Dehydrogenative coupling between hydrosilanes and alkynes catalyzed by alkoxides, alkylmetals, and metalamides

Article abstract of DOI:10.1006/jcat.1999.2530

The selective dehydrogenative coupling reaction between phenylsilane and ethynylbenzene occurred in the presence of some homogeneous base catalysts such as alkoxides, alkyl compounds, and the amides of alkali metals or barium. The order of the catalytic activities was Ba(OR)₂ > LiN(SiMe₃)₂ ～ n -BuLi > LiOEt. Barium alkoxide showed the highest activity and selectivity for the reaction, and gave the polymer poly[(phenylsilylene)ethynylene-1,3-phenyleneethynylene] in the reaction of phenylsilane with m-diethynylbenzene. The correlation between the catalytic activities and the catalyst basicities was discussed, and a reaction mechanism involving both the metal acetylide and the metal hydride was proposed.

Full text of DOI:10.1006/jcat.1999.2530

Journal of Catalysis 185, 454–461 (1999)
Article ID jcat.1999.2530, available online at http://www.idealibrary.com on
Dehydrogenative Coupling between Hydrosilanes and Alkynes Catalyzed
by Alkoxides, Alkylmetals, and Metalamides
Jun-ichi Ishikawa and Masayoshi Itoh1
Organic Performance Materials Laboratory, Mitsui Chemicals, Inc., 1190 Kasama-cho, Sakae-ku, Yokohama City 247-8567, Japan
Received January 4, 1999; accepted April 16, 1999
same reaction (11). Moreover, we have recently found that
LiAlH4 shows a much higher activity than MgO (12, 13).
We have obtained a new highly heat- and burn-resistant
thermosetting polymer, poly[(phenylsilylene)ethynylene-
1,3-phenyleneethynylene] (abbreviated MSP) by applying
the reaction to the polymerization (10, 13).
Theselectivedehydrogenativecouplingreaction between phenyl-
silane and ethynylbenzene occurred in the presence of some ho-
mogeneous base catalysts such as alkoxides, alkyl compounds,
and the amides of alkali metals or barium. The order of the
catalytic activities was Ba(OR)2 > LiN(SiMe3)2 n-BuLi > LiOEt.
Barium alkoxide showed the highest activity and selectivity for the
reaction, and gave the polymer poly[(phenylsilylene)ethynylene-
1,3-phenyleneethynylene] in the reaction of phenylsilane with m-
diethynylbenzene. The correlation between the catalytic activities
and the catalyst basicities was discussed, and a reaction mecha-
nism involving both the metal acetylide and the metal hydride was
c
The dehydrogenative coupling reaction can be written as
PhSiH3 + Ph–C C–H MgO or LiA→lH4 Ph–C CSi(Ph)H2.
&
H 2
[1]
proposed.
1999 Academic Press
The dehydrogenative coupling polymerization reaction
can be written as
Key Words: dehydrogenative coupling; hydrosilane; alkyne;
alkoxide; alkylmetal; metalamide; base catalyst.
PhSiH3 + H–C C–C6H4–C C–H MgO or LiAlH
→
4
&
H 2
INTRODUCTION

 

Ph
Ph
|
|


 
 


There have been some reports on the dehydrogena-
tive cross-coupling reactions of hydrosilanes with mono-
substituted alkynes to produce alkynylsilanes. H2PtCl6/LiI,
I (1–3), and IrH2(SiEt3)(COD)(AsPh3), iridium cata-
lysts formed by adding triarylarsines or triarylphosphines
to [Ir(OMe)(diolefin)]2 (4), [IrH(H2O)(bp)L2]SbF6, or
RhClL3 (L = PPh3, bp = 7,8-benzoquinolinato), (5) have
been used as catalysts. In these cases, the dehydrogena-
tive cross-coupling reactions were found to accompany
the significant hydrosilylation reactions to produce the
corresponding alkenylsilanes. Calas and Bourgeois ob-
tained alkynylsilane (n-Bu–C C–SiEt3) from Et3SiH and
1-hexyne using Na or NaH as the catalyst (6). Liu and
Harrod reported that the CuCl/amine catalyst led to the
highly selective dehydrogenative cross-coupling reactions
of hydrosilanes with alkynes (7).
–
–
–C C–C H –C C
–
–
Si
Si–C C–C6H4–C C
6
4

