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A.P. Lothian et al. / Tetrahedron 67 (2011) 2788e2793
petroleum ether (bp 60e80 ꢃC)) to give compound 1 (R¼NO2)
(1.69 g, 35%), colourless prisms, mp 36e7 ꢃC [lit.,38 38e9 ꢃC]; 1H
aryltrimethylsilane 1 (0.74 mmol) in the appropriate solvent (5 mL)
was stirred at room temperature and xenon difluoride (250 mg,
1.48 mmol) was quickly added. Stirring was continued (1 h) and the
mixture then analysed by 1H NMR and GCeMS. If required, the aryl
fluoride 4 was isolated by evaporation followed by chromatography
(silica gel: eluent, petroleum ether (bp 60e80 ꢃC)/ethyl acetate).
Typically, this procedure using hexafluorobenzene as solvent gave
4-tert-butyl-fluorobenzene 4 (R¼tBu) (96 mg, 85%), colourless oil,
identical with an authentic sample, and using dichloromethane as
solvent gave 1,4-difluorobenzene 4 (R¼F) (62 mg, 73%), colourless
liquid, identical with an authentic sample.
NMR (CDCl3) d 0.3 (s, 9H, SiMe3), 7.65 (d J 7.8 Hz, 2H, ArH) and 8.15
(d J 7.8 Hz, 2H, ArH); 13C NMR (CDCl3) -1.60 (SiMe),122.1 (CH),134.0
(CH), 147.4 (CSi) and 149.8 (CNO2); IR (KBr) nmax/cmꢀ1 1519, 1352,
1251 and 837; MS (EI) m/z 195 (Mꢂþ), 180 (100%), 164, 134, 119 and
105, and 1,4-bis(trimethylsilyl)-2-nitrobenzene (0.15 g, 3%), colour-
less crystals, mp 30e35 ꢃC; 1H NMR (CDCl3)
d 0.3 (s, 9H, SiMe3), 0.4
(s, 9H, SiMe3), 7.7 (m, 2H, ArH) and 8.4 (m, 1H, ArH); 13C NMR
(CDCl3) ꢀ1.4 (SiMe), 0.5 (SiMe), 128.3 (CH), 135.5 (CH), 137.9 (CH),
144.0 (CSi) and 153.2 (CNO2); IR (KBr) nmax/cmꢀ1 3067, 2955, 1700,
1664, 1653, 1533, 1506, 1353, 1252, 836 and 749; MS (EI) m/z 252
(MꢂþꢀMe), 207, 177, 133 and 83. Anal. Calcd for C12H21NO2Si2: C,
53.89; H, 7.91; N, 5.24. Found: C, 53.90; H, 8.29; N, 5.31%.
4.4. Fluoroarenes 4
Authentic samples of the fluoroarenes 4 (R¼H, Me, F, Cl, OMe,
SiMe3, NO2, COMe, COPh, SMe) were purchased from SigmaeAldrich,
UK. 40-Fluoroacetanilide 4 (R¼NHCOMe) was purchased from Apollo
Scientific Ltd, UK.
4.2.3. N-[4-(Trimethylsilyl)phenyl]acetamide 1 (R¼NHCOMe). A so-
lution 4-nitrophenyltrimethylsilane 1 (R¼NO2) (0.5 g, 2.6 mmol) in
toluene (50 mL) together with 10% Pd/C catalyst (100 mg) was
stirred under hydrogen (1 atm pressure) until uptake of hydrogen
was complete. After filtration through Celite and evaporation,
Kugelrohr distillation of the residue gave 4-(trimethylsilyl)aniline 1
(R¼NH2) (3.71 g, 89%), pale yellow oil, bp 240 ꢃC at 350 mmHg
4.4.1. 4-tert-Butyl-fluorobenzene
4
(R¼tBu). 4-tert-Butylaniline
(4.5 g, 0.03 mol) and water (20 mL) were cooled in an ice bath and
HCl (37%) (6.0 g, 0.06 mol) in water (40 mL) was then added with
stirring at 0 ꢃC. After 10 min, NaNO2 (2.08 g, 0.03 mmol) in water
(12 mL) was added slowly, keeping the temperature below 5 ꢃC,
and the mixture turned yellow. After a further 30 min, piperidine
(11.73 g, 0.138 mol) in water (10 mL) was added. Orange crystals
formed on top of the yellow liquid and these were collected,
washed, dried under vacuum and identified as 1-(4-tert-butyl-
phenyl)-3,3-(1,5-pentadienyl)triazine (6.86 g, 93%), orange prisms;
[lit.,39 130e132 ꢃC at 30 mmHg]; 1H NMR (CDCl3)
d0.1(s, 9H, SiMe3),
3.3 (s, 2H, NH2), 6.35 (d J 8.3 Hz, 2H, ArH) and 7.05 (d J 8.3 Hz, 2H,
ArH); MS (EI) m/z 165 (Mꢂþ), 133, 120, 106, 77, 65 and 43. The
product 1 (R¼NH2) (0.19 g, 1.2 mmol) in dry toluene (5.0 mL) was
stirred vigorously and acetic anhydride (0.11 mL, 1.2 mmol) and
pyridine (0.10 mL, 1.2 mmol) were added. After stirring at room
