Rhenium-mediated dehydrogenative silylation and highly regioselective hydrosilylation of nitrile substituted olefins

Article abstract of DOI:10.1016/j.jorganchem.2013.10.052

The rhenium (I) complex [Re(CH₃CN)₃Br ₂(NO)] catalyzes the homogeneous hydrosilylation of a variety of substituted acrylonitriles, which were converted into the corresponding silyl-substituted alkanes with high regioselectivity of up to 94%. The products were analyzed by ¹H NMR and GC-MS. A rhenium specific mechanism is proposed for the hydrosilylation of olefins.

Full text of DOI:10.1016/j.jorganchem.2013.10.052

Journal of Organometallic Chemistry 750 (2014) 17e22
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Rhenium-mediated dehydrogenative silylation and highly
regioselective hydrosilylation of nitrile substituted olefins
*
Hailin Dong, Yanfeng Jiang, Heinz Berke
Anorganisch-chemisches Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
The rhenium (I) complex [Re(CH3CN)3Br2(NO)] catalyzes the homogeneous hydrosilylation of a variety of
substituted acrylonitriles, which were converted into the corresponding silyl-substituted alkanes with
high regioselectivity of up to 94%. The products were analyzed by 1H NMR and GCeMS. A rhenium
specific mechanism is proposed for the hydrosilylation of olefins.
Received 19 March 2013
Received in revised form
28 October 2013
Accepted 29 October 2013
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Acrylonitrile
Alkene
Homogeneous catalysis
Hydrosilylation
Dehydrogenative silylation
Rhenium
1. Introduction
moiety to the double bond of acrylonitrile [4]. The reaction be-
tween acrylonitrile and silane with prevailing formation of the a-
Hydrosilylation of carbonecarbon double bonds catalyzed by
transition metals is of great importance in the production of silicon
derived compounds and in the potential use of the transformation
in organic synthesis [1]. A variety of methods can be applied to
initiate this transformation in an atom-efficient way. Catalysts for
the 1,2-addition of hydrosilanes to alkenes are mainly based on late
transition metals including those with platinum, rhodium or
ruthenium centers [2]. In comparison, middle transition metals
such as rhenium are scarcely involved in such reactions simply due
to their high tendency to obey the 18-electron rule. The metal
ligand bonds are often too strong and not labilized for the initiation
of metal-centered reactivity. Such drawbacks have to be overcome
by proper tuning of the ancillary using for instance the cis-
isomer was detected to occur with only a few catalysts, like
peroxide compounds and platinum-based catalysts [5].
During our explorations on the feasibility of low-valent rhenium
catalysts [6], we found that the nitrosyl rhenium complex
[Re(CH3CN)3Br2(NO)] exhibited high catalytic activity and regiose-
lectivity in the hydrosilylation of substituted alkenes. We present
here the [Re(CH3CN)3Br2(NO)] catalyzed hydrosilylation of
substituted olefins and acrylonitrile to give predominantly the a-
addition product with a regioselectivity of up to 94% with a rela-
tively low catalyst loading (1.5 mol %) at 115 ꢀC in CH3CN. As
mentioned before, nitrile substituted olefins, in particular, are
difficult substrates for hydrosilylations, because of a poisoning or
partial poisoning of the catalysts by strong binding of the nitrile
group to the metal center. However, in the [Re(CH3CN)3Br2(NO)]
complex at least one nitrile ligand seemed to be labile, because it
can easily be replaced by THF [6b]. This labile site is prone for acting
as a catalytic site.
labilization effect of
ence effect of strong
p
s
donor ligands (i.e. Br) or the trans influ-
donors (i.e. NO) [2d].
