B(C6F5)3-Catalyzed Silation of Alcohols: A Mild, General Method for Synthesis of Silyl Ethers

Article abstract of DOI:10.1021/jo9903003

The commercially available borane tris(pentafluorophenyl)borane, B(C₆F₅)₃, is an effective catalyst for the dehydrogenative silation of alcohols using a variety of silanes, R₃SiH, R₂SiH₂, and R₂R′SiH. Generally, the reactions occur in a convenient time frame at room temperature using 2 mol % of the borane and are clean and high yielding, with dihydrogen as the only byproduct. Primary aliphatic alcohols are silated cleanly but slowly, with reaction times ranging from 20 to 144 h. Faster reaction times can be achieved by increasing the catalyst loading to 8 mol % or by heating the reaction to ～60°C. Secondary and tertiary alcohols react more rapidly, with most reactions being complete in 0.5-2 h. The reaction is tolerant of many functional groups including C=C, C=C, -Br, aliphatic ketones, C(O)OR, lactones, furans, OBn, OMe, and NO₂; examples of each are given. Using the phenolic substrate 2,4,6-trimethylphenol, a number of different silanes were tested. Only the most bulky silanes (Bn₃SiH and Prⁱ₃SiH) were not reactive under these conditions. The selectivity of the silation reactions are roughly governed by the relative basicity of the alcohols (and other functions in the molecule) with more basic groups being selectively silated. These observations are rationalized on the basis of a mechanism that invokes borane activation of the silane by hydride abstraction. The resulting intermediate silylium/hydridoborate ion pair then reacts with alcohols to give the observed silyl ether and dihydrogen products.

Full text of DOI:10.1021/jo9903003

J . Org. Chem. 1999, 64, 4887-4892
4887
B(C6F 5)3-Ca ta lyzed Sila tion of Alcoh ols: A Mild , Gen er a l Meth od
for Syn th esis of Silyl Eth er s
J ames M. Blackwell, Katherine L. Foster, Victoria H. Beck, and Warren E. Piers*
Department of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary, Alberta T2N 1N4, Canada
Received February 17, 1999
The commercially available borane tris(pentafluorophenyl)borane, B(C6F5)3, is an effective catalyst
for the dehydrogenative silation of alcohols using a variety of silanes, R3SiH, R2SiH2, and R2R′SiH.
Generally, the reactions occur in a convenient time frame at room temperature using 2 mol % of
the borane and are clean and high yielding, with dihydrogen as the only byproduct. Primary aliphatic
alcohols are silated cleanly but slowly, with reaction times ranging from 20 to 144 h. Faster reaction
times can be achieved by increasing the catalyst loading to 8 mol % or by heating the reaction to
∼60 °C. Secondary and tertiary alcohols react more rapidly, with most reactions being complete in
0.5-2 h. The reaction is tolerant of many functional groups including CdC, CtC, -Br, aliphatic
ketones, C(O)OR, lactones, furans, OBn, OMe, and NO2; examples of each are given. Using the
phenolic substrate 2,4,6-trimethylphenol, a number of different silanes were tested. Only the most
bulky silanes (Bn3SiH and Pri SiH) were not reactive under these conditions. The selectivity of the
3
silation reactions are roughly governed by the relative basicity of the alcohols (and other functions
in the molecule) with more basic groups being selectively silated. These observations are rationalized
on the basis of a mechanism that invokes borane activation of the silane by hydride abstraction.
The resulting intermediate silylium/hydridoborate ion pair then reacts with alcohols to give the
observed silyl ether and dihydrogen products.
In tr od u ction
tertiary or otherwise hindered alcohols, necessitating the
use of more expensive silyl triflate reagents.8 Alcohols
with other, base-sensitive functions are difficult to protect
chemoselectively. From an environmental perspective,
the frequent need for the use of DMF and the production
of stoichiometric quantities of ammonium hydrochloride
byproducts is less than desirable.
