Synthesis and rearrangement of stable NHC→silylene adducts and their unique reactivity towards cyclohexylisocyanide

Article abstract of DOI:10.1002/asia.200900434

The syntheses and reactivity of the two N-heterocyclic carbine (NHC)→ silylene complexes 2 and 4 have been investigated. The latter are easily accessible by reaction of the zwitterionic, N-heterocyclic silylene LSi: 1 [L=Ar-N-CACHTUNGTRENUNG(=CH2)CH=C(Me)-N-Ar, Ar=2,6-iPr2C6H3] with 1,3,4,5-tetramethylimidazol-2-ylidene and 1,3-diisopropyl-4,5-dimethylimidazol- 2-ylidene, respectively. While compound 2 undergoes facile rearrangement above -20°C to give the unsymmetrical Nheterocyclic silylcarbene 3, the derivative 4 remains unchanged even after boiling in benzene. The remarkable reactivity of 3 and 4 towards cyclohexyl-isocyanide has been examined which leads in a unique series of C-H, Si-H, and C-N bond activations to the new triaminosilanes 5 and 6, respectively. The novel compounds 3, 4, 5, and 6 were fully characterized by 1H, 13C, and 29Si NMR spectroscopy, EI-MS, elemental analysis, and single-crystal Xray diffraction.

Full text of DOI:10.1002/asia.200900434

FULL PAPERS
DOI: 10.1002/asia.200900434
Synthesis and Rearrangement of Stable NHC!Silylene Adducts and Their
Unique Reactivity towards Cyclohexylisocyanide
Yun Xiong, Shenglai Yao, and Matthias Driess*[a]
Abstract: The syntheses and reactivity
of the two N-heterocyclic carbene
(NHC)! silylene complexes 2 and 4
have been investigated. The latter are
easily accessible by reaction of the
zwitterionic, N-heterocyclic silylene
undergoes facile rearrangement above
ꢀ208C to give the unsymmetrical N-
heterocyclic silylcarbene 3, the deriva-
tive 4 remains unchanged even after
boiling in benzene. The remarkable re-
activity of 3 and 4 towards cyclohexyl-
ACHTUNGTRNEiNUNG socyanide has been examined which
ꢀ
ꢀ
leads in a unique series of C H, Si H,
ꢀ
and C N bond activations to the new
triaminosilanes 5 and 6, respectively.
The novel compounds 3, 4, 5, and 6
were fully characterized by H, 13C, and
1
LSi: 1 [L=Ar-N-C
N
29Si NMR spectroscopy, EI-MS, ele-
mental analysis, and single-crystal X-
ray diffraction.
N-Ar, Ar=2,6-iPr2C6H3] with 1,3,4,5-
tetramethylimidazol-2-ylidene and 1,3-
diisopropyl-4,5-dimethylimidazol-2-yli-
dene, respectively. While compound 2
Keywords: C-H activation
· C-N
activation · hydrosilylation · keteni-
mines · silicon
Introduction
heterocyclic carbenes (NHCs) as depicted in Scheme 1. It is
noteworthy that using less bulky isocyanides such as tBuNC:
and MesNC: (Mes=2,4,6-trimethylphenyl) leads to transient
There has been increasing interest over the last two decades
in the study of stable silylene species (divalent, two-coordi-
nate silicon compounds) in part owing to their carbene-like
reactivites.[1] Silylenes, in contrast to the carbene homo-
logues, have a singlet ground state with a lone pair of elec-
trons (mainly 3s character) and a vacant 3p orbital at the
SiII site and thus can act either as Lewis bases or Lewis
acids toward substrates.[2] In terms of their Lewis base reac-
tivity, silylenes can serve as strong donor ligands as shown in
numerous metal–silylene complexes which represent versa-
tile functional systems in catalysis and beyond.[3] On the
other hand, although many complexes in which silylenes
serve as Lewis acids have been reported, they are too reac-
tive to be isolable at ambient temperature and thus could
only be observed spectroscopically in cryogenic matrices.[4]
Rare exceptions of stable examples of the latter type com-
prise complexes with bulky substituted isocyanides and N-
ꢀ
ꢀ
silylene–isocyanide complexes which afford C H or C C
[5a]
ꢀ
and Si C activation species as final products.
[a] Dr. Y. Xiong, Dr. S. Yao, Prof. Dr. M. Driess
Technische Universitꢀt Berlin
Institute of Chemistry:
Metalorganic and Inorganic Materials
Sekr. C2, Strasse des 17. Juni 135, 10623 Berlin (Germany)
Fax : (+49)30-314-29732
Scheme 1. Several NHC- and RNC-stabilized low-coordinate silicon spe-
cies and a silaketenimine.
