C-N bond cleavage and ring expansion of N-heterocyclic carbenes using hydrosilanes

Article abstract of DOI:10.1002/anie.201204333

Ring expansion of NHCs! The reaction of N-heterocyclic carbenes (NHCs) with hydrosilanes Ph_4-nSiH_n (n=1, 2, 3) results in complete rupture of the heterocycle and silylene insertion into one of the C-N bonds of the carbene (see scheme

Full text of DOI:10.1002/anie.201204333

Angewandte
Chemie
DOI: 10.1002/anie.201204333
N-Heterocyclic Carbenes
À
C N Bond Cleavage and Ring Expansion of N-Heterocyclic Carbenes
using Hydrosilanes**
David Schmidt, Johannes H. J. Berthel, Sabrina Pietsch, and Udo Radius*
Dedicated to Dieter Fenske on the occasion of his 70th birthday
Since the isolation of stable N-heterocyclic carbenes (NHCs)
in 1991,[1] the chemistry of NHCs and related molecules[2] has
grown rapidly, and numerous applications of NHCs in main-
group[3] and transition-metal chemistry,[4] as well as catalysis,[5]
have been reported. In particular, NHCs are extensively used
as ligands for transition-metal complexes because of their
strong two-electron donor character.[6] Furthermore, NHCs
have found wide application in main-group chemistry
research in recent years. Among others, there are three
important fields in main-group chemistry where NHCs and
related molecules had a significant impact over the last few
years: 1) the stabilization of low-coordinate main-group
element compounds;[7,8] 2) element–element bond activa-
tion;[9] and 3) the application of NHCs in FLPs (molecules
with “frustrated” Lewis pairs).[10] The strong s-donation of
NHCs has been applied to stabilize low-coordinate main-
group element compounds, as exemplified by the isolation of
side products at higher reaction temperatures that originate
from the dissociation of the NHC ligand and the reaction of
the NHC with the hydrosilane employed. We report herein
À
the C N bond cleavage of NHC imidazoline cores accom-
panied by reductive ring opening and silylene ring expansion
of the NHCs, which should be of particular relevance for
NHC-based catalyst degradation as well as for main-group
NHC chemistry. During the final stage of our work, Hill
et al.[19] reported the closely related insertion of a BeH2 unit
À
into the C N bond of Dipp2Im under mild conditions.
Based on our observations from nickel chemistry, and
recent reports that some carbenes may undergo insertion into
the Si H bond of several silanes,[9] we were interested in the
À
reactivity of selected NHCs towards phenyl-substituted
silanes of the type Ph4ÀnSiHn (n = 1, 2, 3). Treatment of the
alkyl-substituted NHCs tBu2Im, iPr2Im, and nPr2Im with one
equivalent of phenylsilane PhSiH3 in toluene at 1008C for
three days afforded in all cases pale yellow residues, which
were obtained in low yields after work-up. These compounds
were identified as derivatives of 3,4-dehydro-2,5-diazasili-
nanes 1 (R = tBu 1a, iPr 1b, nPr 1c; Scheme 1). Formally, two
=
NHC-supported :Si Si: compounds with formally zero-valent
silicon by Robinson et al.[7a] NHC-stabilized dihalosilylenes
=
(SiX2) as well as a NHC-stabilized silanones with a Si O
double bond have also been reported.[8] Similarly interesting
is the activation of small molecules such as dihydrogen,
ammonia, and silanes, which was achieved using the closely
related cyclic alkyl amino carbenes (CAACs)[9] or “frus-
trated” NHC–borane Lewis pairs.[11]
In all these cases, the NHCs seem to provide a robust
environment. Within the coordination sphere of transition
metals, however, constructive and destructive decomposition
pathways of NHC ligands are well-established, which mainly
[12]
[13]
[14]
involve C H, C N, C C, and N H[15] bond activation
of the nitrogen alkyl, aryl, or hydrogen substituents. During
our studies on the reactivity of [Ni2(iPr2Im)4(cod)][16]
À
À
À
À
Scheme 1. Reaction of different 1,3-dialkylimidazolin-2-ylidenes with
PhSiH3 and Ph2SiH2.
(iPr2Im = 1,3-diisopropylimidazolin-2-ylidene,
cyclooctadiene) with hydrosilanes HnSiR4Àn
cod = 1,5-
and on the
[17]
catalytic hydrodefluorination of polyfluorinated aromatics
using hydrosilanes,[18] we became aware of the formation of
hydrogen atoms of the phenylsilane have been transferred to
the NHC carbene carbon atom and the remaining silylene
À
fragment has been inserted into one of the NHC C N bonds
with the formation of a six-membered heterocycle.