Silyl ethers are employed widely as protecting groups
for alcohols in organic synthesis.1 Variation in the bulk
of the alkyl substituents on silicon allows for acheivement
of a balance between the stability of the silyl ether and
the ease with which it can be removed at a later stage.1,2
Silyl groups such as ButMe2Si- (TBDMS),3 ButPh2Si-
Silanolysis of alcohols using R3SiH9 is an attractive
alternative, since the only byproduct of the reaction is
H2 (eq 1). While thermodynamically quite favorable, this
(TBDPS),4 Pri Si- (TIPS),5 and Et3Si- (TES) are among
3
the most effective in striking this balance and are
commonly encountered trialkylsilyl moieties for alcohol
protection.
The standard method for introducing these groups
involves treatment of the alcohol to be protected with a
silyl chloride in the presence of an excess of a nitrogen
Lewis base (such as imidazole) in a polar solvent (com-
monly DMF). In some instances, catalytic amounts of the
strong base can be employed,6 with Et3N serving to
neutralize the HCl byproduct. Verkade et al. have shown
that DMF can be avoided in favor of less polar and more
convenient solvents (CH3CN or CH2Cl2) via the use of a
superbase.7 While generally effective, these methods for
introducing silyl ether functions using silyl chlorides have
some drawbacks. They often fail for the protection of
reaction requires a catalyst to occur at convenient rates,
and many transition-metal-based catalysts have been
reported that mediate the process.10 None of them have
enjoyed widespread use because they each suffer from
one or more disadvantages, including poor functional
group tolerance, slow rates with bulky (desirable) silanes
and tertiary alcohols, the need for rigorously anaerobic
and water-free conditions, and the lack of a commercial
source for the catalyst.
Herein we report the use of the commercially available
highly Lewis acidic borane tris(pentafluorophenyl)borane,
B(C6F5)3, as a catalyst for the silanolysis of alcohols.11 In
addition to being effective for primary, secondary, and
tertiary alcohols alike, it is tolerant of a wide variety of
* To whom correspondence should be addressed. Phone: 403-220-
5746. Fax: 403-289-9488. E-mail: wpiers@chem.ucalgary.ca.
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10.1021/jo9903003 CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/09/1999

4888 J . Org. Chem., Vol. 64, No. 13, 1999
Blackwell et al.
Ta ble 1. B(C6F 5)3-Ca ta lyzed Sila tion of Un fu n ction a lized
Alcoh ols
functional groups and is effective for a number of differ-
ent silanes, including some of the more commonly
employed silyl ether protecting groups.
Resu lts a n d Discu ssion
B(C6F 5)3-Ca ta lyzed Sila tion of Alcoh ols. In general,
the reactions reported here are carried out by adding the
borane catalyst to a toluene or CH2Cl2 solution containing
the substrate alcohol and the silane. The borane catalyst
may be easily prepared12 or purchased and used as
received. In most instances, clean reactions (quantitative
by 1H NMR) occur and are signaled by an observable
evolution of H2. Isolated yields are high and workup
procedures are trivial since the only byproduct is dihy-
drogen. Although we used scrupulously dried borane to
effect the reactions reported, special treatment of the
borane is not entirely necessary. When “off the shelf”
borane is employed, this wet material is found to be only
moderately less active, with typical reactivity being
observed after an initial induction period over which time
the water is silated to R3SiOSiR3. Since the presence of
siloxane dimers can complicate product purification, we
prefer to use dry borane. Similarly, although we generally
used carefully dried solvents, use of conventionally
distilled solvents or freshly opened bottles is not ulti-
mately detrimental to the procedures reported.
As summarized in Table 1, the B(C6F5)3/R3SiH reagent
combination leads efficiently to the silation of primary,
secondary, tertiary, and phenolic aliphatic alcohols. Un-
like conventional base-mediated methods for alcohol
silation, the relative reactivity order found in these
studies follows an inverse trend with respect to alcohol
size; that is, sterically bulky alcohols are silated more
rapidly than less hindered alcohols. For instance, the
triphenylsilation of decyl alcohol (Table 1, entry 1)
requires significantly more time for the reaction to reach
completion compared to cyclohexanol (Table 1, entry 4)
for which vigorous hydrogen evolution occurs immedi-
ately upon addition of B(C6F5)3 to the solution of alcohol
a
Conditions A: ROH (5 mmol), B(C6F5)3 (0.1 mmol), toluene,
rt. Conditions B: ROH (5 mmol), B(C6F5)3 (0.05 mmol), toluene,
b
rt. Time (h). c Isolated yield (%).
and triphenylsilane. The sluggishness of primary alcohols
toward silation is also demonstrated by benzyl alcohol
and 1-phenethyl alcohol (Table 1, entries 2 and 3). The
reaction times required for less active substrates can be
lowered significantly by increasing the catalyst loading
(8%) or heating (50-60 °C) gently.
