Thermodynamic: Versus kinetic control in substituent redistribution reactions of silylium ions steered by the counteranion

Article abstract of DOI:10.1039/c8sc01833b

An in-depth experimental and theoretical study of the substituent exchange reaction of silylium ions is presented. Apart from the substitution pattern at the silicon atom, the selectivity of this process is predominantly influenced by the counteranion, wh

Full text of DOI:10.1039/c8sc01833b

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Thermodynamic versus kinetic control in substituent redistribu-
tion reactions of silylium ions steered by the counteranion†
Lukas Omann,a Bimal Pudasaini,b Elisabeth Irran,‡a Hendrik F. T. Klare,a Mu-Hyun Baik*,b,c and
Received 00th January 20xx,
Accepted 00th January 20xx
Martin Oestreich*,a
DOI: 10.1039/x0xx00000x
www.rsc.org/
An in-depth experimental and theoretical study of the substituent exchange reaction of silylium ions is presented. Aside
from the substitution pattern at the silicon atom, the selectivity of this process is predominantly influenced by the
counteranion, which is introduced with the trityl salt in the silylium ion generation. In contrast to Müller’s protocol for the
synthesis of triarylsilylium ions under kinetic control, the use of Reed’s carborane anions leads to contact ion pairs,
allowing for the selective formation of trialkylsilylium ions under thermodynamic control. DFT calculations finally revealed
an unexpected mechanism for the rate-determining alkyl exchange step, which is initiated by an unusual 1,2-silyl migration
in the intermediate ipso-disilylated arenium ion. The resulting ortho-disilylated arenium ion can then undergo an alkyl
transfer via a low-barrier five-center transition state.
2011
Müller (
):
Introduction
Ph3C+[B(C6F5)4]–
Ar3Si+[B(C6F5)4]–
29Si NMR (C6D6)
Silylium ions (R3Si+) have recently emerged as useful and
versatile catalysts for synthetically attractive transfor-
mations.1,2 The most commonly used approach to generate
silylium ions is the Bartlett‒Condon‒Schneider reaction,3 that
is the silicon-to-carbon hydride transfer from a hydrosilane to
the trityl cation (Ph3C+) paired with a weakly coordinating
counteranion.4,5 However, substituent redistribution of the
hydrosilane starting material can occur under these highly
Lewis acidic reaction conditions, leading to undesired mixtures
of various silicon compounds.6–8 Hence, hydrosilanes
containing three identical substituents, e.g. Et3SiH or iPr3SiH,
are usually employed in this reaction.9 Conversely, Müller and
co-workers have turned this unselective process into a useful
synthetic route to triarylsilylium ions (Scheme 1, top).10 When
MeAr2SiH
(1.6 equiv)
C6D6
rt, 1 h
– Ph3CH
– Me3SiH
δ
216–230 ppm
Ar = 2,6-disubstituted phenyl
this work:
Me2RSiH
(2 equiv)
Ph3C+[CHB11H5Br6]–
Me3Si+[CHB11H5Br6]–
toluene
rt, overnight
– Ph3CH
29Si NMR (o-Cl2C6D4)
δ
93 ppm
– MexRySiHz
R = aryl, benzyl
Scheme 1 Divergence in the generation of silylium ions by substituent
sterically demanding methyl(diaryl)silanes MeAr2SiH are used redistribution (x+y+z = 4).
in the hydride abstraction with Ph3C+[B(C6F5)4]‒, the formation
Herein, we report that treatment of hydrosilanes of type
of otherwise difficult to prepare triarylsilylium ions
Ar3Si+[B(C6F5)4]‒ is observed.11 Notably, the use of less bulky
hydrosilanes such as MePh2SiH or Me(o-Tol)2SiH do not give
triarylsilylium ions but mixtures of different silicon cations.12
Me2RSiH (R = aryl, benzyl) with Reed’s carborane-based trityl
salt Ph3C+[CHB11H5Br6]–13 results in substituent exchange
reactions selectively forming the elusive trimethylsilylium ion
Me3Si+[CHB11H5Br6]– (Scheme 1, bottom). This method thus
complements Müller’s approach and offers a practical route to
Me3Si+, avoiding the use of gaseous and highly flammable
Me3SiH.14
A systematic experimental and computational
investigation was performed to gain a full mechanistic picture
of this phenomenon. DFT calculations revealed an unexpected
mechanism and suggest an active role of the carborane
counteranion in the outcome of these reactions.
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Results and discussion
Generation of the trimethylsilylium ion by substituent
redistribution
When a mixture of Me2PhSiH and Ph3C+[CHB11H5Br6]‒ in
toluene was stirred overnight at room temperature, a white
suspension was obtained. The solid was collected by filtration,
washed with n-pentane, and dissolved in o-Cl2C6D4 for NMR
spectroscopic analysis. Unexpectedly, only a singlet at 0.83
ppm was detected in the 1H NMR spectrum, while no aromatic
resonances except for those of the deuterated solvent were
observed. The low-field 29Si NMR chemical shift of 93 ppm in
the corresponding 1H/29Si HMQC spectrum, which is
characteristic for trialkylsilylium ions, indicated clean
formation of Me3Si+[CHB11H5Br6]‒ (Fig. 1). The structural
integrity of the carborane counteranion was confirmed by 11B
NMR spectroscopy.
