Trifluoromethylation of nonenolizable carbonyl compounds with a stable piperazino hemiaminal of trifluoroacetaldehyde

Article abstract of DOI:10.1002/1099-0690(200104)2001:8<1467::AID-EJOC1467>3.0.CO;2-A

A new stable hemiaminal of fluoral (1) can easily be obtained from the methyl hemiketal of fluoral and N-benzylpiperazine. This white crystalline compound can be used under basic conditions as an efficient nucleophilic trifluoromethylating reagent towards nonenolizable carbonyl compounds.

Full text of DOI:10.1002/1099-0690(200104)2001:8<1467::AID-EJOC1467>3.0.CO;2-A

FULL PAPER
Trifluoromethylation of Nonenolizable Carbonyl Compounds with a Stable
Piperazino Hemiaminal of Trifluoroacetaldehyde[‡]
[a]
[a]
[a]
¨
Thierry Billard,* Bernard R. Langlois,* and Gaelle Blond
Keywords: Nitrogen heterocycles / Fluorine / Aldehydes / Nucleophilic addition / Fluoral
A new stable hemiaminal of fluoral (1) can easily be obtained
sic conditions as an efficient nucleophilic trifluoromethylat-
from the methyl hemiketal of fluoral and N-benzylpiperaz- ing reagent towards nonenolizable carbonyl compounds.
ine. This white crystalline compound can be used under ba-
Introduction
Because of the intrinsic properties of fluorine, fluorinated
organic compounds present unique properties and often ex-
hibit unusual reactivities.[1] Among them, trifluoromethyl-
substituted molecules are of great interest for different
applications.[2Ϫ4] Nevertheless, the efficient introduction of
the CF3 moiety to organic substrates always constitutes an
Scheme 1. Synthesis of the hemiaminal of fluoral 1
important challenge for chemists.
At present, many reliable methods are available[5] but the
nucleophilic strategy, which is the most promising one, still
suffers from a limited number of efficient reagents able to
overcome the great instability of the CF3 anion, which is
known to decompose rapidly into fluoride ion and difluoro-
carbene.
Presently, the most popular commercial reagent is Rup-
pert’s reagent (CF3SiMe3).[6] However, this compound is
prepared from bromotrifluoromethane (CF3Br), the indus-
trial production of which is now restricted.[7] This drawback
prompted us to investigate new reagents useful for the nu-
cleophilic trifluoromethylation and easily accessible from
simple, cheap and readily available starting materials.
This led us to study the trifluoromethylating ability of 1
under basic conditions. First, we determined which basic
system was most suitable for the trifluoromethylation of
benzophenone with 1. (Table 1)
Ϫ
Table 1. Determination of the best basic system for trifluoromethyl-
ation of 1
Entry
1 (equiv.)
Base
Trifluoromethyl carbinol
2a (%)[a]
Results and Discussion
1
2
1
2
2
2
2
2
2
2
2
2
2
2
tBuOK
tBuOK
tBuONa
tBuOLi
KH
50
90
0
Trifluoroacetaldehyde (fluoral), in its hemiketal form,
fulfils these latter conditions and we recently showed that
it reacts very easily with N-benzyl piperazine to provide,
after silylation, the corresponding O-silylated hemiami-
nal.[8] This new stable compound is able to trifluoromethyl-
ate nonenolizable carbonyl compounds by fluoride activa-
tion. However, as shown in Scheme 1, its synthesis needs
the intermediate preparation of the free hemiaminal 1,
which can easily be purified by precipitation from its dichlo-
romethane solution with pentane.
3
4
0
5
95
0
6
NaH
7
KN(SiMe3)2
BuLi
PhMgCl
DBU[b]
Imidazole
DABCO[c]
90
0
8
9
0
10
11
12
0
0
0
[a]
Determined by 19F NMR spectroscopy with internal standard
[b]
[c]
(PhOCF3). Ϫ
DABCO: 1,4-Diazabicyclo[2.2.2]octane.
DBU: 1,8-Diazabicyclo[5.4.0]undec-7-ene. Ϫ
[‡]
New Stable Reagents for Nucleophilic Trifluoromethylation, 3.
Ϫ Part 2: Ref.[8]
[a]
´
Laboratoire SERCOF, Universite Claude Bernard-Lyon 1,
Bat. Chevreul (303), 43 Bd du 11 novembre 1918, 69622
Villeurbanne, France
It appears from Table 1 that two equivalents of 1 and
base are necessary to obtain good yields (entries 1 and 2).
