A new approach to the synthesis of unsymmetrical disilenes and germasilene: Unusual 29Si NMR chemical shifts and regiospecific methanol addition

Article abstract of DOI:10.1021/om010419d

The reaction of dilithiosilanes (ⁱPr₃Si)₂SiLi₂ (1a) and (^tBu₂MeSi)₂SiLi₂ (1b) with diaryldichloro-silanes [Ar₂SiCl₂, Ar = Mes (2,4,6-trimethylphe

Full text of DOI:10.1021/om010419d

Organometallics 2001, 20, 4141-4143
4141
A New Ap p r oa ch to th e Syn th esis of Un sym m etr ica l
29
Disilen es a n d Ger m a silen e: Un u su a l Si NMR Ch em ica l
Sh ifts a n d Regiosp ecific Meth a n ol Ad d ition
Masaaki Ichinohe, Yoriko Arai, and Akira Sekiguchi*
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, J apan
Nozomi Takagi and Shigeru Nagase
Department of Theoretical Studies, Institute for Molecular Science,
Myodaiji, Okazaki 444-8585, J apan
Received May 21, 2001
Summary: The reaction of dilithiosilanes (iPr3Si)2SiLi2
(1a ) and (tBu2MeSi)2SiLi2 (1b) with diaryldichloro-
silanes [Ar2SiCl2, Ar ) Mes (2,4,6-trimethylphenyl) and
Tip (2,4,6-triisopropylphenyl)] quantitatively produced
the corresponding unsymmetrical 1,1-diaryl-2,2-bis(tri-
silylene, Ar2Si:; (B) photochemical cleavage of cyclo-
trisilanes; and (C) reductive coupling of dihalosilanes.
These general methods allow the preparation of di-
silenes of the type A2SidSiA2 and ABSidSiAB. Of
course, disilenes having differently substituted silicon
atoms, such as A2SidSiB2, are also accessible by these
methods; however, the resulting disilene A2SidSiB2 is
usually produced in a statistical mixture together with
A2SidSiA2 and B2SidSiB2.3c,4 Methods for the selective
synthesis of unsymmetrically substituted disilenes are
currently very limited.5 For a germasilene, a regio-
i
alkylsilyl)disilenes, Ar2SidSi(SiR3)2 (2, R3Si ) Pr3Si;
t
3, R3Si ) Bu2MeSi). A stable germasilene, Mes2Ged
Si(SiMetBu2)2 (4), was also synthesized by the reaction
of 1b with dimesityldichlorogermane. These compounds
reveal unusual 29Si NMR shifts and regioselective
methanol additions.
t
specific generation of Bu2SidGeMes2 (Mes ) 2,4,6-
Since tetramesityldisilene was first isolated as a
stable SidSi doubly bonded compound in 1981,1 many
other stable disilenes bearing various substituents such
as alkyl-, aryl-, silyl-, and heteroatom-containing groups
have been synthesized and characterized.2,3 Almost all
stable disilenes have been synthesized by three meth-
ods: (A) photolysis of linear trisilanes such as (Me3Si)2-
SiAr2 and consequent dimerization of the intermediate
trimethylphenyl) is reported by the photochemical reac-
tion of Si,Si′-di-tert-butyltetramesitylsiladigermirane.6
Recently, we have reported the successful preparation
and isolation of bis(triisopropylsilyl)dilithiosilane (1a )
by the reaction of 3,3-bis(triisopropylsilyl)-1,2-bis(tri-
methylsilyl)-3-silacyclopropene with lithium metal in
THF.7 We have also described some reactions of di-
lithiosilane with electrophiles such as chlorosilanes, and
dilithiosilane was found to be a promising reagent for
the synthesis of a variety of organosilicon compounds.7,8
We report here the reaction of dilithiosilane 1b with
dihalosilanes as electrophiles, which provides a new,
highly effective route for the selective synthesis of
unsymmetrical disilenes4,5 and germasilene.6,9 The un-
(1) West, R.; Fink, M. J .; Michl, J . Science 1981, 214, 1343.
(2) For the recent reviews on metallenes and dimetallenes of group
14 elements, see: (a) Weidenbruch, M. Eur. J . Inorg. Chem. 1999, 373.
(b) Power, P. P. Chem. Rev. 1999, 99, 3463. (c) Escudie´, J .; Ranaivon-
jatovo, H. Adv. Organomet. Chem. 1999, 44, 113.
(3) For tetraaryldisilenes: (a) Masamune, S.; Hanzawa, Y.; Mu-
rakami, S.; Bally, T.; Blount, J . F. J . Am. Chem. Soc. 1982, 104, 1150.
