Hydrosilylation of olefins and dehydrogenative double silylation of conjugated dienes catalyzed by lanthanide-imine complexes

Article abstract of DOI:10.1016/S0040-4039(01)02027-5

Divalent lanthanide-imine complexes and a related species catalyzed the hydrosilylation of olefins with phenyl- and diphenylsilane. On the other hand, conjugated dienes were converted to 1,4-bissilyl-2-butenes and 3-silacyclopentenes, accompanied with hyd

Full text of DOI:10.1016/S0040-4039(01)02027-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 9211–9214
Hydrosilylation of olefins and dehydrogenative double silylation
of conjugated dienes catalyzed by lanthanide–imine complexes
Ken Takaki,* Kentaro Sonoda, Takeshi Kousaka, Go Koshoji, Tetsuya Shishido and
Katsuomi Takehira
Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University, Kagamiyama,
Higashi-Hiroshima 739-8527, Japan
Received 20 September 2001; revised 22 October 2001; accepted 26 October 2001
Abstract—Divalent lanthanide–imine complexes and a related species catalyzed the hydrosilylation of olefins with phenyl- and
diphenylsilane. On the other hand, conjugated dienes were converted to 1,4-bissilyl-2-butenes and 3-silacyclopentenes, accompa-
nied with hydrogen evolution, under similar conditions. © 2001 Elsevier Science Ltd. All rights reserved.
Catalytic hydrosilylation and related reactions are
important and efficient methods for the synthesis of
organosilicon compounds.1 In the last decade, trivalent
lanthanocene catalysts have been used in this field,
including cyclization/silylation of diene and triene sys-
tems, and exhibited unique regio-, chemo- and site-
selectivities through the fine tuning of their steric and
electronic environment.2 These reactions have been
proved to proceed via insertion of olefins to lanthanide
hydride, followed by s-bond metathesis as depicted in
Scheme 1 (path a).3 In contrast to late transition metal
catalysts, there is no precedent for the process initiated
by a lanthanide silyl species (path b). However, if the
silyl species are added to olefins, this process may lead
to double silylation, instead of hydrosilylation, owing
to the polarity of hydrosilane in the s-bond metathesis
(path c).4
With such consideration in mind, we studied the reac-
tion of olefinic compounds with hydrosilanes by using
divalent
lanthanide–imine
complexes,
[Ln(h2-
PhNCPh2(hmpa)n] (1), which can be simply prepared in
situ from lanthanide metals (Yb and Sm) and aromatic
imines,5 and different catalytic activities from trivalent
lanthanocenes in various reactions were exhibited; for
example, dehydrogenative silylation of terminal alky-
nes,6 hydrosilylation of imines7 and intermolecular
hydrophosphination of alkynes.8 We report herein these
results.
When styrene was treated with an equimolar amount of
PhSiH3 in the presence of 1a (Ln=Yb, n=4, 5 mol%)9
in THF at −35°C for 20 h, 2-(phenylsilyl)ethylbenzene
(2a) was formed in 85% yield together with polyphenyl-
silane after evolution of hydrogen, but the other
regioisomer 3a, a major product by lanthanocene cata-
lysts,2 was not detected (Eq. (1)). The reaction at room
temperature gave 2a in lower yield (33%) probably
because of the predominant polymerization of the
silane. No noticeable reaction was observed with SmI2
(10 mol%) and without the catalyst 1a even under
refluxing conditions. Results on the hydrosilylation by
using 1a are summarized in Table 1 (method A).
Phenylsilane could be substituted by diphenylsilane to
afford 2-(diphenylsilyl)ethylbenzene (2b) in 76% yield
(run 2), but triphenylsilane was inactive. In contrast to
the reaction of styrene and 4-fluorostyrene (runs 1–3),
this method was not applicable to electron-rich and
substituted styrenes as well as a-olefins (runs 4–8).
Ln H
Ln Si
Si H
(a)
C C
C C
Si H
Ln H
C C
(b)
(c)
C C
C C
Si Si
Si Ln
Scheme 1.
Keywords: hydrosilylation; double silylation; lanthanide catalyst;
olefins; conjugated dienes.
* Corresponding author. E-mail: ktakaki@hiroshima-u.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02027-5

