Functionalized cyclic disilenes via ring expansion of cyclotrisilenes with isocyanides

Article abstract of DOI:10.1021/om400054u

The reaction of cyclotrisilenes 1 with 1 equiv of alkyl and aryl isocyanides at 25 C affords the four-membered trisilacyclobutenes 2 with an exocyclic imine functionality as the major products of formal insertion into one of the Si-Si single bonds of 1. M

Full text of DOI:10.1021/om400054u

Communication
pubs.acs.org/Organometallics
Functionalized Cyclic Disilenes via Ring Expansion of Cyclotrisilenes
with Isocyanides
Yu Ohmori,† Masaaki Ichinohe,† Akira Sekiguchi,*,† Michael J. Cowley,‡ Volker Huch,‡
and David Scheschkewitz*,‡
†Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
‡Saarland University, Krupp-Chair for General and Inorganic Chemistry, 66123 Saarbrucken, Germany
̈
S
* Supporting Information
ABSTRACT: The reaction of cyclotrisilenes 1 with 1 equiv of alkyl and aryl
isocyanides at 25 °C affords the four-membered trisilacyclobutenes 2 with
an exocyclic imine functionality as the major products of formal insertion
into one of the Si−Si single bonds of 1. Minor quantities of the
iminotrisilabicyclo[1.1.0]butanes 3 are obtained as side products, formally
resulting from [1 + 2] cycloaddition of the isocyanides to the Si−Si double
bond of 1. The bicyclo[1.1.0]butanes 3 become dominant at lower
temperatures and may react with an additional 1 equiv of isonitriles to give
the diiminotrisilabicyclo[1.1.1]pentanes 4.
Scheme 1. Adducts between SiSi and SiSi Species and
he chemistry of silicon multiple bonds has matured
a
1
N-Heterocyclic Carbenes or Isocyanides
Tdramatically in the past few decades. Even functionalized
disilenes (SiSi) and silenes (SiC) are becoming
increasingly available and show considerable promise as
synthetic tools for the transfer of intact silicon-containing
double-bond moieties.2 Conversely, small, unsaturated sila-
cycles3 are relatively rare due to their less straightforward
synthesis. To date, the few known examples have been obtained
by the reduction of halosilanes,4 reactions of metallosilyl
reagents,5 cycloadditions of disilynes with alkenes,6 and thermal
or photochemical interconversions.7 In particular, cyclo-
trisilenes are attractive precursors: the incorporation of the
silicon-containing double bond into the highly strained three-
membered ring allows for functionalizing interventions via ring
opening.5b,d,7c
The relatively shallow energy profile of species with silicon−
silicon multiple bonds manifests itself in the facile polarizability
of electron density,8,9 which makes disilenes and disilynes
susceptible to coordination by strong Lewis bases. The
coordination of an N-heterocyclic carbene (NHC)10 to the
SiSi bond of a bulky silyl-substituted disilyne9 affords
complex A (Scheme 1), thus generating a nonbonding electron
pair at the second silicon atom available for further
functionalization: e.g., with ZnCl2.11 Another remarkable
example was very recently reported by Scheschkewitz and
Jutzi et al. with the reversible coordination of cyclotrisilene
Tip3Cp*Si3 (Tip = 2,4,6-iPr3C6H2) to give the corresponding
cyclotrisilene−NHC complex B.5b
Previously, West and co-workers reported that the reaction
of tetraxylyldisilene R2SiSiR2 (R = Xyl = 2,6-dimethylphenyl)
with XylNC yields the [2 + 1] cycloaddition adduct C as the
sole product.12 Very recently, the Scheschkewitz group
reported the reaction of the unsymmetrically substituted
disilene Tip2SiSi(Ph)Tip13 with isocyanides to initially
a
Legend: A, Dsi = CH(SiMe3)2; B, Tip = 2,4,6-iPr3C6H2, Cp* =
1,2,3,4,5-pentamethylcyclopentadienyl; C, Xyl = 2,6-Me2C6H2; D, Dsi
= CH(SiMe3)2, R = Bu, Oct.
t
t
yield a similar adduct.14 The differing coordination modes in
B and C can be regarded as a reflection of the stronger π-
accepting character of the isocyanide in comparison to NHCs.
