First application of ionic liquids in electrophilic fluorination of arenes; Selectfluor (F-TEDA-BF4) for "green" fluorination

Article abstract of DOI:10.1039/b111725d

The NF fluorinating agent F-TEDA-BF₄ dication salt (Selectfluor) 1 dissolves in imidazolium-based ionic liquids [emim][OTf] 7, [emim][BF₄] 8, [bmim][PF₆] 9 and [bmim][BF₄] 10 (assisted by sonication), providing a convenient medium for fluorination of arenes under essentially acid-free conditions in a simple set-up (no volatile solvent; simple extraction of the aromatics without aqueous work-up), from which the ionic liquid can be easily recycled and reused. Comparative studies in [emim][OTf] 7 with anisole as substrate show that 1 is superior to NFTh-BF₄ (Accufluor) 2 and that the N-fluoropyridinium salt NFPy-B₂F₇ 4 is least effective. The scope of the reaction has been surveyed. Substrate selectivity (k _mesitylene: k _durene = 10) measured in competitive experiments in 7 is clearly indicative of a conventional polar mechanism. Substrate selectivity measured without the ionic liquid in MeCN solvent is also indicative of a polar mechanism but exhibits lower magnitude (k _mesitylene: k_durene = 6). Addition of dicyclohexano-24-crown-8 to the fluorination reaction mixture (1 and anisole) in 7 reduced the conversion but did not change the isomer distribution. AM1 minimization was used to model the complexation of 1 with this crown. With reactive aromatics optimal fluorination yields in ionic liquids (using 1 equivalent of the NF reagent) are around 50% (higher for naphthalene). A key control experiment suggests that the free base (produced upon transfer-fluorination) could complex to unreacted 1 (generating a bulky dimer complex which may be ineffective for fluorine transfer) in competition to N-protonation.

Full text of DOI:10.1039/b111725d

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First application of ionic liquids in electrophilic fluorination of
arenes; Selectfluor (F-TEDA-BF4) for “green” fluorination
Kenneth K. Laali* and Gennady I. Borodkin†
2
Department of Chemistry, Kent State University, Kent, OH 44242, USA
Received (in Cambridge, UK) 2nd January 2002, Accepted 6th March 2002
First published as an Advance Article on the web 28th March 2002
The NF fluorinating agent F-TEDA-BF4 dication salt (Selectfluor) 1 dissolves in imidazolium-based ionic liquids
[emim][OTf] 7, [emim][BF4] 8, [bmim][PF6] 9 and [bmim][BF4] 10 (assisted by sonication), providing a convenient
medium for fluorination of arenes under essentially acid-free conditions in a simple set-up (no volatile solvent; simple
extraction of the aromatics without aqueous work-up), from which the ionic liquid can be easily recycled and reused.
Comparative studies in [emim][OTf] 7 with anisole as substrate show that 1 is superior to NFTh-BF4 (Accufluor®)
2 and that the N-fluoropyridinium salt NFPy-B2F7 4 is least effective. The scope of the reaction has been surveyed.
Substrate selectivity (kmesitylene : kdurene = 10) measured in competitive experiments in 7 is clearly indicative of a
conventional polar mechanism. Substrate selectivity measured without the ionic liquid in MeCN solvent is also
indicative of a polar mechanism but exhibits lower magnitude (kmesitylene : kdurene = 6).
Addition of dicyclohexano-24-crown-8 to the fluorination reaction mixture (1 and anisole) in 7 reduced the
conversion but did not change the isomer distribution. AM1 minimization was used to model the complexation of
1 with this crown.
