Novel fluoride ion mediated synthesis of unsymmetrical siloxanes under phase transfer catalysis conditions

Article abstract of DOI:10.1016/S0022-328X(03)00286-9

Unsymmetrical siloxanes have been prepared from silanols or hydrosilanes using phase transfer catalytic (PTC) systems Me₃ SiN₃-CsF-18-crown-6-toluene or Me₃SiN ₃-CsF-18-crown-6-H₂O-toluene, correspond

Full text of DOI:10.1016/S0022-328X(03)00286-9

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Novel fluoride ion mediated synthesis of unsymmetrical siloxanes
under phase transfer catalysis conditions
Ramona Abele, Edgars Abele *, Mendel Fleisher, Solveiga Grinberga,
Edmunds Lukevics *
Catalytic Synthesis Laboratory, Latvian Institute of Organic Synthesis, 21 Aizkraukles Street, Riga LV-1006, Latvia
Received 20 February 2003; received in revised form 9 April 2003; accepted 9 April 2003
Abstract
Unsymmetrical siloxanes have been prepared from silanols or hydrosilanes using phase transfer catalytic (PTC) systems
Me3SiN3ꢀCsFꢀ18-crown-6ꢀtoluene or Me3SiN3ꢀCsFꢀ18-crown-6ꢀH2Oꢀtoluene, correspondingly. The target unsymmetrical
siloxanes were prepared in yields up to 100%. Quantum chemical calculation of mechanism of unsymmetrical siloxanes formation
/
/
/
/
/
/
/
has been performed.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Fluoride ion; Unsymmetrical siloxanes; Phase transfer catalysis
1. Introduction
Fluoride ion as activator of silicon bonds is widely
used in silylation reactions [39ꢀ42]. Recently O-silyla-
tion of alcohols and phenols by Me3SiN3 in the presence
/
Siloxanes [1,2] are of interest as potential materials
of Bu4NBr [43] and carboxylic acids in the system
with specific properties [3ꢀ6] and biologically active
/
Me3SiN3ꢀ
of this work was to examine the phase transfer catalytic
(PTC) system Me3SiN3ꢀCsFꢀ18-crown-6ꢀtoluene con-
taining water (or without it) for the synthesis of
unsymmetrical siloxanes from silanols and hydrosilanes.
/
CsFꢀ
/
18-crown-6 [44] was described. The aim
compounds [7]. Unsymmetrical disiloxanes were usually
obtained in the reaction of corresponding silanol with
/
/
/
chlorosilaneꢀ
chlorosilaneꢀ
alkoxysilane [15ꢀ
HClꢀdioxane [25]. Hydrosilanes and silanols were
converted to unsymmetrical siloxanes in the presence
of zinc and cadmium halidesꢀDMF [26ꢀ29], rhodium
complex catalysts [30ꢀ32], PdꢀC [33] or dibutyltin
/
pyridine [8,9], Naꢀ
NaOHꢀEtOH [12], aminosilane [8,13,14],
23], acyloxysilane [24] or silanolꢀ
/
chlorosilane [9ꢀ11],
/
/
/
/
/
/
/
/
2. Results and discussion
/
/
diacetate, dilaurate or di(2-ethyl)hexanoate [34]. Un-
We have found that reaction of hydrosilanes
(R?RƒR§SiH) with azidotrimethylsilane in the system
symmetrical siloxanes were also obtained in the reaction
of chlorosilanes in presence of symmetric siloxaneꢀ
FeCl3 [35], symmetric siloxane under elevated pressure
at 300 8C [36] or chlorosilaneꢀPhHꢀH2O [37], and by
/
CsFꢀ
50 8C
/
18-crown-6ꢀ
/
H2Oꢀ
/
toluene readily proceeds at
unsymmetrical siloxanes
to afford
/
/
(R?RƒR§SiOSiMe3) as main products, along with a
small amount of corresponding symmetrical siloxanes
(R?RƒR§SiOSiR?RƒR§). Formation of disiloxane Me3-
oxidation of hydrosilanes with bis(trimethylsilyl)perox-
ide [38]. However, the yields and selectivity in the
synthesis of unsymmetrical siloxanes usually not high.
SiOSiMe3 was not detected (GCꢀMS data).
/
The influence of amount of CsF and water in the
reaction of dimethylphenylsilane (2) with azidotri-
methylsilane (Table 1) was studied. It was found that
one equivalent of CsF and water was optimal amount of
reagents for the selective synthesis of unsymmetrical
* Corresponding authors. Tel.: ꢁ
0338.
E-mail addresses: abele@osi.lv (E. Abele), sinta@osi.lv (E.
Lukevics).
/
371-7-55-1822; fax: ꢁ371-7-55-
/
0022-328X/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00286-9