[*] D. Schmidt, J. H. J. Berthel, S. Pietsch, Prof. Dr. U. Radius
Institut fꢀr Anorganische Chemie
The reaction products can be easily distinguished spec-
troscopically from the starting material. Most notably, in the
proton NMR spectra of the compounds 1a–c, two sets of
multiplets can be observed in the range between 2.29 ppm and
2.50 ppm for the diastereotopic NCH2Si protons. The remain-
ing SiH protons of 1a–c give rise to signals in the region
between 5.25 and 5.46 ppm, with JSiH coupling constants of
205.9 (1a), 205.5 (1b), and 206.9 (1c) Hz, respectively. The
characteristic NHC carbene signals at approximately 212 ppm
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: u.radius@uni-wuerzburg.de
Homepage: http://www.ak-radius.de
[**] This work was supported by the Julius-Maximilians-Universitꢁt
Wꢀrzburg, and the Deutsche Forschungsgemeinschaft (DFG).
1
Supporting information for this article (additional figures and
experimental details) is available on the WWW under http://dx.doi.
org/10.1002/anie.201204333.
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in the 13C{1H} NMR spectra of the starting compounds are
absent and these carbon atoms give rise to significantly high-
field shifted signals at 32.4 (1a), 31.7 (1b), and 36.7 ppm (1c)
for the NCH2Si carbon atoms of the product. In the 29Si NMR
spectra, signals were observed at À25.4 (1a), À24.9 (1b), and
NHC C N bond. The bond lengths Si C1 (1.872(2) ꢀ) and
À
À
À
À
Si N2 (1.718(2) ꢀ) lie in the range typically observed for Si
[20]
À
À
C or Si N single bonds in six-membered rings. The C C
À
double bond of the NHC backbone is still intact (C2 C3
1.337(3) ꢀ), and the torsion angle N1-C2-C3-N2 of 1.995(40)8
confirms the coplanar alignment at the C2 C3 double bond.
The carbon atom C1 lies 0.576 ꢀ above, and the silicon atom
Si 0.121 ꢀ below, the plane defined by the atoms N1-C2-C3-
N2, leading to a twisted six-membered ring.
As no reaction between diphenylsilane and the tert-butyl-
substituted NHC tBu2Im was observed, the influence of the
NHC alkyl substituents and the number of phenyl groups of
the silane used for the reaction was further analyzed. Treat-
ment of asymmetrically substituted 1-isopropyl-3-methylimi-
dazolin-2-ylidene (iPrMeIm) with mono- and diphenylsilane
led to two different insertion products (denoted as A and B in
Scheme 2). The use of PhSiH3 leads to the isomers 3a and 3b
À
À
À21.6 ppm (1c), and Si H stretch vibrations were detected in
the IR spectra of the compounds at 2129 (1a), 2115 (1b), and
2115 (1c) cmÀ1. Furthermore, the composition of all of the
compounds was confirmed by mass spectrometry (EI-MS)
and elemental analysis.
To demonstrate the generality of this reaction, the same
set of NHCs (tBu2Im, iPr2Im, and nPr2Im) was reacted with
diphenylsilane under identical conditions (3 d at 1008C). For
iPr2Im and nPr2Im, the corresponding insertion products 2b
and 2c were isolated in moderate yields (48 and 61%). In the
case of tBu2Im, no conversion was observed, which presum-
ably arises from the steric hindrance of the bulky tert-butyl
substituents and the phenyl groups of the silane in the course
of the reaction. However, for the sterically much less-
hindered NHC 1,3-dimethyl-imidazolin-2-ylidene (Me2Im)
the corresponding methyl-substituted 3,4-dehydro-2,5-diaza-
silinane 2d was formed (Scheme 1). In the proton NMR
spectra, the NCH2Si signals for the products appear as singlets
at 2.59 (2b), 2.89 (2b), and 2.56 ppm (2d), and no signals for
1
in a ratio of 0.88:1.00 according to H NMR spectroscopy,
affording the sterically unfavorable 3b as the major product.
The reaction of iPrMeIm with Ph2SiH2 gave a 1.00:0.53
mixture of 4a and 4b in favor of the sterically less-crowded
4a.
À
Si H could be found. Similarly to 1a-c, the carbon nuclei
signals of NCH2Si were detected at 33.0 (2b), 51.4 (2c), and
41.5 ppm (2d) in the 13C{1H} NMR spectra, and the silicon
atoms gave rise to signals at À20.8 (2b), À20.4 (2b), and
À19.4 ppm (2b) in the 29Si NMR spectra.
