Disilylfluoronium ions - Synthesis, structure, and bonding

Article abstract of DOI:10.1021/om2003128

The synthesis of disilylfluoronium ions 4 with a naphthalene-1,8-diyl backbone via the corresponding arenium ions 3 is reported. The cations were isolated in the form of their [B(C₆F₅)₄] ^- salts. The borates 3[B(C₆F₅)₄] and 4[B(C₆F₅)₄] are active in catalytic hydrodefluorination reactions using fluorodecane as substrate. Methylphenyl-substituted arenium ions 3c,d undergo an interconversion reaction via a formal 1,3-methyl group migration at room temperature. This rearrangement was shown by DFT methods to proceed by a multiple-step sequence involving a methonium-like transition state. The NMR parameters for disilylfluoronium ions 4 indicate, in agreement with DFT calculations, the presence of a symmetric Si-F-Si linkage in these cations. QTAIM, NBO, and VB analyses reveal that the high ionic contributions to the bonding in the SiFSi moiety are responsible for the symmetric structures of these cations.

Full text of DOI:10.1021/om2003128

ARTICLE
pubs.acs.org/Organometallics
Disilylfluoronium Ions—Synthesis, Structure, and Bonding
Nicole L€uhmann,† Hajime Hirao,‡,§ Sason Shaik,‡ and Thomas M€uller*,†
†Institut f€ur Reine und Angewandte Chemie, Carl von Ossietzky University Oldenburg, Carl von Ossietzky Strasse 9-11,
D-26129 Oldenburg, Federal Republic of Germany
‡Department of Organic Chemistry and The Lise Meitner-Minerva Center For Computational Quantum Chemistry,
The Hebrew University, 91904 Jerusalem, Israel
S Supporting Information
b
ABSTRACT: The synthesis of disilylfluoronium ions 4 with a
naphthalene-1,8-diyl backbone via the corresponding arenium
ions 3 is reported. The cations were isolated in the form of their
[B(C6F5)4]ꢀ salts. The borates 3[B(C6F5)4] and 4[B(C6F5)4]
are active in catalytic hydrodefluorination reactions using
fluorodecane as substrate. Methylphenyl-substituted arenium
ions 3c,d undergo an interconversion reaction via a formal 1,3-
methyl group migration at room temperature. This rearrange-
ment was shown by DFT methods to proceed by a multiple-step sequence involving a methonium-like transition state. The NMR
parameters for disilylfluoronium ions 4 indicate, in agreement with DFT calculations, the presence of a symmetric SiꢀFꢀSi linkage
in these cations. QTAIM, NBO, and VB analyses reveal that the high ionic contributions to the bonding in the SiFSi moiety are
responsible for the symmetric structures of these cations.
Scheme 1. Synthesis of Precursor Disilanes (R1 = R2 = Ph,
’ INTRODUCTION
5a; R1 = R2 = Tol, 5b; R1 = Me, R2 = Ph 5c)
One decade after the successful synthesis and structural
characterization of silylium ions, the chemistry of these
extremely electrophilic compounds has now reached a turning
point.1,2 While the taming of the high reactivity was the focus
in the beginning,2 the target is now the ingenious use of exactly
this high electrophilicity of silylium ions for novel, syntheti-
cally useful applications in synthesis.3 The first examples
already demonstrated that the enormous Lewis acidity can
be successfully applied in DielsꢀAlder cyclizations4 and in
Mukaiyama aldol4c condensation reactions. Particularly suc-
cessful was the use of silyl cation salts in the CꢀF activation
reactions. Pioneering work by the group of Ozerov5 and by our
group6 demonstrated that silyl arenium ions 1 and disilyl
cations 2 can be used in highly efficient catalytic hydrode-
fluorination reactions of aliphatic organofluorides and in CꢀC
scrambling in arenium ions 3 and give an account on the
bonding in fluoronium ions 4, with particular emphasis on a
comparison to its carbon congener.
cross-coupling reactions between silyl arenes and aromatic
fluorides. Quite recently, Siegel and co-workers extended this
concept and developed an intriguing catalytic FriedelꢀCrafts
arylation chemistry.7 In our CꢀF activation work, we used
disilyl cations of type 2 with a naphthalene-1,8-diyl backbone
as catalysts.6 The electron deficiency of the silicon atoms in
these disilyl cations can be modified and regulated by the
bridging substituent X and by the substituents R1ꢀR4, and
thereby their reactivity becomes controllable. As an extension
of our previous work on disilyl arenium ions 38,9 and fluor-
onium ions 4,6a we present here the synthesis of several new
fluoronium ions via the corresponding arenium ions that can
be applied in catalytic CꢀF activation reactions. In addition,
Received: May 14, 2011
Published: June 28, 2011
we report on a surprisingly high tendency for substituent
r
2011 American Chemical Society
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|

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Scheme 2. Synthesis of Arenium (3) and Fluoronium Ions (4)a
a See Table 1 for the assignment of R1ꢀR5.
Table 1. Significant NMR Parameters of Cations 3, 4, and 6
compd
3aa
3ba
R1 R2 R3 R4 R5
δ
29Si (1JSiH (Hz))
δ
19F (1JSiF (Hz))
δ1H(SiH) (1JHSi (Hz))
δ
13C1; δ13C2,6; δ13C4
Ph Ph Ph
H
ꢀ2.2 (SiPh2); ꢀ9.7 (SiHPh, 248)
5.21 (247)
5.34 (243)
4.82 (237)
4.93 (229)
5.35 (233)
95.1; 164.7, 165.8; 153.1
89.9; 163.5, 164.5; 170.7
95.7; c
Tol Tol Tol
Me Ph Me
Me Ph Me
Me Me Ph
CH3 ꢀ3.3 (SiTol2); ꢀ11.4 (SiHTol, 249)
H
H
H
cis-3c
trans-3c
3d
ꢀ10.2 (SiHMe); 4.8 (SiMePh)
ꢀ11.8 (SiHMe); 6.1 (SiMePh)
ꢀ13.5 (SiHPh); 12.9(SiMe2)
40.0 (253)
95.3; c
95.6; 165.4, 166.0; 153.3
4a
Ph Ph Ph Ph
Tol Tol Tol Tol
ꢀ130.8 (253)
4b
40.2 (253)
ꢀ129.1 (253)
cis-/trans-4cd Me Ph Me Ph
58.4 (250); 58.3 (250) SiMePh
ꢀ134.1 (250); ꢀ133.2 (250)
4d
6b
Me Me Ph Ph
76.8 (SiMe2, 245); 41.2 (SiPh2, 250) ꢀ137.1 (250)e
77.2 (242)
Me Me Me Me
ꢀ144.0
a From ref 8b. b From ref 6a. No clear assignment of the carbon atoms C2/6 and C4 could be made, due to overlapping signals from the second
stereoisomer and from 3d; see the Experimental Section and Supporting Information. d Only obtained as a mixture with 4d. e The splitting of the 29Si
satellites due to the coupling of the 19F nuclei with two different 29Si atoms could not be resolved.
