Utility of silicon tetrafluoride as a catalyst of reactions with xenon difluoride: Fluorinations of phenyl alkenes and benzaldehydes

Article abstract of DOI:10.1016/S0022-1139(99)00103-7

Application of silicon tetrafluoride to fluorinations with xenon difluoride as a catalyst is investigated. It was found that vic-difluorination of phenyl alkenes and transformation of benzaldehydes to difluoromethoxybenzenes using xenon difluoride are enhanced by silicon tetrafluoride.

Full text of DOI:10.1016/S0022-1139(99)00103-7

Journal of Fluorine Chemistry 98 (1999) 163±166
Utility of silicon tetra¯uoride as a catalyst of reactions with xenon
di¯uoride: ¯uorinations of phenyl alkenes and benzaldehydes
Masanori Tamura*, Toshiyuki Takagi, Heng-dao Quan, Akira Sekiya
National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Received 29 March 1999; received in revised form 10 May 1999; accepted 17 May 1999
Abstract
Application of silicon tetra¯uoride to ¯uorinations with xenon di¯uoride as a catalyst is investigated. It was found that vic-di¯uorination
of phenyl alkenes and transformation of benzaldehydes to di¯uoromethoxybenzenes using xenon di¯uoride are enhanced by silicon
tetra¯uoride. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Silicon tetra¯uoride; Xenon di¯uoride; Fluorination; Phenyl alkene; Benzaldehydes
1. Introduction
and transformation of benzaldehydes to di¯uoromethoxy-
benzenes.
Silicon tetra¯uoride (SiF4) is a colorless gas which is
easily obtained by the reaction of silicon dioxide with
hydro¯uoric acid and is also obtained as a by-product in
the manufacture of phosphate fertilizers from phosphate
rock [1]. Concerning applications in industry, SiF4 is used
in the ®eld of manufacturing ®lms of semiconductor devices,
optical ®ber manufacture, and so on [2]. Recently, we have
investigated the utility of SiF4 for synthesis of organic
¯uorine compounds and disclosed new ¯uorination reac-
tions using SiF4 [3±7]1. In the course of this study, we have
investigated the application of SiF4 as a catalyst of ¯uorina-
tions with xenon di¯uoride (XeF2).
XeF2 is a useful ¯uorinating agent for many types of
¯uorination reactions such as addition of ¯uorine to alkenes,
aromatic ring ¯uorination, and so on [8±10]. On these
¯uorinations with XeF2, a catalyst has a signi®cant in¯u-
ence. It was reported that some reactions do not proceed or
are very slow without catalyst [11,12], and that the products
and the mechanism of the reaction are in¯uenced by the
catalyst used [13,14]. Acidic compounds, such as hydrogen
¯uoride (HF), tri¯uoroacetic acid, boron tri¯uoride etherate,
etc. have been used as the catalyst [13]. In this paper, we
describe that SiF4 works as a catalyst for the ¯uorinations
with XeF2 and enhances vic-di¯uorination of phenyl alkenes
2. Results and discussion
2.1. vic-Difluorination of phenyl alkenes
vic-Di¯uorination of phenyl alkenes is one of the most
typical ¯uorination reactions using XeF2 [8±10]. The reac-
tion of trans-stilbene with XeF2 was initially attempted to
examine the effect of SiF4 on vic-di¯uorination of phenyl
alkene (Scheme 1). Without catalyst, vic-di¯uorination of
trans-stilbene proceeded, however the reaction was slow and
was not completed for 3 h (Table 1, entry 1). On the other
hand, the reaction proceeded faster and trans-stilbene was
consumed within 3 h in the presence of SiF4, which shows
that SiF4 is effective as a catalyst and enhances this reaction.
The yield depended on the amounts of XeF2 and SiF4 (entry
2±7), and the product 1,2-di¯uoro-1,2-diphenylethane was
obtained in 78% yield by the reaction with slightly excess
amount of XeF2 in the presence of a catalytic amount of SiF4
(entry 6).
The reaction of various phenyl alkenes was examined, and
the results are summarized and compared with the reaction
catalyzed by HF in Table 2. The corresponding vic-di¯uoro
compounds were obtained in good yields, and the diaster-
eoselectivities were almost same as the reaction catalyzed by
HF.
*Corresponding author. Tel.: +81-298-54-4570; fax: +81-298-54-4570;
e-mail: mtamura@home.nimc.go.jp
1Several investigations had been reported concerning the application of
SiF4 to fluorination reaction [6, 7].
The mechanism of vic-di¯uorination with XeF2 had been
discussed in the previous papers [18], and is considered as
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 1 0 3 - 7

