Hydrosilylation catalysis by an earth alkaline metal silyl: Synthesis, characterization, and reactivity of bis(triphenylsilyl)calcium

Article abstract of DOI:10.1039/c3cc49308c

Bis(triphenylsilyl)calcium [Ca(SiPh₃)₂(thf)] obtained in high yield as a crystalline ether adduct catalyzes the hydrosilylation of activated C-C double bonds efficiently and regioselectively. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc49308c

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Hydrosilylation catalysis by an earth alkaline metal
silyl: synthesis, characterization, and reactivity of
bis(triphenylsilyl)calcium†
Cite this: Chem. Commun., 2014,
50, 2311
Received 7th December 2013,
Accepted 7th January 2014
Valeri Leich,a Thomas P. Spaniol,a Laurent Maronb and Jun Okuda*a
DOI: 10.1039/c3cc49308c
www.rsc.org/chemcomm
Bis(triphenylsilyl)calcium [Ca(SiPh3)2(thf)] obtained in high yield as a
crystalline ether adduct catalyzes the hydrosilylation of activated
C–C double bonds efficiently and regioselectively.
Catalytic hydrosilylation of alkenes is a commercially utilized
process for the efficient synthesis of organosilicon compounds.1
Potent catalysts such as Speier’s2 and Karsted3 catalysts are
based on late transition metals. Recently attention has shifted
to more inexpensive alternatives based on main group metals
including AlCl3 and B(C6F5)3.4 The first series of well-defined
catalysts based on s-block metals (K, Ca, and Sr), with presum-
ably a metal hydride as the catalytic active species, was intro-
duced by Harder et al.5 A metal silyl was also implicated as active
species in the hydrosilylation of C–C double bonds. Although used
Scheme 1 Synthesis of calcium silyls 2a and 2a.
2a crystallizes in the monoclinic space group P21/c with Z = 2.
in synthetic chemistry,6 metal silyls have never been applied as The octahedral calcium atom acts as the inversion center with two
hydrosilylation catalysts before. Herein we report that structurally silyl ligands (Ca–Si: 3.1503(8) Å) in the axial and four THF ligands
well-characterized bis(triphenylsilyl)calcium (2a) catalyzes the regio- in the equatorial positions. The angles Si1–Ca1–Si10 of 180.001,
selective hydrosilylation of aromatic olefins.
Si1–Ca1–O1 of 89.64(5)1 and Si1–Ca1–O2 of 86.56(5)1 suggest an
Bis(triphenylsilyl)calcium, [Ca(SiPh3)2(thf)4] (2a), was prepared octahedral geometry (Fig. 1). Distorted trigonal bipyramidal
in quantitative yield by salt metathesis of anhydrous CaI2 and two coordination geometry was observed for [Ca{Si(SiMe3)3}2(thf)3].8
equivalents of [KSiPh3(thf)] (1) (Scheme 1).7 The pale yellow Metal-to-ligand distances of 2a are similar to those of the
product is soluble in THF and benzene, but insoluble in aliphatic calcium-silyl [Ca{Si(SiMe3)3}2(thf)3].
hydrocarbons. It decomposes with a half life of t1/2 = 7 d in THF
The THF ligands in 2a are labile and easily replaced with
and t1/2 = 40 d in benzene. HSiPh3 was identified by H NMR triglyme to give [Ca(SiPh3)2(triglyme-k4)(thf)] (2b) in 70% yield
spectroscopy as the major decomposition product in solution. (Scheme 1). The solid state structure of the adduct 2b shows,
Single crystals of 2a were obtained from a saturated THF/pentane according to single crystal X-ray analysis, a pentagonal bipyramidal
solution upon cooling to ꢀ30 1C and characterized by X-ray coordination geometry with the silyl ligands in the axial and
1
diffraction.
triglyme and one THF ligand in the equatorial positions (ESI†).
Ca–Si bond lengths are 3.175(3) and 3.242(3) Å. Other bond lengths
and angles are comparable to those of known calcium triglyme
complexes.9 Compound 2b decomposes with a half life of t1/2 = 12 h
in THF, methyltriphenylsilane was identified as a major decom-
a Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1,
52056 Aachen, Germany. E-mail: jun.okuda@ac.rwth-aachen.de
b
´
Universite de Toulouse et CNRS INSA, UPS, CNRS, UMR 5215, LPCNO,
1
135 avenue de Rangueil, 31077 Toulouse, France.
position product by H NMR spectroscopy.
E-mail: Laurent.maron@irsamc.ups-tlse.fr
DFT calculations at the B3PW91 level were carried out to
understand the nature of the Ca–Si bond in 2a. The optimized
structure compares well with the crystal structure, represented
by a Ca–Si bond length of 3.215 Å (0.06 Å longer than in 2a).
