An Isolable Potassium Salt of a Borasilene–Chloride Adduct

Article abstract of DOI:10.1002/anie.201612545

Among the variety of isolable compounds with multiple bonds involving silicon, examples of compounds that contain silicon–boron double bonds (borasilenes) still remain relatively rare. Herein, we report the synthesis of the potassium salt of a chloride adduct of borasilene 1 ([2]^?), which was obtained as an orange crystalline solid. Single-crystal X-ray diffraction analysis and reactivity studies on [2]^? confirmed the double-bond character of the Si=B bond as well as the reduced Lewis acidity, which is due to the coordination of Cl^? to the boron center. A thermal reaction of [2]^? afforded a bicyclic product by formal intramolecular C?H insertion across the Si=B bond of 1, which was corroborated by a theoretical study.

Full text of DOI:10.1002/anie.201612545

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201612545
German Edition: DOI: 10.1002/ange.201612545
Borasilenes
An Isolable Potassium Salt of a Borasilene–Chloride Adduct
Yuko Suzuki, Shintaro Ishida, Sota Sato, Hiroyuki Isobe, and Takeaki Iwamoto*
Abstract: Among the variety of isolable compounds with
multiple bonds involving silicon, examples of compounds that
contain silicon–boron double bonds (borasilenes) still remain
relatively rare. Herein, we report the synthesis of the potassium
salt of a chloride adduct of borasilene 1 ([2]ꢀ), which was
obtained as an orange crystalline solid. Single-crystal X-ray
diffraction analysis and reactivity studies on [2]ꢀ confirmed the
=
double-bond character of the Si B bond as well as the reduced
Lewis acidity, which is due to the coordination of Clꢀ to the
boron center. A thermal reaction of [2]ꢀ afforded a bicyclic
ꢀ
product by formal intramolecular C H insertion across the
=
Si B bond of 1, which was corroborated by a theoretical study.
Figure 1. Borasilenes and related compounds.
N
umerous low-coordinate silicon compounds stabilized by
bulky protecting groups and/or the coordination of Lewis
bases and alkali-metal halides have been synthesized, and
these species represent an important and active research area
in silicon chemistry.[1,2] Among the compounds that contain
multiple bonds involving silicon, those with silicon–boron
bond, are still unknown. On account of the low-lying vacant
2p orbital on the boron atom,[4] and owing to the weakness of
[6]
=
the Si B double bond, such compounds are expected to
behave as electron acceptors and strong Lewis acids.
We have synthesized various stable compounds with
silicon–heteroatom double bonds that bear bulky cyclic
=
double bonds (borasilenes, R2Si BR’) are relatively rare.
=
Moreover, only one such compound has been isolated.
Sekiguchi and Nakata reported compound A, which is
stabilized by an electron-donating amino substituent on the
boron center (Figure 1).[3] An acetylide adduct of A (A’) was
synthesized by treatment of A with the corresponding
acetylide[3] while B was proposed as an intermediate in the
reaction between (tBu2MeSi)2SiLi2 and MesBCl2 (Mes =
2,4,6-Me3C6H2) in THF.[4] Tokitoh et al. have suggested partial
alkyl units, R2Si X [R2Si = 2,2,5,5-tetrakis(trimethylsilyl)sila-
cyclopentan-1,1-diyl (RH2Si), X = NR’,[7] S,[8] Se,[8] Te ;[8] R2Si =
2,2,5,5-tetrakis(isopropyldimethylsilyl)silacyclopentan-1,1-
diyl, X = O[9]] and investigated the intrinsic properties of their
=
Si X double bonds. The results prompted us to synthesize
stable base-free borasilene 1, featuring an RH2Si moiety and
a Mes group on the boron atom, to determine the intrinsic
structure and properties of the Si B double bond. During our
=
=
double-bond character for the Si B bonds in (diarylboryl)-
synthetic study on 1, we unexpectedly obtained the chloride
adduct of 1 as a potassium salt, which was isolated as an
orange crystalline solid ([2][K(18-c-6)], 18-c-6 = 18-crown-6
ether).[10] A thermal reaction of [2]ꢀ afforded an intramolec-
silyl anions.[5] Nevertheless, base-free borasilenes, which
should possess the intrinsic properties of a Si B double
=
ꢀ
ular C H insertion product (5), which is an isomer of 1. This
result suggests that 5 is formed via intermediate 1. Herein, we
[*] Y. Suzuki, Prof. Dr. S. Ishida, Prof. Dr. T. Iwamoto
Department of Chemistry
report the synthesis, structure, and reactivity of [2]ꢀ.
