Silylation of N-heterocyclic carbene with aminochlorosilane and -disilane: Dehydrohalogenation vs. Si-Si bond cleavage

Article abstract of DOI:10.1039/c1dt11592h

Reactions of the aminochlorosilane RSiHCl₂ and disilane R ₂Si₂HCl₃ (R = (2,6-iPr₂C ₆H₃)(SiMe₃)N) with an excess of 1,3-bis(tert-butyl)imidazol-2-ylidene resulted in th

Full text of DOI:10.1039/c1dt11592h

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PAPER
Silylation of N-heterocyclic carbene with aminochlorosilane and -disilane:
dehydrohalogenation vs. Si–Si bond cleavage†
Haiyan Cui and Chunming Cui*
Received 18th June 2011, Accepted 25th August 2011
DOI: 10.1039/c1dt11592h
Reactions of the aminochlorosilane RSiHCl2 and disilane R2Si2HCl3 (R = (2,6-iPr2C6H3)(SiMe3)N)
with an excess of 1,3-bis(tert-butyl)imidazol-2-ylidene resulted in the silylation of the NHC while
reaction with the less hindered 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene yielded an
NHC-stabilized aminochlorosilylene.
Introduction
Results and discussion
Stable N-heterocyclic carbenes (NHCs) have been widely used as
ligands for transition metal catalysis and stabilization of molecules
with unusual bonding and reactivity.1 It has been shown that
the substituents on the 4 and 5 positions of Arduengo type
carbenes have pronounced effects on their properties. Thus, the
functionalization of NHCs with various groups has attracted
considerable recent attention for tuning their electronic properties.
Bertrand and co-workers have reported an efficient method for
the introduction of several types of groups to the 4 and 5
positions via abnormal carbene intermediates.2 Gates and Roesky
have reported 4-phosphino- and 4-silyl-substituted NHCs by
the reactions of phosphaalkenes with NHCs and an NHC-
stabilized dichlorosilylene with an organic azide, respectively.3
A few carbene-silylene adducts have also been synthesized and
structurally characterized.4b,4c,9 We have previously shown that
a 4-diaminosilyl-substituted NHC can be obtained as a side
product in the dehydrochlorination of diaminochlorosilanes for
the generation of silylenes.4a However, this reaction appeared to
be complicated since silylenes, disilanes and other unsaturated
silicon species can be obtained in some cases.4a,5–9 In order to
develop a practical and selective method for the silylation of
NHCs and synthesis of silylenes from simple chlorosilanes, the
detailed investigation of the reaction is highly desirable since
both silylated NHCs and silylenes are desired useful products.
Herein, we report on reaction of the aminochlorosilane RSiHCl2
(1, R = (2,6-iPr2C6H3)(SiMe3)N) and the disilane (R)2Si2HCl3
(2) with different NHCs, leading to the selective silylation of
an NHC and formation of an NHC-stabilized aminochlorosi-
lylene, respectively, under optimized conditions (Schemes 1
and 3).
We recently reported that reaction of 1 with 1,3-bis(tert-butyl)-
imidazol-2-ylidene (ItBu2) proceeded rapidly at room temperature
to yield the disilane 2 in an excellent yield.6 It is anticipated that 2
could be the suitable precursor for the synthesis of aminochlorosi-
lylene or its oligomer by dehydrochlorination reaction with NHCs.
Heating a mixture of 2 with 1 equiv of ItBu2 in C6D6 led to the
formation of a new species and regeneration of 1 as indicated
by the NMR analysis. Thus, the reaction of 2 with an excess of
ItBu2 was investigated for the clean synthesis of the new species.
Refluxing a mixture of 2 and 3 equiv of ItBu2 in THF yielded
the new compound, which was isolated as colorless crystals in
good yield and has been identified as the silylated NHC 3 by 1H,
13C and 29Si NMR spectroscopy, elemental analysis and an X-ray
single-crystal analysis (Scheme 1).
