Dehydrochlorination to silylenes by N-heterocyclic carbenes

Article abstract of DOI:10.1021/om900444z

Reaction of cyclic diaminochlorosilanes with 1,3-bis(terr-butyl)imidazol-2- ylidene resulted in the facile formation of the corresponding stable and transient diaminosilylenes. This novel dehydrochlorination route could be applied for the generation of fo

Full text of DOI:10.1021/om900444z

Organometallics 2009, 28, 5191–5195 5191
DOI: 10.1021/om900444z
Dehydrochlorination to Silylenes by N-Heterocyclic Carbenes
Haiyan Cui, Yanjun Shao, Xiaofei Li, Lingbing Kong, and Chunming Cui*
State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin 300071, People’s Republic
of China
Received May 26, 2009
Reaction of cyclic diaminochlorosilanes with 1,3-bis(tert-butyl)imidazol-2-ylidene resulted in the
facile formation of the corresponding stable and transient diaminosilylenes. This novel dehydro-
chlorination route could be applied for the generation of four- and five-membered N-heterocyclic
silylenes with a range of different substituents under very mild conditions. Activation of an olefinic
C-H bond and reduction of a cyclic diaminochlorosilane have been observed for these new transient
silylenes.
Introduction
The reaction of trichorosilane with various amines for the
generation of SiCl3 anion has been exclusively discussed in
a review.5 The mechanistic studies suggested that [SiCl3]-
was generated by the initial deprotonation of HSiCl3 by
an appropriate amine. Recently, Karsch and co-workers
demonstrated that the reaction of trichlorosilane with tert-
iary amines in the presence of diazabutadienes yielded
1,4-diazasilacyclopentene derivatives. In this system, di-
chlorosilylene has been proposed to be the key intermediate
generated via the dissociation of one chloride ligand from
[SiCl3]-.6 In view of these earlier reports on the deprotona-
tion of trichlorosilane with organic bases, it is anticipated
that substituted hydrocholorosilanes could also be deproto-
nated by suitable strong bases to yield the corresponding
silylenes.
N-Heterocyclic carbenes are well-known strong bases and
have been used as HCl acceptors by Roesky’s group for the
preparation of β-diketiminato divalent germanium species
and aluminum diamide from the corresponding halides.7,8
Herein, we report on the facile route to cyclic silylenes via
dehydrochlorination of cyclic diaminohydrochlorosilanes
by the bulky heterocyclic carbene 1, 3-bis(tert-butyl)imida-
zol-2-ylidene. Our preliminary results showed that this
method can be applied for the generation of stable and
transient four- and five-membered heterocyclic silylenes with
a range of different substituents on the ring backbones under
very mild conditions. To the best our knowledge, this
method represents the first approach for the generation of
heterocyclic silylenes under mild conditions in the absence of
metallic reagents.
Silylenes are of considerable current interest owing to their
fundamental interest and potential applications in transition
metal catalysis.1 Since the first stable N-heterocyclic silylene
was reported by West in 1994,2 several other cyclic silylenes
have been synthesized and fully characterized by Lappert,
Kira, and others.3 At present, the number of stable
N-heterocyclic silylenes is still very small and the factors
that control the stability and chemical behavior of silylenes
have yet to be investigated. This situation may be largely
associated with very limited available synthetic approaches.
The earlier routes to transient silylenes commonly involved
photolytic and high-temperature exclusion of small organic
fragments from appropriate silanes,4 but the most versatile
route is reduction of substituted silicon dihalides with highly
reducing reagents such as alkali metals and naphthalene
lithium in solution phase.2,3 All of the known synthetic
routes require either harsh conditions or highly reactive
metallic reagents. Hence, the development of alternative
routes to silylenes under mild conditions is highly desirable.
*Corresponding author. E-mail: cmcui@nankai.edu.cn.
(1) For leading reviews on silylenes and silylene metal complexes, see:
(a) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem. Res. 2000, 33, 704.
(b) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165. (c) Gehrus,
B.; Lappert, M. F. J. Organomet. Chem. 2001, 617-618, 209. (d) Kira, M. J.
Organomet. Chem. 2004, 689, 4475. (e) Waterman, R.; Hayes, P. G.; Tilley,
T. D. Acc. Chem. Res. 2007, 40, 712.
(2) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Belyakov, A. V.;
Verne, H. P.; Haaland, A.; Wagner, M.; Metzler, N. J. Am. Chem. Soc.
1994, 116, 2691.
(3) (a) West, R.; Denk, M. Pure Appl. Chem. 1996, 68, 785. (b)
Gehrhus, B.; Lappert, M. F.; Heinicke, J.; Boese, R.; Blaser, D. J. Chem.
