The effect of substituents and operating conditions on the electrochemical fluorination of alkyl phenylacetates in Et3N·4HF medium

Article abstract of DOI:10.1016/j.jfluchem.2007.10.007

Selective electrochemical fluorination of alkyl phenylacetates (Ph-CH₂-COOR, where R is methyl, ethyl, n-propyl, n-butyl, i-propyl and sec-butyl) under galvanostatic conditions were reported in Et₃N·4HF medium. Preparative electrolysis experiments were carried out both in pre-electrolysed dry Et₃N·4HF and the same electrolyte medium without pre-electrolysis. Very little hydrolysed fluorinated products were obtained in pre-electrolysed medium where as significant quantities of hydrolysed products leading to fluorinated phenylacetic acid were obtained from Et₃N·4HF without pre-electrolysis. Under optimum experimental conditions up to 87% selectivity of monofluoro ester could be achieved. Difluoro alkyl phenylacetate, monofluoro and difluoro phenylacetic acids were the other predominant side products obtained. The hydrolysis appears to be initiated by tautomeric transformation of proton after the initial electro oxidative formation of the cation radical. ¹⁹F as well as ¹H NMR spectroscopy have been employed to identify the minor constituents formed during the electro oxidative process.

Full text of DOI:10.1016/j.jfluchem.2007.10.007

Available online at www.sciencedirect.com
Journal of Fluorine Chemistry 129 (2008) 185–192
www.elsevier.com/locate/fluor
The effect of substituents and operating conditions on the electrochemical
fluorination of alkyl phenylacetates in Et3NÁ4HF medium
*
N. Ilayaraja, A. Manivel, D. Velayutham, M. Noel
Electroorganic Division, Central Electrochemical Research Institute, Karaikudi 630006, India
Received 3 August 2007; received in revised form 31 October 2007; accepted 31 October 2007
Available online 4 November 2007
Abstract
Selective electrochemical fluorination of alkyl phenylacetates (Ph–CH2–COOR, where R is methyl, ethyl, n-propyl, n-butyl, i-propyl and sec-
butyl) under galvanostatic conditions were reported in Et3NÁ4HF medium. Preparative electrolysis experiments were carried out both in pre-
electrolysed dry Et3NÁ4HF and the same electrolyte medium without pre-electrolysis. Very little hydrolysed fluorinated products were obtained in
pre-electrolysed medium where as significant quantities of hydrolysed products leading to fluorinated phenylacetic acid were obtained from
Et3NÁ4HF without pre-electrolysis. Under optimum experimental conditions up to 87% selectivity of monofluoro ester could be achieved. Difluoro
alkyl phenylacetate, monofluoro and difluoro phenylacetic acids were the other predominant side products obtained. The hydrolysis appears to be
initiated by tautomeric transformation of proton after the initial electro oxidative formation of the cation radical. 19F as well as 1H NMR
spectroscopy have been employed to identify the minor constituents formed during the electro oxidative process.
# 2007 Elsevier B.V. All rights reserved.
Keywords: Selective electrochemical fluorination; Et3NÁ4HF; Alkyl phenylacetates; a-Fluoro phenylacetates
1. Introduction
this powerful fluorinating agent, the yield obtained was around
56% [4]. Quite recently Yoneda co-workers have reported 17%
yield during the selective electrochemical fluorination of ethyl
phenylacetate in dichloromethane using different supporting
electrolytes [11]. A brief report by Cao and Fuchigami is also
available [12].
a-Fluoro carbonyl derivatives themselves are of importance
and these can also be used as building blocks to synthesize more
complex drug molecules, in which the a-fluorine is able to
modify the biological activity [1–4]. Despite the availability of
multi-step chemical routes, efforts to synthesize these
molecules through single step electrochemical partial fluorina-
tion route also continue.
Solvent-free electrolytes have been used as the media for
compounds, which are relatively more difficult to oxidize [5,7].
Et3NÁ3HF [13,14], Et4NFÁnHF [15–20] and Et3NÁ5HF [21,22]
are the most commonly employed solvent-free electrolyte
media for partial fluorination. The selective fluorination of
benzyl cyanide [23] and cyclic ethers [24] has been achieved in
these systems.
For partial fluorination of aromatic compounds by electro-
chemical method in fluoride containing solvent-free electrolyte
systems has been extensively studied and reviewed [5–8].
