Oxidation of primary aliphatic and aromatic aldehydes with difluoro(aryl)-λ3-bromane

Article abstract of DOI:10.1021/ol202248x

Oxidation of primary aliphatic aldehydes with p- trifluoromethylphenyl(difluoro)-λ³-bromane in dichloromethane at 0 °C afforded acid fluorides selectively in good yields, while that of aromatic aldehydes in chloroform at room temperature produced aryl difluoromethyl ethers. A larger migratory aptitude of aryl groups compared to primary alkyl groups during a 1,2-shift from carbon to an electron-deficient oxygen atom in bromane(III) Criegee-type intermediates will result in these differences in the reaction courses.

Full text of DOI:10.1021/ol202248x

ORGANIC
LETTERS
2011
Vol. 13, No. 20
5568–5571
Oxidation of Primary Aliphatic and
Aromatic Aldehydes with Difluoro(aryl)-
λ3-bromane
Masahito Ochiai,* Akira Yoshimura, Md. Mahbubul Hoque, Takuji Okubo,
Motomichi Saito, and Kazunori Miyamoto
Graduate School of Pharmaceutical Sciences, University of Tokushima, 1-78 Shomachi,
Tokushima 770-8505, Japan
mochiai@ph.tokushima-u.ac.jp
Received August 25, 2011
ABSTRACT
Oxidation of primary aliphatic aldehydes with p-trifluoromethylphenyl(difluoro)-λ3-bromane in dichloromethane at 0 °C afforded acid fluorides
selectively in good yields, while that of aromatic aldehydes in chloroform at room temperature produced aryl difluoromethyl ethers. A larger
migratory aptitude of aryl groups compared to primary alkyl groups during a 1,2-shift from carbon to an electron-deficient oxygen atom in
bromane(III) Criegee-type intermediates will result in these differences in the reaction courses.
The BaeyerꢀVilliger oxidation of ketones with peracids
directly affords more complex and valuable esters or
lactones.1 Recently, we developed a conceptually distinct,
modern strategy for the BaeyerꢀVilliger oxidation:2 the
method involves an initial hydration of water to carbonyl
compounds, followed by ligand exchange of hypervalent
Frohn reagent p-trifluoromethylphenyl(difluoro)-λ3-bro-
mane (1)3 on bromane(III) withthe resulting hydrate(gem-
diol), yielding a new type of activated Criegee intermediate
2 as depicted in Scheme 1. The alkoxy-λ3-bromane inter-
mediate 2 undergoes BaeyerꢀVilliger rearrangement and
produces rearranged ester 3 via facile reductive elimination
of an aryl-λ3-bromanyl group, because of the excellent
hypernucleofugality.4,5 This novel strategy makes it possi-
Scheme 1
(1) (a) Baeyer, A.; Villiger, V. Ber. Dtsch. Chem. Ges. 1899, 32, 3625.
(b) Hassall, C. H. Org. React. 1957, 9, 73. (c) Krow, G. R. Org. React.
1993, 43, 251. (d) Strukul, G. Angew. Chem., Int. Ed. 1998, 37, 1198. (e)
Renz, M.; Meunier, B. Eur. J. Org. Chem. 1999, 737. (f) Brink, G.-J.;
Arends, I. W. C. E.; Sheldon, R. A. Chem. Rev. 2004, 104, 4105.
(2) Ochiai, M.; Yoshimura, A.; Miyamoto, K.; Hayashi, S.; Nakanishi,
W. J. Am. Chem. Soc. 2010, 132, 9236.
(3) (a) Frohn, H. J.; Giesen, M. J. Fluorine Chem. 1998, 89, 59. (b)
Ochiai, M.; Nishi, Y.; Goto, S.; Shiro, M.; Frohn, H. J. J. Am. Chem.
Soc. 2003, 125, 15304. (c) Frohn, H.-J.; Hirschberg, M. E.; Wenda, A.;
Bardin, V. V. J. Fluorine Chem. 2008, 129, 459.
(4) (a) Okuyama, T.; Takino, T.; Sueda, T.; Ochiai, M. J. Am. Chem.
Soc. 1995, 117, 3360. (b) Ochiai, M. In Chemistry of Hypervalent
Compounds; Akiba, K.-y., Ed.; Wiley-VCH: New York, 1999; Chapter 12.
