Convenient and selective preparation of mono-alkoxyphenylsilanes from phenylsilane and alcohols

Article abstract of DOI:10.1246/cl.2006.714

The selective dehydrogenative coupling of phenylsilane and alcohols, including primary, secondary, and tertiary alcohols, was smoothly catalyzed by bis(hexafluoroacetylacetonato)-copper(II) complex to afford the corresponding mono-alkoxysilanes in good-to

Full text of DOI:10.1246/cl.2006.714

714
Chemistry Letters Vol.35, No.7 (2006)
Convenient and Selective Preparation of Mono-alkoxyphenylsilanes
from Phenylsilane and Alcohols
Yasuhiko Gunji, Yoshihiro Yamashita, Taketo Ikeno, and Tohru YamadaÃ
Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi, Kohoku-ku, Yokohama 223-8522
(Received March 23, 2006; CL-060359; E-mail: yamada@chem.keio.ac.jp)
Table 1. Various acetylacetonato complexesa
The selective dehydrogenative coupling of phenylsilane and
alcohols, including primary, secondary, and tertiary alcohols,
was smoothly catalyzed by bis(hexafluoroacetylacetonato)-
copper(II) complex to afford the corresponding mono-alkoxy-
silanes in good-to-high yield after distillation.
M(acac)n
rt
+
+
O
PhSiH
PhSiH3
HO
PhSiH2O
2
mono-alkoxysilane bis-alkoxysilane
Entry
Metal
Time/h
Conv./%b
Mono:Bis
Yield/%b
1
2
3
4
None
Mn(III)
Fe(II)
2.5
3.0
7.0
0.67
1
21
32
53
1
7
4
9
36:64
12:88
17:83
Hydrosilanes are some of the most versatile reducing agents
in organic synthesis.1 Their combined use with various transi-
tion-metal complexes has provided many chemoselective or
stereoselective synthetic reactions, for example, the hydrosilyla-
tion of ketones, imines,2 or alkenes,3 the silylformylation of
alkenes,4 etc. Hydrosilanes are also employed as a hydride
source to generate metal hydrides.5 In these synthetic reactions,
the reactivities and selectivities can be tuned by the structures of
the hydrosilanes, e.g., after the screening of various alkoxy-
silanes, the high yield and high enantioselectivity have been
achieved for the enantioselective conjugated reaction catalyzed
by Rh complex.6 It was also noted that polymethylhydrosiloxane
was employed in the place of tetramethyldisiloxane as the silane
reducing agent to drastically improve the observed ee value for
the hydrosilylation of imines catalyzed by Cu complex.7 There-
fore, preparative methods for the varieties of hydrosilane
reagents are assumed to take more advantage for the tuning of
these synthetic reactions and for the development of new meth-
odologies using hydrosilanes. The dehydrogenative coupling of
hydrosilanes and alcohols is one of the most preferable methods
to prepare the alkoxysilanes, and various catalysts were reported
for the dehydrogenative reaction including transition-metal com-
plexes, inorganic salts, and Lewis acids.8 Although the highly se-
lective silylation of a primary alcohol over a secondary alcohol
was recently reported with the catalytic use of Cu(I)–phosphine
complex,9 few reports have noted the mono-alkoxylation of tri-
hydrosilanes to afford the mono-alkoxydihydrosilanes, and nei-
ther of them seemed to provide the efficient preparative methods
for various alkoxysilanes.10 It was also reported that PdCl2 or
NiCl2 was used as a catalyst for the mono-alkoxylation of phen-
ylsilane on the basis of its high reactivity but was unsuccessful.11
In this communication, we report the convenient preparation
of various alkoxydihydrosilanes from phenylsilane and the
correspondig alcohols in the presence of a catalytic amount of
the bis(hexafluoroacetylacetonato)copper(II) complex.