Fig. 2 Molecular structure of Me3Si+[CHB11H5Br6]‒ (thermal ellipsoids at 50%
probability level; H atoms omitted for clarity).
Influence of the substituent pattern at the silicon atom on the
selectivity of the substituent redistribution reaction
To understand the differences between Müllers’s protocol10
and our findings, we systematically studied the hydride
transfer reaction of various hydrosilanes of type MeAr2SiH and
Unambiguous
evidence
for
the
structure
of
Me2ArSiH
using
trityl
salts
Ph3C+[B(C6F5)4]–
and
Me3Si+[CHB11H5Br6]‒ was eventually provided by its
crystallographic characterization (Fig. 2).15 Single crystals
suitable for X-ray diffraction analysis were obtained by vapor
diffusion with n-hexane from a solution of the silylium salt in o-
F2C6H4 at room temperature. In accordance with reported
molecular structures of silylium carboranes,16 one bromine
atom at the pentagonal belt of the icosahedral anion is bound
to the silicon cation. Both the Si‒Br bond distance of 2.435(6)
Å and the sum of all C‒Si‒C bond angles of 346.3(6)° are
comparable to the larger Et3Si+[CHB11H5Br6]‒.
Ph3C+[CHB11H5Br6]– (Table 1). Depending on the counteranion,
slightly modified procedures were applied for the generation
of the silicon caꢀons (see the see the ESI† for details). For all
reactions, an excess of hydrosilane (4 equiv) was used, thereby
excluding any influence of stoichiometry on the product
formation. In accordance with Müller’s report, bulky
methyl(diaryl)silane Me(C6Me5)2SiH was converted to the
corresponding triarylsilylium ion, regardless of which
counteranion was used (entries 1 and 2). In contrast, hydride
abstraction of sterically less hindered MePh2SiH with
Ph3C+[B(C6F5)4]– led to a complex reaction mixture as a result
of anion decomposition (entry 3).12,17 The use of the carborane
counteranion [CHB11H5Br6]– furnished the unscrambled
silylium ion MePh2Si+[CHB11H5Br6]–, as confirmed by X-ray
diffracꢀon analysis (entry 4; see the ESI† for the molecular
structure of MePh2Si+[CHB11H5Br6]–).15
In contrast to the clean formation of Me3Si+, the unpolar n-
pentane filtrate contained several tri- and tetraorganosilanes,
such as Ph4Si, MePh3Si, Ph3SiH, Me2Ph2Si, MePh2SiH, Me3PhSi,
and Me2PhSiH, as verified by GC-MS analysis. Since silylium
ions are known to promote substituent redistribution,8 this
result did not come as a surprise but raised the question why
Me3Si+ was selectively formed in this reaction mixture,
whereas Müller’s conditions cleanly afford sterically congested
triarylsilylium ions.10
Table 1 Silylium ion generation by substituent redistribution: Effect of the hydrosilane
and counteranion (Si = Triorganosilyl).
Ph3C+[X]–
Si+[X]–
Si
H
– Ph3CH
– MexArySiHz
ppm
δ(29Si)
[ppm]b
217
217
‒
-300
-250
-200
-150
-100
-50
Si‒H
Entrya
[X]‒
Si+
(4 equiv)
1
2
Me(C6Me5)2SiH
Me(C6Me5)2SiH [CHB11H5Br6]‒
[B(C6F5)4]‒
(C6Me5)3Si+
(C6Me5)3Si+
c
3
MePh2SiH
MePh2SiH
[B(C6F5)4]‒
[CHB11H5Br6]‒ MePh2Si+/Me2PhSi+d
[B(C6F5)4]‒
[CHB11H5Br6]‒
[B(C6F5)4]‒
‒
4
57/76
‒
c
5
Me2PhSiH
‒
0
6
Me2PhSiH
Me3Si+
93
7
8e
Me2(C6Me5)SiH
Me2(C6Me5)SiH [CHB11H5Br6]‒
(C6Me5)3Si+
Me3Si+
217
93
50
100
150
200
a All reactions were performed according to the General Procedure (GP) 1 for X‒
=
[B(C6F5)4]‒ (C6D6, room temperature, 60 min) or GP 2 for X‒ = [CHB11H5Br6]‒
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
ppm
(toluene, room temperature, 18‒24 h). See the ESI† for details. b Measured in o-
Fig. 1 1H/29Si HMQC NMR spectrum (500/99 MHz, o-Cl2C6D4, 298 K, optimized for
Cl2C6D4. c A complex mixture was obtained as
a result of counteranion
J
= 7
Hz) of Me3Si+[CHB11H5Br6]‒ from the reaction of Me2PhSiH with
decomposition.17 d Ratio of 79:21 determined by 1H NMR spectroscopy. e Reaction
Ph3C+[CHB11H5Br6]‒.
performed at 50 °C for 72 h.