E-mail: billard@univ-lyon1.fr; Bernard.Langlois@univ-lyon1.fr Furthermore, the nature of the base counter-cation plays a
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Eur. J. Org. Chem. 2001, 1467Ϫ1471
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1467

T. Billard, B. R. Langlois, G. Blond
FULL PAPER
crucial role. Indeed, if we assume, according to our previous on the trifluoromethylation efficiency. Nevertheless, the ion-
work,[8] that the trifluoromethylating species is the hemi- bonding of lithium salts is so strong that even DMF or
aminolate 3 (Scheme 2), the better the ion pair is separated, TMEDA are not chelating enough to allow trifluoromethyl-
the better the generation of CF3Ϫ: reagents 3 forming tight ation (entries 4, 5).
ionic pairs [Li salts (entries 4 and 8) or Na salts (entries 3
As chelating phenomena could also shed some light on
and 6)] or too covalent OϪM bonds [Mg salts (entry 9)] do the necessity of using two equivalents of 1 and base to reach
not lead to the expected product 2a. Negative results were good trifluoromethylation yields, we had a deeper insight
also obtained with amines (entries 10Ϫ12), which are prob- into solvent effects on the stoichiometry of the reaction
ably not basic enough to deprotonate 1.
(Table 3).
Table 3. Solvent effect on the reactivity of 3 with benzophenone
Entry
3 (equiv.)
Solvent
2a (%)[a]
Ϫ
Scheme 2. Generation of a CF3 equivalent from 1
1
2
3
4
5
6
2
1
1
1
1
1
THF
90
91
68
93
85
68
THF/DMF (1:1: v/v)
THF ϩ DMF (1 equiv.)
DMF
To verify the influence of the pair separation we carried
out the same reaction in solvents exhibiting higher polari-
ties and basicities than THF, or in the presence of chelating
agents. (Table 2)
THF ϩ 18-c-6 (1 equiv.)[b]
DME[c]/THF (1:1: v/v)
[a]
Determined by 19F NMR spectroscopy with internal standard
[b]
[c]
(PhOCF3). Ϫ
thane.
18-c-6: 18-crown-6. Ϫ DME: 1,2-Dimethoxye-
Table 2. Effect of solvation on the trifluoromethylation of 1
It appears from Table 3 that the use of DMF as solvent
Entry
Base
Solvent
2a (%)[a] or co-solvent leads to the same yield as in pure THF but
with only one equivalent of 1 and tBuOK instead of two
1
2
3
4
5
NaH
DMF
90
90
91
0
tBuONa
tBuOK
tBuOLi
BuLi
DMF
DMF
DMF
THF ϩ TMEDA (2 equiv.)[b]
0
[a]
Determined by 19F NMR spectroscopy with internal standard
[b]
(PhOCF3). Ϫ TMEDA: N,NЈ-tetramethylethylenediamine.
Table 2 shows that solvents with high donor-numbers like
DMF (DMF: DNN ϭ 0.8 vs. THF: DNN ϭ 0.52),[9] which
is able to chelate cations more efficiently than THF, lead to
the same results with sodium and potassium salts (entries
1Ϫ3). This underlines the importance of ion-pair separation Scheme 3. Self-activation of 3 through a dimeric form
Scheme 4. Mechanism of the trifluoromethylation reaction of 1
1468
Eur. J. Org. Chem. 2001, 1467Ϫ1471

Trifluoromethylation of Nonenolizable Carbonyl Compounds
FULL PAPER
(entries 2, 4). A similar result can be obtained by adding 18- those of THF and DMF, (entry 6) also increased the yield
crown-6 to THF as a specific chelating agent for potassium but to a lesser extent.
cations (entry 5). The use of only one equivalent of DMF
These observations led us to propose that, in THF, 3
(entry 3), or of a co-solvent like DME, with a DN between (M ϭ K) exists in a dimeric form (4) which allows the self-
activation of ion pairs. After reaction with benzophenone,
4 is transformed into complex 5 in which one equivalent of
3 is replaced by the conjugate base of 2a (Scheme 3). Obvi-
ously, the potassium hemiaminolate moiety included in 5 is
Table 4. Trifluoromethylation of nonenolizable carbonyl com-
pounds with 1
no longer activated by chelation and remains inactive to-
wards benzophenone. This could explain the necessity of
two equivalents of 1 for getting good trifluoromethylation
yields.
