Nickel-Catalyzed Selective Cross-Coupling of Chlorosilanes with Organoaluminum Reagents

Article abstract of DOI:10.1002/cctc.201900047

Nickel-catalyzed cross-coupling reactions of chlorosilanes with organoaluminum reagents were developed. An electron-rich Ni(0)/PCy₃ complex was found to be an effective catalyst for the desired transformation. The reaction of dichlorosilanes 1 proceeded to give the corresponding monosubstituted products 2. Trichlorosilanes 4 underwent selective double substitution to furnish the corresponding monochlorosilanes 2. Overall, the selective synthesis of a series of alkylmonochlorosilanes 2 from di- and trichlorosilanes was achieved using the present catalytic systems.

Full text of DOI:10.1002/cctc.201900047

Accepted Article
Title: Nickel-Catalyzed Selective Cross-Coupling of Chlorosilanes with
Organoaluminum Reagents
Authors: Yuki Naganawa, Haiqing Guo, Kei Sakamoto, and Yumiko
Nakajima
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10.1002/cctc.201900047
ChemCatChem
COMMUNICATION
Nickel-Catalyzed Selective Cross-Coupling of Chlorosilanes with
Organoaluminum Reagents
Yuki Naganawa,* Haiqing Guo, Kei Sakamoto, and Yumiko Nakajima*
Dedication ((optional))
Abstract: Nickel-catalyzed cross-coupling reactions of chlorosilanes
with organoaluminum reagents were developed. An electron-rich
Ni(0)/PCy3 complex was found to be an effective catalyst for the
desired transformation. The reaction of dichlorosilanes 1 proceeded
to give the corresponding monosubstitued products 2.
Trichlorosilanes 4 underwent selective double substitution to furnish
the corresponding monochlorosilanes 2. Overall, the selective
synthesis of a series of alkylmonochlorosilanes 2 from di- and
trichlorosilanes was achieved using the present catalytic systems.
who reported silyl-Heck reaction[8] with olefins as well as silyl-
Negishi coupling with organozinc reagents[9] and silyl-Kumada–
Tamao coupling with Grignard reagents[10] of halosilanes
(Scheme 1c). In these reactions, the scope of the substrates is
mainly limited to the use of monohalosilanes (R3SiX), and the
product selectivity was not discussed in detail.
Chlorosilanes are important raw materials for the production of
various organosilicon materials, such as silicones and silane
coupling reagents. The performance of the organosilicon
materials is highly dependent on the substituent groups on the
silicon atom; thus, the synthesis of chlorosilanes bearing various
organic substituents (organochlorosilanes) occupies
a
fundamental position in the silicon industry.[1,2] There has been
increasing demand for high-performance silicon materials,
especially in recent years. Thus, the development of new methods
for the precise introduction of organic substituents into
chlorosilanes is of great importance.
The representative strategies of C–Si bond formation[3-11] was
classified in Scheme 1. Industrially, copper-catalyzed reactions of
metal silicon with alkyl chlorides (i.e., the Müller–Rochow “direct
process” method),[3] are performed to furnish a mixture of
organochlorosilanes (RSiCl3, R2SiCl2, R3SiCl, etc) in large-scale
quantities (Scheme 1a). However, the product selectivity of this
process is generally poor and subsequent fractional distillation of
the reaction mixture is usually required to isolate the desired
product.
Common preparative methods that are used on a laboratory scale
to access organochlorosilanes include alkylation and arylation of
di-, tri-, and tetrachlorosilanes with organometallic reagents, such
as organolithium or Grignard reagents (Scheme 1b). In this case,
the low reaction selectivity with respect to the introduction of
organic groups, stemming from the high reactivity of
organometallic reagents, is a troublesome issue.[4,5]
Transition-metal-catalyzed cross-coupling of halosilanes with
various coupling partners are attractive methods to synthesise
organochlorosilanes with various substituents selectively.[6-11]
Recent research in this field has been conducted by Watson et al.,
Scheme 1. Strategies of C–Si bond formation for syntheses of
organochlorosilanes.