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However, the formation of the MePh2Si+ cation was Mechanism of the substituent redistribution reaction with
accompanied by a substantial amount of a second silylium ion, Me2PhSiH
which was assigned as Me2PhSi+ cation.18 Notably, longer
To gain insight into the reaction mechanism and to understand
why the treatment of Me2PhSiH with Ph3C+[CHB11H5Br6]–
reaction times (7 days) or elevated temperatures (50 °C for 72
exclusively gives Me3Si+[CHB11H5Br6]–, we constructed
a
h) did not significantly change the product ratio of
shown). In all cases, the generation of Me3Si+ was not
observed. We then turned our attention to
~79:21 (not
complete reaction energy profile using DFT calculations at the
M06/cc-pVTZ(-f)//6-31G** level of theory (Fig. 3; see the ESI†
for details of the computational method).20 The hydride
abstraction of Me2PhSiH with Ph3C+[CHB11H5Br6]– was found to
have a barrier of 15.5 kcal∙mol–1 and is therefore expected to
occur rapidly at room temperature (not shown). In the
dimethyl(aryl)silanes (entries 5–8). The reaction of Me2PhSiH
with Ph3C+[B(C6F5)4]– again resulted in decomposition of the
borate counteranion (entry 5).17 Conversely, treatment of
Me2PhSiH with trityl carborane Ph3C+[CHB11H5Br6]– exclusively
afforded silylium salt Me3Si+[CHB11H5Br6]– without detectable
formation of neither MePh2Si+ nor Me2PhSi+ (entry 6).
Strikingly, hydride abstraction from sterically more demanding
Me2(C6Me5)SiH led to the corresponding triarylsilylium ion in
the presence of the borate counteranion (entry 7), while
substituent redistribution into the ‘opposite direction’ was
induced by the carborane anion, now affording
Me3Si+[CHB11H5Br6]– (entry 8).19 However, heating of the
reaction at 50 °C for 72 h was necessary.
condensed phase, the resulting silylium ion Me2PhSi+
(6A),
which is located at a relative free energy of 6.5 kcal∙mol–1, is
stabilized through coordination by the solvent, another
hydrosilane molecule, or by the counteranion (see the ESI† for
a comparison of the association energies).8e,21 Coordination of
one of the bromine atoms of the carborane counteranion to
the silicon cation results in the highest binding energy, and the
resulting ion pair 6A’ is predicted to be at a relative free
energy of
with another equivalent of Me2PhSiH to form hydride-bridged
adduct 7A 21 mol–1. Note that these
located at 6.5 kcal
‒24.1 kcalꢀ
mol–1. Silylium ion 6A can also interact
Overall, these results indicate that hydride abstraction of
hydrosilanes of type Me2ArSiH with a carborane-based trityl
salt tends to form the trimethylsilylium ion, whereas
hydrosilanes of type MeAr2SiH with a bulky aryl substituent
favor triarylsilylium ion generation.
,
‒
ꢀ
energies are not adjusted for the different concentrations of
the components and assume normal conditions. Given that
Me2PhSiH (1A) is present in excess, these normal energies
suggest that adduct 7A will be encountered easily in significant
quantities.
Fig. 3 Energy (kcal∙mol‒1) profile of the substituent redistribution in the reaction of Me2PhSiH (1A) with Ph3C+[CHB11H5Br6]‒ (2A). The energies are relative to the starting material
1A and 2A.
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Hydride-bridged adduct ion 7A can undergo a phenyl group As shown in Fig. 4, the silylium ions can either be bound to the
transfer to arrive at phenyl-bridged adduct 8A7c,8b,22 via the apical or one of the equatorial bromine atoms of the
four-center transition state 7A-TS, associated with a barrier of carborane counteranion, with a slight preference of 1.1
13.4 kcal
ꢀ
mol–1. Surprisingly, the subsequent methyl group kcal∙mol–1 for the apical position in Me3Si+[CHB11H5Br6]–
12A’’). This result is in contrast to the molecular structure in
transfer does not proceed via another typical four-membered
(
transition state.23 Instead, our calculations suggest that 1,2- the solid state, which shows the equatorial conformer (cf. Fig.