Tertiary alcohols are highly reactive under typical
silation conditions, but for these substrates, use of Ph3-
SiH leads to mixtures of products that consist of the
desired silyl ether contaminated with significant quanti-
ties of olefinic side products. However, silyl ether forma-
tion is favored cleanly when Et3SiH is employed (Table
1, entries 8 and 9). Addition of as little as 1 mol % of
B(C6F5)3 to a toluene solution of triethylsilane and
1-methylcyclohexanol or 1-adamantol resulted in im-
mediate evolution of hydrogen, which subsided after a
few minutes. The high rate of reactivity of tertiary
alcohols vs secondary and primary substrates is not
entirely unprecedented10g and has mechanistic implica-
tions that will be discussed below.
To obtain a picture of the functional group tolerance
of the silation conditions, a variety of functionalized
alcohols were protected as their triphenylsilyl ethers
using B(C6F5)3 catalysis, Table 2. The first three examples
demonstrate that alkene and alkyne functionality is not
affected under these conditions. This is an important
distinction from several of the transition-metal-based
catalysts for this reaction, for which competitive alkene
and alkyne hydrosilation is often observed.10 Further-
more, B(C6F5)3-catalyzed hydrostannation of alkynes has
been recently reported.13 As entry 3 of Table 2 shows,
sluggish reactions can be encouraged with higher catalyst
loadings. Halogens are also tolerated (Table 2, entries 4
(10) (a) Chang, S.; Scharrer, E.; Brookhart, M. J . Mol. Catal. A 1998,
130, 107. (b) Wang, X., Ellis, W. W.; Bosnich, B. Chem. Commun. 1996,
2561. (c) Lorenz, C.; Schubert, U. Chem. Ber. 1995, 128, 1267. (d)
Gregg, T. B.; Cutler, A. R. Organometallics 1994, 13, 1039. (e) Burn,
M. J .; Bergman, R. G. J . Organomet. Chem. 1994, 472, 43. (f) Barber,
D. E.; Lu, Z.; Richadson, T.; Crabtree, R. H. Inorg. Chem. 1992, 31,
4709. (g) Bedard, T. C.; Corey, J . Y. J . Organomet. Chem. 1992, 428,
315. (h) Barton, D. H. R.; Kelly, M. J . Tetrahedron Lett. 1992, 33, 5041.
(i) Doyle, M. P.; High, K. G.; Bagher, V.; Pieters, R. J .; Lewis, P. J .;
Pearson, M. M. J . Org. Chem. 1990, 55, 6082. (j) Luo, X.; Crabtree, R.
H. J . Am. Chem. Soc. 1989, 111, 2527. (k) Yamamoto, K.; Takemae,
M. Bull. Chem. Soc. J pn. 1989, 62, 2111. (l) Caseri, W.; Pregosin, P.
S. Organometallics 1988, 7, 1373. (m) Oehmichen, U.; Singer, H.; J .
Organomet. Chem. 1983, 243, 199. (n) Davies, J . A.; Hartley, F. R.;
Murray, S. G.; Marshall, G. J . Mol. Catal. 1981, 10, 171. (o) Blackburn,
S. N.; Haszeldine, R. N.; Parish, R. V.; Stchfi, J . H. J . Organomet.
Chem. 1980, 192, 329. (p) Haszeldine, R. N.; Parish, R. V.; Riley, B. F.
J . Chem. Soc., Dalton Trans. 1980, 705. (q) Archer, N. J .; Haszeldine,
R. N.; Parish, R. V. J . Chem. Soc., Dalton Trans. 1979, 695. (r) Corriu,
R. J . P.; Moreau, J . J . E. J . Organomet. Chem. 1977, 127, 7. (s) Corriu,
R. J . P.; Moreau, J . J . E. J . Organomet. Chem. 1976, 120, 337. (t)
Corriu, R. J . P.; Moreau, J . J . E. J . Organomet. Chem. 1976, 114, 135.