 

|
%
H
y
x,
[x/y = 9/1(MgO), 6/1(LiAlH4)]. [2]
As MgO is easily separated by filtration from the reac-
tion solution after the completion of the polymerization
reaction, the polymer is very pure and stable in air. How-
ever, the catalytic activity of MgO is low, so a large amount
of MgO is needed to obtain the product in high yield. We
have been searching for a more active catalyst.
In this paper, we report some new catalysts for the above
dehydrogenative coupling reaction and discuss their reac-
tion mechanisms.
We have found that a solid base such asMgO catalyzesthe
selective dehydrogenative coupling reactions (8–10). Baba
et al. have also recently reported KNH2 loaded on Al2O3,
which isa solid base, and showed the catalyticactivityfor the
EXPERIMENTAL
Reaction Procedures
Phenylsilane, diphenylsilane, triphenylsilane, hexylsi-
lane, and triethylsilane were used as the hydrosilanes. Ethy-
nylbenzene (abbreviated EB), 1-octyne, and m-diethy-
nylbenzene (abbreviated m-DEB) were used asthe alkynes.
1 To whom correspondence should be addressed. Fax: 81-045-895-8240.
E-mail: masayoshi.itoh@mitsui-chem.co.jp.
454
0021-9517/99 $30.00
c
Copyright
1999 by Academic Press
All rights of reproduction in any form reserved.

