Synthesis of difunctional borasiloxanes and their behavior in metathesis reactions

Article abstract of DOI:10.1016/S0022-328X(98)00498-7

New divinyl- and diallyl-borasiloxanes have been synthesized in good yield from corresponding boronic acids. They have been characterized by ¹H-, ¹³C-, ²⁹Si- and ¹¹B-NMR, as well as by MS. Their behavior in metathesis reactions initiated by two different catalysts ([RhCl(COD)]₂ for vinylsiloxanes and [Mo] Schrock's catalyst for allylsiloxanes) has been studied. This has led to new cycloborasiloxanes that have been characterized.

Full text of DOI:10.1016/S0022-328X(98)00498-7

Journal of Organometallic Chemistry 560 (1998) 109–115
Synthesis of difunctional borasiloxanes and their behavior in
metathesis reactions
Anne-Franc¸oise Mingotaud *, Vale´rie He´roguez, Alain Soum
Laboratoire de Chimie des Polyme`res Organiques, Unite´ Mixte de Recherche CNRS-ENSCPB, A6. Pey Berland, B.P. 108,
33402 Talence Cedex, France
Received 17 November 1997; received in revised form 2 February 1998
Abstract
New divinyl- and diallyl-borasiloxanes have been synthesized in good yield from corresponding boronic acids. They have been
1
characterized by H-, 13C-, 29Si- and 11B-NMR, as well as by MS. Their behavior in metathesis reactions initiated by two different
catalysts ([RhCl(COD)]2 for vinylsiloxanes and [Mo] Schrock’s catalyst for allylsiloxanes) has been studied. This has led to new
cycloborasiloxanes that have been characterized. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Borasiloxanes; Cyclocarboborasiloxanes; Ring-closing metathesis
1. Introduction
usually leads to a mixture of different products. Re-
cently, Wagener and collaborators showed that poly-
carbosiloxanes could be synthesized through acyclic
diene metathesis (ADMET) polymerization using
Schrock’s catalyst [7].
In our research towards new organometallic poly-
mers, we investigated whether polyborasiloxanes could
be obtained following the same method. Indeed, it has
been shown that these polymers could not be synthe-
sized through ring-opening polymerization, since this
reaction leads to a mixture of polysiloxanes and cy-
cloboroxins [8] and never to the expected polymer.
We report here the results obtained in metathesis
reactions of unsaturated borasiloxanes, which did not
give polymers as we hoped, but instead well-defined
cyclocarboborasiloxanes.
Metathesis reactions of alkenes and dienes (Scheme
1) have been limited for a long time to organic com-
pounds without any functional groups. However, recent
developments have shown that catalysts can be found
to accommodate such groups [1,2]. For example,
Marciniec and collaborators reported the possibility of
metathesis of vinyl or allyl silanes in the presence of
ruthenium or rhodium catalysts [3–6]. This reaction
2. Results and discussion
Scheme 1. Metathesis reactions of alkenes and dienes.
2.1. Synthesis of borasiloxanes 1–4
Several dialkenylborasiloxanes 1–4 were synthesized
* Corresponding author. Tel.: +33 556848486; fax: +33
556848487; e-mail: dirlcpo@enscpb.u-bordeaux.fr
following the method described by Wannagat et al. for
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00498-7

