Meerwein's reagent mediated, significantly enhanced nucleophilic fluorination on alkoxysilanes

Article abstract of DOI:10.1055/s-0031-1290757

We developed a new facile method to fluorosilanes from alkoxysilanes using Meerwein's reagent. Our protocol afforded fluorosilanes in excellent yields in various organic solvents including acetonitrile under mild reaction conditions at room temperature. We also proposed a reaction mechanism with the probable silyloxonium intermediates. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290757

1064▌
LETTER
Letter
Meerwein’s Reagent Mediated, Significantly Enhanced Nucleophilic
Fluorination on Alkoxysilanes
Nucleophilic Fluorination on Alkoxysilanes
Yogesh R. Jorapur,a,b Toyoshi Shimada*a,b
a
Department of Chemical Engineering, Nara National College of Technology, 22 Yata-cho Yamatokoriyama, Nara, 639-1080, Japan
b
Core Research for Evolutional Science and Technology (CREST), JST Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Fax +81(743)556154; E-mail: shimada@chem.nara-k.ac.jp
Received: 24.01.2012; Accepted after revision: 14.02.2012
egory various methods have been reported, such as the use
of SbF3,20 zinc fluoride,21 NH4F,22 CuF2,23 sodium fluoro-
silicate,24 sodium hexafluorophosphate, sodium hexafluo-
roantimonate, sodium tetrafluoroborate, trimethyltin
Abstract: We developed a new facile method to fluorosilanes from
alkoxysilanes using Meerwein’s reagent. Our protocol afforded fluo-
rosilanes in excellent yields in various organic solvents including
acetonitrile under mild reaction conditions at room temperature. We
also proposed a reaction mechanism with the probable silyloxonium fluoride, and perfluorinated complex anions.25 Most of the
intermediates.
above reported methods required either metal catalyst or
reagents that are often expensive, toxic, pyrophoric, and
difficult to handle. Also, few of the above-mentioned
methods are performed at very high temperature.19,25a
Key words: Meerwein’s reagent, nucleophilic fluorination, silyl-
oxonium, fluorine, silicon
We recently discovered Meerwein’s reagent26 (MR)-cata-
lyzed sol–gel polycondensations to silica from tetraeth-
oxysilane (TEOS) in acetonitrile (MeCN) as the reaction
medium.27 Interestingly, during our preliminary investi-
gation we found that the condensation reaction with
monoethoxy silane 1a28 instead of TEOS in the presence
of trimethyoxonium tetrafluoroborate (Me3OBF4, 0.3
equiv) afforded an unexpected fluorosilane 2a in 28%
yield (Scheme 1). In continuation of our previous report
here we present our detailed studies on the enhanced nu-
cleophilic fluorination on alkoxysilanes (SN2–Si) using
MR and proposed a plausible reaction mechanism of fluo-
rination. To our knowledge this is the first report of an
MR-mediated nucleophilic fluorination proceeding via
silyloxonium intermediate.27,29
Halosilanes, particularly fluorosilanes are useful starting
materials in synthetic organosilicon chemistry.1 They are
used for the synthesis of highly coordinated silicon com-
pounds and highly hindered organosilanes.2 They are also
used in the semiconductor industry and in optical fiber
production.3 Halosilanes are found to be a good starting
material for the preparation of polysiloxanes.4 Chudesova
also recently used halosilanes to cleave siloxane bond in
organosilanes.5 In addition, physical properties including
heats of formation and bond dissociation energies of the
halosilanes are also well studied.6
Eventually, their significance led to several reports on the
synthesis of fluorosilanes. The three most common routes
to prepare fluorosilanes include (i) direct conversion of
hydrosilanes to fluorosilanes, (ii) conversion of alkoxysi-
lanes to fluorosilanes, and (iii) via halogen exchange of
chlorosilanes. In the first category several methods have
been reported in the literatures. Tang et al. reported the
monofluorination of hydrosilanes with highly toxic anti-
mony trifluoride (SbF3).7,8 Olah and co-workers used
highly reactive nitrosonium tetrafluoroborate or nitroni-
um tetrafluoroborate to generate fluorosilanes.9 Other
methods include the use of triphenylcarbenium tetrafluo-
roborate,10 phosphorous pentafluoride,11 silver fluoride,12
copper(II) fluoride (CuF2)–CCl4,13 and copper(II) chlo-
ride (copper iodide)/potassium iodide.14 The methods to
generate fluorosilanes from alkoxysilanes include the use
of ammonium fluoride (NH4F) in H2SO4,15 hydrofluoric
acid,16 and boron trifluoride diethyl etherate.17 Carbonyl
fluorides have also been used as fluorination reagents.18
Recently, alkali-metal salts of perfluorinated complex an-
ions have been used to effect nucleophilic fluorination of
alkyl-, arylalkyl-, and arylalkoxysilanes.19 In the last cat-
R = allyl
R
R
R
R
Me3OBF4
Si
Si
OEt
F
Br
O
Br
1a
2a
R3Si OR'
n-Si(OEt)4
Meerwein's
Reagent
R3Si–F
Si
Si
n
R = aryl, alkyl
ref. 27
R' = OMe, OEt, Oi-Pr, Ot-Bu
Scheme 1 MR-mediated nucleophilic fluorination
We began our studies with the control reaction; in the ab-
sence of mediator, the reaction did not proceed, and the
starting material was recovered (Table 1, entry 1). To find
the optimal conditions, we next examined the reaction
with varying amounts of Me3OBF4 (Table 1, entries 2–6).