c
95.6) for the sp3-hybridized ipso carbon atom are detected. Two
’ RESULTS AND DISCUSSION
distinct signals in the 29Si{1H} NMR spectra (δ29Si ꢀ13.5, 12.9)
The symmetric precursor disilanes 5aꢀc were prepared along
indicate the asymmetric substitution of the arenium ion 3d. The
a synthetic route reported previously from 1-bromonaphthalene
NMR spectroscopic identification of the arenium ion 3c is
hampered by the formation of both possible stereoisomers cis-
via 1,8-dilithionaphthalene (Scheme 1) by a salt metathesis
reaction.8b The disilanes 5 were obtained in low to moderate
and trans-3c upon ionization. According to the integration of the
yields. The substitution pattern at the silicon atom in compound
5c generates two centers of chirality, and therefore it was
obtained as a nonseparable mixture of two diastereomers. The
SiH 1H NMR signals the stereoisomers are formed in a 3:2 ratio.
1
The assignments of the H and 29Si NMR signals to the indi-
vidual isomers (see Table 1 and the Supporting Information) was
unsymmetrical peri-silyl-substituted naphthalene 5d was ob-
tained in a stepwise lithiation/silylation reaction sequence
(Scheme 1). Disilanes 5 were transformed quantitatively into
the corresponding arenium ions 3 by reaction with trityl tetrakis-
(pentafluorophenyl)borate, [Ph3C][B(C6F5)4], in benzene or
toluene (Scheme 2).8 The borates 3[B(C6F5)4] were obtained
after washing with benzene and pentane as a slightly yellow,
glassy material. The newly obtained arenium borates 3c[B-
(C6F5)4] and 3d[B(C6F5)4] were fully characterized by NMR
spectroscopy. The most significant data are summarized in
Table 1, along with those reported previously for 3a,b. The
reaction of arenium borates 3[B(C6F5)4] with trifluorotoluene in
benzene yields quantitatively, according to NMR spectroscopy,
the fluoronium salts 4[B(C6F5)4] (Scheme 2). The colorless
amorphous salts were washed with pentane and characterized by
NMR spectroscopy (Table 1).
1
supported by two-dimensional NOESY, H/1H COSY, and
29Si/1H HMBC spectroscopy. The NOESY spectrum was cen-
tral to the assignment. It allowed distinguishing between the trans
isomer, trans-3c, which features two methyl groups at the silicon
atoms in 1,3-diaxial positions on the same side of the newly
formed six-membered ring. In this configuration a NOE contact
between both methyl groups is possible. In contrast, in stereo-
isomer cis-3c, with an anti orientation of these methyl groups,
their spatial separation is too large for significant NOE transfer.
The 13C NMR spectra of a benzene-d6 solution of the salt
3d[B(C6F5)4] show a chemical shift pattern which is character-
istic for arenium ions.8 That is, three signals at low field (δ13C2,6
166.0, 165.4; δ13C4 153.3) for the deshielded methine carbon
atoms at the ortho, ortho0, and para positions of the bridging aryl
group and one signal at a relatively high field position (δ13C1
NMR spectra of arenium borates 3a[B(C6F5)4] and 3b[B-
(C6F5)4] showed no significant change upon standing at room
temperature or slightly elevated temperatures.8b In contrast,
upon prolonged standing of a benzene-d6 solution of arenium
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Scheme 3. Independent Synthesis of Arenium Ions 3c,d and Their Interconversion
Figure 1. 500 MHz 1H NMR spectra of 3d[B(C6F5)4] in benzene at 300 K (a) shortly after preparation (only the region of the SiH resonances is
shown) and (b) after 1 h at 300 K (spectrum resolution enhanced by gauss multiplication). (c) Calculated reaction coordinate for the isomerization
reaction 3d f cis-3c (at the B3LYP/6-311G(d,p) level). Free enthalpies at 298 K, G298, are given relative to the conformer of arenium ion 3d with the
phenyl substituent in the equatorial position (3d(Ph eq)).
Scheme 4. Solvolysis of Tris(trimethylsilyl)methylsilyl Halides 7 According to Eaborn and Co-workers10
borate 3d[B(C6F5)4] additional signals in the NMR spectra
indicate the formation of both stereoisomers of arenium ion 3c
(Scheme 3 and Figure 1). Similarly, warming of a benzene
solution of a freshly prepared cis/trans mixture of 3c[B(C6F5)4]
yields 3d in equilibrium with cis-/trans-3c. From the integration
of the signals of the SiH hydrogen atoms at 300 K the relative
equilibrium concentrations 3d:cis-3c:trans-3d are 1.5:1.6:1
(Figure 1b). Formally, the isomerization 3d f 3c requires a
1,3-shift of one methyl group across the six-membered ring. This
alkyl group migration bears a close resemblance to seminal work
by Eaborn and colleagues.10 They previously noted that silyl
halides 7 underwent dissociative solvolysis reactions and gave in
addition to the expected product 8 also rearranged silanes 9
(Scheme 4). These 1,3-methyl migrations are thought to proceed
via methyl-bridged intermediates 10, although equilibrium
between trivalent silyl cations 11 and 12 could not be excluded
on the basis of the experimental data at that time (Scheme 4).
A related 1,3-methyl migration was noticed by Sekiguchi and
co-workers in a polysilyl silylium ion,11 and similar methyl
migrations are important steps in Lewis acid catalyzed rearrange-
ments of polysilanes.12 In this connection the reversible intra-
molecular transformation 3c f 3d presented here is of principal
interest, and therefore, we conducted in addition to the experi-
mental investigations a DFT study.13 The results of the DFT
calculations at the B3LYP/6-311G(d,p) level suggests that the
intramolecular isomerization 3c f 3d is a multiple-step reaction
involving the two high-lying intermediates 13 and 14 (see
Scheme 5 and Figure 1c).14 The calculations predict an overall
free enthalpy barrier of 90 kJ molꢀ1 at room temperature, with
the highest barrier being that connected with the opening of the
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ARTICLE
Scheme 5. Suggested Mechanism for the Isomerization 3d f 3c Based on DFT Calculations at B3LYP/6-311G(d,p)
Figure 2. Calculated molecular structures of pertinent stationary points along the reaction coordinate 3d f cis-3c (B3LYP/6-311G(d,p) level, bond
lengths and interatomic distances given in pm, bond angles and sum of bond angles (∑R) given in deg, hydrogen atoms bonded to aryl groups not
shown).
aryl bridge in cation 3d and formation of the secondary silyl
cation 13 (Figures 1c and 2). The relatively high calculated
barrier is in qualitative agreement with the observed slow
conversion at room temperature. The computed structure of
silyl cations 13 and 14 indicate the stabilization of the positively
charged silicon atom by an agostic interaction with one CꢀH
bond of the methyl group of the second silyl group. This is
suggested by the calculated molecular structure of cation 13,
which features an elongated CꢀH bond (by 2.1 pm compared to
CH bonds of the same methyl group; see Figure 2) in the methyl
group directed toward the secondary silylium ion and a signifi-
cantly pyramidalized tricoordinated silicon center (∑R = 345.9°).