These studies indicate high selectivity towards fluorination of
active methylene group attached to sulfur atom [5–8].
Relatively few reports on partial fluorination of phenylacetic
acid and its derivatives are available in the literature. Selective
fluorination at benzylic position and chiral substituents was
reported by Laurent and co-workers [9,10]. Ethyl phenylacetate
was chemically fluorinated with perchloryl fluoride. Even with
In the present work, partial fluorination of methyl, ethyl, n-
propyl, n-butyl, i-propyl and sec-butyl esters of phenylacetic
acid in solvent-free Et3NÁ4HF medium is reported. Efforts are
made to optimize the galvanostatic experimental condition to
achieve higher selectivity of fluorinated products namely a-
fluoro esters and a-fluoro phenylacetic acid. In addition to
evaluating the effect of pre-electrolysis of solvent-free
electrolyte system, the present work also attempts to study
the effects of alkyl substituents as well as excessive electric
charge on the product selectivity and distribution.
* Corresponding author. Tel.: +91 4565 227772; fax: +91 4565 227779.
E-mail address: yemenoel@yahoo.co.in (M. Noel).
0022-1139/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2007.10.007

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2. Results and discussion
Selectivity between fluorinated esters and fluorinated free
acid was found to be highly dependent on moisture content in
the reaction medium. The effects of pre-electrolysis on the
voltammetric responses were evaluated. Fig. 1(a) shows the
cyclic voltammetric response of Et3NÁ4HF prior to pre-
electrolysis. Slightly higher background current is noticed
above 0.5 V in this medium. A small anodic wave is also
observed around 1.7 V. After pre-electrolysis noticeable
background current was observed only above 1.0 V
(Fig. 1(b)). The anodic wave also disappears completely after
the pre-electrolysis.
Typical cyclic voltammograms for different concentration of
ethyl and butyl phenylacetates on platinum electrode in pre-
electrolysed Et3NÁ4HF medium are presented in Figs. 2 and 3,
respectively, along with the background current. The electro
oxidation occurs along with supporting electrolyte oxidation
and hence the anodic peaks are found to be merged with
background oxidation current. However, the oxidation process
for butyl phenylacetate is initiated at around 1.35 Vas shown in
Fig. 3, whereas the oxidation process for ethyl phenylacetate
occurs only beyond 1.45 V as shown in Fig. 2. This suggests
that increasing the size of alkyl substituents facilitates the
electrochemical oxidation process. Under identical concentra-
tion and sweep rate, the anodic peak current for butyl
phenylacetate was found to be lower than that of the
corresponding ethyl phenylacetate. This may be due to either
blocking type of adsorption or decrease in the diffusion
coefficient as a result of increase in the molecular size.
Monofluoro alkyl esters (2a–f) were observed to be the
predominant products during the electrochemical fluorination
in pre-electrolysed Et3NÁ4HF medium (Scheme 1, Table 1).
Fig. 2. Typical cyclic voltammogram of 1b in Et3NÁ4HF on Pt electrode at
different concentrations b–e = 10, 20, 30, 40 mM, respectively, n = 10 mV/s.
Where (a) is background current.
Fig. 1. Typical background cyclic voltammogram of Et3NÁ4HF on Pt electrode
with out (a) and with (b) pre-electrolysed electrolyte at a scan rate (n) of
10 mV/s.
Fig. 3. Typical cyclic voltammogram of 1d in Et3NÁ4HF on Pt electrode at
different concentrations b–e = 10, 20, 30, 40 mM, respectively, n = 10 mV/s.
Where (a) is background current.

N. Ilayaraja et al. / Journal of Fluorine Chemistry 129 (2008) 185–192
187
Table 1
Selectivity of products obtained during anodic fluorination of 1a–fa in pre-electrolysed Et3NÁ4HF medium
Compound
Conversionb (%)
Selectivity of fluorinated productsc (mole%)
Mono fluoro ester
Mono fluoro acid
Difluoro ester
Difluoro acid
Othersd
1a
1b
1c
1d
1e
1f
83
86
88
89
84
87
61.4
87.2
68.8
82.4
81.7
26.8
25.1
<1
3.1
8.1
5.6
<1
6.1
4.8
3.7
13.1
<1
6.5
5.5
10.2
10.7
3.3
<1
<1
2.8
6.4
<1
29.1
6.6
38.0
a
Charge passed = 2 F/mole.
b
c
d
Based on HPLC data.