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K. Takaki et al. / Tetrahedron Letters 42 (2001) 9211–9214
SiH2Ph
i.e. linear silane 2a with HMPA and branched silane 3a
without the ligand were formed exclusively.
SiH2Ph
+
+
PhSiH3
Ph
Ph
Ph
2a
3a
Next, hydrosilylation of conjugated dienes was investi-
gated (Table 2). The reaction of isoprene with phenylsi-
lane (3 equiv.) in the presence of 1a at 0°C for 20 h in
THF gave 1,4-bis(phenylsilyl)-2-methyl-2-butene (5a)
and 1-phenyl-3-methyl-3-silacyclopentene (6a) in 61 and
13% yields, respectively (run 1). The normally expected
1,4- and 1,2-hydrosilylation products were not detected
at all, in contrast to trivalent lanthanocene catalysts
that were reported to yield three 1,4-hydrosilylation
products.12 Use of equimolar or less amounts of the
silane only caused a decrease of 5a (ca. 20%) with
similar yields of 6a. The reaction with diphenylsilane
also produced the products 5b and 6b in 57 and 16%
yields, respectively (run 2). Similarly, the dehydrogena-
tive double silylation took place in the reaction of
2-octyl- and 2,3-dimethylbutadiene (runs 3 and 4),
wherein the silacyclopentene 6c was formed as a major
product in the former case.13 Unfortunately, alkyl sub-
stituents on the terminal position seemed to signifi-
cantly decrease the reaction efficiency as shown by
1,3-pentadiene (run 5). All products 5 were preferen-
tially obtained as (E)-isomers. In addition, similar or
somewhat better results were obtained with the diamide
catalyst 4a.14
(1)
The limitation was significantly improved by the addi-
tion of equimolar amounts of primary and secondary
amines, particularly diphenylamine, whereby the imine
complexes 1 would be changed to diamides 4 (Eq.
(2)).10 Thus, use of 4a (n=4, 10 mol%) for the reaction
of styrene with phenylsilane resulted in the reduction of
the reaction time and an increase of the yield of 2a to
95%.11 As can be seen in Table 1 (method B), all
styrene derivatives, including a- and b-methylstyrene,
were converted to the expected products 2 in good
yields, except for 4-methoxystyrene (run 5). In the
reaction of 1-decene, the hydrosilylation did not occur
with 4a, but it could be conducted with the samarium
catalyst 4b (n=0) to afford the product 2h in 79% yield
(run 8).
Ph
Ph2CH
N
+
Ph2NH
N Ln NPh2 (hmpa)n
Ln (hmpa)n
Ph
Ph
Ph
1a: Ln = Yb, 1b: Ln = Sm
4a, b
(2)
Various features of the reaction were elucidated
through the improvement mentioned above. With
respect to the catalyst activity of 4, Yb was superior to
Sm for the hydrosilylation of styrene, but the order was
reversed for 1-decene. The HMPA ligand showed the
promotive and inhibitory effect in the reaction of sty-
rene and 1-decene, respectively. Moreover, regiochem-
istry in the reaction of styrene depended on the ligand,
These results clearly indicate that the reaction mode is
completely different between olefins and conjugated
dienes, which would be interpreted as follows. At first,
since dehydrogenative oligomerization/polymerization
Table 2. Dehydrogenative double silylation of conjugated
dienesa
Table 1. Hydrosilylation of olefins
R1
1a (n= 4,10 mol%)
R3
+
2 R'R"SiH2
R'R"HSi
THF, 0 ˚C, 20 h
R2
R1
R'
R2
R1 R3
SiHR'R"
+
R3
R"
Si
R2
5
6
product, yield (%)b and E / Z ratio
run
diene
hydrosilane
5
6
1
PhSiH3
Ph2SiH2
PhSiH3
PhSiH3
PhSiH3
5a 61 (93 / 7)
5b 57 (93 / 7)
5c 18 (97 / 3)
5d 60 (96 / 4)
5e 19 (71 / 29)
6a 13
6b 16
6c 41
2c
3
C8H17
4
6d
6e
9
0
5
a Silane / diene = 3. b GC yield based on the diene. c Carried out at rt.

