B(C6F5)3 catalyzed hydrosilation of enones and silyl enol ethers

Article abstract of DOI:10.1016/S0040-4020(02)00974-2

The 1,4 hydrosilation of a variety of simple α,β-unsaturated enones as catalyzed by B(C₆F₅)₃ (1-2%) is described. For substrates with no steric hindrance near the β-carbon, 1,4 addition of silane is very clean; in other instances, 1,2 hydrosilation is competitive. The reaction is facile with five commercially available silane reagents. For two examples, a novel hydrosilation of the resulting silylenol ethers was also observed. The net trans stereochemistry of H-Si addition to the silylenol ether C=C bond was established and points to a stepwise mechanism for this reaction. This was supported by the observation and full spectroscopic characterization of a silylcarboxonium ion intermediate with an [HB(C₆F₅)₃]^- counteranion in the hydrosilation of the silylenol ether derived from 4,4-dimethyl-2-cyclohexen-1-one and PhMe₂SiH.

Full text of DOI:10.1016/S0040-4020(02)00974-2

Tetrahedron 58 (2002) 8247–8254
B(C6F5)3 catalyzed hydrosilation of enones and silyl enol ethers
*
James M. Blackwell, Darryl J. Morrison and Warren E. Piers
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alta., Canada T2N 1N4
Received 2 May 2002; accepted 22 May 2002
Abstract—The 1,4 hydrosilation of a variety of simple a,b-unsaturated enones as catalyzed by B(C6F5)3 (1–2%) is described. For substrates
with no steric hindrance near the b-carbon, 1,4 addition of silane is very clean; in other instances, 1,2 hydrosilation is competitive. The
reaction is facile with five commercially available silane reagents. For two examples, a novel hydrosilation of the resulting silylenol ethers
was also observed. The net trans stereochemistry of H–Si addition to the silylenol ether CvC bond was established and points to a stepwise
mechanism for this reaction. This was supported by the observation and full spectroscopic characterization of a silylcarboxonium ion
intermediate with an [HB(C6F5)3]2 counteranion in the hydrosilation of the silylenol ether derived from 4,4-dimethyl-2-cyclohexen-1-one
and PhMe2SiH. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
A number of strategies in which hydrosilanes serve as
the stoichiometric reductant have been reported7 – 9
although Lewis acid catalyzed processes are rare.2a,10
These reactions are noteworthy since the enolate
produced is masked as a stable silyl enol ether which
could be subsequently employed in a number of
important C–C and C–X bond forming reactions.6b,11
Developing new, experimentally convenient methods for
the preparation of silyl enol ethers via conjugate
reduction remains an important synthetic endeavor
particularly using commercially available Lewis acid
catalysts.
The selective reduction of a,b-unsaturated carbonyl com-
pounds is an important transformation in organic synthesis
and a number of methods exist for carrying out either
selective 1,21,2 or 1,4-reductions2 (Scheme 1). For the latter
purpose, stoichiometric copper hydride3 and alumino- or
borohydride reductants1,4 are particularly effective. Various
strategies for effecting asymmetric conjugate reductions
have also recently been described.5 The resulting masked
enolates have the potential to be synthetically elaborated
by reaction with a variety of electrophiles and indeed,
tandem reactions involving conjugate hydride addition fol-
lowed by reaction with an electrophile have been effected
successfully.6
Tris-pentafluorophenylborane, B(C6F5)3, has been shown
by us and others to be a versatile Lewis acid capable of
catalyzing a number of organic transformations.12 In
particular, this stable, highly electrophilic organometallic
Lewis acid can catalyze the condensation or dehydro-
coupling reactions of various group 14 reagents with
common organic functions. For example, B(C6F5)3 cata-
lyzes the hydrosilation of carbonyl,13 imine14 and alkene15
functions and the reductive silation of alcohols16 and
ethers.17 The condensation of allylstannanes and hydro-
stannanes with alkyne18 and carbonyl functions19 has also
been demonstrated. Finally, aldol-type reactions between
silyl enol ethers or silylketene acetals with aldehydes and
aldimines have been reported.20
Two general mechanistic pathways have been shown to
be operative in B(C6F5)3 catalyzed reactions depending
on the particular group 14 reagent and/or organic
compound being used. These pathways differ depending
on whether B(C6F5)3 first activates the organic function
to nucleophilic attack by the Group 14 reagent, Scheme
2 path (a) or vice versa, Scheme 2 path (b). The former,
Scheme 1.
Keywords: Lewis acid catalysis; hydrosilation; enones; silylenol ethers;
tris-(pentafluorophenyl)borane.