(b) Michalczyk, M. J .; West, R.; Michl, J . Organometallics 1985, 4, 826.
(c) Yokelson, H. B.; Maxka, J .; Siegel, D. A.; West, R. J . Am. Chem.
Soc. 1986, 108, 4239. (d) Watanabe, H.; Takeuchi, K.; Fukawa, N.;
Kato, M.; Goto, M.; Nagai, Y. Chem. Lett. 1987, 1341. (e) Tokitoh, N.;
Suzuki, H.; Okazaki, R. J . Am. Chem. Soc. 1993, 115, 10428. For
tetraalkyldisilenes: (f) Masamune, S.; Eriyama, Y.; Kawase, T. Angew.
Chem., Int. Ed. Engl. 1987, 26, 584. For 1,2-dialkyl-1,2-diaryldi-
silenes: (g) Michalczyk, M. J .; West, R.; Michl, J . J . Am. Chem. Soc.
1984, 106, 821. (h) Fink, M. J .; Michalczyk, M. J .; Haller, K. J .; Michl,
J .; West, R. Organometallics 1984, 3, 793. (i) Shephered, B. D.; Powell,
D. R.; West, R. Organometallics 1989, 8, 2664. For tetrasilyldisilenes:
(j) Kira, M.; Maruyama, T.; Kabuto, C.; Ebata, K.; Sakurai, H. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1489. (k) Kira, M.; Ohya, S.; Iwamoto,
T.; Ichinohe, M.; Kabuto, C. Organometallics 2000, 19, 1817. For
heteroatom-substituted disilenes: (l) Michalczyk, M. J .; West, R.;
Michl, J . Organometallics 1985, 4, 826. (m) Schmedake, T. A.; Haaf,
M.; Apeloig, Y.; Mu¨ller, T.; Bukalov, S.; West, R. J . Am. Chem. Soc.
1999, 121, 9479. For tetrasila-1,3-butadienes: (n) Weidenbruch, M.;
Willms, S.; Saak, W.; Henkel, G. Angew. Chem., Int. Ed. Engl. 1997,
36, 2503. For cyclotrisilenes: (o) Iwamoto, T.; Kabuto, C.; Kira, M. J .
Am. Chem. Soc. 1999, 121, 886. (p) Ichinohe, M.; Matsuno, T.;
Sekiguchi, A. Angew. Chem., Int. Ed. 1999, 38, 2194. For 1- and
2-disilagermirenes: (q) Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A. J . Am.
Chem. Soc. 2000, 122, 9034. For cyclotetrasilenes: (r) Kira, M.;
Iwamoto, T.; Kabuto, C. J . Am. Chem. Soc. 1996, 118, 10303. (s)
Wiberg, N.; Auer, H.; No¨th, H.; Knizek, J .; Polborn, K. Angew. Chem.,
Int. Ed. 1998, 37, 2869.
(4) Unsymmetrical tetraaryldisilenes of the A2SidSiB2 type were
generated by cophotolysis of two individual trisilane precursors: See
ref 3c and Yokelson, H. B.; Siegel, D. A. Millevolte, A. J .; Maxka, J .;
West, R. Organometallics 1990, 9, 1005.
(5) Unsymmetrical disilenes have been prepared by the reductive
dehaloganation of the corresponding dihalodisilane derivatives. For
Mes2SidSiTip2, see: (a) Weidenbruch, M.; Pellmann, A.; Pan, Y.; Pohl,
S.; Saak, W. J . Organomet. Chem. 1993, 450, 67. (b) For (tBuMe2Si)2Sid
Si(SiMeiPr2)2, see ref 3k.
(6) Kolleger, G. M.; Stibbs, W. G.; Vittal, J . J .; Baines, K. M. Main
Group Met. Chem. 1996, 19, 317.
(7) Sekiguchi, A.; Ichinohe, M.; Yamaguchi, S. J . Am. Chem. Soc.
1999, 121, 10231.
(8) (a) Tokitoh, N.; Hatano, K.; Sadahiro, T.; Okazaki, R. Chem. Lett.
1999, 931. (b) Hatano, K.; Tokitoh, N.; Takagi, N.; Nagase, S. J . Am.
Chem. Soc. 2000, 122, 4829.
(9) (a) Baines, K. M.; Cooke, J . A. Organometallics 1991, 10, 3419.
(b) Baines, K. M.; Cooke, J . A. Organometallics 1992, 11, 3487. (c)
Baines, K. M.; Cooke, J . A. Dixon, C. E.; Liu, H. W.; Netherton, M. R.
