Reaction of the Ruppert-Prakash reagent with perfluorosulfonic acids

Article abstract of DOI:10.1016/j.jfluchem.2009.05.003

A new property of Me₃SiCF₃ to undergo C-F bond activation upon interaction with perfluorosulfonic acids is described. The reaction is catalyzed by titanium tetrachloride and affords difluoromethyl perfluorosulfonates in good yields.

Full text of DOI:10.1016/j.jfluchem.2009.05.003

Journal of Fluorine Chemistry 130 (2009) 667–670
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Reaction of the Ruppert–Prakash reagent with perfluorosulfonic acids
*
Vitalij V. Levin, Alexander D. Dilman , Pavel A. Belyakov, Marina I. Struchkova, Vladimir A. Tartakovsky
N. D. Zelinsky Institute of Organic Chemistry, 119991 Moscow, Leninsky Prospect 47, Russia
A R T I C L E I N F O
A B S T R A C T
Article history:
A new property of Me3SiCF3 to undergo C–F bond activation upon interaction with perfluorosulfonic
acids is described. The reaction is catalyzed by titanium tetrachloride and affords difluoromethyl
perfluorosulfonates in good yields.
Received 7 April 2009
Received in revised form 30 April 2009
Accepted 3 May 2009
ß 2009 Elsevier B.V. All rights reserved.
Available online 12 May 2009
Keywords:
Trifluoromethyltrimethylsilane
Perfluorosulfonic acids
C–F bond activation
1. Introduction
Analysis of the reaction mixture by 1H and 19F NMR spectro-
scopy indicated that none of silane 1 was formed. Instead,
The Ruppert–Prakash reagent (Me3SiCF3) [1] has found
extensive applications as a convenient equivalent of trifluoro-
methyl carbanion in reactions with carbonyl compounds, imines,
and heteroatom-centered electrophiles [2]. The key feature of
these processes is the activation of the silane with a Lewis base
followed by transfer of the CF3-group from pentacoordinate
complex to an electrophilic species [3].
At the same time, reactions of Me3SiCF3 under conditions of
acidic activation have remained virtually unexplored [4]. Indeed,
there is only one reported example of the process of this type, the
interaction of Me3SiCF3 with sulfur trioxide leading to Me3SiOTf
through the insertion of SO3 into C–Si bond [5].
difluoromethyltriflate 2a and Me3SiF were observed as the major
products together with a small amount of Me3SiOTf (path b). The
rest of material was unreacted Me3SiCF3 and triflic acid.
Despite the fact that the reaction occurs exothermically upon
mixing, it does not go to completion at room temperature even for
a longer period (Table 1, entry 2). To achieve complete conversion
of starting silane the reaction has to be performed with at least two
equivalents of triflic acid at 40–45 8C affording difluoromethyltri-
flate 2a in 80% yield after distillation (entry 3) [8].
It was foundthat the presence of catalytic amounts (1–5 mol%) of
Lewis acid provides significant rate acceleration (Table 1, entries
4–12). Boron tribromide gave quite clean reactions, while AlCl3 and
BF3ÁOEt2 had virtually no influence on the process. With antimony
pentafluoride the reaction was most violent though decreased yield
of difluoromethyltriflate was observed. Titanium tetrachloride
turned out to be most efficient Lewis acid leading to rapid and
clean reactions. Under optimal conditions the desired product was
obtainedwithin1 hwithonly1.2equivalentsoftriflicacid(entry12).
Other perfluorosulfonic acids were also involved in reaction
with the Ruppert–Prakash reagent affording corresponding
difluoromethylsulfonates 2b,c in reasonable yields (Scheme 2).
Disappointingly, non-fluorinated methanesulfonic acid proved to
be absolutely unreactive under similar conditions.
Of special note is the facile C–F bond activation in M3SiCF3
occurring with perfluorosulfonic acids, while for conventional
fluorocarbons the use of superacidic systems such as HF/SbF5 or
CF3CO2H/SbF5 has been reported [9]. Only recently it was shown
that activation of C–F bond in CF3-substituted aromatics can be
induced by large excess of triflic acid [10].
Herein we report a new property of the Ruppert–Prakash
reagent to undergo concomitant C–F and C–Si bond activation in
the presence of perfluorosulfonic acids.
2. Results and discussion
Within the context of our studies on the reactivity of silyl
triflates bearing perfluorinated group [6] we attempted the
preparation of silyl triflate 1 by reaction of Me3SiCF3 with triflic
acid with the hope to effect the protonation of Me3SiCF3 at the
methyl group [7] (Scheme 1, path a). The reaction was performed
by addition of Me3SiCF3 to 2.0 equivalents of triflic acid without
solvent at À20 8C followed by stirring at room temperature for 1 h.
