Synthesis and characterization of an isolable base-stabilized silacycloprop-1-ylidene

Article abstract of DOI:10.1002/anie.201201581

Strained but stable: An isolable silacycloprop-1-ylidene stabilized by intramolecular complexation with an iminophosphorus ylide fragment was successfully synthesized and fully characterized. The formation of this small highly strained cyclic silylene inv

Full text of DOI:10.1002/anie.201201581

Angewandte
Chemie
DOI: 10.1002/anie.201201581
Silicon Chemistry
Synthesis and Characterization of an Isolable Base-Stabilized
Silacycloprop-1-ylidene**
Ricardo Rodriguez, Thibault Troadec, Tsuyoshi Kato,* Nathalie Saffon-Merceron,
Jean-Marc Sotiropoulos, and Antoine Baceiredo*
The chemistry of stable silylenes has been the subject of
considerable research since the isolation of silicocene[1] and
N-heterocyclic silylene.[2] To date, several stable silylenes,[3–5]
including kinetically stabilized dialkylsilylene,[6] have been
synthesized and their chemical properties have been exten-
sively investigated.[7] A novel silylene stabilized by ylidic
carbon centers has also recently been reported.[8] However,
the structural modifications of stable silylenes are still far
more limited than those of their lighter analogues, the
carbenes.[9]
Herein, we report the first synthesis of a base-stabilized
silacycloprop-1-ylidene 3.
We recently reported the synthesis of the first phosphine-
stabilized silicon(II) hydride 1 and studied the reactivity of
the species with olefins.[16] Compound 1 similarly reacts at
room temperature with diphenylacetylene through a [2+1]
cycloaddition reaction to give the corresponding pentacoor-
dinate silirene 2, which was fully characterized by NMR
spectroscopy in solution. A 29Si NMR analysis of silirene 2
shows a high-field doublet (ꢀ135.2 ppm, 1JSiP = 58.9 Hz),
which is characteristic of this type of structure.[19] In the
1
ꢀ
H NMR spectrum, the Si H fragment appears as a doublet at
6.25 ppm (2JPH = 96.4 Hz) with the characteristic 29Si satellites
(ca. 4.7%, 1JHSi = 286.1 Hz), confirming a direct silicon–
hydrogen bond in 2 (Scheme 1).
Recently, silylenes I stabilized by the coordination of
a donating ligand to the silicon(II) atom have attracted much
attention owing to their high reactivity, which is strongly
related to the nature of the ligands.[10–13] In particular, this
stabilization method allows for easy functionalization of the
ꢀ
Si(II) atom. Indeed, silylenes with reactive Si X bonds II
(X = Cl, Br)[11,14] and even dihalogenosilylene complexes
III[12] have been isolated as stable crystalline materials and
have been used as precursors for preparing unique mole-
cules.[14,15] We have also successfully synthesized the first base-
stabilized silicon(II) hydride species IV,[16] which are difficult
to prepare in the absence of steric protection.[17] In this series,
the small highly strained cyclic silylenes, such as the silacy-
cloprop-1-ylidenes V, remain challenging target molecules.[18]
Scheme 1. Synthesis of base-stabilized silacycloprop-1-ylidene 3.