*
Corresponding author. Tel.: þ1-403-220-5746; fax: þ1-403-289-9488;
e-mail: wpiers@ucalgary.ca
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00974-2

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J. M. Blackwell et al. / Tetrahedron 58 (2002) 8247–8254
2. Results and discussion
2.1. 1,4-Hydrosilation of enones
B(C6F5)3 is an effective catalyst for carrying out 1,4-
hydrosilation of unfunctionalized a,b-unsaturated ketones,
Table 1. In general, these exothermic reactions are high
yielding and only require small catalyst loadings. In cases
where high yields of the 1,4-addition products are isolated,
GC–MS analysis does indicate the presence of small
amounts of an isomer which is presumably the 1,2-addition
product. Reactions of 3-methyl-2-cyclopentenone (entry 3)
Table 1. B(C6F5)3 catalyzed hydrosilation of enones
Scheme 2.
more conventional case, is tacitly assumed to occur in
many cases such as in the addition of silyl enol ethers
and silylketene acetals to aldehydes and aldimines.20
We have shown that B(C6F5)3 activates aromatic
aldehydes to nucleophilic attack by allylstannane but
that subsequent O–Sn bond formation with release of
B(C6F5)3 does not occur.21 Instead, the resulting
aldehyde-coordinated stannylium species propagates
C–C bond making by reaction with another molecule
of allylstannane, the initially formed alkoxyborate anion
remaining intact throughout the catalytic cycle. This
study illustrates that even when conventional mechan-
isms are operative, more complicated behavior may be
occurring.
The second mode of reactivity involves initial Lewis
acidic activation of the group 14 reagent to nucleophilic
attack by the organic function, as illustrated by path b
in Scheme 2. We have found this mechanism to be
operative in B(C6F5)3 catalyzed hydrosilation reactions.
Extensive mechanistic studies show that B(C6F5)3
polarizes the Si–H bond of hydrosilanes rendering the
silicon susceptible to nucleophilic attack (ionization) by
carbonyl functions.13 The resulting hydridoborate anion,
[H–B(C6F5)3]2, consummates hydrosilation regenerating
B(C6F5)3. Evidence has been provided that this type of
mechanism likely occurs in the hydrosilation of imine,14
alkene15 and alcohol16 functions as well.
In this paper, we show that 1,4-hydrosilation of a,b-
unsaturated ketones and the hydrosilation of silyl enol ethers
can be appended to the list of useful reactions catalyzed by
B(C6F5)3. Both of these hydrosilation reactions are
proposed to occur via a mechanistic pathway akin to that
shown in Scheme 2(b).

J. M. Blackwell et al. / Tetrahedron 58 (2002) 8247–8254
8249
and methyl vinyl ketone (entry 11) lead to complex reaction
mixtures containing both 1,2 addition products and
oligomers.22 Some oligomerization/polymerization23 likely
occurs in all cases, but the reactions can be biased towards
the desired 1,4 reduction by manipulating reaction con-
ditions. For example, in the case of 2-cyclopentenone (entry
1), clean 1,4-hydrosilation occurs at room temperature,
whereas when the reaction is performed at low temperature,
a complex reaction mixture indicative of these side
reactions is observed. While linear a,b-unsaturated ketones
undergo 1,4 hydrosilation under these conditions (entries 8
and 9), the corresponding aldehyde, trans-cinnamaldehyde,
undergoes predominant 1,2-hydrosilation (entry 10); only
small amounts of 1,4-hydrosilation are indicated by GC–
MS analysis of the reaction mixtures. 1,2-Hydrosilation is
also observed to predominate in substrates which incorpor-
ate steric bulk in the b-position (entry 3) but when both
pathways are equally sterically problematic, the 1,4 addition
is favored (for example, entry 4). The product mixture for
the substrate of entry 4 consists of a 9:1 mixture of
diastereomers (GC/MS and NMR spectroscopy), but both
can be assigned as 1,4 addition products on the basis of the
presence of only one vinylic methyl resonance for both
diastereomers. The lower yield in this case was due to
unavoidable product decomposition in the work-up pro-
cedure. Finally, as shown in Table 2, a variety of other
commercially available silanes can be employed, although
iPr3SiH does not react. We have previously noted the
ineffectiveness of this silane in the silation of alcohols.16
have not been done, previous work in our lab established
that the B(C6F5)3/silane reduction system is compatible with
a variety of functions, provided they are not more basic than
the enone moiety. This is exemplified here by the substrate
in entry 7, which also contains a potentially reducible C–C
double bond.15
Based on the mechanistic work performed on saturated
carbonyl13 and imine14 functions, we presume these
reactions proceed similarly, i.e. via a silane activated
pathway akin to that shown in Scheme 2(b). Thus, as
depicted in Scheme 3, activation of silane by B(C6F5)3 leads
to silylcarboxonium intermediates stabilized by the hydrido-
borate counteranion [HB(C6F5)3]2; this ion pair then
collapses to product via delivery of hydride from the
hydridoborate. However, 19F NMR analysis of a mixture of
the substrate of entry 6, Table 1, 4,4-dimethyl-2-cyclo-
hexen-1-one, B(C6F5)3 and PhMe2SiH at 2408C (a
temperature at which reaction is occurring) reveals only
the presence of the borane/ketone adduct, and no evidence
for formation of a hydridoborate stabilized silylcarboxo-
nium ion pair is observed. Nonetheless, formation of the
silylcarboxonium ion pair is the likely path to reduction, and
hydride delivery to the silylcarboxonium ion can occur via
three resonance structures, as shown in the Scheme. Attack
at the silicon (a) results merely in starting materials, while
hydride delivery which effects net 1,2 or 1,4 addition occurs
via resonance structures b and c, respectively. This appears
to be governed to some extent by steric effects in that 1,2
addition competes effectively for b-substituted enones
(Table 1, entries 3 and 11) or aldehydic substrates (entry 10).