Scheme 1
The 29Si NMR spectrum of 3 exhibits two resonances at d
-29.24 and 9.71 ppm. The former resonance is noticeably high-
field shifted compared to that observed in 1 due to the less
electronegative olefinic carbon atom compared to the chloride
ligand. The 13C NMR spectrum signal for the central carbene
carbon atom appears at 213.4 ppm, which is comparable to the
value found for the free carbene.10 Single crystals of 3 suitable
for an X-ray diffraction study were obtained from n-hexane. The
structure of 3 is shown in Fig. 1 with relevant bond parameters.
The C–C and C–N bond lengths in the five-membered ring are
consistent with those observed in free carbenes. 3 represents a
rare example of a chlorosilyl-substituted NHC and could be
further derivatised by simple metalation reactions. Attempted
State Key Laboratory of Elemento-organic Chemistry, Nankai University,
Tianjin, 300071, China. E-mail: cmcui@nankai.edu.cn; Fax: +86-22-
23503461; Tel: +86-22-23503461
† CCDC 830376 (3) and CCDC 830375 (4). For crystallographic data in
CIF or other electronic format, see DOI: 10.1039/c1dt11592h
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slowly decomposed at the high temperature, the alternative low
temperature route starting from 1 by dehydrohalogenation with
the NHC has been investigated for the isolation of the new
compound. As expected, reaction of 1 with two equiv of IiPr2
in THF from low temperature to room temperature afforded the
same compound as that obtained at high temperature with 2
(Scheme 3). The new compound can be isolated as yellow powder
in 75% yield from the reaction. It has been fully characterized by
1H, 13C and 29Si NMR spectroscopy and elemental analysis. The
structure of 4 has been confirmed as an NHC-silylene complex by
an X-ray single crystal analysis.
Fig. 1 Ortep drawing of 3 with ellipsoids given at the 30% probability
level. Hydrogen atoms have been omitted for clarity. Selected bond
˚
lengths (A) and angles (deg): Cl1–Si1 2.0833(10), Si1–N1 1.713(2),
Si1–C1 1.851(2), Si1–H1 1.40(2), C1–C2 1.357(3), N2–C1 1.419(3),
N2–C3 1.367(3), N3–C3 1.365(3), N3–C2 1.377(3), N1–Si1–C1 113.37(10),
N1–Si1–Cl1 111.50(7), C1–Si1–Cl1 106.03(9).
dehydrohalogenation of 3 with NHCs has been unsuccessful to
date.
The most reasonable explanation for the silylation reaction is
the formation of the aminochlorosilylene intermediate RClSi (A,
Scheme 2) by the reaction of 2 with the NHC. The silylation of the
NHC with 2 and regeneration of RSiHCl2 as observed in NMR
scale reaction suggested that the reaction proceeded through the
disproportionation of 2 rather than the dehydrohalogenation of
2 by the NHC. The regenerated 1 can be converted to 2 by
the dehydrogenation pathway. Therefore, it is expected that the
silylation reaction could be more conveniently and practically
furnished by the reaction of 1 with an excess of the NHC at high
temperature. Indeed, the reaction of 1 with 3 equiv of ItBu2 in
refluxing THF afforded 3, which was isolated as colorless crystals
from n-hexane in good yield. Although the disproportionation of
disilanes to silylenes and silanes in the presence of Lewis bases
has been proposed previously,11 no Lewis base stabilized silylenes
have been isolated from these reactions. In addition, reactions of
disilanes with N-heterocyclic carbenes have not been described in
the literature. Therefore, it is interesting and desirable to design a
suitable route and select appropriate precursors for the isolation
of proposed intermediates.
