Nonhydrolytic synthesis of silanols by the hydrogenolysis of benzyloxysilanes

Article abstract of DOI:10.1246/cl.131079

The hydrogenolysis of benzyloxysilanes was smoothly catalyzed by Pd/C in THF to give corresponding silanols under nonhydrolytic conditions. The reaction proved to be applicable to various benzyloxysilanes giving silanemonools, diol, and triol.

Full text of DOI:10.1246/cl.131079

Received: November 17, 2013 | Accepted: December 6, 2013 | Web Released: April 5, 2014
CL-131079
Nonhydrolytic Synthesis of Silanols by the Hydrogenolysis of Benzyloxysilanes
Masayasu Igarashi, Tomohiro Matsumoto, Kazuhiko Sato, Wataru Ando, and Shigeru Shimada*
National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565
(E-mail: s-shimada@aist.go.jp)
Table 1. Hydrogenolysis of 1 using various types of Pd/Csa
The hydrogenolysis of benzyloxysilanes was smoothly
Ph
catalyzed by Pd/C in THF to give corresponding silanols under
nonhydrolytic conditions. The reaction proved to be applicable to
various benzyloxysilanes giving silanemonools, diol, and triol.
H2 (1 atm)
Ph Si OBn
+
Ph
Ph
1
10 mol% Pd/C
PhCH3
Ph
Si OH
+
Silanols are important chemical species that are widely used
as building blocks for silicon-based polymeric materials and are
also important intermediates generated from halosilanes and
alkoxysilanes in sol-gel-type processes.1,2 They are also used as
reagents and catalysts in organic synthesis; aryl-, alkenyl-, and
alkynylsilanols are useful cross-coupling reagents3 and silane
diols and bis(silanols) are new types of organocatalysts based on
their strong hydrogen-bonding formation.4 Extensive hydrogen-
bonding networks formed by silanols would also be useful as
building blocks for supramolecular organometallic chemistry.5
Introduction of a silanol moiety in a molecule attracts consid-
erable interests in medicinal chemistry.6
Two methods are mainly used for the preparation of
silanols. One is hydrolysis of compounds containing Si-X
groups such as chlorosilanes and alkoxysilanes.2 The resulting
silanols are often further condensed without isolation to form
oligomeric and polymeric materials. This process is very
important in the silicone industry. The other general method is
the oxidation of hydrosilanes using oxidizing agents, such as
dioxiranes.2 This oxidation method is mostly limited to the
preparation of silanemonools. Recently, highly efficient catalysts
for the oxidation of hydrosilanes using water as an oxidant have
been developed.7
Sterically less demanding silanols are difficult to isolate
because they easily condense to form siloxanes, in particular
upon concentration. As mentioned above, most silanol syntheses
are conducted in the presence of water. Therefore, it is difficult
to use the resulting silanols for reaction with moisture-sensitive
compounds such as halosilanes and metal alkoxides, if the
silanols cannot be isolated. We are interested in developing
nonhydrolytic preparation of silanols, so that they can be
directly used for reaction with moisture-sensitive compounds.
Herein, we report nonhydrolytic synthesis of silanols by the Pd/
C-catalyzed hydrogenolysis of benzyloxysilanes.
Benzyl ethers are commonly used to protect hydroxy groups
in organic syntheses.8 The benzyl groups can be readily removed
by hydrogenolysis catalyzed by Pd/C. However, this procedure
is rarely used for the protection of silanols.9 We thought this
procedure is useful for the nonhydrolytic synthesis of silanols.
