Palladium-catalyzed silylation of alcohols with hexamethyldisilane

Article abstract of DOI:10.1039/b608958e

The combination of hexamethyldisilane and a catalytic amount of [PdCl(η³-C₃H₅)]₂-PPh ₃ was found to be effective for the trimethylsilylation of alcohols, where both of the two trimethylsilyl groups of hexamethyldisilane were transferred to alcohols without coproduction of any stoichiometric amount of byproduct but H₂. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b608958e

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Palladium-catalyzed silylation of alcohols with hexamethyldisilane{
Eiji Shirakawa,* Koji Hironaka, Hidehito Otsuka and Tamio Hayashi*
Received (in Cambridge, UK) 23rd June 2006, Accepted 14th July 2006
First published as an Advance Article on the web 4th August 2006
DOI: 10.1039/b608958e
The combination of hexamethyldisilane and a catalytic amount
of [PdCl(g3-C3H5)]2–PPh3 was found to be effective for the
trimethylsilylation of alcohols, where both of the two
trimethylsilyl groups of hexamethyldisilane were transferred
(1)
To get an insight into how the silyl groups are transferred from
hexamethyldisilane (1a) to alcohols, we performed several
stoichiometric reactions using [PdCl(g3-PhCHCHCHPh)]2 (3b)
to alcohols without coproduction of any stoichiometric amount
of byproduct but H2.
Silylation of alcohols is an important process not only as a
protecting method of alcohols, but also for synthesis of functional
organosilicon compounds.1 The treatment of alcohols with a
trialkyl(chloro)silane in the presence of a stoichiometric amount of
an amine is certainly the most widely used method, where
coproduction of HCl?amine is inevitable. One of the most effective
alternatives free from such salts is the use of trialkyl(hydro)silanes
as silyl sources, often in combination with a transition metal
catalyst, giving H2 as the sole byproduct.2,3 However, the method
becomes impractical when the corresponding hydrosilane is
difficult to handle, as in the case with trimethylsilylation, which
requires the use of volatile trimethylsilane. Here we report
palladium-catalyzed trimethylsilylation of alcohols using readily
available hexamethyldisilane.4 To the best of our knowledge, there
has been only one precedent for use of hexamethyldisilane as a
silylation reagent, where tetrabutylammonium fluoride is used as a
catalyst.5,6
Table 1 Palladium-catalyzed silylation of alcohols with hexamethyl-
disilanea
Entry
Alcohol
Yield (%)b
1
2
95
93
3
4
95
95
5c
6c
7
91d
91d
93
We have previously reported that the combination of hexam-
ethyldisilane (1a) with D2O reduces alkynes into 1,2-dideuterioalk-
enes with the aid of a palladium catalyst, where D2O is silylated
on the O–D bonds to be transformed to hexamethyldisiloxane
via trimethylsilanol.7 We expected that the replacement of D2O
with alcohols should lead to formation of trimethylsilyl ethers.
Thus, treatment of 1-hexadecanol (2a: 1.0 equiv.) with 1a
(0.60 equiv.) in the presence of [PdCl(g3-C3H5)]2 (3a: 5 mol%
of Pd) and PPh3 (10 mol%) in DMA at 80 uC for 3 h gave
1-(trimethylsiloxy)hexadecane (4a) in 95% yield (eqn (1) and entry 1
of Table 1). Worthy of note is that only 0.60 equiv. of 1a is
required, implying that both of the two trimethylsilyl groups of
disilane 1a are used as the silyl source. The silylation is applicable
also to secondary and tertiary alcohols (entries 2–4). Both trans-
and cis-1,2-cyclohexanediol reacted equally with 1a to give bis-
silylation products (entries 5 and 6). The reaction is compatible
with functional groups such as carbonyl, haloalkane and alkene
(entries 7–9), and a sugar having a free hydroxy group underwent
the silylation (entry 10).
8
9
95
91
10
90
a
The reaction was carried out in DMA (1.0 mL) at 80 uC for 3 h
using an alcohol (0.80 mmol) and Me3SiSiMe3 (0.48 mmol) in the
presence of [PdCl(g3-C3H5)]2 (0.040 mmol of Pd) and PPh3
Department of Chemistry, Graduate School of Science, Kyoto
University, Sakyo, Kyoto, 606-8502, Japan.