Scheme 3
The resonance for the Si(II) atom in the 29Si NMR spectrum
of 4 was found at d = 3.14 ppm, which is slightly downfield
shifted compared to those observed for the NHC-stabilized
arylchlorosilylenes (0.77 to 1.34 ppm) that have been reported.9
The 13C NMR signal for the NHC central carbon atom in 4 appears
at 164.08 ppm, consistent with those found in the NHC→Si
donor–acceptor complexes.7–9 The molecular structure of 4 (Fig. 2)
features the trigonal-pyramidal three-coordinate silicon atom (the
sum of angles at the Si1 atom = 305.22◦). The Si1–C20 bond length
˚
(2.0023(19) A) is slightly longer than those reported for NHC-
stabilized dihalosilylenes (1.985(4) and 1.989(3) A).7,12 Compound
4 is the first example of a donor-stabilized aminochlorosilylene, as
˚
far as we know.
Scheme 2
Fig. 2 Ortep drawing of 4 with ellipsoids drawn at 30% probability.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
Since ItBu2 easily undergoes a C–H bond insertion reaction, the
less bulky 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr2),
which does not possess olefinic C–H bonds on the NHC ring,
was chosen for trapping the proposed silylene intermediate.
Furthermore, the steric effects of the N-substituents of NHCs
on this type of reaction can be explored by employing different
carbenes. It was found that 2 did not react with an excess of
IiPr2 in C6D6 at room temperature. However, upon heating the
mixture to 80 ◦C for 12 h, the formation of a new compound
was observed by NMR analysis. Since the new compound was
˚
(A) and angles (deg): Si1–N1 1.7861(15), Si1–Cl1 2.2274(7), Si1–C20
2.0023(19), Si2–N1 1.7603(15), N2–C20 1.369(2), N3–C20 1.358(2);
N1–Si1–C20 103.18(7), N1–Si1–Cl1 105.92(5), C20–Si1–Cl1 96.12(5).
The isolation of 4 strongly supports the aminochlorosilylene
A (Scheme 2) as the intermediate for the silylation of ItBu2.
In addition, the steric factor afforded by the substituents on
the NHC nitrogen atoms has noticeable effects on the reaction
11938 | Dalton Trans., 2011, 40, 11937–11940
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conditions. In the case of ItBu2, the reaction of 1 below room
temperature yielded the disilane 2 as an isolable product, with the
formation of a very small amount of the silylated product 3 that
can be only observed by the proton NMR analysis of the crude
product and could not be isolated, indicating the initial formed
intermediate A by the dehydrohalogenation of 1 is prone to react
with 1 to give the disilane 2 rather than ItBu2. In contrast, the
less hindered IiPr2 can trap the intermediate A at low temperature
to afford the NHC-stabilized aminochlorosilylene 4 in good yield.
Reactions of the disilane 2 with NHCs proceed through initial
Si–Si bond cleavage and disproportionation to generate A and
1 at more harsh conditions. The reaction of 2 with the bulky
ItBu2 only proceeds at high temperature and does not follow
the initial dehydrohalogenation pathway. On the other hand, the
similar reaction of 2 with IiPr2 appears to be more complicated
since 4 is thermally sensitive and partially decomposed at the high
temperature.
1.01, (d, 3H, CH(CH3)2), 1.24 (d, 3H, CH(CH3)2), 1.39 (d, 3H,
CH(CH3)2), 1.26 (s, 9H, C(CH3)3), 1.72 (s, 9H, C(CH3)3), 3.11 (m,
1H, CH(CH3)2), 3.91 (m, 1H, CH(CH3)2), 6.17 (s, 1H, CH ),
6.44 (s, 1H, Si-H), 6.82–7.08 (m, 3H, Ar-H) ppm; 13C NMR
(100.61 MHz, C6D6): d 1.61 (Si(CH3)3), 23.88, 24.69, 25.43, 26.49
(CH(CH3)2), 28.35, 28.44 (CH(CH3)2), 31.17, 32.42 (C(CH3)3),
56.05, 58.33 (C(CH3)3), 130.34 (CH ), 120.34 (SiC ), 124.58,
124.84, 126.26, 140.34, 147.26, 147.66 (Ar-C), 213.40 (carbene)
ppm; 29Si NMR (59.62 MHz, C6D6): d -29.24 (Si-H), 9.71
(Si(CH3)3) ppm; Elemental analysis (%) calcd for C26H46ClN3Si2:
C, 63.43; H, 9.42; N, 8.54; Found: C, 62.99; H, 9.90; N, 8.34;
IR (cm-1): nSi–H 2196.88.