On the contrary, when the reaction was carried out in
the presence of DMF, DME or 18-crown-6, 4 cannot exist
because of the external chelation of the potassium cation
with such species. Consequently, the following refined
mechanism can be proposed for the trifluoromethylation of
benzophenone with 1 in THF. (Scheme 4)
In this mechanism, it can be assumed that the transfer of
the CF3 moiety from 4 to the ketone occurs in a concerted
way through a six-membered transition state 6, in order to
Ϫ
account for the fact that no free CF3 was formed, as
indicated by the absence of fluoride ions in the crude mix-
ture (19F NMR spectroscopy). After acidic hydrolysis, 5
gives the trifluoromethyl carbinol 2a and, theoretically, one
equivalent of 1; this compound, however, is too acid sensit-
ive and could not be recovered.
After model studies with benzophenone, this trifluorome-
thylation reaction has been extended to other nonenolizable
carbonyl compounds. (Table 4)
As shown in Table 4, good yields were usually obtained
from various non enolizable ketones or aldehydes (entries
1Ϫ6, 9) even in heterocyclic series (entries 7, 8, 10). Phthali-
mide could also be trifluoromethylated without opening the
cyclic imide moiety (entry 11). After reaction, both free al-
cohols or silylated ethers could be obtained, depending on
the treatment of the crude mixture either with acid or
chlorotrimethylsilane.
Conclusion
In conclusion, we have shown that it is possible to obtain,
in one step from the industrially available methyl hemiketal
of trifluoroacetaldehyde, a new stable crystalline com-
pound, which acts as an efficient nucleophilic trifluorome-
thylating agent towards nonenolizable carbonyl compounds
under mild conditions. It can also react with enolizable car-
bonyl substrates but in a completely different way. In this
case, the reaction occurs at the α-carbon center and will be
described in future papers.
Experimental Section
General Remarks: Solvents were distilled prior to use. Aldehydes
were also distilled or recrystallized prior to use. Other reagents were
1
used as received. H, 13C and 19F NMR spectra were recorded in
[a]
Isolated yield; in parentheses: crude yield determined by 19F
NMR spectroscopy with internal standard (PhOCF3).
CDCl3 at 300, 75 and 188 MHz, respectively. Chemical shifts are
Eur. J. Org. Chem. 2001, 1467Ϫ1471
1469

T. Billard, B. R. Langlois, G. Blond
FULL PAPER
given in ppm relative to TMS (1H, 13C) or CFCl3 (19F) as internal
references. Coupling constants are given in Hz. Mass spectrometry
was performed at 70 eV. Flash chromatography was performed on
silica gel 60M (0.04Ϫ0.063 mm). Melting points (uncorrected) were
determined in capillary tubes on a Büchi apparatus. Elemental ana-
lyses were carried out by the ‘‘Centre de Microanalyse’’ of the
CNRS at Solaize.
NMR: δ ϭ Ϫ75.19. Ϫ MS: m/z ϭ 253 [Mϩ·], 184, 176, 106, 78, 77,
69, 51. Ϫ C13H10F3NO: C 61.66, H 3.98, N 5.53; found C 62.01,
H 4.35, N 5.36.
2,2,2-Trifluoro-1,1-di(2-pyridyl)ethyl Trimethylsilyl Ether (2e): Pet-
roleum ether/acetone (4:1). Oil (196 mg, 60%). Ϫ 1H NMR: δ ϭ
8.54Ϫ8.57 (m, 2 H), 7.70Ϫ7.81 (m, 4 H), 7.21Ϫ7.26 (m, 2 H), 0.06
(s, 9 H). Ϫ 13C NMR: δ ϭ 158.9, 148.7, 136.9, 125.0 (q, J ϭ 287.7),
123.6, 123.2 (q, J ϭ 1.7), 83.6 (q, J ϭ 27.6), 2.3. Ϫ 19F NMR: δ ϭ
Ϫ73.55. Ϫ MS: m/z ϭ 326 [Mϩ·], 311, 257, 242, 215, 187, 168, 150,
121, 106, 78, 73, 51. Ϫ C15H17F3N2OSi (326.39): C 55.20, H 5.25,
Si 8.60; found C 55.29, H 5.64, Si 8.25.
Synthesis of 1-(4-Benzylpiperazino)-2,2,2-trifluoroethanol (1): Mo-
˚
lecular sieves (4 A) and the methyl hemiketal of fluoral (12 g,
92 mmol) were added to a solution of N-benzylpiperazine (14.77 g,
˚
84 mmol) in CH2Cl2 (80 mL; dried over 4 A molecular sieves). The
reaction mixture was stirred at room temperature for 48 h then fil-
tered and the solvents evaporated in vacuo at room temperature.