With reactive aromatics optimal fluorination yields in ionic liquids (using 1 equivalent of the NF reagent) are
around 50% (higher for naphthalene). A key control experiment suggests that the free base (produced upon transfer-
fluorination) could complex to unreacted 1 (generating a bulky dimer complex which may be ineffective for fluorine
transfer) in competition to N-protonation.
functional groups. However, direct fluorination of aromatics
Introduction
with NF reagents represents a continuing challenge. By and
Rapidly growing interest in fluorinated organic compounds,
large, transfer-fluorination to aromatics with the NF reagents is
dictated by an ever-increasing demand for fluorinated pharma-
only possible with activated arenes and yields are variable
ceuticals, agrochemicals, dyestuffs, liquid crystals, materials and
depending on the reagents, the arene substrates and the solvent
polymers,1,2 has placed paramount importance on the devel-
system.3–6,8 The use of excess fluorinating agent is necessary in
opment of selective fluorination methods and reagents that can
the case of 1 to increase conversion. Reactions are typically
deliver electrophilic (Fϩ)1,3–6 or nucleophilic fluorine (FϪ)1,7 to
performed in MeCN (usually under reflux), MeOH or TFA.3–6
target molecules under mild, nondestructive, and very desirably
Intervention by a radical cation mechanism has been inferred in
“eco-friendly” conditions.
some cases.3
Among the “Fϩ” reagents (Fig. 1), the NF fluorinating agents
An interesting recent study employed TfOH as solvent which
substantially increased the yields.9 The observed ktoluene/kbenzene
=
13.8 clearly supports an electrophilic reaction with a reactive
electrophile proposed to be the O-protonated CF3SO2OF
formed in situ.9
With 5a, several examples of arene fluorination under mild
conditions have been reported,10–12 whereas with 3, fluorin-
ation of anisole required high temperatures and prolonged
reaction times.12 Reagent 6 exhibited some success in PAH
fluorination, which is especially difficult.13
There is tremendous current interest in the utility and appli-
cation of room temperature ionic liquids as designer solvents
(and catalysts) for organic14–19 and organometallic transform-
ations.20 The increasingly popular imidazolium-based ionic
liquids [emim][X] (emim = 1-ethyl-3-methylimidazolium) and
[bmim][X] (bmim = 1-butyl-3-ethylimidazolium), especially
those with less nucleophilic counterions (Fig. 2), promise to be
Fig. 1
F-TEDA-BF4 (Selectfluor)3–6 1 and NFTh (Accufluor®)3–5
2, N-fluoropyridinium salts3–5 (such as 3 and 4) and sulfon-
imides3–5,8 of type 5 (Fig. 1; all commercially available except
5a) have shown promise in selective fluorination of various
† Visiting scientist on leave from Novosibirsk Institute of Organic
Chemistry, Russia.
Fig. 2
DOI: 10.1039/b111725d
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This journal is © The Royal Society of Chemistry 2002
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Table 1 Electrophilic fluorination of anisole with the NF reagents in ionic liquid solvents
Isomer distribution (%)
NMRb
Yield (%)a
GC
ArH : NF ratio
Ionic liquid
(molar ratio)
NMR
GC
ortho
para
ortho
para
F-TEDA-BF4 (1)
d
d
d
d
d
d
d
d
d
d
d
d
d
d
[emim][CF3SO3]
Recycled once
Recycled twice
Recycled 3rd time
[emim][BF4]
[bmim][PF6]
[emim][CF3SO3] ϩ dicyclohexano-24-crown-8
9 : 1
10 : 1
11 : 1
11 : 1
10 : 1
12 : 1
9 : 1
53
41
45
42
37
43
13
56
36
46
40
42
39
11
47
48
47
46
43
48
47
53
52
53
54
57
52
53
NFTh-BF4 (2)
d
d
d
d
[emim][CF3SO3]
10 : 1
10 : 1
28
11
30
56
44
NFPy-B2F7 (4)
d
[emim][CF3SO3]
54c
46c
a Yields for fluorinated products were determined from 1H NMR and GC using mesitylene as internal standard. b Determined from 1H NMR
spectra. c Determined from 19F NMR spectra. d Incomplete GC peak separation for anisole and 4-fluoroanisole.
ideal media for performing ionic reactions involving electron-
deficient intermediates (carbocations and onium ions) under
“green” chemistry.
