Catalysis by cationic oxorhenium(v): Hydrolysis and alcoholysis of organic silanes

Article abstract of DOI:10.1039/b822783g

The cationic [2-(2′-hydroxyphenyl)-2-oxazolinato(-2)]oxorhenium(v) complex 1 promotes oxidative dehydrogenation of organosilanes with water and alcohols in a catalytic manner to give excellent yields of silanols and silyl ethers, respectively. The reactions proceed conveniently under ambient and open-flask conditions with low catalyst loading (≤1 mol%). The scope of the reaction with water is quite broad and includes aliphatic, aromatic, tertiary, secondary and primary silanes. The rate of reaction depends on the catalyst and silane concentrations and kinetic isotope effect measurements demonstrate involvement of the Si-H bond in the activated complex. The most influential factor on the silane affecting reactivity is steric hindrance and a quantitative correlation with the Taft steric parameter (E) is presented. A combination of kinetic data and isotope labelling results are used to discuss plausible mechanisms for the oxidative dehydrogenation reaction pathway.

Full text of DOI:10.1039/b822783g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Catalysis by cationic oxorhenium(V): hydrolysis and alcoholysis
of organic silanes†
Rex A. Corbin, Elon A. Ison‡ and Mahdi M. Abu-Omar*
Received 18th December 2008, Accepted 3rd February 2009
First published as an Advance Article on the web 25th February 2009
DOI: 10.1039/b822783g
The cationic [2-(2¢-hydroxyphenyl)-2-oxazolinato(-2)]oxorhenium(V) complex 1 promotes oxidative
dehydrogenation of organosilanes with water and alcohols in a catalytic manner to give excellent yields
of silanols and silyl ethers, respectively. The reactions proceed conveniently under ambient and
open-flask conditions with low catalyst loading (≤1 mol%). The scope of the reaction with water is
quite broad and includes aliphatic, aromatic, tertiary, secondary and primary silanes. The rate of
reaction depends on the catalyst and silane concentrations and kinetic isotope effect measurements
demonstrate involvement of the Si–H bond in the activated complex. The most influential factor on the
silane affecting reactivity is steric hindrance and a quantitative correlation with the Taft steric
parameter (E) is presented. A combination of kinetic data and isotope labelling results are used to
discuss plausible mechanisms for the oxidative dehydrogenation reaction pathway.
Introduction
An efficient and versatile method for the preparation of organic
silanols is highly valuable because silanols are widely employed in
the production of silicon-based polymers as well as intermediates
1,2
in organic synthesis. The conventional method of preparation
is hydrolysis of silyl halides under strictly buffered conditions
to avoid condensation of the silanol to give the less desirable
2 3
Fig. 1 Cationic oxorhenium(V) catalyst (1); solv. = H O or CH CN.
3
siloxane product. The latter reaction is catalyzed by acid or
base and especially difficult to control for unhindered silanols. An
alternative approach is oxidation of the Si–H bond in organosi-
lanes. Most available oxidation methods require stoichiometric
conditions for silanol formation from the reaction of organosilane
and water (eqn (1)).
10
4
oxidant and present the problem of wasteful byproducts. Some
R SiH + H O Æ R SiOH+H
3
2
3
2
transition-metal catalysts have been reported for the oxidation of
silanes with varying range of substrates and levels of siloxane
(1)
R = alkyl,aryl,etc.
5
byproduct formation. Noteworthy of special mention are the
systems based on the precious metals ruthenium and iridium that
The analogous reaction with alcohols affords silyl ethers
(eqn (2)) and constitutes a widely used protecting group reaction
6
7
utilize water and air, and the MTO/H
2
2
O system. Adam and
11
co-workers have optimized the latter to favour silanol formation
in organic synthesis. The alcoholysis of organosilanes requires a
catalyst because alcohols are not sufficiently nucleophilic to react
with Si–H bonds under normal conditions. The standard method
is to treat silyl chloride with alcohol in the presence of a base such
8
by employing urea hydrogen peroxide in organic media. These
systems are desirable because they utilize green oxidants, air and
hydrogen peroxide. Another approach to inserting an oxygen
atom into Si–H is hydrolytic oxidation, eqn (1), where the atom
12
as imidazole in polar organic solvents such as DMF. Verkade and
co-workers have shown that convenient organic solvents (such as
economy is excellent (98.5% for Et
3
SiH to Et
3
Si–OH) and the
13
only byproduct is dihydrogen. [(Ph P)Cu(H)]
3
6
catalyzes hydrolysis
CH
3
CN and CH
2
Cl ) can be used with a super base. However,
2
of silanes in polar organic solvents such as THF with moderate
yields of silanol when the organosilane is added slowly. We have
reported recently an air and water-stable oxorhenium(V) catalyst
base-sensitive functional groups cannot be tolerated and the
production of ammonium hydrochloride salt byproduct makes this
approach less appealing. Cationic iridium and iron catalysts have
9
14,15
(
Fig. 1) that exhibits high reactivity and selectivity under ambient
been employed for the alcoholysis of silanes.