(u) Ojima, I.; Kogure, T.; Nihonyanagi, M.; Kono, H.; Inaba, S.; Nagai,
Y. Chem. Lett. 1973, 501. (v) Chalk, A. J . J . Chem. Soc. Chem.
Commun. 1970, 847.
(11) Other applications of B(C6F5)3 in organic synthesis: (a) Ishi-
hara, K.; Hananki, N.; Yamamoto, H. Synlett 1993, 577. (b) Ishihara,
K.; Funahasi, M.; Hanaki, N.; Miyata, M.; Yamamoto, H. Synlett 1994,
963. (c) Ishihara, K.; Hananki, N.; Yamamoto, H. Synlett 1995, 721.
(d) Ishihara, K.; Hanaki, N.; Funahasi, M.; Miyata, M.; Yamamoto,
H. Bull. Chem. Soc. J pn. 1995, 68, 1721. (e) Gevorgyan, V.; Liu, J .-X.;
Yamamoto, Y. J . Org. Chem. 1997, 62, 2963.
(12) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1966, 5, 218.

B(C6F5)3-Catalyzed Silation of Alcohols
J . Org. Chem., Vol. 64, No. 13, 1999 4889
Ta ble 2. B(C6F 5)3-Ca ta lyzed Sila tion of F u n ction a lized
Alcoh ols
Ta ble 3. B(C6F 5)3-Ca ta lyzed Sila tion of F u n ction a lized
P h en ols
E
X
Y
conda
timeb
yieldc
1
2
3
4
5
6
7
8
9
OMe
OBn
H
H
H
NO2
H
A
A
A, B
A
A, B
C
C
C
C
20
20
20
2
240
95
92
87
91
12
d
d
d
d
CO2Me
H
CH2CtN
C(O)H
C(O)Me
H
H
H
C(O)H
C(O)Me
H
a
Conditions A: ROH (2.5 mmol), B(C6F5)3 (0.125 mmol), toluene
or CH2Cl2, rt. Conditions B: same as A, except 40 °C (refluxing
CH2Cl2). Conditions C: ROH (0.1 mmol), B(C6F5)3 (0.005 mmol),
b
d
CDCl3, rt. Time (h). c Isolated yield (%). Competitive reduction
of X or Y groups.
ketone carbonyl groups (Table 2, entry 10); since this
substrate is a tertiary alcohol, the use of triethylsilane
was necessary (vide supra).
The results summarized in Table 3 further probe the
selectivity of this reaction using a series of phenolic
substrates. Both methyl and benzyl ethers (Table 3,
entries 1 and 2) did not interfere with alcohol silation,
although we have observed facile cleavage of these ethers
under typical reaction conditions in the absence other
functionality.16 Again, ester groups do not compete for
silane, as the efficient protection of the phenolic OH of
methyl 4-hydroxybenzoate demonstrates (Table 3, entry
3). The presence of a nitro group did not interfere with
silyl ether formation, but the nitrile group again hindered
the reaction by slowing the reaction to impractical rates.
A more functionalized substrate is also cleanly protected,
showing again tolerance for an ester and also the com-
monly employed acetonide protecting group, eq 2.
a
Conditions A: ROH (5 mmol), B(C6F5)3 (0.1 mmol), toluene,
rt. Conditions B: ROH (5 mmol), B(C6F5)3 (0.4 mmol), toluene,
rt. Conditions C: ROH (2.5 mmol), B(C6F5)3 (0.125 mmol), CH2Cl2,
b
rt. Time (h). c Isolated yield (%).
and 5), but nitrile functions prove problematic in that
reactions are inconveniently slow, even with higher
catalyst loadings (Table 2, entry 6). An independent NMR
tube experiment revealed that no competing processes
involving the nitrile group were occurring during the
reaction, even with heating. Although for this particular
nitrile-substituted substrate the reaction proceeded to
completion, the isolated yield was consistently low,
perhaps a result of losses/decomposition during purifica-
tion of the silyl ether by column chromatography. Again,
primary alcohols are silated comparatively slowly, al-
though, curiously, propargyl alcohol and 2-bromoethanol
are silated much more rapidly than the other primary
alcohols (vide infra).
