High yield access to silylene RSiCl (R = PhC(NtBu)2 and its reactivity toward alkyne: Synthesis of stable disilacyclobutene

Article abstract of DOI:10.1021/ja9091374

Two new approaches for synthesizing RSiCl, (R ) PhC(NtBu)₂) are reported by the reaction of RSiHCl₂ with bis-trimethyl silyl lithium amide and N-heterocyclic carbene respectively. In the former method silylene is produced in 90% yield. The silylene was treated with biphenyl alkyne to afford the disilacyclobutene system. This is a rare example of two five-coordinate silicon centers arranged adjacent to each other in a four-membered ring. Furthermore, we fluorinated the four-membered ring by trimethyltin fluoride to obtain the fluoro substituted disilacyclobutene.

Full text of DOI:10.1021/ja9091374

Published on Web 12/31/2009
High Yield Access to Silylene RSiCl (R ) PhC(NtBu)2) and Its
Reactivity toward Alkyne: Synthesis of Stable Disilacyclobutene
Sakya S. Sen, Herbert W. Roesky,* Daniel Stern, Julian Henn, and Dietmar Stalke
Institut fu¨r Anorganische Chemie der UniVersita¨t Go¨ttingen, Tammannstrasse 4,
37077 Go¨ttingen, Germany
Received October 28, 2009; E-mail: hroesky@gwdg.de
Abstract: Two new approaches for synthesizing RSiCl, (R ) PhC(NtBu)2) are reported by the reaction of
RSiHCl2 with bis-trimethyl silyl lithium amide and N-heterocyclic carbene respectively. In the former method
silylene is produced in 90% yield. The silylene was treated with biphenyl alkyne to afford the disilacyclobutene
system. This is a rare example of two five-coordinate silicon centers arranged adjacent to each other in a
four-membered ring. Furthermore, we fluorinated the four-membered ring by trimethyltin fluoride to obtain
the fluoro substituted disilacyclobutene.
Introduction
Benkeser and his co-workers discussed the formation of the
SiCl3 anion when trichlorosilane is treated with amines.5 We
showed that N-heterocyclic carbene could function as a dehy-
drochlorinating agent and we were able to isolate a Lewis base
stabilized dichloro silylene.6 Subsequently Cui et al. reported
the isolation of four- and five-membered silylenes using the
same approach.7 We employed these two techniques further and
found out two new routes for the preparation of LSiCl in good
yields. No silylenes have been isolated so far by using lithium
amide as dehydrochlorinating agent.
In the past 20 years N-heterocyclic silylenes attracted much
attention because of their structure, bonding, and potential
applications in transition metal catalysis.1 A handful of room
temperature stable silylene complexes were since then synthe-
sized and structurally characterized.2 In 2006, we successfully
prepared the first heteroleptic chloro silylene stabilized by a
benzamidinato ligand with tBu substituents on nitrogen atoms.
It was prepared by reduction of LSiCl3 (L ) PhC(NtBu)2) with
finely divided potassium in 10% yield.3 The reactivities of bare
silylenes are well documented but little is known about the
chemistry of substituted silylenes.4 However, due to the low
yield of LSiCl, further investigations were limited. Hence, there
is a high quest for an alternative route to silylene under mild
conditions with good yield.
Results and Discussion
Previously we used SiCl4 for the preparation of LSiCl3, which
we further selectively reduced with finely divided potassium to
LSiCl.3 In the new procedure we reacted HSiCl3 with tert-butyl
carbodiimide and phenyl lithium to yield LSiHCl2. The compound
was characterized by NMR spectroscopy, EI-mass spectrometry,
and elemental analysis. A toluene solution of LSiHCl2(1) and 1,3-
di-tert-butylimidazol-2-ylidene under stirring immediately changed
the color from colorless to yellow and finally to brown-red with
the formation of a white precipitate. The insoluble white precipitate
was identified as 1,3-di-tert- butylimidazolium chloride and the
soluble part as the silylene 2 with 35% yield, which was confirmed
by NMR spectroscopy, EI-mass spectrometry, and compared to a
previously reported sample (Scheme 1). However 2 is easily
accessible in 90% yield from the direct reaction of 1 with
LiN(TMS)2 as a base in molar ratio of 1:1. The increase in the
yield for 2 with the new method allows investigating its reactivity.
