Formation of a unique fluorosilene-KF complex bearing bulky substituents

Article abstract of DOI:10.1246/cl.2011.196

We report here the attempted synthesis of a stable 1-fluoro-2- hydrosilene bearing a Bpq group, which is a bulky aryl group bearing a cavity-shaped framework. We have successfully synthesized an overcrowded benzyldifluorosilane as a precursor. Treatment o

Full text of DOI:10.1246/cl.2011.196

doi:10.1246/cl.2011.196
196
Published on the web January 22, 2011
Formation of a Unique Fluorosilene-KF Complex Bearing Bulky Substituents
Eiko Mieda,1 Takahiro Sasamori,*1 Shohei Sase,2 Kei Goto,2 and Norihiro Tokitoh*1
1Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
2Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551
(Received December 6, 2010; CL-101030; E-mail: sasamori@boc.kuicr.kyoto-u.ac.jp,
tokitoh@boc.kuicr.kyoto-u.ac.jp)
We report here the attempted synthesis of a stable 1-fluoro-2-
H
F
H
Cl
Bpq C Si (t-Bu)
Cl
(2)
(1)
hydrosilene bearing a Bpq group, which is a bulky aryl group
bearing a cavity-shaped framework. We have successfully
synthesized an overcrowded benzyldifluorosilane as a precursor.
Treatment of the benzyldifluorosilane with 2 equiv of t-BuOK in
THF afforded the corresponding benzyl anion, the structure of
which showed a unique feature as a fluorosilene-KF complex.
BpqCH2Br
Bpq C Si (t-Bu)
H
F
H
1
(85%)
2 (95%)
F–
F
Si
K+
F
Bpq
Bpq
H
(3)
2
C
Si (t-Bu)
F
K
C
H
(t-Bu)
3 (54%)
X
Multiply bonded silicon species have been studied over
a long period and continue to be of substantial interest.1
Generally, such species are too reactive to be synthesized and
isolated as stable compounds, but they can be isolated as stable
species when they are well kinetically stabilized by using bulky
substituents. Especially, a disilene (Si=Si) or a silene (Si=C)
bearing an Si-X (X = halogen) bond is a fascinating species as
a simple-disilene or silene and as a building block of a low-
coodinated Si moiety. The first stable disilene bearing Si-Cl
bonds, RSi(Cl)Si=Si(Cl)RSi [RSi = SiMe(Sit-Bu3)2], was report-
ed by Wiberg et al., but it was inert toward water, methanol,
or hydrogen fluoride probably due to the extremely bulky
substituents.2 On the other hand, we3 and Tamao’s group4
independently reported the synthesis of stable 1,2-dibromodi-
silenes bearing two =Si-Br bonds by taking advantage of bulky
aryl groups, Bbt5 and Eind,6 respectively. In particular, our
stable dibromodisilene, Bbt(Br)Si=Si(Br)Bbt, was found to
undergo unique substitution reactions with organometallic
reagents such as MeMgBr on the Si=Si moiety to give the
corresponding disilene, Bbt(R)Si=Si(Br)Bbt (R = Me, Bu, or
Ph), showing its potential as a good precursor for several types
of disilenes.3 In the case of a silene, a stable difluoro-substituted
silene on the central carbon atom was synthesized by Sekiguchi
et al., which was characterized by NMR spectra.7 However,
there is no example of a halogen-substituted silene on a doubly
bonded silicon atom. We report here the attempted synthesis of
a stable 1-fluorosilene bearing bulky substituents. During the
course of our studies of the synthesis of a 1-fluorosilene bearing
Bpq and t-Bu groups (Scheme 1) on the C and Si atoms,
respectively, unique benzyl anion species, K+[BpqCHSiF2-
BpqX
Scheme 1. Reagents and conditions: (1) t-BuSiCl3, THF, reflux;
(2) AgBF4, toluene, reflux; (3) t-BuOK, THF, rt.
F(2)
DME
Si
Si
F(1)
F(1)
C
K
C
F(2)
Bpq
Bpq
K
DME
Figure 1. Molecular structure of the core moiety of [32¢(dme)1.5 2
]
(30% probability). The hydrogen atoms other than those on the
benzyl position, substituents of the Bpq groups, and one of the DME
molecule were omitted for clarity.
signal at ¹147.0 ppm as a pseudo-triplet (3JFH = 4.2 Hz). The
29Si NMR signal of 2 (C6D6) was observed at ¹9.2 ppm as a
triplet with JFSi = 320 Hz. When 2 was treated with 2 equiv of
1
t-BuOK at rt in THF, it was completely converted to a new
product as judged by 1H and 19F NMR spectra. Recrystallization
of the crude mixture from DME gave benzyl anion 3 as colorless
crystals. X-ray crystallographic analysis of 3 revealed its unique
dimeric structure with two potassium cations intermolecularly
bridging two fluorine atoms as shown in Figure 1, where the two
K+ ions are coordinated by the two DME molecules. Interest-
ingly, the two Si-F bonds are not identical, where one fluorine
atom [F(1)] is coordinated by the potassium atom and the other
one [F(2)] is bonded with the silicon atom without any
coordination. The Si-F(2) bond [1.805(19) and 1.86(2) ¡] is
apparently longer than the Si-F(1) bond [1.602(18) and
1.527(15) ¡]. Compared with the Si-F bond lengths of com-
pound 2 [1.5851(11) and 1.5868(12) ¡], it was suggested that
¹
(t-Bu)] (3), was obtained as a stable compound. Compound 3
was found to feature a silene-KF complex character.
A primary alkyl bromide with a cavity-shaped framework,
BpqCH2Br, was synthesized according to literature procedure.8
The reaction of BpqCH2Br with 3 equiv of Mg metal and t-
BuSiCl3 in THF under reflux afforded BpqCH2SiCl2(t-Bu) (1) in
85% yield (Scheme 1).
Dichlorosilane 1 was treated with an excess amount of
AgBF4 in toluene under reflux to give BpqCH2SiF2(t-Bu) (2)
in 95% yield. The 19F NMR spectrum of 2 (C6D6) showed a
Chem. Lett. 2011, 40, 196-197
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

