Isolable 1,1-disubstituted silole dianion: A homogeneous two-electron-transfer reducing reagent

Article abstract of DOI:10.1021/ic500820v

The 1,1-disubstituted silole dianion 2 has been isolated and characterized by single-crystal X-ray analysis for the first time. 2 can be used as a two-electron-transfer reducing reagent for the reduction of organic compounds and inorganic salts with regeneration of the corresponding neutral silole in nearly quantitative yields, indicating that it is an excellent reducing reagent. Reduction of (Mes)₂SiCl₂ with 2 selectively yielded the cyclotrisilane (Mes₂Si)₃ in high yield, which has not been isolated in pure form with the existing methods.

Full text of DOI:10.1021/ic500820v

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Isolable 1,1-Disubstituted Silole Dianion: a Homogeneous Two-
Electron-Transfer Reducing Reagent
Zhengang Han, Jianfeng Li, Hongfan Hu, Jianying Zhang, and Chunming Cui*
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, and Collaborative Innovation Center of
Chemical Science and Technology, Tianjin 300071, People’s Republic of China
S
* Supporting Information
Scheme 1. Synthesis of the Silole Alkali-Metal Salts 2−4
ABSTRACT: The 1,1-disubstituted silole dianion 2 has
been isolated and characterized by single-crystal X-ray
analysis for the first time. 2 can be used as a two-electron-
transfer reducing reagent for the reduction of organic
compounds and inorganic salts with regeneration of the
corresponding neutral silole in nearly quantitative yields,
indicating that it is an excellent reducing reagent.
Reduction of (Mes)2SiCl2 with 2 selectively yielded the
cyclotrisilane (Mes2Si)3 in high yield, which has not been
Reduction of 1 with 2 equiv of alkali metals (Li, Na, and K) was
isolated in pure form with the existing methods.
carried out in tetrahydrofuran (THF) at room temperature. The
reaction with lithium for 2 h afforded the dilithium salt 2,
[Li+(THF)]2[12−], as orange crystals in 98% yield. Similarly,
he development of homogeneous electron-transfer re-
agents has a great impact on synthetic chemistry. For
reduction of 1 with sodium and potassium yielded the sodium
salt 3, [Na+(THF)]2[12−], as purple powders in 95% yield and
potassium salt 4, [K+(THF)]2[12−], as blue powders in 95%
yield, respectively. All of these alkali-metal salts have been
T
example, Birch reduction, a naphthalene radical anion, and SmI2
have been widely employed in synthetic chemistry.1 These
homogeneous reducing reagents have been proven to be superior
to alkali metals in terms of their chemical selectivity and
stereoselectivity.
Siloles (Chart 1) exhibit unique chemical and optical
properties because of their low-lying lowest unoccupied
1
characterized by H, 13C, and 29Si NMR spectroscopy. The
molecular structure of 2 has been established by single-crystal X-
ray analysis.
The 29Si NMR spectra of 2−4 all exhibited two lines at −17.2
and 8.8, −19.7 and 12.1, and −20.0 and 13.8 ppm, respectively.
The negative values, assigned to the silicon atom in the silole ring,
are upfield for 1 (ca. −10.4 ppm), while the chemical shifts for the
SiMe3 group are downfield from that for 1 (ca. 3.0 ppm).7b The
7Li NMR spectrum of 2 exhibits a singlet at −2.43 ppm in C6D6,
while two resonances at −0.36 and −2.31 ppm were observed in
THF-d8, indicating that the structure of the lithium salt in
solution is solvent-dependent. X-ray crystallographic analysis of
the lithium salt 2 revealed that there are four independent
molecules in the asymmetric unit. Although some geometry
variations among the four molecules cannot simply be ignored,
their structural features as a whole are very similar. For simplicity,
only one of the molecules is shown in Figure 1. The SiC4 ring in 2
is not planar, and the silicon atom is ca. 15° above the C4 plane.
The two lithium atoms are located above and below the SiC4 ring
and coordinated to the silole ring in an η5 fashion. This
coordination mode is different from those of dilithium 2,3,4,5-
tetraphenylsilole and 3-n-butyl-2-phenyl-1-silaindene, in which
one of the lithium atoms is η1-coordinated to the silicon atom
while the other lithium is η5-coordinated to the silole ring.5c,d,j
The Li1 atom in 2 is also coordinated to the N1 atom with a Li1−
N1 bond distance of 2.132(5) Å, leading to the relatively short
Li1−Si1 distance of 2.492(5) Å in comparison with a normal Li−
Chart 1
molecular orbital (LUMO) levels resulting from the orbital
overlap between the π* orbitals of the butadiene segment and the
σ* orbitals associated with the exocyclic σ bond on the silicon
atom.2 Because of their high electron affinity, it is possible to
generate the corresponding silole dianions and radical anions by
reduction. Although the reduction of boroles and alumoles
yielded the isolable dianions or radical anions,3 chemical
reduction of siloles has only been confined to NMR-scale
reactions and the silole dianions (Chart 1, A) have not been
isolated and structurally characterized so far.4 It is noted that the
silole dianions with a divalent silicon atom (Chart 1, B) have
been reported.2b,5a,6 Herein, we report the first isolation and X-
ray crystal structure of a silole dianion of type A (Chart 1) .
We envisioned that a 2,5-disilyl-substituted silole could be a
suitable precursor to the corresponding 1,1-disubstituted silole
dianion because the resulting carbanion could be stabilized by the
α-silyl groups. Thus, the silyl-substituted silole 1 (Scheme 1) was
prepared by the protocol developed by the Tamao group.7
Received: April 16, 2014
Published: May 30, 2014
© 2014 American Chemical Society
5890
dx.doi.org/10.1021/ic500820v | Inorg. Chem. 2014, 53, 5890−5892

