Synthesis and NMR characterization of a spirosilole tetra-anion: A homogeneous four-electron reductant

Article abstract of DOI:10.1515/mgmc-2018-0008

The development of highly selective homogeneous electron transfer reagents is highly desirable for synthetic chemistry. An isolable spirosilole tetra-anion described in this paper has been proved to be a promising soluble electron transfer reagent. The re

Full text of DOI:10.1515/mgmc-2018-0008

Main Group Met. Chem. 2018; aop
a,
a
Zhengang Han *, Lei Bai , Shuhui Huo, Jing Chen* and Xiaoquan Lu*
Synthesis and NMR characterization of a
spirosilole tetra-anion: a homogeneous four-
electron reductant
https://doi.org/10.1515/mgmc-2018-0008
Received March 2, 2018; accepted May 7, 2018
molecular orbital (LUMO) level due to the electronic inter-
*
action between the π orbital of the butadiene moiety and
*
the σ orbital of the exocyclic Si-R bond (for reviews on
Abstract: The development of highly selective homoge-
neous electron transfer reagents is highly desirable for
synthetic chemistry. An isolable spirosilole tetra-anion
described in this paper has been proved to be a promis-
ing soluble electron transfer reagent. The reduction of
octaphenyl-1,1′-spirobisilole (1) with lithium resulted in
the formation of the isolable spirosilole tetra-anion in
good yield. The tetra-anion (2) has been characterized by
nuclear magnetic resonance (NMR) and cleanly under-
went electron transfer reactions with dioxygen, carbonyl
groups, and transition metal halides to yield reduction
products with the regeneration of 1.
siloles, see Yamaguchi and Tamao, 1996, 1998; Lee et al.,
2
000; Shirota and Kageyama, 2007; Zhan et al., 2009). They
can be reduced by alkali metals to form the corresponding
dianions. To date, the ‘silole’ dianions have been isolated
and characterized completely (Saito and Yoshioka, 2005).
Spirosiloles that contain a silicon center surrounded
by four carbon atoms have attracted much attention
because they may serve as starting materials for the prepa-
ration of macromolecules and polymers with interesting
physical properties (Lee et al., 2005). On the other hand,
there is a rare report on its anion compounds. Inspired
by the generation of the ‘silole’ dianions (Janzen et al.,
1967; O’Brien and Breeden, 1981; Hong, 2013; Han et al.,
Keywords: anion; homogeneous; reductant; spirosilole.
2
014), here, we report the first isolation of a spirosilole
tetra-anion.
Considering octaphenyl-1,1′-spirobisilole (1) with
the multiply phenyl groups, we selected compound 1
Introduction
Homogeneous reductants have played a great role on as the suitable precursor to the corresponding spiro-
synthetic chemistry. For example, Birch reduction, naph- silole tetra-anion. Following published procedures,
thalene radical anion, and SmI have been widely used octaphenyl-1,1′-spirobisilole were synthesized by treat-
2
in synthetic chemistry (Birch, 1947; Molander, 1992; Con- ment of 1,4-dilithiotetraphenylbutadiene with 0.5
nelly and Geiger, 1996). These homogeneous reducing equivalent tetrachlorosilane with the reported low yield
reagents have exhibited superior to traditional heteroge- (4~10%; Timokhin et al., 2005). In order to improve the
neous reductants owing to their high stereo and chemical yield of compound 1, we attempt the reaction (Scheme 1)
selectivity. Hence, the development of new homogeneous in dimethoxyethane and precise stoichiometric ratio to
reductants is very essential.
afford the high yield (60%), which illustrated that strong
Siloles (Chart 1) show unique chemical and optical cooperative oxy-solvent and precise stoichiometric ratio
properties because of its low-lying lowest unoccupied between diphenylacetylene and lithium were crucial in
obtaining the spirobisilole (1).
a
Zhengang Han and Lei Bai: These authors contributed equally to
this work.
*
Results and discussion
Corresponding authors: Zhengang Han, Jing Chen and Xiaoquan Lu,
Key Laboratory of Bioelectrochemistry and Environmental Analysis
of Gansu Province, College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou 730070, People’s Republic of
China, e-mail: 0610911@mail.nankai.edu.cn (Z. Han); jchen@nwnu.
edu.cn (J. Chen); luxq@nwnu.edu.cn (X. Lu)
The choosing of the solvent is important for the isolation
of the air- and moisture-sensitive anions. It is said that
oxy-solvents like tetrahydrofuran (THF), diethyl ether,
dimethoxyethane could contribute to the stabilization of
the anions for its strong coordination to the metallic ions.