(Table 1). AM1 minimizations on the F-TEDA dication and
dicyclohexano-24-crown-8 showed that both N–F and N–CH2-
Cl could complex, with the energy difference between the free
and bound forms being 48.2 and 47.0 kcal molϪ1 respectively. It
is, therefore, conceivable that the two host–guest forms rapidly
interconvert in solution resulting in rate retardation but no
noticeable overall change in regioselectivity. Attempts to detect
a host–guest cation–molecule cluster (either intact 12ϩؒcrown or
intact 1ϩؒcrown) by electrospray mass spectrometry were unsuc-
cessful due to rapid defluorination of 12ϩ forming abundant
monocation (m/z 161), with intact 1ϩ (m/z 180) being observed
in very low abundance.
Recent examples of electrophilic reactions include Friedel–
Crafts acylation of arenes,15 alkylation of aromatics with
alkenes,16 halogenation of alkenes and alkynes,17 nitration of
aromatics,18 and fluorodediazoniation of arenediazonium
salts.19 The present study was undertaken to determine the
efficacy of transfer-fluorination of arenes with NF salts in ionic
liquids, a process that could take “tamed” fluorination into the
realm of “green” chemistry.
Table 2 summarizes the results of the reactivity survey for
simple aromatics using only one equivalent of the NF reagent.
The reported yields may be increased by addition of an extra
equivalent of the NF reagent to the reaction mixture after the
completion of the first fluorination cycle, as exemplified for
p-xylene (Table 2). With p-chloroanisole there was no drop in
yield relative to parent anisole or p-methylanisole, but p-fluoro-
anisole gave lower conversions. With the more deactivated
aromaticssuchasnitrotoluene, nofluorinationtookplace, aswas
found previously with nitrobenzene in TfOH solvent.9
Table 3 summarizes examples of bicyclic and polycyclic
arenes and the resulting isomer distributions and conversions.
The ionic liquids used in this study were all used samples which
had been recovered. Fluorination of naphthalene with 1 in
MeCN was reported to give a 30% conversion, but in hot TFA
the yield increased to 75%.22 The isomer distribution was
reported as 1-F : 2-F = 3 : 1. The latter is at variance with the
regioselectivity observed in ionic liquids in the present study
(13 : 1). Fluorination of 1-methylnaphthalene with the F-TEDA-
BF4 reagent had not been studied before. For dibenzofuran,
the conversion observed in 7 is superior to those previously
reported in MeCN, TFA and MeCN ϩ TFA as solvent.21 Chart
1 is a cumulative summary of the 19F data for the products
obtained in this survey.
Results and discussion
Dication salt 1 slowly dissolves in excess ionic liquids [emim]-
[OTf] 7, [emim][BF4] 8, [bmim][PF6] 9 and [bmim][BF4] 10 at
room temperature. In the case of ionic liquids 7 and 9 dis-
solution of 1 is accompanied by counterion exchange,‡ but
this does not create any limitation relative to fluorination. In
all cases, sonication at rt increases solubility, leading to homo-
geneous (or nearly homogeneous) liquids, which eventually
become clear solutions upon warming to 80 ЊC (reaction
temperature for fluorination). Anisole was used as model sub-
strate to compare different ionic liquids and to determine the
efficiency of recycling. The results are summarized in Table 1.
Yields were determined both by NMR and by GC. The isomer
distribution for fluoroanisoles (47% ortho, 53% para) closely
resembles that reported for fluorination of anisole with 1 in
TfOH (45% ortho, 55% para)9 but differs from the data
reported in refluxing MeCN and TFA solvents.21 The ionic
liquid was recycled and reused three times for the same
reaction. This led to about 10% decrease in conversion (see
Table 1). Comparative fluorination of anisole with 2 and 4 by
using 7 resulted in decreased conversions, with 4 showing the
lowest yield (Table 1). Isomer distribution observed in the case
of 2 is closely analogous to that reported in MeCN under
reflux.21
A noteworthy feature is that optimal fluorination yields
(using 1 equivalent of the NF reagent) in ionic liquids are
around 50% (except for naphthalene which was higher). In a
control experiment (Scheme 1), the 19F resonance of F-TEDA-
BF4 shifted downfield by over 2 ppm upon addition of auth-
entic 1-benzyl-4-aza-1-azoniabicyclo[2.2.2]octane chloride (ca.
