Tris(pentafluorophenyl)borane-Catalyzed Reactions of Siloxanes: A Combined Experimental and Computational Study

Article abstract of DOI:10.1002/ejoc.201700760

The reaction of 1,1,3,3-tetramethyldisiloxane with 1-octene as a model reaction of silicone curing catalyzed by B(C₆F₅)₃ resulted in the redistribution of the disiloxane into dimethylsilane and cyclic oligosiloxanes, and the subsequent hydrosilylation reaction of dimethylsilane afforded dimethyldioctylsilane. To obtain insights into the reaction mechanism and possibility alter the reaction pathway to favor the hydrosilylation over the redistribution, mechanistic analysis of the reaction between a hydrosiloxane (1,1,3,3-tetramethyldisiloxane, silox-H) and a vinylsiloxane (1,1,3,3-tetramethyl-1,3-divinyldisiloxane, silox-vin) in the presence of B(C₆F₅)₃ was performed through density functional theory calculations. The results of the calculations indicate that the activation of a Si–H bond in silox-H by B(C₆F₅)₃ initiates the reaction to form the B(C₆F₅)₃–silox-H complex with a Lewis acidic silicon atom and a hydridic hydrogen atom. The B(C₆F₅)₃–silox-H complex can undergo two different reaction pathways, that is, trisiloxane formation and the hydrosilylation of silox-vin by silox-H. The trisiloxane formation involves trisilyloxonium ions as intermediates and can lead to either the homotrisiloxane of silox-H or a mixed trisiloxane of silox-H and silox-vin. The energetics of the reaction pathways predict the preference of trisiloxane formation over hydrosilylation, and the fine tuning of the steric and electronic natures of the substrates could alter the thermodynamic and kinetic favorability.

Full text of DOI:10.1002/ejoc.201700760

A Journal of
Accepted Article
Title: Tris(pentafluorophenyl)borane-Catalyzed Reaction of Siloxanes:
A Combined Experimental and Computational Study
Authors: Jomon Mathew, Katsuya Eguchi, Yumiko Nakajima, Kazuhiko
Sato, Shigeru Shimada, and Yoong-Kee Choe
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700760
Link to VoR: http://dx.doi.org/10.1002/ejoc.201700760
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10.1002/ejoc.201700760
European Journal of Organic Chemistry
Tris(pentafluorophenyl)borane-Catalyzed Reaction of Siloxanes:
A Combined Experimental and Computational Study
Jomon Mathew†, Katsuya Eguchi¶, Yumiko Nakajima¶, Kazuhiko Sato¶, Shigeru
Shimada¶*, Yoong-Kee Choeǂ*
†Department of Chemistry, St. Joseph's College, Devagiri, Calicut, Kerala 673 008,
India
ǂResearch Center for Computational Design of Advanced Functional Materials, National
Institute of Advanced Industrial Science and Technology (AIST), Central-2, Tsukuba,
305-8568, Japan.
¶Interdisciplinary Research Center for Catalytic Chemistry, National Institute of
Advanced Industrial Science and Technology (AIST), Central-5, Tsukuba, 305-8568,
Japan.
Keywords: hydrosilylation, tris(pentafluorophenyl)borane, reaction mechanism
*Corresponding authors: Shigeru Shimada (email: s-shimada@aist.go.jp), Yoong-Kee
Choe (email: yoongkee-choe@aist.go.jp)
Abstract: The reaction of 1,1,3,3-tetramethyldisiloxane with 1-octene as a model
reaction of silicone curing catalyzed by B(C6F5)3 resulted in redistribution of the
disiloxane into dimethylsilane and cyclic oligosiloxanes, and the subsequent
hydrosilylation reaction of dimethylsilane afforded dimethyldioctylsilane. To obtain
insights into the reaction mechanism and the possibility to alter the reaction pathway
favoring the hydrosilylation over the redistribution, mechanistic analysis of the reaction
between a hydrosiloxane (1,1,3,3-tetramethyldisiloxane, silox-H) and a vinyl siloxane
(1,1,3,3-tetramethyl-1,3-divinyldisiloxane, silox-vin) in the presence of B(C6F5)3 was
carried out using density functional theory calculations. Results of the calculation indicate
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10.1002/ejoc.201700760
European Journal of Organic Chemistry
that activation of a Si–H bond in silox-H by B(C6F5)3 initiates the reaction leading to the
B(C6F5)3–silox-H complex with a Lewis acidic silicon atom and a hydridic hydrogen.