Soc., Chem. Commun. 1995, 1931. (c) Heinicke, J.; Oprea, A.; Kindermann,
M. K.; Karpati, T.; Nyulaszi, L.; Veszprem, T. Chem.;Eur. J. 1998, 4, 541.
(d) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1999,
121, 9722. (e) Driess, M.; Yao, S.; Brym, M.; van W€ullen, C.; Lentz, D. J.
Am. Chem. Soc. 2006, 128, 9628. (f) So, C. W.; Roesky, H. W.; Magull, J.;
Oswald, R. B. Angew. Chem. 2006, 118, 4052. Angew. Chem., Int. Ed.
2006, 45, 3948.
(4) (a) Atwell, W. H.; Weyenberg, D. R. Angew. Chem., Int. Ed. Engl.
1969, 8, 469. (b) Tamao, K.; Nagata, K.; Asahara, M.; Kawachi, A. Q.; Ito, Y.;
Shiro, M. J. Am. Chem. Soc. 1995, 117, 11592. (c) Gaspar, P. P.; Holten, D.;
Konieczny, S.; Corey, J. Y. Acc. Chem. Res. 1987, 20, 329.
(5) Benkeser, R. A. Acc. Chem. Res. 1971, 4, 94.
(6) Karsch, H. H.; Schluter, P. A.; Bienlein, F.; Herker, M.; Witt, E.;
€
Sladek, A.; Heckel, M. Z. Anorg. Allg. Chem. 1998, 624, 295.
(7) After our submission of the manuscript, Roesky and co-workers
reported on a carbene-stabilized dichlorosilylene by the reaction of
trichlorosilane with an N-heterocyclic carbene; see: Ghadwal, R. S.;
Roesky, H. W.; Merkel, S.; Henn, J.; Stalke, D. Angew. Chem., Int. Ed.
2009, DOI: 10.1002/anie.200901766.
(8) (a) Jana, A.; Objartel, I.; Roesky, H. W.; Stalke, D. Inorg. Chem.
2009, 48, 798. (b) Jancik, V.; Pineda, L. W.; Pinkas, J.; Roesky, H. W.;
Neculai, D.; Neculai, A. M.; Herbst-Irmer, R. Angew. Chem., Int. Ed. 2004,
43, 2142. (c) Pineda, L. W.; Jancik, V.; Roesky, H. W.; Neculai, D.; Neculai,
A. M. Angew. Chem., Int. Ed. 2004, 43, 1419.
r
2009 American Chemical Society
Published on Web 08/05/2009
pubs.acs.org/Organometallics

5192 Organometallics, Vol. 28, No. 17, 2009
Cui et al.
Experimental Section
(s, 6H, Si(CH3)2), 1.03, 1.22, 1.34, 1.39 (d, 24H, CH(CH3)2),
1.40, 1.63 (s, 18H, C(CH3)3), 3.88, 4.05 (sept, 4H, CH(CH3)2),
6.66 (s, 1H, SiH), 7.92 (s, 1H, CdCHN). 13C NMR (100 MHz,
C6D6): δ 1.07, 4.98 (SiMe2), 25.17, 25.29, 26.05 (C(CH3)2),
28.36, 28.61 (C(CH3)2), 31.61, 32.54 (C(CH3)3), 56.22, 57.90
(C(CH3)3), 124.25, 125.00, 128.74, 136.73, 146.65 (Ar-C), 217.90
(C:). 29Si NMR (C6D6): δ -29.9 (d, SiH, 1JSi-H=302 Hz), 10.0
(SiMe2). IR (cm-1): ν 2188 (SiH).
All operations were carried out under an atmosphere of
dry argon or nitrogen by using modified Schlenck line and
glovebox techniques. All solvents were freshly distilled from
Na and degassed immediately prior to use. Elemental analyses
1
were carried out on an Elemental Vario EL analyzer. The H,
13C, and 29Si NMR spectra were recorded on Bruker AV300 and
AV400 spectrometers. Infrared spectra were recorded on a Bio-
Rad FTS 6000 spectrometer. Me2Si(ArNH)2 (Ar=2,6-i-Pr2C6-
H3),9 PhB(ArNLi)2,10 (CH)2(t-BuN)2,11 (CH)2(NCy)2,12 1,2-[N-
(SiMe3)]2C6H4,13 and 1,3-bis(tert-butyl)imidazol-2-ylidene14
were synthesized according to published procedures.
Synthesis of Me2Si(ArN)2(Cl)SiSi(H)(NAr)2SiMe2 (4). A so-
lution of I(t-Bu) (0.18 g, 1 mmol) in toluene (20 mL) was added
to a solution of Me2Si(ArN)2SiHCl (0.94 g, 2 mmol) in toluene
(20 mL) at -20 °C. The mixture was warmed to room tempera-
ture and stirred for 10 h. After filtration, the filtrate was pumped
to dryness. The residue was extracted with n-hexane (2ꢀ30 mL).