E-mail: matthias.driess@tu-berlin.de
322
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Chem. Asian J. 2010, 5, 322 – 327

In line with that, the reactions of stable N-heterocyclic si-
lylenes (NHSis) with tBuNC furnish silacyanides and/or 1-
ꢀ
cyano-2-organo-disilanes under Me3C NC bond cleava-
ge.[5b–d] Remarkably, using an electronically less disturbed di-
alkylsilylene, Kira et al. were able to obtain stable silakete-
nimines (Scheme 1, compound b).[6] In 1999 Gehrhus and
Lappert reported a stable NHC-silylene adduct.[7] Very re-
cently, the utilization of NHCs to stabilize low-valent silicon
species has achieved a great progress. Presumably the most
striking example is the synthesis of an isolable NHC!:Si=
!
Si: NHC species with silicon in the formal oxidation state
zero (Scheme 1, compound d).[8a] Accordingly, the first relat-
ed NHC-stabilized dihalosilylenes NHC!SiX2 (X=Cl, Br)
have also been synthesized (Scheme 1, compound e).[8b–c]
We have been engaged in exploring the chemistry of the
thermally stable, partially zwitterionic, N-heterocyclic sily-
lene LSi: 1 [L=Ar-N-CACTHNUTRGNE(UGN =CH2)CH=C(Me)-N-Ar, Ar=2,6-
iPr2C6H3][9] which shows also a striking reactivity toward iso-
cyanides and reacts with an excess of cyclohexylisocyanide,
ꢀ
Scheme 2. Tautomerization of 2 to 3 via the proposed pathway (1) or (2).
affording an unusual cycloaddition and C H activation
product along with a silacyanide.[10] More recently, we com-
municated the synthesis and structure of the 1:1 adduct 2,
starting from 1 and a NHC (Scheme 1).[11] We have now
learned that 2 is thermolabile and undergoes unexpected
tautomerization to give the unsymmetric N-heterocyclic si-
lylcarbene 3. The existence of 2 and 3 prompted us to ex-
plore its reactivity toward organoisocyanides which turned
out to be very different from that of 1. Herein we report on
the formation of 3, the synthesis and structure of the new
NHC!silylene homologue 4, and their unusual reactivity
towards cyclohexylisocyanide.
of a NMe group to give B, which transforms to 3 under the
cleavage of the NHC!Si dative bond.
Compound 3 has been fully characterized by 1H, 13C,
29Si NMR spectroscopy, EI-MS, and elemental analysis. In
1
the H NMR spectrum of 3 the singlet at d=6.17 ppm can
ꢀ
be unequivocally assigned to the Si H proton, and the re-
spective 29Si nucleus resonates at d=ꢀ38 ppm in the
29Si{H} NMR spectrum, indicating a four-coordinated silane
species. The 13C{H} NMR spectrum exhibits a characteristic
low-field resonance for the two-coordinate sp2 carbon atom
of the NHC at d=208.7 ppm. The structure of the silyl-sub-
stituted unsymmetrical NHC 3 has been confirmed by X-ray
diffraction analysis (Figure 1). It crystallizes in the triclinic
space group P1 with two independent molecules in the
asymmetric unit. The carbene moieties of both molecules
are disordered over two orientations (60:40). Compound 3
consists of a six-membered, puckered SiC3N2 ring and a
five-membered, planar NHC skeleton which are bridged by
a methylene group.
Results and Discussion
As reported previously,[11] the NHC-silylene adduct 2 is ac-
cessible at ꢀ608C by direct conversion of silylene 1 with
1,3,4,5-tetramethylimidazol-2-ylidene in toluene and can be
isolated in high yield by crystallization at low temperature.
Dissolution of 2 in toluene revealed that it undergoes slowly
ꢀ
rearrangement under intramolecular C H activation above
ꢀ208C to give the N-heterocyclic unsymmetric silylcarbene
In order to learn something about the influence of steric
congestion on adduct formation, the more crowded 1,3-di-
1
3 (Scheme 2). Monitoring the reaction progress by H NMR
spectroscopy revealed that the conversion can be completed
within 12 h at ambient temperature, and finally compound 3
can be isolated in the form of colorless crystals in 75%
yield.
