Regioselective Hydrosilylation of Olefins Catalyzed by a Molecular Calcium Hydride Cation

Article abstract of DOI:10.1002/anie.201909585

Chemo- and regioselectivity are often difficult to control during olefin hydrosilylation catalyzed by d- and f-block metal complexes. The cationic hydride of calcium [CaH]⁺ stabilized by an NNNN macrocycle was found to catalyze the regioselective hydrosilylation of aliphatic olefins to give anti-Markovnikov products, while aryl-substituted olefins were hydrosilyated with Markovnikov regioselectivity. Ethylene was efficiently hydrosilylated by primary and secondary hydrosilanes to give di- and monoethylated silanes. Aliphatic hydrosilanes were preferred over other commonly employed hydrosilanes: Arylsilanes such as PhSiH₃ underwent scrambling reactions promoted by the nucleophilic hydride, while alkoxy- and siloxy-substituted hydrosilanes gave isolable alkoxy and siloxy calcium derivatives.

Full text of DOI:10.1002/anie.201909585

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Accepted Article
Title: Regioselective Hydrosilylation of Olefins Catalyzed by a
Molecular Calcium Hydride Cation
Authors: Jun Okuda, Danny Schuhknecht, Thomas P. Spaniol, and
Laurent Maron
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http://dx.doi.org/10.1002/ange.201909585

10.1002/anie.201909585
Angewandte Chemie International Edition
Regioselective Hydrosilylation of Olefins Catalyzed by a
Molecular Calcium Hydride Cation
Danny Schuhknechta, Thomas P. Spaniola, Laurent Maronb and Jun
Okudaa,*
[a] D. Schuhknecht, Dr. T. P. Spaniol, Prof. Dr. J. Okuda*, Institute of Inorganic Chemistry,
RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany, email:
jun.okuda@ac.rwth-aachen.de
[b] Prof. Dr. L. Maron, CNRS, INSA, UPS, UMR 5215, LPCNO, Université de Toulouse 135
avenue de Rangueil, 31077 Toulouse, France
Supporting information for this article is available on the WWW under
http//:www.angewandte.chemie.org or from the author
Keywords: calcium hydride • hydrosilylation • alkaline earth metal • regioselective catalysis • Lewis
acids
Table of Contents
The calcium hydride cation [CaH]+ catalyzes the regioselective hydrosilylation of ethylene, α-
olefins and styrene derivatives. Aliphatic hydrosilanes are preferred over commonly employed
silanes such as PhSiH3 as the latter undergoes scrambling reactions.
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10.1002/anie.201909585
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ABSTRACT: Chemo- and regioselectivity are often difficult to control during olefin
hydrosilylation catalyzed by d- and f-block metal complexes. The cationic hydride of calcium
[CaH]+ stabilized by an NNNN macrocycle was found to catalyze the regioselective
hydrosilylation of aliphatic olefins to give anti-Markovnikov products, while aryl substituted
olefins were hydrosilyated with Markovnikov regioselectivity. Ethylene was efficiently
hydrosilylated by primary and secondary hydrosilanes to give di- and mono-ethylated silanes.
Aliphatic hydrosilanes were preferred over other commonly employed hydrosilanes:
Arylsilanes such as PhSiH3 underwent scrambling reactions promoted by the nucleophilic
hydride, while alkoxy- and siloxy-substituted hydrosilanes gave isolable alkoxy and siloxy
calcium derivatives.