Organometallics 1994, 13, 631. Quite recently, we have reported the
synthesis of 1,1,2,3-tetrakis[di-tert-butyl(methyl)silyl]-4-phenyl-1,2-
disila-3-germacyclopenta-2,4-diene, which has a silole-type structure
with SidGe and CdC double bonds; see: (d) Lee, V. Ya.; Ichinohe, M.;
Sekiguchi, A. J . Am. Chem. Soc. 2000, 122, 12604. For 2-disila-
germirene with a SidGe double bond, see ref 3q.
10.1021/om010419d CCC: $20.00 © 2001 American Chemical Society
Publication on Web 09/05/2001

4142 Organometallics, Vol. 20, No. 20, 2001
Communications
Sch em e 1
Sch em e 2
These unsymmetrical disilenes are quite stable even in
solution, and over a couple of months at room temper-
ature no isomerization has been observed. This is in
contrast to the behavior of both unsymmetrical tetra-
aryl- and tetrasilyl-substituted disilenes of the A2Sid
SiB2 type, which isomerize at room temperature to give
an equilibrium mixture of the E- and Z-isomers of the
ABSidSiAB type.3c,k,4 The germasilene 4 is also quite
stable in solution, in constrast to the facile germasilene-
to-silylgermylene rearrangement by a 1,2-mesityl shift
of tetramesitylgermasilene.9b
usual 29Si NMR chemical shift of the sp2 silicon atoms
and the regiospecific addition of methanol are also
presented.
In the 29Si NMR spectrum of 2b, measured by an
inverse-gate pulse sequence, three signals are observed
at +8.2, +24.1, and +148.6 ppm with relative intensities
of 1:2:1. On the basis of 2D Si-H NMR spectroscopy,
the largest signal at +24.1 ppm is assigned to the
tBu2MeSi groups; the other two signals having equal
intensity at +8.2 and +148.6 ppm are assigned to the
silyl- and aryl-substituted sp2 silicon atoms, respec-
tively. The other disilenes also show similar 29Si NMR
chemical shifts of the sp2 silicon atoms: +152.3 (2a ),
+137.2 (3a ), +142.0 ppm (3b) for the aryl-substituted
sp2 silicon atoms; -0.8 (2a ), +14.0 (3a ), +14.9 ppm (3b)
for the silyl-substituted sp2 silicon atoms. The germa-
silene also exhibits a 29Si NMR resonance of the
tBu2MeSi-substituted sp2 silicon atom at +22.4 ppm,
which supports the validity of the above assignments.
The tetraaryldisilenes and tetrakis(trialkylsilyl)di-
silenes are reported to give signals of the sp2 silicon
atoms around 53-661,3d,4 and 131-156 ppm3j,k, respec-
tively. The resonances of sp2 silicon atoms substituted
by the aryl groups in 2 and 3 are largely downfield
shifted relative to that of tetraaryldisilene; in contrast,
the chemical shifts of sp2 silicon atoms substituted by
Dry oxygen-free THF was introduced by vacuum
transfer to a mixture of bis[di-tert-butyl(methyl)silyl]-
dilithiosilane (1b)10 and dichlorodimesitylsilane, and the
reaction mixture was stirred for 1.5 h at room temper-
ature. The color of the reaction mixture immediately
changed to red due to the formation of 1,1-dimesityl-
2,2-bis[di-tert-butyl(methyl)silyl]disilene (2b), which was
isolated quantitatively as a red-brown oil (Scheme 1).11
In a similar manner, 1,1-diaryl-2,2-bis(trialkylsilyl)-
disilenes (2a , 3a , and 3b) were also prepared by the
reaction of 1b with the corresponding diaryldichloro-
silanes in 92-96% yield. The germasilene 4 with a
SidGe double bond was also synthesized by the reaction
of the dilithiosilane 1b with dimesityldichlorogermane
in 94% yield.12
1
The structures of 2-4 were determined by mass, H,
13C, and 29Si NMR spectroscopies11-13 as well as by prod-
uct analysis of the reaction with methanol (vide infra).
(10) Dilithiosilane 1b was prepared from 3,3-bis[di-tert-butyl(meth-
yl)silyl]-1,2-bis(trimethylsilyl)-3-silacyclopropene in a manner similar
to that for 1a . See ref 7.