DEHYDROGENATION BETWEEN HYDROSILANES AND ALKYNES
455
˚
These liquid reactants were dried over 3-A molecular sieves Product Characterization
prior to use. Metal compounds, including alkoxides, alkyl-
1 [Ph–C CSi(Ph)H2], 2 [(Ph–C C)2Si(Ph)H], 3 [Ph–
==
metals, and metalamides were used as the catalysts (see
Table 1). Alkali and alkaline earth metal compounds were
mainly used. The solvents diglyme, monoglyme, toluene,
THF, dioxane, tetrahydropirane, diethoxyethane, and
CH CHSi(Ph)H2], 4 [Ph–CH(SiPhH2)–CH2Si(Ph)H2],
==
and 5 [PhC CSi(Ph) (CH CH–Ph)H]: see references 9
and 12.
Ph–C CSi(Ph)2H: [MS(m/z) (EI-mode) 284 ((M)+, base
peak), 206, 181, 129, 105]. (Ph–C C)2Si(Ph)2: [MS(m/z)
(EI-mode) 307 (base peak), 384 (M)+, 129, 205, 278].
Ph–C CSi(C6H13)H2: [MS(m/z) (EI-mode) 131 (base
peak), 105, 160, 84, 216(M)+]. (Ph–C C)2Si(C6H13)H:
[MS(m/z) (EI-mode) 231 (base peak), 129, 260, 274,
316(M)+]. C6H13–Ph–C CSi(Ph)H2: [MS(m/z) (EI-mode)
81 (base peak), 105, 67, 146, 216(M)+]. (C6H13–Ph–C
C)2Si(Ph)H: [MS(m/z) (EI-mode) 81 (base peak), 67, 105,
95, 324(M)+]. Ph–CH(SiPhH2)–CH(SiMe3)[Si(Ph)H2]:
[MS(m/z) (EI-mode) 238 (base peak), 131, 73, 161, 207,
285, 390(M)+].
˚
anisol were dried over 3-A molecular sieves.
All experiments were carried out in a nitrogen atmo-
sphere. The reactions were performed in a 100-ml glass flask
under atmospheric pressure. The prescribed amount of hy-
drosilane, ethynyl compound, catalyst, and solvent were
charged into the glass flask (see Tables 1, 3, and 6) and
the reaction was carried out under homogeneous condi-
tion. After the reactions, the products were removed and
analyzed as follows.
Reaction Products of Hydrosilanes
with Ethynyl Compounds
Poly[(phenylsilylene)ethynylene-1,3-phenyleneethynyl-
ene] (run No. 8 in Table 6): IR (neat) (Si–H, C C)
2156, (Si–H) 796 cm
After the completion of the reaction, the amounts of the
reaction products and unreacted monomers were analyzed
by GC (Shimadzu Co.; capillary column-CBPI-M25-025)
using tetradecane as the internal standard. The liquid prod-
ucts were separated by distillation under reduced pressure,
and assignments were made based on the GC-MS, IR, and
1H, 13C, and 29Si NMR spectra (8, 12, 13).
1
;
1H NMR(CDCl3/TMS) 3.09
(s, C CH), 4.74 (s, (Ph)SiH2), 5.05 (s, (Ph)SiH), 7.1–8.0
(m, PhH); 13C NMR(CDCl3/CDCl3) 78.3 (C CH), 82.4
(CCH), 86.6 (C C–C6H4–), 107.1 (C C–C6H4–), 123–136
(Ph); 29Si NMR(CDCl3/TMS)
((Ph)SiH, 69.1 ((Ph)Si).
59.4((Ph)SiH2), 63.1
RESULTS AND DISCUSSION
Polymerization Reaction Products of Phenylsilane
with m-DEB
Reactions of Hydrosilanes with Ethynyl Compounds
The reaction products and unreacted monomers were an-
alyzed by GC and gel permeation chromatography (GPC)
(Shimadzu Co.; column-shodex KF-802 and 806L). The re-
acted solution was then added to 20 ml of toluene and 20 ml
of 1 N HCl aq. in a vessel with stirring and separated into
two layers [(I) and (II)]. The bottom layer (II) was added
to 5 more ml of toluene and the toluene solution (III) layer
was separated. The upper layer (I) was added above the
toluene solution (III) and washed with water three times.
The upper layer (I) was then dried over calcium sulfate
and evacuated for 50 h at 60 C to give the liquid or solid
polymer.
In a reaction (run 8 in Table 6), 30 ml of the reacted
solution was separated and stirred with 0.5 g of a
cation exchange resin (Mitsubishi Chemical Co., DIAION
(RCP160M), particle size 0.39 mm, specific surface area
30 m2 g 1, average pore size 20–30 nm), which had been
dried for 24 h at 110 C at room temperature. After the
treatment, the resin was removed by filtration and the so-
The results of the reaction of phenylsilane with EB in the
presence of some alkoxides, alkylmetals, and alkylamides
are shown in Table 1. The catalytic activities depended on
the types of metals and substitutions of the catalysts. The
dehydrogenated products (1, 2) were produced with a very
small amount of the hydrosilylated products (3, 4, and 5) in
the presence of the alkali metals, barium, and lanthanum
compounds, which have strong basicities:
PhSiH3 + Ph–C C–H Ca→ta. Ph–C CSi(Ph)H2
1
==
+ (Ph–C C)2Si(Ph)H + Ph–CH CHSi(Ph)H2
2
3
+ Ph–CH(SiPhH2)–CH2Si(Ph)H2
4
==
+ (Ph–C C)(Ph–CH CH)Si(Ph) H.
[3]
5
lution was evacuated for 50 h at 60 C.
Products having higher molecular weight than those of
1
IR, and H, 13C, and 29Si NMR spectra were employed products (4, 5) were detected by using GPC. Almost
to determine the structure of the polymers (9, 13). Molec- no products were observed by GPC except for runs
ular weights were obtained by GPC with retention times 9, 12, and 19 in Table 1. Barium 2-ethylhexoxide (Ba
calibrated against polystyrene samples.
[OCH2CH(C2H5)(CH2)3CH3], abbreviated Ba(OC8H17)2),

