Syntheses and characterization of bis(silacyclopropene) and disilabenzvalene

Full text of DOI:10.1021/ja9637412

J. Am. Chem. Soc. 1997, 119, 3629-3630
3629
Syntheses and Characterization of
Bis(silacyclopropene) and Disilabenzvalene
Wataru Ando,* Tadahiro Shiba, Takahiro Hidaka,
Kenji Morihashi, and Osamu Kikuchi
Department of Chemistry, UniVersity of Tsukuba
Tsukuba, Ibaraki 305, Japan
Figure 1. Crystal structure of 1. Selected bond distances (Å) and
angles (deg): 2.313(5) [Si(1)-Si(2)], 1.877(5) [Si(1)-C(1)], 1.880(5)
[Si(1)-C(2)], 1.560(7) [C(1)-C(2)], 1.520(8) [C(1)-C(5)], 1.504(7)
[C(1)-C(6)], 1.522(7) [C(2)-C(7)], 1.513(6) [C(2)-C(8)], 49.1 [C(1)-
Si(1)-C(2)], 65.6 [Si(1)-C(1)-C(2)], 65.4 [Si(1)-C(2)-C(1)].
ReceiVed October 28, 1996
In continuation of our program on the syntheses and studies
of small-ring silicon compounds,1 we were desired to prepare
1,2-bissilylene, which is an equivalent of disilyne. Earlier
papers have described the isolation of trapped products believed
to arise from disilyne2 and 1,4-disilabenzene3,4 in the decom-
position of di(7-silanorbornadin-7-yl) derivatives. However,
there is no unambiguous evidence on the intermediacy of the
disilynes. Herein, we report the syntheses and structural
characterizations of the first isolable bis(silacyclopropene) and
disilabenzvalene.
The synthesis of the precursor bis(silacyclopropane) 1 is
carried out as follows: diphenyltetrakis(1-bromo-1-methylethyl)-
disilane was allowed to react with magnesium by using a
modification of the method of Seyferth.5 Pure, neat 16,7 is stable
but slowly decomposes or polymerizes in solution (half-life of
17 min at 65 °C) (eq 1, Figure 1).
Figure 2. Crystal structure of 2. Selected bond distances (Å) and
angles (deg): 2.296(3) [Si(1)-Si(2)], 1.835(9) [Si(1)-C(1)], 1.835(9)
[Si(1)-C(2)], 1.84(1) [Si(2)-C(3)], 1.80(1) [Si(1)-C(4)], 1.32(1)
[C(1)-C(2)], 1.33(1) [C(3)-C(4)], 119.2(3) [Si(2)-Si(1)-C(1)], 116.3(3)
[Si(2)-Si(1)-C(2)], 116.3(3) [Si(2)-Si(1)-C(3)], 120.3(3) [Si(2)-
Si(1)-C(4)], 68.9(6) [Si(1)-C(1)-C(2)], 68.8(6) [Si(1)-C(2)-C(1)],
67.3(6) [Si(2)-C(3)-C(4)], 70.0(6) [Si(2)-C(4)-C(3)], 42.3(5) [C(1)-
Si(2)-C(2)], 42.7(5) [C(3)-Si(2)-C(2)].
Scheme 1
Thermolysis of 1 with bis(trimethylsilyl)acetylene at 60 °C
affords bis(silacyclopropene) 2 (61%, unstable with respect to
hydrolysis and oxidation) (Scheme 1): 8 m/e 550.2215, calcd
for C28H46Si6, m/e 550.2215, mp 72-75 °C; 1H NMR (C6D6,
300 MHz) δ 0.30 (s, 36H), 7.05-7.07 (m, 6H), 7.54-7.57 (m,
4H); 13C NMR (C6D6, 75 MHz) δ -0.1 (q), 128.0 (d), 129.2
(d), 134.6 (d), 137.9 (s), 173.8 (s); 29Si NMR (C6D6, 60 MHz)
δ -143.2, -9.4; m/e 550 (M+), 535 (M+ - Me). The 29Si
(1) (a) Kabe, Y.; Ando, W. AdV. Strain Org. Chem. 1993, 3, 59. (b)
Hojo, F.; Ando, W. Synlett 1995, 880. (c) Ando, W. Bull. Chem. Soc. Jpn.
1996, 69, 1.
(2) Sekiguchi, A.; Zigler, S. S.; West, R. J. Am. Chem. Soc. 1986, 108,
4241.
(3) Sekiguchi, A.; Gillet, G. R.; West, R. Organometallics 1988, 7, 1226.
(4) Rich, J. D.; West, R. J. Am. Chem. Soc. 1982, 104, 6884.
(5) (a) Seyferth, D.; Annarelli, D. C. J. Am. Chem. Soc. 1975, 97, 2273.
(b) Seyferth, D.; Lambert, R. L., Jr.; Annarelli, D. C. J. Organomet. Chem.
1976, 122, 311.