To unequivocally determine the connectivity of the
reaction products obtained, single crystals of 2b were grown
by cooling a boiling hexane solution of the compound slowly
to room temperature. The result of the X-ray analysis as well
as selected bond lengths and bond angles are given in
Figure 1.
The molecular structure of 2b confirms that the ring
expansion is the result of a formal twofold hydrogen atom
transfer from diphenylsilane to the carbene carbon atom with
insertion of the remaining formal silylene fragment into the
Scheme 2. Reaction of 1-isopropyl-3-methylimidazolin-2-ylidene with
PhSiH3 and Ph2SiH2.
NMR spectroscopy experiments were performed to
elucidate mechanistic details of the reaction of iPr2Im and
Ph2SiH2. A vacuum-sealed NMR tube containing equimolar
amounts of iPr2Im and Ph2SiH2 in C6D6 was slowly heated in
steps of five degrees per 30 min until the reaction initiated at
a temperature of approximately 758C. Furthermore, the
reaction was monitored at 908C in [D8]toluene (Supporting
Information, Figure S1). In these spectra, only signals for
iPr2Im, Ph2SiH2, and the reaction product 2b were observed.
This finding basically confirms the quantitative formation of
2b from the starting material on an NMR scale, but gave no
information on possible intermediates of the reaction. How-
ever, concerning the mechanism, these experiments suggest
that the initial step of the reaction has a high activation
barrier. In another experiment, we reacted iPr2Im with
deuterated Ph2SiD2 and obtained [D2]-2b (Scheme 3), exclu-
sively deuterated at the NHC carbene carbon atom (C1 in
Figure 1). The reaction of iPr2Im with a 1:1 ratio of Ph2SiH2
and Ph2SiD2 afforded a mixture of 2b and [D2]-2b, and no H/
D cross products were observed.
Figure 1. ORTEP diagram of the molecular structure of 2b[21] in the
solid state (ellipsoids set at 50% probability). Hydrogen atoms have
been omitted for clarity, with the exception of those located at C1 (the
NHC carbene carbon atom of the starting compound). Selected bond
lengths [ꢂ], angles [8], and dihedral angles [8]: Si–C1 1.872(2), C1–N1
1.463(3), N1–C2 1.398(3), C2–C3 1.337(3), C3–N2 1.410(3), N2–Si
1.718(2); C1-Si-N2 101.83(9), N1-C1-Si 110.72(15), N1-C1-Si-N2
46.915(17), N1-C2-C3-N2 1.995(40), C3-C2-N1-C1 27.352(34), C2-C3-
N2-Si 6.013(31).
Bertrand and co-workers[9] previously reported that some
CAACs and the saturated NHC bis(2,6-diisopropylphenyl)-
2
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Scheme 3. Reaction of 1,3-diisopropylimidazolin-2-ylidene with
Ph2SiD2.
imidazolidin-2-ylidene (Dipp2ImH2) react with hydrosilanes
À
with the formation of the corresponding Si H bond-activa-
À
tion products. As it seemed likely that Si H activation
Figure 2. ORTEP diagram of the molecular structure of 5b[21] in the
products are decisive intermediates for the reaction observed
here, we were led to react the NHCs with triphenylsilane in
the hope of avoiding the transfer of two hydrogen atoms to
solid state (ellipsoids set at 50% probability). Hydrogen atoms have
been omitted for clarity, with the exception of that located at C1 (the
former NHC carbene carbon atom). Selected bond lengths [ꢂ], angles
[8], and dihedral angles [8]: Si–C1 1.912(2), C1–N1 1.474(2), N1–C2
1.388(2), C2–C3 1.337(2), C3–N2 1.421(2), N2–Si 1.717(2); C1-Si-N2
101.95(7), N1-C1-Si 107.90(11), N1-C1-Si-N2 48.785(12), N1-C2-C3-N2
5.865(30), C3-C2-N1-C1 6.760(27), C2-C3-N2-Si 17.825(33).
À
the carbene carbon atom, and thus observing a product of Si
H addition to the NHC. Surprisingly, in all cases, analogues of
1 and 2 were isolated, which were presumably formed by
transfer of one silane hydrogen atom and one silane phenyl
substituent to the NHC carbene carbon atom with insertion of
À
the remaining {SiPh2} fragment into one of the C N bonds of
the NHCs (Scheme 4). By using the unsymmetrically sub-
À
stituted NHC iPrMeIm, insertion into the C N(Me) bond was
observed exclusively with formation of isomer 5a.
Scheme 5. Reaction of aryl-substituted NHCs with PhSiH3. Dipp=2,6-
iPr2C6H3.
7 was observed after 5 h in toluene at 1008C. As already
reported by Bertrand et al., Dipp2ImH2 cleanly reacts with
Scheme 4. Reaction of various 1,3-dialkylimidazolin-2-ylidenes with
Ph3SiH.