1 equiv.).§
In an effort to use complexation as a means to create a more
sterically crowded electrophile, which might lead to increased
para selectivity, fluorination of anisole with 1 in 7 as solvent
was re-examined in the presence of dicyclohexano-24-crown-8.
This led to decreased yield (consistent with lowered reactivity
of the electrophile) but did not influence the isomer distribution
‡ For examples of counterion exchange (metathesis) observed in elec-
trophilic nitration of aromatics in [emim][X] solvents see ref. 18.
§ The positive charge on fluorine is larger in the complex (0.086) than in
free F-TEDA (0.065) (AM1 calculations).
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J. Chem. Soc., Perkin Trans. 2, 2002, 953–957

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Table 2 Electrophilic fluorination of ArH with F-TEDA-BF4
Yield (%)a
ArH : NF
ArH
Ionic liquid
(molar ratio)
NMR
56
GC
Composition of reaction mixture (%)
p-Methylanisole
[emim][CF3SO3]
4 : 1
46b
2-Fluoro-4-methylanisole (93), 3-fluoro-4-methylanisole (6),c
2,6-difluoro-4-methylanisole (1)c
p-Chloroanisole
p-Fluoroanisole
p-Xylene
p-Xylene
Mesitylene
p-Nitrotoluene
p-Nitrotoluene
[emim][CF3SO3]
[emim][CF3SO3]
[emim][BF4]
[emim][CF3SO3]
[bmim][PF6]
[bmim][BF4]
[bmim][CF3SO3]
5 : 1
4 : 1
10 : 1
10 : 1
4 : 1
4 : 1
4 : 1
50
24
50b
22
2-Fluoro-4-chloroanisole (95), 2,6-difluoro-4-chloroanisole (5)c
2,4-Difluoroanisole (∼100)
24
21b c
46b d
50b e
0
2-Fluoro-1,4-dimethylbenzene (∼100)
2-Fluoro-1,4-dimethylbenzene (∼100)
2-Fluoromesitylene (∼100)
51d
52e
<1
<1
0
a Yields of fluorinated products were calculated from 1H NMR and GC using mesitylene as internal standard. b No authentic fluorinated sample was
available as GC standard. c Specific assignment tentative. d Yield of 2-fluoro-1,4-dimethylbenzene after repeating the fluorination reaction in the same
reaction vessel by adding a second equivalent of F-TEDA-BF4. e Yield of 2-fluoromesitylene was calculated from 1H NMR and GC using anisole as
internal standard.
Table 3 Electrophilic fluorination of bicyclic and polycyclic aromatic hydrocarbons with F-TEDA-BF4
Yield (%)a
ArH : NF
ArH
Ionic liquid
(molar ratio)
NMR
GC
Composition of reaction mixture (%)b
f
Naphthalene
[emim][CF3SO3]
(recycled once)
4 : 1
88
1-Fluoronaphthalene (91), 2-fluoronaphthalene (7),
1,8-difluoronaphthalenec (2)
f
1-Methylnaphthalene
Dibenzofuran
[bmim][PF6]
(recycled twice)
2 : 1
4 : 1
23
1-Fluoro-4-methylnaphthalene (53), 2-fluoro-
1-methylnaphthalene (21), 5-fluoro-1-methylnaphthalenec
(11), 1-fluoro-8-methylnaphthalenec (15)
[emim][CF3SO3]
(recycled 3-times)
49d
38d e
1-Fluorodibenzofuran (20, NMR; 18, GC),
2-fluorodibenzofuran (41, NMR; 42, GC),
3-fluorodibenzofuran (39, NMR; 40, GC)
a Yields of fluorinated products were calculated from 1H NMR and GC using mesitylene as internal reference. b Determined from 19F NMR spectra.
1
c Specific assignment tentative. d Yields of fluorinated products were calculated from H NMR and GC using 4-fluoroanisole as internal reference.
e No authentic fluorinated dibenzofuran isomers were available as GC standard. f No isomer separation.