(6) A solution of 8.26 g (11.8 mmol) of diphenyltetrakis(1-bromo-1-
methylethyl)disilane in 50 mL of THF was added dropwise to an excess of
pure magnesium (99.95% from Aldrich, 895 mg, 36.8 mmol) in 10 mL of
THF. Upon workup, 1 was isolated a white solid (1.48 g, 33 %); the pure
compound was obtained by recrystallization from hexane/benzene (mp 124
°C (dec.), unstable for air and water).
chemical shifts for the ring silicon atoms are highly unusual.9
The crystal structure was solved by direct methods, and the
molecular structure of 2 is shown in Figure 2.10 The observed
silicon-silicon bond distance is significantly shorter than found
for a normal silicon-silicon single bond. Bis(trimethylsilyl)-
(7) The crystal structure of 1 is shown in Figure 1. Crystal data for 1:
fw ) 378.1, monoclinic, a ) 39.096(6) Å, b ) 6.766 (2) Å, c ) 17.125(3)
Å, â ) 97.81(1)°, V ) 4581.3 Å3, space group C2/c, Z ) 8, µ (Mo KR) )
1.5 cm-1, F (calcd) ) 1.01 g/cm3. The 4541 independent reflections (|Fo|
2
> 3.0σ|Fo |, 2θ e 50.0°) were measured on an Enraf-Nonius CAD4
(9) Seyferth, D.; Annarelli, D. C.; Vick, S. C. J. Am. Chem. Soc. 1976,
98, 6382.
diffractometer using Mo KR irradiation and an ω - 2θ scan. An empirical
absorption correction based on a series of ψ scans were applied to the data
0.88/1.00. The structure was solved by direct methods, and hydrogen atoms
were added to the structure factor calculations but their positions were not
refined anisotropically to R ) 0.052 (Rw ) 0.065).
(10) Crystal data for 2: fw ) 551.20, triclinic, a ) 9.797(1) Å, b )
10.310(7) Å, c ) 19.188(9) Å, R ) 87.24(5)°, â ) 79.49(6)°, γ ) 72.94(7)°,
V ) 4581.3 Å3, space group P-1, Z ) 2, µ (Mo KR) ) 2.4 cm-1, F (calcd)
) 1.00 g/cm3. The 6787 independent reflections (|Fo| > 3.0σ|Fo |, 2θ e
2
(8) The benzene solution of 1 (200 mg, 0.53 mmol) with bis(trimeth-
ylsilyl)acetylene (1.3 g, 7.6 mmol) in a degassed sealed tube was stirred at
60 °C for 15 h. The volatile materials were removed under reduced pressure,
and the residue was chromatographed with a Florisil column eluted by
hexane to give 2 (170 mg). The solid 2 was recrystalized from hexane at
-20 °C.
50.0°) were measured on an Enraf-Nonius CAD4 diffractometer using Mo
KR irradiation and an ω - 2θ scan. An empirical absorption correction
based on a series of ψ scans were applied to the data 0.78/1.00. The
structure was solved by direct methods, and hydrogen atoms were added
to the structure factor calculations but their positions were not refined
anisotropically to R ) 0.078 (Rw ) 0.088).
S0002-7863(96)03741-9 CCC: $14.00 © 1997 American Chemical Society

3630 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Communications to the Editor
Scheme 2
Figure 3. Crystal structure of 7. Selected bond distances (Å) and
angles (deg): 1.858(5) [Si(1)-C(1)], 1.897(5) [Si(1)-C(2)], 1.680(7)
[C(1)-C(1′)], 1.344 [C(2)-C(2′)], 2.549 [Si(1)-Si(1′)], 108.5 [C(1)-
Si(1)-C(2)], 54.2 [C1)-Si(1)-C(1′)], 108.5 [Si(1)-C(2)-C(2′)], 62.0
[Si(1)-C(1)-C(1′)], 87.4 [Si(1)-C(1)-Si(1′)].
acetylene is an efficient trapping reagent for silylene 3.
Evidence for the intermediacy of silylene 3 was obtained by
thermolyzing 1 in the presence of triethylsilane or anthracene,
which afforded the 2,3-diphenyltetrasilane 4 (79%) or the adduct
6 (101 mg, 85%).11 Silylene 3, in the presence of an inefficient
trapping reagent undergoes the ring expansion to disilene 5
which upon Diels-Alder reaction affords 6. The possible
involvement of disilyne may not be considered under the
reaction conditions.
Scheme 3
Bis(silacyclopropene) 2 is expected to be an intermediate in
a number of interesting isomerizations of disilabenzene. Ther-
molysis of 2 in a degassed sealed tube at 120 °C in benzene
afforded the disilabenzvalene 7 (82%) (Scheme 2). The
structure of 7 (mp 161-164 °C, air and moisture sensitive) was
initially deduced from its HRMS (calcd for C28H46Si6 550.2203,
found 550.2215): 1H NMR (C6D6, 300 MHz) δ 0.08 (s, 18H),
0.20 (s, 18H), 7.15-7.17 (m, 6H), 7.96-7.99 (m, 4H); 13C NMR
(C6D6, 75 MHz) δ 2.39 (q), 2.83 (q), 37.9 (s), 128.1 (d), 130.7
(d), 131.3 (s), 137.6 (d), 182.8 (s); 29Si NMR (C6D6, 60 MHz)
δ -61.9, -6.5, -4.5; UV 312 nm (ꢀ 4495). The crystal
structure was solved by direct methods, and the molecular
structure of 7 is shown in Figure 3.12 The silicon-silicon
internuclear distance is 2.549 Å, which due to a transannular
bond between two silicon atoms.
