Dispersion-energy-driven Wagner-Meerwein rearrangements in oligosilanes

Article abstract of DOI:10.1021/jacs.6b03560

The installation of structural complex oligosilanes from linear starting materials by Lewis acid induced skeletal rearrangement reactions was studied under stable ion conditions. The produced cations were fully characterized by multinuclear NMR spectroscopy at low temperature, and the reaction course was studied by substitution experiments. The results of density functional theory calculations indicate the decisive role of attractive dispersion forces between neighboring trimethylsilyl groups for product formation in these rearrangement reactions. These attractive dispersion interactions control the course of Wagner-Meerwein rearrangements in oligosilanes, in contrast to the classical rearrangement in hydrocarbon systems, which are dominated by electronic substituent effects such as resonance and hyperconjugation.

Full text of DOI:10.1021/jacs.6b03560

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Dispersion-Energy-Driven Wagner−Meerwein Rearrangements in
Oligosilanes
†
,⊥
†
,‡
,§
ller*,†
̈
Lena Albers, Saskia Rathjen, Judith Baumgartner,* Christoph Marschner,* and Thomas Mu
†
Institute of Chemistry, Carl von Ossietzky University Oldenburg, Carl von Ossietzky-Str. 9-11, 26129 Oldenburg, Federal Republic
of Germany
‡
Institute of Chemistry, Karl Franzens University Graz, Stremayergasse 9, 8010 Graz, Austria
Institute of Inorganic Chemistry, Technical University Graz, Stremayergasse 9, 8010 Graz, Austria
§
*
S Supporting Information
ABSTRACT: The installation of structural complex oligosi-
lanes from linear starting materials by Lewis acid induced
skeletal rearrangement reactions was studied under stable ion
conditions. The produced cations were fully characterized by
multinuclear NMR spectroscopy at low temperature, and the
reaction course was studied by substitution experiments. The
results of density functional theory calculations indicate the
decisive role of attractive dispersion forces between neighbor-
ing trimethylsilyl groups for product formation in these
rearrangement reactions. These attractive dispersion interactions control the course of Wagner−Meerwein rearrangements in
oligosilanes, in contrast to the classical rearrangement in hydrocarbon systems, which are dominated by electronic substituent
effects such as resonance and hyperconjugation.
30
INTRODUCTION
Scheme 1. Synthesis of Persilaadamantane 1
■
A major impetus for the development of microelectronics is the
constant quest for ever smaller integrated circuits defined by
1
Moore’s law. A useful bottom-up approach to semiconductor
devices could be the synthesis of small silicon clusters,
molecular entities of defined structures that resemble parts of
crystalline or amorphous silicon. Silicon clusters of diverse
structural complexity with intriguing electronic and bonding
properties have been synthesized by reductive oligomerization
2
−20
of polyhalosilanes
or disproportionation reactions of
Scheme 2. Lewis Acid Catalyzed Rearrangement of Linear
Oligosilane 2 To Give Branched Oligosilanes 3 and 4
2
1
2
5,31
Si Cl . A different synthetic approach to silicon cluster
structures is the rearrangement of linear poly- or oligosilanes
catalyzed by Lewis acids. Pioneering work of Kumada’s and
2
6
22
West’s groups showed that by using AlCl as the catalyst, linear
3
oligosilanes can be transformed to structures of higher
23−27
complexity such as branched or cyclic systems.
Recently,
we have been able to document the activity of carbocations,
is a profound understanding of the generation of branched
oligosilanes from linear starting materials, which increases the
number of tetrasila-substituted silicon atoms. Scheme 2
provides an example for this type of reaction in which the
linear oligosilane 2 is transformed initially into its branched
such as the trityl cation, in these catalytic rearrangement
2
8
reactions. Furthermore, the use of a trityl cation paired with a
weakly coordinating anion in stoichiometric amounts allowed
the detection of oligosilanylsilyl cations under carefully
2
8,29
controlled reaction conditions.
The most prominent
25,31
isomers 3 and 4.
We have studied the reaction shown in
example for this type of Lewis acid catalyzed sila-Wagner−
Meerwein rearrangement is the synthesis of persilaadamantane
Scheme 2 by ionizing close derivatives of oligosilane 2 at low
temperature and characterized important cationic intermedi-
ates, which has allowed the formulation of a detailed
mechanism for this reaction. In addition, the accompanying
1
from a bicyclic precursor that was reported recently by two of
30
us (Scheme 1). During this transformation, the complexity of
the bicyclic compound increases significantly by increasing the
number of tetrasila-substituted silicon atoms. While the
formation of the persilaadamantane is certainly a multiple-
step reaction, the key for controlling this type of rearrangement
Received: April 15, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b03560
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Journal of the American Chemical Society
Article
computational studies have provided clear indications that
this signal also reveals a 1J(SiH) coupling of 43 Hz.
