Unprecedented insertion reaction of a silylene into a B-B bond and generation of a novel borylsilyl anion by boron-metal exchange reaction of the resultant diborylsilane

Article abstract of DOI:10.1039/b409575h

A kinetically stabilized diarylsilylene, Tbt(Mes)Si: (1, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes = mesityl), thermally generated from overcrowded disilene Tbt(Mes)Si=Si(Mes)Tbt (2) or stable silylene-isocyanide complex (3a), was found to in

Full text of DOI:10.1039/b409575h

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Unprecedented insertion reaction of a silylene into a B–B bond and
generation of a novel borylsilyl anion by boron–metal exchange reaction
of the resultant diborylsilane{
Takashi Kajiwara, Nobuhiro Takeda, Takahiro Sasamori and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.
E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp; Fax: 181 774 38 3209; Tel: 181 774 38 3200
Received (in Cambridge, UK) 24th June 2004, Accepted 20th July 2004
First published as an Advance Article on the web 20th August 2004
A kinetically stabilized diarylsilylene, Tbt(Mes)Si: (1, Tbt ~
2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes
~
mesityl),
disilene
thermally generated from overcrowded
Tbt(Mes)SiLSi(Mes)Tbt (2) or stable silylene–isocyanide
complex (3a), was found to insert into a B–B bond of
bis(pinacolato)diboron, B2(pin)2 (4), and the boron–lithium
exchange reaction of the resulting diborylsilane gave the first
borylsilyl anion.
Scheme 1 Synthesis of diborylsilane 5.
and B–Cl bonds. On the other hand, the reaction of silylene–
isocyanide complex 3a with 4 did not proceed at all even after
heating at 60 uC for 5 days. The reaction of 3a with 4 at 80 uC for
3 days gave the insertion product 5 in only 12% yield (Scheme 1).
Since we have reported that 1 is more easily generated from 3a than
from 2,4,5 one may feel strange about this lower reactivity to 4 of 3a
compared with 2. These results can be explained as follows. In this
slow reaction, the insertion of 1 into the B–B bond of 4 is
considered to be the rate-determining step The lower concentration
of 1 in the equilibrium state in the case of 3a than in that of 2 results
in the lower yield of 5 from 3a.
Although some examples have been reported for the insertion of
a silylenoid into a B–B bond,7,8 there have been no reports on the
reaction of a silylene with diboron compounds so far. Thus, the
formation of 5 by the reaction of 2 (or 3a) with 4 should be of great
interest as the first example of the insertion of a silylene into a B–B
bond. Moreover, there have been only a few reports on the
compounds containing a B–Si–B skeleton,7,9 while silyl-bridged
boranes and silaboranes, i.e., cage compounds containing 3-center
B–Si–B interactions, have been well-known.10
In recent years, silylboranes have attracted much attention from the
standpoints of both fundamental and applied chemistry.1 However,
their chemistry has not been fully disclosed yet mainly due to the
limited synthetic methods for them. Although silylboranes are most
commonly prepared by the reactions of silyllithium with halo-,
hydro- or alkoxyboranes,2 only a few examples are known to date
due to the high reactivity of silylboranes and the difficulty of the
preparation of precursors.
On the other hand, we have recently reported the synthesis of
stable silylborane derivatives utilizing a kinetically stabilized silylene
1.3 Reactions of silylene 1, thermally generated from overcrowded
disilene 24 or stable silylene–isocyanide complexes 3,5 with hydro-
boranes or haloboranes proceeded smoothly to afford the corres-
ponding (hydrosilyl)boranes or (halosilyl)boranes, respectively. The
reaction mechanism was reasonably interpreted in terms of the
insertion of 1 into the B–H or B–halogen bonds rather than
the electrophilic attack of the boron reagents. In this paper, we
present that the insertion of a silylene is feasible toward the B–B
bond as well as giving the corresponding diborylsilane. In addition,
the boron–metal exchange reaction of the resulting diborylsilane
was examined to generate an unprecedented borylsilyl anion.
The 29Si NMR spectrum of 5 showed a broad signal at
267.6 ppm. This value is similar to those of the related
hydrosilanes, Tbt(Mes)SiHB(pin) (262.7 ppm)3b and Tbt(Mes)-
SiH2 (261.3 ppm).4b The molecular structure of 5 was unambigu-
ously determined by X-ray crystallographic analysis, and the
ORTEP drawing of 5 is shown in Fig. 1.{ The Si–B bond lengths
˚
˚
(2.038(3) A for Si1–B1 and 2.054(3) A for Si1–B2) are comparable
to those of previously reported silylborane derivatives.2f,3,9c,11
Interestingly, one of the two dioxaborolane rings in 5 is slightly
distorted, while the other is almost planar. The ring puckering
