Dehydrogenative coupling of styrene with trisubstituted silanes catalyzed by nickel complexes

Article abstract of DOI:10.1016/S1381-1169(97)00310-5

Trisubstituted silanes, e.g., Me,.(EtO)(3-n)SiH (where n = 0-2) and Me₂PhSiH in the presence of nickel complexes, e.g., [Ni(acac)₂] and [Ni(cod)₂], undergo two reactions of dehydrogenative silylation of styrene to yield in

Full text of DOI:10.1016/S1381-1169(97)00310-5

Ž
.
Journal of Molecular Catalysis A: Chemical 135 1998 223–231
Dehydrogenative coupling of styrene with trisubstituted silanes
1
catalyzed by nickel complexes
)
Bogdan Marciniec , Hieronim Maciejewski, Ireneusz Kownacki
Department of Organometallic Chemistry, Adam Mickiewicz UniÕersity, Grunwaldzka 6, 60-780 Poznan, Poland
´
Received 2 September 1997; accepted 27 November 1997
Abstract
Ž
.
Ž
.
Trisubstituted silanes, e.g., Men EtO 3ynSiH where ns0–2 and Me2 PhSiH in the presence of nickel complexes, e.g.,
w
Ž
. x . x
w Ž
Ni acac and Ni cod , undergo two reactions of dehydrogenative silylation of styrene to yield in both cases an
2
2
unsaturated product—E-1-phenyl-2-silyl-ethene as well as products of styrene hydrogenation—ethylbenzene DS-1 and of
hydrogenative dimerization of styrene—1,3-diphenylbutane—DS-2. The two reactions are accompanied by the hydrosilyla-
tion products H as well as redistribution of the silanes containing at least one ethoxy substituent. The catalytic examinations
Ž .
Ž .
and identification of nickel square planar complexes suggest that the intermediates containing Ni–Si I , Ni–H II and Ni–C
Ž
.
III bonds are responsible for the catalysis. q 1998 Elsevier Science B.V. All rights reserved.
Keywords: Dehydrogenative coupling; Silylation; Styrene; Nickel-complex catalyst
1. Introduction
The dehydrogenative coupling of hydrosilanes with olefins proceeds via two pathways yielding
Ž
.
Ž
.
unsaturated product vinylsilane and hydrogen dehydrocondensation andror a hydrogenated olefin
Ž
.
Ž
Ž ..
dehydrogenative silylation as shown below Eq. 1 , and is usually accompanied by hydrosilylation
w x
products 2 ;
Ž .
1
Complexes of iron- and cobalt-triad members—i.e., Fe, Ru, Os and Co, Rh, Ir—appeared to be
w x
extremely favourable catalysts for the heterocoupling of hydrosilanes with olefins 3 .
Ž
.
However, the number of examples of exclusive or highly selective formation of vinylsilanes is
w
Ž
.
x w
Ž
.
x w
x
w x
still limited. Ru3 CO , Fe3 CO
4–6 and cationic rhodium complexes 7 catalyzed dehydro-
12
12
)
Corresponding author.
Part XXXII in the series ‘Catalysis of Hydrosilylation’, for Part XXXI see Ref. 1 .
1
w x
1381-1169r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S1381-1169 97 00310-5

(
)
224
B. Marciniec et al.rJournal of Molecular Catalysis A: Chemical 135 1998 223–231
w x
w
Ž
.
x w x
9 and
genative silylation of styrene and of vinylsilane with HSiEt3 8 as well as Fe CO
5
w
Ž
. Ž
. x w x
particularly RuH2 H2 PCy3 2 10 catalyzed reaction of ethylene with HSiEt3 are the most
2
convincing examples.
w
x
w
x
w
x
Pt 11 , Pd 12 and Ni 13,14 complexes are exceptionally applied for catalysis of dehydrogenative
coupling as the main direction of the complex reactions. Also, the early transition metal compounds
w
x
have recently been reported as efficient catalysts of this reaction 15,16 .
Our recent contribution to the field is the first successful attempt of nickel 0 and nickel II
catalyzed dehydrogenative silylation of vinylsubstituted silanes by triethoxysilane and triethylsilane.