3
2
29
favorable attractive London dispersion forces in these
intermediates drive these types of skeletal rearrangement
reactions in the direction of branched products such as 4.
Interestingly, the low-field Si resonance also shows a
1
correlation to a singlet in the H NMR spectrum, which we
assigned to a SiMe group based on the relative intensity in the
2
1
H NMR spectrum. Supported by additional one- and two-
RESULTS AND DISCUSSION
The starting point of our experimental investigation was the
ionization of 2,5-dihydridohexasilane 5 as a mimic for
oligosilane 2. The use of the dihydrido compound 5 in our
study instead of the dimethyl compound 2 has two advantages:
■
dimensional NMR studies, we assigned the structure of bis-
silylhydronium ion 8 to the produced cation (see Figure 2,
Scheme 3, and Supporting Information). In particular, the
33
1
analysis of the J(SiSi) coupling pattern was highly instructive,
which established the connectivity of the silicon atoms and
secured our structural assignment (see Figures 1 and 2). Cation
(i) the position for the cation formation is clearly defined by
the position of the accessible Si−H bonds, and (ii) the second
Si−H linkage serves as an autoscavenge reagent, stabilizing the
incipient cation by formation of a Si−H−Si bond. The
increased stability of the so-formed bis-silylhydronium ion
suggests that its NMR detection and characterization are
possible without affecting the topology of the originally formed
silyl cation. Using this methodology, we are able to identify
otherwise only transient cationic species in rearrangement
8
was transformed into the branched dihydrido compound 9 by
reaction with Na[Et BH] at −30 °C (Scheme 3). Compound 9
3
29
29
was identified by its Si NMR chemical shifts (δ Si = −8.8
SiMe ), −30.2 (SiHMe ), and −132.9 (SiSi )) and by
(
3
2
4
46
comparison with literature data.
Finally, our structural assignment for the formed cation was
corroborated by the good agreement between the experimental
29
Si NMR parameters and those computed by quantum
29
reactions of oligosilanes.
From the reaction of dihydrido-oligosilane 5 with trityl
47,48
mechanical calculations (see Table 1).
The largest deviation
was found for the tetrasila-substituted silicon atoms (calculated
borate, [Ph C][B(C F ) ], we thus expected the formation of
29
29
3
6 5 4
δ Si = −124.8 vs experimental δ Si = −120.9). Noteworthy is
bis-silylhydronium ion 7 as the self-trapping product of the
logically first-formed cation 6 (Scheme 3). The reaction was
the fine accordance between calculated and experimental data
29
for the central Me Si−H−SiMe unit (calculated δ Si = 105.9,
2
2
1
29
1
J(SiH) = 40 Hz vs experimental δ Si = 108.9, J(SiH) = 43
Scheme 3. Ionization of Dihydrido-oligosilane 5 To Give
Hydronium Ion 8 and Its Trapping Reaction To Give
Oligosilane 9
Hz). In addition, the results of calculations for cation 7 deviate
significantly from those found in the experiment, suggesting
that cation 7 is not present in the reaction mixture. The
immediate and selective formation of cation 8 was surprising to
us. It showed, however, the pronounced tendency of the
formed polysilanyl cations to favor structural arrangements with
tetrasila-substituted silicon atoms.
Next, we turned our attention to the actual reaction
mechanism for the formation of cation 8. Substitution of the
trisila-substituted silicon atoms by germanium atoms in
hexasilane 5 is not expected to change the reactivity
significantly, but it allows following the reaction course
(
Scheme 4). Based on the experience with oligosilane 5, we
expected that hydride transfer from the 2,4-digermahexasilane
1
1
0 should result either in formation of bis-silylhydronium ion
1 or in the generation of bis-germylhydronium ion 12. The
first cation is produced by complete skeletal rearrangement
with disruption of the central Si−Si bond in 10 and
reorientation of the fragments.
