Three step procedure for the preparation of aromatic and aliphatic difluoromethyl ethers from phenols and alcohols using a chlorine/fluorine exchange methodology

Article abstract of DOI:10.1016/j.jfluchem.2014.01.018

Difluoromethyl ethers are prepared from phenols in three steps via their respective formate ester derivatives. The formates are first converted to dichloromethyl ethers by treatment with PCl5. These ethers are then induced to undergo chlorine/fluorine exchange to form the respective difluoromethyl ethers. The chlorine/fluorine exchange is carried out by either a room temperature, solvolytic process using THF-5HF or Et₃N-3HF as exchange medium, where HF is the ultimate source of fluorine, or by a direct displacement process in sulfolane at 125 C, where KF is the source of fluorine. By one or another of these processes, virtually all phenols, electron-rich and electron-poor, can be converted to their respective difluoromethyl ethers in good yields. Aliphatic alcohols are also able to be converted to their difluoromethyl ether derivatives using the Et₃N-3HF exchange medium.

Full text of DOI:10.1016/j.jfluchem.2014.01.018

Journal of Fluorine Chemistry 160 (2014) 72–76
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Three step procedure for the preparation of aromatic and aliphatic
difluoromethyl ethers from phenols and alcohols using a chlorine/
fluorine exchange methodology
a
a
a
b
William R. Dolbier Jr.a, , Fei Wang , Xiaojun Tang , Charles S. Thomoson , Linhua Wang
*
a Department of Chemistry, University of Florida, Gainesville, FL 32608, United States
b Syngenta Crop Protection LLC, 410 Swing Road, Greensboro, NC 27409, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
Difluoromethyl ethers are prepared from phenols in three steps via their respective formate ester
derivatives. The formates are first converted to dichloromethyl ethers by treatment with PCl5. These
ethers are then induced to undergo chlorine/fluorine exchange to form the respective difluoromethyl
ethers. The chlorine/fluorine exchange is carried out by either a room temperature, solvolytic process
using THF-5HF or Et3N-3HF as exchange medium, where HF is the ultimate source of fluorine, or by a
direct displacement process in sulfolane at 125 8C, where KF is the source of fluorine. By one or another of
these processes, virtually all phenols, electron-rich and electron-poor, can be converted to their
respective difluoromethyl ethers in good yields. Aliphatic alcohols are also able to be converted to their
difluoromethyl ether derivatives using the Et3N-3HF exchange medium.
Received 17 December 2013
Received in revised form 21 January 2014
Accepted 24 January 2014
Available online 3 February 2014
Keywords:
Difluoromethyl ethers
Anhydrous HF
Chlorine/fluorine exchange
Synthesis
ß 2014 Elsevier B.V. All rights reserved.
1. Introduction
obtained when using these methods, interest remains in the
development of new approaches to the synthesis of aryl
A broad spectrum of fluoroorganic molecules has gained
prominence in all areas of synthesis where biologically active
compounds are of interest [1,2]. In particular, the difluoromethyl
ether functionality has attracted considerable importance
within both the pharmaceutical and agrochemical research
communities [3].
Aryl difluoromethyl ethers are of particular interest, and
almost all of the methodologies that have been used for their
preparation start from the phenol and involve the use of a
difluorocarbene-generating reaction, under basic conditions, in
the presence of a proton donor. An early classic example of this
type of reaction used chlorodifluoromethane (CHF2Cl, F22) as the
difluorocarbene precursor, as shown in Scheme 1 [4], but this
process has fallen out of favor because CHF2Cl is a significant
ozone-depleting compound and, as a result, its availability is
highly limited.
difluoromethyl ethers. In this paper we will describe a three-step
approach from phenols that includes use of classic Swarts
chemistry, that is chlorine–fluorine exchange methodology, for
the incorporation of fluorine into the molecules.
One of the most common and economical ways to introduce
multiple fluorines into organic molecules is by replacement of
chlorine with fluorine, a process that is perhaps best exemplified
by the conversion of trichloromethyl aromatics into trifluoro-
methyl aromatics by treatment with HF, often in the presence of a
Lewis acid or metal oxide catalyst [8–10]. Conversions of
trichloromethoxy- to trifluoromethoxyarenes have been carried
out in a similar manner [11,12].