456
ISHIKAWA AND ITOH
TABLE 1
Reactions of Phenylsilane with Ethynylbenzene (EB) in the Presence of Alkoxide, Alkylmetals, and Alkylamidesa
Conversion (% )
Product yield (% )b
Run
No.
PhSiH3
(mmol)
EB
(mmol)
Catalyst
LiOEt
LiO–i–Pr
NaOEt
(mmol)
PhSiH3
EB
1
2
3
4
5
1
2
53.7
51.3
50.8
49.5
51.2
50.6
49.4
50.5
51.3
52.5
52.6
51.9
51.1
51.4
51.8
51.4
54.2
49.8
0.26
0.34
0.31
0.33
0.27
0.29
0.27
0.28
0.26
46.2
57.9
62.0
9.9
43.5
75.8
85.4
6.5
34.3
47.4
46.1
5.9
48.8
53.2
51.3
53.1
1.1
6.2
14.6
17.8
0.1
0
0
0
0
0
0
0
0.5
0.2
0.1
0
3c
4d
5
KOEt
0
Ba(OMe)2
Ba(O–i–Pr)2
Ba(O–t–Bu)2
73.9
74.8
73.4
75.8
93.6
(89.1
72.1
66.3
42.7
51.9
55.9
51.3
8.8
98.4
99.3
97.5
99.6
99.8
99.8
98.7
74.4
95.1
50.9
66.8
62.5
3.3
24.7
29.5
29.7
29.8
0
0
0
0
0
0
0
6
7
8
9
0
0
0
0
Ba(OC2H4OMe)2
0
0
e
Ba(OC8H17
)
0
0.1
0
0
0.1
0) f
0
2
3.7
0.4
0
10g
11g,h
12i
13
14
15
16
17
18
19
50.6
50.2
49.9
51.1
53.5
51.6
49.5
48.9
49.8
50.1
50.2
49.1
50.3
52.1
51.5
51.2
50.9
50.2
50.6
50.3
Ba(OC8H17
Ba(OC8H17
Cu(OEt)2
)
)
0.29
0.31
0.27
0.27
0.32
0.34
0.35
0.22
0.31
0.37
40.8
72.8
0.9
35.4
40.3
45.3
5.7
54.0
42.7
8.2
39.5
25.3
23.3
41.9
19.2
5.7
0
0
2
2
0
0
0.4
5.6
7.3
10.2
0
30.4
13.1
1.4
20.0
3.5
48.4l
9.2
20.2
0.6
0.4
0
0.8
0
0.5
0
0
0
0
1.4
0.6
0.1
0.5
0
La(O–i–Pr)3
n-BuLi
PhLi
Bu2Mg
(C CPh)2Ba
LiN(SiMe3)2
NaN(SiMe3)2
74.6
52.7
83.5
(68.4
28.5
94.0
52.8
98.9
69.6
99.1
93.9
30.8
74.7
56.4
0
0
0
0
0
0.4
0.3
0
0
1.0
0.9
0.3
0
1.4
1.2) j
0
20
21k
22m
51.8
4.9
52.8
53.9
11.1
53.4
KN(SiMe3)2
MgO
LiAlH4
0.28
25
2.7
0
0
1.0
0
a Approx. 10 ml of diglyme was used as the solvent in each experiment and the reaction was carried out for 8 h at 80 C.
b
==
The mole percent ratio of the product vs. the charged PhSiH3. [1, Ph–C CSi(Ph)H2; 2, (Ph–C C)2Si(Ph)H; 3, Ph–CH CHSi(Ph)H2; 4, Ph–
==
CH(SiPhH2)–CH2Si(Ph)H2; 5, Ph–C CSi(Ph)(CH CH–Ph)H.]
c 10 ml of diglyme and 1 ml of THF were used as the solvents.
d 10 ml of diglyme and 1 ml of Et2O were used as the solvents.
e Barium 2-ethylhexoide.
f The data at a 4 h reaction time.
g The reaction was carried out at 40 C.
h 10 ml of toluene was used as the solvent.
i
+
==
(Ph–CH CH)2Si(Ph)H was also produced. [MS (m/z) (EI mode) 209 (base peak), 130, 105, 312 (M ), 183.]
j The data at a 2 h reaction time.
k 10 ml of benzene was used as the solvent and the reaction was at 80 C for 4 h.
l (Ph–C C)3SiPh was also produced (yield 5.5% ).
m The reaction was carried out for 20 h at 120 C (Ref. 13).
showed the highest activity. The activity of Ba(O–i–Pr)2 was
about 100 times that of LiAlH4 and about 4000 times that
of MgO (see Table 2).
TABLE 2
On the other hand, the compounds of the elements be-
longing to Groups 2 (alkaline earth metals, excluding bar-
ium), 4, 12, and 13 of the periodic table such as Mg(OEt)2,
n-Bu2Mg, n-BuMgCl, Ca(OEt)2, Sr(O–i–Pr)2, Zr(O–i–Pr)4,
Ti(OEt)4, Et2Zn, Al(OEt)3, and Ga(OEt)3, which have
higher ionization potential energies than alkali metals,
showed almost no catalytic activity. Cu(OEt)2 showed a
higher catalytic activity against hydrosilylation than dehy-
drogenation (run no. 12 in Table 1).
Relative Catalytic Activities vs the Dehydrogenative Reaction
of Phenylsilane with EBa
Catalyst
activityb Ba(O–i–Pr)2 LiN(SiMe3)2 n-BuLi LiOEt LiAlH4 MgO
1
2
4000
630
600
140
470
530
310
240
40
40
1
1
a The reaction was carried out for 4 h at 80 C in diglyme (Table 1).
b
Figure 1 shows the changes in the conversions of the re-
actants and the yields of the main products with time when
Relative catalytic activity calculated from the conversion of EB and
the catalyst amount. 1, Per unit catalyst mole; 2, per unit catalyst gram.