The reaction was found to be slow and incomplete when
≤0.5 equivalents of Me3OBF4 were used (Table 1, entries
2-4). In the presence of one and two equivalents of the me-
diator, nucleophilic fluorination to the silicon center was
significantly enhanced to afford 2a within 15 and 5 min-
SYNLETT 204012, 237, 1064–1068
Advanced online publication: 05.04.2012
0
9
3
6
-
5
2
1
4
1
4
3
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2
0
9
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DOI: 10.1055/s-0031-1290757; Art ID: ST-2012-U0072-L
© Georg Thieme Verlag Stuttgart · New York

LETTER
Nucleophilic Fluorination on Alkoxysilanes
1065
utes in excellent yields (89% and 91%, Table 1, entries 5 the phenylethoxysilane did not show any adverse effect on
and 6, respectively).30 We also examined the effect of the fluorination reaction and afforded excellent yields of
temperature on the rate of fluorination. At ambient tem- fluorosilanes in the presence of both MR (Table 3, entries
perature, the reaction proceeded completely after 60 min- 2–4).
utes to provide 90% of 2a (Table 1, entry 7). Conducting
the reaction at elevated temperature resulted in the forma-
tion of 2a in a short time (Table 1, entries 8 and 9). Fur-
Table 2 Nucleophilic Fluorination Using Trimethyloxonium Tetra-
fluoroborate in Various Solvent Systemsa
thermore, Et3OBF4 in the place of Me3OBF4 was also
Yield of 2a (%)b
Entry
Solvent
Time (min)
successfully used to give 2a in excellent yield (90%, Ta-
ble 1, entry 10). It has previously been reported that an al-
kali-metal salt such as KBF4 can be used in a halogen-
exchange nucleophilic fluorination.25 However, the reac-
tion of ethoxysilane 1a with KBF4 and KF did not proceed
(Table 1, entries 11 and 12).
1
1,4-dioxane
toluene
15
60
60
15
10
90
85
85
89
89
2
3
t-BuOH
MeCN
4c
5d
Table 1 Nucleophilic Fluorination at Various Reaction Conditionsa
MeCN
Entry Mediator
(equiv)
Time
Yield of 1a Yield of 2a
a All reactions were carried out on a 1.0 mmol reaction scale of
(4-bromophenyl)ethoxydi(prop-2-enyl)silane (1a) using Me3OBF4
(1.0 mmol) in various organic solvents (5.0 mL) at 50 °C.
b Isolated yield.
(%)b
(%)b
1
2
3
4
5
6
–
12 h
92
90
58
35
–
–
c Reaction in H2O (0.5 mL) as co-solvent.
Me3OBF4 (0.1)
Me3OBF4 (0.25)
Me3OBF4 (0.5)
Me3OBF4 (1.0)
Me3OBF4 (2.0)
Me3OBF4 (1.0)
Me3OBF4 (1.0)
Me3OBF4 (1.0)
Et3OBF4 (1.0)
KBF4 (1.0)
12 h
trace
25
55
89
91
90
87
89
90
–
d Reaction in the presence of PEG-1000 (500 mg).
12 h
Similarly, we prepared various other fluorosilanes 2f–i in
high yields (Table 3, entries 5–8). Furthermore, we per-
formed fluorination reactions on substrates with various
alkoxy groups and obtained good to excellent yields of
fluorosilanes 5–8 (Table 3, entries 9–12). Astonishingly,
even sterically bulky isopropoxy and tert-butoxy groups
also afforded good yields (Table 3, entry 12).