migration of the silicon group in 8A occurs via the low barrier 2). We speculate that either packing effects or a statistical
transition state 8A-TS, leading to ortho-disilylated arenium ion preference for the equatorial conformer are the reason for this
9A. This seemingly unfavorable intermediate is only 4.1 discrepancy. Notably, the equatorial conformer 12A’ is still 1.8
kcalꢀ
mol–1 higher in energy than arenium ion 8A. Finally, 9A kcal∙mol–1 lower in energy than the equatorial conformer of
facilitates the exchange of one methyl group, passing through MePh2Si+[CHB11H5Br6]– (13A’). The higher ion pairing energy in
five-center transition state 9A-TS with an overall barrier of 12A’ can be ascribed to the low steric demand of Me3Si+,
24.2 kcalꢀ . This energetically most leading to a closer carborane coordination and to attractive
mol–1 relative to 7A
demanding reaction step forms methonium ion 10A, which is van der Waals interactions between the methyl moieties and
metastable and rapidly rearranges to hydride-bridged adduct the carborane anion. Especially in the apical position, the
11A via low barrier transition state 10A-TS. The hydrosilane- methyl functionality can interact with the highly polarizable
stabilized silylium ions 7A and 11A are almost isoenergetic (
= 0.4 kcal
mol–1), suggesting that both structures coexist in more demanding silylium ions Me2PhSi+
equilibrium. The formal dissociation of 11A either gives Me3Si+
13A) with the carborane counteranion is less tight, and the
or MePh2Si+, the former being calculated to be 2.8 kcal mol–1 ion pairing is therefore slightly less favorable. This trend is
∆
G
bromine atoms. In contrast, the molecular fit of the sterically
ꢀ
(
6A) and MePh2Si+
(
ꢀ
higher in energy. However, coordination by the carborane reflected in the corresponding Si–Br bond lengths of these
anion changes the energy landscape decisively, as ion pair silylium carborane salts, which were computed to be shortest
formation reverses the energy ordering. Me3Si+[CHB11H5Br6]– in both conformers of Me3Si+[CHB11H5Br6]– (12A’ and 12A’’).
(
12A’’), which is located at
lower in energy than MePh2Si+[CHB11H5Br6]–
4.5 kcal
mol–1 more stable than Me2PhSi+[CHB11H5Br6]–
‒
28.5 kcal
ꢀ
mol–1, is 2.9 kcal
13A’) and also
6A’),
ꢀ
mol–1
(
ꢀ
(
thus predicting the silylium salt 12A’’ as the major product of
the substituent redistribution reaction.
It should be noted that silylium ions are significantly more
stabilized by coordination of the carborane counteranion than
by
formation
of
solvent
adducts
such
as
R3Si(toluene)+[CHB11H5Br6]–. Moreover, the energy differences
between these arenium ions are small, predicting a mixture of
different silylium ions in the absence of the carborane
counteranion (see the ESI† for details).24 This result was
supported by independent control experiments (Scheme 2).
The hydride abstraction of Me2PhSiH with borate-based trityl
salt Ph3C+[B(C6F5)4]– was repeated but stopped after stirring for
10 min in toluene (cf. Table 1, entry 5). NMR spectroscopic
analysis of the polar phase in o-Cl2C6D4 revealed formation of a
mixture of Me3Si+[B(C6F5)4]– and Me2PhSi+[B(C6F5)4]– in a ratio
of ~51:49 along with small amounts of byproducts arising from
counteranion decomposition. In contrast, stopping the
reaction of Me2PhSiH with Ph3C+[CHB11H5Br6]– after stirring for
10 min in toluene furnished Me3Si+[CHB11H5Br6]– as the major
product along with only small amounts of unscrambled
Me2PhSi+[CHB11H5Br6]– (ratio
~84:16). In both reactions, full
conversion of the trityl salt was observed.
Fig. 4 Computed apical and equatorial conformers of Me2PhSi+[CHB11H5Br6]‒
(top), Me3Si+[CHB11H5Br6]‒ (middle) and MePh2Si+[CHB11H5Br6]‒ (bottom). Si‒Br
bond distances given in Å and relative free energy differences (kcal∙mol‒1) shown
in parenthesis.
Scheme 2 Influence of the counteranion on the selectivity of the
trimethylsilylium ion formation.
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Hence, this ion pair is the most stable silylium salt despite the scrambled hydride-bridged adduct 11B. The transformation of
lack of stabilizing phenyl groups. Both conformers of 7B to 11B via intermediates 8B 9B, and 10B is again reversible,
Me2PhSi+[CHB11H5Br6]– (6A’ and 6A’’) are higher in energy than since 7B and 11B have similar free energies ( mol–
G = 0.7 kcal
the corresponding MePh2Si+[CHB11H5Br6]– 13A’ and 13A’’), 1). As before, the methyl group transfer via five-membered
indicating that the stabilization of these silylium carborane transition state 9B-TS shows the highest barrier, which is 24.2
salts is determined by a delicate balance of electronic and kcal
mol–1 relative to 7B. In this equilibrium, unscrambled
steric effects. It should be also noted here that the DFT MePh2Si+[CHB11H5Br6]–
6B’) with a relative free energy of
optimized structures for Me3Si+[CHB11H5Br6]– mol–1 is predicted to be the major species, followed
12A’ and 25.9 kcal
MePh2Si+[CHB11H5Br6]– (13A’) are in good agreement with the by Me2PhSi+[CHB11H5Br6]–
scrambled 12B’ and
corresponding molecular structures obtained by X-ray Ph3Si+[CHB11H5Br6]– (13B’’), which are basically isoenergetic at
,
∆
ꢀ
(
ꢀ
(
‒
(
)
ꢀ
(
)
diffraction analysis (see the ESI† for details).
