Hydrogenation of silyl formates: sustainable production of silanol and methanol from hydrosilane and carbon dioxide

Article abstract of DOI:10.1039/c8cc02276c

A new process for simultaneously obtaining two chemical building blocks, methanol and silanol, was realized starting from silyl formates which can be derived from silane and carbon dioxide. Understanding the reaction mechanism enabled us to improve the reaction efficiency by the addition of a small amount of methanol.

Full text of DOI:10.1039/c8cc02276c

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Hydrogenation of Silyl Formate: Sustainable Production of Silanol
and Methanol from Hydrosilane and Carbon Dioxide
Received 00th January 20xx,
Accepted 00th January 20xx
Jangwoo Koo, Seung Hyo Kim, and Soon Hyeok Hong*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new process for simultaneously obtaining two chemical building
blocks, methanol and silanol, was realized starting from silyl
formates which can be derived from silane and carbon dioxide.
Understanding the reaction mechanism enabled us to improve the
reaction efficiency by the addition of small amount of methanol.
Previous work
A. Methanol synthesis via reduction of carbonates
H2
O
[Ru]
H2
CO2
CH3OH
CH3OH
+
CH3OH
X
O
R
NH2
(X=-H,-OCH3,-NHCH R)
2
Selective reduction of carbon dioxide (CO2) is a fascinating
approach for the production of methanol,1 which is a versatile
commodity chemical2 and a future energy source.3 Methanol is
commercially produced from fossil-fuel-based syngas at high
temperatures and pressures.4 Research on the catalytic
hydrogenation of carbon dioxide5 and alternative approaches
such as reduction of carbonates6 and formic acid7 is being
pursued to realize a highly efficient and economically viable
conversion of carbon dioxide to methanol (Scheme 1A). In
heterogeneous catalysis, Cu-based catalysts have been actively
investigated for the industrial scale production of methanol by
the hydrogenation of CO2.8 In homogeneous catalysis,
improvement of the overall catalytic efficiency for the selective
production of methanol from CO2 is a current challenge.
Silanol is widely utilized in industry as a monomer for
polysiloxane synthesis,9 precursor for cross-coupling
reactions,10 and key functional group on silicon surfaces.11
Silanol can be produced by the hydrolysis of silyl halides
(Scheme 1B),12 however in this case, toxicological and
environmental problems may arise because of the toxic
reagents required for the process, i.e., acid, or base and the
generated halide waste. To address this issue, catalytic
oxidation of hydrosilanes with water producing silanols has
been developed using homogeneous13 and heterogeneous14
catalysts (Scheme 1B).
B. Conventional silanolsynthesis
H2
O
H2O
[M] or NP
acid/base
RnSiX(4-n)
RnSi(OH)(4-n)
RnSiH(4-n)
-HX
air
C. Zhang & Ying's work
NaOH/H2
O
NHC
CO2
+
3Ph3SiH
Ph3SiOCH3
Ph3SiOH
+ CH3OH
-Ph3SiOSiPh3
• Limited scope only with Ph3SiH
• Low atom economy on silanolproduction
This work
O
H
[M]
[Ru]
R
3Si
CO2
R3Si-OH
+
CH3OH
R3SiH
H2
O
• CO2 as a C1 source
• Base-, oxidant-, and halide-free
• Simultaneous generation of two highly valuable chemicals
Scheme 1 Synthesis of methanol and silanols.
The use of silane derivatives to fixate CO2 has been actively
In 2009, the Zhang and Ying group15a
16
investigated.15,
reported N-heterocyclic carbene (NHC)-catalyzed reduction of
CO2 to methanol using hydrosilane, where silanol was
generated as a coproduct (Scheme 1C). However, the major
silicon product was siloxane (two-third molar equiv of [Si]), the
competitive side product in NHC-catalyzed reaction;
consequently, the atom economy for the production of silanol
was reduced. Besides, the direct catalytic Si-H oxidation is only
applicable to diphenylsilane, and this limits the utility of the
method.
