Disilenyl Silylene Reactivity of a Cyclotrisilene

Article abstract of DOI:10.1002/anie.201711833

The highly reactive silicon congeners of cyclopropene, cyclotrisilenes (c-Si₃R₄), typically undergo either π-addition to the Si=Si double bond or σ-insertion into the Si?Si single bond. In contrast, treatment of c-Si₃Tip₄ (Tip=2,4,6-ⁱPr₃C₆H₂) with styrene and benzil results in ring opening of the three-membered ring to formally yield the [1+2]- and [1+4] cycloaddition product of the isomeric disilenyl silylene to the C=C bond and the 1,2-diketone π system, respectively. At elevated temperature, styrene is released from the [1+2]-addition product leading to the thermodynamically favored housane species after [2+2] cycloaddition of styrene and c-Si₃Tip₄.

Full text of DOI:10.1002/anie.201711833

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Accepted Article
Title: Disilenyl silylene reactivity of a cyclotrisilene
Authors: David Scheschkewitz, Kinga Leszczyńska, Hui Zhao, Lukas
Klemmer, Volker Huch, and Michael Zimmer
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Disilenyl silylene reactivity of a cyclotrisilene
Hui Zhao, Kinga Leszczyńska, Lukas Klemmer, Volker Huch, Michael Zimmer, David Scheschkewitz*
Abstract: The highly reactive silicon congeners of cyclopropene,
cyclotrisilenes (c-Si
3 4
R ), typically undergo either -addition to the
Si=Si double bond or -insertion into the Si-Si single bond. In
i
3 4 3 6 2
contrast, treatment of c-Si Tip (Tip = 2,4,6-Pr C H ) with styrene
and benzil results in ring opening of the three-membered ring to
formally yield the [1+2] and [1+4] cycloaddition product of the
isomeric disilenyl silylene to the C=C bond and the 1,2-diketone 
system, respectively. At elevated temperature, styrene is released
from the [1+2] addition product leading to the thermodynamically
favored housane species after [2+2] cycloaddition of styrene and c-
3 4
Si Tip .
Scheme 1. Generic reactivity pathways of cyclotrisilenes I.
The heavier congers of cyclopropenes (c-C
3 4
R ), namely
[
1]
cyclotrisilene (c-Si ), continue to attract interest from theory
3
H
4
and experiment alike due to their unique structure and reactivity.
A significant number of stable cyclotrisilenes I have been
isolated taking advantage of kinetic stabilization by sterically
One of the most investigated SiSi reactivities in general is
the [2+2] cycloaddition of carbonyl species to afford
[
8]
disiloxetanes. In the case of cyclotrimetallenes of heavier
Group 14 elements, however, just a limited number of reactions
[
2]
demanding silyl and aryl substituents.
The extensive
[
3a,b,9]
investigations into the reactivity of strained, unsaturated
silacycles revealed two main generic reaction pathways
with carbonyl compounds is known.
As the only example in
the case of cyclotrisilenes, a [1+2] cycloaddition with carbon
[
3b]
(
Scheme 1): (1) The -addition to the SiSi double bond leading
monoxide itself was reported.
While the reaction of a 1-
[3,4]
to products of type II
and (2) the -insertion into one of the
disilagermirene with benzaldehyde affords the [2+2]
[
3a]
endocyclic SiSi single bonds effectively resulting in ring
cycloaddition product,
the same species gives rise to a 1,2-
[
4]
[
9]
expansion products of type III. In addition, a few reactions
OH addition product with the enolizable ketone, acetophenone.
under (3) formal insertion into an exocyclic -bond of one of the
Building on the divergent reactivity of cyclotrisilenes towards
[
5]
substituents have been reported, for example the insertion of
isonitriles ([1+2] cycloaddition leading to the kinetic and 
[5b]
[
3b]
sulfur into one of the bonds to the silyl-substituents.