The B(C6F5)3–silox-H complex can undergo two different reaction pathways, viz.,
trisiloxane formation and the hydrosilylation of silox-vin by silox-H. Trisiloxane
formation involves the intermediacy of a trisilyloxonium ion and can lead to either the
homotrisiloxane of silox-H or a mixed trisiloxane of silox-H and silox-vin. The
energetics of the reaction pathways predict the preference of trisiloxane formation over
hydrosilylation and the fine tuning of steric and electronic nature of substrates can alter
the thermodynamic and kinetic favorability.
Introduction
Hydrosilylation is one of the most important reactions in silicon chemistry.[1] In the
silicone industry, hydrosilylation is used to produce functional silicon compounds as well
as for the curing of silicone materials, where hardening of the silicone is attained by the
cross-linking hydrosilylation reaction of hydrosiloxanes with vinylsiloxanes (Scheme 1).
As a catalyst for hydrosilylation, platinum compounds are mostly used in the industry. It
was estimated that the silicone industry consumed 5.6 tonnes of platinum in 2007, which
is ca. 3% of annual platinum production.[2] Therefore, there is a high demand for
technology that enables the reduction of the platinum consumption in the silicone industry
by developing platinum catalysts with higher catalytic activity or nonplatinum catalysts.[3]
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10.1002/ejoc.201700760
European Journal of Organic Chemistry
Scheme 1. Curing of polysiloxanes by the hydrosilylation reaction of hydrosiloxanes with
vinylsiloxanes.
B(C6F5)3 has emerged as a powerful Lewis acid catalyst for a large number of organic
transformations as well as polymerization reactions.[4] B(C6F5)3 is also an efficient
catalyst for the transformation of organosilicon compounds, including hydrosilylation
reactions.[5] Piers et al. reported the hydrosilylation of carbonyl groups in aldehydes,
ketones, and esters using B(C6F5)3 catalyst.[6] It was considered that the reaction is
initiated by the activation of carbonyl groups by B(C6F5)3 to form borane–carbonyl
adducts as an intermediate. This mechanistic pathway was supported by the crystal
structure of such an adduct.[7] Whereas, quantitative kinetic studies by Piers et al. showed
a different reaction mechanism, in which borane activates the Si–H bond of silane and
forms a silane–borane adduct providing a Lewis acidic silicon atom and a hydridic
hydrogen atom.[6b] Binding of carbonyl oxygen to the electrophilic silicon center followed
by the attack of H– from [B(C6F5)3H]– on the carbonyl carbon leads to the hydrosilylated
product (Scheme 2). Extensive mechanistic investigations by Oestreich et al. support the
silane activation mechanism for the hydrosilylation of both carbonyl compounds and
imines.[8] Recent computational analysis of B(C6F5)3-catalyzed hydrosilylation of acetone
(at the M06-2x(PCM)/6-311++G**//M06-2X/6-311G** level) by Sakata and Fujimoto
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also confirms the silane activation mechanism.[9]
Scheme 2. Mechanism of B(C6F5)3-catalyzed hydrosilylation of ketone.
Regarding olefin hydrosilylation, B(C6F5)3 is reported to be effective for the reaction
of simple olefins such as styrenes or aliphatic olefins with R3SiH (R = alkyl or Ph) or
Ph2SiH2,[10] whereas it is also reported to catalyze the redistribution reaction of
hydrosiloxanes
such
as
1,1,3,3-tetramethyldisiloxane
and
1,1,1,3,3-
pentamethyldisiloxane.[11] Therefore, it is not clear whether B(C6F5)3 is effective for the
curing of hydrosiloxanes with vinylsiloxanes. In this paper, we first briefly describe
experimental results on the reaction of 1,1,3,3-tetramethyldisiloxane with 1-octene or
1,1,3,3-tetramethyl-1,3-divinyldisiloxane catalyzed by B(C6F5)3 as a model for the curing
reaction, which resulted in a redistribution/hydrosilylation reaction. Then, the main part
of this paper, a theoretical study on the mechanism of the B(C6F5)3-catalyzed reaction of
1,1,3,3-tetramethyldisiloxane with
described.