The extract was concentrated (ca. 10 mL) and stored overnight
in a freezer at -25 °C to afford a white solid of 4 (0.56 g, 62%),
mp 285 °C. Anal. Calcd for C52H81ClN4Si4 (908.52): C, 68.63;
Synthesis of Me2Si(ArN)2SiHCl (1). A solution of n-BuLi (8 mL,
2.5 M in n-hexane) was added to a stirred solution of Me2Si-
(ArNH)2 (4.10 g, 10 mmol) in n-hexane (50 mL) at -78 °C. The
mixture was slowly warmed to room temperature and stirred for
6 h. It was cooled to -78 °C, and neat HSiCl3 (1.34 g, 10 mmol)
was added. The mixture was allowed to warm to room tempera-
ture and stirred for 12 h. It was filtered and evaporated. The
residue was extracted with n-hexane (30 mL ꢀ 2). The combined
extracts were pumped to dryness to afford a white powder of 1
(3.6 g, 76%), mp 205 °C. Anal. Calcd for C26H41ClN2Si2
(472.25): C, 65.99; H, 8.73; N, 5.92. Found: C, 66.17; H, 8.54;
N, 6.10. 1H NMR (300 MHz, C6D6): δ 0.17, 0.51 (s, 6H, SiMe2),
1.25 (d, 12H, CH(CH3)2), 1.34 (d, 12H, CH(CH3)2), 3.92 (sept,
4H, CH(CH3)2), 6.26 (s, 1H, SiH). 13CNMR (75 MHz, C6D6):
δ 0.79 (SiMe2), 4.05 (SiMe2), 25.51 (CH(CH3)2), 28.35
(CH(CH3)2), 124.25 (Cmeta), 134.42 (Cipso), 146.90 (Cpara).
1
H, 8.97; N, 6.16. Found: C, 69.03; H, 9.75; N, 6.30. H NMR
(C6D6): δ 0.00, 0.38 (s, 12H, Si(CH3)2), 1.13, 1.19, 1.23 (d, 24H,
CH(CH3)2), 3.72, 3.89 (sept, 8H, CH(CH3)2), 5.48 (s, 1H, SiH).
13C NMR (100 MHz, C6D6): δ 0.14, 4.34 (SiMe2), 24.35,
24.56, 25.33, 26.40 (C(CH3)2), 27.78, 28.32 (C(CH3)2), 123.89,
124.00, 125.08, 135.94, 146.44, 146.63 (Ar(C)). IR (cm-1):
ν 2179 (Si-H). MS (ESI): 909.5 [M þ 1]þ.
Synthesis of PhB(ArN)2(Cl)SiSi(H)(NAr)2BPh (5). A solu-
tion of I(t-Bu) (0.36 g, 2.0 mmol) in n-hexane (20 mL) was added
to a solution of 2 (2.0 g, 4.0 mmol) in n-hexane (40 mL). The
mixture was stirred for 12 h at room temperature, and a white
precipitate was formed. After filtration, the colorless filtrate was
concentrated (ca. 10 mL) and stored at -40 °C for 24 h to give
white crystals of 5 (1.5 g, 77.5%), mp 206-208 °C. Anal. Cacld
for C60H79B2ClN4Si2: C, 74.33; H, 8.21; N, 5.78. Found: C,
74.12; H, 8.05; N, 5.72. 1H NMR (C6D6): δ 0.64 (d, 3H,
CH(CH3)2), 0.67 (d, 3H, CH(CH3)2), 1.09 (d, 3H, CH(CH3)2),
1.10 (d, 3H, CH(CH3)2), 1.17 (d, 3H, CH(CH3)2), 1.19 (d, 3H,
CH(CH3)2), 1.42 (d, 3H, CH(CH3)2), 1.47 (d, 3H, CH(CH3)2),
3.09 (sept, 1H, CH(CH3)2), 3.63 (sept, 1H, CH(CH3)2), 3.836
(sept, 1H, CH(CH3)2), 4.066 (sept, 1H, CH(CH3)2), 6.63 (s, 1H,
Si-H), 6.67 (t, 2H, Ar-H), 6.82 (qua, 1H, Ar-H), 7.01 (d, 1H, Ar-
H), 7.05 (t, 1H, Ar-H), 7.11 (d, 1H, Ar-H), 7.18 (d, 2H, Ar-H),
7.21 (t, 2H, Ar-H). 13C NMR (C6D6, 100.6 MHz): 21.50, 21.96,
23.91, 24.29, 24.42, 24.81, 25.53, 25.83 (s, CH(CH3)2), 28.70,
29.36, 30.47, 31.96 (s, CH(CH3)2), 123.58, 123.94, 124.48,
125.05, 126.00, 126.10, 127.48, 27.58, 131.29, 131.71, 135.28,
136.17, 137.90,, 138.82, 143.24, 145.04, 145.34, 146.15 (s, Ar-C).