Table 2. B(C6F5)3 catalyzed hydrosilation of 4,4-dimethyl-2-cyclohexen-1-
one with various silanes
Entry
Silane
Yield (%)
1
2
3
4
5
6
Ph2MeSiH
tBuMe2SiH
PhMe2SiH
Et3SiH
Ph3SiH
iPr3SiH
96
86
88
91
86
–
Although there exist many methods to effect the conjugate
addition of hydride to enones, this method is attractive for a
number of reasons. Small catalyst loadings are used (1%)
and the products are readily purified by column chroma-
tography using Et3N/hexanes as eluent or by distillation.
Unlike many other 1,4-reduction protocols, the products
formed retain the ‘enolate’ functionality in the useful form
of a silyl enol ether, an effective functional group for C–C
bond formation via Lewis acid catalyzed aldol conden-
sations.20 The potential thus formally exists for developing
tandem one-pot 1,4-hydrosilation/aldol or other alkylation
reaction protocols where both reactions are catalyzed by
B(C6F5)3. Alternatively, the silyl enol ether products can be
readily hydrolyzed leading to the saturated ketone deriva-
tives providing a convenient method for the selective
reduction of the ‘ene’ portion of enones. Finally, although
extensive studies on more heavily functionalized substrates
Scheme 3.
2.2. B(C6F5)3-catalyzed hydrosilation of silyl enol ethers
In the above reactions, we found that careful control of the
equivalency of the silane reagent is necessary since the silyl
enol ether products are themselves susceptible to hydro-
silation. This is particularly true for the more reactive
silanes, for example PhMe2SiH. With this silane, 1,4
hydrosilation of 2-cyclopenten-1-one or 4,4-dimethyl-2-
cyclohexen-1-one is rapid, forming the silyl enol ethers

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J. M. Blackwell et al. / Tetrahedron 58 (2002) 8247–8254
Scheme 4.
products la and 1b, respectively, Scheme 4. These
compounds can be elaborated further through a second,
stereoselective hydrosilation step, to give the b-siloxy
alkylsilanes 2a and 2b in good yields. Complete hydrosila-
tion of the cyclohexyl silyl enol ether 1b requires .40 h at
room temperature with 2% catalyst loading and is
accompanied by some PhMe2SiOSiMe2Ph as a byproduct.
By contrast, an NMR tube experiment has shown that
hydrosilation of 1a occurs rapidly even at 2708C. The
products 2 were purified by column chromatography; 2b
isolated in this fashion was contaminated with PhMe2
SiOSiMe2Ph, which in addition to being produced in the
reaction, may be a result of partial decomposition of 2b on
the silica gel.
Scheme 5.
The formation of an ion pair, I, can be observed by 19F
NMR spectroscopy (Fig. 1). The ketone 4,4-dimethyl-2-
cyclohexen-1-one was mixed with approximately 20%
B(C6F5)3 in C7D8 in an NMR tube at 2408C, Fig. 1(a).
Under these conditions, the borane is mostly sequestered
as the ketone adduct. Addition of one equivalent of
PhMe2SiH leads rapidly to the silyl enol ether lb as
described above (Table 2) and, as shown in Fig. 1(b),
signals for free B(C6F5)3 are observed indicating that at
2408C, an adduct between lb and B(C6F5)3 is not
formed. In light of the poor basicity of silyl enol ethers,
this is perhaps not too surprising. Addition of a second
equivalent of PhMe2SiH to the sample resulted in the
formation of another species, present as an undissolved
oil at the bottom of the NMR tube. This behavior is
common to many reactions where a perfluoroaryl borate
counterion is formed.27 Spectroscopic analysis of this
oil revealed that, in the 19F NMR spectrum (Fig. 1(c)) a
new set of signals has arisen. Significantly, a much
The siloxy and silyl groups in 2a example are assigned a cis-
disposition by nOe experiments and the magnitude of the
3JHH coupling constant. Irradiation of Ha leads to enhance-
ment in Hb and vice versa, while the coupling constant of
5.5 Hz is most consistent with a cis-orientation; vicinal
trans-hydrogens on a cyclopentane ring are expected to
experience little coupling compared to <8 Hz coupling
expected for vicinal cis-hydrogens.24 For comparison,
Fleming has prepared cis-2-(dimethylphenylsilyl)cyclopen-
3
tanol via another route and observed a JHH of 4.8 Hz.25
Treatment of 2a with TBAF in THF gave cis-2-(dimethyl-
phenylsilyl)cyclopentanol with identical data to that
reported by Fleming.25 Similar considerations for product
2b indicate a cis stereochemistry (see Section 4). Thus, the
Si–H bond has added to the enol ether CvC bond in a trans
fashion, implying a stepwise reaction for this reduction.
