Benzodisilacyclobutadienes: 8π-Electron Systems with an Antiaromatic Silicon Ring

Article abstract of DOI:10.1002/anie.201707553

Benzodisilacyclobutadienes 2 a–c were isolated as blue to green crystalline solids from the reaction of stable disilyne 1 and 1,2-dibromobenzenes in the presence of potassium graphite. In the solid state, substantial bond alternation was observed within the benzene rings of 2 a–c. In hexane, 2 a–c showed remarkable bathochromic shifts of the π→π* (HOMO→LUMO) absorption bands at 625–670 nm. NMR spectra and theoretical calculations indicated that the diamagnetic ring currents of the benzene rings of 2 a–c are considerably reduced by contributions from the antiaromatic 1,2-disilacyclobutadienes. In their entirety, the obtained results indicate that 2 a–c represent 8π-electron systems that contain an antiaromatic 1,2-disilacyclobutadiene.

Full text of DOI:10.1002/anie.201707553

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International Edition: DOI: 10.1002/anie.201707553
German Edition: DOI: 10.1002/ange.201707553
Aromaticity
Benzodisilacyclobutadienes: 8p-Electron Systems with an
Antiaromatic Silicon Ring
Shintaro Ishida,* Yoshifumi Misawa, Shohei Sugawara, and Takeaki Iwamoto*
Abstract: Benzodisilacyclobutadienes 2a–c were isolated as
blue to green crystalline solids from the reaction of stable
disilyne 1 and 1,2-dibromobenzenes in the presence of
potassium graphite. In the solid state, substantial bond alter-
nation was observed within the benzene rings of 2a–c. In
hexane, 2a–c showed remarkable bathochromic shifts of the
p!p* (HOMO!LUMO) absorption bands at 625–670 nm.
NMR spectra and theoretical calculations indicated that the
diamagnetic ring currents of the benzene rings of 2a–c are
considerably reduced by contributions from the antiaromatic
1,2-disilacyclobutadienes. In their entirety, the obtained results
indicate that 2a–c represent 8p-electron systems that contain
an antiaromatic 1,2-disilacyclobutadiene.
D.[4–6] According to these experimental and theoretical
studies,[7] the diamagnetic ring current on the benzene rings
of the benzocyclobutadienes is reduced owing to the para-
magnetic ring current of the fused cyclobutadiene moieties.
Progress in main-group-element chemistry has meanwhile
enabled the incorporation of heavier Group 14 elements into
neutral cyclobutadienes and their charged (dianionic and
dicationic) derivatives.[8–13] Among these compounds, espe-
cially neutral 4p-electron systems that involve Si and Ge
atoms show characteristics that are unexpected from the
perspective of classic organic chemistry. For example, 1,2-
digermacyclobutadienes E and F exhibit trapezoid four-
=
membered rings with long Ge Ge double bonds, which
characterizes them as nonaromatic species,[12] whereas tetra-
silacyclobutadiene G forms a non-aromatic charge-separated
rhomboid to avoid an antiaromatic structure.[13] Herein, we
report the synthesis and properties of the isolable benzodi-
silacyclobutadienes 2a–c. In contrast to the previously
reported analogues of cyclobutadienes with heavier
Group 14 elements, 2a–c show pronounced antiaromatic
character of their 1,2-disilacyclobutadiene moieties.
B
enzannulated cyclobutadienes (benzocyclobutadienes)
form a unique class of cyclic 8p-conjugation systems that
contain two subunits with opposing character, that is, an
antiaromatic cyclobutadiene ring and an aromatic benzene
ring.[1] Even though the parent benzocyclobutadiene A
(Figure 1) readily dimerizes, even at temperatures as low as
75 K,[2] A can still be observed by fast-flow NMR spectros-
copy techniques.[3] The introduction of sterically demanding
The reduction of an equimolar mixture of dialkyldisilyne
substituents allowed the isolation of benzocyclobutadienes B– 1,[14] which contains bulky Rs protecting groups [Rs = 1,1-
bis(trimethylsilyl)-3,3-dimethylbutyl], and 1,2-dibromoben-
zene with potassium graphite (KC8; 2.2 equiv) in 1,2-dime-
thoxyethane (DME) at À408C afforded 2a as a blue crystal-
line solid in 40% yield (Scheme 1). Similarly, the correspond-
Scheme 1. Synthesis of benzodisilacyclobutadienes 2a–c.