29Si NMR (CDCl3): δ -54.7 (d, SiH, 1JSi-H=295 Hz), -22.0
(SiCl). IR (cm-1): ν 2109 (Si-H).
1
29Si NMR (CDCl3): δ 11.2, -34.4 (d, JSi-H=307 Hz). IR: ν
2181 cm-1 (Si-H).
Synthesis of PhB(ArN)2SiHCl (2). Neat SiHCl3 (0.75 g,
5.5 mmol) was slowly added to a solution of PhB(ArNLi)2
(2.26 g, 5.0 mmol) in n-hexane (60 mL) at -78 °C. The solution
was slowly warmed to room temperature, whereupon a white
precipitate formed. After 12 h, it was filtered, and the filtrate was
concentrated (ca. 20 mL) and stored at -40 °C for 24 h to give
white crystals of 2 (2.1 g, 83.5%), mp 83-85 °C. Anal. Calcd for
C30H40BClN2Si: C, 71.63; H, 8.02; N, 5.57. Found: C, 71.45; H,
7.89; N, 5.34. 1H NMR (CDCl3, 300 MHz): δ 1.07 (d, 3H,
CH(CH3)2, 1.12 (d, 3H, CH(CH3)2), 1.19 (d, 3H, CH(CH3)2),
1.35 (d, 3H, CH(CH3)2), 3.59 (sept, 1H, CH(CH3)2), 3.91 (sept,
1H, CH(CH3)2), 6.35 (s, 1H, Si-H), 6.71 (t, 2H, Ar-H), 6.87 (t,
1H, Ar-H), 7.09-7.19 (7H, Ar-H), 7.27 (d, 2H, Ar-H).
13C NMR (CDCl3, 75.4 MHz): δ 23.52, 24.00, 24.85, 25.13 (s,
CH(CH3)2), 28.97, 29.33 (s, CH(CH3)2), 123.91, 124.42, 126.24,
131.66, 135.09, 136.38, 144.69, 145.78 (s, Ar-C). 29Si NMR
1
(CDCl3): δ -26.1 (d, JSi-H=299 Hz). IR (cm-1): ν 2216 (Si-
Synthesis of [(CH)2(NMes)2]SiHCl (7). Lithium (0.18 g,
25 mmol) was added to a solution the diimine (CH)2(NMes)2
(Mes=2,4,6-Me3C6H2, 2.92 g, 10 mmol) in THF (30 mL). The
mixture was stirred for 2 days to give an orange-red suspension.
It was filtered, and the filtrate was cooled to -78 °C. To the
cooled solution was added neat HSiCl3 (10 mmol) in 5 min. The
resulting solution was warmed to room temperature and stirred
for 12 h. It was filtered, and the volatiles were removed in vacuo.
The residue was extracted with n-hexane (50 mL), and the
extract was stored in a freezer at -40 °C overnight to give white
crystals of 7 (2.5 g, 70%), mp 125-127 °C. Anal. Calcd for
C20H24N2Si: C, 67.29; H, 7.06; N, 7.85. Found: C, 66.93; H,
7.35; N, 7.42. 1H NMR (CDCl3): δ 2.35 (s, 6H, p-CH3), 2.46 (s,
12H, o-CH3), 5.78 (s, 2H, CHCH), 6.41 (s, 2H, Si-H), 6.99 (s,
4H, Ar-H). 13C NMR (CDCl3): δ 18.7 (p-CH3), 20.9 (o-CH3),
117.9 (HCCH), 128.9, 129.4, 136.0, 136.7 (Ar-C). 29Si NMR
(CDCl3): δ -39.3 (d, 1JSi-H=285 Hz).
H). MS: m/z 502.1 [PhB(NAr)2SiHCl]þ.
Synthesis of Me2Si(ArN)2SiH[(C3N2)H(t-Bu)2] (3). A solu-
tion of Me2Si(ArN)2SiHCl (0.47 g, 1.0 mmol) in THF (30 mL)
was slowly added to a solution of I(t-Bu) (=1,3-bis(tert-butyl)-
imidazol-2-ylidene) (0.36 g, 2 mmol) in THF (10 mL) at
-20 °C in 10 h. The mixture was allowed to warm to room
temperature and stirred for 10 h. It was filtered and all volatiles
were removed in vacuo. The residue was extracted with
n-pentane (10 mL ꢀ 2), and the combined extracts were con-
centrated (ca. 5 mL) and stored in a freezer at 4 °C for 5 days to
give colorless crystals of 3(0.26g,40%),mp183-185 °C. Anal. Calcd
for for C37H6N4Si2 (616.44): C, 72.02; H, 9.80; N, 9.08. Found: C,
1
71.95; H, 9.84; N, 8.82. H NMR (400 MHz, C6D6): δ 0.25, 0.54
(9) Hill, M. S.; Hitchcock, P. B. Organometallics. 2002, 21, 3258.