Based on 19F NMR.
Other products include compounds 6–28 depending on the reactants.
Table 2
Except for sec-butyl phenylacetate (1f), good yields of
monofluoro esters (above 60% yield) were obtained for alkyl
phenylacetates (1a–e) (Table 1). In the cases of ethyl (1b), n-
butyl (1d) and isopropyl (1e) esters, the yield of 2b, 2d and 2e
was found to be above 80%. In the cases of methyl (1a), propyl
(1c) and sec-butyl (1f) esters, a-fluoro phenylacetic acid (4)
was obtained in substantial quantities even in the pre-
electrolysed Et3NÁ4HF medium. In the case of 1f, other
nuclear fluorinated products were also obtained in substantial
yield.
Selectivity of products obtained during anodic fluorination of 1a
Charge
passed
Conversiona(%)
Selectivity of fluorinated products (mole%)b
(F/mole)
2a
4
3a
0.8
5
Othersc
2
3
4
5
6
84
92
95
96
98
24.0
57.2
68.2
43.2
23.3
26.6
18.4
11.7
12.6
13.6
1.4
3.2
1.5
5.7
2.6
47.2
17.0
16.0
31.2
48.2
4.3
2.7
7.4
12.4
a
Based on HPLC data.
Based on 19F NMR.
The above results suggest that ester hydrolysis occurs as the
side reaction even in pre-electrolysed Et3NÁ4HF medium. All
further electrochemical fluorination reactions were carried out
in Et3NÁ4HF medium without pre-electrolysis and hence the
fluorinated esters as well as free acids are monitored under
different experimental conditions.
b
c
Other products include compounds 6–28 depending on the reactants.
other fluorinated compounds were accounted for more than
47% yield.
Additional experiments were carried out using the same
starting material and passing more than 2 F per mole of over all
electric charge. These results (Table 2) suggest that the
selectivity of 2a increased from 24% to a maximum of 68%
after the passage of 4 F per mole of electric charge. Further
increase in overall electric charge leads to decrease in the
selectivity of 2a, while the yields of other products were found
to be higher. Formation of 2a probably occurs through the
conventional ECBECN mechanism (Scheme 2) proposed for
selective electrochemical fluorination, where CB refers to
abstraction of proton by conjugate base and CN refers to
nucleophilic fluoride attack [4–7]. Further fluorination without
the cleavage of the ester group may also follow similar
mechanistic pathway leading to 3a (Scheme 2).
Typical results obtained during the electrochemical partial
fluorination of methyl phenylacetate (1a) at constant current
density of 15 mA/cm2 are summarized in Table 2. After passing
2 F per mole, only 24% yield of methyl-a-fluoro phenylacetate
(2a) was obtained. Almost equal quantity of the carboxylic acid
4 was also obtained. Substantially smaller amount of methyl-
a,a-difluoro phenylacetate (3a) could also be identified by 19
F
NMR. Major products obtained during anodic fluorination of 1a
are given in Scheme 1. Other compounds including reactant and
Scheme 1. Predominant products obtained during anodic fluorination of alkyl
phenylacetates in Et3NÁ4HF medium.
Scheme 2. ECBECN mechanism of selective electrochemical fluorination [4–7].

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N. Ilayaraja et al. / Journal of Fluorine Chemistry 129 (2008) 185–192
of monofluoro acid. It appears that by controlling the moisture
content one can enhance the yield of fluorinated ester rather
than the acid.
As indicated in Scheme 1, 1a–f investigated in this work
were found to give measurable quantities of esters 2a–f and
3a–f as well as acids 4 and 5. Typical results obtained during the
selective fluorination of 1a–f after passing 2 F of electric charge
at the same current density of 15 mA/cm2 are summarized in
Table 3. The selectivity for the formation of monofluoro esters
was found to increase with increase in alkyl chain length, thus a
maximum selectivity of 63% is noted for the monofluoro ester
during the electrochemical fluorination of 1d. In the case of
branched alkyl chain with the same number of carbon atoms
(1f), the selectivity of monofluoro product was found to be
lower. Thus the selectivity of monofluoro ester for substrates 1c
and 1d was found to be significantly higher than that for the
corresponding 1e and 1f.
