The effect of an acylphosphine ligand on the rhodium-catalyzed hydrosilylation of alkenes

Article abstract of DOI:10.1016/j.jorganchem.2017.12.004

We synthesized a series of acylphosphines and investigated the hydrosilylation of alkenes that were catalyzed using RhCl₃/acylphosphine. The results indicated that RhCl₃/(diphenylphosphino) (phenyl)methanone exhibited higher activity as well as higher levels of β–adduct selectivity.

Full text of DOI:10.1016/j.jorganchem.2017.12.004

Journal of Organometallic Chemistry 855 (2018) 7e11
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
The effect of an acylphosphine ligand on the rhodium-catalyzed
hydrosilylation of alkenes
Jiayun Li*, Chuang Yang, Ying Bai, Xiaoling Yang, Yu Liu, Jiajian Peng**
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 311121, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We synthesized a series of acylphosphines and investigated the hydrosilylation of alkenes that were
catalyzed using RhCl3/acylphosphine. The results indicated that RhCl3/(diphenylphosphino) (phenyl)
Received 9 October 2017
Received in revised form
29 November 2017
Accepted 4 December 2017
Available online 6 December 2017
methanone exhibited higher activity as well as higher levels of
b
eadduct selectivity.
© 2017 Elsevier B.V. All rights reserved.
Keywords:
Acylphosphine
Hydrosilylation
Rhodium complex
1. Introduction
Although a wide range of catalysts have been tested for hydro-
silylation, most industrial syntheses and studies have been carried
out in the presence of platinum or rhodium complexes [14,15]. New
selective hydrosilylation catalysts based on Fe or Co have also been
studied [16e18]. However, low conversions and low selectivities of
the product are usually observed with terminal alkenes in partic-
ular. In addition, the catalytic hydrosilylation of styrene using
hydrosilane is a model reaction, which has been widely reported
and often used to check the catalytic activity of catalysts, shows
unsatisfactory catalytic activity selectivity. It has been reported that
Wilkinson's catalyst (Rh(PPh3)3Cl) can effectively catalyze the
hydrosilylation of alkynes or alkenes [19]. In this catalytic process,
the phosphine ligands also play a significant role in the hydro-
silylation reaction [20e22]. In this paper, the synthesis of a series of
acylphosphines and their application in the hydrosilylation reac-
tion are described.
Transition metal catalysts play an important role in organic
chemistry [1e3]. Apart from the metal center used, the ligands used
to contain the metal complexes also play an essential role in the
reaction [4e6]. Various ligands based on heteroatoms or coordi-
native functional groups have been designed and synthesized. The
ligands used result in the diverse catalytic abilities of number-
limited transition metals via their stereoelectronic effects on the
metal center. Phosphorus ligands occupy an important position in
organometallic chemistry and homogenous catalysis, and play an
irreplaceable role [7e9]. The electronic properties of phosphorus
ligands, which may be tuned by an adjoining functional group,
immensely influence the activity of the catalyst. It is well known
that electron-rich phosphorus ligands can enhance the activity of a
catalyst in a range of organometallic reactions such as hydrogena-
tion and hydroformylation. Recently, there have been many reports
using electron-deficient groups as a new type of electron tuning
group. Acylphosphines have been developed as a type of carbonyl
group tuned electron-deficient phosphorus ligand and found to be
efficient in rhodium-catalyzed arylation of aldehydes [10].
2. Experimental
2.1. General information
Hydrosilylation is usually performed to prepare functionalized
silanes, polysilane and other organosilicon compounds [11e13].
Styrene was washed with 5% NaOH and dried with Na2SO4, and
then was distilled under reduced pressure after filtration. All other
materials were purchased from Aldrich and were used as received.
Gas Chromatography: Trace DSQ GC Column ¼ DB-5 30 m ꢀ 2.5
mm ꢀ 0.25
m
m, split ¼ 50:1, flow ¼ 1 mL minꢁ1 constant flow, inlet
* Corresponding author.
** Corresponding author.
temperature ¼ 260 ꢂC, column temperature ¼ 50 ꢂC (hold 1 min)
then 15 ꢂC$minꢁ1 up to 260 ꢂC (hold 10 min). GC-MS: Trace DSQ
E-mail addresses: jiayun1980@hznu.edu.cn (J. Li), jjpeng@hznu.edu.cn (J. Peng).
https://doi.org/10.1016/j.jorganchem.2017.12.004
0022-328X/© 2017 Elsevier B.V. All rights reserved.

8
J. Li et al. / Journal of Organometallic Chemistry 855 (2018) 7e11
GC-MS Column.
d
(ppm):13.47; m/z: 308.1.