The starting materials, benzyloxysilanes, can be prepared by
the reaction of corresponding chlorosilanes with benzyl alcohol
in the presence of triethylamine and a catalytic amount of
4-(dimethylamino)pyridine.10
THF-d8, r.t., 1.5 h
Ph
2
Conv. Yield Selectivity
Entry Pd/C (wt %)
/%b,c /%d
/%e
1
2
3
4
OH type (Pd 10 wt %)
ASCA-2 (Pd 4.5 wt %, Pt 0.5 wt %) 98
PE type (Pd 10 wt %)
NX type (Pd 5 wt %)
8
6
95
13
6
75
97
93
75
14
8
aReaction conditions: 1 (0.150 mmol) and Pd/C (10 mol %)
were stirred under hydrogen atmosphere (1 atm) at room
temperature for 1.5 h in THF-d8 (1.0 mL). bConvresion of
Ph3SiOBn. cConversion and yield based on 1 were determined
by integration of 29Si NMR spectra using inverse-gated
decoupling pulse sequence with 1,4-bis(trimethylsilyl)benzene
as an internal standard. dYield of Ph3SiOH. eSelectivity =
Yield/conversion (%).
acids of Pd/Cs (depending of suppliers) cause solvolytic
cleavage of silyl ethers.11 Thus, in order to avoid hydrolytic
cleavage of benzyloxysilanes, all Pd/Cs used in this study were
dried by heating at 120 °C for 48 h under vacuum before use.
Initial attempts by using OH-type Pd/C (N. E. CHEMCAT),
which is recommended for the deprotection of benzyl ethers by
the supplier, proved that it was not a very effective catalyst for
the deprotection of Ph3SiOBn (Bn: benzyl) (1) (Table 1,
Entry 1). Therefore, we surveyed several other types of Pd/Cs
available from the same supplier for the reaction. The hydro-
genolysis of 1 was conducted by stirring a mixture of 1
(0.150 mmol) and Pd/C (10 mol %) in THF-d8 (1 mL) under
1 atm of H2 at room temperature for 1.5 h in a sealed tube. As
shown in Table 1, ASCA-2 (N. E. CHEMCAT), which contains
Pt (0.5 wt %) and Pd (4.5 wt %), completed the deprotection of 1
within 1.5 h (95% yield and 97% selectivity, Table 1, Entry 2).
Simultaneous generation of an approximately equimolar amount
of toluene (93%) was confirmed by H and 13C NMR spectra of
1
the reaction mixture. Other Pd/Cs, PE- and NX-types (N. E.
CHEMCAT), which are also recommended for benzyl ether
deprotection, are also much less effective than ASCA-2
(Table 1, Entries 3 and 4).
Table 2 summarizes the solvent effects in this reaction by
using ASCA-2-type Pd/C as a catalyst. THF is the solvent of
choice in this reaction, giving the desired product nearly
quantitatively within 1.5 h at room temperature (Entry 2). On
Commercially available Pd/Cs are generally sold moistened
with water for safety. Sajiki and Hirota reported that residual
Chem. Lett. 2014, 43, 429–431 | doi:10.1246/cl.131079
© 2014 The Chemical Society of Japan | 429

the other hand, in the other five solvents, MeCN, CH2Cl2,
EtOAc, toluene, and hexane, the reaction proceeded slowly with
lower selectivities. The reason of this specific preference for
THF is not clear at the moment.
The deprotection reaction of 1 did not proceed in the
absence of H2 or the catalyst at all. The hydrogenolysis of 1
catalyzed by Pd/C (ASCA-2) was further confirmed by the
experiment using D2 instead of H2 (Scheme 1). Treatment of 1
under 1 atm of D2 at room temperature for 1.5 h in the presence
of ASCA-2 (10 mol %) in anhydrous THF-d8 almost quantita-
tively afforded the expected products Ph3SiOD and PhCH2D
(respectively 98% and 96% by NMR).
This nonhydrolytic procedure is well applicable to various
benzyloxysilanes (Table 3).12 Not only arylsilanes, but also an
alkylsilane, i-Pr3SiOBn, efficiently converted to the correspond-
ing silanol (Table 3, Entry 2). Replacement of one or two Ph
groups of 1 with Me groups considerably affected the
selectivities partly because of the ease of self-condensation to
form disiloxanes (Table 3, Entries 3 and 4). Sajiki and Hirota
reported that a solution treated with Pd/Cs (depending on
suppliers) under H2 became acidic due to the residual acids and/
or PdCl2 in Pd/Cs.11 Therefore, it is possible that the acid
coming from Pd/C accelerates self-condensation of silanols. In
Entry 4, yield of silanol considerably decreased (4%). This is
probably attributed not only to the acceleration of silanol
condensation but also to the adsorption of silanol on Pd/C,
because total recovery of products was considerably low in this
case. Actually, reduction of the catalyst amounts from 10 to
1 mol % considerably increased the silanol yield from 4 to 51%
and slightly decreased the disiloxane yield from 55 to 44%
Table 2. Solvent effect toward the hydrogenolysis of 1a
Conv.