The product distributions for 1a–f after passing the charge of
4 F per mole of reactants are summarized in Table 4. It is noted
that the selectivity of monofluoro esters decreases steadily with
increasing the chain length for all the four n-alkyl esters. In
these cases, further passage of electric current lead to
significantly higher concentration of hydrolysed monofluoro
acids. In the case of branched chain alkyl groups, slightly
higher yields of monofluoro ester could still be obtained. From
the above experimental results, it is apparent that the actual
faraday per mole of the reactant may be different for different
Scheme 3. Schematic representation of hydrolysis of ester during selective
electrochemical fluorination.
Formation of significant quantities of fluorinated acids
during the partial fluorination of 1a–f was an important
observation made during this work. The hydrolysis of ester does
not occur in blank experiments in the same electrolyte medium,
as long as no electric charge was passed. This indicates that the
hydrolysis is induced by electron transfer. It appears that the
electro-generated cation radical undergoes a tautomerism with
adjacent carbonyl group as proposed in Scheme 3, which
facilitates hydrolysis with trace level water present in the
electrolyte medium.
ꢀ
The intermediate Ph–CH–COOH undergoes further electron
transfer followed by fluoride addition leading to the formation
Table 3
Selectivity of products obtained during anodic fluorination of 1a–fa
Compound
Conversionb (%)
Selectivity of fluorinated productsc (mole%)
Mono fluoro ester
Mono fluoro acid
Difluoro ester
Difluoro acid
Othersd
1a
1b
1c
1d
1e
1f
80
82
82
85
81
83
24.1
49.9
54.5
62.7
33.0
45.6
26.6
30.3
23.4
16.0
23.4
40.5
0.8
4.2
5.4
5.7
3.1
5.0
1.4
2.6
2.3
0.6
1.6
4.3
47.2
13.0
14.5
15.0
38.9
4.5
a
Charge passed = 2 F/mole.
b
c
d
Based on HPLC data.
Based on 19F NMR.
Other products include compounds 6–28 depending on the reactants.
Table 4
Selectivity of products obtained during anodic fluorination of 1a–fa
Compound
Conversionb (%)
Selectivity of fluorinated productsc (mole%)
Mono fluoro ester
Mono fluoro acid
Difluoro ester
Difluoro acid
Othersd
1a
1b
1c
1d
1e
1f
95
96
99
99
96
98
68.2
48.7
18.8
13.5
58.7
39.8
11.7
21.5
58.4
43.1
12.2
38.2
2.7
8.8
1.5
8.1
6.8
2.5
5.5
5.1
16.0
13.0
8.4
7.6
5.6
35.3
12.6
11.1
11.0
5.8
a
Charge passed = 4 F/mole.
b
c
d
Based on HPLC data.
Based on 19F NMR.
Other products include compounds 6–28 depending on the reactants.

N. Ilayaraja et al. / Journal of Fluorine Chemistry 129 (2008) 185–192
189
Table 5
19F and 1H NMR spectral data of fluorinated phenylacetic acid and its derivatives obtained during anodic fluorination of 1a–f [11,25]
Products Ph–CHF–COOR Ph–CF2–COOR
2
2
R
R
R
R
R
R
R
H
À180.96 (d, 1F) JHF = 47.80 Hz, 5.84 (d, 1H) JHF = 46.80 Hz, 10.05 (s, 1H)
À105.45 (s, 2F), 10.05 (s, 1H)
À104.27 (s, 2F)
2
2
CH3
À180.33 (d, 1F) JHF = 47.87 Hz, 5.83 (d, 1H) JHF = 47.60 Hz
2 2
À179.78 (d, 1F) JHF = 47.44 Hz, 5.80 (d, 1H) JHF = 48.00 Hz
C2H5
À103.91 (s, 2F)
2
2
C3H7
À179.99 (d, 1F) JHF = 45.18 Hz, 5.81 (d, 1H) JHF = 47.60 Hz
À104.02 (s, 2F)
2
2
C4H9
À180.02 (d, 1F) JHF = 45.18 Hz, 5.80 (d, 1H) JHF = 47.60 Hz
À104.05 (s, 2F)
2
2
CH(CH3)2
CH(CH3)(C2H5)
À179.65 (d, 1F) JHF = 45.18 Hz, 5.77 (d, 1H) JHF = 48.00 Hz
À104.09 (s, 2F)
2
2
À179.52 (d, 1F) JHF = 45.18 Hz, 5.77 (d, 1H) JHF = 48.00 Hz
À104.12 (s, 2F)
alkyl groups. However, the selective fluorinated products
generally increase with the increase in chain length of the alkyl
In the present work, an attempt was made to identify minor
fluorinated products through systematic analysis by 19F NMR
data with and without proton decoupling. Wherever possible,
supporting literature data are also provided. As indicated by the
NMR data, a variety of competitive fluorination processes are
possible, of course, with very low selectivity (Table 6). Nuclear
fluorination of ortho, meta and para positions (6–8) can occur
even before the fluorination of the active methylene group.
