Synthesis of bromohydrosilanes: Reactions of hydrosilanes with CuBr2 in the presence of CuI

Article abstract of DOI:10.1246/cl.2001.1228

Reactions of hydrosilanes, R_4-nSiH_n (R = alkyl or phenyl, n = 1-3), with 2 equiv of CuBr₂ in the presence of a catalytic amount of CuI led to selective replacement of an H-Si bond with a Br-Si bond giving R₃SiBr, R₂SiHBr, or RSiH₂Br, while treatment of R₂SiH₂ and RSiH₃ with 4 equiv of the reagent produced R₂SiBr₂ and RSiHBr₂, respectively. Similar reaction of HEt₂SiSiEt₂H afforded HEt₂SiSiEt₂Br.

Full text of DOI:10.1246/cl.2001.1228

1228
Chemistry Letters 2001
Synthesis of Bromohydrosilanes: Reactions of Hydrosilanes
with CuBr2 in the Presence of CuI
Atsutaka Kunai,* Takahiko Ochi, Arihiro Iwata, and Joji Ohshita
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,
1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527
(Received October 18, 2001; CL-011023)
Reactions of hydrosilanes, R4-nSiHn (R = alkyl or phenyl, n
presence of a catalytic amount of CuI (thus CuBr2(CuI)
reagent), in a way analogous to chlorination.1 When Et3SiH
was treated with 2 equiv of the CuBr2(CuI) reagent in diethyl
ether at room temperature for 1.5 h, Et3SiBr was obtained in
82% yield after distillation of the reaction mixture (run 1 in
Table 1).
= 1–3), with 2 equiv of CuBr2 in the presence of a catalytic
amount of CuI led to selective replacement of an H–Si bond
with a Br–Si bond giving R3SiBr, R2SiHBr, or RSiH2Br, while
treatment of R2SiH2 and RSiH3 with 4 equiv of the reagent pro-
duced R2SiBr2 and RSiHBr2, respectively. Similar reaction of
HEt2SiSiEt2H afforded HEt2SiSiEt2Br.
Organohalosilanes are important starting materials for
building up organosilicon compounds as well as useful reagents
in synthetic organic chemistry. Of these, organohalohydro-
silanes have both Si–H and Si–halogen bonds and thus enables
us to carry out independent reactions, such as hydrosilylation
and nucleophilic substitution. Previously, we reported a con-
venient method for the synthesis of chlorohydrosilanes from
hydrosilanes with the use of CuCl2 in the presence of a catalytic
amount of CuI,1 and extended this method to the selective syn-
theses of fluorohydrosilanes2 and chlorohydrogermanes.3 We
also reported Pd-catalyzed H/I exchange reactions of hydrosi-
lanes with alkyl iodides to afford iodosilanes.4
This reagent could be applied for the monobromination of
dihydrosilanes.15 Thus, when n-Hex2SiH2 was treated in
diethyl ether with 2 equiv of the CuBr2(CuI) reagent at room
temperature, the starting silane disappeared within 1 h, and dis-
tillation of the resulting mixture afforded n-Hex2SiHBr in 73%
yield as the sole volatile product (run 2). Similar treatment of
MePhSiH2 for 9 h afforded MePhSiHBr in 68% yield, while the
reaction of Ph2SiH2 in toluene for 7.5 h produced Ph2SiHBr in
64% yield (runs 4 and 5).
As to bromosilanes, there have been reported several syn-
thetic methods, such as reactions of polybromosilanes with
Grignard reagents,5 cleavage of Si–O bonds with PBr3,6 cleav-
age of Si–Ph bonds7 or Si–Si bonds8 with Br2, Cl/Br exchange
6a
of chlorosilanes with AlBr3 or MgBr2,9 bromination of
polyhydrosilanes with Br2,
To obtain more information, we carried out dibromination
of dihexylsilane and monitored the progress by GLC. When n-
Hex2SiH2 was treated with 4 equiv of the CuBr2(CuI) reagent in
ether at room temperature, dihexylsilane disappeared soon after
to generate n-Hex2SiHBr as the primary product. The mono-
bromide then diminished slowly and disappeared after 44 h,
during which n-Hex2Si(OEt)H and n-Hex2Si(OEt)Br began to
increase and became the final products, indicating that the
6a,10
HBr,11 HgBr2,12 or NBS,13 and
partial reduction of 1,2-dibromodisilanes with trialkyltin
hydrides,14 but it is difficult to obtain selectively a desired type
of bromohydrosilane by these methods.
In our continuing study on halogenation of hydrosilanes,
we found that hydrosilyl groups can be transformed into bro-
mosilyl groups with the use of CuBr2, instead of CuCl2, in the
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1229
monobromination proceeds quite fast, while dibromination
requires much longer time, and the bromide once formed reacts
gradually with ether to give ethoxy derivatives.16 To suppress
undesired reactions, we changed the solvent from ether to ben-
zene. When n-Hex2SiH2 was treated with 4 equiv of the
CuBr2(CuI) reagent in benzene under reflux for 34 h, n-
Hex2SiBr2 was obtained in 70% isolated yield (run 3).