The mechanism
is believed to proceed through nucleophilic attack by the alcohol
2
onto an h -silane adduct. Drawbacks of these catalysts include
the need for rigorously anaerobic and water-free conditions, slow
rates especially with bulky tertiary silanes, and poor functional
Brown Laboratory, Department of Chemistry, Purdue University, 560 Oval
Drive, West Lafayette, Indiana, 47907, USA. E-mail: mabuomar@
purdue.edu; Fax: +1-765-494-0239; Tel: +1-765-494-5302
†
Electronic supplementary information (ESI) available: Schematic figure
of RGA-MS and description of general instrument design, figure of log k
vs. nSi–H, and log k vs. the Taft constant Rs*. See DOI: 10.1039/b822783g
Current address: Department of Chemistry, North Carolina State
University, Box 8204, 2620 Yarbrough Drive, Raleigh, NC 27695, USA.
group tolerance. The highly Lewis acidic borane B(C
6
F
5
3
) has
1
been shown to effect the dehydrogenative silylation of alcohols
1
16
with various silanes albeit at very slow rates. Reaction times at
room temperature with 2 mol% catalyst loading range from 1–6 d.
‡
2
850 | Dalton Trans., 2009, 2850–2855
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PhMe
Et
this solution with Re(O)(hoz)
green solution that contains 1, triphenyl methane, and PhMe
2
SiH (0.35 mL, 2.22 mmol) to form the silylium cation
R SiH + R¢OH Æ R SiOR¢ + H
3
3
2
18
[
3
Si(CD
3
CN)][B(C
6
F
5
)
4
] as a brown solution. Treatment of
Cl, 2, (1.25 g, 2.22 mmol) yields a
SiCl.
R = alkyl,aryl,etc.
(2)
2
R¢ = alkyl
2
The solvent was removed under vacuum and the resulting green
In this study, we report on the use of cationic oxorhenium(V)
complex 1 (Fig. 1) as a highly effective and selective catalyst for
the hydrolysis and alcoholysis of organosilanes including tertiary
substrates under ambient open-flask conditions. The reactions at
oil was washed repeatedly with pentane, to yield 1, as a green
1
powder in 90% yield (2.5 g). H NMR d (300 MHz, CD
3
CN),
7
7
5
.93 (d, 1H, 6.8 Hz), 7.59 (t, 1H, 7.1 Hz), 7.10 (t, 1H, 7.1 Hz),
+
.03(d, 1H, 6.8 Hz). MS(ESI ) m/z calcd For C18
H
16
N
2
5
O Re:
1
mol% catalyst loading are complete in ca. 1–2 h. The scope of our
-
25.06/527.06. Found 524.90/526.88. MS(ESI ) m/z calcd For
system with different substrates is defined. The kinetics have been
investigated following dihydrogen evolution using an in-house
built mass spectrometer. We report the preparation, isolation,
and characterization of a rhenium oxo hydride, and its potential
involvement in catalysis. We discuss viable mechanistic pathways
that are consistent with our collective experimental observations
including kinetics, linear free energy relationships, kinetic isotope
effects, and isotope labelling experiments.
C24BF20: 678.98. Found 679.14.
Alternatively, 1 can be prepared in situ as follows: trityl
tetra(pentaflourophenyl) borate, (0.050 g, 0.053 mmol) was dis-
solved in 1 mL of CD CN in a nitrogen-filled glove-box and
treated with Et SiH (8.7 mL, 0.053 mmol) to form the silylium
cation [Et Si(CD CN)][B(C ] as a brown solution. Treatment
of this solution with Re(O)(hoz) Cl, 2, (0.030 g, 0.053 mmol) yields
a green solution that contains 1, triphenyl methane and Et SiCl.
3
3
3
3
6
F
)
5 4
2
3
1
8
18
O-enriched complex 1 (Re= O). In a nitrogen-filled glove box
trityl tetra(pentaflourophenyl) borate, (0.050 g, 0.053 mmol) was
dissolved in 1 mL of CD CN and treated with Et SiH (8.7 ml,
.053 mmol) to form the silylium cation. The brown solution was
Experimental
Materials and instrumentation
3
3
0
Organosilanes were purchased from Gelest or Aldrich and
used as received. [Re(O)(hoz)
hydroxyphenyl)-2-oxazolinato(-2)) was prepared according to a
then treated with 2 (0.030 g, 0.053 mmol) and the green solution
2
Cl] (2) (where, hoz = 2-(2¢-
removed from the glove box. OH
18
(95% O, 30.0 mL, 1.66 mmol)
18
2
was added to this solution and the solution stirred for 1 h. Analysis
17
published procedure. HPLC grade acetonitrile was used for
kinetic measurements. Trityl tetra(pentaflourophenyl) borate was
purchased from Strem Chemicals and used as received. NMR
spectra were recorded on a Varian Inova300 spectrometer. Mass
spectrometry was performed by the Purdue University Campus
Wide Mass Spectrometry Center on a FinniganMAT LCQ mass
spectrometer system (ESI) or a Hewlett Packard Engine mass
spectrometer (GC/MS). A commercial gas evolution apparatus
18
by mass spectrometry (ESI) revealed 1, as 84% O enriched.