197
H
OH
the Si-F(1) bond is fastened and the Si-F(2) bond is weakened
by the coordination of K+ atom toward F(1) atom.9 In addition,
the Si-C(benzyl) bond lengths are 1.832(15) and 1.827(16) ¡,
which are slightly shorter than those of 2 [1.856(6) ¡].
Accordingly, it was suggested that compound 3 should have
structural features as a fluorosilene-KF complex rather than a
silyl-substituted benzyl anion as in the case of [Li(12-crown-
air
Bpq C Si (t-Bu)
3
C6D6, rt
H
F
4
Scheme 2. Hydrolysis of 3.
¹
¹
4)2]+{FMe2Si-C(SiMe3)[SiMe(t-Bu)2]} showing F -coordi-
In summary, we have succeeded in the synthesis of 1-
fluorosilene-KF complex 3 as a stable crystalline compound. Its
unique structure was characterized by the X-ray crystallographic
nated silene character.10
In C6D6 solution, the 29Si NMR spectrum of 3 showed a
1
¹
doublet signal at ¹21.7 ppm with JSiF = 293.5 Hz, that is, the
and spectroscopic analyses, showing F coordinating silene
central Si nuclei was found to be coupled with only one fluorine
atom. In the 19F NMR spectrum of 3 in C6D6, two independent
signals were observed at ¹134.73 (d, 3JFH = 11.2 Hz) and
¹134.97 (d, 3JFH = 11.2 Hz) ppm with negligible F-F J-cou-
pling.11 Thus, one of the Si-F bonds of compound 3 is apparently
weakened in solution as in the case of the crystalline state. On the
basis of these structural and spectral features of compound 3, it
can be concluded that compound 3 exhibits unique character as
a 1-fluorosilene-KF complex, that is, a 1-fluorosilene weakly
coordinated by a fluoride anion at the silicon center.
character of 3.16 Further investigation of the elimination of
KF from 3 leading to the formation of the corresponding 1-
fluorosilene, Bpq(H)C=SiF(t-Bu), and reactivity of 3 are
currently in progress.
This work was partially supported by Grants-in-Aid for
Scientific Research (B) (No. 22350017) and the Global COE
Program B09 from Ministry of Education, Culture, Sports,
Science and Technology, Japan. E. M. is grateful for JSPS
fellowship for young scientists.
Unfortunately, 1H and 13C NMR spectra of 3 are very
broadened and/or complicated probably due to the restricted
rotation of the bulky aryl groups of the Bpq group and several
types of coupling with 19F and 1H nuclei. Such complexity of the
spectra could not be solved by variable temperature NMR spectra
(¹100 - +60 °C in toluene-d8 or C6D6). In order to assign the
13C NMR chemical shift of the benzyl carbon of 3, 13C-labeled
benzyl anion 3 (3-13C) was prepared according to a similar
procedure for 3 with using 13C-labeled methyl acetate. In the
13C NMR spectrum of 3-13C in C6D6, a characteristic doublet
signal was observed at 18.9 ppm with 2JCF = 21 Hz, where the J-
coupling with only one fluorine nucleus was observed. The Si-C
coupling constant, 1JSiC, of 3 is 58.2 Hz, which is slightly larger
than those of typical Si-C single-bond compounds (ca, 50 Hz)12
but smaller than that of 2-13C (1JSiC = 68 Hz).13 Interestingly,
19F NMR spectrum of 3-13C showed two double-doublet signals,
both of which were found to be coupled with the 13C nucleus to
similar extent (2JCF = 19.6 and 19.4 Hz). The 19F NMR spectrum
of compound 3-13C seems to be inconsistent with that of the
13C NMR spectra showing not a triplet signal but a doublet signal
coupled with one 19F nucleus. Although we have no clear
explanation for the observed NMR spectra at present, it can be
thought that the two fluorine atoms (F1 and F2) would rapidly