A methyl-bridged species with a pentacoordinated carbon atom
analogous to neutral methyl-bridged dialuminium compounds15
or to those suggested by Eaborn et al.10 does not correspond to a
minimum along the reaction coordinate for the transformation
3c/3d. Such a molecular arrangement was identified, however, as
transition state TS(13/14), which interconverts the two silyl
cations 13 and 14 (see Figure 2).
Table 2. Catalytic Hydrodefluorination Reactions Using
Arenium and Fluoronium Borates as Catalystsa
entry
cat.
RF
RH
amt, mol % TON
1
3a [B(C6F5)4] n-C10H21
3b [B(C6F5)4] n-C10H21
3d [B(C6F5)4] n-C10H21
4a [B(C6F5)4] n-C10H21
4d [B(C6F5)4] n-C10H21
F
F
F
F
F
F
F
n-C10H22
n-C10H22
n-C10H22
n-C10H22
n-C10H22
n-C10H22
n-C10H22
2.0
4.3
1.0
1.0
1.0
2.9
2.2
48
23
39
45
51
35
45
2
3
4
5
6b
7b
6 [B(C6F5)4]
n-C10H21
18 [B(C6F5)4] n-C10H21
a For exact reaction conditions, see the main text and the Experimental
Section. The temperature in all cases was 25 °C. b From ref 6a.
slightly smaller than in neutral fluorosilanes (i.e., 1JSiF in Ph3SiF is
281 Hz and in PhMe2SiF is 278 Hz).18 In particular, it is
1
significantly larger than the JSiF coupling constant found
for the dynamic molecule 17 (1JSiF = 127 Hz).19 In this case,
the JSiF coupling constant is considerably reduced due to a
degenerate fluorine exchange which is fast on the NMR time
Aryl-substituted disilylfluoronium ions 3 are identified by their
typical 29Si and 19F NMR parameters (Table 1), which can be
compared with data for the related cations 6, 15, and 16.6a,16,17
The silicon atoms of the SiFSi group in cations 4 are strongly
deshielded (δ29Si 40.0ꢀ76.8), which indicates considerable
charge accumulation at the tetracoordinated silicon atom. For
a more detailed analysis this chemical shift region can be further
subdivided into segments that are characteristic for diaryl-sub-
stituted (δ29Si 40ꢀ41), for arylmethyl-substituted (δ29Si 58),
and for dimethyl-substituted silicon atoms (δ29Si 77ꢀ78). The
19F NMR resonances of the bridging fluorine atoms in aryl-
substituted disilylfluoronium ions 4 (δ19F ꢀ130.8 to ꢀ137.1)
are deshielded compared to those of neutral triarylfluorosilanes
(e.g., δ19F(Ph3SiF) = ꢀ170.4)18 and fall in the chemical shift region
reported previously for alkyl-substituted disilylfluoronium ions 6
1
scale even at 240 K. In addition, the very similar JSiF coupling
constants in the asymmetrically substituted cation 4d (1JMe SiꢀF
=
245 Hz; 1JPh SiꢀF = 250 Hz) suggest a similar bonding sit2uation
2
for both SiF linkages in this particular cation. Thus, the large 1JSiF
coupling constants in cations 4 rule out the possibility of a fast
equilibrium between unsymmetrically bridged cations and suggest
for these aryl-substituted disilyl fluoronium ions a symmetrical
SiꢀFꢀSi moiety, similar to those firmly established by X-ray
diffraction experiments for their alkyl-substituted counterparts
6 and 16.6a,17
1
(δ19Fꢀ144.0)6a and 16 (δ19F ꢀ132.0).17 The JSiF constant
between the central fluorine atom and the neighboring silicon
atoms in cations 4 (1JSiF = 245 ꢀ253 Hz; see Table 1) is only
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Table 3. Significant Computed Structural Parameters for Fluoronium Ions 4 and 6 and Related Compoundsa
2
j
entry
compd
point group
SiꢀF (pm)
181.6, 182.7
FꢀSiꢀF (deg) NBO charge at F (e)//WBI (E-F)i F (e bohrꢀ3)//r F (e bohrꢀ5
)
1
4a
C1
C1
C1
C2
C1
C2
C2
C1
C3
C1
C1
C2
C3
129.2
129.1
130.6
131.0
128.0
128.7
127.9
129.9(1)
ꢀ0.60//0.322, 0.312 (SiFSi)
0.07, 0.06//0.429, 0.400
2
4b
181.5, 182.7
3
cis-4c
trans-4c
4d
181.8, 182.3
4
182.0
5
178.9 (Me2SiF), 185.8 (Ph2SiF)
6
6
6b
6c,d
182.0
7
180.7
8
175.5(8), 176.3(5)
163.9
143.2,g 249.0h
142.4(2)g, 244.4(2)h
175.3(9)
9
19
ꢀ0.63//0.573 (SiF)
0.08//0.862
10
11
12
13
20
112.7
ꢀ0.41//0.759, 0.029 (CꢀF/C+)
0.201, 0.018//0.233, 0.077
20e,f
16k
22
111.1(1)
163.0(3)
142.7
ꢀ0.42//0.779
0.207//0.185
a At the B3LYP/6-311G(d,p) level. b Calculated at the B3LYP/6-311+G(2d,p) level. c Experimental data from XRD of the [B(C6F5)4] salt. d From ref 6a.
e Experimental data from XRD of the [BF4] salt. From ref 20. g CꢀF bond length. h CF C+ distance. At the B3LYP/6-311+G(d,p) level.
f
i
3 3 3
j Calculated using an HF/6-311G(d,p) wave function. k From ref 17.
The disilylarenium borates 3a[B(C6F5)4], 3b[B(C6F5)4], and
3d[B(C6F5)4] and disilylfluoronium borates 4a[B(C6F5)4] and
4d[B(C6F5)4] are active in catalytic CꢀF activation processes.
