Synthesis, structure and reactions of triarylgermyl anion with α,δ-ambiphilic character

Article abstract of DOI:10.1039/b923606f

Benzosilagermacyclobutene bearing two Ar groups on the germanium atom [Ar = o-(fluorodimethylsilyl)phenyl] undergoes cleavage of the Ge-Si bond with KF in the presence of a cryptand to form Ar₃Ge^-[K ⁺(cryptand)] with a,d-ambiphilic character consisting of the germyl anion center and the fluorosilyl moieties. In the presence of LiBPh ₄·3dme or BF₃·Et₂O, the germylpotassium is converted back into the benzosilagermacyclobutene. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b923606f

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Synthesis, structure and reactions of triarylgermyl anion with
a,d-ambiphilic characterw
Atsushi Kawachi,* Koji Machida and Yohsuke Yamamoto
Received (in Cambridge, UK) 10th November 2009, Accepted 11th December 2009
First published as an Advance Article on the web 13th January 2010
DOI: 10.1039/b923606f
Benzosilagermacyclobutene bearing two Ar groups on the
germanium atom [Ar = o-(fluorodimethylsilyl)phenyl] under-
goes cleavage of the Ge–Si bond with KF in the presence of a
cryptand to form Ar3Geꢀ[K+(cryptand)] with a,d-ambiphilic
character consisting of the germyl anion center and the fluorosilyl
moieties. In the presence of LiBPh4ꢁ3dme or BF3ꢁEt2O, the
germylpotassium is converted back into the benzosilagerma-
cyclobutene.
triarylgermyl anion 3 (a,d-ambiphilic species III (n = 2)), in
which a germyl anion center and three electrophilic fluorosilyl
groups are linked to each other through o-phenylene frame-
works, and also that the reaction of 3 with a Lewis acid
reversibly leads to a ring-closing reaction to regenerate 2.
Treatment of 2 with KF in the presence of [2.2.2]cryptand in
tetrahydropyran (THP) afforded a pale yellow solution of 3,
which was isolated in 77% yield as pale yellow crystals after
recrystallization from THP–hexane, as shown in Scheme 1.6,7
z
The cryptand is essential for this reaction in order to activate a
fluoride ion and stabilize the resulting germylpotassium.
Compound 3 was also trapped with CH3COOH to give
hydrogermane 4 in 62% yield (eqn (1)). The selective fluoride
attack on the silicon atom, forming the fluorosilyl moeity and
the germyl anion moiety, is controlled by the following steric
and electronic factors: (i) the silicon site is sterically less
crowded; (ii) the Ge–Si bond is polarized as Gedꢀ–Sid+ due
to electronegativity;8 (iii) the covalent bond energy of Si–F is
higher than that of Ge–F;8 and (iv) the resulting germyl anion
is stabilized by three aryl groups whereas the silyl anion to be
formed is stabilized by only one aryl group. The choice of THP
as solvent was essential to increase the yield of 3.9 In THF, 3
gradually abstracted an a-proton of a THF molecule to
produce 4 in the course of the recrystallization of 3.
