76
V. Jairaj, D.J. Burton / Journal of Fluorine Chemistry 121 (2003) 75–77
reaction mixture was stirred at room temperature for 3 h. 19F
NMR analysis of the reaction mixture indicated that the
zinc reagent was the major product, with trace quantities of
trifluoroethene and starting material. The concentration of
5,6
the zinc reagent was calculated to be 0.62 M, while the
conversion was 74%. In all subsequent trials, the yield of the
zinc reagent prepared following the above procedure ranged
from 65 to 75%.
1
9
F NMR (DMF solvent, PhCF reference): ꢀ96.3 ppm
3
a
(
dd, Jab ¼ 93 Hz, Jac ¼ 33 Hz, 1F ); ꢀ130.6 ppm (dd,
b
Jbc ¼ 104 Hz, Jab ¼ 93 Hz, 1F ); ꢀ195.0 ppm (dd,
Fig. 1. Condensation of 1-bromo-1,2,2-trifluoroethene using a dry ice
condenser.
c
J
¼ 104 Hz, J ¼ 33 Hz, 1F ) [25].
bc
ac
available, low temperature processes are avoided, no electro-
chemical cells needs to be constructed, toxic reagents are
avoided, and the overall scheme provides for each work-up
and isolation of trifluorovinyltrimethylsilylethene.
3. Preparation of 1,1,2-trifluoro-2-
trimethylsilylethene
A 250 ml three necked round bottomed flask equipped
with a side arm, PTFE coated magnetic stir bar, cold water
condenser attached to a nitrogen-T, thermometer, and rubber
septum was charged with the zinc reagent prepared above
(154 mmol) (Fig. 2, Scheme 1). To this zinc reagent was
added 34.9 g (323 mmol, 2.1 equivalents) of dry chlorotri-
2. Preparation of 1,1,2-trifluoroethenyl zinc—a
general procedure
A 1 l three necked round bottomed flask was equipped with a
PTFE coated magnetic stir bar, dry ice condenser cooled with
dry ice and isopropanol attached to a source of nitrogen, rubber
7
methylsilane. The system was placed in an ice water bath.
Then, 2.2 g (15 mmol, 0.10 equivalent) of copper(I) bro-
8
mide was added all at once to the reaction mixture. After the
addition of the copper(I) bromide was complete, the reaction
septum and thermometer. The flask was charged with 16.23 g
(
1
249.69 mmol) of activated zinc powder and 250 ml of dry
2
3
DMF. 1-Bromo-1,2,2,-trifluoroethene (34 g, 210.67 mmol)
was added dropwise via the dry ice condenser (Fig. 1). The
reaction generally initiated after a short induction period and was
4
5
The NMR yield and concentration are calculated by addition of a
exothermic. After the addition of the ethene was completed, the
known amount (usually 5 ml) of benzotrifluoride to 0.5 ml of the reaction
mixture in an NMR tube. The NMR spectrum is obtained in the usual
manner and the integral ratios are obtained as accurately as possible. The
concentration is calculated by the following relation:
1
Zinc was activated by stirring 50 g of zinc powder with 500 ml of distilled
water. To this slurry was added ꢁ10 ml of concentrated hydrochloric acid
drop-wise till the metal began to coagulate. After stirring the slurry for about
ꢀ0
ꢁ
:041 ꢂ 3
b
2
0 min, the zinc was washed repeatedly with water (3 ml ꢂ 500 ml), acetone
ꢂ
¼ c
a
0:5
(
3 ml ꢂ 250 ml) and finally with ether (3 ml ꢂ 100 ml). The activated zinc
powder, which had a silvery white appearance, was dried under full vacuum
for at least 8 h. This activated zinc powder can be stored either under an
atmosphere of nitrogen or vacuum for ꢁ1 month. The zinc powder can also be
activated by Knochel’s method, cf. [25].
where ‘a’ is the integral value of the standard, ‘b’ the normalized integral
value of the sample, and ‘c’ the concentration of the sample in mmol/ml.
The product of ’c’ and the total volume of the reaction mixture in ml gives
the millimoles of the zinc reagent. This zinc reagent can be stored under
nitrogen for at least 1 month with minimal loss of activity.
2
Stirred over calcium hydride overnight and distilled under reduced
˚
pressure. The dried DMF was stored in amber bottles over 4 A MS.
6
It is important that the concentration of the zinc reagent be less than
3
Weight calculated by weighing the tank before and after the addition.
1 M. Highly concentrated zinc reagents (typically greater than 1 M) do not
readily exchange with copper (I) halides.
4
When the reaction was carried out on a larger scale, it is advisable to add
7
part of the 1-bromo-1,2,2-trifluoroethene, wait until the reaction initiates, and
then bleed the remaining ethene slowly into the reaction mixture. This avoids
an uncontrollable exotherm. If the reaction does not initiate, the stirring is
stopped; and the zinc is allowed to settle, and ꢁ0.2 ml of chlorotrimethylsilane
is added. Once the surface of the reaction mixture turns pale yellow, the
reaction has initiated and stirring is again commenced. Freezing of the gas
inside the condenser and thus blocking it is avoided by using dry ice/
isopropanol as the cooling mixture. The temperature of this mixture is ꢀ78 8C,
much higher than the freezing point of 1-bromo-1,2,2-trifluoroethene. In the
unlikely event of an uncontrollable exotherm, rapid pressure build-up is
prevented by both the nitrogen flowing through the reaction flask as well as
efficient cooling of the condenser.
Stirred over calcium hydride for 1 h, distilled at ambient pressure and
used immediately.
8
Copper (I) bromide was activated as follows: 50 g of copper (I)
bromide was stirred with 20 ml of distilled water. To this slurry was added
50 ml of 48% hydrobromic acid. Once the copper (I) bromide had
dissolved, it was re-precipitated by addition of ꢁ500 ml of water. The
precipitated copper (I) bromide was washed repeatedly with 500 ml
portions of water till the washings were neutral to a pH paper. The copper
(I) bromide was then washed with acetone (3 ml ꢂ 250 ml), ether
(3 ml ꢂ 100 ml), and dried under full vacuum for at least 8 h. The off-
white colored copper (I) bromide can be stored for at least 1 month in the
absence of air or oxygen.