(10) Chivers, T.; Fedorchuk, C.; Parvez, M. Inorg. Chem. 2004, 43,
2643.
(11) Dieck, H. T.; Zettlitzer, M. Chem. Ber. 1987, 120, 795.
(12) Boncella, J. M.; Wang, S.; VanderLende, D. D.; Huff, R. L.;
Abboud, K. A.; Vaughn, W. M. J. Organomet. Chem. 1997, 530, 59.
(13) Kliegman, J. M.; Barnes, R. K. Tetrahedron 1970, 26, 2555.
(14) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.;
Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516.
Synthesis of [(CH)2(NCy)2]SiHCl (8). Lithium chunks (0.25 g,
36.3 mmol) were added to a solution of the diimine (CHNCy)2
(4.0 g, 18.2 mmol) in THF (50 mL) at -78 °C. The mixture was
allowed to warm to room temperature and stirred overnight to
give a red solution. Neat HSiCl3 (1.83 mL, 18.2 mmol) was
added to the solution at -78 °C, and the mixture was allowed to

Article
Organometallics, Vol. 28, No. 17, 2009 5193
warm to room temperature in 20 h. It was filtered, and the
volatiles were removed in vacuo. The residue was extracted with
60 mL of n-hexane. After filtration and removal of solvents,
the remaining colorless oil was distilled under reduced pressure
(101 °C, 0.3 mbar) to afford 8 as a colorless oil (1.45 g, 28%).
Anal. Calcd for C14H25ClN2Si: C, 59.02; H, 8.84; N, 9.83.
procedure to that described for 11, mp 166 °C. Anal. Calcd for
C23H42N4Si3: C, 60.20; H, 9.23; N, 12.21. Found: C, 59.90; H,
9.39; N, 12.25. 1H NMR (C6D6): δ 7.34 (s, 1H, CH), 6.93 and
6.82 (m, 4H, Ar-H), 6.13 (s, 1H, Si-H), 1.59 and 1.48 (s, 9H,
C(CH3)3), 0.22 (s, 18H, Si(CH3)3). 13C NMR (C6D6): δ 222.0
(carbene), 142.8, 119.5, and 114.7 (Ar-C), 133.1 (CH), 57.8 and
56.0 (CMe3), 32.3 and 31.3 (C(CH3)3), 0.44 (Si(CH3)3).
1
Found: C, 58.76; H, 8.76; N, 9.24. H NMR (CDCl3): δ 6.48
1
29Si NMR (CDCl3): δ -22.3 (d, JSi-H = 229 Hz), 3.59. IR
(s, 1H, Si-H), 5.68 (s, 2H, CHdCH), 2.87 (m, 2H, cyclohexyl-
CH), 2.07, 1.93, 1.55, 1.40, 1.00 (m, 20H, cyclohexyl-CH2).
13C NMR(CDCl3): δ 114.9 (CHdCH), 56.3, 35.4, 34.8, 25.9
(cyclohexyl). 29Si NMR (CDCl3): δ -36.1 (d, 1JSi-H=320 Hz).
MS (EI): 284.3 (Mþ).
(cm-1): ν 2156, 2098 (Si-H). MS (EI): 458.3 (Mþ).
X-ray Structural Determination. All intensity data were col-
lected with a Bruker SMART CCD diffractometer using gra-
˚
phite-monochromated Mo KR radiation (λ = 0.71073 A) at
Synthesis of [(CH)2(Nt-Bu)2]Si (9). A solution of I(t-Bu)
(0.19 g, 1.1 mmol) in THF (5 mL) was added to a solution of
[(CH)2(t-BuN)2]Si HCl (0.25 g, 1.1 mmol) in THF (5 mL) at
room temperature. The mixture was stirred overnight. It was
filtered, the filtrates were removed under vacuum, and the
remaining solid was extracted with 30 mL of n-hexane. The
extract was concentrated (ca. 2 mL) and stored in a freezer at
-40 °C overnight to give colorless crystals of 9 (0.15 g, 71.5%).
1H NMR (C6D6): δ 6.75 (s, 2H, CH), 1.40 (s, 18H, CH3).