Hydrosilylation of olefins is of considerable importance, since organosilicon intermediates and
fine chemicals can be synthesized by addition of a Si-H function to unsaturated C=C double
bonds.[1] This reaction is efficiently catalyzed by transition metal catalysts,[2] in particular
platinum (Speier and Karstedt catalysts).[3] In the context of current quests for inexpensive,
innocuous, and earth-abundant alternatives, base metal catalysts containing Mn, Fe, Co, and
Ni have been reported.[2b, 4] Systems based on rare earth metals[5] were also introduced as
hydrosilylation catalysts, but operate through the combination of hydrometalation and σ-bond
metathesis steps instead of the sequence of oxidative addition/reductive elimination as
commonly observed for late transition metal catalysts (Chalk-Harrod mechanism).[6] Catalysts
based on p-block elements also promote this reaction.[7] Harder et al. reported on the use of
group 1 and 2 metal benzyl complexes for the hydrosilylation of styrene derivatives with
arylsilanes (Scheme 1);[8] the molecular calcium hydride [(DIPPBDI)(thf)Ca(µ-H)]2 (DIPPBDI =
CH[C(Me)N-DIPP]2, DIPP = 2,6-iPr2C6H3) was found to catalyze the hydrosilylation of 1,1-
diphenylethylene (1,1-DPE) with phenylsilane.[9] The triphenylsilyl complex [Ca(SiPh3)2(thf)4]
hydrosilylated styrene derivatives with anti-Markovnikov selectivity as did related alkali metal
silanide and hydrosilicate complexes.[10] Although simple Na[HBEt3] was reported to
hydrosilylate styrene derivatives with aryl hydrosilanes, no activity was observed when
aliphatic olefins like 1-hexene or silanes such as Et3SiH were used.[11]
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10.1002/anie.201909585
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a) Previously reported hydrosilylation by s-block metal catalysts
Hydrosilanes limited to aryl hydrosilanes
M = Li, Na, K, Mg, Ca, Sr
SiH2Ph
Me
[M]-H
- metal-hydride catalyst
- Markovnikov products
+
+
PhSiH3
PhSiH3
Ph
Ph
Ph
[M]-SiR3
H
- metal-silyl catalyst
- anti-Markovnikov products
SiH2Ph
Ph
b) This work: Hydrosilylation of
-olefins and styrene derivatives with alkyl hydrosilanes
[CaH]+
[CaH]+
SiH2nOct
Me
H
SiH2nOct
+
nOctSiH3
R
alkyl
Ph
R = alkyl
R = Ph
Scheme 1. a) Previous reports of s-block metal catalyzed hydrosilylation. b) Hydrosilylation catalyzed
by calcium hydride cation [CaH]+.
In contrast to olefin hydrogenation by molecular alkaline earth metal hydrides,[13]
hydrosilylation of unactivated alkenes using aliphatic silanes by s-block metal catalysts
remains elusive.[12] Herein we report on the hydrosilylation of ethylene, α-olefins, and styrene
derivatives using a molecular calcium hydride cation [CaH]+ stabilized by the macrocyclic
polyamine ligand Me4TACD (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane).
2
N
N
N
N
N
N
[Ca(C3H5)2]
N
N
N
H
H
THF/n-pentane
+ RSiH3
N
+
Ca
Ca
O
Ca
2 [BAr4]
O
[BAr4]
0.5
N
N

(C3H5)RSiH2
THF
30 °C
[(Me4TACD)H][BAr4]

1
70 °C  25 °C
Ar = C6H3-3,5-Me 2
2
3
(89%)
(83%)
Scheme 2. Improved synthesis of the cationic calcium hydride 3 starting from the allyl calcium cation 2.
Figure 1. Structure of the molecular cations in 2 and 3. Displacement parameters are shown at the 50%
probability level. Anions, lattice solvents, and hydrogen atoms except for the hydrides are omitted for
clarity.
Recently we isolated cationic calcium hydrides by hydrogenolysis of benzyl complexes
[(L)Ca(η1-CH2Ph)x(thf)(2-x)][BAr4](2-x) (L = Me4TACD; x = 1,2; Ar = C6H3-3,5-Me2), which were
limited by their solubility, stability, and scalability.[13a] When bis(allyl)calcium[14] was treated with
the conjugated Brønsted acid of the macrocyclic ligand [LH][BAr4] (1),[15] the allyl calcium
cation [(L)Ca(η3-C3H5)(thf)][BAr4] (2) was obtained in high yield (Scheme 2).[16] Although 2 was
inert towards H2 even at 70 °C, the stoichiometric reaction with RSiH3 (R = Ph, nOct) in THF
gave the dimeric calcium hydride cation [(L)2Ca2(-H)2(thf)][BAr4]2 (3) alongside the
corresponding allylsilane (C3H5)RSiH2 after 5 min at 25 °C. A crystal structure analysis of 3 by
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10.1002/anie.201909585
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X-ray diffraction revealed a non-symmetrical dimer with one coordinated THF as observed for
the Me4TACD-stabilized hydride of divalent ytterbium.[17] The 1H NMR spectrum of 3 in [D8]THF
shows a Ci symmetric structure even at –80 °C, indicating fast reversibility of the THF
coordination (see SI). Calculations on the DFT level suggest exothermic THF coordination (ca.