Ring-
insertion to the thermodynamic product), we investigated the
behavior of 1 towards carbonyl compounds with a view to
possible disilenyl silylene-like reactivity.
opening reactions of cyclotrisilenes were notably absent,
although well-studied in the cases of carbon-based
[6]
cyclopropenes and even applied for the synthesis of polymers.
Recently, we reported the first example of (4) the reversible ring-
opening of a cyclotrisilene: the equilibrium reaction of aryl-
substituted cyclotrisilene with an N-heterocyclic carbene
[
7]
(
NHC).
Disilenyl silylenes such as the resulting NHC-
coordinated example V (Scheme 1) have also been proposed as
[1a,2f]
transient intermediates.
Clear-cut preparative manifestations
of disilenyl silylene reactivity of cyclotrisilenes, however, remain
elusive. We now report the reactions of homoleptic cyclotrisilene
Scheme 2. Reactions of cyclotrisilene 1 with benzaldehyde, benzophenone
i
c-Si
3
Tip
4
(1, Tip  2,4,6-Pr
3
6
C H2, Scheme 2) with substrates
and benzil to give housanes 2a,b and disilenyl-substituted 2,5-dioxasilol 3 (R'
i
containing CC or CO double bond(s) that allow for the
detection and in two cases even isolation and full
 H (2a), Ph (2b); R  Tip  2,4,6- Pr
3
C
6
H
2
).
–
–
characterization of the assumed kinetic product of disilenyl
silylene-like reactivity of 1.
Treatment of 1 with one equivalent of benzaldehyde or
benzophenone in benzene at room temperature, however, leads
to rapid conversion to the [2+2] cycloaddition products, housane
2
9
derivatives 2a,b (Scheme 2). Three sharp Si NMR signals
each (2a: 43.87, 7.87 and −65.83 ppm; 2b: 27.61, −1.66 and
−67.14 ppm) are comparable to those of the aforementioned
[*]
M. Sc. H. Zhao, Dr. K. Leszczyńska, M. Sc. L. Klemmer,
Dr. V. Huch, Dr. M. Zimmer, Prof. Dr. D. Scheschkewitz
Krupp-Lehrstuhl für Allgemeine und Anorganische Chemie
Universität des Saarlandes
[
3b]
imino trisilabicyclo[1.1.0]butane (45.0, 27.9 and −78.7 ppm).
29
Campus, C4.1, 66123 Saarbrücken (Germany)
E-mail: scheschkewitz@mx.uni-saarland.de
1
29
On the basis of H/ Si HMBC spectrum, the two low-field Si
NMR signals are assigned to the bridgehead silicon atoms, while
Supporting information and ORCID(s) identification numbers for the
authors of this article are available under:
http://dx.doi.org/10.1002/anie.xxxxxxxxx
1
the high-field signal is due to the SiTip
2
bridge. In the H NMR
spectrum of 2a, the benzylic hydrogen atom gives rise to a
singlet at 6.34 ppm, very similar to that of trans-
1
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COMMUNICATION
[
3a]
29
disilagermabicyclo[2.1.0]pentane (6.01 ppm).
2
Both, 2a and
as an orange powder in acceptable yield (60). In the Si NMR,
13
2
b show diagnostic C NMR resonances (2a: 77.38; 2b:
the two signals at low field are assigned to the formally sp

92.31ppm) in the typical range for the ring carbon atoms in
hybridized silicon atoms. The signal at 14.40 ppm corresponds
to the tetracoordinate silicon atom of the dioxasilole ring. As
typical for disilenes, the UV/vis spectrum of 3 in hexane shows a
longest wavelength absorption at max  437 nm (  14950
[
3a]
disilaoxetanes.