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Table 1
Products of PTC reaction of dimethylphenylsilane (2) with azidotrimethylsilane at 50 8C during 20 h
CsF (equivalent)
Me3SiN3 (equivalent)
H2O (equivalent)
Relative content after reaction completion (%) (GC data)
PhMe2SiH
PhMe2SiOSiMe3
PhMe2SiOSiMe2Ph
1
3
3
3
3
3
3
2
4
1
0
90
71
82
40
61
0
10
26
10
1
0.5
0.2
1
1
3
1
8
0.2
2
59
0
1
39
0
0
1
100
0
0 a
1
1
74
41
26
22
1
1
a
37% of PhMe2SiN3.
pentamethylphenyldisiloxane (7) (selectivity 90%, pre-
parative yield 70%). In the absence of cesium fluoride
the formation of siloxane 7 was not observed.
Table 2
Synthesis of siloxanes 6ꢀ
system CsFꢀ18-crown-6ꢀH2Oꢀ
/
9 from hydrosilanes and Me3SiN3 in the
PhMe
/
/
/
The selectivity of pentamethylphenyldisiloxane (7)
synthesis was influenced also by amount of azidotri-
methylsilane. Thus, interaction of hydrosilane with three
equivalents of Me3SiN3 leads to desired disiloxane 7 in
90% selectivity. In the presence of two equivalents of
azide the selectivity of disiloxane 7 formation was lower.
Interestingly, that reaction of PhMe2SiH with large
excess of azidotrimethylsilane (four equivalents) in the
system (above mentioned) leads to formation of three
products: PhMe2SiOSiMe3 (41%), PhMe2SiOSiMe2Ph
Hydrosilane
Reaction time (h)
Product
Yield (%)
1
2
3
4
19
20
25
20
6 [8,35,37]
7 [38,47]
70
63
45
33
8 [9,10,15,25]
9 [48,49]
The reaction of triethylsilanol (5) with azidotrimethyl-
silane in the PTC system CsFꢀ18-crown-6ꢀtoluene
(molar ratio 5:Me3SiN3:CsF:18-crown-6ꢂ
1:1:0.05:0.05) at room temperature led to 1,1,1-triethyl-
3,3,3-trimethyldisiloxane (6) in quantitative yield during
30 min. Silylation of silanol 5 in the absence of fluoride
ion source also occurred at room temperature. However,
in this case only about 60% yield of desired product 6
was obtained during 12 h.
/
/
/
(22%) and PhMe2SiN3 (37%) (GCꢀMS data). We
/
supposed that the formation of azidodimethylphenylsi-
lane [45,46] occurs from corresponding hydrosilane
(PhMe2SiH) and Me3SiN3 by fluoride ion mediated
transfer of azido group.
The catalytic system Me3SiN3ꢀ
(molar ratio hydrosilane:Me3SiN3:CsF:18-crown-6ꢂ
1:3:0.1:0.1) as the most active was used in the synthesis
of unsymmetric siloxanes 6ꢀ9 from hydrosilanes 1 to 4
(Scheme 1). The siloxanes 6ꢀ9 were obtained in 45ꢀ70%
/
solid CsFꢀ/18-crown-6
/
Quantum chemical calculations of mechanism of
unsymmetrical siloxanes formation in the PTC system
˚
was carried out. Selected distances in A are shown in
/
/
/
figures.
We suppose that at the first step of the reaction with
water the following complex with crown ether and
yields (Table 2). This method was used also in the
synthesis of trisiloxane 9 (yield 33%) from diphenylsi-
lane (4) and six equivalents of Me3SiN3. Unfortunately,
the yields of corresponding siloxanes 7 and 8 were not
high due to sterical hindrance.
cesium fluoride is formed [HOꢃ ꢀ
ꢃ ꢀ
transported into organic phase.
/
2(18-crown-6)Csꢁ ꢀ
/
F
/
Hꢁ]. Thus, dissociated water molecule has been
The starting state of proton and hydroxyl anion
interaction with hydrosilane molecule is shown in Fig.
1. The distance between hydrosilane negative charged
˚
0.232 e) and proton is 2.53 A. The distance
hydrogen (ꢃ
/
between silicon atom of this molecule and hydroxyl
˚
anion is 5.50 A. At the beginning of the process the
proton approaches to negatively charged hydrogen.
Then cleavage of siliconꢀ/hydrogen bond proceeds with
formation of hydrogen molecule. The next step is
addition of hydroxyl anion to positively charged silicon
and formation of silanol molecule. Final state is shown
Scheme 1.
in the Fig. 2. The heat of reaction is ꢃ .
378.1 kcal molꢃ1
/

