Salt-free preparation of trimethylsilyl ethers by B(C6F 5)3-catalyzed transfer silylation by using a Me 3SiH surrogate

Article abstract of DOI:10.1002/ejoc.201301840

An unprecedented transfer silylation of alcohols catalyzed by the strong Lewis acid B(C₆F₅)₃ is described. Gaseous Me₃SiH is released in situ by B(C₆F₅) ₃-catalyzed decomposition of 3-trimethylsilylcyclohexa-1,4-diene and subsequently reacts with an alcohol in a dehydrogenative Si-O coupling promoted by the same boron catalyst. Benzene and dihydrogen are formed during the reaction, but no salt waste is. This expedient protocol is applicable to several silicon groups, and the preparation of trimethylsilyl ethers presented here is potentially useful for alcohol derivatization prior to GLC analysis. Copyright

Full text of DOI:10.1002/ejoc.201301840

FULL PAPER
DOI: 10.1002/ejoc.201301840
Salt-Free Preparation of Trimethylsilyl Ethers by B(C6F5)3-Catalyzed Transfer
Silylation by Using a Me3SiH Surrogate
Antoine Simonneau,[a] Jonas Friebel,[a] and Martin Oestreich*[a]
Keywords: Si–H activation / Silicon / Silane transfer / Dehydrogenative coupling / Lewis acids
An unprecedented transfer silylation of alcohols catalyzed by
the strong Lewis acid B(C6F5)3 is described. Gaseous Me3SiH
is released in situ by B(C6F5)3-catalyzed decomposition of 3-
trimethylsilylcyclohexa-1,4-diene and subsequently reacts
are formed during the reaction, but no salt waste is. This ex-
pedient protocol is applicable to several silicon groups, and
the preparation of trimethylsilyl ethers presented here is po-
tentially useful for alcohol derivatization prior to GLC analy-
with an alcohol in a dehydrogenative Si–O coupling pro- sis.
moted by the same boron catalyst. Benzene and dihydrogen
Introduction
During the course of our research program devoted to
the chemistry of electron-deficient boranes and application
thereof as catalysts in hydrosilylation reactions,[1] our labo-
ratory recently disclosed an efficient protocol for the cata-
lytic in situ generation of Me3SiH, an otherwise gaseous
(b.p. 6.7 °C), highly flammable, and potentially explosive
reagent. The new method is based on the B(C6F5)3-pro-
moted decomposition of 3-silylated cyclohexa-1,4-dienes I
(Scheme 1, top).[2] The abstraction of a bisallylic hydride
triggers silane release with concomitant formation of a
benzene molecule, and the thus-formed Si–H bond is sub-
sequently activated by B(C6F5)3 (IIǞIV, presumably via
equilibrium III). The feasibility of this new approach was
demonstrated in an ionic transfer hydrosilylation of alkenes
(Scheme 1, bottom).[2,4]
Scheme 1. Proposed pathway for the release of R3SiH from 3-sil-
ylated cyclohexa-1,4-dienes I promoted by B(C6F5)3 (top) and the
B(C6F5)3-catalyzed ionic transfer hydrotrimethylsilylation (bot-
tom).
[3]
These results encouraged us to implement this “silane
transfer” strategy to other silylation reactions B(C6F5)3 is
known to catalyze. Among them, the dehydrogenative cou-
pling of hydrosilanes with alcohols[5] to produce silyl ethers
appeared attractive, as the latter are an important class of
chemicals that are applied in a broad array of domains. In
synthetic organic chemistry, the temporary protection of
alcohols along a multistep sequence might be their most
prominent use.[6] Of the available methods for their synthe-
sis, catalytic dehydrogenative Si–O coupling is an atom-
economic alternative to commonly employed procedures
that rely on either silyl chlorides or expensive silyl triflates.
The formation of silyl ethers by using the latter reagents is
burdened with the stoichiometric formation of ammonium
salts, whereas the sole byproduct of dehydrogenative Si–O
couplings is dihydrogen. Numerous transition-metal-,[7,8]
metal–alkoxide,[9,10] and main-group-based[11] catalysts have
been shown to efficiently promote Si–O couplings with vari-
ous hydrosilanes. However, this method is totally impracti-
cal for the preparation of trimethylsilyl-protected alcohols
because of the hazardousness of Me3SiH gas.[12] Although
a rather labile protecting group for alcohols and especially
phenols owing to its facile cleavage in either basic or slightly
acidic media, the trimethylsilyl group still finds meaningful
application;[13] the protection of tertiary alcohols is one of
them. Likewise, polar, hydroxy-containing molecules are
made amenable to GLC analysis by trimethylsilylation,[14]
and methods avoiding the use of Me3SiCl or Me3SiOTf (Tf
= trifluoromethanesulfonyl) and, hence, the formation of
stoichiometric salt waste were developed. The use of hexa-
methyldisilazane (HMDS)[15] and bis(trimethylsilyl)acet-
amide (BSA)[16] are representative solutions to that prob-
lem. We now report a catalytic and salt-free preparation of
[a] Institut für Chemie, Technische Universität Berlin,
Strasse des 17. Juni 115, 10623 Berlin, Germany
E-mail: martin.oestreich@tu-berlin.de
http://www.organometallics.tu-berlin.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301840.
