Alkoxyallylsilanes: Functional protecting groups

Article abstract of DOI:10.1016/S0040-4020(99)01062-5

Allyl-t-butylmethylsilyl groups were shown to function as alcohol protecting groups whose hydrolytic stability was greater than t- butyldimethylsilyl (TBS) and Si(SiMe₃)₃ (sisyl) groups. Pseudo-first-order rate constants for the acid

Full text of DOI:10.1016/S0040-4020(99)01062-5

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 1617–1622
Alkoxyallylsilanes: Functional Protecting Groups
†
*
Sonya Balduzzi and Michael A. Brook
Department of Chemistry, McMaster University, 1280 Main St. W., Hamilton, Ontario, Canada L8S 4M1
Received 26 October 1999; revised 12 November 1999; accepted 6 December 1999
Abstract—Allyl-t-butylmethylsilyl groups were shown to function as alcohol protecting groups whose hydrolytic stability was greater than
t-butyldimethylsilyl (TBS) and Si(SiMe3)3 (sisyl) groups. Pseudo-first-order rate constants for the acidic hydrolysis of primary, benzylic, and
secondary allyl-t-butylmethylsilyl ethers in AcOD/THF-d8/D2O were determined to be 2:94×10Ϫ3; 8:26×10Ϫ4; and 8:26×10Ϫ4 sϪ1
;
respectively. The regioselectivity of acidic hydrolysis of allylbenzyloxy-t-butylmethylsilane 1 was examined under strong acid (p-TsOH/
CD2Cl2) and weak acid (AcOD/THF-d8/D2O) conditions. In both cases, benzyl alcohol was initially produced exclusively from silicon–
oxygen bond cleavage: allylic cleavage was only subsequently observed over time. However, the allyl group of the silyl ether could be
hydrosilylated. The resulting alkoxy-functionalized disiloxane had greater hydrolytic stability under acidic conditions than the starting
alkoxyallyl-t-butylmethylsilane. ᭧ 2000 Elsevier Science Ltd. All rights reserved.
Introduction
lipophilicity of trimethylsilylated a-hydroxyacids (e.g.
Me3SiOCH2COOSiMe3, an anti-oxidant for skin care)4 or
testosterone over the parent compounds led to improved
delivery of the active agent and, in the latter case, to more
potent androgenic activity.5
The silyl ether has become the most commonly used alcohol
protecting group in organic synthesis due to its ease of
formation and the selectivity with which it can be removed.1
The latter process is typically carried out either with a
‘naked’ fluoride source such as tetrabutylammonium
fluoride, or via acidic hydrolysis.2 The stability of silyl
groups to hydrolysis and other nucleophilic conditions is
generally proportional to the degree of steric hindrance at
both the silicon atom and the alcohol carbon atom.3 As a
result, a large number of alkylsilanes possessing various
Usually, silyl ethers are prepared from chlorosilanes or silyl
triflates and an alcohol in the presence of base. However, the
protection of alcohols as silyl ethers has also been effected
with allyltrimethylsilane, whereby the allyl moiety serves as
the leaving group on silicon, giving a trimethylsilyl ether,
and gaseous propene as the only by-product (Scheme 1A).6
Scheme 1.
degrees of steric bulk around the silicon atom have
previously been prepared and utilized as alcohol protecting
groups, particularly when the selective cleavage of one of
two or more different silyl ether groups within the same
molecule is required.
This cleavage of the allylic silicon–carbon bond, which
follows electrophilic addition to the double bond, has
found significant synthetic application as a method of
introducing the allyl group onto carbon electrophiles such
as carbonyl compounds, acetals, ketals, and iminium ions
(Scheme 1B).7
Silyl ethers can, in addition, serve quite a different purpose.
The silylation of hydrophilic drugs is a method of imparting
lipophilic character to them. For instance, the increased
We were interested in determining if the utility of allyl-
silanes could be combined with that of silyl ethers. That
is, to learn whether the reactivity of each of the functional
groups remains unchanged when both are present within the
same molecule. Towards this end, we have prepared several
allyloxysilanes and examined their reactivity under acidic
conditions to compare the lability of the allyl substituent
Keywords: hydrolysis; kinetics; protecting groups; silicon and compounds.