Acrylonitrile is widely used in the production of acrylic and
methacrylic fibers, resins and rubbers, and as a chemical interme-
diate [3]. Although there is an abundance of catalysts which are
effective in promoting the addition of the SieH bond across alkenes
and alkynes, relatively few show significant activity in the hydro-
silylation of acrylonitrile [3]. Agents such as amines, Raney nickel,
2. Results and discussion
2.1. Dehydrogenative silylation of substituted styrenes catalyzed by
rhenium (I) complexes
amides and phosphines, normally result in b-addition of the silyl
We first examined the reactions of 4-methylstyrene with
dimethylphenylsilane in the presence of various rhenium (I)
* Corresponding author. Tel.: þ41 1 63 546 81; fax: þ41 1 63 568 02.
E-mail address: hberke@aci.uzh.ch (H. Berke).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.10.052

18
H. Dong et al. / Journal of Organometallic Chemistry 750 (2014) 17e22
complexes and solvents at 110e115 ꢀC, which are summarized in
Table 1. The progress of each reaction according to Table 1 was
monitored by 1H NMR, and the identity of the hydrosilylation
product was assured by 1H NMR spectroscopy.
loadings (8e10 mol %), no obvious improvement of the yield was
observed.
The scope of the dehydrogenative silylation of substituted sty-
renes including both electron-rich and electron-deficient groups
with dimethylphenylsilane was then investigated at 110e115 ꢀC in
toluene using [Re(CH3CN)3Br2(NO)] (3 mol %) as a catalyst (Table 2).
All the reactions were complete in 2e6 h with conversion of almost
100%, and no catalyst precipitationwas observed and the color of the
reaction solution color turned a little darker after reaction. For the
substituted styrenes bearing electron-withdrawing groups, such as
fluoro- and chloro-substitution, the dehydrogenative silylation
completed with conversion around 99% in 2e6 h and the relatively
higher yields of 86%, 86% and 88% (entries 2, 6 and 7). A reaction time
of 2 h was required for 4-methylstyrene, 3-methylstyrene and sty-
rene to obtainyields of 85%, 82% and 83%, respectively (entry 4, 5 and
1). 3-Methoxystyrene gave the dehydrogenative silylation products
with relatively low yield of 78% in 2 h (entries 3). Among all the
reactions, the ratios of the yields between dehydrogenative silyla-
tion and hydrosilylation stayed approximately constant at 85:15.
The dehydrogenative silylation of various substituted styrenes
catalyzed by [Re(CH3CN)3Br2(NO)] proceeded following the similar
mechanism we reported before [7a].
Appropriate solvents were expected to have high boiling points
and low polarity. Toluene turned out to be the solvent of choice,
and it dissolves the rhenium catalyst [Re(CH3CN)3Br2(NO)]
reasonably well at 110e115 ꢀC. In toluene, [Re(CH3CN)3Br2(NO)]
provided the best activity after 2 h with a conversion of 100% and a
TOF value of 16.7 hꢁ1 (entry 1). The reaction took mainly the
course of dehydrogenative silylation [7] and hydrosilylation (85/
15). The reaction can also be carried out in polar solvents, but with
loss in yield. In CH2Cl2, CH3CN, THF and CHCl3 yields of 66%, 59%,
45% and 50% were obtained (entries 2e5). Bromobenzene and
DMSO gave the lowest yields in comparison with the other sol-
vents (entries 6 and 7).
Screening the type of rhenium catalyst, [Re(CH3CN)3Br2(NO)]
(entry 1) showed the best activity from the selection of the rhenium
complexes given in Table 1. Various phosphine substituted com-
plexes were tested for catalytic hydrosilylation. In the case of the
rhenium complex [Re(PTA)2Br2(NO)(CH3CN)] (PTA ¼ 1,3,5-triaza-7-
phospha-adamantane) containing two phosphine ligands, the re-
action in toluene gave a yield of 47%, while in CH3CN and dioxane
very low yields were obtained after 24 h with TOF values of 0.7, 0.07
and 0.13 hꢁ1 (entries 8, 9 and 11). No conversion was observed
Rhenium-mediated hydrosilylations of acrylonitrile and its de-
rivatives with alkyl and aryl silanes in a 1:1 ratio.