Graduate School of Science, Tohoku University
Aoba-ku, Sendai 980-8578 (Japan)
E-mail: iwamoto@m.tohoku.ac.jp
Treating silane 4, which was obtained in 86% yield from
the reaction of dialkylsilylene 3[11] with MesBCl2[12] in hexane
(Scheme 1),[5,13] with KC8 and 18-c-6 in 1,2-dimethoxyethane
(DME) at room temperature induced an immediate color
change to orange. After stirring for 3 h, [2][K(18-c-6)] was
obtained in 73% yield as air-sensitive and thermally unstable
orange crystals,[14] which were characterized by multinuclear
NMR spectroscopy, high-resolution mass spectrometry, ele-
mental analysis, and single-crystal X-ray diffraction (XRD).
Homepage: http://www.ssoc.chem.tohoku.ac.jp/en_index.html
Prof. Dr. S. Sato, Prof. Dr. H. Isobe
Isobe Degenerate p-Integration Project
Exploratory Research for Advanced Technology (ERATO)
Japan Science and Technology Agency
Aoba-ku, Sendai 980-8577 (Japan)
and
Advanced Institute for Materials Research
Tohoku University
Aoba-ku, Sendai 980-8577 (Japan)
and
Department of Chemistry
University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201612545.
Scheme 1. Synthesis of [2][K(18-c-6)].
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The coordination of Clꢀ to the unsaturated boron center
suggests that the Lewis acidity of borasilene 1 is very high. In
the absence of 18-c-6, a similar chloride adduct with a DME-
coordinated potassium cation ([2][K(dme)2]) was obtained,
the structure of which was determined only by preliminary
XRD analysis (see the Supporting Information). As
[2][K(dme)2] is thermally less stable than [2][K(18-c-6)], it
could not be fully characterized.
although an equilibrium between these species cannot be
ruled out.[18,19] Compared to the dSi and dB values of A’ (dSi =
23.4 ppm, dB = 55.3 ppm) in [D8]THF at 298 K, those of
[2][K(18-c-6)] are downfield- and upfield-shifted, respec-
tively.[3]
The electronic structure of [2]ꢀ was also investigated by
DFT calculations. The structural parameters of the optimized
structure of RH2Si BMesCl···K(18-crown-6) ([2][K(18-c-
=
The molecular structure of [2][K(18-c-6)] shows a contact
ion pair in the solid state (Figure 2). The Cl1···K1 distance
6)]opt) at the B3PW91-D3/6-31 + G(D) level of theory are in
good agreement with the experimental values of 2 obtained
=
ꢀ
from the XRD analysis. For [2][K(18-c-6)]opt, Si B, B Cl, and
Cl···K distances of 1.857, 1.889, and 3.018 ꢀ, respectively,
were calculated as well as angle sums of 359.78 (Si) and 359.98
=
(B). The geometry around the Si B double bond in the anion
[RH2Si BMesCl] ([2] opt) is also similar to that of [2][K(18-c-
ꢀ
ꢀ
=
=
6)]opt. The Wiberg bond index for the Si B double bond in
[2]ꢀ (1.70) is almost twice that for the corresponding Si B
ꢀ
opt
bond in 4opt (0.88). The HOMO of [2]ꢀopt is a p(Si B) orbital,
=
and its energy level is much higher than that of base-free
=
borasilene 1opt owing to interactions between the p(Si B)
orbital and the lone pair orbital on Cl (Figure S69). These
results support the notion that [2]ꢀ should be interpreted as an
=
anionic Si B species rather than as a borylsilyl anion
(RH2Si BMesCl). The addition of Cl to borasilene 1 was
calculated to be exergonic (DG = ꢀ28.9 kJmolꢀ1 in THF;
DG = ꢀ77.5 kJmolꢀ1 in toluene; T= 298.15 K; same level of
theory), which is in good agreement with the experimental
observation that [2]ꢀ was obtained rather than 1.
