Hydrogenation of chlorosilanes by NaBH4

Article abstract of DOI:10.1246/cl.160808

Hydrogenation of chlorosilane was achieved in acetonitrile using NaBH₄, a safe and easy-to-handle reagent. This reaction converted Si-Cl portion(s) in organosilanes into Si-H portion(s) without hydrogenation of cyano, chloro, and aldehyde groups on an alkyl substituent of the Si reagents. In addition, the Si-Cl/Si-H exchange reaction was applicable to dichlorodisilane without Si-Si bond cleavage.

Full text of DOI:10.1246/cl.160808

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Hydrogenation of Chlorosilanes by NaBH4
Masaki Ito, Masumi Itazaki, Takashi Abe, and Hiroshi Nakazawa*
Advance Publication on the web October 13, 2016
doi:10.1246/cl.160808
© 2016 The Chemical Society of Japan
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1
Hydrogenation of Chlorosilanes by NaBH4
Masaki Ito,1 Masumi Itazaki,1 Takashi Abe2 and Hiroshi Nakazawa1*
Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan
Advanced Materials Research Center, Nippon Shokubai Co., Ltd., 5-8 Nishi Otabi-cho, Suita, Osaka 564-8512, Japan
1
2
(E-mail: < nakazawa@sci.osaka-cu.ac.jp)
Hydrogenation of chlorosilane was achieved in
acetonitrile using NaBH4 being safe and easy to handle. This
reaction converted Si-Cl portion(s) in organosilanes into Si-H
portion(s) without hydrogenation of cyano, chloro, and
aldehyde groups on an alkyl substituent of the Si reagents. In
addition, the Si-Cl/Si-H exchange reaction was applicable to
dichlorodisilane without Si−Si bond cleavage.
When THF was used instead of acetonitrile under the
reaction conditions depicted in Scheme 1, the hydrogenation
of chlorosilane took place although 5 h were needed to
complete the reaction (Table 1, entry 2; 78% isolated yield).
When the reaction was performed in other organic solvents
such as toluene, dichloromethane and diethyl ether, the
starting chlorosilane was not consumed at all (entries 3−5).
The reaction under neat conditions also did not proceed (entry
6). Therefore, we believe that acetonitrile is the best solvent in
this reaction and used it as a solvent hereafter.
LiAlH4 is one of the essential reagents for reduction of
various chemicals.1 However, the use of LiAlH4 needs great
care because of ignition and explosive nature. In addition,
LiAlH4 often causes undesired reduction at several functional
groups.
Hydrosilane is known as a mild reducing reagent,2 and
therefore has been widely used as a starting compound in, for
example, hydrosilylation,3 C−H silylation4 in order to obtain
silyl-substituted organic compounds. Transition metal-, group
13 metal- and organo compound-catalyzed reactions using
hydrosilane instead of LiAlH4 as a reducing reagent have been
developed toward alkyl halide,5 aldehyde,6 ketone,7 ester,8
amide,9 epoxide,10 nitro,11, CO2,12 nitrile,7b, 8a, 13 etc. mainly
from the safety reason. However, hydrosilane is generally
prepared by hydrogenation of chlorosilane with LiAlH4.
Therefore, the use of hydrosilane in place of LiAlH4 is not a
basic solution for the purpose of avoiding the use of LiAlH4.
Now, safe and easy handling reagent for hydrogenation has
been required. Sodium borohydride (NaBH4) seems to be one
of the candidates because it is a more handy reagent (much
less risky than LiAlH4). However, NaBH4 has not been
reported to hydrogenate chlorosilanes under normal
conditions. Here, we describe the first example of preparation
of hydrosilane from chlorosilanes using NaBH4.
Diphenylmethylchlorosilane Ph2MeSiCl (0.5 mmol) was
added dropwise to a suspension of NaBH4 (1.0 mmol) in
acetonitrile (0.5 mL) under nitrogen atmosphere, and the
mixture was stirred at room temperature for 15 min (Scheme
1). After removal of the volatile materials under reduced
pressure, n-hexane was added to the residue. The suspension
was passed through a silica gel pad and removal of n-hexane
under reduced pressure from the eluate afforded 84% of pure
diphenylmethylhydrosilane Ph2MeSiH. When the amount of
NaBH4 was reduced from 1.0 to 0.50 mmol, the yield of the
product was diminished (65% isolated yield).14
Table 1. Solvent screening for the hydrogenation of chlorosilane by
using NaBH4.a
Reaction time /h
Entry
Solvent
acetonitrile
THF
Isolated yield /%
0.25
5
1
2
3
4
5
6
84
78
−
24
24
24
24
Toluene
CH2Cl2
Et2O
−
−
−
−
aChlorosilane (0.5 mmol) and NaBH4 (1.0 mmol) in 0.5
mL of an organic solvent.
In order to obtain insight into the role of the cation (Na+),
we examined the reaction using LiBH4 instead of NaBH4
under the conditions similar to those in entry 1 in Table 1; the
reaction afforded the corresponding hydrosilane in 82% yield
(Scheme 2). It strongly suggests that the cation in [M][BH4]
does not play an important role in our reduction system.
Scheme 2. Reaction of Ph2MeSiCl with LiBH4.
Next, several chlorosilanes were examined to explore the
scope of our hydrogenation reaction (Table 2). The desired
reaction proceeded effectively for mono and trichlorosilanes
(entries 1−3 and 6). Dichlorosilanes were converted into the
Scheme 1. Hydrogenation of Ph2MeSiCl by NaBH4 in acetonitrile.