E-mail: shirakawa@kuchem.kyoto-u.ac.jp; Fax: +81 75 753 3988
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b608958e
b
c
(0.080 mmol). Isolated yield based on the alcohol. Me3SiSiMe3
(0.96 mmol), [PdCl(g3-C3H5)]2 (0.080 mmol of Pd) and PPh3
(0.16 mmol) were used. Yield of the bis-silylation product.
d
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and 1,1,2,2-tetramethyl-1,2-diphenyldisilane (1b) instead of
[PdCl(g3-C3H5)]2 (3a) and 1a, where phenyl substituents facilitate
the tracking of the allyl and the silyl moieties. On treatment of 3b
(0.400 mmol of Pd) and PPh3 (0.800 mmol) with 1b (0.190 mmol)
in the presence of 1-phenylethanol (2b: 3.99 mmol) in DMA at
80 uC for 1 h, we observed the formation of PdH(Cl)(PPh3)2 (5),
which was confirmed by 31P NMR (bs, 28.1 ppm).8 The reaction
mixture was analyzed by GC, and was found to contain 1,3,4,6-
tetraphenyl-1,5-hexadiene (6b as a mixture of diastereomers: 0.187
mmol, 94% yield based on 3b) and 1-[dimethyl(phenyl)siloxy]-1-
phenylethane (49b: 0.352 mmol, 93% yield based on 1b) (eqn (2)).9
PdH(Cl)(PPh3)2 (5) thus generated was found to catalyze the
trimethylsilylation of 2b faster than 3a–PPh3 (1 : 2): the reaction
completed within 1 h under the same conditions as entry 2 of
Table 1 to give 4b in 94% yield.10 It is interesting that the two silyl
groups of 1b were used for the silylation of alcohol during the
conversion of p-allylpalladium 3b into active H–Pd–Cl complex 5.
Scheme 1
(2)
H–Pd–Cl 5 with the use of Me3SiSiMe3 (1a) and [PdCl(g3-C3H5)]2
(3a) are likely to be depicted as shown in Scheme 1. In Cycle A,
H–Pd–Cl 5 first reacts with 1a to give H–Pd–SiMe3 and Me3SiCl
(8a), which silylates ROH 2. Reductive elimination from H–Pd–
SiMe3 gives Me3SiH (7a) and Pd0, which accepts oxidative
addition of HCl coproduced on the silylation to regenerate H–Pd–
Cl 5. Hydrosilane 7a thus generated participates in a similar cycle
(Cycle B), transmetalating with H–Pd–Cl 5 to give Me3SiCl (8a)
and H–Pd–H. The cycle goes along a similar scheme to Cycle A
but gives H2 instead of the hydrosilane.
(3)
In conclusion, we have disclosed a new method of trimethylsi-
lylation of alcohols with the aid of a palladium catalyst, The most
striking feature is high atom-economy, where H2 is a sole
byproduct in use of just a slightly excess amount of a silyl source.{
Notes and references
Next, we examined how the silyl groups of disilanes are
transferred to alcohols under the influence of PdH(Cl)(PPh3)2 (5)
(eqn (3)). The reaction of PhMe2SiSiMe2Ph (1b) with 5 (1.0 equiv.)
in the presence of alcohol 2b (10 equiv.) at 0 uC for 1.5 h proceeded
with 87% conversion to give 49b (100% yield) and PhMe2SiH (7b:
72% yield), which also was efficiently transformed further to 49b
upon heating at 80 uC for 1 h.3,11 Silyl ether 49b observed before
heating is possibly generated by the reaction of initially formed
PhMe2SiCl (8b) with alcohol 2b. Although we failed to detect 8b in
the reaction mixture of eqn (3) in the absence of 2b due to its
instability in DMA,12 chlorosilane 8b (60%) and hydrosilane 7b
(34%) were observed by GC analysis on treatment of 1b with 5
(1.0 equiv.) in THF at 25 uC for 10 min, with 48% conversion. As
chlorosilanes are known to be obtained from hydrosilanes and
HCl in the presence of a palladium catalyst,13 silyl ether 49b
derived from hydrosilane 7b upon heating (eqn (3)) is likely to be
produced through chlorosilane 8b in the presence of H–Pd–Cl
complex 5.
{ General procedure for the trimethylsilylation of alchohols: To a solution
of [PdCl(g3-C3H5)]2 (3a: 7.4 mg, 0.040 mmol of Pd) and PPh3 (21 mg,
0.080 mmol) in DMA (1.0 mL) were added successively an alcohol (2:
0.80 mmol) and hexamethyldisilane (1a: 70 mg, 0.48 mmol). After stirring
at 80 uC for 3 h, the resulting mixture was diluted with diethyl ether
(20 mL), washed with water (10 mL 6 5) and brine (10 mL), and dried
over anhydrous sodium sulfate. After filtration, followed by evaporation of
the solvent at atmospheric pressure, the resulting mixture was filtered
through an alumina plug (10 mL) using hexane (30 mL) as an eluent.