ꢀ
ꢀ
ꢀ
Figure 2. Molecular structure of [2][K(18-c-6)].[35] Thermal ellipsoids set
at 30% probability, hydrogen atoms omitted for clarity. Selected bond
lengths [ꢀ] and angles [8]: Si1–B1 1.859(2), B1–Cl1 1.879(2), Cl1···K1
3.0150(6); Si1-B1-C5 132.69(13), C5-B1-Cl1 113.03(12), Cl1-B1-Si1
114.25(10).
As Clꢀ is a good leaving group, we expected that Clꢀ could
be eliminated from [2]ꢀ to generate free borasilene 1.
However, when a C6D6 solution of [2][K(18-c-6)] was left to
stand at room temperature for one day, bicyclic compound 5,
which is an isomer of borasilene 1, was obtained unexpectedly
(Scheme 2). Interestingly, in this thermal reaction, cis-5
[3.0150(6) ꢀ] is shorter than the sum of the ionic radii of Clꢀ
ꢀ
and K+ (3.14 ꢀ).[15] The Si1 B1 bond in [2] [1.859(2) ꢀ] is
ꢀ
similar in length to that in the neutral borasilene A [1.8379-
(17) ꢀ] and considerably shorter than that in borasilene–
acetylide adduct A’ [1.933(3) ꢀ].[3] The geometry around the
Si1 and B1 atoms is almost planar, which is reflected by the
angle sums around the Si1 (359.318) and B1 (359.978)
ꢀ
atoms.[16] The B1 Cl1 bond in [2] [1.879(2) ꢀ] is significantly
ꢀ
longer than that in precursor 4 [1.7751(17) ꢀ], which suggests
that the electron donation from the chloride to the boron
atom is weaker than in 4. These structural parameters show
Scheme 2. Thermal reaction of [2][K(18-c-6)].
ꢀ
=
that the Si B bond in [2] exhibits double-bond character,
with little contribution from a silyl anion (RH2Si BMesCl)
resonance structure.
ꢀ
ꢀ
[(1S*,5R*)-5] was formed selectively, which was confirmed
by NMR spectroscopy and XRD analysis.[20] At lower
temperatures (ꢀ80 to 08C) in C7D8, [2][K(18-c-6)] did not
react, but at room temperature, 5 was formed in a similar
fashion. During this reaction, 1H NMR resonances assignable
to intermediates such as 1 were not observed.