1,1,3,3-tetramethyl-1,3-divinyldisiloxane, is
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European Journal of Organic Chemistry
Experimental
General Information. 1H and 29Si NMR spectra were recorded using a Bruker AVANCE
III HD 600 spectrometer. 1H NMR (600 MHz) and 29Si NMR (119 MHz) chemical shifts
1
were referenced to the residual solvent signal for H (benzene-d6 = 7.16 ppm) and to
tetramethylsilane for 29Si (0.00 ppm). The GC–MS analysis was conducted on a
Shimadzu QP-2010 Plus spectrometer. Size-exclusion chromatography (SEC) was
performed at 45 °C using a Waters ACQUITY advanced polymer chromatography (APC)
system consisting of Isocratic Solvent Manager (Model AIS), Sample Manager pFTN
(Model ASM), Column Manager-S (Model AZC), PDA TS Detector (Model ADT),
Refractive Index (RI) Detector (Model URI) equipped with a Waters APCTM XT45
column (linear, 4.6 mm × 150 mm; pore size, 4.5 nm; bead size, 1.7 μm; exclusion limit,
5000), a Waters APCTM XT200 column (linear, 4.6 mm × 150 mm; pore size, 20.0 nm;
bead size, 2.5 μm; exclusion limit, 70 000), and a Waters APCTM XT450 column (linear,
4.6 mm × 150 mm; pore size, 45.0 nm; bead size, 2.5 μm; exclusion limit, 400 000) in
toluene at the flow rate of 0.70 mL min−1.
Reaction of 1,1,3,3-Tetramethyldisiloxane with 1-Octene Catalyzed by B(C6F5)3.
B(C6F5)3 (41 mg, 0.08 mmol) was added to a CH2Cl2 solution (2 mL) of 1,1,3,3-
tetramethyldisiloxane (55 mg, 0.41 mmol), 1-octene (94 mg, 0.84 mmol), and mesitylene
(42 mg, 0.35 mmol, standard compound for NMR analysis) under N2. The mixture was
stirred at room temperature for 24 h. A part (ca. 0.1 mL) of the reaction mixture was taken
and diluted with C6D6 (0.3 mL) to make an NMR sample, which was analyzed by 1H and
29Si NMR spectroscopies, showing the formation of dimethyldioctylsilane in 84% yield
as well as octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5).
The sample was also analyzed using GC–MS, confirming the formation of these products.
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European Journal of Organic Chemistry
(CH3)2{CH3(CH2)7}2Si: 1H NMR (600 MHz):  0.02 (s, 6H), 0.51–0.57 (m, 4H), 0.90 (t,
29
6H, J = 7.0 Hz), 1.25–1.38 (br m, 24H); Si NMR (119 MHz):  2.41. {(CH3)2SiO}4:
29Si NMR (119 MHz): .12 {(CH3)2SiO}5: 29Si NMR (119 MHz): -20.99.12
Computational Details
All calculations have been carried out at the MPW1K/6-31G(d) level of density functional
theory.[13] The MPW1K functional was selected because of its accurate description of
borane complexes.[14] The Gaussian 09 suite of programs was used for all calculations.[15]
Transition states were located using the synchronous transit guided quasi-Newton method
(STQN) implemented in Gaussian 09 and are characterized by a single imaginary
frequency. Dispersion corrections were accounted for by performing single-point
calculations of all MPW1K/6-31G(d) optimized geometries at the M06-2X/6-
311++G(d,p) level of theory.[16] Solvent effects (CH2Cl2) were treated using the SMD
solvation model[17] for all optimizations and single-point calculations. Free energy values
(Gsol) calculated at the M06-2X/6-311++G(d,p)(SMD)//MPW1K/6-31G(d) (SMD)
level are used for the discussion.[18]
Results and Discussion
Experimental study on the reaction of a hydrosiloxane with an olefin catalyzed by
B(C6F5)3
The reaction of 1,1,3,3-tetramethyldisiloxane (silox-H) with 1-octene was examined as a
model reaction in the presence of B(C6F5)3. As shown in Scheme 3, dimethyldioctylsilane
was obtained in 89% yield together with octamethylcyclotetrasiloxane D4 and
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European Journal of Organic Chemistry
decamethylcyclopentasiloxane D5 as the main siloxane products. Dimethyldioctylsilane
is probably produced by the initial formation of dimethylsilane by a B(C6F5)3-catalyzed
redistribution reaction of silox-H,[11] which also forms D4 and D5, and a subsequent
B(C6F5)3-catalyzed hydrosilylation reaction with 1-octene. 1H and 29Si NMR
spectroscopies, as well as GC–MS analysis of the crude reaction mixture, clearly showed
the formation of these products.
Scheme 3. Reaction of a hydrosiloxane with an alkene catalyzed by B(C6F5)3.
