Reactivity of an aryl-substituted silicon-silicon triple bond: 1,2-disilabenzenes from the reactions of a 1,2-diaryldisilyne with alkynes

Article abstract of DOI:10.1039/c0dt00115e

The reactivity of a diaryl-substituted disilyne, Ar-Si≡Si-Ar, with alkynes was examined. Reaction of the disilyne with acetylene yielded a 1,2-disilabenzene as the sole product. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0dt00115e

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Reactivity of an aryl-substituted silicon–silicon triple bond: 1,2-disilabenzenes
from the reactions of a 1,2-diaryldisilyne with alkynes†
Joon Soo Han,‡ Takahiro Sasamori, Yoshiyuki Mizuhata and Norihiro Tokitoh*
Received 11th March 2010, Accepted 9th June 2010
DOI: 10.1039/c0dt00115e
≡
The reactivity of a diaryl-substituted disilyne, Ar–Si Si–Ar,
with alkynes was examined. Reaction of the disilyne with
acetylene yielded a 1,2-disilabenzene as the sole product.
In 2004, the groups of Sekiguchi and Wiberg independently
reported a successful synthesis of overcrowded silyl-substituted
disilynes (RSi SiR: R = Si(i-Pr)[CH(SiMe3)2]2 (I),1a R =
≡
SiMe[Si(t-Bu)3]2 (II)1b). Although more than 6 years have passed
since then, only a few reactions of disilynes are known.1–3
Recently, we reported the synthesis and isolation of a stable
≡
1,2-diaryldisilyne, BbtSi SiBbt (1, Bbt
=
2,6-bis[bis(tri-
methylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]-phenyl), along
with its unique properties.3 In addition, we demonstrated that the
1,2-diaryldisilyne 1 has a reactivity with alkenes similar to that of
the silyl-substituted disilynes I and II, with the exception of its
unique reactivity towards a conjugated diene.3c
Since our successful synthesis of a stable 2-silanaphthalene in
1997, we have continued to explore the chemistry of group 14 met-
allaaromatic compounds (heavy aromatics).4 Recently, Sekiguchi
et al. reported the first synthesis of diphenyl-substituted 1,2-
disilabenzene derivatives from the reaction of 1,2-bis(silyl)disilyne
I with phenylacetylene.2c They reported that I reacted with 2
equivalents of phenylacetylene to produce a mixture of two regioi-
somers, 4,5-diphenyl-1,2-disilabenzene (IIIa) and 3,5-diphenyl-
1,2-disilabenzene (IIIb) in 2 : 3 ratio. On the other hand, Power
and co-workers reported that the reaction of a digermyne bearing
bulky terphenyl substituents underwent [2 + 2] cycloaddition with
alkynes, such as diphenylacetylene or trimethylsilylacetylene, to
give the corresponding 1,2-digermacyclobutadiene derivatives.5
Following these results, we turned our attention to the reactions
of diaryldisilyne 1 with several alkynes, from which the corre-
sponding 1,2-diaryl-1,2-disilabenzenes were obtained as major
products. Herein, we report the preliminary results obtained for
1,2-diaryl-1,2-disilabenzene 2, which exhibits the interesting trans-
bent nature of the Si–Si bond.
Scheme 1 Reactivity of 1 towards alkynes.
that 1,2-disilabenzene 2 was produced as the sole product. From
the concentrated hexane solution, pure 1,2-disilabenzene 2 was
isolated in 63% yield as light-yellow needles.
Treatment of
1
with trimethylsilylacetylene gave 3,5-
bis(trimethylsilyl)-1,2-disilabenzene 3 as the major product. Two
peaks at 64.1 and 65.4 ppm (C6D6) were observed for Si
atoms of the central ring in the 29Si NMR spectrum. While a
product mixture of 4,5- and 3,5-substituted isomer (2 : 3) has
been reported from the reaction of silyl-substituted disilyne I
with phenylacetylene,2c no significant signals for the 4,5- or 3,6-
substituted isomer could be observed in the NMR spectra of the
crude product. Although 3 was formed as the major product,
pure 3 could be isolated in only 25% yield due to its low
crystallinity and high solubility in organic solvents. Reaction of
1 with phenylacetylene in hexane gave an orange solution after
2 h at room temperature, where the complete consumption of 1
was confirmed by NMR analysis of the reaction mixture. As in
the reaction of Sekiguchi’s disilyne (I) with phenylacetylene,2c two
peaks at 55.0 and 61.7 ppm (C6D6) in 29Si NMR spectra strongly
suggested the formation of the corresponding 3,5-diphenyl-1,2-
disilabenzene 4 as the major product. However, the isolation of 4
as a pure compound was unsuccessful due to its low crystallinity.
