Borohydride catalyzed redistribution reaction of hydrosilane and chlorosilane: A potential system for facile preparation of hydrochlorosilanes

Article abstract of DOI:10.1039/d0ra03536j

Various borohydrides were found to catalyze the redistribution reaction of hydrosilane and chlorosilane in different solvents to produce hydrochlorosilanes efficiently and facilely. The redistribution reaction was affected by solvent and catalyst. The substrate scope was investigated in HMPA with LiBH₄ as catalyst. A possible mechanism was proposed to explain the redistribution process.

Full text of DOI:10.1039/d0ra03536j

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Borohydride catalyzed redistribution reaction of
hydrosilane and chlorosilane: a potential system for
facile preparation of hydrochlorosilanes†
ab
Cite this: RSC Adv., 2020, 10, 17404
Yi Chen,ab Liqing Ai,ab Yongming Lia and Caihong Xu
*
Received 19th March 2020
Accepted 27th April 2020
Various borohydrides were found to catalyze the redistribution reaction of hydrosilane and chlorosilane in
different solvents to produce hydrochlorosilanes efficiently and facilely. The redistribution reaction was
affected by solvent and catalyst. The substrate scope was investigated in HMPA with LiBH4 as catalyst. A
possible mechanism was proposed to explain the redistribution process.
DOI: 10.1039/d0ra03536j
rsc.li/rsc-advances
Hydrochlorosilanes, with both Si–H and Si–Cl bonds on the in past decades. In 1992, Kunai reported a simple method for
silicon center, have attracted much attention in past decades for selective chlorination of hydrosilane using stoichiometric CuCl2
their importance in the production of functional polysiloxanes, in the presence of CuI.5a,b The catalytic efficiency was further
polysilanes, polysilazanes, and other silicon-containing mate- improved by adding ceramic spheres into the system.5c Chulsky
rials.1 However, the facile and highly selective preparation of and Dobrovetsky studied the selective chlorination of Si–H
this type of compound has remained a great challenge, espe- bond with HCl gas in the presence of B(C6F5)3 or B(C6F5)3/Et2O
cially for bulk production. The important commercially avail- catalyst, and proposed the corresponding mechanisms in
able hydrochlorosilanes, MeSiHCl2 and MeSiH2Cl, are obtained 2017.6a Recently, Sturm reported that the Si–H can be activated
2
¨
as by-products from the Muller–Rochow Direct Process.
by Lewis base, such as ethers, amines, and chloride ions. The
Regarding the preparation of hydrochlorosilanes, much activated Si–H was then selectively chlorinated by the HCl/ether
research work has been focused on the redistribution reaction solution.6b Though various catalytic systems for selective chlo-
between hydrosilane and chlorosilane, selective chlorination of rination of hydrosilane have been developed, most of them have
hydrosilane, and partial reduction of chlorosilane.3 In 1947, been limited in laboratory synthesis, due to disadvantages of
Sommer et al. rst reported the redistribution reaction between high cost or strict operation conditions. It is still an important
hydrosilane and chlorosilane catalyzed by AlCl3.3a Then the issue to achieve simple and economic preparation of
quaternary ammonium salt and tertiary amine were found also hydrochlorosilanes.