In contrast, in cation 12, the central backbone of germasilane
0 is conserved and the reorientation of the silyl and methyl
conducted in chlorobenzene at T = −30 °C and produced
1
almost selectively a polysilanyl cation with a highly symmetric
29
groups must have taken place by successive 1,2-shifts. The
reaction of digermahexasilane 10 with trityl cation results in the
formation a thermally unstable cation. Even at reaction
temperatures as low as −40 °C, we noted severe decomposition
of the formed product (see Supporting Information). Only the
use of dichloromethane as solvent and reaction temperatures as
low as −95 °C allowed the clean and selective formation of a
new cation, which was identified by 29Si NMR spectroscopy at
−80 °C.
structure. The formed cation is characterized by only three Si
2
9
1
29
NMR resonances in the Si{ H} NMR spectrum at δ Si =
08.9, −6.4, and −120.9 (Scheme 3 and Figure 1). In the
1
29
proton-coupled Si INEPT NMR spectrum, the high-field
29
signal at δ Si = −120.9 appears in a spectral region that is
typical for tetrasila-substituted silicon atoms and is only
broadened by unresolved couplings to distant hydrogen atoms.
The resonance at δ Si = −6.4 is a multiplet in the Si INEPT
spectrum. Its chemical shift region suggests an assignment as a
SiMe group. The low-field signal at δ Si = 108.9 reveals a
doublet of septets pattern with coupling constants of J(SiH) =
3 Hz and J(SiH) = 5 Hz. This chemical shift and the small
size of the J(SiH) coupling constant are characteristic of
34
29
29
2
9
29
29
29
Only two Si NMR signals at δ Si = 117.6 and δ Si = 0.9
suggested a highly symmetric structure for the produced cation.
Decisive for the structural assignment was the low-field signal
3
1
3
4
1
29
with a Si NMR chemical shift, which is characteristic for a
2
9,35−45
1
29
29
cationic Si−H−Si groups.
spectrum, this signal correlates with a broad singlet in the H
NMR spectrum at δ H = 2.15. The analysis of Si satellites for
In the H Si HMQC
cationic Si−H−Si unit (δ Si = 117.6) and which is a doublet in
1
29
the Si INEPT spectrum with a Si−H coupling constant of
1
29
1
35
29
J(SiH) = 45 Hz. In addition, this Si resonance shows cross-
B
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Figure 1. 29Si{ H} NMR spectrum (99 MHz, C D Cl, T = −30 °C) of the reaction mixture after hydride abstraction from oligosilane 5. Insets are
1
6
5
29
29
1
taken from the Si INEPT (top trace) and the Si{ H} NMR (bottom trace) spectrum (*unidentified compound).
Scheme 4. Ionization of Digermaoligosilane 10 To Give Bis-
silylhydronium Ion 11
Figure 2. Structure elucidation of cation 8 based on the analysis of
1
1
29
detected J couplings (left) and H/ Si correlations based on HMQC
and HMBC spectra.
Table 1. Experimental and Calculated (in Italics) NMR
a
Parameters of Cations 7, 8, 11, and 12
2
9
1
29
2
29
3
cation
δ Si(Si )
δ Si(Si )
δ Si(Si )
and experimental data further substantiated the assignment of
the signals (see Table 1).
b
1
8
8
8
7
1
1
1
1
108.9, J(SiH) = 43 Hz
−120.9
−124.8
−122.0
−33.3
−6.4
−5.0
−5.7
−3.7
0.4
c
1
f
105.9, J(SiH) = 40 Hz
Based on these experimental results, we suggest the following
mechanism for the formation of the characterized cation 8 and
likewise for its digerma analogue 11 (Scheme 5). The logical
starting compound for the reaction sequence is cation anti-6,
which undergoes a 1,2-silyl shift with disruption of the central
Si−Si bond to give the open cation 13 in its syn-conformation.
The subsequent formation of the hydrogen-bridged cation 14
allows for a facile 1,4-hydride shift to give the open cation syn-
c
(PhCl)
c
103.4
44.8
d
1
1
117.3, J(SiH) = 45 Hz
e
1
1
117.6, J(SiH) = 45 Hz
0.9
c
1
f
1
120.8, J(SiH) = 42 Hz
−3.4
−11
c
2
−93
a
b
For the assignments, see Schemes 3 and 4. At −30 °C in C D Cl.
Calculated at GIAO/M06-L/6-311G(2d,p)//M06-2X/6-311+G-
d,p). At −40 °C in C D Cl. At −80 °C in CD Cl . Calculated
at B3LYP/IGLOIII (Si,C,H), 6-311+G(d,p) (Ge).
6
5
c
1
5. A second 1,2-silyl shift followed by rotation around the
d
e
f
(
6
5
2
2
central Si−Si bond in anti-16 results in the formation of the
cyclic bis-silylated hydronium ion 8.
