Potassium-Catalyzed Hydrosilylation of Activated Olefins: Evidence for a Silyl Migration Mechanism

Article abstract of DOI:10.1021/acs.organomet.6b00160

The alkali-metal silyl [K(L)SiPh₃] (1; L = 18-crown-6 ether) catalyzed the hydrosilylation of activated C=C double bonds. Isolation and characterization of an addition product is in agreement with the anti-Markovnikov selectivity. Second-order

Full text of DOI:10.1021/acs.organomet.6b00160

Communication
pubs.acs.org/Organometallics
Potassium-Catalyzed Hydrosilylation of Activated Olefins: Evidence
for a Silyl Migration Mechanism
Valeri Leich, Thomas P. Spaniol, and Jun Okuda*
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany
S
* Supporting Information
ABSTRACT: The alkali-metal silyl [K(L)SiPh3] (1; L = 18-
crown-6 ether) catalyzed the hydrosilylation of activated CC
double bonds. Isolation and characterization of an addition
product is in agreement with the anti-Markovnikov selectivity.
Second-order kinetics for the hydrosilylation of 1,1′-diphenyl-
ethylene and the kinetic isotope effect of kH/kD = 3.1 indicate
that a silyl migration mechanism is operative.
Hydrosilylation of olefins is an efficient method to synthesize
and modify organosilicones.1 Catalysts for industrial applica-
tions usually are based on late transition metals such as
platinum.2 Alternative catalysts could avoid the use of this
expensive and toxic metal and lead to different chemo- and
regioselectivities. Several catalysts based on inexpensive and
earth-abundant main-group metals have been developed over
the past decade.3 Using well-defined hydrosilylation catalysts of
group 1 and 2 metals (K, Ca, and Sr),4 Harder and co-workers
observed both Markovnikov and anti-Markovnikov products.5a
The formation of the unusual anti-Markovnikov products was
explained by the silyl migration (Scheme 1), for which the
formation of a metal silyl is assumed.4 Hydrosilylation of CC
double bonds catalyzed by d0 and rare-earth metals occur by
the metal hydride mechanism.5b Recently we have obtained
exclusively anti-Markovnikov hydrosilylation products using
molecular alkali-metal and alkaline-earth-metal silyls.6 We have
now examined the mechanism using the structurally well-
characterized potassium triphenylsilyl [K(L)SiPh3] (1; L = 18-
crown-6 ether)6a and the substrate scope of this regioselective
hydrosilylation reaction. Isolation, characterization, and single-
crystal X-ray diffraction of an intermediate along with kinetic
studies suggest the silyl migration mechanism to be operative.
Potassium triphenylsilyl [K(L)SiPh3)] (1; L = 18-crown-6
ether) hydrosilylated 1,1′-diphenylethylene (1,1′-DPE) as a
model substrate. HSiPh3 led to full conversion within 24 h at 60
°C (Table 1, entry 1).6a The deuterated silane DSiPh3 gave the
corresponding anti-Markovnikov product also on a preparative
scale, but it took ca. 120 h for completion (Table 1, entry 2).
Using 1, styrene and trans,trans-1,4-diphenyl-1,3-butadiene
were polymerized within 0.1 and 2 h, respectively; 4-phenyl-
1-butene and 1,3-cyclohexadiene were not hydrosilylated at 60
°C (Table 1, entries 3−6). 1-Phenylcyclohexene reacted with
H3SiPh to give the cis anti-Markovnikov product in 53% yield
within 1.5 h (Table 1, entry 7). The reaction time and
conversion are much lower than for the reported hydro-
silylation of 1-phenylcyclohexadiene with H3SiPh (16 h and
90%).4 Sterically hindered triphenylethylene slowly reacted
with HSiPh3 to give the anti-Markovnikov product (Table 1,
entry 8). After 24 h, only 10% of the hydrosilylated product was
Scheme 1. Silyl Migration Mechanism Proposed by Harder4
7
1
observed in the H NMR spectrum. With the silanes H2SiPh2
and H3SiPh shorter reaction times of 3 and 1.5 h gave
quantitative conversion (Table 1, entries 9 and 10). The
hydrosilylation catalyzed by the silyl 1 is limited to activated
CC double bonds.