Hydrosilation of silyl enol ethers has not, to our knowledge,
been reported. Larson and co-workers have shown that
hydroboration of silyl enol ethers is possible but this
reaction did not require Lewis acid activation and gave trans
products resulting from concerted cis addition of B–H to the
silyl enol ether CvC bond.26 Given that hydrosilation
catalyzed by B(C6F5)3 effects trans addition of Si–H to the
substrate double bond, a likely mechanism again involves
borane activation of silane followed by reaction with the
silyl enol ether, particularly since the silyl enol ethers do not
appear to interact with B(C6F5)3 in solution as judged by 19
F
NMR spectroscopy (vide infra). A stepwise mechanism
such as that proposed in Scheme 5 would also account for
the observed stereochemistry of the products 2. Also
consistent with this proposal is the spectroscopic obser-
vation of a silylcarboxonium intermediate when a mixture
of B(C6F5)3, 2b and PhMe2SiH is probed via multinuclear
NMR spectroscopy.
Figure 1. 282 MHz 19F NMR spectra at 2408C showing (a) 4,4-dimethyl-
2-cyclohexen-1-one and 20% B(C6F5)3; (b) sample in (a) plus 1 equiv. of
PhMe2SiH; (c) sample in (b) plus a second equivalent of PhMe2SiH.

J. M. Blackwell et al. / Tetrahedron 58 (2002) 8247–8254
8251
lowered value for Ddp,m of 2.8 ppm indicates that an
anionic borate had been formed, which we take as
evidence for the formation of an intermediate like I.28
4. Experimental
4.1. Starting materials and solvents
The 11B NMR chemical shift for I was measured to be
225 ppm, again consistent with an anionic borate.29
Comparison of spectral data of I to separately prepared
[Bu4N]þ[H–B(C6F5)3]2, which showed identical 11B and
19F NMR data to that observed for I (see Section 4), helped
to confirm that this species indeed contained the hydrido-
borate anion [H–B(C6F5)3]2. The 29Si NMR spectrum of I
showed two signals at 46 and 7 ppm. The former signal is
consistent with the silicon from a silylcarboxonium
intermediate,30 while the latter is in the expected region
for tetraalkyl substituted silanes. The corresponding values
for the hydrosilated product, 2b are 3.4 ppm (O–Si) and
21.1 ppm (C–Si). Clearly, a significant shift in the value
for the O–Si NMR shift is reflective of the partial positive
charge on the Si atom. Species with similar structural
characteristics to I have been proposed and generated in situ
previously.31,32 In the present case, spectroscopic obser-
vation of a chemically stable silylcarboxonium intermediate
is unusual. As it is warmed, I proceeds to products by
selective attack of the hydride trans to the bulky silyl group
leading to the cis products in the second part of this stepwise
process.
B(C6F5)3 was purchased from Boulder Scientific and was
dried with Cl(H)SiMe2, sublimed under vacuum at ,l008C
and stored in a glove box. Other chemicals were purchased
from Aldrich and used as received or purified using standard
procedures. Toluene was purified using the Grubbs/Dow
method.34 C7D8 and C6D6 were purchased from Cambridge
Isotopes and were distilled from Na/benzophenone. NMR
spectra were obtained on either a Bruker AMX300 or
Bruker AM-400 spectrometer. All 1H and 13C spectra were
referenced externally to Me4Si at 0 ppm by referencing the
solvent peak. 11B NMR spectra were referenced relative to
BF3·Et2O at 0 ppm. 19F NMR spectra were referenced
externally to C6F6 at 2163 ppm relative to CFC13 at 0 ppm.
29Si NMR spectra were referenced relative to Me4Si at
0 ppm. NMR tube reactions were generally carried out by
charging an NMR tube with the initial reagents in glove box
under an argon atmosphere using scrupulously dried
deuterated solvents. Subsequent reagents were introduced
by syringe to the septa-sealed NMR tubes at the appropriate
temperature. For low temperature reactions, reactants were
typically mixed at 2788C and the placed in NMR probe at
appropriate temperature. In some cases, the NMR tube, after
being submerged in dry ice/acetone, was shaken once to
ensure proper mixing mindful of the potential for premature
heating.35 In cases where premature reaction occurs, the
reactions were repeated. IR spectra were obtained on neat
samples for liquids and as KBr disks for solids using a
Matteson Instruments 4030 Galaxy Series spectrometer.
Elemental analyses were performed using a Control
Equipment Corporation 440 Elemental Analyzer. High
resolution mass spectra were obtained using a Kratos MS-
80 spectrometer.