Ž .
Ž .
w
Ž
.
x w
Ž
.
2 x
In the excess of vinylsilanes over triethoxysilane, the reaction catalyzed by Ni acac , Ni cod
2
and other Ni-complexes yields in the temperature range 80–1208C predominantly products of
w
x
dehydrogenative silylation according to the following equations 13,14,17 :
Ž .
2
The latter reaction is quite a new pathway of dehydrogenative silylation leading to unsaturated
product and the product of hydrogenative dimerization of vinylsilane.
Most side reactions such as regular hydrosilylation, redistribution of hydrosilanes and reactions of
unsaturated products with hydrosilanes can be practically excluded under the optimum conditions
w
x
Ž
.
2
17 . Separate study on the reaction of Ni acac with triethoxysilane in the presence of PPh3
w
Ž
.
Ž
.x Ž . w x
permitted isolation of Ni acac Et PPh3 A 18 which appeared to be an intermediate in the
w
Ž
.
x
Ž .
Ni acac qPPh3 catalyzed reaction. The A is very active at room temperature, but only after
2
w
x
preliminary oxygenation of the coordinated triphenylphosphine 19 .
Ž .
All the stoichiometric and catalytic reactions of A with substrates allow us to propose the
catalytic cycle involving competitive insertion of vinylsilane molecule into Ni–Si, Ni–H as well as
w
x
Ni–C bonds 19 .
The aim of this work is to examine the catalytic systems based on Ni acac and Ni cod in the
w
Ž
.
x
w
Ž
. x
2
2
reaction of trisubstituted silanes, mostly containing at least one alkoxy substituent, with styrene in
order to find products of dehydrogenative silylation.
2. Results and discussion
The reaction of styrene with trisubstituted silanes containing at least one alkoxy group at silicon,
Ž
.
Ž .
Ž .
e.g., HSi OEt , in the presence of Ni 0 and Ni II complexes in the range 20–1208C occurs
3
according to the following equations;
Ž .
3

(
)
B. Marciniec et al.rJournal of Molecular Catalysis A: Chemical 135 1998 223–231
225
Ž .
The organosilicon products are numbered. Besides, tetrasubstituted silanes SiR4 5 and siloxanes are
also observed as products of the HSiR3 disproportionation as well as oxygenation and hydrolysis of
the [Si–O–C– bond, followed by further condensation.
Thus, two reactions of dehydrogenative silylation DS-1 and DS-2 compete with regular hydrosily-
Ž .
lation H. E-1-phenyl-2-silyl-ethene 2 , the main product yielded, comes from the two reactions—DS-1
Ž .
and DS-2. The amount of 2 was almost equal to a sum of the second products of the reactions DS-1
and DS-2, i.e., ethylbenzene and 1,3-diphenylbutane. The latter is an exclusive isomer formed by
hydrogenative dimerization of styrene. Our previous experience in the reaction of vinylsilane with
Ž
.
hydrosilanes indicated it was worth using triethoxysilane 1a as a initial substance.
It could be expected that the kind of the nickel precursor as well as the ratio of styrene and
triethoxysilane affected markedly the selectivity of the above mentioned reactions as well as, finally,
their products. The effect is summarized in Table 1.
The hydrogenative silylation pathway DS dominates over the hydrosilylation one H only in the
w
Ž
.
x
w
Ž
. x
presence of Ni acac and Ni cod as the initial catalysts. The excess of styrene leads not only to
2
2
Ž
.
an increase in the total conversion of triethoxysilane 1a but obviously, to increase in the yield of the
Ž
.
Ž
.
x
Ž
.
unsaturated product 2a , and also the ratio of 2a to the hydrosilylation products 3aq4a . Under
w
x w
the optimum conditions, i.e., when SiH : styrene s1:2™1:3 the dehydrogenative silylation takes
o
o
place along both pathways DS-1 and DS-2. The amounts of ethylbenzene and 1,3-diphenylbutane as
Ž
Ž ..
the second products of the DS-1 and DS-2 pathways, respectively, Eq. 3 are also proportionally
Ž
.
w
Ž
.
x
w
x
increased see Section 3 . NiI2 PPh3 2 and NiCp2 are more favourable catalysts of the hydrosilyla-
tion H and the redistribution of 1a than the dehydrogenative silylation DS. In the presence of nickel
Ž
.
w
Ž
.
x
w
Ž
. Ž
.
x
w
Ž
. Ž
. x
phosphine complexes such as NiCl2 PPh3 2
,
Ni SCN PPh3 2
,
Ni NO3 2 PPh3 2 and
2
w
Ž
. Ž
. x
Ni CO PPh3 2 only products of styrene polymerization and the redistribution of silane are
2
observed.
w
Ž
.
