832
P. Clavel et al.
PAPER
with an aluminium rod as the anode and a concentric cylindrical
stainless steel grid as the cathode. These two electrodes were previ-
ously chemically scoured by a 10% HCl solution, then rinsed out
several times with distilled water and with acetone. The dried cell
containing 0.8 mmol of supporting electrolyte: NBu4Br (0.25 g),
NBu4BF4 (0.25 g), NBu4PF6 (0.30 g), (CF3SO2)2NLi (0.22 g),
CF3SO3Li (0.12 g), Aliquat 336 (0.31 g), AlCl3 (0.10 g) + LiCl (0.03
g) was deaerated twice under vacuum and then with anhyd N2. THF
(55 mL), DMPU (1.8 mL, 14.9 mmol) or TDA-1 (4.8 mL, 14.9
mmol) and TMSCl (9 mL, 68.5 mmol) were introduced under light
N2 pressure. HCl resulting from the reaction between TMSCl and
the residual H2O was removed by preelectrolysing the solution (i =
0.1 A; j = 0.4 A.dm–2). The other hydrolysis product, Me6Si2O, re-
mains electrochemically inert. When evolution of H2 ceased, TFMB
(2 g, 13.7 mmol) was introduced through a septum by syringe. The
electrolysis was then performed (i = 0.1 A; j = 0.4 A.dm–2) over 9
hours, until the required charge (2.4 F◊mol–1) has been passed. The
progress of the reaction was monitored by gas chromatography. At
the end of the electrolysis, the mixture was poured into 250 mL of
cold water. The organic layer was extracted with Et2O (3 x 100 mL)
and washed with cold H2O (2 x 100 mL). After drying (MgSO4),
Et2O was evaporated off. Fractional distillation over a Vigreux col-
umn gave with DMPU 1.9 g (70%) and with TDA-1 1.8 g (66%) of
PhCF2TMS; bp = 80 °C / 2.5 kPa.
tions: NBu4Br (0.25 g) as the supporting electrolyte, THF
(15 mL) as the solvent, DMPU (15 mL) as the cosolvent,
and letting the molar ratio of TFMB/TMSCl = 1 (20 g; 2.0
mol◊L–1 of TFMB and 14.7 g; 2.0 mol◊L–1 of TMSCl). To
avoid the protonation reaction that occurs at the end of the
electrolysis due to the low concentration of the residual
TMSCl, the current was stopped after passing 1.3
F◊mol–1. In these conditions, we acquired 13 g (65 mmol)
of pure PhCF2TMS.
An extrapolation of these conditions to a larger scale is in
progress.
In order to check the ability of PhCF2TMS as a (difluo-
rophenyl)methylating agent, we reacted three carbonylat-
ed compounds with this intermediate in the presence of
TBAF according to Table 3:
It should be observed that in our conditions, the electro-
phile was used in stoichiometric proportions, instead of
the large excess recommended by M. Yoshida et al.5 With
benzaldehyde and octanal, our results are in complete
agreement with those of these authors. With cyclohex-
anone, the corresponding alcohol had never been de-
scribed.
(b) High Concentration Procedure
The cell, the electrodes and the solvent were prepared as described
in the General Procedure. The dried cell containing NBu4Br (0.25 g,
0.8 mmol) was deaerated twice under vacuum and then with anhyd
N2. THF (15 mL), DMPU (15 mL, 0.12 mol) and TMSCl (18 mL,
0.14 mol) were introduced under light N2 pressure. After the pre-
electrolysis, the TFMB (20 g, 0.14 mol) was introduced through a
septum by syringe. The electrolysis was then performed (i = 0.1 A;
j = 0.4 A.dm–2) over 48 hours, until the required charge (1.3
F◊mol–1) has been passed. The mixture was then treated as described
in the General Procedure. Fractional distillation over a Vigreux col-
umn gave 7.2 g of unreacted TFMB and 13.1 g of pure PhCF2TMS
(75% versus to converted TFMB), bp = 80 °C / 2.5 kPa.
Therefore, without any chemical equivalent, this electro-
chemical procedure offers an easy, highly selective and
safe, large scale method for the synthesis of PhCF2TMS,
a PhCF2 equivalent, from the readily available trifluo-
romethylbenzene.