(4-Chlorophenyl)(diphenylphosphino)methanone
1H, 13C and 31P NMR spectra were measured using a Bruker
AV400 MHz spectrometer operating at 400.13, 100.62 and
161.97 MHz, respectively.
6
95.2
%
yield. Yellow solid. 1H NMR (400 MHz, CDCl3)
d
(ppm): 7.90-7.86 (m,
2H), 7.58-7.56(m, 2H), 7.43-7.40 (m, 5H), 7.36-7.33 (m, 5H); 13C
NMR (100 MHz, CDCl3) (ppm): 212.0, 211.7, 135.1, 134.9, 132.0,
129.8, 129.7, 129.6, 129.0, 128.9; 31P NMR (162 MHz, CDCl3)
d
2.2. Synthesis of acylphosphine
d(ppm):13.69; m/z: 324.1.
(Diphenylphosphino) (p-tolyl)methanone 7 94.3 % yield. Yellow
To a solution of substituted benzoyl chloride (3.3 mmol) in
10 mL of Et2O was added triethylamine (3.3 mmol), followed by
dropwise addition of diphenylphosphine (3 mmol). The mixture
was stirred at room temperature and stirred for another 6 h. The
mixture was quenched with H2O (10 mL) and the product was
extracted with CH2Cl2 (2 ꢀ 50 mL). The extract were washed with
brine (20 mL), dried over Na2SO4, and evaporated. The crude
product was recrystallized from MeOH (Schemes 1 and 2).
solid. 1H NMR(400 MHz, CDCl3)
d
(ppm): 7.82-7.79(m, 2H), 7.41-
7.38(m,5H), 7.26-7.24(m, 5H), 7.10-7.08(m, 2H), 2.24(s, 3H, CH3); 13
NMR (100 MHz, CDCl3) (ppm): 212.2, 211.9, 135.1, 134.9, 130.7,
129.8, 129.7, 129.5, 128.8, 128.7, 21.8; 31P NMR (162 MHz, CDCl3)
C
d
d
(ppm): 12.71; m/z: 304.1.
(Diphenylphosphino) (4-methoxyphenyl)methanone 8 92.0 %
yield. Yellow solid. 1H NMR(400 MHz, CDCl3)
d
(ppm): 7.78-7.76(m,
2H), 7.40e7.36(m, 5H), 7.32-7.28 (m, 5H), 7.16 (m, 2H), 3.70 (s, 3H,
OCH3); 13C NMR (100 MHz, CDCl3)
(ppm): 210.4, 210.1,135.0,134.9,
132.9, 130.9, 130.8, 129.5, 128.8, 128.7, 55.4; 31P NMR (162 MHz,
CDCl3) (ppm): 11.41; m/z: 320.1. Yield: 92.0%
(Diphenylphosphino) (4-fluorophenyl)methanone
yield. Yellow solid. 1H NMR (400 MHz, CDCl3)
(ppm): 7.95-7.92(m,
2H), 7.66-7.62(m, 2H), 7.39-7.35 (m, 5H), 7.32-7.25 (m, 5H); 13C
NMR (100 MHz, CDCl3) (ppm): 211.3, 210.9, 135.1, 134.8, 130.1,
129.7, 129.1, 129.0, 128.9, 128.8; 31P NMR (162 MHz, CDCl3)
(ppm):13.53; m/z: 308.1.
(Diphenylphosphino) (phenyl)methanone 1 96.2% yield. Yellow
solid. 1H NMR (400 MHz, CDCl3)
d
(ppm): 7.90e7.85 (m, 2H), 7.58
(m, 1H), 7.43 (m, 2H), 7.37e7.28 (m, 10H); 13C NMR (100 MHz,
CDCl3) : 213.1, 212.8, 135.0, 134.9, 130.6, 128.9, 128.7, 128.6, 128.4,
128.3; 31P NMR (162 MHz, CDCl3)
: 13.51; m/z: 290.1.
(2-Chlorophenyl)(diphenylphosphino)methanone
yield. Yellow solid. 1H NMR(400 MHz,CDCl3)
(ppm): 7.89-7.87 (m,
1H), 7.51-7.48 (m, 1H), 7.38-7.24 (m, 10H), 7.21(m, 2H); 13C
NMR(100 MHz, CDCl3) (ppm): 216.4, 216.1, 134.9, 134.7, 131.9,
129.8, 129.7, 129.6, 128.7, 128.5; 31P NMR (162 MHz, CDCl3)
d
d
d
9
93.4 %
d
d
2
96.1
%
d
d
d
d
d(ppm):13.42; m/z: 324.1.