Evaporation of the solvent at atmospheric pressure followed by evacuation
at 35 uC under ca. 20 mmHg gave trimethylsilyl ether 4.
1 T. W. Greene and P. G. M. Wuts, Protective Groups in Organic
Synthesis, Wiley & Sons, New York, 3rd edn, 1999.
2 For a review, see: E. Lukevics and M. Dzintara, J. Organomet. Chem.,
1985, 295, 265–315.
3 A wide variety of transition metals are known to catalyze the silylation
of alcohols. Cu: B. N. Dolgov, N. P. Kharitonov and M. G. Voronkov,
Zh. Obshch. Khim., 1954, 24, 1178–1188; Pd: L. H. Sommer and
J. E. Lyons, J. Am. Chem. Soc., 1967, 89, 1521–1522; L. H. Sommer and
J. E. Lyons, J. Am. Chem. Soc., 1969, 91, 7061–7067; K. Yamamoto
and M. Takemae, Bull. Chem. Soc. Jpn., 1989, 62, 2111–2113;
W. J. Leigh, R. Boukherroub and C. Kerst, J. Am. Chem. Soc., 1998,
120, 9504–9512; Y. Li and Y. Kawakami, Macromolecules, 1999, 32,
Considering the generation of a chlorosilane and a hydrosilane
as the intermediates for silyl ether 4, catalytic cycles starting with
3928 | Chem. Commun., 2006, 3927–3929
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3871–6873; S. Sudhakar and T.-Y. Luh, J. Org. Chem., 2002, 67,
6860–6862; M.-K. Chung, G. Orlova, J. D. Goddard, M. Schlaf,
R. Harris, T. J. Beveridge, G. White and F. R. Hallett, J. Am. Chem.
Soc., 2002, 124, 10508–10518.
8 A similar broad singlet was observed at 28.1 ppm in 31P NMR of
authentic trans-5 prepared according to the literature. K. Kudo,
M. Hidai, T. Murayama and Y. Uchida, J. Chem. Soc. D, 1970,
1701–1702.
4 Yamamoto and Kumada reported that the PdCl2(PEt3)2-catalyzed
reaction of PhMe2SiSiMe3 with ethanol at 90 uC for 26 h proceeded
with 65% conversion to give PhMe2SiOEt with evolution of molecular
hydrogen. However, hexaalkyldisilanes are considered to be inert under
these conditions. K. Yamamoto, M. Kumada, I. Nakajima, K. Maeda
and N. Imaki, J. Organomet. Chem., 1968, 13, 329–341; K. Yamamoto
and M. Kumada, J. Organomet. Chem., 1972, 35, 297–302.
5 Aliphatic alcohols are trialkylsilylated with trialkyl(hydro)silane
(Et3SiH, t-BuMe2SiH, and others: 1.5 equiv.) or disilanes
(Me3SiSiMe3, MePh2SiSiPh2Me: 1.0 equiv.) in the presence of
20 mol% of tetrabutylammonium fluoride. Y. Tanabe, H. Okumura,
A. Maeda and M. Murakami, Tetrahedron Lett., 1994, 35, 8413–8414.
6 Recently, the rhodium-catalyzed trimethylsilylation of alcohols with
trimethyl(vinyl)silane has been reported. J.-W. Park, H.-J. Chang and
C.-H. Jun, Synlett, 2006, 771–775.
9 p-Allyl complex 3b and disilane 1b were found to be as equally effective
as 3a and 1a, respectively, as a palladium precursor and a silylating
reagent: the reaction of 2b with 1b (0.60 equiv.) in the presence of 3b–
PPh3 (1:2, 5 mol% of Pd) at 80 uC for 3 h gave 49b in 92% yield.
10 The higher catalytic activity of PdH(Cl)(PPh3)2 (5) was demonstrated by
comparison of the reactions stopped at 10 min: 37% 4b (5) vs. ,1% 4b
(3a–PPh3).
11 Treatment of 7b with 5 (1.0 equiv.) and 2b (10 equiv.) at 80 uC for 1 h
gave 49b in 89% yield.
12 The peak corresponding to 8b was observed on GC analysis of a THF
solution, whereas it was not observed in a DMA solution.
13 For the palladium-catalyzed reaction of hydrosilanes with HCl to give
chlorosilanes, see: L. H. Sommer and J. D. Citron, J. Org. Chem., 1967,
32, 2470–2472. The reaction mechanism of H–Pd–Cl with a Si-chiral
hydrosilane is discussed based on the stereochemical outcome.
L. H. Sommer and J. E. Lyons, J. Am. Chem. Soc., 1969, 91,
7061–7067.
7 E. Shirakawa, H. Otsuka and T. Hayashi, Chem. Commun., 2005,
5885–5886.
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