/%
Yield
/%b
Selectivity
Entry
Solvent
/%c
1
2
3
4
5
6
MeCN
THF
CH2Cl2
AcOEt
Toluene
Hexane
18
98
19
20
13
11
11
95
16
18
8
61
97
84
90
62
73
Ph
D2 (1 atm)
Ph Si OBn
+
8
Ph
Ph
aReaction conditions: 1 (0.150 mmol) and Pd/C (ASCA-2
type, 10 mol %) were stirred under hydrogen atmosphere
(1 atm) at room temperature for 1.5 h in various solvents
1
10 mol % ASCA-2
PhCH2D
96 %
Ph
Si OD
+
THF-d8, r.t., 1.5 h
Ph
b
(1.0 mL). Conversion and yield based on 1, were determined
2-D
98 %
by integration of 29Si NMR spectra using inverse-gated
decoupling pulse sequence with 1,4-bis(trimethylsilyl)benzene
c
Scheme 1. Deuteriolysis of 1 with D2.
as an internal standard. Selectivity = Yield/conversion (%).
Table 3. Hydrogenolysis of various benzyloxysilanes using Pd/C (ASCA-2 type)a
ASCA-2
R4−nSi(OBn)n
H2 (1 atm)
{R4–n(HO)n–1Si}2O + n PhCH3
+
+
R
4−nSi(OH)n
THF-d8
n = 1, 2, 3
Pd/C
Yield/%b
R4¹nSi(OH)n {R4¹n(HO)n¹1Si}2O
(mol % Pd for
each benzyloxy
group)
Temp.
/°C
Time
/h
Conv.
/%
Selectivity
Entry
Benzyloxysilane
/%c
1
2
3
4
5
6
7
8
9
10
11f
12
13
14
Ph3SiOBn
10
10
10
10
1
r.t.
r.t.
r.t.
r.t.
r.t.
¹25
r.t.
r.t.
r.t.
r.t.
1.5
1.5
1.5
1.5
1.5
1.5
1.5
20
98
100
100
100
100
86
24
74
85
100
100
98
85
90
95 (90)
82
60
4
51
66
19
69
80
70e
58
89
22
0
0
97
82
60
4
i-Pr3SiOBn
Ph2MeSiOBn
PhMe2SiOBn
13
55
44
17
0
0
0
5
0
51
77
79
93
94
70
58
91
26
36
1
Ph2t-BuSiOBn
10
10
10
10
10
5
40
Ph2ViSiOBnd
Ph2Si(OBn)2
1.5
1.5
1.5
1.5
1.5
r.t.
r.t.
r.t.
¹25
5
54
40
PhSi(OBn)3
1
3
32
aReaction conditions: Benzyloxysilane (0.150 mmol) and Pd/C (ASCA-2 type) were stirred under hydrogen atmosphere (1 atm) in THF-
b
d8 (1.0 mL). Conversion and yield based on each benzyloxysilane were determined by integral value of 29Si NMR analysis using
inverse-gated decoupling pulse sequence with 1,4-bis(trimethylsilyl)benzene as an internal standard. The values in parentheses show the
c
d
e
f
yield of isolated product. Selectivity = Yield of R4¹nSi(OH)n/conversion (%). Vi: vinyl. The product is Ph2EtSiOH. Ph2Si(OBn)2
(0.075 mmol) was used.