164
M. Tamura et al. / Journal of Fluorine Chemistry 98 (1999) 163±166
Table 1
a
vic-Difluorination of trans-stilbene with XeF2 and SiF4
Entry
Amount of
XeF2 (mmol)
Amount of
SiF4 (mmol)
Yield
(%)b,c
1
2
3
4
5
6
7
1
0
17
73
70
63
56
78
71
1
0.1
0.4
1
1
1
1
2
1.2
2
0.1
0.1
a trans-Stilbene (1 mmol), XeF2, and SiF4 were agitated in dichloro-
methane (3 ml) at room temperature for 3 h.
b Yield was determined by 19F-NMR of the crude product (C6H5CF3 was
used as internal standard).
c Diastereoselectivity of the product, anti:syn, was 64:36±66:34, which
was determined by 19F-NMR.
Scheme 1. vic-Difluorination of phenyl alkenes.
shown in Scheme 2. XeF2 is polarized by a catalyst (A) and
reacts with phenyl alkene as an electron de®cient species to
form a ꢀ-complex-like intermediate, which is further trans-
formed into a b-¯uorocarbonium ion by direct ¯uorine
transfer or an electron transfer to a cation radical followed
by ¯uorine transfer. Attack of ¯uoride anion on the b-
¯uorocarbonium ion affords a vicinal di¯uoride. Here,
the catalyst A plays an important role, since the reaction
proved to be very slow without it [12]. A variety of
acids have been used as the catalyst, and HF is the most
common one [8±10]. anti-Diastereoselectivity had been
explained by nucleophilic attack of ¯uoride anion to the
preferable conformation of the b-¯uorocarbonium ion
intermediate [16].
In the case of the reaction using XeF2 and SiF4, the
mechanism would be same as mentioned above, which is
suggested from the result that the diastereoselectivities were
almost same as those catalyzed by HF, and SiF4 would work
as a catalyst. Although SiF4 is a weak Lewis acid, it has
strong af®nity to ¯uorine and forms ¯uorosilicate species
with ¯uoride anion [19], consequently it can polarize xenon
di¯uoride and enhance the reaction.
2.2. Transformation of benzaldehydes to
difluoromethoxybenzenes
Fluorination reaction of aromatic aldehydes with XeF2
and HF was reported in 1994 [20]. In this reaction, ¯uor-
ination was accompanied by skeletal rearrangement, and
di¯uoromethyl aryl ethers were obtained. This reaction
depends on the concentration of HF. It was reported that
conversion of the substrate was very low if the amount of HF
was small and that large excess amount of HF gave poly-
meric material. The best result was obtained when the
amount of HF was 4 or 5 mol equivalent of the substrate.
Here, the reaction of aromatic aldehydes with XeF2 in the
presence of SiF4 was attempted to investigate the effect of
SiF4 on this reaction (Scheme 3).
The reaction of p-nitrobenzaldehyde with XeF2 was
attempted in the presence of various amount of SiF4. As
shown in Table 3, the reaction was slow when the amount of
SiF4 was small, and the reaction became faster with increas-
ing the amount of SiF4. This result suggests that SiF4 works
as an activator of XeF2 like HF.
Table 2
vic-Difluorination of phenyl alkenes
a
Entry
Substrate
Catalyzed by SiF4
Catalyzed by HFb
Yield
Yieldc
anti:synd
anti:syn
1
2
3
4
5
6
Ph2C=CH2
72e
64f
71f
78
±
90
75
75
78
78
70
±
trans-PhCH=CHCH3
cis-PhCH=CHCH3
trans-PhCH=CHPh
cis-PhCH=CHPh
1,2-Dihydronaphthalene
61:39
64:36
66:34
55:45
25:75g
60:40
64:36
62:38
53:47
14:86g
87
80
a Phenyl alkene (1 mmol), XeF2 (1.2 mmol), and SiF4 (0.1 mmol) were agitated in dichloromethane (3 ml) at room temperature for 3 h.
b Data from Refs. [15±17].
c Isolated yield.
d Determined by 19F-NMR.
e Ph2CFCF2H (16%) and Ph2C=CFH (4%) were obtained.
f Yield determined by 19F-NMR of the crude product (C6H5CF3 was used as internal standard).
g cis:trans.