Scheme 1
On the basis of this observation it is suggested that the free
base (produced as transfer-fluorination progresses) could com-
plex to unreacted 1 (generating a bulky dimer complex which
may be ineffective for fluorine transfer) in competition with
N-protonation (Scheme 2).
For competitive reactions, a 1 : 1 mixture of mesitylene and
durene was allowed to react with 1 in ionic liquid 7. The kMES
:
kDUR values determined by 19F NMR and by GC were 9 and 12,
respectively. An identical experiment performed in MeCN sol-
vent gave the kMES : kDUR ratio as 5 (by NMR) and 7 (by GC)
respectively. The observed substrate selectivities in both media
are consistent with a polar mechanism (σ-complex) involving a
reactive electrophile, with slightly greater degree of polar char-
acter developing in the ionic liquid solvent. As a comparison,
the kMES : kDUR ratio recently measured for iodination with ICl
in MeCN solvent is 46.23 Therefore, 1 is a more reactive (less
selective) electrophile which nevertheless relies heavily on the
nucleophilicity of the arene nucleophile (consistent with lack of
reaction with deactivated arenes).
Scheme 2
comparable and in some cases exceed those reported in MeCN
and TFA but are lower that those found in TfOH solvent.
However, the present process, which uses only one equivalent
of the NF reagent, avoids the use of strong acids and involves
no aqueous work up. The ionic liquid solvent can be recycled
and reused. The yields can be increased by repeating the
fluorination cycle as was demonstrated for p-xylene.
In summary, the feasibility of aromatic fluorination with NF
reagent 1 in ionic liquid solvents has been tested. The yields are
J. Chem. Soc., Perkin Trans. 2, 2002, 953–957
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ment utilizing a 25 m OV101 capillary column. ES-MS analysis
was performed on a Bruker Esquire LCQ-MS instrument in
infusion mode.
Typical fluorination procedure
The [emim][BF4] ionic liquid (1.55 g, 7.8 mmol) was transferred
to a Schlenk tube and F-TEDA-BF4 (110 mg, 0.31 mmol) was
added (under argon atmosphere). The closed Schlenk tube was
placed inside an ultrasonic bath (frequency 40 kHz; Branson
1510R-MTH) at room temperature for 2 h (to increase solu-
bility). After addition of anisole (337 mg, 3.12 mmol) at rt the
mixture was stirred at 80 ЊC for 15 h. The organics were
extracted with ether (5 mL × 4), washed with H2O (5 mL × 2),
dried (CaSO4) and filtered. The organic extract was concen-
trated by slowly bubbling a slow stream of nitrogen through the
reaction mixture. The residue was directly analyzed by GC and
NMR (1H and 19F) using internal standards (see footnotes
in Tables 1–3). The procedure was similar for fluorinations
with [emim][OTf] and [bmim][PF6] (in these cases counterion
exchange takes place but does not interfere with the subsequent
step).
Competitive experiments
a) In [emim][OTf]. The ionic liquid (1.15 g, 4.43 mmol) was
transferred to a Schlenk tube. A homogeneous solution of
durene (44 mg, 0.33 mmol) and mesitylene (39 mg, 0.33 mmol)
in CH2Cl2 (1.3 g) was added to the ionic liquid under argon.
The CH2Cl2 solvent was then removed under vacuum and
F-TEDA-BF4 (58 mg, 0.16 mmol) was added. The reaction
mixture was stirred at 80 ЊC for 16 h, following which the
aromatics were extracted as described above and analyzed by
NMR and GC. The product ratio (1-fluoro-2,4,6-trimethyl-
benzene : 1-fluoro-2,3,5,6-tetramethylbenzene) was 9 (based on
19F NMR) and 12 (based on GC).
b) In CH3CN. F-TEDA-BF4 (131 mg, 0.37 mmol), durene
(98 mg, 0.73 mmol) and mesitylene (88 mg, 0.73 mmol) were
transferred to a Schlenk tube and dry MeCN (5 mL) was added
under argon. The solution was stirred at 80 ЊC for 16 h and was
analyzed directly by 19F NMR (product ratio: 1-fluoro-2,4,6-
trimethylbenzene : 1-fluoro-2,3,5,6-tetramethylbenzene = 5). A
portion of the solution was then passed through a short silica-
gel column (ether eluent) to remove the TEDA-derived mono
and dication salts by-products (see Scheme 2) (these remained
on the column). The eluent was analyzed directly by GC (pro-
duct ratio: 1-fluoro-2,4,6-trimethylbenzene : 1-fluoro-2,3,5,6-
tetramethylbenzene = 7).