* Corresponding author. Tel.: ϩ1-905-525-91-40; fax: ϩ1-905-522-25-09;
e-mail: mabrook@mcmaster.ca
†
E-mail: balduzs@mcmaster.ca
0040–4020/00/$ - see front matter ᭧ 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(99)01062-5

1618
S. Balduzzi, M. A. Brook / Tetrahedron 56 (2000) 1617–1622
versus the alkoxy substituent. The allyloxysilanes were also
investigated as precursors for the preparation of lipophilic,
silicone-modified ethers.
correlation of increased hydrolytic stability with increased
steric hindrance at the alcohol carbon (Fig. 1).1–3 The
decreased lability in protic media on going from sisyl to
TBS to allyl-t-butylmethylsilyl ethers reflects an increase
in steric bulk at silicon.
Results and Discussion
Two possible reaction pathways for acidic hydrolysis of
allylsilyl ethers are illustrated in Scheme 2. Path A involves
nucleophilic substitution at silicon with cleavage of the
silicon–oxygen bond, for which considerable evidence has
been put forth to support a mechanism involving extra-
coordination at silicon.12 Path B involves cleavage of the
allylic silicon–carbon bond via electrodesilylation of the
allyl group.7,10 Since the versatility of the allyl-t-butyl-
methylsilyl ether as a protecting group would be altered in
the case of electrophilic substitution or electrophilic
addition at the allyl group, the ability for this to occur was
examined.
Allyl-t-butylchloromethylsilane was prepared by the reaction
of t-butylmagnesium chloride with allyldichloromethyl-
silane. This route was more efficacious in our hands than
the complementary approach involving substitution at
t-butyltrichlorosilane by allylmagnesium bromide, followed
by methylmagnesium chloride. The presence of catalytic
quantities of CuCN affords high yields of organosilanes
under very mild conditions from the reaction of organomag-
nesium reagents and chlorosilanes.8 The t-butyl and methyl
substituents were chosen in order to create a steric environ-
ment at silicon which is comparable to that found in the
t-butyldimethylsilyl group—the most frequently utilized
member of the repertoire of alcohol protecting groups,
whose reactivity is well defined. The allyl-t-butylmethyl-
silyl ethers of representative primary, benzylic and secondary
alcohols, 2-phenylethanol, benzyl alcohol and cyclopentanol,
respectively, were then prepared from allyl-t-butylchloro-
methylsilane using the standard conditions for silylation of
alcohols.9
An analysis of the reaction products from the hydrolysis of
allylbenzyloxy-t-butylmethylsilane 1 with both a weak acid
(AcOD/THF-d8/D2O) and a strong acid (p-toluenesulfonic
acid/CD2Cl2), revealed that in both cases benzyl alcohol was
initially produced exclusively (Scheme 2A). No loss of the
allyl group was observed under weakly acidic conditions,
although the silyl by-product 2 subsequently reacted further
to give the disiloxane 3 (Scheme 2). Under strongly acidic
conditions, 4 subsequently underwent protiodesilylation:
there was no evidence for Path B (Scheme 2). Strong
acids rather than moderate nucleophiles are clearly required
for the hydrodesilylation of these allylsilanes.
Initially, an examination of the relative reactivity of the two
functional groups on the allylsilyl ethers was attempted: it
was important to distinguish between acidic hydrolysis at
the Si–O group and cleavage of the allyl group via the
b-effect under acidic conditions.10 Thus, the hydrolytic
stability of these silyl ethers was compared with that of
the corresponding t-butyldimethylsilyl ethers (TBS), and
with the corresponding tris(trimethylsilyl)silyl (sisyl)
ethers, the latter being susceptible to both hydrolytic and
photolytic cleavage.11 This was accomplished by measuring
the rates of acid-catalyzed hydrolysis in AcOD/THF-d8/D2O
under pseudo-first-order conditions using a modification of
the procedure established by Corey et al.9 The observed rate
constants are given in Table 1.
From the preceding experiments it is clear that protonation
preferentially occurs at oxygen rather than the allyl group of
1. It seemed possible that this bias could be overcome by the
use of a bulky electrophile which, for steric reasons, would
be prone to attack electrophilically at the allyl group rather
than at the alkoxy group. Although allylsilanes typically
react with aldehydes and ketones in the presence of Lewis
acids to form homoallylic alcohols (Scheme 1), 1 did not
react with benzaldehyde in the presence of boron trifluoride
etherate. In a competition reaction between 1 equiv. of each
of 1, allyltrimethylsilane, and benzaldehyde, only allyl-
trimethylsilane gave allylic transposition onto benzaldehyde.