Inspired by the recent development of readily available
16
silyl formate synthesis from CO2 and the Ru-catalyzed
hydrogenation of carbonates,6 we envisioned a new process
for the simultaneous production of highly valuable silanol and
methanol via the reduction of silyl formates, which can serve
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as a novel process for the concurrent production of the value- Table S1 in the Supporting Information) under 500 ppm
added products from economical and sustainable hydrosilane catalyst loading. Tri-n-propylsilyl formate 7b and dimethyl(2-
and CO2. Additionally, our developed reaction does not methylphenyl)silyl formate 7d required a higher catalyst
generate waste and requires no additive such as base, oxidant, loading (2 mol%) to provide good yields, presumably due to
or halide.
the steric hindrance at the silicon center. Aryl-substituted
To realize the aforesaid goal, hydrogenation of triethylsilyl compounds such as dimethylphenylsilyl formate 7c reacted
formate 7a was first investigated with ruthenium complexes, smoothly to produce methanol and silanol in good yields. To
and excellent activity for the hydrogenation of carbonyl investigate electronic effects, silyl formates with electron-
compounds was observed.17 Milstein catalyst 118 was not withdrawing or electron-donating groups at the para-position
effective for this reaction (entry 1). Ru-MACHO catalyst 219 did of the phenyl group were prepared. Electron-rich compounds,
not show any notable reactivity in the presence of a base methoxyphenyl- and tolyl- substituted silyl formates (7e and
(entry 2). When Ru-MACHO-BH
3 was used as the precatalyst, 7f), exhibited limited reactivity for the desired products,
the two desired products were obtained in quantitative generating byproducts such as methoxysilane (22%, 7e; 42%,
amounts without the need of any base or additive (entry 3). 7f) and dimerized siloxanes (10%, 7e; 24%, 7f). A further
Complexes
4 and 5, which showed better performance in the increase in H2 pressure to 80 bar enabled the selective
hydrogenation of ester derivatives20 were further tested, but reduction of 7e and 7f in good yields. Reduction of
they were not active for this reaction (entries 4 and 5). Among methyldiphenylsilyl formate 7g required an elevated pressure
the tested solvents, THF showed the highest yields for both of 40 bar H2 to achieve good yields of the products. Relatively
products (entries 3, 6−9). When we decreased the H2 pressure electron-deficient substrates with para trifluoromethyl- or
to 10 bar, the product was isolated in excellent yield (entry chloro- substituted phenyl groups (7h and 7i) showed fair to
10).
good yields under 10 bar H2. When the H2 pressure was
increased to 40 bar, nearly quantitative yields of both
methanol and silanol were obtained.
Table 1 Optimization of reaction conditions.a
Table 2 Hydrogenation of silyl formate to silanol and methanol catalyzed by 3.a
yield (%)
8a
entry [Ru]
base
solvent
H2 (bar)
CH3OH
75
1
1
2
3
4
5
3
3
3
3
3
-
THF
THF
THF
THF
THF
40
40
40
40
40
56
2
KOtBu
11
50
3
-
99
99
4
KOtBu
16
43
5
KOtBu
11
54
6
-
-
-
-
-
toluene 40
ethanol 40
dioxane 40
79
99
7
0
0
8
68
81
9
hexane
THF
40
10
46
92
10
90
91
a
Reaction conditions: triethylsilyl formate 7a (0.5 mmol), [Ru] (1 mol%),
base (1 mol%), THF (20 mL), 150 °C, 12 h, H2 (40 bar). Yields were
determined by GC analysis using p-xylene as an internal standard. THF =
tetrahydrofuran.
a Reaction conditions: silyl formate 7 (0.5 mmol), 3 (1 mol%), THF (20 mL), 150 °C,
12 h, H2 (10 bar). Methanol was checked by GC analysis using p-xylene as an
Various silyl formates, which were synthesized from silane
16c
were
and carbon dioxide (Scheme S1 and S2),16a,
b
internal standard. Silanol was isolated by silica column chromatography. 3 (0.1
mol%). c 3 (2 mol%). d 3 (5 mol%), H2 (80 bar). e H2 (40 bar). N.D.=not detected.
