8894
C. Chiappe et al. / Tetrahedron Letters 47 (2006) 8893–8895
1603; (d) Wiberg, N.; Niedermayer, W.; Polborn, K.;
Mes
Mes
Mes
ii
iii
Cl
Cl
Mes
Mayer, P. Chem. Eur. J. 2002, 8, 2730; (e) Hajgato, B.;
Takahashi, M.; Kira, M.; Veszpremi, T. Chem. Eur. J.
2002, 8, 2126; (f) Bravo-Zhivotovskii, D.; Melamed, S.;
Kapon, M.; Apeloig, Y. Organometallics 2002, 21, 2049;
For recent theoretical studies, see: (g) Auer, D.; Strohmann,
C.; Arbuznikov, A. V.; Kaupp, M. Organometallics 2003,
22, 2442; (h) Takahashi, M.; Sakamoto, K. Organometallics
2002, 21, 4212.
i
Si
Si
Mes
Mes
1
Mes Br
Si
Si
Mes
n
Mes
2
3
4a n=1
4b n=2
4c n=3
Scheme 1. Reagents and conditions: (i) tert-BuLi (2 mol), THF, À78
to 0 ꢁC, then SiCl4; (ii) KC8, THF, 0 ꢁC; (iii) hexane, UV irradiation
with 70 W high pressure mercury lamp, 0 ꢁC.
2. For a recent overview see: (a) Lenoir, D.; Chiappe, C.
Chem. Eur. J. 2003, 9, 1036; (b) Chiappe, C.; Detert, H.;
Lenoir, D.; Pomelli, C. S.; Ruasse, M. F. J. Am. Chem. Soc.
2003, 125, 2864; (c) Chiappe, C.; De Rubertis, A.; Detert,
H.; Lenoir, D.; Schleyer, P. von R.; Wannere, C. S. Chem.,
Eur. J. 2002, 8, 967.
3. Tan, R.; Yokelson, H. B.; Gillette, G. R.; West, R. Inorg.
Synth. 1992, 29, 19.
4. Kira, M.; Ohya, S.; Iwamoto, T.; Ichinohe, M.; Kabuto, C.
Organometallics 2000, 19, 1817, See also Ref. 1b.
5. Masamune, S.; Murakami, S.; Hanzawa, Y.; Bally, T.;
Blount, J. F. J. Am. Chem. Soc. 1982, 104, 1150.
6. West, R. Pure. Appl. Chem. 1984, 56, 163.
7. Fink, M. J.; De Young, D. J.; West, R. J. Am. Chem. Soc.
1983, 105, 1070.
8. All reactions were carried under an argon atmosphere and
the solvents were freshly distilled from sodium/benzophe-
none under argon. For the photochemical reaction, a 70 W
Hg high-pressure immersion lamp (Hanau model TQ81)
was used. HPLC–UV–ESI-MS spectra: compounds were
separated on a C-18 reverse phase column (Phenomenex
Luna C 18(2); 3; 150 · 2 mm.); the eluant method consisted
of a 30-min linear gradient from 90:10 to 95:5 water
(containing 10 mmol/l ammonium acetate) and 2-propanol;
UV apparatus was set at 230 nm; mass spectra were
obtained using a Thermo Finnigan TSQ 7000 triple-
quadrupole mass spectrometer equipped with an electro
spray ionization source and a closed collision gas cham-
ber. UV spectra were recorded on a Cary 2200 spectro-
photometer.
filtration through silica gel using hexane as the eluant.