3). Thus, the disilabenzvalene 7a and the Dewar benzene 8a
have been found to have similar energies and are slightly less
stable than 1,4-disilabenzene (9a).13,14 Since large basis sets
and inclusion of electron correlation generally tend to favor
tricyclic over bicyclic or monocyclic geometries,15 7a, 8a, and
9a may have quite similar energies; the stability order may even
be reversed. An alternative mechanism involving the 1,2-
migration of silyl group to adjacent olefinic carbon and
formation of silylene 10a is proposed to account for bis-
(silacyclopropene) rearrangement. Since the structure retains
one silacyclopropene ring, the true energy change involved in
the silacyclopropene by silylene is expected to be very small.
This is supported by HF/6-31G* calculations.13 Interestingly,
the isomerization of 2 to 7 proceeds much more rapidly in
AgBF4 which functions as a catalyst (half-life of 5 min at 20
°C). The reaction is proposed as an alternative route for the
silylene intermediate via a cationic species. We are continuing
to explore this system in search of new information.
The isomerization of 2 could be considered as occuring via
the initial formation of an intermediate Dewar silabenzene or
1,4-disilabenzene and their subsequent isomerization to disila-
benzvalene. To evaluate the stabilities of 2, the relative energies
of the corresponding valence isomers (2a, 7a-10a) were
determined by 6-31G* molecular orbital calculations (Scheme
Acknowledgment. This work was supported by Grant-in-Aid for
Scientific Research for the Ministry of Education, Science, and Culture,
Japan. We thank Shin-Etsu Chemical Co., Ltd., for a gift of
organosilicon reagents.
(11) A mixture of 1 (100 mg, 0.264 mmol) and anthracene (471 mg,
2.64 mmol) in 5 mL of benzene was heated in a degassed sealed tube at
150 °C for 2 h, producing tetramethylethylene (∼70 %) as well as of 1,2-
diphenyl-1,2-disilacyclobutene 5 (Scheme 1). Compound 6: yellow crystal,
Supporting Information Available: Crystallographic details tables
of atomic positional parameters with estimated standard deviations (54
pages). See any current masthead page for ordering and Internet access
instructions.
1
mp 145 °C (dec.); H NMR (C6D6, 300 MHz) δ 0.53 (s, 6H), 0.8 (s, 6H),
4.60 (s, 2H), 6.74-6.87 (m, 2H), 7.08-7.46 (m, 14H); 13C NMR (C6D6,
75 MHz) δ 21.8 (q), 24.8 (q), 34.3 (s), 43.8 (d), 124.6 (d), 125.0 (d), 125.7
(d), 126.3 (d), 127.7 (d), 129.5 (d), 131.6 (s), 136.1 (d), 139.9 (s), 140.1
(s); 29Si NMR (C6D6, 60 MHz) δ 9.3.
JA9637412
(13) Calculated relative energies of 1,4-disilabenzene isomers by HF/6-
31G* (for optimized structures in Scheme 2): 2a is 32.7 kcal/mol higher
than a planar form of 1,4-disilabenzene (9a), while the disilabenzvalene
(7a) and the Dewar form (8a) are only 8.5 and 2.3 kcal/mol higher than
9a. Isomers, 7a, 8a, and 9a have quite similar energies; the stabilities order
may even be reversed. Silylene (10a) is less stable by 3.6 kcal/mol than
2a (Scheme 3).
(14) Chandrasekhar, J.; Schleyer, P. v. R. J. Organomet. Chem. 1985,
289, 51.
(15) Witeside, R. A.; Krishnan, R.; Defrees, D. J.; Pople, J. A.; Schleyer,
P. v. R. Chem. Phys. Lett. 1981, 78, 538.
(12) Crystal data for 7: fw ) 551.20, monoclinic space group P2/n, a
) 9.251(3) Å, b ) 11.884(4) Å, c ) 16.814(6) Å, â ) 101.54(3)°, V )
1811.1 Å3, Z ) 2, F ) 1.01 g/cm3, µ (Mo KR) ) 4.6 cm-1
.
The 3448
2
independent reflections (|Fo| > 3.0σ|Fo |, 2θ e 50.0°) were measured on
an Enraf-Nonius CAD4 diffractometer using Mo KR irradiation and an ω
- 2θ scan. An empirical absorption correction based on a series of ψ
scans were applied to the data 0.78/1.00. The structure was solved by direct
methods, and hydrogen atoms were added to the structure factor calculations
but their positions were not refined anisotropically to R ) 0.054 (Rw
0.058).
)