† Electronic supplementary information (ESI) available: Experimental section,
details of DFT calculations, NMR spectra, and X-ray crystallographic data. CCDC
963862 (2a) and 963863 (2b). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3cc49308c
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Fig. 1 Molecular structure of 2a. Displacement ellipsoids are shown at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths
[Å] and angles [1]: Ca1–Si1 3.1503(8), Ca1–O1 2.438(2), Ca1–O2 2.3838(19),
Si1–Ca1–Si10 180.00, Si1–Ca1–O1 89.64(5), Si1–Ca1–O2 86.56(5),
Ca1–Si1–C1 108.73(8), Ca1–Si1–C7 122.11(8), Ca1–Si1–C13 115.38(8).
Scheme 2 Reaction of 2a with water and Me3SiCl at 25 1C in C6D6.
Activation of dihydrogen, regioselective addition to DPE, and silylation of
pyridine with 2a at 60 1C in C6D6.
t
(R = Bu, SiMe3), a solution of 2a in C6D6 reacts with water by
protonation.13 In a nucleophilic substitution reaction of 2a with
chlorotrimethylsilane, 1,1,1-trimethyl-2,2,2-triphenyldisilane
was identified as the only product of the reaction.12,13
Bis(triphenylsilyl)calcium 2a is able to cleave dihydrogen at
elevated temperatures within 4 h. After full consumption of 2a
the 1H NMR spectra showed triphenylsilane as the sole product
of the reaction with dihydrogen. As HSiPh3 was formed in
quantitative yield, a calcium hydride [CaH2(L)n]m (3, L = neutral
Lewis base) appeared to have formed. A very broad proton
resonance (3.9–5.4 ppm) was assigned to the hydride signal of
3, but this compound could not be crystallized from the
reaction mixture. To verify the existence of a hydride species
in solution, 9-fluorenone was added to the degassed reaction
mixture. The solution turned yellow-green with concomitant
evolution of H2. Although no proton resonances could be
Fig. 2 Selected molecular orbitals of complex 2a.
The bonding nature was analyzed by molecular orbital (MO) analysis. assigned due to overlap with the signals of triphenylsilane, a
Two MOs corresponding to the Ca–Si interaction, HOMO ꢀ 1 and fluorenone ketyl complex [Ca(OC13H8)2(thf)n] is proposed to be the
the HOMO, were found. HOMO ꢀ 1 corresponds to a bonding product of the reaction. This reaction path is in contrast to the
interaction between Ca and Si, involving the dz2 orbital of Ca. The reactivity of the fully characterized calcium hydride [{CaH(DIPP-
HOMO corresponds to an antibonding interaction between Ca and nacnac)(thf)}2] (DIPP-nacnac = (2,6-iPr2Ph)NC(Me)C(H)C(Me)N(2,6-
Si, involving the pz orbital of Ca (Fig. 2). This suggests no fully iPr2Ph)), which reacts with benzophenone by hydride transfer.14
covalent interaction between Ca and Si. Therefore natural bond Fluorenone ketyl complexes of calcium have been reported before.15
orbital (NBO) analysis was carried out to determine the amount of
1,1-Diphenylethylene (DPE) reacted with 2a at 60 1C. The
donor–acceptor interaction. A Wiberg index of 0.363 for the Ca–Si formation of the insertion product 4 was verified by 1H NMR
bond, which would be 1 for a covalent single bond, indicates a spectroscopy. Attempts to crystallize the insertion product failed.
substantial donor–acceptor interaction. Analyzing the second The regioselective addition of alkaline metal silyls to DPE is
order donor–acceptor interaction, a 70 kcal molꢀ1 interaction known.16 High regio- and stereoselectivities in the addition of silyl
is found between a filled sp3 lone pair on Si and an empty hybrid compounds to activated C–C double bonds can be achieved by
sd orbital of Ca. It is noteworthy that the carbon homologue of using silylcuprates.17 Main group silyls such as lithium silyls are
2a, [Ca(CPh3)2(thf)6], was characterized as a solvent-separated useful silylating reagents in organic synthesis.6d
ion-pair.10 The involvement of d orbitals of Ca11 and the inter-
DPE was selectively hydrosilylated by HSiPh3 using 2a as the
action between Ca and Si represents a unique bonding situation. catalyst within 62 h (Table 1, entry 1).18 Increase of the tempera-
Reactivity studies of 2a were monitored by NMR spectro- ture to 100 1C gave full conversion to the hydrosilylated product
scopy (Scheme 2). In analogy to the bulky supersilyl anions SiR3 within 2 h (Table 1, entry 3).
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Table 1 Catalytic hydrosilylation of activated olefins using catalyst 2a
Notes and references
1 (a) B. Marciniec, Coord. Chem. Rev., 2005, 249, 2374; (b) B. Marciniec,
Hydrosilylation: A Comprehensive Review on Recent Advances, Springer,
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2 J. L. Speier, Adv. Organomet. Chem., 1979, 17, 407.