(11) Procedure for synthesis of 2b: Dry oxygen-free tetrahydrofuran
(ca. 2 mL) was introduced by vacuum transfer to a mixture of 3,3-bis-
[di-tert-butyl(methyl)silyl]-1,2-bis(trimethylsilyl)-3-silacyclopropene (98
mg, 0.19 mmol) and lithium metal (ca. 10 mg, 1.4 mmol), and then
the reaction mixture was stirred at room temperature for 1.5 h to give
a red solution of bis[di-tert-butyl(methyl)silyl]dilithiosilane (1b). After
the removal of solvent and bis(trimethylsilyl)acetylene in vacuo, 1b
was obtained as a pale yellow powder. In a glovebox, the lithium metal
pieces were removed and then dichlorodimesitylsilane (62 mg, 0.19
mmol) was added. Dry oxygen-free tetrahydrofuran (ca. 2 mL) was
introduced into the glass reaction vessel containing 1b and dichloro-
dimesitylsilane, and then the reaction mixture was stirred at room
temperature for 1 h to give a red-brown solution of 2b. The solvent
was removed in vacuo and then dry oxygen-free C6D6 (0.6 mL) was
introduced into the reaction mixture. After filtration of the resulting
lithium salt, the solvent was removed in vacuo to give pure 2b as a
red-brown oil (118 mg, 99%): 1H NMR (C6D6, δ) 0.09 (s, 6 H), 1.18 (s,
36 H), 2.06 (s, 6 H), 2.80 (s, 12 H), 6.72 (s, 4 H); 13C NMR (C6D6, δ)
-3.5, 21.1, 22.1, 26.0, 30.4, 128.7, 137.8, 139.4, 144.1; 29Si NMR (C6D6,
δ) 8.2, 24.1, 148.6; HRMS m/z calcd for C36H64Si4 608.4085, found
608.4089; UV-vis (hexane) λmax/nm (ꢀ) 289 (6700), 437 (3200).
(12) Spectroscopic data for 4: 94% yield; 1H NMR (C6D6, δ) 0.12 (s,
6 H), 1.16 (s, 36 H), 2.08 (s, 6 H), 2.71 (s, 12 H), 6.74 (s, 4 H); 13C NMR
(C6D6, δ) -3.7, 21.0, 21.9, 26.1, 30.3, 128.8, 137.5, 142.4, 145.2; 29Si
NMR (C6D6, δ) 22.4, 25.1; HRMS m/z calcd for C36H64GeSi3 654.3536,
found 654.3535; UV-vis (hexane) λmax/nm (ꢀ) 285 (7400), 424 (3200).
t
iPr3Si or Bu2MeSi groups are largely upfield shifted
relative to that of tetrakis(trialkylsilyl)disilene. The
reason for such unusual large upfield and downfield
shifts of the sp2 silicon atoms is not clear, but similar
trends are reproduced by a GIAO-B3LYP calculation of
the model compound (Me3Si)2SidSiMes2; the chemical
shift of the Me3Si-substituted silicon atom (47.8 ppm)
is far more upfield shifted than that of the mesityl-
substituted one (156.1 ppm).14
The disilenes and germasilene undergo regioselective
additions of methanol.15 Disilene 2b readily reacted with
methanol in THF to give the corresponding adduct 5 as
(13) For the spectral data of 2a , 3a , and 3b, see the Supporting
Information.
(14) The GIAO-B3LYP calculation was carried out using the Gauss-
ian 98 program with basis sets 6-311G(3d) for Si and 6-311(d) for C
and H. Geometry optimization was performed at the B3LYP/6-31G(d)
level.

Communications
Organometallics, Vol. 20, No. 20, 2001 4143
the difference in electronegativity of Si and C. Actually,
the calculated natural atomic charges on sp2 silicon
atoms in the model (Me3Si)2SidSiMes2 are -0.355 for
(Me3Si)2Si and +0.958 for Mes2Si at the B3LYP/6-31(d)
level. Thus, the methoxy group is attached to the more
positive silicon atom with mesityl substituents.
The germasilene 4 also reacted with methanol to give
a single product, 6,17 in which the methoxy group is
unexpectedly bonded to the germanium atom, as shown
in Figure 1. This result shows regioselectivity opposite
that of tetramesitylgermasilene, which gives [dimesityl-
(methoxy)silyl]dimesitylgermane, in which the methoxy
group is attached to the silicon atom.9a,b The regio-
selectivity in 4 can be well understood in terms of the
polarized Siδ-dGeδ+ double bond, caused by the differ-
ence in electronegativity of the substituents, as found
in the methanol addition to the disilenes 2 and 3. The
calculated atomic charges (-0.332 for (Me3Si)2Si and
+0.834 for Mes2Ge) in the model (Me3Si)2SidGeMes2 at
the B3LYP/6-31G(d) level also support the experimental
result.