The reaction of silox-H (1 equiv) with 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (silox-
vin, 1 equiv) was also examined under the similar reaction conditions (20 mol% of
B(C6F5)3. in CH2Cl2 at room temperature for 24 h). SEC analysis of the reaction mixture
showed the formation only of oligomeric species with low molecular weights but not of
polymers. GC-MS analysis of the reaction mixture showed many signals including D4,
D5 and higher cyclic oligomers. 1H, 13C{1H}, and 29Si{1H} NMR analysis of the reaction
mixture showed complete consumption of both starting compounds. 29Si{1H} NMR
spectrum showed the signal of dimethylsilane at -38 ppm. 1H NMR spectrum showed the
broadened signals corresponding to vinyl groups with ca. 70% integral intensity of that
of the initial silox-vin. These results probably suggest that silox-H was converted to
dimethylsilane and cyclic siloxanes such as D4 and D5 as in the case of the reaction of
silox-H and 1-octene. A part of dimethylsilane reacted with silox-vin, but it was less
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10.1002/ejoc.201700760
European Journal of Organic Chemistry
efficient than the reaction with 1-octene. Silox-vin was also involved in the oligosiloxane
formation with silox-H.
Theoretical study on the reaction of hydrosiloxanes with vinylsiloxanes catalyzed by
B(C6F5)3
The analysis is centered on the oligomerization of silox-H (HMe2SiOSiMe2H) in the
presence of B(C6F5)3 reported by Chojnowski et al.[11] and on the possibility of the
hydrosilylation reaction of silox-vin by silox-H using B(C6F5)3 catalyst (Scheme 4).
Scheme 4. The reaction between silox-H and silox-vin in the presence of B(C6F5)3.
In the reaction system of B(C6F5)3, silox-H, and silox-vin, three adducts can be
formed; two O–B adducts by the donation of oxygen lone pairs of silox-H and silox-vin
to the empty 2p orbital of the boron atom and one Si–H–B adduct by the activation of a
Si–H bond in silox-H by B(C6F5)3. Calculations showed that silox-vin does not form an
adduct by O–B interaction because of the steric bulkiness of the substituents on both the
silicon and boron atoms. silox-H is able to form an adduct by O–B interaction while its
formation is endothermic by 13.5 kcal/mol. The Si–H–B adduct, designated as B(C6F5)3–
silox-H, is only 2.2 kcal/mol higher in energy than the dissociated silox-H and B(C6F5)3
(Figure 1). The r(B–H) and r(Si–H) in B(C6F5)3–silox-H are 1.44 and 1.58 Å,
respectively, suggesting a relatively strong interaction between silox-H and B(C6F5)3.
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European Journal of Organic Chemistry
Figure 1. Activation of Si–H in silox-H by B(C6F5)3. Relative Gibbs free energy values
(at the M06-2X/6-311++G(d,p) (SMD)//MPW1K/6-31G(d) (SMD) level of theory) are
in kcal/mol and bond lengths are in Å.
The Si–H bond activation of silox-H by B(C6F5)3 leads to a Lewis acidic silicon atom
and hydridic hydrogen atom in B(C6F5)3–silox-H (NBO charges on Si and H are +2.09
and –0.24e, respectively), which opens up the three possible below mentioned reaction
pathways (Figure 2).
path A—Interaction of oxygen lone pairs of silox-H with the Lewis acidic silicon atom
path B—Interaction of oxygen lone pairs of silox-vin with the Lewis acidic silicon atom
path C—Interaction of the vinyl group of silox-vin with the Lewis acidic silicon atom
Figure 2. Three possible reaction pathways for the B(C6F5)3–silox-H complex
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Paths A and B lead to trisiloxane formation reaction[7] while path C leads to the
hydrosilylation reaction. The free energy profiles of paths A and B are depicted in Figure