4
7,48
The results of quantum mechanical computations
provided (i) important insights with respect to the thermody-
namic driving force for the formation of the branched cations 8
and 11 and (ii) the basis for a deeper mechanistic discussion of
the reaction sequence shown in Scheme 5. In the following
discussion, we will concentrate on the rearrangements in the
oligosilanyl system. The results for the digerma analogue are
1
1
1
peaks to H NMR signals at δ H = 2.66 and δ H = 1.02 in the
1
29
1
H/ Si HMBC NMR spectrum (Figure 3). The broad H
2
9
1
NMR signal shows characteristic Si satellites of J(SiH) = 45
Hz. This clearly indicates that the formed species is cation 11.
The good agreement between computed NMR chemical shifts
C
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Figure 3. 29Si{ H} NMR spectrum (99 MHz, CD Cl , T = −80 °C) of the reaction mixture after hydride abstraction from digermaoligosilane 10.
1
2
2
29
1
29
Insets are taken from the Si INEPT spectrum and the H/ Si HMBC NMR spectrum.
Scheme 5. Suggested Reaction Mechanism for the
Formation of Cation 8
quite similar with respect to the relative energies of the isomers
and their calculated structures and are given in the Supporting
Information. The close similarities between the persila- and the
digermasila systems indicate that the perturbation induced by
the replacement of silicon atoms by germanium atoms is not
likely to change our conclusions. As expected, the results of
density functional theory (DFT) calculations at the M06-2X/6-
Figure 4. Relative energies of isomers 7, 8, and 14, calculated using
different density functionals. Values obtained at B3LYP (red) do not
include dispersion interactions. B3LYP/D3 (black) correct the B3LYP
values for dispersion interaction using Grimme’s D3 function. The
M06-2X functional includes dispersion interaction (blue). All
calculations use the 6-311+G(d,p) basis set.
311+G(d,p) level for the gas-phase indicate a significant
stabilization of all investigated polysilanyl cations by 54−62
−1
kJ mol due to the formation of Si−H−Si linkages (6/7, 54 kJ
substituent effects of the trimethylsilyl groups in cations 7 and
8 suggests that cation 7 is more stable than the branched cation
8. This fact raises the question about the driving force for the
selective formation of cation 8 in the experiment. Furthermore,
the accumulation of four trimethylsilyl groups at the central Si−
Si unit in cation 8 seemed to us to be a rather unfavorable
situation for steric reasons. On the contrary, attractive
dispersion forces between the vicinal trimethylsilyl groups in
cation 8 could stabilize this cation compared to its isomers 7
−1
−1
−1
mol ; 13/14, 62 kJ mol ; 16/8, 58 kJ mol ).
The most stable hydrogen-bridged cation, the branched
−1
cation 8, is more stable by 16 kJ mol than the intermediate
−1
cation 14 and more stable by 30 kJ mol than cation 7, and it
is the most stable cation that we located on the potential energy
surface (PES) (see Figure 4, blue). We found this result rather
surprising because the quantitative evaluation of substituent
effects on the stability of cations having Si−H−Si units
indicated that substitution with four trimethylsilyl groups at the
two silicon atoms (α-substitution, as in cation 7) stabilizes this
32,49−51
and 14.
The decisive role of attractive dispersion forces
between large, polarizable silyl groups for the formation of
−1
50
type of cation by 37 kJ mol compared to tetramethyl
tetrylene dimers was demonstrated recently by our groups
48
51
substitution, as detailed in the Supporting Information.
and by Nagase and Power. In order to quantify the relative
Substitution with four trimethylsilyl groups in the β-position
to the Si−H−Si (as in cation 8) is less favorable and stabilizes
stabilization of cations 7, 8, and 14 by attractive dispersion
forces, we optimized their molecular structures at the B3LYP/
6-311+G(d,p) level. This DFT method practically neglects
−1
only by 24 kJ mol . Consequently, comparison of the different
D
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Figure 5. Approximate reaction coordinate for the formation of the hydrogen-bridged cation 8 starting from the complex between cation 6 and
rel
rel
PhCl. The relative energies E (black) and the relative Gibbs energies G (298) at 298 K (blue) of the aggregates between cations and solvent are
computed using the M06-2X/6-311+G(d,p) functional and are given relative to the energy of the complex 6(PhCl). The influence of the solvent was
modeled by SCRF calculations using chlorobenzene as solvent. The barriers shown are drawn arbitrarily; see the Supporting Information for details.
For cation 14, only complex 14b(PhCl) with the lowest energy is shown. Complex 14a(PhCl) (coordination via the SiMe group) is higher in
2
−1
energy by 5 kJ mol .