(c) Ochiai, M. In Topics in Current Chemistry; Wirth, T., Ed.; Springer:
Berlin, 2003; Vol. 224, p 5.
ble to selectively induce the BaeyerꢀVilliger oxidation of
straight chain primary aliphatic as well as aromatic
r
10.1021/ol202248x
Published on Web 09/15/2011
2011 American Chemical Society

aldehydes, which is missing in the classical BaeyerꢀVilliger
methods.2
Table 1. Oxidative Fluorination of Aliphatic Aldehydes with
Difluoro-λ3-bromane 1a
The presence of a small amount of water plays a pivotal
role in the λ3-bromane-induced BaeyerꢀVilliger oxidation
of carbonyl compounds: thus, rearranged formate 3 (R =
n-C9H19) was producedin a good yield from the reaction of
decanal in the presence of 1 to 10 equiv of water, while no
formation of 3 was detected under anhydrous conditions.2
We report herein an oxidation reaction of aldehydes with
Frohn reagent 1 without adding external water under mild
conditions (at 0 °C or ambient temperature): in the reac-
tion of primary aliphatic aldehydes, carboxylic acid fluor-
ides 4 were predominantly produced in good yields. In
marked contrast, reaction of difluoro-λ3- bromane 1 with
aromatic aldehydes under anhydrous conditions afforded
selectively rearranged difluoromethyl aryl ethers 7 in good
yields.
Environmentally friendly hypervalent aryl-λ3-iodanes
(ArILL0) with two heteroatom ligands (L/L0) enjoy their
rich chemistry, especially for oxidative transformations of
various kinds of functionalities in modern organic
synthesis.6 On the other hand, oxidation reactions using
hypervalent organo λ3-bromanes (ArBrLL0) remain vir-
tually unexplored,7,8 although their oxidizing power seems
to be greater than that of aryl-λ3-iodanes.9 In fact, the
ionization potential of bromobenzene (8.98 eV) is larger
compared to that of iodobenzene (8.69 eV).10
product
bromane 1
t
entry
aldehyde
(equiv)
(h)
4
yield (%)b
δc
1
n-C9H19CHO
n-C9H19CHO
n-C9H19CHO
n-C9H19CHO
MeCHO
1.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3
3
1
3
3
3
3
3
3
4a 37
2
4a 73 (73)
44.9
3d
4e
5
4a
4a
4
0
4b 60 (50)
4c 68 (67)
4d 56 (42)
4e 69 (53)
4f 81 (70)
51.0
44.8
47.4
45.0
45.0
44.9
45.0
53.9
36.2
44.4
6
BuCHO
7
i-BuCHO
8
n-C5H11CHO
TsO(CH2)5CHO
Cl(CH2)5CHO
Br(CH2)5CHO
t-BuCH2CHO
c-C6H11CHO
PhCH2CHO
9
10
11
12
13
14
24 4g 81 (72)
36 4h 76 (63)
3
4
3
4i 25 (16)
4j 36 (36)
4k tracef
a Conditions: aldehyde (0.05 M)/bromane 1/CH2Cl2/0 °C/Ar. b 1H
NMR yields. Parentheses are isolated yields of benzyl esters 5. c 19F
NMR chemical shifts (ppm, CDCl3) of 4. d Water (2 equiv) was used.
BaeyerꢀVilliger product 3 (R = n-C9H19) was produced as a major
product (80% yield). e Instead of difluoro-λ3-bromane 1, p-CF3C6H4IF2
was used. f PhCH2F (57%) and PhCH2OCHF2 (5%) were obtained.
selectively (Scheme 2 and Table 1, entry 2). Acid fluoride
4a showed a characteristic 19F NMR signal at δ 44.9 ppm
as a singlet in CDCl3, which is in good agreement with the
reported values for dodecanoyl (δ 44.92 ppm) and octa-
noyl fluoride (δ 44.9 ppm).12 Further conversion to the
known benzyl ester 5a through treatment of the reaction
mixture with benzyl alcohol/Et3N13 strongly suggests the
formation of acid fluoride 4a in the reaction.