Test reactions were performed at room temperature using a small
quantity of the disilyl cation borate, n-fluorodecane as substrate,
and triethylsilane as the solvent and hydride source. After 30ꢀ60
min the alkyl fluoride was consumed and the reaction was
completed. The determined turnover numbers (TON) of
23ꢀ51 clearly indicated a catalytic reaction (Table 2). Arenium
ions 3 and fluoronium ions 4 showed comparable activities which
are practically identical with those reported previously for the
related fluoronium and hydronium ions 6 and 18.6
In the absence of XRD data, reliable structural parameters for
fluoronium ions 4 were obtained from density functional compu-
tations.13 The most significant computed molecular structural
data for cations 4 are summarized in Table 3 and are compared
with experimental data found previously for the tetramethyl-
substituted fluoronium ion 6 and the bis(trimethylsilyl)fluoro-
nium species 16.6a,17 The calculated structural data for cation 6
indicate an overall good agreement with the experimental data;
however, the SiꢀF bonds are computed too long by Δδ =
5.7ꢀ6.5 pm (at the B3LYP/6-311G(d,p) level; see Table 3,
entries 6 and 8). Basis set extension to a 6-311+G(2d,p) basis set
improves the situation (Δδ = 4.4ꢀ5.2 pm; Table3, entry 7);
however, the size of the studied cations and the extent of the
study excluded the use of this large basis set throughout. The
computed structure of the tetraphenyl-substituted fluoronium
ion 4a is shown in Figure 3. All theoretical structures show in all
significant structural aspects a close resemblance to the experi-
mental molecular structure of cations 6 and 16. The most
prominent feature of cations 4 is a nearly symmetric bent
SiꢀFꢀSi linkage (bend angles R = 128.0ꢀ130.8°) with SiꢀF
bond lengths that are clearly elongated compared to the com-
puted SiꢀF bond length in triphenylsilyl fluoride (Ph3SiF, 19)
(181.5ꢀ182.7 pm in cations 4aꢀc and 163.9 pm in 19; see
Table 3 and Figure 3). Even in the unsymmetrically substituted
cation 4d the two computed SiꢀF bond lengths differ only by
6.9 pm and the shortest SiꢀF bond is elongated by 9% compared
to the SiF single bond in silyl fluoride 19. Therefore, the
combined experimental NMR spectroscopic and theoretical data
Figure 3. Calculated molecular structure of fluoronium ion 4a
(B3LYP/6-311G(d,p), bond lengths given (in pm) in boldface and
bond angles in italics, hydrogen atoms omitted): (black) carbon; (gray)
silicon; (white) fluorine.
Figure 4. (a) Contour plots of the calculated electron density of 4a in
the Si(1)ꢀFꢀSi(2) plane. The bond paths are indicated as bold lines
and the corresponding bond critical points as red circles. (b) Contour
2
plot of the Laplacian of the electron density, r F(r), in the same plane.
2
Red contours indicate regions of local charge accumulations (r F (r) < 0)
2
and blue contours regions of local charge depletion (r F(r) > 0).
suggest that bis-silyl-substituted fluoronium ions 4 exist as single
minima on their respective potential energy surfaces (PES).
These findings are in contrast with the results reported by
Gabbaï and co-workers for the closely related carbocation 20.20
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Scheme 6. Bonding in Silyl-Substituted Fluoronium Ions 4 and in Carbocation 20
According to XRD measurements, NMR spectroscopic investi-
gations, and quantum mechanical studies, carbocation 20 adopts
a molecular structure with a nonsymmetrical CꢀF/C+ linkage,
consisting of one covalent CꢀF bond and one dative F/C+
interaction. The symmetrical fluoronium ion 21 is a local
maximum on this PES, separating two equivalent minimum
structures corresponding to carbocation 20.20 This intriguing
difference in particular between the silicon compound 4a and its
carbon congener 20 aroused our interest and prompted us to
compare both cations more closely by means of natural bond
orbital (NBO) analysis and quantum theory of atoms in mol-
ecules (QTAIM) and by considering valence bond correlation
mixing diagrams (VBCMD) for a closely related model.
during the fluorine migration between the cations H3E+ and
the fluoride FꢀEH3 (E = C, Si). The energies of VB structures
for the distorted and bond-symmetric states were computed at
the VBSCF/6-31G(d) level using the XMVB program25 linked
to GAMESS (see Figures S1 and S2 in the Supporting Informa-
tion for details).26 The final (adiabatic) energy curve was
inferred from the relative energies and mixing patterns of the
VB structures.
Figures 5 depicts the energy run of the covalent Heitlerꢀ
London (HL) and the ionic structures for a fluoride ion
+
transfer process between two EH3 cations. It is seen that for
both E = Si and E = C, the ionic structure lies below the covalent
structures. Furthermore, at the symmetric geometry the electro-
static stabilization is maximized and this leads to a symmetric
single-minimum structure with a triple-ion character, H3E+Fꢀ +EH3.
The triple ionic structure marks the main difference between
E = C and E = Si. Thus, the energy gap from the triple-ionic to the
covalent structures is much larger for E = Si, and the correspond-
ing curve is also much deeper at the symmetric geometry. These
features of the triple-ionic structures in the case of silicon are a
result of two factors: (i) the lower ionization potential (IP) of the
silicon radical center, which lowers the triple-ionic curve relative
According to the results of the NBO analysis,21 the negative
charge at the fluorine atom is virtually the same in fluoronium ion
4a as in the neutral model fluorosilane Ph3SiF (19) (ꢀ0.63e
(19); ꢀ0.60e (4a)). This indicates only insignificant charge
transfer from the fluorine atom to the silicon atoms in cation 4a.
The calculated bond orders (as gauged by the Wiberg bond index
(WBI))22 are nearly identical for both SiꢀF bonds in fluoronium
ion 4a, and they are significantly smaller than those computed for
the model compound 19 (see Table 3), in agreement with the
expected multiple-center bonding in cation 4a. In the framework
of the QTAIM23 analysis the SiꢀF bonds in both silyl fluoride 19
and fluoronium ion 4a are highly ionic, as indicated by their small
electron densities F and by their substantial positive Laplacian
to the HL curves (i.e. IP = 786 kJ molꢀ1 for SiH3 and IP = 951 kJ
•
molꢀ1 for CH3 , at the B3LYP/6-311G(d,p) level) and (ii) the
•
high positive charge localization at the silicon atom in SiH3+ with
even negative charge on the corresponding hydrogen atoms (see
Figure 6). In contrast, in the case of E = C, the positive charge is
delocalized over all the atoms of the cation.
2
r F(r) at the bond critical points (see Figure 4 and Table 3).