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A. Simonneau, J. Friebel, M. Oestreich
FULL PAPER
trimethylsilyl ethers by using the corresponding bisallylic standard. Preliminary experiments had shown that addition
silane (Scheme 2, top). Notably, more than three decades of the alcohol to the reagent–catalyst combination must be
ago Morita and co-workers showed that allyltrimethylsilane delayed by at least 30 min to allow for the quantitative re-
also acts as a transfer silylation reagent for alcohols upon lease of the silane. The presence of the free alcohol pre-
treatment with catalytic amounts of a Brønsted acid. Tri- vented the B(C6F5)3 Lewis acid from activating the cy-
methylsilyl ethers are produced through “deallylative” cou- clohexa-1,4-diene reagent. Although alcohol/B(C6F5)3 ad-
pling along with propane gas (Scheme 2, bottom).[17] Also, ducts are known to be labile,[19] the bisallylic C–H bond is
combined catalytic amounts of rhodium(I) and HCl were too weak of a donor to replace the Lewis basic oxygen
recently shown to promote transfer silylation from vinyltri- atom. Conversely, the Si–H bond of the silane is not out-
methylsilane onto alcohols through the generation of competed by hydroxy groups, which allows its activation
Me3SiCl.[18] Although related, our method is conceptually by B(C6F5)3 in the presence of equimolar quantities of the
different, as hydride abstraction by the non-metallic, strong alcohol. Moreover, the reaction temperature had to be kept
Lewis acid occurs to deliver Me3SiH in situ prior to its de- at 0 °C to avoid deoxygenation.[20] Conducting the silylation
hydrogenative Si–O coupling.
of 2a in [D8]toluene with 1a (1.4 equiv.) and of the boron
catalyst (5 mol-%) afforded 3aa in modest yield, but it was
contaminated with substantial amounts of n-octane
(Table 1, entry 1). The side reaction, that is, the partial de-
oxygenation, was suppressed by decreasing the excess
amount of 1a, and improved yields were seen with 1.2 and
even 1.1 equiv. of 1a (Table 1, entries 2–4); an equimolar
quantity of 1a was detrimental though (Table 1, entry 5).
Screening of a few other solvents typical for this chemistry
(C6D6 and CD2Cl2) led to even higher yields (Table 1, en-
tries 6 and 7), and there was no detectable amount of the
corresponding alkane in CD2Cl2.
Scheme 2. Allyltrimethylsilanes as trimethylsilyl transfer reagents:
planned transfer silylation of alcohols through dehydrogenative
coupling (top) and Morita’s acid-catalyzed transfer silylation of
alcohols by using allyltrimethylsilane (bottom).
With the optimized protocol in hand, we probed the
scope of the new methodology (Table 2). We selected a few
primary alcohols bearing functional groups and were
Results and Discussion
Table 2. Scope of the transfer silylation.[a]
Our study commenced with the trimethylsilylation of a
primary alcohol. Octan-1-ol (2a) was treated with transfer
reagent 1a in the presence of catalytic amounts of
B(C6F5)3 (Table 1). As resulting trimethylsilyl ether 3aa is
rather fragile and volatile, we found it convenient to deter-
1
mine the yields by H NMR spectroscopy with an internal
Table 1. B(C6F5)3-catalyzed transfer silylation: optimization of the
reaction conditions.[a]
Entry
Equiv. 1a
Solvent
Yield [%][b]
1
2
3
4
5
1.4
1.3
1.2
1.1
1.0
1.1
1.1
1.1
[D8]toluene
[D8]toluene
[D8]toluene
[D8]toluene
[D8]toluene
C6D6
62[c]
70[c]
74[c]
75[c]
69[c]
84
[a] Yields were determined by 1H NMR spectroscopy by using mes-
itylene as an internal standard. [b] By using the optimized pro-
cedure of Table 1. [c] 2 (0.20 mmol), 1a (0.22 mmol, 1.10 equiv.),
and B(C6F5)3 (2.5 mol-%) in [D8]toluene (0.2 mL) at 0 °C for 1 h
(see the Exp. Sect. for further details). [d] Yield under the optimized
conditions of Table 1 was 46%. [e] Յ10% of deoxygenated com-
pounds was observed. [f] 2j (50 μmol), 1a (55 μmol, 1.1 equiv.), and
B(C6F5)3 (5.0 mol-%) in CD2Cl2 (0.5 mL) at room temperature for
2 h (see the Exp. Sect. for further details).
6
7
CD2Cl2
CD2Cl2
89
0
8[d]
[a] Reactions were run on a 0.1 mmol scale. [b] Determined by
1H NMR spectroscopy by using mesitylene as an internal standard.
[c] Deoxygenation but quantification of the amount of n-octane
failed as a result of signal overlapping in the aliphatic region.
[d] No catalyst.
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Salt-Free Preparation of Trimethylsilyl Ethers
pleased to find that the reaction was tolerant of C–C mul-
tiple bonds (as in 3ab and 3ac, no hydrosilylation[21]) as well
as a methyl ether (as in 3ad, no deoxygenation) and a
C(sp3)–Cl bond (as in 3ae, no hydrodechlorination[22]).
Upon switching to secondary alcohols, we were disap-
pointed to notice significantly deteriorated yields, and de-
oxygenation was again an issue. A brief reoptimization al-
lowed us to identify a satisfying protocol that furnishes the
desired compounds in high yields. A quantitative yield was
recorded for trimethylsilyl-protected cyclohex-1-ylethanol
3af, and good yields were also obtained for 3ag–ai. Minor
quantities (Յ10%) of deoxygenated alcohols were detected
in a few cases. Encouraged by the protection of (–)-menthol
(2ai), we also tested another terpenoid alcohol, cholesterol
(3aj). An excellent yield was obtained under slightly modi-
fied conditions. We successfully implemented the same pro-
tocol to tertiary and benzylic alcohols 3ak and 3al, respec-
tively; minor amounts of toluene were seen for the latter.
The initial optimized protocol (cf. Table 1) is also applicable
to phenols, for example, 3am and 3an.
