An efficient solvent-free route to silyl esters and silyl ethers

Article abstract of DOI:10.1002/adsc.200900230

Dinuclear metal complexes, especially (p-cymene)ruthenium dichloride dimer {[RuCl₂(p-cymene)]₂}, have been found to exhibit high catalytic performance for the dehydrosilylation of various kinds of carboxylic acids and alcohols. The dehydrosilylation with [RuCl₂(p-cymene)] ₂ proceeded efficiently with only one equivalent of silane with respect to substrate (carboxylic acids or alcohols) under solvent-free conditions to give the corresponding silyl esters and ethers in excellent yields with a high turnover number (TON) and frequency (TOF). The ¹H NMR spectrum of a toluene-d₈ solution of [RuCl₂(p-cymene)] ₂ and a silane showed a signal assignable to the ruthenium hydride species. In contrast, no new signals were detected in the ¹H NMR spectrum of a toluene-d₈ solution of [RuCl₂(p-cymene)] ₂ and a carboxylic acid or an alcohol. There-fore, the ruthenium metal in [RuCl₂(p-cymene)]₂ activates a silane to afford the hydride intermediate, possibly a silylmetal hydride species. Then, the nucleophilic attack of a substrate (carboxylic acid or alcohol) to the hydride intermediate proceeds to give the corresponding silylated product. The present dehydrosilylation with an optically active silane proceeded exclusively under inversion of stereochemistry at the chiral silicon center, suggesting that the nucleophilic attack of a substrate to the hydride intermediate occurs from the backside of the ruthenium-silicon bond.

Full text of DOI:10.1002/adsc.200900230

FULL PAPERS
DOI: 10.1002/adsc.200900230
An Efficient Solvent-Free Route to Silyl Esters and Silyl Ethers
Yuko Ojima,a Kazuya Yamaguchi,a,b and Noritaka Mizunoa,b,*
a
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
113-8656, Japan
Fax : (+81)-3-5841-7220; phone: (+81)-3-5841-7272; e-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
Core Research for Evolutional Science and Technology (CREST) (Japan) Science and Technology Agency (JST),
b
4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Received: January 29, 2009; Revised: April 6, 2009; Published online: June 3, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900230.
Abstract: Dinuclear metal complexes, especially (p- cymene)]2 and a carboxylic acid or an alcohol. There-
cymene)ruthenium dichloride dimer {[RuCl2ACTHNUGTRENN(UG p- fore, the ruthenium metal in [RuCl2ACHTUNGTREN(NGUN p-cymene)]2 ac-
cymene)]2}, have been found to exhibit high catalytic tivates a silane to afford the hydride intermediate,
performance for the dehydrosilylation of various possibly a silylmetal hydride species. Then, the nucle-
kinds of carboxylic acids and alcohols. The dehydro- ophilic attack of a substrate (carboxylic acid or alco-
silylation with [RuCl2ACHTNUGTRENUNG(p-cymene)]2 proceeded effi- hol) to the hydride intermediate proceeds to give the
ciently with only one equivalent of silane with re- corresponding silylated product. The present dehy-
spect to substrate (carboxylic acids or alcohols) drosilylation with an optically active silane proceed-
under solvent-free conditions to give the correspond- ed exclusively under inversion of stereochemistry at
ing silyl esters and ethers in excellent yields with a the chiral silicon center, suggesting that the nucleo-
high turnover number (TON) and frequency (TOF). philic attack of a substrate to the hydride intermedi-
1
The H NMR spectrum of a toluene-d8 solution of ate occurs from the backside of the ruthenium-silicon
[RuCl2ACHTUNGTRENNUNG(p-cymene)]2 and a silane showed a signal as- bond.