For each of the three silyl groups, an increased rate of
hydrolysis of the silyl ether derived from the primary
alcohol, when compared with the secondary alcohol, was
observed, which is consistent with the well-documented
However, when 1 was allowed to react with the sterically
hindered and strongly electrophilic triphenylcarbenium
hexafluorophosphate, a small quantity of 4,4,4-triphenyl-
1-butene 5, the product of allylic cleavage, was observed.
The major product was triphenylmethyl benzyl ether. This
suggests that a bias towards allyl cleavage might only be
established with alkoxy groups that are bulkier than benzyl-
oxy, and perhaps, with even bulkier electrophiles than
Ph3Cϩ. This is clearly a significant synthetic limitation to
the use of these bifunctional silanes as both allylsilanes and
alkoxysilanes.
Table 1. Rate constants (sϪ1) for acidic hydrolysis of silyl ethers ROSiR03
SiR0
R
3
CH2CH2Ph
CH2Ph
C5H9
a
Si(Si(CH3)3)3
3.74×10Ϫ2
2.94×10Ϫ3
6.04×10Ϫ3
1.94×10Ϫ2
8.26×10Ϫ4
3.53×10Ϫ3
1.30×10Ϫ2
8.26×10Ϫ4
3.49×10Ϫ3
Si(CH3)(CH2CHyCH2)t-Bu
1
Si(CH3)2t-Bua
a Rate constants previously reported, see Ref. 11.
Figure 1. Relative rates of hydrolytic cleavage of silyl ethers in AcOD/THF-d8/D2O.

S. Balduzzi, M. A. Brook / Tetrahedron 56 (2000) 1617–1622
1619
Scheme 2.
Although the allyl group does not undergo selective acidic
cleavage, it remains reactive under other non-acidic con-
ditions. In particular, as is widely practiced in the silicone
industry, it readily undergoes hydrosilylation with hydro-
silanes. It was anticipated that the resulting product would
be approximately as stable towards hydrolysis as the parent
allylsilane. To establish whether allylalkoxysilanes could
provide a general route to lipophilic, silicone-modified alco-
hols, 1 was subjected to hydrosilylation with the model
silicone pentamethyldisiloxane and Karstedt’s catalyst
(Ptn((H2CyCHSiMe2)2O)m typically nˆ2, mˆ313) to give
6 (Scheme 3).
reactivity order of hydrolysis of the benzyloxysilyl ethers
was determined in CDCl3 using p-toluenesulfonic acid
monohydrate.11 The hydrolysis rates under identical
reaction conditions for benzyloxy-t-butyldimethylsilane,
benzyloxy-(tris(trimethylsilyl)silyl), 1, and 6 were deter-
mined to be 2.5, 0.9, 0.8, and 0.7 (×10Ϫ4 mol LϪ1 minϪ1),
respectively (Fig. 2). The reversal of reactivity observed
between TBS and sisyl ethers when the solvent was changed
from protic to aprotic has previously been reported to be
attributed to a changeover in mechanism due to the protic
solvent’s ability to facilitate extracoordination at silicon.11
These results are consistent with our expectation that the
steric bulk associated with the distal silicon atom (Siء,
Fig. 2), which is separated by a propyl unit from the silicon
As 6 was insoluble in the aqueous solvent system used for
the measurement of rate constants (Table 1), a relative
Scheme 3.

1620
S. Balduzzi, M. A. Brook / Tetrahedron 56 (2000) 1617–1622
Figure 2. Reactivity order of silyl ethers with p-TsOH in CDCl3.
at which hydrolysis takes place, does not affect the rate of
alkoxysilane hydrolysis. It further suggests that hydro-
silylation of allylalkoxysilanes could serve as a useful,
practical, and predictable method of improving the lipo-
philicity of hydroxy-containing drugs, and as a strategy
for the introduction of organic functional groups onto
silicones. These are research directions that are currently
being investigated in our laboratory, and will be reported
in due course.
MS (EI, m/z): 176 (10), 135 (20), 119 (20); MS (CI, NH3,
m/z): 194 (50), 176 (30), 152 (50); HRMS (m/z) calcd for
C8H17Si1Cl1 (Mϩ): 176.0788, found: 176.0778.