Our protocol provides fast and user-friendly access to tri-
methylsilyl ethers. The fact that no salt waste is generated
avoids tedious extraction–washing purification steps, and
that makes this procedure attractive for derivatization prior
to GLC analysis. As illustrated by comparison of the chro-
matograms of (–)-menthol (2i) and its trimethylsilyl-deriv-
atized analog 3ai, peak tailing is reduced upon injecting at
similar concentrations (Figure 1).
As demonstrated by us in our previous study,[2] this
method also allows the transfer of other silicon groups
(Scheme 3). Reagent 1b transferred its dimethyl(phenyl)silyl
group under the optimized conditions of Table 1 to afford
3ba in good yield. Transfer silylation with Et3SiH surrogate
Figure 1. Peak tailing in GLC analysis of alcohols: (–)-menthol be-
1c required a longer reaction time at room temperature, but fore (top) and after (bottom) trimethylsilylation.
nonetheless triethyl(oct-1-yloxy)silane (3ca) was furnished
in excellent yield. However, protection of octan-1-ol (2a)
with the bulky triisopropylsilyl group by using cyclohexa-
1,4-diene 1d could not be achieved even under forcing con-
ditions. Notably, decomposition of 1d into iPr3SiH was ob-
served. These examples are synthetically less competitive
than those with Me3SiH but illustrate the generality of the
approach.
The use of Me2SiH2 surrogate 1e is particularly interest-
ing, as the use of Me2SiH2 (b.p. –20 °C) is afflicted with the
same practical and safety issues as those encountered with
the use of Me3SiH. Unfortunately, our attempts to perform
the twofold alcoholysis of 1e with 2a failed. Dimethylbis-
(oct-1-yloxy)silane was detected as the major component,
but the reaction was plagued with deoxygenation and the
formation of heavier siloxanes (not shown). Also, we were
not able to find conditions to selectively perform the ring-
closure reaction of catechol with 1e; mixtures of presumably
oligomeric material were obtained instead (not shown).
However, we accomplished the selective twofold silylation
of phenol 2o with a substoichiometric amount of 1e, which
furnished disiloxane 4eo in high yield (Scheme 4). To per-
form the alcoholysis monoselectively, we had to use a
Scheme 3. B(C6F5)3-catalyzed dehydrogenative Si–O coupling with
silane surrogates 1b–d. [a] Yields were determined by 1H NMR
spectroscopy by using mesitylene as an internal standard. [b] By
using the optimized procedure of Table 1. [c] 2a (0.10 mmol), 1c
(0.11 mmol), and B(C6F5)3 (5.0 mol-%) in CD2Cl2 (0.2 mL) at
room temperature for 24 h (see the Exp. Sect. for further details).
[d] No product formation was seen in [D8]toluene at 60 °C after
phenol carrying a bulky substituent in the ortho position. 24 h, but decomposition of 1d into iPr3SiH occurred.
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A. Simonneau, J. Friebel, M. Oestreich
FULL PAPER
General Procedure 2 (GP2)
Indeed, reaction of 1e with 2-(tert-butyl)phenol (2p) re-
sulted in the formation of phenyl(hydrido)silyl ether 3ep in
excellent yield. Similar alkyl- and aryl(hydrido)silyl ethers
Synthesis of Trimethylsilyl Ethers 3af–ai and 3ak: In a glove box,
B(C6F5)3 (2.6 mg, 5.0 μmol, 2.5 mol-%) was weighed into a 1.3 mL
have been recently demonstrated to act as directing groups GLC vial and dissolved in [D8]toluene (0.2 mL). Another 1.3 mL
in transition-metal-catalyzed C–H bond activation.[23]
GLC vial was charged with cyclohexa-2,5-dien-1-yltrimethylsilane
(1a; 33.5 mg, 0.220 mmol, 1.10 equiv.) and mesitylene (9.30 μL,
66.6 μmol, 33.3 mol-%, internal standard). After the addition of a
stir bar, the freshly prepared B(C6F5)3 solution was added, and the
vial was capped. The resulting mixture was stirred for 1 h at room
temperature outside the glove box and then cooled to 0 °C by using
an ice–water bath. The alcohol (0.20 mmol, 1.0 equiv.) was added
by microsyringe through the septum cap, which resulted in vigorous
hydrogen evolution (pressure must be equalized on larger scale). The
reaction mixture was stirred for approximately 1 h and then filtered
through a short plug of neutral alumina directly into an NMR
tube. The plug was washed with C6D6 (3ϫ ca. 0.15 mL), and the
NMR tube was capped, shaken, and immediately analyzed by
NMR spectroscopy as well as by GLC and GLC–MS analysis.
Trimethyl(oct-1-yloxy)silane[24] (3aa): Prepared from octan-1-ol (2a;
Scheme 4. Twofold and monosilylation of phenols with dimethyl-
silane precursor 1e. [a] Yields were determined by H NMR spec-
troscopy by using mesitylene as an internal standard.
1
15.7 μL, 0.100 mmol, 1.00 equiv.) according to GP1, 89% yield
1
(NMR, Table 1). GLC: tR = 8.4 min. H NMR (400 MHz, C6D6):
δ = 3.50 (t, J = 6.6 Hz, 2 H), 1.55–1.44 (m, 2 H), 1.35–1.16 (m, 10
H), 0.86 (t, J = 6.6 Hz, 3 H), 0.07 (s, 9 H) ppm. 13C NMR
(126 MHz, C6D6): δ = 62.8, 33.3, 32.3, 29.9, 29.8, 26.3, 23.1, 14.3,
–0.4 (3 C) ppm. GLC–MS (EI): m/z = 187.2 [M+ – CH3].