Meanwhile, we also reported nickel-catalyzed silyl-Heck reaction
of chlorosilanes.[11] This research demonstrated 1) the first
example of direct catalytic transformation of strong Si–Cl bonds
(ca. 113 kcal/mol) into Si–C bonds and 2) the selective
substitution of di- and trichlorosilanes to give monoalkenyl
chlorosilanes. The key finding in this reaction was the efficient
catalytic system composed of electron-rich Ni(0) supported by
PCy3 and organoaluminum reagents. We postulate that the Si–Cl
bond cleavage could be facilitated by an SN2-type process[12] with
electron-rich Ni(0) species. At the same time, Lewis acidic
organoaluminum electrophilically activates the Si–Cl bond to
assist the oxidative addition.[13]
[a]
Dr. Y. Naganawa, Dr. H. Guo, K. Sakamoto, Dr. Y. Nakajima
Interdisciplinary Research Center for Catalytic Chemistry
National Institute of Advanced Industrial Science and Technology
Tsukuba, Ibaraki 305-8565, Japan
In this context, our interest focused on the use of the
E-mail: yuki.naganawa@aist.go.jp/yumiko-nakajima@aist.go.jp
organoaluminum reagent itself as
a coupling partner of
chlorosilanes.[14,15] Herein, we report the nickel-catalyzed cross-
coupling reaction of chlorosilanes with organoaluminum reagents
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/cctc.201900047
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Table 2. Scope and limitations of nickel-catalyzed cross-coupling reaction
of dichlorosilanes 1 with trialkylaluminum.[a]
for the selective introduction of alkyl substituents into
chlorosilanes (Scheme 1d).
Table 1. Reactions of phenylmethyldichlorosilane (1a) with various
methylation reagents.[a]
R1, R2
(1)
T
(°C)
% Yield (2’)[c]
(ratio of 2/3)[d]
Entry
R[b]
Entry
Me-[M][b]
(x equiv)
Ni cat.
%Conv.
% Yield
1
2
Ph, Me (1a)
Me
Et
60
90
99 (2’a) (>20/1)
91 (2’b) (>20/1)
>99 (2’c) (>20/1)
<5 (–/–)
(1a)[c]
(2a/3a)[c]
1a
1
2
3
4
5
6
7
Me3Al (1)
Me3Al (2)
Me2AlCl (2)
Me3Al (2)
MeLi (1)
Ni(cod)2/2PCy3
Ni(cod)2/2PCy3
Ni(cod)2/2PCy3
Ni(cod)2/2PPh3
None
>95
>95
<5
>95/<5
>95/<5
<5/trace
<5/trace
33/43
3
1a
C8H17
iBu
120
120
60
4
1a
5
1a
Ph
68 (2’d) (>20/1)
89 (2’b) (>20/1)
80 (2’e) (>20/1)
50 (2’f) (>20/1)
>99 (2’d) (>20/1)
78 (2’g) (>20/1)
99 (2’h) (>20/1)
88 (2’i) (5/1)
<5
6
Ph, Et (1b)
Me
90
76
7[e]
8[e]
9
1b
Et
120
120
60
MeMgBr (1)
Me3Al (2)
None
90
78/12
1b
C8H17
Me
None
n.r.
–
Ph, Ph (1c)
[a] Reaction conditions: 1a (0.25 mmol), methylation reagent (0.5 or 0.25
mmol), Ni(cod)2 (5 mol%), PCy3 (10 mol%), dioxane (0.5 mL), 60 C, 24 h.
[b] Concentration of methylation reagents; Me3Al (1.4 M in hexane), Me2AlCl
(1.1 M in hexane), MeLi (1 M in Et2O), MeMgBr (3 M in Et2O). [c] Based on
1H NMR spectroscopic analysis with mesitylene (0.25 mmol) as internal
standard. n.r. = no reaction
10
11
12
13
14
15
16
17
18
19
20
1c
Et
120
120
60
1c
C8H17
Me
p-Tol, Me (1d)
1d
Et
120
120
60
82 (2’j) (>20/1)
94 (2’k) (>20/1)
>99 (2’l) (>20/1)
87 (2’m) (>20/1)
82 (2’n) (>20/1)
11 (2o)[f] (1/2.1)
14 (2p)[f] (2.3/1)
36 (2q)[f] (1.3/1)
1d
C8H17
Me
o-Tol, Me (1e)
First, we examined reactions of methylphenyldichlorosilane (1a)
with trimethylaluminum or dimethylaluminum chloride, which were
used in the previous silyl-Heck reactions (Table 1). The reaction
of 1a with trimethylaluminum proceeded smoothly at 60 C in the
presence of a Ni(0) catalyst prepared in situ from Ni(cod)2 (5
mol%) and PCy3 (10 mol%) to provide the corresponding
methylated products (Table 1, entry 1). Monomethylated product
2a was predominantly formed (>95% yield) accompanied by the
dimethylated product 3a as a minor product. To our surprise, even
with 2 equivalents of trimethylaluminum, the formation of
dimethylated product 3a was almost completely suppressed
(Table 1, entry 2). Reflective of these results, dimethylaluminum
chloride did not work as a suitable coupling partner (Table 1, entry
3). It was confirmed that Ni(0) species with triphenylphosphine as
a less donating ligand did not catalyze the reaction (Table 1, entry
4). Reactions of 1a with an equimolar amount of highly reactive
1e
Et
120
120
60
1e
C8H17
Me
Me, Me (1f)
1f
1f
Et
90
C8H17
90.