2
corresponding dihydrosilanes (entries 4 and 5). These yields
were moderate, but the starting dichlorosilanes were
completely consumed. In the reaction of Ph2SiCl2 with NaBH4,
the reduction of the amount of NaBH4 from 4 equiv (Table 2,
entry 4) to 2 equiv against Ph2SiCl2 revealed that the products
were a mixture of Ph2SiHCl and Ph2SiH2 (1.3:1 molar ratio).
The results show no big difference in the reaction rate
between the first and the second reductions.
Next, we examined the hydrogenation of chlorosilanes
having a functional group such as a cyano, chloro, or
aldehyde group on an alkyl substituent of the Si reagent
(Table 3).15 These groups are well-known to be reduced by
LiAlH4. It should be noted that these groups could tolerate the
hydrogenation by NaBH4 with hydrogenation of the Si−Cl
portion.
It was noteworthy that 1, 2−dichlorodisilane containing a
Si−Si bond was also converted into the corresponding
dihydrodisilane efficiently (Scheme 3) and no silyl
compounds caused by Si-Si bond cleavage were detected.
Moreover, monohydrogenated product (HMe2Si−SiMe2Cl)
was not observed in the 1H NMR spectrum. It should be noted
in the reduction of an Si-Si bond containing compound that
the quality of NaBH4 is crucial for hydrogenation to occur, as
NaBH4 of lesser purity provided some undesired products.15
The hydrogenation by NaBH4 is expected to be a useful
reduction method for polysilanes because a Si−Si bond being
important in the field of materials chemistry remains intact.16
Scheme 3. Hydrogenation of 1,2−dichlorodisilane using NaBH4 in
acetonitrile.
Table 2. Hydrogenation of chlorosilane by using NaBH4.a
In summary, we have developed a simple, efficient and
safety reaction to access hydrosilanes from chlorosilanes.
Moreover, some functionalized chlorosilanes can be
converted into the corresponding hydrosilanes. The real
reason why chlorosilanes can be converted into the
corresponding hydrosilanes by NaBH4 in acetonitrile is not
clear at the moment. Further investigation of the reaction
involving mechanistic consideration is in progress.
Entry
Chlorosilane
Ph2MeSiCl
Ph3SiCl
Hydrosilane
Ph2MeSiH
Ph3SiH
Yieldb,c /%
84
1
2
3
4
5
6
65
Et3SiCl
Et3SiH
(88)
45
Ph2SiCl2
Ph2SiH2
This work was supported by a Challenging Exploratory
Research Grant (No. 15K13662), a Grant-in-Aid for Science
Research Japan (C) (No. 16K05728), a Grant-in-Aid for
Scientific Research on Innovative Area “Stimuli-responsive
Chemical Species for the Creation of Function Molecules (No.
2408)” (JSPS KAKENHI Grant Number JP15H00957) from
MEXT Japan and the Sasakawa Scientific Research Grant
from The Japan Science Society.
(n-C6H13)2SiCl2
(PhC2H4)SiCl3
(n-C6H13)2SiH2
(PhC2H4)SiH3
54
(82)
aChlorosilane (0.5/n mmol) and NaBH4 (1.0 mmol) in
0.5/n mL of acetonitrile. Isolated yield. cYield based upon
b
1H NMR in parentheses.
Table 3. Hydrogenation of chlorosilane containing a functional group
using NaBH4.a
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ChemCatChem
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accepted
(DOI:
10.1002/cctc.201600940R1).
14 When 0.5 mmol NaBH4 was used, the chlorosilane was not
consumed completely and some unidentified signals in the 1H
NMR spectrum of the reaction mixture were observed.
15 Supporting Information is also available electronically on the
CSJ-Journal
Web
site,
http://www.csj.jp/journals/chem-
lett/index.html.
16 R. D. Miller, J. Michl, Chem. Rev. 1989, 89, 1359.