Anionic species of Group 14 elements with ambiphilic
character, which exhibit both nucleophilicity and electrophilicity,
have received increasing attention in the last few decades
because of their unique reactivity and potential utility as
building blocks for main group element compounds.1–3
Reaction modes of the ambiphilic species may be governed
by the position of the electrophilic center. For examples,
silylenoids that have an electrophilic center at the a-position
(a,a-ambiphilic species I) undergo self-condensation reaction
to give disilanyl anions or a-elimination to afford silylenes.2
b-Functionalized silyl anions that possess an electrophilic
center at the b-position (a,b-ambiphilic species II) undergo
dimerization to form four-membered ring compounds or
b-elimination to produce disilenes.3 Ambiphilic species bearing
an electrophilic center at the position further than b have been
less investigated in spite that such species are postulated as key
intermediates in cyclization or polymerization reactions.4
ð1Þ
The 29Si{1H} NMR spectra of 3 showed a doublet at d(29Si) =
15.5 (1J(29Si–19F) = 274 Hz). In the 19F{1H} NMR spectra, a
broad signal appeared at d(19F) = ꢀ163.0. The molecular
structure was unambiguously determined to be 3 by single-
crystal X-ray diffraction analysis (Fig. 1).10 The potassium
cation is incorporated into the cryptand cavity and completely
separated from the germyl anion center (K(1)–Ge(1) =
8.1621(9) A). The germyl anion is surrounded by three
o-(fluorodimethylsilyl)phenyl groups that form a pyramidal C3
propeller. The C–Ge–C bond angles (av. 99.11) and the Ge–C
bond lengths (av. 2.02 A) are similar to those of triarylgermyl
We have recently reported that the reaction of [o-(fluorodi-
methylsilyl)phenyl]lithium (1) as an a,g-ambiphilic species III
(n = 1) with GeCl2 produced benzosilagermacyclobutene 2
having two o-(fluorodimethylsilyl)phenyl groups on the
germanium atom in the four-membered ring.5 Compound 2
is expected to exhibit unique reactivity because of the highly
strained four-membered ring and the highly electrophilic
fluorosilyl functionality. Here we report that the reaction of
2 with KF leads to a ring-opening reaction to give a novel
Department of Chemistry, Graduate School of Science,
Hiroshima University, Kagamiyama 1-3-1,
Higashi-Hiroshima 739-8526, Japan.
6
ꢀ
metals reported so far. The unit cell (P1) includes a pair of
enantiomers, in one of which the three aromatic rings are
leaning clockwise, and anticlockwise in the other. The three
Si–F bonds are projected away from the germanium center.
The Si–F bond lengths (1.6194(17)–1.6232(17) A) are consistent
with normal Si–F single-bond lengths whereas the Geꢁ ꢁ ꢁSi
E-mail: kawachi@sci.hiroshima-u.ac.jp
w Electronic supplementary information (ESI) available: Experimental
details, tables of crystallographic data for 2 and computational work.
CCDC 733620. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b923606f
ꢂc
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Scheme 1 Formation of 3 and regeneration of 2 by reversible ring-
opening/closing reactions: (a) KF (ꢃ1)/[2.2.2]cryptand ( = L)/THP/
r.t., 0.5 h; 77% yield. (b) LiBPh4ꢁ3dme (ꢃ3)/THP/r.t., 20 h; 60% yield.
(c) BF3ꢁEt2O (ꢃ1.1)/THP/r.t., 0.5 h; 39% yield.
distances (3.4176(8)–3.4767(8) A) are within the sum of the
van der Waals radii of the two elements (3.91 A).11
Thus, the nucleophilic germyl anion is ‘‘close’’ but ‘‘inert’’ to
the highly electrophilic fluorosilyl moieties. The inertness may
be due to complete encapsulation of the potassium cation,
which inhibits a,d-elimination of KF. In order to confirm this
point, we treated 3 in THP with several metal salts and found
that LiBPh4ꢁ3dme is effective for the regeneration of 2. On the
other hand, LiF and LiCl are ineffective perhaps due to their
low solubility in THP. Upon addition of LiBPh4ꢁ3dme to a
pale yellow solution of 3 in THP, the solution became a white
suspension within 20 h (Scheme 1). After the insoluble materi-
als were removed by filtration, the filtrate was concentrated
and the residue was recrystallized from hexane to provide 2 in
60% yield. A non-metallic Lewis acid, BF3ꢁEt2O, is also
effective for the formation of 2 (39% yield) within 0.5 h
(Scheme 1). It is postulated that the coordination of the Lewis
acid to the fluorine to be displaced polarizes the Si–F bond
and lowers the energy of the s*(Si–F) orbital, making the
silicon more receptive toward nucleophiles and facilitating the
departure of the leaving group.12
Fig. 2 HOMO of the anion part of 3 optimized at B3LYP/6-31G(d)
level.
the HOMO and the s*(Si–F) orbital was confirmed, which is
consistent with the inertness of 3.