As mentioned in the introductory part hydrosilylations of
acrylonitrile and its derivatives often suffer from coordination of
the acrylonitrile to the catalytic center. Perhaps it can even bind in
when bromobenzene was used as
[Re(PTA)3Br2(NO)] gave only 6% yield of the desired product after
24 (entry 12). The protonated species [Re(PTAH)2Br2(-
a solvent (entry 10).
h
an h
4-fashion and thus attain affinity to the rhenium center higher
NO)(CH3CN)][Br]2, [Re(PTAH)(PTA)Br2(NO)-(CH3CN)][Br], did not
show any catalytic activity even after 24 h (entries 13e16).
Catalysts were generally employed at the 3 mol % level relative
to the substrate. Lower catalyst loading could be used, but required
longer. In the case of the rhenium complexes containing phosphine
ligands such as PTA, PCy3 and PiPr3, even with much higher
than two separate acetonitrile ligands. We anticipated that for the
[Re(CH3CN)3Br2(NO)] molecule as a catalyst this blockage problem
would be less significant owing to the fact that we deal with a labile
nitrile substituted species.
We tested the hydrosilylation of acrylonitrile and its de-
rivatives using this rhenium (I) catalyst in a sealed Young NMR
Table 1
Various rhenium complex-catalyzed dehydrogenative silylation of 4-methylstyrene with HSiMe2Ph.a
Cat.[Re] 3 mol %
solvent
SiMe2Ph
SiMe2Ph
+
+
H-SiMe2Ph
4-MePh
+
4-MePh
4-MePh
4-MePh
110-115 o
C
Entry
Catalyst
Solvent
t (h)
Conversion
TOF
c
(%)b
(hꢁ1
)
1
2
3
4
5
6
7
8
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(PTA)2Br2(NO)
(CH3CN)]
Toluene
CH2Cl2
CH3CN
THF
CHCl3
Bromobenzene
DMSO
2
12
12
12
12
12
12
24
100
66
59
45
50
15
<10
47
16.7
1.8
1.6
1.2
1.3
0.4
0.28
0.7
Toluene
9
[Re(PTA)2Br2(NO)
(CH3CN)]
[Re(PTA)2Br2(NO)
(CH3CN)]
[Re(PTA)2Br2(NO)-
(CH3CN)]
[Re(PTA)3Br2(NO)]
[Re(PTAH)2Br2(NO)-
(CH3CN)][Br]2
CH3CN
24
24
24
5
0
0.0
10
11
Bromobenzene
Dioxane
e
10
0.13
0.08
12
13
Toluene
Toluene
24
24
6
0
e
14
[Re(PTAH)(PTA)Br2
(NO)(CH3CN)][Br]
Toluene
24
0
e
a
All reactions were carried out with 4-methylstyrene (0.5 mmol), HSiMe2Ph (0.25 mmol), and the rhenium catalyst (0.0075 mmol, 3 mol %) in the appropriate solvent
(0.8 ml) at 110e115 ꢀC for given time under a nitrogen atmosphere. Progress of the reactions was monitored by 1H NMR spectroscopy.
b
Conversion of silane based on the integration of the 1H NMR spectrum.
Defined as mol product per mol of catalyst per hour.
c

H. Dong et al. / Journal of Organometallic Chemistry 750 (2014) 17e22
19
Table 2
Rhenium-mediated dehydro/hydro-silylation of substituted olefins with HSiMe2Ph.a
Table 4
Rhenium mediated hydrosilylation of acrylonitrile in various solvents.a
c
Entry
Substrate
t (h)
Conversion (%)b
Dehydro/hydroc
Entry Solvent
Silane
t (h) Yield (%)b TOF (hꢁ1
)
1
2
3
4
5
6
7
8
9
Isopropenylnitrile
Isopropenylnitrile
Isopropenylnitrile
2,4,5-Trimethylbenzonitrile PhMe2SiH 48
2,4,5-Trimethylbenzonitrile Et3SiH 48
2,4,5-Trimethylbenzonitrile ClMe2SiH 48
Benzonitrile
Benzonitrile
Benzonitrile
PhMe2SiH 36
Et3SiH 36
ClMe2SiH 36
90
84
78
32
20
0
49
38
9
1.7
1.5
1.4
0.44
0.28
e
1
2
2
100
100
83/17
2
86/14
PhMe2SiH 48
Et3SiH 48
ClMe2SiH 48
0.68
0.53
0.13
3
2
100
78/22
a
All reactions were performed with acrylonitrile (0.5 mmol), chlorosilane
MeO
(0.6 mmol), and the rhenium complex [Re(CH3CN)3Br2(NO)] (0.0075 mmol, 1.5 mol
%) in the proper solvent (0.8 ml) at 115e120 ꢀC for given time under a nitrogen
atmosphere. Progress of the reaction was monitored by 1H NMR spectroscopy.
