Organoelement compounds in the electrochemical synthesis of fluoroorganics

Article abstract of DOI:10.1016/S0022-1139(02)00041-6

Electrochemical generation of organic anion-radicals in the presence of fluoroorganosilanes causes the chain addition reaction. Adducts of CFCl₂SiMe₃, CFCl=CFSiMe₃ and CF₃SiMe₃ with benzaldehyde were

Full text of DOI:10.1016/S0022-1139(02)00041-6

Journal of Fluorine Chemistry 114 (2002) 225–228
Organoelement compounds in the electrochemical
synthesis of fluoroorganics
A.A. Stepanov
A.N. Nesmeyanov Institute of Organoelement Compounds, RAS 28, Vavilov Street, 119991 Moscow, Russia
Received 18 September 2001; received in revised form 23 January 2002; accepted 23 January 2002
Abstract
Electrochemical generation of organic anion-radicals in the presence of fluoroorganosilanes causes the chain addition reaction. Adducts of
CFCl SiMe , CFCl=CFSiMe and CF SiMe with benzaldehyde were obtained with conversion efficiency up to 8000%.
Electroreduction of bis-(trifluoromethyl)mercury (II) was established to be the route for the intermediate formation of trifluoromethyl
2
3
3
3
3
anion, which was trapped by the reactions with benzaldehyde, Me SiCl and bromobenzonitrile.
3
The use of salen Ni(II) complex as mediator allows the electrochemical reduction of polyfluoroalkylchlorides at the potentials more than
V higher than their reduction potentials. # 2002 Published by Elsevier Science B.V.
1
Keywords: Electrochemistry; Reduction; Organofluoro compounds; Organosilicon compounds; Organomercury compounds
1
. Introduction
Starting from 5–10 mM of silane, this reaction is comple-
ꢀ
1
The use of organoelement compounds gives many possi-
ted in 1–2 h and charge consumption is 0.02–0.33 F mol
bilities for the synthesis of fluoroorganics. Organofluoro-
trialkylsilanes and polyfluoro oganometallics (Zn, Cd, Cu,
etc.) are convenient reagents for the synthesis of partially
fluorinatedcompounds. Transitionmetalcomplexesareeffec-
tive catalysts in the addition reactions of fluoroalkylhalides.
Here will be considered some examples of the application of
elementoorganic compounds in preparative organic electro-
chemistry for synthesis of fluororganics.
(i.e. conversion efficiency is 300–8000%) relative to the
starting silane (Table 1), and depends mainly ontheimpurities
in the electrolyte (traces of moisture and oxygen).
The isomer ratio in the difluorochlorovinylsilane addition
product (entry 4) does not change in comparison with that in
the starting silane (E:Z ¼ 1:5).
Zinc powder in dimethylformamide may induce the same
reactions. Addition of fluorodichloromethyltrimethylsilane
(I) to trifluoroacetophenone proceeds exothermically and the
product was isolated in about 60% yield.
The yields from the Zn induced reaction of benzaldehyde
with (I), strongly depend on the quality of the Zn powder
2
. Results and discussion
2
.1. Organosilicon compounds
(
(
30–70%). The reduction potential of benzaldehyde
ꢀ1.83 V vs SCE) is about 0.3 V lower than that of trifluor-
The perfluoroalkylation of carbonyl compounds by fluor-
oacetophenone (ꢀ1.57 V vs SCE), so if the Zn powder is not
active enough, the generation of the anion-radical occurs
slowly, and the following reaction takes place
oorganotrimethylsilanes proceeds under catalysis with
nucleophilic initiators (usually fluoride ion). We have found
that the electrochemically generated anion-radical of a car-
bonyl compound causes the same reaction:
Zn; DMF
CFCl Tms
2
! Tms-F ð95%Þ
Destruction of (I) by Zn in DMF is the reason for the
failure of our earlier attempts [1] to prepare this silane by the
reaction of CFCl with Zn in the presence of chlorotri-
3
methylsilane—obviously, the rates of silane formation and
destruction are similar.
E-mail address: stepanov@ineos.ac.ru (A.A. Stepanov).
0022-1139/02/$ – see front matter # 2002 Published by Elsevier Science B.V.
PII: S 0 0 2 2 - 1 1 3 9 ( 0 2 ) 0 0 0 4 1 - 6