3. Conclusions
In summary, we have observed clean 1,4 hydrosilation of
several a,b unsaturated enones utilizing B(C6F5)3 as a
catalyst. Mechanistically, the reactions are proposed to be
related to the reduction of saturated carbonyl functions and
imines by the same B(C6F5)3/silane reagent system. Since
the silyl enol ether products of these reactions are
themselves susceptible to B(C6F5)3 catalyzed addition to,
for example, aldehydes, the potential for tandem reactions
exists.
4.2. General procedure for enone hydrosilation (Tables 1
and 2)
A round bottom flask or 10 dram vial was charged with the
enone (5.0 mmol), B(C6F5)3 (51 mg, 0.01 mmol) and
toluene (5.0 mL). The silane (5.0 mmol) was then added
to the reaction mixture via syringe (except for Ph3SiH which
is a solid) all at once. Typically, after 1–5 min period of
time, the reaction mixture heated up considerably and
turned yellow or orange in color. Stirring was continued for
30–60 min and the reaction mixtures were analyzed by
GC–MS, which usually indicated the reaction was com-
plete. The products were purified by column chromato-
graphy (silica gel, 2% Et3N/hexanes as eluent) and
Perhaps more intriguing is the observation that the cyclic
silyl enol ether products can also undergo B(C6F5)3
mediated hydrosilation, providing new avenues for one-
pot transformations of enone substrates. For example, the b-
silylalkoxy functionality is prone to acid or base induced
elimination reactions,33 as indicated by the loss of PhMe2
SiOSiMe2Ph upon attempted silica gel purification of 2b.
Thus, this protocol could provide a means of effecting a
novel transformation, enone to alkene with vinylic group
transposition in one pot.
1
characterized by H and 13C NMR spectroscopy and high-
Reactions of silyl enol ethers with electrophiles are
generally thought to take place via initial C-attack by the
electrophile. Hence, silylcarboxonium species are the
presumed intermediates, the electrophilic Si subsequently
being attacked by a nucleophile to generate a CvO double
bond. However, until now, direct spectroscopic observation
of these intermediates by NMR spectroscopy or other means
is without precedent. The characterization of intermediate
ion pair I supports the general mechanistic picture we have
developed for the B(C6F5)3/silane reagent system in which
the key function of the borane is to activate the silane by
partial abstraction of the silane hydride.
resolution mass spectroscopy or by comparison of 1H NMR
data to literature values. The purity of the samples was
generally greater than 95%. In some cases, small amounts of
R3SiOSiR3 formed via hydrolysis of the silyl enol ether was
observed.
4.2.1. Table 1, entry 1. Yield: 81% of colorless oil. HRMS:
mass calcd for C18H20OSi, 280.1283; found, 280.1294.
Identified by comparison to published data.36
1
4.2.2. Table 1, entry 2. Yield: 90% of colorless oil. H

8252
J. M. Blackwell et al. / Tetrahedron 58 (2002) 8247–8254
1
NMR (CDCl3, 400 MHz): d 7.74–7.69 (m, 4H), 7.51–7.40
(m, 6H), 2.25 (app. td, 4H, J¼1.6, 7.2 Hz), 1.79 (m, 2H),
1.65 (s, 3H), 0.79 (s, 3H). 13C NMR (CDCl3, 100 MHz): d
146.12, 136.27, 134.19, 129.85, 127.78, 113.29, 33.58,
33.39, 19.73, 11.92, 21.99. HRMS: mass calcd for
C19H22OSi, 294.1440; found, 294.1413.
4.2.9. Table 1, entry 10. Yield: 88% of colorless oil. H
NMR (CDCl3, 400 MHz): d 7.80–7.67 (m, 2H), 7.55–7.26
(m, 6H), 6.69 (dt, 1H, J¼1.5, 15.9 Hz, Ha), 6.39 (dt, 1H,
J¼5.3, 15.9 Hz, Hb), 4.52 (dd, 1H, J¼1.6, 5.3 Hz, CH2O),
0.80 (s, 3H, SiCH3). 13C NMR (CDCl3, 100 MHz): d
136.92, 135.81, 134.37, 133.95, 130.26, 129.86, 128.44,
127.87, 127.37, 126.38, 64.22, 22.85. HRMS: mass calcd
for C22H22OSi, 330.1268; found, 330.1281.