The properties of the electron densities for both bond critical
points of the SiꢀFꢀSi linkage in 4a are nearly identical, as
expected for a symmetrical structure. It is noteworthy, however,
that the ionicity of the SiꢀF bonds in cation 4a is significantly
smaller than that of the SiꢀF bond in the neutral fluorosilane 19,
As a consequence of this high positive charge, the ionic radii
+
+
for the SiH3 cation is smaller than for the CH3 cation (e.g.,
r(CH3 ) = 64 pm and r(SiH3 ) = 35 pm),27a and this allows a
closer contact between anion and the silicon cation, thereby
maximizing the electrostatic interaction between the silicon
cations and the fluoride anion. In contrast, with the charge-
delocalized carbenium ions, the electrostatic interaction is weak-
er and the curve is less deep. In both cases in Figures 5, the
covalent (HL) structures mix with the triple-ionic structure to
shape the final (adiabatic) energy curve.24,27 Since the covalentꢀ
ionic energy separation at the reactant and product geometries
for E = C are small, the mixing is significantly larger than in the
symmetrical F- - -C- - -F structure. This, in turn, leads to a single
transition state separating the two unsymmetrical fluorine-
bridged clusters (Figure 5a). The fluoride transfer process be-
tween two silylium ions (Figure 5b), however, clearly dominates
along the complete reaction coordinate by the very favorable
triple-ionic structure. This is because the energy gap between the
ionic and the HL structures is much larger than in the case of
methylium ions. With such large gaps between the ionic and
covalent structures, their mixing is smaller and the ionic curve
retains its shape with a minimum at the symmetric geometry. In
conclusion, the silylium ions in our model behave more like
protons having a concentrated positive charge and unlike carbo-
cations which possess delocalized charges, a fact which underlines
+
+
2
as indicated by the less positive Laplacian r F(r) (see Table 3).
In summary, the results of preceding analyses for 4a suggest that
the ionic canonical structure 4(B) (Scheme 6) is important for
the understanding of the bonding in fluoronium ions 4.
The character of the CꢀF bonds in the model trityl fluoride 22
as well as in the carbocation 20 is clearly less ionic than in their
silicon analogues cation 3a and silyl fluoride 19. This is indicated
by the higher electron density F(r) and the less positive Laplacian
2
r F(r) at the bond critical points of the CꢀF bonds. The
bonding characteristics of the CꢀF bond in cation 20 are very
similar to those in the model compound 22 (see Table 3). In
addition, only a relative small electron density F(r) is computed
at the bond critical point for the F/C+ interaction. Therefore, the
results of the QTAIM suggest that the CꢀF/C+ linkage in
carbocation 20 consists of a covalent CꢀF bond and a dative
F/C+ interaction, in agreement with the previous bonding
analysis by Gabbaï, Hall, and co-workers.20
For a complementary understanding of the differences be-
tween the two obviously so closely related cations 4a and 20, we
used the valence bond configuration mixing diagram (VBCMDs)
model24 and analyzed the principal VB structures involved
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Figure 5. Valence bond configuration mixing diagrams (VBCMDs) for the fluoride transfer (a) between two methyl cations and (b) between two silyl
cations: (red) energy profiles of the covalent HeitlerꢀLondon (HL) structures of reactant and product (ΦHLR, ΦHLP); (blue) energy profile of the
ionic structures (ΦIon); (black) qualitative adiabatic energy curves.
linkage similar to those found for the closely related cations 66a
and 1617 by XRD measurements.
Arenium and fluoronium borates 3[B(C6F5)4] and 4[B-
(C6F5)4] are active catalysts in CF activation processes. As
already noted for related silyl cation based catalysts, aliphatic
CꢀF groups were exclusively reduced.5,6 The determined TONs
Figure 6. Calculated NBO charges (in italics) for methylium and
show the catalytic course of the reaction.
silylium cations (at the B3LYP/6-311G(d,p) level).
The intriguing difference in structure between the closely
related bissilylfluoronium ion 4a and the carbocation 2020 was
analyzed applying NBO theory, QTAIM, and valence bond
schemes. The theoretical analysis indicates that the high ionicity
of the SiꢀF bond is the main factor which determines the
the close relationship of fluoronium ions such as 4 with proto-
nated hydrogen fluoride [HꢀFꢀH]+.17
symmetrical Si
F
Si arrangement in cation 4a.
3 3 3 3 3 3
’ CONCLUSION
’ EXPERIMENTAL SECTION
The synthesis of bis-silylated fluoronium ion salts 4[B-
(C6F5)4] via the corresponding arenium borates 3[B(C6F5)4]
is reported. The arenium ions 3 were formed by hydride transfer
reaction between the peri-substituted naphthylsilanes 5 and trityl
borate, Ph3C[B(C6F5)4]. Subsequent reaction of the so obtained
arenium borates 3[B(C6F5)4] with trifluorotoluene yields the
fluoronium ions 4. The disilyl cations 3 and 4 were characterized
by multinuclear NMR spectroscopy.
The bis(methylphenylsilyl)- and (dimethylsilyl)(diphenyl-
silyl)-substituted arenium ions 3c,d, respectively, undergo at
room temperature a slow interconversion via 1,3-methyl group
transfer. The results of DFT calculations suggest for this reaction
a multiple-step mechanism via the high-lying unsymmetrical
methyl-bridged intermediates 13 and 14 and a methonium-like
transition state (TS13/14).
General Procedures. Diethyl ether and tetrahydrofuran were
distilled from sodium/benzophenone. Benzene, benzene-d6, hexane,
and pentane were distilled from sodium. TMEDA, Me2HSiCl, and
Ph2HSiCl were distilled freshly from calcium hydride. 1-Fluorodecane,
R,R,R-trifluorotoluene, and hexafluorobenzene were freshly destilled
from P4O10. All reactions were carried out under inert conditions. NMR
spectra were recorded on Bruker DPX-250, DPX 400, Avance 300
1
and Avance 500 spectrometers at 305 K, if not stated otherwise. H
and 13C NMR spectra were calibrated using residual solvent signals
δ1H(CHCl3) 7.24, δ13C(CDCl3) 77.0, δ1H(C6D5H) 7.20, and
δ13C(C6D6) 128.0. 29Si NMR spectra were calibrated using external
Me2HSiCl (δ29Si 11.1 vs TMS) and 19F NMR against external CFCl3
(δ19F(CFCl3) 0.0). 29Si NMR spectra were recorded using the INEPT
pulse sequence. Mass spectra were recorded on a Finnigan MAT 212
instrument with electron ionization. 1,8-Dibromonaphthalene and
[Ph3C][B(C6F5)4] were synthesized as described in the literature.28,29
Compound 5c. n-Butyllithium (12 mmol,1.6 M in hexane) was
added slowly to a stirred solution of 10 mmol of 1-bromonaphthalene in
For all fluoronium ions 4, the obtained NMR spectroscopic
data, in particular the large determined 1JSiF coupling constants
(1JSiF = 242ꢀ253 Hz), and the results of DFT calculations
indicate a molecular structure with a symmetric Si
F
Si
3 3 3 3 3 3
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30 mL of dry diethyl ether at ꢀ40 °C. After a further 30 min of stirring
15 mL of dry hexane was added at ꢀ75 °C and the supernatant clear
solution was decanted off via a flexible Teflon tube. The precipitate was
washed three times with 20 mL portions of dry hexane and dried under
vacuum. Then 14 mmol of n-butyllithium (1.6 M in hexane) and
14 mmol of TMEDA were added to the resulting white precipitate.
