Selective dehydrogenative silylation-hydrogenation reaction of divinyldisiloxane with hydrosilane catalyzed by an iron complex

Article abstract of DOI:10.1021/ja209436s

A hydride and a silyl group of hydrosilane is introduced into 1,3-divinyldisiloxane in the presence of a catalytic amount of (η⁵-C₅H₅)Fe(CO)₂Me. Instead of the product expected from the well-known hydrosilylatio

Full text of DOI:10.1021/ja209436s

Communication
pubs.acs.org/JACS
Selective Dehydrogenative Silylation−Hydrogenation Reaction of
Divinyldisiloxane with Hydrosilane Catalyzed by an Iron Complex
Roman N. Naumov, Masumi Itazaki, Masahiro Kamitani, and Hiroshi Nakazawa*
Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan
S
* Supporting Information
from a green and sustainable viewpoint, because it is abundant
ABSTRACT: A hydride and a silyl group of hydrosilane is
introduced into 1,3-divinyldisiloxane in the presence of a
catalytic amount of (η5-C5H5)Fe(CO)2Me. Instead of the
product expected from the well-known hydrosilylation
reaction, the product obtained is that characteristic of
dehydrogenative silylation at one vinyl group and
hydrogenation at the other vinyl group of 1,3-divinyldisi-
loxane. Based on deuterium labeling experiments, a
catalytic cycle for this new reaction has been proposed.
in nature and can be metabolized into nontoxic products.
Several iron complexes have been reported in the catalytic
hydrosilylation of CO bonds in aldehydes and ketones.2 In
contrast, the number of reports on the use of iron complexes
for catalyzing hydrosilane−alkene reactions is limited. In 1962,
Nesmeyanov et al. reported that Fe(CO)5 serves as a catalyst in
the reaction of R3SiH with alkenes to give a hydrosilylation
(HySi) product and a dehydrogenative silylation (DHySi)
product depending on the substrate and the reaction
conditions.3 Wrighton et al. examined the reaction of
Cp*Fe(CO)2R (Cp* = η5-C5Me5, R = alkyl, silyl) with
hydrosilane and alkene and highlighted the implications of
inserting a CC double bond into an FeSi bond on the
mechanism of transition-metal-catalyzed hydrosilylation.4
Murai et al. reported that Fe3(CO)12 catalyzes dehydrogenative
silylation (DHySi) in the reaction of Et3SiH with styrene and
its derivatives, although it forms both a dehydrogenative
silylation (DHySi) and a hydrogenation (Hy) product in the
reaction of Et3SiH with 1-hexene.5 Adams et al. reported that a
t r i n u c l e a r c o m p l e x c o n t a i n i n g F e a n d P t ,
FePt2(CO)5(PPh3)2(PhC2Ph), shows activity in catalyzing the
hydrosilylation (HySi) of alkynes, but the role of Fe might be
unimportant in the catalytic system because its activity is much
less than that of Pt(PPh3)2(PhC2Ph).6 Marciniec et al. showed
that the reaction of CpFe(CO)2(SiMe3) (Cp = η5-C5H5) with
CH2CHR (R = SiMe3, Ph, H) gives dehydrogenative
silylation (DHySi) products but the iron complex does not
show catalytic activity.7 However, the Fe(CO)5 + Et3SiH
system catalyzes the reaction of CH2CHSi-
Me2OSiMe2(CHCH2) with Et3SiH to give dehydrogenative
silylation (DHySi) and hydrogenation (Hy) products.7
Recently, Chirik et al. reported that bis(imino)pyridine iron
complexes perform as excellent catalysts for alkene hydro-
silylation reactions.8
rganosilicon compounds are present in many widely used
Oproducts such as oil, grease, rubbers, cosmetics, medicinal
chemicals, etc. Because the above-mentioned are not naturally
occurring substances, the organosilicon compounds in them are
also manufactured artificially. The addition of the Si−H bond in
hydrosilane across a CC double bond or a CC triple bond
(hydrosilylation (HySi) reaction, Scheme 1) is a very
Scheme 1. Reaction of Alkene with Hydrosilane in the
Presence of Catalyst
commercially useful reaction in yielding a Si−C bond. A
transition metal catalyst is required to drive this reaction.1
The use of many transition-metal-based catalysts has been
reported for the hydrosilylation of alkenes. However, the
hydrosilylation reaction is often accompanied by side reactions
such as dehydrogenative silylation (DHySi) and hydrogenation
(Hy) of alkenes (Scheme 1), in addition to isomerization,
oligomerization, polymerization of alkenes, and redistribution
of silicon hydride. Therefore, the discovery of new catalysts
with higher selectivity as well as activity is important. In
addition to hydrosilylation, the metal-catalyzed dehydrogen-
ative silylation of alkenes has emerged as a powerful method to
yield Si−C bonds.