‒24.6 kcalꢀ ‒24.7 kcalꢀ
mol–1 and mol–1, respectively. This finding
is in good agreement with the experimental observation of
unscrambled MePh2Si+[CHB11H5Br6]– being the main product of
the reaction (cf. Table 1, entry 4).25
Mechanism of the substituent redistribution reaction with
MePh2SiH
To understand why the reaction of MePh2SiH with Our calculations suggest that a subsequent methyl exchange
Ph3C+[CHB11H5Br6]– does not furnish Me3Si+[CHB11H5Br6]–, we reaction leading to Me3Si+ is unlikely (11B
18B, gray energy
constructed again a complete energy profile employing DFT profile in Fig. 5). The transition state for this methyl group
simulations (Fig. 5). The initial hydride transfer of the transfer, 16B-TS, is located 26.7 kcal∙mol–1 relative to 11B
hydrosilane to the trityl cation has a calculated barrier of 14.3 which is 1.8 kcal∙mol–1 higher in energy than the barrier of the
kcal
mol–1 (not shown), which is 1.2 kcal mol–1 lower in energy backward reaction via transition state 9B-TS. Consequently,
→
,
ꢀ
ꢀ
as in the case of Me2PhSiH due to the slightly higher hydride the reaction of MePh2SiH with Ph3C+[CHB11H5Br6]– stops at the
donor strength of MePh2SiH (see Table S1 in the ESI† for above mentioned mixture of silicon cations rather than
details). The resulting silylium ion MePh2Si+ (6B) with a relative undergoing exhaustive substituent redistribution to furnish
free energy of 0.75 kcalꢀ
mol–1 is almost isoenergetic to the low energy Me3Si+[CHB11H5Br6]–.
reactant state. Adduct formation with another equivalent of
MePh2SiH affords hydrosilane-stabilized silylium ion 7B, which
undergoes a methyl/phenyl exchange reaction following a very
similar reactivity pattern as described above, leading to
Fig. 5 Energy (kcal∙mol‒1) profile of the substituent redistribution in the reaction of MePh2SiH (1A) with Ph3C+[CHB11H5Br6]‒
(2B). The energies are relative to the
starting material 1B and 2B
.
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Table 3 Silylium ion generation from hydrosilanes of type Me2RSiH.
Entrya
R
Si+
δ(29Si) [ppm]b
1
2
3
Ph
Bn
tBu
Me3Si+
Me3Si+
Me2tBuSi+
93
93
98
a All reactions were performed according to GP 2. See the ESI† for details.
b Measured in o-Cl2C6D4.
Scheme 3 Probing kinetic inhibition in the substituent redistribution reaction
with MePh2SiH.
To investigate whether the phenyl group in Me2PhSiH can be
replaced by other `leaving groups´, we also tested a benzyl and
alkyl substituent in Me2RSiH (Table 3). As in the case of
Me2PhSiH (entry 1), clean formation of Me3Si+[CHB11H5Br6]‒
was observed with Me2BnSiH (entry 2), showing that the
phenyl group is not essential for the exchange process. In
contrast, the bulky tert-butyl group in Me2tBuSiH completely
prevented substituent redistribution, and silylium ion
Me2tBuSi+[CHB11H5Br6]‒ was formed as the only product (entry
3). This result again demonstrates that the intermolecular
substituent exchange reaction is sensitive towards sterically
demanding alkyl groups (cf. entry 3 in Table 2).
This kinetic inhibition was further proven by another
mechanistic control experiment (Scheme 3). When a mixture
of Ph3C+[CHB11H5Br6]– and MePh2SiH in toluene was stirred
overnight at room temperature, a pale yellow suspension was
obtained, which is characteristic for silylium ions with aromatic
substituents (cf. Table 1, entry 4). Addition of less bulky
Me2PhSiH to this mixture resulted in a quick decolorization and
formation of a white suspension. NMR spectroscopic analysis
of the solid now confirmed exclusive formation of
Me3Si+[CHB11H5Br6]–.