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To the best our knowledge, rare carbanions could selectively
exhibit electron-transfer reactivity toward a variety of substrates,
except for the naphthalene radical anion and its derivatives.1b,9 It
was found that the silole 1 could be reduced by lithium
naphthalenide to form the lithium salt 2, but the reverse
reduction reaction with 2 did not occur, indicating that 2 is a
relatively mild reducing reagent, compared to lithium
naphthalenide.
Alkali metals have been used for the synthesis of low-valent
main-group and transition-metal complexes. However, over-
reduction and other side reactions limited their wide
applications.10 The silole dianion 2 possesses a number of
unique features that make it an appealing alternative for
reduction chemistry. For example, it is thermally stable and
decomposed above 200 °C, it is soluble in a wide range of organic
solvents, it can be easily prepared in nearly quantitative yield on
gram scales, and it does not present any significant fire or toxicity
hazards. Notably, the silole precursor 1 can be recovered in
excellent yields.
To examine the reduction potential of 2 in synthetic chemistry,
reduction of (Mes)2SiCl2 (Mes = 2,4,6-Me3C6H2) with 2 has
been investigated. Since the first disilene Mes2SiSiMes2, a
fascinating compound that stimulated great interest in main-
group multiple-bonding species,11 was prepared via photolysis of
Mes2Si(SiMe3)2 by West et al. in 1981,12 reduction of Mes2SiCl2
and Dmp2SiCl2 (Dmp = 2,6-Me2C6H3) with alkali metals and
lithium naphthalenide has been studied by several groups.13,14 In
2006, Napolitano and co-workers reported that reduction of
Mes2SiCl2 with alkali metals yielded a mixture containing a
number of products. Under optimized conditions with potassium
graphite as the reductant, the cyclotrisilane (Mes2Si)3 was
obtained as a major product containing other higher oligomers
and has not been isolated in pure form.14 The problems might
mainly lie in the stepwise electron-transfer process that may lead
to a number of intermediates. The dianion 2 is a relatively mild
reducing reagent and possesses two-electron-transfer capacity.
Thus, it might be a good choice to avoid the formation of
monohalide intermediates.
Figure 1. Molecular structure of one of the four independent molecules
of 2 in the asymmetric unit. Thermal ellipsoids were drawn at 30%
probability. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (deg): Si−C1 1.839(3), Si−C4 1.843(3),
Si−N1 1.812(2), Si−N2 1.741(2), C1−C2 1.497(3), C2−C3 1.399(4),
C3−C4 1.492(3), Li1−Si1 2.492(5), Li2−Si1 2.628(5), Li1−C1
2.679(6), Li1−C2 2.360(5), Li1−C3 2.237(6), Li−C4 2.386(6), Li−
N1 2.132(5), Li2−C1 2.223(6), Li2−C2 2.115(5), Li2−C3 2.101(5),
Li2−C4 2.249(5); C1−Si1−C4 94.17(12), Si1−C1−C2 105.28(18),
Si1−C4−C3 106.15(19); C4−Si1−C1−C2−15.98(18).
Si distance of 2.60 Å.2b,c,5c,6a The C1−C2 [1.497(3) Å] and C3−
C4 [1.492(3) Å] bond lengths are ca. 0.1 Å longer than that of
C2−C3 [1.399(4) Å], indicating that electron delocalization in
the C4 unit is not significant. The nonplanar structure and C−C
bond distances observed in the silole ring indicated that 2 does
not possess noticeable aromaticity.
The molecular geometry predicted by DFT calculations on 2
at the RB3LYP/6-31G(d) basis set is in line with its X-ray crystal
structural results. The calculated highest occupied molecular
orbital (HOMO) and LUMO are given in Figure S1 in the
Supporting Information (SI). The HOMO of 2 is mainly
dominated by the π orbitals originating from the CC double
