Convenient access to monosilicon epoxides with pentacoordinate silicon

Article abstract of DOI:10.1002/anie.201000835

From intermediate to stable molecule: Stable monosilicon epoxides with pentacoordinate silicon were prepared for the first time by [2+1] cycloaddition reactions of Lewis base stabilized chlorosilylenes with ketones. Molecular structures of these compounds

Full text of DOI:10.1002/anie.201000835

Communications
DOI: 10.1002/anie.201000835
Silicon Chemistry
Convenient Access to Monosilicon Epoxides with Pentacoordinate
Silicon**
Rajendra S. Ghadwal, Sakya S. Sen, Herbert W. Roesky,* Markus Granitzka, Daniel Kratzert,
Sebastian Merkel, and Dietmar Stalke*
Dedicated to Professor S. S. Krishnamurthy on the occasion of his 70th birthday
“If carbonyl compounds have been said to be virtually the
backbone of organic synthesis, the epoxides correspond to at
least one of the main muscles”—D. Seebach
Epoxides play an important role in synthetic organic
chemistry.[1] They undergo nucleophilic ring-opening reac-
Scheme 1. Silaoxirane A and silacarbonyl ylide B.
tions with high stereo- and regioselectivity. Unequivocally,
analogous organosilicon species would be interesting reagents
for organic synthesis, although these compounds are not
easily accessible. Formation of silicon–carbon bonds by
reaction of silylenes with organic substrates has attracted
considerable interest in the synthesis of organosilicon com-
pounds.[2] Strained small ring compounds containing a silicon
atom are of importance due to their molecular structure and
high reactivity resulting from ring-strain energy.[3] Among
three-membered silicon ring compounds, silacyclopropanes
and silacyclopropenes have been prepared and studied
extensively.[4,5] They are much more reactive than their
carbon analogues. Likewise there has been considerable
theoretical interest[6] in the formation and stability of
silaoxiranes, which suggested that strain energy alone may
not be the only criterion for isolating these compounds.
Computational studies revealed that neutral silaoxiranes
should be strongly stabilized by adding an anion like
fluoride.[6a] Therefore, we were interested whether this
stabilization could be achieved by a neutral carbon- or
nitrogen-donor ligand to isolate uncharged silaoxiranes,
which are otherwise assumed to be very unstable compounds.
Furthermore, silaoxirane A and the silacarbonyl ylide B
(Scheme 1) are assumed to be intermediates[7,8] in the
reaction of transient or stable silylenes with carbonyl
compounds. The first evidence for the formation of transient
silaoxirane (siloxacyclopropane) was presented in 1977 by
Ando et al.[7a] in the reaction of thermally generated silylene
with benzophenone. In 1982 the same group succeeded in the
isolation of a heavily substituted stable silaoxirane,[9] con-
taining a four-coordinate silicon atom, by reaction of photo-
chemically generated silylene with a cyclic ketone. To the best
of our knowledge, this is the only structurally characterized
silaoxirane to date. Formation of cyclic silaoxanes (or
silylenol ethers) has been reported[8] by the reaction of
silylenes with carbonyl compounds. The involvement of either
silaoxirane (A) or silacarbonyl ylide (B) as intermediates was
proposed. Surprisingly, there is no report on the isolation of a
stable silaoxirane. Recently, we isolated N-heterocyclic car-
bene (NHC) stabilized dichlorosilylene L·SiCl2 (1a; L = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene) by dehydro-
chlorination of HSiCl3.[10] Accordingly,[6a] NHC-stabilized
silylene 1a should react with ketones to give silaoxiranes.
Herein, we report on the preparation of stable silaoxiranes 2
and 3 by reaction of 1a with ketones (Scheme 2). The
monosilicon epoxides 2 and 3 are the first examples contain-
ing a pentacoordinate silicon atom and are stabilized by the s-
[*] Dr. R. S. Ghadwal, S. S. Sen, Prof. Dr. H. W. Roesky, M. Granitzka,
D. Kratzert, S. Merkel, Prof. Dr. D. Stalke
Institut fꢀr Anorganische Chemie
Georg-August-Universitꢁt Gꢂttingen
Tammannstrasse 4, 37077 Gꢂttingen (Germany)
Fax: (+49)551-393-373
E-mail: hroesky@gwdg.de
dstalke@chemie.uni-goettingen.de
[**] Support of the Deutsche Forschungsgemeinschaft is highly
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000835.
