Synthesis of unsaturated starburst compounds with a boron atom in the core

Article abstract of DOI:10.1021/om201098w

A highly effective method for regio- and stereoselective synthesis of unsaturated starburst compounds with a boron atom core and a trans configuration of all double bonds via silylative coupling of selected olefins (styrenes, vinylboronates, and vinylsilanes) with boron tris(dimethylvinylsiloxide) catalyzed by [Ru(CO)ClH(PCy₃)₂] is described. All products could be potential precursors for new highly branched materials with unique mechanical, thermal, and optoelectronic properties.

Full text of DOI:10.1021/om201098w

Article
pubs.acs.org/Organometallics
Synthesis of Unsaturated Starburst Compounds with a Boron Atom
in the Core
Jędrzej Walkowiak and Bogdan Marciniec*
́
Center of Advanced Technologies and Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
S
* Supporting Information
ABSTRACT: A highly effective method for regio- and
stereoselective synthesis of unsaturated starburst compounds
with a boron atom core and a trans configuration of all double
bonds via silylative coupling of selected olefins (styrenes,
vinylboronates, and vinylsilanes) with boron tris-
(dimethylvinylsiloxide) catalyzed by [Ru(CO)ClH(PCy3)2]
is described. All products could be potential precursors for new
highly branched materials with unique mechanical, thermal,
and optoelectronic properties.
endrimers constitute a class of highly branched,
symmetric and precisely designed molecules, whose
complexes containing or generating hydride ([TM]-H) or silyl
D
([TM]-Si) ligands (silicometallics) (where TM = Ru, Rh, Ir,
Co). This mode of reactivity seems to be general and is also
exhibited by vinylboranes (trans-borylation)9 and vinyl-
germanes (trans-germylation).10 These reactions have become
valuable synthetic tools in the preparation of vinyl-function-
alized organometalloid compounds. The general scheme is
shown by eq 1.6
properties depend on the core of the molecule as well as on
the types of functional groups attached.1 Over the last few
decades a new class of dendrimers, called metallodendrimers,
containing main-group elements and transition metals, has
attracted much attention because of their unique chemical and
physical properties. Metallodendrimers can be used in many
branches of chemistry, biology, medicine, and materials science.
Their structure determines potential applications of these
highly branched compounds in catalysis, photoelectronic, light-
harvesting, host−guest chemistry, and new hybrid inorganic/
organic materials.2
There are only a few known examples of dendrimers
containing a boron atom in the core or at the ends of
dendrimer arms. Carboborane-modified star-shaped molecules
synthesized via cobalt-catalyzed cycloaddition are used in
neutron capture therapy in cancer disease.3 Boron-modified
dendrimers are also used as active Lewis acid catalysts for,
among others, hydrosilylation reactions of ketones.4 Such
catalysts can be easily separated from the reaction mixture,
recycled, and used in the subsequent catalytic process.
The presence of boron and silicon atoms in dendrimer
structures determines the physical and chemical properties of
new materials. Such dendrimers enhance the thermal and
mechanical properties and can be used as precursors for new
types of ceramic or optoelectronic materials.5
The investigation of new, highly selective protocols for the
synthesis of branched organometallic compounds containing
boron and silicon is of great interest.