Chart 1
Experimental
The [emim][X] ionic liquids (X = BF4, OTf ) were purchased
from Aldrich, whereas [bmim][BF4] and [bmim][PF6] were
synthesized from [bmim][Cl] by metathesis with NaBF4 and
HPF6 respectively as previously reported.18 The Selectfluor
reagent (F-TEDA-BF4) was a sample from Aldrich. Accufluor®
(NFTh-BF4/alumina blend) was a gift sample from Allied
Signal from which the alumina was removed by extracting the
Accufluor® with CH2Cl2 under argon and removal of solvent
under high vacuum. The N-fluoropyridinium pyridine hepta-
fluoroborate (NRPy-B2F7) was a gift from Allied Signal.
Dicyclohexano-24-crown-8 and the aromatics were all high
purity commercial samples purchased from Aldrich or Acros.
The 1-benzyl-4-aza-1-azonia-bicyclo[2.2.2]octane chloride was
prepared as previously described.24 Liquid arenes were dried
over molecular sieves. Ether was dried over sodium. Fluorin-
ation reactions in ionic liquids were carried out in Schlenk tubes
under an argon atmosphere.
ComplexationofF-TEDA-BF4 to1-benzyl-4-aza-1-azoniabicyclo-
[2.2.2]octane chloride
The 19F NMR spectrum of F-TEDA-BF4 (25.4 mg, 0.072
mmol) in acetone-d6 ϩ D2O (92 : 8) solvent gives a singlet at
δ 49.13. Addition of 1-benzyl-4-aza-1-azonia-bicyclo[2.2.2]-
octane chloride (18.3 mg, 0.077 mmol) to the NMR tube caused
the fluorine resonance to shift to δ 51.17(∆δF = 2.04 ppm).
Effect of crown ether
[emim][OTf] (1.13 g, 4.35 mmol) was transferred into a Schlenk
tube and F-TEDA-BF4 (73 mg, 0.20 mmol) was added under
argon. After sonication, dicyclohexano-24-crown-8 (78 mg,
0.17 mmol) and anisole (192 mg, 1.77 mmol) were added and
the mixture was stirred at 80 ЊC for 15 h (for subsequent steps
see general procedure).
NMR spectra, GC analysis and electrospray MS
1H and 19F NMR spectra were recorded in acetone-d6 and
chloroform-d3 at room temperature on a Varian INOVA
500 MHz instrument using a 5 mm broad-band probe. GC
analyses were performed with an HP-5890 capillary GC instru-
Recycling of ionic liquid [emim][BF4] and [bmim][BF4]
This was accomplished by extraction with dry CH2Cl2. The
dichloromethane layer was separated by filtration and the
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11 J. Zhang and D. D. DesMarteau, J. Fluorine Chem., 2001, 111, 253.
12 T. Umemoto, S. Fukami, G. Tomizawa, K. Harasawa, K. Kawada
and K. Tomita, J. Am. Chem. Soc., 1990, 112, 8563.
solvent was removed reduced pressure to afford the pure ionic
liquid (assayed by 1H NMR).
13 K. K. Laali, M. Tanaka, F. Forohar, M. Cheng and J. C. Fetzer,
J. Fluorine Chem., 1998, 91, 185.
14 J. Howarth, K. Hanlon, D. Fayne and P. McCormac, Tetrahedron
Lett., 1997, 38, 3097; M. J. Earle, P. B. McCormac and K. R.
Seddon, Chem. Commun., 1998, 2245.
15 C. J. Adams, M. J. Earle, G. Roberts and K. R. Seddon, Chem.
Commun., 1998, 2097.