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A.A. Stepanov / Journal of Fluorine Chemistry 114 (2002) 225–228
Table 1
Electrochemically induced addition of fluoroalkyltrimethylsilanes to
0
0
carbonyl compounds: RR C¼O þ RfTms ! RfRR CꢀOTms
0
ꢀ1
F mol
Entry
R
R
R
f
Substance
yield (%)
2
(ꢁ10 )
1
2
3
4
Ph
Ph
Ph
Ph
H
CFCl
CFCl
2
62
50
84
49
1.9
1.5
1.2
32
CF
H
3
2
CF
CFCl=CF
3
H
Stainless steel cathode, Al anode, in 10–20 ml DMF, 5–10 mM silane and
carbonyl compound 5–10% excess, constant current 5–20 mA.
Contrary to the electrochemical process, where the pre-
sence of moisture only decreases the current efficiency,
in the reduction of (I) by Zn, it results in the formation
of fluorodichloromethane.
Unfortunately, our attempts at the electrochemically
induced trifluoromethylation of ethylmethacrylate and acry-
lonitrile by trifluoromethyltrimethylsilane failed; as with the
fluoride ion initiated reaction [2], only polymeric materials,
containing according to the NMR data, no fluorine were
obtained.
3 2 4 4
Fig. 1. Voltammogram of Hg(CF ) (DMF, 0.1 M Et NBF , glassy carbon
ꢀ1
electrode, SCE, 100 mV s ).
Reaction of nitrobenzene with fluorodichloromethyl- and
trifluoromethyltrimethylsilanes was completed (disappear-
ance of the silane) after consumption of less than 0.005 F
per one electron process. The following Scheme 1 may be
suggested.
The dimerization of fluorinated radicals and hydrogen
capture are the main process. Fluoromethylation of nitro-
benzene was observed only in traces. Similar results were
precursors of the trifluoromethyl anion. These substances
are available by pyrolysis of mercury trifluoroacatate with
near quantitative yields. They possess very high boiling
points. The voltammetry of bis -(trifluoromethyl)mercury
(II) shows a very complicated process (Fig. 1), nevertheless,
the reduction takes place already at the available potential
ꢀ1.5 V.
Preparative electrolysis of (II) in the presence of benzal-
dehyde, chlorotrimethylsilane and p-bromobenzonitrile
allows the preparation of corresponding tifluoromethylated
products (Scheme 2):
obtained for the reaction of m-dinitrobenzene with CF Tms
3
under electroreduction conditions, and the yields of fluor-
oalkylated aromatics were also very low.
The yields in these reactions are 40–50% and the con-
version of starting mercury compound was about 50%. The
reason of low current efficiency was that the electrolyses
were carried out in a one compartment cell with a sacrificial
anode. The copper anode was acceptable only, because
standard zinc or Al anodes, even positively polarized, caused
the decomposition of the starting mercurial (II). For this
reason, a side reaction was the cathodic deposition of
copper, arising from the anodic process. Trifluoromethyl-
mercury trifluoroacetate was used in these reactions with
approximately the same result.
2
.2. Organomercury compounds
Trifluoromethylhalides (bromides and iodides) are excel-
lent sources of the trifluoromethylanion, both in organic
and electroorganic [3,4] syntheses. In the latter case, their
low boiling points are substantial inconvenience, because
the special electrochemical equipment for working under
pressure is not readily available. We have suggested that
bis-(trifluoromethyl)mercury (II) or trifluoromethylmercury
trifluoroacetate, when electrochemically reduced, may be
Scheme 1. Reaction of electrochemically induced anion-radical of nitrobenzene with organofluorotrimethylsilane.