4
5
2
2
100
100
85/15
82/18
b
Yield on the basis of GCeMS.
Defined as mol product per mol of catalyst per hour.
c
the reaction temperatures from 115 ꢀC to 60 ꢀC when Et3SiH was
applied as a silane source (entries 1e3). Similar results were
obtained for the cases of PhMe2SiH (entries 4e6). The hydro-
silylation of acrylonitrile required 48 h to achieve appropriate
yields at 115 ꢀC and when Ph2MeSiH was used (entry 7). The
hydrosilylation of 2-chloroacrylonitrile proceeded slowly and
gave low yields even at 110 ꢀC after 24 h and also in the case of
Ph2MeSiH (22%) after 24 h (entries 8e10). 2-Methylacrylonitrile
provided a little better yields compared to 2-chloroacrylonitrile
within 12 h of reaction time (entries 11e13). The highest TOF
value (8.3) was obtained in the reaction between acrylonitrile
and triethylsilane at 115 ꢀC (entry 1).
6
4
6
99
99
86/14
88/12
Cl
7
Cl
a
All reactions were carried out with the alkene (0.5 mmol), HSiMe2Ph
(0.25 mmol), and the rhenium catalyst [Re(CH3CN)3Br2(NO)] (0.0075 mmol, 3 mol %)
in toluene (0.8 ml) at 110e115 ꢀC for given time under a nitrogen atmosphere.
Progress of the reactions was monitored by 1H NMR spectroscopy.
b
Conversion of silane based on the integration in 1H NMR spectrum.
c
Ratio determined by the integration in 1H NMR spectrum.
Considering the results of Table 3, we concluded that the activity
of the substrates is in the order of acrylonitrile
> 2-
tube (Table 3). Various temperatures (60e115 ꢀC) were applied
and acetonitrile was regarded as the prime choice of solvent due
to the high solubility of the rhenium catalyst. An alkyl silane
(Et3SiH) and aryl silanes (PhMe2SiH, Ph2MeSiH) were chosen as
silyl reagents to examine the activity of the rhenium catalyst. The
reaction times were dramatically prolonged with a decrease of
methylacrylonitrile > 2-chloroacrylonitrile, while the order for
the silane activities is triethylsilane > dimethylphenylsilane >
diphenylmethylsilane. The regioselectivity ratios of all the reactions
were found to be a:b z 3:1 almost identical irrespective of the
substrates, reaction temperature and reaction time. Also, various
basic additives (TMEDA, KOtBu, CF3CO3H, K2CO3, HCOONa and
Table 3
Rhenium-mediated hydrosilylation of acrylonitrile and its derivatives with alkyl and aryl silanes.a
R
R
R'Si
Cat.[Re] (1.5 mol %)
R'SiH
+
N
+
N
N
CH3CN
R
R'Si
Cat=[ReBr2(NO)(CH3CN)3]
R=H, Me, Cl
( )
( )
R' = Et3,
PhMe2, Ph2Me
c
d
Entry
Substrate
Silane
T (ꢀC)
t (h)
Conversion (%)b
TOF (hꢁ1
)
Selectivity (a:b)
1
2
3
4
5
6
7
8
Acrylonitrile
Acrylonitrile
Acrylonitrile
Acrylonitrile
Acrylonitrile
Acrylonitrile
Acrylonitrile
2-Chloroacrylonitrile
2-Chloroacrylonitrile
2-Chloroacrylonitrile
2-Methylacrylonitrile
2-Methylacrylonitrile
2-Methylacrylonitrile
Et3SiH
Et3SiH
Et3SiH
115
90
60
115
90
60
115
110