ACHTUNGTRENiNUNG sopropyl-4,5-dimethylimidazol-2-ylidene has been employed
as NHC ligand. In fact, while the latter reacts with equimo-
lar amounts of 1 in toluene at ꢀ608C, leading to the corre-
sponding adduct 4 (Scheme 3), further increase of the steric
congestion of the NHC ligand precludes adduct formation
with 1. Thus, 1,3-bis(tert.-butyl)imidazol-2-ylidene and its
heavier homologue, the silylene L’Si: (L’=tBu-N-CH=CH-
N-tBu), are chemically inert towards 1 (Scheme 3).
In contrast to 2, the adduct 4 is even stable in boiling ben-
zene. Compound 4 can be isolated as yellow crystals in 87%
yield. Its composition is proven by multinuclear NMR, IR
spectroscopy, elemental analysis, and EI-MS. The 29Si NMR
spectrum of 4 exhibits a slight shielding for the three-coordi-
nate, divalent 29Si atom in comparison to the situation ob-
Although the mechanism is still unknown, we propose the
ꢀ
two possible pathways (1) and (2) for the C H activation
and tautomerization of 2 which are sketched in Scheme 2.
Pathway (1) involves deprotonation of a NMe group of the
NHC donor in close proximity to the nucleophilic exocyclic
methylene group of the partially zwitterionic silylene 1
which yields intermediate A. Subsequently, the latter under-
goes HACHTUNGTRENNUNG[1,4]- and SiACHTUNGTRENNUNG[1,3]-shifts to yield the final product. Al-
ternatively and according to pathway (2), the reactive sili-
ꢀ
N
Chem. Asian J. 2010, 5, 322 – 327
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323

M. Driess et al.
FULL PAPERS
Figure 1. Molecular structure of 3. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms (except for that at Si1) are omitted for
clarity. Two independent molecules in the asymmetric unit were ob-
served. Additionally, the carbene moieties are statistically disordered
over two orientations in a population ratio of 60:40 and 44:56, respective-
ly. Only one of the two molecules with one orientation is shown. Selected
bond lengths (pm) and bond angles (8): Molecule 1: Si1-N1 172.8(4), Si1-
N2 173.1(4), Si1-C30a 187.2(5); N1-Si1-N2 104.1(2); Molecule 2: Si1–N1
176.3(4), Si1–N2 175.7(4), Si1–C30a 185.0(6); N1-Si1-N2 102.1(2).
Figure 2. Molecular structure of 4. Thermal ellipsoids are drawn at 50%
probability level. H atoms and a cocrystalized toluene molecule are omit-
ted for clarity. Selected distances (pm) and angles (8): Si1-N2 179.4(2),
Si1-N1 182.0(2), Si1-C30 206.5(2), N1-C2 139.6(3), N2-C4 140.3(3), N3-
C30 136.4(3), N3-C31 139.2(3), N4-C30 136.4(3), N4-C32 139.8(3), C1-C2
136.7(3), C2-C3 144.8(3), C3-C4 135.4(3), C4-C5 148.9(3), C31-C32
134.3(3); N2-Si1-N1 97.64(8), N2-Si1-C30 105.4(1), N1-Si1-C30 97.47(8).
action mixture was allowed to warm up to ambient tempera-
ture, 2 and the isocyanide react solely to the tricyclic triami-
nosilane 5 (Scheme 4), which can be isolated in the form of
Scheme 3. Reactivity of 1 toward bulkier NHCs and an NHSi.
served in 2 (d=ꢀ7.6 for 4 vs. ꢀ12 ppm for 2). The molecu-
lar structure of 4 has been established by X-ray analysis
(Figure 2). As expected, the silicon atom in 4 features a pyr-
amidal coordination similar to the structure of 2, with a five-
membered, planar NHC ring and a six-membered SiN2C3
ring which is slightly puckered. In comparison to 2, both the
ꢀ
longer Si1 C30 distance of 206.5(2) pm (vs. 201.6(3) pm in
Scheme 4. The formation of 5 from 2 via 3.