The composition of the mixture was determined on the
basis of HPLC–MS analysis: cyclic trisilane 4a turned
out to be the main component (79%), whereas tetrasi-
lane 4b (7%) and pentasilane 4c (14%) were present in
lower but still significant proportions. The attempts to
separate 4a from the mixture of cyclic polysilanes 4 by
recrystallization were unsuccessful. We found, however
that the above mixture of cyclic silanes 4 can be used
to produce disilene 1 in a photochemical reaction; this
is consistent with the general tendency of polysilanes
to generate silylenes, whose dimerization leads to disil-
enes. Thus, when an hexane solution of 4 in a quartz
Schlenk flask was irradiated with a high pressure mer-
cury lamp at 0 ꢁC, a permanent yellow colouration
developed after 30 min; UV analysis showed the absorp-
tion maximum at 422 nm typical of 1 and, from its
intensity, an almost complete conversion (>97% yield)
of 4a–c into 1 was calculated.6 The solution of 1 from
the photolysis of 4a–c under the above conditions
turned out to be remarkably stable up to 7 days at
0 ꢁC, as shown by UV monitoring. No attempt was
made to isolate pure 1 from this solution. Further in-
direct evidence for the completeness of the photochemi-
cal conversion of 4a–c into 1 was provided by quenching
experiments: the reaction of 1 generated from 4a–c with
air or methanol gave a product distribution (NMR,
HPLC) practically identical to those obtained from
authentic 1.7,8
Dichlorodimesitylsilane (3). To a THF solution (200 mL) of
1 (14.93 g, 75.0 mmol), magnetically stirred while cooled in
a dry ice/acetone bath, tert-BuLi (100 mL of a 1.5 M
pentane solution, 150.0 mmol) was added dropwise and the
reaction mixture was further stirred at À78 ꢁC for 30 min.
A solution of tetrachlorosilane (6.37 g, 37.5 mmol) in THF
(50 mL) was then added dropwise. The cooling bath was
removed and the mixture was allowed to stir for 6 h. The
precipitate was removed by filtration and the filtrate
evaporated to dryness; the residue was digested in hexane
(50 mL); any undissolved material was removed by filtra-
tion and the filtrate was partially evaporated and cooled to
0 ꢁC, which caused the separation of 3 (9.08 g; 95%) as a
colourless crystalline solid, with a mp of 124 ꢁC (Lit.3 mp
121).
In conclusion, the mixture of cyclic polysilenes 4a–c,
cleanly obtained by the reductive coupling of dichloride
3, is a suitable precursor to 1 via photolysis. Preliminary
data have shown that solutions of 1 produced in this
way are suitable for accurate kinetic studies of Si@Si
double bond reactivity; these results will appear in due
course.
Mixture of cyclic silanes 4a–c. Potassium graphite was
prepared by heating potassium (0.87 g, 22.3 mmol) and
graphite (2.15 g, 180 mmol) together in Schlenk flask. THF
(20 mL) was added and to the resulting suspension mag-
netically stirred while cooled in a ice–water bath; dichloride
3 (3.37 g, 10 mmol) dissolved in THF (15 mL) was then
added dropwise. After 1 h stirring, external cooling was
discontinued and, after 1 h stirring at room temperature,
the precipitate was filtered off and the filtrate evaporated to
dryness. The residue was digested in hexane and the solid
material was filtered off; the filtrate was evaporated to give
a residue from which 4 (2.2 g, 83%) was obtained by flash
chromatography (eluant: hexane) as a white powder. Calcd
for C18H22Si: C 81.14, H 8.32; found: C 81.02, H 8.25. 29Si
NMR: (79.49 MHz): À51.3, À52.38, À52.95. HPLC–MS
Acknowledgement
We are indebted to Dr. Rudolf Vasold (University of
Regensburg, Germany) for HPLC-Mass Spectra.
References and notes
1. Reviews: (a) Okazaki, R.; West, R. Organomet. Chem.
1996, 39, 233; For recent synthetic and reactivity studies,
see: (b) Ichinohe, M.; Kinjo, R.; Sekiguchi, A. Organomet-
allics 2003, 22, 4261; (c) Samuel, M. S.; Jenkins, H. A.;
Hughes, D. W.; Baines, K. M. Organometallics 2003, 22,