Silation of alcohols can also be carried out in the
presence of functionalities that are known to be reactive
under B(C6F5)3/R3SiH conditions.14 Entries 7 and 8 of
Table 2 demonstrate tolerance of the ester functionality,
including a lactone, which also illustrates the suitability
of this protocol for base-sensitive substrates. Despite the
observed propensity of the silating reagents to ring open
tetrahydrofuran,15 selective alcohol silation of 3-hy-
droxytetrahydrofuran is observed (Table 2, entry 9). The
protocol cannot be extended to include smaller oxygen-
containing heterocycles, however. For example, at-
tempted protection of the hydroxy epoxide glycidol re-
sulted in competitive ring-opening of the epoxide11c in
addition to silyl ether formation. Finally, nonphenolic
hydroxy groups are selectively silated in the presence of
For phenolic substrates, the presence of ketone or
aldehyde functions in the 4-position of the ring resulted
in poor chemoselectivity using the B(C6F5)3/Ph3SiH re-
agent combination. Indeed, when monitored by NMR
spectroscopy, it is clear that the major products are
derived from carbonyl hydrosilation (Table 3, entries 6
and 7). Because the para substitution pattern results in
increased basicity of the carbonyl groups with a concomi-
tant decrease in the basicity of the phenolic oxygen, we
also investigated the meta-substituted isomers; however,
taking the carbonyl groups out of resonance with the
phenolic -OH has no effect on the selectivity of the
reaction.
With the exception of the deployment of Et3SiH for
tertiary substrates, triphenylsilane was used because of
(13) Gevorgyan, V.; Liu, J .-X.; Yamamoto, Y. Chem. Commun. 1998,
37.
(14) Parks, D. J .; Piers, W. E. J . Am. Chem. Soc. 1996, 118, 9440.
(15) Parks, D. J . Ph.D. Dissertation, University of Calgary, 1998.
(16) The products of these reactions are a silyl ether and either
methane or toluene depending on the ether used. We are exploring
the B(C6F5)3/R3SiH system as a deprotection protocol for these groups.

4890 J . Org. Chem., Vol. 64, No. 13, 1999
Blackwell et al.
Ta ble 4. B(C6F 5)3-Ca ta lyzed Sila tion of
2,4,6-Tr im eth ylp h en ol w ith Va r iou s Sila n es
Sch em e 1
E
silane
HSiEt3
conda
timeb
yieldc
1
2
3
4
5
6
7
8
A
A
A
A
A
A
B
B
2
2
12
1
2
<1
95
72
95
80
79
95d
HSiPh3
HSiMe2t-Bu
HSiMe2Ph
HSiMe2Cl
H2SiPh2
HSiBn3
HSii-Pr3
a
Conditions A: ROH (5 mmol), B(C6F5)3 (0.1 mmol), toluene,
rt. Conditions B: ROH (5 mmol), B(C6F5)3 (0.1 mmol), toluene,
b
d
110 °C. Time (h). c Isolated yield (%). Crude yield.
the stability of the silyl ether products and the ease with
which they can be handled. However, the triphenylsilyl
group is not the most widely used protecting group in
synthetic chemistry, and to more fully demonstrate the
scope of this silation method, the silation of 2,4,6-
trimethyl phenol was carried out with a number of
silanes. The results are summarized in Table 4. Of the
eight silanes examined, six of them were shown to be
effective for the silation of this phenol. In addition to the
triphenylsilyl and triethylsilyl groups, TBDMS and di-
methylphenylsilyl groups were easily installed using
B(C6F5)3 and the appropriate silane. However, the more
sterically bulky silanes triisopropylsilane and tribenzyl-
silane were not reactive even under forcing conditions
with this particular substrate (Table 4, entries 7 and 8).