Even difluoro aromatic derivative 9 without side chain
fluorination could be observed. Similar nuclear fluorinated
products 10–13 with mono fluorinated active methylene groups
were also obtained.
group and decrease with chain branch. The complete 1H and 19
F
NMR data for all the major fluorinated products discussed in
Tables 2–4 are summarized in Table 5.
Electrochemical partial fluorination is also commonly
termed as selective electrochemical fluorination implying that
the process itself is highly selective and yields a single product
under ideal experimental conditions. Experimentally, however,
high selectivity is observed only in some sulfur linked active
methylene groups and lactone-based heterocyclic compounds.
Especially in solvent-free Et4NFÁnHF system, Momota et al.
have identified a variety of side products during the fluorination
of benzene and substituted benzene [15–20]. Similar reports on
solvent-free Et3NÁnHF medium have not been reported so far.
Addition of fluorine to the aromatic nuclei also seems to
occur during electrochemical fluorination. Simple 2, 5
additions to the reactant molecule lead to 16. 1, 4 additions
Table 6
19F NMR data of nuclear substitution and addition products [25–27]
No
Structure
Chemical shifts (d, ppm)
À117.5 (m, 1F)
6
7
À113.21 (m, 1F)
8
9
À111.69 (m, 1F)
À118.07 (m, 1F). À123.78 (m, 1F)
4
2
4
10
11
À123.21 (dm, 1F) JFF = 15.1, À197.03 (dd, 1F) JHF = 48.2, JFF = 15.1
2
6
À111.34 (dm, 1F), À178.92 (dd, 1F) JHF = 46.7, JFF = 3.4

190
N. Ilayaraja et al. / Journal of Fluorine Chemistry 129 (2008) 185–192
Table 6 (Continued )
No
Structure
Chemical shifts (d, ppm)
2
6
12
À117.50 (m, 1F), À122.93 (m, 1F), À178.94 (dd, 1F) JHF = 46.3, JFF = 3.76
2
4
13
14
À117.47 (m, 1F), À123.15 (m, 1F), À196.30 (dd, 1F) JHF = 48.2, JFF = 15.1
2
5
À123.51 (m, 1F), À157.70 (m, 1F), À178.33 (dd, 1F) JHF = 45.2, JFF = 3.8
15
À150.00 (br s, 1F), À88.19 (m, 1F) JAB = 320.0, À89.04 (m, 1F) JAB = 320.0
2
2
16
17
À180.72 (d, 1F) JHF = 47.1, À182.87 (d, 1F) JHF = 46.3
2
5
2
À150.45 (br s, 1F), À178.88 (dd, 1F) JHF = 46.7, JHF = 3.6, À181.24 (d, 1F) JHF = 45.2
2
3
2
18
À130.71 (dd, 2F) JHF = 52.7, JFF = 15.1, À156.46 (m, 1F), À183.32 (d, 1F) JHF = 45.2
2
2
2
19
20
À129.14 (d, 2F) JHF = 56.5, À181.68 (d, 1F) JHF = 48.9, À182.16 (d, 1F) JHF = 58.9
2
2
À92.45 (m, 1F) JAB = 323.8, À93.31 (m, 1F) JAB = 323.8, À129.30 (d, 2F) JHF = 56.5, À180.70 (d, 1F) JHF = 48.9
2
21
À129.59 (d, 2F) JHF = 56.5, À103.45 (s, 2F), À105.05 (s, 2F)
2
22
23
24
25
26
Ph–CH2–COO–CH2F
À183.74 (t, 1F) JHF = 46.3
2
2
Ph–CHF–COO–CH2F
Ph–CH2–COO–CF(CH3)2
Ph–CHF–COO–CF(CH3)2
Ph–CF2–COO–CF(CH3)2
À179.53 (d, 1F) JHF = 46.3, À181.63 (t, 1F) JHF = 47.8
À155.97 (br s, 1F)
2
À148.62 (br s, 1F), À180.75 (d, 1F) JHF = 48.9
À145.71 (br s, 1F), À103.81 (s, 2F)

N. Ilayaraja et al. / Journal of Fluorine Chemistry 129 (2008) 185–192
191
Table 6 (Continued )
No
Structure
Chemical shifts (d, ppm)
2
27
À206.98 (t, 1F) JHF = 48.1, À111.64 (m, 1F)
2
28
À110.54 (d, 2F) JHF = 56.5, À109.01 (m, 1F)
and nuclear substitution lead to 14, 15 and 17. Fluorination can
also occur in the alkyl groups of ester part identified as 22–26.
