Persistent bissilylated arenium ions

Article abstract of DOI:10.1002/1521-3765(20020301)8:5<1163::AID-CHEM1163>3.0.CO;2-M

A series of bissilylated arenium ions 1 with different substitution patterns on the aryl ring have been synthesized by hydride abstraction from 2-aryl-substituted 2,6-dimethyl-2,6-disilaheptanes (2) via transient silylium ions. The arenium ions have been

Full text of DOI:10.1002/1521-3765(20020301)8:5<1163::AID-CHEM1163>3.0.CO;2-M

FULL PAPER
Persistent Bissilylated Arenium Ions
Rita Meyer, Karla Werner, and Thomas M¸ller*[a]
Abstract: A series of bissilylated areni-
um ions 1 with different substitution
patterns on the aryl ring have been
synthesized by hydride abstraction from
2-aryl-substituted 2,6-dimethyl-2,6-disi-
laheptanes (2) via transient silylium
ions. The arenium ions have been iden-
tified by their characteristic NMR chem-
ical shifts, (d29Si ˆ 19.1 25.6, d13Cipso
ˆ 89.0 102.4, d13Cortho ˆ 160.9 182.0,
d13Cmeta ˆ 132.5 146.9, d13Cpara ˆ 150.2
169.9) supported by quantum mechan-
ical calculations of structures, energies,
and magnetic properties at the B3LYP/
6-311G(d,p)//B3LYP/6-31G(d)
‡
temperature for periods ranging from a
few hours to days. This unusual stability
is attributed to: 1) the thermodynamic
stabilization of the arenium ion by two
b-silyl substituents and 2) the essentially
non-nucleophilic reaction conditions
(the use of the weakly coordinating
[B(C6F5)4]À anion and aromatic hydro-
carbons as solvents). Addition of stron-
ger nucleophiles than aromatic hydro-
carbons (for example, acetonitrile) re-
sults in desilylation of the arenium ion 1
and recovery of the 2-aryl-2,6-disilahep-
tane moiety.
DZPVE level of theory. The calculations
clearly reveal the charge dispersing and
stabilizing effect of the silyl substituents
in arenium ions 1. The bissilylated
benzenium ion 1a is more stable than
the parent benzenium ion (C6H7 ) by
37.6 kcalmolÀ1. The synthesized arenium
ions 1 are stable in solution at room
‡
Keywords: carbocations
functional calculations
¥
density
¥ NMR
spectroscopy ¥ silicon ¥ silyl effects
allowed the isolation of a silylarenium ion,[30] chloroarenium
ions,[31] and even protonated arenes[32] as crystalline materials
and their characterization by X-ray structure analysis.
Recent achievements in silyl cation chemistry[33] have
cleared the way for a novel approach to stable, silyl-
Introduction
Arenium ions,[1] formed by the protonation or alkylation of
arenes, are key intermediates in electrophilic aromatic sub-
stitution reactions and in rearrangements in acidic solutions
and the gas phase.[2 7] The history of stable arenium ions dates
back to pioneering work by the groups of Olah,[8, 9]
McLean,[10, 11] Doering,[12] and Gillespie[13] in the late 1950s
37]
substituted carbocations[34] at ambient temperatures.[30, 35
The hydride-transfer reaction between trityl cations and
trialkylsilanes in aromatic hydrocarbons, originally errone-
ously interpreted as resulting in stable silylium ions,[30, 39]
yields persistent silylated arenium ions.[40] The high electro-
philicity of these silylated arenium ions has been utilized in
the synthesis of novel non-oxidizing superacids[32, 41] and
stable silylium ions.[42] They are also suitable reagents for
1
to early 1960s. 13C and H NMR spectroscopic investigations
in superacidic media by Olah and co-workers greatly elabo-
18]
rated this early work,[14 and recently even the eigenvalues
of the 13C chemical shift tensors of simple arenium ions have
been determined on solid metal halide superacids.[19] Mech-
anisms and rates of gas-phase reactions involving arenium
‡
the transfer of R3Si equivalents to molecules with multiple
ions have been extensively studied by mass spectroscopy,
bonds or lone pairs. This is highlighted by the synthesis of b-
silyl-substituted carbocations,[35] which are persistent at room
temperature and by the recent synthesis of persilylated
phosphonium and arsonium ions.[43] Similarly, we have shown
28]
ICR, and related methods.[20
Although the isolation of
tetrafluoroborate salts of simple alkylbenzenium ions was
achieved by Olah and Kuhn[8] in the mid-1950s and the
structural characterization of the heptamethyl benzenium ion
as the tetrachloroaluminate salt[29] dates back to the late 1960s,
only the recent progress in experimental techniques has
ˆ
that intramolecular addition of transient silylium ions to C C
[37]
ꢀ
double and C C triple bonds yields silanorbornyl cations
and b-silyl-substituted vinyl cations,[36] respectively. The
success of this approach to silyl-substituted carbocations over
the traditional methods using magic acid or related super-
acidic media[44, 45] clearly rests on the reaction conditions. The
omnipresence of fluorine- or oxygen-containing nucleophiles
in those superacids results in the rapid decomposition of silyl-
[a] Dr. T. M¸ller, R. Meyer, K. Werner
Institut f¸r Anorganische Chemie der Goethe Universit‰t Frankfurt
Marie Curie-Str. 11, 60439 Frankfurt/Main (Germany)
Fax : (‡49)6979829188
E-mail: dr.thomas.mueller@chemie.uni-frankfurt.de
À
substituted carbocations by favorable cleavage of Si C bonds
Supporting information for this article is available on the WWW under
http://www.wiley-vch.de/home/chemistry/ or from the author.
even at temperatures as low as À1008C.[44, 45] In contrast, the
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1163

T. M¸ller et al.
FULL PAPER
reaction conditions for the initial hydride transfer reaction,
that is, involving weakly coordinating anions (e.g., the
tetrakis(pentafluorophenylborate) anion) and aromatic hy-
drocarbons, exclude this type of successive reaction, thereby
increasing the kinetic stability of the silylated cation. The
advantages of these very weakly nucleophilic reaction con-
ditions are highlighted by the recent synthesis of stable
silicon halides with the {(Me2ArSi)(Me3Si)2C} group.[46] We
describe herein the synthesis of bissilylated arenium ions
under experimental conditions that allow their NMR charac-
terization in solution at ambient temperature. The results of
quantum-mechanical investigations on structures, energies,
and magnetic properties are reported; these not only corrob-
orate the experimental findings but also give a detailed
description of the bonding and charge distribution in arenium
ions 1.
‡
protonated arenes, including benzenium (C6H7 ), which were
isolated as crystalline materials at ambient conditions with
carborate or tetraarylborate anions[32] and protonated C60
fullerene.[41]
Following our previous work[36, 37] on the addition of
Results and Discussion
À
transient silylium ions to C C multiply bonded systems, we
embarked on a systematic study on the intramolecular
addition to aryl groups using 2-aryl-substituted 2,6-disilahep-
tanes 2 as precursors for the synthesis of bissilyl-substituted
arenium ions 1. The cyclic arenium ions 1 might be viewed as
NMR studies: The starting materials for the synthesis of
bissilylated arenium ions 1a g, that is, 2-aryl-2,6-disilahep-
tanes 2a g, were obtained from the silyl chlorides 4 by
reactions summarized in Scheme 1. The structures of com-
pounds 2 were verified by using standard one-dimensional 1H,
13C, 29Si and two-dimensional 1H/13C NMR techniques.