can catalyze the redistribution.3b–d However, both of them
Herein we demonstrate a borohydride catalyzed redistribu-
require relatively high temperature and strict operation condi- tion reaction system, which leads to a facile and exible prep-
tions, which cause extra cost and limitation of available aration of hydrochlorosilane. During the preparation of
substrates. Preparation of hydrochlorosilane by partial reduc- Cl2CHSiMeH2 (1) by reduction of Cl2CHSiMeCl2 (2) using
tion of chlorosilane has been an attractive subject. NaBH4 was borohydrides in THF, hydrochlorosilane Cl2CHSiMeHCl (3) was
reported to partially reduce dialkyldichlorosilane to dialkyl- unexpectedly detected when chlorosilane 2 was accidentally
chlorosilane in hexamethyl phosphoric triamide (HMPA), but mixed with an ether solution of hydrosilane 1 that contained
the mechanism of the selectivity in this reduction system was some LiBH4 residue. We guessed that the formation of 3 may be
not well understood.4d Attempts to produce hydrochlorosilane from the redistribution reaction of 1 and 2, while LiBH4 served
by reduction of chlorosilane with LiAlH4, the commonly used as a catalyst. To verify the conjecture and to comprehend the
reductants, have not been realized facilely and efficiently possible solvent effect, we examined the reaction of ClCH2SiCl3
because of the intractable over reduction.4
and ClCH2SiH3 in different solvents in the presence of LiBH4
The research on the preparation of hydrochlorosilanes by (Table 1). As expected, the redistribution reaction proceeded in
selective chlorination of hydrosilanes has made great progress various solvents, particularly those with relatively high polarity,
e.g., tetrahydrofuran (THF), CH3CN, diethylene glycol dimethyl
ether (diglyme), 1,3-dimethyl-2-imidazolidinone (DMI), and
aBeijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry,
HMPA (Table 1, entries 2–6). The relatively high conversion and
Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: caihong@iccas.ac.
selectivity encouraged us to further investigate the redistribu-
cn
bUniversity of Chinese Academy of Sciences, Beijing, 100049, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra03536j
tion reaction, which may be a useful way to synthesize hydro-
chlorosilane. Although CH3SiHCl2 was unexpectedly formed in
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a
Table 1 Solvent optimization of redistribution reaction of ClCH2SiH3/ClCH2SiCl3
LiBH4ðcat:Þ
ClCH2SiH3 þ ClCH2SiCl3
ClCH SiH Cl þ ClCH SiHCl
ƒƒƒƒƒƒƒƒƒ!
2
2
2
2
RT; 16 h
Entry
Solvent
Product (yield)b
No reaction
CH3SiHCl2: 6%, ClCH2SiHCl2: 63%, ClCH2SiH2Cl: 12%
ClCH2SiHCl2: 63%, ClCH2SiH2Cl: 16%
ClCH2SiHCl2: 71%, ClCH2SiH2Cl: 11%
ClCH2SiHCl2: 71%, ClCH2SiH2Cl: 14%
ClCH2SiHCl2: 72%, ClCH2SiH2Cl: 9%
No reaction
1
2
3
4
5
6
7
8
9
None
Diglyme
THF
CH3CN
DMI
HMPA
Bu2O
Et2O
Toluene
No reaction
No reaction
a
Reaction conditions: ClCH2SiH3 (0.01 mol), ClCH2SiCl3 (0.02 mol), LiBH4 (3.0 mol/%), THF (5 mL), room temperature. b Yields were determined
1
by H NMR.
a yield of 6% in diglyme, suggesting the C–Cl bond also NaBH4 in diglyme, which is related to the character of metal
participated in the redistribution reaction with Si–H bond ion, and (b) the different solubility of the formed metal chlo-
(Table 1, entry 2), there were few detected CH3SiHCl2 in THF, ride, LiCl and NaCl.7
DMI, or HMPA. It may be attributed to the different solvation