Scope of the substituent redistribution reaction
The hydride abstraction of various dialkyl(phenyl)silanes with
Ph3C+[CHB11H5Br6]‒ finally revealed that the redistribution
reaction is not restricted to methyl groups (Table 2). Although
Et2PhSiH reacted much slower compared to Me2PhSiH,
exclusive formation of trialkylsilylium ion Et3Si+[CHB11H5Br6]‒
was observed (entries 1 and 2). Employing more bulky
iPr2PhSiH led to clean generation of unscrambled
dialkyl(aryl)silylium ion iPr2PhSi+[CHB11H5Br6]‒, as verified by X-
ray crystallography (entry 3; see the ESI† for for the molecular
structure of iPr2PhSi+[CHB11H5Br6]‒).15 These results are in
accordance with our calculations, predicting high energy
barriers for the transfer of bulky alkyl groups. Sterically even
more shielded tBu2PhSiH then completely thwarted the
hydride abstraction, and only the trityl salt was recovered from
the reaction mixture (entry 4).
Conclusion
It has been known for decades, that silylium ions can undergo
redistribution reactions of their substituents.8 The present
combined experimental and detailed computational study
finally provides a full mechanistic picture of this phenomenon.
The mechanism involves a series of phenyl and alkyl exchange
reactions, the latter being calculated to be the energetically
most demanding steps. While the transfer of phenyl groups
proceeds via common four-center transition states, the
corresponding alkyl exchange was found to pass unusual five-
membered transition states. These are accessible after 1,2-silyl
migration at the stage of the intermediate disilylated arenium
ions.
Additionally, our DFT calculations revealed that the silicon
cations are significantly more stabilized by ion pair formation
with the carborane counteranion (R3Si+[CHB11H5Br6]‒) than by
formation of toluenium (R3Si(toluene)+[CHB11H5Br6]‒) or
Table 2 Silylium ion generation from hydrosilanes of type R2PhSiH.
hydrosilane-stabilized
silylium
ions
([R3Si‒H‒
SiR3]+[CHB11H5Br6]‒). More importantly, purely aliphatic
silylium carboranes with small substituents, i.e., methyl or
ethyl groups, were found to be distinctly lower in energy than
the corresponding mixed aliphatic/aromatic or purely aromatic
silylium ion pairs as a result of stronger attractive interactions
(ΔG ≥ 2.9 kcal∙mol‒1 for R = Me). These energy differences
Entrya
R
Si+
δ(29Si) [ppm]b
1
2c
Me
Et
Me3Si+
Et3Si+
iPr2PhSi+
‒
93
100
76
‒
account for
the highly selective
formation
of
3
iPr
tBu
Me3Si+[CHB11H5Br6]‒ and Et3Si+[CHB11H5Br6]‒ from the reaction
of the corresponding hydrosilanes R2PhSiH (R = Me, Et) with
Ph3C+[CHB11H5Br6]‒ under thermodynamic control.
4
a All reactions were performed according to GP 2. See the ESI† for details.
b Measured in o-Cl2C6D4. c With 4 equiv of Et2PhSiH and 7 days reaction
time. d No reaction, only Ph3C+[CHB11H5Br6]‒ was recovered.
6 | J. Name., 2012, 00, 1-3
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ARTICLE
I. Manners, J. Am. Chem. Soc., 2000, 122, 2126‒2127 (ring-
opening protonolysis); (c) A. Schäfer, M. Reißmann, A.
Schäfer, M. Schmidtmann and T. Müller, Chem. Eur. J., 2014,
20, 9381‒9386 (silylene protonation); (d) A. Simonneau, T.
Biberger and M. Oestreich, Organometallics, 2015, 34, 3927‒
3929 (cyclohexadienyl-leaving-group approach); (e) Q.-A.
Chen, H. F. T. Klare and M. Oestreich, J. Am. Chem. Soc.,
2016, 138, 7868‒7871 (hydrosilane protonation).
For a review of substituent redistribution reactions at silicon,
see: D. R. Weyenberg, L. G. Mahone and W. H. Atwell, Ann.
N. Y. Acad. Sci., 1969, 159, 38‒55.
For Lewis acid-catalyzed substituent redistribution reactions
of hydrosilanes, see: (a) J. L. Speier and R. E. Zimmerman, J.
Am. Chem. Soc., 1955, 77, 6395‒6396; (b) M. Khandelwal
The phenyl group in Me2PhSiH turned out to be exchangeable
by other `leaving groups´, such as a benzyl or even a sterically
demanding C6Me5 group. However, two alkyl groups must be
preinstalled in the hydrosilane starting material to steer the
reaction towards formation of Me3Si+[CHB11H5Br6]‒. In
contrast, hydride abstraction of MePh2SiH with only one alkyl
substituent leads to a mixture of different silylium ions, as
exhaustive scrambling to Me3Si+ is kinetically inhibited.
Exchanging the phenyl groups in MePh2SiH by 2,6-
disubstituted aryl groups (e.g. C6Me5) eventually provides
access to sterically congested triarylsilylium ions, as previously
demonstrated by Müller and co-workers.10
6
7
and R. J. Wehmschulte, Angew. Chem. Int. Ed., 2012, 51
7323‒7326; (c) A. Feigl, I. Chiorescu, K. Deller, S. U. H.