The contribution of our group was the development of a
unique, convenient, effective, highly regio- and stereoselective
method for the functionalization of molecular and macro-
molecular compounds containing one or a few vinyl groups
connected to the silicon atom.6−8 The process, named silylative
coupling (SC), occurs via cleavage of the C−Si bond in
vinylsilane and the C−H bond in olefin and is catalyzed by
The regio- and stereoselectivities of these processes are
strongly dependent on the reaction conditions and the type of
reagents. For example, for the silylative coupling of vinylsilanes
with vinylboranes at low (0−25 °C) temperatures the
formation of 1,1-borylsilylethene is strongly recommended. A
higher temperature increases the amount of 1-boryl-2-
silylethene.7c,11
Silylative coupling is a very powerful tool for the synthesis of
unsaturated, highly conjugated compounds. The process was
used to modify tris(dimethylvinylsilyl)benzene with substituted
styrenes, giving as a product starburst unsaturated compounds
with silylene−vinylene−arylene sequences with high yields and
selectivities. Such compounds show photo- and electro-
luminescence properties.12
Modification of multi-vinyl-substituted cyclosiloxanes and
silsesquioxanes with highly π-conjugated olefins was recently
carried out using this coupling reaction.13
This coupling reaction is often an alternative to metathesis
for the synthesis of silyl-substituted olefins, especially when
methyl-substituted vinylsilanes are applied as reagents. As was
Received: November 9, 2011
Published: May 8, 2012
© 2012 American Chemical Society
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previously proved, when the number of methyl substituents in
vinylsilanes is increased, the amount of the desired metathetic
product decreases. Substitution of OEt or OSiMe3 by methyl
groups increases competition to metathesis β-SiR3 elimination
in β-SiR3-substituted ruthenacyclobutanes followed by reduc-
tive elimination and formation of allylsilane as the main
product. Such a transformation reduces significantly the
number of species active in metathesis and slows down or
almost completely inhibits the formation of ethenylsilanes.14,15
Palladium-catalyzed Heck reactions of vinylsilanes with
olefins are other routes to silyl-substituted ethenes. This
coupling protocol, which has found many applications in
organic synthesis and industry, can be applied to any kind of
alkene, but for vinylsilanes it is quite problematic. When typical
Heck conditions were applied to the coupling of vinylsilanes
with aryl or alkenyl iodides, the main products were styrene
derivatives instead of silyl-substituted alkenes. Silicon−carbon
bond cleavage, in the presence of Pd complexes, is responsible
for the formation of undesirable products. This effect was
suppressed by the addition of silver salts (i.e., AgNO3), but the
parallel formation of biaryls made them the major product.
Also, the addition of strong bases and other components
necessary for the proper course of the process might be
responsible for the cleavage of the bonds to the silicon
atom.16−18
molar excess of styrene (and 1.5 molar excess of vinylboronate)
for each vinyl group of tris(dimethylvinylsilyl)borate (I) is
sufficient for total conversion of the substrate. Such an excess
causes no problem during the isolation of the products 1−7.
The best results were obtained when 2 mol % of the ruthenium
catalyst in relation to I was used. A lower concentration of the
catalyst extends the time necessary for total conversion of the
substrate. When 3 mol % of ruthenium complex was used in the
reaction of I with vinyloboronate, the presence of bisbor-
ylethene was detected in the reaction mixture. This is a product
of a competitive homocoupling reaction (trans-borylation), in
which the C−B bond in one molecule and the C−H bond
in another vinylboronate molecule are catalytically activated. It
was also shown that the reactions should be carried out for 24 h
if a high yield of the products is to be expected.19
In the reactions with vinylsilanes, a 2- or 3-fold excess of
vinylsilane for each vinyl group in boron siloxide I is required to
obtain a desirable product, but difficulties in the process
selectivity arise (Table 2). When vinylsilane is used as a reagent,
two parallel reactions of homocoupling of vinylsilane (eq 3)
and its cross-coupling with tris(dimethylvinylsilyl)borate (I) are
observed.
RESULTS AND DISCUSSION
■
In this paper we present the application of the silylative
coupling protocol for the stereo- and regioselective synthesis of
branched products with a boron atom in the core and π-
conjugated substituents or functional organometallic groups at
the ends of branches (eq 2).
The formation of homocoupling products follows from two
reasons. When two different vinyl-substituted silyl molecules
are used in the reaction, a variety of silylative coupling products
(cross-coupling as well as homocoupling) is possible. The
excess of another vinylsilane molecule is necessary for the
complete conversion of all vinyl groups in tris-
(dimethylvinylsilyl)boronate, and therefore homocoupling
products are also observed in the reaction mixture, often as a
major product.