2) and the larger sum of bond angles of 300.518 (vs. 295.98
in 2) reflect the larger steric congestion of the NHC and the
weaker coordination of the NHC to the silicon(II) atom in
4. However, owing to the electron donation of the NHC, the
yellow crystals in 81% yield. The formation of 5 can be ex-
plained through prior rearrangement of 2 to 3 and subse-
quent conversion of 3 with isocyanide. This was proven by
authentic experiments upon treatment of equimolar
amounts of 3 with cyclohexylisocyanide under similar reac-
tion conditions. Although the mechanism is currently un-
known, we reason that the ketenimine C is an intermediate
which than undergoes intramolecular hydrosilylation of the
C=N double bond and ring closure to furnish 5. In line with
that, it is well known that carbenes can react with organoiso-
cyanides to form ketenimines,[12a,b] and even a silaketeni-
ꢀ
two Si N bonds in 4 (182.0(2) and 179.4(2) pm) become
longer than in 1 (173.4 and 173.5(1) pm),[9a] but are similar
to those in 2 (180.2 and 180.5(3) pm).[11]
The marked nucleophilic character of the SiII center in 2
and 4 prompted us to investigate the reactivity towards or-
ganoisocyanides, which is expected to be different from that
of 1. At first, we probed the reactivity of 2 with equimolar
amounts of cyclohexylisocyanide at ꢀ408C, but no conver-
sion could be detected after several days. However, if the re-
324
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Chem. Asian J. 2010, 5, 322 – 327

Stable NHC!Silylene Adducts and Their Reactivity towards Cyclohexylisocyanide
mine, related to the proposed
intermediate C, has been iso-
lated previously (Scheme 1).[6]
The proposed mechanism to
give 5 is reminiscent of a hy-
drosilylation of a imine cata-
lyzed by NHC which has been
reported recently.[12c]
1
The H NMR spectrum of 5
shows, besides other signals, a
characteristic singlet at d=
Scheme 5. Formation of 6 from 4 and cyclohexylisocyanide, probably via D.
3.98 ppm which can be assigned to the proton of the
RNCH=C moiety. The corresponding 13C nucleus of the
RNCH=C subunit resonates at d=65.9 ppm in the
13C{1H} NMR spectrum, and the singlet at d=ꢀ43.2 ppm in
the 29Si NMR spectrum indicates the presence of a four-co-
ordinate silicon species. An X-ray diffraction analysis re-
vealed that the compound consists of a tricyclic, N-heterocy-
clic triaminosilane skeleton with a tetrahedrally coordinated
silicon atom as a spirobicyclic center (Figure 3). Both six-
donor ability of the NHC, the cyclohexylisocyanide here
acts as electron acceptor to give the proposed intermediate
D (Scheme 5). The latter involves insertion of the terminal
ꢀ
carbon(II) atom of the isocyanide into the C H bond of an
NCHMe2 group to give D, which rearranges to form the
ꢀ
final product 6. The C H bond cleavage of a NHC alkyl
group is reminiscent of the formation of 3 as shown in
ꢀ
Scheme 2. A related base-induced but metal-mediated C H
activation has also been described recently.[12d]
1
ꢀ
In the H NMR spectrum of 6 the C H proton of the
RNCH=CMe2 moiety is observed at d=5.52 ppm, and the
respective carbon atom exhibits a resonance at d=62.8 ppm
in the 13C NMR spectrum. The 29Si NMR spectrum of 6
shows as expected a high-field resonance relative to the pre-
cursor 4 (d=ꢀ58 ppm for 6 vs. ꢀ43 ppm for 5, and
ꢀ7.6 ppm for 4). An X-ray analysis revealed that compound
6 contains a tetrahedrally coordinated silicon atom as part
of a SiN3C group (Figure 4). The transferred CMe2 group
from N4 of the former NHC ligand is now part of a C=C
ꢀ
double bond with a C38 C39 distance of 133.1(3) pm. The
ꢀ
N4 C30 distance of 133.8(3) pm in the modified C3N2 het-
ꢀ
erocycle is indicative of a C=N double bond. The Si1 N5
distance of 170.4(2) pm is slightly shorter than those of Si1
ꢀ
ꢀ
ꢀ
N1 (176.0(2) pm) and Si1 N2 (175.0(2) pm). The Si1 C30
ꢀ
distance of 189.4(2) pm falls into the normal range for a Si
Figure 3. Molecular structure of 5 Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms (except for those at C1 and C37) are
omitted for clarity. Selected bond lengths (pm) and angles (8): Si1-N1
174.0(2), Si1-N2 174.2(1), Si1-N5 172.0(2), Si1-C30 186.3(2), N5-C37
143.7(3), C37-C33 133.4(3), C33-N4 139.2(3), C33-N3 141.0(3), N3-C30
146.3(3); N5-Si1-C30 103.3(1), N1-Si1-N2 102.7(1).
membered, spirobicyclic connected SiN2C3 rings are puck-
ered whereas the five-membered C3N2 ring is planar. The
ꢀ
short C37 C33 distance of 133.4(3) pm is indicative of a
ꢀ
new-formed C=C double bond. The C33 N3 distance of
ꢀ
141.0(3) pm as well as the C33 N4 distance of 139.2(3) pm
are significantly shorter than a C N single bond but longer
than a C=N double bond, presumably because of p delocali-
zation within the five-membered, heterofulvene-like C3N2
ring.