* Corresponding author. Fax: +7 499 135 53 28.
E-mail address: adil25@mail.ru (A.D. Dilman).
0022-1139/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2009.05.003

668
V.V. Levin et al. / Journal of Fluorine Chemistry 130 (2009) 667–670
Scheme 1.
Table 1
Reaction of Me3SiCF3 with TfOH.
Entry
TfOH, equiv.
Additive (mol %)
Time, h
Temperature, 8C
Conversion, %a
Yield of 2a, %b
c
c
1
2
1.5
2
–
3
18
3
40–45
r.t.
71
96
–
3
2
–
40–45
r.t.
100
91
80
c
4
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
BBr3 (5)
BBr3 (5)
AlCl3 (5)
AlCl3 (5)
BF3ÁOEt2 (5)
SbF5 (5)
TiCl4 (5)
TiCl4 (1)
1
5
18
1
r.t.
100
61
83
c
6
r.t.
c
c
7
18
18
1
r.t.
88
8
r.t.
98
9
r.t.
100
100
100
72
83
83
10
11
1
r.t.
2
r.t.
12
1.2
TiCl4 (1)
1
r.t.
100
80
The last row is the most efficient procedure among all entries.
a
Determined by 19F NMR spectroscopy.
b
After distillation.
c
The isolation of product was problematic, since unreacted Me3SiCF3 cannot be removed by distillation.
Concerning the mechanism of this reaction, it can be proposed
that the final product is produced by protonation of difluorocar-
bene, which is generated either by stepwise or by concerted
elimination of HF and Me3SiOTf (Scheme 3).
To probe the stepwise mechanism, we evaluated other triflur-
omethylated compounds—PhCF3 and PhOCF3, which upon abstrac-
tion of fluoride anion would give phenyl and phenoxy stabilized
carbocations. However, these substances proved to be absolutely
unreactive in reaction with TfOH even in the presence of 5% of TiCl4.
Another argument against stepwise mechanism in the reaction of
Me3SiCF3 is that the generation of carbocationic intermediate would
be unfavorable since trimethylsilyl group does not exert significant
Indeed, it is known that C–F bond can be heterolitically
activated with Lewis acids [15–17]. In particular, various poly-
fluoroolefins interact with boron triflate B(OTf)3 to give products of
substitution of a fluorine atom by TfO-group [16]. When we treated
the Ruppert–Prakash reagent with boron triflate the only
observable product was Me3SiOTf, which was isolated with the
yield of 54% (Scheme 4). We believe that in this case the reaction
also proceeds in a concerted fashion with the expulsion of
difluorocarbene and generation of another boron Lewis acid
FB(OTf)2. The latter species may further react with the silane
providing, finally, boron trifluoride.
We also studied the behavior of pentafluoroethyl-substituted
silane Me3SiC2F5 in the presence of triflic acid (Scheme 5). The
reaction was very slow furnishing silyl triflate 3 as a result of the
protonation of the methyl group.
cation-stabilizing effect on
a-carbon atom [11].
These data suggest that the concerted pathway for the reaction
of the Ruppert–Prakash reagent, involving simultaneous C–F and
C–Si bond cleavage along with H–F and O–Si bond formation, is
more likely.
The role of Lewis acids deserves special comment. It is expected
that the Lewis acid interacts with triflic acid to give corresponding
metal triflates [12,13]. Such highly Lewis acidic substance may
accelerate the reaction of Me3SiCF3 with acid by one of the
following ways. Thus, it may either increase the Bronsted acidity of
the acid [14] or increase the rate of the equilibrium regeneration of
triflic acid (Scheme 3, bottom equation). Another opportunity is
that Lewis acid accelerates the generation of difluorocarbene from
Me3SiCF3 through the activation of C–F bond.
Scheme 2.
Scheme 3.

V.V. Levin et al. / Journal of Fluorine Chemistry 130 (2009) 667–670
669
Scheme 4.
The collected liquid contains compounds 2b,c, which were >98%
pure according to NMR spectroscopy. Subsequent vacuum
distillation afforded analytically pure substances.
3.2.1. Difluoromethyl trifluoromethanesulfonate (2a)
Scheme 5.