,
These general trends provide a solid foundation for the
mechanistic understanding of the substituent redistribution of
silylium ions, thereby enabling the prediction of the outcome
of these exchange reactions. Thus, this process can be used as
a reliable synthetic route not only to triaryl- but also to
trialkylsilylium ions by deliberate choice of the hydrosilane and
counteranion of the trityl salt.
Heidsieck, M. R. Buchner, V. Karttunen, A. Bockholt, A.
Genest, N. Rösch and B. Rieger, Chem. Eur. J., 2013, 19
12526‒12536; (d) R. J. Wehmschulte, M. Saleh and D. R.
Powell, Organometallics, 2013, 32 6812‒6819; (e) R.
,
,
Labbow, F. Reiß, A. Schulz and A. Villinger, Organometallics,
2014, 33, 3223‒3226; (f) J. Chen and E. Y.-X. Chen, Angew.
Chem. Int. Ed., 2015, 54, 6842‒6846; (g) Y. Ma, L. Zhang, Y.
Luo, M. Nishiura and Z. Hou, J. Am. Chem. Soc., 2017, 139
12434–12437.
,
8
For substituent redistribution reactions of silicon cations,
see: (a) C. Eaborn, P. D. Lickiss, S. T. Najim and W. A.
Stańczyk, J. Chem. Soc., Chem. Commun., 1987, 19, 1461‒
1462; (b) N. Choi, P. D. Lickiss, M. McPartlin, P. C. Masangane
and G. L. Veneziani, Chem. Commun., 2005, 6023‒6025; (c)
N. Lühmann, H. Hirao, S. Shaik and T. Müller,
Organometallics, 2011, 30, 4087–4096; (d) K. Müther, P.
Hrobárik, V. Hrobáriková, M. Kaupp and M. Oestreich, Chem.
Eur. J., 2013, 19, 16579‒16594; (e) S. J. Connelly, W.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
L.O. thanks the Fonds der Chemischen Industrie for
a
predoctoral fellowship (2015‒2017), and M.O. is indebted to
the Einstein Foundation (Berlin) for an endowed professorship.
B.P. and M.-H.B. are grateful for the financial support (IBS-R10-
D1) from the Institute of Basic Science. DFT calculations were
performed using high performance computing facility located
in the KAIST campus.
Kaminsky and D. M. Heinekey, Organometallics, 2013, 32
7478–7481. (f) Ref 7e; (g) L. Albers, S. Rathjen, J.
,
Baumgartner, C. Marschner and T. Müller, J. Am. Chem. Soc.,
2016, 138, 6886–6892.
For seminal reports, see: (a) J. B. Lambert, S. Zhang, C. L.
Stern and J. C. Huffmann, Science, 1993, 260, 1917‒1918; (b)
9
C. A. Reed, Z. Xie, R. Bau and A. Benesi, Science, 1993, 262
402‒404.
,
10 (a) A. Schäfer, M. Reißmann, A. Schäfer, W. Saak, D. Haase
and T. Müller, Angew. Chem. Int. Ed., 2011, 50, 12636‒
12638; (b) A. Schäfer, M. Reißmann, S. Jung, A. Schäfer, W.
Notes and references
1
For general reviews of silylium ion chemistry, see: (a) V. Y.
Lee and A. Sekiguchi, in Organosilicon Compounds, ed. V. Y.
Lee, Academic Press, Oxford, 2017, Vol. 1, pp. 197–230; (b)
T. Müller, in Structure and Bonding, ed. D. Scheschkewitz,
Springer, Berlin, 2014, Vol. 155, pp. 107‒162; (c) T. Müller, in
Science of Synthesis Knowledge Updates 2013/3; ed. M.
Oestreich, Thieme, Stuttgart, 2013, pp. 1‒42.
Saak, E. Brendler and T. Müller, Organometallics, 2013, 32
4713‒4722.
,
11 (a) J. B. Lambert and Y. Zhao, Angew. Chem. Int. Ed. Engl.,
1997, 36, 400–401; (b) K.-C. Kim, C. A. Reed, D. W. Elliot, L. J.
Mueller, F. Tham, L. Lin and J. B. Lambert, Science, 2002, 297
825‒827. (c) J. B. Lambert and L. Lin, J. Org. Chem., 2001, 66
8537–8539.
,
,
2
3
4
For silylium ions in catalysis, see: (a) H. F. T. Klare, ACS Catal.,
2017, 7, 6999‒7002; (b) A. Schulz and A. Villiger, Angew.
Chem. Int. Ed., 2012, 51, 4526‒4528; (c) H. F. T. Klare and M.
Oestreich, Dalton Trans., 2010, 39, 9176‒9184.
(a) P. D. Bartlett, F. E. Condon and A. Schneider, J. Am. Chem.
Soc., 1944, 66, 1531–1539; (b) J. Y. Corey and R. West, J. Am.
Chem. Soc., 1963, 85, 2430–2433; (c) J. Y Corey, J. Am. Chem.