Received: February 26, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00160
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Table 1. Catalytic Hydrosilylation of Olefins Catalyzed by Alkali-Metal Silyl [K(L)SiPh3] (1)
a
b
1
Reaction conditions: cat. 1 (2.5 μmol), olefin (0.1 mmol), silane (0.11 mmol) in THF-d8 (0.6 mL). L = 18-crown-6 ether. Determined by H
c
NMR spectroscopy. One diastereomer, cis product.
Hydrosilylation catalyzed by alkali or alkaline-earth metals
(K, Ca, and Sr) gave both Markovnikov and anti-Markovnikov
products, with presumably a metal hydride as the catalytically
active species.4,8 Potassium triphenylsilyl 1 led to anti-
Markovnikov products selectively with a metal silyl as the
catalytically active species. Harder explained the opposite
regioselectivity by a mechanism that involves silyl migration,9
a process well established for transition-metal-catalyzed hydro-
silylation of CC double bonds.10
soluble in aliphatic hydrocarbons. Elemental analysis and NMR
spectroscopic data agree with the proposed formula (see the
Supporting Information).
A single crystal of 2 was obtained from a concentrated
benzene solution at 25 °C. The crystal structure from X-ray
diffraction reveals a one-dimensional coordination polymer in
the solid state formed by alternating units of [K(L)-
Ph2CCH2SiPh3] (Figure 1 and the Supporting Information
for a graphical representation of the coordination polymer).
The potassium atom is coordinated by six oxygen atoms of the
18-crown-6 ether within the plane of the crown ether (C5−
K1−Oav = 90.60(4)°). K···CPh interactions are indicated by K−
C distances of 3.0817(16)−3.5535 (15) Å, which fall in the
To find out if the proposed product B of formal addition is
formed, we treated the alkali-metal silyl 1 with 1,1′-DPE in
THF at 25 °C. The solution immediately turned red. The
product [K(L)Ph2CCH2SiPh3] (2; L = 18-crown-6 ether) was
isolated from THF/n-pentane solution after 12 h at −30 °C
(Scheme 2). 2 is soluble in THF and benzene but sparingly
11
range of unsymmetrical K···CPh and K−Colefin interactions.12
Ionic interactions of the potassium ion with the phenyl groups
of the 1,1′-DPE moiety could stabilize the transition state
leading to the anti-Markovnikov product.
Scheme 2. Formation of the Addition (“Insertion”) Product
1
a
The H NMR spectrum of 2 in THF-d8 agrees with the
[K(L)Ph2CCH2SiPh3] (2)
proposed formula, and the 29Si{1H} NMR spectrum of 2 shows
one singlet at δ −17.35 ppm (see the Supporting Information).
Next, we investigated the rate-determining C−H bond
formation of the addition (“insertion”) product 2 with silanes
(Scheme 3). The σ-bond metathesis proceeded more slowly
than the addition. Heating a mixture of [K(L)Ph2CCH2SiPh3]
a
L = 18-crown-6 ether.
B
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Communication
decomposition, in agreement with the selectivity of this process
and the stability of the insertion product 2.
We explain this reactivity by a mechanism that involves
formal silyl migration. 1,1′-DPE inserts irreversibly into the
metal−silyl bond of [K(L)SiPh3] (1). The resonance-stabilized
addition (insertion) product [K(L)Ph2CCH2SiPh3] (2) reacts
with HSiPh3 under σ-bond metathesis (protonolysis) to
regenerate the catalyst [K(L)SiPh3] (1) and to give the anti-
Markovnikov product 3. This step is rate-determining with k1
≫ k2; the C−H bond forming reaction is reversible. A value of
K = k2/k−2 = 1.91 was estimated for the reversible σ-bond
metathesis step.
