Dehydrogenative coupling of alcohol with hydrosilane catalyzed by an iron complex

Article abstract of DOI:10.1016/j.ica.2015.02.019

Silane alcoholysis of triethylsilane (Et₃SiH) with alcohol (ROH) with the help of CpFe(CO)₂Me (1) has been achieved to produce triethylsilyl ether (ROSiEt₃) under the thermal condition. For some alcohols, the iron complex

Full text of DOI:10.1016/j.ica.2015.02.019

Accepted Manuscript
Dehydrogenative Coupling of Alcohol with Hydrosilane Catalyzed by an Iron
Complex
Kozo Fukumoto, Michiho Kasa, Hiroshi Nakazawa
PII:
S0020-1693(15)00112-7
http://dx.doi.org/10.1016/j.ica.2015.02.019
ICA 16437
DOI:
Reference:
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Inorganica Chimica Acta
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Revised Date:
Accepted Date:
10 December 2014
11 February 2015
15 February 2015
Please cite this article as: K. Fukumoto, M. Kasa, H. Nakazawa, Dehydrogenative Coupling of Alcohol with
Hydrosilane Catalyzed by an Iron Complex, Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.
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015.02.019
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Dehydrogenative Coupling of Alcohol with Hydrosilane Catalyzed by
an Iron Complex
1
Kozo Fukumoto , Michiho Kasa , Hiroshi Nakazawa *
2
2
1
2
Department of Chemistry, Faculty of Education, University of the Ryukyus,kinawa 903-0213, Japan
Department of Chemistry, Graduate School of Science, Osaka City University, Osaka 558-8585, Japan
*
Corresponding author. Tel.: +6 6605 2547; fax: +6 6605 2522. E-mail address: nakazawa@sci.osaka-cu.ac.jp (H. Nakazawa)
Received ***
Table 1. Thermal Coupling Catalyzed by an Iron
a
Complex
ABSTRACT: Silane alcoholysis of triethylsilane (Et
3
SiH) with
alcohol (ROH) with the help of CpFe(CO)
achieved to produce triethylsilyl ether (ROSiEt
condition. For some alchols, the iron complex served as a catalyst. A
6e silyl iron complex (CpFe(CO)(SiEt )) was proposed to be one of
active iron species.
2
Me (1) has been
∆
3
) under the thermal
CpFe(CO) Me(ꢁ) + PhEH + Et SiH ꢃ PhE−SiEt
ꢀ
ꢂ
ꢂ
d
e
1
3
entry
1
PhEH
PhOH
[M]:[E]:[Si]
TON
1 : 1 : 1
Trace
0
2
3
PhSH
PhOH
PhSH
PhSH
1 : 1 : 1
1
.
INTRODUCTION
1 : 10 : 100
1 : 10 : 100
1 : 100 : 1000
0.3
5.8
6.8
Silyl ethers have been widely used as agents in the synthesis of
b
organic-inorganic hybrid materials, protection groups in organic
synthesis, external electron donors in Ziegler-Natta catalytic
polymerization processes, and precursors for the synthesis of silicon
polymers, and thus are very useful silicon compounds [1-11]. In
general, silyl ethers have been synthesized through the reactions of
chlorosilanes [12] or disilazanes [13-16] with alcohols in the
presence of a base such as pyridine. However, chlorosilanes are
often not convenient for handling due to moisture sensitive and
corrosive reagents. In addition, these reactions produce
stoichiometric amount of salts as a byproduct. An alternative
method for the synthesis of silyl ethers is catalytic dehydrogenative
coupling of hydrosilanes with alcohols. The merit of the reaction is
4
c
5
a
Reactions were carried out for 24 h at 80 °C by using
CpFe(CO) Me (0.21 mmol) and substrates in toluene (The
amount of toluene was adjusted to keep the concentration of the
2
b
c
Fe complex at 0.025 M in entries 1-4). See reference 46.
d
solvent free. Molar ratio of an iron complex, (thio)alcohol and
e
Et SiH. Calculated from the H NMR spectrum. The values are
1
3
based on the concentration of an iron complex.
2
the formation of only H as the co-product. To date, several metal
2
.
Results and discussion
catalysts derived from copper(II) [17, 18], alkali metal(I) [19, 20],
rhodium(I) [21, 22], palladium(II) [23, 24], gold(III) [25-28],
platinum(II) [29], ruthenium(II) [30, 31], zinc(II) [32], indium(III)
A solution of an equimolar amount of CpFe(CO)
2
Me (1) (0.21
), PhOH (0.21 mmol), and Et SiH (0.21
mmol) in toluene (8.2 mL) was heated at 80 °C for 24 h (Table 1,
entry 1). The solvent and Et SiH were removed under reduces
pressure, and then the residue was dissolved in C
measurement of the solution at room temperature revealed the
formation of a small amount PhOSiEt . This result shows that 1
hardly helps the formation of PhOSiEt . A stoichiometric reaction of
1, PhSH, and Et SiH did not produce PhSSiEt at all (entry 2). In
5
mmol) (Cp = η -C
5
H
5
3
[
33], titanium(II) [34-36], manganese [37], rhenium [38], iron [39],
iridium [40], nickel [41, 42] complexes and organocatalysts [43-45]
have been reported.
3
1
6 6
D . The H NMR
Recently, we examined the dehydrogenative coupling reaction of
hydrosilane with thiols in the presence of CpFe(CO)
2
Me (1) and
3
found that (i) the dehydrogenative coupling could be achieved by
the iron complex, (ii) the iron complex served as a catalyst
irrespective of the existence of sulfur-containing compounds, and
3
3
3
contrast and surprisingly, the use of a catalytic amount of 1 showed
much better results. When 10 mol% of 1 based on PhOH was used
(
iii) only silyl thioether was formed and homodehydrogenative
coupling products such as disulfide and disilane were not produced
46]. These results prompted us to investigate reactions of alcohols
3 3
in the reaction of PhOH with Et SiH, PhOSiEt was formed in 30%
[
yield (entry 3). This observation seems peculiar at a glance, but it
can be understood if one thinks that large excess substances make
their coordination to the active iron species easy, causing
acceleration of dehydrogenative coupling and also retardation of
decomposition of the iron active species. This tendency has been
instead of thiols with hydrosilane in the presence of 1. The
following points were examined: (i) whether 1 serves as a catalyst in
this reaction similar to the reaction between thiols and hydrosilane,
(
ii) whether substrates such as alcohols as starting materials and
silylethers as expected products work as catalyst poisons or not, and
iii) whether the selective formation of silyl ether is achieved or not.
reported in the reaction of excess amounts of PhSH and Et
form PhSSiEt (entries 3-5) [46]. And this effect is higher for PhSH
than PhOH.
3
SiH to
(
3
3
This paper reports the results of the reaction of alcohols with Et SiH
in the presence of 1.
1