On the other hand, the occurrence of homocoupling
products is related to the fact that the smaller vinylsilane
molecule more readily undergoes insertion into the ruthe-
nium−hydrogen bond than does the more sterically hindered
tris(dimethylvinylsilyl)borate (I). Subsequently the insertion of
the second vinylsilane molecule into the Ru−Si bond occurs
more quickly and bissilylethene is observed as a main product.
The chemical similarity of both coupling productsbissilyle-
thene and boronateis responsible for difficulties in the
isolation of silyl-substituted boron siloxide products by column
chromatography (only one product was isolated with high
purity, 8). The presence of vinylmetalloid compounds (vinyl-
silane, vinylboronate, or vinylgermane) is essential for the
coupling procedure, because the formation of the TM−E bond
(where E = Si, B, Ge) is required for such catalysis, and
therefore the reaction between two organic terminal olefins
does not occur.
All possible catalytic routes that occurred in the Ru−H-
catalyzed coupling of tris(dimethylvinylsilyl)borate (I) with a
variety of vinyl compounds are presented in Scheme 2.
Pathway A shows the cross-coupling of I with organic olefins
or vinylboronates. In this process the insertion of I begins the
catalytic cycle and no side reactions are observed under
optimized reaction conditions. When vinylsilanes are used as
the reagents, the competitive parallel homocoupling reaction of
vinylsilanes occurs, which often becomes the main catalytic
A spectrum of olefins was tested for the functionalization of
tris(dimethylvinylsilyl)borate (I), including styrene, styrenes
substituted at the para position, 9-vinylanthracene, 4-vinyl-
biphenyl, 2-vinyl-1,3,2-dioxaborinane, and a broad range of
vinylsilanes. Scheme 1 shows the number of isolated
trisubstituted products, which were characterized by NMR
spectra and elemental analysis.
The coupling reaction was examined in the presence of
complexes containing the Ru−H bond, particularly [Ru(CO)-
ClH(PCy3)2]. This five-coordinate ruthenium complex is
known as the most effective catalyst in silylative coupling
reactions.6 The influence of reaction conditions (temperature,
molar ratio of reagents, and concentration of catalyst) on the
conversion of I with select olefins (styrene and vinylboronate)
was investigated. The reactions were carried out in toluene or
1,4-dioxane at elevated temperatures.19 The optimized results
of the efficient silylative coupling of tris(dimethylvinylsilyl)-
borate (I) with styrene, substituted styrenes as well as other
highly π-conjugated olefins, and vinylboronate are compiled in
Table 1.
The yield of desired products is influenced by the molar ratio
of the reagents. It has been proved that for olefins only a 1.1
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Scheme 1. Isolated Functionalized Borates Synthesized via a Silylative Coupling Reaction
Table 1. Isolated Yields of Starburst Compounds with a
Table 2. Silylative Coupling of
Boron Siloxide Core and a Trans Configuration of the CC
Tris(dimethylvinylsilyl)borate (I) with Vinylsilanes
a
Bond
conversn of
b
conversn of selectivity of CC/HC
b
cd
,
no.
SiR3
SiEt3
silane (%)
I (%)
products
e
1
2
3
95
97
94
91 (8)
1:1.5
1:1.6
1:1.4
SiMe2Ph
94
Si(OSiMe3)3
97
a
Reaction conditions: [Ru]:[(I)]:[vinylsilane] = (2 × 10−2):1:6;
argon; open system; T = 80 °C; t = 24 h; [Ru(CO)ClH(PCy3)2];
b
c
d
toluene. Determined by GC. Determined by GC and GC-MS. CC
= cross-coupling product; HC = homocoupling product. The
e
numbering of isolated products described in the Experimental Section
is given in parentheses.
process (pathway C). The formation of a desirable starburst
product is still possible (pathway B), but the process is much
less selective in comparison to the reaction with organic olefins
(pathway A). The scheme, for simplicity, presents only the
coupling of one of the vinylsilyl groups in tris-
(dimethylvinylsilyl)borate (I), but the following coupling with
the remaining groups occurs analogously.