General procedure for protection of the alcohol
To a stirring solution of the alcohol (1 mmol) and
4-dimethylaminopyridine (or imidazole) (1.20 mmol) in
CH2Cl2 (or DMF) was added a solution of the silyl chloride
(1 mmol) in CH2Cl2 (or DMF) (1 M). The solution was
stirred overnight at room temperature under N2. Water
was added and the aqueous layer was extracted with
CH2Cl2. The organic layer was dried over anhydrous sodium
sulfate, filtered and the solvent was removed under reduced
pressure. Separation by flash chromatography gave the
products in 80–90% yields.
Experimental
1H and 13C NMR spectra were recorded on a Bruker AC-200
spectrometer with CDCl3 as an internal standard. 29Si NMR
spectra were recorded on a Bruker AC-300 spectrometer (at
59.6 Hz). Infrared spectra were acquired on samples
prepared as KBr pellets or as liquid films on NaCl discs,
on a Perkin–Elmer 283 spectrometer. Electron impact (EI)
and chemical ionization (CI, NH3) mass spectra were
recorded at 70 eV with a source temperature of 200ЊC on
a VG analytical ZAB-R mass spectrometer equipped with a
VG 11-250 data system. High-resolution mass spectral
(HRMS) data were obtained using the EI method.
Allylbenzyloxy-t-butylmethylsilane 1. Benzyl alcohol
(0.75 mL, 7.2 mmol), imidazole (1.0 g, 14.7 mmol), DMF
(1.5 mL), allyl-t-butylchloromethylsilane (0.90 g, 5.1 mmol).
Purification by chromatography with eluent pentane/diethyl
etherˆ19:1 yielded 0.93 g, 74% of a colorless oil. 1H NMR
(CDCl3): d 7.27 (m, 5H), 5.84 (m, 1H), 4.86 (m, 2H), 4.75
(s, 2H), 1.66 (m, 2H), 0.92 (s, 9H), 0.09 (s, 3H); 13C NMR
(CDCl3): d 141.2, 134.5, 128.1, 126.8, 125.9, 113.7, 65.2,
26.2, 20.7, 18.9, Ϫ7.45; 29Si NMR (CDCl3): d 15.12; IR
(neat, NaCl): n 2956, 2930, 2858, 1631, 1470, 1255, 1096,
1070, 833, 770, 728 cmϪ1; MS (EI, m/z): 248 (3), 207 (55),
191 (15), 91 (85); MS (CI, NH3, m/z): 266 (5), 224 (100);
HRMS (m/z): calcd for C15H24O1Si1 (Mϩ): 248.1596, found:
248.1600.
The starting alcohols, DMAP, and imidazole were obtained
from Aldrich and used without further purification. Allyl-
dichloromethylsilane, pentamethyldisiloxane and platinum–
divinyltetramethyldisiloxane complex (Karstedt’s catalyst)
were obtained from Gelest and used without further purifi-
cation. CH2Cl2 was distilled from P2O5 prior to use. Tris-
(trimethylsilyl)silyl chloride (sisyl chloride) was prepared
using the procedure reported by Bu¨rger et al.14
Allyl-t-butylmethyl-(2-phenyl)ethanoxysilane. 2-Phenyl-
ethanol (0.65 mL, 5.4 mmol), DMAP (0.86 g, 7.0 mmol),
CH2Cl2 (5 mL), allyl-t-butylchloromethylsilane (0.77 g,
4.3 mmol). Purification by chromatography with eluent
pentane/diethyl etherˆ9:1 yielded 0.87 g, 78% of a color-
Allyl-t-butylchloromethylsilane. A solution of t-butylmag-
nesium chloride in THF (1.0 M, 100 mL, 100 mmol) was
added dropwise to a flask containing allyldichloromethyl-
silane (13 mL, 90.1 mmol) and copper (I) cyanide (0.45 g,
5.0 mmol) in THF (90 mL) at 0ЊC, over a period of 2 h.
After stirring at 0ЊC for 2 h, the solution was warmed to
25ЊC and stirring was continued for an additional 24 h.
