An Isolable Silicon Analogue of a Ketone that Contains an Unperturbed Si=O Double Bond

Article abstract of DOI:10.1002/anie.201905198

Despite tremendous efforts to synthesize isolable compounds with an Si=O bond, silicon analogues of ketones that contain an unperturbed Si=O bond have remained elusive for more than 100 years. Herein, we report the synthesis of an isolable silicon analogu

Full text of DOI:10.1002/anie.201905198

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Accepted Article
Title: An Isolable Silicon Analogue of a Ketone that Contains an
Unperturbed Si=O Double Bond
Authors: Ryo Kobayashi, Shintaro Ishida, and Takeaki Iwamoto
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201905198
Angew. Chem. 10.1002/ange.201905198
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Angewandte Chemie International Edition
10.1002/anie.201905198
COMMUNICATION
An Isolable Silicon Analogue of a Ketone that Contains an
Unperturbed Si=O Double Bond
[
a]
[a]
[a]
Ryo Kobayashi , Shintaro Ishida , and Takeaki Iwamoto*
Abstract: Despite tremendous efforts to synthesize isolable
compounds with an Si=O bond, silicon analogues of ketones that
contain an unperturbed Si=O bond have remained elusive for more
than 150 years. Herein, we report the synthesis of an isolable silicon
analogue of a ketone that exhibits a three-coordinate silicon center
and an unperturbed Si=O bond, thus representing the first example of
a genuine silanone. Most importantly, this silanone does not require
coordination of Lewis bases and acids and/or the introduction of
electron-donating groups to stabilize the Si=O bond. The structure
and properties of this unperturbed Si=O bond were examined by a
single-crystal X-ray diffraction analysis, nuclear magnetic resonance
spectroscopy, and theoretical calculations. Bimolecular reactions
revealed high electrophilicity on the Si atom and high nucleophilicity
on the O atom of this genuine Si=O bond.
coordinated only by a donor on the Si atom (IB) to stabilize the
polar Si=O bond, have been reported.[
17-38]
Recently, the
synthesis of isolable Si=O species containing a three-coordinate
unsaturated silicon atom such as silaacylium complex IIA and
silaamide IIB, has been reported by the introduction of a
transition-metal fragment and an N-heterocyclic carbene or by
electron-donating groups (e.g. amido, imino, and ylide) on the
Si=O bond (Figure 1b).[
39-42]
However, to elucidate the intrinsic nature of an unperturbed
Si=O bond, isolable compounds containing an Si=O bond without
the aforementioned electronic stabilization such as a silicon
analogue of ketone (silanone) with an unperturbed Si=O moiety
(Figure 1c) are required, and the thermodynamic stabilization
approach used for I and II cannot be applied. The synthetic
difficulties associated with the isolation of such compounds is not
least reflected in the fact that these have remained elusive for
more than 150 years. Accordingly, an alternative approach is
required to synthesize isolable silanones with an unperturbed
Si=O moiety.
Ever since the pioneering synthetic studies on Si=O species
by Friedel and Crafts in the mid-19th century,[1] as well as those
by Kipping[2] and Dilthey in the early 20th century,[3] silicon
analogues of carbonyls with a Si=O double bond have attracted
substantial attention as heavier-group-14-element analogues of
carbonyl compounds. Theoretical calculations have predicted a
(
a) Donor-/acceptor-coordinated Si=O species
ex.
N
δ
−
donor
Dip
Dip
δ+
substantial polarization of the Si=O bond (Si –O ) due to the
large difference in electronegativity between Si and O [Si: 1.7; O:
R
N
N
N
H
O
Si
O
Si
Si O
R
N
N
3
(
.5; Allred–Rochow scale] and a remarkably weak π(Si–O) bond
58.5 kJ/mol) compared to the σ(Si–O) bond (119.7 kJ/mol).[4]
(acceptor)
6 5 3
B(C F )
donor: Lewis base
Dip
Dip
acceptor: Lewis acid
IA
IB
Indeed, compounds with a Si=O double bond are intrinsically
susceptible to auto-oligomerization, and compounds with an Si=O
double bond have so far only been postulated as fleeting
intermediates during the formation of the corresponding
oligomers (polysiloxanes, silicones), and been detected as short-
lived species in low-temperature matrices.[
(
b) Three-coordinate Si=O species with electron-donating substituents
ex.