signable to the ruthenium hydride species. In con-
1
trast, no new signals were detected in the H NMR Keywords: alcohols; carboxylic acids; dehydrosilyla-
spectrum of a toluene-d8 solution of [RuCl2ACTHUNRTGNE(UNG p- tion; ruthenium; silyl esters; silyl ethers
Introduction
If the synthesis of silyl esters and silyl ethers direct-
ly from silanes and carboxylic acids or alcohols could
Silyl esters and silyl ethers are an important class of be performed (catalytic dehydrosilylation), it would
chemicals that have widely been used for the produc- be more economical and environmentally-friendly be-
tion of silicon-based polymeric materials in industry cause it produces only H2 as a co-product. Although
as well as for intermediates in organic syntheses.[1] many transition metal catalysts have been reported
The silylation of carboxylic acid derivatives or alco- for the dehydrosilylation of alcohols with silanes,[4]
hols with expensive and reactive silylating reagents there are only a few reports for the dehydrosilylation
such as hexamethyldisilazane and aminosilanes usual- of carboxylic acids.[5] In addition, the reported systems
ly requires continuous removal of NH3 and amines have shortcomings; low TON and TOF and/or narrow
formed.[2] Although silyl esters and silyl ethers have applicability to the limited silanes and carboxylic
been synthesized by the cross-coupling of carboxylic acids. In this paper, we report that simple dimeric
acid derivatives or alcohols with chlorosilanes,[3] these metal complexes, especially [RuCl
2ACTHNUTRGNEUNG(p-cymene)]2, act
procedures require high reaction temperatures to as efficient homogeneous catalysts for the dehydrosi-
attain high yields of the corresponding silylated prod- lylation of carboxylic acids with only one equivalent
ucts with concomitant formation of by-products such of silanes under solvent-free conditions [Eq. (1)]. The
as silanols and siloxanes.[3] In addition, HCl is formed
in the cross-coupling and at least stoichiometric
amounts of bases such as NH3 and amines are neces-
sary to neutralize it.
Adv. Synth. Catal. 2009, 351, 1405 – 1411
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Yuko Ojima et al.
FULL PAPERS
applicability of the present system to the dehydrosily- [RuCl2(CO)3]2,
lation of alcohols is also reported [Eq. (2)]. The syn- [IrCl2Cp*]2, and [PdCl
and [RuCl2A
[RhCl2Cp*]2,
(p-allyl)]2 efficiently proceeded
CTHUNGTRENN(UNG p-cymene)]2 showed the highest catalytic
[RhClACHTUNGTRENNUNG(cod)]2,
ACHTUNGTRENNUNG
activity and chemoselectivity among the transition
metal catalysts tested. For example, in the presence of
thesis of silyl esters and ethers sometimes meets with 0.5 mol% of [RuCl2ACTHUNTRGNE(UGN p-cymene)]2, the dehydrosilyla-
difficulty because of their instability during isolation tion was almost completed within 2 h at 508C and the
and purification processes. The dehydrosilylation re- corresponding silyl ester 3a was obtained in 94%
ported herein would be ideal in that it produces H2 yield with 98% chemoselectivity. In this case, the stoi-
gas as the only by-product and typically needs no sol- chiometric amount of gaseous H2 with respect to 3a
vents. The use of rather simple, easily available cata- was formed. Although other transition metal catalysts
lysts also benefits synthetic applications of this such as RuCl
N
2ACHTGNUERTNGNUN(PPh3)4, Ru3(CO)12,
system.
RuCl3, RhCl
N
ACHTUNGTRENNUNG
lytic activity for the transformation, the chemoselec-
tivities to 3a (70–82%) were lower than the 98% ob-
Results and Discussion
tained with [RuCl
2ACHTUNGTRENUN(NG p-cymene)]2.
Next, the scope of the present [RuCl2ACHNUTTGRENN(GUN p-cymene)]2-
First, the dehydrosilylation of acetic acid (1a) with catalyzed system toward various kinds of structurally
one equivalent of dimethylphenylsilane (2a) was ex- diverse carboxylic acids and silanes was examined
amined in the presence of various transition metal (Table 2).[6] The reactions were carried out with equi-
catalysts under solvent-free conditions. The results are molar amounts of silanes with respect to carboxylic
summarized in Table 1. The sterically less-hindered acids under solvent-free conditions.[7] The reactions
silane 2a was chosen as a model substrate because its were typically carried out under an air atmosphere.