In the case of i-propyl group, the secondary carbon attached to
the carbonyl group was preferably fluorinated 24–26. Cleavage
can also occur between the active methylene group and
carboxylic acid group. Fluorine substituted toluenes 27, 28 are
formed in this cases. Side chain cleavage, nuclear substitution
and addition can also occur. In such processes 1, 4 addition 18
and 3, 4 addition 19–21 are predominant resulting in the
formation of cyclo-hexadiene derivatives.
The following procedure was adopted for the preparation of
Et3NÁ4HF. A long jar of high-density polypropylene was cooled
with ice salt mixture in a well-ventilated fume chamber.
Slightly excess quantity of (>4 mole) anhydrous hydrogen
fluoride (AHF) was taken to account for possible loss due to
volatility of AHF. The reaction vessel was provided with
magnetic stirrer and closed with a lid containing a provision for
inlet and an outlet tube for the release of AHF vapours during
the reaction. One mole of pre-cooled triethylamine was added
drop wise using separating funnel. After the addition of each
drop, sufficient time was given to ensure cooling of the reactant
mixture. The overall addition was completed in 3 h. The HF
content was determined by acid–base titration. Excess of AHF
if any beyond 1:4 mole ratio of Et3N:HF in the mixture was
appropriately compensated by the addition of triethylamine.
Partial fluorination of alkyl phenylacetates thus proceeds at
nuclear as well as side chain positions. Selectivity in such cases,
however, may be substantially improved by optimizing the
experimental conditions including current density, charge
passed per mole of the reactant and temperature.
3. Conclusion
4.2. Preparation of alkyl phenylacetates
Selective electrochemical fluorination of side chain methy-
lene group in alkyl phenylacetates can be conveniently
achieved under galvanostatic conditions. The amount of
electric charge passed and the nature of the alkyl group are
found to substantially influence the product distribution pattern.
Hydrolysis through tautomeric transformation of electro-
generated cationic radical is an important factor for side
product formation. However, in pre-electrolysed Et3NÁ4HF
medium, very high yields of mono fluorinated esters may still
be obtained under galvanostatic conditions.
Appropriate alcohols (0.05 M) were mixed with 0.5 ml of
conc. H2SO4 and 0.1 M of phenylacetic acid in a 100 ml round
bottom flask fitted with a reflux condenser capped with calcium
chloride tube to prevent moisture during the course of reaction.
The mixture was refluxed for 4 h. After completion of reaction,
the mixture was poured into 25 ml of ice-cold water. The
bottom organic layer was removed and then washed with 20 ml
of saturated sodium hydrogen carbonate solution and 25 ml of
water. Then the crude ester was dried with anhydrous
magnesium sulfate, and it was filtered to remove the salt and
distilled under vacuum. Alkyl phenylacetates were obtained
with a yield of 70–75% and purity of >99%. Purity of the esters
was checked using HPLC and NMR. Similar method was
adopted for the preparation of all the other esters.
4. Experimental
4.1. Preparation of Et3NÁ4HF
Et3NÁ3HF is a well-known conventional electrolyte medium
for selective electrochemical fluorination. However, during
galvanostatic experiments, HF content would decrease during
pre-electrolysis as well as selective fluorination process. If the
HF content in Et3NÁ3HF goes below this mole ratio, free base
Et3N may be liberated and its redox reaction can complicate the
over all process. To ensure that the HF content through out
the electrolysis is more than 3 moles, Et3NÁ4HF was selected as
the electrolyte medium. Earlier workers also pointed out this
advantage of Et3NÁnHF (n = 4 and 5) [28].