ꢀ
Figure 4. Molecular structure of 6. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms (except for those at C1 and C38) and
two cocrystallized toluene molecules are omitted for clarity. Selected
bond lengths (pm) and angles (8): Si1-N1 176.0(2), Si1-N2 175.0(2), Si1-
N5 170.4(2), Si1-C30 189.4(2), C30-N4 133.8(3), C30-N3 138.9(3), N5-C38
144.3(3), C38-C39 133.1(3); N1-Si1-N2 101.2(1), C30-Si1-N5 105.8(1).
Interestingly, treatment of 4 with an equimolar amount of
cyclohexylisocyanide at ambient temperature takes a com-
pletely different course, affording the novel compound 6 in
70% yield (Scheme 5). Usually an isocyanide acts as elec-
tron donor;[5a,6,10] however, because of the strong electron
Chem. Asian J. 2010, 5, 322 – 327
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325

M. Driess et al.
FULL PAPERS
¯
4: Triclinic, space group P1, a=11.1382(4), b=14.2145(4), c=
14.2274(5) ꢂ, a=75.283(3), b=80.647(3), g=86.899(4)8, V=
2149.48(12) ꢂ3, Z=2, 1calcd =1.108 Mgmꢀ3, m(MoKa)=0.090 mmꢀ1, 18120
C single bond. The bond angles of the SiN3C core are in the
range of 101.2(1)–114.0(1)8 and thus slightly distorted from
a regular tetrahedron.
AHCTUNGTRENNUNG
collected reflections, 7522 crystallographically independent reflections
[Rint =0.0439], 5157 reflections with I>2s(l), qmax =258, R(Fo)=0.0560
2
(I>2s(l)), wR
N
¯
Conclusions
5: Triclinic, space group P1, a=11.7216(5), b=12.0002(6), c=
17.0485(9) ꢂ, a=101.360(4), b=97.438(4), g=116.781(5)8, V=
ACTHNUTRGNEUNG
2032.62(17) ꢂ3, Z=2, 1calcd =1.108 Mgmꢀ3, m(MoKa)=0.093 mmꢀ1, 14803
collected reflections, 7073 crystallographically independent reflections
[Rint =0.0599], 4156 reflections with I>2s(l), qmax =258, R(Fo)=0.0599
We prepared the new bulky substituted NHC!silylene
adduct 4. While the less sterically congested NHC-silylene
adduct 2 undergoes rearrangement via C H activation by
2
ꢀ
(I>2s(l)), wR
(Fo )=0.0996 (all data), 454 refined parameters.
the nucleophilic silicon(II) center to give the unsymmetrical
N-heterocyclic silylcarbene 3 above ꢀ208C, the bulkier
adduct 4 remains stable even in boiling benzene. Because of
the strong nucleophilic character of the SiII atom, the ad-
ducts 2 and 4 possess a very different reactivity towards cy-
clohexylisocyanide in comparison to 1. At low temperature,
compound 2 does not react with cyclohexylisocyanide. In-
stead compound 2 rearranges to 3 at elevated temperature,
prior to reacting with cyclohexylisocyanide. However, the
N-heterocyclic silylcarbene 3 reacts with cyclohexylisocya-
nide to give under formation of a ketenimime intermediate
and subsequent intramolecular hydrosilylation the tricyclic
triaminosilane 5, bearing a spirocyclic tetracoordinate Si
atom. The reaction of 4 with cyclohexylisocyanide proceeds
at room temperature and leads in a unique intramolecular
rearrangement sequence to the novel triaminosilane 6, in-
6: Monoclinic, space group P21/n, a=12.1569(5), b=25.0641(9), c=
16.2116(7) ꢂ, b=93.454(4)8, V=4930.7(3) ꢂ3, Z=4, 1calcd =1.113 Mgmꢀ3
(MoKa)=0.087 mmꢀ1, 25314 collected reflections, 8666 crystallographi-
cally independent reflections [Rint =0.0532], 4468 reflections with I>
2s(l), qmax =258, R(Fo)=0.0492 (I>2s(l)), wRACTHNUTRGENUGN(Fo )=0.0994 (all data), 622
refined parameters.