The conversion of dichloromethyl- to difluoromethyl aromatics
by treatment with HF has been reported [13,14], but such
chemistry is much less frequently encountered in the literature.
Unlike the trichloromethyl case, processes involving nucleophilic
replacement of chloride by fluoride have also been successful for
dichloromethyl aromatic systems [15]. To our knowledge, there
has been no attempt made until the present work to convert
dichloromethoxy aromatics to difluoromethoxy aromatics, either
by treatment by HF-Lewis base mixtures or by direct nucleophilic
substitution by fluoride.
Recently it has been reported that CHF2OTf, fluoroform (CHF3)
and TMSCF2Br can be used under similar conditions as effective
replacements for CHF2Cl [5–7]. In spite of the excellent results
*
Corresponding author. Tel.: +1 352 335 4896.
E-mail address: wrd@chem.ufl.edu (W.R. Dolbier Jr.).
0022-1139/$ – see front matter ß 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jfluchem.2014.01.018

W.R. Dolbier Jr. et al. / Journal of Fluorine Chemistry 160 (2014) 72–76
73
ArO K
H2O
The results for overall conversion of the formate esters to the
difluoromethoxy-arenes are given in Table 1.
KOH
- HCl
Ar-O-CF2H
CHF2Cl
CF2
Ar-O-CF2
Very good results were obtained for conversion of relatively
electron-deficient aryl formates using THF-5HF as both fluorinat-
ing agent and reaction medium [20,21], stirring its mixture with
crude dichloromethyl ether for 12 h at room temperature.
Although we have published only two papers on the subject, it
is our experience that a 5–6 to 1 ratio of HF to THF provides an
optimal mixture for a large variety of reactions that are otherwise
carried out either in pure HF or in other HF-Lewis base mixtures
[22,23].
The results when using the THF-5HF mixture with even
modestly electron-rich substrates were less satisfactory. That
being the case, it was decided to try the alternative HF reagent,
Et3N-3HF, which we have observed will often exhibit significantly
different behavior as a fluorinating agent. As seen in Table 1, the
conversion of electron rich aryl formates proceeded much more
effectively when using Et3N-3HF as the fluorinating reagent.
Using classic conditions for nucleophilic displacement by
fluoride ion, heating the crude product with 4-fold excess of KF
at 125–130 8C for 15 h in sulfolane, in the presence of phase-
transfer catalyst, tetrabutylammonium bromide often gave results
that were competitive with the use of the HF-Lewis base mixtures,
but the SN2 procedure generally proved less satisfactory.
Unfortunately, it was found that the dichloromethoxy ethers
formed from the very electron-deficient formate derivatives of
either 2-hydroxy- or 3-hydroxypyridine were not sufficiently
stable to allow their in situ conversion to the respective pyridyl
difluoromethyl ethers.
Scheme 1. Difluoromethyl ethers from difluorocarbene reagents.
2. Results and discussion
2.1. Aryl difluoromethyl ethers
Our new approach to the preparation of difluoromethoxy
aromatics is depicted generally in Scheme 2. It involves initial
conversion of phenols to aryl formates (1) by reaction with mixed
anhydride, HCO2COCH3 [16,17]. The aryl formates were then
converted to their respective dichloromethoxy derivatives (2) by
treatment with PCl5. Conversion of these dichloromethyl ethers to
their respective difluoromethyl ethers was examined using two
mechanistically different approaches for achieving chlorine/
fluorine exchange, using either an HF-Lewis base fluorination
medium, which one assumes proceeds via carbocation intermedi-
ates, or by direct substitution of the chlorines by use of fluoride ion
in an SN2 process: The former method involved use of either THF-
5HF or Et3N-3HF as fluorinating agent at room temperature,
whereas the latter involved heating the dichloromethyl ethers in
sulfolane at 125 8C in the presence of KF.