Most products with functional groups at the end of the
branch of the starburst compound (see Scheme 1, products 1−
8) with a boron atom in the core were isolated and purified by
liquid chromatography, and their structures were confirmed
using NMR spectroscopy (1H, 13C, 29Si, 11B NMR). The
chemical shifts in 11B NMR spectra for boron bonded to three
a
Reaction conditions: [Ru]:[I] = (2 × 10−2):1; argon; open system; T
b
= 80 °C; t = 24 h; [Ru(CO)ClH(PCy3)2]; toluene. All isolated
c
products have an E configuration at the double CC bond. The
numbering of isolated products described in the Experimental Section
is given in parentheses.
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Scheme 2. Proposed Mechanism of the Ruthenium-Catalyzed Coupling of Tris(dimethylvinylsilyl)borate (I) with Organic
Olefins, Vinylsilanes, and Vinylboronate
oxygen atoms are in the typical range (∼27 ppm) for all
products 1−7 and are similar to the signal due to
tris(vinyldimethylsilyl)borate. In case of product 7, the
borinane group has a characteristic shift for a boron atom
attached to ethenyl and two alkoxy groups (31.7 ppm). Signals
in the 29Si NMR spectra in tris(dimethylvinylsilyl)borate as well
as in the products have positive shift values due to the presence
of a boron atom close to a silyl group, which causes changes in
the shielding of the silicon nucleus. The J(H−H) coupling
constant (approximately 19−20 Hz) for protons connected to
functionalized borates can be potentially used as precursors for
new highly branched materials with unique mechanical,
thermal, and optoelectronic properties. The physicochemical
properties and reactivity of the products are now under
investigation.
EXPERIMENTAL SECTION
■
1
General Considerations. H NMR (300 MHz), 13C NMR (75
MHz), 11B NMR (96 MHz), and 29Si NMR (79 MHz) spectra were
recorded on a Varian XL 300 MHz spectrometer in CDCl3 solution.
Chemical shifts are reported in ppm with reference to the residual
1
sp2 carbon atoms shows exactly in the H NMR spectrum that
1
solvent (CH3Cl) peak for H and 13C, to BF3·Et2O for 11B, and to
only E isomers are obtained during the coupling process under
select reaction conditions. This confirms that the chosen
method distinguishes high regio- and stereoselectivity and is a
clean way (only gaseous ethylene is obtained as a byproduct) to
functionalize boron siloxide. The preferential formation of the
E isomer can be explained by the higher thermodynamic
stability of this isomer, which is formed exclusively in the
process when elevated temperatures are applied. It is
commonly known that at high temperatures Z isomers often
undergo thermal isomerization to the more stable E isomer. As
was already mentioned, the regio- and stereoselectivity of some
of the silylative coupling procedures (i.e., with vinylboronates)
are temperature dependent and different isomers can be
obtained by controlling the process parameters. The steric
hindrance of the substituents at the silicon atom and the type of
olefin can also influence the formation of a specific stereo-
isomer. The alternative metathesis route cannot be used due to
deactivation of ruthenium alkylidene Grubbs catalysts, when
methyl groups are directly attached to the silicon atom.14,15 The
Heck coupling with the corresponding aryl iodides can also be
problematic because of the necessity of using the base and
additives that on one hand accelerate the reaction but on the
other hand can cleave the B−O−Si bond.