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A.-F. Mingotaud et al. / Journal of Organometallic Chemistry 560 (1998) 109–115
Scheme 2. Reactions between aryl/alkylboronic acid and a chlorosi-
lane.
the synthesis of cycloborasiloxanes [9]. They were ob-
tained from a simple reaction between aryl- or alkyl-
boronic acid and a chlorosilane (Scheme 2). All
experimental conditions and the corresponding results
are reported in Table 1. NMR data (Table 2) and MS
spectra (Fig. 1) are consistent with the structure.
2.2. Synthesis of borasiloxanes 7 and 8
Scheme 3. Borasiloxanes 7 and 8.
For the longer borasiloxanes, 7 and 8, the appropri-
ate chlorosiloxanes 5 and 6 were first synthesized
(Scheme 3). This was realized by reacting an excess of
dimethyldichlorosilane with the appropriate silanol ob-
tained by the hydrolysis of the corresponding chlorosi-
lane [10]. The chlorosiloxanes were obtained from the
silanol in good yields. The reaction with an arylboronic
acid then leads to the borasiloxanes. Unfortunately, the
crude products displayed a slow decomposition upon
heating during the purification process. This led to
mixtures of borasiloxanes, boroxins and siloxanes.
Therefore, the metathesis reaction of these borasilox-
anes could not be studied.
With catalyst 9, Schrock and collaborators have
shown that vinylsiloxanes are unsuitable reactants since
they lead to the formation of a stable metallacyclobu-
tane [11]. However, diallylsilanes react via an usual
ADMET reaction with Schrock’s catalyst to give a
disilane having a 1,2-disubstituted bridge. Therefore, in
this work, Schrock’s catalyst has only been tested on
diallylborasiloxanes, whereas [RhCl(COD)]2 has been
used on divinylborasiloxanes.
The experimental conditions and the corresponding
results are reported in Table 3; NMR characteristics of
the products (10–12) synthesized are presented in Table
4. It is noteworthy that compounds 10 and 11 are the
only isomers obtained from the metathesis reactions of
1 and 3 (Scheme 5), since there is no evidence of a
1,2-dialkenyl bridge formation from NMR and MS
spectra (Fig. 1). This is contrary to what Marciniec et
al. [5] reported for ADMET reactions of vinylsilanes.
Our result is likely to be due to a favorable conforma-
tion (i.e. a favorable position of the double bonds),
either in the starting product or in the corresponding
carbenic intermediate, leading mostly to the six-mem-
bered ring.
2.3. Metathesis reactions of borasiloxanes 1–4
The behavior of borasiloxanes 1–4 in metathesis
reactions was studied under two different conditions:
(1) with [Mo] Schrock’s catalyst 9, (2) and
[RhCl(COD)]2 (Scheme 4). This latter system is particu-
larly effective in divinylsiloxane ADMET reactions [5],
whereas with diallylsiloxanes, it mainly gives rise to the
isomerization of an allyl into a propenyl group. In
order to explain the metathesis activity, Marciniec and
collaborators proposed the formation of a carbenic
intermediate by reaction of the catalyst with the vinyl
group [4]. They showed that this [Rh] system generally
leads to 1,1- as well as 1,2-disubstituted alkenyl bridges
[5].
The reaction of Schrock’s catalyst 9 with 2 gives a
mixture of cis and trans isomers of cycloborasiloxanes
1
12/12’ (Table 4), for which the H- and 13C-NMR data
are consistent with those reported by Wagener for
siloxane 13. Their formation by ring-closing metathesis
Table 1
Experimental conditions for the synthesis of linear borasiloxanes 1–4
Compound Et2O (ml)
R–B(OH)2 g (mmol)
Chlorosilane g (mmol)
NEt3 g (mmol)
Product g (mmol)
Yielda (%)
1
2
3
4
90
40
100
50
4.1 (34)
2.1 (17)
1.95 (19)
2.05 (20)
8.2 (68)
4.6 (34)
4.6 (38)
5.4 (40)
8.6 (85)
4.3 (43)
4.4 (44)
5.1 (50)
8.3 (29)
3.9 (12)
4.5 (17)
5.1 (17)
84
71
88
82
a Yield of purified product.