Silicon-containing small ring compounds are of interest
because of their versatile role as building blocks in organosilicon
chemistry.8 Therefore we turned our attention toward alkyne
to prepare cyclobutene analogues of silicon since alkynes are
well-known labile and effective trapping reagents for main group
(1) (a) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem. Res. 2000, 33,
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9
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J. AM. CHEM. SOC. 2010, 132, 1123–1126 1123

A R T I C L E S
Sen et al.
tive6 and also with those known for cyclic silicon compounds.12
In the EI-MS spectrum, only fragments were observed. All
ions are in accordance with the proposed formula of 3.
Scheme 1. Preparation of 1 and 2
The molecular structure of 3 is shown in Figure 1.13
Compound 3 crystallizes in the monoclinic space group P21/c
(Table 1). The Si-Si bond length of 2.36(4) Å is comparable
with those of compounds containing Si-Si single bonds.14 The
distance between the two carbon atoms in the ring (1.36(12)
Å) corresponds to a carbon carbon double bond. The Si-C bond
distances are (1.92(9) Å and 1.93(9) Å) in the expected range.15
The most important feature is the four-membered Si2C2ring that
is almost planar (sum of the internal angles 357.82°). The
amidinate ligands and chlorine atoms are disposed above and
below the Si2C2 ring in such a way that the Si centers exhibit
trigonal bipyramidal coordination sites. To the best of our knowl-
edge, two five-coordinate silicon centers arranged adjacent to each
other in a four-membered ring have not been reported so far.
To give a mechanistic insight of the reaction we postulate
that initially there is an oxidative addition between chloro
silylene and biphenyl alkyne resulting in the formation of a
strained three-membered ring. Usually such type of cycloaddi-
tion or oxidative addition reactions is very common for heavier
group 14 elements with unsaturated hydrocarbon.16 The strained
three-membered ring undergoes a facile rearrangement by the
insertion of another silylene molecule, thus giving rise to a stable
four-membered disilacyclobutene system with formation of a
Si-Si bond (Scheme 3). This is an oxidative addition followed
by insertion reaction which is novel in the case of a Si(II)
system. Although we were not able to trap the silacyclopropene,
we were recently successful in characterizing the corresponding
alumacyclopropene.11 Therefore, the formation of silacyclopro-
pene as an intermediate proceeds with very high plausibility.
To obtain the complexation energy of the reaction, the
energy of the isolated moieties 2, 3, and biphenyl alkyne
were calculated. All the calculations were performed on the
B3LYP/TZVP17-19 level of calculation with Turbomole.20
Individual optimization and energy calculations on each part
were performed. Subsequent calculations of frequencies
yielded only positive frequencies for all parts. (Calculated
energy of 2 -1444.399479989 au, energy of biphenyl alkyne
Scheme 2. Preparation of 3
carbene analogues.9 The formation of disilacyclobutene was
commonly achieved by the reduction of 1,2-bis(chloroalkylsilyl)-
benzene with sodium.10 But a promising alternative strategy
represents the utilization of unsaturated silicon compounds and
alkyne to obtain disilacyclobutene system. We already reported
the successful preparation of the cyclopropene analogue of
aluminum.11 This prompted us to investigate whether LSiCl is
also capable of giving novel strained silicon-carbon cycles.
LSiCl was reacted with biphenyl alkyne in toluene at ambient
temperature under stirring overnight (Scheme 2). The solution
was concentrated and kept for crystallization. After 4 days
colorless crystals of 1,2-disilacyclobutene 3 were obtained
suitable for X-ray crystallography. The structure of the com-
pound was confirmed by NMR spectroscopy, EI-mass spec-
trometry, X-ray crystallography, and elemental analysis. The
29Si NMR showed a resonance at -109.53 ppm. The value is
consistent with that of the reported trisilacyclopentane deriva-
(12) Ando, W.; Shiba, T.; Hidaka, T.; Morihashi, K.; Kikuchi, O. J. Am.