Figure 1. Benzocyclobutadienes A–D, 1,2-digermacyclobutadienes E
and F, and tetrasilacyclobutadiene G [Mes=2,4,6-trimethylphenyl,
Dip=2,6-diisopropylphenyl, Dsi=bis(trimethylsilyl)methyl].
ing dimethyl (2b) and dimethoxy derivatives (2c) were
isolated as blue and green crystalline solids in 27 and 23%
yield, respectively. The molecular structures of 2a–c were
unequivocally determined by multinuclear NMR spectrosco-
py, mass spectrometry, elemental analysis, and single-crystal
X-ray diffraction (XRD) analysis. Although the detailed
reaction mechanism that leads to the formation of 2a–c
remains unclear at this point, the reaction could be initiated
by the reduction of disilyne 1, given that Sekiguchi and co-
workers have reported that the one-electron reduction of
disilynes affords the corresponding anion radical.[15] More-
over, we found trace amounts of biphenylene 3, silacyclopro-
pabenzene 4, and benzodisilacyclobutene 5 as by-products
[*] Prof. Dr. S. Ishida, Y. Misawa, S. Sugawara, Prof. Dr. T. Iwamoto
Department of Chemistry, Graduate School of Science
Tohoku University
Aoba-ku, Sendai 980–8578 (Japan)
E-mail: sishida@m.tohoku.ac.jp
iwamoto@m.tohoku.ac.jp
Homepage: http://www.ssoc.chem.tohoku.ac.jp/en index.html
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201707553.
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À
C4, and C5 C6 bonds [1.371(3)–1.386(3) Š]. The bond-
alternation pattern of 2a is similar to that of benzocyclobu-
tadiene C,[6] which indicates that 2a-I (Scheme 2) should be
the predominant resonance structure. Since the benzene rings
Figure 2. By-products formed during the synthesis of 2a.
(Figure 2), which suggests the intermediacy of benzyne and
Br–K exchange during the formation of 2a.[16,17] Similar to
previously reported disilenes,[18] 2a reacted with 1,2-dibromo-
ethane to give trans-dibrominated benzodisilacyclobutene 5
in 40% yield.
The molecular structures of 2a–c in the solid state were
determined by single-crystal XRD analysis (Figure 3; see also
Figures S20 and S21 in the Supporting Information).[19]
Scheme 2. Resonance structures of 2a.
in 5 (Figure 2) and cis-9c[21c] (Figure 4) do not exhibit any
bond alternation (see Table S6 in the Supporting Informa-
tion), the observed structural characteristics of 2a should be
due to a reluctance to adopt the 1,2-disilacyclobutadiene
structures (2a-II and 2a-III).[22,23] The [C6Si2] core in 2a is
essentially planar,[24] although the geometry around the
unsaturated silicon atoms is substantially pyramidalized
[sum of the angles: 335.68(7)8 (Si1) and 340.47(7)8 (Si2)]
=
and the Si Si double bond adopts a distinct trans-bent
geometry [bent angles: 40.48 (Si1) and 35.38 (Si2)].[25]
Benzodisilacyclobutadienes 2b and 2c exhibit structural
features that are similar to those of 2a (see Table S6).
The UV/Vis spectra of 2a–c in hexane at room temper-
ature (see Figures S21–23) showed the longest wavelength
absorption band (band-I) of 2a at labs = 625 nm, which
corresponds to a bathochromic shift (2b: 635 nm; 2c:
670 nm) upon the introduction of electron-donating substitu-
ents on the benzene ring. The bathochromic shift of band-I in
2a–c is quite remarkable as compared to that of 8[26]
considering the similar substitution pattern (labs = 427 nm;
Figure 4). These results indicate effective orbital interactions
between the disilene and the annulated aryl moieties. As the
band maxima and oscillator strength values obtained from
TDDFT calculations[27] on the optimized structures of 2a–c
(2aopt–copt; see Figures S21–S23) were in good agreement with
the experimental absorption spectra of 2a–c, band-I was
assigned to the HOMO!LUMO (p!p*) transition. The
frontier Kohn–Sham (KS) orbitals of 2aopt (Figure 5) suggest
that the HOMO consists predominantly of an out-of-phase
Figure 3. ORTEP representations of 2a (thermal ellipsoids displayed at
50% probability).