[a] Reaction conditions: 1 (0.5 mmol), trialkylaluminum (1.0 mmol), Ni(cod)2
(5 mol%), PCy3 (10 mol%), dioxane (1.0 mL), 60–120 C, 24 h. [b] Hexane
or nBu2O solution (see SI). [c] Isolated yield. [d] Based on 1H NMR
spectroscopic analysis with mesitylene (0.5 mmol) as internal standard. [e]
Ni(cod)2 (10 mol%), PCy3 (20 mol%). [f] NMR yield.
Next, we moved to clarify the scope of the substrates in the nickel-
catalyzed cross-coupling reaction of dichlorosilanes 1 with 2
equivalents of trialkylaluminum (Table 2). We determined the
isolated yields of each product after the conversion of 2 into the
corresponding alkoxysilanes 2’, which are stable in the
purification process by column chromatography. For example, the
standard reaction of 1a with trimethylaluminum and the
continuous workup with cyclohexanol and triethylamine in the
organometallic
reagents,
such
as
methyllithium
or
methylmagnesium bromide, furnished the methylated products 2a
and 3a under the non-catalytic reaction conditions, albeit with
poor reaction selectivity (Table 1, entries 5 and 6). We also
confirmed that the reaction did not proceed without a catalyst
(Table 1, entry 7).
presence of
a
catalytic amount of DMAP (N,N-
dimethylaminopyridine) gave the corresponding product 2’a in a
quantitative yield (Table 2, entry 1). The reaction with
alkylaluminum with longer alkyl chains such as triethylaluminum
and trioctylaluminum similarly proceeded at higher reaction
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10.1002/cctc.201900047
ChemCatChem
COMMUNICATION
temperatures to form the corresponding 2’b and 2’c in 91% and
>99% yields, respectively (Table 2, entries 2 and 3). The use of
bulky triisobutylaluminum resulted in the formation of trace
amounts of products (Table 2, entry 4). The introduction of phenyl
group was also performed using triphenylaluminum to give 2’d in
68% yield (Table 2, entry 5). Ethylphenyldichlorosilane (1b) and
diphenyldichlorosilane (1c) were also tolerated and
predominantly provided the corresponding monoalkylated
products 2’ in good yields (Table 2, entries 6–11). Considering the
steric effect of the substituent in 1, we tested the reaction of
furnish the di-alkylated monochlorosilanes 2 (Table 3). For
example,
tolyltrichlorosilane (4b) and o-tolyltrichlorosilanes (4c) with
trialkylaluminum provided the corresponding
reactions
of
phenyltrichlorosilane (4a),
p-
dialkylarylmonochlorosilanes 2 in a selective manner (Table 3,
entries 1 and 3–9). Selective formation of monosubstituted
product 1a was not achieved either even in the reaction with 1.2
equiv. of trimethylaluminum (Table 3, entry 2). The octylation of
sterically-hindered 4c only led to the formation of monooctyl
product 1v as a major product (Table 3, entry 10). Almost no
trialkylated 3 was formed with any of substrates. After alcoholic
workup, the corresponding alkoxysilanes 2’ were obtained in
good yields.
methyl(p-tolyl)dichlorosilane
(1d)
and
methyl(o-
tolyl)dichlorosilane (1e). Both 1d and 1e reacted with various
trialkylaluminum compounds to give the products 2’ in good yields
without any difficulties (Table 2, entries 12–17). Unfortunately, the
reactions of dimethyldichlorosilane (1f) were not successful and
resulted in poor product selectivity (Table 2, entries 18–20).
