Synthesis of a base-stabilized 1-hydrosilanimine via NHC-mediated dehydrohalogenation of hydrochlorosilane

Article abstract of DOI:10.1002/asia.201100022

Covering all the bases: Donor-stabilized 1-hydrosilanimine was prepared by dehydrohalogenation of Ar(SiMe₃)NSiHCl₂ with a less-hindered N-heterocyclic carbene. The steric effects of NHCs have significant influences on the dehydrohalo

Full text of DOI:10.1002/asia.201100022

COMMUNICATION
DOI: 10.1002/asia.201100022
Synthesis of a Base-Stabilized 1-Hydrosilanimine via NHC-Mediated
Dehydrohalogenation of Hydrochlorosilane
Haiyan Cui and Chunming Cui*[a]
In recent years, unsaturated silicon compounds (silylenes
and silicon multiple-bonding species) have attracted much
attention owing to their significant roles in the development
of silicon chemistry.[1] Silanimines (R2Si=NR), as silicon an-
alogues of imines, are, in general, short-lived and highly re-
active species in the absences of efficient protecting groups
on both the silicon and nitrogen atoms.[2] By taking advant-
age of donor–acceptor stabilization concepts, a fair number
Scheme 1. Synthesis of 2 and 3. IMe4 =1,3,4,5-tetramethylimidazol-2-yli-
dene.
of donor-stabilized silanimines have been isolated and struc-
turally characterized since the first isolable silanimine was
reported in 1986.[3,4] It is well-known that organosilicon hy-
drides have been widely used as hydrosilylation reagents in
organic synthesis. However, unsaturated silicon hydrides, in
which the hydride ligands are bonded to an unsaturated sili-
con atom, are extremely rare.[5] The lack of these species
may be related to their intrinsic high reactivity arising from
a reactive hydride ligand being attached to the unsaturated
silicon atom. Among these, hydrosilanimines (RHSi=NR)
are interesting and attractive synthetic targets because of
their fundamental interest considering the relative stability
of hydrosilanimines and their corresponding silylene isomers
(RSiNHR).[6] Furthermore, it has been recently shown that
transition-metal–hydrosilanimine complexes are useful for
some stoichiometric and catalytic transformations.[7] Howev-
er, stable hydrosilanimines are still elusive, presumably
owing to their high reactivity and lack of synthetic methods
for their synthesis.
steric effects of the substituents on the NHC nitrogen
atoms.
We and others have previously reported NHC-mediated
dehydrochlorination reactions for the generation of stable
and transient silylenes.[8] To extend that methodology, we ex-
amined the reactions of the silylamino-substituted dichloro-
silane RSiHCl2 (1) with NHCs. As previously observed in
the reactions of various N-heterocyclic chlorosilanes with
NHCs,[8a] the products varied largely depending on the reac-
tion conditions and the substrates employed. Therefore, it is
essential to probe the steric effects of NHCs on the reaction.
Thus, the less-hindered 1,3,4,5-tetramethylimidazol-2-ylidene
(IMe4) and the bulky 1,3-bis(tert-butyl)imidazol-2-ylidene
(ItBu2) were chosen for this study.[9] The reaction of 1 with
one equivalent of IMe4 in C6D6 at room temperature gave a
complicated mixture of products as indicated by NMR anal-
ysis. However, unexpectedly, the reaction of 1 with two
equivalents of IMe4 in tetrahydrofuran at low temperature
gave 1-hydrosilanimine 2 (Scheme 1) as yellow crystals in
26% yield along with some insoluble materials. Compound
2 is air and moisture sensitive, and it is soluble in common
hydrocarbon solvents such as n-hexane and toluene. The
Herein, we report the first donor-stabilized hydrosilani-
mine prepared by the reaction of RSiHCl2 (1, R=Ar-
ACHTUNGTRENNUNG
(SiMe3)NÀ, Ar=2,6-iPr2C6H3) with N-heterocyclic carbenes
(NHCs; Scheme 1). In addition, we demonstrate that the re-
actions of 1 with NHCs are significantly influenced by the
proton NMR spectrum of 2 indicated the presence of one
1
À
Si H (d=6.21 ppm, JSi-H =218 Hz) bond, two Ar rings, one
[a] H. Cui, Prof. Dr. C. Cui
State Key Laboratory of Elemento-organic Chemistry
Nankai University
SiMe3 group, and one IMe4 group in the structure. The de-
tailed structure of 2 was finally determined by single-crystal
X-ray analysis.
Tianjin, 300071 (China)
Fax : (+86)-22-23503461
X-ray structural analysis of 2 revealed that it was an
NHC-stabilized 1-hydrosilanimine (Figure 1). The central
silicon atom was four-coordinate and adopted a distorted
E-mail: cmcui@nankai.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100022.
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Chem. Asian J. 2011, 6, 1138 – 1141

Figure 1. ORTEP of 2. Selected bond lengths (ꢁ) and angles (deg): Si1–
N4 1.6209(13), Si1–N3 1.7442(13), Si1–C34 1.9429(16), Si1–H1 1.376(18),
N4–C19 1.363(2), N3–C7 1.4490(19), N1–C34 1.349(2), N2–C34
1.3557(18), C2–C3 1.360(2); Si1-N4-C19 148.09(11), N3-Si1-N4 112.49(6),
N3-Si1-C34 108.30(6), N4-Si1-C34 110.23(7), N3-Si1-H1 102.4(7), N4-Si1-
H1 123.9(7), C34-Si1-H1 97.8(7).
Figure 2. ORTEP of 3. Selected bond lengths (ꢁ) and angles (deg): Si1–
N1 1.7211(17), Si1–N3 1.7144(16), Si1–Cl1 2.0682(8), Si2–N1 1.721(2),
Si2–N2 1.726(2), Si2–Cl2 2.0532(16), Si3–N2 1.7184(17), Si3–N3
1.7168(16), Si3–Cl3 1.9921(11); Si1-N1-Si2 127.23(10), N1-Si2-N2
108.67(10), Si3-N2-Si2 127.68(10), N3-Si3-N2 108.72(8), Si1-N3-Si3
127.66(10), N3-Si1-N1 110.49(8).
À
tetrahedral
geometry.
The
Si1 N4
bond
length
(1.6209(13) ꢁ) was the longest observed so far for structur-
ally characterized free- and donor-stabilized silanimines
NMR and IR spectroscopy, and X-ray single-crystal analysis
(Figure 2). The Si3N3 six-membered ring is twisted in a simi-
lar fashion to those in the known cyclotrisilazanes.[13] The
aryl rings are almost perpendicular to the mean Si3N3 ring.
For comparison, the reaction of 1 with the bulky N-heter-
ocyclic carbene ItBu2 was investigated. As with our previ-
ously reported results, disilane 4 was isolated from the reac-
(1.568–1.611 ꢁ);[3,4] however, it was shorter than other Si N
À
single bonds (1.64–1.78 ꢁ) in a variety of silanamines. The
À
À
Si1 C34 bond length (1.9429(16) ꢁ) was longer than Si C
single bonds in organosilanes (average 1.86 ꢁ) but in the re-
ported range for NHC-supported silicon halides (1.935–
2.0088 ꢁ).[8d,10] As is the case for all known stable silani-
1
À
À
mines, the Si1 N4 C19 bond angle (148.09(11)8) was much
larger than the analogous C-N-C bond angles in imines
tion (Scheme 2). Compound 4 has been characterized by H
À
owing to highly polarized Si N double bonds. However, it is
much smaller than those in silyl-substituted (both on the sili-
con and nitrogen atoms of the Si=N moiety) silanimines, in
which the interaction of the lone pair on the nitrogen atom
with the silyl group on the nitrogen atom has been proposed
by calculations as well as based on X-ray crystal analy
sis.[4a,11] The N4 C19 bond length (1.363 (2) ꢁ) was signifi-
À
À
cantly shorter than a C N single bond, thus suggesting an
Scheme 2. Synthesis of 2 from 4. ItBu2 =1,3-bis(tert-butyl)imidazol-2-yli-
dene.
interaction of the nitrogen electrons with the aryl ring. Com-
pound 2 is the first example of donor-stabilized 1-hydrosi-
lanmines. Recent studies have shown that NHCs could stabi-
lize some interesting silicon multiple-bonding species, such
and 13C NMR and IR spectroscopy, and elemental analysis.
The different products observed in the reactions with small
IMe4 and bulky ItBu2 indicated either that the reactions pro-
ceeded through different initial mechanisms or that product
4 underwent further reactions with the less-hindered IMe4.
To test the latter pathway, the reaction of 4 with IMe4 was
studied. Indeed, the reaction of 4 with one equivalent of
IMe4 in tetrahydrofuran afforded a mixture containing 2 and
3 in the molar ratio of 1:0.57, as estimated from the proton
NMR spectrum. In contrast, the reaction of 4 with two
equivalents of IMe4 exclusively yielded 2 as a major product,
as the formally silicon(0) species (NHC)Si=SiACHTNUTRGNE(NUG NHC) report-
ed by Robinson and co-workers and a NHC-supported sila-
none reported by Driess and co-workers.[12]
In order to better understand the reaction mechanism for
the formation of 2, various experimental conditions (differ-
ent stoichiometries, temperatures, and addition sequences)
were been examined. In most cases, it is impossible to iso-
late pure products even from repeated crystallizations from
different solvents. However, compound 3 was obtained from
the slow addition of a solution of IMe4 in tetrahydrofuran to
two equivalents of 1 in tetrahydrofuran at low temperature.
Compound 3 has been fully characterized by multinuclear
1
whilst 3 was not observed by H NMR analysis. These re-
sults strongly indicated that 2 was formed following the ini-
Chem. Asian J. 2011, 6, 1138 – 1141
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www.chemasianj.org
1139

H. Cui and C. Cui
COMMUNICATION
(400 MHz, C6D6): d=0.05 (d, 3H, Me), 0.51 (s, 9H, SiMe3), 0.10 (s, 6H,
NMe), 1.17 (d, 3H, Me) 1.35 (d, 3H, Me), 1.48 (dd, 12H, CHMe2), 1.57
(d, 3H, Me), 3.21 (m, 1H, CHMe2), 4.00 (m, 1H, CHMe2), 4.13 (m, 2H,
CHMe2), 6.21 (s, 1H, 1JSiÀH =218 Hz, SiH), 6.86–7.19 (m, 4H, ArH),
7.36 ppm (d, 2H, ArH); 13C NMR (100.61 MHz, C6D6): d=3.34, 7.39,
22.56, 22.71, 24.84, 25.04, 25.65, 26.06, 26.41, 27.84, 28.58, 28.71,113.3,
123.0, 123.3, 124.9, 125.1, 125.7, 139.7, 140.5, 148.1, 149.6, 151.3,
154.3 ppm; 29Si NMR (59.62 MHz, C6D6): d=À72.01 (SiH), 2.74 ppm (s,
SiMe3); EI-MS (m/z): 576.4 [M+]; UV/Vis (lmax): 282 nm (toluene, e=
tial formation of 4 as an intermediate in the reaction of 1
with IMe4. In this case, the small size of IMe4 may attack
the disilane 4, thereby resulting in the elimination of insolu-
ble (SiCl2)n and Me3SiCl (Scheme 3). The extrusion of 1
1200 cm2 mmolÀ1); IR: n˜ =1623, 2962 cmÀ1
.
4: A solution of ItBu2 (0.09 g, 0.5 mmol) in THF (5 mL) was slowly
added to a stirred solution of 1 (0.35 g, 1 mmol) in THF (5 mL) at
À788C. The mixture was slowly warmed to room temperature and stirred
overnight, before being filtered and the volatile compounds removed. 4
was obtained as a colorless solid by sublimation at 2198C. 1H NMR
(400 MHz, C6D6): d=0.17 (s, 9H, Me3Si), 0.28 (s, 9H, Me3Si), 1.15–1.42
(m, 24H, CHMe2), 3.66 (m, 1H, CH), 3.76 (m, 2H, CH), 3.98 ppm (m,
2H, CH), 5.40 (s, 1H, SiH), 7.05 (m, 6H, ArH); 13C NMR (100.61 MHz,
C6D6,): d=1.01, 2.36, 2.59, 23.88, 24.30, 24.35, 25.02, 25.24, 26.01, 26.67,
26.80, 27.34, 27.42, 28.07, 28.27, 29.70, 77.20, 124.1, 124.3, 124.4, 125.9,
126.0, 146.8, 147.6, 162.4 ppm; EI (m/z): 660.2 [M+]; IR: n˜ =2169 cmÀ1
Scheme 3. Proposed initial reactions of 1 with IMe4.
À
(Si H).
Further details for the synthesis and characterization of compounds 1–4
are given in the Supporting Information. CCDC 761526 (2) and
CCDC 761525 (3) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
from 4 to form the chlorosilylene RSiCl could also be oper-
ating. Therefore, an excess of IMe4 favors the formation of
2, as 1 could again be converted into 2 in this case. These
observations were consistent with the conditions for the
preparation of 2 from 1. Compound 3 can be viewed as the
elimination product from 1 through loss of Me3SiCl, as the
small IMe4 may coordinate to 1 to form a hypervalent sili-
con species (Scheme 3), which facilitates the elimination re-
action to give B followed by its trimerization.
Acknowledgements
We are grateful to the National Natural Science Foundation of China
(Grant No. 20725205) for the finicial support.
The proposed mechanism for the formation of 2 is further
supported by the previous observation that a disilane easily
À
undergoes Si Si bond cleavage and disproportionation in
Keywords: carbene
acceptor · silanimine · silicon
·
dehydrohalogenation
·
donor-
the presence of a Lewis base.[14] It is reasonably assumed
that the reaction of 1 with IMe4 may proceed through two
distinct pathways because of its small size (Scheme 3). In
contrast, the reaction of 1 with ItBu2 can only proceed
through the dehydrohalogenation pathway to give the inter-
mediate A, as proposed previously by ourselves, owing to
the bulkiness of the ItBu2 group.
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Experimental Section
2: A solution of 1 (0.35 g, 1.00 mmol) in THF (5 mL) was added to a so-
lution of IMe4 (0.25 g, 2 mmol) in THF (5 mL) at À788C. The mixture
was allowed to warm up to room temperature and stirred overnight. All
volatile compounds were removed, and the remaining residual was ex-
tracted with n-hexane. Removal of solvents and crystallization from tolu-
ene at À308C yielded yellow crystals of 2 (26%). M.p.: 1778C; 1H NMR
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Received: January 13, 2011
Published online: March 17, 2011
Chem. Asian J. 2011, 6, 1138 – 1141
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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