Conclusions
In summary, we disclosed that 3-silylated cyclohexa-1,4-
dienes 1 are valuable silane surrogates in the Lewis acid
catalyzed dehydrogenative Si–O coupling of aliphatic
alcohols and phenols. The boron Lewis acid B(C6F5)3 plays
the double role of promoting the decomposition of 1 into a
silane and a benzene molecule and catalyzing the Si–O cou-
pling through activation of the Si–H bond of the released
silane. This method appears to be useful for the salt-free
preparation of trimethylsilyl ethers without the necessity of
handling gaseous Me3SiH, and this thereby provides a
straightforward protocol for the derivatization of hydroxy-
containing molecules prior to GLC analysis.
Trimethyl[(2-methylbut-3-en-1-yl)oxy]silane (3ab): Prepared from 2-
methylbut-3-en-1-ol (2b; 10.3 μL, 0.100 mmol, 1.00 equiv.) accord-
ing to GP1, 72% yield (NMR, Table 2). GLC: tR = 3.5 min. 1H
NMR (500 MHz, C6D6): δ = 5.74 (ddd, J = 17.3, 10.4, 6.9 Hz, 2
H), 5.00 (d, J = 17.3 Hz, 1 H), 4.95 (d, J = 10.4 Hz, 1 H), 3.41 (dd,
J = 9.7, 6.1 Hz, 1 H), 3.30 (dd, J = 9.7, 6.9 Hz, 1 H), 2.28 (tddd,
J = 6.6, 6.6, 6.6, 6.6 Hz, 1 H), 0.96 (d, J = 6.8 Hz, 3 H), 0.05 (s, 9
H) ppm. 13C NMR (126 MHz, C6D6): δ = 141.7, 114.1, 67.6, 40.5,
16.3, –0.4 (3 C) ppm. GLC–MS (EI): m/z = 143.1 [M+ – CH3],
103.1 [M+ – C4H7].
(But-3-yn-1-yloxy)trimethylsilane[25] (3ac): Prepared from but-3-yn-
1-ol (2c; 7.8 μL, 0.10 mmol, 1.0 equiv.) according to GP1, 84%
yield (NMR, Table 2). GLC: tR = 3.3 min. 1H NMR (500 MHz,
C6D6): δ = 3.54 (t, J = 7.0 Hz, 2 H), 2.24 (td, J = 7.0, 2.6 Hz, 2
H), 1.76 (t, J = 2.6 Hz, 1 H), 0.04 (s, 9 H) ppm. 13C NMR
(126 MHz, C6D6): δ = 81.7, 69.7, 61.4, 23.1, –0.5 (3 C) ppm. GLC–
MS (EI): m/z = 127.1 [M+ – CH3], 103.1 [M+ – C3H3].
Experimental Section
General Methods: See the Supporting Information.
(2-Methoxyethoxy)trimethylsilane (3ad): Prepared from 2-methoxy-
ethanol (2d; 7.9 μL, 0.10 mmol, 1.0 equiv.) according to GP1, 86%
yield (NMR, Table 2). GLC: tR = 3.4 min. 1H NMR (500 MHz,
C6D6): δ = 3.57 (t, J = 5.0 Hz, 2 H), 3.25 (t, J = 5.0 Hz, 2 H), 3.12
(s, 3 H), 0.06 (s, 9 H) ppm. 13C NMR (126 MHz, C6D6): δ = 74.4,
62.3, 58.7, –0.4 (3 C) ppm. GLC–MS (EI): m/z = 133.1 [M+ – CH3],
103.1 [M+ – C2H5O].
General Procedure 1 (GP1)
Synthesis of Trimethylsilyl Ethers 3aa–ae: In
a glove box,
B(C6F5)3 (2.6 mg, 5.0 μmol, 5.0 mol-%) was weighed into a 1.3 mL
GLC vial and dissolved in CD2Cl2 (0.2 mL). Another 1.3 mL GLC
vial was charged with cyclohexa-2,5-dien-1-yltrimethylsilane (1a;
16.8 mg, 0.110 mmol, 1.10 equiv.) and mesitylene (4.60 μL,
33.3 μmol, 33.3 mol-%, internal standard). After the addition of a
stir bar, the freshly prepared B(C6F5)3 solution was added, and the
vial was capped. The resulting mixture was stirred for 30 to 45 min
at room temperature outside the glove box and then cooled to 0 °C
by using an ice–water bath. The alcohol (0.10 mmol, 1.0 equiv.) was
added by microsyringe through the septum cap, which resulted in
vigorous hydrogen evolution (pressure must be equalized on larger
scale). The reaction mixture was stirred for approximately 30 min
and then filtered through a short plug of neutral alumina directly
into an NMR tube. The plug was washed with C6D6 (3ϫ ca.
0.15 mL), and the NMR tube was capped, shaken, and immediately
analyzed by NMR spectroscopy as well as by GLC and GLC–MS
analysis.
(3-Chloropropoxy)trimethylsilane (3ae): Prepared from 3-chloro-
propan-1-ol (2e; 8.4 μL, 0.10 mmol, 1.0 equiv.) according to GP1,
84% yield (NMR, Table 2). GLC: tR = 4.7 min. 1H NMR
(500 MHz, C6D6): δ = 3.49 (t, J = 5.7 Hz, 2 H), 3.38 (t, J = 6.4 Hz,
2 H), 1.68 (tt, J = 6.1, 6.1 Hz, 2 H), 0.01 (s, 9 H) ppm. 13C NMR
(126 MHz, C6D6): δ = 58.5, 41.4, 35.2, –1.1 (3 C) ppm. GLC–MS
(EI): m/z = 151.0 [M+ – CH3], 93.0 [M+ – Si(CH3)3].