Housanes 2a,b, were isolated as single crystals suitable for
X-ray diffraction in acceptable yields (2a 60, yellow blocks,
m.p.  200°C; 2b 55, colorless blocks, m.p.  200°C). The
solid state structures of 2a,b were confirmed by X-ray
crystallography (2a: Figure 1, 2b: see SI).
display a three- and four-membered ring fused skeleton with the
−1
−1
[8f]
M cm ), which is assigned to the -* transition.
Crystals of 3 suitable for X-ray crystallography were obtained
[10]
[10]
Housanes 2a,b
from hexane solution (Figure 2). The SiSi double bond length
[
12]
of 2.179(4) Å is within the typical range (2.138-2.289 Å). The
angle between the two planes being 115.9(4)°in 2a and
twisted SiSi double bond (twist angle   23.5(4)°) is only
1
13.4(1)° in 2b which is slightly larger than that of the trans-
2
moderately trans-bent (  12.4(3)° at SiTip , 6.7(2)° at
[
3a]
disilagermabicyclo[2.1.0]pentane (107.8°),
smaller than the dihedral angle in imino-1,2,3-trisila-
but significantly
Si(Tip)Si). The dioxasilole ring of 3 exhibits an envelope
confirmation with an angle of 6.3(4)° between the planes of O1-
C61-C62-O2 and O1-Si3-O2, slightly more than in the reported
[
3b]
bicyclo[1.1.0]butane (134.2°).
three-membered rings of 2a,b (2a: 2.333(6), 2.347(6) and
.364(7) Å, 2b: 2.338(2), 2.349(3) and 2.403(3) Å) are within the
The SiSi bond lengths in the
[
11g]
1,3,2-dioxasilole (3.3°).
The two phenyl groups of the former
2
benzil moiety twist from the dioxasilole ring by interplane angles
typical range for SiSi single bonds.
of39.9(4)° and 37.3(4)°.
Figure 1. Molecular structure of 2a in the solid state. Hydrogen atoms are
omitted for clarity (thermal ellipsoids at 50). Selected bond lengths (Å) and
angles (°): Si1-Si2 2.347(6), Si2-Si3 2.333(6), Si1-Si3 2.364(7), Si1-O1
Figure 2. Molecular structure of disilenyl-substituted 2,5-dioxasilole 3·C H in
6
14
the solid state. Co-crystallized C H and hydrogen atoms are omitted for
6
14
clarity (thermal ellipsoids at 50). Selected bond lengths (Å) and angles (°):
Si1-Si2 2.179(4), Si1-Si3 2.333(4), Si3-O1 1.679(8), Si3-O2 1.679(8), C61-O1
1.396(1), C62-O2 1.393(1), C61-C62 1.345(2); Si2-Si1-Si3 136.78(2), Si1-Si3-
O1 122.40(3), O1-Si3-O2 94.17(4), Si3-O1-C61 109.06(6), O1-C61-C62
1.675(1), Si2-C1 1.975(2), C1-O1 1.453(2), C1-C2 1.505(2), C1-H1 0.964(2);
Si1-Si3-Si2 59.95(2), Si3-Si1-O1 109.14(5), Si3-Si2-C1 98.99(5), Si1-O1-C1
104.76(8), Si2-C1-O1 102.85(9), C2-C1-H1 106.40(1).
113.59(9).
As the [2+2] cycloaddition of an isolated CO moiety prevails
over the targeted [1+2] cycloaddition with the postulated
transient disilenyl silylene 6 (Scheme 4), we speculated that the
1+4] cycloaddition of the silylene moiety with a 1,2-diketone
It is well-established that polar substrates facilitate the [2+2]
[
13]
cycloaddition to SiSi bonds. As the lower polarity of the CO
bonds in benzil might contribute to the prevalence of the [1+4]
cycloaddition pathway, we extrapolated that a reaction of
cyclotrisilene 1 with the inherently less polar CC bond of
alkenes might also result in disilenyl silylene reactivity. While a
cis/trans isomeric mixture of stilbene (1,2-diphenyl ethene) did
not react with 1 even at elevated temperatures, presumably due
to steric hindrance, the treatment of cyclotrisilene 1 with one
equivalent of styrene (phenyl ethene) at room temperature
[
might be sufficiently favored compared to any reaction of the
SiSi fragment. Reactions of isolated silylenes with 1,2-
diketones were previously reported to give dioxasilole
[
11]
derivatives.