3 (a) B. D. Karsted, US Pat., 3 715 334, 1973; (b) B. D. Karsted, US Pat.,
Entrya Substrate
Silane
T (1C) t (h) Yieldb (%)
1
2
3
4
5
6
7
8
R1QR2QPh
R1QR2QPh
R1QR2QPh
R1QR2QPh
R1QR2QPh
R1QPh, R2QMe HSiPh3
R1QPh, R2QH
R1QBu, R2QH
HSiPh3
HSiPh3
HSiPh3
H2SiPh2
H3SiPh
60
80
100
60
62
27
2
96c
3
715 452, 1973; (c) P. B. Hitchcock, M. F. Lappert and
97c
N. J. W. Warhurst, Angew. Chem., Int. Ed. Engl., 1991, 30, 438.
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V. Y. Lee and M. Nanjo, Coord. Chem. Rev., 2000, 210, 11.
>99
85
3
60
60
2
2
80
96c
HSiPh3
HSiPh3
25
80
—
—
Polymerization
—
a
Reaction conditions: catalyst (2.5 mmol), olefin (0.1 mmol), silane
7 Preparation of [Ca(SiPh3)2(thf)4] (2a). A solution of [KSiPh3(thf)] (1)
(556 mg, 1.5 mmol) in THF (5 mL) was added to a suspension of
anhydrous CaI2 (220 mg, 0.75 mmol) in THF (5 mL). The colorless
precipitate was filtered off and the solvent was removed under
reduced pressure. After washing with pentane (3 ꢁ 2 mL) and
subsequent drying, [Ca(SiPh3)2(thf)4] (2a) (635 mg, 0.75 mmol,
>99%) was obtained as a pale yellow powder. Crystals of 2a suitable
for single crystal X-ray analysis were grown from THF–pentane
within 1 h at ꢀ30 1C. 1H NMR ([D8]THF, 400.1 MHz): d 1.77
(m, 16H, thf), 3.62 (m, 16H, thf), 6.94–6.97 (m, 6H, para-Ph), 7.01-7.04
(m, 12H, meta-Ph), 7.37–7.39 (m, 12H, ortho-Ph). 13C{1H} NMR
([D8]THF, 100.6 MHz): d 26.39 (thf), 68.21 (thf), 125.46 (para-Ph),
127.27 (meta-Ph), 137.12 (ortho-Ph), 144.97 (ipso-Ph). 29Si{1H} NMR
([D8]THF, 25 1C, 79.5 MHz): d ꢀ13.99 (CaSi). Anal. calc. for
(0.11 mmol). Determined by 1H NMR spectroscopy. Catalyst deacti-
b
c
vation after time indicated.
Using H2SiPh2 or H3SiPh shortened reaction times drastically,
but decreased the yield of the desired products to 85% and 80%,
respectively (Table 1, entries 4 and 5). The substrate scope of catalytic
hydrosilylation was further investigated by using a-methylstyrene
and styrene as substrates. The hydrosilylation of a-methylstyrene
with HSiPh3 yielded 96% of the product (Table 1, entry 6), whereas
styrene polymerized at 25 1C within 2 min (Table 1, entry 7). The
catalytic activity and selectivity of 2a as the hydrosilylation catalyst is
comparable to other calcium complexes.19 As implied by Harder
et al. earlier,19a the catalytic cycle involves insertion of DPE into the
calcium silicon bond to give an alkyl as exemplified by the formation
of 4, which then undergoes reaction with HSiPh3 to give the silyl and
the hydrosilylated product. Noteworthy is the fact that formally
silane acts as a Brønsted acid rather than a latent silylium ion with
reversed polarity of the silicon–hydrogen bond. Therefore neither
oxidative addition following the Chalk–Harrod mechanism nor a
hydride mechanism as known for lanthanides takes place here.20
Finally, 2a reacted with an excess of pyridine within 5 min
selectively to give 4-triphenylsilylpyridine and bis(1,4-dihydro-1-
pyridyl)calcium (5), which were characterized by 1H NMR spectroscopy
(Scheme 2). A fast silylation at the ortho position of pyridine
followed by rearrangement generates the 1,4-insertion product.
Re-aromatization upon calcium hydride formation, instead of
the C–H bond activation reported for pyridine derivatives,21 gave
4-triphenylsilyl pyridine. This compound was reported by Gilman
et al. previously as the product of the reaction of triphenylsilyl lithium
with an excess of pyridine.22 The calcium hydride formed reacted with
pyridine to give 5. Such a reactivity pattern is known for magnesium
and zinc hydrides23 and seems to apply for calcium hydride 3 as well.
In conclusion, earth alkaline metal silyl 2a was synthesized by
salt metathesis and characterized by spectroscopic, crystal diffrac-
tion and computational methods. It catalyzes the hydrosilylation of
activated olefins, supporting the active role of calcium silyls in this
catalysis. Furthermore, 2a activates dihydrogen under mild condi-
tions and undergoes with pyridine a dehydrogenative silylation.
This reactivity pattern reflects the rather unique polarization of the
calcium–silicon bond.
C
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We thank the Cluster of Excellence RWTH Aachen ‘‘Tailor-Made
Fuels from Biomass’’ for financial support and the Alexander von
Humboldt Foundation for a fellowship to L.M.
18 Hydrosilylation experiments were performed in [D8]THF and C6D6.
Catalyst 2a does not show any activity in C6D6.
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