transformation to the desired silyl ester 3a is very sen- The reaction rate for the dehydrosilylation under Ar
sitive to the reaction conditions and catalysts used, or O2 atmosphere was almost the same as that under
and the decomposition to the silanol and the conden- an air atmosphere. A number of carboxylic acids in-
sation to the disiloxane easily proceed. Under the cluding aliphatic (1a–1g), aromatic (1h–1k), and unsa-
present conditions, no reactions proceeded in the ab- turated (1l) ones efficiently reacted with silanes (2a–
sence of catalysts. The dehydrosilylation with dimeric 2c) and all combinations gave the corresponding silyl
metal complexes such as [RuCl2ACTHNUTRGNEN(UG p-cymene)]2, esters (3a–3u) in good yields. In the case of chloro-
benzoic acid (1i), the reaction efficiently proceeded to
afford the desired silyl ester without formation of the
dechlorinated product. Also, no reduction of the nitro
group was observed for nitrobenzoic acid (1j). Under
the present conditions, the desired unsaturated silyl
ester was obtained by the reaction of a silane with an
unsaturated carboxylic acid (1l) with the formation of
a small amount of an over-reduced product. In the
case of 4-acetylbenzoic acid (1m), no hydrogenation
and hydrosilylation of the carbonyl group proceeded
to give the desired silyl ester in high yield.
Table 1. Dehydrosilylation of 1a with 2a by various cata-
ACHTUNGTRENNUNG
lysts.[a]
Run
Catalyst
[RuCl2A(p-cymene)]2
Conv. [%]
Select. [%]
1
2
3
4
5
6
7
8
G
G
96
91
>99
96
>99
50
98
92
80
82
74
70
67
67
–
G
N
Ru3(CO)12
RuCl3·nH2O
RuH
K2RuCl5·H2O
Ru(acac)3
[Ru(NH3)6]Cl3
(NH4)2RuCl6
RuCp2
[RhCl2Cp*]2
[RhCl(cod)]2
RhCl(PPh3)3
[IrCl2Cp*]2
[PdCl(p-allyl)]2
Pd(OAc)2
[CuH(PPh3)]6
2ACHTUNGTRENNUNG(PPh3)4
In order to establish the applicability of the present
system to larger scale productions, a 50-mmol scale
reaction of 1a with 2a (one equivalent with respect to
6
3
E
9
G
<1
<1
<1
>99
94
>99
76
>99
>99
4
1a) was carried out with 0.007 mol% of [RuCl2ACHTUNGTRENNUNG(p-
10
11
12
13
14
15
16
17
18
19
20
T
–
–
cymene)]2. This solvent-free larger-scale reaction
showed a TOF of 380 hÀ1 and the TON reached up to
12,000 [Eq. (3)]. These TOF and TON values were
E
92
93
76
95
86
75
75
–
A
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
A
U
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
[Mn(CO)5]2
blank
<1
<1
–
[a]
Reaction conditions: 1a (5 mmol), 2a (5 mmol), catalyst
(metal: 1 mol%), 508C, 2 h.
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An Efficient Solvent-Free Route to Silyl Esters and Silyl Ethers
FULL PAPERS
Table 2. Dehydrosilylation of various carboxylic acids with
Table 3. Dehydrosilylation of 4a with 2a by various cata-
silanes catalyzed by [RuCl
2A
lysts.[a]
ACHTUNGTRENNUNG
Run
Catalyst
[RuCl2A(p-cymene)]2
Conv. [%]
Select. [%]
1
2
3
4
5
6
7
8
G
G
>99
90
4
2
2
<1
<1
<1
<1
<1
<1
>99
73
92
92
89
62
88
–
–
–
–
–
ACHTUNGTRENNUNG
R
Run Carboxylic acid
Silane
Silyl
t
Yield
ester
[h] [%]
E
1
2
3
MeCOOH (1a)
PhMe2SiH 3a
(2a)
Ph2MeSiH 3b
(2b)
2
6
6
94
82
89
E
1a
1a
Ru3(CO)12
K2RuCl5·H2O
R
9
Et3SiH
(2c)
2a
2b
2a
2b
2a
2a
3c
10
11
12
13
14
15
16
17
18
E
–
4
5
6
7
8
9
10
11
EtCOOH (1b)
1b
n-PrCOOH (1c)
1c
i-PrCOOH (1d)
n-BuCOOH (1e)
1e
3d
3e
3f
3g
3h
3i
2
6
3
6
3
3
6
3
76
83
75
84
74
75
85
76
N
95
99
–
97
97
77
–
A
ACHTUNGTRENNUNG
U
<1
95
95
88
<1
N
A
U
N
2b
2a
3j
3k
n-C5H11COOH
[a]
Conditions: 4a (5 mmol), 2a (5 mmol), catalyst (metal:
1 mol%), 08C (ice bath), 5 min.
