N-heterocyclic carbene organocatalysts for dehydrogenative coupling of silanes and hydroxyl compounds

Article abstract of DOI:10.1002/chem.201301893

Go organic! N-Heterocyclic carbene (NHC) 1,3-diisopropyl-4,5- dimethylimidazol-2-ylidene (IiPr) has been found to be an efficient and selective catalyst for the dehydrogenative coupling of a wide range of silanes and hydroxyl groups to form Si-O bonds under mild and solvent-free conditions (see scheme). Mechanistic studies indicated that the activation of hydroxyl groups by the NHC is the most plausible initial step for the process. Copyright

Full text of DOI:10.1002/chem.201301893

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DOI: 10.1002/chem.201301893
N-Heterocyclic Carbene Organocatalysts for Dehydrogenative Coupling of
Silanes and Hydroxyl Compounds
Dongjing Gao and Chunming Cui*[a]
Silyl ethers have been widely used as coupling agents in
the synthesis of organic–inorganic hybrid materials, protec-
tion groups in organic synthesis, external electron donors in
Ziegler–Natta catalytic polymerization processes, and pre-
cursors for the synthesis of silicone polymers, and thus are
inarguably among the most useful silicon compounds.[1–4]
The development of synthetic routes to silyl ethers has
therefore attracted considerable attention. In the past sever-
nents simultaneously.[10c] Herein, we present the detailed
studies on the dehydrogenative coupling of silanes with hy-
droxyl groups by using an NHC as a single component cata-
À
lyst for the clean formation of Si O bonds [Eq. (1)]. The
catalyst is highly efficient and chemoselective for a range of
functionalized alcohols under solvent-free conditions. In
addition, the catalytic process can be significantly accelerat-
ed in CH3CN and has been amended for the synthesis
of diphenylprolinol silyl ethers, one type of versatile
organocatalyst.[11]
À
al decades, several approaches for the formation of Si O
bonds by reactions of alcohols with different types of silyla-
tion reagents, such as halosilanes, HNACHTUNGRTENUNG(SiMe3)2, and hydrosi-
lanes, have been developed.[5] Among these, catalytic dehy-
drogenative silylation of alcohols with hydrosilanes repre-
sents the most attractive and atom-economic route since this
process generates the clean fuel hydrogen as the sole by-
product. Despite the fact that some transition-metal com-
plexes, main group metal alkoxides, and strong Lewis acids
have been successfully employed as catalysts,[6–8] only a few
of them exhibit high reactivity and selectivity as well as sat-
isfactory functional-group tolerance. Therefore, the develop-
ment of highly active and selective catalysts for the process
is still highly desirable.
Initially, we examined the effects of employing different
NHCs as catalysts on the dehydrogenative coupling of etha-
nol and Et3SiH (Table 1) under the same conditions. The re-
action catalyzed by IiPr resulted in the formation of
Table 1. Comparison of different stable carbenes as catalysts for dehy-
drogenative coupling of Et3SiH with EtOH.[a]
Recently, intense interest has emerged in the development
of environmentally benign and efficient metal-free organo-
catalysts. These types of catalysts also provided distinct
mechanistic pathways from metal-catalyzed reactions. How-
ever, to the best our knowledge, organocatalysts that could
selectively catalyze dehydrogenative coupling of a range of
silanes with functional alcohols has not been reported.
Recent studies have shown that N-heterocyclic carbenes
(NHCs) can catalyze addition reactions of alcohols with un-
saturated substrates, in which NHCs may act as a Brçnsted
bases to activate alcohols.[9] On the other hand, the employ-
ment of NHCs as catalysts for a wide variety of silane trans-
formations has been reported.[10] One of the most notable
results is the NHC-catalyzed reduction of CO2 by silanes.[10b]
Theoretical calculations on the mechanism predicted the ac-
tivation mode via a transition state involving three compo-
Entry
Cat.[b]
t [h]
Conv. [%][c]
1
2
3
4
5
6
7
IiPr
IMe4
ItBu
IMes
IDipp
MIC
CAAC
9
10
10
10
10
10
10
90
41
78
2
trace
39
trace
[a] Conditions: 5% mol of catalyst, room temperature, in C6D6. [b] IiPr=
1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; IMe4 =1,3,4,5-tetrame-
thylimidazol-2-ylidene; ItBu, IMes and IDipp=1,3-disubstituted-imid-
azol-2-ylidene (Mes=2,6-Me2C6H3, Dipp=2,6-iPr2C6H3); MIC=1-(2,6-
AHCTUNGTRENNUNG
diisopropylphenyl)-3-methyl-4-phenyl-1H-1,2,3-triazol-5-ylidene;
CAAC=1-(2,6-diisopropylphenyl)-2,2,4,4-tetramethylpyrrolidine-5-yli-
dene. [c] Determined by NMR spectroscopy.