We first select the THF and diethyl ether as solvent for the
reduction of 2. Unfortunately, they led to the formation of
Lei Bai and Shuhui Huo: Key Laboratory of Bioelectrochemistry and
Environmental Analysis of Gansu Province, College of Chemistry
and Chemical Engineering, Northwest Normal University, Lanzhou
7
30070, People’s Republic of China
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ꢀꢀꢀꢀꢁꢀZ. Han et al.: An isolable spirosilole tetra-anion
1
R′
R′′
R′
R′′
According to the H NMR spectra, there are four DME
R4
R4
R5
R4
Si
Si
Si
molecules in one unit of tetra-anion 2, indicating the DME
was efficient to the stabilization of the 2. The Si NMR
spectra of 2 exhibited at −13.7 ppm in C D , are upfield
for 1 (ca. −8.3 ppm), which attributed a four-coordinate
silicon atom and was similar to the 1,1-disubstituted silole
dianion (−15~20 ppm) reported by the Cui group (Han
2
9
Silole
Spirosilole
Silole dianion
6
6
Chart 1:ꢀChemical structures of silole, spirosilole and silole dianion.
Ph
Ph
Ph
1
3
et al., 2014). The C NMR spectra of 2 appears a peak at
78.6 ppm, which is obvious in the range of carbanions,
0
.5 eq. SiCl4
Li
Ph
Ph
Ph
Ph + Li
Si
DME
DME
Ph belonged to the attribution of negative charge in the
α-C atoms of the spirosilole tetra-anion 2 and is upfield
for 1 (128.4 ppm). The result indicates the change of the
C-C bonds in the spirosilole rings and is consistent with
those of the reported silolenides with 2,5-carbodianions
Ph
Ph
1
Scheme 1:ꢀSynthesis of the octaphenyl-1,1′-spirobisilole.
−2
[
1
Ph C Si(R )(R )] (77.4~80 ppm; O’Brien and Breeden,
981; Hong, 2013; Han et al., 2014). The C NMR chemi-
4 4 1 2
1
3
a mixture that cannot be easily purified and isolated. The cal shifts of two C , C , C , C , and C show at 127.7 ppm;
β
i
o
m
p
example inferred that the spirosilole tetra-anion may be 127.4, 128.3 ppm; 129.5, 131.6 ppm; 132.3, 135.4 ppm; and
more reactive and needs the strong cooperative solvent. 136.7, 154.8 ppm for 2, respectively (Table 1), which are
Dimethoxyethane contains two oxygen atoms in one similar to that of the related 2,5-carbanion silole danions
molecule and exhibits the strong cooperation in the iso- (O’Brien and Breeden, 1981; Hong, 2013; Han et al., 2014).
lation of some reactive compounds. Therefore, the reduc- In order to investigate the coordination behavior of the
7
tion of 1 was tried in dimethoxyethane (DME).
lithium in solution, the Li NMR spectrum of 2 was deter-
Treating 1 with four equivalents of lithium in DME mined and exhibits a single resonance at −8.5 ppm in
at room temperature for 2 h afforded the tetra-anion 2 C D , which illustrated a single environment for the four
6
6
+
4−
7
[
Li (DME)] [1 ] as air- and moisture-sensitive orange lithium atoms in solution. Even though the Li NMR reso-
4
powders in 95% yield. The compound 2 had been charac- nance at −8.5 ppm means the possible aromaticity, the α-C
1
13
29
13
terized by H, C, and Si NMR spectroscopy (Scheme 2).
atoms show obvious carbanions according to the C NMR
spectra of spirosilole tetra-anion 2 (78.6 ppm). If the nega-
13
tive charge delocalizes in the ring of the spirosilole, the C
Ph
Ph
Ph
Ph
Ph
NMR spectra of α-C atoms would appear at 120~130 ppm
+
4
Li (DME)
(
Saito and Yoshioka, 2005). Cui and the coworkers have
DME, rt
Ph
Ph
Ph
Ph
Ph
Ph
also reported a 2,5-carbanion silole dianions [Ph (SiMe)
Si
+ 4 Li
Si
2
Ph
−2
+
C Si(NEt) ] ꢀ·ꢀ2[Li] (Han et al., 2014). The silole dianion
2
4
2
7
also shows the negative Li NMR resonance at −2.4 ppm,
but the change of the C-C bond distances observed in the
single-crystal X-ray analysis indicated that the silole dian-
ions did not possess noticeable aromaticity. So it is hard to
Ph
Ph
Ph
Ph
1
2
Scheme 2:ꢀSynthesis of the spirosilole tetra-anion 2.