(1f)
1f
1f
12
13
2b
2c
2a
2a
2c
2c
3l
7
6
4
2
3
6
84
73
84
88
96
85
3m
3n
3o
3p
3q
14[b] AdCOOH[c] (1g)
15[b] PhCOOH (1h)
16[b] 1h
dehydrosilylation with dimeric metal complexes such
as [RuCl
[RhCl(cod)]2, [IrCl2Cp*]2, and [PdCl
ciently proceeded (Table 3). When 2a (2.5 mmol) was
added to methanol solution of [RuCl2A(p-cymene)]2
(1.0 mM, 2.5 mL) at 328C, the reaction was completed
within 1 min [Eq. (4)]. In this case, the TOF (deter-
(p-cymene)]2, [RuCl2(CO)3]2, [RhCl2Cp*]2,
17[d] 4-ClC6H4COOH
(1i)
A
ACHTUNGTRENNUNG
18[d] 4-NO2C6H4COOH 2c
(1j)
3r
3s
7
2
75
84
CTHUNGTRENNUNG
19[b] Ph
20[b] 1k
N
2a
2c
3t
3u
3
7
97
21[b] PhCH=CHCOOH 2c
(1l)
76[e]
22[f] 4-acetyl-
2c
3v
7
90
C6H4COOH (1m)
[a]
[b]
Reaction conditions: carboxylic acid (5 mmol), silane
(5 mmol), [RuCl2A(p-cymene)]2 (0.5 mol%), 508C. Yields
CHTUNGTRENNUNG
1
were determined by H NMR or GC.
Reaction conditions: carboxylic acid (1 mmol), silane
(1 mmol), [RuCl2ACTHNUGTRNEUNG(p-cymene)]2 (1 mol%), toluene (3 mL),
508C.
Ad=1-adamantyl.
Reaction conditions: carboxylic acid (1 mmol), silane
(1 mmol), [RuCl2ACHTNUTRGNE(NUG p-cymene)]2 (1 mol%), 1,4-dioxane
(3 mL), 508C.
Compound 3t was formed as a by-product (7%).
Reaction conditions: carboxylic acid (1 mmol), silane of 4b with 2a (one equivalent with respect to 4b)
(1 mmol), [RuCl2ACHTNUTRGNE(NUG p-cymene)]2 (1 mol%), 1,4-dioxane
mined by the initial rate) reached up to 367,000 hÀ1
and the value was the highest among those previously
reported dehydrosilylation of alcohols (TOF: 1–
130,000 hÀ1).[4] Furthermore, the catalyst amount
could be much reduced: In a 150-mmol scale reaction
[c]
[d]
[e]
[f]
using 0.0013 mol% of [RuCl2ACHTUNRTGNEUNG(p-cymene)]2, the TON
(5 mL), 508C.
reached up to 60,000 [Eq. (5)]. This value was of the
highest level among those with previously reported
systems (TON: 50–7,000)[4] except for the [Au]-
the highest among those for the previously reported SMAP-Rh system, which was recently reported by Sa-
dehydrosilylation reactions (TOF: 0.25–168 hÀ1, TON: wamura and co-workers (12,000–90,000).[4p]
6–192).[5]
As shown in Table 4, various kinds of structurally
In addition, the present system could be applied to diverse primary and secondary alcohols including ali-
the dehydrosilylation of alcohols with silanes.[6,8] The phatic (4a–4e and 4j), aromatic (4f), alkenic (4g), alk-
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Yuko Ojima et al.