–
For electrolysis in a 70 mL cell, THF (SDS) was distilled over sodi-
um-benzophenone ketyl. The cosolvents HMPA, DMPU (Fluka),
TDA-1 (Aldrich) were used without any treatment. The supporting
electrolytes were pumped off over 48 h at r.t.. Trimethylchlorosi-
lane was distilled over Mg powder just before use. Gas chromatog-
raphy was performed with a temperature-programmable Hewlett-
Packard 5890A apparatus equipped with a 25 m ¥ 0.25 mm CP-Sil
5CB capillary column. 1H NMR spectra were recorded in CDCl3 at
250 MHz with a Brucker AC 250 spectrometer, using residual
CHCl3 (d = 7.27 ppm) as the internal standard. The signals are des-
ignated s (singlet), d (doublet), t (triplet), q (quartet), and m (mul-
tiplet). 13C NMR spectra were obtained at 62.86 MHz with a
Brucker AC 250 using CDCl3 (d = 77.70 ppm) as the internal stan-
dard. The signals are designated s (singlet), d (doublet), t (triplet), q
(quartet), and m (multiplet). 29Si NMR spectra were recorded in
CDCl3 at 39.73 MHz with a Brucker AC 200 spectrometer. 19F
NMR spectra were recorded in CDCl3 at 282 MHz with a Brucker
AC 200 spectrometer. Electron impact mass spectra were recorded
at an ionisation voltage of 70 eV with a VG Micromass 16F spec-
trometer coupled with a gas chromatograph equipped with a 25 m ¥
0.25 mm CP-Sil capillary column. IR spectra were recorded with a
Perkin Elmer 1420 spectrophotometer in pure liquids films (NaCl
sheets). Elementary microanalyses were performed by the “Service
Central de Microanalyses” of CNRS (France). Solvents, PhCF3,
NBu4F in THF (1 mol◊L–1) are purchased from Aldrich, and SiO2
(9385) from Merck.
(c) Large Scale Synthesis Using a Tubular Flow Cell
The electrolytic equipment previously described by Thiebault et
al.14 (Figure 3) comprises of: a 2 L jacketed tank cooled by H2O at
r.t., a 50 mL stainless steel cylinder (125 cm2 surface area cathode)
fitted with a 20 mm diameter aluminium rod (the anode) which con-
stitutes the electrolytic cell, an Iwaki magnet pump which insures
the circulation of the electrolytic medium (imposed flow = 65
L◊min–1) through the electrolytic cell, and a racking valve to take
samples for GC analysis.
Before any electrolysis, both of the electrodes were previously
scoured by a 10% HCl solution, fitted together and rinsed out by a
circulating, in the whole setting, a dimethylformamide/acetic acid
mixture (50/50), and then rinsed twice with commercial THF.
Solvents, cosolvents, TMSCl and supporting electrolyte were used
without any treatment. TFMB (150 g, 0,5 mol◊L–1), TMSCl (600
mL, 2,5 mol◊L–1), DMPU (180 mL, 0.75 mol◊L–1) or TDA-1 (480
mL, 0,75 mol◊L–1), tetrabutylammonium bromide (6.5 g, 0,01
mol.L–1) and THF (1.2 L) were introduced into the tank. The mix-
ture was pumped into the electrolytic cell and an electric charge
(i = 1 A, j = 0.45 A.dm–2) was administered for the required time.
The complete or partial conversion of TFMB according to the cho-
sen option was monitored by GC. For example, for the complete
conversion of 150 g of TFMB, it required 86 hours of electrolysis.
(Trimethylsilyldifluoro)methylbenzene, PhCF2TMS, in a 70
mL Laboratory Cell
At the conclusion of the electrolysis, the mixture was transferred
into a flask and volatile products (TMSCl in excess, THF and
Me6Si2O) were evaporated off. The viscous residue was poured into
600 mL of a cold aq. HCl (2mol◊L–1) and the organic products were
(a) General Procedure with a 0.2 mol◊L–1 TFMB Concentration
The electrolysis of magnetically stirred solutions was performed
under nitrogen, in a previously described6,13 undivided cell fitted
Synthesis 1999, No. 5, 829–834 ISSN 0039-7881 © Thieme Stuttgart · New York