hydrogenated to silanol and methanol in good to excellent
yields under the optimized conditions (Table 2). Triethylsilyl
formate 7a was efficiently reduced in excellent yield even with
reduced loading of the catalyst (0.1 mol%). Additionally, 7a
exhibited good turnover numbers (8a: 1680, CH3OH: 1900, see
During the reactions depicted in Table 2, a small amount of
methyl formate
9 was observed in the case of 7b, 7d, 7f, 7g,
2 | J. Name., 2012, 00, 1-3
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and 7i. Under the reaction conditions,
9
could be generated with two-third of the molar equivalents of carbon being
consumed to afford CO2, suggesting that this is a minor and
with methanol, which is initially obtained from the direct undesirable pathway (eq 2). Additionally, hydrolysis of by
hydrogenation of To demonstrate the postulation, (eq 3) or
water16b generated from the dimerization of silanol
stoichiometric reaction between 7a and methanol was disproportionation of formic acid 117b (eq 2) may occur,
conducted (Scheme 2). The reaction proceeded via two inducing the regeneration of 11 and . Thus, it is essential to
pathways—one involving carbon attack (SNA) to produce control the reaction pathway preferably to DH and SNA for a
triethylsilanol 8a and , and the other involved silicon attack more efficient production of the desired silanol and methanol.
via a simple nucleophilic substitution (SN) to silyl formate
7
7
7.
a
8
8
9
(SNB) pathway to produce triethyl(methoxy)silane 10a (Table
S2 and Figure S1). The results indicated that the initially
formed methanol can react with the starting silyl formate,
leading to divergent pathways.
Table 3 Improvement of reactivity by the addition of methanola
Scheme 2 Reaction of silyl formate 7a with methanol.
H2
O
[Ru]
HO
H
11
R3SiOH
CO2, H2
O
8
R3Si OCH3
10
Hydrolysis
SN
B
CH3OH
H2
O
O
[Ru]
R3SiOH
8
CH3OH
+
H2
R3Si
O
H
7
Direct hydrogenation
O
R3Si
SiR3
12
SN
A
CH3OH
H2
O
Autocatalysis
reaction
O
O
CH3OH
R3Si
SiR3
12
[Ru]
H3CO
H
9
H2
Scheme 3 Proposed mechanism for hydrogenation of silyl formate.
a
Reaction conditions: 7 (0.5 mmol, 1.0 equiv.), 3 (1 mol%), THF (20 mL), 150 °C,
Based on this observation, we propose a plausible reaction
mechanism (Scheme 3). The initially postulated direct
12 h, H2 (10 bar), CH3OH (0.1 equiv.). Yields were determined by GC analysis using
e
p-xylene as an internal standard. b 3 (0.1 mol%), 4 h. c 3 (5 mol%). d H2 (80 bar).
H2 (40 bar). N.D. = not detected.
hydrogenation (DH) of silyl formate
7
can generate methanol
simultaneously. The initially formed methanol can
via nucleophilic substitution (SN). If methanol
(SNA), methyl formate and
can be generated. Then, can be reduced to
methanol under our catalytic conditions. A control experiment
confirms that reduction of is efficient under our catalytic
conditions (eq 1). Instead of SNA, if methanol reacts with the
silicon center of (SNB), methoxysilane 10 and formic acid 11
and silanol
react with
8
7
Inspired by the investigated positive SNA pathway, we
suspect that methanol as an additive could aid the catalysis. In
a preliminary test with 7a, methanol was found to accelerate
the production of both methanol and silanol 8a (Table 3).
Upon the addition of 0.1 equiv methanol to the reaction of 7e
and 7f, which required a high H2 pressure of 80 bar to obtain
acceptable yields, silanol and methanol were furnished in
about 70% yield even at 10 bar H2 pressure, which is typically
ineffective for product formation. However, no significant
attacks the carbon center of
silanol
7
9
8
9
9
7
can be formed. Although 11 can be hydrogenated to
methanol,7b lower efficiency is observed under the conditions
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obtained from carbon dioxide and silane. In addition to the
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using methanol as the additive.
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There are no conflicts to declare.
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■ Table of Contents Entry
Simultaneous production of methanol and silanols
was achieved by hydrogenation of silyl formates
readily obtained from silanes and CO2.