As noted above, in the presence of 2 equiv of water,
reaction of decanal with difluoro-λ3-bromane 1 (1.5 equiv)
in dichloromethane at 0 °C for 1 h under argon resulted in
the selective formation of BaeyerꢀVilliger product 3 (R =
n-C9H19) in 80% yield, along with the formation of a small
amount of decanoyl fluoride (4a) (4%) (Table 1, entry 3).
In marked contrast, in the absence of water no rearranged
formate ester 3 was produced by the reaction of decanal
with 1, but instead the reaction afforded moisture sensitive
carboxylic acid fluoride 4a11 (73%, 1H NMR yield)
Scheme 2
(5) Ochiai, M.; Tada, N.; Okada, T.; Sota, A.; Miyamoto, K. J. Am.
Chem. Soc. 2008, 130, 2118.
(6) For reviews of aryl-λ3-iodanes, see: (a) Merritt, E. A.; Olofsson, B.
Synthesis 2011, 517. (b) Uyanik, M.; Ishihara, K. Chem. Commun. 2009,
2086. (c) Zhdankin, V. V.; Stang, P. J. Chem. Rev. 2008, 108, 5299. (d)
Quideau, S.; Pouysegu, L.; Deffieux, D. Synlett 2008, 467. (e) Ochiai,
M.; Miyamoto, K. Eur. J. Org. Chem. 2008, 4229. (f) Zhdankin, V. V.
Sci. Synth. 2007, 31a, Chapter 31.4.1, 161. (g) Wirth, T. Angew. Chem.,
Int. Ed. 2005, 44, 3656. (h) Tohma, H.; Kita, Y. Adv. Synth. Catal. 2004,
346, 111. (i) Hypervalent Iodine Chemistry; Wirth, T., Ed.; Topics in Current
Chemistry; Springer: Berlin, 2003; Vol. 224. (j) Togo, H.; Sakuratani, K. Synlett
2002, 1966. (k) Ochiai, M. In Chemistry of Hypervalent Compounds; Akiba,
K., Ed.; VCH: New York, 1999; p 359. (l) Varvoglis, A. The Organic
Chemistry of Polycoordinated Iodine; VCH: New York, 1992.
(7) (a) Nguyen, T. T.; Martin, J. C. J. Am. Chem. Soc. 1980, 102,
7382. (b) Nguyen, T. T.; Wilson, S. R.; Martin, J. C. J. Am. Chem. Soc.
1986, 108, 3803.
(8) For reactions of bromine trifluoride BrF3, see: (a) Rozen, S. Acc.
Chem. Res. 2005, 38, 803. (b) Hagooly, Y.; Rozen, S. J. Org. Chem. 2008,
73, 6780. (c) Cohen, O.; Sasson, R.; Rozen, S. J. Fluorine Chem. 2006,
127, 433. (d) Sasson, R.; Rozen, S. Org. Lett. 2005, 11, 2177.
(9) For reviews of aryl-λ3-bromanes, see: (a) Ochiai, M.; Miyamoto,
K.; Hayashi, S.; Nakanishi, W. Chem. Commun. 2010, 46, 511. (b)
Ochiai, M. Synlett 2009, 159. (c) Farooq, U.; Shah, A. A.; Wirth, T.
Angew. Chem., Int. Ed. 2009, 48, 1018.
Difluoro-λ3-bromane-induced oxidative fluorination
of aliphatic aldehydes such as acetaldehyde, pentanal,
(10) CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC:
Boca Raton, FL, 1992.
(11) Lal, G. S.; Pez, G. P.; Pesaresi, R. J.; Prozonic, F. M. Chem.
(12) Oxidation of primary aliphatic alcohols with BrF3 afforded a
mixture of acyl fluorides (major products) and esters. See: Rozen, S.;
Ben-David, I. J. Fluorine Chem. 1996, 76, 145.
Commun. 1999, 215.
(13) Chen, C.; Chien, C.-T.; Su, C.-H. J. Fluorine Chem. 2002, 115, 75.