DEHYDROGENATION BETWEEN HYDROSILANES AND ALKYNES
457
reactants were converted. Some compounds with a molecu-
lar weight of 400–500 were detected by GPC. Products 1 and
2 would be produced during an early stage of the reaction,
and when all the EB was consumed, the hydrosilylation re-
actions of 1 and 2 and phenylsilane would proceed to pro-
duce the high-molecular-weight compounds. This presump-
tion could be supported by the facts that products 1 and 2
were the main ones for the low conversions of the reactants
(runs 10, 11, and 19 in Table 1), and the hydrosilylation re-
action occurred between Ph–C CSiMe3 and PhSiH3 in the
presence of Ba(OC8H17)2 (run 7 in Table 3).
Mechanism of Dehydrogenative Coupling Reaction
In Table 4 and Fig. 2, the correlation between the first
ionization potentials (abbreviated IP) of the metals of the
alkoxides and their catalytic activities is shown. Alkoxides
with a low IP, which are strong bases, have high catalytic
activities, except for KOEt. The facts that the strong ba-
sic catalysts showed higher catalytic activities and the hy-
drogen of the ethynyl compound is acidic suggest that the
reaction would proceed via an ionic intermediate. Bis(2-
phenylethynyl)barium [(C CPh)2Ba] showed a high cata-
lytic activity and selectivity like the other barium com-
FIG. 1. Reaction of phenylsilane with EB in the presence of Ba
(OC8H17 (run 10 in Table 1). [1, Ph–C CSi(Ph)H2; 2, (Ph–C C)2
Si(Ph)H.]
)
2
Ba(OC8H17)2 was used as the catalyst. The dehydro- pounds (run 17 in Table 1). Therefore, we presumed that
genative reaction rapidly occurred even at 40 C, and the reaction would proceed involvinga metalacetylide asan
PhSiH3 and EB decreased to produce 1 and 2 (run 10 in intermediate as shown in Fig. 3. The metal alkoxide would
Table 1).
be converted into a metal hydride by the reaction with the
During the experiments of runs 9 and 19 in Table 1, very hydrosilane as reported in the literature (Eq. (a) in Fig. 3)
fewproductscould be detected byGC, though almost allthe (15). Moreover, it is well known that an alkyl metal and
TABLE 3
a
Reactions of Hydrosilanes with Alkynes in the Presence of Ba(OC8H17)2
Conversion (% )
Hydrosilane Alkyne
Product yield (% )b
Run
No.
Ba(OC8H17
(mmol)
)
2
Hydrosilane
mmol
Alkyne
mmol
a
b
c
1c
2
3
4
5
6
7
PhSiH3
Ph2SiH2
Ph3SiH
C6H13SiH3
Et3SiH
50.6
49.9
47.0
48.6
49.9
49.9
51.0
EB
EB
EB
EB
EB
50.2
52.2
50.6
51.0
50.8
51.0
50.0
0.29
0.25
0.25
0.26
0.27
0.26
0.24
72.1
76.8
0
81.4
9.1
98.7
99.1
4.4
99.3
1.7
40.8d
57.9f
0
19.2e
31.3g
—
0
0
0
0
0
0
7.3l
58.7h
0
31.7i
—
PhSiH3
PhSiH3
C6H13–C CH
Ph–C CSiMe3
78.9
25.3
99.2
7.3
39.0j
0
23.1k
0
a Approx. 10 ml of diglyme was used as the solvent in each experiment and the reaction was carried out for 8 h at 80 C.
b The mole percent ratio of the product vs the charged PhSiH3. [a, b, dehydrogenation reaction products, c, hydrosilylation reaction products.]
c The reaction was carried out at 40 C.
d 1, Ph–C CSi(Ph)H2.
e 2, (Ph–C C)2Si(Ph)H.
f Ph–C CSi(Ph)2H.
g (Ph–C C)2Si(Ph)2.
h Ph–C CSi(C6H13)H2.
i (Ph–C C)2Si(C6H13)H.
j C6H13–Ph–C CSi(Ph)H2.
k (C6H13–Ph–C C)2Si(Ph)H.
l Ph–CH(SiPhH2)–CH(SiMe3)[Si(Ph)H2], Ph2SiH2 (yield 3.9% ), and SiH4 (yield 3.0% ) were also produced.