bond in the ring and the lone pairs on the silyl carbon atoms with
small contributions from the three silicon σ orbitals. The HOMO
of 2 displays a ring surface similar to that of the LUMOs of
neutral 1,1-disubstituted siloles.8
Reduction of Mes2SiCl2 with 1 equiv of 2 in THF at 0 °C
proceeded completely in 4 h. Analysis of the crude product by the
1H NMR spectrum (Figure S2 in the SI) indicated that the
reaction is clean and yielded 1 and a new product (Scheme 3).
The new product can be obtained as yellow-green crystals in 70%
yield after crystallization from n-hexane, and the silole 1 can be
recovered in 90% yield by column chromatography. The new
product was identified as the cyclotrisilane 5 by 1H, 13C, and 29Si
NMR spectra and elemental analysis. To the best our knowledge,
5 has not been reported to be isolated in pure form. Single-crystal
The reaction chemistry of 2 has been investigated (Scheme 2).
Solid 2−4 can be oxidized by oxygen to regenerate 1; in toluene,
Scheme 2. Reduction of Diphenyl Ketone, 1,4-Benzoquinone,
Dioxygen, MeI, and Pd(PPh3)2Cl2 with the Silole Dianion 2
Scheme 3. Comparison of Reduction of Mes2SiCl2 with
Different Reductants
2 was cleanly oxidized by oxygen to give 1 in nearly quantitative
yield. The reaction of 2 with diphenyl ketone and 1,4-
benzoquinone almost quantitatively yielded 1 with formation
of the corresponding ketyl radical anion and dilithium hydro-
quinone (Scheme 2). These reactions are very similar to those
reduced by alkali metals. The reaction of 2 with MeI exclusively
yielded MeLi and 1; reduction of Pd(PPh3)2Cl2 yielded a
palladium powder and 1. In all of these reactions, 2 acted as a
two-electron donor to the substrates and was converted to 1 in an
almost quantitative yield.
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Inorganic Chemistry
Communication
K.; Sugiyama, Y.; Takagi, N.; Nagase, S.; Hosoi, Y.; Furukawa, Y.;
Tokitoh, N. J. Am. Chem. Soc. 2008, 130, 13856.
X-ray analysis of 5 further confirmed its cyclotrisilane skeleton,
which is shown in Figure 2. The Si−Si bond lengths lie in a very
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Figure 2. Molecular structure of 5. Thermal ellipsoids were drawn at
30% probability. Hydrogen atoms have been omitted for clarity.
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Si1−Si3 2.4278(15), Si2−Si3 2.4428(16); Si3−Si2−Si1 59.39(5), Si1−
Si3−Si2 60.62(5), Si3−Si1−Si2 59.99(4).
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silole dianion with a four-coordinate silicon atom for the first
time. The silole dianion 2 selectively underwent two-electron-
transfer reactions with a number of substrates. 2 can be used as a
selective and mild reductant, as demonstrated by reduction of
Mes2SiCl2. This work demonstrated that 2 is a promising
electron-transfer reducing reagent. We are continuing to explore
applications of 2 for the synthesis of low-valent main-group
compounds and the electron-transfer properties of 2 toward
organic substrates.
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ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format and further details of
the synthesis and characterization of compounds 1−5. This
material is available free of charge via the Internet at http://pubs.
acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: cmcui@nankai.edu.cn.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China and 973 Program (Grant 2012CB821600).
■
(14) Chiappe, C.; Imperato, G.; Lenoirb, D.; Napolitano, E.
Tetrahedron Lett. 2006, 47, 8893.
(15) Weidenbruch, M. Chem. Rev. 1995, 95, 1479.
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