Heterogeneous-gold-catalyzed acceptorless cross-dehydrogenative coupling of hydrosilanes and isocyanic acid generated in situ from urea

Article abstract of DOI:10.1002/anie.201303132

Support structure: In the presence of gold nanoparticles supported on alumina (Au/Al₂O₃), various hydrosilanes can be converted into the corresponding silyl isocyanates using urea as an isocyanate source. The observed catalysis is truly heterogeneous, and the retrieved Au/Al ₂O₃ catalyst can be reused several times without any loss of its high catalytic performance. Copyright

Full text of DOI:10.1002/anie.201303132

Angewandte
Chemie
Communications
Cross-Coupling
K. Taniguchi, S. Itagaki, K. Yamaguchi,
N. Mizuno*
&&&& — &&&&
Support structure: In the presence of gold
nanoparticles supported on alumina (Au/
Al2O3), various hydrosilanes can be con-
verted into the corresponding silyl iso-
cyanates using urea as an isocyanate
source. The observed catalysis is truly
Heterogeneous-Gold-Catalyzed
heterogeneous, and the retrieved Au/
Al2O3 catalyst can be reused several times
without any loss of its high catalytic
performance.
Acceptorless Cross-Dehydrogenative
Coupling of Hydrosilanes and Isocyanic
Acid Generated in situ from Urea
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DOI: 10.1002/anie.201303132
Cross-Coupling
Heterogeneous-Gold-Catalyzed Acceptorless Cross-Dehydrogenative
Coupling of Hydrosilanes and Isocyanic Acid Generated in situ from
Urea**
Kento Taniguchi, Shintaro Itagaki, Kazuya Yamaguchi, and Noritaka Mizuno*
Cross-coupling reactions (e.g., Suzuki, Negishi, Heck, Stille,
Kumada, and Buchwald-Hartwig reactions) are of paramount
importance and have proven useful for the design of
molecules.[1] Although cross-coupling reactions can precisely
present, the vast majority of silyl isocyanates have been
synthesized using chlorosilanes and metal cyanates (e.g.,
silver cyanate) [Eq. (2)].[14]
À
À
À
construct new C C, C X, and X X bonds (X = heteroatom),
they usually utilize preactivated substrates (e.g., halides,
tosylates, and triflates) and require transmetalation steps,
which concurrently generate at least stoichiometric amounts
of metal salts as waste.[1] Recently, cross-dehydrogenative
À
À
coupling reactions by direct activation of C H or X H bonds
have been emerging as synthetic tools because they are more
atom efficient and environmentally benign than classical
cross-coupling reactions.[2] To date, several efficient cross-
dehydrogenative coupling reactions using hydrogen acceptors
(oxidants) such as tert-butyl hydroperoxide,[3] hydrogen
peroxide,[4] and molecular oxygen[5] have been developed.
Acceptorless cross-dehydrogenative coupling reactions have
However, toxic and moisture-sensitive chlorosilanes and
expensive metal cyanates are required in this antiquated
procedure. In addition, the atom efficiency is low because of
the inevitable formation of stoichiometric amounts of metal
salts as waste. For example, the atom efficiency of the reaction
of dimethylphenylchlorosilane (R1, R2 = CH3, R3 = Ph) with
silver cyanate to form dimethylphenylsilyl isocyanate is 55%
[Eq. (2)]. Thus, the development of efficient catalytic syn-
thetic procedures using alternative starting materials is very
important for green silyl isocyanate synthesis. The use of
hydrosilanes and urea as starting materials would be more
desirable because 1) they are readily available, inexpensive,
and less-toxic, 2) coproducts are NH3 and H2, and 3) the atom
efficiency reaches up to 90% (for R1, R2 = CH3, R3 = Ph)
[Eq. (1)].
To realize the reaction in Equation (1), hydrosilanes
should electrophilically be activated with appropriate cata-
lysts. With regard to activation of hydrosilanes, several
hydrolytic oxidation systems using gold,[15] ruthenium,[16]
rhenium,[17] platinum,[18] iridium,[19] and silver catalysts[20]
have been reported where electrophilic silicon species are
possibly generated. If the efficient synthetic procedures for
silyl isocyanates using recoverable and reusable heteroge-
neous catalysts could be developed, they would be more
desirable from the standpoint of green chemistry.[21]
Therefore, we initially prepared various kinds of sup-
ported metal catalysts (gold, silver, copper, ruthenium,
rhodium, palladium, and platinum on Al2O3; see the Support-
ing Information for the preparation), and their catalytic
performances were evaluated by the reaction of dimethyl-
phenylsilane (1a) with urea (1.1 equivalents with respect to
1a) to form dimethylphenylsilyl isocyanate (2a; Table 1). The
reactions were carried out under reflux conditions in toluene
using 0.2 mol% of catalyst with respect to 1a (see also
Table S1 in the Supporting Information).[22] The catalysts and
urea were carefully dried before use for the transformation
(to avoid the undesirable hydrolysis of urea and in situ formed
isocyanic acid, see the Supporting Information). Under the
present reaction conditions, the reaction did not proceed at all
also been developed.[6] Quite recently, we have also reported
C N (terminal alkynes and amides), P N (H-phosphonates
[7]
À
À
and amides),[8] Si N (hydrosilanes and indoles), and Si C
(hydrosilanes and terminal alkynes)[10] bond-forming reac-
tions by cross-dehydrogenative coupling strategy. Herein, we
successfully developed a novel green synthetic route to silyl
isocyanates through heterogeneous-gold-catalyzed, acceptor-
less cross-dehydrogenative coupling of hydrosilanes and
[9]
À
À
isocyanic acid (generated by in situ thermolysis of urea[11]
[Eq. (1)].