Re(O)(hoz)
.77 mmol) in acetonitrile was added Bu
.1 equiv.). The suspension was allowed to stir overnight and
2
(H), 3. To a green suspension of 2 (1.00 g,
1
2
3
SnH (1.0 mL, 3.79 mmol,
then the green powder was filtered and washed several times with
1
pentane to yield 3 (0.78 g, 84% yield). H NMR (300 MHz,
CD
2
Cl
2
): d 7.93 (dd, J = 8.2, 1.1 Hz, 1H), 7.69 (dd, J = 8.2,
1
1
8
1
1
.8 Hz, 1H), 7.46 (td, J = 7.0 1.8 Hz, 1H), 7.24 (td, J = 8.8,
(
ChemGlass) was used to determine the yields of hydrogen gas
evolved.
Dihydrogen gas analyses were performed using an in-house built
.7 Hz, 1H), 6.99–6.74 (overlapping, 4H), 5.32–4.01 (overlapping,
1
3
H), 1.53 (s, 1H); CNMR (300 MHz, CD
79.7, 168.5, 137.0, 135.5, 130.0, 128.1, 122.5, 120.0, 117.1, 116.9,
11.6, 75.3, 71.1, 70.5, 69.1, 59.5. Elemental analysis, calcd For:
2
2
Cl ): d 175.5, 172.1,
residual gas analyzer (Stanford Research Systems RGA100) elec-
tron impact ionization quadrupole mass spectrometer. Typically,
the reaction solution (1–2 mL) was stirred in a custom made
glass RGA cell with a minimum (1–2 mL) head space. An inert
C
18
H
17
N
2
5
O Re, C 40.98, H 3.25, N 5.31; Found C 40.83, H 3.22,
N 5.38.
carrier gas (Ar or N ) was drawn over the reaction head space
2
at 2 mL min-1 by a Varian model SH 100 vacuum pump and
Typical catalytic procedure
Hydrolytic oxidation of PhMe
analyzed by a Stanford Research Systems RGA 100 mass spec-
trometer equipped with an Alcatel ATH31 Series turbopump. The
RGA-MS was customized for real time gas evolution observation.
2
SiH. An acetonitrile solution of
[
Re(O)(hoz)
water (3 mL, 166 mmol) in a Schlenk tube. Phenyldimethyl silane
1 mL, 6.53 mmol) was then added and the evolution of hydrogen
2
][B(C
6
F )], 1, (0.050 g, 0.040 mmol) was treated with
5
The relative intensity of H in terms of partial pressure as directly
2
(
monitored at m/z = 2 with respect to time provided for rate
determinations. Carrier gas regulated at 4 psi (over atmospheric
pressure) was passed over the reaction flask atmosphere at a
flow rate of ª 2 cm min through 0.007 inch internal diameter
capillary tubing into a series of pressure reduction apertures for
mass analyzer system introduction. A detailed schematic of the
RGA instrument setup is provided in the ESI.†
was monitored using a glass evolution apparatus. Upon cessation
of hydrogen evolution (1.5 h) the reaction mixture was poured
onto ice and treated with pentane to precipitate the catalyst. The
mixture was filtered and the organic layer was extracted with
diethyl ether and pentane. The solution was dried with anhydrous
2
-1
MgSO
afford 88% isolated yield of PhMe
5%).
4
and filtered. Pentane and ether were removed in vacuo to
2
Si(OH) (95%) and (PhMe Si)
2
2
O
(
Synthesis
Kinetics
[
Re(O)(hoz)
phenyl) borate, (2.05 g, 2.22 mmol) was dissolved in 25 mL
of CH CN in a nitrogen-filled glove-box and treated with
2
(Solv)]B(C
6
F
5
)
4
,
1. Trityl tetra(pentaflouro-
Pseudo first-order conditions, with 10-fold excess water or alcohol
concentration, were utilized. In a typical run the total sample
3
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Dalton Trans., 2009, 2850–2855 | 2851

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volume was 1 mL in CH
maintained constant at 25.0 ± 0.2 C. Reagent concentrations were
3
CN solvent and the temperature was
The conversion to the silanols is excellent for aryl, alkyl, and
alkoxy silanes. Isolated yields of silanol are quite good (Table 1,
entry 3) because the subsequent conversion of silanol to silyl
ether is slow. Secondary and primary organosilanes are reactive
and afford high conversions. In the case, of secondary silanes a
mixture of mono- and di-silanol is obtained (Table 1, entries 5
and 6). However, primary silanes give multiple silicon containing
products (Table 1, entry 7). This finding is not surprising because
organosilanetriols tend to condense more readily than silanediols
◦
as follows: [organosilane] = 0.10 M, [H O] or [ROH] = 2.0–5.0 M,
2
and [1] = 0.0020-0.018 M. Catalyst stock solutions were prepared
fresh and used within one day. A solution of the catalyst (1) and
H O or ROH was incubated in the RGA-MS reaction vessel,
2
equipped with magnetic stirring bar, and purged with solvent
saturated Ar carrier gas until the solution and headspace were
effectively degassed. Confirmation was provided by monitoring
19
the decline of N
2
, O
2
and H
2
gas intensity within the carrier gas
to give siloxane polymers and/or rings.