F igu r e 1. Molecular structure of 6 with thermal ellipsoids
drawn at the 30% level (hydrogen atoms are omitted for
the clarity). Selected bond distances [Å]: Si1-Ge2 2.4614(8),
Si1-Si3 2.423(1), Si1-Si4 2.431(1), Ge2-O1 1.804(2),
Ge2-C19 2.003(3), Ge2-C28 1.986(3), Selected bond angles
[deg]: Ge2-Si1-Si3 106.1(0), Ge2-Si1-Si4 120.7(0), Si3-
Si1-Si4 120.9(0), Si1-Ge2-O1 97.6(1), Si1-Ge2-C19
124.6(1), Si1-Ge2-C28 112.0(1), O1-Ge2-C19 107.8(1),
O1-Ge2-C28 110.9(1) C19-Ge2-C28 103.7(1).
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research (Nos. 12042213,
13029015, 13440185) from the Ministry of Education,
Science and Culture of J apan, and TARA (Tsukuba
Advanced Research Alliance) fund.
a single product in quantitative yield, the structure of
which was determined by spectroscopic and X-ray
crystallographic methods.16 The methoxy group is re-
gioselectively bonded to the silicon atom substituted by
the two mesityl groups. In the unsymmetrical disilenes,
the sp2 silicon atom connected to the two mesityl groups
is more positive than the other sp2 silicon atom, due to
Su p p or tin g In for m a tion Ava ila ble: Tables giving the
details of the X-ray structure determination, thermal ellipsoid
plots, fractional atomic coordinates, anisotropic thermal pa-
rameters, bond lengths, and bond angles for 5 and 6, 1H NMR,
13C NMR, and 29Si NMR charts of 2b, and spectral data of 2a ,
3a , and 3b. This material is available free of charge via the
Internet at http://pubs.acs.org.
(15) For the mechanistic investigation of the addition of alcohols to
disilenes, see: Sakurai, H. The Chemistry of Organic Silicon Com-
pounds; Rappoport, Z., Apeloig, Y., Eds.; J ohn Wiley & Sons Ltd.: New
York, 1998; Vol. 2, Chapter 15, and references cited therein.
OM010419D
(16) Spectroscopic data for 5: Mp 203-204 °C; 1H NMR (C6D6, δ)
0.35 (s, 6 H), 1.12 (s, 18 H), 1.17 (s, 18 H), 2.09 (s, 6 H), 2.47 (s, 12 H),
3.35 (s, 3 H), 3.78 (s, 1 H), 6.70 (s, 4 H); 13C NMR (C6D6, δ) -1.7, 21.8,
25.5, 30.3, 30.4, 51.8, 129.8, 135.5, 138.8, 143.9; 29Si NMR (C6D6, δ)
-107.1, 5.4, 19.1. Anal. Calcd for C37H68OSi4: C, 69.30; H, 10.69.
Found: C, 69.61; H, 10.37. Crystal data of 5: C37H68OSi4, fw ) 641.27,
triclinic, space group P1h, a ) 10.9930(8) Å, b ) 11.2640(5) Å, c )
18.314(1) Å, R ) 103.009(4)°, â ) 97.239(3)°, γ ) 113.272(4)°, V )
1970.4(2) Å3, Z ) 2, dcalc ) 1.081 g‚cm-3, temperature 120 K, R )
0.0557 (I > 2σ(I)), Rw ) 0.1634 (all data), GOF ) 1.026.
(17) Spectroscopic data for 6: Mp 177-178 °C; 1H NMR (C6D6, δ)
0.38 (s, 6 H), 1.11 (s, 18 H), 1.17 (s, 18 H), 2.08 (s, 6 H), 2.52 (s, 12 H),
3.53 (s, 3 H), 4.08 (s, 1 H), 6.70 (s, 4 H); 13C NMR (C6D6, δ) -1.9, 21.7,
23.0, 30.22, 30.24, 53.2, 129.7, 138.4, 139.3, 143.3; 29Si NMR (C6D6, δ)
-92.9, 19.7. Anal. Calcd for C37H68GeOSi3: C, 64.80; H, 9.99. Found:
C, 64.94; H, 9.94. Crystal data of 6‚(toluene)0.5: C37H68OSi4‚(C7H8)0.5
,
fw ) 731.89, triclinic, space group P1h, a ) 11.0570(7) Å, b ) 11.2600(9)
Å, c ) 18.539(1) Å, R ) 81.261(4)°, â ) 85.511(4)°, γ ) 67.188(4)°, V )
2102.4(3) Å3, Z ) 2, dcalc ) 1.083 g‚cm-3, temperature 120 K, R )
0.0520 (I > 2σ(I)), Rw ) 0.1389 (all data), GOF ) 0.944.