Org. Lett., Vol. 13, No. 20, 2011
5569

hexanal, and substituted hexanals with an electron-with-
drawing tosyloxy (TsO), chloro, or bromo functionality at
the terminal methyl group under anhydrous conditions
afforded good yields (56ꢀ81%) of acyl fluorides 4bꢀh
selectively (entries 5ꢀ11). In marked contrast, tert-butyla-
cetaldehyde, cyclohexanecarboxaldehyde, and phenylace-
taldehyde resulted in the formation of very low to modest
yields (3ꢀ36%) of fluorination products 4iꢀk (entries
12ꢀ14). This low efficiency for the oxidative fluorination
of these aldehydes, yielding acid fluoride 4, will be due
to the occurrence of a competing BaeyerꢀVilliger-type
rearrangement, being evoked by the reported greater
migratory aptitude of neopentyl, cyclohexyl, and benzyl
groups compared to that of simple linear primary alkyl
groups.1d,14 In fact, formation of rearranged products was
detected in the difluoro-λ3-bromane-induced oxidative
fluorination: for instance, in the reaction of phenylacetal-
dehyde both benzyl fluoride in a large amount (57%) and
benzyl difluoromethy ether (5%) were produced (entry
14). The excellent hypernucleofugalityof aryl-λ3-bromanyl
groups5 suggests that the formation of these fluorine
compounds probably involves a BaeyerꢀVilliger-type 1,2-
shift of the benzyl group from a carbon to oxygen atom
(Supporting Information, Scheme S1).15
bromane, has been proposed for the oxidation of ethanol
to acetaldehyde with 1.18
In marked contrast to the reaction of R-hydroxyalkoxy-
λ3-bromane 2 (R = n-C9H19), which predominantly un-
dergoes a 1,2-shift of the alkyl group to afford rearranged
BaeyerꢀVilliger ester 3 (Scheme 1 and Table 1, entry 3),2
the reductive β-elimination pathway leading to the forma-
tion of acid fluoride 4a constitutes a major reaction course
for R-fluoroalkoxy-λ3-bromane 6 (R = n-C9H19, entry 2).
The presence of the electron-withdrawing R-fluorine atom
with a Hammett substituent constant σp of 0.06 in alkoxy-
λ3-bromane 6,19 instead of the electron-donating R-hydro-
xy group (σp = ꢀ0.37) in 2, probably not only slows down
the rate of the 1,2-shift of the alkyl group, yielding formate
3, but also enhances the rate of reductive β-elimination
producing 4a, because of the increased acidity of the R-
hydrogen atom in 6.
Scheme 3
Instead of Frohn reagent 1, use of difluoro(aryl)-λ3-
iodane p-CF3C6H4IF2 did not undergo oxidation of de-
canal to acid fluoride 4a under our conditions: thus, the
aldehyde (98%) and a large amount of difluoro-λ3-iodane
were recovered unchanged, indicatingthe higher activity of
the hypervalent difluoro-λ3-bromane 1 in the oxidation of
decanal (entry 4). Acid fluorides are generally prepared by
a halogen exchange reaction of acyl chlorides or by
fluorination of carboxylic acids.16 We showed that λ3-
bromane-induced oxidative fluorination of simple linear
primary aldehydes provides a new direct method for access
to acid fluorides.17
A reaction pathway involving the initial formation of R-
fluorohydrin from aldehyde through the addition of HF
generated, in situ, its facile ligand exchange with difluoro-
λ3-bromane 1 on bromane(III), and finally reductive β-
elimination of the resulting R-fluoroalkoxy(fluoro)-λ3-
bromane 6 will reasonably explain the selective formation
of acid fluoride 4 from aldehyde (Scheme 2). In a close
parallel to this, a reaction sequence consisting of the ligand
exchange of an alcohol with Frohn reagent 1, followed by
the reductive β-elimination of intermediate alkoxy-λ3-
λ3-Bromane-induced BaeyerꢀVilliger oxidation of
benzaldehydes in the presence of a small amount of
water exclusively produced aryl formates 8 in good to
excellent yields.2 In marked contrast to the reaction of
aliphatic aldehydes (Table 1), even in the absence of
water the reactions of benzaldehydes with difluoro-λ3-
bromane 1 hold their tendency to undergo Baeyerꢀ
Villiger-type oxidative rearrangement, but the major
products in the reactions were changed from aryl for-
mates 8 to aryl difluoromethyl ethers 7 (Scheme 3). Thus,
reaction of benzaldehyde with Frohn reagent 1 (1.3
equiv) in chloroform at room temperature afforded
difluoromethyl phenyl ether (7a)20 (85%) as a major
product and a small amount of formate ester 8a (11%).