The reaction mixture was heated to reflux for 3 h. The supernatant
brown solution was decanted off via a syringe. The precipitate was
washed once with hexane and cooled to ꢀ30 °C. After addition of 10 mL
of diethyl ether, 25 mmol of methylphenylchlorosilane was added. The
suspension was warmed to room temperature and heated to reflux for
1 h. The solvent was removed under vacuum, and hexane was added.
The suspension was filtered and the solvent removed under vacuum.
The crude product was purified by column chromatography on silica
using THF: Rf = 0.9. Yield: 8.7 mmol as a colorless oil, 87%. 1H NMR
(499.87 MHz, C6D6): δ 0.59 (d, 6H, CH3, 3JHH = 3.6 Hz), 0.68 (d, 6H,
[Ph3C][B(C6F5)4] in 2 mL of benzene at 5ꢀ10 °C via a syringe. The
mixture was stirred for 30 min at 20 °C. Stirring was stopped, and the
two phases were allowed to separate. The lower phase was transferred
into an NMR tube and investigated by NMR spectroscopy. The brown
salt was purified by washing with 2 mL of pentane and then with 1 mL of
benzene to give cis-/trans-3c[B(C6F5)4] in a 3:2 mixture as a glassy solid.
1
Data for cis-3c[B(C6F5)4]: H NMR (499.87 MHz, C6D6) δ ꢀ0.60
3
(d, 3H, CH3, JHH = 3.2 Hz), ꢀ0.35 (s, 3H, CH3), 4.82 (m, 1H,
SiH 1JSiH = 236.6 Hz); 13C NMR (125.77 MHz, C6D6) δ ꢀ5.5, ꢀ2.6,
95.7 (Cipso);30 29Si NMR (99.30 MHz, C6D6) δ ꢀ10.2 (SiH), 4.8
(SiMePh). Data for trans-3c[B(C6F5)4]: 1H NMR (499.87 MHz, C6D6)
δ ꢀ0.42 (d, 3H, CH3, 3JHH = 3.2 Hz), 0.57 (s, 3H, CH3), 4.82 (m, 1H,
SiH, 1JSiH =228.6 Hz); 13C NMR (125.77 MHz, C6D6) δ ꢀ5.1, ꢀ1.1,
95.3 (Cipso));30 29Si NMR (99.30 MHz, C6D6) δ ꢀ11.8 (SiH), 6.1
(SiMePh).
Arenium Borate 3d[B(C6F5)4]. A 2 mL portion of benzene was
added to a mixture of 0.5 mmol of solid 5d and 0.5 mmol of
[Ph3C][B(C6F5)4] via vacuum transfer. The solution was warmed to
room temperature for 30 min. Stirring was stopped, and the two phases
were allowed to separate. The lower phase was transferred into an NMR
tube and investigated by NMR spectroscopy. The brown salt was
purified by washing with 2 mL of pentane and then with 1 mL of
benzene to give 3d[B(C6F5)4] as a glassy solid. 1H NMR (499.87 MHz,
C6D6): δ ꢀ0.55 (s, 3H, CH3), 0.37 (s, 3H, CH3), 5.35 (s, 1H, SiH,
1JSiH = 233.4 Hz), 6.67ꢀ7.34 (m, ArH), 7.60ꢀ7.63 (m, ArH),
7.72ꢀ7.79 (m, ArH), 8.08ꢀ8.11 (m, ArH). 13C NMR (125.77 MHz,
C6D6, T = 305 K): δ ꢀ2.0 (CH3), 1.4 (CH3), 95.6 (C1), 122.2 (Cq),
126.1 (Cq), 126.3, 126.4, 126.4, 126.1, 128.5, 128.6, 128.8 (Cq), 129.9,
3
1
CH3, JHH = 3.6 Hz), 5.89ꢀ5.93 (m, 4H, SiH, JSiH = 198.8 Hz),
7.13ꢀ7.25 (m), 7.49ꢀ7.51 (m, 5H), 7.54ꢀ7.56 (m, 6H), 7.69ꢀ7.77
(m, 9H), 7.83ꢀ7.85 (m, 4H). 13C NMR (125.77 MHz, C6D6): δ 0.0
(CH3), 0.3 (CH3), 126.8, 127.5, 127.9, 131.2, 131.3, 131.3, 131.7, 132.9,
134.0, 135.7, 136.5, 137.2, 137.3, 137.4, 137.6, 137.8, 137.9, 139.8, 139.9,
140.2. 29Si NMR (99.30 MHz, C6D6): δ ꢀ20.3 (1JSiH = 198.8 Hz).
12
HRMS (m/z): calcd for
C
1H2428Si2 368.1217, found 368.1406, GC/
24
MS (m/z (%)): 368.2 [M]+ (2), 247.1 (100) [M ꢀ SiCH3C6H5], 197.1
(60), 169.1 (35), 121.1 (22), 105.0 (30).
1-Bromo-8-(dimethylsilyl)naphthalene. n-Butyllithium
(10 mmol,1.6 M in hexane) was added to a solution of 10 mmol of
1,8-dibromonaphthalene in tetrahydrofuran at ꢀ78 °C, and the
mixture was stirred for 90 min. After 10 mmol of Me2HSiCl was
added via a syringe, the mixture was warmed to room temperature.
After aqueous workup the crude product was purified by column
chromatography on silica using hexane: Rf = 0.52. Yield: 9.5 mmol as
a colorless oil, 95%. 1H NMR (499.87 MHz, C6D6): δ 0.61 (d, 6H,
132.8, 133.6 (Cq), 134.2, 134.2, 134.5, 134.8, 135.0, 136.6, 136.7 (d, 1JCF
=
240.4 Hz, [B(C6F5)4]), 138.4, 138.5 (d, 1JCF = 240.4 Hz, [B(C6F5)4]),
139.6 (Cq), 148.8 (d, 1JCF = 240.4 Hz, [B(C6F5)4]), 153.3 (C4), 165.4,
166.0. 29Si NMR (99.30 MHz, C6D6): δ ꢀ13.5 (SiHPh), 12.9 (SiMe2).
Synthesis of Fluoronium Borates 4[B(C6F5)4]. General Pro-
cedure. A solution of 0.5 mmol of 3[B(C6F5)4] in benzene was cooled to
8 °C. After 0.3 mmol of neat R, R, R-trifluortoluene was slowly added,
the cooling was stopped. The reaction mixture was stirred at 50 °C for 3
h. After the stirring was stopped, a two-phase reaction mixture was
obtained. The lower phase was transferred into an NMR tube and
investigated by NMR spectroscopy. The dark red [B(C6F5)4] salts were
purified by washing with pentane.