FULL PAPERS
ene-d8 solution of [RuCl
(or 1a) (50 mM). These results suggest that the [RuCl2
(p-cymene)]2 catalyst cannot activate carboxylic acids
and alcohols, and that the reaction is initiated by the
activation of silanes by the [RuCl2A(p-cymene)]2 cata-
lyst. To a toluene-d8 solution of [RuCl2A(p-cymene)]2
2ACHTNUGTREN(UNGN p-cymene)]2 (5 mM) and 4b
AHCTUNGTRENNUNG
CTHUNGTRENNUNG
CTHUNGTRENNUNG
(5 mM) and 2a (50 mM) at 258C, ten equivalents of
4b (or 1a) with respect to 2a were added, resulting in
the disappearance of the hydride signal and the pro-
duction of the corresponding silylated compound and
the evolution of H2. On the basis of these results, we
ACHTUNGTRENNUNG
CTHUNGTRENNUNG
present conditions, the desired unsaturated silyl hydride intermediate, possibly the silylmetal hydride
ethers (5e and 5n–5p) were obtained with the forma- species.[11] Then, the nucleophilic attack of a substrate
tion of small amounts of over-reduced products. Inter- (carboxylic acid or alcohol) to the hydride intermedi-
estingly, the desired silyl ether was obtained by the re- ate proceeds to give the corresponding silylated prod-
action of a silane with an epoxy alcohol (4i) without uct.[10] The present [RuCl
2ACHTNUTRGNENG(U p-cymene)]2-catalyzed de-
the formation of ring-opening and hydrosilylation hydrosilylation with an optically active silane pro-
products. Although it took a longer reaction time ceeded exclusively with inversion of stereochemistry
(900 min), the dehydrosilylation of a tertiary alcohol at the chiral silicon center: the silylation of 4b (1.5
4j with 2a gave the corresponding silyl ether 5r in equivalents with respect to 2f) with R-(+)-methyl(1-
good yield.
naphthyl)phenylsilane (92% ee) (2f) afforded R-(À)-
The H NMR spectrum of a toluene-d8 solution of butoxymethyl(1-naphthyl)phenylsilane (5s) in 84% ee,
[RuCl2A
(p-cymene)]2 (5 mM) and 2a (50 mM) at 258C for example [Eq. (6)].[12–14] This inversion of the ste-
1
CHTUNGTRENNUNG
showed a signal at À10.6 ppm assignable to the ruthe- reochemistry suggests that the nucleophilic attack of a
nium hydride species.[9] In contrast, no new signals substrate to the hydride intermediate occurs from the
1
were detected in the H NMR spectrum of the tolu- backside of the ruthenium-silicon bond.
Table 4. Dehydrosilylation of various alcohols with silanes catalyzed by [RuCl2ACTHNUTRGNEUNG
(p-cymene)]2.[a]
Run
Alcohol
Silane
Silyl ether
t [min]
Yield [%]
1
2
MeOH (4a)
4a
4a
4a
2a
2b
2c
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
5q
5r
5
5
5
30
5
5
30
5
60
5
30
5
30
5
30
5
15
900
91
93
99
82
83[c]
98
96
92
92
91
86
88
92
88
95
90
85
68
3
4[b]
5
Ph2SiH2 (2d)
PhMe
2a
2b
2a
2b
2a
2a
2a
2b
2a
4a
ACHTUNGTREN(NUNG CH2=CH)SiH (2e)
6
7
8
9
10
11
12
13
14
15
16
17
18
n-BuOH (4b)
4b
n-C6H13OH (4c)
4c
cyclopentanol (4d)
cyclohexanol (4e)
BnOH (4f)
4f
allyl alcohol (4g)
4g
4-pentyn-2-ol (4h)
trans-2,3-epoxy-1-hexanol (4i)
t-BuOH (4j)
2b
2a
2a
2a
[a]
Reaction conditions: alcohol (5 mmol), silane (5 mmol), [RuCl
termined by H NMR or GC.
Reaction conditions: alcohol (10 mmol), silane (5 mmol), [RuCl
Ethylmethoxymethylphenylsilane was formed as a by-product (10%).
U
1
[b]
[c]
N
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An Efficient Solvent-Free Route to Silyl Esters and Silyl Ethers
FULL PAPERS
Synthesis of R-(+)-Methyl(1-naphthyl)phenylsilane
(2f)
Conclusions
Compound 2f (92% ee) was synthesized according to the lit-
erature procedures.[17] The Grignard reagent was prepared
from 1-bromonaphthalene (14.8 g, 71.5 mmol) with magnesi-
um turnings (2.09 g, 86.0 mmol) in a mixed solvent of ether
(5 mL), toluene (10 mL), and THF (5 mL). To the mixture
was added
a solution of dimethoxymethylphenylsilane
(13.0 g, 71.3 mmol) in THF (10 mL) and the reaction mix-
ture was stirred under reflux. After 13 h, the reaction mix-
ture was cooled to ca. 208C and treated with saturated aque-
ous NH4Cl (20 mL). The aqueous phase was extracted with
ether (20 mL ꢁ 2). The ether solution was washed with
water (20 mL ꢁ 2), dried over Na2SO4, and ether was re-
moved under reduced pressure. The crude product was dis-
tilled, followed by crystallization to give purified methoxy-
methyl(1-naphthyl)phenylsilane as colorless crystals; yield:
16.1 g (89% yield based on 1-bromonaphthalene).