Thus, a series of aryl formate esters were prepared and
converted to their crude dichloromethoxy derivatives. These
dichloromethoxy aromatics could be isolated, but they proved
to be relatively unstable and in general they were not isolated, with
the crude products being subjected directly to the appropriate
fluorination procedure. Aryl dichloromethyl ethers have only
rarely been prepared and characterized [18,19]. Their formation
was demonstrated, based on the characteristic singlets for their
OCCl2H protons at about 7.5 ppm in their 1H NMR spectra.
To our knowledge, there are no instances reported for
conversion of dichloromethyl ethers to difluoromethyl ethers.
All of the difluoromethoxy arene products have been previously
reported, and they were characterized mainly by their character-
istic 19F NMR spectra, which gave doublets between À79 and
À86 ppm with 2JHF values around 73 Hz, and their proton spectra,
which exhibited the expected triplets at about 6.5 ppm. The yields
of the aryl difluoromethyl ethers were determined by 19F NMR,
using trifluorotoluene as internal standard.
2.2. Aliphatic difluoromethyl ethers
The same general synthetic approach could also be applied to
the preparation of aliphatic difluoromethyl ethers, which are
encountered in the literature much less frequently than their
aromatic analogs. Chen demonstrated that the difluorocarbene
approach could be used for aliphatic difluoromethyl ether
synthesis (Scheme 3a) [24], and more recently, Rozen reported a
three-step process from alcohols involving initial formation of the
chlorodifluoromethyl ethers [25] followed by reduction of the Cl by
use of n-Bu3SnH (Scheme 3b) [26].
In our case, yields with alcohols were inferior to those obtained
with phenols. Aliphatic dichloromethyl ethers were obtained in
Scheme 2. Three step process for synthesis of difluoromethoxy arenes.
Scheme 3. Chen and Rozen processes for difluoromethyl ether synthesis.

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W.R. Dolbier Jr. et al. / Journal of Fluorine Chemistry 160 (2014) 72–76
Scheme 4. Conversion of aliphatic formates to difluoromethyl ethers.
Table 1
rich substrates. For the preparation of aliphatic difluoromethyl
ethers, only the Et3H-3HF method provided acceptable yields.
Preparation of aryl difluoromethyl ethers from aryl formates.
Substrate
Ar
Ph
Fluorinating agent
Yields of 3 (%)
4. Experimental
1a
THF-5HF
KF
35
41
82
20
60
65
15
50
77
47
80
94
Trace
37
76
14
50
11
33
71
0
1b
1c
1d
1e
4-Br-Ph
4-Cl-Ph
4-Ph-Ph
4-NO2-Ph
THF-5HF
Et3N-3HF
KF
4.1. Typical procedure for synthesis of aryl and alkyl formates
Acetic anhydride (40 mL) was added to a three-necked flask at
room temperature. Then, formic acid (20 mL) was added under N2
atmosphere. The mixture was stirred at 60 8C for 1 h. After cooling
to room temperature, phenol (4.7 g, 50 mmol) and NaHCO3 (8.5 g,
100 mmol) were added. The mixture was stirred at room
temperature overnight, after which it was poured into ice water.
The organic phase was extracted with CH2Cl2, washed with brine
(three times) and saturated NaHCO3 solution (one time), and then
dried with MgSO4. After the solution was filtered and the solvent
was evaporated under vacuum, the residue was subjected to silica
gel column chromatography to give product phenyl formate (1a)
THF-5HF
Et3N-3HF
KF
THF-5HF
Et3N-3HF
KF
THF-5HF
Et3N-3HF
KF
1f
2-NO2-Ph
4-CN-Ph
4-CH3-Ph
THF-5HF
KF
1g
1h
THF-5HF
KF
(4.58 g, yield: 75%) as a colorless liquid: 1H NMR:
d 8.29 (s, 1H),
THF-5HF
Et3N-3HF
KF
7.43–7.34 (m, 2H), 7.28–7.21 (m, 1H), 7.14–7.07 (m, 2H). The
spectroscopic data is consistent with those in the literature [16,27].