exchange with each other in solution. Indeed, 19F, 13C, and
29Si NMR spectra of 3-13C at low temperature in toluene (¹60 °C
to rt) showed dynamic behavior. For example, the two signals of
19F NMR spectra (¤F = ¹134.41 and ¹134.60) were shifted to
¤F = ¹138.41 and ¹146.77 at ¹60 °C.14 Thus, there would be
some dynamic change of the structure in solution, though the
geometry around C-Si-F moiety is not clear at present.
References and Notes
1
For recent reviews, see: a) M. Kira, T. Iwamoto, Adv. Organomet.
Chem. 2006, 54, 73. b) T. Sasamori, N. Tokitoh, in Encyclopedia of
Inorganic Chemistry, 2nd ed., ed. by R. B. King, John Wiley &
Sons, Chichester, 2005, p. 1698. c) M. Weidenbruch, in The
Chemistry of Organic Silicon Compounds, ed. by Z. Rappoport, Y.
Apeloig, John Wiley & Sons, Chichester, 2001, Vol. 3, p. 391.
N. Wiberg, S. K. Vasisht, G. Fischer, P. Mayer, Z. Anorg. Allg.
Chem. 2004, 630, 1823.
2
3
T. Sasamori, K. Hironaka, Y. Sugiyama, N. Takagi, S. Nagase, Y.
Hosoi, Y. Furukawa, N. Tokitoh, J. Am. Chem. Soc. 2008, 130,
13856.
4
5
K. Tamao, T. Matsuo, K. Suzuki, Jpn. Kokai Tokkyo Koho JP
2009215237 (A) 20090924, 2009.
Bbt: 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)meth-
yl]phenyl).
6
7
Eind: 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl.
M. Igarashi, M. Ichinohe, A. Sekiguchi, 18th. Symposium on
Fundamental Organic Chemistry, Japan, 2006, p. B24.
S. Sase, Y. Aoki, N. Abe, K. Goto, Chem. Lett. 2009, 38, 1188.
Since the central Si moieties were found to be disordered, the
structural parameters of the major part were shown here. Although
the standard deviations of structural parameters are relatively large
due to very weak reflections in the high-resolution area of the data,
the characteristic features of these structural parameters can be
discussed with sufficient accuracy.
8
9
10 N. Wiberg, G. Wagner, G. Reber, J. Riede, G. Müller, Organo-
metallics 1987, 6, 35.
11 The multiplicity and the coupling constants were determined by
using 19F{1H} NMR and also by measurement with different
frequencies.
12 E. A. Willians, in The Chemistry of Organic Silicon Compounds, ed.
by S. Patai, Z. Rappoport, Wiley, Chichester, 1989, Vol. 1, p. 511.
13 While the reason for such large JSiC value of 2-13C is unclear at
1
present, the Si-F bonds with high p-character would lead to a Si-C
bond showing high s-character.
Compound 3 was found to be thermally stable at 80 °C in
C6D6 solution and up to 156.5 °C in the crystalline state
(decomp). Interestingly, exposure of 3 toward air and moisture
in C6D6 solution afforded not the protonated compound 2 but the
corresponding fluorohydroxysilane 4, BpqCH2SiF(OH)(t-Bu),
quantitatively as judged by the NMR spectra, suggesting that
one Si-F bond would be weakened to undergo facile hydrolysis
(Scheme 2). Thus, compound 3 was found to show the reactivity
as a 1-fluorosilene.15
14 Unfortunately, the coupling constants of the 19F signals could not be
determined due to broadening of the signals. In the 29Si spectrum of
3-13C, the coupling constants could not be found probably due to the
low solubility and broadening under these conditions. The 13C NMR
signals were broadened at ¹60 °C probably due to the slow dynamic
process and the coupling with 19F atoms.
15 A. G. Brook, M. A. Brook, Adv. Organomet. Chem. 1996, 39, 71.
16 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
Chem. Lett. 2011, 40, 196-197
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/