TMS for 29Si. Analytical gas chromatographic (GC) analyses were
performed on a Varian Star 400CX instrument with a DB-5 fused silica
capillary column (30 m × 0.15 mm) and TCD. Mass spectra were
obtained by GC-MS analysis (VarianSaturn 2100T instrument,
equipped with a BD-5 capillary column (30 m) and an ion trap
detector). Elemental analyses were carried out on a Vario EL III
instrument. Thin-layer chromatography (TLC) was carried out on
plates coated with 250 μm thick silica gel (Aldrich and Merck), and
the column chromatography was performed with silica gel 60 (70−230
mesh; Fluka). Toluene and 1,4-dioxane were dried by distillation from
sodium, and hexane was dried by distillation from sodium hydride.
Liquid substrates were also dried and degassed by bulb-to-bulb
distillation. The reactions were carried out under a dry argon
atmosphere. The chemicals were obtained from the following sources:
toluene, 1,4-dioxane, dodecane, hexane, benzene-d6, chloroform-d,
styrene, 4-chlorostyrene, 4-bromostyrene, 4-methoxystyrene, 9-vinyl-
anthracene and 4-vinylbiphenyl from Aldrich, ethyl acetate from
ChemPur, tris(dimethylvinylsilyl)borate (I)20 from ABCR GmbH &
Co.KG. 2-Vinyl-1,3-dioxaborinane was synthesized according to the
literature procedure with some modifications.21,22 The ruthenium
complex [Ru(CO)ClH(PCy3)2] (A)23 was prepared according to
literature procedures.
General Procedure for the Silylative Coupling of Tris-
(dimethylvinylsilyl)borate (I). In a typical test, the ruthenium
catalyst [Ru(CO)ClH(PCy3)2)] (A; 2 mol %) was dissolved in toluene
or 1,4-dioxane and placed in a glass vessel under argon. The reagents
and dodecane as internal standard (5% by volume all components)
used in the appropriate molar ratios (see Tables 1 and 2) were added.
The reaction was conducted in an open system, under argon.
Subsequently, the vessel was heated at 80 °C and maintained at this
temperature for 6−48 h. The progress of the reaction was monitored
by GC (GC-MS). The conversion of reagents and chemoselectivity of
the reactions was calculated by using the internal standard method.
After the reaction, the crude product was purified by silica gel modified
with 5% of HMDS column chromatography with hexane/ethyl acetate
CONCLUSIONS
■
A highly selective method for modification of tris-
(dimethylvinylsilyl)borate (I) with styrenes and other π-
conjugated olefins is presented. The reactions take place with
high regio- and stereoselectivitythe trans product is observed
exclusively. For vinylsilanes, due to their high reactivity, two
parallel reactions of homocoupling of vinylsilanes and their
cross-coupling with I are observed. Olefin-, boroxine-, or silyl-
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1
(1/1) as eluent, and the structure of the product was proved by H,
13C, 11B, and 29Si NMR and elemental analysis.
CH3SiCHCH, JHH = 19.8 Hz), 6.53 (3H, s, CH3SiCHCH, JHH
=
20 Hz). 13C NMR (75 MHz, CDCl3): δ 0.15 (Si(CH3)2), 3.2
(SiCH2CH3), 7.3 (SiCH2CH3), 140.5 (Et3SiCHCH), 148.8
(Et3SiCHCH). 11B NMR (96 MHz, CDCl3): δ 27.2. 29Si NMR
(79 MHz, CDCl3): δ 4.1, 6.2. Anal. Calcd for C30H69BO3Si6: C, 54.83;
H, 10.58. Found: C, 55.10; H, 10.73.
Spectroscopic Data of Isolated Products. Tris-
1
(dimethylstyrylsilyl)borate (1). Isolated yield: 88%. H NMR (300
MHz, CDCl3): δ 0.31 (18H, s, Si(CH3)2), 6.46 (3H, d,
(CH3)2SiCHCHPh, JHH = 19.5 Hz), 7.02 (3H, d, (CH3)2SiCH
CHPh, JHH = 19.3 Hz), 7.27−7.40 (9H, m, C6H5), 7.44−7.47 (6H, m,
C6H5). 13C NMR (75 MHz, CDCl3): δ 0.2 (Si(CH3)2), 126.5 (C6H5),
127.3 (C6H5), 128.4 (C6H5), 137.7 (C6H5), 140.5 ((CH3)2SiCH
CHPh), 145.0 ((CH3)2SiCHCHPh). 11B NMR (96 MHz, CDCl3):
δ 26.9. 29Si NMR (79 MHz, CDCl3): δ 2.1. Anal. Calcd for
C30H39BO3Si3: C, 66.39; H, 7.24. Found: C, 66.57; H, 7.30.