effects of LiBH4, which resulted in their different reductive substrate scope of the redistribution reaction in the LiBH4/
activity toward the chloromethyl group. HMPA system (Table 3). The redistribution reaction of Si–H and
Then under the optimized condition, we investigated the
We further optimized the catalyst with the substrate system Si–Cl proceeded efficiently for various substituted hydrosilane
of ClCH2SiCl3/ClCH2SiH3 as a model (Table 2). It is found that and chlorosilane substrates. The substituents include chlor-
in THF, all examined lithium salt, LiBH4, LiBEt3H, and LiAlH4, oalkyl, alkyl, and phenyl groups. Especially, for a-chloromethyl
can catalyze the redistribution reaction (Table 2, entries 1–5), silane substrates, e.g., ClCH2SiCl3/ClCH2SiH3, ClCH2SiMeCl2/
but NaBH4 and KBH4 which have poor solubility cannot (Table ClCH2SiMeH2, Cl2CHSiMeCl2/Cl2CHSiMeH2, the redistribution
2, entries 6 and 7). Changed the solvent from THF to diglyme, was completed in half an hour (Table 3, entries 1–4). The
a better solvent for NaBH4 and KBH4, the redistribution reac- stronger electron-withdrawing effect of the substituents on
tion proceeded, though the yield of product is low (see ESI, silicon center may be the reason. For the reaction system of
Fig. 35 and 36†). This result indicated that the solubility of RSiCl3/RSiH3, namely, the substrate has more available redis-
catalyst in solvent is important for the reaction. With the tribution groups, different predominated product was achieved
increase of catalyst dosage, the yield of hydrochlorosilane through adjustment of the proportion of hydrosilane and
increased (Table 2, entries 1–4). Besides, the metal counterion chlorosilane in reactants (Table 3, entries 1, 2, 6 and 7). The
of BH4ꢀ also affects the redistribution reaction. With 4 mol% of LiBH4 catalyzed redistribution reaction in HMPA has yields
LiBH4 or NaBH4 as catalyst, the conversion efficiencies for the ranging from 56% to 85%. Further work is still needed to realize
redistribution reaction of Cl2CHSiMeCl2/Cl2CHSiMeH2 are 67% a thorough and selective redistribution, however, the advan-
and 15%, respectively, though both of them can be dissolved in tages of its very simple and mild reaction condition, easily
diglyme completely (see ESI, Fig. 36 and 37†). The possible acquired catalysts, relatively fast reaction rate, and high
reason may be (a) the different solvation effects of LiBH4 and conversion efficiency suggest
a great potential of the
a
Table 2 Catalyst optimization of redistribution reaction of ClCH2SiH3/ClCH2SiCl3
catalyst
THF; RT; 20 h
ClCH2SiH3 þ ClCH2SiCl3
ClCH SiH Cl þ ClCH SiHCl
ƒƒƒƒƒƒƒƒƒ!
2
2
2
2
Entry
Cat.
Cat. (mol%)
Product (yield)b
1
2
3
4
5
6
7
8
LiBH4
LiBH4
LiBH4
LiBH4
LiBEt3H
NaBH4
KBH4
0.4
0.8
1.5
3.0
3.0
3.0
3.0
3.0
ClCH2SiHCl2: 10%, ClCH2SiH2Cl: 20%
ClCH2SiHCl2: 32%, ClCH2SiH2Cl: 27%
ClCH2SiHCl2: 55%, ClCH2SiH2Cl: 18%
ClCH2SiHCl2: 63%, ClCH2SiH2Cl: 16%
ClCH2SiHCl2: 68%, ClCH2SiH2Cl: 12%
No reaction
No reaction
ClCH2SiHCl2: 18%, ClCH2SiH2Cl: 21%
LiAlH4
a
1
Reaction conditions: ClCH2SiH3 (0.01 mol), ClCH2SiCl3 (0.02 mol), THF (5 mL), room temperature. b Yields were determined by H NMR.
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a
Table 3 Redistribution reaction between hydrosilane and chlorosilane in HMPA catalyzed by LiBH4
LiBH4ðcat:Þ
R4ꢀxSiHx þ R4ꢀxSiClx
R4ꢀxSiHxꢀyCly
ƒƒƒƒƒƒƒƒƒ!