CH3, 3JHH = 3.7 Hz), 5.44 (sept, 1H, SiꢀH, 3JHH = 3.7 Hz, 1JSiH
=
198.6 Hz), 6.88 (m, 1H), 7.17 (m, 1H), 7.45 (m, 1H), 7.53 (m, 1H),
7.67 (m, 1H), 7.96 (m, 1H). 13C NMR (125.77 MHz, C6D6): δ 1.1
(CH3), 123.5 (CꢀBr), 125.6 (CH), 125.9 (CH), 129.6 (CH), 131.5
(CH), 132.5 (CH), 136.4 (Cq), 137.0 (Cq), 137.1 (Cq), 138.9 (CH).
29Si NMR (99.30 MHz, C6D6): δ ꢀ13.3. Mass spectrum (EI;
m/z (%)): 265.9(20) [M + 2]+, 263.9 (27) [M]+, 265.0 (28) [M +
2 ꢀ H]+, 262.9 (26) [M ꢀ H]+, 250.9 (100) [M + 2 ꢀ Me]+,
4a[B(C6F5)4]. 1H NMR (499.87 MHz, C6D6): δ 7.04ꢀ7.07 (m, 8H),
248.8 (98) [M ꢀ Me]+, 169.0 (85). HRMS (m/z) calcd for
7.12ꢀ7.13 (m, 8H), 7.26ꢀ7.30 (m, 6H), 7.32ꢀ7.34 (m, 2H), 7.90ꢀ7.92
12
1H1379Br28Si 263.9970, found 263.9964.
4
(m, 2H). 13C NMR (125.77 MHz, C6D6): δ 121.9 (d, JCF = 5.6 Hz,
C
12
0
0
0
0
C
2 /7 ), 123.4 (d, JCF = 19.1 Hz, C1 /8 ), 126.5, 129.4, 134.0, 134.8, 135.9,
Compound 5d. n-Butyllithium (7 mmol,1.6 M in hexane) was
added to a solution of 7 mmol of 1-bromo-8-(dimethylsilyl)naphthalene
in diethyl ether at ꢀ78 °C, and the mixture was stirred for 90 min at this
temperature. After 7 mmol of Ph2HSiCl was added via a syringe, the
mixture was warmed to room temperature. The solvent was removed
under vacuum, and hexane was added. The resulting suspension was
filtered. The filtrate was concentrated at ꢀ20 °C until the product
136.1, 136.9 (d, JCF = 246.3 Hz, CF, [B(C6F5)4]ꢀ) 137.8 (d, JCF
=
=
=
1
14.2 Hz, C1), 138.9 (d, 1JCF = 246.3 Hz, [B(C6F5)4]ꢀ), 149.1 (d, 1JCF
242.6 Hz, [B(C6F5)4]ꢀ). 29Si NMR (99.30 MHz, C6D6): δ 40.0 (d, 1JSiF
253.1 Hz). 19F NMR (470.27 MHz, C6D6): δ ꢀ166.4 (m, CF), ꢀ162.4
(t, 3JFF = 20 Hz), ꢀ132.2 (m, CF), ꢀ130.8 (s, 1JSiF = 253 Hz, SiF).
4b[B(C6F5)4]. 1H NMR (250.131 MHz, C6D6): δ 2.13 (s, 12H, CH3),
6.93ꢀ6.96 (m, 8H), 7.16ꢀ7.20 (m, 8H), 7.27ꢀ7.34 (m, 2H), 7.45ꢀ
7.43 (m, 2H), 7.89ꢀ7.92 (m, 2H). 13C NMR (62.902 MHz, C6D6):
δ 21.04 (CH3), 120.4 (d, 2JCF = 19.08 Hz, C1), 122.7 (d, 2JCF = 14.4 Hz,
1
started to crystallize. Yield: 1.8 mmol of a white solid, 25%. H NMR
(499.87 MHz, C6D6): δ 0.30 (d, 6H, CH3, 3JHH =3.4 Hz), 5.40 (sept,
1H, SiꢀH, 3JHH = 3.4 Hz, 1JSiH = 193.7 Hz), 6.44 (s, 1H, SiꢀH, 1JSiH
=
0
0
0
C
1 /8 ), 126.4 (d, 5JCF = 1.8 Hz, C3 /60 ), 130.2 (C3), 135.6 (C10 ), 136.9
203.2 Hz), 7.09ꢀ7.20 (m, 9H,), 7.27ꢀ7.30 (m, 1H), 7.59ꢀ7.61 (m,
4H), 7.72ꢀ7.73 (m, 1H), 7.94ꢀ7.96 (m, 1H). 13C NMR (125.77 MHz,
C6D6): δ ꢀ0.8 (CH3), 124.7 (CH), 124.9 (CH), 129.6 (CH), 131.6
(CH), 132.4 (CH), 133.3 (Cq), 134.5 (Cq), 135.5 (CH), 136.2 (Cq),
136.3 (CH), 138.7 (Cq), 140.3 (CH), 142.7 (Cq). 29Si NMR (99.30 MHz,
0
(C2), 137.8 (d, JCF = 5.6 Hz, C2 /7 ), 136.9 (d, JCF = 248.8 Hz,
4
1
[B(C6F5)41]ꢀ), 138.8 (d, JCF = 245.9 Hz, [B(C6F5)4]ꢀ), 146.1 (C4),
1
149.1 (d, JCF = 241.6 Hz, [B(C6F5)4]ꢀ). 29Si NMR (49.694 MHz,
C6D6): δ 40.2 (d, SiF, 1JSiF = 253 Hz). 19F NMR (235.325 MHz, C6D6):
δ ꢀ166.7 (t, JFF = 18 Hz, [B(C6F5)4]ꢀ), ꢀ162.7 (t, JFF = 20 Hz,
3
3
C6D6): δ ꢀ19.5 (SiH(CH3)2), ꢀ19.2 (SiHPh2). HRMS (m/z): calcd for
1H2428Si2 368.1217, found 368.1425, GC/MS (EI; m/z (%)): 368
12
[B(C6F5)4]ꢀ), ꢀ132.0 (m, [B(C6F5)4]ꢀ), ꢀ129.1 (d, SiF, 1JSiF = 253 Hz).
C
24
[M]+ (13), 309 (100), 275 (7), 247 (13), 231 (55), 182 (26), 135 (12),
105 (9), 78 (3).