110
110
110
110
110
8
9
100
95
92
100
93
80
95
66
53
22
80
72
36
8.3
7.0
1.3
6.7
5.2
1.1
1.3
1.8
1.5
0.6
4.5
4.0
2.0
73:27
73:27
73:27
73:27
73:27
73:27
73:27
75:25
75:25
75:25
73:27
73:27
73:27
48
10
12
48
48
24
24
24
12
12
12
PhMe2SiH
PhMe2SiH
PhMe2SiH
Ph2MeSiH
Et3SiH
PhMe2SiH
Ph2MeSiH
Et3SiH
9
10
11
12
13
PhMe2SiH
Ph2MeSiH
a
All reactions were carried out with the substrate (0.5 mmol), HSiMe2Ph (0.6 mmol), and the rhenium complex [Re(CH3CN)3Br2(NO)] (0.0075 mmol, 1.5 mol %) in
acetonitrile (0.8 ml) at proper temperature for given time under a nitrogen atmosphere. Progress of the reaction was monitored by 1H NMR spectroscopy.
b
Yield on the basis of GCeMS.
Defined as mol product per mol of catalyst per hour.
Selectivity was measured by GCeMS.
c
d

20
H. Dong et al. / Journal of Organometallic Chemistry 750 (2014) 17e22
pyridine) were introduced into the reactions. However, no obvious
influence on the hydrosilylation was observed.
regioselectivity ratio of around 73:27, which was found to be in-
dependent of temperature (Table 3). To accomplish a better regio-
selectivity, we thought to test hydrosilylations also in 1:2 and 2:1
ratios with a rhenium catalyst loading of 1.5 mol % in acetonitrile at
60e115 ꢀC for the given times (Table 5).
2.2. Solvent effects in the rhenium-mediated hydrosilylation of
acrylonitrile
The ratios between acrylonitrile and PhMe2SiH were varied
from 4:1 to 1:4. When the ratio between substrate and PhMe2SiH
was 1:1, the regioselectivity of the reaction indeed remained
practically constant independent of temperature and the silane
source (entries 1e7). Higher regioselectivity (86:14) in favor of the
silyl substitution and relatively high TOF values (14.5 and 11) were
obtained when the hydrosilylation reactions were carried out with
1:2 or 2:1 ratios (entries 8 and 9). The reaction times were greatly
reduced to 4.5 and 6 h. The regioselectivity of the hydrosilylation
reaction changed also with unequal ratios of the substrates and
could reach 92:8 and 94:6 when the reactants were loaded in a
ratio of 1:4 and 4:1, respectively. In these cases the reaction times
still decreased to 1.3 and 1.0 h (entries 10e14). These observations
of accelerated catalyzes are however difficult to interpret, in
particular with respect to the fact that the increase of the concen-
trations of both the unsaturated compound and the silane did not
effect much better catalytic performance. This almost excludes the
possibility of an influence of second order terms in the rate deter-
mining step or steps.
A variety of solvents, from polar to nonpolar, from aliphatic to
aryl substituted ones were also examined in the hydrosilylation of
acrylonitrile. However, it was found that only nitrile solvents
worked in the hydrosilylation reactions to an acceptable extent.