33 kJ/mol, see SI) to the solvent-free dimer,[13a] in line with labile solvation of other molecular
calcium hydrides.[18]
The molecular cation in 3 was highly reactive toward hydrosilanes: Treating a solution of 3 with
the commonly employed hydrosilylation reagent PhSiH3 led to broad resonances for the
calcium- and silicon-bonded hydrides in the 1H NMR spectrum. Cross peaks appeared in an
EXSY NMR experiment (see SI), indicating their exchange on the NMR time scale. After 30
min, signals for SiH4 and Ph2SiH2 were observed as the result of reversible aryl exchange.[19]
Addition of D2 (1 bar) lowered the intensity of the hydride resonances and signals of H2 and
HD appeared within 3 d. These exchange processes may be explained by reversible
coordination of the metal hydride to the silicon center to form a hypervalent silicate.[20] Although
such a species was not observed by 1H and 29Si NMR spectra even at –90 °C, NOESY signals
indicate close proximity of the silicon hydrides to the methyl protons in the Me4TACD ligand
(see SI). Transfer of a hydride from [CaH]+ to the hydrosilane to give a solvent-separated ion
pair with [PhSiH4]- anion remains undetected but cannot be ruled out.[8] An experiment
performed with nOctSiH3 under identical conditions did not show any formation of nOct2SiH2 or
n
SiH4 after 24 h; however, deuterolysis of OctSiH3 in the presence of 3 (10 mol%) to give
nOctSiH3-xDx (x = 0–3) and HD/H2 was observed after 7 d at 25 °C under 1 bar of D2. When
alkoxy hydrosilanes were used, irreversible hydride transfer from calcium to silicon occurred
(Scheme 3). Reaction with MeSiH(OMe)2 gave the dimeric methoxy calcium complex 4 which
was independently synthesized from 3 and methanol. Commonly used O(SiMe2H)2 also
underwent Si-O cleavage to selectively give the dimethylsiloxy complex 5 and Me2SiH2 within
2 h at 70 °C. Coordination of the silyl ether to the Lewis acidic calcium center, as recently
observed for a cationic magnesium complex, might facilitate the ether cleavage.[21]
Scheme 3. Reaction of calcium hydride 3 with alkoxy- and siloxy-substituted hydrosilanes.
To assess the suitability of different hydrosilanes, complex 3 was tested in the hydrosilylation
of the parent olefin ethylene. At ambient conditions (25 °C, 1 bar ethylene), PhSiH3 gave
Et2PhSiH within 6 h (Table 1, entry 1). Because of competing aryl exchange, ca. 5% of
Ph2EtSiH and Et3SiH were also formed at 70 °C (Table 1, entry 2). Higher selectivity was
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10.1002/anie.201909585
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observed with electron-rich (para-R-C6H4)SiH3 (Table 1, entry 3, R = MeO and 4, R = Me2N),
while electron-deficient (para-CF3-C6H4)SiH3 showed immediate scrambling followed by
decomposition of the catalyst (Table 1, entry 5). Alkyl-substituted primary hydrosilanes were
selectively converted within 30–35 min (Table 1, entries 6–9). Monitoring the reaction by NMR
spectroscopy revealed full consumption of the primary hydrosilane prior to the second addition
of ethylene. While the selectivity with Ph2SiH2 was lower due to scrambling (Table 1, entry 10),
alkyl-substituted secondary silanes were fully converted within 15–20 min (Table 1, entries 11–
14). Hydrosilylation of sterically more demanding iBu2SiH2 was incomplete after 60 min (Table
1, entry 15), while bulkier hydrosilanes Mes2SiH2 and tBu2SiH2 as well as tertiary hydrosilanes
Et3SiH and Me2EtSiH did not react at all (Table 1, entries 16 and 17).
Table 1. Hydrosilylation of ethylene by complex 3.a
Entry Silane
Productb
t
Conv.
TOF
[h-1]d
[min] [%]c
1[e]
2
PhSiH3
PhSiH3
Et2PhSiH
Et2PhSiH
360
30
90
90
99
12
144
160
3
(para-MeO-
Et2(para-MeO-
30
C6H4)SiH3
C6H4)SiH
4
(para-Me2N-
Et2(para-Me2N-
30
30
99
0
160
0
C6H4)SiH3
C6H4)SiH
5[e]
(para-CF3-
–
[f]
C6H4)SiH3
n
6
nBuSiH3
Et2 BuSiH
35
30
30
30
15
15
15
15
20
60
60
99
99
99
90
90
99
99
99
99
28
0
144
160
160
144
144
160
160
160
120
11
n
7
nHexSiH3
nOctSiH3
CySiH3
Et2 HexSiH
n
8
Et2 OctSiH
9
Et2CySiH
EtPh2SiH
EtPhMeSiH
EtnOctMeSiH
EtCyMeSiH
Et3SiH
10
11
12
13
14
15
16
Ph2SiH2
PhMeSiH2
nOctMeSiH2
CyMeSiH2
Et2SiH
iBu2SiH2
EtiBu2SiH
–
Mes2SiH2
tBu2SiH2
/
0
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17
Et3SiH
Me2EtSiH
/
–
60
0
0
a) 0.1 mmol of substrate in 0.6 mL of [D8]THF, 25 µmol 1,4-
(SiMe3)2C6H4. b) Characterized by NMR spectroscopy and GC MS.