The residue was dissolved in CH2Cl2. Compound 1 was precipit-
ated from this solution by addition of pentane to yield 17.56 g of
a white solid (76%). M.p. 101Ϫ102 °C. Ϫ 1H NMR: δ ϭ 7.28Ϫ7.38
(m, 5 H), 4.68 (broad s, 1 H), 4.49 (q, J ϭ 5.9, 1 H), 3.56 (s, 2 H),
2.87 (t, J ϭ 4.85, 4 H), 2.50 (broad t, J ϭ 4.85, 4 H). Ϫ 13C NMR:
δ ϭ 137.4, 129.9, 128.7, 127.7, 124.0 (q, J ϭ 287.4), 84.5 (q, J ϭ
32.0), 63.3, 53.3, 47.2. Ϫ 19F NMR: δ ϭ Ϫ76.1 (d, J ϭ 5.8). Ϫ
MS: m/z ϭ 274 [Mϩ·], 257, 176, 134, 91. Ϫ C13H17F3N2O (274.28):
C 56.93, H 6.25, N 10.21; found C 57.13, H 6.22, N 10.23.
2,2,2-Trifluoro-1-(2-naphthyl)-1-phenylethanol
(2f):
Petroleum
ether/diethyl ether (30:1). Oil (242 mg, 80%). Ϫ 1H NMR: δ ϭ 8.21
(s, 1 H), 7.84Ϫ7.96 (m, 3 H), 7.56Ϫ7.65 (m, 5 H), 7.41Ϫ7.54 (m,
3 H), 3.25 (s, 1 H). Ϫ 13C NMR: δ ϭ 139.7, 137.0, 133.5, 133.1,
129.2, 129.1, 128.8, 128.6, 128.0 (q, J ϭ 1.7), 128.0, 127.4, 127.0,
127.0 (q, J ϭ 1.5), 125.9 (q, J ϭ 286.6), 125.6 (q, J ϭ 1.5), 80.1 (q,
J ϭ 28.5). Ϫ 19F NMR: δ ϭ Ϫ74.38. Ϫ C18H13F3O (302.29): C
71.52, H 4.33; found C 71.28, H 4.66.
2,2,2-Trifluoro-1-phenyl-1-(2-thienyl)ethanol (2g): Petroleum ether/
1
acetone (9:1). Oil (121 mg, 47%). Ϫ H NMR: δ ϭ 7.64Ϫ7.70 (m,
2 H), 7.38Ϫ7.44 (m, 4 H), 7.25Ϫ7.27 (m, 1 H), 7.04Ϫ7.07 (m, 1
H), 3.32 (s, 1 H). Ϫ 13C NMR: δ ϭ 143.6, 138.2, 129.5, 128.6,
127.6 (q, J ϭ 2.1), 127.4 (q, J ϭ 1.5), 127.3, 127.2, 125.2 (q, J ϭ
286.4), 78.2 (q, J ϭ 29.9). Ϫ 19F NMR: δ ϭ Ϫ76.70. Ϫ MS: m/z ϭ
258 [Mϩ·], 189, 111, 105, 77, 69, 51, 39. Ϫ C12H9F3OS (258.26): C
55.81, H 3.51, S 12.42; found C 55.56, H 3.68, S 12.10.
General Procedure for the Trifluoromethylation of 1: In a two-
necked flask 1 (0.548 g, 2 mmol), the carbonyl substrate (1 mmol)
and dry THF (1 mL) were successively introduced, under nitrogen.
After solubilization, tBuOK (2 mL of a 1  solution in THF,
2 mmol) was added. The reaction mixture was stirred at room tem-
perature for 24 h. After reaction, ClSiMe3 (0.5 mL) or aqueous 1
 HCl (1 mL) was dropped into the reaction mixture, which was
then extracted twice with brine and ether. The combined organic
layers were dried over Na2SO4 and evaporated in vacuo at room
temperature. The crude residue was purified by flash chromato-
graphy. Eluents are indicated for each compound.