We then tested various types of nitrile solvents to trace any
refined influence on the hydrosilylation with a rhenium catalyst
loading of 1.5 mol % (Table 4). Hydrosilylation of acrylonitrile was
first attempted in isopropenylnitrile with various silanes. Rela-
tively high yields were achieved after 36 h at 115 ꢀC ranging from
78% to 90% (entries 1e3). 2,4,5-Trimethylbenzonitrile which
possesses enhanced steric hindrance was then introduced into
the reactions. Unfortunately, the hydrosilylation proceeded
sluggishly to give yields lower than 32% after 48 h at 120 ꢀC
(entries 4e6). Benzonitrile provided relatively better yields
compared to those in 2,4,5-trimethylbenzonitrile with yields
between 49% and 9% at 120 ꢀC after 48 h (entries 7e9). For the
same solvent, PhMe2SiH with medium hydridic and steric
congestion gave the best conversion among all the three silane
candidates (PhMe2SiH, Et3SiH and ClMe2SiH). In contrast to the
results of Table 1, acetonitrile revealed to be the best performing
solvent for hydrosilylations.
2.4. Investigation of the mechanism of the rhenium based
hydrosilylation
2.3. Rhenium-catalyzed hydrosilylation of acrylonitrile with various
silane ratios
A generalized catalytic cycle for the hydrosilylation with the
rhenium complex [Re(CH3CN)3Br2(NO)] is proposed in Scheme 1.
[Re(CH3CN)3Br2(NO)] takes the role of a pre-catalyst via primary
dissociation of one of the CH3CN ligands. In the 1H NMR spectrum
(CD3CN) this dissociation became evident by a singlet at 1.98 ppm
Hydrosilylation of acrylonitrile in acetonitrile in a ratio of 1:1
gave
a and b silyl substituted products with a relatively constant
Table 5
Rhenium mediated hydrosilylation of acrylonitrile in acetonitrile with various ratios.a
SiR3
N
Cat.[Re] (1.5 mol %)
N
N
+
SiR3
R3SiH
+
CH3CN
Cat=[ReBr2(NO)(CH3CN)3]
( )
( )
c
d
Entry
Substrate: Silane (mmol)
Ratio
Silane
T (ꢀC)
t (h)
Conversion (%)b
TOF (hꢁ1
)
Selectivity (a:b)
1
2
3
4
5
6
7
8
0.5:0.5
0.5:0.5
0.5:0.5
0.5:0.5
0.5:0.5
0.5:0.5
0.5:0.5
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:2
2:1
4:1
4:1
4:1
1:4
4:1
6:1
1:6
Et3SiH
Et3SiH
Et3SiH
115
90
60
115
90
60
115
115
115
115
115
115
115
115
115
115
115
115
8
9
100
95
92
100
100
90
95
96
98
98
99
99
99
99
100
99
99
8.3
7.0
1.3
6.7
5.6
1.3
3.2
6.7
7.3
14.5
11
33
73
55
51.3
66
73:27
73:27
73:27
72:28
72:28
72:28
73:27
72:28
73:27
88:12
86:14
88:12
90:10
89:11
92:8
48
10
12
48
20
PhMe2SiH
PhMe2SiH
PhMe2SiH
Ph2MeSiH
PhMe2SiH
PhMe2SiH
PhMe2SiH
PhMe2SiH
Ph2MeSiH
Et3SiH
9.5
9
9
2:2
10
11
12
13
14
15
16
17
18
0.5:1.0
1.0:0.5
1.0:0.25
1.0:0.25
1.0:0.25
0.25:1.0
1.0:0.25
1.5:0.25
0.25:1.5
4.5
6.0
2.0
0.9
1.2
1.1
1.0
1.2
1.5
Et3SiH
PhMe2SiH
PhMe2SiH
PhMe2SiH
PhMe2SiH
94:6
90:10
88:12
55
44
99
a
All reactions were carried out with the substrate (0.5e1.0 mmol), silane (0.5e1.0 mmol), and the rhenium complex [Re(CH3CN)3Br2(NO)] (0.0075 mmol, 1.5 mol %) in
acetonitrile (0.8 ml) at proper temperature for given time under a nitrogen atmosphere. Progress of the reactions was monitored by 1H NMR spectroscopy.
b
Conversion on the basis of integration of the 1H NMR spectrum.
Defined as mol product per mol of catalyst per hour.