3. Intermediates and transition states located for path A are given in Figure 4.
In path A, silox-H approaches the electrophilic silicon atom of B(C6F5)3–silox-H
from the back side of the activated Si–H bond to form the reactant complex 3-A. As can
be seen in the figure, the O–Si length in 3-A is 5.14 Å, suggesting negligible O–Si
interaction. Upon complex formation, the intra-Si-O bond lengths of silox-H became
asymmetric. The Si-O bond length is computed to be 1.61 Å in case of the Si-O moiety
interacting with B(C6F5)3 where another one is computed to be 1.66 Å. Such a feature is
likely to arise from the activation of the Si-H bond due to the presence of B(C6F5)3. The
complex 3-A is 9.6 kcal/mol higher in energy than 2. Nucleophilic attack of the oxygen
lone pair of silox-H on the Lewis acidic silicon leads to the formation of complex 4-A via
the transition state TS1-A. The activation barrier for this transformation is computed to
be 5.9 kcal/mol and is exothermic by 8.3 kcal/mol. In TS1-A, the attacking oxygen atom
comes closer to the electrophilic silicon (r(O–Si) = 2.34 Å) and the Si–H bond elongates
to 1.86 Å from 1.59 Å in 3-A. The complex 4-A is an ion pair complex with a
trisilyloxonium ion and [B(C6F5)3H]–. The reaction proceeds by hydride ion abstraction
from [B(C6F5)3H]– by either of the two SiHMe2 groups of the trisilyloxonium ion to
cleave the Si–O bond. For the abstraction of H– by SiHMe2, [B(C6F5)H]– moves to the
vicinity of one of the SiHMe2 groups and forms another ion pair, 5-A, which is 2.4
kcal/mol more stable than 4-A.[19] Hydride ion transfer occurs via the transition state TS2-
A leading to the product complex 6-A, a trisiloxane, and an adduct of SiH2Me2 and
B(C6F5)3. The activation barrier for the hydride ion abstraction is 9.7 kcal/mol and the
reaction is endothermic by 4.3 kcal/mol but the relative free energy of the final product
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European Journal of Organic Chemistry
7-A, (trisiloxane, SiH2Me2, and the catalyst B(C6F5)3), is –3.5 kcal/mol with respect to
the reactants (1). The overall reaction is exothermic by 3.5 kcal/mol and the activation
barriers are achievable at ambient temperature, suggesting the feasibility of path A.
Further oligomerization continues by the activation of the Si–H bond in the trisiloxane.
In path B, the oxygen atom of silox-vin attacks the electrophilic silicon atom in the
B(C6F5)3–silox-H complex and follows a similar mechanism to that of path A leading to
a
mixed trisiloxane HMe2SiOSiMe2OSiMe2(CH=CH2) and
a
hydrosilane
Me2Si(CH=CH2)H as coproducts (intermediates and transition states located for path B
are given in the Supporting Information). It can be seen that all the intermediates,
transition states, and the final product in path B are higher in energy than those in path A,
suggesting the preference for path A over path B (Figure 3). Structural bulkiness of silox-
vin in comparison with silox-H causes the energetics of transition state to be higher than
corresponding ones in path A.
TS1-B
20.0
path A
TS2-B
path B
16.1
TS1-A
17.2
3-B
11.8
6-B
10.5
3-A
11.3
5-B
7.8
4-B
6.3
TS2-A
10.3
6-A
4.9
4-A
3.0
2
2.2
7-B
-1.9
5-A
0.6
1
0.0
7-A
-3.5
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Figure 3. Gibbs free energy profile (at the M06-2X/6-311++G(d,p) (SMD)//MPW1K/6-
31G(d) (SMD) level of theory) for paths A and B. Relative free energies are in kcal/mol
and bond lengths are in Å.
1.43
1.59
1.21
3.79
1.29
1.86
2.34
1.79
5.14
1.79
TS1-A
3-A
4-A
1.77
1.78
1.79
2.39
1.71
1.49
4.53
1.56
TS2-A
5-A
6-A
Figure 4. Optimized structures of intermediates and transition states for path A at the
MPW1K/6-31G(d) (SMD) level of theory. Bond lengths are given in Å.
Path C leads to the hydrosilylation of silox-vin by silox-H. The free energy profile
for path C is given in Figure 5 and the intermediates and transitions states located are
shown in Figure 6. silox-vin forms the reactant complex 3-C with B(C6F5)3–silox-H that
is 8.2 kcal/mol higher in energy than 2. Interaction of the -electrons of the vinyl group
with the electrophilic silicon atom leads to an ion pair complex 4-C through the transition
state TS1-C. The activation barrier for the formation of 4-C is 13.6 kcal/mol and is
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10.1002/ejoc.201700760
European Journal of Organic Chemistry
endothermic by 12.8 kcal/mol (formation of 4-C is endothermic by 15.9 kcal/mol at the
MPW1K/6-31G(d) level). Rearrangement of 4-C leads to 5-C, in which [B(C6F5)3H]– is
in the vicinity of the C=C double bond. Hydride ion abstraction from [B(C6F5)3H]– by the
vinylic CH group via the transition state TS2-C leads to the hydrosilylated product and
B(C6F5)3 (6-C). The activation barrier for the hydride ion transfer is 11.1 kcal/mol and
the reaction is exothermic by 29.1 kcal/mol. A transition state for the hydride ion transfer
from silox-H either directly or with the assistance of [B(C6F5)3H]– could not be located.