A solution of [2][K(18-c-6)] in [D8]THF showed the 29Si
NMR resonance (dSi) of the central unsaturated silicon
nucleus at 56.8 ppm and the 11B NMR resonance (dB) as
a broad signal at 38.0 ppm at 263 K. These values are in good
agreement with the corresponding theoretical gauge inde-
pendent atomic orbital (GIAO) values for the optimized
The formation of 5 from [2][K(18-c-6)] was accelerated in
C6D6 at room temperature upon addition of one equivalent of
triethylsilylium tetrakis(pentafluorophenyl)borate ([Et3Si]-
[TPFPB]),[21] which is an effective Clꢀ scavenger.[22,23] The
reaction mixture turned from orange to pale yellow, and two
layers formed within 2 h. In the upper layer, the formation of
5 (67%), Et3SiCl (70%), a small amount of Et3SiH,[24] and
other unidentified compounds was observed by 1H NMR
spectroscopy. The formation of 5 and Et3SiCl suggests that the
ꢀ
structures of [RH2Si BMesCl] ([2]ꢀopt; dSi = 59.6 ppm, dB =
=
33.6 ppm) and RH2Si BMesCl···K(18-crown-6) ([2][K(18-c-
=
6)]opt; dSi = 65.0 ppm, dB = 30.4 ppm) while they differ sub-
stantially from those of RH2Si BMes(thf) (dSi = 86.8 ppm,
=
dB = 41.5 ppm; for details of the theoretical study, see the next
paragraph and the Supporting Information). These results
suggest that in solution, [2]ꢀ is present as a borasilene–
chloride adduct rather than as a borasilene–THF adduct
2
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abstraction of Clꢀ accelerates the generation of borasilene
intermediate 1.
The mechanism for the transformation of [2]ꢀ into 5 was
investigated theoretically.
A
free energy change of
+ 77.5 kJmolꢀ1 was calculated at the B3PW91-D3/6–31 +
G(d) level of theory for the dissociation of Clꢀ from [2]ꢀ
opt
([2]ꢀopt!1opt + Clꢀ) at 298.15 K in toluene, which is consistent
ꢀ
with the experimental results. The stereospecific C H inser-
tion reaction of model compound 1’, in which the SiMe3 and
Mes groups were replaced with hydrogen atoms and a Me
group, respectively, to the corresponding bicyclic product 5’,
involves a remarkably small activation energy (47.9 kJmolꢀ1)
in toluene at the B3PW91/6-31 + G(D) level of theory
(Figure 3a). This activation barrier is much smaller than
Scheme 3. Reactivity of [2][K(18-c-6)] towards S8 and Ph3CCl.
should be oxidized with another equivalent of Ph3CCl (see the
Supporting Information).[28] Similar chlorine abstraction
reactions have already been reported for other compounds
=
with multiple bonds to silicon, such as disilenes (R2Si
SiR2).[29]
Reactions of [2][K(18-c-6)] with typical Lewis bases such
as PPh3 or N,N-dimethyl-4-aminopyridine did not proceed,
which indicates that the Lewis acidity of [2]ꢀ was substantially
reduced by the absence of the vacant 2p orbital on the boron
atom after the coordination of Clꢀ.[30]
Interestingly, [2]ꢀ exhibited unexpected reactivity towards
9,10-phenanthrenequinone (PQ), which is a typical trapping
agent for compounds with multiple bonds involving silicon.
Treating [2][K(18-c-6)] with two equivalents of PQ in C6D6
led to an immediate color change to dark red, which gradually
faded. The formation of the silylene–PQ adduct 7 (98%) and
1,3,2-dioxaborolane 8[31] (81%) instead of the expected [4+2]
cycloadduct was observed by 1H NMR spectroscopy
(Scheme 4). Further information on this reaction was
ꢀ
ꢀ
Figure 3. Possible C H insertion reactions of 1’ and [2’] . Whereas
both the cis and trans isomers of [2’]ꢀ are possible, they exhibit very
similar reaction routes with virtually identical activation energies; for
further details see the Supporting Information.
ꢀ
that for the corresponding C H insertion reaction from
model compound [2’]ꢀ to 5’·Clꢀ (188.7 kJmolꢀ1; Figure 3b).
Elimination of Clꢀ from 5’·Clꢀ is endergonic at 298.15 K in
toluene (DG =+ 15.1 kJmolꢀ1). These results support the
notion that the transformation of [2]ꢀ into 5 proceeds via free
ꢀ
borasilene 1. Although C H insertions have been reported
for methylene boranes[25] and other heavier multiple-bond
3
ꢀ
compounds, C(sp ) H insertions have not been observed in
other base-free double-bonded compounds bearing the same
RH2Si moiety, suggesting that the reactivity of base-free
borasilenes is relatively high.