as determined by the RGA-MS instrument. The reaction was
initiated by addition of the organosilane substrate using a gas
tight syringe. Gas evolution was followed by monitoring m/z = 2.
Even though the catalytic reaction can be run neat (no solvent)
as a biphasic reaction to high conversion (Table 1, entry 4), the
isolated product is disiloxane, which arises most likely from the
coupling of PhMe
should be noted that while secondary silanes afford 2 equivalents
of H , the primary silane PhSiH does not afford 3 equivalents of
(Table 1, entry 7). In fact, the H yield from PhSiH indicates
that precisely 2 molar equivalents of H are given. In other words,
2
SiH and PhMe
2
SiOH under neat conditions. It
Results
2
3
H
2
2
3
Reaction scope
2
once the silandiol is formed the third Si–H bond is markedly less
reactive.
Typically the reactions reported here are carried out in CH
3
CN or
CH Cl by adding 1 mol% of catalyst 1 to a solution containing
2
2
The alcoholysis reaction proceeds to high conversions with
aliphatic alcohols. The reaction need not be dry as silylation of
primary and secondary alcohols are quite competitive with the
water reaction. However, tertiary (t-BuOH) and aromatic (ArOH)
the organosilane substrate and water or alcohol. Clean reactions
occur with quantitative conversion of the organosilane as observed
1
29
by H and Si NMR and quantitative evolution of H . Since
2
catalyst 1 is air- and water-stable, no extra precaution is needed
and the described reactions proceed smoothly under open-flask
conditions. Furthermore, the catalyst can be prepared in situ
starting with the rhenium chloride precursor complex 2 and
adding to the reaction mixture 1 molar equivalent relative to
alcohols are not effective and only trace amounts of H
is detected.
2
evolution
Kinetics
2
[Ph
3
C][B(C
6
F
5
)
4
]. Formation of the active cationic catalyst is
We have chosen to evaluate the kinetics by following H
2
evolution
rapid compared to the oxidative hydrolysis or alcoholysis of
organosilane. The results for silanol and silyl ether formation from
the reaction of organosilanes with water and alcohols as catalyzed
by 1 are given in Table 1.
in real time using RGA-MS. The employed conditions have water
or alcohol in excess and the organosilane substrate limiting. This
was selected because it provides for pseudo-first-order conditions
and hence simplified rate equation. Additionally, reversal of the
a
Table 1 Rhenium-catalyzed hydrolysis and alcoholysis of organosilanes
b
%
Yield
Entry
Silane
ROH
Catalyst
Silanol (SiOH)
Siloxane (SiOSi)
Silyl ether (SiOR)
H
2
+
c
1
2
3
Ph
Ph
PhMe
PhMe
2
MeSiH
MeSiH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
O
O
O
2/Ph
3
C
>95
>95
88
—
>95
<2
<2
—
70
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
>98
>98
>98
95
98
78
95
82
88
66
95
95
97
95
95
95
95
98
98
98
2
1
d
2
SiH
SiH
1
1
e
d
4
2
+
+
c
c
f
5
6
7
8
9
0
1
2
3
4
5
6
7
Ph
Ph
2
SiH
SiH
2
2/Ph
3
C
C
f
2
2
1
g>95
—
g
PhSiH
EtMe SiH
3
2/Ph
3
2
1
1
1
1
1
1
1
1
1
1
>95
>95
92
>95
>95
—
>95
—
—
—
—
8
—
—
—
—
—
—
—
Et
Et
Et
2
3
2
MeSiH
SiH
PrSiH
1
1
1
1
1
1
1
1
i
n
Pr
3
SiH
(OEt)SiH
BuMe SiH
Me
2
t
2
Et
Et
Et
3
3
3
SiH
SiH
SiH
MeOH
EtOH
i
PrOH
—
a
Conditions: 1 mol% 1 or 2:[Ph
3
C][B(C
6
F
5
)
4
b
] (1 : 1), 1.0 M silane substrates, 2–10 equivalents of H
2
O or ROH in CH
3
CN or CH
2
Cl
2
under ambient
yields
6 5 4
F ) ] to precursor rhenium chloride 2.