Method b. A mixture of 2 (0.41 g, 0.622 mmol) and ItBu2
(0.34 g, 1.87 mmol) was treated with 20 mL of THF. The resulting
suspension was refluxed for 24 h. And the reaction processed
quantitatively. The volatiles were removed under vacuum. The
residue was extracted with n-hexane (40 mL). Concentration to ca.
2 mL and storage at room temperature afforded colorless crystals
of 3 (0.3 g, 49%).
Conclusions
We have established that the disilane 2 generated by the dehy-
drohalogenation of 1 with NHCs at low temperatures does not
undergo a dehydrohalogenation reaction at ambient temperature
but a Si–Si bond cleavage reaction at high temperatures to yield
1 and the aminochlorosilylene RClSi, the key intermediate for
silylation of NHCs that can be trapped and stabilized by IiPr2 to
give the NHC stabilized aminochlorosilylene 4. The mechanistic
information provided by the results should contribute to the
selective and facile access to desired silylenes or silylated NHCs
from simple chlorosilanes.
Synthesis of RSiCl(IiPr2) (4, R = (2,6-iPr2C6H3)(SiMe3)N).
A
solution of 1 (0.42 g, 1.19 mmol) in THF (10 mL) was added to
a stirred solution of IiPr2 (0.43 g, 2.38 mmol) in THF (5 mL)
at -78 ◦C. The mixture was slowly warmed to room temperature
and stirred overnight whereupon a large quantity of precipitate
formed. The volatiles were removed under vacuum. The residue
was extracted with toluene (40 mL). After filtration and removal
solvents, the remaining residue was washed with n-hexane (5
mL ¥ 3) to afford a light yellow powder of 4.(0.44 g, 75.16%).
◦
1
M.p. 149–150 C; H NMR (400 MHz, C6D6): d 0.33 (d, 3 H,
CH(CH3)2), 0.54 (s, 3H, Si(CH3)3), 1.21, (d, 3H, CH(CH3)2),
1.25 (d, 6H, CH(CH3)2), 1.41 (d, 3H, CH(CH3)2), 1.48 (s, 6H,
CCH3), 1.56 (d, 3H, CH(CH3)3), 3.30 (m, 1H, CH(CH3)2),
4.55 (m, 1H, CH(CH3)2), 5.06 (br, CH(CH3)2), 6.93–7.03 (m, 2H,
Ar-H), 7.23–7.25 (d, 1H, Ar-H) ppm; 13C NMR (100.61 MHz,
C6D6): d 3.86, 9.90, 23.89, 25.88, 25.94, 26.44, 27.87, 28.46, 124.02,
124.25, 124.87, 126.40, 144.51, 147.67, 148.36, 164.08 ppm; 29Si
NMR (59.62 MHz, C6D6): d 6.99 (Si(CH3)3), 3.14 (SiCl) ppm;
Elemental analysis (%) calcd for C26H46ClN3Si2: C, 63.43; H, 9.42;
N, 8.54; Found: C, 63.45; H, 9.70; N, 8.49; UV-VIS: e220 = 1.63
¥ 104 L mol-1 cm-1 (THF); IR (cm-1): n 441.64, 541.87, 596.68,
664.25, 755.03, 802.72, 843.46, 932.26, 1054.91, 1192.06, 1250.83,
1450.58, 1553.56, 1628.75, 2180.91, 2870.54.