3
2
dispersion interactions. These results were compared with
those obtained using the same functional but including
Grimme’s D3 function, which adds dispersion corrections to
driving force. Nevertheless, the selective and exclusive
formation of cation 8 with no indications for the presence of
cation 14 contrasts the experimental finding for the actual
rearrangement reaction (Scheme 2), with both oligosilanes 3
52,53
the original B3LYP functional.
Interestingly, the B3LYP
31
calculations predict cation 14, which is putatively the sterically
least congested one, to be the most stable compound in this
series (see Figure 4, red). The outcome of the B3LYP/D3
calculations alters the relative order of stability between cation
and 4 formed initially in equal amounts. In an attempt to
tackle this aspect, we started a detailed computational
investigation of the PES connecting the isomeric cations 7, 8,
and 14. The result of these calculations revealed a delicate
balance between inter- and intramolecular stabilization of the
silyl cation center. As shown above, the formation of Si−H−Si
hydrogen bridges stabilizes the cations 7, 8, and 14 significantly.
For these cations, direct coordination of the positively charged
silicon atom to solvent molecules is not of chemical significance
1
4 and the branched cation 8 (see Figure 4, black). In addition,
we note that the relative stability order for the three cations is
the same at B3LYP/D3 as at the M06-2X level (see Figure 4,
blue). Both methods predict the branched cation 8 to be the
most stable one, which is in qualitative agreement with the
exclusive NMR detection of cation 8 in the experiment. The
fact that only DFT methods, which explicitly take dispersion
energy contributions into account, are able to confirm the
experimentally determined relative stability order hints at the
importance of dispersion energy contributions in this system.
The comparison between the three isomeric structures suggests
that London dispersion forces are maximized in cation 8 and
overrule unfavorable substituent effects (compared to cation 7)
and disadvantageous steric repulsion (relative to cation 14). As
a consequence, the attractive dispersion forces between the
vicinal trimethylsilyl substituents in the branched cation 8
provide the thermodynamic driving force for its exclusive
formation in the experiment. There is a clear relation also to the
rearrangement reaction shown in Scheme 2. In the framework
of our model chemistry, which replaces selected methyl groups
with hydrogen atoms, cations 7, 8, and 14 are placeholders for
the linear starting material 2 and the branched oligosilanes 3
and 4. Therefore, the determined relative stability order in the
cations 7, 8, and 14 clearly reflects the observed rearrangement
chemistry shown in Scheme 2. This fact suggests that
dispersion energy contributions strongly influence the relative
stability order of the intermediate cations of this rearrangement
and are at least important components to its thermodynamic
diss
even in solution. The calculated dissociation energies ΔE for
the complexes of the three hydrogen-bridged cations with
diss
−1
chlorobenzene (PhCl) are small (ΔE = 39 kJ mol (8), 43
−1
kJ mol (14a, coordination via the SiMe group) and 47 kJ
mol (14b, coordination via the Si(SiMe ) group), and 44 kJ
2
−
1
3
2
−1
54
mol (7)). In all three cases, the main structural features of
the hydrogen-bridged cations are conserved also in the PhCl
complex. This is shown, for example, by the close agreement
between the calculated NMR parameter for the gas-phase
structure of cation 8 and for the complex 8(PhCl) (see Table 1
and Supporting Information for a structural comparison). In
addition, entropy effects favor the dissociation of these
aggregates into cation and solvent molecule, and their Gibbs
diss
energy of dissociation, ΔG (298), is calculated to be negative
diss
−1
−1
(ΔG (298) = −14 kJ mol (8), −9 and −10 kJ mol (14),
−
1
and −11 kJ mol (7)), which negates their chemical
55
significance.
The situation is different, however, for their open isomers
with tricoordinated positively charged silicon atoms. Structure
optimizations for cations such as anti-6 resulted either in
rearrangements (e.g., to cation syn-13) or in cations which
show clear structural indications for stabilization by remote
methyl or silyl groups (see Supporting Information for details
E
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Article
and examples). The interaction energies for the latter
stabilization modes are significantly smaller than that computed
for the formation of Si−H−Si bridges. Although these
calculated structures are important for gas-phase investigations,
in solution, intermolecular interaction with solvent molecules
represses the intramolecular stabilization. As a consequence, for
these non-hydrogen-bridged cations, explicit coordination to
solvent molecules has to be taken into account. Therefore, we
determined the relative energy of the intermediates for the
proposed reaction pathway shown in Scheme 5 in the form of
their PhCl complexes and used the polarized continuum
maximized in the detected products. The course of Wagner−
Meerwein rearrangements in carbon chemistry is usually
discussed in terms of carbocation stabilities that are defined
by substituent effects, such as resonance and hyperconjugation.