Soc., 1975, 97, 3237–3238.
For recent reviews of weakly coordinating anions, see: (a) I.
M. Riddlestone, A. Kraft, J. Schaefer and I. Krossing, Angew.
Chem. Int. Ed., 2018, 57, DOI: 10.1002/anie.201710782; (b)
T. A. Engesser, M. R. Lichtenthaler, M. Schleep and I.
Krossing, Chem. Soc. Rev., 2016, 45, 789–899.
12 Hydride abstraction of MePh2SiH with Ph3C+[B(C6F5)4]‒ was
reported as a clean reaction: J. B. Lambert, S. Zhang, S. M.
Ciro, Organometallics, 1994, 13, 2430‒2443. However, this
result could not be reproduced neither by the Müller (cf. Ref
10b) nor our group.
13 (a) C. A. Reed, Acc. Chem. Res., 1998, 31, 133‒139; (b) C. A.
Reed, Acc. Chem. Res., 2010, 43, 121–128.
14 For the synthesis and crystallographic characterization of
related trimethylsilylium salts, see: (a) Me3Si+[CRB11F11]‒ (R =
H, Et): T. Küppers, E. Bernhardt, R. Eujen, H. Willner and C.
W. Lehmann, Angew. Chem. Int. Ed., 2007, 46, 6346‒6349;
(b) Me3Si(arene)+[B(C6F5)4]‒: M. F. Ibad, P. Langer, A. Schulz
and A. Villiger, J. Am. Chem. Soc., 2011, 133, 21016‒21027.
15 CCDC 1818576 for Me3Si+[CHB11H5Br6]‒, CCDC 1818582 for
5
For further strategies to generate silylium ions, see: (a) J. B.
Lambert, Y. Zhao, H. Wu, W. C. Tse and B. Kuhlmann, J. Am.
Chem. Soc., 1999, 121, 5001‒5008 (allyl-leaving-group
MePh2Si+[CHB11H5Br6]‒,
and
contain
CCDC
the
1818581 for
supplementary
iPr2PhSi+[CHB11H5Br6]‒
approach); (b) M. J. MacLachlan, S. C. Bourke, A. J. Lough and
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crystallographic data for this paper. These data are provided
Journal Name
8d). However, the calculated barriers for the transition states
were relatively high. For the calculated mechanism of an
intramolecular substituent exchange reaction at a silicon
cation with a rigid naphthalene-1,8-diyl backbone, see: Ref
8c.
free of charge by The Cambridge Crystallographic Data
Centre.
16 Z. Xie, R. Bau, A. Benesi and C. A. Reed, Organometallics,
1995, 14, 3933‒3941.
17 The decomposition of the [B(C6F5)4]‒ counteranion is likely to 21 (a) S. P. Hoffmann, T. Kato, F. S. Tham and C. A. Reed, Chem.
proceed via an SEAr reaction of the formed silylium ions with
Commun., 2006, 767–769; (b) M. Nava and C. A. Reed,
Organometallics, 2011, 30, 4798–4800.
the borate. The formation of B(C6F5)3 was verified by 19F
NMR spectroscopic analysis, and GC-MS analysis revealed 22 R. Meyer, K. Werner and T. Müller, Chem. Eur. J., 2002,
formation of several silanes containing a C6F5 unit. 1163–1171.
18 The generated silylium ions were converted to the 23 We were not able to locate a four-center transition state
8,
corresponding fluorosilanes using (C6F5)3PF2 (1.0 equiv),
thereby facilitating product characterization by both NMR
from 8A to directly arrive at 10A. See the Figure S67 in the
ESI† for geometric scan calculations.
spectroscopic and GC-MS analysis. For the preparation of 24 Me3Si(toluene)+[CHB11H5Br6]‒ was calculated to be only 0.7
(C6F5)3PF2, see: C. B Caputo, L. J. Hounjet, R. Dobrovetsky and
D. W. Stephan, Science, 2013, 341, 1374‒1377.
kcal∙mol‒1 lower in energy than MePh2Si(toluene)+
[CHB11H5Br6]‒ (see the ESI† for details).
19 Small
amounts
of
the
triarylsilylium
ion 25 Although our calculations predict formation of small
amounts of Ph3Si+[CHB11H5Br6]‒ in the reaction of MePh2SiH
with Ph3C+[CHB11H5Br6]‒, we were not able to detect this
silylium ion by 1H/29Si HMQC NMR.
(C6Me5)3Si+[CHB11H5Br6]‒ were also detected (cf. Ref 10).
20 The mechanism of intermolecular substituent exchange
reactions at related ferrocene-stabilized silylium ions had
already been studied by quantum-chemical analyses (cf. Ref
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TOC
Substituent exchange reactions of silylium ions can be steered in opposite directions. The judicious choice of the
hydrosilane and the counteranion enables the selective formation of either triaryl- or trialkylsilylium ions.
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