Experimental section
General considerations
All operations were carried out under an atmosphere of dry
argon or nitrogen by using modified Schlenk line and glovebox
techniques. All solvents were freshly distilled from Na and
degassed immediately prior to use. Elemental analyses were carried
out on an Elemental Vario EL analyzer. The H, 13C and 29Si
1
NMR spectroscopic data were recorded on Bruker Mercury Plus
300, 400 and 600 MHz NMR spectrometers. Infrared spectra were
recorded on a Bio-Rad FTS 6000 spectrometer. The UV-vis spectra
were recorded on a Shimadzu UV-2450 spectrometer and emis-
sion spectra on an Edinburgh Analytical Instruments FL900CD
spectrometer. 1,3-Bis(tert-butyl)-imidazol-2-ylidene ItBu2,13 1,3-
diisopropyl-4,5-dimethylimidazol-2-ylidene (IiPr2)14 and disilane
4a
X-Ray structural determinations
R2Si2HCl3 (R = (2,6-iPr2C6H3)(SiMe3)N) were synthesized ac-
cording to published procedures.
Intensity data for compound 3 was collected with a Bruker
SMART CCD diffractometer, and compound 4 was collected
with a Rigaku Saturn 724 CCD diffractometer, using graphite-
Synthesis of RSiHCl[(C3N2)H(But)2] (3, R = (2,6-iPr2C6H3)-
(SiMe3)N): Method a. A solution of 1 (3.46 g, 10 mmol) in
THF (30 mL) was added to a stirred solution of ItBu2 (3.61 g,
20 mmol) in THF (30 mL) at -78 ◦C. Soon a white suspension
formed. The mixture was slowly warmed to room temperature, and
refluxed for 24 h. The volatiles were removed under vacuum. The
residue was extracted with n-hexane (40 mL ¥ 2). Concentration
to ca. 2 mL and storage at room temperature afforded colorless
crystals of 3 (2.3 g, 46.7%). M.p. 162–164 ◦C; 1H NMR (400 MHz,
C6D6): d 0.24 (s, 9 H, Si(CH3)3), 0.46 (d, 3H, CH(CH3)2),
˚
monochromated Mo-Ka radiation (l = 0.71073 A). The structure
was solved by direct methods (SHELXS-97)15 and refined by
full-matrix least squares on F2. H1 atom for 3 was obtained
from difference Fourier map Q1. All non-hydrogen atoms were
refined anisotropically and hydrogen atoms by a riding model
(SHELXL-97).16 Crystals of 3 suitable for X-ray analysis were
grown from hexane, and crystals of 4 were obtained from toluene.
Crystallographic data for 3 and 4 are given in Table 1.
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Table 1 Crystallographic data for 3 and 4
Chem. Rev., 2009, 109, 3783; (h) B. Alcaide, P. Almendros and A.
Luna, Chem. Rev., 2009, 109, 3817; (i) K. M. Hindi, M. J. Panzner,
C. A. Tessier, C. L. Cannon and W. J. Youngs, Chem. Rev., 2009, 109,
3859; (j) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008,
47, 3122; (k) F. Glorius, N-Heterocyclic Carbenes in Transition Metal
Catalysis; Topics in Organometallic Chemistry, Springer Verlag, 2007,
vol. 21; (l) D. Bourissou, O. Guerret, F. P. Gabba¨ı and G. Bertrand,
Chem. Rev., 2000, 100, 39.
Compound
3
4
CCDC number
Formula
830376
C26H46ClN3Si2
492.29
Orthorhombic
10.129(2)
17.177(3)
34.277(7)
90.00
90.00
90.00
5964(2)
Pbca
8
0.0561
0.0573
0.1318
0.0662
0.1364
1.145
830375
C26H46ClN3Si2
492.29
Monoclinic
10.3126(17)
17.824(3)
15.921(3)
90.00
102.561(2)
90.00
2856.4(8)
P21/n
4
0.0545
0.0448
0.0964
0.0627
0.1040
1.016
M
Crystal system
˚
2 D. Mendoza-Espinosa, B. Donnadieu and G. Bertrand, J. Am. Chem.
Soc., 2010, 132, 7264–7265.
a/A
˚
b/A
˚
3 (a) J. I. Bates, P. Kennepohl and D. P. Gates, Angew. Chem., Int. Ed.,
2009, 48, 9844–9847; (b) R. S. Ghadwal, H. W. Roesky, M. Granitzka
and D. Stalke, J. Am. Chem. Soc., 2010, 132, 10018–10020.