2 x
w
Ž
.2 x
The results of the catalysis by Ni acac
and Ni cod clearly show that styrene acts as a
hydrogen acceptor. The dehydrogenative silylation pathways DS-1 and DS-2 are competitive with the
hydrosilylation. Thus, we have focused on developing a highly selective dehydrogenative silylation as
well as increased total yield of the reaction. The selectivity is slightly sensitive to the reaction
Ž
.
temperature Table 2 . However, the reaction occurs effectively even at room temperature although it
is accompanied not only by hydrosilylation but also by silane redistribution products.
Table 1
Effect of the kind of nickel catalyst and the molar ratio of substrates on competitive dehydrogenative silylation and hydrosilylation of
styrene with triethoxysilane
Yielda
%
Product distributionb
%
Ž .
Ž
.
Ž
.
Catalyst
Styrene mmol
2a
3a
4a
5a
3.5
6
2
6.5
30
6.5
6.5
2
w
Ž
. x
Ni acac
6
12
18
6
6
6
12
18
6
55
80
78
80
59
52
76
84
69
74
76
81
51
19
70
79
82
19
14
8
9
31
42
14
8
3.5
2
2
4.5
0
4.5
2
2
w
w
Ž
.2 x
NiI2 PPh3
. x
Ž
Ni cod
2
9
25
2
0
w
Ž
. x
Ni Cp
44
2
y3
w
x w
x
w
Ž
. x
3
Reaction conditions: 1208C, 1 h, argon, without solvent, HSi[ : Ni s1:5=10 , HSi OEt s6 mmole.
a
Ž
.
The yield is calculated from the HSi OEt consumed, determined by GLC.
3
b For products notation see text.

(
)
226
B. Marciniec et al.rJournal of Molecular Catalysis A: Chemical 135 1998 223–231
Table 2
The effect of temperature on the yield and selectivity of the nickel-complex catalyzed dehydrogenative silylation of styrene with
triethoxysilane
Ž
.
Ž .
Product distribution %
Conditions
Yield %
2a
3a
4a
5a
w
Ž
. x
Ni acac
2
1208C, 4 h
808C, 4 h
808C, 6 h
RT, 100 h
RT, 100 ha
Reflux, 1 h
92
86
90
0
74
66
77
80
82
0
64
80
12
11
12
0
12
7
1
1.5
0
0
2.5
5
6
3
3.5
0
16
3.5
w
Ž
. x
Ni cod
2
1208C, 4 h
808C, 4 h
808C, 6 h
RT, 50 h
83
64
65
32
8
83
83
83
83
53
91
83
10
1.5
1.5
1.5
1.5
4.5
3.5
6
5
2
2.5
9
25
9.5
9.5
4
12
4.5
7
RT, 50 ha
Reflux, 1 h
Reflux, 2 h
60
67
1
5
Reaction conditions: HSi[ : styrene : Ni s1:2:5=10y3, under argon.
x w
w
x w x
aw
x w
x
HSi[ : styrene s1:1.
It is well known that the reactivity of the Si–H bond in the competitive silylation and hydrosilyla-
w x
tion reactions as well as their selectivity can be influenced by substituents at the silicon atom 2 .
Therefore, some trisubstituted silanes were used in the reaction studied in the presence of both
w
Ž
.
x
w
Ž
. x
catalysts Ni acac and Ni cod . Results are summarized in Table 3.