Although a light-yellow solution was obtained from the reaction
of 1 with 1,7-octadiyne, the resulting NMR spectra suggested the
formation of a complex mixture. On the other hand, no reaction
was observed when 1 was treated with bis(trimethylsilyl)acetylene
for 8 h at room temperature, likely due to the steric hindrance.
The reactivity of 1 with various alkynes is summarized in
Scheme 1. When a hexane solution of 1 was treated with 1
atm acetylene gas at room temperature for 5 min, the dark-
yellow colour of 1 immediately disappeared and a light-yellow
solution was obtained. Using NMR analysis, it was evidenced
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto,
611-0011, Japan. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp; Fax: +82 774
38 3209; Tel: +81 774 38 3200
† Electronic supplementary information (ESI) available: Experimental
details, characterization data, and NMR spectra for 2 and 6. CCDC
reference numbers 768830 and 768831. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0dt00115e
‡ Current address: Nano Materials Research Center, Korea Institute of
Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-
791, Korea.
9238 | Dalton Trans., 2010, 39, 9238–9240
This journal is
The Royal Society of Chemistry 2010
©

Single crystals of 2 were obtained by recrystallization from
toluene and its molecular structure is shown in Fig. 1.§ The
central ring skeleton of 1,2-disilabenzene 2 has an almost planar
structure (the sum of the internal bond angles = 719.8◦). One
of the most unique structural features of 2 is the large torsion
angle of C(Bbt)–Si–Si–C(Bbt) (45.6◦), which is likely due to
the trans-bent character of the Si–Si bond (trans-bent angles =
12.1◦, 13.7◦). The sums of the bond angles around skeletal Si
atoms are 357.5◦ and 356.9◦. Such feature is in sharp contrast to
that of 1,2-bis(silyl)-4,5-diphenyl-1,2-disilabenzene IIIa, showing
virtually planar geometries around the two skeletal Si atoms [Si–
Si–Si–Si = 13.1(2)◦].2c
seen as an intermediate of a single and a double bond. Previously,
we reported the structure of 1-Tbt-silabenzene (Tbt = 2,4,6-
[CH(SiMe3)2]3-C6H2), showing C–C bond lengths of 1.381(6)–
4c
˚
1.399(6) A. The skeletal C–C bond lengths of 2 are in the range
˚
of 1.37–1.40 A and closely resemble that of silabenzene, suggesting
the magnitude of the p-electron delocalization is similar to the case
of a monosilabenzene. The longest wavelength absorption maxima
of 2 in n-hexane was observed at 378 nm (e 11 000), which is further
red-shifted than the value of 1-Tbt-silabenzene (331 nm).4c
The disilabenzene 2 is highly oxygen-sensitive. Exposure of 2
under 1 atm molecular oxygen produced a colourless product
6 in quantitative yield (Scheme 2). Single crystals suitable for
X-ray structure analysis§ were obtained from a toluene and hexane
solution, and its crystal structure is shown in Fig. 2. Based on
the analysis, it was evident that the Si–Si bond was oxidized
to form a 1,3,2,4-dioxadisiletane ring, which is bridged by a
butadiene chain. Consistent with previous findings,1b,7 a short
˚
distance [2.3777(7) A] between the two Si atoms in the 1,3,2,4-
dioxadisiletane ring was observed, which is comparable to a typical
˚
Si–Si single bond length (2.34 A). Although the ring connected
with the butadiene chain is slightly distorted,8 the relatively short
distance between the two Si atoms and symmetrical structure of
6 provide a good model to compare with 2. We also performed
theoretical calculations for 1,2-diaryl-1,2-disilabenzene 2-A, which
has two 2,6-bis[bis(trimethylsilyl)methyl]benzene groups instead
of Bbt groups of 2.
Fig. 1 Thermal ellipsoid (50%) drawing of [2·0.5(C7H8)] [(a) top view
of the disilabenzene ring; (b) side view of the disilabenzene ring].