Scheme 4. Reactivity of [2][K(18-c-6)] towards 9,10-phenanthrenequi-
none.
Compound [2][K(18-c-6)] exhibits a double-bond charac-
=
ter for the Si B bond that is similar to that in borasilene A,
and also reflected in its reactivity. Treatment of [2][K(18-c-6)]
with S8 (0.28 equiv) in toluene immediately produced the
boracycle 1,3,2,4-dithiasilaboretane 6, which could be isolated
in 39% yield as colorless crystals (Scheme 3).[26] A similar
borathiacycle, resulting from the insertion of an S atom into
obtained from the stepwise addition of PQ to [2][K(18-c-
6)]. Treatment of [2][K(18-c-6)] with one equivalent of PQ
afforded 7 (21%), 3 (7%), 4 (43%), and other unidentified
products, including a dark red precipitate. After addition of
another equivalent of PQ to the mixture, this precipitate was
consumed, and the color of the mixture turned to pale yellow.
=
the Si B double bond, was obtained from the reaction of
borasilene A with S8.[27] Furthermore, [2][K(18-c-6)] reacted
with two equivalents of Ph3CCl. Upon addition, the color of
the reaction mixture immediately turned to an intense purple,
but faded quickly. The formation of 4 and the trityl dimer was
observed by 1H NMR spectroscopy (Scheme 3). One con-
ceivable mechanism for this reaction involves the initial
abstraction of a ClC radical from Ph3CCl to generate a trityl
radical and the radical anion of 4. Subsequently, the latter
1
The H NMR spectrum of the mixture showed that 3, 4, and
the unidentified products were consumed with concomitant
formation of 7 (81%) and 8 (88%). The mechanism for this
reaction remains unclear at this stage but the initial formation
of 4 suggests an electron transfer from [2]ꢀ to PQ[32] and
a subsequent disproportionation, which would lead to 4, while
the precipitate should contain unidentified anionic species.
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Am. Chem. Soc. 2003, 125, 3414 – 3415; b) G. Molev, D. Bravo-
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Another equivalent of neutral PQ should then react with the
anionic species to provide 7, 8, and the radical anion of PQ
(PQCꢀ), the latter of which should then convert 4 into 7 and
8
[33] (for details, see the Supporting Information, Scheme S1).
The reaction of [2][K(18-c-6)] with PQ thus demonstrates the
electron-rich nature of [2]ꢀ.
In summary, borasilene–chloride adduct [2]ꢀ was synthe-
sized and isolated as [2][K(18-c-6)] in the form of orange
=
crystals. Its molecular structure exhibits a Si B bond and low
Lewis acidity. The generation of borasilene 1 from [2]ꢀ was
proposed on the basis of the stereoselective formation of
ꢀ
intramolecular C H insertion product 5 and theoretical
calculations on model compounds. This reaction is very
interesting as it includes the activation of relatively inert
3
ꢀ
C(sp ) H bonds. This, in turn, suggests unusually high
reactivity for the base-free borasilene. As theoretical calcu-
lations predicted an absorption band for base-free borasilene
1 in the near-infrared region arising from the HOMO!
LUMO transition,[34] isolation of the base-free borasilene
stabilized by carefully designed protecting groups should be
an important step towards unravelling the unusual reactivity
[11] M. Kira, S. Ishida, T. Iwamoto, C. Kabuto, J. Am. Chem. Soc.
1999, 121, 9722 – 9723.
=
and electronic nature of base-free Si B double bonds.
[12] a) E. M. Meyer, S. Gambarotta, C. Floriani, A. Chiesi-Villa, C.
Guastini, Organometallics 1989, 8, 1067 – 1079; b) A. Sundarara-
man, F. Jꢂkle, J. Organomet. Chem. 2003, 681, 134 – 142.