+
Dip
N
Si
X
O
O
Si
O
OC
Cr
OC CO
Si
Dip
P
N
t-Bu
5-15]
N
Si
N
NR'
t-Bu
IIB
2
X'
N
To elicit the intrinsic features of the Si=O bond, a substantial
number of studies have been devoted to the synthesis of isolable
compounds containing an Si=O bond. But despite tremendous
efforts, the electronic stabilization of the Si atom by additional
electron donation, i.e., via coordination of Lewis bases to the Si
atom, or via introduction of strong electron-donating groups at the
Si atom, are still indispensable to isolate such Si=O species.
Following the seminal study on the synthesis of an isolable silicon
analogue of formamide IA (Figure 1a) by Driess et al. in 2007, [
several isolable Si=O species that are either coordinated by an
electron donor on the Si atom and an acceptor on the O atom, or
Dip
IIA
X, X' = metal,
donor
(
e.g. NR , ylide) Dip: 2,6-diisopropylphenyl R' = i-Pr or cyclohexyl
2
(c) Si analogue of a ketone with an unperturbed Si=O moiety
MeO
t-But-Bu OMe
t-Bu
cf.
t-Bu
R
3
Si SiR
3
R
R
Si
O
Si
O
Si O
16]
R
3
Si SiR3
R = alkyl and/or aryl
t-Bu
t-Bu
IIIA: SiR3 = SiMe i-Pr
2
MeO t-But-Bu OMe
(This work)
IIIB: SiR
= SiMe
3 3
1
[a]
R. Kobayashi, Prof. Dr. S. Ishida, Prof. Dr. T. Iwamoto
Department of Chemistry,
Figure 1. Thermodynamically stabilized (a) donor- and/or acceptor-
coordinated Si=O species and (b) Si=O species with a three-coordinate silicon
atom containing electron-donating N-heterocyclic carbene, transition-metal,
and heteroatom substituents. Kinetically stabilized (c) silicon analogues of
ketones containing an unperturbed three-coordinate Si=O moiety.
Graduate School of Science, Tohoku University,
Aoba-ku, Sendai 980-8578, Japan
E-mail: takeaki.iwamoto@tohoku.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
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COMMUNICATION
Herein, we report the synthesis and isolation of such a
kinetically stabilized silicon analogue of a ketone with the
unperturbed Si=O moiety (1; Figure 1c), which does not contain
any coordinating Lewis bases and/or acids on the Si=O moiety,
thus representing the first example of the isolable genuine
silanone. Bimolecular reactions of 1 confirm the intrinsic high
electrophilicity on the Si atom and the high nucleophilicity on the
O atom as the unperturbed Si=O moiety.
The molecular structure of 1 in the solid state was
unequivocally determined by a single-crystal XRD analysis
(Figure 2). Colorless platelets of 1 suitable for XRD analysis were
obtained from a recrystallization from benzene. The asymmetric
6 6
unit contains one molecule of 1 and 3.5 molecules of C H . The
distances between the silanone Si and O atoms of the closest
neighboring molecules [10.336 (2) Å] (Figure 2b) is much larger
than the sum of the van der Waals radii of the Si and O atoms
[Si+O: 3.40 Å].[43] The atoms intermolecularly closest to the Si1
and O1 atoms are carbon atoms in the t-Bu groups of a
neighboring molecule, with distances [Si1ꞏꞏꞏC: 4.494 (3) Å;
O1ꞏꞏꞏC: 3.472 (4) Å] that are significantly longer than the sum of
the van der Waals radii [Si+C: 3.85 Å; O+C: 3.25 Å].[43] These
results suggest that in the crystal, significant intermolecular
interactions are absent between the Si=O moiety of 1 and its
We have previously generated IIIA (Figure 1c), which
persists for several hours at –80 °C and was characterized by
multinuclear NMR and IR spectroscopy.[15] Although the large
protecting groups substantially increase the thermal stability of
IIIA compared to that of other silanones that have only been
observed in low-temperature argon matrices (e.g. IIIB),[ IIIA
14]
2
readily undergoes isomerization via 1,3-migration of the i-PrMe Si
group from the carbon to the oxygen atom at room temperature.
To further improve the thermal stability of silanones, we designed
a more robust substituent that contains bulky carbon-based
protecting groups with a diminished migratory propensity relative
6 6
nearest neighbor or a molecule of C H . Accordingly, it is feasible
to consider 1 as an isolated Si=O species. The angle sum around
the three-coordinate silicon atom [359.99(14)°] indicates that the
Si atom adopts a virtually ideal trigonal-planar structure. The Si–
O distance [1.518 (2) Å] is much shorter than that in Si–O single
bonds (1.63 Å)[44] and represents the shortest hitherto reported
Si–O bond for crystalline Si=O species with a three-coordinate
silicon atom (1.526–1.543 Å).[39-42] It is also very close to the Si–
to that of the i-PrMe
2
Si group. This feature should prevent oligo-
/
polymerization and isomerization, and thus preserve the intrinsic
electronic structure of the Si=O bond.