Difluoromethyl ether 7a is rather labile toward hydro-
lysis, and the pure sample of 7a was obtained by
(14) Winnik, M. A.; Stoute, V. Can. J. Chem. 1973, 51, 2788.
(15) For the 1,2-shift of aryl groups from a carbon to oxygen atom in
benzyloxy-λ3-bromanes, see: Ochiai, M.; Yoshimura, A.; Miyamoto, K.
Tetrahedron Lett. 2009, 50, 4792.
(16) (a) Carpino, L. A.; Beyermann, M.; Wenschuh, H.; Bienert, M.
Acc. Chem. Res. 1996, 29, 268. (b) Lal, G. S.; Pez, G. P.; Pesaresi, R. J.;
Prozonic, F. M.; Cheng, H. J. Org. Chem. 1999, 64, 7048. (c) Olah, G. A.;
Nojima, M.; Kerekes, I. Synthesis 1973, 487. (d) White, J. M.; Tunoori,
A. R.; Turunen, B. J.; Georg, G. I. J. Org. Chem. 2004, 69, 2573.
(17) Radical oxidation of aldehydes with cesium fluoroxysulfate
(CsSO4F) in acetonitrile has been reported to give acid fluorides. See:
(a) Stavber, S.; Planinsek, Z.; Zupan, M. J. Org. Chem. 1992, 57, 5334.
(b) Stavber, S.; Kosir, I.; Zupan, M. J. Org. Chem. 1997, 62, 4916.
(18) Ochiai, M.; Yoshimura, A.; Mori, T.; Nishi, Y.; Hirobe, M.
J. Am. Chem. Soc. 2008, 130, 3742.
(19) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(20) (a) Zhang, L.; Zheng, J.; Hu, J. J. Org. Chem. 2006, 71, 9845. (b)
Langlois, B. R. J. Fluorine Chem. 1988, 41, 247.
(21) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry;
Wiley-VCH: Weinheim, 2003.
5570
Org. Lett., Vol. 13, No. 20, 2011

preparative GC. Benzaldehydes with electron-donating
(o- and p-Me, and p-MeO) and moderately electron-
withdrawing substituents (p-F, p-Cl, and p-Br) selec-
tively afforded aryl difluoromethyl ethers 7bꢀg in good
to high yields (Table 2, entries 2ꢀ7). The greater migra-
tory aptitude of these aryl groups compared to that
of linear primary alkyl groups will be responsible for
the selective 1,2-aryl shift from a carbon to oxygen atom
in R-fluorobenzyloxy-λ3-bromanes 9, instead of their
reductive β-elimination of the aryl-λ3-bromanyl group
to give aromatic acid fluorides.1
benzylic hydrogen abstraction of R-fluoro-p-nitrobenzy-
loxy-λ3-bromane 9h during reductive β-elimination, lead-
ing to the formation of p-NO2C6H4COF. Thus, solvent
hydrogen bonding toward the fluoride anion will be more
effective in chloroform solution than in dichloromethane
solution, which was further supported by the fact that the
former is more acidic with an estimated pKa value of ca.
24.22 Therefore, the rate of reductive β-elimination of 9h,
compared to that of the oxidative 1,2-aryl shift, will be
slowed down more remarkably in chloroform solution.15
For this reason, dichloromethane was used as a solvent in
the λ3-bromane-induced oxidative fluorination of primary
aliphatic aldehydes to acyl fluorides 4 (Table 1), where the
fluoride anion (or hydrogen fluoride) may participate in
the reductive β-elimination of R-fluoroalkoxy-λ3-bromane
6. Addition of a small amount of 2,6-di-tert-butylpyridine
slightly increased the rate of β-elimination of R-fluoro-
benzyloxy-λ3-bromane 9h and afforded the acid fluoride
in 70% yield (entry 10).23
Stavber and co-workers reported oxidative rearrange-
ment of benzaldehydes to aryl difluoromethyl ethers 7
using xenon difluoride in the presence of large excessive
amounts (4ꢀ5 equiv) of hydrogen fluoride, where initial
formation of R-fluorohydrins via the addition of hydrogen
fluoride to aldehydes and the subsequent ligand exchange
of xenon difluoride with formation of alkoxyxenon fluor-
ide were proposed.24
Thus, the reaction of aldehydes with difluoro-λ3-bromane
1 seems to involve the initial formation of R-fluorohydrins
and then their ligand exchange reaction on Br(III) of 1. The
follow-up reaction is dependent on the magnitude of the
migratory aptitude of substituents: thus, R-fluorobenzy-
loxy-λ3-bromanes 9 mostly undergo a 1,2-shift of aryl
groups with a high migratory aptitude to give aryl difluor-
omethyl ethers 7, while in R-fluoroalkoxy-λ3-bromanes
6 reductive β-elimination leading to the formation of acid
fluorides 4 constitutes a major reaction pathway, because
of the decreased migratory aptitude of alkyl groups.