,
mACHTUNGTRENNUNG
2
Syntheses
3: To
a
solution of 1,3,4,5-tetramethylimidazol-2-ylidene (0.14 g,
1.12 mmol) in toluene (5 mL) was added a solution of silylene 1 (0.50 g,
1.12 mmol) in toluene (5 mL) at ꢀ608C. The reaction mixture was al-
lowed to warm to room temperature. After two days the reaction was
completed and the colorless crystals of 3 at ꢀ208C isolated. Yield: 0.48 g
(0.84 mmol, 75%); m.p.: 858C (decomp.); 1H NMR (200.13 MHz,
[D6]benzene, 258C): d=1.00–1.45 (m, 21H; CHMe2), 1.20 (s, 3H, Me),
1.35 (s, 3H; Me), 1.55 (s, 3H, NCMe), 1.57 (d, 3J (H,H)=7.0 Hz, 3H;
CHMe2), 2.79 (s, 3H; NMe), 3.08 (s, 1H; NCH2), 3.12 (s, 1H; NCH2),
3.32 (s, NCCH2), 3.72 (m, 4H; CHMe2), 3.99 (s, 1H; NCCH2), 5.44 (s,
1H; g-H), 6.17 (s, 1H; SiH), 7.02–7.19 ppm (m, br, 6H; 2,6-iPr2C6H3);
13C{1H} NMR (100.61 MHz, [D6]benzene, 258C): d=8.18, 8.57 (C2Me2),
22.1, 23.3, 24.3, 24.6, 25.0, 25.2, 25.4, 26.1, 26.6 (NCMe, CHMe2), 28.3,
28.4, 28.6, 28.9 (CHMe2), 33.9, 36.9 (NCH2Si, NMe), 86.7 (NCCH2), 104.2
(g-C), 122.0, 122.4, 123.5, 123.9, 124.4, 124.8, 126.9, 139.5, 140.3, 141.8,
146.9, 148.4, 148.8, 148.9, 149.5 (NCMe, NCCH2, 2,6-iPr2C6H3, C2Me2),
208.7 ppm (carbene C); 29Si{1H} NMR (79.49 MHz, [D6]benzene, 258C):
d=ꢀ38.4 ppm (s); EI-MS: m/z (%):568 (55) [M+], 553 (100) [M+ꢀMe],
525 (77) [M+ꢀiPr]; elemental analysis calcd (%) for C36H52N4Si: C 76.00,
H 9.21, N 9.85, found: C 75.87, H 9.26, N 9.53.
ꢀ
ꢀ
volving C H and C N bond activation and C=C coupling
processes.
Experimental Section
General Considerations
All experiments and manipulations were carried out under dry oxygen-
free nitrogen using standard Schlenk techniques or in an MBraun inert
atmosphere dry-box containing an atmosphere of purified nitrogen. Sol-
vents were dried by standard methods and freshly distilled prior to use.
The starting material silylene 1, 2, NHCs, and NHS were prepared ac-
cording to literature procedures.[9a,11,13a–c] NMR spectra were recorded
with Bruker spectrometers ARX200, AV400 and with residual solvent
signals as internal reference (1H and 13C{1H}) or with an external refer-
ence (SiMe4 for 29Si). Abbreviations: s=singlet; d=doublet; t=triplet;
sept=septet; m=multiplet; br=broad.
4: 1,3-Diisopropyl-4,5-dimethylimidazol-2-ylidene (0.13 g, 0.72 mmol) in
toluene (10 mL) was added to a solution of silylene 1 (0.32 g, 0.72 mmol)
in toluene (10 mL) at ꢀ608C. After 10 min the reaction was completed
and the solution was concentrated to about 10 mL and cooled at ꢀ208C.
The product 4 crystallized as yellow crystals. Yield: 0.39 g (0.63 mmol,
87%); m.p.: 788C (decomp.); 1H NMR (200.13 MHz, [D6]benzene,
258C): d=0.38 ꢀ1.66 (m, 45H; CHMe2, C2Me2, NCHMe2, NCMe), 3.20–
4.27 (m, 6H; NCHMe2, CHMe2), 3.32 (s, 1H; NCCH2), 3.94 (s, 1H;
NCCH2), 5.55 (s, 1H; g-CH), 7.00–7.27 ppm (m, br, 6H; 2,6-iPr2C6H3);
13C{1H} NMR (100.61 MHz, [D6]benzene, 258C): d=9.7 (C2Me2), 21.4–
28.4 (NCHMe2, NCMe, CHMe2), 49.0 (NCHMe2), 82.9 (NCCH2), 107.4
(g-C), 123.9–148.2 (NCMe, NCCH2, 2,6-iPr2C6H3, C2Me2); 149.7 ppm
(SiC); 29Si{1H} NMR (79.49 MHz, [D6]benzene, 258C): d=ꢀ7.6 ppm (s);
EI-MS: m/z (%): 444 (5 [(M-NHC)+]), 429 (100, [(M-NHC-Me)+]), 180
(11, NHC+); elemental analysis calcd (%) for C40H60N4Si·C7H8: C 78.71,
H 9.56, N 7.81, found: C 78.37, H 9.70, N 7.57.