1
4.2.3. Table 1, entry 4. Yield: 55% of colorless oil. H
NMR (CDCl3, 300 MHz): d 7.66–7.62 (m, 4H), 7.42–7.37
(m, 6H), 2.30 (m, 1H), 1.96 (m, 1H), 1.76 (m, 1H), 1.40
(dm, 2H, J¼2.05 Hz), 0.93 (d, 3H, J¼6.87 Hz), 0.85 (d,
3H, J¼7.16 Hz), 0.74 (d, 3H, J¼7.02 Hz), 0.70 (s, 3H,
SiCH3). 13C NMR (CDCl3, 100 MHz): d 149.2, 136.2,
134.4, 129.9, 127.9, 118.2, 45.4, 42.2, 41.5, 16.8, 14.4,
13.2, 10.9, 22.2. HRMS: mass calcd for C22H28OSi,
336.19094; found, 336.19133. The isolated product was
determined to be an 8.7:1 mixture of two diastereomers
by GC/MS analysis. The NMR data reported above is
for the major diastereomer. The enone starting material,
2,3,4,5-tetramethyl-2-cyclopentenone (Aldrich), was deter-
mined to be an 8:1 ratio of trans/cis diastereomers by
1
4.2.10. Table 2, entry 2. Yield: 86% of colorless oil. H
NMR (CDCl3, 400 MHz): d 4.75 (t, 1H, J¼3.9 Hz, CvCH),
1.98 (m, 2H), 1.79 (dt, 2H, J¼2.2, 4.0 Hz), 1.38 (t, 2H,
J¼6.5 Hz), 0.90 (s, 9H), 0.90 (s, 6H, CH3), 0.64 (s, 6H). 13
C
NMR (CDCl3, 100 MHz): d l49.44, 103.20, 37.94, 35.97,
28.62, 28.03, 27.55, 25.77, 18.02, 24.38. HRMS: mass
calcd for C14H28OSi, 240.1909; found, 240.1912.
1
4.2.11. Table 2, entry 3. Yield: 88% of colorless oil. H
NMR (C7D8, 400 MHz): d 7.60–7.50 (m, 2H), 7.20–7.10
(m, 3H), 4.82 (tt, 1H, J¼1.3, 4.0 Hz), 2.02 (m, 2H), 1.71 (dt,
2H, J¼2.1, 4.2 Hz), 1.26 (t, 2H, J¼6.6 Hz), 0.82 (s, 6H),
0.39 (s, 3H). Identified by comparison to published data.41
1
GC/MS and H NMR analysis.37
1
4.2.4. Table 1, entry 5. Yield: 82% of colorless oil. HRMS:
mass calcd for C19H22OSi, 294.1256; found, 294.1439.
Identified by comparison to published data.38
4.2.12. Table 2, entry 4. Yield: 91% of colorless oil. H
NMR (CDCl3, 400 MHz): d 4.74 (tt, 1H, J¼1.2, 4.0 Hz,
CvCH), 1.99 (m, 2H), 1.78 (dt, 2H, J¼2.2, 4.1 Hz), 1.38 (t,
2H, J¼6.6 Hz), 0.96 (t, 9H, J¼8.0 Hz, SiCH2CH3), 0.90 (s,
6H, CH3), 0.64 (q, 6H, J¼8.0 Hz, SiCH2CH3). 13C NMR
(CDCl3, 100 MHz): d 149.31, 102.72, 37.90, 35.97, 28.58,
27.94, 27.46, 6.69, 4.92. HRMS: mass calcd for C14H28OSi,
240.1909; found, 240.1918.
4.2.5. Table 1, entry 6; Table 2, entry 1. Yield: 96% of
colorless oil. 1H NMR (CDCl3, 400 MHz): d 7.75–7.70 (m,
4H), 7.48–7.42 (m, 6H), 4.90 (tt, 1H, J¼1.3, 4.1 Hz), 2.13
(m, 2H), 1.84 (td, 2H, J¼2.3, 3.8 Hz), 1.46 (t, 2H,
J¼6.4 Hz), 0.95 (s, 6H), 0.81 (s, 3H). 13C NMR (CDCl3,
100 MHz): d 149.19, 136.26, 134.28, 129.79, 127.78,
104.04, 37.80, 35.84, 28.52, 27.85, 27.51, 22.48. HRMS:
mass calcd for C21H26OSi, 322.1753; found, 322.1751.
1
4.2.13. Table 2, entry 5. Yield: 86% of white solid. H
NMR (CDCl3, 400 MHz): d 7.69–7.61 (m, 6H), 7.46–7.33
(m, 9H), 4.84 (t, 1H, J¼4.1 Hz), 2.01 (m, 2H), 1.68 (dt, 2H,
J¼2.2, 3.8 Hz), 1.32 (t, 2H, J¼6.4 Hz), 0.78 (s, 6H). 13C
NMR (CDCl3, 100 MHz): d 149.16, 135.43, 134.34, 129.99,
127.74, 104.69, 37.76, 35.81, 28.46, 27.80, 27.56. HRMS:
mass calcd for C26H28OSi, 384.1909; found, 384.1947.
4.2.6. Table 1, entry 7. Yield 92% of colorless oil. 1H NMR
(CDCl3, 300 MHz): d 7.65–7.62 (m, 4H), 7.44–7.34 (m,
6H), 4.64 (m, 1H, vinyl CH2), 4.58 (m, 1H, vinyl CH2),
2.17–1.82 (m, 5H, 2CH2 groups and CH), 1.60 (s, 3H), 1.59
(s, br, 3H), 1.31 (m, 2H), 0.71 (s, 3H, SiCH3). 13C NMR
(CDCl3, 100 MHz): d 149.1, 142.4, 136.5, 134.2, 129.8,
127.8, 111.8, 108.6, 42.3, 35.6, 30.1, 27.7, 20.7, 16.3, 21.8.