After filtration of the precipitate under N2, distillation
under reduced pressure yielded 7.1 g, 40% of a colorless
oil (bp 55ЊC/25 mmHg). 1H NMR(CDCl3): d 5.78 (m,
1H), 4.99 (m, 2H), 1.82 (m, 2H), 0.97 (s, 9H), 0.32 (s,
3H); 13C NMR (CDCl3): 132.7, 115.1, 25.6, 22.3, 19.6,
Ϫ3.7; 29Si NMR (CDCl3): d 32.14; IR (neat, NaCl): n
1
less oil. H NMR (CD2Cl2): d 7.23 (m, 5H), 5.82 (m, 1H),
4.85 (m, 2H), 3.83 (t, Jˆ6.9 Hz, 2H), 2.79 (t, Jˆ6.9 Hz,
2H), 1.60 (m, 2H), 0.88 (s, 9H), 0.01 (s, 3H); 13C NMR
(CD2Cl2): d 139.7, 135.2, 129.5, 128.5, 126.3, 113.5, 65.1,
39.8, 26.3, 20.9, 19.1, Ϫ7.4; 29Si NMR (CDCl3): d 14.36; IR
(neat, NaCl): n 3076, 3022, 2953, 2928, 2887, 2859, 1733,
1633, 1473, 1254, 1105, 896, 832, 768, 693, 610 cmϪ1; MS
(EI, m/z): 221 (40), 179 (75), 163 (50), 105 (75), 75 (60), 41
(100); MS (CI, NH3, m/z): 238 (20), 221 (100), 179 (40), 105
(40).
2959, 2934, 2862, 1633, 1471, 1256, 901, 826, 776 cmϪ1
;

S. Balduzzi, M. A. Brook / Tetrahedron 56 (2000) 1617–1622
1621
Allyl-t-butylcyclopentanoxymethylsilane. Cyclopentanol
(0.50 mL, 5.5 mmol), DMAP (0.90 g, 7.4 mmol), CH2Cl2
(5 mL), allyl-t-butylchloromethylsilane (0.77 g, 4.3 mmol).
Purification by chromatography with eluent pentane/diethyl
(CDCl3): d(139.2, 129.2, 128.20, 126.1, 64.6, 39.7, 25.9,
18.3, Ϫ5.4; 29Si NMR (CDCl3): d 18.80; IR (neat, NaCl):
n 3023, 2953, 2930, 2854, 1474, 1254, 1102, 827, 781,
692 cmϪ1; MS (EI, m/z): 221 (10), 179 (100), 161 (25),
105 (50), 75 (25); MS (CI, NH3, m/z): 254 (35), 237
(100), 196 (35), 179 (15), 122 (15), 105 (15); HRMS (m/z):
calcd for C13H21O1Si1 (MϩϪCH3): 221.1361, found:
221.1361.
1
etherˆ9:1 yielded 0.80 g, 80% of a colorless oil. H NMR
(CD2Cl2): d 5.84 (m, 1H), 4.85 (m, 2H), 4.27 (m, 1H), 1.65
(m, 2H), 1.52 (m, 2H), 0.89 (s, 9H), 0.06 (s, 3H); 13C NMR
(CD2Cl2): d 135.5, 113.3, 75.1, 36.1, 26.3, 23.5, 21.5, 19.0,
6.9; 29Si NMR (CDCl3): d 11.63; IR (neat, NaCl): n 3075,
2955, 2929, 2895, 2858, 1633, 1474, 1362, 1248, 1054, 895,
824, 767, 610 cmϪ1; MS (EI, m/z): 185 (25), 143 (30), 101
(20), 75 (100), 41 (70); MS (CI, NH3, m/z): 227 (10), 202
(100), 185 (25), 134 (40), 92 (40), 75 (40); HRMS (m/z):
calcd for C13H26OSi (Mϩ): 226.1752, found: 226.1752.
1-(t-Butyldimethylsilyl)benzyl alcohol. Benzyl alcohol
(2 ml, 19.3 mmol), imidazole (1.57 g, 23.1 mmol), DMF
(20.0 mL), t-butylchlorodimethylsilane (2.9 g, 19.3 mmol).
Purification by chromatography with eluent petroleum
ether/diethyl etherˆ39:1 yielded 3.69 g, 86% of a colorless
1
oil. H NMR (CDCl3): d 29 (m, 5H), 4.73 (s, 2H), 0.92 (s,
1-[Tris(trimethylsilyl)silyl]cyclopentanol. Cyclopentanol
(0.12 mL, 1.3 mmol), DMAP (0.18 g, 1.6 mmol), CH2Cl2
(2.0 mL), sisyl chloride (0.38 g, 1.3 mmol). Purification by
chromatography with eluent pentane yielded 0.33 g, 90% of
9H), 0.08 (s, 6H); 13C NMR (CDCl3); d 141.4, 128.2, 126.8,
126.0, 64.9, 25.9, 18.4, Ϫ5.3; 29Si NMR (CDCl3): d 19.44;
IR (neat, NaCl): n 2955, 2930, 2857, 1470, 1254, 1096,
1070, 838, 776, 728 cmϪ1; MS (EI, m/z): 165 (100), 135
(40), 91 (45); MS (CI, NH3, m/z): 240 (75), 223 (15), 182
(15), 108 (100), 91 (20); HRMS (m/z): calcd for
C13H22Si1O1 (Mϩ): 222.1439, found: 222.1429.
