Synthesis of a phosphine-stabilized silicon(II) hydride and its addition to olefins: A catalyst-free hydrosilylation reaction

Article abstract of DOI:10.1002/anie.201105639

No cat.s allowed: A stable and isolable tricoordinate silicon(II) hydride stabilized by a phosphine ligand was successfully synthesized and fully characterized (see structure, Sigreen, Pyellow, Nblue, Cgray, Hwhite). Interestingly, this silicon hydride adds to olefins in an unprecedented catalyst-free hydrosilylation reaction in very mild conditions.

Full text of DOI:10.1002/anie.201105639

Communications
DOI: 10.1002/anie.201105639
Hydrosilylation
Synthesis of a Phosphine-Stabilized Silicon(II) Hydride and Its
Addition to Olefins: A Catalyst-Free Hydrosilylation Reaction**
Ricardo Rodriguez, David Gau, Yohan Contie, Tsuyoshi Kato,* Nathalie Saffon-Merceron, and
Antoine Baceiredo*
À
Hydrosilanes (R3Si H) are powerful chemical tools and are
often used as hydride donors or as reducing agents owing to
the labile silicon–hydrogen bond, which has a significant
À
base-stabilized three-coordinate silicon(II) hydride remains
elusive,[19] although the corresponding heavier germanium
and tin analogues have already been synthesized.[20] Herein,
we present the synthesis and reactivity of the first stable and
negative polarization toward the hydrogen atom (Sid+
H
dÀ).[1]
Particularly, transition-metal-catalyzed addition of silicon
hydrides to unsaturated substrates such as olefins or
ketones—the hydrosilylation reaction—is one of the most
important reactions in synthetic chemistry.[2] Furthermore,
the versatile reactivity of hydrosilanes also allows for their use
as precursors of various highly reactive intermediates, such as
silylium cations,[3–5] silyl radicals,[6] and silylenes.[7]
isolable phosphine-stabilized silicon(II) hydrides
(Scheme 1).
2
Although a broad range of tetravalent hydrosilanes are
known, divalent species (silylenes) with a terminal hydride
are highly reactive, and their chemistry is very poorly
developed. Indeed, only a few stable and isolable silicon(II)
hydride species are known, and most of them are stabilized
transition-metal complexes.[8] Very recently stable bis-adducts
(Lewis acid)(Lewis base)SiIIH I, have also been reported.[9]
Scheme 1. The synthesis of phosphine-stabilized silicon(II) hydride 2.
The phosphine-stabilized silicon(II) hydrides 2 were
readily prepared by reduction of the corresponding dichloro-
silane derivatives 1 using magnesium metal and were isolated
as pale yellow crystals (70% (2a), 79% (2b)). The silicon(II)
hydride 2a appeared to be highly reactive, but it is thermally
robust. Indeed, no degradation of 2a was observed after
heating a toluene solution under argon at 1108C for several
hours. In the 31P NMR spectrum, derivative 2a displays two
singlet signals (d = 91.2 and 90.4 ppm), in agreement with the
presence of two diastereomers (60:40), as expected for this
type of phosphonium silaylide.[17] In the 29Si NMR spectrum
two doublets were observed in the same ratio, both having a
typically large phosphorus–silicon coupling constant (d =
Base-stabilized silylenes II have attracted much attention
because they are thermally stable and they keep their
reactivity as SiII species.[10–15] The efficiency of this method-
ology has been well demonstrated by the isolation of
silicon(II) halides (DSiX2, X = Cl, Br) as N-heterocyclic
carbene complexes and their use in molecular silicon(II)
chemistry.[15a,16] We also recently reported the synthesis of a
stable phosphine-stabilized silylene III (Dipp = 2,6-
iPr2C6H3),[17] which displays a unique ability to reversibly
react with ethylene gas.[18] However, the synthesis of isolable
À44.8 ppm (1JSiP = 143.2 Hz) and À38.0 ppm (1JSiP
=
140.1 Hz)), thus indicating a direct silicon–phosphorus inter-
action. The signals corresponding to the terminal hydride
1
appear as doublets in the H NMR spectrum at a somewhat
lower field (d = 5.99 ppm (2JPH = 3.3 Hz) and 5.76 ppm (2JPH
=
2.9 Hz)). The 29Si satellites (ca. 4.7%) for the terminal
hydrogen atoms appear as a doublet of doublets with a
large silicon–hydrogen coupling constant (1JH–Si = 85.1 and
85.7 Hz), which indicates a direct silicon–hydrogen bond in
2a. However, this value is much smaller than those observed
for tetravalent silicon hydrides (150–380 Hz)[21] or for silicon-
(II) species stabilized by the push–pull complexation system I
(235 Hz).[9]
[*] Dr. R. Rodriguez, Dr. D. Gau, Y. Contie, T. Kato, A. Baceiredo
Universitꢀ de Toulouse, UPS, and CNRS
LHFA UMR 5069, F-31062 Toulouse (France)
E-mail: baceired@chimie.ups-tlse.fr
Homepage: http://hfa.ups-tlse.fr
Dr. N. Saffon-Merceron
The molecular structure of 2b was unambiguously deter-
mined by X-ray crystallography,[22] which reveals that the
molecule is monomeric in the solid state (Figure 1). Although
the geometric features of 2b are quite similar to those of the
reported phosphine-stabilized silylene III, which bears a
phenyl group on the silicon atom, the tricoordinate SiII center
(ꢀ8Sia = 2758) is much more pyramidalized and has a more
Universitꢀ de Toulouse, UPS, and CNRS
ICT FR2599, F-31062 Toulouse (France)
[**] We are grateful to the CNRS and the ANR (NOPROBLEM) for
support of this work. R.R. acknowledges the Ministerio de
Educaciꢁn (MEC, Spain) for a postdoctoral fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105639.
11492
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 11492 –11495