12 h
15 min
5 min
60 min
30 min
5 min
15 min
12 h
–
7c
8d
9e
–
–
In order to gain insight into the reaction mechanism, we
performed the reactions A and B in the presence of
Me3OBF4 and Et3OBF4 in MeCN-d3 at 25 °C using NMR
tubes equipped with J. Young valves (Scheme 2).31 The
formation of MeOMe and EtOMe together with fluorosi-
lane 2a in reaction A, and EtOEt together with 2a in reac-
tion B was observed, respectively (see Supporting
Information). The elimination of EtOMe in reaction A
strongly supports the existence of the silyloxonium inter-
mediate 3. Based on these findings, we propose a mecha-
nism for the MR-mediated fluorination on 1a with two
possible pathways as shown in Scheme 3. In path I, 1a re-
acts with Me3OBF4 to form intermediate 3 which gets at-
tacked by the ‘naked’ fluoride ion to provide fluorosilane
2a together with MeOMe and EtOMe, whereas in path II
ethoxysilane 1a reacts with Et3OBF4 to form intermediate
4 which gets attacked by the ‘naked’ fluoride ion to give
2a with the elimination of EtOEt. In both pathways, there
is a possibility of the involvement of the hypervalent sili-
con intermediate because of the high affinity of silicon to-
ward fluoride ion.32
–
10
–
11
12
92
91
KF (1.0)
12 h
–
a All reactions were carried out on a 1.0 mmol reaction scale of (4-bro-
mophenyl)ethoxydi(prop-2-enyl)silane (1a) in MeCN (5.0 mL) at
80 °C.
b Isolated yield.
c The reaction was carried out at ambient temperature (25 °C).
d The reaction was carried out at 50 °C.
e The reaction was carried out at 100 °C.
Further, we also examined the MR-mediated fluorination
in various solvents (Table 2). The use of nonpolar aprotic
and polar protic solvents also afforded 2a in high yields
(Table 2, entries 1–3). The reaction performed in water as
co-solvent provided 89% of 2a (Table 2, entry 4). Entry 5
(Table 2) in the presence of 500 mg of PEG-1000 in
MeCN furnished 89% of 2a.
In conclusion, we have developed a new facile method to
generate fluorosilanes from commercially available
and/or easily synthesizable alkoxysilanes using MR. Fur-
thermore, this novel nucleophilic fluorination via silyl-
oxonium intermediates is explained with the additional
experiments with evidences to provide a plausible reac-
To determine the scope and limitations of the MR-medi-
ated nucleophilic fluorination, we performed the reactions
with various alkoxysilanes 1b–o (Table 3). In entry 1, eth-
oxysilane 1b reacted with Me3OBF4 and Et3OBF4 to give
fluorosilane 2b in 92% and 90% yields, respectively. The
para substituents such as iodo, methyl, and methoxy on
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1064–1068

1066
Y. R Jorapur, T. Shimada
LETTER
tion mechanism. In addition, the reported procedure did smoothly generate high purity fluorosilanes even at ambi-
not use any toxic metal catalyst or hazardous chemicals ent temperature. This protocol has the additional merit
which are detrimental to the environment. Our process that it can be performed in various organic solvents.
does not require harsh reaction conditions and can
Table 3 Nucleophilic Fluorination on Various Alkoxy Silanes in the Presence of Meerwein’s Reagenta
Fluorosilane Yield (%)b
Entry
Alkoxysilane 1
Fluorosilane
Method A
Method B
R
R
R
R
Si
Si
F
OEt
1
92
90
2b
1b
R
R
R
R
Si
Si
OEt
F
2
3
4
85
91
90
87
90
92
I
I
1c
2c
R
R
R
R
Si
Si
F
OEt
Me
Me
1d
2d
R
R
R
R
Si
Si
OEt
F
MeO
MeO
1e
2e
R
R
R
R
Si
R
EtO Si
R
Si OEt
R
F
Si
R
F
5
6
88
89
89
89
1f
2f
R
R
R
I
Si OEt
R
I
Si
F
1g
2g
F
OEt
Si
R
R
R
R
R
Si
R
Si
F
Si
R
7
86
85
R
OEt
1h
1i
2h
2i
SiR2F
SiR2OEt
8
9
90
91
R
R
Ph
Ph
Ph
Ph
Si
Si
OEt/OMe
F
90/89
90/88
1j/1k
5
Me
Me
F
Ph
Si
Ph
Ph
Si
Ph
OEt/OMe
10
11
85/83
82/81
84/80
82/78
6
1l/1m
R3Si
R3Si
F
OEt/OMe
1n/1o
7
Synlett 2012, 23, 1064–1068
© Georg Thieme Verlag Stuttgart · New York

LETTER
Nucleophilic Fluorination on Alkoxysilanes
1067
Table 3 Nucleophilic Fluorination on Various Alkoxy Silanes in the Presence of Meerwein’s Reagenta
Fluorosilane Yield (%)b
Entry
12
Alkoxysilane 1
Fluorosilane
Method A
Method B
Ph3Si
Ph3Si
F
OEt/OMe/Ot-Bu/Oi-Pr
83/81/72/78
83/82/70/74
1p/1q/1r/1s
8
a All reactions were carried out on a 1.0 mmol reaction scale of alkoxysilane 1 using Me3OBF4 (1.0 mmol, method A) and Et3OBF4 (1.0 mmol,
method B) in MeCN (5.0 mL) at 50 °C; R = allyl.
b Isolated yield.