2
2
w
Ž
. x
2
Under the conditions studied, triethylsilane causes a reduction of Ni acac and decomposition of
w
Ž
. x
Ni cod to give metallic nickel, which does not catalyze the hydrosilylation andror dehydrogena-
2
Ž
.
tive silylation. The rest of silanes include at least one electronwithdrawing substituent EtO and Ph at
Ž .
the silicon. While in the presence of Ni 0 complex there is no change in catalytic activity and
Table 3
Ž
.
The effect of substituents at silicon on the nickel-complex catalyzed dehydrogenative silylation of styrene with trisubstituted silanes R3SiH
Ž
.
Ž .
Product distribution %
R3
Yield %
2a
3a
4a
5a
w
Ž
Et3
Me2 EtO
Me EtO
EtO
Me2 Ph
. x
Ni acac
2
0
10
35
80
41
0
18
92
76
71
0
0
7
8
14
0
0
1
2
15
0
40
0
6
0
Ž
.
Ž
.
2
Ž
.
3
w
Ž
Et3
Me2 EtO
Me EtO
EtO
Me2 Ph
. x
2
Ni cod
0
71
76
76
65
0
83
85
79
84
0
8
8
8
9
0
1
1
2
7
0
4
3.5
6.5
0
Ž
.
Ž
.
2
Ž
.
3
Reaction conditions: 1208C; 1 h; HSi[ : styrene : Ni s1:2:5=10y3, under argon.
x w
w
x w x

(
)
B. Marciniec et al.rJournal of Molecular Catalysis A: Chemical 135 1998 223–231
227
Ž
.
selectivity of products for all trisubstituted silanes except Et3SiH , the total yield of DS and H in
w
Ž
.
x
Ž
Ni acac catalyzed reaction strongly depends on the presence of electronegative substituents e.g.,
2
.
EtO at the silicon.
Lowering the number of EtO group in the hydrosilane implies a decrease also in their yield in the
X
X
Ž
reaction with styrene. Replacement of ethoxy on phenyl group in Me2R SiH where R sEtO andror
.
w
Ž
. x
Ph activates particularly Ni acac precursor in the reaction examined.
Earlier examinations of Ni acac –HSi OEt –PPh3 system, allowed an isolation and identifica-
tion of the square planar red complex Ni acac PPh3 Et A being an active intermediate in
2
w
Ž
.
x
Ž
w
w
.
2
3
Ž
x
.Ž
. x Ž .
Ž
.
.
dehydrogenative silylation of vinylsilanes 19 after prior oxygenation of PPh3 .
Ž
Regardless of the method used for preparation of complex A A1 or A2 appeared to be an
intermediate also in the reaction of trisubstituted silanes containing alkoxy group with styrene, but
Ž
.
only after elimination of phosphine by its oxygenation Table 4 .
So, similarly to the mechanism of dehydrogenative silylation of vinylsilanes under catalytic
Ž
.
Ž .
conditions of the reaction of tri methyl, ethoxy silanes with styrene, the following complexes I and
Ž . Ž
Ž .
Ž ..
II Eq. 4 can be generated which are directly responsible for the catalytic cycle postulated in Eq.
6 .
Ž .
4
w
Ž
.
x
Contrary to silylation of vinylsilanes catalyzed by Ni acac , such a reaction of trisubstituted
2
silanes with styrene occurs also at room temperature. Apparently a formation of the square planar Ni
Ž .
Ž
.
complex of the A andror I, II type structure can be generated and this complex can remain in
Ž
solution even in the absence of PPh3. So, such system can be active even at room temperature in
.
argon .
w
Ž
.
x
Another difference between silylation of vinylsilane and styrene in the presence of Ni acac is
the inactivity of dimethylphenylsilane with the former and its relatively mild activity with latter one
2
Ž
.
w
Ž
. x
Table 3 . Direct formation of Ni–Si intermediates in the reaction of Ni acac with HSiR3 seems to
2
Table 4
The catalytic activity of A1 and A2 in the dehydrogenative silylation of styrene with triethoxysilane
Ž .