1
3
29
7
Table 1:ꢀ C, Si, and Li NMR chemical shifts of reported 2,5-carbanion silole anions.
ꢁ
,5-Carbanion silole anionsꢂ [Ph C Si(Me) ]ꢃ−ꢃꢁꢃ·ꢃꢁ[Li]ꢃ+ꢃ
ꢂ
[Ph C Si(n-Bu) ]ꢃ−ꢃꢁꢃ·ꢃꢁ[Li]ꢃ+ꢃ
ꢂ
[Ph (SiMe) C Si(NEt) ]ꢃ−ꢃꢁꢃ·ꢃꢁ[Li]ꢃ+ꢃꢂ [Ph C Si ]ꢃ−ꢃ4ꢃ·ꢃ4[Li]ꢃ+ꢃ
4
4
ꢁ
4
4
ꢁ
ꢁ
ꢁ
4
ꢁ
8
8
ꢁ
Cα
Cβ
Cp
Cm
Co
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢃꢃ.4
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢃꢄ.ꢅ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢆꢃ.ꢇꢈ
ꢉꢅꢅ.ꢈꢅ
ꢉꢅꢉ.ꢅ
ꢉꢅꢊ.ꢄ
ꢉꢄꢈ.ꢃ
ꢉ4ꢊ.ꢊ
−ꢉꢃ.ꢅ
−ꢅ.4
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢃꢇ.ꢆ
ꢉꢅꢇ.ꢊ
ꢉꢅꢇ.ꢉ
ꢉꢅꢃ.ꢃ
ꢉꢈꢃ.ꢇ, ꢉꢅꢈ.ꢊ
ꢉꢅꢄ.ꢄ, ꢉꢅꢊ.ꢇ
ꢉꢅꢆ.ꢊ, ꢉꢄꢅ.4
ꢉꢊꢈ.ꢆ, ꢉ4ꢃ.ꢉ
ꢄ.ꢅ
ꢉꢈꢇ.ꢊ, ꢉꢅꢈ.ꢇ
ꢉꢅꢄ.ꢆ, ꢉꢅꢆ.ꢆ
ꢉꢄꢅ.ꢋ, ꢉꢅꢆ.ꢆ
ꢉꢊꢉ.ꢊ, ꢉ4ꢃ.ꢃ
−ꢈ.ꢅꢃ
ꢉꢅꢃ.4, ꢉꢅꢇ.ꢄ
ꢉꢅꢋ.ꢊ, ꢉꢄꢉ.ꢆ
ꢉꢄꢅ.ꢄ, ꢉꢄꢊ.4
ꢉꢄꢆ.ꢃ, ꢉꢊ4.ꢇ
−ꢉꢄ.ꢃ
−ꢇ.ꢊ
This work
C
i
ꢅꢋ
Si ring
ꢃLi
Reference
–
–
O’Brien and Breeden (ꢉꢋꢇꢉ)ꢂ Hong (ꢅꢈꢉꢄ)
Han et al. (ꢅꢈꢉ4)
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Z. Han et al.: An isolable spirosilole tetra-anionꢌꢁꢀꢀꢀꢌ3
demonstrate the spirosilole tetra-anion 2 shows obvious
Ph
Ph
Ph
Ph
+
4
Li (DME)
O , rt
aromaticity.
13
29
7
Ph
Ph
Ph
Ph
Ph
Ph
2
Ph
Ph
As a contrast, a table with regard to C, Si, and Li
NMR chemical shifts of the related 2,5-carbanion silole
anions has been built in Table 1. The NMR spectra of the
tetra-anion 2 appear to be similar to those of the related
silole dianions reported. All of these supply sufficient evi-
dence to confirm the structure of tetra-anion 2 (Figure 1).
The molecular geometry predicted of 2 by DFT calcu-
lations at B3LYP/6-31G(d) basis set. The calculated highest
Si
Si
+
2 Li2O
Ph
Ph
Ph
Ph
2
1
Scheme 3:ꢀReaction of 2 with dioxygen.
occupied molecular orbital (HOMO) and LUMO are given double bond in the silole ring and the lone pairs on the
in Figure 2 in the ‘Experimental’ section. The HOMO of 2 is α-carbon atoms with contribution from the silicon σ orbit-
mainly formed by the π orbitals originating from the C=C als. The HOMO of 2 displays a ring surface similar to that
of the LUMOs of neutral 1,1-disubstituted siloles.