A
(7) Hong, C. M.; Witt, S. D.; Tang, Y. N. J. Fluorine Chem.
1983, 23, 359.
(8) Damrauer, R.; Simon, R. A.; Kanner, B. Organometallics
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B
Me3OBF4
Et3OBF4
2a
EtOEt
1a
2a
+
MeOMe
EtOMe
MeCN-d3, 25 °C
MeCN-d3, 25 °C
(9) Prakash, G. K. S.; Wang, Q.; Li, X.; Olah, G. A. New J.
Chem. 1990, 14, 791.
Scheme 2 Additional experiments
(10) Bulkowski, J. E.; Stacy, R.; Van Dyke, C. H. J. Organomet.
Chem. 1976, 87, 137.
R
R
(11) Finch, M. A.; Marcus, L. H.; Smirnoff, C.; Van Dyke, C. H.;
Viswanathan, N. Synth. Inorg. Met.-Org. Chem. 1971, 103.
(12) Anderson, H. H. J. Am. Chem. Soc. 1958, 80, 5083.
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Organometallics 1996, 15, 2478. (b) Kunai, A.; Ohshita, J.
J. Organomet. Chem. 2003, 686, 3.
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B. O.; Sommer, L. H.; Goldberg, G. M.; Kerr, G. T.;
DiGregio, P. A.; Whitmore, F. C. J. Am. Chem. Soc. 1948,
70, 433.
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Chem. Soc. 1951, 73, 5127. (b) Eaborn, C. J. Chem. Soc.
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1950, 72, 2847. (e) Spialter, L.; Towers, R. S.; Kant, M. M.
Tetrahedron Lett. 1960, 1, 11.
Et3O+BF4– + MeCN
Me3O+BF4– + MeCN
MeOMe
Si
OEt
Br
1a
EtOEt
path II
path I
MeCN+Me
–
BF4
R
R
R
R
MeCN+Et
Si
Et
Si
Et
–
BF4
O
O
Me
Et
Br
Br
4
3
+
Me2O⋅BF3
F–
Et2O⋅BF3
R
R
F–
+
Si
F
EtOMe
EtOEt
Br
R = allyl
2a
Scheme 3 Plausible mechanism
(17) (a) Knoth, W. H.; Lindsay, R. V. J. Org. Chem. 1958, 23,
1392. (b) Horner, L.; Mathias, J. J. Organomet. Chem. 1985,
282, 155.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.onmorinftIangoimnuSpinorttfoIangiStupor
(18) Müller, R.; Mross, D. Z. Chem. 1971, 11, 382.
(19) (a) Farooq, O. J. Chem. Soc., Perkin Trans. 1 1998, 661.
(b) Farooq, O. J. Fluorine Chem. 1997, 86, 189.
(20) Booth, H. S.; Suttle, J. F. J. Am. Chem. Soc. 1946, 68, 2658.
(21) Hengge, E.; Schrank, F. J. Organomet. Chem. 1986, 299, 1.
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(23) Tamao, K.; Yoshida, J.; Yamamoto, H.; Kakui, T.;
Matsumoto, H.; Takahashi, M.; Kurita, A.; Murata, M.;
Kumada, M. Organometallics 1982, 1, 355.
(24) Lickiss, P. D.; Lucas, R. J. Organomet. Chem. 1996, 510,
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(25) (a) Farooq, O.; Tiers, G. V. D. J. Org. Chem. 1994, 59, 2122.
(b) Roesky, H. W.; Herzog, A.; Keller, K. Z. Naturforsch.,
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(27) Jorapur, Y. R.; Mizoshita, N.; Maegawa, Y.; Nakagawa, H.;
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2012, 41, 280.
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References and Notes
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Silicon Compounds; Patai, S.; Rappoport, Z., Eds.; Wiley:
Chichester, 1989, Chap. 10, 655. (b) Larson, G. L. The
Chemistry of Organic Silicon Compounds; Patai, S.;
Rappoport, Z., Eds.; Wiley: Chichester, 1989, Chap. 11, 763.