a
Table 2. Dehydrogenative Coupling of Alcohol and Triethylsilane
∆
CpFe(CO) Meꢄ(ꢁ) + 10ꢄeq. ROH + 100ꢄeq. Et SiH ꢃ RO−SiEt
ꢀ
ꢂ
ꢂ
b
entry
1
ROH
RO-SiEt
3
TON
2.1
2
3
4
5
6
7
1.6
1.4
0.3
0.7
0.4
0.7
a
Reactions were carried out for 24 h at 80 °C by using CpFe(CO)
b
2
Me complex (0.112 mmol), ROH (1.12 mmol), Et
3
SiH (11.2 mmol)
1
in toluene (0.46 mL, 4.3 mmol). Calculated from the H NMR spectrum. The values are based on the concentration of an iron
complex.
To explore the scope and limitation of the catalytic activity of 1 the
dehydrogenative coupling of several alcohols and Et SiH were
from 1 induces the oxidative addition of Et
by the dissociation of MeCHO to give CpFe(CO)(SiEt
3
SiH or PhOH, followed
) (a) or
3
3
examined (Table 2). The corresponding dehydrogenative coupling
products were observed in all cases. Compared with entries 1-3, a
bulky substituent on the alcohol seems to reduce the catalytic
activity to some extent. An electron-donating substituent in alcohols
exhibits better TONs (entry 1 vs 6 and 4 vs 5). The alcohol having a
furan ring capable of coordination to the metal indicated the small
TON (entry 7), although the corresponding thiol did not undergo the
dehydrogenative coupling at all [46].
CpFe(CO)(OPh) (a’), respectively. Complex a undergoes the
oxidative addition of PhOH and the reductive elimination of
PhOSiEt
obtained from the reaction of a’ with Et
CpFe(CO)(H)(SiEt ) (b). The 16e complex c may exhibit two type
additions with Et SiH and PhOH. The former addition regenerates
complex a involving H gas release (cycle A). In the latter reaction,
a’ is regenerated via the oxidative addition of PhOH and the
reductive elimination of H (cycle B)
In order to obtain insight into catalytic cycle, the thermal reaction
of CpFe(CO)(py)(SiEt ) (py = pyridine) (2), considered as a synthon
of a 16e complex a [46-51], with excess EtOH was examined
3
to produce the hydride complex c. Complex c is also
3
SiH via
3
3
2
2
Scheme 1. Proposed Catalytic Cycles
3
1
(
Scheme 2). The H NMR spectrum of the reaction mixture showed
the formation of EtOSiEt (21% yields based on 2). This product is
probably formed according to the reaction sequence of
→a→b→EtOSiEt . This result shows that the reaction sequences
3
2
3
of cycle A in Scheme 1 proceeds to some extend although a reaction
pathway according to cycle B cannot be ruled out. As a 16e species
CpFe(CO)(SiEt
cycle in the reaction of thiols with Et
CpFe(CO)(SiEt ) is also one of active Fe species in the reaction of
alcohol with Et SiH.
3
) has been proposed to be involved in a catalytic
3
SiH, we propose here that
3
3
Scheme 2. Thermal reaction of EtOH in the presence of a
pyridine iron complex.
In summary, iron complex 1 was found to help dehydrogenative
coupling in a stoichiometric reaction of PhOH with Et
3
SiH, although
not in the reaction of PhSH with Et SiH. Complex 1 also showed
3
For the dehydrogenative coupling of alcohol and Et
silyl ether, we proposed a reaction mechanism (Scheme 1) similar to
that of dehydrogenative coupling of thiols and Et SiH [46]. Two
plausible catalytic cycles were proposed. The 16e species 1’ formed
3
SiH to give
catalytic activity in dehydrogenative coupling reaction of several
alcohols. The catalytic activities (TON) were in the range of 0.3 to
3
2
.1. As the activity toward alcohol is lower than that toward
2

thioalcohol, the active iron species may suffer more catalytic
activity loss from alcohol than thioalcohol. Selective formation of
silyl ether was observed because no other compounds were detected
in our system.
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D
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4

5

Highlights
•
Silane alcoholysis of Et
3
SiH with ROH was achieved to produce ROSiEt
Me was served as the catalyst precursor.
This dehydrogenative coupling reaction was effective for several alcohols.
3
.
5
•
•
•
5 5 2
(η -C H )Fe(CO)
5
5 5
A 16e species, (η -C H )Fe(CO)Me, was proposed to be one of active species.
6