture was heated and stirred at
508C for 4 h, when a second identical portion of the nitrating
mixture was added and the mixture was then heated at 508C
for a further 17 h. Finally, a third portion of the nitrating
mixture was added and the whole mixture heated for a
further 48 h. After this time the mixture was worked up as
for the Eaborn method above to yield an oil (3.88 g).
Puri®cation by column chromatography (hexane and then
hexane/ether 10:1 on silica) gave unreacted starting material
(0.53 g, 19%), 2,4-di¯uoronitrobenzene (0.39 g, 16.2%),
1,3-di¯uoro-4,6-dinitrobenzene (0.01 g, 0.03%), 3-¯uoro-
4,6-dinitrophenol (0.02 g, 0.7%) identical with authentic
samples and 2,4-di¯uoro-5-nitrophenyltrimethylsilane
MS m/z 266/264 [M] , 251/249 [M±Me] , ꢀH 0.38 (t, 9H
5JHF1.5 Hz), 6.73 (m,1H) 7.47 (m,1H) ꢀF 91 (m,1F),
98.9 (m,1F). Accurate mass: requires 263.999814, Found:
263.998009.
3. Results and discussion
The preparation of the desired trimethylsilanes has been
described previously using the reaction of the corresponding
¯uoroaryllithium or magnesium reagents [8]. Bromodesi-
lylations have been carried out in a variety of ways but most
work has been done either with elemental bromine in acetic
acid [3±5] or by using elemental bromine alone.[9] We
initially chose to study the reactions in acetic acid, as used
by Eaborn et al., and the reactions were monitored by GC. In
each of the cases studied, the reactions appeared to proceed
in good yields, by GC analysis, but there were considerable
losses in workup by the literature method, which involved
washing away the acetic acid with large volumes of water.
This problem was partially overcome by pouring the pro-
duct into water followed by a standard ether extraction/
sodium bicarbonate washing procedure. In this way, mod-
erate yields of the desired bromo¯uoro-compounds were
obtained. Thus, 2,4-di¯uorophenyltrimethylsilane afforded
bromo-2,4-di¯uorobenzene in 18% yield, 2,6-di¯uorophe-
nyltrimethylsilane gave bromo-2,6-di¯uorobenzene in 25%
yield and interestingly 3-bromo-2,6-di¯uorophenytri-
methylsilane was also obtained in 21% yield from 1,3-
di¯uoro-2,4-bis (trimethylsilyl)benzene. This latter reaction
showed that it is possible to remove trimethylsilyl groups in
this series selectively, con®rming related observations by
Bennetau [7].
5
(0.12 g, 3.4%) ꢀH0.37 (d 9H, JHF1 Hz, SiMe3), 6.95
3
3
(dd, 1H, JHF11 Hz, JHF7.8 Hz, 3-H), 8.13 ( dd, 1H,
4
4JHF9 Hz, JHF5.5 Hz, 6-H), ꢀF 86.4 ( m, 1F, 2-F),
111.7 (ddd, 1F 4JFF16.3 Hz, 3JHF10.6 Hz, 4JHF9 Hz,
4-F), the 13C NMR spectrum was consistent with the
structure.
2.7. Nitrodesilylation of 2,6-difluorophenyltrimethylsilane
by Chvalovsky's method
Nitric acid (70%, 1.46 g, 0.016 mol) was added dropwise
over 30 min to a solution of 2,6-di¯uorophenyltrimethyl-
silane (1.86 g, 0.01 mol) in acetic anhydride at room tem-
perature. The reaction mixture was heated at 658C for 5 h,
when a second identical portion of nitric acid was added
over 15 min. The mixture was further heated at 658 for 24 h.
The mixture was worked up as above to yield an oil (0.66 g),
which on puri®cation by column chromatography (hexane/
ether 20:1 on silica) afforded 1,3-di¯uorobenzene (0.15 g,
13.4%), 2,4-di¯uoronitrobenzene (0.04 g, 2.6%), a trace of
2,6-di¯uoronitrobenzene, all identical with authentic sam-
ples and 2,6-di¯uoro-3-nitrophenyltrimethylsilane (0.27 g,