[13] A. Gackstatter, H. Braunschweig, T. Kupfer, C. Voigt, N.
Arnold, Chem. Eur. J. 2016, 22, 16415 – 16419.
Acknowledgements
[14] Sekiguchi and Nakata have synthesized borasilene A by differ-
This work was supported by MEXT KAKENHI grant
JP24109004 (T.I.; Grant-in-Aid for Scientific Research on
Innovative Areas “Stimuli-Responsive Chemical Species”).
We thank Prof. Makoto Yamashita (Nagoya University) for
helpful discussions, Naohiko Akasaka for helpful discussions
and XRD analyses, and Shunsuke Kayamori (Technical
Division, School of Engineering, Tohoku University) for
recording ESR spectra.
ent methods, for example, by treating
a 1,1-dilithiobis-
(silyl)silane with an aminodichloroborane; see Ref. [3].
[15] J. Emsley, The Elements, 2nd ed., Clarendon Press, Oxford, 1991.
ꢀ
=
[16] As expected from the calculated geometry of [R2Si BR’2]
[17]
species[5] and R2Si CR’OR’’,
model compound [Me2Si
=
=
BMeCl]ꢀ ([2Me]ꢀ) should exhibit a pyramidalized Si atom
ꢀ
(angle sum: 336.88) and a long Si B bond (1.912 ꢀ; B3PW91/
6-311 + G(D) level of theory). However, [Me2Si BMeCl]ꢀ with
=
a planar Si atom and a shorter Si B bond (1.867 ꢀ; [2Me’’]ꢀ),
ꢀ
which was located as a transition state, is only 7.1 kJmolꢀ1 higher
in energy than [2Me]ꢀ, indicating a shallow potential energy
surface for the pyramidalization of the Si atom and the
Conflict of interest
ꢀ
elongation of the Si B distance. The observed planar geometry
ꢀ
=
around the Si B bond in [2] should thus be ascribed to the
The authors declare no conflict of interest.
Keywords: boron · C-H activation · multiple bonds · silicon
bulky RH group. For details, see the Supporting Information.
2
[17] a) T. Guliashvili, I. El-Sayed, A. Fischer, H. Ottosson, Angew.
Chem. Int. Ed. 2003, 42, 1640 – 1642; Angew. Chem. 2003, 115,
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[18] The 1H NMR resonances of [2][K(18-c-6)] broadened upon
increasing the temperature, which is probably due to rotation
[1] For examples of low-coordinate silicon species stabilized by the
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=
around the Si B double bond. The calculated activation barrier
ꢀ
ꢀ
=
=
for rotation around the Si B bond in [Me2Si BMeCl] ([2Me] )
at the B3PW91/6-311 + G(D) level of theory is 60.1 kJmolꢀ1
.
The considerably lower solubility of [2][K(18-c-6)] in toluene
compared to THF prevented the recording of 29Si NMR spectra
of [2][K(18-c-6)] in [D8]toluene.
[19] The ESI spectrum of [2][K(18-c-6)] in DME showed peaks that
were assigned to [RH2Si BMesCl]ꢀ (negative mode) and [K(18-
=
c-6)]+ (positive mode), which may support the presence of
a solvent-separated ion pair in solution.
[20] Compound 5 could also be obtained directly from (bromome-
sitylboryl)bromosilane 4’, which in turn was obtained from the
treatment of 3 with MesBBr2 and KC8; for details, see the
Supporting Information.
[2] For isolable silylenoids, R2SiMX (M = alkali metal, X = halo-
gen), which are formal metal halide adducts of silylenes, see:
a) R. Fischer, J. Baumgartner, G. Kickelbick, C. Marschner, J.
4
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Communications
Chemie
[21] a) J. B. Lambert, S. Zhang, S. M. Ciro, Organometallics 1994, 13,
[32] The reaction mixture exhibited a strong and broad ESR signal
2430 – 2443; b) V. J. Scott, R. Celenligil-Cetin, O. V. Ozerov, J.