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A.-F. Mingotaud et al. / Journal of Organometallic Chemistry 560 (1998) 109–115
113
is somewhat surprising, since nine-membered rings are
usually not favored. Moreover, Wagener and collabora-
tors have shown that ADMET reactions on siloxane 14
(without any boron atom) led to polymers [7]. In our
case, it can be supposed that the small radius of the
boron atom, as well as the sterically demanding phenyl
group, enables a folded conformation leading prefer-
ably to cycles (which is confirmed by examining theo-
retical models of the molecules). In order to reduce the
interactions between the phenyl group and the methyl
substituents of the silicon atoms, the molecule has to
adopt a folded conformation, thus leading to a smaller
distance between the alkenes in 12 compared with the
siloxane analog 14 described by Wagener [7].
Recently, some examples of ring-closing metathesis
leading to medium-size cycles have been reported [12–
16], but only with organic compounds. Moreover, in
these examples, the reaction took place in favorable
conditions, i.e. diluted solutions. What is surprising
here is the fact that even in bulk, the dienes do not lead
to ADMET polymerization, but to ring-closing
metathesis.
Scheme 4. The behavior of vinylsilanes 1–4 in metathesis reactions
under different conditions.
Under the same conditions, borasiloxane 4 (with
butyl on the boron atom) has an ambiguous behavior.
Indeed, if some gas evolution was clearly observed
upon its mixing with Schrock’s catalyst, it stopped after
a few minutes; this was still observed even after re-
peated purifications of 4. So, apparently, only arylbo-
rasiloxanes exhibit efficient metathesis reactions
initiated by Schrock’s catalyst 9. At this time, there is
no explanation for this observation.
3. Experimental
Dichlorosilanes and chlorosilanes (Roth-Sochiel)
were stirred in the presence of magnesium for 18 h, and
trap-to-trap distilled. Phenylboronic acid (Fluka),
butylboronic acid (Aldrich), [RhCl(COD)]2 and [Mo]
Schrock’s catalyst (Strem Chemicals) were used without
further purification. Silanols were prepared according
to a previously published procedure [10]. Triethylamine
Table 3
Ring-closing metathesis reactions of various borasiloxanes at 25°C in bulk conditions
Compound
Reactant g (mmol)
Catalyst g (mmol)
Product g (mmol)
Yield (%)
10
11
12
1 6.9 (24)
3 0.24 (0.9)
2 0.5 (1.6)
[RhCl(COD)]2 0.17 (0.34)
[RhCl(COD)]2 0.1 (0.2)
9 0.1 (0.02)
5.0 (19)
0.2
Mixture 12/12’/residual 2
80a
85b
90b
a Yield of purified product; b yield of crude product.
Table 4
NMR chemical shifts of resulting products of ADMET cyclization in CDCl3 at 20°C (ppm)
Com-
1H
13C
29Si
11B
pound
10
0.31 (s, 12H, SiMe); 6.37 (s, 2H, ꢀCH2); 7.39
(m, 3H, m, p BPh); 7.89 (dd, 3J (o, m)=7.9
Hz, 4J (o, p)=1.7 Hz, 2H, o BPh)
0.23 (s, 12H, SiMe); 0.72 (t, J=7.2 Hz, 2H,
ꢁCH2ꢁB); 0.87 (t, J=6.8 Hz, 3H, CH3ꢁ);
1.15–1.45 (m, 4H, ꢁCH2ꢁ); 6.3 (s, 2H, ꢀCH2)
0.23 (s, 12H, SiMe); 1.65 (m, 4H, ꢁCH2ꢁꢀ);
0.17 (SiMe); 127.3 (o BPh); 130.7 (p BPh);
135.0 (m BPh); 140.4 (ꢀCH2); 153.5 (Cꢀ)
5.59 (SiMe)
4.2 (SiMe)
26.7 (BPh)
11
0.87 (SiMe); 14.7 (CH3ꢁ); 26.1 (ꢁCH2ꢁ); 27.4
(ꢁCH2ꢁB); 140.8 (ꢀCH2); 154.0 (ꢀC)
0
12/12’ a
0.31 (SiMe); 24.7 (CH2ꢁꢀ); 124.9 (CHꢀCH);
5.26–5.53 (m, 2H, CHꢀCH ratio cis/trans 1/4); 128.1 (o BPh); 131.2 (p BPh); 135.6 (m BPh)
7.39 (m, 3H, m, p BPh); 7.75 (m, 2H, o BPh)
a Crude product.