HRMS: mass calcd for C23H28OSi, 348.19094; found
348.19003.
1
4.2.14. 1-(Dimethylphenylsiloxy)-1-cyclopentene, 1a. H
NMR (CDC13, 400 MHz): d 7.69–7.61 (m, 6H), 7.46–7.33
(m, 9H), 4.84 (t, 1H, J¼4.1 Hz), 2.01 (m, 2H), 1.68 (dt, 2H,
J¼2.2, 3.8 Hz), 1.32 (t, 2H, J¼6.4 Hz), 0.78 (s, 6H). d
HRMS: mass calcd for C13H18OSi, 218.1127; found,
218.1137.
4.2.7. Table 1, entry 8. Yield: 85% as 3:2 mixture of E/Z
isomers. Both identified by comparison to published data.38
4.2.15. 1-(Dimethylphenylsilyl)-2-(dimethylphenylsiloxy)
cyclopentane, 2a. A round-bottom flask was charged with
B(C6F5)3 (51 mg, 0.10 mmol), 2-cyclopenten-1-one
(420 mL, 5.0 mmol) and toluene (5.0 mL). PhMe2SiH
(1.61 mL, 10.5 mmol) was added via syringe all at once.
An exothermic reaction ensues after which the mixture was
stirred at room temperature for 30 min. The mixture was
concentrated in vacuo and then purified by column
chromatography (silica gel, hexanes then 2% ethyl acetate/
hexanes as eluent) affording a colorless oil (1.5 g, 85%). A
small amount of PhMe2SiOSiMe2Ph was present but could
be removed by purifying a second time by column
4.2.8. Table 1, entry 9. Yield 90% of colorless oil. 1H NMR
(CDCl3, 300 MHz): d 7.62–7.58 (m, 4H), 7.42–7.19 (m,
16H), 5.32 (t, 1H, J¼7.18 Hz, vinyl CH), 3.38 (d, 2H,
J¼7.18 Hz, CH2), 0.56 (s, 3H, SiCH3). 13C NMR (CDCl3,
100 MHz): d 149.6, 141.3, 138.6, 135.6, 134.4, 130.0,
128.4, 128.2, 127.9, 127.8, 127.6, 126.0, 125.7, 110.6, 32.4,
22.2. HRMS: mass calcd for C28H26OSi, 406.17529; found,
406.17358. The product mixture was found to be a 10:1
mixture of Z/E isomers by NMR and GC/MS analysis. Only
data for the major Z isomer is reported above. Assignment
of the major isomer as Z was made on the basis of nOe
experiments and comparison of data to the closely related
silyl enol ethers containing (Z)-dimethylphenyl silyloxy,39
and (Z)-trimethyl silyloxy groups.40
1
chromatography. H NMR (C6D6, 400 MHz): d 7.55 (dd,
2H, J¼1.9, 7.3 Hz), 7.48 (dd, 2H, J¼3.6, 6.6 Hz), 7.29–
7.17 (m, 6H), 4.36 (ddd, 1H, J¼2.2, ,5.0, ,5.0 Hz, Ha),
1.85–1.67 (m, 2H), 1.65–1.51 (m, 2H), 1.47–1.29 (m, 2H),

J. M. Blackwell et al. / Tetrahedron 58 (2002) 8247–8254
8253
1.12 (ddd, 1H, J¼5.5, 8.8, 10.4 Hz, Hb), 0.41 (s, 3H), 0.36
(s, 3H), 0.27 (s, 3H), 0.25 (s, 3H). 13C NMR (C6D6,
100 MHz): d 141.10, 139.08, 134.59, 134.26, 130.08,
129.21, 128.33 (one aryl C missing), 78.36, 38.05, 35.43,
26.94, 25.29, 20.28, 21.14, 21.72, 22.42. Anal. calcd for
C16H30OSi2: C, 71.12%; H, 8.53%. Found: C, 71.45%;
8.23%. 1H NMR decoupling experiments showed that
J(Ha/Hb)¼5.5 Hz. In nOe experiments, irradiation of Ha
led to enhancement in Hb (8.0%) and vice versa (11.0%).
into new compound with the following 19F and 11B NMR
spectroscopic data. 19F NMR (C7D8, 282 MHz, 2608C): d
2132.0 (br., 2F, o-F’s), 262.8 (br., lF, p-F’s), 2165.6 (br.,
2F, m-F’s). 11B NMR (C7D8, 128.4 MHz, 2608C): d 225.
1
29Si NMR (C7D8, 76.5 MHz, 2608C): d 46, 7. H and 13C
NMR spectra are very broad in toluene, but can be recorded
at 2808C in CD2Cl2.