Et3SiOEt in approximately 90% yield (Table 1, entry 1).
The reaction catalyzed by ItBu also led to a good conversion
(entry 3). However, the reaction catalyzed by the other car-
benes listed in Table 1 only led to very low to modest con-
versions. It appears that N-alkyl-substituted NHCs (en-
tries 1–3) are more effective catalysts than N-aryl substitut-
ed ones (entries 4 and 5) and that the steric factors of the
N-alkyl group have notable effects on the catalytic activity.
[a] D. Gao, Prof. Dr. C. Cui
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (P. R. China)
Fax : (+86)22-23503461
E-mail: cmcui@nankai.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301893.
Chem. Eur. J. 2013, 19, 11143 – 11147
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11143

1
Table 3. Solvent-free dehydrogenative coupling of selected hydrosilanes
with different hydroxyl groups.[a]
The H NMR spectrum of a mixture of IiPr and the same
amount of EtOH in C6D6 displayed a significant low-field
singlet at d=7.14 ppm for the OH proton, whereas the reso-
nance for IDipp and EtOH appeared at d=4.80 ppm, which
indicated their different hydrogen bond strength, which
might be related to their different catalytic activity (entries 1
and 5).
With IiPr as the optimized catalyst, the reactions of
EtOH, iPrOH, and tBuOH with PhSiH3, Ph2SiH2, and
PhMeSiH2 catalyzed by 2.5% molar loading of the catalyst
have been studied under solvent-free conditions (Table 2).
Entry Hydrosilane Product
t
T
Yield
[h]
[8C] [%][b]
1
2
3
4
PhMe2SiH
Ph3SiH
Ph3SiH
PhMe2SiOEt
Ph3SiOEt
Ph3SiOCH2CH2Ph
PhMe2SiOCH2CH2Ph
1
3
1
1
RT
RT
RT
RT
75
93
95
95
PhMe2SiH
5
6
7
PhMe2SiH
PhMe2SiH
PhMe2SiH
0.5 RT
80
84
95
2
1
RT
RT
Table 2. Solvent-free reactions of selected alcohols and silanes catalyzed
8
9
PhMe2SiH
PhMe2SiH
2
5
RT
RT
91
95
by IiPr at room temperature.[a]
Entry
Hydrosilane
Alcohol
Product
PhSi(OEt)3
PhSiH(OiPr)2
PhSiH2OtBu
PhMeSi(OEt)2
Ph2Si(OEt)2
t [h]
Yield [%][b]
1[c]
2[d]
3[e]
4[d]
5[d]
PhSiH3
PhSiH3
PhSiH3
PhMeSiH2
Ph2SiH2
EtOH
iPrOH
tBuOH
EtOH
EtOH
G
1
0.5
1
2
2
100 (70)
90 (78)
90 (80)
100 (85)
90 (83)
G
10
11
12
PhMe2SiH
PhMe2SiH
Et3SiH
5
7
5
RT
RT
50
95
90
96
N
N
[a] Conditions: IiPr (2.5 mol%, 0.012 mmol), hydrosilane (0.468 mmol).
[b] Determined by NMR spectroscopy, isolated yields given in parenthe-
ses. [c] Alcohol: 1.403 mmol (3 equiv). [d] Alcohol: 0.936 mmol (2 equiv).
[e] Alcohol: 0.468 mmol (1 equiv).
13
Et3SiH
Et3SiH
12
50
ACHTUNGTRENNUG
Reaction of EtOH with primary and secondary silanes pre-
dominantly led to the conversion of all SiH groups to tri-
14
15
16
Et3SiOCH2CH2Ph
15
5
10
50
60
50
94
96
tBuPh2SiH tBuPh2SiOCH2CH2Ph
PhMe2SiH
PhOSiMe2Ph
ACHTUNTGRENUNG
and bis
with PhSiH3 yielded the corresponding bis
and the monosubstituted silane PhSiH2A(OtBu), respectively.