1
c) Determined by H NMR spectroscopy. d) Calculated based on
the amount of ethylene consumed. e) 25 °C. f) Decomposition of
catalyst.
The Lewis acidity of the silicon center of the hydrosilane could promote the nucleophilic
addition of the metal hydride to form a hypervalent silicate, but also facilitates the aryl
exchange.[19b] Alkyl groups that lower the Lewis acidity increase the chemoselectivity and are
favored for hydrosilylation catalyzed by 3. As the homologous magnesium hydride cation
[MgH]+ did not show any reaction with PhSiH3 under comparable conditions,[22] the
combination of the Lewis acidic metal center with the nucleophilicity of the hydride ligand
appears crucial.
1
While monitoring the catalysis by H NMR spectroscopy, no calcium ethyl species was
observed. Only after the hydrosilane was fully consumed or when hydride 3 was dissolved in
[D8]THF and pressurized with ethylene (1 bar), was formation of an ethyl calcium species
detected by characteristic signals at  –1.02 (q) and 1.26 (t) ppm for the methylene and methyl
protons, respectively.[24] After 10 min at 25 °C, higher n-alkyl calcium species were also evident
from their characteristic resonances, indicating additional insertion of ethylene into the
calcium-n-alkyl bond (see SI). While oligomerization of ethylene was not observed for solid
and [(DIPPBDI)Ca(µ-H)]2,[24] strontium hydride [(DIPePBDI)Sr(µ-H)]2 (DIPeP = 2,6-
[23]
[CaH2]∞
(pent-3-yl)2-phenyl) formed oligoethylene at room temperature.[20a] Unlike the BDI-stabilized
ethyl complexes, the highly reactive cationic n-alkyl calcium derivative of 3 could not be
isolated as it readily decomposed in THF solution (t1/2 <10 min at 25 °C, 20 min at –20 °C) to
give short alkanes (C2–C6) and other undefined species. Only fully protonated alkanes were
detected when the reaction was carried out in [D8]THF, indicating that the reaction with the
ligand backbone as in related lanthanide complexes[25] is favored over solvent deprotonation.
Higher -olefins such as 1-octene and 1-hexene were hydrosilylated at 70 °C to give the anti-
Markovnikov products with high regioselectivity. Depending on the stoichiometry and the
hydrosilane, secondary (Table 3, entries 1 and 2) or tertiary (Table 3, entries 3 and 4) silanes
formed, while no reaction occurred with the tertiary products or Et3SiH (Table 3, entry 5). The
silanes could be readily isolated after the catalyst 3 was precipitated with n-pentane and
filtered off (see SI). Hydrosilylation of 1,4-hexadiene only gave 4-alkenylsilane, and as for 2-
hexene, no reaction of the internal double bond was detected even after prolonged reaction
time or with an excess of hydrosilane (Table 3, entries 6 and 7). Likewise, the internal double
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10.1002/anie.201909585
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bond in 4-vinylcyclohexene, cyclohexene, norbornene, as well as the 1,1-disubstituted double
bond in 2-ethyl-1-butene (Table 3, entries 8–11) were not hydrosilylated. Allyl ethers were not
hydrosilylated as the nucleophilic hydride was readily converted into the methoxy complex 4
under formation of propene (Table 3, entry 12).
Table 2. Regioselective hydrosilylation of aliphatic olefins by complex 3.a
Entry
1
Olefin
Silane
Productb
t
Conv.
[%]c
TOF
[h-1]
[h]
n
Et2SiH2
Et2 HexSiH
24
96
0.8
2
nOctSiH3
nOctSiH3
Et2SiH2
Et3SiH
nOct2SiH2
nOct3SiH
24
24
24
24
24
95
87
95
0
0.8
0.7
0.8
0
3d
4
n
Et2 OctSiH
5
–
–
6
Et2SiH2
95
0.8
7
8
nOctSiH3
nOctSiH3
24
24
0
0
70
0.6
9
nOctSiH3
nOctSiH3
–
–
24
24
0
0
0
0
10
11
12
nOctSiH3
nOctSiH3
–
24
0
0
0
0
[e]
–
0.1
a) Substrate (0.1 mmol) in [D8]THF (0.6 mL), 1,4-(SiMe3)2C6H4 (25 µmol) as
internal standard. b) Characterized by NMR spectroscopy and GC MS. c)
1
Determined by H NMR spectroscopy. d) 0.5 equiv. of silane e) Formation of
methoxide complex 4 and propene.