◦
temperature (ca. 20 C), reaction time 2–4 h. For silicon containing compounds, determined by NMR and/or GC unless otherwise indicated. H
2
c
determined by volumetric displacement apparatus. Catalyst prepared in situ by addition of 1 equiv. [Ph
3
C][B(C
d
e
f
29
g
Isolated yields. Neat in the absence of solvent. 26% Ph
silicon containing products result.
SiH(OH) mono-silanol and 74% Ph Si(OH) silanediol determined by Si NMR. Multiple
2 2 2
2
852 | Dalton Trans., 2009, 2850–2855
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Table 2 Rate constants for the oxorhenium-catalyzed dehydrogenative
oxidation of organosilanes with water, nSi–H stretching frequencies, and
Taft electronic (s*) and steric (E) parameters
a
/L mol-1 s-1
Si–H/cm-1 Rs*b
c
Entry Silane
k
1
n
RESi
1
2
3
4
5
6
7
8
9
1
1
EtMe
2
SiH
1.27 ± 0.07
0.71 ± 0.08
0.30 ± 0.02
0.18 ± 0.02
0.023 ± 0.002
0.063 ± 0.005
0.85 ± 0.09
1.2 ± 0.2
2120
2113
2107
2109
-0.10
-0.20
-0.30
-0.36
-0.39
-0.30
0.09
+0.60
+1.20
—
-0.15
-0.30
-0.45
-0.65
-0.854
-1.46
-0.15
—
Et
2
MeSiH
SiH
Et
Pr
3
n
3
i
SiH
Et
BuMe
2
PrSiH
SiH
(EtO)Me SiH
PhMe SiH
MeSiH
SiH
t
2
2117
2
2
2129
2132
2139
2160
Ph
Ph
2
2
0.11 ± 0.01
0.10 ± 0.02
0.19 ± 0.01
—
—
—
0
1
2
PhSiH
3
—
1
Fig. 3 A typical H NMR kinetic profile for oxorhenium catalyzed
a
◦
b
c
In CH
3
CN at 25.0 C. Ref. 21. Ref. 22.
hydrolysis of organosilanes. Conditions: Ph
.0 M, and [1]
= 0.0090 M in CH
circles), Ph MeSiOH (closed squares) and (Ph
are fitted to a consecutive reactions model Ph
MeSiH → Ph
(Ph MeSi) O.
2
MeSiH = 1.00 M, H
2
O =
◦
2
0
3
CN at 25.0 ± 0.2 C. Ph
2
MeSiH (open
excess reagent, that is silane in excess and water or alcohol limiting,
is not feasible because it leads to reduction in oxorhenium(V) to
an inactive form and thus catalyst degradation. Indeed the time
profiles follow first-order dependence on [silane]. A plot of the
2
2
MeSi)
2
O (open diamonds)
MeSi(OH) →
2
2
10
2
2
evolution and illustrated nicely that subsequent condensation of
silanol is slower than its formation. Detailed kinetics of disiloxane
formation was not investigated.
Kinetics of the alcoholysis reaction was studied using Et SiH as
the prototypical silane substrate and different alcohols. As men-
observed first-order rate constant k
Re] for one of the substrates, Et SiH, defines a straight line that
passes through the origin (Fig. 2). Variation in [H O] has no effect
y
vs. catalyst concentration
[
T
3
2
3
on the observed rate constant. Therefore, under these conditions
the experimental rate constant is given in eqn (3). The slope from
the plot of k
t
tioned above, tertiary alcohols ( BuOH) and phenolic substrates
ArOH) were not reactive and only trace amount of dihydrogen
y
vs. [Re] affords the second-order rate constant
T
(
k
1
. Other substrates behave similarly. The values of the second-
was detected. Therefore, the kinetics of Et
investigated with CH OH, CH CH
follow an experimental rate law that is identical to that observed
with water, first-order in [Et SiH] and [Re] , and zero-order in
ROH], eqn (3). The second-order rate constants k (in CH CN at
5 C) for the three alcohols are as follows: 0.26 ± 0.04 (MeOH),
3
SiH alcoholysis was
OH, and PrOH. All three
order rate constants k
Table 2.
1
for all the silanes studied are presented in
i
3
3
2
-
d[silane] d[H2 ]
3
T
=
= k [silane][Re]T
(3)
1
[
2
0
1
3
dt
dt
◦
i
-1 -1
.18 ± 0.01 (EtOH), and 0.11 ± 0.01 ( PrOH) L mol s .