1
a colorless oil. H NMR (CDCl3) d 3.90 (m, 1H), 1.64 (m,
2H), 1.58 (m, 2H), 1.08 (m, 4H), 0.16 (s, 27H); 13C NMR
(CDCl3): d 78.9, 35.8, 36.0, 23.4, 23.3; 29Si NMR (CDCl3):
d Ϫ2.72, 11.30; IR (neat, NaCl): n 2952, 2895, 1439, 1246,
1060, 836 cmϪ1; MS (EI, m/z): 263 (20), 191 (20), 175
(30), 131 (35), 73 (100); MS (CI, NH3, m/z): 264
(Si(TMS)ϩ3 ϩNH3, 8).
1-(t-Butyldimethylsilyl)cyclopentanol.
Cyclopentanol
(1.6 mL, 17.6 mmol), DMAP (2.58 g, 21.1 mmol), CH2Cl2
(18.0 mL), t-butylchlorodimethylsilane (2.65 g, 17.6 mmol).
Purification by chromatography with eluent pentane
yielded 2.61 g, 75% of a colorless oil. 1H NMR
(CDCl3): d 4.20 (m, 1H), 1.66 (m, 4H), 1.47 (m, 4H),
0.85 (s, 9H), 0.01 (s, 6H); 13C NMR (CDCl3): d 74.4,
35.7, 25.9, 23.1, 18.2, Ϫ4.7; 29Si NMR (CDCl3): d
15.82; IR (neat, NaCl): n 2951, 2930, 2855, 1468,
1252, 1060, 832, 775 cmϪ1; MS (EI, m/z): 185 (5),
143 (50), 75 (100), 59 (10), 43 (15); MS (CI, NH3,
m/z): 201 (100), 162 (20), 133 (35), 92 (55), 74 (15);
HRMS (m/z): calcd for C10H21O1Si1 (MϩϪCH3):
185.1361, found: 185.1361.
1-[Tris(trimethylsilyl)silyl]-2-phenylethanol. 2-Phenyl-
ethanol (0.24 mL, 2.1 mmol), DMAP (0.29 g, 2.5 mmol),
CH2Cl2 (3.0 mL), sisyl chloride (0.61 g, 2.1 mmol). Purifi-
cation by chromatography with eluent pentane/diethyl
1
etherˆ9:1 yielded 0.50 g, 80% of a colorless oil. H NMR
(CDCl3): d 7.27 (m, 5H), 3.74 (t, Jˆ6.9 Hz, 2H), 2.85 (t,
Jˆ6.9 Hz, 2H), 0.24 (s, 27H); 13C NMR (CDCl3): d 139.0,
129.0, 128.0, 126.0, 69.0, 39.8, 0.3; 29Si NMR (CDCl3): d
1.47, Ϫ16.55; IR (neat, NaCl): n 3029, 2950, 2893, 1396,
1376, 1245, 1088, 1078, 836.9 cmϪ1; MS (EI, m/z): 263
(15), 175 (10), 131 (15), 105 (100), 73 (60); MS (CI,
NH3, m/z): 369 (100), 263 (50), 138 (50), 122 (60), 105
(70), 90 (50).
1-(Benzyloxy-t-butylmethylsilyl)-3-(pentamethyldisilox-
ane)-propane 6. To a flask containing pentamethyl-
disiloxane (0.28 g, 2 mmol) and allylbenzyloxy-t-
butylmethylsilane (0.5 g, 2 mmol) in hexanes (5 mL), was
added platinum–divinyltetramethyldisiloxane complex
(one drop) at 25ЊC. After stirring overnight, purification
by chromatography with eluent pentane/ether (19:1) yielded
0.75 g, 96% of a colorless oil. 1H NMR (CDCl3): d 7.46 (m,
5H), 4.92 (s, 2H), 1.65 (m, 2H), 1.12 (s, 9H), 1.05 (m, 2H),
0.79 (m, 2H), 0.27 (s, 3H), 0.23 (s, 9H), 0.20 (s, 6H); 13C
NMR (CDCl3): d 141.5, 128.1, 126.8, 126.0, 65.3, 26.4,
23.2, 18.8, 17.8, 17.1, 2.0, 0.5, Ϫ7.0; 29Si NMR (CDCl3):
d 18.43, 9.07, 6.55; IR (neat, NaCl): n 2957, 2930, 2859,
1254, 1064, 842 cmϪ1; MS (EI, m/z): 181 (12), 91 (100); MS
(CI, NH3, m/z): 207 (26), 91 (100), 72 (22), 43 (16); HRMS
(m/z): calcd for C20H39O2Si3 (MϩϪH): 395.2257, found:
395.2215.