(1-Cyclohexylethoxy)trimethylsilane (3af): Prepared from cyclohex-
1-ylethanol (2f; 27.9 μL, 0.200 mmol, 1.00 equiv.) according to
1
GP2, Ͼ95% yield (NMR, Table 2). GLC: tR = 8.2 min. H NMR
(500 MHz, C6D6): δ = 3.46 (dq, J = 6.1, 6.1 Hz, 1 H), 1.90 (dd, J
= 12.9, 2.0 Hz, 1 H), 1.73 (m, 2 H), 1.64 (d, J = 11.1 Hz, 2 H),
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Salt-Free Preparation of Trimethylsilyl Ethers
1.30–1.12 (m, 4 H), 1.09 (d, J = 6.2 Hz, 3 H), 0.98 (q, J = 12.0 Hz, box at room temperature for 2 h. The vial was transferred outside
2 H), 0.13 (s, 9 H) ppm. 13C NMR (126 MHz, C6D6): δ = 72.8, the glove box, and the solution was filtered through neutral alu-
45.9, 29.5, 28.9, 27.1, 26.8, 26.8, 21.3, 0.5 (3 C) ppm. GLC–MS
mina directly into an NMR tube. The filter cake was washed with
CD2Cl2, and the filtrate was immediately analyzed by NMR spec-
troscopy as well as by GLC and GLC–MS analysis, Ͼ95% yield
(NMR, Table 2). GLC: tR = 30.0 min. 1H NMR (500 MHz,
CD2Cl2): δ = 5.38–5.28 (m, 1 H), 3.48 (tt, J = 11.0, 4.7 Hz, 1 H),
2.25–2.21 (m, 1 H), 2.16 (ddd, J = 13.2, 5.1, 2.3 Hz, 1 H), 2.07–
1.94 (m, 2 H), 1.90–1.79 (m, 2 H), 1.75–1.67 (m, 1 H), 1.63–1.43
(m, 7 H), 1.44–1.33 (m, 3 H), 1.33–1.24 (m, 1 H), 1.24–1.04 (m, 10
H), 1.01 (s, 3 H), 0.93 (d, J = 6.5 Hz, 3 H), 0.88 (dd, J = 6.6,
2.0 Hz, 6 H), 0.70 (s, 3 H), 0.10 (s, 9 H) ppm. 13C NMR (126 MHz,
CD2Cl2): δ = 142.4, 122.2, 73.3, 57.8, 57.2, 51.3, 43.7, 43.3, 40.9,
40.5, 38.4, 37.2, 36.8, 33.1, 33.0, 32.9, 29.2, 29.0, 25.2, 24.8, 23.5,
23.3 (2 C), 22.1, 20.2, 19.5, 12.6, 0.9 (3 C) ppm. GLC–MS (EI):
m/z = 443.4 [M+ – CH3].
(EI): m/z = 185.1 [M+ – CH3].
Trimethyl[(trans-2-methylcyclohexyl)oxy]silane (3ag): Prepared
from trans-2-methylcyclohexanol (2g; 24.7 μL, 0.200 mmol,
1.00 equiv.) according to GP2, 75% yield (NMR, Table 2). GLC:
tR = 6.7 min. 1H NMR (500 MHz, C6D6): δ = 3.07 (td, J = 9.9,
4.2 Hz, 1 H), 1.89–1.80 (m, 1 H), 1.60 (ddd, J = 13.6, 7.9, 3.0 Hz,
2 H), 1.52–1.45 (m, 1 H), 1.46–1.29 (m, 2 H), 1.23–1.04 (m, 2 H),
1.02 (d, J = 6.5 Hz, 3 H), 0.95–0.81 (m, 1 H), 0.15 (s, 9 H) ppm.
13C NMR (126 MHz, C6D6): δ = 77.3, 40.6, 36.5, 33.9, 26.1, 25.5,
19.5, 0.6 (3 C) ppm. GLC–MS (EI): m/z = 186.1 [M+], 171.1 [M+
–
CH3].
Trimethyl[(1-phenylpropan-2-yl)oxy]silane[18a] (3ah): Prepared from
1-phenylpropan-2-ol (2h; 28.0 μL, 0.200 mmol, 1.00 equiv.) accord-
ing to GP2, 79% yield (NMR, Table 2). GLC: tR = 9.3 min. 1H
NMR (500 MHz, C6D6): δ = 7.20–7.13 (m, 2 H), 7.12–7.05 (m, 3
H), 3.92–3.79 (m, 1 H), 2.69 (dd, J = 13.1, 7.5 Hz, 1 H), 2.55 (dd,
J = 13.1, 5.2 Hz, 1 H), 1.11 (d, J = 6.0 Hz, 3 H), –0.03 (s, 9 H)
ppm. 13C NMR (126 MHz, C6D6): δ = 139.9, 130.0 (2 C), 128.3 (2
C), 126.3, 70.2, 46.8, 24.0, 0.0 (3 C) ppm. GLC–MS (EI): m/z =
193.1 [M+ – CH3].
Trimethyl(2-methyl-1-phenylpropan-2-yl)oxysilane (3ak): Prepared
from 2-methyl-1-phenylpropan-2-ol (2k; 30.9 μL, 0.200 mmol,
1.00 equiv.) according to GP2, Ͼ95% yield (NMR, Table 2). GLC:
tR = 10.3 min. 1H NMR (500 MHz, C6D6): δ = 7.23–7.08 (m, 5 H),
2.66 (s, 2 H), 1.14 (s, 6 H), 0.11 (s, 9 H) ppm. 13C NMR (126 MHz,
C6D6): δ = 139.1, 131.1 (2 C), 127.9 (2 C), 126.4, 74.3, 51.4, 29.8
(2 C), 2.7 (3 C) ppm. GLC–MS (EI): m/z = 207.1 [M+ – CH3].