Due to the increased size of the silicon atoms, these effects are
not as pronounced in polysilanyl cations as in carbocations. The
results of our combined experimental/computational inves-
tigation indicate instead that attractive dispersion forces
between large polarizable silyl substituents play a dominant
role in skeletal rearrangement reactions of oligosilanes and
determine the configuration of the products.
5
6
model to include the general effects of solvation (Figure 5).
In an extended computational search for transition states
connecting these cationic intermediates, we faced, on the one
hand, a very flat potential energy surface on which no transition
state relevant to the investigated chemical transformations
could be located. All barriers found belonged to rotations of
methyl or silyl groups. On the other hand, on a PES on which
topology is dominated by weak rotation modes, no other large
barriers are to be expected. This allows an approximate
discussion of the PES based on the different energies of the
intermediates of this reaction sequence. The computed reaction
coordinate shown in Figure 5 reveals an energy difference of
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b03560.
■
*
S
Computed structures of Figure 4 (XYZ)
Computed structures of Figure 5 (XYZ)
Computed structures of Figure S15 (XYZ)
Computed structures of Figure S16 (XYZ)
Computed structures of Figure S17 (XYZ)
Computed structures of Figure S18 (XYZ)
Experimental and theoretical characterization, including
preparation of all compounds of interest and computa-
tional details (PDF)
−1
about 30−40 kJ mol between the more stable hydrogen-
bridged cations 7, 8, and 14 and their open isomers 6, 13, 15,
and 16 (all in the form of their PhCl complexes). The least
stable isomer in this series is 6(PhCl), which is higher in energy
by 60 kJ mol− compared to the final product, the hydrogen-
bridged cation 8. This relative small energy difference suggests
that even at temperatures as low as −30 °C (the temperature of
the experiment) the thermodynamic equilibrium between the
hydrogen-bridged cations via their open isomers is fully
established and explains the selective formation of the most
stable cation, the branched hydrogen-bridged cation 8. In order
to put the results for our model system into relation to the
preparative important rearrangement chemistry of Scheme 2, it
is interesting to note that the energy difference between cation
solvent complexes 14(PhCl) and 8(PhCl) is relatively small
1
AUTHOR INFORMATION
Corresponding Authors
thomas.mueller@uni-oldenburg.de
christoph.marschner@tugraz.at
baumgartner@tugraz.at
Present Address
Lena Albers) School of Chemistry, University of Edinburgh,
Edinburgh EH9 3FJ, UK.
■
*
*
*
⊥
(
Notes
The authors declare no competing financial interest.
−
1
(
ΔE = 11 kJ mol in favor of cation 8). Thermal and entropy
effects favor the dissociation of both PhCl complexes, and the
Gibbs energy difference at 298 K, ΔG(298), between the cation
4 and 8 decreases to 6 kJ mol . Against the background
ACKNOWLEDGMENTS
This work was supported by the ERA-chemistry program
DFG Mu1440/8-1, FWF-I00669) and by the Carl von
Ossietzky University Oldenburg. The simulations were
performed at the HPC Cluster HERO (High End Computing
Resource Oldenburg), located at the University of Oldenburg
■
(
−1
57
1
that cation 14 and 8 are placeholders for the oligosilanes 3 and
4
, it is reasonable to assume that at higher temperatures or
58
different solvents a mixture of products is obtained.
(
Germany), and funded by the DFG through its Major
CONCLUSIONS
■
Research Instrumentation Program (INST 184/108-1 FUGG)
and the Ministry of Science and Culture (MWK) of the Lower
Saxony State.
We investigated the Lewis acid induced rearrangement reaction
of linear oligosilanes to give new structures of higher
complexity. We used dihydrogen-substituted starting silanes
for our stoichiometric model reactions, which allow the
targeted formation of cationic intermediates by cleavage of
one Si−H bond and their stabilization by formation of
Si−H−Si bonds with the second Si−H bond. The cationic
products of the rearrangement reaction were characterized at
low temperatures by multinuclear NMR spectroscopy.
Substitution experiments identified the cleavage of the central
Si−Si bond of the oligosilane backbone as the key step for the
production of an oligosilane structure with a higher number of
tetrasila-substituted silicon atoms. According to accompanying
DFT calculations, a major contributors to the thermodynamic
driving force for this reaction are attractive dispersion forces
between the polarizable trimethylsilyl groups, which are
REFERENCES
(1) Moore, G. E. Electronics 1965, 38, 4.
■
(2) Sekiguchi, A.; Nagase, S. In The Chemistry of Organic Silicon
Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, 1998;
Vol. 2, p 119.