Compound 2 is extremely air sensitive and can be
slowly oxidized in sealed NMR tubes to generate 1. Thus,
the reaction of tetra-anion 2 with dioxygen has been inves-
tigated (Scheme 3). The reaction at room temperature
cleanly generated 1 in nearly quantitative yield.
As a common oxidizing agent, 1,4-benzoquinone can
reveal the reactivity of some reductants. Similarly, the
reaction of tetra-anion 2 with 1,4-benzoquinone resulted
in the quantitative regeneration of 1 with the formation of
the corresponding dilithium benzoquinone (Scheme 4).
All these reactions indicated that tetra-anion 2 prefers
electron transfer rather than nucleophilic addition to
these substrates. The electron transfer property of tetra-
anion 2 is further supported by its reaction with PdCl .
2
Reaction of 2 with PdCl yielded a black precipitate and 1
2
o
Figure 1:ꢀX-ray photoelectron spectroscopy (XPS) spectra of Pd 3d.
(
Scheme 5). In order to identify the black powder, we have
done the X-ray photoelectron spectroscopy (XPS) to dem-
o
onstrate the Pd 3d (335.1 and 340.9 eV).
All of these reactions converted 2 to 1 in an almost
quantitative yield, and thus certified that the tetra-anion 2
acted as a homogeneous four-electron transfer reductant.
Figure 2 shows cyclic voltammograms (CV) of spi-
rosilole 1 in dichloromethane solution containing 0.1 ꢁ
tetrabutylammonium perchlorate as supporting electro-
lyte. As the potential is scanned from 1.8 V to the negative
region (−1.8 V), the reduction of spirosilole 1 showed four
potential peaks (1.28, 0.85, −0.82, and 1.27 V), indicating
spirosilole 1 could get four electrons to form the corre-
sponding spirosilole tetra-anion 2.
Since tetra-anion 2 undergoes electron transfer reac-
tions with most of electrophiles, we tried its reaction with
dilute hydrochloric acid to further confirm the negative
charge of alpha carbon atoms (Scheme 6), and got the
Figure 2:ꢀCyclic voltammograms of 1.0 mm of spirosilole 1 in
CH Cl with 0.1 ꢍ tetrabutylammonium perchlorate as supporting
2
2
1
,2,3,4,6,7,8,9-octaphenyl-5-silaspiro[4.4] nona-2,7-diene
electrolyte.
(
3) as the hydrolyzed product. Compound 3 was character-
A Pt disk embedded in a glass tube was used as a working electrode
1
13
29
2
ized by H, C, and Si NMR spectra and mass spectrum.
The result illustrated that the alpha carbon atoms were
(
0.03 cm ), a Pt wire as a counterelectrode, and an Ag wire as a
−1
quasi-reference electrode. The scan rate was at 0.1 V s .
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ꢄꢀꢀꢀꢀꢁꢀZ. Han et al.: An isolable spirosilole tetra-anion
Ph
Ph
4
Ph
Ph
Ph
+
OLi
OLi
Li (DME)
Ph
Ph
O
O
Ph
Ph
O2, rt
Ph
Ph
Si
+
2
Si
+ 2
Ph
Ph
Ph
Ph
Ph
2
1
Scheme ꢄ:ꢀReaction of 2 with benzoquinone.
Ph
Ph
4
Ph
Ph
+
Li (DME)
Ph
Ph
Ph
DME, rt
Ph
Ph
Ph
Si
+ 2 PdCl2
Si
2 Pdo
+
+ 2 LiCl
Ph
Ph
Ph
Ph
Ph
Ph
2
1
Scheme ꢅ:ꢀReaction of 2 with PdCl2.
Ph
H
Ph
H
toluene; it can be easily prepared in the nearly quantita-
tive yield on gram scales; and it does not present any sig-
nificant fire or toxic hazards. Most importantly, the silole
precursor 1 can be recovered in excellent yields.
Ph
Ph
4
+
Li (DME)
Ph
Ph
Ph
Ph
Ph
Ph
HCl/H2O
DME, r.t.