(Benzyloxy)trimethylsilane[28] (3al): Prepared from benzyl alcohol
(2l; 20.7 μL, 0.200 mmol, 1.00 equiv.) according to GP2, 70% yield
{[(1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl]oxy}trimethyl-
silane[26] (3ai): As (–)-menthol (2i) is a solid, we had to slightly
modify GP2 to perform the synthesis of 3ai. In a glove box,
B(C6F5)3 (2.6 mg, 5.0 μmol, 2.5 mol-%) was weighed into a 1.3 mL
GLC vial and dissolved in [D8]toluene (0.1 mL). Another 1.3 mL
GLC vial was charged with cyclohexa-2,5-dien-1-yltrimethylsilane
(1a; 33.5 mg, 0.220 mmol, 1.10 equiv.) and mesitylene (9.30 μL,
66.6 μmol, 33.3 mol-%, internal standard). After the addition of a
stir bar, the freshly prepared B(C6F5)3 solution was added, the vial
was capped, and the resulting solution was stirred for 1 h. (–)-Men-
thol (2i; 31.3 mg, 0.200 mmol, 1.00 equiv.) was dissolved in [D8]-
toluene (0.1 mL) in another vial. Outside the glove box, the
B(C6F5)3/1a solution was cooled to 0 °C by using an ice–water
bath, and the (–)-menthol solution was added by using a 250 μL
microsyringe through the septum cap, which resulted in vigorous
hydrogen evolution (pressure must be equalized on larger scale). The
end of the procedure was in accordance with GP2, 78% yield
1
(NMR, Table 2). GLC: tR = 8.0 min. H NMR (500 MHz, C6D6):
δ = 7.30 (d, J = 7.7 Hz, 2 H), 7.21–7.17 (m, 2 H), 7.13–7.07 (m, 1
H), 4.56 (s, 2 H), 0.10 (s, 9 H) ppm. 13C NMR (126 MHz, C6D6):
δ = 141.7, 128.5 (2 C), 127.3, 126.7 (2 C), 64.8, –0.3 (3 C) ppm.
GLC–MS (EI): m/z = 180.1 [M+], 165.1 [M+ – CH3], 91.1 [M+
–
OSi(CH3)3].
(4-Chlorophenoxy)trimethylsilane (3am): As 4-chlorophenol (2m) is
a solid, we had to slightly modify GP1 to perform the synthesis of
3am. In a glove box, B(C6F5)3 (2.6 mg, 5.0 μmol, 5.0 mol-%) was
weighed into a 1.3 mL GLC vial and dissolved in CD2Cl2 (0.1 mL).
Another 1.3 mL GLC vial was charged with cyclohexa-2,5-dien-1-
yltrimethylsilane (1a; 16.8 mg, 0.110 mmol, 1.10 equiv.) and mesity-
lene (4.6 μL, 33.3 μmol, 33.3 mol-%, internal standard). After the
addition of a stir bar, the freshly prepared B(C6F5)3 solution was
added, the vial was capped, and the resulting solution was stirred
for 30 min. 4-Chlorophenol (2m; 12.9 mg, 0.100 mmol, 1.00 equiv.)
was dissolved in CD2Cl2 (0.1 mL) in another vial. Outside the glove
box, the B(C6F5)3/1a solution was cooled to 0 °C by using an ice–
1
(NMR, Table 2). GLC: tR = 9.6 min. H NMR (500 MHz, C6D6):
δ = 3.41 (td, J = 10.2, 4.3 Hz, 1 H), 2.36 (dsept, J = 7.0, 2.2 Hz, 1
H), 1.99–1.89 (m, 1 H), 1.58–1.49 (m, 2 H), 1.29–1.20 (m, 2 H),
1.12 (q, J = 12.2, 11.7 Hz, 1 H), 0.95 (d, J = 7.1 Hz, 3 H), 0.87 (d, water bath, and the solution of 4-chlorophenol was added by using
J = 6.5 Hz, 3 H), 0.82 (d, J = 7.0 Hz, 3 H), 0.76 (qd, J = 14.1,
a 250 μL microsyringe through the septum cap, which resulted in
vigorous hydrogen evolution (pressure must be equalized on larger
13.1, 3.5 Hz, 1 H), 0.16 (s, 9 H) ppm; one proton overlapped with
the CH3 peaks. 13C NMR (126 MHz, C6D6): δ = 72.7, 50.6, 46.1, scale). The end of the procedure was in accordance with GP1, 76%
35.0, 31.9, 25.8, 23.4, 22.6, 21.5, 16.4, 0.8 (3 C) ppm. GLC–MS
yield (NMR, Table 2). GLC: tR = 9.2 min. 1H NMR (400 MHz,
C6D6): δ = 7.02 (d, J = 9.0 Hz, 2 H), 6.59 (d, J = 8.7 Hz, 2 H),
0.10 (s, 9 H) ppm. 13C NMR (126 MHz, C6D6): δ = 154.3, 129.8
(2 C), 126.8, 121.6 (2 C), 0.0 (3 C) ppm. GLC–MS (EI): m/z =
200.0 [M+], 185.0 [M+ – CH3].
(EI): m/z = 228.2 [M+], 213.2 [M+ – CH3].