1i
3-CH3-Ph
THF-5HF
Et3N-3HF
KF
15
50
10
58
40
36
0
4.2. 4-Methylphenyl dichloromethyl ether (2h)
1j
2-I-Ph
THF-5HF
KF
PCl5 (2.3 g, 11 mmol) was added to 4-methylphenyl formate
(1.4 g, 10 mmol) in a plastic bottle and the mixture was allowed to
stir overnight (approximately 16 h) at 40 8C. The resulting mixture
was then diluted with CH2Cl2 and washed with saturated NaHCO3
(one time) and water (3 times), after which the organic layer was
dried with MgSO4. The dried solution was then concentrated under
vacuum to give 4-methylphenyl dichloromethyl ether (2h): 1H
1k
1l
2-t-Bu-Ph
2-naphthyl
4-OCH3-Ph
THF-5HF
KF
THF-5HF
KF
51
42
51
1m
Et3N-3HF
NMR:
d 2.35 (s, 3H), 7.05 (m, 2H), 7.18 (m, 2H), 7.42 (s, 1H), which
the same manner as the aromatic analogs, using PCl5. However, for
aliphatic systems the only fluorination procedure that has thus far
proven successful is fluorine/chlorine exchange using Et3N-3HF
(Scheme 4). The inefficiency of the benzyl formate in this reaction
is likely due side reactions involving formation of the stable benzyl
cation.
Use of THF-5HF led to no observable product, and pyridine-HF
provided only 10% yield. The SN2 method, using KF in sulfolane at
120 8C also did not lead to significant product formation from
aliphatic dichloromethyl ethers.
was consistent with literature data [18].
4.3. Preparation of aryl difluoromethyl ethers from aryl formates
4.3.1. THF-5HF method
PCl5 (4.16 g, 20 mmol) was added into a solution of 4-
bromophenyl formate 1b (2.0 g, 10 mmol) and DCM (10 mL) under
N2 atmosphere. The mixture was stirred at 35 8C for 12 h. H NMR
showed that the conversion was complete and the –CCl2H
compound was formed. After filtration and washing solid with
CH2Cl2, the combined organic phase was washed with saturated
NaHCO3 solution and water several times, and then dried with
MgSO4. After the solution was filtered and the solvent was
evaporated under vacuum, the residue was added to a solution of
THF-5HF (10 mL) in a 25 mL plastic bottle. The mixture was stirred
3. Conclusions
Aryl difluoromethyl ethers have been prepared in three steps
from phenols via their formate esters by conversion of the formate
esters to dichloromethyl ethers followed by chlorine/fluorine
exchange. Two general methods were utilized for conversion of the
dichloromethyl to the difluoromethyl ethers, one involving
solvolytic exchange at room temperature for 12–16 h in an HF/
Lewis base medium (either THF-5HF or Et3N-3HF), and the other a
direct, SN2 nucleophilic displacement of chloride by fluoride ion
using KF in sulfolane at 125 8C for 15 h. Yields ranged from 0 to 80%
for the KF procedure, and from 35 to 94% for the generally more
effective solvolytic processes, with THF-5HF proving superior for
electron-poor substrates and Et3N-3HF being superior for electron
for 12 h at room temperature. The product 3b was observed by 19
F
NMR to be formed in about 82% yield using trifluorotoluene as the
internal standard. The reaction was quenched by adding excess
amount of H2O, followed by extraction with CH2Cl2. The organic
phase was washedsuccessivelywithsaturatedNaHCO3 solution and
brine, and then dried over anhydrous MgSO4. After the solution was
filtered and the solvent was evaporated under vacuum, the residue
was subjected to silica gel column chromatography to give product
3b (1.78 g, yield: 80%) as a colorless liquid: 1H NMR:
(d, J = 8.7 Hz, 2H), 7.02 (d, J = 8.7 Hz, 2H), 6.48 (t, J = 73.2 Hz, 1H);
d 7.48

W.R. Dolbier Jr. et al. / Journal of Fluorine Chemistry 160 (2014) 72–76
75
19F NMR:
d
À86.8 (d, J = 73 Hz, 2F). The characterization data was
4.3.11. 3-Methyl-(difluoromethoxy)benzene (3i)
19F NMR:
À81.5 (d, J = 73 Hz, 2F) [31].
consistent with previous reports [28,29].
d
4.3.2. Et3N-3HF method
4.3.12. 2-Iodo-(difluoromethoxy)benzene (3j)
19F NMR:
was consistent with the previous report [32].