Activation parameters
The second-order rate constant k was evaluated for the hydrolysis
1
and alcoholysis with MeOH of Et
3
SiH over the temperature
◦
range 15–45 C. Kinetic data for both reactions was collected
◦
at 5 intervals and analyzed by the Arrhenius and transition-
state theory equations, eqn (4) and (5), respectively. The results
are summarized in Table 3 for hydrolysis and methanolysis of
Et SiH.
3
-
Ea/RT
k
1
= Ae
(4)
(5)
kBT
∓
∓
RT
R
k1 =
eDS e-DH
h
Kinetic isotope effect
The variation of k with deuterium substitution on silicon in tri-
Fig. 2 Rhenium dependence on the pseudo-first-order rate constants for
◦
the hydrolysis of Et
3
SiH catalyzed by 1 in CH
3
CN at 25.0 C (R = 0.982).
1
ethylsilane and on oxygen in water and methanol was investigated.
The choice to follow H
2
production also removes complications
from the subsequent condensation reaction of silanol to give
disiloxane. This point is pertinent only for the water reaction and
not alcoholysis. To confirm the reliability of our kinetic measure-
Table 3 Activation parameters
ROH
E
a
/kJ mol-
1
DH /kJ mol
‡
-1
DS /J mol K-1
‡
-1
ments, we have followed the reaction progress for two substrates
1
H
MeOH
2
O
34.9(3)
34.6(1)
32.4(3)
31.9(2)
-148(1)
-151(1)
(
Ph
2
MeSiH and PhMe
2
SiH) by H NMR. Representative time
profiles are shown in Fig. 3. These verified the kinetics from H
2
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The use of deutero-water (D
methanol (CH OD, 95% enrichment) has no effect on the rate
of reaction with Et SiH. The collected values were as follows:
(H O)/k (D (MeOH)/k (MeOD) =
O) = 1.03 ± 0.03 and k
.1 ± 0.1. This finding is expected since the experimental rate
laws do not exhibit dependence on the concentrations of water
and alcohol. The value of k for Et SiD (97% enrichment) in
comparison with the same quantity for Et SiH gave a primary
KIE for the reactions with water and methanol as follows:
(Et SiH)/k (Et O), and 1.4 ± 0.2 (with
SiD) = 1.3 ± 0.1 (with H
MeOH).
2
O, 95% enrichment) or deuteron-
flask conditions. The oxorhenium complex also catalyzes silylation
of alcohols with high efficiency. The versatility of the reactions is
quite good. Tertairy, secondary, and primary silanes are suitable
substrates. While secondary silanes give mixtures of mono-silanol
and silanediol, primary silanes give complex mixtures of silicon-
containing products and are not selective for discrete silanetriols.
Aliphatic and aromatic tertairy silanes are effective; however,
3
3
k
1
1
2
1
2
1
1
1
3
3
triaryl silane such as Ph
3
SiH is not reactive. Only trace amounts of
H
Pr
2
was detected. As for aliphatic silanes, substrates bulkier than
i
k
1
3
1
3
2
3
SiH are not reactive either. For the alcohol silylation reaction,
t
the only limitations are tertiary alcohols ( BuOH) and phenolic
substrates (ArOH). These are sluggish and afford only small yields
Isotope labelling experiments
of H .
2
Our kinetics results show that the substituents on the silicon
have an effect on the rate of reaction. The trend for alkyl
The source of hydrogen atoms in the H
2
product was established by
employing Et
3
SiD with H
2
O and Et
3
SiH with D O. The produced
2
silanes follows this order of reactivity: EtMe
2
SiH > Et
2
MeSiH >
gas was exclusively HD (94 and 98%, respectively) as determined
by mass spectrometry.
To ascertain the source of the oxygen in the silanol product is
water and not air, we conducted the catalytic reaction in open
n
t
i
Et
3
SiH > Pr
3
SiH > BuMe
2
SiH ≥ PrEt
2
SiH. We took the Si–
H stretching vibration as measure for the electronic variation
in the Si–H bond strength (Table 2). However, a plot of log(k
1
)
1
8
18
against nSi–H revealed no correlation. The expectation was that
silane compounds with the smallest vibrational frequencies (that
is the weakest Si–H bonds) would be most reactive since the
experimental rate law depended on [silane]. Since the substituents
we employed are aliphatic, aromatic, and one mesomeric substrate,
we tested the Taft linear-free energy relationship. Adding the s*
values for each of the substituents on silicon gives Rs*, which
air with H
Et SiOH product. To investigate the role of the multiply bonded
oxo ligand on the elementary steps of the reaction with silane,
2
O. Quantitative O-enrichment was observed in the
3
1
8
we conducted O isotope labelling experiments under single-
turnover conditions. Equimolar amounts of 1, Ph MeSiH, and
CN and the silanol product analyzed
2
1
8
H
2
O were reacted in CH
3
1
8
by mass spectrometry gave high O-enrichment (86%), eqn (6).
