Synthesis and reactions of a stable 1,2-diaryl-1,2-dibromodisilene: A precursor for substituted disilenes and a 1,2-diaryldisilyne

Article abstract of DOI:10.1021/ja8061002

Synthesis and isolation of the stable diaryldibromodisilene, Bbt(Br)Si-Si(Br)Bbt, has been accomplished for the first time. The dibromodisilene underwent substitution reactions with organometallic reagents on the low-coordinated silicon atom to afford the corresponding substituted disilenes. Furthermore, the reaction of 1 with t-BuLi afforded the corresponding 1,2-diaryldisilyne, BbtSi≡SiBbt, the characters of which were revealed by spectroscopic and crystallographic analyses. Copyright

Full text of DOI:10.1021/ja8061002

Published on Web 09/26/2008
Synthesis and Reactions of a Stable 1,2-Diaryl-1,2-dibromodisilene: A
Precursor for Substituted Disilenes and a 1,2-Diaryldisilyne
Takahiro Sasamori,† Koji Hironaka,† Yusuke Sugiyama,† Nozomi Takagi,‡ Shigeru Nagase,‡
Yoshinobu Hosoi,§ Yukio Furukawa,§ and Norihiro Tokitoh*,†
Institute for Chemical Research, Kyoto UniVersity, Gokasho, Uji, Kyoto 611-0011, Japan, Department of Theoretical
Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan, and Department of Chemistry,
School of Science and Engineering, Waseda UniVersity, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
Received August 4, 2008; E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp
There has been much interest in the chemistry of multiply bonded
organosilicon compounds, i.e., disilenes and disilynes.1-4 While
numbers of kinetically stabilized disilenes have been synthesized
and characterized since the isolation of the first stable disilene,
Mes2SidSiMes2 (Mes ) mesityl), by West and his co-workers,2
only two examples of stable disilynes have been known up to now.
Wiberg et al.3 and Sekiguchi et al.4 independently reported the
synthesis of disilynes bearing bulky silyl groups, while theoretical
studies suggest that a silyl group should afford thermodynamic
stabilization toward the low-coordinated organosilicon compounds
due to the electropositive silicon atom.5 Although a carbon-
substituted disilyne has been desired as a new standard of a SitSi
Figure 1. Structures of (a) dibromodisilene 1 and (b) diaryldisilyne 9.
Displacement ellipsoids were drawn at the 30% probability level.
triple-bond compound, no example has been reported so far for a
Scheme 1
diaryl- or dialkyl-substituted disilyne. On the other hand, unique
properties of disilenes prompted many chemists to explore the
chemistry of novel systems containing SidSi units with extended
π-conjugation from the standpoint of material science.6 Accord-
ingly, a proper disilene unit as a “building block” has been desired
for the synthetic studies of functionalized disilenes. Recently, the
stable disilyne, RSiSitSiRSi (RSi ) Si[CH(SiMe3)2]2(i-Pr)), was
Scheme 2
evidenced to afford unique disilenes by addition reactions.7
Although R*(Cl)SidSi(Cl)R* [R* ) SiMe(Si(t-Bu)3)2],3,8 which
has been known as the only stable dihalodisilene, should be of great
interest as a good precursor for the functionalized disilenes via
substitution reaction, it is too protected to undergo substitution
reaction on the central silicon atoms. We report here the synthesis
and structure of a new, stable 1,2-dibromodisilene 1. Furthermore,
it was demonstrated that 1 was transformed into a variety of
substituted disilenes and the corresponding stable 1,2-diaryldisilyne
by reaction with alkyl metals.
Reduction of tribromosilane 2 with lithium naphthalenide af-
forded dibromodisilene 1 as stable yellow crystals (Scheme 1).9
X-ray crystallographic analysis (Figure 1a) revealed the structural
parameters of 1 with a highly trans-bent structure (trans-bent angles
) 32.4°, 39.8°) and a relatively longer SidSi double bond
[2.2264(8) Å], which are in sharp contrast to the planar structure
and a relatively shorter SidSi bond length [2.163(4) Å] of
R*(Cl)SidSi(Cl)R*,8 where the central SidSi moiety might be
sterically perturbed by the bulky silyl substituent. In the solid state
signals were observed for the SiMe3 groups at the ortho-positions
of Bbt groups even at 60 °C, suggesting the trans-bent structure of
1 in solution. The UV-vis spectra of 1 in hexane showed
characteristic absorption at λmax ) 434 nm (ꢀ 25 000) assignable
to the π-π* transition, which did not change at variable temperature
(-60 to 40 °C). Thus, dibromodisilene 1 did not undergo any
thermal dissociation into the corresponding silylenes in contrast to
the case of the germanium analogue, Bbt(Br)GedGe(Br)Bbt.10
However, dibromodisilene 1 was found to start gradual decomposi-
tion at 60 °C (Scheme 2), and further heating of 1 at 100 °C for
10 h afforded the cyclization product 3 quantitatively. The formation
of 3 is most likely interpreted in terms of the intermediacy of
dibromosilylsilylene 4 generated by the migration of the bromine
atom.11
Raman spectra, 1 exhibited an intense Raman line at 541 cm-1
,
which should predominantly correspond to the SidSi vibration.1
In the 29Si NMR spectrum (C6D6) of 1, the characteristic, low-
field shifted signal was observed at 79.4 ppm, indicating its SidSi
double-bond character. Although the 1H NMR spectrum (toluene-