(3β)-3-[(Trimethylsilyl)oxy]cholest-5-ene[27] (3aj): In a glove box,
B(C6F5)3 (1.3 mg, 2.5 μmol, 5.0 mol-%) was weighed into a 1.3 mL
GLC vial and dissolved in CD2Cl2 (0.2 mL). Another 1.3 mL GLC
vial was charged with cyclohexa-2,5-dien-1-yltrimethylsilane (1a;
8.4 mg, 55 μmol, 1.1 equiv.) and mesitylene (2.30 μL, 16.6 μmol,
33.3 mol-%, internal standard). After the addition of a stir bar, the
freshly prepared B(C6F5)3 solution was added, the vial was capped,
and the resulting solution was stirred for 30 min. Cholesterol (2j;
19.3 mg, 50.0 μmol, 1.00 equiv.) was dissolved in CD2Cl2 (0.3 mL)
in another vial. That solution was added by using a 250 μL micro-
syringe to the B(C6F5)3/1a solution, which resulted in vigorous hy-
drogen evolution (pressure must be equalized on larger scale). The
vial was capped, and the reaction mixture was stirred in a glove
Trimethyl[4-(trifluoromethyl)phenoxy]silane (3an): Prepared from 4-
(trifluoromethyl)phenol (2n; 16.2 mg, 0.100 mmol, 1.00 equiv.) ac-
cording to the modified GP1 described for the synthesis of 3am,
79% yield (NMR, Table 2). GLC: tR = 6.9 min. 1H NMR
(400 MHz, C6D6): δ = 7.31 (d, J = 8.4 Hz, 2 H), 6.68 (d, J = 7.0 Hz,
2 H), 0.11 (s, 9 H) ppm. 13C NMR (126 MHz, C6D6): δ = 158.5,
127.3 (q, J = 3.8 Hz, 2 C), 125.3 (q, J = 271.2 Hz), 123.9 (q, J =
32.8 Hz), 120.3 (2 C), 0.0 (3 C) ppm. 19F NMR (470 MHz, C6D6):
δ = –61.3 ppm. GLC–MS (EI): m/z = 234.1 [M+], 219.0 [M+
–
CH3].
Eur. J. Org. Chem. 2014, 2077–2083
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2081

A. Simonneau, J. Friebel, M. Oestreich
FULL PAPER
Dimethyl(oct-1-yloxy)(phenyl)silane[29] (3ba): Prepared from octan-
1-ol (2a; 15.7 μL, 0.100 mmol, 1.00 equiv.) according to GP1 with
C) ppm. GLC–MS (EI): m/z = 208.1 [M+], 193.1 [M+ – CH3].
HRMS (EI): calcd. for C20H29O2Si [M]+· 208.1278; found
208.1273.
cyclohexa-2,5-dien-1-yldimethyl(phenyl)silane
(1b;
23.6 mg,
0.110 mmol, 1.10 equiv.) as the silane precursor instead of 1a, 82%
yield (NMR, Scheme 2). GLC: tR = 15.3 min. 1H NMR (500 MHz,
C6D6): δ = 7.60–7.48 (m, 2 H), 7.29–7.17 (m, 3 H), 3.52 (t, J =
6.5 Hz, 2 H), 1.49 (quint, J = 6.6 Hz, 2 H), 1.32–1.12 (m, 10 H),
0.85 (t, J = 6.9 Hz, 3 H), 0.30 (s, 6 H) ppm. 13C NMR (126 MHz,
C6D6): δ = 138.6, 133.8 (2 C), 129.7, 128.1 (2 C), 63.2, 33.1, 32.2,
29.7, 26.2, 23.1, 14.3, –1.6 (3 C) ppm. GLC–MS (EI): m/z = 249.2
[M+ – CH3].
Supporting Information (see footnote on the first page of this arti-
cle): 1H NMR, 13C NMR, and 19F NMR spectra of all compounds,
as well as GLC traces.
Acknowledgments
This research was supported by the Technische Universität Berlin.
M. O. is indebted to the Einstein Foundation (Berlin) for an en-
dowed professorship.
Triethyl(oct-1-yloxy)silane[29] (3ca): Prepared from octan-1-ol (2a;
15.7 μL, 0.100 mmol, 1.00 equiv.) according to the modified
GP1 with cyclohexa-2,5-dien-1-yltriethylsilane (1c; 21.4 mg,
0.110 mmol, 1.10 equiv.) as the silane precursor instead of 1a. The
reaction was performed at room temperature for 24 h, Ͼ95% yield
(NMR, Scheme 2). GLC: tR = 12.8 min. 1H NMR (400 MHz,
C6D6): δ = 3.55 (t, J = 6.5 Hz, 2 H), 1.56–1.47 (m, 2 H), 1.39–1.18
(m, 10 H), 0.99 (t, J = 7.9 Hz, 9 H), 0.86 (s, 3 H), 0.59 (q, J =
8.0 Hz, 6 H) ppm. 13C NMR (126 MHz, C6D6): δ = 63.1, 33.4,
32.3, 29.9, 29.8, 26.3, 23.1, 14.3, 7.1 (3 C), 4.9 (3 C) ppm. GLC–
MS (EI): m/z = 215.2 [M+ – C2H5].
[1] a) S. Rendler, M. Oestreich, Angew. Chem. 2008, 120, 6086–
6089; Angew. Chem. Int. Ed. 2008, 47, 5997–6000; b) D. T. Hog,
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Bis(3-isopropylphenoxy)dimethylsilane (4eo): In
a
glove box,
[3] For detailed mechanistic studies on the reactivity of Si–H
bonds activated by B(C6F5)3, see: a) D. J. Parks, J. M.
Blackwell, W. E. Piers, J. Org. Chem. 2000, 65, 3090–3098; b)
see ref.[1a]; for a general overview, see: c) W. E. Piers, A. J. V.
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for reviews of the chemistry of B(C6F5)3, see: d) T. Robert, M.
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[7] J. Y. Corey in Advances in Silicon Chemistry, vol. 1 (Ed.: G.
Larson), JAI Press, Greenwich, 1991, p. 327–387.