)
Silyl isocyanates have been utilized as silane coupling and
surface finishing agents,[12] and are potentially useful as
carbamoyl synthons for organic synthesis.[12,13] Up to the
[*] K. Taniguchi, S. Itagaki, Dr. K. Yamaguchi, Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
[**] We thank Dr. Y. Kuroda and H. Kobayashi (The University of Tokyo)
for their help with preliminary experiments and helpful discussions.
This work was supported in part by the Grants-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303132.
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Table 1: Synthesis of dimethylphenylsilyl isocyanate (2a) by the reaction
of dimethylphenylsilane (1a) with urea in the presence of various
catalysts.[a]
can rule out any contribution to the observed catalysis from
gold species which leached into the reaction solution, and the
observed catalysis for the present transformation is truly
heterogeneous in nature.[24]
After the reaction of 1a with urea was completed, Au/
Al2O3 could easily be retrieved from the reaction mixture by
simple filtration with greater than 95% recovery. The
retrieved Au/Al2O3 catalyst could be reused at least six
times for the same reaction without significant loss of its high
catalytic performance. Even for the sixth reuse experiment,
95% yield of 2a was still obtained (see Figure S2 in the
Supporting Information), though the formation rate of 2a for
the sixth reuse experiment (2.2 mm minÀ1, determined by the
yield for 1 h) was somewhat smaller than that of the first run
with fresh Au/Al2O3 (2.6 mm minÀ1; Figure S3 in the Support-
ing Information).[23] The average turnover number (TON;
TON = 2a produced [mol]/Au used [mol]) for one run was
474, and the total TON reached up to 3320 after the sixth
reuse experiment.
Next, the scope of the present Au/Al2O3-catalyzed syn-
thesis of silyl isocyanates was examined. As shown in Table 2,
various kinds of hydrosilanes could be converted into the
corresponding silyl isocyanates. The reaction of dimethylphe-
nylsilane (1a) and its derivatives with electron-donating (1b
and 1c) as well as electron-withdrawing (1d and 1e)
substituents at the para-position of the phenyl rings efficiently
Entry
Catalyst
Conv. [%]
Yield [%]
2a
Others[b]
1
Au/Al2O3
Au/Al2O3
Pd/Al2O3
Rh/Al2O3
Ag/Al2O3
Pt/Al2O3
Ru/Al2O3
Cu/Al2O3
Au/SiO2
Au/TiO2
66
>99
54
5
60
91
48
4
4
2
<1
<1
52
45
4
46
22
3
6
9
6
2[c]
3
4
5
6
7
8
9
10
11
12[d]
13[d]
14[d]
15[d]
16
1
4
2
<1
<1
<1
<1
3
2
2
7
4
<1
<1
55
47
6
53
26
12
<1
<1
Au/CeO2
Au/CeO2 + Al2O3
Au/CeO2 + SiO2
Au/CeO2 + MgO
Al2O3
9
<1
<1
<1
<1
None
[a] Reaction conditions: Catalyst (metal: ca. 0.2 mol%), 1a (0.5 mmol),
urea (0.55 mmol), toluene (2 mL), reflux (bath temperature: 1358C), Ar
(1 atm), 1 h. Conversions and yields were determined by GC analyses.
[b] Silanols, disiloxanes, and disilazanes were formed as byproducts (see
also Table S1 in the Supporting Information). [c] 3 h. [d] Metal oxide
additives (40 mg).
Table 2: Scope of the present Au/Al2O3-catalyzed synthesis of silyl
isocyanates.[a]
in the absence of the catalysts or the presence of Al2O3 alone
(Table 1, entries 15 and 16). The supported gold catalyst was
the most effective for the transformation among the catalysts
examined (Table 1, entries 1 and 2), and the supported
Entry
1
Hydrosilane
t [h]
Yield [%]
91
palladium catalyst also showed
a good performance
(Table 1, entry 3). The reaction hardly proceeded in the
presence of supported rhodium, silver, platinum, ruthenium,
and copper catalysts (Table 1, entries 4–8). Al2O3 was the best
support (Table 1, entry 1), and SiO2 and TiO2 were also good
supports for the transformation (Table 1, entries 9 and 10). In
contrast, the reaction hardly proceeded with gold on rela-
tively basic CeO2 (Table 1, entry 11). Thus, gold supported on
Al2O3, Au/Al2O3,[23] was the best catalyst. For example, when
the reaction of 1a with urea was carried out in the presence of
Au/Al2O3 for 3 hours, the desired silyl isocyanate 2a was
obtained in 91% yield (Table 1, entry 2).