The observed enrichment is less than the expected theoretical value
starting with ca. 95% H
21
reflects the total electronic environment at silicon. However, the
1
8
experimentally measured second-order rate constants (k ) showed
1
2
O because of some oxo exchange between
1
8
16
no correlation with the Taft Rs* values. The trend observed
H
2
O and Re= O, eqn (7).
for the aliphatic silanes at least follows steric hinderance. For
n
i
example, Pr
3
SiH is more reactive than Et PrSiH. Therefore,
2
we took an analogue to the Taft steric parameter (E) that has
22
been developed for organosilicon compounds (ESi). The plot
of log(k
1
) against RESi, which is the sum of the steric ESi value
t
(
(
6)
7)
for each of the substituents, is given in Fig. 4. BuMe
2
SiH was
excluded because it was the only outlier. The relationship is
linear with a slope d* = 2.2 ± 0.3 with a correlation factor
R = 0.97. The positive value is consistent with a transition-
state that is sensitive to steric congestion of the silane substrate
and the value is larger than unity, which is observed for ester
hydrolysis.
Reactivity of the rhenium hydride complex 3
The reaction of silane with complex 1 does not yield a rhenium
hydride. Instead the rhenium is reduced slowly to an uncharacter-
10
ized rhenium(III) species. Nevertheless, the potential involvement
of an oxorhenium(V) hydride in the catalytic reaction is worth
consideration. We prepared the rhenium hydride 3 from the
reaction of the rhenium chloride 2 and tin hydride. Complex 3
20
is not very hydridic. It is stable towards water, alcohols, and even
acetic acid. However, 3 reacts with strong acids such as HOTf
(
OTf = trifluoromethane sulfonate) in acetonitrile to give H
2
and
[Re(O)(hoz) (CH CN)][OTf] (complex 1).
2
3
Discussion
Complex 1 is an effective catalyst for the preparation of silanol
from the reaction of organosilanes and water under ambient open-
Fig. 4 Taft steric correlation plot, log kobs vs. Taft steric substituent
constants. The correlation coefficient (d*) value is 2.2 (R = 0.97).
2
854 | Dalton Trans., 2009, 2850–2855
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The kinetic isotope effects (kie) of 1.3 and 1.4 for Et
reactions with H O and MeOH, respectively, are consistent with
a primary isotope effect, and comparable to the kie of 1.23
3
Si–H/D
Acknowledgements
2
This work was supported by the Chemical Sciences, Geosciences
and Biosciences Division, Office of Basic Energy Sciences, U.S. De-
partment of Energy (grant no. DE-FG02–06ER15794).
23
observed for dichlorocarbene insertion into Bu
3
SiH. Therefore,
we postulate insertion of the Si–H bond in the activated complex.
1
8
The O-labelling studies show that even under single turnover
conditions the water oxygen is exclusively transferred to the silicon
atom of silane. Thus, we suggest attack of Si, the electrophilic
site of the Si–H bond, onto a hydroxo or alkoxo ligand (a
nucleophilic site on rhenium) with concurrent hydride transfer
Notes and references
1 I. Ojima, Z. Li, J. Zhu, The Chemistry of Organic Silicon Compounds,
S. Rappoport, Y. Apeloig, ed., Wiley, New York 1998, ch. 29.
P. D. Lickiss, Adv. Inorg. Chem., 1995, 42, 147.
2
3
to afford silanol or sily ether and oxorhenium(V) hydride. The
E. G. Rochow and W. F. Gilliam, J. Am. Chem. Soc., 1941, 63, 798; G. I.
Harris, J. Chem. Soc., 1963, 5978.
+
latter reacts with solvated [H ] to evolve H
2
and regenerate the
4
5
W. Adam, R. Mello and R. Curci, Angew. Chem., Int. Ed. Engl., 1990,
cationic active catalyst. This mechanism is outlined in Scheme 1.
The activation parameters support this mechanistic model. The
2
9, 890.
(a) L. H. Sommer and J. E. Lyons, J. Am. Chem. Soc., 1969, 91, 7061;
b) E. Matarasso-Tchiroukhine, J. Chem. Soc., Chem. Commun., 1990,
‡
negative values of DS are consistent with a bimolecular reaction
(
‡
and the modest DH values are indicative of compensating
681; (c) W. Adam, H. Garcia, C. M. Mitchell, C. R. Saha-M o¨ ller and
D. Weichold, Chem. Commun., 1998, 2609.
bond making and bond breaking processes that are involved in
the insertion process. It is interesting to note that while other
6
(a) M. Lee, S. Ko and S. Chang, J. Am. Chem. Soc., 2000, 122, 12011;
(
b) K. Mori, M. Tano, T. Mizugaki, K. Ebitani and K. Kaneda,
24
25
metal oxo systems such as OsO
been shown to undergo Si–H addition across dioxo ligands
3 + 2], we do not detect Si–H insertion into the oxo ligand for
this monooxo rhenium(V) complex 1, nor for other mono-oxo
4
and Re(O)
2
(I)(PPh
3
)
2
have
New J. Chem., 2002, 26, 1536; (c) Y. Lee, D. Semoon, S. Kim, H.