To a solution of methoxymethyl(1-naphthyl)phenylsilane
(10.0 g, 35.9 mmol) in toluene (10 mL) were added (À)-men-
thol (5.66 g, 36.2 mmol) and solid NaOH (0.131 g,
2.34 mmol). The reaction mixture was maintained at 1408C
for 11 h, while the methanol-toluene azeotrope was distilled
over a Vigreux column. After cooling to ca. 208C, NaOH
was removed by passing the reaction mixture through a
short column of silica gel with ether as an eluent. The ex-
tract was concentrated to afford a pale yellow oil. The pale
yellow oil containing a mixture of the diastereomers was di-
luted with twice its volume of pentane and chilled to
À508C. After several days, colorless needle-like crystals
were formed, followed by recrystallization from pentane to
afford (À)-menthoxymethyl(1-naphthyl)phenylsilane; yield:
1.25 g (17% yield based on methoxymethyl(1-naphthyl)phe-
nylsilane).
We have demonstrated that [RuCl2ACHTNUTRGNE(NUG p-cymene)]2 could
act as an efficient homogeneous catalyst for the dehy-
drosilylation of carboxylic acids or alcohols with si-
lanes. The dehydrosilylation of various kinds of struc-
turally diverse carboxylic acids or alcohols with si-
lanes efficiently proceeded to afford various kinds of
the corresponding silylated products in good yields
and selectivities, and showed high TON and TOF
values. A wide variety of functional groups such as
phenyl, chloro, nitro, alkenic, alkynic, carbonyl, and
epoxy groups remained intrinsically intact under the
present conditions. The reaction mechanism involving
the formation of a silylmetal hydride species followed
by the nucleophilic attack of a substrate (carboxylic
acid or alcohol) has been proposed for the present de-
hydrosilylation.
To a slurry of LiAlH4 (0.277 g, 7.30 mmol) in dry ether
(2 mL) was added a solution of (À)-menthoxymethyl(1-
naphthyl)phenylsilane (0.772 g, 1.92 mmol) in n-butyl ether
(2 mL) and the resulting mixture was stirred vigorously at
80–908C. After 24 h, the mixture was cooled to ca. 208C
and unreacted LiAlH4 was slowly decomposed by the addi-
tion of water. To the mixture was added concentrated HCl
and the mixture was extracted with ether (10 mL ꢁ 3). The
combined extract was dried with Na2SO4 and evaporated.
The crude product was purified by a column chromatogra-
phy on silica gel with n-hexane as an eluent to give R-(+)-
methyl(1-naphthyl)phenylsilane (2f) as colorless crystals;
yield: 0.44 g [92% yield based on (À)-menthoxymethyl(1-
naphthyl)phenylsilane]. MS (EI): m/z (%)=249 (22), 248
(86) [M+], 233 (33), 171 (18), 170 (100), 169 (21), 167 (14),
156 (10), 155 (62), 129 (15), 121 (15), 120 (88), 105 (54);
1H NMR (270 MHz, chloroform-d1, 258C, TMS): d=0.742
Experimental Section
General Remarks
GC analyses were performed on a Shimadzu GC-2014 with
an FID detector equipped with a TC-1 or TC-5 capillary
column. The mass spectra were recorded on a Shimadzu
GCMS-QP2010 at an ionization voltage of 70 eV. LC analy-
ses were performed on a Shimadzu Prominence with a CD
(Jasco CD-2095 Plus) detector equipped with a DAICEL
CHILALCEL OD-H column. The liquid-state NMR spectra
were recorded on a JEOL JNM-EX-270. The 1H and
13C NMR spectra were measured at 270 and 67.8 MHz, re-
spectively, with TMS as an internal standard. Dynamic light
scattering (DLS) measurements of the reaction solutions
were performed on a Malvern Zetasizer Nano. The complex
3
3
(d, JH,H =3.79 Hz, 3H, SiCH3), 5.36 (q, JH,H =3.96 Hz, 1H,
SiH), 7.28–8.07 (m, 12H, aryl H); 13C {1H} NMR (67.8 MHz,
chloroform-d1, 258C, TMS): d=À4.50
ACHTUNGRTEN(NUGN SiCH3), 125.2, 125.6,
[RuCl2ACHTUNGTRENNUNG(p-cymene)]2 was obtained from Aldrich (reagent
126.0, 128.0, 128.9, 129.5, 130.5, 133.2, 133.3, 134.9, 135.2,
135.4, 137.0; HPLC: tRR =25.6 min (cf. tRS =29.8 min,
DAICEL CHIRALCEL OD-H, 0.46 cm fꢁ0.25 cm, eluent:
n-hexane).