To verify whether the observed catalysis is truly caused by
solid Au/Al2O3 or leached gold species, the following control
experiment was carried out. The reaction of 1a with urea was
carried out under the reaction conditions described in Table 1,
and the Au/Al2O3 was removed from the reaction mixture by
filtration; the yield of 2a was 70%. Urea (0.55 mmol) was
then added to the filtrate, and the resulting mixture was again
heated at 1358C (bath temperature) under 1 atm of Ar. In this
case, no further production of 2a was observed (see Figure S1
in the Supporting Information). In addition, we confirmed by
the inductively coupled plasma atomic emission spectroscopy
(ICP-AES) analysis that no gold species was detected in the
filtrate (below the detection limit of 3.0 ppb). These results
1a
1b
1c
1d
1e
3
2
3
4
5
3
3
3
3
94
91
87
97
6
7
8
9
10
11
Ph2MeSi-H
Ph3Si-H
Et3Si-H
(nPr)3Si-H
(EtO)3Si-H
(nBuO)3Si-H
1 f
1g
1h
1i
1j
1k
3
4.5
6
15
9
24
93
97
99
69
56
43
[a] Reaction conditions: Au/Al2O3 (Au: 0.2 mol%), hydrosilane
(0.5 mmol), urea (0.55 mmol), [D8]toluene (2 mL), reflux (bath temper-
ature: 1358C), Ar (1 atm), 1 h. Yields were determined by 1H NMR and
GC analyses. Conversions of 1a–h were greater than 99%. Conversions
of 1i, 1j, and 1k were 72%, 87%, and 65% respectively. The major
byproducts were the corresponding silanols, disiloxanes, and disila-
zanes. As for entry 10, tetraethoxysilane (23%) was also formed as
a byproduct. As for entry 11, tetra(n-butoxy)silane (18%) was also
formed as a byproduct.
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proceeded to afford the corresponding substituted dimethyl-
phenylsilyl isocyanate derivatives in high yields (Table 2,
entries 1–5). The reactions of sterically more hindered
diphenylmethylsilane (1 f) and triphenylsilane (1g) also
efficiently proceeded (Table 2, entries 6 and 7). Besides
hydrosilanes having phenyl groups, trialkylsilanes (1h and
1i) could be converted into the corresponding trialkylsilyl
isocyanates, though the steric effect was significant (Table 2,
entries 8 and 9). Notably, alkoxysilyl isocyanates could be
obtained in moderate yields by the reaction of alkoxysilanes
(1j and 1k) with urea (Table 2, entries 10 and 11). In these
cases, tetraalkoxysilanes were formed as by products.
proceeds, thus giving the corresponding silyl isocyanate with
the concurrent formation of H2 (step 3) to complete the
catalytic cycle.
In summary, we have successfully developed for the first
time an efficient heterogeneous catalytic system for synthesis
of silyl isocyanates through acceptorless cross-dehydrogen-
ative coupling of hydrosilanes and isocyanic acid obtained by
the in situ thermolysis of urea. This novel synthetic procedure
with the heterogeneous Au/Al2O3 catalyst could completely
avoid utilization of stoichiometric reagents (e.g., metal
cyanates) and formation of vast amounts of byproducts
(e.g., metal halides).
We confirmed in separate experiments that Au/Al2O3
could also act as an efficient heterogeneous catalyst for
hydrolytic oxidation of hydrosilanes, silylation of alkynes, and
hydrosilylation of alkynes and ketones (see Figure S4 in the
Supporting Information). It is well known that hydrolytic
silane oxidation,[15–20] silylation,[6b,10] and hydrosilylation[25]
proceed through formation of electrophilically activated
hydrosilanes. Thus, in the present case, the formation of
electrophilic silicon species on Au/Al2O3 is suggested. The MS
analyses of the gas phase for the reaction of 1a with urea
showed formation of NH3 and H2 during the transformation.
Received: April 15, 2013
Revised: May 29, 2013
Published online: && &&, &&&&
Keywords: cross-coupling · gold · heterogeneous catalysis ·
.
isocyanates · silanes
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À
(Scheme 1). The Si H bond of a hydrosilane is activated by
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Scheme 1. Possible reaction mechanism for the present Au/Al2O3-
catalyzed synthesis of silyl isocyanates by the reaction of hydrosilanes
with urea. Si-Hꢀhydrosilane, AuꢀAu/Al2O3.
4
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Chemie
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presence Al2O3 at 1358C (estimated form the TG curve,
Figure S6) was approximately three times larger than that in
the absence of Al2O3. In contrast, in the case of MgO, the
evolution of isocyanic acid was not observed because MgO likely
promoted the condensation of urea to polymeric compounds
such as biuret and cyanuric acid.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!