9-(Trifluoromethyl)-9H-thioxanthen-9-ol (2h): Petroleum ether/acet-
1
one (19:1). Beige solid (200 mg, 71%). M.p. 121 °C. Ϫ H NMR:
δ ϭ 7.96Ϫ7.99 (m, 2 H), 7.28Ϫ7.39 (m, 6 H), 3.06 (s, 1 H). Ϫ 13C
NMR: δ ϭ 132.2, 130.3, 129.7, 128.9, 126.7, 126.3, 125.2 (q, J ϭ
288.1), 74.3 (q, J ϭ 30.7). Ϫ 19F NMR: δ ϭ Ϫ81.11. Ϫ MS: m/z ϭ
282 [Mϩ·], 213, 184, 152, 77, 69. Ϫ C13H17F3N2O (274.28): C 59.57,
H 3.21, S 11.36; found C 59.81, H 3.29, S 11.45.
2,2,2-Trifluoro-1,1-diphenylethanol (2a): Petroleum ether/acetone
(19:1). Oil (202 mg, 80%).1H NMR: δ ϭ 7.46Ϫ7.50 (m, 4 H),
7.31Ϫ7.34 (m, 6 H), 3.09 (s, 1 H). Ϫ 13C NMR: δ ϭ 139.5, 128.7,
127.5 (q, J ϭ 1.6), 125.5 (q, J ϭ 286.0), 79.6 (q, J ϭ 28.7). Ϫ 19F
NMR: δ ϭ Ϫ74.70. Ϫ MS: m/z ϭ 252 [Mϩ·], 189, 105, 77, 51, 39.
Ϫ C14H11F3O (252.23): C 66.67, H 4.39; found C 66.68, H 4.55.
2,2,2-Trifluoro-1-phenylethyl Trimethylsilyl Ether (2i): Petroleum
ether. Oil (161 mg, 65%). Ϫ 1H NMR: δ ϭ 7.53Ϫ7.57 (m, 2 H),
7.42Ϫ7.47 (m, 3 H), 5.01 (q, J ϭ 6.6, 1 H), 0.21 (s, 9 H). Ϫ 13C
NMR: δ ϭ 135.9 (q, J ϭ 1.2), 129.5, 128.7, 128.0 (q, J ϭ 1.2),
124.7 (q, J ϭ 282.2), 73.7 (q, J ϭ 31.8), 0.1. Ϫ 19F NMR: δ ϭ
Ϫ78.90 (d, J ϭ 7.7). Ϫ MS: m/z ϭ 248 [Mϩ·], 179, 137, 109, 77, 73.
9-(Trifluoromethyl)-9H-fluoren-9-ol (2b): Petroleum ether/acetone
(20:1). Beige solid (160 mg, 64%). M.p. 91 °C. Ϫ 1H NMR: δ ϭ
7.67 (m, 4 H), 7.48 (m, 2 H), 7.36 (m, 2 H), 2.99 (broad s, 1 H). Ϫ
13C NMR: δ ϭ 141.4, 141.3, 131.2, 128.8, 125.6 (q, J ϭ 1.7), 125.5
(q, J ϭ 284.1), 120.8, 81.8 (q, J ϭ 30.8). Ϫ 19F NMR: δ ϭ Ϫ79.1.
Ϫ MS: m/z ϭ 250 [Mϩ·], 181, 152, 76. Ϫ C14H9F3O (250.22): C
67.20, H 3.63; found C 66.94, H 3.70.
2,2,2-Trifluoro-1-(1-methyl-1H-pyrrol-2-yl)ethyl
Trimethylsilyl
1
Ether (2j): Petroleum ether. Oil (241 mg, 96%). Ϫ H NMR: δ ϭ
6.66 (dd, J ϭ 2.6, J ϭ 2, 1 H), 6.27 (dd, J ϭ 3.7, J ϭ 1.8, 1 H),
6.13 (dd, J ϭ 2.7, J ϭ 3.7, 1 H), 5.08 (q, J ϭ 7.1, 1 H), 3.73 (s, 3
H), 0.16 (s, 9 H). Ϫ 13C NMR: δ ϭ 125.5, 125.2, 124.6 (q, J ϭ
282.0), 111.5 (q, J ϭ 1.2), 107.6, 68.8 (q, J ϭ 33.9), 35.2 (q, J ϭ
2.3), Ϫ0.1. Ϫ 19F NMR: δ ϭ Ϫ77.66 (d, J ϭ 7.3). Ϫ MS: m/z ϭ
251 [Mϩ·], 182, 162, 140, 73. Ϫ C10H16F3NOSi (251.32 : C 47.79,
H 6.42, N 5.57; found C 47.94, H 6.41, N 5.30.