Entries 5 and 6 of Table 4 show that difunctional
silanes can also be employed; i.e., both dimethylchlorosi-
lane (possessing a functionalizable Si-Cl bond) and
diphenylsilane reacted cleanly to give the corresponding
silyl ether. The use of diphenylsilane offers the potential
for further reactivity involving the remain Si-H bond
and another group. Equations 3 and 4 offer illustrations
of this principle for two diols that can be protected as
cyclic siloxanes in moderate yield as shown. These
Furthermore, the second group need not be limited to
ROH; we have performed examples of directed carbonyl
reductions using B(C6F5)3/Ph2SiH2 and keto alcohol sub-
strates.
Finally, the scalability of the reaction was tested by
performing a silation of 2,6-dimethylphenol on a 0.1
molar scale. This experiment was performed by dropwise
addition of Et3SiH to a toluene solution of the phenol
containing 5% B(C6F5)3 catalyst. The rate of hydrogen
evolution was controlled by modulating the rate of silane
addition, which was complete in 5 min. Workup led to a
95% isolated yield of the silyl ether product.
Mech a n istic Con sid er a tion s. In the foregoing dis-
cussion, we have focused primarily on the utility of this
B(C6F5)3-catalyzed reaction rather than on the mecha-
nistic aspects of the process. Although we have not
engaged in extensive mechanistic studies on this par-
ticular reaction, the qualitative evidence strongly sug-
gests that the mechanism is related to that observed in
the B(C6F5)3-catalyzed hydrosilation of carbonyl func-
tions, a reaction we have studied in detail from a
mechanistic perspective.14,15 In that study, kinetic, isotope
labeling, and computational studies indicate that, while
the borane readily forms adducts with carbonyl contain-
ing substrates,18 it is borane activation of the silane (via
hydride abstraction) that is the key step in the addition
of Si-H to CdO.
Thus, while the ROH substrates undoubtedly form
19
adducts with B(C6F5)3 (either through the alcohol func-
tion or another, more basic group in the molecule), it is
likely that dissociation of the borane is necessary so that
it can interact with R3SiH and effect silation of the ROH
functionality (Scheme 1). This notion is consistent with
the qualitative observation that primary alcohols are
(17) Tanino, K.; Shimizu, T.; Kuwahara, M.; Kuwajima, I. J . Org.
Chem. 1998, 63, 2422.
(18) Parks, D. J .; Piers, W. E.; Parvez, M.; MacQuarrie, D. C.;
Zaworotko, M. J . Organometallics 1998, 17, 1369.
(19) While we have not studied these adducts in any detail, they
have been claimed as olefin polymerization cocatalysts and cationic
initiators: Siedle, A. R.; Lamanna, W. M. (3M) U.S. Pat. 5416177, 1995.
Additionally, the water adduct of B(C6F5)3 has recently been re-
ported: Danopoulos, A. D.; Galsworthy, J . R.; Green, M. L. H.;
Cafferkey, S.; Doerrer, L. H.; Hursthouse, M. B. Chem Commun. 1998,
2529.
procedures have not as yet been optimized, but the lower
yield stems mainly from the instability of the products
toward our purification procedures. The experiments
illustrate the potential of the B(C6F5)3/R3SiH system for
tandem protection of 1,2-diols and, in connection with
other protocols,17 should be effective in protecting second-
ary alcohols in the presence of primary -OH functions.

B(C6F5)3-Catalyzed Silation of Alcohols
J . Org. Chem., Vol. 64, No. 13, 1999 4891
Sch em e 2
silated more slowly by B(C6F5)3 than secondary or
tertiary, a trend that has also been observed in certain
transition-metal catalysts.10j Since primary alcohols are
more basic toward sterically significant Lewis acids than
the more bulky secondary or tertiary substrates, there
is less free borane present to activate the silane via
hydride abstraction. Indeed, this mechanistic picture of
the reaction is not unlike what has been proposed (and
in some instances strongly substantiated) for the general
mechanism of transition-metal-catalyzed alcohol silation
reactions (Scheme 2).10a,d,h,j,v Instead of a transition-metal
center activating the Si-H bond (either via oxidative
addition or by formation of a σ-complex20), the highly
electrophilic borane center is able to fulfill this role. The
polarization induced in the Si-H bond upon electrophilic
activation renders the silicon center susceptible to nu-
cleophilic attack by the alcohol in both mechanisms,
giving essentially an alcohol adduct of a trialkyl-
substituted silylium cation.21 Subsequent proton transfer
from this acid to the “M - H” species (in the case of the
B(C6F5)3-catalyzed reaction, [HB(C6F5)3]-, Scheme 1)
completes the cycle, releasing dihydrogen and regenerat-
ing the catalyst.