Tris((4-chlorostyryl)silyldimethyl)borate (2). Isolated yield: 81%.
1H NMR (300 MHz, CDCl3): δ 0.30 (18H, s, Si(CH3)2), 6.42 (3H, d,
(CH3)2SiCHCH, JHH = 19.2 Hz), 6.95 (3H, d, (CH3)2SiCHCH,
JHH = 19.2 Hz), 7.29−7.39 (12H, m, C6H4Cl). 13C NMR (75 MHz,
CDCl3): δ 0.18 (Si(CH3)2), 127.7 (C6H4Cl), 128.6 (C6H4Cl), 133.9
(C6H4Cl), 136.2 (C6H4Cl), 128.2 ((CH3)2SiCHCH), 143.6
((CH3)2SiCHCH). 11B NMR (96 MHz, CDCl3): δ 28.6. 29Si
NMR (79 MHz, CDCl3): δ 7.3 . Anal. Calcd for C30H36BCl3O3Si3: C,
55.77; H, 5.72. Found: C, 55.67; H, 5.74.
Tris((4-bromostyryl)dimethylsilyl)borate (3). Isolated yield: 84%.
1H NMR (300 MHz, CDCl3): δ 0.30 (18H, s, Si(CH3)2), 6.43 (3H, d,
(CH3)2SiCHCH, JHH = 19.3 Hz), 6.91 (3H, d, (CH3)2SiCHCH,
JHH = 19.5 Hz), 7.19−7.47 (12H, m, C6H4Br). 13C NMR (75 MHz,
CDCl3): δ 0.17 (Si(CH3)2), 128.0 (C6H4Br), 128.4 (C6H4Br), 131.6
(C6H4Cl), 136.8 (C6H4Br), 143.2 ((CH3)2SiCHCH), 143.7
((CH3)2SiCHCH). 11B NMR (96 MHz, CDCl3): δ 26.9. 29Si
NMR (79 MHz, CDCl3): δ 7.2. Anal. Calcd for C30H36BBr3O3Si3: C,
46.23; H, 4.66. Found: C, 46.19; H, 4.60.
ASSOCIATED CONTENT
■
S
* Supporting Information
A table detailing the optimization of the silylative coupling
reaction of tris(dimethylvinylsilyl)borate (I) with styrene and
2-vinyl-1,3,2-dioxaborinane by the influence of the process
temperature, time, molar ratio of reagents, and catalyst on the
substrate conversion and product selectivity. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: marcinb@amu.edu.pl. Fax: +48 (61) 82 91 508.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Ministry of Science and Higher
Education of Poland (Grant NN 204265538) is gratefully
acknowledged.
Tris((4-methoxystyryl)dimethylsilyl)borate (4). Isolated yield: 82%.
1H NMR (CDCl3, 300 MHz): δ 0.34 (18H, s, Si(CH3)2), 3.82 (9H, s,
OCH3), 6.45 (3H, d, (CH3)2SiCHCH, JHH = 19.7 Hz), 6.95 (3H, d,
(CH3)2SiCHCH, JHH = 19.5 Hz), 7.35−7.48 (4H, m, C6H4OMe).