K. Takaki et al. / Tetrahedron Letters 42 (2001) 9211–9214
9213
In summary, it has been found that the imine com-
plexes 1 and related diamide species 4 exhibit unique
catalytic activity for the reaction of olefinic compounds
with hydrosilanes, wherein the reaction mode is selected
by the substrates. Particularly, double silylation of con-
jugated dienes with hydrosilanes, not with disilanes, has
been achieved, though it involves restrictions on the
substituents.
of PhSiH3 and Ph2SiH2 was induced by the catalysts 1
and 4, lanthanide hydride A and silyl species B would
be generated in the reaction mixture (Scheme 2).15 In
the case of olefins, it is likely that the reaction was
initiated by olefin insertion to the hydride species A,
based on the regiocontrol by the HMPA ligand
observed for the reaction of styrene. That is, p-coordi-
nation of the Ph ring to benzylic lanthanide metal after
styrene insertion to Ln–H, which has been suggested to
be a key factor to produce the branched product 3a,3
was disturbed by HMPA and, thus, linear product 2a
was formed instead. If styrene inserted to the silyl
species B, a reverse ligand effect to produce 3a and 2a
in the presence and absence of HMPA, respectively,
could be observed.
Acknowledgements
This work was partially supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education,
Science, Sports and Culture, Japan.
On the other hand, the dehydrogenative double silyla-
tion would be initiated by the reaction of diene with the
silyl species B to yield allylic lanthanide C (Scheme 3).
Silylation of the syn isomer of C gave the product 5 and
the hydride A, and the latter was converted finally to B
by the reaction with another molecule of the silane with
hydrogen evolution. Similarly, intramolecular silylation
of the anti isomer of C would yield the silacyclopentene
6. The reason for the change in the reaction mode by
the two substrates still remains unclear. Alternatively,
the two reactions may be explained by the addition of
a silyl radical, followed by H abstraction from Si–H for
olefin and Si abstraction for diene. However, this pro-
cess seems less likely, because a radical reaction of
dienes with hydrosilane was reported to yield hydrosilyl-
ation products,16 and in fact, 5 and 6 were not formed
by the reaction with AIBN.
References
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Si
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R'R"SiH2
1 or 4
[Ln]
H
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Scheme 2.
[Ln] Si
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9. The isolated complex 1a contains three HMPA ligands,
see: Ref. 5. However, the real coordination number of 1
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this additive used for their preparation are tentatively
shown as the coordination number (n).
[Ln]
H
SiHR'R"
2
[Ln]
R
A
C
anti isomer
R1
R1
R'
R2
R"
R'R"HSi
R'R"SiH2
SiHR'R"
10. Although the exact structure of 4 was not clear, forma-
tion of diamide species was confirmed by NMR. For
example, on addition of PhMeNH to 1a in THF-d8, a
signal at l 160.2 (CꢀN) changed to l 62.9 (CH–N) in 13C
NMR spectra and a new peak of the methine proton
R2
5
Si
6
1
Scheme 3.
appeared at l 5.5 in H NMR, together with the change

9214
K. Takaki et al. / Tetrahedron Letters 42 (2001) 9211–9214
of the Me group of the amine: l 30.3 to l 19.1 in 13C
15. It has been reported that dehydrogenative polymeriza-
tion of PhSiH3 with trivalent lanthanocenes proceeds
through these two intermediates, see: (a) Sakakura, T.;
Lautenschlager, H.-J.; Nakajima, M.; Tanaka, M.
Chem. Lett. 1991, 913–916; (b) Forsyth, C. M.; Nolan,
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and lithium amides (2 equiv.), showed similar catalyst
activities to a certain extent.
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2a decreased to 47% yield.
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13. As shown in Scheme 3, preferential formation of the
anti isomer of C over the syn isomer, owing to the octyl
substituent, would be responsible for this selectivity.
14. For example, the reaction with 4a (n=4, 10 mol%, 0°C,
7 h) gave 5a (69%) and 6a (12%), 5d (65%) and 6d
(8%), and 5e (23%).
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Adams, R.; Blatt, A. H.; Boekelheide, V.; Cairns, T. L.;
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