Table 3. Scope and limitations of nickel-catalyzed double cross-coupling
reaction of trichlorosilanes 4 with trialkylaluminum.[a]
R1 (1)
R[b]
T
(°C)
% Yield (2’)[c]
(ratio of 1/2/3)[d]
Entry
1
2[e]
3[f]
4[f]
5
Ph (4a)
Me
Me
60
60
73 (2’a) (1/>20/–)
57 (2a)[g] (1/8.1/–)
49 (2’e) (1/>20/–)
52 (2’r) (1/5.7/–)
88 (2’i) (1/>20/–)
57 (2’s) (1/>20/–)
62 (2’t) (1/3.0/–)
91 (2’l) (1/>20/–)
41 (2’u) (1/>20/–)
33 (2’v) (2.5/1/–)
4a
4a
4a
Et
120
120
60
Scheme 2. (a) Gram-scale synthesis of 2c. (b) Decrease in the amount of
trimethylaluminum in the presence of triisobutylaluminum. (c) Alkylation and
silyl-Heck reaction sequence.
C8H17
Me
p-Tol (4b)
4b
6[f]
7[f]
8
Et
120
120
60
4b
C8H17
Me
To further demonstrate the utility of the developed protocol, we
o-Tol (4c)
4c
performed the following experiments. First,
a gram-scale
9[f]
10[f]
Et
120
120
synthesis of monochlorosilane 2c from 1a (10 mmol) and
trioctylaluminum was performed. After the simple distillation of the
crude reaction mixture, 2c was obtained in 64% isolated yield
(Scheme 2a)
Interestingly, it was revealed that the monomethylation of 1a
successfully proceeded using the reduced amount of
trimethylalminum (0.5 equiv) in the presence of 0.5 equiv of
triisobutylalminum (Scheme 2b). In the reaction, the resulting
dimethylaluminiumchloride should undergo alkyl ligand exchange
4c
C8H17
[a] Reaction conditions: 4 (0.5 mmol), trialkylaluminium (solution in hexane
(see SI), 1.0 mmol), Ni(cod)2 (5 mol%), PCy3 (10 mol%), dioxane (1.0 mL),
60–120 C, 24 h. [b] Hexane solution (see SI). [c] Isolated yield. [d]
Determined by 1H NMR with mesitylene (0.5 mmol) as an internal standard.
[e] 1.2 equiv. of trimethylaluminium was used. [f] Ni(cod)2 (10 mol%), PCy3
(20 mol%). [g] NMR yield.
with
triisobutylalminum
in
situ
to
form
reactive
dimethylisobutylalminium.
Finally, sequential reactions of the initial methylation of 1a with
trimethylaluminum and the subsequent silyl-Heck reaction with
We then conducted the reactions of trichlorosilanes 4, in which
the double cross-coupling reaction of 4 occurred selectively to
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Figure 1. Possible catalytic cycle.
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[7]
styrene, both of which are promoted by the same nickel
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A possible mechanism of the nickel-catalyzed cross-coupling
reaction of chlorosilanes with trialkylaluminum is illustrated in
Figure 1. The overall catalytic cycle involves a similar process to
that of well-known C–C cross-coupling reactions, namely: i) an
oxidative addition of chlorosilanes to Ni(0) species, ii) a
transmetalation with the coupling partner, and iii) a reductive
elimination to form Si–C bonds. As proposed in our previous study
on silyl-Heck reaction,[10] the oxidative addition of strong Si–Cl
bonds could be assisted by Lewis acidic organoaluminum
reagents.
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In summary, we have developed a novel method for the
preparation of monochlorosilanes with alkyl groups via nickel-
catalyzed cross-coupling reaction of di- and trichlorosilanes with
trialkylaluminum, which are hardly obtained via conventional
nucleophilic additions of di- or trichlorosilanes with organometallic
reagents.[3,4] It should be noted that the incorporation of a C1 unit
(i.e., a methyl group) is well demonstrated with the present
method, whereas hydrosilylation of olefins, which isone of reliable
C–Si bond forming reactions, is in principle not applicable for this
purpose.[16] Further investigations on catalytic transformations of
chlorosilanes with other coupling partners should open up new
opportunities for the synthesis of organochlorosilanes and studies
in this direction are under way in our laboratory
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Acknowledgements
This research was supported by Grants-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science
(No. 18K05115 and No. 18H01986).
Keywords: Nickel • Silicon • Aluminum •Cross-coupling •
Chlorosilanes
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[1]
[2]
a) S. J. Clarson, M. J. Owen, S. D. Smith, M. E. Van Dyke, Advances in
Silicones and Silicone-Modified Materials; American Chemical Society:
Washington, DC, 2010; b) J. E. Mark, D. W. Schaefer, G. Lin, The
Polysiloxanes; Oxford University Press: New York, 2015.
Si–Cl bond forming reactions have been also reported as a means of the
preparation of organochlorosilanes. Examples, see: a) W. Wang, Y. Tan,
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