ACHTUNGTRENNUNG
17[c]
PhMe2SiH
36
70
AHCTUNGTRENNUNG
CTHUNGTRENNUNG
These results indicated that the steric effects of alcohols
have significant effects on the products. The reactions of
tBuOH with secondary and tertiary silanes were proved to
be very slow and only very low conversions were observed
over several hours. The activity observed in the present cat-
[a] Conditions: IiPr (2.5 mol%, 0.012 mmol), ROH (0.468 mmol), hydro-
silane (0.515 mmol). [b] Isolated yields, NMR spectroscopic yields given
in parentheses. [c] IiPr (5 mol%, 0.024 mmol), PhMe2SiH (0.936 mmol).
alytic system contrasts that found in the B
(C6F5)3-catalyzed
508C in 10 h (entry 16), probably due to the less nucleophil-
ic character of phenoxides than alkoxides.
À
Si O coupling reaction, for which the tertiary alcohols are
much more reactive than primary alcohols.[7a]
Diphenylprolinol silyl ethers are ubiquitous organocata-
lysts and have been widely used in asymmetric synthesis.
Reaction of dipehylprolinol with chlorosilanes or silyl trif-
Tertiary silanes have been frequently employed for the
protection of hydroxyl groups in organic synthesis.[2] Thus,
the dehydrogenative coupling reactions of several tertiary si-
lanes with different types of alcohols were investigated
under solvent-free conditions (Table 3). Most of the reac-
tions led to high conversions at room temperature over sev-
eral hours, and the coupling products could be isolated in
good to excellent yields. Notably, the reactions of ethanol
and 1-phenylethyl alcohol with Ph3SiH and PhMe2SiH
(Table 3, entries 1–4) resulted in almost complete conver-
sions at room temperature over one hour. However, the re-
actions of 1-phenylethyl alcohol with Et3SiH and tBuPh2SiH
(entries 14 and 15) required higher temperatures (508C) and
longer reaction times. Comparison of these results indicated
that phenylsilanes are more reactive than alkylsilanes. En-
tries 6–10 demonstrate the tolerance of the C–C double and
triple bonds, amino, imino, ketone, and ester groups. How-
ever, the catalyst is not efficient for the coupling of phenol
with PhMe2SiH, as only an 18% conversion was observed at
AHCTUNGTRENNUNG
late represents a straightforward route to the silyl ethers.[12]
However, these reactions require stoichiometric strong
bases to trap the acidic byproducts. Since the NHC-cata-
lyzed Si–O coupling reaction is selective in the presence of a
À
N H group, we investigated the dehydrogenative process
for the synthesis of diphenylprolinol silyl ethers. Initially,
the reaction of diphenylprolinol with PhMe2SiH was carried
out under solvent-free conditions with 5% loading of IiPr as
the catalyst. Heating the mixture at 708C for 36 h led to the
isolation of the coupling product in 85% yield (Table 3,
entry 17). To shorten the reaction time and improve the
yield, the reaction was amended to be conducted in several
organic solvents, such as toluene, THF, and CH3CN. It was
found that the reactions in toluene and THF are very slow.
However, the reaction in CH3CN is significantly improved
and a complete conversion was observed at 508C in 6 h.
Thus, the similar reactions with Ph3SiH and Et3SiH have
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Dehydrogenative Coupling of Silanes and Hydroxyl Compounds
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chemical shift with those observed for free ethanol in organ-
ic solvents (d=1.32–3.39 ppm)[14] suggested the strong hy-
drogen-bonding interaction of this proton with the NHC.[15]
1
In contrast, the H NMR spectrum of a mixture of IiPr and
Ph3SiH showed two sets of signals for the two compounds,
which indicated that there were no interactions between
them. Addition of Ph3SiH to the mixture of EtOH and IiPr
in C6D6 led to the immediate evolution of hydrogen gas and
the hydrogen resonance for Ph3SiH disappeared in the
1H NMR spectrum. For comparison, the reaction of IiPr
with the less hindered silane PhSiH3 has also been studied
in C6D6 on NMR spectroscopic scales. In this case, a new
singlet at d=À39.5 ppm in the 29Si NMR spectrum may sug-
gest the activation of PhSiH3 by the NHC.[16] However, this
reaction is quite slow: only 9% conversion was observed in
30 min and 32% in 23 h. It has been shown that reaction of
CAAC with phenylsilane resulted in the complete oxidative
addition of the silane to CAAC in 14 h.[16a] However, our ex-
periment indicated that CAAC is almost inactive for the de-
hydrogenative coupling reaction (Table 1, entry 7). There-
fore, it can be concluded that the activation of EtOH by
IiPr is much more favorable than that of silanes under the
experimental conditions for the dehydrogenation process.