Time h
Ž
.
w x
Product distribution %
Catalyst
Yield %
2a
3a
4a
5a
Siloxanes
A1
1a
22b
0
20
32
51
0
0
50
65
72
0
0
4.5
5
5
0
0
6
8
8
0
0
14.5
12.5
10
0
0
9
8
8
0
29
17
11
0
1.5
2.0
2
0
0
3
4
4
0
44b
110b
110a
1a
A2
0
0
0
18b
17
25
36
0
64
68
71
0
18
12
9
50b
110b
110a
0
0
0
b
Reaction conditions: RT. aargon; air; HSi[ : styrene : Ni s1:2:5=10
.
y3
w
x w
x w x

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)
228
B. Marciniec et al.rJournal of Molecular Catalysis A: Chemical 135 1998 223–231
be a key step for all the reactions examined, at least for the silanes containing no alkoxy groups and
occurring in the absence of PPh3. It can proceed according to the following equation:
Ž .
5
Ž
.
Elimination of MeCOCH2CH OSiR3 Me is a result of protonation of one acetylacetone ligand by
w
x
trisubstituted silane followed by its fast hydrosilylation as was found earlier 18 .
Ž .
Ž .
The mechanism involving two types of intermediates, i.e., containing Ni–Si I and Ni–H II , is
Ž .
illustrated in Eq. 6 .
Ž .
6
Ž .
The insertion of styrene into the nickel silicon bond of
I
leads to a complex with
Ž
.
Ž
.
Ž .
a phenyl b silyl ethyl ligand bonded to nickel. Elimination of 1-phenyl-2-silylethene 2 —the
product of the dehydrogenative silylation—leads to generation of the active intermediate II .
Insertion of the styrene molecule into Ni–H bond gives complex containing s- phenyl ethyl III or
s- a-phenyl ethyl III ligands, respectively. The subsequent step involves an oxidative addition of
hydrosilane to form unstable complex IV andror IV , respectively, which can undergo elimination
of the hydrosilylation products 3 and 4 and to regenerate the hydride complex II . However, in the
reaction with styrene the favoured pathway for IV and for IV is a reductive elimination of the
ethylbenzene and regeneration of the Ni–Si intermediate I . In competition with oxidative addition of
Ž .
.
Ž
.
Ž
X
Ž
.
Ž
.
X
Ž
Ž .
.
Ž
.
Ž .
Ž .
X
Ž
.
Ž
.
Ž .
Ž
.
Si–H particularly in an excess of olefin insertion of styrene into Ni–C bond occurs to yield the
complex V and V starting from both III and III complexes, respectively and finally—after
reductive elimination of only one isomer 1,3-diphenylbutane to regenerate the active intermediate
X
Ž .
Ž
X. Ž
Ž
.
Ž
.
.
Ž
.
X
X
Ž .
Ž
.
Ž
.
Ž
.
Ž .
I . Pathway III ™ V seems to be more probably than III ™ V to yield corresponding
s-carbyl-metal intermediate, responsible for selective formation of 1,3-diphenylbutane. It is worth to

(
)
B. Marciniec et al.rJournal of Molecular Catalysis A: Chemical 135 1998 223–231
229
Ž
.
emphasize that this is a novel pathway of the dehydrogenative silylation of styrene olefin DS-2
occurring via competitive insertion of olefin into Ni–C bond giving the product of hydrogenation of
Ž
.
dimeric olefin e.g., butyl derivatives , beside the unsaturated product. Products of redistribution of
trisubstituted silanes containing at least one alkoxy group occur under the reaction conditions studied
competitively with main reactions. In oxygen atmosphere catalytic oxygenation of Si–H to Si–OH
and hydrolysis of the Si–O–C bond followed by further condensation give siloxanes observed mostly
w
x
as by-products. The scheme for their formation was presented earlier 19 .
3. Experimental
3.1. Materials
All reagents were dried and purified before use by the usual procedures. Triethoxysilane,
methyldiethoxysilane and dimethylethoxysilane were obtained by alcoholysis of SiHCl3, MeSiHCl2
and Me2SiHCl, respectively, according to the standard procedure. Alkoxysilanes were thoroughly
purified to avoid the presence of traces of HCl.
Ž
.
Ž .
w
x
Ž
.
Ž . w x
Bis 2,4-pentanodionato nickel II 20 and bis cyclooctadiene nickel 0 21 were prepared as
Ž
.