Hydrogen atoms, solvate, and methyl groups on the Bbt gro◦ups have been
Fig. 2 Thermal ellipsoid (50%) drawing of [6·C7H8·C6H14]. Hydrogen
˚
omitted for clarity. Selected bond lengths (A) and angles ( ): Si(1)–Si(2)
atoms, solvates, and methyl groups on the Bbt groups have been omitted for
2.2334(7), Si(1)–C(1) 1.8047(19), C(1)–C(2) 1.377(3), C(2)–C(3) 1.400(3),
C(3)–C(4) 1.372(3), C(4)–Si(2) 1.806(2), Si(1)–C(1)–C(2) 131.04(15),
C(1)–C(2)–C(3) 126.51(18), C(2)–C(3)–C(4) 126.02(18), C(3)–C(4)–Si(2)
130.90(15), C(4)–Si(2)–Si(1) 102.98(7), Si(2)–Si(1)–C(1) 102.37(6),
C(1)–Si(1)–C(5) 106.48(8), C(5)–Si(1)–Si(2) 148.67(6), C(4)–Si(2)–C(35)
106.96(8), C(35)–Si(2)–Si(1) 146.98(6), C(5)–Si(1)–Si(2)–C(35) 45.6(2).
◦
˚
clarity. Selected bond lengths (A) and angles ( ): Si(1)–O(1) 1.6910(14),
O(1)–Si(2) 1.6904(14), Si(2)–O(2) 1.6931(14), O(2)–Si(1) 1.6888(13),
Si(1)–Si(2) 2.3777(7), Si(1)–C(1) 1.862(2), C(1)–C2 1.342(3), C(2)–C(3)
1.467(3), C(3)–C(4) 1.343(3), C(4)–Si(2) 1.857(2), Si(1)–C(1)–C(2)
127.45(16), C(1)–C(2)–C(3) 129.3(2), C(2)–C(3)–C(4) 129.7(2),
C(3)–C(4)–Si(2) 127.84(18), C(4)–Si(2)–Si(1) 101.93(7), Si(2)–Si(1)–C(1)
102.58(7), Si(1)–O(1)–Si(2) 89.37(7), O(1)–Si(2)–O(2) 87.77(7),
Si(2)–O(2)–Si(1) 89.35(7), O(2)–Si(1)–O(1) 87.89(7), C(1)–Si(1)–C(5)
109.78(9), C(5)–Si(1)–Si(2) 147.56(6), C(4)–Si(2)–C(35) 109.32(9),
C(5)–Si(1)–Si(2)–C(35) 18.16(19).
In our previous studies, we reported the structure of 1,2-
disilacyclobutene 5, which has two Bbt groups in cis geometry,
˚
=
a trans-bent structure with a Si Si bond distance of◦2.213(3) A,
3c
=
and a C(Bbt)–Si Si–C(Bbt) torsion angle of 103.3(8) .
Scheme 2 Oxidation of 2.
Although 2 has a similar cis-1,2-bis(Bbt) geometry as 5, the
Si–Si bond distance [2.2334(7) A] is slightly longer than those
˚
In Table 1, selected structural features and chemical shifts were
compared among 2, 2-A, and 6. Although optimized geometry
of 2-A has asymmetrical Si atoms, the calculated 29Si chemical
of disilenes having aromatic substituents in cis geometry, which
vary from 2.140 to 2.21 A.3c,6 Therefore, the Si–Si bond of 2 can be
˚
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9238–9240 | 9239
©

˚
Table 1 Selected chemical shifts (ppm), bond lengths (A), and dihedral
Notes and references
angles (◦)
§ Crystal data for [2·0.5(C7H8)]. C67.5H142Si16, M = 1403.25, monoclinic,
3
˚
˚
2-Aa
6
a = 18.524(2), b = 12.0037(14), c = 40.568(5) A, U = 8836.5(19) A , T =
103(2) K, space group P21/c (no. 14), Z = 4, 80 148 reflections measured,
15 370 independent reflections (Rint = 0.0709), R1 = 0.0441 [I > 2s(I)],
wR2 = 0.1284 (all data).
2
d Sib
57.1
61.40, 63.43
8.40, 8.33
8.02, 8.06
2.245
1.812
1.813
-10.1
dH(1)b
7.73–7.79 (m)
7.61–7.68 (m)
2.2334(7)
1.8047(19)
1.806(2)
1.377(3)
1.372(3)
6.64–6.71 (m)
6.12–6.19 (m)
2.3777(7)
1.862(2)
Crystal data for [6·C7H8·C6H14]. C77H160O2Si16, M = 1567.49, triclinic, a =
dH(2)b
3
˚
˚
16.8221(2), b = 17.7685(2), c = 18.7611(2) A, U = 4879.28(10) A , T =
dSi(1)–Si(2)
dSi(1)–C(1)
dSi(2)–C(4)
dC(1)–C(2)
dC(3)–C(4)
dC(2)–C(3)
∠Ar–Si–Si–Ar
¯
103(2) K, space group P1 (no. 2), Z = 2, 40 744 reflections measured,
16 924 independent reflections (Rint = 0.0296), R1 = 0.0452 [I > 2s(I)],
1.857(2)
1.342(3)
1.343(3)
1.467(3)
wR2 = 0.1244 (all data).