1-[Tris(trimethylsilyl)silyl]benzyl alcohol. Benzyl alcohol
(0.085 mL, 8.1 mmol), DMAP (1.33 g, 10.9 mmol), CH2Cl2
(8.0 mL), sisyl chloride (2.29 g, 8.1 mmol). Purification by
chromatography with eluent petroleum ether/diethyl
etherˆ39:1 yielded 2.33 g, 82% of a colorless oil. 1H
NMR (CDCl3): d 7.26 (m, 5H), 4.59 (s, 2H),18 (s, 27H);
13C NMR (CDCl3,): d 141.7, 128.1, 126.8, 125.6, 69.3,
Ϫ0.1; 29Si NMR (CDCl3,): d 3.47, Ϫ15.61; IR (neat,
NaCl): n 2952, 2893, 2857, 1245, 1085, 1063, 836,
690 cmϪ1; MS (EI, m/z): 339 (10), 263 (90), 248 (15), 189
(30), 175 (40), 131 (25), 117 (25), 91 (100), 73 (75); MS (CI,
NH3, m/z): 372 (15), 355 (100), 264 (70), 108 (20), 90 (25);
HRMS (m/z): calcd for C15H31O1Si1 (MϩϪCH3): 339.1451,
found: 339.0750.
1-(t-Butyldimethylsilyl)-2-phenylethanol. 2-Phenylethanol
(1.5 mL, 12.5 mmol), DMAP (1.84 g, 15.0 mmol), CH2Cl2
(13.0 mL), t-butylchlorodimethylsilane (1.88 g, 12.5 mmol).
Purification by chromatography with eluent pentane/diethyl
General procedure for acidic hydrolysis
(A) Relative rates in aqueous media. To an NMR tube
containing the silyl ether (0.076 mmol) under N2 at 25ЊC
was added THF-d8 (0.26 mL), D2O (0.12 mL), and
CD3CO2D (0.38 mL), to make a 0.1 M solution of the
1
etherˆ19:1 yielded 2.6 g, 88% of a colorless oil. H NMR
(CDCl3): d 7.20 (m, 5H), 3.78 (t, Jˆ7.1 Hz, 2H), 2.80 (t,
Jˆ7.1 Hz, 2H), 0.85 (s, 9H), Ϫ0.03 (s, 6H); 13C NMR
1
silyl ether. The H NMR spectra were acquired initially

1622
S. Balduzzi, M. A. Brook / Tetrahedron 56 (2000) 1617–1622
within 2 min of addition of CD3CO2D, and subsequently
every 10 min. The presence of benzyl alcohol was
confirmed by injecting an authentic sample into the NMR
tube containing the reaction mixture. The presence of allyl-
t-butylmethyldisiloxane was confirmed by comparison with
an authentic sample that was prepared by hydrolyzing allyl-
t-butylchloromethylsilane.
Acknowledgements
We gratefully acknowledge the financial support of the
Natural Sciences and Engineering Council of Canada. The
authors would like to thank Krista Kerr for experimental
assistance.
(B) Organic media. To an NMR tube containing allyl-
benzyloxy-t-butylmethylsilane (0.022 g, 0.091 mmol) in
CD2Cl2 (0.5 mL) under N2 at 25ЊC was added p-toluene-
sulfonic acid, which had been dried by azeotropic dis-
References
1. van Look, G.; Simchen, G.; Heberle, J. Silylating Agents,
Fluka: Buchs, Switzerland, 1995.
1
tillation (0.091 mmol in 0.5 mL CD2Cl2). The H NMR
spectra were acquired initially within 5 min, and subse-
quently every 15 min. This was then repeated using 0.4,
0.8, and 1.5 molar equivalents of p-toluenesulfonic acid.