The hydrosilylation of 1,1′-DPE to give 3 was monitored by
1H NMR spectroscopy. In THF-d8 at 60 °C, second-order
kinetics was found with rate constants between kH = 8.9(5) ×
10−4 and [4.44(3)] × 10−3 L mol−1 s−1 (Table 2, entries 1−4).
Figure 1. Molecular structure of 2 in the solid state. Displacement
ellipsoids are shown at 50% probability; all hydrogen atoms are
omitted for clarity. Selected interatomic distances (Å) and angles
(deg): K1−C4 3.5535(15), K1−C5 3.0817(16), K1−C6 3.3400(16),
K1−C12 3.3095(16); C4−C5−C6 121.80(15), C5−K1−Oav
90.60(4), C1−C2−C3−C8 172.53(12), C1−C2−C9−C10
150.62(13).
Table 2. Rate Constants of the Hydrosilylation of 1,1′-DPE
with (H/D)SiPh3
Scheme 3. Stoichiometric Reactions Involving C−H Bond
a
Formation
a
b
entry
silane
1 (mol %)
k (L mol−1 s−1
8.9(5) × 10−4
)
1
2
3
4
5
HSiPh3
HSiPh3
HSiPh3
HSiPh3
DSiPh3
4.5
10
1.97(5) × 10−3
3.15(5) × 10−3
4.44(3) × 10−3
8.2(1) × 10−4
19
27
17
a
Conditions: 100 μmol of 1,1′-DPE, 105 μmol of (H/D)SiPh3, 33.3
b
μmol of C6Me6 in THF-d8 (total volume 0.7 mL). Determined from
logarithmic plots (up to 60% conversion).
a
Results with different catalyst loadings indicate direct
involvement of the metal silyl [K(L)SiPh3] (1) by a reaction
rate that has a first-order dependence on the concentration of
the catalyst (see the Supporting Information). Rate constants
between kobs = 0.67 × 10−4 and 3.93 × 10−3 L mol−1 s−1 (T =
0−30 °C) were observed for the hydrosilylation of styrene with
HSiMe2Ph catalyzed by a silica-supported platinum catalyst15
and kobs = 83 × 10−3 and 252 × 10−3 L mol−1 s−1 (T = 25 °C)
were reported for the hydrosilylation of (allyloxy)benzene with
1,1,1,3,3,5,5-heptamethyltrisiloxane catalyzed by dichloro-
(dicyclopentadiene)platinum.16
With 1,1′-DPE and 10 equiv of HSiPh3, we observed pseudo-
first-order dependence (kobs = 3.69(5) × 10−3 s−1). A rate
constant of kobs = 1.7 × 10−3 s−1 (T = −38 °C) was reported for
the hydrosilylation of 3,3′-dimethyl-1-butene with HSiEt3 (10−
30 equiv) and a palladium complex.10
L = 18-crown-6 ether. Legend: (a) reaction of 2 with (H/D)SiPh3
(60 °C, 2 h) and the reverse reaction (25 °C, 10 min) in THF-d8; (b)
reaction of 2 with (H/D)SiPh3 in the presence of 1,1′-DPE in THF-d8.
(2) with the silanes (H/D)SiPh3 in THF-d8 for 2 h at 60 °C led
to 3 and 3-d1 in yields up to 40% (Scheme 3a). Since prolonged
reaction times did not lead to higher conversion, the process
appears to be reversible. The alkali-metal silyl [K(L)SiPh3] (1)
reacted with the hydrosilylation products 3 and 3-d1 in THF-d8
within 10 min at 25 °C to give the same equilibrium mixture
1
(see the Supporting Information for in situ H NMR spectra).