458
ISHIKAWA AND ITOH
TABLE 4
Correlation between the First Ionization Potential (IP) and the
Catalytic Activities of the Alkoxide during the Dehydrogenative
Reaction of Phenylsilane with EB
Conversion (% )
Yield (% )
Run
No.
Catalyst
KOEt
NaOEt
Ba(O–i–Pr)2
LiOEt
LiO–i–Pr
La(O–i–Pr)3
Sr(O–i–Pr)2
Al(OEt)3
Ga(OEt)3
Ca(OEt)2
Mg(OEt)2
Metal IP (eV) PhSiH3
EB
1
2
1
2
3
4
5
6
7
8
9
K
4.341
5.139
5.212
5.392
5.392
5.577
5.695
5.986
5.999
6.113
7.646
9.9
62.0
74.8
46.2
57.9
51.9
3.8
6.5
5.9
0.1
17.8
29.5
6.2
14.6
5.6
0
Na
Ba
Li
Li
La
Sr
Al
Ga
Ca
Mg
85.4 46.1
99.3 53.2
43.5 34.3
75.8 47.4
50.9 35.4
0.1
2.1
0
FIG. 3. Postulated mechanism of the dehydrogenative coupling reac-
tion of hydrosilane with alkyne.
0.1
0
7.0
3.7
0
The reason for the low catalytic activity of KOEt is not
obvious, but we think it is probably caused by the low sol-
ubility of potassium hydride in the organic solvent. Ethers
such as diglyme and monoglyme were better solvents for
the reaction than toluene (runs 10 and 11 in Table 1). The
polarity of the ethers would be advantageous for the for-
mation of the ionic intermediate. The activities of Ph3SiH
and Et3SiH were low, as shown in Table 3, which would be
caused by the steric hindrances.
In Table 5 and Fig. 5, the correlation between the catalytic
activities of the lithium compounds and pKas of their conju-
gate acids is shown. The catalytic activities of n-BuLi, PhLi,
LiN(SiMe3)2, and LiOEt were much higher than those of
LiOPh and LiOCOPh, which showed almost no catalytic
activity. This would be caused by the low basicity of PhO
and PhCOO . Probably, n-BuLi, PhLi and LiN(SiMe3)2
would form the acetylide by the reaction with EB ( pKa =
21), and LiOEt would form the hydride by the reaction
with PhSiH3. On the other hand, LiOPh and LiOCOPh
would not give the metal hydride by the reaction with an
PhSiH3.
0
0
10
11
0
0
0
0
2.2
3.4
0
0
a
The reaction was carried out for 8 h at 80 C in diglyme (runs 1–4, 5;
see Table 1).
b 1, Ph–C CSi(Ph)H2; 2, (Ph–C C)2Si(Ph)H.
metalamide react with ethynyl compounds to produce the
metal acetylide at 78 C (Eqs. (b) and (c) in Fig. 3) (16,
17). The metal hydride or metal acetylide would then react
with an ethynyl compound or hydrosilane, respectively, as
shown in Eq. (d) in Fig. 3. From the results of the energy
diagrams obtained by an ab initio calculation method using
the 6-31G basis set for the two elementary processes of
the above mechanism as shown in Fig. 4, the endothermic
elementary process (I) would be the rate-determining step.
It is well known that the R3Si–H bond can be activated by
transition metal complexes, and many reactions concerning
hydrosilanes have been studied. We have shown that ionic
(basic) compounds such as alkoxides, alkylmetals, and met-
alamides containing alkali metals or barium can activate
the Si–H bond.
Polymerization Reactions of Phenylsilane with m-DEB
Barium alkoxide showed a much higher activity than
MgO or LiAlH4 as shown above. Therefore, we studied
the polymerization reaction using Ba(OC8H17)2 as a
catalyst (see Table 6). The dehydrogenative coupling
polymerization reaction occurred and the solid polymer
poly[(phenylsililene)ethynylene-1,3-phenylenethynylene]
was obtained. Figure 6 shows the changes in the conver-
sions of the monomers and the yield and molecular weight
of the polymer. The monomers rapidly decreased and the
oligomer including the adduct of two monomers were pro-
duced during an early stage of the reaction. The molecular
FIG. 2. Correlation between the first ionization potential (IP) and the
catalytic activities of the alkoxide during the dehydrogenative reaction of
phenylsilane with EB. 1, KOEt; 2, NaOEt; 3, Ba(OiPr)2; 4, LiOEt; 5, LiO–
i–Pr; 6, La(OiPr)3; 7, Sr(OiPr)2; 8, Al(OEt)3; 9, Ga(OEt)3; 10, Ca(OEt)2;
11, Mg(OEt)2.