À
PhSiH3 at room temperature in hexane to give the Si H
activation product 9, which can be isolated in pure form. After
heating compound 9 for 5 h at 1008C in toluene, the clean
conversion to 7 was also observed (Scheme 5), which confirms
the hypothesis that the reported ring expansion proceeds
À
À
Compared to compounds 1 and 2, the N CH(C6H5) Si
protons are shifted downfield in the proton NMR spectra of
compounds 5 (5b 4.27, 5c 4.23, 5d 4.03 ppm) owing to the
electron-withdrawing effect of the phenyl substituent. In the
À
initially by insertion of the NHC into the Si H bond, followed
13
1
À
À
À
C{ H} NMR spectra, the N CH(C6H5) Si signals were
by a rearrangement of the Si H bond activation product into
observed at 53.1 (5b), 53.5 (5c), and 56.4 ppm (5d). The
silicon nuclei of these compounds give rise to signals at À23.8
(5a), À22.7 (5b), and À22.2 ppm (5c) in the 29Si NMR
spectrum. X-ray analyses were performed on single crystals of
5b (Figure 2) and 5c (Supporting Information, Figure S3) to
confirm phenyl group transfer from silicon to the NHC
carbon atom. The substitution of C1 with a phenyl group has
no significant influence on the diazasilinane core. The kinking
within the ring is more pronounced (deviation of Si and C1
from the N1-C2-C3-N2 plane 0.580 ꢀ and 0.249 ꢀ, respec-
the corresponding derivatives of the diazasilinane. However,
no intermediates were detected using NMR techniques for
the transformation of 9 to 7.
For the formation of the diazasilinanes, we propose the
pathway shown in Scheme 6 for the reaction of 1,3-dimethyl-
imidazolin-2-ylidene (Me2Im, I) with diphenylsilane (II) to
give 1,1-diphenyl-2,5-dimethyl-3,4-dehydro-2,5-diazasilinane
(2d). The first step of the reaction involves activation of one
À
of the Si H bonds of diphenylsilane at the NHC, that is, the
insertion of the NHC carbene carbon atom of Me2Im into the
À
À
tively) and the Si C1 distance is slightly elongated (5b
Si H bond of the silane to give intermediate A, followed by
1.912(2) ꢀ, 2b 1.872(2) ꢀ).
amide transfer to the silicon atom to yield intermediate B. The
final step of the reaction sequence involves hydrogen atom
transfer from the silicon atom to the carbon atom to give 2d.
In summary, the direct insertion of silylene moieties into
À
In the search for isolable intermediates from Si H bond
activation, the reactions of the diisopropylphenyl-substituted
imidazolin-2-ylidene Dipp2Im and imidazolidin-2-ylidene
(Dipp2ImH2; saturated at the NHC backbone) with silanes
were also investigated (Scheme 5). Dipp2Im does not react
with PhSiH3 in boiling toluene. However, a reaction leading
to diazasilinane 6 was observed in neat phenylsilane at higher
temperatures, which was complete after 5 h at 1608C. In the
case of Dipp2ImH2, complete conversion to the diazasilinane
À
the C N bond of N-heterocyclic carbenes with ring expansion
and formation of diazasilinanes has been achieved. These
reactions are feasible using primary, secondary, and tertiary
silanes Ph4ÀnSiHn and a variety of NHCs; that is, saturated and
unsaturated NHCs with N-alkyl and N-aryl substituents. This
ring expansion should be of general interest for the different
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Scheme 6. Proposed reaction path for the formation of 2d from
Me2Im (I) and diphenylsilane (II).
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Received: June 4, 2012
Published online: && &&, &&&&
À
Keywords: C N bond activation · N-heterocyclic carbenes ·
silanes
.
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[21] CCDC 884932 (2b), 884933 (5b), 884934 (5c), and 884935 (7)
contain the supplementary crystallographic data for this paper.
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www.angewandte.org
5
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.
Angewandte
Communications
Communications
N-Heterocyclic Carbenes
Ring expansion of NHCs! The reaction of
N-heterocyclic carbenes (NHCs) with
hydrosilanes Ph4ÀnSiHn (n=1, 2, 3)
results in complete rupture of the het-
erocycle and silylene insertion into one of
D. Schmidt, J. H. J. Berthel, S. Pietsch,
U. Radius*
&&&& — &&&&
À
À
C N Bond Cleavage and Ring Expansion
of N-Heterocyclic Carbenes using
Hydrosilanes
the C N bonds of the carbene (see
scheme; R=alkyl, aryl).
6
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!