PCl5 (2.3 g, 11 mmol) was added to a 4-methylphenyl formate
(1h) (1.4 g, 10 mmol) in a plastic bottle and the mixture was left to
stir overnight (approx. 16 h) at 40 8C. 1H-NMR indicated that
conversion was complete, and that the dichloromethoxy-com-
pound was formed. The mixture was then placed on the rotovap to
remove any volatile compounds. The resulting residue was placed
in a 25 mL plastic bottle at room temperature, and Et3N-3HF (4.8 g,
30 mmol) was added at room temperature, and the mixture stirred
for 4 h. At that time, an internal standard (trifluorotoluene) was
added and the yield of 3h was determined by 19F NMR to be 30%.
The mixture was then left to stir for an additional 12 h, after which
time the yield was determined to be 71%. The 19F NMR also
indicated the presence of some PF6.
d
À82.1 (d, J = 73 Hz, 2F). The characterization data
4.3.13. 2-Tert-butyl-(difluoromethoxy)benzene (3k)
19F NMR:
d
À79.2 (d, J = 74 Hz, 2F); 1H NMR:
d 1.42 (s, 9H), 6.52
(t, J = 71 Hz, 1H), 6.60 (dd, J = 8.1, 1.5 Hz, 1H), 6.88 (m, 1H), 7.08 (m,
1H), 7.80, J = 7.8, 1.8, 1H). This compound has not been previously
reported in the literature.
4.3.14. 2-(Difluoromethoxy)naphthalene (3l)
19F NMR:
d
À81.8 (d, J = 73 Hz, 2F). The characterization data
was consistent with the previous report [6,29].
4.3.15. 4-Methoxy-(difluoromethoxy)benzene (3m)
4.3.3. KF method
19F NMR:
d
À80.4 (d, J = 74 Hz, 2F); 1H NMR:
d 3.78 (s, 3H), 6.42
PCl5 (4.16 g, 20 mmol) was added into a solution of 4-
bromophenyl formate 1b (2.0 g, 10 mmol) and CH2Cl2 (10 mL)
under N2 atmosphere. The mixture was stirred at 35 8C for 12 h,
after which time the 1H NMR indicated that the conversion was
complete and that the –OCHCl2 compound had been formed. After
filtration and washing of the solid with CH2Cl2, the combined
organic phase was washed with NaHCO3 solution and water
several times, and then dried over with MgSO4. After the solution
was filtered and the solvent was evaporated under vacuum, the
residue was added to mixture of KF (2.32 g, 40 mmol), TBAB
(0.258 g, 0.8 mmol) and sulfolane (15 mL) in a 50 mL three-
necked flask under nitrogen. The mixture was then stirred at
125 8C for 15 h. The reaction was quenched by adding excess
amount of H2O, followed by extraction with CH2Cl2. The organic
phase was washed brine three times, and then dried over
anhydrous MgSO4. After the solution was filtered and the solvent
evaporated under vacuum, the residue was subjected to silica gel
column chromatography to give product 3b (1.38 g, yield: 62%) as
a colorless liquid.
(t, J = 74 Hz, 1H), 6.87 (d, J = 9 Hz, 2H), 7.06 (d, J = 9 Hz, 2H). The
characterization data was consistent with the previous report
[6,29].