This interpretation is lent additional support by the reaction of
a disilyne9 with alkyl isocyanides to afford the disilyne−
isocyanide adducts D, which are best described as bis-
(silaketenimines), highly functional unsaturated silicon com-
pounds.15
Bearing in mind the high strain in cyclotrisilenes, we decided
to investigate the coordinative properties of isocyanides toward
aryl- and silyl-substituted representatives of this class of
Received: January 23, 2013
Published: February 22, 2013
© 2013 American Chemical Society
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compounds: 1a with triisopropylphenyl (Tip) substituents and
1b5b with MetBu2Si groups.4b Here we report the reactions of
alkyl and aryl isocyanides with cyclotrisilenes 1a,b affording the
four-membered cyclic disilenes 2a,d with an exocyclic imine
functionality in potential conjugation with the SiSi double
bond. In addition, the isomeric bicyclic imines 3a−c were
isolated as potential intermediates as well as representative
examples (4c,d) of reaction products with a second equivalent
of isocyanide.
hexane solvate at room temperature (Figure 1). The folded
bicyclobutane-type structure exhibits a long bond between the
Cycloaddition Reactions of Isocyanides with Cyclo-
trisilenes. Treatment of cyclotrisilenes 1a,b with 1 equiv of
xylyl or cyclohexyl isocyanide, respectively, resulted in
formation of the bicyclic compounds 3a−c (Scheme 2).
a
Scheme 2. Reactions of iIocyanides with Cyclotrisilenes
Figure 1. ORTEP diagram of 3b·C6H14 (ellipsoids at the 50%
probability level). Hydrogen atoms and hexane solvent molecule are
omitted for clarity. Selected bond distances (Å): Si1−Si2 = 2.3264(6),
Si1−Si3 = 2.2905(6), Si2−Si3 = 2.4255(6), Si2−C1 = 1.8667(16),
Si3−C1 = 1.8681(16), C1−N1 = 1.282(2). Selected bond angles
(deg): Si1−Si2−Si3 = 57.589(17), Si1−Si3−Si2 = 59.034(18), Si3−
Si1−Si2 = 63.377(19), C1−Si2−Si3 = 49.53(5), C1−Si3−Si2 =
49.48(5), Si2−C1−Si3 = 81.00(6).
a
Legend: 1a, R = Tip = 2,4,6-iPrC6H2; 1b, R = SiMetBu2; 2a, R = Tip,
R′ = tBu; 2d, R = SiMetBu2, R′ = Xyl = 2,6-Me2C6H2; 3a, R = Tip, R′
= tBu; 3b, R = Tip, R′ = Xyl; 3c, R = SiMetBu2, R′ = cyclohexyl = Cy;
[3d], R = SiMetBu2, R′ = Xyl; 4c, R = SiMetBu2, R′ = Cy; 4d, R =
SiMetBu2, R′ = Xyl.
two bridgehead atoms: Si2−Si3 = 2.4255(6) Å. The fold angle
between the plane of the trisilacyclopropane and the
disilacyclopropanimine ring is 134.2°. The two bonds between
the bridgehead silicon atoms and the imine carbon have
essentially identical distances (Si2−C1 = 1.8667(16) Å, Si3−
C1 = 1.8681(16) Å). The imine C1−N1 double-bond distance
is unremarkable at 1.282(2) Å.17
The formal [2 + 1] cycloaddition reactions of isocyanides
yielding 3b,c are akin to the aforementioned reaction by West
and co-workers, forming the disilacyclopropanimine C.12
Indeed, the solid-state structure of 3b bears close similarities
to that of C; the disilacyclopropane ring and the exocyclic C
N double bond of 3b are virtually coplanar (dihedral angle
13.4°; for C this is reported as “∼13°”), and the N-xylyl group
is orthogonal to the CN double bond in both compounds.
Despite the similar aryl substituents, the bridging Si−Si bond in
3b is significantly longer than the equivalent bond in C
(2.4255(6) Å vs 2.328(3) Å), presumably due to the
significantly increased ring strain arising from the bicyclobutane
structure of 3b. Despite the ring strain and long bridging bond,
the two bridgehead silicon atoms of 3b possess chemical shifts
(δ 40.5, 27.9) in the same region of the 29Si NMR spectrum as
for the equivalent atoms in C (δ 37.2 ppm).
Compound 3a was obtained as a powder of near-spectroscopic
purity by evaporation of the solvent (hexane) in 77% yield; 3b,c
crystallize in good yields (3b, 85% 3c, 58%) from hexane or
pentane, respectively. The 29Si NMR spectra of 3a,b exhibit
each two signals at relatively low field and one at higher field
(3a, δ −17.42, −25.05, −91.49; 3b, δ 40.5, 27.9, and −78.7
ppm). In both cases, the two lower field resonances are
extremely broad and are assigned to the bridgehead silicon
atoms, while the high-field resonance is due to the Tip2Si
group. The silyl-substituted product 3c shows a similar NMR
spectrum, albeit with all three resonances shifted to
substantially higher field (δ −93.9, −105.4, and −127.9
ppm). Again, the two high-field resonances are assigned to
the bridging silicon atoms. The chemical inequivalence of all
three silicon atoms in 3a−c is readily explained by the presence
of the exocyclic imine functionality: the bridgehead atoms are
located in E and Z positions relative to the substituent at the
imine nitrogen atom. The imine 13C resonances of 3a−c are
found at low field, as is usual for cyclic bis(silyl)imines12 (3a, δ
180.8; 3b, 193.2; 3c, 198.2 ppm).