5: Method 1: Cyclohexylisocyanide (0.18 mL, d=0.88 gmLꢀ1, 1.46 mmol)
was added to a solution of 3 (0.83 g, 1.46 mmol) in diethyl ether (15 mL)
at ꢀ208C. After three days yellow crystals of 5 formed at ꢀ208C with
yield of 0.80 g (1.18 mmol, 81%). Method 2: A solution of 1,3,4,5-tetra-
methylimidazol-2-ylidene (0.16 g, 1.30 mmol) in toluene (5 mL) was
added to a solution of silylene 1 (0.59 g, 1.30 mmol) in toluene (10 mL)
at ꢀ408C under stirring. After 30 min. the reaction was completed. Cy-
clohexylisocyanide (0.16 mL, d=0.88 gmLꢀ1) was added in situ to the re-
action mixture at ꢀ408C. The reaction temperature was allowed to warm
to room temperature and the solvent was changed from toluene to dieth-
yl ether (5 mL), and the solution was cooled at ꢀ208C. After two days
the product 5 crystallized at ꢀ208C as yellow crystals. Yield: 0.66 g
Single-Crystal X-ray Structure Determination
Crystals were each mounted on a glass capillary in perfluorinated oil and
measured in a cold N2 flow. The data of 3, 4, 5, and 6 were collected on
an Oxford Diffraction Xcalibur S Sapphire at 150 K (MoKa radiation, l
=0.71073 ꢂ). The structures were solved by direct methods and refined
on F2 with the SHELX-97[14] software package. The positions of the H
atoms were calculated and considered isotropically according to a riding
model. CCDC 739318 (3), CCDC 731580 (4), CCDC 739319 (5),
CCDC 739317 (6) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre at www.ccdc.cam.ac.uk/data_
request/cif.
3: Triclinic, space group P1, a=8.7223(4), b=12.1105(5), c=
17.1446(4) ꢂ, a=103.360(3), b=101.828(3), g=100.769(4)8, V=
ACTHNUTRGNEUNG
1671.61(11) ꢂ3, Z=1, 1calcd =1.130 Mgmꢀ3, m(MoKa)=0.100 mmꢀ1, 13949
collected reflections, 8441 crystallographically independent reflections
[Rint =0.0295], 7025 reflections with I>2s(l), qmax =258, R(Fo)=0.0639
2
(I>2s(l)), wR
N
326
www.chemasianj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 322 – 327

Stable NHC!Silylene Adducts and Their Reactivity towards Cyclohexylisocyanide
(0.97 mmol, 73%); m.p.: 1718C (decomp.); 1H NMR (200.13 MHz,
[D6]benzene, 258C): d=1.24–1.82 (m, 44H; C2Me2, CHMe2, C6H11,
NCMe), 2.27 (s, 3H; NMe), 3.03–3.10 (b, 2H; NCH2Si), 3.32 (s, 1H;
NCCH2), 3.55 (m, 1H; CHMe2), 3.78 (m, 1H, CHMe2), 3.89 (m, 1H,
CHMe2), 3.98 (s, 1H; RNCH=C), 4.02 (m, 1H; CHMe2), 4.04 (s, 1H;
NCCH2), 5.45 (s, 1H; g-H), 7.08–7.27 ppm (m, br, 6H; 2,6-iPr2C6H3);
13C{1H} NMR (100.61 MHz, [D6]benzene, 258C): d=8.67 (C2Me2), 22.3,
24.6, 25.0, 25.3, 25.4, 25.7, 26.1, 26.4, 26.6, 26.9, 27.0, 28.4, 28.6, 28.8, 29.7,
Int. Ed. 2009, 48, 3170; c) A. G. Avent, B. Gehrhus, P. B. Hitchcock,
M. F. Lappert, H. Maciejensk, J. Organomet. Chem. 2003, 686, 321;
d) W. A. Herrmann, P. G. Harter, W. K. Chsistian, F. Bielert, N. See-
both, P. Sirsch, J. Organomet. Chem. 2002, 649, 141.
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2270; b) G. R. Gillette, G. H. Noren, R. West, Organometallics 1987,
6, 2617; c) W. Ando, A. Sekiguchi, K. Hagiwara, A. Sakakibara, H.
Yoshida, Organometallics 1988, 7, 558; d) G. R. Gillette, G. H.
Noren, R. West, Organometallics 1989, 8, 487; e) G. Levin, P. K.
Das, C. Bilgrien, C. L. Lee, Organometallics 1989, 8, 1206; f) C. A.