temperature (3 h) a white crystalline solid had separated from the
orange solution. This was collected and identified as compound 1
(R¼NHCOMe)(0.13 g). The solution was evaporated and the orange
residue was purified by chromatography (silica gel: eluent, petro-
leum ether (bp 60e80 ꢃC)/ethyl acetate 4:1) to give an additional
batch (0.04 g). The batches were combined to give compound 1
(R¼NHCOMe) (0.17 g, 71%), colourless plates, mp 170e171 ꢃC [lit.,40
1H NMR (CDCl3)
d 1.3(s, 9H, CMe3),1.6 (m, 6H, CH2), 3.7 (br s, 4H,
NCH2) and 7.3 (s, 4H, ArH), which was used as follows without
further purification. The triazine (4.48 g, 0.018 mol) was dissolved
in a minimum amount of toluene in a diaflon (polytrifluoromono-
chloroethylene) vessel and HFepyridine complex (2.61 g, 5 equiv)
was added slowly. The mixture was stirred (30 min) and then
heated to 50 ꢃC (10 min). After cooling, the mixture was neutralised
using aq NaOH (ca. 4 mL) and water (25 mL) was added. The
aqueous mixture was extracted with ether (3ꢁ25 mL) and the
combined ether layers were washed with dil HCl, dried (MgSO4)
and distilled to afford 4-tert-butyl-fluorobenzene 4 (R¼tBu) (0.53 g,
19%), colourless oil, bp 167e172 ꢃC [lit.,41 bp 172 ꢃC]; 1H NMR
169e170 ꢃC]; 1H NMR (CDCl3)
d 0.25(s, 9H, SiMe3),2.15 (s, 3H, Me)
and 7.5 (m, 4H, ArH); IR (KBr) nmax/cmꢀ1 3255, 1670, 1590, 1540,
1248 and 840; HRMS m/z calcd for C11H17NOSi 207.1079, found
207.1082.
4.2.4. 2-(But-3-en-1-yl)phenyltrimethylsilane 10. To a stirring solu-
tion of 2-trimethylsilyl-(bromomethyl)benzene16 (1.0 g, 4.0 mmol)
in THF (5.0 mL), under an argon atmosphere, allyl magnesium
bromide (21.0 mL, 21.0 mmol) was added dropwise. The solution
darkened with formation of a white precipitate. The mixture was
then heated under reflux (1 h), cooled to 0 ꢃC and carefully
quenched with 2 M H2SO4. Water was then added and the two
phases separated. The aqueous layer was extracted with Et2O, dried
(MgSO4) and evaporated to give a yellow oil (0.58 g), which was
Kugelrohr distilled to give compound 10 (0.51 g, 61%), pale yellow
(CDCl3) d 1.35 (s, 9H, CMe3),7.05(m, 2H, ArH) and 7.40 (m, 2H, ArH);
19F NMR (CDCl3) ꢀ119.0 (s, CF); IR (liquid film) nmax/cmꢀ1 3060,
2965,1392,1364 and 1230; MS (EI) m/z 152 (Mꢂþ) (20%), 137 (100%),
121, 115, 109, 101, 95, 80, 74, 63 and 57.
Acknowledgements
We thank the EPSRC for the award of studentships (to M.M.S.
and R.G.S.) and the EPSRC National Mass Spectrometry Service
Centre for high-resolution mass spectra.
oil, bp 135e140 ꢃC at 40 mmHg; 1H NMR (CDCl3)
d 0.2 (s, 9H,
SiMe3), 2.3 (m, 2H, CH2), 2.8 (m, 2H, CH2), 4.95 (m, 2H, CH2), 5.85
(m, 1H, CH) and 7.0e7.4 (m, 4H, ArH); 13C NMR (CDCl3) 0.4 (SiMe),
35.4(CH2), 36.5 (CH2), 114.8 (CH2), 125.2 (CH), 128.5 (CH), 129.2
(CH), 134.5 (CH), 138.0 (CCH2) and 147.6 (CSi); IR (KBr) nmax/cmꢀ1
3058, 2956, 1438, 1262, 1249, 1128, 912, 849, 837, 752 and 621; MS
(EI) m/z 204 (Mꢂþ), 189, 163, 129, 105, 59(100%) and 31. Anal. Calcd
for C13H20Si: C, 76.40; H, 9.86. Found: C, 76.15; H, 9.60%.
References and notes
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4.3. Fluorodesilylations using xenon difluoride
Xenon difluoride was purchased from Apollo Scientific Ltd, UK.
Reactions were carried out in conventional round bottom PyrexÒ
flasks (10 mL). In early studies reactions were carried out in a glove
box under an atmosphere of dry nitrogen but it was subsequently
found that this was unnecessary. In a typical procedure, the