When the silyl-substituted cyclotrisilene 1b is reacted with 2
equiv of cyclohexyl isocyanide, the diiminotrisilabicyclo[1.1.1]-
pentane 4c is formed, presumably via a second equivalent of
isocyanide reacting with 3c at the strained Si−Si bridging bond
(Scheme 2). Compound 4c has two resonances in the 29Si
NMR spectrum at δ −19.4 and −108.1 ppm, reflecting its
higher symmetry in comparison to 3c. A single resonance for
the two imine carbon atoms was detected at δ 194.2 ppm in the
13C NMR spectrum.
Single crystals of 3b,c were obtained from hexane and
pentane, respectively. X-ray crystallography confirmed the
formulation of the two compounds as the products of formal
[2 + 1] cycloadditions between 1 and isocyanides. The
structure of 3c suffers from severe disorder (details are given
in the Supporting Information); therefore, only the structure of
3b will be discussed here.16 Compound 3b crystallized as the
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Iminotrisilacyclobutenes. When the cyclotrisilenes 1a,b
were reacted with tert-butyl and xylyl isocyanide, respectively,
divergent reactivity was observed. Addition of xylyl isocyanide
to solutions of 1b at room temperature afforded the ring-
expanded iminotrisilacyclobutene 2d as dark red crystals in
moderate (48%) yield (Scheme 2). Similarly, 2a was formed via
of the SiSi and CN double bonds in 2d, the bond lengths
of SiSi (2.1975(7) Å) and CN (1.286(2) Å) are very
similar to those observed in other four-membered cyclic
disilenes (2.163−2.174 Å)4a,6,18 and nonconjugated imines C
N (1.279 Å).17 Moreover, the skeletal Si1−C1 bond length of
1.9169(18) Å is very similar to that of the single Si−C bonds in
the bicyclo[1.1.0]trisilabutane derivatives (1.906−1.920 Å).7a
For 2a, the SiSi and CN double-bond lengths (2.1505(6)
and 1.279(2) Å, respectively) are both slightly shorter than
those observed for 2d, as is the SiSi−C single bond at
1.906(2) Å.
Despite the relative coplanarity of the SiSi and CN
double bonds in 2a,d, there is no clear structural evidence for
conjugation. We therefore recorded UV−vis spectra of 2a,d in
hexane to further clarify the issue. While solutions of 2a are
bright yellow, with an absorbance at λmax 402 nm fairly typical
of an aryl-substituted disilene, 2d is deep red. The UV−vis
absorption spectrum of 2d in hexane has a strong absorption at
455 nm (ε = 11000 cm2 mol−1), which was assigned to the
π−π* (SiSi) electronic transition by TD-DFT calculations.
This absorption has a shoulder corresponding to the n (lone
pair on nitrogen) to π* (SiSi) forbidden transition. The
maximum wavelength of 2d is very close to that of other four-
membered disilenes (420−455 nm).4a,6,18
t
the reaction of 1a with BuNC at 60 °C for 16 h. In the 29Si
NMR spectrum of 2b, two low-field resonances (δ 173.9 and
165.1 ppm) are characteristic of the presence of a silyl-
substituted SiSi double bond, although these values are
somewhat downfield in comparison with other four-membered
disilenes (with chemical shifts between δ 141.3 and 167.6
ppm).4a,6,18 The imino carbon atom was observed at δ 206.7
ppm in the 13C NMR spectrum, in the same region as that for 3
and 4. The 29Si NMR spectrum of the Tip-substituted 2a
reveals two signals at δ 131.63 and 82.77 ppm for the SiSi
double bond, upfield of those observed for 2b, as would be
expected for an aryl-substituted disilene in comparison to a
silyl-substituted example. The four-coordinate silicon atom is
found at δ −11.86 ppm. In the 13C NMR spectrum, the imino
carbon shows a resonance in the typical downfield region at δ
194.8 ppm.
The molecular structures of 2a (see the Supporting
Information) and 2d in the solid state were determined by
single-crystal X-ray diffraction analyses (Figure 2). The four-
The possibility of SiSi and CN conjugation in 2d was
also examined by DFT calculations at the B3LYP/6-31G(d)
level on the model compound IIb (in which the SiMetBu2
groups of 2d are replaced by SiMe3 groups). The optimized
structural parameters of IIb are in good agreement with the X-
ray crystallographic data for 2d.