4.3. Electrochemical fluorination
A 5 ml capacity undivided polypropylene tube served as
preparative cell for partial fluorination of alkyl phenylacetates.
The anode and cathode were a rectangular Platinum foil
(3 cm2). Pre-electrolysis of 5 ml of Et3NÁ4HF solution was
carried out at a constant cell voltage of 2.5 V under nitrogen
atmosphere. Current at the start of pre-electrolysis was around
1.5 mA/cm2 and the current dropped to 20% of initial value
(0.3 mA/cm2) with in 90 min. Alkyl phenylacetate (300 mmol)

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N. Ilayaraja et al. / Journal of Fluorine Chemistry 129 (2008) 185–192
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was added subsequently and the electrolysis was continued. In
experiments involving without pre-electrolysis, 5 ml of
Et3NÁ4HF containing 300 mmol of the substrate was introduced
into the cell. Electrolysis was carried out galvanostatically
(15 mA/cm2) at 25 Æ 2 8C and pre-determined electric charge
was passed. After completion of electrolysis, the electrolyte
solution was poured into 50 ml of cold water and extracted with
diethyl ether. The extract was washed with brine solution and
dried over anhydrous magnesium sulfate. The ether was
distilled out to get partially fluorinated alkyl phenylacetates. 1H
and 19F NMR spectra were used to analyse the crude product.
[6] K.M. Dawood, Tetrahedron 60 (2004) 1435–1451.
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4.4. Analytical instruments for characterization of the
products
[14] J.H.H. Meurs, W. Eilenberg, Tetrahedron 47 (1991) 705–714.
[15] K. Momota, M. Morita, Y. Matsuda, Electrochim. Acta 38 (1993) 1123–
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The purity of the reactant samples as well as the conversions
was checked by HPLC (Shimadzu 10VP) with ODS column
using UV–vis detector (254 nm). Methanol–water mixture
[16] K. Momota, K. Kato, M. Morita, Y. Matsuda, Electrochim. Acta 39 (1994)
41–49.
1
(70:30) was used as eluent. H NMR spectra were recorded
[17] K. Momota, K. Kato, M. Morita, Y. Matsuda, Electrochim. Acta 39 (1994)
681–690.
with 400 MHz Bruker NMR Spectrometer with CDCl3 and TMS
as solvent and reference, respectively. 19F NMR (376.5 MHz) of
theproducts were taken usingCFCl3 as internal reference. Cyclic
voltammograms were recorded using a single compartment
polythene beaker as cell. Et3NÁ4HF was used as supporting
electrolyte. Platinum wires were used as working (0.0942 cm2)
and counter electrodes. Pd/H2 reference electrode was polarised
cathodically before cyclic voltammetry experiments. Electrolyte
was deaerated using Iolar grade nitrogen for about 15 min.
[18] K. Momota, H. Horio, K. Kato, M. Morita, Y. Matsuda, Electrochim. Acta
40 (1995) 233–240.
[19] H. Horio, K. Momota, K. Kato, M. Morita, Y. Matsuda, Electrochim. Acta
41 (1996) 1611–1618.
[20] K. Momota, K. Mukai, K. Kato, M. Morita, Electrochim. Acta 43 (1998)
2503–2514.
[21] N. Yoneda, S.Q. Chen, T. Hatakeyama, S. Hara, T. Fukuhara, Chem. Lett.
23 (1994) 849–850.
[22] S. Hara, S.Q. Chen, T. Hatakeyama, T. Fukuhara, M. Sekiguehi, N.
Yoneda, Tetrahedron Lett. 36 (1995) 6511–6514.
[23] V. Suryanarayanan, M. Noel, J. Fluorine Chem. 92 (1998) 177–180.
[24] M. Hasegawa, H. Ishii, T. Fuchigami, Tetrahedron Lett. 43 (2002) 1503–
1505.
Acknowledgements
[25] E. Laurent, B. Marquet, R. Tardivel, J. Fluorine Chem. 49 (1990) 115–126.
[26] K. Momota, T. Yonezawa, Y. Hayakawa, K. Kato, M. Morita, Y. Matsuda,
J. Appl. Electrochem. 25 (1995) 651–658.
The authors wish to thank CSIR, New Delhi and Ministry of
Environment and Forests, Govt. of India for financial support.
[27] K. Momota, T. Yonezawa, K. Mukai, M. Morita, J. Fluorine Chem. 87
(1998) 173–178.
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