In summary, we have prepared (triarylgermyl)potassium 3
with a,d-ambiphilic character by the reaction of 2 with
KF/cryptand and regenerated 2 by exposure of a Lewis acid
to 3. The highly strained four-membered ring in 2 readily
captures a fluoride ion to open the ring under mild reaction
conditions while the a,d-ambiphilic character in 3 leads to
the intramolecular nucleophilic substitution reaction, giving
the four-membered ring. Thus, the unique reversible inter-
conversions between 3 and 2 have been achieved.
This work was partly supported by a Grant-in-Aid for
Scientific Research on Priority Area ‘‘Advanced Molecular
Transformations of Carbon Resources’’ (No. 19020048) from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
DFT calculations were performed on an anion part of 3 at
the B3LYP/6-31G(d) level.13 The optimized structure is in
P
good agreement with the crystal structure ( (C–Ge–C)3
=
299.51; Si–F = 1.649 A; Geꢁ ꢁ ꢁSi = 3.555–3.559 A). The
HOMO is the lone electron pair on the germanium atom,
as shown in Fig. 2. The vector of the HOMO is not directed
towards the silicon atoms and no orbital interaction between
Notes and references
z All manipulations were carried out under anaerobic and anhydrous
conditions. Details of experiments and characterization are described
in ESI.w
[Ar3Geꢀ][K+(cryptand)] 3: Compound 2 (770 mg, 1.50 mmol),
KF (89 mg, 1.50 mmol) and [2.2.2]cryptand (565 mg, 1.50 mmol)
were dissolved in THP (5.0 mL) at room temperature, and the reaction
mixture was stirred at the same temperature for 0.5 h to give a yellow
solution of 3. After concentration, the resulting yellow solid
was recrystallized from THP–hexane (1 : 1) at ꢀ30 1C to give 3
(1.10 g, 77% yield) as yellow crystals.
Reaction of 3 with BF3ꢁEt2O: BF3ꢁEt2O (27 mL, 0.22 mmol) was
added to 3 (190 mg, 0.20 mmol) in THP (4.0 mL) at room temperature
and the reaction mixture was stirred at the same temperature for 0.5 h.
After concentration, hexane (3 mL ꢃ 3) was added to the residue and
the resulting insoluble materials were filtered off. The filtrate was
concentrated in vacuo. The residue was recrystallized from hexane at
ꢀ30 1C to give 2 (40 mg, 39% yield) as colorless crystals.
1 A review for carbenoids: G. Boche and J. C. W. Lohrenz,
Chem. Rev., 2001, 101, 697, and references therein.
Fig. 1 Molecular structure of 3 with thermal ellipsoids set at 30%
probability; hydrogen atoms are omitted for clarity. Selected bond
lengths (A) and angles (1): Ge(1)–C(1) 2.023(2), Ge(1)–C(9) 2.024(2),
Ge(1)–C(17) 2.027(2), Si(1)–F(1) 1.6206(17), Si(2)–F(2) 1.6194(17),
Si(3)–F(3) 1.6232(17); C(1)–Ge(1)–C(9) 99.23(9), C(1)–Ge(1)–C(17)
100.23(9), C(9)–Ge(1)–C(17) 97.78(9).