2,2,2-Trifluoro-1,1-bis(4-fluorophenyl)ethanol (2c): Petroleum ether/
diethyl ether (19:1) Oil (216 mg, 75%). Ϫ 1H NMR: δ ϭ 7.49
(broad dd, J ϭ 5.2, J ϭ 8.3, 4 H), 7.07 (t, J ϭ 8.3, 4 H), 3.13 (s, 1
H). Ϫ 13C NMR: δ ϭ 163.2 (d, J ϭ 248.3), 135.4 (d, J ϭ 3.5),
129.8 (dq, J ϭ 8.1, J ϭ 1.7), 125.5 (q, J ϭ 286.4), 115.7 (d, J ϭ
21.8), 79.2 (q, J ϭ 28.9). Ϫ 19F NMR: δ ϭ Ϫ75.01 (s, 3F), Ϫ113.04
(m, 2F). Ϫ MS: m/z ϭ 288 [Mϩ·], 219, 123, 95, 75, 69. Ϫ C14H9F5O
(288.21): C 58.34, H 3.15; found C 57.98, H 3.36.
2-Methyl-3-(trifluoromethyl)-3-[(trimethylsilyl)oxy]-1-isoindolinone
(2k): Pure after extraction. Brown solid (230 mg, 76%). M.p. 80Ϫ83
1
°C. Ϫ H NMR: δ ϭ 7.85 (m, 1 H), 7.60Ϫ7.65 (m, 3 H), 3.08 (s,
3 H), Ϫ0.07 (s, 9 H). Ϫ 13C NMR: δ ϭ 167.4, 140.1, 133.4, 132.3,
131.3, 127.3 (q, J ϭ 1.5), 123.7, 122.9 (q, J ϭ 287.0), 88.5 (q, J ϭ
39.0), 25.2, 0.8. Ϫ 19F NMR: δ ϭ Ϫ80.76. Ϫ MS: m/z ϭ 234, 214,
164, 151, 73.
2,2,2-Trifluoro-1-phenyl-1-(2-pyridyl)ethanol (2d): Petroleum ether/
diethyl ether (9:1) Beige solid (202 mg, 80%). M.p. 60Ϫ61 °C. Ϫ
1H NMR: δ ϭ 8.61Ϫ8.63 (m, 1 H), 7.70Ϫ7.78 (m, 3 H), 7.50Ϫ7.55
(m, 1 H), 7.32Ϫ7.47 (m, 4 H), 7.06 (broad s, 1 H).13C NMR: δ ϭ
155.3, 147.6, 138.7, 137.9, 129.0, 128.9, 127.4 (q, J ϭ 2.1), 125.5
(q, J ϭ 286.4), 124.4, 123.3 (q, J ϭ 1.9), 78.1 (q, J ϭ 28.9). Ϫ 19F
[1]
U. Groß, S. Rüdiger, in Organo-Fluorine Compounds (Eds.: B.
Baasner, H. Hagemann, J. C. Tatlow), Houben-Weyl: Methods
1470
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Trifluoromethylation of Nonenolizable Carbonyl Compounds
FULL PAPER
M. A. McClinton, D. A. McClinton, Tetrahedron 1992, 48,
[5]
of Organic Chemistry, Thieme, Stuttgart, 1999, Vol. E10a,
18Ϫ26.
6555Ϫ6666.
[6]
[7]
[8]
[2]
G. K. S. Prakash, A. K. Yudin, Chem. Rev. 1997, 97, 757Ϫ786.
As for ref.[4] pp. 1092Ϫ1100.
T. Billard, B. R. Langlois, G. Blond, Tetrahedron Lett. 2000,
R. Filler, Y. Kobayashi, Y. L. Yagulpolskii, Organofluorine
Compounds in Medicinal Chemistry and Biomedical Applica-
tions, Elsevier, Amsterdam, 1993.
[3]
R. E. Banks, B. E. Smart, J. C. Tatlow, Organofluorine Chem-
istry: Principles and Commercial Applications, Plenum Press,
New York, 1994.
41, 8777Ϫ8780.
[9]
C. Reichardt, Solvents and Solvents Effects in Organic Chem-
istry, VCH, New York, 1999, 20Ϫ21.
Received November 10, 2000
[4]
M. Hudlicky, A. E. Pavlath, Chemistry of Organic Fluorine
Compounds II. A critical Review, ACS Monograph 187, Amer-
ican Chemical Society, Washington DC, 1995.
[O00572]
Eur. J. Org. Chem. 2001, 1467Ϫ1471
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