Am. Chem. Soc. 2005, 127, 2852 – 2853.
[22] Y. Shoji, N. Tanaka, K. Mikami, M. Uchiyama, T. Fukushima,
Nat. Chem. 2014, 6, 498 – 503.
[23] When [Ag(C6H6)3][TPFPB] was used, 1 and 5 were not formed
(see the Supporting Information).
accompanied by two doublet satellite signals (Figure S63).
Although the observed paramagnetic species has not yet been
fully characterized, the ESR signal should be assigned to a silyl
radical bearing an RH2Si group and a boryl group such as [RH2Si
ꢀ
BClMes]C, which is consistent with the proposed one-electron
oxidation of [2][K(18-c-6)] with PQ.
[24] Et3SiH was formed during the reaction between the triethylsi-
lylium ion and 18-crown-6 ether.
[25] a) A. Berndt, Angew. Chem. Int. Ed. Engl. 1993, 32, 985 – 1009;
Angew. Chem. 1993, 105, 1034—1058; b) J. J. Eisch, Adv.
Organomet. Chem. 1996, 39, 355 – 391.
[26] Treatment of [2][K(18-c-6)] with dry O2 afforded a complex
product mixture.
[27] N. Nakata, A. Sekiguchi, Chem. Lett. 2007, 36, 662 – 663.
[28] Other mechanisms involving an electron-transfer reaction
cannot be ruled out at this point; for further details see the
Supporting Information.
[33] The radical anion of PQ (PQCꢀ) reacts with 4 to furnish 7 (77%)
and 8 (78%). The formation of PQCꢀ from PQ and KC8 was
confirmed by ESR spectroscopy (Figure S62); see: K. S. Chen,
R. T. Smith, J. K. S. Wan, Can. J. Chem. 1978, 56, 2503 – 2507.
[34] TD-DFT calculations on 1opt at the B3LYP-D3/6-311 + G(d)
[hexane]//B3PW91-D3/6-31 + G(d) level of theory predicted
that the longest-wavelength absorption band, arising from the
HOMO!LUMO transition, should appear at 1593 nm.
[35] CCDC 1520340 ([2][K(18-c-6)]), 1520341 ([2][K(dme)2]),
1520342 (4), 1520343 (4’), 1520344 (5), 1520345 (6), and
1530520 ([S3][Li(CH3CN)4]) contain the supplementary crystal-
lographic data for this paper. These data are provided free of
charge by The Cambridge Crystallographic Data Centre.
[29] a) T. Iwamoto, H. Sakurai, M. Kira, Bull. Chem. Soc. Jpn. 1998,
71, 2741 – 2747; b) M. Kira, T. Ishima, T. Iwamoto, M. Ichinohe,
J. Am. Chem. Soc. 2001, 123, 1676 – 1682.
[30] The reaction of [2][K(18-c-6)] with MeLi provided a product that
was determined to be a methylborate resulting from the addition
of a methyl anion to the boron atom of 5 ([S3]ꢀ), and not fully
characterized (see the Supporting Information).
Manuscript received: December 26, 2016
Revised: February 2, 2017
[31] T. Umemoto, K. Adachi, J. Org. Chem. 1994, 59, 5692 – 5699.
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Borasilenes
Y. Suzuki, S. Ishida, S. Sato, H. Isobe,
T. Iwamoto*
&&&& — &&&&
An Isolable Potassium Salt of
a Borasilene–Chloride Adduct
The potassium salt of a borasilene–chlo-
ride adduct was synthesized and isolated.
Its reactivity and single-crystal X-ray dif-
fraction analysis revealed the double-
reduced Lewis acidity. This adduct pro-
vides a cyclic product by formal intra-
ꢀ
=
molecular C H insertion across the Si B
bond, possibly via the corresponding
base-free borasilene.
=
bond character of its Si B bond and
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!