dissolved in C
solution almost instantly, as opposed to the pale yellow to
Indeed, an equimolar mixture of benzil and 1
6
D
6
or toluene afforded an intensely bright red
2
9
colorless reaction mixtures of 2a,b. Three new signals in Si
NMR spectrum at  99.69, 36.01 and 14.40 ppm served as first
indication for the formation of 1,3,2-dioxasilole 3 with an
exocyclic SiSi double bond and thus the desired disilenyl
silylene reactivity of cyclotrisilene 1. The product 3 was isolated
2
9
results in the uniform appearance of three new signals in the Si
NMR spectrum at 103.42, 36.16 and −94.15 ppm (Scheme 4).
As in case of 3, the two low-field signals are diagnostic of an
acyclic SiSi unit. The signal at high field falls well within the
2
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typical region for the endocyclic silicon atom of three-membered
C Si rings: for instance, siliranes obtained by the reaction of an
2
acyclic iminosilylsilylene and a silylsilylene with ethylene were
Given that the steric requirements of styrene and
benzaldehyde are fairly comparable, we considered the
possibility of isomerization of the disilenyl-substituted silirane 4
to an isomeric saturated housane structure akin to 2a,b. Indeed,
heating of solid 4 slightly above the melting point for 20 minutes
under argon atmosphere affords trisilabicyclo[2.1.0]pentane 5
(Scheme 3) in 32 yield after crystallization from hexane.
Colorless single crystals suitable for X-ray diffraction analysis
reported to show resonances at  −100.90and −80.76 ppm,
[
14]
respectively. With the help of a DEPT-135 spectrum (Figure
1
3
S14-15), the C NMR signal at  10.84 ppm is assigned to the
CH moiety in the three-membered ring of 4, while the phenyl
2
substituted ring carbon gives rise to a signal at 23.59 ppm. As
in the case of 3, the longest wavelength absorption in the UV/Vis
(Figure 4) were obtained from hexane at 5°C confirming the
−
1
−1
[10]
spectrum of 4 at max 426 nm (  17430M cm ) confirms the
presence of an acyclic SiSi unit as it is almost identical to that
constitution of a housane structure.
The angle between the
mean planes of the three- and four-membered rings is
119.0(9)°,which is slightly larger than that in 2a,b (2a: 115.9(4)°;
[
15]
of a previously reported chlorosilyl disilene (max 427 nm).
2
b: 113.4(1)°). The Si-Si bond lengths vary between 2.311(2) Å
2
9
and 2.392(2) Å. Curiously, in the Si NMR spectrum of 5 in C
6 6
D
solution at 25°C only one signal at 14.28 ppm is observed,
2
9
while in the solid state CP-MAS Si NMR revealed the expected
2
9
three signals at 14.72, 8.39 and −88.75 ppm. Similarly, the Si
NMR spectrum in solution at −40°C displays three signals at 
13.19, 9.28 and −84.22 ppm, which coalesce in a rather
undefined manner upon warming to room temperature. These
findings suggest that the two signals at 9.28 and −84.22 ppm are
involved in an unknown dynamic process. Despite the high
probability that the high-field signal at −84.22 ppm belongs to the
Scheme 3. Synthesis of disilenyl-substituted silirane 4 (kinetic product) and its
conversion to housane 5 (thermodynamic product; R  Tip  2,4,6-Pr C H ).