Ž .
described in the literature. Bis 2,4-pentanodionato nickel II was dehydrated by maintaining a
temperature 1008C while under vacuum for 24 h. Other chemicals were purchased as follows:
Ž
triethylsilane, dimethylphenylsilane, triphenylphosphine, styrene, triethylaluminium, tetrakis triphen-
.
Ž .
ylphosphine nickel 0 and nickelocene from Fluka; nickel trifluoroacetylacetonate from Strem Chemi-
cals. Benzene Fluka , n-pentane, cyclooctadiene Merck , acetylacetone and all of used solvents
Ž
.
Ž
.
Ž
.
Aldrich were purified by standard methods and distilled prior to use.
Ž
.Ž
.
Ž .
.
2,4-pentanedionato triphenylphosphine ethylnickel II was prepared by two methods: A1—A
solution of diethylaluminium monoethoxide 20 mmol in hexane was added dropwise to a stirred
mixture of nickel acetylacetonate 20 mmol and triphenylphosphine 20 mmol in 50 ml of ether
Ž
Ž
.
Ž
.
cooled to y308C. The temperature was allowed slowly to increase to room temperature and was
maintained there for 1 h. The mixture was filtered off and than placed in the freezer overnight. The
1
w
x
Ž
. Ž
.
Ž
.
reddish-yellow crystals were washed with hexane 22,23 . H NMR d, ppm C6D6 0.79 s, C2H5 ;
13
Ž
.
.
Ž
.
.
Ž
Ž
.
Ž
.
Ž
. Ž
.
.
1.40 s, CH3 ; 1.90 s, CH3 ; 5.27 s, CH ; 6.9–7.8 m, C6H5 . C NMR d, ppm C6D6 7.1
Ž
Ž
Ž
.
Ž
.
Ž
.
Ž
CH3 ; 15.0 –CH2– ; 27.0 CH3-acac ; 27.9 CH3-acac ; 100.9 CH-acac ; 189.6 CO ; 127–129
.
Ž
Ž
.
Ž
.
C6H5 . A2—To a stirred mixture, cooled to y308C under argon, of Ni acac 0.1 mmol and of
2
.
Ž
.
Ž
.
0.5 mmol was added. The solution was
PPh3 0.3 mmol in 10 ml of benzene, HSi OEt
3
concentrated by evaporation and yellow-red crystals were precipitated with pentane. Analytical data
showed that this solid contains a mixture of various complexes. Apart from the signals attributed to
1
w
Ž
.
Ž
.
x
w
Ž
.
Ž
.
Ž
.
Ž
Ni acac C2 H5 PPh3 in H NMR spectra 0.90 s, C2 H5 ; 1.32 s, CH3 ; 1.93 s, CH3 ; 5.35 s,
.
x
w
Ž
.
x w
Ž
.
Ž
.
Ž
.
x
w
Ž
. x
CH , the signals of Ni acac
6.8–7.8 m, C6H5 and in P NMR y608C y22 ppm and a few other unidentified signals were
1.18 s, CH3 ; 1.38 s, CH3 ; 5.30 s, CH and of Ni PPh3 3
2
31
w
Ž
.
Ž
.
x
observed.
3.2. Equipment and analytical measurements
1H NMR, 13C NMR and 31P NMR spectra were recorded on Varian XL 300 spectrometer. Samples
were dissolved in C6D6 solutions. GC analysis were performed on Varian 3400 chromatograph
Ž
.
equipped with Megabore column DB-1, 30 m . GC-MS analyses were carried out with Varian 3300

(
)
230
B. Marciniec et al.rJournal of Molecular Catalysis A: Chemical 135 1998 223–231
Ž
.
chromatograph equipped with a DB-1 capillary column, 30 m connected to a Finnigan Mat 700
mass detector.
3.3. General procedure for the dehydrogenatiÕe silylation of styrene
Ž
The nickel complex precursors were placed in glass ampoules or in a flask equipped with a
.
Ž
.
condenser and filled with a mixture of styrene and one of various silanes in different molar ratio .