Si
Si
Ph
H
Ph
H
Ph
Ph
Ph
Ph
2
3
Conclusion
Scheme 6:ꢀReaction of 2 with dilute hydrochloric acid.
In summary, we have isolated and NMR characterized
spirosilole tetra-anions with a four-coordinate silicon
atom for the first time. The silole tetra-anion 2 selectively
underwent four-electron transfer reactions with a number
of substrates and nucleophilic addition and substitution
reactions that are common to carbanions have not been
observed. This work demonstrated that 2 is a promising
electron transfer reducing reagent that could be used in
synthetic chemistry. We are continuing to explore the
applications of tetra-anion 2 to the reduction of main
group compounds and will report on this in due course.
negatively charged and further confirm the structure of
tetra-anion 2.
To the best of our knowledge, rare carbanions could
selectively display electron transfer reactivity with all of
these substrates, except for naphthalene radical anion
and its derivatives (Garst, 1971). We found that the spiro-
silole 1 could be reduced by naphthalene lithium to form
the tetra-anion 2. But the reverse reaction did not occur,
indicating that 2 is a relatively mild reducing reagent com-
pared to naphthalene lithium.
It is well-known that alkali metals and lithium naph-
thalene as well as magnesium have been widely used as
reductants for the synthesis of low-valent main group
and transition metal complexes. The choice of the reduct-
ants is crucial to obtain the desired products due to over-
reduction and the occurrence of other side reactions
Experimental
Materials and measurements
All operations were carried out under an atmosphere of dry argon by
using modified Schlenk line and glovebox techniques. All solvents
were freshly distilled from Na and degassed immediately prior to use.
(
Fischer and Power, 2010). In this respect, the spirosilole
tetra-anion 2 possesses a number of unique features that
make it an appealing alternative to the above-mentioned
reductants. For example, it is thermally stable and soluble
1
13
29
The H, C, and Si NMR spectroscopic data were recorded on Varian
Mercury Plus 400 MHz NMR spectrometers. Chemical shifts are refer-
1
13
29
in a wide range of organic solvents like THF, DME, and enced against external Me Si ( H, C, and Si).
4
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Z. Han et al.: An isolable spirosilole tetra-anionꢌꢁꢀꢀꢀꢌꢅ
Synthesis of octaphenyl-1,1′-spirobisilole (1), tetra-
anion (2)
Octaphenyl-1,1-spirobisilole (1):ꢂTo a solution of diphenylacety-
lene (7.0 g, 40.0 mmol) in dry DME (50 mL) was added clean lithium
shavings (0.28 g, 40.0 mmol). The reaction mixture was stirred at
room temperature for 12 h in a dry nitrogen atmosphere. The mix-
ture was then followed by the addition of 1.7 g (10.0 mmol) silicon
tetrachloride at room temperature. After refluxing for 5 h, the reac-
tion mixture was cooled and filtered, and the filtrate was washed
with water. The organic layer was extracted with toluene and dried
over magnesium sulfate. The solvent was removed and the residue
was recrystallized from ethanol gave a faintly greenish–yellow crys-
HOMO
Figure 3:ꢀThe calculated HOMO and LUMO of 2.
LUMO
1
tal in 60% yields (3.2 g). m.p.ꢀ=ꢀ263°C. H NMR (C D ): δꢀ=ꢀ6.93–7.26
6
6
1
3
the mixture became dark green immediately. The solution was con-
centrated and toluene (25 mL) was added. The mixture was filtered
and the filtrate was concentrated to give the silole precursor 1 (0.16 g,
(
m, 40H, Ar). C NMR (C D ): δꢀ=ꢀ124.8, 125.8, 126.2, 126.9, 127.3,
6 6
2
9
1
27.2, 127.5, 128.6, 130.2, 150.9. Si NMR (C D ): δꢀ=ꢀ−8.3. Analysis
6 6
calculated for C H Si, Cꢀ=ꢀ90.76%, Hꢀ=ꢀ5.44%; found, Cꢀ=ꢀ90.72%,
5
6
40
9
3% recovery). The residues were hydrolyzed with aqueous HCl. The
Hꢀ=ꢀ5.66%.
resulting product was extracted with diethyl ether to afford dihydro-
quinone (0.04 g, 75%).