Scheme 2. Synthesis of silaoxiranes 2–4 (1a=L·SiCl2; L=1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene and 1b=PhC(NtBu)2SiCl)
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Angewandte
Chemie
donating NHC ligand. Inspired by the successful isolation of
monosilicon epoxides 2 and 3, we extended our investigation
to N-donor base stabilized monochloro silylene[11] PhC-
(NtBu)2SiCl (1b) to afford silaoxirane 4 by reaction with
benzophenone (Scheme 2). Formation of 2–4 as stable com-
pounds indicates that silaoxiranes can be electronically
stabilized by neutral s-donor ligands. Moreover, 2,5-dioxa-
1-silacyclopent-1-enes were prepared by reaction of a stable
silylene[8a] as well as thermally or photochemically generated
silylenes[12] with diketones. For comparison we treated 1a with
1,2-diphenyl-1,2-ethanedione (benzil) and obtained 2,5-
dioxa-1-silacyclopent-1-ene 5 (Scheme 3), even when an
excess of 1a was used.
Figure 1. Molecular structure of 2; anisotropic displacement parame-
ters depicted at 50% probability. A toluene molecule is omitted for
ꢀ
clarity. Selected bond lengths [ꢃ] and angles [8]: Si1 O1 1.6520(10),
ꢀ
ꢀ
ꢀ
ꢀ
Si1 C2 1.8834(15), Si1 C1 1.9653(15), Si1 Cl1 2.0690(6), Si1 Cl2
2.1175(6); O1-Si1-C2 50.70(6), O1-Si1-C1 94.54(5), C28-Si1-C1
143.06(6), O1-Si1-Cl1 117.74(4), C28-Si1-Cl1 106.22(5), C1-Si1-Cl1
101.11(4), O1-Si1-Cl2 133.21(5), C28-Si1-Cl2 104.28(5), C1-Si1-Cl2
90.95(4), Cl1-Si1-Cl2 106.55(2).
Scheme 3. Synthesis of 5 (1a=L·SiCl2; L=1,3-bis(2,6-diisopropylphe-
nyl)imidazol-2-ylidene)
Compounds 2–5 form colorless crystals, are stable under
inert atmosphere, and are soluble in common organic
solvents. The molecular structures of 2–5 were established
1
by single-crystal X-ray diffractions studies.[13] The H NMR
spectra of 2, 3, and 5 show one set of resonances for the NHC
1
ligand coordinated to the silicon atom. The H NMR spec-
trum of 4 exhibits two resonances for the amidinate moiety. In
addition, 2–5 show 1H NMR signals for the phenyl group
attached to the OSiC or O2SiC2 ring. The 29Si NMR spectra of
2–5 exhibit sharp signals (d = ꢀ123.85, ꢀ123.39, ꢀ115.53, and
ꢀ99.50 ppm, respectively) consistent with pentacoordinate[14]
silicon.
The molecular structures of silaoxiranes 2 and 3 are shown
in Figures 1 and 2, respectively.[13] Compound 2 crystallizes in
the monoclinic space group P21/c with one toluene molecule
in the asymmetric unit, and 3 in the orthorhombic space group
Pbca with half a molecule of n-hexane in the asymmetric unit.
The silicon atom in each of 2 and 3 is pentacoordinate with a
distorted trigonal-bipyramidal geometry. The NHC ligands in
2 and 3 occupy an axial position with C28-Si1-C1 as largest
Figure 2. Molecular structure of 3; anisotropic displacement parame-
ters depicted at 50% probability. Half of an n-hexane molecule is
ꢀ
omitted for clarity. Selected bond lengths [ꢃ] and angles [8]: Si1 O1
ꢀ
ꢀ
ꢀ
1.6486(13), Si1 C28 1.8924(18), Si1 C1 1.9724(17), Si1 Cl1
!
bond angle. The Si C bond lengths in 2 (1.9653(15) ꢀ) and 3
(1.9724 (17) ꢀ) are comparable with that of 1a (1.985(4) ꢀ).
Both chlorine atoms in 2 and 3 occupy the equatorial
ꢀ
2.0759(6), Si1 Cl2 2.0942(7); O1-Si1-C28 50.31(6), O1-Si1-C1
92.40(7), C28-Si1-C1 142.35(7), O1-Si1-Cl1 123.28(5), C28-Si1-Cl1
105.16(6), C1-Si1-Cl1 100.04(5), O1-Si1-Cl2 127.56(5), C28-Si1-Cl2
105.93(6), C1-Si1-Cl2 92.59(5), Cl1-Si1-Cl2 107.03(2).