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32.3, 32.5, 35.4 (NCMe, NCH2Si, NMe, CHMe2, CHMe2, NCH
ACHTUGNRTNE(NUNG CH2)5),
57.1 (NCH(CH2)5), 65.9 (RNCH=C), 87.3 (NCCH2), 107.8 (g-C), 116.9,
ACHTUNGTRENNUNG
118.0, 124.6, 124.7, 124.9, 125.0, 139.0, 139.2, 142.9, 143.3, 148.1, 148.4,
148.5, 148.6, 150.3 ppm (NCMe, NCCH2, C2Me2, C=CHN, 2,6-iPr2C6H3);
29Si{1H} NMR (79.49 MHz, [D6]benzene, 258C): d=ꢀ43.2 ppm (s); EI-
MS: m/z (%): 677.63 (100) [M+], 662 (5) [M+ꢀMe], 594 (70) [M+ꢀiPr];
elemental analysis calcd (%) for C43H63N5Si: C 76.17, H 9.36, N 10.33,
found: C 76.29, H 9.34, N 10.16.
6: 1,3-Diisopropyl-4,5-dimethylimidazol-2-ylidene (0.25 g, 1.39 mmol) was
added to a solution of silylene 1 (0.62 g, 1.39 mmol) in toluene (10 mL)
at ꢀ208C. The reaction mixture was allowed to warm to room tempera-
ture. After 20 min. cyclohexylisocyanide (0.17 mL, d=0.88 gmLꢀ1) was
added to the reaction mixture at ꢀ208C and the reaction was finished
after 24 h. Colorless crystals of 6 result from the concentrated toluene so-
lution at ꢀ208C. Yield: 0.71 g (0.97 mmol, 70%); m.p.: 3148C (decomp.);
1H NMR (200.13 MHz, [D6]benzene, 258C): d=1.02–1.54 (m, 50H;
C2Me2, NCHMe2, CHMe2, C6H11, NCMe), 2.12 (s, 3H;=CMe2), 2.19 (s,
3H;=CMe2), 2.90 (m, 2H; CHMe2), 3.30 (s, 1H, NCCH2), 3.81 (m, 2H,
CHMe2), 3.99 (s, 1H; NCCH2), 5.31 (s, 1H; g-H), 5.40 (m, 1H; CHMe2),
5.52 (s, 1H; CH=CMe2), 6.96–7.18 ppm (m, br, 6H; 2,6-iPr2C6H3);
13C{1H} NMR (100.61 MHz, [D6]benzene, 258C): d=11.6, 12.9 (C2Me2),
19.6, 21.4, 23.2, 23.4, 23.6, 23.7, 24.4, 24.6, 24.7, 25.4, 26.4, 26.6, 26.7, 27.2,
27.3, 27.6, 28.0, 28.4, 29.1, 29.5, 32.5, 33.2 (CHMe2, NCHMe2, NCMe,
[6] T. Abe, T. Iwamoto, C. Kabuto, M. Kira, J. Am. Chem. Soc. 2006,
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NCHACHTUNGTRENNUNG(CH2)5), 49.8 (NCHAHCUTGNTRENN(UNG CH2)5), 62.8 (CH=CMe2), 89.8 (NCCH2), 105.1
(g-C), 123.4, 124.1, 124.8, 125.2, 125.3, 125.4, 125.6, 127.2, 128.3, 129.3,
136.7, 136.8, 139.7, 139.9, 142.7, 148.0, 148.2, 148.3, 149.0, 150.4 ppm
(NCMe, NCCH2,=CMe2, 2,6-iPr2C6H3, CSi); 29Si{1H} NMR (79.49 MHz,
[D6]benzene, 258C): d=ꢀ58.0 ppm (s); EI-MS: m/z (%): 733 (100) [M+
], 718 (18) [M+ꢀMe], 690 (70) [M+ꢀiPr]; elemental analysis calcd (%)
for C47H71N5Si·C7H8: C 78.49, H 9.64, N 8.48, found: C 78.46, H 9.66,
N 8.68.
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S. A. Macgregor, M. F. Mahon, M. K. Whittlesey, J. Am. Chem. Soc.
2009, 131, 4604.
Acknowledgements
Financial support from the Cluster of Excellence “Unifying Concepts in
Catalysis” (EXC 314/1), funded by the Deutsche Forschungsgemeinschaft
and administered by the Technische Universitꢀt Berlin, is gratefully ac-
knowledged.
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Received: September 8, 2009
Published online: December 22, 2009
Chem. Asian J. 2010, 5, 322 – 327
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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