−
ꢄ
+
ꢄ
[
Octaphenyl-1,1-spirobisilole] [Li (DME)] (2):ꢂLithium shavings
(
(
0.30 g, 44.0 mmol) was added to a DME solution (25 mL) of silole 1
7.4 g, 10.0 mmol) at room temperature, and the mixture was stirred
Reaction of 2 with dichloropalladium(II):ꢂTo a mixture of 2 (0.40 g,
0
.25 mmol) and dichloropalladium(II) (0.10 g, 0.50 mmol) was added
for 2 h. An excess of lithium was separated by filtration, and the fil-
trate was concentrated. The crude product was recrystallized from
DME/n-hexane (2:1) to afford 2 as orange powders (11.3 g, 95%).
DME (10 mL) at room temperature. After 2 h, the mixture became light
yellow and the formation of dark powder was observed and demon-
1
strated by XPS (340.9 eV for Pd3d3/2 and 335.1 eV for Pd3d ). The solu-
H NMR (C D ): δꢀ=ꢀ2.96 (24H, CH -dme), 3.07 (36H, CH -dme), 6.93–
5/2
6
6
2
3
1
3
tion was concentrated and toluene (25 mL) was added. The mixture
was filtered, and the filtrate was concentrated to give the spirosilole
precursor 1 (0.14 g, 90% recovery).
7
7
(
(
.26 (m, 40H, Ar). C NMR (C D ): δꢀ=ꢀ58.6 (CH -dme), 71.0 (CH -dme),
6
6
2
3
8.6 (C ), 127.7 (C ), 127.4 (C ), 128.3 (C ), 129.5 (C ), 131.6 (C ), 132.3
α
β
p p m m
2
9
7
C ), 135.4 (C ), 136.7 (C ), 154.8 (C ). Si NMR (C D ): δꢀ=ꢀ−13.7. Li NMR
o o i i 6 6
C D ): δꢀ=ꢀ−8.5.
6 6
Computational details
Reduction reactions of the tetra-anion 2
All calculations were performed using the Gaussian 03 suite of pro-
Reaction with dilute hydrochloric acid:ꢂAn aqueous 5% HCl solu- grams, revision C. 02 (Frisch et al., 2004). The geometries and har-
tion (10 mL) was slightly added to a solution of 2 (0.40 g, 0.25 mmol) monic vibration frequency of 2 were optimized in DFT method with
in DME (10 mL) at room temperature, and the mixture became color- B3LYP method and 6-31+G(d) basis set. The calculated HOMO and
less. The solution was concentrated and toluene (25 mL) was added. LUMO for 2 are given in Figure 3.
The mixture was extracted with diethyl ether (3ꢀ×ꢀ25 mL). After its
concentration to 10 mL under reduced pressure, the solution was
Acknowledgments: This work was supported by the
stored in the freezer (−40°C) and 1,2,3,4,6,7,8,9-octaphenyl-5-silas-
National Natural Science Foundation of China.
piro[4.4] nona-2,7-diene (3) was afforded as a white powder (0.13 g,
1
6
7%). m.p.ꢀ=ꢀ247°C. H NMR (C D ): δꢀ=ꢀ7.30–7.33 (m, 4H, Ar), 7.82–7.11
6
6
1
3
(
(
1
m, 36H, Ar), 2.87 (s, 4H, SiCH ). C NMR (C D ): δꢀ=ꢀ147.7 (C ), 142.1
2
6
6
i
C ), 137.7 (C ), 134.0 (C ), 132.0 (C ), 128.2 (C ), 127.6 (C ), 126.8 (C ),
i
o
o
m
m
p
p
2
9
24.4 (C ), 50.1 (C ). Si NMR (C D ): δꢀ=ꢀ11.9. MS(EI): m/zꢀ=ꢀ743.2
β
α
6
6
References
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solutions. J. Chem. Soc. 19ꢄ7, 102–125.
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tallic chemistry. Chem. Rev. 1996, 96, 877–910.
+
[
M ]. Analysis calculated for C H Si, Cꢀ=ꢀ90.28%, Hꢀ=ꢀ5.95%; found,
5
6
40
Cꢀ=ꢀ90.32%, Hꢀ=ꢀ5.96%.
Reaction with dioxygen:ꢂThe solid tetra-anion 2 (0.40 g, 0.25 mmol)
was exposed to an excess of dioxygen at room temperature. It was
stirred for 2 h until the color had become light yellow. To the solid
was added toluene (25 mL) and the solution was filtered. The filtrate
was concentrated to give the spirosilole precursor 1 (0.18 g, 95%
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of 2 (0.40 g, 0.25 mmol) in DME (10 mL) at room temperature, and
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