Characteristic for all disilaheptanes 2 are two 29Si NMR
signals, at d29Si ˆ À14.1 to À14.5 (SiH) and d29Si ˆ À3.9 to
Me2HSi
SiMe2
Me2Si
SiMe2
1
À4.6 (Si-aryl), and the multiplet H NMR resonance for the
SiH proton, at d ˆ 3.84 3.79. Reactions of 2a g with one
R
equivalent
of
trityltetrakis(pentafluorophenyl)borate
R
(TPFPB) in [D6]benzene at room temperature (RT) gave in
1
2
the first step the silylium ions 5a g and triphenyl methane;[38]
1
this last compound was unequivocally identified from its H
Me2
Si
Me3Si
Me3Si
and 13C NMR spectra.[47] The silylium ions 5a g are only
transient species and undergo an intramolecular reaction
yielding bissilylated arenium ions 1a g (Scheme 1).[38]
The arenium ions 1a g (Table 1) were identified by their
characteristic NMR spectra (see Figure 1 for an example).[48]
One single line in the 29Si NMR spectra in [D6]benzene at
d29Si ˆ 19.1 25.6 indicates in each case the formation of a
symmetric species (see Table 2). The observed chemical shift
range is similar to the 29Si NMR chemical shifts found for bis-
b-silyl-substituted vinyl cations (d29Si ˆ 22 24)[36] and can be
R
Si
Me2
3
possible intermediates for a 1,5-aryl shift and thereby bear a
close resemblance to the 1,3-bridged cations 3, suggested by
Eaborn and co-workers as intermediates in the solvolysis of
3
1
2
R4
R2
Me2HSi
SiMe2
Me2Si
SiMe2
R2
R4
R3
R5
R2
R4
R3
R5
(i), (ii)
(iii)
R1
Br
R5
R3
R1
R1
2
5
2
3
1
Me2Si
SiMe2
R2
R3
R4
R5
R1
1
Scheme 1. Synthesis of arenium ions 1 (see also Table 1): i) Mg, THF (a d, f) or nBuLi, hexanes (e,g); ii) Me2HSi(CH2)3SiMe2Cl (4),THF, reflux;
iii) TPFPB, C6D6, RT.
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1164

Arenium Ions
1163 1172
Table 1. Substitution pattern of the aryl ring in substituted arenium ions
1a g (see Scheme 1).
clearly distinguished from the
expected resonances of mono-
silylated benzenium ions, 6a
g, at d29Si ˆ 70 100,[30b, 39]
which might be formed by
intermolecular reaction of the
intermediate silylium ions
5a g with the solvent ben-
zene. For the benzenium ion
Me2Si
SiMe2
R1
R2
R3
R4
R5
R2
R4
R3
R5
H
a
b
c
d
e
f
H
H
H
CH3
CH3
CH3
H
H
H
H
H
CH3
H
CH3
H
H
H
H
H
CH3
H
H
H
H
CH3
H
CH3
CH3
H
H
H
R1
6a-g
CH3
H
g
CH3
CH3
1a, nearly the same 29Si NMR
chemical shift is observed in
[D8]toluene as in [D6]benzene
(d29Si ˆ 26.9 and 25.6, respec-
tively, see Table 2), indicating
negligible solvent cation inter-
actions. This contrasts the be-
havior of cations of type 6 for
which solvent effects on the 29Si
NMR chemical shift of more
than 10 ppm are reported under
the same conditions.[30b] The
downfield shift of the 29Si
NMR signal, Dd29Si, which oc-
curs upon ionization of 2, is
significant (23 29 ppm and
À
33 40 ppm, for the Si aryl
À
and the Si H resonance, re-
spectively) and indicates an ac-
cumulation of positive charge
at silicon. There is a clear
dependence of Dd29Si on the
Figure 1. 49.7 MHz 29Si {1H} Inept NMR spectrum (upper trace) and 62.9 MHz 13C {1H} NMR spectrum of 1c in
À
*
*
[D6]benzene (lower trace, triphenylmethane, [B(C6F5)4] ).
Table 2. Experimental,[a,d] calculated (at GIAO/B3LYP/6-311G(d,p)[b] in parenthesis), and corrected, theoretical NMR data[c] (in italics and parenthesis) of
silylated arenium ions and related compounds.
d29Si
d13Cipso
d13Cortho
d13Cmeta
d13Cpara
d13C1/3
d13C2
d13CH3
1a
25.6
(39.7)
102.4
(106.1)
(101.3)
102.6
162.9
(178.3)
(171.9)
163.0
165.8
(176.3)
(170.0)
182.0, 165.1
(192.3),(174.7)
(185.6), (167.8)
180.0, 164.2
(193.9),(173.5)
(187.2), (158.2)
180.6
(194.7)
(188.0)
160.9
(174.8)
133.9
(140.0)
(134.5)
133.2
135.6
(141.2)
(135.7)
136.7, 132.5
(143.9),(138.6)
(138.3),(133.2)
142.4, 135.9
(150.7),(142.8)
(145.0),(137.3)
134.3
(140.3)
(134.8)
146.9
(153.6)
150.3
(160.0)
(154.0)
150.3
14.6
19.8
(22.1)
(19.4)
19.9
À 2.9
1a[b]
1a[c]
1a[d]
1b
(18.9)
(16.3)
14.6
(2.2)
(0.0)
26.9
22.1
(34.9)
À 3.0
93.3
169.9
14.4
17.1
23.5, À1.8
1b[b]
1b[c]
1c
(98.8)
(94.3)
89.8
(96.9)
(92.5)
96.2
(102.0)
(97.4)
95.1
(100.2)
(95.7)
102.9
(103.7)
(99.1)
89.0
(94.3)
(89.9)
(139.7)
(141.2)
138.2
(180.2)
(173.8)
169.0
(178.4)
(172.0)
152.9
(160.8)
(154.9)
150.2
(157.7)
(151.8)
153.6
(163.3)
(157.3)
167.3
(176.8)
(170.5)
(138.0)
(136.9)
129.4
(18.7)
(16.1)
15.4
(19.6)
(17.0)
15.5
(19.8)
(17.2)
16.6
(20.1)
(17.5)
15.1
(18.8)
(16.2)
16.7
(20.1)
(17.5)
(39.8),(27.1)
(37.0), (24.6)
19.7, 17.2
(19.9),(17.5)
(22.2)
(19.5)
16.8
(22.9)
(20.2)
16.8
(22.7)
(20.0)
17.4
(21.7)
(19.0)
17.1
(22.2)
(19.5)
16.8
(21.6)
(18.9)
(21.0)
(25.6)
18.0
(27.3), (1.5)
(24.5), (À0.7)
26.0, 23.1, À0.3, À2.6
(28.4), (26.4), (1.6), (0.8)
(25.6), (23.6), (À0.6), (À1.4)
25.6, 19.5, À0.3, À2.3
(28.7), (23.0), (1.9), (1.2)
(25.9), (20.3), (À0.3), (À1.0)
27.0, À0.1
20.4
(33.1)
1c[b]
1c[c]
1d
22.7
(35.8)
1d[b]
1d[c]
1e
21.6
(35.9)
1e[b]
1e[c]
1f
(31.9), (2.3)
(29.0), (0.1)
24.3
(35.9)
20.0, À1.4
1f[b]
1f[c]
1g
(23.2), (2.0)
(168.5)
179.7
(192.2)
(185.5)
(139.4),(137.1)
(138.5)
133.8
(147.8)
135.7
(141.4)
(135.9)
(135.5),(134.6)
(134.5)
128.2
(20.5), (À0.2)
27.8, 22.6, À0.5
(31.5), (26.3), (2.0)
(28.6), (23.5), (À0.2)
(12.8), (1.9)
19.1
(31.2)
1g[b]
1g[c]
5a
(388.1),(À0.7)
(115.5), (À3.7)
35.0, À4.2
6a
10
11
(1.3), (À1.7)
0.0, À3.5, À3.7
(2.5), (1.7)
(49.6), (32.7)
(107.2)
(177.5), (177.4)
(140.1), (139.0)
(158.9)
(21.9)
[a] In [D6]benzene at RT, versus TMS. [b] Calculated using B3LYP/6-31G(d) geometries versus TMS; s29Si(TMS) ˆ 339.9, s13C(TMS) ˆ 183.8. [c] Calculated
from theoretical data (see [b]) using the empirical correlation: dcalcd ˆ (2.234 Æ 1.158) ‡ (1.019 Æ 0.008) dexptl , obtained from a set of 8 standard compounds;
see Supporting Information for details. [d] In [D8]toluene at RT.