the cyclopentane substituted silicon(II)-phosphine complex 3
was obtained as a mixture of two diastereomers (40% yield of
isolated product, Scheme 2). Formally, this result can be
À
Scheme 2. The olefin insertion reaction into the Si H bond of 2a.
rationalized by the addition of the silicon(II) hydride 2a to
the alkene functionality. The formation of 3 was unambigu-
ously established by NMR spectroscopy as well as by an X-ray
diffraction analysis (Figure 2). To our knowledge, this is the
first example of a hydrosilylation reaction of an alkene
involving a stable silicon hydride derivative without any
catalyst. This type of reactivity has been postulated for the
parent molecule (DSiH2) at much higher temperatures (ca.
600–1000 K).[23]
In marked contrast, silicon(II) hydride 2a reacts with an
excess of vinyltrimethylsilane (58 equiv) at room temperature
in a [2+1] cycloaddition reaction to give the corresponding
pentacoordinate silirane 6 as a mixture of two diastereomers
(2a:6 = 1:3, 258C; Scheme 3). Silirane 6 has been identified in
Figure 1. Molecular structure of 2b. Thermal ellipsoids represent 30%
probability. H atoms (Except for that on Si1) are omitted for clarity.
À
The Si H unit in 2b was disordered over two positions, and only one
molecular arrangement is shown for clarity. Selected bond lengths [ꢂ]
and angles [8]: Si1–H 1.44(5), N1–Si1 1.840(13), P–Si1 2.318(15),
P–C1 1.7344(13), C1–C2 1.3776(18), C2–N1 1.3593(17); N1-Si1-H
98.7(19), P-Si1-H 90.0(19), N1-Si1-P 86.8(6), Si1-P-C1 96.0(3), P-C1-C2
114.90(10), C1-C2-N1 122.84(12), C2-N1-Si1 119.2(5).
acute silylene angle (N1-Si1-H = 98.78) than other silylene
phosphine complexes (ꢀ8Sia = 2978, N1-Si1-C3 = 104.788 for
3, Figure 2). This difference is probably caused by a decrease
of the steric hindrance around the silicon center induced by
the small hydrogen substituent. The SiII-containing five-
membered ring is quasi-planar, as indicated by the sum of
the interior angles (597.58).
We have recently reported the reversible [2+1] cyclo-
addition reaction of the silicon(II)-phosphine complex III
with ethylene gas to form a pentacoordinate silirane.[18] The
silicon(II) hydride 2a also reacts with olefins such as cyclo-
pentene at 1108C, but in this case, instead of a cycloadduct,
Scheme 3. Reaction of 2a with vinyltrimethylsilane.
solution by the characteristic upfield 29Si NMR chemical
shifts for pentacoordinate siliranes[18] (d = À72.3 and
1
À70.2 ppm, JSi–P = 40.7 and 36.2 Hz). At room temperature,
cycloadduct 6 appears to be in equilibrium with 2a, which was
quantitatively recovered after removal of the olefin under
vacuum. A vanꢀt Hoff analysis of the variable-temperature
31P NMR spectroscopy data affords a Gibbs free energy
(DG208C) of (0.803 Æ 0.128) kcalmolÀ1 for the olefin addition
reaction to 2a. This small endothermic value explains the
reversibility of the reaction as well as the equilibrium
preferentially displaced to 2a. Upon heating the mixture in
toluene at 1108C for 2 h, 6 transforms into the corresponding
ethylene insertion products (4TMS and 5TMS), suggesting that
silirane 6 is an intermediate of the hydrosilylation reaction.
Similarly, hydrosilylation of other monosubstituted olefins
has been performed, which leads to the formation of two
regioisomers: Markovnikov-type products 4 as well as the
anti-Markovnikov-type adducts 5. The regioselectivity of the
reaction appears to be strongly dependent on the nature of
the olefin used (Table 1).
Figure 2. Molecular structure of 3. Thermal ellipsoids represent 30%
probability. H atoms are omitted for clarity. Selected bond lengths [ꢂ]
and angles [8] for 3: P–Si1 2.3272(6), P–C1 1.7245(17), Si1–N1
1.8458(14), Si1–C3 1.921(2), N1–C2 1.346(2), C1–C2 1.388(2);
N1-Si1-C3 104.78(7), P-Si1-C3 105.14(6), C1-P-Si1 93.52(6), N1-Si1-P
87.67(5), C2-N1-Si1 115.52(11), N1-C2-C1 124.06(15).
The hydrosilylation reactions involving 2a mainly give
anti-Markovnikov selectivity, suggesting that the reaction
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Communications
Table 1: Regioselectivity in hydrosilylation reactions of different mono-
substituted olefins with 2a.
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2
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phosphine ligand. Interestingly, this silicon hydride adds to
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Received: August 9, 2011
Published online: October 5, 2011
Keywords: hydrides · hydrosilylation · phosphorus · silicon ·
.
silylenes
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Angew. Chem. Int. Ed. 2011, 50, 11492 –11495

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