1.383
1.383
1 (a) A. Sekiguchi, R. Kinjo and M. Ichinohe, Science, 2004, 305, 1755–
1757; (b) N. Wiberg, S. K. Vasisht, G. Fischer and P. Mayer, Z. Anorg.
Allg. Chem., 2004, 630, 1823–1828.
1.400(3)
45.6(2)
1.414
60.59710
18.16(19)
2 (a) A. Sekiguchi, M. Ichinohe and R. Kinjo, Bull. Chem. Soc. Jpn.,
2006, 79, 825–832; (b) R. Kinjo, M. Ichinohe and A. Sekiguchi, J. Am.
Chem. Soc., 2007, 129, 26–27; (c) R. Kinjo, M. Ichinohe, A. Sekiguchi,
N. Takagi, M. Sumimoto and S. Nagase, J. Am. Chem. Soc., 2007, 129,
7766–7767; (d) K. Takeuchi, M. Ichinohe and A. Sekiguchi, J. Am.
Chem. Soc., 2008, 130, 16848–16849; (e) K. Takeuchi, M. Ichinohe, A.
Sekiguchi, J.-D. Guo and S. Nagase, Organometallics, 2009, 28, 2658–
2660; (f) A. Sekiguchi, R. Kinjo and M. Ichinohe, Synth. Met., 2009,
159, 773–775; (g) K. Takeuchi, M. Ichinohe, A. Sekiguchi, J.-D. Guo
and S. Nagase, J. Phys. Org. Chem., 2010, 23, 390–394; (h) K. Takeuchi,
M. Ikoshi, M. Ichinohe and A. Sekiguchi, J. Am. Chem. Soc., 2010, 132,
930–931.
3 (a) T. Sasamori, K. Hironaka, Y. Sugiyama, N. Takagi, S. Nagase, Y.
Hosoi, Y. Furukawa and N. Tokitoh, J. Am. Chem. Soc., 2008, 130,
13856–13857; (b) T. Sasamori, J. S. Han, K. Hironaka, N. Takagi, S.
Nagase and N. Tokitoh, Pure Appl. Chem., 2010, 82, 603–612; (c) J. S.
Han, T. Sasamori, Y. Mizuhata and N. Tokitoh, J. Am. Chem. Soc.,
2010, 132, 2546–2547.
a For calculations, 2,6-bis[bis(trimethylsilyl)methyl]benzene group was
substituted instead of Bbt; structural optimizations: B3PW91/6-311G(3d)
for Si, 6-31G(d) for C, H; GIAO calculations: B3PW91/6-311+G(3df) for
Si, 6-311G(2d,p) for C, H. b 300 MHz for 1H and 59 MHz for 29Si; C6D6
was used as a solvent for 2 and 6.
shifts of the central silicon atoms of 2-A are close to the signal
observed from 2 (d 57.1). As expected, a 29Si NMR signal of
the ring Si atoms of 6 was observed at highly upfield region
(d -10.1). The central ring protons of 2 were observed more than
1 ppm downfield compared to 6. These downfield shifts should be
interpreted as a result of ring current effect of the 1,2-disilabenzene
ring. Although, the central rings of 2 and 6 have structural
similarities, remarkable differences in the bond distances of their
central rings were observed. Compared to 6, 2 has significantly
shortened Si–C and C(2)–C(3) bond distances, which can be
interpreted as indicative of its aromatic property. Furthermore, the
good agreement in bond distances with the theoretical calculations
supports the delocalization of p-electrons of 2. The calculated
NICS(1) (nucleus-independent chemical shift)9 values of 2-A are
-8.15 and -8.14 of which the significant negative values provide
evidence for its aromaticity.