The relative free energy of the free silylated product and B(C6F5)3 (7-C) is –16.9 kcal/mol
with respect to the reactant molecules (1). The hydrosilylation reaction is highly
exothermic compared with the trisiloxane formation reactions (paths A and B) but the
high-energy intermediates and transition states in the potential energy surface of path C
suggest the lower feasibility of hydrosilylation at ambient temperatures.
TS2-C
33.9
TS1-C
24.0
4-C
23.2
5-C
22.8
3-C
10.4
2
2.2
6-C
-6.3
1
0.0
7-C
-16.9
Figure 5. Gibbs free energy profile (at the M06-2X/6-311++G(d,p) (SMD)//MPW1K/6-
31G(d) (SMD) level of theory) for path C. Relative free energies are in kcal/mol and bond
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10.1002/ejoc.201700760
European Journal of Organic Chemistry
lengths are in Å.
2.39
1.59
1.43
1.23
2.54
1.34
1.21
4.82
3.56
2.44
2.80
1.35
2.31
1.33
3-C
TS1-C
4-C
1.27
1.99
3.22
1.36
1.97
2.22
5-C
TS2-C
6-C
Figure 6. Optimized structures of intermediates and transition states for path C at the
MPW1K/6-31G(d) (SMD) level of theory. Bond lengths are given in Å.
The free energy profiles for the three pathways show that the preference of trisiloxane
formation reaction over hydrosilylation mainly lies in the low-energy intermediates in
paths A and B. The electron density in the lone-pair region of oxygen atoms in siloxanes
is higher than in the -electron region of the vinyl group. Figure 7 shows the molecular
electrostatic potential (MESP) at the lone-pair region of oxygen and olefin moiety of both
silox-H and silox-vin. The higher electron density for the oxygen lone pair than that of
the olefinic -electrons is visible from the larger spatial distribution of the MESP
isosurface in the lone-pair region of the oxygen atoms. Therefore, Si–O interactions are
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10.1002/ejoc.201700760
European Journal of Organic Chemistry
stronger than those of Si–vinyl interactions and stabilize the intermediates 4-A and 4-B
in paths A and B.
Figure 7. MESP isosurface at –0.03 a.u. in the lone-pair regions of the oxygens and -
electron region of the olefin moiety.
The energetics of paths A, B, and C show that the thermodynamic and kinetic
favorability of paths A and B should be lowered for the hydrosilylation reaction to occur.
The reaction is possible only by lessening the accessibility of the oxygen lone pairs to
interact with the Lewis acidic Si atom, and a potential strategy would be the introduction
of sterically bulky substituents in silox-H and silox-vin. Preliminary analysis shows that
the thermodynamic stability of 4-A and 4-B decreases as the steric nature of the
substituents on silox-H increases (Supporting Information), while an extensive analysis
is required for the fine tuning of steric and electronic natures of substrates as well as the
borane catalyst because the bulky substituents in silox-H may prevent the Si–H activation
by the borane catalyst and it is difficult to increase the kinetic favorability of path C.
Conclusion
The reaction of the hydrosiloxane (silox-H) with the vinyl siloxane (silox-vin) in the
presence of B(C6F5)3 can follow two different pathways: trisiloxane formation of
siloxanes and hydrosilylation of silox-vin by silox-H. Both the trisiloxane formation and
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10.1002/ejoc.201700760
European Journal of Organic Chemistry
the hydrosilylation are initiated by the activation of the Si–H bond of silox-H by B(C6F5)3.
Trisiloxane formation can lead to either the homotrisiloxane of silox-H (path A) or a
mixed trisiloxane of silox-H and silox-vin (path B). Trisiloxane formation reactions
involve the intermediacy of trisilyloxonium ion and proceed via cleavage of the Si–O
bond. Hydrosilylation (path C) involves high-energy intermediates and transition states
compared with paths A and B; the energetics favor path A over paths B and C. Fine tuning
of the stereoelectronic nature of the substrates and catalyst can enable the favorability of
the hydrosilylation reaction over trisiloxane formation.
Acknowledgment
This work was supported by the “Development of Innovative Catalytic Processes for
Organosilicon Functional Materials” project from the New Energy and Industrial
Technology Development Organization (NEDO). We thank Drs. V. Srinivas, K. Hayashi,
and K. Fuchise for their help.
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