Chem. Soc. 1997, 119, 3629–3630.
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9
1124 J. AM. CHEM. SOC. VOL. 132, NO. 3, 2010

Silylene RSiCl (R ) PhC(NtBu)2)
A R T I C L E S
Figure 1. Crystal structure of 3. Hydrogen atoms are not shown for clarity. Selected bond distances (Å) and bond angles (deg): Si(1)-N(2) 1.8389(8), Si(1)-C(32)
1.9285(9), Si(1)-N(1) 1.9943(8), Si(1)-Cl(1) 2.1688(3), Si(1)-Si(2) 2.3659(4), Si(2)-N(3) 1.8284(8), Si(2)-C(31) 1.9348(9), Si(2)-N(4) 2.0370(9), Si(2)-Cl(2)
2.1551(3), C(31)-C(32) 1.3694(12); N(2)-Si(1)-C(32) 108.30(4), N(2)-Si(1)-N(1) 68.46(3), C(32)-Si(1)-N(1) 173.99(4), N(2)-Si(1)-Cl(1) 117.83(3),
C(32)-Si(1)-Cl(1) 90.40(3), N(1)-Si(1)-Cl(1) 86.88(3), N(2)-Si(1)-Si(2) 124.71(3), C(32)-Si(1)-Si(2) 74.76(3), N(1)-Si(1)-Si(2) 111.25(3), Cl(1)-Si(1)-Si(2)
117.322(14), N(3)-Si(2)-C(31) 109.78(4), N(3)-Si(2)-N(4) 67.91(3), C(31)-Si(2)-N(4) 175.62(4), N(3)-Si(2)-Cl(2) 114.46(3), C(31)-Si(2)-Cl(2) 91.59(3),
N(4)-Si(2)-Cl(2) 86.18(3), N(3)-Si(2)-Si(1) 126.47(3), C(31)-Si(2)-Si(1) 74.54(3), N(4)-Si(2)-Si(1) 109.83(3), Cl(2)-Si(2)-Si(1) 118.779(14).
Table 1. X-ray Data for Compound 3
of the aforementioned moieties was calculated in its final
geometry once with basis set contributions from the other
parts of the molecule, where the nuclear charge of these other
parts was set to zero, and once for the isolated parts in their
final geometry. Subtracting these values leads to the coun-
terpoise energy of ∆Ecp -3.69048 kcal/mol. This is an
estimate for the artificial energy lowering the whole molecule
due to the basis set contributions from 2, 3, and biphenyl
alkyne. The corrected final complexation energy with coun-
terpoised corrected complexation energy is -4.08839 kcal/
mol. In summary, the calculation does not give an answer
for the preferred formation of the four-membered ring system.
Compound 3 is easily converted to the corresponding fluorine
derivative 4 using trimethyltin fluoride as a fluorinating agent
(Scheme 4). Compound 4 is a colorless solid and soluble in
solvents like toluene, ether and THF. 4 was characterized by
1H, 19F, and 29Si NMR spectroscopy. In the 19F NMR spectrum
of 4, a resonance appeared as a sharp singlet at -71.73 ppm
with silicon satellite (JSi-F ) 254.22 Hz). The values are
consistent with those reported in literature.22
3
empirical formula
molecular weight
color
CCDC-No.
crystal system
space group
a [Å]
C44H56Cl2N4Si2
768.01
colorless
748995
monoclinic
P21/c
13.1830(4)
13.2275(4)
24.4548(8)
90
94.5010(1)
90
4251.2(2)
4
b [Å]
c [Å]
R [deg]
ꢀ [deg]
γ [deg]
V [Å3]
Z
Fcalc [g m-3
]
1.200
0.244
1640.0
µ [mm-1
F(000)
]
wR2 (all data)
0.1388 (25354)
-539.2917962348 au and of 3 -3428.103152781 au) The
overall stabilization energy is -0.0123966 au (-7.77885
kcal/mol). This value, however, is too low due to a basis set
superposition error. A counterpoise correction21 was applied,
and therefore 6 additional calculations were necessary. Each
Experimental Section
All manipulations were carried out in an inert atmosphere of
dinitrogen using standard Schlenk techniques and in a dinitrogen
(21) van Duijneveldt, F. B.; van de Rijdt, J. G. C. M.; van Lenthe, J. H.