B.p. 48.5–49 8C. 1H NMR (300 MHz, CDCl3),
d
: 6.85 (t, 1H,
: 115.0 (t, J = 276.1,
CHF2), 118.3 (qt, J = 319.6, 1.9, CF3) 19F NMR (282 MHz, CDCl3),
J = 67.9, CHF2). 13C NMR (75 MHz, CDCl3),
d
We briefly examined the reactivity of difluoromethyl triflate
2a with typical nucleophiles. No reaction was observed with
pyridine and triphenylphosphine for 24 h at room temperature
(in CDCl3, NMR control). With more nucleophilic N-methylimi-
dazole the rapid decomposition of 2a was noted to give
several species, presumably, N-methylimidazolium fluoride
and CF3SO2F. The inability of trilate 2a to serve as alkylating
reagent is similar to that of trifluoromethyl triflate [18] and
1,1-difluoroethyl triflate [19]. At the same time, such
behavior stands in sharp contrast with that of difluoromethyl-
sulfonium salts, which are competent difluoromethylating
reagents [20].
d:
À82.9 (d, 2F, J = 67.9, CHF2), À75.2 (s, 3F, CF3). Material contains ca.
1.5% of Me3SiF, 1H NMR,
d
: 0.23 (d, J = 7.1); 19F NMR,
d
: À158.59
(deqaplet, J = 7.1).
3.2.2. Difluoromethyl 1,1,2,2,3,3,4,4,4-nonafluorobutane-1-sulfonate
(2b)
B.p. 75–80 8C (bath temperature)/65 Torr. 1H NMR (300 MHz,
CDCl3), d d:
: 6.85 (t, 1H, J = 67.9, CHF2). 13C NMR (75 MHz, CDCl3),
108.5 (tqtt, J = 271.5, 39.8, 32.6, 1.6, CF2CF3), 109.9 (tquint,
J = 271.5, 31.0, CF2CF2CF2), 114.5 (tt, J = 302.7, 36.5, CF2S), 115.1
(t, J = 276.2, CHF2), 117.3 (qtt, J = 288.4, 32.7, 1.5, CF3). 19F NMR
In summary, a new reaction of the Ruppert–Prakash reagent
proceeding upon interaction with perfluorosulfonic acids and
affording difluoromethyl perfluorosulfonates has been described.
The simultaneous C–F and C–Si bond activation is believed to be
the key feature responsible for the efficiency of the reaction.
(282 MHz, CDCl3),
d
: À129.7 (m, 2F, CF2), À124.6 (m, 2F, CF2),
À113.4 (t, 2F, J = 13.8, CF2), À85.9 (d, 2F, J = 67.9, CHF2), À84.8 (t,
3F, J = 9.5, CF3). Anal. Calcd for C5HF11O3S (350.11): C, 17.15; H,
0.29. Found: C, 17.29; H, 0.21.
3.2.3. Difluoromethyl 1,1,2,2-tetrafluoro-2-
3. Experimental
(pentafluoroethoxy)ethanesulfonate (2c)
B.p. 70–75 8C (bath temperature)/65 Torr. 1H NMR (300 MHz,
3.1. General experimental procedures
CDCl3), d d:
: 6.84 (t, 1H, J = 67.9, CHF2). 13C NMR (75 MHz, CDCl3),
112.4 (tt, J = 301.5, 38.0), 114.1 (tq, J = 288.9, 44.4, CF2CF3), 115.0 (t,
J = 276.2), CHF2), 115.3 (tt, J = 290.8, 30.5), 115.7 (qt, J = 285.3, 40.5,
All reactions were performed under an argon atmosphere.
Commercially available silanes (P&M) and perfluorsulfonic acids
(P&M, Aldrich) were purified by distillation. NMR spectra were
recorded on a Bruker AM-300 or AC-200 instruments.
CF2CF3). 19F NMR (282 MHz, CDCl3),
d
: À115.0 (s, 2F, CF2), À89.3 (t,
2F, J = 12.7, CF2), À87.8 (s, 3F, CF3), À83.1 (d, 2F, J = 67.9, CHF2),
À82.7 (t, 2F, J = 12.7, CF2). Anal. Calcd for C5HF11O4S (366.11): C,
16.40; H, 0.28. Found: C, 16.59; H, 0.24.