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A.-F. Mingotaud et al. / Journal of Organometallic Chemistry 560 (1998) 109–115
together with the excess of dimethyldichlorosilane. The
resulting product was trap-to-trap distilled under vac-
uum to yield 8.1 g of a colorless liquid (82%).
1H-NMR (CDCl3; l ppm): 5.65–5.9 (m, 1H, CHꢀ);
3
3
4.78–4.95 (2dd J (trans)=16.2 Hz, J (cis)=10.2 Hz,
2H, ꢀCH2); 1.62 (d, J=8 Hz, 4H, CH2ꢁꢀ); 0.43 (s, 6H,
SiMeCl); 0.15 (s, 6H, SiMe).
3.3. Synthesis of 6inyltetramethylchlorodisiloxane 6
Compound 6 was synthesized following the same
procedure as for 5 (yield 51%).
1H-NMR (CDCl3; l ppm): 5.65–6.28 (m ABM sys-
2
3
3
tem J (gem)=4.8 Hz, J (trans)=19.4 Hz, J (cis)=
14.9 Hz, 3H, CHꢀCH2); 0.43 (s, 6H, SiMeCl); 0.21 (s,
6H, SiMe). 13C-NMR (CDCl3; l ppm): 138.9 (CHꢀ);
133.1 (ꢀCH2); 4.86 (SiMeCl); 0.71 (SiMe).
3.4. Synthesis of borasiloxanes 7 and 8
The procedure was the same as for 1.
1
7: H-NMR (CDCl3; l ppm): 7.88 (m, 2H, o BPh);
7.3–7.5 (m, 3H, m and p BPh); 5.69–5.95 (m, 2H,
3
2
CHꢀ); 4.8–5.0 (2dd J (trans)=18 Hz, J (gem)=0.8
Hz, 3J (cis)=9.5 Hz, 4H, ꢀCH2); 1.63 (d 3J=8 Hz, 4H,
CH2ꢁꢀ); 0.25 and 0.15 (2s, 24H, SiMe). 13C-NMR
(CDCl3; l ppm): 136.1 (m BPh); 134.8 (CHꢀ); 131.4 (p
BPh); 128.1 (o BPh); 114.2 (ꢀCH2); 26.9 (CH2ꢁꢀ); 1.73
(OSiO); 0.40 (Siꢁallyl).
Scheme 5. Metathesis reactions of borasiloxanes 1–3.
was stored under CaH2 and trap-to-trap distilled before
use. When useful, diethylether was distilled on CaH2.
1H- and 13C-NMR spectra were recorded on a
Bruker AC 200 at 200 and 50.32 MHz, respectively.
29Si- and 11B-NMR were carried out on a Bruker
DPX200, respectively, at 39.8 and 64.21 MHz. MS
spectra were realized on an AutospecEQ spectrometer
(electron impact mode, 70 eV).
1
3
8: H-NMR (CDCl3; l ppm): 7.84 (dd J (o, m)=7.9
4
Hz J (o, p)=1.7 Hz, 2H, o BPh); 7.3–7.5 (m, 3H, m,
2
p BPh); 5.69–6.25 (m, ABM system, J (gem)=4.5 Hz,
3
3J (trans)=19.6 Hz, J (cis)=14.8 Hz, 6H, CHꢀCH2);
0.21 and 0.18 (2s, 24H, SiMe). 13C-NMR (CDCl3; l
ppm): 139.7 (ꢀCH); 136.2 (m BPh); 132.6 (ꢀCH2); 131.4
(p BPh); 128.1 (o BPh); 1.76 (OSiO); 0.93 (SiVi). 29Si-
NMR (l ppm): −3.09 (SiVi); −18.9 (OSiO).
3.1. Synthesis of borasiloxanes 1–4
3.5. Metathesis reactions of borasiloxanes 1–4
To a cooled solution (0°C) of aryl- or alkylboronic
acid and triethylamine in diethylether, a solution of
alkenyldimethylchlorosilane in 10 ml of diethylether
was added dropwise. The solution was stirred over 4 h
at ambient temperature. Filtration of triethylammo-
nium chloride and evaporation of the solvent followed
by trap-to-trap distillation under high vacuum afforded
a colorless viscous oil.
A small quantity of the desired catalyst was added to
the dialkenylborasiloxane under nitrogen. The mixture
was stirred under vacuum for 18 h. Ethylene evolution
immediately occurred and was continuously eliminated
by pumping until no gas formation was observed. The
crude product was vacuum trap-to-trap distilled under
high vacuum.
3.2. Synthesis of allyltetramethylchlorodisiloxane 5
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