4.4. Preparation of aliphatic difluoromethyl ethers
3-Phenylpropyl formate (4) (0.49 g, 3 mmol) was added to a
10 mL Schlenk tube filled with N2. After being cooled in an ice bath,
PCl5 (0.68 g, 3.3 mmol) was added slowly and the mixture stirred
at 40 8C until disappearance of the PCl5. After removing POCl3 by
rotary evaporation, Et3N-3HF (1.5 g, 9 mmol) was added slowly,
and the resulting solution was stirred at room temperature
overnight after which 20 mL Et2O was added. The mixture was
washed with saturated NaHCO3, water and brine, and then dried
over Na2SO4. After removing the solvent, the residue was purified
by flash chromatography on a silica gel column using pet ether and
CH2Cl2 (5:1) as the eluent. The product, (3-difluoromethoxypro-
pyl)benzene (6a) was obtained in 56% yield as a colorless oil: 19F
NMR:
d
À84.1 (d, J = 74 Hz); 1H NMR:
d 1.91–2.00 (m, 2H), 2.71
(t, J = 7.6 Hz, 2H), 3.84 (t, J = 6.3 Hz, 2H), 6.19 (t, J = 75 Hz, 1H),
4.3.4. (Difluoromethoxy)benzene (3a)
7.17–7.32 (m, 5H); 13C NMR:
d 30.7, 31.8, 62.8 (t, J = 4 Hz), 116.2
19F NMR:
d
À81.7 (d, J = 73 Hz, 2F). The characterization data
(t, J = 260 Hz), 126.1, 128.4, 141.1, 155.0; MS-EI: M+ 186; HRMS
was consistent with the previous report [28,29].
(EI): Calcd C9H10OF2 186.0856, found 186.0851.
4.3.5. 4-Chloro-(difluoromethoxy)benzene (3c)
4.4.1. (2-difluoromethoxyethyl)benzene (6b)
19F NMR:
d
À82.5 (d, J = 73 Hz, 2F). The characterization data
(46% yield); 19F NMR:
d
À84.6 (d, J = 76 Hz); 1H NMR:
d 2.96 (t,
was consistent with the previous report [28,29].
J = 7 Hz, 2H), 4.06 (t, J = 7 Hz, 2H), 6.18 (t, J = 75 Hz, 1H), 7.21–7.34
(m, 5H); 13C NMR:
d 35.6, 64.9 (t, J = 6 Hz), 116.0 (t, J = 260 Hz),
4.3.6. 4-Phenyl-(Difluoromethoxy)benzene (3d)
126.6, 128.5, 128.8, 137.4; MS-EI: M+ 172; HRMS (EI): Calcd
C9H10OF2 172.0700, found 172.0703.
19F NMR:
d
À81.9 (d, J = 74 Hz, 2F). The characterization data
was consistent with the previous report [30].
Acknowledgment
4.3.7. 4-Nitro-(difluoromethoxy)benzene (3e)
19F NMR:
d
À84.0 (d, J = 73 Hz, 2F). The characterization data
Support of this research by Syngenta Crop Protection is
gratefully acknowledged by the authors.
was consistent with the previous report [5].
4.3.8. 2-Nitro-(difluoromethoxy)benzene (3f)
References
19F NMR:
d
À83.1 (d, J = 74 Hz, 2F). The characterization data
was consistent with the previous report [5].
[1] J.-P. Begue, D. Bonnet-Delpon, Bioorganic and Medicinal Chemistry of Fluorine,
Wiley, Hoboken, NJ, 2008.
[2] P. Kirsch, Fluoroorganic Chemistry: Synthesis, Reactivity Applications, Wiley-
VCH, Weinheim, 2004.
4.3.9. 4-Cyano-(difluoromethoxy)benzene (3g)
19F NMR:
d
À83.5 (d, J = 73 Hz, 2F). The characterization data
[3] J. Hu, W. Zhang, F. Wang, Chem. Commun. (2009) 7465–7478.
[4] T.G. Miller, J.W. Thanassi, J. Org. Chem. 25 (1960) 2009–2012.
[5] P.S. Fier, J.F. Hartwig, Angew. Chem. Int. Ed. 52 (2013) 2092–2095.
[6] C.S. Thomoson, W.R. Dolbier Jr., J. Org. Chem. 78 (2013) 8904–8908.