13C NMR (C6D6): δ 120.0 (CH), 54.1 (C(CH3)3), 33.1
(C(CH3)3). 29Si NMR (C6D6): δ 77.8 (s).
113(2) K. The structures were resolved by direct methods and
refined by full matrix least-squares on F2. Hydrogen atoms were
considered in calculated positions. All non-hydrogen atoms
were refined anisotropically. Crystals of 3 suitable for X-ray
analysis were grown from n-hexane. Crystallographic data for 3:
˚
˚
monoclinic, space group P21/c, a=29.013(6) A, b=11.349(2) A,
3
˚
˚
c=23.425(5) A, β=99.15(3)°, V=7615(3) A , Z=8, 50 498
reflections, 13 373 unique (Rint = 0.0511), R1 = 0.0549 (I >
2σ(I)), wR2 = 0.1460 (all data). CCDC-731412 contains the
supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Synthesis of [(CH)2(NMes)2]Si (10). To a solution of 7 (1.78 g,
5 mmol) in n-hexane (20 mL) was added a solution of I(t-Bu)
(0.9 g, 5 mmol) in n-hexane (20 mL). The reaction mixture was
stirred for 10 h. Filtration and subsequent concentration and
storage at -40 °C overnight afforded yellow crystals of 10 (1.3 g,
81.2%), mp 110-112 °C. Anal. Calcd for C20H24N2Si: C, 74.95;
Results and Discussion
1
We have recently reported the isolation of two N-aryl-
substituted heterocyclic silylenes via low-temperature reduc-
tion of their corresponding dihalides.15 As a continuation of
our search for stable silylenes featuring other frameworks,
we designed to prepare four-membered heterocyclic silylenes
of the type [E(NR)2]Si (E=Me2Si, PhB). It has been reported
that the silylene [Me2Si(Nt-Bu)2]Si could be observed by low-
temperature matrix isolation spectroscopy,16 and we at-
tempted to study this type of silylenes in condensed phases.
After our unsuccessful attempts using a reduction route with
alkali metals, we turned our attention to dehydrohalogena-
tion reaction with N-heterocyclic carbenes.
H, 7.55; N, 8.74. Found: C, 74.92; H, 7.41; N, 8.68. H NMR
(C6D6): δ 2.17 (s, 6H, p-CH3), 2.25 (s, 12H, o-CH3), 6.29 (s, 2H,
CHCH), 6.83 (s, 4H, Ar-H). 13C{1H} NMR (C6D6): δ 18.5
(p-CH3); 21.0 (o-CH3); 124.4 (HCdCH), 129.3, 134.8, 135.8,
140.3 (Ar-C). 29Si NMR (C6D6): δ 77.8.
Synthesis of [(CH)2(NCy)2]SiH[(C3N2)H(t-Bu)2] (11). I(t-Bu)
(0.47 g, 2.58 mmol) in THF (5 mL) was added to a solution of 8
(0.37 g, 1.29 mmol) in THF (5 mL) at room temperature. The
mixture was stirred overnight. After filtration and removal of
volatiles in vacuo, the residues were extracted with 30 mL of
n-hexane. The extract was concentrated and stored in a freezer at
-40 °C overnight to afford colorless crystals of 11 (0.33 g, 60%),
mp 136 °C. Anal. Calcd for C25H44N4Si: C, 70.04; H, 10.34; N,
13.07. Found: C, 69.90; H, 10.39; N, 12.75. 1H NMR (CDCl3):
δ 7.43 (s, 1H,), 6.46 (s, 1H), 5.72 (s, 2H), 2.90 (m, 2H), 1.96, 1.54,
1.34, 0.96 (m, 20H). 13C NMR (CDCl3): δ 221.97, 132.19,
115.09, 57.91, 56.19, 55.97, 34.85, 34.60, 32.33, 31.35, 26.05,
25.78. 29Si NMR (C6D6): δ -38.7 (d, 1JSi-H=238 Hz).