Co(II)
5
6
Co(III)
Ru(III)
Pt(II)
6.0
2.0
4.0
6.0
2.5
1.0
1.0
67
67
14:86
48:52
50:50
10:90
95%c
55%c
89:11
9
28
18
6
7
49
8
Ni(II)
100
100
100
75
9
Rh(III)
Pd(II)
Cu(II)
0
10
11
0
67
aPhenylsilane, 1 mmol; 2-propanol, 10 mmol; metal, 0.1 mmol.
bDetermined by GC analysis and based on phenylsilane. The
yield of diisopropyloxyphenylsilane.
c
generated by over-condensation of isopropyloxyphenylsilane
with 2-propanol. Ni(II), Pd(II), and Rh(III) were active enough
to achieve the complete consumption of phenylsilane, but the
desired mono-alkoxysilane was scarcely remained. When bis-
(acetylacetonato)copper was employed, in contrast, the dehydro-
genative coupling smoothly proceeded in good selectivity to
afford isopropyloxyphenylsilane in 67% yield.
In the presence of 10 mol % of the various copper12 com-
plexes, various ligands were then screened (Table 2). Copper
chloride, bromide, iodide, and acetate catalyzed the dehydrogen-
ative reaction to afford the mono-alkoxysilane as the major prod-
uct while the yields were moderate or low. Various 1,3-diketo-
nato-type complexes were then examined. In each case, the se-
lectivity and yield of the mono-alkoxyphenylsilane were not
improved compared to the bis(acetylacetonato)copper. Reducing
the loading amounts of the catalyst and the alcohol enabled one
to control the reactivity in the reaction catalyzed by bis(hexa-
fluoroacetylacetonato)copper(II) to afford the monoalkoxyphen-
ylsilane with good selectivity. The optimization of the reaction
conditions resulted in an improved product yield up to 86%.
The dehydrogenative coupling of phenylsilane and 2-propa-
nol was successfully applied to the 10 mmol scale preparation
with bis(hexafluoroacetylacetonato)copper(II) and the obtained
crude material could be purified by distillation to give the de-
sired product in 77% isolated yield.13 The other secondary alco-
hols and primary alcohols could also be transformed to the cor-
responding mono-alkoxysilanes in good yields after distillation
(Table 3). Although the bulky alcohol like t-butyl alcohol was
less reactive, the increased use of the copper catalyst and alcohol
afforded full conversion of the phenylsilane to provide the
Various acetylacetonato metal complexes were first exam-
ined for the dehydrogenative coupling of phenylsilane and 2-
propanol as a model reaction (Table 1). Without metal com-
plexes, no reaction occurred. The Mg(II), Al(III), Ti(IV),
V(IV), Cr(III), and Mn(II) acetylacetonato complexes had little
activity for the reaction, which resulted in the slight conversion
of phenylsilane. Although Mn(III), Fe(II), Co(II), Co(III),
Ru(III), and Pt(II) were found to catalyze the dehydrogenative
reaction, the major product was diisopropyloxyphenylsilane,
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.7 (2006)
715
Table 2. Various copper complexesa
8.0 mol %Cu(hfa)2
Ph2SiH2
+
+
HO
HO
Ph2SiHO
cat.CuLn
0 °C, 2 h
+
PhSiH3
HO
PhSiH2O
89% yield
rt
3.6 mol %Cu(hfa)2
Entry
CuLn
CuCl
Conv./%b
Mono:Bisb Yield/%b
n-C8H17SiH3
n-C8H17SiH2O
CH2Cl2, 0 °C, 1 h
1
2
3
4
5
92
88
93
50
9
87:13
74:26
68:32
85:15
97: 3
73
49
59
40
8
80% yield
CuCl2
CuBr
Scheme 1.
CuBr−Me2S
CuI
alkoxysilane with 89% yield was observed (Scheme 1).
It is noted that trihydrosilanes were selectively transformed
into dihydroalkoxysilanes by the dehydrogenative coupling with
primary, secondary, and tertiary alcohols in the presence of a
catalytic amount of bis(hexafluoroacetylacetonato)copper(II).
6
7
8
9
Cu(OAc)2
Cu(paa)2
64
53
86:14
93: 7
80:20
27%c
47
39
65
0
Cu(bza)2
Cu(dpm)2
Cu(tfh)2
85
100
References and Notes
10
85
98
3:97
65:35
77:23
33%c
88:12
91: 9
2
44
46
0
1
2
I. Ojima, Z. Li, J. Zhu, in The Chemistry of Organic Silicon
Coompounds, ed. by Z. Rappoport, Wiley, Chichester, 1998,
Vol. 2, Chap. 29, pp. 1687–1792.
a) H. Nishiyama, K. Itoh, in Catalytic Asymmetric Synthesis,
ed. by I. Ojima, Wiley-VCH, New York, 2000, Chap. 2,
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1989, 111, 2332.