(Diphenylphosphino) (o-tolyl)methanone 3 95.6 % yield. Yellow
2.2.1. Catalytic hydrosilylation of alkene with triethoxysilane
A 10 mL three-necked flask equipped with a magnetic stirrer
was charged with RhCl3$3H2O (8.0 ꢀ 10ꢁ3 mmol) and acylphos-
phines prepared (4.0 ꢀ 10ꢁ2 mmol) under argon atmosphere. Then
alkene (4 mmol) and silane (4.4 mmol) were added via syringe. The
hydrosilylation reaction proceeded with constant stirring under an
appropriate temperature for 5 h. At the end of the reaction, the
conversion of alkene and the selectivity of product were deter-
mined by GC (Scheme 3).
solid. 1H NMR(400 MHz, CDCl3)
d
(ppm): 7.80-7.77 (m,1H), 7.47-7.40
(m, 1H), 7.33-7.25 (m, 10H), 7.20(m, 2H), 2.34 (s, 3H, CH3); 13C NMR
(100 MHz, CDCl3) (ppm): 216.7, 216.4, 134.9, 134.7, 131.5, 130.2,
130.0, 129.5, 128.7, 128.6, 20.9; 31P NMR (162 MHz, CDCl3)
d
d
(ppm):17.53; m/z: 304.1.
(Diphenylphosphino) (2-methoxyphenyl)methanone 4 89.7 %
yield. Yellow solid. 1H NMR(400 MHz, CDCl3)
d
(ppm): 7.63 (m, 1H),
7.53-7.45 (m, 1H), 7.37-7.29 (m, 10H), 7.03-7.01 (m, 2H), 3.47 (s, 3H,
OCH3); 13C NMR (100 MHz, CDCl3)
(ppm): 214.4, 214.0, 134.9,
134.6, 132.6, 129.0, 128.8, 128.7, 128.4, 128.3, 54.9; 31P NMR (162
MHz, CDCl3) (ppm):13.97; m/z: 320.1.
(Diphenylphosphino) (2-fluorophenyl)methanone
yield. Yellow solid. 1H NMR(400 MHz, CDCl3)
(ppm): 8.01-7.97 (m,
1H), 7.54-7.52 (m, 1H), 7.39-7.31 (m, 10H), 7.19-7.16(m, 2H); 13C
d
3. Results and discussion
d
5
92.8 %
3.1. The effects of different acylphosphines on the hydrosilylation
reaction
d
NMR (100 MHz, CDCl3) d(ppm): 212.4, 212.0, 135.1, 134.9, 132.3,
The catalytic properties of RhCl3/acylphosphines in the hydro-
silylation reaction of styrene using triethoxysilane were investi-
gated and the results are listed in Table 1. The results in Table 1
129.5, 129.4, 128.7, 128.6, 128.5; 31P NMR (162 MHz, CDCl3)
O
indicate that low selectivity towards the
RhCl3/PPh3 as the catalyst. However, excellent catalytic activity and
selectivity towards the -adduct were obtained using RhCl3/acyl-
b-adduct when using
P
R
b
phosphines (1e9) as the catalyst system. To compare all these li-
gands with PPh3, it was found that the acylphosphines promote the
hydrosilylation process and improve the selectivity towards the b-
adduct. Among the acylphosphines tested, a lower selectivity of the
1: R=H; 2: R=o-Cl; 3: R=o-CH3; 4: R=o-OCH3; 5: R=o-F; 6: R=p-Cl; 7: R=p-CH3;
8: R=p-OCH3; 9: R=p-F.
Scheme 1. The formula of a series of acylphosphines.
Si(R1)3
α-adduct
O
R
H
P
Si(R1)3
RhCl3
Acylphosphine Ligand
Cl
Et3N,Et2
r.t.,Ar
O
HSi(R1)3
R
β-adduct
O
R
+
+
P
R
alkane
R
Si(R1)3
R
unsaturated-adduct
R
R= Ph,n-C4H9, n-C6H13, n-C10H21;
R1= OCH2CH3.
1: R=H; 2: R=o-Cl; 3: R=o-CH3; 4: R=o-OCH3; 5: R=o-F; 6: R=p-Cl; 7: R=p-CH3;
8: R=p-OCH3; 9: R=p-F.
Scheme 2. Synthesis of a series of acylphosphines.
Scheme 3. The hydrosilylation reaction of alkenes catalyzed with rhodium complex.

J. Li et al. / Journal of Organometallic Chemistry 855 (2018) 7e11
9
Table 1
Effect of acylphosphine ligands on hydrosilylation catalyzed with RhCl3.