silylium ion formed from borane abstraction of H- from
the silane. In competition, it will be the most basic group
that prevails in this task.22 The overall rate, however, is
governed by the level of free borane present, which in
turn is dependent on the basicity of the substrate, i.e.,
the position of the equilibrium between B(C6F5)3 and
ROH and the alcohol adduct of B(C6F5)3. Lower chemose-
lectivity is observed when other functions in the substrate
reach a comparable basicity to the alcohol moiety, as in
the phenolic substrates with ketone or aldehyde func-
tionality present23 (Table 3, entries 6-9).
It is unclear at present why substrates with nitrile
functions perform so poorly in this reaction (Table 2,
entry 5, and Table 3, entry 5). While the nitrile group is
24
known to coordinate relatively strongly to B(C6F5)3 it
should be labile enough to allow for reaction. Either these
substrates sequester too much borane, lowering its
effectiveness, or the long reaction times and higher
temperatures required lead to catalyst degredation. For
example, treatment of B(C6F5)3 with Et3SiH at 60 °C for
3 days is a preparative route to bis-pentafluorophenyl-
borane, HB(C6F5)2.25 This borane reacts rapidly with
ROH to form borinate esters ROB(C6F5)2, which are
significantly poorer Lewis acids and do not activate
silanes effectively.
In conclusion, the use of B(C6F5)3 for the formation of
silyl ethers from a wide variety of alcohols has been
demonstrated. Many features of this reaction, including
the ready availability of the catalyst, the high functional
group tolerance, and the easy workup procedures in-
volved, make this method an attractive alternative to
traditional procedures for protecting alcohols as silyl
ethers.
An interesting aspect of this mechanism is that while
more basic substrates react more slowly than less basic
functions, it is the more basic groups that are favored in
terms of selectivity. Thus, although 1-decanol is silated
over the course of 24 h as opposed to less than 2 h for
cyclohexanol (Table 1), in a competition experiment it is
the primary substrate that is silated almost to the
exclusion of the secondary alcohol (eq 5).
Exp er im en ta l Section
Gen er a l Con sid er a tion s. NMR spectra were recorded (1H
NMR, 200 MHz; 13CNMR, 50 MHz) using deuterated chloro-
form as the solvent. 13C NMR and 1H NMR spectra were
referenced to the deuterated chloroform resonance at δ 77.0
and 7.24, respectively. IR spectra were recorded neat. The
(22) The anomolously high rates observed for the silation of
propargyl alcohol and 2-bromoethanol may be due to the lower basicity
of these substrates relative to the other primary alcohols studied.
Alternatively, the presence of donor groups in these positions may aid
in hydridoborate displacement from the silicon by chelation, i.e.,
hypercoordinate silicon.
(23) Relative basicity can be approximated by comparison of the
This arises because a key step in the process is
coordination of the substrate to the silicon atom in the
+
pKa’s of the conjugate acids. pKa for PhOH2 ) -6.7; pKa for PhC(d
OH)CH3 ) -6.2; pKa for PhC(dOH)H+ ) -7.1. Although basicity
+
toward acids other than proton is largely dictated by steric factors, to
the extent that these functions are sterically similar, the pKa values
suggest that they have very similar inherent basicities, accounting for
the poor chemoselectivity observed in these cases. pKa values taken
from: Streitwieser, A.; Heathcock, C. H.; Kosower, E. M. Introduction
to Organic Chemistry, 4th ed.; Macmillan: New York, 1992; p A-6.
(24) Li, L.; Marks, T. J . Organometallics 1998, 17, 3996.
(25) Parks, D. J .; Piers, W. E.; Yap, G. P. A. Organometallics 1998,
17, 5492.
(20) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789.