(22) Ghadwal, R. S.; Sen, S. S.; Roesky, H. W.; Tavcar, G.; Merkel, S.;
Chem. ReV. 1994, 94, 1873–1885.
Stalke, D. Organometallics 2009, 28, 6374–6377.
9
J. AM. CHEM. SOC. VOL. 132, NO. 3, 2010 1125

A R T I C L E S
Sen et al.
Scheme 3. Suggested Mechanism for the Formation of 3
Scheme 4. Preparation of 4
Alternative Method. Toluene (50 mL) was added to a mixture
of 1 (1.00 g, 3.03 mmol) and 1,3-di-tert-butylimidazol-2-ylidene
(0.6 g, 3.33 mmol) at ambient temperature. The resulting mixture
was stirred overnight. The solvent was then removed in Vacuo, and
the residue was extracted with toluene (50 mL). The filtrate was
concentrated to yield colorless crystals of 2 (0.32 g, 35%). Mp
159-162 °C. Elemental analysis (%) calcd for C15H23ClN2Si: C,
61.10; H, 7.87; N, 9.51; found: C, 60.95; H, 7.64; N, 9.35; 1H NMR
(200 MHz, C6D6, 25 °C): δ 1.08 (s, 18 H, tBu), 6.78-7.05 ppm
(m, 5 H, Ph); 13C{1H} NMR (125.75 MHz, C6D6, 25 °C): δ 31.4
(CMe3), 53.7 (CMe3), 126.32, 127.4, 127.9, 128.4, 129.8, 133.0
(Ph), 166.7 ppm (NCN); 29Si{1H} NMR (99.36 MHz, C6D6, 25
°C): δ 14.6; EI-MS: m/z: 295 [M+].
filled glovebox. Solvents used were purified by MBRAUN solvent
purification system MB SPS-800. 1,3-Di-tert-butylimidazol-2-
ylidene23 and Me3SnF24 were prepared by the literature methods.
All chemicals purchased from Aldrich were used without further
Preparation of 3. To the mixture of 2 (0.30 g, 1.02 mmol) and
biphenyl alkyne (0.10 g, 0.60 mmol) toluene (20 mL) was added
at room temperature. The mixture was stirred overnight. The solid
was filtered off and the solution was concentrated and kept at room
temperature for 4 days to yield colorless crystals of 3 (0.11 g, 42%).
Mp 165-170 °C. Elemental analysis (%) calcd for C44H56Cl2N4Si2:
1
purification. H, 13C, 19F, 29Si NMR spectra were recorded using
Bruker Avance DPX 200 or Bruker Avance DRX 500 spectrom-
eters. The NMR spectra were recorded in C6D6. The chemical shifts
δ are given relative to SiMe4. EI-mass spectra were obtained using
a Finnigan MAT 8230 instrument. Elemental analyses were
performed by the Institut fu¨r Anorganische Chemie, Universita¨t
Go¨ttingen. Melting point was measured in a sealed glass tube on a
Bu¨chi B-540 melting point apparatus and was uncorrected.