grade) and used as received. Other metal salts and com-
plexes were obtained from Wako Pure Chemical Industries,
Kanto Chemical, TCI, or Aldrich (reagent grade) and used
as received. Alcohols, carboxylic acids, silanes, and solvents
were obtained from TCI or Aldrich (reagent grade) and pu-
rified prior to the use.[15] Compounds 1m,[16] 2f,[17] and 4i[18]
were synthesized according to the literature procedures.
Adv. Synth. Catal. 2009, 351, 1405 – 1411
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1409

Yuko Ojima et al.
FULL PAPERS
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6, 2773; l) A. Biffis, M. Braga, M. Basato, Adv. Synth.
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Kagi, M. Sawamura, Angew. Chem. 2008, 120, 5709;
Angew. Chem. Int. Ed. 2008, 47, 5627.
Catalytic Dehydrosilylation
The catalytic dehydrosilylation of carboxylic acids was car-
ried out as follows: To a mixture of 1a (5 mmol) and
ACHTUNGTRENNUNG[RuCl2ACHTUNGTRENNUNG(p-cymene)]2 (0.035 mmol, 0.7 mol%) was added 2c
(5 mmol) at 508C under an air atmosphere. The reaction
mixture was stirred at 508C for 4 h and the progress was
1
monitored by GC and H NMR. The desired silyl ester 3c
was obtained in 79% isolated yield after Kugelrohr distilla-
tion under reduced pressure.
The catalytic dehydrosilylation of alcohols was carried out
as follows: To a mixture of 4b (5 mmol) and [RuCl2ACTHNUTRGNE(UNG p-
cymene)]2 (0.025 mmol, 0.5 mol%) was added 2a (5 mmol)
at 08C (ice bath) under an air atmosphere. The reaction
mixture was stirred at 08C for 5 min and the progress was
[5] a) L. H. Sommer, J. E. Lyons, J. Am. Chem. Soc. 1969,
91, 7061; b) Y. Nagai, I. Ojima, S. Inaba, Jpn. Kokai
Tokkyo Koho JP49–110634, 1974; c) T. Fuchigami, Jpn.
Kokai Tokkyo Koho JP05–228372, 1993; d) C. Lorenz,
U. Schubert, Inorg. Chem. 1997, 36, 1258; e) Y. Kizaki,
Jpn. Kokai Tokkyo Koho JP10–182666, 1998; f) M.
Chauhan, B. P. S. Chauhan, P. Boudjouk, Org. Lett.
2000, 2, 1027; g) G.-B. Liu, Synlett 2006, 9, 1431; h) C.
Xu, J. Fang, Faming Zhuanli Shenqing Gongkai Shuo-
minghu, CN2006–10141770, 2006; i) G.-B. Liu, H.-Y.
Zhao, T. Thiemann, Synth. Commun. 2007, 37, 2727;
j) G-B. Liu, H-Y. Zhao, T. Thiemann, Adv. Synth.
Catal. 2007, 349, 807.
[6] It was confirmed by the DLS measurements of the re-
action solutions that no ruthenium clusters (detection
limit: 0.6 nm) were detected during the dehydrosilyla-
tion.
[7] For solid carboxylic acids, toluene or 1,4-dioxane was
used as a solvent.
1
monitored by GC and H NMR. The desired silyl ester 5f
was obtained in 95% isolated yield after Kugelrohr distilla-
tion under reduced pressure.
Acknowledgements
This work was supported in part by the Global COE Pro-
gram (Chemistry Innovation through Cooperation of Science
and Engineering), the Core Research for Evolutional Science
and Technology (CREST) program of the Japan Science and
Technology Agency (JST), and Grants-in-Aid for Scientific
Researches from Ministry of Education, Culture, Sports, Sci-
ence and Technology.