3 6 2
i
SiTip
2
moiety on grounds of similarity with 2a, we confirmed the
After workup, disilenyl-substituted silirane 4 is isolated in
4 yield (m.p. 168-170°C) as an orange powder. Orange
assignment by calculating the NMR shifts at the M06-
5
2
9
2X/def2TZVPP level of theory. The predicted results ( Sicalc
blocks suitable for X-ray structure analysis were obtained at
20°C from toluene solution. The molecular structure of 4 in the
solid state confirmed the connectivity deduced from the
−79.04 (SiTip ), 29.43 (SiCPh), 20.90 (SiCH ) ppm) are in
2
2
−
qualitative agreement with the experiments. We tentatively
explain the peculiar NMR behavior with hindered rotation at the
two more congested silicon atoms.
[
10]
spectroscopic data (Figure 3).
.180(1) Å displays moderate twisting and trans-bending (twist
angle   17.9(9)°, trans-bent angle   13.2(7)° at SiTip
The SiSi double bond of
2
2
,
18.4(6)° at Si(Tip)Si). Interestingly, the Si1Si2 distance of
2
.300(1) Å is rather short for the typical SiSi single bond
[
16]
suggesting a certain degree of hyperconjugation
with the
adjacent SiSi unit.
Figure 4. Molecular structure of housane 5. Hydrogen atoms are omitted for
clarity (thermal ellipsoids at 50%). Selected bond lengths (Å) and angles (°):
Si1-Si2 2.392(2), Si1-Si3 2.311(2), Si2-Si3 2.371(1), Si1-C1 1.905(4), Si2-C2
1
6
.916(4), C1-C2 1.546(7), C1-H1 1.05(4), 0.92(5), C2-H2 0.97(3); Si3-Si1-Si2
0.54(5), Si1-Si3-Si2 61.41(5); C1-Si1-Si2 75.38(2), Si1-C1-C2 106.0(3), Si2-
Figure 3. Molecular structure of disilenyl-substituted silirane 4. Hydrogen
atoms are omitted for clarity (thermal ellipsoids at 50%). Selected bond
lengths (Å) and angles (°): Si1-Si2 2.300(1), Si2-Si3 2.180(1), C1-C2 1.533(4),
Si1-C1 1.844(3), Si1-C2 1.890(2); Si1-Si2-Si3 125.37(4), C1-Si1-C2 48.44(1),
Si1-C2-C1 64.21(2).
C2-C1 99.6(3).
Finally, we pondered the question whether the formation of 5
proceeds (i) as an intramolecular isomerization or (ii) through the
dissociation of styrene from the kinetic product 4. A few
3
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COMMUNICATION
reversible [1+2] cycloadditions involving silylenes and ethylene
have indeed been reported, e.g. the reaction of ethylene with a
styrene react through the usual [2+2] cycloaddition pattern to
give the housane product 5. Thermolysis of 4 under vacuum
strongly supports the dissociative nature of the transformation to
5.
[
17]
[14b,18]
phosphonium silaylide and with acyclic silylenes.
Melting
of 4 was therefore repeated under dynamic vacuum in order to
remove any liberated styrene rapidly. The melt residue of 4 after
having been kept in vacuum at 170°C for two minutes showed
2
9
the characteristic Si NMR signals of cyclotrisilene 1 at  42.82,
Acknowledgements
1
−22.98 ppm. From the
H NMR spectrum the yield of
regenerated 1 was estimated to about 78 (Figure S15-16),
accompanied by small quantities of  addition product 5 and
other by-products.
Support by Saarland University, the China Scholarship Council
(
Fellowship H.Z.), and COST Action CM1302 (Smart Inorganic
Polymers) is gratefully acknowledged.
On this basis, we propose the following mechanistic scenario
for the formation of 5: the kinetic product of [1+2] addition 4 re-
dissociates to give free styrene and the transient disilenyl
silylene 6, which should rapidly isomerize to cyclotrisilene 1
even at elevated temperature. Under thermodynamic control,
Keywords: Group 14 elements • silicon • low-valent species •
ring-opening • cycloaddition
[1]
[2]
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956-964; b) B. Pintér, A. Olasz, K. Petrov, T. Veszprémi,
Organometallics 2007, 26, 3677-3683.