HMPA
Entry
R4ꢀxSiHx/R4ꢀxSiClx
Ratio
Cat. (mol%)
t (h)
Product (yield)b
1
2
3
4
5
6
7
8
9
ClCH2SiH3/ClCH2SiCl3
ClCH2SiH3/ClCH2SiCl3
ClCH2SiMeH2/ClCH2SiMeCl2
Cl2CHSiMeH2/Cl2CHSiMeCl2
Et2SiH2/Et2SiCl2
PhSiH3/PhSiCl3
PhSiH3/PhSiCl3
PhMeSiH2/PhMeSiCl2
Ph2SiH2/Ph2SiCl2
1 : 2
2 : 1
1 : 1
1 : 1
1 : 1
1 : 2
2 : 1
1 : 1
1 : 1
3
3
2
4
4
3
3
4
4
0.5
0.5
0.5
0.5
8
2
2
2
8
ClCH2SiHCl2: 75%, ClCH2SiH2Cl: 9%
ClCH2SiHCl2: 23%, ClCH2SiH2Cl: 56%
ClCH2SiMeHCl: 74% (62%c)
Cl2CHSiMeHCl: 85% (72%c)
Et2SiHCl:72%
PhSiHCl2: 61%PhSiH2Cl: 10%
PhSiHCl2:12%PhSiH2Cl: 61%
PhMeSiHCl: 66%
Ph2SiHCl: 63%
a
Reaction conditions: [entry 3–5, 8 and 9] hydrosilane (0.01 mol), chlorosilane (0.01 mol); [entry 1 and 6] hydrosilane (0.01 mol), chlorosilane (0.02
mol); [entry 2 and 7] hydrosilane (0.02 mol), chlorosilane (0.01 mol); HMPA (5 mL), room temperature. b Yields determined by 1H NMR. c Isolated
yield in a larger scale reaction. Reaction conditions: hydrosilane (0.2 mol), chlorosilane (0.2 mol), HMPA (10 mL), LiBH4 (2 mol%), room
temperature.
redistribution method in achieving more economic and effi- be easily achieved with LiAlH4 as reductant, if the solvents
cient preparation of hydrochlorosilanes.
which are effective for the redistribution system are used.
To demonstrate the utility of the redistribution strategy,
To make clear the mechanism, we further investigated the
larger scale syntheses of hydrochlorosilane ClCH2SiMeHCl and redistribution reaction. The Si–Cl bond of chlorosilane
ClCH2SiMeHCl were carried out. Aer nishing the reaction, all substrates can be reduced to Si–H bond by borohydrides, with
the volatiles were rst removed by vacuum distillation to give BH3 and chloride salt as the by-products (see ESI, Fig. 51†).
a silane mixture, which was then submitted to a packed column Thus, our initial consideration is that BH3, which is formed in
fractional distillation to produce pure hydrochlorosilane situ during the reduction reaction, may play the role of catalyst.
product, such as ClCH2SiMeHCl or Cl2CHSiMeHCl. The simple However, the experiment proved that BH3 alone cannot catalyze
manipulation and good yield indicated its high potential for the redistribution reaction (Table 4, entry 1). The Si–Cl reduc-
industrial application.
tion process of chlorosilane substrate with borohydrides is
On the other hand, the redistribution system also provides easily understood by referring to the well-studied substitution
a possible reference for the partial reduction of chlorosilane to reacꢀtion at silicon center using nucleophiles,8 i.e., the negative
prepare hydrochlorosilane with NaBH4, LiAlH4, and other BH4 attacks to the partial positive silicon center to form
common reductants. For the NaBH4/HMPA partial reduction a pentacoordinate intermediate of chlorosilane in the rst step,
system reported by Hiiro,4d the reduction selectivity of chlor- then looses BH3 and Clꢀ to form corresponding silane product.
osilane can be easily understood from the perspective of the We assumed that the chlorination process of Si–H may also
redistribution system herein, i.e., in the presence of decient undergo a similar process, i.e., Si–H was rst activated through
amount of borohydrides, hydrochlorosilanes can be produced the formation of pentacoordinate intermediate with Clꢀ that
from the redistribution reaction between the in situ formed formed in the Si–Cl reduction process. Then the in situ formed
hydrosilane products and the chlorosilane substrates. It is also Lewis acid BH3 during the Si–Cl reduction process interacted
easy to comprehend that partial reduction of chlorosilane can with the activated Si–H bond, which further promoted the Si–H
bond dissociation. The early reported research on the ability of
B(C6F5)3 and Clꢀ to activate Si–H bond6b,9 provides a good proof
for reasonability of the envisaged chlorination process.