Preparation of 1. PhLi (6.86 mL, 13.72 mmol, 1.8 M in diethyl
ether) was added to a solution of tBuNdCdNtBu (2.12 g, 13.72
mmol) in diethyl ether (80 mL) at -78 °C. The solution was raised
to ambient temperature and stirred for 4 h. To the solution HSiCl3
was added drop by drop (4.25 mL, 17.17 mmol) at -78 °C. The
reaction mixture was warmed to room temperature and was stirred
for 12 h. The precipitate was filtered off, and after removal of all
volatiles the residue was extracted with toluene (20 mL). Storage
of the extract at -32 °C in a freezer for 1 day afforded colorless
crystals of 1. Mp 145-150 °C. Elemental analysis (%) calcd for
C15H24Cl2N2Si (331.36): C, 54.37; H, 7.30; N, 8.45; found: C,54.66;
1
C, 68.81; H, 7.35; N, 7.29; found: C, 67.95; H, 7.04; N, 8.35; H
NMR (300 MHz, C6D6, 25 °C): δ 1.15 (s, 36 H, tBu), 6.78-7.05
(m, 10 H, Ph), 7.28-7.55 ppm (m, 10 H, Ph); 13C{1H} NMR
(125.75 MHz, C6D6, 25 °C): δ 32.83 (CMe3), 55.23 (CMe3), 128.5,
128.9, 129.4, 129.9, 130.2, 133.0 (Ph), 156.45 ppm (NCN); 29Si{1H}
NMR (99.36 MHz, C6D6, 25 °C): δ -109.53 ppm. EI-MS: m/z:
294 [M+](100%).
Preparation of 4. To the mixture of 3 (0.5 g, 0.65 mmol) and
trimethyltin fluoride (0.24 g, 1.31 mmol) toluene (25 mL) was added
and stirred until the solution becomes transparent. After filtration
the solution was concentrated and kept at - 32 °C to get pure
compound 4 (0.2 g, 41%). Mp 182 - 185 °C. 1H NMR (300 MHz,
C6D6, 25 °C): δ 1.23 (s, 36 H, tBu), 7.12-7.28 (m, 10 H, Ph),
7.48-7.75 ppm (m, 10 H, Ph); 13C{1H} NMR (125.75 MHz, C6D6,
25 °C): δ 32.4 (CMe3), 53.7 (CMe3), 127.4, 127.9, 128.4, 129.8,
133.0 (Ph), 166.7 ppm (NCN); 19F (188.31 MHz, CFCl3, C6D6, 25
°C): δ -71.73 (JSi-F ) 254.22 Hz) ppm: 29Si{1H} NMR (99.36
MHz, C6D6, 25 °C): δ -117.82, - 115.26 (doublet JSi-F ) 254.22
Hz) EI-MS: m/z: 283 [M+](100%).
1
H, 7.63; N,8.06. H NMR (200 MHz, C6D6, 25 °C): δ 1.08 ppm
(s, 18H, tBu), 6.70 ppm (s, 1H, Si-H), 7.41-7.48 ppm (m, 5H,
Ph); 13C{1H}NMR (125.75 MHz, C6D6, 25 °C): δ 31.8 (CMe3),
54.2 (CMe3), 128.6, 128.9, 129.1, 129.9, 130.6, 135.6 (Ph), 173.3
ppm (NCN); 29Si{1H}NMR (99.36 MHz, C6D6, 25 °C): δ -96.834
ppm. EI-MS: m/z(%): 330 [M+], (100).
X-Ray Crystallography. Shock cooled crystals were selected
and mounted under nitrogen atmosphere using the X-TEMP2. The
structure was solved by direct methods (SHELXS) and refined on
F2 using full matrix least-squares methods of SHELXL.
Preparation of 2. Toluene (50 mL) was added to a mixture of 1
(0.31 g, 1.00 mmol) and bis-trimethyl silyl lithium amide (0.17 g, 1.01
mmol) at ambient temperature. Immediately the solution turned to a
red color with the formation of LiCl. The resulting mixture was stirred
overnight. The solvent was then removed in Vacuo, and the residue
was extracted with toluene (20 mL). The filtrate was concentrated to
yield colorless crystals of 2 (0.26 g, 90%).
Acknowledgment. The Deutsche Forschungsgemeinschaft sup-
ported this work.
Supporting Information Available: CIF and theoretical
calculation for 3. This material is available free of charge via
the Internet at http://pubs.acs.org.
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