We have been engaged in chemistry of iron complexes
having silyl ligand(s).9 While studying the catalytic property of
these iron complexes in the formation of a Si−C bond, we
found that CpFe(CO)2Me shows unprecedented catalytic
activity in the reaction of 1,3-divinyldisiloxane with hydrosilane.
We describe our findings in this paper.
A toluene solution containing 1,1,3,3-tetramethyl-1,3-divinyl-
disiloxane and pentamethyldisiloxane in a 1:3 molar ratio was
heated at 80 °C for 24 h in the presence of 4 mol % of
C p F e ( C O ) 2 M e , c a u s i n g t h e f o r m a t i o n o f
Speier’s catalyst and Karstedt’s catalyst are known as very
powerful and commercially used catalysts. Both of these are Pt-
based complexes. Replacement of precious metal catalysts such
as these by broadly applicable metal catalysts is becoming
extremely important. Iron, for example, offers some advantages
Received: October 12, 2011
Published: December 29, 2011
© 2011 American Chemical Society
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Journal of the American Chemical Society
Communication
a
Table 1. Reaction of Olefin with Hydrosilane Catalyzed by CpFe(CO)2Me in Toluene at 80 °C
a
Ratio of olefin/R′3SiH is 1/3 (1/2 in entry 7). The amount of CpFe(CO)2Me is 4 mol % based on olefin concentration (10 mol % in entry 4).
C2H5SiMe2OSiMe2CHCHSiMe2OSiMe3 at an isolation yield
of 81% (Table 1, entry 1). In this reaction, one of the vinyl
groups of the divinyldisiloxane was dehydrogenatively silylated;
during this process, the SiMe2OSiMe3 group is selectively
introduced into the trans position relative to the original silyl
group. The other vinyl group was hydrogenated. Consequently,
this constitutes a sort of hydrosilylation (HySi) reaction,
because both a hydrogen and a silyl group are introduced into
the substrate. However, the reaction described here is not the
well-known hydrosilylation, but a combination of DHySi and
Hy reactions, wherein one vinyl hydrogen is replaced by a silyl
group and this removed hydrogen and the hydrogen from
hydrosilane are added to the other alkene portion to form an
alkyl group. Therefore, this reaction should be referred to as a
DHySi-Hy reaction. Because this type of reaction is
unprecedented, we examined further instances of this reaction
by utilizing different combinations of substances. The use of
different hydrosilanes resulted in the formation of DHySi-Hy
products in good yields (entries 2 and 3). Phenyl-substituted
divinyldisiloxane also exhibited the same reaction (entry 4)
although the yield was 14%; however, increasing the amount of
catalyst from 4 to 10 mol % resulted in a yield of 98%. When 4
mol % of the iron catalyst was used, the yield was 14%
presumably due to its low reactivity. Using divinyldisiloxane
with methyl and phenyl substituents on the silicons also
resulted in the DHySi-Hy reaction. In this case, because the Si
is a chiral center, we used a diastereomeric mixture and no
diastereoselectivity was observed in the DHySi-Hy reaction,
suggesting that the catalytically active Fe species has no chiral
center. The products of entries 1−5 were isolated and
characterized by 1H, 13C, 29Si, HRMS(EI) and elemental
analyses. In contrast, CH2CHSiMe2CH2SiMe2CHCH2
and Me3SiOSiMe2CHCH2 were not converted into the
corresponding DHySi-Hy products (entries 6 and 7), and the
starting vinyl compounds remained unchanged under the
reaction conditions. These results indicate that the oxygen
atom connecting the two vinyldimethylsilyl groups and the two
vinyl groups are essential in DHySi-Hy reaction. The general
scheme of the reaction we discovered is described in Scheme 2.
Scheme 2. Iron-Catalyzed Dehydrogenative Silylation−
Hydrogenation Reaction of Divinyldisiloxane with
Hydrosilane
In order to understand the DHySi-Hy reaction mechanism,
deuterium labeling studies were conducted using a reaction
similar to that of entry 3, in which MePh2SiD was used in place
of MePh2SiH (entry 8). The reaction was stopped after 24 h to
avoid undesired deuterium scrambling, and the Si-containing
1
compound (6) was isolated in 53% yield. In the H and 13C
NMR spectra, the chemical shifts and the coupling patterns
were identical to those of 3 (within experimental errors) except
1
for the ethyl portion. In the H NMR of 6, the methylene
group showed a multiplet with one proton intensity and the
methyl group exhibited a doublet, indicating that the methylene
carbon has only one proton. Moreover, the methylene carbon
of 6 showed a coupling pattern caused by a C−D bond at 9.99
ppm in the 13C{1H} NMR. These spectroscopic data
d e m o n s t r a t e t h e s e l e c t i v e f o r m a t i o n o f
CH3CHDSiMe2OSiMe2CHCHSiPh2Me. Therefore, the hy-
drogenation process in the discovered DHySi-Hy reaction
consists of two selective hydrogen introductions: the hydrogen
introduced at the vinyl α-carbon comes from the hydrosilane
added, and the hydrogen introduced at the vinyl β-carbon
originates from the hydrogen on the β-carbon in the other vinyl
group.