Based on GCeMS measurement.
c
d

H. Dong et al. / Journal of Organometallic Chemistry 750 (2014) 17e22
21
methylstyrene suggesting a structure as shown in the catalytic cy-
cle. The resonance of freed CH3CN ligand was observed as a singlet at
0.73 ppm. One of the acetonitrile ligands of [Re(CH3CN)3Br2(NO)] is
apparently labilized and can be substituted by alkene. The stoi-
chiometric reaction between [Re(CH3CN)3Br2(NO)] and Et3SiH
(6 equiv.) was also examined. However, the reaction in toluene-d8
afforded a dark brown solution and a dark precipitate within 1 h at
105 ꢀC. 1H NMR spectroscopy indicated the formation of several
silane species in solution including a major one which most likely
bears a Et2HSi-unit. The SieH signal is high-field shifted from
3.78 ppm (heptet) to 2.60 ppm (quart). A definite structural
assignment of this major component failed, however. The reaction
between [Re(CH3CN)3Br2(NO)] and acrylonitrile (6 equiv.) was also
investigated. At 110 ꢀC for 1 h, a doublet of doublets at 3.69 ppm
(3J ¼ 12 Hz, 3J ¼ 9 Hz) and two doublets at 3.42 ppm (3J ¼ 9 Hz) and
3.27 ppm (3J ¼ 12 Hz) were observed in 1H NMR spectra, which are
assigned to alkene protons. The high-field shift of the alkene reso-
nance indicated the formation of an alkene coordinated rhenium
species.
NCCH3
NO
Br
Br
Re
NCCH3
NCCH3
1
CH3CN
NCCH3
SiR3
R'
Br
Br
NO
6
R'
Re
NCCH3
CH3CN
2
NCCH3
NCCH3
Br
Br
NO
Br
Br
NO
Re
Re
NCCH3
SiR3
R'
R'
5
3
3. Conclusion
CH3CN
NCCH3
In summary, we have studied the reactions of silanes with
various substituted styrenes proceeding in highly selective dehy-
drogenative silylation reactions affording vinyl silanes in good to
excellent yields. Furthermore, we have developed a rhenium-
mediated hydrosilylation of acrylonitrile and its derivatives,
which often are reluctant to undergo hydrosilylation. The reactions
were extensively investigated by evaluating various parameters in
detail, such as solvents, stoichiometrics and various silanes.
[Re(CH3CN)3Br2(NO)] was found to be an effective catalyst for the
reduction of acrylonitrile and its derivatives, when the reactions
were carried out with aryl silanes in acetonitrile and in the ratios of
4:1 or 1:4. Excellent yields and high regioselectivity were observed.
At higher temperatures (115 ꢀC) the catalyst remained stable and
provided even better performance.
HSiR3
Br
Br
NO
SiR3
Re
H
R'
4
Scheme 1. Proposed pathway for the hydrosilylation of an olefin, catalyzed by the
[Re(CH3CN)3Br2(NO)] complex. R ¼ Et, PhMe2, Ph2Me; R0 ¼ nitrile and nitrile con-
taining groups.
assigned to the CH3 group of free CH3CN. Then an olefin molecule
coordinates to the rhenium center of 2, forming the olefin adduct 3.