The molecular orbitals of IIb (Figure 3) show that the
HOMO consists of the Si−Si π bond and some minor
Figure 3. Relevant molecular orbitals of IIb.
Figure 2. ORTEP diagram of 2d (50% probability level). Hydrogen
atoms are omitted for clarity. Selected bond distances (Å): Si1−Si2 =
2.1975(7), Si1−C1 = 1.9169(18), Si2−Si3 = 2.3835(7), Si3−C1 =
1.9567(18), N1−C1 = 1.286(2). Selected bond angles (deg): Si2−
Si1−Si4 = 142.95(3), Si2−Si1−C1 = 88.91(6), Si4−Si1−C1 =
125.78(6), Si1−Si2−Si3 = 83.26(2), Si1−Si2−Si5 = 134.78(3), Si3−
Si2−Si5 = 141.93(3), Si2−Si3−Si6 = 103.89(2), Si2−Si3−Si7 =
122.04(3), Si6−Si3−Si7 = 118.52(3), Si2−Si3−C1 = 82.81(5), Si6−
Si3−C1 = 111.27(5), Si7−Si3−C1 = 112.83(5), C1−N1−C38 =
125.51(15), Si1−C1−Si3 = 103.72(8), Si1−C1−N1 = 135.10(14),
Si3−C1−N1 = 120.66(13).
contributions from the Si−SiMe3 σ bonds. The LUMO consists
of the π* orbital of the SiSi bond slightly mixed with the π*
orbital of the CN bond. The CN π bonding orbital is
primarily located in the HOMO−15. These computational
results are consistent with both the spectroscopic and structural
properties of 2d, which indicate only minor interactions
between the SiSi and CN double bonds.
Mechanism for the Formation of 2a,d. In an attempt to
understand the factors controlling formation of the iminobicy-
clobutane products 3 vs the iminocyclobutenes 2, calculations
on the model compounds iminobicyclobutane I and
iminocyclobutene II were performed at the B3LYP/6-31G(d)
level (Figure 4). Model compounds IIa,b, bearing alkyl and aryl
substituents, respectively, at nitrogen, are both found to be
more stable than their bicyclobutane isomers I, with the aryl-
substituted cyclobutene IIb showing the greater relative
stability vs its bicyclobutane isomer (IIa, −2.4 kcal mol−1 vs
Ia; IIb, −5.1 kcal mol−1 vs Ib). These results suggest that the
membered rings of the two compounds are almost planar, with
the sum of the internal angles being close to 360° (2a, 358.9°;
2d, 358.7°). For 2d, the SiSi and CN double bonds lie in
the same plane, as seen from the values for the sum of angles at
Si1, Si2, and C1, which are all close to 360°. In 2a, the CN
bond lies somewhat out of the plane of the SiSi double bond,
with an angle of 9.6° between the plane defined by C−NC
and that defined by the four ring atoms. Despite the coplanarity
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ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 23108701, 23655027, 24245007, 90143164)
from the Ministry of Education, Science, Sports, and Culture of
Japan as well as by the EPSRC (EP/H048804/1), the Alfried
Krupp Foundation, and the European Commission with a
Marie-Curie Fellowship (M.J.C.).
Figure 4. Relative energies of trimethylsilyl-substituted Si3C
bicyclo[1.1.0]butanes I and cyclobutenes II (optimized at the
B3LYP/6-31G(d) level): Ia, IIa, R′ = Cy; Ib, IIb, R′ = xylyl.
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ASSOCIATED CONTENT
■
S
* Supporting Information
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Text, figures, tables, and CIF files giving experimental
procedures and spectral data for 2−4, computational results
for I and II, and crystallographic data including atomic
positional and thermal parameters for 2a,d. This material is
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(16) We will not discuss details of the structural parameters of
iminotrisilabicyclo[1.1.0]butane 3c and [1.1.1]pentanes 4c,d because
the refinement of their crystal structures was of insufficient quality.
(17) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.
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Organometallic Compounds of Low-Coordinate Si, Ge, Sn and Pb; Wiley:
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AUTHOR INFORMATION
■
Corresponding Author
*A.S.: tel, +81-29-853-4314; fax, +81-29-853-4314; e-mail,
sekiguch@chem.tsukuba.ac.jp. D.S.: tel, +49-681-302-71641;
fax, +49-681-302-71642; e-mail, scheschkewitz@mx.uni-
saarland.de.
Notes
The authors declare no competing financial interest.
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