2 Examples of silylenoids: (a) K. Tamao and A. Kawachi,
Angew. Chem., Int. Ed. Engl., 1995, 34, 818; (b) A. Kawachi,
Y. Oishi, T. Kataoka and K. Tamao, Organometallics, 2004, 23,
2949; (c) M. E. Lee, H. M. Cho, Y. M. Lim, J. K. Choi, C. H. Park,
S. E. Jeong and U. Lee, Chem.–Eur. J., 2004, 10, 377; (d) G. Molev,
ꢂc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1890–1892 | 1891

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D. Bravo-Zhivotovskii, M. Karni, B. Tumanskii, M. Botoshansky
and Y. Apeloig, J. Am. Chem. Soc., 2006, 128, 2784; (e) Y. M. Lim,
M. E. Lee, J. Lee and Y. Do, Organometallics, 2008, 27, 6375.
3 Examples of b-functional silyl anions: (a) A. Kawachi and
K. Tamao, Organometallics, 1996, 15, 4653; (b) R. Fischer,
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2110; (d) M. Zirngast, M. Flock, J. Baumgartner and
C. Marschner, J. Am. Chem. Soc., 2008, 130, 17460.
4 (a) T. Tsumuraya, S. A. Batcheller and S. Masamune, Angew.
Chem., Int. Ed. Engl., 1991, 30, 902; (b) H. Watanabe, K. Higuchi,
T. Goto, T. Muraoka, J. Inose, M. Kageyama, Y. Iizuka,
M. Nazaki and Y. Nagai, J. Organomet. Chem., 1981, 218, 27.
5 A. Kawachi, K. Machida and Y. Yamamoto, Organometallics,
2009, 28, 6347.
6 For reviews of triarylgermyl–alkali metals: (a) P. Riviere,
M. R. Baudet and J. Satge, in Comprehensive Organo-
metallic Chemistry, ed. G. Wilkinson, F. G. A. Stone and
E. W. Abel, Pergamon Press, Ltd., New York, 1982, ch. 10, vol.
2, pp. 399–518.; (b) P. Riviere, M. R. Baudet and J. Satge, in
Comprehensive Organometallic Chemistry II, ed. A. G. Davies,
Elsevier Science, Ltd., Oxford, 1995, ch. 5, vol. 2, pp. 137–216.
Recent examples of triarylgermyl-alkali metals and related
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G. W. Bushnell, J. Organomet. Chem., 1995, 493, 47;
(d) A. Kawachi, Y. Tanaka and K. Tamao, Eur. J. Inorg. Chem.,
8 Pauling’s electronegativity: 3.98 for F; 1.90 for Si; and 2.01 for Ge.
Covalent bond energies: ca. 580 kJ molꢀ1 for Si–F; and ca.
460 kJ molꢀ1 for Ge–F: J. Emsley, The Elements, Oxford
University Press, Oxford, 2nd edn, 1992.
9 The inertness of THP for radical reactions was demonstrated by
Ryu et al.: H. Yasuda, Y. Uenoyama, O. Nobuta, S. Kobayashi
and I. Ryu, Tetrahedron Lett., 2008, 49, 367.
10 Crystallographic data for 3 recorded at 173 K with Mo-Ka
radiation (l = 0.71073 A): C42H66F3GeKN2O6Si3, Mr = 947.93,
ꢀ
triclinic, space group P1, a = 11.5904(9), b = 13.6336(10),
c
=
16.5928(12) A,
a
=
77.929(2),
b
=
83.078(2),
g = 71.543(2)1, V = 2427.9(3) A3, Z = 2, R1 = 0.0449
(I > 2s(I)), wR2 = 0.1069 (all data).
11 van der Waals radii estimated by Pauling’s approximation are
1.40 A for F, 1.93 A for Si, and 1.98 A for Ge: A. Bondi, J. Phys.
Chem., 1964, 68, 441.
12 Theoretical studies on metal-assisted ionization of silicon–heteroatom
bonds: (a) Y. Tanaka, M. Hada, A. Kawachi, K. Tamao and
H. Nakatsuji, Organometallics, 1998, 17, 4573; (b) M. Flock and
C. Marschner, Chem.–Eur. J., 2005, 11, 4635. For experimental
studies, see refs. 2a,d and 3b.
13 All calculations were performed with the Gaussian 98 program:
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7 Examples of Si–Ge bond cleavage with
a
nucleophile:
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ꢂc
This journal is The Royal Society of Chemistry 2010
1892 | Chem. Commun., 2010, 46, 1890–1892