The silanes are presumably then coordinated to the rhenium center
with loss of another acetonitrile ligand giving species 4, which re-
acts subsequently with H transfer to the olefin ligand leading to the
formation of complex 5. A 1H-NOE experiment provided some ev-
idence for the presence of a side-on rhenium bonded SieH unit,
since saturation of the dissociated acetonitrile methyl group at
around 2 ppm gives rise to a very broad (>100 Hz) NOE signal,
4. Experimental section
4.1. General information
which was assigned to a Re attached SieH group. a-Regioselectivity
All manipulations were carried out using standard vacuum line,
Schlenk and cannula techniques or in a dry box (M. Braun 150B-G-
II) containing an atmosphere of purified nitrogen. Solvents were
initially distilled under N2 atmosphere using standard procedures
and were degassed by freezeethaw cycles prior to use. The
following complexes: Re(NO)(PCy3)2(CH3CN)Br2] [6d], [Re(NO)(-
PiPr3)2(CH3CN)Br2] [6d], [Re(NO)(CH3CN)3Br2] [6b], [Re(PTA)2Br2(-
NO)(CH3CN)] [6b], [Re(PTA)3Br2(NO)] [9], Re(PTAH)2Br2(NO)-
(CH3CN)][Br]2 [9], [Re(PTAH)(PTA)Br2(CH3CN)(NO)][Br] [9] were
prepared according to known procedures. All other chemicals
were purchased from Aldrich Chemical Co. or Fluka and,
unless otherwise noted, used without further purifica-
tion. 1H and 11B NMR spectra were recorded on a Varian
was preferred for R0 ¼ CN. We assume that the silane oxidatively
adds to the rhenium center and a hydride is transferred to the
incipient carbocationic end of the olefin [8]. Alternatively, we had
to assume that the silane in 4 appears acidified by coordination
initiating a proton transfer to the olefin. The
a-regioselectivity
would however not cope with the proton transfer step due to the
expected opposite polarization of the olefin based on the stability of
the intermediate incipient carbocation. The catalytic cycle “closes”
with reductive elimination of the product 6 and re-formation of
complex 2. Attempts to isolate the
h
2-R3SiH complex 4 were not
successful presumably due to its instability. Further detailed studies
are currently underway to achieve full understanding of the reac-
tion mechanism.
Gemini 300 (300.08 MHz), or on
a Varian Mercury 200
A stoichiometric reaction between [Re(CH3CN)3Br2(NO)] and 4-
methylstyrene (6 equiv.) was carried out in toluene-d8 which
afforded at 105 ꢀC a yellow solution within 20 h 1H NMR spectros-
copy indicated the formation of a new species assigned to the 4-
(199.96 MHz) spectrometer. GCeMS analysis were carried out on a
CP-3800 Saturn 2000 MS/MS spectrometer (CP-Sil8CB low bleed/
MS 30m, ID 0.25 mm, OD 0.39 mm, Film thickness 0.25
Chrompack).
mm from
methylstyrene coordinated Re(I) complex [Re(Br)2(CH3CN)2(h2
-
CH2 ¼ CH(4-tolyl))(NO)]. The resonances of the alkene protons were
observed as a doublet of doublet and a multiplet in the range of
4.99e5.10 and 3.92e4.01 ppm. The signals of the two acetonitrile
ligands were observed as two singlets at 1.13 and 1.08 ppm due to
different chemical environment caused by the coordinated 4-
4.2. General procedure for the catalytic hydrosilylations according
to Tables 1e4
A solution of the appropriate substrate (0.5 mmol), the silane
(0.6 mmol) and the proper catalytic amount of the rhenium

22
H. Dong et al. / Journal of Organometallic Chemistry 750 (2014) 17e22
complex in the given solvent (0.8 ml) was stirred for an appropriate
period of time at proper temperature under nitrogen atmosphere.
Upon completion, the reaction mixture was filtered over Celite. The
resulting solution was analyzed by NMR spectroscopy and GCeMS.
The yields are listed in Tables 1e4.
J(HH) ¼ 7.4 Hz, SieH, 1H), 0.85 (t, J(HH) ¼ 7.4 Hz, 9H, CH3), 0.56e0.67
(m, 6H, CH2).
Acknowledgments
We gratefully acknowledge financial support from the Swiss Na-
tional Science Foundation and the funds of the University of Zurich.
4.3. General procedure for the catalytic hydrosilylations according
to Table 5
References
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silylations of Tables 1e4. The reaction mixture was heated to the
proper temperature and stirred for the required reaction time.
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conversion and selectivity was analyzed by GCeMS. 3-(Dime-
thyl(phenyl)silyl)propanenitrile: 1H NMR (300.08 MHz, toluene-d8,
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m
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h
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3
MHz, toluene-d8, ppm): 4.99e5.10 (dd, J(HH)
¼
13 Hz,
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137.3, 137.2, 128.5, 127.2, 78.0, 52.9, 2.3, 1.9. The resonance of freed
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