Synthesis of [C6H4(NSiMe3)2-1,2]SiHCl (12). n-BuLi
(6.85 mL, 17.1 mmol, 2.5 M solution in n-hexane) was slowly
added to a solution of [C6H4(HNSiMe3)2-1,2] (2.2 g, 8.56 mmol)
in 50 mL of THF at -78 °C. The mixture was allowed to warm to
room temperature and stirred overnight. To this precooled
solution (-78 °C) was added HSiCl3 (0.86 mL, 8.56 mmol),
and the mixture was allowed to warm to room temperature and
stirred for 12 h to yield a light yellow solution. All volatiles were
removed in vacuo. The remaining residue was extracted with
60 mL of THF. After filtration and removal of the volatiles, the
remaining light green oil was distilled under reduced pressure
(105 °C, 0.2 mbar) to give colorless oil of 12 (2.1 g, 77.9%),
which solidified in a couple of days, mp 58 °C. Anal. Calcd for
C12H23ClN2Si3: C, 45.75; H, 7.36; N, 8.89. Found: C, 45.27; H,
7.87; N, 8.25. 1H NMR (CDCl3): δ 6.89 and 6.78 (m, 2H, Ar-H),
6.14 (s, 1H, Si-H), 0.43 (s, 18H, Si(CH3)3). 13C NMR (CDCl3):
δ 140.6, 119.2, and 114.2 (Ar-C), 0.15 (Si(CH3)3). 29Si NMR
(CDCl3): δ -16.3 (d, SiH, 1JSi-H=315 Hz), 5.40 (SiMe3). IR:
2250.4 (Si-H). MS (EI): 314.4 (Mþ).
The four-membered cyclic diaminochlorosilanes [Me2Si-
(NAr)2]SiHCl (Ar=2,6-iPr2C6H3, 1) and [PhB(NAr)2]SiHCl
(2) were obtained by the respective reaction of Me2Si(Ar-
NLi)2 and PhB(ArNLi)210 with trichlorosilane. Reaction of
the N-heterocyclic carbene I(t-Bu) (I(t-Bu) = 1,3-bis(tert-
butyl)imidazol-2-ylidene)14 with 1 in toluene at room tem-
perature resulted in the rapid formation of a white precipi-
tate, which was identified as 1,3-di-tert-butylimidazolium
chloride. Analysis of the soluble materials by proton NMR
spectroscopy indicates the formation of new species. Two
species were isolated in pure form under the optimized
conditions (Scheme 1); slow addition of a solution of 1 to 2
equiv of I(t-Bu) in THF or toluene followed by crystal-
lization of the crude product from n-hexane yielded 3 in
good yield, while addition of I(t-Bu) to 2 equiv of 1 in toluene
or n-hexane gave the disilane 4. Compounds 3 and 4 have
1
been characterized by H and 13C NMR and IR spectro-
scopies and elemental analysis. The presence of silicon
hydride in 3 and 4 was evident from their 1H NMR and IR
spectra. The IR bands at 2188 cm-1 for 3 and 2179 cm-1 for
4 can be assigned to Si-H stretch vibrations. The 1H NMR
signals due to the Si-H for 3 and 4 appear at δ 6.66 and
Synthesis of [C6H4(NSiMe3)2-1,2]SiH[(C3N2)H(t-Bu)2] (13).
I(t-Bu) (0.51 g, 2.8 mmol) in THF (5 mL) was added to a solution
of 12 (0.44 g, 1.4 mmol) in THF (5 mL) at room temperature.
13 was obtained as colorless crystals (62%) following a similar
(15) Kong, L.; Zhang, J.; Song, H.; Cui, C. Dalton Trans. 2009, 5444.
€
(16) Veith, M.; Werle, E.; Lisowsky, R.; Koppe, R.; Schnockel, H.
€
Chem. Ber. 1992, 125, 1375.

5194 Organometallics, Vol. 28, No. 17, 2009
Cui et al.
Scheme 1
Scheme 2
reactivity observed for the two systems may arise from steric
18
˚
effects since the relatively short B-N bond (ca. 1.43 A)
would result in the more open structure of the borylene-
bridged diaminosilylene, which can be more easily attacked
by 2. Monitoring the reactions in C7D8 did not provide
any information on the possible intermediates due to the
rapid formation of the imidazolium chloride even at low
temperature.
In order to gain further support for the formation of the
silylene intermediates by the dehydrochlorination reaction,
we attempted to prepare the known five-membered hetero-
cyclic silylenes 9 and 10 via this route.2,15 Indeed, the reac-
tions of the diaminochlorosilanes 66 and 7 with 1 equiv of I(t-
Bu) in THF or n-hexane at room temperature cleanly yielded
9 and 10 in ca. 72% and 81% yields, respectively (Scheme 2).
The spectroscopic data and physical properties of 9 and 10
are in good agreement with those reported. Obviously, this
route is superior to the potassium reduction reaction pre-
viously employed for the synthesis of 9 and 10 in that the
dehydrochlorination reaction can be easily carried out at
room temperature in THF, n-hexane, and toluene, and the
high yields are reproducible. It is noted that the reaction rate
for the formation of the imidazolium chloride in n-hexane
and toluene is slower that that in THF, suggesting that the
dehydrohalogenation may go through a polarized intermedi-
ate. We postulate that the reaction involves the initial
deprotonation of the diaminochlorosilanes by I(t-Bu) due
to the strong basicity of the N-heterocyclic carbene. This
could lead to the formation of cyclic diaminocholorosilyl
anions, which then undergo dissociation of the chloride to
give the silylenes. The near inertness of the dichorosilane
[(CH)2(t-BuN)2]SiCl2 to I(t-Bu) under similar conditions
strongly supports the deprotonation pathway.
The dehydrohalogenation reaction can also be applied for
the generation of a transient heterocyclic silylene with less
hindered N-substituted groups and a benzo-fused hetero-
cyclic silylene (Scheme 2). The reaction of the N-cyclohexyl-
substituted diaminochlorosilane 8 with I(t-Bu) in THF
yielded the C-H activation product 11. Similarly, the reac-
tion of the N-silyl-substituted substrate 12 with I(t-Bu) gave
13 in good yield. The new compounds 8 and 11-13 have been
characterized by spectroscopic methods and elemental anal-
ysis. The 29Si NMR spectra of the C-H activation products
display the characteristic doublet (δ -38.7 (1JSi-H=238 Hz)
and -22.3 ppm (1JSi-H =229 Hz) for 11 and 13, respecti-
vely) for the central silicon atom. In the five-membered
Figure 1. ORTEP drawing of 3 with 30% probability ellipsoids.
Hydrogen atoms have been omitted for clarity. Selected bond
lengths (A) and angles (deg): N1-Si1 1.7402(18), N2-Si1
1.7298(18), Si1-C27 1.852(2), N3-C29 1.369(3), N4-C29
1.352(3); N1-Si1-N2 87.02(9), N3-C29-N4 103.52(18).
˚
5.48 ppm. The structure of 3 has been confirmed by an X-ray
single-crystal analysis (Figure 1). It is notable that the central
silicon atom is bonded to one of the olefinic carbon atoms on
the imidazol-2-ylidene ring. Most likely, the formation of 3
involves cleavage of one olefinic C-H bond on the ring.
Reaction of [PhB(NAr)2]SiHCl (2) with I(t-Bu) in n-hex-
ane also resulted in the rapid formation of the corresponding
imidazolium chloride. However, the exclusive product is the
disilane 5, irrespective of the addition sequences, molar ratio,
and solvents employed (Scheme 1). The disilane 5 has been
ambiguously characterized by NMR, IR, and mass spectro-
scopies and elemental analysis. The 29Si NMR spectrum of 5
gives one doublet at δ -54.7 (1JSi-H = 295 Hz) and one
singlet at -22.0 ppm due to the two different silicon atoms. It
has been reported that disilanes can also be generated by the
reaction of a stable heterocyclic silylene with halocarbons.17
The most plausible explanation for the formation of the
C-H activation product 3 and disilanes 4 and 5 may involve
the initial generation of the transient four-membered hetero-
cyclic silylenes A via a dehydrochlorination reaction
(Scheme 1). The diaminosilylenes can either insert one
of the olefinic C-H bonds on the imidazol-2-ylidene ring
or reduce the hydrochlorosilanes 1 and 2. The different
€
(17) Moser, D. F.; Naka, A.; Guzei, I. A.; Muller, T.; West, R. J. Am.
Chem. Soc. 2005, 127, 14730.
(18) Chivers, T.; Fedorchuk, C.; Parvez, M. Inorg. Chem. 2004, 43, 2643.

Article
Organometallics, Vol. 28, No. 17, 2009 5195
heterocyclic system, the formation of a disilane has not been
observed probably due to the more hindered steric environ-
ment in the five-membered cyclic silylenes, which suppress
the attack of the resulting silylene to the silicon center of the
hydrochlorosilanes. Both the C-H activation products have
been characterized by standard spectroscopic methods.
The formation of these C-H activation products and disi-
lanes indicates that the sterically hindered environments
around a divalent silicon center are essential for the protection
of the heterocyclic silylene from attack by other substrates in
the reaction medium. In the case of the N-silyl-substituted
benzo-fused heterocyclic silylene, the destabilization may result
from the reduced electron density on the nitrogen atoms due to
the electron-withdrawing property of the silyl substituent.
silylenes under very mild conditions has been developed.
This route has allowed the generation of stable and
transient four- and five-membered cyclic diaminosilyl-
enes with a range of different substituents on the ligand
backbones in high yields. Most remarkably, these silyl-
enes were obtained by a non-metallic route under very mild
conditions for the first time. We are currently extend-
ing this methodology for the generation of other types
of silylenes with various N-heterocyclic carbenes and
investigating a detailed mechanism for this type of new
reaction.
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (20725205).
Conclusion
Supporting Information Available: A cif file for compound 3.
This material is available free of charge via the Internet at http://
pubs.acs.org.
A novel and facile dehydrohalogenation route by a stable
N-heterocyclic carbene for the generation of N-heterocyclic