430 | Chem. Lett. 2014, 43, 429–431 | doi:10.1246/cl.131079
© 2014 The Chemical Society of Japan

(Table 3, Entries 4 and 5). Furthermore, the selectivity to
PhMe2SiOH increased to 77% by conducting the reaction at
lower temperature, ¹25 °C (Table 3, Entry 6). Introduction of
a bulkier substituent, t-Bu, to 1 considerably decreased the
conversion under the same reaction conditions (24% conversion
and 19% silanol yield, Table 3, Entry 7). As expected, longer
reaction time increased the conversion (74% after 20 h and 85%
after 40 h, Table 3, Entries 8 and 9) while keeping good
selectivity. In the case of a vinyl-substituted compound, hydro-
genation of the vinyl group simultaneously took place to give
Ph2EtSiOH (Table 3, Entry 10). This methodology can also be
applied to the synthesis of silanediol and -triol, although
condensation of the resulting silanols to the dimers and higher
oligomers becomes more serious. Thus, in the case of
Ph2Si(OBn)2, decreasing the catalyst amounts from 10 mol %
to 5 mol % increased the yield and the selectivity of Ph2Si(OH)2
respectively from 58% to 89% and from 58% to 91% (Table 3,
Entries 11 and 12). It was difficult to suppress the condensation
to the siloxanes in the case of PhSi(OBn)3 by reducing the
catalyst amounts and/or by lowering the reaction temperature;
1 mol % of the catalyst at room temperature or 3 mol % of the
catalyst at ¹25 °C respectively resulted in 26% and 36%
selectivities to PhSi(OH)3 (Table 3, Entries 13 and 14).13
In summary, we found a versatile and effective Pd/C-
catalyzed hydrogenolysis of benzyloxysilanes to the correspond-
ing silanols under nonhydrolytic mild conditions. This synthetic
method is applicable to the synthesis of silanemonools, -diol,
and -triol. Further study to improve the selectivity as well as the
application of this procedure for the reaction with moisture
sensitive compounds, such as halosilanes and metal alkoxides, is
underway in our laboratory.
Eur. J. 2011, 17, 9897. c) N. T. Tran, S. O. Wilson, A. K.
Franz, Org. Lett. 2012, 14, 186. d) C. Beemelmanns, R.
Husmann, D. K. Whelligan, S. Özçubukçu, C. Bolm, Eur. J.
Org. Chem. 2012, 3373.
5
6
a) G. Prabusankar, R. Murugavel, R. J. Butcher, Organo-
metallics 2004, 23, 2305. b) J. Beckmann, A. Duthie, G.
Reeske, M. Schürmann, Organometallics 2004, 23, 4630. c)
Y. Kawakami, Y. Sakuma, T. Wakuda, T. Nakai, M.
Shirasaka, Y. Kabe, Organometallics 2010, 29, 3281.
a) S. M. Sieburth, T. Nittoli, A. M. Mutahi, L. Guo, Angew.
Chem., Int. Ed. 1998, 37, 812. b) R. Tacke, T. Schmid, M.
Hofmann, T. Tolasch, W. Francke, Organometallics 2003,
22, 370. c) D. H. Juers, J. Kim, B. W. Matthews, S. M.
Sieburth, Biochemistry 2005, 44, 16524. d) R. Tacke, B.
Nguyen, C. Burschka, W. P. Lippert, A. Hamacher, C.
Urban, M. U. Kassack, Organometallics 2010, 29, 1652. e)
A. K. Franz, S. O. Wilson, J. Med. Chem. 2013, 56, 388.
a) M. Lee, S. Ko, S. Chang, J. Am. Chem. Soc. 2000, 122,
12011. b) T. Mitsudome, A. Noujima, T. Mizugaki, K.
Jitsukawa, K. Kaneda, Chem. Commun. 2009, 5302. c) M.
Jeon, J. Han, J. Park, ChemCatChem 2012, 4, 521. d) K.
Shimizu, T. Kubo, A. Satsuma, Chem.®Eur. J. 2012, 18,
2226. e) J. John, E. Gravel, A. Hagège, H. Li, T. Gacoin, E.
Doris, Angew. Chem., Int. Ed. 2011, 50, 7533. f) N. Asao, Y.
Ishikawa, N. Hatakeyama, Menggenbateer, Y. Yamamoto,
M. Chen, W. Zhang, A. Inoue, Angew. Chem., Int. Ed. 2010,
49, 10093. g) Y. Kikukawa, Y. Kuroda, K. Yamaguchi, N.