A.A. Stepanov / Journal of Fluorine Chemistry 114 (2002) 225–228
227
Scheme 2. Electroreductive reactions of bis-(trifluoromethyl)mercury (II).
2
.3. Organometallic mediators
chloride to the electrolyte, causes an irreversible process and
the peak current increases by five times (Fig. 2, curve 2), the
molar ratio chloride: complex being 1.4. It should be noted
that the catalytic effect on the fluorinated alkylchlorides is
very high. In the case of non-fluorinated species, the addition
of even a 10-fold excess of halide is followed by the increase
of the peak current up to 1.5 times [5]. Preparative electro-
lysis of CF CH Cl in acetonitrile on a platinum cathode in
Polyfluoroalkyl chlorides are less expensive than corre-
sponding bromides and iodides, but they were not used in
electrochemical synthesis because of their low reduction
potential. We find out that the salen Ni(II) complex is an
effective mediator for reduction of CF CH Cl (reduction
3
2
potential is more negative than –2.7 V) and H(CF ) Cl
2
6
3
2
(
complex (Fig. 1, curve 1) shows one electron reversible
reduction at ꢀ1.65 V. The addition of the polyfluoroalkyl
reduction potential, ꢀ2.65 V). Voltammetry of salen Ni(II)
the presence of catalytic amounts of salen Ni(II) complex
at ꢀ1.6 V results in 1,1-difluoroethene formation. On the
addition of benzaldehyde (reduction potential ꢀ1.8 V) to
the electrolyte, a low yield mixture of trifluoroethylated
aromatics, forming
obviously, both via trifluoroethyl radical and anion, was also
observed.
Preparative electroreduction of H(CF ) Cl in acetonitrile
2
6
(
pKa ¼ 25), using the salen Ni(II) complex leads to hydro-
genolysis of the carbon–chlorine bond and H(CF ) H was
2
6
obtained with 50% current efficiency. So, as the conversion
of starting material was lower than 100%, the substance
yield is apparently higher. In dimethylformamide (pK ¼ 32)
a
solution, proton abstraction is hindered and defluorination
product 6-H-perfluorohexene-1 also forms.
3. Experimental
Electrochemically induced addition of fluoroorganosi-
lanes to carbonyl compounds were carried out in a 30 ml
glass cell, equipped with cooling jacket, magnetic stirrer,
2
inert gas inlet, stainless steel cathode (20 cm ), and Al anode
(6 mm diameter rod). The cell was charged with 20 ml DMF,
0
5
.2 g of Et NBF , 5–10 mM silane and carbonyl compound,
4 4
–10% excess, and electrolyzed under intensiostatic condi-
2
tions with current density 0.25–1 mA/cm , till the silane
disappearance (controlled by GLC). The electrolyte was
poured into 20 ml of water and extracted with ether (3 ꢁ
Fig. 2. Voltammogram of salen Ni(II) complex (acetonitrile, 0.1 M
ꢀ3
ꢀ1
Et
4
NBF
4
, glassy carbon, SCE, 100 mV s ). Curve 1, 2 ꢁ 10 M; curve
2
, 2 ꢁ 10ꢀ salen Ni(II) complex and 2:8 ꢁ 10ꢀ3 M of H(CF
3
2
)Cl.
15 ml), dried over MgSO . The pure silyl esters were
4

228
A.A. Stepanov / Journal of Fluorine Chemistry 114 (2002) 225–228
obtained by the column chromatography (Al O , hexane).
2
Acknowledgements
3
The silyl esters were transferred by acidic hydrolysis into
corresponding alkohols and identified by comparison with
authentic samples [6].
Author thanks the French Ministry of National Education
and ESFC-13 Organizing Committee for the financial sup-
port of this presentation.
Fluoroorganomercurials were reduced in the same cell,
using copper anode (6 mm diameter rod). The cell was
charged with 20 ml DMF, 6 ml of tetramethylethylenedia-
mine, 0.2 g of Et NBF , 10 mM of (II) and 20–30 mM of
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