(21) It should be noted that the trityl cation (which is isoelectronic
to B(C6F5)3) is an effective reagent for abstracting H- from trialkyl
silanes to form silylium ions in the condensed phase: Lambert, J . B.;
Zhang, S.; Ciro, S. M. Organometallics 1994, 13, 2430. Furthermore,
coordination of an Si-H bond to a borane center has recently been
conclusively demonstrated: Wrackmeyer, B.; Tok, O. L.; Bubnov, Y.
N. Angew. Chem., Int. Ed. Engl. 1999, 38, 124.

4892 J . Org. Chem., Vol. 64, No. 13, 1999
Blackwell et al.
silanes were purchased from Aldrich-Sigma and degassed
before use. B(C6F5)3 was purchased from Boulder Scientific and
was dried by stirring with excess Me2ClSiH for ∼20 min. The
silane was removed under vacuum and the borane purified
by sublimation at 80 °C under full dynamic vacuum. This
procedure was generally carried out on 5-10 g batches that
were then stored in a drybox. Alcohol substrates were pur-
chased from Aldrich-Sigma; solids were used as received,
while liquid alcohols were generally passed through basic
alumina prior to use. Allyl alcohol, benzyl alcohol, and cyclo-
hexanol were dried over CaH2 and then distilled. Toluene was
purified by the Grubbs method26 and stored over titanocene27
in an evacuated bomb. Methylene chloride was dried over CaH2
and vacuum transferred to the reaction vessel. All reactions
were performed under an atmosphere of argon, and yields refer
to the amount of isolated product unless otherwise noted.
Gen er a l P r oced u r e for th e B(C6F 5)3-Ca ta lyzed Sila tion
of Alcoh ols. To a solution of equimolar quantities of the
alcohol and silane in toluene or dichloromethane (0.1-1.0 M)
at room temperature was added B(C6F5)3 (1-8%) as a solid
under a purge of argon. In many cases, visible hydrogen
evolution ensued upon borane addition. The reactions were
monitored by either GC-MS, TLC, or 1H NMR spectroscopy
for the absence of either alcohol or silane. Typical workup
procedures involved concentration of the reaction mixture
followed immediately by purification using flash column
chromatography, recrystallization, or a combination thereof.
Silyl ether products were identified using NMR spectroscopy,
mass spectrometry, elemental analysis, and comparison to
literature data where available. In the Supporting Information,
data for each product is given along with more specific
information concerning the conditions employed in each
individual case. Refer to Tables 1-4 for reaction times and
isolated yields.
Sca led -Up Syn th esis of 2,6-Dim eth yl-1-tr ieth ylsiloxy-
ben zen e. To a mixture of 2,6-dimethylphenol (12.24 g, 0.10
mol) and B(C6F5)3 (2.57 g, 0.005 mol) was added 100 mL of
dry toluene. Et3SiH (16.0 mL, 0.10 mol) was immediately
added by syringe over the course of 5 min. Vigorous evolution
of H2 and gentle warming was noted, but a cautious rate of
addition kept the exotherm in check. After completion of silane
addition, the reaction was stirred for 30 min, at which time
TLC analysis showed the starting phenol to be completely
consumed. The reaction mixture was concentrated to ∼50 mL,
and a portion of silica gel was added to the solution. The slurry
was pumped to dryness, and the resulting white powder was
poured into a large column containing hexanes-packed silica;
the product was eluted with 2% ethyl acetate/hexanes. The
fractions were concentrated in vacuo leading to 22.4 g (95%)
of the silyl ether product in >95% purity as judged by 1H NMR
spectroscopy and gas chromatography.
Ack n ow led gm en t. Financial support for this work
was provided by the Natural Sciences and Engineering
Council of Canada in the form of a Research Grant
(W.E.P.) and a Postgraduate B Fellowship (J .M.B.).
W.E.P. also thanks the Alfred P. Sloan Foundation for
a Research Fellowship (1996-98). The authors also
thank Dr. Dan Parks for the preliminary observations
that inspired this work.
Su p p or tin g In for m a tion Ava ila ble: Complete charac-
terization data (1H, 13C{1H}, IR, elemental analysis) for all the
compounds prepared in this study. This material is available
free of charge via the Internet at http://pubs.acs.org.
(26) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
(27) Marvich, R. H.; Brintzinger, H. H. J . Am. Chem. Soc. 1971,
93, 2046.
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