4 (a) H. Cui, Y. Shao, X. Li, L. Kong and C. Cui, Organometallics, 2009,
28, 5191–5195; (b) Y. Xiong, S. Yao and M. Driess, Chem.–Asian J.,
2010, 5, 322–327; (c) W. M. Boesveld, B. Gehrhus, P. B. Hitchcock, M.
F. Lappert and P. von R. Schleyer, Chem. Commun., 1999, 755–756.
5 S. S. Sen, H. W. Roesky, D. Stern, J. Henn and D. Stalke, J. Am. Chem.
Soc., 2010, 132, 1123–1126.
6 H. Cui and C. Cui, Chem.–Asian J., 2011, 6, 1138–1141.
7 R. S. Ghadwal, Herbert W. Roesky, S. Merkel, J. Henn and D. Stalke,
Angew. Chem., Int. Ed., 2009, 48, 5683–5686.
8 Y. Gao, J. Zhang, H. Hu and C. Cui, Organometallics, 2010, 29, 3063–
3065.
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Space group
Z
Rint
R1 (I > 2s(I))
wR(F2) (I > 2s(I))
R1 (all data)
wR(F2) (all data)
GOF
9 A. C. Filippou, O. Chernov, B. Blom, K. W. Stumpf and G. Schnaken-
burg, Chem.–Eur. J., 2010, 16, 2866–2872.
10 A. J. Arduengo III, H. Bock, H. Chen, M. Denk, D. A. Dixon, J. C.
Green, W. A. Herrman, W. L. Jones, M. Wagner and R. West, J. Am.
Chem. Soc., 1994, 116, 6641–6649.
11 (a) R. Richter, G. Roewer, U. Bohme, K. Busch, F. Babonneau, H. P.
Martin and E. Muller, Appl. Organomet. Chem., 1997, 11, 71; (b) H.
Hildebrandt and B. Engels, Z. Anorg. Allg. Chem., 2000, 626, 400.
12 A. C. Filippou, O. Chernov and G. Schnakenburg, Angew. Chem., Int.
Ed., 2009, 48, 5687–5690.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Grant No. 20725205) for the financial support.
Notes and references
1 For reviews, see: (a) D. Tapu, D. A. Dixon and C. Roe, Chem. Rev.,
2009, 109, 3385; (b) P. L. Arnold and I. J. Casely, Chem. Rev., 2009,
109, 3599; (c) S. D´ıez-Gonza´lez, N. Marion and S. P. Nolan, Chem.
Rev., 2009, 109, 3612; (d) M. Poyatos, J. A. Mata and E. Peris, Chem.
Rev., 2009, 109, 3677; (e) C. Samojo¨wicz, M. Bieniek and K. Grela,
Chem. Rev., 2009, 109, 3708; (f) W. A. L. van Otterlo and C. B. de
Koning, Chem. Rev., 2009, 109, 3743; (g) S. Monfette and D. E. Fogg,
13 N. M. Scott, R. Dorta, E. D. Stevens, A. Correa, L. Cavallo and S. P.
Nolan, J. Am. Chem. Soc., 2005, 127, 3516.
14 N. Kuhn and T. Kratz, Synthesis, 1993, 561–562.
15 G. M. Sheldrick, SHELXS-90/96, Program for Structure Solution,
Acta Crystallogr., Sect. A: Found. Crystallogr., 1990, 46, 467.
16 G. M. Sheldrick, SHELXL 97, Program for Crystal Structure Refine-
ment, University of Goettingen, Goettingen, Germany, 1997.
11940 | Dalton Trans., 2011, 40, 11937–11940
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The Royal Society of Chemistry 2011
©