[8] Prominent examples include catalysis by metal hydrides, see: a)
C. Lorenz, U. Schubert, Chem. Ber. 1995, 128, 1267–1269; for
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315.
B(C6F5)3 (2.6 mg, 5.0 μmol, 2.5 mol-%) was weighed into a 1.3 mL
GLC vial and dissolved in [D8]toluene (0.2 mL). Another 1.3 mL
GLC vial was charged with cyclohexa-2,5-dien-1-yldimethylsilane
(1e; 15.2 mg, 0.110 mmol, 0.550 equiv.) and mesitylene (4.6 μL,
33.3 μmol, 16.7 mol-%, internal standard). After the addition of a
stir bar, the previously prepared B(C6F5)3 solution was added, and
the vial was capped. The resulting mixture was then stirred for 1 h
at room temperature outside the glove box and cooled to 0 °C by
using an ice–water bath. 3-Isopropylphenol (2o; 26.8 μL,
0.200 mmol, 1.00 equiv.) was added by microsyringe through the
septum cap, which resulted in vigorous hydrogen evolution (pres-
sure must be equalized on larger scale). The reaction mixture was
stirred for approximately 1.5 h and then filtered through a short
plug of neutral alumina directly into an NMR tube. The plug was
washed with C6D6 (3ϫ ca. 0.15 mL), and the NMR tube was
capped, shaken, and immediately analyzed by NMR spectroscopy
as well as by GLC and GLC–MS analysis, 90% yield (NMR,
1
Scheme 3). GLC: tR = 18.6 min. H NMR (500 MHz, C6D6): δ =
7.06 (t, J = 7.8 Hz, 2 H), 7.02 (s, 2 H), 6.94 (d, J = 8.0 Hz, 2 H),
6.78 (d, J = 7.6 Hz, 2 H), 2.67 (sept, J = 6.8 Hz, 2 H), 1.11 (d, J
= 6.9 Hz, 12 H), 0.33 (s, 6 H) ppm. 13C NMR (126 MHz, C6D6):
δ = 155.0 (2 C), 151.0 (2 C), 129.9 (2 C), 120.6 (2 C), 118.4 (2 C),
117.6 (2 C), 34.4 (2 C), 24.1 (4 C), –2.2 (2 C) ppm. GLC–MS (EI):
m/z = 328.2 [M+], 313.2 [M+ – CH3]. HRMS (APCI): calcd. for
C20H29O2Si [M + H]+ 329.1931; found 329.1924.
[2-(tert-Butyl)phenoxy]dimethylsilane (3ep): Prepared from 2-(tert-
butyl)phenol (2p; 31.0 μL, 0.200 mmol, 1.00 equiv.) according to a
procedure similar to that used for 4eo by using cyclohexa-2,5-dien-
1-yldimethylsilane (1e; 30.4 mg, 0.110 mmol, 1.10 equiv.). As 3ep
was highly moisture sensitive in the presence of B(C6F5)3, we could
not filter the reaction mixture without converting it into the disilox-
ane. Analysis of the crude mixture was performed by diluting it
with C6D6 under an atmosphere of N2 and transferring the re-
sulting solution directly into an oven-dried NMR tube filled with
[10] a) A. Weickgenannt, M. Oestreich, Chem. Asian J. 2009, 4,
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[11] a) see ref.[5]; b) D. Gao, C. Cui, Chem. Eur. J. 2013, 19, 11143–
11147.
1
N2, Ͼ95% yield (NMR, Scheme 3). GLC: tR = 9.8 min. H NMR
[12] We are not aware of any report on dehydrogenative Si–O cou-
pling by employing trimethylsilane.
[13] The facile cleavage of trimethylsilyl ethers makes them useful
when it comes to orthogonal protection of different alcohols.
For two selected recent, elegant total syntheses featuring a pro-
(400 MHz, C6D6): δ = 7.29 (dd, J = 7.7, 1.8 Hz, 1 H), 7.07–6.98
(m, 1 H), 6.92–6.84 (m, 2 H), 5.15 (sept, J = 2.8 Hz, 1 H), 1.44 (s,
9 H), 0.21 (d, J = 2.8 Hz, 6 H) ppm. 13C NMR (126 MHz, C6D6):
δ = 154.8, 139.7, 127.5 (2 C), 121.6, 118.5, 34.9, 30.1 (3 C), –1.5 (2
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Eur. J. Org. Chem. 2014, 2077–2083

Salt-Free Preparation of Trimethylsilyl Ethers
tection step with a trimethylsilyl group, see: a) B. B. Liau, B. C.
Milgram, M. D. Shair, J. Am. Chem. Soc. 2012, 134, 16765–
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geheuer, J. Felding, P. S. Baran, Science 2013, 341, 878–882.
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W. W. Wells, J. Am. Chem. Soc. 1963, 85, 2497–2507; b) C. A.
Bruynes, T. K. Jurriens, J. Org. Chem. 1982, 47, 3966–3969.
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88.
8919–8922; b) V. Gevorgyan, M. Rubin, S. Benson, J.-X. Liu,
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[21]
[22]
Deoxygenation is likely to occur instead of hydrosilylation of
the unsaturated C–C bond if an excess amount of the silane is
present, given the rate differences between these two processes:
deoxygenation occurs at 0 °C in less than 1 h, whereas hydro-
silylation with the same system[2] requires several hours at room
temperature.
Notably, hydrodefluorination with silanes catalyzed by
B(C6F5)3 was reported: C. B. Caputo, D. W. Stephan, Organo-
metallics 2012, 31, 27–30.
[23]
[24]
E. M. Simmons, J. F. Hartwig, Nature 2012, 483, 70–73.
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Received: December 10, 2013
Published Online: January 30, 2014
Eur. J. Org. Chem. 2014, 2077–2083
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2083