d8) of 1 suggested that the Bbt groups should freely rotate without
any steric restriction at rt and even at -40 °C, two independent
Interestingly, the reaction of dibromodisilene 1 with MeLi in
THF at -80 °C resulted in the formation of bromomethyldisilene
5, Bbt(Me)SidSi(Br)Bbt, in 48% yield (Scheme 3). The treatment
of 1 with MeMgBr, EtMgBr, n-BuLi, and PhMgBr under the same
conditions also afforded the corresponding substituted disilenes, 5
† Kyoto University.
‡ Institute for Molecular Science.
§ Waseda University.
9
13856 J. AM. CHEM. SOC. 2008, 130, 13856–13857
10.1021/ja8061002 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
triple-bond character even in solution, though the 29Si NMR signal
for the triply bonded silicon atoms of 9 was observed in relatively
upper field region as compared with those of the silyl-substituted
disilynes and tetraaryldisilenes probably due to the large contribu-
tion of the paramagnetic terms in 9.16
Scheme 3
In summary, the synthesis and isolation of stable diaryldibro-
modisilene 1 has been accomplished for the first time. It should be
noted that 1 was found to undergo substitution reactions with
organometallic reagents on the low-coordinated silicon atom to
afford the corresponding substituted disilenes. Furthermore, the
reaction of 1 with t-BuLi afforded the corresponding 1,2-diaryld-
isilyne, BbtSidSiBbt (9), the structure and properties of which were
established by X-ray crystallographic and spectroscopic analyses
together with theoretical calculations.
(46%), 6 (ca. 40%), 7 (ca. 50%), and 8 (ca. 20%), respectively.
The formal substitution reaction on the sp2-silicon atom of 1 may
occur via the addition-elimination mechanism, where RM under-
goes an addition reaction toward the SidSi bond of 1 leading to
the formation of Bbt(Br)(R)SisSi(M)(Br)Bbt followed by the 1,2-
elimination of MBr.12 Thus, it was demonstrated that 1 should be
a candidate for the building block of novel disilenes by reactions
with nucleophiles. Since it is well-known that bromoethene
derivatives undergo no substitution reaction with an organometallic
reagent such as MeLi or MeMgBr, the observed substitution
reactions of dibromodisilene 1 should reflect unique properties of
a low-coordinated organosilicon species, i.e., lower LUMO of a
disilene.1 However, further addition of MeLi toward bromometh-
yldisilene 5 under the same conditions afforded a complicated
mixture without any disilene species probably due to the slightly
higher LUMO of 5 than that of 1.13
When dibromodisilene 1 was treated with t-BuLi at -100 °C in
THF, not t-butyl-substituted disilene but 1,2-diaryldisilyne 9 was
obtained (55% yield as judged by 1H NMR spectra).14 In this case,
t-BuLi may attack not the sp2-silicon atom but the bromine atom
of 1 due to the bulkiness of t-butyl group (halophilic reaction) to
afford 9 via the elimination of LiBr from the intermediately formed
Bbt(Li)SidSi(Br)Bbt.15 In the 29Si NMR spectrum, diaryldisilyne
9 showed a singlet signal for the triply bonded silicon atoms at
18.7 ppm, the chemical shift of which is in the apparently upper
field than that of the silyl substituted disilynes (91.5 and 89.9
ppm).3,4 Similarly, tetraaryl disilenes were reported to show their
29Si NMR chemical shifts in the upper field than those of tetrasilyl-
substituted disilenes.1 Since the slightly remaining 1 and the slow
decomposition of 9 prevent 9 from further purification, disilyne 9
could not be completely purified. However, patient efforts to
recrystallize 9 gave us the single crystals of 9 for X-ray crystal-
lographic analysis.
Diaryldisilyne 9 has a crystallographic C2 axis through the central
SitSi bond; that is, the two BbtSi units of 9 are inherently identical.
As in the case of the previously reported disilyldisilyne,
RSiSitSiRSi,4 9 exhibited a bent structure with the SisSisC bond
angles of 133.0(3)° and the CsSisSisC dihedral angle of
164.1(8)°. The SitSi bond length of 9 [2.108(5) Å] is reasonably
shorter than the typical SisSi and SidSi bond lengths, featuring
its triple-bond character. In addition, the slightly longer SitSi bond
length of 9 than that of RSiSitSiRSi [2.0622(9) Å]4 should reflect
the electronic feature of the aryl substituents on the triply bonded
silicon atoms. Theoretical calculations (B3PW91/6-311+G(2df) for
Si; 6-31G(d) for C,H) for 1,2-diaryldisilyne 9 strongly supported
the observed structure and chemical shifts in the 29Si NMR
spectrum. The structural parameters of 9 optimized with C2
symmetry exhibits the SidSi bond length of 2.119 Å and SisSisC
bond angles of 135.9°. The 29Si NMR chemical shift for the triply
bonded silicon atoms was computed as 16.7 ppm, supporting the
observed value in C6D6. That is, diaryldisilyne 9 should feature a
Acknowledgment. This work was partially supported by Grants-
in-Aid for Scientific Research (Nos. 17GS0207, 20036024) and the
Global COE Program, from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
Supporting Information Available: Experimental procedures and
spectral data for new compounds. Theses materials are available free
of charge via the Internet at http://pubs.acs.org.
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