Styrene and PhSiH3 reacted with moderate selectivity to the Markovnikov-silylated product,
while by-products included unreacted Ph2SiH2 and the dibenzylsilane (PhCHMe)2SiH2, which
were formed from SiH4 through competing silane scrambling (Table 3, entry 1). Electron-rich
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(para-MeO-C6H4)SiH3 gave a similar product mixture with higher selectivity for the expected
monoalkylated arylsilane (Table 3, entry 2). Slower, but highly selective conversion was
observed for nOctSiH3, as no silane scrambling was evident after 90 min (Table 3, entry 3). In
contrast to the hydrosilylation of α-alkenes, no hydrosilylation using secondary Et2SiH2 was
detected, giving only oligostyrene after 6 h (Table 3, entry 4). 1,1-DPE was selectively
n
hydrosilylated with primary OctSiH3 after 4 h at 70 °C (Table 3, entry 5), slower than
hydrosilylation by [(DMAT)2Ca(thf)2] (DMAT = α-Me3Si-2-Me2N-benzyl) using PhSiH3 in
benzene.[8] While this calcium catalyst gave the anti-Markovnikov product when the reaction
was carried out in THF, complex 3 gave the Markovnikov product exclusively. This indicates
that the hydride-insertion mechanism is operative for 3 even in THF.[8, 10c] Again, the sterically
demanding tertiary carbanion [Ph2CMe]– readily formed by insertion remained unreacted after
6 h in presence of secondary Et2SiH2 (Table 3, entry 6), in line with a metal-centered σ-bond
metathesis. α-Methylstyrene was also selectively hydrosilylated as were the internal double
bonds in E- and Z-stilbene, but longer reaction times were required (Table 3, entries 7–9).
Higher substituted 1-phenylcyclohexene as well as tri- and tetraphenylethylene did not show
any conversion (Table 3, entries 10–12). Triphenyl(vinyl)silane gave a mixture of both
regioisomers (Table 3, entries 13), formed through competing insertion to give either the
sterically favored linear or the α-silicon stabilized branched carbanion (Ph3SiCHCH3)–.[11, 26]
Table 3. Regioselective hydrosilylation of activated olefins by complex 3.a
Conv.
[%]c
t
TOF
[h-1]
Entry
Olefin
Silane
Productb
[h]
1
2
PhSiH3
0.5
0.5
60d
24
32
(para-MeO-
81d
C6H4)SiH3
Ph
3
4
nOctSiH3
Et2SiH2
1.5
6
99
0
13.3
0
oligostyrene
5
nOctSiH3
4
99
5
6
7
8
9
Et2SiH2
nOctSiH3
nOctSiH3
nOctSiH3
–
6
0
0
48
22
22
94
99
99
0.6
0.9
0.9
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10
11
nOctSiH3
nOctSiH3
–
–
24
24
0
0
0
0
12
13
nOctSiH3
nOctSiH3
–
24
16
0
0
72/28
1.3
a) Substrate (0.1 mmol) in [D8]THF (0.6 mL) 1,4-(SiMe3)2C6H4 (25 µmol) as internal
standard. b) Characterized by NMR spectroscopy and GC MS. c) Determined by 1H
NMR spectroscopy. d) Ar2SiH2 + (MeCHPh)2SiH2 (diastereomers) as by-products.
In conclusion, the cationic calcium hydride 3 catalyzed the hydrosilylation of ethylene and -
olefins with anti-Markovnikov selectivity as well as styrene derivatives with Markovnikov
selectivity. Selective catalysis was observed with (alkyl)hydrosilanes, whereas PhSiH3 as well
as alkoxy- and siloxy-substituted hydrosilanes underwent nucleophilic substitution of the
hydride. Using nOctSiH3, the following order of TOF for 3 was established: ethylene > styrene
> 1-octene. All experimental evidence suggests that [CaH]+ forms an alkyl calcium complex[24]
as the result of hydrometalation of olefin followed by σ-bond metathesis with the hydrosilane
to give the hydrosilylated product and [CaH]+.
Notes
The authors declare no competing financial interest.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft through the International Research Training
Group “Selectivity in Chemo- and Biocatalysis” (GRK 1628) for financial support, F. Ritter and
M. Paul for assistance with X-ray crystallography, and Dr. G. Fink for NMR measurements.
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