̈
(3) Hengge, E.; Stuger, H. In The Chemistry of Organic Silicon
Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, 1998;
Vol. 2, p 2177.
4) Wiberg, N.; Finger, C. M. M.; Polborn, K. Angew. Chem., Int. Ed.
Engl. 1993, 32, 1054.
5) Ichinohe, M.; Toyoshima, M.; Kinjo, R.; Sekiguchi, A. J. Am.
(
(
Chem. Soc. 2003, 125, 13328.
(6) Sekiguchi, A.; Yatabe, T.; Kabuto, C.; Sakurai, H. J. Am. Chem.
Soc. 1993, 115, 5853.
F
DOI: 10.1021/jacs.6b03560
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(
7) Abersfelder, K.; Russell, A.; Rzepa, H. S.; White, A. J. P.;
Haycock, P. R.; Scheschkewitz, D. J. Am. Chem. Soc. 2012, 134, 16008.
8) Matsumoto, H.; Higuchi, K.; Hoshino, Y.; Koike, H.; Naoi, Y.;
Nagai, Y. J. Chem. Soc., Chem. Commun. 1988, 1083.
9) Sekiguchi, A.; Yatabe, T.; Kamatani, H.; Kabuto, C.; Sakurai, H. J.
Am. Chem. Soc. 1992, 114, 6260.
10) Matsumoto, H.; Higuchi, K.; Kyushin, S.; Goto, M. Angew.
Chem., Int. Ed. Engl. 1992, 31, 1354.
11) Nied, D.; Koppe, R.; Klopper, W.; Schno
Am. Chem. Soc. 2010, 132, 10264.
12) Iwamoto, T.; Tsushima, D.; Kwon, E.; Ishida, S.; Isobe, H.
Angew. Chem., Int. Ed. 2012, 51, 2340.
13) Fischer, G.; Huch, V.; Mayer, P.; Vasisht, S. K.; Veith, M.;
Wiberg, N. Angew. Chem., Int. Ed. 2005, 44, 7884.
(45) Kordts, N.; Borner, C.; Panisch, R.; Saak, W.; Mu
Organometallics 2014, 33, 1492.
(46) Stella, F. Synthesis, Rearrangement and Further Reactions of
Organosilanes and Organostannes, Dissertation, Technical University
Graz, 2016.
(47) All calculations were done using the Gaussian 09 program,
version B1.
(48) For details, see the Supporting Information.
(49) Grimme, S.; Schreiner, P. R. Angew. Chem. 2011, 123, 12849.
̈
ller, T.
(
(
(
(
̈
̈
ckel, H.; Breher, F. J.
(50) Arp, H.; Baumgartner, J.; Marschner, C.; Zark, P.; Mu
Am. Chem. Soc. 2012, 134, 6409.
̈
ller, T. J.
(
(51) Guo, J.-D.; Liptrot, D. J.; Nagase, S.; Power, P. P. Chem. Sci.
2015, 6, 6235.
(52) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys.
(
(
(
14) Scheschkewitz, D. Angew. Chem., Int. Ed. 2005, 44, 2954.
15) Abersfelder, K.; White, A. J. P.; Berger, R. J. F.; Rzepa, H. S.;
2010, 132, 154104.
(53) Grimme, S. WIREs Comput. Mol. Sci. 2011, 1, 211.
diss
(
54) ΔE is calculated as the difference between the electronic
Scheschkewitz, D. Angew. Chem., Int. Ed. 2011, 50, 7936.
16) Abersfelder, K.; White, A. J. P.; Rzepa, H. S.; Scheschkewitz, D.
Science 2010, 327, 564.
17) Suzuki, K.; Matsuo, T.; Hashizume, D.; Fueno, H.; Tanaka, K.;
Tamao, K. Science 2011, 331, 1306.
18) Ishida, S.; Otsuka, K.; Toma, Y.; Kyushin, S. Angew. Chem., Int.
Ed. 2013, 52, 2507.
19) Tsurusaki, A.; Koganezono, M.; Otsuka, K.; Ishida, S.; Kyushin,
S. Chem. - Eur. J. 2014, 20, 9263.
20) Tsurusaki, A.; Iizuka, C.; Otsuka, K.; Kyushin, S. J. Am. Chem.
Soc. 2013, 135, 16340.
21) Tillmann, J.; Wender, J. H.; Bahr, U.; Bolte, M.; Lerner, H. W.;
Holthausen, M. C.; Wagner, M. Angew. Chem., Int. Ed. 2015, 54, 5429.