The absence of benzyl p-toluenesulfonate was confirmed
by comparison with an authentic sample that had been
prepared by the method of Tipson.15
2. Lalonde, M.; Chan, T. H. Synthesis 1985, 817.
3. Hwu, J. R.; Tsay, S.-C.; Cheng, B.-L. Steric Effects of Silyl
Groups. In The Chemistry of Organic Silicon Compounds,
Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998;
Vol. 2, p 431 (Chapter 8).
4. Archimica Inc., http://www.silcare.com
5. (a) Tacke, R.; Linoh, H. Bio-organosilicon Chemistry. In The
Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z.,
Eds.; Wiley: Chichester, UK, 1998; Vol. 1, Chapter 19, p 1143. (b)
Tacke, R.; Wagner, S. A. Chirality in Bioorganosilicon Chemistry.
In The Chemistry of Organic Silicon Compounds; Rappoport, Z.,
Apeloig, Y., Eds.; Wiley: Chichester, UK, 1998; Vol. 2, Chapter
41, p 2363.
(C) Relative rates in organic media. To the silyl ether in a
flask (0.07 mmol) with CDCl3 (3 mL), was added p-toluene-
sulfonic acid monohydrate (0.28 mmol). The reaction was
1
monitored by H NMR every 10 min.
Reactions with electrophiles
6. Morita, T.; Okamoto, Y.; Sakurai, H. Tetrahedron Lett. 1980,
21, 835.
(A) Carbenium ion. To an NMR tube containing allyl-
benzyloxy-t-butylmethylsilane (0.02 g, 0.08 mmol) in
CD2Cl2 (0.8 mL) under N2 at 25ЊC was added triphenyl-
carbenium hexafluorophosphate (0.031 g, 0.08 mmol). The
1H NMR spectra were acquired initially within 5 min, and
subsequently every 15 min. The presence of 4,4,4-triphenyl-
1-butene and triphenylmethyl benzyl ether was confirmed
by comparison with authentic samples that were prepared
according to the procedures of Bachmann et al.16 and
Chaudhary et al.,17 respectively.
`
7. Fleming, I.; Dunogues, J.; Smithers, R. Org. React. 1989, 37,
57.
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1989, 8, 1121.
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Polymer Chemistry; Wiley: New York, 2000. (b) Colvin, E. W.
Silicon in Organic Synthesis; Butterworths: London, 1981. (c)
Weber, W. P. Silicon Reagents in Organic Chemistry; Springer:
Weinheim, 1983.
11. (a) Brook, M. A.; Balduzzi, S.; Mohamed, M.; Gottardo, C.
Tetrahedron 1999, 55, 10027. (b) Brook, M. A.; Gottardo, C.;
Balduzzi, S.; Mohamed, M. Tetrahedron Lett. 1997, 38, 6997.
12. (a) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem.
Rev. 1993, 93, 1371. (b) Cartledge, F. K.; McKinnie, B. G.;
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Chem., Int. Ed. Engl. 1991, 30, 438.
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Obshch. Khim. 1955, 25, 2448.
(B) Aldehyde. To a flask containing benzaldehyde (0.1 mL,
0.98 mmol) and BF3·OEt2 (0.12 mL, 0.98 mmol) in CD2Cl2
(10 mL) was added allylbenzyloxy-t-butylmethylsilane
(0.24 g, 0.96 mmol) at 0ЊC. After stirring at 25ЊC for 24 h,
only the starting material was detected by TLC.
(C) Competition reaction with allyltrimethylsilane. To a
flask containing benzaldehyde (0.1 mL, 0.98 mmol) and
BF3·OEt2 (0.12 mL, 0.98 mmol) in CD2Cl2 (5 mL) was
added a solution of allylbenzyloxy-t-butylmethylsilane
(0.23 g, 0.93 mmol) and allyltrimethylsilane (0.15 mL,
0.93 mmol) in CD2Cl2 (5 mL) at 0ЊC. When benzaldehyde
could no longer be detected using TLC, a saturated aqueous
solution of NaHCO3 was added at 0ЊC, followed by extrac-
tion using CH2Cl2, drying over MgSO4, and removal of
solvent under reduced pressure. The presence of 1-phenyl-
3-butene-1-ol was confirmed by comparison with an
authentic sample. This had been prepared by the reaction
of benzaldehyde with allytrimethylsilane in the presence
of BF3·OEt2, and compared with previously reported
characterization.18