4d[B(C6F5)4]. 1H NMR (499.87 MHz, C6D6): δ 0.16 (d, 6H, 3JHF
=
13.1 Hz, CH3), 7.11ꢀ7.12 (m, 1H), 7.23 (m, 2H), 7.25ꢀ7.28 (m, 1H),
Arenium Borate cis-/trans-3c[B(C6F5)4]. A 0.5 mmol portion of
5c was slowly added to a stirred solution of 0.5 mmol (460 mg) of
7.29ꢀ7.31 (m, 5H), 7.35ꢀ7.38 (m, 4H). 13C NMR (125.77 MHz,
2
C6D6): δ ꢀ0.8 (d, JCF = 14.2 Hz), 121.0 (d, JCF = 14.8 Hz), 123.2
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ARTICLE
(d, JCF = 19.6 Hz), 125.9 (d, JCF = 10.7 Hz), 126.4 (d, JCF = 2.0 Hz),
126.5 (d, JCF = 3.1 Hz), 129.7, 133.7, 134.4 (d, JCF = 7.6 Hz), 135.0,
127, 2852. (c) Gu, W.; Haneline, M. R.; Douvris, C.; Ozerov, O. V.
J. Am. Chem. Soc. 2009, 131, 11203. (d) Douvris, C.; Nagaraja, C. M.;
Chen, C.-H.; Foxman, B. M.; Ozerov, O. V. J. Am. Chem. Soc. 2010,
132, 4946.
(6) (a) Panisch, R.; Bolte, M.; M€uller, T. J. Am. Chem. Soc. 2006,
128, 9676. (b) L€uhmann, N.; Panisch, R.; M€uller, T. Appl. Organomet.
Chem. 2010, 24, 533.
(7) (a) Duttwyler, S.; Douvris, C.; Fackler, N. L. P.; Tham, F. S.;
Reed, C. A.; Baldridge, K. K.; Siegel, J. S. Angew. Chem., Int. Ed. 2010,
49, 7519. (b) Alleman, O.; Duttwyler, S.; Romanato, P.; Baldridge, K. K.;
Siegel, J. S. Science 2011, 332, 574.
(8) (a) Meyer, R.; Werner, K.; M€uller, T. Chem.—Eur. J. 2002, 8, 1163.
(b) Panisch, R.; Bolte, M.; M€uller, T. Organometallics 2007, 26, 3524.
(9) Choi, N.; Lickiss, P. D.; McPartlin, M.; Masangane, P. D.;
Veneziane, G. L. Chem. Commun. 2005, 6023.
1
135.2, 135.9, 136.1, 138.9 (d, JCF = 246.3 Hz, [B(C6F5)4]ꢀ), 142.1,
1
149.1 (d, JCF = 242.6 Hz, [B(C6F5)4]ꢀ). 29Si NMR (99.30 MHz,
1
1
C6D6): δ 41.2 (d, SiPh2F, JSiF = 249.5 Hz), 76.8 (d, SiMe2F, JSiF
=
245.9 Hz). 19F NMR (470.27 MHz, C6D6): δ ꢀ166.5 (m, 8F), ꢀ162.6
(m, 4F), ꢀ137.1 (s, 1F, SiF, 1JSiF = 245.9 Hz), ꢀ131.9 (m, 8F).
Typical Procedure for the Catalytic Hydrodefluorination
of 1-Fluorodecane. A mixture of 5 mmol of 1-fluorodecane, 1 mL of
triethylsilane, and 100 μL of hexafluorobenzene as internal standard was
added to 0.05 mmol of 3[B(C6F5)4] or 4[B(C6F5)4] at 0 °C. The
mixture was warmed to room temperature and stirred overnight. The
progress of the reaction was followed by 19F NMR spectroscopy.
The product Et3SiF was detected by NMR spectroscopy and GC/MS.
Decane was identified by GC/MS. Et3SiF: 19F NMR (235.334 MHz,
C6D6) δ ꢀ175.2 (s, 1JSiF = 288.7 Hz); 29Si NMR (49.696 MHz, C6D6):
δ 31.6 (d, 1JSiF = 288.7 Hz). Decane: GC/MS (EI; m/e (%)) 142 (5),
113 (3), 99 (4), 98 (4), 85 (15), 84 (6), 71 (30), 70 (13), 57 (88), 55
(27), 43 (100), 41 (82), 29 (50), 27 (30).
(10) (a) Eaborn, C.; Happer, D. A. R.; Kazem, D. S.; Walton,
D. R. M. J. Organomet. Chem. 1978, 157, C50. (b) Eaborn, C.; Happer;
Hopper, S. P.; D., A. R.; Kazem, D. S. J. Organomet. Chem. 1980,
188, 179. (c) Eaborn, C.; Lickiss, P. D.; Najim, S. T.; Stꢀnczyk, W. A.
Chem. Commun. 1987, 1461.
(11) Nakamoto, M.; Fukawa, T.; Sekiguchi, A. Chem. Lett. 2004,
33, 38.
’ ASSOCIATED CONTENT
(12) (a) Ishikawa, M.; Iyoda, J.; Ikeda, H.; Kotake, K.; Hashimoto,
T.; Kumada, M. J. Am. Chem. Soc. 1981, 103, 4845. (b) Ishikawa, M.;
Watanawe, M.; Iyoda, J.; Ikeda, H.; Kumada, M. Organometallics 1982,
1, 317. (c) Blinka, A.; West, R. Organometallics 1986, 5, 128. (d) Fischer,
J.; Baumgartner, J.; Marschner, C. Science 2005, 310, 825. (e) Wagner,
H.; Wallner, A.; Fischer, J.; Flock, M.; Baumgartner, J.; Marschner, C.
Organometallics 2007, 26, 6704. (f) Wagner, H.; Baumgartner, J.; M€uller,
T.; Marschner, C. J. Am. Chem. Soc. 2009, 131, 5022.
(13) The Rev. D.02 version of the Gaussian 03 program was used.
See the Supporting Information for details.
S
Supporting Information. Text, tables, and figures giving
b
a complete description of the computations, including optimized
molecular structures in Cartesian coordinates and absolute
energies for all calculated compounds, and spectroscopic data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
(14) The reaction coordinate forming the trans isomer is similar;
however, due to steric hindrance the intermediates corresponding to
cation 13 and 14 are somewhat higher in energy (by 12.9 and 7.5 kJ
molꢀ1; see the Supporting Information). For the sake of clarity, only the
lowest energy path 3d f cis-3c is shown and discussed here.
(15) (a) Lewis, P. H.; Rundle, R. E. J. Chem. Phys. 1963, 21, 986. (b)
Vranka, R. G.; Amma, E. L. J. Am. Chem. Soc. 1967, 89, 3121. (c) Byram,
S. K.; Fawcett, J. K.; Nyburg, S. C.; O'Brien, R. J. Chem. Commun.
1970, 16–17. (d) Huffman, J. C.; Streib, W. E. Chem. Commun.
1971, 911.
Corresponding Author
*E-mail: thomas.mueller@uni-oldenburg.de.
Present Addresses
§Division of Chemistry & Biological Chemistry, School of
Physical & Mathematical Sciences, Nanyang Technological
University, 21 Nanyang Link, Singapore 637371.
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’ ACKNOWLEDGMENT
This work was supported by the DFG (No. Mu1440/6-1).
The Center for Scientific Computation (CSC) is thanked for
providing computational resources.
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