22) Marschner, C. In Science of Synthesis, Knowledge Updates 2013;
Oestreich, M., Ed.; Thieme Verlag: Stuttgart, 2013; Vol. 2, p 109.
energy of the optimized structure of the complex between the cation
and PhCl and the sum of the electronic energies of the constituents
(
(
cation and PhCl).
(
(
55) ΔGdiss(298) is calculated as the difference between the Gibbs
energy of the optimized structure of the complex between the cation
and PhCl at 298 K and the sum of the Gibbs energy of the
constituents (cation and PhCl).
(
(
(
2
(
56) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105,
999.
57) The Gibbs energy difference between cation 8 and cation 14 of
(
−1
6
kJ mol suggests that at 243 K ≈5% of cation 14 could be present in
(
29
equilibrium. The quality of the Si NMR spectra shown in Figure 1
does not allow, however, a clear assignment. A graphical comparison
between calculated 29Si NMR spectra for cations 8 and 14 with the
experimentally spectrum obtained for the reaction mixture is provided
in the Suppporting Information (Figure S20).
(
(
(
(
23) Ishikawa, M.; Kumada, M. J. Chem. Soc. D 1969, 567.
24) Ishikawa, M.; Kumada, M. J. Chem. Soc. D 1970, 157.
25) Ishikawa, M.; Iyoda, J.; Ikeda, H.; Kotake, K.; Hashimoto, T.;
(58) The Gibbs free energy differences between silanes 3 and 4
calculated at 298 K for the gas phase also do not differ significantly;
Kumada, M. J. Am. Chem. Soc. 1981, 103, 4845.
26) Ishikawa, M.; Watanabe, M.; Iyoda, J.; Ikeda, H.; Kumada, M.
Organometallics 1982, 1, 317.
298
that is, G values relative to silane 2 are calculated as follows:
(
298
−1
298
−1
ΔG (3) = −6 kJ mol and ΔG (4) = −24 kJ mol .
(
(
27) Blinka, T. A.; West, R. Organometallics 1986, 5, 128.
28) Albers, L.; Meshgi, M. A.; Baumgartner, J.; Marschner, C.;
Mu
29) Albers, L.; Baumgartner, J.; Marschner, C.; Mu
J. 2016, DOI: 10.1002/chem.20160116.
30) Fischer, J.; Baumgartner, J.; Marschner, C. Science 2005, 310,
25.
31) Wagner, H.; Baumgartner, J.; Marschner, C.; Poelt, P.
Organometallics 2011, 30, 3939.
32) Wagner, J. P.; Schreiner, P. R. Angew. Chem., Int. Ed. 2015, 54,
2274.
33) Kayser, C.; Kickelbick, G.; Marschner, C. Angew. Chem., Int. Ed.
002, 41, 989.
34) Krempner, C. Polymers 2012, 4, 408.
35) Muller, T. In Structure and Bonding; Scheschkewitz, D., Ed.;
Springer: Berlin, 2014; Vol. 155, p 107.
̈
ller, T. Organometallics 2015, 34, 3756.
(
̈
ller, T. Chem. Eur.
(
8
(
(
1
(
2
(
(
̈
(
36) Mu
̈
ller, T. Angew. Chem., Int. Ed. 2001, 40, 3033.
ller, T. J. Am. Chem. Soc. 2006, 128,
(
37) Panisch, R.; Bolte, M.; Mu
̈
9
(
2
(
676.
38) Sekiguchi, A.; Murakami, Y.; Fukaya, N.; Kabe, Y. Chem. Lett.
004, 33, 530.
39) Khalimon, A. Y.; Lin, Z. H.; Simionescu, R.; Vyboishchikov, S.
F.; Nikonov, G. I. Angew. Chem., Int. Ed. 2007, 46, 4530.
40) Hoffmann, S. P.; Kato, T.; Tham, F. S.; Reed, C. A. Chem.
Commun. 2006, 767.
(
(
41) Nava, M.; Reed, C. A. Organometallics 2011, 30, 4798.
(
42) Connelly, S. J.; Kaminsky, W.; Heinekey, D. M. Organometallics
2
(
013, 32, 7478.
43) Bolli, C.; Derendorf, J.; Jenne, C.; Scherer, H.; Sindlinger, C. P.;
Wegener, B. Chem. - Eur. J. 2014, 20, 13783.
44) Schafer, A.; Reißmann, M.; Schafer, A.; Schmidtmann, M.;
Muller, T. Chem. - Eur. J. 2014, 20, 9381.
(
̈
̈
̈
G
DOI: 10.1021/jacs.6b03560
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX