Facile access to silyl-functionalized N-heterocyclic olefins with HSiCl3

Article abstract of DOI:10.1039/c3cc45652h

N-heterocyclic olefins (NHOs), IPrCH₂ (1) and SIPrCH₂ (2) (IPrCH₂ = {N(2,6-iPr₂C₆H ₃)CH}₂CCH₂ and SIPrCH₂ = {N(2,6-iPr₂C₆H₃)CH₂} ₂CCH₂), react with HSiCl₃ and afford IPrCH(SiHCl₂) (3) and SIPrCH(SiHCl₂) (4), respectively. Compounds 3 and 4 have been isolated in almost quantitative yield. Interestingly, treatment of the silylene IPr·SiCl₂ with 1 also affords 3, where silylene insertion into a C-H bond is observed. Computational analysis shows a high energy barrier for silylene insertion, therefore a protonation-deprotonation mechanism is more likely.

Full text of DOI:10.1039/c3cc45652h

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Facile access to silyl-functionalized N-heterocyclic
olefins with HSiCl3†
Rajendra S. Ghadwal,*a Sven O. Reichmann,a Felix Engelhardt,a Diego M. Andradab
and Gernot Frenkingb
Cite this: Chem. Commun., 2013,
49, 9440
Received 25th July 2013,
Accepted 13th August 2013
DOI: 10.1039/c3cc45652h
www.rsc.org/chemcomm
N-heterocyclic olefins (NHOs), IPrCH2 (1) and SIPrCH2 (2) (IPrCH2
=
{N(2,6-iPr2C6H3)CH}2CCH2 and SIPrCH2 = {N(2,6-iPr2C6H3)CH2}2CCH2),
react with HSiCl3 and afford IPrCH(SiHCl2) (3) and SIPrCH(SiHCl2) (4),
respectively. Compounds 3 and 4 have been isolated in almost quanti-
tative yield. Interestingly, treatment of the silylene IPrÁSiCl2 with 1 also
affords 3, where silylene insertion into a C–H bond is observed.
Computational analysis shows a high energy barrier for silylene inser-
tion, therefore a protonation–deprotonation mechanism is more likely.
Investigation of intriguing properties, reactivity and applica-
tions of the compounds with low-valent main group elements is
an active area of current research.1–4 Among group 14 elements,
carbenes and silylenes are the most studied reactive species.5
Scheme 1 NHC (A), NHSi (B), NHC stabilized silylenes (C), and NHO (D).
They insert into s-bonds or undergo oxidative cycloaddition applications not only as ligands in transition-metal catalysis
reactions with unsaturated organic compounds to form tetra- and organometallic chemistry, but also as organocatalysts in
valent derivatives of carbon and silicon.2–6 Carbenes were their own right.10,11 Furthermore, NHCs have been found to be
believed to be non-isolable highly reactive intermediates. This very efficient neutral ligands in stabilizing highly reactive main
situation changed when Bertrand et al. reported a phosphino group species.3,4,12 Among dihalosilylenes, some trapping reac-
stabilized carbene7 in 1989. In 1991, Arduengo and co-workers tions of condensed SiCl2 with acetylene and benzene were
reported the first thermally stable carbene as a N-heterocyclic already carried out by Timms in 1968.13 In 2009, the first
carbene (NHC) (Scheme 1A).8 However, it took almost three monomeric dihalosilylenes were reported, which were stabi-
years to isolate a silicon analogue of a NHC. In 1994, West and lized by a NHC (Scheme 1C).14,15 Besides the classical method
co-workers succeeded in isolating the first stable N-heterocyclic for preparing compounds with low-valent main group elements
silylene (NHSi) (Scheme 1B).9 Now, a number of stable carbenes using alkali metals, a novel route was also disclosed to prepare
and silylenes with variable structural motifs are known and can NHC-stabilized dichlorosilylene (Scheme 1C; X = Cl).14 Reduc-
be readily synthesized.2–4
tive dehydrochlorination of HSiCl3 with a NHC affords a NHC
The electron richness and structure of the NHCs provide a stabilized dichlorosilylene.14 Silylene C (X = Cl) has been found
unique class of s-donor ligands. NHCs have found widespread to be a strong s-donor ligand for transition metals.16,17 Silylene
C (X = Cl) behaves as a Lewis base and some Lewis acid–base
adducts of C with boranes have also been isolated.18,19
a
¨
¨
¨
Institut fur Anorganische Chemie, Georg-August-Universitat Gottingen,
N-heterocyclic olefin (NHO) (Schemes 1D and 2) and its Lewis
acid–base adducts were reported by Kuhn et al. in 1994.20
Scheme 2 illustrates various mesomeric forms (a, b, and c) of a
NHO. Therefore, NHO (D) can be considered as a lighter con-
gener of C. Interest in this seemingly unusual molecule aroused
very recently. Rivard and co-workers have used NHOs (Scheme 2)
with sterically demanding substituents and have isolated inter-
esting compounds with low-valent Ge and Sn.21a Recently, NHOs
have been shown to stabilize interesting boron compounds.21c,d
¨
Tammannstrasse 4, 37077 Gottingen, Germany.
E-mail: rghadwal@uni-goettingen.de
b
¨
Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Straße,
35032 Marburg, Germany
† Electronic supplementary information (ESI) available: General experimental
procedures, X-ray crystallographic information on compound 3, cartesian coor-
dinates (in Å) and total BP86/def2-TZVP energies (in au, noncorrected zero-point
vibrational energies included) of all stationary points discussed. CCDC 951884.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3cc45652h
c
9440 Chem. Commun., 2013, 49, 9440--9442
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compounds 3 and 4 shows a doublet for the SiH proton accom-
panied by silicon satellite signals (JSi–H = 291.13 Hz (3) and JSi–H
=
295.67 Hz (4)). Imidazoline ring NCH protons in 3 are magnetically
non-equivalent, and each appears as a doublet. Similarly, NCH2
protons of 4 exhibit a multiplet. Olefinic CH proton each in
compounds 3 and 4 appears as a doublet due to coupling with
the SiH proton. 13C NMR spectra of the compounds 3 and 4 show
resonances consistent with their 1H NMR spectral data.22 The
resonances are shifted downfield when compared with those of
respective NHO 1 or 2. Each of the compounds 3 (d À9.83 ppm)
and 4 (d À7.15 ppm) shows a signal in the 29Si NMR spectrum,
which is consistent with those observed for four-coordinate orga-
nosilicon compounds.2–4 The EI-mass spectrum of 3 exhibits the
molecular ion peak at 500 (m/z).
Scheme 2 Different mesomeric forms of a NHO.
Base induced disproportionation of HSiCl3 to generate
dichlorosilylene is well known.14,23 Use of NHO as a Lewis base
has been shown by Kuhn, Rivard and others.20,21 We decided to
use NHO as a base to generate silylene with HSiCl3.14 In analogy
with the reaction of NHCs and HSiCl3,14 dehydrochlorination of
HSiCl3 with a NHO base and subsequent silylene insertion into
a C–H bond of NHO to afford 3 or 4 were assumed. However,
computational analysis shows a high energy barrier for SiCl2
insertion (Fig. 1).22 Therefore, formation of adducts I and II and
subsequent deprotonation with the second molecule of the
NHO base to afford 3 and 4 seem more plausible (Scheme 3).
We also carried out the reaction of IPrÁSiCl2 (IPr = {(N(2,6-
iPr2C6H3)CH}2C:)) with 1. Formation of compound 3 with the
1
liberation of free IPr was readily observed from the H and 29Si
NMR spectral studies (Scheme 4). However, in this case, high
solubility of 3 and IPr in common organic solvents impedes their
separation. In general, silylenes insert into the O–H, Si–H, S–H,
Scheme 3 Synthesis of compounds 3 and 4.
Surprisingly, the chemistry of NHOs with silicon has not been C–Cl, and metal–hydrogen bonds.24 Insertion of a thermally
explored so far.
stable silylene into a C–H bond is rather rare.25 Irrespective of
Here, we report on the facile formation of silyl-functiona- the mechanism involved, reaction of IPrÁSiCl2 with 1 to afford 3 is
lized NHOs 3 and 4 (Scheme 3) by the reaction of NHOs 1 and 2 a clear silylene insertion into a C–H bond (Scheme 4). In this
with HSiCl3. Compound 3 can also be prepared by the reaction case, formation of an intermediate (III) and subsequent 1,2-
of NHC stabilized dichlorosilylene IPrÁSiCl2 with 1 (Scheme 4). hydrogen migration to give 3 are plausible. However, the role of
Compounds 3 and 4 have been prepared in almost quantitative IPr as a proton transfer agent via protonation–deprotonation
yield by the reaction of a NHO (1 or 2) with HSiCl3 (Scheme 3). The cannot be ruled out.
insoluble side products 5 and 6 can be readily isolated by filtration
Suitable single crystals of 3 for X-ray diffraction studies were
and can be recycled to form NHOs 1 and 2. Compounds 3 and 4 grown from a saturated benzene solution at room temperature
crystallize as colourless crystals and are soluble in common organic
solvents. Formation of compounds 3 and 4 can be readily observed
1
from their NMR spectra.22 The H NMR spectrum of each of the
Fig. 1 Calculated energy profiles at BP86/def2-TZVP for the silylene insertion
into a C–H bond. Values are the electronic energy (corrected with the ZPVE) given
Scheme 4 Proposed mechanism for the formation of 3.
in kcal molÀ1 22
.
c
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9442 Chem. Commun., 2013, 49, 9440--9442
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