Table 2. Oxidative Rearrangement of Aromatic Aldehydes with
Difluoro-λ3-bromane 1a
product, yield (%)b
entry
aldehyde
PhCHO
7
δc
8
1
7a
7b
7c
7d
7e
7f
85
66
84
66
88
89
84
26
27
20
ꢀ81.1
ꢀ80.5
ꢀ80.9
ꢀ80.9
ꢀ81.5
ꢀ81.7
ꢀ81.7
ꢀ82.5
ꢀ
8a
8b
8c
8d
8e
8f
11
17
6
2
o-MeC6H4CHO
p-MeC6H4CHO
p-MeOC6H4CHO
p-FC6H4CHO
3
4
4
5
12
9
6
p-ClC6H4CHO
p-BrC6H4CHO
p-NO2C6H4CHO
p-NO2C6H4CHO
p-NO2C6H4CHO
7
8d
9d,e
10d,e,f
7g
7h
7h
7h
8g
8h
8h
8h
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
a Conditions: aldehyde (0.05 M)/bromane 1 (1.3 equiv)/CHCl3/room
temperature/2 h/Ar. b 1H NMR yields. c 19F NMR chemical shifts (ppm,
CDCl3) of 7. d Yields of acid fluoride p-NO2C6H4COF: 41% (entry 8),
65% (entry 9), and 70% (entry 10). e In CH2Cl2 as a solvent. f Bromane 1
(1.5 equiv) and 2,6-di-tert-butylpyridine (0.1 equiv) were used.
In fact, p-nitrobenzaldehyde with a highly electron-
deficient aryl group, exhibiting a poor migratory aptitude,
produced a large amount of acid fluoride p-NO2C6H4COF
(41%) at the expense of the formation of difluoromethyl
ether 7h (26%) (entry 8). Changing the solvent from
chloroform to dichloromethane increased the yield of the
acid fluoride to 65% (entry 9).
Chloroform with a larger solvent acceptor number (AN)
of 23.1 than that of dichloromethane (20.4)21 more effi-
ciently solvates and deactivates an anionic species such as
the fluoride anion, which probably participates in the
Acknowledgment. We gratefully acknowledge the Minis-
try of Education, Culture, Sports, Science, and Technology of
Japan for financial support in the form of a grant. We thank
Central Glass Co. Ltd., Japan for a generous gift of BrF3.
Supporting Information Available. Typical experimen-
tal procedures and spectral data. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(22) (a) Margolin, Z.; Long, F. A. J. Am. Chem. Soc. 1973, 95, 2757.
(b) Bohme, D. K.; Lee-Ruff, E.; Young, L. B. J. Am. Chem. Soc. 1972,
94, 5153.
(23) Addition of an equimolar amount of 2,6-di-tert-butylpyridine
completely inhibited the oxidation of p-nitrobenzaldehyde. This is
probably due to deactivation of difluoro-λ3-bromane 1 through the
coordination of the pyridine base to the hypervalent bromane(III) with
formation of a tetracoordinated λ3-bromane species.
(24) (a) Stavber, S.; Koren, Z.; Zupan, M. Synlett 1994, 265. (b) Tius,
M. A. Tetrahedron 1995, 51, 6605. (c) Tamura, M.; Takagi, T.; Quan, H.;
Sekiya, A. J. Fluorine Chem. 1999, 98, 163.
Org. Lett., Vol. 13, No. 20, 2011
5571