Entry
ligand
Conv. (%)
Selectivity (%)
b
a
Ethylbenzene
Dehydrogenative silylation
1
2
3
4
5
6
7
8
9
PPh3
95.4
99.8
99.6
99.9
99.7
99.5
96.4
95.1
95.8
99.5
66.1
88.6
84.0
84.9
82.9
72.1
79.5
82.7
87.8
78.9
6.0
1.6
1.6
1.3
3.3
2.2
1.3
1.6
1.0
3.5
16.7
6.9
9.3
11.2
2.9
5.1
4.4
3.4
8.7
5.4
4.3
2.9
6.7
1
2
3
4
5
6
7
8
9
9.4
10.4
17.0
13.8
11.4
8.3
10
10.9
Reaction conditions: styrene 4 mmol, triethoxysilane 4.4 mmol, 70 ꢂC, 10 h.
Catalyst: RhCl3 0.25% (based on styrene); Ligand: 1.25% (based on styrene).
product was observed when using RhCl3/5 as the catalyst system.
This indicates that the substituent attached to the benzene ring of
the acylphosphines has an impact on the catalytic process. In
particular, when 1 was used as the ligand, it exhibited greater
% RhCl3, respectively. The conversion of styrene was close to 100%
and the selectivity towards the -adduct was about 90% in the
presence of 0.25 mol% RhCl3 (Fig. 3). The conversion of styrene and
b
the selectivity towards the
amount of RhCl3 used.
b-adduct increased upon increasing the
catalytic activity and higher selectivity towards the b-adduct, and a
small quantity of the dehydrogenative silyation product was
detected.
The effects of varying the molar ratio of 1 to RhCl3 were
examined using 1:1, 3:1, 5:1, 7:1, 9:1, 15:1 and 25:1 of 1/RhCl3,
respectively (Fig. 4). Upon increasing the molar ratio of 1 to RhCl3,
the conversion of styrene and the selectivity towards the
increased. When the ratio of 1 to RhCl3 was 5:1, the conversion of
styrene was close to 100% and the selectivity towards the -adduct
was about 90%. When the molar ratio of 1 to RhCl3 was greater than
5:1, the conversion of styrene and the selectivity towards the
adduct quickly reduced. The role of the acylphosphine was thought
to improve the catalytic activity and selectivity of the rhodium
complex. However, it is not conducive to the reaction when the
ratio of 1 to RhCl3 was greater than 5:1.
b-adduct
3.2. The effects of the reaction temperature and time on the
hydrosilylation reaction
b
The effects of the reaction temperature and time on the catalytic
activity are illustrated in Figs. 1 and 2. There was no remarkable
induction period using the different temperatures studied and the
activity of the RhCl3/1 catalyst system increased upon increasing
the reaction temperature. Using the same amount of catalyst, a
99.8% conversion was obtained at 70 ꢂC after 10 h, while a >99.9%
conversion of styrene was obtained within 6 h at 80 ꢂC. At 90 ꢂC, the
conversion of styrene reached >99.9% within 4 h. However, an
excessive reaction temperature can reduce the selectivity towards
b
-
3.4. Scope of the hydrosilylation reaction
the b-adduct.
With the optimized reaction conditions in hand, we next carried
out the hydrosilylation reaction using a variety of other alkenes
using triethoxysilane. When other alkenes such as 1-hexene, 1-
octene, 1-dodecene, 1-tetradecene, 1-(allyloxy)benzene, 1-(vinyl-
oxy)butane, ethoxyethene, 2-methylstyrene, 4-methylstyrene and
3.3. The effects of the amount of the RhCl3/1 catalyst system on the
hydrosilylation reaction
The hydrosilylation reaction was examined in the presence of
0.50 mol%, 0.25 mol%, 0.10 mol%, 0.05 mol%, 0.025 mol% and 0.1 mol
Fig. 1. The effect of reaction temperature on hydrosilylation catalyzed with RhCl3.
Reaction conditions: styrene 4 mmol, triethoxysilane 4.4 mmol, 10 h;.
Catalyst: RhCl3 0.25% (based on styrene); Ligand: 1 1.25% (based on styrene).
Fig. 2. The effect of reaction time on hydrosilylation catalyzed with RhCl3.
Reaction conditions: styrene 4 mmol, triethoxysilane 4.4 mmol;.
Catalyst: RhCl3 0.25% (based on styrene); Ligand: 1 1.25% (based on styrene).

10
J. Li et al. / Journal of Organometallic Chemistry 855 (2018) 7e11
4-methoxystyrene were used as the substrate, excellent conver-
sions and selectivities were obtained with the RhCl3/1 catalyst
system. At the same time, excellent conversions and selectivities
were obtained when ligands 2e5 were employed in the hydro-
silylation of 1-hexene. Also, when triethylsilane or diphenylsilane
were used to replace triethoxysilane, dehydrogenative silylation
was observed, which was attributed to triethylsilane and diphe-
nylsilane being easier to dehydrogenate when compared to trie-
thoxysilane (Table 2).
3.5. The reaction mechanism
On the basis of our results, we propose the following reaction
mechanism (Scheme 4). A reaction may occur between the acyl-
phosphine ligand and the Rh metal center to weaken the com-
plexing ability of chloride, which will help activate the alkene with
the Rh-acylphosphine ligand stabilizing the intermediate formed. It
can also be speculated that steric hindrance of the acylphosphine
ligands has some effect on the reaction pathway and the increased
Fig. 3. The effect of amount of RhCl3 used on hydrosilylation.
Reaction conditions: styrene 4 mmol, triethoxysilane 4.4 mmol, 70 ꢂC, 10 h;.
Catalyst: n (Rh): n (1) ¼ 1:5.
selectivity observed towards the formation of the
same time, some by-products are produced.
b-adduct. At the
4. Conclusions
In summary, a series of acylphosphines (1e9) have been syn-
thesized and the hydrosilylation reaction of alkenes using trie-
thoxysilane catalyzed by RhCl3/acylphosphine investigated. When
comparing all the catalysts studied, RhCl3/(diphenylphosphino)
(phenyl)methanone (1) exhibited higher activity as well as higher
levels of beadduct selectivity. The substituents in the acylphos-
phine ligands have a significant impact on the rhodium-catalyzed
hydrosilylation reaction. Further study on the structures of the
rhodium-acylphosphine complexes and their reaction mechanism
are ongoing in our laboratory.
Fig. 4. The effect of amount of 1 used on hydrosilylation catalyzed with RhCl3.
Reaction conditions: styrene 4 mmol, triethoxysilane 4.4 mmol, 70 ꢂC, 10 h;.
Catalyst: RhCl3 0.25% (based on styrene).
Table 2
Results of the hydrosilylation reaction of alkylene with silane.
Entry
Alkene
Silane
Conv. (%)
Selectivity (%)
b
a
Alkane
Dehydrogenative silylation
1
2
3
4
5
6
7
8
CH3(CH2)3CH ¼ CH2
CH3(CH2)5CH ¼ CH2
CH3(CH2)9CH ¼ CH2
CH3(CH2)11CH ¼ CH2
PhOCH2CH ¼ CH2
CH3(CH2)3OCH ¼ CH2
CH3CH2OCH ¼ CH2
2-Methylstyrene
4-Methylstyrene
4-Methoxystyrene
CH3(CH2)3CH ¼ CH2
CH3(CH2)3CH ¼ CH2
CH3(CH2)3CH ¼ CH2
CH3(CH2)3CH ¼ CH2
styrene
triethoxysilane
99.2
99.5
99.1
99.1
90.0
99.4
99.6
99.8
96.6
>99.9
99.0
>99.9
99.7
98.7
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
84.2
>99.9
99.7
88.0
e
e
e
e
e
e
e
e
e
e
e
e
2.4
e
13.4
e
e
e
e
e
0.3
3.7
3.1
3.0
e
1.5
1.2
1.5
e
6.8
6.0
6.8
e
9
89.7
88.7
10
11a
12b
13c
14d
15
16
>99.9
>99.9
>99.9
>99.9
76.4
e
e
e
e
e
e
e
e
e
diphenylsilane
triethysilane
1.1
1.3
5.1
5.7
17.4
21.1
styrene
71.9
Reaction conditions: alkene 4 mmol, triethoxysilane 4.4 mmol, 70 ꢂC, 10 h.
Catalyst: RhCl3 0.25% (based on styrene); Ligand: 1 1.25% (based on styrene).
a
Ligand: 2 1.25% (based on styrene).
Ligand: 3 1.25% (based on styrene).
Ligand: 4 1.25% (based on styrene).
Ligand: 5 1.25% (based on styrene).
b
c
d

J. Li et al. / Journal of Organometallic Chemistry 855 (2018) 7e11
11
Scheme 4. The reaction mechanism.
6290e6298.
Acknowledgements
[10] J.F. Yang, X.Y. Chen, Z.Q. Wang, Tetrahedron Lett. 56 (2015) 5673e5675.
[11] J. Sun, L. Deng, ACS Catal. 6 (2016) 290e300.
This work was supported by Zhejiang Provincial Technologies
R&D Program of China (grant no. 2017C31105).
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