According to the above hypothesis, we proposed a possible
Si–H/Si–Cl redistribution mechanism demonstrated as
Table 4 Control experiments of redistribution reaction of ClCH2SiH3/
ClCH2SiCl3
a
catalyst
RT; THF
Scheme 1, in which the process of Si–H chlorination and Si–Cl
reduction occur simultaneously. To verify it, the redistribution
reaction of ClCH2SiCl3/ClCH2SiH3 catalyzed by BH3/LiCl or LiCl
ClCH2SiH3 þ ClCH2SiCl3
ClCH SiH Cl þ ClCH SiHCl
ƒƒƒƒƒƒ!
2
2
2
2
t
Entry
Catalyst
(h)
Product (yield)b
1
2
3
4
BH3
LiCl
BH3, LiCl
LiBH4
20
5
5
No reaction
ClCH2SiHCl2: 40%, ClCH2SiH2Cl: 23%
ClCH2SiHCl2: 62%, ClCH2SiH2Cl: 18%
ClCH2SiHCl2: 62%, ClCH2SiH2Cl: 18%
5
a
Reaction conditions: ClCH2SiH3 (0.01 mol), ClCH2SiCl3 (0.02 mol),
b
catalyst (3.0 mol/%), THF (5 mL), room temperature. Yields were
determined by 1H NMR.
Scheme 1 The possible mechanism of borohydrides catalyzed Si–H/
Si–Cl redistribution reaction.
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alone was performed. As expected, LiCl can catalyze the redis-
tribution of ClCH2SiCl3/ClCH2SiH3, and introduction of BH3
can further accelerate the reaction obviously (Table 4, entry 2
and 3). Besides, almost identical results were achieved when the
same amount of LiBH4 or BH3/LiCl was used (Table 4, entry ꢀ3
and 4). The results indicate the similar catalytic effect of BH4
and BH3/LiCl, which can be well explained by the mechanism
proposed in Scheme 1.
3 (a) F. C. Whitmore, E. W. Pietrusza and L. H. Sommer, J. Am.
Chem. Soc., 1947, 69, 2108–2110; (b) A. Benouargha,
D. Boulahia, B. Boutevin, G. Caporiccio, F. Guida-
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J. S. Han and W.-C. Lim, US Pat., US5965762A, 1999; (d)
W. Katsuyu and T. Hidenori, European Patent, EP2308884
(A1), 2011.
To get a better understanding of the possible mechanism in
Scheme 1, a DFT calculation at the B3LYP/6-311G (d, p)10 level of
theory was performed on the pentacoordinate intermediates
[ClCH2SiCl3–H–BH3]ꢀ (I) and [ClCH2SiCl3–H]ꢀ (II), which was
formed in the redistribution of ClCH2SiH3/ClCH2SiCl3 accord-
ing to the mechanism we proposed. The solvation effect of THF
was also considered with SMD model. The calculation result
shows that the apical Si–H bond in intermediate II lengthens
4 (a) P. A. McCusker and E. L. Reilly, J. Am. Chem. Soc., 1953,
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ꢀ
ꢀ
ꢀ
ꢀ
from l.47 A to 1.49 A, while it is elongated from 1.49 A to 1.65 A
in intermediate I. Calculation results on the other possible
pentacoordinate intermediates in the redistribution of ClCH2-
SiH3/ClCH2SiCl3 were similar. It may reect the role BH3 or Clꢀ
plays in the activation of Si–H bond in the chlorination process
as the mechanism we proposed.
In conclusion, we have demonstrated that borohydrides
catalyzed redistribution reaction between hydrosilane and
chlorosilane in different solvents efficiently. The new catalytic
redistribution system works for a broad scope of substituted
hydrosilane and chlorosilane substrates. The very simple and
mild reaction condition, the easily acquired catalysts, the rela-
tively fast reaction rate, and high conversion efficiency are all
advantages of the redistribution system. Further improvement
of the redistribution system is currently under investigation in
our laboratory.
Chem. - Eur. J., 2018, 24, 17796–17801.
ˇ
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Conflicts of interest
There are no conicts to declare.
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