Mizuno, Angew. Chem., Int. Ed. 2012, 51, 2434. h) M. Jeon,
J. Han, J. Park, ACS Catal. 2012, 2, 1539.
7
8
9
For a review, see: P. G. M. Wuts, T. W. Greene, Greene’s
Protective Groups in Organic Synthesis, 4th ed., John Wiley
& Sons, New York, 1998, pp. 106-109. doi:10.1002/
9780470053485.ch2.
To the best of our knowledge, only one example is reported:
K. Tomooka, A. Nakazaki, T. Nakai, J. Am. Chem. Soc.
2000, 122, 408.
This work was supported by the Future Pioneering Projects
“Development of Innovative Catalytic Processes for Organo-
silicon Functional Materials” from the Minister of Economy,
Trade and Industry, Japan (METI).
10 S. K. Chaudhary, O. Hernandez, Tetrahedron Lett. 1979, 20,
99.
References and Notes
1
V. Chandrasekhar, R. Boomishankar, S. Nagendran, Chem.
Rev. 2004, 104, 5847.
P. D. Lickiss, Adv. Inorg. Chem. 1995, 42, 147.
11 a) T. Ikawa, H. Sajiki, K. Hirota, Tetrahedron 2004, 60,
6189. b) K. Hattori, H. Sajiki, K. Hirota, Tetrahedron Lett.
2000, 41, 5711.
2
3
a) K. Hirabayashi, Y. Nishihara, A. Mori, T. Hiyama,
Tetrahedron Lett. 1998, 39, 7893. b) K. Hirabayashi, J.
Kawashima, Y. Nishihara, A. Mori, T. Hiyama, Org. Lett.
1999, 1, 299. c) K. Hirabayashi, J. Ando, J. Kawashima, Y.
Nishihara, A. Mori, T. Hiyama, Bull. Chem. Soc. Jpn. 2000,
73, 1409. d) K. Hirabayashi, A. Mori, J. Kawashima, M.
Suguro, Y. Nishihara, T. Hiyama, J. Org. Chem. 2000, 65,
5342. e) S. E. Denmark, D. Wehrli, Org. Lett. 2000, 2, 565.
f) A. Mori, Y. Danda, T. Fujii, K. Hirabayashi, K. Osakada,
J. Am. Chem. Soc. 2001, 123, 10774. g) S. E. Denmark, R. F.
Sweis, J. Am. Chem. Soc. 2001, 123, 6439. h) S. E.
Denmark, S. A. Tymonko, J. Org. Chem. 2003, 68, 9151. i)
S. E. Denmark, C. S. Regens, Acc. Chem. Res. 2008, 41,
1486. j) H. Zhou, C. Moberg, J. Am. Chem. Soc. 2012, 134,
15992.
12 General procedure for the hydrogenolysis of benzyloxysi-
lane: A stirred mixture of the benzyloxysilane (0.150 mmol)
and Pd/C (0.2-10 mol % Pd for each benzyloxy group) in
THF-d8 (1.0 mL) was treated with hydrogen gas (1 atm) at
room temperature for an appropriate time. The reaction
mixture was filtered through a membrane filter (GE Health-
care Puradisc 13, 0.45 ¯m) to remove the catalyst. 1,4-
Bis(trimethylsilyl)benzene as an internal standard for NMR
analysis and a trace amount of [Cr(acac)3] as a relaxation
agent were added to the filtrate. The mixture was subjected
to 1H and 29Si NMR (inverse-gated decoupling pulse
sequence) analysis to determine the conversion and the
yield. The details of the quantitative 29Si NMR analysis are
shown in the Supporting Information.13
13 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
4
a) A. G. Schafer, J. M. Wieting, A. E. Mattson, Org. Lett.
2011, 13, 5228. b) N. T. Tran, T. Min, A. K. Franz, Chem.®
Chem. Lett. 2014, 43, 429–431 | doi:10.1246/cl.131079
© 2014 The Chemical Society of Japan | 431