DEHYDROGENATION BETWEEN HYDROSILANES AND ALKYNES
459
TABLE 5
Correlation between the Catalytic Activities of Lithium Compounds and pKa of their Conjugate Acids of
the Catalysts during the Dehydrogenative Reaction of Phenylsilane with EBa
Conversion (% )
Yield (% )b
Run
No.
Conjugate
acid
pKa of
conjugate acid
Catalyst
n-BuLi
PhLi
LiN(SiMe3)2
LiOEt
CpNa
LiOPh
LiOCOPh
PhSiH3
EB
1
2
1
2
3
4
5
6
7
n-Bu–H
Ph–H
H–N(SiMe3)2
EtO–H
Cp–H
PhO–H
50
43
35
17
15–18
10
55.9
51.3
52.7
46.2
11.7
2.4
66.8
62.5
69.6
43.5
6.6
40.3
45.3
42.7
34.3
8.8
0
7.3
10.2
13.1
6.2
0.3
0
0.0
PhCOO–H
4
6.0
4.2
0
0
a The reaction was carried out for 8 h at 80 C in diglyme (runs 1–3; see Table 1).
b 1, Ph–C CSi(Ph)H2; 2, (Ph–C C)2Si(Ph)H.
TABLE 6
a
Polymerization Reactions between Phenylsilane and m-DEB in the Presence of Ba(OC8H17)2
Polymerb,d
yield
Adductc
yield
(% )
Conversion (% )
Molecular weightd
Run
No.
PhSiH3
(mmol)
m-DEB
(mmol)
Ba(OC8H17
(mmol)
)
Temp.
( C)
2
Solventa
PhSiH3
m-DEB
(% )
Mw
Mn
Mw/Mn
1
2
3
39
41
40
40
41
43
41
42
42
42
0.40
0.40
0.41
0.20
0.21
Diglyme
Monoglyme
Toluene
THF
Toluene
Diglyme
Toluene
Diglyme
Monoglyme
Monoglyme
80
80
80
67
60
91
82
67
78
66
(83
91
(91
92
95
89
55
73
66
88
96
100
100
100
88
78
30
61
75
85
87
102
93
93
5
11
38
21
7
5
7
3
3
5,560
1,380
410
650
690
1,760
2,230
20,100
21,880
12,470
1320
750
370
490
530
850
920
2040
2110
1720
4.2
1.8
1.1
4
1.3
5e
1.3
2.1)f
2.4
9.9)f
10.4
7.3
6e
41
42
0.22
80
7
41
523
43
516
0.20
2.60
75
75
8g
93
3
a 30 ml of the solvent was used in each experiment and the reaction was carried out for 8 h.
b The mole percent ratio of the weight of the polymer versus the total weight of the charged two monomers.
c The reaction product of PhSiH3 with m-DEB [HC C–Ph–C CSi(Ph)H2].
d The adduct was excluded.
e 30 ml of toluene and 4 ml of diglyme were used as solvents.
f The data at 20 h reaction time.
g 375 ml of monoglyme was used.
FIG. 4. Energy diagrams of elementary processes of the mechanism shown in Fig. 3.