The silyl anion (SiPh3)−, as a strong Brønsted base
(pKa(HSiPh3) = 35.1),13 deprotonates weaker acids such as
3. We estimate a pKa value of 34−35 for 3, as pKa = 33.4 is
reported for Ph2CH213 in combination with the acid-weakening
effect of the β-silicon atom.14 Similar results were found when
arylmethanes were metalated with [KSiPh3].13
Deuterium labeling experiments showed a kinetic isotope
effect (KIE). Since deuterium was not incorporated into the
phenyl groups, the hydrosilylation of 1,1′-DPE with DSiPh3 is
highly selective. A value for KIE of kH/kD = 3.1 (Table 2, entry
5) indicates that a Si−H bond is cleaved during the rate-
determining step, in agreement with the proposed silyl
mechanism (Scheme 4). Similar results were found for the
hydrosilylation of CC double bonds catalyzed by the
Furthermore, stoichiometric reactions revealed that the
potassium silyl 1 acts as the active species and the addition
product 2 as the resting state of the catalyst. To avoid the back-
reaction of the metal silyl 1 with the hydrosilylation product 3,
we added 1 equiv of 1,1′-DPE to the addition product 2 and
(H/D)SiPh3. The metal silyl 1 was expected to quickly react
with 1,1′-DPE to regenerate 2. After the mixture was heated in
THF-d8 for 2 h at 60 °C, we observed a 1:1 composition of 2
and 3 (or 3-d1) (Scheme 3b). The 1H NMR spectrum does not
show signals from other products due to side reactions or
zerovalent platinum complex [Pt(PhCHCH2)3] (kH/kD
=
3.6)17 and the cationic ruthenium silylene [Cp*(iPr3P)-
RuH2(SiHMes)] [B(C6F5)4] (kH/kD = 1.5).18
C
DOI: 10.1021/acs.organomet.6b00160
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[SiH2Ph3] with 1,1′-DPE gave [K(18-crown-6)][SiPh3] and
Ph2CHCH3. Therefore, we believe that a metal silyl species is
responsible for the anti-Markovnikov regioselectivity.
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Scheme 4. Modified Silyl Migration Mechanism
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In conclusion, the alkali-metal silyl [K(L)SiPh3] (1; L = 18-
crown-6 ether)6a was shown to catalyze the hydrosilylation of
activated CC double bonds. We have provided experimental
evidence that a silyl migration mechanism, as first proposed by
Harder et al. for s-block metal catalysts,4 is responsible for the
anti-Markovnikov regioselectivity. The structure of the isolated
addition product [K(L)Ph2CCH2SiPh3] (2) supports this
regioselectivity. A value for KIE of kH/kD = 3.1 confirms σ-
bond metathesis (protonolysis) as the rate-determining,
reversible step in the hydrosilylation cycle. This model could
be useful for the design of other earth-abundant metal
catalysts3,4 for selective hydrosilylation of CC double
bonds leading to anti-Markovnikov products.
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ASSOCIATED CONTENT
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S
* Supporting Information
(13) Buncel, E.; Venkatachalam, T. K. J. Organomet. Chem. 2000, 604,
208−210.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00160.
(14) Bausch, M. J.; Gong, Y. J. Am. Chem. Soc. 1994, 116, 5963−
5964.
(15) Miao, Q. J.; Fang, Z.-P.; Cai, G. P. Catal. Commun. 2003, 4,
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269.
Experimental details, details of the kinetic experiments,
NMR spectra for all compounds, and crystallographic
data for 2 (CCDC reference number 1454773) (PDF)
Crystallographic data for 2 (CIF)
(18) Fasulo, M. E.; Lipke, M. C.; Tilley, T. D. Chem. Sci. 2013, 4,
3882−3887.
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.O.: jun.okuda@ac.rwth-aachen.de.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Cluster of Excellence “Tailor-Made Fuels from
Biomass” for financial support and C. Lhotzky for experimental
assistance.
REFERENCES
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D
DOI: 10.1021/acs.organomet.6b00160
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