3.2. Reaction of Me3SiCF3 with perfluorosulfonic acids—general
procedure
3.3. Preparation of silyl triflate 3
Titanium tetrachloride (11
m
L, 0.1 mmol) was added dropwise
Titanium tetrachloride (55 mL, 0.5 mmol) was added dropwise
to perfluorosulfonic acid (12 mmol) under vigorous stirring at
room temperature, and the mixture was kept for 5 min. The
homogeneous yellow solution was evacuated at 10–15 Torr until
gas evolution ceased (ca. 5–10 min). The mixture was cooled to
À20 8C, Me3SiCF3 (1.48 mL, 10 mmol) was added, and the mixture
was kept for 2 min. The cooling bath was replaced first by ice/water
bath, which was kept for 2 min, and then by water bath with 20 8C,
and the mixture was stirred at room temperature (1 h for 2a or 24 h
for 2b,c).
For the isolation of 2a, volatile materials were distilled off under
vacuum (100 Torr) in a cold trap (liq. nitrogen). The collected liquid
contains 2a and Me3SiF in a molar ratio 1:1, which cannot be
separated by small-scale distillation. The sample of 2a containing
traces of Me3SiF was obtained by fractional distillation through a
15-cm column with glass filling.
to TfOH (1.33 mL, 15 mmol) under vigorous stirring at room
temperature, and the mixture was kept for 5 min. The homo-
geneous yellow solution was evacuated at 10–15 Torr until gas
evolution ceased (ca. 5–10 min). Trimethylpentafluoroethylsilane
(1.69 mL, 10 mmol) was added, the reaction flask was immersed in
the bath preheated to 60 8C, and the mixture was stirred at this
temperature for 3 h. The mixture was cooled to À20 8C, diluted
with diethyl ether (1.56 mL, 15 mmol), and subjected to vacuum
distillation (the addition of ether prior to distillation is necessary
for the efficient separation of the product from excess triflic acid).
Silyl triflate 3 was obtained in 31% yield, 0.988 g.
3.3.1. Dimethyl(pentafluoroethyl)silyl trifluoromethanesulfonate (3)
B.p. 83–85 8C/65 Torr. 1H NMR (300 MHz, CDCl3),
d
: 0.79 (s, 6H).
13C NMR (75 MHz, CDCl3),
d
: À4.1 (m, (CH3)2), 115.5 (tq, J = 270.1,
For the isolation of 2b,c, volatile materials (mostly Me3SiF) were
distilled off under vacuum (20 Torr). The residue was distilled off
under vacuum (ca. 1 Torr, oil pump) in a cold trap (liq. nitrogen).
42.4, CF2CF3), 118.4 (q, J = 317.5, O2SCF3), 119.8 (qt, J = 284.2, 29.6,
CF2CF3). 19F NMR (282 MHz, CDCl3),
3F, CF3), À80.1 (s, 3F, CF3).
d
: À135.8 (s, 2F, CF2), À85.4 (s,

670
V.V. Levin et al. / Journal of Fluorine Chemistry 130 (2009) 667–670
[5] H. Holfter, R.L. Kirchmeier, J.M. Shreeve, Inorg. Chem. 33 (1994) 6369–6372.
3.4. Reaction of Me3SiCF3 with B(OTf)3
[6] V.V. Levin, A.D. Dilman, P.A. Belyakov, A.A. Korlyukov, M.I. Struchkova, V.A.
Tartakovsky, Eur. J. Org. Chem. (2004) 5141–5148.
Triflic acid (0.89 mL, 10 mmol) was added dropwise to BBr3
(834 mg, 3.33 mmol) at 0 8C. The mixture was kept for 10 min at
room temperature, and dried in a vacuum of an oil pump. The
resulting white solid was cooled to À25 8C and treated with
Me3SiCF3 (1.72 mL, 11.66 mmol). The mixture was allowed to
warm to room temperature and stirred for additional 1 h. Volatile
materials were distilled off under vacuum (10 Torr) in a cold trap
(À20 8C) to give 1.12 g of Me3SiOTf (54% yield).
[7] It is known that Me4Si readily reacts with TfOH affording Me3SiOTf with evolution
of methane, see: M. Demuth, G. Mikhail, Synthesis, (1982) 827.
[8] Compound 2a is produced along with Me3SiF, which can be separated only by
using efficient distillation column.
[9] Y.V. Zonov, V.M. Karpov, V.E. Platonov, J. Fluorine Chem. 128 (2007) 1058–1064.
[10] After submitting this manuscript we became aware of the paper describing the
interaction of PhCF3 with benzene solvent promoted by 10 equivalents of TfOH
leading to benzophenone. However, no studies on the reaction of PhCF3 with
TfOH without benzene were reported, see: F. Wang, J. Hu, Chin. J. Chem. 27
(2009) 93–98.