[8] Lee and co-workers have reported the efficient synthe-
sis of siloxacycles from terminal alkenyl alcohols and
alkynylsilanes.[4k] The synthesis consists of two consecu-
tive reactions of silylation of the terminal alkenyl alco-
hols with alkynylsilanes followed by the intramolecular
References
[1] a) A. E. Pierce, Silylation of Organic Compounds,
Pierce Chemical Co., Rockford, IR, 1968; b) M. V. Ka-
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Rev. 1975, 44, 733; c) E. Haslam, Tetrahedron 1980, 36,
2409; d) E. Colvin, Silicon in Organic Synthesis, Butter-
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[2] a) R. O. Sauer, W. Patnod, J. Am. Chem. Soc. 1945, 67,
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Chem. 1984, 62, 336; e) X. Huang, C. Craita, L. Awad,
P. Vogel, Chem. Commun. 2005, 1297.
methathesis, and [RuCl2ACHTNUTRGENNUG(p-cymene)]2 is used for the
former silylation.[4k] Although the silylation of alkenyl
and alkynyl alcohols is reported in the literature, the
applicability to aliphatic and benzylic alcohols as well
as carboxylic acids is not mentioned.[4k].
[9] In this case, the stoichiometric amounts of H2 with re-
spect to the corresponding silyl ethers were also
formed.
[10] J. A. Ayllon, S. F. Sayers, S. Sabo-Eitenne, B. Donna-
dieu, B. Chaudret, Organometallics 1999, 18, 3981.
[11] A similar mechanism has been reported for the hydro-
lytic oxidation of silanes with water: a) M. Lee, S. Ko,
S. Chang, J. Am. Chem. Soc. 2000, 122, 12011; b) K.
Mori, M. Tano, T. Mizugaki, K. Ebitani, K. Kaneda,
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Pieters, P. J. Lewis, M. M. Pearson, J. Org. Chem. 1990,
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R
[12] The ee value of 5r was determined by HPLC (tR
=
23.0 min, tRS =19.9 min, DAICEL CHIRALCEL OD-
H, 0.46 cm fꢁ0.25 cm, eluent: n-hexane) with a CD-
detector: S. Shinke, T. Tsuchimoto, Y. Kawakami, Sili-
con Chem. 2005, 3, 243.
[13] Under the conditions described in Eq. (6) (1.5 equiva-
lents of 4b with respect to 2f), the reaction rate was
very slow and the corresponding silyl ether was ob-
tained in 30% yield after 48 h. In this case, the ee
values of the product were almost unchanged during
the reaction (84–88%ee).
1410
asc.wiley-vch.de
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 1405 – 1411

An Efficient Solvent-Free Route to Silyl Esters and Silyl Ethers
FULL PAPERS
[14] When the reaction of 2f with ten equivalents of 4b with
respect to 2f was carried out for 8 h [Other conditions
were the same as those described in Eq. (6)], the corre-
sponding silyl ether was obtained in 80% yield with
18% ee of the R-isomer. In addition, the reaction of a
butyl ether 5f with 4a (five equivalents) gave the corre-
sponding methyl ether 5a in 50% yield under the con-
ditions described in Eq. (7). All these results show that
the transetherification of silyl ethers proceeds in the
presence of [RuCl2ACHTNUGRTNE(NUG p-cymene)]2 and excess amounts of
[16] A. Dijksman, A. Marino-Gonzꢂlez, A. Mairata i Paye-
ras, I. W. C. E. Arends, R. A. Sheldon, J. Am. Chem.
Soc. 2001, 123, 6826.
alcohols. Therefore, the optical purity of the R-isomer
5s was decreased by the transetherification (racemiza-
tion)in the presence of large excess amounts of 4b [Eq.
(7)].
[17] L. H. Sommer, C. L. Frye, G. A. Parker, K. W. Michael,
J. Am. Chem. Soc. 1964, 86, 3271.
[18] K. Kamata, K. Yamaguchi, N. Mizuno, Chem. Eur. J.
2004, 10, 4728.
[15] Purification of Laboratory Chemicals, 3rd edn., (Eds.:
D. D. Perrin, W. L. F. Armarego), Pergamon Press,
Oxford, U.K., 1988.
Adv. Synth. Catal. 2009, 351, 1405 – 1411
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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