ꢀ
positions. The differences in Si Cl bond lengths for 2
(2.1175(6), 2.0690(6) ꢀ) and 3 (2.0942(7), 2.0759(6) ꢀ) may
ꢀ
be due to steric hindrance. The Si Cl distances in 2 and 3 are
noticeably different and shorter than the average bond length
bond lengths in 2 (1.8834(15) ꢀ) and 3 (1.8924(18) ꢀ) are
comparable. The three-membered strained rings in 2 and 3
each consist of non-isosceles triangles.
ꢀ
in 1a (2.1664(16) ꢀ). The Si O bonds in 2 (1.6520(10) ꢀ) and
3 (1.6486(13) ꢀ) are slightly shorter than that in the reported
silaoxirane,[9] presumably due to the presence of two electro-
The molecular structure of 4 is shown in Figure 3. It
negative chlorine atoms at the silicon atom. The Si1 C28
crystallizes in the monoclinic space group P21/c.[13] In the
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Communications
The molecular structure of 2,5-dioxa-1-silacyclopent-1-
ene 5 is shown in Figure 4. Compound 5 crystallizes in the
monoclinic space group P21/c with two molecules in the
asymmetric unit[13] and features a trigonal-bipyramidal geom-
etry at the pentacoordinate silicon atom. The chlorine atoms
in 5 occupy one axial (Cl2) and one equatorial position (Cl1),
and as expected their bond lengths differ significantly (axial
2.2034(8), equatorial 2.0814(8) ꢀ). The oxygen atoms of the
planar five-membered ring chelate axial and equatorial
positions. The NHC ligand occupies an equatorial position,
!
and the Si C distance (1.939(2) ꢀ) is comparable to those in
2 and 3.
In summary, we have reported for the first time a
convenient method for the preparation of stable silaoxiranes
by the reaction of silylenes with ketones. Silaoxiranes 2–4 are
stable compounds and were fully characterized. They are the
first of their type stabilized by neutral s-donor ligands, and
each contains a pentacoordinate silicon atom. They may serve
as potential precursors for the preparation of novel organo-
silicon compounds. Their formation proves that base stabili-
zation allows isolation of silaoxiranes which are otherwise
assumed to exist only as intermediates.
Figure 3. Molecular structure of 4; anisotropic displacement parame-
ters depicted at the 50% probability level. Two toluene molecules are
ꢀ
omitted for clarity. Selected bond lengths [ꢃ] and angles [8]: Si1 O1
ꢀ
ꢀ
1.6534(13), Si1 C16 1.8641(19), Si1 Cl1 2.0708(6), C16-O1-Si1
71.70(9), O1-Si1-N1 133.01(7), O1-Si1-N2 100.28(6), O1-Si1-C16
50.93(7), O1-Si1-Cl1 118.15(5).
spirocyclic structure the Si atom is part of a four- and a three-
membered ring. The Si atom exhibits a distorted square-
pyramidal geometry. The coordination sites of the Si atom are
occupied by the N atoms of the amidinato ligand and one
oxygen and one carbon atom from the epoxide ring. The fifth
Received: February 10, 2010
Published online: April 15, 2010
Keywords: carbene ligands · cycloaddition · heterocycles ·
ꢀ
coordination site is occupied by a chlorine atom. The Si O
.
silicon · small-ring systems
ꢀ
and Si C distances of 1.6534(13) and 1.8641(19) ꢀ, respec-
ꢀ
tively, are comparable to those in 2 and 3. The Si Cl distance
of 2.0708(6) ꢀ in 4 is shorter than that in 1b (2.156(1) ꢀ) and
in agreement with those of 2 and 3.
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CCDC 763281 (2), 763282 (3), 762176 (4) and 763283 (5) contain
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_
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[I > 2s(I)]: 2: P21/c, a = 11.6851(14), b = 22.919(3), c =
16.2246(19) ꢀ, b = 99.855(2)8, wR2 = 0.0846, R1 = 0.0345; 3:
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0.0828, R1 = 0.0352; 4: P21/c, a = 10.763(5), b = 15.487(7), c =
22.345(11) ꢀ, b = 94.3340(10)8, wR2 = 0.1028, R1 = 0.0370; 5:
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