The synthesis of 1 was achieved by oxidation of cyclic
dialkylsilylene 2 (Scheme 1; for synthetic details, see the
O bond length in H
2
Si=O (1.515 Å), which has been estimated
based on rotational spectroscopy.[
10]
Supporting Information). Treatment of solid 2 with gaseous N
2
O
at room temperature provided 1 in 100% yield as a white solid.
The generation of 1 was confirmed by multinuclear NMR
spectroscopy, high-resolution mass spectrometry (HRMS), and
elemental analysis (EA). The 29Si NMR resonance of the three-
6 6
coordinate silicon nucleus in C D appears in the low-field region
(
90.0 ppm), similar to that of metastable silanone IIIA (128.7
[
15]
ppm), and substantially downfield-shifted compared to those of
the donor- and/or acceptor-coordinated neutral Si=O species (I; –
[
16-38]
1
4 to –91.5 ppm)
electron donation (II; 28.8 to 71.3 ppm).
was recorded in C to gain further insight into the structure of
and a neutral Si=O species stabilized by
[
40-42]
The IR spectrum of
1
6 6
H
the Si=O moiety; however, the band potentially assignable to the
Si–O vibration was difficult to discern. Using DFT calculations, the
calculated Si–O vibration frequency for the optimized structure of
–1
1
(1opt; ~1168 cm ) would overlap with the strong bands of the
aryl groups (Figure S2). The UV-vis absorption spectrum of 1 in
exhibits strong π→π* transitions from the aryl moieties
~300 nm), thus preventing the observation of the n(O)→π*(Si=O)
6 6
C H
(
transition band, which was predicted by DFT calculations to
appear at ~260 nm (Figure S3 and Table S6).
Scheme 1. Synthesis of 1
Figure 2. Molecular structure of 1 in the solid state. (a) Molecular structure of 1
with thermal ellipsoids at 50% probability. (b) Closest intermolecular contacts
between the three-coordinate silicon atom and the terminal oxygen atom in the
crystal. Hydrogen atoms have been omitted for clarity. Selected bond lengths
(
Å) and angles (°): O1–Si1, 1.518(2); Si1–C1, 1.887(3); Si1–C4, 1.892(3); C1–
C2, 1.548(4); C2–C3, 1.536(4); C3–C4, 1.562(4); O1–Si1–C1, 129.55(14); O1–
Si1–C4, 129.95(14); C1–Si1–C4, 100.50(14); C2–C1–Si1, 99.46(19); C3–C2–
C1, 110.3(2); C2–C3–C4, 109.8(2); C3–C4–Si1, 98.99(19).
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COMMUNICATION
Scheme 2. Bimolecular reactions of 1 with small molecules in benzene
To obtain further insight into the nature of the Si=O bond in
1
1
, DFT calculations were carried out on the optimized structure of
(1opt) at the B3LYP-D3/6-31G(d) level of theory. In 1opt, the Si=O
Ar
Ar
Ar
Ar
bond length (1.537 Å) and the angle sum around the three-
coordinate silicon atom (359.9°) are in good agreement with those
Ar Ar
O
Si Si
O
29
observed in the solid state [1.518(2) Å; 359.99(14)°]. The Si
NMR chemical shift of 1 (90.0 ppm) was also reproduced well by
gauge independent atomic orbital (GIAO) calculations on 1opt at
the B3LYP/6-311+G(2df,p) level of theory (79.9 ppm). The
HOMO-13, LUMO, and HOMO-8 orbitals correspond to the π-
and π*-orbitals of the Si=O bond, as well as the non-bonding
orbital of the terminal oxygen atom, respectively (Figure S8). The
natural population analysis of the charges at the silicon (2.08 e)
and oxygen (−1.10 e) atoms of 1opt confirm a substantially
polarized Si –O bond, similar to those previously predicted for
other Si=O species. The results of these DFT calculations thus
support the presence of a polar Si=O double bond in 1.