=
Although the length of the Si Si bond in 2a [2.2386(8) Š] is
=
comparable to those of the Si Si bonds in 1,2-disila-1-
cyclobutenes 6 [2.213(3) Š], trans-7a [2.1632(10) Š], and 7b
[2.175(1) Š] (Figure 4),[20] it is far shorter than those in
=
combination of the p Si Si orbital and a p orbital of the
benzene ring, whereas the LUMO consists of an in-phase
=
combination of the p*(Si Si) orbital and a p* orbital. Thus,
the HOMO and LUMO are delocalized over the entire p-
electron system as in benzocyclobutadiene A (Figure 5b).[27]
The 29Si NMR signals of the disilene moieties (dSi) in 2a–c
appeared at 135.9, 140.5, and 151.2 ppm, respectively. As the
Figure 4. Related compounds 6–9 [Tsi=tris(trimethylsilyl)methyl].
benzodisilacyclobutenes 5 [2.4319(10) Š] and cis-9c [2.355-
(1) Š],[21c] which clearly suggests a disilene structure for 2a.
À
The lengths of the Si C bonds connecting the disilene with
the benzene moiety [1.902(2) and 1.915(2) Š] are slightly
À
elongated as compared to typical Si C single bonds (ca.
1.87 Š). Substantial bond alternation was observed in the
À
À
À
benzene ring of 2a, that is, the C1 C6, C2 C3, and C4 C5
À
À
bonds [1.407(3)–1.432(3) Š] are longer than the C1 C2, C3
Figure 5. Frontier KS orbitals of a) 2aopt and b) benzocyclobutadiene A.
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observed signals are consistent with those of typical dis-
pletely different from those of 10, 11, and 12 and resembles
ilenes,[18,28] the Si Si double-bond character is preserved in
that of all-carbon analogue A: The NICSzz curves of A and 13
positively increase on the benzene rings (“a”!“c” in
=
1
solution. Noticeably, the H NMR signals for the hydrogen
atoms on the aromatic rings of 2a–c were observed at 6.34– Figure 7) to À10 ppm and exhibit maxima with large positive
6.69 ppm in C6D6, which is an approximately 0.5 ppm upfield
values (ca. + 14 ppm) at the center of the C4 and the four-
shift relative to those of benzodisilacyclobutenes 9a (7.03– membered [C2Si2] rings (“d” in Figure 7). This result suggests
7.63 ppm), 9b (7.29–7.50 ppm), and 9c (7.31–7.46 ppm) in
CDCl3[21] as well as those of 5 (7.13 and 7.89 ppm) in C6D6. As
similar upfield shifts have been reported for benzocyclobu-
tadienes (5.8–6.3 ppm for A and B),[3,6] the diamagnetic ring
currents in benzodisilacyclobutadienes 2a–c must be dimin-
ished by the fused 1,2-disilacyclobutadiene moieties.
To examine the ring current of the benzodisilacyclobuta-
dienes, we employed the NICSzz scan method developed by
Stanger and co-workers.[29] The NICSzz value represents the
out-of-plane components of the magnetic shielding tensor
that appropriately reflects the induced ring current at the
specific positions. We chose model compounds 10–13 and A
(Figure 6) and NICSzz scan curves along the long axis of the
molecules (NICSzz-X scans; Figure 7).[30] According to the
that the antiaromatic character of the fused 1,2-disilacyclo-
butadiene unit in 13 perturbs the diatropic ring current of the
benzene ring; that is, the paratropic global ring current in 13
due to the 8p-electron system partially erodes the diatropic
ring current of the benzene ring.
In summary, benzodisilacyclobutadienes 2a–c were syn-
thesized and fully characterized. On the basis of a combina-
tion of systematic structural, spectroscopic, and theoretical
analyses, 2a–c can be described as 8p-electron systems that
contain antiaromatic 1,2-disilacyclobutadiene moieties. Thus,
2a–c represent potential precursors to silicon analogues of
poly(o-phenylenevinylene) after ring-opening polymeri-
zation.[31] Studies on the reactivity of 2a–c, including their
ring opening, are currently under way in our laboratories.
Acknowledgements
This research was supported by JSPS KAKENHI grants
JP25708004 and JP15K13635 (S.I.), as well as the Collabo-
rative Research Program of the Institute for Chemical
Research (Kyoto University; grant 2017-98; T.I.).
Figure 6. Model compounds for the NICSzz scans.
Conflict of interest
The authors declare no conflict of interest.
Keywords: aromaticity ·bond alternation ·fused-ring systems ·
multiple bonds ·silicon
How to cite: Angew. Chem. Int. Ed. 2017, 56, 13829–13832
Angew. Chem. 2017, 129, 14017–14020
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Manuscript received: July 25, 2017
Accepted manuscript online: August 28, 2017
Version of record online: October 2, 2017
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