4.2.18. Separate synthesis of [Bu4N]1[H–B(C6F5)3]2.
This ion pair was generated in situ by mixing B(C6F5)3,
Bu4NBr and Et3 SiH in 1:1:1 molar ratio (0.20 mmol) in
C6D6. 1H NMR (400 MHz): d 4.16 (br. d, 1H, BH), 2.44 (m,
4.2.16. 4,4-Dimethyl-1-(dimethylphenylsilyl)-2-
(dimethylphenylsiloxy)cyclohexane, 2b. A round-bottom
flask was charged with B(C6F5)3 (51 mg, 0.10 mmol), 4,4-
dimethyl-2-cyclohexen-1-one (658 mL, 5.0 mmol) and tolu-
ene (5.0 mL). PhMe2SiH (1.61 mL, 10.5 mmol) was added
via syringe all at once. An exothermic reaction ensues after
which the mixture was stirred at room temperature for 40 h.
The mixture was concentrated in vacuo and then purified by
column chromatography (silica gel, 2% ethyl acetate/
hexanes as eluent) affording a colorless oil (1.41 g).
PhMe2SiOSiMe2Ph was present but could not be removed
by purifying again by column chromatography. The yield of
2b is approximately 60% taking this into account. 1H NMR
(C6D6, 400 MHz): d 7.60 (m, 4H), 7.54–7.49 (m, 6H), 4.03
(m, 1H, Ha), 1.84 (t, 1H, J¼13.1 Hz), 1.66 (td, 1H, J¼4.1,
13.7 Hz), 1.50 (dq, 1H, J¼3.2, 14.5 Hz), 1.32 (tdd, 1H,
J¼2.6, 3.8, 14.2 Hz), 1.21–1.09 (m, 2H, one of them Hb),
0.92 (s, 3H), 0.80 (s, 3H), 0.35 (s, 6H), 0.33 (s, 3H), 0.31 (s,
3H). 13C NMR (C6D6, 100 MHz): d 140.29, 139.84, 134.67,
134.29, 130.09, 130.02, 129.38, 128.48, 69.63, 35.19, 33.81,
33.61, 31.31, 30.44, 28.20, 24.14, 20.12, 20.61, 22.90,
8H, NCH2), 1.01 (m, 16H), 0.74 (br. t, 12H, J¼6.9 Hz). 13
C
NMR (100 MHz): d 149.4 (dm, J¼235 Hz, C–F), 138.9
(dm, J¼240 Hz, C–F), 137.5 (dm, J¼246 Hz, C–F), 126.7
(br., C–B), 58.98 (NCH2), 24.02, 19.97, 13.58. 19F NMR
(282 MHz): d 2133.0 (d, 2F, o-F’s), 2163.9 (t, 1F, p-F’s),
2166.9 (m, 2F, m-F’s). 11B NMR (128.34 MHz): d 225.5
(d, J¼58 Hz). The ion pair can be isolated as a solid by
mixing reactants together (1:1:1) in CH2Cl2 under argon
atmosphere, removing CH2Cl2 in vacuo, and adding hexane
to the resulting oily residue which leads to a white solid
upon sonication. The hexane is removed by filtration, more
hexane is added and removed by filtration. 19F NMR and
elemental analysis of the resulting white powder confirmed
that it was [Bu4N]þ[H–B(C6F5)3]2: This material decom-
poses at room temperature over weeks. Anal. calcd for
C34H37NBF15: C, 54.05%; H, 4.94%; N, 1.85%. Found: C,
53.68%; 4.65%, N, 1.85%.
23.44. 29Si NMR: (C6D6, 79.5 MHz): d 3.4, 21.1. 1H, 13
C
Acknowledgements
NMR and 29Si resonances from PhMe2SiOSiMe2Ph are not
included. HRMS: calcd for C24H34OSi2, 396.2305; found,
396.2312. 1H NMR nOe experiments were used to establish
the cis-relationship of Ha and Hb. First, Hb was assigned
using a DEPT 13C NMR experiment to identify the two CH
carbons and an HMQC NMR experiment to identify the
attached 1H. Irradiation of Ha led to a 6.3% nOe in Hb. The
coupling constant between Ha and Hb could not be obtained,
but is less than 3 Hz based on the appearance of Ha in the 1H
NMR spectrum. If the silyl and siloxy groups were trans-
disposed they would be expected to occupy equatorial sites
whereas Ha and Hb would both occupy axial sites; the
expected coupling constant would be 10–14 Hz.24 In a cis-
substituted cyclohexane, one of Ha/Hb would be axial and
the other equatorial leading to a small coupling constant (0–
3 Hz).
Financial support for this work was provided by the NSERC
of Canada in the form of a Discovery Grant and an E. W. R.
Steacie Memorial Fellowship (2001–2003) to W. E. P., a
PGS B fellowship to J. M. B. and a PGSA fellowship to D. J.
M. J. M. B. also thanks the Killam Foundation for a
Fellowship and the Alberta Heritage Foundation for a
Steinhauer Award. W. E. P. is the S. Robert Blair chair of
Chemistry (2000–2005).
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