The electrophilic and nucleophilic nature of 1 is reflected in
the following bimolecular reactions (Scheme 2). While 1 is stable
Ar Ar
3
59%
Ar
Ar
Ar
Ar
Si
Ar
Si
H
OH
OH
OH
1
) LiAlH4
heat
2) hydrolysis
Ar
in C H
Ar
6
6
2
H O
Ar
5
4
1%*
5
62%*
1
EtMe SiH
B(C F )
6 5 3
2
Ar
O
Ar
Ar
Si
Ar
Si
δ+
δ−
O
H
6 5 2
B(C F )
OSiEtMe2
Ar
C6F
5
Ar
Ar
Ar
Ar
Ar
6
Ar
Si
Ar
O
6
1%*
Si
9
OH
69%*
O
Ar
Ar
6
Ar
in the solid state and in C
at 60 °C to furnish the head-to-tail dimer (3) over the course
of one day. Similar to other stable Si=O species, 1 reacts with H O,
ethyldimethylsilane, LiAlH and acetone to provide the
6
H
6
at 25 °C, it gradually dimerizes in
Ar
6
t-Bu
OMe
t-Bu
*: Yield based on NMR
7
C
6
H
6
8
Ar =
3%*
2%*
2
4
,
corresponding hydrolysis, hydrosilylation, reduction, and ene-type
reaction products (4–7).[39-41] Noticeably, 1 also reacts with
styrene, which contains a C=C double bond of relatively low
polarity, to regioselectively furnish the formal [2+2] adduct 8. It
should be noted here, that such reactions with isolable Si=O
species have not yet been investigated so far. The regioselectivity
of this reaction suggests that the electrophilic addition of the Si
atom to the terminal carbon atom of styrene leads to an
intermediate that contains a benzyl cation or to a transition state
with a similar structure. More strikingly, the reaction of 1 with
In conclusion, we have synthesized the first example of an
isolable silicon analogue of a ketone that contains an unperturbed
Si=O double bond and that is thermally stable at room
temperature. A single-crystal XRD analysis, DFT calculations,
and relevant bimolecular reactions of 1 with small molecules have
provided insight into the molecular and electronic structure as well
as the properties of the unperturbed Si=O moiety in that silanone,
which should open new avenues in the chemistry of silicon
analogues of carbonyl species. Moreover, 1 represents the
missing member in the family of isolable compounds containing
an unperturbed group-14-element-group-16-element double
6 5 3
B(C F ) affords aryl(boryloxy)silane 9, which is different from the
electronically stabilized Si=O species I and II, as these furnish the
corresponding borane complex of the Si=O species
).[16,22,24,25,41] The initial step for the formation
bond (such as Si=S and Ge=O species).
[45,46]
(R
2
Si=O→B(C
6
F
)
5 3
of 9 is most likely the formation of a R
2 6 5 3
Si=O→B(C F ) species
similar to those previously reported. The strong electron-deficient
character of Si in the initial complex obtained from 1 may allow
further 1,3-migration of a C F substituent from the B to the Si
6 5
Acknowledgements
atom. These results clearly indicate that the electron donation of
the carbon-based substituents in 1 is substantially lower than that
in I and II. Accordingly, the unperturbed Si=O moiety in 1 exhibits
both an intrinsically high electrophilicity on the Si atom and
nucleophilicity on the O atom of the genuine Si=O bond.
This work was supported by JSPS KAKENHI grant JP17H03015
(T.I.) and the Division for Interdisciplinary Advanced Research
and Education (DIARE) Tohoku University (R.K.). We thank Profs.
Hiromi Tobita and Hisako Hashimoto for the IR measurements,
Dr. Kenji Yoza (Bruker Japan) for helpful discussions during the
XRD analysis, and Prof. Eunsang Kwon (Research and Analytical
Center for Giant Molecules, Graduate School of Science, Tohoku
University) for helpful support during the XRD measurements.
Keywords: carbonyl compounds • ketone • silanone • Si=O
bond • silicon
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Angewandte Chemie International Edition
10.1002/anie.201905198
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
The first example of an isolable
genuine silanone, i.e., an isolable
silicon analogue of a ketone that
contains an unperturbed Si=O bond,
was synthesized. The structure and
properties of this silanone were
examined by a single-crystal XRD
analysis, NMR spectroscopy, and
theoretical calculations. Bimolecular
reactions revealed high electrophilicity
on the Si atom and high nucleophilicity
on the O atom of the genuine Si=O
bond.
Ryo Kobayashi, Shintaro Ishida, and
Takeaki Iwamoto*
Page No. – Page No.
An Isolable Silicon Analogue of a
Ketone that Contains an Unperturbed
Si=O Double Bond
This article is protected by copyright. All rights reserved.