4 (a) N. Tokitoh, K. Wakita, R. Okazaki, S. Nagase, P. v. R. Schleyer
and H. Jiao, J. Am. Chem. Soc., 1997, 119, 6951–6952; (b) K. Wakita,
N. Tokitoh, R. Okazaki and S. Nagase, Angew. Chem., Int. Ed., 2000,
39, 634–636; (c) K. Wakita, N. Tokitoh, R. Okazaki, N. Takagi and
S. Nagase, J. Am. Chem. Soc., 2000, 122, 5648–5649; (d) N. Tokitoh,
Acc. Chem. Res., 2004, 37, 86–94; (e) N. Tokitoh, Bull. Chem. Soc. Jpn.,
2004, 77, 429–441.
5 C. Cui, M. M. Olmstead and P. P. Power, J. Am. Chem. Soc., 2004, 126,
5062–5063.
6 (a) M. J. Fink, M. J. Michalczyk, K. J. Haller, R. West and J. Michl,
Organometallics, 1984, 3, 793–800; (b) S. Masamune, S. Murakami,
J. T. Snow, H. Tobita and D. J. Williams, Organometallics, 1984, 3,
333–334; (c) H. Watanabe, K. Takeuchi, N. Fukawa, M. Kato, M.
Goto and Y. Nagai, Chem. Lett., 1987, 1341–1344; (d) H. Suzuki, N.
Tokitoh, R. Okazaki, J. Harada, K. Ogawa, S. Tomoda and M. Goto,
Organometallics, 1995, 14, 1016–1022.
Gathering all spectral and structural features of 2 discussed
above, it is clear that 2 has delocalized p-electrons. Because
2 is the first example of 1,2-disilabenzene having the trans-
bent structural feature, it will be interesting to study how the
1
conjugation takes place through a trans-bent bond. In H and
13C NMR spectra, two independent signals were observed for the
SiMe3 groups at the ortho-positions of Bbt groups even at 70 ◦C,
suggesting that 2 retains the trans-bent conformation even in warm
solution.11
7 (a) R. S. Grev and H. F. Schaefer, III, J. Am. Chem. Soc., 1987, 109,
6577–6585; (b) H. B. Yokelson, A. J. Millevolte, B. R. Adams and R.
West, J. Am. Chem. Soc., 1987, 109, 4116–4118; (c) K. L. McKillop, G.
R. Gillette, D. R. Powell and R. West, J. Am. Chem. Soc., 1992, 114,
5203–5208; (d) A. J. Millevolte, D. R. Powell, S. G. Johnson and R.
West, Organometallics, 1992, 11, 1091–1095.
In summary, the 1,2-diaryldisilyne 1 showed high reactivity
with alkynes, especially unhindered alkynes. Both polar and non-
polar alkynes reacted with 1 to give 1,2-disilabenzene as the major
product. Crystal structure analysis showed that the central ring of
1,2-diaryl-1,2-disilabenzene 2 is almost planar, and most notably,
the Si–Si bond has significant trans-bent character. Evidence
supporting the aromatic structure of 2 was collected by both
experimental and theoretical means.
8 The sum of the internal bond angles for a formal 6-membered ring
connected through Si(1)–C(1)–C(2)–C(3)–C(4)–Si(2) is 718.8◦.
˚
9 Reverse signed magnetic shielding computed at 1 A above the ring
center as a probe of aromaticity, see: (a) P. v. R. Schleyer, C. Maerker,
A. Dransfeld, H. Jiao and N. J. R. v. E. Hommes, J. Am. Chem. Soc.,
1996, 118, 6317–6318; (b) P. v. R. Schleyer, H. Jiao, N. J. R. v. E.
Hommes, V. G. Malkin and O. L. Malkina, J. Am. Chem. Soc., 1997,
119, 12669–12670; (c) P. v. R. Schleyer, M. Manoharan, Z.-X. Wang,
B. Kiran, H. Jiao, R. Puchta and N. J. R. v. E. Hommes, Org. Lett.,
2001, 3, 2465–2468.
10 The theoretically optimized structure of the less hindered model, 1,2-
diphenyl-1,2-disilabenzene, exhibits completely planar structure with
C(Ph)–Si–Si–C(Ph) torsion angle of 0◦, suggesting the trans-bent
structure of 2 would be due to the severe steric repulsion of extremely
bulky Bbt groups.
11 When a Bbt group is connected directly to a chiral center, the two
trimethylsilyl groups at the ortho-benzyl position are in diastereomeri-
cally non-identical situation in NMR spectroscopy, see ref. 3a.
Acknowledgements
This work was supported by Grants-in Aid (17GS0207 and
20036024) and the Global COE Program from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan.
9240 | Dalton Trans., 2010, 39, 9238–9240
This journal is
The Royal Society of Chemistry 2010
©