The plausible mechanism for the reaction is shown in
Scheme 2. The initial step may involve the formation of the
À
Scheme 1. Catalytic dehydrogenative Si O coupling reaction of diphenyl-
prolinol with silanes in CH3CN.
been investigated in CH3CN. As expected, the reactions at
508C for 10 and 19 h gave the expected silyl ethers with
high conversions, respectively (Scheme 1). These silyl ethers
can be isolated in excellent yields.
It appears that the dehydrogenative reaction is much
more favorable in CH3CN than that under solvent-free con-
ditions. This effect can be attributed to the hydrogen-bond-
ing interactions between nitriles and alcohols.[13] The reac-
tion of phenol with PhMe2SiH was significantly improved in
CH3CN and led to the 95% conversion at 508C in 20 h
(Table 4, entry 1). Comparison of the reactions in Table 4
with those with the same reactants under solvent-free condi-
tions (Table 3, entries 14–16) showed that the reactions con-
ducted in CH3CN have much shorter reaction times and
milder conditions.
Table 4. Dehydrogenative coupling in CH3CN.
Entry Hydrosilane Product
t [h] T [8C] Yield [%][a]
1
2
3
PhMe2SiH
Et3SiH
tBuPh2SiH
PhOSiMe2Ph
Et3SiOCH2CH2Ph
tBuPh2SiOCH2CH2Ph
20
3
1
50
50
RT
95
100
96
[a] NMR specroscopic yield.
For practical applications, the catalytic activity of the
NHC generated in situ (see Table S1 in the Supporting In-
formation) for the reaction of EtOH with PhMe2SiH has
been investigated. The results showed that IiPr·HCl alone
could not catalyze the coupling reaction even at 708C for
12 h under solvent-free conditions. However, addition of
one equivalent of KOtBu to the mixture of PhMe2SiH
(40 equiv) and IiPr·HCl followed by addition of EtOH
(40 equiv) led to a complete reaction in 1 h under solvent-
free conditions. This reactivity is comparable to that found
for the free NHC (IiPr). However, other bases, such as
DBU, only led to a low conversion (entry 5 in Table S1 of
the Supporting Information, 56% after 12 h at 508C).
KOtBu has been reported to catalyze dehydrogenative cou-
pling of hydrosilanes and alcohols.[8b] For the present reac-
tion, the use of KOtBu as the catalyst only resulted in 22%
conversion in 1 h under the same conditions.
À
Scheme 2. Proposed mechanism for the dehydrogenative Si O coupling
reaction.
intermediate A with a hydrogen bond followed by the nucle-
ophilic attack to a silane to give the hypervalent silicon hy-
dride via transition-state B, followed by the elimination of
À
hydrogen to give the Si O coupling product and regenera-
tion of the catalyst.
The kinetic studies on the model reaction of EtOH with
Et3SiH catalyzed by IiPr indicated first-order dependence
on the concentrations of the catalyst, EtOH and Et3SiH.
The reaction rate increases linearly with the increase of the
amount of the NHC (Figure 1). The increase of the amount
of EtOH or Et3SiH also led to the linear acceleration of the
reaction (see Figures S1 and S2 in the Supporting Informa-
tion). These studies are in line with the proposed mechanism
shown in Scheme 2.
To elucidate the dehydrogenative mechanism, the reaction
of IiPr with EtOH in C6D6 has been investigated. The
1H NMR spectrum exhibited a sharp singlet at d=7.41 ppm
(see Figure S4 in the Supporting Information), which can be
In summary, we have performed the first systematic stud-
ies on the employment of stable carbenes as organocatalysts
À
assigned to an OH proton resonance. Comparison of this
Chem. Eur. J. 2013, 19, 11143 – 11147
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C. Cui and D. Gao
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for the dehydrogenative coupling of silanes and alcohols.
The structures and substituents of carbenes have pro-
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the reactions described in this work, very high conversions
can be obtained under mild and solvent-free conditions with
IiPr as the catalyst. Furthermore, it was found that the reac-
tion can be significantly accelerated in CH3CN and is suit-
ACHTUNGTRENNUNGable for the dehydrogenative coupling of hindered sub-
strates with excellent conversions. The mechanism studies
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Experimental Section
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The Schlenk tube was moved out from the glovebox and the reaction was
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Acknowledgements
We are grateful to the National Natural Science Foundation of China
and 973 Program (grant no. 2012CB821600) for the support of this work.
Keywords: alcohols
·
dehydrogenative
coupling
·
N-heterocyclic carbenes · organocatalysts · silane
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Dehydrogenative Coupling of Silanes and Hydroxyl Compounds
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Received: May 16, 2013
Published online: July 12, 2013
Chem. Eur. J. 2013, 19, 11143 – 11147
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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