(2) (a) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem.
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2099. (d) Damrauer, R.; Danahey, S. E. Organometallics
1986, 5, 1490. (e) Nakadaira, Y.; Ohara, K.; Sakurai, H.
J. Organomet. Chem. 1986, 309, 247.
(3) (a) Ahn, D.; Rolley, P. A. US 20050282959, 2005; Chem.
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A. JP 2011033933, 2011; Chem. Abstr. 2011, 154, 272044f.
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1986; Chem. Abstr. 1986, 105, 7050.
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1064–1068

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Y. R Jorapur, T. Shimada
LETTER
(30) Typical Procedure for the Synthesis of Fluorosilanes:
A dry and nitrogen-flushed 10 mL screw-capped vial was
charged with alkoxysilane (1, 1.0 mmol), MeCN (5.0 mL)
followed by the addition of Me3OBF4 (147.9 mg, 1.0 mmol)
or Et3OBF4 (189.9 mg, 1.0 mmol). The reaction mixture was
stirred at 50 °C for 30 min and quenched by dropwise
addition of H2O. It was then diluted with CH2Cl2; the organic
layer was washed with brine, dried over anhyd MgSO4, and
evaporated under reduced pressure. The crude mixture was
purified by chromatography on silica gel (5% EtOAc–
hexane as eluent) to give the corresponding fluorosilane as a
colorless liquid.
7.58–7.62 (m, 2 H). 13C NMR (68 MHz, CDCl3): δ = 21.1,
21.3, 115.9, 128.0, 130.6, 131.3, 133.5, 133.6. 19F NMR
(470 MHz, CDCl3): δ = –172.82. HRMS (FAB+): m/z [M –
H]+ calcd for C12H15FSi: 206.0927; found: 206.0921.
Fluoro[diphenyl(prop-2-enyl)]silane (5): colorless liquid.
1H NMR (400 MHz, CDCl3): δ = 2.22–2.25 (m, 2 H), 4.95–
5.04 (m, 2 H), 5.79–5.86 (m, 1 H), 7.38–7.47 (m, 6 H), 7.61–
7.63 (m, 4 H). 13C NMR (100 MHz, CDCl3): δ = 21.7, 21.8,
116.2, 128.0, 130.7, 131.2, 132.7, 132.8, 134.25, 134.27.
19F NMR (376 MHz, CDCl3): δ = –170.8. HRMS (EI+):
m/z [M]+ calcd for C15H15FSi: 242.0927; found: 242.0927.
Fluorotri(prop-2-enyl)silane (7): colorless liquid. 1H NMR
(400 MHz, CDCl3): δ = 1.76–1.79 (m, 6 H), 4.97–5.02 (m,
6 H), 5.75–5.82 (m, 3 H). 13C NMR (100 MHz, CDCl3): δ =
20.5, 20.7, 115.6, 131.4. 19F NMR (376 MHz, CDCl3): δ =
–170.2. CAS registry no. 429-89-0.
(4-Bromophenyl)di(prop-2-enyl)fluorosilane (2a):
colorless liquid. 1H NMR (270 MHz, CDCl3): δ = 1.96–2.01
(m, 4 H), 4.97–5.06 (m, 4 H), 5.70–5.86 (m, 2 H), 7.44 (d,
J = 8.1 Hz, 2 H), 7.56 (d, J = 8.1 Hz, 2 H). 13C NMR (68
MHz, CDCl3): δ = 21.0, 21.2, 116.3, 125.6, 130.8, 130.85,
131.2, 135.06, 135.09. 19F NMR (376 MHz, CDCl3): δ =
–171.93. HRMS (FAB+): m/z [M – H]+ calcd for
(31) Role of MeCN is currently unclear. For reports on the
formation of N-methyl- and N-ethylnitrilium ions with
MeCN in the presence of MR, see: Gordon, J.; Turrell, G.
J. Org. Chem. 1959, 24, 269.
C12H13BrFSi: 282.9954; found: 282.9935.
Fluoro[phenyldi(prop-2-enyl)]silane (2b): colorless
liquid. 1H NMR (270 MHz, CDCl3): δ = 1.98–2.04 (m, 4 H),
4.97–5.06 (m, 2 H), 5.74–5.89 (m, 2 H), 7.38–7.50 (m, 3 H),
(32) For hypervalent silicon species, see: Tilgner, I. C.; Fischer,
P.; Bohnen, F. M.; Rehage, H.; Maier, W. F. Microporous
Mater. 1995, 5, 77.
Synlett 2012, 23, 1064–1068
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