[11] J.B. Lambert, Tetrahedron 46 (1990) 2677–2689.
[12] For example, formation of M(OTf)3 from MX3 (M = B, Al) and TfOH is well
documented. Furthermore, it was demonstrated that boron triflate is stronger
Lewis acid than aluminum triflate, see: G.A. Olah, O. Farooq, S.M.F. Farnia, J.A.
Olah, J. Am. Chem. Soc. 110 (1988) 2560–2565.
[13] (a) For the interaction of TiCl4 with TfOH to give titanium triflates TiCln(OTf)4-n
see: R.E. Noftle, G.H. Cady, Inorg. Chem. 5 (1966) 2182–2184;
(b) M. Schmeisser, P. Sartori, B. Lippsmeier, Chem. Ber. 103 (1970) 868–879.
[14] (a) A. Engelbrecht, E. Tschager, Z. Anorg, Allg. Chem. 433 (1977) 19–25;
(b) G.A. Olah, K. Laali, O. Farooq, J. Org. Chem. 49 (1984) 4591–4594.
[15] (a) V.A. Petrov, A. Marchione, W. Marshall, J. Fluorine Chem. 129 (2008) 1011–
1017;
Acknowledgments
This work was supported by the Ministry of Education and
Science, Russian Academy of Sciences, Russian Science Support
Foundation, and the Russian Foundation for Basic Research (project
08-03-00428).
References
(b) C.G. Krespan, V.A. Petrov, Chem. Rev. 96 (1996) 3269–3302.
[16] (a) V.A. Petrov, J. Org. Chem. 63 (1998) 2988–2992;
(b) V.A. Petrov, J. Fluorine Chem. 73 (1995) 17–19.
[17] H. Amii, K. Uneyama, Chem. Rev. 109 (2009) 2119–2183.
[18] (a) S.L. Taylor, J.C. Martin, J. Org. Chem. 52 (1987) 4147–4156;
(b) Y. Kobayashi, T. Yoshida, I. Kumadaki, Tetrahedron Lett. 20 (1979) 3865–
3866;
[1] (a) I. Ruppert, K. Schlich, W. Volbach, Tetrahedron Lett. 25 (1984) 2195–2198;
(b) P. Ramaiah, R. Krishnamurti, G.K.S. Prakash, Org. Syn. 72 (1995) 232–240.
[2] (a) G.K.S. Prakash, A.K. Yudin, Chem. Rev. 97 (1997) 757–786;
(b) G.K.S. Prakash, M. Mandal, J. Fluorine Chem. 112 (2001) 123–131;
(c) R.P. Singh, J.M. Shreeve, Tetrahedron 56 (2000) 7613–7632;
(d) N. Shibata, S. Mizuta, H. Kawai, Tetrahedron: Asymm. 19 (2008) 2633–2644.
[3] (a) N. Maggiarosa, W. Tyrra, D. Naumann, N.V. Kirij, Y.L. Yagupolskii, Angew.
Chem. Int. Ed. 38 (1999) 2252–2253;
(c) A.A. Kolomeitsev, M. Vorobyev, H. Gillandt, Tetrahedron Lett. 49 (2008) 449–
454.
(b) A. Kolomeitsev, G. Bissky, E. Lork, V. Movchun, E. Rusanov, P. Kirsch, G.-V.
Ro¨schenthaler, Chem. Commun. (1999) 1017–1018.
[19] L. Marival-Hodebar, M. Tordeux, C. Wakselman, J. Chem. Res. (S) (1998) 192–
193.
[4] (a) Thoughsome reactionsofnucleophilictrifluromethylationwithMe3SiCF3 inthe
presence of Bronsted or Lewis acids were reported, they likely proceed through
initial nucleophilic activation of silicon, see: V.V. Levin, A.D. Dilman, P.A. Belyakov,
M.I. Struchkova, V.A. Tartakovsky, Eur. J. Org. Chem. (2008) 5226–5230;
(b) S. Mizuta, N. Shibata, S. Ogawa, H. Fujimoto, S. Nakamura, T. Toru, Chem.
Commun. (2006) 2575–2577.
[20] (a) It also was noted, that difluoromethylsulfonium salts may react with triflate
anion to give difluoromethyltriflate, which was observed by 19F NMR spectro-
scopy, see: G.K.S. Prakash, C. Weber, S. Chacko, G.A. Olah, Org. Lett. 9 (2007) 1863–
1866;
(b) G.K.S. Prakash, C. Weber, S. Chacko, G.A. Olah, J. Comb. Chem. 9 (2007) 920–
923.