[7] L. Li, F. Wang, C. Ni, J. Hu, Angew. Chem. Int. Ed. 52 (2013) 12390–12394.
[8] R.D. Chambers, Fluorine in Organic Chemistry, Blackwell Publishing, Oxford,
2004.
was consistent with the previous report [28].
4.3.10. 4-Methyl-(difluoromethoxy)benzene (3h)
19F NMR:
d
À81.6 (d, J = 74 Hz, 2F). The characterization data
was consistent with the previous report [28].

76
W.R. Dolbier Jr. et al. / Journal of Fluorine Chemistry 160 (2014) 72–76
[9] M. Hudlicky, A.E. Pavlath, Chemistry of Organic Fluorine Compounds II, American
[20] S. Hamman, C.G. Bequin, J. Fluorine Chem. 23 (1983) 515–524.
[21] N. Yoneda, Tetrahedron 47 (1991) 5329–5365.
Chemical Society, Washington, DC, 1995.
[10] A.K. Barbour, L.J. Belf, M.W. Buxton, in: M. Stacey, J.C. Tatlow, A.J. Sharpe (Eds.),
Adv. Fluor. Chem., vol. 3, Butterworths, Washington, 1963, pp. 181–270.
[11] L.M. Yagupolskii, Dokl. Akad. Nauk SSSR 105 (1955) 100–102.
[12] J. Salome, C. Mauger, S. Brunet, V. Schanen, J. Fluorine Chem. 125 (2004) 1947–1950.
[13] J.M. Kremsner, M. Rack, C. Pilger, C.O. Kappe, Tetrahedron Lett. 50 (2009) 3665–
3668.
[14] A. Piou, S. Celerier, S. Brunet, J. Fluorine Chem. 134 (2012) 103–106.
[15] W.R. Dolbier Jr., J.-X. Duan, X.X. Rong, J. Fluorine Chem. 128 (2007) 1091–1093.
[16] Y. Katafuchi, T. Fujihara, T. Iwai, J. Terao, Y. Tsuji, Adv. Synth. Catal. 352 (2012)
475–482.
[22] W.R. Dolbier Jr., L. Celewicz, K. Ohnishi, Tetrahedron Lett. 30 (1989) 4929–
4930.
[23] W.R. Dolbier Jr., T.A. Gray, K. Ohnishi, Synthesis (1987) 956–958.
[24] Q.-Y. Chen, S.-W. Wu, J. Fluorine Chem. 44 (1989) 433–440.
[25] Y. Hagooly, O. Choen, S. Rozen, Tetrahedron Lett. 50 (2009) 392–394.
[26] Y. Hagooly, R. Sasson, M.J. Welch, S. Rozen, Eur. J. Org. Chem. (2008) 2875–
2880.
[27] H. Konishi, K. Manabe, T. Ueda, Org. Lett. 14 (2012) 3100–3103.
[28] M. Ochiai, A. Yoshimura, M.M. Hoque, T. Okubo, M. Saito, K. Miyamoto, Org. Lett.
13 (2011) 5568–5571.
[17] J. Martinez, J. Laur, Synthesis (1982) 979–981.
[29] J. Zheng, Y. Li, L. Zhang, J. Hu, G.J. Meuzelaar, H.-J. Federsel, Chem. Commun.
(2007) 5149–5151.
[30] F. Wang, L. Zhang, J. Zheng, J. Hu, J. Fluorine Chem. 132 (2011) 521–528.
[31] G. Hamprecht, M. Witschel, B. Wuerzer, Chimia 58 (2006) 117–122.
[32] F. Wang, W. Huang, J. Hu, Chin. J. Chem. 29 (2011) 2717–2721.
[18] M. Consuelo Jimenez, M.A. Miranda, R. Tormos, Tetrahedron 51 (1995) 5825–5830.
[19] E. Alvarez-Manzaneda, R. Chahboun, E. Cabrera, E. Alvarez, A. Haidour, J.M.
Ramos, R. Alvarez-Manzaneda, Y. Charrah, H. Es-Samti, Org. Biomol. Chem. 7
(2009) 5146–5155.