A reaction similar to that of entry 1 was performed in
toluene-d8 and monitored by 1H NMR. A plot of product yield
as a function of time (Figure 1) shows that there is an induction
period, followed by an increase in the formation of the DHySi-
Hy product over time. During the induction period, resonances
due to the starting iron complex were observed; thereafter,
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Journal of the American Chemical Society
Communication
associates with the Fe) or with e (produced when the siloxy
oxygen associates with the Fe). Coordinatively unsaturated c
undergoes β-hydride elimination to give f, which may be in
equilibrium with g via olefin exchange. Olefin insertion into the
Fe−H bond of g gives h, which may be in equilibrium with
either i or j, in a manner similar to that described for c.
Complex h is a 16e species that reacts with R′3SiH to give an
oxidative addition product k. Reductive elimination of the alkyl
and hydride ligands in k generates the DHySi-Hy product, in
addition to the regeneration of a. According to this reaction
mechanism, the hydrogen in R′3SiH is selectively introduced
into the methylene carbon of the DHySi-Hy product. The
complexes situated next to each other in the cycle, with the
exception of k and a, might equilibrate. The reactions from b to
c and g to h involve processes in which an 18e species forms a
16e species. The coordination of olefin or siloxy oxygen to the
Fe center might help drive the reaction. Because CH2
CHSiMe2CH2SiMe2CHCH2 did not react with pentam-
ethyldisiloxane (Table 1, entry 6), siloxy oxygen coordination,
as shown in e and j, may play an important role in promoting
the catalytic cycle. During the DHySi reaction, only the trans
isomer around the olefin portion was produced. Examples of
selective trans isomer formation by DHySi reaction using iron
complexes have been reported by Murai5 and Marciniec.7
Another mechanism containing two independent catalytic
cycles is conceivable, i.e., one cycle involving the DHySi
reaction catalyzed by [Fe]−SiR′3 and the other containing the
Hy reaction catalyzed by [Fe]−H. However, because the
DHySi product was not detected, the catalytic cycle proposed
in Scheme 3 is more likely.
Figure 1. Plot of DHySi-Hy Product Yield As a Function of Time in
CpFe(CO)2Me-Catalyzed Reaction of (CH2CHSiMe2)2O with
Me2SiOSiMe2H.
these resonances disappeared and small resonances assignable
to CpFe(CO)(SiMe2OSiMe3)2(H) were detected. Resonances
from the DHySi product, CH2CHSiMe2OSiMe2CHCH-
(SiMe2OSiMe3), were not observed.
A proposed catalytic cycle of the unprecedented DHySi-Hy
reaction is shown in Scheme 3. In the initial step, a thermal
activation of the catalyst precursor, CpFe(CO)2Me, generates a
16-electron species, a. This process requires some time. When a
reacts with R′3SiH, the oxidative addition product CpFe(CO)-
(SiR′3)2(H) is formed. It has been reported that CpFe(CO)-
(py)(EEt3) reacts with Et3E′H to form CpFe(CO)(EEt3)-
(E′Et3)(H) (E, E′ = Si, Ge, Sn), and the product CpFe(CO)-
(EEt3)(E′Et3)(H) is in equilibrium with the reductive
elimination products, Et3E′H and CpFe(CO)(EEt3).9d,e Anoth-
er reaction of a is coordination of one vinyl group of 1,3-
divinyldisiloxane to form b followed by the insertion of the
vinyl group into the Fe−Si bond to give c, which may be in
equilibrium either with d (produced when the other vinyl group
In conclusion, we found an unprecedented type of
hydrosilylation in the reaction of divinyldisiloxane with
hydrosilane catalyzed by an iron complex. In this reaction, H
and SiR3 are introduced into divinyldisiloxane, but a simple H
Scheme 3. Proposed Catalytic Cycle of Iron-Catalyzed Dehydrogenative Silylation−Hydrogenation Reaction
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Journal of the American Chemical Society
Communication
and SiR3 addition at the olefin portion does not occur. Instead,
dehydrogenative silylation at one vinyl group and hydro-
genation at the other vinyl group of divinyldisiloxane take place.
This discovery of a highly selective DHySi-Hy reaction is
important from a synthetic viewpoint because the separation of
hydrosilylation products, dehydrogenative silylation products,
and hydrogenation products is generally difficult.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures and characterization of
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
nakazawa@sci.osaka-cu.ac.jp
■
ACKNOWLEDGMENTS
■
This work was supported by a Challenging Exploratory
Research (No. 23655056) grant from the Ministry of
Education, Culture, Sports, Science and Technology of Japan,
and by Innovation Promotion Program in 2010 from NEDO.
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