Ti-H: a) X. Verdaguer, U. E. W. Lange, S. L. Buchwald,
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46, 2037.
11
Cu(dmh)2
Cu(tfa)2
12
60
13
Cu(hfa)2
100
81
14d
15e
Cu(hfa)2 (1.0 mol %)
Cu(hfa)2 (1.5 mol %)
67
86
98
aPhenylsilane, 1 mmol; 2-propanol, 10 mmol unless otherwise
noted. Determined by GC analysis. The yield of diisopropyl-
3
b
c
d
e
oxyphenylsilane. 2-Propanol, 1 equiv. 2-Propanol, 1.5 equiv.
O
O
O
O
O
O
O
O
O
O
O
4
5
N
t
t
t
t
Bu
Bu
CF
3
Bu
Ph
Bu
Ph
Hpaa
O
Hbza
Htfh
Hdmh
Hdpm
Table 3. Preparation of mono-alkoxyphenylsilanesa
cat.Cu(hfa)2
+
PhSiH3
ROH
PhSiH2OR
Entry
1
Alkoxyphenylsilane
PhSiH2O
Conv./%
95
Yield/%
83
(Isolated)
(77)
6
7
8
9
Y. Tsuchiya, Y. Kanazawa, T. Shiomi, K. Kobayashi, H.
Nishiyama, Synlett 2004, 2493.
B. H. Lipshutz, H. Shimizu, Angew. Chem., Int. Ed. 2004, 43,
2228.
J. M. Blackwell, K. L. Foster, V. H. Beck, W. E. Piers, J. Org.
Chem. 1999, 64, 4887, and references therein.
H. Ito, A. Watanabe, M. Sawamura, Org. Lett. 2005, 7, 1869.
2b
3c
93
88
83
75
(66)
(64)
PhSiH2O
PhSiH2O
4
97
86
91
75
71
74
(62)d
(65)
(68)
PhSiH2O
PhSiH2O
PhSiH2O
5c
6e
10 a) W. S. Miller, J. S. Peake, W. H. Nebergall, J. Am. Chem.
Soc. 1957, 79, 5604. b) D. H. R. Barton, M. J. Kelly,
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11 J. Ohshita, R. Taketsugu, Y. Nakahara, A. Kunai, J. Organo-
met. Chem. 2004, 689, 3258.
12 Copper complex catalysts for preparation of alkoxysilane
derivatives, see: a) C. Lorenz, U. Schubert, Chem. Ber. 1995,
128, 1267. b) D. R. Schmidt, S. J. O’Malley, J. L. Leighton,
J. Am. Chem. Soc. 2003, 125, 1190.
13 Typical procedure: To the stirred solution of bis(hexafluoro-
acetylacetonato)copper(II) (143 mg, 0.3 mmol) in 2-propanol
(2.30 mL, 30 mmol) and CH2Cl2 (7 mL) was added PhSiH3
(2.47 mL, 20 mmol) in one portion at 0 ꢀC under N2. After
stirring for 0.75 h at 0 ꢀC, hexane (15 mL) was added and the
existing solid was filtered off. The residue was concentrated
under reduced pressure, then fractionally distilled using bulb-
to-bulb distillation apparatus to give PhSiH2OiPr (2.56 g,
77%) as a colorless oil.
7f
97
95
(76)
PhSiH2O
aThe reaction was carried out using 1.5 equiv. alcohol, 1.5 mol %
of catalyst, and CH2Cl2 at 0 ꢀC unless otherwise noted.
bCatalyst, 2.3 mol %. cCatalyst, 2.0 mol %. d5.5:1 mixture of
PhSiH2(OEt) and PhSiH(OEt)2. Catalyst, 3.0 mol %. Catalyst,
5.0 mol %; t-butanol, 3.0 equiv.
e
f
mono-alkoxysilane with a high selectivity and yield. Octylsilane
was also applicable for the dehydrogenative condensation, and
the pure isopropyloxyoctylsilane was obtained in 80% yield in
the same manner. While the coupling of diphenylsilane and
2-propanol needed both a greater excess of catalyst loading
and neat conditions, the highly selective generation of the mono-
Published on the web (Advance View) May 30, 2006; doi:10.1246/cl.2006.714