Silirene 2 is thermally labile and quantitatively isomerizes
in toluene at 808C into silacycloprop-1-ylidene 3, in which the
SiII atom is stabilized by the intramolecular coordination of
the imine fragment. Compound 3 was successfully isolated as
colorless crystals in 92% yield from pentane at room temper-
ature (Scheme 1). Although the mechanism of the reaction is
still unclear, this transformation formally involves two 1,2-
shifts of silicon ligands, hydride and phosphine, to each of the
[*] Dr. R. Rodriguez, T. Troadec, Dr. T. Kato, Dr. A. Baceiredo
Universitꢀ de Toulouse, UPS, and CNRS, LHFA
118 route de Narbonne, 31062 Toulouse (France)
E-mail: kato@chimie.ups-tlse.fr
carbon atoms of the silirene ring. Contrary to the previous
II
cases, neither an alkyne-insertion into the Si H bond[15] nor
ꢀ
tricyclic phosphine formation[19] was observed. In the
baceired@chimie.ups-tlse.fr
Homepage: http://hfa.ups-tlse.fr
31P NMR spectrum,
3 displays two singlets (46.5 and
Dr. N. Saffon-Merceron
Universitꢀ de Toulouse, UPS, and CNRS, ICT
118 route de Narbonne, 31062 Toulouse (France)
42.8 ppm), which is in agreement with the presence of two
diastereomers (92:8). In the 29Si NMR spectrum, two high-
field doublets were observed in the same ratio (ꢀ87.5 ppm,
2JSiP = 2.9 Hz, for the major one), which is in agreement with
a silicon atom in a small three-membered ring. The cyclo-
Dr. J.-M. Sotiropoulos
Universitꢀ de Pau et des Pays de l’Adour, IPREM
Technopꢁle Hꢀlioparc
propylidene ring proton appears as doublets (3.57 ppm, 3JPH
=
2 avenue du Prꢀsident Angot, 64053 Pau (France)
30.8 Hz, and 3.02 ppm, 3JPH = 31.1 Hz) in the 1H NMR
spectrum.
[**] We are grateful to the CNRS and the ANR (NOPROBLEM, LEGO)
for financial support of this work.
The crystal structure of 3 reveals a cyclopropylidene ring
featuring a divalent silicon atom complexed by the imine
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201581.
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fragment (Figure 1).[20] The three-membered ring has a small
bond angle around the silicon center (C1-Si1-C2 43.338) and
atom (s: 75.4%, p: 24.6%). Despite this result, the energy
level of the HOMO (ꢀ4.49 eV) remains quite high, suggest-
ing a strong nucleophilic character. The LUMO does not
localize on the SiII atom, but rather corresponds to the
p conjugate system of the a-iminophosphorus ylide fragment.
The vacant orbital of the SiII atom is much higher in energy
(LUMO + 7), resulting in a large energy gap between
LUMO + 7 and HOMO (5.45 eV), which suggests a decreas-
ing silylenoid character for 3.
ꢀ
the two Si C bonds (Si1–C1 1.993 ꢀ, Si1–C2 1.920 ꢀ) are
In contrast to the phosphine-stabilized silylene 1, which
shows a silylene-like reactivity,[16,19,25] 3 does not react with
olefins or alkynes. However, it readily reacts with the Lewis
acid BH3·THF to give the corresponding borane complex 4,
which was isolated as colorless crystals in 93% yield. The
crystal structure of 4 reveals the coordination of BH3 to the
SiII atom[26] (Figure 3).[20] The three bonds around the silicon
Figure 1. Crystal structure of 3. Thermal ellipsoids set at 30% proba-
bility. H atoms omitted for clarity. Selected bond lengths [ꢂ] and
angles [8]: Si1–C1 1.993(3), Si1–C2 1.920(3), C1–C2 1.572(3), Si1–N1
1.857(2), C1–P1 1.763(2), P1–C3 1.728(3), C3–C4 1.382(3), C4–N1
1.343(3); C1-Si1-C2 47.33(10), Si1-C1-C2 63.90(13), Si1-C2-C1
68.77(13), N1-Si1-C1 102.02(10), N1-Si1-C2 98.96(10).