Acknowledgements
We thank Dr George Shea (Allied Signal) for gift samples of
NFTh-BF4 and NFPy-B2F7. This work was supported in part
by the NCI of NIH (1 R15 CA78235-01A1).
16 C. E. Song, W. H. Shim, E. J. Roh and J. H. Choi, Chem. Commun.,
2000, 1695.
17 C. Chiappe, D. Capraro, V. Conte and D. Pieraccini, Org. Lett.,
2001, 3, 1061.
References
1 J. H. Clark, D. Wails and T. W. Bastock, Aromatic Fluorination,
CRC Press, Boca Raton, USA, 1996.
18 K. K. Laali and V. J. Gettwert, J. Org. Chem., 2001, 66, 35.
19 K. K. Laali and V. J. Gettwert, J. Fluorine. Chem., 2001, 107, 31.
20 J. K. D. Surette, L. Green and R. D. Singer, Chem. Commun., 1996,
2753; C. J. Adams, M. J. Earle and K. R. Seddon, Chem. Commun.,
1999, 1043; C. J. Mathews, P. J. Smith and T. Welton, Chem.
Commun., 2000, 1249; A. J. Carmichael, D. M. Haddleton,
S. A. F. Bon and K. R. Seddon, Chem. Commun., 2000, 1237;
P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000,
39, 3772; C. E. Song, C. R. Oh, E. J. Roh and D. J. Choo,
Chem. Commun., 2000, 1743; H. Okazaki, Y. Kawannami and K.
Yamamoto, Chem. Lett., 2001, 650; G. Bar, A. F. Parsons and
C. B. Thomas, Chem. Commun., 2001, 1350; P. Wasserscheid, C. M.
Gordon, C. Hilgers, M. J. Muldoon and I. R. Dunkin, Chem.
Commun., 2001, 1186; R. J. C. Brown, P. J. Dyson, D. J. Ellis and
T. Welton, Chem. Commun., 2001, 1862.
2 R. L. Powell, in Methods of Organic Chemistry (Houben-Weyl),
B. Baasner, H. Hagemann and J. C. Tatlow (eds.), Georg Thieme
Verlag, Stuttgart, New York, 1999, Vol. E10a, chap. 6.
3 G. S. Lal, G. P. Pez and R. G. Syvret, Chem. Rev., 1996, 96, 1737.
4 S. D. Taylor, C. C. Kotoris and G. Hum, Tetrahedron, 1999, 55,
12431.
5 G. G. Furin, in Methods of Organic Chemistry (Houben-Weyl),
B. Baasner, H. Hagemann and J. C. Tatlow (eds.), Georg Thieme
Verlag, Stuttgart, New York, 1999, Vol. E10a, chap. 9.
6 R. E. Banks, J. Fluorine Chem., 1998, 87, 1.
7 L. R. Subramanian and G. Siegemund, in Methods of Organic
Chemistry (Houben-Weyl), B. Baasner, H. Hagemann and J. C.
Tatlow (eds.), Georg Thieme Verlag, Stuttgart, New York, 1999,
Vol. E10a, chap. 14.
8 J. Zhang and D. D. DesMarteau, J. Fluorine Chem., 2001, 111,
21 M. Zupan, J. Iskra and S. Stavber, Tetrahedron, 1996, 52, 11341.
22 M. Zupan, J. Iskra and S. Stavber, J. Fluorine Chem., 1995, 70, 7.
23 M. Fabbrini, C. Galli, P. Gentili, D. Macchitella and H. Petride,
J. Chem. Soc., Perkin Trans. 2, 2001, 1516.
253.
9 T. Shamma, H. Buchholz, G. K. S. Prakash and G. A. Olah,
Isr. J. Chem., 1999, 39, 207.
10 S. Singh, D. D. DesMarteau, S. S. Zuberi, M. Witz and H.-N.
Huang, J. Am. Chem. Soc., 1987, 109, 7194.
24 A. R. Hajipour, I. Mohammadpoor-Baltork and G. Kianfar, Bull.
Chem. Soc. Jpn., 1998, 71, 2655.
J. Chem. Soc., Perkin Trans. 2, 2002, 953–957
957