coordinates defined by Cremer and Pople12 were q2 ~ 0.04 A and
˚
˚
ø2 ~ 115.80u for the O1–B1–O2–C2–C1 ring and q2 ~ 0.31 A and
ø2 ~ 128.00u for the O3–B2–O4–C8–C7 ring. The ø2 value of the
latter is close to the value (126u) appropriate to one of the twist
forms with twist axis through B2. Similarly distorted BO2C2 rings
have been reported for R2C[B(pin)]2 compounds, though both of
the two rings are distorted.8b
Although boron-stabilized carbanions have been actively investi-
gated,13 their silicon analogues, i.e., borylsilyl anions, have been
completely unprecedented species. Since boron–metal exchange
When a THF or benzene suspension of disilene 2 and
bis(pinacolato)diboron 46 was heated at 60 uC, the original
orange color faded very slowly giving a colorless solution in 5 days.
Separation of the crude products by silica gel column chromato-
graphy afforded the corresponding B–B insertion compound,
Tbt(Mes)Si[B(pin)]2 (5), in 45% yield (Scheme 1). Since the
reactions of 2 with pinacolborane or B-chloropinacolborane
yielded the corresponding (hydrosilyl)- or (chlorosilyl)pinacolbor-
anes in high yields within a much shorter reaction time (ca. 17 h),3b
the B–B bond of 4 is less reactive toward silylene 1 than the B–H
{ Electronic supplementary information (ESI) available: spectral data for 5
and 6. See http://www.rsc.org/suppdata/cc/b4/b409575h/
Scheme 2 Reaction of 5 with n-BuLi.
2 2 1 8
C h e m . C o m m u n . , 2 0 0 4 , 2 2 1 8 – 2 2 1 9
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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T ~ 103(2) K, GOF ~ 1.079. The structures were solved by a direct
method (SIR-97)14 and refined by full-matrix least-squares procedures on
F2 for all reflections (SHELXL-97).15 CCDC 243326 and 243327. See
http://www.rsc.org/suppdata/cc/b4/b409575h/ for crystallographic data in
.cif or other electronic format.
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Fig. 1 ORTEP drawing of 5 (50% probability).
reactions of silylboranes have been reported by Tamao et al.,1c
diborylsilane 5 might be a good precursor for a borylsilyl anion.
When diborylsilane 5 was treated with 4 molar equivalents of
n-BuLi in hexane at 278 uC, the reaction mixture turned deep red.
Addition of chlorotrimethylsilane to this solution gave the
trimethylsilylated silylborane, Tbt(Mes)Si(SiMe3)B(pin) 6, in 61%
yield, the structure of which was confirmed spectrographically and
crystallographically.{ The formation of TMS-substituted silylbor-
ane 6 is most likely interpreted in terms of the reaction of borylsilyl
anion 7, which is generated by a boron–lithium exchange reaction
of 5,1c with chlorotrimethylsilane. It should be noted that the
formation of 7 by the boron–lithium exchange reaction of 5 is the
first example for the generation of a borylsilyl anion.
In summary, we have found a novel reactivity of an overcrowded
diarylsilylene 1, generated from disilene 2 or silylene–isocyanide
complex 3a, with bis(pinacolato)diboron leading to the first
example of the insertion reaction of a silylene into a B–B bond.
Furthermore, the reaction of diborylsilane 5 thus obtained with
n-BuLi resulted in the generation of borylsilyl anion 7, which
was trapped by chlorotrimethylsilane to give the trimethylsilyl-
substituted silylborane 6. Further studies on the structure, stability,
and reactivities of the borylsilyl anion 7, especially on the applica-
tion toward the synthesis of functionalized silylboranes, are
currently in progress.
This work was partially supported by Grants-in-Aid for COE
Research on Elements Science (No. 12CE2005), Scientific Research
on Priority Area (No. 14078213), and the 21st Century COE
Program on Kyoto University Alliance for Chemistry from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan. We thank Central Glass Co., Ltd. for the generous gift of
tetrafluorosilane.
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Notes and references
{ Crystallographic data for 5: C48H94B2O4Si7, MW ~ 953.48, triclinic,
˚
˚
space group P–1 (no. 2), a ~ 12.189(4) A, b ~ 13.204(5) A, c ~
˚
19.744(7) A, a ~ 79.858(14)u, b ~ 72.051(13)u, c ~ 78.938(16)u, V ~
3
2943.3(17) A , Z ~ 2, Dcalc ~ 1.076 g cm21, R1(I w 2s(I)) ~ 0.0523,
˚
wR2(all data) ~ 0.1221, T ~ 103(2) K, GOF ~ 1.075. Crystallographic
data for 6: C45H91BO2Si8, MW ~ 899.71, triclinic, space group P–1 (no. 2),
˚
˚
˚
3
a ~ 13.100(7) A, b ~ 20.193(10) A, c ~ 24.014(11) A, a ~ 65.154(12)u,
˚
15 G. M. Sheldrick, SHELX-97, Program for the Refinement of Crystal
Structures, University of Go¨ttingen, Go¨ttingen, Germany, 1997.
b ~ 78.169(12)u, c ~ 88.869(18)u, V ~ 5626(5) A , Z ~ 4, Dcalc
~
1.062 g cm21, R1(I w 2s(I)) ~ 0.0605, wR2(all data) ~ 0.1481,
C h e m . C o m m u n . , 2 0 0 4 , 2 2 1 8 – 2 2 1 9
2 2 1 9