Han, S. Chang and P. H. Lee, J. Org. Chem., 2004, 69, 1741.
H. Tan, A. Yoshikawa, M. S. Gordon and J. H. Espenson,
Organometallics, 1999, 18, 4753.
7
8
9
[
W. Adam, C. M. Mitchell, C. R. Saha-M o¨ ller and O. Weichold, J. Am.
Chem. Soc., 1999, 121, 2097.
26
systems.
U. Schubert and C. Lorenz, Inorg. Chem., 1997, 36, 1258.
1
1
0 E. A. Ison, R. A. Corbin and M. M. Abu-Omar, J. Am. Chem. Soc.,
2
005, 127, 11938.
1 T. W. Greene, P. G. Wuts, Protective Groups in Organic Synthesis, 2nd
edn, John Wiley and Sons, New York, 1991.
1
1
2 S. Kim and H. Chang, Bull. Chem. Soc. Jpn., 1985, 58, 3669.
3 B. A. D’Sa, D. McLeod and J. G. Verkade, J. Org. Chem., 1997, 62,
5
057.
1
4 (a) For Ir catalysts, see: X.-L. Luo and R. H. Crabtree, J. Am. Chem.
Soc., 1989, 111, 2527; (b) L. D. Field, B. A. Messerle, M. Rehr, L. P.
Soler and T. W. Hambley, Organometallics, 2003, 22, 2387.
+
1
1
1
1
1
2
5 For [CpFe(CO)(PPh
3
)] , see: S. Chang, E. Scharrer and M. Brookhart,
J. Mol. Catal. A: Chem., 1998, 130, 107.
6 J. M. Blackwell, K. L. Foster, V. H. Beck and W. E. Piers, J. Org. Chem.,
1
999, 64, 4887.
7 L. D. McPherson, M. Drees, S. I. Khan, T. Strassner and M. M. Abu-
Omar, Inorg. Chem., 2004, 43, 4036.
8 (a) J. B. Lambert, L. Kania and S. Zhang, Chem. Rev., 1995, 95, 1191;
(
b) C. A. Reed, Acc. Chem. Res., 1998, 31, 325.
9 R. Murugavel, V. Chandrasekhar and H. W. Roesky, Acc. Chem. Res.,
1
996, 29, 183.
0 E. A. Ison, E. R. Trivedi, R. A. Corbin and M. M. Abu-Omar, J. Am.
Chem. Soc., 2005, 127, 15374.
Scheme 1 Postulated mechanism for rhenium-catalyzed hydrolysis and
alcoholysis of organic silanes.
21 (a) J. H. Espenson, Chemical Kinetics and Reaction Mechanisms,
McGraw Hill, New York, 2002, p. 229; (b) L. Spialter, L. Pazdernik, S.
Bernstein, W. A. Swansiger, G. R. Buell and M. E. Freeburger, J. Am.
Chem. Soc., 1971, 93, 5682.
Another indistinguishable mechanism would involve a rate-
determining oxidative addition of the organosilane to give a
transient rhenium(VII) complex (Scheme 1). Water or alcohol
would attack the electrophilic Si atom on rhenium to give
2
2
2 F. K. Cartledge, Organometallics, 1983, 2, 425.
3 L. Spialter, W. Swansiger, L. Pazdernik and M. Freeburger,
J. Organomet. Chem., 1971, 27, C25.
2
2
4 K. Valliant-Saunders, E. Gunn, G. R. Shelton, D. A. Hrovat, Weston,
T. Borden and J. M. Mayer, Inorg. Chem., 2007, 46, 5212.
5 K. A. Nolin, J. R. Krumper, M. D. Pluth, R. G. Bergman and F. D.
Toste, J. Am. Chem. Soc., 2007, 129, 14684.
silanol or silyl ether, respectively. The cycle is completed by H
2
formation from the reaction of the rhenium oxo hydride and
+
H . Cationic rhenium(VII) has been characterized for this ligand
26 (a) G. Du and M. M. Abu-Omar, Curr. Org. Chem., 2008, 12, 1185;
b) G. Du, P. E. Fanwick and M. M. Abu-Omar, J. Am. Chem. Soc.,
(
2
system in the context of atom transfer reactions, and indeed the
resulting rhenium(VII) species was shown to exhibit electrophilic
007, 129, 5180.
2
7 E. A. Ison, J. E. Cessarich, N. E. Travia, P. E. Fanwick and M. M.
Abu-Omar, J. Am. Chem. Soc., 2007, 129, 1167.
27
reactivity.
This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 2850–2855 | 2855