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R. Abele et al. / Journal of Organometallic Chemistry 686 (2003) 52ꢀ57
/
Fig. 1. The initial state of silanol formation.
Fig. 4. The silanol anion formation.
Fig. 5. The initial state of azidotrimethylsilane attack to silanol anion.
Fig. 2. The formation of hydrogen and silanol molecules.
The calculations show that the silanol formation reac-
tion proceeds spontaneously without an activation
barrier.
The Figs. 3 and 4 show the following process: the
interaction between silanol and fluoride ion. At first the
distance between fluoride anion and positively charged
˚
0.211 e) in hydroxyl group is 3.85 A
hydrogen atom (ꢁ
/
Fig. 6. Azidotrimethylsilane and silanol anion complex formation.
(Fig. 3). Formation of HF molecule and silanol anion
occurred (Fig. 4) as the result of this interaction. The
beginning (Fig. 5) the distance between silicon atom in
˚
azidotrimethylsilane and oxygen atom is 5.90 A. Then
negative charge is located on the oxygen atom (ꢃ
e) in silanol anion. The heat of reaction is equal ꢃ
kcal molꢃ1
/
0.937
the formation of complex of azidotrimethylsilane with
silanol anion occurred (Fig. 6). Stabilization energy of
/
81.5
.
this complex is ꢃ
35.7 kcal molꢃ1. Bond between silicon
/
The next reaction step is interaction between silanol
anion and azidotrimethylsilane (Figs. 5 and 6). At the
in azidotrimethylsilane and bridge type oxygen atom is
week because its order is only 0.473. Then negatively
charged complex reacted with proton (Fig. 7) from 2(18-
crown-6)Csꢁ ꢀ
/F
ꢃ ꢀ
/
Hꢁ species. During the reaction
addition of proton to nitrogen, which is bonded with
silicon atom, proceeds. The result of this process is
cleavage of Siꢀ/N bond, and formation of HN3 and
unsymmetrical disiloxane (Fig. 8). The heat of reaction
290.3 kcal molꢃ1
is ꢃ
The quantumꢀ
/
.
chemical calculations of the unsymme-
/
trical siloxanes synthesis allows to propose the reaction
mechanism described in Fig. 9.
Fig. 3. The fluoride ion attack to silanol hydroxyl group.

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55
control*
/
180ꢀ250 8C) for 20 h. The products were
/
identified by MS spectra. The results see in Table 1.
3.3. General procedure of hydrosilane reaction with
azidotrimethylsilane under PTC conditions
Hydrosilane 1ꢀ3 (1 mmol) was added to the mixture
/
of freshly calcinated CsF (0.152 g, 1 mmol), azidotri-
methylsilane (0.397 ml, 3 mmol or 0.796 ml, 6 mmol for
synthesis of trisiloxane 9), water (0.018 ml, 1 mmol) and
18-crown-6 (0.026 g, 0.1 mmol) in 1.5 ml of dry toluene
under argon atmosphere. Reaction was carried out at
Fig. 7. The interaction of negatively charged complex and proton.
70 8C temperature (GC control*
/
180ꢀ250 8C) for 20 h.
/
The products were purificated by column chromatogra-
phy (eluent hexane:ethyl acetate in different mixtures).
The products were identified by spectroscopic data. The
results see in Table 2. The formation of HN3 (b.p. 36 8C,
Caution: explosive on heating) as side product of
silylation occurred.
3.4. Synthesis of 1,1,1-triethyl-3,3,3-trimethyldisiloxane
(6) from triethylsilanol (5) under PTC conditions
Triethylsilanol (5) (0.306 ml, 2 mmol) was added to
the mixture of freshly calcinated CsF (0.0152 g, 0.1
mmol), azidotrimethylsilane (0.264 ml, 2 mmol) and 18-
crown-6 (0.026 g, 0.1 mmol) in 1.5 ml of dry toluene
under argon atmosphere. Reaction was carried out at
Fig. 8. The unsymmetrical siloxane formation.
20 8C (GC control*
/
150 8C) for 0.5 h. Yield 100%
(GCꢀMS data).
/
3. Materials and methods
3.5. Theoretical calculations
3.1. Instrumental
All calculations were carried out using the semiempi-
rical AM1 [50] method as implemented in MOPAC 6
[51]. All structures were fully optimized using the
eigenvector following routine under the more rigorous
criteria of the keyword PRECISE. Starting states of
molecular systems (Figs. 1, 3, 5 and 7) preceding
optimization were selected without constraints and
with sufficient distances between reacting particles.
The frequencies analysis has shown that all optimum
structures present the minimum points on the potential
energy surface. The heat of reactions was calculated as a
difference between the heat of the final system forma-
tion and the sum of heat of formation of reagents. All
studied reactions proceed spontaneously (barrierless).
To obtain the data on the change in geometry during the
optimization process, calculations were performed using
keyword FLEPO. Post processing animation was car-
ried out with JMOL [52] program. Computer design of
the reaction system was realized by means of the
CHEMCRAFT software package [53].
MS spectra were registered on a GCꢀMS HP 6890 (70
/
eV) apparatus. GC analysis was performed on a Chrom-
5 instrument equipped with a flame-ionization detector
using a glass column packed with 5% OV-101/Chromo-
sorb W-HP (80ꢀ
/
100 mesh) (1.2 mꢄ3 mm). Azidotri-
/
methylsilane, hydrosilanes, 18-crown-6 (Acros) were
used without additional purification. Toluene was dis-
tilled over metallic sodium.
3.2. General procedure of dimethylphenylsilane reaction
with azidotrimethylsilane under PTC conditions
Dimethylphenylsilane (0.155 ml, 1 mmol) was added
to the mixture of freshly calcinated CsF (for amount see
Table 1), azidotrimethylsilane (0.397 ml, 3 mmol), water
(for amount see Table 1) and 18-crown-6 (0.026 g, 0.1
mmol) in 1.5 ml of dry toluene under argon atmosphere.
Reaction was carried out at 50 8C temperature (GC