[
2+2] cycloaddition of 1 with styrene affords housane 5 (Scheme
4
). DFT calculations at the dispersion-corrected M062X/def2-
−1
a) T. Iwamoto, C. Kabuto, M. Kira, J. Am. Chem. Soc. 1999, 121, 886-
SVP level of theory indeed reveal 4 to be about 16.8 kcal mol
8
87; b) T. Iwamoto, M. Tamura, C. Kabuto, M. Kira, Science 2000, 290,
04-506; c) M. Ichinohe, T. Matsuno, A. Sekiguchi, Angew. Chem. Int.
higher in G298 than 5 (Scheme 4). While this difference is only
slightly lowered at 443 K, dissociation of 4 to free disilenyl
5
Ed. 1999, 38, 2194-2196, d) K. Uchiyama, S. Nagendran, S. Ishida, T.
Iwamoto, M. Kira, J. Am. Chem. Soc. 2007, 129, 10638-10639; e) V. Y.
Lee, H. Yasuda, A. Sekiguchi, J. Am. Chem. Soc. 2007, 129, 2436-
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Am. Chem. Soc. 2014, 136, 12896-12898.
silylene
6 is thermodynamically significantly more feasible
−
1
−1
(
G298 = +28.2 kcal mol vs. G443 = +21.4 kcal mol ).
According to Hammond's postulate, the kinetics of an equilibrium
between 1 and 4 under these conditions (G443 = +5.7 kcal
−
1
mol ) can be approximated by the energy of the transient
intermediate 6.
[
3]
4]
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[
2
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1
56, 125-202; c) D. Scheschkewitz, Chem. Lett. 2011, 40, 2-11; d) V. Y.
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[9]
Scheme 4. Mechanism proposed for the formation of housane 5 from the
i
kinetic product disilenyl-substituted silirane 4 (R  Tip  2,4,6-Pr
3
C
6
H
2
;1Gibb's
−
enthalpies at the M062X/def2-SVP level of theory are given in kcal mol ).
[10] CCDC 1586039 (2a), 1586040 (2b), 1586041 (3), 1586042 (4),
586043 (5) contain the supplementary crystallographic data for this
1
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
11] Examples see: a) I. Masayuki, A. Wataru, J.C.S. Chem. Comm. 1979,
In conclusion, with species with one isolated carbonyl group
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bonds of benzil and styrene at room temperature likely proceeds
via ring-opening to transient disilenyl silylene 6 to give [1+4] and
[
6
55-656; b) J. Heinicke, B. Gehrhus, J. Organomet. Chem. 1992, 423,
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1
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[
1+2] addition products 3 and 4, respectively, which represent
1
21-129; f) R. Azhakar, R. S. Ghadwal, H. W. Roesky, J. Hey, D. Stalke,
the first examples of disilenyl silylene reactivity of any
cyclotrisilene. Under thermodynamic control, however, 1 and
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Iwamoto, Heteroatom Chem. 2014, 25, 348-353.
4
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[
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8
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[
4
5
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Entry for the Table of Contents
Layout 2:
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Hui Zhao, Kinga Leszczyńska, Lukas
Klemmer, Volker Huch, Michael Zimmer,
David Scheschkewitz*
Page No. – Page No.
Disilenyl silylene like reactivity of a
cyclotrisilene
Speed is of the essence in order to ring-open the peraryl-substituted cyclotrisilene
i
(
R = 2,4,6-Pr
3
C
6
H
2
) and coax disilenyl silylene reactivity towards benzil and styrene
from it. In case of styrene, the kinetic product with a residual R
2
SiSiR- substituent
is transformed into the saturated thermodynamic product via the re-detachment of
the styrene reagent at elevated temperature as proven by a control experiment
under vacuum.
6
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