M. Tamura et al. / Journal of Fluorine Chemistry 98 (1999) 163±166
165
Scheme 2. Mechanism of vic-difluorination with XeF2.
¯uorination on benzene ring when electron-donating group
is substituted on benzene ring. Especially, in the case of
anisaldehyde, which has strong electron donating group,
tarry material was obtained. These results are corresponding
to the result using HF as an activator [20].
Concerning of this transformation of benzaldehyde to
di¯uoromethoxy benzene, the mechanism has been pro-
posed as shown in Scheme 4 [10]. The product would be
obtained via rearrangement of alkoxy¯uoroxenon (C). In
the case of the reaction using HF as a catalyst (A), two way
has been assumed to form the intermediate C. One is the
reaction of benzaldehyde with XeF2 activated by the catalyst
to give a cation B followed by attack of ¯uoride ion on B
(route a). The other is the direct addition of HF to benzal-
dehyde giving ¯uorohydrin followed by its reaction with
XeF2 (route b). When SiF4 is used as the catalyst, the former
can be postulated. Migration of phenyl group to oxygen
Scheme 3. Transformation of benzaldehydes to difluoromethoxyben-
zenes.
The reaction was examined using various benzaldehydes,
and the results are summarized in Table 4. The result was
in¯uenced by substituents on the benzene ring. The product
was obtained in relatively high yield in the case of substrates
substituted by electron-withdrawing group. However, the
yield was relatively low because of side-reactions such as
Table 3
The effect of SiF4 on the transformation of p-nitrobenzaldehyde to p-
(difluoromethoxy)nitrobenzenea
Table 4
Transformation of benzaldehydes to difluoromethoxybenzenesa
Entry
Amount of SiF4 in
the reactor (mol/l)
Yield of the
product (%)b
Recovery of the
substrate (%)b
Entry
Substrate
Yield (%)b
1
2
3
4
5
6
7
0.002
0.04
0.1
11
35
49
57
77
79
76
69
34
22
11
1
1
2
3
4
5
PhCHO
57
p-ClC6H4CHO
p-NO2C6H4CHO
p-CH3C6H4CHO
p-CH3OC6H4CHO
62
79
0.2
42
0.5
0.9
Trace
<1
2
aSubstrate (1 mmol), XeF2 (1.2 mmol), SiF4 (8±9 mmol), and dichlor-
omethane (3 ml) were placed in a reactor (10 ml) and agitated at room
temperature for 16 h.
bDetermined by 19F-NMR of the crude product (C6H5CF3 was used as
internal standard).
1.1
a p-Nitrobenzaldehyde (1 mmol), XeF2 (1.2 mmol), and SiF4 were agitated
in dichloromethane (3 ml) at room temperature for 16 h.
b Determined by 1H- and/or 19F-NMR of the crude product (C6H5CF3 was
used as internal standard).
Scheme 4. Mechanism of transformation of benzaldehydes to difluoromethoxybenzenes.

166
M. Tamura et al. / Journal of Fluorine Chemistry 98 (1999) 163±166
atom proceeds with loss of xenon and attack of ¯uoride
anion to afford di¯uoromethoxybenzene.
identi®ed by comparison of their IR, 1H-NMR, and
19F-NMR spectra with those of authentic samples.
3. Experimental details
4. Conclusion
General. All of the reagents are commercially available.
All of the organic reagents and solvents were puri®ed prior
to use. XeF2 and SiF4 were used without puri®cation. IR
spectra were measured with FT-IR 8900 (Japan Spectro-
scopic). 1H- and 19F-NMR were measured with JNM-
EX270 (JEOL, 270 MHz) using TMS and CFCl3 as internal
standard and CDCl3 as solvent.
We have demonstrated the effect of SiF4 on ¯uorination
reactions with XeF2. SiF4 enhances vic-di¯uorination of
phenyl alkenes and transformation of benzaldehydes to
di¯uoromethoxybenzenes with XeF2, which suggests the
utility of SiF4 as a catalyst of XeF2. Further investigation of
¯uorination using XeF2 and SiF4 is in progress.
A typical experimental procedure for vic-di¯uorination of
phenyl alkenes. Phenyl alkene (1.0 mmol) dissolved in
CH2Cl2 (3 ml) and XeF2 (1.2 mmol) were placed in a
stainless-steel reactor equipped with a stop valve (volume:
80 ml). SiF4 (0.1 mmol) was introduced into the reactor at
1968C from a vacuum line. The reactor was warmed up to
ambient temperature and shaken vigorously for 3 h. The
reaction mixture was poured into aqueous sat. NaHCO3. The
aqueous phase was extracted with CH2Cl2 and the combined
organic phase was dried with MgSO4. After evaporation of
the solvent, the crude product was puri®ed by preparative
thin-layer chromatography. The products were identi®ed by
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comparison of their IR, H-NMR, and 19F-NMR spectra
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product was determined by 19F-NMR of the crude product.
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