13C NMR (CDCl3, 75 MHz): δ 0.18 (Si(CH3)2), 55.2 (OCH3), 113.8
(C6H4), 127.3 (C6H4), 130.4 (C6H4), 136.2 (CHCH), 143.5 (CH
CH), 159.2 (C6H4). 11B NMR (96 MHz, CDCl3): δ 27.4. 29Si NMR
(79 MHz, CDCl3): δ 7.0. Anal. Calcd for C33H45BO6Si3: C, 62.64; H,
7.17. Found: C, 62.10; H, 7.34.
REFERENCES
■
(1) Newkome, G. R.; Moorefield, C, N.; Vogtle, F. Dendrimers and
Dendrons: Concepts, Syntheses, Applications; Wiley-VCH: Weinheim,
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Tris(((E)-2-(anthracen-9-yl)vinyl)dimethylsilyl)borate (5). Isolated
1
yield: 73%. H NMR (300 MHz, CDCl3): δ 0.24 (18H, s, Si(CH3)2),
6.44 (d, 3H, SiCHCH, JHH = 19.4 Hz), 7.52 (d, 3H, SiCHCH,
JHH = 19.4 Hz), 7.49−7.56 (m, 9H, anthracene), 8.00−8.04 (m, 3H,
anthracene), 8.35−8.41 (m, 6H, anthracene). 13C NMR (75 MHz,
CDCl3): δ 0.3 (Si(CH3)2), 130.2 (SiCHCH), 127.2, 127.6, 128.0,
128.4, 132.1, 136.1, 139.0 (anthracene), 146.2 (SiCHCH). 11B
NMR (96 MHz, CDCl3): δ 27.1. 29Si NMR (79 MHz, CDCl3): δ 5.4.
Anal. Calcd for C54H51BO3Si3: C, 76.93; H, 6.10. Found: C, 76.66; H,
6.31.
Tris(((E)-2-(biphenyl-4-yl)ethenyl)dimethylsilyl)borate (6). Isolated
yield: 77%. 1H NMR (300 MHz, CDCl3): δ 0.49 (18H, s, (Si(CH3)2),
6.78 (d, 3H, SiCHCH, JHH = 19.3 Hz), 7.70 (d, 3H, SiCHCH,
JHH = 19.3 Hz), 7.35−7.90 (m, 27H, C6H4−Ph). 13C NMR (75 MHz,
CDCl3): δ 0.16 (Si(CH3)2), 128.4 (SiCHCH), 127.1, 128.2, 128.7,
129.0, 136.74, 140.5, 141.2 (C6H4-C6H5), 144.1 (SiCHCH). 11B
NMR (96 MHz, CDCl3): δ 27.0. 29Si NMR (79 MHz, CDCl3): δ 4.8.
Anal. Calcd for C48H51BO3Si3: C, 74.78; H, 6.67. Found: C, 74.55; H,
6.89.
Tris(((E)-2-(1,3,2-dioxaborinan-2-yl)ethenyl)dimethylsilyl)borate
(7). Isolated yield: 76%. 1H NMR (300 MHz, CDCl3): δ 0.18 (18H, s,
Si(CH3)2), 1.86 (6H, br, BOCH2CH2), 3.99 (t, BOCH2), 6.23 (3H, d,
(CH3)2SiCHCH, JHH = 17.8 Hz), 6.51 (3H, d, (CH3)2SiCHCH,
JHH = 19.3 Hz). 13C NMR (75 MHz, CDCl3): δ 0.24 (Si(CH3)2), 27.2
(BOCH2CH2), 62.7 (BOCH2CH2), 139.9 (SiCHCH). 11B NMR
(96 MHz, CDCl3): δ 27.1, 31.6. 29Si NMR (79 MHz, CDCl3): δ 7.9.
Anal. Calcd for C21H42B4O9Si3: C, 44.56; H, 7.48. Found: C, 44,68; H,
7.56.
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1
yield: 63%. H NMR (300 MHz, CDCl3): δ 0.23 (18H, s, Si(CH3)2),
0.57 (q, 18H, SiCH2CH3), 0.92 (t, 27H, SiCH2CH3), 6.31 (3H, d,
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