longer than usual for Si C bonds (ca. 1.89 ꢀ).[21] The silicon–
ꢀ
nitrogen distance is also quite long (Si–N1 1.857 ꢀ). The long
bond distances around the tri-coordinate silicon center can
probably be rationalized by the enhanced p character of these
bonds as a consequence of the strong pyramidalization of the
ꢀ
silicon center (S8Si = 248.38). The P C bond (P1–C1 1.763 ꢀ)
is on the short end of the range expected for phosphoniocy-
clopropanes (1.759–1.780 ꢀ).[22] The geometry of the a-
iminophosphorus ylide fragment (P1-C1-C2-N1) complexing
the Si(II) center is similar to that observed in the correspond-
ing transition-metal complexes.[23]
To obtain more information about 3, calculations were
performed at the B3LYP/6-31G(d,p) level of theory
(Figure 2). The optimized structure of 3 agrees quite well
with the experimental data.[24] The HOMO of 3 corresponds
mainly to the lone pair orbital on the silicon atom. Natural
bonding orbital (NBO) analysis reveals a very high-level of
s character for the nonbonding orbital localized on the silicon
Figure 3. Crystal structure of 4. Thermal ellipsoids set at 30% proba-
bility. H atoms (except for H atoms on B1) are omitted for clarity.
Selected bond lengths [ꢂ] and angles [8]: Si1–B1 1.987(2), Si1–C1
1.932(2), Si1–C2 1.868(2), C1–C2 1.610(2), Si1–N1 1.796(2), C1–P1
1.778(2), P1–C3 1.733(2), C3–C4 1.370(2), C4–N1 1.363(2); C1-Si1-C2
50.11(7), C2-C1-Si1 62.89(9), C1-C2-Si1 67.00(9), N1-Si1-C1 107.32(7),
N1-Si1-C2 106.93(7).
atom are significantly shorter (Si1–C1: 1.932 ꢀ, Si1–C2:
1.868 ꢀ, Si1–N1: 1.796 ꢀ) than those in 3. These shorter
bond lengths are probably due to the reduced p character of
these bonds, which is induced by the coordination of the lone
pair. Indeed, the sum of the bond angles for the SiX3 subunit
(X = N1, C1, and C2) in 4 (264.48) is much larger than that
observed for 3 (248.38). Silacycloprop-1-ylidene 3 also reacts
with polarized unsaturated molecules, such as ethyl vinyl
ketone, to give spirocyclic silane 5 in 92% yield (Scheme 2).
The reaction probably involves an initial [4+1] cycloaddition
reaction at the SiII atom of 3 followed by a ring opening
reaction of the three-membered silacyclopropane ring to
afford 5. This system features a bicyclic structure with a novel
seven-membered ring that includes a phosphonium ylide
fragment. The structure of this molecule was unambiguously
confirmed by X-ray diffraction analysis.[20,27]
Figure 2. Calculated molecular orbitals of 3 at a 0.05 cut-off level.
Energy levels of orbitals are given in parentheses.
2
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The isomerization of silirene 2 into silacycloprop-1-
ylidene 3 involves a reduction of the silicon center from SiIV
to SiII (Figure 4). Such an isomerization to give a stable lower-
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Figure 4. Relative energies (kcalmolꢀ1) of isomers 2 and 3.
valent silicon species is, to the best of our knowledge,
unprecedented. Calculations indicate that isomers 2 and 3
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probably due to the efficient stabilization of the SiII center in 3
by coordination of the iminophosphorus ylide fragment.
In conclusion, we have successfully synthesized an N-
donor-stabilized silacycloprop-1-ylidene 3 through the iso-
merization of tetravalent silirene 2 under mild conditions.
This process involves an unprecedented SiIV!SiII rearrange-
ment. Further investigations on the reactivity of this new
species are underway.
Received: February 27, 2012
Published online: && &&, &&&&
Keywords: complexes · phosphorus · silicon · silylenes ·
.
small ring systems
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Silicon Chemistry
Strained but stable: An isolable silacy-
cloprop-1-ylidene stabilized by intramo-
lecular complexation with an iminophos-
phorus ylide fragment was successfully
synthesized and fully characterized. The
formation of this small highly strained
cyclic silylene involves an unprecedented
SiIV!SiII rearrangement under very mild
conditions.
R. Rodriguez, T. Troadec, T. Kato,*
N. Saffon-Merceron, J.-M. Sotiropoulos,
A. Baceiredo*
&&&& — &&&&
Synthesis and Characterization of an
Isolable Base-Stabilized Silacycloprop-1-
ylidene
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!