Mono- and dibrominations of trihydrosilanes are also pos-
sible (runs 6–9). When n-HexSiH3 was treated with 2 equiv of
the CuBr2(CuI) reagent in ether at room temperature for 12 h,
n-HexSiH2Br was obtained in 64% yield as the sole product
after distillation. When n-HexSiH3 was treated with 4 equiv of
the CuBr2(CuI) reagent in benzene at room temperature for 1
week, n-HexSiHBr2 was obtained in 55% isolated yield.
Moreover, the reaction of PhSiH3 with 2 equiv of the reagent in
benzene at room temperature for 6 h afforded PhSiH2Br in 72%
yield, while treatment with 4 equiv of the reagent in benzene at
room temperature for 40 h produced PhSiHBr2 in 67% yield.
References and Notes
1
a) A. Kunai, T. Kawakami, E. Toyoda, and M. Ishikawa,
Organometallics, 11, 2708 (1992). b) M. Ishikawa, E.
Toyoda, M. Ishii, A. Kunai, Y. Yamamoto, and M.
Yamamoto, Organometallics, 13, 808 (1994).
2
3
4
5
6
A. Kunai, T. Sakurai, E. Toyoda, and M. Ishikawa,
Organometallics, 15, 2478 (1996).
J. Ohshita, Y. Toyoshima, A. Iwata, H. Tang, and A.
Kunai, Chem. Lett., 2001, 886.
A. Kunai, T. Sakurai, E. Toyoda, M. Ishikawa, and Y.
Yamamoto, Organometallics, 13, 3233 (1994).
a) H. H. Anderson, J. Am. Chem. Soc., 73, 5804 (1951). b)
M. Kumada, Kogyo Kagaku Zasshi, 55, 373 (1952).
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1583 (1953). b) L. H. Sommer, H. D. Blankman, and P. C.
Millen, J. Am. Chem. Soc., 73, 3542 (1951).
7
a) J. J. McBride, Jr. and H. C. Beachell, J. Am. Chem. Soc.,
74, 5247 (1952). b) B. O. Pray, L. H. Sommer, G. M.
Goldberg, G. T. Kerr, P. A. DiGrogio, and F. C. Whitmore,
J. Am. Chem. Soc., 70, 433 (1948).
8
9
M. Kumada, K. Shiina, and M. Yamaguchi, Kogyo Kagaku
Zasshi, 57, 230 (1954).
U. Krüerke, Chem. Ber., 95, 174 (1962).
10 W. H. Nebergall, J. Am. Chem. Soc., 72, 4702 (1950).
11 a) W. Steudel and H. Gilman, J. Am. Chem. Soc., 82, 6129
(1960). b) M. C. Harvey, W. H. Nebergall, and J. S. Peak,
J. Am. Chem. Soc., 79, 1437 (1957).
12 H. H. Anderson, J. Am. Chem. Soc., 82, 1323 (1960); 81,
4785 (1959); 80, 5083 (1958).
13 P. B. Gansle, B. C. Gruber, J. T. Jarvis, A. Slaitas, S. D.
Jesus, and K. D. Jesus, Microchem. J., 55, 222 (1997).
14 U. Herzog and G. Roewer, J. Organomet. Chem., 527, 117
(1997).
On the other hand, attempted tribromination with this
reagent did not take place. When PhSiH3 was heated under
reflux in benzene with 6 equiv of the CuBr2(CuI) reagent,
dibromination completed within 3 h,17 much faster than the case
using 4 equiv of the reagent at room temperature. However, the
reaction stopped at this stage, and no tribromide would be
formed even after 1 week. Presumably, accumulation of elec-
tronegative bromine atoms on the same silicon center decreases
the reactivity of the silicon center, as has been observed previ-
ously for the chlorination of fluorohydrosilanes.2
We also performed selective bromination of 1,2-dihydro-
disilanes. Thus, when HEt2SiSiEt2H was treated with 2 equiv
of the reagent in benzene at room temperature for 4 h,
HEt2SiSiEt2Br was obtained in 58% isolated yield, together
with a 10% yield of Et2SiHBr (run 10).
15 A typical procedure is as follows. In a 300-mL flask, was
placed a mixture of MePhSiH2 (6.94 g, 56.7 mmol), CuBr2
(25.66 g, 114.9 mmol), and CuI (0.48 g, 2.5 mmol) in 70
mL of diethyl ether under Ar, and the mixture was stirred
at room temperature for 9 h. The mixture was filtered to
remove Cu salts and concentrated. The residue was dis-
tilled under reduced pressure to give 7.75 g (38.6 mmol,
68%) of MePhSiHBr: b.p. 83–86 °C (16 mmHg); MS m/z
1
200 (M+), 185 (M+–Me), 123 (M+–Ph), 121 (M+–Br); H
NMR (δ in CDCl3) 0.47 (3 H, d, J = 3.5 Hz, Me), 5.31 (1
H, q, J = 3.5 Hz, SiH), 7.42–7.71 (5 H, m, phenyl H); 13C
NMR (δ in CDCl3) 0.36, 127.87, 128.25, 130.94, 133.84;
29Si NMR (δ in CDCl3) –2.98. Anal. Calcd for C7H9BrSi:
C, 48.40; H, 4.51%. Found: C, 48.52; H, 4.60%.
The selectivity of the bromodisilane diminished in this
case, but two products could be readily separated by distillation.
The monosilane may be produced by the cleavage of an Si–Si
bond of disilane with HBr generated during the reaction.
16 Nevertheless, diethyl ether can be used as the solvent for
bromination, unless the reaction requires longer reaction
time. The reaction in ether is faster than in benzene, and
subsequent work up is easy due to the low boiling point.
17 Dibromination of hexylsilane (run 7) may complete also
faster under reflux conditions.
Dedicated to Prof. Hideki Sakurai on the occasion of his
70th birthday.