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electron donating ability of the aryl group, that is, Dd29Si is
deshielded (d13Cortho ˆ 162.9 179.7 and d13Cpara ˆ 150.3
29
largest for the benzenium ion 1a [Dd Si ˆ 29.7 (Si aryl) and
169.9) than found for the arenium cations 7 9 (d13Cortho
ˆ
À
40.1 (Si H)] and smallest for the mesitylenium ion 1g
181.2 193.6 and d13Cpara ˆ 178.1 201.9), while the ipso car-
bons in 1a,b,g are less shielded (d13Cipso ˆ 89.0 102.4 (1a,b,g)
and d13Cipso ˆ 49.5 52.2 (7 9)). The changes of d13Cipso and
d13Cortho can not be straightforwardly interpreted since the
electronic effects are obscured by substituent effects of the
silyl groups on d13C; however, the remote d13Cpara is a suitable
probe for the charge distribution in arenium ions.[44c] The high-
field shift of d13Cpara for the pairs of equally substituted
arenium ions 7/1a, 8/1b, and 9/1g indicates that less positive
charge is located in the aryl ring in cations 1a,b,g than in the
protonated arenes 7 9, in accord with the well-established
charge dispersing effect of b-silyl groups.[36, 49] Similar high-
field shifts of the 13C resonance of the positively charged
carbons in carbocations upon b-silyl substitution have been
reported for vinyl cations and related compounds.[34, 36, 43a,b]
For the asymmetrically substituted 2,4-xylylenium ion 1c,
two clearly separated signals for the equatorial and axial
methyl groups at silicon are observed in the 1H and 13C N MR
spectra recorded at 303 K (see Figure 1 for 13C NMR). The
1H NMR signals for the equatorial and axial protons of the
central methylene group (C2H2) of the 1,3-disilacyclohexane
ring were also clearly separated. The signals have no kinetic
line broadening, indicating that the ring-flip process of the 1,3-
disilacyclohexane ring in 1c is slow on the NMR timescale at
RT. In contrast, for the 2,5-xylylenium ion 1d, kinetic line
broadening of the respective signals at 303 K was detected.
Additional NMR measurements at 285 and 318 K allowed an
approximate calculation of the enthalpy of activation for the
ring-flip process (see Scheme 3), which amounts to (15.1 Æ
2.9) kcalmolÀ1 (see Supporting Information for details).[50]
À
29
À
À
[Dd Si ˆ 23.7 (Si aryl) and 33.5 (Si H)].
Two multiplets in the ratio 1:2 in the 1H NMR spectrum and
two signals in the 13C NMR spectrum at d13C ˆ 19.8 16.8 (C2)
and d13C ˆ 14.4 16.7 (C1/3) for the three methylene groups of
1
2
3
À
À
the carbon backbone (C
C C ) confirmed the formation of
a symmetric cation in each case. The typical 13C N MR
19]
chemical shift pattern for arenium ions[14
was found for
each of the cations 1a g (see Table 2 and Figure 1). The ortho
and para carbons of the aryl substituents in cations 1a g are
strongly deshielded relative to the starting silanes
(Dd13Cortho ˆ 39.6 28.9 and Dd13Cpara ˆ 35.1 21.6), while the
meta carbons are only slightly affected by the ionization
(Dd13Cmeta ˆ 9.9 6.3). The 13C NMR chemical shifts for the
Cipso are in the range 89.0 102.9 and are highly diagnostic of
the structure of the ions. The strong high-field shifts of the
Cipso relative to the precursor silanes (Dd13Cipso ˆ À36.5 to
À 49.4) are consistent with the rehybridization of Cipso from sp2
to sp3, which occurs upon addition of the silyl cation to the aryl
substituent. These characteristic changes in the 13C N MR
chemical shifts of the arene substituents gives evidence for the
static nature of the ions 1a g, and, therefore, we can discard
the possibility of a conceivable fast (on the NMR timescale)
1,5-aryl shift between two equivalent silylium ions 5a g.
Furthermore, they indicate the delocalization of positive
charge into the aryl substituent as rationalized by the dienyl
cation-like representations 1A,B (see Scheme 2).
Me2Si
SiMe2
Me2Si
SiMe2
Me2Si
SiMe2
H
H
Si
Si
R
Si
R
R
Si
H
1A
1B
1C
Scheme 2. Schematic representation of the delocalization of positive
charge in arenium ions 1.
H
1d
Scheme 3. Ring-flip process in xylylenium ion 1d.
1d
Comparison of the 13C NMR data for the bissilylated
arenium ions 1a,b,g with reported data for non-silylated
arenium ions, 7 9,[15, 19] (Table 2, Figure 2), clearly reveal the
influence of the two silyl substituents in cations 1. The ortho
and para positions of the aryl substituent in 1a,b,g are less
Arenium ions 1a g are stable at RT; however, there is a
clear dependence of the stability of the ion on the number of
substituents and the substitution pattern at the aryl ring. The
benzenium ion 1a gradually decomposes during a period of
several hours in solution at RT, while the 2,4-xylylenium ion
1c and the mesitylenium ion 1g are stable for days under the
same conditions. Addition of stronger coordinating solvents
than aromatic hydrocarbons leads to the collapse of the
intramolecular interaction, for example, the addition of
acetonitrile to a solution of 1a in benzene, instantaneously
gives the acyclic silyl nitrilium species 10, which is unequiv-
ocally identified by its characteristic 29Si NMR spectra (see
Scheme 4 and Table 2).[30b, 36, 51]
H
H
H
H
H
H
52.2
51.9
49.5
186.5
136.9
181.2
139.4
193.6
133.8
192.1
201.9
178.1
7
8
9
Figure 2. 13C NMR chemical shifts of arenium ions 7 9.[15, 19]
1166
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Arenium Ions
1163 1172
the cyclic arenium ion 1a is favored over the intermolecular
reaction yielding the monosilylated arenium ion 6a; this is in
qualitative agreement with the experimental results. In
addition, 1a is further stabilized by weak interactions with
the solvent. The interaction energy between benzene and 1a,
calculated for 11, is, however, relatively small, EA ˆ
À 2.6 kcalmolÀ1, suggesting that at room temperature no
significant cation solvent interactions are present.[56] This is
in agreement with the negligible solvent effects on the 29Si and
13C NMR chemical shifts found experimentally for 1a (see
Table 2).
Me2Si
SiMe2
Me2Si
SiMe2
H3CCN
N
C
CH3
10
1a
Scheme 4. Formation of the acyclic silyl nitrilium species 10.
Theory: Calculations[52] of structures and energies of bissilyl-
ated arenium ions and related compounds were performed by
using the nonlocal DFT level of theory and Becke×s three
parameter hybrid functional and the LYP correlation func-
tional (B3LYP), along with the standard 6-31G(d) basis set
(B3LYP/6-31G(d)).[53] Subsequent frequency calculations at
the B3LYP/6-31G(d) level were carried out to verify the
stationary points as minima.[54] Finally, relative energies were
calculated at the B3LYP/6-311G(d,p) level with B3LYP/6-
31G(d) geometries (B3LYP/6-311G(d,p)//B3LYP/6-31G(d)),
and those energies were further improved by adding zero-
point vibration energy differences (DZPVE). Relative ener-
gies, reaction energies, and association energies EA quoted
in the text are all computed at this level, unless otherwise
stated. NMR chemical shifts were calculated by using the
GIAO/B3LYP method[55] and the 6-311G(d,p) basis set for all
atoms.
Quantum-mechanical methods for the calculation of NMR
chemical shifts have been extensively applied in carbo-[57] and
silyl-cation chemistry,[33a, 58] and the validity of theoretical
structures is frequently established by comparing the com-
puted NMR chemical shifts against the experimental data.
The 29Si and 13C NMR chemical shifts calculated at GIAO/
B3LYP/6-311G(d,p)//B3LYP/6-31G(d) for 1a g are summar-
ized along with the experimental data in Table 2. The general
trend of the experimental data is faithfully reproduced by the
computations; however, the individual errors between the
computed and experimental NMR chemical shift are large. In
general all nuclei are predicted by the calculations to be too
deshielded, for example, for 1a by 2 15 ppm. In particular,
the deviations for the unsaturated carbons are large: ‡9.6
15.4 for Cortho and ‡7.7 10.3 for Cpara. This is clearly due to
deficiencies of the applied method and basis set. DFT-based
methods like GIAO/DFT calculations are known to over-
estimate paramagnetic contributions to the chemical shield-
ing, giving, in critical cases, overly deshielded chemical
‡
The results of calculations for isomeric C13H23Si2 cations
indicate that the silylium ion 5a in its anti,anti conformation is
a high-lying minimum on the potential energy surface (PES),
while the syn,syn conformer is not a stationary point on the
PES and collapses during the optimization procedure to the
symmetrically bridged ion 1a. The intramolecular stabiliza-
shifts.[58a, 59] In a series of papers Gauss, Siehl, and Schleyer
63]
and co-workers[60
have demonstrated the importance of
including electron correlation in chemical shift calculations
for unsaturated carbocations like vinyl,[61] allyl,[62] and ar-
enium[14, 63] ions. For these cations, MP2[14, 19, 62, 63] or even
higher correlated methods like CCSD(T)[61] must be applied.
The size of the cations studied and the extent of our
investigation prevent, however, the use of these highly
accurate methods. Instead, an empirical correction, which is
derived from a linear regression between experimental and
calculated 13C NMR chemical shifts for a set of eight closely
related compounds (giving 34 data points, for details, see
Supporting Information) was applied to account for some of
the deficits of the less accurate GIAO/DFT method. This
procedure gives improved theoretical 13C NMR chemical
shifts, for example, for 1a deviations of À0.4 9 ppm are
found. However, the overall agreement between these
corrected theoretical and the experimental data is good (see
Table 2); for example, the 13C NMR chemical shift of the
structural highly diagnostic Cipso is nicely reproduced by the
calculations (deviations from experimental d13Cipso are À3.8
2.7). Thus, the NMR chemical shift calculations corroborate
the validity of the computed structures.
Me2Si
SiMe2
Me2Si
SiMe2
anti, anti 5a
syn, syn 5a
Me2Si
SiMe2
Me2Si
SiMe2
H
11
6a
The calculated structures of cations 1a g clearly identify
them as arenium ions and the particular structural features
can be straightforwardly discussed in terms of valence-bond
formulas 1A C (see Scheme 2). The structure of 1a is shown
in Figure 3 and the important geometrical parameter of
cations 1a g and 7 9 are summarized in Table 3. All cations
tion of 5a by the phenyl substituent, which is the energy
difference between the arenium ion 1a and the silylium ion
5a, is 22.1 kcalmolÀ1. Intermolecular interaction between the
silylium ion 5a and benzene, calculated for the ion 6a (EA ˆ
À 17.9 kcalmolÀ1), is smaller (by 4.2 kcalmolÀ1). This suggests
that even in benzene solution the intramolecular formation of
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T. M¸ller et al.
FULL PAPER
shown by the shorter Cipso
C
ortho bonds in 1a,b,g relative to the
À
protonated arenes 7, 8, and 9 (1.441, 1.439 (1a), 1.473 (7);
1.442, 1.442 (1b), 1.475 (8); 1.476, 1.469 (1g) 1.488 ä (9)).
ipso
À
Finally, the Si C bonds in all arenium ions 1a g (2.038
2.066 ä) are strongly elongated [relative to the average
r(Si Caryl) ˆ 1.879 ä[65] in aryl silanes and to the average
À
r(Si Calkyl) ˆ 1.860 ä[65] in alkylsilanes or to rˆ 1.944 and
À
À
1.928 ä, calculated for the Si C bonds in 1a(H)] and is best
described by the no-bond resonance structure 1C (see
Scheme 2).
The structures of silylated areniumions, for example,
triethylsilyltoluenium (12),[30] have been discussed controver-
sially. In particular, their formulation as s complexes, that is,
arenium ions, which is based mostly on the theoretical analysis
of the structure and bonding in these cations,[40] has been
severely questioned. Reed[66] and later Kochi[67] have alter-
natively put forward the idea of
Figure 3. Calculated structure of 1a (B3LYP/6-31G(d), bond lengths in ä,
hydrogen atoms are omitted for clarity).
a continuum of structures be-
tween classical s-complexes
Table 3. Calculated geometrical parameters of arenium ions (at B3LYP/6-31G(d)).
SiCipso
CipsoCortho
CorthoCmeta
CmetaCpara
A
a(SiCipsoSi)
b(CorthoCipsoCortho
)
(arenium ions) and p com-
plexes. A measure for the s/p
character is the angle e between
the electrophile, the Cipso, and
the plane of the arene ring.
Pentamethylbenzenium, a typi-
cal s complex has e ˆ 508,[32, 67]
while for p complexes, for ex-
ample, arene/nitrosonium com-
plexes, e ꢁ 908.[67] Thus, these
authors assigned a rather high
degree of p character to the
1a
1b
1c
1d
1e
1 f
1g
7
2.049, 2.049
2.038, 2.038
2.045, 2.045
2.049, 2.047
2.072, 2.055
2.041, 2.041
2.066, 2.049
1.439, 1.441
1.442, 1.442
1.457, 1.452
1.452, 1.450
1.466, 1.475
1.437, 1.438
1.476, 1.469
1.473
1.387, 1.386
1.381, 1.382
1.392, 1.377
1.399, 1.384
1.394, 1.389
1.392, 1.391
1.385, 1.389
1.372
1.402, 1.402
1.412, 1.409
1.404, 1.410
1.411, 1.391
1.390, 1.396
1.407, 1.407
1.404, 1.400
1.413
0.94
0.93
0.90
0.92
0.85
0.96
0.84
0.81
0.77
0.73
104.1
104.6
105.1
104.6
104.6
104.4
105.1
100.6[a]
101.3[a]
102.4[a]
115.5
114.7
114.9
115.9
115.5
115.9
114.8
116.9
116.0
117.9
8
9
1.475
1.488
1.366
1.371
1.425
1.420
[a] HCipsoH angle.
À
1a g are characterized by alternating C C bond lengths in
the six-membered carbocycle (long Cipso
ortho, short Cortho
bonds). This bond length alterna-
À
C
À
Me2
Si
Et3Si
para
Cmeta, and long Cmeta
À
C
ε
Me2Si
SiMe2
H
ε
tion in the six-membered ring can be quantified by using Julg×s
parameter A,[64] which is defined by the difference between
the individual C C bond length (ri) and the average C C
bond length (r) of the n bonds in the six-membered ring
[Eq. (1)].
H
R
Si
H
À
À
Me2
1(H)
1a-g
12
A ˆ 1 À (225/n)S(1 À ri/r)2
(1)
monosilylated 12 (e ˆ 768).[66, 67] In contrast, for the cations
1a g, e ˆ 52.1 52.68 were calculated. This is in agreement
with the NMR results and with other structural parameters
and these criteria clearly identify 1a g as s complexes
(arenium ions).
The structural and electronic differences between the
silylated arenium ions and their non-silylated counterparts
are accompanied by a strong stabilization of the silylated ions
1a g, that is, 1a is more stable than the parent benzenium ion
7 by 37.6 kcalmolÀ1 as revealed by the isodesmic Equation (2);
it is even more stable than trityl cation by 23.7 kcalmolÀ1
[Eq. (3)].
For benzene, A ˆ 1 and the scaling factor 225 in Equa-
À
¬
tion (1) is used to set A ˆ 0 for Kekule-benzene (r(C C) ˆ
[64]
ˆ
1.520 and r(C C) ˆ 1.330). For the arenium ions 1a g, A
values of 0.96 0.84 are calculated (Table 3). The bond length
alternation is in agreement with experimental structures for
alkyl-substituted arenium ions[29 32] and can be rationalized by
the dienyl cation resonance structures 1A and 1B (Scheme 2).
The influence of the silyl substituents in 1a g is revealed by
comparison of the calculated structures of 1a,b,g with those of
the non-silylated cations 7 9. Qualitatively, the stabilizing b
effect of the silyl groups favors the resonance structure 1A,
leading to an attenuation of the bond length alternation (i.e.,
larger A values) for the cations 1a, 1b, and 1g, (A ˆ 0.94, 0.93,
and 0.84, respectively) relative to their non-silylated counter-
parts 7, 8, and 9 (A ˆ 0.81, 0.77, and 0.73, respectively, see
Table 3). The hyperconjugative effect of the silyl groups is also
1a
1a
+
+
1a(H)
1a(H)
+
+
(2)
(3)
7
Ph3C+
Ph3CH
1168
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Chem. Eur. J. 2002, 8, N o. 5

Arenium Ions
1163 1172
NMR (79.495 MHz, [D1]chloroform, 300 K, d29Si((H3C)2SiHCl) ˆ 11.1,
ext.): d ˆ À4.3 (brs; Si(CH3)2Aryl), À14.4 (dm, 1J(Si,H) ˆ 173.0 Hz,
2J(Si,H) ˆ 7 Hz; Si(CH3)2H).
Conclusion
Bissilylated arenium ions 1 have been synthesized in benzene
at RT by hydride-transfer reactions from 2-aryl-2,6-disilahep-
tanes (2) to trityl cation via transient silylium ions. The cations
1 have been characterized by NMR spectroscopy, which was
supported by quantum-mechanical calculations of structures,
energies, and NMR chemical shifts. The experimental data, as
well as the theoretical results, reveal the arenium-type nature
of the cations 1. The unusual stability of the synthesized
cations results from two factors: 1) an increased thermody-
namic stability owing to the hyperconjugative effect of two
silyl substituents in 1 and 2) an increased kinetic stability as a
result of the use of essential non-nucleophilic reaction
conditions, that is, weakly coordinating anions and hydro-
carbon solvents.
Compound 2c: Colorless oil; yield: 2.66 g (10.1 mmol, 50%); 1H N MR
(250 MHz, [D1]chloroform, 300 K, d1H(CHCl3) ˆ 7.24): d ˆ 7.29 (m, 1H;
Hortho), 6.94 (s, br, 2H; Hmeta), 3.79 (sept, 3J(H,H) ˆ 3.6 Hz, 1H; SiH), 2.37
(s, 3H; CH3), 2.26 (s, 3H; CH3), 1.43 1.30 (m, 2H; C1H2), 0.87 0.81 (m,
2H; C2H2), 0.65 0.58 (m, 2H; C3H2), 0.25 (s, 6H; SiMe2Aryl), À0.01 (d,
3J(H,H) ˆ 3.6 Hz, 6H; Si(CH3)2H); 13C NMR (62.90 MHz, [D1]chloroform,
300 K, d13C(CDCl3) ˆ 77.0): d ˆ 143.5 (Cortho), 138.8 (Cipso), 134.8 (Cortho),
133.9 (Cpara), 130.1 (Cmeta), 125.7 (Cmeta), 23.0 (CH3), 21.2 (CH3), 20.3 (C1),
19.1 (C2), 18.7 (C3), À1.71 (Si(CH3)2Aryl), À4.37 (SiH(CH3)2); 29Si NMR
(49.7 MHz, [D1]chloroform, 300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ
À4.0 (brs; Si(CH3)2Aryl), À14.1 (dm, 1J(Si,H) ˆ 173.1Hz, 3J(Si,H) ˆ
7.0 Hz; Si(CH3)2H).
Compound 2d: Colorless oil; yield: 4.28 g (16.2 mmol, 81%); 1H N MR
(250 MHz, [D1]chloroform, 300 K, d1H(CHCl3) ˆ 7.24): d ˆ 7.20 (brs, 1H;
Haryl), 7.01 (brs, 2H; Haryl), 3.79 (sept, 3J(H,H) ˆ 3.6 Hz, 1H; SiH), 2.35 (s,
3H; CH3), 2.26 (s, 3H; CH3), 1.41 1.30 (m, 2H; C1H2), 0.88 0.79 (m, 2H;
C2H2), 0.65 0.58 (m, 2H; C3H2), 0.25 (s, 6H; SiMe2Aryl), À0.01 (d,
3J(H,H) ˆ 3.6 Hz, 6H; Si(CH3)2H); 13C NMR (62.90 MHz, [D1]chloroform,
300 K, d13C(CDCl3) ˆ 77.0): d ˆ 140.4 (Cortho), 137.4 (Cipso), 135.3 (Cortho),
133.8 (Cmeta), 129.7 (Cpara), 128.7 (Cmeta), 22.5 (CH3), 21.1 (CH3), 20.2 (C1),
19.1 (C2), 18.7 (C3), À1.81 (Si(CH3)2Aryl), À4.40 (SiH(CH3)2); 29Si NMR
(49.7 MHz, [D1]chloroform, 300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ
À3.9 (brs, 1J(Si,Caryl) ˆ 64.9 Hz, 1J(Si,Calkyl) ˆ 52.1 Hz; Si(CH3)2Aryl),
À14.1 (dm, 1J(Si,H) ˆ 173.2 Hz, 3J(Si,H) ˆ 7.0 Hz, 1J(Si,C) ˆ 50.3 Hz;
Si(CH3)2H).
Experimental Section
General: All reagents were obtained from commercial suppliers and were
used without further purification. THF was distilled from sodium/
potassium alloy/benzophenone. Benzene, [D6]benzene, toluene, and
[D8]toluene were distilled from sodium. Acetonitrile was distilled from
P4O10. All reactions were carried out in oven-dried glassware under inert
argon atmospheres. Reactions were monitored by thin-layer chromatog-
raphy on 0.20 mm Machery-Nagel silica-coated aluminum plates. ICN silica
32 63, 60 ä silica gel was used for column chromatography. NMR spectra
were recorded on Bruker AM-400 and DPX-250 instruments. 1H N MR
spectra were calibrated from residual non-deuterated solvents as internal
reference; 13C NMR spectra used the central line of the solvent signal. 29Si
NMR spectra were recorded by using the INEPT pulse sequence and were
optimized on Si(CH3)2 or SiH groups. The calibration was done by using
external (H3C)2SiHCl. 19F NMR were calibrated against external CF2Cl2.
2-Chloro-2,6-dimethyl-2,6-disilaheptane, (4) and TPFPB were prepared as
Compound 2e: Colorless oil; yield: 1.48 g (5.6 mmol, 28%); 1H N MR
(250 MHz, [D1]chloroform, 300 K, d1H(CHCl3) ˆ 7.24): d ˆ 7.17 7.09 (m,
1H; Hpara), 7.00 6.94 (m, 2H; Hmeta), 3.84 (sept, 3J(H,H) ˆ 3.6 Hz,
1J(Si,H) ˆ 180.0 Hz, 1H; SiH), 2.46 (s, 6H; CH3), 1.50 1.36 (m, 2H;
C1H2), 1.00 0.88 (m, 2H; C2H2), 0.72 0.61 (m, 2H; C3H2), 0.42 (s, 6H;
SiMe2Aryl), 0.05 (d, 3J(H,H) ˆ 3.8 Hz, 6H; Si(CH3)2H); 13C
N MR
(62.90 MHz, [D1]chloroform, 300 K, d13C(CDCl3) ˆ 77.0): d ˆ 144.2 (Cortho),
136.1 (Cipso), 128.7 (Cpara), 128.1 (Cmeta), 24.9 (CH3), 22.2 (C1), 19.2 (C2), 18.6
(C3), 2.47 (Si(CH3)2Aryl), À4.40 (SiH(CH3)2); 29Si NMR (49.7 MHz,
[D1]chloroform, 300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ À4.2 (Si-
(CH3)2Aryl), À14.1 (Si(CH3)2H).
[68, 69]
described in the literature.
Compound 2 f: Colorless oil; yield: 4.78 g (18.1 mmol, 90.5%); 1H N MR
(250 MHz, [D1]chloroform, 300 K, d1H(CHCl3) ˆ 7.24): d ˆ 7.14 (brs, 2H;
Hortho), 7.02 (brs, 2H; Hpara), 3.88 (sept, 3J(H,H) ˆ 3.8 Hz, 1J(Si,H) ˆ
180.4 Hz, 1H; SiH), 2.35 (s, 6H; CH3), 1.54 1.39 (m, 2H; C1H2), 0.91
0.80 (m, 2H; C2H2), 0.75 0.68 (m, 2H; C3H2), 0.27 (s, 6H; SiMe2Aryl), 0.08
Synthesis of 2-aryl-2,6-disilaheptanes (2a g): A 100 mL Schlenk flask was
charged with of a solution of the metalated arene (1.1 equivalent, 22 mmol)
in THF (50 mL), which was freshly prepared from the respective arene
bromide and Mg (for the synthesis of 2a d,f) or nBuLi (in hexanes) (for
the synthesis of 2e,g). 2-Chloro-2,6-dimethyl-2,6-disilaheptane (4)
(1.0 equivalent, 20 mmol) was then slowly added by syringe at 08C. The
solution was heated to reflux for 3 hours. After hydrolysis (NH4Cl, ice,
diethyl ether) the organic layer was separated, the volatile components
were removed using a rotary evaporator, and the residue was distilled
under reduced pressure (10À1 bar). All materials in the boiling point range
from 40 1408C were collected and then further purified on a silica gel
column with hexanes as eluent.
3
(d, J(H,H) ˆ 3.6 Hz, 6H; Si(CH3)2H); 13C NMR (62.90 MHz, [D1]chloro-
form, 300 K, d13C(CDCl3) ˆ 77.0): d ˆ 139.4 (Cortho), 137.0 (Cmeta), 131.3
(Cortho), 130.5 (Cpara), 21.4 (CH3), 19.8 (C1), 19.0 (C2), 18.7 (C3), À2.86
(Si(CH3)2Aryl), À4.40 (SiH(CH3)2); 29Si NMR (49.7 MHz, [D1]chloroform,
300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ À4.1 (1J(Si,Caryl) ˆ 65 Hz,
1J(Si,Calkyl) ˆ 53 Hz; Si(CH3)2Aryl), À14.1 (1J(Si,C) ˆ 50.4 Hz; Si(CH3)2H).
Compound 2g: Colorless oil; yield: 3.39 g (12.2 mmol, 61%); 1H N MR
(250 MHz, [D1]chloroform, 300 K, d1H(CHCl3) ˆ 7.24): d ˆ 6.80 (brs, 2H;
Hmeta), 3.82 (sept, 3J(H,H) ˆ 3.6 Hz, 1H; SiH), 2.40 (s, 6H; CH3), 2.24 (s,
3H; CH3), 1.53 1.35 (m, 2H; C1H2), 0.94 0.88 (m, 2H; C2H2), 0.68 0.61
(m, 2H; C3H2), 0.38 (s, 6H; SiMe2Aryl), 0.04 (d, 3J(H,H) ˆ 3.7 Hz, 6H;
Si(CH3)2H); 13C NMR (62.90 MHz, [D1]chloroform, 300 K, d13C(CDCl3) ˆ
77.0): d ˆ 144.3 (Cortho), 138.4 (Cipso), 132.5 (Cpara), 132.5 (Cmeta), 24.8 (CH3),
22.3 (CH3), 20.8 (C1), 19.3 (C2), 18.7 (C3), 2.4 (Si(CH3)2Aryl), À4.4
(SiH(CH3)2); 29Si NMR (79.495 MHz, [D1]chloroform, 300 K,
d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ À4.6 (brs; Si(CH3)2Aryl), À14.4
Compound 2a:[70] Colorless oil; yield: 4.53 g (19.2 mmol, 96%); H N MR
1
(250 MHz, [D1]chloroform, 300 K, d1H(CHCl3) ˆ 7.24): d ˆ 7.50 (m, 2H;
Haryl), 7.35 (m, 3H; Haryl), 3.83 (sept, 3J(H,H) ˆ 3.6 Hz, 1H; SiH), 1.50 1.35
(m, 2H; C1H2), 0.90 0.80 (m, 2H; C2H2), 0.70 0.59 (m, 2H; C3H2), 0.25 (s,
6H; SiMe2Aryl), 0.03, (d, 3J(H,H) ˆ 3.6 Hz, 6H; Si(CH3)2H); 13C N MR
(62.90 MHz, [D1]chloroform, 300 K, d13C(CDCl3) ˆ 77.0): d ˆ 139.7 (Cipso),
133.5 (Cortho), 128.7 (Cpara) 127.6 (Cmeta), 19.8 (C1), 18.9 (C2), 18.6 (C3), À3.0
(Si(CH3)2aryl), À4.4 (SiH(CH3)2); 29Si NMR (79.495 MHz, [D1]chloro-
form, 300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ À4.1 (brs, J(Si,Calkyl) ˆ
1
1
2
52 Hz; Si(CH3)2Aryl), À14.5 (dm, 1J(Si,H) ˆ 188.0 Hz, 2J(Si,H) ˆ 7 Hz;
(dm, J(SiH) ˆ 173.0 Hz, J(SiH) ˆ 7 Hz; Si(CH3)2H).
Si(CH3)2H).
Synthesis of cations 1: Arylsilane 2 (0.5 mmol) was slowly added to a
vigorously stirred solution of TPFPB (460 mg (0.5 mmol) in [D6]benzene
(2 mL) at RT. The color (owing to the trityl cation) disappeared after the
addition of one equivalent of silane. Stirring was stopped and the reaction
mixture was allowed to separate into two layers, a yellow-brown highly
viscous lower phase and a colorless upper phase. The upper phase,
containing the byproduct triphenyl methane, was separated off and the
lower layer was transferred by cannula into a NMR tube and was then
investigated by NMR spectroscopy.
Compound 2b: Colorless oil; yield: 4.25 g (17 mmol, 85%); 1H N MR
(250 MHz, [D1]chloroform, 300 K, d1H(CHCl3) ˆ 7.24): d ˆ 7.45 (m, 2H;
Hortho), 7.10 (m, 2H; Hmeta), 3.80 (sept, 3J(H,H) ˆ 3.6 Hz, 1H; SiH), 2.30 (s,
3H; CH3), 1.48 1.29 (m, 2H; C1H2), 0.83 0.78 (m, 2H; C2H2), 0.65 0.53
(m, 2H; C3H2), 0.20 (s, 6H; SiMe2aryl), 0.01 (d, 3J(H,H) ˆ 3.6 Hz, 6H;
Si(CH3)2H); 13C NMR (62.90 MHz, [D1]chloroform, 300 K, d13C(CDCl3) ˆ
77.0): d ˆ 142.2 (Cpara), 138.5 (Cipso), 135.9 (Cortho), 129.1 (Cmeta), 21.4 (CH3),
19.8 (C1), 18.9 (C2), 18.7 (C3), À2.9 (Si(CH3)2Aryl), À4.4 (SiH(CH3)2); 29Si
Chem. Eur. J. 2002, 8, No. 5
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T. M¸ller et al.
FULL PAPER
Compound 1a ¥ [B(C6F5)4]À: 1H NMR (400 MHz, [D6]benzene, 300 K,
d1H(C6D5H) ˆ 7.20): d ˆ 8.01 (2H; Hortho), 7.54 (1H; Hpara), 7.20 (Hmeta), 1.68
(br, 2H; H2), 0.64 (m, 4H; H1/3), À0.36 (12H; Si(CH3)2); 13C N MR
(100 MHz, [D6]benzene, 300 K, d13C(C6D6) ˆ 128.0): d ˆ 162.9 (Cortho),
150.3 (Cpara), 149.0 (d, 1J(C,F) ˆ 251.6 Hz; Cmeta [B(C6F5)4]À), 138.7 (d,
1J(C,F) ˆ 241.5 Hz; Cpara [B(C6F5)4]À), 136.9 (d, 1J(C,F) ˆ 251.5 Hz; Cortho
[B(C6F5)4]À), 133.9 (Cmeta), 125.2 (br; Cipso [B(C6F5)4]À), 19.8 (C2), 102.4
(Cipso) 14.6 (C1/3), À2.9 (Si(CH3)2); 29Si NMR (79.5 MHz, [D6]benzene,
300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ 25.6; 19F NMR (254 MHz,
[D6]benzene, 300 K, d19F(CF2Cl2) ˆ 0): d ˆ À136.9 (d, J(F,F) ˆ 8.4 Hz, 2F;
Fmeta), À167.7 (t, J(F,F) ˆ 20.7 Hz, 1F; Fpara), À171.5 (t, J(F,F) ˆ 16.8 Hz,
2F; Fortho).
Synthesis of the nitrilium salt 10 ¥ [B(C6F5)4]À: Dry acetonitrile (20 mg,
0.5 mmol) was added at to a solution of freshly prepared 1a ¥ [B(C6F5)4]À
(0.5 mmol) in [D6]benzene at RT, and the resulting solution was inves-
tigated by NMR spectroscopy. 10 ¥ [B(C6F5)4]À: for NMR data of the
[B(C6F5)4]À anion, see 1a. 1H NMR (400 MHz, [D6]benzene, 300 K,
d1H(C6D5H) ˆ 7.20): d ˆ 7.5 (2 H ; Haryl), 7.40 7.20 (3H; Haryl), 1.30 1.20
(m, 2H; H1), 1.12 (s; CH3), 0.80 0.73 (m, 2H; H2), 0.68 0.62 (m, 2H; H3),
‡
0.30 (6H; Si(CH3)2Aryl), 0.04 (6H, Si(CH3)2N CCH3); 13C
N MR
(100 MHz, [D6]benzene, 300 K, d13C(C6D6) ˆ 128.0): d ˆ 138.2 (Cipso),
133.8 (Cortho), 129.4 (Cpara), 128.2 (Cmeta), 19.7 (C1), 18.0 (C2), 17.2 (C3), 0.0
(CH3CN ), À3.5 (Si(CH3)2), À3.7 (Si(CH3)2); 29Si NMR (79.5 MHz,
‡
[D6]benzene, 300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ 35.0 (m,
2J(Si,H) ˆ 7 Hz; Si(CH3)2N CCH3), À4.2 (brs; Si(CH3)2Aryl).
‡
Compound 1b ¥ [B(C6F5)4]À: For NMR data of the [B(C6F5)4]À anion see 1a.
1H NMR (400 MHz, [D6]benzene, 300 K, d1H(C6D5H) ˆ 7.20): d ˆ 7.95 (d,
3J(H,H) ˆ 8.1 Hz, 2H; Hortho), 7.11 (3J(H,H) ˆ 8.1 Hz, 2H; Hmeta), 2.08 (s,
3H; CH3), 1.69 (br, 2H; H2), 0.64 (m, 4H; H1/3), À0.35 (12H; Si(CH3)2);
13C NMR (100 MHz, [D6]benzene, 300 K, d13C(C6D6) ˆ 128.0): d ˆ 169.9
(Cpara), 165.8 (Cortho), 135.6 (Cmeta), 93.3 (Cipso), 23.5 (CH3), 17.1 (C2), 14.4
(C1/3), À1.8 (Si(CH3)2); 29Si NMR (79.5 MHz, [D6]benzene, 300 K,
Acknowledgement
This work was supported by the German Israeli Foundation (GIF) and the
DFG (Scholarship to T.M.). We thank Prof. N. Auner for his support and
continuing interest in this work.
2
d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ 22.1 (m, J(SiH) ˆ 5.6 Hz).
Compound 1c ¥ [B(C6F5)4]À: For NMR data of the [B(C6F5)4]À anion see 1a.
1H NMR (250 MHz, [D6]benzene, 300 K, d1H(C6D5H) ˆ 7.20): d ˆ 7.82 (d,
3J(H,H) ˆ 8.2 Hz, 1H; Hortho), 6.90 (d, 3J(H,H) ˆ 8.2 Hz, 1H; Hmeta), 6.69 (s,
1H; Hmeta), 2.07 (s, 3H; CH3), 2.00 (s, 3H; CH3), 1.83 (br, 1H; He,2), 1.48 (br,
1H; Ha,2), 0.59 (m, 4H; H1/3), À0.03, À0.64 (26H; Si(CH3)2); 13C N MR
(62.90 MHz, [D6]benzene, 300 K, d13C(C6D6) ˆ 128.0): d ˆ 182.0 (q,
2J(C,H) ˆ 5 Hz; Cortho), 169.0 (q, 2J(C,H) ˆ 6 Hz; Cpara), 165.1 (d,
1J(C,H) ˆ 164.8 Hz, Cortho), 136.7 (d, 1J(C,H) ˆ 164.4 Hz; Cmeta), 132.5 (d,
1J(C,H) ˆ 168.4 Hz; Cmeta), 89.8 (s; Cipso), 26.0 (q, 1J(C,H) ˆ 128.7 Hz; CH3),
23.1 (q, 1J(C,H) ˆ 129.3 Hz; CH3), 16.8 (t, 1J(C,H) ˆ 131 Hz; C2), 15.4 (t,
1J(C,H) ˆ 121 Hz; C1/3), À0.27 (q, 1J(C,H) ˆ 123 Hz; Si(CH3)2), À2.55 (q,
1J(C,H) ˆ 123.4 Hz; Si(CH3)2); 29Si NMR (49.7 MHz, [D6]benzene, 300 K,
d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ 20.3 (1J(SiC) ˆ 55.1 Hz).
[1] Arenium ions are also known as s complexes, Wheland or Pfeifer
Wizinger intermediates, and cyclohexadienyl ions. In this contribu-
tion, the nomenclature arenium ion is used throughout, reflecting the
trivalent carbenium ion nature of these ions.
[2] R. Taylor, Electrophilic Aromatic Substitution, Wiley, New York, 1990.
[3] J. March, Advanced Organic Chemistry, 4th ed., Wiley, New York,
1992.
[4] G. A. Olah, Friedel Crafts Chemistry, Wiley, New York, 1973.
[5] D. M. Brouwer, E. L. Mackor, C. McLean in Carbonium Ions, Vol. 2
(Eds.: G. A. Olah, P. v. R. Schleyer), Wiley, 1970, p. 837.
[6] D. Farcasiu, Acc. Chem. Res. 1982, 15, 46.
Compound 1d ¥ [B(C6F5)4]À: For NMR data of the [B(C6F5)4]À anion see 1a.
1H NMR (250 MHz, [D6]benzene, 300 K, d1H(C6D5H) ˆ 7.20): d ˆ 7.83 (s,
1H; Hortho), 7.27 (d, 3J(H,H) ˆ 7.6 Hz, 1H; Hpara), 6.69 (d, 3J(H,H) ˆ 7.6 Hz,
1H; Hmeta), 2.09 (s, 3H; CH3), 1.92 (s, 3H; CH3), 1.68 (br; H2), 0.62 (m, 4H;
H1/3), À0.10, À0.56 (br, 26H; Si(CH3)2); 13C NMR (62.90 MHz, [D6]ben-
zene, 300 K, d13C(C6D6) ˆ 128.0): d ˆ 179.9 (s; Cortho), 164.2 (d, 1J(C,H) ˆ
162.8 Hz; Cortho), 152.9 (d, 1J(C,H) ˆ 164.8 Hz; Cpara), 142.4 (s; Cmeta), 135.9
(d, 1J(C,H) ˆ 165.2 Hz; Cmeta), 96.2 (s; Cipso), 25.6 (q, 1J(C,H) ˆ 129.4 Hz;
CH3), 19.5 (q, 1J(C,H) ˆ 129.5 Hz; CH3), 16.8 (t, 1J(C,H) ˆ 130.9 Hz; C2),
15.5 (t, 1J(C,H) ˆ 119 Hz; C1/3), À0.26 (brq, 1J(C,H) ˆ 120 Hz; Si(CH3)2),
À2.28 (brq, 1J(C,H) ˆ 120 Hz; Si(CH3)2); 29Si NMR (49.7 MHz, [D6]ben-
zene, 300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ 22.7.
[7] ™Contemporary Problems in Carbonium Ion Chemistry III∫: V. A.
Koptyug, Top. Curr. Chem. 1984, 122, 1.
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Olah, S. Kuhn, J. Am. Chem. Soc. 1958, 80, 6535.
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Wadley, W. R. Edwards, G. Laber, Tetrahedron 1958, 4, 178.
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Mateescu, J. Am. Chem. Soc. 1972, 94, 2034.
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1985, 24, 693.
Compound 1e ¥ [B(C6F5)4]À: For NMR data of the [B(C6F5)4]À anion see 1a.
1H NMR (250 MHz, [D6]benzene, 300 K, d1H(C6D5H) ˆ 7.20): d ˆ 7.19 (br;
Harom), 2.10 (br; CH3), 1.76 (br; H2), 0.62 (br; H1/3), À0.37 (br; Si(CH3)2);
13C NMR (62.90 MHz, [D6]benzene, 300 K, d13C(C6D6) ˆ 128.0): d ˆ 180.6
(Cortho), 150.2 (Cpara), 134.3 (Cmeta), 95.1 (Cipso), 27.9 (CH3), 23.1 (CH3), 17.4
(C2), 16.6 (C1/3), À0.1 (Si(CH3)2); 29Si NMR (49.7 MHz, [D6]benzene,
300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ 21.6.
Compound 1f ¥ [B(C 6F5)4]À: For NMR data of the [B(C6F5)4]À anion see 1a.
1H NMR (250 MHz, [D6]benzene, 300 K, d1H(C6D5H) ˆ 7.20): d ˆ 7.86 (s,
2H; Hortho), 7.23 (s, 1H; Hpara), 1.97 (s, 6H; CH3), 1.68 (br, 2H; H2), 0.63 (m,
4H; H1/3), À0.34 (12H; Si(CH3)2); 13C NMR (62.90 MHz, [D6]benzene,
1
300 K, d13C(C6D6) ˆ 128.0): d ˆ 160.9 (d, J(C,H) ˆ 164.9 Hz; Cortho), 153.6
(d, 1J(C,H) ˆ 158.3 Hz; Cpara), 146.9 (s; Cmeta), 102.9 (s; Cipso), 20.0 (q,
1J(C,H) ˆ 129 Hz; CH3), 17.1 (t, 1J(C,H) ˆ 135 Hz; C2), 15.1 (t, 1J(C,H) ˆ
119.1 Hz; C1/3), À1.4 (q, 1J(C,H) ˆ 122.3 Hz; Si(CH3)2); 29Si NMR
(49.7 MHz, [D6]benzene, 300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.): d ˆ 24.3
(1J(Si,C) ˆ 54.8 Hz).
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Compound 1g ¥ [B(C6F5)4]À: For NMR data of the [B(C6F5)4]À anion see 1a.
1H NMR (400 MHz, [D6]benzene, 300 K, d1H(C6D5H) ˆ 7.20): d ˆ 6.6 (s,
2H; Hmeta), 2.05 (s, 6H; CH3), 1.94 (s, 3H; CH3) 1.68 (br, 2H; H2), 0.67 (m,
4H; H1/3), À0.35 (12H; Si(CH3)2); 13C NMR (100 MHz, [D6]benzene,
300 K, d13C(C6D6) ˆ 128.0): d ˆ 179.7 (Cortho), 167.3 (Cpara), 135.7 (Cmeta),
89.0 (Cipso), 27.8 (CH3), 22.6 (CH3) 16.8 (C2), 16.9 (C1/3), À0.5 (Si(CH3)2);
29Si NMR (79.5 MHz, [D6]benzene, 300 K, d29Si((H3C)2SiHCl) ˆ 11.1, ext.):
d ˆ 19.1 (2J(SiH) ˆ 5.6 Hz).
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1170
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0805-1170 $ 17.50+.50/0
Chem. Eur. J. 2002, 8, N o. 5

Arenium Ions
1163 1172
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(7.4 kcalmolÀ1 at B3LYP/6-311G(d,p)//B3LYP/6-31G(d) ‡ DZPVE).
We attribute this difference to the additional ring substituent in 1d,
which will certainly increase the barrier for simple steric reasons.
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of theory, is À0.6 kcalmolÀ1
.
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Received: August 15, 2001 [F3492]
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