460
ISHIKAWA AND ITOH
FIG. 7. TGA–DTA traces of poly[(phenylsilylene)ethynylene-1,3-
phenyleneethynylene] prepared using Ba(OC8H17)2 under argon and air.
FIG. 5. Correlation between the catalytic activities of lithium com-
pounds and pKa of their conjugate acids of the catalysts during the
dehydrogenative reaction of phenylsilane with EB. 1, n-BuLi; 2, PhLi;
3, LiN(SiMe3)2; 4, LiOEt; 5, CpNa; 6, LiOPh; 7, LiOCOPh.
1
The spectral data (IR and H, 13C, and 29Si NMR) of
the polymer were very similar to those of the polymers ob-
tained using MgO or LiAlH4 as the catalyst, though the
value of x/y in formula (2) was 7/4 (run 8 in Table 6), which
was determined from the 29Si NMR spectrum. The ther-
mal properties were measured using TGA-DTA (Fig. 7).
The T d5 (temperature of 5% weight loss) in argon was
848 C [T d5 = 860 C (MgO), 760 C (LiAlH4)]. The poly-
mer showed a high heat and burn resistance, similar to the
polymer obtained using MgO (9, 14) or LiAlH4 (13) as the
catalyst.
weight of the polymer gradually increased when the
monomers were consumed. The solvents of diglyme and
monoglyme gave a polymer with a high molecular weight,
but THF and toluene gave a polymer with a low molec-
ular weight. No polymer could be obtained in dioxane,
tetrahydropyran, diethoxyethane, and anisol.
The Si–H bond in the polymer is easily hydrolyzed in
the presence of moisture to form a siloxane (Si–O) bond
in the air when the polymer was contaminated with ba-
sic compound; therefore the catalyst residues must be re-
moved thoroughly from the polymer. The reacted solution,
which contained 2980 ppm of barium versus the polymer,
was treated with the cation exchange resin, and almost all
the barium compounds were removed from the reacted so-
lution (less than 0.5 ppm). These results would enable the
economical preparation process of MSP.
CONCLUSION
Alkoxides, alkyl compounds, or amides of alkali metals
and barium, such as LiOR, NaOR, La(OR)3, RLi, R2Ba,
LiN(SiMe3)2, NaN(SiMe3)2, and KN(SiMe3)2, showed high
catalytic activity and selectivity vs the dehydrogena-
tive coupling reaction between hydrosilanes and ethynyl
compounds. Barium alkoxide showed the highest ac-
tivity and selectivity for the reaction and gave the
polymer poly[(phenylsilylene)ethynylene-1,3-phenylene-
ethynylene] in the reaction of phenylsilane with m-diethy-
nylbenzene. The residue of barium catalyst was easily
FIG. 6. Polymerization reaction of phenylsilane with m-DEB in the
presence of Ba(OC8H17)2 (run 7 in Table 6). Adduct: the reaction product
of PhSiH3 with m-DEB (HC C–Ph–C CSi(Ph)H2).

DEHYDROGENATION BETWEEN HYDROSILANES AND ALKYNES
461
removed from the polymer by using the cation exchange
resin.
4. Fernandes, M. J., Oro, L. A., and Manzano, B. R., J. Mol. Catal. 45, 7
(1988).
5. Jun, C. H., and Crabtree, R. H., J. Organomet. Chem. 447, 177
(1993).
6. Calas, R., and Bourgeois, P., C. R. Acad. Sci., Paris, Ser. C 268, 72
(1969); Chem. Abstr. 70, 78070n (1969).
7. Liu, H. Q., and Harrod, J. F., Can. J. Chem. 68, 1100 (1990).
8. Itoh, M., Mitsuzuka, M., Utsumi, T., Iwata, K., and Inoue, K.,
J. Organomet. Chem. 476, C30 (1994).
The first ionization potential of the metal in an alkoxide
and the pKa of the conjugate acid in a lithium compound
had good correlations with the catalytic activity. A reaction
mechanism involving both the metal acetylide and metal
hydride was proposed.
9. Itoh, M., Mitsuzuka, M., Iwata, K., and Inoue, K., Macromolecules 27,
7917 (1994).
ACKNOWLEDGMENTS
10. Itoh, M., Inoue, K., Iwata, K., and Mitsuzuka, M., Macromolecules 30,
694 (1997).
11. Baba, T., Takahashi, H., Kato, A., Yuasa, H., Handa, H., and Ono, Y.,
Catal. Catal. 39, 530 (1997).
This work was performed by Mitsui Chemicals, Inc., under the manage-
ment of the Japan Chemical Innovation Institute as part of the Industrial
Science and Technology Frontier Program supported by the New Energy
and Industrial Technology Development Organization.
12. Itoh, M., Kobayashi, M., and Ishikawa, J., Organometallics 16, 3068
(1997).
13. Ishikawa, J., Inoue, K., and Itoh, M., J. Organomet. Chem. 552, 303
(1998).
14. Itoh, M., Inoue, K., Iwata, K., Ishikawa, J., and Takenaka, Y., Adv.
Mater. 9, 1187 (1997).
15. Meals, R. N., J. Am. Chem. Soc. 68, 1880 (1946).
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