All manipulations were carried out using standard Schlenk and high vacuum techniques. The reactor
Ž
.
was evacuated and filled with argon or dry air . The mixture was stirred and heated to the required
reaction temperature. The progress of the reaction was monitored by GLC analysis. After the reaction,
most products were identified by GC-MS analysis, by comparing spectra and the retention time of the
peaks with standards. The yield and distribution of the reaction products were detected and calculated
by GLC analysis.
Ž
.
Ž
.
Example: The catalyst Ni cod
8 mg, 0.03 mmol was placed in a flask equipped with a
2
Ž
.
condensor. The reactor was evacuated and filled with argon, and then a mixture of styrene 12 mmol
and triethoxysilane 6 mmol was added. The reactor was heated at 1208C for 4 h. After the reaction
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completion, a mixture was analyzed; 5.3 mmol triethoxysilane and 11.5 mmol styrene were con-
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sumed. The following products were detected: 4.09 mmol 1- triethoxysilyl -2-phenylethene, 0.56
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.
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.
mmol 1- triethoxysilyl -2-phenylethane, 0.07 mmol 1- triethoxysilyl -1-phenylethane, 0.29 mmol
tetraethoxysilane, 0.31 mmol siloxanes, 3.17 mmol ethylbenzene and 1.09 mmol 1,3-diphenylbutane.
All products were distilled under vacuum and characterized spectroscopically.
1
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.
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.
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.
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.
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.
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E -1- Triethoxysilyl -2-phenylethene: H NMR C6D6 d ppm 1.32 t, 9H , 3.93 q, 6H , 6.28
1
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.
.
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.
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.
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d, 1H , 7.33–742 m, 6H ; 1- Triethoxysilyl -2-phenylethane: H NMR C6D6 d ppm 0.91–1.00
m, 2H , 1.21 t, 9H , 2.68–2.73 m, 2H , 3.80 q, 6H , 7.21–752 m, 5H ; 1- Triethoxysilyl -1-phen-
ylethane: H NMR C6D6 d ppm 1.14 t, 9H , 1.42 d, 3H , 2.3 q, 1H , 3.71 q, 6H , 7.2 m, 5H ;
Ethylbenzene: H NMR CDCl3 d ppm 1.23 t, 3H , 2.6 q, 2H , 7.1 s, 5H ; 1,3-Diphenylbutane:
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.
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1
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1
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1
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H NMR CDCl3 d ppm 1.12 d, 3H , 1.77 m, 2H , 2.39 t, 2H , 2.53 m, 1H , 7.0–7.2 m, 10H .
4. Conclusions
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x
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. x
2
1. Ni acac and Ni cod are effective precursors of dehydrogenative silylation of styrene with
2
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.
trisubstituted silanes containing electron withdrawing non-halogen substituents at silicon, e.g.,
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.
EtO SiH and PhMe2SiH.
3
2. Two pathways, DS-1 and DS-2, of the dehydrogenative silylation of styrene are observed
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involving formation of E-1-phenyl-2-silyl-ethene as the main unsaturated product as well as
ethylbenzene and 1,3-diphenylbutane as products of hydrogenation for DS-1 and hydrogenative
dimerization for DS-2 of styrene. The reactions are accompanied by two products of the styrene
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hydrosilylation H and the redistribution of silanes containing alkoxy group s .
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.
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.x
3. Ni acac Et PPh3 appeared to be an intermediate of the DS-1 and DS-2 with triethoxysilane but
only after elimination of phosphine by oxygenation. However, contrary to the corresponding reaction
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.
2 x
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.
3
with vinylsilanes 19 effective catalysis by Ni acac
of the reaction with EtO SiH at room
temperature as well as relatively high activity of Me2 PhSiH in this reaction suggest that direct
formation of Ni–Si intermediates can be a key step for an initiation of catalytic cycle.
4. All results of the catalytic and analytical studies allow us to propose the general scheme of DS-1,
DS-2 and H catalysis by nickel precursors involving the insertion of styrene into Ni–Si, Ni–H and
Ni–C bonds.

(
)
B. Marciniec et al.rJournal of Molecular Catalysis A: Chemical 135 1998 223–231
231
Acknowledgements
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This work was supported by State Committee for Scientific Research Poland , Project No.
13T09A074.
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